Drawn heat treated steel wire for high strength spring use and pre-drawn steel wire for high strength spring use

Abstract

Drawn heat treated steel wire for high strength spring use is provided containing, by mass %, C: 0.67% to less than 0.9%, Si: 2.0 to 3.5%, Mn: 0.5 to 1.2%, Cr: 1.3 to 2.5%, N: 0.003 to 0.007%, and Al: 0.0005% to 0.003%, having Si and Cr satisfying the following formula:0.3%≦Si−Cr≦1.2%,and having a balance of iron and unavoidable impurities, having as impurities, P: 0.025% or less and S: 0.025% or less, furthermore having a circle equivalent diameter of undissolved spherical carbides of less than 0.2 μm, further having, as a metal structure, at least residual austenite in a volume rate of over 6% to 15%, having a prior austenite grain size number of #10 or more, and having a circle equivalent diameter of undissolved spherical carbides of less than 0.2 μm.

Description

TECHNICAL FIELD[0001]The present invention relates to drawn heat treated steel wire for high strength spring use which can be used as a material for high strength springs produced by cold coiling and to pre-drawn steel wire.BACKGROUND ART[0002]The springs which are used for automobile engines, clutches, etc. are being required to offer more advanced performance and higher durability in order to deal with the trend toward lighter weights and higher performances of automobiles. For this reason, their materials, that is, drawn heat treated steel wire for high strength spring use, are also being required to offer high material strength. In general, when producing such small sized, high strength springs, the material of the drawn heat treated steel wire for high strength spring use is quenched and tempered to impart higher material strength in the drawn heat treated steel wire for high strength spring use, then is cold coiled to obtain a coil spring shape. Furthermore, stress-relief anne...

AI Technical Summary

Benefits of technology

[0044]According to the present invention, in particular, due to the excellent cold coiling ability and temper softening resistance, even with nitriding at 500° C. for 1 hour, drawn heat treated steel wire for high strength spring use with a high surface layer hardness and internal hardness and, furthermore, high strength spring excellent in durability can be provided. The contribution in industry is extremely great.

Problems solved by technology

However, if drawn heat treated steel wire for high strength spring use becomes higher in strength, in cold coiling, breakage may occur and the spring shape may not be able to be formed.

However, undissolved carbides or nitride easily form, so even if suppressing the nitrogen (N), the control of precipitation has to be performed precisely.

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