Causes and countermeasures of longitudinal depression and crack formation on X65 line pipe continuous casting billet surfaces

X65 line pipe continuous casting billet surfaces were analyzed by optical microscopy, scanning electron microscopy and energy dispersive spectrometry to deal with their defects of longitudinal depression and cracks. It is found that causes for these surface defects are mainly due to carbonitride segregation of micro alloying elements Nb, Ti and V at grain boundaries. These surface defects can be efficiently improved through optimizing the mold flux composition and the second cooling curve, controlling the tundish superheat, regulating the continuous casting operation process and checking the continuous casting equipment timely.