JPH05202417A - Production of steel wire for ultrahigh strength spring - Google Patents

Abstract

PURPOSE:To produce a steel wire for ultrahigh strength spring capable of being suitably used for the production of valve spring, etc., in internal combustion engine and having high toughness and ductility by specifying a composition and wiredrawing conditions. CONSTITUTION:A wire rod which has a composition consisting of, by weight, 0.30-0.80% C, 0.80-2.50% Si, 0.10-1.50% Mn, <=0.10% Al, 0.30-2.00% Cr, at least one element selected from the group consisting of 0.01-0.50% W, 0.01-O.50% Mo, 0.01-0.50% V, and 0.01-0.50% Nb, and the balance iron with inevitable impurities is subjected to cold wiredrawing, to quench-and-temper treatment by inline, and further to cold wiredrawing at 10-30% reduction of area.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a steel wire for an ultra-high strength spring, and more particularly, for an ultra-high strength spring which can be suitably used for manufacturing a valve spring for an internal combustion engine or the like. The present invention relates to a method for manufacturing a steel wire.

[0002]

2. Description of the Related Art Steel wires for springs, which are strongly required to have fatigue resistance and sag resistance, such as valve springs, are typically
An oil tempered wire of Si-Cr steel is used. The tensile strength of such an oil-tempered wire is TSmax = 71d ^{-1 where} TSmax is the maximum tensile strength (Kgf / mm ² ) and the wire wire diameter is d (mm), as a schematic formula according to the wire diameter. It is given by ^{/ 2} +170. For example, for a wire with a diameter of 4 mm, the maximum tensile strength calculated based on the above formula is about 205 Kgf / mm ²
Is.

Even if an alloying element is added to Si-Cr steel, the maximum tensile strength obtained is limited to TSmax = 71d ^-1/2 +190. For example, for a wire with a diameter of 4 mm, the maximum tensile strength calculated based on the above formula is about 225 kgf / mm ²
Is.

[0004]

Generally, in a compression / tensile coil spring, the height H (mm) of the spring and the torsional stress τ (Kgf / Kgf / Kgf / Kgf /) generated in the wire when an axial force acts on the spring.
mm ² ) are respectively given by the following equations. H = A · Na · d (1) τ = 8 · P · d / (π · d ³ ) (2) where A is a constant, Na is the effective number of turns, and d is the wire diameter (m
m), P is the load applied to the spring (Kgf), and D is the average coil diameter (mm).

In general, by lowering the height of the valve spring, not only the valve spring but also the entire valve drive system and the block protecting it can be made lighter. In order to lower the value, it is necessary to reduce the effective number of turns Na and reduce the wire diameter d, as understood from the above formula (1). However, in this case, as is clear from the equation (2), the repeated stress acting on the spring becomes large, and the fatigue life of the spring is shortened.

On the other hand, in an engine or the like, it is known that increasing the engine speed is the most effective in order to improve the output. Therefore, it is often desired to increase the natural frequency of the spring used in the main part of the engine. According to JIS B 2704, the natural frequency f _I is given by the following formula f _I = 3.56 × 10 ⁵ · d / (Na · D ² ) (3), so the natural frequency f I _In order to increase _I , it is necessary to reduce the effective number of turns Na and increase the wire diameter of the spring.

However, since it is not desirable to increase the wire diameter d from the viewpoint of weight reduction, the wire diameter d is kept constant and the effective winding number Na is reduced. Since the cyclic stress acting on the spring increases, the fatigue life of the spring decreases. From the above description, it is basically necessary to improve the strength of the spring material in order to guarantee the fatigue life of the spring.

[0008] Therefore, an object of the present invention is to solve the above-mentioned various problems in the conventional spring steel and to provide a method for manufacturing an ultra-high strength steel wire for spring having high toughness and ductility.

[0009]

The method for producing a steel wire for an ultra-super high strength spring according to the present invention is (a) C 0.30-by weight%.
0.80%, Si 0.80-2.50%, Mn 0.10-1.
50%, Al 0.10% or less, and Cr 0.30 to 2.0
0%, (b) W 0.01-0.50%, Mo 0.01-
0.50%, V 0.01-0.50%, and Nb 0.01
A wire rod containing at least one element selected from the group consisting of .about.0.50% and the balance of iron and unavoidable impurities is cold-drawn, and then in-line quenched and tempered. After that, cold drawing is further performed at a surface reduction rate of 10 to 30%.

In the steel material used for manufacturing the ultra-high strength spring steel wire according to the present invention, C is an element necessary for obtaining the required strength, and in the present invention, at least 0.3.
It is necessary to add 0%. However, if added too much, it deteriorates the toughness, so the upper limit of the addition is 0.
80%. Si is effective for deoxidizing steel, and
When the obtained wire is oil tempered, the strength is greatly improved. In order to bring out these effects effectively, it is necessary to add at least 0.80%. However, when it is added in excess of 2.50%, it not only promotes decarburization and lowers the strength of the surface of the obtained wire, but Si is also a source of Al incorporation, so Increase the amount more than necessary. As described later, the increase in the amount of Al causes the formation of non-ductile inclusions and deteriorates the fatigue characteristics of the spring. Therefore, in the present invention, the Si content is 0.80 to
The range is 2.50%.

[0011] Mn has the effect of enhancing the hardenability of steel, and further has the effect of fixing S, which is harmful to the toughness and ductility of steel. However, when added in excess, it deteriorates the ductility of the treated steel and makes it difficult to obtain a high strength wire. Therefore,
In the present invention, the amount of Mn added is in the range of 0.10 to 1.50%. Al produces a large amount of Al ₂ O ₃ when it is added in excess of 0.10%. This Al ₂ O ₃ is not only non-ductile but also extremely hard,
This leads to early destruction of the spring. Thus, Al is an element to be suppressed as much as possible, and in the present invention, it is 0.10% or less.

[0012] Cr is effective in preventing decarburization,
In addition, the sag resistance of the spring during warming is improved. In order to effectively bring out these effects, it is necessary to add 0.30% or more. However, if added in excess of 2.00%, the toughness deteriorates, so the addition amount is 0.30 ~.
The range is 2.00%. In the present invention as well, in addition to the above-mentioned elements, at least one element selected from the group consisting of W, Mo, V and Nb can be added to the wire rod.

W forms a carbide in the steel and precipitates during tempering to impart fine precipitation strengthening. However, when the addition amount is less than 0.01%, the amount of precipitation is insufficient, and as a result, the above-mentioned strengthening effect is insufficient. However, even if added too much, the above effect is saturated. Therefore,
In the present invention, the amount of W added is in the range of 0.01 to 0.50%.

Mo also forms carbides in steel, precipitates fine alloy carbides during tempering, and promotes secondary hardening, thereby improving sag resistance and fatigue resistance. However, when the addition amount is less than 0.01%, the above-mentioned effect is insufficient, and on the other hand, even when it is added in excess, the above effect is saturated. Therefore, in the present invention, Mo is 0.
The range is from 01 to 0.50%.

V has the effect of improving the sag resistance by refining the crystal grains and increasing the proof stress ratio.
To obtain such an effect effectively, it is necessary to add 0.01% or more. However, when it is added in excess of 0.50%, the amount of alloy carbide that is not dissolved in austenite increases. , Remains as large lumps, and becomes nonmetallic inclusions, impairing fatigue resistance. Therefore, the upper limit of the amount of V added is 1.0%.

Similar to V, Nb also has the effect of improving the sag resistance by increasing the yield strength ratio. In order to effectively obtain such effects, it is necessary to add 0.01% or more. However, even if added in excess of 0.50%, the above effects saturate, and economically. However, the upper limit of the amount of Nb added is 0.5%. According to the present invention, a wire rod having a chemical component as described above is cold-drawn, and then subjected to quenching and tempering treatment in-line, and then cold-drawn at a surface reduction rate of 10 to 30%. By drawing, an ultra-high strength steel wire for spring is obtained.

Therefore, it is necessary that the wire has sufficient toughness and ductility. Therefore, the tensile strength TS (Kg / cm ² ) of the wire is d (mm). In this case, it is preferable to adjust the tensile strength so as to satisfy the relationship of 71d ^−1/2 +160 ≦ TS ≦ 71d ^−1/2 +190. Quenching and tempering is 30-50 ° C higher than the Ac ₃ temperature
After heating and holding at a high austenite temperature, oil quenching is performed, and then tempering is performed using lead or the like to obtain a predetermined strength.

According to the method of the present invention, after the quenching and tempering treatment is performed in this manner, the wire is pickled and surface-treated, and then cold wire drawing is performed in a range of 10 to 30% in area reduction. To do. As a result, it is possible to obtain a steel wire for spring which has high strength and is excellent in drawing. When the area reduction rate is less than 10%, the strength is not sufficiently improved, while when it exceeds 30%, the toughness and ductility of the wire deteriorates.

As described above, the tensile strength of the wire is T
S (Kg / cm ² ), wire diameter d (mm), and area reduction rate ε (true strain), 71d ^-1/2 +160 + (80 to 90) ε TS ≤ 71d ^{-1 /} It is possible to obtain a steel wire for an ultra-super high strength spring that satisfies ² +190 + (80 to 90) ε (4).

[0020]

As described above, according to the method of the present invention,
A spring steel wire excellent in toughness and ductility can be obtained while having a high strength of 220 Kgf / mm ² or more.

[0021]

The present invention will be described below with reference to examples.
The present invention is not limited to these examples. A wire rod having the chemical composition shown in Table 1 was cold-drawn, quenched and tempered in-line, and further cold-drawn at an area reduction ratio shown in Table 1 to obtain a spring steel wire. .. Quenching
Table 1 shows the mechanical properties of the tempered material and the spring steel wire obtained by cold drawing.

[0022]

[Table 1]

Comparative Example A1 has a tensile strength of 230 Kgf / by cold drawing and subsequent in-line quenching and tempering.
obtained by a mm ^2, the diaphragm is low. In Comparative Example A2, after quenching and tempering, cold drawing was performed at a surface reduction rate of more than 30%, so that the deterioration of the drawing was remarkable. On the other hand, according to the present invention, it is possible to obtain a steel wire for spring having high strength and ductility toughness.

Claims (1)

[Claims] 1. A weight percentage of (a) C 0.30 to 0.80%, S
i 0.80 to 2.50%, Mn 0.10 to 1.50%, Al
0.10% or less and Cr 0.30 to 2.00%, (b)
W 0.01 to 0.50%, Mo 0.01 to 0.50%, V
Cold drawing of a wire rod containing at least one element selected from the group consisting of 0.01 to 0.50% and Nb 0.01 to 0.50% and the balance iron and unavoidable impurities. Then, after performing quenching and tempering treatment in-line, cold wire drawing is further performed at a surface reduction rate of 10 to 30%, and a method for producing a steel wire for ultra-high strength springs. ..