JP4868935B2 - High strength spring steel wire with excellent sag resistance - Google Patents

Description

  The present invention relates to a steel wire for a high-strength spring used as a material for a valve spring of an automobile engine, a suspension spring of a suspension, a clutch spring, a brake spring, etc., and particularly for a high-strength spring excellent in sag resistance. It relates to steel wires.

  With the reduction in weight and output of automobiles, high-stress design is directed to springs used in the various applications. In particular, as the load stress of the spring increases, those having excellent fatigue strength and sag resistance are required. For example, when the sag resistance is low, the amount of sag of the spring increases during a high stress load, causing the problem that the engine speed does not increase as designed and the responsiveness deteriorates. .

  In order to improve the sag resistance of the spring, it is known that the strength of the spring material may be increased. In addition, increasing the strength of the spring material (spring steel wire) can be expected to improve fatigue characteristics from the viewpoint of the fatigue limit. From the viewpoint of increasing the strength of the steel wire for springs, various techniques have been proposed so far. As one of them, a technique for improving the sag resistance by adjusting the chemical composition and increasing the tensile strength after oil quenching and tempering (after oil temper treatment) has been proposed. In addition, a technique for improving sag resistance by adding a large amount of an alloy element such as Si as a chemical component composition has been proposed (Patent Documents 1 and 2).

However, even in these technologies, the characteristics sufficient to meet the recent demand for higher strength are not always sufficiently exhibited, and it is desired to realize a spring steel wire that can exhibit further excellent sag resistance. It is the actual situation.
Japanese Patent No. 2898472 Patent Claim etc. JP, 2000-169937, A Claims etc.

  The present invention has been made in view of such circumstances. The purpose of the present invention is to prevent the spring from being deformed even when used under a high load stress, and to be resistant to the recent demand for higher strength. An object of the present invention is to provide a spring steel wire having excellent stretchability.

The steel wire for a spring according to the present invention that has achieved the above object is a spherical equivalent of 10 to 100 nm in a circle equivalent diameter of 30 pieces / μm ² or more and a Cr concentration in the cement tie of 20% or more ( Meaning of mass%, the same applies hereinafter), and the V concentration is 2% or more. The “equivalent circle diameter” means the diameter of spherical cementite when converted to a circle having the same area. In this sense, the “spherical cementite” in the present invention includes not only a perfect circle but also an elliptical form. The “steel wire” refers to a material that has been processed and heat-treated after hot rolling, and is distinguished from a “wire material” that has not been processed and heat-treated after hot rolling.

  Although it does not specifically limit about the chemical component composition of the steel wire for springs of this invention, About Cr and V at least, Cr: 1.0-3.0%, V: 0.05-0.5 % Is preferred.

  As a basic component composition in the steel wire for high-strength springs of the present invention, including the preferable contents of Cr and V, C: 0.5 to 0.7%, Si: 1.5 to 2.5% , Mn: 0.2 to 1.0%, Cr: 1.0 to 3.0%, V: 0.05 to 0.5%, and Al: 0.05% or less (0% included) And the remaining Fe and inevitable impurities.

  The steel wire for high-strength springs of the present invention may further include other elements as necessary: (a) Ni: 0.3% or less (excluding 0%), (b) Nb: 0.1% or less ( (C) Mo: 0.5% or less (not including 0%), Co: 0.5% or less (not including 0%), and: W 0.5% or less (0% It is also preferred to contain one or more selected from the group consisting of (not contained), etc., and the properties of the steel wire are improved according to the contained components.

  According to the present invention, high strength that exhibits excellent sag resistance is obtained by finely dispersing cementite of a predetermined size in a steel wire and setting the Cr amount and V amount in the cementite to a predetermined concentration or more. A spring steel wire can be realized, and such a spring steel wire is optimal as a material for various springs used under high load stress.

  The present inventors have studied from various angles without being constrained by the existing concept in order to realize a steel wire with better sag resistance. As a result, spherical cementite having an equivalent circle diameter of about 10 to 100 nm is finely dispersed as much as possible, and if the Cr concentration and V concentration in the cementite are set to a predetermined concentration or more, strain relief annealing and nitriding heat treatment during spring production are performed. The present inventors have found that the decomposition and disappearance of carbide (cementite) can be suppressed during heat treatment, and that excellent sag resistance can be exhibited thereby.

  In the steel wire of the present invention, the particle diameter (equivalent circle diameter) of the target spherical cementite needs to be 10 to 100 nm. If the particle size is less than 10 nm, it decomposes and disappears during heating and does not exhibit a function as a carbide that contributes to sag resistance. On the other hand, even if the particle size of cementite exceeds 100 nm, the properties are not affected so much, but the presence of a large amount reduces the amount of spherical cementite having an effective size (10 to 100 nm). Thus, the effect of finely dispersing becomes difficult to be exhibited.

The spherical cementite as described above can exhibit the effects of the present invention by being finely dispersed as much as possible, but the amount needs to be 30 / μm ² or more. That is, when the amount of spherical cementite is less than 30 pieces / μm ^2, it becomes solid solution in the matrix at the time of heat treatment, and the effect of improving sag resistance is hardly exhibited. The number of spherical cementite is preferably 35 / μm ² or more. However, if the amount is too large, the amount of C in the matrix decreases, the strength of the material decreases, and the fatigue strength and sag resistance deteriorate. 50 / μm ² or less is preferable.

  In the steel wire of the present invention, it is also an important requirement that the target spherical cementite contains Cr and V at a predetermined concentration. These elements prevent the carbides from decomposing and disappearing during the heat treatment, and as a result, make the spring steel wire excellent in sag resistance. In order to exert such an effect, the Cr concentration in cementite needs to be 20% or more and the V concentration needs to be 2% or more. However, if excessive, it becomes coarse Cr carbide and V carbide, It is preferable to set it to 40% or less and V concentration to 15% or less.

  The steel wire of the present invention is not particularly limited with respect to its component composition, but particularly from the viewpoint of controlling the Cr and V contents in the cementite as described above, the contents in these steel wires. About Cr, it is preferable to set it as Cr: 1.0-3.0%, V: 0.05-0.5%. Further, C, Si, Mn, Al and the like are included in the same manner as general spring steel wires as shown below. The reasons for limiting the preferable ranges of these components (including the above Cr and V) are as follows.

[C: 0.5 to 0.7%]
C is an element useful for securing a sufficient strength as a spring steel to which a high stress is applied, and exerts an effect of precipitating fine cementite. In order to exert such effects, the C content is preferably 0.5% or more. However, if the C content is excessive, the toughness and ductility of the steel wire are extremely deteriorated, so 0.7% or less is preferable. In addition, the more preferable minimum of C content is 0.55%, and a more preferable upper limit is 0.67%.

[Si: 1.5-2.5%]
Si is an element having a deoxidizing action, and has an effect of increasing temper softening resistance and improving sag resistance. In order to exert such effects, it is preferable to contain Si by 1.5% or more. More preferably, it should be 1.7% or more. However, if the amount of Si becomes excessive, not only the toughness and ductility of the spring steel will deteriorate, but also the occurrence of surface decarburization and flaws will increase and the fatigue resistance will deteriorate, so 2.5% or less (more preferably Is preferably 2.3% or less).

[Mn: 0.2 to 1.0%]
Mn is an element having a deoxidizing action like Si, and is an element contributing to an increase in strength by enhancing hardenability. In order to exert such effects, it is preferable to contain 0.2% or more of Mn. More preferably, it is good to set it as 0.3% or more. However, when the amount of Mn is excessive, an overcooled structure such as bainite is easily generated during hot rolling or patenting treatment, and the wire drawing property is remarkably deteriorated. 8% or less).

[Cr: 1.0 to 3.0%]
Cr is an element useful for improving hardenability and improving temper softening resistance. It is also an element useful for improving the sag resistance by suppressing the dissolution and disappearance of cementite during heat treatment by solid solution in spherical cementite. In order to exert such an effect, Cr is preferably contained in an amount of 1.0% or more. However, if excessively contained, the patenting time becomes too long, and the toughness and ductility are also deteriorated. It is preferable that In addition, the more preferable lower limit of Cr content is 1.2%, and the more preferable upper limit is 2.5%.

[V: 0.05 to 0.5%]
V, like Cr, is an element useful for improving the softening resistance of a steel wire and for dissolving in cementite to improve sag resistance. In order to exhibit such an effect, the content is preferably 0.05% or more. However, if it is excessively contained, a martensite or bainite structure is formed, and the workability is deteriorated. A more preferable lower limit of the V content is 0.10%, and a more preferable upper limit is 0.4%.

[Al: 0.005% or less (excluding 0%)]
Al is an unavoidable impurity that produces non-metallic inclusions such as Al ₂ O ₃ and reduces fatigue strength, so it is preferable to keep it at 0.005% or less (more preferably 0.003% or less).

  The preferable elements contained in the basic component in the steel wire of the present invention are as described above, and the balance is iron and inevitable impurities (for example, P, S, Cu, N, Ca, Mg, O, etc.). As a result, mixing of elements brought in depending on the situation of raw materials, materials, manufacturing facilities, etc. can be allowed. Of the above inevitable impurities, P, S, Cu, and N include P: 0.025% or less, S: 0.025 or less, Cu: 0.30% or less, and N: 0.010% or less. It will be. Furthermore, it is also effective to further improve the characteristics by positively containing the following elements.

[Ni: 0.3% or less (excluding 0%)]
Ni is an element useful for enhancing the hardenability and suppressing the formation of ferrite, and such an effect increases as the content thereof increases. The site structure is generated, the wire drawing property is remarkably deteriorated, and the toughness and ductility are deteriorated. In order to achieve the above effect, a preferable lower limit is 0.05%.

[Nb: 0.1% or less (excluding 0%)]
Nb is an element useful for producing fine carbonitrides and making crystal grains fine to improve sag resistance. Such an effect increases as the content thereof increases, but if it is excessively contained, coarse nitrides are formed and adversely affected. In order to exhibit the above effect, a preferable lower limit is 0.005%.

[Mo: 0.5% or less (not including 0%), Co: 0.5% or less (not including 0%) and W: 0.5% or less (not including 0%)]
These elements are useful elements for improving the softening resistance of the steel wire. However, if excessively contained, a martensite or bainite structure is generated, and the workability is deteriorated.

  In order to manufacture the steel wire of the present invention, it is preferable to appropriately control the manufacturing conditions. In particular, in order to ensure the Cr concentration and the V concentration in cementite at a predetermined amount or more, (a) increase the austenitizing heating temperature during oil tempering (for example, 970 ° C. or more), (b) increase the tempering temperature. It is effective to perform (for example, 460 ° C. or more), (c) increase the tempering time (for example, 80 seconds or more), and the like.

In addition, in order to disperse 30 or more μm ² of cementite having a circle equivalent diameter of 10 to 100 nm, the austenitizing heating temperature during oil tempering is controlled to be high (for example, 970 ° C. or more), and the tempering time is appropriately set. It is effective to control to.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

[Example 1]
Various steel materials (steel types A to J) having chemical composition shown in Table 1 below were melted in a small vacuum furnace, and a steel piece having a cross section of 155 mm × 155 mm was obtained by hot forging. Then, it heated at 1100 degreeC and obtained the φ8.0mm steel wire by hot rolling.

About the obtained steel wire, after softening annealing (660 ° C. × 2 hours), surface cutting (φ8.0 mm → φ7.4 mm), lead patenting treatment (heating temperature: 980 ° C., lead furnace temperature: 620 ° C.) And cold drawing to φ3.5 mm (φ7.4 mm → φ3.3 mm: total Red = 77.6%). Thereafter, oil temper treatment (heating temperature: 950 to 980 ° C., quenching oil temperature: 60 ° C., tempering temperature: 450 to 480 ° C.) was performed to produce an oil temper wire (steel wire). With respect to the oil tempered wire, the properties of spherical cementite were measured under the following conditions.

[Measurement of spherical cementite properties]
Observation sample: A sample was taken from an oil tempered wire D / 4 part (D: diameter) by the extraction replica method, and the following measurement was performed.
(A) Measurement of precipitation amount of spherical cementite Observed with a transmission electron microscope (observation magnification: 150000 times), and by analyzing the photographed photograph, the number of spherical cementite (carbide) having a circle equivalent diameter of 10 to 100 nm was determined. It was measured.
(B) Measurement of Cr concentration and V concentration in cementite The deposited cementite is observed with a field emission transmission electron microscope, and each element concentration in cementite is measured with an energy dispersive X-ray detector (EDX). It was measured.

The oil tempered wire is cold spring molded (average coil diameter: 21.3 mm, effective number of turns: 4.5), strain relief annealing (400 ° C. × 20 minutes), seat polishing, nitriding treatment (nitriding conditions: 80%) NH ₃ + 20% N ₂ atmosphere, 450 ° C. × 3 hours), double shot peening treatment, low temperature annealing (230 ° C. × 20 minutes) and cold setting were performed to obtain a spring. The obtained spring was tested under the following conditions to evaluate sag resistance.

[Sag test]
A tightening test was performed on each spring, and the residual shear strain was measured. The temperature at this time was 120 ° C., and clamping was performed at a clamping stress of 1300 MPa for 48 hours. Then, those having a residual shear strain of 0.10% or less were evaluated as having excellent sag resistance.

  The oil tempering treatment conditions (heating temperature, tempering temperature, tempering time) for each steel type are shown in Table 2 below, and the number of spherical cementite, Cr concentration in cementite, V concentration and sag resistance test results are shown in Table 3 below. Based on these results, the relationship between the Cr concentration in cementite and the residual shear strain is shown in FIG. 1, and the relationship between the V concentration in cementite and the residual shear strain is shown in FIG.

  These results can be considered as follows (note that the following No. indicates the test No. in Tables 1 and 2).

  No. Since 1, 3, and 5-9 satisfy the requirements specified in the present invention, it can be seen that excellent sag resistance (residual shear strain of 0.10% or less) is exhibited.

  In contrast, no. Since 2, 4, and 10-13 did not satisfy the provisions of the present invention, the results were poor in sag resistance. Specifically, no. 2 and 4, although the chemical component composition satisfies the preferred range of the present invention, the oil tempering conditions are not appropriate, and the present invention is different in terms of the number of spherical cementite, the Cr concentration in the cementite, and the V concentration. It falls outside the specified range, and the sag resistance has deteriorated (residual shear strain exceeds 0.10%). No. In the case of Nos. 10 to 13, the chemical composition is outside the preferable range defined in the present invention, and the sag resistance is deteriorated.

It is the graph which showed the relationship between Cr density | concentration in cementite and a residual shear strain. It is the graph which showed the relationship between V density | concentration in cementite and a residual shear strain.

Claims (4)

C: 0.5-0.7% (meaning of mass%, the same applies hereinafter), Si: 1.5-2.5%, Mn: 0.2-1.0%, Cr: 1.0-3. 0%, V: 0.05 to 0.5%, respectively, and Al: 0.005% or less (not including 0%), the balance being Fe and inevitable impurities,
And a circle equivalent diameter 10~100nm spherical cementite 30 / [mu] m ² or more, and Sementai Cr concentration in preparative 20% or more, sag resistance, wherein the V concentration is more than 2% Excellent steel wire for high strength springs. The steel wire according to claim 1 , further comprising Ni: 0.3% or less (not including 0%) as another element. The steel wire according to claim 1 or 2 , further comprising Nb: 0.1% or less (not including 0%) as another element. Still other elements, Mo: 0.5% or less (not including 0%), contact and W: is intended to include one or more selected from the group consisting of 0.5% or less (not including 0%) The steel wire in any one of Claims 1-3 .

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