Q355热轧厚板的Z向拉伸层状撕裂缺陷分析

Analysis of lamellar tearing defects in the Z-directional tensile fracture of Q355 hot-rolled thick plates

为了提高Q355厚板的Z向拉伸塑性,通过拉伸断口形貌观察、夹杂物观察与统计、金相组织观察以及连铸坯凝固组织模拟,得出了Z向拉伸缺陷的形成原因,并探究了MnS夹杂物在厚板中心聚集出现的原因。结果发现,拉伸断口的脆断平台上布满MnS夹杂物,这些MnS只在厚板中心与马氏体一起出现,且伴随着严重的C、Mn偏析。厚板中心大量聚集性的MnS会形成一个大范围的脆裂影响域,从而造成厚板Z向拉伸缺陷。而MnS在厚板中心聚集出现的根本原因是连铸坯柱状晶过分发达形成严重的中心偏析,最终导致大尺寸的聚集性MnS和马氏体组织一起出现。避免过分发达的柱状晶出现、减少连铸坯中心偏析是改善Z向拉伸塑性的有效方法。

In order to improve the Z-directional tensile plasticity of Q355 thick plates,the reasons for the formation of Z-directional tensile defects were investigated through tensile fracture morphology observation,inclusions observation and statistics,microstructure observation and solidification structure simulation of continuous casting slab.In addition,the reasons for the appearance of MnS inclusions aggregated in the centre of the plate were further investigated.It is found that the brittle fracture platform of the tensile fracture is covered with MnS inclusions.These MnS only appear in the centre of the plate together with martensite and are accompanied by severe C and Mn segregation.The large amount of aggregated MnS in the centre of the plate create a large area of brittle fracture influence,which results in Z-directional tensile defects in the plate.The root cause of the aggregation of MnS in the centre of the plate is the overdeveloped columnar crystal of the continuous casting slab.The overdeveloped columnar crystals form a severe central segregation,which eventually leads to large aggregates of MnS and martensite together.Avoiding the appearance of overdeveloped columnar crystals and reducing the central segregation of the continuous cast slab is an effective way to improve Z-directional tensile plasticity.