JPH11246938A - High strength steel excellent in hydrogen embrittlement resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength steel exhibiting excellent hydrogen embrittlement resistance with certainty by determining a fine compound effective for the improvement of hydrogen embrittlement resistance. SOLUTION: In the high strength steel, the amount of increase of hardness by precipitation hardening is regulated to >=10 Hv. It is desirable that a metallic compound, containing one or more elements selected from the group consisting of Mo, Ti and V, or a complex compound is precipitated in a base phase. Further, as the composition of the high strength steel satisfying the above required condition, it is recommended that one or more kinds selected from the group consisting of <=2.00% (not including 0%) Mo, <=0.20% (not including 0%) Ti, and <=0.20% (not including 0%) V are contained in the steel.

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 resistance to delayed fracture due to excellent hydrogen embrittlement resistance.

[0002]

2. Description of the Related Art It is difficult to identify the cause of delayed fracture that occurs after a certain period of time has passed since a stress was applied to a steel material, because various factors are considered to occur in a complicated manner. However, there is a common understanding that hydrogen embrittlement is generally involved. Factors affecting the hydrogen embrittlement phenomenon include tempering temperature,
The effects of microstructure, material hardness, crystal grain size, various alloying elements, etc. have been recognized for some time. Particularly, in order to improve the hydrogen embrittlement resistance, a fine compound is precipitated and this is used as a trap site for diffusible hydrogen. It is known that it is good.

[0003] However, the precipitation amount of the fine compound varies depending on various factors such as the composition of the component and the tempering temperature, and it is difficult to quantify the fine compound. The fact is that various methods have not been established but are merely proposed by trial and error.

[0004]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above circumstances, and provides excellent hydrogen embrittlement resistance by quantifying fine compounds effective for improving hydrogen embrittlement resistance. It is intended to provide a high-strength steel that reliably exerts its performance.

[0005]

The high-strength steel excellent in hydrogen embrittlement resistance according to the present invention, which has achieved the above-mentioned objects, is characterized in that the increase in hardness due to precipitation hardening is 10 Hv or more. In addition, it is preferable that a metal compound or a composite compound containing at least one element selected from the group consisting of Mo, Ti, and V is precipitated in the parent phase.

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 (not including 0%) It is recommended that the composition contains at least one selected from the group consisting of: Cr: 2.00% or less ( 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 (excluding 0%) It is desirable to contain at least one selected from the group consisting of: C: 0.10 to 0.60% N: 0.001 -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.

Further, the steel further contains, as other components, P: 0.025% or less (excluding 0%) and / or Mn: 0.70% or less (excluding 0%). Desirably.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, one of the causes of deterioration of delayed fracture characteristics is hydrogen (diffusible hydrogen) moving around in steel. In order to reduce this diffusible hydrogen, it is effective to actively precipitate fine compounds in the steel to trap hydrogen, especially in the case of high-strength steel,
Precipitation of a fine alloy-based compound in a matrix by adding a precipitation hardening type alloying element and performing high-temperature tempering treatment is very effective in preventing delayed fracture.

However, it is considered very difficult to quantify the precipitated fine alloy-based compound (hereinafter, sometimes simply referred to as a precipitate), and the relationship between the amount of precipitation and the delayed fracture characteristics is considered. Was not disclosed. Therefore, the present inventors have conducted intensive studies on a method for quantifying precipitates that is effective in preventing delayed fracture. As a result, (a) by subtracting the matrix hardness before tempering determined by the method described later from the actual hardness of the steel material, it is possible to determine the increase in hardness due to precipitates, and (b) obtained A very high correlation is found between the increased hardness and the delayed fracture property, and it has been found that the occurrence of delayed fracture can be reliably prevented by setting the hardness increase to a certain value or more. I arrived. That is, the present invention has completed the technique of grasping the delayed fracture prevention effect by indirectly estimating the precipitate amount from the increase in hardness due to the precipitate, rather than grasping the delayed fracture prevention effect from the precipitate amount itself. It is. The matrix hardness and the amount of increase in hardness may be determined by the following procedures.

First, after quenching the steel material, 300 to 700
Tempering is performed by changing the temperature in the range of ° C., and the hardness of the steel material treated at various temperatures is measured to create a tempering performance curve as shown in FIG. In ordinary carbon steel, the hardness tends to gradually decrease as the tempering temperature rises, but in the case of a steel material containing a precipitation hardening type alloy element, as shown in FIG. A phenomenon in which the tendency of the hardness decrease slows down in the temperature range is observed. That is,
The actual material hardness is the hardness obtained by adding the hardness of the precipitation hardening to the matrix hardness.

[0011] A tempering temperature range in which precipitation hardening is not recognized indicates matrix hardness. In the tempering performance curve of FIG. 1, the results at 300 ° C. and 700 ° C. show the matrix hardness without precipitation hardening.
A matrix hardness line indicating the matrix hardness can be obtained by connecting two points of ℃ with a straight line, and the matrix hardness is determined at each tempering temperature based on the matrix hardness line. Can be obtained by reading the value of

Therefore, at each tempering temperature, the value obtained by subtracting the matrix hardness (value of the matrix hardness line) obtained by the above method from the actual material hardness (value of the tempering performance curve) is determined by the precipitate. This is the amount of increase in hardness.

The amount of increase in hardness due to the precipitates increases as the number of fine precipitates increases, and particularly when the amount of increase in hardness is 10 Hv or more, the effect of preventing delayed fracture occurrence is remarkable. The increase in hardness is preferably 20 Hv or more, and more preferably 30 Hv or more.

In addition, it is finely and efficiently precipitated in steel,
Compounds exhibiting high hydrogen trapping ability include alloy-based compounds. In particular, the above-mentioned alloy-based compounds are Mo,
In the case of a metal compound or a composite compound containing at least one element selected from the group consisting of Ti and V, a very excellent effect on improvement of hydrogen embrittlement resistance is exhibited. Moreover, these compounds are effective in preventing austenite crystal grains from being coarsened, and are effective in improving the strength and toughness of steel. Next, the reasons for limiting the chemical components in the present invention will be described.

(A) Mo: 2.00% or less, Ti: 0.
20% or less, V: 0.20% or less These elements are effective elements for efficiently depositing fine compounds in steel, and are at least one element selected from the group consisting of Mo, Ti, and V. Is an element necessary for precipitating a metal compound or a composite compound containing the above element.
Further, these elements are effective elements for improving hardenability, and also exhibit effects for improving the strength and toughness of the steel material. Therefore, it is preferable that each element contains Mo: 0.3% or more, Ti: 0.01% or more, and V: 0.15% or more.
o: 0.6% or more, Ti: 0.02% or more, V: 0.0
More preferably, it is 2% or more.

However, the effect of the addition of Mo saturates at about 2.00% and, if too large, the deformation resistance increases, thereby shortening the life of the forging tool. 0.20% of Ti and V
If it is added in excess of, it produces huge nitrides and carbides,
Mo: 2.00% or less, Ti:
0.20% or less, V: 0.20% or less. The preferred upper limit contents of each element are Mo: 1.5%, T
i: 0.15%, V: 0.15%, More preferably, Mo: 1.05% or less, Ti: 0.1% or less, V:
0.1% or less.

(B) Cr: 2.00% or less, Al: 0.
05% or less, Nb: 0.20% or less, W: 0.20% or less
Lower, B: 0.003% or less These elements are also effective in precipitating fine compounds in steel efficiently, and the compounds containing these elements prevent austenite crystal grains from becoming coarse. Thus, it has the effect of improving the strength and toughness of steel. In particular, Cr also contributes to the improvement of corrosion resistance and also has the effect of increasing hydrogen embrittlement resistance, and B also has the effect of scattering at the grain boundaries to increase the hardenability of steel. Thus, each element is Cr:
0.3% or more, Al: 0.01% or more, Nb: 0.01
%, W: 0.01% or more, B: 0.0005% or more, Cr: 0.5% or more, A
l: 0.02% or more, Nb: 0.02% or more, W: 0.
More preferably, it is 02% or more and B: 0.001% or more.

However, the effect of Cr saturates at about 2.0%,
When Al, Nb, W, and B are added in large amounts, huge nitrides and carbides are generated, and the toughness is reduced.
2.00% or less, Al: 0.05% or less, Nb: 0.2
0% or less, W: 0.20% or less, B: 0.003% or less. In addition, each preferable upper limit content is Cr:
1.5%, Al: 0.045%, Nb: 0.15%,
W: 0.15%, B: 0.0025%, more preferably Cr: 1.05% or less, Al: 0.04% or less, Nb: 0.1% or less, W: 0.1% Hereinafter, B: 0.
0020% or less.

(C) C: 0.10 to 0.60%, N:
0.001 to 0.010%, O: 0.005% or less,
S: 0.025% or less C, N, O, and S are effective elements for precipitating compounds in steel, and form carbides, nitrides, oxides, and sulfides, respectively.

C is an element that forms carbides and is indispensable for securing the required tensile strength as a high-strength steel, and must be contained in an amount of 0.10% or more. On the other hand, when the C content exceeds 0.60%, coarsening of carbides is caused, and toughness is reduced, thereby deteriorating delayed fracture resistance. Therefore, the content of C is 0.10 to 0.60%.
It was decided. The preferred lower limit of the C content is 0.20%, and more preferably 0.30% or more. On the other hand, a preferred upper limit is 0.45%, and more preferably 0.40% or less.

N is an element that has a favorable effect on the refinement of crystal grains and the improvement in delayed fracture resistance by forming nitrides. To obtain these effects, addition of 0.001% or more is required. is necessary. However, if the amount of N exceeds 0.010%, the amount of solute N increases, which is detrimental to delayed fracture resistance.
Therefore, the content of N is determined to be 0.001 to 0.010%. The preferred lower limit of the amount of N is 0.002%,
More preferably, it is 0.004% or more. On the other hand, a preferable upper limit is 0.007%, and more preferably 0.006% or less.

O forms oxides and is finely dispersed in steel. However, if the O content exceeds 0.005%, a coarse oxide precipitates, causing a decrease in toughness and deteriorating delayed fracture resistance. Therefore, the O content is set to 0.005% or less. still,
The preferred O amount is 0.003% or less, and 0.001%.
% Is more desirable.

S forms sulfide and is finely dispersed in steel. However, if the S content exceeds 0.025%, coarse Mn
Since S and the like are formed to be stress concentration points and deteriorate delayed fracture resistance, the S content is determined to be 0.025% or less. Incidentally, a preferable S amount is 0.010% or less,
More preferably, it is 0.005% or less.

(D) P: 0.025% or less, Mn: 0.
70% or less P is an element that segregates at the grain boundary and lowers the grain boundary strength,
It is preferable to reduce as much as possible, and it is necessary to make it 0.025% or less. The preferred P content is 0.01
0% or less, and more preferably 0.005% or less.

Mn is an element that promotes grain boundary segregation, and may reduce the grain boundary strength. Therefore, Mn is preferably reduced as much as possible, and must be limited to 0.70% or less. It is preferably 50% or less.
More preferably, it is 30% or less.

The steel of the present invention may contain unavoidable impurities in production, but these are permissible as long as the effects of the present invention are not impaired.

Further, in producing the steel of the present invention, it is desirable to control the cooling rate in the solidification process in producing the steel material in order to efficiently and finely precipitate the compound.
Specifically, in the solidification process (during cooling from 1500 ° C. to 1300 ° C.), by cooling at a rate of 10 ° C./min or more, many fine compounds can be precipitated,
Hydrogen embrittlement resistance is improved. Preferred cooling rate is 20 ° C
/ Min or more, and a more preferable cooling rate is 30 ° C / min or more.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following Examples are not intended to limit the present invention. It is included in the technical range of.

[0029]

EXAMPLE In a vacuum melting furnace, 150 kg of a steel material having the composition shown in Table 1 was melted, cast into an ingot, and cooled. For the purpose of changing the cooling rate during the solidification process, the ingot was cast in various sizes in the range of 50 to 150 kg, and was cooled while keeping a part of the temperature.

[0030]

[Table 1]

The above ingot was forged to a diameter of 25 mm,
After performing a solution treatment at 200 ° C. for 30 minutes, a normalizing treatment was performed. After quenching at a predetermined temperature, the tensile strength is adjusted to about 1000 to 2000 N / mm ^2.
Tempering by changing the temperature in the range of 0 to 700 ° C,
A tempering performance curve as shown in FIG. 1 was created. FIG. 1 shows a tempering performance curve of the test steel a.

With respect to the obtained steel material, the matrix hardness and the increase in hardness were calculated by the above-mentioned methods (1) to (4), and the tensile strength and the delayed fracture strength were measured. The delayed fracture strength is
The delayed fracture test specimen shown in FIG. 2 was subjected to stress loading in water and the delayed fracture strength after 100 hours was measured using a loop-type constant strain delayed fracture tester. The results are shown in Table 2.

[0033]

[Table 2]

The steel No. of the present invention having a hardness increase of 10 Hv or more. Comparative steel Nos. 1 to 9 all show high delayed fracture strength, but the hardness increase is less than 10 Hv. 1
It can be seen that all the samples Nos. 0 to 13 have low delayed fracture strength.
In addition, all the steel materials with a material hardness of 360 Hv or less showed high delayed fracture strength.

[0035]

Since the present invention is constituted as described above, a high-strength steel which reliably exhibits excellent hydrogen embrittlement resistance by quantifying fine compounds effective for improving hydrogen embrittlement resistance. Can be provided.

[Brief description of the drawings]

FIG. 1 is a graph showing a tempering performance curve of test steel a in an example.

FIG. 2 is an explanatory diagram showing dimensions and shapes of test pieces used for delayed fracture strength measurement.

Claims (6)

[Claims] 1. A high-strength steel excellent in hydrogen embrittlement resistance, characterized in that an increase in hardness due to precipitation hardening is 10 Hv or more. 2. The high-strength steel according to claim 1, wherein a metal compound or a compound compound containing at least one element selected from the group consisting of Mo, Ti, and V is precipitated in a matrix. 3. Steel: Mo: 2.00% (mass%, the same applies hereinafter) or less (excluding 0%) Ti: 0.20% or less (excluding 0%) V: 0.20% or less The high-strength steel according to claim 1, comprising one or more selected from the group consisting of (not including 0%). 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 steel according to claim 1, wherein the steel further contains P: 0.025% or less (excluding 0%) and / or Mn: 0.70% or less (excluding 0%). The high-strength steel according to any one of claims 3 to 5.