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摘要
Mg-bearing composite inclusions were introduced into Q345GJ steel by Mg-treated,and the effects of Mg-bearing composite inclusions on the microstructure and fracture toughness of coarse-grained heat affected zone (CGHAZ) with high heat input welding (energy input 100 kJ/cm) were studied. The results show that the CGHAZ toughness of test steel after Mg-treated is significantly improved (impact energy increases from 56 J to 108 J). Because of in the process of weld thermal cycle cooling,Mg-bearing composite inclusions are preferentially formed in the CGHAZ,the presence of Mn depletion zone and high-energy strain field around the mg-bearing inclusions with high surface energy promotes the nucleation of acicular ferrite (AF),with the AF ratio of (82.9±2.0)%(volume percent) and the width of (0.96±0.1) μm((32.4±1.5)% and (3.13±0.2) μm for the Mg-free steel). The formation of micropores and crack propagation in CGHAZ impact specimens after high heat input welding were investigated. There are a large number of dislocations near the inclusions in test steel. During the deformation process,stress concentration is caused by the accumulation of dislocation loops. When the dislocation source of stress concentration to adjacent grains cannot is activated. Micropores are formed on the orientation which is not conducive to slip and then connected to form cracks under external force. Micropores of Mg-free steel are denser,which provides favorable conditions for crack initiation. The proportion of high angle grain boundaries(HAGBS) in Mg-treated steel is 80.2%,the geometric necessary dislocation density (GND) is 0.806,and Kernel Average Misorientation (KAM) is 0.912. (with 71.9%,0.896 and 0.956 for the Mg-free steel). The small and self-locking AF in CGHAZ improves the impact toughness of Mg-treated steel due to hinders the expansion of micropores and inhibits the propagation of cracks. The crack propagates along the HAGBS,the crack propagation path is prolonged and the energy consumption increases when the crack encounters AF. ? 2023 Chinese Society for Metals.
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