JP2022114890A - Ferrite-austenite duplex stainless steel welded structures, ferrite-austenite duplex stainless steel welded joints, and methods of welding ferrite-austenite duplex stainless steel materials - Google Patents

Abstract

To provide ferrite-austenite duplex stainless steel welded structures having a welded metal part with excellent corrosion resistance.SOLUTION: Provided is a ferrite-austenite duplex stainless steel welded structure comprising a base material made of a ferrite-austenite duplex stainless steel material and a weld, wherein a reduction margin of an austenite phase ratio (γBM-γWM) in a surface layer of a weld metal in the weld, which is the difference between an austenite phase ratio γBM in a metallographic structure of the base metal and an austenite phase ratio γWM in the surface layer of the weld metal, is less than 20 area%, and a pitting potential reduction margin (PBM - PWM), which is the difference between a pitting potential PBM of the base metal and a pitting potential PWM of the weld, is less than 150 mV.SELECTED DRAWING: None

Description

The present invention relates to a ferrite-austenite duplex stainless steel welded structure, a ferrite-austenite duplex stainless steel welded joint, and a welding method for ferrite-austenite duplex stainless steel materials.

Duplex stainless steel is stainless steel having both austenite phase and ferrite phase in the structure of the steel. Compared to austenitic stainless steel, which generally has the same corrosion resistance, duplex stainless steel is attracting attention as a material that can be made thinner with lower alloying costs because of its low Ni content and high strength. . Due to its high corrosion resistance, it has been used as a thick plate for petrochemical equipment, pumps, and chemical tanks. progressing.

There are many types of duplex stainless steel, such as SUS821L1 with a low alloy content, SUS329J4L with a relatively medium amount of alloy, and SUS327L1 with a relatively large amount of alloy. These currently widely used steel types contain about 0.1 to 0.3% of N (nitrogen), which is excellent in alloy cost, together with Cr, Ni, and Mo as elements that improve corrosion resistance. .

Non-consumable electrode welding or consumable electrode welding is applied as a welding method for constructing a duplex stainless steel welded structure. Non-consumable electrode welding includes TIG welding, plasma welding, laser welding, and the like. Examples of consumable electrode welding include MIG welding, gas shielded arc welding using flux-cored wire, shielded arc welding, and submerged arc welding. Of these, non-consumable electrode welding is inferior to consumable electrode welding in terms of welding efficiency, but since pure Ar gas is used as the shielding gas, the amount of oxygen in the weld metal is extremely low and the toughness of the weld metal is excellent. It is suitable for the construction of welded structures with strict quality requirements.

When applying welded structures with ferritic/austenitic duplex stainless steel as the base material to the required applications, there is a problem due to the precipitation of σ phase or chromium nitrides in the weld metal and weld heat affected zone. This is a decrease in corrosion resistance. Therefore, when duplex stainless steel is used as the base material for welding, it should be used only in applications where corrosion resistance is not a major concern, the welding method should be restricted, reheat treatment should be performed after welding, and special welding gas should be used. In many cases, welding workability is sacrificed, such as by changing to a different material.

By the way, as mentioned earlier, ferritic-austenitic stainless steels, which are represented by SUS821L1, ASTM S32101, NSSC 2351, etc., have relatively low Cr, Ni, and Mo contents and high N content, and are excellent in economic efficiency. It is called alloy-saving duplex stainless steel. Alloy-saving duplex stainless steel is sometimes used as a substitute for SUS304 and SUS316L, which are general-purpose austenitic stainless steels, depending on its corrosion resistance level. These alloy-saving duplex stainless steels often have a problem of decreased corrosion resistance due to the formation of chromium nitrides. The deterioration of corrosion resistance is caused by the following mechanism.

In duplex stainless steel, the phase ratio between the ferrite phase and the austenite phase varies depending on the heating temperature. Therefore, when duplex stainless steel is welded, the rate of ferrite phase increases and the rate of austenite phase decreases in the weld metal zone and the weld heat-affected zone due to heating for melting the base material. On the other hand, during cooling when the weld metal zone and weld heat affected zone are formed, the austenite phase increases. Generally, however, the rate of cooling during the formation of the weld metal zone and the weld heat affected zone is high, so the ratio of the austenite phase in the weld metal zone and the weld heat affected zone is less than that in the base metal.

In addition, N easily evaporates from the welded metal part under high temperature conditions during welding. Since N acts as an austenite stabilizing element and has the effect of increasing the amount of austenite, a decrease in N in the weld metal works to increase the amount of ferrite.

Most of N in the duplex stainless steel is dissolved in the austenite phase. However, in the weld metal zone and the weld heat-affected zone, the ratio of the austenite phase is smaller than that of the base metal, so the N content in the ferrite phase is high. Since the solid solubility limit of N in the ferrite phase is very small compared to that in the austenite phase, even if there is a slight reduction due to the evaporation of N during welding, the weld metal during cooling during welding and the weld heat effect N, which cannot completely dissolve in the ferrite phase, precipitates as chromium nitrides in the ferrite phase or at the grain boundaries between the ferrite phases.

As a result of the consumption of Cr in the steel accompanying the formation of chromium nitrides, a so-called chromium-deficient layer is formed, resulting in deterioration of corrosion resistance. Therefore, it is important to reduce the amount of chromium nitride that precipitates in the weld metal zone and the weld heat-affected zone in order to improve the corrosion resistance of the weld zone.

As a general duplex stainless steel welding method, Patent Literature 1 describes a technique for suppressing the precipitation of chromium nitrides by controlling the welding method in the final welding pass.

In addition, Patent Document 2 describes a technique for recovering the corrosion resistance of the weld heat affected zone by heat-treating the weld heat affected zone at 700° C. to 1000° C. after welding to reprecipitate the austenite phase.

Furthermore, in Patent Document 3, by using a mixed gas of argon and nitrogen as a welding gas instead of pure argon as a shielding gas during welding, the amount of austenite phase in the weld metal is increased, and nitrogen is added to the austenite phase. A technique for suppressing deterioration in corrosion resistance of the weld heat-affected zone by forming a solid solution is described.

JP-A-62-199272 JP 2015-217434 A Japanese Patent No. 6726499

However, in Patent Document 1, condition control during welding is important, and in Patent Document 2, heat treatment after welding is required. For thin plate applications, large quantities are often welded, and there are cases where restrictions on the welding method or post-welding heat treatment are difficult.

In Patent Document 3, it is necessary to add nitrogen to the welding gas, but compared to the case of using normal pure argon gas, the shape of the welding arc changes, so it may take time to optimize the welding conditions. In addition, since nitrogen has higher reactivity than argon, the welding electrode may wear out more quickly in TIG welding or the like.

Furthermore, in general, thin plate applications do not use a filler material during welding, so it is difficult to improve corrosion resistance with a filler material. Therefore, it is conceivable to improve the corrosion resistance of the weld metal zone and the weld heat affected zone by adding alloying elements to the base metal, but this raises another problem of increased alloy costs.

Due to the above background, from the viewpoint of reducing the cost of welding processes and welding structures, welding of duplex stainless steel with less restrictions on welding methods, no need for reheating after welding, and less consumption of welding electrodes. A method is desired. In addition, from the viewpoint of improving corrosion resistance, a welded structure and a welded joint having excellent corrosion resistance of welded portions are desired.

The present invention is a duplex stainless steel having two phases of an austenite phase and a ferrite phase, in which the corrosion resistance of the weld metal part and the weld heat affected zone is less reduced when welded, which can be a bottleneck when using the steel. An object of the present invention is to provide a method for welding ferrite-austenite duplex stainless steel materials that can improve welding workability and reduce welding costs.
Another object of the present invention is to provide a ferrite-austenite duplex stainless steel welded structure and a ferrite-austenite duplex stainless steel welded joint in which the weld metal portion is excellent in corrosion resistance.

The present inventors conducted various tests on steels having various compositions and plate thicknesses in order to evaluate the factors affecting the corrosion resistance of the weld metal zone and weld heat affected zone when welded.

Regarding the above test results, the present inventors discovered that the vicinity of the surface layer of the weld metal portion tends to have a structure with lower corrosion resistance than the inside. In welded structures, the weld metal part is not necessarily polished, and often the weld burn is simply removed with a chemical such as a scorch remover. In this case, the structure near the surface layer of the weld metal part determines the corrosion resistance.

Therefore, we focused on the corrosion resistance in the vicinity of the surface layer of the weld metal and investigated the factors that improve the corrosion resistance in the vicinity of the surface layer of the weld metal. It was found that the corrosion resistance can be improved by increasing the nitrogen concentration in the surface layer of the weld metal.

The present invention has been made based on the above knowledge, and the gist thereof is the welding method, welded structure, and welded joint of ferrite-austenite duplex stainless steel materials described below.

[1] A base material made of a ferritic/austenitic duplex stainless steel material;
a weldment, and
_Decrease in the austenite phase ratio of the surface layer of the _weld metal portion (γ _BM -γ _WM ) is less than 20 area%,
A ferrite-austenite duplex stainless steel in which a pitting potential drop (P _BM −P _WM ), which is a difference between the pitting potential P _BM of the base metal and the pitting potential P _WM of the weld zone, is less than 150 mV. Welded structures.
[2] A base material made of a ferrite/austenite duplex stainless steel material;
a weldment, and
_Decrease in the austenite phase ratio of the surface layer of the _weld metal portion (γ _BM -γ _WM ) is less than 20 area%,
A ferrite-austenite duplex stainless steel in which a pitting potential drop (P _BM −P _WM ), which is a difference between the pitting potential P _BM of the base metal and the pitting potential P _WM of the weld zone, is less than 150 mV. welded joints.
[3] A method of welding a base material made of a ferrite-austenite duplex stainless steel material by a gas-shielded arc welding method, comprising:
A method for welding ferritic and austenitic duplex steels, wherein a gas containing 60 to 100% by volume of _N2 is used as either or both of the after gas and the back gas.
[4] The method for welding ferritic and austenitic duplex steel materials according to [3], wherein the torch shield gas is a gas containing less than 60% by volume of _N2 .
[5] The ferrite-austenitic duplex steel material according to [3] or [4], wherein a gas containing 60% by volume or more of _N2 and the balance Ar is used as either or both of the after gas and the back gas. Welding method.

According to the welding method of the present invention, when a duplex stainless steel having two phases of an austenite phase and a ferrite phase is welded, the corrosion resistance of the weld metal part and the weld heat-affected zone is less reduced, thereby It is possible to improve welding workability and reduce welding costs, which can be a bottleneck during use.
Further, according to the welded structure and the welded joint of the present invention, the corrosion resistance of the welded metal portion can be improved.

FIG. 1 is a photograph showing the metallographic structure of the surface layer of the weld metal part obtained by welding the base metal made of steel type B of the example, and (a) is an example using Ar gas as the after gas. (Sample No. 27) and (b) are photographs showing an example (Sample No. 5) using N ₂ as the after gas. FIG. 2 is a graph showing the distribution of nitrogen concentration in the depth direction of the surface layer of the weld metal obtained by welding the base material made of steel type B of the example. is a graph of an example (sample number 32) in which Ar is used as the after gas, and (b) is an example in which Ar + 5% _N2 gas is used as the torch shield gas and Ar gas is used as the after gas (sample number 15). graph. FIG. 3 is a schematic diagram for explaining the welding method in the example. FIG. 4 is a graph showing the relationship between the γ-phase ratio decrease amount and the pitting potential decrease amount of the surface layer of the weld metal portion in the example.

Hereinafter, a ferrite/austenite duplex stainless steel welded structure (which may be referred to as a welded structure in the following description) and a ferrite/austenite duplex stainless steel welded joint (hereinafter referred to as a welded joint) which are embodiments of the present invention and a method of welding ferrite/austenite duplex stainless steel sheets (which may be referred to as a welding method in the following description).

The welded structure of the present embodiment includes a base material made of a ferrite- _austenitic duplex stainless steel material, and a welded portion. The decrease in the austenite phase ratio of the surface layer of the weld metal (γ _BM −γ _WM ), which is the difference from the austenite phase ratio γ _WM , is less than 20 area%, and the pitting potential P _BM of the base metal and the hole of the weld zone The welded steel structure has a pitting potential drop (P _BM −P _WM ), which is a difference from the corrosion potential P _WM , of less than 150 mV.

In addition, the welded joint of the present embodiment includes a base material made of a ferrite- _austenite duplex stainless steel material, and a welded portion. The decrease in the austenite phase ratio of the surface layer of the weld metal (γ _BM −γ _WM ), which is the difference from the austenite phase ratio γ _WM of the weld metal part, is less than 20 area%, and the pitting potential P _BM of the base metal and The welded joint has a pitting potential drop (P _BM −P _WM ), which is a difference from the pitting potential P _WM , of less than 150 mV.
Each requirement will be described in detail below.

(Welded structures and welded joints)
In this embodiment, a welded structure and a welded joint formed by butt welding will be described as an example, but the present invention does not need to be limited to butt welding, and welded by lap welding, fillet welding, etc. It is also applicable to welded structures and welded joints. Welded structures and welded joints formed by butt welding include, for example, welded structures and welded joints in which steel plates are butt-welded, and welded structures and welded joints in which ends of steel pipes are butt-welded. I can give an example.

(base material)
The base material according to this embodiment is a ferrite-austenite duplex stainless steel material. A ferrite-austenite duplex stainless steel material contains a duplex structure of a ferrite phase and an austenite phase in the metal structure. The shape of the base material is not particularly limited and may be a plate material, a tube material, a bar material, a wire material, or the like.

The chemical composition of the ferrite-austenite duplex stainless steel will be described later.

(Regarding welded parts and weld metal parts)
The welded portion according to this embodiment includes a welded metal portion and a welded heat affected zone. In the present embodiment, from the viewpoint of improving the corrosion resistance of the weld zone, attention is focused on the amount of decrease in the austenite phase ratio in the weld metal part and the amount of decrease in the pitting potential in the weld zone.

(Amount of decrease in austenite phase ratio of the surface layer of the weld metal part: less than 20% by area)
The decrease in the austenite phase ratio of the surface layer of the weld metal zone (γ _BM −γ _WM ), which is the difference between the austenite phase ratio γ _BM in the metal structure of the base metal and the austenite phase ratio γ _WM of the surface layer of the weld metal zone, is Less than 20 area %. The reduction is desirably less than 10%. By setting the reduction margin to less than 20 area %, the corrosion resistance of the welded metal portion can be increased to the same level as the corrosion resistance of the base material.

The reason for paying attention to the austenite phase ratio γ _BM in the surface layer of the weld metal part is based on the findings of the present inventors. That is, the present inventors discovered that the vicinity of the surface layer of the welded metal part after welding tends to have a structure with lower corrosion resistance than the inside. In this case, the structure near the surface layer of the welded metal portion determines the corrosion resistance. In addition, in welded structures and welded joints, the surface layer of the weld metal portion is not polished, and the weld burn is only removed with a chemical such as a scorching agent as necessary, leaving the surface layer as it is after welding. There are many cases. Therefore, in order to improve the corrosion resistance, it is necessary to reduce the decrease in the austenite fraction near the surface layer of the weld metal portion.

In addition, although the lower limit of the decrease does not need to be set in particular, it is preferably 0%, that is, the difference between the austenite phase ratio γ _BM in the metal structure of the base metal and the austenite phase ratio γ _WM in the surface layer of the weld metal is 0. %.

The surface layer of the welded metal portion is defined as a region from the surface of the welded metal portion to a depth of 30 μm. The content of the austenite phase in this region is defined as the austenite phase ratio _γWM .

Also, the austenite phase ratio γ _BM in the metal structure of the base metal is defined as the content of the austenite phase in the region from the surface of the base metal to a depth of 200 µm.

The method for measuring the austenite fraction γ _BM in the metallographic structure of the base metal is as follows. When the base material is a steel plate, the metallographic structure of a 200 μm wide×200 μm deep region from the surface layer of a cross section (TD cross section) perpendicular to the rolling length direction at the center position in the rolling width direction is observed with an optical microscope. The structure image of the observation surface is classified into ferrite phase and austenite phase by image analysis, and the austenite phase ratio γ _BM of the base metal is obtained.

The method for measuring the austenite phase ratio γ _WM in the metallographic structure of the surface layer of the weld metal is as follows. A cross section perpendicular to the welding direction at the center of the width of the weld metal portion is observed with an optical microscope for a metal structure of a region of 200 μm width×surface layer to 30 μm depth. The structure image of the observed surface is classified into ferrite phase and austenite phase by image analysis, and the austenite phase ratio γ _WM of the surface layer of the weld metal portion is obtained.

The decrease in the austenite phase ratio of the surface layer of the weld metal portion (γ _BM −γ _WM ) is the value obtained by subtracting the austenite phase ratio γ _WM of the surface layer of the weld metal portion from the austenite phase ratio γ _BM of the base metal.

(Welded pitting potential drop: less than 150 mV)
When chromium nitride precipitates in the welded portion, the corrosion resistance of the welded portion deteriorates more than the corrosion resistance of the base metal portion. On the other hand, the pitting potential of steel varies depending on the degree of precipitation of chromium nitrides, and the pitting potential decreases as the precipitation of chromium nitrides progresses. Therefore, the inventors of the present invention paid attention to the amount of decrease in the pitting potential in the weld zone. The allowable pitting potential drop of the weld zone in this embodiment is less than 150 mV. It is more desirable to be less than 100 mV. Since it is desirable that the pitting corrosion potential decrease is as small as possible, there is no lower limit. By setting the pitting potential drop at the welded portion to less than 150 mV, the corrosion resistance at the welded portion can be enhanced.

The decrease in the pitting potential of the weld zone is the difference (P _BM −P _WM ) between the pitting potential P _BM of the base material and the pitting potential P _WM of the weld zone.

The pitting potential _PBM of the base material is obtained by polishing the surface of the base material to a depth of 0.2 mm and then measuring the pitting potential of the polished surface.
In addition, the pitting potential _PWM of the weld zone is obtained by removing the weld scale if it adheres to the surface of the weld zone, masking the areas other than the weld zone, and applying the masking treatment to the weld zone. obtained by measuring the pitting corrosion potential.

More specifically, after wet polishing the surface of the base material at a position of 0.2 mm below the surface with an abrasive grain size of #600, it is subjected to a 1 mol/NaCl solution at 30° C. by the method specified in JIS G 0577. and a saturated silver chloride electrode (SSE), the pitting potential ( _{V' c100} ) corresponding to a current density of 100 μA/cm ² is measured, and the value obtained by averaging the six measurement data is the pitting potential of the base material P _BM and

In addition, 1 mol at 30 ° C. was performed by the method specified in JIS G 0577 except that the weld was carefully dry-polished with P600 abrasive paper immediately before the measurement after removing the weld scale using a burn-off material. /L NaCl solution and a saturated silver chloride electrode (SSE) were used to measure the pitting corrosion potential ( _V'c100 ) corresponding to a current density of 100 μA/cm ² , and the value obtained by averaging the six measurement data was taken as the surface layer of the weld. is the pitting potential _PWM . As the burn-off material, for example, Lathnonwell JL-500 (manufactured by Mansho Co., Ltd.) can be exemplified, but other chemicals or other methods may be used as long as the weld scale can be sufficiently removed.

In addition, in order to evaluate the corrosion resistance inside the welded metal part of the welded joint, the central part in the plate thickness direction of the welded part was exposed as an evaluation surface, and was heated to 30 ° C. by the method specified in JIS G 0577. 1 mol/L NaCl solution and a saturated silver chloride electrode (SSE), the pitting corrosion potential ( _{V' c100} ) corresponding to a current density of 100 μA/cm ² was measured, and the value obtained by averaging the six measurement data was taken as the weld zone. Let the internal pitting potential be _Pmid .

In this embodiment, the difference (P _BM −P _mid ) between the pitting potential P _BM of the base material and the pitting potential P _mid inside the weld may be 200 mV or less. Thereby, the corrosion resistance of the weld can be further enhanced. Desirably, it is 150 mV or less.

The pitting potential _Pmid inside the weld zone was measured by exposing the central part of the weld zone in the plate thickness direction as an evaluation surface, and using a 1 mol/L NaCl solution at 30°C and saturated silver chloride by the method specified in JIS G 0577. Using an electrode (SSE), the pitting potential ( _V'c100 ) corresponding to a current density of 100 μA/cm ² is measured, and the value obtained by averaging the six measured data is taken as the pitting potential _Pmid inside the weld. good.

In the welded structure and the welded joint of the present embodiment, either the surface side of the welded portion (the welding torch side during welding) or the back side of the welded portion (the side opposite to the welding torch side during welding) In one or both of them, it is sufficient that the surface layer of the weld metal zone has a lower austenite phase ratio of less than 20 area % and a lower pitting potential of the weld zone of less than 150 mV. That is, it is not necessary that the austenite fraction reduction and the pitting potential reduction on both the front and back surfaces of the welded portion satisfy the scope of the present invention. This is because the welded structure and the welded joint according to the present embodiment may be satisfied if the corrosion resistance of either the front surface or the back surface of the welded portion is improved depending on the installation environment.

Next, the welding method of this embodiment will be described.
The welding method of this embodiment is a method of welding a base material made of a ferrite-austenite duplex stainless steel material by a gas-shielded arc welding method. It is a welding method using a gas containing vol% _N2 .
Moreover, in the welding method of the present embodiment, it is preferable to use a gas containing less than 60% by volume of N ₂ as the torch shield gas.
In addition, in the welding method of the present embodiment, it is preferable to use a gas containing 60% by volume or more of N ₂ and the balance Ar as one or both of the after gas and the back gas.

In the welding method of the present embodiment, the welding method, welding conditions, and whether or not to use a filler material can be appropriately set, and are not limited in any way. Also, the shape of the base material and the shape of the welded joint are not limited at all.

Welding is preferably performed by a gas shielded arc welding method as a welding method. Among gas-shielded arc welding methods, non-consumable electrode welding methods are preferred. Also, a filler material may or may not be used. Whether or not to use a filler material may be appropriately selected according to the shape of the base material, the thickness of the base material, the shape of the weld groove, and the type of weld joint. For example, when butt-welding relatively thin duplex stainless steel plates, a non-consumable electrode welding method that does not use a filler material is preferable.

(After gas and back gas)
In the welding method of this embodiment, a gas containing 60 to 100% by volume of N ₂ (hereinafter referred to as a nitrogen-containing gas) is used as either one or both of the after gas and the back gas. The aftergas is used to control the atmosphere of the hot weld zone immediately after welding facing the welding torch during welding.

The back gas is used to control the atmosphere of the welded part in a high temperature state immediately after welding, which faces the opposite side of the steel material from the welding torch during welding.

When the nitrogen-containing gas is used as the aftergas, the corrosion resistance of the welded portion facing the welding torch side during construction is improved. Further, when a nitrogen-containing gas is used as the back gas, the corrosion resistance of the welded portion facing the side opposite to the welding torch side during construction is improved. Furthermore, when the nitrogen-containing gas is used as both the after gas and the back gas, both the corrosion resistance of the welds facing the welding torch side and the opposite side during construction are improved.

The range in which the atmosphere is controlled by the after gas and/or the back gas may be the range until the temperature of the weld zone drops to, for example, 600° C. or lower, preferably 500° C. or lower. Since the cooling rate of the welded portion is relatively fast, it is sufficient to control it for a maximum of about 20 seconds immediately after welding. By controlling the atmosphere with aftergas or back gas to this range, the welded joint and welded structure of the present embodiment can be manufactured.

The nitrogen-containing gas is a gas containing 60 to 100% by volume of N ₂ , preferably an industrial pure nitrogen gas containing 99% by volume or more of N ₂ . In addition, when using a gas containing 60% by volume or more and less than 100% by volume of _N2 , the remainder is preferably a non-oxidizing gas, more preferably an inert gas, and an Ar gas, from the viewpoint of suppressing oxidation of the weld zone. is more preferred. The nitrogen concentration in the nitrogen-containing gas is not particularly specified as long as the decrease in the austenite fraction of the surface layer of the weld metal portion is less than 20% by area, but at least 60% or more of _N2 must be contained. A mixed gas containing N ₂ may be used as necessary from a viewpoint other than corrosion resistance.

Other than using a nitrogen _- containing gas containing 60 to 100 vol.

The mechanism by which the use of a nitrogen-containing gas containing 60% by volume or more of _N2 as the aftergas and/or the back gas can suppress the deterioration of the corrosion resistance of the surface of the weld zone is considered as follows.

FIG. 1 shows the metallographic structure of the surface layer of the weld metal portion of the TIG welded joint with pure Ar or pure _N2 as the aftergas when TIG welding is performed using steel type B of the example as the base material. Both FIGS. 1(a) and 1(b) show the structure of the surface layer of the welded metal portion on the side to which the torch shield gas and the after-shield gas are blown. FIG. 1(a) is an example using Ar gas as the after gas (sample number 27), and FIG. 1(b) is an example using N ₂ as the after gas (sample number 5). The weld joint using Ar gas as the after gas has an austenite phase ratio of 28.4 area%, whereas the weld joint using _N2 as the after gas has an austenite phase ratio of 38.7 area %. The welded joint using _N2 as the gas has a higher proportion of the austenite phase in the surface layer of the weld metal portion. This is probably because the use of N ₂ gas as the aftergas inhibited desorption of nitrogen after solidification of the weld metal portion.

As mentioned above, the solid solubility limit of N in the ferrite phase is much smaller than that in the austenite phase, so during cooling during welding, N in the ferrite phase of the weld metal and the weld heat-affected zone or at the grain boundary between the ferrite phases N, which cannot completely dissolve in the ferrite phase, precipitates as chromium nitride. At this time, Cr is consumed to form a so-called chromium-deficient layer, which lowers the corrosion resistance. Conversely, when the surface layer of the weld metal portion has a large proportion of the austenite phase, nitrogen dissolves in the austenite phase, thereby suppressing precipitation of chromium nitrides and suppressing deterioration of corrosion resistance.

Further, FIG. 2 shows a graph showing the nitrogen concentration distribution in the depth direction from the surface layer of the weld metal portion obtained by welding the base material made of steel type B of the example. FIG. 2(a) is a graph when Ar is used as the torch shield gas and Ar is used as the after gas (Sample No. 32). FIG. 2(b) is a graph in the case of using Ar+5% _N2 gas as the torch shield gas and Ar gas as the after gas (Sample No. 15). Both FIGS. 2(a) and 2(b) show the nitrogen concentration distribution of the surface layer of the weld metal portion on the side to which the torch shield gas is blown. The nitrogen concentration in the figure was determined by EPMA measurement. The dotted line in the figure indicates the nitrogen concentration (0.17%) of the base material.

As shown in FIG. 2(b), the welded joint in which the torch shield gas is Ar + 5% N ₂ gas, compared to the welded joint in which the torch shield gas is pure Ar as shown in FIG. The average nitrogen concentration is high at a depth of about 100 μm or more. On the other hand, in the depth region of 0 to less than 100 μm, both welded joints show a sharp decrease in nitrogen concentration.

The process by which nitrogen decreases in the weld metal is divided into two stages: (A) the liquid phase diffusion process during melting, which is mainly exposed to torch shield gas, and (B) the solid phase diffusion process after solidification, which is mainly exposed to after gas. should be considered. When Ar + 5% N ₂ gas is used as the torch shield gas, nitrogen concentration can be suppressed in the above process (A), so that the nitrogen concentration is about the same as that of the base material at a depth of 100 μm or more from the surface layer. , Since the aftergas is pure Ar, the decrease in nitrogen in the process (B) cannot be suppressed, and the nitrogen concentration in the surface layer is considered to have decreased.

The present inventors have confirmed that the nitrogen concentration in the surface layer of the weld metal agrees well with the simulated nitrogen diffusion in the solid phase using the cooling temperature history after welding.

As described above, in order for the welded portion according to the present embodiment to achieve the desired surface texture and corrosion resistance, a nitrogen-containing gas containing 60% by volume or more of _N2 may be used as the after gas and/or the back gas. Although the components of the torch shield gas are not particularly specified, the nitrogen content in the torch shield gas is preferably less than 60%, more preferably 5% or less, and more preferably pure Ar.

The reason why the nitrogen content in the torch shield gas is preferably less than 60% is that nitrogen is an element that stabilizes the austenite phase and improves corrosion resistance, and at the same time promotes the precipitation of chromium nitrides to prevent chromium deficiency. This is because it is an element that deteriorates the corrosion resistance by forming a layer. That is, when the nitrogen concentration in the torch shield gas is increased, in the above-mentioned process (A) (liquid phase diffusion process during melting in contact with the torch shield gas), nitrogen is introduced from the torch shield gas side into the weld metal, resulting in poor welding. Nitrogen concentration in the metal increases. If a large amount of nitrogen is introduced into the weld metal at this time, the concentration of nitrogen in the weld metal becomes higher than it can be solid-dissolved, which in turn promotes the precipitation of chromium nitrides and reduces the corrosion resistance of the weld metal surface and interior. It may deteriorate. Also, in welding that uses non-consumable electrodes such as TIG welding, the nitrogen gas in the torch shield gas reacts with the electrodes and wears the electrodes, so the frequency of electrode replacement increases, reducing workability and increasing electrode costs. Invite. Therefore, the torch shield gas shall be as described above.

Next, the ferrite/austenite duplex stainless steel applicable to the present embodiment has, for example, a chemical composition in mass % of C: 0.10% or less, Si: 2.00% or less, Mn: 0.50 to 6.00%, P: 0.050% or less, S: 0.050% or less, Ni: 0.10-8.00%, Cr: 17.0-30.0%, N: 0.05-0 .30%, Mo: 0-3.50%, Cu: 0-2.0%, Nb: 0-0.10%, Sn: 0-1.00%, W: 0-1.00%, V : 0-1.00%, Ti: 0-0.05%, B: 0-0.0050%, Ca: 0-0.0050%, Mg: 0-0.0050%, Al: 0-0. 05%, REM: 0 to 0.50%, the remainder being Fe and impurities, and the PREN_Mn value calculated by the following formula (i) is less than 40.0. This chemical composition is merely an example, and the present invention is not limited thereto. The reason for mentioning this chemical composition is as follows. In addition, "%" about content in the following description means "mass %."

PREN_Mn value=Cr+3.3Mo+16N−Mn (i)

C: 0.10% or less C is an element that forms a solid solution in the austenite phase to increase the strength. However, if the C content exceeds 0.050%, the strength of the steel material increases and workability deteriorates. In addition, intergranular corrosion is caused in order to promote the precipitation of Cr carbides. Therefore, the C content should be 0.10% or less. The C content may be 0.050% or less, or may be 0.040% or less. Also, from the viewpoint of corrosion resistance, it is preferable to lower the C content, but in existing steelmaking facilities, reducing the C content to 0.002% or less causes a large increase in cost. Therefore, the C content is preferably 0.002% or more.

Si: 2.00% or less Si is sometimes used as a deoxidizing element or added to improve oxidation resistance. However, when the Si content exceeds 2.00%, the steel sheet is hardened and the toughness and workability are deteriorated. Therefore, the Si content should be 2.00% or less. The Si content is preferably 1.50% or less, more preferably 1.00% or less. Moreover, in order to reduce the Si content to a very small amount, the cost increases during steel refining. Therefore, the Si content is preferably 0.03% or more.

Mn: 0.50-6.00%
Mn has the effect of increasing the austenite phase and increasing the solid solubility of nitrogen to suppress bubble defects during production. However, a large amount of Mn reduces corrosion resistance and hot workability. Therefore, the Mn content should be 0.50 to 6.00%. The Mn content is preferably 1.00% or more, more preferably 2.50% or more. Also, the Mn content is preferably 4.00% or less.

P: 0.050% or less P is an element that is unavoidably mixed in steel and is also contained in raw materials such as Cr, so it is difficult to reduce it. Reduces moldability. The lower the P content, the better, and it is set to 0.050% or less. The P content is preferably 0.040% or less. A lower P content is desirable, but reducing the P content results in a significant increase in cost, so the P content may be 0.0005% or more.

S: 0.050% or less S is an element that is unavoidably mixed in steel, and may combine with Mn to form inclusions and serve as a starting point for rust generation. Therefore, the S content should be 0.050% or less. Since the lower the S content, the better the corrosion resistance, the S content is preferably 0.0030% or less. A lower S content is desirable, but reducing the S content results in a significant increase in cost, so the S content may be 0.0001% or more.

Ni: 0.10-8.00%
Ni is an austenite stabilizing element and is an important element for increasing the austenite fraction of the surface layer. Moreover, Ni has an effect of improving corrosion resistance. However, if Ni is contained in a large amount, the cost of raw materials increases and problems such as stress corrosion cracking may occur. Therefore, the Ni content should be 0.10 to 8.00%. The Ni content is preferably 1.00% or more. Also, the Ni content is preferably 6.00% or less, more preferably 4.00% or less, and even more preferably 3.00% or less.

Cr: 17.0-30.0%
Cr is an element necessary to ensure corrosion resistance. However, a large amount of Cr causes hot work cracking and increases the amount of chromium nitride precipitation in the weld metal zone and the weld heat affected zone. Therefore, the Cr content should be 17.0 to 30.0%. The Cr content is preferably 20.0% or more, more preferably 21.0% or more. Also, the Cr content is preferably 25.0% or less, more preferably 23.0% or less, and even more preferably 22.0% or less.

N: 0.05-0.30%
N is an element that forms a solid solution in the austenite phase to increase strength and corrosion resistance, thereby contributing to alloy saving. However, N is an element that greatly affects precipitation of chromium nitrides during welding cooling. If the content exceeds 0.30%, the amount of chromium nitride precipitated in the weld metal zone and the weld heat-affected zone increases, increasing the difference in corrosion resistance between the base metal and the weld zone. Therefore, the N content should be 0.05 to 0.30%. From the viewpoint of strength and corrosion resistance, the N content may be 0.08% or more, preferably 0.10% or more, and more preferably 0.15% or more. Moreover, from the viewpoint of suppressing precipitation of chromium nitride, the N content is preferably 0.25% or less, more preferably 0.20% or less.

Mo: 0-3.50%
Since Mo is an element that improves corrosion resistance, it may be contained as necessary. However, if a large amount of Mo is contained, the cost of raw materials increases, and the corrosion resistance deteriorates due to the precipitation of the σ phase in the weld zone. Therefore, the Mo content should be 3.50% or less. In order to obtain the above effects, the Mo content is preferably 0.10% or more. Also, the Mo content is preferably 2.50% or less, more preferably 1.00% or less, and even more preferably 0.60% or less.

Cu: 0-2.0%
Cu is a very effective element for improving sulfuric acid resistance, and may be added as necessary. In order to obtain the above effects, the Cu content is preferably 0.1% or more. More preferably, the Cu content is 0.3% or more. On the other hand, Cu is an element that increases the activity of N and facilitates the precipitation of chromium nitrides in the weld metal portion, so the Cu content is made 2.0% or less. The Cu content is preferably 1.5% or less, more preferably 1.0% or less.

Nb: 0-0.10%
Nb has the effect of suppressing precipitation of chromium nitride by forming a compound with N, so it may be contained as necessary. However, a large amount of Nb reduces the workability of the steel sheet. Therefore, the Nb content should be 0.10% or less. In order to obtain the above effects, the Nb content is preferably 0.01% or more, more preferably 0.04% or more.

Sn: 0-1.00%
Since Sn is an element that improves corrosion resistance, it may be contained as necessary. However, a large amount of Sn deteriorates hot workability. Therefore, the Sn content should be 1.00% or less. In order to obtain the above effects, the Sn content is preferably 0.010% or more.

W: 0-1.00%
Since W is an element that improves corrosion resistance, it may be contained as necessary. However, when a large amount of W is contained, the load during rolling is increased, and manufacturing flaws are likely to occur. Therefore, the W content should be 1.00% or less. In order to obtain the above effects, the W content is preferably 0.01% or more. Also, the W content is preferably 0.50% or less.

V: 0-1.00%
Since V is an element that improves corrosion resistance, it may be contained as necessary. However, when V is contained in a large amount, the load during rolling is increased, and manufacturing flaws are likely to occur. Therefore, the V content is set to 1.00% or less. In order to obtain the above effects, the V content is preferably 0.01% or more. Also, the V content is preferably 0.50% or less.

Ti: 0-0.05%
Ti, like Nb, has the effect of preventing coarsening of the weld heat-affected zone and further making the solidification structure into fine equiaxed grains, so it may be contained as necessary. However, a large amount of Ti reduces uniform elongation and local elongation. Therefore, the Ti content should be 0.05% or less. In order to obtain the above effects, the Ti content is preferably 0.005% or more.

B: 0 to 0.0050%
B has the effect of improving the hot workability, so it may be contained as necessary. However, when B is contained in a large amount, the corrosion resistance is significantly deteriorated. Therefore, the B content should be 0.0050% or less. In order to obtain the above effects, the B content is preferably 0.0003% or more. Also, the B content is preferably 0.0030% or less.

Ca: 0-0.0050%
Ca may be contained as necessary for desulfurization and deoxidation. However, if a large amount of Ca is contained, hot working cracks are likely to occur, and corrosion resistance is lowered. Therefore, the Ca content should be 0.0050% or less. In order to obtain the above effects, the Ca content is preferably 0.0001% or more.

Mg: 0-0.0050%
Since Mg has the effect of not only deoxidizing but also refining the solidified structure, it may be contained as necessary. However, containing a large amount of Mg causes an increase in cost in the steelmaking process. Therefore, the Mg content should be 0.0050% or less. In order to obtain the above effects, the Mg content is preferably 0.0001% or more.

Al: 0-0.05%
Al may be contained as necessary for desulfurization and deoxidation. However, a large amount of Al causes an increase in manufacturing flaws and an increase in raw material costs. Therefore, the Al content should be 0.05% or less. In order to obtain the above effects, the Al content is preferably 0.0030% or more.

REM: 0-0.50%
Since REM (rare earth element) has an effect of improving hot workability, it may be contained as necessary. However, a large amount of REM impairs manufacturability and increases costs. Therefore, the REM content should be 0.50% or less. In order to obtain the above effects, the REM content is preferably 0.005% or more. The REM content is preferably 0.020% or more and preferably 0.20% or less.

REM is a general term for a total of 17 elements including Sc, Y and 15 elements (lanthanoids) from La to Lu, and the content of REM means the total content of these elements. Incidentally, lanthanoids are industrially added in the form of misch metals.

In the chemical composition of the steel of the present invention, the balance is Fe and impurities. Here, the term "impurities" refers to components mixed in by various factors in raw materials such as ores, scraps, etc., and in the manufacturing process when steel is manufactured industrially. means something

PREN_Mn value: less than 40.0 The PREN_Mn value is a general index indicating the pitting corrosion resistance of a stainless steel plate, and is calculated from the chemical composition of the stainless steel using the following formula (i).

PREN_Mn value=Cr+3.3Mo+16N−Mn (i)

However, the element symbol in the above formula (i) is the content (% by mass) of each element contained in the steel, and 0 is substituted when it is not contained.

An increase in the PREN_Mn value may cause problems such as an increase in alloy cost due to an increase in Cr and Mo contents and a decrease in corrosion resistance due to precipitation of the σ phase. Furthermore, generation of nitrogen bubbles due to an increase in N content and a decrease in Mn content becomes a problem. Therefore, the PREN_Mn value should be less than 40.0. Preferably, the PREN_Mn value is less than 30.0. Although the lower limit does not have to be specified, it is preferably 18.0 or more, more preferably 20.0 or more, in order to obtain corrosion resistance equivalent to SUS304.

The welded structure according to this embodiment is obtained by welding under the above conditions. The welded structure thus obtained has a pitting potential drop of less than 150 mV with respect to the base metal, and therefore can be applied as a welded structure with good corrosion resistance.

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

A ferrite/austenite duplex stainless steel having the chemical composition shown in Table 1 was melted into a steel slab, hot rolled to a thickness of 5 mm, and then annealed. Cold-rolled steel sheets having thicknesses of 1 mm and 3 mm were produced by subjecting the hot-rolled and annealed sheets to cold rolling, and then subjected to final annealing.

After that, one-pass TIG lick welding was performed on the produced cold-rolled and annealed steel sheets to produce welded joints. Behind the welding torch, an aftershield box with a length of 140 _mm , a width of 40 mm, and a height of 40 mm is installed as shown in FIG. changed in various ways. The gas flow rate was set to torch shield gas: 10 L/min, back gas: 15 L/min, and after gas: 30 L/min. Appropriate welding conditions changed according to the composition of torch shield gas. Therefore, the welding current and welding speed were adjusted so that the arc length was fixed at 1 mm and the width of the back bead was about 2 to 3 times the plate thickness. Welding was performed for a length of 300 mm or more with the rolling direction as the welding direction without using a filler material.

For the steel plate before welding, after wet polishing with a polishing grain size of #600 on the surface at 0.2 mm below the surface, by the method specified in JIS G 0577, 1 mol / NaCl solution and saturated silver chloride at 30 ° C. Using an electrode (SSE), the pitting potential ( _{V' c100} ) corresponding to a current density of 100 μA/cm ² was measured, and the value obtained by averaging six measurement data was taken as the pitting potential _PBM of the base material.

Furthermore, in order to evaluate the corrosion resistance of the surface layer of the welded metal part of the welded joint, after removing the weld scale using a burning material instead of the usual wet polishing, it was carefully dried with P600 abrasive paper immediately before the measurement. ^The pitting potential (V ' _c100 ) was measured, and the value obtained by averaging the six measurement data was taken as the pitting potential _PWM of the surface layer of the weld zone. In order to limit the measurement surface of the pitting potential to the surface of the welded portion, the area other than the welded portion was masked. As the scorching material, Lathnonwell JL-500 (manufactured by Mansho Co., Ltd.) was used, but other chemicals or other methods may be used as long as the weld scale can be sufficiently removed.

In addition, in order to evaluate the corrosion resistance inside the welded metal part of the welded joint, the central part in the plate thickness direction of the welded part was exposed as an evaluation surface, and was heated to 30 ° C. by the method specified in JIS G 0577. 1 mol/L NaCl solution and a saturated silver chloride electrode (SSE), the pitting corrosion potential ( _{V' c100} ) corresponding to a current density of 100 μA/cm ² was measured, and the value obtained by averaging the six measurement data was taken as the weld zone. The internal pitting potential P _mid was used.

A corrosion resistance test piece of a welded joint was prepared from a stationary welded portion which was 50 mm or more away from the welding start point or end point, was melted to the back surface, and had no defects such as burn-through. A corrosion resistance test piece was prepared so that the weld length was 10 mm in a measurement area of 1 cm ² with the welded part at the center. Therefore, the measured area of the corrosion resistance test piece includes the weld metal zone, the weld heat affected zone and the base metal part. After measuring the pitting potential, check the location of pitting corrosion on the corrosion resistance test piece. When the weld scale remained because it was sufficient, it was excluded from the measurement data. In the present invention, the portion of the corrosion resistance test piece from which the measurement data was collected was defined as the welded portion. A weld consists of a weld metal zone and a weld heat affected zone.

Then, the value (P _BM −P _WM ) [(mV vs SSE)] obtained by subtracting the pitting potential P _WM of the weld zone from the pitting potential P _BM of the base metal was obtained as the pitting potential reduction margin.

In addition, in order to investigate the structure morphology of the steel plate before welding, the metal structure of the 200 μm width × 200 μm depth region from the surface layer of the cross section (TD cross section) perpendicular to the rolling length direction at the center position in the rolling width direction was investigated with an optical microscope. did. Similarly, in order to investigate the structural morphology of the welded metal zone, the metallographic structure of a region of 200 µm width × surface layer to 30 µm depth was examined with an optical microscope on the cross section perpendicular to the welding direction at the center of the width of the welded metal zone. . The microstructure images obtained were classified into ferrite phase and austenite phase by image analysis, and the austenite phase ratios γ _BM and γ _WM of the surface layers of the base metal and weld metal were evaluated, respectively. Then, the value (γ _BM −γ _WM ) (area %) obtained by subtracting the austenite phase ratio γ _WM of the surface layer of the weld metal portion from the austenite phase ratio γ _BM of the base metal was obtained as the austenite phase ratio reduction margin.

In addition, in order to evaluate the influence of back gas, the corrosion resistance and austenite fraction of the surface layer were evaluated in the same manner as described above for the weld metal part on the back side of the welded joint.

Table 2 summarizes the above production conditions and measurement results. In addition, the meaning of the code|symbol in Table 2 is as showing below.

_Plate thickness: Plate thickness of base metal Evaluation surface: The effect of after gas was evaluated on the front surface, and the effect of back gas was evaluated on the back surface. γ phase ratio γ _WM (%)
Surface pitting potential drop: base metal pitting potential P _BM (mV) - weld surface pitting potential P _WM (mV)
Internal pitting potential drop: base metal pitting potential P _BM (mV) - pitting potential P _mid (mV) inside the weld zone

The above results are also shown in Table 2.

Sample Nos. 1 to 30 are examples of the present invention. By using N ₂ gas (100% N ₂ ) as the after gas and/or back gas, the γ-phase ratio decrease in the surface layer of the evaluation surface is small, and the pitting potential is low. The decrease was good with less than 150 mV. However, since sample numbers 25 to 30 used 100% N ₂ gas or Ar + 60% N ₂ gas as the torch shield gas, welding was more difficult than when 100% Ar gas or Ar + 5% N ₂ gas was used as the torch shield gas. The corrosion resistance inside the metal part was slightly inferior.

Sample Nos. 31 to 38 are comparative examples, and by using 100% Ar gas as the after gas and back gas, the γ phase ratio decrease in the surface layer of the evaluation surface is large, and the pitting potential decrease is 150 mV or more, which is unsatisfactory. Met.

As described above, in the examples of the present invention, the decrease in the γ-phase ratio of the surface layer was small, and a good pitting potential of the weld zone was obtained. On the other hand, in the comparative example, the decrease in the γ-phase ratio of the surface layer was large, and the corrosion resistance was poor. In addition, in the present invention, restrictions on the welding method are loose except for restrictions on aftergas and back gas, reheat treatment after welding is unnecessary, and wear of the welding electrode is small.

FIG. 4 is a diagram showing the relationship between the γ-phase rate reduction amount of the surface layer and the pitting potential reduction amount of the weld zone. As shown in FIG. 4, it can be seen that there is a relationship between the γ-phase rate decrease of the surface layer and the pitting potential decrease of the weld zone.

According to the present invention, it is possible to obtain a ferrite-austenite duplex stainless steel welded structure and a welded joint having excellent corrosion resistance of the weld zone in the alloy-saving duplex stainless steel material.

Claims (5)

a base material made of a ferrite/austenite duplex stainless steel material;
a weldment, and
_Decrease in the austenite phase ratio of the surface layer of the _weld metal portion (γ _BM -γ _WM ) is less than 20 area%,
A ferrite-austenite duplex stainless steel in which a pitting potential drop (P _BM −P _WM ), which is a difference between the pitting potential P _BM of the base metal and the pitting potential P _WM of the weld zone, is less than 150 mV. Welded structures. a base material made of a ferrite/austenite duplex stainless steel material;
a weldment, and
_Decrease in the austenite phase ratio of the surface layer of the _weld metal portion (γ _BM -γ _WM ) is less than 20 area%,
A ferrite-austenite duplex stainless steel in which a pitting potential drop (P _BM −P _WM ), which is a difference between the pitting potential P _BM of the base metal and the pitting potential P _WM of the weld zone, is less than 150 mV. welded joints. A method of welding a base material made of a ferrite-austenite duplex stainless steel material by a gas-shielded arc welding method, comprising:
A method for welding ferritic and austenitic duplex steels, wherein a gas containing 60 to 100% by volume of _N2 is used as either or both of the after gas and the back gas. The method for welding ferritic-austenitic duplex steel according to claim 3, wherein a gas containing less than 60% by volume of _N2 is used as the torch shield gas. 5. The welding method of ferrite-austenitic duplex steel materials according to claim 3 or 4, wherein a gas containing 60% by volume or more of _N2 and the balance Ar is used as one or both of the after gas and the back gas.

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