冷却模式对NM400相变塑性及残余应力的影响

Effect of cooling mode on transformation plasticity and residual stress of NM400

轧后快速冷却已成为制备耐磨钢NM400的重要手段,但快速冷却过程会在带钢内引入高幅值残余应力,导致带钢出现板形缺陷,并在后续加工以及使用过程中出现畸变。因此,弄清NM400在连续冷却过程的相变行为以及冷却速度对其相变行为的影响规律,是干预带钢相变行为、改善连续冷却过程带钢内残余应力水平及分布的关键。针对某厂热轧连续冷却过程采取的两种冷却模式造成的板形差异问题,通过热模拟试验、基于断裂力学原理的裂纹柔度法以及建立ABAQUS有限元仿真模型,计算了连续冷却过程的温度场、应力应变场,揭示了冷却模式对NM400相变塑性及残余应力的影响规律。研究结果表明,连续冷却过程残余应力的形成分为3个阶段,即热应力主导阶段、表面相变主导阶段和心部相变主导阶段;相变塑性应变的大小及方向与加载在相变瞬间的应力直接相关;加大冷速会提高带钢心部相变开始时带钢表面相变的体积分数,该体积分数越大,心部开始相变时所受到的拉应力水平越高,所产生的相变塑性应变越大,与相变应变两者叠加后在材料内会引入高幅值残余应力。研究成果可为耐磨钢NM400连续冷却过程残余应力调控提供数据基础和理论依据。

Rapid cooling after rolling has become essential for preparing wear-resistant steel NM400.However,the rapid cooling introduces high-amplitude residual stress in the strip,leading to plate defects and distortions during subsequent processing and use.Therefore,clarifying the phase transformation behavior of NM400 and the effect of the cooling rate on its phase transformation behavior is the key to intervening in the phase transformation behavior and improving the level and distribution of residual stress.Aiming at plate shape difference caused by the two cooling modes adopted in the continuous cooling process,the temperature field and stress-strain field were calculated to reveal the effect of cooling mode on phase transformation plasticity and residual stress of NM400 through thermal simulation experiments,the crack compliance method based on the principle of fracture mechanics and the establishment of ABAQUS finite element simulation model.The results show that the formation of residual stress in the continuous cooling process is divided into three stages,the dominant stage of thermal stress,the dominant stage of surface transformation and the dominant stage of core transformation.The size and direction of phase transformation plastic strain are directly related to the stress loaded at the moment of phase transformation.At the beginning of phase transformation in the core,increasing the cooling rate will increase the volume percent of the phase transformation on the surface.The larger the volume percent is,the larger the tensile stress and the phase transformation plastic strain are,the superposition of which will introduce high-amplitude residual stress in the material.The research results can provide data basis and theoretical basis for the regulation of residual stress in the continuous cooling process of wear-resistant steel NM400.