Cold-formed spring having high fatigue strength and high corrosion fatigue strength, steel for such spring, and method of manufacturing such spring

Abstract

The present invention provides a cold-formed spring having high fatigue strength and high corrosion fatigue strength, a specific type of steel for such a spring, and a method of manufacturing such a cold-formed coil spring. The spring according to the present invention is made from a steel material containing, in weight percentage, 0.45 to 0.52% of C, 1.80 to 2.00% of Si, 0.30 to 0.80% of Ni, 0.15 to 0.35% of Cr and 0.15 to 0.30% of V, with Fe substantially constituting the remaining percentage. A wire is produced from the steel, and the wire is subjected to a high-frequency heating process, whereby the wire is hardened at a temperature of 920 to 1040° C. for 5 to 10 seconds, and then tempered at a temperature of 450 to 550° C. for 5 to 20 seconds so that its hardness becomes 50.5 to 53.5 HRC. Finally, the wire undergoes a shot peening process so that its residual stress at 0.2 mm depth from the surface becomes −600 MPa or higher.

1 . A cold-formed spring having high fatigue strength and high corrosion fatigue strength, which is made of a wire made from a steel material containing, in weight percentage, 0.45 to 0.52% of C, 1.80 to 2.00% of Si, 0.30 to 0.80% of Ni, 0.15 to 0.35% of Cr and 0.15 to 0.30% of V, with Fe substantially constituting the remaining percentage, and which is hardened and tempered by a high-frequency heating process.  
     
     
         2 . The cold-formed spring according to  claim 1 , wherein the percentage of P is 0.025% or lower and the percentage of S is 0.020% or lower.  
     
     
         3 . The cold-formed spring according to  claim 2 , wherein the wire has a tensile strength of 1800 to 2000 MPa and a reduction of area of 35% or higher after being hardened and tempered by the high-frequency heating process.  
     
     
         4 . The cold-formed spring according to  claim 1 , wherein the wire has a hardness of 50.5 to 53.5 HRC after being hardened and tempered, and the spring is subject to a shot peening process so that the residual stress at 0.2 mm depth from the surface becomes −600 MPa or higher.  
     
     
         5 . A method of manufacturing a coil spring having high fatigue strength and high corrosion fatigue strength, wherein the spring is made from a steel material containing, in weight percentage, 0.45 to 0.52% of C, 1.80 to 2.00% of Si, 0.30 to 0.80% of Ni, 0.15 to 0.35% of Cr and 0.15 to 0.30% of V, with Fe substantially constituting the remaining percentage, and the method comprises the steps of making a wire from the steel material, hardening and tempering the wire by a high-frequency heating process and cold-coiling the wire into the spring.  
     
     
         6 . The method according to  claim 5 , wherein the high-frequency heating process includes the steps of hardening the wire at a temperature of 920 to 1040° C. for 5 to 20 seconds, rapidly cooling the wire, and tempering the wire at a temperature of 450 to 550° C. for 5 to 20 seconds.  
     
     
         7 . The method according to  claim 6 , wherein the hardening temperature is within the range from 940 to 1020° C. and the tempering temperature is within the range from 480 to 520° C.  
     
     
         8 . The method according to  claim 6 , wherein the wire is rapidly cooled after being tempered.  
     
     
         9 . A type of steel material for cold-forming a spring hardened and tempered by a high-frequency heating process, containing, in weight percentage, 0.45 to 0.52% of C, 1.80 to 2.00% of Si, 0.30 to 0.80% of Ni, 0.15 to 0.35% of Cr and 0.15 to 0.30% of V, with Fe substantially constituting the remaining percentage.  
     
     
         10 . The steel material according to  claim 9 , wherein the percentage of P is 0.025% or lower and the percentage of S is 0.020% or lower.  
     
     
         11 . The cold-formed spring according to  claim 2  wherein the wire has a hardness of 50.5 to 53.5 HRC after being hardened and tempered, and the spring is subject to a shot peening process so that the residual stress at 0.2 mm depth from the surface becomes −600 MPa or higher.  
     
     
         12 . The cold-formed spring according to  claim 3  wherein the wire has a hardness of 50.5 to 53.5 HRC after being hardened and tempered, and the spring is subject to a shot peening process so that the residual stress at 0.2 mm depth from the surface becomes −600 MPa or higher.