Cr-Mn-Si-C-N-RE新型超高温耐热不锈钢的基础研究

Due to the spalling and cracking induced by poor oxidation resistance and insufficient strength, the traditional heat resistant stainless steels, such as expensive high Cr-Ni austenitic stainless steels, are difficult to meet the severe component requirements for long-term service at ultra-high temperature over 1000℃. Therefore, the improvement of high temperature oxidation resistance and strength is the key to exploit heat resistant stainless steels applicable at ultra-high temperature. By breaking through the alloy design route of traditional Cr-Ni heat resistant steel, a novel type of Cr-Mn-Si-C-N-RE austenitic heat resistant steels is designed in this proposal. The entire oxidation resistance is modified by the comprehensive effect of Cr, Si and RE elements. The local oxidation resistance is promoted by the inhibition effect of intergranular film-shaped carbides on the internal oxidation along grain boundaries. Meanwhile, the high temperature strength is elevated by the precipitation strengthening effect of dispersive precipitates in the alloy matrix and the stabilization effect of RE elements on grain boundaries and precipitates. This steel is considered to be resource conservation, low cost, excellent heat-resistant properties and long-term life expectancy. In the study, the microstructure evolution, high temperature oxidation behavior and high temperature mechanical properties are systemically investigated in the thermodynamic and first principle theoretical calculation, optical microscopy, scanning electronic microscopy, transmission electronic microscopy and three-dimensional atom probe experiments. Lots of work will be done for revealing the effect of the variation of C and N contents on the precipitation mechanism of second phases, the inhibition effect of intergranular film-shaped carbides on the internal oxidation along grain boundaries, the precipitation strengthening mechanism of dispersive precipitates, as well as clarifying the comprehensive effect of Cr, Si and RE elements on the oxidation behavior and the role of RE elements on microstructure and mechanical properties. On the basis of the research results above, a new theory of the alloy design for heat resistant stainless steels are expected to be formed.

在1000℃以上超高温服役条件下，由于沿晶内氧化严重和高温强度不足，现有传统耐热不锈钢难以满足耐热部件苛刻的选材要求，成为制约高温装备发展的瓶颈。本项目提出了成分设计与组织调控相结合的合金设计理念，拟开发资源节约、成本可控、性能优异的Cr-Mn-Si-C-N-RE新型超高温耐热不锈钢合金体系。借助热力学、第一性原理计算与透射电镜、三维原子探针等实验手段，研究合金成分、组织调控与高温性能的内在关联，明确组织状态、内氧化机制与服役性能的对应关系；重点分析C、N含量及比率对析出相特征的影响规律，揭示第二相形态与分布对沿晶内氧化的作用机理，阐明Cr、Si和RE对高温氧化行为的微观作用机制，探讨固溶RE对高温组织和高温强度的影响，从而系统阐述C、N共合金化和RE微合金化，抑制沿晶内氧化和提升高温强度的微观机理，为新型超高温耐热不锈钢开发与应用奠定理论基础。

在1000℃以上超高温服役条件下，由于沿晶内氧化严重和高温强度不足，现有传统耐热不锈钢难以满足高温装备的大型化和高效化发展对耐热部件的选材要求，成为制约高温装备发展的瓶颈。本项目通过成分设计与组织调控相结合，优化设计出资源节约、成本可控、性能优异的Cr-Mn-Si-C-N-RE新型超高温耐热不锈钢合金体系（简称CNRE，下同），系统研究了CNRE合金体系的相变特征、高温氧化行为，探讨了CNRE合金体系的第二相析出机制、高温氧化机制及其沿晶内氧化抑制机理，揭示了固溶RE对高温组织与高温强度的影响规律，初步构建了“C+N+RE”共合金化设计理论，为新型超高温耐热不锈钢开发与应用奠定理论基础与数据积累，也为探索少合金化钢种、减少资源消耗、形成自主特色品种，提供了新思路。主要工作与重要结果如下：.（1）CNRE合金体系相变规律：通过热力学理论计算，优化设计了CNRE合金体系成分规范，明确了CNRE合金体系的各类平衡相及其相变特征。借助扫描电镜和透射电镜等实验手段，深入研究了CNRE合金经固溶后，时效过程中第二相的析出机制。1150℃固溶处理后，经600~1000℃温度范围时效，CNRE合金中析出相除了少量含Nb的细小弥散颗粒状MX型碳氮化物之外，主要为富Cr的M23C6碳化物。随着时效温度的升高和时效时间的延长，M23C6碳化物的析出序列为“晶界膜状析出→胞状组织内层片状析出→晶内颗粒状析出”。.（2）CNRE合金体系氧化机制：对比分析CNRE合金与传统选材的高温氧化行为发现，CNRE合金并未产生沿晶内氧化。以Fe-25Cr-1.5Si-xC合金为原型材料，揭示了沿晶内氧化抑制机理。高温氧化过程中，晶界作为快速扩散通道，O、Si元素在晶界处，发生内氧化。而晶界析出碳化物，能够阻碍O、Si元素沿晶扩散，有效延缓或抑制沿晶内氧化。.（3）固溶RE对组织性能影响：以316LN单相奥氏体耐热不锈钢为原型材料，阐明了固溶RE对高温组织和蠕变性能的作用机制。固溶RE，能够促进晶内第二相析出，增强基体高温变形抗力；并且，能够降低界面能，促进蠕变空洞形核，抑制蠕变空洞生长，从而有效改善高温蠕变性能，提升高温强度。