JPS6237108B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a spring steel with excellent resistance to fatigue. 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 reaction conditions, problems arise such as decreased durability and increased sag. This appears as a decrease in the bumper height, 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 CUP6.
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 more of V, Nb, and Mo to spring steel, it has even better fatigue resistance than SUP7, and also has the same fatigue resistance and toughness as SUP7, which is necessary for spring steel. Developed and applied for spring steel with equivalent performance (patent application 1982-108020)
No.). Against this background, the present invention was developed as a result of repeated research by the inventors and others, and was developed by adding an appropriate amount of V or V and Nb to high-Si spring steel, as well as adding A1 to refine the grain size. As a result, we succeeded in further improving the resistance to sagging. In addition, the present invention improves hardenability by adding B as needed, and improves toughness in addition to hardenability by adding Ni. It also has excellent hardenability and 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 C0.50 to 0.80% by weight,
Si1.50~2.50%, Mn0.70~1.50%, V0.20~0.50
% or V0.20-0.50%, Nb0.05-0.15%, and Al0.03-0.10%,
The second invention is B0.0005 to 0.0020 in addition to the first invention.
% and 0.20 to 0.50% of Ni to improve the hardenability and toughness of the first invention steel. 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) are dissolved in austenite during heating during quenching, and become a supersaturated solid solution in martensite by quenching. . When this is tempered, fine alloy carbides re-precipitate during the process, causing secondary hardening, which works to improve the resistance to settling by inhibiting 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 temperature range to a wider range compared to conventional steel, and the targeted hardness can be stably obtained. Furthermore, Al is a grain refining element and 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 make it difficult to roll, 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 hardenability-improving elements such as 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 why the C content is set at 0.50 to 0.80% is that if it is less than 0.50%, sufficient strength cannot be obtained as a steel for high stress springs through quenching and tempering. This is because the decrease in toughness becomes significant. 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 0.70 to 1.50% is because if it is less than 0.70%, hardenability is insufficient and sufficient strength cannot be obtained, and if it is contained in excess of 1.50%, toughness will be inhibited. . Both V and Nb are elements that improve the sag resistance in the steel of the present invention. Regarding the content of V and Nb, which have these functions, we set the content to 0.20 to 0.50% for V and 0.05 to 0.15% for Nb because the above effects are sufficient when V is less than 0.20% and Nb is less than 0.05%. However, even if W exceeds 0.50% and Nb exceeds 0.15%, the effect is saturated, and the amount of alloy carbides that are not dissolved in austenite increases, resulting in large lumps. This is because the fatigue strength of the steel may be reduced due to the action of nonmetallic inclusions. These V and Nb can be added individually or by adding the two in combination.
Compared to when Nb is added alone, dissolution into austenite starts at a lower temperature, and the precipitation of fine alloy carbides during the tempering process is
By further promoting secondary curing, the sagging resistance is further improved. Al forms fine nitrides in steel, which prevent the movement of austenite grain boundaries, thereby achieving refinement of grain size. The reason why Al is set at 0.03 to 0.10% is because if it is below the lower limit, it is not possible to obtain enough nitride to achieve sufficient grain refinement, and even if it is added above 0.10%, the grain refinement effect will not be obtained. This is because as it becomes saturated, the nitride becomes coarser, which is unfavorable from the viewpoint of durability. B is an element that improves hardenability. The amount of B is 0.0005% to 0.0020%, which means 0.0005%.
This is because the effect of improving hardenability and the effect of improving setting resistance cannot be sufficiently obtained if the content is below 0.0020%, and if the content exceeds 0.0020%, 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 why Ni is set at 0.20 to 0.50% is because steel made in an electric furnace normally contains about 0.110% Ni as an impurity, and in the present invention, if it is less than 0.20%, it has an effect on improving hardenability and toughness. is insufficient,
This is because even if the content exceeds 0.50%, the effect will be saturated and there is a risk of forming retained austenite. 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.

[Table] In Table 1, A1, A3, and A6 steels are the steels of the present invention.
A1 and A3 steels are the first invention steels, A6 steels are the second invention steels,
B1 steel is a conventional steel with SUP7. After casting, the first part was hot rolled at a rolling ratio of 50 or more.
Among the test steels in the table, A1, A3 steel, and B1 steel are used as raw materials to form a coil spring having the specifications shown in Table 2.
After quenching and tempering so that the final hardness is HRC45-55, the shear stress of the wire is τ = 115
A setting test piece was prepared by setting the weight to Kg/mm ² . Then, a load was applied to this test piece at a constant temperature of 20°C so that the shear stress of the wire was τ = 105Kg/ ^mm2 , and the amount of fatigue of the coil spring after 96 hours (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, when adding V and Nb, which is the steel of the present invention,
It is recognized that both A1 and A3 steels to which A1 has been added have superior fatigue resistance compared to B1 steel, which is a conventional 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 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 the difference △P
(=P ₁ −P ₂ ) using the following formula, and 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, a fatigue test was conducted by repeatedly applying a load with a shear stress varying from 10 to 110 Kgf/mm ² to coil spring wires having the same specifications as above for A1, A3 steel, and B1 steel of the present invention steel. As a result, both coil springs
It did not break even after being repeated 200,000 times. Next, using A6 steel and B1 steel as materials, a torsion with a parallel part diameter of 30 mmφ and having the specifications shown in Table 3 was prepared.
A bar was manufactured, quenched and tempered to a final hardness of HRC45 to 55, and shot peened to obtain a set test piece. Prior to the settling test, a torque was applied to both ends of the parallel portion of the test piece so that a shear stress τ = 110 Kgf/mm ² appeared on the surface of the parallel portion, and setting was performed. After setting, a torque was applied to make the 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 amount of decrease in the twist angle according to YR = △θ・d/2. .

[Table] Figure 3 shows the amount of set in relation to the hardness of the above test piece. As is clear from FIG. 3, the amount of sag of the test piece with a diameter of 30 mm in the parallel portion made from the steel A6 of the present invention containing B is much better than that of the conventional steel B1. This is because by including B, even in a 30 mmφ torsion bar, a hardened martensite structure can be obtained completely up to the core through quenching, and the resistance to set- ting is not impaired. It is thought that this is effective in reducing fatigue by entering into the crystal and near dislocations as an interstitial type, making it difficult for dislocations to move. Furthermore, among the test steels, the Giyominy curves of A6 steel and B1 steel are shown in Fig. 4. As is clear from FIG. 4, the hardenability of A6 steel containing B is dramatically improved compared to B1 steel that does not contain B. Furthermore, among the test steels, A1, A3 steel, and B1 steel were heated at a quenching temperature of 85 to 1100℃, and the austenite grain size was measured by an oxidation method.
Shown in the figure. As is clear from Fig. 5, V,
The A1 and A3 steels containing Nb and A1 have superior austenite grain size compared to the B1 steel that does not contain them. Furthermore, the above torsion bars made from A6 steel, conventional steel, and B1 steel, which are the steels of the present invention, were subjected to a fatigue test by repeatedly applying a shear stress of 60±50 Kgf/ ^mm2 . Bar too
No breakage occurred even after repeated loading 200,000 times, confirming 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 singly or in combination to conventional high-Si spring steels, and also contains Al, and further contains B, if necessary.
It contains two types of Ni, and has succeeded in significantly improving the fatigue resistance and hardenability of conventional high-Si spring steels, and also has the fatigue resistance and toughness required for spring steels. It is also comparable to conventional steel,
In particular, it has extremely high practicality as a steel for suspension springs for passenger cars.

[Brief explanation of the drawing]

Figures 1 to 3 are graphs showing the amount of settling of test specimens with hardness of HRC45 to 55 after quenching and tempering for the present invention steel and conventional steel, and Figure 4 is for A6 and B1 steels after quenching. Diagram showing gender, Figure 5 is A1~
It is a diagram showing the austenite grain size when A3 and B1 steels are heated at a quenching temperature of 850 to 1100°C.

Claims (1)

[Claims] 1. C0.50-0.80%, Si1.50-2.50 in terms of weight ratio
%, Mn0.70~1.50% and V0.20~0.50% or
A spring steel with excellent fatigue resistance characterized by containing 0.20 to 0.50% of V, 0.05 to 0.15% of Nb, and further containing 0.03 to 0.10% of Al, with the remainder substantially consisting of Fe. 2 C0.50~0.80%, Si1.50~2.50 by weight
%, Mn0.70~1.50%, V0.20~0.50%, and Al0.03~0.10%, B0.0005~0.0020
%, Ni 0.20-0.50%, and the rest is substantially Fe.
Spring steel with excellent fatigue resistance.