Abstract Boron segregation characteristic was analysed based on segregation model and the original position statistic distribution analysis, and surface horizontal crack was observed by using metallographic microscope and scanning electron microscope. Theoretical calculation indicates that boron element has a strong segregation tendency when the solidification fraction is greater than 0.9, at the end of solidification, the boron content is about 70 times higher than the initial boron content, and the cooling rate has no obvious effect on boron segregation. The segregation in the external region was larger, and the maximum segregation of four test samples is 32.372, 18.008, 7.285 and 18.392, respectively. The micro-morphology of crack shows that the extension direction of the crack is vertical to the billet surface and the billet side, and the horizontal crack is formed by the expansion of subcutaneous micro-cracks and small bubbles in billets during rolling. The crack section is attached with metal particles and granular inclusions.