低碳纳米无碳化物贝氏体钢强韧化的残余奥氏体多尺寸效应研究

Low-carbon nanostructured carbide-free bainitic steels show a combination of high strength and high ductility, thus satisfying the demands for weight reduction in the automotive and railway industries. However, the influence of complex evolution behavior of the multi-size retained austenite in microstructure on mechanism of strengthening and toughening is remains unclear during deformation. This project aims to research the laws of deformation, phase transformation, interface composition, dislocation configuration and strain field distribution by tensile and bending deformation tests. And then, base on the phase transformation theory, dislocation theory, interface theory and fracture mechanics theory, the law of non-uniform phase transformation of multi-size retained austenite is analyzed, the correlation between multi-size retained austenite and strain field is established, the coordinated deformation behavior between multi-size retained austenite and other constituent phases is revealed, and the influence mechanism of multi-size retained austenite on crack initiation, micro-crack aggregation, and crack propagation path and rate is clarified. Finally, the microcosmic mechanism of coupling relationship between the non-uniform evolution behavior of multi-size retained austenite and the deformation at different stages on strengthening and toughening of low-carbon nanostructured carbide-free bainitic steels is confirmed. This project will provide an important theoretical and experimental evidence for the development of carbide-free bainitic steels with better mechanical properties by adjusting the microstructure.

低碳纳米无碳化物贝氏体钢兼具高的强度和良好的韧性，因而成为了汽车和铁路等工业轻质结构用材的重要选择。但其微观组织中多尺寸残余奥氏体在变形过程中复杂的演化行为对强韧化的影响机理尚不明确。本项目拟以不同加载条件下的拉伸和弯曲变形试验为前提，对多尺寸残余奥氏体在塑性变形、裂纹萌生和裂纹扩展过程中的变形、相变、界面构成、位错组态及应变场分布规律进行研究；在此基础上，通过相变理论、位错理论、界面理论和断裂力学理论，对多尺寸残余奥氏体不均匀相变的规律进行分析，构建其与应变场之间的关联性，揭示其与组成相的协同变形行为，阐明其对裂纹萌生点、微裂纹聚集形式及裂纹扩展路径和速率的影响机理；最终，从多尺寸残余奥氏体不均匀演化行为与各变形阶段之间的耦合关系确立低碳纳米无碳化物贝氏体钢强韧化的微观机理。本项目的实施，将为通过调整微观组织结构来研发机械性能更加优异的无碳化物贝氏体钢提供重要的理论和实验依据。

无碳化物贝氏体钢优良的强度和韧性匹配与其微观组织中多尺寸残余奥氏体密切相关，但是这些残余奥氏体在变形过程中的演化行为对其强韧化的影响机理问题仍未得到明确解答。本项目通过合金设计和等温淬火工艺制备得到了含有细小贝氏体铁素体板条和不同尺寸残余奥氏体的无碳化物贝氏体钢。通过微观组织分析、拉伸和冲击试验，分别对实验钢的微观组织特征和力学性能，及拉伸和冲击断裂试样中微裂纹、微孔洞和裂纹路径附近微观组织演化行为进行了研究。结果表明，随温度增加，残余奥氏体的尺寸和含量均增加；当铝元素添加之后，随温度升高，残余奥氏体的含量和尺寸均有所降低，但残余奥氏体多尺寸分布形成的梯度结构更加合理。添加合理铝元素和适当提高等温温度可以提升无碳化物贝氏体钢的强度和韧性配合度。大尺寸块状残余奥氏体在变形过程中因其容易转变成脆性马氏体而促进孔洞和裂纹萌生，以及加速裂纹扩展。通过三点弯曲和拉伸变形试验、并结合DIC和微观组织表征技术，研究了实验钢不同类型试样在不同加载速率下的变形行为，以及不同变形状态下试样表面的应变场和残余奥氏体演化行为。结果表明，应变场在试样表面具有不均匀性，且在裂纹尖端形成局部高应变集中区。随加载速率增加，U型缺口和预制裂纹两种试样的断裂吸收功均增加，具有相同的加载速率敏感性；随加载速率增加，光滑试样的拉伸断裂功逐渐降低，而预制裂纹试样的拉伸断裂收功则逐渐增加，表现出相反的加载速率敏感性。这主要与残余奥氏体在不同加载速率下的相变量、相变程度和相变位置密切相关。本项目从微观上揭示了多尺寸残余奥氏体对无碳化物贝氏体钢强韧化的影响机理，为进一步研制性能更加优异的无碳化物贝氏体钢种提供了理论与实验依据。

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