Formation of coarse columnar grains (CCG) in as-cast austenite structure of peritectic carbon steels is one of the serious problems in continuous casting processes. It was recently elucidated that the formation of CCG is ascribed to a discontinuous grain growth.Furthermore, the critical condition for the discontinuous growth to occur was elicited on the basis of phase field simulations and a theory of grain growth. In this study, by means of the phase field simulations, the detailed investigation is carried out for grain coarsening of as-cast austenite structure. It is demonstrated in the two-dimensional simulations that the coarsest grain structure emerges by the discontinuous growth in the vicinity of the critical condition. In addition, a model for predicting the upper limit of grain size during the discontinuous growth is proposed. The model successfully describes the experimental result with reasonable accuracy.
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I. INTRODUCIONAs-cast γ-austenite structure in continuously cast slabs of peritectic carbon steels consists of coarse columnar grains (CCG) in the vicinity of the slab surfaces. Since the size of CCG is closely related to the occurrence of surface cracking and low ductility of the slabs, [1][2][3][4] the precise description and prediction of the as-cast γ grain size are quite important issues in the field of casting of steels.A number of attempts have been made for the description and prediction of size of the CCG during continuous casting processes. [5][6][7][8][9][10] In the early works, the CCG structure has been supposed to originate from continuous grain growth which occurs during cooling below a temperature for completion of transformation to γ single phase, T γ . [5][6][7][8] However, our recent study by means of rapid unidirectional solidification experiment revealed that the formation of CCG structure is actually ascribed to a discontinuous grain growth. [11] Figure 1 shows the as-cast γ grain structure observed in a 0.2 mass% carbon steel quenched during the unidirectional solidification. The CCG develop from the mold side to the upper part.Importantly, fine columnar γ grains (FCG) exist ahead of the CCG region. The boundary between FCG and CCG regions, which in this paper is termed the FCG/CCG region boundary (FCRB), is identified in Figure 1. From temperature measurements, it was found that the temperature at FCRB is always T γ during the solidification and, hence, the FCG region corresponds to liquid + γ two phase field, while the CCG region is γ single phase field.
3Although the FCG always form ahead of the growing CCG during the solidification, the FCG near FCRB shrink with the growth of CCG. More specifically, coarse γ grains, which formed in the vicinity of the mold wall, preferentially develop along the temperature gradient below T γ at the expense of FCG, finally growing to CCG. This process is characterized by vertical motion of FCRB in Figure 1 and is classified as a discontinuous grain growth. Since the liquid phase in FCG region prevents the migration of FCRB, the FC...