Surface Crack Analysis and Microstructure of C-Mn Thin Slab Steel Produced by CSP

C-Mn thin slab steel had been produced at low cost by controlled rolling and cooling method in CSP factory of Wuhan Iron and Steel (Group) Corporation. The mechanical properties of tested steels were measured as: the yield strength of 313MPa, the tensile strength of ≥400MPa and the elongation of ≥37%. The YR was below 0.83 and ductility was good. However, the surface cracks often appeared. The particles with size of 83nm can cause cracks by thermodynamics analysis, while the particles in the experimental steel was about 30nm, which cannot bring about cracks. Meanwhile, mixed-grain was observed in the vicinity of the crack. The stress concentration calculated to be 120MPa, which was more than the strength of the steel rolled by F3 pass, may probably lead to crack. The rate of surfacial crack reduces from 3.67% to 1.34% with higher start rolling temperture as well as lower finishing rolling temperture and coiling temperature.