Transformation and Stress Evolution in Hot Rolled Strip Coil of X70 Pipeline Steel in Cooling Process

For the purpose of analyzing affect of coil cooling on strip flatness,the hot rolled strip coil cooling process of X70 pipeline steel was studied by finite element method.The thermal-stress coupling model was established for the hot rolled strip coil.The temperature,the volume fraction of phase transition and the residual stress of the coil were calculated in the case of coiling temperature of 550 ℃ and 500 ℃ respectively.Not only the affect of the layers contact on the heat transfer,but also the heat transfer coefficient differences among the inner diameter wall,outer diameter wall and two ends of the coil were considered in the model.The results show that the uneven temperature distribution and uneven phase transition of the coil lead to the edge tensile stress and the center compressive stress in the tangential direction of coil.After coiled at 550 ℃,the tensile stress at the edge of 20 mm in width exceeds the yield stress of the steel,leading to the strip edge wave trend.Lower coiling temperature at 500 ℃ promotes the full bainite phase transition and it is conductive to the reduction of residual stress in the coil.After decoiling,the edge wave and transverse C-shaped wrap of X70 pipeline steel strip are the results of both uneven laminar cooling and uneven coil cooling.