Abstract: For crack problem on legs of high strength weathering steel YQ450NQR1 used for train center sill, the morphology of surface cracks was analyzed by the use of optical microscopy, scanning electron microscopy (SEM) and energy disperse spectroscopy (EDS). The conditions for decarburization of crack sides were investigated by the design of decarburization simulation experiments to determine formation process of cracks. The quality defects of YQ450NQR1 bloom, corresponding to cracks on legs, were identified by macroscopic examination and the causes for subcutaneous net cracks were analyzed based on the test results. The results showed that the crack defects on legs of high strength weathering steel YQ450NQR1 were caused by serious subcutaneous net cracks at the edge of bloom and uneven cooling of molten steel in mold, high level of residual elements as well as big fluctuations of nitrogen content contributed to the formation of subcutaneous net cracks. The occurrence ratio of cracks on legs of YQ450NQR1 steel, which fluctuated between 0.5 %～3.5 % originally, was stabilized below 1 % after some measures were taken.

Key words: YQ450NQR1 steel, continuous casting bloom, decarburization, net cracks, copper, precipitate