Effect of Equalisation Time on the Austenite Grain Size of Simulated Thin Slab Direct Charged (TSDC) Vanadium Microalloyed Steels

Abstract

The thin slab direct charging process (TSDC) as applied to microalloyed steels has had a considerable development due to its relatively low production costs and ability to produce steels with good mechanical and toughness properties, comparable to those manufactured by the conventional cold charging process (CCC). This research is concerned with the effect of equalisation time on the microstructures. Three equalisation times (53 min, 318 min, and 1 333 min) at 1 150°C were investigated for a vanadium microalloyed steel, which contained a low carbon content (0.073 wt%) and a high nitrogen content (0.021 wt%). The experimental results show that MnS precipitates provided the main pinning forces during casting due to the high solubility temperature, but MnS particles did not provide a sufficient pinning force to prevent the austenite grains from growing. The austenite grain growth appears more likely to be abnormal in this stage. AlN precipitates were only identified at the longest equalisation time due to a low precipitation rate. The AlN pinning effect was evident at 1 333 min equalisation time, and was able to prevent further austenite grain growth by providing an additional pinning force. The study shows that vanadium does not have any effect on austenite grain growth at 1 150°C because of its low solubility temperature in austenite. When the steel was water-quenched, vanadium remained supersaturated in the solution but tempering in the ferrite phase field (650°C) can promote VN or VCN precipitation often using AlN as the nuclei.