多变量梯度条件下中锰钢相变机理及强韧性控制研究

There are significant multivariate gradients, including hot-rolling deformation reduction, quenching cooling rate, tempering heating rate, and alloy composition, along the thick direction of the extra heavy gauge steel plate, therefore, the control of its microstructure and mechanical property homogeneity has long been the key and common technical problem. Moreover, conventional extra heavy plates have the shortcomings of high alloy cost, high yield ratio, low center toughness, and complex heat treatment process. In this program, medium Mn alloying for enhancing the stability of austenite is creative applied to increase the microstructure and mechanical property homogeneity of the extra heavy plate, and the weldability is improved by ultra-low C addition. The austenite recrystallization behavior and martensite transformation behavior under the condition of different deformation reduction and cooling rate gradients along the thick direction of the extra heavy plate are studied. Based on the low heat transmission rate from the surface to the center of the extra heavy plate during tempering process in two phase region, the martensite recrystallization and austenite reverse transformation thermodynamics and kinetics mechanisms under the condition of different heat rate gradients in extra heavy plate are studied. The strain partition behavior of tempered martensite plate and reverse transformed austenite film duplex microstructure is explored, and the role of gain size submicron and meta-stable austenite TRIP effect on enhancing the strength and toughness in the center of extra heavy plate is elucidated. The development of this program offers theoretical basis for alloy composition design and microstructure and mechanical property homogeneity control of new type extra heavy plate steel. The short treatment process of online quenching after hot-rolling and short time tempering meets the demand of national iron and steel green development.

特厚板厚度方向存在显著的热轧变形量、淬火冷却速率、回火升温速率、合金成分等多变量梯度，其组织性能均匀性控制是长久以来关键的共性技术难题，而且传统特厚板存在合金成本高、屈强比高、心部强韧性差、热处理工序复杂等不足。为此，本课题创新地将中锰合金化显著提高奥氏体稳定性作用应用于增强特厚板组织性能均匀性，并采用超低碳含量改善钢板焊接性能。研究特厚板厚度方向不同变形量及冷却速率梯度条件下的奥氏体再结晶行为及马氏体相变行为。基于特厚板两相区回火过程中表层向心部传热速率低的特点，研究不同升温速率梯度条件下马氏体再结晶及奥氏体逆转变热/动力学机制。探索回火马氏体板条-逆转变奥氏体薄膜双相组织的应力配分行为，揭示晶粒亚微米化、亚稳奥氏体TRIP效应对提高特厚板心部强韧性的作用。本课题的开展为新型特厚板合金成分设计及组织性能均匀性控制提供理论基础，热轧后在线淬火及短时回火短流程工艺符合国家钢铁业绿色发展要求。

特厚板厚度方向存在显著的热轧变形量、淬火冷却速率、回火升温速率、合金成分等多变量梯度，其组织性能均匀性控制是长久以来关键的共性技术难题，而且传统特厚板存在合金成本高、屈强比高、心部强韧性差、热处理工序复杂等不足。为此，本课题创新地将中锰合金化显著提高奥氏体稳定性作用应用于增强特厚板组织性能均匀性，并采用超低碳含量改善钢板焊接性能。确定了0.04~0.06%C、5.0~5.5%Mn成分体系及轧后直接淬火回火的工艺。研究了80~100mm中锰特厚钢板不同热轧变形量梯度条件下的奥氏体再结晶行为、不同冷却速率梯度条件下的马氏体相变行为，变形速率为5s-1时，低于950℃为加工硬化型，冷却速率为0.1~60°C/s时，变形奥氏体向马氏体相变开始温度为370~390°C，高淬透性有助于提高特厚板组织性能均匀性。分析了淬火马氏体板条形态、位错组态及晶体学特征，并重点研究了回火温度、时间对组织性能的影响，定量测定逆转变奥氏体体积分数、分布、形态以及元素富集。通过低温等温及间断拉伸实验，定量分析了奥氏体的低温稳定性及机械稳定性，随着回火温度的提高，逆转变奥氏体尺寸增大、C-Mn元素富集程度降低，奥氏体热稳定性及机械稳定性减弱。中锰合金化提高了特厚板厚度方向的性能均匀性，1/2厚度的屈服强度、抗拉强度、延伸率分别为714MPa、814MPa、24.2%，屈强比0.88，而且-60℃低温冲击功＞80J。揭示了亚稳奥氏体TRIP效应机理及其对强度、塑性、韧性的作用，适度稳定的逆转变奥氏体大幅提高加工硬化率，缓解尖端应力集中，延迟裂纹萌生，其形变诱发马氏体转变引起的体积膨胀起到钝化裂纹扩展作用，从而提高韧塑性能。纳米尺度TiC析出相通过细晶强化、析出强化作用可显著提高屈服强度，进一步提高Ti含量，奥氏体中的C富集程度减弱，不但强度增加量有限，而且降低奥氏体稳定性，从而恶化韧塑性能。针对多变量梯度条件下中锰钢的相变机理及强韧性控制方法研究所形成的规律及机理可为特厚板组织性能控制提供理论基础及工艺参考，在线淬火及短时两相区回火满足国家钢铁行业绿色发展的要求。