连续冷却条件下低碳高强贝氏体钢的残余奥氏体调控及韧化机理

Taking into account that the low temperature impact toughness of low carbon high strength bainitic steel in heavy plate greatly decreases as the strength increases, matrix structure is proposed to introduce retained austenite acting as toughening mode. The key problems consisting of the controlling of retained austenite with different characteristics by two-stage continuous cooling heat treatment processes in intercritical region combined with TMCP-UFC and toughening mechanism are systematically investigated. The controlling of retained austenite with different characteristics is achieved by two-stage continuous cooling heat treatment processes in intercritical region combined with TMCP-UFC, and regulation mechanism is clarified. The relationship of heat treatment processes, retained austenite with different characteristics and stability of retained austenite is defined. By analyzing substructure (including dislocation, twin crystal, etc.) and martensite with different lattice structure in retained austenite during low temperature impact process, the effect of transformation of retained austenite and its influencing factors of stability on low temperature impact toughness is clarified. The acting of transformation of retained austenite on enhancing plasticity of matrix, crack initiation and propagation under low temperature conditions is illuminated, and the mechanism toughening is revealed. The results will provide theoretical basis and production guidance to developing low carbon high strength bainitic steel in heavy plate with high low temperature impact toughness, to realize improvement of production efficiency and reduction of energy consumption. The application of low carbon high strength bainitic steel in high strength structural heavy steel is expanded under condition of cryogenic regions.

本项目针对低碳高强贝氏体钢中厚板低温冲击韧性随着强度级别的增加而下降的现状，拟在其基体组织中引入残余奥氏体作为韧化方式，围绕通过TMCP-UFC结合两阶段临界区连续冷却热处理工艺对不同特征残余奥氏体的调控及韧化机理等关键问题开展研究。实现TMCP-UFC结合两阶段临界区连续冷却热处理工艺对不同特征残余奥氏体的调控，弄清调控机理，明确热处理工艺、不同特征残余奥氏体及其稳定性的关系；通过重点分析低温冲击过程中残余奥氏体内的微结构(位错、孪晶等)和不同晶格结构的马氏体，明确残余奥氏体的转变及稳定性影响因素对低温韧性的影响；阐明低温冲击过程中残余奥氏体的转变在改善塑性及裂纹形核和扩展中的微观作用，揭示韧化机理。该项目的研究结果可以为高低温韧性中厚板用低碳高强贝氏体钢的开发提供理论依据和生产指导，实现生产效率的提高和能源消耗的降低，扩大其在低温领域中厚板高强结构钢中的应用。

高低温韧性中厚板低碳高强钢的研究是钢结构在恶劣环境条件下服役的安全性和抗脆断性的重要保障，改善低碳高强贝氏体钢低温韧性的研究尤为重要。本项目在微合金化贝氏体钢的基础上，添加适当的奥氏体稳定化元素Mn、Ni等，通过TMCP-UFC结合两阶段连续冷却热处理工艺，调控一定量较稳定的残余奥氏体以及M/A组元，以此来改善低碳高强贝氏体钢的低温韧性。本研究取得以下主要结果：(1)利用B、Mo等合金元素对多边形铁素体的抑制作用、Mn、Cu等合金元素提高淬透性的作用以及Ti、Nb元素的复合微合金化，在较大的冷速范围内获得精细的贝氏体组织，基于TMCP-UFC新型工艺路线，调控终冷温度为480℃时，增加点状M/A岛的数量和高角度取向差晶界比例，从而提高裂纹的偏转频率，增加裂纹扩展吸收功；同时，通过降低终冷温度可以合理调控板条贝氏体和粒状贝氏体配比，细化晶粒尺寸，实现了高强度和高韧性的良好平衡。(2) 粗大的残余奥氏体+孪晶马氏体(尺寸9um)恶化冲击韧性，严重降低了裂纹萌生过程中的冲击吸收能量，对于粗大的贝氏体，高角度取向晶界几乎消失。裂纹萌生吸收能决定了低碳高强贝氏体钢解理断裂的微观机制。(3) 建立了两阶段临界热处理工艺-不同特征残余奥氏体-奥氏体稳定性-冲击韧性的关系，较高的临界回火温度使逆转变奥氏体不稳定，块状的M/A转变为孪晶马氏体，由于不充分的TRIP效应，恶化冲击韧性，对加工硬化性能和塑性的贡献相对有限；而适当的临界热处理工艺得到稳定的残余奥氏体以及外层为Mo的细小(Nb, Mo)C，使裂纹扩展吸收功明显升高，提高韧性。开发了具有良好低温韧性的低碳高强贝氏体钢，为实现工业生产提供依据和指导，扩大其在低温领域中厚板高强结构钢中的应用。