Prediction of As-cast Austenite Grain Size for Near-net-shape CC

Tsuchiya

2012

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...

“…It should be noted that the main concern in early works on grain growth of as-cast γ structure [5][6][7][8] has been the cooling process below T γ .…”

mentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

See 2 more Smart Citations

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

Tsuchiya

2012

“…The CCG structure causes the deterioration of the mechanical properties of the cast and also the occurrence of the surface cracking which is one of the serious problems reducing the yield ratio of continuously-cast slabs. Therefore, elucidation of the formation mechanism of CCG structure and also prevention of formation of CCG structure have been one of the longstanding and important issues in the field of the solidification and casting of steels [5][6][7][8] .…”

Section: Introductionmentioning

confidence: 99%

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

Maruyama

2015

Experimental verification of a critical condition for the formation of coarse columnar γ grain (CCG) structure in as-cast hyperperitectic carbon steels, which was put forward based on theories of grain growth and phase-field simulations in early studies, is carried out by means of a Bridgman-type directional solidification experiment. The occurrence of the discontinuous and continuous grain growth processes and the resulting formation of CCG and equiaxed γ grain structures, respectively, are demonstrated. Importantly, these changes of the as-cast microstructures and the grain growth modes are in excellent agreement with the previously-proposed critical condition of the CCG formation.

Phase-Field Simulations and Analysis of Effect of Dispersed Particles on Migration of Delta to Gamma Transformation Interface

Sato

2014

Retardation effect of dispersed inert particles on delta-gamma interface migration in carbon steels during isothermal delta to gamma transformation is analyzed by two-dimensional phase-field simulations. The effect is systematically investigated for different values of particle radius, r, particle spacing, l, and initial carbon concentration of delta phase. The retardation effect becomes stronger when the pinning parameter described by r/l 2 is larger and the carbon concentration of delta phase is higher, indicating that delta to gamma transformation kinetics can be retarded in a similar way to the pinning effect on grain growth kinetics.