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 t8/3, and it decreases slowly as t8/3 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 t8/3 of 8s, the high-angle grain boundary density reaches the top value, resulting in sound impact toughness. © 2022 Editorial Office of Chinese Journal of Mechanical Engineering. All rights reserved.