X70管线钢热轧钢卷冷却过程中的相变与应力变化

为了分析钢卷冷却对带钢板形的影响,运用有限元方法研究了层流冷却后的X70管线钢热轧钢卷冷却过程,建立了热轧钢卷热力耦合模型,计算了卷取温度分别为500℃和550℃时冷却过程中的钢卷温度、相变体积分数、残余应力.模型考虑了钢卷径向层间接触对传热的影响,以及钢卷内径壁、外径壁和2个端面的换热系数的差异.结果表明:冷却过程中,钢卷的温度和相变不均匀使得钢卷的切向上边部受拉应力,而中部受压应力;钢卷在550℃卷取后,带钢边部20 mm以内的拉应力超过基体的屈服应力,板形有边浪趋势;降低带钢的卷取温度至500℃促进相变充分进行,有利于钢卷冷却后残余应力的降低.开卷后X70管线钢的边浪和横向C型弯曲板形是不均匀的层流冷却和不均匀钢卷冷却共同作用的结果.

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.