HOW A BLAST FURNACE WORKS

Introduction

The purpose of a blast furnace is to chemically reduce and physically convert iron oxides into liquid iron called "hot metal". The blast furnace is a huge, steel stack lined with refractory brick, where iron ore, coke and limestone are dumped into the top, and preheated air is blown into the bottom. The raw materials require 6 to 8 hours to descend to the bottom of the furnace where they become the final product of liquid slag and liquid iron. These liquid products are drained from the furnace at regular intervals. The hot air that was blown into the bottom of the furnace ascends to the top in 6 to 8 seconds after going through numerous chemical reactions. Once a blast furnace is started it will continuously run for four to ten years with only short stops to perform planned maintenance.

The Process

Iron oxides can come to the blast furnace plant in the form of raw ore, pellets or sinter. The raw ore is removed from the earth and sized into pieces that range from 0.5 to 1.5 inches. This ore is either Hematite (Fe2O3) or Magnetite (Fe3O4) and the iron content ranges from 50% to 70%. This iron rich ore can be charged directly into a blast furnace without any further processing. Iron ore that contains a lower iron content must be processed or beneficiated to increase its iron content. Pellets are produced from this lower iron content ore. This ore is crushed and ground into a powder so the waste material called gangue can be removed. The remaining iron-rich powder is rolled into balls and fired in a furnace to produce strong, marble-sized pellets that contain 60% to 65% iron. Sinter is produced from fine raw ore, small coke, sand-sized limestone and numerous other steel plant waste materials that contain some iron. These fine materials are proportioned to obtain a desired product chemistry then mixed together. This raw material mix is then placed on a sintering strand, which is similar to a steel conveyor belt, where it is ignited by gas fired furnace and fused by the heat from the coke fines into larger size pieces that are from 0.5 to 2.0 inches. The iron ore, pellets and sinter then become the liquid iron produced in the blast furnace with any of their remaining impurities going to the liquid slag.

The coke is produced from a mixture of coals. The coal is crushed and ground into a powder and then charged into an oven. As the oven is heated the coal is cooked so most of the volatile matter such as oil and tar are removed. The cooked coal, called coke, is removed from the oven after 18 to 24 hours of reaction time. The coke is cooled and screened into pieces ranging from one inch to four inches. The coke contains 90 to 93% carbon, some ash and sulfur but compared to raw coal is very strong. The strong pieces of coke with a high energy value provide permeability, heat and gases which are required to reduce and melt the iron ore, pellets and sinter.

The final raw material in the ironmaking process in limestone. The limestone is removed from the earth by blasting with explosives. It is then crushed and screened to a size that ranges from 0.5 inch to 1.5 inch to become blast furnace flux . This flux can be pure high calcium limestone, dolomitic limestone containing magnesia or a blend of the two types of limestone.

Since the limestone is melted to become the slag which removes sulfur and other impurities, the blast furnace operator may blend the different stones to produce the desired slag chemistry and create optimum slag properties such as a low melting point and a high fluidity.

All of the raw materials are stored in an ore field and transferred to the stockhouse before charging. Once these materials are charged into the furnace top, they go through numerous chemical and physical reactions while descending to the bottom of the furnace.

The iron ore, pellets and sinter are reduced which simply means the oxygen in the iron oxides is removed by a series of chemical reactions. These reactions occur as follows:

1) 3 Fe2O3 + CO = CO2 + 2 Fe3O4 Begins at 850° F

2) Fe3O4 + CO = CO2 + 3 FeO Begins at 1100° F

3) FeO + CO = CO2 + Fe Begins at 1300° F or FeO + C = CO + Fe

At the same time the iron oxides are going through these purifying reactions, they are also beginning to soften then melt and finally trickle as liquid iron through the coke to the bottom of the furnace.

The coke descends to the bottom of the furnace to the level where the preheated air or hot blast enters the blast furnace. The coke is ignited by this hot blast and immediately reacts to generate heat as follows: C + O2 = CO2 + Heat

Since the reaction takes place in the presence of excess carbon at a high temperature the carbon dioxide is reduced to carbon monoxide as follows:

CO2+ C = 2CO

The product of this reaction, carbon monoxide, is necessary to reduce the iron ore as seen in the previous iron oxide reactions.

The limestone descends in the blast furnace and remains a solid while going through its first reaction as follows:

CaCO3 = CaO + CO2

This reaction requires energy and starts at about 1600°F. The CaO formed from this reaction is used to remove sulfur from the iron which is necessary before the hot metal becomes steel. This sulfur removing reaction is:

FeS + CaO + C = CaS + FeO + CO

The CaS becomes part of the slag. The slag is also formed from any remaining Silica (SiO2), Alumina (Al2O3), Magnesia (MgO) or Calcia (CaO) that entered with the iron ore, pellets, sinter or coke. The liquid slag then trickles through the coke bed to the bottom of the furnace where it floats on top of the liquid iron since it is less dense.

Another product of the ironmaking process, in addition to molten iron and slag, is hot dirty gases. These gases exit the top of the blast furnace and proceed through gas cleaning equipment where particulate matter is removed from the gas and the gas is cooled. This gas has a considerable energy value so it is burned as a fuel in the "hot blast stoves" which are used to preheat the air entering the blast furnace to become "hot blast". Any of the gas not burned in the stoves is sent to the boiler house and is used to generate steam which turns a turbo blower that generates the compressed air known as "cold blast" that comes to the stoves.

In summary, the blast furnace is a counter-current realtor where solids descend and gases ascend. In this reactor there are numerous chemical and physical reactions that produce the desired final product which is hot metal. A typical hot metal chemistry follows:

Iron (Fe) = 93.5 - 95.0% Silicon (Si) = 0.30 - 0.90% Sulfur (S) = 0.025 - 0.050% Manganese (Mn) = 0.55 - 0.75% Phosphorus (P) = 0.03 - 0.09% Titanium (Ti) = 0.02 - 0.06% Carbon (C) = 4.1 - 4.4%

The Blast Furnace Plant

Now that we have completed a description of the ironmaking process, let s review the physical equipment comprising the blast furnace plant.

There is an ore storage yard that can also be an ore dock where boats and barges are unloaded. The raw materials stored in the ore yard are raw ore, several types of pellets, sinter, limestone or flux blend and possibly coke. These materials are transferred to the "stockhouse/hiline" (17) complex by ore bridges equipped with grab buckets or by conveyor belts. Materials can also be brought to the stockhouse/hiline in rail hoppers or transferred from ore bridges to self-propelled rail cars called "ore transfer cars". Each type of ore, pellet, sinter, coke and limestone is dumped into separate "storage bins" (18). The various raw materials are weighed according to a certain recipe designed to yield the desired hot metal and slag chemistry. This material weighing is done under the storage bins by a rail mounted scale car or computer controlled weigh hoppers that feed a conveyor belt. The weighed materials are then dumped into a "skip" car (19) which rides on rails up the "inclined skip bridge" to the "receiving hopper" (6) at the top of the furnace. The cables lifting the skip cars are powered from large winches located in the "hoist" house (20). Some modern blast furnace accomplish the same job with an automated conveyor stretching from the stockhouse to the furnace top.

At the top of the furnace the materials are held until a "charge" usually consisting of some type of metallic (ore, pellets or sinter), coke and flux (limestone) have accumulated. The precise filling order is developed by the blast furnace operators to carefully control gas flow and chemical reactions inside the furnace. The materials are charged into the blast furnace through two stages of conical "bells" (5) which seal in the gases and distribute the raw materials evenly around the circumference of the furnace "throat". Some modern furnaces do not have bells but instead have 2 or 3 airlock type hoppers that discharge raw materials onto a rotating chute which can change angles allowing more flexibility in precise material placement inside the furnace.

Also at the top of the blast furnace are four "uptakes" (10) where the hot, dirty gas exits the furnace dome. The gas flows up to where two uptakes merge into an "offtake" (9). The two offtakes then merge into the "downcomer" (7). At the extreme top of the uptakes there are "bleeder valves" (8) which may release gas and protect the top of the furnace from sudden gas pressure surges. The gas descends in the downcomer to the "dustcatcher", where coarse particles settle out, accumulate and are dumped into a railroad car or truck for disposal. The gas then flows through a "Venturi Scrubber" (4) which removes the finer particles and finally into a "gas cooler" (2) where water sprays reduce the temperature of the hot but clean gas. Some modern furnaces are equipped with a combined scrubber and cooling unit. The cleaned and cooled gas is now ready for burning.

The clean gas pipeline is directed to the hot blast "stove" (12). There are usually 3 or 4 cylindrical shaped stoves in a line adjacent to the blast furnace. The gas is burned in the bottom of a stove and the heat rises and transfers to refractory brick inside the stove. The products of combustion flow through passages in these bricks, out of the stove into a high "stack" (11) which is shared by all of the stoves.

Large volumes of air, from 80,000 ft3/min to 230,000 ft3/min, are generated from a turbo blower and flow through the "cold blast main" (14) up to the stoves. This cold blast then enters the stove that has been previously heated and the heat stored in the refractory brick inside the stove is transferred to the "cold blast" to form "hot blast". The hot blast temperature can be from 1600°F to 2300°F depending on the stove design and condition. This heated air then exits the stove into the "hot blast main" (13) which runs up to the furnace. There is a "mixer line" (15) connecting the cold blast main to the hot blast main that is equipped with a valve used to control the blast temperature and keep it constant. The hot blast main enters into a doughnut shaped pipe that encircles the furnace, called the "bustle pipe" (13). From the bustle pipe, the hot blast is directed into the furnace through nozzles called "tuyeres" (30) (pronounced "tweers"). These tuyeres are equally spaced around the circumference of the furnace. There may be fourteen tuyeres on a small blast furnace and forty tuyeres on a large blast furnace. These tuyeres are made of copper and are water cooled since the temperature directly in front of the them may be 3600°F to 4200°F. Oil, tar, natural gas, powdered coal and oxygen can also be injected into the furnace at tuyere level to combine with the coke to release additional energy which is necessary to increase productivity. The molten iron and slag drip past the tuyeres on the way to the furnace hearth which starts immediately below tuyere level.

Around the bottom half of the blast furnace the "casthouse" (1) encloses the bustle pipe, tuyeres and the equipment for "casting" the liquid iron and slag. The opening in the furnace hearth for casting or draining the furnace is called the "iron notch" (22). A large drill mounted on a pivoting base called the "taphole drill" (23) swings up to the iron notch and drills a hole through the refractory clay plug into the liquid iron. Another opening on the furnace called the "cinder notch" (21) is used to draw off slag or iron in emergency situations. Once the taphole is drilled open, liquid iron and slag flow down a deep trench called a "trough" (28). Set across and into the trough is a block of refractory, called a "skimmer", which has a small opening underneath it. The hot metal flows through this skimmer opening, over the "iron dam" and down the "iron runners" (27). Since the slag is less dense than iron, it floats on top of the iron, down the trough, hits the skimmer and is diverted into the "slag runners" (24). The liquid slag flows into "slag pots" (25) or into slag pits (not shown) and the liquid iron flows into refractory lined "ladles" (26) known as torpedo cars or sub cars due to their shape. When the liquids in the furnace are drained down to taphole level, some of the blast from the tuyeres causes the taphole to spit. This signals the end of the cast, so the "mudgun" (29) is swung into the iron notch. The mudgun cylinder, which was previously filled with a refractory clay, is actuated and the cylinder ram pushes clay into the iron notch stopping the flow of liquids. When the cast is complete, the iron ladles are taken to the steel shops for processing into steel and the slag is taken to the slag dump where it is processed into roadfill or railroad ballast. The casthouse is then cleaned and readied for the next cast which may occur in 45 minutes to 2 hours. Modern, larger blast furnaces may have as many as four tapholes and two casthouses. It is important to cast the furnace at the same rate that raw materials are charged and iron/slag produced so liquid levels can be maintained in the hearth and below the tuyeres. Liquid levels above the tuyeres can burn the copper casting and damage the furnace lining.

CONCLUSION

The blast furnace is the first step in producing steel from iron oxides. The first blast furnaces appeared in the 14th Century and produced one ton per day. Blast furnace equipment is in continuous evolution and modern, giant furnaces produce 13,000 tons per day. Even though equipment is improved and higher production rates can be achieved, the processes inside the blast furnace remain the same. Blast furnaces will survive into the next millenium because the larger, efficient furnaces can produce hot metal at costs competitive with other iron making technologies. THE BASIC OXYGEN STEELMAKING (BOS) PROCESS

By John Stubbles, Steel Industry Consultant

I INTRODUCTION

Accounting for 60% of the world's total output of crude steel, the Basic Oxygen Steelmaking (BOS) process is the dominant steelmaking technology. In the U.S., that figure is 54% and slowly declining due primarily to the advent of the "Greenfield" electric arc furnace (EAF) flat-rolled mills. However, elsewhere its use is growing.

Figure 1: Charging aisle of a Basic Oxygen Steelmaking Plant showing scrap being charged into the BOF vessel. A ladle full of hot metal is seen to the right.

There exist several variations on the BOS process: top blowing, bottom blowing, and a combination of the two. This study will focus only on the top blowing variation.

The Basic Oxygen Steelmaking process differs from the EAF in that it is autogenous, or self- sufficient in energy. The primary raw materials for the BOP are 70-80% liquid hot metal from the blast furnace and the balance is steel scrap. These are charged into the Basic Oxygen Furnace (BOF) vessel. Oxygen (>99.5% pure) is "blown" into the BOF at supersonic velocities. It oxidizes the carbon and silicon contained in the hot metal liberating great quantities of heat which melts the scrap. There are lesser energy contributions from the oxidation of iron, manganese, and phosphorus. The post combustion of carbon monoxide as it exits the vessel also transmits heat back to the bath.

The product of the BOS is molten steel with a specified chemical anlaysis at 2900°F-3000°F. From here it may undergo further refining in a secondary refining process or be sent directly to the continuous caster where it is solidified into semifinished shapes: blooms, billets, or slabs.

Basic refers to the magnesia (MgO) refractory lining which wears through contact with hot, basic slags. These slags are required to remove phosphorus and sulfur from the molten charge. BOF heat sizes in the U.S. are typically around 250 tons, and tap-to-tap times are about 40 minutes, of which 50% is "blowing time". This rate of production made the process compatible with the continuous casting of slabs, which in turn had an enormous beneficial impact on yields from crude steel to shipped product, and on downstream flat-rolled quality.

II BASIC OPERATION

BOS process replaced open hearth steelmaking. The process predated continuous casting. As a consequence, ladle sizes remained unchanged in the renovated open hearth shops and ingot pouring aisles were built in the new shops. Six-story buildings are needed to house the Basic Oxygen Furnace (BOF) vessels to accommodate the long oxygen lances that are lowered and raised from the BOF vessel and the elevated alloy and flux bins. Since the BOS process increases productivity by almost an order of magnitude, generally only two BOFs were required to replace a dozen open hearth furnaces.

Some dimensions of a typical 250 ton BOF vessel in the U.S. are: height 34 feet, outside diameter 26 feet, barrel lining thickness 3 feet, and working volume 8000 cubic feet. A control pulpit is usually located between the vessels. Unlike the open hearth, the BOF operation is conducted almost "in the dark" using mimics and screens to determine vessel inclination, additions, lance height, oxygen flow etc.

Once the hot metal temperature and chemical analaysis of the blast furnace hot metal are known, a computer charge models determine the optimum proportions of scrap and hot metal, flux additions, lance height and oxygen blowing time. Figure 2: BOF Vessel in Its Operating Positions. (Ref: Making, Shaping, and Treating of Steel, 11th Edition, Steelmaking And Refining Volume. AISE Steel Foundation, 1998, Pittsburgh PA)

A "heat" begins when the BOF vessel is tilted about 45 degrees towards the charging aisle and scrap charge (about 25 to 30% of the heat weight) is dumped from a charging box into the mouth of the cylindrical BOF. The hot metal is immediately poured directly onto the scrap from a transfer ladle. Fumes and kish (graphite flakes from the carbon saturated hot metal) are emitted from the vessel's mouth and collected by the pollution control system. Charging takes a couple of minutes. Then the vessel is rotated back to the vertical position and lime/dolomite fluxes are dropped onto the charge from overhead bins while the lance is lowered to a few feet above the bottom of the vessel. The lance is water-cooled with a multi-hole copper tip. Through this lance, oxygen of greater than 99.5% purity is blown into the mix. If the oxygen is lower in purity, nitrogen levels at tap become unacceptable.

As blowing begins, an ear-piercing shriek is heard. This is soon muffled as silicon from the hot metal is oxidized forming silica, SiO2, which reacts with the basic fluxes to form a gassy molten slag that envelops the lance. The gas is primarily carbon monoxide (CO) from the carbon in the hot metal. The rate of gas evolution is many times the volume of the vessel and it is common to see slag slopping over the lip of the vessel, especially if the slag is too viscous. Blowing continues for a predetermined time based on the metallic charge chemistry and the melt specification. This is typically 15 to 20 minutes, and the lance is generally preprogrammed to move to different heights during the blowing period. The lance is then raised so that the vessel can be turned down towards the charging aisle for sampling and temperature tests. Static charge models however do not ensure consistent turndown at the specified carbon and temperature because the hot metal analysis and metallic charge weights are not known precisely. Furthermore, below 0.2% C, the highly exothermic oxidation of iron takes place to a variable degree along with decarburization. The "drop" in the flame at the mouth of the vessel signals low carbon, but temperature at turndown can be off by +/- 100°F.

Figure 3: Section through the BOF vessel during oxygen blowing. (Ref: Making, Shaping, and Treating of Steel, 11th Edition, Steelmaking And Refining Volume. AISE Steel Foundation, 1998, Pittsburgh PA)

In the past, this meant delays for reblowing or adding coolants. Today, with more operating experience, better computer models, more attention to metallic input quality, and the availability of ladle furnaces that adjust for temperature, turndown control is more consistent. In some shops, sublances provide a temperature-carbon check about two minutes before the scheduled end of the blow. This information permits an "in course" correction during the final two minutes and better turn-down performance. However, operation of sublances is costly, and the required information is not always obtained due to malfunctioning of the sensors.

Once the heat is ready for tapping and the preheated ladle is positioned in the ladle car under the furnace, the vessel is tilted towards the tapping aisle, and steel emerges from the taphole in the upper "cone" section of the vessel. The taphole is generally plugged with material that prevents slag entering the ladle as the vessel turns down. Steel burns through the plug immediately. To minimize slag carryover into the ladle at the end of tapping, various "slag stoppers" have been designed. These work in conjunction with melter's eyeballs, which remain the dominant control device. Slag in the ladle results in phosphorus reversion, retarded desulfurization, and possibly "dirty steel". Ladle additives are available to reduce the iron oxide level in the slag but nothing can be done to alter the phosphorus.

Figure 4: A ladle of molten steel leaving for the ladle metallurgical facility or the caster.

After tapping steel into the ladle, and turning the vessel upside down and tapping the remaining slag into the "slag pot", the vessel is returned to the upright position. In many shops residual slag is blown with nitrogen to coat the barrel and trunion areas of the vessel. This process is known as "slag splashing". Near the end of a campaign, gunning with refractory materials in high wear areas may also be necessary. Once vessel maintenance is complete the vessel is ready to receive the next charge.

III BASIC CHEMISTRY AND HEAT BALANCE

A heat size of 250 tons is used as the basis for the following calculations. This is close to the average heat size for the 50 BOFs which were operable in the U.S. in 1999. The following charge chemistry is assumed:

%C %Si %Mn %S %P %Al Residuals Hot metal 4.5 .75 1.0 .01 .05 0 0 Scrap .05 .05 .4 .015 .01 .03 0.1

Table 1 illustrates the heat balance PER TON OF HOT METAL. It assumes a 75% hot metal in a total charge of 275 tons which yields 250 tons of liquid steel (without alloys). If the oxygen were supplied as air, the heat required to take N2 from room temperature to 2900°F would be about 500,000 Btu per NTHM, which illustrates that the BOS is a Bessemer process with cold scrap substituted for cold nitrogen. (NTHM one short ton or 2000 pounds of hot metal). TABLE I. HEAT BALANCE PER NET TON OF HOT METAL 75% HOT METAL IN CHARGE

HEAT Btu Btu HEAT REQUIRED AVAILABLE (000's) (000's) H.M 2400—>2900 C —> CO 366 220 F _ Si —> SiO2 204 FLUXES —>2900 F 110 Mn —> MnO 60 O2 —>2900 F 120 P —>P2O5 10 HEAT LOSSES 50 Fe—>FeO 110 SCRAP —>2900 F 415 CO—>CO2 130 SLAG 35 FORMATION TOTAL 915 915

The actual percentage of hot metal in the charge is very sensitive to the silicon content and temperature of the hot metal and obviously increases as these decrease.

The oxygen required per heat is shown in Table II, as #/NTHM and as a percentage for the various reactions. 181#/NTHM corresponds to about 18.6 tons/per heat or 1800 scf/tapped ton. Oxygen consumption increases if end-point control is poor and reblows are necessary.

TABLE II. OXYGEN REQUIREMENTS PER NTHM

REACTION #/NTHM % OF TOTAL C —>CO 120 66 Si—>SiO2 17 9 Fe—>FeO (SLAG) 16 9 CO—>CO2 12 7 Fe—>FeO (FUME) 8 4 Mn,P—>MnO,P2O5 7 4 DISSOLVED OXYGEN 1 1 181 100

The final calculation for yield losses is shown in TABLE III. The metalloids and Mn are oxidized out of the hot metal, the scrap is often coated with Zn which volatilizes, and iron units are lost to the slag, fume, and slopping. To tap 250 tons of liquid steel, 250/0.91 or 275 charge tons are required, of which 206 will be hot metal, and the balance scrap. TABLE III YIELD LOSSES IN A BOF HEAT

LOSS % CHARGE METALLOIDS IN HM (6.3%) 4.7 DEBRIS,COATINGS ON SCRAP ( 2.5 %) 0.6 IRON TO SLAG 2.2 IRON TO FUME 1 IRON TO SLOPPING 0.5 TOTAL 9

IV RAW MATERIALS i) HOT METAL

Hot metal is liquid iron from the blast furnace saturated with up to 4.3% carbon and containing 1% or less silicon, Si. It is transported to the BOF shop either in torpedo cars or ladles. The hot metal chemistry depends on how the blast furnace is operated and what burden (iron-bearing) materials are charged to it. The trend today is to run at high productivity with low slag volumes and fuel rates, leading to lower silicon and higher sulfur levels in the hot metal. If BOF slag is recycled, P and Mn levels rise sharply since they report almost 100% to the hot metal. U.S. iron ores are low in both elements.

The sulfur level from the blast furnace can be 0.05% but an efficient hot metal desulfurizing facility ahead of the BOF will reduce this to below .01%. The most common desulfurizing reagents, lime, calcium carbide and magnesium - used alone or in combination - are injected into the hot metal through a lance. The sulfur containing compounds report to the slag; however, unless the sulfur-rich slag is skimmed before the hot metal is poured into the BOF, the sulfur actually charged will be well above the level expected from the metal analysis. ii) SCRAP

In autogenous BOS operation, scrap is by far the largest heat sink. At 20 - 25% of the charge it is one of the most important and costly components of the charge.

Steel scrap is available in many forms. The major categories are "home scrap", generated within the plant. With the advent of continuous casting, the quantity of home scrap has diminished and it is now necessary for integrated mills to buy scrap on the market. Flat rolled scrap is generally of good quality and it's impact on the chemistry of BOF operations can almost be ignored. There is a yield loss of about 2% due to the zinc coating on galvanized scrap. "Prompt scrap" is generated during the manufacturing of steel products. It finds its way into the recycling stream very quickly. Many steel mills have agreements with manufacturers to buy their prompt scrap. "Obsolete" or "post consumer" scrap returns to the market after a product has ended its useful life. Cans return to the market very quickly but autos have an average life of 12 years.

Scrap also comes in many sizes, varying chemical analyses and a variety of prices. All of which makes the purchase and melting of scrap a very complex issue. Very large pieces of scrap can be difficult to melt and may damage the vessel when charged. Some scrap may contain oil or surface oxidation. Obsolete scrap may contain a variety of other objects which could be hazardous or explosive. Obviously the chemical analysis of obsolete scrap is imprecise. Scrap selection is further complicated by the wide variety of steel products. Deep drawing steels limit the maximum residual (%Cu +%Sn + %Ni +%Cr +%Mo) content to less than 0.13%. While other products allow this to range as high as 0.80%. Since these elements cannot be oxidized from the steel, their content in the final product can only be reduced by dilution with very high purity scrap or hot metal. The use of low residual hot metal in the BOS, with its inherent dilution effect, is one of the features that distinguish BOF from EAF steelmaking. iii) FLUXES

Fluxes serve two important purposes. First they combine with SiO2 which is oxidized from the hot metal to form a "basic" slag that is fluid at steelmaking temperatures. This slag absorbs and retains sulfur and phosphorus from the hot metal.

Lime (95+% CaO) and dolomite (58%CaO, 39% MgO) are the two primary fluxes. They are obtained by calcining the carbonate minerals, generally offsite in rotary kilns. Calcining CaCO3 and MgCO3 liberates CO2 leaving CaO or MgO. Two types, "soft" and "hard" burned lime, are available. A lump of soft burned lime dissolves quickly in a cup of water liberating heat. Hard burned material just sits there. Soft burned fluxes form slag more quickly than hard-burned, and in the short blowing cycle, this is critical for effective sulfur and phosphorus removal. The amount of lime charged depends on the Si content of the hot metal.

In BOS steelmaking a high CaO/SiO2 ratio in the slag is desirable, e.g. 3. A rule of thumb is 6 X the weight of Si charged. The MgO addition is designed to be about 8 to 10% of the final slag weight. This saturates the slag with MgO, thus reducing chemical erosion of the MgO vessel lining. iv) COOLANTS

Limestone, scrap, and sponge iron are all potential coolants that can be added to a heat that has been overblown and is excessively hot. The economics and handling facilities dictate the selection at each shop. v) ALLOYS

Bulk alloys are charged from overhead bins into the ladle. The common alloys are ferromanganese (80%Mn, 6%C, balance Fe), silicomanganese (66%Mn, 16%Si, 2%C, balance Fe), and ferrosilicon (75% Si, balance Fe). Aluminum can be added as shapes and/or injected as rod. Sulfur, carbon, calcium, and special elements like boron and titanium are fed at the ladle furnace as powders sheathed in a mild steel casing about 1/2 inch in diameter.

V REFRACTORIES

The basis for most refractory bricks for oxygen steelmaking vessels in the U.S. today is magnesia, MgO, which can be obtained from minerals or seawater. Only one dolomite (MgO + CaO) deposit is worked in the U.S (near Reading, PA). For magnesia, the lower the boron oxide content, and the lower the impurity levels (but with a CaO/SiO2 ratio above 2 to avoid low melting point intergranular phases), the greater the hot strength of the brick. Carbon is added as pitch (tar) or graphite.

The magnesia lime type refractories used in lining oxygen steelmaking vessels are selected mainly for their compatibility with the highly basic finishing slags required to remove and retain phosphorus in solution. During refining, the refractories are exposed to a variety of slag conditions ranging from 1 to 4 basicity as silicon is oxidizes from the bath and combines with lime. The iron oxide, FeO, content of the bath increases with blowing time especially as the carbon in the steel falls below 0.2 % and Fe is oxidized. Although all refractory materials are dissolved by FeO, MgO forms a solid solution with FeO, meaning they coexist as solids within a certain temperature range. The high concentrations of FeO formed late in the blow, however, will oxidize the carbon in the brick.

The original bricks were tar bonded, where the MgO grains were coated with tar and pressed warm represented a great step forward for the BOS process. Tempering removed volatiles. In service, the tar was coked and the residual intergranular carbon resisted slag wetting and attack by FeO. In addition, as the tar softened during vessel heat-up, the lining was relieved of expansive stresses. Hot strength was increased by sintering bricks made from pure MgO grains at a high temperature and then impregnating them with tar under a vacuum. However, for environmental reasons these types of bricks are no longer used in oxygen steelmaking.

Today's working lining refractories are primarily resin-bonded magnesia-carbon bricks made with high quality sintered magnesite and high purity flake graphite. Resin-bonded brick are unfired and contain 5% to 25% high purity flake graphite and one or more powdered metals. These brick require a simple curing step at 350 to 400°F to "thermoset" the resin that makes them very strong and therefore easily handled during installation. Further refinements include using prefused grains in the mix. Small additions of metal additives (Si, Al, and Mg) protect the graphite from oxidation because they are preferentially oxidized. Metallic carbides, nitrides, and magnesium-aluminate spinel form in service at the hot face of the brick filling voids, and adding strength and resistance to slag attack.

The rate of solution of a refractory by the slag is dependent on its properties. These properties are directly related to the purity and crystal sizes of the starting ingredients as well as the manufacturing process. Additions of up to 15% high purity graphite to MgO- carbon refractories provide increased corrosion resistance. Beyond 15% this trend is reversed due to the lower density of the brick. Ultimately, the cost per ton of steel for brick and gunning repair materials, coupled with the need for vessel availability, dictate the choice of lining.

The penetration of slag and metal between the refractory grains, mechanical erosion by liquid movement, and chemical attack by slags all contribute to loss of lining material. Over the years, there have been numerous operating developments designed to counteract this lining wear: i) Critical wear zones (impact and tap pads, turndown slag lines, and trunion areas) in furnaces have been zoned with bricks of the highest quality. ii) "Slag splashing" whereby residual liquid slag remaining after the tap is splashed onto the lining with high pressure nitrogen blown through the oxygen lance. This seemingly simple practice has increased lining life beyond all expectations, from a few thousand to over 20,000 heats per campaign. iii) Instruments are now available to measure lining contours in a short time period, to maximize gunning effectiveness using MgO slurries. iv) Dolomite (40%MgO) is added to the flux addition to create slags with about 8% MgO, which is close to the MgO saturation level of the slag. v) Improved end-point control resulting in lower FeO levels and shorter oxygen-off to charge intervals have reduced refractory deterioration. None of the above would be significant however, without the improvements in quality and type of basic brick available to the industry.

Today, the refractory industry is undergoing major structural changes. Companies are being continually acquired and the total number of North American suppliers is greatly reduced. A very high percentage of refractory materials are being produced off shore, with China being the most significant newcomer.

VI ENVIRONMENTAL ISSUES

Environmental challenges at BOS shops include: (1) the capture and removal of contaminants in the hot and dirty primary off-gas from the converter; (2) secondary emissions associated with charging and tapping the furnaces; (3) control of emissions from ancillary operations such as hot metal transfer, desulfurization, or ladle metallurgy operations; (4) the recycling and/or disposal of collected oxide dusts or sludges; and (5) the disposition of slag.

In the U.S., most BOF primary gas handling systems are designed to generate plant steam from the water-cooled hood serving the primary system. About half of the systems are open combustion designs where excess air is induced at the mouth of the hood to completely burn the carbon monoxide. The gases are then cooled and cleaned either in a wet scrubber or a dry electrostatic precipitator. The remainder of U.S. systems are suppressed combustion systems where gases are handled in an uncombusted state and cleaned in a wet scrubber before being ignited prior to discharge. In both cases, the cleaned gases must meet EPA-mandated levels for particulate matter.

Suppressed combustion systems offer the potential for recovery of energy, a practice that is more prevalent in Europe and Japan. However, in the U.S., other than steam generation, no attempt is made to capture the chemical or sensible heat in the off-gas leaving the vessel. While this represents the loss of a considerable amount of energy (about 0.7 million Btu/ton), the pay-back on capital required, either for the conversion of open combustion to suppressed combustion systems or the addition of necessary gas collection facilities for suppressed combustion systems, is over 10 years. In addition, the necessity of taking shops out of service to make these changes is not practical. Most BOF shops in the U.S. pre-date the energy crises of the 1970s, and even today, energy in the U.S. is relatively less expensive than it is abroad.

Secondary fugitive emissions associated with charging and tapping the BOF vessel, or emissions escaping the main hood during oxygen blowing, may be captured by exhaust systems serving local hoods or high canopy hoods located in the trusses of the shop or both. Typically a fabric collector, or baghouse, is use for the collection of these fugitive emissions. Similarly, ancillary operations such as hot metal transfer stations, desulfurization, or ladle metallurgy operations are usually served by local hood systems exhausted to fabric filters.

The particulate matter captured in the primary system, whether in the form of sludge from wet scrubbers or dry dust from precipitators, must be processed before recycling. Sludge from wet scrubbers requires an extra drying step. Unlike EAF dust, BOF dust or sludge is not a listed hazardous waste. If the zinc content is low enough, it can be recycled to the blast furnace or BOF vessel after briquetting or pelletizing. Numerous processes for recycling the particulate are in use or under development.

BOF slag typically contains about 5% MnO and 1% P2O5 and are often can be recycled through the blast furnace. Because lime in steel slag absorbs moisture and expands on weathering, its use as an aggregate material is limited, but other commercial uses are being developed to minimize the amount that must be disposed.

VII CONCLUSION

The BOS has been a pivotal process in the transformation of the U.S. steel industry since World War II. Although it was not recognized at the time, the process made it possible to couple melting with continuous casting. The result has been that melt shop process and finishing mill quality and yields improved several percent, such that the quantity of raw steel required per ton of product decreased significantly.

The future of the BOS depends on the availability of hot metal, which in turn depends on the cost and availability of coke. Although it is possible to operate BOFs with reduced hot metal charges, i.e. < 70%, there are productivity penalties and costs associated with the supply of auxiliary fuels. Processes to replace the blast furnace are being constantly being unveiled, and the concept of a hybrid BOF-EAF is already a reality at the Saldahna Works in South Africa. However, it appears that the blast furnace and the BOS will be with us for many decades into the future.

The American Iron and Steel Institute acknowledges, with thanks, the contributions of Teresa M. Speiran, Senior Research Engineer, Refractories and Bruce A. Steiner, Senior Environmental Advisor, Collier Shannon Scott PLLC.

ADDENDUM

HISTORY OF THE BASIC OXYGEN STEELMAKING PROCESS

Basic Oxygen Steelmaking is unquestionably the "son of Bessemer", the original pneumatic process patented by Sir Henry Bessemer in 1856. Because oxygen was not available commercially in those days, air was the oxidant. It was blown through tuyeres in the bottom of the pear shaped vessel. Since air is 80% inert nitrogen, which entered the vessel cold but exited hot, removed so much heat from the process that the charge had to be almost 100% hot metal for it to be autogenous. The inability of the Bessemer process to melt significant quantities of scrap became an economic handicap as steel scrap accumulated. Bessemer production peaked in the U.S. in 1906 and lingered until the 1960s.

There are two interesting historical footnotes to the original Bessemer story:

William Kelly was awarded the original U.S. patent for pneumatic steelmaking over Bessemer in 1857. However, it is clear that Kelly's "air boiling" process was conducted at such low blowing rates that the heat generation barely offset the heat losses. He never developed a commercial process for making steel consistently.

Most European iron ores and therefore hot metal was high in sulfur and phosphorus and no processes to remove these from steel had been developed in the 1860s. As a result, Bessemer's steel suffered from both "hot shortness" (due to sulfur) and "cold shortness" (due to phosphorus) that rendered it unrollable. For his first commercial plant in Sheffield, 1866, Bessemer remelted cold pig iron imported from Sweden as the raw material for his hot metal. This charcoal derived pig iron was low in phosphorus and sulfur, and (fortuitously) high in manganese which acted as a deoxidant. In contrast the U.S. pig iron was produced using low sulfur charcoal and low phosphorus domestic ore. Therefore, thanks to the engineering genius of Alexander Holley, two Bessemer plants were in operation by 1866. However, the daily output of remotely located charcoal blast furnaces was very low. Therefore, hot metal was produced by remelting pig iron in cupolas and gravity feeding it to the 5 ton Bessemer vessels. The real breakthrough for Bessemer occurred in 1879 when Sidney Thomas, a young clerk from a London police court, shocked the metallurgical establishment by presenting data on a process to remove phosphorus (and also sulfur) from Bessemer's steel. He developed basic linings produced from tar-bonded dolomite bricks. These were eroded to form a basic slag that absorbed phosphorus and sulfur, although the amounts remained high by modern standards. The Europeans quickly took to the "Thomas Process" because of their very high- phosphorus hot metal, and as a bonus, granulated the phosphorus-rich molten slag in water to create a fertilizer. In the U.S., Andrew Carnegie, who was present when Thomas presented his paper in London, befriended the young man and cleverly acquired the U.S. license, which squelched any steelmaking developments in the South where high phosphorus ores are located.

Although Bessemer's father had jokingly suggested using pure oxygen instead of air (U.K.patent 2207, Oct 5,1858), this possibility was to remain a dream until "tonnage oxygen" became available at a reasonable cost. A 250 ton BOF today needs about 20 tons of pure oxygen every 40 minutes. Despite its high cost, oxygen was used in Europe to a limited extent in the 1930's to enrich the air blast for blast furnaces and Thomas converters. It was also used in the U.S for scarfing, and welding.

The production of low cost tonnage oxygen was stimulated in World War II by the German V2 rocket program. After the war, the Germans were denied the right to manufacture tonnage oxygen, but oxygen plants were shipped to other countries. The bottom tuyeres used in the Bessemer and Thomas processes could not withstand even oxygen-enriched air, let alone pure oxygen. In the late 1940s, Professor Durrer in Switzerland pursued his prewar idea of injecting pure oxygen through the top of the vessel. Development now moved to neighboring Austria where developers wanted to produce low nitrogen, flat-rolled sheet, but a shortage of scrap precluded open hearth operations. Following pilot plant trials at Linz and Donawitz, a top blown pneumatic process for a 35 ton vessel using pure oxygen was commercialized by Voest at Linz in 1952. The nearby Dolomite Mountains also provided an ideal source of material for basic refractories.

The new process was officially dubbed the "LD Process" and because of its high productivity was seen globally as a viable, low capital process by which the war torn countries of Europe could rebuild their steel industries. Japan switched from a rebuilding plan based on open hearths to evaluate the LD, and installed their first unit at Yawata in 1957.

Two small North American installations started at Dofasco and McLouth in 1954. However, with the know-how and capital invested in 130 million tons of open hearth capacity, plans for additional open hearth capacity well along, cheap energy, and heat sizes greater by an order of magnitude (300 versus 30 tons), the incentive to install this untested, small-scale process in North America was lacking. The process was acknowledged as a breakthrough technically but the timing, scale, and economics were wrong for the time. The U.S., which manufactured about 50% of the world's total steel output, needed steel for a booming post- war economy.

There were also acrimonious legal actions over patent rights to the process and the supersonic lance design, which was now multihole rather than single hole. Kaiser Industries held the U.S. patent rights but in the end, the U.S. Supreme Court supported lower court decisions that considered the patent to be invalid.

Nevertheless, the appeal of lower energy, labor, and refractory costs for the LD process could not be denied and although oxygen usage in the open hearth delayed the transition to the new process in the U.S., oxygen steelmaking tonnage grew steadily in the 1960's. By 1969, it exceeded that of the open hearth for the first time and has never relinquished its position as the dominant steelmaking process in the U.S. but the name LD never caught on in the U.S. Technical developments over the years include improved computer models and instrumentation for improved turn-down control, external hot metal desulfurization, bottom blowing and stirring with a variety of gases and tuyeres, slag splashing, and improved refractories. Electric Arc Furnace Steelmaking By Jeremy A. T. Jones, Nupro Corporation

Courtesy of Mannesmann Demag Corp.

FURNACE OPERATIONS

The electric arc furnace operates as a batch melting process producing batches of molten steel known "heats". The electric arc furnace operating cycle is called the tap-to-tap cycle and is made up of the following operations:

• Furnace charging • Melting • Refining • De-slagging • Tapping • Furnace turn-around Modern operations aim for a tap-to-tap time of less than 60 minutes. Some twin shell furnace operations are achieving tap-to-tap times of 35 to 40 minutes.

Furnace Charging

The first step in the production of any heat is to select the grade of steel to be made. Usually a schedule is developed prior to each production shift. Thus the melter will know in advance the schedule for his shift. The scrap yard operator will prepare buckets of scrap according to the needs of the melter. Preparation of the charge bucket is an important operation, not only to ensure proper melt-in chemistry but also to ensure good melting conditions. The scrap must be layered in the bucket according to size and density to promote the rapid formation of a liquid pool of steel in the hearth while providing protection for the sidewalls and roof from electric arc radiation. Other considerations include minimization of scrap cave-ins which can break electrodes and ensuring that large heavy pieces of scrap do not lie directly in front of burner ports which would result in blow-back of the flame onto the water cooled panels. The charge can include lime and carbon or these can be injected into the furnace during the heat. Many operations add some lime and carbon in the scrap bucket and supplement this with injection.

The first step in any tap-to-tap cycle is "charging" into the scrap. The roof and electrodes are raised and are swung to the side of the furnace to allow the scrap charging crane to move a full bucket of scrap into place over the furnace. The bucket bottom is usually a clam shell design - i.e. the bucket opens up by retracting two segments on the bottom of the bucket. The scrap falls into the furnace and the scrap crane removes the scrap bucket. The roof and electrodes swing back into place over the furnace. The roof is lowered and then the electrodes are lowered to strike an arc on the scrap. This commences the melting portion of the cycle. The number of charge buckets of scrap required to produce a heat of steel is dependent primarily on the volume of the furnace and the scrap density. Most modern furnaces are designed to operate with a minimum of back-charges. This is advantageous because charging is a dead-time where the furnace does not have power on and therefore is not melting. Minimizing these dead-times helps to maximize the productivity of the furnace. In addition, energy is lost every time the furnace roof is opened. This can amount to 10 - 20 kWh/ton for each occurrence. Most operations aim for 2 to 3 buckets of scrap per heat and will attempt to blend their scrap to meet this requirement. Some operations achieve a single bucket charge. Continuous charging operations such as CONSTEEL and the Fuchs Shaft Furnace eliminate the charging cycle.

Melting

The melting period is the heart of EAF operations. The EAF has evolved into a highly efficient melting apparatus and modern designs are focused on maximizing the melting capacity of the EAF. Melting is accomplished by supplying energy to the furnace interior. This energy can be electrical or chemical. Electrical energy is supplied via the graphite electrodes and is usually the largest contributor in melting operations. Initially, an intermediate voltage tap is selected until the electrodes bore into the scrap. Usually, light scrap is placed on top of the charge to accelerate bore-in. Approximately 15 % of the scrap is melted during the initial bore-in period. After a few minutes, the electrodes will have penetrated the scrap sufficiently so that a long arc (high voltage) tap can be used without fear of radiation damage to the roof. The long arc maximizes the transfer of power to the scrap and a liquid pool of metal will form in the furnace hearth At the start of melting the arc is erratic and unstable. Wide swings in current are observed accompanied by rapid movement of the electrodes. As the furnace atmosphere heats up the arc stabilizes and once the molten pool is formed, the arc becomes quite stable and the average power input increases. Chemical energy is be supplied via several sources including oxy-fuel burners and oxygen lances. Oxy-fuel burners burn natural gas using oxygen or a blend of oxygen and air. Heat is transferred to the scrap by flame radiation and convection by the hot products of combustion. Heat is transferred within the scrap by conduction. Large pieces of scrap take longer to melt into the bath than smaller pieces. In some operations, oxygen is injected via a consumable pipe lance to "cut" the scrap. The oxygen reacts with the hot scrap and burns iron to produce intense heat for cutting the scrap. Once a molten pool of steel is generated in the furnace, oxygen can be lanced directly into the bath. This oxygen will react with several components in the bath including, aluminum, silicon, manganese, phosphorus, carbon and iron. All of these reactions are exothermic (i.e. they generate heat) and supply additional energy to aid in the melting of the scrap. The metallic oxides that are formed will end up in the slag. The reaction of oxygen with carbon in the bath produces carbon monoxide, which either burns in the furnace if there is sufficient oxygen, and/or is exhausted through the direct evacuation system where it is burned and conveyed to the pollution control system. Auxiliary fuel operations are discussed in more detail in the section on EAF operations.

Once enough scrap has been melted to accommodate the second charge, the charging process is repeated. Once the final scrap charge is melted, the furnace sidewalls are exposed to intense radiation from the arc. As a result, the voltage must be reduced. Alternatively, creation of a foamy slag will allow the arc to be buried and will protect the furnace shell. In addition, a greater amount of energy will be retained in the slag and is transferred to the bath resulting in greater energy efficiency.

Once the final scrap charge is fully melted, flat bath conditions are reached. At this point, a bath temperature and sample will be taken. The analysis of the bath chemistry will allow the melter to determine the amount of oxygen to be blown during refining. At this point, the melter can also start to arrange for the bulk tap alloy additions to be made. These quantities are finalized after the refining period.

Refining

Refining operations in the electric arc furnace have traditionally involved the removal of phosphorus, sulfur, aluminum, silicon, manganese and carbon from the steel. In recent times, dissolved gases, especially hydrogen and nitrogen, been recognized as a concern. Traditionally, refining operations were carried out following meltdown i.e. once a flat bath was achieved. These refining reactions are all dependent on the availability of oxygen. Oxygen was lanced at the end of meltdown to lower the bath carbon content to the desired level for tapping. Most of the compounds which are to be removed during refining have a higher affinity for oxygen that the carbon. Thus the oxygen will preferentially react with these elements to form oxides which float out of the steel and into the slag.

In modern EAF operations, especially those operating with a "hot heel" of molten steel and slag retained from the prior heat, oxygen may be blown into the bath throughout most of the heat. As a result, some of the melting and refining operations occur simultaneously.

Phosphorus and sulfur occur normally in the furnace charge in higher concentrations than are generally permitted in steel and must be removed. Unfortunately the conditions favorable for removing phosphorus are the opposite of those promoting the removal of sulfur. Therefore once these materials are pushed into the slag phase they may revert back into the steel. Phosphorus retention in the slag is a function of the bath temperature, the slag basicity and FeO levels in the slag. At higher temperature or low FeO levels, the phosphorus will revert from the slag back into the bath. Phosphorus removal is usually carried out as early as possible in the heat. Hot heel practice is very beneficial for phosphorus removal because oxygen can be lanced into the bath while its temperature is quite low. Early in the heat the slag will contain high FeO levels carried over from the previous heat thus aiding in phosphorus removal. High slag basicity (i.e. high lime content) is also beneficial for phosphorus removal but care must be taken not to saturate the slag with lime. This will lead to an increase in slag viscosity, which will make the slag less effective. Sometimes fluorspar is added to help fluidize the slag. Stirring the bath with inert gas is also beneficial because it renews the slag/metal interface thus improving the reaction kinetics.

In general, if low phosphorus levels are a requirement for a particular steel grade, the scrap is selected to give a low level at melt-in. The partition of phosphorus in the slag to phosphorus in the bath ranges from 5 to 15. Usually the phosphorus is reduced by 20 to 50 % in the EAF.

Sulfur is removed mainly as a sulfide dissolved in the slag. The sulfur partition between the slag and metal is dependent on slag chemistry and is favored at low steel oxidation levels. Removal of sulfur in the EAF is difficult especially given modern practices where the oxidation level of the bath is quite high. Generally the partition ratio is between 3 and 5 for EAF operations. Most operations find it more effective to carry out desulfurization during the reducing phase of steelmaking. This means that desulfurization is performed during tapping (where a calcium aluminate slag is built) and during ladle furnace operations. For reducing conditions where the bath has a much lower oxygen activity, distribution ratios for sulfur of between 20 and 100 can be achieved.

Control of the metallic constituents in the bath is important as it determines the properties of the final product. Usually, the melter will aim at lower levels in the bath than are specified for the final product. Oxygen reacts with aluminum, silicon and manganese to form metallic oxides, which are slag components. These metallics tend to react with oxygen before the carbon. They will also react with FeO resulting in a recovery of iron units to the bath. For example:

Mn + FeO = MnO + Fe

Manganese will typically be lowered to about 0.06 % in the bath.

The reaction of carbon with oxygen in the bath to produce CO is important as it supplies a less expensive form of energy to the bath, and performs several important refining reactions. In modern EAF operations, the combination of oxygen with carbon can supply between 30 and 40 % of the net heat input to the furnace. Evolution of carbon monoxide is very important for slag foaming. Coupled with a basic slag, CO bubbles are tapped in the slag causing it to "foam" and helping to bury the arc. This gives greatly improved thermal efficiency and allows the furnace to operate at high arc voltages even after a flat bath has been achieved. Burying the arc also helps to prevent nitrogen from being exposed to the arc where it can dissociate and enter into the steel.

If the CO is evolved within the steel bath, it helps to strip nitrogen and hydrogen from the steel. Nitrogen levels in steel as low as 50 ppm can be achieved in the furnace prior to tap. Bottom tapping is beneficial for maintaining low nitrogen levels because tapping is fast and a tight tap stream is maintained. A high oxygen potential in the steel is beneficial for low nitrogen levels and the heat should be tapped open as opposed to blocking the heat.

At 1600 C, the maximum solubility of nitrogen in pure iron is 450 ppm. Typically, the nitrogen levels in the steel following tapping are 80 - 100 ppm. Decarburization is also beneficial for the removal of hydrogen. It has been demonstarted that decarburizing at a rate of 1 % per hour can lower hydrogen levels in the steel from 8 ppm down to 2 ppm in 10 minutes.

At the end of refining, a bath temperature measurement and a bath sample are taken. If the temperature is too low, power may be applied to the bath. This is not a big concern in modern meltshops where temperature adjustment is carried out in the ladle furnace.

De-Slagging

De-slagging operations are carried out to remove impurities from the furnace. During melting and refining operations, some of the undesirable materials within the bath are oxidized and enter the slag phase.

It is advantageous to remove as much phosphorus into the slag as early in the heat as possible (i.e. while the bath temperature is still low). The furnace is tilted backwards and slag is poured out of the furnace through the slag door. Removal of the slag eliminates the possibility of phosphorus reversion.

During slag foaming operations, carbon may be injected into the slag where it will reduce FeO to metallic iron and in the process produce carbon monoxide which helps foam the slag. If the high phosphorus slag has not been removed prior to this operation, phosphorus reversion will occur. During slag foaming, slag may overflow the sill level in the EAF and flow out of the slag door.

The following table shows the typical constituents of an EAF slag :

Composition Component Source Range

CaO Charged 40 - 60 %

SiO2 Oxidation product 5 - 15 % FeO Oxidation product 10 - 30 %

MgO Charged as dolomite 3 - 8 %

CaF2 Charged - slag fluidizer

MnO Oxidation product 2 - 5%

S Absorbed from steel

P Oxidation product

Tapping

Once the desired steel composition and temperature are achieved in the furnace, the tap- hole is opened, the furnace is tilted, and the steel pours into a ladle for transfer to the next batch operation (usually a ladle furnace or ladle station). During the tapping process bulk alloy additions are made based on the bath analysis and the desired steel grade. De- oxidizers may be added to the steel to lower the oxygen content prior to further processing. This is commonly referred to as "blocking the heat" or "killing the steel". Common de- oxidizers are aluminum or silicon in the form of ferrosilicon or silicomanganese. Most carbon steel operations aim for minimal slag carry-over. A new slag cover is "built" during tapping. For ladle furnace operations, a calcium aluminate slag is a good choice for sulfur control. Slag forming compounds are added in the ladle at tap so that a slag cover is formed prior to transfer to the ladle furnace. Additional slag materials may be added at the ladle furnace if the slag cover is insufficient.

Furnace Turn-around

Furnace turn-around is the period following completion of tapping until the furnace is recharged for the next heat. During this period, the electrodes and roof are raised and the furnace lining is inspected for refractory damage. If necessary, repairs are made to the hearth, slag-line, tap-hole and spout. In the case of a bottom-tapping furnace, the taphole is filled with sand. Repairs to the furnace are made using gunned refractories or mud slingers. In most modern furnaces, the increased use of water-cooled panels has reduced the amount of patching or "fettling" required between heats. Many operations now switch out the furnace bottom on a regular basis (2 to 6 weeks) and perform the hearth maintenance off-line. This reduces the power-off time for the EAF and maximizes furnace productivity. Furnace turn-around time is generally the largest dead time (i.e. power off) period in the tap-to-tap cycle. With advances in furnace practices this has been reduced from 20 minutes to less than 5 minutes in some newer operations.

Furnace Heat Balance

To melt steel scrap, it takes a theoretical minimum of 300 kWh/ton. To provide superheat above the melting point of 2768 F requires additional energy and for typical tap temperature requirements, the total theoretical energy required usually lies in the range of 350 to 370 kWh/ton. However, EAF steelmaking is only 55 to 65 % efficient and as a result the total equivalent energy input is usually in the range of 560 to 680 kWh/ton for most modern operations. This energy can be supplied from a variety of sources as shown in the table below. The energy distribution is highly dependent on local material and consumable costs and is unique to the specific meltshop operation. A typical balance for both older and more modern EAFs is given in the following Table:

Low to Medium UHP FURNACE Power Furnace

Electrical Energy 50 - 60 % 75 - 85 %

INPUTS Burners 5 - 10 %

Chemical Reactions 30 - 40 % 15 - 25 %

TOTAL INPUT 100% 100%

OUTPUTS Steel 55 - 60 % 50 - 55 %

Slag 8 - 10 % 8 - 12 % Cooling Water 8 - 10 % 5 - 6 % Miscellaneous 1 - 3 % 17 - 30 %

Offgas 17 - 28 % 7 - 10 %

Of course the above figures are highly dependent on the individual operation and vary considerably from one facility to another. Factors such as raw material composition, power input rates and operating practices (e.g. post-combustion, scrap preheating) can greatly alter the above balance. In operations utilizing a large amount of charge carbon or high carbon feed materials, up to 60 % of the energy contained in the offgas may be calorific due to large quantities of un-combusted carbon monoxide. Recovery of this energy in the EAF could increase energy input by 8 to 10 %. Thus it is important to consider such factors when evaluating the energy balance for a given furnace operation.

The International Iron and Steel Institue (IISI), classifies EAFs based on the power supplied per ton of furnace capacity. For most modern operations, the design would allow for at least 500 kVA per ton of capacity. The IISI report " The Electic Furnace - 1990" indicates that most new installations allow 900 - 1000 kVA per ton of furnace capacity. Most furnaces operate at a maximum power factor of about 0.85. Thus the above transformer ratings would correspond to a maximum power input of about 0.75 to 0.85 MW per ton of furnace capacity.

MECHANICAL SYSTEMS

Mechanical systems are integral to the operation of the EAF and many are inter-related. To gain a better perspective of the importance of various systems in the furnace operation, it is good to step back and evaluate the function of the electric arc furnace itself. The EAF has several primary functions:

1. Containment of steel scrap 2. Heating and melting of steel scrap 3. Transfer of molten steel to the next processing stage

It is easy to see that the first function, scrap containment can only be properly carried out if the furnace shell is properly maintained. The furnace shell consists of a refractory lined bottom that helps contain the liquid steel and typically, a water-cooled upper section that only comes into contact with scrap and slag. Heating and melting of the scrap are accomplished by supplying electrical energy through the electrodes and chemical energy through the use of burners and oxygen lances. Transfer of the liquid steel to the ladle is accomplished by tilting the furnace and opening either a tapping spout or a bottom tap-hole to allow the steel to flow from the furnace. It is apparent that many sub-systems come into play throughout the tap-to-tap cycle. Many of these systems are dependent of the following systems in order to be able to function properly:

• Hydraulic system • Cooling water system • Lubrication System

Hydraulic system The hydraulic system provides motive power for almost all EAF movements including roof lower/raise, roof swing, electrode arms up/down/regulation/swing, furnace tilt forward/backward, slag door raise/lower and movement of any auxiliary systems such as the burner lance. The hydraulic system consists of a central reservoir, filters, an accumulator, hydraulic valves and hydraulic piping. As hydraulic fluid passes through valves in one of two directions within a given circuit, hydraulic cylinders are extended or contracted to provide movement of various mechanical components. Without sufficient fluid flow and pressure within a circuit, movement is impossible. Thus issues such as low fluid level, low accumulator pressure, system leaks, fluid degradation due to over-heating, solids build-up in valves or in hydraulic lines and wear in mechanical components can lead to poor system performance and in some cases, system failure.

Cooling water system

Another system that is integral to EAF operation is the cooling water system. Typically, there are several cooling systems. Some operations require extremely clean, high quality cooling water. Transformer cooling, delta closure cooling, bus tube cooling and electrode holder cooling are all such applications. Typically, these systems will consist of a closed loop circuit, which conducts water through these sensitive pieces of equipment. The water in the closed loop circuit passes through a heat exchanger to remove heat. The circuit on the open loop side of the heat exchanger typically flows to a cooling tower for energy dissipation. Other water cooled elements such as furnace side panels, roof panels, offgas system ducting, furnace cage etc. will typically receive cooling water from a cooling tower.

The cooling circuit typically consists of supply pumps, return pumps, filters, a cooling tower cell or cells and flow monitoring instrumentation. Sensitive pieces of equipment normally have instrumentation installed to monitor the cooling water flow rate and temperature. For most water-cooled equipment, interruption of the flow or inadequate water quantities can lead to severe thermal over loading and in some cases catastrophic failure.

Lubrication System

Many modern furnaces have an automatic system that provides lubrication to various moving parts based on various "events" occurring during the tap-to-tap cycle. For example, some parts are lubricated every three roof swings, following tapping, etc. Some components such as roller bearings are critical to furnace operation and are lubricated periodically by hand. Some hard to reach locations are serviced using tubing and remote blocks.

AUXILIARY SYSTEMS

In addition to the major mechanical systems associated with the EAF, there are also many auxiliary systems that are integral to furnace operation and performance.

Oxygen lance system

Over the past 20 years, the use of oxygen in EAF steelmaking has grown considerably. In the past when oxygen consumption of less than 300 cubic feet per ton of steel were common, lancing operations were carried out manually using a consumable pipe lance. Most modern operations now use automatic lances and most facilities now use a non- consumable, water-cooled lance for injecting oxygen into the steel. Many of these lances also have the capability to inject carbon as well.

Carbon injection system

Carbon injection is critical to slag foaming operations, which are necessary for high power furnace operations. Carbon reacts with FeO to form CO and "foam" the slag.

Oxy-fuel burner system

Oxy-fuel burners are now almost standard equipment on large high-powered furnaces. In operations with short tap-to-tap times, they provide an important function by ensuring rapid melting of the scrap in the cold spots. This ensures that scrap cave-ins are kept to a minimum and as a result, electrode breakage is minimized. In large diameter furnaces, burners are essential to ensure a uniform meltdown. Non-uniform scrap meltdowns may result in operating delays and lost productivity. The biggest maintenance issue for burners is to ensure that they do not get plugged with metal or slag. The closer burners are mounted to the bath, the greater the risk of them becoming plugged while in a low-fire mode. Some burners are mounted directly in the water-cooled panel while others are mounted in a copper block. If burners are fired at high rates against large pieces of scrap, the flame can blow back on the furnace shell damaging the water-cooled panel. Thus the panel area should be inspected for wear around the burner port. If a copper block is used, it will be more resistant to flame blow back but should still be inspected regularly for wear and cracks.

Electrode spray cooling system

It is common for electrodes to have a spray cooling system in order to reduce electrode oxidation. Spray rings direct water sprays at the electrode below the electrode clamp and the water runs down the electrode thus cooling it. Sprays rings can reduce overall electrode consumption by as much as 10-20%. In addition, spray cooling usually results in improved electrode holder life and surrounding insulation. Due to the reduction in radiation from the electrode, power cable, air hose and hydraulic hose life is also greatly improved.

Temperature Sampling System

The modern disposable thermocouple was introduced to steelmaking almost 40 years ago and temperature measurement had become an integral part of tracking progress throughout the tap-to-tap cycle in steelmaking. Expendable probes are also used for tracking bath carbon content and dissolved oxygen levels in the steel. These tools have enabled the tap-to-tap cycle to be accelerated by eliminating long waiting periods for lab results, thus increasing productivity. Disposable probes are typically mounted in cardboard sleeves that slide on to a steel probe(pole) which has internal electrical contacts. The disposable probe transmits an electrical signal to the steel pole, which in turn transmits the signal to an electronic unit for interpretation. Almost all probes rely on an accurate temperature measurement to precisely calculate carbon or oxygen levels. Most facilities keep several spare poles on hand so that they can be quickly replaced if they have reading problems.

Offgas Direct Evacuation System

Early offgas evacuation systems were installed so that the furnace operators could better see what was happening in and around the furnace. Since the early days of EAF steelmaking, the offgas system has evolved considerably and most modern EAF shops now use a "fourth hole" direct furnace shell evacuation system (DES). The term fourth hole refers to an additional hole other than those for the electrodes, which is provided for offgas extraction. On DC furnaces with only one electrode, the fume extraction port is sometimes referred to as the "second hole". It is important to maintain sufficient draft on the furnace for the following reasons:

1. To provide adequate pollution control. 2. Excessive shop emissions make it difficult for the crane operator to charge the furnace. 3. Excessive emissions around the electrode ports can result in damage of hoses, cables, the electrode holder, the furnace delta, roof refractory, accelerated electrode wear, damage to the electrode spray cooler etc. 4. Emissions at the roof ring can result in warping of the roof ring structure. 5. Excessive emissions of carbon monoxide to the secondary canopy system may result in explosions in the ductwork downstream. 6. Excessive dust build-up may cause arcing between electrode phases.

Most DES systems consist of water-cooled duct, spray cooling, dry duct and may or may not have a dedicated DES booster fan.

ELECTRICAL SYSTEMS

Electrical systems in an EAF meltshop usually consist of a primary system which supplies power from the electrical utility; and the secondary electrical system which steps down the voltage from the utility and supplies the power to the EAF. The primary system may include a yard step-down transformer as part of the steelmaker's facility and this transformer will feed several other transformers with-in the facility including the EAF transformer. Regardless, there will be a main breaker which isolates all of the steelmaking facility's electrical systems from the power utility. On the secondary side of the primary electrical system, a vacuum switch and motorized disconnect are used to isolate the secondary furnace transformer from the primary power supply.

Vacuum Switch

The vacuum switch is a long life switch that is generally used in all electric furnace applications. The traditional vacuum switch allows for the secondary electrical circuit to be broken either under load or without load. Most vacuum switches are rated for 40,000 operations or four years. In practice, it is not unusual for such switches to achieve 200,000 operations without maintenance. The primary cause of failure in these units is a metallic bellows, which is used to provide a seal for the moving contact, which is enclosed in a vacuum. Once this seal begins to wear (typically after 100,000 operations), a vacuum leak will occur thus making it difficult to adequately isolate the primary power from the secondary.

Motorized Disconnect Switch

THE MOTORIZED DISCONNECT SWITCH (MDS) IS TYPICALLY A MOTORIZED KNIFE GATE SWITCH WHICH IS CAPABLE OF PHYSICALLY ISOLATING THE EAF FROM THE PRIMARY POWER SUPPLY. THE KNIFE SWITCHES ARE RETRACTED WHEN THE FURNACE IS NOT UNDER LOAD (VACUUM SWITCH OPEN, ELECTRODES RAISED) SO THAT ARCING DOES NOT OCCUR BETWEEN THE BLADES ON EITHER SIDE OF THE SWITCH.

EAF Transformer

The power flow from the utility's generators, through their network, arrives at the steel plant at very high voltage and must therefore be converted to low voltage suitable for the furnacearcs. Transformers perform this task. The EAF transformer receives the primary low current, high voltage power and transforms this to a high current, low voltage power for use in the EAF. Reliable operation of the EAF is totally dependent on reliable operation of the EAF transformer. Many large furnace transformers are rated 100MVA or greater.

Transforming the power from the kV level at the incoming utility line to the voltage level needed in the EAF is usually done in two stages. A first transformer (occasionally two transformers in parallel) steps the voltage down from the high-voltage line to a medium voltage level which is generally standardized for each country. In the USA this medium voltage is usually 34.5 kV, while in Europe, Japan and other areas the voltages are not very different, often 30 to 33 kV. From the 34.5 kV busbar, the arc furnace is powered by a special, heavy-duty furnace transformer. The secondary voltage of this furnace transformer is designed to allow operation of the arcs in the desired range of arc voltages and currents. Since there are varying requirements of arc voltage/current combinations through the heat it is necessary to have a choice of secondary voltages. The furnace transformer is equipped with a tap-changer for this purpose.

Tap changer

The purpose of a tap changer is to allow a choice of different combinations of volts and amps for different stages of a heat. This is achieved by changing the number of turns of primary coil. ( The primary takes lower current so it is simpler to change the number of turns on this coil rather than the high current secondary coil). Basically the tap changer takes the form of a motorized box of contacts which switch the primary current to different parts of the coil around the iron core. Most tap changers are designed to operate "on-load" meaning switching the primary current, usually in 2 kA steps, at 34.5 kV. A 'make-before- break' contact movement is used to avoid current interruption. These contacts are subject to heavy erosion due to arcing and therefore require preventative maintenance.

Some steelmakers choose an 'off-load' tap changer in order to avoid the heavy duty of on- load switching. However, such a tap changer requires that the steelmaker break the arc by lifting the furnace electrodes and this procedure may take as long as one minute. Today such a delay each time a tap is changed is intolerable and such designs are becoming rare.

SECONDARY ELECTRICAL CIRCUIT

The secondary circuit of the EAF electrical system consists of five major components: delta closure, power cable, bus tube/conducting arm, electrode clamp/holder and the electrode.

Delta Closure

The secondary circuit of the EAF transformer terminates at low voltage bushings, which are attached to the delta closure, which consists of a series of copper plates, tubes or both. These are arranged do that the secondary windings of the transformer are joined to form a closed circuit. Most of this equipment is located within the transformer vault to assure a secure, clean environment. The delta closure protrudes through the wall of the vault adjacent to the EAF and connectors are provided to attach to one end of the furnace power cables; the other end being attached to either the current conducting arms of the furnace or the busbar.

In the case of the direct current EAF, the thyristor will have two copper terminations; one of which is attached to the EAF power cable, and other is attached to the bottom furnace electrode. The bottom furnace electrode is usually rigid, as no movement is required during operation of the furnace. In principle, the termination on the thyristor is analogous to the delta closure, though physically, it differs considerably. With respect to the maintenance issues for the delta closure, however, the same concepts can be applied to the DC operation.

Bus systems are typically supported at the transformer vault wall and may also be supported from stainless steel hangers suspended from the vault ceiling. Suspension systems for secondary bus or delta closures are frequently supported at the vault wall with kiln dried timbers. Secondary bus systems and delta closures are insulated in order to prevent arcing from phase to phase and from phase to ground especially at the support members.

Furnace Power Cables

The water-cooled furnace power cables provide the only flexible connection in the secondary circuit. These cables must be flexible to permit movement of the electrode arms up and down and to allow swinging the electrode arms and roof when charging of the furnace. The connections from the delta closure, which are on the outside of the transformer vault, are silver plated to provide a clean contact for the power cables. The power cables consist of copper wire strandings forming a cylindrical construction, which is soldered to copper terminals at either end of the cable. A rubber jacket around the outside of the cable permits cooling water for the cable. The rubber hose is attached at either end of the cable using stainless steel clamps, vulcanized bumpers or an anti-chaffing hose. The cooling water hose is covered with a protective sleeve which may be fabricated of fiberglass, vulcanized material, and silicon or aluminum glass fiber sleeves. As cable design advanced, it was noted that due to the "skin effect" typical of AC operations, the current was carried predominantly by the outer portion of the copper strands. Therefore the center strands were replaced with a hollow rubber tube which reduced the cable weight, the reactance and the cost of the cable. At a later date, some operations used this inner channel for water cooling as well.

In DC furnace operations, the inner rubber tube in the cable is used for cooling because DC operations do not experience the "skin effect" and the whole cross section of the copper cable carries the current uniformly. However, DC cables are cooled more effectively from the center and cooling from the outside is not always used.

Bus Bar / Current Conducting Arm

Several designs now exist for the electrode arm and bus-bar assembly. Many older furnaces utilize an arm structure that supports an electrically insulated bus-bar. The bus-bar provides the electrical connection between the power cables and the electrode holder. Bus- bars consist of a rigid, round, copper pipe. Typically the bus tube is supported by one or two bolted connections. Good insulation must be installed between the bus tube and its' supporting members to ensure that arcing which could destroy the bust tube does not take place. Bus tubes are usually attached to the power cables using removable, cast copper terminals or in some cases, permanent fabricated copper terminal plates and pads.

Several configurations are available for the bus tube termination at the electrode holder and contact pad. These include flanged connection to the contact pad, flat blade joined to the tube for parallel connection with the holder and a round copper tube contact point with the connector. The bus tubes may be bolted to the holder or contact pad or a fused permanent joint may be used.

Many modern furnaces utilize current conducting arms in which the arm itself transmits electricity to the electrode holder and contact pad. Current conducting arms are usually fabricated from copper clad steel or aluminum alloys. Due to the reduced weight of conducting arms as opposed to conventional arm and bus tube assemblies there is somewhat less mechanical wear. However, many of the same maintenance issues apply both to bus tube assemblies and current conducting arms. Electrode Heads/Contact pads

The Electrode heads and contact pads provide the final connection between the power supply and the graphite electrode. They are exposed to extreme mechanical conditions (vibration, torsion etc.) and thermal cycling and as a result are the weakest link in the secondary circuit. Traditional electrode holders are either cast or fabricated from copper plates. Contact pads are smaller and incorporate only the electrode contact area. In traditional electrode holders, the electrode is pushed forward into the contact area. In the case of contact pads, the electrode is pulled back with steel housing equipment to make contact with the pad. The current transfer between the contact pad and the electrode occurs in the lower 3 or 4 inches. Proper clamping for is a necessity in order to prevent arcing between the electrode and the contact area. Any dirt build-up in this area will result in resistance to current flow and will cause over-heating and damage to the electrode holder/contact pad.

Typically cooling water requirements will vary from 2 to 40 gallons per minute depending on the electrode size, water quality, clamping force and maintenance of a clean contact area. The contact area should be cleaned regularly to remove oxidation, carbon build-up and other material build-up in this area.

Electrode Regulation

Typically, the electrode/arm/mast/cable assembly weighs in the range of 20 tons. This is moved vertically for control purposes by a hydraulic cylinder incorporated in the mast. (In some older furnaces the movement is effected by an electric motor/cable winch arrangement). Since the arc length is dependent, amongst other things, on the ever changing level of scrap or liquid under the electrode it is necessary to have an automatic control over electrode position -- the regulation system.

The regulation system influences many important aspects of furnace performance, such as energy input, mean current, arc stability, scrap melting pattern, energy losses to water- cooled panels, energy, electrode and refractory consumption. All these parameters are interrelated in a complex manner and there are many differences of opinion on 'optimum' control strategies.

The accepted "standard" handling of the electrical signals is to form an "impedance control". This method attempts to hold the ratio of voltage to electrical current constant, hence its description as 'impedance' control. A voltage signal taken from the phase to ground and a current signal are each separately rectified and their dc values are compared "back-to back". If the voltage and current are each at a desired level - the set point, chosen by the steelmaker - the output from this comparison of signals is arranged to be zero. If however the current exceeds this level its signal increases and simultaneously the voltage decreases. Then the two back-to back voltages do not balance and an output voltage is generated. This signal goes to the regulating valve in such a way to command the electrode to raise, aimed at reducing current

ELECTRODES

(By William A. Obenchain, AISI and Steve Casto, UCAR Corp) One of the most important elements in the electric circuit and consumable cost in electric furnace steelmaking are the electrodes The electrodes deliver the power to the furnace in the form of an electric arc between the electrode and the furnace charge. The arc itself is a plasma of hot, ionic gasses in excess of 6,000°F. Electrodes come in two forms: amorphous and graphitic carbon, or graphite. Since only graphite electrodes are used in steelmaking only they will be discussed here.

Courtesy of UCAR Corp.

Graphite electrodes are composed of a mixture of finely divided, calcined petroleum coke mixed with about 30% coal tar pitch as a binder, plus proprietary additives unique to each manufacturer. This mixture is extruded at approximately 220°F, the softening temperature of pitch, to form a cylindrical rod known as a "green electrode". The green electrode is now given a controlled bake in a reducing atmosphere at temperatures as high 1800°F and again impregnated with pitch to increase its strength and density and lower the electrical resistivity. The electrodes are now ready to be graphitized, i.e. converting the amorphous carbon into crystalline graphite. This is accomplished by passing an electric current through them and heating them to as much as 5000°F. The graphitizing consumes as much as 3000-5000kWH/ton of electrode. The final product is strong, dense, and has a low electrical resistivity. Lastly the electrode is machined to its final shape. Into each end of the electrode is a recess in which threads are machined. These are used to accept a threaded nipple manufactured in the same way so that the electrode column can be lengthened as it is consumed.

Historically, electrode consumption has been as high as 12-14 pounds per tons of steel, but through continuous improvement in electrode manufacturing and steelmaking operations, this has been reduced to the neighborhood 3.5 to 4.5 pounds per ton. Most electrode consumption is through oxidation and tip sublimation, with some small pieces lost around the connecting joint. A considerable portion is also lost to mechanical breakage caused by scrap scrap cave-ins in the furnace or crushing the electrode into the charge.

Electrodes are commonly available in sizes from 15 - 30 inches in diameter varying lengths to 10 feet. They come in three grades: regular and premium and the newer DC grade. Continuous Casting of Steel: Basic Principles

By Bruce Kozak, SMS Demag & Joseph Dzierzawski, SMS Demag

Background

Continuous Casting is the process whereby molten steel is solidified into a "semifinished" billet, bloom, or slab for subsequent rolling in the finishing mills. Prior to the introduction of Continuous Casting in the 1950s, steel was poured into stationary molds to form "ingots". Since then, "continuous casting" has evolved to achieve improved yield, quality, productivity and cost efficiency. Figure 1 shows some examples of continuous caster configurations.

Figure 1 - Examples of Continuous Casters

Steel from the electric or basic oxygen furnace is tapped into a ladle and taken to the continuous casting machine. The ladle is raised onto a turret that rotates the ladle into the casting position above the tundish. Referring to Figure 2, liquid steel flows out of the ladle (1) into the tundish (2), and then into a water-cooled copper mold (3). Solidification begins in the mold, and continues through the First Zone (4) and Strand Guide (5). In this configuration, the strand is straightened (6), torch-cut (8), then discharged (12) for intermediate storage or hot charged for finished rolling.

Figure 2 - General Bloom/Beam Blank Machine Configuration

1:Ladle Turret, 2:Tundish/Tundish Car, 3:Mold, 4:First Zone (Secondary Cooling), 5:Strand Guide (plus Secondary Cooling), 6:Straightener Withdrawal Units, 7:Dummy Bar Disconnect Roll, 8:Torch Cut-Off Unit, 9:Dummy Bar Storage Area, 10:Cross Transfer Table, 11:Product Identification System, 12:Product Discharge System

Figure 3 depicts a Slab Caster layout. Note the extended roller containment compared to that for a Bloom/Beam Blank (as in Figure 2), required to maintain product shape through final solidification.

Depending on the product end-use, various shapes are cast (Figure 4). In recent years, the melting/casting/rolling processes have been linked while casting a shape that substantially conforms to the finished product. The Near-Net-Shape cast section has most commonly been applied to Beams and Flat Rolled products, and results in a highly efficient operation. The complete process chain from liquid metal to finished rolling can be achieved within two hours.

Figure 3 -Slab Caster Layout

Figure 4 - Continuous Cast Shapes (sizes in millimeters)

Production and Feasibility Study

This is the first step in designing a continuous caster. First, the product end-use dictates the quality, grade and shape of the cast product (billet, bloom, slab, beam blank, and/or round). Considerations are then made based on desired annual tonnage, liquid steel availability, and anticipated operating hours. Then, the machine design considerations can be made for the number of strands and cast speeds to match the liquid metal supply from the melt shop. Quality and grade considerations are then utilized in determining various design parameters of the casting machine such as its length, vertical height, curved or straight mold, water versus water/air secondary cooling, electromagnetic-stirring, etc.

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Casting Overview

To start a cast, the mold bottom is sealed by a steel dummy bar, which is held in place hydraulically by the Straightener Withdrawal Units (Figure 2, item 6). This bar prevents liquid steel from flowing out of the mold. The steel poured into the mold is partially solidified, producing a steel strand with a solid outer shell and a liquid core. In this primary cooling area, once the steel shell has a sufficient thickness, about 0.4 - 0.8 inches (10 to 20 mm), the Straightener Withdrawal Units are started, and proceed to withdraw the partially solidified strand out of the mold along with the dummy bar. Liquid steel continues to pour into the mold to replenish the withdrawn steel at an equal rate. The withdrawal rate depends on the cross-section, grade and quality of steel being produced, and may vary between 12 and 300 inches per minute. Casting time is typically 1.0 - 1.5 hours per heat to avoid excessive ladle heat losses.

Upon exiting the mold, the strand enters a roller containment section and secondary cooling chamber (Figure 2, items 4 & 5) in which the solidifying strand is sprayed with water, or a combination of water and air (referred to as Air-Mist) to promote solidification. This area preserves cast shape integrity and product quality. Larger cross-sections require extended roller containment (Figure 3). Once the strand is fully solidified and has passed through the Straightener Withdrawal Units, the dummy bar is disconnected, removed and stored. Following the straightener, the strand is cut into individual pieces of the following as-cast products: slabs, blooms, billets, rounds, or beam blanks, depending on machine design.

Billets have cast section sizes up to about 7 inches square. Bloom sections sizes typically range from approximately 7 inches square to about 15 inches by 23 inches. Round castings include diameters of approximately 5 to 20 inches. Slab Castings range in thickness from 2 to 16 inches, and over 100 inches wide. Beam Blanks are shaped like dog bones, and are subsequently rolled into I-Beams. The width-to-thickness ratio, referred to as the "Aspect Ratio", is used to determine the dividing line between blooms and slabs. An Aspect Ratio of 2.5:1 or greater constitutes an as-cast product referred to as a Slab.

To summarize, the casting process is comprised of the following sections:

• A tundish, located above the mold to feed liquid steel to the mold at a regulated rate

• A primary cooling zone or water-cooled copper mold through which the steel is fed from the tundish, to generate a solidified outer shell sufficiently strong enough to maintain the strand shape as it passes into the secondary cooling zone

• A secondary cooling zone in association with a containment section positioned below the mold, through which the still mostly-liquid strand passes and is sprayed with water or water and air to further solidify the strand

• Except straight Vertical Casters, an Unbending & Straightening section

• A severing unit (cutting torch or mechanical shears) to cut the solidified strand into pieces for removal and further processing

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Liquid Steel Transfer

There are two steps involved in transferring liquid steel from the ladle to the molds. First, the steel must be transferred (or teemed) from the ladle to the tundish. Next, the steel is transferred from the tundish to the molds. Tundish-to-mold steel flow regulation occurs through orifice devices of various designs: slide gates, stopper rods, or metering nozzles, the latter controlled by tundish steel level adjustment.

Tundish Overview

The shape of the tundish is typically rectangular, but delta and "T" shapes are also common. Nozzles are located along its bottom to distribute liquid steel to the molds. The tundish also serves several other key functions:

• Enhances oxide inclusion separation

• Provides a continuous flow of liquid steel to the mold during ladle exchanges

• Maintains a steady metal height above the nozzles to the molds, thereby keeping steel flow constant and hence casting speed constant as well (for an open-pouring metering system).

• Provides more stable stream patterns to the mold(s)

Mold

The main function of the mold is to establish a solid shell sufficient in strength to contain its liquid core upon entry into the secondary spray cooling zone. Key product elements are shape, shell thickness, uniform shell temperature distribution, defect-free internal and surface quality with minimal porosity, and few non-metallic inclusions.

The mold is basically an open-ended box structure, containing a water-cooled inner lining fabricated from a high purity copper alloy. Mold water transfers heat from the solidifying shell. The working surface of the copper face is often plated with chromium or nickel to provide a harder working surface, and to avoid copper pickup on the surface of the cast strand, which can facilitate surface cracks on the product.

Mold heat transfer is both critical and complex. Mathematical and computer modeling are typically utilized in developing a greater understanding of mold thermal conditions, and to aid in proper design and operating practices. Heat transfer is generally considered as a series of thermal resistances as follows:

• Heat transfer through the solidifying shell • Heat transfer from the steel shell surface to the copper mold outer surface • Heat transfer through the copper mold • Heat transfer from the copper mold inner surface to the mold cooling water

Mold Oscillation

Mold oscillation is necessary to minimize friction and sticking of the solidifying shell, and avoid shell tearing, and liquid steel breakouts, which can wreak havoc on equipment and machine downtime due to clean up and repairs. Friction between the shell and mold is reduced through the use of mold lubricants such as oils or powdered fluxes. Oscillation is achieved either hydraulically or via motor-driven cams or levers which support and reciprocate (or oscillate) the mold.

Mold oscillating cycles vary in frequency, stroke and pattern. However, a common approach is to employ what is called "negative strip", a stroke pattern in which the downward stroke of the cycle enables the mold to move down faster than the section withdrawal speed. This enables compressive stresses to develop in the shell that increase its strength by sealing surface fissures and porosity.

Secondary Cooling

Typically, the secondary cooling system is comprised of a series of zones, each responsible for a segment of controlled cooling of the solidifying strand as it progresses through the machine. The sprayed medium is either water or a combination of air and water.

Figure 5 - Secondary Cooling

Three (3) basic forms of heat transfer occur in this region:

• Radiation

The predominant form of heat transfer in the upper regions of the secondary cooling chamber, described by the following equation:

• Conduction

As the product passes through the rolls, heat is transferred through the shell as conduction and also through the thickness of the rolls, as a result of the associated contact. This form of heat transfer is described by the Fourier Law:

For conductive heat transfer through the steel shell, k is the shell's thermal conductivity, whereas A and DX are the cross-sectional area and thickness of the steel shell, respectively, through which heat is transferred. Ti and To are the shell's inner and outer surface temperatures, respectively (Figure 6). As shown in Figure 6, this form of heat transfer also occurs through the containment rolls.

Figure 6 - Solidification Profile Through Steel Shell & Roll

• Convection

This heat transfer mechanism occurs by quickly-moving sprayed water droplets or mist from the spray nozzles, penetrating the steam layer next to the steel surface, which then evaporates. This convective mechanism is described mathematically by Newton's Law of Cooling:

Specifically, the spray chamber (Secondary Cooling) heat transfer serves the following functions:

• Enhance and control the rate of solidification, and for some casters achieve full solidification in this region • Strand temperature regulation via spray-water intensity adjustment • Machine Containment Cooling

Shell Growth

Shell growth can be reliably predicted from Fick's Law:

This equation can be used also to calculate the casting distance (L) where the product is fully-solidified (i.e. no liquid core remaining); solving for "L":

Strand Containment

The containment region is an integral part of the secondary cooling area. A series of retaining rolls contain the strand, extending across opposite strand faces. Edge roll containment may also be required. The focus of this area is to provide strand guidance and containment until the solidifying shell is self-supporting.

In order to avoid compromises in product quality, careful consideration must be made to minimize stresses associated with the roller arrangement and strand unbending. Thus, roll layout, including spacing and roll diameters are carefully selected to minimize between-roll bulging and liquid/solid interface strains. Strand support requires maintaining strand shape, as the strand itself is a solidifying shell containing a liquid core, that possesses bulging ferrostatic forces from head pressure related to machine height. The area of greatest concern is high up in the machine. Here, the bulging force is relatively small, but the shell is thinner and at its weakest. To compensate for this inherent weakness and avoid shell rupturing and resulting liquid steel breakouts, the roll diameter is small with tight spacing. Just below the mold all four faces are typically supported, with only the broad faces supported at regions lower in the machine.

Bending and Straightening

Equally important to strand containment and guidance from the vertical to horizontal plane are the unbending and straightening forces. As unbending occurs, the solid shell outer radius is under tension, while the inner radius is under compression. The resulting strain is dictated by the arc radius along with the mechanical properties of the cast steel grade. If the strain along the outer radius is excessive, cracks could occur, seriously affecting the quality of the steel. These strains are typically minimized by incorporating a multi-point unbending process, in which the radii become progressively larger in order to gradually straighten the product into the horizontal plane.

Figure 7 - Curved Section of Multi-Strand Beam Figure 8 - Straightener Withdrawal Units for Strand Blank Caster prior to Unbending Unbending

After straightening, the strand is transferred on roller tables to a cut off machine, which cuts the product into ordered lengths. Sectioning can be achieved either via torches or mechanical shears. Then, depending on the shape or grade, the cast section will either be placed in intermediate storage, hot-charged for finished rolling or sold as a semi-finished product. Prior to hot rolling, the product will enter a reheat furnace to adjust its thermal conditions to achieve optimum metallurgical properties and dimensional tolerances.

Summary

Continuous Casting has evolved from a batch process into a sophisticated continuous process. This transformation has occurred through understanding principles of mechanical design, heat-transfer, steel metallurgical properties and stress-strain relationships, to produce a product with excellent shape and quality. In recent years, the process has been optimized through careful integration of electro-mechanical sensors, computer-control, and production planning to provide a highly-automated system designed for the new millenium. STEEL MAKING PROCESS FLOW

STEEL FINISHING PROCESS FLOW