基于传统连退工艺高强塑Q&P钢多元增塑机制及组织调控技术

Q&P steel is a kind of advanced high-strength automobile steel based on TRIP effect and fully utilized phase transformation strengthening and retained austenite plasticization. It is the typical representative of the third generation advanced high-strength automotive steel with low cost and high performance. However, since the conventional continuous annealing process which is difficult to meet the requirements of high temperature and short time partitioning, limiting its application and popularization. Therefore, this project proposes a design idea of Nb micro-alloyed fine grained Q&P steel, through the control of hot-rolled microstructure, dissolving and precipitation of particles, recrystallization and austenite phase transformation during the annealing process to refine the grain, shorten the diffusion distance of carbon, so that the carbon can fully diffuse, partition and enrich to the retained austenite at lower temperature. The project is to adopt a combination of experimental and Thermo-Calc thermodynamic/kinetic calculations. Focusing on fine grained Q&P steel microstructure modification and multi-component plasticizing mechanism research, exploring the kinetics and its interaction mechanism of recrystallization and phase transformation during the intercritical annealing process, revealing the influence of low temperature partitioning process on the content, morphology, distribution and carbon content of retained austenite, clarifying the dissolution and precipitation behavior of Nb in the whole process and strengthening and plasticization mechanism of second phase particle. The research results provide theoretical guidance for the development of high performance Q&P steel, which has important theoretical significance and practical value.

Q&P钢是基于TRIP效应并充分利用相变强化、残余奥氏体增塑的一种先进高强汽车钢，是第三代先进高强汽车钢的典型代表，具有低成本、高性能的特点。然而由于传统连续退火工艺难以满足其高温短时配分要求，限制了其应用推广。因此本项目提出一种Nb微合金化细晶Q&P钢的设计思路，通过控制热轧组织、粒子回溶析出、退火过程再结晶及奥氏体相变来细化晶粒，缩短C的扩散自由程，使C在较低温度下能充分向残余奥氏体中扩散、配分与富集。项目拟采用实验和基于Thermo-cal热力学/动力学计算相结合的研究方法，重点开展细晶Q&P钢的组织调控和多元增塑机理研究，探讨两相区退火过程再结晶与相变的动力学规律及其相互作用机制，揭示低温配分工艺对残余奥氏体含量、形貌与分布及碳含量的影响规律，阐明Nb的全流程回溶析出行为及第二相粒子增强增塑机制。研究结果为开发高性能Q&P钢提供理论指导，具有重要的理论意义和实用价值。

基于细晶、多相、亚稳、第二相粒子增强增塑耦合作用可实现高强塑钢的制备原理。本项目在传统CSiMn合金体系基础上，开展了12种不同Si、Al、Mn、Nb试验钢的组织性能调控及强塑机制研究。首先研究了不同合金试验钢的热机轧制及奥氏体连续冷却转变规律，探讨了不同热力工艺下主要合金元素对组织类型、形态及大小的作用机制；其次，深入研究了不同Nb含量试验钢的再结晶与奥氏体转变热力学与动力学机制，探究了Nb对再结晶与相变的影响规律；最后，从热处理工艺对不同Si、Al、Mn、Nb含量试验钢的组织性能进行了调控，并对产生的微观结构进行了表征与分析，探讨了其对强塑性的内在作用机制。结果表明：1）Nb的加入显著扩大了铁素体与贝氏体转变区，而Mn的加入使C曲线明显右移；相比其他合金来说，Nb的加入显著细化热轧组织，并更容易获得细小铁素体与珠光体平衡相，有利于后续退火过程合金元素的扩散与改善奥氏体的稳定性。2）合金元素的加入会提高试验钢的再结晶激活能，抑制钢的再结晶过程；并且Nb的作用大于Si等其他合金元素；Nb与Si的加入会促进奥氏体化进程，其中Si的作用更显著。3）Nb含量的增加，有效细化了退火过程的显微组织，尤其是马氏体束与板条得到明显细化，有效提高了钢的强度；同时降低了最大残余奥氏体所需的淬火温度，并提高了残余奥氏体的稳定性与工艺敏感性。通过本项目研究，共发表各类论文20篇(均标注资助号)，其中SCI收录论文9篇，申请发明专利6项，授权4项，培养研究生4名，参加国际会议7人次，参加国际学术合作研究1人次。本研究工作不仅为探索高强塑积冷轧带钢制备和组织性能调控提供了理论依据，还可在传统连续退火生产线推广应用产生重要启示。