低碳纳米结构超级贝氏体钢的组织调控及强韧化机理研究

High-carbon, high-silicon nanobainitic steel shows prospects for wide applications in mechanical engineering and structures because of its ultra-high strength and excellent plasticity and ductility; however, the super long isothermal transformation time seriously restricts its industrialized production. The project proposed here aims to solve the bottle-neck issue of the super long transformation time for obtaining nanostructured ultra-fine baintic microstructure, by taking advantage of the feature of fast transformation of low-carbon steel and the synergetic role in refining bainitic structure via super-cooled austenite deformation and below-Ms austempering. In the proposed study, low-carbon, high-silicon bainitic steels with different carbon contents are to be designed and prepared based on the design idea of low carbon content; then, through investigating the influences of super-cooled austenite deformation, austempering below Ms and their combined process on bainitic transformation kinetics and the final microstructures, the dominant mechanisms and the control conditions for refining the low-carbon bainitic microstructure will be clarified; and finally, through exploring the tensile deformation behavior, ductility and toughness and their comprehensive influencing factors of low-carbon ultra-fine bainitic steels with different carbon contents, the correlations between chemical composition, thermal-mechanical process, microstructure and strength/toughness are to be revealed, the fundamental rules for composition design and microstructure control will be elucidated, and finally the key issue of the contradiction between ultrafine bainitic microstructure and rapid bainitic transformation will be solved. The expected results from this project are of guiding importance and theoretical value for the development, production and application of nanostructured, suprebainitc steel.

高碳高硅纳米贝氏体钢由于其超高强度、高塑韧性而具有广阔应用前景，但其超长的等温相变时间严重制约着其工业化生产。本项目拟通过发挥低碳钢贝氏体相变速度快的特性，同时利用奥氏体变形与Ms点以下等温淬火对贝氏体组织的协同细化作用，来解决“纳米结构超细贝氏体组织—相变时间长”这一瓶颈难题。基于低碳成分设计思路，设计不同碳含量的低碳高硅贝氏体钢；通过奥氏体变形、Ms点以下等温淬火及其复合工艺对贝氏体相变动力学及微观组织影响规律的研究，弄清热—机械协同细化贝氏体组织的机理和控制条件；通过对不同碳含量低碳超细贝氏体钢的拉伸变形行为、塑韧性及其综合影响机理的探究，揭示化学成分、热机械工艺、微观组织、强韧性之间的内在关系，明晰低碳纳米贝氏体钢的成分设计与组织控制原理，最终突破“纳米结构超细贝氏体组织—相变速度快”相矛盾的关键技术问题。本项目研究成果对纳米贝氏体钢的研发、生产与应用，具有重要指导意义和理论价值。

高碳高硅纳米贝氏体钢由于其超高强度、高塑韧性而具有广阔应用前景，但其超长的等温相变时间严重制约着其工业化生产。本项目设计并制备了三种不同碳含量的低碳高硅/富铝钢。研究了温变形、Ms点以下等温淬火及其复合工艺对低碳、贝氏体相变动力学、微观组织、拉伸变形行为、塑韧性、影响规律和机理，揭示了热机械工艺、微观组织、强韧性之间的内在关系，解决了“纳米结构超细贝氏体组织—相变速度快”相矛盾的技术问题。. 结果表明，在Ms点以下等温淬火时，先形成的马氏体能够缩短、甚至消除贝氏体相变孕育期，加速贝氏体转变。Ms点以下等温淬火得到的贝氏体块/板条和残余奥氏体的尺寸比Ms点以上等温淬火试样的细很多。Ms点以下等温淬火试样中不同组成相间的应变配分相对均匀，应变集中程度小；Ms点以上等温淬火试样存在M/A岛，在M/A岛附近或相邻M/A岛之间的贝氏体区存在高度局部应变集中。在不损失拉伸性能的情况下，Ms点以下等温淬火试样的冲击韧性远高于Ms点以上等温淬火试样的相应数值，达到Ms点以上等温淬火试样的2.5倍左右。. 预先温变形、以及降低温变形的温度均加速贝氏体相变，减小贝氏体板条厚度、细化贝氏体组织。预先温变形与Ms点以上等温淬火相结合，增加粗大、脆性M/A岛的数量；但与Ms点以下等温淬火工艺相结合，减少块状M/A岛的数量和尺寸。对应预先温变形+Ms点以上等温淬火的试样，其强塑积为~33 GPa%；与此形成鲜明对比的是，温变形+Ms点以下等温淬火试样的强塑积高达~43 GPa%，表现出强度和塑性的良好组合。此外，温变形+Ms点以下等温淬火试样的冲击韧性为~180 J/cm2，是温变形+Ms点以上等温淬火试样的（~80 J/cm2）两倍以上。. 本项目的研究成果表明，采用预先温变形及Ms点以下等温淬火的耦合工艺可以制备出高性能、纳米结构的低碳贝氏体组织，与中、高碳纳米贝氏体钢相比，大大缩短等温相变时间，这对纳米贝氏体钢的研发、生产与应用，具有重要指导意义和理论价值。