Q355热轧板山峰状表面裂纹形貌特征和形成原因

Morphology characteristic and formation cause of mountain-like surface crack in Q355 hot-rolled plate

为了减少Q355热轧厚板的山峰状表面裂纹,通过酸洗、金相组织观察和夹杂物分析,讨论了裂纹的形貌特征和形成原因。结果发现,山峰状裂纹顶部主要为大尺寸的外来夹杂物,裂纹侧面的夹杂物主要为Al_(2)O_(3),裂纹皮下均为铁氧化物,夹杂物分布有明显的位置特征。山峰状裂纹周围存在大面积异常组织,且仅位于裂纹1侧。结论认为,山峰状裂纹出现的根本原因是外来夹杂物。在连铸坯表面就已经出现微裂纹缺陷,轧制过程中,裂纹1侧向轧制(RD)方向快速变形,1侧的皮下组织被拉伸至表面,形成仅位于1侧的表面异常组织。裂纹表面的两端沿RD上的Al_(2)O_(3)夹杂物扩展,最终形成以外来夹杂物为顶部、以Al_(2)O_(3)为两侧的山峰状裂纹。裂纹皮下经过两个不同的氧化过程,形成FeO-Fe_(2)O_(3)-Fe的分布特征。

For reducing the mountain-like surface cracks in Q355 hot-rolled thick plates, the morphology characteristic and formation cause of the cracks were discussed by surface acid pickling, microstructure observation and inclusions analysis. It turns out that the top of the mountain-like crack are foreign inclusions with large size. The sides of the mountain-like crack are Al_(2)O_(3) inclusions, and the crack below surface is full of iron oxides. The distribution of inclusions in mountain-like crack has a positional character. Large areas of unusual microstructure existed around the mountain-like crack and were only located on side-1 of the crack. It is concluded that the root cause of mountain-like crack is the foreign inclusion. Microcracking defects are already present on the surface of continuous casting billet. During rolling, side-1 of the crack deforms rapidly in the direction of RD. The microstructure below surface on side-1 is stretched to the surface, and forms the surface unusual microstructure on side-1. The ends of crack surface expands along Al_(2)O_(3) inclusions at RD. Eventually a mountain-like crack is formed with the foreign inclusions on top and Al_(2)O_(3) on both sides. The cracks below surface undergo two different oxidation processes, and forms a FeO-Fe_(2)O_(3)-Fe distribution feature.