JPS5867847A - Spring steel excellent in fatigue resistance - Google Patents

Abstract

PURPOSE:To obtain the titled steel having improved workability and the easy processing of a base material due to cutting grinding in order to remove surface decarburization and surface defects when a spring is manufactured, by lowering the C content of high Si spring steel having V, Nb and Mo added thereto to enable water hardening. CONSTITUTION:Spring steel contains, on the basis of wt% 0.35-0.45 C, 1.50- 2.50 Si, 0.50-1.50 Mn, 0.05-0.50 one or more of V, Nb and Mo, according to necessity, 0.02-0.10 one or more of Ti and Zr, in addition, one or more of 0.0005-0.0100 B, 0.20-1.00 Cr, 0.20-2.00 Ni and 0.30 or less rare earth element and comprises the remainder of substantially Fe. In this case, when the C content is below the lower limit, strength is not developed sufficiently at hardening and annealing and, when the C exceeds the upper limit, crack may be generated during water hardening and thereby, grinding or cutting processing become difficult. Al, Ti and Zr further enhance fatigue resistance and B and Cr enhance hardenability and the rare earth metal elements enhance the tenacity.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a spring steel having 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 under high stress conditions to reduce weight. However, if the current copper for springs is used under high stress conditions, Problems arise with durability and increase in sagging, and the latter one
"Sagging" appears as a decrease in spring height, which in turn causes a decrease in vehicle height, which causes a serious safety problem due to the decrease in spring height. There is a need for copper for springs with excellent properties.

Traditionally, copper for springs has excellent resistance to fatigue.

As it is known that Si in spring steel is an effective element for fatigue resistance, 5UP7, which has an even higher Si content than 5UP6, has come to be used more frequently.
There are very strict requirements for reducing the weight of suspension springs, and there has been a strong desire to develop copper for springs that is even more resistant to fatigue than 5UP7.

Against this background, the applicant of the present application first added an appropriate amount of one or more of v, Nb, and Mo to high-Si spring copper, thereby creating a material with even better fatigue resistance than 5UP7. In addition, we developed a copper for springs that has the same fatigue resistance and toughness as 5UP7, which is necessary for copper for springs.

Against this background, the present invention was developed as a result of repeated research by the present inventors, and was developed by adding appropriate amounts of V, Nb, and Mo to copper for high-Si springs.
One or more types are added, and the amount of C is 0.85 to 0.
Water quenching is possible by reducing the temperature to 45%.
We have succeeded in developing copper for springs with excellent fatigue resistance, which improves workability and is easy to process by cutting and grinding to remove surface decarburization and surface flaws during spring manufacturing.

Further, the present invention provides A1 and Tj for refining the crystal grain size.

The setting resistance is further improved by adding Zr or two or more of them, the hardenability is further improved by adding B and Cr, and the hardenability is further improved by adding Ni and rare earth elements. The present invention has excellent not only fatigue resistance but also hardenability and toughness, and also has the fatigue resistance required for copper for springs.
It has the same performance as P7.

The present invention will be explained in detail below.

The first invention steel has CO of 85 to 0.45 in terms of weight ratio.

Si 1.50-. 50%, Mn 0 , 50-
1, 50%, Vo, 05-0.50%, NbO,
0.05-0.50%, Mo 0.05-0.50%, and the second invention steel has an AI of 0.0 (~0.0. 10%,
TiO, 02-0.10%, Zr 0.02-0.10
% (7) Uchi 1 fi fr I L contains two or more types to further improve the settling resistance of the first invention steel,
The third invention steel is the first invention steel E SaralCB 0.0005
-0.0100%, CrO, 20-1,00%, Ni
Hardenability of the first invention steel containing one or more of 0.020 to 2.00% and 0.80% or less of rare earth elements,
The fourth invention steel has improved toughness, and the fourth invention steel has a higher toughness than the second invention steel) n B O, 0005-0,0100%, Cr O
, 20-1,0O1゜Ni O, 20-2,00%, and rare earth elements O, aO% or less, to improve the hardenability and II properties of the second invention steel. It is something.

The mechanism for improving the settling resistance of V, Nb, and Mo in the steel of the present invention will be explained below.

V and Nb9Mo form carbides in steel.

This V carbide + Nb carbide, Mo carbide (hereinafter).

The alloy carbide (alloy carbide) 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, which serves to improve the resistance to settling by inhibiting the movement of dislocations in the steel.

Furthermore, by eliminating austenite during heating during quenching, the resistance to settling is improved.

Furthermore, because the steel of the present invention contains Nb and VlMo, even when quenched from 900° C., which is the quenching temperature of normal spring copper, they re-precipitate during the subsequent tempering process, resulting in secondary hardening. This means that when aiming for the same tempering hardness range, it is possible to use a wider tempering temperature range of 4, 4, 14 compared to conventional steel, and the target hardness can be stably obtained. Become.

In addition, grain-refining elements such as AI, Ti, and Zr often form nitrides in steel, and these nitrides can prevent austenite grains from coarsening during heating during quenching. Such fine crystal grains improve the resistance to settling by reducing the amount of movement of dislocations.

Furthermore, among B, Cr, Ni, and rare earth elements that improve hardenability, B is an element that is particularly effective in improving 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 t'r-B has invaded in this way become difficult to move, they are effective in reducing the fatigue.

In addition, hardenability improving elements such as B, Cr, Ni, and rare earth elements are added to large coil springs, torsion bars, and thick leaf springs used in relatively large automobiles. As a result, a martensitic structure can be obtained up to the core portion during heat treatment, 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 amount of C is set to 0.35 to 0.45% is because if it is less than 0.35%, sufficient strength as copper for high stress springs cannot be obtained through quenching or tempering. This is because if the content exceeds 100%, there is a risk of quench cracking occurring during water quenching, and it becomes difficult to grind or cut the material during spring manufacturing.

The reason for setting the Si amount to 1.50 to 2.50% is 1.50
This is because if the Si content is less than 2.50%, the effect of increasing the strength of the substrate and improving the resistance to settling cannot be sufficiently obtained by solid solution in the ferrite of Si. 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 reason for setting the Mn amount to 0.50-1.50% is 0.50
This is because if the content is less than 1.501, the strength as copper for springs is insufficient and the hardenability is also insufficient, and if the content exceeds 1.501, the toughness is impaired. VNb,
Mo is an element that improves the sag resistance in the steel of the present invention.

The content of V, Nh, and Mo, which play this role, is set to 0.05 to 0.50% each, which is 0.05%.
% or less, the above effect cannot be obtained sufficiently.

Even if the content exceeds 0.50%, the effect is saturated.

In addition, the amount of alloy carbides that are not dissolved in the austenite increases and forms large lumps, which may reduce the fatigue strength of the steel due to the action of nonmetallic inclusions.

In addition to adding each of these V, Nb, and Mo individually, two or three types can be added in combination.As a result, the temperature is lower than when V, Nb, and Mo are added alone. The precipitation of fine alloy carbides during the tempering process further promotes secondary hardening and further improves the fatigue resistance.
This is to improve this.

In the steel of the present invention, AI, Ti, and Zr are elements that refine the crystal grains and improve the settling resistance. AIo, 08-0
, 10S, Ti 0.02-0.10%, ZrO,
The reason why 02-0 and 10% are set is that below the lower limit, the effect of improving the settling resistance is insufficient, and when the content exceeds the upper limit, the amount of nitrides of AI, Ti, and Zr increases.
This is because the formation of large lumps may reduce the fatigue strength of the steel due to the action of non-metallic inclusions.

B, Cr, Ni, and rare earth elements are elements that improve hardenability.

The reason for setting the B amount to 0.0005 to 0.0100% is because
This is because if it is less than 0.0005 inches, the effect of improving hardenability and reducing the resistance to setting cannot be sufficiently obtained, and 0.01
If the content exceeds 0.00%, boron compounds will precipitate,
This is because hot brittleness appears.

The amount of Cr is 0.20-1. OO'% is 0.20
% or less, there is no sufficient hardenability effect.1.
This is because if the content exceeds 0O1, the uniformity of the structure will be impaired in high-Si steels as in the present invention, and the resistance to settling will be impaired.

Ni and rare earth elements are elements that improve hardenability and toughness in the steel of the present invention.

Setting Ni to 0.20 to 2.00 is 0.20%
If the hardenability and toughness improvement effect is insufficient,
．． This is because even if the content exceeds 0.00%, the effect may be saturated and retained austenite with a large amount of t may be formed.

The reason why the rare earth element is set to 0.80% or less is that if it is contained more than that, the crystal grains may become coarse.

The characteristics of the steel of the present invention will be clearly demonstrated by comparing it with conventional steel through examples.

Table 1 In Table 1, A1-A15 m is the steel of the present invention.
8 steel is the first invention steel, A4 to A6m is the second invention steel.

A7 to A10 steels are the third invention steels, A11 to A15 steels are the fourth invention steels, and Bl steel is the conventional steel and is a SUP demon.

Table 2 shows coil springs having the specifications shown in Table 2 using A1 to A6 steels and Blm of the test steels in Table 1 that were hot-rolled at a rolling ratio of 50 or more after casting. After quenching and tempering so that the final hardness is HRC45-55, setting is added so that the shear stress of the strand is 211549/-, and the set test piece is kept at a constant temperature of 20℃. A load such that the shear stress of the wire is τ = 105&p/- is applied at a temperature of 96 hours (hereinafter referred to as long-term load).
The amount of fatigue of the coil spring was measured after this.

Table 2 shows the amount of set in the hardness of the above test piece.
Shown in the figure. As is clear from Figures 1 and 2, V and Nb, which are the steels of the present invention, are added, and the amount of C is 0.35 to 0.
A lowered to 45 cm and added with AI, Ti, and Ni
It can be seen that all of the steels No. 1 to A6m have excellent resistance to aging compared to Bl steel, which is a conventional steel.

The amount of setback is determined by measuring the load PJ 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. The residual shear strain was calculated using the following formula from the difference p (=p+-pg), and was evaluated using a value called residual shear strain.

G: Transverse elastic modulus (*gr/-) D: Coil center diameter (
fl) d Wire diameter (厘-) K: Whirl correction coefficient (constant determined by the shape of the coil spring) Also, the coil spring wire having the same g specifications as above for the A1 to A6 steels of the present invention steel and 81w4. 5. The shear stress is l
As a result of conducting a fatigue test by repeatedly applying a load varying from 9f/- to θ~110, none of the coin springs broke even after 200,000 repetitions.

Next, using the A7 to A15 steel test steels as raw materials, the eighth
A toss-hardened par with a parallel part diameter of 80flφ having the specifications shown in the table was manufactured, and then hardened and tempered so that the final hardness was HQC45-55.

It was subjected to shift peening treatment 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 of Z = 110 = 9 r/- was applied to the surface of the parallel portion of the test piece to set it. After senoching, a torque is applied to make the shear stress T, = 100t9f/-, and it is left as it is for 96 hours.
The amount of residual shear strain was determined according to YR-Δe-d/2.

Table 8 Also, among the test steels, the Jominy curves of A7 to AIO steel and Bl steel are shown in FIG. As is clear from Fig. 5, the A7 to AIO steels containing B, Ni, and Cr are
It can be seen that the hardenability is dramatically improved compared to Bl steel that does not contain it.

Furthermore, among the test steels, A4 to A6 steels and one Bl steel were heated at a quenching temperature of 850 to 1100°C.

The austenite grain size measured by the oxidation method is shown in Figure 6. As is clear from Figure 6.

V, Nb OJ-hi Al, Ti wo-containing case A4~
A6 steel.

It has superior austenite grain size compared to Bl steel that does not contain them.

Furthermore, one steel of the present invention, A7 to A15m, is a conventional steel. As a result of conducting a fatigue test by repeatedly applying a shear stress of 60 ± 50 klV- to the above-mentioned Toshine bars made from Bl steel, it was found that both Tosseen bars
It was confirmed that there was no breakage even after repeated loading 00,000 times, and that the addition of B had no effect on fatigue life.

As mentioned above, the steel of the present invention has an appropriate amount of V added to the conventional high-Si spring copper.
, NblMo is added or combined to lower the C amount to 0.35 to 0.451, and if necessary, one or more of AI, Ti, and Zr are added, and further B, Cr, 19. Containing one or more rare earth elements, it greatly improves the fatigue resistance and hardenability of conventional copper for high-Si springs, making water quenching possible and greatly improving workability. We succeeded in improving stiffness and making cutting and grinding easier during spring manufacturing, and the fatigue resistance and toughness required for copper for springs are similar to that of conventional steel. Therefore, it has extremely high practicality, especially as copper for suspension springs for passenger cars.

[Brief explanation of the drawing]

Figures 1 to 4 show specimens of hardness HRC 4'4 to 55 after quenching and tempering for the steel of the present invention and conventional steel.
10 A diagram showing the amount of settling, Fig. 5 is a diagram showing the hardenability of A8 to A8, Bl steel, Fig. 6 is a diagram showing the hardenability of A4 to AiB1m
It is a diagram showing the austenite crystal grain size in the case of heating at a quenching temperature of 850 to 1100°C. Representative Patent Applicant Patent Applicant Chuo Spring Co., Ltd. Representative Masaharu Minoura

Claims (1)

[Claims] 1. Weight ratio tr L -c c o, a5-o, 45*
, Si 1.50-2.50chi, Mn 0 , 50-1
, 50% total content, Sarani V0.05-0.50%
, NbO,050,50%, MoO,05-0
, 50qb (1) of which IM! or contains two or more types,
A spring steel with excellent fatigue resistance, characterized in that the remainder is substantially made of Fe. 2. Weight ratio EL, "CCO, 85-0.45%,
Si 1.50-2.50~0.50chi JJ 0.0
Contains one or more of 5 to 0.50%, and further contains EAI 0.08-0.10%, TiO, 02-0,
1 or 2 of 10chi, zro, o2~0.10%
1. A spring steel having excellent fatigue resistance, characterized in that it contains at least 50% Fe, with the remainder substantially consisting of Fe. 3. Weight ratio ICL” (CO, 85-0.45%, Si
1.50-2.50chi, MnO, 50-1, 50%
Gt, VQ, 05-0.50%, Nb O, 05-0.
50%, one or two of MO0.05~0.50φ
BO,0005-0-,01
00%, Cro, 20-1.00chi, Nip, 20
-2.00% Rare earth element 0.30% or less of one kind or two or more kinds is contained, and the remainder substantially consists of Fe, and has excellent fatigue resistance. 4. Weight ratio t CO, 85 to 0.45 t, Six,
5o~2.50%1MnO,5-1,5(l and V
O,05-0,50%, Nb O,05-0,50
9b, MO 0.05 to 0.50 chi, and further contains AI 0.0 (~0.10 chi, T
i O, 02-0.10%, Zr 0.02-0.1
(One or more of l and B O,0005-
0,0100%, CrO,20-71,0(1,Ni
0.020 to 2,00chi, one type of rare earth element 0.80chi or less