JPS6041699B2 - Spring steel with excellent hardenability and fatigue resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a spring steel having excellent hardenability and 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.

Conventionally, as a spring steel with excellent resistance to setting, Si in the spring steel is an effective element for resistance to setting.
,-tsu,゛-mu゛ CITDQ1, 7 to 11 + 1-
SUP7, which has a high q-book of 1, has come to be widely 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 of the present application first added appropriate amounts of one or two types of V and Nb to high-Si spring steel, thereby creating a spring with even better fatigue resistance than SUP7. Developed and applied for a spring steel with performance equivalent to SUP7 in terms of fatigue resistance and toughness required for a spring steel (Patent Application No. 10802/1983)
did. However, in large coil springs, torsion bars, and thick stacked leaf springs used in relatively large automobiles, it is difficult to harden down to the core during heat treatment, so the structure of the core is bainite or The structure becomes ferrite pearlite, which has lower hardness than a martensitic structure, and the effect of improving the fatigue resistance due to V and Nb is significantly impaired.

The present inventors have conducted research to obtain sufficient fatigue resistance even for the above-mentioned large or thick springs, and have found that
By adding an appropriate amount of glue, V, and Nb to the steel for springs, and further adding B, and making the amount less than 0.080%, it can be used even in large coil springs, torsion bars, and thick stacked leaf springs 5. During heat treatment, a martensitic structure is obtained up to the core, and we have successfully developed a spring steel with excellent hardenability that does not impair set resistance.

The present invention will be explained in detail below. The present invention is CO. 5O-0.80%, Sil. 5O~2.50%,
MnO. 5O to 1.50% and VO. O5~0.50%
Or VO. O5~0.50%, NbO. O5~0.
50% and BO. Contains OOO5-0.01%,
Furthermore, NO. This is a spring steel with an OO of 80% or less and the remainder being substantially Fe, which has excellent hardenability and set resistance, as well as good fatigue resistance and toughness.

The mechanism for improving the setting resistance and hardenability of V, Nb and B 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 are supersaturated solid dissolved in martensite by quenching.

When this is tempered, fine alloy carbides re-precipitate during the process, causing secondary hardening, which functions to improve the resistance to settling by inhibiting the movement of dislocations in the copper. 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 resistance to settling by reducing the amount of movement of dislocations. In addition, atomic B has an effective effect on hardenability.

This atomic B forms a solid solution in the crystals of steel as an interstitial type, and particularly penetrates near dislocations. easy. Since the dislocations into which B has invaded in this way become difficult to move, they are effective in reducing fatigue. Furthermore, the steel of the present invention has N
By containing b and ■, the quenching temperature of normal spring copper is 900℃! 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 target hardness can be stably obtained.
In order to further clarify this and explain the effect of B, we used high-Si steel as a base and 0.
A1 steel containing 28% ■ and 0.0029% B,
0.21% V and 0.09% Nb. l5O. OO2l
A2 steel containing 0.26% B, B1 steel containing 0.26% ■, and C1 steel, which is a currently used high S1 steel,
Tempering was performed at a temperature between 0 and 600°C, and the hardness was measured. The results are shown in Figure 1.

As is clear from Figure 1, ■, Al containing Nb and B, A? Even in the normal quenching from 900° C., a peak in hardness increase indicating the occurrence of secondary hardening is observed at a tempering temperature of around 550° C. for both B1 steel and B1 steel containing V alone. As a result, it is clear that the addition of B does not inhibit the occurrence of secondary hardening, and at the tempering temperature to obtain a hardness near the hardness regulation range (HRC 45 to 51) of current spring steels, precipitation occurs. It is also clear that reinforcement can be fully utilized. Further, the hardness of the above-mentioned steel is shown in FIG. 2 when the hardenability temperature is gradually increased in the range of 850 to 1100°C and tempered at 550'C. This shows that, with the exception of C1 steel, the hardness of Al, M rust, and B1 steels increases as the quenching temperature increases. This indicates that the amount of alloy carbides dissolved in the austenite phase increases as the quenching temperature rises, resulting in significant secondary hardening. Furthermore, for the above steel, 850 to 11
The austenite grain size measured by the oxidation method after heating at a quenching temperature of 00° C. is shown in FIG.

As is clear from Figure 3, the Al, A steel containing V, Nb (5B) has the same austenite grain size as the B1 steel containing V alone, and from this it can be seen that the addition of B , it can be seen that the effect of the alloy carbides on refining the crystal grains and inhibiting the coarsening of the austenite crystal grains is not significantly impaired.Furthermore, the Jominy curve of the above-mentioned steel is shown in FIG.

As is clear from FIG. 4, the hardenability of Al and A steel containing B is dramatically improved compared to B1 steel and C1 steel that do not contain B. As mentioned above, the steel of the present invention has a high S1 spring steel containing an appropriate amount of ■, Nb (5B), thereby improving secondary hardening and grain refinement by alloy carbides and hardenability by atomic B. A spring steel with excellent hardenability and set resistance was obtained by utilizing the effects of hardening and fixation of dislocations.

Next, the reasons for limiting the composition of the steel of the present invention will be explained.

The reason why the C content is set to 0.50 to 0.80% is because if it is less than 0.50%, sufficient strength cannot be obtained as a steel for high stress springs by quenching and tempering. This is because if it is contained, it becomes hypereutectoid steel, resulting in a significant decrease in toughness.

The reason for setting the Sl amount to 1.50 to 2.50% is 1.50%.
This is because the effect of increasing the strength of the base material and improving the resistance to settling by solid solution in the ferrite of S] cannot be sufficiently obtained, and even if it is contained in an amount exceeding 2.50%. This is because there is a possibility that the effect of improving the settling resistance is saturated and that free carbon is generated due to heat treatment.

The Mn amount was set to 0.50 to 1.50% by 0.50%.
This is because if the content is less than 1.5%, the strength as a spring steel is insufficient, and the hardenability is also insufficient, and if the content exceeds 1.50%, the toughness will be impaired.

(2) and Nb are both elements that improve the sag resistance in the steel of the present invention. The content of ■ and Nb, which play this role, was set at 0.05% to 0.50%, respectively.
．． This is because if the content is less than 0.50%, the above effect cannot be obtained sufficiently, and if the content exceeds 0.50%, the effect will be saturated, and the amount of alloy carbides that will not be dissolved in the austenite will increase, resulting in large lumps. This is because the fatigue strength of the steel may be reduced due to the action of nonmetallic inclusions.

By adding these two types in combination, V and Nb can be
, Nb starts to dissolve into austenite at a lower temperature than when it is added alone, and the precipitation of fine alloy carbides during the tempering process promotes secondary hardening, which improves the resistance to setting. This further improves the

The amount of B was set to 0.0005 to 0.01% because 0.00
This is because if the content is less than 0.05%, the effect of improving hardenability and reducing the fatigue resistance cannot be sufficiently obtained, and if the content exceeds 0.01%, boron compounds will precipitate and hot embrittlement will appear. . The reason why the N amount was set to 0.0080% or less was because
By combining B with N and reducing atomic B, B
This is to prevent the effects from being impaired.

At the same time, the characteristics of the steel of the present invention will be clarified by comparing it with comparative steels and conventional steels through examples.

Table 1 shows the chemical composition of these test steels.

In Table 1, Al and A are the steels of the present invention, Bl and B2 are comparative steels made of high S] copper with ■ and Nb added, and C1 is the conventional steel SUP7. In addition, 0.11 to 0.13% Cr contained in Al, A2 steel, Bl, B2 steel and C1 steel
All amounts are amounts of impurities mixed in from raw materials.

Table 2 shows the test steels in Table 1 that were hot-rolled at a rolling ratio of 50 or more after casting, and were heat-treated to have a tempering hardness of about I (RC48). 0.2% proof stress, elongation, reduction of area, impact value, and torsional strength are shown.

For tensile strength, 0.2% proof stress, elongation, and area of area, please refer to JI
It was measured using a No. S4 test piece, and the impact value was J
Measured using an IS3 test piece, and the torsional strength was 9 in the parallel part.
This was measured using a test piece with a TWL circle.

As is clear from Table 2, B-added steels of the present invention, Al and A'$u, exhibit mechanical properties equivalent to Bl and B2 steels, which are comparison steels with ■ and Nb added, and the conventional steels It has a 0.2% yield strength superior to that of C1 steel.

Next, using the above-mentioned test steel as a material, the diameter of the parallel part is 307S with the specifications shown in Table 3! A torsion bar of lφ was manufactured and quenched to a final hardness of HRC45 to 55.
After the tempering treatment, shot peening treatment was performed to obtain a set test piece. Prior to the settling test, a shear stress γ = 110k9f/WrlL was applied to the surface of the parallel part of the specimen.
Torque was added to both ends to create a setting. Shear stress after setting τ=100k9f/i
A torque of 961 was applied, and the sample was left as it was, and then the amount of residual shear strain was determined from the amount of decrease in twist angle according to YR=ΔQ−d/21. FIG. 5 shows the amount of set in relation to the hardness of the test piece.

As is clear from FIG. 5, the present invention steel Al containing B,
The amount of settling of a test piece with a parallel part diameter of 30Twtφ made from A2 is much better than that of C1 steel, which is a conventional steel.
It also shows good values when compared with B1 steel, which is a comparative steel. This is due to the inclusion of B, resulting in 30Tf$tφ
In the torsion bar, a hardened martensite structure was obtained completely down to the core through quenching, and the fatigue resistance was not impaired. This is thought to be effective in reducing fatigue by making it difficult for dislocations to move. Furthermore, the results of a fatigue test were conducted by repeatedly applying a shear stress of 60±50 kgf/Tf7lt to the above-mentioned torsion bars made from Al and A2 steels, which are the steels of the present invention, and Bl and B@, which are comparative steels. , all torsion bars do not break even after being repeatedly loaded 200,000 times.B
It was confirmed that the addition had no effect on fatigue life.

As mentioned above, the steel of the present invention has an appropriate amount of ■
Or ■, by adding Nb and containing B, and further reducing N to 0.0080% or less, the hardenability and settling resistance of conventional high S1 splash steel can be greatly improved. Furthermore, it has the same fatigue resistance and toughness as conventional steels, which are necessary for spring steel, and has extremely high practicality, especially as a steel for passenger car suspension springs.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the hardness of the steel of the present invention, comparative steel, and conventional steel after being quenched and then tempered between 300 and 600°C. ~120
A diagram showing the hardness when quenched in oil from a temperature of 0℃ and then tempered at 550'C. Figure 3 shows the hardness of the steel shown in Figure 1 heated at a quenching temperature of 850 to 1100'C. Figure 4 is a diagram showing the austenite grain size when
Figure 5 is a diagram showing the hardenability of the steel shown in the figure. Figure 5 shows the amount of set in test pieces with a hardness of 11RC45 to 55 after quenching and tempering for the inventive steel, comparative steel, and conventional steel. It is a line diagram.

Claims (1)

[Claims] 1 Weight ratio: C0.50-0.80%, Si1.5
0 to 2.50%, Mn 0.50 to 1.50%, and V0.
05-0.50% or V0.05-0.50%, N
b0.05-0.50%, and B0.0005-0.
A spring steel having excellent hardenability and fatigue resistance, characterized in that it contains N0.001%, N0.0080% or less, and the remainder substantially consists of Fe.