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The effect of non-metallic particles on as-cast austenitic structure of low carbon steel Komatsu, Nobuyuki

Abstract

The effect of second-phase particles and cooling rate on the as-cast austenite grain size of peritectic plain-carbon steel has been examined. Remelting tests followed by continuous cooling and helium quenching were performed on a Gleeble 1500 Thermo mechanical Simulator. Two low alloy types of steel were used; both had basically the same basic chemical composition, but contained different amounts of titanium and calcium. The as-cast austenite grain size was characterized in terms of both the cooling rate, which was obtained by the Gleeble measuring system through the thermocouples welded on the sample surface, and the volume fraction and size distribution of second-phase particles which were obtained by electron metallography. The classical Zener equation and Gladman model were applied to explain the experimental results relating to the pinning effect of second-phase particle against as-cast austenitic grain growth. It was found that at relatively high cooling rates (4.5 °C/s — 17.5 °C/s), the effect of cooling rate was dominant in austenite grain growth behavior, while at slower cooling rates (0.4°C/s — 1.2 °C/s), the effect of second-phase particles in grain boundary pinning could be observed. Regarding the effect of second-phase particles, it was found that the titanium nitride, which is an effective precipitate for inhibiting austenite grain growth in the conventional continuous casting and reheating process, was not effective for pinning the coarse as-cast austenite grain growth. This was due to both the small volume fraction and the small size of titanium nitrides. On the other hand, oxides, which mainly were Ca-aluminates, were effective for pinning the as-cast austenite grain boundaries due to the large volume fraction and the relatively large particle size of oxides. The results were explained using the Gladman model.

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