Transition of solidification mode and the as-cast γ grain structure in hyperperitectic carbon steels

Abstract

Formation processes of as-cast γ grain structures during casting of hyperperitectic carbon steels with 0.15-0.45 mass% carbon concentrations have been studied by means of a rapid unidirectional solidification technique. In steels with 0.15-0.41 mass% carbon concentrations, coarse columnar γ grains (CCG) with a minor axis diameter of 1-3mm developed along the direction of temperature gradient. In a steel with 0.38 mass% carbon, importantly, columnar γ grains (CG) whose minor axis diameter is less than 500 μm form before the formation of CCG and the grain structure discontinuously changes from CG to CCG. The fraction of CG region increases with increase in the carbon concentration. In the samples with the carbon concentration higher than 0.43 mass%, the as-cast structure consists of CG over almost the entire ingots. Analyses on a relation between γ grain and dendrite structures and their crystallographic orientations indicate that the formation of CG originates from the primary solidification of γ phase instead of δ phase. This is supported by numerical analysis on the dendrite growths.

A critical condition for the formation of a coarse columnar γ grain structure in a peritectic solidified carbon steel

“…The mechanism of formation of CCGs in hyperperitectic carbon steels was investigated in our recent works [7,8] using rapid unidirectional solidification equipment which realizes the cooling conditions similar to those near the slab surface in the practical continuous casting. The schematic illustration of the microstructure during the formation process of CCGs is shown in Fig.…”

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

“…Our recent investigations revealed that the formation of the CCG structure originates from the discontinuous grain growth process from fine columnar γ grain (FCG) structure which forms in liquid + γ two-phase field [9,10] . In addition, the theoretical relationship for the critical condition for the occurrence of the discontinuous grain growth given by Eq.…”

“…Fig. 1 Schematic illustrations of (a) rapid unidirectional solidification equipment used in the previous study [9,10] and (b) typical microstructure during the discontinous grain growth proces observed in the quenched sample. …”

“…The formation process of CCG structure in hyperperitectic carbon steels was investigated by means of a rapid unidirectional solidification experiment, the cooling condition of which is very similar to the one near the slab surface in the practical continuous casting processes [9,10] . Figure 1(a) shows the schematic illustration of the rapid unidirectional solidification equipment in which MgO pipe on water-cooled copper mold was preheated to 1843 K (1570 o C) (melt temperature) and it plays a role of a sidewall of the cast.…”

“…Our experimental investigations on the evolution process of solidification microstructure in hyperperitectic carbon steels revealed that the formation of CCG is ascribable to a discontinuous grain growth process from fine γ grains forming in the δ dendrite [9,10] . 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] .…”