JP2001220650A - Steel wire, spring and producing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide steel wire in which the low cost equal to that in pearlitic steel wire is realized in the production cost, and also, high heat resistance and high fatigue resistance equal to those of Si-Cr steel oil tempered steel wire, which is the conventional steel wire for a heat resistant spring, can be realized in heat resistance in a high temperature region and the fatigue resistance of a spring, to provide a spring and to provide a method for producing the same. SOLUTION: This steel wire contains, by mass, 0.60 to 0.95% C, 0.1 to 1.2% Si and 0.30 to 0.90% Mu and has a bainitic structure of 90% or more in volume %. Moreover, one or more kinds selected from the group consisting of 0.1 to 1.0% Mo, 0.1 to 1.0% Nb, 0.1 to 1.0% Ti and 0.1 to 1.0% W are preferably incorporated therein.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel wire, a spring and a method for manufacturing the same. In particular, the present invention relates to a steel wire and a spring which have a bainite structure and are excellent in heat resistance and fatigue resistance, and which are optimal for a spring, and a method for producing the same.

[0002]

2. Description of the Related Art A high-strength oil-tempered wire mainly composed of Si-Cr steel has been used as a material for a spring component used in an exhaust system of an automobile engine in a service temperature range of up to 250 ° C.
Oil-tempered wire has high fatigue resistance and is extremely excellent as a spring steel wire. However, it requires two heat treatments of quenching and tempering with oil after drawing in manufacturing, and the process is very complicated and expensive. Easy to be. Further, there is a problem that delayed fracture easily occurs as a material. Therefore, a high-carbon steel wire called a piano wire is used as a low-cost material. This has a pearlite structure as a metal structure, but its use is limited due to lack of fatigue resistance and heat resistance.

Therefore, as a method of improving the heat resistance,
Precipitation strengthening is performed by adding a carbide-forming element such as W, Mo, V, and Ti. However, in order to effectively precipitate these carbides, heat treatment at an aging temperature of 350 to 500 ° C. is required. By performing this heat treatment, the cementite, which is fibrous in the drawing direction, which is a factor in the strength and toughness of the pearlite steel wire, is interrupted, and in some cases, spheroidization occurs, and mechanical properties deteriorate, resulting in precipitation The effect of reinforcement cannot be obtained.

[0004] The heat treatment at 350 to 500 ° C is also a temperature condition used in low-temperature annealing which is generally performed after spring processing to remove processing strain generated in a steel wire during processing. That is, in the case of the conventional pearlite steel, it was necessary to set the heat treatment conditions under conditions where the spring characteristics were not lost.

[0005] In order to prevent the strength and toughness from being lowered by the heating, a method of using bainite as the metal structure of the matrix may be considered. Bainite is a metal structure obtained by isothermally transforming austenitized wire at low temperature mainly in heat treatment, and has a structure in which cementite is finely precipitated in ferrite ground by transformation in the low temperature region where only C diffuses. . Therefore, there is a strengthening effect similar to the above-described carbide precipitation strengthening. Further, even if aging heat treatment (low-temperature annealing) is performed by further adding a carbide-forming element, since the temperature range of the aging heat treatment is almost the same as the temperature range of the isothermal transformation described above, fine precipitation occurs in the parent phase. Breaking and spheroidization of cementite is difficult to occur.

[0006] As a prior art relating to a steel wire or a method for producing the same, which has a metal structure of bainite as the matrix,
These include:

Japanese Patent Publication No. Sho 60-18729: Method for producing a high-tensile wire excellent in ductility and cold forgeability JP-A-7-258787, JP-A-7-258797: Drawing workability and fatigue properties and Method for producing hard drawn steel wire for cold drawing with excellent torsion characteristics Japanese Patent Application Laid-Open No. 11-293400: High-strength netting wire with excellent wire breakage resistance while taking advantage of the excellent drawability of bainite

[0008] Each of these prior arts utilizes the properties of bainite which are excellent in ductility and wire drawing workability. Also in other, higher temperature range (near the A ₁ transformation point)
There is a method for producing a high-carbon steel wire for tools for the purpose of making cementite spheroidized by using the method described in Japanese Patent Application Laid-Open No. 4-246129, which utilizes the structure and arrangement of carbides having a bainite structure.

[0009]

However, all of the above-mentioned conventional techniques aim at achieving the intended strength, and in the use and purpose, the high temperature range (150 ° C. or more and 250 ° C. None of the following has been positively improved in heat resistance (high-temperature sag resistance) and fatigue strength.

Accordingly, the main object of the present invention is to realize a conventional heat-resistant spring steel wire which realizes a low cost equivalent to that of a pearlite steel wire in terms of manufacturing cost, and has high heat resistance and high spring fatigue resistance in a high temperature range. An object of the present invention is to provide a steel wire and a spring capable of realizing high heat resistance and high fatigue resistance equivalent to those of a Si-Cr steel oil-tempered wire, and a method for manufacturing the same.

[0011]

According to the present invention, the above object is attained by using bainite as the metal structure of the parent phase or by strengthening the precipitation of fine carbides by adding W, Mo, V and Ti.

The characteristics of the steel wire of the present invention are as follows: C: 0.60 to 0.95%, Si: 0.1 to 1.2%, Mn: 0.30 to 0.9 by mass% as chemical components.
0%, and the base metal structure has a bainite structure of 90% or more by volume%. It goes without saying that the remaining component of the steel wire of the present invention is mainly composed of Fe and may contain unavoidable impurities.

Furthermore, Mo: 0.1
~ 1.0%, Nb: 0.1 ~ 1.0%, Ti: 0.1 ~ 1.0% and W: 0.
It is preferable to include at least one selected from the group consisting of 1 to 1.0%. This is mainly intended to promote carbide aging precipitation of the additional component and to further improve the heat resistance.
In this case, aging precipitation occurs due to general low-temperature annealing after spring processing, and therefore, it is not necessary to add a new heat treatment step. The low-temperature annealing conditions after the spring processing will be described later in detail.

The tensile strength of the steel wire of the present invention is 1300 N / mm ² or more and 240
0 N / mm ² or less is preferable. This is because, in order to perform spring processing (coiling) as a spring steel wire, a tensile strength of 1300 N / mm ² or more is required, and considering toughness, it is preferably 2400 N / mm ² or less. In particular, 1700N / mm ² or more 2400N
/ mm ² or less is more preferable.

In order to more effectively obtain high heat resistance and high fatigue strength in a steel wire having the above strength, the thickness of the ferrite phase in the metal structure in the vertical section of the steel wire must be an average.
Preferably, it is in the range of 10 to 200 nm. When the thickness of the ferrite phase is 10 nm or more, dislocations contained in the ferrite phase can stably remain without moving to the interface and disappearing even in the heat treatment (low-temperature annealing). However, the thickness of the ferrite phase is also related to the tensile strength of the steel wire. When the thickness is 200 nm or more, the tensile strength decreases, and the mechanical properties also decrease. Therefore, the thickness of such a ferrite phase is 10 to 200 n
m.

In the steel wire of the present invention, the Vickers hardness distribution in the depth direction from the wire surface to the center of the wire falls within a range of an average value ± 50, and the Vickers hardness distribution in the circumferential direction of the wire surface is small. It is desirable that the average value be within the range of ± 50. By this hardness limitation, fatigue characteristics can be improved.

In the steel wire of the present invention, a higher improvement in fatigue strength can be obtained by reducing the surface roughness. Specifically, the ten-point average roughness Rz (JIS B 0601-199
In 4), when the thickness is 15 μm or less, particularly improvement in fatigue strength is expected. The smaller the surface roughness at this time, the more effective.

[0018] The steel wire of the present invention remains in the ferrite phase.
The dislocation density is 5.0 × 10 ⁵cm^-2Exists
Is preferred. If such a steel wire is used as a spring,
The fatigue strength of the skin shows a high value. This dislocation density is theoretically
Should be higher, but actually 5.0 × 10⁷cm
^-2Above, no further improvement in characteristics can be obtained.
You. This means that the total amount of fine precipitates fixes all dislocations.
And the dislocations are too stuck together
Because it cannot be 100% immobile dislocations
I have. So 5.0 × 10⁵~ 5.0 × 10⁷cm^-2The dislocation density range
It was enclosed. The interface between ferrite and cementite is
Observe that there may be many dislocations
Is extremely difficult, so only in the ferrite phase
Limited.

Further, in the steel wire of the present invention, it is desirable that cementite in the metal structure is microcrystallized to a crystal grain size of about 3 to 30 nm. When a steel wire having such a structure is used for a spring, the spring exhibits extremely high fatigue strength.

Further, the steel wire of the present invention has a tensile residual stress on the wire surface of 392 MPa (40 kgf / mm ² ) or less, and a residual stress distribution in the circumferential direction on the wire surface having an average value of ± 98 MPa (10 kgf / mm ² ).
Is preferably within the range. This is because when the tensile residual stress on the wire surface is high or the dispersion is large, excellent characteristics are not always obtained. In particular, regarding the fatigue strength, when the tensile residual stress is high, the performance is significantly lowered. Therefore, as an upper limit that does not significantly affect the fatigue strength, the tensile residual stress is set to 392 MPa or less. The lower limit is not particularly provided because the negative direction, that is, the higher the compressive residual stress is, the more preferable. Also, this tensile residual stress is
Even if the value is low, if there is variation in the circumferential direction of the wire surface, the fatigue strength is reduced. Therefore, the residual stress distribution is defined as an average value ± 98 MPa as a range where the fatigue strength does not decrease.

The spring of the present invention is characterized by having the above-mentioned characteristics of the steel wire of the present invention alone or in combination. A spring having these characteristics is excellent in heat resistance and fatigue resistance, and is suitable as a spring used in a valve spring of an automobile engine, particularly in a high temperature environment of 150 ° C or more and 250 ° C or less.

On the other hand, the method for producing the steel wire of the present invention
C: 0.60 ~ 0.95%, Si: 0.1 ~ 1.2%, Mn: 0.30 ~ 0.90 containing steel material transformation to bainite-based structure, wire drawing of 40 ~ 99.9% area reduction, tensile Forming a steel wire having a strength of 1300 N / mm ² or more and 2400 N / mm ² or less.

In the step of transforming bainite into a predominant structure, a heat treatment for austenitizing by heating and then isothermal transformation is performed. The heating temperature for austenitizing is about 900 to 1000 ° C. The constant temperature transformation temperature is 270 ~
About 550 ° C. is preferable. If the temperature is lower than 270 ° C., the formation of martensite increases, and if it exceeds 550 ° C., the generation of pearlite increases. More preferably, it is about 360 to 450 ° C. The preferred cooling rate from the austenitizing heating temperature to the isothermal transformation temperature is 2 to 100 ° C / sec. The isothermal transformation time is preferably from 20 seconds to 10 minutes from the viewpoint of sufficient transformation and productivity.

The reason why the area reduction rate is set to 40 to 99.99% is as follows.
This is because dislocations can be introduced into the metal structure of the steel wire to improve the tensile strength. Of course, also in the method of the present invention, Mo: 0.1 to 1.0%, Nb: 0.1 to 1.0%, Ti: 0.1 to 1.0% by mass%.
And W: desirably contains one or more selected from the group consisting of 0.1 to 1.0%.

Further, in this method for producing a steel wire, it is preferable that at least one of before and after the step of transforming the structure into a structure mainly composed of bainite is provided with a skinning step having a depth of 50 to 300 μm from the wire surface. By this peeling process,
The decarburized layer formed on the wire surface during rolling or bainite heat treatment is removed. The peeling step is effective whether performed after rolling (before the bainitic heat treatment) or after the bainitizing heat treatment. It is still effective if you do both. By this peeling step, the hardness distribution of the steel wire described above can be easily realized. The stripping step can be performed by passing a steel wire through a stripping die.

The method of manufacturing a spring according to the present invention includes, in addition to the above-described method of manufacturing a steel wire, a step of processing into a spring shape and a step of heating at a temperature of more than 300 ° C. and 600 ° C. or less for 30 seconds or more and 5 hours or less. And characterized in that:

Here, the "spring shape" is not limited to a coil shape, but includes any shape that can be processed from a steel wire such as a wire spring.

The reason why the low-temperature annealing step after the spring processing is performed at a temperature of more than 300 ° C. and 600 ° C. or less and 30 seconds or more and 5 hours or less is as follows. When the temperature exceeds 300 ° C., the dislocations in the metal structure move efficiently, and the dislocations are entangled with each other in the ferrite phase around fine carbonized precipitates, thereby improving the heat resistance of the spring. This temperature range is Mo, Nb, Ti and W
This is also a temperature range in which carbide precipitation due to such additives is expected. The reason why the temperature is set to 600 ° C. or less is that if the temperature exceeds 600 ° C., any dislocation moves to the cementite / ferrite interface, and the effect of improving the properties due to the dislocation fixation cannot be expected.
Similarly, the heat treatment time is 30
This is because, when held for more than one second, fixation of the above-mentioned transition and precipitation of fine carbides can be expected. On the other hand, if the heat treatment is performed for 5 hours or more, no further improvement in the characteristics can be obtained,
Alternatively, the dislocation may move more than necessary and the characteristics may be slightly deteriorated.

Further, in the spring manufacturing method of the present invention, any of the existing methods for reinforcing a spring is effective. For example, shot peening, nitriding, and electropolishing can be used. Regarding shot peening such as shot peening and two-step shot peening, the characteristics of springs are all improved, such as imparting compressive residual stress on the wire surface, reducing the variation in circumferential distribution of residual stress on the wire surface, and smoothing the surface roughness of the wire. Is effective. The increase in surface hardness due to nitriding seems to be contrary to the steel wire of the present invention, which has a limited hardness distribution in the depth direction. However, strengthening the wire surface based on a steel wire having a uniform hardness distribution is an extremely effective strengthening means, and is particularly effective for improving fatigue strength. The electric field polishing is effective in reducing the surface roughness of the steel wire or the spring.

Hereinafter, the reasons for selecting the constituent elements and the reasons for limiting the component ranges in the present invention will be described. (C: 0.6~0.95mass%) C is an important element that determines the mechanical properties of the steel, the matrix phase bainite, an element constituting the cementite (Fe ₃ C) for fine precipitation. The parent phase is bainite, and a content of 0.6 mass% or more is required to exhibit strength and heat resistance required for the spring. However, an excessive C content tends to cause deterioration in toughness due to wire drawing. Therefore, the upper limit of the C content is set to 0.95 mass%.

(Si: 0.1 to 1.2 mass%) Si is used as a deoxidizing agent at the time of melting and refining. It also has the effect of forming a solid solution in ferrite and strengthening it. Therefore, the lower limit is set to 0.1 mass% or more to have a deoxidizing effect. However, excessive addition leads to lack of toughness, which is likely to cause breakage during spring processing, so the upper limit is 1.2 mass to prevent lack of toughness.
% Or less.

(Mn: 0.3 to 0.90 mass%) Mn is also used as a deoxidizing agent at the time of melting and refining, similarly to Si. However, Mn is also an element that tends to cause the center segregation of the wire, and generates martensite at the center segregation part during the heat treatment, which significantly increases the disconnection rate during the wire drawing. Therefore, the lower limit of the deoxidizing effect is 0.30 mass% or more, and the upper limit that does not cause toughness deterioration is
0.90 mass%.

(Mo, Nb, Ti, W: 0.1 to 1.0 mass%) W, M
o, V, and Ti have an effect of generating carbides in steel and strengthening precipitation. However, in either case, excessive addition causes an increase or increase in precipitates, and deteriorates toughness. Therefore, the content of each element is Mo: 0.1 to 1.0 mass%, Nb: 0.1 to 1.0 mass%,
Ti: 0.1 to 1.0 mass%, W: 0.1 to 1.0 mass%.

[0034]

Embodiments of the present invention will be described below. (Test Example 1) Steel materials having the chemical components shown in Table 1 were melt-cast,
After forging, a wire rod having a diameter of 9.5 mm was produced by hot rolling. Then 9
After heating to 50 ° C. to austenite, it was subjected to constant temperature transformation at 400 ° C. for 3 minutes. Further, wire drawing was performed with a reduction in area of 82.3% to prepare a test piece having a wire diameter of 4.0 mm. Then, the ratio of bainite and the tensile strength in the obtained steel wire were measured. The bainite ratio is measured by etching the cross section and longitudinal section of the sample with picric acid alcohol before and after the drawing process, and then observing it with an optical microscope. Was estimated and calculated as volume%. For reference, there was no change in the volume% of bainite before and after the wire drawing.

In addition, a comparative example in which the chemical composition and the ratio of bainite are different, and a piano wire (SW
A comparative example using PB pearlite steel) and a comparative example using a Si-Cr steel oil-timber wire for spring (SWOSC-B) were also prepared, and the ratio of bainite and the tensile strength were measured in the same manner. In Table 1,
The bainite volume ratio and tensile strength are also shown.

[0036]

[Table 1]

In Examples 1 to 3, the ratio of bainite is 90% by volume or more, and the tensile strength is 1700 N / mm ² or more. In addition, the tensile strength is 1300 N / mm ² or more in order to evaluate each of the comparative examples as a spring by coiling.

Test Example 2 The stainless steel wire produced in Test Example 1 was evaluated for fatigue strength and sag resistance at high temperatures. Fatigue strength was evaluated in the form of a steel wire having a wire diameter of 3.5 mm x length of 700 mm, and high temperature sag resistance was evaluated in the form of a coil spring having a wire diameter of 1.0 mm.
All the samples for measuring the fatigue strength were stripped from the wire surface to a depth of 150 μm after rolling, and then subjected to bainite heat treatment and wire drawing. In the comparative example, an oil tempering treatment was performed after peeling. The specifications of the spring processed for evaluation of high-temperature set resistance are as follows.

Wire diameter: 1.0 mm Coil average diameter: 4.0 mm Free height: 45.0 mm Effective number of sheets: 10.0 Total number of windings: 12.0 Winding direction: right I've been doing good.

As the low-temperature annealing conditions of each sample, general conditions were used. The temperature was 400 ° C x 30 min except for the Si-Cr steel oil-tempered wire for spring (SWOSC-B) of the comparative example, and the 425 ° C x 30 min for the Si-Cr steel oil-tempered wire for spring (SWOSC-B).
Minutes. After this low-temperature annealing, the volume percentage of bainite was determined, but no change was observed in each sample.

The fatigue strength was measured using a Nakamura-type rotary bending tester. The test was performed up to 1 × 10 ⁷ times at a rotation speed of 7000 rpm, and the undestructed amplitude stress was defined as the maximum fatigue strength.

As shown in FIG. 1, the test method for high-temperature set resistance is to apply a compressive load (load shear stress is 700 MPa) to the produced spring and hold it at a test temperature of 200 ° C. for 2 hours.
The residual shear strain was calculated from the set amount measurement after the test. Table 2 shows the fatigue strength and the residual shear strain after the high-temperature sag resistance evaluation test of the examples and comparative examples.

[0043]

[Table 2]

All of the examples had higher fatigue strength and high-temperature sag resistance than piano wire B (comparative example), which is a common hard steel wire. In particular, Examples-in which addition of carbide-forming elements such as W, Mo, V, and Ti were performed were used for spring Si-C
r It was confirmed that high fatigue strength and high-temperature sag resistance equivalent to steel oil-tempered wire (comparative example) were achieved.
On the other hand, the comparative example with low bainite volume fraction and C
In Comparative Examples having a low content of, it was not possible to obtain excellent fatigue strength and high-temperature set resistance.

(Test Example 3) Next, using the example of Table 1, a process of transforming bainite into a structure mainly composed of bainite without peeling the wire after rolling was carried out, and a diameter of 9.5 mm to 3.5 mm was obtained.
mm, and 20 μm, 50 mm from the wire surface after rolling.
After peeling the skin of μm, 150 μm, and 300 μm, wire drawing (area reduction rate 82%) was performed. Then, the average Vickers hardness of these samples, the hardness distribution in the depth direction from the line surface to the center of the line (Vickers hardness) and the hardness distribution in the circumferential direction of the line surface (Vickers hardness)
In addition, the fatigue strength was determined. The hardness in the depth direction was measured by measuring the hardness of 10 points between the center and the surface in the cross section of the steel wire, obtaining the average value, and calculating the difference between each measured value and the average value. . The hardness in the circumferential direction was determined by measuring the hardness at eight points in the circumferential direction on the surface of the steel wire, calculating the average value thereof, and calculating the difference between each measured value and the average value. The measurement conditions of the fatigue strength are the same as in Test Example 2. Table 3 shows the results.

[0046]

[Table 3]

According to the test results, in Example-1 in which the skin was not stripped, the hardness variation in the depth direction from the wire surface to the wire center and in the circumferential direction of the wire surface, which are considered to be due to the decarburization of the wire surface. And lower fatigue strength than other stripped steel wires. Also, in Example-2 in which the skin was peeled off by 20 μm, it was not enough to remove the decarburized layer, and variation in hardness in the circumferential direction was confirmed. The fatigue strength was also lower than that of the skinned skin of 50 μm or more. On the other hand, those peeled by 50 μm or more had high fatigue strength and almost the same characteristics were obtained.

(Test Example 4) The heat treatment temperature of the low-temperature annealing treatment was set to 300 ° C. to 650 for the example manufactured in Test Example 1.
C, the heat treatment time was changed from 30 seconds to 6 hours, the dislocation density existing in each ferrite phase, the presence or absence of microcrystallization of the cementite phase, the tensile residual stress value of the wire surface and the variation in the circumferential direction, Then, their fatigue strength and high-temperature set resistance were examined. Conditions such as the degree of work and the wire diameter of the steel wire were the same as in Test Examples 1 and 2.

Regarding the dislocation density, a TEM (Transmission Electron Microscope) of a cross section of a sample having a constant thickness (measured by a contamination method) manufactured by electrolytic polishing.
From the image, the length of dislocations in the ferrite per unit volume was measured and calculated. That is, this measurement method does not accurately measure the dislocation existing at the cementite / ferrite interface. It is defined only by the dislocation density existing in the ferrite phase.

The residual stress on the sample surface was calculated from the shift of the diffraction peak obtained from the X-ray diffraction image. More specifically,
Residual stresses at four points in the circumferential direction were calculated, their average values were determined, and the difference between each calculated value and the average value was also determined.

Regarding the microcrystallization of the cementite phase, TE
It was confirmed by obtaining the average particle size of the cementite phase from the M image. When the average particle size was 30 nm or less, microcrystallization was determined to be “present”, and when the average particle size exceeded 30 nm, “presence (coarsening)” was determined. When no crystal grain boundaries were found in the cementite phase from the TEM image, it was determined that microcrystallization was “absent”.

The test conditions for fatigue strength and sag resistance at high temperatures are the same as in Test Example 2. Table 4 shows the test results.

[0053]

[Table 4]

From the test results, those without low-temperature annealing (Example-6), those with low temperature (Example-7),
It was confirmed that the samples with shorter time (-11 and 12) had lower fatigue strength and lower resistance to high-temperature sag than those of other sufficiently heated samples. In addition, as can be seen by comparing Example-7 and Example-12, it was confirmed that microcrystallization of the cementite phase was also important for improving these properties. However, it was confirmed that when the heat treatment was performed at an excessively high temperature or for a long time, the crystal grain size of the cementite was coarsened and the properties were deteriorated. Furthermore, it was also confirmed that the smaller the value and the smaller the variation in the tensile residual stress on the wire surface, the more the fatigue strength and the high-temperature resistance were improved.

(Test Example 5) Next, for the example manufactured in Test Example 1, the austenitizing temperature (950 ° C. or 1000 ° C.) and the conditions of the bainitizing treatment (austempering) were used in order to change the thickness of the ferrite phase. (360-450 at temperature
℃, constant temperature transformation time is 3 minutes)
The thickness of the ferrite phase was measured from a TEM image of a longitudinal section of each sample. Table 5 shows the austempering heat treatment conditions, the thickness of the ferrite phase, the tensile strength, the fatigue strength, and the residual shear strain. Conditions such as the working ratio and the wire diameter were the same as those in Test Examples 1 and 2.

[0056]

[Table 5]

According to the test results, the thickness of the ferrite phase was changed by changing the heat treatment conditions. As the thickness of the ferrite phase became larger, the tensile strength and fatigue strength decreased, and the high-temperature sag resistance was improved (residual resistance). (The value of the shear strain decreases). However, it was confirmed that there was a peak in the sag resistance at a high temperature, and the peak was reduced when the thickness of the ferrite phase exceeded 300 nm. This result is considered to be due to the effect that the austempering performed at a relatively high temperature increased the pearlite volume% and decreased the bainite volume%. It can be confirmed that the fatigue strength is greatly reduced when the thickness of the ferrite phase exceeds 200 nm.

(Test Example 6) Next, with respect to the embodiment manufactured in Test Example 1, the conditions such as the degree of work and the wire diameter were the same as those in Test Example 1.
As in the case of No. 2, shot peening, nitriding, and electropolishing were performed. The conditions of shot peening were as follows: shot used: 0.3 mm S.B., processing time: 30 minutes, and strain relief annealing after shot: 200 ° C. × 30 minutes. The nitriding conditions are
Treatment temperature 420 ° C, treatment time 2hrs. The gas nitrocarburizing method was used. Next, regarding the electrolytic polishing method, the chemical components of the electrolytic solution: a mixed solution of H ₃ PO ₄ , H ₂ SO ₄ , and H ₂ O, a voltage of 15 V, a current density of 250 A / dm ² , and a polishing rate of 20 μm / min. did. And the amount of electrolytic polishing 20μm (electrolytic polishing) and 5μm (electrolytic polishing)
And two types. Originally, as a method of strengthening the spring, the above methods are often performed in combination, but in this test, they are performed separately in order to see their effects. Table 6 shows the test results.

[0059]

[Table 6]

From the test results, it was found that the characteristics were particularly improved in Example 20 in which shot peening was performed. With respect to fatigue strength, it is considered that the two effects of lowering the tensile residual stress by shot peening and improving the wire surface roughness were combined. In addition, it is considered that the improvement in tensile strength by shot peening slightly affects the residual shear strain.

It was also confirmed that the fatigue strength was improved by nitriding. This is thought to be due to the fact that although the tensile strength is decreased by the nitriding treatment, the hardness of the wire surface is increased, and thus the fatigue strength is improved. However, when the metallographic structures are exactly the same, the reduction in tensile strength appears to slightly degrade high temperature set resistance. From this, it is considered that if the tensile strength is not reduced by nitriding at a lower temperature, the high-temperature resistance can be improved.

Next, regarding the effect of electrolytic polishing,
In Example-22 where the polishing amount was large, a particularly large improvement in fatigue strength was confirmed. Including shot peening results, it was confirmed that the fatigue strength was dramatically improved by reducing the surface roughness to Rz: 15 μm or less. It is clear that further improvement in characteristics can be obtained by combining these processes.

It should be noted that the steel wire, the spring, and the method of manufacturing the same according to the present invention are not limited to the above-described specific examples, and various changes can be made without departing from the scope of the present invention. It is.

[0064]

As described above, in the steel wire of the present invention, the base metal structure of the high carbon steel wire has a bainite structure of 90% or more by volume%, and furthermore, the carbide forming elements such as Mo, Nb, Ti, and W are formed. By performing precipitation strengthening by addition, a steel wire for a heat-resistant spring that is less expensive than a Si-Cr steel oil-timber wire for a spring and has the same heat resistance characteristics can be obtained. In addition, by using this steel wire for a spring, fatigue strength can be improved. Therefore, it is expected to be used as a compression / tensile coil spring or torsion bar used for home appliances and automobile parts.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a test method of a high temperature set resistance test.

Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat II (Reference) C22C 38/04 C22C 38/04 38/14 38/14 (72) Inventor Teruyuki Murai 1-1-1, Kunyokita, Itami-shi, Hyogo Prefecture No. 1 Sumitomo Electric Industries, Ltd. Itami Works F term (reference) 4K042 AA01 AA02 AA03 BA04 CA08 CA09 CA12 DA02 DA03 DA04 DA06 DC05

Claims (15)

[Claims] (1) C: 0.60 to 0.95% by mass, Si: 0.1 to 1.
A steel wire containing 2%, Mn: 0.30 to 0.90, and having a bainite structure of 90% or more by volume. 2. Mo: 0.1 to 1.0% by mass%, Nb: 0.
The steel wire according to claim 1, wherein the steel wire contains at least one selected from the group consisting of 1 to 1.0%, Ti: 0.1 to 1.0%, and W: 0.1 to 1.0%. 3. The steel wire according to claim 1, wherein the tensile strength is 1300 N / mm ² or more and 2400 N / mm ² or less. 4. The Vickers hardness distribution in the depth direction from the line surface to the center of the line falls within the range of an average value of ± 50, and the Vickers hardness distribution in the circumferential direction of the line surface has an average value of ± 50.
The steel wire according to claim 1, wherein the steel wire falls within the range. 5. The steel wire according to claim 1, wherein the wire surface roughness has a ten-point average roughness Rz of 15 μm or less. 6. The density of dislocations contained in a ferrite phase is
5.0 × 10⁵~ 5.0 × 10 ⁷(cm^-2)
The steel wire according to claim 1. 7. The cementite phase has an average crystal grain size of 3 to 30.
The steel wire according to claim 1, wherein the steel wire is microcrystallized to nm. 8. The tensile residual stress on the wire surface is 392 MPa or less, and the residual stress distribution in the circumferential direction of the wire surface has an average value of ± 98.
The steel wire according to claim 1, wherein the steel wire falls within a range of MPa. 9. The steel wire according to claim 1, wherein the ferrite phase has an average thickness in a range of 10 nm to 200 nm in a longitudinal sectional structure of the steel wire. 10. Mass%: C: 0.60 to 0.95%, Si: 0.1 to
A process of transforming steel containing 1.2% and Mn: 0.30 to 0.90 into a structure mainly composed of bainite, and a wire drawing process with a surface reduction rate of 40 to 99.9% to have a tensile strength of 1300 N / mm ² or more and 2400 N / mm ^Two
A method for producing a steel wire, comprising the following steps: 11. Mo: 0.1 to 1.0% by mass%, Nb:
11. The method for producing a steel wire according to claim 10, comprising one or more selected from the group consisting of 0.1 to 1.0%, Ti: 0.1 to 1.0%, and W: 0.1 to 1.0%. 12. At least one of before and after the step of transforming bainite into a structure mainly composed of bainite has a depth from a line surface.
The method for producing a steel wire according to claim 10 or 11, further comprising a peeling step of 50 to 300 µm. 13. A spring comprising the steel wire according to claim 1. Description: 14. A mass% of C: 0.60 to 0.95%, Si: 0.1 to
A process of transforming steel containing 1.2% and Mn: 0.30 to 0.90 into a structure mainly composed of bainite, and a wire drawing process with a surface reduction rate of 40 to 99.9% to have a tensile strength of 1300 N / mm ² or more and 2400 N / mm ^Two
A step of forming the following steel wire, a step of processing into a spring shape,
Heating at a temperature higher than 300 ° C. and lower than or equal to 600 ° C. for 30 seconds or more and 5 hours or less. 15. The spring manufacturing method according to claim 14, further comprising a combination of at least one step selected from the group consisting of shot peening, nitriding, and electropolishing.