JP2000026934A - Steel excellent in delayed fracture characteristic and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make better the delayed fracture characteristics of steel and to increase its strength, at the time of immersing steel into an acidic soln., by allowing it to occulude a specified amt. of hydrogen to be eliminated by activation energy. SOLUTION: Diffusible hydrogen causing delayed fracture is generated by corrosion and electroplating and intrudes into a steel at room temp. Therefore, its structure is controlled to the one capable of occuluding >=0.1 wt.ppm hydrogen of 25 to 50 kJ/mol trap energy in an acidic soln. of pH <=4.0. As for hydrogen in which trap energy lies in this range, in the case the steel sheet is heated at a rate of 100 deg.C/hr, the emission peak can be obtd. in the temp. range of 180 to 600 deg.C. Moreover, the structure is controlled to the one essentially consisting of martensite or tempered martensite and contg. at least one kind among the single or composite precipitates of oxides, carbides and nitrides essentially consisting of Si, Mn, Ti or the like, in which the average grain size is 0.05 to 1.0 μm and the average grain interval is 3 to 30 times that of the average grain size.

Description

DETAILED DESCRIPTION OF THE INVENTION

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material having excellent delayed fracture resistance, and more particularly to a steel material for high strength members having a tensile strength of 1200 MPa or more and having excellent delayed fracture resistance.

[0001]

2. Description of the Related Art High-strength steels widely used in machines, automobiles, bridges and buildings are, for example, JISG 4104, JI.
It is manufactured by quenching and tempering using medium carbon steel with a C content of 0.20 to 0.35% such as SCr and SCM specified in SG4105. However, it is well known that when any one of the varieties has a tensile strength exceeding 1300 MPa, the risk of delayed fracture increases. For example, the strength of building steel currently used is 115.
At present, the 0 MPa class is the upper limit.

[0002] As conventional knowledge for improving delayed fracture characteristics of high-strength steel, for example, Japanese Patent Publication No. 3-243744 proposes that it is effective to make old austenite grains fine and to make the structure bainite. are doing.
Certainly, the bainite structure is effective against delayed fracture, but the bainite treatment increases the manufacturing cost. Regarding the refinement of old austenite grains, see JP-B-64-456.
No. 6 and Japanese Patent Publication No. 3-243745. Japanese Patent Publication No. 61-64815 discloses Ca
It is proposed to add However, none of the proposals has led to a significant improvement in delayed fracture characteristics in the tests of the present inventors.

[0003] As described above, in the prior art, there is a limit in producing a high-strength steel with drastically improved delayed fracture characteristics.

[0004]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and is intended to provide a steel material having good delayed fracture characteristics, in particular, a steel material having good delayed fracture characteristics and a strength of 120.
It is an object of the present invention to realize a high-strength steel of 0 MPa or more and to provide a method for producing the steel.

[0005]

Means for Solving the Problems The present inventors first analyzed in detail the delayed fracture behavior using steel materials of various strength levels manufactured by quenching and tempering. It is already evident that delayed fracture has entered the steel from the external environment and has been caused by diffusible hydrogen that can diffuse through the steel at room temperature. And the diffusible hydrogen is used to make the steel material 100 ° C / ho.
In the curve of the temperature obtained when heating at the rate of ur and the hydrogen release rate from the steel material, it can be measured as a curve having a peak at a temperature of about 100 ° C. FIG. 1 shows an example of the measurement. ● Immediately after the hydrogen charge, □ indicates a hydrogen charge of 48 hours, and 試 料 indicates a sample left for 72 hours after the hydrogen charge.

[0006] Therefore, if hydrogen invading from the environment is prevented from diffusing by trapping it in some part of the steel material, it is possible to make the hydrogen harmless, and it is possible to reduce the amount of hydrogen invading from a larger amount of the environment. , Delayed fracture is suppressed. The amount of invading hydrogen in the sample was determined by the difference between the area values of a hydrogen release curve obtained by heating a 10 mmφ steel material before and after hydrogen charging at 100 ° C./hour. The presence of a hydrogen trapping site (hereinafter referred to as a hydrogen trapping site) can be determined from the peak temperature and peak height of the above hydrogen release curve, and the amount of hydrogen trapped at a certain hydrogen trapping site (hereinafter referred to as a hydrogen trapping capacity) is a peak. The activation energy (hereinafter referred to as hydrogen trap energy) E required for hydrogen to desorb from the trap site is given by
It can be determined from the following equation describing the hydrogen release behavior from steel. Eφ / RT2 = Aexp (−E / RT) Equation (1) where φ is the heating rate, A is the reaction constant of trap desorption of hydrogen, R is the gas constant, and T is the peak temperature of the hydrogen release curve. Since the hydrogen trap energy E is a constant determined by the material, the variables in equation (1) are φ and T.
By taking the logarithm of equation (1) and rearranging it, ln (φ / T2) = − (E / R) / T + ln (AR / E) Equation (2) Accordingly, hydrogen analysis is performed at a plurality of heating rates. Was measured, and ln (φ / T2) and -1 / T
By calculating the slope of the straight line indicating the relationship, E can be obtained.

[0007] Therefore, the delayed fracture characteristics were evaluated by determining the "invasive hydrogen amount" at which delayed fracture does not occur. In this method, after a notched round bar specimen contains various levels of diffusible hydrogen by electrolytic hydrogen charging, hydrochloric acid immersion, and a hydrogen annealing furnace, hydrogen escapes from the sample to the atmosphere during the delayed fracture test. In order to prevent this, Cd plating is performed, and thereafter, a predetermined load is applied in the atmosphere, and the amount of invading hydrogen at which delayed fracture does not occur is evaluated.

FIG. 2 shows an example in which the relationship between the amount of invading hydrogen and the rupture time until delayed fracture is analyzed. The smaller the amount of invading hydrogen contained in the sample, the longer the time required for delayed fracture is reached. If the amount of invading hydrogen is below a certain value, delayed fracture does not occur. This amount of hydrogen is defined as "critical intrusion hydrogen amount". The higher the critical penetration hydrogen amount, the better the delayed fracture resistance properties of the steel material, which is a value specific to the steel material determined by the steel composition and the manufacturing conditions such as heat treatment. The amount of invading hydrogen in the sample was 100 ° C. /
The value is obtained from the difference between the area values of the hydrogen release curve obtained by heating with an hour, and includes the amount of hydrogen captured at the hydrogen trap site.

As a result, the hydrogen trap energy becomes 25
５０50 kJ / mol and a hydrogen trapping capacity of 0.1 wt. ppm or more, if a structure having at least one of oxides, carbides, and nitrides that can be hydrogen trap sites alone or as a composite precipitate is formed, the critical intrusion hydrogen even in a high-strength region exceeding 1200 MPa. It was found that the amount increased and the delayed fracture resistance was significantly improved. In addition, a technique was established in which it was possible to form a structure having a single or complex precipitate of oxides, carbides, and nitrides of the type and form that could serve as the hydrogen trap site by selecting the steel material component.

[0010] Based on the above examination results, it was concluded that a high-strength bolt excellent in delayed fracture characteristics can be realized by optimally selecting a steel material composition and a structure form, and made the present invention.

The present invention has been made based on the above findings, and the gist thereof is as follows. That is, (1) When a steel material is immersed in an acidic solution having a pH of 4.0 or less, hydrogen that is desorbed by an activation energy of 25 to 50 kJ / mol is 0.1 wt. A steel material excellent in delayed fracture characteristics characterized by storing at least ppm.

(2) When immersed in an acidic solution having a pH of 4.0 or less and then heated at a rate of 100 ° C./hour,
A hydrogen release peak is obtained in a temperature range of 180 ° C. to 600 ° C., and the amount of released hydrogen is 0.1 wt. pp
m or more, which is excellent in delayed fracture characteristics.

(3) The hydrogen trap energy is 25 to 5
0 kJ / mol, and the hydrogen trap capacity is 0.1
wt. A steel material having excellent delayed fracture characteristics, characterized in that it has at least one of oxides, carbides, and nitrides that can serve as hydrogen trap sites, alone or in combination, which is at least ppm.

(4) It has a structure mainly composed of martensite or tempered martensite, and has an average particle size of 0.1.
Si, which is not less than 05 μm and not more than 1.0 μm and whose average particle interval is not less than 3 times and not more than 30 times the average particle diameter.
The delayed fracture according to any one of claims 1 to 3, characterized in that it has at least one of oxides, carbides and nitrides mainly composed of Mn, Ti, Ai and V, or a composite precipitate. Steel with excellent properties

(5) It has a structure mainly composed of bainite or tempered bainite, and has an average particle size of 0.05 μm.
Si, Mn, T having an average particle size of 3 to 30 times the average particle size.
The delayed fracture characteristic according to any one of claims 1 to 3, further comprising at least one of oxides, carbides, and nitrides mainly composed of i, Ai, and V, or composite precipitates. Excellent steel material

(6) C: 0.15 to 0.5% by weight
0%, Si: 0.05-2.0%, Mn: 0.2-2.
0%, Al: 0.005 to 0.1%, Ti: 0.005
-0.05%, O: 1-500 ppm, N: 20-
6. The composition according to claim 1, wherein the composition contains 150 ppm, with the balance being Fe and unavoidable impurities.
High-strength steel material with excellent delayed fracture resistance described in the item.

(7) In weight%, C: 0.15 to 0.5
0%, Si: 0.05-2.0%, Mn: 0.2-2.
0%, Al: 0.005 to 0.1%, Ti: 0.005
To 0.05%, Mg: 0.1 to 100 ppm, O: 1
５００500 ppm, N: 20-150 ppm,
The high-strength steel material excellent in delayed fracture resistance according to any one of claims 1 to 5, wherein the balance consists of Fe and unavoidable impurities.

(8) Cr: 0.05 to 2.0% by weight
%, Mo: 0.05-1.0%, Ni: 0.05-5.
8. Excellent delayed fracture resistance according to claim 6, further comprising one or more of 0% and Cu: 0.05 to 1.0%. High strength steel.

(9) V: 0.05-0.5% by weight
%, Nb: 0.005 to 0.2%, Ta: 0.005 to
The high-strength steel material excellent in delayed fracture resistance according to any one of claims 6 to 8, comprising one or more kinds of 0.5%.

(10) By weight%, W: 0.05-0.
10% or more of B: 0.0003% to 0.0050%.
A high-strength steel material excellent in delayed fracture resistance according to any one of the above.

(11) C: 0.15-0.
50%, Si: 0.05 to 2.0%, Mn: 0.2 to
2.0%, Al: 0.005 to 0.1%, N: 20 to
A molten steel containing 150 ppm, the balance being Fe and unavoidable impurities, was subjected to a deoxidation treatment to reduce the amount of dissolved oxygen from 1 to 5
After being adjusted to 00 ppm by weight, Ti is further deoxidized, the Ti content is 0.005 to 0.05% by weight, and -0.04 ≦
A molten steel satisfying a relationship of Ti% -O% ≦ 0.04 is cast, and the molten steel is cast and cooled from a solidification temperature of a slab to 1000 ° C at a cooling rate of 0.5 ° C / sec or more. The method for producing a high-strength steel material having excellent delayed fracture resistance according to claim 6.

(12) C: 0.15-0.
50%, Si: 0.05 to 2.0%, Mn: 0.2 to
2.0%, Al: 0.005 to 0.1%, Mg: 0.1
-100 ppm, N: 20-150 ppm,
The molten steel whose balance consists of Fe and inevitable impurities is adjusted to a dissolved oxygen content of 1 to 500 ppm by weight by deoxidation treatment, and then further deoxidized with Ti to obtain a Ti content of 0.005 to 0.5 wt.
05% by weight, and -0.04 ≦ Ti% -O% ≦ 0.0
8. The molten steel according to claim 7, wherein the molten steel is cast, and the molten steel is cast and cooled from a solidification temperature of the slab to 1000 ° C. at a cooling rate of 0.5 ° C./sec or more. A method for manufacturing high-strength steel with excellent delayed fracture resistance.

(13) In the production method according to any one of (11) and (12), the molten steel is
Cr: 0.05 to 2.0%, Mo: 0.05 to 1.0
%, Ni: 0.05-5.0%, Cu: 0.05-1.
9. The method for producing a high-strength steel material excellent in delayed fracture resistance according to claim 8, further comprising 0% or one or more types.

(14) Any one of claims 11 to 13
In the production method described in the paragraph, the molten steel is V
: 0.05-0.5%, Nb: 0.005-0.2
The method for producing a high-strength steel material having excellent delayed fracture resistance according to claim 9, further comprising one or more of Ta and 0.005 to 0.5% of Ta.

(15) Any one of claims 11 to 14
In the production method described in the paragraph, the molten steel is W
: 0.05-0.5%, B: 0.0003-0.0
The method for producing a high-strength steel material having excellent delayed fracture resistance according to claim 10, further comprising one or more of 050%.

[Detailed Description of the Invention]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material having excellent delayed fracture resistance, and more particularly to a steel material for high strength members having a tensile strength of 1200 MPa or more and having excellent delayed fracture resistance.

[0027]

Next, an embodiment of the present invention will be described. (Hydrogen Trap Site) First, the reason for limiting the hydrogen trap site, which is the most important point for improving the delayed fracture characteristics of high-strength steel, which is the object of the present invention, will be described. Diffusible hydrogen causing delayed fracture is generated by corrosion or electroplating and penetrates into steel at room temperature. When hydrogen is allowed to penetrate into steel by immersing in a 20 ° C. acidic solution having a pH of 4.0 or less, preferably for 6 hours or more, assuming hydrogen intrusion due to corrosion, the trap energy becomes 25%.
５０50 kJ / mol, desirably 30 kJ / mol〜5
0 kJ / mol of hydrogen at 0.1 wt. ppm or more, preferably 0.3 wt. ppm or more, more preferably 0.5 wt. By controlling the structure to be capable of storing at least ppm, delayed fracture characteristics can be improved. Note that the trap energy is 25 to 50 kJ /
mol of hydrogen, when a steel material is heated at a rate of 100 ° C./hour, 3% in a temperature range of 180 ° C. to 600 ° C.
For hydrogen at 0 kJ / mol to 50 kJ / mol, an emission peak is obtained in a temperature range from 200 ° C. to 600 ° C. FIGS. 3A and 3B show examples of analyzing the effects of the hydrogen trap energy and the hydrogen trap capacity on the delayed fracture characteristics.

(Structure morphology) Si, Mn, Ti, A having an average particle size of 0.05 to 1.0 μm and an average particle interval of 3 to 30 times the average particle size.
By controlling to a structure mainly composed of martensite or tempered martensite having at least one of oxides, carbides and nitrides alone or composite precipitates mainly composed of i and V, or bainite or tempered bainite structure, It is possible to improve delayed fracture characteristics. In order to improve the strength and the delayed fracture characteristics, the precipitates present in the above structure have an average particle size of 0.05 μm or more and 1.0 μm or less and an average particle interval of 3 μm or less of the average particle size. It is desirably at least twice and at most 30 times. This is because if the average particle size is more than 1 μm, the mechanical properties are reduced, and if the average particle size is more than 30 times the average particle size, the hydrogen trapping effect is small. More desirable conditions are that the average particle size is 0.3 μm or less,
And the average particle interval is 10 times or less of the average particle diameter.

(Steel Material Components) Next, the reasons for limiting the components of the steel to which the present invention is applied will be described. C: C is an essential element for securing the strength of the steel material, but if it is less than 0.15%, the required strength cannot be obtained.
If the content exceeds 0.50%, the toughness is deteriorated and the delayed fracture resistance is also deteriorated. Therefore, the range is limited to 0.15 to 0.50%.

Si: Si has an effect of increasing the strength by a solid solution hardening effect. If it is less than 0.05%, the above effect cannot be exerted. On the other hand, if it exceeds 2.0%, an effect commensurate with the added amount cannot be expected. Therefore, the content is limited to the range of 0.05 to 2.0%.

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 the content exceeds 2.0%, segregation at the grain boundary during heating in the austenite region causes embrittlement of the grain boundary and deterioration of the delayed fracture resistance.
Limited to the range of ２．０2.0%.

Al: Al has an effect of preventing austenite grains from coarsening by forming AlN during deoxidation and heat treatment, and also has an effect of fixing N, but if it is less than 0.005%, these effects are obtained. Is not exhibited,
Even if it exceeds 0.1%, the effect is saturated,
The range was limited to 0.1%.

Ti: Like Ti, Ti forms TiN during deoxidation and heat treatment, thereby preventing the austenite grains from becoming coarser and also has the effect of fixing N, but 0.005% If it is less than 0.05%, these effects are not exhibited, and if it exceeds 0.05%, the effect is saturated. Therefore, the range is limited to 0.005 to 0.05%.

O: O forms an oxide with Si, Mn, Ti, Al, and Mg and serves as a hydrogen trap site.
If it is less than pm, the effect is small, and if it exceeds 500 ppm, the effect is saturated.
Desirable conditions are 5 to 500 ppm.

N: For N, Al, V, Nb, Ti
The formation of the nitride has the effect of refining the prior austenite grains and increasing the yield strength. If it is less than 20 ppm, the effect is small, and if it exceeds 150 ppm, the effect is saturated. Preferably, it is 50 to 100 ppm.

The above are the basic components of the steel which is the subject of the present invention. In the present invention, the steel is further added with a first group Mg: 0.1 to 100 ppm and a second group Cr: 0.05 to 2.0. 0%, Mo: 0.05 to 1.0%, Ni: 0.05 to 5.0%, Cu: 0.05 to 1.0%, Third group V: 0.05 to 0.5%, Nb: 0.005 to 0.2%, Ta: 0.005 to 0.5% Fourth group W: 0.05 to 0.5%, B: 0.0003 to 0.0050% 1 or 2 groups One or more of each of the above can be included.

Mg: Mg has a desulfurizing effect and also becomes a fine oxide, and has an effect of finely dispersing an oxide or nitride of Ti. This effect is not exhibited at 0.1 ppm or less, and the effect saturates at more than 100 ppm. It is highly preferable to add Ti coexisting with Ti in order to effectively reduce the size of the oxide.

The second group of additional elements is an element group effective for increasing the strength. Cr: Cr is an element effective for improving hardenability and increasing softening resistance at the time of tempering treatment.
If the amount is less than 2.0%, the effect cannot be sufficiently exhibited.
If it exceeds 0.005%, the toughness and the cold workability are deteriorated.

Mo: Mo, like Cr, has a strong tempering softening resistance and is an effective element for increasing the tensile strength after heat treatment. If it exceeds 0%, the effect is saturated and the cost is increased, so that it is limited to 0.05 to 1.0%.

Ni: Ni is added in order to improve ductility, which deteriorates with increasing strength, to improve hardenability during heat treatment, and to increase tensile strength.
If it is less than 0.05%, the effect is small. On the other hand, if it exceeds 5.0%, the effect corresponding to the added amount cannot be exhibited.
Limited to 5.0% range.

Cu: Cu is an effective element for increasing the tempering softening resistance. However, if it is less than 0.05%, the effect cannot be exhibited, and if it exceeds 1.0%, hot workability is deteriorated. 0.05% to 1.0%.

The third group of additional elements is a group of elements that are effective mainly for generating fine precipitates other than oxides. V: V has the effect of reducing the size of austenite grains by forming carbonitrides during the quenching treatment, but if less than 0.05%, the effect of the above-mentioned effects cannot be obtained, while 0.5% Since the effect is saturated even if it exceeds, 0.05
０．５0.5%.

Nb: Like V, Nb is an effective element for forming carbonitrides to refine austenite grains, but if it is less than 0.005%, the above effect is insufficient. , 0.2%, the effect saturates, so it was limited to 0.005 to 0.2%.

Ta: Ta also has an effect of refining austenite grains like Nb. However, if it is less than 0.005%, the above-mentioned effect is not exhibited. Is saturated, so the content is limited to 0.005 to 0.5%.

The fourth group of elements is an effective group of elements for strengthening grain boundaries and improving delayed fracture characteristics. W: W is an element effective for improving delayed fracture characteristics of high-strength bolts. However, if it is less than 0.05%, the above-mentioned effect is not exhibited. Since the effect is saturated, 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.
If it is less than 0003%, the above effect is not exhibited, and
Even if it exceeds 50%, the effect is saturated.
It was limited to 0.0050%.

Although P and S are not particularly limited,
From the viewpoint of improving delayed fracture characteristics,
A preferred range is 5% or less.

(Manufacturing method) In the manufacturing method, in the step of deoxidizing steel having the above components, Ti and oxygen are set within the above-specified range of the components, and at the same time, the balance between Ti and oxygen in the molten steel is -0.04 ≦ Ti. After adjusting so as to satisfy the relationship of% -O% ≦ 0.04, cooling from solidification to 1000 ° C. is performed at a cooling rate of 0.5 ° C./sec or more. -0.04 ≦ balance of Ti and oxygen in molten steel
When the relationship of Ti% -O% ≦ 0.04 is not satisfied, the average particle spacing of the composite precipitate to be a hydrogen trap site does not fall within the range of 3 to 30 times the average particle diameter. If the cooling rate from solidification to 1000 ° C. is less than 0.5 ° C./sec, the average particle size of the composite precipitate exceeds 1.0 μm. Therefore, the cooling rate is 0.5 ° C./sec or more. The average particle size of the composite precipitate can be made 0.3 μm or less by preferably setting the temperature to 20 ° C./sec or more.

[0049]

EXAMPLES Hereinafter, the effects of the present invention will be described more specifically with reference to examples. Test materials having the chemical compositions shown in Table 1 were heat-treated under various conditions to adjust the structure to martensite, tempered martensite, bainite, and tempered bainite, and then heated to various temperatures. Table 2 shows the results of evaluation of the mechanical properties, microstructure, and delayed fracture characteristics using the above samples. The delayed fracture characteristics were evaluated based on the above-mentioned limit amount of invading hydrogen, and the applied stress was 90% of the tensile strength.
%. The hydrogen charge was performed by immersing in an acidic solution at 20 ° C. having a pH of 4.0 or less for 100 hours or more, assuming hydrogen intrusion due to corrosion.

[0050]

[Table 1]

[0051]

[Table 2]

Test No. 2 in Table 2 1 to 12 are examples of the present invention,
Others are comparative examples. As can be seen from the table, all of the examples of the present invention have a tensile strength of the bolt of 1200 MPa or more and have a martensite, tempered martensite, bainite, and tempered bainite structure. In these, the delayed fracture type is transgranular cracking, the critical penetration hydrogen amount is higher than that of the conventional bolt, and a bolt having excellent delayed fracture characteristics is realized.

On the other hand, the comparative example No. 13 is C
1200M which is the object of the present invention because the content is too low
A high-strength bolt of Pa or higher has not been realized.

The comparative example No. In Nos. 14 and 17, since the O content was lower than the specified value or Ti was not added, the predetermined precipitate dispersion density was not obtained, and the hydrogen trapping effect could not be confirmed. Therefore, the delayed fracture mode is grain boundary cracking, the critical diffusible hydrogen content is low, and the delayed fracture characteristic is poor.

The comparative example No. In No. 15, the Ti content was lower than the prescribed value, the predetermined precipitate dispersion density was not obtained, and the hydrogen trapping effect was extremely small. For this reason,
This is an example in which the delayed fracture mode is grain boundary cracking, the critical diffusible hydrogen content is low, and the delayed fracture characteristics are poor.

The comparative example No. In No. 16, although the Mn content was higher than the specified value and a predetermined precipitate dispersion density was obtained, the delayed fracture mode was grain boundary cracking, the critical diffusible hydrogen content was low, and the delayed fracture characteristics were poor. It is an example.

In the comparative example No. Nos. 18 to 20 satisfy the requirements in terms of components, but because the cooling rate of the slab was lower than the requirement, a predetermined average particle size of the precipitate was not obtained, and as a result, a predetermined dispersion density was also obtained. This is an example in which the hydrogen trapping effect is small, the critical amount of invading hydrogen is low, and the delayed fracture characteristics are poor.

[0058]

As is clear from the above embodiments, the present invention reduces the layered structure of ferrite and martensite or tempered martensite to reduce the delayed fracture mode of bolts from intergranular cracks to intragranular cracks. Let me
A method for manufacturing the high-strength bolt having a tensile strength of 1200 MPa or more, by greatly improving the delayed fracture characteristics of the high-strength bolt, and by optimally selecting the chemical composition of the steel, the heat treatment conditions, and the structure before the heat treatment. The industrial effects are extremely remarkable.

[Brief description of the drawings]

FIG. 1 is a diagram showing a hydrogen release curve during heating.

FIG. 2 is a diagram schematically showing the relationship between the amount of invading hydrogen and the rupture time.

FIGS. 3A and 3B are diagrams showing a relationship between hydrogen trapping energy, hydrogen trapping capacity, and a limit intrusion hydrogen amount.

Claims (15)

[Claims] 1. A method in which a steel material is immersed in an acidic solution having a pH of 4.0 or less and contains 0.1 wt. pp
A steel material with excellent delayed fracture characteristics, characterized in that it absorbs at least m. 2. After immersion in an acidic solution having a pH of 4.0 or less and heating at a rate of 100 ° C./hour, 18
A peak of hydrogen release is obtained in a temperature range of 0 ° C. to 600 ° C., and the amount of released hydrogen is 0.1 wt. A steel material excellent in delayed fracture characteristics, characterized in that it is not less than ppm. 3. A hydrogen trap energy of 25 to 50 k.
J / mol and a hydrogen trap capacity of 0.1 w
t. A steel material having excellent delayed fracture characteristics, characterized in that it has at least one of oxides, carbides, and nitrides that can serve as hydrogen trap sites, alone or in combination, which is at least ppm. 4. A structure mainly composed of martensite or tempered martensite and having an average particle size of 0.05.
Si, M having an average particle size of not less than 3 μm and not more than 30 times the average particle size.
The delayed fracture according to any one of claims 1 to 3, further comprising at least one of oxides, carbides, and nitrides mainly composed of n, Ti, Ai, and V, or composite precipitates. Steel with excellent properties 5. A structure mainly composed of bainite or tempered bainite, having an average particle size of 0.05 μm or more and 1.0 μm or less, and an average particle interval of 3 to 30 times the average particle size. Na, Si, Mn, Ti,
4. An excellent delayed fracture characteristic according to any one of claims 1 to 3, characterized in that it has at least one of oxides, carbides and nitrides mainly composed of Ai and V, or composite precipitates. Steel 6. In% by weight, C: 0.15 to 0.50%, Si: 0.05 to 2.0%, Mn: 0.2 to 2.0%, Al: 0.005 to 0.5%. The composition according to any one of claims 1 to 5, comprising 1%, Ti: 0.005 to 0.05%, O: 1 to 500 ppm, and N: 20 to 150 ppm, with the balance being Fe and unavoidable impurities. 2. A high-strength steel material having excellent delayed fracture resistance according to item 1. 7. In% by weight, C: 0.15 to 0.50%, Si: 0.05 to 2.0%, Mn: 0.2 to 2.0%, Al: 0.005 to 0.5%. 1%, Ti: 0.005 to 0.05%, Mg: 0.1 to 100 ppm, O: 1 to 500 ppm, N: 20 to 150 ppm, the balance being Fe and unavoidable impurities. The high-strength steel material having excellent delayed fracture resistance according to any one of claims 1 to 5. 8. In weight%, Cr: 0.05-2.0%, Mo: 0.05-1.0%, Ni: 0.05-5.0%, Cu: 0.05-1. The high-strength steel material excellent in delayed fracture resistance according to any one of claims 6 and 7, further comprising one or more of 0%. 9. One or more of V: 0.05 to 0.5%, Nb: 0.005 to 0.2%, and Ta: 0.005 to 0.5% by weight%. The high-strength steel material according to any one of claims 6 to 8, wherein the high-strength steel material has excellent delayed fracture resistance. 10. The composition according to claim 6, comprising one or more of W: 0.05 to 0.5% and B: 0.0003 to 0.0050% by weight. A high-strength steel material excellent in delayed fracture resistance according to any one of the above. 11. In% by weight, C: 0.15 to 0.50%, Si: 0.05 to 2.0%, Mn: 0.2 to 2.0%, Al: 0.005 to 0.5%. 1%, N: 20 to 150 ppm, and the balance of molten steel consisting of Fe and unavoidable impurities is deoxidized to reduce the dissolved oxygen content to 1 to 500 wt pp.
m, and further deoxidized with Ti so that the Ti content is 0.00
5 to 0.05% by weight, and -0.04 ≦ Ti% -O%
A molten steel satisfying a relationship of ≦ 0.04, casting the molten steel,
0.5 ° C / s between the solidification temperature of the slab and 1000 ° C
The method for producing a high-strength steel material excellent in delayed fracture resistance according to claim 6, wherein cooling is performed at a cooling rate of ec or more. 12. In% by weight, C: 0.15 to 0.50%, Si: 0.05 to 2.0%, Mn: 0.2 to 2.0%, Al: 0.005 to 0.5%. 1%, Mg: 0.1 to 100 ppm, N: 20 to 150 ppm, and the balance of molten steel consisting of Fe and unavoidable impurities is deoxidized to reduce the dissolved oxygen content to 1 to 500 parts per weight pp.
m, and further deoxidized with Ti so that the Ti content is 0.00
5 to 0.05% by weight, and -0.04 ≦ Ti% -O%
A molten steel satisfying a relationship of ≦ 0.04, casting the molten steel,
0.5 ° C / s between the solidification temperature of the slab and 1000 ° C
The method for producing a high-strength steel material excellent in delayed fracture resistance according to claim 7, wherein the cooling is performed at a cooling rate of ec or more. 13. The method according to claim 11, wherein:
In the production method described in the section, the molten steel is, by weight%, Cr: 0.05 to 2.0%, Mo: 0.05 to 1.0%, Ni: 0.05 to 5.0%, Cu: The method for producing a high-strength steel material having excellent delayed fracture resistance according to claim 8, further comprising one or more of 0.05 to 1.0%. 14. The manufacturing method according to claim 11, wherein the molten steel is V: 0.05 to 0.5%, and Nb: 0.005 to 0.2% by weight%. The method for producing a high-strength steel material having excellent delayed fracture resistance according to claim 9, further comprising one or more of Ta: 0.005 to 0.5%. 15. The manufacturing method according to claim 11, wherein W: 0.05 to 0.5% and B: 0.0003 to 0.0050% by weight of the molten steel. The method for producing a high-strength steel material having excellent delayed fracture resistance according to claim 10, further comprising one or more of the following. [0001]