JP2003105485A - High strength spring steel having excellent hydrogen fatigue cracking resistance, and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a high strength spring steel which has a high tensile strength of >=1,800 MPa, and excellent hydrogen fatigue cracking resistance, and to provide a production method therefor. SOLUTION: The high strength spring steel has a composition containing, by mass, 0.4 to 0.9% C, 0.5 to 3.0% Si, 0.1 to 2.0% Mn, <=0.02% P and <=0.02% S, and if required, further containing one or more metals selected from Mo, V, Cr, Ni, Cu, W, Al, Ti, Nb, B, N, Ca, Mg, Zr and rare earth metals, and the balance Fe with inevitable impurities, has a layered structure consisting of martensite and ferrite, and has a tensile strength of 1,800 to 2,400 MPa. Steel having the above composition is heated to a two phase region, is thereafter cooled at >=20 deg.C/sec, and is subsequently tempered at a temperature of an AC1 transformation point or lower, or the steel is heated to a two phase region, is thereafter cooled at >=20 deg.C/sec, and, plastic strain of >=0.003 is applied thereto, and the steel is subsequently tempered at the AC1 transformation point or lower.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has a high strength which has a tensile strength of 1800 to 2400 MPa used for valve springs, suspension springs, stabilizers, torsion bars, etc. of engines of automobiles and the like, and which is excellent in hydrogen fatigue resistance. The present invention relates to spring steel and a method for manufacturing the steel.

[0002]

2. Description of the Related Art In manufacturing a spring, for example, JI
A method in which a wire rod hot-rolled using spring steel specified in SG 3560, 3561 and 4801 etc. is drawn to a specified wire diameter and then spring processed after oil tempering (cold forming) or drawing It is general to employ a method of heating and springing after processing and then performing quenching and tempering (hot forming). In recent years, weight reduction of automobiles has been desired for the purpose of reducing carbon dioxide emission and fuel consumption in order to cope with environmental problems. As part of this, there is a demand for a spring having a tensile strength after quenching and tempering increased to 1800 MPa or more.
However, generally, when the strength of the spring is increased, the notch sensitivity is increased and the environment is likely to be adversely affected. Especially in a corrosive environment, when a corrosion pit is formed on the surface, it becomes a stress concentration source and becomes brittle due to hydrogen generated as the corrosion reaction progresses, so there is a problem that fatigue characteristics deteriorate and early breakage occurs. It was As a method of preventing embrittlement due to hydrogen, a method of refining crystal grains or a method of forming fine precipitates has been considered, but both methods show significant hydrogen fatigue resistance in the tests of the inventors. The characteristics have not been improved.

As described above, in the conventional technique, 1800
It was difficult to manufacture a high-strength spring having a tensile strength of MPa or more and excellent hydrogen fatigue resistance.

[0004]

DISCLOSURE OF THE INVENTION The present invention is intended to solve the above-mentioned problems, and is to provide 1800
An object of the present invention is to provide a high-strength spring steel having a tensile strength of ˜2400 MPa and excellent hydrogen fatigue resistance, and a manufacturing method thereof.

[0005]

[Means for Solving the Problems] The present inventors first analyzed hydrogen fatigue behavior in detail using spring steels of various strength levels produced by quenching and tempering. As a result, it was clarified that the fatigue life was shortened by hydrogen in the steel under stress below the fatigue limit. In addition, it was clarified that the decrease in fatigue life was caused by diffusible hydrogen that could penetrate into the steel from the external environment and diffuse in the steel at room temperature. Diffusible hydrogen is used for steel materials at 100 ° C / hou
In the curve of temperature-hydrogen release rate from steel obtained when heating at a rate of r, it can be measured as a curve having a peak at a temperature of about 100 ° C (Fig. 1). Therefore, if hydrogen that has entered from the environment is trapped in some part of the steel material so that it does not diffuse, it becomes possible to render the hydrogen harmless and suppress the fatigue life reduction. Therefore,
The hydrogen fatigue resistance was evaluated by determining the "fatigue limit diffusible hydrogen content" at which hydrogen fatigue does not occur. In this method, various amounts of diffusible hydrogen are contained by electrolytic hydrogen charging, and then Cd plating is performed to prevent hydrogen from being released from the sample into the atmosphere during a rotating bending fatigue test, and then the atmosphere is subjected to Cd plating. Among them, the amount of diffusible hydrogen at which a predetermined load is applied and fatigue fracture does not occur is evaluated. FIG. 2 shows an example of analysis of the relationship between the amount of diffusible hydrogen and fatigue life. As the amount of diffusible hydrogen contained in the sample decreases, the fatigue life becomes longer, and fatigue fracture does not occur if the amount of diffusible hydrogen is below a certain value. This amount of hydrogen is defined as "amount of fatigue limit diffusible hydrogen". Since hydrogen trapped by (Mo, V) ₂ C, etc. is unlikely to adversely affect the fatigue properties, steel materials containing such trap sites have a low fatigue strength due to hydrogen even if the "diffusible hydrogen content" is high. Is less likely to occur. Therefore, the fatigue limit diffusible hydrogen content becomes high. Since the amount of diffusible hydrogen that penetrates into steel in a real environment increases as the environment becomes more severe, a steel material with a higher fatigue limit diffusible hydrogen content has a more severe environment (environment with more diffusible hydrogen content).
However, hydrogen does not reduce fatigue strength. Therefore,
The higher the fatigue limit diffusible hydrogen content, the better the hydrogen fatigue resistance of the steel material, and the fatigue limit diffusible hydrogen content is a value peculiar to the steel material which is determined by the steel material components, manufacturing conditions such as heat treatment.

The present inventors have conducted extensive research on hydrogen fatigue resistance characteristics of various materials having different components by the above-mentioned method for determining the fatigue limit diffusible hydrogen content. As a result, the layered structure of tempered martensite and ferrite is obtained. It was found that the fatigue limit diffusible hydrogen content can be significantly increased by forming it. It was also found that the fatigue limit diffusible hydrogen content can be further increased by precipitating carbonitrides in the layered structure of tempered martensite and ferrite. As a result of further research, by appropriately controlling the composition of the steel material and the manufacturing conditions such as heat treatment, the above-described structure was formed, the tensile strength was 1800 to 2400 MPa, and the hydrogen fatigue resistance was excellent. We have found that steel can be obtained.

The present invention is constructed on the basis of such knowledge, and the gist thereof is (1)% by mass, C: 0.4 to 0.9%, Si: 0.5
~ 3.0%, Mn: 0.1-2.0%, P: 0.02%
Hereafter, S: 0.02% or less is contained, the balance is Fe and unavoidable impurities, the steel structure is tempered martensite and ferrite layered structure, and the tensile strength is 1
High-strength spring steel with excellent hydrogen fatigue resistance, which is 800-2400 MPa.

(2) It contains the component described in (1) above, and further, in mass%, Mo: 0.4 to 3.0%, V: 0.02.
A high-strength spring steel excellent in hydrogen fatigue fracture resistance, characterized by containing 1 to 2% of 1.0% to 1.0%.

(3) The composition according to (1) or (2) above is contained, and further, in mass%, Cr: 0.05 to 3.0%,
Ni: 0.05 to 5.0%, Cu: 0.05 to 2.0
%, W: 0.05 to 3.0% of 1 type or 2 types or more, The high strength spring steel excellent in hydrogen fatigue fracture resistance characterized by the above-mentioned.

(4) The composition contains the components described in (1) to (3) above, and further, in mass%, Al: 0.005 to 0.1%,
Ti: 0.005 to 0.3%, Nb: 0.005 to 0.
3%, B: 0.0003 to 0.05%, N: 0.001
~ 0.05%, 1 type or 2 or more types of containing, The high strength steel for springs excellent in hydrogen fatigue fracture resistance.

(5) The composition according to any one of (1) to (4) above is contained, and further, in mass%, Ca: 0.0.
01-0.01%, Mg: 0.0005-0.01%,
Zr: 0.001-0.05%, REM: 0.001-
A high-strength spring steel excellent in hydrogen fatigue fracture resistance, which is characterized by containing one or more of 0.05%.

(6) Fatigue limit diffusible hydrogen content is 0.2 pp
The high-strength spring steel excellent in hydrogen fatigue fracture resistance according to any one of (1) to (5) above, characterized in that it is m or more.

(7) Any one of the above (1) to (6), characterized by containing one or two of retained austenite, pearlite and bainite in an area ratio of 10% or less.
A high-strength spring steel excellent in hydrogen fatigue resistance.

(8) A method for manufacturing the spring steel according to any one of (1) to (7) above, which comprises:
~ (5) A _C1 steel containing components according to any one of
After heating to transformation point to A _C3 transformation point, cooling from the heating temperature at 20 to 300 ° C. / sec to below the martensite transformation start temperature, resistance to hydrogen fatigue, characterized in that subsequently tempered at 200 ° C. to A _C1 transformation point A method for producing high strength spring steel excellent in fracture characteristics.

(9) A method for manufacturing the spring steel according to any one of (1) to (7), which comprises:
~ (5) A _C1 steel containing components according to any one of
After heating to transformation point to A _C3 transformation point, the heating temperature to below the martensite transformation start temperature was cooled at 20 to 300 ° C. / sec, then applying a plastic strain of 0.003 to 0.1, subsequently 200 ° C. ~ A method for producing a high-strength spring steel excellent in hydrogen fatigue fracture resistance, characterized by tempering at a _C1 transformation point.

(10) Any one of (1) to (7) above
A method for producing a spring steel according to item 1, wherein the steel comprising the component according to any one of (1) to (5) is heated to an A _C1 transformation point to an A _C3 transformation point and then heated. temperature martensitic transformation start temperature was cooled at 20 to 300 ° C. / sec to below from then heated to 200 ° C. to a _C1 transformation point 0.00
A method for producing a high-strength spring steel having excellent hydrogen fatigue fracture resistance, which is characterized by imparting a plastic strain of 3 to 0.1.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The significance of each requirement and the reason for limitation in the present invention will be specifically described below.

First, the reasons for limiting the organizational form will be described.
The present inventors produced various steel materials having the same strength level but different structures by changing the components and heat treatment conditions, and measured the fatigue limit diffusible hydrogen content. An example of the analysis of the relationship between the fatigue limit diffusible hydrogen content and the structure is shown in FIG. The fatigue limit diffusible hydrogen content is low in the single structure of tempered martensite by the conventional quenching and tempering method, but the fatigue limit diffusible hydrogen content is greatly increased by making the layered structure of tempered martensite and ferrite, and the hydrogen fatigue resistance It can be seen that the fracture characteristics are improved. In order to have high strength and to significantly improve hydrogen fatigue fracture resistance, in the layered structure of tempered martensite and ferrite, the average distance between tempered martensites is preferably 10 μm or less, and more preferably 5 μm or less. The average interval can be measured by the cutting method. Further, even if one or more of retained austenite, bainite, and pearlite are present in the tempered martensite having a layered structure in an area ratio of 10% or less, there is no problem with respect to hydrogen fatigue resistance. Further, in the layered structure of tempered martensite, the ferrite content is preferably in the range of 10 to 70%.
This is because if the ferrite fraction is less than 10%, the effect of improving the hydrogen fatigue fracture resistance is small, while if it exceeds 70%, it is 1
This is because it becomes difficult to achieve a high strength of 800 MPa or more.

On the other hand, when the tensile strength exceeds 2400 MPa, sufficient hydrogen fatigue fracture resistance cannot be obtained even with a layered structure of tempered martensite and ferrite, so the tensile strength is set to 2400 MPa or less.

In the present invention, the area ratio of each structure of tempered martensite, ferrite, retained austenite, bainite, and pearlite is the C-section t / 4 part of the steel rod or the C-section t / 4 part of the spring, which is measured by an optical microscope or It is defined as the average value when 10 fields of view are observed with a scanning electron microscope at 200 to 1000 times.

The limit diffusible hydrogen content is 0.2 ppm
If it is less than 0.2 ppm, the hydrogen fracture resistance may not be sufficient and hydrogen fatigue fracture may occur in a typical environment in which it is actually used.

Next, the reasons for limiting the components of the high strength spring steel according to the present invention will be described.

C: C is added as an element that increases the strength of steel. If it is less than 0.4%, it is difficult to secure the strength required for the spring steel, and if it is added in excess of 0.9%, the toughness is significantly deteriorated. Therefore, the C content is 0.
It was set to 4 to 0.9%. Si: Si is added as a deoxidizer and as an element that increases the strength of steel. If it is less than 0.5%, the effect of improving the strength cannot be effectively exhibited, and if it is added in excess of 3.0%, a coarse oxide is formed to deteriorate ductility and toughness. Therefore, the Si content is 0.5 to 3.0%
And Mn: Mn is an element effective for improving the hardenability, but is a harmful element that embrittles the grain boundaries and deteriorates the hydrogen fatigue fracture resistance. If it is less than 0.1%, the effect of enhancing the hardenability is not exhibited, and if it is added in excess of 2.0%, the hydrogen fracture fatigue resistance is deteriorated. Therefore, the Mn content is set to 0.1 to 2.0%. In order to obtain better hydrogen fatigue fracture resistance, the Mn content is preferably 0.5% or less.

P: P is an impurity element that segregates at the grain boundaries to lower the grain boundary strength and deteriorates the toughness. It is desirable that the level is as low as possible, but in consideration of the attainable level and cost of the present refining technology. The upper limit was 0.02%. S: S is an impurity element that deteriorates hot workability and toughness, and it is desirable that the level be as low as possible. However, the upper limit was set to 0.02% in consideration of the attainable level and cost of the current refining technology. The above are the basic components of the present invention.
And unavoidable impurities, depending on the desired strength level and other required properties, Mo, V, Cr, Ni, C
u, W, Al, Ti, Nb, B, N, Ca, Mg, Z
One or two or more of r and REM may be added.

Mo: Mo together with V and C (Mo, V)
It is an element that improves hydrogen fatigue fracture resistance by forming ₂ C and trapping diffusible hydrogen, but if it is less than 0.4%, its effect is not exhibited. To lower the Mo content, the Mo content is 0.3 to
It was set to 3.0%. V: V is an element that forms (Mo, V) ₂ C together with Mo and C, and improves hydrogen fatigue fracture resistance by trapping diffusible hydrogen, but if it is less than 0.02%, its effect is exhibited. However, since the excessive addition of more than 1.0% lowers the toughness, the V content was set to 0.02 to 1.0%. Cr, Ni, Cu, W: Cr, Ni, Cu and W are all effective elements for improving corrosion resistance and strength. This effect is not exhibited when the content is less than 0.05%. Cr is 3%, Ni is 5%, Cu is 2%, and W is excessively added in excess of 3% to deteriorate toughness. Therefore, the Cr content should be 0.
05-3.0%, Ni content 0.05-5.0%,
The Cu content is 0.05 to 2.0%, and the W content is 0.
It was set to 05 to 3.0%. Al: Al acts as a deoxidizer and has the effect of forming AlN and suppressing crystal grain coarsening, but if it is less than 0.005%, that effect is not exhibited, and if added in excess of 0.1%, toughness is exhibited. Is deteriorated, the Al content is 0.005
It was set to 0.1%.

Ti: Ti has the effect of forming TiN and suppressing crystal grain coarsening, but if it is less than 0.005%, that effect is not exhibited, and if it is added in excess of 0.3%, toughness deteriorates. Therefore, the content of Ti is 0.005-0.3%
And Nb: Nb has the effect of forming fine carbonitrides and suppressing crystal grain coarsening, but if it is less than 0.005%, that effect is not exhibited, and if added in excess of 0.3%, toughness deteriorates. Therefore, the content of Nb is set to 0.005 to 0.3%.

B: B segregates itself at the grain boundaries to improve the grain boundary bonding force, suppresses the grain boundary segregation of P, S and Cu, enhances the grain boundary strength, and improves delayed fracture characteristics and toughness. It is also an effective element for improving the hardenability. These effects are 0.0003
If it is less than 0.05%, if it is excessively added in excess of 0.05%, coarse precipitates are formed in the grain boundaries and the hot workability and toughness are deteriorated. Therefore, the content of B is 0.0003 to 0.05. %. N: N has an effect of forming a nitride and suppressing crystal grain coarsening, but if it is less than 0.001%, the effect is not exhibited,
If added over 0.05%, the toughness deteriorates, so N
The content was set to 0.001 to 0.05%.

Ca, Mg, Zr, REM: Ca, Mg,
Both Zr and REM are effective elements that suppress the deterioration of hot workability and toughness due to S. This effect is 0.
Less than 001%, Mg less than 0.0005%, Zr less than 0.005%.
If it is less than 001%, REM is less than 0.001%, Ca is not 0.01%, Mg is 0.01%, and Zr is less than 0.001.
Excess addition of more than 0.05% and REM of 0.05% deteriorates toughness. Therefore, the Ca content is 0.001 to 0.
01%, Mg content 0.0005-0.01%, Z
The content of r was 0.001 to 0.05%, and the content of REM was 0.001 to 0.05%.

Next, the reasons for limiting the manufacturing conditions will be described.

The method for producing a high-strength spring steel according to the present invention comprises heating the steel composed of the above-mentioned components to a two-phase region and subsequently performing a tempering treatment.

First, heating conditions will be described. When the heating temperature exceeds _C1 less than transformation point or A _C3 transformation point A, since the final tempered martensite and ferrite lamellar structure is not obtained, the heating temperature is a temperature range of A _C1 transformation point to A _C3 transformation point Limited In addition, (A _C1 transformation point +10) ~ (A
A temperature range of _C3 transformation point-10) ° C is a more desirable condition. Further, in order to refine the layered structure of tempered martensite and ferrite, the steel before heating is preferably adjusted to a structure mainly composed of martensite, tempered martensite, bainite or pearlite,
At this time, it is desirable that the ferrite fraction is less than 10%. As a means for controlling the structure before heating, there are adjustments of the cooling rate after hot rolling, ordinary quenching and tempering, adjustments of the cooling rate after austenitizing treatment, and any method is acceptable.

After heating to the two-phase region, cooling is started from the heating temperature. If the cooling rate is less than 20 ° C./sec, a large amount of ferrite, pearlite, and bainite may be generated during cooling, and the strength may decrease. Therefore, the lower limit of the cooling rate was limited to 20 ° C./sec. A more desirable cooling rate is 50 ° C./second or more from the viewpoint of preventing as much as possible ferrite, pearlite, and bainite that are generated during cooling. On the other hand, if the cooling rate exceeds 300 ° C./second, quench cracking tends to occur, so the cooling rate is set to 300 ° C./second or less. Incidentally, the end temperature of the cooling for producing a martensite is less martensite transformation start temperature (M _S point).

Next, the tempering treatment conditions will be described. The steel after the dual phase heat treatment has a dual phase structure of martensite and ferrite. Excessive dislocations and residual stress in martensite are eliminated by recovery, and supersaturated carbon atoms are precipitated as carbides, whereby tempering is performed to improve toughness and ductility. In particular, in steel containing V, Mo, Nb, and Ti,
The tempering treatment produces carbonitrides of these alloying elements, and improves hydrogen fatigue resistance. In this tempering treatment, if the heating temperature exceeds the A _C1 transformation point, reverse transformation occurs and finally a layered structure of tempered martensite and ferrite cannot be obtained, so the heating temperature was limited to the A _C1 transformation point or lower.
On the other hand, if the heating temperature is lower than 200 ° C., the above effect cannot be obtained, so the heating temperature is set to 200 ° C. or higher. In addition,
From the viewpoint of improving hydrogen fatigue resistance, it is desirable that the heating rate during tempering be 5 ° C / sec or more, and the cooling rate after tempering be 20 ° C.
/ Sec or more is desirable.

In the present invention, 0.003 after the two-phase region heat treatment
The above plastic strain may be applied and then tempered, or a plastic strain of 0.003 or more may be applied during tempering. The purpose is to increase the yield strength and improve the sag resistance. If the plastic strain is less than 0.003, these effects are small, so the lower limit of the plastic strain is set to 0.003. On the other hand, if the plastic strain exceeds 0.1, the toughness deteriorates, so the upper limit of the plastic strain is 0.1. The means for imparting plastic strain may be any method such as tensile strain and strain due to straightening.

In the present invention, before the heating in the temperature range of A _C1 transformation point to A _C3 transformation point, after heating / cooling or after tempering treatment, light wire drawing or plastic working is performed for the purpose of adjusting wire diameter, deforming, etc. Even if it is carried out, there is no deterioration of hydrogen fatigue resistance, and there is no restriction.

[0036]

EXAMPLES The effects of the present invention will be described more specifically below with reference to examples.

Steels having the compositions shown in Table 1 were heat-treated in the two-phase region under the conditions shown in Table 1, and then tempered at the temperatures shown in Table 1. As shown in Table 1, some steels were subjected to plastic strain before or during tempering. The structure and tensile strength of each steel piece after heat treatment are shown in Table 1 together. In the present invention examples (No. 1 to 5), all are 1800 to 240
A tensile strength of 0 MPa is obtained. The hydrogen fatigue fatigue resistance of these steel pieces was evaluated by the fatigue limit diffusible hydrogen content described above. The load stress at the time of performing the fatigue test was 90% of the fatigue limit in the atmosphere.

[0038]

[Table 1]

[0039]

[Table 2]

From Table 1, in the present invention examples (Nos. 1 to 5), the fatigue limit diffusible hydrogen content is 0.2 ppm or more, and the hydrogen fatigue fracture resistance is excellent. In particular, Mo,
Those containing V (Nos. 2 and 3) all have a fatigue limit diffusible hydrogen content of 0.8 ppm or more, and are markedly excellent in hydrogen fatigue fracture resistance. In addition, a high yield stress was obtained for those to which plastic strain was added (Nos. 4 and 5). On the other hand, in the comparative example (No. 6) in which the heating temperature of the two-phase heat treatment exceeds the A _C3 transformation point, the fatigue limit diffusible hydrogen content is 0.1 pp because of the tempered martensite single phase structure.
It is as low as m or less, and it is understood that the hydrogen fatigue resistance is inferior. Further, in the comparative example (No. 7) in which the heating temperature of the two-phase heat treatment is less than the A _C1 transformation point, the fatigue limit diffusible hydrogen content was high because the cementite had a high temperature tempered martensite single phase structure. It is as low as 0.1 ppm or less, which means that the hydrogen fatigue resistance is inferior. Further, a comparative example (No. 4) in which the cooling rate after the two-phase region heat treatment is less than 20 ° C./sec.
In 8), ferrite and 20% in area ratio were used during cooling.
%, The fatigue limit diffusible hydrogen content was as low as 0.1 ppm or less, and the hydrogen fatigue fracture resistance was inferior. In addition, one or more of the steel components deviate from the scope of the present invention (No. 9, 1)
In 0), the tensile strength is less than 1800 MPa, which means that the tensile strength required for high strength spring steel is not obtained. From the above, by specifying the steel composition in the range shown in the present invention, and by manufacturing under the two-phase region heat treatment conditions and the tempering conditions shown in the present invention, the tempered martensite and the ferrite layered structure are formed, and 1800 to 2400 MPa It is clear that a steel having the above tensile strength and excellent hydrogen fatigue resistance can be obtained.

[0041]

As described above, according to the present invention, 1800
It is possible to obtain a high-strength spring steel having a tensile strength of ˜2400 MPa and excellent hydrogen fatigue resistance and a method for producing the same.

[Brief description of drawings]

FIG. 1 is a diagram showing a hydrogen release curve by a temperature rise analysis and a diffusible hydrogen amount.

FIG. 2 is a diagram showing an example of the relationship between the amount of diffusible hydrogen and fatigue life.

FIG. 3 is a diagram showing a relationship between a fatigue limit diffusible hydrogen content and a structure.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C22C 38/58 C22C 38/58 F term (reference) 4K032 AA01 AA02 AA05 AA06 AA08 AA11 AA12 AA14 AA15 AA16 AA19 AA20 AA21 AA22 AA23 AA24 AA27 AA29 AA31 AA32 AA35 AA36 AA37 AA39 AA40 CA00 CA01 CD03 CF01

Claims (10)

[Claims] 1. In mass%, C: 0.4 to 0.9%, Si: 0.5 to 3.0%, M
n: 0.1 to 2.0%, P: 0.02% or less, S:
0.02% or less, the balance consisting of Fe and unavoidable impurities, the steel structure consisting of tempered martensite and ferrite layered structure, and the tensile strength of 1800 to
High strength spring steel with excellent hydrogen fatigue resistance, characterized by 2400 MPa. 2. Further, in mass%, Mo: 0.4 to 3.0%, V: 0.02 to 1.0%,
1 or 2 types of are contained.
High-strength spring steel with excellent hydrogen fatigue resistance. 3. Further, in mass%, Cr: 0.05-3.0%, Ni: 0.05-5.0
%, Cu: 0.05 to 2.0%, W: 0.05 to 3.
The high-strength spring steel excellent in hydrogen fatigue fracture resistance according to claim 1 or 2, containing 0% of one kind or two or more kinds. 4. Further, in mass%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.
3%, Nb: 0.005 to 0.3%, B: 0.000
3 to 0.05%, N: 0.001 to 0.05%, 1
1. The composition according to claim 1, which contains one or more kinds.
High-strength spring steel excellent in hydrogen fatigue fracture resistance according to any one of 3 to 3. 5. Further, in mass%, Ca: 0.001-0.01%, Mg: 0.0005-
0.01%, Zr: 0.001 to 0.05%, REM:
High strength spring steel excellent in hydrogen fatigue fracture resistance according to any one of claims 1 to 4, containing 0.001 to 0.05% of one or more kinds. . 6. A high strength spring steel excellent in hydrogen fatigue fracture resistance according to claim 1, wherein the fatigue limit diffusible hydrogen content is 0.2 ppm or more. 7. The hydrogen fatigue fracture resistance property according to any one of claims 1 to 6, which contains one or more of retained austenite, pearlite and bainite in an area ratio of 10% or less. High strength steel for springs. 8. A method for producing the spring steel according to any one of claims 1 to 7, wherein the steel comprising the components according to any one of claims 1 to 5 is transformed into A _C1. after heating to the point to a _C3 transformation point, cooled at 20 to 300 ° C. / sec from the heating temperature to below the martensite transformation start temperature, then 200 ° C. ~
A method for producing a high-strength spring steel excellent in hydrogen fatigue fracture resistance, characterized by tempering at an A _C1 transformation point. 9. A method for producing the spring steel according to any one of claims 1 to 7, wherein the steel composed of the components according to any one of claims 1 to 5 is transformed into A _C1. after heating to the point to a _C3 transformation point, cooling martensite transformation start temperature to below 20 to 300 ° C. / sec from the heating temperature, then 0.00
Grant plastic strain of 3 to 0.1, continued to 200 ℃ ~A _C1
A method for producing a high-strength spring steel having excellent hydrogen fatigue resistance, which is characterized by tempering at a transformation point. 10. A method for producing the spring steel according to any one of claims 1 to 7, wherein the steel comprising the components according to any one of claims 1 to 5 is transformed into A _C1. after heating to the point to a _C3 transformation point, cooled at 20 to 300 ° C. / sec from the heating temperature to below the martensite transformation start temperature, then 200 ° C.
Method of producing a high strength spring steel superior in resistance to hydrogen fatigue fracture properties, characterized in that heating below to A _C1 transformation point to impart plastic strain of 0.003 to 0.1.