Patent ID: 7404865

Claim:
A steel wire for heat-resistant springs, the steel wire containing: (a) 0.01 to 0.08 wt % C, 0.18 to 0.25 wt % N, 0.5 to 4.0 wt % Mn, 16 to 20 wt % Cr, 8.0 to 10.5 wt % Ni, and 0.2 to 2.0 wt % Co; (b) at least one constituent selected from the group consisting of 0.1 to 3.0 wt % Mo, 0.1 to 2.0 wt % Nb, 0.1 to 2.0 wt % Ti and 0.3 to 2.0 wt % Si; and (c) mainly Fe and unavoidable impurities both of which constitute the remainder; the steel wire having: (d) a tensile strength of at least 1,300 N/mm 2 and less than 2,000 N/mm 2 before being treated by low-temperature annealing; (e) a maximum crystal-grain diameter of less than 12 μm in the γ phase (austenite) in a transverse cross section of the wire; and (f) a surface roughness, expressed as Rz, of 1 to 20 μm, the surface roughness being defined as a roughness in the direction of wire drawing.