预形变-预相变对纳米贝氏体组织形成及热稳定性的影响机制

Both good strength and ductility nano-bainite steel not to be scale produced and application, the important reasons are its transformation time too long and microstructure metastable. Among factors affecting the efficiency of nano bainite microstructures change there must be interconnected, thorough understanding the coupling effects of these factors of its nucleation and growth mechanism is a prerequisite for enhancing the conversion efficiency. This application plans for parsing the TTT curve of typical composition of nano bainite steel, respectively, were obtained from the nucleation kinetics and growth kinetics curve. On this basis, finding the effect of high-temperature pre deformation and martensite pre-transformation multi factors coupling effect on nano bainite transformation, put forward a kind of pre deformation and pre phase change composite nano bainite microstructures to accelerate the transformation of new technologies. At the same time, using SEM, EBSD, TEM, 3DAP and ECCI fine observation means to locate tracking to observe the formation of nano bainite microstructures and during reheating the process of evolution, reveal of various factors on the formation of nano bainite and thermal stability of the impact mechanism, the establishment of nucleation, growth and stability of the thermodynamic model of nano bainite. The results of this study not only to enrich the existing bainite transformation theory, also will of nano bainite steel for the industrialization of preparation and reasonable application of has important guiding significance.

相变时间过长及组织亚稳是限制兼具良好强塑性匹配的纳米贝氏体钢规模生产和应用的重要原因之一。诸多影响纳米贝氏体组织转变效率的因素之间必然存在相互关联，透彻了解这些因素的耦合效应对纳米贝氏体形核和长大的影响机制是有效提升其转变效率的前提。本项目拟对典型纳米贝氏体钢的TTT曲线进行解析，分别获得其形核动力学和长大动力学曲线。找到奥氏体预形变和马氏体预相变等多因素对纳米贝氏体相变的影响规律，提出一种预形变-预相变复合加速贝氏体转变的新技术。同时，利用SEM、EBSD、TEM、3DAP、ECCI 等精细观测手段分别定位跟踪观察纳米贝氏体组织的形成及其在受热过程中的演变过程，揭示各种因素对纳米贝氏体形成和热稳定的影响机制，建立纳米贝氏体形核、长大及稳定性热力学模型。研究成果不仅有利于丰富现有贝氏体相变理论，同时将对纳米贝氏体钢的工业化制备和合理化应用具有重要指导意义。

纳米贝氏体钢因兼具超高的强度和良好的塑韧性，而成为近年来先进高强度钢的研究热点之一。然而，从热力学角度考虑，细晶的组织实际处于亚稳态，在受热过程中有自发粗化而降低界面能的倾向，组织一旦粗化，则优良的性能将不复存在。本项目从Nb微合金化设计、预相变铁素体和预相变马氏体三方面着手，系统的研究纳米贝氏体的相变动力学和回火过程中组织性能演变规律。主要研究内容包括：（1）中碳纳米贝氏体钢合金成分设计及组织热稳定性的研究；2) 直接等温相变工艺的研究；(3) 预相变铁素体加速贝氏体相变工艺及其对纳米贝氏体显微组织热稳定性的影响；(4) 预相变马氏体加速贝氏体相变工艺及其对纳米贝氏体钢显微组织热稳定的影响。.0.02wt.%Nb元素的添加可明显缩短纳米贝氏体相变孕育期，340°C等温淬火时贝氏体相变完成时间由784.8s缩短到687.0s，在细化贝氏体铁素体板条的同时提高了残余奥氏体的含碳量，从从而提高纳米贝氏体钢的强度和韧性。回火过程中Nb原子与C原子形成稳定性良好的MC型碳化物，能够抑制贝氏体铁素体的回复和铁素体晶粒的长大。预相变铁素体的形成引入了α/γ界面，为为后续贝氏体相变提供更多的形核位置，从而细化贝氏体铁素体板条，加速了贝氏体相变。此外，预相变铁素体的形成使得未转变奥氏体富碳，降低了纳米贝氏体钢中薄膜状残余奥氏体的含含碳量，从而降低了回火过程中碳化物的析出动力，延缓了残余奥氏体的分解。而预相变马氏体的引入通过改变相邻奥氏体基体的应变场，增加贝氏体形核位置，导致相邻贝氏体铁素体板条厚度小于100nm。回火过程中预相变马氏体晶粒内碳化物分解，使得相邻薄膜状残余奥氏体在600°C回火时仍维持原有的形貌，提高了纳米贝氏体组织的热稳定性。该工作的成果对中碳纳米贝氏体钢的工业生产和实际应用提供了理论依据和技术指导。