2022 年 108 巻 12 号 p. 911-925
The 0.2C-2Si (mass%) steels with the addition of 0-4 mass% Cr were prepared by hot rolling followed by subsequent annealing for normalization. The steels were subjected to friction stir welding (FSW) conducted above A3 temperature. For all the steels, sound FSWed joints were obtained. Microstructures and tensile properties using small tensile specimens were investigated for both base materials and stir zones. The base materials showed a relatively good balance of strength and ductility when the Cr content is over 3 mass% presumably owing to the relatively fine microstructures of ferrite and martensite. The tensile properties of stir zones were substantially enhanced by FSW, and the stir zone of the 0.2C-2Si-4Cr joint with fully martensitic structure exhibited the surprisingly high tensile strength of 1720 MPa compared with that of the conventional martensitic steel of 0.2 mass%C together with the excellent balance of ductility. This is assumed to be caused by the refinement of block size in the fresh lath martensite and/or the formation of ausformed martensite induced by the dynamically recrystallized fine austenite grains by FSW of the Cr added steels.
