bof japan developments

DEVELOPMENT OF THE JAPANESE STEEL REFINING TECHNOLOGY IN THESE 15 YEARS

Shin-ya KITAMURA, IMRAM, Tohoku University, Sendai, Japan

Abstract In order to meet the drastic change in the economic situation in these 15 years, Japanese steel manufacturing industries have developed various new technologies. The highest investment has been made in the construction of a new steelmaking shop in Wakayama Works, Sumitomo, in 1999. Moreover, significant investments were also made in the following technologies: 1) hot metal pretreatment, 2) vacuum degasser, 3) scrap melting devices, 4) dust and sludge treatment. In this paper, the change in the steel refining technology especially hot metal dephosphorization using converter, hot metal desulfurization by mechanical stirrer, top blow lance for high speed decarburization, new type degasser to produce ultra-low carbon steel, and mixer with heating devices are shown. Introduction This March, tremendously strong earthquake and tumani attacked north eastern Japan. It reminded us the strong earthquake attacked western Japan in 1995. It is a very sad thing to summaries the development of steel refining technology between these two disasters. About five years after the collapse of the so- economy (due to asset inflation) in Japan in 1995, the economic situation remains grim. Some large banks have been put under state control, and some big steel manufacturing companies are also in the red. The stock prices of steel manufacturing companies were low, and in some case, the stock have sold under par value in the market. In 2008, food crisis and global warming attracted peoples The economic situation was not bad until October. Owing to the large demand for steel from developing countries, especially China, the annual steel production in the world is increasing by more than 6% every year, and currently, it exceeds 1.3 billion tons. Many steel manufacturing companies have made all-time high profits. In October of this year, the subprime loan problems in the U.S. evolved into a global financial crisis, and the economic situation suddenly became unpredictable. After that Japanese steelmaking industry gradually recovered from the financial crisis. Although the damage of the earthquake and tumani for the Japanese economy is very large, there is no doubt that Japan will recover, become an even more marvelous country. In order to meet the drastic change in the economic situation in these 15 years, Japanese steel manufacturing industries have developed various new technologies. In this paper, the change in the steel refining technology in Japan is summarized.

The 6th European Oxygen Steelmaking Conference - Stockholm 2011 Programme No. JS-3

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Change in economic circumstances Figure 11) shows the change is the annual crude steel production and production ratio of EF steel in these 15 years. After the 1973 oil crisis, the annual crude steel production was constant by almost 100 million tons; however, it started increasing after 2000 and reached 120 million tons in 2007. On the other hand, the production ratio of EF steel gradually decreased. Due to the economic development of the developing countries in Asia, especially China, the demand of high-grade steel, which cannot be produced in the developing countries, has increased in Japan. On the other hand, the price of scrap increased due to the high demand for low-grade steel in these countries. As a result, the production of high-grade steel produced by the integrated mills increased. Figure 2 shows the change in the annual operating profit of some Japanese steel manufacturing

Figure 2 Change in annual operating profit of some Japanese steel manufacturing companies.

Figure 1 Change in annual crude steel production and ratio of EF steel production in Japan (based on data ob




companies. After 2003, the profit made by all steelmaking companies started increasing. The profits of the integrated steelmaking mills and special steel mills are high until 2007. Recovery trend after the financial crisis can be seen. On the contrary, that of the EF mill that produce plain carbon steel become to decrease from 2003 except 2008. Figure 3 shows the change in steel and scrap prices in Japan. Steel and scrap prices started to increase after 2001. In 2008, their prices increased to an unbelievable level and even though the economic crisis, they are still increasing.

By the strong demand to produce high-grade steel for the integrated steelmaking mills, the hot metal charging ratio of BOF decreases, in other words, the scrap charging ratio of BOF increased, as shown in Figure 41). Further, the vacuum treatment ratio has been increased up to 75% in order to produce high-grade steel.

Figure 4 Change in hot metal ratio of BOF and vacuum treatment ratio in Japan (based on data obtained




Outline of developments in steel refining technology The Iron and Steel Institute of Japan publishes the article which review the situation of the steelmaking technologies every year in Feramu (bulletin of the Iron and Steel Institute of Japan). Table 11) summarizes the epoch-making technologies in the field of steel refining developed during period 1997 2010. The highest investment has been made in the construction of a new steelmaking shop in Wakayama Works, Sumitomo, in 1999. Moreover, significant

Table 1 Epoch-making technologies developed in these 10 years in Japan (based on information obtained from (H; Hot metal pretreatment, V; Vacuum degassing, E; Environmental

problem, M; Scrap melting etc)




investments were also made in the following technologies: 1) hot metal pretreatment 2) vacuum degasser 3) scrap melting devices 4) dust and sludge treatment

In the following section, the details of some interesting technologies are presented. Hot metal dephosphorization using converter The hot metal dephosphorization process was first invented in Japan. Further development of this process is still underway, since it can be used to decrease slag generation and production costs and increase productivity during the production of high-purity steel. The reason for carrying out dephosphorization under the hot metal conditions is attributed to the strong temperature dependence of the reaction.

2[P] + 5[O] = (P2O50

Figure 5 shows the relationship between the equilibrium phosphorus content and the oxygen activity. The activity of P2O5 in the slag is calculated using a regular solution model2). The temperatures at the end of decarburization in a BOF (LD) and hot metal (HM) pretreatment are assumed to be 1923 and 1623 K, respectively. It is clear that at the hot-metal temperature, the oxygen activity or basicity can be decreased in order to obtain the phosphorus content similar to that obtained at the end of decarburization. In other words, for carrying out dephosphorization at the hot-metal temperature, only one of the following two conditions is necessary:

low oxygen activity with high basicity high oxygen activity with low basicity

Figure 6 shows the slag composition in the phase diagram of CaO-SiO2-FetO for each case of hot metal dephosphorization.

Figure 5 Relationship between oxygen activity and equilibrium phosphorus content.




Initially, a highly basic slag (Region a in Figure 6) is formed by the injection of CaO-CaF2 flux into desiliconized hot metal. This process was started in 1982 at Kimitsu Works, Nippon Steel3). However, recently, the treatment with a less basic slag containing a high amount of T·Fe (Region b in Figure 6) has become more common, as the use of fluorspar has been eliminated because of concerns for the environment (Table 21)).

The basic concept of hot metal dephosphorization using a converter is the same as that of the -slag process. This process was started in 1983 at Kobe Works, Kobe Steel4), followed

by Sumitomo Metals at Kashima Steel Works in 19835). As compared to the conventional double-slag process, hot metal dephosphorization is carried out at lower temperature and higher carbon content. Since the slag basicity is not very high, the slag composition is in the 2CaO·SiO2 saturated region. Further, since the FetO content is high, the solid fraction of the

Figure 6 Slag compositions in hot metal dephosphorization.

Table 2 Hot metal pretreatment process used in Japan. (TPC; Torpedo car, LD; Basic oxygen furnace)




slag is low, and the addition of fluorspar is not imperative. In order to control the FetO content, the optimization of the oxygen top blowing rate and the bottom stirring intensity is necessary. This process has the following advantages:

desiliconization is not necessary before dephosphorization short treatment time and high productivity the scrap melting capacity does not decrease because the latent heat of desiliconization

reaction can be used, and the scrap can be charged in the dephosphorization process the recycling of the decarburization slag to the dephosphorization process is very easy

On the other hand, the disadvantage of this process is that desulfurization does not occur, and the manganese yield in the dephosphorization process is low. In addition, the cost of building a

new converter for the dephosphorization process is high. Therefore, the process is carried out using a stand-by converter or continuous dephosphorization and decarburization processes using intermediate deslagging (MURC; multi-refining converter) at Nippon Steel6) (Figure 7). Hot metal desulfurization by mechanical stirring This process was invented by Dr. Kanbara, Nippon Steel, in 1965 and named the KR process (Kanbara Reactor; Figure 8)7). This method and the equipment used in this process are very simple; the desulfurization flux is added on the mechanically stirred surface of hot metal. Until the 1990s, flux injection process was commonly used. The KR process was constructed one after another in the first half of the 2000s, as shown in Table 31). In most case, process was

Figure 7 Hot metal dephosphorization using BOF.




changed from the lime injection to the KR. The lime injection process is very popular. The investment and operation cost are low. However, since the reaction rate is low it takes a long time. In Europe, the Mg injection process is popular, but the price of the flux is very high in Japan. Further, in this process, slag removal after the treatment is not easy, since the amount of desulfurization slag is small and the slag is not in the liquid state. If the slag is not removed, the sulfur-containing slag will be charged in the BOF, and the sulfur content at the end point of blowing increase. In contrast, although the investment cost of the KR process is rather high, high reaction rate can

be achieved by using cheap lime-based flux. Further, slag can be removed easily. These factors will aid in meeting increasing demand for low-sulfur steel. Many fundamental researches have been carried out by JFE steel8).

Figure 8 Hot metal desulfurization by using KR process

Table 3 Hot metal desulfurization process of each company and its investment year. (TPC; Torpedo car, KR; Mechanical stiring)




Top blow lance for high-speed decarburization9,10) At new Wakayama Works, Sumitomo Metals, high-speed decarburization is carried out. The oxygen blowing rate is 5 Nm3/(t min) and the decarburization time is only 9 min. To carry out high-speed decarburization, a new top blow lance has been developed. This lance has two sets of nozzles: one set with a large diameter and a large inclination angle and the other set with a small diameter and a small inclination angle. Each set consists of three nozzles, as shown in Figure 9. In order to eliminate the overlap of the hot spots formed by each nozzle, the

inclination angle is controlled, and the dust formation can be suppressed. The large-diameter nozzles contribute to the strong stirring of steel bath, and the small nozzles aid in decreasing spitting. Vacuum Degasser As shown in Figure 4, it is surprising that the vacuum treatment ratio has been increased up to 75%. To meet this demand, the increase in the productivity to produce ultra low carbon steel becomes necessary. One of the major drawbacks of vacuum treatment is the formation of skulls. Skull formation and melting during the treatment cause carbon pick up and increase the decarburization time. Further, as the operation of the furnace has to be suspended for skull cleaning, the productivity of the process decreases considerably. In order to prevent skull formation, an online burner has been installed. A multifunctional burner lance11) has been developed by Nippon Steel; it is used for oxygen blowing during decarburization and used for an LNG burner at a stand-by position to heat the vessel. DH-type degassers are commonly used in vacuum treatment; their decarburization rate is considerably slower than that of RH-type degassers. A new degassing process, named REDA

Figure 9 Design of top blow lance used in high-speed decarburization




(revolutionary degassing activator) has been invented by Nippon Steel12,13). This process consists of large size single snorkel and bottom bubbling ladle, as shown in Figure 10. By the bottom bubbling, large scale circulation flow is formed in the steel bath and a large spout area is formed at the surface exposed to the vacuum atmosphere. This process is also applied for stainless steel refining.

Mixer with heating units Previously, steelmaking shops had a mixer to hold hot metal from BF. However, with the development of steelmaking technologies, most of these mixers have been disappeared in Japan. In these ten years, the following mixers with heating units have been developed:a) IRB (iron reserve barrel)14): In order to meet the increasing demand for steel, it is important

to increase the scrap melting capacity. However, in Japan, hot metal pretreatment, which decreases the latent heat of hot metal considerably, is operation. In order to overcome this disadvantage, Yawata Works, Nippon Steel, has developed a mixer with heating units.

b) J-FIRST (iron reservoir for stainless steel)15): At JFE Steel, the smelting reduction process is used to produce stainless steel from chromium ore. In order to match the refining rates of the smelting reduction and decarburization processes and to increase the scrap melting capacity, a mixer with heating units has installed (Figure 11).

c) AN (anticipate the next) process16): At Daido Steel, to melt the low grade scrap in the day time, mixers with heating devices have been installed. In the mixer, the molten steel which was produced in night, using inexpensive electricity is reserved. As a result, electricity cost has decreased by 50%, and the melting capacity of low-grade scrap has increased.

Figure 10 Concept of new vacuum degassing process using large snorkel and bottom bubbling ladle.




Conclusions In these 15 years, according to the drastic change in the economic situation and the increase in the demand for high-quality steel, Japanese steel industries have developed various new technologies. In order to meet the future demands, the following three points have to be considered: 1) The demand for steel has decreased in 2008 due to the economic crisis. However, the

increasing trend in the demand for steel is expected to continue for a long term in developing countries, e.g., India, Southwest Asian countries, etc. Flexible technologies, that can be used even if the products amount changes widely, have to be developed.

2) Doubtlessly, the demand for high-grade steel will increase in the future. Hence, the mass production techniques of high-purity steel at low cost have to be developed.

3) Environmental friendly technologies with less CO2 emission have to be developed. Techniques for using waste energy and recycling waste materials have to be developed.

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Figure 11 Mixer with induction heater for stainless steel refining.




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