Metal treatment – Stock – Ferrous
Reexamination Certificate
1999-05-03
2001-03-27
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S908000, C148S598000, C148S580000, C420S110000
Reexamination Certificate
active
06206984
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a non-heat treated wire or bar steel for springs which possesses a high tensile strength of about 120-150 kgf/mm
2
and exhibits good cold bending properties in its as-hot rolled state even though it does not undergo heat treatment (such as quenching and tempering) after hot rolling.
2. Description of the Related Art
Springs are divided into hard-drawn ones and heat-treated ones if they are made of a high-strength steel wire or bar with a tensile strength greater than 100 kgf/mm
2
. Hard-drawn springs are manufactured from intensively cold-drawn rod, typically piano wire made from eutectoid steel. Heat-treated springs are manufactured from rolled (or hot-rolled) and drawn rod by hot bending and ensuing heat treatment (quenching and tempering) or from previously heat-treated rod by cold bending.
Production of hard-drawn springs needs intense drawing and production of heat-treated springs needs heat treatment, as mentioned above, and they are only possible with large-scale facilities and heavy energy consumption and long processing time. If springs can be made from as-rolled rod without intense working and heat treatment, it would be possible to greatly save facilities and raw materials and to shorten the delivery period, and hence such a technology would be very useful.
The problem involved with as-rolled material is that it cannot be used as such (with a tensile strength in excess of 120 kgf/mm
2
) because it is too poor in toughness and ductility to undergo cold bending and it breaks springs with insufficient impact resistance.
OBJECT AND SUMMARY OF THE INVENTION
The present invention was completed in view of the foregoing. It is an object of the present to provide a non-heat treated wire or bar steel for springs which possesses a high tensile strength and exhibits good bending properties in its as-rolled state.
According to the present invention, the non-heat treated wire or bar steel for springs (abbreviated as “the steel of the present invention” or “non-heat treated steel” hereinafter) is characterized by having in its as-rolled state a tensile strength of 120-150 kgf/mm
2
and a bending breakage rate no higher than 15% when tested according to JIS Z-2248 under the condition of r/d=2.8 where r (mm) denotes the inside radius for the bending curvature and d (mm) denotes the diameter of the as-rolled stock.
According to a preferred embodiment of the present invention, the non-heat treated steel is characterized by having a bending breakage rate no higher than 15% or still no higher than 6% when tested under the condition of r/d=1.4 where r and d are defined as above.
The steel of the present invention basically comprises 0.13-0.35% C, 0.1-1.8% Si, and 0.8-1.8% Cr (% means % by weight hereinafter). It further comprises 0.8-2.5% Mn and up to 0.08% Al (excluding 0%). It further comprises 0.005-0.15% Nb, 0.01-0.1% Ti, and 0.0005-0.01% B. It contains Ti and Nb such that their total amount is no less than 0.08%. It further comprises up to 0.2% V (excluding 0%). It further comprises up to 0.018% S (excluding 0%). According to a preferred embodiment, it comprises 0.13-0.35% C, 0.1-1.8% Si, 0.8-1.8% Cr, 0.8-2.5% Mn, up to 0.08% Al (excluding 0%), 0.005-0.15% Nb, 0.01-0.1% Ti, 0.0005-0.01% B, up to 0.2% V (including 0%), up to 0.018% S (including 0%), and no less than 0.08% Nb+Ti, with the remainder being Fe and unavoidable impurities.
Incidentally, the scope of the present invention embraces springs and stabilizers manufactured from the steel of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present inventors carried out a series of researches in order to provide a non-heat treated wire or bar steel for springs which possesses a high tensile strength and exhibits good bending properties in its as-hot rolled state. As a result, it was found that the conventional technologies are mostly intended to improve tensile strength and are not intended to provide non-heat treated steel which is superior in both tensile strength and bending properties. It was also found that it is possible to improve tensile strength simply by growing the martensite-based structure or bainite-based structure but such structure control alone is not enough if it is desirable to improve both tensile strength and bending properties.
There is disclosed in Japanese Patent Laid-open Nos. 30653/1986, 30650/1986, and 87749/1989 a non-heat treated spring steel which has a tensile strength as high as about 200 kgf/mm
2
because it has the martensite-based structure grown in its as-rolled state. Unfortunately, a steel with such an extremely high tensile strength is remarkably poor in bending properties. It is capable of bending to some extent but is subject to occasional breakage due to bending when it is bent lightly such that the ratio of r/d is 2.8 (for example), where r is the radius of bending curvature and d is the diameter of stock, as in the case of automotive suspension springs. It is also subject to frequent breakage or cracking due to bending when it is bent sharply such that the ratio of r/d is 1.4 (for example).
On the other hand, there is disclosed in Japanese Patent Laid-open No. 239589/1993 a high-strength non-heat treated steel which has both hardness and toughness owing to its bainite-based structure. However, this steel is poor in tensile strength (100 kgf/mm
2
at the highest), and nothing is disclosed about bending properties.
All of the above-mentioned disclosures are intended mainly to improve tensile strength but are not intended to improve both tensile strength and bending properties.
The present inventors carried out extensive studies to provide “a non-heat treated steel superior in both tensile strength and bending properties.” As a result, it was found that good bending properties are attained by a comparatively low carbon content (say, 0.13-0.35%) and by the bainite-based structure, that the bainite structure which is stable under various rolling conditions for various stock diameters is obtained effectively by incorporation with Nb, B, and Ti; that Nb particularly contributes to improvement in tensile strength due to the formation of bainite structure; that improvement in bending properties is effectively achieved by grain refinement by Nb and Ti; and that further improvement in bending properties is achieved if retained austenite remains in an adequate amount owing to incorporation with Si. Studies based on the above-mentioned findings led to another finding that it is necessary that the upper limit of tensile strength should be 150 kgf/mm
2
(particularly 120-150 kgf/mm
2
) if the non-heat treated steel is to have both high tensile strength and good bending properties. Thus it was found that the steel having a tensile strength adjusted in such an extent has also good bending properties and hence is capable of bending in its as-rolled state without breaking even in the case of sharp bending (with a small radius of curvature, say, 1.4 times the diameter). The present invention was completed on the basis of these findings.
The present inventors studied the factors that affect bending properties in connection with tensile strength. As a result, it was found that it is possible to improve both tensile strength and bending properties if the steel has bainite as the main structure and contains subtly controlled chemical components (especially Nb, Ti, and B). The point of the present invention resides in this finding. In other words, the present invention is technically significant because of the finding that the non-heat treated steel can possess both high tensile strengths and good bending properties only when its tensile strength is adjusted within a range of 120-150 kgf/mm
2
.
According to the present invention, the non-heat treated wire or bar steel for springs is characterized by possessing a high tensile strength of 120-150 kgf/mm
2
in its as-rolled state and also possessing a bending breakage rate no higher than 15% when tested according to JIS Z-2248 under the cond
Ibaraki Nobuhiko
Inada Atsushi
Yoshihara Nao
Kabushiki Kaisha Kobe Seiko Sho
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Yee Deborah
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