JP2005220392A - Steel wire for spring - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide steel wire for a spring provided with excellent fatigue properties and toughness in a well balance, to provide a spring produced by the steel wire for a spring, and to provide a method for producing steel wire for a spring optimum for the production of the steel wire for a spring. <P>SOLUTION: The steel wire for a spring is obtained by being subjected to quenching and tempering. The steel wire has a tempered martensitic structure. The value in the reduction of area after quenching and tempering is ≥40%, and its shear yield stress after being subjected to heat treatment at 420 to 480°C for ≥2 hrs after quenching and tempering is ≥1,000 MPa. As the steel wire, the one comprising, by mass, 0.50 to 0.75% C, 1.80 to 2.70% Si, 0.1 to 0.7% Mn, 0.70 to 1.50% Cr and 0.02 to 1.00% Co, and the balance Fe with impurities, or the one comprising, by mass, 0.50 to 0.75% C, 1.80 to 2.70% Si, >0.7 to 1.5% Mn and 0.70 to 1.50% Cr, and the balance Fe with impurities is preferable. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a spring steel wire having a tempered martensite structure that has been tempered and tempered, a method of manufacturing a spring steel wire suitable for manufacturing the spring steel wire, and a spring manufactured from the steel wire. In particular, the present invention relates to a steel wire for springs having high strength and excellent fatigue characteristics suitable for springs used in engine valve springs and transmissions of automobiles.

  In response to the reduction in fuel consumption of automobiles, parts such as automobile engines and transmissions have been reduced in size and weight in recent years. Along with this, stresses applied to springs such as engine valve springs and transmission springs are becoming severer year by year, and further improvement in fatigue characteristics is required for the spring materials used. Conventionally, silicon chrome-based oil temper wires have been used for valve springs and transmission springs of these engines. For example, those described in Patent Documents 1 to 3 are known.

Japanese Patent No. 2842579 JP 2002-194496 A Japanese Patent No. 3045795

  However, the characteristics required of springs such as engine valve springs and transmission springs have become severe in recent years, and further improvements are required for spring steel wires and springs. In particular, it is desired to provide a better balance between fatigue properties and toughness.

  In recent years, with a demand for improving fatigue strength (fatigue limit), heat treatment (nitriding treatment) at a high temperature (specifically, about 420 to 480 ° C.) is performed after spring processing. In the technique described in Patent Document 1, toughness is improved by setting the C (carbon) content to 0.3-0.5% by weight, but heat resistance is improved by making the C content less than 0.50% by weight. Therefore, when the above high-temperature nitriding treatment is performed, the fatigue strength is reduced, which causes internal breakage when used as a spring.

  In the technique described in Patent Document 2, the fatigue strength is improved by setting the average crystal grain size of austenite after quenching to a refined structure of 1.0 to 7.0 μm. However, if the temperature at the time of quenching is made low in order to make the austenite crystal grain size smaller, undissolved carbides remain and cause toughness to decrease. Moreover, since the toughness is reduced, breakage is likely to occur during spring processing, which adversely affects mass productivity.

  In the technique described in Patent Document 3, the surface hardness is reduced by intentionally decarburizing the surface during oil tempering to improve spring workability, but it is difficult to obtain a uniform decarburized layer. Yes, unsuitable for mass production. In addition, the oxygen concentration must be controlled during heating, which increases costs.

  Furthermore, none of the techniques described in any of the documents discusses the proof stress in the torsional direction inside the material, that is, the shear yield stress after nitriding performed after spring processing.

  Therefore, a main object of the present invention is to provide a high-strength steel wire for springs that is excellent in both fatigue strength and toughness. Another object of the present invention is to provide a spring made of the spring steel wire and a manufacturing method suitable for manufacturing the spring steel wire.

  The present invention achieves the above object by defining the drawing value after quenching and tempering and the shear yield stress after heat treatment corresponding to the nitriding treatment performed after quenching and tempering.

  That is, the present invention is a spring steel wire obtained by quenching and tempering. This steel wire for spring has a tempered martensite structure, the drawing value after quenching and tempering is 40% or more, and the shear yield stress after heat treatment at 420 to 480 ° C for 2 hours or more after quenching and tempering. It is characterized by being over 1000MPa.

More preferably, the steel wire for spring contains any one of the following elements 1 to 4 and the balance is Fe and impurities.
1. By mass% C: 0.50-0.75%, Si: 1.80-2.70%, Mn: 0.1-0.7%, Cr: 0.70-1.50%, Co: 0.02-1.00%
2. By mass%, C: 0.50 to 0.75%, Si: 1.80 to 2.70%, Mn: more than 0.7 to 1.5%, Cr: 0.70 to 1.50%
3. By mass% C: 0.50-0.75%, Si: 1.80-2.70%, Mn: more than 0.7-1.5%, Cr: 0.70-1.50%, Ni: 0.1-1.0% and Co: 0.02-1.00% on the other hand
4. In any of the above elements 1 to 3 and mass%, V: 0.05 to 0.50%, Mo: 0.05 to 0.50%, W: 0.05 to 0.15%, Nb: 0.05 to 0.15%, and Ti: 0.01 to 0.20 At least one selected from the group consisting of

Moreover, the following is proposed as a manufacturing method suitable for manufacturing the steel wire for a spring of the present invention. That is, the manufacturing method of the steel wire for a spring of the present invention includes a step of patenting a steel material having a chemical component described in any of the following (A) to (C), and a step of drawing the patented steel material. And a step of quenching and tempering the drawn steel wire. The patenting includes an austenitizing step of heating at 900 to 1050 ° C. for 60 to 180 seconds, and an isothermal transformation step of heating at 600 to 750 ° C. for 20 to 100 seconds after the austenitizing step.
(A) Containing 0.5% to 0.75% by mass, Si: 1.80 to 2.70%, Mn: 0.1 to 0.7%, Cr: 0.70 to 1.50%, Co: 0.02 to 1.00% by mass%, the balance being Fe and impurities Steel material
(B) Steel material containing C: 0.50 to 0.75% by mass, Si: 1.80 to 2.70%, Mn: more than 0.7 to 1.5%, Cr: 0.70 to 1.50%, the balance being Fe and impurities
(C) By mass%: C: 0.50 to 0.75%, Si: 1.80 to 2.70%, Mn: more than 0.7 to 1.5%, Cr: 0.70 to 1.50%, Ni: 0.1 to 1.0% and Co: 0.02 to 1.00% In addition to the chemical components (A) to (C) above, in addition to the chemical components (A) to (C), V: 0.05 to 0.50%, Mo: 0.05 to 0.50%, W May contain one or more elements selected from the group consisting of: 0.05 to 0.15%, Nb: 0.05 to 0.15%, and Ti: 0.01 to 0.20%.

Hereinafter, the present invention will be described in more detail.
(Improvement of fatigue properties)
In order to improve the fatigue characteristics, it is desired to suppress fatigue fracture. When the spring is used repeatedly, the spring is repeatedly subjected to stress in the shearing direction simultaneously with the tensile direction and the compression direction. Such repeated externally applied stress causes local or concentrated repeated slip deformation (plastic deformation), causing irregularities in the vicinity of the surface of the spring, resulting in cracks and failure, that is, fatigue failure It becomes. Therefore, in order to suppress fatigue failure, it is effective to suppress the local or intensive plastic deformation. In order to suppress such plastic deformation, conventionally, heat treatment such as nitriding treatment is performed after spring processing to increase the surface hardness and improve the fatigue limit. However, in recent years when springs have been used with high stress, it may not be possible to use them simply by increasing the fatigue limit. This is presumably because the strength inside the spring is lowered even if the high-hardness nitride layer formed on the spring surface layer does not disappear due to the heat treatment such as nitriding. Therefore, it is desired for a high-strength spring to improve not only the fatigue limit but also the torsional strength, that is, the shear yield stress itself. Thus, as a result of various studies by the present inventors, it was found that the inside of the material should have an appropriate torsional strength after the heat treatment such as the nitriding treatment. Specifically, it was found that if the shear yield stress after heat treatment such as nitriding is 1000 MPa or more, the fatigue characteristics can be improved. Based on this knowledge, the present invention defines the shear yield stress after performing a specific heat treatment after quenching and tempering to 1000 MPa or more.

(High toughness)
No matter how high the strength, if the toughness is low, it will cause breakage during spring processing and hinder mass production. In addition, fatigue characteristics are also reduced due to a decrease in toughness. As a result of various studies by the present inventors, it has been found that setting the drawing value after quenching and tempering to 40% or more is effective in preventing breakage and is excellent in mass productivity. Based on this knowledge, the present invention defines the drawing value after quenching and tempering to 40% or more. If the aperture value is less than 40%, breakage tends to occur during spring processing, which may hinder mass productivity. The drawing value may be slightly reduced by performing a specific heat treatment equivalent to the nitriding treatment after quenching and tempering, but if the drawing value after quenching and tempering is 40% or more, 35% or more after the heat treatment. Can be maintained, and high fatigue characteristics can be obtained.

  As described above, the present invention seeks to achieve both high fatigue strength and high toughness by defining the drawing value after quenching and tempering and the shear yield stress after the heat treatment corresponding to the nitriding treatment.

In order to obtain the spring steel wire of the present invention which is excellent in both the fatigue characteristics and the toughness, the optimum chemical composition and production conditions, particularly patenting conditions, are specified.
<Chemical composition>
First, the fatigue limit of the spring can be improved by improving the surface hardness by heat treatment such as nitriding after the spring processing, but the internal hardness is reduced, and internal breakage may occur when the spring is used. is there. Therefore, in the present invention, C and Si are included in a predetermined range in order to improve the heat resistance of the matrix. In addition, a predetermined amount of Cr is included in order to form carbides during tempering and increase softening resistance. In order to increase the softening resistance, it is also effective to add predetermined amounts of Mo, V, Nb, W, and Ti. It was found that it is effective to contain Co: 0.02 to 1.00% by mass or to contain a large amount of Mn (over 0.7 to 1.5% by mass) for improving the shear yield stress. Therefore, the contents of Mn and Co are specified. The detailed component range and the reason for limiting the range will be described later.

<Production conditions>
The spring steel wire of the present invention can be obtained by melting, hot forging, hot rolling, patenting, wire drawing, quenching and tempering a steel material having the above chemical components.

(Patenting conditions)
In the present invention, by performing patenting under specific conditions before wire drawing, undissolved carbides are dissolved by sufficient austenitization, and a uniform pearlite structure is obtained by appropriate isothermal transformation. Insufficient austenitization causes a reduction in toughness and shear yield stress after quenching and tempering. Therefore, austenitization is suitably performed at a temperature of 900 to 1050 ° C. for 60 to 180 seconds. When the heating temperature is less than 900 ° C., or when the heating temperature is 900 to 1050 ° C. and the heating time is less than 60 seconds, sufficient austenite cannot be formed and undissolved carbides remain. In addition, when the heating temperature is higher than 1050 ° C, or when the heating temperature is 900 to 1050 ° C and the heating time is longer than 180 seconds, the austenite grains are coarsened, and martensite is likely to be generated at the time of transformation. Will be disturbed.

  The isothermal transformation after austenitization is suitably heated at 600 to 750 ° C. for 20 to 100 seconds. When the heating temperature is higher than 750 ° C., or when the heating temperature is 600 to 750 ° C. and the heating time is longer than 100 seconds, the cementite is spheroidized and becomes a factor that inhibits wire drawing. On the other hand, when the heating temperature is lower than 600 ° C., or when the heating temperature is 600 to 750 ° C. and the heating time is shorter than 20 seconds, the transformation to pearlite is not completed and martensite is generated, thereby reducing the drawability. It becomes a factor that obstructs.

(Quenching and tempering)
When the temperature at the time of quenching is too low, undissolved carbides remain and lower toughness. Moreover, when the temperature at the time of quenching is too high, the austenite crystal grains grow and become larger, thereby reducing the fatigue limit. Accordingly, the temperature during quenching is preferably more than 850 ° C. and less than 1050 ° C.

<Organization>
The spring steel wire of the present invention has a tempered martensite structure. In addition, when the austenite crystal grains (old austenite crystal grains) after quenching and tempering are refined, even if repeated stress is applied, local and concentrated slip deformation is unlikely to occur. That is, since the shear yield stress can be improved, it can contribute to the improvement of fatigue characteristics as a result.

  Specifically, the average crystal grain size of austenite crystal grains (former austenite crystal grains) is preferably set to 3.0 to 7.0 μm. The average crystal grain size can be changed by changing the temperature of patenting. More specifically, when the austenitizing temperature is lowered in patenting, the crystal grain size tends to decrease, and when the temperature is increased, the crystal grain size tends to increase. If the average crystal grain size is less than 3.0 μm, the austenitizing temperature is low, so that undissolved carbides remain and the toughness tends to decrease. Further, when the average crystal grain size exceeds 7.0 μm, the fatigue limit is hardly improved. The average crystal grain size is a value after quenching and tempering.

  Hereinafter, the reason for limiting the selection of constituent elements and the component range in the present invention will be described. In addition, the unit of the numerical value described next to an element is the mass%.

C: 0.50 ~ 0.75
C is an important element that determines the strength of steel. If less than 0.50%, sufficient strength cannot be obtained, and if it exceeds 0.75%, the toughness is impaired, so 0.50 mass% or more and 0.75 mass% or less.

Si: 1.80-2.70
Si is used as a deoxidizer during melting and refining. In addition, it has the effect of improving the heat resistance by solid solution in ferrite and preventing the hardness reduction inside the steel wire due to heat treatment such as strain relief annealing and nitriding after spring processing. In order to maintain heat resistance, 1.80% by mass or more is necessary, and if it exceeds 2.70% by mass, the toughness decreases, so it is 1.80% by mass or more and 2.70% by mass or less.

Mn: 0.1-1.5
Mn, like Si, is used as a deoxidizer during melting and refining. Therefore, the lower limit of the amount of addition necessary for the deoxidizer is 0.1% by mass. Moreover, it has the effect of improving hardenability, increasing the strength after quenching and tempering, and improving the shear yield stress. However, if it exceeds 1.5 mass%, martensite is likely to be generated during patenting, and this may cause disconnection during wire drawing, so the upper limit is made 1.5 mass%. In particular, when adding Co to be described later, Mn may be as low as 0.1 to 0.7% by mass. When Co is not added, it is preferable to add Mn as much as 0.7 to 1.5% by mass. A large amount of Mn may be added and Co may be added.

Cr: 0.70 to 1.50
Cr improves the hardenability of the steel and increases the softening resistance after quenching and tempering, so it is effective in preventing softening during heat treatment such as tempering and nitriding after spring processing. If it is less than 0.70% by mass, a sufficient effect for preventing softening cannot be obtained, so it is 0.70% by mass or more. If it exceeds 1.50% by mass, martensite is likely to occur during patenting, causing wire breakage during wire drawing. It becomes a factor to reduce the toughness after oil tempering. Therefore, it is specified to be 0.70 to 1.50%.

Co: 0.02 to 1.00
Co improves the shear yield stress by adding a small amount. In addition, it has an effect of improving heat resistance, and is effective in preventing tempering after spring processing and softening after nitriding. Furthermore, in the case of addition of a specific amount, the toughness is not lowered. If the amount is less than 0.02% by mass, the above effect is difficult to obtain. Even if it exceeds 1.00% by mass, the effect does not change and the cost increases. In addition, when adding Co, you may make the addition amount of Mn low as 0.1-0.7 mass% as mentioned above.

Ni: 0.1-1.0
It has the effect of improving corrosion resistance and toughness. If the amount is less than 0.1% by mass, the above effect is difficult to obtain. Even if the amount exceeds 1.0% by mass, the cost is increased and a further effect of improving toughness cannot be obtained.

Mo, V: 0.05-0.50
W, Nb: 0.05-0.15
These elements tend to form carbides during tempering and increase softening resistance. If it is less than 0.05% by mass, it is difficult to obtain the above effect. Further, if Mo and V exceed 0.50% by mass and W and Nb exceed 0.15% by mass, the toughness tends to be lowered.

Ti: 0.01-0.20
Ti has the effect of forming carbides during tempering and increasing softening resistance. If the amount is less than 0.01% by mass, the above effect cannot be obtained, and if it exceeds 0.20% by mass, refractory non-metallic inclusions TiO are formed and the toughness tends to be lowered. Therefore, the content is set to 0.01% by mass to 0.20% by mass.

  The steel wire for spring of the present invention can be applied not only to a steel wire having a circular cross section perpendicular to the longitudinal direction (drawing direction) of the steel wire, but also to a steel wire having an odd-shaped cross section such as an ellipse, a trapezoid, a square, and a rectangle. .

  The spring of the present invention can be obtained by subjecting the spring steel wire to spring processing such as coiling. In particular, after the spring processing, heat treatment such as nitriding can be performed to improve the surface hardness and have an excellent fatigue limit.

Embodiments of the present invention will be described below.
Steel materials having chemical components (remaining Fe and impurities) shown in Table 1 were melted in a vacuum melting furnace, and a φ6.5 mm wire was produced by hot forging and hot rolling. Thereafter, patenting (austeniteization → constant temperature transformation), skinning, annealing, and wire drawing were performed to obtain a wire of φ3.0 mm. Table 2 shows the patenting conditions. In this example, for patenting, as shown in Table 2, a plurality of austenitizing conditions with different heating temperatures and holding times, and subsequent isothermal transformation conditions were prepared.

  The obtained wire was quenched and tempered. Quenching was performed under the conditions shown in Table 3, and tempering was performed at 450 to 530 ° C. for all samples. After quenching and tempering, the average crystal grain size (average γ grain size) of the drawn (RA) and austenite crystal grains (old austenite crystal grains) was measured. The results are shown in Table 3. Note that the average crystal grain size was changed by changing the quenching temperature. The average crystal grain size of the austenite crystal grains was calculated by a cutting method defined in JIS G0522.

  Further, the shear yield stress and fatigue characteristics (fatigue limit) of the steel wire subjected to a heat treatment (420 ° C. × 2 hours or 480 ° C. × 2 hours) corresponding to nitriding after quenching and tempering were measured. Table 3 shows the results. The shear yield stress was obtained from a torque-θ curve by conducting a torsion test with a sample length of 100d (d: sample diameter). The fatigue limit was evaluated by the Nakamura rotary bending fatigue test.

  As shown in Table 3, sample Nos. 13 to 21 with a drawing value after quenching and tempering of 40% or more and a shear yield stress after heat treatment equivalent to nitriding treatment of 1000 MPa or more have high fatigue limits. Understand. Moreover, since the shear yield stress is high, it is thought that it is excellent also against sag. Therefore, it turns out that this invention is excellent in a fatigue characteristic, having high toughness.

  In contrast, Samples Nos. 1 to 4, 6, and 8 had low shear yield stress after heat treatment and low fatigue limit. In particular, Samples Nos. 2 and 4 also had low drawing values after quenching and tempering, and were poor in toughness. In Samples Nos. 5 and 7, martensite was generated at the time of patenting, and the experiment was stopped because disconnection occurred frequently in the next process. In Sample No. 11, the shear yield stress after heat treatment was low and the amount of V added was large, so that the drawing after quenching and tempering was lowered and the fatigue limit was lowered. Sample No. 12 had low fatigue yield stress due to breakage due to Ti inclusions due to the low shear yield stress after heat treatment and the large amount of Ti added. From these, it can be seen that it is preferable to use a specific component.

  Sample No. 9 had a low shear yield stress after heat treatment and a low average drawing diameter after quenching and tempering because the average γ grain size was small. On the other hand, Sample No. 10 had a low shear yield stress after heat treatment and a large average γ grain size, resulting in a low fatigue limit. From these facts, it can be seen that the average γ particle size is preferably in a specific range.

  For the steel material having the chemical composition of sample K in Table 1, a φ6.5 mm wire was prepared in the same manner as described above, and a φ3.0 mm wire was prepared in the same manner as described above. At this time, the patenting conditions were changed as shown in Table 2. The obtained wire was subjected to quenching and tempering (quenching: 940 ° C., tempering: 450 to 530 ° C.), and the drawing (RA) and the average γ particle size were measured. The results are shown in Table 4. Further, the shear yield stress and fatigue properties (fatigue limit) of the steel wire subjected to the heat treatment (420 ° C. × 2 hours or 480 ° C. × 2 hours) corresponding to the nitriding treatment after quenching and tempering were measured. The results are also shown in Table 4. Each physical property was measured in the same manner as described above.

  As shown in Table 4, sample Nos. 22 and 23 were subjected to patenting under specific conditions (austenitization: 90 to 1050 ° C. for 60 to 180 seconds, isothermal transformation: 600 to 750 ° C. for 20 to 100 seconds). It can be seen that the fatigue limit is high.

  On the other hand, in each of sample Nos. 25 and 27 to 29, martensite was generated, and disconnection occurred frequently in the wire drawing process, so the experiment was stopped. In Samples Nos. 24 and 26, undissolved carbide remained, so that the squeezing after quenching and tempering decreased and the fatigue limit decreased. Shear yield stress was also low. In Samples Nos. 30 and 31, cementite was spheroidized, so that undissolved carbide remained after quenching and tempering, resulting in a decrease in drawing after quenching and tempering, and a low shear yield stress.

  Since the steel wire for springs of the present invention is excellent in fatigue characteristics and toughness, it is most suitable as a spring material used for a portion requiring fatigue strength.

Claims (8)

A spring steel wire obtained by quenching and tempering,
Has a tempered martensite structure,
The aperture value after quenching and tempering is 40% or more,
A steel wire for a spring characterized by having a shear yield stress of 1000 MPa or more after heat treatment at 420 ° C. to 480 ° C. for 2 hours or more after quenching and tempering.   Containing 0.5% to 0.75% by mass, Si: 1.80 to 2.70%, Mn: 0.1 to 0.7%, Cr: 0.70 to 1.50%, Co: 0.02 to 1.00%, with the balance being Fe and impurities 2. The spring steel wire according to claim 1, wherein:   It contains by mass%, C: 0.50 to 0.75%, Si: 1.80 to 2.70%, Mn: more than 0.7 to 1.5%, Cr: 0.70 to 1.50%, and the balance consisting of Fe and impurities. The steel wire for spring according to 1.   4. The spring steel wire according to claim 3, further comprising at least one of Ni: 0.1 to 1.0% and Co: 0.02 to 1.00% by mass, with the balance being Fe and impurities.   Furthermore, at least one selected from the group consisting of V: 0.05 to 0.50%, Mo: 0.05 to 0.50%, W: 0.05 to 0.15%, Nb: 0.05 to 0.15%, and Ti: 0.01 to 0.20% by mass. The spring steel wire according to any one of claims 1 to 4, which is contained.   6. The spring steel wire according to claim 1, wherein an average crystal grain size of the austenite crystal grains (old austenite crystal grains) after quenching and tempering is 3.0 to 7.0 μm. A step of patenting a steel material having a chemical composition according to any one of the following (A) to (C);
Drawing the patented steel material; and
A step of quenching and tempering the drawn steel wire,
The patenting is
An austenitizing step of heating at 90 to 1050 ° C. for 60 to 180 seconds;
A method for producing a steel wire for a spring, comprising: a constant temperature transformation step of heating at 600 to 750 ° C. for 20 to 100 seconds after the austenitizing step.
(A) Containing 0.5% to 0.75% by mass, Si: 1.80 to 2.70%, Mn: 0.1 to 0.7%, Cr: 0.70 to 1.50%, Co: 0.02 to 1.00% by mass%, the balance being Fe and impurities Steel material
(B) Steel material containing C: 0.50 to 0.75% by mass, Si: 1.80 to 2.70%, Mn: more than 0.7 to 1.5%, Cr: 0.70 to 1.50%, the balance being Fe and impurities
(C) By mass%: C: 0.50 to 0.75%, Si: 1.80 to 2.70%, Mn: more than 0.7 to 1.5%, Cr: 0.70 to 1.50%, Ni: 0.1 to 1.0% and Co: 0.02 to 1.00% Steel material containing at least one and the balance being Fe and impurities   A spring manufactured using the spring steel wire according to any one of claims 1 to 6.