Hot ductility trough elimination through single cycle of intense cooling and reheating for microalloyed steel casting

Abstract

Surface transverse cracking, especially corner cracking, is prone to generate in continuously cast slabs of microalloyed steels. The method of surface structure control (SSC) was supposed to the best way to avoid the detrimental defects. However, the mechanism of improving hot ductility by SSC and the specific parameters to control the process are still unclear for the reasonable adoption in production. In the present work, the impact of cooling rate, holding temperature and holding time on austenite decomposition, and the austenite grain size before and after intense cooling were investigated by thermal simulation method. With the increase of cooling rate, it is observed that the phase is transformed from austenite → grain boundary film-like alltromorph ferrite → Widmanstätten ferrite plates (or intragranular ferrite plates) → bainite+martensite. Mostly important, the film-like ferrite can be eliminated through intense cooling and the following reheating, but the austenite grain size is not observed to be refined through the single γ → α → γ cycle. Even though, the reduction of area (RA) is improved drastically to over 70% in the third ductility trough, whereas the RA value is just < 40% for the conventional cooling. The reason can be deduced by eliminating grain boundary film-like ferrite, and carbonitrides were precipitated inside the grain, which simultaneously act as the nucleus of intragranular ferrite. Thus, the principle of SSC process was established, and based on heat transfer calculation, water flow adjustment was put forward to implement the intense secondary cooling, which is a meaningful exploration for reducing transverse cracking.