Spring steel for suspension and method for producing same

Abstract

Provided is spring steel for suspension suppressing or not requiring addition of expensive alloy elements, having a large tensile strength, and excellent in cold formability and delay fracture resistance, wherein at a cross-section parallel to a rolling direction, 90% or more of the metal microstructures by area fraction is tempered martensite, and at a cross-section parallel to the rolling direction, in a range of 10% of diameter or thickness from the surface, a ratio of a length in a long axis direction of prior austenite grains and a length in a direction perpendicular to the long axis direction of the prior austenite grains is 1.5 or more and a ratio of <011> fraction / <111> fraction of martensite texture as seen from the rolling direction is 3.0 or more.

Description

TECHNICAL FIELD[0001]The present disclosure relates to spring steel used for suspension springs for automobiles etc., preferably high strength spring steel for suspension springs having a 1800 MPa or more tensile strength, and a method for producing the same.BACKGROUND ART[0002]A spring is usually manufactured by the cold spring making method of which drawing a hot rolled wire rod to a predetermined wire size, quench hardening it, tempering it, then shaping it in cold, or the method of which shaping a drawn wire rod while hot, quench hardening it, and tempering it. In recent years, high strength springs with a tensile strength of 1800 MPa or more are being developed in order to deal with environmental problems, further lightening of weight and lowering of fuel consumption have been sought from automobiles.[0003]The cold spring making method has a high degree of freedom of design of spring shape and is also high in precision, so is a work method particularly suitable for high strengt...

AI Technical Summary

Benefits of technology

The present invention provides a way to make spring steel for suspension that does not require expensive alloy elements, has a high tensile strength, and is excellent at withstanding cold formability and delay fracture. This means that even strong spring steel can be used for suspension without needing additional elements to improve its performance.

Problems solved by technology

Further, in general, if making a spring high in strength, the sensitivity to hydrogen penetrating into the steel in a corrosive environment and the sensitivity to corrosion pits and other defects rise and the spring is liable to break due to delayed fracture.

However, if adding alloy elements, the cost rises, so spring steel obtained by stretching the prior austenite grains at the surface in the rolling direction to increase the critical hydrogen content and improve the delayed fracture resistance and hydrogen fatigue resistance is proposed (PLT 4).

Images