JPH1017985A - High strength steel excellent in hydrogen embrittlement resistance and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a high strength steel showing improved delayed fracture resistance by making the hydrogen embrittlement resistance excellent even in the case of a high strength steel and to provide a method for producing the same. SOLUTION: Fine compounds with <=50nm dimension are largely dispersed into a steel. Concretely, by allowing them to exist by >=20 pieces/(599nm)<2> in the steel, the high strength steel excellent in hydrogen embrittlement resistance can be obtd. As the fine compounds, any of Mo base compounds, Ti base compounds, V base compounds and composite compounds contg. >= two kinds of elements selected from Mo, Ti and V can be adopted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel having improved delayed fracture resistance due to excellent hydrogen embrittlement resistance and a method for producing the same.

[0002]

2. Description of the Related Art Various causes are considered to be complicatedly intertwined with respect to the cause of delayed fracture which occurs after a certain period of time has passed since stress was applied to a steel material. , It is difficult to determine the cause. However, in general, there is a common understanding that hydrogen embrittlement is involved. on the other hand,
Factors affecting the hydrogen embrittlement phenomenon include tempering temperature,
Although the structure, material hardness, crystal grain size, and the effects of various alloying elements have been recognized for some time, means for preventing hydrogen embrittlement have not been established, and various methods have been proposed by trial and error. The fact is that it is only a matter of fact.

[0003] Also, JP-A-60-114551, JP-A-2-267243, JP-A-3-243745, etc.
It is disclosed that by adjusting various main alloying elements, a high strength bolt steel having excellent delayed fracture resistance even if the tensile strength is 140 kgf / mm ² or more. However, the danger of delayed failure has not been completely eliminated and their scope is limited.

[0004]

Accordingly, the present inventors have investigated in detail the relationship between various compounds in the steel and the resistance to hydrogen embrittlement of conventional high-strength steels. As a result, a good correlation can be seen between the amount of fine compounds and delayed fracture characteristics, and the fact that the occurrence of delayed fracture can be truly prevented by focusing on extremely fine compounds and defining the number thereof. I found it. In other words, one of the causes of the deterioration of delayed fracture characteristics is that hydrogen (diffusible hydrogen) moving around in the steel has an adverse effect. It is effective to positively precipitate the compound and trap hydrogen by the compound. At this time, it seems that compounds of any size exhibit hydrogen trapping ability, but according to the study of the present inventors, it has been revealed that compounds having smaller hydrogen trapping ability are superior. . That is, a fine compound of 50 nm or less, which has not been noticed at all up to now, is extremely effective in improving delayed fracture resistance as compared with a compound larger than 50 nm. The prevention effect becomes remarkable. In particular, in the case of a quenched and tempered material of an alloy steel, after quenching, high-temperature tempering precipitates a large number of fine alloy-based compounds, which is effective in improving delayed fracture.

That is, in the present invention, the total number of fine compounds having a size of 50 nm or less is determined to be 20 / (500 nm) ² or more. These compounds include Mo-based compounds, Ti-based compounds, V-based compounds, or Mo, Ti,
A composite compound containing two or more metals selected from V is shown, and the presence of these fine compounds results in a high-strength steel having better hydrogen embrittlement resistance.

As the components of the high-strength steel of the present invention satisfying the above requirements, Mo: 2.00% or less (not including 0%) Ti: 0.20% or less (not including 0%) ) V: 0.20% or less (excluding 0%) It is recommended that the composition contains at least one member selected from the group consisting of: Cr: 2.00 % Or less (excluding 0%) Al: 0.05% or less (excluding 0%) Nb: 0.20% or less (excluding 0%) W: 0.20% or less (excluding 0%) ) B: 0.003% or less (not including 0%) One or more selected from the group consisting of: and / or as other components, C: 0.10 to 0.60% N: 0. 001 to 0.010% O: 0.005% or less (excluding 0%) S: 0.025% or less (0% It is recommended from the group consisting included not) are those containing at least one selected.

[0007]

Next, the configuration of the present invention will be described in more detail. The total number of fine compounds having a size of 50 nm or less is 2
Requirement of 0 / (500 nm) ² or more: The cause of deterioration of delayed fracture resistance is that hydrogen (diffusible hydrogen) moving around in steel has an adverse effect as described above. To reduce this diffusible hydrogen, it is necessary to increase the ability to trap hydrogen by precipitating many fine compounds in the steel. Furthermore, these compounds have the effect of increasing the toughness of the steel itself and increasing the resistance to hydrogen. Particularly fine compounds (50 nm in size)
It has been found that, when the total number is below 20 / (500 nm) ² , the effect of improving hydrogen embrittlement resistance is remarkably exhibited. Further, the number is preferably 30 / (500 nm) ² or more.

[0008] These larger compounds are described below.化合物 Compounds with a size of 300 nm or more If a softening step such as spheroidizing annealing is included before the quenching and tempering treatment, if the quenching heating is insufficient, a coarse compound having a spheroidized structure will remain. There is. The size of these compounds is 300 nm or more, and if these compounds are large, as shown in Table 5 below, the toughness of the steel tends to decrease, and the delayed fracture strength tends to decrease. It is desirable to reduce compounds of these sizes as much as possible.

Compounds having a size of 50 to 300 nm The precipitates having these sizes are formed of Fe ₃ C (cementite).
And alloy-based precipitates (Mo ₂ C, Cr ₇ C _3, etc.). Although there is no significant difference in the hydrogen trapping effect between the Fe ₃ C-based and alloy-based precipitates, the amount of Fe ₃ C
C-based compounds predominate. However, the Fe ₃ C-based compound may precipitate in a plate shape at a grain boundary or the like, and may rather decrease the grain boundary strength and decrease the delayed fracture strength.

[0010] For compounds of this level, the correlation with delayed fracture strength is unclear as shown in Table 6 below, and even if these amounts are specified, hydrogen embrittlement cannot be prevented. .

The method for measuring the size of a compound will be described in Examples. Kind of compound: As a compound which can be efficiently precipitated as a fine compound in steel and exhibit a high hydrogen trapping ability, an alloy-based compound can be mentioned, in particular, a Mo-based compound,
In the case of a Ti-based compound, a V-based compound, and a composite compound containing two or more kinds of metals selected from Mo, Ti, and V, it is found that a particularly excellent effect for improving hydrogen embrittlement resistance is exhibited. Was. Further, these compounds are effective in preventing austenite crystal grains from being coarsened, and are also effective in improving the strength and toughness of steel.

Important chemical components constituting the steel of the present invention: -1 Mo: at most 2.00%, Ti: at most 0.20%, V: at least one selected from the group consisting of at most 0.20% Is an element effective for precipitating a fine compound efficiently in steel, and is a Mo compound, a Ti compound, a V compound, and two or more metals selected from Mo, Ti, and V. Is an element necessary for precipitating a complex compound or the like containing. Further, these elements are effective elements for improving hardenability and exert an effect for improving the strength and toughness of the steel material.

[0013] The effect of Mo not only saturates at about 2.00%, but if it is added beyond that, the life of the forging tool is reduced due to an increase in deformation resistance. If Ti or V is added in excess of 0.20%, huge nitrides and carbides are generated, and the toughness is reduced. Therefore, Mo: 2.00%
Hereinafter, it is determined that Ti: 0.20% or less and V: 0.20% or less. In addition, each preferable upper limit content is Mo:
1.5% or less, Ti: 0.15% or less, V: 0.15%
Or less, more preferably Mo: 1.05% or less,
Ti: 0.1% or less, V: 0.1% or less. On the other hand, each preferred lower limit content is Mo: 0.3% or more,
Ti: 0.01% or more, V: 0.15% or more, more preferably Mo: 0.6% or more, Ti: 0.02%
As described above, V is 0.02% or more.

-2 Cr: 2.00% or less, Al:
0.05% or less, Nb: 0.20% or less, W: 0.20
% Or less, B: at least one element selected from the group consisting of 0.003% or less. These elements are also effective elements for efficiently precipitating fine compounds in steel. The containing compound also has an effect of preventing austenite crystal grains from being coarsened, and is therefore effective in improving the strength and toughness of steel. Further, Cr contributes to the improvement of corrosion resistance and also has an effect of increasing hydrogen embrittlement resistance, and B also diffuses at grain boundaries to enhance the hardenability of steel.

The effect of Cr saturates at about 2.0%,
When Nb, W, and B are added in a large amount, a large amount of nitride or carbide is generated, and the toughness is reduced. Therefore, Cr: 2.00% or less, Al: 0.05% or less, Nb: 0.20% or less,
W: 0.20% or less, B: 0.003% or less.
In addition, each preferable upper limit content is Cr: 1.5% or less, Al: 0.045% or less, Nb: 0.15% or less,
W: 0.15% or less, B: 0.0025% or less,
More preferably, Cr: 1.05% or less, Al: 0.0
4% or less, Nb: 0.1% or less, W: 0.1% or less,
B: 0.0020% or less. On the other hand, preferable lower limit contents are Cr: 0.3% or more, Al: 0.01% or more, and N:
b: 0.01% or more, W: 0.01% or more, B: 0.0
005% or more, and more preferably, Cr: 0.5%.
Above, Al: 0.02% or more, Nb: 0.02% or more,
W: 0.02% or more, B: 0.001% or more.

-3 C: 0.10-0.60%, N:
0.001 to 0.010%, O: 0.005% or less,
S: 0.025% or less These elements are necessary for precipitating a compound in steel, and form carbides, nitrides, oxides, and sulfides. C is an element that forms carbides and is indispensable for securing the tensile strength necessary for high-strength steel, and must be contained in an amount of 0.10% or more. On the other hand, 0.
C exceeding 60% causes coarsening of carbides,
It causes a decrease in toughness and deteriorates delayed fracture resistance. Therefore C
Was determined to be 0.10 to 0.60%. Note that a preferable lower limit C amount is 0.20%, and a more preferable lower limit C amount is 0.30%. On the other hand, the preferable upper limit C amount is 0.4
5%, and a more preferable upper limit C amount is 0.40%.

N forms nitrides and has a favorable effect on the refinement of crystal grains and the improvement in delayed fracture resistance. To obtain these effects, it is necessary to add 0.001% or more. On the other hand, if the N content exceeds 0.010%, the amount of solid solution N increases, which is detrimental to delayed fracture resistance. Therefore, the content of N is 0.0
It was determined as 01 to 0.010%. Note that a preferable lower limit N amount is 0.002%, and a more preferable lower limit N amount is 0.002%.
4%. A preferred upper limit N amount is 0.007%,
A more preferred upper limit N amount is 0.006%.

O forms oxides and is finely dispersed in steel. However, if the O content exceeds 0.005%, coarse oxides are precipitated, leading to a decrease in toughness and deterioration in delayed fracture resistance. Therefore, the content of O is determined to be 0.005% or less.
In addition, a preferable O amount is 0.003% or less, and a more preferable O amount is 0.001% or less.

S forms sulfide and is finely dispersed in steel. However, when the amount of S exceeds 0.025%, coarse M
By forming nS or the like, it becomes a stress concentration point, and deteriorates delayed fracture resistance. Therefore, the content of S is determined to be 0.025% or less. In addition, a preferable S amount is 0.010% or less, and a more preferable O amount is 0.005% or less. The steel of the present invention may contain unavoidable impurities in production, but these are allowed as long as the effects of the present invention are not impaired. Also, if the manufacturing conditions of steel materials are adjusted as follows,
The useful compound can be finely precipitated efficiently,
High strength steel with excellent hydrogen embrittlement resistance is easily obtained.

That is, a preferable condition is to define a cooling rate in a solidification process in producing a steel material. That is, in the solidification process (during cooling from 1500 ° C. to 1300 ° C.), by cooling at a rate of 10 ° C./min or more, the precipitation of coarse compounds is suppressed, and many fine compounds are precipitated. A more preferred cooling rate is 20 ° C /
Min or more, and a preferable cooling rate is 30 ° C./min or more. Next, examples of the present invention will be described.

[0021]

EXAMPLE A steel material having the composition shown in Table 1 was melted in a 150 kg vacuum melting furnace, cast into a 150 kg ingot, and cooled. In order to change the cooling rate during the solidification process, a part was cast into an ingot of 50 to 150 kg and cooled while keeping the temperature partially. Then forged to 25mmφ, 120
After performing a soaking process at 0 ° C. for 30 minutes, a normalizing process was performed, and a final heat treatment was performed so that the tensile strength became about 1000 to 2000 N / mm ² .

[0022]

[Table 1]

With respect to the obtained steel material, the number of compounds was measured, and at the same time, the tensile strength and the delayed fracture strength were measured.
The results shown in Tables 2 to 6 below were obtained. In addition, the compound measurement and the measurement of delayed fracture strength were measured by the following methods.

<Measurement of number of compounds>: Compounds extracted by the ordinary extraction replica method were subjected to a scanning electron microscope (TEM) at an accelerating voltage of 200 KV and a voltage of 200 KV.
Take a 150,000x photo.

50 nm observed in a visual field of 500 nm × 500 nm (75 mm × 75 mm at 150,000 times)
The following fine compounds are counted. An example of a photograph taken on each upper side of FIGS. 2, 3 and 4 (the length of one side is 500 nm)
Is shown on the lower side, the field of view and the size of the compound are faithfully transferred, and the number thereof (both 50 n as apparent from the figure)
When measured, the number was 36 in FIG. 2, 24 in FIG. 3, and 10 in FIG. 4 and 5 also show the results of composition analysis by EDX.
FIG. 4 shows the case where the precipitate composition is the Mo—Ti—V composite, and FIG. 5 shows the case where the precipitate composition is the Mo—Ti—V—Cr—Al composite.

The composition of the precipitate in FIG. 5 is mainly Mo-Ti-V
6 is mainly composed of Mo.
-A Ti-V-Cr-Al-based composite compound. However,
The reason why the peak of Cu is high is that a Cu mesh was used when fixing the sample in the measurement.

Photographing was performed at arbitrary five locations per sample, and the average value of the measured five locations was defined as the total number of compounds, and represented by ○ / (500 nm) ² .

<Measurement of Delayed Fracture Strength> Using a loop-type constant strain delayed fracture tester, a stress load was applied to the delayed fracture test specimen shown in FIG. 1 in water, and the delayed fracture strength for 100 hours was measured.
The results are as shown in Tables 2, 3 and 4, and all the steels having a large number of fine compounds show high delayed fracture strength.

[0029]

[Table 2]

[0030]

[Table 3]

[0031]

[Table 4]

[0032]

[Table 5]

[0033]

[Table 6]

As shown in Tables 2, 3 and 4, the fine compound was 5
A compound having a thickness of 0 nm or less is particularly effective, and as shown in Table 2, it can be seen that a smaller value of 30 nm or less, and further 10 nm or less, is more effective in improving delayed fracture resistance.
On the other hand, for a compound having a size of 50 nm or more, 50 to
As shown in Table 6, no good correlation between the compound having a size of 300 nm and the quality of delayed fracture resistance can be found, and the number of compounds having a size of 300 nm or more is reduced as shown in Table 5. This is effective for improving delayed fracture resistance. Thus, the present invention has revealed that the correlation between the size (and the number) of the compound and the delayed fracture resistance shows a very unique aspect. On the other hand, as shown in Table 4, it was found that controlling the solidification rate was effective as a means for precipitating a large number of compounds having a size of 50 nm or less.

[0035]

The present invention is configured as described above.
A high-strength bolt steel having a level of 200 N / mm ² or more and having excellent hydrogen embrittlement resistance has been provided.

[Brief description of the drawings]

FIG. 1 is a diagram showing dimensions and shapes of test pieces used for delayed fracture strength measurement.

FIG. 2 is a photograph showing a measurement example of the size and number of compounds and a display diagram of precipitates [36 precipitates / (500 nm)
² ].

FIG. 3 is a photograph showing a measurement example of the size and number of compounds and a display diagram of precipitates [24 precipitates / (500 nm)
² ].

FIG. 4 is a photograph showing a measurement example of the size and number of compounds and a display diagram of precipitates [10 precipitates / (500 nm)
² ].

FIG. 5 is a photograph showing an example of analyzing the compound composition by EDX with respect to 1 in FIG. 2 (precipitate composition: Mo-T).
i-V composite).

6 is a photograph showing an example of analyzing a compound composition by EDX for 2 in FIG. 2 (precipitate composition: Mo-T).
i-V-Cr-Al composite).

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[Procedure amendment]

[Submission date] August 23, 1996

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Brief description of drawings

[Correction method] Change

[Correction contents]

[Brief description of the drawings]

FIG. 1 is a view showing dimensions and shapes of test pieces used for delayed fracture strength measurement.

FIG. 2 is a photomicrograph (TEM image) showing an example of measuring the size and number of compounds, and can be read from the photo.
That [precipitate is a display diagram of the precipitates 36 / (500n
m) ² ].

FIG. 3 is a photomicrograph (TEM image) showing a measurement example of the size and number of compounds and can be read from the photo.
It is a display diagram of that precipitates [precipitates 24 / (500n
m) ² ].

FIG. 4 is a photomicrograph (TEM image) showing a measurement example of the size and number of compounds and can be read from the photo.
It is a display diagram of that precipitates [precipitates 10 / (500n
m) ² ].

5 is a photograph showing an example of a spectrum image obtained by analyzing a compound composition by EDX for 1 in FIG. 2 (precipitate composition: Mo—Ti—V composite).

FIG. 6 is a photograph showing an example of a spectrum image obtained by analyzing the compound composition by EDX for 2 in FIG. 2 (precipitate composition: Mo—Ti—V—Cr—Al composite).

Claims (6)

[Claims] 1. A high-strength steel excellent in hydrogen embrittlement resistance, characterized in that compounds having a size of 50 nm or less are present in the steel in an amount of 20 pieces / (500 nm) ² or more. 2. A compound having a size of 50 nm or less is Mo
Compounds, Ti compounds, V compounds, Mo, T
The high-strength steel according to claim 1, wherein the high-strength steel is any one of a composite compound containing two or more elements selected from i and V. 3. Steel: Mo: 2.00% or less (excluding 0%) Ti: 0.20% or less (excluding 0%) V: 0.20% or less (excluding 0%) The high-strength steel according to claim 1, comprising one or more selected from the group consisting of: 4. The steel further contains Cr: 2.00% or less (not including 0%) Al: 0.05% or less (not including 0%) Nb: 0.20% or less (not including 0%) W: 0.20% or less (excluding 0%) B: 0.003% or less (excluding 0%) Contains one or more selected from the group consisting of: The high-strength steel according to claim 3. 5. The steel further contains, as another component, C: 0.10 to 0.60% N: 0.001 to 0.010% O: 0.005% or less (excluding 0%) S: The high-strength steel according to claim 3 or 4, comprising at least one selected from the group consisting of 0.025% or less (excluding 0%). 6. The method of manufacturing a high-strength steel according to claim 1, wherein a cooling rate during cooling from 1500 ° C. to 1300 ° C. during casting is adjusted to 10 ° C./min or more for cooling. The method for producing a high-strength steel excellent in hydrogen embrittlement resistance characterized by the following.