Coil spring steel

Abstract

a coil spring steel which includes: 0.4 to 0.9 wt % of carbon (C); 1.3 to 2.3 wt % of silicon (Si), 0.5 to 1.2 wt % of manganese (Mn); 0.1 to 0.5 wt % of molybdenum (Mo); 0.05 to 0.80 wt % of nickel (Ni); 0.05 to 0.50 wt % of vanadium (V); 0.01 to 0.50 wt % of niobium (Nb); 0.05 to 0.30 wt % of titanium (Ti); 0.6 to 1.2 wt % of chromium (Cr); 0.0001 to 0.3 wt % of aluminum (Al); less than or equal to 0.3 wt % but greater than 0% of copper (Cu); less than or equal to 0.3 wt % but greater than 0% of nitrogen (N); 0.0001 to 0.0030 wt % of oxygen (O); and a balance of iron (Fe) and unavoidable impurities.

What is claimed is: 
     
         1 . A coil spring steel comprising:
 0.4 to 0.9 wt % of carbon (C);   1.3 to 2.3 wt % of silicon (Si);   0.5 to 1.2 wt % of manganese (Mn);   0.1 to 0.5 wt % of molybdenum (Mo);   0.05 to 0.80 wt % of nickel (Ni);   0.5 to 0.50 wt % of vanadium (V);   0.01 to 0.50 wt % of niobium (Nb);   0.05 to 0.30 wt % of titanium (Ti);   0.6 to 1.2 wt % of chromium (Cr);   0.0001 to 0.3 wt % of aluminum (Al);   less than or equal to 0.3 wt % but greater than 0% of copper (Cu);   less than or equal to 0.3 wt % but greater than 0% of nitrogen (N);   0.0001 to 0 0030 wt % of oxygen (O); and   a balance of iron (Fe) and unavoidable impurities.   
     
     
         2 . The coil spring steel according to  claim 1 , wherein the coil spring steel has a tensile strength of 2150 MPa or more and a hardness of 690 HV or more. 
     
     
         3 . The coil spring steel according to  claim 1 , wherein a wire rod made of the coil spring steel has a fatigue life 280 thousand cycles or more under a condition of a bending moment of 20 kgfm and a load of 100 kgf. 
     
     
         4 . The coil spring steel according to  claim 1 , wherein a coil spring single-part made of the coil spring steel has a complex corrosion fatigue life of 360 thousand cycles or more under a complex corrosion environment in which salt of 50±5 (g/L) is sprayed and to which a bending moment of 20 kgfm and a load of 100 kgf are applied.