PURPOSE: To remarkably improve the fracture life in low cycle fatigue by increasing Si content in a low carbon steel sheet and performing solid-solution strengthening while maintaining a ferritic structure.
CONSTITUTION: The steel sheet has a composition containing 0.5-2.5wt.% Si and also has a structure composed of ferritic and pearlitic structures, and further, retained austenite of ≥2.5vol.% is contained in the structure in the region from the front and rear of the steel sheet to the positions at a depth one-fourth the thickness of the sheet, respectively. At the time of producing this steel sheet, the conventional component range and rolling conditions can be used to obtain the fundamental strength and toughness of the steel sheet. Further, it is desirable that, in order to form retained austenite, water cooling is started from a temp. in the region where the two phases of ferrite and austenite exist in the steal sheet after rolling, and stopped at a temp. not lower than the Ms point. In the case of a low Mn steel sheet, the number of bends before fracture occurs in the repeated bend test can be increased by about 50% by increasing Si content.
ISHIKAWA TADASHI
NOMIYAMA YUJI
