STEEL WIRE

Abstract

A steel wire is provided that has excellent cold-wound workability and an excellent fatigue limit when made into a spring. The chemical composition of the steel wire according to the present embodiment contains, in % by mass, C: 0.50 to 0.80%, Si: 1.20 to less than 2.50%, Mn: 0.25 to 1.00%, P: 0.020% or less, S: 0.020% or less, Cr: 0.40 to 1.90%, V: 0.05 to 0.60%, and N: 0.0100% or less, the remainder being Fe and impurities. In the steel wire, the number density of V-based precipitates with a maximum diameter of 2 to 10 nm is 5000 to 80000 pieces / µm³.

Description

STEEL WIRE FIELD OF INVENTION

[0001] The present invention relates to a steel wire, and more particularly to a steel wire that serves as raw material for springs typified by shock absorber springs and valve springs. STATE OF THE ART

[0002] Many springs are used in automobiles and machinery in general. Among the springs used in automobiles and machinery in general, shock absorber springs have the function of absorbing impacts or vibrations from the outside. A shock absorber spring is used, for example, in a torque converter that transmits the driving force of a car to the transmission. In a case where a shock absorber spring is used in a torque converter, the shock absorber spring absorbs the vibrations of an internal combustion engine (e.g., a car engine). Therefore, the shock absorber spring must have a high fatigue limit.

[0003] Furthermore, among the springs used in automobiles and machinery in general, a valve spring performs the function of regulating the opening and closing of an internal valve in the automobile or machinery. A valve spring is used, for example, to control the opening and closing of an air supply valve in an internal combustion engine (engine) of an automobile. To regulate the opening and closing of the valve, the compression of the valve spring is repeated several thousand times per minute. Therefore, like a shock absorber spring, a valve spring must also have a high fatigue limit. In particular, the compression of a valve spring is repeated several thousand times per minute, and this compression frequency is much higher than the compression frequency of a shock absorber spring.Consequently, a valve spring is required to have an even higher fatigue limit compared to a shock absorber spring. Specifically, while a shock absorber spring is required to have a high fatigue limit of 10⁷ cycles, a valve spring is required to have a high fatigue limit of 10⁸ cycles.

[0004] An example of a method for producing a spring classified as a shock absorber spring or valve spring is as follows. A steel wire is quenched and tempered. After quenching and tempering, the steel wire is cold-wound to form a coil-shaped intermediate steel material. This intermediate steel material is then subjected to stress-relief annealing. After stress-relief annealing, nitriding is performed if necessary. Nitriding may or may not be required. After stress-relief annealing, or after nitriding as needed, shot peening is performed to impart compressive residual stress to the outer layer. A spring is then produced by the above process. ινΐΛ / a / zuzz / ui υζυ i

[0005] Recently, there has been a demand for an improvement in the fatigue limit of the springs.

[0006] Techniques related to improving the fatigue limit of springs are described in Japanese Patent Application Publication No. 2-57637 (Patent Document 1), Japanese Patent Application Publication No. 2010-163689 (Patent Document 2), Japanese Patent Application Publication No. 2007-302950 (Patent Document 3), and Japanese Patent Application Publication No. 2006-183137 (Patent Document 4).

[0007] A steel wire for a spring having a high fatigue limit described in Patent Document 1 is produced by subjecting a steel having a chemical composition containing, in % by mass, C: 0.3 to 1.3%, Si: 0.8 to 2.5%, Mn: 0.5 to 2.0% and Cr: 0.5 to 2.0%, and containing one or more types of elements among Mo: 0.1 to 0.5%, V: 0.05 to 0.5%, Ti: 0.002 to 0.05%, Nb: 0.005 to 0.2%, B: 0.0003 to 0.01%, Cu: 0.1 to 2.0%, Al: 0.01 to 0.1% and N: 0.01 to 0.05% as optional elements, the remainder being Fe and unavoidable impurities, air cooling or rapid cooling after holding for 3 seconds to 30 minutes at 250 to 500 °C after an austenitizing treatment, and has a yield ratio of 0.85 or less.In this Patent Document, the steel wire for a spring having a high fatigue limit having the aforementioned composition is proposed on the basis of the finding that the fatigue limit of a spring depends on the elastic limit of the spring, with the fatigue limit of the spring increasing as the elastic limit of the spring increases (see lines 1 to 5 in the upper right column on page 2 of Patent Document 1).

[0008] A spring described in Patent Document 2 is produced using an oil-tempered wire having an annealed martensitic structure. The oil-tempered wire is composed, by mass percent, of C: 0.50 to 0.75%, Si: 1.50 to 2.50%, Mn: 0.20 to 1.00%, Cr: 0.70 to 2.20%, and V: 0.05 to 0.50%, the remainder being Fe and unavoidable impurities. When this oil-tempered wire is subjected to soft gas nitriding for two hours at 450 °C, the lattice constant of a nitrided layer formed on a portion of the surface of the oil-tempered wire is 2.881 to 2.890 Å. Furthermore, when this oil-tempered wire is heated for two hours at 450 °C, the tensile strength reaches 1974 MPa or more, the yield strength reaches 1769 MPa or more, and the reduction in area reaches more than 40%.This Patent Document defines an oil-quenched wire to be used as the raw material for a spring produced by nitriding. In the production of a spring by nitriding, as the nitriding time increases, the yield strength and tensile strength of the spring's steel material decrease. Consequently, the internal hardness of the steel material decreases, and the fatigue limit decreases. Therefore, Patent Document 2 describes that by using an oil-quenched wire in which the yield strength of the steel material does not decrease, even with a long nitriding treatment time, a spring with a high fatigue limit can be produced (see paragraphs

[0025] and

[0026] of Patent Document 2).

[0009] A high-strength spring steel wire described in Patent Document 3 has a chemical composition containing C: 0.5 to 0.7%, Si: 1.5 to 2.5%, Mn: 0.2 to 1.0%, Cr: 1.0 to 3.0%, and V: 0.05 to 0.5%, wherein Al is controlled to 0.005% or less (excluding 0%), the remainder being Fe and unavoidable impurities. In the steel wire, the number of spherical cementite particles having an equivalent circular diameter ranging from 10 to 100 mm is 30 pieces / pm2 or more, and the concentration of Cr in the cementite is, by mass percent, 20% or more, and the concentration of V is 2% or more. Patent Document 3 describes that increasing the strength of the steel wire is effective in improving the fatigue limit and settlement resistance (see paragraph

[0003] of Patent Document 3).Furthermore, it is described that by making the number of fine spherical cementite particles having an equivalent circular diameter ranging from 10 to 100 nm is 30 pieces / pm2o or more, and making the concentration of Cr in the cementite 20% or more and the concentration of V in the cementite 2% or more by mass, the decomposition and removal of the cementite during a heat treatment such as a stress-relief annealing or nitriding treatment during the production process can be suppressed, and the strength of the steel wire can be maintained (see paragraph

[0011] of Patent Document 3).

[0010] A steel wire serving as raw material for a spring described in Patent Document 4 has a chemical composition consisting of, in % by mass, C: 0.45 to 0.7%, Si: 1.0 to 3.0%, Mn: 0.1 to 2.0%, P: 0.015% or less, S: 0.015% or less, N: 0.0005 to 0.007% and tO: 0.0002 to 0.01%, the remainder being Fe and unavoidable impurities, and has a tensile strength of 2000 MPa or more. On a microscopic observation surface, the area fraction occupied by spherical carbides and cementite-based alloy carbides having an equivalent circular diameter of 0.2 pm or more is 7% or less, the density of spherical carbides and cementite-based alloy carbides having an equivalent circular diameter ranging from 0.2 to 3 pm is 1 piece / pm2 or less, the density of spherical cementite-based carbides and alloy carbides having an equivalent circular diameter of more than 3 pm is 0.0.01 pieces / pm2 or less, the grain size number of the previous austenite is 10 or more, the amount of retained austenite is 15% by mass or less, and the area fraction of a dispersed region where the density of cementite-based spherical carbides having an equivalent circular diameter of 2 pm or more is low is 3% or less. Patent Document 4 describes that it is necessary to further increase the strength to further improve the spring's performance with respect to fatigue, settling, and the like. Patent Document 4 also describes that by controlling the microstructure and controlling the distribution of fine cementite-based carbides, an improvement in the spring's strength is achieved, and the spring's performance with respect to fatigue, settling, and the like is improved (see paragraphs

[0009] and

[0021] of Patent Document 4). LIST OF REFERENCES PATENT DOCUMENT

[0011] Patent Document 1: Publication of Japanese Patent Application No. 2-57637 Patent Document 2: Publication of Japanese Patent Application No. 2010-163689 Patent Document 3: Publication of Japanese Patent Application No. 2007-302950 Patent Document 4: Publication of Japanese Patent Application No. 2006-183137 SUMMARY OF THE INVENTION TECHNICAL PROBLEM

[0012] In the respective techniques described in Patent Documents 1 to 4 above, an approach is adopted in which the characteristics of the spring, such as the fatigue limit or the settling characteristics, are improved by increasing the strength (hardness) of a steel material that serves as raw material for a spring and the strength (hardness) of the spring. However, the fatigue limit of a spring can be increased by adopting another approach.

[0013] Furthermore, in a process for producing a spring, as described above, a steel wire that will serve as the spring's raw material is subjected to cold winding. Therefore, in some cases, a steel wire that will serve as the raw material for a spring is required to have excellent cold-winding workability.

[0014] An objective of the present invention is to provide a steel wire that has excellent cold-wound workability, and that exhibits an excellent fatigue limit when converted into a spring.

[0015] A steel wire according to the present invention has a chemical composition containing, in % by mass, C: from 0.50 to 0.80%, Yes: 1.20 to less than 2.50%, Mn: 0.25 to 1.00%, P: 0.020% or less, S: 0.020% or less, Cr: 0.40 to 1.90%, V: 0.05 to 0.60%, and N: 0.0100% or less, the remainder being Fe and impurities, ινΐΛ / a / zuzz / ui υζυ i where the number density of V-based precipitates with a maximum diameter of 2 to 10 nm is 5000 to 80000 pieces / pm3. ADVANTAGEOUS EFFECTS OF THE INVENTION

[0016] A steel wire according to the present invention has excellent cold winding workability, and exhibits an excellent fatigue limit when a spring is produced using the steel wire as raw material. BRIEF DESCRIPTION OF THE FIGURES

[0017] Figure 1A is an example of a TEM image of a plane (001) in ferrite from a thin-film sample. Figure IB is a schematic diagram of a TEM image of a (001) plane in ferrite from a thin-film sample. Figure 2 is a graph illustrating the relationship between a numerical ratio of Ca sulfides and a fatigue limit at a cycle count of 108 cycles (high cycle fatigue limit) with respect to a valve spring having a chemical composition of the present modality. Figure 3 is a flow diagram illustrating a process for producing a steel wire of this type. Figure 4 is a flowchart illustrating a process for producing a spring using the steel wire of the present modality. DESCRIPTION OF THE MODALITIES

[0018] As described in Patent Documents 1 to 4, conventional spring techniques have been based on the idea that the strength and hardness of the steel material constituting a spring have a positive correlation with the spring's fatigue limit. Thus, the idea that there is a positive correlation between the strength and hardness of (the steel material constituting) a spring and the spring's fatigue limit is common technical knowledge with respect to spring techniques. Therefore, conventionally, as a substitute for an extremely time-consuming fatigue test, the fatigue limits of springs have been predicted based on the strength of the steel material obtained by a tensile test completed in a short time, or based on the hardness of the steel material obtained by a hardness test completed in a short time.In other words, the fatigue limits of springs have been predicted based on the results of a tensile test or a hardness test that do not take much time, without performing the fatigue tests that do take time.

[0019] However, the present inventors considered that the strength and hardness of (the steel material constituting) a spring and the fatigue limit of the spring are not necessarily always correlated. Therefore, the present inventors investigated methods for increasing the fatigue limit of a spring by a technical idea other than increasing the fatigue limit of a spring by increasing the strength and hardness of the spring.

[0020] In this case, the present inventors focused their attention on V-based precipitates, as typified by V carbides and V carbonitrides. In the present description, the term V-based precipitates means precipitates containing V or containing V and Cr. V-based precipitates need not contain Cr. The present inventors considered that by forming a large number of fine nanometer-sized V-based precipitates on a steel wire, the fatigue limit of a spring produced using the steel wire as a raw material will be increased.

[0021] Furthermore, in some cases, a steel wire intended as the raw material for a spring is required to have excellent cold-workability. To increase cold-workability, it is effective to reduce the Si content. Therefore, the present inventors first conducted studies on a steel wire that increases the fatigue limit of a spring by using nanometer-sized precipitates based on V and with which excellent cold-workability is obtained from the standpoint of chemical composition. As a result, the present inventors considered that a chemical composition consisting of, in % by mass, C: 0.50 to 0.80%, Si: 1.20 to less than 2.50%, Mn: 0.25 to 1.00%, P: 0.020% or less, S: 0.020% or less, Cr: 0.40 to 1.90%, V: 0.05 to 0.60%, N: 0.0100% or less, Ca: 0 to 0.0050%, Mo: 0 to 0.50%, Nb: 0 to 0.050%, W: 0 to 0.60%, Ni: 0 to 0.500%, Co: 0 to 0.30%, B: 0 to 0.The chemical composition of a steel wire with the following compositions is suitable: 0.050% copper, 0 to 0.050% aluminum, 0 to 0.0050% titanium, and 0 to 0.050% titanium, the remainder being iron and impurities. This composition is suitable for use as a raw material for a spring. The present inventors then produced steel wires by subjecting a steel material having the aforementioned chemical composition to heat treatment at various temperatures after quenching. Furthermore, they produced springs using these steel wires. The present inventors then investigated the fatigue limit of the springs, as well as a fatigue limit relationship defined by the ratio of the fatigue limit to the spring hardness (i.e., fatigue limit relationship = fatigue limit / spring hardness).

[0022] As a result of such investigations, the present inventors obtained the following novel finding with respect to a steel wire having the aforementioned chemical composition. As described in the background of the art, when producing springs, nitriding is sometimes performed and sometimes not. In one instance where nitriding is performed in the conventional process for producing a spring, in a heat treatment (stress-relieving annealing step or similar) after a quenching and tempering step, a heat treatment is performed at a temperature lower than the nitriding temperature used for nitriding. This is because the conventional process for producing a spring is based on the technical principle that the fatigue limit of a spring is increased while maintaining high strength and hardness.In the case of nitriding, it is necessary to heat the material to the nitriding temperature. Therefore, in the conventional production process, the reduction in spring strength has been mitigated by setting a heat treatment temperature in a step separate from nitriding, at a temperature, as far as possible, lower than the nitriding temperature.

[0023] However, for the steel wire of the present embodiment, instead of the technical idea of ​​increasing the fatigue limit of a spring by increasing the strength of the spring, the present inventors adopted the technical idea of ​​increasing the fatigue limit of a spring by forming a large number of nanometer-sized fine precipitates based on V.For this reason, the research of the present inventors has revealed that, during the production process, if a heat treatment is carried out at a heat treatment temperature within the range of 540 to 650 °C to precipitate a large number of fine nanometer-sized V-based precipitates, even if the heat treatment temperature for the precipitation of the V-based precipitates is higher than the nitriding temperature and, as a result, decreases the strength of a portion of the spring core (i.e., even if the hardness of the portion of the spring core is low), an excellent fatigue limit will be obtained, and a fatigue limit ratio, defined by the ratio of the fatigue limit to the hardness of the portion of the spring core, will be high.More specifically, it has been revealed for the first time through the research of the present inventors that, in a steel wire that is to serve as raw material for a spring, if the number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm is 5000 pieces / pm3 or more, a sufficient fatigue limit is obtained in a spring produced using the steel wire.

[0024] As described above, the steel wire of the present modality is a steel wire derived from a technical idea completely different from the conventional technical idea, and is composed as described below.

[0025] [1] A steel wire, with a chemical composition containing, in % by mass, C: from 0.50 to 0.80%, Yes: from 1.20 to less than 2.50%, Mn: 0.25 to 1.00%, P: 0.020% or less, S: 0.020% or less, Cr: 0.40 to 1.90%, V: 0.05 to 0.60%, and N: 0.0100% or less, the remainder being Fe and impurities, inal / a / zuzz / ui where the number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm is 5000 to 80000 pieces / pm3.

[0026] Here, the term V-based precipitates refers, as mentioned above, to carbides or carbonitrides containing V, or carbides or carbonitrides containing V and Cr, and, for example, refers to one or more types of V carbides and V carbonitrides. V-based precipitates may be compound precipitates containing one of the V carbides and a V carbonitride, and one or more types of another element. V-based precipitates precipitate in a plate-like form along a {001} plane in ferrite (body-centered cubic lattice). Therefore, in a TEM image of a (001) plane in ferrite, V-based precipitates appear as line segments (edge ​​portions) extending linearly parallel to the

[100] or

[010] orientation.Precipitates other than V-based precipitates are not observed as line segments (edge ​​portions) extending linearly parallel to the

[100] or

[010] orientation. In other words, only V-based precipitates are observed as line segments (edge ​​portions) extending linearly parallel to the

[100] or

[010] orientation. Therefore, when observing a TEM image of an (001) plane in ferrite, V-based precipitates can be easily distinguished from Fe carbides, such as cementite, and V-based precipitates can be identified. That is, in the present description, in a TEM image of an (001) plane in ferrite, line segments extending along the

[100] or

[010] orientation are defined as V-based precipitates.

[0027] [2] The steel wire described in [1], where: The chemical composition contains: Ca: 0.0050% or less, and when, among the inclusions, inclusions in which, by mass %, an O content equal to or greater than 10.0% are defined as oxide-based inclusions; inclusions in which, by mass %, an S content equal to or greater than 10.0% and an O content less than 10.0% are defined as sulfide-based inclusions; and among the sulfide-based inclusions, inclusions in which, by mass %, a Ca content equal to or greater than 10.0%, an S content equal to or greater than 10.0%, and an O content less than 10.0% are defined as Ca sulfides; a numerical proportion of Ca sulfides with respect to a total number of oxide-based inclusions and sulfide-based inclusions is 0.20% or less. ινΐΛ / a / zuzz / ui υζυ i

[0028] As mentioned previously, the compression of a valve spring is repeated several thousand times per minute, and the compression frequency of a valve spring is much higher than the compression frequency of a damper spring. Therefore, a valve spring is required to have an even higher fatigue limit than a damper spring. Specifically, while a damper spring requires a high fatigue limit at a cycle count of 10⁷, a valve spring requires a high fatigue limit at a cycle count of 10⁸. Hereafter, in this description, a fatigue limit at a cycle count of 10⁸ cycles is referred to as the high-cycle fatigue limit.

[0029] Among inclusions, particularly calcium sulfides, influence the high-cycle fatigue limit. As mentioned previously, among inclusions, those in which the oxygen content (by mass percent) is 10.0% or more are defined as oxide-based inclusions. Inclusions in which the sulfur content (by mass percent) is 10.0% or more and the oxygen content is less than 10.0% are defined as sulfide-based inclusions. Among sulfide-based inclusions, those in which the calcium content (by mass percent) is 10.0% or more, the sulfur content (by mass percent) is 10.0% or more, and the oxygen content is less than 10.0% are defined as calcium sulfides. Calcium sulfides are a type of sulfide-based inclusion. In a valve spring, if the numerical proportion of Ca sulfides in the oxide-based inclusions and in the sulfide-based inclusions is low, the fatigue limit in a high cycle (108 cycles) increases.More specifically, when the numerical proportion of Ca sulfides with respect to the total number of oxide-based inclusions and sulfide-based inclusions is 0.20% or less, the high-cycle fatigue limit increases particularly.

[0030] One conceivable reason for this is the following. In a valve spring, where the numerical ratio of Ca sulfides to the total number of oxide-based and sulfide-based inclusions is low, the Ca dissolves sufficiently in the oxide-based inclusions and in the sulfide-based inclusions that are not Ca sulfides. In this case, the oxide-based and sulfide-based inclusions soften sufficiently and become fine. Therefore, cracks that originate from oxide-based or sulfide-based inclusions are difficult to develop, and the fatigue limit at a high cycle (10⁸ cycles) increases.

[0031] [3] The steel wire described in [1] or [2], where: The chemical composition contains one or more types of elements selected from the group consisting of: Mo: 0.50% or less, Nb: 0.050% or less, W: 0.60% or less, Ni: 0.500% or less, ινΐΛ / a / zuzz / ui uzm Co: 0.30% or less, and B: 0.0050% or less.

[0032] [4] The steel wire described in any one of [1] to [3], where: The chemical composition contains one or more types of elements selected from the group consisting of: Cu: 0.050% or less, To: 0.0050% or less, and Ti: 0.050% or less.

[0033] The steel wire of the present modality is described in detail below. The symbol % in relation to an item means percentage by mass unless specifically stated otherwise.

[0034] [Chemical composition of steel wire] The steel wire of this embodiment serves as raw material for springs. The chemical composition of the steel wire of this embodiment contains the following elements.

[0035] C: 0.50 to 0.80%. Carbon (C) increases the fatigue strength of a spring produced using steel as raw material. If the C content is less than 0.50%, even if the contents of other elements are within the range specified herein, the aforementioned effect will not be sufficiently achieved. On the other hand, if the C content exceeds 0.80%, coarse cementite will form. In this case, even if the contents of other elements are within the ranges specified herein, the ductility of the steel used as raw material for the spring will decrease. Furthermore, the fatigue strength of a spring produced using the corresponding steel as raw material will decrease. Consequently, the C content is between 0.50% and 0.80%. A preferred lower limit for the C content is 0.51%, more preferably 0.52%, more preferably 0.54%, and most preferably 0.56%.A preferred upper limit for the C content is 0.79%, more preferably 0.78%, more preferably 0.76%, more preferably 0.74%, more preferably 0.72%, and more preferably 0.70%.

[0036] Yes: from 1.20 to less than 2.50%. Silicon (Si) increases the fatigue strength of a spring produced using steel as the raw material, and also increases the spring's settling resistance. Si also deoxidizes the steel. Furthermore, Si increases the temper softening resistance of the steel. Therefore, even after quenching and tempering in the spring production process, the spring's strength can be maintained at a high level. If the Si content is less than 1.20%, even if the content of other elements is within the range specified here, the aforementioned effects will not be sufficiently achieved. On the other hand, if the Si content is 2.50% or higher, even if the content of other elements is within the range specified here, the strength of the steel used as the spring's raw material will increase, and the steel's cold workability will decrease.Therefore, the Si content is from 1.20% to less than 2.50%. A preferred lower limit for the Si content is 1.25%, more preferably 1.30%, more preferably 1.40%, more preferably 1.50%, more preferably 1.60%, more preferably 1.70%, and more preferably 1.80%. A preferred upper limit for the Si content is 2.48%, more preferably 2.46%, more preferably 2.45%, more preferably 2.43%, and more preferably 2.40%.

[0037] Mn: 0.25 to 1.00%. Manganese (Mn) improves the hardenability of steel and increases the fatigue strength of the spring. If the Mn content is less than 0.25%, even if the content of other elements is within the range specified herein, the aforementioned effect will not be sufficiently achieved. On the other hand, if the Mn content is greater than 1.00%, even if the content of other elements is within the range specified herein, the strength of the steel material used as the spring's raw material will increase, and its cold workability will decrease. Therefore, the Mn content is between 0.25% and 1.00%. A preferred lower limit for the Mn content is 0.27%, more preferably 0.29%, more preferably 0.35%, more preferably 0.40%, more preferably 0.50%, and more preferably 0.55%. A preferred upper limit for the Mn content is 0.98%, more preferably 0.96%, more preferably 0.90%, more preferably 0.85% and more preferably 0.80%.

[0038] P: 0.020% or less. Phosphorus (P) is an impurity. P segregates at the grain boundaries and lowers the spring's fatigue limit. Therefore, the P content is 0.020% or less. A preferred upper limit for P content is 0.018%, more preferably 0.016%, more preferably 0.014%, and most preferably 0.012%. It is preferable for the P content to be as low as possible. However, excessively reducing the P content will increase the cost of production. Therefore, considering normal industrial production, a preferred lower limit for P content is greater than 0%, more preferably 0.001%, and most preferably 0.002%.

[0039] S: 0.020% or less Sulfur (S) is an impurity. S segregates at the grain boundaries similarly to P and combines with Mn to form MnS, decreasing the spring's fatigue limit. Therefore, the S content is 0.020% or less. A preferred upper limit for the S content is 0.018%, more preferably 0.016%, more preferably 0.014%, and most preferably 0.012%. It is preferable for the S content to be as low as possible. However, excessively reducing the S content will increase production costs. Therefore, considering normal industrial production, a preferred lower limit for the S content is greater than 0%, more preferably 0.001%, and most preferably 0.002%.

[0040] Cr: 0.40 to 1.90% Chromium (Cr) improves the hardenability of steel and increases the fatigue limit of the spring. If the Cr content is less than 0.40%, even if the content of other elements is within the range specified herein, the aforementioned effect will not be sufficiently achieved. On the other hand, if the Cr content is greater than 1.90%, even if the content of other elements is within the range specified herein, excessively coarse Cr carbides will form, and the fatigue limit of the spring will decrease. Therefore, the Cr content is between 0.40% and 1.90%. A preferred lower limit for the Cr content is 0.42%, more preferably 0.45%, more preferably 0.50%, more preferably 0.60%, more preferably 0.80%, more preferably 1.00%, and more preferably 1.20%. A preferred upper limit for the Cr content is 1.88%, more preferably 1.85%, more preferably 1.80%, more preferably 1.70% and preferably 1.60%.

[0041] V: from 0.05 to 0.60%. Vanadium (V) combines with carbon (C) and / or nitrogen (N) to form fine V-based precipitates, increasing the spring's fatigue limit. If the V content is less than 0.05%, even if the content of other elements is within the range specified herein, the aforementioned effect will not be sufficiently achieved. On the other hand, if the V content exceeds 0.60%, even if the content of other elements is within the range specified herein, the V-based precipitates will become coarser, forming a large number of V-based precipitates with a maximum diameter exceeding 10 nm. In such a case, the spring's fatigue limit will decrease. Therefore, the V content is between 0.05% and 0.60%. A preferred lower limit for the V content is 0.06%, more preferably 0.07%, more preferably 0.10%, more preferably 0.15%, and most preferably 0.20%. A preferred upper limit for the V content is 0.59%, more preferably 0.58%, more preferably 0.55%, more preferably 0.50%, more preferably 0.45%, and more preferably 0.40%.

[0042] N: 0.0100% or less. Nitrogen (N) is an impurity. N combines with Al or Ti to form AlN or TiN, and decreases the spring's fatigue limit. Therefore, the N content is 0.0100% or less. A preferred upper limit for the N content is 0.0090%, more preferably 0.0080%, more preferably 0.0060%, and most preferably 0.0050%. It is preferable for the N content to be as low as possible. However, excessively reducing the N content will increase production costs. Therefore, a preferred lower limit for the N content is greater than 0%, more preferably 0.0001%, and most preferably 0.0005%.

[0043] The remainder of the chemical composition of the steel wire conforming to the present modality is Fe and impurities. Herein, the term impurities refers to elements that, during the industrial production of the steel wire, are mixed in from the ore or scrap used as raw material, or from the production environment or the like, and are permitted within a range that does not adversely affect the steel wire of the present modality.

[0044] [Regarding optional elements] The chemical composition of the steel wire in accordance with the present modality may also contain Ca instead of a portion of Fe.

[0045] Ca: 0.0050% or less Calcium (Ca) is an optional element and is not required. That is, the Ca content can be 0%. When present, meaning the Ca content is greater than 0%, the Ca is contained within oxide-based and sulfide-based inclusions, softening them. These softened oxide-based and sulfide-based inclusions elongate and split during hot rolling, thus refining the spring. Consequently, the spring's fatigue limit increases, particularly the high-cycle fatigue limit. However, if the Ca content exceeds 0.0050%, coarse Ca sulfides and coarse oxide-based inclusions (Ca oxides) will form, and the spring's fatigue limit will decrease. Therefore, the Ca content is from 0 to 0.0050%, and when Ca is present, the Ca content is 0.0050% or less. A preferred lower limit for the Ca content is 0.0.0001%, more preferably 0.0002%, more preferably 0.0003%, more preferably 0.0004%, and more preferably 0.0005%. A preferred upper limit for the Ca content is 0.0048%, more preferably 0.0046%, more preferably 0.0040%, more preferably 0.0035%, more preferably 0.0025%, and more preferably 0.0020%.

[0046] The chemical composition of the steel wire conforming to the present modality may contain, in lieu of a portion of Fe, one or more types of elements selected from the group consisting of Mo, Nb, W, Ni, Co, and B. These elements are optional, and each of these elements increases the fatigue limit of a spring produced using the steel wire as raw material.

[0047] Mo: 0.50% or less Molybdenum (Mo) is an optional element and its presence is not mandatory. In other words, the Mo content can be as low as 0%. When present, meaning the Mo content is greater than 0%, Mo improves the hardenability of the steel and increases the spring's fatigue strength. Mo also increases the steel's resistance to temper softening. Therefore, even after quenching and tempering during spring production, the spring's strength can be maintained at a high level. Even a small amount of Mo produces some of these effects. However, if the Mo content is greater than 0%, the effects will be significantly reduced.50%, even if the content of other elements is within the range of the present modality, the strength of the steel material that will serve as raw material for the spring will increase, and the cold workability of the steel material will decrease. Therefore, the Mo content is 0 to 0.50%, and when Mo is present, the Mo content is 0.50% or less. A preferred lower limit for the Mo content is more than 0%, more preferably 0.01%, more preferably 0.05%, and more preferably 0.10%. A preferred upper limit for the Mo content is 0.45%, more preferably 0.40%, more preferably 0.35%, and more preferably 0.30%.

[0048] Nb: 0.050% or less Niobium (Nb) is an optional element and its presence is not required. That is, the Nb content can be 0%. When present, meaning the Nb content is greater than 0%, Nb combines with carbon and / or nitrogen to form carbides, nitrides, or carbonitrides (hereafter referred to as Nb carbonitrides and the like). Nb carbonitrides and the like reinforce the austenite grains, thereby increasing the spring's fatigue limit. Even a small amount of Nb produces this effect to some extent. However, if the Nb content exceeds 0.050%, coarse Nb carbonitrides and the like form, and the spring's fatigue limit decreases. Therefore, the Nb content is between 0 and 0.050%, and when Nb is present, the Nb content is 0.050% or less. A preferred lower limit for Nb content is more than 0%, more preferably 0.001%, more preferably 0.0.05%, and more preferably 0.010%. A preferred upper limit for Nb content is 0.048%, more preferably 0.046%, more preferably 0.042%, more preferably 0.038%, more preferably 0.035%, more preferably 0.030%, and more preferably 0.025%.

[0049] W: 0.60% or less Tungsten (W) is an optional element and is not required. In other words, the W content can be 0%. When present, meaning the W content is greater than 0%, W improves the hardenability of the steel material and increases the spring's fatigue limit. W also increases the steel material's resistance to temper softening. Therefore, even after quenching and tempering during spring production, the spring's strength can be maintained at a high level. Even a small amount of W produces these effects to some extent. However, if the W content exceeds 0.60%, even if the content of other elements is within the specified range, the strength of the steel material used as the spring's raw material will increase, while its cold workability will decrease.Therefore, the W content is from 0 to 0.60%, and when W is present, the W content is 0.60% or less. A preferred lower limit for the W content is more than 0%, more preferably 0.01%, more preferably 0.05%, and more preferably 0.10%. A preferred upper limit for the W content is 0.55%, more preferably 0.50%, more preferably 0.45%, more preferably 0.40%, more preferably 0.35%, and more preferably 0.30%.

[0050] Ni: 0.500% or less Nickel (Ni) is an optional element and is not required. That is, the Ni content can be 0%. When present, meaning the Ni content is greater than 0%, Ni improves the hardenability of the steel material and increases the fatigue limit of the spring. Even a small amount of Ni produces this effect to some extent. However, if the Ni content exceeds 0.500%, even if the content of other elements is within the range specified herein, the strength of the steel material used as the spring's raw material will increase, and its cold workability will decrease. Therefore, the Ni content is between 0 and 0.500%, and when Ni is included, it is 0.500% or less. A preferred lower limit for the Ni content is greater than 0%, more preferably 0.001%, more preferably 0.005%, and most preferably 0.0.10%, more preferably 0.050%, more preferably 0.100%, and more preferably 0.150%. A preferred upper limit for Ni content is 0.450%, more preferably 0.400%, more preferably 0.350%, more preferably 0.300%, and more preferably 0.250%.

[0051] Co: 0.30% or less. Cobalt (Co) is an optional element and its presence is not required. That is, the Co content can be 0%. When present, meaning the Co content is above 0%, Co increases the temper softening resistance of the steel material. Therefore, even after quenching and tempering during the spring production process, the spring's strength can be maintained at a high level. Even a small amount of Co provides this effect to some extent. However, if the Co content exceeds 0.30%, even if the content of other elements is within the specified range, the strength of the steel material used as the spring's raw material will increase, but its cold workability will decrease. Therefore, the Co content is between 0% and 0.30%, and when Co is included, it must be 0.30% or less.A preferred lower limit for Co content is more than 0%, more preferably 0.01%, more preferably 0.05%, and most preferably 0.10%. A preferred upper limit for Co content is 0.28%, more preferably 0.26%, and most preferably 0.24%.

[0052] B: 0.0050% or less. Boron (B) is an optional element and is not required to be present. That is, the B content can be 0%. When it is present, meaning the B content is greater than 0%, the B ML / a / ZUZZ / U 1 UZ» 1 improves the hardenability of the steel material and increases the fatigue limit of the spring. Even a small amount of B will produce a certain effect. However, if the B content exceeds 0.0050%, even if the content of other elements is within the specified range, the strength of the steel material used as the spring's raw material will increase, and its cold workability will decrease. Therefore, the B content should be between 0 and 0.0050%, and when B is included, it should be 0.0050% or less. A lower preferred B content is greater than 0%, more preferably 0.0001%, more preferably 0.0010%, more preferably 0.0015%, and more preferably 0.0020%. A preferred upper limit for the content of B is 0.0049%, more preferably 0.0048%, more preferably 0.0046%, more preferably 0.0.0044% and more preferably 0.0042%.

[0053] The chemical composition of the steel wire according to the present modality may also contain, as an impurity, instead of a portion of Fe, one or more types of elements selected from the group consisting of Cu: 0.050% or less, Al: 0.0050% or less, and Ti: 0.050% or less. If the contents of these elements are within the ranges mentioned, the advantageous effects of the steel wire according to the present modality and of a spring produced using the steel wire will be obtained.

[0054] Cu: 0.050% or less Copper (Cu) is an impurity and its presence is not required. That is, the Cu content can be 0%. Cu decreases the cold workability of the steel. If the Cu content exceeds 0.050%, even if the content of other elements is within the range specified herein, the cold workability of the steel will be significantly reduced. Therefore, the Cu content should be 0.050% or less. Since the Cu content can be 0%, it falls within the range of 0 to 0.050%. A preferred upper limit for the Cu content is 0.045%, more preferably 0.040%, more preferably 0.030%, more preferably 0.025%, more preferably 0.020%, and more preferably 0.018%. As mentioned previously, the Cu content is preferably as low as possible. However, excessively reducing the Cu content will increase production costs.Therefore, a preferred lower limit of Cu content is more than 0%, more preferably 0.001%, more preferably 0.002%, and most preferably 0.005%.

[0055] Al: 0.0050% or less Aluminum (Al) is an impurity and its presence is not required. That is, the Al content can be 0%. Al forms coarse oxide-based inclusions and decreases the spring's fatigue limit. If the Al content exceeds 0.0050%, even if the content of other elements is within the range specified herein, the spring's fatigue limit will be significantly reduced. Therefore, the Al content is 0.0050% or less. Since the Al content can be 0%, it falls within the range of 0 to 0.0050%. A preferred upper limit for the Al content is 0.0045%, more preferably 0.0040%, more preferably 0.0030%, more preferably 0.0025%, and most preferably 0.0020%. As mentioned above, the Al content is preferably as low as possible. However, an excessive reduction in Al content will increase the cost of production.Therefore, a preferred lower limit for Al content is more than 0%, more preferably 0.0001%, more preferably 0.0003%, and most preferably 0.0005%.

[0056] Ti: 0.050% or less. Titanium (Ti) is an impurity and its presence is not required. That is, the Ti content can be 0%. Ti forms a coarse TiN. TiN readily becomes a starting point for fracture and thus decreases the spring's fatigue limit. If the Ti content exceeds 0.050%, even if the content of other elements is within the range specified herein, the spring's fatigue limit will decrease significantly. Therefore, the Ti content should be 0.050% or less. Since the Ti content can be 0%, it falls within the range of 0 to 0.050%. A preferred upper limit for the Ti content is 0.045%, more preferably 0.040%, more preferably 0.030%, and most preferably 0.020%. As mentioned previously, the Ti content is preferably as low as possible. However, excessively reducing the Ti content will increase production costs.Therefore, a preferable lower limit for the Ti content is more than 0%, and more preferably 0.001%.

[0057] [Microstructure of steel wire] The microstructure of the steel wire of the present embodiment is composed primarily of martensite. Here, the phrase "the microstructure is composed primarily of martensite" means that the area fraction of martensite in the microstructure is 90.0% or more. Note that the term "martensite," as used herein, means quenched martensite. The phases other than martensite in the microstructure of the steel wire are precipitates, inclusions, and retained austenite. Note that, among these phases, the precipitates and inclusions are sufficiently small compared to the other phases that they can be disregarded.

[0058] The martensite area fraction can be determined by the following method. The steel wire conforming to the present embodiment is cut in a direction perpendicular to the longitudinal direction of the steel wire, and a test specimen is taken. Among the surfaces of the taken test specimen, a surface corresponding to a cross-section perpendicular to the longitudinal direction of the steel wire is adopted as the observation surface. After mirror polishing the observation surface, it is etched with 2% nitric acid-alcohol (nital etching reagent). On the etched observation surface, the average position of a line segment (i.e., a radius R) from the surface of the steel wire to the center of the wire is defined as a position R / 2.The R / 2 position of the observation surface is observed using an optical microscope with a magnification of 500x, and photographic images of five arbitrary fields of view are generated. The size of each field of view is set to 100 pm x 100 pm.

[0059] In each field of view, the contrast differs for the respective phases of martensite, retained austenite, precipitates, inclusions, and the like. Consequently, martensite is identified based on contrast. The gross area (pm2) of identified martensite in each field of view is determined. The ratio of the gross martensite area in all fields of view to the gross area (10000 pm2 x 5) of all fields of view is defined as the area fraction (%) of martensite.

[0060] [Numerical density of V-based precipitates on steel wire] In the steel wire of the present embodiment, the number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm is 5000 to 80000 pieces / pm3. In the present description, the term number density of V-based precipitates means the number of V-based precipitates per unit volume (1 pm3 in the present description).

[0061] In the present description, the term V-based precipitates refers to precipitates containing V, or V and Cr. V-based precipitates are, for example, V carbides and V carbonitrides. V-based precipitates may be compound precipitates containing one of the V carbides and a V carbonitride and one or more other elements. As mentioned above, V-based precipitates need not contain Cr. V-based precipitates precipitate in a plate-like form along a {001} plane in ferrite.Therefore, in a TEM image of a (001) plane in ferrite, V-based precipitates are observed as line segments (edge ​​portions) that extend linearly parallel to the

[100] or

[010] orientation. Therefore, when observing a TEM image of the (001) plane in ferrite, V-based precipitates can be easily distinguished from Fe carbides such as cementite, and V-based precipitates can be identified.

[0062] Note that, in a steel wire in which the content of each element in the chemical composition is within the range of the present modality and which is produced by a production method described below, in a TEM image of the (001) plane in the ferrite, the fact that a precipitate observed as a line segment (edge ​​portion) extending along the

[100] or

[010] orientation is a V-based precipitate can be confirmed by an analysis using energy-dispersive X-ray spectroscopy (EDS) and nanobeam electron diffraction (NBD).

[0063] Specifically, in a TEM image of the (001) plane in ferrite, when a precipitate is observed as a line segment extending along the

[100] or orientation

[010] is subjected to compositional analysis by EDS, and V or V and Cr are detected. Furthermore, when the precipitate is subjected to crystal structure analysis by NBD, the crystal structure of the precipitate is cubic, and the lattice constant is a = b = c = within the range of 0.4167 nm ± 5%. Note that, in the International Centre for Diffraction Data (ICDD) database, the crystal structure of V-based precipitates (V carbides and V carbonitrides) is cubic, and the lattice constant is 0.4167 nm (ICDD No. 065-8822).

[0064] By inducing the precipitation of a large number of nanometer-sized V-based precipitates with a maximum diameter ranging from 2 to 10 nm on the steel wire of the present embodiment, the fatigue limit of a spring produced from the steel wire is increased. If the number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm is less than 5000 pieces / pm3, the V-based precipitates that contribute to improving the fatigue limit will be too few. In this case, a sufficient fatigue limit will not be obtained in the spring. If the number density of V-based precipitates having a maximum diameter of 2 to 10 nm is 5000 pieces / pm3 or more, there will be sufficient V-based precipitates present on the steel wire. Consequently, the fatigue limit and the fatigue limit ratio of the spring will be significantly increased.A preferred lower limit of the number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm is 6000 pieces / pm3, more preferably 7000 pieces / pm3, more preferably 8000 pieces / pm3, more preferably 10000 pieces / pm3, more preferably 11000 pieces / pm3, more preferably 12000 pieces / pm3, more preferably 13000 pieces / pm3, more preferably 14000 pieces / pm3, and more preferably 15000 pieces / pm3.

[0065] Note that the upper limit of the number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm is not particularly restricted. However, in the case of the chemical composition described above, the upper limit of the number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm is, for example, 80,000 pieces / pm3. The upper limit of the number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm may be 75,000 pieces / pm3, or it may be 73,000 pieces / pm3.

[0066] [Method for measuring the number density of V-based precipitates] In steel wire according to the present embodiment, the number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm can be determined by the following method. The steel wire according to the present embodiment is cut perpendicular to the longitudinal direction of the steel wire, and a disk is extracted having a surface (cross-section) perpendicular to the longitudinal direction of the steel wire and having a thickness of 0.5 mm. Both sides of the disk are ground and polished using iviA / a / zuzz / ui emery paper so that the disk thickness is 50 µm. A sample 3 mm in diameter is then extracted from the disk. The sample is immersed in a 10% perchloric acid-glacial acetic acid solution for electrolytic polishing, thus preparing a thin-film sample with a thickness of 100 nm.

[0067] The prepared thin-film sample is observed using a transmission electron microscope (TEM). Specifically, an analysis of the Kikuchi lines is first performed with respect to the thin-film sample to identify the crystal orientation of the thin-film sample. The thin-film sample is then tilted according to the identified crystal orientation, and the thin-film sample is adjusted so that the (001) plane in the ferrite (body-centered cubic lattice) can be observed. Specifically, the thin-film sample is placed in the TEM, and the Kikuchi lines are observed. The tilt of the thin-film sample is adjusted so that the

[001] direction of the ferrite in the Kikuchi lines coincides with the direction of incidence of an electron beam. After adjustment, when the actual image is observed, the observation will be from a direction perpendicular to the (001) plane in the ferrite.After performing the adjustment, the observation fields of view are identified at four arbitrary locations on the thin-film sample. Each observation field is observed using an observation magnification of 200,000x and an acceleration voltage of 200 kV. The observation field of view is adjusted to 0.09 μιη χ 0.09 pm.

[0068] Figure 1A is an example of a TEM image of a (001) plane in ferrite from a thin-film sample, and Figure 1B is a schematic diagram of a TEM image of a (001) plane in ferrite from a thin-film sample. An axis denoted by

[100] a in the figures signifies the

[100] orientation in the ferrite that is the generating phase. An axis denoted by

[010] a in the figures signifies the

[010] orientation in the ferrite that is the generating phase. V-based precipitates precipitate in a plate-like fashion along a {001} plane in the ferrite. In ferrite grains in the (001) plane, V-based precipitates are observed as line segments (edge ​​portions) that extend linearly with respect to the

[100] or

[010] orientation. In a TEM image, the precipitates are shown with a different brightness contrast compared to the generating phase.Therefore, in a TEM image of a (001) plane in ferrite, line segments extending along the

[100] or

[010] orientation are considered V-based precipitates. The length of the line segment of an identified V-based precipitate in the field of view is measured, and this measured length is defined as the maximum diameter (nm) of the corresponding V-based precipitate. For example, reference number 10 (a black line segment) in Figure 1A and Figure 1B denotes a V-based precipitate.

[0069] The total number of V-based precipitates with a maximum diameter ranging from 2 to 10 nm in the visual fields of observation at the four locations is determined by the aforementioned measurement. The numerical density (pieces / pm3) of V-based precipitates having a maximum diameter ranging from 2 to 10 nm is determined based on the total number of V-based precipitates thus determined and the total volume of the observation fields of view at the four locations.

[0070] [Area of ​​preferred numerical ratio of Ca sulfides] In this modality, oxide-based inclusions, sulfide-based inclusions, and Ca sulfides in steel wire are defined as follows: Oxide-based inclusions: inclusions that have, by mass %, an O content of 10.0% or more. Sulfide-based inclusions: inclusions that have, by mass %, a S content of 10.0% or more and an O content of less than 10.0%. Calcium sulfides: inclusions that have, in % by mass, a Ca content equal to or greater than 10.0%, a S content equal to or greater than 10.0% and an O content less than 10.0%.

[0071] Oxide-based inclusions are, for example, one or more types selected from the group consisting of S1O2, MnO, Al2O3, and MgO. Oxide-based inclusions may be composite inclusions containing one or more types selected from the group consisting of S1O2, MnO, Al2O3, and MgO, and another alloying element. Sulfide-based inclusions are, for example, one or more types selected from the group consisting of MnS and CaS, and may also be composite inclusions containing one or more types selected from the group consisting of MnS and CaS, and another alloying element. Calcium sulfides are, for example, CaS, and may be composite inclusions containing CaS and another alloying element.

[0072] In steel wire, the numerical proportion of calcium sulfides to the total number of oxide-based and sulfide-based inclusions is defined as the numerical proportion of calcium sulfides (%).That is, Rea is represented by the following equation. Rea = number of Ca sulfides / total number of oxide-based inclusions and sulfide-based inclusions x 100 (1)

[0073] In the present embodiment, preferably, the steel wire contains Ca: 0.0050% or less, and the numerical ratio (Rea) of Ca sulfides in the steel wire is 0.20% or less. Herein, the phrase "Rea of ​​numerical ratio of Ca sulfides in the steel wire" means the numerical ratio (Rea) of Ca sulfides at a position R / 2 from the surface of the steel wire when, in a cross section including the centerline of the steel wire (a cross section parallel to the longitudinal direction of the steel wire), a distance from the surface of the steel wire to the centerline is defined as R (i.e., a radius of a cross section perpendicular to the longitudinal direction of the steel wire is defined as R) (mm).

[0074] Figure 2 is a graph illustrating the relationship between the numerical ratio of calcium sulfides (Rea) and the fatigue limit at a cycle count of 108 cycles (high-cycle fatigue limit) for a valve spring produced using steel wire having the chemical composition of the present embodiment and in which the calcium content is 0.0050% or less as raw material. Referring to Figure 2, when the numerical ratio of calcium sulfides (Rea) is greater than 0.20%, the high-cycle fatigue limit increases markedly as the numerical ratio of calcium sulfides (Rea) decreases. On the other hand, when the numerical ratio of calcium sulfides (Rea) is 0.20% or less, even as the numerical ratio of calcium sulfides (Rea) decreases, the high-cycle fatigue limit does not increase much and remains approximately constant.That is, in Figure 2, there is an inflection point at the position where around the numerical proportion of Ca sulfides Rea = 0.20%.

[0075] As described above, when the numerical ratio of calcium sulfides (Rea) is greater than 0.20%, the fatigue limit at 10⁸ cycles (high-cycle fatigue limit) decreases rapidly. When the Rea of ​​Ca sulfides is 0.20% or less, an excellent high-cycle fatigue limit is obtained. Therefore, in the steel wire of the present embodiment, the calcium content is preferably within the range of greater than 0 to 0.0050%, and the numerical ratio of calcium sulfides in the steel wire is 0.20% or less. A preferred upper limit for the numerical ratio of calcium sulfides is 0.19%, more preferably 0.18%, and even more preferably 0.17%.Note that, although a lower limit of the numerical ratio of Ca sulfides is not particularly restricted, in the case of the chemical composition described above, the lower limit of the numerical ratio of Ca sulfides is, for example, 0%, or for example, 0.01%.

[0076] The numerical ratio of calcium sulfides is measured by the following method. A test specimen is extracted from a cross-section that includes the centerline of the steel wire in accordance with the present modality. Among the surfaces of the extracted test specimen, a surface corresponding to a cross-section that includes the centerline of the steel wire is adopted as the observation surface. The observation surface is mirror-polished. On the mirror-polished observation surface, fields of view (each field of view: 100 pm x 100 pm) are observed at 10 arbitrary locations at a position R / 2 from the surface of the steel wire using a scanning electron microscope (SEM) with a magnification of 1000x.

[0077] Inclusions in each field of view are identified based on the contrast of each field of view. Each identified inclusion is subjected to EDS to identify oxide-based inclusions, sulfide-based inclusions, and Ca sulfides. Specifically, based on the elemental analysis results obtained by EDS with respect to the inclusions, inclusions with a mass percent oxygen content of 10.0% or more are identified as oxide-based inclusions. Among the inclusions, those with a mass percent sulfur content of 10.0% or more and an oxygen content of less than 10.0% are identified as sulfide-based inclusions. In addition, among the identified sulfide-based inclusions, those inclusions that have, in % by mass, a Ca content of 10.0% or more, a S content of 10.0% or more, and an O content of less than 10.0% are identified as Ca sulfides.

[0078] The inclusions subject to the aforementioned identification are inclusions with an equivalent circular diameter of 0.5 sqm or more. Here, the term equivalent circular diameter means the diameter of a circle if the area of ​​each inclusion were converted into a circle with the same area. If the inclusions have an equivalent circular diameter that is two or more times the beam diameter in the EDS, the accuracy of the elemental analysis is increased. In the present modality, the beam diameter in the EDS used for the identification of the inclusions is assumed to be 0.2 sqm. In this case, inclusions with an equivalent circular diameter less than 0.5 sqm cannot increase the accuracy of the elemental analysis in the EDS. Furthermore, inclusions with an equivalent circular diameter less than 0.5 sqm have an extremely small influence on the fatigue limit of a spring.Therefore, in this modality, inclusions with an equivalent circular diameter of 0.5 µm or more are considered the target for identification. The upper limit of the equivalent circular diameter for oxide-based inclusions, sulfide-based inclusions, and Ca sulfides is not particularly restricted, and is, for example, 100 µm.

[0079] The numerical ratio of Ca sulfides (%) is determined using equation (1) based on the total number of oxide-based inclusions and sulfide-based inclusions identified in the aforementioned visual observation fields at 10 locations, and the total number of Ca sulfides identified in the aforementioned visual observation fields at 10 locations. Rea = number of Ca sulfides / total number of oxide-based inclusions and sulfide-based inclusions xl00 (1)

[0080] As described above, in the steel wire of the present embodiment, the respective elements in the chemical composition are within the range specified herein, and the number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm is within the range of 5000 to 80000 pieces / qm³. Therefore, a spring produced using the steel wire of the present embodiment has an excellent fatigue limit. Specifically, a high fatigue limit is obtained at a cycle count of 10⁷ cycles. In this case, the steel wire of the present embodiment is particularly suitable for use in a shock absorber spring.

[0081] Preferably, the steel wire of the present embodiment further contains Ca in an amount of 0.0050% or less (i.e., the Ca content is greater than 0 to 0.0050%), and the numerical ratio of Ca sulfides (Rea) is 0.20% or less. Therefore, an excellent additional fatigue limit is obtained in a spring produced using the steel wire of the present embodiment. Specifically, a high fatigue limit (high cycle fatigue limit) is obtained at a cycle count of 10⁸ cycles. In this case, the steel wire of the present embodiment is particularly suitable for use in a valve spring.

[0082] [Method for producing steel wire] The following describes an example of a method for producing the steel wire of this embodiment. Note that, provided the steel wire of this embodiment is constituted as described above, the production method is not limited to the production method described below. However, the production method described hereafter is a favorable example for producing the steel wire of this embodiment.

[0083] Figure 3 is a flow diagram illustrating an example of a process for producing steel wire of the present embodiment. Referring to Figure 3, the method for producing steel wire of the present embodiment includes a wire rod preparation step (S10) and a steel wire production step (S20). Each of these steps is described below.

[0084] [Wire rod preparation step (S10)] The wire rod preparation step (S10) includes a raw material preparation step (SI) and a hot working step (S2). In the wire rod preparation step (S10), wire rod is produced that will serve as raw material for steel wire.

[0085] [Raw material preparation step (SI)] In the raw material preparation step (SI), a raw material with the chemical composition mentioned above is produced. The term "raw material" used here refers to a billet or ingot. In the raw material preparation step (SI), molten steel with the aforementioned chemical composition is first produced using a known refining method. The molten steel produced is then used to produce a raw material (billet or ingot). Specifically, a billet is produced by a continuous casting process using the molten steel. Alternatively, an ingot is produced by an ingot-making process using the molten steel. The hot working step (S2), which is the next step, is carried out using the billet or ingot.

[0086] [Hot work step (S2)] In the hot working step (S2), the raw material (billet or ingot) prepared in the raw material preparation step (SI) is subjected to hot rolling to produce a wire rod.

[0087] The hot working step (S2) includes a rough rolling process and a finishing rolling process. In the rough rolling process, the raw material is first heated. A reheating furnace or holding pit is used to heat the raw material. The raw material is heated to a temperature of 1200 to 1300 °C in the reheating furnace or holding pit. For example, the raw material is held for 1.5 to 10.0 hours at a furnace temperature of 1200 to 1300 °C. ML / a / zuzz / ui υ϶υ i After heating, the raw material is removed from the reheating furnace or holding pit and subjected to hot rolling. For example, in the rough rolling process for hot rolling, a roughing mill is used. The roughing mill is used to subject the raw material to a roughing process to produce a bar. If a continuous rolling mill is available after the roughing mill, it can be used to further hot roll the bar obtained after hot rolling, thus producing an even smaller bar. In the continuous mill, for example, horizontal stands with a pair of horizontal rolls and vertical stands with a pair of vertical rolls are arranged alternately in a row. Through the above process, a bar is produced from the raw material in the rough rolling process.

[0088] In the finishing rolling process, the bar obtained from the rough rolling process is hot-rolled to produce wire rod. Specifically, the bar is loaded into a reheating furnace and heated to a temperature of 900 to 1250 °C. The heating time at this furnace temperature is, for example, 0.5 to 5.0 hours. After heating, the bar is removed from the reheating furnace. The removed bar is then hot-rolled using a continuous mill to produce wire rod. The diameter of the wire rod is not specifically limited. The diameter of the wire rod is determined based on the diameter of the spring wire, which is the final product. Wire rod is produced using the same production process described above.

[0089] [Steel wire production step (S20)] In the steel wire production step (S20), steel wire of the present form is produced, which will serve as raw material for a spring. In this case, the term steel wire means a steel material obtained by subjecting wire rod, which is a hot-work material (hot-rolled material), to wire drawing one or more times. The steel wire production step (S20) includes a patenting treatment step (S3) performed as required, a drawing step (S4), a quenching and tempering step (S5), and a V-based precipitate forming heat treatment step (S100).

[0090] [Patenting treatment step (S3)] In the plating treatment step (S3), the wire rod produced by the wire rod preparation step (S10) undergoes plating to give the wire rod a ferrite-pearlite microstructure, thereby softening it. It is sufficient to perform the plating treatment using a known method. The heat treatment temperature in the plating treatment is, for example, 550 °C or higher, and more preferably 580 °C or higher. The upper limit for the heat treatment temperature in the plating treatment is 750 °C. Note that the plating treatment step (S3) is not an essential step; it is an optional step. That is, the plating treatment step (S3) does not have to be performed.

[0091] [Drawing step (S4)] If the patenting treatment step (S3) is performed, the wire rod after the patenting treatment step (S3) is drawn in the drawing step (S4). If the patenting treatment step (S3) is not performed, the wire rod after the hot working step (S2) is drawn in the drawing step (S4). Drawing produces a steel wire of the desired diameter. The drawing step (S4) can be performed using a known method. Specifically, the wire rod undergoes a lubrication treatment, and a lubricating coating, such as a phosphate coating or a metallic soap layer, is formed on its surface. After the lubrication treatment, the wire rod is drawn at normal temperature. A well-known wire drawing machine can be used for the drawing. A wire drawing machine is equipped with dies to subject the wire rod to drawing.

[0092] [Tempering and annealing step (S5)] In the quenching and tempering step (S5), the steel wire, after the drawing step (S4), undergoes a quenching and tempering treatment. The quenching and tempering step (S5) includes a quenching process and an annealing process. In the quenching process, the steel wire is first heated to the Ac3 transformation point or higher. For example, heating is carried out using a high-frequency induction heating apparatus or a radiant heating device. The heated steel wire is then rapidly cooled. The rapid cooling method can be with water or oil. During the quenching process, the microstructure of the steel wire is transformed into a structure composed primarily of martensite.

[0093] After the quenching process, the steel wire undergoes an annealing process. The annealing temperature is at or below the Acl transformation point. For example, the annealing temperature is 250 to 520 °C. During the annealing process, the microstructure of the steel wire is transformed into a structure consisting primarily of tempered martensite.

[0094] [V-based precipitate formation heat treatment step (S100)] In the V-based precipitation heat treatment step (S100), a heat treatment (V-based precipitation heat treatment) is performed on the steel wire after the quenching and tempering step (S5) to form fine V-based precipitates on the steel wire. By performing the V-based precipitation heat treatment step (S100), the number density of V-based precipitates with a maximum diameter ranging from 2 to 10 nm on the steel wire is 5,000 to 80,000 pieces / pm³.

[0095] In the V-based precipitate formation heat treatment, a heat treatment temperature is established within the range of 540 to 650 °C. A holding time t (min) at the heat treatment temperature T (°C) is not particularly limited, and, for example, is within the range of 5 / 60 (i.e., 5 seconds) to 50 minutes. The above-mentioned heat treatment temperature and holding time are adjusted so that the number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm on the steel wire is 5000 to 80000 pieces / pm3.

[0096] If a nitriding step (S8) is performed in a process for producing a spring described below, the heat treatment temperature in the V-based precipitation heat treatment may be higher than the nitriding temperature in the nitriding step (S8). In the conventional process for producing a spring, a heat treatment (stress-relieving annealing step or similar) following a quenching and tempering step is performed at a temperature lower than the nitriding temperature in the case of performing the nitriding step (S8). This is because the conventional process for producing a spring is based on the technical principle that the fatigue limit is increased by maintaining a high level of strength and hardness in the steel material that constitutes the spring.If the nitriding step (S8) is performed, heating to the nitriding temperature is necessary. Therefore, in the conventional production process, the reduction in strength of the spring (the steel material constituting the spring) is minimized by ensuring that the heat treatment temperature in any heat treatment step other than nitriding is lower than the nitriding temperature. Furthermore, for the steel wire of the present embodiment, instead of increasing the fatigue limit of a spring by increasing the strength of the spring (the steel material constituting the spring), the technique of increasing the fatigue limit of a spring by forming a large number of fine, nanometer-sized V-based precipitates was adopted.Therefore, in the heat treatment for forming V-based precipitates, the heat treatment temperature is set between 540 and 650 °C, a temperature range in which V-based precipitates readily form. A preferred lower limit for the heat treatment temperature in the V-based precipitation heat treatment is 550 °C, more preferably 560 °C, more preferably 565 °C, and more preferably 570 °C. A preferred upper limit for the heat treatment temperature in the V-based precipitation heat treatment is 640 °C, more preferably 630 °C, more preferably 620 °C, and more preferably 610 °C.

[0097] Furthermore, the V-based precipitate formation heat treatment is carried out so that Fn, defined by the following equation (2), is within the range of 29.5 to 38.9. Fn = {T3 / 2x {0.6t1 / 8+ (Cr + Mo + 2V)1 2}} / l000 (2) iviA / a / zuzz / ui uzm T in equation (2) represents a heat treatment temperature (°C) in the V-based precipitate formation heat treatment, and t represents a holding time (min) at the heat treatment temperature T. The content (mass %) of a corresponding element in the chemical composition of the steel wire is substituted for each element symbol in equation (2).

[0098] The amount of V-based precipitates that form is influenced not only by the heat treatment temperature T (°C) and the holding time t (min), but also by the respective contents of Cr, Mo, and V, which are elements that contribute to the formation of V-based precipitates.

[0099] Specifically, the formation of V-based precipitates is facilitated by Cr and Mo. Although the reason for this is not clear, the following explanation is conceivable.In a temperature region below the range where vanadium-based precipitates form, chromium forms iron-based carbides, such as cementite or chromium carbides. Similarly, in a temperature region below the range where vanadium-based precipitates form, molybdenum forms molybdenum carbides (Mo₂C). As the temperature increases, the iron carbides, chromium carbides, and molybdenum carbides dissolve and serve as nucleation sites for vanadium-based precipitates. As a result, at the heat treatment temperature T, the formation of vanadium-based precipitates is facilitated.

[0100] Assuming that the content of each element in the chemical composition of the steel wire is within the range specified herein, if Fn is less than 29.5, the formation of V-based precipitates will be insufficient in the V-based precipitate formation heat treatment. In this case, in the resulting steel wire, the number density of V-based precipitates with a maximum diameter between 2 and 10 nm will be less than 5000 pieces / pm3. On the other hand, if the content of each element in the chemical composition of the steel wire is within the range specified herein, and Fn is greater than 38.9, the V-based precipitates formed will be coarser. In this case, in the resulting steel wire, the number density of V-based precipitates with a maximum diameter between 2 and 10 nm will be less than 5000 pieces / pm3.

[0101] Under the premise that the content of each element in the chemical composition of the steel wire is within the present modality, when Fn is within the range of 29.5 to 38.9, in the steel wire produced, the number density of the V-based precipitates having a maximum diameter ranging from 2 to 10 nm will be within the range of 5000 to 80000 pieces / pm3.

[0102] A preferred lower bound of Fn is 29.6, more preferably 29.8, and most preferably 30.0. A preferred upper bound of Fn is 38.5, more preferably 38.0, more preferably 37.5, more preferably 37.0, more preferably 36.5, more preferably 36.0, and most preferably 35.5.

[0103] The steel wire of the present embodiment can be produced by the above production process. Note that, in the above production process, the quenching and tempering step (S5) and the V-based precipitation heat treatment step (SI00) are performed separately from each other. However, the tempering process in the quenching and tempering step (S5) can be omitted, and the V-based precipitation heat treatment step (SI00) can be performed after the quenching process. In this case, the steel wire, after the quenching process, is subjected to a heat treatment (V-based precipitation heat treatment) in which the heat treatment temperature T is set between 540 and 650 °C, and which is performed such that Fn falls within the range of 29.5 to 38.9.In this way, the tempering process can be omitted, and the V-based precipitation heat treatment step can be performed after the quenching process. In this case, during the V-based precipitation heat treatment, the precipitation of V-based precipitates and the tempering can be carried out simultaneously.

[0104] [Preferred production process for manufacturing Rea of ​​numerical proportion of Ca sulfides in steel wire 0.20% or less] In the case that the steel wire contains Ca: 0.0050% or less and to make the numerical ratio of Ca sulfides Rea of ​​0.20% or less in the steel wire, preferably, in the raw material preparation step (SI), a raw material is prepared which is produced by carrying out the following refining and smelting process.

[0105] [Refining Process] In the refining process, molten steel is refined and its components are adjusted. The refining process includes primary and secondary refining. Primary refining is refining using a converter, and the resulting refinement is known. Secondary refining is refining using a ladle, and the resulting refinement is known. In secondary refining, various types of ferroalloys and auxiliary raw materials (slag-forming agents) are added to the molten steel. Ferroalloys and auxiliary raw materials typically contain calcium in various forms. Therefore, to control the calcium content and the numerical ratio (Rea) of calcium sulfides in a valve spring to be produced using steel wire, (A) controlling the calcium content of the ferroalloys and (B) the timing of the addition of the auxiliary raw materials are important.

[0106] [Regarding (A)] With regard to the above (A), the Ca content in ferroalloys is high. Furthermore, in the case of molten steel subjected to Si deoxidation, the Ca content in the molten steel is high. Therefore, in secondary refining, if ferroalloys with a high Ca content are added, excessive Ca sulfides will form in the molten steel, increasing the numerical ratio (Rea) of Ca sulfides. Specifically, in secondary refining, if the Ca content in the ferroalloys added to the molten steel exceeds 1.0% by mass, the numerical ratio (Rea) of Ca sulfides will increase. Ca will be greater than 0.20%. Therefore, the Ca content in ferroalloys added to molten steel in secondary refining is made 1.0% or less.

[0107] [With respect to (B)] Furthermore, with regard to point (B) above, auxiliary raw materials (slag-forming agents) are added to the molten steel. Slag-forming agents are quicklime, dolomite, or recycled slag containing calcium oxides or similar compounds. The calcium from the slag-forming agents added to the molten steel during the secondary refining stage is contained within the agents as calcium oxides. Therefore, the calcium from the slag-forming agents is incorporated into the slag during secondary refining. However, if the slag-forming agents are added to the molten steel during the final stage of secondary refining, the calcium will not float sufficiently and will remain in the molten steel without being incorporated into the slag. In this case, the numerical ratio (Rea) of calcium sulfides will increase. Therefore, the slag-forming agents are added to the molten steel before the final stage of secondary refining.Here, the phrase "before the final stage of secondary refining" means, in a case where the refining time of secondary refining is defined as t (min), at least within a time corresponding to 4t / 5 minutes from the moment secondary refining began. That is, slag-forming agents are added to the molten steel before a time corresponding to 0.801 minutes from the start of secondary refining in the refining process.

[0108] [Smelting process]. A raw material (billet or ingot) is produced using the molten steel produced by the refining process described above. Specifically, a billet is produced by a continuous casting process using the molten steel. Alternatively, an ingot can be produced by an ingot-making process using the molten steel. The hot working step (S2), which is the next step, is carried out using the billet or ingot (raw material). The subsequent steps are the same as those described above.

[0109] By performing the above-described production process, a steel wire can be produced in which the content of each element in the chemical composition is within the range of the present modality, Ca is contained and the Ca content is 0.0050% or less, the number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm is 5000 to 80000 pieces / pm3, and the numerical ratio of Ca sulfides Rea is 0.20% or less.

[0110] [Method for producing springs using steel wire] Figure 4 is a flow diagram illustrating an example of a method for producing a spring using the steel wire of the present embodiment. The method for producing a spring using the steel wire of the present embodiment includes a cold winding step (S6), a stress-relieving annealing treatment step (S7), a nitriding step (S8) performed as required, and a shot peening step (S9).

[0111] [Cold winding step (S6)] In the cold winding step (S6), the steel wire of the present embodiment produced by the steel wire production step (S20) is subjected to cold winding to produce an intermediate spring steel material. The cold winding is carried out using a known winding apparatus. The winding apparatus is equipped with, for example, a plurality of transfer roller sets, a wire guide, a plurality of coil forming tools (winding pins), and a mandrel having a semicircular cross-section. Each transfer roller set includes a pair of rollers facing each other. The plurality of transfer roller sets are arranged in a row. Each transfer roller set interleaves the steel wire between the pair of rollers and carries the steel wire in the direction of the wire guide. The steel wire passes through the wire guide.The steel wire passing through the wire guide is bent into an arc shape by the plurality of winding pins and by the mandrel, thus forming an intermediate steel material in the form of a coil.

[0112] [Stress Relief Annealing Treatment Step (S7)] The stress-relief annealing treatment step (S7) is essential. In this step, an annealing treatment is performed to eliminate the residual stress generated in the intermediate steel material by the cold winding step. The treatment temperature (annealing temperature) in the annealing treatment is set, for example, between 400 and 500 °C. The holding time at the annealing temperature is not particularly limited; for example, the holding time is 10 to 50 minutes. After the holding time has elapsed, the intermediate steel material is allowed to cool or is slowly cooled to normal operating temperature.

[0113] [Nitriding step (S8)] The nitriding step (S8) is optional and not essential. That is, the nitriding step may or may not be performed. If the nitriding step (S8) is performed, it is carried out on the intermediate steel material after the stress-relieving annealing treatment step (S7). In nitriding, nitrogen is introduced into the outer layer of the intermediate steel material, and a nitrided (hardened) layer forms on the outer layer through solid solution hardening caused by the soluble nitrogen and precipitation hardening caused by the formation of nitrides.

[0114] It is sufficient to carry out the nitriding in accordance with known conditions. The nitriding is performed at a treatment temperature (nitriding temperature) that is not higher than the transformation point Aci. The nitriding temperature is, for example, from 400 to 530 °C. The holding time at the nitriding temperature is between 1.0 hour and 5.0 hours. The atmosphere inside the furnace in which the nitriding is carried out is not particularly restricted, provided that the atmosphere is one in which the chemical potential of nitrogen is sufficiently high. The furnace atmosphere for nitriding can be, for example, an atmosphere in which a gas with carburizing properties (RX gas or similar) is mixed, as in the case of soft nitriding.

[0115] [Shot blasting step (S9)] The shot blasting step (S9) is an essential step. In the shot blasting stage (S9), shot blasting is performed on the surface of the intermediate steel material after the stress-relieving annealing treatment stage (S7) or on the surface of the intermediate steel material after the nitriding stage (S8). This imparts a compressive residual stress to the outer layer of the spring, and the spring's fatigue limit can be further increased. Shot blasting can be performed using a known method. For example, shot blasting abrasives with a diameter of 0.01 to 1.5 mm are used. Common shot blasting abrasives, such as steel shot or steel beads, can be used as shot blasting abrasives.The residual compressive stress imparted to the spring is adjusted based on the shot diameter, the shot speed, the blasting period (duration), and the amount of blasting abrasive impacted on a unit of surface per unit of time.

[0116] A spring is produced by the production process described above. The spring is, for example, a shock absorber spring or a valve spring. Note that, in the production process of a spring, as mentioned above, the nitriding step (S8) may or may not be performed. In summary, a spring produced using the steel wire in this embodiment may or may not be nitrided.

[0117] [Spring configuration for shock absorber] In the case of a shock absorber spring, the shock absorber spring is coil-shaped. The wire diameter, average coil diameter, inner coil diameter, outer coil diameter, free height, number of active coils, total number of coils, helix direction, and spring pitch are not particularly limited.

[0118] Among shock absorber springs, a shock absorber spring that has undergone nitriding is called a nitrided shock absorber spring. Among shock absorber springs, a shock absorber spring that has not undergone nitriding is called a non-nitrided shock absorber spring. A nitrided shock absorber spring includes a nitrided layer and a core portion. The nitrided layer includes a composite layer and a diffusion layer that forms deeper than the composite layer. The nitrided layer need not include a composite layer. The core portion is a portion of base material that is deeper than the nitrided layer and is not substantially affected by the nitrogen diffusion caused by nitriding. It is possible to distinguish between the nitrided layer and the core portion in the nitrided shock absorber spring by observing the microstructure. A non-nitrided shock absorber spring does not have a nitrided layer.

[0119] When a nitrided damper spring is produced using the steel wire of the present embodiment, the chemical composition of the core portion of the nitrided damper spring is the same as the chemical composition of the steel wire of the present embodiment, and the number density of the V-based precipitates having a maximum diameter ranging from 2 to 10 nm is within the range of 5000 to 80000 pieces / pm3. Therefore, an excellent fatigue limit is obtained in the damper spring. Note that the microstructure of the core portion of the nitrided damper spring is the same as the microstructure of the steel wire, and the area fraction of martensite is 90.0% or more.

[0120] When a non-nitrided damper spring is produced using the steel wire of the present embodiment, within the non-nitrided damper spring (at an arbitrary position R / 2 (R represents the radius) of a cross-section in the wire diameter direction), the chemical composition is the same as that of the steel wire of the present embodiment, and at the R / 2 position, the number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm is within the range of 5000 to 80000 pieces / pm3. Therefore, even in the case of a non-nitrided damper spring, an excellent fatigue limit is obtained. Note that the microstructure at the R / 2 position of the non-nitrided damper spring is the same as that of the steel wire, and the martensite area fraction is 90.0% or more.

[0121] [Valve spring configuration] In the case of a valve spring, the valve spring is coil-shaped. The wire diameter, mean coil diameter, inner coil diameter, outer coil diameter, free height, number of active coils, total number of coils, helix direction, and pitch of the valve spring are not particularly limited.

[0122] Among valve springs, a valve spring subjected to nitriding is called a nitrided valve spring. Among valve springs, a valve spring not subjected to nitriding is called a non-nitrided valve spring. A nitrided valve spring includes a nitrided layer and a core portion. The nitrided layer includes a composite layer and a diffusion layer that forms deeper than the composite layer. The nitrided layer need not include a composite layer. The core portion is a portion of base material that is deeper than the nitrided layer and is not substantially affected by the nitrogen diffusion caused by nitriding. It is possible to distinguish between the nitrided layer and the core portion in the valve spring by observing the microstructure. A non-nitrided valve spring does not have a nitrided layer.

[0123] When a nitrided valve spring is produced using the steel wire of the present embodiment, the chemical composition of the core portion of the nitrided valve spring is the same as the chemical composition of the steel wire of the present embodiment, and the number density of the V-based precipitates having a maximum diameter ranging from 2 to 10 nm is within the range of 5000 to 80000 pieces / pm3. Furthermore, in the core portion, the numerical ratio (Rea) of Ca sulfides is 0.20% or less. Therefore, an excellent high-cycle fatigue limit is obtained in the nitrided valve spring. Note that the microstructure of the core portion of the nitrided valve spring is the same as the microstructure of the steel wire, and the area fraction of martensite is 90.0% or more.

[0124] When a non-nitrided valve spring is produced using the steel wire of the present embodiment, within the non-nitrided valve spring (at an arbitrary position Rz2 (R represents the radius) of a cross-section in the wire diameter direction), the chemical composition is the same as that of the steel wire of the present embodiment, and at position R / 2, the number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm is within the range of 5000 to 80000 pieces / pm3. Furthermore, at position R / 2, the numerical ratio of Ca sulfides (Rea) is 0.20% or less. Therefore, even in the case of a non-nitrided valve spring, an excellent high-cycle fatigue limit is obtained.Note that the microstructure at the R / 2 position of the non-nitrided valve spring is the same as the microstructure of the steel wire, and the area fraction of martensite is 90.0% or more.

[0125] Note that a steel wire producer of the present modality may receive a wire supply from a third party and may produce the steel wire using the prepared wire.

[0126] [Example 1] The advantageous effects of the steel wire of the present embodiment will now be described more specifically by way of examples. The conditions adopted in the following examples are illustrative of the conditions adopted to confirm the feasibility and advantageous effects of the steel wire of the present embodiment. Consequently, the steel wire of the present embodiment is not limited to this illustrative set of conditions.

[0127] [Steel wire production] In Example 1, steel wires were produced to serve as raw material for shock absorber springs. Additionally, nitrided and non-nitrided shock absorber springs were produced using the steel wires, and their characteristics (fatigue limit) were investigated. MA / a / zuzz / ui υζυ i shock absorber springs. Specifically, cast steels were produced with the chemical compositions shown in Table 1.

[0128] [Table 1] Núm. of maple type Chemical composition (unit is % of mass; the rest is Fe and impurezas) C Si Mn P s Cr VN Ca Mo Nb w Ni Co B Cu Al Ti A 0.62 2.18 0.72 0.009 0.008 1.51 0.35 0.0041 - - - - - - - - - B 0.60 2.25 0.70 0.008 0.009 1.55 0.32 0.0042 - - - - - - 0.012 0.0012 0.001 C 0.62 2.40 0.76 0.010 0.007 1.56 0.52 0.0037 - - - - - - 0.011 0.0009 0.001 D 0.61 1.30 0.69 0.008 0.007 1.53 0.28 0.0040 - - - - - - 0.010 0.0011 0.002 E 0.61 2.21 0.74 0.009 0.009 1.47 0.30 0.0039 - 0.26 - - - - 0.009 0.0010 0.001 F 0.62 2.20 0.73 0.009 0.008 1.53 0.31 0.0043 - - 0.013 - - - 0.008 0.0009 0.002 G 0.63 2.21 0.70 0.007 0.008 1.49 0.29 0.0038 - - - 0.13 - - 0.012 0.0008 0.001 H 0.61 2.19 0.72 0.008 0.007 1.56 0.08 0.0040 - - - - 0.251 - 0.010 0.0012 0.001 1 0.61 2.22 0.73 0.009 0.008 1.47 0.30 0.0040 - - - - - 0.16 - 0.009 0.0012 0.001 J 0.60 2.17 0.74 0.007 0.008 1.53 0.29 0.0043 - - - - - 0.0039 0.010 0.0009 0.001 K 0.58 2.15 0.81 0.008 0.008 1.87 0.28 0.0046 - - - - - - - - - L 0.62 2.18 0.75 0.009 0.007 0.44 0.30 0.0044 - - - - - - - - - M 0.62 2.53 0.69 0.010 0.006 1.43 0.27 0.0041 - - - - - - 0.009 0.0010 0.001 N 0.61 2.31 0.72 0.008 0.008 1.54 0.03 0.0039 - - - - - - 0.012 0.0012 0.002 0 0.52 2.20 0.68 0.007 0.007 1.48 0.28 0.0042 - - - - - - - - - P 0.79 2.18 0.71 0.008 0.008 1.52 0.30 0.0044 - - - - - - - - - Q 0.61 2.46 0.71 0.008 0.007 1.53 0.31 0.0038 - - - - - - - - - R 0.62 2.23 0.26 0.007 0.007 1.50 0.27 0.0040 - - - - - - - - - S 0.61 2.21 0.69 0.009 0.009 1.47 0.07 0.0037 - - - - - - - - - T 0.60 2.19 0.72 0.010 0.009 1.52 0.59 0.0043 - - - - - - - - -.

[0129] In Table 1, the symbol means that the content of the corresponding element was below the detection limit. That is, it means that the corresponding element was not present. For example, with respect to the Nb content of steel type number A, the symbol means that the content was 0% when rounded to three decimal places. In the chemical compositions of the steel type numbers listed in Table 1, the remainder, apart from the elements listed in the Table 1 shows the iron and impurities. Each of the listed cast steels was used to produce a casting (brick) using a continuous casting process. After heating, the billet underwent a roughing process, which is a coarse rolling process, and was then rolled on a continuous rolling mill to produce a bar with a cross-section perpendicular to the longitudinal direction of 162 mm x 162 mm. The heating temperature used for roughing was 1200 to 1250 °C, and the holding time at the heating temperature was 2.0 hours.

[0130] The produced bar was subjected to a finishing rolling process to produce a wire with a diameter of 5.5 mm. The heating temperature in a reheating furnace for each test number in the finishing rolling process was 1150 to 1200 °C, and the holding time at the heating temperature was 1.5 hours.

[0131] The produced wire underwent a patenting treatment. The heat treatment temperature for the patenting treatment was 650 to 700 °C, and the holding time at the heat treatment temperature was 20 minutes. After the patenting treatment, the wire was drawn to produce a steel wire with a diameter of 4.0 mm. The produced steel wire was then tempered. The tempering temperature was 950 to 1000 °C. The steel wire, held at the tempering temperature, was then water-quenched. After tempering, the steel wire was annealed. The annealing temperature was 480 °C. After tempering, the steel wire was subjected to a V-based precipitation heat treatment.The heat treatment temperature T (°C), the holding time t (min) at the heat treatment temperature T, and the Fn value in the V-based precipitation heat treatment were as indicated in Table 2. Note that, in tests number 24 and 25, a V-based precipitation heat treatment was not performed. The steel wires of the respective test numbers were produced by the previous process.

[0132] [Table 2 Test Number Steel Type Number Winding Possible / Not Possible V-Based Precipitate Forming Heat Treatment Step Martensite Area Fraction (%) V-Based Precipitate Numerical Density (pieces / pm3) Nitrided Not Nitrided Comments Heat Treatment Temperature T (°C) Holding Time t (min) Fn Fatigue Limit (MPa) Fatigue Limit Ratio Fatigue Limit (MPa) Fatigue Limit Ratio 1 AO 590 15 334 98.2 34877 1480 2.57 1435 2.47 Example of Invention 2 BO 590 15 33.3 98.4 33333 1485 2.58 1435 2.48 Example of Invention 3 CO 590 15 35.2 98.7 76235 1495 2.59 1445 2.49 Example of invention 4 DO 590 15 32.8 98.2 25617 1475 2.56 1430 2.46 Example of invention 5 EO 590 15 33.9 98.0 70679 1495 2.59 1445 2.49 Example of invention 6 FO 590 15 33.1 98.6 24691 1480 2.57 1430 2.47 Example of invention 7 GO 590 15 32.7 98.0 78395 1495 2.59 1445 2.49 Example of invention 8 HO 590 15 30.9 98.1 14506 1470 2.55 1420 2.47 Example of invention 9 IO 590 15 32.7 98.3 33333 1480 2.56 1445 2.48 Example of invention 10 JO 590 15 32.9 98.2 30247 1485 2.57 1440 2.48 Example of invention 11 KO 590 15 344 98.5 56790 1490 2.59 1445 2.48 Example of invention 12 LO 615 35 29.8 98.4 18519 1475 2.56 1430 2.47 Example of invention 13 AO 630 5 35.1 98.6 26852 1475 2.58 1435 2.48 Example of invention 14 AO 570 30 32.7 98.1 30864 1480 2.58 1440 2.48 Example of invention 15 0 O 590 15 32.5 98.3 33951 1480 2.57 1430 2.47 Example of invention 16 PO 590 15 32.9 98.2 38889 1485 2.58 1435 2.48 Example of invention 17 QO 590 15 33.1 98.0 36111 1485 2.58 1435 2.47 Example of invention 18 RO 590 15 32.5 98.4 32716 1480 2.57 1430 2.46 Example of invention 19 SO 590 15 30.2 98.5 16975 1475 2.56 1425 2.47 Example of invention 20 TO 590 15 35.6 98.2 78086 1495 2.60 1445 2.49 Example of invention 21 AO 550 12 29.7 98.0 5864 1470 2.55 1420 2.47 Example of invention 22 MX 590 15 32.2 98.3 41358 - - - Comparative Example 23 N o 590 15 30.2 98.8 4630 1390 2.41 1365 2.30 Comparative Example 24 A o - - - 98.0 - 1430 2.19 1385 2.19 Comparative Example 25 B o - - - 98.5 - 1435 2.20 1390 2.16 Comparative Example 26 A o 520 15 27.6 98.2 - 1430 2.31 1400 2.25 Comparative Example 27 B o 520 15 27.5 98.4 - 1430 2.33 1405 2.27 Comparative Example 28 B o 500 15 26.0 98.5 - 1440 2.20 1390 2.19 Comparative Example 29 A or 660 15 39.5 99.2 3704 1410 2.45 1365 2.42 Comparative Example 30 B or 660 15 39.4 99.1 3086 1405 2.49 1365 2.45 Comparative Example 31 A or 680 15 41.3 99.0 2778 1400 2.44 1355 2.45 Comparative Example 32 A or 640 35 39.2 99.0 3395 1410 2.44 1370 2.40 Comparative Example 33 A or 560 5 294 98.2 926 1395 2.41 1360 2.29 Comparative example.

[0133] [Production of shock absorber springs] Nitrided and non-nitrided shock absorber springs were manufactured from the produced steel wires. The nitrided shock absorber springs were produced using the following method. The steel wire from each test batch was cold-wound under the same conditions to produce a wound intermediate steel material. The average coil diameter D of the wound intermediate steel material was 26.5 mm, and the wire diameter d of the wound intermediate steel material was 4.0 mm. A stress-relief annealing treatment was performed on the intermediate steel material. The annealing temperature for the stress-relief annealing treatment was 450 °C, and the holding time at the annealing temperature was 20 minutes. After the holding time had elapsed, the intermediate steel material was allowed to cool.The intermediate steel material, after stress-relieving annealing, was subjected to nitriding. The nitriding temperature was set at 450 °C, and the holding time at the nitriding temperature was set at 5.0 hours. After nitriding, shot blasting was performed under known conditions. Initially, shot blasting was carried out using 0.8 mm diameter cut wire as the blasting abrasive. Subsequently, 0.2 mm diameter steel shot was used as the blasting abrasive. The shot rate, shot blasting time (duration), and the amount of shot fired per unit area per unit time in the respective blasting were made the same for each test run. The nitrided shock absorber springs were produced using the aforementioned production method.

[0134] The non-nitrided shock absorber springs were manufactured using the following production method. The steel wire for each test number was cold-wound under the same conditions to produce a wound intermediate steel material. The intermediate steel material underwent stress-relief annealing. The annealing temperature for the stress-relief annealing treatment was 450 °C, and the holding time at the annealing temperature was 20 minutes. After the holding time, the intermediate steel material was allowed to cool. Following the stress-relief annealing treatment, nitriding was not carried out, and shot peening was performed under the same conditions as for the nitrided shock absorber springs. The non-nitrided shock absorber springs were manufactured using the same production method as described above.Shock absorber springs (nitrided and non-nitrided) were produced using the previous production process.

[0135] [Assessment tests] The steel wire produced from each test number was subjected to a cold winding workability test, a microstructure observation test, and a test to measure the number density of V-based precipitates. In addition, the shock absorber springs produced (nitrided and non-nitrided) from each test number were subjected to a microstructure observation test, a test to measure the number density of V-based precipitates, a Vickers hardness measurement test, and a fatigue test.

[0136] [Cold winding workability test] iviA / a / zuzz / ui The cold winding of the steel wire for each test number was carried out under the following conditions, and it was investigated whether or not the cold winding work was possible. The mean coil diameter D (= (inner coil diameter + outer coil diameter) / 2) of the wound intermediate steel material was fixed at 12.1 mm, and the wire diameter d of the wound intermediate steel material was fixed at 4.0 mm. Whether the cold winding work was possible or not is shown in the Winding Possible / Not Possible column of Table 2. The symbol O indicates that the cold winding work could be performed, and the symbol x indicates that the cold winding work could not be performed.

[0137] [Microstructure observation test] The steel wire of each test number was cut perpendicular to its longitudinal direction, and a test sample was extracted. The surface corresponding to a cross-section perpendicular to the longitudinal direction of the steel wire was selected as the observation surface. After mirror polishing, the observation surface was etched with 2% nitric acid-alcohol (nital etching reagent). An R / 2 position of the etched observation surface was observed using an optical microscope at 500x magnification, and photographic images of five arbitrary fields of view were generated. The size of each field of view was set at 100 pm x 100 pm. In each field of view, the contrast differed for the respective phases of martensite, retained austenite, precipitates, inclusions, and the like. Therefore, martensite was identified based on contrast.The gross area (pm2) of identified martensite was determined in each field of view. The proportion of the gross martensite area in all fields of view relative to the gross area (10,000 pm2 x 5) of all fields of view was defined as the martensite area fraction (%). The martensite area fraction thus determined is shown in Table 2. The nitrided damper spring from each test number was cut along the wire diameter, and a test sample was extracted. In addition, the non-nitrided damper spring from each test number was cut along the wire diameter, and a test sample was extracted. Each of the extracted test samples was subjected to the microstructure observation test described above.The results of the microstructure observation test showed that the martensite area fraction of the central part of the nitrided shock absorber spring of each test number, and the martensite area fraction of the non-nitrided shock absorber spring of each test number were equal to the martensite area fraction of the steel wire of the corresponding test number.

[0138] [Test for measuring the number density of V-based precipitates]. The steel wire for each test number was cut perpendicular to its longitudinal direction, and a disc with a surface (cross-section) perpendicular to the longitudinal direction and 0.5 mm thick was extracted. Both sides of the disc were ground and polished using emery paper to achieve a thickness of 50 µm. A 3 mm diameter sample was then taken from the disc. This sample was immersed in a 10% perchloric acid-glacial acetic acid solution for electrolytic polishing, resulting in a thin-film sample 100 nm thick.

[0139] The prepared thin-film sample was observed using a TEM. Specifically, an analysis of the Kikuchi lines was first performed with respect to the thin-film sample to identify the crystal orientation of the thin-film sample. The thin-film sample was then tilted based on the identified crystal orientation and adjusted so that the (001) plane in the ferrite (body-centered cubic lattice) could be observed. Specifically, the thin-film sample was introduced into the TEM, and the Kikuchi lines were observed. The tilt of the thin-film sample was adjusted so that the

[001] direction of the ferrite in the Kikuchi lines coincided with the direction of incidence of an electron beam. After adjustment, when the actual image was observed, the observation was made from a vertical direction toward the (001) plane of the ferrite.After adjusting the thin-film sample, the observation fields of view were identified at four arbitrary locations on the thin-film sample. Each observation field of view was observed using an observation magnification of 200,000x and an acceleration voltage of 200 kV. The observation field of view was set at 0.09 pm x 0.09 pm.

[0140] As mentioned previously, V-based precipitates precipitate in a plate-like form along a {001} plane in ferrite. In ferrite grains in the (001) plane, V-based precipitates are observed as line segments (edge ​​portions) that extend linearly with respect to the

[100] or

[010] orientation. In a TEM image, the precipitates appear with a different brightness contrast compared to the generating phase. Therefore, in a TEM image of a (001) plane in ferrite, line segments extending along the

[100] or

[010] orientation were considered V-based precipitates. The line segment length of the respective V-based precipitates identified in each of the observation fields was measured, and the measured line segment length was defined as the maximum diameter (nm) of the corresponding V-based precipitate.

[0141] The total number of V-based precipitates with a maximum diameter ranging from 2 to 10 nm in the observation fields of view at the four locations was determined by the aforementioned measurement. The numerical density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm (pieces / pm3) was determined based on the total number of V-based precipitates and the total volume of the observation fields of view at the four locations. The determined numerical density of V-based precipitates is shown in the V-based Precipitate Numerical Density (pieces / pm3) column of Table 2. The symbol in the V-based Precipitate Numerical Density (pieces / pm3) column means that the numerical density of V-based precipitates was 0 pieces / pm3.Note that the number density of V-based precipitates in the nitrided shock absorber spring of each test number was also measured using the same method as that used to determine the number density of V-based precipitates in the steel wire. The results showed that the number density of V-based precipitates in the core of the nitrided shock absorber spring of each test number was the same as the number density of V-based precipitates in the steel wire of the corresponding test number. Furthermore, the number density of V-based precipitates in the non-nitrided shock absorber spring of each test number was measured using the same method as that used to determine the number density of V-based precipitates in the steel wire.The results showed that the number density of V-based precipitates in the non-nitrided shock absorber spring of each test number was the same as the number density of V-based precipitates in the steel wire of the corresponding test number.

[0142] [Vickers Hardness Measurement Test]. The hardness of the core portion of the nitrided damper spring for each test number was determined by a Vickers hardness test. Specifically, a Vickers hardness test was performed in accordance with JIS Z 2244 (2009) at three arbitrary points at a position R / 2 of a cross-section in the wire diameter direction of the nitrided damper spring for each test number. The test force was set at 0.49 N. The arithmetic mean of the Vickers hardness values ​​obtained at the three locations was taken as the Vickers hardness of the core portion of the nitrided damper spring for the corresponding test number.

[0143] Similarly, the hardness of the non-nitrided shock absorber spring of each test number was determined by a Vickers hardness measurement test. Specifically, a Vickers hardness measurement test was performed in accordance with JIS Z 2244 (2009) at three arbitrary locations at a position R / 2 of a cross-section in the wire diameter direction of the non-nitrided shock absorber spring of each test number. The test force was set at 0.49 N. The arithmetic mean of the Vickers hardness values ​​obtained at the three locations was taken as the Vickers hardness of the non-nitrided shock absorber spring of the corresponding test number.

[0144] [Fatigue Test] A fatigue test, described below, was performed using the shock absorber springs (nitrided and non-nitrided) from each test batch. For the fatigue test, a compression fatigue test was conducted in which a repeated load was applied along the central axis of the wound shock absorber springs (nitrided and non-nitrided). An electro-hydraulic servo fatigue testing apparatus (500 kN load capacity) was used as the testing machine. ινΐΛ / a / zuzz / ui υζυ i

[0145] As test conditions, a stress ratio of 0.2 was set as the load and the frequency was set from 1 to 3 Hz. The test was performed until the damper spring fractured, with a cycle count of 107 cycles set as the upper limit. If the damper spring did not fracture before reaching 107 cycles, the test was stopped at 107 cycles and the test result was determined to be no fracture. In this case, the maximum value of the test stress when the damper spring did not fracture at 107 cycles was defined as Fm, and the minimum value of the test stress when the damper spring fractured before reaching 107 cycles at no less than Fm was defined as Fb. The arithmetic mean value of Fm and Fb was defined as Fa, and the value of Fa in a case where (Fb - Fm) / Fa < 0.10 was defined as the fatigue limit (MPa).Furthermore, in a case where all the shock absorber springs fractured as a result of the test—that is, a case where Fm could not be obtained—a test stress corresponding to a life of 107 cycles was extrapolated based on the relationship between fracture life and test stress, and the resulting test stress was defined as the fatigue limit (MPa). In this case, the test stress corresponded to the surface stress amplitude at the fracture location. A fatigue limit (MPa) was determined for the shock absorber springs of each test number, based on the aforementioned definitions and the evaluation tests.In addition, the obtained fatigue limit and Vickers hardness were used to determine a fatigue limit ratio (= fatigue limit / Vickers hardness of the core portion) of the nitrided damper spring and a fatigue limit ratio (= fatigue limit / Vickers hardness) of the non-nitrided damper spring.

[0146] [Test Results] The test results are shown in Table 2. Referring to Table 2, in tests 1 through 21, the chemical composition and production process were appropriate. Therefore, in the microstructure of the steel wire in each of these test numbers, the martensite area fraction was 90.0% or higher. Furthermore, in each of these test numbers, the number density of the V-based precipitates with a maximum diameter ranging from 2 to 10 nm was 5,000 to 80,000 pieces / pm³. Therefore, the fatigue strength of the nitrided shock absorber spring produced using the steel wire as raw material was 1470 MPa or higher, and the fatigue strength ratio (fatigue strength / Vickers hardness of the core portion) of the nitrided shock absorber spring was 2.55 or higher.Furthermore, the fatigue limit of the non-nitrided shock absorber spring produced using the steel wire was 1420 MPa or more, and the fatigue limit ratio (= fatigue limit / Vickers hardness) of the non-nitrided shock absorber spring was 2.46 or more.

[0147] On the other hand, in test number 22, the Si content was too high. Therefore, the workability in cold winding was low.

[0148] In test number 23, the V content was too low. Therefore, in the steel wire, the number density of V-based precipitates with a maximum diameter ranging from 2 to 10 nm was too low. Consequently, the fatigue limit of the nitrided damper spring was less than 1470 MPa, and the fatigue limit ratio was less than 2.55. Furthermore, the fatigue limit of the non-nitrided damper spring was less than 1420 MPa, and the fatigue limit ratio was less than 2.46.

[0149] In tests 24 and 25, although the chemical composition was adequate, the steel wire was not subjected to the V-based precipitate formation heat treatment. Therefore, in the steel wire, the number density of V-based precipitates with a maximum diameter ranging from 2 to 10 nm was too low. Consequently, the fatigue limit of the nitrided damper spring was less than 1470 MPa, and the fatigue limit ratio was less than 2.55. Furthermore, the fatigue limit of the non-nitrided damper spring was less than 1420 MPa, and the fatigue limit ratio was less than 2.46.

[0150] In tests 26 to 28, although the chemical composition was adequate, the heat treatment temperature for the V-based precipitate formation heat treatment was too low. Therefore, in the steel wire, the number density of V-based precipitates with a maximum diameter ranging from 2 to 10 nm was too low. Consequently, the fatigue limit of the nitrided damper spring was less than 1470 MPa, and the fatigue limit ratio was less than 2.55. Furthermore, the fatigue limit of the non-nitrided damper spring was less than 1420 MPa, and the fatigue limit ratio was less than 2.46.

[0151] In tests 29 to 31, although the chemical composition was adequate, the heat treatment temperature for the V-based precipitate formation heat treatment was too high. Therefore, in the steel wire, the V-based precipitates thickened, and the number density of V-based precipitates with a maximum diameter ranging from 2 to 10 nm was too low. As a result, the fatigue limit of the nitrided damper spring was less than 1470 MPa, and the fatigue limit ratio was less than 2.55. Furthermore, the fatigue limit of the non-nitrided damper spring was less than 1420 MPa, and the fatigue limit ratio was less than 2.46.

[0152] In test number 32, although the chemical composition was adequate, in the V-based precipitate formation heat treatment, the Fn defined by equation (2) was greater than 38.9. As a result, in the steel wire, the number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm was too low. Consequently, the fatigue limit of the nitrided damper spring was less than 1470 MPa, and the fatigue limit ratio was less than 2.55. Furthermore, the fatigue limit of the non-nitrided damper spring was less than 1420 MPa, and the fatigue limit ratio was less than 2.46. ινΐΛ / a / zuzz / ui υζυ i

[0153] In test number 33, although the chemical composition was adequate, in the V-based precipitate formation heat treatment, Fn defined by equation (2) was less than 29.5. As a result, the number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm in the steel wire was too low. Consequently, the fatigue limit of the nitrided damper spring was less than 1470 MPa, and the fatigue limit ratio was less than 2.55. Furthermore, the fatigue limit of the non-nitrided damper spring was less than 1420 MPa, and the fatigue limit ratio was less than 2.46. [Example 2]

[0154] [Steel wire production] In Example 2, steel wires were produced to serve as raw material for valve springs. Additionally, nitrided and non-nitrided valve springs were produced using the steel wires, and the characteristics (fatigue limit) of the valve springs were investigated. Specifically, cast steels with the chemical compositions indicated in Table 3 were produced. [Table 3] Steel Type Number Chemical Composition (assembled is % by mass; the remainder is Fe and impurities) C Si Mn PS Cr VN Ca Mo Nb w Ni Co B Cu Al Ti A 0.60 2.20 0.75 0.010 0.009 1.52 0.30 0.0041 0.0010 - - - - - - - - B 0.62 2.21 0.74 0.009 0.008 1.56 0.32 0.0039 0.0007 - - - - - 0.015 0.0010 0.002 C 0.62 2.40 0.70 0.009 0.007 1.50 0.51 0.0042 0.0009 - - - - - 0.012 0.0009 0.001 D 0.60 1.31 0.68 0.007 0.007 1.49 0.31 0.0038 0.0006 - - - - - 0.011 0.0012 0.001 E 0.61 2.14 0.72 0.008 0.009 1.48 0.29 0.0042 0.0008 0.24 - - - - 0.008 0.0010 0.001 F 0.60 2.23 0.71 0.009 0.008 1.52 0.33 0.0040 0.0010 - 0.018 - - - 0.009 0.0008 0.001 G 0.60 2.20 0.69 0.006 0.006 1.50 0.30 0.0038 0.0009 - - 0.21 - - 0.011 0.0009 0.001 H 0.63 2.18 0.67 0.007 0.008 1.49 0.09 0.0046 0.0008 - - - 0.215 - - 0.008 0.0012 0.002 1 0.62 2.17 0.75 0.008 0.008 1.53 0.29 0.0043 0.0009 - - - 0.24 - 0.009 0.0011 0.001 J 0.62 2.23 0.71 0.009 0.007 1.55 0.32 0.0038 0.0008 - - - - 0.0044 0.012 0.0010 0.001 K 0.58 2.22 0.72 0.008 0.009 1.88 0.31 0.0044 0.0009 - - - - - - - - L 0.61 1.98 0.68 0.008 0.007 0.43 0.30 0.0040 0.0010 - - - - - - - - M 0.62 2.51 0.75 0.008 0.009 1.53 0.28 0.0040 0.0008 - - - - - 0.009 0.0014 0.001 N 0.63 2.15 0.74 0.008 0.008 1.51 0.02 0.0042 0.0009 - - - - - 0.012 0.0008 0.001 0 0.62 2.21 0.69 0.011 0.009 1.51 0.31 0.0039 - - - - - - 0.013 0.0010 0.002 P 0.60 2.20 0.68 0.009 0.009 1.47 0.33 0.0042 0.0051 - - - - - 0.010 0.0011 0.001 Q 0.51 2.19 0.68 0.008 0.009 1.48 0.27 0.0043 0.0005 - - - - - - - - R 0.79 2.21 0.69 0.007 0.008 1.52 0.32 0.0042 0.0045 - - - - - - - - S 0.60 2.46 0.72 0.007 0.008 1.47 0.30 0.0039 0.0010 - - - - - - - - T 0.61 2.22 0.27 0.010 0.007 1.53 0.29 0.0040 0.0012 - - - - - - - - u 0.58 2.18 0.74 0.010 0.008 1.50 0.06 0.0045 0.0009 - - - - - - - - V 0.62 2.24 0.70 0.009 0.009 1.53 0.60 0.0036 0.0007 - - - - - - - -.

[0155] In Table 3, the symbol means that the content of the corresponding element was below the detection limit. In the chemical compositions of the steel type numbers listed in Table 3, the distinct balance of the elements listed in Table 3 was Fe and impurities. The refining conditions (Ca content (% by mass) in the ferroalloys added to the molten steel in the refining process and, when refining time is defined as t (min), the time elapsed from the start of the refining process until the addition of slag-forming agents) when producing the molten steel were as indicated in Table 4.

[0156] [Table 4] Test No. Steel Type No. Ca content in ferroalloys (% by mass) Time until addition of slag-forming agents (min) Winding possible / not possible Heat treatment step for V-based precipitate formation Martensite area fraction (%) Numerical density of V-based precipitates (pieces / pm3) Numerical ratio area of ​​Ca sulfides (%) Nitrided Not nitrided Comments Heat treatment T (°C) Holding time t (min) Fn Fatigue limit (MPa) Fatigue limit ratio Fatigue limit (MPa) Fatigue limit ratio 1 A 0.7 0.70t 590 15 32.9 98.4 35494 0.12 1440 2.47 1385 2.39 Example of invention 2 B 0.7 0.70t 0 590 15 33.3 98.3 37346 0.13 1435 2.46 1385 2.38 Example of invention 3 C 0.7 0.70to 590 15 34.8 98.3 72531 0.11 1445 2.49 1395 2.39 Example of invention 4 D 0.7 0.70to 590 15 32.9 98.6 35802 0.09 1435 2.47 1375 2.38 Example of invention 5 E 0.6 0.70to 590 15 33.8 98.0 73765 0.13 1445 2.49 1395 2.39 Example of invention 6 F 0.6 0.70to 590 15 33.2 98.6 33951 0.12 1435 2.47 1370 2.37 Example of invention 7 G 0.6 0.70to 590 15 32.8 99.0 77469 0.11 1445 2.49 1395 2.39 Example of invention 8 H 0.7 0.70t 0 590 15 30.6 98.2 19753 0.09 1395 2.45 1350 2.36 Example of invention 9 I 0.6 0.70to 590 15 32.9 98.1 32716 0.11 1435 2.46 1375 2.38 Example of invention 10 J 0.6 0.70 to 590 15 33.3 98.3 30864 0.12 1435 2.46 1370 2.38 Example of invention 11 K 0.7 0.70 to 590 15 34.7 98.4 52469 0.11 1445 2.49 1395 2.39 Example of invention 12 L 0.6 0.70 to 615 35 29.8 98.3 16975 0.13 1435 2.47 1375 2.37 Example of invention 13 A 0.6 0.70 to 630 5 34.6 99.0 29630 0.11 1435 2.46 1375 2.37 Example of invention 14 A 0.7 0.70to 570 30 32.3 98.4 33642 0.13 1440 2.47 1380 2.39 Example of invention 15 Q 0.6 0.70to 590 15 32.4 98.5 32407 0.09 1435 2.47 1385 2.39 Example of invention 16 R 0.7 0.70to 590 15 33.1 98.6 40123 0.12 1440 2.47 1380 2.39 Example of invention 17 S 0.6 0.70to 590 15 32.7 98.2 37654 0.11 1440 2.46 1385 2.38 Example of invention 18 T 0.7 0.70to 590 15 32.9 98.0 34568 0.13 1435 2.47 1385 2.39 Example of invention. 19 U 0.7 0.70t O 590 15 30.3 98.4 15432 0.13 1430 2.45 1370 2.38 Example of invention 20 V 0.7 0.70t O 590 15 35.7 98.6 75617 0.12 1445 2.49 1395 2.39 Example of invention 21 A 0.7 0.70t O 550 15 29.6 98.3 5556 0.10 1430 2.45 1365 2.37 Example of invention 22 M 0.6 0.70t X 590 15 32.8 98.2 43827 0.11 - - - - Example Comparative Example 23 N 0.7 0.70 590 15 29.9 98.1 4321 0.09 1330 2.35 1280 2.27 Comparative Example 24 0 - 0.70 590 15 33.0 98.3 29630 - 1310 2.31 1275 2.20 Comparative Example 25 P 0.6 0.70 590 15 33.0 98.7 62346 0.22 1335 2.34 1300 2.23 Comparative Example 26 A 0.7 0.70 - - - 98.1 - 0.12 1325 2.13 1295 2.04 Comparative Example 27 B 0.7 0.70to - - - 98.0 - 0.11 1330 2.15 1300 2.05 Comparative Example 28 C 0.6 0.70t 0 - - - 98.3 - 0.12 1325 2.11 1270 2.04 Comparative Example 29 A 0.6 0.70to 500 15 25.7 98.5 - 0.10 1325 2.11 1290 2.09 Comparative Example 30 B 0.7 0.70to 500 15 26.0 98.3 - 0.10 1330 2.12 1280 2.08 Comparative Example 31 C 0.6 0.70to 500 15 27.2 98.6 - 0.09 1345 2.14 1295 2.09 Comparative Example 32 A 0.7 0.70to 660 15 39.0 99.0 4012 0.12 1310 2.38 1300 2.28 Comparative Example 33 B 0.7 0.70to 660 15 39.4 98.7 4630 0.12 1310 2.40 1305 2.32 Comparative Example 34 A 0.6 0.70t 0 680 15 40.7 99.1 2778 0.11 1310 2.41 1300 2.30 Comparative Example 35 A 1.3 0.70to 590 15 32.9 98.5 32716 0.24 1325 2.33 1280 2.27 Comparative Example 36 B 1.2 0.70 590 15 33.3 98.3 30247 0.23 1325 2.34 1290 2.29 Comparative Example 37 A 0.6 0.85 590 15 32.9 98.4 34259 0.27 1310 2.32 1280 2.26 Comparative Example 38 C 0.6 0.85 590 15 34.8 98.1 70062 0.28 1315 2.31 1285 2.24 Comparative Example 39 A 0.7 0.70to 645 35 39.2 99.2 4630 0.12 1310 2.40 1300 2.31 Comparative example 40 A 0.6 0.70to 560 5 29.0 98.3 1543 0.10 1310 2.13 1285 2.09 Comparative example. iviA / a / zuzz / ui uzm

[0157] Each of the molten steels, after refining, was used to produce a billet by a continuous casting process. After heating the billet, it was rough-rolled and then rolled on a continuous rolling mill to produce a bar with a cross-section perpendicular to the longitudinal direction of 162 mm x 162 mm. The heating temperature used for roughing was 1200 to 1250 °C, and the holding time at the heating temperature was 2.0 hours.

[0158] The produced bar was subjected to a finishing rolling process to produce a wire with a diameter of 5.5 mm. The heating temperature in a reheating furnace for each test number in the finishing rolling process was 1150 to 1200 °C, and the holding time at the heating temperature was 1.5 hours.

[0159] The produced wire rod was subjected to a patenting treatment. The heat treatment temperature for the patenting treatment was 650 to 700 °C, and the holding time at the heat treatment temperature was 20 minutes. After the patenting treatment, the wire rod was drawn to produce a steel wire with a diameter of 4.0 mm. The produced steel wire was then tempered. The tempering temperature was 950 to 1000 °C. The steel wire, held at the tempering temperature, was water-quenched. After tempering, the steel wire was annealed. The annealing temperature was 480 °C. After annealing, the steel wire was subjected to a V-based precipitation heat treatment.The heat treatment temperature T (°C), the holding time t (min) at the heat treatment temperature T, and the Fn value in the V-based precipitation heat treatment were as indicated in Table 4. Note that, for tests number 26 to 28, a V-based precipitation heat treatment was not performed. The steel wires of the respective test numbers were produced by the previous process.

[0160] [Valve spring production] Nitrided and non-nitrided valve springs were produced using the manufactured steel wires. The nitrided valve springs were produced using the following method. The steel wire from each test batch was cold-wound under the same conditions to produce a wound intermediate steel material. The average coil diameter (D) of the wound intermediate steel material was 26.5 mm, and the wire diameter (d) of the wound intermediate steel material was 4.0 mm. A stress-relief annealing treatment was performed on the intermediate steel material. The annealing temperature for the stress-relief annealing treatment was 450 °C, and the holding time at the annealing temperature was 20 minutes. After the holding time, the intermediate steel material was allowed to cool.The intermediate steel material, after stress-relieving annealing, was subjected to nitriding. The nitriding temperature was set at 450 °C, and the holding time at the nitriding temperature was set at 5.0 hours. After nitriding, shot blasting was performed under known conditions. Initially, shot blasting was carried out using 0.8 mm diameter cut wire as the blasting abrasive. Subsequently, 0.2 mm diameter steel shot was used as the blasting abrasive. The blasting speed, blasting time (duration), and amount of blasting abrasive applied per unit area per unit time in the respective blasting were made the same for each test run. The nitrided valve springs were produced using the previously described production method.

[0161] The non-nitrided valve springs were produced using the following production method. The steel wire for each test number was cold-wound under the same conditions to produce a wound intermediate steel material. The intermediate steel material underwent stress-relief annealing. The annealing temperature for the stress-relief annealing treatment was 450 °C, and the holding time at the annealing temperature was 20 minutes. After the holding time, the intermediate steel material was allowed to cool. Following the stress-relief annealing treatment, nitriding was not carried out, and shot peening was performed under the same conditions as for the nitrided valve springs. The non-nitrided valve springs were produced using the same production method as described above.Valve springs (nitrided and non-nitrided) were produced using the above production process.

[0162] [Assessment tests] The steel wire produced from each test number was subjected to a cold winding workability test, a microstructure observation test, a measurement test of the numerical ratio of Ca sulfides and a test to measure the numerical density of V-based precipitates. In addition, the valve springs produced (nitrided and non-nitrided) from each test number were subjected to a microstructure observation test, a test to measure the numerical density of V-based precipitates, a Vickers hardness measurement test and a fatigue test.

[0163] [Cold winding workability test] The cold winding of the steel wire for each test number was carried out under the following conditions, and it was investigated whether or not the cold winding work was possible. The mean coil diameter D (= (inner coil diameter + outer coil diameter) / 2) of the wound intermediate steel material was fixed at 12.1 mm, and the wire diameter of the wound intermediate steel material was fixed at 4.0 mm. Whether the cold winding work was possible or not is shown in the Winding Possible / Not Possible column of Table 4. The symbol O indicates that the cold winding work could be performed, and the symbol x indicates that the cold winding work could not be performed.

[0164] [Microstructure observation test] The martensite area fraction of the steel wire from each test number was determined using the same method as that adopted in the microstructure observation test performed in Example 1. The martensite area fractions thus determined are shown in Table 4. Note that the nitrided valve spring from each test number was cut along the wire diameter and a test sample was taken. In addition, the non-nitrided valve spring from each test number was cut along the wire diameter and a test sample was taken. Each of the extracted test samples was subjected to the microstructure observation test described above.The results of the microstructure observation test showed that the martensite area fraction of the nitrided valve spring core portion of each test number, and the martensite area fraction of the non-nitrided valve spring of each test number, were equal to the martensite area fraction of the steel wire of the corresponding test number.

[0165] [Test to measure the number density of V-based precipitates] The number density of the V-based precipitates on the steel wire of each test number was determined using the same method as that used in the test to measure the number density of V-based precipitates performed in Example 1. Specifically, the steel wire of each test number was cut perpendicular to the longitudinal direction of the steel wire, and a disk was extracted with a surface (cross-section) perpendicular to the longitudinal direction of the steel wire and with a thickness of 0.5 mm. Both sides of the disk were ground and polished using emery paper to achieve a disk thickness of 50 µm. A 3 mm diameter sample was then taken from the disk. This sample was immersed in a 10% perchloric acid / glacial acetic acid solution for electrolytic polishing to prepare a thin-film sample with a thickness of 100 nm.

[0166] The prepared thin-film sample was used to determine the number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm (pieces / pm3) by the same method as that used in Example 1. The determined number density of the V-based precipitates is shown in the V-based Precipitate Number Density (pieces / pm3) column of Table 4. The symbol in the V-based Precipitate Number Density (pieces / pm3) column means that the V-based precipitate number density was 0 pieces / pm3. Note that the V-based precipitate number density on the nitrided valve spring of each test number was also measured by the same method as that used to determine the V-based precipitate number density on the steel wire.The results showed that the number density of V-based precipitates in the nitrided valve spring core of each test number was the same as the number density of V-based precipitates in the steel wire of the corresponding test number. Furthermore, the number density of V-based precipitates in the non-nitrided valve spring of each test number was measured using the same method as that used to determine the number density of V-based precipitates in the steel wire. The results showed that the number density of V-based precipitates in the non-nitrided valve spring of each test number was the same as the number density of V-based precipitates in the steel wire of the corresponding test number.

[0167] [Area measurement test of numerical ratio of Ca sulfides] A sample was extracted from the cross-section of each test sample, including the central axis of the steel wire. From the surfaces of the extracted sample, a cross-sectional area including the central axis of the steel wire was selected as the observation surface. This observation surface was then mirror-polished. On this mirror-polished observation surface, 100 pm x 100 pm fields of view were observed at arbitrary locations, positioned R / 2 from the steel wire surface, using a scanning electron microscope (SEM) with a magnification of 1000x.

[0168] Inclusions in each field of view were identified based on contrast. Each identified inclusion was subjected to EDS to identify oxide-based inclusions, sulfide-based inclusions, and Ca sulfides. Specifically, based on the elemental analysis results obtained by EDS, inclusions with an oxygen content of 10.0% or more (by mass percent) were identified as oxide-based. Inclusions with a sulfur content of 10.0% or more (by mass percent) and an oxygen content of less than 10.0% were identified as sulfide-based. In addition, among the sulfide-based inclusions identified, those inclusions that have, by mass %, a Ca content of 10.0% or more, an S content of 10.0% or more, and an O content of less than 10.0% were identified as Ca sulfides.

[0169] The inclusions that were the subject of the aforementioned identification were inclusions that had an equivalent circular diameter of 0.5 pm or more. The beam diameter in the EDS used for the identification of the inclusions was set at 0.2 pm. The numerical ratio of Ca sulfides (%) was determined using equation (1) based on the total number of oxide-based inclusions and sulfide-based inclusions identified in the aforementioned observation fields at 10 locations, and the total number of Ca sulfides identified in the aforementioned observation fields at 10 locations. Rea = number of Ca sulfides / total number of oxide-based inclusions and sulfide-based inclusions x 100 (1)

[0170] [Vickers Hardness Measurement Test] The hardness of the core portion of the nitrided valve spring for each test number was determined by a Vickers hardness test. Specifically, a Vickers hardness test was performed in accordance with JIS Z 2244 (2009) at three arbitrary locations at a position R / 2 of a cross-section in the wire diameter direction of the nitrided valve spring for each test number. The test force was set at 0.49 N. The arithmetic mean of the Vickers hardness values ​​obtained at the three locations was taken as the Vickers hardness of the core portion of the nitrided valve spring for the corresponding test number.

[0171] Similarly, the hardness of the non-nitrided valve spring of each test number was determined by a Vickers hardness measurement test. Specifically, a Vickers hardness measurement test was performed in accordance with JIS Z 2244 (2009) at three arbitrary locations at a position R / 2 of a cross-section in the wire diameter direction of the non-nitrided valve spring of each test number. The test force was set at 0.49 N. The arithmetic mean of the Vickers hardness values ​​obtained at the three locations was taken as the Vickers hardness of the non-nitrided valve spring of the corresponding test number.

[0172] [Fatigue Test] A fatigue test, described below, was performed using the valve springs (nitrided and non-nitrided) from each test batch. For the fatigue test, a compression fatigue test was conducted in which a repeated load was applied along the central axis of the wound valve springs (nitrided and non-nitrided). An electro-hydraulic servo apparatus (500 kN load capacity) was used as the test machine.

[0173] As test conditions, a stress ratio of 0.2 was set as the load, and the frequency was set from 1 to 3 Hz. The test was carried out until the valve spring fractured, with a cycle count of 10⁸ cycles set as the upper limit. If the valve spring did not fracture before reaching 10⁸ cycles, the test was stopped at 10⁸ cycles and the test result was determined to be no fracture. Here, the maximum value of the test stress when the valve spring did not fracture at 10⁸ cycles was defined as Fm, and the minimum value of the test stress when the valve spring fractured before reaching 10⁸ cycles at no less than FM was defined as Fb. The arithmetic mean value of Fm and Fb was defined as Fa, and the value of Fa in a case where (Fb - Fm) / Fa < 0.10 was defined as the fatigue limit (MPa).Furthermore, in a case where all valve springs fractured as a result of the test, i.e., where Fm could not be obtained, a test stress corresponding to a life of 108 cycles was extrapolated based on the relationship between fracture life and test stress, and the resulting test stress was defined as the fatigue limit (MPa). In this case, the test stress corresponded to the surface stress amplitude at the fracture location. For the valve springs of each test number, a fatigue limit (MPa) at a high cycle was determined based on the aforementioned definitions and evaluation tests.Furthermore, the obtained fatigue limit and Vickers hardness were used to determine a fatigue limit ratio (= fatigue limit / Vickers hardness of the core portion) of the nitrided valve spring, and a fatigue limit ratio (= fatigue limit / Vickers hardness) of the non-nitrided valve spring.

[0174] [Test results]. The results of the tests are shown in Table 4. Referring to Table 4, in tests number 1 to 21, the chemical composition was appropriate and the production process was also appropriate. Therefore, in the microstructure of the steel wire from each test number, the martensite area fraction was 90.0% or higher. Furthermore, in each of these test numbers, the number density of the V-based precipitates with a maximum diameter ranging from 2 to 10 nm was 5,000 to 80,000 pieces / pm³. Additionally, the numerical ratio (R²a) of Ca sulfides was 0.20% or lower. Therefore, the fatigue strength of the nitrided valve spring produced using the steel wire as raw material was 1390 MPa or higher, and the fatigue strength ratio (fatigue strength / Vickers hardness of the core portion) of the nitrided valve spring was 2.45 or higher. Furthermore, the fatigue limit of the non-nitrided valve spring produced using the steel wire was 1340 MPa or more and the fatigue limit ratio of the non-nitrided valve spring (= fatigue limit / Vickers hardness) was 2.35 or more.

[0175] On the other hand, in test number 22, the Si content was too high. Therefore, the workability in cold winding was low.

[0176] In test number 23, the V content was too low. Therefore, in the steel wire, the number density of V-based precipitates with a maximum diameter ranging from 2 to 10 nm was too low. Consequently, the fatigue limit of the nitrided valve spring was less than 1390 MPa, and the fatigue limit ratio was less than 2.45. Furthermore, the fatigue limit of the non-nitrided valve spring was less than 1340 MPa, and the fatigue limit ratio was less than 2.35.

[0177] In test number 24, the Ca content was too low. Consequently, the fatigue limit in a high cycle (108 cycles) of the nitrided valve spring was less than 1390 MPa, and the fatigue limit ratio was less than 2.45. Furthermore, the fatigue limit in a high cycle (108 cycles) of the non-nitrided valve spring was less than 1340 MPa, and the fatigue limit ratio was less than 2.35.

[0178] In test number 25, the Ca content was too high. Therefore, in the steel wire, the numerical ratio of Ca sulfides was too high. Consequently, the fatigue limit of the nitrided valve spring was less than 1390 MPa, and the fatigue limit ratio was less than 2.45. Furthermore, the fatigue limit of the non-nitrided valve spring was less than 1340 MPa, and the fatigue limit ratio was less than 2.35.

[0179] In tests 26 to 28, although the chemical composition was adequate, the V-based precipitate formation heat treatment was not performed. Therefore, in the steel wire, the number density of V-based precipitates with a maximum diameter ranging from 2 to 10 nm was too low. Consequently, the fatigue limit of the nitrided valve spring was less than 1390 MPa, and the fatigue limit ratio was less than 2.45. Furthermore, the fatigue limit of the non-nitrided valve spring was less than 1340 MPa, and the fatigue limit ratio was less than 2.35.

[0180] In tests number 29 to 31, although the chemical composition was adequate, the heat treatment temperature in the V-based precipitate formation heat treatment was too low. Therefore, in the steel wire, the number density of V-based precipitates with a maximum diameter of 2 to 10 nm was too low. Consequently, the fatigue limit of the nitrided valve spring was less than 1390 MPa, and the fatigue limit ratio was less than 2.45. Furthermore, the fatigue limit of the non-nitrided valve spring was less than 1340 MPa, and the fatigue limit ratio was less than 2.35.

[0181] In tests 32 to 34, although the chemical composition was adequate, the heat treatment temperature for the V-based precipitate formation heat treatment was too high. Therefore, in the steel wire, the V-based precipitates thickened, and the number density of V-based precipitates with a maximum diameter ranging from 2 to 10 nm was too low. Consequently, the fatigue limit of the nitrided valve spring was less than 1390 MPa, and the fatigue limit ratio was less than 2.45. Furthermore, the fatigue limit of the non-nitrided valve spring was less than 1340 MPa, and the fatigue limit ratio was less than 2.35.

[0182] In tests number 35 and 36, during the refining process, the Ca content in the ferroalloys added to the molten steel exceeded 1.0%. Therefore, in the steel wire, the numerical ratio of Ca sulfides was too high. Consequently, the fatigue limit of the nitrided valve spring was less than 1390 MPa, and the fatigue limit ratio was less than 2.45. Furthermore, the fatigue limit of the non-nitrided valve spring was less than 1340 MPa, and the fatigue limit ratio was less than 2.35.

[0183] In tests number 37 and 38, during the refining process, the time elapsed from the start of the refining process until the addition of slag-forming agents was greater than 4t / 5 (0.80 t) (min). Therefore, in the steel wire, the numerical ratio of Ca sulfides was too high. Consequently, the fatigue limit of the nitrided valve spring was less than 1390 MPa, and the fatigue limit ratio was less than 2.45. Furthermore, the fatigue limit of the non-nitrided valve spring was less than 1340 MPa, and the fatigue limit ratio was less than 2.35.

[0184] In test number 39, although the chemical composition was adequate, in the V-based precipitate formation heat treatment, the Fn defined by equation (2) was greater than 38.9. As a result, in the steel wire, the number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm was too low. Consequently, the fatigue limit of the nitrided valve spring was less than 1390 MPa, and the fatigue limit ratio was less than 2.45. Furthermore, the fatigue limit of the non-nitrided valve spring was less than 1340 MPa, and the fatigue limit ratio was less than 2.35.

[0185] In test number 40, although the chemical composition was adequate, in the V-based precipitate formation heat treatment, Fn defined by equation (2) was less than 29.5. As a result, in the steel wire, the number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm was too low. Consequently, the fatigue limit of the nitrided valve spring was less than 1390 MPa, and the fatigue limit ratio was less than 2.45. Furthermore, the fatigue limit of the non-nitrided valve spring was less than 1340 MPa, and the fatigue limit ratio was less than 2.35.

[0186] The embodiments of the present invention have been described above. However, the above embodiments are merely examples for implementing the present invention. Accordingly, the present invention is not limited to the above embodiments, and the above embodiments may be modified and implemented appropriately within a range that does not depart from the essence of the present invention.

Claims

1. A steel wire with a chemical composition containing, in % by mass, C: 0.50 to 0.80%, Si: 1.20 to less than 2.50%, Mn: 0.25 to 1.00%, P: 0.020% or less, S: 0.020% or less, Cr: 0.40 to 1.90%, V: 0.05 to 0.60%, and N: 0.0100% or less, the remainder being Fe and impurities, wherein the number density of the V-based precipitates having a maximum diameter ranging from 2 to 10 nm is 5000 to 80000 pieces / pm3.

2. The steel wire according to claim 1, wherein: 15 the chemical composition contains: Ca: 0.0050% or less, and where, among the inclusions, inclusions in which, by mass %, an O content equal to or greater than 10.0% are defined as oxide-based inclusions; 20 inclusions in which, by mass %, an S content equal to or greater than 10.0% and an O content less than 10.0% are defined as sulfide-based inclusions, and among the sulfide-based inclusions, inclusions in which, by mass %, a Ca content equal to or greater than 10.0%, an S content equal to or greater than 10.0%, and an O content less than 10.0% are defined as Ca sulfides; 25 a numerical proportion of the Ca sulfides with respect to a total number of the oxide-based inclusions and the sulfide-based inclusions are 0.20% or less.

3. The steel wire according to claim 1 or claim 2, wherein: the chemical composition contains one or more types of elements selected from the group consisting of: Mo: 0.50% or less, Nb: 0.050% or less, W: 0.60% or less, Ni: 0.500% or less, Co: 0.30% or less, and β: 0.0050% or less.

4. The steel wire according to any of claims 1 to 3, wherein the chemical composition contains one or more types of elements selected from the group consisting of: 5 Cu: 0.050% or less, Al: 0.0050% or less, and Ti: 0.050% or less.