EP2832876B1

EP2832876B1 - High-strength stainless steel wire having excellent heat deformation resistance, high-strength spring, and method for manufacturing same

Info

Publication number: EP2832876B1
Application number: EP13767507.0A
Authority: EP
Inventors: Masayuki Tohjoh; Kohji Takano; Haruhiko Kajimura; Tsuneo Akiura; Gou TOYODA
Original assignee: Nippon Seisen Co Ltd; Nippon Steel and Sumikin Stainless Steel Corp
Current assignee: Nippon Seisen Co Ltd; Nippon Steel Stainless Steel Corp
Priority date: 2012-03-29
Filing date: 2013-03-27
Publication date: 2019-11-20
Anticipated expiration: 2033-03-27
Also published as: JP2013227662A; EP2832876A4; EP2832876A1; TW201346044A; TWI491745B; CN104136645B; JP6259579B2; KR20140117568A; KR101615844B1; HK1205769A1; WO2013146876A1; CN104136645A

Description

TECHNICAL FIELD

The present invetion relates to a high-strength stainless steel wire used as parts which require heat resistance and high-strength properties, such as automobile engine exhaust system parts and electrical components, and mainly used as a heat resistant steel wire rod for such as a heat setting resistant spring, and a heat resistant rope. The present invetion relates to a precipitation hardening metastable austenitic stainless steel wire with high-strength having a metallographic structure of austenite (γ) phase + deformation induced martensite (α') phase, and fine precipitates are controlled by cold working and aging treatment with addition of Mo, Al, and the like. Particularly, the present invetion relates to a high-strength heat resistant stainless steel wire, a high-strength spring using the same, and particularly to, a high-strength heat resistant spring, and a method for manufacturing the same.

BACKGROUND ART

Conventionally, as a high-strength spring material, a piano wire, and a high-strength stainless steel wire of SUS304, and SUS 301, and the like have been used. However, conventional spring products have sufficient strength at room temperature. However, for example, in a case of a piano wire, since heat setting resistance is rapidly reduced to 0.01% or more in terms of a residual shearing strain, which will be described later, in a warm temperature region in which an environment temperature is in a range of about 100°C to 300°C, there has been a limitation in application. This tendency is also found in a case of a stainless steel wire. Thus, for example, an austenitic stainless steel wire to which Mo, Al, Ti, and the like are added has been proposed (Patent Documents 1 and 2). The heat setting resistance is improved by such component adjustment. However, the amount of deformation induced martensite is small and the tensile strength is less than 1800 MPa. Thus, the strength is insufficient and is not sufficient as a high-strength spring product.
In addition, martensite stainless steel obtained by using precipitation hardening of Mo, Al, and the like has been also proposed (Patent Document 3). However, since the content of C is high, and martensite is already formed in the stainless steel after heat treatment, workability is deteriorated. Further, a large amount of work hardening cannot be expected and the strength is not sufficient as a high-strength spring product.
Further, precipitation hardening austenitic steel with high-strength obtained by using precipitation hardening of Mo, Al, Cu, and the like also has been proposed (Patent Document 4). However, since a large amounts of Ni and Cu are contained in the stainless steel, high material costs are required. In addition, since deformation induced martensite is suppressed in the stainless steel, heat setting resistance is not satisfactory.
In this manner, both of high strength and excellent heat setting resistance cannot be obtained in the conventional high-strength spring stainless steel wire.
Patent Document 5 discloses precipitation hardenable martensitic stainless steels for the production of stainless springs with a composition consising of (in wt.%) up to 0.08 C, 0.5-4 Si, up to 4.0 Mn, 5-9 Ni, 10-17 Cr, more than 0.3-2.5 Mo, 0.15-1 Ti, up to 1 Al, up to 0.03 N, balance Fe and unavoidable impurities.
Patent Document 6 discloses precipitation hardenable stainless steel of such composition balance as to be martensitic in the solution-treated condition and precipitation-hardenable both by single heat-treatment and by double heat-treatment.

PRIOR ART DOCUMENTS

Patent Documents

Patent Document 1: Japanese Patent No. 4163055
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. H10-68050
Patent Document 3: Japanese Patent No. 3482053
Patent Document 4: Japanese Patent No. 4327601
Patent Document 5: US5035855 A
Patent Document 6 : FR1402682 A

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

An object of the present invention is to provide a high-strength stainless steel wire having both of sufficient high-strength properties and heat setting resistance (heat deformation resistance) even under a temperature environment on the assumption that the stainless steel wire is used as a heat resistant material for particularly a heat setting resistant spring which is frequently used in the warm temperature region, and to provide a high-strength spring made of the steel wire, and a method for manufacturing the same. Means for Solving the Problems
As a result of various studies for solving the above problem, the inventors have reached a conclusion that it is effective to further greatly improve strength and heat setting resistance in a precipitation hardening metastable austenitic stainless steel wire by the following points, and have completed the present invention.

1) A large amount of a deformation induced martensite (ausformed martensite) structure is formed from a structure mainly including austenite by controlling austenite stability through hard working such as cold wire drawing is performed before forming a wire into a spring shape or the like. Thus, strength is improved while maintaining ductility.
2) The amounts of C and N are controlled to be in a range of 0.05 ≤ (C + N) ≤ 0.13; and thereby, ductility is secured while maintaining strength.
3) Fine compounds of Ni, Al, and Mo are uniformly dispersed particularly in a hard worked deformation induced martensite structure near the surface layer of the steel wire by adding Al and Mo in combination of hard working and aging treatment condition.

That is, an aspect of the present invention is provided with the following features.

(1) A high-strength stainless steel wire having excellent heat setting resistance including, by mass%: C: 0.04 % to 0.12%; N: 0.005% to 0.03%; 0.05% ≤ (C + N) ≤ 0.13%; Si: 0.1% to 2.0%; Mn: 0.1% to 2.0%; Ni: 6.8% to 9.0%; Cr: 12.0% to 14.4%; Mo: 1.0% to 3.0%; and Al: 0.5% to 2.0%, further optionally comprising one or more selected from V: 0.01% to 1.0%, Nb: 0.01% to 1.0%, Ti: 0.01% to 1.0%, W: 0.05% to 2.0%, Ta: 0.05% to 2.0%, Cu: 0.8% or less, Co: 0.1% to 2.0%, B: 0.0005% to 0.015%, Ca: 0.0005% to 0.01%, Mg: 0.0005% to 0.01% and REM: 0.0005% to 0.1% and/or P:0.015 to 0.045%, and/ or S: 0.0001% to 0.01%, and with a balance being Fe and unavoidable impurities, wherein a deformation induced martensite formation index MdS value expressed by Expression (1) is in a range of 15 to 60, an amount of deformation induced martensite is in a range of 80 vol% to 99 vol% in a matrix, and a tensile strength is in a range of 1800 MPa to 2200 MPa, $MdS = 551 - 462 (C + N) - 9.2 Si - 8.1 Mn - 29 (Ni + Cu) - 13.7 Cr - 18.5 Mo$
wherein chemical symbols in the expression represent contents (mass%) of respective elements.
(2) The high-strength stainless steel wire having excellent heat setting resistance according to (1), wherein the stainless steel wire has a high torsional performance in which a torsion value that causes fracture without vertical cracks is in a range of 5 times or more when a torsion test is performed in which the stainless steel wire is held in a gage length that is 100 times longer than an equivalent wire diameter, and one end of the steel wire is twisted and rotated.
(3) A high-strength stainless steel wire having excellent heat setting resistance and a tensile strength in a range of 2100 MPa to 2600 MPa, obtainable from aging treatment of the stainless steel wire according to (1) or (2).
(4) The high-strength stainless steel wire for a heat resistant spring having excellent heat setting resistance according to any one of (1) to (3), wherein a proof stress ratio {(σ_0.2/σ_B) × 100} of a tensile strength (σ_B) and a 0.2% proof stress (σ_0.2) is in a range of 80% to 95 %.
(5) A high-strength spring having excellent heat setting resistance, wherein the spring includes the stainless steel wire according to any one of (1) to (4), and a residual shearing strain ε expressed by Expression (2) at an environment temperature of 200°C satisfies ε ≤ 0.008%, $Residual shearing strain ε = \{8 Δ PD / {πd}^{3} G\} \times 100$
wherein, ΔP: a load loss (N), D: a central diameter of spring (mm), d: an equivalent wire diameter of the steel wire (mm), and G: a transverse elastic coefficient of the steel wire (N/mm²).
(6) The high-strength spring having excellent heat setting resistance according to (5), wherein fine NiAl-based compound particles having particle sizes of 50 nm or less are included in a matrix of the steel wire.
(7) A method for manufacturing a high-strength spring having excellent heat setting resistance the method comprising: subjecting a cast steel having the component composition according to (1) in which a deformation induced martensite formation index MdS value expressed by Expression below is in a range of 15 to 60, to casting and hot rolling to form a wire rod; subjecting the wire rod to cold working and solution treatment repeatedly and setting a total working ratio to be in a range of 60 area% to 90 area% in cold working after final solution treatment so as to manufacture a stainless steel wire of which a wire diameter is reduced to be a target wire diameter; and forming the stainless steel wire into a predetermined spring shape, and then performing aging treatment at a temperature of 300°C to 600°C, wherein a high-strength spring in which a residual shearing strain ε expressed by Expression below at an environment temperature of 200°C satisfies ε ≤ 0.008%, is manufactured by the manufacturing steps, MdS = 551 - 462(C + N) - 9.2Si - 8.1Mn - 29(Ni + Cu) - 13.7Cr -18.5Mo wherein chemical symbols in the expression represent contents (mass%) of respective elements, Residual shearing strain ε = {8ΔPD/πd3 G} x 100 ,wherein, ΔP: a load loss (N), D: a central diameter of spring (mm), d: an equivalent wire diameter of the steel wire (mm), and G: a transverse elastic coefficient of the steel wire (N/mm2).
(8) The method for manufacturing a high-strength spring having excellent heat setting resistance according to (7), wherein in the manufacturing of the stainless steel wire, aging treatment is performed after the final solution treatment and final cold working.
(9) The method for manufacturing a high-strength spring having excellent heat setting resistance according to (7) or (8), wherein the aging treatment is performed under the condition in which an aging treatment factor of Expression below is in a range of 100 to 10000, and thereby, fine NiAl-based compound particles having particle sizes of 50 nm or less are precipitated in the matrix of the steel wire, $Aging treatment factor = \{temperature (° C) \times treatment time (\min)\} / 2 \sqrt \{equivalent wire diameter (mm) \times deployed length (mm) of spring\}$

Effects of the Invention

The precipitation hardening high-strength stainless steel wire having excellent heat setting resistance according to the aspect of the present invention has a large amount of deformation induced martensite (α') and a predetermined tensile strength in a wire drawing stage. In addition, the high-strength stainless steel wire according to the aspect of the present invention is formed into a spring shape and then subjected to aging treatment to facilitate formation of fine compounds, particularly, precipitation of fine compounds uniformly dispersed in the surface layer of the steel wire. Accordingly, it is possible to impart higher strength and heat setting resistance, particularly in a warm temperature region. Therefore, it is possible to provide a high-strength spring product having both of high strength and excellent heat setting resistance, which are hardly achieved in a conventional steel wire. Accordingly, the high-strength stainless steel wire according to the aspect of the present invention is suitable for a high-strength spring which particularly requires severe quality properties.
Further, the method for manufacturing a spring according to the aspect of the present invention can be performed in a normal low-temperature heat treatment range and can be performed stably without particular cost increase by continuously performing treatments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of an enlarged image of a fracture surface by a torsion test, (a) shows a good torsion fracture surface, and (b) shows a twisted and cracked fracture surface.
FIG. 2 is an illustration illustrating a method for measuring spring properties, (a) shows a spring before a compression load is applied, (b) shows a spring in a state in which a compression load is applied, and (c) shows a spring in a state in which a compression load is released.
FIG. 3 is a photomicrograph showing an example of a precipitated compound formation state by aging treatment and a molecular model of NiAl. (a) is a bright field image, (b) is a diffraction image, (c) is a dark field image, and (d) is a molecular model of NiAl having a B2 structure.
FIG. 4 is a diagram showing an example of an evaluation result, (a) is a diagram showing a relationship between an aging treatment temperature and tensile strength, and (b) is a diagram showing an example of a relationship between an aging treatment temperature and residual shearing strain properties.

EMBODIMENTS OF THE INVENTION

A high-strength stainless steel wire having excellent heat setting resistance according to an embodiment contains, by mass%, C 0.04% to 0.12%, N: 0.005% to 0.03%, 0.05% ≤ (C + N) ≤ 0.13%, Si: 0.1% to 2.0%, Mn: 0.1% to 2.0%, Ni: 6.8% to 9.0%, Cr: 12.0% to 14.4%, Mo: 1.0% to 3.0%, and Al: 0.5% to 2.0%, further optionally comprising one or more selected from V: 0.01% to 1.0%, Nb: 0.01% to 1.0%, Ti: 0.01% to 1.0%, W: 0.05% to 2.0%, Ta: 0.05% to 2.0%, Cu: 0.8% or less, Co: 0.1% to 2.0%, B: 0.0005% to 0.015%, Ca: 0.0005% to 0.01%, Mg: 0.0005% to 0.01% and REM: 0.0005% to 0.1% and/or P:0.015 to 0.045%, and/or S: 0.0001% to 0.01%, and with a balance being Fe and unavoidable impurities. A deformation induced martensite (α') formation index (hereinafter, simply referred to as a "formation index") MdS value expressed by the following Expression (1) is in a range of 15 to 60.
The amount of deformation induced martensite (α') in the matrix is in a range of 80 vol% to 99 vol%, and the tensile strength is in a range of 1800 MPa to 2200 MPa. The high-strength stainless steel wire according to the embodiment is a heat resistant stainless steel wire having high-strength, and is suitable for, for example, a spring wire rod, particularly, a wire rod that is used in a warm temperature region in which an environment temperature is in a range of 100°C to 300°C. $MdS = 551 - 462 (C + N) - 9.2 Si - 8.1 Mn - 29 (Ni + Cu) - 13.7 Cr - 18.5 Mo$
Wherein, the chemical symbols in the expression represent the contents (mass%) of the respective elements. In addition, in the case where there is no element necessary for calculation, or in the case where there is an element whose content is not clear, 0 is substituted for the content of the element.
The shape of the stainless steel wire is not particularly limited and the stainless steel wire of the embodiment is frequently used as a normal wire rod, for example, as a fine wire for secondary working having a wire diameter of 6 mm or less, more specifically, having a wire diameter of about 0.05 mm to 3 mm. In addition, the shape thereof is not particularly limited, and for example, the stainless steel wire of the embodiment is used as a non-circular wire rod such as a flat wire or a square wire, in addition to a round wire. However, there is no limitation thereto and the wire can be applied to various shapes. In this manner, the shape of the stainless steel wire of the embodiment also includes a wire rod whose cross section shape is not circular, and thus, regarding the wire diameter indication, for example, an equivalent wire diameter (d) calculated from an area of an arbitrary transverse cross section of the wire is used.
In the embodiment, a case in which the round wire is used as a target for manufacturing a wire by wire drawing is mainly described, but, for example, a combined working in which rolling and the wire drawing are combined can be employed, instead of only the wire drawing.
In addition, the stainless steel wire has a precipitation hardening function, and fine compound particles are precipitated and distributed in the matrix by aging treatment which is performed in the final stage. In the embodiment, in addition to a precipitation element such as Al or Mo, an appropriate amounts of N and C are added to the composition so as to exhibit the precipitation hardening function. Then, NiAl or Mo based compound particles are uniformly dispersed and precipitated in the deformation induced martensite phase near the surface layer of the steel wire which is subjected to hard working according to conditions in drawing such as cold wire drawing, cold rolling, and the like. Thus, it is possible to provide a high-strength heat resistant spring product having high strength and excellent heat setting resistance.
Generally, it is well known that austenitic stainless steel is work-hardened by cold working, and one of the factors is an influence of a deformation induced martensite phase induced by the working. However, the amount of deformation induced martensite (the amount of formed deformation induced martensite) varies greatly depending on the balance of component composition of each element constituting the steel and the working conditions. For example, in stable SUS316 stainless steel, even in the case where normal working treatment is performed, the amount of formed deformation induced martensite is only about several percents. In contrast, in the embodiment, formation of deformation induced martensite due to cold working is actively facilitated and the composition is adjusted such that the amount of the formed deformation induced martensite is increased to 80 vol% to 99 vol%. Accordingly, the tensile strength of the steel wire itself is increased to 1800 MPa to 2200 MPa in a state of being subjected to cold working such as wire drawing, to achieve high strength, which is one of the features of the embodiment.
As a further method for improving heat setting resistance of a spring product with the high-strength properties, the composition is adjusted so that a deformation induced martensite formation index MdS value is in a range of 15 to 60, and the stainless steel is subjected to wire drawing under predetermined working conditions. Thus, formation of deformation induced martensite which becomes precipitation nuclei of fine precipitates is facilitated. The formation index MdS value is an index indicating the balance of each component composition.
The MdS value refers to a temperature at which 50% of the structure is transformed into a martensite phase when 30% of tensile deformation is applied to the stainless steel, and a level of the amount of deformation induced martensite formed during working is grasped from a relationship with the component elements.
Accordingly, it is possible to increase the amount of deformation induced martensite during the wire drawing, which contributes to high-strengthening.
In the embodiment, the reason why MdS value is set in the above range is as follows. In the case where the MdS value is less than 15, stabilization of the austenite phase is increased and the amount of deformation induced martensite is decreased to less than 80 vol% after wire drawing, and thus, high-strengthening is hardly achieved. In addition, the amount of precipitation hardening by aging treatment at a temperature of 300°C to 600°C is reduced and heat setting resistance is also deteriorated. On the other hand, in the case where the MdS value is more than 60, surplus deformation induced martensite of more than 99 vol% is formed in predetermined wire drawing, and ductility and toughness is decreased after the wire drawing, and thus, manufacturability is deteriorated. The range of the MdS value is more preferably in a range of 20 to 50.
By adjusting the components in that manner, the amount of deformation induced martensite can be set to be in a range of 80 vol% to 99 vol% and improvement of each property is facilitated in the stainless steel wire of the embodiment. That is, in the case where the amount of deformation induced α' (martensite) is less than 80 vol% in the matrix, even when aging treatment is performed, required high-strength properties cannot be obtained in the spring product. Contrarily, in the case where the amount of deformation induced α' (martensite) is more than 99 vol%, structure stability is poor, and corrosion resistance and toughness are hardly satisfied. In addition, deterioration in spring fatigue resistance is expected. The amount of deformation induced martensite is preferably in a range of 83 vol% or more and more preferably in a range of 85 vol% or more. Further, the amount of deformation induced martensite is preferably in a range of 95 vol% or less, and more preferably in a range of 90 vol% or less.

[Measurement of Amount of Martensite]

As a method for measuring an amount of martensite, for example, various methods such as a method using a ferrite scope, a magnetic method, and a method using an X-ray can be employed and a test sample which is arbitrarily collected from the stainless steel wire is used in the measurement. Regarding the magnetic method, there are many descriptions in "Iron and Steel" (81-S1163) of The Iron and Steel Institute of Japan, and the like.
In the embodiment, a saturation magnetization value of the wire rod was measured at 1.0 × 10⁴ Oe using a DC flux meter, and the amount of deformation induced martensite (α') was calculated by using the following Expressions (4) to (6). $Amount of stain induced α' (vol %) = σ_{s} / σ_{s} (bcc) \times 100$
σ_s: a saturation magnetization value (T), σ_s(bcc): a saturation magnetization value (calculated value) when the structure is completely transformed into α' $σ_{s} (bcc) = 1.83 - 0.030 {Cr}_{eq}$
${Cr}_{eq} = Cr + 1.8 Si + Mo + 0.5 Ni + 0.9 Mn + 3.6 (C + N) + 1.25 P + 2.91 S + 1.85 Al$
Wherein, the chemical symbols in the expression represent the contents (mass%) of the respective elements.
The stainless steel wire has high-strength properties in which tensile strength (σ_B) is in a range of 1800 MPa to 2200 MPa in a state in which the steel wire is subjected to cold wire drawing. The tensile strength can be measured according to, for example, JIS-Z2241. In the case where the tensile strength is less than 1800 MPa, significant improvement in strength properties cannot be expected even by the following aging treatment. In addition, in the case where the tensile strength is more than 2200 MPa, problems occur in terms of product quality in a spring forming stage, for example, variations in the shape of the spring increases, brittle fracture is easily induced, or the like arises. The tensile strength is more preferably in a range of 1900 MPa to 2100 MPa.
On the other hand, when the stainless steel wire of the embodiment, which is subjected to cold wire drawing, is subjected to aging treatment, strength properties are further rapidly improved. Depending on the aging treatment condition, a preferable value of a tensile strength of 2100 MPa to 2600 MPa can be obtained. Accordingly, for example, in the case where the spring product is used in a linear state such as a microshaft part, the wire is subjected to the wire drawing and then subjected to shape correction, and the shape-corrected wire is subjected to continuous aging treatment to form a long spring material. In this manner, it is possible to further increase mechanical properties in the wire state. These treatments can be continuously performed.
The embodiment includes the stainless steel wire subjected to aging treatment after the cold wire drawing as another configuration. The tensile strength in the case where the stainless steel wire is subjected to aging treatment is in a range of 2100 MPa to 2600 MPa. The lower limit of the tensile strength is more preferably 2200 MPa, and the upper limit thereof is more preferably 2500 MPa. The aging treatment condition of the steel wire can be appropriately set so that the tensile strength is within the above range after the aging treatment is performed. Examples of the condition include the aging treatment condition after spring forming which will be described later.
Further, a proof stress ratio {(σ_0.2/σ_B) × 100} of the tensile strength (σ_B) to 0.2% proof stress (σ_0.2) can be obtained with the tensile strength (σ_B). The proof stress ratio is preferably in a range of 80% to 95%. Such stainless steel wire is effective as a heat resistant spring material in which the strength is high and the fatigue fracture is improved. In the case where the proof stress ratio is less than 80%, predetermined elastic properties cannot be obtained. In the case where the proof stress ratio is more than 95%, there is a concern of adverse influence on a yield during severe spring working. The lower limit of the proof stress ratio is more preferably 83%, and the upper limit thereof is more preferably 91%.

[Torsion Test 1]

Further, as other properties for evaluating spring workability, twisting properties measured by the following torsion test can be mentioned. The twisting properties are measured as follows. While a test sample collected from the stainless steel wire is held in a gage length which is 100 times longer than the equivalent wire diameter, one end of the test sample is twisted and rotated. Then, a torsion number is measured until the stainless steel wire is fractured. The torsion number (torsion value) indicates torsional performance. For example, when the cold-worked stainless steel wire has high torsional performance in which the torsion number is 5 times or more, for example, about 5 times to 10 times or more without vertical cracks, the steel wire can be widely used in various spring products.
In the stainless steel wire which is subjected to the aging treatment, and the stainless steel wire whose proof stress ratio is more than 95%, the torsional performance is about 2 times or 3 times, or vertical cracks easily occur. Therefore, for example, when the steel wire is worked into a spring under severe conditions as in the case where the steel wire is worked into a coil spring in which a ratio D/d of a wire diameter (d) to an average coil diameter (D) is 4 times or less, there is a concern of adverse influence on a yield. That is, a spring can be formed irrespective of the torsion value, but a stainless steel wire exhibiting a torsion value of 5 times or more without vertical cracks is preferable for spring forming and the torsion value is more preferably 6 times or more.

[Torsion Test 2]

In torsion test 2, for example, as described in JIS-G4314, the stainless steel wire is twisted and rotated until the steel wire is fractured. Then, the toughness state of the stainless steel wire is evaluated by observing the fracture surface.
FIG. 1 shows an example of a fracture surface. FIG. 1(a) shows an almost uniform fracture surface and the fracture surface is good. On the other hand, in FIG. 1(b), torsional cracks and brittle fracture are observed in a part of the transverse cross section. In a stainless steel wire in which a good fracture surface can be obtained as in the former case, the above torsion number can be satisfied.
Next, the reason why each constitutional element of the stainless steel wire that is a target in the embodiment is limited will be described. In the embodiment, unless otherwise specified, the unit of the content of each element is mass%.
0.04% or more (hereinafter, all mass%) of C is added so as to obtain high strength after wire drawing. However, in the case where the added amount of C is more than 0.12%, sensitization occurs, corrosion resistance is deteriorated, and manufacturability is also deteriorated. Therefore, the upper limit of the amount of C is set to 0.12%. The amount of C is preferably less than 0.10%, and more preferably in a range of 0.04% to 0.09%.
N is an element which contributes to increasing strength and is effective in refining grains of the material before cold working during solution treatment by forming carbonitrides. Therefore, 0.005% or more of N is added. However, in the case where the added amount of N is more than 0.03%, coarse nitrides such as AlN and the like are formed, and ductility and toughness are deteriorated. Thus, manufacturability is greatly deteriorated. Accordingly, the upper limit of the amount of N is set to 0.03%. The lower limit of the N content is preferably 0.01%, and the upper limit thereof is preferably 0.025%.
Both of C and N are interstitial elements and cause strain to contribute to solute strengthening acting on strengthening. In addition, C and N are effective in forming a Cottrell atmosphere or fine carbonitrides and fixing dislocations in the metallographic structure. In order to obtain these effects, a total amount (C + N) of added C and N is 0.05% or more. However, in the case where a total amount (C + N) of added C and N is more than 0.13%, ductility and toughness are deteriorated. Therefore, the upper limit of C + N is set to 0.13%. The amount of C + N is preferably in a range of 0.08% to 0.11%.
Since Si causes deoxidation, 0.1% or more of Si is added. However, in the case where the added amount of Si is more than 2.0%, the effect is saturated and also manufacturability is deteriorated. Thus, the upper limit of the amount of Si is set to 2.0%. The amount of Si is preferably in a range of 0.3% to 1.0%.
Since Mn causes deoxidation, 0.1% or more of Mn is added. However, in the case where the added amount of Mn is more than 2.0%, corrosion resistance is deteriorated. In addition, the amount of deformation induced martensite (α') is reduced and thus, strength is decreased. Also, heat setting resistance is deteriorated. Thus, the upper limit of the amount of Mn is set to 2.0%. The amount of Mn is preferably in a range of 0.5% to 1.5%.
6.8% or more of Ni is added to secure the ductility and toughness of the material and to obtain an appropriate amount of deformation induced martensite in wire drawing. However, in the case where the added amount of Ni is more than 9.0%, the MdS value is decreased and the amount of deformation induced martensite is reduced, and thus, strength is decreased. Further, heat setting resistance is also deteriorated. Therefore, the upper limit of the amount of Ni is set to 9.0%. The amount of Ni is preferably in a range of more than 7.0% and 8.5% or less, and more preferably in a range of 7.5% to 8.2%.
12.0% or more of Cr is added to secure corrosion resistance and to obtain an appropriate amount of deformation induced martensite. However, in the case where the added amount of Cr is more than 14.4%, the MdS value is decreased and the amount of deformation induced martensite is reduced, and thus, strength is decreased. Further, heat setting resistance is also deteriorated. Therefore, the upper limit of the amount of Cr is set to 14.4%. The amount of Cr is preferably in a range of 13.0% to 14.0%.
Mo is solid-solubilized in an austenite matrix to increase the hardness of the matrix and further Mo alleviates heat setting caused by temperature rising when the steel wire is used. Moreover, Mo allows fine Mo-based metal clusters to be finely precipitated in the deformation induced martensite by aging treatment at a temperature of 300°C to 600°C when a spring is manufactured. Accordingly, strength is increased and heat setting resistance is improved. Therefore, Mo is an effective element in increasing strength and improving heat setting resistance, and 1.0% or more of Mo is added. However, in the case where the added amount of Mo is more than 3.0%, the effect is saturated and the MdS value is decreased. Thus, the amount of deformation induced martensite is reduced and strength is decreased. Also, heat setting resistance is deteriorated. Therefore, the upper limit of the amount of Mo is set to 3.0%. The amount of Mo is preferably in a range of 1.5% to 2.6%, and more preferably in a range of 1.7% to 2.3%.
A1 allows fine NiAl-based metal compounds to be finely precipitated in the deformation induced martensite by, for example, aging treatment at a temperature of 300°C to 600°C when a spring is manufactured. Accordingly, strength is increased and heat setting resistance is improved. Therefore, A1 is an effective element in increasing strength and improving heat setting resistance, and 0.5% or more of A1 is added. However, in the case where the added amount of A1 is more than 2.0%, the effect is saturated and manufacturability is deteriorated. Therefore, the upper limit of the amount of A1 is set to 2.0%. The amount of A1 is preferably in a range of 0.7% to 1.5%, and more preferably in a range of 0.9 % to 1.2%.
The stainless steel wire contains these constitutional elements, and the components are adjusted such that the MdS value is in a range of 15 to 60, and the stainless steel wire includes Fe and unavoidable impurities as the balance. Examples of the unavoidable impurities include O: 0.001% to 0.01%, Zr: 0.0001% to 0.01%, Sn: 0.001% to 0.1%, Pb: 0.00005% to 0.01%, Bi: 0.00005% to 0.01%, Zn: 0.0005% to 0.01%, materials contained in a raw material and a heat resistance material, which are mixed in stainless steel during manufacturing of normal stainless steel, and a total amount of the elements and the materials of 2.0% or less is allowable.
Further, in addition to the above-described constitutional elements, the embodiment can contain at least one selected from the following elements.
A first group includes V, Nb, Ti, W, and Ta, and these elements respectively form fine carbonitrides. Accordingly, these elements contribute to increasing strength and improving heat setting resistance by refining grains. The effects can be obtained by adding at least one selected from V: 0.01% to 1.0% (preferably 0.05% to 0.6%), Nb: 0.01% to 1.0% (preferably 0.05% to 0.4%), Ti: 0.01% to 1.0% (preferably 0.02% to 0.2%), W: 0.05% to 2.0% (preferably 0.05% to 0.5%), and Ta: 0.05% to 2.0% (preferably 0.1% to 0.5%). However, in the case where the added amount of each element is more than each upper limit, carbonitrides are coarsened to deteriorate manufacturability. Accordingly, more preferably, it is recommended to add the elements within the above-described preferable ranges.
A second group includes the following elements, and these elements increase the corrosion resistance, toughness, and workability of the stainless steel wire as an incidental effect. Therefore, at least one selected from the following elements may be added as required.
Cu is an effective element in improving corrosion resistance, and Cu is added as required. However, in the case where the added amount of Cu is more than 0.8%, the amount of work hardening is small. Thus, the steel wire is softened and also heat setting resistance is decreased. Therefore, the upper limit of the amount of Cu is set to 0.8% or less. The amount of Cu is preferably in a range of 0.1% to 0.6%.
Since Co secures ductility and toughness to improve heat setting resistance, 0.1% or more of Co is added as required. However, in the case where the added amount of Co is more than 2.0%, strength is decreased and heat setting resistance is deteriorated. Thus, the upper limit of the amount of Co is set to 2.0%. The amount of Co is preferably in a range of 0.5% to 1.5%.
In addition, since B improves the hot manufacturability and toughness of the stainless steel, 0.0005% or more of B is added as required. However, in the case where the added amount of B is more than 0.015%, boride is formed, and thus, conversely, ductility and toughness are decreased and manufacturability is deteriorated. Thus, the upper limit of the amount of B is set to 0.015%. The amount of B is preferably in a range of 0.001% to 0.01%.
Further, as a third group, Ca, Mg, and REM are selected. These elements can be contained for deoxidation, and one or more of Ca: 0.0005% to 0.01%, Mg: 0.0005% to 0.01%, and REM: 0.0005% to 0.1% are added as required. However, in the case where the added amount of each element is more than each upper limit, coarse inclusions are generated and thus, manufacturability is decreased.
In the embodiment, from the view point of hot workability, ductility, and toughness, the amounts of P and S are preferably adjusted in specific ranges as other elements. The allowable ranges are P: 0.015% to 0.045%, and S: 0.0001% to 0.01%. A reduction of the content more than required leads a rather cost increase. Contrarily, in the case where a large amounts of P and S are contained, nonmetallic inclusions or the like cause deterioration in quality. Among the respective groups, an element can be selected from a single group and added, and an element can be selected from two or more groups and added.
The stainless steel wire of the embodiment thus constituted is manufactured by, for example, the following method. A cast steel having the predetermined component composition is subjected to casting and hot rolling to form a wire rod. Next, the diameter of the wire rod is reduced while the wire rod is subjected to cold working repeatedly. Solution treatment may be performed between the cold working steps. By reducing the diameter, a stainless steel wire having a target wire diameter can be formed. The cold working includes the wire drawing and rolling, and, for example, continuous wire drawing using a drawing die or a roller die, and rolling using a rolling roller are employed. Particularly, in cold working after the final solution treatment, a total working ratio may be in a range of 60% to 90%. Accordingly, the amount of deformation induced martensite (α') in the matrix and the tensile strength defined in the embodiment can be realized and the torsion value and proof stress ratio of the stainless steel wire defined in the embodiment can be realized in the same manner. The total working ratio of the final cold working is preferably in a range of about 65% to 85%, and more preferably in a range of 70% to 83%, and the final cold working may be performed within a range in which the total working ratio is relatively suppressed.
In addition, as a more preferable form of these cold workings, for example, a working temperature is preferably adjusted so that the surface temperature of the steel wire on the inlet side of a final finish die or a final roll is in a range of 70°C or lower (preferably in a range of 10°C to 50°C). In addition, the working ratio in the final finish die or the final rolling is set to be in a range of 20% or less, and preferably in a range of 10% or less, and uniform hard working of the surface layer is preferably performed. Thus, heat setting resistance can be further improved.
The surface temperature of the steel wire on the inlet side of the final finish die and the working ratio in the final finish die or in the final rolling are controlled; and thereby, heat setting resistance is further improved. The mechanism is not clear at this point. However, the inventors have found that the steel wire in the case where these conditions were controlled and the steel wire the case where these conditions were not controlled were respectively subjected to aging treatment, and the vicinities of the surface layers of the steel wires after the aging treatment were observed and compared to each other. As a result, it has been found that fine compounds were uniformly distributed in the steel wire in the case where the conditions were controlled. Thus, it can be assumed that the fine compounds being more uniformly precipitated in the vicinities of the surface layer of the steel wire have influence on further improvement of heat setting resistance.
It is also effective to increase lubricity by applying Ni plating or the like to the surface of the stainless steel wire, if required, and accordingly, the yield can be improved.
The working ratio is expressed by a change ratio of the area of the transverse cross section of the stainless steel wire by the working and is calculated by the following expression. $Working ratio (%) = \{(cross section area before working - cross section area after working) / cross section area before working\} \times 100$

[Method for Manufacturing Spring Product]

Next, a spring product of an embodiment will be described. The spring product is made of the stainless steel wire of the embodiment and can be formed in various shapes such as a coil spring, a torsion spring, and a linear spring. Further, spring properties can be improved by performing aging treatment which will be further described later. In the embodiment, on the assumption that the spring is used in the warm temperature region, particularly, the spring properties are set based on an environment temperature of 200°C, and the residual shearing strain at the temperature is in a range of 0.008% or less.
The heat setting resistance of the spring properties is expressed by a load loss in some cases. For example, as shown in FIG. 2, the spring is deformed to a height corresponding to an arbitrary stress (for example, 400 MPa) and is heated under a predetermined environmental test condition while maintaining the deformed state. Next, a load loss is calculated by dividing a load difference between applied loads corresponding to the heights of the spring before and after the test by an applied load before the test.
However, the load loss measured by the method varies depending on the shape of the spring and is not always standard. Therefore, in the embodiment, instead of the load loss, the residual shearing strain ratio is used. In addition, the environment temperature is also set to 200°C as described above.
A residual shearing strain ε is defined as follows. A predetermined spring is deformed by applying a fixed load or a torque to the spring. Next, the load or the torque is released. A shearing strain ratio remaining when the load or the torque is released is a residual shearing strain ε, and is calculated by, for example, the following Expression (7). That is, for example, a case in which the spring is a compression coil spring will be described. As shown in FIG. 2, a predetermined compression load is applied to the coil spring, and a height of the spring is displaced from S to S1. While the state is maintained, the spring is heated to 200°C. Next, the spring is cooled to room temperature and the compression load is released. Then, a height of the spring when the compression load is released is set to S0, and a load when the height of the spring is returned from S1 to S0 is used to calculate a load loss (ΔP). Specifically, as shown in FIG. 2(b), the height of the spring S1 when the compression load is applied is set to a predetermined set height. FIG. 2(c) shows a spring after being heated in a state in which a predetermined compression load is applied and then cooled and the compression load has been released, and the height of the spring is S0. FIG. 2(a) shows a spring before a test in which a predetermined compression load is applied, and the height of the spring is S. With respect to each spring shown in FIGS. 2(a) and 2(b), a load required for displacement to the height of S1 is measured using a spring load testing machine. A difference between the required loads of the springs is calculated and the value is set to a load loss (ΔP). Then, a residual shearing strain ε is calculated by the following Expression (7) using the load loss. heat setting resistance can be evaluated from the residual shearing strain ε. $Residual shearing strain ε = \{8 Δ PD / {πd}^{3} G\} \times 100$

ΔP: a load loss (N)
D: a central diameter of spring (mm) and a distance between facing center points of the steel wire as shown in FIG. 2(a)
d: an Equivalent wire diameter of steel wire (mm)
G: a transverse elastic coefficient of steel wire (N/mm²), (MPa)

Conventionally, in order to reduce function degradation of spring products while the spring products are used, for example, heat setting treatment is performed. A spring product in which the residual shearing strain is in a range of 0.008% or less and the heat setting resistance is excellent has an advantage of the heat setting treatment being omitted. The residual shearing strain is more preferably in a range of 0.005% or less.
In order to further improve such spring properties, it is recommended to perform, for example, aging treatment. Specifically, a spring product is subjected to heat treatment at a predetermined temperature in advance, and fine compound particles are uniformly precipitated particularly in the vicinity of the surface layer in the structure of the stainless steel wire. In the aging treatment, for example, a heating time is preferably set to be in a range of about 3 minutes to 10 hours in a temperature range of 300°C to 600°C. Thus, for example, as shown in FIG. 3, it is possible to form and distribute fine and hard compounds. As a result, the residual shearing strain of the high-strength spring defined in the embodiment can be realized. Particularly, it is desirable that the components thereof are adjusted in advance so that the aforementioned compounds are precipitated to form precipitation hardening stainless steel by subjecting the stainless steel wire to hard working.
The more preferable condition of the aging treatment is as follows. The shape and the distribution state of the compound particles precipitated by the aging treatment are influenced by the volume and the shape of the spring product. It is desirable that a setting temperature and a heating time are adjusted considering the volume and the shape of the spring product. For example, the setting temperature and the heating time are desirably adjusted such that an aging treatment factor in the following Expression (3) becomes in a range of 100 to 10000, and preferably in a range of 150 to 3000. $Aging treatment factor = \{temperature (° C) \times treatment time (\min)\} / 2 \sqrt \{equivalent wire diameter of spring (mm) \times deployed length (mm) of spring\}$
The deployed length means a total length of a stainless steel wire constituting the spring product.
By such aging treatment, precipitation of the desired compounds in the matrix is realized and material properties are improved.
In the case where a heating temperature of the aging treatment is lower than 300°C, a sufficient amount of the compounds are not formed even in the case where the wire is heated for a long period of time. In the case where a heating temperature of the aging treatment is higher than 600°C, the stainless steel wire is softened and the strength is decreased. It is recommended that the aging treatment is more preferably performed at a temperature of about 400°C to 580°C. In addition, the formation and the precipitation state of the compounds depend on the heating time, and the particle size and the density change. Therefore, heating is preferably performed for at least 3 minutes or longer. Proper ranges of the heating temperature and the heating time are set by the aforementioned Expression (3) including the condition. The proper range of the heating temperature is more preferably in a range of 400°C to 550°C.
Since the aforementioned compounds are very fine, it is difficult to define the presence of the compounds specifically throughout most of the aforementioned condition range of the aging treatment. However, it is possible to confirm the presence of the compounds with a three-dimensional atom probe or a transmission electron microscope. Particularly, as the temperature of the aging treatment increases and the heating time becomes longer, the compounds grows gradually, and thus, the presence of the compounds formed under a treatment condition close to the upper limit can be confirmed with a transmission type electron microscope.
For example, FIG. 3(a) is an enlarged image of the transverse cross section of the stainless steel wire obtained by the aging treatment at 600°C for 30 minutes at high magnification. Fine compounds of NiAl having an average particle size of 50 nm or less are precipitated at a high density in the matrix of the martensite. In addition, FIG. 3(b) is an electron beam diffraction image thereof and it has been confirmed that the compounds have a B2 structure. The average particle size of the compounds is expressed by, for example, an average value of particle sizes of respective compound particles confirmed in an arbitrary observation visual field of the diffraction image, and a more proper particle size is in a range of 20 nm or less.
FIG. 3(a) is a bright field image of a thin film sample collected from the stainless steel wire obtained by a transmission electron microscope and is an image of the deformation induced martensite structure. FIG. 3(b) is a diffraction image (which is obtained by Fourier-transforming the structure of the sample) in the region, and the presence of NiAl having the B2 structure as shown in FIG. 3(d) and a BCC structure of the deformation induced martensite can be confirmed. FIG 3(c) shows a dark field image in which only the NiAl precipitates having the B2 structure are projected. It is observed that the compound particles tend to be distributed more uniformly by controlling the aforementioned surface temperature of the steel wire on the inlet side of the final finish die and the working ratio in the final finish die or the final rolling.
As described above, the shape and the distribution state of the compounds are highly dependent on the heating temperature and the heating condition, the working condition and the constitutional elements of the steel wire. For example, the reaction is facilitated by high temperature heating and long-time heating, and thus, the particle sizes of the compounds can be increased and the density can be increased. Accordingly, it is desirable that while a preliminary test is performed, the treatment is performed so as to obtain a desired compound configuration.
In other stainless steel wires, piano wires, and the like, that are conventionally used, a preheating adjustment (heat setting) step is performed before a spring is used. Contrarily, the spring product that can be obtained by the embodiment has high strength and excellent heat setting resistance. Therefore, cost reduction can be expected by omitting the preheating adjustment (heat setting) step. As described above, the properties of a spring product made of a piano wire are deteriorated in a warm temperature region in which the wire is in a slightly heated state. Contrarily, the stainless steel wire of the embodiment is suitable for a spring product which is heat-resistant in the warm temperature region in which the wire is in a slightly heated state. Further, the stainless steel wire of the embodiment is expected to be applied to heat resistance uses such as use in a general high temperature environment of 400°C or higher, and thus, the utilization range is extended.
Hereinafter, examples of the embodiment will be described further.

[Example 1]

<<Manufacture of Stainless Steel Wire>>

The chemical components of the stainless steels used as examples are shown in Tables 1 and 2 and Comparative Steels are also shown in the Tables together. In both Tables 1 and 2, numerical values out of the ranges defined in the embodiment are underlined. Steel type C and Example No. 3 have C out of the scope of the present invention and are therefore reference examples.
Steel having these components was melted in a vacuum melting furnace and casted to form a cast steel having a diameter of φ 178 mm. The cast steel was subjected to hot forging to form a steel bar having a diameter of φ 62 mm. Next, the steel bar was heated to 1250°C and extruded using a hot extrusion simulator to form a wire rod having a diameter of φ 10.7 mm. Then, the wire rod was subjected to solution treatment and pickling, and then, the wire rod was subjected to wire drawing to form a wire rod having a diameter of φ 5.5 mm.
Then, the wire rod was used as a raw material and worked into a soft wire having a diameter of rod of 2.2 mm while cold wire drawing and solution treatment were repeatedly performed. Next, a fine hard wire (drawn steel wire) having a wire diameter of φ 1.0 mm was formed by the final cold wire drawing. The final cold wire drawing was performed under conditions where a final total wire drawing ratio was set to 80%. In addition, a reduction (working ratio) of the final wire drawing die was adjusted to be in a range of 8% to 25%, and the surface temperature of the steel wire on the inlet side of the die was controlled to be in a range of 0°C to 80°C. Then, a Ni plating layer having a thickness of 1.2 µm was formed on the surface of the steel wire (drawn steel wire) after the working.
In all Invention Examples according to the embodiment, working of diameter reduction was able to be performed without problems to obtain a fine high-strength wire having a tensile strength of 1800 MPa to 2200 MPa (N/mm²), a proof stress ratio of 80% to 95%, and a torsion value of 5 times or more. The amount of deformation induced martensite (α') was in a range of 80 vol% to 95 vol%.
The tensile strength and the 0.2% proof stress were measured according to JIS-Z2241. The amount of the deformation induced martensite was measured by the magnetic method described in the [Measurement of Amount of Martensite]. The torsion value was measured by the methods described in the [Torsion Test 1] and [Torsion Test 2]. The results are shown in Tables 3 and 4.

[Example 2]

<<Verification of Aging Properties>>

Next, in order to evaluate changes in the properties of each stainless steel wire (drawn steel wire) described in Example 1 due to aging treatment, each stainless steel wire (drawn steel wire) after the final wire drawing in Example 1 was cut to have a length of 150 mm; and thereby, samples were obtained. Then, each of the samples was subjected to aging treatment at 500°C for 30 minutes. The aging treatment factor expressed by the above-described Expression (3) was 612.
Then, the tensile strength, the proof stress, the proof stress ratio, the torsion value, and the rigidity of the stainless steel wire (drawn wire, aging-treated material) after the aging treatment were evaluated. The results are show in Tables 5 and 6. The rigidity was evaluated by a torsion pendulum method.
The aging-treated steel wires of Invention Examples according to the embodiment had excellent high-strength properties in which the tensile strength was in a range of 2100 MPa to 2600 MPa, the proof stress ratio was in a range of 80% to 95%, and the rigidity was in a range of 77000 MPa or more. As a result of observing an arbitrary transverse cross section thereof with a microscope, similar to FIG. 3, precipitated compounds consisting of NiAl particles having average particle sizes of about 3 nm to 10 nm were able to be confirmed.
Regarding the torsion value, in all the aging-treated stainless steel wires, vertical cracks occurred when the stainless steel wire was twisted 5 times.

[Example 3]

<<Verification of Spring Product>>

Next, in order to further verify the effect of Example 2, each stainless steel wire (drawn steel wire) before aging treatment was subjected to coiling working to form a compression coil spring having an average coil diameter: 7 mm, an effective coiling number: 4.5, a spring free length: 25 mm, and a deployed length: 100 mm. Next, aging treatment was performed at 500°C for 30 minutes. Then, the heat setting resistance of an actual spring product was evaluated. The heat setting resistance (residual shearing strain ε) was measured by the method described in the [Method for Manufacturing Spring Product]. Specifically, while maintaining a state in which a compression stress of 600 MPa was applied, the spring product was held at 200°C for 96 hours. Then, the residual shearing strain ε was calculated by Expression (7).
The obtained results are shown in Tables 5 and 6. In all Invention Examples, it was confirmed that the residual shearing strain was in a range of 0.008% or less, the strength was high, and the heat setting resistance was excellent. On the other hand, all Comparative Examples except No. 51 had large values of a residual shearing strain of more than 0.008%. Accordingly, the effect of the embodiment was recognized. No. 51 had insufficient strength while the residual shearing strain was small.
Regarding the manufacturability, when cracking, wire cutting, and breakage occurred in the wire rod rolling, the wire drawing, and the spring working, it was evaluated to be unmanufacturable. In Invention Examples, the steel wire was able to be manufactured to a spring product without problems.

[Example 4]

<<Influence of Aging Condition>>

Next, in order to evaluate the influence of the aging treatment condition for the aforementioned stainless steel wire and spring material (compression coil spring), steels A and D in Table 1 which were Steels of Invention Examples and steel AP in Table 2 which was Comparative Steel were prepared. Then, a cold-drawn stainless steel wire having a diameter of φ 1.0 mm was manufactured by the method described in the <<Manufacture of Stainless Steel Wire>> of Example 1. A compression coil spring before aging treatment was manufactured from the cold-drawn stainless steel wire by the method described in the <<Verification of Spring Product>> of Example 3. Next, each of the cold-drawn stainless steel wire and the compression coil spring was subjected to aging treatment at a temperature of 250°C to 650°C for 2 minutes to 10 hours. Then, the tensile strength of the stainless steel wire and the heat setting resistance of the compression coil spring after the aging treatment were evaluated. Some of the results are shown in Table 7, FIGS. 4(a) and 4(b).
The peak of the tensile strength was observed particularly around a temperature of 450°C to 550°C, and the strength was slightly softened at 600°C. In the same manner, it was recognized that the residual shearing strain of almost 0.008% or less was obtained in all Invention Examples but in the temperature range increased up to around 600°C, the properties were slightly deteriorated. In addition, in the steel wires having an aging treatment factor of about 150 to 825, the residual strain properties were in a range of 0.005% or less, which was very preferable.

[Example 5]

Next, each of steels A and D in Table 1 was subjected to wire drawing by the method described in Example 1 to obtain a soft wire having a wire diameter of φ 1.8 mm. A metallic soap lubricant was applied to the surface of the soft wire, and then, working of diameter reduction was performed by a cold wire drawing apparatus to form a fine hard wire having a wire diameter of 1.0 mm. Subsequently, cold rolling was performed by a multistage rolling apparatus, and pressing to a thickness of 0.2 mm was finally performed to manufacture a hard flat wire. In the rolling, the optimal cooling method was employed such that the surface temperature of the steel wire on the inlet side of the rolling roll in the final finish was 45°C.
It was confirmed that a total working ratio after the solution treatment was 83% and the stainless steel wire had good workability without problems such as material cracking and wire cutting due to the multistage cold rolling.
First, the lubricant attached on the surface was removed with a solvent so as to evaluate the properties when a flat wire was worked into a spring product. Next, aging treatment was performed at 500°C for 30 minutes as was the case with Example 2, and the properties of the flat wire were evaluated before and after the heat treatment.

The results are shown in Table 8.

Table 8

Test Items	Steel type A		Steel type D
Test Items	Before heat treatment	After heat treatment	Before heat treatment	After heat treatment
Amount of deformation induced α' (Vol.%)	90	-	89	-
Tensile strength (N/mm²)	2030	2550	1910	2350
Proof stress ratio (%)	92	89	91	88
Residual shearing strain (%)	0.009	0.004	0.007	0.004

Here, the tensile strength was evaluated by the tensile test method as was the case with Example 1. In addition, the residual shearing strain was evaluated as the properties at a temperature of 200°C as was the case with Example 3 in the following manner. Torsion stress was applied to both ends of the flat wire having a predetermined length. The wire was heated to 200°C while maintaining this state. Next, the wire was cooled to room temperature and the torsion stress was released. The residual shearing strain was evaluated based on a change in the return angle at this time.
Specifically, the residual shearing strain of the flat wire was calculated using a load loss, an elastic coefficient, and a cross section area as was the case with the spring. With regard to the flat wire, unlike the case of the spring, a load loss was calculated as described below. An arbitrary distance was set as a gage length in a range of, for example, about 5 times to 50 times of the width of the flat wire. A predetermined stress was applied to the both ends of the flat wire having the gage length to twist the wire. The wire was heated to 200°C while maintaining this state. Next, the wire was cooled to room temperature and the stress was released. With regard to the flat wire on which a series of operations was performed and the flat wire before the operations (at an initial stage of the test), loads required to have the same torsion angle were measured. The difference between the loads was calculated, and the value was used as a load loss (ΔP).
As seen from the result, the flat stainless steel wire has excellent mechanical properties which can be used as, for example, a spring material for a wave spring. In addition, the surface properties were preferable since the bright surface having excellent smoothness due to fine grains was obtained.

Industrial Applicability

As described above, the stainless steel wire according to the embodiment has a tensile strength of 1800 MPa to 2200 MPa in a state in which the wire is subjected to wire drawing. In addition, the amount of deformation induced martensite is in a range of 80 vol% to 99 vol%. Therefore, spring properties are greatly improved by the subsequent aging treatment. Particularly, high strength and excellent heat setting resistance can be obtained. Thus, the stainless steel wire according to the embodiment can be applied to, for example, a compression coil spring, a tension coil spring, a torsion spring, and other various spring products, and a spring product having high strength and excellent heat setting resistance can be obtained.
As specific applications, for example, the stainless steel wire is suitably applied to a spring product used in a warm temperature region in a heated state of the periphery of an engine or an electrical system of an automobile, and a heat resistant spring for home electric appliances. In addition to the above-described applications, for example, the embodiment can also be used in various linear products, such as a heat-resistant high-strength rope, a heat resistant shaft, a heat resistant pin, and the like, which are used in a high temperature region and have high strength and heat resistance, and the embodiment is industrially useful.

Claims

A high-strength stainless steel wire having excellent heat setting resistance consisting of, by mass%:
C: 0.04% to 0.12%;

N: 0.005% to 0.03%;

0.05% ≤ (C + N) ≤ 0.13%;

Si: 0.1% to 2.0%;

Mn: 0.1% to 2.0%;

Ni: 6.8% to 9.0%;

Cr: 12.0% to 14.4%;

Mo: 1.0% to 3.0%; and

Al: 0.5% to 2.0%, further optionally comprising

one or more selected from V: 0.01% to 1.0%, Nb: 0.01% to 1.0%, Ti: 0.01% to 1.0%, W: 0.05% to 2.0%, Ta: 0.05% to 2.0%, Cu: 0.8% or less, Co: 0.1% to 2.0%, B: 0.0005% to 0.015%, Ca: 0.0005% to 0.01%, Mg: 0.0005% to 0.01%, and REM: 0.0005% to 0.1% and/or P: 0.015 to 0.045%, and /or S: 0.0001% to 0.01%, and

a balance of Fe and unavoidable impurities,

wherein a deformation induced martensite formation index MdS value expressed by Expression (1) is in a range of 15 to 60,

an amount of deformation induced martensite is in a range of 80 vol% to 99 vol% in a matrix, and

a tensile strength is in a range of 1800 MPa to 2200 MPa, $MdS = 551 - 462 (C + N) - 9.2 Si - 8.1 Mn - 29 (Ni + Cu) - 13.7 Cr - 18.5 Mo$
wherein chemical symbols in the expression represent contents (mass%) of respective elements.
The high-strength stainless steel wire having excellent heat setting resistance according to claim 1,
wherein the stainless steel wire has a high torsional performance in which a torsion value that causes fracture without vertical cracks is in a range of 5 times or more when a torsion test is performed in which the stainless steel wire is held in a gage length that is 100 times longer than an equivalent wire diameter, and one end of the steel wire is twisted and rotated.
A high-strength stainless steel wire having excellent heat setting resistance and a tensile strength in a range of 2100 MPa to 2600 MPa obtainable from aging treatment of the stainless steel wire according to claim 1 or 2.
The high-strength stainless steel wire for a heat resistant spring having excellent heat setting resistance according to any one of claims 1 to 3,
wherein a proof stress ratio {(σ_0.2/σ_B) × 100} of a tensile strength (σ_B) and a 0.2% proof stress (σ_0.2) is in a range of 80% to 95%.
A high-strength spring having excellent heat setting resistance,
wherein the spring comprises the stainless steel wire according to any one of claims 1 to 4, and a residual shearing strain ε expressed by Expression (2) at an environment temperature of 200°C satisfies ε ≤ 0.008%, $Residual shearing strain ε = \{8 Δ PD / {πd}^{3} G\} \times 100$
wherein, ΔP: a load loss (N), D: a central diameter of spring (mm), d: an equivalent wire diameter of the steel wire (mm), and G: a transverse elastic coefficient of the steel wire (N/mm²).
The high-strength spring having excellent heat setting resistance according to claim 5,
wherein fine NiAl-based compound particles having particle sizes of 50 nm or less are included in a matrix of the steel wire.
A method for manufacturing a high-strength spring having excellent heat setting resistance, the method comprising:
subjecting a cast steel having the component composition according to claim 1 in which a deformation induced martensite formation index MdS value expressed by Expression (1) is in a range of 15 to 60, to casting and hot rolling to form a wire rod;

subjecting the wire rod to cold working and solution treatment repeatedly and setting a total working ratio to be in a range of 60 area% to 90 area% in cold working after final solution treatment so as to manufacture a stainless steel wire of which a wire diameter is reduced to be a target wire diameter; and

forming the stainless steel wire into a predetermined spring shape, and then performing aging treatment at a temperature of 300°C to 600°C,

wherein a high-strength spring in which a residual shearing strain ε expressed by Expression (2) at an environment temperature of 200°C satisfies ε ≤ 0.008%, is manufactured by the manufacturing steps, $MdS = 551 - 462 (C + N) - 9.2 Si - 8.1 Mn - 29 (Ni + Cu) - 13.7 Cr - 18.5 Mo$

wherein chemical symbols in the expression represent contents (mass%) of respective elements, $Residual shearing strain ε = \{8 Δ PD / {πd}^{3} G\} \times 100$
wherein, ΔP: a load loss (N), D: a central diameter of spring (mm), d: an equivalent wire diameter of the steel wire (mm), and G: a transverse elastic coefficient of the steel wire (N/mm²).
The method for manufacturing a high-strength spring having excellent heat setting resistance according to claim 7,
wherein in the manufacturing of the stainless steel wire, aging treatment is performed after the final solution treatment and final cold working.
The method for manufacturing a high-strength spring having excellent heat setting resistance according to claim 7 or 8,
wherein the aging treatment is performed under the condition in which an aging treatment factor of Expression (3) is in a range of 100 to 10000, and thereby, fine NiAl-based compound particles having particle sizes of 50 nm or less are precipitated in the matrix of the steel wire, $Aging treatment factor = \{temperature (° C) \times treatment time (\min)\} / 2 \sqrt \{equivalent wire diameter (mm) \times deployed length (mm) of spring\}$