Ultra-high-strength spring steel

Abstract

An ultra-high-strength spring steel, for use as a valve spring steel in a vehicle engine, includes 0.5 to 0.7% by weight of C, 1.2 to 1.5% by weight of Si, 0.6 to 1.2% by weight of Mn, 0.6 to 1.2% by weight of Cr, 0.1 to 0.5% by weight of Mo, 0.05 to 0.8% by weight of Ni, 0.05 to 0.5% by weight of V, 0.05 to 0.5% by weight of Nb, 0.05 to 0.3% by weight of Ti, 0.3% or less by weight of Cu (but not 0%), 0.0001 to 0.3% by weight of Al, 0.03% or less by weight of N (but not 0%), 0.0001 to 0.003% by weight of O, and a remainder of Fe and other unavoidable impurities, based on 100% by weight of the ultrahigh-strength spring steel.

What is claimed is: 
     
         1 . An ultra-high-strength spring steel, for use as valve spring steel in a vehicle engine, comprises:
 0.5 to 0.7% by weight of C, 1.2 to 1.5% by weight of Si, 0.6 to 1.2% by weight of Mn, 0.6 to 1.2% by weight of Cr, 0.1 to 0.5% by weight of Mo, 0.05 to 0.8% by weight of Ni, 0.05 to 0.5% by weight of V, 0.05 to 0.5% by weight of Nb, 0.05 to 0.3% by weight of Ti, 0.3% or less by weight of Cu (but not 0%), 0.0001 to 0.3% by weight of Al, 0.03% or less by weight of N (but not 0%), 0.0001 to 0.003% by weight of O, and a remainder of Fe and other unavoidable impurities, based on 100% by weight of the ultrahigh-strength spring steel.   
     
     
         2 . The ultra-high-strength spring steel according to  claim 1 , wherein the spring steel has a tensile strength of 2300 MPa or more. 
     
     
         3 . The ultra-high-strength spring steel according to  claim 1 , wherein the spring steel has a fatigue strength of 1100 MPa or more. 
     
     
         4 . The ultra-high-strength spring steel according to  claim 1 , wherein the spring steel has a yield strength of 2800 MPa or more. 
     
     
         5 . The ultra-high-strength spring steel according to  claim 1 , wherein the spring steel has a hardness of 710 HV or more. 
     
     
         6 . The ultra-high-strength spring steel according to  claim 1 , wherein the sizes of inclusions present in the spring steel are 15 μm or less. 
     
     
         7 . The ultra-high-strength spring steel according to  claim 1 , wherein, in the inclusions, a fraction of inclusions having sizes of 10 to 15 μm is 10% or less and a fraction of inclusions having sizes of less than 10 μm is 90% or more.