Kinetics of both slag/metal reactions and metal/inclusion reactions were investigated experimentally using 20 kg vacuum induction furnace in order to clarify the mechanism of the formation of MgAl 2 O 4 spinel inclusions in aluminum killed ferritic stainless steel (SUS430). The results obtained are summarized as follows : 1) By reducing CaO/SiO 2 and CaO/Al 2 O 3 ratio of top slag, MgO contents in Al 2 O 3 based inclusions decreased.2) The two film theory was employed to analyze the rate determining step of slag/metal reaction (reduction of MgO in top slag). By this model, it was found that the rate determining step of the reaction was the mass transfer of Mg through the film in molten steel. The increase rate of Mg in molten steel is determined by the activities of soluble oxygen and MgO at the slag/metal interface, and hence by slag composition.3) The unreacted core model was employed to analyze the rate determining step of metal/inclusions reaction.The analysis showed that the rate determining step of the reaction in the case of 20 kg vacuum induction furnace was the diffusion of Mg in molten steel.

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In order to increase the heat efficiency of the chromium ore smelting reduction furnace, a heated ore addition technology using a burner in the converter was developed. 5 ton scale pilot converter tests were conducted. Based on the results of the pilot converter tests, this technology has been applied to the actual process. The results are summarized as follows:1) Compared with the conventional process (without burner), the amount of effective heat transfer increased by 18% with addition of heated ore using the burner in 5 ton converter.2) A decrease in the off-gas temperature and reduction of the thermal load on the refractory were also confirmed in 5 ton converter.3) The heat transfer ratio of burner combustion with addition of heated ore increased up to about 90%, depending on the increase of the ore feeding rate in 5 ton converter. 4) From the results of a numerical simulation, it was revealed that the total sensible heat of all particles increases as the ore feeding rate increases. As a result, the ore particles heated by the flame function as a medium of heat transfer from the flame to the molten metal and slag.5) This technology was applied to an actual smelting reduction furnace at JFE Steel East Japan Works (Chiba). As in the tests with 5 ton converter, the heat transfer ratio of burner combustion with addition of heated ore was about 90%. The supplied energy per unit of chromium ore added to the furnace decreased by 17%.

In this paper, the history and latest trends in the converter-based steelmaking process in Japan were summarised. After World War II, Japanese steel companies introduced the LD converter, and various original technologies have been developed to increase productivity, reduce the production cost, and improve the quality of steel. About 20 years after the introduction, the necessity of the additional stirring was recognised and various types of top-and-bottomblowing converters were developed. By using this process, hot-metal dephosphorisation and smelting reduction were developed. In addition, fundamental research related to the converter technology is shown. Japanese steelmaking technology has been developed by the collaboration of university and industry. In industry, each company has an R&D centre where the applied and practical research is conducted. The research in academia is concentrated on the fundamentals. Recently, ISIJ organised the research projects on 'multiphase slag refining' and 'enhancement of lime dissolution'.