2021 年 107 巻 8 号 p. 661-671
Grain boundary segregation of Sn in 16.5 mass%Cr-0.15 mass%Ti steel and 16.5 mass%Cr-0.40 mass%Nb steel was investigated along with its toughness effects. Comparing ductile-brittle transition temperature (DBTT) after aging at 973 K for 3.6 ks, the increase in DBTT per 0.1 mass% Sn addition was twice as large for 0.15 mass%Ti steel as for 0.4 mass%Nb steel, and their brittle fracture surfaces were mainly intergranular in 0.15 mass%Ti steel and intragranular in 0.4 mass%Nb steel, respectively. The grain boundary segregation of Sn was also recognized in the 0.4 mass%Nb steel, suggesting that the intergranular embrittlement was suppressed by the intergranular reinforcement of Nb. In 16.5 mass%Cr-0.15 mass%Ti-0.3 mass%Sn-P steels after various aging time and temperature, it was found that the amount of the grain boundary segregation of Sn changed by the precipitation amount of the phosphorous compound and P content, indicating that there is a site competition effect (SC effect) in the grain boundary segregation of Sn and P. The grain boundary segregation energy of Sn estimated from this study was ‐46 kJ/mol, which was similar to the reported value of the carbon steel. As P content increased from 0 to 0.03 mass%, the grain boundary segregation amount of Sn after aging at 773 K for 604.8 ks was decreased from 4.3 at% to the bulk concentration, which is lower than the segregation amount of Sn estimated from the grain boundary segregation energy of P in the carbon steel, suggesting that due to the effect of the Cr content, the grain boundary segregation energy of P decreased by about 1.4-fold compared to the carbon steel.
