EP2677056A1

EP2677056A1 - Duplex stainless steel

Info

Publication number: EP2677056A1
Application number: EP12747362.7A
Authority: EP
Inventors: Kenta Yamada; Hiroyuki Nagayama; Masahiko Hamada; Daisuke MOTOYA; Hisashi Amaya
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2011-02-14
Filing date: 2012-02-10
Publication date: 2013-12-25
Anticipated expiration: 2032-02-10
Also published as: JP5206904B2; EP2677056B1; BR112013017647B1; CN103370435A; US20130315776A1; WO2012111537A1; MX2013008518A; AU2012218661A1; CN103370435B; CA2826893C; MX351782B; EP2677056A4; AU2012218661B2; JPWO2012111537A1; CA2826893A1; BR112013017647A2

Abstract

Provided is a duplex stainless steel that can suppress precipitation of a σ phase at the time of high heat input welding, is excellent in SCC resistance under high-temperature chloride environments, and has a high strength. The duplex stainless steel according to the present invention includes a chemical composition containing, in mass percent, C: at most 0.030%, Si: 0.20 to 1.00%, Mn: at most 8.00%, P: at most 0.040%, S: at most 0.0100%, Cu: more than 2.00% and at most 4.00%, Ni: 4.00 to 8.00%, Cr: 20.0 to 28.0%, Mo: 0.50 to 2.00%, N: 0.100 to 0.350%, and sol. Al: at most 0.040%, the balance being Fe and impurities, and satisfying Expression (1) and Expression (2); a structure having a ferrite rate of at least 50%; and a yield strength of at least 550 MPa or more:

2.2 Cr + 7 Mo + 3 Cu > 66

Cr + 11 Mo + 10 Ni < 12 (Cu + 30 N)

Description

Technical Field

The present invention relates to a duplex stainless steel, and, more particularly, to a duplex stainless steel that is suitable for a steel material for a line pipe.

Background Art

Petroleum oil and natural gas produced from oil fields and gas fields contain associated gas. The associated gas contains corrosive gas such as carbon dioxide gas (CO₂) and hydrogen sulfide (H₂S). Line pipes transport the associated gas together with the petroleum oil and the natural gas. Hence, the line pipes suffer from problems of stress corrosion cracking (SCC), sulfide stress corrosion cracking (sulfide stress cracking: SSC), and general corrosion cracking that causes a decrease in wall thickness.
The propagation speeds of SCC and SSC are high. Hence, SCC and SSC penetrate through the line pipes in a short time from the occurrence thereof. Moreover, SCC and SSC locally occur. Hence, steel materials for line pipes are required to have an excellent corrosion resistance (a SCC resistance, a SSC resistance, and a general corrosion resistance), and are required to have, particularly, a SCC resistance and a SSC resistance.
WO 96/18751 and JP 2003-171743A each propose a duplex stainless steel excellent in corrosion resistance. The duplex stainless steel according to WO 96/18751 contains 1 to 3% of Cu. WO 96/18751 describes that this increases the corrosion resistance of the duplex stainless steel under chloride and sulfide environments.
A method of producing the duplex stainless steel according to JP 2003-171743A involves properly adjusting the contents of Cr, Ni, Cu, Mo, N, and W and controlling the area fraction of a ferrite phase in the duplex stainless steel to 40 to 70%. JP 2003-171743A describes that this increases the strength, toughness, and seawater corrosion resistance of the duplex stainless steel.

Disclosure of the Invention

Unfortunately, in the duplex stainless steel disclosed in WO 96/18751 , the corrosion resistance of a portion near a weld zone easily decreases, and the portion near the weld zone easily embrittles, at the time of high heat input welding. Similarly in the duplex stainless steel disclosed in JP 2003-171743A , the corrosion resistance of a portion near a weld zone easily decreases, and the portion near the weld zone easily embrittles, at the time of high heat input welding. Such a decrease in the corrosion resistance of the portion near the weld zone and such an embrittlement thereof are caused by a sigma phase (σ phase) precipitating in the portion near the weld zone at the time of the high heat input welding. The σ phase is an intermetallic compound.
In the duplex stainless steel disclosed in JP 2003-171743A , moreover, the SCC resistance is low under high-temperature chloride environments containing the associated gas and having a temperature range of 120 to 200°C.
Moreover, nowadays, steel materials for line pipes are required to have a high strength. Specifically, the steel materials are required to have a yield strength of 80 ksi (550 MPa or more).
The present invention has an objective to provide a duplex stainless steel that can suppress precipitation of a σ phase at the time of high heat input welding, is excellent in SCC resistance under high-temperature chloride environments, and has a high strength.
A duplex stainless steel according to the present invention includes: a chemical composition containing, in mass percent, C: at most 0.030%, Si: 0.20 to 1.00%, Mn: at most 8.00%, P: at most 0.040%, S: at most 0.0100%, Cu: more than 2.00% and at most 4.00%, Ni: 4.00 to 8.00%, Cr: 20.0 to 28.0%, Mo: 0.50 to 2.00%, N: 0.100 to 0.350%, and sol. Al: at most 0.040%, the balance being Fe and impurities, and satisfying Expression (1) and Expression (2); a structure having a ferrite rate of at least 50%; and a yield strength of at least 550 MPa: $2.2 Cr + 7 Mo + 3 Cu > 66$
$Cr + 11 Mo + 10 Ni < 12 (Cu + 30 N)$

where a content (mass percent) of each element in the steel is substituted into a symbol of each element in Expression (1) and Expression (2).
The duplex stainless steel according to the present invention can suppress precipitation of a σ phase at the time of high heat input welding and is excellent in SCC resistance under high-temperature chloride environments. Moreover, the duplex stainless steel according to the present invention has a high strength.
Preferably, the above-mentioned duplex stainless steel further satisfies Expression (3): $0.6 X - Y - 5.2 + (T - 1, 070) \times 0.007 \geq 0$

where T represents a solution treatment temperature (°C), X is defined by Expression (4), and Y is defined by Expression (5): $X = Cr + 1.5 Si + Mo$
$Y = Ni + 0.5 Mn + 30 C + 30 N$

where a content (mass percent) of a corresponding element in the steel is substituted into a symbol of each element in Expression (4) and Expression (5).
The chemical composition of the above-mentioned duplex stainless steel may contain one or more types of element selected from at least one group of the following first group to third group, instead of part of the Fe.

First group: V: at most 1.50%
Second group: Ca: at most 0.0200%, Mg: at most 0.020%, and B: at most 0.0200%
Third group: rare earth metal (REM): at most 0.2000%

Brief Description of Drawings

[Figure 1] Figure 1 is a graph showing a relation between: the Cr content, the Mo content, and the Cu content in a steel; and the SCC resistance of the steel.
[Figure 2] Figure 2 is a graph showing a relation between the ferrite rate and the yield strength of a duplex stainless steel;
[Figure 3] Figure 3 is a graph showing a relation between the content of a ferrite forming element in the steel, the content of an austenite forming element in the steel, the solution treatment temperature, and the ferrite rate.
[Figure 4A] Figure 4A is a plan view of a plate material made in Example.
[Figure 4B] Figure 4B is a front view of the plate material illustrated in Figure 4A.
[Figure 5A] Figure 5A is a plan view of a welded joint made in Example.
[Figure 5B] Figure 5B is a front view of the welded joint illustrated in Figure 5A.
[Figure 6] Figure 6 is a perspective view of a four-point bending specimen collected from the welded joint illustrated in Figure 5A and Figure 5B.

Description of Embodiments

Hereinafter, an embodiment of the present invention is described in detail with reference to the drawings. Hereinafter, "%" in the content of an element means mass percent.
The inventors of the present invention carried out various experiments and detailed studies to obtain the following findings.

(a) In order to suppress precipitation of a σ phase at the time of high heat input welding, it is necessary to suppress the Cr content and the Mo content. However, molybdenum (Mo) strengthens a passivation film containing chromium (Cr) as its major ingredient, and increases the SCC resistance of a duplex stainless steel. Accordingly, if the Cr content and the Mo content are low under high-temperature chloride environments containing carbon dioxide gas and hydrogen sulfide and having an atmospheric temperature of 120°C to 200°C, the duplex stainless steel may not be provided with an excellent SCC resistance.
(b) Cu decreases the corrosion speed of a steel material under high-temperature chloride environments. Accordingly, if the Cr content and the Mo content are suppressed to be low while Cu is contained, the passivation film can be strengthened.

Figure 1 is a graph showing the SCC resistance of each duplex stainless steel to the Cr content, the Mo content, and the Cu content. Figure 1 is obtained according to an examination method of Example to be described later. The ordinate of Figure 1 represents a "7Mo + 3Cu" value. The "7Mo + 3Cu" value is obtained on the basis of the Mo content and the Cu content of the duplex stainless steel. Specifically, the Mo content (mass percent) and the Cu content (mass percent) of the corresponding duplex stainless steel are respectively substituted into "Mo" and "Cu" in "7Mo + 3Cu". The abscissa of Figure 1 represents the Cr content (mass percent) of the duplex stainless steel. "○" marks of a plurality of marks in Figure 1 represent that SCC did not occur in the duplex stainless steel during a SCC test described in Example. "●" marks therein represent that SCC occurred. A number given to the upper right of each mark corresponds to a test number in Table 1 in Example to be described later. That is, each mark represents a result of the SCC test for the corresponding test number in Table 1.
With reference to Figure 1, SCC did not occur in the duplex stainless steels with the marks located above a straight line P0 defined by 7Mo + 3Cu = -2.2Cr + 66. In comparison, SCC occurred in the duplex stainless steels with the marks located below the straight line P0.
From the result described above, if the duplex stainless steel satisfies Expression (1), the passivation film is strengthened, and the SCC resistance is improved: $2.2 Cr + 7 Mo + 3 Cu > 66$

where the content (mass percent) of each element in the steel is substituted into the symbol of each element in Expression (1).

(c) In the case where the Cu content is equal to or less than 2.00%, a sufficient corrosion resistance (a SCC resistance, a SSC resistance, and a general corrosion resistance) cannot be obtained. Accordingly, the Cu content needs to be more than 2.00%.

(d) At the time of welding the duplex stainless steel, a portion near a weld zone is heated in a short time, and is cooled in a short time. Such short-time heating and cooling cause the σ phase to more easily precipitate. In order to suppress the precipitation of the σ phase, it is preferable to suppress the nucleation and nuclear growth of the σ phase.

(e) As the Ni content increases, the driving force of the nucleation of the σ phase increases. Accordingly, in order to suppress the nucleation of the σ phase, Ni should not be contained. However, if Ni is not contained, the toughness of the duplex stainless steel and the corrosion resistance (including the SCC resistance) thereof decrease. Accordingly, in order to suppress the decrease in the toughness and corrosion resistance of the duplex stainless steel while suppressing the precipitation of the σ phase, it is preferable to adjust the Ni content in accordance with the Cu content and the N content. Specifically, if the duplex stainless steel satisfies Expression (2), the decrease in the toughness and corrosion resistance of the duplex stainless steel is suppressed, while the precipitation of the σ phase is suppressed: $Cr + 11 Mo + 10 Ni < 12 (Cu + 30 N)$

The left-hand side of Expression (2) "Cr + 11mo + 10Ni" represents the magnitude of the precipitation driving force of the σ phase. In the duplex stainless steel, Cr, Mo, and Ni increase the driving force of the nucleation of the σ phase precipitation. The contribution ratio of the Mo content to the precipitation driving force of the σ phase is 11 times that of the Cr content. The contribution ratio of the Ni content to the precipitation driving force of the σ phase is 10 times that of the Cr content.
Meanwhile, the right-hand side of Expression (2) "12(Cu + 30N)" represents the magnitude of the precipitation inhibiting force of the σ phase. The contribution ratio of the Cu content to the precipitation inhibiting force of the σ phase corresponds to 12 times the contribution ratio of the Cr content to the precipitation driving force of the σ phase. The contribution ratio of the N content to the precipitation inhibiting force of the σ phase corresponds to 30 times the contribution ratio of the Cu content.
The reason why Cu and N suppress the σ phase precipitation is estimated as follows. A boundary plane between ferrite and austenite (hereinafter, referred to as ferrite/austenite boundary plane) is a nucleation site of the σ phase. When a Cu atom or a N atom is placed in the vicinity of a Ni atom placed in a crystal lattice, a decrease in interfacial energy on the ferrite/austenite boundary plane is suppressed. If the decrease in interfacial energy is suppressed, the amount of decrease in free energy at the time of the σ phase precipitation becomes smaller. Hence, the driving force of the nucleation of the σ phase becomes smaller.
Moreover, Cu extremely finely precipitates as a Cu concentrated phase in a matrix. The precipitated Cu is dispersed in the matrix. The precipitated Cu can be a nucleation site of the σ phase. The large amount of Cu dispersed and precipitated in the matrix competes with the ferrite/austenite boundary plane that is the original nucleation site of the σ phase. The nuclear growth of the σ phase on the ferrite/austenite boundary plane is faster than the nuclear growth of the σ phase on the dispersed and precipitated Cu. Accordingly, the nuclear growth of the σ phase on the ferrite/austenite boundary plane is delayed by the dispersed and precipitated Cu, so that the precipitation of the σ phase is suppressed.

(f) If the Ni content satisfies Expression (2), a Cu atom and a N atom are easily placed in the vicinity of a Ni atom placed in a crystal lattice. Hence, the nucleation of the σ phase is suppressed.

(g) If the ferrite rate of the duplex stainless steel is equal to or more than 50%, the yield strength of the duplex stainless is equal to or more than 550 MPa (80 ksi). Note that the ferrite rate refers to the area fraction of a ferrite phase in the steel. The yield strength (MPa) refers to an offset yield stress of 0.2% based on ASTM A370.

Figure 2 is a graph showing a relation between the ferrite rate (%) and the yield strength (MPa) of the duplex stainless steel within the range of the above-mentioned chemical composition according to the present invention. With reference to Figure 2, the yield strength of the duplex stainless steel increases as the ferrite rate increases. Then, if the ferrite rate is equal to or more than 50%, the yield strength is equal to or more than 550 MPa. "○" marks in Figure 2 fall within the range of the present invention, and "●" marks fall outside of the range of the present invention.

(h) The ferrite rate of the duplex stainless steel is related to a ferrite forming element in the steel, an austenite forming element in the steel, and a solution treatment temperature (°C). Specifically, if Expression (3) is satisfied, the ferrite rate of the duplex stainless steel is equal to or more than 50%: $0.6 X - Y - 5.2 + (T - 1, 070) \times 0.007 \geq 0$

where X is defined by Expression (4), and Y is defined by Expression (5): $X = Cr + 1.5 Si + Mo$
$Y = Ni + 0.5 Mn + 30 C + 30 N$

where the content (mass percent) of the corresponding element in the steel is substituted into the symbol of each element in Expression (4) and Expression (5).

X means a Cr equivalent, and is formed of a ferrite forming element. X represents the contribution ratio to ferrite production in the steel. Y means a Ni equivalent, and is formed of an austenite forming element. Y represents the contribution ratio to austenite phase production in the steel.
It is defined that F3 = 0.6X - Y - 5.2 + (T - 1,070) x 0.007. Figure 3 is a graph showing a relation between F3 and the ferrite rate. Figure 3 was obtained according to the following method. A plurality of duplex stainless steels within the range of the chemical composition according to the present invention were produced at various solution treatment temperatures. The graph of Figure 3 was obtained on the basis of the chemical compositions of the produced duplex stainless steels.
With reference to Figure 3, "○" marks fall within the range of the present invention, and "●" marks fall outside of the range of the present invention. If the F3 value is equal to or more than 0, the ferrite rate is equal to or more than 50%. Accordingly, if the duplex stainless steel having the above-mentioned chemical composition satisfies Expression (3), the ferrite rate is equal to or more than 50%. As a result, as shown in Figure 2, the yield strength of the duplex stainless steel is equal to or more than 550 MPa.
On the basis of the above findings, the duplex stainless steel according to the present invention is completed. Hereinafter, the duplex stainless steel according to the present invention is described.

[Chemical Composition]

The duplex stainless steel according to the present invention has the following chemical composition.

C: 0.030% or less

Carbon (C) stabilizes austenite. That is, C is an austenite forming element. Meanwhile, if C is excessively contained, carbides more easily precipitate, and the corrosion resistance decreases. Accordingly, the C content is equal to or less than 0.030%. The upper limit of the C content is preferably 0.025% and more preferably 0.020%.

Si: 0.20 to 1.00%

Silicon (Si) suppresses a decrease in the flowability of molten metal at the time of welding, and suppresses the occurrence of a weld defect. Moreover, Si is a ferrite forming element. Meanwhile, if Si is excessively contained, an intermetallic compound typified by the σ phase is more easily produced. Accordingly, the Si content is 0.20 to 1.00%. The upper limit of the Si content is preferably 0.80% and more preferably 0.65%. The lower limit of the Si content is preferably 0.30% and more preferably 0.35%.

Mn: 8.00% or less

Manganese (Mn) desulfurizes and deoxidizes the steel, and increases the hot workability of the steel. Moreover, Mn increases the solubility of nitrogen (N). Moreover, Mn is an austenite forming element. Meanwhile, if Mn is excessively contained, the corrosion resistance decreases. Accordingly, the Mn content is equal to or less than 8.00%. The upper limit of the Mn content is preferably 7.50% and more preferably 5.00%. The lower limit of the Mn content is preferably 0.03% and more preferably 0.05%.

P: 0.040% or less

Phosphorus (P) is an impurity. P decreases the corrosion resistance and toughness of the steel. Accordingly, it is preferable that the P content be low. The P content is equal to or less than 0.040%. The P content is preferably equal to or less than 0.030% and more preferably equal to or less than 0.020%.

S: 0.0100% or less

Sulfur (S) is an impurity. S decreases the hot workability of the steel. Moreover, S forms sulfides. The sulfides become pitting occurrence origins, and thus decrease the pitting resistance of the steel. Accordingly, it is preferable that the S content be low. The S content is equal to or less than 0.0100%. The S content is preferably equal to or less than 0.0050% and more preferably equal to or less than 0.0010%.

Cu: more than 2.00% and equal to or less than 4.00%

Copper (Cu) strengthens a passivation film, and increases the corrosion resistance including the SCC resistance, under high-temperature chloride environments. Moreover, Cu extremely finely precipitates in the base material at the time of high heat input welding, and suppresses the precipitation of the σ phase at the ferrite/austenite phase boundary. If the Cu content is more than 2.00%, an excellent corrosion resistance is obtained, and the precipitation of the σ phase is suppressed. Meanwhile, if Cu is excessively contained, the hot workability of the steel decreases. Accordingly, the Cu content is more than 2.00% and equal to or less than 4.00%. The lower limit of the Cu content is preferably 2.20% and more preferably 2.40%.

Ni: 4.00 to 8.00%

Nickel (Ni) stabilizes austenite. That is, Nickel is an austenite forming element. Moreover, Ni increases the toughness of the steel, and increases the corrosion resistance including the SCC resistance of the steel. Meanwhile, if Ni is excessively contained, an intermetallic compound typified by the σ phase is more easily produced. Accordingly, the Ni content is 4.00 to 8.00%. The lower limit of the Ni content is preferably 4.20% and more preferably 4.50%. The upper limit of the Ni content is preferably 7.00% and more preferably 6.00%.

Cr: 20.0 to 28.0%

Chromium (Cr) increases the corrosion resistance of the steel, and particularly increases the SCC resistance of the steel under high-temperature chloride environments. Moreover, Cr is a ferrite forming element. Meanwhile, if Cr is excessively contained, an intermetallic compound typified by the σ phase is produced. Hence, the weldability of the steel decreases, and the hot workability thereof decreases. Accordingly, the Cr content is 20.0 to 28.0%. The lower limit of the Cr content is preferably 22.0% and more preferably 24.0%. The upper limit of the Cr content is preferably 27.5% and more preferably 27.0%.

Mo: 0.50 to 2.00%

Molybdenum (Mo) increases the SCC resistance of the steel. Moreover, Mo is a ferrite forming element. Meanwhile, if Mo is excessively contained, an intermetallic compound typified by the σ phase is produced. Hence, the weldability of the steel decreases, and the hot workability thereof decreases. Accordingly, the Mo content is 0.50 to 2.00%. The lower limit of the Mo content is preferably 0.80% and more preferably 1.00%.

N: 0.100 to 0.350%

Nitrogen (N) is a strong austenite forming element, and increases the thermal stability and corrosion resistance of the steel. The duplex stainless steel according to the present invention contains Cr and Mo that are ferrite forming elements. If the balance of the amount of ferrite and the amount of austenite in the duplex stainless steel is taken into consideration, the N content is equal to or more than 0.100%. Meanwhile, if N is excessively contained, blow holes that are weld defects occur. If N is excessively contained, moreover, nitrides are more easily produced at the time of welding, and the toughness and corrosion resistance of the steel decrease. Accordingly, the N content is 0.100 to 0.350%. Moreover, the lower limit of the N content is preferably 0.120% and more preferably 0.150%. Moreover, the upper limit of the N content is preferably 0.330% and more preferably 0.300%.

Sol. Al: 0.040% or less

Aluminum (Al) deoxidizes the steel. Meanwhile, if Al is excessively contained, aluminum nitride (AlN) is formed, and the toughness and corrosion resistance of the steel decrease. Accordingly, the Al content is equal to or less than 0.040%. The Al content herein means the content of acid-soluble Al (sol. Al).
The lower limit of the Al content is preferably 0.003% and more preferably 0.005%. The upper limit of the Al content is preferably 0.035% and more preferably 0.030%.
The balance of the duplex stainless steel according to the present invention consists of Fe and impurities. The impurities in this context mean elements mixed in for ores and scraps used as raw materials for the steel or various factors in a production process. Note that tungsten (W) is an impurity in the present invention. Specifically, the W content is equal to or less than 0.1%.

[With regard to Expression (1) and Expression (2)]

Moreover, the chemical composition of the duplex stainless steel according to the present invention satisfies Expression (1) and Expression (2): $2.2 Cr + 7 Mo + 3 Cu > 66$
$Cr + 11 Mo + 10 Ni < 12 (Cu + 30 N)$

where the content (mass percent) of each element in the steel is substituted into the symbol of each element in Expression (1) and Expression (2).

[With regard to Expression (1)]

In the duplex stainless steel according to the present invention, the Cr content and the Mo content are restricted in order to suppress the precipitation of the σ phase. Accordingly, it is preferable that a proper amount of Cu be contained, in order to strengthen a passivation film.
It is defined that F1 = 2.2Cr + 7Mo + 3Cu. In the case where F1 is equal to or less than 66, the SCC resistance is low under high-temperature chloride environments. If F1 is more than 66, a sufficiently excellent SCC resistance can be obtained even under high-temperature chloride environments.

[With regard to Expression (2)]

As described above, "Cr + 11mo + 10Ni" in Expression (2) represents the magnitude of the precipitation driving force of the σ phase. "12(Cu + 30N)" therein represents the magnitude of the precipitation inhibiting force of the σ phase.
It is defined that F2 = Cr + 11mo + 10Ni - 12(Cu + 30N). In the case where F2 is less than 0, that is, in the case where Expression (2) is satisfied, the precipitation inhibiting force of the σ phase is larger than the precipitation driving force of the σ phase. Hence, it is possible to sufficiently suppress the σ phase from precipitating at the ferrite/austenite phase boundary at the time of high heat input welding.

[With regard to Selective Element]

The chemical composition of the duplex stainless steel according to the present invention may contain, instead of Fe, one or more types of element selected from at least one group of the following first group to third group. That is, the elements in the first group to the third group are selective elements that can be contained as needed.

First group: V: 1.50% or less
Second group: Ca: 0.0200% or less, Mg: 0.020% or less, and B: 0.0200% or less
Third group: rare earth metal (REM): 0.2000% or less

[First Group]

V: 1.50% or less

Vanadium (V) is a selective element. V increases the corrosion resistance of the duplex stainless steel, and particularly increases the corrosion resistance under acid environments. More specifically, if V is contained together with Mo and Cu, the crevice corrosion resistance of the steel increases. Meanwhile, if V is excessively contained, the amount of ferrite in the steel excessively increases, and the corrosion resistance of the steel decreases. Accordingly, the V content is equal to or less than 1.50%, and preferably less than 1.50%. If the V content is equal to or more than 0.05%, the above-mentioned effect can be remarkably obtained. However, even if the V content is less than 0.05%, the above-mentioned effect can be obtained to some extent. The upper limit of the V content is preferably 0.50% and more preferably 0.10%.

[Second Group]

Ca: 0.0200% or less

Mg: 0.020% or less

B: 0.0200% or less

Calcium (Ca), magnesium (Mg), and boron (B) are selective elements. Ca, Mg, and B immobilize S and ○ (oxygen) in the steel, and increase the hot workability of the steel. The S content of the duplex stainless steel according to the present invention is low. Accordingly, even if Ca, Mg, and B are not contained, the hot workability of the steel is high. However, for example, in the case where a seamless steel pipe is produced according to a skew rolling method, a higher hot workability may be required. If one or more types selected from the group consisting of Ca, Mg, and B are contained, a higher hot workability can be obtained.
Meanwhile, if one or more types of Ca, Mg, and V are excessively contained, non-metallic inclusions (such as oxides and sulfides of Ca, Mg, and B) increase. The non-metallic inclusions become pitting origins, and thus decrease the corrosion resistance of the steel. Accordingly, the Ca content is equal to or less than 0.0200%, the Mg content is equal to or less than 0.020%, and the B content is equal to or less than 0.0200%.
In order to remarkably obtain the above-mentioned effect, it is preferable that the content of at least one type of Ca, Mg, and B or the total content of two or more types thereof be equal to or more than S (mass percent) + 1 / 2 × ○ (mass percent). However, if one or more types of Ca, Mg, and B are contained even a little, the above-mentioned effect can be obtained to some extent.
In the case where two types of Ca, Mg, and B are contained, the total content of these elements is equal to or less than 0.04%. In the case where all of Ca, Mg, and B are contained, the total content of these elements is equal to or less than 0.06%.

[Third Group]

Rare earth metal (REM): 0.2000% or less

Rare earth metal (REM) is a selective element. Similarly to Ca, Mg, and B, REM immobilizes S and O (oxygen) in the steel, and increases the hot workability of the steel. Meanwhile, if REM is excessively contained, non-metallic inclusions (such as oxides and sulfides of rare earth metal) increase, and the corrosion resistance of the steel decreases. Accordingly, the REM content is equal to or less than 0.2000%. In order to remarkably obtain the above-mentioned effect, it is preferable that the REM content be equal to or more than S (mass percent) + 1 / 2 × ○ (mass percent). However, if REM is contained even a little, the above-mentioned effect can be obtained to some extent.
REM is a collective term including 15 elements of lanthanoid, Y, and Sc. One or more types of these elements are contained. The REM content means the total content of one or more types of these elements.

[Structure]

The structure of the duplex stainless steel according to the present invention includes ferrite and austenite, and the balance thereof consists of precipitates and inclusions.
In the structure of the duplex stainless steel according to the present invention, the ferrite rate is equal to or more than 50%. Note that the ferrite rate refers to the ferrite area fraction, and is measured according to the following method. A sample is collected from a given portion of the duplex stainless steel. The collected sample is mechanically polished, and then the polished sample is electrolytically etched in a 10% oxalic acid solution. The electrolytically etched sample is further electrolytically etched in a 10% KOH solution. An image of the electrolytically etched sample surface is analyzed using an optical microscope, and the ferrite rate is obtained.
If the ferrite rate is equal to or more than 50%, as shown in Figure 2, a strength of 550 MPa or more (80 ksi or more) is obtained.

[Production Method]

The duplex stainless steel having the above-mentioned chemical composition is molten. The duplex stainless steel may be molten using an electric furnace, and may be molten using an Ar-O₂ gaseous mixture bottom blowing decarburization furnace (AOD furnace). Alternatively, the duplex stainless steel may be molten using a vacuum decarburization furnace (VOD furnace). The molten duplex stainless steel may be formed into an ingot according to an ingot-making process, and may be formed into a cast piece (a slab, a bloom, or a billet) according to a continuous casting process.
The duplex stainless steel material is produced using the produced ingot or cast piece. Examples of the duplex stainless steel material include a duplex stainless steel plate and a duplex stainless steel pipe.
The duplex stainless steel plate is produced according to, for example, the following method. Hot working is performed on the produced ingot or slab, whereby the duplex stainless steel plate is produced. Examples of the hot working include hot forging and hot rolling.
The duplex stainless steel pipe is produced according to, for example, the following method. Hot working is performed on the produced ingot, slab, or bloom, whereby a billet is produced. Hot working is performed on the produced billet, whereby a duplex stainless steel pipe is produced. Examples of the hot working include piercing-rolling according to a Mannesmann process. Hot extrusion may be performed as the hot working, and hot forging may be performed thereas. The produced duplex stainless steel pipe may be a seamless pipe, and may be a welded steel pipe.
In the case where the duplex stainless steel pipe is a welded steel pipe, for example, bending work is performed on the above-mentioned duplex stainless steel pipe, to be thereby formed into an open pipe. Both the end faces in the longitudinal direction of the open pipe are welded according to a well-known welding method such as submerged arc welding, whereby the welded steel pipe is produced.
Solution treatment is performed on the produced duplex stainless steel material. Specifically, the duplex stainless steel material is housed in a heat treatment furnace, and is soaked at a solution treatment temperature (°C). After the soaking, the duplex stainless steel is rapidly cooled by water-cooling or the like.
A solution treatment temperature T (°C) satisfies Expression (3): $0.6 X - Y - 5.2 + (T - 1, 070) \times 0.007 \geq 0$

where X is defined by Expression (4), and Y is defined by Expression (5). $X = Cr + 1.5 Si + Mo$
$Y = Ni + 0.5 Mn + 30 C + 30 N$

where the content (mass percent) of the corresponding element in the steel is substituted into the symbol of each element in Expression (4) and Expression (5).
It is defined that F3 = 0.6X - Y - 5.2 + (T - 1,070) × 0.007. If the F3 value is equal to or more than 0, that is, if Expression (3) is satisfied, the ferrite rate of the duplex stainless steel material is equal to or more than 50%. Hence, the yield strength of the duplex stainless steel is equal to or more than 550 MPa (80 ksi).
The soaking time in the solution treatment is preferably 2 to 60 minutes.
The duplex stainless steel material according to the present invention remains in a solution state (so-called as-solution-treated material). That is, after the solution treatment, the duplex stainless steel material is used as a product without performing thereon other heat treatment and other cold working (cold drawing and Pilger rolling) than cold straightening.

Example

Duplex stainless steels having various chemical compositions were molten using a vacuum furnace having a capacity of 150 kg. A plurality of duplex stainless steel plates were produced using the molten duplex stainless steels according to various production conditions. The ferrite rate, the yield strength, the SCC resistance, and whether or not the σ phase precipitated at the time of high heat input welding were examined using the produced steel plates.

[Examination Method]

Duplex stainless steels having chemical compositions of the steel A to the steel Z shown in Table 1 were molten.
[Table 1]
The contents (mass percents) of the corresponding elements in the steel with each steel symbol (the steel A to the steel Z) are shown in the chemical composition section in Table 1. The balance (components other than the elements shown in Table 1) in the chemical composition of each steel symbol consists of Fe and impurities. "-" in Table 1 represents that the content of the corresponding element is in an impurity level. Selective elements other than W contained in the corresponding steel are shown in the "Others" section in Table 1. For example, ".023B-.0026Ca" represents that the B content is 0.023% and that the Ca content is 0.0026%.
The chemical compositions of the steel A to the steel L, the steel O, and the steel T fell within the range of the chemical composition according to the present invention. Meanwhile, any element(s) in the chemical compositions of the steel M, the steel N, the steel P to the steel S, and the steel U to the steel Y fell outside of the range of the present invention.
The molten duplex stainless steels were cast, whereby ingots were produced. The produced ingots were each heated to 1,250°C. Hot forging was performed on the heated ingots, whereby plate materials were produced. The produced plate materials were heated again to 1,250°C. Hot rolling was performed on the heated plate materials, whereby steel plates each having a thickness of 15 mm were produced. The surface temperature of each steel material at the time of the rolling was 1,050°C. Solution treatment was performed on the produced steel plates. The solution treatment temperature was 1,070°C to 1,200°C, and the soaking time was 30 minutes. After the soaking, the steel plates were water-cooled to reach a normal temperature (25°C), whereby materials under test with the test numbers 1 to 32 were produced.

[Making of Specimens]

Two plate materials 10 illustrated in Figure 4A and Figure 4B were made from each material under test. Figure 4A is a plan view of the plate material 10, and Figure 4B is a front view thereof. In Figure 4A and Figure 4B, numerical values with "mm" represent dimensions (the unit is millimeter).
As illustrated in Figure 4A and Figure 4B, the plate material 10 had a thickness of 12 mm, a width of 100 mm, and a length of 200 mm. Moreover, the plate material had a V-type groove surface 11 on its longer side, and the V-type groove surface 11 had a groove angle of 30°. The plate material 10 was made by machine processing.
The V-type groove surfaces 11 of the two made plate materials 10 were placed so as to be opposed to each other. The two plate materials 10 were welded according to tungsten inert gas welding, whereby a welded joint 20 illustrated in Figure 5A and Figure 5B was made. Figure 5A is a plan view of the welded joint 20, and Figure 5B is a front view thereof. The welded joint 20 had a front surface 21 and a back surface 22, and included a weld zone 30 in its center. The weld zone 30 was formed from the front surface 21 side according to multi-layer welding, and extended in the longer-side direction of the plate materials 10. All the weld zones 30 with their respective test numbers were formed using a weld material having the same chemical composition as that of the steel A and having an outer diameter of 2 mm. The heat input in the tungsten inert gas welding was 30 kJ/cm.
A plate-shaped specimen 40 including the weld zone 30 was collected from the back surface 22 side of the welded joint 20. A broken line portion of the welded joint 20 in Figure 5B shows a portion from which the specimen 40 was collected. Figure 6 is a perspective view of the collected specimen. In Figure 6, numerical values with "mm" represent dimensions (the unit is millimeter). With reference to Figure 6, the specimen 40 had a plate-like shape. An upper surface 41 of the specimen 40 corresponded to the back surface 22 of the welded joint (see Figure 5). The longitudinal direction of the specimen 40 was orthogonal to the longitudinal direction of the weld zone 30. As illustrated in Figure 6, one of two boundary lines 30B between the weld zone 30 and the plate materials 10 was placed in the center of the specimen 40.

[SCC Test]

A four-point bending test was performed using the specimen 40, and the SCC resistance of each material under test was evaluated. An actual yield stress (the yield stress of each material under test) in conformity to ASTM G39 was applied to the specimen 40 using a four-point bending jig. The specimen 40 to which the stress was applied was immersed in a 25%-NaCl aqueous solution (150°C) into which CO₂ was injected at 3 MPa, and the immersed specimen 40 was held for 720 hours without any change. After the elapse of 720 hours, whether or not SCC occurred on a surface of the specimen 40 was visually observed. Moreover, the specimen 40 was cut in a direction perpendicular to the upper surface 41. The cross-section of the specimen 40 was observed using a 500x optical microscope, and whether or not SCC occurred was determined.

[Area Fraction Measurement Test of σ Phase]

The welded joint 20 with each test number was cut in a direction perpendicular to the weld line and the front surface 21 thereof. After the cutting, the cross-section of the welded joint 20 was mirror-polished and etched. After the etching, a welding heat affected zone (HAZ; a portion near the weld zone) of the etched cross-section was selected for four visual fields, and an image in each visual field was analyzed, using an optical microscope with x500 field. The area of each visual field used for the image analysis was about 40,000 µm². The area fraction (%) of the σ phase for each visual field (HAZ) was obtained through the image analysis. The average of the area fractions (%) obtained for the four visual fields was defined as the area fraction (%) of the σ phase in the HAZ for each test number. In the case where the area fraction of the σ phase was equal to or more than 0.5%, it was determined that the σ phase precipitated. In the case where the area fraction of the σ phase was less than 0.5%, it was determined that the σ phase did not precipitate.

[Tension Test]

A round bar tensile specimen was collected from each material under test. The round bar tensile specimen had an outer diameter of 6.35 mm and a parallel part length of 25.4 mm. The parallel part thereof extended in the rolling direction of the material under test. A tensile test was performed on the collected round bar specimen at a normal temperature. An offset yield stress of 0.2% based on ASTM A370 was defined as the yield strength (YS).

[Measurement of Ferrite Rate]

The ferrite rate of each material under test was obtained according to the following method. A specimen for structure observation was collected from each material under test. The collected specimen was mechanically polished. The polished specimen was electrolytically etched in a 10% oxalic acid solution. The electrolytically etched specimen was further electrolytically etched in a 10% KOH solution. The etched sample surface was selected for four visual fields, and an image in each visual field was analyzed, using an optical microscope (500x). At this time, the area of the observed region was about 40,000 µm². The ferrite rate (%) in the observed region was obtained.

[Test Results]

The test results are shown in Table 1. The X value that is obtained for each test number according to Expression (4) is inputted to the "X" section in Table 1. The Y value that is obtained for each test number according to Expression (5) is inputted to the "Y" section. A solution treatment temperature (°C) is inputted to the "Solution Treatment Temperature" section. A ferrite rate (%) is inputted to the "Ferrite Rate" section. A yield strength (MPa) is inputted to the "YS (MPa)" section. A yield strength (ksi) is inputted to the "YS (ksi)" section. The F3 value (the left side of Expression (3)) is inputted to the "F3" section. The F1 value (F1 = 2.2Cr + 7Mo + 3Cu) is inputted to the "F1" section. The F2 value (F2 = Cr + 11mo + 10Ni - 12(Cu + 30N)) is inputted to the "F2" section. "Not Found" in the "SCC" section represents that SCC was not observed in the material under test with the corresponding test number. "Found" therein represents that SCC was observed in the material under test with the corresponding test number. "Not Found" in the "σ Phase" section represents that the area fraction of the σ phase was less than 0.5%. "Found" therein represents that the area fraction of the σ phase was equal to or more than 0.5%.
With reference to Table 1, the chemical compositions of materials under test with test numbers 1 to 8 fell within the range of the present invention. Moreover, the materials under test with the test numbers 1 to 8 satisfied Expression (1) and Expression (2). Hence, SCC was not observed in the materials under test with the test numbers 1 to 8, and the σ phase did not occur therein. Moreover, the materials under test with the test numbers 1 to 8 satisfied Expression (3). Hence, the ferrite rates of the materials under test with the test numbers 1 to 8 were equal to or more than 50%, and the yield strengths thereof were equal to or more than 550 MPa.
The chemical compositions of materials under test with test numbers 9 to 18 fell within the range of the present invention. Moreover, the materials under test with the test numbers 9 to 18 satisfied Expression (1) and Expression (2). However, the materials under test with the test numbers 9 to 18 did not satisfy Expression (3). Hence, the ferrite rates of the materials under test with the test numbers 9 to 18 were less than 50%, and the yield strengths thereof were less than 550 MPa.
The Cr content of a material under test with a test number 19 was less than the lower limit of the Cr content according to the present invention. Hence, SCC occurred in the material under test with the test number 19. The N content of a material under test with a test number 20 was less than the lower limit of the N content according to the present invention. Then, the material under test with the test number 20 did not satisfy Expression (1) and Expression (2). Hence, the σ phase occurred in a HAZ of the material under test with the test number 20, and SCC occurred in the material under test with the test number 20.
The chemical composition of a material under test with a test number 21 fell within the range of the present invention. However, the material under test with the test number 21 did not satisfy Expression (1). Hence, SCC occurred in the material under test with the test number 21.
The Ni content of a material under test with a test number 22 was less than the lower limit of the Ni content according to the present invention. Moreover, the test number 22 did not satisfy Expression (1). Hence, SCC occurred in the material under test with the test number 22. The C content of a material under test with a test number 23 was more than the upper limit of the C content according to the present invention, and the Ni content thereof was less than the lower limit of the Ni content according to the present invention. Moreover, the material under test with the test number 23 did not satisfy Expression (1). Hence, SCC occurred in the material under test with the test number 23.
The Cu contents of materials under test with test numbers 24 and 25 were less than the lower limit of the Cu content according to the present invention. Hence, SCC occurred in the materials under test with the test numbers 24 and 25.
The chemical composition of a material under test with a test number 26 fell within the range of the present invention. However, the material under test with the test number 26 did not satisfy Expression (1) and Expression (2). Hence, the σ phase occurred in the material under test with the test number 26, and SCC occurred therein.
The Cu content of a material under test with a test number 27 was less than the lower limit of the Cu content according to the present invention, and the Mo content thereof was more than the upper limit of the Mo content according to the present invention. Hence, SCC occurred in the material under test with the test number 27, and the σ phase occurred therein.
The Mo contents of materials under test with test numbers 28, 29, and 31 were less than the lower limit of the Mo content according to the present invention. Hence, SCC occurred in the materials under test 28, 29, and 31.
The Cu content of a material under test with a test number 30 was less than the lower limit of the Cu content according to the present invention. Hence, SCC occurred in the material under test with the test number 30, and the σ phase occurred therein.
The chemical composition of the material under test with the test number 32 fell within the range of the present invention, and satisfied Expression (1). However, the material under test with the test number 32 did not satisfy Expression (2). Hence, the σ phase occurred for the material under test with the test number 32.
Hereinabove, the embodiment of the present invention has been described, and the above-mentioned embodiment is given as a mere example for carrying out the present invention. Accordingly, the present invention is not limited to the above-mentioned embodiment, and can be carried out by appropriately modifying the above-mentioned embodiment within a range not departing from the gist thereof.

Industrial Applicability

A duplex stainless steel according to the present invention can be widely applied to environments that are required to have a SCC resistance. In particular, a duplex stainless steel according to the present invention is applicable as a steel material for a line pipe provided under chloride environments.

Claims

A duplex stainless steel comprising:
a chemical composition containing, in mass percent, C: at most 0.030%, Si: 0.20 to 1.00%, Mn: at most 8.00%, P: at most 0.040%, S: at most 0.0100%, Cu: more than 2.00% and at most 4.00%, Ni: 4.00 to 8.00%, Cr: 20.0 to 28.0%, Mo: 0.50 to 2.00%, N: 0.100 to 0.350%, and sol. Al: at most 0.040%, the balance being Fe and impurities, and satisfying Expression (1) and Expression (2);

a structure having a ferrite rate of at least 50%; and

a yield strength of at least 550 MPa: $2.2 Cr + 7 Mo + 3 Cu > 66$
$Cr + 11 Mo + 10 Ni < 12 (Cu + 30 N)$

where a content (mass percent) of each element in the steel is substituted into a symbol of each element in Expression (1) and Expression (2).
The duplex stainless steel according to claim 1, further satisfying Expression (3): $0.6 X - Y - 5.2 + (T - 1, 070) \times 0.007 \geq 0$

where a solution treatment temperature (°C) is substituted into T in Expression (3), X is defined by Expression (4), and Y is defined by Expression (5): $X = Cr + 1.5 Si + Mo$
$Y = Ni + 0.5 Mn + 30 C + 30 N$

where a content (mass percent) of a corresponding element in the steel is substituted into a symbol of each element in Expression (4) and Expression (5).
The duplex stainless steel according to claim 1 or claim 2, wherein
the chemical composition contains V: at most 1.50%, instead of part of the Fe.
The duplex stainless steel according to any one of claim 1 to claim 3, wherein
the chemical composition contains at least one type selected from the group consisting of Ca: at most 0.0200%, Mg: at most 0.020%, and B: at most 0.0200%, instead of part of the Fe.
The duplex stainless steel according to any one of claim 1 to claim 4, wherein
the chemical composition contains rare earth metal: at most 0.2000%, instead of part of the Fe.