- The reason for an award -

In the course of hot forming process of steel products, precise prediction of flow stress is essential for the purpose of achieving the products’ required dimension preciseness and minimizing the manufacturing cost. To reduce the heat loss and to optimize the forming efficiency, a multi-pass deformation is preferred. In this case, the metallurgical phenomena, such as work hardening, recovery and recrystallization, is inevitably affected by the previous deformation process. This makes extremely difficult the precise prediction of the flow stress.

This paper aims at modeling and parameter identification of the duplex stainless steel taking advantage of the experimental and numerical schemes previously developed by the authors. This state-of-art methodology has been extended in the article to a more complicated system by using the mixture law of the stresses of each phase.

The paper successfully modelled the complicated metallurgy phenomena of the duplex stainless steel and well identified the parameters which appear to be in good agreement with the experimental results, confirming the model justification. As the reason described above, this extraordinary paper is recognized as one of the most valuable papers in terms of both industrial and academic point of view, and thus is well worthy of the Sawamura Award.

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In eutectoid steel, lamellar pearlite consisting of a (cementite + ferrite) lamellar microstructure is formed, whereas degenerate pearlite with dispersed cementite particles is known to be formed at the lower carbon concentration than the eutectoid one. Non-cooperative growth of ferrite and cementite is proposed to induce the formation of degenerated pearlite, but the conditions and mechanism of its formation have been unclear in detail. This paper clarifies the transition carbon composition from degenerate pearlite formed under low-carbon conditions and lamellar pearlite formed under high-carbon conditions through sophisticated experiments in which the carbon concentration in austenite is systematically varied using proeutectoid ferrite formation. Furthermore, by modeling the competitive reaction between ferrite growth and lamellar perlite growth, they have succeeded in deriving the conditions under which ferrite and cementite cannot grow cooperatively, explaining the transition carbon compositions observed in the experiments. Regarding mechanical properties, the paper shows that lamellar pearlite softens more slowly upon annealing than degenerate pearlite and points out that this is due to the constraint on Ostwald growth by the lamellar microstructure.

As described above, this paper clarified the formation conditions and mechanism of degenerate pearlite, which had been unclear, from both experimental and theoretical aspects, and is of high academic and engineering significance, making it suitable for the Sawamura Award.

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In the decarburization of high chromium special steel, there are incentives to reduce environmental burden and production cost because chromium has a tendency to be easily oxidized. The key to suppressing the oxidation loss of chromium is a better understanding of the phase relationship among the oxides and the activities of the primary oxides in the refining slag with chromium oxides. However, there are few reports on reliable thermochemical data applicable to the operating conditions in the actual production process.

In the paper, equilibrium experiments were carried out at 1573K and in a mixed gas atmosphere (Ar+H₂＋CO) using CaO–SiO₂–Cr₂O₃ system slag and molten copper. The activity coefficient of chromium in molten copper and activity of Cr₂O₃ in the two-phase region of CaO+ CaCr₂O₄ and the three-phase region of CaSiO₃+ SiO₂+Ca₃Cr₂Si₃O₁₂ were determined with high accuracy. The obtained results were verified with XRD measurement. Moreover, the Gibbs energy changes of CaCr2O4 and Ca₃Cr₂Si₃O₁₂ and activities of oxides, CaO, SiO₂, and Cr₂O₃ in all three-phase regions were derived based on the values noted above.

It will be expected that the fundamental properties obtained in the elaborated experiments are applied to the computational thermodynamics and broaden the applicability to the actual process. Thus, the paper has an absolute value with academic and technical significance, and it is worth well to the Sawamura Award.

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Growing attention has been paid and growing demand is expected for Fe-Cr-Ni-Mo-Cu alloy applicable to the use in severe corrosion environments; However, the investigation is quite limited to the component segregation behavior during solidification which greatly affects castability, hot-workability as well as weldability. The authors took up a uni-directional solidification experiment which can freeze the microstructure and component during the solidification, to evaluate the distribution coefficient of each element from the inter-dendritic region to the center of the dendrite by random sampling method for their concentration. The accuracy is comparable with in-situ measurement by X-ray method, indicating the extensive application of the present method to other high alloys creation and designing. Also, this paper is highly appreciated for contributing to academic progress, and is worth well to the Sawamura Award.

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This paper describes the formation mechanism of Al oxides in the Zn galvanized coating, which greatly affects the formability, weldability, and chemical conversion in the manufacturing process of steel components for automobiles, in terms of microstructural changes in the galvanized coating and diffusion of Al atoms. The present study demonstrated that the formation rate of Al oxides was suppressed by applying the temper rolling process to the hot-dip galvanized steel sheet. Detailed microstructural characterization revealed that recrystallization and grain growth of the Zn galvanized coating could promote the diffusion of solute Al atoms in the Zn galvanized coating. In industrial aspects, this study is highly evaluated for providing new insights for controlling various properties of the hot-dip Zn galvanized steel sheets for automotive use. In addition, the paper presents a fundamental finding that the diffusion of Al atoms could be promoted through microstructural control of the Zn galvanized coating by using the temper rolling process.

Based on the abovementioned points, it can be concluded that the present findings indicate a new possibility for controlling functionalities of the hot-dip Zn galvanized steel sheets through microstructural control of the Zn coating using the plastic forming process, and will greatly contribute to the advancement of processing technologies for hot-dip Zn galvanized steel sheets. Therefore, this paper is worth well the Sawamura Award.

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The Al deoxidation process of molten steel, which is vital for producing high-quality steel, is significantly affected by the mutual reactions among inclusions, molten steel, slag, and refractory materials. Therefore, accurate control and prediction of these reactions are essential for ensuring steel quality and optimizing the steelmaking process. Against this background, many studies have been conducted on the Al deoxidation equilibrium of molten iron. On the other hand, there have been limited studies on the Al deoxidation equilibrium of highly-alloyed molten steels such as stainless steels, and studies on the effect of molten steel composition and temperature, which covers the refining to the solidification processes, are insufficient.

In this paper, Al deoxidation equilibrium experiments were conducted on Fe-Cr-Ni melts over a wide composition range and analyzed using the sub-regular solution model and the Redlich-Kister type polynomial approximation. Various efforts were made to obtain highly accurate results in the deoxidation equilibrium experiments. New interaction parameters, including ternary and higher order parameters, were derived using the obtained results and reliable thermodynamic data from previous studies. Reported data can reproduce the measured results with high accuracy. Using these parameters, Al deoxidation equilibrium data over a wide composition range and a wide temperature range which is applicable to refining and solidification processes were calculated.

This excellent study contributes to the development of the steel refining process from both academic and technological perspectives and is worth well to the Sawamura Award.

- The reason for an award -

The effect of grain size on the deformation-induced martensitic transformation has not been understood in metastable austenitic steels with ultrafine-grain. It is important to know the relationship between the mechanical stability and grain size in metastable austenitic steels because fine-grained retained austenite is used for TRIP-aided steels. This paper clarified the relationship between the mechanical stability and grain size in metastable austenitic steels based on the crystallographic characteristics of deformation-induced martensitic transformation. Athermal martensitic transformation was suppressed when the grain size is below 20μm. On the other hand, no effect of grain size on the deformation induced martensitic transformation behavior during tensile deformation was confirmed. In case of athermal martensitic transformation, 24 variants were formed (multi-variant transformation) when the grain size is large, while one variant was selected (single-variant transformation) for ultrafine grain. The strain energy by martensitic transformation is not accommodated for ultrafine grain, leading to suppression of martensitic transformation. For deformation-induced martensitic transformation, some specific variants were formed (near single-variant transformation) so that the anisotropic transformation strain is accommodated by tensile strain even when the grain size is large. This leads to no grain size dependence of mechanical stability of austenite. This paper provides fundamental knowledge for discussion of thermal and mechanical stability austenitic steels and has a useful long-term impact on the field. Therefore, it is judged to be worthy of the Distinguished Article Award.