Abstract: Due to high alloy content in the hot rolled coil plate and the large difference in strength between the coil plate and the casing, the forming welding of high grade SEW pipe is difficult to control and prone to flattening and cracking. The causes of cracking of 110 ksi grade SEW pipe blank in flattening test were studied from the view of material and process. The results show that the causes of cracking of SEW pipe blank are the serious strip segregation of coil plate which affects the continuity of the material and a lot of reflow inclusions in the welding which results in large stress concentration during flattening.