Effects of microstructure on crack resistance and low-temperature toughness of ultra-low carbon high strength steel

The effects of microstructure on crack resistance and toughening mechanism of an ultra-low carbon steel were investigated. The microstructures were controlled via thermal-mechanical control processing (TMCP) and heat-treatments. Distribution of stress concentration, microcracks formation and propagation during Charpy impacting were investigated in detail. The results indicate that the lath martensitic structure provided a higher yield stress together with a better impact property, compared to the polygonal ferritic structure. The high strength can be attributed to the high density of dislocations in the lath martensitic structure introduced by quenching. The instrumented Charpy impact results indicated that the crack initiation energy in the lath martensitic structure was similar to that in the ferritic structure while the crack propagation energy was significantly greater than that in the ferritic structure, leading to the high toughness of the steel with the lath martensitic structure. Local stress concentration distributed uniformly in lath martensitic structure, leading to the homogeneous nucleation of microcrack. The high crack propagation energy in the lath martensitic structure can be attributed to the high fraction of high angle grain boundaries and fine effective grains, which deflected the cleavage crack propagation direction.