JP2002327235A - Steel for machine structure superior in hydrogen fatigue fracture resistance, and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel for machine structure superior in hydrogen fatigue fracture resistance having tensile strength of 1800 MPa or higher, and a manufacturing method therefor. SOLUTION: The steel for machine structure superior in hydrogen fatigue fracture resistance, is characterized by including, by mass%, 0.1-0.6% C, less than 0.5% Si, 0.1-0.5% Mn, 0.4-3.0% Mo, 0.02-0.5% V, 0.02% or less P, and 0.02% or less S, satisfying 5<=(Mo/V)<=20, and further including a predetermined amount of one or more sort among Cr, Ni, Cu, Al, Ti, Nb, B, or N, as needed, and the balance Fe with unavoidable impurities. The method for manufacturing the steel for machine structure superior in hydrogen fatigue fracture resistance is characterized by quenching the steel consisting of the above components, and then tempering it at 500 deg.C or higher.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for machine structural use which has a tensile strength of 1800 MPa or more and is excellent in hydrogen fatigue resistance and is used for automobile parts and the like, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, it has been desired to reduce the weight of automobiles for the purpose of reducing carbon dioxide emission and fuel efficiency in order to respond to environmental problems. Increasing the strength of steel is an effective means for reducing the weight of automobiles, and there is a demand for steel for machine structural use in which the tensile strength after quenching and tempering is increased to 1800 MPa or more.

[0003] However, in general, when the strength of a steel material is increased, notch sensitivity is increased and the steel material is easily affected by the environment. Particularly in a corrosive environment, if corrosion pits are formed on the surface, they become a source of stress concentration, and they become brittle due to hydrogen generated as the corrosion reaction progresses. Was. As a method of preventing embrittlement due to hydrogen, a method of refining crystal grains and a method of generating fine precipitates have been considered,
Neither method has led to any significant improvement in hydrogen fatigue resistance in the tests of the present inventors.

[0004] As described above, in the conventional technology, 1800 MPa
It has been difficult to produce a mechanical structural steel having the above tensile strength and excellent hydrogen fatigue resistance.

[0005]

SUMMARY OF THE INVENTION The present invention is to solve the above-mentioned problems, and is intended to solve the above-mentioned problems.
It is an object of the present invention to provide a mechanical structural steel having the above tensile strength and excellent hydrogen fatigue resistance, and a method for producing the same.

[0006]

Means for Solving the Problems In order to solve the above problems, the present inventors conducted research on hydrogen fatigue resistance characteristics of various materials having different components by a method of obtaining a fatigue limit diffusible hydrogen amount described later. As a result, they found that (Mo, V) ₂ C is very effective as a hydrogen trap site and greatly increases the amount of diffusible hydrogen at the fatigue limit. In addition, Si
It has been found that reducing the amount of hydrogen can increase the amount of diffusible hydrogen at the fatigue limit. As a result of further research,
The addition ratio of Mo and V (Mo / V) is set to 5 to 20, the content of Si is set to less than 0.5%, and the content of Mn, which lowers the grain boundary strength, is set to 0.5% or less.
It has been found that a steel having the above tensile strength and excellent in hydrogen fatigue resistance can be obtained.

The present invention has been made based on such findings, and the gist thereof is as follows: (1) In mass%, C: 0.1 to 0.6%;
Less than 5%, Mn: 0.1-0.5%, Mo: 0.4-
3.0%, V: 0.02 to 0.5%, P: 0.02% or less, S: 0.02 or less, satisfying 5 ≦ (Mo / V) ≦ 20, and the balance Is a steel for machine structural use having excellent resistance to hydrogen fatigue fracture, characterized in that it is composed of Fe and unavoidable impurities. (2) The composition according to the above (1), further containing
, Cr: 0.05-3.0%, Ni: 0.05-5.
A steel for machine structural use having excellent hydrogen fatigue fracture resistance, characterized by containing one or more of 0% and Cu: 0.05 to 2.0%. (3) The composition according to (1) or (2), further containing 0.005 to 0.1% of Al and 0.1 to 0.1% by mass of Al.
005-0.3%, Nb: 0.005-0.3%, B:
0.0003-0.05%, N: 0.001-0.05
%, Characterized in that the steel for machine structural use has excellent resistance to hydrogen fatigue fracture. (4) The steel for machine structural use having excellent hydrogen fatigue fracture resistance according to any one of the above (1) to (3), wherein the fatigue limit diffusible hydrogen amount is 0.2 ppm or more. (5) A mechanical structure excellent in hydrogen fatigue fracture resistance, characterized in that after quenching a steel comprising the component according to any one of (1) to (3), the steel is tempered at 500 ° C. or more. Method of manufacturing steel. It is in.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors first analyzed in detail the hydrogen fatigue behavior using mechanical strength 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 is caused by diffusible hydrogen that can enter the steel from the external environment and diffuse in the steel at room temperature. Diffusible hydrogen is used for steel at 100 ° C / h.
In the curve of “temperature-hydrogen release rate from steel material” obtained when heating at a rate of our, it can be measured as a curve having a peak at a temperature of about 100 ° C. (FIG. 1). Therefore, if hydrogen that has invaded from the external environment is trapped in some part of the steel material so as not to diffuse in the steel material, the hydrogen can be rendered harmless, and a reduction in fatigue life can be suppressed.

Therefore, the hydrogen fatigue resistance was evaluated by determining the "fatigue limit 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 “the fatigue limit diffusible hydrogen amount”. Since hydrogen trapped by (Mo, V) ₂ C or the like is unlikely to adversely affect the fatigue properties, the steel containing such trap sites has a reduced fatigue strength due to hydrogen even if the “diffusible hydrogen amount” is high. Is less likely to occur. Therefore, such a steel material has a high fatigue limit diffusible hydrogen amount. In a real environment, the amount of diffusible hydrogen that penetrates into steel in an actual environment increases as the environment becomes harsher. Therefore, steel with higher fatigue limit diffusible hydrogen has a lower fatigue strength due to hydrogen even in a more severe environment (diffusible hydrogen: large). Since this does not occur, the higher the fatigue limit 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.

Hereinafter, the significance of each requirement and the reason for limitation in the present invention will be specifically described.

The present inventors have found that 0.5% C-0.06% S
Steel having i-0.2% Mn as a base component and various addition ratios of Mo and V was tempered to the same strength level by quenching and tempering, and the fatigue limit diffusible hydrogen content was measured. Mo
FIG. 3 shows the relationship between the addition ratio of Mo and V (Mo / V) and the amount of diffusible hydrogen at the fatigue limit. From FIG. 3, it has been found that when Mo / V is 5 or more and 20 or less, the amount of diffusible hydrogen at the fatigue limit is significantly improved. Therefore, Mo / V is set to 5 or more and 20 or less.

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 for increasing the strength of steel. If it is less than 0.1%, it is difficult to secure the strength required for the steel for machine structural use, and if it is added in excess of 0.6%, the toughness is significantly deteriorated. Therefore, the C content is set to 0.1 to 0.6%.

Si: Si is added as a deoxidizing agent, but an excessive addition of 0.5% or more decreases the amount of diffusible hydrogen at the fatigue limit, thereby deteriorating the hydrogen fatigue characteristics. Therefore, Si
The smaller the content, the better, the content being less than 0.5%.

Mn: Mn is an element effective for enhancing hardenability, but is a harmful element that, on the other hand, embrittles grain boundaries and deteriorates hydrogen fatigue fracture resistance. If it is less than 0.1%, the effect of enhancing hardenability will not be exhibited, and if it is added in excess of 0.5%, the hydrogen fracture fatigue resistance will deteriorate. Therefore, the Mn content is set to 0.1 to 0.5%. In order to obtain better hydrogen fatigue fracture resistance, the Mn content should be set to 0.1.
The content is desirably 3% or less, and more desirably 0.2% or less.

Mo; Mo together with V and C (Mo,
V) An essential element for improving hydrogen fatigue fracture resistance by forming ₂ C and trapping diffusible hydrogen.
If the content is less than 0.4%, the effect is not exhibited, and an excessive addition exceeding 3.0% lowers the toughness. Therefore, the Mo content is set to 0.4 to 3.0%.

V; V is Mo and V together with (Mo, V)
It is an essential element that improves hydrogen fatigue fracture resistance by forming ₂ C and trapping diffusible hydrogen.
If the content is less than 02%, the effect is not exhibited, and if the addition exceeds 0.5%, the toughness is reduced.
02 to 0.5%.

P: P is an impurity element that segregates at the grain boundaries to lower the grain boundary strength and degrades the toughness, and it is desirable that the level be as low as possible. Therefore, the upper limit was made 0.02%.

S: S is an impurity element that deteriorates hot workability and toughness, and is desirably as low as possible. However, considering the achievable level and cost of the current refining technology, the upper limit is 0.02%. And

The above are the basic components of the present invention. Usually, other than the above, Fe and unavoidable impurities, but depending on the desired strength level and other necessary properties, Cr, Ni, C
One or more of u, Al, Ti, Nb, B, and N may be added.

Cr, Ni, Cu: Cr, Ni and Cu are all effective elements for improving corrosion resistance and strength.
This effect is not manifested at less than 0.05%, respectively.
An excessive addition of more than 3% of r, 5% of Ni and 2% of Cu deteriorates toughness. Therefore, the content of Cr is set to 0.05 to
3.0%, the content of Ni was set to 0.05 to 5.0%, and the content of Cu was set to 0.05 to 2.0%.

Al; Al is used as a deoxidizing agent and AlN
Has the effect of suppressing crystal grain coarsening.
If the content is less than 05%, the effect is not exhibited. If the content exceeds 0.1%, the toughness is deteriorated. Therefore, the content of Al is set to 0.005 to 0.1%.

Ti: Ti has the effect of forming TiN and suppressing the coarsening of the crystal grains, but the effect is not exhibited if it is less than 0.005%, and the toughness is deteriorated if it is excessively added in excess of 0.3%. Therefore, the content of Ti is 0.005 to 0.3
%.

Nb: Nb forms fine carbonitrides and has the effect of suppressing the coarsening of crystal grains. However, if less than 0.005%, the effect is not exhibited. Since the toughness is deteriorated, the Nb content is set to 0.005.
-0.3%.

B; B itself segregates at the grain boundaries to improve the grain boundary bonding force, suppresses the grain boundary segregation of P, S and Cu, increases the grain boundary strength, and improves the delayed fracture characteristics and toughness. It is also an element effective for improving the hardenability. These effects are 0.000
If it is less than 3%, it is not expressed, and if it exceeds 0.05%, coarse precipitates are formed at the grain boundaries to deteriorate hot workability and toughness. 05%
And

N: N has the effect of forming nitrides and suppressing the coarsening of crystal grains. However, if less than 0.001%, the effect is not exhibited, and if more than 0.05% is added, toughness is deteriorated. Therefore, the N content is set to 0.001 to 0.05%.

The fatigue limit diffusible hydrogen content is 0.2 p
If it is less than pm, the resistance to hydrogen destruction is not sufficient, and hydrogen fatigue destruction may occur in a typical environment actually used.

Next, the reasons for limiting the manufacturing conditions will be described.
In the present invention, it is important that the tempering temperature at the time of performing the quenching and tempering treatment is 500 ° C. or higher, and other manufacturing conditions do not need to be particularly limited. This becomes a hydrogen trap site when the tempering temperature is lower than 500 ° C. (Mo,
V) fatigue critical diffusible hydrogen amount to the amount deposited of ₂ C is not sufficiently obtained is becomes lower. More preferred conditions are 550 ° C. or higher. The upper limit of the tempering temperature does not need to be particularly defined. However, when the tempering temperature is 650 ° C. or higher, the precipitates are coarsened and the effect as a hydrogen trap site is reduced.

[0029]

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

After quenching steel having the composition shown in Table 1,
Tempering was performed at the temperatures shown in Table 1. Table 1 shows the tensile strength of each steel slab after the heat treatment. All are 1800MP
The tensile strength of a or more is obtained. The hydrogen fatigue fracture resistance of these steel slabs was evaluated by the above-mentioned fatigue limit diffusible hydrogen content. In addition, the load stress at the time of performing the fatigue test was performed under the condition of 90% of the fatigue limit in the atmosphere.

[0031]

[Table 1]

From Table 1, it can be seen that all of the examples of the present invention (Nos. 1 to 5) have a fatigue limit diffusible hydrogen content of 0.2 ppm or more and are excellent in hydrogen fatigue fracture resistance. In particular, those having a tempering temperature of 550 ° C. or higher (Nos. 1, 4, and 5) all have a fatigue limit diffusible hydrogen amount of 1.0 ppm or higher, and have excellent hydrogen fatigue fracture resistance.

On the other hand, in the comparative examples (Nos. 6, 7, and 8) in which one or more of the Mo amount, the V amount, and (Mo / V) deviate from the range of the present invention, the fatigue limit diffusion is all. It can be seen that the amount of reactive hydrogen is as low as 0.1 ppm or less, and the hydrogen fatigue fracture resistance is poor. In addition, Mo amount, V amount, (Mo / V)
Is in the range of the present invention, but in the comparative example (No. 9) in which the amounts of Si and Mn deviate from the component ranges shown in the present invention, the fatigue limit diffusible hydrogen amount is as low as 0.1 ppm or less, It turns out that it is inferior in hydrogen fatigue fracture characteristics. Further, in the comparative example (No. 10) in which the amount of Mo and the tempering temperature were out of the range of the present invention, the amount of fatigue-limited diffusible hydrogen was 0.1 pp.
m or less, which indicates that the resistance to hydrogen fatigue fracture is inferior.

As described above, the amounts of Mo, V, Si, Mn and the addition ratio of Mo and V (Mo / V) are specified in the range shown in the present invention, and the alloy is manufactured under the tempering conditions shown in the present invention. It is apparent that a steel having a tensile strength of 1,800 MPa or more and excellent in hydrogen fatigue resistance can be obtained.

[0035]

As described above, according to the present invention, 1800
A steel for machine structural use having a tensile strength of not less than MPa and excellent hydrogen fatigue resistance can be obtained.

[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 graph showing the relationship between the addition ratio of Mo and V (Mo / V) and the amount of diffusible hydrogen at the fatigue limit.

Claims (5)

[Claims] C: 0.1 to 0.6%, Si: less than 0.5%, Mn: 0.1 to 0.5%, Mo: 0.4 to 3.0% by mass%. V: 0.02 to 0.5%, P: 0.02% or less, S: 0.02% or less, and satisfying 5 ≦ (Mo / V) ≦ 20, with the balance being Fe and inevitable Steel for machine structural use, which is excellent in hydrogen fatigue fracture resistance, characterized by being composed of chemical impurities. 2. One or more of Cr: 0.05 to 3.0%, Ni: 0.05 to 5.0%, Cu: 0.05 to 2.0% by mass%. The steel for machine structural use excellent in hydrogen fatigue fracture resistance according to claim 1, characterized in that it contains: 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 The steel for machine structural use having excellent hydrogen fatigue fracture resistance according to claim 1 or 2, comprising one or more of 0.05% and N: 0.001 to 0.05%. 4. The steel for machine structural use excellent in hydrogen fatigue fracture resistance according to claim 1, wherein the fatigue limit diffusible hydrogen content is 0.2 ppm or more. 5. A mechanical structure excellent in hydrogen fatigue fracture resistance, characterized in that after quenching a steel comprising the component according to claim 1, the steel is tempered at 500 ° C. or more. Steel production method.