Abstract: Aiming at the problem of serious surface craze cracks (incidence rate 7.92 %) of 50 steel plate rolled by 200 mm thick continuous casting slab, the morphology, distribution and composition of surface cracks were studied by means of scanning electron microscopy and spectral analysis. The results showed that surface cracks randomly appeared on the steel plate without fixed position in the width direction of the steel plate, and the crack depth was shallow, and the two sides of them showed the characteristics of high-brightness structure and oxidation dots. Spectral analysis showed that the carbon mass fraction at the crack was 0.2 %-0.3 % higher than that of the matrix, which showed that the slab of 50 steel was partially carbonized during the continuous casting process by mold flux with some carbon content. Subsequently, cracks and obvious high temperature oxidation characteristics were caused by uneven thermoplasticity of the slab during heating and rough rolling. According to the carburizing mechanism of continuous casting mold flux, the continuous casting process parameters such as mold slag line adjustment mode, casting speed, nozzle shape, mold flux composition, etc. were optimized, which effectively reduced the occurrence of local carburization on the surface of the continuous casting slab and cracking of the 50 steel plate.

Key words: 50 steel, surface defects, local carburization, mold liquid level fluctuation, mold slag