US20240209481A1

US20240209481A1 - Steel wire and spring

Info

Publication number: US20240209481A1
Application number: US17/802,486
Authority: US
Inventors: Taichi Okada; Takashi Watanabe; Takafumi Uwano; Kazuhiro Goto; Hiromu Izumida
Original assignee: Sumitomo Electric Industries Ltd
Current assignee: Sumitomo Electric Industries Ltd
Priority date: 2021-06-08
Filing date: 2002-02-03
Publication date: 2024-06-27
Also published as: JP7173410B1; KR20240019005A; JPWO2022259606A1; CN115943225A

Abstract

A steel wire is composed of a steel containing: not less than 0.5 mass % and not more than 0.7 mass % carbon, not less than 1 mass % and not more than 2.5 mass % silicon, not less than 0.3 mass % and not more than 1 mass % manganese, and not less than 0.5 mass % and not more than 2 mass % chromium, with the balance being iron and unavoidable impurities. The steel wire has a wire diameter of not less than 0.5 mm and not more than 2 mm, and a tensile strength of not less than 2000 N/mm2 and not more than 2700 N/mm2. The steel has a pearlite structure. A value obtained by dividing a sum of 5.8885×10−3 and a lattice strain S of the steel by the tensile strength T, (S+5.8885×10−3)/T, the tensile strength T being in N/mm2, is not less than 4.8×10−6.

Description

TECHNICAL FIELD

The present disclosure relates to a steel wire and a spring.
The present application claims priority based on Japanese Patent Application No. 2021-095849 filed on Jun. 8, 2021, the entire contents of which are incorporated herein by reference.

BACKGROUND ART

In a steel wire containing a pearlite structure, a technique for improving settling resistance of springs and fatigue strength of springs has been proposed (see, for example, Japanese Patent Application Laid-Open No. 2012-117129 (Patent Literature 1)).

CITATION LIST

Patent Literature

Patent Literature 1: Japanese Patent Application Laid-Open No. 2012-117129

SUMMARY OF INVENTION

A steel wire according to the present disclosure is composed of a steel containing: not less than 0.5 mass % and not more than 0.7 mass % carbon, not less than 1 mass % and not more than 2.5 mass % silicon, not less than 0.3 mass % and not more than 1 mass % manganese, and not less than 0.5 mass % and not more than 2 mass % chromium, with the balance being iron and unavoidable impurities. The steel wire has a wire diameter of not less than 0.5 mm and not more than 2 mm, and a tensile strength of not less than 2000 N/mm²and not more than 2700 N/mm². The steel has a pearlite structure. A value obtained by dividing a sum of 5.8885×10⁻³and a lattice strain S of the steel by the tensile strength T, (S+5.8885×10⁻³)/T, the tensile strength T being in N/mm², is not less than 4.8×10⁻⁶.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic perspective view showing the structure of a steel wire:

FIG. 2 is a schematic cross-sectional view showing the structure of a steel constituting the steel wire in a cross section perpendicular to a longitudinal direction of the steel wire;

FIG. 3 is a schematic perspective view showing the structure of a spring:

FIG. 4 is a flowchart schematically illustrating a method of producing a steel wire and a spring:

FIG. 5 is a perspective view showing the structure of a raw material wire;

FIG. 6 is a diagram showing a relationship between (S+5.8885×10⁻³)/T and a twist count: and

FIG. 7 is a diagram showing a relationship between the percentage of ferrite structure and (S+5.8885×10⁻³)/T.

DESCRIPTION OF EMBODIMENTS

Problem to be Solved by Present Disclosure

One of the objects of the present disclosure is to provide a steel wire and a spring capable of achieving both high strength and high toughness.

Advantageous Effects of Present Disclosure

According to the steel wire of the present disclosure, high strength and high toughness can both be achieved.

Description of Embodiments of Present Disclosure

First, embodiments of the present disclosure will be listed and described. A steel wire of the present disclosure is composed of a steel containing: not less than 0.5 mass % and not more than 0.7 mass % carbon, not less than 1 mass % and not more than 2.5 mass % silicon, not less than 0.3 mass % and not more than 1 mass % manganese, and not less than 0.5 mass % and not more than 2 mass % chromium, with the balance being iron and unavoidable impurities. The steel wire has a wire diameter of not less than 0.5 mm and not more than 2 mm, and a tensile strength of not less than 2000 N/mm²and not more than 2700 N/mm². The steel has a pearlite structure. A value obtained by dividing a sum of 5.8885×10⁻³and a lattice strain S of the steel by the tensile strength T, (S+5.8885×10⁻³)/T, the tensile strength T being in N/mm², is not less than 4.8×10⁻⁶. Here, the “steel diameter” is defined as an equivalent circle diameter of a cross section perpendicular to the longitudinal direction of the steel wire. In the case where the cross section is circular, the equivalent circle diameter means the diameter of the circle. In the case where the cross section is not circular, the equivalent circle diameter means the diameter of a circle having the same area as the cross-sectional area of the cross section.
The present inventors investigated measures to achieve both strength and toughness in a steel wire. If the degree of working of a wire is increased in a wire drawing step when producing a steel wire, the strength of the steel wire can be increased. However, this is accompanied by a decrease in toughness due to age hardening.
According to the studies conducted by the present inventors, when the lattice strain of a steel and the tensile strength satisfy a certain relationship, the decrease in toughness due to the age hardening would not likely occur, allowing high strength to be achieved while maintaining toughness. Specifically, in the steel wire of the present disclosure, the value obtained by dividing a sum of 5.8885×10⁻³and the lattice strain S of the steel by the tensile strength T, (S+5.8885×10⁻³)/T, is not less than 4.8×10⁻⁶. Therefore, according to the steel wire of the present disclosure, high strength and high toughness can both be achieved. The above value, (S+5.8885×10⁻³)/T, may be 4.84×10⁻⁶or more. This facilitates achieving both high strength and high toughness.
In the above steel wire, the steel may have a ferrite structure with a percentage of 20% or less. Setting the percentage of the ferrite structure to 20% or less can improve the homogeneity of the steel structure, in which case the decrease in toughness due to age hardening would not likely occur. Thus, sufficient toughness is easily ensured. Here, the percentage of the ferrite structure refers to the percentage, %, of a total area of the ferrite structure to a sum of the total area of the ferrite structure and a total area of the pearlite structure in a square region of 200 μm per side in a cross section perpendicular to the longitudinal direction of the steel wire. The percentage of the ferrite structure may be 15% or less. This further facilitates ensuring sufficient toughness.
In the above steel wire, the steel may further contain at least one element selected from the group consisting of: not less than 0.05 mass % and not more than 0.5 mass % vanadium, not less than 0.02 mass % and not more than 1 mass % cobalt, not less than 0.02 mass % and not more than 1 mass % nickel, and not less than 0.05 mass % and not more than 0.5 mass % molybdenum.
A spring of the present disclosure is composed of the above steel wire. According to the spring of the present disclosure, by being composed of the steel wire of the present disclosure, a spring capable of achieving both high strength and high toughness can be provided.
Here, the reasons for limiting the component composition of the steel constituting the steel wire to the above-described ranges will be described.
Carbon: Not Less than 0.5 Mass % and not More than 0.7 Mass %
Carbon is an element that greatly affects the strength of the steel wire having a pearlite structure. For obtaining sufficient strength as the steel wire, the carbon content is required to be not less than 0.5 mass %. An increased carbon content leads to reduced toughness and degraded workability when coiling the wire into a spring shape or when drawing the wire. For ensuring sufficient workability, the carbon content is required to be not more than 0.7 mass %. For further increasing the strength, the carbon content is preferably not less than 0.62 mass %. For improving the toughness and facilitating the working, the carbon content is preferably not more than 0.68 mass %.
Silicon: Not Less than 1 Mass % and not More than 2.5 Mass %
Silicon is an element necessary as a deoxidizing agent at the time of steel refining. Silicon imparts resistance to softening, which is the property to suppress softening by heating, to the steel. For suppressing softening in heat treatment such as nitriding treatment performed after the steel wire is coiled into a spring shape, the silicon content is required to be not less than 1 mass %. For further improving the resistance to softening against heating, the silicon content is preferably not less than 1.5 mass %, and more preferably not less than 1.95 mass %. An increased silicon content may lead to reduced toughness. For ensuring sufficient toughness, the silicon content is required to be not more than 2.5 mass %.
Manganese: Not Less than 0.3 Mass % and not More than 1 Mass %
Manganese, as with silicon, is an element necessary as a deoxidizing agent at the time of steel refining. In order to ensure sufficient effectiveness as the deoxidizing agent, the manganese content is required to be not less than 0.3 mass %. In order to further ensure sufficient effectiveness as the deoxidizing agent, the manganese content is preferably not less than 0.5 mass %. If manganese is added in an excessive amount, in the case where patenting is performed prior to the wire drawing step, a martensite structure may be generated during cooling after heating. The martensite structure thus generated would degrade the workability at the time of wire drawing. Thus, the manganese content is required to be not more than 1 mass %. For further reducing the generation of the martensite structure, the manganese content is preferably not more than 0.8 mass %.
Chromium: Not Less than 0.5 Mass % and not More than 2 Mass %
Chromium contributes to the refinement of the steel structure and the suppression of softening during heating. For realizing such effects sufficiently, the chromium content is required to be not less than 0.5 mass % and preferably not less than 0.7 mass %. If chromium is added in an excessive amount, in the case where patenting is performed prior to the wire drawing step, a martensite structure may be generated during cooling after heating. The martensite structure thus generated would degrade the workability at the time of wire drawing. Further, chromium added in an excessive amount leads to reduced toughness. Thus, the chromium content is required to be not more than 2 mass %. For reducing the generation of the martensite structure and improving the toughness, the chromium content is preferably not more than 1.5 mass %.

Unavoidable Impurities

During the process of producing a steel wire, phosphorus, sulfur, and others are inevitably mixed as unavoidable impurities into the steel constituting the steel wire. Phosphorus and sulfur contained in an excessive amount will cause grain boundary segregation and produce inclusions, thereby degrading the properties of the steel wire. Therefore, the phosphorus content and sulfur content are each preferably not more than 0.025 mass %. The total content of the unavoidable impurities, including phosphorus and sulfur, is preferably not more than 0.3 mass %.
Vanadium: Not Less than 0.05 Mass % and not More than 0.5 Mass %
Vanadium functions as a carbide-forming element in the steel, and contributes to the suppression of softening during heating by the generation of carbides. For realizing such effects sufficiently, the vanadium content may be not less than 0.05 mass %. Vanadium added in an excessive amount will reduce toughness. For ensuring sufficient toughness, the amount of vanadium added may be not more than 0.5 mass % and further not more than 0.2 mass %.
Cobalt: Not Less than 0.02 Mass % and not More than 1 Mass %
Cobalt contributes to the improvement in thermal resistance of the steel and the suppression of softening during heating. For realizing such effects sufficiently, the cobalt content may be not less than 0.02 mass % and further not less than 0.05 mass %. The above effects of cobalt will be saturated even if the steel contains cobalt exceeding 1 mass %. Therefore, the cobalt content is preferably not more than 1 mass %. From the standpoint of reducing the cost, the cobalt content may be not more than 0.5 mass %.
Nickel: Not Less than 0.02 Mass % and not More than 1 Mass %
Adding nickel improves corrosion resistance and toughness. For realizing this function sufficiently, the nickel content may be not less than 0.02 mass % and further may be 0.1 mass %. The above effects of nickel will be saturated even if the steel contains nickel exceeding 1 mass %. Further, nickel, which is an expensive element, contained in the steel in an amount exceeding 1 mass % will increase the production cost of the steel wire. Therefore, the nickel content is preferably not more than 1 mass %. From the standpoint of reducing the cost, the nickel content may be not more than 0.5 mass %.
Molybdenum: Not Less than 0.05 Mass % and not More than 0.5 Mass %
Molybdenum functions as a carbide-forming element in the steel, and contributes to the suppression of softening during heating by the generation of carbides. For realizing such effects sufficiently, the molybdenum content may be not less than 0.05 mass %. For ensuring sufficient toughness, the molybdenum content may be not more than 0.5 mass % and further not more than 0.25 mass %.

Details of Embodiments of Present Invention

Embodiments of the steel wire and spring according to the present disclosure will be described below with reference to the drawings. In the drawings referenced below; the same or corresponding portions are denoted by the same reference numerals and the description thereof will not be repeated.
FIG. 1 is a perspective view showing the structure of a steel wire. FIG. 1 also illustrates a cross section of the steel wire perpendicular to a longitudinal direction Y thereof. Referring to FIG. 1 , a steel wire 1 in the present embodiment is a steel wire that has a cross section of a circular shape perpendicular to the longitudinal direction Y and an outer peripheral surface 11 of a cylindrical surface shape. The steel wire 1 has a wire diameter D of not less than 0.5 mm and not more than 2 mm. The cross section of the steel wire 1 perpendicular to the longitudinal direction Y may be other than circular: it may be elliptical, for example.
The steel wire 1 is composed of a steel containing: not less than 0.5 mass % and not more than 0.7 mass % carbon, not less than 1 mass % and not more than 2.5 mass % silicon, not less than 0.3 mass % and not more than 1 mass % manganese, and not less than 0.5 mass % and not more than 2 mass % chromium, with the balance being iron and unavoidable impurities.
FIG. 2 is a schematic cross-sectional view showing a portion of a cross section of the steel wire 1 perpendicular to the longitudinal direction Y. In FIG. 2 , boundaries 21A between pearlite structure 21 regions and boundaries 22A between a pearlite structure 21 region and a ferrite structure 22 region are shown in solid lines. Referring to FIG. 2 , the steel constituting the steel wire 1 includes a pearlite structure 21 and a ferrite structure 22. In the present embodiment, the percentage of the pearlite structure 21 in the steel constituting the steel wire 1 is not less than 80%. The percentage of the pearlite structure 21 is preferably 85% or more and more preferably 90% or more. In the present embodiment, the percentage of the ferrite structure 22 in the steel constituting the steel wire 1 is not more than 20%. The percentage of the ferrite structure 22 is preferably 15% or less and more preferably 10% or less. Setting the percentage of the ferrite structure to 20% or less can improve the homogeneity of the steel structure, in which case the decrease in toughness due to age hardening would not likely occur. Thus, sufficient toughness is easily ensured. The percentages of the pearlite structure 21 and the ferrite structure 22 are derived, for example, with the following method. First, a sample is taken from the steel wire 1. A cross section of the obtained sample perpendicular to its longitudinal direction Y is polished. The polished surface is observed, for example, with an electron microscope. The images obtained from the observation are processed using appropriate software, to thereby derive the percentages of the pearlite structure 21 and the ferrite structure 22.
The steel wire 1 has a tensile strength T of not less than 2000 N/mm²and not more than 2700 N/mm². The lower limit for the tensile strength T of the stee wire 1 is preferably 2050 N/mm²and more preferably 2100 N/mm². The upper limit for the tensile strength T of the stee wire 1 is preferably 2600 N/mm²and more preferably 2500 N/mm². The tensile strength T is measured, for example, based on JIS Z 2241.
When a lattice strain in the steel constituting the steel wire 1 is represented as S, a value obtained by dividing a sum of S and 5.8885×10⁻³by the tensile strength T, (S+5.8885×10⁻³)/T, is not less than 4.8×10⁻⁶. For example, when S is 0.005, the value as a sum of S and 5.8885×10⁻³is 0.0108885 (10.8885×10⁻³). Further, when the tensile strength T is 2230 N/mm², (S+5.8885×10⁻³)/T is 4.88×10⁻⁶. It should be noted that the figure is rounded off to two decimal places. (S+5.8885×10⁻³)/T is preferably 4.84×10⁻⁶or more. The lattice strain is derived, for example, by using the Williamson-Hall method indicated by the following expression (1). In the expression (1), S represents lattice strain, β represents full width at half maximum (radian) of a diffraction ray, λ represents wavelength (×10⁻¹nm) of X rays for measurement, θ represents Bragg angle (radian) of the diffraction ray, and a dimensionless number, &, represents a constant.
$\begin{matrix} β \cos θ / λ = 2 S \times (\sin θ / λ) + (1 / ε) & [Math . 1] \end{matrix}$
FIG. 3 is a perspective view showing the structure of a spring. Referring to FIG. 3 , a spring 2 in the present embodiment is composed of the steel wire 1.
An exemplary method of producing the steel wire 1 and the spring 2 will now be described. Referring to FIG. 4 , in the method of producing the steel wire 1 and the spring 2 in the present embodiment, a raw material wire preparing step is first performed as S10. FIG. 5 is a perspective view showing the structure of a raw material wire. FIG. 5 also illustrates a cross section of the raw material wire 5 perpendicular to its longitudinal direction Y. In S10, a raw material wire 5 is prepared which is composed of a steel containing: not less than 0.5 mass % and not more than 0.7 mass % carbon, not less than 1 mass % and not more than 2.5 mass % silicon, not less than 0.3 mass % and not more than 1 mass % manganese, and not less than 0.5 mass % and not more than 2 mass % chromium, with the balance being iron and unavoidable impurities. The steel constituting the raw material wire 5 may further contain at least one element selected from the group consisting of: not less than 0.05 mass % and not more than 0.5 mass % vanadium, not less than 0.02 mass % and not more than 1 mass % cobalt, not less than 0.02 mass % and not more than 1 mass % nickel, and not less than 0.05 mass % and not more than 0.5 mass % molybdenum.
Next, a patenting step is performed as a step S20. In this step S20, the material steel wire prepared in step S10 is subjected to patenting. Specifically, heat treatment is carried out in which the material steel wire is heated to a temperature range not lower than the temperature at which the steel constituting the steel wire is austenitized (austenitizing treatment), rapidly cooled to a temperature range higher than the MS point, which is the temperature at which martensitic transformation of the steel starts, and then held at the temperature range (isothermal transformation treatment). With this, the metal structure of the material steel wire becomes a fine pearlite structure with small lamellar spacing. This step is an important step in order for the value of (S+5.8885×10⁻³)/T to be 4.8×10⁻⁶or more. In the austenitizing treatment in the present embodiment, the material steel wire is heated to a temperature range not lower than the A3 point at which the steel becomes single-phase austenite. The austenitizing treatment is preferably performed at a temperature immediately above the A3 point. Specifically, the austenitizing treatment is preferably performed in a temperature range of not lower than the A3 point and not higher than the A3 point +20° C. The temperature corresponding to the A3 point can be calculated, for example, using the following computational expression:
$910 - 203 \times [C %] - 5.2 \times [Ni %] + 44.7 \times [Si %] + 104 \times [V %] + 31.5 \times [Mo %] (°C),$
where [C %], [Ni %], [Si %], [V %], and [Mo %] are contents (mass %) of C, Ni, Si, V, and Mo, respectively. In the above austenitizing treatment, from the standpoint of suppressing the occurrence of decarburization, the material steel wire is heated within an inert gas atmosphere. In the isothermal transformation treatment, the material steel wire is held at a high temperature range compared to the case of typical isothermal transformation treatment for piano wire or the like. Specifically, in the present embodiment, the material steel wire is held at a temperature range of 650° C. or higher. Thus by setting the temperature for the isothermal transformation treatment to a high temperature range compared to the case of typical isothermal transformation treatment for piano wire or the like, although the lamellar spacing in the obtained pearlite structure slightly increases, it is possible to further reduce the percentage of the ferrite phase that would slightly exist after the isothermal transformation treatment. With this, the value of (S+5.8885×10⁻³)/T can be set to be 4.8×10⁻⁶or more.
Next, a wire drawing step is performed as a step S30. In this step S30, the material steel wire that has undergone patenting in step S20 is subjected to wire drawing. In the present embodiment, the lower limit for the reduction of area as the degree of working in the wire drawing step is 85%. Setting the reduction of area to 85% or more can increase the strength of the steel wire 1. In the present embodiment, the upper limit for the reduction of area is 95%. By performing the wire drawing step, the steel wire 1 as shown in FIG. 1 , having the pearlite structure, is obtained. It should be noted that when a cross-sectional area of a cross section perpendicular to the longitudinal direction of the material steel wire before being drawn is represented as V1 and a cross-sectional area of a cross section perpendicular to the longitudinal direction of the material steel wire that has been drawn is represented as V2, then the reduction of area is the percentage obtained by dividing a difference between V1 and V2 by V1.
A method of producing a spring 2 using the steel wire 1 obtained in S30 will now be described. Following S30, a coiling step is performed as a step S40 in which the steel wire is coiled into a spring shape. In this S40, referring to FIGS. 1 and 3 , the steel wire 1 is plastically worked into a helical shape, as shown for example in FIG. 3 , and thus formed into the shape of a spring. Next, an annealing step is performed as a step S50. In this S50, the steel wire 1 formed into the spring shape in S40 is subjected to annealing. Specifically, the steel wire 1 formed into the spring shape is heated, so that the strain in the steel wire 1 caused in S40 is reduced. The spring 2 of the present embodiment is completed through the above steps.
Here, in the steel wire 1 and the spring 2 in the present embodiment, with regard to the lattice strain S of the steel and the tensile strength T (in N/mm²), the ratio obtained by dividing a sum of S and 5.8885×10⁻³by T, (S+5.8885×10⁻³)/T, is not less than 4.8×10⁻⁶. Therefore, according to the steel wire 1 and the spring 2 in the present embodiment, high strength and high toughness can both be achieved.
In the above embodiment, the steel wire 1 that has undergone annealing in S50 may be subjected to shot peening. Although the shot peening is not an indispensable step in the method of producing the spring 2 in the present embodiment, this step when conducted imparts compressive stress to the region including the surface of the spring 2, contributing to improved fatigue strength.
In the above embodiment, prior to the wire drawing step of S30, a shaving step of removing the decarburized layer in the raw material wire 5 and/or an annealing step may be performed. The heating temperature in the annealing step is, for example, not lower than 550° C. and not higher than 650° C. The processing time in the annealing step is, for example, not shorter than 120 minutes and not longer than 240 minutes. Further, a preliminary wire drawing step may be performed prior to the patenting step of S20. Performing the preliminary wire drawing step facilitates adjustment of the degree of working in the wire drawing step of S30.

EXAMPLES

Samples of the above-described steel wire 1 of the present disclosure were fabricated and subjected to evaluation to confirm the effect of achieving both high strength and high toughness. The evaluation procedure was as follows.
Sample I was fabricated in a similar procedure as the method of producing the steel wire 1 explained in the above embodiment. The component composition of the steel in Sample I is shown in Table 1. The time (for austenitizing treatment) during which Sample I is held at a temperature range not lower than the A3 point in the patenting step was set to 60 seconds. The temperature for the isothermal transformation treatment of Sample I in the patenting step was set to 650° C. Samples II to VI were fabricated in a similar manner as Sample I, except that the steel component compositions were changed as shown in Table 1. Samples VII and VIII were fabricated in a similar manner as Sample I, except that the steel component compositions were changed as shown in Table 1 and the temperature for the isothermal transformation treatment was set to 600° C. Samples IX and X were fabricated in a similar manner as Sample I, except that the steel component compositions were changed as shown in Table 1 and the austenitizing treatment temperature in the patenting step was set to be lower than the A3 point. The figures in Table 1 indicate the contents of respective elements in mass %. The components of the steel other than the component compositions shown in Table 1 are iron and unavoidable impurities.

TABLE 1

(mass %)

	C	Si	Mn	Cr	V	Co	Mo	Ni

I	0.63	2.35	0.56	1.22	0.11	0.2	—	—
II	0.66	1.32	0.8	0.75	—	—	—	—
III	0.63	2.1	0.78	0.81	0.15	—	0.22	—
IV	0.65	1.98	0.83	0.89	0.12	—	—	0.15
V	0.63	2.02	0.77	0.68	0.09	—	—	—
VI	0.54	1.45	0.55	0.6	—	—	—	—
VII	0.64	2.2	0.57	1.18	0.11	—	—	—
VIII	0.62	1.28	0.77	0.65	0.11	—	—	—
IX	0.54	1.45	0.55	0.6	—	—	—	—
X	0.54	1.45	0.55	0.6	—	—	—	—

For Samples I to X, tensile strength T, lattice strain S, percentage of the ferrite structure 22, and twist count were measured. The tensile strength T, the lattice strain S, and the percentage of the ferrite structure 22 were measured after a lapse of two weeks from the fabrication of Samples I to X. The lattice strain S was measured in the following manner.
Data obtained with X-ray diffraction measurement was plotted based on the Williamson-Hall method, to calculate the strain values. For the X-ray diffraction measurement, X rays monochromatized using a silicon double-crystal monochromator were used. Specifically, diffraction was made to occur on the (111) plane of the silicon single crystal, and the wavelength of the X rays was adjusted to 0.0689 nm. Since the intensity of X rays tends to be attenuated significantly in a double-crystal monochromator, an X-ray source from a synchrotron radiation facility that can provide high intensity was used. Specifically, BL16 of the Kyushu Synchrotron Radiation Research Center (SAGA-LS) was used. As the beamlines of synchrotron radiation facilities equipped with similar double-crystal monochromators, for example, SPring-8 BL16B2, SPring-8 BL16XU, Spring-8 BL19B2, SPring-8 BL46XU, Aichi-SR BL5SI, SAGA-LS BL15, and other equipment can also be used for similar measurement. The diffraction measurement was performed so as to be able to measure two diffraction rays from the iron contained in the steel wire. Specifically, the (110) diffraction ray of iron and the (220) diffraction ray of iron were observed. The obtained diffraction rays were subjected to shape analysis to obtain the full width at half maximum of the diffraction ray, wavelength, and Bragg angle of the diffraction ray, and they were substituted into the expression (1), Williamson-Hall formula, to thereby calculate the lattice strain S.
The measured tensile strength T and lattice strain S were used to calculate (S+5.8885×10⁻³)/T. The twist count was measured in the following manner. With one end of a sample fixed, the other end of the sample was twisted to measure the number of times of twisting until the sample broke as the twist count. The gage length was 100×D and the twisting speed was 60 rpm. A first twist count was measured immediately after the samples were fabricated, and a second twist count was measured two weeks after the fabrication of the samples to check the influence of age hardening to the toughness. A sample with both the first and second twist counts of 20 times or more was considered acceptable. A sample with the first twist count of 20 times or more and the second twist count of less than 20 times was considered unacceptable as having undergone age hardening. It should be noted that any sample with the first twist count of less than 20 times was considered unacceptable. The experimental results are shown in Table 2 and FIGS. 6 and 7 . In Table 2, only the second twist count is shown. In the overall evaluation in Table 2, any sample with both the first and second twist counts being 20 times or more and the percentage of the ferrite structure 22 being 20% or less is indicated with A as acceptable. Any sample with the first twist count being 20 times or more and the second twist count being less than 20 times is indicated with B as unacceptable.

TABLE 2

Tensile Strength	Lattice Strain	(S % +	Ferrite Area Ratio	Second Twist Count	Overall
T (N/mm²)	S	0.58885%)/T	(%)	(times)	Evaluation

I	2230	0.0050	4.88 × 10⁻⁶	7	37	A
II	2100	0.0045	4.95 × 10⁻⁶	20	34	A
III	2320	0.0054	4.87 × 10⁻⁶	8	41	A
IV	2331	0.0054	4.84 × 10⁻⁶	3	32	A
V	2270	0.0055	5.02 × 10⁻⁶	13	38	A
VI	2100	0.0054	5.38 × 10⁻⁶	2	36	A
VII	2450	0.0058	4.77 × 10⁻⁶	20	5	B
VIII	2300	0.0050	4.73 × 10⁻⁶	22	8	B
IX	2276	0.0049	4.74 × 10⁻⁶	32	11	B
X	2435	0.0051	4.51 × 10⁻⁶	40	3	B

In Samples I to X, the first twist count was 20 times or more. Referring to Table 2, in Samples I to X, the tensile strength T is not less than 2000 N/mm²and not more than 2700 N/mm². Therefore, Samples I to X have high strength. In Samples VII and VIII, the ferrite area ratio is high, conceivably because the temperature for the isothermal transformation treatment was set low: In Samples IX and X, the ferrite area ratio is high, conceivably because the austenitizing temperature was set low: The ferrite area ratio also varies depending on the cooling rate from the austenitizing treatment temperature to the isothermal transformation treatment temperature in the patenting step. Referring to FIG. 6 , in Samples I to VI for which (S+5.8885×10⁻³)/T is 4.8×10⁻⁶or more, the second twist count was 20 times or more. That is, in Samples I to VI, the reduction in toughness due to age hardening would not likely occur, so high toughness is maintained. In Samples VII to X for which (S+5.8885×10⁻³)/T is less than 4.8×10⁻⁶, the second twist count was less than 20 times. That is, in Samples VII to X, age hardening has occurred, resulting in reduced toughness. It can thus be said that Samples I to VI achieve both high strength and high toughness. Referring to FIG. 7 , in Samples I to VI for which (S+5.8885×10⁻³)/T is 4.8×10⁻⁶or more and the percentage of the ferrite structure is 20% or less, the reduction in toughness due to age hardening would not likely occur, so high toughness is maintained. It is therefore more preferable that (S+5.8885×10⁻³)/T is 4.8×10⁻⁶or more and the percentage of the ferrite structure is 20% or less. As such, according to the steel wire of the present disclosure, high strength and high toughness can both be achieved.
It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

REFERENCE SIGNS LIST

1: steel wire: 2: spring: 5: raw material wire: 11: outer peripheral surface: 21: pearlite structure: 21A, 22A: boundary: 22: ferrite structure: D: wire diameter: I, II, III, IV, V, VI, VII, VIII, IX, X: sample: S: lattice strain: S10, S20, S30, S40, S50: step: T: tensile strength: V1, V2: cross-sectional area: and Y: longitudinal direction.

Claims

1. A steel wire composed of a steel containing:

not less than 0.5 mass % and not more than 0.7 mass % carbon,

not less than 1 mass % and not more than 2.5 mass % silicon,

not less than 0.3 mass % and not more than 1 mass % manganese, and

not less than 0.5 mass % and not more than 2 mass % chromium,

with the balance being iron and unavoidable impurities;

the steel wire having a wire diameter of not less than 0.5 mm and not more than 2 mm,

the steel wire having a tensile strength of not less than 2000 N/mm²and not more than 2700 N/mm²,

the steel having a pearlite structure,

a value obtained by dividing a sum of 5.8885×10⁻³and a lattice strain S of the steel by the tensile strength T, (S+5.8885×10⁻³)/T, where the tensile strength T is in N/mm², being not less than 4.8×10⁻⁶.

2. The steel wire according to claim 1, wherein said (S+5.8885×10⁻³)/T is not less than 4.84×10⁻⁶.

3. The steel wire according to claim 1, wherein the steel has a ferrite structure with a percentage of 20% or less.

4. The steel wire according to claim 1, wherein the steel has a ferrite structure with a percentage of 15% or less.

5. The steel wire according to claim 1, wherein the steel has the pearlite structure with a percentage of 80% or more.

6. The steel wire according to claim 1, wherein the steel further contains at least one element selected from the group consisting of: not less than 0.05 mass % and not more than 0.5 mass % vanadium, not less than 0.02 mass % and not more than 1 mass % cobalt, not less than 0.02 mass % and not more than 1 mass % nickel, and not less than 0.05 mass % and not more than 0.5 mass % molybdenum.

7. A spring composed of the steel wire according to claim 1.

8. A steel wire composed of a steel containing:

not less than 0.5 mass % and not more than 0.7 mass % carbon,

not less than 1 mass % and not more than 2.5 mass % silicon,

not less than 0.3 mass % and not more than 1 mass % manganese, and

not less than 0.5 mass % and not more than 2 mass % chromium,

with the balance being iron and unavoidable impurities;

the steel having a pearlite structure,

a value obtained by dividing a sum of 5.8885×10⁻³and a lattice strain S of the steel by the tensile strength T, (S+5.8885×10⁻³)/T, where the tensile strength T is in N/mm², being not less than 4.8×10⁻⁶,

the steel having a ferrite structure with a percentage of 15% or less,

the steel having the pearlite structure with a percentage of 80% or more.

9. A spring composed of the steel wire according to claim 8.