Favorable Impact Toughness of High Heat Input Coarse-Grained HAZ in an Ultra-Low Carbon High-Strength Microalloyed Steel

An ultra-low carbon (0.035 wt%) high-strength steel plate was produced. The coarse-grained heat-affected Zone (CGHAZ) was simulated with different heat inputs of 25-200 kJ/cm. The low-temperature impact toughness and microstructure of CGHAZ were analyzed. The results indicated that the impact toughness of CGHAZ exhibited a favorable value at -20 degrees C under high heat input conditions. The Ti-containing complex inclusions stimulated the crack initiation and induced the nucleation of acicular microstructure. With the increase of heat inputs, the morphology of carbides or martensite-austenite (M-A) constituents changed from strip to block gradually, and the volume fraction decreased gradually, which increased the crack initiation absorbed energy. The coarse grain boundary ferrite (GBF) occupied by massive low-angle grain boundaries was not conducive to the crack propagation absorbed energy. In addition, the kinetic effective activation energy of GBF and acicular ferrite/granular bainite (AF/GB) were calculated to be similar to 175 kJ/mol and similar to 227 kJ/mol respectively, which revealed the kinetic reason for the preferential precipitation of GBF.