钴-铜复合马氏体时效硬化不锈钢强韧化机理及富铜纳米析出相控制

Maraging hardening can improve the strength of the stainless steel, while it can also worse ductility and toughness at the same time. This cannot meet the requirements of super-strength, corrosion-resistant and good toughness and ductility for some components in aircraft manufacturing. Aiming to resolve this problem, the strengthening and toughening mechanism and the control technique of the microstructure of the material can be thoroughly investigated in this application based on the chemical composition of 15-5PH stainless steel by reducing the content of C, regulation of Co and maintaining the other element suchas Cu, Cr, Ni, Nb content unchangeable with the help of the simulation prediction, laboratory test, advanced characterization analysis and testing methods. The influence of cobalt on complex phase matrix structure of lath martensite and reversed austenite are studied with cobalt alloying as a starting point, and the coupling formation mechanism of the complex phases can be proposed. On this basis, the nucleation,growth, distribution, boundary feature of ε-Cu precipitated phase are investigated under Co element and coupled thermomechanical process. Whereafter, the influence rules of the multiphase microstructure changes on the strong performance can be revealed, Moreover, the relative contribution relation rates due to different strengthening mechanism can be clarified. The mechanism of strengthening-toughening can be established. The control methods can be mastered for Cu-rich precipitation phase and the most optimal complex phase matrix structure. Finally, a new super-maraging hardening stainless steel can be developed and obtained with Rm>1550Mpa and KIC>90MPa.m1/2

鉴于马氏体时效硬化不锈钢强度进一步提高的同时其塑韧性会下降，无法满足大飞机制造中某些结构件超高强度、不锈性和良好塑韧性要求的问题。本申请拟以15-5PH马氏体沉淀硬化不锈钢成分为基础，降低碳含量，调控钴元素，保持Cu、Cr、Ni、Nb等其它元素含量不变，采用模拟预测、实验室中试，先进表征手段分析与实验测试结合的方法，深入研究该材料的强韧化机理及组织控制方法。以钴合金化为出发点，探讨钴对板条马氏体与逆变奥氏体复相基体结构的影响规律，提出复相基体结构的耦合形成机制及共生特点；在此基础上，研究钴元素及热—力耦合共同作用下，富铜相的析出行为、演变机制及与复相基体结构的相关性；提出多相微观结构变化对强韧性能的影响规律，明确不同强化机制的相对贡献率，阐明强韧化机理；掌握富铜析出相及复相基体结构最优的控制方法，研发出抗拉强度>1550Mpa，断裂韧度>90MPa.m1/2的新型超级马氏体时效硬化不锈钢。

本项目提出的Co-Cu复合马氏体时效硬化不锈钢是一种以期满足国家重大需求的功能型高强度钢。该类钢的微观组织中包含超低碳板条马氏体、逆变奥氏体和弥散析出相等多相结构，阐明多相微结构协同配合的强韧化机理及获得组织控制措施是解决的关键科学和技术问题。本项目以15-5PH马氏体沉淀硬化不锈钢成分为基础，降低碳含量，调控钴元素，并结合热力学和动力学计算结果，制备了Co含量分别为3、6、9、10 wt.%的马氏体时效硬化不锈钢。研究了板条马氏体与逆变奥氏体复相基体结构的共生析出特点，发现在时效过程中纳米级逆变奥氏体（约50~250 nm）沿超细板条马氏体（约80~500 nm）边界呈薄膜状析出，与板条马氏体基体之间具有[110]γ∥[111]M和(111)γ∥(011)M晶体学取向，二者复合K-S关系。Co的添加对逆变奥氏体作用明显，即促进了逆变奥氏体形成，增加了其薄膜的宽度，当其含量达到9 wt.%时，组织中出现明显的块状奥氏体，其含量随着回火温度的升高而增大。同时，Co合金化抑制了高Cr马氏体不锈钢中铁素体的沿晶析出。研究了固溶温度、固溶时间、冷却方式、时效温度和时效时间在热处理阶段对ε-Cu相析出和复相基体组织演变的影响规律，在此基础上，确定了最优热处理工艺，有效控制复相基体组织及纳米富铜相的析出、长大及其分布。ε-Cu相主要在马氏体内部呈球形或椭球形弥散析出，与其存在共格的[011]Cu∥[001]M和(111)Cu∥(110)M取向关系。Co的添加抑制了ε-Cu相呈棒状长大、粗化的过程，增强了其热力学稳定性。通过拉伸、冲击、硬度和断裂韧度等性能测试实验，研究多相微观结构变化对强韧性能的影响规律，明确了不同强化机制的相对贡献率。阐明了马氏体时效硬化不锈钢的析出强化、固溶强化、位错强化、细晶强化和逆变奥氏体增韧的复合强韧化机理，从Co合金化和热处理角度掌握了富铜析出相及复相基体结构最优控制方法，获得了高强韧性能。通过本项目的研究，深化了合金化理论和多相组织演变方面的认识，为优化马氏体时效硬化不锈钢合金成分及热处理工艺制度等提供了理论依据。