JPS6338419B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a spring steel with excellent resistance to settling. In recent years, as part of efforts to reduce the weight of automobiles, there has been a strong demand for lighter suspension springs. In response to this demand, it is considered effective to increase the design stress of the spring and use it in a high stress state to reduce the weight. However, when current spring steel is used under high stress, problems arise in terms of durability and increased sagging. This results in a reduction in the height of the bumper, which poses a major safety problem. Therefore, in recent years, there has been a demand for spring steel with excellent fatigue resistance that enables high-stress designs. Traditionally, spring steels with excellent resistance to fatigue include:
As it is known that Si in spring steel is an effective element for fatigue resistance, Si is used even more than SUP6.
SUP7, which has a high volume, is becoming more and more used. However, there are strict requirements for reducing the weight of suspension springs, and there has been a strong desire to develop a steel for springs that is even more resistant to fatigue than SUP7. Against this background, the applicant first developed a high-Si
By adding appropriate amounts of one or two types of V and Nb to spring steel, it has even better fatigue resistance than SUP7, and has the same performance as SUP7 in terms of fatigue resistance and toughness required for spring steel. Developed and applied for spring steel with
No.). Against this background, the present invention was developed as a result of repeated research by the present inventors, and was developed by adding an appropriate amount of V to high-Si spring steel.
Add one or two types of Nb and increase the amount of C from 0.35 to 0.45%
For springs with excellent fatigue resistance, which enables water quenching by lowering the hardness and improves workability, and also facilitates material processing by cutting and grinding to remove surface decarburization and surface flaws during spring manufacturing. This was the successful development of steel. In addition, the present invention further improves the settling resistance by adding Al that refines the grain size, and improves the hardenability by adding B.
The present invention improves not only the hardenability but also the toughness by adding
It also has excellent toughness and has the same performance as SUP7 in terms of fatigue resistance, which is necessary for spring steel. The present invention will be explained in detail below. The first invention steel has a weight ratio of C 0.35 to 0.45.
%, Si 1.50-2.50%, Mn 0.50-1.50%, V
0.05-0.50%, Nb 0.05-0.50%, and the second invention steel further contains Al 0.03-0.10% in the first invention steel to improve the fatigue resistance of the first invention steel. The third invention steel has further improved B 0.0005 to 0.0100 than the first invention steel.
%, Ni 0.20 to 2.00% to improve the hardenability and toughness of the first invention steel.
The hardenability and toughness of the second invention steel are improved by containing one or two of 0.0005 to 0.0100% and 0.20 to 2.00% Ni. The mechanism for improving the settling resistance of V and Nb in the steel of the present invention will be explained below. V and Nb form carbides in steel, and these V carbides and Nb carbides (hereinafter referred to as alloy carbides)
is dissolved in austenite during heating during quenching, and is dissolved as a supersaturated solid solution in martensite by quenching. When this is tempered, fine alloy carbides re-precipitate during the process, causing secondary hardening.
This works to improve the resistance to sag by blocking the movement of dislocations in the steel. In addition, alloy carbides that are not dissolved in austenite during heating during quenching refine the austenite crystal grains and prevent them from becoming coarser. The grain boundaries refined in this manner improve the settling resistance by reducing the amount of movement of dislocations. Furthermore, by containing Nb and V, the steel of the present invention can be heated to a temperature of 900℃, which is the quenching temperature of ordinary spring steel.
Even in the case of hardening, redeposition occurs during the subsequent tempering process, resulting in secondary hardening. This means that when aiming for the same tempering hardness range, it is possible to set the tempering range to a wider range compared to conventional steel, and the targeted hardness can be stably obtained. Furthermore, Al is a grain refining element that often forms nitrides in steel, and these nitrides can prevent austenite grains from becoming coarse during heating during quenching. Such fine crystal grains improve the resistance to settling by reducing the amount of movement of dislocations. Furthermore, among B and Ni, which improve hardenability,
In particular, B is an element that is effective for resistance to settling. That is, atomic B forms a solid solution in the crystals of steel as an interstitial type, and is particularly likely to enter near dislocations. Since the dislocations into which B has invaded in this way become difficult to move, they are effective in reducing fatigue. Furthermore, even in thick coil springs, torsion bars, and thick stacked leaf springs used in relatively large automobiles, by adding elements that improve the hardenability of B and Ni, martin is added to the core during heat treatment. A site structure can be obtained without impairing the resistance to settling. The reasons for limiting the composition of the steel of the present invention will be explained below. The reason for setting the C content to 0.35 to 0.45% is that if it is less than 0.35%, sufficient strength cannot be obtained as steel for high stress springs through quenching and tempering. This is because there is a risk of cracking and it becomes difficult to grind or cut the material during spring manufacturing. The reason why the amount of Si is set at 1.50 to 2.50% is because if it is less than 1.50%, the effect of increasing the strength of the base material and improving the resistance to settling cannot be obtained sufficiently by solid solution of Si in the ferrite. This is because even if the content exceeds 2.50%, the effect of improving the resistance to settling is saturated, and there is a risk that free carbon may be generated by heat treatment. The reason for setting the Mn content to 0.50 to 1.50% is that if it is less than 0.50%, the strength as a spring steel is insufficient, and the hardenability is also insufficient.If the Mn content exceeds 1.50%, the toughness This is to inhibit the V,
Nb is an element that improves the sag resistance in the steel of the present invention. The reason why we set the content of V and Nb, which have these functions, at 0.05 to 0.50% each is because the above effects cannot be obtained sufficiently below 0.05%.
Even if the content exceeds %, the effect will be saturated, and the amount of alloy carbides that are 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 and Nb can be added individually or by adding the two in combination.
Compared to the case where Nb is added alone, dissolution into austenite starts at a lower temperature, and the precipitation of fine alloy carbides during the tempering process further promotes secondary hardening, further improving the fatigue resistance. It is something that improves. In the steel of the present invention, Al is an element that refines the crystal grains and improves the sag resistance. Al0.03~0.10%
This is because if the content is below the lower limit, the effect of improving the settling resistance is insufficient, and if the content exceeds the upper limit,
This is because the amount of Al nitride increases and becomes large lumps, which may reduce the fatigue strength of the steel due to the action of non-metallic inclusions. B and Ni are elements that improve hardenability. The amount of B is 0.0005% to 0.0100%, which means 0.0005%.
This is because the hardenability improvement effect and the setting resistance effect cannot be sufficiently obtained if the content is less than 0.0100%, and if the content exceeds 0.0100%, boron compounds will precipitate and hot embrittlement will appear. Ni is an element that improves hardenability and toughness in the steel of the present invention. The reason for setting Ni to 0.20 to 2.00% is that if it is less than 0.20%, the effect of improving hardenability and toughness is insufficient.
This is because even if the content exceeds %, the effect may be saturated and a large amount of retained austenite may be formed. Next, the characteristics of the steel of the present invention will be clarified through examples in comparison with conventional steel.

【table】

[Table] In Table 1, A1 to A3, A21, and A22 steels are the first invention steels, A4 steel is the second invention steel, A7, A9,
A10, A23, A24 steel is the third invention steel, A11, A12
The steel is the fourth invention steel, and the B1 steel is a conventional steel with SUP7. Table 2 shows A1 to A4, A21, and A22 of the test steels in Table 1 that were hot-rolled at a rolling ratio of 50 or higher after casting.
A coil spring with the specifications shown in Table 2 is formed using steel, B1 steel as a material, and the final hardness is HR C45 to 55.
After quenching and tempering the wire, the wire was set so that the shear stress τ = 115 Kg/ ^mm2 , and the test specimen was sheared at a constant temperature of 20℃. Add a load that makes stress τ = 105Kg/mm ² , 96
The amount of fatigue of the coil spring after the passage of time (hereinafter referred to as long-term load) was measured.

[Table] Figures 1 and 2 show the amount of set in relation to the hardness of the test piece. As is clear from Figures 1 and 2, A1 to A4, A21, and A22 steels, which are the steels of the present invention, are made by adding V and Nb, reducing the amount of C to 0.35 to 0.45%, and adding Al as necessary. It is recognized that all of these steels have superior fatigue resistance compared to the conventional steel B1 steel. It can be seen that among the steels of the present invention, the steel to which V and Nb are added in combination has even better resistance to settling than the steel to which V is added alone. The amount of setback is the load required to compress the coil spring to a certain height before applying the long-term load.
Measure P ₁ and the load P ₂ required to compress to the same height after applying the long-term load, and calculate from the difference △P (=P ₁ − P ₂ ) using the following formula. It has a unit of shear strain, and was evaluated using a value called residual shear strain. γR=1/G・K8D/πd ³ △PG G: Transverse elastic modulus (kgf/mm ² ) D: Coil center diameter (mm) d: Wire diameter (mm) K: Whirl correction coefficient (shape of coil spring In addition, A1 to A4, A21, A22 steel, B1 of the steel of the present invention
A fatigue test was conducted on steel coil spring wires having the same specifications as above, by repeatedly subjecting them to loads varying in shear stress from 10 to 110 kg/ ^mm2 . There was no breakage. Next, using the A7, A9~A12, A23, and A24 steel test steels as raw materials, we manufactured a torsion bar with a parallel part diameter of 30 mmφ having the specifications shown in Table 3, and the final hardness was HR C45~55. After quenching and tempering the specimen, it was subjected to shot peening treatment to obtain a set test piece. Prior to the settling test, a torque was applied to both ends of the specimen so that a shear stress τ = 110 Kgf/mm ² appeared on the surface of the parallel portion of the specimen, and setting was performed. After setting, a torque was applied to give a shear stress τ = 100Kgf/mm ² , and it was left as it was for 96 hours, and then the amount of residual shear strain was determined from the decrease in twist angle according to YR = △θ・d/2l. .

[Table] Among the sample steels, A7, A9, A10, A23,
Figure 5 shows the geominy curves of A24 steel and B1 steel. As is clear from Figure 5, the hardenability of A7, A9, A10, A23, and A24 steels containing B and Ni is dramatically improved compared to B1 steel that does not contain B and Ni. I understand that. Further, among the test steels, A4 steel and B1 steel were heated at a quenching temperature of 850 to 1100°C and measured by an oxidation method, and the austenite grain size is shown in FIG. As is clear from Figure 6, V and
A4 steel containing Al is B1 steel that does not contain them.
It has superior austenite grain size compared to steel. Furthermore, A7, A9 to A12, A23, which are the steels of the present invention,
Shear stress 60±50Kgf/mm ² for the above torsion bar made from A24 steel and B1 steel, which is conventional steel.
As a result of conducting a fatigue test by applying repeated loads, it was confirmed that none of the torsion bars broke even when the loads were applied repeatedly 200,000 times, and that the addition of B had no effect on fatigue life. As mentioned above, the steel of the present invention is made by adding appropriate amounts of V and Nb alone or in combination to the conventional steel for high-Si springs.
The content is reduced to 0.35-0.45%, and it also contains Al as necessary, and one or two of B and Ni, improving the setting resistance and hardenability of conventional high-Si spring steels. This material has significantly improved the steel's properties, enables water quenching, greatly improves workability, and has succeeded in making cutting and grinding easier during spring manufacturing. The fatigue resistance and toughness of this steel are comparable to those of conventional steels, and it has extremely high practicality, especially as a steel for passenger car suspension springs.

[Brief explanation of the drawing]

Figures 1 to 4 are graphs showing the amount of settling of test specimens with hardness of HR C45 to 55 after quenching and tempering for the steel of the present invention and conventional steel.
A diagram showing the hardenability of A23, A24, and B1 steel,
FIG. 6 is a diagram showing the austenite grain size when A4 and B1 steels are heated at a quenching temperature of 850 to 1100°C.

Claims (1)

[Claims] 1. C 0.35~0.45%, Si 1.50~
2.50%, Mn 0.50-1.50%, and V
1. A spring steel with excellent fatigue resistance characterized by containing one or two of 0.05 to 0.50% Nb and 0.05 to 0.50% Nb, with the remainder substantially consisting of Fe. 2 C 0.35~0.45%, Si 1.50~
2.50%, Mn 0.50~1.50%, V 0.05~0.50%
and 0.03 to 0.10% of Al, with the remainder substantially consisting of Fe. A spring steel with excellent fatigue resistance. 3 C 0.35~0.45%, Si 1.50~
2.50%, Mn 0.50-1.50%, V 0.05-0.50% or V 0.05-0.50%, Nb 0.05-0.50%, and further contains one or two of B 0.0005-0.0100% and Ni 0.20-2.00%. and the remaining substantially Fe
Spring steel with excellent fatigue resistance. 4 C 0.35~0.45%, Si 1.50~
2.50%, Mn 0.50-1.50%, V 0.05-0.50% or V 0.05-0.50%, Nb 0.05-0.50%, Al 0.03-0.10%, B 0.0005-0.0100%,
A spring steel with excellent fatigue resistance, characterized by containing 0.20 to 2.00% Ni, with the remainder substantially consisting of Fe.