Investigation on new type of fracture in Cr-Mo-V steel slab

Abstract

Abstract The causes of through-thickness cracking in a continuous casting steel slab with low carbon and high alloys were investigated by observation of the fractured surface and microstructures of a fractured slab, and analysis of phase transformation through a continuous cooling transformation (CCT) diagram. Intergranular brittle fracture and ductile fracture were observed on the surface of the slab and inside the slab, respectively. A bainite and martensite microstructure was transformed in the vicinity of the slab surface where intergranular fracture occurred due to excessive water-cooling during continuous casting, and ferrite and pearlite were transformed inside the slab due to a slow cooling rate and relatively high temperature. Immediately after continuous casting, the temperature and the cooling rate for the surface of the slab were 300–450 °C and about 0.1 °C/s, respectively, and thus bainite and martensite on the surface of the slab were tempered simultaneously during transfer to the slab yard. As a result, the tempered bainite and martensite on the surface of the slab and the mixed microstructure of ferrite and pearlite inside the slab were transformed. However, segregation of P and S at grain boundaries was observed in the tempered bainite and martensite at the surface of the slab. The segregation of P and S at grain boundaries could occur by tempering at the temperature between 300–450 °C and the cooling rate as slow as 0.1 °C/s at the surface of the slab, and is a critical cause on ‘temper embrittlement’. Therefore, it is believed that the intergranular fracture at the surface of the slab was due to the temper embrittlement, and ductile fracture occurred inside the slab having a mixed microstructure of ferrite and pearlite, which is ductile.