Effects of growth rate and amount of Ca–Si modifier on the microstructure scales of granular eutectics in an austenite medium Mn steel

Results presented in this paper contribute to investigation of the effects of growth rate (V) and the amount of added Ca–Si modifier (αk) on the microstructure scales of granular eutectics such as the volume fraction (f) and diameter (d) in an austenite medium Mn steel (AMMS). Directional solidification of AMMS has been carried out using vertical Bridgman method. There are three critical growth rates for each AMMS sample, |V|1, |V|2, and |V|3. If V<|V|1, the microstructure of AMMS is composed of pearlite, granular (Fe,Mn)3C carbides in grain, network carbides surrounding grain, and retained austenite. If |V|1<V<|V|2, the granular γ-(Fe,Mn)3C eutectics and (Fe,Mn)3C carbides are unevenly distributed in the grain surrounded by inter-boundary carbides. The needle-like carbides may appear when αk≤1.0%. If |V|2<V<|V|3, the microstructure is similar to that at |V|1<V<|V|2, but without the granular/needle-like carbides in grain. If |V|>|V|3, the microstructure is like that at |V|2<V<|V|3 although the inter-boundary (Fe,Mn)3C carbides are absent. Each critical growth rate decreases with increasing αk. The granular eutectics nucleate in the liquid pockets among the primary austenite dendrites at the later stage of solidification. For a constant αk, f initially increasing with V attains a peak and then follows a decreasing trend, and d always decreases with the increment of V. For a given V, f increases, but d decreases with increasing αk. The growth rate (V0) at which f attains its peak for one AMMS decreases as αk increases.