JPH07228945A - High strength spring steel excellent in corrosion resistance - Google Patents

Abstract

PURPOSE:To produce a spring steel, capable of providing a high strength spring having superior corrosion resistance while maintaining conventional strength, by incorporating specific percentages of C, Si, Mn, and Al into iron. CONSTITUTION:A spring steel, having a composition consisting of, by weight, 0.3-0.8% C, 0.1-4% Si, 0.1-2% Mn, 0.1-2% Al, and the balance iron with inevitable impurities and containing, if necessary, 0.01-1% Cu, 0.1-4% Ni, 0.1-5% Cr, 0.1-2% Mo, 0.1-0.5% V and/or 0.01-0.5% Nb, or 0.1-5% Co, is prepared. By this method, the spring steel for producing a high strength spring increased in corrosion resistance and improved in corrosion fatigue characteristic can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to steel for high strength springs used for valve springs and suspension springs of internal combustion engines such as automobile engines, and more particularly to high strength steels having improved corrosion resistance and improved corrosion fatigue properties. The present invention relates to spring steel for manufacturing springs.

[0002]

[Prior Art] The chemical composition of spring steel is JIS G3565-356.
74801, etc., rolled materials manufactured from them are drawn to a specified wire diameter, then oil tempered and then spring processed (cold working), or rolled materials are drawn. Various springs are manufactured by processing, heating and spring forming, and then quenching and tempering (hot working). In recent years, as the demand for higher strength of springs has become more and more strict, there is concern about deterioration of corrosion fatigue characteristics, and alloy steels containing various alloy elements have been used to solve this. Therefore, there is a problem that the material cost becomes very high and the manufacturing process becomes so complicated.

[0003]

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and an object thereof is to keep the strength and hardness of the wire constituting the spring as usual and to endure it. It is an object of the present invention to provide a spring steel that provides a high-strength spring with further improved corrosion resistance represented by corrosion fatigue characteristics.

[0004]

The high-strength spring steel of the present invention capable of achieving the above object is C: 0.3 to 0.8%, S
i: 0.1-4%, Mn: 0.1-2%, Al: 0.1
Each of them has a gist of being contained in the balance of iron and inevitable impurities.

[0005]

It is known that increasing the strength of a material increases the susceptibility to defects. This also applies to springs, and it is feared that if the stress is increased, the susceptibility to defects such as inclusions and flaws is increased, and the durability life of the spring is shortened. In a corrosive environment (for example, in the atmosphere,
Even in springs used in seawater, mud, etc., it is not easy to increase the strength.

The cause of the decrease in corrosion fatigue life is that when used in a corrosive environment, corrosion pits are formed on the surface of the material, and the corrosion pits gradually grow (especially grow in the depth direction), which is a stress concentration source. This is considered to be the place where fatigue cracks occur. When repeatedly subjected to stress in a corrosive environment, a new surface due to slippage appears on the material surface,
Corrosion pits are likely to be preferentially formed in that portion due to the local battery effect, and eventually become large pits. In order not to reduce the corrosion fatigue life, it is necessary to suppress the formation of corrosion pits or to slow the formation / growth rate of corrosion pits, that is, to improve the corrosion resistance.

[0007] Therefore, the present inventors have made repeated studies from another viewpoint to improve the corrosion resistance of the steel for springs, without being caught by the existing concept. As a result, they have found that the corrosion resistance of spring steel can be remarkably improved by adjusting Al in the steel material to an appropriate range, and completed the present invention. The reason for limiting the chemical composition in the high-strength spring steel according to the present invention is
It is as follows.

C: 0.3 to 0.8% C is an element necessary to secure the strength after quenching and tempering. If the C content is less than 0.3%, the strength after quenching and tempering is insufficient. Also, if added in excess of 0.8%, not only will the toughness after quenching and tempering deteriorate, but
The desired fatigue characteristics and sag resistance cannot be obtained.

Si: 0.2-4% Si is necessary as a solid solution strengthening element, and if it is less than 0.2%, the strength of the matrix becomes insufficient. Further, the sag resistance, which is one of the spring characteristics, deteriorates. However, 4%
If it is added in excess, the melting of carbide during heating during quenching becomes insufficient, and if it is not heated to a high temperature, not only austenite does not form uniformly, the strength after quenching and tempering decreases, but also the sag resistance of the spring deteriorates.

Mn: 0.1 to 2% Mn is required to be 0.1% or more as a hardenability improving element. However, if added in excess of 2%, there is residual austenite after quenching and tempering, which rather decreases the strength.

Al: 0.1 to 2% It is known that Al is used as a deoxidizing element. It is also known that it has an effect of refining crystal grains. As a result of studying such Al, the present inventors have found that Al is very effective as an element for improving the corrosion resistance of spring steel as described above. The reason why such an effect is obtained by adding Al is as follows.
Although not all of them have been clarified, it is considered that this is probably because addition of Al achieves densification and homogenization of the rust layer formed on the surface of the steel material. Such an effect cannot be achieved when the Al content is less than 0.1%, and excessive addition exceeding 2% causes the steel structure to become mixed grains during quenching and tempering and ductility after quenching and tempering. Becomes extremely low. The preferable range of Al is about 0.2 to 1.0%.

The spring steel of the present invention contains the above elements as basic components, and the balance is Fe and inevitable impurities, but if necessary, Cu, Ni, Cr, Mo, V, N.
It is also effective to contain b, Co or the like. The reasons for limiting the chemical components when these elements are contained are as follows.

Cu: 0.01 to 1% Cu is an element that is electrochemically nobler than Fe, and has the action of enhancing corrosion resistance by densifying the rust that is generated. These effects are exhibited with the addition of 0.1%,
Even if added in excess of 1%, no further effect can be obtained, and rather the material may become brittle during hot rolling.

Ni: 0.1 to 4% Ni has the effect of improving the toughness of the material after quenching and tempering, and also has the effect of significantly improving the sag resistance, which is important as spring characteristics. In order to exert these effects effectively, at least 0.1% or more must be contained. However, when the content exceeds 4% and is excessively contained, the Ms point is lowered, and the predetermined tensile strength cannot be obtained due to the influence of the retained austenite.

Cr: 0.1 to 5% Cr, like Mn, is an element effective for improving hardenability.
Cr is also an element that improves heat resistance. These effects are effectively exhibited by containing 0.1% or more, but if the amount is too large, the toughness after quenching and tempering tends to decrease, so the upper limit was set to 5%.

Mo: 0.1 to 2% Mo is a carbide-forming element, which improves the sag resistance and fatigue resistance by precipitating fine alloy carbides after tempering and promoting secondary hardening. If the Mo content is less than 0.1%, the effects are insufficient, and if it exceeds 2%, the effects are saturated.

V: 0.01 to 0.5% and / or N
b: 0.01 to 1% V and Nb are effective in refining the crystal grain size to improve the yield strength ratio and improving the sag resistance of the spring. In order to exert this effect effectively, it is necessary to add 0.01% or more in all cases. However, when V is added in excess of 0.5% and Nb is added in excess of 1%, the amount of alloy carbide that is not dissolved in austenite during quenching and heating increases and remains as large lumps, which reduces fatigue life. Let

Co: 0.1 to 5% Co is a solid solution strengthening element, has the characteristic of not reducing toughness, and also has the function of increasing corrosion resistance. These effects are exhibited by the addition of 0.1% or more, but the upper limit was set to 5% in consideration of saturation of the effects and an expensive element.

The present invention will be described in more detail with reference to the following examples, but the following examples are not intended to limit the present invention and can be carried out with appropriate modifications within a range that is compatible with the gist of the preceding and the following. All of these are included in the technical scope of the present invention.

[0020]

[Example] As shown in Table 1, the current JISSUP6, J
20 mm (W) x 50 mm (L) x 3 mm by ISSUP7, JISSUP12, JISSUP9, and alloy type spring steels with various amounts of Al added after vacuum melting, hot rolling, quenching and tempering, and grinding.
A plate state test piece of (T) was prepared. At this time, the quenching heating temperature was 950 ° C. × 15 min for oil quenching, and the tempering temperature was 400 ° C. (1 hour). Quenching
Table 2 shows the tensile strength and the drawing value after tempering. Al
The steel added with 2.5% (No. 14) is
It can be seen that the aperture value is extremely low.

With respect to the obtained test piece, (8 hours of salt water spraying → 16 hours of drying) was set as one cycle, and the depth of the corrosion pit was measured after 7 cycles of the cycle. The results are also shown in Table 2. Compared with the comparative example in which Al is not added (No. 1 to 8), the example of the present invention in which Al is added (No. 9 to 13) is compared. It can be seen that the progress of the corrosion pits has been suppressed.

[0022]

[Table 1]

[0023]

[Table 2]

[0024]

The present invention is constructed as described above, and it is possible to provide a spring steel capable of obtaining a high-strength spring having excellent corrosion resistance while maintaining the conventional strength. .

(72) Inventor Takenori Nakayama 1-5-5 Takatsukadai, Nishi-ku, Kobe-shi, Hyogo Inside Kobe Research Institute of Kobe Steel, Ltd. (72) Inventor Shigeaki Miyauchi 1-chome, Takatsuka-shi, Nishi-ku, Kobe No. 5-5 Inside Kobe Research Institute, Kobe Steel, Ltd.

Claims (7)

[Claims] 1. C: 0.3-0.8% (meaning% by weight,
The same shall apply hereinafter), Si: 0.1 to 4%, Mn: 0.1 to 2
%, Al: 0.1 to 2%, respectively, and a balance of iron and inevitable impurities, which is a high-strength spring steel excellent in corrosion resistance. 2. The high-strength spring steel according to claim 1, which further contains Cu: 0.01 to 1%. 3. The high-strength spring steel according to claim 1, further comprising Ni: 0.1 to 4%. 4. The high strength spring steel according to claim 1, further comprising Cr: 0.1 to 5%. 5. The high-strength spring steel according to claim 1, which further contains Mo: 0.1 to 2%. 6. The high-strength spring according to claim 1, further comprising V: 0.1 to 0.5% and / or Nb: 0.01 to 0.5%. For steel. 7. The high-strength spring steel according to claim 1, further containing Co: 0.1 to 5%.