JPS5941502B2 - Spring steel with excellent fatigue resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to copper for springs having excellent resistance to fatigue.

Conventionally, SUP6 and SUP9 have been the main spring copper used in suspension systems for automobiles and the like.

In recent years, there has been a demand for lighter automobiles, and there has also been a strong demand for lighter suspension systems themselves. To deal with this, various methods have been tried for suspension systems in general, but
Among them, means to increase the design stress of the spring are considered to be effective. As described above, with the high stress design, when a spring is manufactured using the above-mentioned conventional spring material, a problem arises in that the set-off increases. Particularly when used in passenger cars, increased sag leads to a reduction in bumper height, posing a serious safety problem. As a result of various studies, it was discovered that increasing the Si content in spring steel improves the fatigue properties. Among the steels for springs, SUP7, which has the highest Si content, has come to be widely used as copper for suspension springs for passenger cars. However,
There are strict requirements for reducing the weight of suspension springs, and S
There has been a strong desire to develop copper for springs that is even more resistant to fatigue than UP7.

Against this background, as a result of repeated research by the present inventors, the present invention has been developed by adding appropriate amounts of one or more of V, Nb, and Mo to copper for high-Si springs. We have developed copper for springs that has excellent fatigue resistance and also has the same performance as SUP7 in terms of fatigue resistance and toughness required for copper for springs. V, Nb, and Mo form carbides in steel, and vanadium carbide, niobium carbide, and molybdenum carbide (hereinafter referred to as alloy carbides) are dissolved in austenite during heating during quenching.

When this is rapidly cooled and quenched, martensite containing these elements in a supersaturated solid solution is obtained. When this is tempered, fine alloy carbides begin to re-precipitate during the process, which blocks the movement of dislocations in the steel and causes secondary hardening, resulting in V,
It works to increase the hardness compared to copper for springs that does not contain Nb or Mo, and also improves the resistance to fatigue. In addition, alloy carbides that are not dissolved in austenite during heating during quenching can refine austenite crystal grains and prevent them from becoming coarser. Moreover, such fine crystal grains improve the resistance to settling by reducing the amount of movement of dislocations. In addition, the fact that secondary hardening occurs as mentioned above means that when aiming for the same tempering hardness range, it is possible to set the tempering temperature range to a wider range compared to conventional steel, and the desired hardness can be achieved. This means that a stable amount of energy can be obtained.

To clarify this further, the 0.20%
A2 steel containing V of 0.25, V of 0.22
A6 steel containing Nb, 0.23% V, 0.2
Figure 1 shows the results of tempering A9 and SUP7 Bl steel containing 1% (7) Nb and 0.22% MO at 300 to 600°C and measuring their hardness. Indicated. The figure also shows the spring hardness range H specified in the automobile standard JASO72ll "Automotive suspension coil springs".
The range corresponding to B4l5-495 is also shown. V,Nb
As is clear from FIG. 1, the tempering temperature range corresponding to hardness of the A2 steel, A6 steel, and A9 steel of the present invention containing appropriate amounts of MO is found to be wider than that of conventional steels. Next, FIG. 2 shows the hardenability of the above-mentioned A2 steel, A6 steel, A9 steel, and Bl steel.

As is clear from FIG. 2, A9 steel containing 0.22% MO has superior hardenability compared to A2 steel, A6 steel, and Bl steel. The chemical composition of the steel of the present invention is CO. 5O~0.80%, Sil. 5O~2.50%,
MnO. 5O to 1.50, and VO. O5~
0.50%, NbO. O5-0.50 section, MOO. O5
-0.50% of one kind or two or more kinds, and the rest consists essentially of Fe.

The reasons for limiting the composition of the steel of the present invention will be explained below.

The reason why the amount of C is set to 0.50 to 0.80% is because if it is less than 0.50%, sufficient strength as a high stress spring steel cannot be obtained by quenching and tempering. This is because if the content exceeds the S0 coefficient, the steel will become hypereutectoid and the toughness will be significantly lowered.

The reason why the amount of Si is set to 1.50 to 2,50 is that if it is less than 1.50, Si dissolves in the ferrite, which increases the strength of the substrate and improves the resistance to settling. This is because even if the content exceeds 2.50%, the effect of improving the sagging resistance will be saturated, and there is a risk that free carbon will be generated by heat treatment.

The reason why the Mn content is set to 0.50-1.50% is because if the modulus is less than 0.50, the strength as a spring steel is insufficient, and the hardenability is also insufficient. This is because if the content exceeds the range, the toughness will be impaired.

V, Nb. Both MOs are elements that improve the sag resistance in the steel of the present invention.

The reason why the content of V, Nb, and MO, which have such functions, is set at 0.05 to 0.50 is that the above effects cannot be obtained sufficiently if the content is less than 0.05. 5
Even if the content exceeds 0%, the effect will be saturated, and the amount of alloy carbide that is not dissolved in austenite will increase, forming large lumps, which may reduce the fatigue strength of steel due to the action of nonmetallic inclusions. This is because there is.

These V, Nb, and MO can be added individually, or by adding two or three types in combination.
b. Compared to the case where MO is added alone, it starts dissolving into austenite at a lower temperature, and the precipitation of fine alloy carbides during the tempering process promotes secondary hardening, making it more resistant to fatigue. This will further improve the quality of the product.

Next, the characteristics of the steel of the present invention will be clarified through examples in comparison with conventional steel. Table 1 shows the chemical composition of these test steels.

In Table 1, A1 to A9 steels are steels of the present invention, and B1:B2
The steel is conventional steel and is SUP7.

Table 2 shows the test steels shown in Table 1 which were hot rolled at a rolling ratio of 50 or higher after casting, and the test steels were as follows: Tl8Okgf/Mt? The 0.2% proof stress, elongation, reduction of area, and impact value and bending strength are shown after quenching and tempering at a temperature at which a tensile strength of approximately t is obtained.

For tensile strength, 0.2% proof stress, elongation, and area of area, I8
It was measured using a No. 4 test piece, and the impact value was JI
Measured using a No. 83 test piece, and the torsional strength was measured using a No. 83 test piece.
This was measured using a test piece of rn7IL circle.

As is clear from Table 2, A1 to A9 are the steels of the present invention.
Steels include those containing V, Nb, and MO alone, and those containing V, Nb, and MO.
B and B, which are conventional steels with composite addition of MO.
1, the value is equal to or higher than that of the B2 steel, and almost no difference is observed between the steels tested in the steel of the present invention.

Next, a coil spring having the specifications shown in Table 3 is formed using the above-mentioned test steel as a raw material, and after being quenched and tempered to a final hardness of HRC 45 to 55, the shear stress of the strands is Setting was added so that τ2 was 115 kg/1 to prepare a fatigue test piece.

Then, a load was applied to this test piece at a constant temperature of 20°C to give a shear stress τ2 of the wire of 105 kg/MA, and after 96 hours (hereinafter referred to as long-term load), the amount of fatigue of the coil spring was was measured. The amount of set in relation to the hardness of the test piece is shown in FIGS. 3 to 6.

As is clear from Figures 3 to 6, V, Nb, which is the steel of the present invention,
, A1 to A5 steels with MO added alone, V, Nb, MO
It is recognized that all of the 86 to A9 steels to which B1 is added in combination have superior sag resistance compared to Bl steel, which is a conventional steel. It can be seen that among the steels of the present invention, the steel to which V, Nb, and MO are added in combination has even better resistance to stagnation than the steel to which V, Nb, and MO are added alone. In addition, the amount of settling is measured by the load P1 required to compress the coil spring to a certain height before applying the long-term load, and the load P2 required to compress the coil spring to the same height after applying the long-term load. It was calculated using the following formula from the difference ΔI) (=P, -P2), which has the unit of shear strain, and was evaluated as a value in seconds of the residual shear strain.

d: Wire diameter<Mm) K: Whirl correction coefficient (constant determined by the shape of the coil spring) and A1 to A9 of the steel of the present invention
As a result of conducting a fatigue test by repeatedly applying a load varying in shear stress of 10 to 110 kgf/MA to coil spring wires having the same specifications as above for steel and Bl steel, all coil springs were tested 200,000 times. It did not break even after repeated use.

As mentioned above, the steel of the present invention has an appropriate amount of V compared to conventional high-Si spring steel.
, Nb, and MO, either singly or in combination, have succeeded in further improving the excellent fatigue resistance of conventional high-Si spring steels, and have achieved the fatigue resistance necessary for spring steels. In terms of toughness, it is comparable to conventional steels, and has extremely high practicality, especially as a steel for suspension springs for passenger cars.

[Brief explanation of the drawing]

Figure 1 shows the steel of the present invention and the conventional steel after being quenched.
This is a diagram showing the hardness of tempered steel of 600 C. Figure 2 is a diagram showing the hardenability of the steel in Figure 1, and Figures 3 to 6 are diagrams showing the hardness of the steel of the present invention. And H after quenching and tempering of conventional steel? It is a diagram showing the amount of settling of test pieces with hardness of C45 to C55.

Claims (1)

[Claims] 1 C0.50-0.8% by weight, Si1.50
~2.50%, Mn0.50~1.50%, further V0.05~0.50%, Nb0.05~0.50
%, Mo0.05 to 0.50%, and the remainder is substantially Fe.