含Cu低碳钢I&Q&P处理中Cu配分行为及增塑机制研究

The chemical composition C and Mn has a strong influence on the stability of the metastable retained austenite at room temperature. In the intercritical annealing - austenitizing - quenching - partitioning process process, Mn element improved the stability of the austenite by partitioning from ferrite to austenite. And the enrichment of Mn in austenite could also impact on the diffusion of C element from martensite to retained austenite in partitioning process. Based on C partitioning, Mn partitioning can further improve the product of strength and elongation, and has no negative effect on weldability of the low carbon high strength steel. However, when Mn content is more than 3% mass fraction, it will cause trouble for the smelting and casting and deteriorate the welding performance. In this case, one low alloy C-Si-Mn steel is studied in present work to explore the implementation the Cu partioning conditions of thermodynamics and kinetics by means of I&Q&P process. The Cu pre-partitioning behavior and its effect on C partitioning and the stability of the retained austenite are studied. In combination of C and Mn element partitioning of retained austenite volume fraction and distribution regulation of the form, the product of srength and elanganation of lw alloy C-Si-Mn steel will be effectively improved. The relationship between the I&Q&P process parameters and retainde austenite volume fraction and the mechanical properties will be established. It will be explored that the effect of Cu element partitioning on retainedl austenite in deformation and plasticity enhancement mechanisms. The influnence of C, Mn, Cu element partitioning on the structure and properties of a Cu bearing low-carbon steel will be clarified. The scientific problems of mechanical properties of Q&P steel being restricted by C、Mn content are solved by means of studying on the comprehensive mechanism of of C、Mn and Cu elements. It can enriches theoretical research in strengthentoughening and provides a new train of thought to preparation of low carbon high strengh automotive steel.

在保证焊接性能及C配分的基础上，低碳高强钢经双相区保温-奥氏体化-淬火-C配分（I&Q&P）处理，由Mn配分获得的富锰奥氏体显著影响了碳配分中C元素由马氏体向奥氏体扩散，明显提高钢的强塑积。但Mn含量超过3%质量分数时，将会给冶炼及铸造造成困难、恶化焊接性能。因此，本研究以含Cu低碳硅锰钢为研究对象，采用I&Q&P处理工艺，探索实现Cu配分的热力学、动力学条件；研究Cu配分行为及对残余奥氏体的影响机理，结合C、Mn元素配分对残余奥氏体体积分数和分布形态的调控，从而有效提高强塑积；建立I&Q&P工艺参数与残余奥氏体体积分数和力学性能的关系；探讨Cu元素配分对变形中残余奥氏体的TRIP效应影响及增塑机制；阐明C、Mn、Cu元素配分对含Cu钢组织性能的综合作用；解决Q&P处理钢力学性能受C、Mn元素含量制约的相关科学问题。该研究可丰富高强钢的强韧化理论，为汽车用低碳高强钢的制备提供一种新思路。

以QP钢为代表的第三代汽车用高强钢，具有低成本、高强塑积等优点。两相区合金元素配分工艺可在不增加合金元素含量与数量的前提下，通过调控钢中合金元素的分布形态以控制热处理后的组织构成，制备出具有一定M/RA体积比的强韧性组织，在变形时诱发TRIP效应，使钢的强塑积达到30 GPa·%以上，满足第三代汽车用钢的性能要求。第三代汽车用钢主要以C、Mn元素强化基体，在保证焊接性能及C配分的基础上，Mn含量不能超过3%(质量分数)，否则冶炼及铸造将会比较困难，而钢中原有的残留Cu元素则可很好的解决配分效果与制备工艺之间的矛盾。因此，本项目以含Cu低碳硅锰钢为研究对象，采用I&Q&P处理工艺，探索实现Cu配分的热力学、动力学条件；研究Cu配分行为及对残余奥氏体的影响机理，利用与C、Mn配分间的协同作用实现对残余奥氏体体积分数和分布形态的调控，以达到有效提高钢的强塑积目的；建立I&Q&P工艺参数与残余奥氏体体积分数、力学性能之间的本征关系，探讨Cu配分对形变残余奥氏体TRIP效应影响及增塑机制。两相区C、Mn、Cu配分热力学、动力学计算表明，温度对配分行为起到关键性作用，两相区温度的中间段为最佳配分温度区间，Cu、Mn、C元素间的协同配分，有利于促进奥氏体晶粒的细化与稳定，计算结果与试验结果吻合。利用两相区形变配分工艺，可缩短配分时间、降低配分温度，进一步提高残余奥氏体的稳定性；经形变I&Q&P工艺处理后，低碳硅锰钢板材的强塑积显著提高，最高为34.1 GPa·%。本项目的开展，完善了QP处理工艺的元素配分热力学、动力学理论，深化了QP处理工艺的强韧化机理认识，提出了一种提高配分效率与强塑积的新工艺；丰富了汽车用高强钢的研发思路与方法，为第三代汽车用低碳高强钢的快速应用与发展夯实了基础。