JP6780426B2 - Duplex stainless steel - Google Patents

Description

The present invention relates to duplex stainless steel.

The oil and natural gases produced from the oil and gas fields contain corrosive gases such as carbon dioxide gas (CO ₂ ) and hydrogen sulfide gas (H ₂ S) as accompanying gases. High corrosion resistance materials such as duplex stainless steel are used for line pipes used for transporting petroleum and natural gas containing such corrosive gas. The same applies to oil well pipes used for oil and natural gas mining.

Japanese Patent No. 5072285 discloses a duplex stainless steel having excellent corrosion resistance used in a corrosive environment such as a chloride environment.

Japanese Patent No. 5870201 describes ferrite / austenitic two-phase stainless steels of the chemical industry and the like for nitric acid environments that require good total corrosion resistance and high strength.

Japanese Patent No. 5072285 Japanese Patent No. 5870201

In Japanese Patent No. 5870201, the total corrosiveness is examined, but the sulfide stress corrosion cracking resistance is not examined.

An object of the present invention is to provide a duplex stainless steel having excellent corrosion resistance in a trace hydrogen sulfide environment.

The duplex stainless steel according to one embodiment of the present invention has a chemical composition of C: 0.03% or less, Si: 0.2 to 1%, Mn: 1.5% or less, P: 0. 040% or less, S: 0.010% or less, Al: 0.001 to 0.040%, Ni: 4% or more and less than 7%, Cr: 24% or more and 28% or less, Mo: 0.5 to 4 0.0%, Cu: more than 2.0% and 4.0% or less, Co: 0.001 to 0.3%, N: more than 0.1% and 0.35% or less, O: 0.010 % Or less, V: 0 to 1.5%, Ca: 0 to 0.02%, Mg: 0 to 0.02%, B: 0 to 0.02%, REM: 0 to 0.2%, balance: The Cr, Ni, Mo, and Cu contents in the ferrite phase, which are Fe and impurities and have a structure containing an austenite phase and a ferrite phase and are measured using an electron beam microanalyzer, are as follows. Satisfy (1).

(0.3 × Cr) ² + (0.2 × Ni) ² + (0.4 × Mo) ² + (2.1 × Cu) ² ≧ 86.0 (1)
The contents of Cr, Ni, Mo, and Cu in the ferrite phase are substituted in% by mass into Cr, Ni, Mo, and Cu of the formula (1).

According to the present invention, a duplex stainless steel having excellent corrosion resistance in a trace hydrogen sulfide environment can be obtained.

FIG. 1 is a map of the Ni content obtained by measuring duplex stainless steel with an electron probe microanalyzer. FIG. 2 shows the region selected as the ferrite phase from FIG. 1 in white.

The present inventors investigated the sulfide stress corrosion cracking resistance of duplex stainless steel in a trace hydrogen sulfide environment of 0.3 bar or less. As a result, the following findings were obtained.

Two-phase stainless steel has a metallic structure containing an austenite phase and a ferrite phase. Each alloying element in steel is partitioned into an austenite phase and a ferrite phase. Specifically, the content of Ni, Cu, N, etc. is higher in the austenite phase, and the content of Cr, Mo, etc. is higher in the ferrite phase, as compared with the chemical composition of the entire steel. This component distribution changes depending on the heat treatment conditions as well as the chemical composition of the steel.

The content of each distributed element is a factor that determines the corrosion resistance of each phase. Corrosion resistance in a trace hydrogen sulfide environment is affected by the contents of Cr, Ni, Mo, and Cu. In addition, stress corrosion cracking of duplex stainless steel in a trace hydrogen sulfide environment basically propagates in the ferrite phase. Therefore, in order to improve the sulfide stress corrosion cracking resistance of duplex stainless steel, it is necessary to increase the Cr, Ni, Mo, and Cu contents in the ferrite phase.

Specifically, the Cr, Ni, Mo, and Cu contents in the ferrite phase obtained by measurement using an electron probe microanalyzer (hereinafter referred to as "EPMA") are expressed by the following formula ("EPMA"). It is necessary to satisfy 1).

(0.3 × Cr) ² + (0.2 × Ni) ² + (0.4 × Mo) ² + (2.1 × Cu) ² ≧ 86.0 (1)
The contents of Cr, Ni, Mo, and Cu in the ferrite phase are substituted in% by mass into Cr, Ni, Mo, and Cu of the formula (1).

Co forms a sulfide film and significantly improves the sulfide stress corrosion cracking resistance of steel. If the content of Cr, Ni, Mo and Cu in the ferrite phase obtained by containing a predetermined amount of Co and measuring using EPMA satisfies the above formula (1), an excellent sulfide resistant product is obtained. Stress corrosion cracking property can be obtained.

Based on the above findings, the present invention has been completed. Hereinafter, the two-phase stainless steel according to one embodiment of the present invention will be described in detail.

[Chemical composition]
The duplex stainless steel according to this embodiment has the chemical composition described below. In the following description, "%" of elemental content means mass%.

C: 0.03% or less Carbon (C) stabilizes austenite. However, if the C content exceeds 0.03%, carbides are likely to precipitate and the corrosion resistance is lowered. Therefore, the C content is 0.03% or less. The lower limit of the C content is preferably 0.002%, more preferably 0.005%. The upper limit of the C content is preferably 0.025%, more preferably 0.02%.

Si: 0.2 to 1%
Silicon (Si) is an element effective in preventing welding defects because it improves the fluidity of the molten metal during welding. If the Si content is less than 0.2%, this effect cannot be sufficiently obtained. On the other hand, when the Si content exceeds 1%, a precipitated phase such as a σ phase is likely to be formed. Precipitated phases such as the σ phase deteriorate the toughness and pitting corrosion resistance of the material. Therefore, the Si content is 0.2 to 1%. The lower limit of the Si content is preferably 0.3%, more preferably 0.4%. The upper limit of the Si content is preferably 0.8%, more preferably 0.7%.

Mn: 1.5% or less Manganese (Mn) improves hot workability by desulfurization and deoxidizing effects. In addition, Mn increases the solubility of N. However, if the Mn content exceeds 1.5%, the corrosion resistance is lowered. Therefore, the Mn content is 1.5% or less. The lower limit of the Mn content is preferably 0.1%, more preferably 0.5%. The upper limit of the Mn content is preferably 1.2%, more preferably 1.0%.

P: 0.040% or less Phosphorus (P) is mixed into steel as an impurity and reduces the corrosion resistance and toughness of steel. Therefore, the P content is 0.040% or less. The P content is preferably 0.030% or less, and more preferably 0.025% or less.

S: 0.010% or less Sulfur (S) is mixed in the steel as an impurity and reduces the hot workability of the steel. In addition, sulfide serves as a starting point for pitting corrosion and reduces the pitting corrosion resistance of steel. Therefore, the S content is 0.010% or less. The S content is preferably 0.005% or less, and more preferably 0.002% or less.

Al: 0.001 to 0.040%
Aluminum (Al) deoxidizes steel. On the other hand, when the N content in the steel is high, Al precipitates as aluminum nitride (AlN), which lowers the toughness and corrosion resistance of the steel. Therefore, the Al content is 0.001 to 0.040%. The lower limit of the Al content is preferably 0.002%, more preferably 0.005%. The upper limit of the Al content is preferably 0.030%, more preferably 0.025%. The Al content in the present embodiment refers to the content of acid-soluble Al (sol.Al).

Ni: 4% or more and less than 7% Nickel (Ni) stabilizes austenite. Ni also improves the toughness of steel. If the Ni content is less than 4%, these effects cannot be sufficiently obtained. On the other hand, when the Ni content is 7% or more, a precipitated phase such as a σ phase is likely to be formed. Therefore, the Ni content is 4% or more and less than 7%. The lower limit of the Ni content is preferably 4.2%, more preferably 4.5%. The upper limit of the Ni content is preferably 6.5%, more preferably 6%.

Cr: More than 24% and less than 28% Chromium (Cr) improves the corrosion resistance of steel. If the Cr content is 24% or less, this effect cannot be sufficiently obtained. On the other hand, when the Cr content exceeds 28%, a precipitated phase such as a σ phase is likely to be formed. Therefore, the Cr content is more than 24% and 28% or less. The lower limit of the Cr content is preferably 24.5%, more preferably 25%. The upper limit of the Cr content is preferably 27%, more preferably 26%.

Mo: 0.5-4.0%
Molybdenum (Mo) improves the corrosion resistance of steel. If the Mo content is less than 0.5%, this effect cannot be sufficiently obtained. On the other hand, when the Mo content exceeds 4.0%, a precipitated phase such as a σ phase is likely to be formed. Therefore, the Mo content is 0.5 to 4.0%. The lower limit of the Mo content is preferably 0.7%, more preferably 1.0%. The upper limit of the Mo content is preferably 3.0%, more preferably 2.0%, still more preferably 1.5%.

Cu: More than 2.0% and 4.0% or less Copper (Cu) reinforces a passivation film containing Cr as a main component in a chloride environment containing a corrosive acid gas. Cu is also finely deposited on the matrix during high heat input welding and suppresses the formation of the σ phase at the interface between the ferrite phase and the austenite phase. If the Cu content is 2.0% or less, this effect cannot be sufficiently obtained. On the other hand, when the Cu content exceeds 4.0%, the hot workability of the steel is lowered. Therefore, the Cu content is more than 2.0% and 4.0% or less. The lower limit of the Cu content is preferably 2.1%, more preferably 2.2%. The upper limit of the Cu content is preferably 3.8%, more preferably 3.5%.

Co: 0.001 to 0.3%
Cobalt (Co) forms a sulfide film and improves the sulfide stress corrosion cracking resistance of steel. When the Co content is 0.001% or more, this effect can be remarkably obtained. On the other hand, Co is expensive and when added in a large amount, the phase balance (ratio of austenite phase and ferrite phase) of the steel material changes, which may affect the performance. Therefore, the Co content is set to 0.3% or less. The lower limit of the Co content is more preferably 0.005%, still more preferably 0.01%. The upper limit of the Co content is preferably 0.2%, more preferably 0.1%.

N: More than 0.1% and less than 0.35% Nitrogen (N) is a strong austenite-forming element, which improves the thermal stability and corrosion resistance of duplex stainless steel. Since the two-phase stainless steel according to the present embodiment contains a large amount of Cr and Mo which are ferrite forming elements, the N content is set to 0.1% or more in order to make the balance between the ferrite phase and the austenite phase appropriate. Also many. On the other hand, if the N content exceeds 0.35%, blow holes will occur during welding. In addition, the toughness and corrosion resistance of the weld metal are reduced by the nitride produced during welding. Therefore, the N content is more than 0.1% and 0.35% or less. The lower limit of the N content is preferably 0.12%, more preferably 0.15%. The upper limit of the N content is preferably 0.3%, more preferably 0.25%.

O: 0.010% or less Oxygen (O) forms oxides that are non-metal inclusions and reduces the toughness of duplex stainless steel. Therefore, the O content is 0.010% or less. The O content is preferably 0.008% or less, and more preferably 0.005% or less.

The rest of the chemical composition of duplex stainless steel according to this embodiment is Fe and impurities. The impurities referred to here refer to elements mixed from ores and scraps used as raw materials for steel, or elements mixed from the environment of the manufacturing process.

The chemical composition of the duplex stainless steel according to the present embodiment may further contain an element described below in place of a part of Fe. The elements described below are all selective elements. That is, the chemical composition of the two-phase stainless steel according to the present embodiment does not have to contain a part or all of the following elements.

V: 0-1.5%
Vanadium (V) is a selective element. V improves the corrosion resistance of duplex stainless steel. More specifically, V is contained in combination with Mo and Cu to improve the crevice corrosion resistance. This effect can be obtained if even a small amount of V is contained. On the other hand, when the V content exceeds 1.5%, the ferrite phase becomes excessive and the toughness and corrosion resistance are lowered. Therefore, the V content is 0 to 1.5%. The lower limit of the V content is preferably 0.01%, more preferably 0.03%. The upper limit of the V content is preferably 1.2%, more preferably 1.0%.

Ca: 0-0.02%
Mg: 0-0.02%
B: 0 to 0.02%
REM: 0-0.2%
Calcium (Ca), magnesium (Mg), boron (B), and rare earth element (REM) are all selective elements. All of these elements fix S and O to improve hot workability. This effect can be obtained if any of these elements are contained. On the other hand, when the contents of each of Ca, Mg, and B exceed 0.02%, non-metal inclusions increase and toughness and corrosion resistance decrease. Therefore, the content of each of Ca, Mg, and B is 0 to 0.02%. Similarly, when the REM content exceeds 0.2%, non-metal inclusions increase and toughness and corrosion resistance decrease. Therefore, the REM content is 0 to 0.2%.

The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0005%. The upper limit of the Ca content is preferably 0.01%, more preferably 0.005%. The lower limit of the Mg content is preferably 0.001%, more preferably 0.002%. The upper limit of the Mg content is preferably 0.01%, more preferably 0.005%. The lower limit of the B content is preferably 0.0001%, more preferably 0.0005%. The upper limit of the B content is preferably 0.01%, more preferably 0.005%.

Note that REM is a general term for 17 elements in which Y and Sc are combined with 15 elements of lanthanoid, and one or more of these elements can be contained. REM content means the total content of these elements. The lower limit of the REM content is preferably 0.0005%, more preferably 0.001%. The upper limit of the REM content is preferably 0.1%, more preferably 0.05%.

[Organization]
The two-phase stainless steel according to the present embodiment is composed of an austenite phase and a ferrite phase, and the balance is precipitates and inclusions.

In the duplex stainless steel according to the present embodiment, the Cr, Ni, Mo, and Cu contents in the ferrite phase obtained by measurement using EPMA satisfy the following formula (1).

(0.3 × Cr) ² + (0.2 × Ni) ² + (0.4 × Mo) ² + (2.1 × Cu) ² ≧ 86.0 (1)
The contents of Cr, Ni, Mo, and Cu in the ferrite phase are substituted in% by mass into Cr, Ni, Mo, and Cu of the formula (1).

Stress corrosion cracking of duplex stainless steel in a hydrogen sulfide environment propagates in a ferrite phase with relatively low corrosion resistance. Therefore, in order to improve the corrosion resistance in the hydrogen sulfide environment, it is necessary that a large amount of elements exhibiting corrosion resistance in the hydrogen sulfide environment are dispersed in the ferrite phase.

More specifically, the Cr, Ni, Mo, and Cu contents in the ferrite phase are determined as follows. First, a test piece is taken from duplex stainless steel. The collected test piece is mechanically polished and then electrolytically polished. The polished specimen is analyzed using EPMA. Specifically, an electron beam having an accelerating voltage of 15 kV is used to measure a region of 200 μm × 200 μm in a grid pattern at intervals of 0.4 μm. That is, 250,000 points of 500 points × 500 points are measured.

FIG. 1 shows No. 1 produced in Examples described later. It is a map of the Ni content obtained by measuring the test piece collected from the steel plate of No. 5 by EPMA. In FIG. 1, the left-right direction of the paper surface is the rolling direction, and the vertical direction of the paper surface is the width direction (direction perpendicular to the rolling direction and the thickness direction). The bright region is a region having a high Ni content, and the dark region is a region having a low Ni content.

The region with a high Ni content corresponds to the austenite phase, and the region with a low Ni content corresponds to the ferrite phase. As shown in FIG. 1, in the structure of two-phase stainless steel, the base material is a ferrite phase, and an austenite phase is generally formed in an island shape.

From the measured 250,000 points of data, 200 or more measurement points that can be clearly determined to be the ferrite phase are selected. FIG. 2 shows the region selected as the ferrite phase from FIG. 1 in white.

The average value of the Ni content at the selected 200 or more measurement points is defined as the Ni content in the ferrite phase. Similarly, the average Cr content, the average Mo content, and the average Cu content at the selected 200 or more measurement points are the Cr content, Mo content, and Mo content in the ferrite phase, respectively. Defined as Cu content.

Although not used in the formula (1), the content of each alloying element in the austenite phase can also be determined by the same method.

The Cr, Ni, Mo, and Cu contents in the ferrite phase vary depending on the heat treatment conditions as well as the chemical composition of the duplex stainless steel. Specifically, the higher the Cr, Ni, Mo, and Cu contents in the duplex stainless steel, the higher the Cr, Ni, Mo, and Cu contents in the ferrite phase tend to be. Further, the higher the heat treatment temperature, the higher the ratio of the volume of the ferrite phase to the metal structure, the lower the concentration of Cr and Mo (ferrite forming element), and the higher the concentration of Ni and Cu (austenite forming element).

The value obtained by substituting the Cr, Ni, Mo, and Cu contents in the ferrite phase into the left side of the equation (1) is defined as the CI-F value. When the CI-F value is 86.0 or more, excellent sulfide stress corrosion cracking resistance can be obtained. The CI-F value is preferably 86.5 or more, and more preferably 87.0 or more.

[Production method]
Hereinafter, an example of a method for producing duplex stainless steel according to the present embodiment will be described. However, the method for producing duplex stainless steel according to the present embodiment is not limited to this.

A material having the above-mentioned chemical composition is prepared. For example, steel is melted using an electric furnace, an Ar—O ₂ mixed gas bottom blowing decarburization furnace (AOD furnace), a vacuum decarburization furnace (VOD furnace), or the like. The molten metal may be cast into an ingot or a rod-shaped billet by a continuous casting method, for example.

The prepared material is hot-processed into a predetermined shape. Hot working is, for example, hot rolling, hot forging, drilling rolling, and hot extrusion. The ingot may be forged to form a steel plate, or the billet may be drilled and rolled to form a seamless steel pipe.

The hot-processed material is subjected to solution heat treatment. Specifically, the material is heated to a predetermined temperature, held for a predetermined time, and then rapidly cooled. The hot-processed high-temperature material may be subjected to solution heat treatment, or the hot-processed material may be cooled to around room temperature and then reheated to undergo solution heat treatment.

The temperature of the solution heat treatment is preferably 950 ° C to 1070 ° C. Even if the temperature of the solidification heat treatment is lower than 950 ° C or higher than 1070 ° C, it becomes difficult to make the CI-F value 86.0 or more. Further, if the temperature of the solidification heat treatment is too low, a precipitated phase such as a σ phase may be formed. The lower limit of the temperature of the solution heat treatment is preferably 960 ° C, more preferably 970 ° C. The upper limit of the temperature of the solution heat treatment is preferably 1040 ° C, more preferably 1030 ° C.

The holding time is not particularly limited, but is preferably 5 minutes or more in the soaking time, and more preferably 10 minutes or more in the soaking time. Even if the heat is soaked for a long time, the effect is almost saturated. From the viewpoint of manufacturing cost, the soaking time is preferably 30 minutes or less, more preferably 20 minutes or less. The quenching after holding is, for example, water cooling.

The duplex stainless steel according to the embodiment of the present invention has been described above. Duplex stainless steel according to this embodiment has excellent sulfide stress corrosion cracking resistance.

Hereinafter, the present invention will be described in more detail with reference to Examples. The present invention is not limited to these examples.

Steels having the chemical compositions shown in Table 1 were melted and hot forged and hot rolled to produce steel sheets. In Table 1, "-" indicates that the content of the corresponding element is at the impurity level.

The produced steel sheet was subjected to solution heat treatment at a predetermined temperature. A test piece is collected from the center of the thickness of each steel sheet after the solution heat treatment, and the content of alloying elements (Cr, Ni, Mo, and Cu) in the ferrite phase and the austenite phase is determined by the method described in the embodiment. It was measured.

A sulfide stress corrosion cracking test was carried out on each steel sheet as follows. A test piece having a thickness of 2 mm, a width of 10 mm, and a length of 75 mm was collected from the center of the thickness of each steel sheet after the solution heat treatment. A 100% actual yield stress was applied to the collected test piece by 4-point bending. _{H 2} S and 0.97bar of CO ₂ 0.03bar was prepared autoclave sealed. In an autoclave, a test piece to which the above-mentioned 100% actual yield stress was applied was immersed in a 10% NaCl aqueous solution by mass for 1 month. After soaking for 1 month, it was investigated whether or not sulfide stress cracking (Sufide Stress Cracking, hereinafter referred to as "SSC") occurred in each test piece. Specifically, the presence or absence of SSC was determined by observing the cross section perpendicular to the longitudinal direction of each test piece with an optical microscope having a magnification of 100 times.

Table 2 shows the temperature of the solution heat treatment, the content of alloying elements in the ferrite phase and the austenite phase, and the results of the sulfide stress corrosion cracking test.

In the columns of "Cr-A", "Ni-A", "Mo-A", and "Cu-A" in Table 2, Cr and Ni in the austenite phase obtained by measurement using EPMA, respectively. , Mo, and Cu contents are listed. Similarly, in the columns of "Cr-F", "Ni-F", "Mo-F", and "Cu-F", Cr and Ni in the ferrite phase obtained by measurement using EPMA, respectively. , Mo, and Cu contents are listed.

In the "CI-F value" column, the values obtained by substituting the Cr, Ni, Mo, and Cu contents in the ferrite phase obtained by measuring using EPMA into the left side of the formula (1) are described. There is.

The results of the sulfide stress corrosion cracking test are described in the "SSC test results" column.

No. The steel plates 4 to 8 and 16 had a CI-F value of 86.0 or more. No. The steel sheets 4 to 8 and 16 did not generate SSC in the sulfide stress corrosion cracking test.

No. The steel sheets 1 to 3 had a CI-F value of less than 86.0, and SSC was generated in the sulfide stress corrosion cracking test. It is considered that this is because the heat treatment temperature was too low.

No. The steel sheet of No. 9 had a CI-F value of less than 86.0, and SSC was generated in the sulfide stress corrosion cracking test. It is considered that this is because the Cr content of the steel material C was too low.

No. The steel sheet of No. 10 had a CI-F value of less than 86.0, and SSC was generated in the sulfide stress corrosion cracking test. It is considered that this is because the Cu content of the steel material D was too low.

No. The steel sheets 11 to 14 had a CI-F value of less than 86.0, and SSC was generated in the sulfide stress corrosion cracking test. It is considered that this is because the heat treatment temperature was too high.

No. Although the CI-F value of the 15 steel sheets was appropriate, SSC was generated in the sulfide stress corrosion cracking test. It is considered that this is because the steel material E did not contain Co.

The embodiments of the present invention have been described above. The above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented within a range that does not deviate from the gist thereof.

Claims (3)

The chemical composition is mass%,
C: 0.03% or less,
Si: 0.2 to 1%,
Mn: 1.5% or less,
P: 0.040% or less,
S: 0.010% or less,
Al: 0.001 to 0.040%,
Ni: 4% or more and less than 7%,
Cr: More than 24% and less than 28%,
Mo: 0.5-4.0%,
Cu: More than 2.0% and less than 4.0%,
Co: 0.01 ~0.3%,
N: More than 0.1% and 0.35% or less,
O: 0.010% or less,
V: 0-1.5%,
Ca: 0-0.02%,
Mg: 0-0.02%,
B: 0 to 0.02%,
REM: 0-0.2%,
Remaining: Fe and impurities,
It has a structure containing an austenite phase and a ferrite phase.
A duplex stainless steel in which the Cr, Ni, Mo, and Cu contents in the ferrite phase obtained by measurement using an electron probe microanalyzer satisfy the following formula (1).
(0.3 × Cr) ² + (0.2 × Ni) ² + (0.4 × Mo) ² + (2.1 × Cu) ² ≧ 86.0 (1)
The contents of Cr, Ni, Mo, and Cu in the ferrite phase are substituted in% by mass into Cr, Ni, Mo, and Cu of the formula (1). The duplex stainless steel according to claim 1.
When the chemical composition is mass%,
V: 0.01-1.5%,
Duplex stainless steel containing. The duplex stainless steel according to claim 1 or 2.
When the chemical composition is mass%,
Ca: 0.0001 to 0.02%,
Mg: 0.001 to 0.02%,
B: 0.0001 to 0.02%, and REM: 0.0005 to 0.2%,
Duplex stainless steel containing one or more selected from the group consisting of.