Austenite Grain Growth in Peritectic Solidified Carbon Steels Analyzed by Phase-Field Simulation
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INTRODUCTION
THE as-cast c-austenite structure in continuously cast slabs of peritectic carbon steels consists of coarse columnar grains (CCG) in the vicinity of the slab surfaces. Because the size of CCG is related closely to the occurrence of surface cracking and low ductility of the slabs,[1–4] the precise description and prediction of the as-cast c grain size are important issues in the field of casting of steels. Several attempts have been made for the description and prediction of size of the CCGs during continuouscasting processes.[5–10] In the early works, the CCG structure has been supposed to originate from continuous grain growth that occurs during cooling below a temperature for completion of transformation to c single phase Tc.[5–8] However, our recent study by means of a rapid unidirectional solidification experiment revealed that the formation of the CCG structure is actually ascribed to a discontinuous grain growth.[11] Figure 1 shows the ascast c grain structure observed in a 0.2 mass pct carbon steel quenched during the unidirectional solidification. The CCGs develop from the mold side to the upper part. Importantly, fine columnar c grains (FCGs) exist ahead of the CCG region. The boundary between the FCG and CCG regions, which in this article is termed the FCG/ CCG region boundary (FCRB), is identified in Figure 1. From the temperature measurements, it was found that the temperature at FCRB is always Tc during the solidification, and hence, the FCG region corresponds to liquid + c two-phase field, whereas the CCG region is MUNEKAZU OHNO, Associate Professor, SHINGO TSUCHIYA, Graduate Student, and KIYOTAKA MATSUURA, Professor, are with the Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan. Contact e-mail: [email protected] Manuscript submitted August 5, 2011. Article published online February 10, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
c single-phase field. Although the FCGs always form ahead of the growing CCGs during the solidification, the FCGs near the FCRB shrink with the growth of CCG. More specifically, coarse c grains, which formed in the vicinity of the mold wall, develop preferentially along the temperature gradient below Tc at the expense of FCGs, finally growing to CCGs. This process is characterized by vertical motion of FCRB in Figure 1 and is classified as a discontinuous grain growth. Because the liquid phase in FCG region prevents the migration of FCRB, the FCRB moves upward together with the position of Tc. In our recent study,[12] the occurrence of the discontinuous growth in various cooling conditions and the CCG formation by this mechanism were verified by means of phase-field simulations. Then, we elicited the critical condition for the discontinuous growth to start and continue based on a theory of grain growth, which is given by[12] 1 VT M0 Tc ½1 ndinitial c position where VT is the migration velocity of the T called the thermal velocity in this article, M0 Tc is the grain boundary mob
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