新型热轧高屈服强度高强塑积的中锰钢的相变机理及组织调控

The development of the 3rd generation high strength steels mostly concerns cold rolled steels and relies on the continuous annealing process. Being one of the most active concepts, the development of medium Mn steels mostly focuses on the intercritical annealing and revert austenite transformation, which results in a high product of tensile strength and total elongation, but relatively low yield strength. In order to develop high gauge 3rd generation high strength steel and to satisfy its requirement on the high yield strength, the current proposal aims to explore fundamental aspects and industrialization potentials of a novel hot rolled medium Mn steels combining high yield strength and high product of tensile strength and total elongation..The proposed research focuses on the hot rolling process and takes full advantages of the texture control and very slow cooling of a hot rolled coil. The prolonged (quasi) isothermal treatment allows the slow transformation to take place and results in microstructure of martensite/bainite embedded with retained austenite. The research will carry out dilatometer experiments systematically, analyse the transformation kinetics and resulting microstructures, and elaborate 1) the complex transformation mechanisms during a prolonged isothermal treatment between Ms and Mf and 2) the mechanisms to accelerate the bainitic transformation kinetics at the low temperature. Mechanical properties will be characterized based on the lab process. The effects of retained austenite on strengthening and toughening will be investigated. And eventually an optimized composition and heat treatment scheme will be proposed. The proposal presents a new concept of medium Mn steels (other than intercritical annealing) and will establish fundamentals for the development and industrialization of the novel medium Mn steels proposed.

第三代高强钢的基础研究与产业化主要基于冷轧和连续退火工艺，作为其热点之一的中锰钢则基本采取亚临界退火概念，虽然强塑积较高，但屈服强度较低。为拓展第三代高强钢更厚规格带钢的发展并满足其对高屈服强度的要求，本项目拟开展以高屈服强度高强塑积为目标的中锰热轧带钢的基础研究与产业化探讨。.本项目充分利用热轧工艺对织构的调控及热轧卷缓慢冷却的特性，实现马氏体相变温度区间的长时间保温，获得贝氏体/马氏体结合残余奥氏体的目标组织，达到高屈服强度与高强塑积的协同实现。通过对相变动力学的系统研究和所获组织的深入表征，阐明马氏体相变温度区间长时间相变的复杂机制及温度相关性，明确贝氏体低温相变的加速机理及有效控制手段。通过中试规模的实验和力学性能的表征，获得残余奥氏体的强韧化规律，实现成分和热处理方案的优化。本项目采用的非亚临界退火概念拓宽了中锰钢研究的思路, 为新一代高强热轧中锰钢的开发奠定理论和产业化基础。

本研究针对传统两相区退火的双相中锰钢屈服强度较低的应用瓶颈，通过热轧卷取工艺，创新性的引入贝氏体组织来调控最终组织并实现高屈服强度的目标。.研究首先详细阐明了贝氏体动力学的控制因素，通过形变热处理，初始马氏体触发，合金元素优化三种调控手段成功实现在中锰钢内引入并加速贝氏体生成的效果，进一步从相变机理角度阐明各因素影响贝氏体动力学的影响本质，如初始马氏体通过提供更多的界面形核位置，显著促进了贝氏体形核，大幅度缩减贝氏体相变孕育期，加速贝氏体相变。.基于对贝氏体相变动力学的机理研究，进一步在多种先进超高强汽车钢体系中阐明贝氏体相变对钢种终态组织，尤其是残余奥氏体的影响。首先构建了0.2C-(1.6/2.82/3.92/5.14)Mn-1.6Si成分体系贝氏体相变工艺窗口，为钢种的设计提供了实验及理论依据。进一步考虑贝氏体对残余奥氏体的影响，对依据Q&P概念建立的CCE模型进行改进，通过引入T0模型修正了传统模型以引入贝氏体相变影响，实现Q&P工艺中精准的预测残余奥氏体，为Q&P工艺下基于贝氏体调控的组织设计提供理论依据。.通过上述系统精准的组织调控与设计，本研究实现了中锰钢的综合力学性能提升。本研究中0.2C-5Mn-1.6Si成分钢在形变热处理工艺后贝氏体体积分数可以增加至23%，在该钢种中调控贝氏体相变来替换部分马氏体组织，同时促进奥氏体的稳定，实现塑性优化；并且贝氏体的引入同样可以部分补偿马氏体量减少造成的强度损失，在预变形作用下由细晶强化等因素进一步提高强度。基于以上组织设计，该钢种强度得到显著提升（屈服强度升至~1400MPa，抗拉强度升至~1900MPa），同时总延伸率基本保持不变。本研究中两步马氏体预相变工艺可使中锰钢性能达到强塑积~24.8GPa%，抗拉强度UTS达~1300MPa，屈服强度YS达~950MPa，总延伸率达TEL为~19%。研究基本实现了基于贝氏体调控理念的先进超高强汽车钢组织性能精准调控及性能提升。.本项目研究结果已在金属材料领域权威期刊Acta Materialia上发表，并为先进超高强汽车钢的设计提供了新的思路和相关理论依据。