基于珠光体的奥氏体逆转变工艺制备新型先进高强钢及其力学性能优化

The advanced high-strength steels (AHSSs) have been developed with the increasing requirement of weight reduction and high safety in the automotive industry. Whereas, with the increased strength, the edge-cracking has been frequently reported during forming process due to a low fracture toughness. For the AHSSs having retained austenite, the crack easily prefers to nucleate and propagate along the blocky retained austenite after it transforms to martensite, resulting in the decreased fracture toughness. It is well-known that the lamellar microstructure can enhance the strength while the film retained austenite is beneficial to the enhancement of fracture toughness. In order to produce the steels having high strength and high fracture toughness, based on the pearlite-to-austenite transformation, the lamellar microstructure consisting of retained austenite and martensite is obtained by the adjustment of the contents of carbon and manganese and of the heat treatment. The effect of chemical compositions and heat treatment on the microstructure evolution is systematically investigated, which is helpful for the precise control of the microstructure. The transformation of retained austenite to martensite under uniaxial tension (low stress triaxiality) and double edge-notched tension (high stress triaxiality) is studied, while the effect of volume fraction and stability of retained austenite on tensile properties and fracture toughness is deeply analyzed. According to the above analysis, the microstructure constituents and corresponding heat treatment can be further improved for the development of AHSSs simultaneously having high strength and high fracture toughness.

汽车轻量化和安全性的需求推动了先进高强钢的开发，在不断提高强度的情况下断裂韧性却没有得到足够的重视，使得在成型过程中因断裂韧性不足而时有裂纹的产生。特别是对于利用残余奥氏体的相变诱导塑性来开发的先进高强钢，其中块状的残余奥氏体在转变为马氏体后，易于裂纹的萌生和扩展。本项目依据片层微观结构有利于获得高强度以及片层残余奥氏体有利于提高断裂韧性，来开发高强高韧的先进高强钢。拟利用珠光体的片层结构，通过调节碳、锰元素含量和控制逆奥氏体化热处理工艺，制备片层残余奥氏体和片层马氏体相互交叠的微观组织。系统研究成分和热处理工艺对微观组织的影响规律，建立微观组织精确控制的理论基础。以单向拉伸力学性能和断裂韧性为双评价机制，分析残余奥氏体在不同应力状态下的转变规律，探索残余奥氏体体积分数和稳定性对综合力学性能的影响规律，指导微观组织的优化和热处理工艺的改进，开发高强高韧的新型先进高强钢。

对汽车轻量化和安全性不断提高的要求，促进了先进高强钢的研发——在减少钢板厚度的同时，不仅降低了汽车重量，也保证了安全性不被削弱。利用多边形铁素体、贝氏体或马氏体，在室温下获得残余奥氏体，已经开发了许多具有相变诱导塑性效应的先进高强钢。但是，珠光体因消耗碳元素导致奥氏体含碳量不足，而需极力避免把它作为使用状态的组织。这导致了珠光体在先进高强钢的开发过程中，没有得到足够的重视。.本项目以珠光体为初始组织，首先通过调节碳、锰含量制备不同的珠光体组织，然后基于珠光体的逆奥氏体化工艺，制备具有片层残余奥氏体和片层马氏体微观组织的新型先进高强钢。通过电阻炉、盐浴炉、扫描电子显微镜、透射电子显微镜、原子探针层析技术和力学测试等实验手段，研究了微观组织和力学性能之间的关系，取得以下成果。.（1）揭示了不同珠光体微观结构（渗碳体片层尺寸和体积分数等）及奥氏体逆转变温度和时间对片层残余奥氏体稳定性和体积分数的影响规律；研究了回火工艺对马氏体微观结构的影响。.（2）揭示了在珠光体逆奥氏体化的过程中碳元素和锰元素随加热温度和保温时间的扩散规律，为精确控制残余奥氏体的体积分数和稳定性提了供理论依据。.（3）在保证一定塑性的前提下，为改善先进高强钢的断裂韧性和提高抵抗裂纹扩展的能力，需要尽可能的减少块状残余奥氏体的体积分数，同时增加片状残余奥氏体的体积分数。研究了微观组织特别是残余奥氏体对单向拉伸力学性能和断裂韧性的影响规律，发现鬼珠光体含量增加提高了冲击韧性，意味着鬼珠光体组织具有阻碍裂纹扩展的能力。.珠光体是钢中常见的一种相组成，却没有在先进高强钢的开发中得到足够的重视和应用。本项目验证了基于片层珠光体开发新型先进高强钢的可行性，拓展了先进高强钢的设计路径。

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