The Effect of the Hyperstoichiometric Ti/N Ratio Due to Excessive Ti on the Toughness of N-Controlled Novel Fire- and Seismic-Resistant Steels

Abstract

The toughness of the base material (BM) and the coarse-grain heat-affected zone (CGHAZ) of N-controlled fire- and seismic-resistant (FSR) steels (FSR1: 0.03Ti, 0.25Mo, 0.018Nb; FSR2: 0.07Ti, 0.09Mo; and FSR3: 0.14Ti, 0.17Mo) were systematically investigated. FSR1 steel exhibited better toughness than FSR2 and FSR3 steels in the BM and CGHAZ. Fractographic analysis of the BM and CGHAZ revealed the presence of coarse TiN precipitates in fracture initiation sites, which are responsible for the low toughness of FSR2 and FSR3 steels. Coarse TiN precipitates with equivalent diameters of 1 to 6 μm are present in all FSR steels. However, FSR1 steel has relatively lower number density and area fraction of TiN precipitates. The formation mechanism of TiN precipitates is elucidated from the theoretical thermodynamic approach and validated with a quantitative metallographic observation; it was found that the mechanism is the same for both the BM and CGHAZ in all FSR steels. The equilibrium solubility temperature of TiN precipitates in FSR1 steel was lower than the solidus temperature, which resulted in fewer coarse TiN precipitates. The lower boundary of both the Ti/N (hypostoichiometric) ratio and TiN solubility product at the solidus temperature is highly recommended for increased toughness of FSR steels containing Ti, Mo, and Nb.