WO2015119082A1

WO2015119082A1 - Spring and spring production method

Info

Publication number: WO2015119082A1
Application number: PCT/JP2015/052876
Authority: WO
Inventors: 英利吉川
Original assignee: 中央発條株式会社
Priority date: 2014-02-04
Filing date: 2015-02-02
Publication date: 2015-08-13
Also published as: JP2015145712A

Abstract

This spring contains a spring wire rod (10) that is coiled into a spiral shape. A plurality of micropockets (16) are formed on a surface of the spring wire rod (10), are extended in the length direction of the spring wire rod, and have a width in the direction orthogonal to the length direction that is equal to or less than 10μm. The depth (d) of each micropocket (16) is equal to or less than 2.0μm.

Description

Spring and spring manufacturing method

The technology disclosed in this specification relates to a spring and a method for manufacturing the spring. Specifically, the present invention relates to a technique for improving the fatigue strength of a spring (for example, a valve spring or a spring for a clutch).

In order to improve the fatigue strength of the spring, a technique for applying compressive residual stress to the surface of the material by shot peening is known (for example, Japanese Patent Application Laid-Open No. 2004-346424). In this technique, the surface of a spirally coiled spring wire is nitrided, and then shot peening is performed using amorphous particles. As a result, the fatigue strength is improved while keeping the surface roughness low.

In the technique of Japanese Patent Application Laid-Open No. 2004-346424, the spring has high fatigue strength by keeping the surface roughness of the spring low and applying compressive residual stress to the surface of the spring. However, according to the study of the present inventor, when the fatigue strength of the spring increases and the stress acting on the spring increases, the effect of surface defects increases, and the technique disclosed in Japanese Patent Application Laid-Open No. 2004-346424 provides sufficient fatigue strength. It has been found that there are cases where it cannot be improved. In particular, a spring having a relatively small diameter (for example, 2.0 to 6.5 mm) requires a high fatigue strength, so that the influence of surface defects becomes extremely large.

This specification aims at providing the technique for improving the fatigue strength of a spring more.

The spring disclosed in this specification has a spring wire coiled in a spiral shape. On the surface of the spring wire, a plurality of micro pockets extending in the longitudinal direction of the spring wire and having a width in a direction perpendicular to the longitudinal direction of 10 μm or less are formed. And the depth of each micro pocket is 2.0 micrometers or less.

In this spring, the depth of the micro pocket formed on the surface of the spring wire is 2.0 μm or less. As a result of intensive studies by the inventor of the present application, the depth d of micro pockets (so-called wire-drawing scratches) formed on the surface of the spring wire during spring production (at the time of wire drawing) is extremely important for the fatigue strength of the spring. It has been found. As will be described in detail later, when the depth d of the micro pocket is 2.0 μm or less, the fatigue strength is remarkably improved. In this spring, since the depth d of the micro pocket is 2.0 μm or less, it can have excellent fatigue strength.

The present specification also discloses a spring manufacturing method for manufacturing the above-described spring. That is, the spring manufacturing method disclosed in the present specification includes a coiling process in which the spring wire is coiled in a spiral shape, a first shot peening process in which the first projecting material is projected onto the surface of the spring wire after the coiling process, After the first shot peening process, there is a nitriding process for nitriding the surface of the spring wire, and a second shot peening process for projecting the second projecting material onto the surface of the spring wire after the nitriding process. On the surface of the spring wire after the coiling step, a plurality of micro pockets extending in the longitudinal direction of the spring wire and having a width in the direction perpendicular to the longitudinal direction of 10 μm or less are formed. After the first shot peening process, the first shot peening process is performed so that the depth of the micro pocket is 2.0 μm or less.

According to this manufacturing method, the first shot peening process is performed before nitriding the surface of the spring wire. For this reason, before the surface of a spring wire hardens | cures, the depth of the micro pocket of the surface of a spring wire can be made small. Thereby, a high fatigue strength spring can be efficiently manufactured.

Sectional drawing of the spring (spring wire) which concerns on a present Example. The figure which shows typically the micro pocket formed in the surface of a spring wire (cross-sectional view which expands and shows the surface of a spring wire). The figure which shows typically the shape of a micro pocket when the surface of a spring wire is planarly viewed. The flowchart which shows the manufacturing process of the spring of a present Example. The figure which shows the experimental result which measured the relationship between the depth of a micro pocket, and 108 ⁸ times fatigue strength. The figure which shows the relationship between the depth of a micro pocket, and projection time.

In the spring disclosed in this specification, the spring wire may have a steel layer and a nitride compound layer formed on the surface of the steel layer. By forming a nitride compound layer on the surface of the spring wire, the strength of the spring can be increased.

In the spring disclosed in the present specification, the steel layer is, in mass%, C: 0.50 to 0.80, Si: 1.30 to 2.50, Mn: 0.30 to 1.00, Cr: 0. .40 to 1.40 may be contained. Furthermore, the steel material layer may further contain at least one of Mo, V, W, Ni, and Nb. According to such a configuration, since the steel material for forming the spring is formed of an appropriate material, the fatigue strength can be further improved.

(Example) The spring 20 which concerns on an Example is demonstrated. The spring 20 is used as a valve spring for an automobile engine. The spring 20 is configured by a spring wire 10 formed in a coil shape (spiral shape), and a predetermined interval is provided between adjacent spring wires 10. In the present embodiment, the diameter of the spring wire is set to φ2.0 mm to 6.5 mm. When the diameter of the spring wire is in the range of φ2.0 to 6.5 mm, micro pockets (described later) formed on the surface of the spring wire greatly affect the fatigue strength of the spring.

As shown in FIG. 1, the spring wire 10 is composed of a steel material layer 12 and a compound layer 14. The steel material layer 12 is formed by heat-treating the spring wire. The steel material layer 12 (namely, spring wire) may contain, for example, C (carbon), Si (silicon), Mn (manganese), Cr (chromium), iron, and inevitable impurities. In this case, the ratio of each element is mass%, C is 0.50 to 0.80%, Si is 1.30 to 2.50%, Mn is 0.30 to 1.00%, and Cr is 0. .40 to 1.40%, and the balance may be Fe (iron) and inevitable impurities. The reason why C is 0.50% or more is that when C is less than 0.50%, it is difficult to satisfy both durability and sag resistance. Further, the reason why C is set to 0.80% or less is that when C exceeds 0.80%, the moldability is lowered, and the possibility of cracking or breakage during processing increases. The reason why Si is 1.30% or more is that when Si is less than 1.30%, sufficient sag resistance cannot be obtained. The reason why Si is set to 2.50% or less is that when Si exceeds 2.50%, the amount of decarburization during heat treatment exceeds the allowable range and adversely affects the durability. The reason why Mn is 0.30% or more is that sufficient strength cannot be obtained when Mn is less than 0.30%. The reason why Mn is set to 1.00% or less is that when Mn exceeds 1.00%, the amount of retained austenite becomes excessive. The reason why Cr is 0.40% or more is that when the Cr is less than 0.40%, sufficient solid solution strength and hardenability cannot be obtained. The reason why Cr is made 1.40% or less is that when Cr exceeds 1.40%, the amount of retained austenite becomes excessive.

The steel material layer 12 may further contain W (tungsten). When the steel material layer 12 contains W, the proportion of W is preferably about 0.08 to 0.20% by mass. The reason why W is made 0.08% or more is that when W is less than 0.08%, the effect of adding W (improving hardenability, increasing strength, etc.) cannot be obtained. Further, the reason why W is made 0.20% or less is that when W exceeds 0.20%, coarse carbides are formed, and mechanical properties such as ductility are deteriorated.

The steel material layer 12 may contain at least one of Mo (molybdenum), V (vanadium), Ni (nickel), and Nb (niobium) together with or in place of W. By containing Mo itself, the strength of steel can be improved and the hardenability can be improved. Moreover, by containing V, the magnitude | size of the carbide | carbonized_material precipitated in the steel material layer 12 can be made fine, and the intensity | strength of the steel material layer 12 can be improved more. Moreover, the toughness and ductility after heat processing can be improved by containing Ni. Moreover, by containing Nb, crystal grains can be refined, and toughness and ductility can be improved. When the steel material layer 12 contains at least one of Mo, V, Ni, and Nb, the ratio of the element is mass%, Mo is in the range of 0.05 to 0.25%, and V is 0.05 to 0. Preferably, the range is .60%, Ni is 0.20 to 0.40%, and Nb is 0.005 to 0.300%. The reason why Mo is 0.05% or more is that sufficient strength cannot be obtained when Mo is less than 0.05%. The reason why Mo is set to 0.25% or less is that when Mo exceeds 0.25%, the stabilizing action of retained austenite cannot be ignored. Further, the reason why V is set to 0.05% or more is that when V is less than 0.05%, a sufficient amount of carbide is not generated and the effect of preventing crystal grain growth cannot be obtained. Further, the reason why V is set to 0.60% or less is that when V exceeds 0.60%, vanadium carbide itself grows and becomes large, which adversely affects durability. The reason why Ni is 0.20% or more is that when Ni is less than 0.20%, the effect of improving toughness and ductility cannot be obtained. The reason why Ni is set to 0.40% or less is that when Ni exceeds 0.40%, the effect of increasing toughness and ductility is saturated. The reason why Nb is 0.005% or more is that when Nb is less than 0.005%, the crystal grains cannot be sufficiently refined. The reason why Nb is 0.300% or less is that when Nb exceeds 0.300%, the hardenability deteriorates.

A compound layer 14 is formed on the entire surface of the steel material layer 12. The thickness of the compound layer 14 is 7 μm or less. Since the thickness of the compound layer 14 is 7 μm or less, a decrease in strength due to the brittleness of the compound layer can be prevented. The compound layer 14 contains N (nitrogen) in addition to C, Si, Mn, Cr, Fe and unavoidable impurities contained in the steel material layer 12, and the compound layer 14 contains Si, Mn, Cr, A compound (nitride) of N and a metal element such as Fe exists. The concentration of N in the compound layer 14 is not particularly limited. For example, N is in a range of 5.0 to 6.1% by mass%.

2 and 3, a micro pocket 16 is formed on the surface of the spring wire 10. The micro pocket 16 is a scratch having a minute depth formed on the surface of the spring wire 10, and a plurality of micro pockets 16 are formed on the surface of the spring wire 10. The micro pocket 16 is a wire wound formed mainly when a material (wire material) is drawn. Since the micro pocket 16 is formed by wire drawing, the micro pocket 16 extends in the longitudinal direction of the spring wire 10 (X-axis direction in FIG. 3).

The width w (see FIG. 3) in the direction orthogonal to the longitudinal direction of each micro pocket 16 is 10 μm or less. That is, when the width w of the micro pocket 16 exceeds 10 μm, the strength of the spring wire 10 is greatly reduced. For this reason, the processing conditions and the like at the time of wire drawing are adjusted so that the width w of the micro pocket does not exceed 10 μm.

In this embodiment, the compound layer 14 is formed on the surface by coiling the drawn spring wire and then nitriding. That is, after a wire wound is formed on the spring wire by wire drawing, nitriding is performed on the spring wire. Therefore, as shown in FIG. 2, scratches 16 (that is, micro pockets 16) are also formed on the surface of the compound layer 14 due to the wire wound formed in the steel material layer 12. For this reason, the depth d of the micro pocket 16 becomes a depth proportional to the depth of the wire wound of the steel material layer 12. As is clear from the above, the depth d of the micro pocket 16 is reduced if the depth of the wire wound of the steel layer 12 is reduced. In the present embodiment, the depth of the micro pocket 16 formed in the spring wire 10 is 2.0 μm or less. As shown in the experimental results described later, the fatigue strength of the spring 20 can be dramatically improved by setting the depth of the micro pocket 16 formed on the surface of the spring wire 10 to 2.0 μm or less.

Next, a method for manufacturing the spring 20 will be described with reference to FIG. As shown in FIG. 4, first, the spring wire is formed into a coil shape by a coiling machine (S12). The spring wire is produced by drawing a material (wire). For this reason, a plurality of wire drawing scratches are formed on the surface of the spring wire. The diameter of the spring wire after drawing is set to φ2.0 to 6.5 mm. The drawn spring wire is formed into a coil shape (spiral shape) by a known coiling machine. Note that the spring wire is in mass%, C: 0.50 to 0.80, Si: 1.30 to 2.50, Mn: 0.30 to 1.00, Cr: 0.40 to 1.40, The balance contains iron and inevitable impurities. The spring wire may further include, by mass%, W: 0.08 to 0.20, and / or Mo: 0.05 to 0.25, and / or V: 0.05 to 0.60, and / or Alternatively, Ni: 0.20 to 0.40 and / or Nb: 0.005 to 0.300 can be contained.

After the spring wire is formed into a coil shape, the end of the spring wire is cut, and then the spring wire formed into a coil shape is subjected to low-temperature annealing, and the end surface of the spring wire formed into a coil shape is further formed. Grind. Thereby, the spring wire is formed into a spring shape.

Next, a first shot peening process (pre-shot peening process) is performed on the surface of the spring wire formed into a spring shape (S14). The first shot peening treatment is not for imparting compressive residual stress to the spring wire, but for removing the wire wound formed on the surface of the spring wire (that is, reducing the depth of the wire wound). Done. For this reason, the projection material used for the first shot peening process, and the projection speed and the projection time thereof are determined according to the depth of the wire wound formed on the surface of the spring wire. That is, if the depth of the wire wound formed on the surface of the spring wire is deep, the hardness of the projection material is relatively high, and / or the projection speed is relatively high, and / or the projection time is reduced. Set long. On the other hand, if the depth of the wire wound formed on the surface of the spring wire is shallow, the hardness of the projection material is relatively low, and / or the projection speed is relatively slow, and / or the projection time is reduced. Set short. Accordingly, the surface roughness of the spring wire due to shot peening is suppressed while reducing the depth of the wire drawing scratch formed on the surface of the spring wire. As a result, the depth of the wire wound after the first shot peening is about 2.0 μm or less, for example. In the first shot peening process, for example, a projection material having a diameter of 0.3 mm and a hardness of 390 HV or more can be used, and the projection speed of the projection material can be set to 60 to 90 m / s. The projection time can be 5 to 60 minutes.

Next, nitriding treatment is performed in an ammonia atmosphere on the spring wire whose depth of the wire drawing scratches has been reduced (S16). Thereby, the compound layer 14 having nitride is formed on the surface of the spring wire, and the steel material layer 12 not containing nitride is formed at the center of the spring wire. In the nitriding treatment, the temperature condition can be 400 ° C. or more and 540 ° C. or less (for example, 450 ° C.), and the treatment time can be 1 to 4 hours (for example, 1.5 hours). When the treatment time is less than 2 hours, the thickness of the compound layer 14 can be adjusted to an appropriate thickness (for example, 3 μm).

Next, in order to improve the fatigue resistance of the spring wire, a second shot peening process is performed on the surface of the spring wire (S16). The second shot peening process can be performed in a plurality of times. By performing shot peening a plurality of times, compressive residual stress can be applied to a deep position of the spring wire. In the second shot peening treatment, for example, the first stage shot peening (for example, the diameter of the projection material φ0.6 mm, the hardness of the projection material 650 to 750 HV) is performed on the surface of the spring wire just after nitriding, and then the second Stage shot peening (for example, projection material diameter φ0.3 mm, projection material hardness 650 to 750 HV) is performed, and third stage shot peening (for example, projection material diameter φ0.1 mm, A hardness of 700 to 1230 HV). Thus, by performing shot peening in multiple stages while changing the diameter and hardness of the projection material, it is possible to effectively apply compressive residual stress to the spring wire. In each of the first stage shot peening, the second stage shot peening, and the third stage shot peening, the projection speed of the projection material is preferably 60 to 90 m / s.

After performing the second shot peening process in S16, the spring wire is subjected to low temperature annealing, and then the spring wire is set. Thereby, the spring 20 is manufactured from a spring wire.

Next, spring wire (mass% C: 0.58, Si: 1.97, Mn: 0.76, P: 0.010, S: 0.008, Cr: 1.00, V: 0.086, The result of measuring the relationship between the depth of the micro pocket 16 and the fatigue strength of a spring actually manufactured using Ni: 0.22, the balance being iron and inevitable impurities) will be described. The diameter of the spring wire used for the measurement was φ3.4 mm. Further, the depth of the micro pocket 16 formed on the surface of the spring wire was changed in the range of 0.8 μm to 4.6 μm by changing the projection time of the projection material used for the first shot peening process. That is, as shown in FIG. 6, the depth of the micro pocket varies depending on the projection time of the projection material, and the depth of the micro pocket decreases as the projection time increases. For this reason, the depth of the micro pocket was adjusted to a range of 0.8 μm to 4.6 μm by changing the projection time of the projection material. In the example shown in FIG. 6, the first shot peening process is performed using a projection material having a diameter of 0.3 mm and a hardness of 650 to 750 HV. Each manufacturing process other than the first shot peening process was performed under the same conditions. Each spring manufactured as described above was subjected to a fatigue test with an average stress τm = 686 MPa and stress amplitudes at two levels of 615 MPa and 645 MPa, and the fatigue strength was measured 10 ⁸ times.

FIG. 5 shows the relationship between the micropocket depth (horizontal axis) and the fatigue strength (vertical axis) of 10 ⁸ times. As is apparent from FIG. 5, the fatigue strength of a spring having a micro pocket depth of 2.0 μm or less is 650 MPa or more, whereas the spring having a micro pocket depth of more than 2.0 μm has a fatigue strength of about 600 MPa. It became. Therefore, it was confirmed that the fatigue strength changes drastically when the depth of the micro pocket is 2.0 μm or less. The depth of the micro pocket was measured by photographing the surface of the spring with a laser microscope and measuring the depth of the micro pocket (within a width of 10 μm) from the photographed image. And the depth of the deepest micropocket among the depths of each micropocket was made into the depth of the micropocket of the spring.

As is clear from the above results, in the spring of this example, the depth of the micro pocket 16 formed on the surface of the spring wire is 2.0 μm or less. For this reason, the deep micro pocket used as the starting point of stress concentration is removed from the surface of a spring wire, and the fatigue strength of a spring can be improved markedly.

Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

For example, the embodiment described above is a valve spring for an automobile engine, but the present invention is not limited to such a form, and can be applied to other springs (for example, a clutch spring). The spring wire may contain inevitable impurities such as P (phosphorus) and S (sulfur). Since these inevitable impurities lead to a decrease in spring strength, the lower the concentration, the better. For example, it is preferable that P contained in the spring wire is 0.025% or less by weight and S is 0.025% or less.

The technical elements described in the present specification or drawings exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technology illustrated in the present specification or the drawings achieves a plurality of objects at the same time, and has technical utility by achieving one of the objects.

Claims

It has spring wire that is coiled spirally,
On the surface of the spring wire, a plurality of micro pockets extending in the longitudinal direction of the spring wire and having a width in a direction perpendicular to the longitudinal direction of 10 μm or less are formed,
A spring characterized in that each micro pocket has a depth of 2.0 μm or less.
2. The spring according to claim 1, wherein the spring wire has a steel material layer and a nitride compound layer formed on the surface of the steel material layer.
The steel layer contains, by mass%, C: 0.50 to 0.80, Si: 1.30 to 2.50, Mn: 0.30 to 1.00, Cr: 0.40 to 1.40. The spring according to claim 1 or 2.
The spring according to claim 3, wherein the steel layer further contains at least one of Mo, V, W, Ni, and Nb.
A method of manufacturing a spring,
A coiling process for spirally coiling the spring wire;
A first shot peening process for projecting the first projecting material onto the surface of the spring wire after the coiling process;
A nitriding step of nitriding the surface of the spring wire after the first shot peening step;
A second shot peening process for projecting the second projecting material onto the surface of the spring wire after the nitriding process,
On the surface of the spring wire after the coiling step, a plurality of micro pockets extending in the longitudinal direction of the spring wire and having a width in the direction perpendicular to the longitudinal direction of 10 μm or less are formed,
A method for manufacturing a spring, wherein the first shot peening step is performed after the first shot peening step so that the depth of the micro pocket is 2.0 μm or less.