Steel wire for heat-resistant spring, heat-resistant spring and method for producing heat-resistant spring

A method for producing a heat-resistant spring, the method comprising the steps of:

• (a) forming a spring by using a steel wire containing:
• (a1) 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;
• (a2) 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
• (a3) mainly Fe and unavoidable impurities both of which constitute the remainder;
• the steel wire having:
• (a4) a tensile strength of at least 1,300 N/mm2 and less than 2,000 N/mm2 before being treated by low-temperature annealing;
• and
• (a5) a maximum crystal-grain diameter of less than 12 μm in the γ phase (austenite) in a transverse cross section of the wire;
• and
• (a6) 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;
• and
• (b) treating the spring with low-temperature annealing at a temperature of 450 to 600° C., wherein
• the step of forming the spring includes performing solution treatment at a temperature of 950 to 1.200° C., a temperature-maintaining period of which is 0.3 to 5 min/mm, the temperature-maintaining period being expressed by a ratio of maintaining period (mm) and wire diameter (mm), and controlling wire drawing reduction of area to be within 50 to 70%.

A method for producing a heat-resistant spring, the method comprising the steps of:

• forming a spring by using a 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/mm2 and less than 2,000 N/mm2 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;
• and
• treating the spring with low-temperature annealing at a temperature of 450 to 600°C.

A steel wire for heat-resistant springs, the steel wire containing:

• (a1) 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;
• (a2) 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
• (a3) mainly Fe and unavoidable impurities both of which constitute the remainder;
• the steel wire having:
• (a4) a tensile strength of at least 1,300 N/mm2 and less than 2,000 N/mm2 before being treated by low-temperature annealing;
• (a5) a maximum crystal-grain diameter of less than 12 μm in the γ phase (austenite) in a transverse cross section of the wire;
• and
• (a6) 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, wherein
• solution treatment is performed at a temperature of 950 to 1,200 0C, a temperature-maintaining period of which is 0.3 to 5 min/mm, the temperature-maintaining period being expressed by a ratio of maintaining period (min) and wire diameter (mm), and
• wire drawing reduction of area is controlled to be within 50 to 70%.