JPS62260020A - Steel wire for spring having excellent resistance to fatigue and its production - Google Patents

Abstract

PURPOSE:To easily produce a steel wire for springs having excellent resistance to fatigue by subjecting a wire contg. a small amt. of Cr as a steel wire for springs to a hardening treatment, then softening the surface layer of the wire by quick heating. CONSTITUTION:The steel products contg., by wt%, 0.3-1.0% C, 0.15-2.5% Si, 0.3-2.0% Mn, and <3.0% Cr are drawn by hot rolling, etc., to a base material having >=2.0mm diameter. The wire is then subjected to hardening and tempering treatments, by which the hardness is adjusted to >=500 Hv; thereafter, the surface layer part is heated to 500-700 deg.C for the treatment time of <=200m sec per time by a laser heating method or high-frequency induction heating method of >=10MHz. The hardness in the range of 10mum from the surface is adjusted to <=450 Hv and the hardness in the deep part of >=100mum to >=500 Hv. The steel wire for springs having the excellent resistance to fatigue is produced without subjecting the wire to a surface treatment such as shot peening treatment and soft nitriding as in the conventional practice.

Description

[Detailed Description of the Invention] <Industrial Field of Application> This defense provides a spring steel wire with excellent fatigue resistance that can be used as a high-strength spring steel wire for engines and other applications that require a high level of fatigue resistance. This is a description of the manufacturing method.

<Prior art> The fatigue resistance of metal materials increases with their hardness, but once the hardness exceeds a certain level, it begins to deteriorate due to the effects of defects such as surface flaws and non-metallic inclusions contained within the material. It is said.

Therefore, it is thought that a material with high strength and few defects is the best for fatigue resistance.

For example, valve springs used in automobile engines are required to have high fatigue resistance and heat resistance due to the harsh environments in which they are used. As the material for this valve spring II, high carbon single wire and low alloy steel wire have been put into practical use in consideration of their heat resistance, coiling formability, economic efficiency, etc. in addition to high strength.

Among them, 5LCr steel wire has better fatigue resistance and heat resistance than others.
It is superior to U-wire, and has recently been highly evaluated as the most suitable material, and its use has become widespread.

In recent years, along with this increase in strength, material defects have been improved by reducing surface flaws through surface care techniques such as peeling and countermeasures against flaw sources, and by reducing non-metallic inclusions through the establishment of clean steel melting technology. The fatigue resistance of the valve spring has been further improved.

<Problems to be Solved by the Invention> However, looking at recent trends in the development of automobile engines, there is a demand for engines that have even higher output than conventional engines and are still lightweight. Therefore, even in valve springs,
In addition, high-stress designs and long lifespans are required, and they tend to be subjected to even more severe conditions. - It is difficult to meet the demands for high output and light weight even if improvements are made by surface treatments such as shot peening and nitrocarburizing, which are usually performed to strengthen fatigue resistance.

To meet this demand, it is necessary to further strengthen the high-strength steel wire currently used for springs, but increasing the strength alone will result in a decrease in fatigue resistance due to material defects. The first premise is to eliminate the effects of material defects. However, as described above, efforts have been made to reduce material defects both on the surface and inside the material, but the level of these efforts is currently close to its limit, and it is difficult to make improvements by reducing material defects.

<Means for Solving the Problems> In view of the problems of the conventional method as described above, the present invention provides a new steel wire for springs that eliminates the influence of surface defects that are a major impediment to the fatigue resistance of W4 wire. and a method for producing it.

That is, this invention contains CO, 3 to 1.0% by weight, and S.

0.15-2.5 heavy metal%, 11n 0.3-2.0
After adjusting the base material hardness of a separate wire having a wire diameter of 2.0 or more, containing less than 3.0 wt% Cr and the balance consisting of Fe and unavoidable impurities to Hv≧500, rapid heating means The fatigue resistance is characterized by heating and softening the wire surface abdomen, and the hardness after treatment is Hv≦450 in the range of 10 μm from the wire surface, and 1-IV≧500 in the deep part of 100 μm or more. We decided to use a method for producing an excellent spring steel wire, in which only the surface layer of the high-strength steel wire is heated and softened.
The susceptibility to surface micro-defects, which were previously difficult to remove, was reduced, and a steel wire for springs with unprecedented fatigue resistance was obtained.

<Function> The fatigue resistance of a steel wire increases with hardness, but conversely decreases when the hardness exceeds a certain level. This is thought to be due to material defects, but especially in the case of coil springs,
When a load is applied, the shear stress is greatest at the wire surface, so the influence of surface defects is large.

This effect is noticeable in the high hardness region.

For example, if a line with relatively many surface flaws is designated as A, and a line with few surface flaws is designated as B, the relationship between hardness and fatigue strength is schematically shown in FIG. 1.

Further, if A is the conventional flaw level and B is the current flaw level, the effect of improving fatigue resistance due to improvement in the surface flaw level during this period can be expressed as D.

According to this invention, since the skin layer in which microscopic flaws of several micrometers in depth, which are difficult to remove at present, exist, is softened, it is possible to significantly weaken the flaw sensitivity in that area. As a result, the fatigue strength increases to B-C. The width of the softened layer needs to be at least 10 μm at the current level of flaws. Moreover, if it exceeds 100 μm, the strength of the entire line will decrease, which is not preferable.

Further, the application of the present invention is limited to steel wires having a wire diameter of 2.0ffi#1 or more. If it is less than 2.0 mm, the ratio of the softened layer to the base layer becomes large, which is not suitable. Heating of the skin layer that satisfies the above conditions is possible by applying high-density energy such as laser, electron beam, plasma, and high-frequency power with a frequency of 10 MHz or more.

The steel wire used in this invention is made of C, Si, Mn, Cr1Fe.
and unavoidable impurities, and the range of each component is preferably as follows. In addition, the following percentages are weight percentages.
It is.

C is preferably 0.3 to 1.0%, and if it is less than 0.3%, sufficient strength cannot be obtained. Moreover, if it exceeds 1.0%, quench cracking is likely to occur during quenching.

S is preferably 0.15 to 2.5%, and if it is less than 0.15%, the heat resistance will be poor. Moreover, if it exceeds 2.5%, flaws are likely to occur on the surface during hot rolling.

1 is preferably 0.3 to 2.0%; if it is less than 0.3%, the hardenability will be poor and the strength will be insufficient, and if it exceeds 2.0%, the workability will be poor.

Cr is preferably less than 3.0%. The presence of Cr improves hardenability,
Although it is very effective in obtaining heat resistance, it was set to less than 3% in consideration of economic efficiency.

In addition, in the case of spring steel wire, the hardness of the ffl material is preferably Hv≧500 in order to ensure fatigue resistance, and conversely, the flaw susceptibility increases rapidly when Hv > 450, so the wire surface hardness It is preferable that Hv≦450. To satisfy this condition, soften! 2! When processing L, the processing section is 50
It is appropriate that the heat treatment be carried out at a temperature in the range of 0 to 700°C.

f! for springs of the present invention obtained as described above! 4P
It was confirmed that il provides fatigue resistance that exceeds that obtained by surface treatments such as shot peening and nitrocarburizing, which are normally applied to improve the fatigue resistance of springs.

<Example> Hereinafter, this invention will be explained in detail with reference to Examples.

Example 1 5WO8C-V (CO, 60%, S.

1.4%, CrO, 70%, tin 0.70%, remainder Fe and unavoidable impurities) gAPil, as-rolled wire rod was stripped, drawn, and pickled to make a diameter 3.0III #l.
After heating at 850°C for 10 minutes, quenching in oil at 50°C, and continuously tempering at 400°C for 30 minutes.

Next item frequency 27MH71? High frequency power of 3m#l5ec per processing time is 2 times per second at Ti pressure 6.5KV.
The treated steel F is applied continuously at a rate of
AA is jq.

Actual Example 5 2 A raw steel wire with the same composition as in Example 1 was quenched and tempered in the same manner as in Example 1, and then 0.5KW C
O2 laser was continuously irradiated. The relative feed speed of the steel wire to the laser is 100 s/sec.
This was done while rotating the 5lll wire so that the

A treated m-line B was obtained by the above treatment.

In both Examples 133 and 2 above, during the softening treatment, the heat source was adjusted so that the temperature of the treated area was 500 to 700°C.

Cross section i of the treated steel wires A and B thus obtained! l1
The fll structure is shown in the micrographs shown in FIGS. 2(a) and (b), respectively. Further, the hardness distribution of the surface layer portion in the cross section of treated steel wires A and B is as shown in FIG. 3, and it can be seen that only the skin portion of both wires has been softened.

Next, treated steel wires A and B and treated steel wire A, [
Steel wire equivalent to 3 was added to steel FA (C51!L material) which had undergone the same quenching and tempering treatment, and to that which had undergone shot peening treatment (S treatment material), and was subjected to Nakamura type rotary bending fatigue. When the fatigue properties of the steels aA and B were evaluated using a testing machine, the results shown in Figure 4 were obtained, and the fatigue resistance of the steels aA and B treated according to the present invention was superior to that of the conventional C-treated steel treated only with heat treatment. S51 treated with shot peening
! L3! It was found that this material was even better than the L material.

<Effects of the Invention> As detailed above, the steel wire for springs of the present invention has higher fatigue resistance than ever before, and especially when used in valve springs of automobile engines, it becomes possible to reduce the weight of the valve springs and improve the engine's performance. It is expected that performance will improve.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the relationship between hardness and fatigue strength of the steel wire made by the method of this invention and the steel wire made by the conventional method, and Fig. 2 (a) and (b) are the copper wires obtained by the method of this invention. Figure 3 is a diagram showing the hardness distribution of the cross-sectional surface layer of the steel wire of the present invention, Figure 4 is the fatigue properties of the steel of the present invention, the steel wire, and the comparative material. 5-Ni! It is an IT diagram. Applicant's agent Patent attorney Kazu 1) Showa 3rd VA
Figure 1 Figure 4 (a) L-1'' 0prn Figure 12 (b) B processing charge m='' 0prn

Claims (4)

[Claims] (1) C0.3-1.0% by weight, Si0.15-2.5
Weight %, Mn 0.3-2.0 weight %, Cr 3.0 weight %
H_v≧5
After adjusting to 00, the surface layer of the wire is heated and softened using a rapid heating means, and the hardness after treatment is H_
v≦450, and H_v≧5 at a depth of 100 μm or more
00, and a method for manufacturing the same. (2) The method for manufacturing a spring steel wire with excellent fatigue resistance as set forth in claim 1, wherein the rapid heating means is a laser heating method. (3) Excellent fatigue resistance as set forth in claim 1, wherein the rapid heating means is an induction heating method that continuously supplies high-frequency power with a frequency of 10 MHz or more and a processing time of 200 msec or less per time. Method for manufacturing steel wire for springs. (4) The steel wire for springs having excellent fatigue resistance as set forth in claim 1, wherein in the heat softening treatment of the surface layer portion of the wire, the treated portion is heated to a temperature in the range of 500 to 700°C. Production method.