Effect of abnormally large prior-austenite grains on the presence of surface intergranular cracks

Surface network cracks that often appear in bloom casting can cause severe longitudinal cracks during the rolling process. Through micro-examination and hot tensile tests, several results were obtained. The surface network cracks are obviously intergranular located at the boundaries of abnormally large prior-austenite grains. It has been recognized that network cracks are associated with large prior-austenite grains, residual Cu element, precipitation of carbides and nitrides (AlN, BN, Nb(C, N)), and ferrite films. The main precondition for crack formation appears to be a coarse microstructure, which leads to critical concentrations of segregated solutes and precipitates along the austenite grain boundaries. This may lead to a combined effect of region Ⅱ-and Ⅲ-embrittlement for network cracking. The cracks occur mostly at the locations where bulging or surface depression appears. The study has shown that the abnormally large prior-austenite grains are caused by higher superheat and uneven cooling.