JP3166798B2 - Duplex stainless steel with excellent corrosion resistance and phase stability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to stress corrosion cracking resistance,
The present invention relates to a stainless steel comprising a ferrite phase and an austenitic phase having excellent corrosion resistance such as pitting corrosion resistance and weldability (hereinafter referred to as a duplex stainless steel).

[0002]

2. Description of the Related Art In recent years, heat exchangers using seawater, chemical equipment and structures requiring seawater resistance, piping for various chemical plants, line pipes, oil well pipes, and the like, have excellent corrosion resistance and weldability. The demand for duplex stainless steels is increasing, especially the requirements for corrosion resistance are becoming more stringent. Conventionally, there are a number of duplex stainless steels that have been put into practical use, but in order to explain them, a comprehensive list of weldable duplex stainless steels (edited by the Dutch Welding Society, duplex stainless steel for welding and super duplex stainless steel). Duplex stainless steel, L. van Nassau, H. Meelk
er, J. et al. Referring to Hilker '91), the duplex stainless steels developed to date can be classified into the following four types (a) to (d). However, PREN is:% Cr + 3.3 ×% Mo + 16 ×% N
Is an index of corrosion resistance given by

(A). 23% Cr, Mo-free Duplex Stainless Steel PREN-25 (b). 22% Cr duplex stainless steel PREN 30-36 (c). 25% Cr (0-2.5% Cu) duplex stainless steel PREN 32-40 (d). 25% Cr super duplex stainless steel PREN> 41 Since corresponding grades are described in the above (a) to (d), (a) to (d) will be described with reference to Tables 1 to 4. .

[0004]

[Table 1]

[0005]

[Table 2]

[0006]

[Table 3]

[0007]

[Table 4]

As shown in Tables 1 to 4, the duplex stainless steels are (a) 23% Cr-Mo-free, (b). (C) a 22% Cr-3% Mo-based alloy having improved corrosion resistance by adding about 3% of Mo, based on the 23% Cr- and Mo-free system; 22% C of (b)
25% Cr-3% Mo-based, in which the corrosion resistance is enhanced by further increasing Cr to 25% based on the r-3% Mo-based, (d).
Further, a super duplex stainless steel having the ultimate corrosion resistance by adding Cu or Mo, N to the 25% Cr-3% Mo system of (c) has been developed. As shown in Table 4, the super duplex stainless steel has a PREN of 40 or more and has a Cr content of 25% or more so as to have good corrosion resistance.
The basic philosophy is to include a large amount of Mo and N based on steel.

As the super duplex stainless steel, C: 0.05% or less, Cr: 23 to 27%, Ni:
5.5-9%, N: 0.25-0.40%, Si: 0.
8% or less, Mn: 1.2% or less, Mo: 3.5 to 4.9
%, Cu: 0.5% or less, W: 0.5% or less, S: 0.
010% or less, V: less than 0.5%, Ce: less than 0.18%, and ordinary impurities and additives in a state where the alloying element content is adjusted so as to satisfy the following conditions (1) to (6). Duplex stainless steel containing residual Fe in addition to (1)% Cr + 3.3 ×% Mo + 16 ×% N-1.6
×% Mn−122 ×% S> 39.1 (2) (% Cr + 0.51 ×% Mn + 0.22 ×% M
o-1.04x% Si-0.22x% Ni-2.89x
% C) / (3.7 ×% N)> 18.9 (3) (% Cr + 0.3 ×% Mn-2 ×% Si-0.
2 ×% Ni) / (4.31 ×% N)> 1000 (4) (% Mn) / (% N) <3 (5) Ferrite content after liquid heat treatment at 1075 ° C. =
30 to 50% (6)% Cr + (% Mo) ^1.5 + 5 ×% Si +% W +
0.2 ×% Mn｝ / (50 ×% N +% ferrite) <
Although 0.75 (Japanese Patent Application Laid-Open No. 62-56556) has been proposed, the basic idea is to substantially contain 24.5% or more of Cr.

[0010]

However, 25%
If a large amount of Mo and N are added to the Cr steel, there is a serious problem that the precipitation of intermetallic compounds mainly composed of the σ phase is remarkably accelerated. Precipitation of intermetallic compounds not only causes agglomeration, hardening of billet after rolling, remarkable reduction in machinability, brittle fracture due to embrittlement of the material, but also lowers the corrosion resistance of the heat-affected zone (HAZ) after welding. cause. Therefore, the duplex stainless steel containing a large amount of Mo and N on the basis of 25% Cr steel not only makes industrial production difficult, but also has a good corrosion resistance of PREN of 40 or more in the welded portion. The fact is that it has not been demonstrated.

An object of the present invention is to further improve the corrosion resistance of the conventional super duplex stainless steel, particularly to improve the corrosion resistance of the heat-affected zone after welding, to have excellent phase stability, and to prevent precipitation of intermetallic compounds. It is to provide a duplex stainless steel.

[0012]

Means for Solving the Problems To achieve the above object, the present inventors investigated the effects of Cr, Mo, Ni, N and W on the precipitation of intermetallic compounds such as the σ phase in Table 5 below.
After melting 14 types of duplex stainless steel having the composition shown in the following, and rolling to a thickness of 100 mm, a water-cooled material such that intermetallic compounds such as σ phase do not precipitate, and an intermetallic compound such as σ phase are precipitated. The difference in Vickers hardness of the air-cooled material was determined, and the increase in hardness due to precipitation of intermetallic compounds such as the σ phase was measured. As a result, when the amount of added Ni changes, the amount of precipitation of intermetallic compounds such as the σ phase greatly changes.
Among the two types of duplex stainless steels, the relationship between the amounts of Cr and Mo and the change in hardness was investigated for 12 types of 7% Ni-based. The result is shown in FIG.

[0013]

[Table 5]

As shown in FIG. 1, the increase in hardness when the amount of Mo added is increased, ie, the amount of precipitation of intermetallic compounds such as the σ phase, largely depends on the amount of Cr added. It turned out to be high. Therefore, since linear regression cannot be used simply, regression analysis was performed on 14 steel types shown in Table 5 by adding a term of% Cr ×% Mo. As a result, Vickers hardness difference (ΔHv (σ)) between a material cooled with water so that intermetallic compounds such as σ phase does not precipitate and a material cooled by air so that intermetallic compounds such as σ phase precipitates
ΔHv (σ) = 4.9 ×% Cr ×% Mo + 4.5 ×% Cr-103.4 ×% Mo + 19.3 ×% Ni-10.7 ×% N-3.6 ×% W-228 . 6 was obtained. From the correspondence between the change in hardness and the machinability in Table 5, ΔHv (σ) ≦ 65 is required to have machinability in a normal billet manufacturing process. That is, in order to aim at a component system in which an intermetallic compound represented by the σ phase is unlikely to be precipitated as represented by the above formula, the amount of Cr is reduced to less than 25%, and a large amount of Mo and N are added. Have found that a method for ensuring corrosion resistance is advantageous.

As a method for evaluating the corrosion resistance of stainless steel, the lowest temperature at which pitting occurs on the test piece surface (Critical Pitting) using ASTM-G48.
A method for determining the temperature (hereinafter referred to as CPT) was introduced, and the corrosion resistance of the base material portion, the bond portion, and the joint heat-affected zone was compared. It was clarified that considering the girth welding, it is necessary to ensure the corrosion resistance of the joint heat affected zone. Therefore, the relationship between the corrosion weight loss and the pitting resistance index (PREW) was determined using a test piece in the joint heat affected zone. The result is shown in FIG. As shown in FIG. 2, it was found that PREW =% Cr + 3.3 × (% Mo + 0.5 ×% W) + 16 ×% N ≧ 43 is necessary to secure the corrosion resistance of the joint heat affected zone.

Further, since the duplex stainless steel has a dual phase structure of ferrite and austenite, it is necessary to ensure corrosion resistance in each phase.
ob Micro Analyzer (hereinafter EPM)
A) analysis showed that Cr, Mo, Ni,
The W and N contents and the ferrite content (%) at that time were measured, and the pitting resistance index (hereinafter referred to as PREW) in the ferrite was measured.
(Α)) and the pitting resistance index in austenite (hereinafter referred to as PREW (γ)) can be calculated by the following formula. PREW (α) = (3 ×% α−1000) / ｛0.03
× (% α) ² -2 ×% α-800｝ ×% Cr + 3.3 ×
(3 ×% α-1360) / ｛0.03 × (% α) ² −
5.6 ×% α−800 ° ×% Mo + 1.65 × (30
0) / (% α + 200) ×% W PREW (γ) = (− 800) / ｛0.03 × (% α)
^{2 -2 ×% α-800}} ×% Cr + 3.3 × (-80
0) / ｛0.03 × (% α) ² −5.6 ×% α−80
0｝ ×% Mo + 1.65 × (200) / (% α + 20
0) ×% W + 16 × (100) / (100−% α) ×%
N% α = 5.8 ×% Cr + 4 ×% Mo + 1.2 ×% W−
5.7 ×% Ni-63.8 ×% N-54.4 Based on the above formulas, PREW (α) and PREW (γ) were calculated, the difference ΔPREW was obtained, and the relationship with the corrosion weight loss was obtained. As shown in FIG. 3, when ΔPREW is equal to or less than a predetermined value, that is, in a range of −3 ≦ ΔPREW ≦ 3, it has been found that extremely excellent corrosion resistance is exhibited, and the present invention has been achieved.

That is, according to the present invention, C: 0.03% or less, Si: 0.10 to 2.00%, Mn: 0.10 to 0.10%
2.00%, P: 0.05% or less, S: 0.005% or less, Cr: 18.0 to 25.0%, Ni: 2.0 to 8.
0%, Mo: 3.0 to 7.0%, Al: 0.001
0.04%, N: 0.10 to 0.40%, the balance consists of unavoidable impurities and Fe, and the pitting resistance index (PREW) given by the following formula (1) is 43 or more, An index Δ of the difference in Vickers hardness between a water-cooled material that does not precipitate an intermetallic compound such as a σ phase and an air-cooled material that precipitates an intermetallic compound such as a σ phase, which is given by the following equation (2).
Hv (σ) is 65 or less, a pitting resistance index (PREW (α)) in ferrite given by the following equation (3) and a pitting corrosion index (PREW) in austenite given by the following equation (4) (Γ)) is −3.0.
It is a duplex stainless steel excellent in corrosion resistance and phase stability, which satisfies the condition of not less than 3.0. PREW =% Cr + 3.3 × (% Mo + 0.5 ×% W)
+ 16 ×% N (1) Expression ΔHv (σ) = 4.9 ×% Cr ×% Mo + 4.5 ×% C
r-103.4 ×% Mo + 19.3 ×% Ni-10.7
×% N−3.6 ×% W−228.6 (2) Formula PREW (α) = (3 ×% α−1000) / ｛0.03
× (% α) ² -2 ×% α-800｝ ×% Cr + 3.3 ×
(3 ×% α-1360) / ｛0.03 × (% α) ² −
5.6 ×% α−800 ° ×% Mo + 1.65 × (30
0) / (% α + 200) ×% W (3) Formula PREW (γ) = (− 800) / ｛0.03 × (% α)
^{2 -2 ×% α-800}} ×% Cr + 3.3 × (-80
0) / ｛0.03 × (% α) ² −5.6 ×% α−80
0｝ ×% Mo + 1.65 × (200) / (% α + 20
0) ×% W + 16 × (100) / (100−% α) ×%
N ... (4) where% α = 5.8 ×% Cr + 4 ×% Mo + 1.2 ×
% W-5.7 ×% Ni-63.8 ×% N-54.4

Further, C: 0.03% or less, Si: 0.1
0 to 2.00%, Mn: 0.10 to 2.00%, P:
0.05% or less, S: 0.005% or less, Cr: 18.
0 to 25.0%, Ni: 2.0 to 8.0%, Mo: 3.
0 to 7.0%, Al: 0.001 to 0.04%, N:
0.10 to 0.40% and Cu: 0.01 to 2.00
%, W: one or two of 0.01 to 1.50%, the balance consisting of unavoidable impurities and Fe, and a pitting corrosion resistance index (PRE) given by the following formula (1).
W) is 43 or more, and is an index of the Vickers hardness difference between a water-cooled material that does not precipitate an intermetallic compound such as a σ phase and an air-cooled material that precipitates an intermetallic compound such as a σ phase, which is given by the following formula (2). ΔHv (σ) is 65 or less, the following (3)
The pitting resistance index (PRE) in ferrite given by the equation
W (α)) and the pitting resistance index (PREW (γ)) in austenite given by the following equation (4) (ΔP
REW) which satisfies the condition of -3.0 or more and 3.0 or less, and is a duplex stainless steel excellent in corrosion resistance and phase stability. PREW =% Cr + 3.3 × (% Mo + 0.5 ×% W)
+ 16 ×% N (1) Expression ΔHv (σ) = 4.9 ×% Cr ×% Mo + 4.5 ×% C
r-103.4 ×% Mo + 19.3 ×% Ni-10.7
×% N−3.6 ×% W−228.6 (2) Formula PREW (α) = (3 ×% α−1000) / ｛0.03
× (% α) ² -2 ×% α-800｝ ×% Cr + 3.3 ×
(3 ×% α-1360) / ｛0.03 × (% α) ² −
5.6 ×% α−800 ° ×% Mo + 1.65 × (30
0) / (% α + 200) ×% W (3) Formula PREW (γ) = (− 800) / ｛0.03 × (% α)
^{2 -2 ×% α-800}} ×% Cr + 3.3 × (-80
0) / ｛0.03 × (% α) ² −5.6 ×% α−80
0｝ ×% Mo + 1.65 × (200) / (% α + 20
0) ×% W + 16 × (100) / (100−% α) ×%
N ... (4) where% α = 5.8 ×% Cr + 4 ×% Mo + 1.2 ×
% W-5.7 ×% Ni-63.8 ×% N-54.4

Further, C: 0.03% or less, Si: 0.
10 to 2.00%, Mn: 0.10 to 2.00%, P:
0.05% or less, S: 0.005% or less, Cr: 18.
0 to 25.0%, Ni: 2.0 to 8.0%, Mo: 3.
0 to 7.0%, Al: 0.001 to 0.04%, N:
0.10 to 0.40%, V: 0.01 to 0.50%,
Ti: 0.01 to 0.50%, Nb: 0.01 to 0.5
0%, one or more of them, the balance consists of unavoidable impurities and Fe, and the pitting resistance index (PREW) given by the following formula (1) is 43 or more, and the following (2) The index ΔH of the difference in Vickers hardness between a water-cooled material and an air-cooled material such that the intermetallic compound such as the σ phase is precipitated, which is given by the following formula:
The pitting resistance index (PREW (α)) in ferrite given by the following equation (3) and the pitting corrosion index (PREW) in austenite given by the following equation (4) when v (σ) is 65 or less. (Γ)) is −3.0.
It is a duplex stainless steel excellent in corrosion resistance and phase stability, which satisfies the condition of not less than 3.0. PREW =% Cr + 3.3 × (% Mo + 0.5 ×% W)
+ 16 ×% N (1) Expression ΔHv (σ) = 4.9 ×% Cr ×% Mo + 4.5 ×% C
r-103.4 ×% Mo + 19.3 ×% Ni-10.7
×% N−3.6 ×% W−228.6 (2) Formula PREW (α) = (3 ×% α−1000) / ｛0.03
× (% α) ² -2 ×% α-800｝ ×% Cr + 3.3 ×
(3 ×% α-1360) / ｛0.03 × (% α) ² −
5.6 ×% α−800 ° ×% Mo + 1.65 × (30
0) / (% α + 200) ×% W (3) Formula PREW (γ) = (− 800) / ｛0.03 × (% α)
^{2 -2 ×% α-800}} ×% Cr + 3.3 × (-80
0) / ｛0.03 × (% α) ² −5.6 ×% α−80
0｝ ×% Mo + 1.65 × (200) / (% α + 20
0) ×% W + 16 × (100) / (100−% α) ×%
N ... (4) where% α = 5.8 ×% Cr + 4 ×% Mo + 1.2 ×
% W-5.7 ×% Ni-63.8 ×% N-54.4

Further, C: 0.03% or less, Si:
0.10 to 2.00%, Mn: 0.10 to 2.00%,
P: 0.05% or less, S: 0.005% or less, Cr: 1
8.0 to 25.0%, Ni: 2.0 to 8.0%, Mo:
3.0 to 7.0%, Al: 0.001 to 0.04%,
N: 0.10 to 0.40%, Ca: 0.0005 to
0.010%, Mg: 0.0005 to 0.010%,
B: 0.0005 to 0.010%, rare earth metal: 0.0
005-0.010%, and the balance consists of unavoidable impurities and Fe, and the pitting resistance index (PREW) given by the following formula (1) is 4
3 or more, an index ΔHv (σ) of the difference in Vickers hardness between a water-cooled material and an air-cooled material such that a σ phase or the like intermetallic compound is precipitated, which is given by the following formula (2). ) Is 65 or less and the pitting resistance index (PREW) in ferrite given by the following equation (3) is given.
(Α)) and the difference (ΔPR) between the pitting resistance index (PREW (γ)) in austenite given by the following equation (4):
EW) satisfying the condition of -3.0 or more and 3.0 or less, is a duplex stainless steel excellent in corrosion resistance and phase stability. PREW =% Cr + 3.3 × (% Mo + 0.5 ×% W)
+ 16 ×% N (1) Expression ΔHv (σ) = 4.9 ×% Cr ×% Mo + 4.5 ×% C
r-103.4 ×% Mo + 19.3 ×% Ni-10.7
×% N−3.6 ×% W−228.6 (2) Formula PREW (α) = (3 ×% α−1000) / ｛0.03
× (% α) ² -2 ×% α-800｝ ×% Cr + 3.3 ×
(3 ×% α-1360) / ｛0.03 × (% α) ² −
5.6 ×% α−800 ° ×% Mo + 1.65 × (30
0) / (% α + 200) ×% W (3) Formula PREW (γ) = (− 800) / ｛0.03 × (% α)
^{2 -2 ×% α-800}} ×% Cr + 3.3 × (-80
0) / ｛0.03 × (% α) ² −5.6 ×% α−80
0｝ ×% Mo + 1.65 × (200) / (% α + 20
0) ×% W + 16 × (100) / (100−% α) ×%
N ... (4) where% α = 5.8 ×% Cr + 4 ×% Mo + 1.2 ×
% W-5.7 ×% Ni-63.8 ×% N-54.4

C: 0.03% or less, Si: 0.1
0 to 2.00%, Mn: 0.10 to 2.00%, P:
0.05% or less, S: 0.005% or less, Cr: 18.
0 to 25.0%, Ni: 2.0 to 8.0%, Mo: 3.
0 to 7.0%, Al: 0.001 to 0.04%, N:
0.10 to 0.40% and Cu: 0.01 to 2.00
%, W: one or more of 0.01 to 1.50%, V: 0.01 to 0.50%, Ti: 0.01 to
0.50%, Nb: contains one or more of 0.01 to 0.50%, with the balance being unavoidable impurities and Fe
And a water-cooled material having a pitting corrosion index (PREW) of 43 or more given by the following formula (1) and an intermetallic compound such as a σ phase given by the following formula (2): The index ΔHv (σ) of the difference in Vickers hardness between the air-cooled material and the intermetallic compound precipitated is 65 or less;
The difference between the pitting resistance index (PREW (α)) in ferrite given by the following equation (3) and the pitting corrosion index (PREW (γ)) in austenite given by the following equation (4) ( ΔPREW) satisfying the condition of -3.0 or more and 3.0 or less is a duplex stainless steel excellent in corrosion resistance and phase stability. PREW =% Cr + 3.3 × (% Mo + 0.5 ×% W)
+ 16 ×% N (1) Expression ΔHv (σ) = 4.9 ×% Cr ×% Mo + 4.5 ×% C
r-103.4 ×% Mo + 19.3 ×% Ni-10.7
×% N−3.6 ×% W−228.6 (2) Formula PREW (α) = (3 ×% α−1000) / ｛0.03
× (% α) ² -2 ×% α-800｝ ×% Cr + 3.3 ×
(3 ×% α-1360) / ｛0.03 × (% α) ² −
5.6 ×% α−800 ° ×% Mo + 1.65 × (30
0) / (% α + 200) ×% W (3) Formula PREW (γ) = (− 800) / ｛0.03 × (% α)
^{2 -2 ×% α-800}} ×% Cr + 3.3 × (-80
0) / ｛0.03 × (% α) ² −5.6 ×% α−80
0｝ ×% Mo + 1.65 × (200) / (% α + 20
0) ×% W + 16 × (100) / (100−% α) ×%
N ... (4) where% α = 5.8 ×% Cr + 4 ×% Mo + 1.2 ×
% W-5.7 ×% Ni-63.8 ×% N-54.4

Further, C: 0.03% or less, Si: 0.
10 to 2.00%, Mn: 0.10 to 2.00%, P:
0.05% or less, S: 0.005% or less, Cr: 18.
0 to 25.0%, Ni: 2.0 to 8.0%, Mo: 3.
0 to 7.0%, Al: 0.001 to 0.04%, N:
0.10 to 0.40% and Cu: 0.01 to 2.00
%, W: one or more of 0.01 to 1.50%, Ca: 0.0005 to 0.010%, Mg: 0.
0005-0.010%, B: 0.0005-0.01
0%, rare earth metal: 0.0005 to 0.010%, one or more of which are inevitable impurities and Fe
And a water-cooled material having a pitting corrosion index (PREW) of 43 or more given by the following formula (1) and an intermetallic compound such as a σ phase given by the following formula (2): The index ΔHv (σ) of the difference in Vickers hardness between the air-cooled material and the intermetallic compound precipitated is 65 or less;
The difference between the pitting resistance index (PREW (α)) in ferrite given by the following equation (3) and the pitting corrosion index (PREW (γ)) in austenite given by the following equation (4) ( ΔPREW) satisfying the condition of -3.0 or more and 3.0 or less is a duplex stainless steel excellent in corrosion resistance and phase stability. PREW =% Cr + 3.3 × (% Mo + 0.5 ×% W)
+ 16 ×% N (1) Expression ΔHv (σ) = 4.9 ×% Cr ×% Mo + 4.5 ×% C
r-103.4 ×% Mo + 19.3 ×% Ni-10.7
×% N−3.6 ×% W−228.6 (2) Formula PREW (α) = (3 ×% α−1000) / ｛0.03
× (% α) ² -2 ×% α-800｝ ×% Cr + 3.3 ×
(3 ×% α-1360) / ｛0.03 × (% α) ² −
5.6 ×% α−800 ° ×% Mo + 1.65 × (30
0) / (% α + 200) ×% W (3) Formula PREW (γ) = (− 800) / ｛0.03 × (% α)
^{2 -2 ×% α-800}} ×% Cr + 3.3 × (-80
0) / ｛0.03 × (% α) ² −5.6 ×% α−80
0｝ ×% Mo + 1.65 × (200) / (% α + 20
0) ×% W + 16 × (100) / (100−% α) ×%
N ... (4) where% α = 5.8 ×% Cr + 4 ×% Mo + 1.2 ×
% W-5.7 ×% Ni-63.8 ×% N-54.4

Further, C: 0.03% or less, Si:
0.10 to 2.00%, Mn: 0.10 to 2.00%,
P: 0.05% or less, S: 0.005% or less, Cr: 1
8.0 to 25.0%, Ni: 2.0 to 8.0%, Mo:
3.0 to 7.0%, Al: 0.001 to 0.04%,
N: 0.10 to 0.40%, V: 0.01 to 0.50
%, Ti: 0.01 to 0.50%, Nb: 0.01 to
One or more of 0.50%, Ca: 0.0
005-0.010%, Mg: 0.0005-0.01
0%, B: 0.0005 to 0.010%, rare earth metal:
One or more of 0.0005 to 0.010% is contained, the balance consists of unavoidable impurities and Fe, and the pitting resistance index (PREW) given by the following formula (1) is 4
3 or more, an index ΔHv (σ) of the difference in Vickers hardness between a water-cooled material and an air-cooled material such that a σ phase or the like intermetallic compound is precipitated, which is given by the following formula (2). ) Is 65 or less and the pitting resistance index (PREW) in ferrite given by the following equation (3) is given.
(Α)) and the difference (ΔPR) between the pitting resistance index (PREW (γ)) in austenite given by the following equation (4):
EW) satisfying the condition of -3.0 or more and 3.0 or less, is a duplex stainless steel excellent in corrosion resistance and phase stability. PREW =% Cr + 3.3 × (% Mo + 0.5 ×% W)
+ 16 ×% N (1) Expression ΔHv (σ) = 4.9 ×% Cr ×% Mo + 4.5 ×% C
r-103.4 ×% Mo + 19.3 ×% Ni-10.7
×% N−3.6 ×% W−228.6 (2) Formula PREW (α) = (3 ×% α−1000) / ｛0.03
× (% α) ² -2 ×% α-800｝ ×% Cr + 3.3 ×
(3 ×% α-1360) / ｛0.03 × (% α) ² −
5.6 ×% α−800 ° ×% Mo + 1.65 × (30
0) / (% α + 200) ×% W (3) Formula PREW (γ) = (− 800) / ｛0.03 × (% α)
^{2 -2 ×% α-800}} ×% Cr + 3.3 × (-80
0) / ｛0.03 × (% α) ² −5.6 ×% α−80
0｝ ×% Mo + 1.65 × (200) / (% α + 20
0) ×% W + 16 × (100) / (100−% α) ×%
N ... (4) where% α = 5.8 ×% Cr + 4 ×% Mo + 1.2 ×
% W-5.7 ×% Ni-63.8 ×% N-54.4

C: 0.03% or less, Si: 0.1
0 to 2.00%, Mn: 0.10 to 2.00%, P:
0.05% or less, S: 0.005% or less, Cr: 18.
0 to 25.0%, Ni: 2.0 to 8.0%, Mo: 3.
0 to 7.0%, Al: 0.001 to 0.04%, N:
0.10 to 0.40% and Cu: 0.01 to 2.00
%, W: one or more of 0.01 to 1.50%, V: 0.01 to 0.50%, Ti: 0.01 to
0.50%, Nb: one or more of 0.01 to 0.50%, Ca: 0.0005 to 0.010%,
Mg: 0.0005-0.010%, B: 0.0005
0.010%, rare earth metal: 0.0005 to 0.01
0% of one or more kinds, the balance consists of unavoidable impurities and Fe, and the pitting corrosion resistance index (PREW) given by the following equation (1) is 43 or more; The index ΔHv (σ) of the difference in Vickers hardness between a material that is water-cooled so as not to precipitate an intermetallic compound such as a σ phase and a material that is air-cooled so as to precipitate an intermetallic compound such as a σ phase is 65 or less, and the following (3) The pitting corrosion resistance index (PREW (α)) in ferrite given by the equation and the pitting corrosion index (PREW) in austenite given by the following equation (4)
(Γ)) is a duplex stainless steel excellent in corrosion resistance and phase stability, characterized by satisfying the condition of -3.0 or more and 3.0 or less. PREW =% Cr + 3.3 × (% Mo + 0.5 ×% W)
+ 16 ×% N (1) Expression ΔHv (σ) = 4.9 ×% Cr ×% Mo + 4.5 ×% C
r-103.4 ×% Mo + 19.3 ×% Ni-10.7
×% N−3.6 ×% W−228.6 (2) Formula PREW (α) = (3 ×% α−1000) / ｛0.03
× (% α) ² -2 ×% α-800｝ ×% Cr + 3.3 ×
(3 ×% α-1360) / ｛0.03 × (% α) ² −
5.6 ×% α−800 ° ×% Mo + 1.65 × (30
0) / (% α + 200) ×% W (3) Formula PREW (γ) = (− 800) / ｛0.03 × (% α)
^{2 -2 ×% α-800}} ×% Cr + 3.3 × (-80
0) / ｛0.03 × (% α) ² −5.6 ×% α−80
0｝ ×% Mo + 1.65 × (200) / (% α + 20
0) ×% W + 16 × (100) / (100−% α) ×%
N ... (4) where% α = 5.8 ×% Cr + 4 ×% Mo + 1.2 ×
% W-5.7 ×% Ni-63.8 ×% N-54.4

[0025]

The duplex stainless steel of the present invention has a pitting resistance index (PREW) of 43 or more and a ΔHv (σ) of 65 or more.
Hereinafter, since ΔPREW satisfies the condition of −3.0 or more and 3.0 or less, the corrosion resistance of the joint heat affected zone is ensured by setting PREW ≧ 43, and by setting ΔHv (σ) ≦ 65, Machinability is secured, -3.0 ≦ ΔPR
By setting EW ≦ 3.0, the corrosion resistance of the ferrite phase and the austenitic phase, and each phase, is secured, and the corrosion resistance, particularly the corrosion resistance of the girth weld, is excellent, and the workability, phase stability, and bulk rolling property are excellent. .

The reason for limiting the chemical composition of the duplex stainless steel in the present invention to the above range will be described. C
Is an unavoidable element contained in steel. However, if it exceeds 0.03%, carbides precipitate in the heat affected zone of welding and the corrosion resistance is remarkably deteriorated. Si is an element acting as a deoxidizing agent, but in order to ensure sufficient corrosion resistance, reduction of oxygen is indispensable. If it is less than 0.10%, the deoxidizing effect is insufficient, and it exceeds 2.00%. And 0.10 to 2.00% to cause embrittlement. Mn is deoxidized,
Element added for the purpose of desulfurization, but 0.10%
If it is less than 2,000%, a sufficient effect cannot be obtained, and if it exceeds 2.00%, the corrosion resistance is adversely affected. P is an unavoidable element contained in steel, but must be as low as possible in order to degrade hot workability and corrosion resistance. S is also an unavoidable element contained in the steel and is the element that most affects the hot workability of the duplex stainless steel. Therefore, it is necessary to make S as low as possible.

Cr is one of the basic components of the duplex stainless steel and is an important element that governs the corrosion resistance. To ensure high corrosion resistance while forming a two-phase structure of ferrite and austenite, Although 18.0% or more is necessary, when a large amount of Mo is added to increase pitting resistance, adding more than 25.0% remarkably accelerates precipitation of intermetallic compounds such as σ phase. 0.0 to 25.0
%. Ni has a Cr content, M
When the content is less than 2.0%, a two-phase structure mainly composed of a ferrite phase is formed, and the corrosion resistance is reduced. When the content exceeds 8.0%, a two-phase mainly composed of an austenite phase is added. Not only organization, but also
2.0 to 2.0 to accelerate the precipitation of intermetallic compounds such as
It was set to 8.0%. Mo is an element for improving the pitting corrosion resistance, and if it is less than 3.0%, it is not enough to obtain the desired pitting corrosion resistance, and if it exceeds 7.0%, the precipitation of intermetallic compounds such as the σ phase may be prevented. In order to accelerate, it was set to 3.0 to 7.0%.

Al is indispensable as a deoxidizing element, and oxygen must be reduced to ensure sufficient corrosion resistance. If the content is less than 0.001%, a sufficient deoxidizing effect cannot be obtained.
On the other hand, when the content exceeds 0.04%, AlN easily precipitates,
0.001 to 0.04 to deteriorate toughness and corrosion resistance
%. N is an important element for forming a two-phase structure and is a very effective element for improving pitting corrosion resistance. However, if it is less than 0.10%, the effect is not sufficient.
On the other hand, if the content exceeds 0.40%, the hot workability is significantly reduced. Cu is an element having an effect of improving the acid resistance and corrosion resistance of steel, and is added as necessary. However, if it is less than 0.01%, the desired effect cannot be obtained, and if it exceeds 2.00%, In order to reduce the hot workability, the content is set to 0.01 to 2.00%.
W is an element having an effect of improving crevice corrosion resistance and improving corrosion resistance, and is added if necessary.
If it is less than 1.5%, the desired effect cannot be obtained, and if it exceeds 1.50%, the hot workability deteriorates.
0%.

V, Ti, and Nb are elements that form stable carbides in steel and improve corrosion resistance. One or more of these elements may be added as necessary. If less than 0.01%, desired effects can be obtained. It cannot be obtained, and even if it is added in excess of 0.50%, it will be in a saturated state. C
a, Mg, and rare earth metals (La, Ce, etc.) are elements that form sulfides in steel to fix S and improve hot workability, and one or more elements are added as necessary.
If it is less than 0.0005%, the desired effect cannot be obtained.
Even if it is added in excess of 0.010%, it becomes saturated,
0.0005 to 0.010%. B is an element that improves hot workability by segregating at grain boundaries in steel, and is added as necessary. However, if it is less than 0.0005%, a desired effect cannot be obtained. Even if added in excess, it will be in a saturated state, so it was made 0.0005 to 0.010%.

[0030]

EXAMPLE Using a high-frequency induction vacuum melting furnace, each duplex stainless steel having the composition shown in Tables 6 and 7 was melted, and 50 kg was melted.
, And hot-rolled to a thickness of 12 mm. Each of the duplex stainless steels is obtained by using the above formulas (1) to (4)
PREW, ΔPREW and ΔHv (σ) were determined based on the formula. Hardness was measured in order to evaluate the machinability of each of the thus obtained duplex stainless steels. In general, it is impossible to cut unless the Shore hardness is less than 40.
The above was evaluated as poor machinability (x). Further, each duplex stainless steel was heated and maintained at 1070 ° C. for 30 minutes, then subjected to a solution treatment under water-cooling conditions, the amount of ferrite was investigated, and the thickness was 3 mm, the width was 50 mm, and the length was 50 mm.
Specimens for the base metal corrosion test were collected. In addition, in order to evaluate the corrosion resistance of the welded portion, each duplex stainless steel was grooved, subjected to fully automatic TIG welding, and corrosion test specimens were taken from the bond portion and the joint heat affected zone. In the corrosion test, each of the test pieces was immersed in a 10% aqueous solution of FeCl ₃ .6H ₂ O at each temperature for 24 hours, and the lowest temperature (CPT) at which pitting occurred on the test piece surface and the corrosion loss were measured. As a result, those showing good corrosion resistance were evaluated as (（), and those showing poor corrosion resistance were evaluated as (x). PREW, ΔPRE
Tables 8 and 9 show the results together with W and ΔHv (σ).

[0031]

[Table 6]

[0032]

[Table 7]

[0033]

[Table 8]

[0034]

[Table 9]

As shown in Tables 8 and 9, in the conventional steel and the comparative steel, since the conditions of the present invention were not satisfied by the portions marked with *, the desired effects were not obtained in the machinability and the pitting corrosion resistance of the welded portion. However, all of the steels of the present invention satisfying the conditions of the present invention have good machinability and pitting corrosion resistance of a welded portion.

[0036]

As described above, the duplex stainless steel of the present invention is excellent in corrosion resistance, especially in the girth weld zone, and is excellent in phase stability and bulk rolling property. For this reason, in a field where high corrosion resistance (seawater resistance) is required as a line pipe for oil wells, it is possible to solve the conventional problems such as deterioration of corrosion resistance at the time of girth welding.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between Cr and Mo contents in a duplex stainless steel and an increase in hardness due to σ phase precipitation.

FIG. 2 is a graph showing the results of a corrosion resistance test on a heat-affected zone of a duplex stainless steel joint at the weld heat-affected zone in relation to PREW and corrosion weight loss.

FIG. 3 .DELTA.PRE of corrosion resistance test result of duplex stainless steel
It is a graph which shows the relationship between W and corrosion weight loss.

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Kazuhiro Ogawa 4-33, Kitahama, Chuo-ku, Osaka-shi, Osaka Sumitomo Metal Industries, Ltd. (56) References JP-A-4-280946 (JP, A) JP-A 62-253755 (JP, A) JP-A-4-165019 (JP, A) JP-A 61-19764 (JP, A) JP-A 3-82740 (JP, A) JP-A 57-47852 (JP-A 57-47852) JP, A) JP-A-61-113749 (JP, A) JP-A-2-111845 (JP, A) JP-A-62-156556 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , (DB name) C22C 38/00 302 C22C 38/44 C22C 38/58

Claims (8)

(57) [Claims] 1. C: 0.03% or less, Si: 0.10 to 0.10
2.00%, Mn: 0.10-2.00%, P: 0.0
5% or less, S: 0.005% or less, Cr: 18.0 to 2
5.0%, Ni: 2.0-8.0%, Mo: 3.0-
7.0%, Al: 0.001 to 0.04%, N: 0.1
0 to 0.40%, the balance being unavoidable impurities and Fe
And a water-cooled material having a pitting corrosion index (PREW) of 43 or more given by the following formula (1) and an intermetallic compound such as a σ phase given by the following formula (2): The index ΔHv (σ) of the difference in Vickers hardness between the air-cooled material and the intermetallic compound precipitated is 65 or less;
The difference between the pitting resistance index (PREW (α)) in ferrite given by the following equation (3) and the pitting corrosion index (PREW (γ)) in austenite given by the following equation (4) ( A duplex stainless steel excellent in corrosion resistance and phase stability, wherein ΔPREW satisfies the condition of -3.0 or more and 3.0 or less. PREW =% Cr + 3.3 × (% Mo + 0.5 ×% W)
+ 16 ×% N (1) Expression ΔHv (σ) = 4.9 ×% Cr ×% Mo + 4.5 ×% C
r-103.4 ×% Mo + 19.3 ×% Ni-10.7
×% N−3.6 ×% W−228.6 (2) Formula PREW (α) = (3 ×% α−1000) / ｛0.03
× (% α) ² -2 ×% α-800｝ ×% Cr + 3.3 ×
(3 ×% α-1360) / ｛0.03 × (% α) ² −
5.6 ×% α−800 ° ×% Mo + 1.65 × (30
0) / (% α + 200) ×% W (3) Formula PREW (γ) = (− 800) / ｛0.03 × (% α)
^{2 -2 ×% α-800}} ×% Cr + 3.3 × (-80
0) / ｛0.03 × (% α) ² −5.6 ×% α−80
0｝ ×% Mo + 1.65 × (200) / (% α + 20
0) ×% W + 16 × (100) / (100−% α) ×%
N ... (4) where% α = 5.8 ×% Cr + 4 ×% Mo + 1.2 ×
% W-5.7 ×% Ni-63.8 ×% N-54.4 2. C: 0.03% or less, Si: 0.10 to
2.00%, Mn: 0.10-2.00%, P: 0.0
5% or less, S: 0.005% or less, Cr: 18.0 to 2
5.0%, Ni: 2.0-8.0%, Mo: 3.0-
7.0%, Al: 0.001 to 0.04%, N: 0.1
0 to 0.40%, Cu: 0.01 to 2.00%, W:
One or two of 0.01 to 1.50% are contained, the balance is made of unavoidable impurities and Fe, and the pitting resistance index (PREW) given by the following formula (1) is 43.
As described above, the index ΔHv (σ) of the Vickers hardness difference between a water-cooled material that does not precipitate an intermetallic compound such as a σ phase and an air-cooled material that precipitates an intermetallic compound such as a σ phase is given by the following equation (2). Is 65 or less, the pitting resistance index (PREW (α)) in ferrite given by the following equation (3):
And a difference (ΔPREW) between the pitting resistance index (PREW (γ)) in austenite given by the following equation (4) satisfies the condition of −3.0 or more and 3.0 or less. Duplex stainless steel with excellent corrosion resistance and phase stability. PREW =% Cr + 3.3 × (% Mo + 0.5 ×% W)
+ 16 ×% N (1) Expression ΔHv (σ) = 4.9 ×% Cr ×% Mo + 4.5 ×% C
r-103.4 ×% Mo + 19.3 ×% Ni-10.7
×% N−3.6 ×% W−228.6 (2) Formula PREW (α) = (3 ×% α−1000) / ｛0.03
× (% α) ² -2 ×% α-800｝ ×% Cr + 3.3 ×
(3 ×% α-1360) / ｛0.03 × (% α) ² −
5.6 ×% α−800 ° ×% Mo + 1.65 × (30
0) / (% α + 200) ×% W (3) Formula PREW (γ) = (− 800) / ｛0.03 × (% α)
^{2 -2 ×% α-800}} ×% Cr + 3.3 × (-80
0) / ｛0.03 × (% α) ² −5.6 ×% α−80
0｝ ×% Mo + 1.65 × (200) / (% α + 20
0) ×% W + 16 × (100) / (100−% α) ×%
N ... (4) where% α = 5.8 ×% Cr + 4 ×% Mo + 1.2 ×
% W-5.7 ×% Ni-63.8 ×% N-54.4 3. C: 0.03% or less, Si: 0.10%
2.00%, Mn: 0.10-2.00%, P: 0.0
5% or less, S: 0.005% or less, Cr: 18.0 to 2
5.0%, Ni: 2.0-8.0%, Mo: 3.0-
7.0%, Al: 0.001 to 0.04%, N: 0.1
0 to 0.40%, V: 0.01 to 0.50%, Ti:
0.01 to 0.50%, Nb: contains one or more of 0.01 to 0.50%, and the balance consists of unavoidable impurities and Fe, and in the following formula (1) A pitting corrosion resistance index (PREW) given is 43 or more, and a water-cooled material and an air-cooled material such that a σ phase or the like intermetallic compound is precipitated by the following formula (2). The index ΔHv (σ) of the difference between Vickers hardness and the pitting corrosion resistance index (PREW (α)) in ferrite given by the following equation (3) and the austenite given in the following equation (4) Pitting resistance index (PREW)
(Γ)), wherein the difference (ΔPREW) satisfies the condition of -3.0 or more and 3.0 or less, a duplex stainless steel excellent in corrosion resistance and phase stability. PREW =% Cr + 3.3 × (% Mo + 0.5 ×% W)
+ 16 ×% N (1) Expression ΔHv (σ) = 4.9 ×% Cr ×% Mo + 4.5 ×% C
r-103.4 ×% Mo + 19.3 ×% Ni-10.7
×% N−3.6 ×% W−228.6 (2) Formula PREW (α) = (3 ×% α−1000) / ｛0.03
× (% α) ² -2 ×% α-800｝ ×% Cr + 3.3 ×
(3 ×% α-1360) / ｛0.03 × (% α) ² −
5.6 ×% α−800 ° ×% Mo + 1.65 × (30
0) / (% α + 200) ×% W (3) Formula PREW (γ) = (− 800) / ｛0.03 × (% α)
^{2 -2 ×% α-800}} ×% Cr + 3.3 × (-80
0) / ｛0.03 × (% α) ² −5.6 ×% α−80
0｝ ×% Mo + 1.65 × (200) / (% α + 20
0) ×% W + 16 × (100) / (100−% α) ×%
N ... Equation (4) where% α = 5.8 ×% Cr + 4 ×% Mo + 1.2%
W-5.7 ×% Ni-63.8 ×% N-54.4 4. C: 0.03% or less, Si: 0.10 to
2.00%, Mn: 0.10-2.00%, P: 0.0
5% or less, S: 0.005% or less, Cr: 18.0 to 2
5.0%, Ni: 2.0-8.0%, Mo: 3.0-
7.0%, Al: 0.001 to 0.04%, N: 0.1
0 to 0.40% and Ca: 0.0005 to 0.010
%, Mg: 0.0005 to 0.010%, B: 0.00
0.05 to 0.010%, rare earth metal: 0.0005 to 0.5%.
010%, one or more of them, the balance being unavoidable impurities and Fe, and a pitting resistance index (PREW) given by the following formula (1) of 43 or more, and the following (2) The index ΔH of the difference in Vickers hardness between a water-cooled material and an air-cooled material such that the intermetallic compound such as the σ phase is precipitated, which is given by the following formula:
The pitting resistance index (PREW (α)) in ferrite given by the following equation (3) and the pitting corrosion index (PREW) in austenite given by the following equation (4) when v (σ) is 65 or less. (Γ)) is −3.0.
A duplex stainless steel excellent in corrosion resistance and phase stability, satisfying the conditions of not less than 3.0 and not more than 3.0. PREW =% Cr + 3.3 × (% Mo + 0.5 ×% W)
+ 16 ×% N (1) Expression ΔHv (σ) = 4.9 ×% Cr ×% Mo + 4.5 ×% C
r-103.4 ×% Mo + 19.3 ×% Ni-10.7
×% N−3.6 ×% W−228.6 (2) Formula PREW (α) = (3 ×% α−1000) / ｛0.03
× (% α) ² -2 ×% α-800｝ ×% Cr + 3.3 ×
(3 ×% α-1360) / ｛0.03 × (% α) ² −
5.6 ×% α−800 ° ×% Mo + 1.65 × (30
0) / (% α + 200) ×% W (3) Formula PREW (γ) = (− 800) / ｛0.03 × (% α)
^{2 -2 ×% α-800}} ×% Cr + 3.3 × (-80
0) / ｛0.03 × (% α) ² −5.6 ×% α−80
0｝ ×% Mo + 1.65 × (200) / (% α + 20
0) ×% W + 16 × (100) / (100−% α) ×%
N ... (4) where% α = 5.8 ×% Cr + 4 ×% Mo + 1.2 ×
% W-5.7 ×% Ni-63.8 ×% N-54.4 5. C: 0.03% or less, Si: 0.10%
2.00%, Mn: 0.10-2.00%, P: 0.0
5% or less, S: 0.005% or less, Cr: 18.0 to 2
5.0%, Ni: 2.0-8.0%, Mo: 3.0-
7.0%, Al: 0.001 to 0.04%, N: 0.1
0 to 0.40%, Cu: 0.01 to 2.00%, W:
One or two of 0.01 to 1.50%, V:
0.01 to 0.50%, Ti: 0.01 to 0.50%,
Nb: One or more of 0.01 to 0.50% is contained, the balance is made of unavoidable impurities and Fe, and a pitting corrosion resistance index (PRE) given by the following formula (1)
W) is 43 or more, and is an index of the Vickers hardness difference between a water-cooled material that does not precipitate an intermetallic compound such as a σ phase and an air-cooled material that precipitates an intermetallic compound such as a σ phase, which is given by the following formula (2). ΔHv (σ) is 65 or less, the following (3)
The pitting resistance index (PRE) in ferrite given by the equation
W (α)) and the pitting resistance index (PREW (γ)) in austenite given by the following equation (4) (ΔP
A duplex stainless steel excellent in corrosion resistance and phase stability, characterized in that REW) satisfies the condition of -3.0 or more and 3.0 or less. PREW =% Cr + 3.3 × (% Mo + 0.5 ×% W)
+ 16 ×% N (1) Expression ΔHv (σ) = 4.9 ×% Cr ×% Mo + 4.5 ×% C
r-103.4 ×% Mo + 19.3 ×% Ni-10.7
×% N−3.6 ×% W−228.6 (2) Formula PREW (α) = (3 ×% α−1000) / ｛0.03
× (% α) ² -2 ×% α-800｝ ×% Cr + 3.3 ×
(3 ×% α-1360) / ｛0.03 × (% α) ² −
5.6 ×% α−800 ° ×% Mo + 1.65 × (30
0) / (% α + 200) ×% W (3) Formula PREW (γ) = (− 800) / ｛0.03 × (% α)
^{2 -2 ×% α-800}} ×% Cr + 3.3 × (-80
0) / ｛0.03 × (% α) ² −5.6 ×% α−80
0｝ ×% Mo + 1.65 × (200) / (% α + 20
0) ×% W + 16 × (100) / (100−% α) ×%
N ... (4) where% α = 5.8 ×% Cr + 4 ×% Mo + 1.2 ×
% W-5.7 ×% Ni-63.8 ×% N-54.4 6. C: 0.03% or less, Si: 0.10
-2.00%, Mn: 0.10-2.00%, P: 0.
05% or less, S: 0.005% or less, Cr: 18.0 to
25.0%, Ni: 2.0-8.0%, Mo: 3.0-
7.0%, Al: 0.001 to 0.04%, N: 0.1
0 to 0.40%, Cu: 0.01 to 2.00%, W:
One or two of 0.01 to 1.50%, Ca:
0.0005-0.010%, Mg: 0.0005-
0.010%, B: 0.0005 to 0.010%, Rare earth metal: 0.0005 to 0.010%, containing one or more kinds, and the balance consisting of unavoidable impurities and Fe,
And a pitting resistance index (PRE) given by the following equation (1):
W) is 43 or more, and is an index of the difference in Vickers hardness between a water-cooled material that does not precipitate an intermetallic compound such as a σ phase and an air-cooled material that precipitates an intermetallic compound such as a σ phase, which is given by the following formula (2). ΔHv (σ) is 65 or less, the following (3)
The pitting resistance index (PRE) in ferrite given by the equation
W (α)) and the pitting resistance index (PREW (γ)) in austenite given by the following equation (4) (ΔP
A duplex stainless steel excellent in corrosion resistance and phase stability, characterized in that REW) satisfies the condition of -3.0 or more and 3.0 or less. PREW =% Cr + 3.3 × (% Mo + 0.5 ×% W)
+ 16 ×% N (1) Expression ΔHv (σ) = 4.9 ×% Cr ×% Mo + 4.5 ×% C
r-103.4 ×% Mo + 19.3 ×% Ni-10.7
×% N−3.6 ×% W−228.6 (2) Formula PREW (α) = (3 ×% α−1000) / ｛0.03
× (% α) ² -2 ×% α-800｝ ×% Cr + 3.3 ×
(3 ×% α-1360) / ｛0.03 × (% α) ² −
5.6 ×% α−800 ° ×% Mo + 1.65 × (30
0) / (% α + 200) ×% W (3) Formula PREW (γ) = (− 800) / ｛0.03 × (% α)
^{2 -2 ×% α-800}} ×% Cr + 3.3 × (-80
0) / ｛0.03 × (% α) ² −5.6 ×% α−80
0｝ ×% Mo + 1.65 × (200) / (% α + 20
0) ×% W + 16 × (100) / (100−% α) ×%
N ... (4) where% α = 5.8 ×% Cr + 4 ×% Mo + 1.2 ×
% W-5.7 ×% Ni-63.8 ×% N-54.4 7. C: 0.03% or less, Si: 0.10 to
2.00%, Mn: 0.10-2.00%, P: 0.0
5% or less, S: 0.005% or less, Cr: 18.0 to 2
5.0%, Ni: 2.0-8.0%, Mo: 3.0-
7.0%, Al: 0.001 to 0.04%, N: 0.1
0 to 0.40%, V: 0.01 to 0.50%, Ti:
0.01 to 0.50%, Nb: one or more of 0.01 to 0.50%, Ca: 0.0005 to 0.5%.
010%, Mg: 0.0005 to 0.010%, B:
0.0005 to 0.010%, rare earth metal: 0.000
5 to 0.010% of one or more kinds, the balance being unavoidable impurities and Fe, and having a pitting resistance index (PREW) given by the following formula (1) of 43 or more and the following ( 2) An index Δ of the Vickers hardness difference between a water-cooled material given by the formula and a water-cooled material such that a σ phase or the like intermetallic compound is precipitated and a material cooled by air so as to deposit a σ phase or other intermetallic compound.
Hv (σ) is 65 or less, a pitting corrosion resistance index (PREW (α)) in ferrite given by the following equation (3) and a pitting corrosion index (PREW) in austenite given by the following equation (4) (Γ)) is −3.0.
A duplex stainless steel excellent in corrosion resistance and phase stability, satisfying the conditions of not less than 3.0 and not more than 3.0. PREW =% Cr + 3.3 × (% Mo + 0.5 ×% W)
+ 16 ×% N (1) Expression ΔHv (σ) = 4.9 ×% Cr ×% Mo + 4.5 ×% C
r-103.4 ×% Mo + 19.3 ×% Ni-10.7
×% N−3.6 ×% W−228.6 (2) Formula PREW (α) = (3 ×% α−1000) / ｛0.03
× (% α) ² -2 ×% α-800｝ ×% Cr + 3.3 ×
(3 ×% α-1360) / ｛0.03 × (% α) ² −
5.6 ×% α−800 ° ×% Mo + 1.65 × (30
0) / (% α + 200) ×% W (3) Formula PREW (γ) = (− 800) / ｛0.03 × (% α)
^{2 -2 ×% α-800}} ×% Cr + 3.3 × (-80
0) / ｛0.03 × (% α) ² −5.6 ×% α−80
0｝ ×% Mo + 1.65 × (200) / (% α + 20
0) ×% W + 16 × (100) / (100−% α) ×%
N ... (4) where% α = 5.8 ×% Cr + 4 ×% Mo + 1.2 ×
% W-5.7 ×% Ni-63.8 ×% N-54.4 8. C: 0.03% or less, Si: 0.10%
2.00%, Mn: 0.10-2.00%, P: 0.0
5% or less, S: 0.005% or less, Cr: 18.0 to 2
5.0%, Ni: 2.0-8.0%, Mo: 3.0-
7.0%, Al: 0.001 to 0.04%, N: 0.1
0 to 0.40%, Cu: 0.01 to 2.00%, W:
One or two of 0.01 to 1.50%, V:
0.01 to 0.50%, Ti: 0.01 to 0.50%,
Nb: one or more of 0.01 to 0.50%, Ca: 0.0005 to 0.010%, Mg: 0.0
005 to 0.010%, B: 0.0005 to 0.010
%, Rare earth metal: 0.0005 to 0.010%, one or more kinds, the balance being unavoidable impurities and Fe, and a pitting corrosion resistance index (PREW) given by the following formula (1). ) Is 43 or more and σ given by the following equation (2).
The index ΔHv (σ) of the Vickers hardness difference between a water-cooled material such that no intermetallic compound such as a phase precipitates and an air-cooled material such that a σ phase or other intermetallic compound precipitates is given by the following formula (3). The difference (ΔPREW) between the pitting resistance index (PREW (α)) in ferrite and the pitting corrosion index (PREW (γ)) in austenite given by the following equation (4) is −3.0. A duplex stainless steel excellent in corrosion resistance and phase stability, satisfying the conditions of not less than 3.0 and not more than 3.0. PREW =% Cr + 3.3 × (% Mo + 0.5 ×% W)
+ 16 ×% N (1) Expression ΔHv (σ) = 4.9 ×% Cr ×% Mo + 4.5 ×% C
r-103.4 ×% Mo + 19.3 ×% Ni-10.7
×% N−3.6 ×% W−228.6 (2) Formula PREW (α) = (3 ×% α−1000) / ｛0.03
× (% α) ² -2 ×% α-800｝ ×% Cr + 3.3 ×
(3 ×% α-1360) / ｛0.03 × (% α) ² −
5.6 ×% α−800 ° ×% Mo + 1.65 × (30
0) / (% α + 200) ×% W (3) Formula PREW (γ) = (− 800) / ｛0.03 × (% α)
^{2 -2 ×% α-800}} ×% Cr + 3.3 × (-80
0) / ｛0.03 × (% α) ² −5.6 ×% α−80
0｝ ×% Mo + 1.65 × (200) / (% α + 20
0) ×% W + 16 × (100) / (100−% α) ×%
N ... (4) where% α = 5.8 ×% Cr + 4 ×% Mo + 1.2 ×
% W-5.7 ×% Ni-63.8 ×% N-54.4