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Home , Mill (grinding), Open hearth furnace, Smelting

Stirring in the Processes*

By Takeshi KATO**

I. Opening Remarks was established, the blowing of pure , called It is indeed a great honor for me to be awarded the bessemerizing, was perfected; this gave rise to with the G. Watanabe Medal, for which I sincerely effective stirring, which made the operation of open thank you all. hearth furnace easier and more efficient and greatly When I was asked to deliver a memorial lecture, raised productivity. however, I was at a loss as to what to talk about in Yet, the demand on the ingenuity of engineers was an orderly manner, because it has been already some quite heavy : for example, in the making of Armco 24 years since I left the field of research and devoted or ingot iron in an , because myself to jobs somewhat sundry. After some ponder- the bath was apt to become excessively hotter ing, I came upon a theme of " Stirring in the Steel- in the upper layer, as proceeded and making Processes," a subject which I have been steel became inactive, all sorts of stirring, including interested in through all these years. manual stirring with a stange (pole or rod), the That is to say, in the evolution of the steelmaking throwing-in of pig , and blowing-in of oxygen, process, it seems to me that the " stirring " to achieve had to be tried. Table 1, which I found in the 1962 the desired states of chemical composition and tem- edition of our Iron and Steel Handbook, tells you its perature most efficiently with regard to the quantity own story. and time available has been occupying a very impor- Though methods of stirring similar to those for tant position. I wish, therefore, to review this evolu- open hearth furnaces were employed for electric tion of steelmaking methods with " stirring " as my arc furnaces, adaption of an electromagnetic stirrer viewpoint. to the furnace bottom became popular with large furnaces to make the slagging-off after oxidation II. Changes of Stirring in the Steelmaking Proc.. and the subsequent reduction refining easier. esses And this method evolved later into the ASEA-SKF I have illustrated in Fig. 1 the evolution of steel- secondary refining process or into the electromagnetic making as viewed from the methods of stirring applied stirring in the continuous casting. to various processes. You will note that even in the Then came the LD converter. As we started past the first process of obtaining steel from hot , production, we became aware of an acute need for the puddle method, stirring was undertaken just as understanding the slopping phenomenon and solving its name implied. As decarburization proceeded, it. The simulation studies on the dynamic behavior however, the melting point of hot metal raised, of the steel bath under an oxygen jet by means of aggravating its fluidity, and with firewood models, gave us the following insight that by became necessary. This stirring by power taking, contrary to the generally accepted idea, the had been used in the open hearth furnaces and electric L/ LO ratio large, namely, by making the oxygen jet arc furnaces of the later age until methods of stirring crater deeper relative to the steel bath depth, slopping by gas were implemented. can be prevented. When the Bessemer converter made its appearance, In the meantime, studies of dephosphorization were intensive stirring by bottom-blown air became a new energetically undertaken for converters of high hot refining means, thereby replacing the puddling. metal ratios, because [P] was apt to become high Due to this strong stirring action, decarburization and unless low [P] were discriminately provided. temperature raising became easy, which made steel- Prevention of slopping and dephosphorization are making much easier. mutually contradicting propositions, and how to With the open hearth furnace, which subsequently satisfy both has been a great concern of steelmaking became the mainstream of steelmaking, the stirring engineers. Notions like the hard blow or the soft became important again to prevent over- blow were advanced, but in actual operations, it was oxidation of molten steel and to augment heat conduc- the control of lance clearance in consideration of the tion. Here, besides the traditional puddling or simi- blowing time, the kind of steel to be made, the life lar methods, the throwing-in of ingot pig or of furnace, and such, that proved most effective. stone, so-called lime boiling, were added as new When enlargement of the LD converter began to stirring methods. As mass production of oxygen be studied, the question was, if one and single lance

* Based on the Special Lecture delivered by Dr . Kato on the commemoration of his receiving G. Watanabe Medal in the l l lth ISIJ Meeting on April 2, 1986, at The University of Tokyo in Tokyo. Manuscript received on May 23, 1986. ©CISIJ 1986 * * Dr . Eng., President, Godo Steel Ltd., Umeda, Kita-ku, Osaka 530.

Special Lecture (851) (852) Transactions 'SIT, Vol. 26, 1986

Fig. 1. Changes in the method of steelmaking and stirring.

Table 1. An example of making Armco iron in a 60 t open hearth furnace.

would be enough for adequate mixing. Simulation lance at the dead center of the Tokyo Bay, do we get studies with water models gave us an answer through a good mixing in Ghiba and Kawasaki ? * So, we dimensional analysis in the affirmative. But I remem- thought a large scale experiment should be made in ber I jokingly asked that if we set up just one big order to make a feasibility study of large converters,

* Two cities that face eac h other diametrically across th e Tokyo Bay, an d that are well known for their great integral steelmaking plants.

Special Lecture Transactions ISIJ, Vol. 26, 1986 (853) and we made a large model. To visualize improved Thomas process, which utilizes oxygen the states of mixing at periphery, we hang, of all enrichment of the bottom-blown air, was intensively things, china marble candies along the wall, because pursued. In this method, steam of CO2 was also they change color as they melt. used to protect the , and in an extension of this, This experiment made us suspect that the stirring the double-tube tuyere was developed. From this, in the periphery would be insufficient, whereupon we furthermore, methods of using fuel as well as pure decided on two courses of research. In one, we oxygen to protect the tuyere were developed, making conducted experiments of tangential bottom-blowing it possible to blow in oxygen from the bottom, namely of inert gas in a small experimental furnace, while giving birth to the bottom blowing oxygen converter. in the other, we pushed forward with the develop- To give theoretical foundation to the features of ment of a multiple-nozzle lance. We found soon the bottom blowing oxygen converter of quiet and enough that, as the use of a multi-nozzle lance gave steady progress of reactions, yet with little formation rise to good mixing in the steel bath even with soft of (FeO), re-examination of stirring was begun. It blow, slopping was prevented, and operation much was found that, when stirring was augmented by stabilized. Thus, implementation of the multi-nozzle bottom blowing, merits such as (1) decreased oxidation lance made enlargement of LD converter possible losses of Fe and Mn in the low range and with one lance, which in turn made the converter (2) improved controllability due to the elimination steelmaking progress by leaps and bounds. As one of inhomogeneities in composition and temperature reason why the converter steelmaking technology of the steel bath was readily be enjoyed. Also, made such remarkable progress here in Japan, I attempts at quantification of the effects of stirring would like to cite the efforts all of us expended in were made. earlier times in controlling the flow in the steel bath In those studies, a notion that for LD converters due to oxygen jet. the stirring by oxygen jets alone is insufficient has As for the inert gas bottom-blowing experiment been put forward. As a difficulty of control that I mentioned, we had to discontinue it even was apparent with bottom blowing oxygen converters, though we did plan on a larger-scale experimentation, however, development of the top and bottom blowing because we did so well with the multi-nozzle lance converter that takes advantage of the merits of both on one hand and we decided to use that experimental was pursued intensively in France, U.K., and Japan converter for the study of converter gas recovery, on almost at the sametime. The result has been that the other hand. We did not know then that we were today in Japan, as may be seen in Fig. 2, most of our to revive this technology 17 or 18 years later to aug- LD converters have been remodeled into the top ment the stirring, which was thought still insufficient and bottom blowing version. for large converters. Incidentally, this bottom blowing oxygen converter was developed by Maxhutte of Germany, and was III. Top and Bottom Blowing Converter named OBM. It was then taken up by U.S. Steel, In Europe, where high phosphorus were used, and they called it Q -BOP. Now, it was about a the Thomas converter was popular, but as oxygen decade ago that I was asked by a Thyssen engineer became widely available, studies of the so-called if I knew what OBM was an abbreviation of. Well,

Fig. 2. Emergence of the top and bottom blowing converter.

Source M. Hanmyo : The 100th-101st Nishiyama Memorial Seminar ISIJ, Tokyo, (1984), 203.

Special Lecture ( 854 ) Transactions ISIJ, Vol. 26, 1986

he told me that it stood for Ober Bayerische Musik, Thyssen remodeled their LD to be bottom blowing, and I thought he meant rather sarcastically that the they left the top-blowing lance intact. process was popular only in the Southern Germany. Whatever early history was, the top and bottom But I have heard that Thyssen too had their LD blowing technology has since evolved into a multitude converter converted to the bottom blowing. Then, of practicable methods, one subtly differing from next in 1978 or thereabout, Maxhi tte conducted experi- in the kind of gas to be blown from bottom as sum- ments of melting a lot of by providing a top- marized in Table 2, or in the form of the tuyere to blowing lance to their OBM, and called it OBM-S, be used as illustrated in Fig. 3, or if GaO is blown in with an S for scrap. People told me then that when or not.

Table 2. Characteristics of various combined-blow processes.

Fig. 3. Bottom-blow nozzle.

Source : M. Hanmyo: The 100th-101st Nishi- yama Memorial Seminar, ISIJ, Tokyo, (1984), 204.

Special Lecture Transactions ISIJ, Vol. 26, 1986 ( 855)

From the quantitative studies of the stirring force stirring in the up and down directions as well. Well, of bottom-blown gases into the steel bath, we have I never have heard his invention materialized. learnt that the stirring action is different with the As the partitioning refining spread, and as the kind of gas used because of the reaction between tapping temperature became higher for the conve- blown-in gas and steel bath. For example, when niences of continuous casting, instances of performing 02 or CO2 is bottom-blown, a strong stirring force dephosphorization with the hot metal pretreatment is developed while the carbon is still high enough, but increased.. To do this, however, two steps, first the stirring action rapidly diminishes in the low carbon oxidizing P then forming slag for it, are necessary. range of, say, C<0.10%. Here, strong stirring means This means either 02 itself or an oxidizer as well as a preferential oxidation of C, or fast decarburization, dephosphorizer have to be supplied efficiently, for and weak stirring implies a delay in C supply to the which stirring will become the critical factor. reaction sites, meaning the oxidation of Fe is apt to Furthermore, the trend is to perform desiliconiza- occur. Based on these notions, parameters that tion outside the furnace. Generally, this is done at determine proper oxygen feeding rates and bottom- the tapping runner by either projecting or spraying blown gas flow rates to attain the target (T.Fe) have the oxidizer, such as a scale, on to the hot metal been proposed. stream. But it is known that desiliconization takes Intensification of stirring has brought about an place only at cascading parts, where the hot metal expectation of further improvements in the yield of and desiliconizer are mixed well, and that the slag- Cr or Mn, or even positive reduction of their ores. metal reaction in the static bath hardly progresses. Indeed, they may be said to be adding new functions You would agree that stirring is the most important to the converter. When applications to such new single factor also for pretreatment. technical field such as gasification of or reduction of ores are considered, however, I must V. Ladle Refining say that there is much to be done in the quest for As purification of converter steel progressed, optimum stirring conditions and in establishing on vacuum degassing became widely spread. Here, optimum nozzle design. I would, therefore, hope methods of analysis of molten steel flow that were for further development in those fields. developed for the LD converter, were applied to the DH and RH to quantify the metallurgical effects and Iv. Hot Metal Pretreatment raise process efficiency. Today, there is another prominent trend in steel- In the DH, for example, the traditional designing making besides the top and bottom blow. It is for a large bath surface area in emphasis of availability partitioning refining, namely, a combination of hot of reaction interface has been changed to designing metal pretreatment-decarburization in the converter for larger stirring force by allowing greater vertical -ladle refining . motion. In the RH, as intensification of recircula- As the LD converter process became the main- tion of molten steel has become known to increase stream of steelmaking, purification of converter treatment effects, steels of lower carbon, though became the subject of study. Whereupon, desulfuri- better homogenized, have become produced in the zation of hot metal was developed energetically and same treating time by enlarging the immersion tube duly put to practice. Along with it, moreover, inner diameter or by increasing the Ar flow rate. vacuum degasing became popular. A consequence is that today both DH and RH have In the desulfurization of hot metal, the old method been recognized as indispensable in making special of adding soda ash to utilize the dynamic motion of steels with a converter. hot metal for stirring became not good enough because As the were improved, furthermore, of low temperature and other reasons, and various the stirring that were developed for hot new methods of stirring were devised and tried in metal desulfurization became applicable to ladle various new desulfurization processes, as may be seen refining. The result has been the development of in Fig. 4. In the end, the porous plug method and various processes as shown in Fig. 5, owing to which the injection lance method, in which powerful stirring making of special steels with a converter has become is achieved by the gas blown in through the porous so much easier. Nowadays, the gas bubbling method plugs provided at the ladle bottom of the former, and the electromagnetic stirring method are available or by the carrier gas on which the desulfurizer agent besides the vacuum treatment method, so that, when is blown into the metal through a lance in the latter, a single method is deemed insufficient, any of these have survived aided by improvements achieved in may be used complementarily. Not only that, various the refractories. measures are being taken, such as performing a part There have been many exotic inventions. One of the decarburization by blowing 02 or compensating such idea that I still remember was what an European for the temperature drop during treatment by equip- gentleman tried to sell me some twenty years ago. ping the ladle with a heating apparatus. He called it the " Resonator ", and according to this As for the Ar bubbling with regard to homogenizing man, if we are to line a ladle in an eliptical cylinder the steel composition and to accelerating separation and rotate it, the hot metal bath within should start and ascension of nonmetallic inclusions, its effective- resonating at a certain rotation speed to generate ness has been demonstrated indubitably by presenting waves. Then by tilting the ladle, we should have a decrease of nonmetallic inclusions in terms of the

Special Lecture (856) Transactions ISIJ, Vol. 26, 1986

Fig. 4. Various hot metal desulfurization processes.

Source : Genzai no Kinzokugaku, Seiren-Hen I (The Modern Metallurgy, Refining Processes I), ed, by The Japan Inst. Metals, Maruzen, Tokyo, (1979), 222.

decrease in the total oxygen in steel. Indeed, to make ability and to lower H2 in steel. clean steel, it used to be in the olden times " keep the So, you will agree that stirring is a basic element steel still ", but nowadays it is " stir it up ". This in ladle refining, too. has revolutionalized the technological notion of deoxi- dation also. VI. Concluding Remarks To meet the demand for continuous casting, on I have briefly described the evolution of steelmaking the other hand, the composition and temperature processes and the changes of the stirring technology of the molten steel had to be controlled in a narrow that were made in it. I have also described how the area. This has increased the importance of ladle progress of stirring was important in establishing the refining, which in turn is being refined mostly by mass production processes of steel and the processes means of improving gas stirring. Speaking of gas of making high grade steels, so important in fact that stirring, even with the ASEA-SKF process, which features of those steelmaking processes that have been utilizes electromagnetic stirring, concurrent applica- industrialized today can hardly be clearly defined tion of Ar bubbling is known to raise the refining without referring to stirring.

Special Lecture Transactions ISIJ, Vol. 26, 1986 (857)

Fig. 5. Various ladle refining processes.

Source : B. Eto : The 27th-28th Nishiyama Memorial Seminar, ISIJ, Tokyo, (1974), 5. T. Ito: The 72nd-73rd Nishiyama Memorial Seminar, ISIJ, Tokyo, (1981), 162.

Though I did not go into any detail, no big words only indirectly. Therefore, in the matters of predict- are needed to see that the simulation experiments ing reaction or designing an apparatus, much still with water models centering on dimensional analysis remains dependent on experiences. and the computer numerical analysis have contributed Now we are in a position where we may expect greatly to the implementation of all those stirring successful development of new steelmaking processes, methods, thus accelerating the industrialization of including smelting reduction. And this is why I hot metal pre-treatment and ladle refining. call for a renewed endeavor to advance the quantifica- Since steelmaking processes are essentially the tion of the flow in steelmaking processes and to batch refining process, and since the temperatures develop, by the force of industry and academy com- and specific gravities concerned are both especially bined, new processes that will allow us to the high, however, no ideal stirring method can be found. world by making an effective use of stirring. Also, the states of the steel bath motion can be grasped I thank you for your attention.

Special Lecture