海洋工程用低密度δ铁素体钢韧化机理研究

Recently the requirement for weight reduction，efficiency improvement and carbon dioxide emission reduction has been proposed in the application of automobile manufacturing and offshore structure construction. Therefore the application and study on the low density steels become more and more attractive since it has lower density and high corrosion resistance. However it has just on the way that the study on the microstructure and properties of low density steels. On the other hand the research works are focused on the microstructure and properties of hot rolled thin plates and cold rolled thin plates for automobile application. The study on the microstructure and properties of low density offshore structural steel with delta ferrite are just a little. In this project the key mechanism for alloying in the low density offshore structural steels with delta ferrite, and the segregation of microalloying elements such as Nb and B at subgrain boundaries and grain boundaries of delta ferrite will be investigated. Also the solubility product for Nb carbonitride or carbide at delta ferrite at higher temperatures need to be modified. The strain induced precipitation behavior of Nb carbide or carbonitride need to be clarified. The interface energy for Nb carbide or carbonitride and the stacking fault energy will be calculated by employing the first principle when the Al is added in the steels. The dynamic and static recrystallization behavior will be studied. More over the interaction between segregation, precipitation of Nb and B with the recrystallization will be also discussed. By the way the texture by deformation and recrystallization will be also detected. The texture is helpful to obstacle the propagation of cracks. This project will be a good supplement for the Nb microalloying theory and also the alloy design and microstructure refinement technology will be developed for low density steels with delta ferrite.

海洋工程、汽车等应用领域都提出了减重、降低排放的需求，低密度钢因密度低、耐蚀性能好的优点引起业界的重点关注。目前国际上对低密度钢的研究集中在汽车用热轧或冷轧薄板的组织性能及变形机制方面，对组织为全δ铁素体的海洋工程用钢的研究匮乏，迫切需要系统的工作。本项目着眼于研究全δ铁素体海洋工程用低密度钢的关键合金化原理，微合金元素Nb、B在δ铁素体亚晶界以及移动晶界非平衡偏聚行为，Nb碳化物或碳氮化物在高温δ铁素体区的溶解度积以及应变诱导析出行为，利用第一性原理计算Nb碳氮化物界面能，计算铝对铁素体层错能的影响，揭示δ铁素体动、静态再结晶行为及其形核长大机理，分析其与微合金元素偏聚、析出等的交互作用。厘清变形和再结晶织构与止裂性能的关系，提出创新的δ铁素体组织细化及韧化的控制工艺。项目完成将使Nb在高温δ铁素体区的微合金化原理更加完善，形成海洋工程用低密度钢的合金设计、工艺、性能控制成套理论基础。

为了满足国内海洋工程设施对低密度，高强度钢材的需求，实现减重、降低排放的国家战略。本项目聚焦于高强韧，低密度全δ铁素体钢合金设计，热变形工艺及再结晶工艺优化控制、组织调控以及δ铁素体断裂特征的研究。提出了全δ铁素体钢合金设计思路，阐明了Al，Nb，Zr，B等合金/微合金元素在全δ铁素体海工钢中的作用机制，揭示了它们对于细化高温δ铁素体，在线与离线热处理工艺控制再结晶δ铁素体的作用机制，提出控制 δ 铁素体的晶粒形态与晶粒尺寸的新原理。同时建立δ铁素体钢热变形本构方程，弄清了δ铁素体动、静态再结晶行为。结合第一性原理计算阐明了随Al含量增加，层错能降低，从而使得回复过程中的软化占到了60%，修正了针对δ铁素体的两次压缩法测量再结晶的方法。构建了Nb碳化物或碳氮化物在高温δ铁素体区的溶解度积公示，阐明了Nb碳化物固溶、析出行为，拓展了Nb在高温δ铁素体区的微合金化原理。形成适用于高性能低密度海洋工程用钢的 TMCP 三阶段控轧和加速冷却以及热处理的性能控制技术。通过对δ铁素体海工钢变形断裂行为的研究，提出了改善全δ-铁素体低密度钢低温韧性的有效途径。发现条带状δ-铁素体组织间细小的再结晶组织和内部较高密度的小角度晶界和亚结构，增加了裂纹扩展路径，提高了抵抗裂纹扩展的能力，在裂纹扩展的过程中利用δ-铁素体组织的界面分离，有效地提高了冲击能量释放速率，可有效地阻碍裂纹的传播，使得试验钢的0℃冲击韧性提高到了29 J（半厚度横向样品），从而提升材料的韧性。解决了全δ铁素体钢发展过程中的卡脖子技术难题。相关的技术已经在两个大型钢铁企业通过了试生产，形成海洋工程用低密度钢的合金设计、工艺、性能控制成套理论基础。项目发表期刊论文7篇，会议论文1篇，培养学生6名，申请专利3件，其中2件已授权。