Cracking reason of Q355B straight seam welded pipe

A company used Q355B hot rolled strip to make straight seam welded pipe. After welding, it was found that the weld area of some welded pipe cracked. The cracking reasons were researched by means of macroscopic observation, metallographic examination, scanning electron microscope and energy spectrum test. The results show that the sulfide content of the strip steel was high during melting, and the segregation of inclusions occured at the 1/4 thickness of the inner arc of the strip steel during continuous casting. The sulfide destroyed the continuity of the matrix, and the stress concentration occured at the micro-cracks, which eventually caused the cracking of the welded pipe.