Steel wire for spring, spring, method for manufacturing steel wire for spring, and method for manufacturing spring

Abstract

A steel wire for a spring is formed of a steel containing from 0.5% by mass to 0.8% by mass of carbon, from 1.0% by mass to 2.5% by mass of silicon, from 0.2% by mass to 1.0% by mass of manganese, and from 0.5% by mass to 2.5% by mass of chromium, the balance being iron and incidental impurities. The steel has a tempered martensite structure. The hardness of a surface region that is a region within 10 μm from an outer surface is from more than 0 HV to 50 HV higher than the hardness of a region other than the surface region.

1 . A steel wire for a spring, comprising a steel containing from 0.5% by mass to 0.8% by mass of carbon, from 1.0% by mass to 2.5% by mass of silicon, from 0.2% by mass to 1.0% by mass of manganese, and from 0.5% by mass to 2.5% by mass of chromium, the balance being iron and incidental impurities, the steel having a tempered martensite structure,
 wherein a hardness of a surface region that is a region within 10 μm from an outer surface is from more than 0 HV to 50 HV higher than a hardness of a region other than the surface region.   
     
     
         2 . The steel wire for a spring according to  claim 1 , wherein the steel wire for a spring has a diameter of 15 mm or less. 
     
     
         3 . The steel wire for a spring according to  claim 1 , wherein a carbon concentration of the surface region is from more than 0% by mass to 0.05% by mass higher than a carbon concentration of the region other than the surface region. 
     
     
         4 . The steel wire for a spring according to  claim 1 , wherein a proportion of a {110} texture in the surface region is from 5% to 35% higher than a proportion of a {110} texture in a central region that is a region within 10 μm from a center of gravity of a cross-section perpendicular to a longitudinal direction of the steel wire for a spring. 
     
     
         5 . The steel wire for a spring according to  claim 1 , wherein the steel wire for a spring has a circularity of 3 μm or less in a cross-section perpendicular to a longitudinal direction 
     
     
         6 . The steel wire for a spring according to  claim 1 , wherein the steel wire for a spring has a variation in diameter of 1% or less per meter in a cross-section perpendicular to a longitudinal direction. 
     
     
         7 . The steel wire for a spring according to  claim 1 , wherein the steel further contains from 0.05% by mass to 0.5% by mass of vanadium 
     
     
         8 . A spring comprising the steel wire for a spring according  claim 1 . 
     
     
         9 . The spring according to  claim 8 , wherein the spring has a surface with a nitrogen concentration of 300 ppm or less. 
     
     
         10 . A method for manufacturing a steel wire for a spring, comprising the steps of:
 providing a starting wire material containing from 0.5% by mass to 0.8% by mass of carbon, from 1.0% by mass to 2.5% by mass of silicon, from 0.2% by mass to 1.0% by mass of manganese, and from 0.5% by mass to 2.5% by mass of chromium, the balance being iron and incidental impurities;   subjecting the starting wire material to first drawing;   subjecting to carburizing treatment the starting wire material subjected to the first drawing such that a carbon concentration of a region including a surface of the starting wire material is from more than 0% by mass to 0.05% by mass higher than a carbon concentration of an interior thereof; and   subjecting to quenching and tempering treatment the starting wire material subjected to the carburizing treatment.   
     
     
         11 . The method for manufacturing a steel wire for a spring according to  claim 10 , wherein the step of subjecting to carburizing treatment the starting wire material subjected to the first drawing comprises performing carburizing treatment by heating the starting wire material in an atmosphere whose composition is controlled to have a CP value of from 0.5% by mass to 1.5% by mass. 
     
     
         12 . The method for manufacturing a steel wire for a spring according to  claim 10 , further comprising a step of subjecting to second drawing the starting wire material subjected to the quenching and tempering treatment. 
     
     
         13 . The method for manufacturing a steel wire for a spring according to  claim 12 , wherein the second drawing is performed at a reduction of area of from 1% to less than 5%. 
     
     
         14 . A method for manufacturing a steel wire for a spring, comprising the steps of:
 providing a starting wire material containing from 0.5% by mass to 0.8% by mass of carbon, from 1.0% by mass to 2.5% by mass of silicon, from 0.2% by mass to 1.0% by mass of manganese, and from 0.5% by mass to 2.5% by mass of chromium, the balance being iron and incidental impurities;   subjecting the starting wire material to first drawing;   subjecting to quenching and tempering treatment the starting wire material subjected to the first drawing; and   subjecting to second drawing the starting wire material subjected to the quenching and tempering treatment,   wherein the second drawing is performed at a reduction of area of from 1% to less than 5%.   
     
     
         15 . The method for manufacturing a steel wire for a spring according to  claim 12 , wherein the second drawing is performed while the starting wire material is heated to a temperature range from 25° C. to 450° C. 
     
     
         16 . The method for manufacturing a steel wire for a spring according to any  claim 12 , wherein the second drawing is performed through a die with an approach angle of from 0.1° to 7°. 
     
     
         17 . The method for manufacturing a steel wire for a spring according to  claim 10 , wherein the starting wire material is drawn to a diameter of 15 mm or less by the first drawing. 
     
     
         18 . A method for manufacturing a spring, comprising the steps of:
 providing a steel wire for a spring by the method for manufacturing a steel wire for a spring according to  claim 10 ; and   coiling the steel wire for a spring.   
     
     
         19 . The method for manufacturing a spring according to  claim 18 , further comprising a step of subjecting the coiled steel wire for a spring to shot peening treatment,
 wherein the steel wire for a spring to be subjected to the shot peening treatment has a surface with a nitrogen concentration of 300 ppm or less.