大应变管线钢溶质配分与组织细化诱发TRIP效应机理及增强增塑规律研究

Ferritic/bainitic high-grade (X70-80) high deformability pipeline steel with good strength-plastic matching, has been successfully applied to earthquakes and other special regions. However, with the further improvement of the steel grade (strength), the plasticity rapidly decreases below the standard value, and it cannot meet the actual demand. In order to solve this problem, based on the coupled TRIP effect and the two-phase coordinating deformation mechanism, then it is supplemented by the refinement strengthening mechanism. Here, design idea of the plasticizing before enhancement is consistent with the mechanical properties of high deformability pipeline steel. Therefore, this project uses low-carbon (≤0.08%) structural steel as the research object. Based on the two solute partitioning, firstly, the retained austenite at room temperature is obtained and TRIP plasticization is realized. In addition, through phase transformation/recrystallization symbiosis, high-temperature fine equiaxed austenite grains are obtained and the ferrite/bainite/retained austenite multiphase structure is refined. Finally, using the macroscopic mechanical properties test, the multi-phase and multi-deformation mechanism synergistic effect of TRIP plasticization and fine-grain strengthening was revealed, and the relationship between processing technology, microstructure, and mechanical properties was established, which was used to obtain the strengthen plasticizing window includes composition, grain size, and strengthen-plasticity phase volume fraction, guiding process technology and microstructure control, and achieving the goal of simultaneous enhancement of strength and plasticity in the ultra-high steel grade (X120) high-strain pipeline steel.

铁素体+贝氏体高钢级（X70-80）大应变管线钢具有良好的强塑性匹配，已成功应用于地震等特殊地域。然而，随着钢级（强度）的进一步提高，塑性快速降低至标准值以下，无法满足实际需求。为此，在耦合TRIP效应与双相协调变形机制的基础上，辅以细晶强化机制，这种先增塑后增强的设计思想，符合大应变管线钢力学性能特点。因此，本项目以低碳（≤0.08%）结构钢为研究对象，基于二次溶质配分，首先获得室温残余奥氏体，实现TRIP增塑。进而，通过相变/再结晶共生现象获得高温细小等轴奥氏体晶粒，细化铁素体/贝氏体/残余奥氏体多相组织。最后，通过宏观力学性能测试，揭示TRIP增塑与细晶增强的多相多机制协同作用机理，建立加工工艺—微观组织—力学性能的关系，并利用此关系获得成分、晶粒尺寸、增强增塑相体积分数等增强增塑窗口，指导工艺与组织调控，实现超高钢级（X120）大应变管线钢强塑性同时提高的目标。

在传统管线钢基础上，通过化学成分优化设计，利用一次回火热处理工艺，将TRIP效应引入管线钢中，实现塑性的提高。基于此，进一步降低碳元素含量，适当增加铜和钼等微合金元素，通过双相区塑性变形，获得细化奥氏体晶粒，进而获得细小的铁素体和贝氏体组织，结合临界退火+二次回火热处理工艺，溶质发生二次配分现象，得到细小弥散的室温残余奥氏体组织，实现TRIP增塑。组织细化和TRIP效应的协同作用，导致材料的强度和塑性的同时提高。与传统X70高应变管线钢相比，塑性基本相同的条件下，屈服强度和抗拉强度得到了显著提高。该设计思路和研究方法可以用于指导工艺与组织调控，实现超高钢级大应变管线钢强塑性同时提高的目标。