JPH07166303A - High strength martensitic stainless steel excellent in stress corrosion cracking resistance and production thereof - Google Patents

Abstract

PURPOSE:To impart excellent properties against stress corrosion cracking to a steel by specifying its compsn. and structure. CONSTITUTION:This steel has a compsn. contg., as essential components, by weight, <=0.06% C, 12 to 16% Cr, <=1.0% Si, <=2.0% Mn, 0.5 to 8% Ni, 0.1 to 2.5% Mo, 0.3 to 4.0%. Cu and 0.05% N and has a structure in which the area ratio of a delta-ferritic phase is regulated to <=10% and the fine precipitates of Cu are dispersed into a matrix. For obtaining this structure, it is austenitized at a temp. from the Ac3 to 980 deg.C, is thereafter cooled and is next subjected to tempering treatment under the conditions in which the tempering temp. T( deg.C) is regulated to either lower one between >=500 deg.C to 630 deg.C or the Ac1 or below and the tempering time t (hr) is regulated to 15200 to 17800 by the value of (20+logt)(273+T). Moreover, the structure may be added with one or more kinds of 0.01 to 0.1% V and 0.01 to 0.1% Nb as well.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength martensitic stainless steel having excellent resistance to stress corrosion cracking and a method for producing the same, more specifically, for example, wet carbon dioxide gas in the drilling and transportation of petroleum and natural gas, The present invention relates to a high-strength stainless steel having high stress corrosion cracking resistance in an environment containing wet hydrogen sulfide and a method for manufacturing the same.

[0002]

2. Description of the Related Art Recently, petroleum and natural gas produced in recent years are increasing in amount containing a large amount of wet carbon dioxide gas and wet hydrogen sulfide. In place of conventional carbon steel, martensitic stainless steel such as 13Cr stainless steel. Has been used. However, although conventional martensitic stainless steels have excellent corrosion resistance to wet carbon dioxide (hereinafter simply referred to as corrosion resistance), they do not have sufficient stress corrosion cracking resistance to wet hydrogen sulfide (hereinafter simply referred to as stress corrosion cracking resistance). Without
A martensitic stainless steel having improved stress corrosion cracking resistance while maintaining strength, toughness and corrosion resistance has been desired.

In addition to strength, toughness, and corrosion resistance, the requirements of stress corrosion cracking resistance are disclosed in JP-B-61-3391.
JP-A-58-199850, JP-A-61-207550
Is disclosed. However, although they show resistance in an environment containing a very small amount of hydrogen sulfide, there is a problem that they cannot be used in an environment containing a large amount of hydrogen sulfide because stress corrosion cracking occurs in an environment where the partial pressure of hydrogen sulfide exceeds 0.01 atm. It was

On the other hand, a martensitic stainless steel having improved resistance to stress corrosion cracking in an environment where the partial pressure of hydrogen sulfide exceeds 0.01 atm has also been proposed.
-174859, JP-A-62-54063 and the like are disclosed. However, these steels cannot completely prevent stress corrosion cracking due to hydrogen sulfide.

Further, from the viewpoint of strength, the toughness and stress corrosion cracking resistance of each of the above-mentioned martensitic stainless steels are remarkably deteriorated when an attempt is made to increase the strength, so that the strength or toughness and stress corrosion cracking resistance are deteriorated. There was also the problem of having to sacrifice one of the sexes. Therefore, for example, there is a drawback that it cannot be applied to a deep oil well pipe that requires high strength, stress corrosion cracking resistance, corrosion resistance, and toughness at the same time.

[0006]

The present invention was devised to solve the above-mentioned problems in the prior art, and simultaneously improves the strength, stress corrosion cracking resistance and toughness of conventional martensitic stainless steel. This provides a high-strength martensitic stainless steel that can be used without causing stress corrosion cracking even in an environment containing a large amount of hydrogen sulfide while maintaining corrosion resistance, and a method for producing the same. Here, the target performance was set as follows in view of the performance required for carbon dioxide, petroleum containing hydrogen sulfide, natural gas excavation, and steel pipes for transportation.

Strength: 75% at 0.2% proof stress
g / mm ² or more Toughness: Absorbed energy in Charpy full size test piece at 0 ° C. (called Charpy impact value) is 10
More than kg-m Stress corrosion cracking resistance: 0.2% on test pieces in 5% saline + 0.5% acetic acid solution saturated with hydrogen sulfide gas at 1 atm
Load 60% of proof stress and withstand 720 hours without breaking

[0008]

[Means for Solving the Problems] Increasing Cr is effective for improving the corrosion resistance of martensitic stainless steel. However, on the other hand, since it forms a δ-ferrite phase and deteriorates the strength and toughness, Ni that is an austenite forming element may be increased to suppress the formation of the δ-ferrite phase, but the increase of Ni is costly. There are restrictions. Also C
Is also effective in suppressing the .delta.-ferrite phase, but carbides are formed during tempering, which deteriorates corrosion resistance. Therefore, its content should be limited. The amount of δ-ferrite phase should be limited to 10% or less because its area ratio exceeds 10%, which adversely affects strength and toughness.

On the other hand, generally, when the strength of steel is increased, the toughness and stress corrosion cracking resistance are deteriorated, but Cu is contained in an appropriate amount, and Cu is dispersed as a fine precipitate in the matrix of this stainless steel by heat treatment. By doing so, it is possible to increase the strength without deteriorating them. However, in order to deposit fine Cu precipitates, it is necessary to strictly control the tempering conditions, and it is necessary to control not only the tempering temperature but also the tempering time at the same time.

The present invention has high toughness, high strength, and excellent stress corrosion cracking resistance, which cannot be realized by the conventional martensitic stainless steel, while taking into consideration the restriction of the metal structure due to the increase of Cr as described above. It succeeded in obtaining a new martensitic stainless steel. The summary is
In the first invention, the main component is, by weight%, C: 0.
06% or less, Cr: 12-16%, Si: 1.0% or less, Mn: 2.0% or less, Ni: 0.5-8.0%, M
o: 0.1-2.5%, Cu: 0.3-4.0%, N:
The area ratio of the δ-ferrite phase is 1 including 0.05% or less.
It is a high-strength martensitic stainless steel excellent in stress corrosion cracking resistance, characterized in that fine precipitates of Cu of 0% or less are dispersed in the matrix, and in the second invention, the first invention In the steel of No. 3, the martensitic stainless steel containing at least one of V: 0.01-0.1% and Nb: 0.01-0.1% by weight% as an additional component. In the fourth aspect of the invention, steels having the composition of the first and second aspects of the invention are each austenitized at a temperature of Ac3 to 980 ° C and then cooled, and then a tempering temperature T (unit: ° C) of 500 ° C to 630 ° C. Or A
c1, whichever is lower, the tempering time t
Excellent stress corrosion cracking resistance, characterized by dispersing fine Cu precipitates in the matrix by tempering under conditions where (unit: time) is (20 + log t) (273 + T) value of 15200 or more and 17800 or less. And a high strength martensitic stainless steel.

[0011]

The reasons for limitation in the present invention will be described below. (1) C: 0.06% or less C combines with Cr at the time of tempering to form a carbide and precipitates to deteriorate corrosion resistance, stress corrosion cracking resistance and toughness. If the C content exceeds 0.06%, the deterioration becomes remarkable, so 0.0
The content is 6% or less.

(2) Cr: 12-16% Cr is a basic element that constitutes martensitic stainless steel, and is an important element that exhibits corrosion resistance, but if the content is less than 12%, sufficient corrosion resistance is obtained. Does not appear, 1
If it exceeds 6%, the amount of δ-ferrite phase produced increases and the strength and toughness deteriorate, no matter how the other alloy elements are adjusted.
2-16%.

(3) Si: 1.0% or less Si is an element necessary as a deoxidizing agent, but it is also a strong ferrite-forming element, and if it exceeds 1.0%, δ
-To accelerate the formation of the ferrite phase, the content is 1.0% or less.

(4) Mn: 2.0% or less Mn is an austenite-forming element that is effective as a deoxidizing and desulfurizing agent and suppresses the appearance of the δ-ferrite phase. Is saturated, so 2.0
% Or less.

(5) Ni: 0.5-8.0% Ni improves the corrosion resistance and is an extremely effective element for forming austenite, but if it is less than 0.5%, its effect is small. Since this element is expensive, the upper limit is 8.0% from the economical point of view.

(6) Mo: 0.1-2.5% Mo is an element particularly effective for corrosion resistance, but if the content is less than 0.1%, its effect does not appear, and if it exceeds 2.5%. And an excessive δ-ferrite phase appears, the upper limit is 2.
5%.

(7) Cu: 0.3-4.0% Cu is an important element in the present invention, and it dissolves in the matrix as a solid solution to improve the corrosion resistance, and at the same time, part of it is finely dispersed in the matrix by tempering. The precipitation has both effects of improving the strength without deteriorating the stress corrosion cracking resistance. However, if the content is less than 0.3%, the effect is not sufficient, and if the content exceeds 4.0%, the effect is saturated and it causes cracking during hot working. 3 to 4.0%.

(8) N: 0.05% or less N is an element effective for improving the corrosion resistance and is also an austenite forming element, but if it is contained in excess of 0.05%, it will combine with Cr during tempering. Corrosion resistance due to precipitation as nitride
0.06% due to deterioration of stress corrosion cracking resistance and toughness
The content is as follows.

(9) Additional components V, Nb (V: 0.01-
0.10%, Nb: 0.01-0.10%) V and Nb are strong carbide-forming elements, and by precipitating fine carbides, the crystal grains are made finer to improve the stress corrosion cracking resistance. . However, it is also a ferrite forming element and δ
Increase the ferrite phase. Content of 0.0
It was set to 1-0.10% and 0.01-0.10%. 0.0
If it is less than 10%, the effect of improving stress corrosion cracking resistance does not appear,
If it exceeds 0.10%, the effect is saturated, and the δ-ferrite phase increases and the toughness is adversely affected, so that the content of both V and Nb is 0.01-0.10%, 0.01-0.
10%.

(10) Area ratio of δ-ferrite phase: 10
% Or less The δ-ferrite phase is a phase that does not transform into martensite during quenching of martensitic steel and remains as ferrite, and if the amount is large, the toughness deteriorates significantly. In this steel, if the area ratio of the δ-ferrite phase exceeds 10%, the toughness deteriorates significantly, so the content is made 10% or less.

(11) Fine Cu Precipitate If the Cu precipitate is fine, the strength is increased by precipitation hardening, and the stress corrosion cracking resistance does not deteriorate due to the increase in strength. Here, fine precipitates can be identified by an electron microscope of 100,000 times and have a diameter of about 0.10 micron or less. However, the Cu precipitates became coarse and the diameter was 0.
When it exceeds 10 microns, the strength increasing effect disappears. Further, if Cu does not precipitate and remains dissolved in the matrix, strength increase due to precipitation hardening cannot be expected. Therefore, Cu precipitates are fine precipitates. Although the amount of dispersion is not particularly limited, it is desirable that 30 or more fine precipitates are present per square micron of the matrix.

(12) Austenitizing temperature: Ac3 or higher and 980 ° C or lower If the temperature is lower than the Ac3 temperature, the austenitization is insufficient and the necessary strength cannot be obtained, and if it exceeds 980 ° C, the crystal grains become coarse and the toughness is increased. Since the deterioration becomes remarkable and the stress corrosion cracking resistance decreases, the temperature is set to Ac3 or higher and 980 ° C or lower.

(13) Tempering temperature T (unit: ° C): 50
The temperature is 0 ° C. or higher and 630 ° C. or less, whichever is lower. Tempering softens martensite to secure toughness, and at the same time, has the effect of finely precipitating Cu in the matrix to increase the strength. However, if the tempering temperature is less than 500 ° C., the softening of martensite is not sufficient and the fine Cu precipitates are insufficient, and the expected performance cannot be obtained. On the other hand, when the tempering temperature is higher than Ac1, a part of the structure is austenitized again and the tempering is not performed and the toughness deteriorates. Also 630 ℃
If it exceeds, even if fine Cu precipitates are deposited, they will not be dissolved again and high strength cannot be obtained. Therefore, the tempering temperature is set to 500 ° C. or more and 630 ° C. or Ac1, whichever is lower.

(14) Tempering time t (unit: hours):
(20 + log t) (273 + T) is 15200 or more, 1
Tempering time of 7800 or less Even if the tempering temperature is the same, if the tempering time is too short, C
Precipitation of u is insufficient and sufficient strength cannot be obtained. Further, if the tempering time is too long, even if a fine Cu precipitate is once deposited, it will be redissolved or agglomerated and coarsened, which will not contribute to the improvement of strength. That is, the tempering time required to achieve an appropriate strength increase is limited to a certain range, but the range depends on the tempering temperature adopted.

FIG. 1 shows the results of examining the relationship between the tempering parameter, which is a variable that combines the tempering temperature and the tempering time, and the 0.2% proof stress and the Charpy impact value.
If between 7800, 0.2% proof stress is 75kg / mm
^It can be seen that the target performance of the present invention is satisfied when the Charpy impact value is ² or more and the Charpy impact value is 10 kg-m or more. Here, the temper parameter is defined by the following equation.

P = (20 + log t) (273 + T) t: tempering time (unit: hours) T: tempering temperature (unit: ° C) Therefore, tempering time is 15 in temper parameter.
It is set to be 200 or more and 17800 or less.

The method for producing the steel of the present invention will be described below. The steel of the present invention has its components adjusted in the component range of the present invention in a converter or an electric furnace, and is cast into a slab by the ordinary ingot casting method or continuous casting method. It is manufactured by hot working it into a seamless steel pipe or steel plate and then heat treating it. The heat treatment method is as described above.

In the components of the steel of the present invention, Al, W, Ti, Zr, Ta, Hf, Ca and REM may be contained as additional components. These elements may be useful for further improving the performance of the steel of the present invention, and the purpose and suitable content of each are as follows.

Al: added for the purpose of deoxidation, the suitable content is 0.01-0.10%. W: It is particularly effective against carbon dioxide corrosion, but if it is contained excessively, the toughness deteriorates, so the maximum content is 4%.

Ti, Zr, Ta, Hf: Effective for improving the corrosion resistance, and the suitable content is 0.2% at maximum. However, if it exceeds 0.2%, coarse precipitates are formed to deteriorate the stress corrosion cracking resistance.

Ca, REM: S which is a harmful impurity in steel
Combined with, it has the effect of significantly reducing the degree of harmfulness and improving stress corrosion cracking resistance. However, since an excessive content has an adverse effect on the stress corrosion cracking resistance, an appropriate content is Ca:
0.01% or less, REM: 0.02% or less.

The unavoidable impurities include P and S, both of which are elements that deteriorate the hot workability and stress corrosion cracking resistance of steel, and the smaller the content, the better. However, if P is 0.04% or less and S is 0.01% or less, the stress corrosion cracking resistance, which is the object of the present invention, can be secured, and in the production of hot-rolled steel plates or seamless steel pipes. No obstacle appears.

[0033]

EXAMPLES Specific examples of the present invention will be described below. The inventors of the present invention produced the invention steels 1 to 16 and the comparative steels a to j as test steels, hot-rolled them into steel plates having a thickness of 12 mm, and then heat-treated them as described below to obtain various test pieces. It was collected. (Example 1) Table 1 shows main components of the steel of the present invention, and Table 2 shows additional components and other components, Ac1 and Ac3 transformation temperatures. This steel was austenitized at 980 ° C. and air-cooled,
Table 3 shows the results of examining the δ-ferrite phase, mechanical properties, and stress corrosion cracking resistance after tempering at 00 ° C for 1 hour. The temper parameter in the tempering of Example 1 was 174.
60. First, the δ-ferrite phase is steel No. 5, 8, 1
No δ-ferrite phase of 10% or less was observed in No. 4 at all. Regarding the Cu deposition state, it was confirmed by electron microscope observation at a magnification of 100,000 that fine Cu deposits having a diameter of about 0.001 to 0.10 micron were uniformly dispersed in the matrix. Regarding the degree of dispersion, 30 to 1 fine Cu precipitates per square micron of base
It was about 00. 0.2% proof stress, Charpy impact value at 0 ° C is all target 75kg / mm ² , 10kg-m
That was all. The stress corrosion cracking resistance was evaluated according to the American Association of Corrosion Engineers Standard TM01-77. That is, 5% saline solution saturated with hydrogen sulfide gas at 1 atm +
60% of 0.2% proof stress on the test piece in 0.5% acetic acid aqueous solution
(For example, in Steel No. 1 in Table 3, 76 × 0.6 = 45.
A stress of 6 kg / mm ² ) was applied and the time until breakage was measured. As shown in the column of "SSC test rupture time" in Table 3, none of the steel numbers 1 to 16 broke within 720 hours.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

Corrosion resistance to carbon dioxide gas corrosion is 200 ° C.
In the autoclave, the carbon dioxide partial pressure was 30 atm and the hydrogen sulfide partial pressure was 0.05 atm, and the sample was immersed in 10% saline for 336 hours to measure the corrosion weight loss. The corrosion loss of any of the steels 1 to 16 is 0.5 g / m ² / hr or less, which is 1.0 g / m ² / h required for the conventional martensitic stainless steel.
It was confirmed that the steel of the present invention had a significantly lower corrosion resistance than r.

(Example 2) Next, steel No. 3 in Tables 1 and 2 was used.
The results of changing the austenitizing temperature (indicated as quenching temperature in Table 4) for the steel No. 3 are shown in part of Table 4. In either case, after air-cooling after austenitization, 60
Tempered at 0 ° C for 1 hour. The tempering parameter for tempering of Example 2 is 17460. When the austenitizing temperature is within the range of the present invention, good performance is exhibited,
At a low temperature of 700 ° C, the target strength has not been reached because of insufficient austenitization. On the other hand, at a high temperature of 1000 ° C., the toughness is low and the stress corrosion cracking resistance is poor.

[0039]

[Table 4]

Example 3 The austenitizing temperature is 95.
The results of changing the tempering temperature while keeping the temperature constant at 0 ° C. are shown in part of Table 4. Also in this case, steel No. 3 was air-cooled after being austenitized, and the tempering time was 1 hour. When the tempering temperature is within the range of the present invention, good performance is exhibited, but at 450 ° C, which is lower than that, martensite remains hard and brittle, resulting in poor toughness and poor stress corrosion cracking resistance. Furthermore, Cu precipitation has not occurred. On the other hand, 650 ℃ higher than Ac1 temperature
Then, it was confirmed that fine precipitates of Cu did not exist because they were redissolved, and as a result, the strength became low.

Example 4 The effect of temper parameters as a tempering condition on the steel of the present invention is examined. Also in this case, steel No. 5 was austenitized at 950 ° C., then cooled and tempered in the range of 450 to 680 ° C. The results are shown in Table 5.
Shown in.

In Table 5, even if the tempering temperature is 500 ° C., the Charpy impact value is lower than the target value when the tempering time is as short as 0.10 hours (temper parameter 14690). On the other hand, it can be seen that when the tempering time is 0.5 hours or more, the temper parameter becomes 15200 or more and sufficient strength toughness and good stress corrosion cracking resistance are exhibited.

When the tempering temperature is 550 ° C., tempering
It can be seen that the target performance is satisfied because the parameters are tempered in the range of 15200 to 17800.
On the other hand, when the tempering temperature is 600 ° C., the tempering time is 1.
Since the tempering parameter of 0 hour has a tempering parameter in the range of 15200 to 17800, the desired performance is obtained, but the tempering parameter of 5 hours has a tempering parameter of more than 17800, and Cu precipitation occurs. It can be seen that the strength was insufficient and the stress corrosion cracking resistance was also insufficient because the material was redissolved or coarsened.

[0044]

[Table 5]

(Comparative Example) Among Comparative Examples, Table 6 and Table 7 show the component composition of the steel and the test results for the steels which were out of the composition range of the present invention as the co-test materials. The austenitizing temperature and the tempering treatment are the same as in Example 1. Since some of the components of the steels in Table 6 are out of the range of the present invention, either the strength or toughness of the test result is outside the target, and as a result, the stress corrosion cracking resistance does not reach the target. Among them, steel numbers a and b have Cu contents of 0.
Since it was less than 3%, Cu precipitates could not be formed, resulting in a strength of less than 75 kg / mm ² . Steel No. c had a Cu content of more than 4.0%, which caused cracks in the steel sheet during hot rolling, significantly impairing commercial value, and also had poor SSC performance.
Further, steel No. d has a low Ni content, steel No. g has a high Cr content and Mo content, and steel No. i has a high Mo content.
A ferrite phase appears and significantly deteriorates toughness. Steel No. e was not suitable for the purpose of the present invention because Ni was more expensive than 9% and was extremely expensive, and the SSC performance was poor.
Steel No. f has a low Cr content and Steel No. h has a low Mo content, and therefore has poor corrosion resistance to carbon dioxide corrosion and SSC performance. Steel No. j had a high C and therefore had poor SSC performance.

[0046]

[Table 6]

[0047]

[Table 7]

[0048]

According to the present invention, it is possible to provide a high-strength martensitic stainless steel which is excellent in stress corrosion cracking resistance in an environment containing a large amount of hydrogen sulfide as well as in corrosion resistance against carbon dioxide gas corrosion. It was

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between 0.2% proof stress and Charpy impact value and temper parameters.

Claims (4)

[Claims] 1. C .: 0.06% or less, Cr: 12-16%, Si: 1.0% or less, Mn:% by weight as a main component.
2.0% or less, Ni: 0.5-8.0%, Mo: 0.1
-2.5%, Cu: 0.3-4.0%, N: 0.05%
A high-strength martensitic stainless steel excellent in stress corrosion cracking resistance, characterized in that the area ratio of the δ-ferrite phase is 10% or less and fine Cu precipitates are dispersed in the matrix. steel. 2. C .: 0.06% or less, Cr: 12-16%, Si: 1.0% or less, Mn:% by weight as a main component.
2.0% or less, Ni: 0.5-8.0%, Mo: 0.1
-2.5%, Cu: 0.3-4.0%, N: 0.05%
Including: V: 0.01-0.1% and Nb:
The stress resistance is characterized by including at least one of 0.01-0.1%, the area ratio of the δ-ferrite phase is 10% or less, and fine precipitates of Cu are dispersed in the matrix. High-strength martensitic stainless steel with excellent corrosion cracking resistance. 3. C .: 0.06% or less, Cr: 12-16%, Si: 1.0% or less, Mn:% by weight as a main component.
2.0% or less, Ni: 0.5-8.0%, Mo: 0.1
-2.5%, Cu: 0.3-4.0%, N: 0.05%
A martensitic stainless steel having a composition including
After austenitizing at a temperature of c3 or more and 980 ° C. or less, it is cooled, and then a tempering temperature T (unit: ° C.) is 500 ° C. or more and 630 ° C. or less of Ac1, whichever is lower, and a tempering time t (unit: hour). ) Is (20 + log t)
Manufacture of high-strength martensitic stainless steel excellent in stress corrosion cracking resistance, which is characterized by tempering under the condition that the value of (273 + T) is 15200 or more and 17800 or less to disperse fine Cu precipitates in the matrix. Method. 4. C .: 0.06% or less, Cr: 12-16%, Si: 1.0% or less, Mn:% by weight as a main component.
2.0% or less, Ni: 0.5-8.0%, Mo: 0.1
-2.5%, Cu: 0.3-4.0%, N: 0.05%
V: 0.0% by weight as an additional component
Martensitic stainless steel containing 1-0.1% and one or more of Nb: 0.01-0.1% is austenitized at a temperature of Ac3 or more and 980 ° C or less, then cooled, and then tempered. As T (unit: ° C), 500 ° C or higher 630
℃ or Ac1 whichever is lower, the tempering time t (unit: hour) is (20 + log t) (273+
A method for producing a high-strength martensitic stainless steel excellent in stress corrosion cracking resistance, characterized by tempering under the condition of T) of 15200 or more and 17800 or less to disperse Cu fine precipitates in a matrix. .