淬火配分钢中碳扩散动力学及其对奥氏体稳定性的影响

Quenching and partitioning (Q&P) steel developed in recent years is becoming a member of the new generation of high strength steels in automobile industry. The excellent mechanical property of Q&P steel is attribute to the appropriate match between martensite and retained austenite in its microstructure. The diffusion behaviour of carbon during Q&P process directly determines the volume fraction and stability of retained austenite in the microstructure, and thus affects the final mechanical property of Q&P steel. In the typical Q&P steel, the various diffusion behaviours of carbon usually overlap during the Q&P process, which makes it difficult to clearly research the kinetics of carbon diffusion. In the present project, in order to separate the carbon diffusion behaviour during the temperature increasing process after quenching, the Q&P steels with their quenching temperatures (between Ms and Mf) nearby the room temperature will be designed by adjusting chemical composition. The carbon diffusion kinetics during the temperature increasing process after quenching will be studied by means of internal friction method which is very sensitive to the diffusion of carbon atoms. Thus, the carbon diffusion behaviours will be verified in the different temperature range, which is beneficial to determine the partitioning technics in the following process. The carbon diffusion behaviour during partitioning process will be researched using in-situ X- ray diffraction method. The effect of carbon diffusion on the volume fraction and stability of retained austenite will be further studied by means of scanning electron microscopy, high resolution transmission electron microscopy and in-situ synchrotron radiation tensile experiments. The research on the carbon diffusion kinetics not only is able to provide the reference for optimizing heat treatment process of Q&P steel, but also has important meanings on the designing and controlling the microstructure and property of Q&P steel.

淬火配分（Q&P）钢是近年来发展起来的新一代汽车用高强度钢，其优异的性能来源于马氏体和残余奥氏体两相组织的合理配比，而碳在Q&P处理过程中的扩散行为直接影响残余奥氏体的含量及稳定性，进而影响Q&P钢的最终性能。典型Q&P钢中碳的不同扩散行为往往叠加在一起，使其动力学研究变得很困难。本项目拟设计不同成分的Q&P钢，其淬火温度(Ms-Mf)位于室温附近，使碳在淬火后升温过程中的不同扩散行为分离。采用内耗等对碳的扩散非常敏感的手段研究碳在淬火后升温过程中的扩散动力学，确定其在不同温度范围内的扩散行为，为配分工艺的制定提供指导；采用原位X射线衍射测量在配分过程中碳的扩散行为，并结合扫描电镜，高分辨率透射电镜，原位拉伸同步辐射测量等手段研究碳扩散对残余奥氏体含量及其稳定性的影响。碳的扩散动力学研究不仅能为Q&P钢的最佳热处理工艺制定提供理论依据，而且对Q&P钢的组织和性能的设计及控制具有重要意义。

淬火配分（Q&P）钢及δ-TRIP钢是近年来发展起来的新一代汽车用高强度钢，其优异的性能来源于马氏体/铁素体和残余奥氏体两相组织的合理配比，而碳在热处理过程中的扩散行为直接影响残余奥氏体的含量及稳定性，进而影响钢的最终性能。典型Q&P钢及TRIP中碳的不同扩散行为往往叠加在一起，使其动力学研究变得很困难。本项目拟设计不同成分的Q&P及TRIP钢，设计不同的热处理工艺，以及采用不同温度下变形等手段，结合不同的表征手段研究碳在淬火后升温过程中的不同扩散行为以及对残余奥氏体含量和稳定性的影响。结果表明：在含15Mn的奥氏体单相TWIP钢中，碳主要固溶于奥氏体基体，淬火态试样在升温过程中，300°C附近的内耗峰表明该温度下碳原子可在基体中扩散并于位错交互作用，形成碳化物；在该钢中加入稀土后快冷，并没有观察到稀土碳化物的析出，稀土元素和碳大部分固溶于奥氏体，细化奥氏体晶粒，促进了细小二次孪晶的形成及孪晶数量的增加，使材料的强度和延伸率均有提高；碳在10Mn马氏体/奥氏体双相组织中的扩散研究表明：淬火后经200°C左右的配分和回火处理后，综合力学性能最佳，归因于在此温度下配分回火，少量过渡性型碳化物析出，强化了基体，使材料的屈服强度及抗拉强度均保持在较高水平，同时，由于在配分过程中，部分碳扩散到了附近的残余奥氏体中，使奥氏体稳定下来，材料的延伸率也保持在较高水平；铁素体/奥氏体两相δ-TRIP钢研究表明：热轧组织沿轧向呈带状分布，第二相带由母相奥氏体在不同热轧卷取温度形成不同的相变产物。冷轧δ-TRIP钢退火板中残余奥氏体在不同拉伸温度下的相变行为表明：在150°C-450°C温度区间拉伸时，首次观察到了应变诱发贝氏体相变的发生，从而导致在300°C具有最佳的强塑性。δ-trip钢的变形机制随着变形温度的增加依次为：SIMT和TWIP→SIBT→GB。.碳的扩散动力学研究不仅能为Q&P钢及δ-trip钢的最佳热处理工艺制定提供理论依据，而且对该类钢种的组织和性能的设计及控制具有重要意义。