Patent ID: 8043444

Claim:
A steel wire for a spring containing C: 0.45-0.54% (mass %, the same is applied hereunder), Si: 1.8-2.5%, Mn: 0.05-0.9% and Cr: 0.05-2.0%, comprising a fraction of martensite and bainite of 60% or less in area percentage, wherein P and S are controlled to 0.020% or less (including 0%), respectively; a martensitic transformation start temperature M S1 , shown by the following expression (1), is in the range from 280° C. to 380° C., where M S1 =550−361[C]−39[Mn]−20[Cr] (1), and [C], [Mn] and [Cr] represent the contents (mass %) of C, Mn and Cr, respectively; and the steel wire has been austenized and tempered such that the prior austenite grain size number N of austenite grains is No. 12 or more, the grain boundary share of carbide precipitated along the austenite grain boundaries is 50% or less, the amount of retained austenite is 20 vol. % or less, and the tensile strength is 2,000 MPa or more.