Effects of boron addition on tensile and Charpy impact properties in high-phosphorous steels

In order to provide a new possibility for improving the steel-making productivity by fabricating plain carbon steels containing high phosphorous (P), effects of microstructures on tensile and Charpy impact properties were investigated in this study. Nine plain carbon steels were fabricated by controlling the addition of P and boron (B), and isothermal or quench heat-treatments were conducted on these steels to make ferrite–bainite-based or martensite-based microstructures. The addition of B positively influenced the grain refinement and the formation of bainites, thereby leading to the increase in strength. The upper shelf energy (USE) decreased with increasing P content, while the energy transition temperature (ETT) increased, in all the steels. The B addition beneficially affected both the USE and ETT as the dimpled ductile fracture mode prevailed in the B-added steels. This was because B preferentially covered grain boundaries, which reduced the grain boundary segregation of P. Thus, it effectively suppressed the intergranular fracture due to the segregation of P. According to the fractographic results, the increased tendency of intergranular fracture mode was observable in the 20-ppm-B-added steels rather than in the 10-ppm-B-added steels. When an excess amount of B, e.g., 20 ppm of B, was added, the severe segregation of B on grain boundaries occurred, and led to the precipitation of boro-carbides, which could act as intergranular crack initiation sites.