JP2002030392A - HIGH Cr MARTENSTIC STAINLESS STEEL EXCELLENT IN CORROSION RESISTANCE, AND ITS MANUFACTURING METHOD - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide high Cr martensitic stainless steel excellent in SCC(sulfide stress corrosion cracking) resistance, CO2 corrosion resistance and uniform corrosion resistance and most suitably used as steel for oil well pipe and line pipe for the production/transportation of gas/petroleum or steel for plant, and its manufacturing method. SOLUTION: The high Cr martensitic stainless steel excellent in corrosion resistance has a composition containing >=0.05% C, <=1% Si, <=3% Mn, <=0.03% P, <=0.01% S, 9.0-15.0% Cr, 3.0-8.0% Ni, 0.5-3% Mo, 0.01-0.3% V, 0.01-0.3% Ti, <=0.2% Al and <=0.05% N and satisfying the prescribed relational expression among the components and also has a metallic structure composed of multiphase structure consisting of >=5% retained austenite and martensite or tempered martensite.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a stainless steel having excellent corrosion resistance, which is most suitable as an oil well pipe or line pipe for producing and transporting gas or petroleum used mainly in the field of energy, or steel for a plant. The present invention relates to a manufacturing method and a corrosion-resistant steel pipe product.

[0002]

2. Description of the Related Art In recent years, gas wells or oil wells for producing gas or oil containing a large amount of carbon dioxide (CO ₂ ) have been developed, and CO _{2 or} CO ₂ has been developed to improve the recovery rate of such gas or oil.
Injection has been widely performed. As a material for a steel pipe used in an environment in which such a material is highly corrosive, a 13% Cr steel excellent in CO ₂ corrosion resistance represented by AISI 420 steel is used. However, since this 13% Cr steel has high susceptibility to hydrogen sulfide stress cracking (SSC) generated in an environment containing hydrogen sulfide, development of a material having excellent overall corrosion resistance including improvement in SSC resistance is desired. In addition, since the line pipe, which is the pipe after coming out to the ground, is joined by welding, materials for oil country tubular goods in which weldability is not taken into account cannot be generally used. Therefore, line pipes made of more expensive duplex stainless steel are used. However, from the viewpoint of economy, it is desired that a material of about 13% Cr can be used for the line pipe.

The important factors required for putting 13% Cr steel into practical use as a line pipe are: 1) good weldability that can be welded without preheating; 2) CO resistance at the operating temperature of the line pipe. _{2 having} corrosion resistance and sulfide stress cracking (SSC) resistance to a small amount of H ₂ S;
3) The material strength can be suppressed lower than that of the duplex stainless steel often used as a welding material. To address these technical problems, for example, in JP-A-9-256115 Publication, reduce C for weldability improvement, Cr for resistance CO ₂ corrosion improve, Cu,
Ni and Mo are contained in appropriate amounts to improve SCC resistance, and the balance between ferrite and austenite forming elements is adjusted to reduce strength appropriately, and at least 10% of retained austenite phase is contained in the structure. A high Cr martensitic stainless steel to be produced and a method for producing the same are disclosed.

Further, in Japanese Patent Application Laid-Open No. 9-268349,
20% austenite phase in tempered martensite phase
A high Cr martensitic stainless steel with particularly improved SSC resistance by being mixed at the above fractions and a method for producing the same are disclosed. However, in recent years, H ₂ S
The use of materials in an environment with a high concentration of sulfide stress cracking (SSC) is increasing, and a material having sulfide stress cracking (SSC) resistance higher than that of conventional materials is required. Have been.

[0005]

SUMMARY OF THE INVENTION In view of the above-mentioned conventional problems, the present invention provides an oil well pipe, a line pipe, or a plant for producing and transporting gas and petroleum mainly used in the field of energy. and an object optimal SCC resistance as use steel, resistance to CO ₂ corrosion resistance, to provide a high-Cr martensitic stainless steel and a manufacturing method thereof and corrosion-resistant steel excellent in general corrosion resistance.

[0006]

Means for Solving the Problems The present inventors have conducted various experiments and made intensive studies, and as a result, have obtained the following findings. That is, according to the results of the study by the present inventors, it has been found that at a temperature of 60 to 120 ° C. which is a typical use environment of line pipes,
₍₂ ) It is effective to contain 9% or more of Cr in steel in order to improve the corrosiveness, and it is necessary to add a certain amount or more of Mo in order to improve SSC resistance. Further, since the SSC of the steel type of the present invention in the corrosive environment is a hydrogen embrittlement type at the pit initiation point, the residual austenite phase of 5% or more and the carbonitrides of Ti and V cause the pitting corrosion to be repaired. It has been found that trapping hydrogen is effective for further improving SSC resistance.

Also, the hot workability, welding crack resistance, and
In consideration of the toughness of the heat affected zone of welding, etc., the steel must be a martensite phase that does not contain a ferrite phase (completely austenite phase at high temperatures), and for that purpose, an austenite-forming element such as Ni must be added. On the other hand, such a steel has a low AC ₁ point and extremely high tempering softening resistance. Therefore, in general, it is effective to utilize the retained austenite phase in order to increase the strength of the base metal and to refine the weld metal so that the strength of the weld metal does not become undermatched.

The present invention has been made based on the above findings, and the gist thereof is as follows. (1) In mass%, C: 0.05% or less, Si: 1% or less, Mn: 3% or less, P: 0.03% or less, S: 0.0
1% or less, Cr: 9.0 to 15.0%, Ni: 3.0 to
8.0%, Mo: 0.5-3%, V: 0.01-0.3
%, Ti: 0.01 to 0.3%, Al: 0.2% or less,
N: 0.05% or less, and the following formula according to the C content between the components, when C ≧ 0.02%, IPS = 40C + 34N + 0.3Cu + Ni-1.1C
r-1.8Mo ≧ -10.5 Or, when C> 0.02%, IPS = 0.8 + 34N + 0.3Cu + Ni-1.1C
r-1.8Mo ≧ -10.5, and the balance is composed of iron and unavoidable impurities, and the metal structure is composed of 5% or more of retained austenite and martensite or tempered martensite. High Cr martensitic stainless steel with excellent corrosion resistance characterized by having a dual phase structure. (2) The retained austenite fraction in the metal structure is 5%
The high Cr martensitic stainless steel having excellent corrosion resistance according to the above (1), which is at least 15%. (3) Further, in mass%, Cu: 2% or less, Zr: 0.
2% or less, Ca: 0.01% or less, REM: 0.02%
High C having excellent corrosion resistance according to the above (1) or (2), characterized by containing one or more of the following:
r Martensitic stainless steel. (4) Further, in mass%, Nb: 0.1% or less, Ta:
0.15% or less, Mg: 0.006% or less, B: 0.0
The high Cr martensitic stainless steel according to any one of the above (1) to (3), which contains 005 to 0.03% of one or more kinds. (5) High C according to any one of the above (1) to (4)
A corrosion-resistant steel pipe obtained by subjecting r-martensite stainless steel to a pipe-forming process. (6) A steel slab having the chemical composition described in any of (1), (3), and (4) above is immediately or hot-worked or Ac
Reheat to 3 transformation points or more and cool, then Ac ₁ point to A
and cooled to a temperature in the range of c ₁ point + 0.99 ° C., further followed Ac excellent high corrosion resistance, characterized in _{that one} point -10 ° C. to Ac is heated to a temperature in the range of ₁ point + 60 ℃ cooled Method for producing Cr martensitic stainless steel.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The effects of the steel components of the steel of the present invention and the reasons for limiting their contents will be described below. Hereinafter, all% are mass%. C: C is an element effective for increasing the strength of steel and is a strong austenite forming element, and has an effect of suppressing the formation of a δ ferrite phase. However, too much Cr
To form carbides and impair corrosion resistance. In the case of welding, it is necessary to reduce C in order to suppress the hardness of the heat affected zone and improve the SSC resistance, and the C content is limited to 0.05% or less. In particular, when it is used for a line pipe, it is preferable to limit it to 0.02% or less. Although the lower limit is not necessarily clear, 0.001% is also effective for suppressing the formation of the δ ferrite phase.

Si: Si is added for deoxidation. However, if the added amount exceeds 1.0%, the low-temperature toughness deteriorates, so the upper limit is made 1.0%. Mn: Mn is an element that lowers the grain boundary strength and impairs crack resistance in a corrosive environment. However, MnS is formed to promote the detoxification of S, and it is also an element effective for making the austenite single phase, so it is added. However, if it exceeds 3.0%, the toughness deteriorates remarkably, so the upper limit of Mn is set to 3.0%.

P: P is present in steel as an impurity element,
The upper limit is set to 0.03% because segregation at the grain boundaries weakens the grain boundary strength and lowers SSC resistance and low-temperature toughness. S: S is also present as an impurity element in steel, embrittles the steel and has an adverse effect on corrosion resistance, so the upper limit is 0.01%.
And Cr: Cr is the most important element that reduces carbon dioxide gas corrosion. If less than 9% is added, sufficient corrosion resistance cannot be obtained,
On the other hand, if it exceeds 15%, a δ ferrite phase is likely to be formed, so the content was set to 9.0 to 15.0%.

Ni: Since Ni is a strong austenite-forming element, it is useful for realizing a martensite structure, forming a retained austenite phase, and improving hot workability. Further, when welding is performed, there is an effect of increasing the toughness of the heat affected zone. If the content is less than 3%, these effects are not sufficient, and if it exceeds 8%, the AC1 transformation point becomes too low,
Since the refining becomes difficult, the range is set to 3 to 8%. In particular, when high SSC resistance is required, the content of Mo is increased, so that 5% or more is desirable from the viewpoint of phase balance.

Mo: Mo has the effect of remarkably improving the SSC resistance, but if it is less than 0.5%, the effect is not sufficient, and if it exceeds 3%, the δ ferrite phase is easily formed. 0.5-3%. When the partial pressure of hydrogen sulfide is high, it is desirable to add 2% or more. Ti, V: These elements are added for improving SSC resistance. It forms carbonitrides in steel and acts as hydrogen trapping sites, similar to retained austenite, to suppress hydrogen embrittlement. By adding Ti and V, the minimum amount of retained austenite required for obtaining good SSC resistance can be kept low at 5%. If the content is less than 0.01%, the effect is not sufficient, and if it exceeds 0.3%, the toughness is impaired. Therefore, Ti: 0.01 to 0.3%, V: 0.01 to 0.
3%.

Al: Al is added for deoxidation. However, if the content exceeds 0.20%, the cleanliness of the steel is lowered and the low-temperature toughness is deteriorated. Therefore, the content of Al is set to 0.20% or less. N: N is an element that remains in the steel as an unremovable element. In the present invention, a carbonitride is formed in the steel together with Ti and V, and serves as a hydrogen trap site to improve SSC resistance. However, if it exceeds 0.05%, the low-temperature toughness significantly deteriorates, so the upper limit was made 0.05%.

The present invention contains the above components as main components, and has a steel structure of 5% as described later.
By using the above-described dual-phase structure composed of the retained austenite phase, the martensite phase, and the tempered martensite phase, good CO ₂ corrosion resistance and SSC resistance, and further, overall corrosion resistance can be obtained. However, in the above components defined in the present invention, Cr, M
When the content of the ferrite-forming element such as o increases, a ferrite phase is formed in the heat affected zone by welding to lower the low-temperature toughness, to cause cold cracking, and to reduce the hot workability. Therefore, it is necessary to consider an appropriate combination of these elements. From conventional knowledge, C, N, and Ni suppress the formation of a ferrite phase, and Cr and Mo promote. Therefore, steels in which the concentration of each element was variously changed were melted, and the contribution ratio was experimentally determined. As a result, when C ≧ 0.02%, IPS = 40C + 34N + 0.3Cu + Ni-1.1C
r-1.8Mo ≧ -10.5 Alternatively, when C <0.02%, IPS = 0.8 + 34N + 0.3Cu + Ni-1.1C
It was found that if the expression of r-1.8Mo ≧ -10.5 was satisfied, no ferrite phase was formed in the metal structure, and a single phase of martensite was obtained in a state of cooling. That is, in the present invention, C, N,
It is necessary that Ni, Cr, and Mo further satisfy this relationship. The IPS defined in the present invention means δ ferrite forming ability.

Based on the above composition and metal structure, one or more of the following elements may be used in order to further improve the CO ₂ corrosion resistance, SSC resistance, or weldability, toughness, and hot workability. Can be selectively added. Cu: Cu coexists with Ni to improve CO ₂ corrosion resistance,
It can be added because it also enhances pitting resistance. The higher the amount of addition, the greater the effect of improving corrosion resistance. However, if added in excess of 2.0%, the hot workability is reduced.
0%.

Zr: A stable compound is formed between P and P which is harmful to SSC resistance, and P is substantially reduced by reducing solid solution P, thereby contributing to improvement of SSC resistance. However, if the content is too large, the toughness and the SSC resistance are reduced due to the formation of a coarse oxide, so the upper limit was made 0.2%. There is no effect with a very small amount, but 0.01% or more is sufficiently effective.

Ca, REM: an element effective for spheroidizing inclusions to render them harmless. However, if the content is too large, the inclusions are increased to lower the low-temperature toughness and the SSC resistance. Therefore, the upper limits are set to 0.01% and 0.02%, respectively. If the amount is too small, the effect is not obtained, but if any of them is contained at 0.001% or more, it is sufficiently effective. Nb, Ta: All of these elements are carbide forming elements and are effective in lowering the hardness of the weld heat affected zone, but the effect is saturated even if they are contained in a large amount. 0.1% and 0.15%.

Mg: Dispersed in the form of oxide, suppresses grain growth in the heat affected zone by welding, and suppresses a decrease in toughness. If the amount is too small, there is no effect. If the amount is excessive, the formation of coarse oxides lowers the toughness. Therefore, when Mg is added, the appropriate range is 0.0005% to 0.006%. B: B is also an effective element for improving hot workability, and 0.0005% or more is added as necessary. However, if the addition amount is large, welding cracks occur, so the upper limit was made 0.03%.

Next, the reasons for limiting the metallographic structure in the present invention will be described below. The final metal structure of conventional martensitic stainless steel is a tempered martensite uniform structure. On the other hand, in the organization of the present invention, the S-resistant
In order to improve the SC property, it is required that 5% or more of the retained austenite phase having a hydrogen trapping effect be contained in the metal structure, similarly to the above-described hydrogen trapping function by the carbon nitride of V or Ti.

In the present invention, since a hydrogen trapping effect by the carbonitrides of V and Ti is obtained, a smaller amount of the retained austenite phase is contained in the metal structure, so that the conventional retained austenite-containing martensitic steel can be obtained. SSC resistance equal to or greater than that can be exhibited, but if the amount of retained austenite is less than 5%, a sufficient improvement effect of SSC resistance cannot be obtained, so the lower limit was set to 5%. The upper limit is SS resistance
Although there is no particular limitation on the corrosion resistance of C, if the residual austenite phase becomes too large, Cr, M
Since the segregation of components such as o and Ni becomes large and the amount of overall corrosion tends to increase slightly, the amount of retained austenite in the metal structure must be less than 15% in order to secure the SSC resistance and the overall corrosion. Is preferred. Here, the retained austenite amount is a value determined from the diffraction peak intensity ratio of the X-ray diffraction lines of the austenite phase and the martensite phase.

In order to obtain the above structure in the present invention, the steel slab is cooled as it is cast, immediately after hot working after reheating, or reheated to a temperature higher than the Ac ₃ transformation point. by heating to a temperature range of AC ₁ point to Ac ₁ point + 150 ℃ cooling, further followed AC ₁ point -10 ° C. to Ac ₁ point + 6
It is necessary to cool by heating to a temperature range of 0 ° C.
That is, two phases of martensite phase and retained austenite phase tempering by heating to the first AC ₁ point to Ac ₁ point + 0.99 ° C.. If the temperature is lower than the AC ₁ point, the austenite phase is not formed or is formed too little. If the temperature is higher than the AC ₁ point + 150 ° C., the amount of the austenite phase transformed into the martensite phase during cooling increases, and the remaining austenite phase increases. Phase decreases. Next, by heating to a two-stage AC ₁ point -10 ° C. to Ac ₁ point + 60 ° C., concentrated elements austenite phase with martensite phase newly generated during after the first heating and cooling a tempered To stabilize the austenitic phase. AC ₁ point -10 ℃
Tempering at a lower temperature cannot sufficiently perform tempering of the martensite phase. On the other hand, when heating to a temperature higher than AC ₁ point + 60 ° C., too much austenite phase is newly formed at a high temperature, and the element concentration in the austenite phase formed by the first heating is diluted, and austenite formed by the first heating is reduced. The phase also transforms to a martensite phase during cooling, the residual austenite phase decreases,
The untempered martensite phase becomes too large. Although the heating time is not particularly specified,
About 60 minutes is desirable. Here, AC ₁ point (° C.) is A
_{C 1 = 812-25 × Ni-670} × C + 3.4 × Mo
Is required.

[0023]

EXAMPLES Steel having the chemical composition shown in Table 1 was melted, hot-rolled, and adjusted to a metal structure containing retained austenite by heat treatment shown in Table 2. Table 2 shows the results of the retained austenite phase fraction, the SSC test, and the overall corrosion test.
The retained austenite phase fraction was determined from the ratio of X-ray diffraction line intensities.

The SSC test (SSC in Table 2) was 100
A 4-point bending test piece provided with an indentation equivalent to the% actual yield stress was equilibrated with a mixed gas of carbon dioxide containing 1% and 10% hydrogen sulfide gas at a temperature of 25 ° C. and atmospheric pressure.
It was immersed in a weight-% saline solution and determined by the presence or absence of cracks after 720 hours. Those which were not cracked by 10% hydrogen sulfide
%, Those that suffered minor cracks but did not crack at 1% were marked as ○, and those that cracked even with 1% hydrogen sulfide were marked X. The overall corrosion test (GC in Table 2) was performed at a hydrogen sulfide partial pressure of 0.001.
20 equilibrated with 4 MPa carbon dioxide containing MPa
A one-week immersion test was conducted at a temperature of 104 ° C. in a sodium chloride solution at a rate of 0.02 mm / y or less.
A sample having a corrosion rate in the range of 0.02 to 0.1 mm / y was evaluated as ○, and a sample having a corrosion rate of 0.1 mm / y or more was evaluated as x.

Steels Nos. 11 to 14 are comparative steels, and No. 11 has a chemical composition in the patent range, but the heat treatment after rolling is inappropriate and the amount of retained austenite is out of the patent range. 12-14
No. steel has a chemical composition out of range. All of the steels of the present invention Nos. 1 to 10 exhibited good SSC resistance and overall corrosion resistance. In all of the comparative steels, the SSC resistance is not sufficient, and the superiority of the steel of the present invention is apparent.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

According to the present invention, it is possible to provide a high Cr alloy steel material having particularly improved SSC resistance and excellent corrosion resistance, a method for producing the same, and a corrosion-resistant steel pipe product. It greatly contributes to the design of equipment.

Continuation of the front page (72) Inventor Shunji Sakamoto 1-1 Niwahata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka Nippon Steel Corporation Yawata Works (72) Inventor Makoto Tamura 1 Tobita-cho, Tobata-ku, Kitakyushu-shi, Fukuoka -1 F-term in Nippon Steel Corporation Yawata Works (reference) 4K032 AA01 AA02 AA04 AA08 AA12 AA13 AA14 AA15 AA16 AA17 AA19 AA20 AA21 AA22 AA31 AA33 AA35 AA36 AA39 AA40 CF02

Claims (6)

[Claims] C .: 0.05% by mass or less, Si:
1% or less, Mn: 3% or less, P: 0.03% or less, S:
0.01% or less, Cr: 9.0 to 15.0%, Ni:
3.0 to 8.0%, Mo: 0.5 to 3%, V: 0.01
-0.3%, Ti: 0.01-0.3%, Al: 0.2
%, N: 0.05% or less, and the following formula according to the C content between those components: When C ≧ 0.02%, IPS = 40C + 34N + 0.3Cu + Ni-1.1C
r-1.8Mo ≧ -10.5 or when C <0.02%, IPS = 0.8 + 34N + 0.3Cu + Ni-1.1C
r-1.8Mo ≧ -10.5, and the balance is composed of iron and unavoidable impurities, and the metal structure is composed of 5% or more of retained austenite and martensite or tempered martensite. High Cr martensitic stainless steel with excellent corrosion resistance characterized by having a dual phase structure. 2. The high Cr martensitic stainless steel with excellent corrosion resistance according to claim 1, wherein the retained austenite fraction in the metal structure is 5% or more and less than 15%. 3. The method according to claim 1, further comprising:
r: 0.2% or less, Ca: 0.01% or less, REM:
The high Cr martensitic stainless steel having excellent corrosion resistance according to claim 1 or 2, which contains one or more of 0.02% or less. Further, in mass%, Nb: 0.1% or less, Ta: 0.15% or less, Mg: 0.006% or less,
B: High Cr martensitic stainless steel having excellent corrosion resistance according to any one of claims 1 to 3, containing one or more of 0.0005 to 0.03%. . 5. The high C according to claim 1, wherein:
A corrosion-resistant steel pipe obtained by pipe forming a martensitic stainless steel. 6. The chemistry according to claim 1, 3 or 4.
The steel slab having the composition is immediately after hot working or
Reheat above the transformation point and cool, then Ac ₁Point to Ac
₁Heated to a temperature in the range of + 150 ° C, cooled,
Then Ac₁Point -10 ° C to Ac₁Temperature in the range of + 60 ° C
Excellent corrosion resistance characterized by heating and cooling each time
Method for producing high Cr martensitic stainless steel.