JPH06299296A - Steel for high strength spring excellent in decarburizing resistance - Google Patents

Abstract

PURPOSE:To obtain steel for a high strength spring capable of producing a spring having high strength, excellent in decarburizing resistance at the time of heating and hardening and good in a fatigue service life and settling resis tance. CONSTITUTION:The steel for a high strength spring having a compsn. contg. 0.3 to 0.6 C, 1.0 to 4.0% Si, 0.1 to 1.0% Mn, 1.0 to 5.0% Cr and 0.1 to 0.5% V, furthermore contg. one or more kinds of elements selected from the group of 0.1 to 4.0% Ni, 0.1 to 2.0% Mo and 0.1 to 5.0% Co and moreover contg., as other elements, 0.1 to 1.0% Cu and/or 0.05 to 1.0% Nb, and the balance iron with inevitable impurities and satisfying the relationship of ([Cr]/([C]X[Si]))>=2.0 (where [the elements] denote the wt.% of each element) is provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to steel for high strength springs used in valve springs, suspension springs, etc. of internal combustion engines, and particularly to enhance fatigue resistance which is an important required property of spring steels. The present invention relates to high strength spring steel with improved decarburization resistance.

[0002]

2. Description of the Related Art The chemical composition of spring steel is JIS G356.
5-3567,4801, etc., the rolled materials produced from them are drawn to a predetermined diameter, then oil tempered and then spring processed (cold working), or rolled materials are drawn. It is manufactured by a method such as wire-working, heating and spring-forming, and then quenching and tempering (hot working). Further, in recent years, as the required characteristics of springs have become more and more strict, various alloy steels subjected to heat treatment are often used.

By the way, the strength of the conventional spring steel after quenching and tempering is generally about 160 to 180 kgf / mm ² , but recently, high strength spring steel of 200 kgf / mm ² or more has been required. There is. It is possible to obtain a strength of 200 kgf / mm ² or more even with conventional spring steel by heat treatment, but if the strength is increased by such a method,
Other important spring properties such as fatigue life and fatigue resistance cannot be satisfied.

Further, addition of Si is a very effective means for increasing the strength to 200 kgf / mm ² or more, but generally, if the amount of Si is increased, decarburization occurs during heat treatment and the fatigue life of the spring is shortened. Cause

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and its purpose is 20
An object of the present invention is to provide a spring steel which has a strength of 0 kgf / mm ² level or more, is excellent in decarburization resistance, and has high fatigue strength and sag resistance.

[0006]

The structure of the high-strength steel for springs according to the present invention, which has been able to achieve the above object, is C:
0.3-0.6%, Si: 1.0-4.0%, Mn:
0.1-1.0%, Cr: 1.0-5.0% and V:
0.1 to 0.5%, or even Ni: 0.
1-4.0%, Mo: 0.1-2.0% and Co:
Cu: 0.1, containing one or more elements selected from the group consisting of 0.1 to 5.0%, or as another element.
˜1.0 and / or Nb: 0.05 to 1.0%, balance iron and unavoidable impurities, and ([Cr] / ([C] × [Si])) ≧ 2.0 ( However, [element] represents the weight% of each element) and has the gist of satisfying the relationship.

[0007]

In order to increase the strength of the material and improve the fatigue life of the spring, it is effective to increase the toughness of the material. On the other hand, in the conventional high-strength spring steel, a steel material having a relatively high carbon content has been used to increase the elastic limit, but from the viewpoint of improving the toughness of the material, it is effective to reduce the carbon content. That is clear. However, if the carbon content is reduced, the strength after quenching and tempering cannot be sufficiently increased, so there is a limit to the reduction of the carbon content, and addition of Si and Cr as other strengthening elements is necessary.

However, the addition of Si is effective in increasing the strength after quenching and tempering, but on the other hand, it tends to cause decarburization in the high temperature quenching step for obtaining a uniform austenite structure, which adversely affects the fatigue life of the spring. Will come.

However, according to the results of various experiments conducted by the present inventors, if the content ratios of the three elements of C, Si and Cr are adjusted so as to satisfy the relationship of the following (1), 20
Along with achieving high strength of 0 kgf / mm ² level or higher,
It was found that decarburization at the time of heating was suppressed and excellent fatigue life and sag resistance were obtained. ([Cr] / ([C] × [Si])) ≧ 2.0 (1)

The reasons for defining the chemical composition of the high strength spring steel according to the present invention will be described below. C: 0.3 to 0.6% C is an element that is indispensable for ensuring the strength after quenching and tempering. If it is less than 0.3%, not only the toughness after quenching and tempering deteriorates, but also it is desired. Fatigue characteristics and sag resistance cannot be obtained. Si: 1.0 to 4.0% Si is an essential element as a solid solution strengthening element, and is 1.0
If it is less than 100%, the strength of the matrix becomes insufficient, and
High reinforcement above the kgf / mm ² level cannot be achieved. However, if it exceeds 4.0%, the dissolution of carbides during quenching heating becomes insufficient, and if it is not heated to a high temperature, a uniform austenite structure cannot be obtained, and not only the strength after quenching and tempering decreases, but also in the spring. The sag resistance when processed is also poor.

Mn: 0.1 to 1.0% Mn must be contained as a hardenability improving element in an amount of 0.1% or more. However, if it exceeds 1.0%, residual austenite tends to be present after quenching and tempering, and rather the strength tends to decrease. Cr: 1.0 to 5.0% Cr, like Mn, contributes to the improvement of hardenability and also exhibits the effect of improving heat resistance. Moreover, according to the confirmation by the present inventors, it was confirmed that Cr exerts an action of significantly improving the decarburizing property during quenching and heating. Further, in order to effectively exert such effects, the content must be 1.0% or more, but if it is too much, the toughness after quenching and tempering tends to be deteriorated, so the upper limit was set to 5.0%.

V: 0.1 to 0.5% V is effective for refining the crystal grain size, increasing the yield strength ratio, and improving the sag resistance. In order to exert such results effectively, it is necessary to contain 0.1% or more,
If it exceeds 0.5%, the amount of alloy carbide that is not solid-dissolved in austenite at the time of quenching and heating increases, and these become large lumps and remain, which reduces the fatigue life.

The high-strength spring steel of the present invention essentially contains the above elements and the balance iron and unavoidable impurities. However, if necessary, one or more of Ni, Mo and Co may be selected. By including an element and further Cu and / or Nb, it is possible to further improve the characteristics. The preferable contents when adding these elements are as follows.

Ni: 0.1-4.0% Ni not only has the effect of increasing the material toughness after quenching and tempering, but also has the effect of significantly improving the sag resistance, which is important as spring characteristics. However, these effects are effectively exerted by containing 0.1% or more. But,
If the content of Al exceeds 4.0%, the Ms point decreases, and the predetermined strength cannot be obtained due to the influence of retained austenite. Mo: 0.1 to 2.0% Mo is a carbide-forming element and has the action of precipitating fine alloy carbides after tempering to promote secondary hardening, and to improve sag resistance and fatigue resistance. ing. 0.1%
If it is less than the above, the effect is insufficient, and if it is 2.0%, the effect is saturated.

Co: 0.1 to 5.0% Co is a solid solution strengthening element, has the characteristic of not degrading toughness, and also has the function of enhancing corrosion resistance. These effects are effectively exhibited by containing 0.1% or more, but 5.0% because it is an expensive element.
Was set as the upper limit. Cu: 0.1 to 1.0% Cu is an element that is electrochemically nobler than iron and has the effect of increasing corrosion resistance. Although such an effect is effectively exhibited when 0.1% or more is added, no further effect is obtained even if the content exceeds 1.0%, and rather the material may be embrittled during hot rolling. Will occur.

Nb: 0.05 to 1.0% Like N, Nb has the effect of refining the crystal grain size and increasing the yield strength ratio, and the effect is effectively obtained by containing 0.05% or more. To be demonstrated. However, even if the content exceeds 1.0%, no further effect is obtained, and rather coarse carbonitrides are formed during quenching and heating, which deteriorates the fatigue life.

In the present invention, in addition to specifying the types of contained elements and the respective content rates as described above, in order to suppress decarburization during quenching heating and to increase the fatigue life of a spring, C, Si and Cr are used. It is an inevitable requirement that the relationship of Eq. (1) be satisfied as the mutual relationship of the respective content ratios. By the way, FIG. 1 shows [Cr] / ([C] × [S] for spring steels having various chemical compositions including Examples described later.
i]) The relationship between the ratio and the decarburization depth is organized and graphed (however, the experimental requirements are shown in the section of Examples).

As is clear from FIG. 1, [Cr] /
When the ([C] x [Si]) ratio is around 2.0, the decarburization depth remarkably changes. When the ratio is less than 2.0, decarburization proceeds from the surface layer to a depth of 0.1 mm or more, and fatigue While the life is adversely affected, if the ratio is 2.0 or more, the decarburization depth can be suppressed to less than 0.1 mm from the surface layer portion, and it is possible to suppress the decrease in fatigue life due to quenching heating.

The manufacturing conditions of the high-strength spring using the spring steel satisfying the above chemical composition requirements are not limited, and forging → rolling → annealing → wire drawing → machining (grinding) →
Although it is sufficient to carry out heating and quenching in sequence, when performing heating and quenching after machining, the ultimate cooling temperature during quenching should be 50
A spring of high strength and high toughness can be obtained more reliably by setting the temperature to not more than 0 ° C. and then performing the tempering treatment.

For quenching of ordinary spring steel, oil quenching is adopted from the viewpoint of preventing quenching cracks, and the temperature is set to 70 to 80 ° C. in consideration of the viscosity of oil and the like. Therefore, it is difficult to set the ultimate cooling temperature to 50 ° C. or lower. However, at the beginning of quenching, cooling with oil was performed
The preferable quenching conditions as described above can be easily achieved by adopting a method of cooling the following temperature range with water or a method of adding a sparingly soluble quenching agent or the like to water to prevent quenching cracks due to water quenching.

[0021]

EXAMPLES Hereinafter, the constitution and effects of the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the following Examples and is not of a nature limiting the present invention, and conforms to the purpose of the above and the following. Any appropriate modification within the range to be obtained is included in the technical scope of the present invention.

Steel materials having chemical compositions No. 1 to 20 shown in Tables 1 and 2 were melted and forged to form a square billet of 155 mm, and then rolled by wire rod to a wire rod of 13 mmφ. Then, after annealing and wire drawing, grinding was carried out by machining to produce a straight bar of φ10 mm. After that, oil quenching was performed under the conditions of quenching and heating at 950 ° C. × 15 min, and the decarburization depth was evaluated by the distribution of hardness from the surface. Further, in order to confirm the tensile properties after quenching and tempering, treatment was performed at a tempering temperature of 350 ° C. × 1 hr, and tensile test pieces were taken from the heat-treated product and subjected to a tensile test. The inspection method in the decarburization depth evaluation was as follows. The results are collectively shown in Table 1.

(Decarburization depth measuring method) Specimen size: φ10 mm × 30 mm length Heat treatment condition: 950 ° C. × 15 min → oil 70 ° C. Heat treatment atmosphere: In air Measurement method: Hardness from the outermost surface to a depth of 0.5 mm Sa (H
v) Distribution Decarburization depth: Depth from the outermost surface where the hardness is lower than the base metal hardness (mm)

[0024]

[Table 1]

[0025]

[Table 2]

From Tables 1 and 2, the following can be considered. Nos. 1 to 12 are examples satisfying the requirements of the present invention, and all have a tensile strength of 200 kgf / mm ² level or more, and the decarburization depth is less than 0.10. It can be seen that it is also excellent in life and sag resistance characteristics. On the other hand, Nos. 13 to 20 are comparative examples lacking any of the prescribed requirements as described below, and either tensile strength or decarburization depth is insufficient.

No. 13: Carburizing depth exceeds 0.1 because Cr and V are not contained and the requirement of the formula (1) is not satisfied. No. 14 to 16: Since the amount of Cr is insufficient and the requirement of the formula (1) is not satisfied, decarburization cannot be suppressed sufficiently. No. 17: The tensile strength is extremely low because the amount of C is insufficient. No. 18: A uniform austenite structure cannot be obtained because the amount of Si is too large, and a target strength of 200 kgf / mm ² level or higher cannot be obtained. No. 19, 20: The individual chemical components satisfy the requirements, but the decarburization suppressing effect is insufficient because the requirement of formula (1) is lacking. No. 21: Since the amount of Mn is too large, the retained austenite increases, and the target strength of 200 kgf / mm ² level or higher cannot be obtained. Also, the decarburization depth tends to be deep. No. 22: The target strength of 200 kgf / mm ² or more cannot be obtained because the amount of V is insufficient. No. 23: 200 due to coarse carbide due to too much V
The target strength above kgf / mm ² cannot be obtained. No. 24: A target strength of 200 kgf / mm ² or more cannot be obtained because the amount of Cr is too large.

[0028]

The present invention is constituted as described above, and by specifying the chemical composition of the steel material, in particular, by containing a specific amount or more of Cr based on the product of the amount of C and the amount of Si, It has become possible to provide a steel material having high strength, excellent decarburization properties, and a spring that has excellent fatigue life and fatigue resistance.

[Brief description of drawings]

FIG. 1 is a graph showing an influence of a [Cr] / ([C] × [Si]) ratio in a steel material on a decarburization depth.

Claims (5)

[Claims] 1. C: 0.3 to 0.6% (meaning% by weight,
The same shall apply hereinafter), Si: 1.0 to 4.0%, Mn: 0.1
1.0%, Cr: 1.0 to 5.0% and V: 0.1
0.5%, balance iron and unavoidable impurities, and ([Cr] / ([C] × [Si])) ≧ 2.0 (where [element] represents the weight% of each element. ) High strength spring steel with excellent decarburization resistance, which satisfies the relationship of 2. Further, as another element, Ni: 0.1 to 0.1
4.0%, Mo: 0.1-2.0% and Co: 0.1
The high-strength spring steel according to claim 1, containing at least one element selected from the group consisting of ˜5.0%. 3. Further, as another element, Cu: 0.1 to 0.1
The high-strength spring steel according to claim 1 or 2, containing 1.0%. 4. Further, as another element, Nb: 0.05-
The high-strength spring steel according to claim 1 or 2, containing 1.0%. 5. Further, as another element, Cu: 0.1
The high strength spring steel according to claim 1 or 2, containing 1.0% and Nb: 0.05 to 1.0%.