2024 年 64 巻 5 号 p. 847-858
The effect of molybdenum (Mo) contents on hardenability and precipitation behaviors in Mo–B simultaneously added steels were investigated placing a focus on high austenitizing temperature. The hardenability of 0.5% Mo - 11 ppm B steel austenitized at 1150°C was decreased compared with that austenitized at 950°C, whereas 1.0% Mo - 10 ppm B and 1.5% Mo - 9 ppm B steels were less affected by high austenitizing temperature than 0.5% Mo - 11 ppm B steel. The Fe23(C, B)6 precipitation by increasing austenitizing temperature was also revealed to be suppressed in 1.5% Mo – 9 ppm B steel. These results indicate that the improved effect of the Mo addition on hardenability by retarding the precipitation of Fe23(C, B)6 still appear in B-added steels austenitized at high temperature. Furthermore, Fe23(C, B)6 precipitation start temperature was increased in Mo–B added steels austenitized at 1150°C. This result implies that non-equilibrium B segregation mechanism during cooling from high austenitizing temperature enhances the amount of segregated B on grain boundaries leading to the promotion of the borides precipitation at high austenitizing temperature region. However, Mo is presumed to fix a part of thermal vacancies as Mo–V complex resulting in the suppression of non-equilibrium B segregation to grain boundaries during cooling, which is speculated to inhibit the Fe23(C, B)6 precipitation. Thus, the effect of Mo–B combined addition on hardenability was presumably maintained even in high austenitizing temperature region.
