Phase-Field Simulations and Analysis of Effect of Dispersed Particles on Migration of Delta to Gamma Transformation Inte
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CONTINUOUS Casting (CC) process is widely used for the production of steels. In carbon steels with carbon concentration lower than 0.17 mass pct, d to c transformation occurs during cooling after completion of peritectic reaction[1,2] or d solidification, followed by the formation of c grain structure. Since the temperature for the completion of d to c transformation is quite high, c grains rapidly grow to be coarse,[3] and the coarse c grain structure causes the deterioration in mechanical properties and the occurrence of surface cracking of the CC slabs.[3,4] Therefore, the grain refinement of c phase is a very important issue to be tackled in the field of casting of steels. One of the key parameters in the grain refinement of the c phase is the temperature for the completion of d to c transformation. This temperature is indicated by the thick line in the Fe-C phase diagram shown in Figure 1. It is called Tc in this paper and it corresponds to the temperature for the onset of c grain growth during cooling. When Tc is higher, c grains grow more rapidly. Hence, the reduction of Tc is effective in retarding the c grain growth[3] because the mobility of the grain boundary is lower at lower temperatures. In general, Tc can be lowered by controlling the cooling rate[5] or by adding alloying elements.[5,6] For example, Tc decreases with an increase in the cooling rates, deviating from the equilibrium value due to the non-equilibrium transformation process. Also the addition of some alloying elements such as P, Al, and Cr DAISUKE SATO, Graduate Student, is with the Graduate School of Engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan. MUNEKAZU OHNO, Associate Professor, and KIYOTAKA MATSUURA, Professor, are with the Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan. Contact e-mail: [email protected] Manuscript submitted November 11, 2013. Article published online November 11, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
reduces the equilibrium value of Tc.[6] In this study, we focus on a different way of decreasing Tc, which is based on a retardation effect of dispersed particles on the migration of d–c interface. It is well known that the existence of second phase particles in polycrystalline materials retards the migration of grain boundaries, which is called pinning effect.[7–10] The origin of the pinning effect corresponds to the reduction of the total grain boundary energy due to the intersection of the boundary with the particles. One can expect that during the migration of d–c interface, the total interfacial energy of d–c interface can be reduced when the interface intersects with the particle. The migration of d–c interface should accordingly be retarded and the d to c transformation should finish at lower temperatures, thus resulting in the reduction of Tc. Hence, the existence of dispersed particles in d phase may result in the reduction of Tc due to the retardati
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