JP2002097551A - High strength steel superior in resistance to hydrogen fatigue, and manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength steel for a spring with adequate resistance to hydrogen fatigue and with a tensile strength of 1700 MPa or more, and a manufacturing method therefor. SOLUTION: The high strength steel for the spring with superior resistance to hydrogen fatigue is characterized by that an upper limit (hereafter referred to as a limited quantity of diffusing hydrogen) of diffusing hydrogen quantity (quantity of released hydrogen when heated from a room temperature to 500 deg.C) in which the fatigue life does not decrease to less than 107 times, is 0.1 ppm or more, when a fatigue test for a steel material is carried out with stress of 90% of fatigue limit in atmosphere, and is further characterized by that a phase of the highest area rate is tempered martensite, and ratio of length to width of an original austenite grain (hereafter referred to as an aspect ratio) is 2 or more.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength spring having a tensile strength of 1700 MPa or more used for valve springs, suspension springs, stabilizers, torsion bars and the like of engines of automobiles and the like. The present invention relates to a high-strength spring steel having excellent hydrogen fatigue properties and a method for producing the same.

[0002]

2. Description of the Related Art High-strength springs, which are widely used in automobiles and the like, are hot-rolled using spring steel specified in, for example, JIS G 3565-3567 and 4801, and then drawn to a predetermined wire diameter. And, after the oil tempering process, spring processing,
Alternatively, it is manufactured by a method of performing spring processing by heating after drawing processing and performing quenching and tempering. In recent years, in order to cope with environmental problems such as reduction of carbon dioxide emission, automobiles have been required to be lighter in weight to reduce fuel consumption. As a part of this, there is a demand for a spring whose tensile strength after quenching and tempering is increased to 1800 MPa or more. However, in general, when the strength of the spring is increased, the fatigue characteristics in a corrosive environment deteriorate, and there is a concern that the spring may be broken at an early stage. One of the causes of deterioration of the corrosion fatigue characteristics is embrittlement due to hydrogen generated as the corrosion reaction progresses. One of the improvement measures is to increase the strength by adding a large amount of various alloying elements. However, this method has a problem that the cost of the material is high. Further, as a method of suppressing the hydrogen fatigue characteristics, a method of refining crystal grains and a method of generating fine precipitates are considered to be promising. The improvement of the hydrogen embrittlement characteristics has not yet been achieved.

As described above, in the prior art, there was a limit in manufacturing a high-strength spring with drastically improved resistance to hydrogen fatigue.

[0004]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above situation, and provides a high-strength spring steel having good hydrogen fatigue resistance and a tensile strength of 1700 MPa or more, and its production. It is intended to provide a method.

[0005]

The present inventors first analyzed the hydrogen fatigue behavior in detail using spring steels of various strength levels manufactured by quenching and tempering. As a result, it has been clarified that the fatigue life is reduced by hydrogen in steel at the stress less than the fatigue limit. In addition, it was clarified that the decrease in fatigue life was caused by diffusible hydrogen that could enter the steel from the external environment and diffuse through the steel at room temperature. Diffusible hydrogen is applied to steel at 100 ° C / hour.
Can be measured as a curve having a peak at a temperature of about 100 ° C. in a curve of temperature versus hydrogen release rate obtained when the steel is heated at the rate of (FIG. 1). Therefore, by preventing hydrogen that has invaded from the environment from diffusing by trapping it in some part in the steel material, it becomes possible to make the hydrogen harmless, and a reduction in fatigue life is suppressed. Therefore, the hydrogen fatigue resistance was evaluated by obtaining the “critical diffusible hydrogen amount” at which hydrogen fatigue does not occur. In this method, various levels of diffusible hydrogen are contained by electrolytic hydrogen charging, and then Cd plating is applied to prevent hydrogen from leaking from the sample into the atmosphere during the rotating bending fatigue test. A predetermined load is applied in the inside, and the amount of diffusible hydrogen at which fatigue fracture does not occur is evaluated. FIG. 2 shows an example of analyzing the relationship between the amount of diffusible hydrogen and the fatigue life. The fatigue life becomes longer as the amount of diffusible hydrogen contained in the sample decreases, and when the amount of diffusible hydrogen is less than a certain value, fatigue fracture does not occur. This amount of hydrogen is defined as “critical diffusible hydrogen amount”. The higher the critical diffusible hydrogen content, the better the hydrogen fatigue resistance characteristics of the steel material, which is a value specific to the steel material determined by the steel composition, heat treatment, and other manufacturing conditions.

[0006] Therefore, in order to increase the critical diffusible hydrogen content of the high-strength spring, that is, to improve the hydrogen fatigue resistance, the inventors examined the effects of the austenite grain size and the conditions of quenching and tempering, and found the following. did.

That is, the hot working finishing temperature is set at 700 ° C. to 900 ° C., which is the non-recrystallization temperature range, and the rolling reduction in this temperature range is
30% or more, preferably 50% or more, by cooling immediately after processing, the old austenite crystal grains from the surface layer to a depth of at least 0.5 mm or more are elongated, the old austenite grains from the surface layer to a depth of at least 0.5 mm It is possible to obtain a structure having an aspect ratio of 2 or more, 4 or more under the above preferable conditions, and a layer having the maximum area ratio of martensite. 1700MPa for such an organization
It has been found that the amount of critical diffusible hydrogen increases significantly even in a high-strength region exceeding the above range, and that hydrogen fatigue resistance is improved. Based on the above examination results, it has been concluded that a high-strength bolt excellent in delayed fracture characteristics can be realized by optimally selecting a steel material composition, a structure form, and a heat treatment condition, and made the present invention.

[0008] The present invention has been made based on the above findings, and the gist thereof is as follows. (1) a fatigue test of the steel material, is released upon heating when performed in 90% stress of the fatigue limit in air, the amount of diffusible hydrogen of fatigue life is not reduced to less than 10 ⁷ times (from room temperature to 500 ° C. Hydrogen amount) (hereinafter referred to as the critical diffusible hydrogen amount)
High strength spring steel excellent in hydrogen fatigue resistance characterized by being 0.1 ppm or more. (2) The phase having the largest area ratio is tempered martensite,
A high-strength spring steel having excellent hydrogen fatigue resistance, wherein the ratio between the length and the width of the prior austenite grains (hereinafter referred to as the aspect ratio) is 2 or more. (3) The phase having the largest area ratio is tempered martensite,
The aspect ratio is 2 or more and the fatigue limit hydrogen content is 0.
High-strength spring steel with excellent hydrogen fatigue resistance, characterized by being at least 1 ppm. (4) By mass%, C: 0.3 to 1% Si: 0.05 to 4% Mn: 0.05 to 2%, and the balance is Fe or any of the above (1) to (3), which consists of unavoidable impurities. A high-strength steel for springs having excellent hydrogen fatigue resistance according to the above item. (5) In mass%, Al: 0.005 to 0.1% Ti: 0.005 to 0.5% Cr: 0.05 to 2%, Mo: 0.05 to 2% Ni: 0.05 to 5%, Cu: 0.05 to 1% V: 0.05 to 2 % Nb: 0.005 to 0.2% Ta: 0.005 to 0.5% W: 0.05 to 0.5% and B: 0.0003 to 0.005% High strength spring steel with excellent properties. (6) A method for producing a high-strength spring steel according to any one of the above (1) to (5), wherein a heat giving a rolling reduction of 30% or more in a temperature range of 700 ° C to 900 ° C. After the cold working process,
Quenching to make the phase with the largest area ratio a martensite structure,
Thereafter, a tempering treatment is carried out.

[0009]

Next, an embodiment of the present invention will be described.

[0010] Critical diffusible hydrogen content: If the critical diffusible hydrogen content is less than 0.1 ppm, the fatigue resistance is insufficient, so the content is set to 0.1 ppm or more.

Structure: In order to obtain high strength, it is preferable that the phase having the largest area ratio is tempered martensite. In the present invention, the area ratio of the tempered martensite is an average value when a C section t / 4 portion of a steel rod or a C section t / 4 of a bolt is observed with an optical microscope at a magnification of 200 to 1000 for 10 fields of view. Other structures can include retained austenite, bainite, ferrite, and pearlite.

Aspect ratio: The reason for limiting the aspect ratio of prior austenite grains, which is the most important point for improving the hydrogen fatigue resistance of the high-strength spring steel intended in the present invention, will be described. FIG. 3 shows an example in which the influence of the aspect ratio of the prior austenite grains on the critical diffusible hydrogen content of the spring having the tempered martensite structure is analyzed. When the aspect ratio is less than 2, the effect of improving the critical diffusible hydrogen amount is small, that is, the effect of improving the hydrogen fatigue resistance is small, so the aspect ratio is limited to 2 or more. Preferably it is 3 or more. Although the effect of the present invention can be obtained without any particular upper limit of the aspect ratio, it is preferably 8 or less in order to obtain good mechanical properties other than delayed fracture characteristics.

The aspect ratio of the prior austenite grains is an average value when the above-mentioned sample is subjected to grain boundary etching and observed in 10 visual fields at 200 to 1000 times with an optical microscope.

Steel composition: Next, the reasons for limiting the composition of the steel targeted by the present invention will be described.

C: C is an essential element for securing the strength of the spring, but if it is less than 0.3%, the required strength cannot be obtained within a predetermined tempering temperature range, while if it exceeds 1%, the toughness is deteriorated. Therefore, it was limited to the range of 0.3 to 1%, preferably 0.5 to 1%.

Si: Si has an effect of increasing the strength by a solid solution hardening effect. If it is less than 0.05%, the above-mentioned effect cannot be exerted. On the other hand, if it exceeds 4%, an effect commensurate with the added amount cannot be expected.

Mn: Mn is an element not only necessary for deoxidation and desulfurization but also effective for enhancing hardenability for obtaining a martensitic structure. On the other hand, if it exceeds 2%, it is segregated at the grain boundary during heating in the austenite region, embrittles the grain boundary and deteriorates the delayed fracture resistance, so that it is limited to the range of 0.05 to 2%.

The above are the basic components of the steel which is the subject of the present invention. In the present invention, the steel further contains Al: 0.005 to 0.1% Ti: 0.005 to 0.5% Cr: 0.05 to 2%, Mo: 0.05 Ni: 0.05 to 5%, Cu: 0.05 to 1% V: 0.05 to 2% Nb: 0.005 to 0.2% Ta: 0.005 to 0.5% W: 0.05 to 0.5% and B: 0.0003 to 0.005% Alternatively, two or more kinds can be contained.

Al: Al has an effect of preventing austenite grains from being coarsened by forming AlN during deoxidation and heat treatment, and also has an effect of fixing N. However, if less than 0.005%, these effects are exhibited. Not 0.1%
Is exceeded, the effect is saturated, so the range is limited to 0.005 to 0.1%.

Ti: Like Ti, Ti forms TiN during deoxidation and heat treatment to prevent the austenite grains from becoming coarser and to fix N. When the content exceeds 0.5%, it is necessary to heat to a high temperature in order to form a solid solution of carbides during quenching, and decarburization which deteriorates fatigue properties occurs. Therefore, the range is limited to 0.005 to 0.5%.

Cr: Cr is an element effective for improving hardenability and increasing softening resistance during tempering, but if it is less than 0.05%, its effect cannot be sufficiently exhibited.
If it exceeds 2%, the toughness and the cold workability deteriorate, so the content is limited to 0.05 to 2%.

Mo: Mo, like Cr, has a strong tempering softening resistance and is an effective element for increasing the tensile strength after heat treatment, but if it is less than 0.05%, its effect is small.
Beyond the limit, the effect is saturated and the cost is increased, so that it is limited to 0.05 to 2%.

Ni: Ni is added in order to improve the ductility, which deteriorates with the increase in strength, and also to improve the hardenability during heat treatment to increase the tensile strength. On the other hand, if the content exceeds 5%, the effect corresponding to the added amount cannot be exhibited, so the content is limited to the range of 0.05 to 5%.

Cu: Cu is an element effective for increasing the tempering softening resistance. However, if it is less than 0.05%, the effect cannot be exhibited, and if it exceeds 1%, the hot workability deteriorates.
%.

V: V has the effect of reducing the size of austenite grains by forming carbonitride during the quenching treatment. However, if it is less than 0.05%, the above effect cannot be obtained. Since the effect is saturated, it was limited to 0.05 to 2%.

Nb: Nb is an effective element for increasing the recrystallization temperature and obtaining prior austenite grains having a large aspect ratio. However, if the content is less than 0.005%, the above effect is insufficient. 0.005 because the effect saturates
Limited to ~ 0.2%.

Ta: Ta also has an austenite grain refinement effect similarly to Nb. However, if the content is less than 0.005%, the above effect is not exerted. Even if added over 0.5%, the effect is saturated. , 0.005 to 0.5%.

W: W is an element effective for improving the delayed fracture characteristics of high-strength bolts. However, if the content is less than 0.05%, the above effect is not exhibited. Is saturated, so the range is limited to 0.05 to 0.5%.

B: B has the effect of suppressing grain boundary fracture and improving delayed fracture characteristics. Further, B segregates at austenite grain boundaries to significantly enhance hardenability.
%, The effect is not exhibited, and if it exceeds 0.005%, the effect is saturated, so the content is limited to 0.0003 to 0.005%.

P and S, which are impurity elements, are not particularly limited, but are each preferably 0.015% or less from the viewpoint of improving hydrogen fatigue resistance. As for N, since the formation of nitrides of Al, V, Nb, and Ti has the effect of refining old austenite grains and increasing the yield strength, 0.002 to 0.1% is a desirable range.

In the method of manufacturing a high-strength spring according to the present invention, after hot working is performed under predetermined conditions, it is immediately quenched to obtain a martensitic structure and then tempered. The reason for limitation will be described.

Hot working temperature: When the hot working temperature exceeds 900 ° C., recrystallization during hot working becomes remarkable, and it is difficult to obtain a martensite structure having an aspect ratio of 2 or more. On the other hand, if the hot working temperature is lower than 700 ° C., it is not possible to secure a sufficient rolling reduction to obtain a structure having a predetermined aspect ratio. Therefore, the hot working temperature is set at 700 ° C to 900 ° C, preferably 70 ° C.
Limited to 0-850 ° C.

Reduction ratio: To obtain a martensite structure having an aspect ratio of 2 or more, a reduction ratio of 30% or more is required in a non-recrystallized region. Therefore, the reduction ratio was limited to 30% or more. The effect of the present invention can be obtained without particularly defining the upper limit of the tensile strength of the spring steel and the spring of the present invention, but is preferably 2200 MPa or less so as not to deteriorate the toughness.

[0034]

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

The steel for spring having the chemical composition shown in Table 1 was quenched and tempered at the temperature shown in Table 2, and the maximum area ratio was adjusted to a structure of tempered martensite.

Table 1 shows the results of the evaluation of the mechanical properties, structure morphology, and hydrogen fatigue resistance of the above samples. Hydrogen fatigue characteristics were evaluated based on the previously described fatigue limit hydrogen amount, and the applied stress was performed under the condition of 90% of the atmospheric fatigue limit.

[0037]

[Table 1]

Test Nos. 1 to 16 in Table 2 are examples of the present invention.
Others are comparative examples. As can be seen from the table, each of the examples of the present invention has a maximum area where the hot working temperature is 700 ° C. to 900 ° C., the rolling reduction is 30% or more, and the aspect ratio of the prior austenite grains is 2 or more. The structure is such that the phase is tempered martensite. These steels have a higher critical diffusible hydrogen content than conventional springs, and springs with excellent hydrogen fatigue resistance have been realized.

On the other hand, the comparative example No. 17 is
This is an example in which the strength of 1700 MPa or more was not obtained because the C content was low, and the steel could not be used as high-strength spring steel.

The comparative example No. Sample No. 18 is an example in which old austenite grains having a predetermined aspect ratio were not obtained because the rolling reduction was low, and the fatigue limit hydrogen amount was low.

In Comparative Example No. 19 is an example in which old austenite grains having a predetermined aspect ratio were not obtained because the hot working temperature was high, and the fatigue limit hydrogen amount was low.

The comparative steel No. No. 20 is an example in which ferrite was precipitated because the hot working temperature was low, and a predetermined strength was not obtained.

The comparative steel No. No. 21 was No. 21 because the Si content was too high. In No. 23, the C content was too high. Sample No. 24 is an example in which the fatigue limit hydrogen amount was low because the Mn content was too high.

The comparative steel No. 22 is an example in which the fatigue limit hydrogen amount was low because the strength was too high.

[0045]

[Table 2]

[0046]

As is apparent from the above examples, the present invention significantly improves the hydrogen fatigue characteristics of a high-strength spring having a tensile strength of 1700 MPa or more by setting the aspect ratio of prior austenite grains to a specific value. The steel for springs and its manufacturing method were established by optimally selecting the chemical composition and hot working conditions of the steel, and the industrial effect was extremely remarkable. .

[Brief description of the 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 the relationship between the aspect ratio of prior austenite grains and the fatigue limit hydrogen content.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Masaharu Oka 20-1 Shintomi, Futtsu Nippon Steel Corporation Technology Development Division F-term (reference) 3J059 AB05 BC02 BC19 EA09 GA02 GA08 4K032 AA01 AA02 AA05 AA06 AA11 AA12 AA14 AA16 AA19 AA20 AA22 AA23 AA24 AA31 AA32 AA33 AA35 AA36 AA37 CB01 CB02 CC02 CC03 CC04 CF01

Claims (6)

[Claims] 1. A fatigue test of a steel material is carried out at a fatigue limit of 9 in the atmosphere.
The upper limit of the amount of diffusible hydrogen (the amount of hydrogen released when heated from room temperature to 500 ° C.) at which the fatigue life does not decrease to less than 10 ⁷ times when the stress is applied at 0% (hereinafter referred to as the critical diffusible hydrogen amount) ) Is 0.1 ppm or more, and is a high strength spring steel excellent in hydrogen fatigue resistance. 2. The phase having the largest area ratio is tempered martensite, and the ratio between the length and the width of the prior austenite grains (hereinafter referred to as aspect ratio) is 2 or more, and the hydrogen fatigue resistance is excellent. High strength spring steel. 3. A phase having a maximum area ratio of tempered martensite, an aspect ratio of 2 or more, and a critical diffusible hydrogen content of 0.1 ppm or more. Steel for strength springs. 4. The method according to claim 1, wherein C: 0.3 to 1% Si: 0.05 to 4% Mn: 0.05 to 2% in mass%, the balance being Fe and unavoidable impurities. A high-strength spring steel having excellent hydrogen fatigue resistance according to any one of the above. 5. In mass%, Al: 0.005 to 0.1% Ti: 0.005 to 0.5% Cr: 0.05 to 2%, Mo: 0.05 to 2% Ni: 0.05 to 5%, Cu: 0.05 to 1% V: 0.05 The hydrogen-resistant material according to claim 4, characterized in that it contains one or more of Nb: 0.005 to 0.2% Ta: 0.005 to 0.5% W: 0.05 to 0.5% and B: 0.0003 to 0.005%. High strength spring steel with excellent fatigue properties. 6. A method for producing a high-strength spring steel according to any one of claims 1 to 5, wherein a hot working step of giving a reduction of 30% or more in a temperature range of 700 ° C. to 900 ° C. After passing
Quenching the phase with the largest area ratio to martensite structure,
Thereafter, a tempering treatment is performed, thereby producing a high-strength spring steel excellent in hydrogen fatigue resistance.