Oil tempered wire for cold forming coil springs

Abstract

[OBJECT] An oil-tempered wire for a cold-formed coil spring having a quality equivalent of or higher than a hot-formed coil spring is provided. A cold-formed coil spring made from the oil-tempered wire is also provided. [MEANS FOR SOLVING THE PROBLEM] Material used is the steel which contains, in weight percentage, 0.35 to 0.55% C, 1.8 to 3.0% Si, 0.5 to 1.5% Mn, 0.5 to 3.0% Ni, and 0.1 to 1.5% Cr. The ferrite fraction in the microscopic structure of this material is set to 50% or less. Hot rolled wire is cold drawn with a predetermined reduction of area, and a heat treatment using high frequency induction heating is conducted. It is preferable to set the maximum heating temperature between 900° C. to 1020° C. (favorably 950° C.) and the holding time between 5 to 20 seconds. It is preferable to make the oil-tempered material to have the grain size number of 9 or more, and the tensile strength from 1830 to 1980 MPa.

1 . An oil-tempered wire for a cold-formed coil spring characterized by being made from steel comprising, in weight percentage, 0.35 to 0.55% C, 1.8 to 3.0% Si, 0.5 to 1.5% Mn, 0.5 to 3.0% Ni, and 0.1 to 1.5% Cr, and by being heat treated with high frequency induction heating.  
     
     
         2 . The oil-tempered wire for a cold-formed coil spring according to  claim 1 , wherein the material is hot-rolled to a wire with a predetermined diameter, the wire is cold drawn with a predetermined reduction of area, and then the drawn wire is heat treated with a high frequency induction heating.  
     
     
         3 . The oil-tempered wire for a cold-formed coil spring according to  claim 1 , wherein the ferrite fraction in the microscopic structure of the steel before the heat treatment with a high frequency induction heating is 50% or less.  
     
     
         4 . The oil-tempered wire for a cold-formed coil spring according to  claim 1 , wherein the maximum heating temperature in the high frequency induction heating is 1020° C. or lower.  
     
     
         5 . The oil-tempered wire for a cold-formed coil spring according to  claim 1 , wherein the maximum heating temperature in the high frequency induction heating is 950° C. or lower.  
     
     
         6 . The oil-tempered wire for a cold-formed coil spring according to  claim 1 , wherein the maximum heating temperature in the high frequency induction heating is 940° C. or higher.  
     
     
         7 . The oil-tempered wire for a cold-formed coil spring according to  claim 1 , wherein the maximum heating temperature in the high frequency induction heating is 900° C. or higher.  
     
     
         8 . The oil-tempered wire for a cold-formed coil spring according to  claim 1 , wherein a holding time at the maximum heating temperature in the high frequency induction heating is 5 seconds or more.  
     
     
         9 . The oil-tempered wire for a cold-formed coil spring according to  claim 1 , wherein a holding time at the maximum heating temperature in the high frequency induction heating is 20 seconds or less.  
     
     
         10 . The oil-tempered wire for a cold-formed coil spring according to  claim 1 , wherein the grain size number after the heat treatment is set to be 9 or more.  
     
     
         11 . The oil-tempered wire for a cold-formed coil spring according to  claim 1 , wherein the tensile strength after the heat treatment is set to be from 1830 to 1980 MPa.  
     
     
         12 . The oil-tempered wire for a cold-formed coil spring according to  claim 1 , wherein the material further comprises 0.01 to 0.025% N and 0.05 to 0.5% V, and the P content is restricted to 0.01% or less, and the S content is restricted to 0.01% or less.  
     
     
         13 . A cold-formed coil spring made from the oil-tempered wire according to  claim 1.