高性能钢中温协变相变的晶体学特征及对韧性影响机理研究

Low carbon low alloy high-performance steels have been applied in many aspects including economic construction and defense industry due to their ideal properties such as the nice strength-toughness matching. In recent years there has been an increasing demand for better performance of low carbon low alloy steels, making the contradiction between strength and toughness become more and more clear. It is necessary to understand the structural factors for toughness control to optimize the strength-toughness balance. The microstructure of low carbon low alloy steels is typical medium temperature coherent transformation products. The key factors to influence toughness and the way to control it are still unclarified. Based on the result and theory of the crystallography of medium temperature coherent transformation, this project is going to find out the influence of key factors (cooling condition, size of austenite grain, and accumulation of strain etc.) on M/A islands-the crack inducer, and on high angle boundary---the crack arrester, investigate the formation regularity of the crystallographic packet containing both M/A island and high angle boundary under different industrial conditions (pipeline, ultra-heavy plate, welding etc.), show the mechanism of crystallographic packet formation and reveal the combining effect of M/A island and high angle boundary on toughness. The results from this project should improve the theoretical principle of strengthening and toughening in steel and could be very beneficial for the strength-toughness balance improving of new generation steel.

高性能低碳低合金钢在经济国防建设的各个方面具有广泛应用，石油、海洋工程领迫切需求高强并且高韧性钢。高强度低碳微金钢的组织中主要部分属于中温转变协变相变产物，该类组织影响韧性的机理始终未能明确。本项目将从中温转变协变相变产物的晶体学结构入手，摸清各种内外部条件，如合金成分，原奥尺寸，应变积累，冷却条件等对中温转变产物M/A岛以及Packet晶体学形貌（有效晶粒单元）的影响，通过揭示裂纹起裂原M/A以及止裂因素大角度晶界的演变规律，结合各类相变组织韧性指标与特征（示波冲击），深入研究M/A及大角晶界的致裂止裂机理，建立控制韧性的协变相变产物的晶体学判据。与此同时，通过开展对M/A和大角晶界的工艺控制，探索屈服强度700MPa（X100管线钢）以上TMCP钢板，海洋工程用100mm以上厚度特厚钢板强韧化工艺技术。

本项目针对高性能钢协变相变晶体学研究方向进行全面展开，探究了晶体学结构与低温韧性的定性、定量化关系。该项目完成了全部课题任务要求，并在以下几个方向取得了突破性进展：（1）低合金钢中晶体学结构信息的高通量、大数据提取及结构的可视化、数字化表征方法——依据中温转变协变相变产物与母相奥氏体的相变关系，并通过MATLAB编程实现了母相奥氏体的重构；通过母相与子相相变关系及欧拉角实现了变体含量、变体对含量及晶界密度的定量化计算。（2）实现了晶体结构可视化表征及其与断裂韧性定量化关联的建立——通过EBSD晶体学数据库的计算及重构奥氏体取向和位向关系信息，解析了（a）焊接粗晶热影响区协变相变组织晶体结构与冲击韧性的定量关系，（b）调质淬火过程奥氏体化温度对低合金钢变体选择的影响机制、对晶界密度的影响及其与低温冲韧性的定量关系。（3）利用中温协变相变理论调控大角度晶界密度及细化脆性相M/A的方法已成功制备出高强高韧的高等级、低温服役、管线用原型钢，且正在逐步向海洋工程用钢、建筑用钢的领域发展。此外，以该项目为依托，本团队与国内各大钢铁企业（沙钢、首钢、邯钢、鞍钢等）密切联系，切实落实产学研的科学发展观，充分运用研究基础理论，逐步深化和拓宽研究领域，开拓新产品，攻克旧难题。