Effect of Thermal History on Hot Ductility of Nb Containing Steel

Hot cracking of Nb containing steels (Nb steel) during HCR (Hot Charge Rolling) process has been investigated by means of laboratory rolling simulation using small ingots and conducting the hot tensile tests simulated HCR process. Hot cracking in Nb containing steels occurs in the case of HCR process with the charging temperature ranging from 650°C to 900°C. This phenomenon can be explained by the precipitation of Nb (CN) at the large austenite grain boundaries succeeded from solidification at the interval of casting process and heating process before rolling. The precipitation of Nb (CN) at such a large austenite grain boundary during rolling process leads to larger strain concentration and promotes the initiation of microvoids due to decohesion of the grain boundary precipitates from the matrix. Final fracture mode is ductile intergranular fracture of austenite by microvoid coalescence. To avoid the hot cracking in Nb containing steels during HCR process, it is favourable to chose the charging temperatures out of above mentioned temperature region. But, even if cast slabs are charged to furnace with such temperature, hot cracking in Nb containing steels can be avoided by controlling the heating temperature before rolling higher than solution temperature of Nb (CN).