汽车用第三代中锰钢电阻点焊组织调控机理及其熔核形核理论研究

Medium manganese steel is the third generation of automobile steel which is developed to meet the need of light weight and safety of automobile. Study on its welding performance is the key to solve its application. The mechanism of the effect of complex alloy elements on the microstructure and mechanical properties of the joints of spot welding under the heat, electricity and force coupling is still unclear. Three pulse welding current and interpulse cooling method were used to ensure the microstructure and properties of resistance spot welding of 1200MPa medium manganese steel in early research. On the basis of previous studies, the theory of nugget crystallization and element partition under precise control of multi-gradient temperature is proposed, which can improve the microstructure of spot welding joints of medium manganese steel and improve the comprehensive mechanical properties. The thermodynamics and kinetics of phase transition of medium manganese steel of multicomponent alloy were studied by monitoring the heat affected zone, the surface temperature of the nugget and the temperature point of phase transformation. The mechanism of microstructure evolution at multi-gradient temperature was clarified, and the correlation model between multi-gradient temperature and its microstructure, characteristic parameters and fracture mechanism was established. The research results of this project will have important theoretical significance and industrial application value for the development and application of the third generation medium manganese steel for automobile.

中锰钢是适应汽车轻量化与安全性需要发展起来的第三代汽车用钢，研究它的焊接性能是解决其应用的关键。中锰钢中复杂合金元素在点焊热、电、力等多场耦合作用下对接头微观组织和力学性能的影响机理目前仍不明确。前期研究采用三脉冲焊接电流，并进行脉冲间冷却方法，保证了1200MPa级中锰钢电阻点焊的组织性能。本研究在前期研究基础上，提出多梯度温度精确调控点焊熔核结晶过程和元素配分理论，达到改善中锰钢点焊接头微观组织和提高综合机械性能的目的。实时监测热影响区、熔核表面温度，建立相变温度点，研究中锰钢多元合金相变热力学与相变动力学。阐明多梯度温度下组织演变机制，揭示高过冷度下各元素配分行为，建立多梯度温度与焊点组织、特征参数、断裂机制的关联模型。本项目研究成果将对第三代汽车用中锰钢的开发应用具有重要的理论意义和工业应用价值。

随着节能减排在全球的大力推广，汽车轻量化已经成为汽车工业发展的重要趋势之一。先进高强钢以其强度高、成型性能好、能量吸收率高以及防撞凹性能好等优势成为汽车轻量化的主要材料。中锰TRIP钢作为第三代汽车高强钢，基体组织由铁素体和奥氏体构成，兼具高强度和高塑性，研究表明其强塑积可达30~60GPa%，具有极高的应用前景，而电阻点焊因其生产效率高，成本低，焊接质量稳定和易实现自动化等优点成为汽车车身连接技术的重要手段之一。目前国内外对中锰TRIP钢的研究较少，故中锰钢一直未广泛应用于汽车车身制造，因而研究中锰钢TRIP钢电阻点焊工艺、组织性能和焊后组织改性对其应用具有重要的参考和指导意义以及工程应用价值。.项目以0.13C-7Mn中锰钢为研究对象，开展了单、双脉冲点焊试验，焊后原位回火和热处理炉内回火工艺研究，结果表明焊接电流对点焊接头性能影响最大，而焊后炉内回火工艺可将力学性能提升100%以上。此外还对0.13C-7Mn中锰钢和DP5590开展了异种材料间点焊实验，系统研究了焊接参数和回火电流对0.13C-7Mn 中锰钢/DP590点焊接头质量特征参数和力学性能的影响。结果表明增加回火电流可使点焊接头拉剪力提升17.13%。开展了基于典型焊接参数下的0.13C-7Mn 钢/DP590异种材料的电阻点焊点焊接头数值模拟，发现随着焊接的进行，0.13C-7Mn钢热影响区出现应力集中，且电流密度在电极和焊板接触面处集中并且模拟和实验测得的熔核直径误差在10%以内，验证了该模型的精确性和可靠性。基于python对电阻点焊飞溅图像进行图像处理，提取飞溅特征，建立数据库，创建了一套先进高强度钢的电阻点焊飞溅监控系统。并且建立的数据库和监控系统对参数进行反馈调节的流程是对任何材料都通用，甚至也适用于点焊以外的其他焊接工艺。