异常粗大的奥氏体晶粒对表面晶间裂纹的影响

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

在大方坯连铸中,经常出现的缺陷是表面网状裂纹.由于铸坯表面存在网状裂纹导致轧制过程中出现严重的纵裂纹.通过微观检测、热模拟等手段分析了表面网状裂纹产生的原因及机理,由实验结果可以看出,表面网状裂纹沿异常粗大的奥氏体晶粒开裂,其实质是奥氏体晶间裂纹;异常粗大的奥氏体晶粒、残余元素Cu的富集以及AlN、BN等氮化物和先共析铁素体薄膜的析出是导致该钢种产生晶间裂纹的主要原因;大的奥氏体晶粒是残余元素富集以及氮化物和先共析铁素体薄膜析出的先决条件,因此表面网状裂纹是第Ⅱ和第Ⅲ脆性区间相互作用的结果,经常出现在存在凹陷和鼓肚的部位;异常粗大的奥氏体晶粒产生的主要原因是高的过热度和不均匀冷却.

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.