JPH0673506A - Coil spring excellent in fatigue resistance and its production - Google Patents

Abstract

PURPOSE:To produce a coil spring having superior fatigue resistance and high hardness by subjecting a steel wire having a specific composition consisting of C, Ni, Si, Mn, V, Mo, Cr, and Fe to spring forming at room temp. and then allowing martensitic transformation to occur. CONSTITUTION:This spring is a cold forming coil spring which has a composition consisting of, by weight, 0.5-1.5% C, 5-25% Ni, 1.0-3.0% Si, 0.5-3.0% Mn, further one or more kinds among <=1.0% V, <=3.0% Mo, and 3.0% Cr, and the balance Fe with inevitable impurities and satisfying inequality 77<=K<=273 (where K=823-361C-17Ni-11Si-39Mn-35V-5Mo-20Cr) and also has >=HMV620 hardness. This spring has excellent fatigue resistance. This spring can be obtained by subjecting a steel wire with this composition, where martensitic transformation is initiated at <=0 deg.C, to spring forming at room temp. and then allowing martensitic transformation to occur by means of cooling down to a temp. not higher than the martensitic transformation initiating temp. Moreover, if necessary, nitriding treatment is done before the cooling treatment, and after that, shot peening treatment is performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a coil spring, such as a valve spring of an automobile engine, which is required to have strict fatigue resistance, and a manufacturing method thereof.

[0002]

2. Description of the Related Art Coil springs, such as valve springs for automobile engines, which are required to have severe fatigue resistance, are currently heated by drawing a wire rod until it becomes austenitic and then quenching it in water or oil. After inducing martensitic transformation, a tempered steel wire, that is, an oil tempered wire is used to secure workability, and this is spring-formed to manufacture. Then, if necessary, a treatment such as nitriding treatment or shot peening is also performed.

As another fatigue resistant coil spring, for example, as shown in Japanese Utility Model Laid-Open No. 1-78157,
After drawing and spring forming the wire, heating and austenitizing, cooling and holding at about 300 ° C just above the martensite transformation start temperature, quenching in water or hot water to cause martensite transformation, and further tempering There is a method of performing (Marquench method).

Further, regarding stainless steel, as described in Metal (Agne Co.) March 1984, P74-75, the amount of martensite induced by working is increased by drawing a stainless wire in a subzero temperature range. There is an example in which the strength is increased by, and the fatigue resistance is improved by spring forming this. Further, as a material whose strength is increased by the sub-zero treatment, there is no application example of a coil spring, but there is, for example, a deep-chill hardening type Fe-Ni alloy as disclosed in JP-A-1-149942.

[0005]

However, in the case of a spring using a conventional oil tempered wire, the tempering temperature must be set high at the time of making the oil tempered wire in order to secure workability during spring forming, and Is reduced. Also, it is necessary to perform tempering for the purpose of strain relief after spring molding,
At this time, martensite is decomposed and the strength is further reduced.

On the other hand, in the case of the marquenching method, after the spring is formed, it is necessary to heat it to 800 ° C. or higher for austenitizing, and at this time, the material strength becomes extremely low and the formed spring is deformed. There is a point.

[0007] Further, in the case of a stainless steel wire processed at a sub-zero temperature and increased in the amount of martensite to improve the strength, there is a problem that the workability is poor because the strength is high and the shape is not stable during spring forming. .

Further, in the case of a deep-chill treatment hardening Fe-Ni alloy, the tensile strength after treatment at a subzero temperature is 175.
Since it is about 0 MPa, it cannot be said that the strength is enough to be used for a coil spring such as a valve spring which requires severe fatigue resistance.

[0009]

The coil spring of the present invention has been made to solve the above-mentioned problems, and its characteristic feature is that C: 0.5 to 1.5% by weight, Ni: : 5
25%, Si: 1.0 to 3.0%, Mn: 0.5 to 3.
0%, further contains 1.0% or less of V, 3.0% or less of Mo, 3.0% or less of one or more kinds of Cr, the balance Fe and unavoidable impurities, In addition to satisfying the following formula, it has a hardness of HMV 620 or higher. 77 ≦ K ≦ 273 where K = 823-361 × [C%] − 17 [Ni%]
-11x [Si%]-39x [Mn%]-35x [V
%] − 5 × [Mo%] − 20 × [Cr%] In this case, it is preferable that the compressive residual stress on the surface is 400 MPa or more.

The method for producing such a coil spring is a component system that causes martensitic transformation and uses a steel wire or an alloy wire whose martensitic transformation start temperature (Ms point) is adjusted to 0 ° C. or less. Of the austenite phase, which is low in workability and excellent in workability, is spring-formed at room temperature, then cooled to a temperature below the martensite transformation start temperature to transform into a martensite phase having high strength. here,
Fatigue resistance can be improved by performing tempering at a low temperature as needed, and further, fatigue resistance can be further improved by using nitriding treatment and shot peening treatment together.

In the present invention, a martensitic transformation start temperature (Ms point) is a component system that causes martensitic transformation.
The steel wire or alloy wire whose temperature is adjusted to 0 ° C. or lower is formed by spring forming in an austenite state with low strength and excellent workability at room temperature, and then cooled to a temperature below the martensitic transformation start temperature to increase the strength. This is to obtain a martensite phase having excellent fatigue resistance.

Next, the reasons for limiting the amount of each constituent element will be described. C is an essential element for obtaining a high-strength martensite phase, and if its amount is less than 0.5%, a martensite phase having sufficient strength cannot be obtained, and conversely 1.5%. This is because if the content exceeds C, the toughness deteriorates because C cannot be completely melted during the solution treatment.

Ni is an essential element for stabilizing the austenite phase at room temperature, but if it is too much, the strength of the martensite phase will be low, and its optimum value is 5 to 25%.

Si and Mn contribute to strengthening the martensite phase as solid solution strengthening elements. However, if the added amount is too large, there is a problem that the toughness of the material decreases, and the optimum values are 1 to 3.0% and 0.5 to respectively.
It is 3.0%.

V, Cr, and Mo are precipitation-strengthening elements that combine with C to form fine carbides and contribute to the improvement of material strength. However, if the amount of addition is too large, the precipitated carbides become coarse and the toughness of the material decreases. The optimum values are 1.0% or less and 3.0, respectively.
% Or less and 3.0% or less.

By satisfying such a component condition, it is possible to obtain high strength having a hardness of HMV 620 or more.
In addition to this condition, in order to cause martensitic transformation by performing subzero treatment with inexpensive liquid nitrogen in an austenite phase at room temperature, the martensitic transformation start temperature (Ms
Point) must be in the range of 0 to -196 ° C. For that, the following formula must be satisfied. 77 ≦ K ≦ 273 where K = 823-361 × [C%] − 17 [Ni%]
-11x [Si%]-39x [Mn%]-35x [V
%]-5 x [Mo%]-20 x [Cr%]

Further, by performing nitriding treatment and / or shot peening treatment, a compressive residual stress can be applied to the surface. The magnitude of this compressive residual stress is 400 MPa
If it is above, fatigue resistance will improve further.

Particularly, after the spring forming, nitriding treatment is performed, then cooling is performed to cause martensite transformation, and then shot peening treatment is performed, as compared with the case where shot peening treatment is performed before martensite transformation is performed. The value of compressive residual stress becomes high, and the fatigue resistance can be effectively improved.

[0019]

EXAMPLES Examples of the present invention will be described below. (Test Example 1) Each sample having the components and K values shown in Table 1 was melted, forged, processed into a wire of 4.0φ, and subjected to solution treatment and aging treatment to obtain a test steel. Then, these sample steels were subjected to sub-zero treatment, and the tensile strength, drawing, and hardness before and after the treatment were examined. Table 2 shows the tensile strength and drawing value before the sub-zero treatment and the tensile strength and hardness after the sub-zero treatment.

[0020]

[Table 1]

[0021]

[Table 2]

From the above results, first, in B1 of the comparative example, the amount of C was small and the amount of Ni was large, and it was not possible to obtain sufficient tensile strength and hardness after the subzero treatment. Next, it is found that B2 has a large amount of C and a small amount of Ni, has a very low aperture value before the sub-zero treatment, and has a problem in workability. Further, B3 has an extremely low K value, and even if the subzero treatment is performed, martensitic transformation does not occur and tensile strength and hardness hardly change.
Further, the amount of Mo in B4 and the amount of Si in B5 are respectively large, and it can be seen that even in these steels, the drawing value before sub-zero treatment is extremely low and the workability is poor.

On the other hand, in all the examples, the drawing value before the sub-zero treatment is 30% or more, and it is understood that the examples have sufficient workability. It was also confirmed that after the sub-zero treatment, the tensile strength was 2000 MPa or more and the hardness was 620 or more, which were sufficient values.

(Test Example 2) Next, in% by weight, C: 0.8
1%, Ni: 15.4%, Si: 1.82%, Mn:
A wire having a metal composition of 0.61%, V: 0.12%, Mo: 0.64%, and the balance of Fe is drawn to 4.0 mmφ, heated to 1150 ° C., immersed in water, and rapidly cooled, followed by aging treatment. The wire was obtained by doing. First, the result of obtaining the martensite transformation start temperature (Ms point) of this wire by electric resistance measurement (DC four-terminal method) is shown in FIG. From the graph of the figure, it can be seen that the Ms point is about −55 ° C.

Next, the tensile strength and hardness (HMV) of this strand were measured. The condition is that the wire is as it is, the wire is immersed in liquid nitrogen and subjected to subzero cooling treatment (−196 ° C.), the wire is subjected to the subzero treatment, and further low temperature tempering (230 ° C. × 20 minutes). ) Was done. The results are shown in Table 3.

[0026]

[Table 3]

As is apparent from Table 3, the tensile strength is increased by 700 MPa or more and the hardness is increased by 200 or more by performing the martensitic transformation by performing the subzero treatment.
It was confirmed that the tensile strength and hardness did not decrease even after the above low temperature tempering.

Next, this strand and Si-C, which is a comparative example, are used.
Using the r-steel oil tempered wire (JIS SWOSC-V), the specifications springs shown in Table 4 were manufactured by the following manufacturing process. Example 1: Wire strand → coiling → temper (400 ° C x 3
0 minutes) → sub-zero cooling (in liquid nitrogen) → tempering (230
℃ × 20 minutes) Comparative Example 1: SWOSC-V → coiling → temper (400 ℃
X 30 minutes) Example 2-1: Element wire → coiling → temper (400 ° C)
X 30 minutes)-> shot peening-> sub-zero cooling (in liquid nitrogen)-> tempering (230 ° C x 20 minutes) Example 2-2: strands->coiling-> temper (400 ° C).
* 30 minutes)-> Sub-zero cooling (in liquid nitrogen)-> Shot peening-> Tempering (230 ° C * 20 minutes) Comparative example 2: SWOSC-V->Coiling-> Temper (400 ° C)
× 30 minutes) → Shot peening → Strain relief annealing (23
0 ° C. × 20 minutes) Example 3-1: Element wire → coiling → nitriding treatment (450 ° C.)
X 2 hours)-> shot peening-> sub-zero cooling (in liquid nitrogen)-> tempering (230 ° C x 20 minutes) Example 3-2: strands->coiling-> nitriding (450 ° C).
× 2 hours) → Sub-zero cooling (in liquid nitrogen) → Shot peening → Tempering (230 ° C × 20 minutes) Comparative Example 3: SWOSC-V → Coiling → Nitriding treatment (450 ° C)
× 2 hours) → Shot peening → Strain relief annealing (23
0 ℃ x 20 minutes)

[0029]

[Table 4]

The residual compressive stress and fatigue characteristics of the obtained coil spring were examined. Fatigue characteristics have an average stress of 65
When the pressure was 0 MPa, the amplitude stress was measured when the fracture did not occur even after 5 × 10 ⁷ times. The results are shown in Table 5.

[0031]

[Table 5]

As is clear from the results shown in Table 5, in each of the examples, the fatigue limit was 80 in comparison with that of the comparative example, not to mention that it was subjected to the nitriding treatment and / or the shot peening treatment. It exceeds 110 MPa, and shows extremely excellent fatigue characteristics. Also, comparing Example 2-1 and Example 2-2 or Example 3-1 and Example 3-2, those obtained by performing shot peening treatment after sub-zero cooling regardless of the presence or absence of nitriding treatment are as follows. The value of residual compressive stress is higher than that performed before the cooling, and the fatigue limit is also increased by 10 to 20 MPa accordingly.

[0033]

As described above, since the coil spring of the present invention has extremely excellent fatigue resistance, it is optimal for applications requiring high reliability such as valve springs for automobile engines. Particularly, the fatigue characteristics can be further improved by using the nitriding treatment and the shot peening treatment together in the manufacturing process.

[Brief description of drawings]

FIG. 1 is a graph showing the results of determining the martensite transformation start temperature of a wire used for a coil spring of the present invention by measuring electric resistance (DC four-terminal method).

Claims (6)

[Claims] 1. By weight%, C: 0.5-1.5%, N
i: 5 to 25%, Si: 1.0 to 3.0%, Mn: 0.
5 to 3.0% inclusive, 1.0% or less V, 3.0
% Or less of Mo and 3.0% or less of Cr, and the balance consists of Fe and unavoidable impurities. The following formula is satisfied and the hardness is HMV 620 or more. Cold forming coil spring. 77 ≦ K ≦ 273 where K = 823-361 × [C%] − 17 [Ni%]
-11x [Si%]-39x [Mn%]-35x [V
%]-5 x [Mo%]-20 x [Cr%] 2. The coil spring according to claim 1, wherein the residual compressive stress on the surface is 400 MPa or more. 3. A component system that causes martensitic transformation,
Using a steel wire or alloy wire whose martensitic transformation starting temperature is adjusted to 0 ° C. or lower, after spring forming at room temperature, cooling to below the martensitic transformation starting temperature to cause martensitic transformation Production method. 4. C: 0.5-1.5%, N by weight%
i: 5 to 25%, Si: 1.0 to 3.0%, Mn: 0.
5 to 3.0% inclusive, 1.0% or less V, 3.0
% Or less of Mo, 3.0% or less of one or more kinds of Cr, the balance consisting of Fe and unavoidable impurities, using a steel wire or alloy wire satisfying the following formula, spring forming at room temperature Then, the method for producing a coil spring is characterized by cooling to a martensitic transformation start temperature or lower to cause martensitic transformation. 77 ≦ K ≦ 273 where K = 823-361 × [C%] − 17 [Ni%]
-11x [Si%]-39x [Mn%]-35x [V
%]-5 x [Mo%]-20 x [Cr%] 5. The method for manufacturing a coil spring according to claim 4, wherein nitriding treatment and / or shot peening treatment is performed in any step after the spring molding. 6. The coil according to claim 4, wherein after the spring is formed, a nitriding treatment is performed and then the martensite transformation start temperature is cooled to a temperature below the martensite transformation start temperature to cause a shot peening treatment. Spring manufacturing method.