JP2022155343A - Ferrite-austenite two-phase system stainless steel material and method for manufacturing the same, and corrosion-resistant member - Google Patents

Abstract

To provide a ferrite-austenite two-phase system stainless steel material which has smooth and glossy surface and has excellent corrosion resistance and fatigue characteristics.SOLUTION: A ferrite-austenite two-phase system stainless steel material 10 has a composition comprising, by mass, 0.001 to 0.150% C, 0.20 to 5.00% Si, 6.00% or less Mn, 0.050% or less P, 0.0300% or less S, 1.00 to 9.00% Ni, 15.00 to 30.00% Cr, 5.00% or less Mo, 2.00% or less Cu, 0.400% or less N, and 3.500% or less Al, wherein Si+2Al is 1.20% or more, and the balance Fe and inevitable impurities. The ferrite-austenite two-phase system stainless steel material 10 has a ratio D2/D1 of a number density D2 of an inclusion 13 on a surface B to a number density D1 of the inclusion 13 in a parent phase 11 at a position C where a depth from the surface B is 1/4 of the thickness of 0.50 or less.SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD The present invention relates to a ferrite-austenite duplex stainless steel material, a method for manufacturing the same, and a corrosion-resistant member.

Stainless steel materials are excellent in various properties such as corrosion resistance, so they are used in a wide range of applications such as automobile parts, construction parts, and kitchen utensils.
Stainless steel materials are classified into steel plates, bar steels, steel strips, steel bars, steel pipes, and the like according to their shapes. A stainless steel plate, which is a general stainless steel material, is manufactured by the following processes. For example, a hot-rolled steel sheet (thick plate material) is manufactured by continuously casting hot metal in which stainless steel raw materials are melted to form a slab, hot-rolling the slab, and then annealing and pickling the slab. A cold-rolled steel sheet (thin sheet material) is produced by cold-rolling a hot-rolled steel sheet, followed by annealing and pickling. In the manufacturing process of such stainless steel materials, pickling is performed to remove oxide scale formed on the surface of the stainless steel materials. Hereinafter, removing the oxide scale formed on the surface of the stainless steel material is referred to as "descaling".

However, the oxide scale may not be sufficiently removed only by pickling.
Therefore, in a general descaling process, a method is used in which mechanical pretreatment such as a scale breaker or shot blasting is applied to crack the oxidized scale, followed by pickling to make the oxidized scale easier to remove. (For example, Patent Documents 1 and 2). Also known is a method of immersing a stainless steel material in a salt bath as a chemical pretreatment.

JP 2014-172077 A JP-A-2-145785

However, in the conventional descaling process using pickling, the surface unevenness formed in the pretreatment and the surface roughening due to pickling cause the surface of the stainless steel material to turn white and lose its luster, degrading the design. . In particular, cold-rolled steel requires a higher level of design (smoothness and gloss) than hot-rolled steel, so it is difficult to obtain the desired design in the conventional descaling process using pickling. . In addition, since the ferrite-austenite duplex stainless steel material has a thick oxide scale formed on the surface, it is difficult to cause cracks by mechanical pretreatment, and uneven pickling is likely to occur. In addition, in the case of a stainless steel material containing a large amount of Si and Al, 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. becomes more difficult to remove. Furthermore, when the oxide scale is removed, inclusions are exposed on the surface of the stainless steel material, and these inclusions are factors that reduce the corrosion resistance and fatigue properties of the stainless steel material.
Therefore, in order to solve these problems, polishing the surface after the descaling process can be considered. Corrosion resistance and design deteriorate due to entrainment of . In addition, since inclusions are also present inside the stainless steel material, even if the surface is polished after the descaling process, new inclusions will be exposed on the surface of the stainless steel material. Therefore, this means cannot be said to be effective.

The present invention has been made in order to solve the above problems, and has a smooth and glossy surface and excellent corrosion resistance and fatigue properties. An object of the present invention is to provide a corrosion-resistant member using this.

The present inventors have made intensive research to solve the above problems, and as a result, controlled the composition of ferrite-austenite duplex stainless steel material and irradiated laser light under specific conditions to improve the surface It was found that descaling while ensuring the smoothness and gloss of the steel and solid solution of inclusions on the surface could improve corrosion resistance and fatigue characteristics. Based on this knowledge, various ferrite-austenite duplex stainless steel materials were produced and investigated. It was found that a ferrite-austenite duplex stainless steel material in which the ratio D2/D1 of the number density D2 of inclusions on the surface to the number density D1 of inclusions in the surface is within a specific range can solve the above problems, and the present invention Completed.

That is, in the present invention, 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% Below, Al: 3.500% or less, Si + 2Al is 1.20% or more, and the balance is Fe and impurities,
The ratio D2/D1 of the number density D2 of the inclusions on the surface to the number density D1 of the inclusions having an equivalent circle diameter of 0.5 to 10 μm in the matrix at a position where the depth from the surface is ¼ of the thickness It is a ferrite-austenite duplex stainless steel material having a ratio of 0.50 or less.

Further, in the present invention, 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% Below, a cold-rolled steel material having a composition containing Al: 3.500% or less, Si+2Al being 1.20% or more, and the balance being Fe and impurities is irradiated with a laser beam to form on the surface of the cold-rolled steel material A method for producing a ferrite-austenite duplex stainless steel material including a descaling step for removing oxide scale formed by
The laser beam irradiation is a production method performed under conditions capable of melting a region of 0.50 to 10.00 μm in depth from the interface between the parent phase and the oxide scale in the cold-rolled steel.

Further, the present invention is a corrosion-resistant member including the ferrite-austenite duplex stainless steel material.

According to the present invention, it is possible to provide a ferrite-austenite duplex stainless steel material having a smooth and glossy surface and excellent corrosion resistance and fatigue properties, a method for producing the same, and a corrosion-resistant member using the same.

1 is a schematic cross-sectional view of a ferrite-austenite duplex stainless steel material according to an embodiment of the present invention. FIG. FIG. 4 is a schematic cross-sectional view for explaining the difference between descaling by laser light irradiation and descaling by a conventional method;

Hereinafter, embodiments of the present invention will be specifically described. The present invention is not limited to the following embodiments, and modifications and improvements can be 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 addition, "%" display regarding a component in this specification means "mass %" unless otherwise indicated.

The ferrite/austenite duplex stainless steel material according to the embodiment of the present invention has 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 is 1.20% or more, and the balance is Fe and impurities.
Here, in this specification, "stainless steel material" means a material formed from stainless steel, and its shape is not particularly limited. Examples of material shapes include plate-like (including belt-like), rod-like, and tubular shapes. In addition, various shaped steels having a cross-sectional shape such as a T-shape and an I-shape may be used. In addition, "impurities" are components that are mixed in by various factors in the manufacturing process, such as raw materials such as ores and scraps, when manufacturing stainless steel materials industrially, and are acceptable within the range that does not adversely affect the present invention. means to be For example, the stainless steel material may contain 0.02% or less of O as an impurity.

Further, the ferrite/austenite duplex stainless steel material according to the embodiment of the present invention has 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%, Co: 0.001 to 1.200%.
Furthermore, the ferrite/austenite duplex stainless steel material according to the embodiment of the present invention has Ca: 0.0001 to 0.0100%, B: 0.0001 to 0.0080%, Sn: 0.001 to 0.500 %, REM: 0.200% or less.
Each component will be described in detail below.

<C: 0.001 to 0.150%>
If the C content is too high, the material becomes hard and has poor workability. In addition, it becomes sensitized when subjected to thermal effects such as welding, and the corrosion resistance of the ferrite-austenite duplex stainless steel decreases. put away. Therefore, the upper limit of the C content is controlled to 0.150%, preferably 0.100%, more preferably 0.080%, still more preferably 0.060%. On the other hand, if the C content is too small, the stability of the austenite phase is lowered, leading to deterioration of workability and an increase in refining cost. Therefore, the lower limit of the C content is controlled to 0.001%, preferably 0.002%, more preferably 0.005%, still more preferably 0.010%.

<Si: 0.20 to 5.00%>
If the content of Si is too high, the workability of the ferrite-austenite duplex stainless steel will be reduced due to hardening. Therefore, the upper limit of the Si content is controlled to 5.00%, preferably 4.00%, more preferably 3.80%, still more preferably 3.50%. On the other hand, if the Si content is too low, the heat resistance of the ferrite-austenite duplex stainless steel will be lowered. 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 ferrite-austenite duplex stainless steel will be lowered. Therefore, the upper limit of the Mn content is controlled to 6.00%, preferably 4.00%, more preferably 2.00%, still 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 still more preferably 0.10%.

<P: 0.050% or less>
If the P content is too high, the corrosion resistance and workability of the ferrite-austenite duplex stainless steel material will deteriorate. Therefore, the upper limit of the P content is controlled to 0.050%, preferably 0.030%, more preferably 0.020%, still 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 still more preferably 0.003%.

<S: 0.0300% or less>
If the content of S is too high, the hot workability is lowered, the manufacturability of the ferrite-austenite duplex stainless steel material is lowered, and the corrosion resistance is also adversely affected. Therefore, the upper limit of the S content is controlled to 0.0300%, preferably 0.0100%, more preferably 0.0050%, still 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 to 9.00%>
Ni, like Mn, is an austenite phase (γ phase) forming element. Since Ni is expensive, an excessive Ni content leads to an increase in manufacturing costs. Therefore, the upper limit of the Ni content is controlled to 9.00%, preferably 8.00%, more preferably 7.50%, still more preferably 7.00%. On the other hand, from the viewpoint of obtaining the effect of Ni, the lower limit of the Ni content is preferably 1.00%, more preferably 2.00%, even more preferably 5.00%, and still more preferably 5.50%. controlled.

<Cr: 15.00 to 30.00%>
If the Cr content is too high, the refining cost will increase, and solid-solution strengthening will harden the steel, thereby deteriorating the workability of the ferrite-austenite duplex stainless steel material. Therefore, the upper limit of the Cr content is controlled to 30.00%, preferably 28.00%, more preferably 26.00%, still more preferably 25.00%. On the other hand, if the Cr content is too small, 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%, still more preferably 22.00%.

<Mo: 5.00% or less>
Mo is an element that improves the corrosion resistance of ferrite-austenite duplex stainless steel. Since Mo is expensive, if the Mo content is too high, it 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%, still 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 still more preferably 1.00%.

<Cu: 2.00% or less>
Cu is an element that improves the workability of the ferrite-austenite duplex stainless steel material. If the Cu content is too high, the corrosion resistance of the ferrite-austenite duplex stainless steel material is lowered, and a low-melting-point phase is formed 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%, still more preferably 1.00%. On the other hand, although the lower limit of the Cu content is not particularly limited, it is preferably 0.01%, more preferably 0.05%, and still more preferably 0.10%.

<N: 0.400% or less>
N is an element that improves corrosion resistance. If the content of N is too high, the workability of the ferrite-austenite duplex stainless steel will be reduced due to hardening. Therefore, the upper limit of the N content is controlled to 0.400%, preferably 0.300%, more preferably 0.280%, still 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%, more preferably 0.10%.

<Al: 3.500% or less>
Al is an element that is added as necessary for deoxidation in the refining process and improves corrosion resistance and heat resistance. If the Al content is too high, the amount of inclusions produced increases and the quality deteriorates. Therefore, the upper limit of the Al content is controlled to 3.500%, preferably 3.000%, more preferably 2.000%, still 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 still more preferably 0.100%.

<Si+2Al: 1.20% or more>
The ferrite-austenite 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. Although the upper limit of Si+2Al is not particularly limited, it is preferably 10.00%, more preferably 8.00%, and still more preferably 5.00%.

<Ti: 0.001 to 0.500%>
Ti is an element that combines with C and N to improve corrosion resistance and intergranular corrosion resistance, and is added as necessary. From the viewpoint of obtaining the effect 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 flaws, resulting in deterioration in quality and deterioration in workability of the ferrite-austenite 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%>
Nb, like Ti, is an element that combines with C and N to improve corrosion resistance and intergranular corrosion resistance, and is added as necessary. From the viewpoint of obtaining the effect of Nb, the lower limit of the Nb content is controlled to 0.001%, preferably 0.004%, more preferably 0.010%. On the other hand, if the Nb content is too high, the workability of the ferrite-austenite duplex stainless steel material is lowered. Therefore, the upper limit of the Nb content is controlled to 1.000%, preferably 0.600%, more preferably 0.060%.

<V: 0.001 to 1.000%>
V is an element that improves corrosion resistance and is added as necessary. From the viewpoint of obtaining the effect 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 ferrite-austenite duplex stainless steel material is lowered. Therefore, the upper limit of the V content is controlled to 1.000%, preferably 0.200%.

<W: 0.001 to 1.000%>
W is an element that improves high-temperature strength and corrosion resistance, and is added as necessary. From the viewpoint of obtaining the effect 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 steel becomes hard and workability deteriorates, and surface defects increase to deteriorate the surface quality of the ferrite-austenite 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 necessary. From the viewpoint of obtaining the effect 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 ferrite-austenite duplex stainless steel material is lowered. Therefore, the upper limit of Zr content is controlled to 1.000%, preferably 0.200%, more preferably 0.050%.

<Co: 0.001 to 1.200%>
Co is an element that improves heat resistance and is added as necessary. From the viewpoint of obtaining the effect 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, an excessive Co content leads to an increase in manufacturing costs. 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 the ferrite-austenite duplex stainless steel material, and is added as necessary. Ca is also an element that forms sulfides and suppresses grain boundary segregation of S, thereby improving grain boundary oxidation resistance. From the viewpoint of obtaining the effect 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, resulting in deterioration of 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 the ferrite-austenite duplex stainless steel material, and is added as necessary. B is also an element that improves the secondary workability of the ferrite-austenite duplex stainless steel material by grain boundary strengthening. From the viewpoint of obtaining the effect of B, the lower limit of the B content is controlled to 0.0001%, preferably 0.0003%, more preferably 0.0005%. On the other hand, if the B content is too high, the weldability and fatigue strength will be lowered. Therefore, the upper limit of the B content is controlled to 0.0080%, preferably 0.0040%, 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 necessary. From the viewpoint of obtaining the effect 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 deteriorates the hot workability of the ferrite-austenite 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>
REM (rare earth element), like B and Ca, is an element that improves the hot workability of ferrite-austenite duplex stainless steel materials, and is added as necessary. REM is also an element that improves corrosion resistance by forming sulfides that are difficult to elute and suppressing the formation of MnS, which is a starting point for corrosion. However, if the REM content is too high, it will lead to an increase in 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 effect of REM.
Note that REM is a general term for two elements, scandium (Sc) and yttrium (Y), and fifteen elements (lanthanides) from lanthanum (La) to lutetium (Lu). These may be used singly or as a mixture.

In the ferrite-austenite duplex stainless steel material according to the embodiment of the present invention, the parent phase (for example, the position C ) to the number density D1 of inclusions with an equivalent circle diameter of 0.5 to 10 μm on the surface (for example, the surface B in FIG. 1), the ratio D2/D1 is 0.50 or less, preferably 0.49 or less, more preferably 0.48 or less. By controlling the number density ratio D2/D1 of inclusions within such a range, the proportion of inclusions existing on the surface can be made smaller than that of inclusions existing in the matrix. Corrosion resistance and fatigue properties of duplex stainless steel can be improved.
The lower limit of the number density ratio D2/D1 of inclusions is not particularly limited because the smaller the ratio, the higher the effect of improving the corrosion resistance and fatigue characteristics, but is, for example, 0.01.

The number density D1 of inclusions with an equivalent circle diameter of 0.5 to 10 μm in the parent phase at a position where the depth from the surface of the ferrite-austenitic duplex stainless steel material is 1/4 of the thickness (hereinafter referred to as “inclusions in the parent phase ) can be calculated as follows. First, an arbitrary point (10 or more fields of view) at a position (mother phase) whose depth from the surface of the ferrite-austenite duplex stainless steel material is 1/4 of the thickness is photographed with an electron microscope. Next, the photographed image is binarized to divide it into inclusions (black) with an equivalent circle diameter of 0.5 to 10 μm and the mother phase (white), and the black regions are counted to obtain the number of inclusions. By dividing the obtained number of inclusions by the area of the observation field, the number density D1 of inclusions in the matrix can be calculated.
Similarly, the number density D2 of inclusions with an equivalent circle diameter of 0.5 to 10 μm on the surface of the ferrite/austenite duplex stainless steel material (hereinafter referred to as “the number density D2 of inclusions on the surface”) is It can be calculated by photographing arbitrary locations (10 or more fields of view) on the surface of the 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 observation field of view. .
Here, since most of the inclusions have an equivalent circle diameter in the range of 0.5 to 10 μm, inclusions having a size in this range were counted as the inclusion number densities D1 and D2. 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.

As an electron microscope, a Schottky scanning electron microscope SU5000 manufactured by Hitachi High-Tech Co., Ltd. can be used. Further, inclusions can be specified by an EDX detector EMAX3.3SP2 manufactured by Oxford Instruments Co., Ltd. attached to the electron microscope. For image processing, analysis software (AZtecSteel) manufactured by Oxford Instruments Co., Ltd. can be used.

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

The number density D2 of inclusions on the surface is preferably 75/mm ² or less, more preferably 60/mm ² or less. By controlling the number density D2 of inclusions on the surface within this range, the amount of inclusions present on the surface of the ferrite-austenite duplex stainless steel material can be reduced. Fatigue strength can be stably improved.
The lower limit of the number density D2 of inclusions on the surface is not particularly limited because the smaller the ^number , the higher the effect of improving the corrosion resistance and fatigue characteristics.

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

The ferrite-austenite duplex stainless steel material according to the embodiment of the present invention preferably has a melt-solidified layer on the surface layer.
Here, as an example, FIG. 1 shows a schematic cross-sectional view of a ferrite-austenite duplex stainless steel material according to an embodiment of the present invention.
As shown in FIG. 1 , a ferrite-austenite duplex stainless steel material 10 includes a mother phase 11 and a melt-solidified layer 12 formed on a surface A of the mother phase 11 . Inclusions 13 are included in the matrix 11 . Although the melt-solidified layer 12 may also contain inclusions 13 , the number of inclusions 13 exposed on the surface B of the melt-solidified layer 12 is small. This is because the inclusions 13 exposed on the surface B were dissolved by descaling by the laser beam.

The thickness of the melt-solidified layer is not particularly limited, but is preferably 0.50 to 10.00 μm, more preferably 1.00 to 10.00 μm, still more preferably 2.00 to 10.00 μm. If the thickness of the melt-solidified layer is less than 0.50 μm, the inclusions are likely to be in a state where they are not fully dissolved. Therefore, the number of inclusions exposed on the surface increases, and the corrosion resistance and fatigue properties of the ferrite-austenite duplex stainless steel tend to deteriorate. On the other hand, if the thickness of the melt-solidified layer exceeds 10.00 μm, the surface tends to be rough, making it difficult to obtain a smooth and glossy surface.

The ferrite/austenite duplex stainless steel material according to the embodiment of the present invention has a surface arithmetic mean roughness Ra of preferably 0.01 to 0.40 μm, more preferably 0.01 to 0.30 μm, further preferably 0.01 to 0.20 μm. By controlling the surface arithmetic mean roughness Ra within such a range, the smoothness of the ferrite-austenite duplex stainless steel material can be ensured.
Here, in this specification, "arithmetic mean roughness Ra" means arithmetic mean roughness Ra measured based on JIS B0601:2013.

The ferrite-austenite 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%. 750%. By controlling the 60-degree specular gloss Gs (60°) of the surface within such a range, the glossiness of the ferrite-austenite duplex stainless steel material can be ensured.
Here, "60-degree specular gloss Gs (60°)" as used herein means 60-degree specular gloss Gs (60°) measured according to JIS Z8741:1997.

The ferrite-austenite duplex stainless steel material according to the embodiment of the present invention is preferably a ferrite-austenite duplex stainless cold-rolled steel material, and more preferably a ferrite-austenite duplex stainless steel cold-rolled steel sheet. In this specification, the cold-rolled steel and cold-rolled steel sheet from which oxide scale has been removed are referred to as "ferrite-austenite duplex stainless steel cold-rolled steel" and "ferrite-austenite duplex stainless cold-rolled steel sheet". Cold-rolled steel materials and cold-rolled steel sheets before scale removal are referred to as "cold-rolled steel materials" and "cold-rolled steel sheets."
Hereinafter, a case where the ferrite-austenite duplex stainless steel material according to the embodiment of the present invention is a ferrite-austenite duplex stainless cold-rolled steel sheet will be described as an example.
The thickness (plate thickness) of the ferrite/austenite duplex stainless steel 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 still more preferably 2 mm or less. be. Also, the lower limit thereof is, for example, 0.1 mm, and may be 0.3 mm.

A ferrite-austenite duplex stainless steel cold-rolled steel sheet according to an embodiment of the present invention is produced by irradiating a cold-rolled steel sheet having the above composition with a laser beam to perform a descaling step of removing oxide scale formed on the surface. Other than that, it can be produced by using a method known in the art.

Although the method for producing the cold-rolled steel sheet having the above composition is not particularly limited, it can be produced, for example, as follows. First, stainless steel having the above composition is melted, and a steel slab is obtained by forging or casting. Next, the 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. The conditions of each step are not particularly limited and may be appropriately adjusted according to the composition of the stainless steel.

The cold-rolled steel sheet is irradiated with laser light at a depth of 0.50 to 10.00 μm, preferably 1.00 to 10.00 μm, more preferably 2.0 μm from the interface between the parent phase and the oxide scale in the cold-rolled steel sheet. The melting is carried out under conditions that allow the region from 00 to 10.00 μm to be melted. By irradiating the laser beam under these conditions, the oxide scale is removed while ensuring the smoothness and gloss of the surface, and the inclusions on the surface are dissolved by the heat effect to form a molten solidified layer on the surface. Corrosion resistance and fatigue properties can be improved by forming the steel. If the irradiation conditions are such that the cold-rolled steel sheet can only be melted to a depth of less than 0.50 μm from the surface, the inclusions do not dissolve sufficiently, so the inclusions exposed on the surface cannot be sufficiently reduced, and ferrite・The corrosion resistance and fatigue properties of the austenitic duplex stainless steel cold-rolled steel sheet are degraded. In addition, if the irradiation conditions are such that a region with a depth of more than 10.00 μm from the surface of the cold-rolled steel sheet is melted, the surface of the ferrite-austenite duplex stainless steel cold-rolled steel sheet becomes rough, smooth and glossy. No surface available.

FIG. 2 shows a schematic cross-sectional view for explaining the difference between descaling by laser light irradiation and descaling by a conventional method.
As shown in FIG. 2( a ), a cold-rolled steel material (cold-rolled steel sheet) 20 has an oxide scale 21 formed on the surface of the parent phase 11 . Inclusions 13 are present at the interface D between the matrix 11 and the oxide scale 21 (the surface of the matrix 11).
When descaling (removing the oxide scale 21) is performed by a conventional method (pickling and/or polishing), as shown in FIG. Object 13 is exposed on surface E. In addition, 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 exist inside the matrix 11. exists, the inclusions 13 inside the matrix 11 are exposed to the surface E. Therefore, it is difficult to reduce the inclusions 13 exposed on the surface E by the conventional method.

On the other hand, when descaling is performed by irradiating laser light, as shown in FIG. 13 dissolves. Further, as a result, a melt-solidified layer 12 is formed on the surface layer of the matrix 11, and the number of inclusions 13 exposed on the surface B is reduced. The reduction in the number of inclusions 13 occurs when the inclusions 13 near the surface melt when the molten solidified layer 12 is formed, become diluted with the mother phase components, and rapidly solidify before reprecipitation. In addition, in the vicinity of the melt-solidified layer 12, solid solution of the inclusions 13 occurs due to the increase in temperature even in the portion that was not directly melted, and the size of the inclusions 13 is reduced and the number of the inclusions 13 is reduced. occurs. Further, even if the inclusions are not completely melted and rendered harmless, they are partially melted to reduce the equivalent circle diameter of the inclusions 13, thereby improving corrosion resistance and fatigue strength.

The irradiation laser conditions may be adjusted in consideration of the following matters according to the device to be used.
(type of laser light)
When using a continuous wave laser beam, the energy required to remove the oxide scale increases, the required power increases, and the range in which the thermal effect occurs is too large, making it difficult to control the thickness of the molten solidified layer. Therefore, a pulsed laser beam that can instantaneously apply heat is preferable.
(wavelength)
In general, the reflectance of a substance with respect to light depends on the wavelength, and if a wavelength with a low reflectance is selected, the heat input increases and transpiration of oxide scale is likely to occur. Therefore, by selecting a wavelength at which the reflectance of the matrix is high and the reflectance of the oxide is low, the oxide scale can be selectively evaporated and removed without melting the matrix 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. If the pulse width is narrow, ablation occurs before the heat input by the laser is transmitted to the surroundings, so the ablation threshold becomes small and the thermal effect on the matrix phase is reduced. When laser irradiation is performed under conditions that cause melting of the matrix phase, the narrower the pulse width, the more rapid cooling occurs and the reprecipitation of melted inclusions is suppressed. However, the pulse width is determined mainly by the performance of the oscillator, and a device capable of oscillating with a short pulse width is expensive, so it is preferable to select a short pulse width within the specification range of the device.
(Oscillation frequency)
The shorter the pulse width, the higher the oscillating frequency, and the higher the oscillating frequency, the more pulses are applied per unit time, and the oxide scale removal speed is improved. Therefore, it is preferable to select a high oscillation frequency within the specification range of the device.

(scan frequency)
The scan frequency represents the moving speed of the pulse irradiation position in the plane direction. The higher the scan frequency, the faster the oxide scale removal speed. Decreases the descaling rate. Therefore, it is preferable to increase the scan frequency within a range in which the descaling rate can be maintained.
(laser beam diameter)
The larger the pulse, the wider the irradiation range, that is, the range that can be descaled with one pulse, and the descaling efficiency improves, but the energy density (fluence) per pulse decreases. It is preferable to increase the beam diameter within a range in which the fluence capable of transpiration removal of oxide scale is maintained.
(fluence)
By irradiating with a laser beam having a fluence exceeding the ablation threshold of the oxides forming the oxide scale, the oxide scale can be removed by evaporation. The higher the fluence, the greater the thickness of the oxidized scale that can be removed. However, if the fluence is too high, not only the oxidized scale but also the mother phase is removed by transpiration. In addition, the pulse laser has less heat effect than the continuous wave laser, but the higher the fluence, the greater the heat input to the matrix phase, and the larger the melted zone and the heat-affected zone. Therefore, considering the balance between the properties (thickness, structure, composition, etc.) of the oxide scale to be removed and the heat input to the matrix, the fluence should be adjusted within a range that does not cause the matrix to melt more than necessary. good. If the fluence distribution is different within the beam spot, it may be controlled using the average fluence.

Since the ferrite-austenite duplex stainless steel material according to the embodiment of the present invention having the above characteristics is excellent in corrosion resistance, it can be used as a corrosion-resistant member. In addition, since this ferrite-austenite duplex stainless steel material is excellent in fatigue characteristics, it is suitable for use in corrosion-resistant members that require fatigue characteristics. Furthermore, since this ferrite-austenite duplex stainless steel material has a smooth and glossy surface and is excellent in designability, it is suitable for use in corrosion-resistant members that require designability.

A corrosion-resistant member according to an embodiment of the present invention includes the ferrite-austenite duplex stainless steel material described above.
The ferrite-austenite 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 can further include a member other than the above ferrite-austenite duplex stainless steel material.
Corrosion-resistant members include, but are not limited to, automobile parts, construction parts, and kitchen utensils.

EXAMPLES The content of the present invention will be described in detail below with reference to Examples, but the present invention is not construed as being limited thereto.

30 kg of stainless steel having the compositions of steel types A to E shown in Table 1 (the balance being Fe and impurities) was melted by vacuum melting, forged into a steel slab with a thickness of 30 mm, and 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 a hot-rolled steel sheet. The hot-rolled steel sheet was cut into pieces of 50 mm (rolling direction)×50 mm (width direction) by processing, and then descaled by pickling. For the pickling descaling, a hydrofluoric-nitric acid aqueous solution containing 50 g/L of hydrofluoric acid and 150 g/L of nitric acid is held at 60°C in a constant temperature bath, and the hot-rolled and annealed sheet is immersed for 240 to 1320 seconds, and then immediately washed with running water. This was done by letting it dry naturally. 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. Thereafter, the hot-rolled steel sheet was cold-rolled from a thickness of 3 mm to 1 mm, and annealed at 1100° C. for 5 minutes in an air atmosphere to obtain a cold-rolled steel sheet. The obtained cold-rolled steel sheets were used in the following examples and comparative examples.

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

(Comparative example 1)
A descaling process by pickling was performed on the cold-rolled steel sheet having the composition of steel type A.
Pickling was performed 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 constant temperature bath.

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

The cold-rolled steel sheets (ferrite-austenite duplex stainless steel cold-rolled steel sheets) after the descaling process obtained in the above Examples and Comparative Examples were evaluated as follows.

(Number density D1, D2 of inclusions)
A test piece of 50 mm square was cut from the center in the width direction and the length direction of the ferrite-austenite duplex stainless steel cold-rolled steel sheets obtained in the above examples and comparative examples, and an arbitrary part of the surface was cut. Photographs were taken at 200× using a Hitech Schottky Scanning Electron Microscope SU5000. In addition, after polishing the surface of this test piece to remove 1/4 of the thickness (250 μm) and exposing the mother phase at a position where the depth from the surface is 1/4 of the thickness, any of this exposed surface were photographed in the same manner as above. The photographing was performed in 10 fields of view, each field having a size of 0.48 mm×0.64 mm (0.3072 mm ² ). Next, using analysis software (AZtecSteel) manufactured by Oxford Instruments Co., Ltd., the photographed image is binarized to include inclusions (black) with an equivalent circle diameter of 0.5 to 10 μm and the matrix (white) ) and divided into The image analysis conditions were as follows.
Resolution: 4096
Minimum detectable size: 8 pixels (0.5 μm)
Inclusion color threshold: 47 to 24057
Those with less than 8 pixels were excluded as noise.
Next, in the binarized image, black regions with equivalent circle diameters of 0.5 to 10 μm are counted to obtain the number of inclusions. Object number densities D1 and D2 were calculated. The results of the number densities D1 and D2 of inclusions were the average values of the results in each visual field.
Also, based on the calculated inclusion number densities D1 and D2, the inclusion number density ratio D2/D1 was calculated.

(Thickness of molten solidified layer)
A ferrite/austenite duplex cold-rolled stainless steel sheet is cut in the thickness direction parallel to the rolling direction (perpendicular to the surface), and the composition image of the cut surface is observed at a magnification of 10,000 using an electron microscope. The molten solidified layer was identified and its thickness was measured. The thickness was measured at 10 arbitrary points, and the average value was used as the result.

(Surface roughness measurement)
The surface of the ferrite/austenite duplex stainless steel cold-rolled steel sheet was measured for arithmetic average roughness Ra according to JIS B0601:2013 using a contact surface roughness meter (Surfcom 2800 manufactured by Tokyo Seimitsu Co., Ltd.). Arithmetic mean roughness Ra was measured at 5 points with a reference length of 4 mm, excluding a range from the edge to 5 mm, and the average value was used as the evaluation result. Note that each measurement position was separated by 5 mm or more.

(Gloss measurement)
Regarding the surface of the ferrite/austenite duplex stainless steel cold-rolled steel sheet, 60 degrees specular gloss Gs (60 degrees) is measured using a gloss meter (PG-1M manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS Z8741: 1997. It was measured. The 60-degree specular gloss Gs (60°) was measured at five points excluding a range of 5 mm from the edge, and the average value was used as the evaluation result. Note that each measurement position was separated by 5 mm or more.

(Corrosion resistance test)
The corrosion resistance test was a combined cycle test according to JASO M609 and M610. Specifically, a repeated salt-dry-wet test was conducted in which salt spray, drying, and wetting were repeated. In the salt dry-wet repeated test, a ferrite/austenite duplex stainless steel cold-rolled steel sheet was sprayed with a 5% NaCl aqueous solution (35°C for 2 hours), dried (relative humidity of 30%, temperature of 60°C for 4 hours), and wetness (relative humidity of 95%, temperature of 50°C for 2 hours) is set as one cycle. An area ratio was calculated. The number of cycles at which the rusted area ratio reached 10% or more was defined as the number of corrosion generation cycles.
Calculation of the rusted area ratio was performed in the following procedure. Photographs of the surface of the ferrite/austenite duplex stainless steel cold-rolled steel sheet after the repeated salt-dry-wet test were taken, and the ratio of the area of the rusted portion in the central 25 mm x 25 mm area excluding the end faces was determined. The area of the rusted portion is obtained by binarizing the photograph of the surface of the ferrite/austenite duplex stainless steel cold-rolled steel sheet by image analysis, calculating the area per pixel, and then counting the number of pixels of the rusted portion. rice field. The rusted area ratio was calculated by the following formula.
Rusted area ratio (%)=Area of rusted portion (mm ² )/Area of entire observed portion (625 mm ² )×100
In this evaluation, the number of corrosion occurrence cycles of the ferrite/austenite duplex stainless steel cold-rolled steel sheet of Comparative Example 2 was used as a reference, and the corrosion occurrence of the ferrite/austenite duplex stainless steel cold-rolled steel sheets of Examples 1 to 5 and Comparative Example 1 was used. The rate of improvement in cycle number was calculated. The calculated rate of improvement in the number of corrosion generation cycles was rated at 20% or more as "◯", at 10% or more and less than 20% as "Δ", and at less than 10% as "x".

(Fatigue property test)
The fatigue property test was a plane bending fatigue test according to JIS Z2275:1978. Specifically, a test piece was obtained by cutting a widthwise length of 30mm×rolling direction length of 90mm from a ferrite/austenite duplex stainless steel cold-rolled steel sheet, and forming R portions with a radius of 30mm at both widthwise ends. This test piece was mounted on a plane bending tester, and a fatigue test was repeated 10 ⁷ times. In this fatigue test, two or more specimens were tested for each stress step to measure the strength over time.
In this evaluation, the fatigue strength of the ferrite/austenite duplex stainless steel cold-rolled steel sheet of Comparative Example 2 was used as a reference, and the time strength of the ferrite/austenite duplex stainless steel cold-rolled steel sheets of Examples 1 to 5 and Comparative Example 1 was evaluated. The improvement rate was calculated. When the calculated improvement rate of strength over time was 10% or more, it was evaluated as "O", and when it was less than 10%, it was evaluated as "X".
Table 2 shows the above evaluation results.

As shown in Table 2, the ferrite/austenite duplex stainless steel cold-rolled steel sheets of Examples 1 to 5, in which the inclusion number density ratio D2/D1 is 0.50 or less, have a surface arithmetic mean roughness Ra is 0.01 to 0.40 μm, and the 60° specular gloss Gs (60°) of the surface is 200 to 750%, confirming that the surface has a smooth and glossy surface. Further, the ferrite/austenite duplex stainless steel cold-rolled steel sheets of Examples 1 to 5 had improved corrosion resistance and fatigue properties compared to the ferrite/austenite duplex stainless steel cold-rolled steel sheet of Comparative Example 2.
On the other hand, the ferrite/austenite duplex cold-rolled stainless steel sheet of Comparative Example 1 was subjected to descaling by pickling, so that the inclusion number density ratio D2/D1 exceeded 0.50. Therefore, the ferrite-austenite duplex stainless steel cold-rolled steel sheet of Comparative Example 1 was not sufficiently improved in corrosion resistance and fatigue properties as compared with the ferrite-austenite duplex stainless steel cold-rolled steel sheet of Comparative Example 2.

As can be seen from the above results, according to the present invention, a ferrite-austenite duplex stainless steel material having a smooth and glossy surface and excellent corrosion resistance and fatigue properties, a method for producing the same, and a corrosion-resistant member using the same can be provided.

REFERENCE SIGNS LIST 10 ferrite-austenite duplex stainless steel 11 parent phase 12 melt-solidified layer 13 inclusions 20 cold-rolled steel 21 oxide scale

Claims (11)

Based on mass, 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: 3.0%. 500% or less, Si + 2Al is 1.20% or more, and the balance is Fe and impurities,
The ratio D2/D1 of the number density D2 of the inclusions on the surface to the number density D1 of the inclusions having an equivalent circle diameter of 0.5 to 10 μm in the matrix at a position where the depth from the surface is ¼ of the thickness A ferrite/austenite duplex stainless steel material having a value of 0.50 or less. Based on mass, 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: The ferrite-austenite duplex stainless steel material according to claim 1, further comprising one or more selected from 0.001 to 1.000% and Co: 0.001 to 1.200%. Based on mass, one 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 The ferrite-austenite duplex stainless steel material according to claim 1 or 2, further comprising the above. The ferrite-austenite duplex stainless steel material according to any one of claims 1 to 3, wherein said surface has an arithmetic mean roughness Ra of 0.01 to 0.40 µm. The ferrite-austenite duplex stainless steel material according to any one of claims 1 to 4, wherein the surface has a 60-degree specular gloss Gs (60°) of 200 to 750%. The ferrite-austenite duplex stainless steel material according to any one of claims 1 to 5, comprising a melt-solidified layer on the surface layer. 7. The ferrite-austenite duplex stainless steel material according to claim 6, wherein the melt-solidified layer has a thickness of 0.50 to 10.00 μm. Based on mass, 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: 3.0%. 500% or less, Si + 2Al is 1.20% or more, and the balance is Fe and impurities. A method for producing a ferrite-austenite duplex stainless steel material including a descaling process,
The manufacturing method, wherein the laser beam irradiation is performed under conditions capable of melting a region of 0.50 to 10.00 μm in depth from the interface between the parent phase and the oxide scale in the cold-rolled steel material. The cold-rolled steel material is based on mass, 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%, Co: 0.001 to 1.200%, further comprising one or more selected from the ferritic and austenitic duplex stainless steel material according to claim 8. manufacturing method. The cold-rolled steel material is based on mass, Ca: 0.0001 to 0.0100%, B: 0.0001 to 0.0080%, Sn: 0.001 to 0.500%, REM: 0.200% or less 10. The method for producing a ferrite-austenite duplex stainless steel material according to claim 8 or 9, further comprising one or more selected from: A corrosion-resistant member comprising the ferrite-austenite duplex stainless steel material according to any one of claims 1 to 7.

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