High Strength Spring Steel, High Strength Springs and Manufacturing Method Thereof

Abstract

A high strength spring steel of this invention is characterized in containing, in percent of mass, of C: 0.36-0.48%; Si: 1.80-2.80%; Mn: 0.20-1.40%; P: 0.015% or less; S: 0.010% or less; Cu: 0.10-0.50%; Ni: 0.10-2.00%; Cr: 0.05-1.20%; s-Al: 0.005-0.040%; N: 0.002-0.012%; O: 0.002% or less, while the remainder is constituted of Fe and inevitable impurities and the quantity of inclusions of 10 μm or larger in diameter per field of vision of 100 mm2 is 10 or less. A manufacturing method of high strength spring of this invention is characterized in that the high strength spring steel which is tempered to HRC52 or higher is formed into a spring shape by hot forming or cold forming and warm shot peening is carried out so as to produce a high strength spring of 1176 MPa or higher in maximum shearing stress.

Description

RELATED APPLICATIONS [0001] This application is word-for-word translation of PCT Patent Application No. PCT / JP2004 / 012277 filed on Aug. 26, 2004, which is incorporated herein by reference.BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The present invention relates to a high strength spring steel, high strength springs and manufacturing method thereof. [0004] 2. Description of the Related Art [0005] In recent years, reduction in amount of emission of carbon dioxide gas has been considered as a problem to be solved from a viewpoint of suppressing global warming. Thus, reduction in weight of a vehicle resulting in enhancement of fuel efficiency has been regarded important in a field of automobiles, so that reduction in weight of a variety of springs such as a suspension coil spring has been sought after. In order to achieve reduction in weight of the spring while maintaining required spring characteristics, it is required to have a high strength spring having desig...

AI Technical Summary

Benefits of technology

[0013] Secondly, the quantity of inclusions (particularly inclusion of oxides) having a diameter of 10 μm or larger, which can become an origin of fracture, is reduced by deoxidizing sufficiently (more specifically deoxidizing by Al) thereby providing high fatigue strength.

[0042] Although in an ordinary spring steel, its C quantity is generally 0.55-0.65%, the quantity of C in the high strength spring steel of the present invention is lower as described above, and thus its yield ratio is small so that a sufficient compressive residual stress can be obtained by shot peening. Further, since the quantity of C is low so that a larger deformation can be introduced, improvement in hardness in the vicinity of the surface by dynamic strain aging is achieved, thereby securing an excellent fatigue strength enhancing effect.

Problems solved by technology

However, conventionally, even if the above-described means is used, the increase in the strength can only be achieved up to HRC52 in spring hardness and 1900 MPa in tensile strength from the durability viewpoint.

That is, if the strength of the spring is intensified, the fracture toughness decreases so that the notch sensitivity increases thereby resulting in dispersion in durability and decrease in reliability.

Further, it is also a problem that inclusions acting as notches decrease the durability.

Further, there are also other problems such that adding any special element costs high in order to prevent decrease in fracture toughness, or that adding a large amount of alloy elements increases hardness after hot rolling so that cracking or breaking of wire occurs when wire is drawn.

However, in conventional spring steels such as SUP7 and SAE9254, increasing the hardness leads to decrease in corrosion fatigue strength and in conventional steel, there is a limit in increasing the hardness more than a current level.

If the hardness of a spring is higher than that of a shot peening (SP) material, compressive residual stress which contributes largely to the enhancement of fatigue strength is not loaded sufficiently.

Additionally, there are other problems such that the SP material becomes easy to break thereby leading to higher cost, and that crashed grains stick into a spring thereby reducing its durability.

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