Nippon Steel Corporation

Nippon Steel Sumitomo Metal Corporation

Koichi Banno

Manabu Kubota

Atsushi Monden

Keisuke Chiba

Shunta Homma

Ryuuichi Seki

Steel for a suspension spring including, as a chemical composition, by % by mass: C: more than 0.40% and 0.65% or less; Si: 1.00% to 3.50%; Mn: more than 2.00% and 3.00% or less; Cr: 0.01% to 2.00%; V: 0.02% to 0.50%; P: 0.020% or less; S: 0.020% or less: N: 0.0100% or less; and a remainder of Fe and impurities, in which Kf is 280 or more, in which a structure contains a tempered martensite of which an area ratio is 90% or more, in which a Fe-based carbide precipitated in the tempered martensite is a cementite, and in which an average of an area ratios of prior austenite grains which stretch in a longitudinal direction of the steel and of which an aspect ratio exceeds 3.0, is 80% or more in a case of depths of 0.1 mm, 0.2 mm, and 0.3 mm in a sheet thickness direction from a surface of the steel.

Steel for suspension spring and method of manufacturing same

A steel bar according to one embodiment of the present invention includes predetermined chemical composition, wherein a quenching deflection in a cross section is 1.5 mm or less, wherein &#x394;max and &#x394;min is 1.5 mm or less, wherein a structure in a surface layer area includes 10 area % or less of ferrite and a remainder including one or more selected from a group consisting of a bainite and a martensite, wherein an average value of the grain size of a bcc phase in the surface layer area is 1.0 to 10.0 &#x3bc;m, wherein an average value of the grain size of the bcc phase in a center area is 1.0 to 15.0 &#x3bc;m, wherein a hardness of a region of which a depth from the surface is 50 &#x3bc;m is Hv200 to Hv500, and wherein a total decarburized layer thickness DM-T is 0.20 mm or less.

Steel bar

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Hokkaido, Japan

Koichi Banno