JP7260838B2 - Steel wire for spring, spring and method for producing them - Google Patents

Description

TECHNICAL FIELD The present invention relates to steel wires for springs suitable for application to suspension springs for automobiles, springs, and methods of manufacturing the same.

High-strength springs used for automobile suspension springs and the like are manufactured from spring wire rods obtained by hot-rolling steel materials with excellent spring properties through one of the following three patterns of processes. Pattern 1 is a step of drawing a spring wire in a straight state to a desired wire diameter, quenching and tempering it to obtain a spring steel wire, and cold working it into a spring shape. Pattern 2 is a step of performing quenching and tempering treatment after cold working the spring wire into a spring shape. Pattern 3 is a step in which the spring wire is heated to 850 to 1000° C., hot-worked into a spring shape, and then quenched and tempered.

High-strength springs, such as suspension springs for automobiles, which are used in severe corrosive environments such as outdoor environments, may develop fatigue cracks originating from corrosion during long-term use. Even if a high-strength spring is coated with anti-corrosion paint, if a high-strength spring is used for a long period of time in a severely corrosive environment such as outdoors, if the paint peels off or corrosion pits originating from the peeled-off part occur, corrosion pits will form. Fatigue crack initiation can occur. Therefore, in the spring itself, it is required to improve the characteristic (corrosion fatigue characteristic) of preventing fatigue cracks caused by corrosion.

As means for improving the corrosion fatigue characteristics of springs, it is conceivable to improve the corrosion resistance of springs to suppress the occurrence of corrosion pits, and to improve the mechanical properties of springs to suppress the occurrence of fatigue cracks. be done.

As means for improving the corrosion fatigue properties of springs, techniques have been reported for controlling corrosion pits by adjusting the components of springs and steel wires for springs (for example, Non-Patent Documents 1 to 3). However, in the case of low-alloy steels such as those applied to automobile suspension springs, it is difficult to sufficiently improve the corrosion fatigue properties so as to suppress the occurrence of corrosion and pitting by adjusting the alloying elements. is. Therefore, in order to improve the corrosion fatigue characteristics of springs, it is considered effective to improve the mechanical properties of steel materials, specifically tensile strength, yield ratio, and toughness.

In order to control the mechanical properties of steel materials having a tempered martensitic structure, such as suspension springs, heat treatment conditions such as tempering are important in order to control the precipitation state of iron carbides that affect the mechanical properties of steel materials. it is conceivable that. A method of analyzing the precipitation state of iron carbide during the tempering process of spring steel using differential scanning calorimetry (DSC) has been reported (Non-Patent Document 4). Utilizing this method, a method of optimizing the tempering conditions has been proposed by evaluating the precipitation state of iron carbide during tempering of spring steel and springs using the DSC exothermic peak (Patent Document 1).

However, the method of Non-Patent Document 4 focuses only on the temperature at the apex of the exothermic peak. Since the shape of the peak curve, especially the width and position of the peak, changes depending on the amount of carbon and alloy addition, the evaluation method that focuses only on the temperature of the exothermic peak apex is insufficient as an evaluation method for the state of fine iron carbide precipitation. was discovered.

Japanese Patent Application Laid-Open No. 2008-202124

Takenori Nakayama et al., "Corrosion Fatigue Characteristics of Steel for High-Strength Suspension Springs and Their Improvements," Kobe Steel Technical Report, Vol. 47, No. 2, July 1997, published by Kobe Steel, Ltd., p. 50-53 Kazuyoshi Kimura et al., "Influence of Alloying Elements on Corrosion Fatigue Life of Spring Steel", Electric Steelmaking, Vol. 75, No. 1, January 2004, Electric Steelmaking Research Group, p. 19-25 Yutaka Kurebayashi, Akio Yoneguchi, "1200 MPa class high-strength spring steel 'ND120S'", Electric Steel, Vol. 71, No. 1, January 2000, Electric Steel Research Institute, p. 95-101 Mamoru Nagao et al., "Evaluation of tempering behavior of Si-added medium carbon steel using DSC", CAMP-ISIJ, Vol. 17, 2004, The Iron and Steel Institute of Japan, p. 359-362

An object of the present invention is to solve the above problems and to provide a high-strength and high-toughness spring suitable for suspension springs of automobiles, a steel wire for the spring, and a method for producing the same.

The gist of the present invention is as follows.

(1) A spring steel wire according to an aspect of the present invention has, in terms of mass%, chemical components of C: more than 0.50% and 0.60% or less, Si: 1.00 to 3.00%, Mn: 0.10-1.50%, Cr: 0.15-1.20%, Al: 0.001-0.050 %, P: 0.015% or less, S: 0.015% or less, Ti: 0 .010 to 0.100%, B: 0.0010 to 0.0060%, and N: 0.0070% or less, the balance being Fe and impurities, and the content of Ti and N is determined by the formula 1 is satisfied, and in the cross section cut perpendicular to the longitudinal direction, the metal structure at points equidistant from both the center and the surface is the tempered martensite structure, and the temperature range from 50 ° C to 600 ° C is 0.25 ° C. The area of the region of 450° C. or lower among the peaks of the exothermic reaction obtained by measuring the differential scanning calorimetry by raising the temperature at a rate of 1/s is 30% or less of the total peak area of the peaks of the exothermic reaction.
[Ti]≧3.5×[N] (Formula 1)
In Formula 1, [Ti] and [N] are substituted with the above contents in mass % of Ti and N.
(2) The steel wire for spring according to (1) above further includes, in mass %, the chemical components of Mo: 0 to 1.00%, Ni: 0 to 1.00%, and Cu: 0 to 1. .00% Nb: 0 to 0.100%, V: 0 to 0.50% Sb: 0 to 0.050%, one or more of these may be contained.
(3) A spring according to another aspect of the present invention is a spring having the chemical composition described in (1) or (2) above, wherein the temperature range from 50 ° C. to 600 ° C. is 0.25 ° C./s The area of the exothermic reaction peak at 450° C. or below obtained by measuring the differential scanning calorimetry by raising the temperature at 30% or less of the total peak area of the exothermic reaction peak.
(4) A method for producing a steel wire for spring according to another aspect of the present invention is the method for producing a steel wire for spring according to (1) or (2) above, wherein A step of heating and quenching the wire made of the chemical composition described in 850 to 1050 ° C., tempering temperature T [K], tempering time t [s], and C content C% [mass%] and and a step of tempering under the condition that the Cr content C% [mass %] satisfies Equation 2.
58000≦(T+700×[C%]+50×[Cr%])×(51.7+logt)≦62000 (Formula 2)
(5) A method for manufacturing a spring according to another aspect of the present invention is the method for manufacturing a spring according to (3) above, wherein the wire rod made of the chemical composition according to (1) or (2) above is A step of cold working into a spring shape, a step of quenching by heating to 850 to 1050 ° C., tempering temperature T [K], tempering time t [s], and C content C% [mass%] and Cr content C % [mass %] satisfies Equation 2, and tempering is performed.
58000≦(T+700×[C%]+50×[Cr%])×(51.7+logt)≦62000 (Formula 2)
(6) A method for manufacturing a spring according to another aspect of the present invention is the method for manufacturing a spring described in (3) above, wherein a wire rod comprising the chemical composition described in (1) or (2) above is A step of heating to 850 to 1050 ° C. and hot working into a spring shape, a step of quenching the wire having the spring shape, tempering temperature T [K], tempering time t [s], and C content C % [mass %] and Cr content C % [mass %] satisfies Equation 2, and a step of tempering.
58000≦(T+700×[C%]+50×[Cr%])×(51.7+logt)≦62000 (Formula 2)
(7) A method for manufacturing a spring according to another aspect of the present invention comprises the step of subjecting the spring steel wire according to (1) or (2) to spring processing.

According to the high-strength and high-toughness spring steel wire, the spring, and the method for producing the same of the present invention, it is possible to reduce the size and weight of suspension springs, which greatly contributes to the improvement of fuel efficiency and performance of automobiles. The industrial contribution is extremely remarkable because it is possible to

After quenching, it is an example of the differential scanning calorimetry result of the test piece before tempering. It is an example of a method for evaluating the total peak area 3 of exothermic reaction peaks in differential scanning calorimetry and the area 4 of the exothermic reaction peaks at 450° C. or less in the present invention. It is a flow chart which shows the manufacturing method of the steel wire for springs concerning this embodiment. It is a flow chart which shows the manufacturing method (including cold working) of the spring concerning this embodiment. It is a flow chart which shows the manufacturing method (including hot working) of the spring concerning this embodiment. It is a flowchart which shows the method of manufacturing a spring using the steel wire for springs which concerns on this embodiment.

EMBODIMENT OF THE INVENTION Below, it demonstrates in detail about the steel wire for springs, a spring, and those manufacturing methods as a form which implements this invention. However, the present invention is not limited to the following description, and those skilled in the art will readily understand that various changes in form and detail may be made without departing from the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited to the descriptions of the embodiments shown below. In addition, mass % in the composition described below is simply described as %.

The spring steel wire according to the present embodiment has, in terms of % by mass, the chemical components of C: more than 0.50% and 0.60% or less, Si: 1.00 to 3.00%, Mn: 0.10 to 1. .50%, Cr: 0.15 to 1.20%, Al: 0.050% or less, P: 0.015% or less, S: 0.015% or less, Ti: 0.010 to 0.100%, B: 0.0010 to 0.0060% and N: 0.0070% or less, the balance being Fe and impurities, the content of Ti and N satisfying the formula A, cut perpendicular to the longitudinal direction In the cross section, the metallographic structure at the point equidistant from both the center and the surface is tempered martensite, and the temperature range from 50 ° C to 600 ° C is increased at 0.25 ° C/s and differential scanning calorimetry is measured. The area 4 of the region of 450° C. or lower among the exothermic reaction peaks obtained by
[Ti]≧3.5×[N] (formula A)

Moreover, the spring according to the present embodiment has the same chemical composition as the steel wire for spring according to the present embodiment, and the temperature range from 50° C. to 600° C. is raised at a rate of 0.25° C./s and subjected to differential scanning. Among the exothermic reaction peaks obtained by measuring the calorific value, the area of the region of 450° C. or less is 30% or less of the total peak area of the exothermic reaction peaks. As long as the spring according to this embodiment satisfies these requirements, the manufacturing method is not particularly limited. However, in the following, a product obtained by cold-working a steel wire for a spring into a spring shape, and a wire rod cold-worked or hot-worked into a spring shape and then subjected to quenching treatment and tempering treatment will be described. What is obtained is shown as a preferred example of the spring according to this embodiment.

First, the new findings of the present inventors that have led to the completion of the present invention will be described.

The present inventors have developed a high-strength suspension spring, specifically a tensile strength of 2000 MPa or more (when the tensile strength cannot be measured due to a spring shape, the Vickers hardness is 573 HV or more when measured with a load of 300 gf). Various factors affecting the corrosion fatigue characteristics of high-strength suspension springs have been extensively studied, and the following findings have been obtained.

(i) In the case of high-strength (tensile strength of 2000 MPa or more) suspension spring steel, if the toughness of the steel material is low, intergranular fracture may appear on the fatigue fracture surface, and the growth rate of the fatigue crack is particularly high. That is, by improving the toughness of the steel material, the fatigue strength of the spring can be increased. In addition, "steel material" means both the steel which comprises the steel wire for springs which concerns on this embodiment, and the steel which comprises the spring which concerns on this embodiment.

(ii) the area of the region below 450°C in the peak of the exothermic reaction when subjected to differential scanning calorimetry (DSC) in which the temperature range from 50°C to 600°C is increased at 0.25°C/s; is less than 30% of the total peak area of the exothermic reaction peak has extremely high toughness.

The method of identifying iron carbide by DSC and the technical significance of the above requirements will be described below. DSC raises the temperature of a sample to cause phase transformation and precipitation of iron carbide in the sample, and detects heat generation and endotherm at this time. Based on the DSC curve thus obtained, the phase state and iron carbide state in the sample before heating the sample can be evaluated. In addition, the measurement by DSC is performed based on JISK0129:2005 in principle. Detailed measurement conditions will be described later. In the following description, the peak is defined as the portion of the DSC curve from when the curve departs from the baseline to when it returns to the baseline again. Baseline is defined as the DSC curve in the temperature range where no change occurs in the specimen. These terms are defined according to JIS K 0129:2005. Peak area is defined as the area bounded by the peak and baseline.

Examples of DSC curves are shown in FIGS. FIG. 1 is a DSC curve that does not meet the above requirements, ie, a DSC curve in which the area of the region below 450° C. of the exothermic reaction peak is more than 30% of the total peak area of the exothermic reaction peak. FIG. 2 is a DSC curve that satisfies the above requirements, that is, a DSC curve in which the area of the exothermic reaction peak at 450° C. or below is 30% or less of the total peak area of the exothermic reaction peak.

When DSC measurement is performed on a steel material that has not been tempered in the manufacturing stage, that is, a steel material as quenched at a heating rate of 0.25° C./s, two peaks are observed as shown in FIG. The peak on the low temperature side is presumed to be the peak of an exothermic reaction (peak 1 in FIG. 1) due to the solid solution carbon being precipitated as ε carbide, which is fine iron carbide, due to heating during DSC measurement. The peak on the high temperature side is presumed to be the peak of an exothermic reaction due to the transition of fine iron carbides to coarse θ carbides (peak 2 in FIG. 1).

The structure of the as-quenched steel material is mainly martensite before the DSC measurement, and C is dissolved in the martensite. When such a steel material is subjected to DSC measurement in which the temperature range from 50 ° C. to 600 ° C. is increased at 0.25 ° C./s, first, C precipitates from martensite in the form of fine iron carbides, that is, ε carbides. do. It is believed that this precipitation reaction is detected as peak 1 in FIG. When the DSC measurement is continued and the temperature of the sample is increased, the fine iron carbides transform into coarse θ carbides. It is believed that this transition reaction is detected as peak 1 in FIG. Note that two peaks as shown in FIG. 1 are also observed in the DSC curve of the steel material that is insufficiently tempered after quenching in the manufacturing stage. Although a small amount of fine iron carbide precipitates in a steel material that is insufficiently tempered at the manufacturing stage, there is still room for further precipitation reactions of iron carbide to occur, so when this is subjected to DSC measurement, , peak 1 is detected.

These peaks have broadening, and the respective tails of peak 1 and peak 2 may overlap. In addition to this, an exothermic reaction peak due to the decomposition of residual γ has also been reported. However, in the case of spring steel, the amount of retained γ is several percent, and the peak of the exothermic reaction is extremely weak.

On the other hand, if a DSC measurement is performed at a heating rate of 0.25° C./s on a steel material that has been quenched in the manufacturing stage and then tempered under the predetermined conditions described later, a DSC curve such as that shown in FIG. 2 is obtained, for example. be done. In this DSC curve, the area 4 of the exothermic reaction peak at 450° C. or lower is 30% or less of the total peak area 3 of the exothermic reaction peak. That is, in the DSC curve of the steel material obtained by the manufacturing method of tempering under predetermined conditions, the DSC measurement shows that the exothermic reaction (corresponding to peak 2 in FIG. 1) due to the transition of fine iron carbides to coarse θ carbides. Compared to the size, the size of the exothermic reaction (corresponding to peak 1 in FIG. 1) in which fine iron carbide precipitates according to DSC measurement is extremely small.

It is presumed that the structure of the steel material that exhibits the DSC curve as shown in FIG. 2 is mainly tempered martensite and that fine ε carbides are precipitated before the DSC measurement. In a steel material in which fine iron carbides are sufficiently precipitated, there is no room for further fine iron carbides to precipitate, so even if this is subjected to DSC measurement, no precipitation reaction of fine iron carbides occurs. Therefore, in the DSC measurement of such a steel material, heat generation due to precipitation of fine iron carbides (corresponding to peak 1 in FIG. 1) is considered to hardly occur. On the other hand, in steel materials in which fine ε-carbides are precipitated, there is still room for fine iron carbides to transition to coarse θ-carbides before DSC measurement. Therefore, in the DSC measurement of such steel, it is presumed that heat generation (corresponding to peak 2 in FIG. 1) due to the transition from fine iron carbides to coarse θ carbides is strongly generated.

On the other hand, even if the DSC measurement is performed on the steel material tempered in the manufacturing stage, there are cases where no clear peak is observed. It is presumed that the structure of such a steel material has been excessively tempered before the DSC measurement, and all of the steel materials have transitioned to coarse θ-carbides. When the precipitated iron carbide consists only of coarse θ-carbides, there is no room for precipitation of fine iron carbides from martensite and no room for a transition reaction from fine iron carbides to coarse θ-carbides. Therefore, when a steel material in which only coarse θ-carbides are precipitated before DSC measurement is subjected to DSC measurement, the DSC curve does not show a clear peak. Such DSC curves also do not meet the above requirements.

The present inventors have confirmed that a steel material exhibiting a DSC curve as shown in FIG. 2 has extremely good toughness. This is presumed to be because fine ε-carbides are precipitated in the steel material exhibiting the DSC curve as shown in FIG. be.

On the other hand, as shown in FIG. 1, a steel material in which both peaks 1 and 2 are observed in the DSC curve has a low yield ratio and poor fatigue properties. This is presumed to be due to insufficient precipitation of fine iron carbides.

In addition, steel materials in which no clear peak is observed in the DSC curve are also inferior in strength and toughness. It is presumed that this is because the transition reaction to θ-carbides progresses too much, causing coarsening of θ-carbides, and this coarse θ-carbides causes deterioration in mechanical properties. That is, in a steel material in which fine ε carbides are sufficiently secured, the exothermic reaction corresponding to peak 2 (reaction with a peak position between 450 and 600 ° C.) is measured as a peak with a height of, for example, 10 mW / g or more. However, this peak is not observed in steel materials in which the transition reaction has proceeded excessively.

(iii) In order to obtain a steel material that exhibits a DSC curve that satisfies the above requirements, it is necessary to control the content of C, which is effective for increasing strength, within an appropriate range, while controlling the content of Cr, which is effective in suppressing temper softening. It is necessary to control the range and temper the steel material under appropriate tempering conditions according to the C content and Cr content.

Hereinafter, the steel wire for spring and the spring of this embodiment will be described in detail. First, the reasons for limiting the chemical components in the spring steel wire and spring of the present embodiment will be described in detail.

C: more than 0.50% and 0.60% or less C is an element necessary for obtaining high strength. Therefore, it is necessary to contain C exceeding 0.50%. On the other hand, when the C content exceeds 0.60%, the toughness is lowered. Also, if C is contained excessively, the tempering temperature for obtaining the desired strength rises, the amount of cementite (θ) generated increases, and it is considered that both high strength and high toughness cannot be achieved. Therefore, the upper limit of the C content is made 0.60%. A preferred upper limit for the C content is 0.58%, 0.57%, 0.56%, or 0.55%. A preferred lower limit for the C content is 0.51%, 0.52%, 0.53%, or 0.54%.

Si: 1.00-3.00%
Si is an element that is effective in strengthening steel and improving settling properties of springs, and also improves temper softening resistance. In order to obtain this effect, it is necessary to contain 1.00% or more of Si. On the other hand, if the Si content exceeds 3.00%, it promotes decarburization during wire rod rolling and heat treatment. Therefore, it is necessary to set the upper limit of the amount of Si to 3.00%. A preferred lower limit for the amount of Si is 1.10%, 1.35%, or 1.50%. A preferred upper limit for the amount of Si is 2.80% or 2.50%.

Mn: 0.10-1.50%
Mn is an element effective in improving tensile strength. In order to obtain this effect, it is necessary to contain 0.10% or more of Mn. On the other hand, when the Mn content exceeds 1.50%, center segregation during casting is promoted and toughness is lowered. Therefore, the Mn content should be in the range of 0.10-1.50%. A preferable lower limit of the Mn content is 0.15% or 0.25%. A preferred upper limit for the Mn content is 1.45%, 1.20%, or 1.00%.

Cr: 0.15-1.20%
Cr has the effect of refining iron carbide and increasing strength. In order to obtain this effect, it is necessary to contain 0.15% or more of Cr. On the other hand, when the Cr content exceeds 1.20%, center segregation during casting is promoted and toughness is lowered. Therefore, the Cr content should be in the range of 0.15-1.20%. In addition, the suitable lower limit of Cr content is 0.20% or 0.30%. A suitable upper limit for the Cr content is 1.15%, or 1.00%.

Al: 0.050% or less In the spring steel wire and spring according to the present embodiment, containing Al is not essential for solving the problem. Therefore, the Al content may be 0%. On the other hand, Al forms oxides to reduce the amount of oxygen in the steel. In addition, Al precipitates AlN and reduces the amount of dissolved N. When B is contained together with Al, the amount of solid solution N is reduced, the formation of BN is prevented, and the effect of improving the hardenability of B is further enhanced. For the above reasons, the spring steel wire may contain Al. In order to fix N in the steel, it is preferable to contain Al of 2.0×N or more (the symbol “N” is the N content in unit mass %). A preferred lower limit for the Al content is 0.015%. However, even if the content of Al exceeds 0.050%, the effect is saturated. Therefore, the upper limit of Al content is 0.050%. Further, in order to suppress deterioration of toughness due to coarsening of AlN, it is preferable to set the upper limit of the Al content to 0.030%.

Ti: 0.010-0.100%
Ti forms TiN and reduces the amount of dissolved N. By reducing the amount of dissolved N, the formation of BN is prevented, and the effect of improving the hardenability of B is enhanced. In order to fix N in the steel, it is preferable to contain Ti in an amount of 3.5×N (the symbol “N” is the N content in unit mass %) or more. A preferred lower limit for the Ti content is 0.015%, 0.020%, or 0.030%. However, even if Ti content exceeds 0.100%, the effect is saturated. Therefore, the upper limit of the Ti content is 0.100%. Further, in order to suppress deterioration in toughness due to coarsening of TiN and Ti(CN), the upper limit of the Ti content is preferably 0.080%, 0.070%, or 0.060%.

B: 0.0010 to 0.0060%
B is an element effective in improving the hardenability of steel even if the content is very small. In addition, B in a solid solution state segregates at the prior austenite grain boundaries to strengthen the grain boundaries and has the effect of improving the toughness. In particular, when B is contained in the steel containing C and Si within the range of the spring steel wire and spring according to the present embodiment, it has the effect of further improving the toughness of the steel. Therefore, it is preferable to contain 0.0010% or more of B. On the other hand, when the B content exceeds 0.0060%, the effect is saturated. Therefore, the upper limit of the B content may be 0.0060%. A preferred lower limit for the B content is 0.0015%, 0.0020%, or 0.0025%. A preferred upper limit for the B content is 0.0050%, 0.0040%, or 0.0030%. In order to obtain the effect of containing B, it is preferable to prevent the formation of BN by reducing the amount of solid solution N and to allow B to be present in the steel as solid solution. Therefore, limiting the N content and adding Ti are extremely effective.

P: 0.015% or less, S: 0.015% or less P and S are impurities. P is an element that segregates at prior austenite grain boundaries to embrittle the grain boundaries and reduce toughness. S forms a sulfide and serves as a corrosion starting point for steel. Therefore, it is necessary to limit P and S to 0.015% or less. Moreover, it is preferable to reduce P and S as much as possible, and the preferable upper limit is 0.010%. Considering the capacity and economy of the current refining process, it is normal for each of P and S to be mixed into steel by about 0.001%. However, it is not prevented to reduce the content of each of P and S further than this.

N: 0.0070% or less N is an impurity that combines with B to form BN, thereby reducing the hardenability improvement effect of solid solution B. Therefore, the N content is limited to 0.0070% or less. Also, the smaller the N content, the smaller the Ti content for fixing N, and the smaller the amount of TiN produced. Therefore, it is preferable to reduce N as much as possible. A preferred upper limit for the N content is 0.0060% or 0.0050%. Considering the capacity and economy of the current refining process, it is normal for N to be mixed into steel by about 0.001%. However, it is not prevented to reduce the N content further.

[Ti]≧3.5×[N]
In the above formula, the contents of Ti and N in mass % are substituted for [Ti] and [N]. Ti has the effect of fixing solid solution N as TiN by forming TiN, which is extremely effective in reducing the amount of solid solution N. Fixing solute N as TiN is an effective measure to prevent loss of B due to BN formation and utilize B contained in steel as solute B. Comparing the atomic weights of Ti and N, in order to fix all the N as TiN, 3.5 times the mass of Ti as N is required. On the other hand, when N is excessive with respect to Ti, coarse TiN is formed, which adversely affects toughness and the like.

The rest of the chemical composition of the spring steel wire and spring according to the present embodiment consists of Fe and impurities. Here, the impurities are, for example, those that are mixed from ore, scrap, or the manufacturing environment as raw materials when the spring steel wire and spring are industrially manufactured. It means a permissible range that does not adversely affect the wire and spring. An example of impurities is O (oxygen), which is related to the amount of oxide-based inclusions in the spring steel wire. By setting the O content to 0.0015% or less, it is possible to suppress the formation of coarse oxide-based inclusions that affect the fatigue fracture of the spring. In addition, impurities other than P, S, N and O include Mg, Co, As, Zr, W, REM (Sc, Y and elements with atomic numbers from 57 to 71), Hf, Ta, Ca, In, Sn, Pb, Bi, Te, and Zn, respectively, Mg: 0.02% or less, (Co, As): 0.1% or less, (W, REM, Hf and Ta): 0.01 % or less (Ca, In, Zr, Te, Bi, Pb, Sn and Zn): it is desirable to regulate to 0.01% or less.

In addition, the spring steel wire and spring manufactured with the above chemical composition are selected from the group consisting of Mo, Ni, Cu, Nb, V, and Sb for the purpose of improving properties such as hardenability. You may contain the above elements as needed. Since the content of these elements is arbitrary, the lower limit of the content of each element is 0%.

Mo: 0-1.00%, Ni: 0-1.00%,
Mo and Ni can be contained in order to obtain the effect of improving hardenability. When it is contained, it is preferable that each of Mo and Ni be contained in an amount of 0.05% or more. On the other hand, containing more than 1.00% of Mo or Ni may increase the alloy cost and impair economy. Therefore, the contents of Mo and Ni are preferably 1.00% or less. The content of Mo and Ni is more preferably 0.10% or more. The content of Mo and Ni is more preferably 0.50% or less.

Cu: 0-1.00%
Cu can be contained in order to obtain the effect of improving the hardenability. When it is contained, Cu is preferably contained in an amount of 0.05% or more. On the other hand, when the Cu content exceeds 1.00%, the hot ductility is lowered, and the occurrence of cracks, scratches, etc. during continuous casting and hot rolling is facilitated, and the manufacturability of the steel may be impaired. Therefore, the Cu content is preferably in the range of 1.00% or less. The Cu content is preferably 0.10% or more. The Cu content is preferably 0.50% or less.

Nb: 0 to 0.100%,
Nb can be contained in order to obtain the effect of improving the toughness by refining the structure. When it is contained, Nb is preferably contained in an amount of 0.010% or more. On the other hand, even if the content of Nb exceeds 0.100%, the effect is saturated. Therefore, the Nb content is preferably in the range of 0.100% or less. More preferably, Nb is 0.015% or more. More preferably, Nb is 0.040% or less.

V: 0-0.50%
V can be contained in order to obtain the effect of improving the toughness by refining the structure. When it is contained, it is preferable to contain 0.05% or more of V. On the other hand, even if the V content exceeds 0.50%, the effect is saturated. Therefore, the V content is preferably in the range of 0.50% or less. More preferably, V is 0.10% or more. More preferably, V is 0.30% or less.

Sb: 0 to 0.050
Sb can be contained because it is an element that segregates on the surface of the steel material and suppresses decarburization that occurs during heating during hot rolling, cooling after rolling, heating for quenching, and the like. When it is contained, it preferably contains 0.001% or more of Sb. On the other hand, even if the content of Sb exceeds 0.050%, the effect is saturated. Therefore, the Sb content is preferably in the range of 0.050% or less. More preferably, Sb is 0.002% or more. More preferably, Sb is 0.030% or less.

Iron Carbide: In order to obtain high-strength and high-toughness spring steel wire and spring, it is considered necessary to appropriately control ε-carbide and θ-carbide. ε-carbides are finer iron carbides than θ-carbides, and are extremely effective in improving strength. Furthermore, the present inventors have found that at a strength level of 2000 MPa or more in TS, control of not only coarse θ carbide but also grain boundary θ is important for improving toughness.

Specifically, θ carbide is iron carbide that transitions from ε carbide and precipitates as the temperature rises during tempering, and precipitates more coarsely than ε carbide, reducing toughness.

Furthermore, at the initial stage of the transition from ε carbide to θ carbide, θ carbide precipitates at grain boundaries. At a strength level of TS of 2000 MPa or more, θ precipitated at grain boundaries causes embrittlement in the same way as coarse θ carbides.

The high-strength and high-toughness spring steel wire and spring that have been properly tempered according to the present invention are provided so that iron carbides can be finely precipitated, and at the beginning of the transition from ε carbides to θ carbides, The tempering conditions are controlled so as to suppress the precipitation of θ-carbides. On the other hand, it is difficult to directly identify fine iron carbides and grain boundary θ-carbides with a microscope or the like. In this embodiment, the iron carbide state is identified by differential scanning calorimetry (DSC) as described below.

Differential scanning calorimetry: Differential scanning calorimetry (DSC) is performed according to JIS K 0129:2005. The measurement method is important in differential scanning calorimetry. The steel wire for spring and the iron carbide of the spring of the present invention are identified by setting the temperature increase rate to 0.25 [° C./s] in the temperature range from 50° C. to 600° C., the reaching temperature to 600° C., and the same sample. Two measurements are taken consecutively. In the steel material after the first measurement, there is no room for precipitation reactions of new iron carbides and no room for transition reactions from ε carbides to θ carbides. Therefore, the DSC curve obtained by the second measurement has a flat shape without a clear peak. Therefore, by subtracting the second measurement data from the first measurement data, the exothermic phenomenon (positive calorific value) detected only during the initial heating is defined as the exothermic reaction of iron carbide precipitation. By such a measurement method, the data after calculation is graphed as X axis = temperature [° C.], Y axis = calorific value [mW / g = mJ / g / s], and the temperature increase rate [° C. / s], the region where the Y-axis = positive value on the graph can be determined as an exothermic peak, and the peak area can be converted into the calorific value "mJ/g" associated with precipitation of iron carbide.

The θ carbides precipitated at the grain boundaries coarsen even in a low temperature range of 450° C. or lower during heating during DSC measurement, and undergo an exothermic reaction. That is, the region of 450° C. or less among the peaks of the exothermic reaction suggests the presence of grain boundary θ-carbides in the material.

Regarding the high-strength and high-toughness spring steel wire and spring according to the present embodiment, the DSC obtained by measuring the differential scanning calorimetry by increasing the temperature range from 50 ° C. to 600 ° C. at 0.25 ° C./s As for the curve, as shown in FIG. 2, the area 4 of the exothermic reaction peak at 450° C. or below is 30% or less of the total peak area 3 of the exothermic reaction peak. Steel wires and springs exhibiting DSC curves having such peaks can exhibit precipitation strengthening due to fine iron carbides, avoid embrittlement caused by grain boundary θ carbides, and achieve both high strength and high toughness. On the other hand, in spring steel wires and springs that do not exhibit a clear exothermic reaction, it is determined that fine iron carbides have all transitioned to coarse θ carbides before DSC measurement. Such spring steel wires and springs have extremely low toughness because excessive θ carbides are formed in the steel.

When the steel material is as quenched or the formation of ε carbide is insufficient, an exothermic reaction with peaks 1 and 2 is shown as shown in FIG. In such a case, since fine ε carbides and θ carbides are not precipitated, the yield ratio is lowered.

In this embodiment, DSC measurement was performed using DSC-60 manufactured by Shimadzu Corporation in accordance with JIS K 0129:2005. However, if the measurement conditions are the same, the difference is small even if other instruments are used, and among the exothermic reaction peaks described above, the area 4 of the region below 450 ° C. is 30 of the total peak area 3 of the exothermic reaction peak. % or less, the spring steel wire and spring of the present invention can satisfy the conditions.

Moreover, the spring according to this embodiment exhibits substantially the same thermal analysis results as the spring steel wire according to this embodiment. This is because even if the steel wire is subjected to spring processing, the processing does not substantially affect the mode of precipitation of iron carbides.

Next, a spring steel wire and a spring manufacturing method according to another aspect of the present invention will be described. According to this manufacturing method, the spring steel wire and the spring according to the present embodiment can be suitably obtained. However, the following description does not limit the spring steel wire and the spring according to this embodiment. That is, the spring steel wire and spring that satisfy the above requirements are considered to be the spring steel wire and spring according to the present embodiment, regardless of the manufacturing method thereof.
The method for manufacturing a steel wire for spring according to the present embodiment includes a step of heating a wire rod composed of the above-described chemical components to 850 to 1050 ° C. and performing a quenching treatment S103, a tempering temperature T [K], a tempering time t [ s], C content C % [mass %], and Cr content Cr % [mass %] satisfy formula (2), and a step of performing tempering S104.
58000≦(T+700×[C%]+50×[Cr%])×(51.7+logt)≦62000 (2)

In the spring manufacturing method according to the present embodiment shown in FIG. 6, a spring shape is obtained by subjecting the spring steel wire according to the present embodiment to cold spring processing S100. In the method for manufacturing the spring according to the present embodiment shown in FIG. 4, in the middle of the method for manufacturing the steel wire for spring according to the present embodiment, the wire rod is subjected to cold working S102 into a spring shape, followed by quenching treatment S103 and tempering. Processing S104 is performed. Alternatively, in the spring manufacturing method of the present invention shown in FIG. Processing S104 is performed.

When the wire rod is cold-worked S102 into a spring shape, specifically, the wire rod composed of the chemical components described above is cold-worked S102 into a spring shape and then heated to 850 to 1050° C. to perform the quenching treatment S103. tempering temperature T [K], tempering time t [s], C content C% [mass%], and Cr content Cr% [mass%] satisfy formula (2). S104 is performed.
58000≦(T+700×[C%]+50×[Cr%])×(51.7+logt)≦62000 (2)
When hot working S101 the wire into a spring shape, specifically, the wire made of the above-mentioned chemical components is heated to 850 to 1050° C. and hot worked S101 into a spring shape, followed by quenching S103. tempering temperature T [K], tempering time t [s], C content C% [mass%], and Cr content Cr% [mass%] satisfy formula (2). S104 is performed.
58000≦(T+700×[C%]+50×[Cr%])×(51.7+logt)≦62000 (2)

(Cold working S102 and hot working S101)
Here, the method of the cold working S102 and the method of the hot working S101 are not particularly limited when the wire is spring-worked to obtain a spring shape. A known method can be used. Also, the heating method is not limited.
In the method of manufacturing the spring according to the present embodiment, it is important to satisfy the above-described chemical composition and satisfy the quenching conditions and tempering conditions described later.

(Quenching process S103)
Quenching conditions: The heating temperature for quenching springs and steel wires for springs is set to 850°C or higher in order to austenite the structure. On the other hand, if the heating temperature for quenching exceeds 1050° C., coarsening of the austenite crystal grains is caused. Therefore, the heating temperature for quenching must be in the range of 850 to 1050°C. A preferable heating temperature range for quenching is 900°C to 990°C. The heating method may be furnace heating, high-frequency induction heating furnace, or the like. The heating time is usually about 5 to 3000 seconds (s). The spring steel wire may be heated and hot formed into a spring shape, and then quenched by cooling for quenching (so-called hot forming spring). The quenching cooling method may be oil cooling, water cooling, or the like. By quenching, a structure mainly composed of martensite is obtained.

(Tempering process S104)
Tempering conditions: After quenching, tempering is performed under the conditions of formula (2) in order to achieve both high strength and high toughness.
58000≦(T+700×[C%]+50×[Cr%])×(51.7+logt)≦62000 Expression (2)
Here, T: tempering temperature (K), t: tempering time (s), [C %]: C content [mass %], [Cr %]: Cr content [mass %]. Note that logt is the common logarithm of t. The unit of tempering temperature is absolute temperature, not Celsius.

When (T + 700 × [C%] + 50 × [Cr%]) × (51.7 + logt) is less than 58000, the tempering is insufficient, the yield ratio is low, the fatigue properties of the spring are reduced, and the temper embrittlement toughness is lowered by When (T+700*[C%]+50*[Cr%])*(51.7+logt) exceeds 62000, cementite ([theta]) precipitates and toughness decreases. The term of the content of C and Cr in the formula takes into consideration the effect of C and Cr to lower the temperature at which the transition from ε carbide to θ starts and shorten the time. A preferred lower limit for the value of (T+700*[C%]+50*[Cr%])*(51.7+logt) in tempering conditions is 59,000. A preferred upper limit is 61,000. The means for cooling after tempering may be either air cooling or water cooling, and is not particularly specified.

As shown in FIG. 3, the steel wire for spring manufactured by performing the quenching treatment S103 and the tempering treatment S104 while the straight wire rod is straight without being subjected to hot spring processing, is cooled as shown in FIG. Spring processing S100 may be performed between them, followed by strain relief annealing. The strain relief annealing may be carried out under the conditions described in the publication A, for example. Springs manufactured by hot working or cold working are used as suspension springs after undergoing processes such as shot peening, painting, and setting.
Publicly Known Literature A: Spring Papers, No. 58 (2013) p. 49

That is, the spring according to the present invention includes the following two forms.
(1) Spring A: A wire rod, which is a raw material, is drawn to a desired wire diameter, subjected to quenching treatment S103 and tempering treatment S104 to obtain a spring steel wire according to the present invention, and then subjected to cold working. A spring manufactured by applying spring processing S100 to obtain a spring shape and then applying strain relief annealing.
(2) Spring B: A wire rod that can be the material for the spring steel wire according to the present invention is subjected to hot working S101 or cold working S102 to form a spring shape, and then to quenching treatment S103 and tempering treatment S104. Spring manufactured.
In the spring A, the state of the metal structure and iron carbide of the spring steel wire formed by the quenching treatment S103 and the tempering treatment S104 is maintained even after the spring processing S100 and strain relief annealing, which are the subsequent treatments. Therefore, the spring A has the metal structure and iron carbide state common to those of the spring steel wire according to the present invention, and exhibits the same level of characteristics.
In the spring B, the quenching treatment S103 and the tempering treatment S104, which are heat treatments that affect the metal structure, match the quenching treatment S103 and the tempering treatment S104 in the spring steel wire manufacturing process. It has the same metal structure and iron carbide state as wires, and exhibits the same level of characteristics.
That is, the springs (spring A and spring B) according to the present invention have the same chemical composition, metallographic structure and state of iron carbide as the steel wire for springs according to the present invention, although their shapes are not steel wires.

Since the spring and the steel wire for the spring according to the present embodiment are quenched and tempered by the above method, the structure thereof is a tempered martensitic structure. The term "tempered martensite structure" as used herein refers to a structure mainly composed of tempered martensite formed by quenching and tempering, and includes retained austenite and iron carbide. In the metal structure, ferrite, pearlite, and bainite, which can be clearly distinguished from the tempered martensite structure, must be suppressed to a total area ratio of 5% or less. Insufficient quenching results in formation of a structure containing more than 5% of non-tempered martensite (ferrite, pearlite, bainite), resulting in insufficient strength. When measuring the metallographic structure, the point (center and surface at a point equidistant from both), and measured by observing an area with a field width of 100 μm×100 μm with an optical microscope.

The invention is further illustrated by the following examples.

Steel materials having chemical compositions shown in Table 1 were produced by converter smelting and continuous casting, or by test furnace vacuum melting and ingot casting. Contents of alloys not intentionally added are left blank in Table 1. In addition, the unit of all chemical components shown in Table 1 is % by mass, and the balance is iron and impurities. In Table 1, values outside the scope of the invention are underlined. In other tables as well, values outside the scope of the invention or values not meeting the pass/fail criteria are underlined.
If necessary, the steel material was subjected to soaking diffusion treatment. After that, the continuously cast material was made into a wire rod shape having a diameter of 13 mm through blooming and hot rolling processes. The ingot cast material was formed into a wire shape with a diameter of 13 mm through hot forging and cutting.

Next, the wire was drawn to a diameter of 12 mm, heated to 950° C. in a heating furnace, held for 30 minutes (1800 s), and then immediately put into oil (oil quenching) to quench the wire. . However, for some wires, instead of oil quenching, they were quenched by heating to 980° C. in a high-frequency induction heating furnace, holding for 10 seconds, and then cooling with a fountain (water quenching). Next, the quenched wire was tempered under the conditions shown in Table 2. The tempering method was furnace heating and air cooling for the oil-quenched material, and high-frequency induction heating and water cooling for the water-quenched material.

From the quenched and tempered spring steel wire, a No. 14A tensile test piece with a parallel part diameter of 6 mm was produced in accordance with JIS Z 2241: 2011, and a length of 55 mm was produced in accordance with JIS Z 2242: 2005. A 2 mm-U notch impact test specimen (subsize, width 5 mm) was prepared. Each of these test pieces was subjected to a tensile test according to JIS Z 2241:2011 and a Charpy impact test according to JIS Z 2242:2005.

Also, a part of each spring steel wire was cut out to confirm the formation state of the tempered martensite structure. Specifically, each spring steel wire was cut perpendicularly to the longitudinal direction and polished to prepare a test piece for metallographic observation. Assuming that the radius of the spring steel wire is R, a range with a field of view of 100 μm×100 μm was confirmed with an optical microscope at a point at a depth of R/2 from the surface, and five structures corresponding to non-tempered martensite were found. % or more was confirmed. As a result, as shown in Table 3, no non-tempered martensite of 5% or more was observed in the metal structure in any of the test pieces, indicating that a tempered martensite structure was formed in any of the test pieces. (In Table 3, "Tempered M" in the structure column indicates that the metal structure is a tempered martensite structure.)

The mechanical properties of the spring steel wire were evaluated as follows. In the tensile test, the tensile strength and 0.2% proof stress were measured to obtain the yield ratio. The tensile strength and yield ratio suitable for high-strength suspension springs are 2000 MPa or more and 85% or more, respectively. If this condition was satisfied, it was determined that the strength and settling properties were good when used as a suspension spring. The test temperature of the Charpy impact test was 20°C. Steel wires with an impact value of 60 J/cm ² or more were considered to have good toughness.

Further, a test piece (3 mm in diameter×1 mm in thickness) for differential scanning calorimetry was sampled from the quenched and tempered spring steel wire. DSC-60 manufactured by Shimadzu Corporation was used as a differential scanning calorimeter. The measurement conditions for differential scanning calorimetry were as follows.
Atmospheric gas: N ₂ (30 ml/min)
・Measurement temperature range: 50°C to 600°C
・Cell: Made of aluminum ・Reference: α-Al ₂ O ₃
・Temperature increase rate: 0.25°C/s
A DSC curve was measured under these conditions, and the area ratio (%) of the peak region below 450° C. in the DSC curve of the exothermic reaction was calculated. These test results are summarized in Table 3. Here, the "450°C or lower peak area ratio (%)" in Table 3 refers to the area of the exothermic reaction peak at 450°C or lower relative to the total area 3 of the exothermic reaction peak. It means the ratio of the area 4 of the region. In addition, as described above, the spring of the present invention has the same chemical composition and properties as the spring steel wire of the present invention. Therefore, in the examples, since the chemical composition and mechanical properties of the spring are the same as those of the spring steel wire, description of these values is omitted.
In addition, test number No. in Table 3. A peak of 10 mW/g or more was observed as an exothermic reaction corresponding to peak 2 in all of 1 to 35 and 2a.

As shown in Table 3, example no. Spring steel wires Nos. 1 to 23 had excellent properties.

On the other hand, No. In No. 24, sufficient tensile strength was not obtained because the C content was below the range of the present invention.
No. In No. 25, the C content exceeded the range of the present invention, so a high impact value could not be obtained.
No. In No. 26, the Si content was below the range of the present invention, so high tensile strength could not be obtained.
No. In No. 27, sufficient tensile strength was not obtained because the Mn content was below the range of the present invention.
No. In No. 28, the Mn content exceeded the range of the present invention, so a sufficient impact value could not be obtained.
No. In No. 29, the Cr content was below the range of the present invention, so sufficient tensile strength could not be obtained.
No. In No. 30, the Cr content exceeded the range of the present invention, so a sufficient impact value could not be obtained.
No. In No. 31, the Ti content was below the range of the present invention, so a sufficient impact value could not be obtained.
No. In No. 32, the B content was below the range of the present invention, so a sufficient impact value could not be obtained.
No. In No. 33, the Ti content and N content were within the range of the present invention, but Ti-3.5N was below the range of the present invention, so sufficient impact value could not be obtained.
No. In No. 34, the chemical composition was within the scope of the present invention, but the tempering conditions were inappropriate. No. In the manufacturing stage of No. 34, as a result of excessive tempering, No. In the DSC curve of No. 34, the high-temperature side peak became smaller and the ratio of the low-temperature side region increased relatively, so the area ratio of the region below 450 ° C. among the exothermic reaction peaks was the total peak area of the exothermic reaction peaks. exceeded 30% of Therefore, No. No. 34 did not have sufficient strength and impact value.
No. In No. 35, although the chemical composition was within the scope of the present invention, the tempering conditions were inappropriate, and sufficient yield ratio and impact value could not be obtained. In addition, No. In the DSC curve of No. 35, the area of the exothermic reaction peak below 450° C. exceeded 30% of the total peak area of the exothermic reaction peak. Therefore, No. It is presumed that the spring steel wire of No. 35 was deficient in ε carbide.

As described above, the invention examples described in the table assume steel that has not been subjected to machining and stress relief annealing after quenching and tempering (spring steel wire and spring that has been quenched and tempered after spring forming: spring B described above). It is what I did. In order to confirm that the spring steel wire belonging to this steel has the necessary mechanical properties even after being subjected to machining and strain relief annealing to make a spring (the above-mentioned spring A), further investigation was carried out. did Specifically, No. The same applies to the sample (denoted as No. 2a) obtained by subjecting the spring steel wire of No. 2 to cold drawing with an area reduction of 5% and then performing strain relief annealing at 400° C. for 30 minutes. I did some research.
As a result, no significant change was observed before and after machining and stress relief annealing with respect to the DSC curve, tensile strength, yield ratio and impact value. That is, the steel wire for spring according to the present invention which is machined and strain relief annealed to make a spring exhibits the same characteristics as the steel wire in the DSC curve and exhibits excellent characteristics as a spring.

1 first apex of peak 2 second apex of peak 3 peak area of exothermic reaction peak 4 area of region below 450° C. among exothermic reaction peaks

Claims (7)

In mass %, as a chemical component,
C: more than 0.50% and 0.60% or less,
Si: 1.00 to 3.00%,
Mn: 0.10-1.50%,
Cr: 0.15-1.20%,
Al: 0.001 to 0.050 %,
P: 0.015% or less,
S: 0.015% or less,
Ti: 0.010 to 0.100%,
B: 0.0010 to 0.0060%, and N: 0.0070% or less,
and the balance consists of Fe and impurities,
The content of Ti and N satisfies Equation 1,
In a cross section cut perpendicular to the longitudinal direction, the metal structure at a point equidistant from both the center and the surface is the tempered martensite structure,
Among the peaks of the exothermic reaction obtained by measuring the differential scanning calorimetry by increasing the temperature from 50°C to 600°C at a rate of 0.25°C/s, the area of the region below 450°C is the peak of the exothermic reaction. 30% or less of the total peak area of the spring steel wire.
[Ti]≧3.5×[N] (Formula 1)
In Formula 1, [Ti] and [N] are substituted with the above contents in mass % of Ti and N. Furthermore, in mass%, as the chemical component,
Mo: 0 to 1.00%,
Ni: 0 to 1.00%,
Cu: 0-1.00%
Nb: 0 to 0.100%,
V: 0-0.50%
Sb: 0-0.050%
The steel wire for a spring according to claim 1, characterized by containing one or more of A spring having the chemical composition according to claim 1 or 2,
Among the peaks of the exothermic reaction obtained by measuring the differential scanning calorimetry by increasing the temperature from 50°C to 600°C at a rate of 0.25°C/s, the area of the region below 450°C is the peak of the exothermic reaction. 30% or less of the total peak area of the spring. A method of manufacturing the steel wire for spring according to claim 1 or 2, wherein the wire rod composed of the chemical composition according to claim 1 or 2 is heated to 850 to 1050° C. and quenched;
A step of tempering under conditions where the tempering temperature T [K], the tempering time t [s], the C content C% [mass%], and the Cr content C% [mass%] satisfy Equation 2; ,
A method for manufacturing a steel wire for a spring.
58000≦(T+700×[C%]+50×[Cr%])×(51.7+logt)≦62000 (Formula 2) A method for manufacturing the spring according to claim 3,
a step of cold-working a wire comprising the chemical composition according to claim 1 or 2 into a spring shape;
A step of heating to 850 to 1050 ° C. and quenching;
Tempering temperature T [K], tempering time t [s], C content C% [mass%] and Cr content C% [mass%] satisfy formula 2. A step of performing tempering treatment A method of manufacturing a spring comprising:
58000≦(T+700×[C%]+50×[Cr%])×(51.7+logt)≦62000 (Formula 2) A method for manufacturing the spring according to claim 3,
A step of hot-working a wire made of the chemical composition according to claim 1 or 2 to a spring shape by heating it to 850 to 1050° C.;
a step of quenching the wire having the spring shape;
A step of tempering under conditions where the tempering temperature T [K], the tempering time t [s], the C content C% [mass%], and the Cr content C% [mass%] satisfy Equation 2. and,
A method of manufacturing a spring comprising:
58000≦(T+700×[C%]+50×[Cr%])×(51.7+logt)≦62000 (Formula 2) A method for manufacturing a spring, comprising the step of subjecting the spring steel wire according to claim 1 or 2 to spring processing.

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