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Studies of NRIM Continuous Steelmaking Process*

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Studies of NRIM Continuous Steelmaking Process* UDC 669.18-932 Studies of NRIM Continuous Steelmaking Process* By Ry uichi NAKAGAWA:* S hiro YOS HIMATSU:* Taklly a UEDA:* Tatsllro M ITSUI, ** Akira FUKUZA WA:* A kira S ATO:* and TS llyoshi OZAKI** Synopsis K.)U- 13) The fundamental aspect q! the development of the NRIM multi-stage (2) Tank type continuous steelmaking process at trough type continuous steelmaking process and the results of its recent IRSID (France )14- 16) operations are presented in this jJaper. Though the scale of the plant used (3) Single stage trough type continuous steel­ was small (7.8 tlhr in hot metal flow rate), a suitable sejJaration of the making process (WORCRA process) at CRA (Austral­ steelmaking reactions to each stage of the continuous steelmaking furnace ia)17 - 20) and the know-how qf its ojJeration were satiifactorily obtained. As the (4) Single stage trough type continuous steel­ result of the separation, that is, silicon and phosphorus were mostly removed making process at Bethlehem Steel Co. (U.S.A. )21) in the first stage so that the final carbon level was controlled mainly in the second stage, the product with phosphorus as low as 0.005% (dephosjJhori­ (5) Single stage multi-chamber type continuous z ation rate 96% ) was obtained with comparable amount of lime to that steelmaking process at MISiS (U.S.S.R.)22) of the conventional batch type steelmaking processes. The industrializ a­ (6) Multi-stage trough type continuous steelmak­ tion of this process is confirmed to be feasible. ing process at NRIM (Japan)23- 34) In NRIM the research on the continuous steelmak­ I. Introduction ing process which takes a part of the integrated con­ One of the d istinguishing characteristics of the con­ tinuous iron and steelmaking plant was started in 1964 tinuous steelmaking process is that the measurement to obtain fundamental informations. The NRIM a nd control of steelmaking reactions are more easily three stage trough type continuous steelmaking equip­ made as compared with other processes. Though con­ ment was designed and installed in 1967 from the view tinuous operation, in general, accompanies some tech­ point that the multi-stage reactor would be more ad­ nical difficulties which are not observed in the case of vantageous for the continuous steelmaking operation. batch operation, it is advantageous in the following As satisfying results have recently been obtained espe­ points: cially on the dephosphorization, fundamental concepts (I) Feasibility for mass production of the NRIM process and its recent results are reported (2) Reduction of installation and running cost here. (3) Uniformity of the quality of product (4) Facility of process control II. Experimental Equip men t of t h e NRIM Con­ Any kinds of experimental plant tests which have so tinuous Steelmaking Process far been done for the technical progress and improve­ ment in the above-mentioned fields, have more or less 1. Fundamental Concepts for the Development of the Ex­ aimed to draw these advantages, and eventually related perimental Equipment to the continuous operation. The experimental equipment was designed with in­ In the iron and steelmaking process, various sorts tent to realize a continuous steelmaking furnace in of continuous operation are adopted: continuous cast­ which complicated steelmaking reactions can be sep­ ing, hot and cold strip mill, surface treatment of sheet arated into the specified reactions at the specified iron, etc. The blast furnace operation is also con­ stage, where those reactions can be controlled and tinuous in essence. molten metal can be transported in the simplest way. Due to the complicated reactions at high tempera­ Through various trials the present unit furnaces and ture, steelmaking process is still in batch operation by their layout have been adopted. electric furnace, open-hearth furnace or converter. At first it was considered that the following condi- Especially the basic oxygen steelmaking process has tions should be fulfilled by every unit furnace: been adopted in many steel plants by virtue of its high (1) Steady state to be easily obtainable productivity. And there is a good chance for the (2) Bath mixing conditions to be changeable continuation of oxygen steelmaking due to the recent (3) Suitable measuring site to be obtainable development in both process control and refractories, (4) Easy slag- metal separation which would result in full continuous operation of the (5) Slag-formation in a short period of time iron and steel industry. Persevere efforts have been (6) Applicability for countercurrent operation made from earlier times to make steel continuously1- 10) between metal and slag and the reports described the technical feasibili ty are A special consideration was paid on the m ixing summarized as follows: 1l- 34 ) condition, since the controllabilities of these reactions (1) Spray steelmaking process at BISRA (U. and the stability of the composition of product were * Originally published in Tetsu-to-Hagane, 59 (1973), 414, in Japanese. English version received March 29, 1973. ** National Research Institute for M etals, Nakameguro, Meguro-ku, Tokyo 153. Re se arch Article ( 333 ) (334 J Transactions ISIJ, Vol. 13, 1973 markedly affected by the mixing conditions. There­ urement and control and also offers the optimum fore, considering the complexity of steelmaking reac­ application of refractori es depending on the type ofreac­ tions, a unit furnace which could provide any level of tion and temperature. This mea ns, for example, that mixing characteristics is indispensable. the use of basic refractories for dephosphorization and Proposed continuous steelmaking processes can be the use of cheaper acid refractories for decarburization classified into two types, namely the converter or tank will be acceptable. From the above reasons, a multi­ type and the trough type. Tank type reactor belongs stage reactor was adopted. to a perfect mixing reactor and seems to be suitable As the consequence of foregoing considerations, for the promotion of bath mixing and formation of steelmaking reactions were supposed to be separated emulsion which play an important role in the steel­ into three groups, that is, desiliconization and dephos­ making process. In this type of process, however, it phorization, decarburization, and the final control of is very hard to maintain stable outputs against the steel grade, so that three-stage cascade type continuous variations of inputs, and is not possible to satisfy the steelmaking furnace was adopted. conditions(3) and (4) mentioned above. In addition, Concepts for the development of our experimental an extra vessel is necessary for the slag- metal separa­ process have been stated here, and the present plant tion. On the contrary, trough type reactor belongs after several alterations is called as the NRIM con­ to a plug flow reactor which is attractive to control tinuous steelmaking furnace- type 16- 2. the reactions occurring inside of the furnace as a factor of position along the reactor. This is advantageous 2. Constitution of the Experimental Plant for the m easurement and control of steelmaking reac­ Though the experimental plant must be able to tions. But it has also some problems, such as the con­ obtain exactly the factors which affect reactions, dis­ struction of furnace and the heat loss caused by its turbances from some uncertain factors are unavoidable rectangular shape. Since these two types have re­ because the present plant is so small as compared with spectively the merits and demerits, it is difficult to judge the industrial one. Namely, the scale factor must be which process is more advantageous. taken into account in setting up the equipment. Es­ An important item considered for the design of a pecially in the case of a small laboratory scale steel­ unit furnace was the mixing characteristics (modified making plant, it is difficult to select the right configura­ Peclet number or the ratio between perfect mixing tion of refractories, and the heat capacity is low in a and plug flow), which could easily be selected by the whole. These facts bring about serious problems for combination of the flow rates of oxygen and hot metal experimental operations. Attention should be paid and the lance condition (position, number, height etc.) for the controllable factors as much as possible in order As the result of this consideration, trough type was to raise the accuracy of experiments. adopted in the current work as the most suitable shape 1. Hot Pig Iron Feeding Equipment (Holding Furnace) for a unit furnace. In the case of trough type the The capacity of the holding furnace is 15 t because existence of settling flow zone provides the part of of the restriction of the shop. The size of holding slag- metal separation and the site for measurment, furnace is 1.4 m in inner diameter and 1.6 m in respectively. Simultaneous supplying system of oxy­ height, and the wall is lined with chamotte and in­ gen and flux (oxy-flux lance method) was adopted for sulating bricks. By using a LPG burner, 12 t of hot the faster slag-formation. Because of widely variable pig iron at I 400°C can be held in the furnace without lance position, slag- metal countercurrent operation any practical temperature drop. The holding fur­ is feasible, which would be advantageous for refining nace is equipped with load cells measuring its gross of high phosphorus hot metal. The shape of unit weight and the fe eding rate of hot metal is regulated furnace was then determined. by tilting the furnace oil-hydraulically in accordance In order to make effective continuous steelmaking with the prescribed time- weight diagram. However, operation, the following conditions were proposed as momentary feeding rate by this manual control method the requisites: deviates considerably, and according to the recorded (1) To minimize the decrease of reaction efficien­ chart the momentary deviations reach to ± 50 kg/min cy which is inherent in continuous operation for the aimed feeding rate, 130 kg/min.
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