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|1. (WO2017018457) STEEL FOR SUSPENSION SPRING AND METHOD FOR MANUFACTURING SAME|
|Applicants:||NIPPON STEEL & SUMITOMO METAL CORPORATION
NETUREN CO., LTD.
|Title:||STEEL FOR SUSPENSION SPRING AND METHOD FOR MANUFACTURING SAME|
Provided is a steel for a suspension spring that has a large tensile strength, excellent delayed fracture resistance and excellent cold workability while requiring little or no addition of expensive alloy elements. The steel for a suspension spring, said steel comprising, in terms of % by mass, 0.40-0.70% of C, 0.80-2.20% of Si, 0.05-1.50% of Mn, 0.05-1.00% of Cr, P in an amount limited to 0.020% or less, S in an amount limited to 0.020% or less and the remainder being Fe and inevitable impurities, is characterized in that: in a cross section parallel to a rolling direction, annealed martensite exists at a ratio by area of 90% or more relative to the metal structure; in a cross section parallel to the rolling direction, the ratio of the length in the major axis direction of prior austenite crystal grains and the length thereof in the direction orthogonal to the major axis direction is 1.5 or more, within a range of 10% of diameter or thickness from the surface; and the ratio [<011> texture ratio/<111> texture ratio] of a textured martensite structure viewed from the rolling direction is 3.0 or more.