Spring and manufacture method thereof

Abstract

A spring consists of, by mass %, 0.5 to 0.7% of C, 1.0 to 2.0% of Si, 0.1 to 1.0% of Mn, 0.1 to 1.0% of Cr, not more than 0.035% of P, not more than 0.035% of S, and the balance of Fe and inevitable impurities. The spring has a structure including not less than 65% of bainite and 4 to 13% of residual austenite by area ratio in a cross section. The spring has a compressive residual stress layer in a cross section from a surface to a depth of 0.35 mm to D/4, in which D (mm) is a circle-equivalent diameter of the cross section. The spring has a high hardness layer with greater hardness than a center portion by 50 to 500 HV from a surface to a depth of 0.05 to 0.3 mm.

1 . A spring consisting of, by mass %, 0.5 to 0.7% of C, 1.0 to 2.0% of Si, 0.1 to 1.0% of Mn, 0.1 to 1.0% of Cr, not more than 0.035% of P, not more than 0.035% of S, and the balance of Fe and inevitable impurities,
 wherein the spring has a structure including not less than 65% of bainite and 4 to 13% of residual austenite by area ratio in a cross section, the residual austenite contains carbon at an average concentration of 0.65 to 1.7%,   the spring has a compressive residual stress layer in a cross section from a surface to a depth of 0.35 mm to D/4, in which D (mm) is a circle-equivalent diameter of the cross section, the compressive residual stress layer has maximum compressive residual stress of 800 to 2000 MPa, and   the spring has a center portion with Vickers hardness of 550 to 650 HV in a cross section and has a high hardness layer with greater hardness than the center portion by 50 to 500 HV from a surface to a depth of 0.05 to 0.3 mm.   
     
     
         2 . The spring according to  claim 1 , wherein the structure includes martensite at 5 to 30% by area ratio in a cross section. 
     
     
         3 . The spring according to  claim 1 , wherein the spring is formed of a wire rod with a diameter of 1.5 to 15 mm. 
     
     
         4 . The spring according to  claim 1 , wherein the residual austenite has an average circle-equivalent grain diameter of not more than 3 μm. 
     
     
         5 . The spring according to  claim 4 , wherein the structure includes martensite at 5 to 30% by area ratio in a cross section. 
     
     
         6 . The spring according to  claim 4 , wherein the spring is formed of a wire material which has an average circle-equivalent diameter of 1.5 to 15 mm in a cross section. 
     
     
         7 . A production method for a spring, comprising:
 a step of preparing a wire rod consisting of, by mass %, 0.5 to 0.7% of C, 1.0 to 2.0% of Si, 0.1 to 1.0% of Mn, 0.1 to 1.0% of Cr, not more than 0.035% of P, not more than 0.035% of S, and the balance of Fe and inevitable impurities;   a step of forming the wire rod into a shape of a spring;   a heat treatment step; and   a shot peening step of shooting shot at the wire rod after the heat treatment step,   wherein in the heat treatment step, the wire rod is austenitized at a temperature of Ac3 point to (Ac3 point+250° C.) and is cooled at a cooling rate of not less than 20° C./second, and then the wire rod is held at a temperature of Ms point to (Ms point+60° C.) for not less than 400 seconds and is cooled to room temperature at a cooling rate of not less than 20° C./second.   
     
     
         8 . The production method for the spring according to  claim 7 , further comprising a setting step of providing permanent strain to the wire rod after the shot peening step. 
     
     
         9 . A production method for a spring, comprising:
 a step of preparing a wire rod consisting of, by mass %, 0.5 to 0.7% of C, 1.0 to 2.0% of Si, 0.1 to 1.0% of Mn, 0.1 to 1.0% of Cr, not more than 0.035% of P, not more than 0.035% of S, and the balance of Fe and inevitable impurities;   a step of forming the wire rod into a shape of a spring;   a heat treatment step; and   a shot peening step of shooting shot at the wire rod after the heat treatment step,   wherein in the heat treatment step, the wire rod is austenitized at a temperature of Ac3 point to (Ac3 point+250° C.) and is cooled at a cooling rate of not less than 20° C./second, and then the wire rod is held at a temperature of (Ms point−20° C.) to (Ms point+60° C.) for not less than 400 seconds and is cooled to room temperature.   
     
     
         10 . The production method for the spring according to  claim 9 , wherein the wire rod is cooled to room temperature at a cooling rate of not less than 20° C./second. 
     
     
         11 . The production method for the spring according to  claim 9 , further comprising a setting step of providing permanent strain to the wire rod after the shot peening step. 
     
     
         12 . The spring according to  claim 2 , wherein the spring is formed of a wire rod with a diameter of 1.5 to 15 mm. 
     
     
         13 . The spring according to  claim 5 , wherein the spring is formed of a wire material which has an average circle-equivalent diameter of 1.5 to 15 mm in a cross section. 
     
     
         14 . The production method for the spring according to  claim 10 , further comprising a setting step of providing permanent strain to the wire rod after the shot peening step.