Steel wire material for spring and its producing method

Abstract

The steel wire material for a spring of the invention contains; C: 0.37-0.54%, Si: 1.7-2.30%, Mn: 0.1-1.30%, Cr: 0.15-1.1%, Cu: 0.15-0.6%, Ti: 0.010-0.1%, Al: 0.003-0.05%, and the balance including iron with inevitable impurities, wherein ferrite decarburized layer depth is 0.01 mm or less, whole decarburized layer depth is 0.20 mm or less, and fracture reduction of area is 25% or more. It alternately may contain; C: 0.38-0.47%, Si: 1.9-2.5%, Mn: 0.6-1.3%, Ti: 0.05-0.15%, Al: 0.003-0.1%, and the balance including iron with inevitable impurities, wherein ferrite decarburized layer depth is 0.01 mm or less, Ceq1 in the equation (1) below is 0.580 or more, Ceq2 in the equation (2) below is 0.49 or less, and Ceq3 in the equation (3) below is 0.570 or less.

Ceq1=[C]+0.11[Si]−0.07[Mn]−0.05[Ni]+0.02[Cr]  (1)

Ceq2=[C]+0.30[Cr]−0.15[Ni]−0.70[Cu]  (2)

Ceq3=[C]−0.04[Si]+0.24[Mn]+0.10[Ni]+0.20[Cr]−0.89[Ti]−1.92[Nb]  (3)

(In the above equations, [ ] shows the content (mass %) of each element in steel.)

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]The present invention relates to a steel wire material for a spring wherein ferrite decarburized layer is not substantially present and workability is excellent, and its producing method.[0003]The present invention further relates to steel for spring (spring steel) useful as a material for a coil spring used in a heat treated (quenched and tempered) condition, and more specifically, to a steel wire material for a spring excellent in corrosion fatigue property.[0004]2. Description of the Related Art[0005]In the steel wire material for a spring which requires high fatigue strength, high alloying is generally directed, and in addition, much Si is added to improve yield strength ratio of an element wire for the spring after quenching and tempering. However, because addition of a large amount of Si narrows the austenitic zone in the phase equilibrium diagram, ferrite decarburization is liable to occur.[0006]To inhibit ferrit...

AI Technical Summary

Benefits of technology

[0077]Mn is the element effective for improving hardenability of steel and securing the hardness after quenching and tempering. If Mn amount is too little, it is difficult to achieve the hardenability required for the wire material for a spring. On the contrary, if Mn amount is excessive, the supercooled structure is generated in cooling after rolling and workability of the wire material is deteriorated. Therefore, Mn amount is set at 0.1% or more (preferably 0.12% or more, and more preferably 0.2% or more) and 1.30% or less (preferably 1.0% or less, more preferably 0.9% or less, and further more preferably 0.8% or less).

[0079]Cr is the element for strengthening the matrix of steel by solid solution strengthening and for improving hardenability. If Cr amount is too little, it is difficult to achieve the hardenability required for the wire material for a spring. On the contrary, if Cr amount is excessive, workability of the wire material is deteriorated. Therefore, Cr amount is set at 0.15% or more (preferably 0.2% or more, more preferably 0.5% or more, and 1.0% or more in particular) and 1.1% or less (preferably 1.05% or less).

[0081]Cu has the action of enhancing corrosion resistance of steel, and is the element inhibiting ferrite decarburization at the time of the heat treatment in hot rolling and spring working. However, if Cu amount becomes excessive, the hot crack possibly occurs. Therefore, Cu amount is set at 0.15% or more (preferably 0.20% or more) and 0.6% or less (preferably 0.5% or less).

[0083]Ti is the element effective for refining the old austenite grains after quenching and tempering and improving durability in the air and hydrogen embrittlement

Problems solved by technology

However, because addition of a large amount of Si narrows the austenitic zone in the phase equilibrium diagram, ferrite decarburization is liable to occur.

However, only adding of these alloy elements enhances hardenability of the wire material too much and the metastable structure such as bainite and martensite is liable to be generated in the cooling process after hot rolling.

This metastable structure exerts bad influence upon wire drawing (especially upon large diameter wire material) and causes cuppy break or transverse crack fracture.

In the sp

Images