中碳微纳结构贝氏体钢中残留奥氏体细化及其韧化机理

Micro/nano-structured bainitic steel provides a unique combination of ultra-high strength and high ductility due to their structure consisting of nano-scale bainitic-ferrite and retained austenite, but the toughness is a little low. The retained austenite plays a leading role for the toughness, it can significantly increase the toughness of micro/nano-structured bainitic steel by refining the size of blocky retained austenite and improving the content of film retained austenite. Simultaneously, the structure of retained austenite affects the stability of retained austenite, even can change the micro-deformation and decide the toughness. This project will plan to refine retained austenite of medium-carbon bainitic steel by using two-step bainite transformation to study phase transformation of retained austenite through heat treatment. The nano-indentation behavior of retained austenite will be investigated to fundamentally understand stability of retained austenite dependence on micro-deformation mechanism. Meanwhile, the impact fracture microstructure of micro/nano-structured bainitic steel will be analyzed to investigate the relationship between toughness and strain behaviors of retained austenite. Through the above-mentioned studies, this research not only precisely refines the retained austenite structure, reveals the effect of retained austenite stability on micro-deformation mechanism and resolves toughness mechanism, but also provides the theoretical guidance for the large scale production of micro/nano-structured bainitic steels combination with good toughness.

微纳结构贝氏体钢由纳米贝氏体铁素体板条和残留奥氏体组成，具有超高强度和高塑性，但韧性较低。对韧性起主要作用的为残留奥氏体，通过细化块状残留奥氏体并提高薄膜状残留奥氏体含量，能显著提高微纳结构贝氏体钢的韧性；同时，残留奥氏体显微形态会影响奥氏体稳定性，甚至改变微观形变并决定韧性。本项目拟对中碳贝氏体钢进行两步贝氏体转变以细化残留奥氏体，研究热处理工艺对残留奥氏体组织转变的影响；分析不同形态残留奥氏体的纳米压痕行为，探讨残留奥氏体稳定性和微观形变机制的内在关联；观察微纳结构贝氏体钢的冲击断裂组织，确立残留奥氏体应变对韧性的影响。这不仅可以细化残留奥氏体，揭示残留奥氏体的微观形变机制，解析韧化机理，而且可以为开发优良韧性的微纳结构贝氏体钢提供理论指导。

微纳结构贝氏体钢由纳米贝氏体铁素体板条和残留奥氏体组成，具有超高强度和高塑性，但韧性较低。对韧性起主要作用的为残留奥氏体，通过细化块状残留奥氏体并提高薄膜状残留奥氏体含量，能显著提高微纳结构贝氏体钢的韧性。本项目采用高碳（200℃等温10days）和中碳（320℃等温1day）钢低温贝氏体转变工艺、中碳钢中温（400–450℃等温2–4hrs）和低温（320–360℃等温12–24hrs）贝氏体转变、中碳钢一步（300℃等温6hrs）和两步（300℃等温2hrs + 250℃等温24hrs）低温贝氏体转变工艺，研究了残留奥氏体对中碳微纳结构钢冲击韧性的影响。中碳钢低温贝氏体转变中存在大量的残留奥氏体，在冲击过程中可以引起残留奥氏体的塑性变形，使断裂时吸收的能量增加，显著提高材料的韧性；同时第二步贝氏体转变时新形成的贝氏体铁素体分割细化块状未转变奥氏体，减少贝氏体等温后淬火过程中块状马氏体形成，在冲击过程中能够更好地使裂纹分叉甚至阻止裂纹的扩展，进一步提高样品的韧性。通过对微纳结构贝氏体钢进行纳米压痕试验结合EBSD分析，发现贝氏体束和残留奥氏体的纳米压痕中载荷-位移曲线存在明显不同，其微观形变也完全不同，探讨了显微组织和微观形变机制的内在关联；通过冲击断口试验结合EBSD研究微纳结构贝氏体钢冲击裂纹，发现裂纹终止于残留奥氏体附近，残留奥氏体可以阻碍裂纹扩展与长大，揭示残留奥氏体的微观形变机制，解析韧化机理，为开发优良韧性的微纳结构贝氏体钢提供理论指导。