具有复相组织特征梯度强化马氏体时效不锈钢的强韧性研究

Without reduction of toughness and ductility while enhancement in strength has become an important bottle-neck problem in the research and development of maraging stainless steels. Therefore, this proposal will design a novel gradient strengthening maraging stainless steel with heterogeneous microstructure. The main characteristic of the steel is as follows: gradient trasition from the strain-induced martensite at the surface to the austenite in the core, which is expected to effectively improve the mechanical property of maraging stainless steel. A self-developed strain-induced martensite stainless steel with composition of Fe-12Cr-12Ni-2Ti-2Cu-2Co-Mo-V-Mn-Al is selected as experimental material, and high-energy shot peening and aging treatment will be employed to study their effects on formation of the gradient microstructure. Based on the tensile tests and microstructural observation, the relationship between gradient microstructure and the mechanical property will be obtained, and the mechanism of strengthening/toughening of the steel with heterogeneous microstructure will be explored. The research findings will establish a principle on the gradient microstructure design for maraging stainless steel and produce a novel strengthening/toughening mechanism for the gradient microstructure, which will further enrich the theory of strengthening/toughening of materials.

如何在提高马氏体时效不锈钢强度的同时不降低其韧塑性，已成为马氏体时效不锈钢研究和发展的主要瓶颈问题。本项目拟采用薄板状样品设计出由表面（马氏体+强化析出相）组织向心部（奥氏体）组织梯度过渡的新型马氏体时效不锈钢，探索通过梯度组织强化提高马氏体时效不锈钢综合力学性能的新途径。本项目以拥有自主专利权的C-12Cr-12Ni-2Ti-2Cu-2Co-Mo-V-Mn-Al形变诱发马氏体时效不锈钢为研究对象，采用高能喷丸工艺获得表层为形变诱发马氏体，心部为奥氏体的梯度强化马氏体时效不锈钢，研究喷丸工艺参数以及时效工艺参数的变化对梯度组织结构的影响规律；通过评价梯度组织强化马氏体时效不锈钢的力学性能，分析试样断口附近组织的变化来研究梯度组织与力学性能之间的关系，探索梯度组织强韧化机理。本项目的研究成果将建立马氏体时效不锈钢的梯度组织设计原理和新颖的梯度组织强韧化机制，丰富材料的强韧化理论。

项目以阐明具有复相结构马氏体时效不锈钢的结构-性能关系和强韧化机理，提高马氏体时效不锈钢的综合力学性能为主要研究目标。采用高能喷丸工艺获得了表层为形变诱发马氏体，心部为奥氏体的梯度强化马氏体时效不锈钢；同时以具有低层错能的AISI304不锈钢为研究对象，通过扭转变形成功制备出梯度强化奥氏体不锈钢，研究了等效应变的大小对基体组织和力学性能影响，以相变发生的热力学为基础，理论分析了形变诱发马氏体相变发生的可能性；最终得到了一种新型具有复相结构多相强化高强度（强度级别可以达到1900MPa）马氏体时效不锈钢，其名义成分为Fe-13Co-12.5Cr-5.5Mo-4.5Ni-0.5Ti，其最大抗拉强度和屈服强度分别为1928MPa和1649MPa，同时保持较高的延伸率和断面收缩率，分别为10.5%和34%，并且断裂韧性达到83.61MPa•m1/2。结合APT和TEM两种分析手段分析时效过程显微组织结构演变规律。经深冷处理后，材料为高密度位错板条马氏体组织；时效初期形成Ni-Ti-Al，富Mo和富Cr三种团簇，时效3h后，η相，R′相和α′相三种析出相从Ni-Ti-Al，富Mo和富Cr团簇中析出，随时效时间的延长，三种析出相的成分趋向于平衡成分，等效半径、数量密度和体积分数发生相应的变化。另外，时效过程中形成逆转变奥氏体，随时效时间延长，板条界和晶界的条带状奥氏体逐渐长大粗化成块状。本项目的研究成果建立了马氏体时效不锈钢的梯度组织设计原理和新颖的梯度组织强韧化机制，丰富相关材料的强韧化理论。