Morphology characteristic and formation cause of mountain-like surface crack in Q355 hot-rolled plate
ZHANG Chuang-ju1, SHEN Wen-jun2, PAN Shi-song1, LIANG Feng1, YANG Hua1, CHENG Guo-guang2
1. Manage Manufacturing Department, Chongqing Iron and Steel Co., Ltd., Chongqing 401258, China; 2. State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China
Abstract：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 Al2O3 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 Al2O3 inclusions at RD. Eventually a mountain-like crack is formed with the foreign inclusions on top and Al2O3 on both sides. The cracks below surface undergo two different oxidation processes, and forms a FeO-Fe2O3-Fe distribution feature.
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2023, Vol. 33 
