JP7744566B2 - Ferrite-austenitic duplex stainless steel material, its manufacturing method, and corrosion-resistant member - Google Patents

Description

The present invention relates to a ferritic-austenitic duplex stainless steel material, a manufacturing method thereof, and a corrosion-resistant component.

Stainless steel materials have various excellent properties such as corrosion resistance, and are therefore used in a wide range of applications such as automobile parts, building parts, and kitchen utensils.
Stainless steel materials are classified into steel plates, steel bars, steel strips, steel bars, steel pipes, etc. depending on their shape. Stainless steel plates, which are common stainless steel materials, are manufactured by the following process. For example, hot-rolled steel plates (thick plates) are manufactured by continuously casting molten iron, which is made by dissolving stainless steel raw materials, into slabs, hot-rolling the slabs, and then annealing and pickling them. Cold-rolled steel plates (thin plates) are manufactured by cold-rolling the hot-rolled steel plates, followed by annealing and pickling them. In these stainless steel manufacturing processes, pickling is performed to remove oxide scale formed on the surface of the stainless steel material. Hereinafter, removing oxide scale formed on the surface of the stainless steel material will be referred to as "descaling."

However, pickling alone may not be sufficient to remove oxide scale.
Therefore, in a typical descaling process, mechanical pretreatment using a scale breaker or shot blasting is performed to create cracks in the oxide scale, and then the oxide scale is pickled to make it easier to remove (see, for example, Patent Documents 1 and 2). Another known chemical pretreatment method is to immerse the stainless steel material in a salt bath.

JP 2014-172077 A Japanese Unexamined Patent Publication No. 2-145785

However, in conventional descaling processes using pickling, the surface irregularities formed by pretreatment and the surface roughness caused by pickling cause the stainless steel material to turn white and lose its luster, resulting in a deterioration of its design. In particular, cold-rolled steel requires a higher level of design (smoothness and gloss) than hot-rolled steel, making it difficult to achieve the desired design using conventional descaling processes using pickling. Furthermore, ferritic-austenitic duplex stainless steel materials have a thick oxide scale formed on their surfaces, making it difficult to create cracks through mechanical pretreatment and prone to uneven pickling. Furthermore, in the case of stainless steel materials with high Si and Al contents, the internal oxide layer containing SiO ₂ and Al ₂ O ₃ formed at the interface between the oxide scale and the stainless steel material is chemically stable, making it even more difficult to remove the oxide scale. Furthermore, removing the oxide scale exposes inclusions on the surface of the stainless steel material, which can reduce the corrosion resistance and fatigue properties of the stainless steel material.
To solve these problems, it is conceivable to polish the surface after the descaling process. However, polishing the surface until it is smooth increases the amount of grinding required, reducing yield, and also reduces corrosion resistance and design properties due to polishing burns and entrapment of polishing debris. Furthermore, since inclusions are also present inside the stainless steel material, polishing the surface after the descaling process will only expose new inclusions on the surface of the stainless steel material. Therefore, this method cannot be said to be effective.

The present invention was made to solve the above-mentioned problems, and aims to provide a ferritic-austenitic duplex stainless steel material with a smooth, glossy surface and excellent corrosion resistance and fatigue properties, a method for manufacturing the same, and a corrosion-resistant component made from the same.

As a result of extensive research aimed at solving the above-mentioned problems, the inventors discovered that by controlling the composition of a ferritic-austenitic duplex stainless steel material and irradiating it with laser light under specific conditions, it is possible to descale the material while maintaining surface smoothness and gloss, and by dissolving surface inclusions, improve corrosion resistance and fatigue properties. Based on this discovery, the inventors produced and examined various ferritic-austenitic duplex stainless steel materials. As a result, they discovered that a ferritic-austenitic duplex stainless steel material with a specified composition and a specific range of D2/D1 (the ratio of the number density D2 of inclusions at the surface to the number density D1 of specified inclusions in the parent phase at a depth of one-quarter of the thickness from the surface) can solve the above-mentioned problems, leading to the completion of the present invention.

That is, the present invention provides a steel sheet having a composition, on a mass basis, containing C: 0.001 to 0.150%, Si: 0.20 to 5.00%, Mn: 6.00% or less, P: 0.050% or less, S: 0.0300% or less, Ni: 1.00 to 9.00%, Cr: 15.00 to 30.00%, Mo: 5.00% or less, Cu: 2.00% or less, N: 0.400% or less, Al: 3.500% or less, Si+2Al being 1.20% or more, and the balance being Fe and impurities;
The ferritic-austenitic duplex stainless steel material has a ratio D2/D1 of 0.50 or less where D1 is the number density D2 of inclusions at the surface and having an equivalent circle diameter of 0.5 to 10 μm in the parent phase at a position a depth of 1/4 of the thickness from the surface, and D2 is the number density D2 of the inclusions at the surface , which is 75 pieces/mm2 or less ^.

The present invention also provides a method for producing the ferritic-austenitic duplex stainless steel material, comprising:
a descaling step of irradiating a cold-rolled steel material having the composition on a mass basis with laser light to remove oxide scale formed on the surface of the cold-rolled steel material,
The laser beam irradiation is carried out under conditions that allow the region of the cold-rolled steel material from the interface between the parent phase and the oxide scale to a depth of 0.50 to 10.00 μm to be melted.

Furthermore, the present invention relates to a corrosion-resistant component comprising the above-mentioned ferritic-austenitic duplex stainless steel material.

The present invention provides a ferritic-austenitic duplex stainless steel material with a smooth, glossy surface and excellent corrosion resistance and fatigue properties, a method for manufacturing the same, and a corrosion-resistant component made from the same.

FIG. 1 is a schematic cross-sectional view of a ferritic-austenitic duplex stainless steel material according to an embodiment of the present invention. 10A and 10B are schematic cross-sectional views for explaining the difference between descaling by laser light irradiation and descaling by a conventional method.

The following is a detailed description of the embodiments of the present invention. The present invention is not limited to the following embodiments, and it should be understood that modifications and improvements made to the following embodiments based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention are also within the scope of the present invention.
In this specification, the "%" designation for components means "% by mass" unless otherwise specified.

A ferritic-austenitic duplex stainless steel material according to an embodiment of the present invention has a composition containing C: 0.001 to 0.150%, Si: 0.20 to 5.00%, Mn: 6.00% or less, P: 0.050% or less, S: 0.0300% or less, Ni: 1.00 to 9.00%, Cr: 15.00 to 30.00%, Mo: 5.00% or less, Cu: 2.00% or less, N: 0.400% or less, Al: 3.500% or less, with Si+2Al being 1.20% or more, and the balance being Fe and impurities.
Here, in this specification, "stainless steel material" refers to a material formed from stainless steel, and the shape of the material is not particularly limited. Examples of the material shape include plate (including strip), rod, and tube. The cross-sectional shape may also be various shaped steels such as T-shaped and I-shaped. Furthermore, "impurities" refer to components that are mixed in during the industrial production of stainless steel material due to raw materials such as ore and scrap, or various factors in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention. For example, the stainless steel material may contain 0.02% or less of O as an impurity.

Furthermore, the ferritic-austenitic duplex stainless steel material according to the embodiment of the present invention may further contain one or more selected from Ti: 0.001 to 0.500%, Nb: 0.001 to 1.000%, V: 0.001 to 1.000%, W: 0.001 to 1.000%, Zr: 0.001 to 1.000%, and Co: 0.001 to 1.200%.
Furthermore, the ferritic-austenitic duplex stainless steel material according to the embodiment of the present invention may further contain one or more selected from Ca: 0.0001 to 0.0100%, B: 0.0001 to 0.0080%, Sn: 0.001 to 0.500%, and REM: 0.200% or less.
Each component will be described in detail below.

<C: 0.001-0.150%>
If the C content is too high, the material becomes hard and reduces workability. In addition, sensitization occurs when the material is subjected to heat effects such as welding, reducing the corrosion resistance of the ferritic-austenitic duplex stainless steel material. Therefore, the upper limit of the C content is controlled to 0.150%, preferably 0.100%, more preferably 0.080%, and even more preferably 0.060%. On the other hand, if the C content is too low, the stability of the austenite phase decreases, leading to deterioration of workability and increased refining costs. Therefore, the lower limit of the C content is controlled to 0.001%, preferably 0.002%, more preferably 0.005%, and even more preferably 0.010%.

<Si: 0.20-5.00%>
If the Si content is too high, the ferritic-austenitic duplex stainless steel material will become hard and the workability will decrease. Therefore, the upper limit of the Si content is controlled to 5.00%, preferably 4.00%, more preferably 3.80%, and even more preferably 3.50%. On the other hand, if the Si content is too low, the heat resistance of the ferritic-austenitic duplex stainless steel material will decrease. Therefore, the lower limit of the Si content is controlled to 0.20%, preferably 1.00%, more preferably 1.50%, and even more preferably 2.5%.

<Mn: 6.00% or less>
Mn is an austenite phase (γ phase) forming element. If the Mn content is too high, the corrosion resistance of the ferritic-austenitic duplex stainless steel material will decrease. Therefore, the upper limit of the Mn content is controlled to 6.00%, preferably 4.00%, more preferably 2.00%, and even more preferably 1.00%. On the other hand, the lower limit of the Mn content is not particularly limited, but is preferably 0.01%, more preferably 0.05%, and even more preferably 0.10%.

<P: 0.050% or less>
If the P content is too high, the corrosion resistance and workability of the ferritic-austenitic duplex stainless steel material will decrease. Therefore, the upper limit of the P content is controlled to 0.050%, preferably 0.030%, more preferably 0.020%, and even more preferably 0.010%. On the other hand, the lower limit of the P content is not particularly limited, but is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%.

<S: 0.0300% or less>
If the S content is too high, the hot workability will be reduced, which will decrease the manufacturability of the ferritic-austenitic duplex stainless steel material, and it will also have an adverse effect on corrosion resistance. Therefore, the upper limit of the S content is controlled to 0.0300%, preferably 0.0100%, more preferably 0.0050%, and even more preferably 0.0010%. On the other hand, the lower limit of the S content is not particularly limited, but is preferably 0.0001%, more preferably 0.0002%.

<Ni: 1.00-9.00%>
Ni, like Mn, is an austenite phase (γ phase) forming element. Because Ni is expensive, if the Ni content is too high, the manufacturing cost increases. Therefore, the upper limit of the Ni content is controlled to 9.00%, preferably 8.00%, more preferably 7.50%, and even more preferably 7.00%. On the other hand, from the viewpoint of obtaining the effects of Ni, the lower limit of the Ni content is controlled to preferably 1.00%, more preferably 2.00%, even more preferably 5.00%, and even more preferably 5.50%.

<Cr:15.00-30.00%>
If the Cr content is too high, it not only increases refining costs but also hardens due to solid solution strengthening, reducing the workability of the ferritic-austenitic duplex stainless steel material. Therefore, the upper limit of the Cr content is controlled to 30.00%, preferably 28.00%, more preferably 26.00%, and even more preferably 25.00%. On the other hand, if the Cr content is too low, sufficient corrosion resistance cannot be obtained. Therefore, the lower limit of the Cr content is controlled to 15.00%, preferably 18.00%, more preferably 20.00%, and even more preferably 22.00%.

<Mo: 5.00% or less>
Mo is an element that improves the corrosion resistance of ferritic-austenitic duplex stainless steel materials. Because Mo is expensive, an excessive Mo content leads to an increase in manufacturing costs. Therefore, the upper limit of the Mo content is controlled to 5.00%, preferably 4.50%, more preferably 4.00%, and even more preferably 3.80%. On the other hand, the lower limit of the Mo content is not particularly limited, but is preferably 0.10%, more preferably 0.50%, and even more preferably 1.00%.

<Cu: 2.00% or less>
Cu is an element that improves the workability of ferritic-austenitic duplex stainless steel materials. If the Cu content is too high, the corrosion resistance of the ferritic-austenitic duplex stainless steel material will decrease, and a low-melting-point phase will form during casting, resulting in a decrease in hot workability. Therefore, the upper limit of the Cu content is controlled to 2.00%, preferably 1.60%, more preferably 1.20%, and even more preferably 1.00%. On the other hand, the lower limit of the Cu content is not particularly limited, but is preferably 0.01%, more preferably 0.05%, and even more preferably 0.10%.

<N: 0.400% or less>
N is an element that improves corrosion resistance. If the N content is too high, the material will harden and the workability of the ferritic-austenitic duplex stainless steel will decrease. Therefore, the upper limit of the N content is controlled to 0.400%, preferably 0.300%, more preferably 0.280%, and even more preferably 0.260%. On the other hand, the lower limit of the N content is not particularly limited, but is preferably controlled to 0.01%, preferably 0.05%, and more preferably 0.10%.

<Al: 3.500% or less>
Al is an element added as needed for deoxidation in the refining process and improves corrosion resistance and heat resistance. If the Al content is too high, the amount of inclusions generated increases, resulting in a decrease in quality. Therefore, the upper limit of the Al content is controlled to 3.500%, preferably 3.000%, more preferably 2.000%, and even more preferably 1.500%. On the other hand, the lower limit of the Al content is not particularly limited, but is preferably 0.001%, more preferably 0.010%, and even more preferably 0.100%.

<Si+2Al: 1.20% or more>
The ferritic-austenitic duplex stainless steel material according to the embodiment of the present invention is intended to have a high content of Si and Al. Specifically, Si+2Al (each element symbol represents the content of each element) is 1.20% or more, preferably 1.30% or more. The upper limit of Si+2Al is not particularly limited, but is preferably 10.00%, more preferably 8.00%, and even more preferably 5.00%.

<Ti: 0.001 to 0.500%>
Ti is an element that bonds with C and N to improve corrosion resistance and intergranular corrosion resistance, and is added as needed. From the viewpoint of obtaining the effects of Ti, the lower limit of the Ti content is controlled to 0.001%, preferably 0.005%. On the other hand, if the Ti content is too high, it causes surface defects, leading to a decrease in quality and a decrease in the workability of the ferritic-austenitic duplex stainless steel material. Therefore, the upper limit of the Ti content is controlled to 0.500%, preferably 0.300%, more preferably 0.100%.

<Nb: 0.001 to 1.000%>
Like Ti, Nb is an element that combines with C and N to improve corrosion resistance and intergranular corrosion resistance, and is added as needed. From the viewpoint of obtaining the effects of Nb, the lower limit of the Nb content is controlled to 0.001%, preferably 0.004%, and more preferably 0.010%. On the other hand, if the Nb content is too high, the workability of the ferritic-austenitic duplex stainless steel material will decrease. Therefore, the upper limit of the Nb content is controlled to 1.000%, preferably 0.600%, and more preferably 0.060%.

<V: 0.001-1.000%>
V is an element that improves corrosion resistance and is added as needed. From the viewpoint of obtaining the effects of V, the lower limit of the V content is controlled to 0.001%, preferably 0.010%. On the other hand, if the V content is too high, the workability of the ferritic-austenitic duplex stainless steel material will decrease. Therefore, the upper limit of the V content is controlled to 1.000%, preferably 0.200%.

<W: 0.001-1.000%>
W is an element that improves high-temperature strength and corrosion resistance and is added as needed. From the viewpoint of obtaining the effects of W, the lower limit of the W content is controlled to 0.001%, preferably 0.010%. On the other hand, if the W content is too high, the material becomes hard and the workability deteriorates, and the surface defects increase, degrading the surface quality of the ferritic-austenitic duplex stainless steel material. Therefore, the upper limit of the W content is controlled to 1.000%, preferably 0.300%.

<Zr: 0.001 to 1.000%>
Zr is an element that combines with C and N to improve oxidation resistance and intergranular corrosion resistance, and is added as needed. From the viewpoint of obtaining the effects of Zr, the lower limit of the Zr content is controlled to 0.001%, preferably 0.010%. On the other hand, if the Zr content is too high, the workability of the ferritic-austenitic duplex stainless steel material will decrease. Therefore, the upper limit of the Zr content is controlled to 1.000%, preferably 0.200%, more preferably 0.050%.

<Co:0.001-1.200%>
Co is an element that improves heat resistance and is added as needed. From the viewpoint of obtaining the effects of Co, the lower limit of the Co content is controlled to 0.001%, preferably 0.010%. On the other hand, since Co is expensive, if the Co content is too high, the manufacturing cost will increase. Therefore, the upper limit of the Co content is controlled to 1.200%, preferably 0.400%.

<Ca: 0.0001 to 0.0100%>
Ca is an element that improves the hot workability of ferritic-austenitic duplex stainless steel materials and is added as needed. Ca also forms sulfides to suppress grain boundary segregation of S, thereby improving intergranular oxidation resistance. From the viewpoint of obtaining the effects of Ca, the lower limit of the Ca content is controlled to 0.0001%, preferably 0.0003%. On the other hand, if the Ca content is too high, the number of inclusions increases, leading to a decrease in workability. Therefore, the upper limit of the Ca content is controlled to 0.0100%, preferably 0.0050%.

<B: 0.0001 to 0.0080%>
B is an element that improves the hot workability of ferritic-austenitic duplex stainless steel materials and is added as needed. B also improves the secondary workability of ferritic-austenitic duplex stainless steel materials by strengthening grain boundaries. From the viewpoint of obtaining the effects of B, the lower limit of the B content is controlled to 0.0001%, preferably 0.0003%, and more preferably 0.0005%. On the other hand, if the B content is too high, it will lead to a decrease in weldability and fatigue strength. Therefore, the upper limit of the B content is controlled to 0.0080%, preferably 0.0040%, and more preferably 0.0025%.

<Sn: 0.001 to 0.500%>
Sn is an element that improves corrosion resistance and high-temperature strength and is added as needed. From the viewpoint of obtaining the effects of Sn, the lower limit of the Sn content is controlled to 0.001%, preferably 0.002%. On the other hand, if the Sn content is too high, a low-melting-point phase is formed, which reduces the hot workability of the ferritic-austenitic duplex stainless steel material. Therefore, the upper limit of the Sn content is controlled to 0.500%, preferably 0.100%, more preferably 0.050%.

<REM: 0.200% or less>
Like B and Ca, REM (rare earth elements) are elements that improve the hot workability of ferritic-austenitic duplex stainless steel materials and are added as needed. REM also improves corrosion resistance by forming sulfides that are difficult to dissolve and suppressing the formation of MnS, which acts as a corrosion starting point. However, an excessive REM content leads to increased manufacturing costs. Therefore, the upper limit of the REM content is controlled to 0.200%, preferably 0.100%. On the other hand, the lower limit of the REM content is not particularly limited, but is preferably 0.001%, more preferably 0.010%, from the viewpoint of obtaining the effects of REM.
REM is a collective term for two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). These may be used alone or as a mixture.

In a ferritic-austenitic duplex stainless steel material according to an embodiment of the present invention, the ratio D2/D1 of the number density D2 of inclusions having an equivalent circle diameter of 0.5 to 10 μm at the surface (e.g., surface B in FIG. 1 ) to the number density D1 of inclusions having an equivalent circle diameter of 0.5 to 10 μm in the parent phase (e.g., position C in FIG. 1 ) at a depth of ¼ of the thickness from the surface (e.g., surface B in FIG. 1 ) is 0.50 or less, preferably 0.49 or less, and more preferably 0.48 or less. By controlling the inclusion number density ratio D2/D1 within this range, the proportion of inclusions present at the surface can be reduced compared to inclusions present in the parent phase, thereby improving the corrosion resistance and fatigue properties of the ferritic-austenitic duplex stainless steel material.
The lower limit of the ratio D2/D1 of the number densities of inclusions is not particularly limited because the smaller the ratio, the greater the effect of improving corrosion resistance and fatigue properties. For example, the lower limit is 0.01.

The number density D1 of inclusions having a circle-equivalent diameter of 0.5 to 10 μm in the matrix at a position from the surface to a quarter of the thickness of a ferritic-austenitic duplex stainless steel material (hereinafter referred to as "the number density D1 of inclusions in the matrix") can be calculated as follows. First, arbitrary locations (10 or more fields of view) at a position from the surface to a quarter of the thickness of the ferritic-austenitic duplex stainless steel material are photographed with an electron microscope. Next, the photographed images are binarized to separate inclusions having a circle-equivalent diameter of 0.5 to 10 μm (black) from the matrix (white), the black regions are counted to determine the number of inclusions, and the number of inclusions thus obtained is divided by the area of the observation field, thereby calculating the number density D1 of inclusions in the matrix.
Similarly, the number density D2 of inclusions having an equivalent circle diameter of 0.5 to 10 μm on the surface of a ferritic-austenitic duplex stainless steel material (hereinafter referred to as "number density D2 of inclusions on the surface") can be calculated by photographing arbitrary locations (10 or more fields of view) on the surface of the ferritic-austenitic duplex stainless steel material with an electron microscope and dividing the number of inclusions obtained in the same manner as above by the area of the observed fields of view.
Here, since most of the inclusions have an equivalent circle diameter in the range of 0.5 to 10 μm, the number of inclusions in this size range was counted and defined as the number densities D1 and D2 of the inclusions. The equivalent circle diameter means the diameter of a circle when the area of each observed black region is converted into a circle having the same area.

The electron microscope used can be a Schottky scanning electron microscope SU5000 manufactured by Hitachi High-Tech Corporation. Furthermore, inclusions can be identified using an EDX detector EMAX3.3SP2 manufactured by Oxford Instruments Ltd., which is attached to the electron microscope. Furthermore, analysis software (AZtecSteel) manufactured by Oxford Instruments Ltd. can be used for image processing.

The number density D1 of the inclusions in the matrix is, for example, about 80 to 300 pieces/mm ² .

The number density D2 of the inclusions on the surface is preferably 75 pieces/ ^mm2 or less, and more preferably 60 pieces/ ^mm2 or less. By controlling the number density D2 of the inclusions on the surface within this range, the amount of inclusions present on the surface of the ferritic-austenitic duplex stainless steel material can be reduced, and therefore the corrosion resistance and fatigue strength of the ferritic-austenitic duplex stainless steel material can be stably improved.
The lower limit of the number density D2 of the inclusions on the surface is not particularly limited because the smaller the value, the greater the effect of improving corrosion resistance and fatigue properties. For example, it is 10 pieces/mm ² .

The inclusions are non-metallic inclusions (particularly sulfide-based or oxide-based inclusions), and specific examples thereof include MnS, TiN, TiC, NbN, NbC, CaO, SiO ₂ , Al ₂ O ₃ and complex compounds thereof.

The ferritic-austenitic duplex stainless steel material according to the embodiment of the present invention preferably has a molten and solidified layer on the surface.
As an example, a schematic cross-sectional view of a ferrite-austenite duplex stainless steel material according to an embodiment of the present invention is shown in FIG.
As shown in Figure 1, a ferritic-austenitic duplex stainless steel material 10 comprises a parent phase 11 and a molten and solidified layer 12 formed on a surface A of the parent phase 11. The parent phase 11 contains inclusions 13. The molten and solidified layer 12 may also contain inclusions 13, but only a small amount of the inclusions 13 is exposed on a surface B of the molten and solidified layer 12. This is because the inclusions 13 exposed on the surface B have been solid-dissolved by descaling with a laser beam.

The thickness of the molten solidified layer is not particularly limited, but is preferably 0.50 to 10.00 μm, more preferably 1.00 to 10.00 μm, and even more preferably 2.00 to 10.00 μm. If the thickness of the molten solidified layer is less than 0.50 μm, the inclusions are likely to be insufficiently dissolved. As a result, more inclusions are exposed on the surface, which tends to reduce the corrosion resistance and fatigue properties of the ferritic-austenitic duplex stainless steel material. On the other hand, if the thickness of the molten solidified layer exceeds 10.00 μm, the surface becomes rough, making it difficult to achieve a smooth, glossy surface.

The ferritic-austenitic duplex stainless steel material according to the embodiment of the present invention preferably has a surface arithmetic mean roughness Ra of 0.01 to 0.40 μm, more preferably 0.01 to 0.30 μm, and even more preferably 0.01 to 0.20 μm. By controlling the surface arithmetic mean roughness Ra within this range, the smoothness of the ferritic-austenitic duplex stainless steel material can be ensured.
Here, in this specification, "arithmetic mean roughness Ra" means the arithmetic mean roughness Ra measured in accordance with JIS B0601:2013.

The ferritic-austenitic duplex stainless steel material according to the embodiment of the present invention has a surface 60-degree specular gloss Gs(60°) of preferably 200 to 750%, more preferably 300 to 750%, and even more preferably 400 to 750%. By controlling the surface 60-degree specular gloss Gs(60°) within this range, the gloss of the ferritic-austenitic duplex stainless steel material can be ensured.
Here, in this specification, "60 degree specular gloss Gs (60°)" means the 60 degree specular gloss Gs (60°) measured in accordance with JIS Z8741:1997.

The ferritic-austenitic duplex stainless steel material according to the embodiment of the present invention is preferably a ferritic-austenitic duplex cold-rolled stainless steel material, and more preferably a ferritic-austenitic duplex cold-rolled stainless steel sheet. In this specification, the cold-rolled steel material and cold-rolled steel sheet after the oxide scale has been removed are referred to as "ferritic-austenitic duplex cold-rolled stainless steel material" and "ferritic-austenitic duplex cold-rolled stainless steel sheet," and the cold-rolled steel material and cold-rolled steel sheet before the oxide scale has been removed are referred to as "cold-rolled steel material" and "cold-rolled steel sheet."
Hereinafter, a case will be described in which the ferritic-austenitic duplex stainless steel material according to the embodiment of the present invention is a ferritic-austenitic duplex cold-rolled stainless steel sheet.
The thickness (sheet thickness) of the ferritic-austenitic duplex stainless cold-rolled steel sheet according to one embodiment of the present invention is not particularly limited, but is preferably less than 3 mm, more preferably 2.5 mm or less, and even more preferably 2 mm or less. The lower limit is, for example, 0.1 mm, and may be 0.3 mm.

A ferritic-austenitic duplex stainless steel cold-rolled sheet according to one embodiment of the present invention can be manufactured using methods known in the art, except for the descaling step in which a cold-rolled steel sheet having the above-described composition is irradiated with laser light to remove oxide scale formed on the surface.

The method for producing cold-rolled steel sheet having the above composition is not particularly limited, but can be, for example, as follows. First, stainless steel having the above composition is melted and forged or cast to obtain a steel billet. Next, the steel billet is hot-rolled, annealed, and then descaled. The descaling method is not particularly limited, and can be carried out using pickling, polishing, or laser light. Next, the descaled hot-rolled steel sheet is cold-rolled and annealed to obtain a cold-rolled steel sheet. Note that the conditions for each step are not particularly limited and can be adjusted appropriately depending on the composition of the stainless steel, etc.

The laser beam is irradiated onto cold-rolled steel sheets under conditions that melt the region from the interface between the matrix and oxide scale to a depth of 0.50 to 10.00 μm, preferably 1.00 to 10.00 μm, and more preferably 2.00 to 10.00 μm. By irradiating with laser beams under these conditions, oxide scale can be removed while maintaining surface smoothness and gloss. The thermal effect dissolves surface inclusions, forming a molten, solidified layer on the surface, improving corrosion resistance and fatigue properties. If the irradiation conditions only melt the region to a depth of less than 0.50 μm from the surface of the cold-rolled steel sheet, the inclusions do not dissolve sufficiently, resulting in insufficient reduction of the inclusions exposed on the surface, and the corrosion resistance and fatigue properties of the ferritic-austenitic duplex stainless cold-rolled steel sheet are reduced. Furthermore, if the irradiation conditions melt the region from the surface of the cold-rolled steel sheet to a depth of more than 10.00 μm, the surface of the ferritic-austenitic duplex stainless cold-rolled steel sheet becomes rough, preventing a smooth, glossy surface.

FIG. 2 shows a schematic cross-sectional view for explaining the difference between descaling by laser light irradiation and descaling by a conventional method.
2(a), a cold-rolled steel material (cold-rolled steel sheet) 20 has an oxide scale 21 formed on the surface of a matrix 11. In addition, inclusions 13 are present at an interface D between the matrix 11 and the oxide scale 21 (on the surface of the matrix 11).
When descaling (removal of oxide scale 21) is performed by a conventional method (pickling and/or polishing), inclusions 13 present at the interface D between the matrix 11 and the oxide scale 21 are exposed at the surface E, as shown in FIG. 2( b). Even if the surface layer of the matrix 11 is removed together with the oxide scale 21 in order to remove the inclusions 13 present at the interface D between the matrix 11 and the oxide scale 21, the inclusions 13 present inside the matrix 11 end up being exposed at the surface E. Therefore, it is difficult to reduce the inclusions 13 exposed at the surface E using conventional methods.

In contrast, when descaling is performed by irradiating a laser beam, as shown in Figure 2(c), the oxide scale 21 is removed and the inclusions 13 present at the interface D between the parent phase 11 and the oxide scale 21 are dissolved. As a result, a molten solidified layer 12 is formed on the surface of the parent phase 11, reducing the number of inclusions 13 exposed at surface B. The reduction in the number of inclusions 13 occurs because the inclusions 13 near the surface melt when the molten solidified layer 12 is formed, become diluted with parent phase components, and rapidly solidify before reprecipitation. In addition, the increase in temperature causes the inclusions 13 to dissolve in parts of the molten solidified layer 12 that were not directly melted, resulting in a reduction in their size and number. Even if the inclusions are not completely melted and rendered harmless, partial melting reduces the circle-equivalent diameter of the inclusions 13, improving corrosion resistance and fatigue strength.

The conditions for the laser irradiation may be adjusted depending on the device used, taking into consideration the following points.
(Type of laser light)
When continuous wave laser light is used, the energy required to remove oxide scale increases, the required power increases, and the range in which heat is generated is too large, making it difficult to control the thickness of the molten and solidified layer. Therefore, pulsed laser light, which can apply heat instantaneously, is preferred.
(wavelength)
Generally, the reflectivity of a material to light is wavelength-dependent, and selecting a wavelength with low reflectivity increases the heat input, making it easier for oxide scale to evaporate. Therefore, by selecting a wavelength with high reflectivity for the parent phase and low reflectivity for the oxide, it is possible to selectively evaporate and remove oxide scale without melting the parent phase more than necessary.
(pulse width)
The pulse width represents the time during which one pulse is irradiated, and the narrower the pulse width, the more instantaneous heating occurs. With a narrow pulse width, ablation occurs before the heat input from the laser is transmitted to the surroundings, lowering the ablation threshold and reducing the thermal impact on the parent phase. When laser irradiation is performed under conditions that cause the parent phase to melt, the narrower the pulse width, the more rapid cooling occurs and the more suppressed the reprecipitation of molten inclusions. However, the pulse width is primarily determined by the performance of the oscillator, and since devices capable of oscillating at short pulse widths are expensive, it is preferable to select a short pulse width within the specifications of the device.
(oscillation frequency)
The shorter the pulse width, the higher the oscillation frequency can be, and the higher the oscillation frequency, the more pulses are emitted per unit time, improving the rate at which oxide scale is removed. Therefore, it is preferable to select a high oscillation frequency within the specifications of the device.

(scan frequency)
The scanning frequency represents the speed at which the pulse irradiation position moves in the planar direction. The higher the scanning frequency, the faster the oxide scale removal rate. However, if the scanning frequency is too high, gaps will form between the pulse irradiation positions, leaving oxide scale behind and reducing the descaling rate. Therefore, it is preferable to increase the scanning frequency within a range that allows the descaling rate to be maintained.
(laser beam diameter)
The larger the beam diameter, the wider the irradiation range, i.e., the range that can be descaled with one pulse, and the better the descaling efficiency, but the lower the energy density (fluence) of one pulse. It is preferable to increase the beam diameter within a range that maintains a fluence that can evaporate and remove oxide scale.
(Fluence)
Oxide scale can be vaporized and removed by irradiating it with laser light with a fluence exceeding the ablation threshold of the oxides that make up the oxide scale. The higher the fluence, the thicker the oxide scale that can be removed. However, if the fluence is too high, not only the oxide scale but also the matrix will be vaporized and removed. Pulsed lasers have less thermal impact than continuous-wave lasers, but the higher the fluence, the greater the heat input to the matrix, resulting in larger melted and heat-affected zones. Therefore, the fluence can be adjusted within a range that does not melt the matrix more than necessary, taking into account the balance between the characteristics of the oxide scale to be removed (thickness, structure, composition, etc.) and the heat input to the matrix. If the fluence distribution within the beam spot varies, the average fluence can be used for control.

The ferritic-austenitic duplex stainless steel material according to the embodiment of the present invention, which has the above-mentioned characteristics, has excellent corrosion resistance and can be used as a corrosion-resistant component. Furthermore, this ferritic-austenitic duplex stainless steel material also has excellent fatigue properties, making it suitable for use in corrosion-resistant components that require fatigue properties. Furthermore, this ferritic-austenitic duplex stainless steel material has a smooth, glossy surface and excellent designability, making it suitable for use in corrosion-resistant components that require designability.

A corrosion-resistant member according to an embodiment of the present invention includes the above-described ferritic-austenitic duplex stainless steel material.
The ferritic-austenitic duplex stainless steel material used for this corrosion-resistant member may be processed into various shapes by methods known in the art.
The corrosion-resistant member according to the embodiment of the present invention may further include members other than the above-mentioned ferritic-austenitic duplex stainless steel material.
Examples of corrosion-resistant members include, but are not limited to, automobile parts, building parts, kitchen utensils, and the like.

The present invention will be explained in detail below using examples, but the present invention should not be construed as being limited to these examples.

30 kg of stainless steel having the composition of steel types A to E shown in Table 1 (the remainder being Fe and impurities) was vacuum melted and forged into a 30 mm thick billet. This was then heated at 1230°C for 2 hours, hot rolled to a thickness of 3 mm, and annealed at 1100°C for 5 minutes in an air atmosphere to obtain hot-rolled steel sheet. The hot-rolled steel sheet was then cut into 50 mm (rolling direction) x 50 mm (width direction) by machining and then pickled and descaled. The pickled and descaled hot-rolled annealed steel sheet was immersed in a hydrofluoric/nitric acid aqueous solution containing 50 g/L of hydrofluoric acid and 150 g/L of nitric acid, held at 60°C in a constant-temperature bath, for 240 to 1320 seconds, immediately rinsed with running water, and air-dried. The specific immersion times were 240 seconds for steel type A, 1200 seconds for steel type B, 1320 seconds for steel type C, 1200 seconds for steel type D, and 1200 seconds for steel type E. The hot-rolled steel sheets were then cold-rolled from 3 mm to 1 mm in thickness and annealed at 1100°C for 5 minutes in an air atmosphere to obtain cold-rolled steel sheets. The obtained cold-rolled steel sheets were used in the following examples and comparative examples.

Examples 1 to 5
Cold-rolled steel sheets having the various steel compositions were subjected to a descaling process by irradiating them with laser light.
The laser beam was irradiated using a commercially available device (LaserClear 50A manufactured by IHI Inspection & Measurement Co., Ltd.). The cold-rolled steel sheet was placed on the movable stage of this device, and while moving at 0.2 m/min along the rolling direction, the cold-rolled steel sheet was scanned at a constant speed in the sheet width direction from above, and irradiated once with pulsed laser beam. The scan width per scan was 25 mm. The pulsed laser beam irradiation conditions were as follows:
Wavelength: 1085nm
Pulse width: 220 ns
Oscillation frequency: 60 kHz
Scan frequency: 100Hz
Laser beam diameter: 90 μm
Average fluence: 8 J/cm ²

(Comparative Example 1)
A cold-rolled steel sheet having the composition of steel type A was subjected to a descaling process by pickling.
The pickling was carried out as follows: A hydrofluoric-nitric acid aqueous solution containing 30 g/L of hydrofluoric acid and 100 g/L of nitric acid was kept at 60° C. in a thermostatic bath, and the cold-rolled steel sheet was immersed for 600 seconds, and then immediately rinsed with running water and air-dried.

(Comparative Example 2)
The cold-rolled steel sheet after the descaling process obtained in Comparative Example 1 was subjected to belt polishing using SiC polishing paper (grit #400) and water-soluble grinding oil. The grinding depth was 20 μm from the surface.

The following evaluations were carried out on the cold-rolled steel sheets (ferritic-austenitic duplex stainless steel cold-rolled steel sheets) obtained in the above examples and comparative examples after the descaling process.

(Number density of inclusions D1, D2)
50 mm square test pieces were cut from the width and length centers of the ferrite-austenite duplex stainless cold-rolled steel sheets obtained in the above examples and comparative examples, and arbitrary locations on the surface were photographed at 200x magnification using a Schottky scanning electron microscope SU5000 manufactured by Hitachi High-Tech Corporation. The surface of this test piece was polished to remove 1/4 of the thickness (250 μm), exposing the parent phase at a depth of 1/4 of the thickness from the surface, and then arbitrary locations on this exposed surface were photographed in the same manner as above. Photography was performed in 10 fields of view, with each field being 0.48 mm x 0.64 mm (0.3072 mm ² ). Next, the photographed images were binarized using analysis software (AZtecSteel) manufactured by Oxford Instruments Ltd. to separate inclusions (black) with a circle equivalent diameter of 0.5 to 10 μm from the parent phase (white). The image analysis conditions were as follows.
Resolution: 4096
Minimum detection size: 8 pixels (0.5 μm)
Inclusion color threshold: 47 to 24057
Anything less than 8 pixels was excluded as noise.
Next, the number of inclusions was determined by counting black regions with a circle-equivalent diameter of 0.5 to 10 μm in the binarized image, and the number of inclusions thus obtained was divided by the area of the observation field to calculate the number densities D1 and D2 of the inclusions. The results of the number densities D1 and D2 of the inclusions were taken as the average values of the results in each field of view.
Furthermore, based on the calculated number densities D1 and D2 of the inclusions, the ratio of the number densities of the inclusions D2/D1 was calculated.

(Thickness of molten and solidified layer)
A ferrite-austenite dual-phase stainless steel cold-rolled sheet was cut in the thickness direction (direction perpendicular to the surface) parallel to the rolling direction, and the composition image of the cut surface was observed using an electron microscope up to 10,000 times to identify the molten and solidified layer based on the difference in contrast, and its thickness was measured. The thickness was measured at 10 random locations, and the average value was used as the result.

(surface roughness measurement)
The arithmetic mean roughness Ra of the surface of the ferritic-austenitic duplex stainless steel cold-rolled steel sheet was measured using a contact surface roughness meter (Surfcom 2800 manufactured by Tokyo Seimitsu Co., Ltd.) in accordance with JIS B0601:2013. The arithmetic mean roughness Ra was measured at five locations, excluding the range from the end to 5 mm, with a reference length of 4 mm, and the average value was used as the evaluation result. The measurement locations were spaced at least 5 mm apart.

(Gloss measurement)
The 60-degree specular gloss Gs (60 °) of the surface of the ferrite-austenite duplex stainless steel cold-rolled steel sheet was measured using a glossmeter (PG-1M manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS Z8741:1997. The 60-degree specular gloss Gs (60 °) was measured at five locations excluding the range of 5 mm from the end, and the average value was used as the evaluation result. The measurement positions were separated by 5 mm or more.

(Corrosion resistance test)
The corrosion resistance test was a combined cycle test in accordance with JASO M609 and M610. Specifically, a salt-dry-wet cyclic test was conducted, in which salt spray, drying, and wetting were repeated. In the salt-dry-wet cyclic test, a ferritic-austenitic duplex stainless steel cold-rolled steel sheet was subjected to one cycle of spraying with a 5% NaCl aqueous solution (at 35°C for 2 hours), drying (at 30% relative humidity and 60°C for 4 hours), and wetting (at 95% relative humidity and 50°C for 2 hours). After each cycle, the sheet was rinsed with water and dried, and the surface of the ferritic-austenitic duplex stainless steel cold-rolled steel sheet was observed to calculate the rust area ratio. The number of cycles at which the rust area ratio reached 10% or more was defined as the number of cycles at which corrosion occurred.
The rust area ratio was calculated using the following procedure. The surface of the ferritic-austenitic duplex stainless cold-rolled steel sheet after the salt-dry-wet cycle test was photographed, and the percentage of the area of the rusted portion in a central 25 mm × 25 mm area excluding the edge was determined. The area of the rusted portion was determined by binarizing the photograph of the surface of the ferritic-austenitic duplex stainless cold-rolled steel sheet using image analysis, calculating the area per pixel, and then counting the number of pixels in the rusted portion. The rust area ratio was calculated using the following formula.
Rust area rate (%) = area of rusted portion (mm ² ) / total area of observed portion (625 mm ² ) × 100
In this evaluation, the improvement rate of the number of cycles to corrosion occurrence for the ferritic-austenitic duplex stainless cold-rolled steel sheet of Comparative Example 2 was used as the standard, and the improvement rate of the number of cycles to corrosion occurrence for the ferritic-austenitic duplex stainless cold-rolled steel sheets of Examples 1 to 5 and Comparative Example 1 was calculated. A calculated improvement rate of the number of cycles to corrosion occurrence of 20% or more was marked "Good," a rate of 10% or more but less than 20% was marked "Good," and a rate of less than 10% was marked "Poor."

(Fatigue property test)
The fatigue property test was performed in accordance with JIS Z2275:1978, and a plane bending fatigue test was conducted. Specifically, a test piece was obtained by cutting a ferritic-austenitic dual-phase stainless steel cold-rolled plate to a width length of 30 mm and a rolling length of 90 mm, and forming a 30 mm radius R section on both ends in the width direction. The test piece was mounted on a plane bending tester, and a fatigue test was conducted with ¹⁰⁷ cycles. This fatigue test was performed on two or more test pieces for each stress stage, and the strength-to-time test was measured.
In this evaluation, the improvement rates of the time strength of the ferritic-austenitic duplex stainless cold-rolled steel sheet of Comparative Example 2 were used as a reference, and the improvement rates of the time strength of the ferritic-austenitic duplex stainless cold-rolled steel sheets of Examples 1 to 5 and Comparative Example 1 were calculated. A calculated improvement rate of the time strength of 10% or more was marked "Good", and a calculated improvement rate of less than 10% was marked "Poor".
The results of the above evaluations are shown in Table 2.

As shown in Table 2, the ferritic-austenite duplex stainless cold-rolled steel sheets of Examples 1 to 5, in which the ratio D2/D1 of the number densities of inclusions was 0.50 or less, had a surface arithmetic mean roughness Ra of 0.01 to 0.40 μm and a surface 60-degree specular gloss Gs(60°) of 200 to 750%, and were confirmed to have a smooth and glossy surface. Furthermore, the ferritic-austenite duplex stainless cold-rolled steel sheets of Examples 1 to 5 had improved corrosion resistance and fatigue properties compared to the ferritic-austenite duplex stainless cold-rolled steel sheet of Comparative Example 2.
In contrast, the ferritic-austenitic duplex stainless cold-rolled steel sheet of Comparative Example 1 was subjected to descaling by pickling, and therefore the ratio D2/D1 of the number densities of inclusions exceeded 0.50. Therefore, the ferritic-austenitic duplex stainless cold-rolled steel sheet of Comparative Example 1 did not have sufficiently improved corrosion resistance and fatigue properties compared to the ferritic-austenitic duplex stainless cold-rolled steel sheet of Comparative Example 2.

As can be seen from the above results, the present invention can provide a ferritic-austenitic duplex stainless steel material with a smooth, glossy surface and excellent corrosion resistance and fatigue properties, a method for manufacturing the same, and a corrosion-resistant component made from the same.

REFERENCE SIGNS LIST 10 Ferrite-austenitic duplex stainless steel material 11 Parent phase 12 Melt-solidified layer 13 Inclusions 20 Cold-rolled steel material 21 Oxide scale

Claims (9)

The composition contains, on a mass basis, C: 0.001 to 0.150%, Si: 0.20 to 5.00%, Mn: 6.00% or less, P: 0.050% or less, S: 0.0300% or less, Ni: 1.00 to 9.00%, Cr: 15.00 to 30.00%, Mo: 5.00% or less, Cu: 2.00% or less, N: 0.400% or less, and Al: 3.500% or less, with Si+2Al being 1.20% or more, and the balance being Fe and impurities;
A ferritic-austenitic duplex stainless steel material in which the ratio D2/D1 of the number density D2 of inclusions at the surface to the number density D1 of inclusions having an equivalent circle diameter of 0.5 to 10 μm in the parent phase at a position at a depth of 1/4 of the thickness from the surface is 0.50 or less, and the number density D2 of the inclusions at the surface is 75 pieces/ ^mm2 or less. The ferritic-austenitic duplex stainless steel material according to claim 1 further contains, by mass, one or more selected from Ti: 0.001-0.500%, Nb: 0.001-1.000%, V: 0.001-1.000%, W: 0.001-1.000%, Zr: 0.001-1.000%, and Co: 0.001-1.200%. The ferritic-austenitic duplex stainless steel material according to claim 1 or 2 further contains, by mass, one or more selected from Ca: 0.0001-0.0100%, B: 0.0001-0.0080%, Sn: 0.001-0.500%, and REM: 0.200% or less. The ferritic-austenitic duplex stainless steel material according to any one of claims 1 to 3, wherein the arithmetic mean roughness Ra of the surface is 0.01 to 0.40 μm. A ferritic-austenitic duplex stainless steel material according to any one of claims 1 to 4, wherein the 60-degree specular gloss Gs(60°) of the surface is 200 to 750%. A ferritic-austenitic duplex stainless steel material according to any one of claims 1 to 5, having a molten and solidified surface layer. The ferritic-austenitic duplex stainless steel material according to claim 6, wherein the thickness of the molten and solidified layer is 0.50 to 10.00 μm. A method for producing the ferritic-austenitic duplex stainless steel material according to any one of claims 1 to 3 , comprising the steps of:
A descaling process includes irradiating a cold-rolled steel material having a composition according to any one of claims 1 to 3 with laser light to remove oxide scale formed on the surface of the cold-rolled steel material,
The laser beam is irradiated under conditions that allow the region of the cold-rolled steel material to melt from the interface between the parent phase and the oxide scale to a depth of 0.50 to 10.00 μm. A corrosion-resistant component comprising the ferritic-austenitic duplex stainless steel material according to any one of claims 1 to 7.