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"Phase Transformation Behavior in the Rapidly Cooled C-Mn Steels" ΄S|€Z^[@θΰO[v@ΰ Ώͺ (KIM, HAN SOO) In the research for the creation of high quality steels from steel scrap, two main topics that should be clearly understood in the high temperature metallurgy scheme are (1) phase transformation behavior from solidification to austenite formation and (2) formation behavior of fine precipitates. For mechanical properties of as-cast steels are greatly influenced by prior austenite grain size and formation of fine precipitates. Effect of steel chemistry on mechanical properties is quite evident even in the high impurity steels as can be seen in the report of Yoshida et al.k1l And it has been recently reported that impurity elements of copper and sulfur can be utilized for improving mechanical properties through precipitation of fine copper sulfide particles in the rapidly cooled high impurity steelk2l. It is expected that effective combination of near-net-shape casting process and inverse utilization technique of impurities can enhancethe feasibility of creating high quality steels from steel scrap. In order to comply with the demands for applying rapid solidification process to high impurity steels, clear understanding of phase transformation and precipitation behaviors in high impurity steels under rapid cooling process is inevitable. For elucidating the relationship between cooling rates and metallurgical phenomena encompassing solidification, austenite formation and precipitation behavior of fine particles in high impurity steels, it has been tried to quantify the cooling rate of a rapid solidification process which uses electron beam as a heat source. Cooling rate in each melting and solidification experiment was
simulated with Rosenthal thick plate solutionk3l. And the simulated cooling rate in each charge showed good agreement with the cooling rate calculated from the measured secondary dendrite arm spacing in each sample. The highest cooling rate realized with the melting technique using electron beam travel was 1.11E05 K/s. And the relationship between austenite grain size and cooling rate were revealed in the recent work of our study. For the analyses of phase transformation behavior with varying impurity contents and cooling rates, rigorous computational simulations were carried out. Computational simulation provided vivid pictures of the composition and phase changes in the system domain along the whole path of phase transformation. And the results of simulation, coupled with austenite grain growth modeling, were successfully employed to analyze Through this work, it is aimed to develop a comprehensive model to predict the final austenite grain size of as-cast steels with various cooling conditions and various impurity elements of varying contents. References k1lN. Yoshida, O. Umezawa and K. Nagai: ISIJ Int., 44(2004), 547. ---------------------------------------------------------------------------@ |