无碳化物贝/马复相高强钢基于Q-P-T工艺的强塑性改善及其机理研究

In order to improve the strength and ductility of the advanced high strength steel (AHSS), the Quenching & Partitioning (Q&P), and Quenching-Partitioning-Tempering (Q-P-T) processes have been studied and used widely. Due to the advantage of combination of carbide free bainite microstructure and Q-P-T process, based on the previous works on the carbide free bainite/martensite (CFB/M) multiphase high-strength low alloy steels, a novel Q-P-T process combined with CFB/M multiphase will be employed to further improve the strength and ductility. Furthermore, the mechanism on the enhanced strength and ductility will be discussed. The alloying and heat treatment process would be designed through using of Thermo-Calc software. Various CFB microstructures with different distribution, size and fraction will form during quenching step of Q-P-T process by controlling quenching cooling rate. The influence of various CFB microstructure and Q-P-T process on the microstructure and mechanical properties would be investigated to improve the strength and ductility of the steels. The design ideas of the novel Q-P-T process combined with CFB/M multiphase will be summarized. The relationship between process, microstructure factors and strength, ductility will be established. The microstructure evolution during partitioning step will be investigated to study the reason why the partitioning time can be prolonged in condition that some CFB microstructure was formed during quenching step of the novel Q-P-T process. The results will be beneficial to expand the application of Q-P-T process. The evolution of various retained austenite with different morphology, size and distribution during deformation will be investigated to study the relationship between retained austenite and mechanical properties. The strength and ductility enhancement mechanism of the CFB/M multiphase steels treated by Q-P-T process will be discussed.

鉴于CFB/M复相组织与Q-P-T工艺结合对高强钢强塑性改善的显著优势，本课题计划以Mn-Si-Cr系CFB/M复相高强钢为基础，采用与CFB/M复相组织相结合的Q-P-T工艺改善复相钢的强塑性，深入分析强塑性改善的机制。利用Thermo-Calc软件设计合金成分和冷却工艺，研究淬火阶段形成的不同分布、尺寸和含量的CFB组织以及Q-P-T工艺参数对CFB/M复相钢组织及强塑性的影响，掌握CFB/M复相组织与Q-P-T工艺结合的设计思路，建立工艺参数、组织参数对强塑性的影响预测模型。研究淬火阶段形成的一定量的CFB组织对分配阶段组织演变的影响规律，探讨CFB/M复相组织与Q-P-T工艺结合而延长分配时间的机理，拓展Q-P-T工艺的工程应用领域。研究变形过程中不同形态、尺寸和分布的残余奥氏体的变化规律，探讨残余奥氏体与强塑性的关系，分析经Q-P-T处理的CFB/M复相钢强塑性改善的机理。

为了实现钢铁构件的轻量化，达到节能减排的目的，设计具有更高强度和塑性的先进高强钢（AHSS），探索其强塑化机理，研发相应的工艺技术，具有重要的科学和工程意义。近年来发展的淬火-分配（Q&P）工艺是通过马氏体向奥氏体中的碳配分以稳定奥氏体至室温，残余奥氏体在变形过程中发挥相变诱导塑性（TRIP）效应，提高钢的强塑性。而Q&P处理过程中是否发生贝氏体相变及其对钢组织和性能的影响一直是Q&P钢研究领域关注的热点。本项目以Mn-Si-Cr系无碳化物贝/马（CFB/M）复相高强钢为基础，系统研究了Q&P工艺中的贝氏体相变及其对CFB/M复相高强钢显微组织及力学性能的影响规律；并在此基础上创新性地提出贝氏体相变与Q&P工艺相结合的BQ&P工艺。分析认为，形成的贝氏体组织可以分割未转变的奥氏体，细化组织；在贝氏体铁素体形成时，合金元素（C）向未转变的奥氏体中富集，稳定奥氏体组织；通过两个过程（贝氏体相变和Q&P过程）实现奥氏体的碳富集，可形成两种类型的RA，分布在贝氏体铁素体板条之间的RA，以及分布在贫碳马氏体板条之间的RA，两种类型的RA的形态、尺寸、合金成分、周围相不尽相同，具有不同的稳定性，可在塑性变形的不同阶段发挥TRIP/TWIP效应，提高钢的强塑性。通过优化BQ&P工艺参数，在中碳CFB/M复相高强钢中获得了抗拉强度为1500-1800MPa，总延伸率为14-32%，强塑积（PSE）最高达到47.5GPa%的强塑性匹配。研究了高强塑性BQ&P钢的塑性变形机制，揭示了其强塑性改善的机理。同时，本项目提出的BQ&P工艺，解决了Q&P工艺“Q阶段淬火冷速快，P阶段配分时间短，不能用于大尺寸构件”的技术难题，成功将BQ&P工艺用于重型钎杆的制备，扩宽了Q&P工艺的工程应用领域。