基于组织遗传的层状超细晶双相钢制备及强韧化机理研究

It is of great significance to make steels both strong and tough because they are always needed to reduce the weight of component and improve safety. But conventional methods such as grain refinement and reduction of impurities have certain limitations on toughness improvement. Many studies have indicated that delamination induced by lamellar structure can significantly improve the toughness of steels, which provides a new way for strengthening and toughening of high strength steel. And it has also been reported that introduction of the second phase is conducive to lamellar structure formation and toughness improvement. In this project, a new method has been proposed to prepare lamellar-structured ultrafine-grained dual-phase steels based on martensite structure heredity, and the pre-research experiment has shown that the room-temperature impact absorbing energy is increased to 139.8J and does not decrease until the temperature is lowered to -196℃, and all specimens show a delamination fracture mode. Moreover, an abnormal inverse temperature dependence of toughness occurs at temperature range from 0℃ to -80℃. However, the controlling factors for inducing delamination toughening are still not clear. Based on systematical study of the effect of martensite phase content, grain morphology, texture and interface properties on inducing delamination toughness in this project, we expect to reveal the controlling factors and mechanism of delamination toughening in the lamellar-structured ultrafine-grained ferrite/martensite steels. By further analyzing characteristics of delamination fractures and combining fracture mechanism theory, a criterion model for low-temperature fracture mode is being proposed to clarify the low-temperature toughening mechanism. It will provide basic evidence and theoretical guidance for designing high-strength and high-toughness lamellar-structured steel.

钢铁材料的强韧化对减轻构件重量和提高安全性具有重要意义。细化晶粒、降低杂质含量等常规方法对提高材料韧性有一定的局限性，而层状结构组织诱发分层断裂可显著提高材料韧性，为高强钢的强韧化提供了新思路。研究表明引入第二相有利于层状组织的获得和韧性提高。本项目提出了基于马氏体组织遗传制备层状超细晶铁素体/马氏体双相钢的思路，预研实验表明该层状双相钢在室温至-196℃韧性不随温度下降而降低，达到138J，并在0℃至-80℃出现了韧性随温度降低反常升高的现象，断口以分层断裂为主，但其中诱发分层的控制因素仍不明确。本项目通过系统研究马氏体含量、晶粒形态、织构及相界面特性对诱发分层增韧的影响规律，揭示层状超细晶双相钢中诱发分层增韧的控制因素及其作用机制；通过进一步对分层断裂特征进行分析并结合断裂力学知识，构建低温断裂方式判据模型，阐明层状超细晶双相钢的低温韧化机制，为高强韧层状材料的设计提供理论指导和依据。

钢铁材料的强韧化对减轻构件重量和提高安全性具有重要意义，而层状超细晶双相结构可同时赋予材料高强度和高韧性，为结构材料强韧化提供了新的思路。本项目利用马氏体钢的组织遗传现象制备了层状超细晶铁素体/马氏体双相钢，系统研究了合金元素对马氏体组织遗传的影响及其作用机制，研究了层状结构特征对材料强韧性的影响，分析了晶粒细化、层状结构及分层等对材料强韧性提升的作用机制。. 研究结果表明合金元素Mn和Si是决定马氏体组织遗传特性的主要元素，揭示了合金元素的偏聚是导致材料产生组织遗传现象的根本原因；阐明了层状超细晶双相钢优异的强塑性主要源于超细晶组织、较高的马氏体含量及层状组织特征导致的持续应变硬化；证实了层状超细晶双相钢优异的室温及低温韧性主要源于塑性变形和稳态裂纹扩展吸收能量，超细晶层状组织抑制了冲击加载过程中的应变局部化和剪切带快速扩展，从而使材料具有良好的塑性变形能力和较高的韧性；证实了晶粒细化及分层断裂均提高材料的韧性，阐明了分层诱发韧性裂纹扩展路径增加是导致韧性提升的关键。. 利用该方法制备的层状材料强度达到1.67GPa，同时可获得11.8%延伸率，室温和液氮温度冲击功达到401J和245J，其强韧性由于文献报道结果。研究结果给出了层状材料低温断裂方式判据模型，并指出提高马氏体相的KIC或降低马氏体片层厚度均可以提高马氏体层的变形能力，从而进一步提高层状双相钢的韧性。这些研究结果可为层状材料的强韧化设计及制备提供理论指导和依据，为新型高强韧钢铁材料的设计提供新的思路。