热成形配分机制对超高强度汽车用钢强塑性能的影响

The introduction of ultra high strength automobile steels (UHSS) represents the effectual approach to reduce the weights of vehicles, lower fuel consumption and gas emission. Hot stamping is an innovative forming technology, developed for its potential in fabrication of high strength steel components for automobiles. While the steels after hot stamping usually have a lower ductility, limiting its substantive applications in the automotive industry as secure structural sheet components. Therefore，the study of a novel heat treatment process, hot stamping and partitioning, is proposed for improving the ductility and the products of the strength and elongation of UHSS. The hot forming processes are to be performed by tool forming techniques. In situ neutron diffraction is to be applied in the detection of the austenite deforming behavior, and microstructure is to be learned by means of various kinds of techniques. This application will develops the study in three ways as follows：1) Effect of crystallographic texture of the deformed austenite, carbon diffusion and segregation during the hot pressing on the subsequent martensitic transformation; 2) Effect of carbon partitioning on the phase constitution and the products of the strength and elongation; 3) Effect of the ratio of the width of retained austenite film and that of martensitic lath on the tailored strength and ductility of the products. The hot forming and partitioning processes are designed to manufacture UHSS products with a high ductility and high products of the strength and elongation achieving 30GPa%. The effect mechanism of the novel forming techniques improving the ductility and the tailored mechanical properties of UHSS would be confirmed. Therefore, the work develops an effective hot forming process for ultra high strength automobile steels with a higher ductility, providing experimental evidences for constructing theoretical foundation and application guidance for hot stamping.

超高强度汽车用钢是汽车轻量化、节能减排的关键材料。热冲压成形为超高强度钢板在汽车上的应用提供了可行的成形方法，但热成形后塑性不足是制约超高强度钢作为汽车安全结构件广泛应用的瓶颈。为此，本项目提出热成形配分(QP)机制提高超高强度汽车用钢的塑性和强塑积的研究。将采用模具热成形试验方法、奥氏体高温形变过程的原位观察、微观组织结构表征等实验手段，在以下3个方面展开系统研究：1) 形变奥氏体内的碳扩散、偏聚行为和形变织构对马氏体相变的影响；2) 配分处理过程中碳的分配机制对最终组织组成和强塑积的影响；3) 残奥薄膜和马氏体板条的尺寸配比对强塑性能的影响，从而优化热成形配分处理工艺，制备高塑性和高强塑积(30GPa%及以上) 的超高强度热成形钢，揭示热成形配分处理提高超高强度马氏体钢的塑性和强塑积的机制。本课题的实施可以为高强钢兼具韧性提供新的有效工艺和理论基础与应用指导。

超高强度汽车用钢是汽车轻量化、节能减排的关键材料。目前国际上通常采用热冲压成形的方法来实现汽车用安全结构件马氏体钢的制备；但热成形后塑性不足是制约超高强度钢作为汽车安全结构件广泛应用的瓶颈。为此，本项目提出热成形配分(QP)机制提高超高强度汽车用钢的塑性和强塑积的研究。.以超高强度汽车用钢22SiMn2TiB、30CrMnSi2Nb为研究对象，采用非等温压缩试验和原位中子试验研究了形变奥氏体中相变规律。建立了温控系统控制模具，在生产线上实现热成形配分工艺。.研究结果表明：.(1)从热力学上解释了非等温压缩形变量对Ms温度的影响机理，建立了形变奥氏体发生马氏体相变的非化学自由能与无形变条件下马氏体相变的非化学自由能和形变储能之间的关系式。.(2)奥氏体的形变促进铁素体的相变，抑制贝氏体和珠光体的转变，形变作用下CCT曲线向左上方移动；形变作用下奥氏体内形成形变织构，在铁素体内形成相变织构。.(3)奥氏体形变影响了奥氏体相变的相变应力，压缩形变在奥氏体晶格内产生了压缩应力平衡了奥氏体相变时受到的拉伸应力。奥氏体形变降低了残余奥氏体内的碳含量，但是对铁素体内的碳含量基本没有影响。.(4)回火温度低于473K，板条马氏体的位错密度基本没有变化；高于473K, 随回火温度的升高板条内的位错密度下降。673K时，残余奥氏体发生分解；马氏体板条粗化，板条内的位错分布由随机分布位错转变为沿小角度晶界分布。.(5)马氏体形成初期，板条内的位错为刃型位错和螺型位错的混合位错。随着回火温度的升高，刃型位错(优先)逐渐消失。板条马氏体在回火温度下的力学性能在573K以上温度随着回火温度的升高逐渐下降。回火马氏体的强度决定于板条内的位错密度和位错的分布。.(6)热冲压Q&P处理可显著提高超高强度钢的塑性和强塑积，30CrMnSi2Nb热成形配分处理件的强塑积最高达26GPa%。且抗拉强度基本不受配分时间的影响。热冲压Q&P处理可以提高残余奥氏体含量和稳定性，残奥含量是决定淬火马氏体钢塑性和强塑积的主要控制单元。