Effect of Ca addition on the brittle-to-ductile transition in low-carbon fully martensitic steels

抄録

This study investigates the effect of Ca addition on the brittle-to-ductile transition (BDT) and grain boundary decohesion by S segregation in as-quenched low-carbon fully martensitic steel. Temperature dependence of the impact absorbed energy was examined in two kinds of steels with different Ca content (Ca-added steel and Ca-free steel). The BDT temperature of the fully martensitic steel was significantly decreased with the Ca addition. The temperature dependence of the 0.2% proof stress was measured to discuss the decrease in the BDT temperature based on shielding theory. The temperature dependence of 0.2% proof stress was compatible between the two steels, indicating that Ca addition did not affect the dislocation mobility regardless of the Ca content. Observations of brittle fracture surface revealed that intergranular fracture was prominent in the Ca-free steel, whereas it was suppressed in the Ca-added steel. Auger electron spectroscopy further revealed that S was segregated at prior austenite grain boundaries in the Ca-free steel. These results suggest that the improvement in low-temperature toughness in the Ca-added steel is attributed to the increase in surface energy for intergranular fracture, resulting from the suppression of S segregation by Ca addition.