JPH0841599A - Martensitic stainless steel excellent in corrosion resistance in weld zone - Google Patents

Abstract

PURPOSE:To prevent the increase in hardness in a weld zone and to provide corrosion resistance to hydrogen sulfide, carbon dioxide gas, and chlorine ion by incorporating specific amounts of C, Si, Mn, P, S, Cr, Mo, Ni, Al, N, and Fe. CONSTITUTION:This steel is a martensitic stainless steel which has a composition consisting of <=0.009% C, <=1% Si, <=1% Mn, <=0.04% P, <=0.005% S, 9-15% Cr, 1.5-7% Mo, 4-8% Ni, 0.001-0.1% Al, <=0.1% N, and the balance essentially Fe and satisfying Cr+Mo>=11 and 30C+Ni-1.1Cr-1.1Mo>=-10.5. Because C content is extremely reduced, this steel is free from increases in hardness even if subjected to the influence of high heat at the time of welding, and adequate strength, toughness, and corrosion resistance can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a martensitic stainless steel having excellent corrosion resistance of a welded portion, and is a martensitic stainless steel mainly used for transporting crude oil or natural gas collected from an oil field or gas field. The present invention relates to stainless steel, and more particularly to martensitic stainless steel suitable for use as a line pipe for crude oil or natural gas containing corrosive impurities such as carbon dioxide, hydrogen sulfide and chlorine ions.

[0002]

2. Description of the Related Art In recent years, oil and gas wells have been developed in the deep oil field, which has been neglected until now, due to the soaring crude oil price and the depletion of oil resources expected in the near future. The sour gas field, which had been abandoned for development, is being actively conducted on a global scale. Oil wells and gas wells like this
Generally, the depth is extremely deep, and the amount of carbon dioxide, hydrogen sulfide, crude oil containing chlorine ions or natural gas is increasing,
Along with this, also for line pipes for transporting crude oil or natural gas, materials having high strength and having both corrosion resistance and stress corrosion cracking resistance are required.

Conventionally, carbon steel or low alloy steel was generally used for the conventional line pipe, but as the environment in which it is used becomes severe, steel having an increased alloy element has come to be used. Is coming. For example, in an environment containing a large amount of carbon dioxide gas, since the addition of Cr significantly improves the corrosion resistance, SUS 420 martensitic stainless steel containing 13% of Cr, which is widely used as a steel pipe for oil wells, is used for the line pipe. There are some examples, but SUS
Since 420 martensitic stainless steel has a significantly hardened welded portion due to circumferential welding, heat treatment after welding is required, resulting in a significant increase in cost and poor resistance to sulfide stress corrosion cracking, which limits its use. It is the actual situation. In such an environment containing carbon dioxide gas and hydrogen sulfide at the same time, at present, it is unavoidable to use duplex stainless steel or super duplex stainless steel with a higher alloying element, but the addition of alloying elements will increase. The cost rise is remarkable.

As a material having high strength and having both corrosion resistance and stress corrosion cracking resistance, C: 0.02% or less, Si: 0.50% or less, Mn: 0.50 to 1.50.
%, S: 0.005% or less, Cr: 12 to 15%, N
i: 3.5 to 6%, Mo: 0.5 to 3%, the balance being Fe and inevitable impurities, martensitic stainless steel (Japanese Patent Laid-Open No. 60-174859),
C: 0.05% or less, Si: 1.0% or less, Mn: 0.
1 to 3.0%, P: 0.04% or less, S: 0.005%
Below, Cr: 15-19%, Ni: 3.5-8.0%,
Al: 0.001-0.1%, N: 0.1% or less, M
o: 0.1-4.0%, the balance being Fe and unavoidable impurities in martensitic stainless steel (Japanese Patent Laid-Open No. 2-
243739), C: 0.05% or less, Si:
1.0% or less, Mn: 0.5 to 3.0%, P: 0.04
% Or less, S: 0.005% or less, Cr: 9.0 to 15
%, Mo: 0.1-7.0%, Ni: 2-8%, Al:
0.001-0.1%, N: 0.1% or less, and T
i: 0.5% or less, Nb: 0.5% or less, V: 0.5%
And one or more of Zr: 0.5% or less, the balance being Fe and inevitable impurities, and 30Cr (%) + 36Mo (%) + 14Si (%)-2.
8Ni (%) ≦ 455 (%) 21Cr (%) + 25Mo (%) + 17Si (%) + 3
Martensitic stainless steel (Japanese Patent Laid-Open No. 2-243740) having a steel composition of 5Ni (%) ≦ 731 (%), C: 0.05% or less, Si: 1.0% or less, Mn: 0. Less than 5%, P:
0.04% or less, S: 0.002% or less, Cr: 8 to 1
5%, Mo: 1.5 to 7%, Ni: 2 to 8%, Al:
0.001-0.1%, N: 0.1% or less, and Cr
+ Mo ≧ 11.0 (%), 30Cr (%) + 36Mo (%) + 14Si (%)-2
8Ni (%) ≦ 455 (%) 21Cr (%) + 25Mo (%) + 17Si (%) + 3
5Ni (%) ≦ 731 (%) is satisfied at the same time, and the balance is a martensitic stainless steel having a steel composition consisting of Fe and unavoidable impurities (JP-A-3-120337), C: more than 0.05% 0 ．
2% or less, Si: 2% or less, Mn: 2% or less, P: 0.
04% or less, S: 0.01% or less, Cr: 8% or more 17
% Or less, Ni: more than 2.5 and 8% or less, Al: 0.001%
0.1% or less, N: 0.1% or less, but 30Cr (%) + 14Si (%)-28Ni (%)-7
90 [C (%) + N (%)] ≦ 455 (%) 21Cr (%) + 17Si (%) + 35Ni (%) + 9
10 [C (%) + N (%)] ≦ 731 (%) Martensitic stainless steel having a steel composition consisting of balance Fe and inevitable impurities (Japanese Patent Laid-Open No. 4-12024).
Many proposals have been made.

[0005]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
The martensitic stainless steel disclosed in Japanese Patent No. 4859 certainly has slightly improved corrosion resistance by the addition of Cr and Mo under an environment containing only carbon dioxide gas, but a large amount of corrosive impurities such as carbon dioxide gas, hydrogen sulfide and chlorine ions. In the environment containing extremely high corrosiveness, the alloy does not have sufficient corrosiveness, and particularly when it is intended to be used for a line pipe, the hardness of the welded portion is significantly increased, and sufficient corrosion resistance cannot be obtained. In addition, JP-A-2-243739,
JP-A-2-243740, JP-A-3-12033
The martensitic stainless steels disclosed in Japanese Patent Laid-Open No. 7-120249 and Japanese Patent Laid-Open No. 4-120249 have improved corrosiveness in an extremely corrosive environment containing a large amount of corrosive impurities such as carbon dioxide, hydrogen sulfide and chlorine ions. As a heat treatment, it has a good corrosion resistance even if it is quenched at about 800 to 1000 ° C. However, these martensitic stainless steels have no problem if they are used for oil country tubular goods, but in order to use them for line pipes, circumferential welding is indispensable, and the vicinity of the welded portion is 1
Since the temperature is high from 000 to 1400 ° C., the hardness is remarkably increased, stress corrosion cracking occurs, and there is a problem that the line pipe cannot be used as it is.

An object of the present invention is to suppress an increase in hardness of a welded part in a member used for performing welding work such as a line pipe, and to have excellent strength, toughness and hydrogen sulfide, carbon dioxide gas, and chlorine ion. It is intended to provide a martensitic stainless steel having stress corrosion cracking resistance under a corrosive environment.

[0007]

SUMMARY OF THE INVENTION In order to achieve the above-mentioned object, the inventors of the present invention have investigated the effects of alloying elements on corrosion resistance centering on stress corrosion cracking resistance in an environment containing carbon dioxide gas, hydrogen sulfide and chlorine ions. As a result of various experiments and investigations to investigate the effect, the steel in which an appropriate amount of Ni is added has the corrosion resistance under the above environment, which can be arranged by (Cr + Mo)% amount and is not a carbide or a nitride. In order to increase the amounts of Cr and Mo, it is only necessary to specify the upper limits of C and N, and in order to prevent an increase in the hardness of the weld, it is absolutely essential to significantly reduce the absolute amount of C. In order to obtain a more industrially stable strength, Ti that matches the C and N contents,
It was found that the addition of Nb and Zr is effective, and that the addition of Ti, Nb, and Zr is effective for suppressing the hardness increase in the weld heat affected zone. As a result of further research, C
By significantly reducing the amount from the conventional level and then adding a trace amount of carbonitride stable elements such as Ti, Nb, and Zr, a high temperature (1000 to 1
The strength can be stabilized even by quenching from 400 ° C), and appropriate strength, toughness, and corrosion resistance can be obtained even if it is quenched without the conventional martensite quenching and tempering processes. Researched and arrived at this invention.

That is, according to the present invention, C: 0.009% or less, Si: 1.0% or less, Mn: 1.0% or less, P:
0.04% or less, S: 0.005% or less, Cr: 9.0
~ 15.0%, Mo: 1.5-7.0%, Ni: 4.0
~ 8.0%, Al: 0.001-0.1%, N: 0.1
% Or less, and Cr (%) + Mo (%) ≧ 11.0 (%) 30C (%) + Ni (%)-1.1 Cr (%)-1.1
A martensitic stainless steel excellent in corrosion resistance of a welded portion, characterized by simultaneously satisfying Mo (%) ≧ -10.5 (%) and having a steel composition in which the balance is Fe and inevitable impurities.

Further, according to the present invention, C: 0.009% or less, Si: 1.0% or less, Mn: 1.0% or less, P:
0.04% or less, S: 0.005% or less, Cr: 9.0
~ 15.0%, Mo: 1.5-7.0%, Ni: 4.0
~ 8.0%, Al: 0.001-0.1%, N: 0.1
% Or less, Ti: 0.2% or less, Nb: 0.2%
Hereinafter, one or more of Zr: 0.2% or less is contained, and Cr (%) + Mo (%) ≧ 11.0 (%) 30C (%) + Ni (%)-1. 1Cr (%)-1.1
Mo (%) ≧ -10.5 (%) C (%)-12 {Ti (%) / 48 + Zr (%) / 91
+ Nb (%) / 93−N (%) / 14} ≦ 0 (%) at the same time, and the balance has a steel composition consisting of Fe and inevitable impurities. A martensitic stainless steel for line pipes. is there.

[0010]

[Function] The martensitic stainless steel of the present invention is
It is excellent in strength, toughness, and corrosion resistance without being subjected to the conventional quenching and tempering treatments of martensitic stainless steel, with little variation in strength even when it is rolled or quenched. Further, since the martensitic stainless steel of the present invention has a small strength variation during quenching, it is easy to control the strength after tempering. Further, the martensitic stainless steel of the present invention, by having an extremely low C, does not increase in hardness even when subjected to a high heat effect such as welding, and can obtain appropriate strength, toughness, and corrosion resistance.

Further, the martensitic stainless steel of the present invention does not undergo the quenching and tempering treatments which are the conventional wisdom of conventional martensitic stainless steels, and has a small strength variation even if it is rolled or quenched, and has excellent strength, toughness and corrosion resistance. Are better. Further, since the martensitic stainless steel of the present invention has a small strength variation during quenching, it is easy to control the strength after tempering. Furthermore, the martensitic stainless steel of the present invention does not increase in hardness even when it is subjected to a high heat effect such as welding when it is made to have an extremely low C, and further, even in a low temperature heat affected zone by the addition of Ti, Nb and Zr. The increase in hardness is suppressed, and more appropriate strength, toughness, and corrosion resistance can be obtained.

Next, the reason for limiting the chemical composition of steel in the present invention will be described in detail. C is an important element of the present invention, and as shown in FIG. 1, when it exceeds 0.009%, the heat-affected zone becomes extremely hard during welding work, which is essential when used as a line pipe, and the toughness and corrosion resistance are improved. In order to deteriorate it, it was set to 0.009% or less. Si is necessary as a deoxidizing agent in the ordinary steelmaking process, but if it exceeds 1.0%, the toughness decreases, so Si was made 1.0% or less. Mn is an element that increases the strength, but since it has the effect of lowering the toughness, it is preferable that its content be small in order to improve the toughness. To maintain both strength and toughness at appropriate levels, Mn is 1.0% or less. And The lower the P, the higher the toughness, but
If it exceeds 0.04%, the toughness is remarkably reduced, so that
It was set to 04% or less. From the viewpoint of hot workability, S is preferably as small as possible, but in consideration of the desulfurization cost, it was set to 0.005% or less.

In order to form a corrosion resistant film, Cr is 9.
It is necessary to add 0% or more, but if it exceeds 15%, Mo
Ferrite is likely to be generated due to the synergistic effect with and the strength is reduced, so the content is set to 9.0 to 15%. Mo has a remarkable effect in improving the corrosion resistance to hydrogen sulfide, but 1.
If it is less than 5%, a sufficient effect cannot be obtained, and if it exceeds 7%, C
Ferrite is easily generated due to the synergistic effect with r,
Since the strength decreases, it was set to 1.5 to 7.0%. Especially M
o markedly improves the corrosion resistance of the weld heat affected zone, and is 2.0
% Or more is preferable. Ni is the required strength, corrosion resistance,
It is added in order to obtain hot workability, but if it is less than 4.0%, its effect is not sufficient due to the balance with Cr and Mo, but on the contrary it decreases, and if it exceeds 8.0%, its effect is saturated. Not because it causes cost increase, 4.0-8.0
%. Al is necessary as a deoxidizer in the ordinary steelmaking process, but if it is less than 0.001%, its effect cannot be obtained,
If it exceeds 0.1%, the amount of inclusions in the steel increases and the corrosion resistance deteriorates, so the content was made 0.001 to 0.1%. N is 0.
If it exceeds 1%, the strength is excessively increased to increase the susceptibility to sulfide stress corrosion cracking, and it is preferable that it is also small from the viewpoint of corrosion resistance, and it is preferably 0.02% or less.

Ti, Nb, and Zr are important elements of the second invention of the present application, and they form a compound with C and N during hot working at high temperature or during solution treatment, so that the free C and N contents in steel can be controlled. It has a controlling effect and can suppress the increase in hardness even if it is affected by heat during welding. However, if the content of C and N exceeds 0.2%, not only the effect will be saturated, but also
On the contrary, since it hardens by forming compounds with elements such as Ni,
It was set to 0.2% or less. Cr (%) + Mo (%) is 11.
If it is less than 0%, the stress corrosion cracking resistance is not sufficient.
0% or more is required, and preferably 14.0% or more, the more the stress corrosion cracking resistance is improved. However, if too much is added, ferrite is likely to be generated, which not only lowers the strength but also increases the cost. Therefore, it does not exceed 22.0%.

Further, the steel composition according to the present invention must satisfy the following equations (1) and (2). 30C (%) + Ni (%)-1.1Cr (%)-1.1Mo (%) ≧ -1 0.5 (%) ……………… Formula (1) C (%)-12 {Ti ( %) / 48 + Zr (%) / 91 + Nb (%) / 9 3-N (%) / 14} ≦ 0 (%) ... Formula (2) In order to easily hot form the subject steel of the present invention, It is desirable to be an austenite single phase at a high temperature, and it becomes an austenite single phase at a normal heating temperature of 900 to 1250 ° C, and it is necessary to transform into a martensitic steel when cooled. In order to perform hot forming in an austenite single phase state without forming δ ferrite at high temperature, the above formula (1)
Need to be satisfied. Further, the target steel type of the present invention is
Welding work is required for line pipes, so stable hardness of the welded part is required to ensure excellent strength, toughness, and corrosion resistance. It is achieved by setting the heat-affected zone of the welded part to extremely low C. In addition, Ti, Nb, and Z are added as elements that stabilize free C during hot working and solution treatment.
In order to achieve stable hardness by adding r, it is essential to satisfy the above formula (2).

[0016]

EXAMPLES Steel Nos. Shown in Table 1 1 to 18 were melted, and each was hot-rolled to obtain a plate material having a plate thickness of 8 mm. Then, after heat-treating at a predetermined temperature, each plate material is subjected to V-groove processing, full-layer TIG welding is performed, and the yield strength, microhardness, toughness, and sulfide stress corrosion cracking resistance of the welded portion are tested. It was In the tensile test, a tensile test piece having a thickness of 4 mm and a parallel portion length of 34 mm was cut out from the plate material, and the yield strength was measured according to the JIS Z2241 metal material tensile test method.
For the micro hardness test, a micro hardness tester of JIS B7734 was used, and as shown in FIG. 2, the center portion of the test piece 1 in the wall thickness direction was tested under a test load of 9.Z according to JIS Z2244 Vickers hardness test method. The Vickers hardness was measured at a pitch of 0.5 mm at 807 N, and the maximum value was evaluated. In addition, 2 shows a welded part and 3 shows a measurement point. Toughness has a size of 1
A Charpy test piece of 0 mm × 5 mm × 55 mm having a 2 mm V notch was cut out and evaluated by a Charpy impact value measured according to the JIS Z2242 metal material impact test method using a Charpy impact tester specified in JIS B7722. Sulfide stress corrosion cracking resistance, as shown in Figure 3,
Two 4-point bending test pieces 11 having a thickness T = 2 mm, a width W = 10 mm, and a length L = 75 mm were prepared, and then the test piece 11 was placed on the upper surface of the bending jig 12 as shown in FIG. Two fulcrums (spacing 60 mm) and two lower fulcrums of the bending mechanism 13 (spacing 1
0 mm), σ = Ety {2 / 3L ₁ ² + L ₁ L ₂ +
The stress represented by ¼L ₂ ² } ⁻¹ is 1σy (σy:
The bending stress was applied so as to obtain a 0.2% proof stress). In addition, E in a formula shows Young's modulus. The bent shape of the test piece 11 at this time was as shown in FIG. The test environment is 5% NaCl + 0.01 atmH ₂ S
The specimen was soaked for 366 hours at +30 atmCO ₂ and 25 ° C., taken out, and examined for the presence or absence of cracks by observing the appearance with the naked eye and observing with an optical microscope. The results of these tests are summarized in Table 2. In Table 2, "○○" in the column of sulfide stress corrosion cracking indicates that no two cracks occurred, and "XX" indicates that both cracked.

[0017]

[Table 1]

[0018]

[Table 2]

As shown in Tables 1 and 2, steel Nos. Outside the composition range of the present invention were used. In each of the comparative steels 13 to 18, stress corrosion cracking occurs even in an environment of hydrogen sulfide of 0.01 atm. Steel No. of comparative steel 13,
In Nos. 14 and 15, the Mo content was not added or was insufficient, and stress corrosion cracking occurred. In addition, steel N of comparative steel
o. Although the levels of 17 and 18 were conventionally called low C, considering the increase in hardness due to the effect of welding heat, C was still C.
The content is high and stress corrosion cracking has occurred. Steel No. of comparative steel In No. 16, since the addition amount of Ti, Nb, and Zr was too large, a compound with Ni or the like was formed and hardened, and stress corrosion cracking occurred. On the other hand, the steel No. 1
In all of Nos. 12 to 12, an increase in hardness of the welded portion is suppressed, stress corrosion cracking resistance is improved, stress corrosion cracking does not occur, and strength and toughness are excellent.

[0020]

As described above, the martensitic stainless steel of the present invention can prevent the hardness increase due to the heat effect of the welded portion, which is indispensable when it is used as a line pipe, and can prevent hydrogen sulfide, carbon dioxide gas, chlorine. It has sufficient corrosion resistance even in a harsh environment containing ions, and
It has excellent strength and toughness, and can be sufficiently used as a martensitic stainless steel member for welding.

[Brief description of drawings]

FIG. 1 is a graph showing the maximum hardness of welds and the occurrence of stress corrosion cracking when the C content in martensitic stainless steel is changed.

FIG. 2 is an explanatory view showing hardness measurement positions of a welded portion used in an example.

FIG. 3 is a perspective view showing the shape of a 4-point bending test piece used in Examples.

4A and 4B show a stress application state of a test piece using a bending jig. FIG. 4A is a state diagram in which the test piece is set in the bending jig, and FIG. 4B is a test piece in a stress application state. FIG.

[Explanation of symbols]

 1, 11 Test piece 2 Welded portion 3 Measurement point 12 Bending jig 13 Bending mechanism

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazuhiro Ogawa 4-53-3 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd.

Claims (2)

[Claims] 1. C: 0.009% or less, Si: 1.0%
Hereinafter, Mn: 1.0% or less, P: 0.04% or less, S:
0.005% or less, Cr: 9.0 to 15.0%, Mo:
1.5-7.0%, Ni: 4.0-8.0%, Al:
0.001 to 0.1%, N: 0.1% or less, and Cr (%) + Mo (%) ≧ 11.0 (%) 30C (%) + Ni (%) − 1.1Cr ( %)-1.1
A martensitic stainless steel having excellent corrosion resistance of a weld, which has a steel composition in which Mo (%) ≧ -10.5 (%) is satisfied at the same time and the balance is Fe and inevitable impurities. 2. C: 0.009% or less, Si: 1.0%
Hereinafter, Mn: 1.0% or less, P: 0.04% or less, S:
0.005% or less, Cr: 9.0 to 15.0%, Mo:
1.5-7.0%, Ni: 4.0-8.0%, Al:
0.001-0.1%, N: 0.1% or less, and T
i: 0.2% or less, Nb: 0.2% or less, Zr: 0.2
% Or less, and contains Cr (%) + Mo (%) ≧ 11.0 (%) 30C (%) + Ni (%)-1.1Cr (%)-1. 1
Mo (%) ≧ -10.5 (%) C (%)-12 {Ti (%) / 48 + Zr (%) / 91
+ Nb (%) / 93-N (%) / 14} ≦ 0 (%) at the same time, and the balance has a steel composition consisting of Fe and unavoidable impurities. Series stainless steel.