复杂应变路径下Q&P980超高强钢的非弹性回复行为
韩飞, 操召兵     
北方工业大学 机械与材料工程学院, 北京 100144
摘要:板材成形过程中复杂加载路径的改变会影响其弹塑性流动行为。以超高强钢淬火—配分(quenching&partitioning,Q&P)980钢为研究对象,在室温下沿着轧制方向的不同角度进行2步拉伸实验,得到不同应力状态下的应力—应变曲线,并根据单位体积塑性功相等原则,确定了板材不同等效塑性应变(0%、1%、4%、6%)下的实验屈服轨迹。结果表明:在应变路径变化上,初始流动应力显著降低,特别在45°和90°方向上,瞬时时段之后的流动应力存在持续的偏移,较大应变条件下各向异性比较明显。实验屈服轨迹呈外凸性,部分屈服轨迹不对称,随变形程度的增加,屈服轨迹向外扩大。通过对比简单加载和循环加载,分析其弹塑性行为,并建立各自的卸载弦数学模型,指出弹性模量随应变的增加而降低,降低到一定程度后趋于平缓。在相同塑性应变下,循环加载时弹性模量的变化值比简单加载时要大。
关键词超高强钢    复杂加载路径    屈服轨迹    非弹性回复    
Inelastic recovery of Q&P980 ultra high strength steel with a complicated deformation path
HAN Fei, CAO Zhaobing     
School of Mechanical and Materials Engineering, North China University of Technology, Beijing 100144, China
Abstract: The loading paths used for sheet metal forming affect its elastic-plastic flow behavior. Quenching & partitioning (Q&P) 980 steel was used here as the research object. Two-step tensile tests using different angles relative to the rolling direction were conducted at room temperature to obtain the stress-strain curves for various stress states. The equal volume plasticity work principle was used to determine the experimental yield loci of different equivalent plastic strain (0%, 1%, 4%, 6%). The results show that the initial flow stress is significantly reduced by changing the strain path, especially in the 45° and 90° directions, the flow stress after the transient period various by a fixed amount, and the anisotropy is quite large for larger strains. The test yield loci line is convex and some of the yield loci are asymmetric. As the deformation increases, the yield loci expand outward. Simple loading and cyclic loading were compared to analyze the elasto-plastic behavior and develop unloading string mathematical models. The elastic modulus decreases quickly with increasing strain, then decreases slowly and then becomes constant. For a given plastic strain, the change in the elastic modulus during cyclic loading is greater than when simply loaded.
Key words: ultra high strength steel     complex strain path     yield loci     inelastic recovery    

随着汽车工业的高速发展,以淬火—配分(quenching & partitioning, Q&P)钢为代表的第三代汽车超高强钢在综合力学性能大幅提升的同时,保持着很好的经济性。采用热处理工艺,得到以马氏体(Martensite)和残余奥氏体(retained Austenite)为主要成分的、具有高强度和良好塑性韧性的组织,增强车身抵抗变形的能力,减轻车身重量,是汽车轻量化的理想材料[1]。然而,高强钢板由于自身强度高,薄板在成形过程变形区域过短,应变历程不同导致局部变形过大,金属板带的纵向伸长和收缩不均,对零件的尺寸精度产生很大影响。因此,准确地描述薄板成形时各向异性的屈服准则及弹塑性行为,对工艺的优化和理论指导具有重要的参考价值[2]

对于高强钢的各向异性及弹塑性行为的研究,国内外学者做了大量的实验和验证。Tarigopula等[3]对DP800钢在不同应变路径下进行了研究,结果表明:初始流动应力在应变路径变化中有相当大的减少。Liao等[4]研究了DP780在复杂应变路径下的力学行为,运用硬化模型HAH并采用Yld2000-2d各向异性屈服函数进行本构建模,结果表明:再屈服应力硬化速率演化取决于重新加载的角度。Yoshida等[5]研究了双相高强钢在循环拉伸压缩时的Bauschinger效应,结果表明:随着预应变的增加,卸载Young's模量随之减小。高付海等[6]设计了2种新型单拉平板剪切试件,实验表明:试件的工作区剪切效果比较明显, 偏置剪切试件适用于研究韧性较好材料剪切失效。余海燕等[7-8]采用2步拉伸方法对TRIP780钢进行非比例加载实验,指出非弹性回复机理是由于塑性变形引起的材料组织结构及相发生了改变。庄京彪等[9]研究了DC06和DP600这2种材料的Bauschinger效应,结果表明:混合强化模型对高强钢回弹精度的预测优于等向强化模型。Kim等[10]研究了Mild270、BH340、DP490和DP590这4种不同强度级别钢塑性变形对弹性模量的影响,分析了应力—应变曲线并用位错模型解释非线性现象。孙珊珊[11]研究了卸载循环对Q&P980钢的微观