JPH0257637A - Manufacture of spring with high fatigue strength and steel wire for spring for use therein - Google Patents

Abstract

PURPOSE:To manufacture a spring having high fatigue strength by subjecting a steel with a specific composition to austenitizing treatment, to heating treatment under specific conditions, and to cooling and further subjecting the above steel to cold spring forming into the prescribed shape and then to strain age hardening treatment at a specific temp. CONSTITUTION:A steel which has a composition consisting of, by weight, 0.3-1.3% C, 0.8-2.5% Si, 0.5-2.0% Mn, 0.5-2.0% Cr, and the balance Fe with inevitable impurities and containing, if necessary, one or >=2 kinds among 0.1-0.5% Mo, 0.05-0.5% V, 0.002-0.5% Ti, 0.005-0.2% Nb, 0.0003-0.01% B, 0.1-2.0% Cu, 0.01-0.1% Al, and 0.01-0.05% N is subjected to austenitizing treatment. Subsequently, the above steel is held at 250-500 deg.C for 3sec-30min, air-cooled or rapidly cooled, and subjected to cold spring forming into the prescribed shape and then to strain age hardening treatment at 200-450 deg.C. By this method, the cold forming coiled spring having >= about 200kgf/mm<2> strength and high fatigue strength can be obtained.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method for manufacturing a cold-formed coil spring with a strength of 200 kgf/mm" 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 stated in "Latest Spring Technology" (Japan Spring Manufacturers Association) 118, but since there are few breakages due to fatigue in the market, design stress is determined based on sag characteristics. The actual situation is what determines. However, in terms of materials, 5UP7, 5AE9254, which is a high Si material, and V,
Nb-added steel has been developed, and with the practical use of hot seso-ching, the settling properties 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 coil springs meet JIS3560°356.
1.3565.3566 or 5AE9254 etc. to the specified dimensions, oil quenching and tempering is performed in the final process to give the specified mechanical properties, and then cold forming is performed. ing. 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. If only the strength is to be increased, it is possible to use steel that meets the current JIS standards and lower the tempering temperature after quenching, but in reality, when manufacturing coil springs using such low-temperature tempered material in the cold, Two problems arise. One is that low-temperature tempered materials have low ductility and are therefore prone to breakage during cold coil spring forming. Another problem is that even if the coil spring can be formed, the higher the strength, the more the tensile residual stress after forming, so even if the yield strength is high, the fatigue strength of the coil spring will decrease. Therefore, in order to reduce the tensile residual stress after forming the spring, it is currently necessary to anneal it for 15 to 30 minutes at a high temperature of 400 to 500°C, which lowers the yield strength.

As described above, there is a limit to increasing the fatigue strength of cold-formed coil springs due to the contradictory characteristics of cold-opening formability and strength and tensile residual stress.

On the other hand, as a conventional finding to improve cold-open formability, Japanese Patent Application Laid-Open No. 60-89553 discloses that by adding 2% or more Ni, residual austenite after quenching can be reduced by 10%.
% or more to improve the cold-opening formability of copper for high-strength springs, but there is a problem in that the cost increases due to the addition of Ni.

On the other hand, since the tensile residual stress generated during cold spring forming is reduced by high-temperature annealing, Mo, which has strong resistance to tempering softening,
Additions of V, Nb, etc. have been attempted to increase strength and fatigue strength, but since the steel is annealed at high temperatures, it depends on how much strength loss can be minimized, and there is a limit to achieving high fatigue strength. was there.

(Problem to be solved by the invention) The present invention was made in view of the above-mentioned circumstances, and
The purpose of this invention is to provide a method for realizing high fatigue strength of coil springs by improving the cold spring formability of high-strength materials with a strength of 200 kgf/mm" or more and reducing the tensile residual stress after spring forming. That is.

(Means and effects for solving the problem) As a result of intensive research to solve the above problems, the present inventors have improved the cold spring formability of high-strength materials with a strength of 200 kgf/IIn++" or more. In order to reduce residual stress, it is necessary to reduce the yield ratio to 0.85 or less, and by optimally selecting the steel composition and heat treatment conditions, the yield ratio can be reduced to 0.85 or less. Furthermore, after cold spring forming, the yield strength can be significantly increased by applying strain age hardening treatment that takes advantage of the strain caused by the spring forming, resulting in higher fatigue strength of the spring. The present invention was made based on new findings.

The present invention has been made based on the above findings,
The gist is that C: 0.3-1.
3% Si: 0.8-2.5% Mn: 0.5-2
．． Contains 0% Cr: 0.5-2.0%, Mo: 0.1-0.5% V: 0.05-0.5 as required
%Ti: 0.002~0.05% Nb: 0.005~
0.2%B: 0.0003~0.01%Cu:
0.1~2.0% 8: o, ot~0.1% N
: For steels containing 0.01 to 0.05% of one or more elements, with the remainder consisting of Fe and unavoidable impurities, after austenitizing treatment, the steel is maintained at 250 to 500°C for 3 seconds to 30 minutes, and then air cooled or Rapid cooling reduces yield ratio to 0.85
After cold spring forming into a predetermined shape, 20
The present invention relates to an m-wire for a high fatigue strength spring and a method for manufacturing a high fatigue strength spring, which is characterized by performing strain age hardening treatment at 0 to 450°C.

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.

CTC is an essential element to give the product the required strength, but if it is less than 0.3%, it will not reach the desired 200kgf/
It is not possible to obtain a strength of mn+'' or higher, and on the other hand, the strength no longer increases even if it exceeds 1.3%, so it is limited to a range of 0.3 to 1.3%.

Si: Si increases strength through solid solution hardening, so it is essential for improving the fatigue resistance of the spring and for reducing the yield ratio to 0.85 or less before cold spring forming, which is the most important aspect of the present invention. However, if it is less than 0.8%, it becomes difficult to reduce the yield ratio to 0.85 or less;
．． If it exceeds 5%, the above effect will be saturated, so 0.8 to 2
．． It was limited to 5%.

Mn: Mn is not only necessary for deoxidation and desulfurization, but also improves the hardenability of steel and further increases the yield ratio to 0.85.
Although it is an effective element to reduce .

Cr: Cr is an effective element for improving hardenability, preventing graphitization of C, and reducing the yield ratio to 0.85 or less, but if it is less than 0.5%, the above effect cannot be obtained. On the other hand, if it exceeds 2.0%, the toughness decreases, so 0.
．． It was limited to 5-2.0%.

The above are the basic components of the steel targeted by the present invention, but in addition to this, the present invention also improves the hardenability of the steel, prevents coarsening of grain size during austenitizing treatment, and prevents coarsening of the grain size before spring forming. Mo, V to lower the yield ratio
,'Ti,,Nb,B,Cut AN.

It is also possible to contain one or more types of N.

Mo: Mo is an effective element for increasing hardenability and temper softening resistance, but 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.

■: Like Mo, ■ has the effect of increasing hardenability and temper softening resistance, and also has the effect of suppressing coarsening of the crystal grain size during austenitizing treatment, but if it is less than 0.05%, it has the effect of increasing the hardenability and temper softening resistance. However, the effect is saturated even if it exceeds 0.5%, so it is limited to 0.05 to 0.5%.

Ti: Ti has the effect of suppressing coarsening of crystal grain size during reaustenitization treatment by combining with N to form TiN, but if it is less than 0.002%, the effect is insufficient; If it exceeds .05%, coarse TiN will be produced which is harmful to spring fatigue, so it is limited to a range of 0.002 to 0.05%.

Nb: Nb has the effect of preventing coarsening of grain size during austenitizing treatment, but if it is less than 0.005%, the effect is insufficient, while if it exceeds 0.2%, the effect is saturated, so It was limited to .005-0.2%.

B: B is an effective element for improving hardenability, but if it is less than 0.0003%, its effect will not be fully demonstrated, while if it exceeds 0.01%, it will become coarse and precipitate compounds, degrading toughness. Therefore, it was limited to 0°0003 to 0.01%.

Cu: Cu is an effective element for improving hardenability and lowering the yield ratio, but if it is less than 0.1%, the expected effect cannot be obtained, while if it exceeds 2.0%, Cu
Since the effect of is saturated, it is limited to 0.1 to 2.0%.

Al: Al has the same effect as Ti, but at 0.01%
If it is less than 0.1%, the effect will not be exhibited, and if it exceeds 0.1%, the effect will be saturated, so it was limited to 0.01 to 0.1%.

NUN combines with 8β, Ti, and V to form AI, respectively.
! N.

It is an element that forms TiN and VN and has the effect of preventing crystal grain coarsening during austenitizing treatment, and also reduces the yield ratio and is effective for strain age hardening after spring forming.
．． If it is less than 0.01%, the curing of the above effect is not noticeable, while if it exceeds 0.05%, the effect is saturated, so 0.01 to 0.
．． It was limited to 0.05%.

Next, in order to make the yield ratio of the steel before cold forming, which is the most important point of the present invention, to 0.85 or less, after the austenitizing treatment, the steel should be held at 250 to 500°C for 3 seconds to 30 minutes, and then air cooled or Needs to be cooled quickly. By setting the yield ratio to 0.85 or less, it is possible to improve cold spring formability and to significantly reduce the tensile and compressive residual stress generated during cold spring forming compared to conventionally used oil-quenched and tempered materials. becomes.

First, the reason why the yield ratio was limited to 0.85 or less will be explained.

Tensile strength is 200kgf/m due to steel material composition and heat treatment.
elongation, which is a measure of cold spring formability, and residual stress after cold spring forming, when the yield ratio is changed over m'' and the yield ratio is changed.
Table 1 shows the results of the investigation of the effects on spring fatigue strength. A11 which is JIS3566: and JIS3566
B with increased amount of C based on .

C steel has been given a predetermined strength by the commonly performed quenching and tempering treatments before cold spring forming. Quenched and tempered materials have a high yield ratio of 0.9 or more, so their elongation at break is small. In particular, C steel was tempered at a low temperature, so its elongation decreased and cracks occurred during cold spring forming. Furthermore, the tensile residual stress generated during cold spring forming of steels A and B becomes extremely high. This means that even if the yield strength is increased, the residual stress increases accordingly, and the fatigue strength conversely decreases. On the other hand, by applying the steel material composition and heat treatment of the present invention, E and Fm have a yield ratio of 0.85 or less, which is lower than that of conventionally processed materials, so they have excellent elongation at break and have extremely low residual stress. As a result, the residual stress of the coil spring according to the present invention is smaller than that of conventional hardened and tempered materials, so that the fatigue strength is much higher than that of hardened and tempered materials, as will be described later.

Furthermore, by lowering the yield ratio, the ductility of the steel increases, so 0.8%, which was previously considered difficult to form into cold springs.
It is possible to form springs even with steel of C or higher, making it possible to further increase the fatigue strength of the spring. As mentioned above, in order to reduce residual stress during cold spring forming and improve spring formability, it is necessary to reduce the yield ratio to 0.85 or less, preferably 0.7.
5 or less is better.

Next, as the heat treatment conditions before cold spring forming in order to make the yield ratio 0.85 or less, after the austenitization treatment 250 ~
After being held at 500°C for 3 seconds to 30 minutes, air cooling or quenching treatment is performed, and by this treatment it is possible to reduce the temperature to 0.85 or less in the component system targeted by the present invention. First, the temperature range is from 250 to 500'j: However, if the temperature exceeds 500°C, pearlite tends to form, so it is not possible to obtain a strength of 200 Kgf/mm'' or more, which is the goal of the present invention. This is because the yield strength increases when the yield ratio is kept at 0.85 or less.

Furthermore, if the holding time is less than 3 seconds, the yield ratio cannot be kept below 0.85, while if the holding time exceeds 30 minutes, the yield ratio will hardly change and productivity will deteriorate, so a limit of 3° minutes was set. .

Although the composition system and heat treatment of the present invention improve spring formability and significantly reduce tensile residual stress after spring forming, the yield ratio is higher than that of quenched and tempered materials manufactured by conventional processes, which have a yield ratio of 0.9 or more. 200 after spring forming because of the small
It is necessary to increase the yield ratio by performing strain aging hardening treatment using cold spring forming strain in the temperature range of ~450°C. - As an example, Figure 1 shows the relationship between strain age hardening temperature, yield strength, and yield ratio when plastic strain is applied. As is clear from the figure, the strain age hardening treatment increases the yield strength and can increase the yield ratio to 0.9 or more. As a result, it is possible to increase the fatigue strength of the spring and reduce fatigue due to the low residual stress and high yield strength. The strain age hardening treatment temperature was set to be in the range of 200 to 450°C since it would take too much time to obtain age hardening if it was less than 200°C, whereas if it exceeded 450°C the strength would decrease. The retention time is not particularly specified, but 5 minutes or more is sufficient.

(Example 1) Table 2 shows the chemical composition of the sample material and the heat treatment conditions after the austenitization treatment. Test numbers 1-3, 5- in the same table
8.10 to 12 are examples of the present invention, and the others are comparative examples.

All of these test materials were manufactured by vacuum melting a 300 kg steel ingot, forging, hot rolling, and cold wire drawing. After austenitizing using these test materials, a predetermined heat treatment was performed, and a tensile test was performed to determine the 0.2% yield strength, 1 tensile strength, and yield ratio. These test results are also listed in Table 2.

As seen in the same table, all of the examples of the present invention have a strength of 200
Kgf/mm2 or more and yield ratio is 0.85 or less. As a result, as described above, the cold spring formability of the high-strength material is improved, and the residual stress generated during spring forming can be reduced compared to conventional hardened and tempered materials.

On the other hand, Comparative Example No. 4 had a high yield ratio of 0.86 due to the low Si content. Comparative examples Nos. 9 and 1.3.14 are all examples in which the heat treatment conditions after the austenitizing treatment were inappropriate.

That is, in No. 9, the heat treatment temperature was too high and the target tensile strength of 200 x gf/ms could not be obtained, and in No. 13, the holding time was too short, and in No. 14, the holding temperature was too low, and the yield ratio was 0.85 in both cases. This is an example where the following was not achieved.

(Example 2) Fatigue test of coil springs for test steels A and B as comparative examples and ■, ■, ■, ■, [phase], ■ as examples of the present invention in Tables 1 and 2 A test was conducted on the tightening of the coil spring to examine its resistance to fatigue. Wire diameter 41, spring diameter 26mm
After cold forming a coil spring with a spring height of 64 mm+ and an effective number of turns of 5, it was heat treated at the temperature shown in Table 3, followed by shot peening and setting, and a fatigue test and a tightening test were conducted. The fatigue test was conducted up to 10 times under the conditions that the maximum shear stress calculated from the spring shape was 70 ± 50 Kgf/mm, and the maximum shear stress was 120
Tighten the coil spring with a load of 9 kgf/mm.
Residual strain, which is an index of fatigue characteristics, was determined from the change in spring height after being left for 6 hours. These results are shown in Table 3.

As can be seen from Table 3, all of the coil springs manufactured according to the present invention have a longer fatigue life than those manufactured by quenching and tempering, which is a comparative example. This is because by limiting the yield ratio to 0.85 or less, the residual stress generated during cold spring forming is smaller than that of quenched and tempered materials, and because it is difficult to cold form high strength materials with quenched and tempered materials. This is due to the fact that cold spring forming is now possible. Further, the coil spring manufactured according to the present invention has less residual shear strain than a hardened and tempered material.

(Effects of the Invention) As is clear from the above examples, the present invention improves the yield ratio of a spring steel wire with a tensile strength of 200 Kgf/mmt or more by optimally selecting the steel material composition and the heat treatment conditions after austenitization. By making it 0.85 or less, it is possible to reduce the residual window and force generated during cold coil spring forming, and to improve cold formability. Furthermore, after cold spring forming, it is possible to increase the yield strength through strain age hardening treatment that utilizes forming strain, and ultimately increases the fatigue strength of the coil spring by reducing residual stress and increasing yield strength. The industrial effect has been extremely remarkable.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between strain age hardening treatment temperature, yield strength change, and yield ratio change when plastic strain is applied to a steel wire. Patent applicant Nippon Steel Corporation

Claims (2)

[Claims] (1) In weight%, C: 0.3-1.3% Si: 0.8-2.5% Mn:
Contains 0.5-2.0% Cr: 0.5-2.0%, Mo: 0.1-0.5% V: 0.05-0.5% Ti
:0.002~0.05% Nb:0.005~0.2
%B: 0.0003-0.01% Cu: 0.1-2.
0% Al: 0.01-0.1% N: 0.01-0.0
For steel containing 5% of one or more of the above, with the remainder consisting of Fe and unavoidable impurities, the steel is kept at 250 to 500°C for 3 seconds to 30 minutes after austenitizing treatment, then air cooled or rapidly cooled, and then shaped into a predetermined shape. After cold spring forming to 20
A method for manufacturing a high fatigue strength spring, which comprises subjecting the spring to strain age hardening treatment at 0 to 450°C. (2) In weight%, C: 0.3-1.3% Si: 0.8-2.5% Mn:
Contains 0.5-2.0% Cr: 0.5-2.0%, Mo: 0.1-0.5% V: 0.05-0.5% Ti
:0.002~0.05% Nb:0.005~0.2
%B: 0.0003-0.01% Cu: 0.1-2.
0% Al: 0.01-0.1% N: 0.01-0.0
For steel containing 5% of one kind or two or more kinds, with the remainder consisting of Fe and unavoidable impurities, the yield ratio can be adjusted by holding at 250 to 500 °C for 3 seconds to 30 minutes after austenitizing treatment, and then air cooling or rapid cooling. A high fatigue strength spring steel wire characterized by having a tensile strength of 0.85 or less.