JP5693126B2 - Coil spring and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength coil spring excellent in fatigue resistance and sag resistance, which is mainly used as a coil spring of an internal combustion engine for automobiles, and a method for manufacturing the same.

As wire rods for valve springs in internal combustion engines, JIS includes oil temper wires for valve springs (SWO-V: JIS G 3561), chrome vanadium steel oil temper wires for valve springs (SWOCV-V: JIS G 3565), and In addition, a silicon chrome steel oil tempered wire (SWOSC-V: JIS G 3566) for valve springs is defined, and conventionally, SWOSC-V having excellent fatigue resistance and sag resistance has been mainly used.
These wires are pre-quenched and tempered at the factory to obtain the required strength, and after this, coiled into the spring of the required shape, nitriding, shot peening, temper, setting It is a general manufacturing method of a valve spring to obtain a spring excellent in fatigue strength and sag resistance characteristics by performing a process such as.

  On the other hand, from the viewpoint of environmental protection and resource protection, there is a high demand for purifying exhaust and improving fuel efficiency for automobiles, but it is the weight reduction of vehicles that greatly contributes to these. However, efforts to reduce weight continue to be made.

The valve spring can be made more compact by further increasing its fatigue strength and lowering the sag, and can contribute to the weight reduction of the engine.
Therefore, various proposals for improving the fatigue strength of the valve spring have been made by increasing the strength of the valve spring wire itself.

On the other hand, it is well known that machine parts that are generally carburized, quenched, and tempered have improved fatigue strength. Using this principle, for example, in Patent Document 1, C: 0.3 to 0.5% by weight, Si: 0.5 to 2.0%, Mn: 0.5 to 1.0%, Cr: A spring having high strength and excellent durability characterized by carburizing, quenching and tempering using steel containing 0.2 to 2.5% and other Fe and unavoidable impurities has been proposed. ).
In addition, in this patent document, a spring using steel further added with one or more of V: 0.02 to 0.5% or Nb: 0.02 to 0.5% to the material steel (Claim 2), In addition, a spring using steel added with Ni: 0.5 to 2.0% or Mo: 0.1 to 0.6% (Claim 3) is disclosed.

Japanese Patent Laid-Open No. 01-165751

  In general, carburizing and tempering are performed to improve wear resistance and fatigue strength by significantly hardening only the surface layer of the material. Therefore, since the surface layer portion (carburized layer) is hard and hardly ductile, it cannot be subjected to plastic working after carburizing and tempering. In other words, it is not possible to coil carburized and tempered wire, so when applying this heat treatment to the spring, carburizing and tempering after forming a wire with good workability into the spring. Will be.

  By the way, what has been disclosed conventionally (Patent Document 1) regulates the amount of C up to 0.5%, and beyond that, the spring becomes too brittle and is no longer used. None added 0.5% or more of C. However, when the C content is up to 0.5%, since the C content of the material is low, the strength of the material dough itself other than the surface layer portion after quenching and tempering is insufficient, and fatigue resistance and resistance to resistance are reduced. There is a limit to the improvement of the slipperiness, and in order to further improve the durability, it is essential to increase the strength by adding C amount exceeding 0.5%.

The present invention has been made in view of the above-described circumstances, and by selecting the optimum material and making the subsequent spring manufacturing process appropriate for the material, the fatigue is compared with the conventional one. It is to provide a spring with improved strength.
Originally, in the state of use of the spring, the spring is used within the elastic limit, so it does not undergo large composition deformation during use. Therefore, in spring products, a large amount of ductility is essential. And not. Therefore, as a result of studying a method for manufacturing a carburized and tempered spring using a material having a C content of more than 0.5%, the spring does not cause breakage or defects during the spring manufacturing, and the spring As a result, it has become clear that the durability and sag resistance during use of the spring can be greatly improved.

The coil spring according to the present invention made to solve the above problems is, by mass ratio, more than 0.5% and 0.9% or less of C, 0.8 to 3.5% of Si, 0 and .3～3.0% of Mn, and 0.5 to 3.5% of Cr, and with 0.05 to 1.5% of Ni, La, 0. The steel is made of steel containing 0.5 to 0.5% V , the balance being Fe and unavoidable impurities, and having a carburized hardened layer with a depth of 0.05 to 1.00 mm on the surface, and 0.1 from the surface. The hardness at a position of 02 mm is 650 to 1000 Hv. However, the hardened layer depth here refers to a range in which the hardness is improved with respect to the base material hardness.

  When the coil spring is manufactured, the steel having the above composition is carburized. The manufacturing method of the present invention is characterized in that the carburizing process is performed in a vacuum state.

  In the spring described above, the hardness at a position 1 / 4d from the outer diameter of the wire is preferably 550 Hv or more. More desirably, it is 700 Hv or more.

  In the spring described above, the grain size number of the base material after carburizing and tempering is preferably 9 or more. More desirably, the grain size number is 11 or more.

  According to the present invention, by using a steel containing C in a mass ratio of more than 0.5% and not more than 0.9%, and carburizing the steel, the depth of the carburized hardened layer is 0.05 to 1. Since the hardness at 0.000 mm and the position 0.02 mm from the surface is 650 to 1000 Hv, it is possible to provide a coil spring that is further excellent in fatigue strength and sag resistance.

  In the high-strength coil spring of the present invention, first, the C content of the raw steel is higher than that of the conventional steel (Patent Document 1), more than 0.5%, and less than 0.9%. C is an essential element for increasing the strength of the steel wire, and in order to further increase the strength, carburizing treatment was performed and the surface layer had a hardened layer. Moreover, since there is a concern that the toughness will decrease as the strength increases, the toughness is improved by refining the crystal grains.

  Under the above basic idea, various chemical components of the spring material having high strength have been studied and the present invention has been achieved. Below, the reason for the limiting conditions of components and tissues will be described.

Hardened layer depth: 0.05 to 1.00 mm, and hardness at 0.02 mm from the surface: 650 Hv to 1000 Hv In the carburizing treatment, C diffuses from the surface and the surface layer is cured. If the thickness is less than 0.05 mm, the hardening of the carburization is small, and if it exceeds 1.00 mm, the carbide is coarsely precipitated, so that the strength is lowered. Further, if the hardness at 0.02 mm from the surface is less than 650 Hv, the absolute hardening amount is small and the fatigue strength is small.

Hardness at a position 1 / 4d from the wire outer diameter: 550Hv or more When a certain external force (load) is applied for a long time, the spring is permanently deformed even if the stress applied to the spring is within the elastic limit. However, if the internal hardness (1 / 4d position) is less than 550 Hv, the sag resistance tends to be inferior, so the hardness is preferably 550 Hv or more.

Hereinafter, the coil spring of the present invention and the manufacturing method thereof will be described in more detail together with the action of each alloy component and the reasons for limiting the numerical values thereof. In the present specification, “%” means mass percentage unless otherwise specified.
First, together with each component element and action in the present invention, the reason for limiting their numerical values will be described.

C: Over 0.5% and 0.9% or less
C is an essential component element for increasing the strength of the steel wire.
That is, when the C content of the steel is 0.5% or less, sufficient strength cannot be obtained. Conversely, when it exceeds 0.9%, the toughness decreases and the wrinkle sensitivity of the steel wire increases, reliability. Therefore, it is necessary to be within the above range.

Si: 0.8 to 3.5%
Si is an element effective for improving the strength of ferrite and martensite and improving sag resistance.
However, when the Si content of the steel is less than 0.8%, the above effects cannot be obtained sufficiently. Conversely, when it exceeds 3.5%, the cold workability is lowered and the hot workability is reduced. In order to promote decarburization by heat treatment or heat treatment, the above range is necessary.

Mn: 0.3 to 3.0%
Mn improves the hardenability of the steel and has the effect of fixing S in the steel and reducing its damage.
However, if the Mn content is less than 0.3%, the effect is hardly obtained, and if it exceeds 3.0%, the toughness decreases, so the above range is set.

Cr: 0.5 to 3.5%
Cr, like Mn, is a component necessary for improving the hardenability of steel and preventing C graphitization in high C and high Si steel.
At this time, if the Cr content in the steel is less than 0.5%, the effect is not sufficient, and conversely if over 3.5%, the solid solution of the carbide is suppressed, causing a decrease in strength, Since the hardenability is excessively increased and the toughness is lowered, it is necessary to be within the above range.

Ni: 0.05 to 1.5%
Ni is added because it is an effective element for imparting toughness to the spring strengthened by C.
However, when the Ni content is less than 0.05%, the intended effect is not exhibited, and when it exceeds 1.5%, the toughness is reduced. Must be within the range.

Mo: 0.05-1.5%
V: 0.05-0.5%
Nb: 0.01 to 0.5%
These elements are all elements that form carbides during tempering and increase the softening resistance. Therefore, V is added, but otherwise , within the respective numerical ranges as described above, any one of these is added . One or a combination of two or more can be added.
However, when each added amount is less than the lower limit of each, the intended effect cannot be sufficiently obtained, and when the upper limit is exceeded, a large amount of carbides are formed during quenching heating, resulting in a decrease in toughness. Therefore, even when it is added, it is necessary to be within the above ranges.

Crystal grain size of base material: 9 or more The refinement of crystal grains improves the toughness of the spring. If the crystal grain size is less than 9, the toughness inside the spring may be insufficient, so this range was adopted.

As described above, the coil spring of the present invention is more than 0.5% 0.9% or less C, 0.8-3.5% Si, 0.3-3.0% Mn, containing 0.5 to 3.5 percent of Cr, containing the balance Ri Fe and unavoidable impurities der, and further 0.05 to 1.5% of Ni, 0.05 to 0.5% V Made of steel. And it has a carburized hardened layer having a depth of 0.05 to 1.00 mm from the surface, and further has a hardness of 650 to 1000 Hv at a position of 0.02 mm from the surface.

Such a coil spring is obtained by hot forging and hot rolling the steel of the above components to form a wire, forming a coil spring after patenting, wire drawing and oil tempering, carburizing treatment, preferably vacuum carburizing. It can manufacture by processing.
Thereafter, from the viewpoint of further improving the fatigue strength, shot peening or setting can be performed as necessary, and it is desirable to generate a compressive residual stress of 750 MPa or more on the surface. Furthermore, as a means for improving the surface hardness, it is also desirable to perform a nitriding treatment after the carburizing treatment if necessary.

  Furthermore, after the carburizing treatment, it is desirable to cool the gas to a temperature below the A1 transformation point, and then to perform a reheating treatment for 10 to 30 minutes in a temperature range of, for example, 830 to 850 ° C. Is possible.

  The coil spring thus obtained can be suitably used as, for example, a valve spring for an automobile engine or a transmission spring.

  EXAMPLES Hereinafter, although this invention is explained in full detail based on an Example and a comparative example, this invention is not limited to these Examples.

Each steel having the chemical composition shown in Table 1 was melted in a vacuum melting furnace, and then a wire rod having a wire diameter of 8.0 mm was formed by hot forging and hot rolling based on a conventional method. Then, after removing the decarburized layer on the surface of the wire by removing 0.15 mm of the surface layer portion of the wire, heating, austenitizing in a neutral gas atmosphere, and so-called lead patenting was performed for pearlite transformation in molten lead. After that, wire drawing was performed to a wire diameter of 4.1 mm, and an oil temper treatment was performed under conditions suitable for the components of the experimental material to produce a spring wire.
Next, this element wire was used to form a coil spring having an average coil diameter of 24.60 mm, a free height of 46.55 mm, and a winding number of 5.75.

Next, each of the obtained coil springs was vacuum carburized under any of the conditions shown in Table 2 and then heated at 850 ° C. for 15 minutes and then quenched in oil maintained at 50 ° C. Thereafter, tempering was performed at 350 ° C. for 90 minutes. Further, hot setting was performed after shot peening. Shot peening was performed in three stages, and the particle size projected gradually from the first stage was reduced. Hot setting was performed at a temperature of 230 ° C. with a stress of 1600 MPa or more.
In Comparative Example 1, nitriding was performed instead of vacuum carburizing, and hot setting was performed after three stages of shot peening. About Comparative Examples 2-9, the vacuum carburizing process of a predetermined condition was performed.

  For each coil spring obtained as described above, the depth of the carburized hardened layer and the hardness at the 0.02 mm position and d / 4 position from the surface were measured, and the austenite grain size was measured based on JIS G 0551.

Further, a fatigue test was performed at room temperature under the condition that the maximum shear stress (τmax) calculated from the spring shape was 760 ± 711 MPa.
In addition, after compressing and holding the spring at a temperature of 120 ° C. for 48 hours at a stress of 1471 MPa, the spring is solved, the amount of sag before and after the test is measured, and the residual shear strain rate is calculated. did. These results are shown in Table 3.

  As is clear from the above results, it was confirmed that all the coil springs according to the examples of the present invention had a long fracture life, high strength and excellent durability.

Claims (5)

By mass ratio, more than 0.5% and 0.9% or less of C, 0.8 to 3.5% Si, 0.3 to 3.0% Mn, 0.5 to 3.5 % Of Cr , 0.05 to 1.5% Ni, 0.05 to 0.5% V, the balance being Fe and unavoidable impurities steel, the austenite crystal grain size of the steel A coil spring having a number of 9 or more, a carburized hard layer having a depth of 0.05 to 1.00 mm, and a hardness of 650 to 1000 Hv at a position of 0.02 mm from the surface. 2. The coil spring according to claim 1 , wherein when the wire diameter is d, the hardness at a position of d / 4 from the surface is 550 Hv or more. 3. A method for manufacturing a coil spring, wherein the carburizing process is performed in a vacuum state when the coil spring according to claim 1 is manufactured. 3. A method of manufacturing a coil spring according to claim 1 or 2 , wherein, after carburizing, gas cooling is performed to a temperature below the A1 transformation point, heat treatment is performed again, and quenching is performed. The coil spring according to claim 1 , wherein the coil spring is a valve spring or a transmission spring for an automobile engine.