Austenite Grain Growth in Peritectic Solidified Carbon Steels Analyzed by Phase-Field Simulation

Abstract

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. 2 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...

Microstructural Features and Formation Processes of As-cast Austenite Grain Structures in Hypoperitectic Carbon Steels

“…As shown in Fig. 11, the increase of x C from 0.2 to 0.25 mass% yields the formation of EG structure near the mold wall, which can be explained by the occurrence of the continuous grain growth 10,11,20) due to the reduction of T γ . A further increase in x C results in the formation of CG structure near the mold wall.…”

Experimental Verification of a Critical Condition for the Formation of As-Cast Coarse Columnar Austenite Grain Structure in a Hyperperitectic Carbon Steel

“…The occurrence of such discontinuous grain growth in the as-cast γ grain structures was confirmed by the phase field simulation of the grain growth from the FCG structure [11,12] . Moreover, the critical condition for the discontinuous growth to start and continue was examined in detail.…”

“…(1) was examined by means of casting experiments using the different types of molds (metallic and ceramic molds) in a recent study [12] . The as-cast γ grain structures in all the samples consist of the CCG and EG regions.…”

“…Furthermore, a theoretical relationship indicating a critical condition for the formation of CCG structure was put forward based on theories of grain growth and phase-field simulations of the discontinuous grain growth [11,12] . The relationship is given by a balance between the cooling condition and the growth rate of CCG and it will serve as a guiding principle for the prevention of CCG formation.…”

Phase-field topology optimization model that removes the curvature effects

“…The successful work of Kobayashi to simulate the complicated and beautiful dendrite growth during solidification opened the phase-field study area (Kobayashi, 1993;Kobayashi, 1994). Since then, in the solidification field (Asta et al, 2009;Steinbach, 2009;Takaki, 2014), the phase-field model has been applied to binary alloys (Warren and Boettinger, 1995;Wheeler et al, 1992), polycrystals (Miyoshi and Takaki, 2016;Steinbach and Pezzolla, 1999), quantitative models (Karma and Rappel, 1996;Ohno and Matsuura, 2009), multi-component alloys (Ohno et al, 2012), coupled models with convection (Beckermann et al, 1999;Rojas et al, 2015;Takaki et al, 2015b), and large-scale computations (Sakane et al, 2015;Shibuta et al, 2015;Takaki et al, 2014;Takaki et al, 2013;Yamanaka et al, 2011). The great success of the phase-field method in material science is due to the advantages of the phase-field method: tracking the interface position is not necessary, the curvature effects are included in the model, the evolution equations can be derived from the free-energy functional based on the second law of thermodynamics, the discretization of the time evolution equations is easy, and so on.…”