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Steelmaking Technologies Contributing to Steel Industries

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Steelmaking Technologies Contributing to Steel Industries Steelmaking Technologies Contributing to Steel Industries Haruyoshi Tanabe* and Masayuki Nakada** * General Manager, Steelmaking Technology Development Dept., Steel Technical Center ** General Manager, Steelmaking Research Dept., Materials & Processing Research Center NKK developed steelmaking technology in the 20th century while considering its social responsibilities. NKK devel- oped and applied highly efficient production technology and produced high quality steel products while reducing the generation of wastes. The 21st century will face a worldwide borderless society. Hence, the newly formed JFE Steel Corporation will develop new steelmaking processes and contribute to meeting diverse demands from around the world. the-world, open-ladle-type desiliconization station was in- 1. Introduction stalled at Fukuyama Works in March 1998. The silicon NKK has continued to develop new steelmaking tech- content of the hot metal was minimized before dephospho- nologies in its never-ending pursuit of the ideal of meeting rization, improving the efficiency of lime for dephospho- the needs of society. The unique, world-leading steelmak- rization. Moreover, slag generation through the entire ing technologies developed and put into practice by NKK steelmaking process was successfully lowered to an ulti- are represented by the dramatic reduction in the amount of mate level. The phosphorous content of the steel was low- slag generation due to efficient dephosphorization in the ered to the level of the final product specification while hot metal pre-treatment process; high-speed and extremely still in the hot metal stage, and slag generation during effective quality control in the continuous casting process; dephosphorization in the converter was nearly eliminated. and the development of useful products made from steel- Therefore, this technology was named the ZSP (Zero Slag making slag. These technologies constitute an important Process) and deployed at Keihin Works as well in May of foundation for the newly formed JFE Steel Corporation to the same year, expanding the application of this process continue responding to social needs into the 21st century. company-wide1). Fig.1 shows the process flow at the Fu- This paper summarizes major steelmaking technologies kuyama Works. Hot metal is transferred through the de- that NKK has developed in recent years, noting their sig- siliconization station, a mechanical-stirring-type desulfu- nificance. rization process called KR, a ladle type dephosphorization station called the NRP (New Refining Process), or an 2. Refining technologies LD-converter-type dephosphorization process called 2.1 Hot metal pre-treatment technology LD-NRP, before finally being charged into a converter. The hot metal pre-treatment method, where hot metal is Each technology for the ZSP is described below. dephosphorized prior to refining in a converter, was ac- 2.1.1 Technology for mass-production ultra-low tively put into practice in the 1980’s by the major steel- silicon hot metal makers of Japan in order to meet customers’ requirements The silicon content of hot metal tapped from the blast for lower phosphorous content in steel products, while re- furnace is already lowered to a level of 0.2% by the low ducing slag generation and increasing the iron yield. silicon operation of the blast furnace. This is achieved by However, dephosphorization by this method is still insuf- methods such as low temperature operation, wherein the ficient, and a more effective process was desired. temperature is measured continuously. The low silicon hot A thorough investigation by NKK on the dephosphori- metal is then sent to the desiliconization station, where it- zation mechanism revealed that lowering the silicon con- becomes ultra-low silicon hot metal with a silicon content tent in the hot metal to an ultimate level leads to a of less than 0.10%. dramatic improvement in the efficiency of lime for At the desiliconization station, oxygen gas is used along dephosphorization. Based on this finding, a first-in- with sintered iron ore (iron oxide) as the oxidizer for de NKK TECHNICAL REVIEW No.88 (2003) –18– Steelmaking Technologies Contributing to Steel Industries De [Si] –ST KR NRP de[P] BF de[Si] de[S] LD-NRP de[C] Fig.1 ZSP process flow at NKK’s Fukuyama Works siliconization. The reaction vessel is a ladle type, and the synchronized with the tap-to-tap time of the decarburiza- hot metal is vigorously stirred by injecting lime through a tion furnace, to which the hot metal is then sent. The effi- submerged lance. This method dramatically improved the cient high-speed dephosphorization achieved by these oxygen efficiency for desiliconization over the conven- technological developments allowed an increase in the ra- tional method of desiliconization, which is performed in tio of hot metal for which the dephosphorization operation hot metal runners on the cast-floor, and provides a highly can be applied. At Fukuyama Works, it is now possible to efficient and stable supply of ultra-low silicon hot metal. apply the ZSP to 100% of hot metal, even at the high pro- 2.1.2 Hot metal dephosphorization technology duction amount of 10 million tons per year. The average Experiments confirmed that reducing the silicon content phosphorous content of hot metal after treatment is con- of hot metal in turn lowers the amount of CaO that reacts sistently less than 0.012%, allowing the decarburization with silica to form calcium silicate (2CaO-SiO2) in the furnace to be operated without the need of performing early stage of desiliconization. Instead, calcium phosphate dephosphorization. Hence, flux consumption at the decar- (3CaO-P2O5) is formed directly. Also, a practical technol- burization furnace was lowered to the minimum level re- ogy was established for performing the dephosphorization quired to protect the furnace refractories. Fig.2 shows slag of the ultra-low silicon hot metal by controlling the oxy- generation before and after the desiliconization station was gen flow rate and temperature. The reduced silicon content installed. The slag, which was previously generated at a increased the efficiency of lime for dephosphorization, rate of more than 100 kg per ton of steel, was decreased by significantly lowering the lime consumption and stabiliz- half. The slag generated in the converter dropped to less ing the phosphorous content in the hot metal after treat- than 10 kg/t. ment. At the Fukuyama No.2 steelmaking shop where the la- 150 dle-type dephosphorization process (NRP) is employed, the reduced slag generation retards the slag foaming phe- Other de[Si] nomenon and other process-hindering factors. Hence, the 100 extent of dephosphorization in the NRP was markedly in- de[P] creased by elimination of freeboard limitation in the hot 50 metal transfer ladle. BOF On contrary, the LD-converter-type dephosphorization kg/t Slag generation , process (LD-NRP) has been in operation at the Fukuyama 0 No.3 steelmaking shop since 1995. The LD converters in Before ZSP ZSP (Present) (Without de［Si］station) this shop are used as a decarburization furnace in the first half of their vessel life and then as a dephosphorization Fig.2 Effect of ZSP on slag generation furnace in the latter half. Using ultra-low silicon hot metal, the dephosphorization furnace performs high-speed The lowered generation of slag brought about various dephosphorization operation on all the hot metal that goes additional benefits. The first is that the direct reduction of through this shop. This dephosphorization operation is manganese ore in the converter became possible. Thus, –19– NKK TECHNICAL REVIEW No.88 (2003) Steelmaking Technologies Contributing to Steel Industries ferromanganese consumption was markedly reduced. The adopted at the Fukuyama No.3 steelmaking shop is sche- second is that the life of the refractory lining of the con- matically shown in Fig.3. With this system, the dust gen- verters was extended from 3000 charges to 8500 to 9000 eration during converter blowing operation is measured charges. In addition, the ZSP had a large effect on im- on-line, allowing optimization of the oxygen blowing pat- proving the quality of the steel produced, such as a sig- tern and other operational parameters. Those data resulted nificant reduction in the generation of alumina, as de- in the rapid development of a new lance nozzle. scribed later. Further, the compositions of the slags were simplified, Primary dust catcher Secondary dust catcher which expanded their effective uses. As also described later, slag from desiliconization is now used effectively as potassium silicate fertilizer, while slag from dephosphori- zation is formed into large blocks by carbonation for con- structing artificial fishing reefs. These slag products have been commercialized by NKK as environmentally friendly products that open the way to the next-generation steel- BOF Online dustmeter making process. (colorimeter) 2.2 New converter technologies 2.2.1 High-speed blowing technology In the 1980’s, a top-bottom-combined blowing technol- ogy (NK-CB) was developed by NKK for steelmaking Fig.3 Schematic view of on-line dust measurement system converters2). Next, the development of the ZSP described above turned a converter into a decarburization furnace (2) Dynamic pressure control of top-blown oxygen that can effectively perform direct reduction of manganese Iron spitting and dust generation rates in a converter are ore3). Major problems associated with this operation were correlated with the dynamic pressure of
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