TWIP/TRIP钢预应变孪生/ε马氏体相变异步协作变形机制研究

TWIP steels with high strength-elongation product and high energy absorption capability offer the promising perspective for automotive industry and have been applied as structure parts, particularly in crash relevant parts. However, relatively low yield strength limits the further application. In order to circumvent this intractable issue, a research program is proposed based on the unique characteristic of deformation mechanisms depending on temperature in TWIP steels. The strategy is to introduce the deformation twin by controlling the pre-strain temperature. Upon that, asynchronous synergy of two deformation mechanisms, namely twinning and ε martensite transformation, will be developed in the material, leading to novel bimodal-microstructured TWIP/TRIP steels with high performance. By systematic research on the influence of pre-strain technology on microstructure, the formation process and the corresponding evolution mechanism of the ε martensite during plastic deformation at room temperature will be investigated, and the interactions of ε martensite with pre-strain deformation microstructures will be studied. The correlations between the flow stress of twinning/ε martensite transformation asynchronous deformation and the microstructures such as deformation twins, ε martensite and dislocations will be established quantitatively or semi-quantitatively. The mechanism of twinning/ε martensite transformation asynchronous deformation and the effective approach of strength-ductility will be revealed. Efforts of this study will provide a complement for not only the theory of plastic deformation but also the theory of strengthening and toughening, which serve as the theoretical guides for control of microstructure and performance for advanced TWIP/TRIP steels.

兼具高强塑积和高能量吸收能力的TWIP钢是最具应用潜力的新型汽车用钢，目前已获得较多的应用，如汽车防撞梁。然而到目前为止开发的TWIP钢通常表现出塑性有余而屈服强度不足的缺点。为弥补这一缺陷，本项目利用TWIP钢的变形机制受变形温度敏感影响的特点，提出通过调控预应变温度引入形变诱发孪生机制，使其与室温形变诱发ε马氏体相变异步协作的设计思路，开发以预应变控制为基础具有混合组织的新型高性能TWIP/TRIP钢。通过系统研究预应变工艺对微观变形组织的影响规律，考察预应变后室温成形过程中ε马氏体的形成演变规律以及其与预应变组织的交互作用，定量或半定量描述孪生/ε马氏体相变异步协作变形方式下流变应力与形变孪晶、ε马氏体及位错的关系，进而揭示孪生/ε马氏体相变异步协作变形机制及强韧化效果。本项目研究将丰富和发展金属材料塑性变形理论和强韧化理论，为新型高性能TWIP/TRIP钢的组织性能控制提供理论基。

TWIP/TRIP钢具有优异的强塑积及高加工硬化率是理想的汽车用钢材料。但较低的屈服强度阻碍了TWIP/TRIP钢的进一步应用。为了弥补这一缺点，本项目通过调控预应变温度引入形变诱发孪生，使其与室温形变诱发ε-马氏体相变异步协作，重点研究了预应变参数对TWIP/TRIP钢微观变形组织和力学性能影响规律，阐明了孪生/ε马氏体相变异步协作变形机制以及微观变形组织之间的交互作用机制，半定量描述流变应力与孪晶、ε马氏体和位错的关系。主要研究内容和结果如下：.（1）明确了预应变轧制温度对组织性能的影响。随着预应变轧制温度的升高，由于层错能升高微观变形组织发生了由ε-马氏体到孪晶，最后位错滑移的转变，同时伴随着位错密度的下降。特别是673 K预应变轧制后，在拉伸变形过程中观测都了大量ε-马氏体形成，实现TWIP和TRIP效应的异步协作。.（2）明确了预应变轧制压下量对组织性能的影响。随着预应变轧制压下量的增加，形变孪晶密度逐渐增加，当压下量为55%时微观组织由位错、形变孪晶及剪切带组成。在拉伸变形过程中产生ε-马氏体的含量随着压下量的增加而下降，最终消失。此外ε-马氏体总是沿着形变孪晶界出现，且与形变孪晶相互平行。特别是压下量为35%时在拉伸过程中观测到ε-马氏体与形变孪晶同时生长，提高了加工硬化率。.（3）明确了异步协作变形方式下流变应力与形变孪晶、ε马氏体及位错的关系。形变孪晶与ε-马氏体可减小有效晶粒尺寸，实现细晶强化，然而对流变应力贡献最大的仍是位错强化。但位错密度的增加会增加有效层错能，抑制拉伸过程中的TRIP效应。因此通过合理设计预应变轧制工艺调控形变孪晶、ε马氏体及位错的比例，可实现在保证TWIP/TRIP钢塑性的前提下改善屈服强度。