High Strength Spring Steel and High Strength Heat Treated Steel Wire for Spring

Abstract

The present invention provides a high strength heat treated steel wire for spring having a tensile strength of 2000 MPa or more which is coiled in the cold state and can achieve both sufficient atmospheric strength and coilability and spring steel used for that steel wire, that is, a high strength heat treated steel wire for a spring characterized by comprising, by mass %, C: 0.5 to 0.9%, Si: 1.0 to 3.0%, Mn: 0.1 to 1.5%, Cr: 1.0 to 2.5%, V: over 0.15 to 1.0%, and Al: 0.005% or less, controlling N to 0.007% or less, further containing one or two of Nb: 0.001 to less than 0.01% and Ti: 0.001 to less 0.005%, and having a tensile strength of 2000 MPa or more, having cementite-based spheroidal carbides and alloy-based spheroidal carbides in a microscopic visual field satisfying an area percentage of carbides with a circle equivalent diameter of 0.2 μm or more of 7% or less and a density of carbides with a circle equivalent diameter of 1 grain/μm 2 or less, having a prior austenite grain size number of #10 or more, and having retained austenite of 15 mass % or less.

1 . High strength spring steel characterized by comprising, by mass %,
 C: 0.5 to 0.9%,   Si: 1.0 to 3.0%,   Mn: 0.1 to 1.5%,   Cr: 1.0 to 2.5%,   V: over 0.15 to 1.0%, and   Al: 0.005% or less,   controlling N to 0.007% or less,   further containing one or two of Nb: 0.001 to less than 0.01% and Ti: 0.001 to less 0.005%, and   having a balance of Fe and unavoidable impurities.   
   
   
       2 . High strength spring steel according to  claim 1  characterized by further containing one or two of
 W: 0.05 to 0.5% and   Mo: 0.05 to 0.5%.   
   
   
       3 . High strength spring steel according to  claim 1  characterized by further containing, by mass %, one or two or more of
 Ni: 0.05 to 3.0%,   Cu: 0.05 to 0.5%,   Co: 0.05 to 3.0%, and   B: 0.0005 to 0.006%.   
   
   
       4 . High strength spring steel according to  claim 1  characterized by further containing, by mass %, one or two or more of
 Te: 0.0002 to 0.01%,   Sb: 0.0002 to 0.01%,   Mg: 0.0001 to 0.0005%,   Zr: 0.0001 to 0.0005%,   Ca: 0.0002 to 0.01%, and   Hf: 0.0002 to 0.01%.   
   
   
       5 . A high strength heat treated steel wire for a spring characterized by having steel compositions according to  claim 1   having a tensile strength of 2000 MPa or more,   having cementite-based spheroidal carbides and alloy-based spheroidal carbides in a microscopic visual field satisfying   an area percentage of carbides with a circle equivalent diameter of 0.2 μm or more of 7% or less and   a density of carbides with a circle equivalent diameter of 1 grain/μm 2  or less,   having a prior austenite grain size number of #10 or more, and   having retained austenite of 15 mass % or less.