JPS5827957A - Spring steel with superior yielding resistance - Google Patents

Abstract

PURPOSE:To obtain a spring steel with superior yielding resistance, hardenability and toughness by adding a specified percentage each of C, Si, Mn, V, Nb and Mo to Fe. CONSTITUTION:A steel consisting of, by weight, 0.50-0.80% C, 1.50-2.50% Si, <=0.45% Mn, 1 or >=2 kinds among 0.05-0.50% V, 0.05-0.50% Nb and 0.05- 0.50% Mo, and the balance essentially Fe is prepared. To the steel may be added 1 or >=2 kinds among 0.0005-0.0100% B, 0.20-1.00% Cr, 0.20-2.00% Ni and <=0.30% rare earth element, and/or 1 or 2 kinds among 0.03-0.10% Al, 0.02- 0.10% Ti and 0.02-0.10% Zr. A spring steel superior to SUP7 in yielding resistance, hardenability and toughness and equal to SUP7 in fatigue resistance is obtd.

Description

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

Conventionally, 5UP6, SU]Fg has been the main copper for springs 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.

In response to this, various measures have been attempted for suspension systems in general, but among these measures, a measure of increasing the design stress of the spring is considered to be effective. With this 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 major safety problem. As a result of various studies, it was discovered that increasing the S+ content in spring steel improves the fatigue resistance. ．． T I
Among the spring steels specified in SG 4.801, S TJ P 7, which has the highest Si, has come to be widely used as copper for passenger car suspension springs.

However, there are strict requirements for reducing the weight of suspension springs (3), and there has been a strong desire to develop copper for springs that has even better fatigue properties than SUT'7.

Against this background, the present invention was developed as a result of repeated research by the present inventors, and was developed as a result of repeated research by the present inventors.

Nl), MO, and one or more of B, Cr, Ni, and rare earth elements, or also kl, T1゜Zr, depending on the purpose.
By adding one or more types of Noji, S
It has better fatigue resistance, hardenability, and toughness than U B7, and also has the fatigue resistance necessary for spring copper.
We have developed copper for springs that has the same performance as P7.

V, Nll, 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 in the process, which blocks the movement of dislocations in the steel (41) and causes secondary hardening.

It works to increase the hardness compared to spring copper without the addition of Nb and Mo, and further improves the resistance to fatigue. Also,
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.

The reason why the amount of Mn is lowered in the steel of the present invention is to improve the toughness of the steel by suppressing the formation of retained austenite during quenching. Recently, there has been a trend to increase the hardness of springs in order to reduce spring fatigue, but increasing the hardness of springs causes a new problem of reduced toughness. Therefore, the steel of the present invention attempts to solve this new problem by limiting the M and n contents to low values.

In order to clarify this, 0.40% Mn
of impact tests carried out on Al steel containing <0.38% Mn, A2 steel containing the same <0.38% Mn, and B1 steel with a conventional steel containing 0.86% Mn with substantially S U P 7. The results are shown in Table 1.

As is clear from Table 1, it can be seen that the invention steels, AI and A2 steels with a small amount of Mn, have higher toughness than the conventional steel, BI steel.

Table 1 In addition, B, Cr, Ni, and rare earth elements are elements that improve the hardenability of steel.In particular, low M and n steels such as the steel of the present invention may have insufficient hardenability. The elements not only act effectively in the sense of supplementing them, but also the elements of the steel of the present invention.
Furthermore, it can be applied to large and thick springs.

To clarify this fact, the 0.19% V,!
= He3 steel containing 0.0025% B, 0.24% ■ and 0.11% N1), A4 steel containing 0.0022% B, 0.21% V, 0. 51% Ni
.

A5 steel containing 0.0023% B, 0.020% V,
0.13% Nb, 0.48% Ni, o, oo
He6 steel containing 27% B, 022% V, O,00
A7 steel containing 25% B, 0.05% rare earth elements, 0.21%(7)V, 0.1.2% N1-1
, A8 steel containing 0.080029% B, 0.05% rare earth elements and Blf, which is STJ P 7 of conventional steel.
Figure 1 shows the results of a comparison of personnel for IA. As is clear from FIG. 1, it can be seen that the addition of hardenability-improving elements makes it possible to obtain hardenability that is twice as high as that of conventional steels.

Next, AI, Ti, and Zr all often combine with N to form nitrides in steel, which refine the austenite grains during hot rolling, and when heated to the austenitizing temperature, the austenite grains become smaller. It has the function of causing coarsening of the particles. Since the amount of movement of dislocations in a structure with finer grains is smaller, the fatigue resistance of the steel can be improved.

(7) In Fig. 2, A9 to A12, which will be described later, have Al and Ti added.
The size of austenite crystal grains when heated and held at each austenizing temperature of 850 to 1100°C for B1 steel and conventional steel is clearly seen, and the effect of adding grain refining elements is clearly recognized. It will be done.

The chemical composition of the steel of the present invention is C0.50-0.80%, 81L
50-2.50%, Mn□, 45% or less, and further contains VO, 05-050%, Nl) 0. Q
5-050%, MOo, 05-050%. O0%, Oro,
Contains one or more of 20-1.00%, NiO, 20-2.00%, rare earth elements 0.30% or less, or also A40.03-010%, Ti 0.0
Zr 0.02-0.10%, Zr 0.02-0.10%, one or more of the following: 1. The rest is essentially Fe
It is more than that.

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 050-080% is that if it is less than 0.050% (81%), sufficient strength as copper for high stress springs cannot be obtained by quenching and tempering, and if it is contained in excess of 0.080%, it will be too strong. This is because the steel becomes an analytical steel and the toughness decreases significantly.

The S+ amount was 1.50% to 250%.
This is because the effects of increasing the strength of the base material and improving the resistance to settling cannot be sufficiently obtained by solid-dissolving Si in the Ferrite 1 to 1 containing Si in an amount exceeding 2.50%. This is because the effect of improving the fatigue resistance is saturated and there is a possibility that free carbon may be generated by heat treatment.

The reason why the Mn amount is 0.45% or less is because the M-n amount is 0°4
By limiting the content to 5% or less, the formation of retained austenite is suppressed to a low level, thereby imparting toughness to the steel.

V, Nb, and Mo are all 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 to 0.05 to 0.50% each is because the above effects cannot be obtained sufficiently below 0.05%.
Even if the content exceeds 100%, the effect will be saturated, and the amount of alloy carbides that will not be dissolved in the austenite will increase, forming large lumps, which may reduce the fatigue strength of the steel due to the action of nonmetallic inclusions. This is because there is.

In addition to adding each of these V, Nl), and No singly, by adding two or three types in combination, a lower temperature can be achieved compared to when V, Nl), M, and o are added alone. In addition, the precipitation of fine alloy carbides during the tempering process further promotes secondary hardening and further improves the resistance to setting.

Also, B and Car are elements that improve the hardenability of steel.

The amounts of Ni and rare earth elements are respectively BO,0005 ~
o, oiooo%, Or 0.20-1.00%, Ni
Q: 20 to 2.00% and rare earth elements 0.30% or less is because B is not sufficient to improve hardenability at 0.0005% or less, and B content exceeding 0.0100% is This is because even if it is allowed to do so, the effect will be saturated and the formation of the B compound may lead to hot embrittlement.

Regarding Or, if it is less than 0.020%, it will not be effective in improving hardenability, and if it is contained in more than 1.00%, the structure will change in high-Sl steel such as the steel of the present invention. This is because there is a risk of non-uniformity.

Regarding Ni, if it is less than 0.20%, the effects of improving hardenability and toughness cannot be sufficiently obtained, and if it is contained in excess of 200%, a large amount of retained austenite will be formed during hardening. This is because there is a risk of

The reason why the amount of rare earth elements is set to 0.30% or less is that if it is contained more than that, the crystal grains may become coarse.

In addition, the crystal grains are made finer to improve the resistance to settling.
The content of e + ” l + Z r is A40, respectively.
03~0.10%, 'f'i0.02~0.10%, Z
The reason why r
11) If the content exceeds 0.10%, it may cause cracking during hot rolling or deteriorate the toughness of the steel as non-metallic inclusions.

Next, the characteristics of the steel of the present invention will be clarified by comparison with conventional steel and examples.

Table 2 shows the chemical composition of these test steels.

→(Down-゛h In Table 2, A1 to A1.4 steels are the steels of the present invention, and BI
The steel used was conventional steel 8' (JP7). All of the sample steels were hot-rolled at a rolling ratio of 50 or higher after casting, and subjected to the sag test described below.

In carrying out the settling test, a coil spring having the specifications shown in Table 3 was molded, and the final hardness was Hη, 045~
After quenching and tempering so that the wire had a shear stress of τ-115 kti/ma, a set test piece was prepared. Then, this test piece was heated at a constant temperature of 20°C, and the shear stress of the strand τ-t 05
A load of kq/- was applied, and the amount of fatigue of the coil spring was measured after 96 hours (hereinafter referred to as long-term load).

Table 3 However, for HE3-A8 steels to which hardenability improving elements have been added, l-john bars having the specifications shown in Table 4 were prepared, and the final hardness was adjusted to ITTt045-55. After quenching and tempering, τ-1, 1, Okqf
Setting is performed with a stress of /-, and then τ-100kO
A stress of r/- was applied for 96 hours, and the amount of settling that occurred during that time was measured.

Table 4 Note that the amount of settling is 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 ΔP (-pr), which has a unit of 2 shear strain, and was evaluated as a value called residual shear strain.

8D rll-π・I()△P (15) G: Transverse elastic modulus (kqf/rgrl) D, Coil center diameter (K) d; Wire diameter (D) K: Wahl's modification coefficient (Coil spring's Also, the amount of set-off from the 1-john bar is determined by converting the amount of decrease in torsion angle △0 to the amount of residual shear strain according to yn-△θ-(1/2β) Ta.

d; wire diameter (mm); l; effective length, mm).
Shown in the figure. As is clear from FIGS. 3 to 6, it is recognized that all of the steels of the present invention have superior sag resistance compared to Bl steel, which is a conventional steel.

Further, for A1, A2, A9 to A, 14 steel, and BI steel of the present invention, a coil spring having the same specifications as above is used, and for A3 to A8 steel, a coil spring of 10 to 110 is used using the above torsion bar. A fatigue test was conducted by repeatedly applying a stress of kgf/vnJ, but all samples
Even after repeating it ten thousand times, it did not break (16).

As mentioned above, the steel of the present invention contains ■9Nb as copper for high Si springs.
, precipitation strengthening elements such as Mo, B, Or, Ni.

Hardenability improving elements such as rare earth elements, and even Al.

By adding crystal grain refining elements such as TsrZr, we have succeeded in further improving the excellent fatigue resistance of conventional high-Sl copper for springs. The fatigue resistance required for copper is also comparable to that of conventional steel, and it has extremely high practicality, especially as copper for suspension springs for passenger cars.

[Brief explanation of the drawing]

Figure 1 is a diagram comparing the hardenability of the present invention steel and conventional steel to which hardenability improving elements have been added. Figures 3 to 6 are diagrams showing the austenite grain size when heated and held at the quenching temperature.
It is a diagram showing the amount of settling of the test piece when the hardness is set to H hole C45 to C55 after tempering. Tempering hardness HRC) Figure 5

Claims (1)

[Claims] 1. CO, 50-080% by weight, si 1°50-250%, Mrlo, 45% or less, and VO
, O5~0.50%, Nll Q, Q 5~0.50
%, M, o Q. Copper for springs with excellent fatigue resistance, characterized in that it contains one or two of the following: 0.05 to 0.50%, and the remainder substantially consists of Fe. 2 0050~080% by weight, Si ]50~
250%, MnO, 45% or less, and ■0.05-0
50%, NbO, 05-0.50%, MoQ. Containing one or two of 05 to 050%,
Sarani BO, 0005~0.0100%T 0r020
-1.00%, Ni 0.20-2.00%, one or two or more of rare earth elements 0.30% or less, CO50-080% by weight, Si 1.50-2.50%,
Mrl 0.45% or more F and ■005~0.50%
, Nb □, Q 5-050%, MOo, 05
Contains one or two insertions -1- from ~050%, and one or more from A40.03~010%, 'l'io, o2~0°10%, Zro, o2~010% -4 Weight ratio: 0050~0.80% S + 1°50
~2.50%, Mn 0.45% or less, VO, O5
~0.50%, NiI Q,Q 5~0.50%,
MOQ. 05 to 0.50%, and further contains no, o o O5 to 0. Ot o o%+
CrO420-1.00%, NiQ, 20-2.00%
, one or more of rare earth elements 0.30% or less and A[0.03~0.10% 41″1002~0.1
0%, Zr 0.02-0.1-0%, one or two inserts] 2, and the remainder substantially consisting of Fe.