JP7825802B2 - Hot-rolled stainless steel material and its manufacturing method - Google Patents

Description

The present invention relates to hot-rolled stainless steel and its manufacturing method.

Due to its excellent corrosion resistance and other properties, stainless steel is used in a wide range of applications, including automotive parts, construction parts, and kitchen utensils. Some of these applications require excellent surface gloss, and polishing the surface of the stainless steel material to achieve the desired gloss.

Depending on their shape, stainless steel materials are classified into steel plates, steel bars, steel strips, steel bars, steel pipes, etc. Stainless steel plates, which are common stainless steel materials, are manufactured using the following process. For example, hot-rolled steel sheets (including strips) are manufactured by continuously casting molten iron, which is made by dissolving stainless steel raw materials, into stainless steel slabs, which are then hot-rolled, annealed, and pickled (see, for example, Patent Documents 1 to 3). Furthermore, if gloss is required, polishing is performed after pickling, which follows annealing. In this common stainless steel manufacturing process, pickling is performed to remove scale formed on the surface of the stainless steel material. Hereinafter, the removal of scale formed on the surface of stainless steel material will be referred to as "descaling."

As a descaling method other than pickling, for example, Patent Document 4 proposes a descaling method in which a scale layer formed on the surface of a stainless steel sheet when the sheet is annealed after rolling is irradiated with laser light to melt, vaporize, and remove the scale.

Japanese Patent Application Publication No. 8-269549 Japanese Patent Application Publication No. 8-060251 Japanese Patent Application Laid-Open No. 2001-47121 Japanese Patent Application Publication No. 4-182020

However, the descaling method of Patent Document 1, which relies solely on pickling, may not be able to sufficiently remove scale. In Patent Documents 2 and 3, preliminary descaling using shot blasting or other methods creates unevenness on the surface. As a result, corrosive substances such as salt water are trapped in these unevenness, making the material more susceptible to rusting, and therefore the corrosion resistance is insufficient.

Furthermore, the descaling method using laser light in Patent Document 4 does not specify the type or conditions of the laser light, so depending on the conditions, scale may not be sufficiently removed. Furthermore, even if pickling is performed after laser light irradiation, surface irregularities caused by the laser light may reduce corrosion resistance.

The present invention was made to solve the above-mentioned problems, and aims to provide hot-rolled stainless steel material with excellent corrosion resistance and a method for manufacturing the same.

As a result of extensive research into the surface properties (surface roughness) of hot-rolled stainless steel obtained by various descaling methods, the inventors discovered that the above-mentioned problems could be solved by controlling the root-mean-square height Sq and developed interface area ratio Sdr within a specified range, leading to the completion of the present invention. The inventors also discovered that hot-rolled stainless steel having such surface properties can be easily obtained by performing laser descaling under specific conditions followed by grinding.

That is, the present invention provides a stainless hot-rolled steel material having a surface roughness in which the root mean square height Sq is 5.0 μm or less and the developed interface area ratio Sdr is 25.0% or less,
In a salt-dry-wet cyclic test, if the pitting corrosion resistance index (PREN) of the hot-rolled stainless steel material is less than 30, the installation angle (surface tilt angle) of the hot-rolled stainless steel material is set to 20° with respect to the vertical direction, and if the pitting corrosion resistance index (PREN) of the hot-rolled stainless steel material is 30 or more, the installation angle (surface tilt angle) of the hot-rolled stainless steel material is set to horizontal (0°) with respect to the vertical direction, and after 10 cycles of spraying (at 35°C for 15 minutes) with a 5% NaCl aqueous solution, drying (at 60°C for 1 hour with a relative humidity of 30%) and wetting (at 50°C for 3 hours with a relative humidity of 95%), the rust area ratio is 1% or less , and the pitting corrosion resistance index (PREN) is calculated by the following formula (2) .
Pitting corrosion resistance index (PREN)=Cr+3.3Mo+16N...(2)
In formula (2), each element symbol represents the content (mass %) of each element symbol.

The present invention also provides a method for producing the hot-rolled stainless steel material,
a hot rolling process for hot rolling the stainless steel billet;
an annealing step of annealing the hot-rolled steel material;
a descaling step of removing scale formed on the surface of the annealed steel material,
the descaling step includes a laser descaling step of irradiating a pulsed laser beam and a grinding step of grinding the workpiece after the laser descaling step,
The pulsed laser light satisfies the following formula (1) when the fluence is F [J/cm ² ] and the pulse width is τ [ns] (where τ is 10 to 1000 ns).
5.0×(τ/100) ^1/2 ≦F≦13.0×(τ/100) ^1/2 ...(1)

The present invention provides hot-rolled stainless steel with excellent corrosion resistance and a method for manufacturing the same.

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 "mass %" unless otherwise specified. Furthermore, "hot-rolled steel material" refers to a steel material that has been hot-rolled (hot-rolled) and annealed, and the material shape and thickness are not particularly limited. Examples of material shapes include plate (including strip), rod, and tube. Furthermore, various types of sectional shapes such as T-shaped and I-shaped steel are also acceptable.

The hot-rolled stainless steel material according to the embodiment of the present invention has a surface roughness in which the root-mean-square height Sq (hereinafter abbreviated as "Sq") is 5.0 μm or less, preferably 4.0 μm or less, and more preferably 3.0 μm or less. Sq is a parameter equivalent to the standard deviation of the distance from the mean plane, as defined in ISO 25178-2:2012, and serves as an index of the number of surface defects. By controlling Sq within the above range, surface defects can be reduced and surface smoothness can be ensured. As a result, corrosive substances are less likely to be retained on the surface of the hot-rolled stainless steel material, thereby improving corrosion resistance. Here, "corrosive substances" as used herein refers to substances that react with the hot-rolled stainless steel material to induce corrosion, including saltwater such as seawater and body fluids, as well as air pollutants such as sulfur dioxide and nitrogen dioxide. For example, hot-rolled stainless steel material can corrode due to the adhesion of sea salt that has drifted outdoors. However, a smoother surface reduces the adhesion of sea salt and makes it easier for it to be washed away by rain. Furthermore, high surface smoothness also leads to improved polishing properties.
The smaller the Sq, the fewer the surface defects, so the lower limit of Sq is not particularly limited, but from the viewpoint of manufacturing costs, the lower limit is, for example, 1.0 μm.
Sq can be measured in accordance with ISO 25178-2:2012.

The hot-rolled stainless steel material according to an embodiment of the present invention has a surface roughness in which the developed interface area ratio Sdr (hereinafter abbreviated as "Sdr") is 25.0% or less, preferably 20.0% or less, and more preferably 15.0% or less. Sdr is a parameter defined in ISO 25178-2:2012 that represents the rate of increase in surface area and serves as an index of the number of surface defects. In other words, Sdr represents the rate of increase in the actual surface area of a given region relative to the area of the surface of that region when viewed in plan. By controlling Sdr within the above range, surface defects are reduced and surface smoothness can be ensured, thereby improving corrosion resistance. Furthermore, high surface smoothness also leads to improved polishability. The salt-dry-wet cycle test described below is one of the accelerated corrosion resistance tests in outdoor environments.
The smaller the Sdr, the fewer the surface defects, so the lower limit of Sdr is not particularly limited, but from the viewpoint of manufacturing costs, the lower limit is, for example, 10.0%.
Sdr can be measured in accordance with ISO 25178-2:2012.

The hot-rolled stainless steel material according to the embodiment of the present invention has excellent corrosion resistance. Here, in this specification, "excellent corrosion resistance" means that in a salt-dry-wet cycle test (hereinafter referred to as "combined cyclic test (CCT)") in which salt spray, drying, and wetting are repeated, the rust area ratio is 1% or less when 10 cycles of salt spray, drying, and wetting are performed.
Furthermore, the corrosion resistance of hot-rolled stainless steel is related not only to the surface properties of the hot-rolled stainless steel but also to its composition. Therefore, the combined cycle test is performed taking into account the composition of the hot-rolled stainless steel. Specifically, when the pitting resistance equivalent number (PREN) of the hot-rolled stainless steel is less than 30, the test conditions were set with reference to the neutral salt spray cycle test specified in JIS H8502:1999. The installation angle of the hot-rolled stainless steel in the combined cycle test machine (the inclination angle of the surface of the hot-rolled stainless steel) was set so that the test surface of the test specimen was tilted 20° from the vertical (so that the test surface of the test specimen was held at an angle of 20° from the vertical). Furthermore, when the pitting corrosion resistance index of the hot-rolled stainless steel material is 30 or more, the installation angle of the hot-rolled stainless steel material in the combined cycle testing machine was such that the test surface of the test piece was set horizontally (0°), and the other conditions were the same as when the hot-rolled stainless steel material has a pitting corrosion resistance index of less than 30. The installation angle when the pitting corrosion resistance index is 30 or more was determined with reference to the method specified in JIS G0597:2017. The pitting corrosion resistance index can be calculated by the following formula (2).
Pitting corrosion resistance index (PREN)=Cr+3.3Mo+16N...(2)
In formula (2), each element symbol represents the content (%) of each element symbol.

In the hot-rolled stainless steel material according to the embodiment of the present invention, the difference ΔHV (hereinafter abbreviated as "ΔHV") between the Vickers hardness of the surface and the Vickers hardness of the center portion in the thickness direction is preferably 80 or less, more preferably 60 or less, and even more preferably 40 or less. If ΔHV can be controlled within such a range, work hardening of the surface can be suppressed, thereby improving polishability.
The lower limit of ΔHV is not particularly limited, but is, for example, 20.
The Vickers hardness of the surface and the center in the thickness direction can be measured in accordance with JIS Z2244: 2009. Furthermore, a test piece for measuring the Vickers hardness of the center in the thickness direction may be one obtained by cutting a hot-rolled stainless steel material in the thickness direction.

In the hot-rolled stainless steel material according to the embodiment of the present invention, the 45-degree specular gloss Gs (hereinafter abbreviated as "Gs(45°)") of the surface after buffing is preferably 500% or more, and more preferably 600% or more. If a Gs(45°) in this range can be ensured, it can be said to have excellent polishability.
Gs (45°) can be measured in accordance with JIS Z8741:1997.

The type of hot-rolled stainless steel material according to the embodiment of the present invention is not particularly limited, and various stainless steel types specified in the JIS standard can be selected, such as austenitic, ferritic, ferrite-austenite dual-phase, and martensitic.
A typical austenitic hot-rolled stainless steel material has a composition of 0.001 to 0.100% C, 5.00% or less Si, 3.00% or less Mn, 15.0 to 26.0% Cr, 6.0 to 26.0% Ni, 0 to 8.0% Mo, 0.350% or less N, 0 to 4.0% Cu, with the balance being Fe and impurities. A preferred austenitic hot-rolled stainless steel material has a composition of 0.005 to 0.080% C, 4.00% or less Si, 2.00% or less Mn, 16.0 to 23.0% Cr, 6.5 to 22.0% Ni, 0.1 to 7.0% Mo, 0.250% or less N, 0 to 2.0% Cu, with the balance being Fe and impurities.
Furthermore, a typical ferritic stainless hot-rolled steel material has a composition consisting of C: 0.001 to 0.100%, Si: 2.00% or less, Mn: 2.00% or less, Cr: 11.0 to 30.0%, Ni: 0 to less than 2.0%, Mo: 0 to 2.5%, N: 0.050% or less, Cu: 0 to 0.6%, Nb: 0 to 1.0%, Ti: 0 to 1.0%, Sn: 0 to 0.5%, and the balance being Fe and impurities. The ferritic stainless hot-rolled steel preferably has a composition of C: 0.002 to 0.020%, Si: 1.00% or less, Mn: 1.00% or less, Cr: 13.5 to 23.0%, Ni: 0 to 0.6%, Mo: 0 to 2.0%, N: 0.020% or less, Cu: 0 to 0.5%, Nb: 0 to 0.6%, Ti: 0 to 0.3%, Sn: 0 to 0.2%, and the balance being Fe and impurities.

A typical ferritic-austenitic duplex stainless steel hot rolled material has a composition of 0.001 to 0.100% C, 1.50% or less Si, 6.00% or less Mn, 15.0 to 30.0% Cr, 1.0 to 9.0% Ni, 0 to 5.0% Mo, 0.370% or less N, 0 to 2.0% Cu, with the balance being Fe and impurities. A preferred ferritic-austenitic duplex stainless steel hot rolled material has a composition of 0.005 to 0.030% C, 1.00% or less Si, 4.00% or less Mn, 20.0 to 26.0% Cr, 1.5 to 7.0% Ni, 0 to 4.0% Mo, 0.200% or less N, 0 to 1.5% Cu, with the balance being Fe and impurities.
Furthermore, a typical martensitic stainless hot-rolled steel material has a composition consisting of C: 0.020 to 0.400%, Si: 1.00% or less, Mn: 1.00% or less, Cr: 10.5 to 14.0%, Ni: 0 to 0.6%, Mo: 0 to 1.0%, N: 0.060% or less, Cu: 0 to 1.0%, Nb: 0 to 1.0%, Ti: 0 to 1.0%, and the balance being Fe and impurities. The martensitic stainless hot-rolled steel preferably has a composition of C: 0.100 to 0.300%, Si: 0.50% or less, Mn: 0.50% or less, Cr: 12.0 to 13.5%, Ni: 0 to 0.5%, Mo: 0 to 0.5%, N: 0.030% or less, Cu: 0 to 0.5%, Nb: 0 to 0.5%, Ti: 0 to 0.5%, and the balance being Fe and impurities.
In this specification, the term "impurities" refers to components that are mixed in during industrial production of hot-rolled stainless steel due to raw materials such as ores and scraps, or various factors in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention. For example, hot-rolled stainless steel may contain, as impurities, P: 0.050% or less and S: 0.0300% or less.

The thickness of the hot-rolled stainless steel material according to the embodiment of the present invention is not particularly limited, but for example, in the case of a plate-shaped hot-rolled stainless steel material (hot-rolled stainless steel plate), the plate thickness is preferably 3 mm or more.

The hot-rolled stainless steel material according to an embodiment of the present invention has excellent corrosion resistance and can therefore be used as a corrosion-resistant component. Furthermore, this hot-rolled stainless steel material also has excellent polishability, so polishing it can give it a smooth, glossy surface, improving its design. Therefore, it is suitable for use in corrosion-resistant components that require design.

The hot-rolled stainless steel material according to an embodiment of the present invention can be manufactured by a method including a hot-rolling process (S1), an annealing process (S2), and a descaling process (S3).

The hot rolling step (S1) is a step of hot rolling a stainless steel billet. Specifically, the stainless steel billet is heated to a predetermined temperature, and once the temperature has reached the predetermined temperature, the stainless steel billet is subjected to multiple stages of rough rolling and finish rolling in a hot rolling mill until it has a predetermined shape.
The hot rolling conditions are not particularly limited and may be adjusted appropriately depending on the composition of the stainless steel billet, etc. The stainless steel billet can be obtained by melting to a predetermined composition and then forging or casting.

The annealing step (S2) is a step of annealing the steel material obtained in the hot rolling step (S1). By annealing the hot-rolled steel material in an annealing furnace, the steel material that has been hardened and has lost its ductility due to the hot rolling can be softened and the ductility of the steel material can be restored.
The annealing conditions are not particularly limited and may be adjusted appropriately depending on the composition of the stainless steel flakes.

The descaling process (S3) is a process for removing scale formed on the surface of the steel material obtained in the annealing process (S2). The descaling process (S3) includes a laser descaling process (S3a) in which pulsed laser light is irradiated, and a grinding process (S3b) in which grinding is performed after the laser descaling process. By performing this descaling process, there is no need to perform descaling by pickling, which eliminates the environmental impact and cost issues associated with pickling.

In the laser descaling step (S3a), the scale formed on the surface of the steel material by annealing is evaporated by irradiating the scale with a pulsed laser beam. The irradiation conditions of the pulsed laser beam are set so as to satisfy the following formula (1) when the fluence is F [J/cm ² ] and the pulse width is τ [ns] (where τ is 10 to 1000 ns):
5.0×(τ/100) ^1/2 ≦F≦13.0×(τ/100) ^1/2 ...(1)
By setting the fluence of the pulsed laser beam within the range of formula (1), it is possible to obtain a sufficient descaling effect while ensuring surface smoothness. If the fluence of the pulsed laser beam is less than the lower limit of formula (1), the descaling effect will be insufficient. On the other hand, if the fluence of the pulsed laser beam exceeds the upper limit of formula (1), the surface will become rough and the polishability will decrease. The lower limit of formula (1) is preferably 6.0 × (τ/100) ^1/2 . The upper limit of formula (1) is preferably 12.0 × (τ/100) ^1/2 .
The pulse width is set in the range of 10 to 1000 ns, preferably 30 to 500 ns, and more preferably 50 to 300 ns.

Here, pulse width is the time per pulse that pulsed laser light exerts a thermal effect on the irradiated object. Furthermore, fluence is the energy density per unit area of pulsed laser light. In this specification, fluence is the value obtained by dividing the energy per pulse of pulsed laser light by the irradiated area of the pulsed laser light. Therefore, if other conditions are the same, the longer the pulse width of the pulsed laser light, the greater the fluence of the pulsed laser light.

The pulsed laser beam can be irradiated using a laser beam irradiation device including a laser oscillator, a laser head, a driving unit, and a control unit.
The type of laser oscillator is not particularly limited as long as it is capable of oscillating pulsed laser light. From the viewpoint of shortening the descaling time, the laser oscillator is preferably a solid-state laser with a large laser output per unit area.
The laser head focuses pulsed laser light generated by a laser oscillator and irradiates the light onto the scale formed on the surface of the annealed steel material.

The driving unit is connected to the laser head and drives the laser head to move the irradiation position of the pulsed laser light.
The control unit is connected to the laser oscillator and the laser head and controls the irradiation conditions of the pulsed laser beam, such as the fluence, pulse width, oscillation period, laser output, irradiation speed, irradiation width, and beam diameter.

The oscillation period of the pulsed laser light is not particularly limited, but is preferably 60 to 120 kHz. By controlling the oscillation period of the pulsed laser light within this range, it is possible to irradiate the scale formed on the surface of the steel material by annealing with pulsed laser light with an optimized fluence. If the oscillation period of the pulsed laser light is less than 60 kHz, the number of times the scale is irradiated per unit time will be too small. As a result, it may not be possible to irradiate the pulsed laser light with a fluence sufficient for descaling. Furthermore, if the oscillation period of the pulsed laser light is greater than 120 kHz, the fluence per pulse of the pulsed laser light will be small. As a result, it may not be possible to irradiate the pulsed laser light with a fluence sufficient for descaling.

In the grinding process (S3b), the surface of the steel material after the laser descaling process is ground to remove Cr-depleted layers that may cause seizure during the laser descaling process and reduce corrosion resistance.
The grinding method is not particularly limited, and can be carried out using a grinding tool known in the art (for example, a grinding brush).
The grinding tool preferably has abrasive grains of #80 or coarser. For stronger grinding, the grinding tool preferably has abrasive grains of #60 or coarser. By using a grinding tool with abrasive grains of this size, it is possible to easily remove seizure and Cr-depleted layers while suppressing the occurrence of surface defects.

The descaling step (S3) may further include a preliminary descaling step (S3c) before the laser descaling step (S3a). For example, the preliminary descaling step (S3c) may include a step of creating cracks or fissures in the scale on the surface of the steel material using shot blasting, a bender, or a leveler. By performing such a mechanical preliminary descaling step (S3c), the scale becomes easier to remove.
Generally, when the preliminary descaling step (S3c) using shot blasting is performed, large irregularities are formed on the surface of the steel material, and even if grinding is performed with an abrasive brush, for example, there may be areas that cannot be ground because the abrasive brush does not come into contact with the recesses. However, if the laser descaling step (S3a) is performed before grinding with the abrasive brush, the large irregularities caused by shot blasting are reduced, making grinding with the abrasive brush possible and removing seizure and Cr-depleted layers.
The type and conditions of the preliminary descaling step (S3c) may be appropriately selected and adjusted depending on the type of steel, the equipment used, etc. Furthermore, as necessary, a pickling step using an acidic aqueous solution may be performed as the preliminary descaling step (S3c). The pickling step may be performed after the grinding step (S3b).

The present invention will be explained in detail below using examples.

30 kg of stainless steel having the compositions of steel types A to G shown in Table 1 (the remainder being Fe and impurities) was vacuum melted and forged into a 30 mm thick stainless steel billet. The billet was then heated at 1230°C for 2 hours and hot-rolled to a thickness of 3 mm. The hot-rolled steel sheet was then cut into a 50 mm (rolling direction) x 50 mm (width direction) piece by cutting. The cut steel sheet was then annealed in an electric furnace in an air atmosphere at the temperatures shown in Tables 2 and 3 for 1 minute and then air-cooled. Next, the descaling steps shown in Tables 2 and 3 were sequentially performed to obtain hot-rolled stainless steel sheets (test materials). In Tables 2 and 3, the descaling step listed on the left indicates the earlier step was performed. "a" and "b" indicate the type of step performed, "〇" indicates that the step was performed, and "--" indicates that the step was not performed. For the laser descaling step, the fluence of the pulsed laser light (the number in parentheses indicates the fluence) is also shown.
The steel types of Nos. A, C, and F are austenitic, the steel types of Nos. B and G are ferritic-austenitic two-phase, and the steel types of Nos. D and E are ferritic.

The detailed conditions for the descaling step are as follows:
<Preliminary descale a>
Preliminary descaling was performed using a laboratory cold rolling mill instead of a bender and/or leveler. Specifically, the annealed steel sheets were rolled using this laboratory cold rolling mill. The cold rolling conditions were a thickness elongation of 2%.

<Preliminary descale b>
Preliminary descaling was performed by shot blasting. Specifically, iron balls with a diameter of approximately 0.3 mm were projected from an impeller onto the surface of the annealed steel sheet for 10 seconds.

<Laser>
Laser descaling was performed by irradiating a pulsed laser beam under the following irradiation conditions (excluding the fluence):
Wavelength: 1085nm
Pulse width: 100 ns
Oscillation frequency: 120 kHz
Irradiation speed: 100Hz
Laser beam diameter: 90 μm

<Grinding>
Grinding was carried out using a wheel-shaped grinding brush made of nylon woven with #80 abrasive grains, at a rotation speed of approximately 800 rpm, a pressure of 2 mm, and a sheet running speed of 20 m/min.

The hot-rolled stainless steel sheets (test materials) obtained as described above were evaluated as follows.
<Descale judgment>
The presence or absence of scale on the surface of the hot-rolled stainless steel sheet was evaluated by observation with a magnifying glass at a magnification of 50. In this evaluation, if no scale was observed and the scale was sufficiently removed, it was designated "◯" (good descaling ability), and if scale was observed and the scale was not sufficiently removed, it was designated "×" (poor descaling ability).

<Surface roughness Sq and Sdr>
Images of the surface of the hot-rolled stainless steel sheet were taken using a laser microscope (LEXT OLS410) manufactured by Olympus Corporation. The captured images were analyzed using analysis software for the laser microscope (LEXT OLS410) manufactured by Olympus Corporation. Measurements of Sq and Sdr were performed in accordance with ISO 25178-2:2012. The temperature during measurement was 23 to 25°C. The measurement results were calculated as the average of values measured at any five locations excluding the range of 5 mm from the end. The measurement positions were separated by 5 mm or more. The main setting conditions for the laser microscope and analysis software are as follows:
Optical zoom magnification: 200x Beam diameter: 0.2 μm
Captured image size: 4669 μm x 4720 μm

<Difference ΔHV between Vickers hardness on the surface and Vickers hardness at the center in the thickness direction>
Test pieces were cut out from the hot-rolled stainless steel plate, and the Vickers hardness was measured on the surface and at the center in the thickness direction (center of the plate thickness) using a Vickers hardness tester in accordance with JIS Z2244:2009. The test force was 0.05 N. The Vickers hardness was measured at five random locations, and the average value was used as the result.
The measurement surface at the center in the thickness direction was a cross section parallel to the rolling direction.

<Corrosion resistance>
Corrosion resistance was evaluated by a salt-dry-wet cycle test (combined cycle test) in which salt spray, drying, and wetting were repeated. The combined cycle test was performed by placing the hot-rolled stainless steel material in a combined cycle tester (CYP-90 manufactured by Suga Test Instruments Co., Ltd.). The cycle conditions were in accordance with the neutral salt spray cycle test specified in JIS H8502:1999, and 10 cycles were performed, each cycle consisting of spraying with a 5% NaCl aqueous solution (35°C for 15 minutes), drying (30% relative humidity, 60°C for 1 hour), and wetting (95% relative humidity, 50°C for 3 hours). The installation angle of the hot-rolled stainless steel material in the combined cycle tester was 20° relative to the vertical when the pitting corrosion resistance index (PREN) of the hot-rolled stainless steel material was less than 30 (Steel Nos. A to E), and horizontal (0°) when the pitting corrosion resistance index (PREN) of the hot-rolled stainless steel material was 30 or more (Steel Nos. F and G). Thereafter, the hot-rolled stainless steel sheets were washed with water and dried, and the rust area ratio of the hot-rolled stainless steel sheets was calculated.
The rust area ratio was calculated using the following procedure. The surface of the hot-rolled stainless steel sheet after the combined cycle test was photographed, and the percentage of the area of the rusted portion in a central 25 mm x 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 hot-rolled stainless 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 part (mm ² ) / total area of observed part (625 mm ² ) × 100
In this evaluation, a rust area rate of 1% or less was evaluated as "◯" (good corrosion resistance), and a rust area rate of more than 1% was evaluated as "×" (poor corrosion resistance).

<Abrasiveness>
Using #400 alumina as an abrasive, the surface of the stainless steel hot-rolled steel sheet was buffed to remove approximately 0.6 μm of the surface layer. The polished surface of the stainless steel hot-rolled steel sheet was measured for 45-degree specular gloss Gs (45°) using a glossmeter (PG-1M manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS Z8741:1997. Gs (45°) was measured at five locations excluding a 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. In this evaluation, a Gs (45°) of 500% or more was represented as "◯", and a Gs (45°) of less than 500% was represented as "X".
The results of the above evaluations are shown in Tables 4 and 5.

As shown in Tables 4 and 5, the hot-rolled stainless steel sheets of the present invention were excellent in descaling properties because scale could be sufficiently removed, and also had excellent corrosion resistance because Sq was 5.0 μm or less and Sdr was 25.0% or less. Furthermore, the hot-rolled stainless steel sheets of the present invention, which had a ΔHV of 80 or less, were excellent in polishability in addition to corrosion resistance.
In contrast, some of the comparative hot-rolled stainless steel sheets had insufficient descaling properties because the scale could not be sufficiently removed (Nos. 1-5, 1-6, 2-5, 3-5, and 4-5). Furthermore, some of the comparative hot-rolled stainless steel sheets had Sq and Sdr within the above ranges but insufficient corrosion resistance (Nos. 1-7 and 1-9). These two comparative examples (Nos. 1-7 and 1-9) were hot-rolled stainless steel sheets made of austenitic stainless steel, and were not ground after laser descaling. It is presumed that this resulted in a Cr-depleted layer remaining, resulting in poor corrosion resistance. Furthermore, the remaining comparative hot-rolled stainless steel sheets had either Sq or Sdr outside the above range, resulting in insufficient corrosion resistance.

As can be seen from the above results, the present invention can provide hot-rolled stainless steel material with excellent corrosion resistance and a method for manufacturing the same.

Claims (5)

A stainless hot-rolled steel material having a surface roughness in which the root mean square height Sq is 5.0 μm or less and the developed interface area ratio Sdr is 25.0% or less,
In a salt-dry-wet cyclic test, if the pitting corrosion resistance index (PREN) of the hot-rolled stainless steel material is less than 30, the installation angle (surface tilt angle) of the hot-rolled stainless steel material is set to 20° with respect to the vertical, and if the pitting corrosion resistance index (PREN) of the hot-rolled stainless steel material is 30 or more, the installation angle (surface tilt angle) of the hot-rolled stainless steel material is set to horizontal (0°) with respect to the vertical, and after 10 cycles of spraying (at 35°C for 15 minutes), drying (at 60°C for 1 hour, relative humidity of 30%) and wetting (at 50°C for 3 hours, relative humidity of 95%) with a 5% NaCl aqueous solution, the rust area ratio is 1% or less , and the pitting corrosion resistance index (PREN) is calculated by the following formula (2) :
Pitting corrosion resistance index (PREN)=Cr+3.3Mo+16N...(2)
In formula (2), each element symbol represents the content (mass %) of each element symbol. The hot-rolled stainless steel product according to claim 1, wherein the difference in Vickers hardness ΔHV between the surface and the center of the thickness is 80 or less. The hot-rolled stainless steel product according to claim 1 or 2, wherein the 45° specular gloss (Gs) of the surface is 500% or more after buffing using #400 alumina as an abrasive to remove 0.6 μm of the surface layer. A method for producing a hot-rolled stainless steel material according to any one of claims 1 to 3,
a hot rolling process for hot rolling the stainless steel billet;
an annealing step of annealing the hot-rolled steel material;
a descaling step of removing scale formed on the surface of the annealed steel material,
the descaling step includes a laser descaling step of irradiating a pulsed laser beam and a grinding step of grinding the workpiece after the laser descaling step,
A method in which the pulsed laser light satisfies the following formula (1) when the fluence is F [J/cm ² ] and the pulse width is τ [ns] (where τ is 10 to 1000 ns):
5.0×(τ/100) ^1/2 ≦F≦13.0×(τ/100) ^1/2 ...(1) A corrosion-resistant component comprising the hot-rolled stainless steel material described in any one of claims 1 to 3.