JPWO2020138490A1 - Welded structure and its manufacturing method - Google Patents

Abstract

The present invention provides a welded structure having excellent toughness and corrosion resistance in a brackish water environment. The welded structure according to the present invention includes a two-phase stainless steel base material having a PREN value of 28 or more defined by the following formula (1), and a welded portion including a welded metal and a heat-affected portion. The PREN value defined by the following formula (1) is 30 or more, the amount of austenite of the two-phase stainless steel base material cut out from the structure is 30 to 70 area%, and the weld metal and the weld heat affected portion. The amount of austenite is 15 to 70 area%, and the pitting potential by the JIS G0577 A method measured at 50 ° C. of the welded portion is 0.30 V vs. SSE or more.

Description

The present invention relates to a welded structure using duplex stainless steel and a method for manufacturing the same.

With the frequent occurrence of natural disasters such as the latest earthquake, construction and renovation of structures in response to tsunamis and floods are being strengthened in various places. The structure of these has become larger due to the recent revision of the assumed water level of tsunami and flood damage. Among these structures, the floodgates built on rivers and the land locks built on the roads of embankments are made of steel or aluminum because they are movable parts.

Recently, austenitic stainless steel or duplex stainless steel has been increasingly applied to these floodgates and land locks.

Of the floodgates, those installed at the estuary are submerged in seawater or water with a high salinity close to it, and high corrosion resistance is required. In the case of austenitic stainless steel, SUS304 often has insufficient corrosion resistance, and SUS316L having better corrosion resistance is often used.

Duplex stainless steel has higher strength than other stainless steels and carbon steels in addition to corrosion resistance and can be made thinner and lighter. rice field.

There are six JIS grades of duplex stainless steel: SUS821L1, SUS323L, SUS329J1, SUS329J3L, SUS329J4L, and SUS327L. Among them, SUS821L1 is a steel grade developed as a substitute for SUS304, SUS323L is a steel grade developed as a substitute for SUS316L, and SUS329J3L, SUS329J4L, and SUS327L are highly corrosion-resistant steel grades having corrosion resistance in a harsher environment.

In the case of duplex stainless steel, it is necessary to consider the decrease in toughness and corrosion resistance of the welded part. N added to duplex stainless steel is precipitated as Cr nitride by heating and cooling during welding. This nitride lowers the toughness of the welded portion by promoting the propagation of cracks, and also lowers the corrosion resistance by consuming Cr by precipitation and forming a so-called Cr-deficient layer.

In the case of a welded structure such as a water gate, in order to weld a thick plate of several tens of mm, dozens of passes may be welded, and as a result, nitride precipitation and accompanying deterioration of the toughness and corrosion resistance of the welded portion are severe. It can be a thing.

The corrosion resistance of the base metal of the above-mentioned SUS323L is equal to or higher than that of SUS316L, but it may be lower than the corrosion resistance level of SUS316L depending on the welding conditions. As shown in Patent Document 1, SUS821L1 is a component system capable of suppressing a decrease in corrosion resistance of a welded portion, but since it is a SUS304 substitute steel, it is not suitable for use in a welded structure in a brackish water environment. Among the higher corrosion resistant steel types, SUS329J3L, SUS329J4L, and SUS327L have very excellent corrosion resistance, but contain 3% or more of expensive Mo and are very costly.

The remaining SUS329J1 is suitable for use in welded structures in a brackish water environment because the corrosion resistance of the base metal is higher than that of SUS323L and the content of Mo is low, but the major problem is that the corrosion resistance of the welded portion is severely deteriorated. As a countermeasure against this, for example, Patent Document 2 describes an improved duplex stainless steel of SUS329J1 in which the corrosion resistance of the welded portion is improved by adding an appropriate N in relation to Ni. However, the composition of the steel is designed on the premise that there is no filler metal by TIG welding, and it is patented how toughness the welded structure manufactured by welding the duplex stainless steel has in the base metal and the welded part. It is not clarified in Document 2.

Further, Patent Document 3 describes a welding method in which nitrogen is mixed in a weld metal using a filler material coated with a coating agent containing nitrogen. However, a special filler metal is required and a base metal is used. No improvement is made for the weld heat affected zone.

Patent Document 4 discloses a duplex stainless steel welded structure having corrosion resistance in an ozone-containing water environment by optimizing the elemental components. However, there is no mention of the weld heat affected zone of the base metal. Further, Patent Document 5 discloses a duplex stainless steel in which a decrease in corrosion resistance due to nitride precipitation in the HAZ portion is suppressed by optimizing the elemental components. However, neither invention is premised on use in a brackish water environment.

Japanese Patent No. 5345070 Japanese Unexamined Patent Publication No. 62-267452 Japanese Unexamined Patent Publication No. 2014-14830 JP-A-2018-168461 Japanese Unexamined Patent Publication No. 2012-197509

In view of the above circumstances of the prior art, the present invention provides a welded structure having excellent corrosion resistance of a welded portion in a steam water environment and excellent toughness as a structure by using duplex stainless steel having a Mo content of less than 3%. The purpose is to do.

In order to solve the above problems, the present inventors have conducted detailed studies on the components of steel materials, the components of weld metals, the manufacturing conditions of steel materials, and the welding conditions from the viewpoint of improving corrosion resistance and toughness in brackish water environments.

Generally, the pitting corrosion resistance of stainless steel is ranked by the pitting corrosion index, but various calculation formulas have been proposed. The pitting corrosion index (PREN) is often expressed by the formula Cr + 3.3Mo + 16N in duplex stainless steel.

Using this formula, the present inventors estimated a method for improving the corrosion resistance of the welded portion of SUS329J1 by including N in the composition range of SUS329J1 by simulation calculation, and confirmed it by an experiment. As a result, the base metal satisfies the required corrosion resistance if the value of the PREN (formula (1) below) is 28 or more, even if the corrosion resistance is lowered due to the precipitation of Cr nitride in the weld heat affected zone. In addition to the above, for weld metals, even if the local corrosion resistance is reduced due to component segregation, the amount of austenite is secured as described later, and the PREN value is 30.0 or more and Mo is appropriately set. It was clarified that by increasing the amount, duplex stainless steel having corrosion resistance equal to or higher than that of SUS316L can be obtained.
PREN = Cr + 3.3Mo + 16N ... (1)

Here, the corrosion resistance equal to or higher than that of SUS316L means that "the pitting potential by the JIS G0577 A method measured at 50 ° C. becomes 0.30 V vs. SSE or higher".

Further, when the steel material is exposed to the ferrite single-phase region for a long time during welding of the steel material, the ferrite phase is promoted to be coarsened, and the toughness of the weld heat-affected zone is lowered. The present inventors have found that the components of the steel material preferably satisfy the following formula (2) in order to prevent a decrease in the toughness of the weld heat-affected zone.
Tα = 1455-13.6Cr + 22.7Ni-11.2Mo + 2.1Mn + 781.8N ≧ 1330 ... (2)

Further, due to gas shield arc welding and tungsten arc welding, the amount of austenite in the weld heat affected zone is less than the amount of austenite in the base metal, the toughness is lowered due to the excess ferrite, and the corrosion resistance in the austenite phase may be lowered.
Since the welding metal has a particularly high cooling rate, not only the time during which the austenite phase can be reprecipitated is limited, but also it is necessary to consider a local decrease in the composition, and the amount of Ni is appropriately increased from the viewpoint of ensuring toughness. From the viewpoint that it is necessary to do so, the present inventors have conducted diligent studies.
As a result, even when the lower limit of the austenite amount of the weld heat affected zone and the weld metal is set to 15%, the composition is adjusted so that the N amount of the steel material and the weld metal satisfies the following formula (3). The present inventors have found that they effectively improve the strength and corrosion resistance of duplex stainless steel.
N ≧ (0.08Cr + 0.08Mo-0.06Ni-1.21) /0.6 × 0.15 ... (3)

In the formula (3), when the lower limit of the amount of austenite in the weld heat-affected zone and the weld metal in the present invention is set to 15%, the amount of N that effectively acts to improve the strength and corrosion resistance of the two-phase stainless steel is determined. This formula is estimated from the contents of Cr, Ni, and Mo, which are the main elements. Based on these findings, the present invention was made, and the gist thereof is as follows.

(1) By mass%
C: 0.001 to 0.050%,
Si: 0.05 to 0.80%,
Mn: 0.10% to 2.00%,
Cr: 21.50 to 26.00%,
Ni: 3.00 to 7.00%,
Mo: 0.50-2.50%,
N: 0.100 to 0.250%,
Al: 0.003 to 0.050%,
Contains,
O is 0.0060% or less,
P is 0.050% or less,
S is limited to 0.0050% or less
And the PREN value defined by the following formula (1) is 28.0 or more.
Duplex stainless steel base material with the balance consisting of Fe and impurities,
A welded structure including a weld metal and a welded zone including a heat-affected zone.
The weld metal is
By mass%
C: 0.001 to 0.060%,
Si: 0.05 to 0.80%,
Mn: 0.10% to 3.00%,
Cr: 21.50 to 28.00%,
Ni: 4.00-10.00%,
Mo: 1.00 to 3.50%,
N: 0.080 to 0.250%,
Al: 0.001 to 0.100%,
Contains,
O is 0.150% or less,
P is 0.050% or less,
S is limited to 0.0200% or less,
And the PREN value defined by the following formula (1) is 30.0 or more.
The rest consists of Fe and impurities
The amount of austenite in the two-phase stainless steel base material is 30 to 70 area%, and the amount of austenite in the weld metal and the weld heat-affected zone is 15 to 70 area%, respectively.
A welded structure characterized in that the pitting potential of the welded portion and the pitting corrosion test piece containing the duplex stainless steel base material measured at 50 ° C. by the JIS G0577 A method is 0.30 V vs SSE or more.
PREN = Cr + 3.3Mo + 16N ... (1)
However, the element symbol in the formula (1) indicates the content (mass%) of each element, and if it is not contained, 0 is substituted.
(2) The components of the duplex stainless steel base material satisfy the formula (2), and the N amounts of the duplex stainless steel base material and the weld metal satisfy the formula (3).
Further, when the duplex stainless steel base material contains Nb, the chromium nitride precipitation temperature TN of the duplex stainless steel base material is 1010 ° C. or lower, and the duplex stainless steel base material does not contain Nb. The welded structure according to (1), wherein the duplex stainless steel base material has a chromium nitride precipitation temperature TN of 980 ° C. or lower.
Tα = 1455-13.6Cr + 22.7Ni-11.2Mo + 2.1Mn + 781.8N ≧ 1330 ... (2)
N ≧ (0.08Cr + 0.08Mo-0.06Ni-1.21) /0.6 × 0.15 ... (3)
However, the element symbol in the formulas (2) and (3) indicates the content (mass%) of each element, and if it is not contained, 0 is substituted.
(3) The welded structure according to (2), wherein the chromium nitride precipitation temperature TN is the following estimated formula (4) or formula (5).
8Cr-20Ni + 30Mo + 50Si-10Mn + 550N + 730 (when the duplex stainless steel base material contains Nb) ... (4)
8Cr-20Ni + 30Mo + 50Si-10Mn + 550N + 700 (when the duplex stainless steel base material does not contain Nb) ... (5)
However, the element symbol in the formulas (4) and (5) indicates the content (mass%) of each element, and if it is not contained, 0 is substituted.
(4) At least one of the duplex stainless steel base material and the weld metal further contains Nb: 0.005 to 0.150%.
Ti: 0.003 to 0.020%
Ta: 0.005 to 0.200%,
Zr: 0.001 to 0.050%
Hf: 0.001 to 0.080%
Sn: 0.005 to 0.100%,
W: 0.01 to 1.00%
Co: 0.01-1.00%
Cu: 0.01 to 3.00%
V: 0.010 to 0.300%
B: 0.0001 to 0.0050%
Ca: 0.0005 to 0.0050%
Mg: 0.0005 to 0.0050%
REM: 0.005 to 0.050%
The welded structure according to any one of (1) to (3), which comprises one or more of the above.
(5) A heat-rolling material having the composition of the duplex stainless steel base material has a reduction ratio of 3.0 or more represented by the following formula (6) and a reduction ratio of 1050 ° C. or lower represented by the following formula (7). (1) to (4), wherein the duplex stainless steel base material is produced by hot rolling so that the temperature is 30% or more and heat-treating at TN + 20 ° C. or higher and 1100 ° C. or lower for 5 minutes or longer. The method for manufacturing a welded structure according to any one of the above.
Thickness of hot rolling material / thickness of duplex stainless steel base material ... (6)
(Thickness when reaching 1050 ° C or lower-thickness of duplex stainless steel base material) / Thickness when reaching 1050 ° C or lower x 100 ... (7)
(6) The weld metal is formed by gas shielded arc welding or tungsten arc welding using a filler rod, and the welding heat input Q defined by the following formula (8) is 5,000 J / cm or more 50. The method for manufacturing a welded structure according to (5), wherein the base metal dilution ratio D defined by the following formula (9) is formed under welding conditions of 000 J / cm or less and 50% or less.
Q = [Welding current (A)] x [Welding voltage (V)] ÷ [Welding speed (cm / s)] ... (8)
D = [Melted volume of duplex stainless steel base metal] / [Total weld metal volume] x 100 ... (9)

The welded structure obtained by the present invention has sufficient corrosion resistance equal to or higher than that of SUS316L in a brackish water environment such as a floodgate near the mouth of a river, and can be reduced in weight due to high strength, resulting in a significant cost reduction. It can contribute to high efficiency, and it contributes significantly to the industrial and environmental aspects.

Welded structure No. It is a partially enlarged sectional view of the welded part of 51-88.

[Composition of duplex stainless steel base material]
First, the composition of the duplex stainless steel base material constituting the welded structure of the present invention and the reason for limiting the structure will be described below. Unless otherwise specified in the present specification,% with respect to a component represents mass%.

[Essential elements]
C is limited to a content of 0.050% or less in order to ensure the corrosion resistance of stainless steel. If it is contained in excess of 0.050%, Cr carbide is generated during hot rolling, and the corrosion resistance and toughness are deteriorated. It is preferably 0.030% or less, and more preferably 0.025% or less.
On the other hand, 0.001% is set as the lower limit from the viewpoint of cost of reducing the amount of C in stainless steel.

Si is contained in an amount of 0.05% or more for deoxidation. It is preferably 0.10% or more, more preferably 0.20% or more.
On the other hand, if it is contained in excess of 0.80%, the toughness deteriorates. Therefore, it should be 0.80% or less. It is preferably 0.50% or less, more preferably 0.40% or less.

Mn has the effect of increasing the austenite phase and improving toughness. It also has the effect of lowering the nitride precipitation temperature TN. It is contained in an amount of 0.10% or more due to the toughness of the base metal and the welded part. It is preferably 0.30% or more, and more preferably 0.50% or more.
On the other hand, since Mn is an element that lowers the corrosion resistance of stainless steel, it is preferable to set Mn to 2.00% or less. It is preferably 1.80% or less, more preferably 1.50% or less.

Cr is contained in an amount of 21.50% or more in order to ensure the basic corrosion resistance of the steel of the present invention. It is preferably 22.00% or more, more preferably 23.00% or more.
On the other hand, when Cr is contained in excess of 26.00%, the ferrite phase fraction increases and the toughness and corrosion resistance of the welded portion are impaired. Therefore, the Cr content was set to 26.00% or less. It is preferably 25.00% or less, more preferably 24.50% or less.

Ni is contained in an amount of 3.00% or more in order to stabilize the austenite structure, improve corrosion resistance to various acids, and improve toughness. By increasing the Ni content, it becomes possible to lower the nitride precipitation temperature. It is preferably 4.00% or more, more preferably 5.00% or more.
On the other hand, Ni is an expensive alloy, and the content of the present invention steel for alloy-saving duplex stainless steel is limited to 7.00% or less from the viewpoint of cost. It is preferably 6.50% or less, more preferably 6.00% or less.

Mo is a very effective element for enhancing the corrosion resistance of stainless steel, and is contained in an amount of 0.50% or more in order to impart corrosion resistance of SUS316 or more. It is preferably 0.80% or more, more preferably 1.00% or more.
On the other hand, Mo is an element that promotes precipitation of intermetallic compounds as well as being expensive, and the steel of the present invention preferably has a low Mo content from the viewpoint of suppressing precipitation during hot rolling and from the viewpoint of economy. It shall be 50% or less. It is preferably less than 2.00%, more preferably 1.80% or less, and more preferably 1.50% or less.

Since N is an effective element that dissolves in the austenite phase to enhance the strength and corrosion resistance of the duplex stainless steel, it is contained in an amount of 0.100% or more. It is preferably 0.120% or more, more preferably 0.150% or more.
On the other hand, the solid solution limit increases according to the Cr content, but in the steel of the present invention, if it is contained in excess of 0.250%, Cr nitride is precipitated and the toughness and corrosion resistance are impaired. Therefore, the N content was set to 0.250% or less. It is preferably 0.230% or less, more preferably 0.200% or less.

Al is an important element for deoxidation of steel, and is contained together with Ca and Mg in order to control the composition of inclusions in the steel. Al may be contained together with Si in order to reduce oxygen in the steel. Al is contained in an amount of 0.003% or more in order to control the composition of inclusions and enhance pitting corrosion resistance. It is preferably 0.005% or more.
On the other hand, Al is an element having a relatively large affinity for N, and when it is added in excess, a nitride of Al is generated and the toughness of stainless steel is impaired. The degree depends on the N content, but if Al exceeds 0.050%, the toughness is significantly reduced, so the content should be 0.050% or less. It is preferably 0.040% or less, more preferably 0.030% or less.

[Remaining]
In the chemical composition of the duplex stainless steel base material constituting the welded structure of the present invention, the balance is Fe and impurities. Here, impurities are those that are mixed in from ore, scrap, manufacturing environment, etc. as raw materials when the steel base material is industrially manufactured, and are allowed as long as they do not adversely affect the steel. Means what is done. Examples of the main impurities include, but are not limited to, P, S, and O, and other elements may be contained as impurities.

O (oxygen) is an impurity and is an element that impairs the hot workability, toughness, and corrosion resistance of stainless steel, so it is preferable to reduce it as much as possible. Therefore, the O content is limited to 0.006% or less. Further, since a very large cost is required for refining in order to extremely reduce oxygen, the amount of oxygen may be 0.001% or more in consideration of economic efficiency.

P is an element that is inevitably mixed from the raw material, and since it deteriorates hot workability and toughness, it is preferably as small as possible, and is limited to 0.050% or less. Preferably, it is 0.040% or less. In order to reduce P to an extremely low amount, the cost at the time of refining becomes high. Therefore, it is preferable to set the lower limit of the amount of P to 0.010% in consideration of the cost.

S is an element that is inevitably mixed from the raw material, and since it also deteriorates hot workability, toughness, and corrosion resistance, it is preferably as small as possible, and the upper limit is limited to 0.0050% or less. It is preferably 0.0020% or less, more preferably 0.0010% or less. In order to reduce S to an extremely low amount, the cost at the time of refining becomes high. Therefore, the lower limit of the amount of S may be set to 0.0001% in consideration of the cost.

[28.0 ≤ PREN; Austenite amount is 30 area% or more to 70 area% or less]
In the environment of natural water such as fresh water and brackish water of rivers, the natural potential of duplex stainless steel increases due to the activity of microorganisms. In an environment with a high natural potential, even a slight decrease in Cr concentration has a great effect on corrosion resistance. Therefore, in an environment to which the steel of the present invention is applied, when Cr nitride is deposited by welding duplex stainless steel, the Cr-deficient layer around the Cr nitride becomes the starting point of pitting corrosion.

Generally, in a two-phase stainless steel, the amount of austenite is preferably close to the amount of ferrite. When the amount of ferrite is excessive, the toughness is lowered and Cr nitrides are likely to be precipitated. On the other hand, when austenite is excessive, stress corrosion cracking and ear cracking during hot rolling are likely to occur. Further, in either case, the component difference between the ferrite phase and the austenite phase becomes large, and the corrosion resistance decreases in either phase. In the present invention, the lower limit of the amount of austenite in which the above problem is unlikely to occur in the component system of the present invention is defined as 30 area%, and the upper limit is defined as 70 area%.

Further, in the case of duplex stainless steel, it is desirable to secure a higher PREN than austenitic stainless steel when aiming for the same corrosion resistance in consideration of the decrease in corrosion resistance of the weld heat affected zone. As a result of the experiment, when the PREN defined in (1) below, which is an index of pitting corrosion resistance, is less than 28.0, the amount of austenite in the duplex stainless steel base material is 30.0 to 70.0 area%. Even if there was, the corrosion resistance was lower than SUS316L in the weld heat affected zone under the steam environment.
PREN = Cr + 3.3Mo + 16N ... (1)
However, the element symbol in the formula (1) indicates the content (mass%) of each element, and if it is not contained, 0 is substituted.

In order to prevent pitting corrosion in the weld metal in an environment such as steam water, the duplex stainless steel base material for the welded structure according to the present invention has an austenite content of 30.0 to 70.0 area% and the above formula. The PREN value defined in (1) is 28.0 or more. The lower limit of the preferable PREN value of the duplex stainless steel base material is 30.0.

However, if the Cr and Mo contents are excessive in order to increase the PREN of the duplex stainless steel base material, the alloy cost will increase, and if the N content is excessive, Cr nitride will be precipitated to improve toughness and corrosion resistance. Inhibit. Therefore, in the present invention, the duplex value of duplex stainless steel is preferably 35.0 or less. The preferable lower limit of the amount of austenite in the duplex stainless steel base material is 40.0 area%, and the preferable upper limit is 60.0 area%.

In the case of a duplex stainless steel base material, the amount of austenite in the present invention is obtained by collecting a cross section parallel to the rolling direction of the thick steel plate from a position corresponding to t / 4 (t is the plate thickness) of the base steel plate and embedding it in the resin. After mirror polishing and electrolytic etching in a KOH aqueous solution, the ferrite fraction (area%) is measured by performing image analysis by observing with an optical microscope, and the remaining portion is determined as the amount of austenite.

Further, the amount of austenite in the weld metal and the weld heat-affected portion is determined by collecting a test piece so as to include the weld portion (weld metal and the weld heat-affected portion) and the base material in the vicinity thereof, and rolling the two-phase stainless steel base material. Weld metal and weld heat-affected parts are each subjected to etching treatment, observation with an optical microscope, and image analysis in the same manner as for a two-phase stainless steel base material using a mirror-polished directional cross section. The amount of austenite in the metal structure of the metal structure is measured.

[Composition of weld metal]
Next, the reason for limiting the component composition of the weld metal formed in the welded structure in the present invention will be described below. In addition, "%" shown below means "mass%" unless otherwise specified.

The content of each component of the weld metal described below is determined by using either a solid wire or a flux-filled wire, taking into consideration the dilution of the components of the duplex stainless steel base metal into the weld metal, and determining the components in the wire. It can be adjusted within a predetermined range by adjusting.

[Essential elements]
Although C is harmful to corrosion resistance, it is preferably contained to some extent from the viewpoint of strength, so the C content is 0.001% or more. Further, when the content exceeds 0.060%, C is combined with Cr to precipitate Cr carbides in the state of being welded and when reheated, and the intergranular corrosion resistance and pitting corrosion resistance are significantly deteriorated. Since the toughness and ductility of the weld metal are significantly reduced, the content thereof is limited to 0.001 to 0.060%.

Si is added as a deoxidizing element, but its effect is not sufficient if it is less than 0.05%, while if its content exceeds 0.80%, the toughness is greatly reduced as the ductility decreases, and at the time of welding. The melt penetration of silicon is also reduced, which becomes a problem in practical welding. Therefore, its content was limited to 0.05 to 0.80%.

Mn is added as a deoxidizing element and as an element that increases the solubility of N, but the effect is not sufficient when the content is less than 0.10%, while the ductility decreases when the content exceeds 3.00%. Therefore, the lower limit of the content was set to 0.10, and the upper limit was limited to 3.00%. The Mn content is preferably 2.00% or less.

Cr forms a passivation film as a main element of stainless steel and contributes to improvement of corrosion resistance. In order to obtain excellent corrosion resistance in a brackish water environment, 21.50% or more is contained. On the other hand, the higher the Cr content, the better the pitting corrosion resistance in a steam environment, but the toughness decreases because brittle intermetallic compounds such as the sigma phase (σ phase) are likely to precipitate. Further, since Cr is a ferrite-forming element, it is necessary to increase the amounts of Ni, Cu, and N in order to secure the austenite phase, which lowers the manufacturability of the wire used for welding and increases the manufacturing cost. The upper limit of the content was set to 28.00%. It is preferably 26.00% or less.

Ni gives remarkable resistance to corrosion in a neutral chloride environment and strengthens the passive film. Therefore, the higher the Ni content, the more effective the corrosion resistance. In addition, Ni is an austenite-forming element, which forms and stabilizes the austenite phase. As described above, the amount of Ni of the weld metal is appropriately adjusted from the viewpoints that the cooling rate is particularly high, the time during which the austenite phase can be reprecipitated is limited, it is necessary to consider a local decrease in the composition, and the toughness is ensured. It is desirable to increase the amount. In the present invention, in order to ensure sufficient austenite formation in the weld metal, Ni is more than the steel base material from the viewpoint of phase balance when the weld metal contains 21.50 to 28.00% of Cr, which is a ferrite forming element. It is preferable to increase the content, and for weld metals, the lower limit is set to 4.00% and the upper limit is set to 10.00%. The reason for limiting the upper limit of the Ni content to 10.00% is that the manufacturing cost of the wire used for welding is high. It is preferably 6.00% or more.

Mo is an extremely effective element for stabilizing the passivation film and obtaining high corrosion resistance, and the improvement in pitting corrosion resistance in a chloride environment is particularly remarkable. Furthermore, for weld metals, in addition to the above, it is necessary to consider the local deterioration of corrosion resistance due to component segregation. As a result of the experiment, it was found that the effect of improving the corrosion resistance was insufficient if it was less than 1.00%. Further, in order to compensate for the decrease in austenite in the weld metal, it is preferable to increase the Mo content in the weld metal as compared with the steel base material.
However, if the content exceeds 3.50%, a brittle intermetallic compound such as a sigma phase is generated and the toughness of the weld metal is lowered. Therefore, the lower limit is set to 1.00 and the upper limit is limited to 3.50%. It is preferably 2.00% or more and 3.00% or less.

N is a strong austenite-forming element, which improves pitting corrosion resistance in a chloride environment. At 0.080% or more, pitting corrosion resistance and crevice corrosion resistance are improved, and the larger the content, the greater the effect. On the other hand, when the N content is increased, blow holes are likely to occur during welding, especially when it exceeds 0.250%. Therefore, the lower limit of the N content is limited to 0.080% and the upper limit is limited to 0.250%. It is preferably 0.100% or more and 0.200% or less.

Al is added as a deoxidizing element and as an element for improving the droplet transfer phenomenon, but if it is less than 0.001%, the effect is not sufficient, while its excessive addition reacts with N to AlN. And inhibit toughness. The degree depends on the N content, but when Al exceeds 0.100%, the toughness is significantly reduced. Therefore, the lower limit of the content is 0.001% and the upper limit is limited to 0.100%.

[Remaining]
In the chemical composition of the weld metal formed in the welded structure of the present invention, the balance is Fe and impurities. Here, impurities are those that are mixed in from ore, scrap, manufacturing environment, etc. as raw materials when the steel base material is industrially manufactured, and are allowed as long as they do not adversely affect the steel. Means what is done. Examples of the main impurities include, but are not limited to, P, S, and O, and other elements may be contained as impurities.

O, P, and S are unavoidable components in weld metals, and are limited to a small number for the following reasons.

Since O produces oxides and excessive content significantly reduces toughness, the upper limit of the content is set to 0.150%.

When a large amount of P is present, the resistance to high temperature weld cracking and toughness during solidification are lowered, so a small amount is preferable, and the upper limit of the content is set to 0.050%.

If a large amount of S is also present, the high temperature cracking resistance, ductility and corrosion resistance are lowered, so a small amount is preferable, and 0.0200% is the upper limit.

[PREN ≧ 30.0; Austenite amount is 15 area% or more and 70 area% or less]
In the environment of natural water such as fresh water and brackish water of rivers, the natural potential of duplex stainless steel increases due to the activity of microorganisms. In an environment with a high natural potential, even a slight decrease in Cr concentration has a great effect on corrosion resistance. Therefore, in an environment to which the steel of the present invention is applied, when Cr nitride is deposited by welding duplex stainless steel, the Cr-deficient layer around the Cr nitride becomes the starting point of pitting corrosion. The present inventors have clarified that when the amount of austenite in the two-phase stainless steel welded portion of the welded structure is less than 15 area% or more than 70 area%, the corrosion resistance is less than SUS316L.

Similar to the steel base material, the amount of austenite in the weld metal is preferably close to the amount of ferrite. However, the amount of austenite phase produced in the weld heat-affected zone and the weld metal tends to be small, and in addition to increasing the amount of austenite phase as much as possible, the amount of austenite in the weld metal is further reduced than that in the weld heat-affected zone. In order to suppress this, the composition is improved by a filler rod such as a steel welding wire. In addition, the amount of austenite that does not cause a problem that the corrosion resistance is lowered as compared with SUS316L is defined as 15 area% or more and 70 area% or less.

Further, regarding PREN, which is an index of pitting corrosion resistance, when the PREN of the weld metal is less than 30.0, even if the amount of austenite in the weld metal is 15 area% or more and 70 area% or less, component segregation occurs to cause locality. Corrosion resistance is lowered, and the corrosion resistance of weld metal is lower than that of SUS316L in a steam environment.

Therefore, in the weld metal, the amount of austenite is 15 area% or more and 70 area% or less, and the PREN of the weld metal is 30.0 or more. The preferable lower limit of the amount of austenite in the weld metal is 18.0 area%, and the more preferable lower limit is 20.0 area%. The preferable upper limit of the amount of austenite in the weld metal is 60.0 area%, and the more preferable upper limit is 50.0 area%. Further, the PREN value of the weld metal is preferably higher than the PREN value of the duplex stainless steel base material. However, if the contents of Cr and Mo are excessive in order to increase the PREN of the weld metal, the alloy cost will increase, and if the content of N is excessive, blow holes are likely to occur during welding. Therefore, in the present invention, the PREN value of the weld metal is preferably 35.0 or less.

Further, in gas shielded arc welding and tungsten arc welding, in order to ensure the corrosion resistance of the welded portion of the welded structure of the present invention, the weld heat affected zone also has an austenite amount of 15 area% or more and 70 areas, similarly to the weld metal. %.

[Arbitrary additive components of duplex stainless steel base metal and weld metal]
Further, the duplex stainless steel base material and weld metal constituting the welded structure of the present invention (hereinafter, also simply referred to as “base material and weld metal of the welded structure of the present invention”) are one of the following elements. Species or two or more species can be contained in an amount of 0% or more as required. However, the object of the present invention can be achieved without containing any of these elements.

Nb is an element that has a strong affinity for N and has an effect of further reducing the precipitation rate of chromium nitride. Therefore, the base material and the weld metal of the welded structure of the present invention may contain 0.005% as the lower limit. It is preferably 0.010% or more, more preferably 0.020% or more, and more preferably 0.030% or more.
On the other hand, when Nb is contained in an amount of more than 0.150%, a large amount of nitride of Nb is precipitated and the toughness is inhibited. Therefore, the content is set to 0.150% or less. It is preferably 0.090% or less, more preferably 0.070% or less, and more preferably 0.050% or less.
Although Nb is an expensive element, the cost of the stainless steel melting raw material can be reduced by positively using Nb contained in low-grade scrap. It is preferable to reduce the melting cost of the Nb-containing steel by such a method.

Ti has a very strong affinity with N and forms a nitride of Ti in steel. Therefore, when Ti is contained, it is desirable to use a very small amount. If the content exceeds 0.020%, the toughness will be inhibited by the nitride of Ti. Therefore, the content thereof is 0.020% or less, preferably 0.015% or less, more preferably 0.010%. It should be as follows. When Ti is contained, it is preferable to contain it in an amount of 0.003% or more, preferably 0.005% or more, and more preferably 0.006% or more in order to obtain the effect.

Ta is an element that improves corrosion resistance by modifying inclusions, and may be contained if necessary. Since the effect is exhibited by the content of Ta of 0.005% or more, the lower limit of the amount of Ta may be set to 0.005% or more. When the Ta amount exceeds 0.200%, the ductility at room temperature is lowered and the toughness is lowered. Therefore, the upper limit of the Ta amount is preferably 0.200% or less, more preferably 0.100% or less. When the effect is exhibited with a small amount of Ta, the amount of Ta is preferably 0.050% or less.

W is an element that improves the corrosion resistance of stainless steel like Mo, and may be contained. It may be contained in the steel of the present invention for the purpose of enhancing corrosion resistance. However, since it is an expensive element, it should be 1.00% or less. It is preferably 0.70% or less, more preferably 0.50% or less. When added, it is preferably contained in an amount of 0.05 or more. When W is contained, the W content is preferably 0.01% or more, preferably 0.05% or more, and more preferably 0.10% or more in order to obtain the effect.

V is an element that has an affinity for N and has an action of lowering the precipitation rate of chromium nitride. Therefore, it may be contained. However, if the content exceeds 0.300%, a large amount of V nitride is precipitated and the toughness is inhibited. Therefore, the V content is 0.300% or less, preferably 0.250% or less. , More preferably 0.200% or less. When V is contained, the V content is preferably 0.010% or more, preferably 0.030% or more, and more preferably 0.080% or more in order to obtain the effect.

Ca and Mg are added to control the composition of inclusions in the steel of the present invention and to enhance the pitting corrosion resistance and hot workability of the steel of the present invention. In steels to which Ca and Mg are added, the Ca content is reduced to 0 by adding it together with Al of 0.0030% or more and 0.0500% or less using a dissolving raw material, or by adjusting the content through deoxidation and desulfurization operations. The Mg content is controlled to 0.0001% or more and 0.0001% or more. It is preferable that Ca is 0.0010% or more, Mg is 0.0003% or more, and more preferably Ca is 0.0015% or more and Mg is 0.0005% or more.

On the other hand, the contents of Ca and Mg should be controlled to 0.0050% or less for Ca and 0.0050% or less for Mg because excessive addition of both Ca and Mg deteriorates hot workability and toughness. .. Ca is preferably 0.0040% or less, Mg is 0.0025% or less, and more preferably Ca is 0.0035% or less and Mg is 0.0020% or less.

Co is an element effective for enhancing the toughness and corrosion resistance of steel, and may be contained. Even if Co is contained in an amount of more than 1.00%, it is an expensive element and therefore an effect commensurate with the cost is not exhibited. Therefore, it is preferable to contain Co in an amount of 1.00% or less. It is preferably contained in an amount of 0.70% or less, more preferably 0.50% or less. When Co is contained, the Co content is preferably 0.01% or more, preferably 0.03% or more, and more preferably 0.10% or more in order to obtain the effect.

Cu may be contained because it is an element that additionally enhances the corrosion resistance of stainless steel to acid and has an action of improving toughness. If Cu is contained in an amount of more than 3.00%, εCu is precipitated and embrittled in excess of the solid solubility during cooling after hot rolling. Therefore, it is preferable to contain Cu in an amount of 3.00% or less. It is preferably contained in an amount of 1.70% or less, more preferably 1.50% or less. When Cu is contained, it is preferable to contain it in an amount of 0.01% or more, preferably 0.33% or more, and more preferably 0.45% or more.

B is an element that improves the hot workability of steel, and may be contained if necessary. Further, it is an element having a very strong affinity for N, and when it is contained in a large amount, the nitride of B is precipitated and the toughness is inhibited. Therefore, the content may be 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less. When B is contained, the B content is preferably 0.0001% or more, preferably 0.0005% or more, and more preferably 0.0014% or more in order to obtain the effect.

REM is an element that improves the hot workability of steel, and may be contained in an amount of 0.005% or more for that purpose. It is preferably contained in an amount of 0.010% or more, more preferably 0.020% or more. On the other hand, excessive addition lowers hot workability and toughness, so the REM should be contained in an amount of 0.050% or less. It is preferably 0.040% or less, more preferably 0.030% or less.
Here, REM is the sum of the contents of lanthanoid rare earth elements such as La and Ce.

Zr, Hf, and Sn segregate at the grain boundaries to suppress coarsening of crystal grains during welding. Further, Zr and Hf are conventionally effective elements for improving hot workability, cleanliness of steel, and oxidation resistance. Sn is concentrated near the surface and suppresses the oxidation of Cr.

In order to obtain these effects, Zr: 0.001% or more, Hf: 0.001% or more, Sn: 0.005% or more may be contained. In the welded structure of the present invention, the weld metal portion contains at least one element of the Zr, Hf, Sn element group instead of the Ni, Cu, Mo, W element group as described above. It may be contained in a range.

On the other hand, since excessive addition of these elements promotes grain boundary destruction due to a decrease in grain boundary strength, Zr: 0.050% or less, Hf: 0.080% or less, Sn: 0.100% or less.

[Ferrite single-phase temperature]
In the present invention, the components of the duplex stainless steel base material preferably satisfy the following formula (2).
Tα = 1455-13.6Cr + 22.7Ni-11.2Mo + 2.1Mn + 781.8N ≧ 1330 ... (2)
Tα is a component formula for estimating the temperature at which austenite disappears and becomes a ferrite single phase when the two-phase stainless steel base material is heated (hereinafter, referred to as “ferrite monophase temperature”; the unit is ° C.). .. If the ferrite single-phase temperature is low, the ferrite single-phase region is exposed for a long time during welding, which promotes coarsening of the ferrite phase and reduces the toughness of the weld heat-affected zone. As a result of the experiment, it was found that the toughness of the heat-affected zone is extremely lowered when the Tα is lower than 1320 ° C. Therefore, it is preferable to set the temperature to 1330 ° C. or higher. More preferably, it is 1340 ° C. or higher.
This formula was obtained by equilibrium calculation using Thermo-Calc's thermodynamic calculation software "Thermo-Calc" (registered trademark), and was corrected experimentally.

[Chromium nitride precipitation temperature and N amount]
In the present invention, the N amount of the duplex stainless steel base material and the weld metal preferably satisfies the following formula (3).
N ≧ (0.08Cr + 0.08Mo-0.06Ni-1.21) /0.6 × 0.15 ... (3)
However, the element symbol in the formula (3) indicates the content (mass%) of each element, and if it is not contained, 0 is substituted.

In the formula (3), when the lower limit of the austenite amount of the welding heat-affected portion and the welding metal in the present invention is set to 15%, the two-phase stainless steel is dissolved in the austenite phase of the welding heat-affected portion and the welding metal. It is an equation that estimates the amount that effectively acts to improve strength and corrosion resistance from the contents of Cr, Ni, and Mo, which are the main elements.

The component formula for estimating the austenite content of the two-phase stainless steel is described in, for example, Ni-bal. All of these estimate the amount of austenite in the solution-heat-treated steel material. In this case, Cr and Mo are distributed and concentrated in the ferrite phase, and Ni and N are distributed and concentrated in the austenite phase to form each phase.

In the case of the weld heat affected zone and the weld metal, a ferrite single phase is once formed during heating, and then an austenite phase is formed during cooling. As a result of the experiment, when the weld heat-affected zone and the weld metal are cooled, Cr, Ni, and Mo are hardly concentrated in the austenite phase, and only N is concentrated in the austenite phase to form the austenite phase. I found out. Further, the amount of N concentrated in the austenite phase during the cooling varies depending on the amount of Cr, Ni, and Mo, and when the austenite-forming element Ni is high, the amount of N is small, and when Cr and Mo are high, the amount of N is small. It was found that the amount of N in the austenite phase was increased. From the above findings, when the amount of N enrichment is small, it is expected that a large amount of austenite can be produced with a small amount of N by increasing the amount of Ni contained in the weld heat affected zone and the weld metal.

The precipitate that mainly affects the material of the duplex stainless steel base material constituting the welded structure of the present invention is chromium nitride.
Chromium nitride is a precipitate in which Cr and N are bonded, and in duplex stainless steel, cubic Cr _{N or hexagonal Cr 2} N is often precipitated in ferrite grains or at ferrite grain boundaries. When these chromium nitrides are formed, the impact characteristics are deteriorated, and the corrosion resistance is lowered due to the chromium-deficient layer formed by the precipitation.

The chromium nitride precipitation temperature TN, which is an index for precipitation of such chromium nitride during hot rolling, is a characteristic value experimentally obtained by the following procedure.
(1) A 10 mm thick test steel is heat-rolled and then heat-treated at 1050 ° C for 20 minutes, then soaked at an arbitrary temperature of 800 to 1100 ° C for 20 minutes, and then water-cooled within 5 seconds. ..
(2) Polish the surface layer of the test steel after cooling with # 500.
(3) A 3 g sample is separated and electrolyzed (100 mV constant voltage) in a non-aqueous solution (containing 3% maleic acid and 1% tetramethylammonium chloride, the balance being methanol) at room temperature to dissolve the matrix.
(4) The residue (that is, the precipitate) is filtered through a filter having a hole diameter of 0.2 μm, and the precipitate is extracted.
(5) Using ICP, the chemical composition of the residue is analyzed to determine the chromium content (mass%) contained in the residue. The chromium content in this residue is used as an index of the amount of chromium nitride precipitated.
(6) The soaking heat treatment temperature of (1) is variously changed, and the lowest temperature among the soaking heat treatment temperatures at which the chromium content in the residue is 0.03% or less is defined as TN.

As the TN is lower, the temperature range in which chromium nitride is deposited is limited to the low temperature side, so that the precipitation rate and amount of chromium nitride are suppressed, and the corrosion resistance of the duplex stainless steel base material is maintained.

Therefore, the duplex stainless steel base material constituting the welded structure of the present invention has a chromium nitride precipitation temperature TN of 1010 ° C. or lower when it contains Nb, and a chromium nitride precipitation temperature TN when it does not contain Nb. Is preferably 980 ° C. or lower.

The above-mentioned chromium nitride precipitation temperature TN may be estimated using the following formula (4) or formula (5).
8Cr-20Ni + 30Mo + 50Si-10Mn + 550N + 730 (when duplex stainless steel base material contains Nb) ... (4)
8Cr-20Ni + 30Mo + 50Si-10Mn + 550N + 700 (when duplex stainless steel base material does not contain Nb) ... (5)
However, the element symbol in the formulas (4) and (5) indicates the content (mass%) of each element, and if it is not contained, 0 is substituted.

[Method for manufacturing welded structure of the present invention]
Next, a method for manufacturing the welded structure of the present invention will be described.
[Manufacturing method of duplex stainless steel base material]

Thick steel materials such as 20 mm and 50 mm are often used for water gates and the like. When these two-phase stainless steels are manufactured, the impact value of the base metal is lowered, and as a result, the toughness of the heat-affected portion is further lowered, which may cause a problem. In order to avoid this phenomenon, the hot-rolling material having the composition of the duplex stainless steel base material described above has a compression reduction ratio of 3.0 or more represented by the following formula (6) and the following formula (7). It is effective to apply appropriate strain to obtain a fine structure by hot rolling so that the rolling reduction ratio is 30% or more at 1050 ° C. or lower.
Thickness of hot rolling material / Thickness of duplex stainless steel base material of welded structure of the present invention ... (6)
(Thickness when reaching 1050 ° C or lower-thickness of duplex stainless steel base metal of the welded structure of the present invention) / Thickness when reaching 1050 ° C or lower x 100 ... (7)
The "thickness when the temperature reaches 1050 ° C. or lower" is obtained by sequentially measuring the surface temperature of the hot rolling material during hot rolling and measuring the thickness when the temperature reaches 1050 ° C. or lower. ..

Further, in order to eliminate intermetallic compounds and chromium nitrides that reduce corrosion resistance in duplex stainless steel, hot rolled steel sheets are heat-treated at a chromium nitride precipitation temperature (TN) + 20 ° C. or higher and 1100 ° C. or lower for 5 minutes or longer. If the heat treatment temperature is less than TN + 20 ° C. or the heat treatment time is less than 5 minutes, the chromium nitride precipitated by hot rolling does not dissolve in solid solution, and the toughness and corrosion resistance are impaired. Further, if the heat treatment temperature exceeds 1100 ° C., the amount of ferrite may become excessive. This heat treatment may be continuously performed from the hot rolling step, or may be performed by reheating the cooled steel sheet after cooling the hot rolled steel sheet.

[Welding process]
In the present invention, it is preferable to limit the welding conditions when forming the weld metal in order to form the welded portion having excellent toughness and corrosion resistance in the seawater environment as follows.

The weld metal of the present invention can be formed by either gas shielded arc welding or tungsten arc welding, but it is preferable to specify the welding heat input amount Q and the base metal dilution ratio D for the following reasons.

[Welding heat input Q]
When a duplex stainless steel containing Cr and Mo is held in a temperature range of about 700 ° C. to 900 ° C., brittle intermetallic compounds such as a sigma phase, which is harmful to toughness, are precipitated, and corrosion resistance and toughness are significantly reduced. .. Similarly, Cr nitride, which is also harmful to corrosion resistance and toughness, precipitates in a temperature range of about 600 ° C to 800 ° C. When the weld metal passes through 900 ° C. to 600 ° C. for a long time in the cooling process after solidification, a large amount of sigma phase or Cr nitride is precipitated. Further, in the weld metal formed by the multi-layer pass welding, the same applies when the front layer pass undergoes a thermal cycle by the subsequent pass and the time to reach the temperature range of 600 ° C. to 900 ° C. becomes long.

In the present invention, by defining the component compositions of the duplex stainless steel base material and the weld metal as described above, the precipitation of intermetal compounds such as sigma phase and Cr nitride is suppressed, and the duplex excellent in toughness and corrosion resistance. A welded structure made of a stainless steel base metal and a weld metal can be obtained. However, in gas shielded arc welding or tungsten arc welding, if the welding heat input Q exceeds 50,000 J / cm, the base metal dilution rate described later increases and the cooling rate decreases, resulting in 900 ° C to 600 ° C. There is a risk that the cooling time will be long and intermetallic compounds such as the sigma phase and Cr nitride will precipitate, resulting in deterioration of corrosion resistance and toughness. Therefore, in order to stably secure the corrosion resistance and toughness of the welded structure, it is preferable to limit the manufacturing conditions of the welded structure, that is, the amount of heat input to the weld at the time of welding to 50,000 J / cm or less.

The welding heat input Q (J / cm) is defined by the following equation (8).
Q (J / cm) = [welding current (A)] x [welding voltage (V)] ÷ [welding speed (cm / s)] ... (8)

On the other hand, if the welding heat input amount is too small, less than 5,000 J / cm, the cooling rate becomes high, and the austenite precipitation amount becomes too small even if the component is specified as in the present invention.

[Base material dilution rate D]
In the present invention, in order to secure the corrosion resistance and the austenite content of the weld metal, it is preferable that the weld metal has a higher Mo content, Ni content, and PREN than the two-phase stainless steel base material. However, if the dilution ratio with the base material is too high, even if an appropriate filler rod is used, the base material is mixed in a large amount, and it becomes difficult to obtain the target component. That is, as a welding condition, the base metal dilution rate at the time of welding is preferably limited to 50% or less. The base metal dilution ratio D is defined by the following formula.
D = [Melted volume of duplex stainless steel base metal] / [Total weld metal volume] x 100 ... (9)

The welded structure of the present invention can also be manufactured by submerged arc welding, plasma welding, etc. on the premise that appropriate filler rods and welding heat input control are performed. Further, the manufacturing method can be applied not only to the manufacture of welded structures but also to repair welding or overlaying of those structures.

In the present invention, when a welded structure composed of a duplex stainless steel base material and a weld metal having a defined component content as described above is manufactured, welding is performed under the above-mentioned welding conditions to obtain excellent toughness. A welded structure having a weld metal whose corrosion resistance is ensured in a steam water environment can be stably obtained.

[Pitting potential of welded part by JIS G0577 A method measured at 50 ° C is 0.30V vs SSE or more]
In the welded structure of the present invention, the pitting potential of the welded portion including the weld metal and the heat-affected zone measured at 50 ° C. by the JIS G0577 A method is 0.30 V vs. SSE or more. As described above, the welded structure of the present invention has corrosion resistance equal to or higher than that of SUS316L in a brackish water environment.

[Toughness of welded structures]
The duplex stainless steel base material constituting the welded structure of the present invention has a Charpy impact value of 100 J / cm ² or more at −20 ° C. measured by the Charpy impact test method specified in JIS Z 2202.
Further, in the weld heat-affected zone and the weld metal of the welded structure of the present invention, the Charpy impact value measured by the Charpy impact test method specified in JIS Z 2202 is 50 J / cm ² or more at −20 ° C. be.

Hereinafter, the present invention will be described with reference to Examples. In the following examples, for convenience, an example of the present invention will be described based on a butt-type joint made of the same steel base material, but the welded structure according to the present invention is limited to the illustrated structure. Not done. The welded structure according to the present invention can have not only a butt joint but also a general welded joint structure such as a T joint, a cross joint, and a lap joint, and has a structure in which different types of welded joints are combined. You may be doing it. Further, as long as the steel base material does not deviate from the scope of the present invention, the welded structure according to the present invention may have a structure in which a steel base material having at least one different steel composition and metal structure is welded.

Duplex stainless steel having the components shown in Table 1-1 and Table 1-2, except for Steel No. 24, was melted in an MgO crucible in a 50 kg vacuum induction furnace in the laboratory and cast into a flat steel ingot. The flat steel ingot was ground so that the surface of the flat steel ingot was smooth to prepare a material for hot rolling of about 100 mm. The hot-rolled material was heated to a temperature of 1180 ° C. for 1 to 2 hours and then rolled so that the reduction ratio of 1050 ° C. or lower was 35% to obtain a hot-rolled thick steel sheet having a plate thickness of 12 mm and a length of about 700 mm. The temperature immediately after hot rolling was spray-cooled from 800 ° C. or higher to 200 ° C. or lower. Then, the cooled steel sheet was heated and heat-treated to equalize the heat at 1050 ° C. for 20 minutes, and after the heat treatment, the steel sheet was water-cooled.

Tα (° C.) in Tables 1-1 and 1-2 is the temperature value defined by the above formula (2), and the “value of the formula (3)” is defined by the above formula (3). It is the amount of N, and the "TN estimated value (° C.)" is the value of the temperature defined by the above formula (4) or the formula (5). The TN measured values shown in Tables 1-1 and 1-2 are the measured values of the chromium nitride precipitation temperature of each steel base material, and the steel Nos. A 10 mm-thick test steel was cut out from each steel base material other than 24, and the cut-out test steel was subjected to soaking heat treatment and precipitation from the test steel after the soaking heat treatment was performed by the above-mentioned procedure. It was measured by determining the lowest temperature among the soaking heat treatment temperatures at which the chromium content in the precipitate was 0.03% or less.

In Tables 1-1 and 1-2, steel Nos. 1 to 8 refer to the welded structure Nos. of the examples of the present invention as shown in Tables 3 and 5. It is a duplex stainless steel base material constituting 51 to 61.

Steel Nos. 9 to 25 are welded structure Nos. of Comparative Examples. It is a duplex stainless steel base material constituting 62 to 73, 81, 84 to 88. Steel No. Reference numerals 9 to 17, 20, 21, and 24 are steel base materials that do not satisfy the requirements for the composition of the steel base material of the welded structure of the present invention. Reference numerals 13, 15 and 21 are duplex stainless steel base materials whose PREN values do not meet the requirements for the steel base material of the welded structure of the present invention. Steel No. Reference numeral 18 denotes a duplex stainless steel base material in which the amount of N does not satisfy the above formula (3). Steel No. Reference numeral 19 denotes a two-phase stainless steel base material in which the amount of ferrite is excessive and the amount of austenite is insufficient (welded structure No. 73 in Tables 3 and 5). Steel No. Reference numeral 22 denotes a two-phase stainless steel base material in which the amount of austenite is excessive (welded structure No. 85 in Tables 3 and 5). Steel No. 24 is a stainless steel base material having a thickness of 12 mm and a length of about 700 mm, which is made of commercially available SUS316L.

Steel Nos. In Table 1-1 and Table 1-2. As shown in FIG. 1, a groove having a groove angle of 90 ° on one side and 35 ° on one side and a root spacing of 4 mm was prepared by using steel base materials 11a and 11b as 1 to 25. In FIG. 1, the steel base materials 11a and 11b have the same steel No. It is a steel base material of.

Tables 2-1 and 2-2 show the welded structure Nos. No. of steel welding wire used to manufacture 51-88. The component compositions of 31 to 43 are shown. The wire diameter is 1.2 mmφ. Welded structure No. Reference numerals 51 to 88 are butt-type welded joints 1 shown in FIG. 1, and the steel Nos. It was manufactured by welding the steel base materials 1 to 25 against each other while attaching a backing metal 2 to the back surface of the steel base material. The welding conditions are as shown in Table 3. In the case of gas shielded arc welding (GMAW), the conditions are: welding current: 150 to 200 A, arc voltage: 23 to 31 V, welding speed: 5 to 40 cm / min, and CO ₂ shield gas flow rate: 20 liters / min. Welded joint 1 was manufactured. In the case of tungsten arc welding (GTAW), the welding current is 180 to 220 A, the arc voltage is 11 to 14 V, the welding speed is 15 to 25 cm / min, and the 100% Ar shield gas flow rate is 15 liters / min. Welded joint 1 was manufactured.

Table 3 shows the welded structure No. The combination of the steel base material and the welding wire used in the production of 51 to 88, the welding method, and the amount of heat input for welding are shown. In the welding method shown in Table 3, GMAW indicates gas shielded arc welding and GTAW indicates tungsten arc welding.

Tables 4-1 to 4-3 show the composition of the weld metal 12 formed under the conditions of Table 3, the dilution ratio of the base metal, PREN, and the amount of N (mass%) defined by the above formula (3) (“Formula”. (3) value ”), the temperature estimated from the above formula (4) or the formula (5) (item“ TN estimated value (° C.) ”in Table 4) is shown.

In Tables 4-1 to 4-3, blanks indicate that the corresponding component was not added. The underline indicates that the composition of the weld metal constituting the welded structure of the present invention is outside the range.

Welded structure No. 62 was manufactured using the same steel base material and steel welding wire as the welded structure No. 61 of the example of the present invention, but the carbon content of the weld metal was excessive because oil and the like were mixed in the welded portion during welding. became.

In addition, the welded structure No. shown in Table 3 Perforated test pieces were collected from the steel base material in the vicinity of the weld heat affected zone and the weld metal so as to include all the weld heat affected zone and the weld metal from 51 to 88, and were contained in a 3.5% NaCl solution at 50 ° C. The pitting potential was measured in accordance with the method specified in JIS G0577.

Furthermore, the welded structure No. From each of 51 to 86 and No. 88, the steel base material of the welded joint, the weld heat affected zone (0.1 mm outside from the weld line) and the weld metal portion are notched at a depth of 1/4 of the plate thickness from the surface layer. A V-notch test piece was sampled in the direction perpendicular to the rolling direction according to the Charpy impact test method specified in JIS Z 2202 so as to correspond to the notch portion of the test piece. Each of these V-notch test pieces was subjected to a Charpy impact test at a test temperature of −20 ° C. The results of the pitting potential and the Charpy impact test are shown in Table 5.

In addition, the welded structure No. The amount of austenite contained in each of the two-phase stainless steel base materials of 51 to 86 and No. 88, the weld metal, and the weld heat-affected zone was measured by the method described above. The results are shown in Table 5. Welded structure No. Each of the metal structures of 51 to 86 and 88 contains the austenite phase area ratio (%) shown in Table 5, and the balance is ferrite.

The underline in Table 5 indicates that it is outside the scope of the present invention. Welded structure No. 87 was manufactured using a commercially available product SUS316L, and the measurement of the austenite phase area ratio and the Charpy impact value was omitted.

From Table 5, it can be seen that Examples No. 51 to 61 of the present invention have sufficient corrosion resistance equal to or higher than that of SUS316L. Further, in Examples No. 51 to 61 of the present invention, the Charpy impact value of the duplex stainless steel base material is 100 J / cm ² or more at −20 ° C., and the Charpy impact value of the weld heat affected zone and the weld metal is −. It is 50 J / cm ² or more at 20 ° C. As described above, it can be seen that Examples No. 51 to 61 of the present invention have excellent toughness in addition to excellent corrosion resistance.

In the welded structure No. 86 of the comparative example, the corrosion resistance of the welded metal does not reach SUS316L because the PREN value of the weld metal is less than 30.0.

Under the manufacturing conditions shown in Table 6-1 5) was obtained. The "pressure reduction ratio" is a value defined by the above formula (6), and the "reduction rate of 1050 ° C. or lower" is a value defined by the above formula (7). "TN (° C.) (actual measurement value)" is the same as that of "TN (° C.) (actual measurement value)" with duplex stainless steel base materials such as steel Nos. 1 to 8 in Table 1-1 and Table 1-2. Measured by method. The manufacturing conditions other than the manufacturing conditions in Table 6-1 were the same as the manufacturing conditions for duplex stainless steel base materials such as Steel Nos. 1 to 8 in Tables 1-1 and 1-2. Next, using the steel base materials Nos. 1, 3, and 5, the steel welding wire Nos. Welded structures No. 101-107 were manufactured using 31 under the conditions shown in Table 6-1. The manufacturing conditions of the welded structure Nos. 101 to 107 are the same as those of the welded structure No. 101 to 107, except for the manufacturing conditions shown in Table 6-1. It is the same as 51-86 and No.88.

For each of the welded structures Nos. 101 to 107, the welded structure Nos. The Charpy impact test and the measurement of the pitting potential were carried out in the same manner as in 51-86 and the like. The results are shown in Table 6-2. The underline in Table 6-2 indicates that it is outside the scope of the present invention.

Welded structure No. Since Nos. 1 and 5 were manufactured by the manufacturing method of the present invention, the Charpy impact value was more than ^{100 J / cm 2.} On the other hand, No. ^{In Nos. 2} , 3, 6 and 7, the Charpy impact value is less than 100 J / cm 2 and the welded structure No. 2 of the comparative example. No. 4 had low corrosion resistance.

According to the present invention, in a brackish water environment such as a sluice near the mouth of a river, it has sufficient corrosion resistance equal to or higher than that of SUS316L, and can be reduced in weight due to its high strength. It is possible to contribute, and the place that contributes to the industrial and environmental aspects is extremely large.

1 Welded joint 11a Steel base material 11b Steel base material 12 Welded metal 12

Claims (6)

By mass%
C: 0.001 to 0.050%,
Si: 0.05 to 0.80%,
Mn: 0.10% to 2.00%,
Cr: 21.50 to 26.00%,
Ni: 3.00 to 7.00%,
Mo: 0.50-2.50%,
N: 0.100 to 0.250%,
Al: 0.003 to 0.050%,
Contains,
O is 0.0060% or less,
P is 0.050% or less,
S is limited to 0.0050% or less
And the PREN value defined by the following formula (1) is 28.0 or more.
Duplex stainless steel base material with the balance consisting of Fe and impurities,
A welded structure including a weld metal and a welded zone including a heat-affected zone.
The weld metal is
By mass%
C: 0.001 to 0.060%,
Si: 0.05 to 0.80%,
Mn: 0.10% to 3.00%,
Cr: 21.50 to 28.00%,
Ni: 4.00-10.00%,
Mo: 1.00 to 3.50%,
N: 0.080 to 0.250%,
Al: 0.001 to 0.100%,
Contains,
O is 0.150% or less,
P is 0.050% or less,
S is limited to 0.0200% or less,
And the PREN value defined by the following formula (1) is 30.0 or more.
The rest consists of Fe and impurities
The amount of austenite in the two-phase stainless steel base material is 30 to 70 area%, and the amount of austenite in the weld metal and the weld heat-affected zone is 15 to 70 area%, respectively.
A welded structure characterized in that the pitting potential of the welded portion and the pitting corrosion test piece containing the duplex stainless steel base material measured at 50 ° C. by the JIS G0577 A method is 0.30 V vs SSE or more.
PREN = Cr + 3.3Mo + 16N ... (1)
However, the element symbol in the formula (1) indicates the content (mass%) of each element, and if it is not contained, 0 is substituted. The components of the duplex stainless steel base material satisfy the formula (2), and the N amounts of the duplex stainless steel base material and the weld metal satisfy the formula (3).
Further, when the duplex stainless steel base material contains Nb, the chromium nitride precipitation temperature TN of the duplex stainless steel base material is 1010 ° C. or lower, and the duplex stainless steel base material does not contain Nb. The welded structure according to claim 1, wherein the duplex stainless steel base material has a chromium nitride precipitation temperature TN of 980 ° C. or lower.
Tα = 1455-13.6Cr + 22.7Ni-11.2Mo + 2.1Mn + 781.8N ≧ 1330 ... (2)
N ≧ (0.08Cr + 0.08Mo-0.06Ni-1.21) /0.6 × 0.15 ... (3)
However, the element symbol in the formulas (2) and (3) indicates the content (mass%) of each element, and if it is not contained, 0 is substituted. The welded structure according to claim 2, wherein the chromium nitride precipitation temperature TN is the following estimated formula (4) or formula (5).
8Cr-20Ni + 30Mo + 50Si-10Mn + 550N + 730 (when the duplex stainless steel base material contains Nb) ... (4)
8Cr-20Ni + 30Mo + 50Si-10Mn + 550N + 700 (when the duplex stainless steel base material does not contain Nb) ... (5)
However, the element symbol in the formulas (4) and (5) indicates the content (mass%) of each element, and if it is not contained, 0 is substituted. At least one of the duplex stainless steel base metal and the weld metal further contains Nb: 0.005 to 0.150%.
Ti: 0.003 to 0.020%
Ta: 0.005 to 0.200%,
Zr: 0.001 to 0.050%
Hf: 0.001 to 0.080%
Sn: 0.005 to 0.100%,
W: 0.01 to 1.00%
Co: 0.01-1.00%
Cu: 0.01 to 3.00%
V: 0.010 to 0.300%
B: 0.0001 to 0.0050%
Ca: 0.0005 to 0.0050%
Mg: 0.0005 to 0.0050%
REM: 0.005 to 0.050%
The welded structure according to any one of claims 1 to 3, wherein the welded structure contains one or more of the two or more. The heat-rolling material having the composition of the duplex stainless steel base material has a reduction ratio of 3.0 or more represented by the following formula (6) and a reduction ratio of 30% at 1050 ° C or lower represented by the following formula (7). Any one of claims 1 to 4, characterized in that the duplex stainless steel base material is produced by hot rolling so as to be as described above and heat-treating at TN + 20 ° C. or higher and 1100 ° C. or lower for 5 minutes or longer. The method for manufacturing a welded structure according to the section.
Thickness of hot rolling material / thickness of duplex stainless steel base material ... (6)
(Thickness when reaching 1050 ° C or lower-thickness of duplex stainless steel base material) / Thickness when reaching 1050 ° C or lower x 100 ... (7) The weld metal is formed by gas shielded arc welding using a filler rod or tungsten arc welding, and the welding heat input Q defined by the following formula (8) is 5,000 J / cm or more and 50,000 J / cm. The method for manufacturing a welded structure according to claim 5, wherein the base metal dilution ratio D defined by the following formula (9) is formed under welding conditions of 50% or less.
Q = [Welding current (A)] x [Welding voltage (V)] ÷ [Welding speed (cm / s)] ... (8)
D = [Melted volume of duplex stainless steel base metal] / [Total weld metal volume] x 100 ... (9)

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