JP7542389B2 - Ferrite-austenitic duplex stainless steel sheet, bent product, bending method for ferritic-austenitic duplex stainless steel sheet, and bending die - Google Patents

Description

The present invention relates to a ferritic-austenitic duplex stainless steel sheet, a bent product, a method for bending a ferritic-austenitic duplex stainless steel sheet, and a die for bending.

Austenitic stainless steel sheets such as SUS304 and SUS316, and ferritic stainless steel sheets such as SUS430 are widely used for exterior and interior building materials. Recently, the use of duplex stainless steel sheets, which are cheaper than austenitic stainless steel sheets and have better corrosion resistance than ferritic stainless steel sheets, has been expanding as a material for exterior and interior building materials.

However, because duplex stainless steel has a higher yield strength than austenitic stainless steel or ferritic stainless steel, the dimensional accuracy of the material after bending may be poor. If the processing conditions are made stricter in order to improve the dimensional accuracy, cracks may occur in the bent portion. Furthermore, if the processing conditions are made stricter, scratches or scoring marks may occur, compromising the design. This is particularly problematic when the material is used as a building material where aesthetics are important.

Patent Document 1 describes a grooved austenitic stainless steel sheet with excellent bending workability, with the objective of providing a grooved steel plate material that can be bent to form sharp ridgelines with excellent design properties. However, as mentioned above, duplex stainless steel sheets have low dimensional accuracy after bending, making it difficult to form sharp ridgelines by bending.

Patent Document 2 describes a lean duplex stainless steel with excellent bending workability. However, as shown in Table 2 of the same document, the duplex stainless steel described in Patent Document 2 has a yield strength YS of less than 500 MPa, so its strength is relatively low and it may not be strong enough for use as an exterior or interior building material.

JP 2008-169423 A Special table 2019-522726 publication

The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide a ferritic-austenitic duplex stainless steel sheet that is capable of consistently producing bent products with high dimensional accuracy, that can prevent the occurrence of cracks in the bent portions, and that can further suppress the occurrence of rust transfer and scoring scratches, and that also has excellent strength.
Another object of the present invention is to provide a bent product which has high dimensional accuracy, is free from cracks at bent portions, is suppressed from developing rust or scoring marks, and has excellent strength.
Furthermore, an object of the present invention is to provide a method for bending a ferritic-austenitic duplex stainless steel sheet, which makes it possible to stably obtain bent products with high dimensional accuracy, and which can prevent the occurrence of cracks in the bent portion and suppress the occurrence of rust transfer and scoring marks, and a die for bending the same.

[1] A ferritic-austenitic duplex stainless steel sheet having a 0.2% yield strength of 500 MPa or more, a work hardening index of less than 0.25 in the elongation range of 5 to 15%, and a Lankford value of 1 or less as specified in JIS Z 2254;
The chemical composition of the ferritic-austenitic duplex stainless steel sheet is, in mass%,
C: 0.10% or less,
Si: 0.01 to 5.0%,
Mn: 0.01 to 8.0%,
P: 0.100% or less,
S: 0.050% or less,
Ni: 0.5-30.0%,
Cr: 20.00-30.00%,
Mo: 0.01-8.0%,
Cu: 0.01 to 5.0%;
The balance is Fe and impurities.
A V-shaped groove is provided on the surface of the steel plate,
When the plate thickness is t (mm), the width of the groove on the surface of the steel plate is w (mm), and the depth of the deepest part of the groove is D (mm), the relationships 0.5t≦D≦0.6t and 0.8t≦w≦1.0t are established,
A ferritic-austenitic duplex stainless steel plate used to form a bend along the groove .
[ 2 ] In mass%,
C: 0.10% or less,
Si: 0.01 to 5.0%,
Mn: 0.01 to 8.0%,
P: 0.100% or less,
S: 0.050% or less,
Ni: 0.5-30.0%,
Cr: 20.00 to 30.00%,
Mo: 0.01-8.0%,
Cu: 0.01 to 5.0%;
The balance is Fe and impurities.
A ferritic-austenitic duplex stainless steel sheet that satisfies the following requirements: 0.2% yield strength of 500 MPa or more, a work hardening index of less than 0.25 in the elongation range of 5 to 15%, and a Lankford value of 1 or less as specified in JIS Z 2254.
[ 3 ] The Ni content is, in mass%,
The ferritic-austenitic duplex stainless steel sheet according to [ 1 ] or [ 2 ], wherein Ni is 1.0 to 10.0% .
[ 4 ] The ferritic-austenitic duplex stainless steel sheet according to any one of [ 1 ] to [ 3 ], further comprising one or more elements selected from one or more of the following first, second and third groups:
Group 1: In mass %, N: 0.05 to 0.8%.
Group 2: one or more selected from, by mass%, Al: 1.0% or less, Ti: 0.40% or less, Nb: 0.01 to 0.40%, V: 0.01 to 0.50%, W: 0.01 to 1.0%, Ta: 0.001 to 0.10%, Sn: 0.001 to 0.50%, Sb: 0.001 to 0.50%, and Ga: 0.001 to 0.50%.
Third group: one or more selected from, by mass%, B: 0.0002 to 0.0050%, Ca: 0.0002 to 0.0050%, Mg: 0.0002 to 0.0050%, and REM: 0.001 to 0.10%.

[ 5 ] A steel plate material made of a ferritic-austenitic duplex stainless steel plate having a 0.2% yield strength of 500 MPa or more, a work hardening index in an elongation range of 5 to 15 being less than 0.25, and a Lankford value defined in JIS Z 2254 being 1 or less, is provided with a bent portion;
The bending angle on the valley fold line side of the bent portion is in the range of 87 to 93 degrees,
A groove or a notch is provided on the valley fold line side of the bent portion ,
The chemical composition of the ferritic-austenitic duplex stainless steel sheet is, in mass%,
C: 0.10% or less,
Si: 0.01 to 5.0%,
Mn: 0.01 to 8.0%,
P: 0.100% or less,
S: 0.050% or less,
Ni: 0.5-30.0%,
Cr: 20.00-30.00%,
Mo: 0.01-8.0%,
Cu: 0.01 to 5.0%;
The remainder of the bent product is Fe and impurities .
[6] The bent product according to claim 5, further comprising one or more elements selected from one or more of the following first, second and third groups:
Group 1: In mass %, N: 0.05 to 0.8%.
Group 2: one or more selected from, by mass%, Al: 1.0% or less, Ti: 0.40% or less, Nb: 0.01 to 0.40%, V: 0.01 to 0.50%, W: 0.01 to 1.0%, Ta: 0.001 to 0.10%, Sn: 0.001 to 0.50%, Sb: 0.001 to 0.50%, and Ga: 0.001 to 0.50%.
Third group: one or more selected from, by mass%, B: 0.0002 to 0.0050%, Ca: 0.0002 to 0.0050%, Mg: 0.0002 to 0.0050%, and REM: 0.001 to 0.10%.

[7] A bending method for forming a bent portion in a steel plate material by subjecting a ferritic-austenitic duplex stainless steel plate to a die bending process using a punch and a die, comprising the steps of:
The steel plate material is made of a ferritic-austenitic duplex stainless steel plate having a 0.2% yield strength of 500 MPa or more, a work hardening index in the elongation range of 5 to 15% of less than 0.25, and a Lankford value defined in JIS Z 2254 of 1 or less, and a V-shaped groove is provided on the surface of the steel plate, and when the plate thickness is t (mm), the width of the groove on the surface of the steel plate is w (mm), and the depth of the deepest part of the groove is D (mm), the relationships of 0.5t≦D≦0.6t and 0.8t≦w≦1.0t are satisfied.
The die has a V-shaped recess consisting of two inclined surfaces on a die surface, the width L (mm) of each of the inclined surfaces is set so as to satisfy the relationship L≧10t with respect to the thickness t (mm) of the steel plate material, and the radius of curvature R (mm) of a corner where the inclined surface and the die surface meet is set so as to satisfy the relationship 0.9t≦R≦1.1t with respect to the thickness t of the steel plate material,
A method for bending a ferritic-austenitic duplex stainless steel sheet, comprising: placing the steel sheet material on the recess of the die with the V-shaped groove facing the punch; and pushing the punch into the recess to form the bent portion along the groove.

[8] A bending die for forming a bent portion in a steel plate material comprising a ferritic-austenitic duplex stainless steel plate having a 0.2% yield strength of 500 MPa or more, a work hardening index in the elongation range of 5 to 15% of less than 0.25, and a Lankford value of 1 or less as defined in JIS Z 2254, and having a V-shaped groove on the surface of the steel plate, the relationship of 0.5t≦D≦0.6t and 0.8t≦w≦1.0t being satisfied when the plate thickness is t (mm), the width of the groove on the surface of the steel plate is w (mm), and the depth of the deepest part of the groove is D (mm), is provided for the steel plate material,
A die for bending, wherein the die surface has a V-shaped recess consisting of two inclined surfaces, the width L (mm) of each of the inclined surfaces is set so that the relationship L≧10t holds with respect to the thickness t (mm) of the steel plate material, and the radius of curvature R (mm) of the corner where the inclined surfaces meet the die surface is 0.9t≦R≦1.1t with respect to the thickness t of the steel plate material.

According to the ferritic-austenitic duplex stainless steel sheet (hereinafter referred to as steel sheet) of the present invention, the yield strength, work hardening index and Lankford value are within predetermined ranges, and grooves of a predetermined shape are provided therein. Therefore, when used as a material for bent products, bent products having a sharp ridgeline on the outside of the bent portion and excellent strength can be stably obtained.
That is, since the work hardening index of the steel sheet is small, less than 0.25, the springback is small, and since the average Lankford value is 1 or less, the occurrence of warping after bending is suppressed. Furthermore, since the steel sheet has a groove of a predetermined shape, the occurrence of wrinkles on the inside of the bend after bending is suppressed. As a result, the variation in the bending angle at the bent portion after bending is within ±3°, and the dimensional accuracy after bending is excellent. Furthermore, the ridge line of the bent portion can be made sharp, and the occurrence of cracks at the bent portion can be prevented.
Furthermore, by using the steel plate according to the present invention as a material for a bent product, despite the high yield strength of 500 MPa or more, a bent product having sharp ridgelines can be manufactured even if the load applied to the steel plate during bending with a punch and die is reduced. Therefore, when the steel plate comes into contact with the die, less iron adheres from the steel material constituting the die, suppressing the occurrence of so-called transfer rust and also suppressing the occurrence of scoring marks.

In addition, the steel plate according to the present invention has a specified chemical composition and therefore has excellent corrosion resistance. By using such a steel plate as the material for a bent product, a bent product with excellent corrosion resistance and design can be obtained. The obtained bent product can be suitably used as a building material part where appearance and weather resistance are important.

Next, according to the bent product of the present invention, the variation in bending angle at the bent portion is within ±3°, and the product has excellent dimensional accuracy after forming.
In addition, since the work hardening index of the steel plate used as the raw material for the bent product is less than 0.25, springback is reduced, the ridge lines of the bent parts can be made sharp, and the occurrence of cracks in the bent parts can be prevented.
Furthermore, despite the material's high yield strength of 500 MPa or more, a bent product having sharp ridgelines can be produced even when the load applied to the steel plate is reduced during bending using a punch and die. This means that when the steel plate comes into contact with the die, less iron adheres from the steel material that makes up the die, suppressing the occurrence of so-called transfer rust and also suppressing the occurrence of scoring scratches, making the material suitable for use as building material parts where appearance and weather resistance are important.

Next, according to the forming method of the present invention, a V-shaped recess consisting of two inclined surfaces is provided, and the relationship between the width L of the inclined surface and the plate thickness t is L≧10t, and the width of the inclined surface is 10 times or more the plate thickness. Therefore, during bending with the punch and die, the load applied from the steel plate to the die is distributed, and adhesion of iron from the steel material that constitutes the die is reduced, suppressing the occurrence of so-called transfer rust and also suppressing the occurrence of scoring scratches, making it possible to easily manufacture parts for building materials where appearance is important.
In addition, the radius of curvature R (mm) of the corner where the inclined surface and the die surface meet is set to 0.9t≦R≦1.1t, where t is the plate thickness of the steel plate material. Therefore, even if the steel plate comes into contact with this corner during bending with the punch and die, the occurrence of scoring scratches is suppressed, and parts for building materials where appearance is important can be easily manufactured.

Next, according to the bending die of the present invention, a V-shaped recess consisting of two inclined surfaces is provided, and the relationship between the width L of the inclined surface and the plate thickness t is L≧10t, and the width of the inclined surface is 10 times or more the plate thickness. Therefore, during bending with the punch and die, the load applied from the steel plate to the die is distributed, and adhesion of iron from the steel material that constitutes the die is reduced, thereby suppressing the occurrence of so-called transfer rust and also suppressing the occurrence of scoring scratches, making it easy to manufacture parts for building materials where appearance is important.
In addition, the radius of curvature R (mm) of the corner where the inclined surface and the die surface meet is set to 0.9t≦R≦1.1t, where t is the plate thickness of the steel plate material. Therefore, even if the steel plate comes into contact with this corner during bending with the punch and die, the occurrence of scoring scratches is suppressed, and parts for building materials where appearance is important can be easily manufactured.

FIG. 1 is a perspective view showing a ferritic-austenitic duplex stainless steel plate according to an embodiment of the present invention. FIG. 2 is a schematic cross-sectional view showing a bent product according to an embodiment of the present invention. FIG. 1 is a schematic cross-sectional view illustrating a bending die and a method for bending a ferritic-austenitic duplex stainless steel sheet according to an embodiment of the present invention.

The following describes the embodiments of the present invention, including a ferritic-austenitic duplex stainless steel sheet (hereinafter sometimes referred to as a steel sheet), a bent product, a method for bending a ferritic-austenitic duplex stainless steel sheet, and a die for bending.

(Steel plate)
The steel plate according to this embodiment satisfies the following requirements: a 0.2% yield strength of 500 MPa or more, a work hardening index in the elongation range of 5 to 15% of less than 0.25, and a Lankford value defined in JIS Z 2254 of 1 or less. The steel plate has a V-shaped groove on the surface thereof, and when the plate thickness is t (mm), the width of the groove on the surface of the steel plate is w (mm), and the depth of the deepest part of the groove is D (mm), the relationships of 0.5t≦D≦0.6t and 0.8t≦w≦1.0t hold, and the steel plate is used for forming a bent portion along the groove.
This steel plate may be a steel plate whose chemical components, in mass %, include C: 0.10% or less, Si: 0.01 to 5.0%, Mn: 0.01 to 8.0%, P: 0.100% or less, S: 0.050% or less, Ni: 0.5 to 30.0%, Cr: 15.00 to 30.00%, Mo: 0.01 to 8.0%, Cu: 0.01 to 5.0%, and the balance being Fe and impurities.

The steel plate according to the present embodiment may contain, by mass%, C: 0.10% or less, Si: 0.01 to 5.0%, Mn: 0.01 to 8.0%, P: 0.100% or less, S: 0.050% or less, Ni: 0.5 to 30.0%, Cr: 15.00 to 30.00%, Mo: 0.01 to 8.0%, Cu: 0.01 to 5.0%, with the balance being Fe and impurities, and may be a steel plate that satisfies the following: a 0.2% yield strength of 500 MPa or more, a work hardening index in an elongation range of 5 to 15% of less than 0.25, and a Lankford value defined in JIS Z 2254 of 1 or less.
The reasons for limiting the steel plate of this embodiment will be described below.

<0.2% Yield Strength>
The 0.2% yield strength of the steel plate according to this embodiment is set to 500 MPa or more. This ensures sufficient strength when the steel plate according to this embodiment is used as a material for exterior and interior building materials. If the 0.2% yield strength is less than 500 MPa, the strength is insufficient.

<Strain hardening index in the elongation range of 5 to 15%>
The work hardening index (n value) is a characteristic value indicating the degree of work hardening of a material, and the larger this value, the greater the work hardening relative to the amount of strain introduced, and the greater the load required to deform during bending. This increases the bending moment generated during bending, and the driving force of springback generated by residual stress increases. In this embodiment, it is necessary to reduce the springback after bending, and for this purpose, it is better for the work hardening index of the steel plate, which is the raw material, to be small. Therefore, the work hardening index in the elongation range of 5 to 15% is set to less than 0.25. The reason why the elongation range is limited to 5 to 15% is because the elongation range of the material at the bent portion when performing bending according to this embodiment is generally in the range of 5 to 15%, and by setting the work hardening index in this elongation range to less than 0.25, it is possible to significantly reduce the springback after bending. This makes it possible to form a sharp shape according to the shape of the die (die) used during bending.

<Lankford value>
The Lankford value (hereinafter sometimes referred to as the r value) is the ratio of the amount of deformation in the width direction and the thickness direction during deformation, and the larger this value is, the greater the width shrinkage during processing and the smaller the thickness reduction. When bending a steel plate, if the r value is large, a difference in the amount of strain occurs between the outside and inside of the bend, and the outside of the bend shrinks more in width than the inside of the bend in response to the amount of strain, which causes warping in the shape after bending, resulting in processing defects. For this reason, the r value of the steel plate is better to be small, specifically, the r value is set to 1 or less. By setting the r value to 1 or less, it is possible to suppress the occurrence of processing defects and enable bending processing as intended.

The Lankford value is measured in accordance with JIS Z 2254:2013 (Test method for plastic strain ratio of thin plate metal materials).

As shown in FIG. 1, the surface of the steel plate 1 according to this embodiment may be provided with a groove 2 that is V-shaped in cross section. The steel plate 1 is bent using this groove 2 as a valley fold line. The groove 2 is preferably provided in a straight line. As shown in FIG. 1, when the thickness of the steel plate 1 is t (mm), the width of the groove 2 on the surface of the steel plate is w (mm), and the depth of the deepest part of the groove 2 is D (mm), the shape of the groove 2 is such that the relationships 0.5t≦D≦0.6t and 0.8t≦w≦1.0t hold.

<0.5t≦D≦0.6t>
If the depth D of the deepest part of the groove 2 is less than 0.5t (the groove is shallow), the pair of inner wall surfaces 2a of the V-shaped groove 2 come into close contact with each other at an early stage of bending, and the material is not pushed toward the inside of the bend. The flow of material is eliminated and the material flows outward, resulting in a large bending radius r at the outer corner of the bend, making it impossible to form a desired sharp ridgeline. Since the elongation and n value of the stainless steel are small, the groove shape required for good bending is limited. In other words, in order to obtain a sharp ridgeline, it is necessary to suppress cracks during bending. However, in the steel plate 1 according to the present embodiment, if the depth D of the groove 2 is less than 0.5t, cracks may occur during bending. Although the bending radius can be made small, when forming a deep groove such that the depth D of the deepest part of the groove 2 exceeds 0.6t, the work hardening due to the groove forming process becomes large and the ductility decreases, so that the bending process is difficult. The crack susceptibility increases when used in a wide range of applications. Furthermore, if the depth D of the deepest part of the groove 2 exceeds 0.6t, the thickness of the steel plate in the groove 2 becomes small, and the strength of the bent product after bending decreases. The depth D of the deepest part of the groove is set in the range of 0.5t to 0.6t in relation to the plate thickness t of the steel plate.

<0.8t≦w≦1.0t>
If the width w of the groove 2 on the surface of the steel sheet exceeds the sheet thickness t, the gap angle of the groove after bending will exceed 10° and become large, resulting in problems such as insufficient strength. If the bending depth is less than 8t, the pair of wall surfaces 2a of the groove will come into close contact with each other at an early stage of bending, causing the material to flow outward as described above, making it impossible to form a sharp ridgeline. The width w is set to be 0.8t or more and 1.0t or less in relation to the plate thickness t of the steel plate 1.

Next, the chemical composition of the steel plate of this embodiment will be described. Note that percentages indicating the steel composition refer to mass percentages unless otherwise specified.

In order to ensure the corrosion resistance of the steel plate, the C content is limited to 0.10% or less. If the C content exceeds 0.10%, Cr carbides are formed, deteriorating the corrosion resistance. On the other hand, C is an element that forms austenite, which constitutes a two-phase structure. For this reason, the lower limit of the C content is preferably 0.005% or more, and more preferably 0.010% or more. The upper limit of the C content is preferably 0.08% or less, and more preferably 0.05% or less.

Si is contained in an amount of 0.01% or more for deoxidation. The lower limit of the Si content is preferably 0.10% or more, and more preferably 0.30% or more. However, if the Si content exceeds 5.0%, the precipitation of the σ phase is promoted. Therefore, the upper limit of the Si content is limited to 5.0% or less. The upper limit of the Si content is preferably 2.0% or less, and more preferably 0.60% or less.

Mn is contained at 0.01% or more as a deoxidizer and an austenite stabilizing element for forming a two-phase structure. The lower limit of the Mn content is preferably 0.10% or more, and more preferably 1.5% or more. However, if the Mn content exceeds 8.0%, the corrosion resistance deteriorates. Therefore, the upper limit of the Mn content is limited to 8.0% or less. The upper limit of the Mn content is preferably 5.0% or less, and more preferably 4.0% or less.

Since P deteriorates hot workability and toughness, the P content is limited to 0.100% or less. The P content is preferably 0.050% or less, and more preferably 0.035% or less. On the other hand, if the P content is reduced too much, refining costs will increase, so it is preferably 0.005% or more.

Since S deteriorates hot workability, toughness, and corrosion resistance, the S content is limited to 0.050% or less. The S content is preferably 0.010% or less, and more preferably 0.001% or less. On the other hand, excessively reducing the S content increases raw material costs and refining costs, so it is preferably 0.0003% or more.

When Ni is contained in the coating of the steel sheet, it has the effect of suppressing the occurrence of pitting corrosion when the Fe concentration of the coating is high, and the effect of suppressing the progression of corrosion when corrosion occurs. If the Ni content is less than 0.5%, sufficient corrosion resistance cannot be obtained. If the Ni content exceeds 30.0%, the Cr concentration of the coating decreases too much, and sufficient corrosion resistance cannot be obtained. Therefore, the Ni content must be in the range of 0.5 to 30.0%. The lower limit of the Ni content is preferably 1.0% or more, 2.0% or more, or 4.0% or more. The upper limit of the Ni content is preferably 15.0% or less, 10.0% or less, or 7.0% or less.

If the Cr content is less than 15.00%, sufficient corrosion resistance cannot be obtained. If the Cr content exceeds 30.00%, the Cr concentration in the film becomes high and sufficient corrosion resistance cannot be obtained in an environment where the natural potential of the stainless steel is high. In addition, precipitation of the σ phase increases, deteriorating corrosion resistance and hot manufacturability. Therefore, the Cr content must be in the range of 15.00 to 30.00%. The lower limit of the Cr content is preferably 18.00% or more, more preferably 20.00% or more, and even more preferably 21.00% or more. The upper limit of the Cr content is preferably 28.00% or less, and more preferably 25.00% or less.

Mo is an element that improves corrosion resistance, and is effective when contained at 0.01% or more. Mo may be contained up to 8.0%, but if the Mo content exceeds 4.0%, the σ phase is likely to precipitate during hot working. For this reason, the lower limit of the Mo content is 0.01% or more, preferably 0.05% or more, and more preferably 1.0% or more. The upper limit of the Mo content is 8.0% or less, preferably 4.0% or less, and more preferably 1.5% or less.

If the content of Cu is 0.01% or more, the effect of suppressing the progression of corrosion when corrosion occurs is obtained. Cu may be contained in an amount of 5.0% or less. Furthermore, if the amount of Cu exceeds 3.0%, cracks may easily occur during casting. Therefore, the lower limit of the amount of Cu is 0.01% or more, preferably 0.05% or more, and more preferably 0.20% or more. The upper limit of the amount of Cu is 5.0% or less, preferably 3.0% or less, and more preferably 0.5% or less.

In this embodiment, in addition to the elements described above, one or more alloying elements selected from the following group may be contained in order to adjust the properties of the steel sheet.

Group 1: In mass %, N: 0.05 to 0.8%.
Group 2: one or more selected from, by mass%, Al: 1.0% or less, Ti: 0.40% or less, Nb: 0.01 to 0.40%, V: 0.01 to 0.50%, W: 0.01 to 1.0%, Ta: 0.001 to 0.10%, Sn: 0.001 to 0.50%, Sb: 0.001 to 0.50%, and Ga: 0.001 to 0.50%.
Third group: one or more selected from, by mass%, B: 0.0002 to 0.0050%, Ca: 0.0002 to 0.0050%, Mg: 0.0002 to 0.0050%, and REM: 0.001 to 0.10%.

When 0.05% or more of N is contained, corrosion resistance is improved. Therefore, N is an effective element for increasing corrosion resistance. N may be contained up to 0.8%. However, when more than 0.3% of N is contained, bubbles may easily occur during casting. Therefore, the lower limit of the N content is 0.05% or more, preferably 0.10% or more, and more preferably 0.12% or more. The upper limit of the N content is 0.8% or less, preferably 0.3% or less, and even more preferably 0.18% or less.

Although Al is useful as a deoxidizing element, it should not be contained in large amounts because it deteriorates workability. It is better to limit the upper limit of Al content to 1.0% or less. The preferred range of Al content is 0.5% or less. The lower limit of Al content is 0.01% or more.

Ti, Nb, V, W, Ta, Sn, Sb, and Ga are elements that improve corrosion resistance, and one or more of them may be contained within the following ranges.
Ti: 0.40% or less, Nb: 0.01 to 0.40%, V: 0.01 to 0.50%, W: 0.01 to 1.0%, Ta: 0.001 to 0.10%, Sn: 0.001 to 0.50%, Sb: 0.001 to 0.50%, Ga: 0.001 to 0.50%.

Ti and Nb have the effect of improving corrosion resistance, particularly suppressing intergranular corrosion, by fixing C and N as carbonitrides, so that one or both of Ti and Nb may be contained.
However, even if they are contained in excess, the effect is saturated, so the upper limit of each of the contents of Ti and Nb is set to 0.40% or less. Here, if the content of at least one of Ti and Nb is 0.001% or more, preferably 0.01% or more, the effect can be exhibited. The appropriate contents of Ti and Nb are preferably such that the total amount of Ti and Nb is 5 times or more and 30 times or less than the total amount of C and N. Preferably, the total content of Ti and Nb is 10 times or more and 25 times or less than the total amount of C and N.

V and W may be included as necessary to improve corrosion resistance, especially crevice corrosion resistance. However, excessive inclusion of V or W reduces workability and saturates the effect of improving corrosion resistance, so the lower limit of each of the amounts of V and W is set to 0.01% or more, the upper limit of the amount of V is set to 0.50% or less, and the upper limit of the amount of W is set to 1.0% or less. The lower limit of the amount of V is preferably 0.04% or more, and the upper limit of the amount of V is preferably 0.30% or less. The lower limit of the amount of W is preferably 0.04% or more, and the upper limit of the amount of W is preferably 0.50% or less.

The inclusion of trace amounts of Sn or Sb improves corrosion resistance. For this reason, Sn and Sb are useful elements for improving corrosion resistance, and are included within a range that does not impair cost. If the amount of Sn or Sb is less than 0.001%, the effect of improving corrosion resistance is not realized, and if the amount of Sn or Sb exceeds 0.50%, the increase in cost becomes apparent and workability also decreases, so the appropriate range for the amount of Sn and Sb is 0.001 to 0.50%. The lower limit of each of the amounts of Sn and Sb is preferably 0.01% or more, and the upper limit of each of the amounts of Sn and Sb is preferably 0.30% or less.

Ga is an element that contributes to improving corrosion resistance and workability, and can be contained in the range of 0.001 to 0.50%. The lower limit of the Ga content is preferably 0.015% or more, and the upper limit of the Ga content is preferably 0.30% or less.

Ta is an element that improves corrosion resistance by modifying inclusions, and may be included as necessary. The effect is exhibited by a Ta content of 0.001% or more, so the lower limit of the Ta content is set to 0.001% or more. If the Ta content exceeds 0.10%, it will lead to a decrease in room temperature ductility and toughness, so the upper limit of the Ta content is preferably 0.10% or less, and more preferably 0.050% or less. If the effect is to be exhibited with a small amount of Ta, it is preferable that the Ta content be 0.020% or less.

B, Ca, Mg, and REM are elements that improve hot workability, and one or more of them may be contained for that purpose. Since the effects of B, Ca, and Mg are apparent at amounts of 0.0002% or more, the lower limit of each of the amounts of B, Ca, and Mg is set to 0.0002% or more. In the case of REM, the lower limit is set to 0.001% or more.

However, excessive content of any of these elements can actually reduce hot workability, so it is preferable to set the upper and lower limits of their contents as follows: the amounts of B, Ca, and Mg are each 0.0002-0.0050%, and the amount of REM is 0.001-0.10%.

The lower limit of each of the amounts of B, Ca, and Mg is preferably 0.0005% or more. The upper limit of each of the amounts of B, Ca, and Mg is preferably 0.0015% or less. The lower limit of the amount of REM is preferably 0.005% or more, and the upper limit of the amount of REM is preferably 0.030% or less.

Here, REM (rare earth elements) is a general term for two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu), according to the general definition. They may be contained alone or as a mixture. The amount of REM is the total amount of these elements.

The steel sheet of this embodiment contains the remainder of the above-mentioned elements as well as Fe and impurities, but other elements besides those described above may also be contained within a range that does not impair the effects of this embodiment.

Next, a method for manufacturing the steel sheet of this embodiment will be described. The steel sheet of this embodiment is basically manufactured by applying a general process for manufacturing stainless steel. For example, molten steel having the above chemical composition is produced in an electric furnace, and refined in an AOD furnace or VOD furnace. Steel billets are produced by continuous casting or ingot casting, and then hot rolling is performed on the steel billets. Next, the hot-rolled steel sheet is annealed and pickled. Furthermore, cold rolling is performed, and then final annealing is performed in a continuous annealing furnace. Intermediate annealing may be performed during cold rolling. Pickling may be performed after final annealing. In this way, the original sheet of the steel sheet according to this embodiment is manufactured.

Hereinafter, manufacturing conditions for achieving a 0.2% yield strength of a steel plate of 500 MPa or more, a work hardening index of less than 0.25, and an r-value of 1 or less will be described.
First, it is preferable that the cold rolling reduction rate is 80% or less. The lower limit of the cold rolling reduction rate is preferably 30% or more.
Next, in the final annealing in the continuous annealing furnace, the residence time at 900°C or higher is set to a range of 20 to 90 seconds. At this time, the furnace temperature and the conveying speed of the steel sheet are adjusted so that the residence time at 1000°C or higher is 1 second or more. In addition, the final annealing is performed while applying a tension of 0.60 to 0.90 kgf/ ^mm2 to the steel sheet.

By satisfying the above manufacturing conditions, it is possible to achieve a 0.2% yield strength of the steel plate of 500 MPa or more, a work hardening index of less than 0.25, and an r-value of 1 or less.

Next, a V-shaped groove having the above-mentioned dimensions is formed in the obtained steel plate (original plate). It is preferable that the processing for forming the groove is not performed by "removal processing" but by "non-removal processing" accompanied by plastic deformation. In the case of removal processing, sufficient work hardening does not occur near the groove, so there is a risk that the bent product after bending will have insufficient strength. In the case of non-removal processing accompanied by plastic deformation, appropriate work hardening occurs during processing, so there is no insufficient strength. A preferred processing method for forming grooves is to press a rotating roll with blades on the outer periphery against the surface of the steel plate while it is being passed through.

In this manner, the steel plate according to this embodiment can be manufactured.

(Bent products)
Next, a bent product according to this embodiment will be described. An example of the bent product is shown in Fig. 2. The bent product 10 according to this embodiment is formed by providing a bent portion 11 on a steel plate material 1a made of a ferritic-austenitic duplex stainless steel plate that satisfies the following requirements: 0.2% proof stress is 500 MPa or more, a work hardening index in the 5 to 15% elongation range is less than 0.25, and a Lankford value is 1 or less. The bent portion 11 has a bend angle θ in the valley fold line side in the range of 87 to 93°, and a groove portion 12 or a notch portion 12 is provided on the valley fold line side of the bent portion 11.

The steel plate material 1a is the steel plate according to the present embodiment described above. The bent product 10 according to the present embodiment is obtained by performing bending along the groove portion 2 of the steel plate according to the present embodiment. The bending angle θ on the valley bend side of the bent portion 11 is set to be within the range of 87 to 93°. This means that when the target bending angle θ is 90°, the bending angle θ in the bent portion 11 is within the range of ±3°. By setting the bending angle θ within the range of 87 to 93°, the variation in the bending angle θ is reduced, and the entire bent portion 11 has a sharp ridgeline. This improves the dimensional accuracy of the bent product 10 itself. A more preferable bending angle θ is in the range of 88 to 92°, and even more preferably in the range of 89 to 91°. When the bending angle θ is measured at 10 measurement points at 10 cm intervals within a range of 1 m in the longitudinal direction of the bent portion, the bending angle θ at each measurement point may be within the range of 87 to 93°. If the shape of the bent product is too small to ensure 10 measurement points, measurements can be taken at as many measurement points as possible. Also, if the shape of the bent product is small, a bent product larger than the desired size can be manufactured in advance so that 10 measurement points can be ensured, and the bending angle can be measured at the 10 measurement points, and then the bent product can be cut to the original desired size.

In addition, it is preferable that the average bending angle on the valley fold line side of the bent portion 11 is within the range of 87 to 93° in terms of high dimensional accuracy. The average bending angle is the average value of the bending angles θ at the multiple measurement points described above.

In addition, a groove portion 12 or a notch portion 12 is provided on the valley fold line side of the bent portion 11. This groove portion 12 or notch portion 12 is formed by deforming the groove 2 provided on the steel plate surface of the above-mentioned steel plate 1, which is the steel plate material 1a, by bending. By providing the groove portion 12 or the notch portion 12, it is possible to provide a sharp bent portion 11.

The bent product 10 of this embodiment is formed by bending the steel plate 1 of this embodiment. The bent product 10 of this embodiment can be suitably used as a material for exterior and interior building materials.

(Bending dies)
Next, a bending die used in manufacturing the bent product of this embodiment will be described with reference to FIG.
The die 20 according to the present embodiment is a die for die bending, and is used together with a punch when bending a metal sheet. In bending processing using die bending, the punch and die are attached to a press machine, a metal sheet is placed between the punch and the die, and the press machine is operated to bring the punch and the die closer to each other, thereby performing a V-bend on the metal sheet to form a bent portion.
The reasons for limiting the dice will be explained below.

Figure 3 shows the cross-sectional shape of the die 20. The die 20 according to this embodiment is made of a steel material such as tool steel, and is configured such that a V-shaped recess 23 consisting of two inclined surfaces 22, 22 is provided on the die surface 21. The relative angle between the two inclined surfaces 22 may be, for example, 90°, or the relative angle may be 85 to 90° in consideration of the amount of springback of the steel plate material 1a after bending. The width L (mm) of each inclined surface 22 is set so that the relationship L ≥ 10t is established with respect to the sheet thickness t (mm) of the steel plate material 1a. In addition, the radius of curvature R (mm) of the corner 24 where the inclined surface 22 and the die surface 21 meet is set to 0.9t ≤ R ≤ 1.1t with respect to the sheet thickness t of the steel plate material 1a.

<L≧10t>
In the die 20 of this embodiment, the width L (mm) of the inclined surface 22 constituting the recess 23 is set to be 10 times or more the plate thickness t (mm) of the steel plate material 1a. During bending by die bending, a load is applied to the die 20 with respect to the steel plate material 1a, but by setting L≧10t, the load applied to the die 20 is distributed, the contact surface pressure between the die 20 and the steel plate material 1a is reduced, and iron powder generated by wear of the die 20 is suppressed. As a result, there is less risk of iron adhering to the steel plate material 1a from the steel material constituting the die 20, the occurrence of rust is suppressed, and the occurrence of scoring flaws is also suppressed. If L<10t, the contact surface pressure between the die 20 and the steel plate material 1a increases, making it difficult to suppress rust and scoring flaws, and the aesthetics of the bent product are reduced, which is not preferable.

<0.9t≦R≦1.1t>
In this embodiment, the radius of curvature R (mm) of the corner 24 where the inclined surface 22 and the die surface 21 contact is set to be in the range of 0.9 to 1.1 times the plate thickness t (mm) of the steel plate material 1a. In the bending process using a die, in the initial stage of the deformation process, the steel sheet material 1a placed on the die 20 is supported at two points by the corners 24 of the die 20, and then is pressed and bent by the punch to cause pure bending deformation. However, when the steel sheet material 1a comes into contact with the corners 24 of the die 20, scoring marks may occur on the surface of the steel sheet material 1a. It is necessary to reduce the contact length with the material 1a. In order to reduce the contact length, the radius of curvature R (mm) of the corner 24 should be reduced. Specifically, the thickness t (mm) of the steel plate material 1a should be reduced. This makes it possible to suppress the occurrence of scoring marks. On the other hand, if the radius of curvature R (mm) of the corner 24 is too small, the corner 24 becomes sharp, and scoring defects are more likely to occur. The radius of curvature R of the corner of the die is set to 0.9 times or more the thickness t (mm) of the steel sheet material 1a. Therefore, the radius of curvature R of the corner of the die is set to 0.9t≦R≦1.1t in relation to the thickness t of the steel sheet material 1a. Let us assume that.

(Bending method)
Next, a bending method for a steel plate according to the present embodiment will be described. In the bending method according to the present embodiment, the ferritic-austenitic duplex stainless steel plate according to the present embodiment is used as a steel plate material 1a, and a bent portion is formed in the steel plate material 1a by performing a die bending process on the steel plate material 1a using a punch and a die. The die used in the bending process is the die according to the present embodiment. There is no particular limitation on the punch used in the bending process.

Specifically, the steel plate material 1a is placed on the recess 23 of the die 20 with the V-shaped groove 2 on the surface of the steel plate material 1a facing the punch, and the punch is pressed into the recess 23 of the die 20 to form the bent portion 11 along the groove 2 of the steel plate material 1a.

As described above, the die 20 is set so that the width L (mm) of the inclined surface 22 satisfies the relationship L≧10t with respect to the thickness t (mm) of the steel plate material 1a, and the radius of curvature R (mm) of the corner 24 where the inclined surface 22 and the die surface 21 meet is set to 0.9t≦R≦1.1t with respect to the thickness t of the steel plate material 1a. The steel plate material 1a also satisfies the following: 0.2% yield strength of 500 MPa or more, work hardening index of less than 0.25, Lankford value of 1 or less, and a V-shaped groove 2 of a predetermined shape is provided. As a result, the bending radius r on the outside of the bent portion 11 is small, the dimensional accuracy is high, there is no cracking in the bent portion, and furthermore, the occurrence of rust and scoring is suppressed, and a bent product with excellent strength can be obtained.

Steel having the chemical composition shown in Table 1 was cast into slabs, which were then hot-rolled, hot-rolled annealed and pickled, cold-rolled, intermediate annealed, and final annealed to produce ferritic-austenitic duplex stainless steel sheets having the thickness shown in Table 1. The manufacturing conditions for the steel sheets are shown in Table 2.

Next, a rotating roll with a blade on the outer periphery was pressed against the surface of the steel plate while it was being passed, creating a V-shaped groove on the surface of the steel plate. The deepest depth of the groove and the width of the groove on the surface of the steel plate are shown in Table 1. The steel plate with the V-shaped groove was used as the steel plate material. The size of the steel plate material was 100 mm wide and 1000 mm long, with the V-shaped groove positioned in the center in the longitudinal direction. The V-shaped groove extended along the width direction of the steel plate material.

A die made of tool steel was also prepared. A V-shaped recess consisting of two inclined surfaces was provided on the surface of the die. The relative angle between the two inclined surfaces was 89°. The width L (mm) of the inclined surface and the radius of curvature R (mm) of the corner where the inclined surface meets the die surface were as shown in Table 3. A punch with a tip angle of 88° was also prepared.

The punch and die were then attached to a press machine, and the steel plate material was placed between the punch and die. The steel plate material was placed on the recess in the die, with the V-shaped groove on the surface of the steel plate material facing the punch. The press machine was then operated to bring the punch and die closer to each other, and the punch was pressed into the recess in the die, thereby V-bending the steel plate material to form a bent portion. In this way, a bent product with a bent portion was manufactured.

The bending angle θ at the bent portion of the obtained bent product was measured. Ten measurement points were set at 10 cm intervals within a range of 1 m in the longitudinal direction of the bent portion, and the bending angle θ was measured on the valley line side of the bent portion at each measurement point. The average, maximum, and minimum values of the bending angle θ at the 10 measurement points were then calculated. In addition, the bending radius r on the outside of the bent portion was measured at each measurement point, and the average value was calculated. These results are shown in Table 4.

Furthermore, the bent parts of the bent products were visually inspected for the presence or absence of cracks, rust, and scoring. If no cracks, rust, or scoring were found, it was judged as "absent," and if they were found, it was judged as "present." If all cracks, rust, and scoring were "absent," the product was deemed to have passed. The results are shown in Table 4.

Test Nos. 1, 2, 5, 6, 9, 10, 12, 15, 16 and 18 used steel plates and forming conditions within the scope of the present invention, and the bending angle of the bent portion of the bent product was in the range of 87 to 93 degrees, resulting in high dimensional accuracy and a sharp ridgeline on the outside of the bent portion. Furthermore, no cracks, rust or scoring occurred. In addition, because the specified chemical composition was present, corrosion resistance was also good.

In Test No. 3, 0.8t≦w was not satisfied, the maximum bending angle in the bent portion was 95°, and a sharp ridgeline was not obtained.
In Test No. 4, 0.5t≦D was not satisfied, the minimum bending angle at the bent portion was 85°, and cracks occurred at the bent portion.
In Test No. 7, D≦0.6t and w≦1.0t were not satisfied, the minimum bending angle in the bent portion was 84°, and the dimensional accuracy of the bent portion was low.
In Test No. 8, w≦1.0t and L≧10t were not satisfied, the maximum bending angle in the bent portion was 96°, the dimensional accuracy was reduced, sharp ridgelines were not obtained in some places, and further, rust transfer and scoring scratches occurred.
In Test No. 11, L≧10t was not satisfied, and transfer of rust occurred.
In test No. 13, 0.9t≦R was not satisfied, and scoring defects occurred.
In Test No. 14, 0.5t≦D and L≧10t were not satisfied, the maximum bending angle in the bent portion was 97°, the dimensional accuracy was reduced, sharp ridgelines were not obtained in some places, and further, rust was generated.
In Test No. 17, L≧10t and R≦1.1t were not satisfied, and transfer flaws and scoring flaws occurred.

1...steel plate (ferritic-austenitic duplex stainless steel plate), 1a...steel plate material, 2...groove, 10...bent product, 11...bent portion, 12...groove portion, 20...die, 21...die surface, 22...inclined surface, 23...recess, 24...corner, t...plate thickness, w...groove width, D...depth of deepest portion of groove.

Claims (8)

The present invention is made of a ferritic-austenitic duplex stainless steel sheet that satisfies the following requirements: 0.2% proof stress is 500 MPa or more, the work hardening index in the elongation range of 5 to 15% is less than 0.25, and the Lankford value specified in JIS Z 2254 is 1 or less;
The chemical composition of the ferritic-austenitic duplex stainless steel sheet is, in mass%,
C: 0.10% or less,
Si: 0.01 to 5.0%,
Mn: 0.01 to 8.0%,
P: 0.100% or less,
S: 0.050% or less,
Ni: 0.5-30.0%,
Cr: 20.00-30.00%,
Mo: 0.01-8.0%,
Cu: 0.01 to 5.0%;
The balance is Fe and impurities.
A V-shaped groove is provided on the surface of the steel plate,
When the plate thickness is t (mm), the width of the groove on the surface of the steel plate is w (mm), and the depth of the deepest part of the groove is D (mm), the relationships 0.5t≦D≦0.6t and 0.8t≦w≦1.0t are established,
A ferritic-austenitic duplex stainless steel plate used to form a bend along the groove. In mass percent,
C: 0.10% or less,
Si: 0.01 to 5.0%,
Mn: 0.01 to 8.0%,
P: 0.100% or less,
S: 0.050% or less,
Ni: 0.5-30.0%,
Cr: 20.00 to 30.00%,
Mo: 0.01-8.0%,
Cu: 0.01 to 5.0%;
The balance is Fe and impurities.
A ferritic-austenitic duplex stainless steel sheet that satisfies the following requirements: 0.2% yield strength of 500 MPa or more, a work hardening index of less than 0.25 in the elongation range of 5 to 15%, and a Lankford value of 1 or less as specified in JIS Z 2254. The Ni content is, in mass%,
The ferritic-austenitic duplex stainless steel sheet according to claim 1 or 2 , wherein Ni is 1.0 to 10.0% . The ferritic-austenitic duplex stainless steel sheet according to any one of claims 1 to 3 , further comprising one or more elements selected from one or more of the following first, second and third groups :
Group 1: In mass %, N: 0.05 to 0.8%.
Group 2: one or more selected from, by mass%, Al: 1.0% or less, Ti: 0.40% or less, Nb: 0.01 to 0.40%, V: 0.01 to 0.50%, W: 0.01 to 1.0%, Ta: 0.001 to 0.10%, Sn: 0.001 to 0.50%, Sb: 0.001 to 0.50%, and Ga: 0.001 to 0.50%.
Third group: one or more selected from, by mass%, B: 0.0002 to 0.0050%, Ca: 0.0002 to 0.0050%, Mg: 0.0002 to 0.0050%, and REM: 0.001 to 0.10%. A steel plate material made of a ferritic-austenitic duplex stainless steel plate that satisfies the following requirements: 0.2% proof stress is 500 MPa or more, a work hardening index in the elongation range of 5 to 15% is less than 0.25, and a Lankford value specified in JIS Z 2254 is 1 or less, a bent portion is provided,
The bending angle on the valley fold line side of the bent portion is in the range of 87 to 93 degrees,
A groove or a notch is provided on the valley fold line side of the bent portion ,
The chemical composition of the ferritic-austenitic duplex stainless steel sheet is, in mass%,
C: 0.10% or less,
Si: 0.01 to 5.0%,
Mn: 0.01 to 8.0%,
P: 0.100% or less,
S: 0.050% or less,
Ni: 0.5-30.0%,
Cr: 20.00-30.00%,
Mo: 0.01-8.0%,
Cu: 0.01 to 5.0%;
The remainder of the bent product is Fe and impurities . The bent product according to claim 5 , further comprising one or more elements selected from one or more of the following first, second and third groups:
Group 1: In mass %, N: 0.05 to 0.8%.
Group 2: one or more selected from, by mass%, Al: 1.0% or less, Ti: 0.40% or less, Nb: 0.01 to 0.40%, V: 0.01 to 0.50%, W: 0.01 to 1.0%, Ta: 0.001 to 0.10%, Sn: 0.001 to 0.50%, Sb: 0.001 to 0.50%, and Ga: 0.001 to 0.50%.
Third group: one or more selected from, by mass%, B: 0.0002 to 0.0050%, Ca: 0.0002 to 0.0050%, Mg: 0.0002 to 0.0050%, and REM: 0.001 to 0.10%. A bending method for forming a bent portion in a steel plate material, the method comprising the steps of: bending a ferritic-austenitic duplex stainless steel plate using a punch and a die;
The steel plate material is made of a ferritic-austenitic duplex stainless steel plate that satisfies the following: a 0.2% yield strength of 500 MPa or more, a work hardening index in the elongation range of 5 to 15% of less than 0.25, and a Lankford value defined in JIS Z 2254 of 1 or less, and has a V-shaped groove on the surface of the steel plate, and when the plate thickness is t (mm), the width of the groove on the surface of the steel plate is w (mm), and the depth of the deepest part of the groove is D (mm), the relationships of 0.5t≦D≦0.6t and 0.8t≦w≦1.0t are satisfied.
The die has a V-shaped recess consisting of two inclined surfaces on a die surface, the width L (mm) of each of the inclined surfaces is set so as to satisfy the relationship L≧10t with respect to the thickness t (mm) of the steel plate material, and the radius of curvature R (mm) of a corner where the inclined surface and the die surface meet is set so as to satisfy the relationship 0.9t≦R≦1.1t with respect to the thickness t of the steel plate material,
A method for bending a ferritic-austenitic duplex stainless steel sheet, comprising: placing the steel sheet material on the recess of the die with the V-shaped groove facing the punch; and pushing the punch into the recess to form the bent portion along the groove. A bending die for forming a bent portion in a steel plate material comprising a ferritic-austenitic duplex stainless steel plate that satisfies the following requirements: a 0.2% proof stress of 500 MPa or more, a work hardening index in the elongation range of 5 to 15% of less than 0.25, and a Lankford value of 1 or less as specified in JIS Z 2254, the steel plate having a V-shaped groove on a surface thereof, the relationship of 0.5t≦D≦0.6t and 0.8t≦w≦1.0t being satisfied when the plate thickness is t (mm), the width of the groove on the surface of the steel plate is w (mm), and the depth of the deepest part of the groove is D (mm), the bending die for forming a bent portion in the steel plate material by performing a die bending process on the steel plate material,
A die for bending, wherein the die surface has a V-shaped recess consisting of two inclined surfaces, the width L (mm) of each of the inclined surfaces is set so that the relationship L≧10t holds with respect to the thickness t (mm) of the steel plate material, and the radius of curvature R (mm) of the corner where the inclined surfaces meet the die surface is 0.9t≦R≦1.1t with respect to the thickness t of the steel plate material.

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