新型高强塑性马氏体不锈钢合金化设计的基础研究

The toughness and ductility of martensitic stainless steels (MSSs) can be improved significantly by quenching and partitioning (Q&P) treatment, in which carbide precipitation inhabitation is much critical. Preliminary studies show that, carbide precipitation can be suppressed through adding non-carbide forming elements; as a result the production of strength and elongation of MSSs can be increased to a high level of 44000MPa%. However, the action mechanisms of these elements and their adding methods have not been clarified and require in-depth investigation. In present proposal, systematic researches on phase transformation and mechanism of strengthening and toughening will be carried out in 13%Cr MSSs subjected to Q&P treatment, by means of microstructure characterization techniques including X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM), in combination with mechanical properties measurement methods such as tensile and Charpy V-notch impact testing. The roles of non-carbide forming elements of Si, Al and Ni added alone or simultaneously on martensitic transformation, carbon diffusion in martensite and retained austenite, carbide precipitation within martensite, and stability of retained austenite are to be investigated respectively, aiming to clarify the mechanisms associated. As well the corrosion resistances of experimental steels are to be evaluated. The optimized alloying design criterion and according process controlling conditions that help to achieve excellent comprehensive properties will be figured out ultimately. The results achieved will benefit extending application field of Q&P theory and enriching alloying principle of MSSs, and lay a solid foundation for optimizing the chemistry & processing as well as controlling microstructures & performances of new-typed MSSs with high strength and ductility.

淬火-配分处理是提高马氏体不锈钢塑韧性的有效手段，而抑制碳化物析出是该工艺成功实施的关键。预研究表明，添加非碳化物形成元素能够有效抑制碳化物析出从而使钢的强塑性配合达到更高的水平（(Rm×A)≥44000MPa%），但这些元素的作用机制和添加方式还不明确。本项目拟通过XRD、SEM、TEM等组织分析手段与拉伸、冲击等力学性能测试方法，针对Cr13型马氏体不锈钢在淬火-配分处理时的组织转变和强韧化机理展开系统研究，探明硅、铝、镍等非碳化物形成元素在单独或复合添加时对马氏体转变、碳在马氏体和奥氏体中的扩散、马氏体内碳化物析出和奥氏体稳定性的影响规律及作用机理，进而对新钢种的耐腐蚀性能做出评估，找到获取最佳综合性能的合金化设计准则和工艺控制方向。研究结果将扩展并丰富淬火-配分理论的应用领域，促进马氏体不锈钢合金化的进步，为新型高强塑性马氏体不锈钢的成分工艺设计和组织性能控制奠定切实的理论基础。

针对传统马氏体不锈钢自身成分和处理工艺难以克服的耐腐蚀性能和塑韧性不足的问题，提出利用“淬火-配分”（Q&P）工艺向马氏体中引入一定体积分数残留奥氏体的解决手段，而通过添加非碳化物形成元素抑制碳化物析出是该工艺成功的关键。以Cr13型马氏体不锈钢为基础，设计制备了20Cr13、10Cr13N、10Cr13NiN、10Cr13Si2N、10Cr13NiSiN等5种成分的实验钢，利用全自动金属相变仪研究了实验钢马氏体相变点与转变动力学、“淬火-配分”过程中组织演变及非碳化物形成元素对相变的影响规律，通过对热处理后的试样进行XRD物相分析和EBSD及TEM组织表征获得了工艺参数对显微组织相组成、比例和形貌的影响规律及合金元素的作用，将组织表征与拉伸、冲击等力学性能测试的结果相结合建立起了成分-工艺-组织-性能关系和强韧化机理，找出获得最佳组织和性能的淬火中止温度、配分温度和配分时间等工艺参数确定原则，通过对实验钢的耐腐蚀性能评估，获得了显微组织和非碳化物形成元素对耐腐蚀性能的影响规律。结果表明：用部分N元素代替C元素有利于耐腐蚀新提高，但是一定程度上减弱了Q&P组织中残留奥氏体的含量；向马氏体不锈钢中加入一定量的Ni或者Si可以有效抑制碳化物析出，提升Q&P效果；Q&P组织中的残留奥氏体体积分数随淬火终止温度的变化存在极值规律，通常在Ms温度以下150℃达到极大值；选择在残留奥氏体极值温度以下20~30℃作为淬火中止温度，能有效避免二次马氏体生成，可以获得最佳力学性能；不完全淬火组织在450℃配分处理可以获得最多的残留奥氏体及强塑性组合，其塑韧性比传统钢种提高一倍以上；Q&P处理比Q&T处理具有更优异的耐腐蚀性能，向钢中添加Ni元素比添加Si元素更有利于耐腐蚀性能提高。本项目研究结果为高强高韧高耐蚀Q&P型MSSs的开发奠定了坚实的理论基础。