JPH02217421A - Production of spring with high fatigue strength and steel wire for use therein - Google Patents

Abstract

PURPOSE:To improve the fatigue strength of a coil spring by subjecting a steel in which respective contents of C, Si, Mn, and Cr are specified to wiredrawing and to torsional plastic working under respectively specified conditions and then to cold spring forming. CONSTITUTION:A steel wire rod which has a composition consisting of, by weight, 0.075-1.1% C, 1.0-2.0% Si, 0.2-1.0% Mn, 0.1-1.0% Cr, and the balance Fe with inevitable impurities and containing, if necessary, one or more kinds among 0.1-0.5% Mo, 0.05-0.5% V, 0.002-0.05% Ti, 0.005-0.2% Nb, and 0.01-0.1% Al is subjected to patenting treatment and then to wiredrawing at >=75% wiredrawing reduction in area. Successively, after subjected, if necessary, to annealing treatment at 400-600 deg.C for 5-300sec, the above wire is subjected to torsional plastic working at >=3 deg. tortion angle, followed by cold spring forming. By this method, strength in a Z direction can be improved, and residual stress generated at the time of coil spring forming can be reduced.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing a cold-formed coil spring with a strength of 200 kgf/mw" or more and high fatigue strength, and a steel wire for the spring used therein. .

[Prior Art] With the need to reduce the weight of vehicles or increase the output of engines, there is a need for copper for coil springs capable of high design stress in suspension springs, valve springs, etc. Design stress generally depends on fatigue strength and sag characteristics, as described in "Latest Spring Technology" (Japan Spring Industry Association, p. 118), but since there are few breakages due to fatigue in the market, design stress depends on sag characteristics. The actual situation was that the design stress was determined. However, with the practical application of hot setting, stress peening, etc., the settling characteristics have been significantly improved. For this reason, there is currently a need for copper for springs with excellent fatigue strength that allows for high design stress.

Conventionally, cold-formed spring steel wires that are finished by drawing at room temperature after patenting are JIS 35.
Piano wire described in No. 22 etc. has been used for a long time.

The fatigue strength of such cold-formed coil springs is basically determined by the yield strength of the spring material and the tensile residual stress on the spring surface caused by cold forming.The higher the yield strength, and the lower the tensile residual stress, the higher the fatigue strength. Intensity increases. Therefore, in order to achieve high fatigue strength of a coil spring, it is necessary to increase the yield strength as much as possible and reduce the residual stress generated during cold spring forming, but two problems arise here. One is that the tensile residual stress after spring forming increases as the strength of the drawn wire material increases, so even if the strength is increased, the fatigue strength of the coil spring itself will hardly increase.

The other problem is that the effect of yield strength on fatigue strength is small even if the residual stress is constant, which is a phenomenon peculiar to coil springs using cold wire-drawn materials. The reason for this is according to (Spring Technology Research Group 1986 Spring Lecture Preprint Collection, Spring Technology Research Group, p. 7), the direction perpendicular to the wire drawing direction (hereinafter Z
This is thought to be because the strength in the direction parallel to the wire drawing direction (hereinafter referred to as the backward direction) is lower than the strength in the direction parallel to the wire drawing direction (hereinafter referred to as the backward direction), and the increase in strength in the Z direction due to wire drawing is smaller than in the L direction.

In this way, there are limits to increasing the fatigue strength of cold-formed coil springs using drawn wire materials due to the contradictory characteristics of 1 strength and tensile residual stress, and the fact that the strength in the Z direction is lower than in the L direction. there were.

[Problems to be solved by the invention] The present invention has been made in view of the above-mentioned circumstances, and has the following problems:
Provides a method for increasing the Z-direction strength of a high-strength wire-drawn material with a strength of 200 kgf/am" or more and reducing the tensile residual stress generated during cold spring forming, thereby increasing the fatigue strength of a coil spring. The purpose is to

[Means and effects for solving the problem] As a result of intensive research to solve the above-mentioned problems, the present inventors have succeeded in increasing the strength in two directions of the wire-drawn material and reducing the tensile residual that occurs during cold spring forming. In order to reduce stress, it is necessary to apply torsional plastic processing to the wire drawn material with a twist angle of 3 degrees or more before cold spring forming, and the steel material composition and heat treatment conditions after wire drawing must be optimally selected. The present invention was made based on the completely new knowledge that torsional plastic working of 3 degrees or more is possible in a drawn wire material having a strength of 200 kgf/am'' or more.

The present invention was made based on the above findings, and
The gist is in weight percent.

C: 0.75-1.1%, Si: 1.0-2.0%
.

Mn: 0.2-1.0%, Cr: 0.1-1
．． Mo: 0.1-0.5%, V: 0.05, including 0%, and others as necessary.
~0.5%.

Ti: 0.002-0.05%, Nb: 0.
005-0.2%.

A Q: Regarding steel wires containing one or more of 0.01 to 0.1%, with the remainder consisting of Fe and unavoidable impurities, after patenting treatment 7
Draw the wire at a wire drawing area reduction rate of 5% or more, and then perform torsional plastic processing at a twist angle of 3 degrees or more, or
The present invention relates to a steel wire for a high fatigue strength spring and a method for manufacturing a high fatigue strength spring, which is characterized by performing annealing at ~600°C for 5 to 300 seconds, torsional plastic working, and then cold spring forming.

The present invention will be explained in detail below.

First, the reasons for limiting the composition of steel, which is the object of the present invention, will be described.

C: C is an essential element to increase the strength of the patented material, but if it is less than 0.75%, the final strength of the drawn wire material of 200 kgf/m2 or more cannot be obtained; If it exceeds 1%, pro-eutectoid cementite will precipitate at the austenite grain boundaries during the patenting process, deteriorating wire drawability and toughness, so it is limited to a range of 0.75 to 1.1%.

Si: Si is effective for reinforcing ferrite in pearlite and deoxidizing steel, and also has the effect of suppressing strength loss when annealed at 400 to 600°C to increase torsional plastic workability. Although I have it.

If it is less than 1.0%, the above effect cannot be expected; on the other hand, if it is 2.0%,
If it exceeds 1.0 to 2.0%, the above effect will be saturated.
%.

Mn: Mn is not only necessary for deoxidation and desulfurization, but is also an effective element for improving the hardenability of steel and increasing the strength of patented materials, but if it is less than 0.2%, the above On the other hand, if the content exceeds 1.0%, the above effects will be saturated and the holding time will be too long to complete the pearlite transformation in the patenting process, which is impractical. % range.

Cr: Cr refines the cementite spacing of pearlite and increases the strength of the patenting material.

Furthermore, it is an effective element for reducing strength loss when annealing is performed after wire drawing and improving torsional plastic workability, but if it is less than 0.1%, the above effect cannot be obtained; If it exceeds 1.0%, the patenting processing time becomes too long, so it is limited to 0.1 to 1.0%.

The above are the basic components of the steel that is the object of the present invention. In addition, the present invention also improves the hardenability of the steel, increases the strength of the patented material, and coarsens the grain size during austenitizing treatment. Mo, V,
It is also possible to contain one or more of Ti, Nb, and AQ.

Mo: Mo increases the strength of the patenting material.

Further, although it is an effective element for reducing strength loss during annealing treatment, it is ineffective if it is less than 0.1.

On the other hand, even if it exceeds 0.5%, there is no effect commensurate with the added amount, so this was set as the upper limit.

v: Like Mo, v increases the strength of the patenting material and reduces the decrease in strength during annealing treatment, and also suppresses coarsening of the crystal grain size during austenitization treatment by forming nitrides. However, if it is less than 0.05%, the above effect cannot be obtained, and on the other hand, if it exceeds 0.5%, the effect is saturated, so it is limited to 0.05 to 0.5%.

Ti: Ti has the effect of suppressing coarsening of crystal grain size during austenitizing treatment by combining with N to form TiN, but if it is less than 0.002%, the effect is insufficient; If it exceeds 0.002 to 0, coarse TiN or TiC will be generated which is harmful to spring fatigue.
．． It was limited to a range of 0.05%.

Nb: Nb has the effect of preventing coarsening of the crystal grain size during austenitizing treatment, but if it is less than 0.00%, the effect is insufficient, while if it exceeds 0.2%, the effect is saturated. ,oos~0.2%.

Afi: AQ-4+Nb has the same effect as Ti, but if it is less than 0.01%, the effect will not be exhibited, while if it exceeds 0.1%, the effect will be saturated, so 0.01 to 0.
Limited to 1%.

Furthermore, the amounts of impurity elements P and S, which are harmful to coil spring fatigue, are not particularly regulated, but are preferably 0.01% or less each.

After the patenting treatment with the above component range, it is necessary to perform wire drawing with a wire drawing area reduction rate of 75% or more.

This is because the strength of 200 kgf/am'' or more, which is the target of the present invention, cannot be obtained with a wire drawing degree of less than 75%.The strength after patenting treatment is 135~
160 kgf/■■2 can be obtained with the above component range.

Next, torsional plastic working before cold spring forming, which is the most important point of the present invention, will be explained. By subjecting the wire-drawn material to torsional plastic processing with a twist angle of 3 degrees or more, it is possible to increase the Z-direction strength of the wire-drawn material and to significantly reduce the tensile residual stress that occurs during cold spring forming.

First, in the present invention, a twist angle of 3 degrees or more means:
This refers to an angle φ of 3 degrees or more as shown in FIG. 1, and represents the amount of torsional plastic working. Table 1 shows examples of the strength change in the Z direction, the residual stress change before cold spring forming, and the residual stress change after spring forming when such torsional plastic processing is applied to drawn wire material at various twist angles. show,
When the torsion angle is O1, that is, as it is after wire drawing in the conventional method, or when the torsion angle is less than 3 degrees, the strength in the Z and L directions is extremely low compared to the L direction, and the fatigue strength is determined by the strength in the two directions. It shows. In addition, the residual stress in the surface layer is tensile when the wire is drawn, and the tensile residual stress generated during cold spring forming is also high.The strength in the Z direction is lower than that in the L direction, and the tensile residual stress after forming the spring is also high. High stress means that the fatigue strength of the coil spring does not increase even if the strength in the L direction is increased. On the other hand, in the steel wire of the present invention, which has undergone torsional plastic working of 3 degrees or more,
The strength in the Z direction increases, and the residual stress in the drawn wire material changes from the tensile side to the compressive side due to torsional plastic processing, so the tensile residual stress generated during cold spring forming is significantly greater than that of steel wire that is not subjected to torsional plastic processing. As a result, the fatigue strength of the coil spring according to the present invention is higher in two directions than the coil spring manufactured by the conventional method, and the residual stress generated during cold spring forming is low, so the fatigue strength is extremely high as will be described later. .

As described above, the strength in the Z direction is increased and the residual stress generated during cold spring forming is reduced. In order to achieve this, it is necessary to perform torsional plastic processing with a twist angle of 3 degrees or more, but the first
As is clear from the table, as the amount of torsional plastic working increases, the strength in two directions increases, and the residual stress after spring forming also decreases, so it is preferably 5 degrees or more.

In addition, in the present invention, the wire drawing material is heated at 400 to 600°C to
Torsional plastic working may be performed after 300 seconds of annealing. The purpose of annealing is twofold. One is to lower the yield strength and reduce the residual stress generated during cold spring forming. . Another reason is that if the strength of the wire-drawn material in the composition system of the present invention exceeds approximately 230 kgf/■ 2, cracks may occur during torsional plastic processing.
This is to prevent this. - As an example, Table 2 shows the effect of annealing on the residual stress generated during cold spring forming and the torsional plastic workability. The annealed steel wire has lower residual stress after spring forming and has better torsional plastic workability. Therefore, the strength of the drawn wire material is approximately 230 kgf/
+a■2 or more, there are fewer problems with the quality of the coil spring if annealing is performed before torsional plastic working.

The annealing treatment conditions were set at a temperature range of 400 to 600°C, since the above effects could not be obtained if the temperature was less than 400°C, and on the other hand, if the temperature exceeded 600°C, the strength would be significantly reduced by the annealing treatment. Further, if the retention time is less than 5 seconds, the above-mentioned effects of annealing cannot be expected, and if it exceeds 300 seconds, the above-mentioned effects will not improve and it is not practical, so it was limited to 5 to 300 seconds.

[Example 1 Table 3 shows the chemical composition, patenting material strength, wire drawing area reduction ratio, annealing conditions after wire drawing, tensile strength, and torsional plastic workability of the test materials, and the test numbers in the same table. 1-5, 7.1
0 is an example of the present invention, and the others are comparative examples. These test materials were made from 300kg steel ingots by vacuum melting.
Manufactured by forging and hot rolling. In addition, the patenting treatment is performed after the austenitization treatment.
The temperature was maintained at 0 to 590°C for 30 seconds. In addition, the wire drawing area reduction rate was varied by changing the material wire diameter, and the final wire diameter was 4IIIlφ for all.

As seen in the same table, all of the examples of the present invention have a strength exceeding the target of 200 kgf/mm" and also have good torsional plastic workability. As a result, as mentioned above,
In addition to increasing the strength in two directions, the residual stress generated during spring forming can be reduced compared to the conventional wire drawing process, and the fatigue strength can be significantly increased.

On the other hand, the comparative example No. 6 has an appropriate chemical composition, but the wire drawing area reduction rate is as low as 70%, so the strength is 200K.
gf/a1m2 and comparative example No.
.

8.9 are all examples of inappropriate annealing conditions after wire drawing. That is, No. 8 is an example in which the annealing temperature was too low and the tensile strength was high, but cracking occurred during torsional plastic working. No. 9, on the other hand, has a too high annealing temperature and has excellent torsional plastic workability, but the target tensile strength is 200 kg.
f/Hokaku 2 not obtained, No again.

11 has a strength of 200 kgf/m because the amount of Si is too low.
This is an example in which the strength of the patenting material is low because the C content is too low, and the strength is 2.
@ No. 13 with a value of less than 00 kgf/am" is an example in which the pearlite transformation could not be completely completed during the patenting process due to the excessive amount of Cr. As a result, the wire drawing area reduction rate 58% or more cannot be drawn,
It has low strength and poor torsional plastic workability. Moreover, No. 14.15 is an example in which the amount of Si is low and it is necessary to add Cr, and the strength of the patenting material is very low compared to the example of the present invention. As a result, No. 14 could not achieve the target strength, and No. 15 cracked during torsional plastic working despite increasing the wire drawing area reduction rate and annealing to increase strength. is occurring.

[Example 2] Tests were conducted on the tightening of the coil springs in order to investigate the fatigue test and fatigue resistance of the coil springs for the test steels 1, 3, 10.14 in Table 3. Wire diameter 4mm, spring diameter 26m
After cold forming a coil spring with a spring height of 64 m and an effective number of turns of 5, it was annealed at a low temperature of 300°C for 10 minutes.

Subsequently, shot peening, aging treatment at 200° C. for 10 minutes and setting were performed, and a fatigue test and a tightening test were conducted. The fatigue test was conducted under the condition that the maximum shear stress calculated from the spring shape was 60±40kgf/++m".
Repeated up to 7 times, maximum shear stress was 90kgf/11m
The coil spring was tightened with a load of 2, and the residual strain, which is an index of the fatigue characteristics, was determined from the change in spring height after leaving it for 96 hours. These results are shown in Table 4.

As is clear from Table 4, all of the coil springs manufactured according to the present invention have a longer fatigue life than the comparative example, which was manufactured from wire-drawn processed material. This is due to the fact that by subjecting the wire-drawn material to torsional plastic working, the strength in the Z direction becomes higher than that of the wire-drawn material as is, and that the tensile residual stress generated during cold spring forming becomes smaller.

In addition, the coil spring manufactured according to the present invention has less residual shear strain than the wire-drawn raw material.

[Effects of the Invention] As is clear from the above examples, the present invention improves the tensile strength by optimally selecting the steel material composition and wire drawing conditions.
By setting the QOkgf/wm or more and applying torsional plastic working, it is possible to increase the strength in the Z direction and reduce the residual stress that occurs during cold coil spring forming.As a result, the yield in the Z direction can be reduced. This makes it possible to increase the fatigue strength of coil springs by increasing strength and reducing residual stress, and the industrial effect is extremely significant.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the definition of the twist angle in the present invention. It is.

Claims (3)

[Claims] (1) In weight%, C: 0.75-1.1%, Si: 1.0-2.0%, M
Contains n: 0.2-1.0%, Cr: 0.1-1.0%, Mo: 0.1-0.5%, V: 0.05-0.5% as necessary. Ti
:0.002~0.05%, Nb:0.005~0.2
%Al: 0.01 to 0.1% of steel wire containing one or more of 0.01 to 0.1%, the remainder being Fe and unavoidable impurities, after patenting treatment 7
A method for producing a high fatigue strength spring, which comprises drawing a wire at a wire drawing area reduction rate of 5% or more, followed by torsional plastic working at a twist angle of 3 degrees or more, and then cold spring forming. (2) In weight%, C: 0.75-1.1%, Si: 1.0-2.0%, M
Contains n: 0.2-1.0%, Cr: 0.1-1.0%, Mo: 0.1-0.5%, V: 0.05-0.5% as necessary. , T
i: 0.002-0.05%, Nb: 0.005-0.
2%, Al: 0.01 to 0.1%, and the remainder is Fe and unavoidable impurities, with a drawing area reduction rate of 75% or more after patenting treatment. A high fatigue strength spring steel wire characterized by being drawn and then subjected to torsional plastic processing at a twist angle of 3 degrees or more. (3) In weight%, C: 0.75-1.1%, Si: 1.0-2.0%, M
Contains n: 0.2-1.0%, Cr: 0.1-1.0%, Mo: 0.1-0.5%, V: 0.05-0.5 as necessary. %, T
i: 0.002-0.05%, Nb: 0.005-0.
A steel wire containing one or more of 2% Al: 0.01 to 0.1%, with the remainder consisting of Fe and unavoidable impurities, and is drawn with a drawing area reduction rate of 75% or more after patenting treatment. 400-600℃
After annealing for 5 to 300 seconds at a twist angle of 3
A high fatigue strength steel wire for springs, which is characterized by having been subjected to torsional plastic working of more than 30 degrees.