中锰汽车钢焊接热影响区组织与韧性

热模拟第三代中锰汽车钢热影响粗晶区的焊接热循环，并利用扫描电镜、透射电镜、显微硬度、冲击和电子背散射衍射（Electron back scatter diffraction，EBSD）试验方法研究了焊接热循环对粗晶区显微组织、硬度和冲击韧性的影响。结果表明：中锰钢热影响粗晶区经过焊接热循环后主要为马氏体组织，当冷却速度较快时，马氏体板条间分布着大量的位错团；随着冷速的减缓，组织中有少量的贝氏体生成，且组织逐渐粗化；粗晶区的硬度随着焊后冷速的减缓呈降低的趋势，在t_8/3>33s后硬度下降缓慢；粗晶区组织中有少量的M-A组元，对冲击韧性影响不大；中锰钢粗晶区中的大角度晶界（>15°）即原奥氏体晶界、马氏体板条束界和板条块界均对裂纹有阻碍作用；大角度晶界密度可作为衡量中锰钢粗晶区的韧性指标，其与冲击韧性成正比，均随着冷速的减缓先增加再逐渐降低，在冷却速度t_8/3为8s时，大角度晶界密度最大，获得的冲击韧性相对较好。

Microstructure and Toughness of Welding Heat Affected Zone of Medium Manganese Automobile Steel

The coarse-grained heat-affected zone (CGHAZ) of the third generation medium manganese automobile steel was simulated by thermal simulation technique, and its microstructure, hardness and impact toughness were investigated using scanning electron microscope, transmission electron microscope, electron back scatter diffraction and Charpy impact test. The results indicate that the microstructure of the CGHAZ is mainly composed of martensite, and a large number of dislocation clusters are distributed in the martensite laths when the cooling rate is fast. As the cooling rate decreases, a small amount of bainite forms and the microstructure is gradually coarsened. The hardness of the CGHAZ decreases with an increase in t_8/3, and it decreases slowly as t_8/3 is over 33s. There is a small amount of M-A constituent in the CGHAZ, which has little effect on the impact toughness. The high-angle grain boundary (>15°) in the CGHAZ of medium manganese steel, namely the prior austenite crystal boundary, packet boundary and block boundary, may hinder the crack. The high-angle grain boundary density can be used as the indicator of the toughness of the CGHAZ, and it is proportional to the impact toughness, which increases first and then decreases gradually as the cooling rate decreases. At t_8/3 of 8s, the high-angle grain boundary density reaches the top value, resulting in sound impact toughness.