Causing a stir

ArcSave® increases productivity and lowers costs in electric arc furnaces

LIDONG TENG, AARON JONES, MICHAEL MEADOR, HELMUT homogenous melt bath, a higher decarburization rate and HACKL – ArcSave® is a new generation of ABB electromag- a higher free opening frequency of the eccentric bottom netic stirrer (EMS) for electric arc furnaces (EAFs) that helps tapping (EBT). ArcSave has also reduced the tapping to improve safety, increase productivity and reduce costs. temperature and tapping oxygen in the steel, which brings The first ArcSave system was installed in 2014 on a 90 t arc a higher scrap yield and saves ferroalloy consumption in furnace. The hot test results show that it has stabilized the the downstream ladle furnace operation. The lower energy arcs and enhanced the heat and mass transfer in the arc consumption, short tap-to-tap time and consistent furnace ­furnace process. This results in a faster scrap melting rate, operation greatly increase the productivity and operation a lowering of the slag superheat during arcing, a more safety.

Causing a stir 25­ ABB has been committed to the development of new electromag- netic products to improve quality, productivity and safety in the steel business over 70 years.

BB has been committed to the SDI believes that every ton of steel has development of new electro- to be produced as efficiently and safely magnetic products to improve as possible. For this reason, SDI entered A quality, productivity and safety into the project with ABB to install in the steel business over 70 years. The ­ArcSave on the EAF. first electromagnetic stirrer for electrical arc furnaces (EAF-EMS) was delivered in Stirring principles 1947 to Uddeholms AB in Sweden and The ArcSave stirrer was placed under a since then more than 150 units have nonmagnetic (austenitic stainless steel) been installed worldwide. Recently, a steel plate bottom ➔ 1a. Low-frequency new generation of EAF-EMS – ArcSave electric current in the stirrer windings – has been developed by ABB to fulfill generates a traveling magnetic field that penetrates the fur- nace bottom and The main difference ArcSave generates physical forces in the mol- makes to the EAF is the ten steel ➔ 1b. Stir- ring can be cus- intensity of convection in the tomized to match melt bath. the needs of differ- ent EAF process steps such as the requirement for a stronger stirring scrap heating, homogenization, melting power in the EAF process for both plain of alloys, decarburization (reduction of carbon and high-alloyed steel produc- carbon content), deslagging and tapping. tion. The designed average melt velocity in- In July 2014, the installation and com- duced is around 0.5 m/s. ➔ 2 shows a missioning of the first ArcSave was com- simulated example of the horizontal Title picture pleted on a 90 t AC EAF at Steel Dynam- cross-section of the mean flow pattern Electromagnetic stirrers are essential for the ics, Inc. (SDI) in Roanoke, United States. for the horizontal plane about 25 cm efficient operation of arc furnaces. ABB’s new SDI is a company with a tradition of im- ­below the melt surface in a 150 t EBT generation of stirrer – ArcSave – has been in operation for a year in a steelworks – what are provement using the best technology tapping EAF. It can be seen that the flow the results so far? available. In order to remain competitive, is slightly asymmetric. However, com-

­26 ABB review 3|15 1 Arc furnace at the SDI site

1a Showing the ABB stirrer mounted underneath 1b Furnace top view schematic showing the stirrer underneath and flow patterns

pared with the more localized bottom crease in surface superheat will reduce gas stirring via porous plugs, the EMS heat losses to the furnace wall and roof, creates a global circulation in the arc fur- and thereby reduce electricity consump- nace bath and thereby provides a more tion. Simultaneously, the stirring will in- efficient mixing of the complete bath crease the scrap melting rate and decar- melt. This mixing effect accelerates the homogenization of the temperature and chemical composition of the steel, as At SDI, the total well as that of the chemical reactions ­between steel and slag. energy saving is

The complete ArcSave system compris- about 14 kWh/t, es the electromagnetic stirrer, a frequen- which is equivalent cy converter, a transformer and a water station ➔ 3. It has the following features: to 4 percent of − No physical contact with the steel melt electrical energy − Normal refractory lining can be used saving. − The stirring direction can be reversed by changing the current direction − Very little maintenance is required burization rate and therefore save fur- ­nace process time, which also reduces The hot test results from SDI show sig- the heat loss. In the ArcSave test at SDI, nificant process benefits have been the total energy saving includes the achieved due to the improved kinetic chemical energy decrease from the low- conditions for heat and mass transfer ered consumption of natural gas, carbon with ArcSave. It is worthwhile to examine and oxygen, and the reduction of FeO in these benefits in detail. the slag. The total energy saving is about 14 kWh/t, which is equivalent to 4 per- Arc heating efficiency and energy cent of the total. Because of the higher saving power input efficiency and higher heating Due to the homogenization, the temper- efficiency, ArcSave reduces the power- ature gradient in the melt with EMS is on time by around 5 percent. reduced to only 25 percent of what it was before. This means that the EMS Scrap melting ­reduces the melt surface superheat and The main difference ArcSave makes to the heat from the arc zone is quickly the EAF is the intensity of convection in transmitted to the melt bulk. The de- the melt bath. The forced convection in-

Causing a stir 27­ EAF bath stirring 2 Melt flow pattern created by electromagnetic stirring [1] can push the ­carbon-oxygen ­reaction closer to the equilibrium value. It is also seen that the

Fe2O3 content in the slag is reduced by 2.5 percent.

Main velocity magnitude:

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

duced by electromagnetic stirring will en- es for a three-bucket melting period hance the melting of larger scrap pieces is reduced by about 50 percent with and bundles, contribute to a homoge- ArcSave ➔ 4b. The reduced current nous temperature and composition dis- swings result in higher power input and, tribution, and make scrap stratification therefore, increased productivity. less significant. Without stirring there is only the very low convection caused by A homogenous temperature distribution in the difference of densities. the melt bath also gives a hot EBT and smooth tapping without delays and makes Stirring also reduced electrode stabili- it possible to obtain an exact tapping tem- zation time (by some 10 percent) after perature for the different steel grades. bucket charging at SDI ➔ 4a. In addition,

it has been found that the EMS stabi- Decarburization and O2 yield lized the arc by melting big scrap bun- After the scrap is completely molten, dles faster and by reducing scrap cave- the refining period starts. This mainly in- volves decarburi- zation by injecting A homogenous temperature oxygen. This injec- tion creates a high- distribution in the melt bath ly turbulent reac- tion zone where also gives a hot EBT and carbon from the smooth tapping without bulk metal can re- act with oxygen or delays and makes it possible FeO. If the steel carbon content is to obtain an exact tapping high, the decarbu- temperature for the different rization reaction rate is determined steel grades. by the rate of oxy- gen supply. How- ever, if it is lower ins (surges in scrap movement in the than a certain level, the rate of carbon melt that disturb process continuity). transport to the reaction zone would The standard deviation of current chang- consequently normally be lower, as

28 ABB review 3|15 3 System overview of ArcSave

Junction box

Deaeration valve

Safety switch Monitoring unit

Converter

Pump unit

Transformer

Customer Customer Customer Customer cooling water supply compressed air supply network power feeding

would then be the decarburization reac- Steel yield tion rate. But ArcSave facilitates carbon Besides the FeO reduction in slag, an- mass transfer to the reaction zone and other big contribution to the steel yield increase is the re- duction of metal- ArcSave facilitates carbon lic scraps in the dumped slag – it is mass transfer to the reaction found that the scrap in the recy- zone and can double the de- cled slags is re- carburization reaction rate, and duced by about 40 percent. This gives reduce oxygen consumption. some 0.6 percent steel yield increase in the EAF. The can double both these rates and reduce reason for the reduction of steel droplets oxygen consumption – by 5 percent in the slag with stirring is under investi- at SDI. gation, but the intensive slag/metal inter- action and the more homogeneous slag Steel deoxidation with ArcSave are possible causes. It is known that EAF bath stirring can push the carbon-oxygen reaction closer Yet another contribution to the steel yield to the equilibrium value [2]. This has been increase is the metallic yield increase due proven by the test results from SDI, to lower tap oxygen in the steel. where the tap oxygen was reduced from 618 ppm to 504 ppm, with a slight tap Vortex formation and slag carryover carbon increase. These results indicate Theoretically, an EBT hole should result in that ArcSave makes it possible to reach slag-free tapping. However, slag carry- lower carbon and oxygen levels at the over is always evident in the tapping ladle, same time. It is also seen that the Fe2O3 the main reason being vortices in the later content in the slag is reduced by 2.5 per- stage of the tapping. Water modeling re- cent with ArcSave. This, according to the sults show that vortex formation can be materials balance calculation, gives a suppressed by the stirring force from 0.2 percent steel yield increase, which ­ArcSave. The slag thickness in the tap reduces scrap and conversion cost. ladle was measured by the aluminium-

Causing a stir 29­ 4 Effect of ArcSave on electrode current swings

80.0 10.0

70.0 9.0

8.0 60.0 7.0 50.0 6.0 40.0 5.0 30.0 4.0 20.0 3.0 Secondary current (kVA) Secondary current

10.0 deviation of current Standard 2.0

0.0 1.0

-10.0 0.0 0.0 50.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 55.0 60.0 First bucket Second bucket Third bucket

Process time (min)

Without stirring With ArcSave Without stirring With ArcSave

4a Comparison of the electrode secondary current without stirring and 4b Standard deviation of current change for a three-bucket melting with ArcSave. The current swings were reduced using ArcSave. period. It is reduced by about 50 percent with ArcSave.

steel pole method. The results show that ­increases the EBT free opening ratio, the carryover slag amount is reduced by thereby increasing furnace safety. 50 percent with ArcSave. The following process disturbances Furnace refractory could be decreased or eliminated with Six months of hot test results at SDI the aid of ArcSave: show that stirring with ArcSave reduced − Scrap cave-ins furnace refractory repairs by some 15 − Residual non-molten big scrap or pig percent. The superheat reduction is iron probably the main contributor to this − Carbon boiling-out (sudden boiling since the most critical refractory damage phenomena in the melt) – a significant is in the slag-line area. The reduction of topic if charging with pig iron the FeO in the slag and the oxygen in the − Off-target tapping weight and steel also helps. A third contribution to ­temperature reduced refractory wear is the lowered − Low EBT opening ratio ➔ 6 tap temperature – down by 14 °C without affecting the ladle furnace arrival tem- Safer, quicker and cheaper steel- perature ➔ 5. making The review of results from the SDI plant Process reliability and safety has shown that ArcSave has produced Safety and reliability are always of great multiple benefits for the steelmaker ➔ 7. importance for EAF operation. The posi- ArcSave helps the melting process make tive effects of ArcSave on the EAF pro- liquid steel more safely, quicker and with cess discussed above will have a signifi- lower cost. ArcSave improves the heat and mass transfer of the EAF pro- The review of results from the cess; speeds up the scrap melt- SDI plant has shown that down; accelerates the homogeniza- ­ArcSave has produced multiple tion of the temper- benefits for the steelmaker. ature and chemical composition of the steel bath; forces cant impact on improving process the metal/slag reactions closer to the reliability and safety. ArcSave decreases equilibrium state; increases the decarbu- the tap temperature and reduces the rization rate; and improves the operation ­superheat in the hotspots area, and safety, reliability, and productivity. The

­30 ABB review 3|15 5 Change of tap temperature after ArcSave 6 Change of EBT opening frequency after ArcSave. High EBT opening frequency is beneficial for operation safety and productivity.

1,650

1,639 100 1,637 97

C) 78 80 1,625 1,626

60

1,615 40 EBT opening ratio (%) Last tap temperature ( ° Last tap temperature 1,604 20

1,593 0

Without stirring With ArcSave Without stirring With ArcSave

7 Process benefits summary

Process items Benefits

Total energy - 3~5%

Electrode - 4~6%

Power on time - 4~6%

Oxygen - 5~8%

Deoxidants -10~15%

Steel yield + 0.5~1.0%

Productivity +4~7%

Six months of hot Lidong Teng test results at SDI ABB Metallurgy, Process Automation show that stirring Västerås, Sweden [email protected] with ArcSave Aaron Jones reduced furnace Michael Meador Steel Dynamics, Inc., Roanoke Bar Division refractory repairs by Roanoke, VA, United States some 15 percent. Helmut Hackl ABB Metals, Process Automation impact of ArcSave will be particularly Västerås, Sweden ­significant in those EAFs requiring a [email protected] ­productivity increase.

References The authors would like to acknowledge the kind sup- [1] O. Widlund et al., “Modeling of electric arc port from and valuable discussions with Paul Schuler furnaces (EAF) with electromagnetic stirring,” and Nuno Vieira Pinto at SDI (Roanoke) during the in Proceedings of 4th International Conference ArcSave hot test work. Thanks also to Chris Curran on Modelling and Simulation of Metallurgical from ABB Metallurgy, Canada, for the kind help with Processes in Steelmaking (SteelSim), Düssel- the carryover slag measurement work. Thanks to dorf, Germany, 2011. Boo Eriksson and Jan Erik Eriksson from ABB in [2] R.J. Fruehan, The Making, Shaping and Treating Sweden for the useful technical discussions during of Steel, Volume 2: Steel Making and Refining, the test work. The AISE Steel Foundation, 1998, pp.125–133.

Causing a stir 31­