STEEL FOR HIGH STRENGTH SPRING, AND HEAT TREATED STEEL WIRE FOR HIGH STRENGTH SPRING

摘要

PROBLEM TO BE SOLVED: To provide a heat treated steel wire for a spring subjected to cold coiling, in which sufficient atmospheric strength and coiling workability can be made consistent, and having a tensile strength of≥2,000 MPa, and to provide steel for a spring used for the steel wire. SOLUTION: The heat treated steel sheet for a high strength spring has a composition comprising, by mass, 0.5 to 0.9% C, 1.0 to 3.0% Si, 0.1 to 1.5% Mn, 1.0 to 2.5% Cr, >0.15 to 1.0% V and≤0.005% Al, and in which the content of N is limited to≤0.007%, and further comprising one or two kinds selected from 0.001 to <0.01% Nb and 0.001 to <0.005% Ti, and has a tensile strength of≥2,000 MPa. Further, regarding cementite based spheroidal carbide and alloy based spheroidal carbide occupying in a microscopic cross section, the occupancy area ratio of the ones having a diameter of the equivalent circle of≥0.2μm satisfies≤7%, and the density of the ones having a diameter of the equivalent circle of≥0.2μm satisfies≤1 piece/μm2, also, old austenite grain size number is≥10, and the content of retained austenite is≤15 mass%. COPYRIGHT: (C)2007,JPO&INPIT