JPWO2018211779A1 - Oil tempered wire - Google Patents

Abstract

C is 0.50% to 0.90% by mass, V is 0.02% to 0.50% by mass, Ta is 0.01% to 0.50% by mass, and 0.01% by mass. % To 0.50% by mass of Nb, 0.02% to 0.50% by mass of Mo, and 0.02% to 1.00% by mass of W. Element, Si is 0.80% to 3.00% by mass, Mn is 0.40% to 1.00% by mass, and Cr is 0.40% to 2.00% by mass. Having a composition in which the balance is Fe and impurities, and includes a carbide containing the reinforcing element, and the total content of the reinforcing element in the carbide containing the reinforcing element is the total amount of the reinforcing element. Is 10% or more by mass with respect to the average particle size of the carbide containing the reinforcing element. Oil-tempered wire is 30nm or less.

Description

The present invention relates to an oil-tempered wire.
This application claims priority based on Japanese Patent Application No. 2017-100377 filed on May 19, 2017, and incorporates all the contents described in the Japanese Application.

  One of the components such as an engine and a transmission of an automobile is a spring. Oil tempered wires are widely used for these spring materials.

  The oil-tempered wire is typically manufactured by drawing steel, followed by quenching and tempering. As a steel type having a high fatigue limit, SWOSC-V (JIS G 3561, 1994) containing carbon (C), silicon (Si), manganese (Mn), and chromium (Cr) is known. Steel types containing vanadium (V) in SWOSC-V and steel types containing V and cobalt (Co) have higher fatigue limits and are used for valve springs and the like.

  Patent Literature 1 discloses an oil-tempered wire made of steel equivalent to SWOSC-V, having no spherical carbide having a diameter of 0.2 μm or more, and having an average crystallite area of a specific size. According to Patent Document 1, if the diameter is less than 0.2 μm, the spherical carbide hardly acts as a starting point of fracture, and the toughness can be improved because the crystallite has a specific size.

JP 2002-180195 A

Oil tempered wire of the present disclosure,
C is 0.50% by mass or more and 0.90% by mass or less,
V of 0.02% by mass to 0.50% by mass, Ta of 0.01% by mass to 0.50% by mass, Nb of 0.01% by mass to 0.50% by mass, 0.02% by mass At least 0.50% by mass of Mo and at least 0.02% by mass and at least 1.00% by mass of W;
0.80 mass% or more and 3.00 mass% or less of Si;
Mn is 0.40% by mass or more and 1.00% by mass or less;
Cr has a composition of not less than 0.40% by mass and not more than 2.00% by mass, with the balance being Fe and impurities,
Comprising a carbide containing the reinforcing element,
The total content of the reinforcing element in the carbide containing the reinforcing element is at least 10% by mass relative to the total amount of the reinforcing element,
The carbide containing the reinforcing element has an average particle size of 30 nm or less.

FIG. 4 is a micrograph of an oil-tempered wire of No. 4. FIG. 1 is a photomicrograph of an oil-tempered wire of No. 1. FIG. 4 is a photomicrograph of the oil-tempered wire No. 4 after a simulation process, and in order from the left, an atomic number contrast image (Z contrast image) by a transmission electron microscope (TEM), and an energy dispersive X-ray analysis attached to the TEM. FIG. 2 is an element mapping image of Fe and an element mapping image of V analyzed using an (EDX) apparatus. FIG. FIG. 1 is a photomicrograph of the oil-tempered wire 1 after a simulation process, in which, from left to right, a Z-contrast image by TEM, an element mapping image of Fe analyzed using a TEM-EDX device, and an element mapping image of V. .

[Problems to be solved by the present disclosure]

  In recent years, in response to demands for low fuel consumption of automobiles, downsizing and weight reduction of engines, transmissions, and the like have been increasingly promoted, and the stress applied to springs used for these engines has been increasing. Therefore, there is a demand for an oil-tempered wire that hardly breaks even when repeatedly subjected to stress and has more excellent fatigue characteristics.

Therefore, it is an object to provide an oil-tempered wire having excellent fatigue characteristics.
[Effects of the present disclosure]

  The above-mentioned oil-tempered wire has excellent fatigue characteristics.

[Description of Embodiment of the Present Invention]
The inventors of the present invention have controlled addition elements that are effective in improving fatigue characteristics, and have studied, in particular, an improvement in fatigue limit by strengthening dispersion of precipitates in crystal grains.

When an oil-tempered wire is used as a spring, a portion where the spring yields when the spring is repeatedly subjected to stress during use and may become a starting point of fracture is considered to be within a crystal grain rather than a crystal grain boundary. This is because in the oil-tempered wire, cementite (Fe ₃ C) is mainly precipitated at the crystal grain boundaries, and it is considered that the crystal grain boundaries are strengthened by the cementite. Therefore, particularly, precipitation strengthening in crystal grains was studied. As a result, a specific strengthening element such as V is included in a specific range, and at least a part of the content is included in the crystal grains as carbide, and if the carbide is very fine, the It has been found that the fatigue limit can be improved by strengthening the dispersion in the crystal grains. The present invention is based on the above findings.
First, embodiments of the present invention will be listed and described.

(1) The oil-tempered wire according to one embodiment of the present invention is:
C is 0.50% by mass or more and 0.90% by mass or less,
V of 0.02% by mass to 0.50% by mass, Ta of 0.01% by mass to 0.50% by mass, Nb of 0.01% by mass to 0.50% by mass, 0.02% by mass At least 0.50% by mass of Mo and at least 0.02% by mass and at least 1.00% by mass of W;
0.80 mass% or more and 3.00 mass% or less of Si;
Mn is 0.40% by mass or more and 1.00% by mass or less;
Cr has a composition of not less than 0.40% by mass and not more than 2.00% by mass, with the balance being Fe and impurities,
Comprising a carbide containing the reinforcing element,
The total content of the reinforcing element in the carbide containing the reinforcing element is at least 10% by mass relative to the total amount of the reinforcing element,
The carbide containing the reinforcing element has an average particle size of 30 nm or less.

  In the above-mentioned oil-tempered wire, the average particle diameter of the carbide containing the reinforcing element (hereinafter sometimes referred to as dispersed carbide) is 30 nm or less, and the dispersed carbide is very fine. Therefore, the dispersed carbide tends to be harder than a carbide containing Cr, for example, but hardly becomes a starting point of destruction when the oil-tempered wire is repeatedly subjected to stress. In addition, the oil-tempered wire contains, as a dispersed carbide, an amount of 10% or more by mass with respect to the total amount of the reinforcing element, and the dispersed carbide is mainly contained in crystal grains. It can be said that the inside of the crystal grain has a dispersion strengthened structure. The above-mentioned oil-tempered wire having such a specific strengthened structure has a high fatigue limit, is excellent in fatigue characteristics, and can be suitably used for various spring materials.

  In the above-mentioned oil-tempered wire, Fe-based carbides such as cementite are mainly precipitated at crystal grain boundaries, and grain boundaries are strengthened by the Fe-based carbides. From this point as well, the oil-tempered wire has a high fatigue limit and is excellent in fatigue characteristics.

  The above-mentioned oil-tempered wire distinguishes the carbide present in the metal structure into the above-mentioned dispersed carbide which mainly strengthens the inside of the crystal grain and the Fe-based carbide which mainly strengthens the crystal grain boundary. As the carbide that contributes to the improvement of the stress, the size and content of the dispersed carbide are set to the above specific ranges. Such an oil-tempered wire is superior in fatigue characteristics as compared with a case where only the maximum diameter is controlled without distinguishing carbides.

(2) As an example of the above oil tempered wire,
An embodiment in which the crystal grain size is 10 or more and 14.5 or less is given.

  In the above embodiment, in addition to the strengthening effect in the crystal grains due to the fine dispersed carbides, the toughness is excellent because the crystal grain size is not too small, and the cementite is appropriately present in the crystal grain boundaries when the crystal grain size is not too large. Thus, the effect of strengthening the crystal grain boundaries by cementite can be obtained favorably. Therefore, the above embodiment is more excellent in fatigue characteristics.

(3) As an example of the above-mentioned oil-tempered wire,
The composition is from 0.10% to 0.30% by mass of V, 0.10% to 0.35% by mass of Ta, and 0.10% to 0.35% by mass of Nb. A form including one or more selected strengthening elements may be mentioned.

  The above embodiment includes at least one strengthening element of V, Ta, and Nb in the above-described specific range. Therefore, in the above embodiment, while the reinforcing element is present as a dispersed carbide, it is easy to prevent a decrease in toughness due to the solid solution of the excess in cementite, and is excellent in fatigue characteristics.

(4) As an example of the above oil tempered wire,
A form in which the total content of carbides is 1% by mass or more and 3% by mass or less is given. The total content of the above carbides is the total amount of dispersed carbides, Fe-based carbides, and other Cr-containing carbides contained in the oil-tempered wire.

  In the above-described embodiment, since the carbide is contained in the above-described specific range, both effects of precipitation strengthening in the crystal grain by the dispersed carbide and precipitation strengthening of the crystal grain boundary by the Fe-based carbide are appropriately obtained, and the excess carbide Low toughness and excellent fatigue properties.

(5) As an example of the above oil tempered wire,
An embodiment in which the total content of the carbide is 2% by mass or more and 4% by mass or less after heat treatment equivalent to nitriding treatment, shot peening, and heat treatment equivalent to low-temperature annealing is sequentially performed.
The heat treatment equivalent to the nitriding treatment is performed at 400 ° C. for 4 hours.
In the shot peening, a process of using a cut wire having a diameter of 0.7 mmφ as a shot material and a process of using a steel ball having a diameter of 0.3 mmφ as a shot material are each performed for 30 minutes (a total of 1 hour).
The heat treatment corresponding to low-temperature annealing is performed at 230 ° C. for 30 minutes.

  The above-described process is a process that simulates a spring manufacturing process (hereinafter, may be referred to as a simulation process). After the simulated treatment, Fe-based carbides such as cementite tend to increase. Therefore, the total content of carbides in the above-described embodiment tends to increase from the amount specified in (4) above, but within the above range, the dispersed carbide and the Fe-based carbide described in (4) above can be used. The effect obtained by including both of them is appropriately obtained, and the fatigue characteristics are excellent. When the grain boundaries are further strengthened with an increase in Fe-based carbides, the fatigue characteristics may be more excellent. The above-described embodiment is excellent in fatigue characteristics after the above-described simulation processing, and thus can be suitably used as a spring material, and can form a spring having excellent fatigue characteristics.

(6) As an example of the above oil tempered wire,
Examples of the composition include a form containing 0.63% by mass or more and 0.68% by mass or less of C.

  In the above embodiment, since the content of C is within the above-described specific range, both the dispersed carbide and the Fe-based carbide are more appropriately contained, and the fatigue characteristics are excellent.

[Details of Embodiment of the Present Invention]
Hereinafter, embodiments of the present invention will be specifically described. The content of the additional element is a mass ratio (mass%) with the composition of the oil-tempered wire being 100%. The measurement conditions such as the content and the size will be described in Test Examples described later.

[Oil tempered wire]
(composition)
The oil-tempered wire of the embodiment contains C, Si, Mn, Cr and a specific strengthening element in a specific range, and the balance is composed of steel that is Fe and impurities. Examples of the impurities include phosphorus (P) and sulfur (S). Hereinafter, the content of each element and the effect of each element will be described.

<C>
C is a steel strengthening element. When the content of C is 0.50% by mass or more, high strength steel can be obtained. The greater the content of C, the higher the strength of the steel. When the content of C is 0.60% by mass or more, and more preferably 0.63% by mass or more, the steel can have higher strength and a higher fatigue limit. When the C content is 0.90% by mass or less, a decrease in toughness caused by excessive precipitation of Fe-based precipitates such as cementite at crystal grain boundaries can be suppressed. When the content of C is 0.80% by mass or less, 0.70% by mass or less, further 0.68% by mass or less, an oil-tempered wire having more excellent toughness can be obtained.

<Si>
Si is an element that contributes to solid solution strengthening of steel and improvement in heat resistance of steel. Further, Si is used as a deoxidizing agent at the time of refining. When the content of Si is 0.80% by mass or more, the above effects can be obtained favorably. Since the higher the Si content, the more excellent the strength and heat resistance, the Si content is set to 0.90 mass% or more, 1.00 mass% or more, 1.20 mass% or more, and 1.30 mass% or more. be able to. When the content of Si is 1.80% by mass or more, 1.90% by mass or more, and more preferably 2.00% by mass or more, the steel is more excellent in strength and has a higher fatigue limit, or the steel is more excellent in heat resistance. Or you can. Excellent heat resistance can prevent the steel from softening due to the heat treatment during the production of oil-tempered wires (particularly tempering) and the heat treatment during the production of springs using oil-tempered wires (nitriding, annealing, etc.). . When the content of Si is 3.00% by mass or less, the effect of solid solution strengthening can be favorably obtained. In addition, a decrease in toughness due to excessive precipitation of Si can be prevented, and cold workability such as wire drawing can be prevented. The decrease is suppressed and the productivity is excellent.
From this point, the content of Si can be set to 2.50% by mass or less, and further 2.40% by mass or less.

<Mn>
Mn improves the hardenability of steel. Mn also has the effect of improving the strength of steel. Further, Mn is used as a deoxidizing agent at the time of melting and refining, like Si. When the content of Mn is 0.40% by mass or more, the above effect can be obtained favorably. The above effect is enhanced as the Mn content increases, and the Mn content can be set to 0.45% by mass or more, and further 0.50% by mass or more. When the Mn content is 1.00% by mass or less, a decrease in toughness can be suppressed. When the content of Mn is 0.90% by mass or less, 0.80% by mass or less, and further 0.70% by mass or less, the decrease in toughness is more easily suppressed.

<Cr>
Cr improves the hardenability of the steel and increases the softening resistance of the steel after quenching and tempering. Due to the increase in softening resistance, as described above, it is possible to prevent the steel from being softened by the heat treatment during the production of the oil-tempered wire or the heat treatment during the production of the spring, thereby contributing to higher strength. When the content of Cr is 0.40% by mass or more, the above effects can be obtained favorably. The above effect is enhanced as the Cr content increases, and the Cr content can be adjusted to 0.60% by mass or more, 0.80% by mass or more, and 0.90% by mass or more. When the content of Cr is 2.00% by mass or less, a decrease in toughness can be suppressed. When the content of Cr is 1.90% by mass or less, and more preferably 1.80% by mass or less, a decrease in toughness can be more easily suppressed.

<Strengthening element>
The oil tempered wire of the embodiment contains one or more types of strengthening elements selected from V, Ta, Nb, Mo, and W in the following range. In the oil-tempered wire of the embodiment, at least a part of the strengthening element is contained as a carbide, and the carbide is very fine and dispersed in the crystal grains to improve the fatigue properties. Even when the reinforcing element contains only one of the five types listed above, at least a part of the content of the element is excellent in fatigue properties because it exists as a fine dispersed carbide. If two or more of the above five elements are contained and at least a part of the total content of these elements is present as fine dispersed carbides, it is expected that fatigue properties will be further improved.
V: 0.02% to 0.50% by mass Ta: 0.01% to 0.50% by mass Nb: 0.01% to 0.50% by mass Mo: 0.02% by mass or more 0.50% by mass or less W: 0.02% by mass or more and 1.00% by mass or less

When the content of the reinforcing element satisfies the lower limit described above, a carbide (dispersed carbide) containing the reinforcing element can be formed, and the effect of improving the fatigue properties by dispersion strengthening can be obtained. As the content of the strengthening element increases, the precipitation amount of the dispersed carbides is easily increased, so that the effect of improving the fatigue properties by the above-described dispersion strengthening is easily obtained. When the content of the reinforcing element satisfies the upper limit described above, it is possible to prevent the excess from being solid-dissolved in cementite while containing dispersed carbides, and to have good toughness. The content of the reinforcing element can be further in the following range.
V: 0.05% by mass or more and 0.35% by mass or less, further 0.10% by mass or more and 0.30% by mass or less Ta, Nb, Mo: 0.05% by mass or more and 0.35% by mass or less 0.10% by mass to 0.35% by mass W: 0.05% by mass to 0.90% by mass, further 0.10% by mass to 0.80% by mass

  Among the strengthening elements, in particular, V of 0.10% to 0.30% by mass, Ta of 0.10% to 0.35% by mass, and 0.10% to 0.35% by mass. When one or more elements selected from the following Nb are included, the fatigue characteristics are more excellent. In the case where V having a relatively small atomic weight is contained, the dispersion strengthening effect is easily obtained when a relatively large amount of fine dispersed carbide is contained. When Ta or Nb having an atomic weight larger than V is contained, the dispersion strengthening effect is easily obtained even if the content of the dispersed carbide is relatively small.

(Organization)
<Crystal size>
The oil tempered wire of the embodiment has a crystal structure. In particular, when the crystal grain size is 10 or more, since the crystal grains (former austenite crystal grains) are fine, the amount of crystal boundaries per unit volume is large, and Fe-based carbides such as cementite can be appropriately present at the crystal boundaries. As a result, the grain boundary strengthening by the Fe-based carbide can be favorably performed, and the fatigue characteristics are excellent. Since the larger the crystal grain size is, the finer the crystal is and the easier the above-mentioned effect is to be obtained, the crystal grain size can be 10.5 or more, 11 or more, and further 12 or more. When the crystal grain size is 14.5 or less, the crystals are not too small and can have good toughness. When the crystal grain size is 14 or less, and more preferably 13.8 or less, it is possible to have the effect of improving the fatigue properties and the appropriate toughness by the grain boundary strengthening described above. A crystal grain size of 10 or more and 14.5 or less corresponds to an average crystal grain size of 2.32 μm or more and 10.0 μm or less.

<Precipitate>
-Dispersion carbide The oil tempered wire of the embodiment includes a carbide (dispersion carbide) containing the above-described reinforcing element as one of the precipitates. In the oil-tempered wire of the embodiment, the dispersed carbides are very fine, and have a structure dispersed and present in the crystal grains, that is, a dispersion strengthened structure by precipitates. Examples of the dispersed carbide include V ₄ C ₃ , TaC, and NbC.

··· Particle size In the oil tempered wire of the embodiment, the average particle size of the dispersed carbide is 30 nm or less. If the dispersed carbide is very fine as described above, it is easy to uniformly disperse and exist in the crystal grains, and a good dispersion strengthening effect can be obtained. As the average particle size is smaller, the dispersed carbides are more uniformly dispersed to easily obtain a dispersion strengthening effect, and the dispersed carbides are less likely to be a starting point of destruction. Therefore, the average particle size is preferably 28 nm or less, more preferably 25 nm or less, 20 nm or less, and 18 nm or less. In order to reduce the dispersed carbides, for example, as described later in the manufacturing process, when the temperature is kept at a predetermined temperature in the process of lowering the temperature from the austenitizing temperature, the holding temperature is made lower. When the average particle size is 1 nm or more, and more preferably 2 nm or more, it is possible to manufacture without excessively lowering the heat treatment temperature in the above-described manufacturing process, and the productivity is excellent.

  The smaller the maximum diameter of the dispersed carbide, the better. This is because, when the oil-tempered wire of the embodiment or the spring using the oil-tempered wire is repeatedly subjected to stress, if the carbide particles are fine, it is difficult to become a starting point of fracture and the fatigue limit can be increased. The maximum diameter is preferably 100 nm or less, more preferably 80 nm or less, 70 nm or less, and 50 nm or less.

··· Content In the oil-tempered wire of the embodiment, the total content of the reinforcing elements in the dispersed carbide is 10% or more by mass, with the total amount of the reinforcing elements in the composition of the oil-tempered wire being 100%. Hereinafter, the total content of the strengthening elements in the dispersed carbide may be referred to as the precipitation amount (% by mass) of the strengthening elements, and the precipitation amount of the strengthening elements with respect to the total amount of the strengthening elements may be referred to as the precipitation ratio. As the precipitation ratio increases, the amount of the strengthening element present as the dispersed carbide increases, so that the effect of improving the fatigue characteristics by the dispersion strengthening in the crystal grains is easily obtained. The ideal upper limit of the precipitation ratio is 100%, that is, the total amount of the strengthening element is present as carbide. In order to increase the deposition rate, for example, it is necessary to secure a long deposition time in the manufacturing process. However, the prolongation of the manufacturing time causes a decrease in productivity. When the precipitation ratio is 70% or less, and more preferably 60% or less, the production can be performed without excessively increasing the production time, and the productivity is excellent. In addition, it is allowed that some of the strengthening elements are dissolved in the matrix.

The present inventors prefer that the precipitation ratio is higher when the reinforcing element contains V having a relatively small atomic weight, and when the element contains Ta, Nb, Mo, and W having a relatively large atomic weight. It has been found that even if the precipitation ratio is about 10%, the effect of improving the fatigue properties can be obtained.
Therefore, when V is contained, the precipitation ratio is preferably 20% or more, more preferably 25% or more, and 30% or more. In the case where Ta, Nb, Mo, and W are contained, when the precipitation ratio is 11% or more, and more preferably 12% or more, the effect of improving the fatigue characteristics is more easily obtained.

The amount of precipitation of the above-mentioned strengthening element is 0.001% by mass or more, assuming that the composition of the oil-tempered wire is 100% by mass, since the lower limit of the content of the strengthening element is 0.01% by mass.
As the precipitation amount of the strengthening element is larger, the effect of improving the fatigue characteristics by the dispersion strengthening in the crystal grains is more easily obtained, and the precipitation amount of the strengthening element is 0.01% by mass or more with respect to the composition of the oil-tempered wire, and It can be 0.02% by mass or more. When V is contained, the lower limit of the V content is 0.02% by mass. Therefore, the amount of V deposited is 0.006% by mass or more, more preferably 0.01% by mass or more, with respect to the composition of the oil-tempered wire. 0.03% by mass or more and 0.05% by mass or more.

  The size of the dispersed carbide, the precipitation amount of the reinforcing element, and the precipitation ratio can be adjusted by the total amount of the reinforcing element in the oil-tempered wire, production conditions (see below), and the like. The larger the total amount of the strengthening element, the easier it is to increase the amount and ratio of the precipitation of the strengthening element.

-Other carbides In the oil tempered wire of the embodiment, typically, a carbide containing Fe (Fe-based carbide) exists at the crystal grain boundary, and the crystal grain boundary is strengthened by the Fe-based carbide. Typically, the Fe-based carbide includes Fe ₃ C. In addition, the oil tempered wire of the embodiment typically includes a carbide containing Cr (such as (Fe, Cr) ₃ C) in the crystal grains.

-Carbide amount The oil tempered wire of the embodiment contains carbide in both the crystal grains and the crystal grain boundaries as described above. When the composition of the oil-tempered wire is 100% by mass and the total content of these carbides is 1% by mass or more, the dispersion strengthening effect in the crystal grains by the dispersed carbides and the grain boundary strengthening effect by the Fe-based carbides are reduced. It is preferable because it can be obtained well. The larger the total content is, the more easily the above-described dispersion strengthening effect in the crystal grains and the strengthening effect of the crystal grain boundaries are obtained, and the fatigue characteristics are easily improved. 0.4 mass% or more, and further 1.5 mass% or more. When the total content is 3% by mass or less, there is not too much carbide, and in particular, it is difficult to cause a decrease in toughness due to too much Fe-based compound. When the total content is 2.8% by mass or less, 2.6% by mass or less, and more preferably 2.5% by mass or less, the above-described dispersion strengthening effect in crystal grains and the effect of strengthening crystal grain boundaries can be obtained favorably. In addition, the toughness can be suppressed from being reduced, and the fatigue characteristics are more excellent. More preferably, the content of the above-mentioned dispersed carbide satisfies 0.01% by mass or more, and the total content of carbides satisfies 1% by mass or more and 3% by mass or less.

  When the oil tempered wire of the embodiment is subjected to a simulation process simulating the manufacturing process of the spring, the Fe-based carbide such as cementite tends to increase, but the amount of the dispersed carbide before and after the simulation process does not substantially change. I have confirmed. However, the size of the dispersed carbide before and after the simulation processing may change, or may increase mainly. However, if the average particle size of the dispersed carbide before the simulation treatment is as small as 30 nm or less, the average particle size of the dispersed carbide after the simulation treatment tends to satisfy 30 nm or less. In other words, the oil-tempered wire after the simulated treatment tends to maintain a structure in which fine dispersed carbides are uniformly dispersed in the crystal grains and Fe-based carbides exist in the crystal grain boundaries.

  For example, if the total content of carbides after heat treatment equivalent to nitriding treatment, shot peening, and heat treatment corresponding to low-temperature annealing is sequentially performed on the oil-tempered wire is 2% by mass or more, the dispersion strengthening effect in the crystal grains by the dispersed carbides is obtained. And the effect of strengthening the crystal grain boundaries by the Fe-based carbide can be favorably obtained. When the total content is 4% by mass or less, the toughness is hardly reduced particularly due to too much Fe-based compound. When the total content is 3.8% by mass or less, further 3.6% by mass or less, 3.5% by mass or less, and 3% by mass or less, the dispersion strengthening effect in the crystal grains and the strengthening of the crystal grain boundaries described above are made. It is possible to suppress the decrease in toughness while obtaining good effects, and it is excellent in fatigue characteristics. When such an oil-tempered wire is used for a spring, this spring is considered to have a structure similar to the structure after the above-described simulation processing, and has a high fatigue limit and excellent fatigue characteristics.

  Those whose total content of carbides in the oil-tempered wire after the above-mentioned simulation treatment satisfies 2% by mass or more and 4% by mass or less typically have a total content of carbides of 1% by mass or more and 3% by mass before the simulation treatment. % Or less.

(Mechanical properties)
As one index indicating that the oil-tempered wire of the embodiment has excellent fatigue properties, the fatigue when performing a fatigue test after sequentially performing heat treatment equivalent to nitriding treatment, shot peening, and heat treatment equivalent to low-temperature annealing on the oil-tempered wire. Satisfies 1045 MPa or more. It can be said that the higher the fatigue limit after the above-described simulation processing, the more the spring using the oil-tempered wire is less likely to break even when subjected to repeated stress, and is more excellent in fatigue characteristics. Therefore, the fatigue limit is preferably 1050 MPa or more, more preferably 1055 MPa or more. The upper limit is not set because the higher the fatigue limit is, the better.

(Shape, size)
The oil tempered wire of the embodiment can take various shapes. Typically, a round wire having a circular cross section is used. In addition, various deformed lines such as a polygonal shape such as a rectangular flat wire, a trapezoidal shape, or an elliptical cross section can be used.

  The oil-tempered wire of the embodiment can take various sizes (cross-sectional area, wire diameter, etc.). For example, a round wire having a wire diameter of about 0.7 mm or more and about 6.0 mm or less may be used. The cross-sectional area and wire diameter can be changed depending on the degree of wire drawing.

(Application)
The oil-tempered wire of the embodiment can be used for various spring materials. More specifically, the oil-tempered wire of the embodiment can be suitably used for various spring materials for automobiles, such as an engine valve spring and a transmission spring.

[Method of manufacturing oil-tempered wire]
Typically, the oil-tempered wire of the embodiment can be manufactured through a process of melting raw steel → hot forging → hot rolling → patenting → peeling → annealing → drawing → hardening and tempering.

In particular, a heat treatment for precipitating at least a part of the strengthening element as carbides and forming fine carbides is performed.
For example, in a quenching step, in a temperature lowering process after heating to an austenitizing (γ-forming) temperature, the temperature may be rapidly cooled after being maintained in a supercooled state at a predetermined temperature of _AC3 point or less. Specifically, an austenitizing step and a precipitation step of lowering the temperature from a gamma temperature to a predetermined temperature selected from a temperature range of 600 ° C. to 800 ° C. at a rate of 20 ° C./sec or more and maintaining the predetermined temperature And a quenching step of quenching after the precipitation step.
Tempering is performed after the above specific quenching.

The above heat treatment is based on the following findings.
The present inventors examined the precipitation state of the dispersed carbide at each temperature in the process of decreasing the temperature from the γ-forming temperature. As a result, the matrix is transformed into a face-centered cubic structure (fcc) by the γ-formation, and in the temperature decreasing process from the γ-formation temperature, in the temperature range where the matrix maintains the fcc, the dispersed carbide easily precipitates stably. It has been found that it is preferable to maintain the temperature below the _AC3 point in a supercooled state so as not to transform into a centered cubic structure (bcc). By maintaining the matrix at a temperature that does not cause pearlite transformation while maintaining fcc, dispersed carbides can be precipitated. In addition, by rapidly cooling from this holding temperature, the matrix can have a martensitic structure, and the dispersed dispersed carbide is difficult to grow, can maintain a fine state, and the fine dispersed carbide is dispersed in the crystal grains. That the existing tissue can be obtained. Further, when tempering is performed after the above specific quenching, a tempered martensite structure is obtained, and toughness is enhanced. In addition, fine dispersed carbides grow to some extent, but a fine state in which the average particle size satisfies 30 nm or less is maintained. I got the knowledge that I can do it.
Hereinafter, the heat treatment will be described in detail. Known conditions other than the following quenching conditions and tempering conditions can be used.

  The γ-forming temperature is, for example, 850 ° C. or more and 1000 ° C. or less. When the temperature is 850 ° C. or higher, the added element can be sufficiently dissolved in solid solution, and as a result, precipitates are easily deposited uniformly. When the temperature is 1000 ° C. or lower, the growth of crystal grains and precipitates is suppressed and a fine crystal structure is easily formed. For example, the average particle size can be 10 or more. When the gamma-forming temperature is 980 ° C. or lower, and more preferably 900 ° C. or higher and 950 ° C. or lower, a good solid solution can be obtained while having a fine crystal structure. If the γ-formation temperature is lower and the holding temperature in the precipitation step is lower in the above range, a fine crystal structure having an average particle size of 11 or more, further 12 or more, and 13 or more can be easily obtained.

  In the process of lowering the temperature from the above-mentioned γ-forming temperature, the dispersed carbide is deposited while maintaining the supercooled state at a predetermined temperature selected from a temperature range of 600 ° C. or more and 800 ° C. or less. For example, the supercooling state may be achieved by increasing the temperature decreasing rate to the above-mentioned predetermined temperature to a relatively high value of 20 ° C./sec or more. The lower the predetermined temperature selected from the above temperature range, the more difficult it is for the dispersed carbide to grow, and the more easily the dispersed carbide becomes finer.

  The holding time of the above-mentioned γ-forming temperature and the holding time for the precipitation of the dispersed carbide are, for example, 1 second or more and less than 1 minute (60 seconds). As the total time is shorter, coarsening of the crystal grains and coarsening of the dispersed carbide can be suppressed, so that the total time can be 30 seconds or less, further 20 seconds or less, and 15 seconds or less. When the total time is at least 1 second, more preferably at least 3 seconds, and at least 5 seconds, gamma can be appropriately performed, and dispersed carbides can be easily precipitated. If the total time is set longer in the above range, the amount of precipitation is easily increased.

  For quenching in a short time as described above, a high-frequency heating furnace can be suitably used. The high-frequency heating furnace can easily increase the rate of temperature rise and the rate of temperature decrease as compared with the atmosphere furnace. Therefore, when a high-frequency heating furnace is used, the holding time of the γ-formation can be easily shortened, and the cooling rate can be increased in the cooling process, and a supercooled state can be formed by rapid cooling.

  Tempering conditions include, for example, a holding temperature of 400 ° C. or more and 650 ° C. or less, and a holding time of 60 seconds or less. It is preferable to lower the tempering holding temperature and shorten the holding time, because the coarsening of the dispersed carbide can be suppressed.

[Spring]
A spring is obtained by subjecting the oil-tempered wire of the embodiment to spring processing. After the spring working, under known conditions, strain relief annealing, nitriding treatment, shot peening, low-temperature annealing and the like can be appropriately performed.

[Test Example 1]
Oil-tempered wires were prepared under various conditions, and the structure and fatigue characteristics were examined.

A raw steel is melted in a vacuum melting furnace, and hot forging and hot rolling are sequentially performed to produce a rolled material having a wire diameter of 6.5 mm. The rolled material is sequentially subjected to patenting, peeling, annealing, wire drawing, and quenching and tempering to obtain an oil-tempered wire having a wire diameter of 3.0 mm.
The components of the steel used in the test were: C: 0.63% by mass, strengthening elements shown in Table 1: 0.15% by mass, Si: 2.20% by mass, Mn: 0.51% by mass, Cr: 1 .18% by mass, with the balance being Fe and unavoidable impurities.
Quenching is performed on each of the prepared samples under the following conditions. Tempering is performed at a holding temperature of 500 ° C. and a holding time of 2 seconds.

(Hardening conditions)
<Sample No. 1> Using an atmosphere furnace After performing γ-forming at a γ-forming temperature of 950 ° C. and a holding time of about 55 seconds, a rapid cooling from the γ-forming temperature to room temperature is performed without performing a later-described precipitation step.
<Sample No. 2> Using a high-frequency heating furnace After performing γ-forming at a γ-forming temperature of 950 ° C. and a holding time of 10 seconds or less, a rapid cooling from the γ-forming temperature to room temperature is performed without performing a precipitation step described below.

<Sample No. 3-No. 7> Using a high-frequency heating furnace A gamma-forming step in which the gamma-forming temperature is set to 950 ° C, and the temperature is lowered from the gamma-forming temperature to the temperature (° C) shown in the quenching conditions in Table 1 at a rate of temperature reduction of 20 ° C / sec or more. (° C.) and a quenching step of rapidly cooling from this holding temperature. The total time of the holding time of the γ-forming temperature and the holding time of the temperature (° C.) shown in the quenching conditions in Table 1 in the precipitation step is set to be 10 seconds or less.
In all samples, the quenching adjusts the cooling rate so as to obtain a martensitic structure.

  A cross section of the oil-tempered wire of each sample subjected to the above-described quenching and tempering was taken, and the cross section was observed with a microscope. FIG. 4 is an optical micrograph of Sample No. 4, and FIG. 1 is an optical micrograph of FIG. 1 and 2, black streaks are crystal grain boundaries, and each area surrounded by the black streaks is a crystal grain.

As shown in FIGS. 1 and 2, each sample has a similar crystal structure in which the crystal grains are fine and streaks exist at the crystal grain boundaries. You can see that it is fortified by The streaks of the grain boundaries were subjected to component analysis by point analysis using an energy dispersive X-ray analyzer (EDX) attached to a transmission electron microscope (TEM). Typically, it has been confirmed that Fe-based carbide such as Fe ₃ C is mainly used.

The following items were examined for the oil tempered wire of each sample.
(Carbide content)
The total content of carbide contained in the oil temper wire of each sample was examined.
Here, the residue obtained by the potentiostatic electrolysis method is subjected to component analysis, and the mass ratio (mass%) of the residue when the oil temper line is set to 100 mass% is determined.
Specifically, a test piece is taken from the oil-tempered wire of each sample, the test piece is dissolved using an appropriate electrolytic solution to extract a residue, and the residue is separated by filtering through a filter medium such as a membrane filter. The components of the separated residue are analyzed by inductively coupled radio frequency plasma (ICP) emission spectroscopy to quantify the components. The crystal structure of the separated residue is analyzed by an X-ray diffraction method. The mass of the test piece is measured in advance.
From the component analysis and the crystal structure analysis of the residue, it was confirmed that the residue was a carbide (a dispersed carbide such as V ₄ C _{3, an} Fe-based carbide such as Fe ₃ C, and other (Fe, Cr) ₃ C). Measure the mass. Of the residue, impurities other than carbides are removed.
The mass of the residue is defined as the total content of the carbide, and the mass ratio of the total content of the carbide to the mass of the test piece (formula 1 below), that is, the total content of the carbide with the composition of the oil-tempered wire being 100% by mass (mass) %).
(Equation 1) [(total content of carbide) / (mass of test piece)] × 100
Similarly, regarding the oil-tempered wire after performing a process (nitriding, shot peening, low-temperature annealing) simulating the manufacturing process of a spring described later, similarly, the total content of carbides with the composition of the oil-tempered wire being 100% by mass. Is determined.
The total content of carbides in the oil-tempered wire not subjected to the simulated treatment is defined as the total amount (% by mass) before the simulated treatment, and the total content of the carbides in the oil-tempered wire after the simulated treatment is calculated as the total amount after the simulated treatment. And the results are shown in Table 1.

(Precipitation amount and precipitation ratio of strengthening element)
From the component analysis and the crystal structure analysis of the residue, a carbide (dispersed carbide) containing a reinforcing element is extracted. The total content of the strengthening elements in the extracted dispersed carbide is determined using the above-described component analysis values. The total content of the reinforcing elements in the dispersed carbide with respect to the mass of the test piece (formula 2 below) is determined. That is, the precipitation amount (% by mass) of the reinforcing element when the composition of the oil-tempered wire was 100% by mass was determined. The results are shown in Table 1.
(Equation 2) [(total content of reinforcing elements in dispersed carbide) / (mass of test piece)] × 100
With respect to the total content of the reinforcing elements in the dispersed carbide, the mass ratio (%) to the total amount of the reinforcing elements in the composition of the oil-tempered wire (here, 0.15% by mass in all samples) was determined, that is, The ratio was determined, and the results are shown in Table 1.

(Average particle size of dispersed carbide)
The measurement of the average particle size of the dispersed carbide is performed using the extraction replica method. Specifically, a cross section of the oil-tempered wire is taken, and after mirror-polishing the cross section, the cross section is corroded by a potentiostatic electrolytic corrosion method to make a precipitate appear, and carbon is further deposited. A carbon film (replica film) formed by vapor deposition is separated, and a precipitate (particle) is extracted.
The extracted precipitate is subjected to elemental analysis using TEM-EDX to examine the composition of the precipitate. In addition, the crystal structure of the precipitate is analyzed by electron beam diffraction to examine the structure of the precipitate. Based on the composition and structure of the precipitate, (Fe, Cr) ₃ C and the like are excluded, and only dispersed carbides such as V ₄ C ₃ , TaC, and NbC are extracted.
Using the TEM observation image of the extracted dispersed carbide, the size of the dispersed carbide is measured. Here, Table 1 shows the diameter of the equivalent area circle of each particle in the TEM observation image of the above-mentioned cross section as the diameter of each particle, and the average of 100 or more particles as the average particle diameter (nm). Table 1 shows the largest particle among the averaged particles as the maximum diameter (nm).

(Average grain size of crystal)
Using the above-mentioned optical micrograph, the crystal grain size is measured in accordance with JIS G 0551 (2013) “Steel—Microscopic Test Method for Grain Size”. The photographing magnification here is 1000 times.

(Characteristic)
The following simulating processes (nitriding, shot peening, low-temperature annealing) simulating the spring manufacturing process are sequentially performed on the oil-tempered wire of each sample, and then a fatigue test is performed to determine a fatigue limit (MPa). Ten test pieces were prepared for each sample to examine the fatigue limit, and the average value of the ten test pieces is shown in Table 1. In the fatigue test, a Nakamura-type rotary bending fatigue test was performed.
[Simulation processing]
Nitriding treatment: 400 ° C × 4 hours, nitrogen atmosphere Shot peening: 0.7mmφ wire cut × 30 minutes → 0.3mmφ steel ball × 30 minutes Low temperature annealing: 230 ° C × 30 minutes

  As shown in Table 1, the sample No. 3-No. In the oil tempered wire of Sample No. 7, the fatigue limit after the above-described simulation test was Sample No. 7. 1, No. Higher than No. 2 and excellent in fatigue properties. In this test, sample no. 3-No. The fatigue limit of Sample No. 7 was as follows. It is improved by about 2.5% or more with respect to 1. In particular, the sample No. 5-No. The fatigue limit of Sample No. 7 was as follows. It is improved by 5.5% or more with respect to 1.

The reason that such a result was obtained is considered as follows.
Sample No. 3-No. As shown in Table 1, the oil temper wire of No. 7 has an average particle size of a carbide (dispersed carbide) containing a reinforcing element of less than 38 nm, and is further less than 30 nm, and is very fine. Further, the total content of the reinforcing elements in the dispersed carbide is at least 10% by mass relative to the total amount of the reinforcing elements (the precipitation ratio is at least 10%). That is, a certain amount of the strengthening element exists in the crystal grains as a dispersed carbide. Then, the sample No. 3-No. The oil-tempered wire of No. 7 has a structure in which very fine dispersed carbides are dispersed in the crystal grains, and fine dispersed carbides exist in the fine crystal grains even after the simulation test as described later. Have an organization. As described above, at least a part of the strengthening element is provided as a carbide, and since the carbide is very fine and is dispersed and present in the crystal grains, the yield stress can be improved by the dispersion strengthening effect, and the repeated bending can be performed. It is considered that it was difficult to break when it was subjected, and the fatigue limit was improved.

Further, in this test, the sample No. 3-No. The maximum diameter of the dispersed carbide in the crystal grains in the oil temper wire of No. 7 is less than 100 nm, and is very small, less than 90 nm, 80 nm or less, and 50 nm or less. Therefore, it is considered that the dispersed carbides in the crystal grains did not easily become the starting point of the fracture, and the fracture became more difficult. In this test, the sample No. 3-No. The crystal grain size in the oil temper wire of No. 7 is as large as 10 or more, further 12 or more, and the crystal is fine.
It is considered that by having such a fine crystal structure, an appropriate amount of crystal grain boundaries was present, and it became difficult to break even by the effect of strengthening the crystal grain boundaries by Fe-based carbide such as cementite. It is considered that the crystal grain size was 14.5 or less, and the crystal was not too small, so that it was excellent in toughness. In addition, in this test, sample no. 3-No. The total content of carbides before the simulated treatment of the oil tempered wire of No. 7 is in the range of 1% by mass to 3% by mass. From the fact that the total content of the carbides is appropriate, it is considered that both the effect of strengthening the precipitation in the crystal grains by the dispersed carbide and the effect of strengthening the precipitation of the crystal grain boundaries by the Fe-based carbide were favorably obtained. It is also considered that the toughness was hardly reduced by excessive carbides.

  Furthermore, when the simulation treatment is performed, the amount of the dispersed carbides does not substantially change, and the amount of Fe-based carbides tends to increase, so that the total content of carbides after the simulation treatment increases. However, sample no. 3-No. The total content of carbides after the simulated treatment of the oil tempered wire of No. 7 was about 2.3% by mass, and the simulated treatment was compared with the total content of carbides (about 1.7% by mass) before the above-described simulation test. The increase in the total carbide content later is small. Sample No. 3-No. The oil-tempered wire of No. 7 has a range in which the total content of carbides after the simulating treatment is higher than that before the simulating treatment as described above, but satisfies 2% by mass or more and 4% by mass or less. It is considered that, while obtaining the effect of strengthening the crystal grain boundaries by increasing the amount of Fe-based carbide, it was difficult to cause a decrease in toughness due to excessive precipitation of Fe-based carbide.

  On the other hand, sample No. 1, No. In No. 2, although the carbide containing the reinforcing element is present in the crystal grains, the average particle size of these carbides is large and the maximum diameter is also large. It is considered that the presence of such coarse carbides in the crystal grains makes it easy for the coarse carbide grains to be broken and becomes a starting point of fracture when repeatedly bent, thereby lowering the fatigue limit.

The structure of each sample will be described in more detail.
3 and 4 are cross-sectional images of the wire after the above-described simulation test, and are TEM observation images of the cross-section. From the left, a Z-contrast image and an Fe element mapping analyzed using a TEM-EDX device are shown. 1 shows an image and an element mapping image of V. In this example, carbon is used as the replica film, and in the element mapping image by the TEM-EDX apparatus, the contrast ratio looks larger and white as the atomic number is larger than carbon. Iron (Fe) and vanadium (V) appear white because their atomic numbers are sufficiently higher than carbon (C). FIG. 4 is an observation image after the simulation test. FIG. 1 is an observation image after a simulation test. By the component analysis (point analysis) described above, Fe shown in the element mapping image of Fe is Fe-based carbide such as Fe ₃ C, and V shown in the element mapping image of V is V-based carbide such as V ₄ C ₃ Make sure that

  As shown in the element mapping images of Fe in FIGS. 3 and 4, it can be seen that Fe-based carbides are scattered around the crystal grains in all the samples, that is, the existing state of Fe is the same. On the other hand, as shown in the element mapping image of V in FIG. It can be seen that the V-based carbide of No. 4 is very fine and is dispersed in crystal grains surrounded by the above-mentioned Fe-based carbide. In this example, even after the simulation test, the V-based carbide has an average particle size of about 15 nm and a maximum diameter of less than 50 nm. On the other hand, as shown in the element mapping image of V in FIG. 1, it is understood that a very large V-based carbide exists locally. In this example, the sample No. V-based carbide of Sample No. 1 It is about twice as large as 4. From this, oil-tempered wire having a structure in which fine dispersed carbides are dispersed in fine crystal grains is easy to maintain this specific structure even after the spring manufacturing process, and is excellent in fatigue limit. Is supported.

In addition, the following can be said from this test.
(1) When the same strengthening element is included, the smaller the average particle size of the dispersed carbide, the higher the fatigue limit. Further, the smaller the maximum diameter of the dispersed carbide, the higher the fatigue limit. (Refer to samples No. 3 to No. 5 for comparison)
(2) In the case where V is contained, when the average particle size of the dispersed carbide is 30 nm or less and the precipitation ratio of the reinforcing element is relatively large (preferably 30% or more in mass ratio), the fatigue limit is improved. easy. (See Sample Nos. 3 to 5)
(3) When a reinforcing element having a relatively large atomic weight, such as Ta or Nb, is included, the fatigue limit can be improved even when the precipitation rate of the reinforcing element is small (about 10% by mass). (Refer to sample Nos. 6 and 7)
(4) An oil-tempered wire having an appropriate amount of fine dispersed carbides in crystal grains and excellent in fatigue properties can be produced by quenching under specific conditions including a gamma-forming step, a precipitation step, and a quenching step.
(5) The average particle size, the maximum particle size, and the crystal particle size of the dispersed carbide can be changed by changing the manufacturing conditions (particularly, the temperature in the precipitation step). As the temperature in the precipitation step is lowered, the average particle size and the maximum particle size of the dispersed carbide are easily reduced, and the crystal grain size is increased to make the crystal finer (see Comparative Examples 3 to 5).

The present invention is not limited to these examples, but is indicated by the appended claims, and is intended to include any modifications within the scope and meaning equivalent to the appended claims.
For example, in the above-described test examples, at least one of the composition of the wire, the wire diameter, and the manufacturing conditions can be changed. For example, the content of one or more elements selected from C, Si, Mn, and Cr is changed, or two or more elements selected from V, Ta, Nb, Mo, and W are set in a specific range. To be included.

Claims (6)

C is 0.50% by mass or more and 0.90% by mass or less,
V of 0.02% by mass to 0.50% by mass, Ta of 0.01% by mass to 0.50% by mass, Nb of 0.01% by mass to 0.50% by mass, 0.02% by mass At least 0.50% by mass of Mo and at least 0.02% by mass and at least 1.00% by mass of W;
0.80 mass% or more and 3.00 mass% or less of Si;
Mn is 0.40% by mass or more and 1.00% by mass or less;
Cr has a composition of not less than 0.40% by mass and not more than 2.00% by mass, with the balance being Fe and impurities,
Comprising a carbide containing the reinforcing element,
The total content of the reinforcing element in the carbide containing the reinforcing element is at least 10% by mass relative to the total amount of the reinforcing element,
An oil-tempered wire in which the carbide containing the reinforcing element has an average particle size of 30 nm or less.   The oil tempered wire according to claim 1, wherein the crystal grain size is 10 or more and 14.5 or less.   The composition is from 0.10% to 0.30% by mass of V, 0.10% to 0.35% by mass of Ta, and 0.10% to 0.35% by mass of Nb. The oil-tempered wire according to claim 1, comprising one or more selected reinforcing elements.   The oil-tempered wire according to any one of claims 1 to 3, wherein a total content of the carbides is 1% by mass or more and 3% by mass or less.   5. The oil-tempered wire according to claim 4, wherein a total content of the carbide becomes 2% by mass or more and 4% by mass or less after heat treatment corresponding to nitriding treatment, shot peening, and heat treatment corresponding to low-temperature annealing are sequentially performed.   The oil-tempered wire according to any one of claims 1 to 5, wherein the composition contains 0.63% by mass or more and 0.68% by mass or less of C.

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