Stirring in the Steelmaking Processes*

Stirring in the Steelmaking Processes* By Takeshi KATO** I. Opening Remarks was established, the blowing of pure oxygen, 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 iron or ingot iron in an open hearth furnace, because myself to jobs somewhat sundry. After some ponder- the steel bath was apt to become excessively hotter ing, I came upon a theme of " Stirring in the Steel- in the upper layer, as decarburization 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 irons, 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.. refining 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 metal, 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 puddling with firewood water models, gave us the following insight that by became necessary. This stirring by human 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] metals 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 technology 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 lime 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 plastic 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 tuyere, 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 carbon 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 slag 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 ores 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.

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