WO2011030709A1 - Two-phase stainless steel - Google Patents

Abstract

Disclosed is a two-phase stainless steel which has outstanding weldability during high heat input welding and excellent stress corrosion cracking resistance in a chloride environment containing corrosive gas. The two-phase stainless steel satisfies the relationship between formula (1) and formula (2) and has a chemical composition which comprises C: 0.03 mass% or less, Si: 0.2 mass% to 1 mass%, Mn: 5.0 mass% or less, P: 0.040 mass% or less, S: 0.010 mass% or less, sol. Al: 0.040 mass% or less, Ni: 4 mass% to 8 mass%, Cr: 20 mass% to 28 mass%, Mo: 0.5 mass% to 2.0 mass%, Cu: over 2.0 mass% but no more than 4.0 mass%, N: 0.1 mass% to 0.35 mass%, with the remainder being Fe and impurities. In addition, the two-phase stainless steel may contain one or more of V, Ca, Mg, B, and rare earth elements instead of some of the Fe. In formulae (1) and (2), each element symbol represents the content of each element in the steel (unit: mass%). (1) 2.2Cr + 7Mo + 3Cu > 66 (2) Cr + 11Mo + 10Ni < 12 (Cu + 30N)

Description

Duplex stainless steel

The present invention relates to a ferritic / austenitic duplex stainless steel excellent in stress corrosion cracking resistance, and more particularly to a duplex stainless steel suitable as a steel material for line pipes for transporting oil / natural gas.

In oil and natural gas produced from oil and gas fields, corrosive gases such as carbon dioxide (CO ₂ ) and hydrogen sulfide (H ₂ S) are present as companion gases. In line pipes that transport highly corrosive oil and natural gas, stress corrosion cracking (SCC), sulfide stress cracking (SSC), and the entire surface that causes a reduction in wall thickness Corrosion becomes a problem. In particular, stress corrosion cracking (SCC) and sulfide stress cracking (SSC) are more serious problems because the speed of progress is high and the time until the crack penetrates the line pipe is short and occurs locally. . Therefore, excellent corrosion resistance is required for steel materials for line pipes as described above.

As a steel material having excellent corrosion resistance, so-called duplex stainless steel composed of a ferrite and austenite phase has been conventionally used. For example, Patent Document 1 describes a duplex stainless steel containing 1 to 3% of Cu and having improved corrosion resistance in a chloride or sulfide environment. Patent Document 2 describes the strength, toughness, and the like by appropriately adjusting the contents of Cr, Ni, Cu, Mo, N and W and controlling the area ratio of the ferrite phase to 40% to 70%. A duplex stainless steel with improved seawater resistance is described.

International Publication No. 96/18751 JP 2003-171743 A

By the way, in the duplex stainless steel described in Patent Document 1, the corrosion resistance of the welded portion tends to deteriorate during high heat input welding. In addition, in the duplex stainless steel described in Patent Document 2, intermetallic compounds are precipitated in the welded part at the time of high heat input welding, so that the welded part is likely to be embrittled and deteriorated in corrosion resistance. Assuming natural gas transportation, the stress corrosion cracking resistance in a chloride environment containing corrosive accompanying gases such as carbon dioxide and hydrogen sulfide is insufficient.

The present invention has been made in order to solve the above-mentioned problems, and is excellent in weldability at the time of large heat input welding and excellent in stress corrosion cracking resistance in a chloride environment containing corrosive gas. It aims to provide stainless steel.

The present inventors conducted various experiments and detailed studies on duplex stainless steels in order to improve the weldability during high heat input welding and the stress corrosion cracking resistance in chloride environments. Piled up. As a result, the following findings (a) to (f) were obtained.

(A) The stress corrosion cracking resistance of the duplex stainless steel can be improved by strengthening a passive film mainly composed of Cr with Mo. On the other hand, in order to prevent precipitation of intermetallic compounds during high heat input welding, it is necessary to regulate the contents of Cr and Mo. However, in a high-temperature chloride environment containing carbon dioxide or hydrogen sulfide, if the Cr and Mo contents are reduced, excellent stress corrosion cracking resistance cannot be obtained in the vicinity of the weld.

(B) In order to improve the stress corrosion cracking resistance while regulating the contents of Cr and Mo, it is sufficient that the passive film containing Cr as a main component can be strengthened by an element different from Mo. Here, Cu is an element having an action of reducing the corrosion rate of the steel material in an acidic environment. Therefore, by adding an appropriate amount of Cu in addition to Cr and Mo, the passive film can be stabilized and the passive film can be strengthened.

FIG. 4 shows the amount (mass%) of “Cr” on the X axis and the quantity (mass%) of “7Mo + 3Cu” on the Y axis for the duplex stainless steels having various chemical compositions used in the examples described later. ) Is plotted. With a straight line of 7Mo + 3Cu = −2.2Cr + 66 as a boundary, it can be classified into “determination that there is no stress corrosion cracking (◯)” on the upper right side and “determination that there is stress corrosion cracking (×)” on the lower left side.

Therefore, it is derived that the passive film can be strengthened by adding Cr, Mo and Cu so as to satisfy the relationship of the following formula (1).
2.2Cr + 7Mo + 3Cu> 66 (1)
However, each element symbol in Formula (1) represents content (unit: mass%) in each steel of each element.

In addition, when the Cu content is 2% by mass or less, sufficient corrosion resistance cannot be obtained. Therefore, Cu needs to be contained exceeding 2%.

(C) When welding duplex stainless steel, the structure in the vicinity of the weld is heated and cooled in a short time. In order to prevent precipitation of an intermetallic compound (sigma phase) in a structure heated and cooled in such a short time, it is important to suppress sigma phase nucleation and growth.

(D) The driving force for sigma phase nucleation increases with increasing Ni content. Therefore, when only considering the suppression of the generation of the sigma phase, Ni should not be contained. However, when Ni is not contained, the ratio of the ferrite phase to the austenite phase greatly deviates from 1: 1, and the toughness and corrosion resistance decrease. Therefore, in order to suppress the formation of the sigma phase while preventing toughness and corrosion resistance from being lowered, an appropriate amount of Ni must be contained according to the contents of Cu and N. Specifically, by containing Ni so as to satisfy the relationship of the following formula (2), the generation of the sigma phase can be suppressed without reducing toughness and corrosion resistance.
Cr + 11Mo + 10Ni <12 (Cu + 30N) (2)
However, each element symbol in Formula (2) represents the content (unit: mass%) of each element in steel.

Here, the left side of the formula (2) represents the driving force for precipitation of sigma phase, and among the components constituting the duplex stainless steel, Cr, Mo and Ni have the driving force for nucleation of precipitation of sigma phase. Various elements have been found that Mo and Ni are 11 times and 10 times as much as Cr, respectively, with respect to Cr.

On the other hand, the right side of equation (2), on the other hand, represents the sigma phase precipitation deterring force, and its contribution is N 30 times that of Cu, and the driving force of Cr is less than that of Cu. It was found by various tests that the deterrent power is 12 times.

The expression mechanism of the deterring power of sigma phase precipitation by Cu and N is as follows. The presence of Cu or N atoms in the vicinity of the Ni atoms present in the crystal lattice suppresses the reduction of the interfacial energy at the ferrite / austenite phase interface, which is the nucleation site of the sigma phase, so during the sigma phase precipitation reaction This is because the amount of decrease in the free energy is reduced and the driving force for crystal nucleation can be reduced. In addition, since Cu precipitates very finely in the matrix as a Cu-concentrated phase, many sigma phase nucleation sites are dispersed and compete with the ferrite / austenite phase interface, which is the original nucleation site. This has the effect of delaying sigma phase formation at the fast-growing ferrite / austenite phase boundary.

(E) It is possible to arrange Cu atoms and N atoms in the vicinity of Ni atoms present in the crystal lattice by containing an appropriate amount of Ni that satisfies the relationship of the above formula (2). In this case, it is possible to suppress a decrease in the interfacial energy at the ferrite phase / austenite phase interface, which is a sigma phase nucleation site. As a result, the amount of decrease in free energy during the precipitation reaction of the sigma phase can be reduced, and the driving force for nucleation of the sigma phase can be reduced. As a result, the generation of sigma phase can be suppressed.

(F) Nuclear growth of the sigma phase can be suppressed by containing an appropriate amount of Cu. Specifically, by containing an appropriate amount of Cu, an extremely fine Cu concentrated phase can be precipitated in the matrix during high heat input welding. Since this Cu enriched phase becomes a nucleation site of the sigma phase, by dispersing and precipitating a large number of Cu enriched phases, the Cu enriched phase is converted into the ferrite phase / austenite phase that is the original nucleation site. Can compete with the interface. As a result, the growth of the sigma phase at the ferrite phase / austenite phase interface can be delayed.

The present invention has been completed based on the above findings, and the gist thereof lies in the following duplex stainless steels (1) to (4).

(1) By mass%, C: 0.03% or less, Si: 0.2-1%, Mn: 5.0% or less, P: 0.040% or less, S: 0.010% or less, sol. Al: 0.040% or less, Ni: 4-8%, Cr: 20-28%, Mo: 0.5-2.0%, Cu: more than 2.0% and 4.0% or less, N: A duplex stainless steel characterized by containing 0.1 to 0.35%, the balance having a chemical composition comprising Fe and impurities, and satisfying the relationship of the following formulas (1) and (2).
2.2Cr + 7Mo + 3Cu> 66 (1)
Cr + 11Mo + 10Ni <12 (Cu + 30N) (2)
However, each element symbol in the formulas (1) and (2) represents the content (unit: mass%) of each element in the steel.

(2) The duplex stainless steel according to (1) above, which contains V: 1.5% or less in mass% instead of part of Fe.

(3) Instead of a part of Fe, by mass%, it contains at least one of Ca: 0.02% or less, Mg: 0.02% or less, B: 0.02% or less The duplex stainless steel according to (1) or (2) above.

(4) The duplex stainless steel according to any one of (1) to (3) above, wherein the rare earth element: 0.2% or less is contained in mass% instead of a part of Fe .

The duplex stainless steel according to the present invention is excellent in weldability during large heat input welding and excellent in stress corrosion cracking resistance in a chloride environment.

It is a figure which shows the board | plate material produced by machining, (a) is a top view, (b) is a front view. It is a figure which shows a welded joint, (a) is a top view, (b) is a front view. It is a perspective view of a test piece. It is a figure which shows the relationship of the chemical composition of the duplex stainless steel which concerns on an Example. ○ indicates “determination that there is no stress corrosion cracking”, and x indicates “determination that there is stress corrosion cracking”.

Hereinafter, the effect of the chemical composition of the duplex stainless steel according to the present invention will be described together with the reason for limiting the content thereof. In addition, "%" regarding content means "mass%".

C: 0.03% or less C is an effective component for stabilizing the austenite phase. However, if the C content exceeds 0.03%, carbides are likely to precipitate, and the corrosion resistance is reduced. Therefore, the C content is 0.03% or less.

Si: 0.2-1%
Since Si can ensure the fluidity of the molten metal during welding, it is an effective component for preventing welding defects. In order to acquire this effect, it is necessary to contain 0.2% or more of Si. On the other hand, when the Si content exceeds 1%, an intermetallic compound (sigma phase or the like) is likely to be generated. Therefore, the Si content is 0.2-1%. A preferable Si content is 0.2 to 0.5%.

Mn: 5.0% or less Mn is an effective component for improving the hot workability by the desulfurization and deoxidation effects at the time of melting duplex stainless steel. Further, Mn has an effect of increasing the solubility of N. However, if the Mn content exceeds 5.0%, the corrosion resistance decreases. Therefore, the Mn content is 5.0% or less.

P: 0.040% or less P is mixed as an impurity in the steel and lowers the corrosion resistance and toughness of the steel. Therefore, the content of P is set to 0.040% or less.

S: 0.010% or less S is mixed as an impurity in the steel and reduces the hot workability of the steel. In addition, the sulfide becomes a starting point of pitting corrosion and reduces the pitting corrosion resistance of the steel. In order to avoid these adverse effects, the S content is set to 0.010% or less. A preferable S content is 0.007% or less.

sol. Al: 0.040% or less Al is an effective component as a deoxidizer for steel. On the other hand, when the amount of N in the steel is large, Al precipitates as AlN (aluminum nitride), which lowers the toughness and corrosion resistance of the steel. Therefore, the Al content is 0.040% or less. The Al content referred to in the present invention refers to the content of acid-soluble Al (so-called sol. Al). Here, in the duplex stainless steel according to the present invention, since the content of Si which is an effective component as a deoxidizer is suppressed, Al is often used as a deoxidizer. However, when producing duplex stainless steel by vacuum melting, Al need not be contained.

Ni: 4-8%
Ni is an effective component for stabilizing austenite. If the Ni content exceeds 8%, it becomes difficult to secure the basic properties of the duplex stainless steel due to the decrease in the ferrite content, and intermetallic compounds (such as sigma phase) are likely to be generated. On the other hand, if the Ni content is less than 4%, the amount of ferrite becomes too large and the characteristics of the duplex stainless steel are lost. Further, since the solid solubility of N in ferrite is small, when the amount of ferrite becomes too large, nitride precipitates and the corrosion resistance decreases. Therefore, the Ni content is 4 to 8%.

Cr: 20 to 28%
Cr is an effective component for maintaining corrosion resistance. In order to obtain the SCC resistance in a chloride environment, it is necessary to contain 20% or more of Cr. On the other hand, if the Cr content exceeds 28%, precipitation of intermetallic compounds (such as sigma phase) becomes prominent, leading to deterioration of hot workability and weldability. Therefore, the Cr content is 20 to 28%.

Mo: 0.5-2.0%
Mo is a very effective component for improving the SCC resistance. In order to acquire this effect, it is necessary to contain Mo 0.5% or more. On the other hand, when the Mo content exceeds 2.0%, the precipitation of intermetallic compounds is remarkably accelerated during high heat input welding, resulting in a decrease in hot workability and weldability. Therefore, the Mo content is 0.5 to 2.0%. A preferable Mo content is 0.7 to 1.8%, and a more preferable Mo content is 0.8 to 1.5%.

Cu: more than 2.0% and 4.0% or less Cu enhances a passive film mainly composed of Cr in a chloride environment containing a corrosive acid gas (carbon dioxide gas, hydrogen sulfide gas, etc.). It is an effective ingredient. Further, Cu precipitates very finely in the matrix during high heat input welding and becomes a nucleation site of an intermetallic compound (sigma phase), and competes with the ferrite / austenite phase interface that is the original nucleation site. As a result, the sigma phase formation at the rapidly growing ferrite / austenite phase interface is delayed. In order to obtain these effects, it is necessary to contain Cu exceeding 2.0%. On the other hand, when Cu is contained exceeding 4.0%, the hot workability of steel is impaired. Therefore, the Cu content is more than 2.0% and 4.0% or less.

N: 0.1 to 0.35%
N is a strong austenite-forming element and is effective in improving the thermal stability and corrosion resistance of the duplex stainless steel. Since the duplex stainless steel according to the present invention contains a large amount of Cr and Mo, which are ferrite forming elements, it is necessary to contain N in an amount of 0.1% or more in order to achieve an appropriate balance between ferrite and austenite. . On the other hand, if the N content exceeds 0.35%, the toughness and corrosion resistance of the steel decrease due to the occurrence of blow holes, which are welding defects, or the formation of nitrides due to the thermal effect during welding. Therefore, the N content is 0.1 to 0.35%.

In addition to the above chemical composition, Cr, Mo, Ni, Cu and N need to satisfy the relationship of the following formulas (1) and (2).
2.2Cr + 7Mo + 3Cu> 66 (1)
Cr + 11Mo + 10Ni <12 (Cu + 30N) (2)
However, each element symbol in the formulas (1) and (2) represents the content (unit: mass%) of each element in the steel.

In the duplex stainless steel according to the present invention, the contents of Cr and Mo are regulated in order to suppress precipitation of intermetallic compounds. Therefore, in order to reinforce the passive film containing Cr as a main component, it is necessary to contain an appropriate amount of Cu separately from Mo. Here, when the value of “2.2Cr + 7Mo + 3Cu” is 66 or less, sufficient resistance to stress corrosion cracking (SCC) in a chloride environment may not be ensured. Therefore, the requirement of the above formula (1) is defined.

When the value of “Cr + 11Mo + 10Ni” is equal to or greater than the value of “12 (Cu + 30N)”, it is not possible to sufficiently suppress the formation of intermetallic compounds at the ferrite / austenite phase boundary during high heat input welding. There is. In consideration of this point, the requirement of the above formula (2) is defined.

The duplex stainless steel according to the present invention has the chemical composition described above, and the balance is Fe and impurities. Here, impurities are components that are mixed due to various factors in the manufacturing process including raw materials such as ore and scrap when industrially producing duplex stainless steel, and have an adverse effect on the present invention. It means what is allowed in the range.

In addition to the above elements, the duplex stainless steel according to the present invention further contains one or more elements selected from at least one of the following first to third groups. May be.

Group 1: V: 1.5% or less Group 2: Ca, Mg, B: 0.02% or less Group 3: Rare earth elements (REM): 0.2% or less Hereinafter, these optional elements will be described in detail. To do.

First group: V: 1.5% or less V can be contained as necessary. V is effective in improving the corrosion resistance (particularly in an acidic environment) of the duplex stainless steel. More specifically, crevice corrosion resistance can be improved by containing V in combination with Mo and Cu. However, if the V content exceeds 1.5%, the amount of ferrite increases excessively and the toughness and corrosion resistance may decrease, so the V content is set to 1.5% or less. In order to stably exhibit the effect of improving the corrosion resistance of the duplex stainless steel by V, it is preferable to contain 0.05% or more of V.

Second group: Ca: not more than 0.02%, Mg: not more than 0.02%, B: not less than 0.02%, one or more selected from Ca, Mg and B is necessary It can be contained according to. Ca, Mg and B have an effect of fixing S (sulfur) or O (oxygen) and improving hot workability, respectively. In the duplex stainless steel according to the present invention, since the S content is specified to be low, the hot workability is good even if Ca, Mg or B is not contained. However, when more hot workability is required under severe processing conditions such as the manufacture of seamless pipes by tilt rolling, duplex stainless steel is obtained by containing one or more of Ca, Mg and B. The hot workability of can be further improved. On the other hand, when the content of these elements exceeds 0.02%, non-metallic inclusions (such as Ca, Mg or B oxides and sulfides) increase, which causes pitting corrosion and lowers corrosion resistance. There is a fear. Therefore, the content of these elements when contained is 0.02% or less. The upper limit of the total content when containing two of Ca, Mg and B is 0.04%, and the upper limit of the total content when containing three of Ca, Mg and B is 0.06. %. In order to stably exhibit the effect of improving hot workability by Ca, Mg or B, it is contained alone or in total “S (mass%) + 1/2 · O (mass%)” or more. preferable.

Group 3: Rare earth elements (REM): 0.2% or less REM can be contained as necessary. Similarly to Ca, Mg and B, the rare earth element also has the effect of fixing S or O and further improving the hot workability of the duplex stainless steel. On the other hand, if the rare earth element content exceeds 0.2%, non-metallic inclusions (rare earth element oxides, sulfides, etc.) increase, which may cause pitting corrosion and decrease corrosion resistance. Therefore, the rare earth element content in the case of inclusion is 0.2% or less. In order to stably exhibit the effect of improving the hot workability by REM, it is preferable to contain “S (mass%) + 1/2 · O (mass%)” or more.

Here, 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. Note that the content of REM means the total content of these elements.

The duplex stainless steel according to the present invention can be produced by a production facility and a production method which are usually used for commercial production. For example, for melting of duplex stainless steel, an electric furnace, an Ar—O ₂ mixed gas bottom blowing decarburization furnace (AOD furnace), a vacuum decarburization furnace (VOD furnace), or the like can be used. The molten metal may be cast into an ingot, or may be cast into a rod-shaped billet by a continuous casting method.

In Table 1 below, duplex stainless steels having chemical compositions (invention examples: trial numbers 1 to 11, comparative examples: trial numbers 12 to 25) were melted using a vacuum melting furnace having a capacity of 150 kg. Cast into ingot. Next, each ingot was heated to 1250 ° C. and forged into a plate having a thickness of 40 mm. Thereafter, each plate was again heated to 1250 ° C. and rolled to 15 mm by hot rolling (working temperature 1050 ° C. or higher). Then, each plate material after rolling was subjected to a solution heat treatment (treatment of holding water soaking at 1070 ° C. for 30 minutes and then water-cooling) to obtain test steel plates.

In order to evaluate the weldability of these test steel plates, first, a plate material having a thickness of 12 mm, a width of 100 mm, a length of 200 mm, and a V groove having a groove angle of 30 degrees on the long side is prepared by machining. did. In FIG. 1, the board | plate material 10 produced by machining is shown. 1A is a plan view and FIG. 1B is a front view.

Next, as shown in FIG. 2, two plate materials 10 having the shape of FIG. 1 are prepared for each test steel, the groove surfaces are brought into contact with each other, and multilayer welding is performed from one side by TIG welding. Was made. 2A is a plan view of the welded joint 20, and FIG. 2B is a front view. As the welding material 30 of each welded joint 20, a welding material having an outer diameter of 2 mm produced from the test steel No. 1 in Table 1 was used in common. Further, the welding was performed under the condition of a heat input of 30 kJ / cm, which is particularly efficient as a general stainless steel welding work.

Next, a test piece was collected from the back side (the first layer side of the weld bead) of the welded joint 20 obtained as described above. Specifically, a test piece having a thickness of 2 mm, a width of 10 mm, and a length of 75 mm was collected with the back bead and the scale during welding remaining. In FIG. 2, a region collected as a test piece is indicated by a broken line.

FIG. 3 shows a perspective view of the collected test piece 40. In addition, in the test piece 40 shown in FIG. 3, an upper surface is a rolling surface (lower surface of the welded joint of FIG. 2). As shown in FIG. 3, the longitudinal direction of the test piece 40 is a direction orthogonal to the weld line. Further, each test piece 40 is sampled so that one of the two boundary lines between the welding material 30 and the plate material 10 on the surface (rolling surface) of the test piece 40 is located at the center of the surface of the test piece 40. did.

A four-point bending test was performed using each obtained test piece. In the 4-point bending test, a stress corresponding to the yield stress of the test piece was loaded in a 25 mass% NaCl aqueous solution (150 ° C.) into which 3 MPa of CO ₂ was injected. The test time for the 4-point bending test is 720 hours.

After the 4-point bending test, the appearance of each specimen is visually observed, and stress corrosion cracking is observed by observation (field of view: 500 times) from the cross-sectional direction (direction perpendicular to the top surface of the specimen in FIG. 3) using an optical microscope. The presence or absence of occurrence was examined. The observation results are shown in Table 2. In Table 2, the case where stress corrosion cracking did not occur is indicated by “◯”, and the case where stress corrosion cracking occurred is indicated by “x”.

Further, in each welded joint (see FIG. 2), a cross section perpendicular to the weld line and the rolling surface was mirror-polished and etched, and then image analysis was performed with an optical microscope in a field of view of 500 times. And the area ratio of the trace amount sigma phase in HAZ (welding heat affected zone) was measured, and when the area ratio of the sigma phase was 1% or more, it was determined that sigma phase was precipitated. Table 2 shows the determination results. In Table 2, the case where it is determined that there is no sigma phase precipitation is indicated by “◯”, and the case where it is determined that there is sigma phase precipitation is indicated by “x”.

FIG. 4 is a diagram showing the relationship between “7Mo (mass%) + 3Cu (mass%)” and “Cr (mass%)” for the duplex stainless steels of trial numbers 1, 4, 6, 13, and 20. is there. Here, as shown in Table 2, no stress corrosion cracking occurred in the test pieces made from the duplex stainless steels of sample numbers 1, 4 and 6, and the duplex stainless steels of sample numbers 13 and 20 In the test piece prepared from the above, stress corrosion cracking occurs. Therefore, as shown in FIG. 4, the values of “7Mo (mass%) + 3Cu (mass%)” of the duplex stainless steels of trial numbers 1, 4, and 6 and “7Mo” of the duplex stainless steels of trial numbers 13 and 20 are used. When a boundary line is drawn between the value (mass%) + 3Cu (mass%), the boundary line is expressed by the following formula (3).
7Mo (mass%) + 3Cu (mass%) = − 2.2Cr (mass%) + 66 (3)

From the relationship shown in FIG. 4, when the value of “7Mo + 3Cu” is larger than the value of “−2.2Cr + 66”, that is, when the duplex stainless steel satisfies the relationship of the above formula (1), the stress It can be seen that the occurrence of corrosion cracking can be prevented. That is, as shown in Tables 1 and 2, in the test pieces made from the duplex stainless steels of trial numbers 1 to 11 that satisfy the chemical composition requirements defined in the present invention and the relationship of the above formula (1), No stress corrosion cracking has occurred. On the other hand, stress corrosion cracking occurs in the test pieces made from the duplex stainless steels Nos. 12 to 18, 20, 22, 23 and 25 that do not satisfy the relationship of the formula (1). In addition, although the duplex stainless steels of the trial numbers 19, 21, and 24 satisfy the relationship of the formula (1), stress corrosion cracking occurs because the Cu content (see Table 1) does not satisfy the requirements of the present invention. It is thought that it occurred.

Further, as shown in Table 2, in welded joints made from the duplex stainless steels of trial numbers 1 to 12, 14 to 19, 22, 23, and 25 that satisfy the relationship of the above formula (2), a trace amount of sigma in HAZ No phase is precipitated. On the other hand, in the welded joint produced from the duplex stainless steels of trial numbers 13, 20, 21 and 24 that do not satisfy the relationship of the formula (2), a trace amount of sigma phase is precipitated in the HAZ.

As is clear from the above results, the duplex stainless steel satisfying the requirements of the present invention can suppress precipitation of intermetallic compounds during high heat input welding and has excellent stress corrosion resistance in a chloride environment. It turns out that it has crackability.

The duplex stainless steel according to the present invention is excellent in weldability during large heat input welding and excellent in stress corrosion cracking resistance in a chloride environment.

10 Plate material 20 Welded joint 30 Welded material 40 Test piece

Claims (4)

In mass%, C: 0.03% or less, Si: 0.2-1%, Mn: 5.0% or less, P: 0.040% or less, S: 0.010% or less, sol. Al: 0.040% or less, Ni: 4-8%, Cr: 20-28%, Mo: 0.5-2.0%, Cu: more than 2.0% and 4.0% or less, N: A duplex stainless steel characterized by containing 0.1 to 0.35%, the balance having a chemical composition comprising Fe and impurities, and satisfying the relationship of the following formulas (1) and (2).
2.2Cr + 7Mo + 3Cu> 66 (1)
Cr + 11Mo + 10Ni <12 (Cu + 30N) (2)
However, each element symbol in Formula (1) and Formula (2) represents content (unit: mass%) in each steel of each element. 2. The duplex stainless steel according to claim 1, wherein the duplex stainless steel contains V: 1.5% or less in mass% instead of a part of Fe. Instead of a part of Fe, it contains at least one of Ca: 0.02% or less, Mg: 0.02% or less, B: 0.02% or less in mass%. Item 3. The duplex stainless steel according to item 1 or 2. The duplex stainless steel according to any one of claims 1 to 3, wherein the duplex stainless steel contains 0.2% or less of a rare earth element in mass% instead of part of Fe.

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