JP7414616B2 - Ferritic/austenitic duplex stainless steel sheets for building materials - Google Patents

Description

The present invention relates to a ferritic-austenitic duplex stainless steel sheet for building materials that has excellent corrosion resistance.

Austenitic stainless steel plates such as SUS304 and SUS316, and ferritic stainless steel plates such as SUS430 are often used for exterior and interior building materials.

In applications for exterior and interior building materials, corrosion resistance against flying sea salt that adheres to the surface of steel is required when manufacturing building material products from steel plates or during construction of building materials. On the other hand, when stainless steel sheets are used as roofing or wall materials, anti-glare properties are often improved by rolling the steel sheets with dull rolls to give the surface irregularities of several μm. However, in an outdoor usage environment, dust and sea salt particles tend to adhere to the steel plate due to the presence of small irregularities on the surface. In addition, the recesses on the surface of the steel sheet formed by dull rolls have been subjected to heavy processing, which weakens the passive film, and concavities are prone to condensation, so the higher the surface roughness, the more difficult it is to use outdoors. It is known that corrosion resistance is extremely reduced.

Therefore, in Patent Document 1, in order to ensure anti-glare properties, the surface is roughened with a dull roll to make Ra 0.5 μm to 5 μm, and the average inclination angle of the unevenness is set to 4° or more and 11° or less, and furthermore, corrosion resistance is ensured. In order to achieve this, the steel composition satisfies 45≧Cr(%)+4Mo(%)≧28, thereby achieving both corrosion resistance and anti-glare property.

In addition, as shown in Patent Document 2, as a stainless steel sheet having both anti-glare properties and corrosion resistance, rolling with dull rolls is performed to make the surface arithmetic mean roughness Ra of 2.8 μm or more, and Cr+Mo: 24.5% or more. , Ni: 0.1% or more and 25% or less, and Cu: 0.01% or more and 3% or less.

However, in the technology described in Patent Document 1 or Patent Document 2, it is essential to increase the alloying elements in the steel, and a significant increase in cost is unavoidable.

Further, when the surface is roughened by dull rolls as in the case of the stainless steel sheet described in Patent Document 1 or Patent Document 2, adhesion of dust and sea salt particles is unavoidable, which may reduce corrosion resistance. Generally, corrosion resistance and surface roughening are opposed to each other, and the rougher the surface, the worse the corrosion resistance may be.

Patent No. 3287302 Patent No. 3499361

The present invention has been made in view of the above circumstances, and aims to provide a ferritic-austenitic duplex stainless steel sheet for building materials that has excellent corrosion resistance even when the surface is roughened to ensure anti-glare properties. Take it as a challenge.

[1] Concave portions and convex portions are provided on the steel plate surface,
The area ratio of the recess is 40 to 60%,
The surface roughness Rz in the center roughness curve of the surface is 3.0 to 20.0 μm,
A ferrite-austenite duplex stainless steel sheet for building materials, characterized in that the maximum diameter of the recess is 400.0 μm or less.
[2] The ferrite/austenite II for building materials according to [1], wherein the difference between the average grain size of the ferrite phase and the average grain size of the austenite phase contained in the steel is 5.0 μm or less. Phase stainless steel plate.

According to the present invention, it is possible to provide a ferrite-austenite duplex stainless steel sheet for building materials that has excellent corrosion resistance even when the surface of the steel sheet is roughened.

FIG. 1 is a diagram showing the shape of irregularities and recesses on the surface of a ferritic-austenitic duplex stainless steel sheet for building materials according to an embodiment of the present invention, measured using a three-dimensional shape measuring machine.

The ferritic-austenitic duplex stainless steel sheet for building materials (hereinafter sometimes referred to as duplex stainless steel sheet) according to the embodiment of the present invention has a surface roughened by shot blasting after final cold rolling and continuous annealing. It is suitable as a building material for roofs, building materials, etc. The duplex stainless steel sheet of this embodiment is controlled to have a predetermined surface texture by shot blasting after finishing cold rolling, and has a surface texture controlled by conventional dull roll rolling. A significant improvement in corrosion resistance is expected.
Hereinafter, the duplex stainless steel plate of this embodiment will be explained in detail.

(steel plate)
The type of duplex stainless steel plate of this embodiment is not particularly limited, and examples thereof include SUS821L1, SUS323L, SUS329J1, SUS329J3L, SUS329J4L, and SUS327L1. Further, the thickness of the steel plate is not particularly limited, but is preferably in the range of 0.1 mm or more and 10.0 mm or less, for example, for outdoor building material applications such as roofing materials.

(Surface texture of steel plate)
Next, the surface properties of the duplex stainless steel plate of this embodiment will be explained. The steel plate surface of the duplex stainless steel plate of this embodiment is provided with concave portions and convex portions, the area ratio of the concave portions is 40 to 60%, and the surface roughness Rz in the center roughness curve of the surface is 3. 0 to 20.0 μm, and the maximum diameter of the recess is 400.0 μm or less. By having such surface properties, it is possible to ensure desirable anti-glare properties as a building material, and it is also possible to improve corrosion resistance. Hereinafter, the relationship between the surface texture and corrosion resistance of the duplex stainless steel sheet of this embodiment will be explained.

From the viewpoint of improving the corrosion resistance of stainless steel used outdoors, aqueous solutions containing chloride ions, such as seawater and condensed water, are considered to be severe corrosive environments. When seawater dries, it produces salts such as sodium chloride (NaCl), and when this salt comes into close contact with the steel base, corrosion is further accelerated.

Therefore, as a result of intensive studies by the present inventors, it was found that controlling the surface to have a predetermined unevenness suppresses the adhesion between the generated salt and the steel base material and improves corrosion resistance.

In the duplex stainless steel sheet according to the present embodiment, the surface quality is defined using three parameters: the area ratio of the recesses, the maximum diameter of the recesses, and the surface roughness Rz, which is an index of the maximum height. was found to be preferable. The arithmetic mean roughness Ra is not appropriate for evaluating the influence of protruding irregularities that can become corrosion starting points, but the maximum height Rz is an index that can appropriately evaluate the relationship with corrosion. I found out. Furthermore, it has been found that a steel plate having a surface roughness Rz within a predetermined range has excellent corrosion resistance when evaluated by surface roughness Rz. The reasons for limiting each parameter will be explained below.

[Area ratio of recesses]
The area ratio of the recesses on the surface of the steel plate is in the range of 40 to 60%. This improves corrosion resistance. When the area ratio of the recesses is less than 40%, salts precipitated from an aqueous solution containing chloride ions are preferentially precipitated in the convex parts and adhere to the steel base, resulting in deterioration of corrosion resistance. On the other hand, when the area ratio of the recesses exceeds 60%, the salt precipitated within the recesses grows, and as a result, the salt adheres to the steel base, reducing corrosion resistance. In addition, the adhesion of salt can be judged by the presence or absence of salt precipitation traces in the dry-wet repeated test described below.

[Surface roughness Rz]
The surface roughness Rz in the center roughness curve is in the range of 3.0 to 20.0 μm. This improves corrosion resistance. If Rz is less than 3.0 μm, the salt and the steel base will adhere to each other, reducing corrosion resistance. Moreover, when Rz exceeds 20.0 μm, dirt tends to adhere to the surface of the steel plate provided with concave portions and convex portions, degrading corrosion resistance. Rz is more preferably in the range of 3.0 to 15.0 μm.

[Maximum diameter of recess]
The maximum diameter of the recess is 400.0 μm or less. This improves corrosion resistance. If the maximum diameter of the recess exceeds 400.0 μm, the salt and the steel base will come into close contact with each other, reducing corrosion resistance.

Rz on the surface of the steel plate is measured by a method according to JIS B 0601:2013. Note that the arithmetic mean roughness Rz is measured three times in a direction perpendicular to the rolling direction, and the average value is calculated and evaluated.

The area ratio of the recesses and the maximum diameter of the recesses on the surface of the steel plate are measured by observing the steel plate surface using a three-dimensional shape measuring machine (manufactured by Keyence: VR-3000) and calculating the area ratio of the recesses. The recess is defined as a place having a height of 0 μm or less and being recessed in a circular shape based on the height of the evaluation surface of 0 μm measured by a three-dimensional shape measuring device. The area other than the concave portion is defined as a convex portion. Note that a reference plane is set when analyzing a three-dimensional profile obtained using a three-dimensional shape measuring machine. This reference plane is automatically determined by the accompanying analysis software. The reference plane corresponds to the median depth of the entire measurement area. In the present invention, the reference plane is set from the height information of the entire field of view measured at a magnification of 120 times. Furthermore, the maximum diameter of the recess is calculated from the measured three-dimensional profile. Note that the observation of the surface of the steel plate is evaluated using an observation magnification of 120 times.

[Average grain size difference between ferrite phase and austenite phase]
The duplex stainless steel sheet of this embodiment includes a ferrite phase and an austenite phase in the steel structure, but in this embodiment, the average grain size of the ferrite phase and the average grain size of the austenite phase contained in the steel are It is preferable that the difference between the two (=ferrite phase average grain size - austenite phase average grain size) is 5.0 μm or less. Thereby, corrosion resistance can be further improved. When the particle size difference is 5.0 μm or less, it is possible to prevent an increase in Rz due to the difference in dissolution rate between the ferrite phase and the austenite phase during pickling, and corrosion resistance is improved.

Next, a method for manufacturing the duplex stainless steel sheet of this embodiment will be explained.

In order to manufacture duplex stainless steel sheets with excellent corrosion resistance, the steel sheets after hot rolling are sequentially subjected to annealing, pickling, final cold rolling, and final annealing. It is important to control the surface to a predetermined property by performing shot blasting. Further, it is preferable to further perform pickling after shot blasting because corrosion resistance is further improved.

First, using a hot-rolled steel plate as a starting material, it is annealed and pickled to remove relatively coarse deposits such as metal and scale. Hot rolling conditions, annealing conditions, and pickling conditions will be described later.

Next, finish cold rolling is carried out at a sufficient rolling rate to smooth out as much as possible the depressions (drop-off marks) generated by pickling and the depressions caused by grain boundary erosion. Further, by performing finish annealing after finish cold rolling, a duplex stainless steel original sheet with excellent corrosion resistance is manufactured. Conditions for final cold rolling, final annealing, and pickling will be described later.

Then, in order to control the area ratio of the recesses, Rz, and the maximum diameter of the recesses, shot blasting is performed on the duplex stainless steel original plate manufactured in this way under predetermined conditions, resulting in a duplex stainless steel sheet with excellent corrosion resistance. Manufacture stainless steel plates. Pickling may be performed after shot blasting. Conditions for shot blasting and pickling will be described later.

In addition, when manufacturing the duplex stainless steel sheet of this embodiment, a hot-rolled steel sheet is used as a starting material, and after performing at least final cold rolling, final annealing and shot blasting may be performed, and other steps may be performed. may be omitted, or the order of the steps may be changed.

An example is procedure (i) in which a hot rolled steel plate is subjected to annealing, pickling, final cold rolling, final annealing, shot blasting, and pickling in this order.

Another procedure may be procedure (ii) in which the steel plate after hot rolling is subjected to annealing, pickling, final cold rolling, final annealing, and shot blasting in this order.

Furthermore, as other steps, the steel plate after hot rolling is subjected to the following treatments: annealing, pickling, cold rolling, annealing, pickling, finish cold rolling, finish annealing, shot blasting, and pickling. Procedure (iii) may also be used.

Furthermore, the steel plate after hot rolling is subjected to annealing, pickling, first cold rolling, first annealing, first pickling, second cold rolling, second annealing, second The process (iv) may be performed in the order of pickling, final cold rolling, final annealing, shot blasting, and pickling.

Alternatively, the procedure (v) may be such that the steel plate after hot rolling is subjected to the following treatments in the order of annealing, pickling, cold rolling, bright annealing, finish cold rolling, finish annealing, shot blasting, and pickling.

Note that in the above steps (i) to (v), a polishing step or a degreasing step may be added as necessary. Further, after the pickling after shot blasting, skin pass rolling may be performed within a range that does not affect the surface properties. Furthermore, after temper rolling, refining processes such as degreasing, tension leveling, slitting, etc. may be performed within a range that does not affect the surface properties.

Hereinafter, specific conditions in each step will be explained.

Hot-rolled steel sheets are manufactured by melting, casting, and hot rolling stainless steel, in the same manner as when manufacturing general duplex stainless steel.

Annealing and pickling of a steel plate after hot rolling are effective treatments for removing coarse foreign matter such as metal and scale attached to the surface of the steel plate.

The conditions for annealing after hot rolling can be selected as appropriate in consideration of the manufacturability and characteristics of the material. Further, the annealing may be performed by either batch annealing or continuous annealing as long as it does not affect the surface properties of the steel sheet, and can be selected depending on the material, for example.

Pickling is performed using a combination of neutral salts and acids such as sulfuric acid, nitric acid, hydrofluoric acid, and hydrochloric acid, and electrolytic pickling may also be performed.

Finish cold rolling is cold rolling performed immediately before final finish annealing. The number of cold rolling passes may be one pass or multiple passes. Furthermore, a plurality of different types of rolling mills, such as a general Sendzimir mill and a mill exclusively for thin plates, may be used in sequence.

The soaking temperature for finish annealing is not particularly limited as long as it is the general condition for finish annealing duplex stainless steel sheets, but in order to control the grain size of the ferrite phase and austenite phase, it is 950 to 1150 °C. It is preferable to do so. Since recrystallization is stabilized when the soaking temperature during annealing is 1000°C or higher, the soaking temperature during annealing is preferably 1000°C or higher. Further, if the soaking temperature during annealing is set to 1100°C or lower, coarsening of the crystal grain size can be prevented, so the soaking temperature during annealing is preferably set to 1100°C or lower.

The soaking time for final annealing is set to 0.05 to 1.00 minutes in order to control the grain size of the ferrite phase and austenite phase. In order to stabilize recrystallization, the soaking time is preferably 0.15 minutes or more. Further, in order to prevent coarsening of the crystal grain size, the soaking time is preferably set to 0.75 minutes or less.

The average particle diameter of the blast material in the shot blasting process is 0.1 to 1.2 mm, the blast speed is 30 to 100 m/s, and the blast amount is 30 to 200 Kg/m ² . Since the area ratio of the recesses increases with the shot amount, the shot amount is preferably 70 to 120 kg/m ² . Further, since Rz and the maximum diameter of the recess increase proportionally with the projection speed, the projection speed is preferably 50 to 70 m/s. The morphology of the surface of the stainless steel plate is controlled by ensuring that the average particle diameter, blasting speed, and blasting amount of the blasting material in shot blasting are within the above ranges. The material of the shot material is preferably hard particles such as iron, alumina, SiC, and ceramics, and any of them may be used.

For pickling after shot blasting, it is preferable to use a mixed acid containing any two of sulfuric acid, nitric acid, hydrofluoric acid, and hydrochloric acid. Preferred concentration ranges for mixed acids are 100 to 300 g/L for sulfuric acid, 20 to 200 g/L for nitric acid, and 10 to 100 g/L for hydrofluoric acid. The temperature of the mixed acid solution is preferably 20°C to 90°C.

Through the above steps, the duplex stainless steel sheet of this embodiment can be manufactured. The duplex stainless steel sheet of this embodiment has excellent corrosion resistance even when the surface of the steel sheet is roughened to provide anti-glare properties.

Hereinafter, the present invention will be explained in more detail with reference to Examples.

[Example 1]
First, SUS821L1 was melted as a duplex stainless steel using an electric furnace, a converter, and an AOD process, and was continuously cast into a slab.

The slab after continuous casting was then hot rolled into a steel plate using a conventional method. Then, using the hot-rolled steel plate as a starting material, annealing, pickling, final cold rolling, final annealing, and shot blasting are performed in this order (step (ii) above), and after shot blasting to a thickness of 1.5 mm, steel plate was obtained.

The soaking temperature for final annealing was 1030°C to 1180°C, and the soaking time was 0.3 minutes. For shot blasting, a 0.4 mm blasting material was used. The material of the shot material was iron. Other shot blasting conditions were as listed in Table 1. In this way, test materials of Inventive Example 1 and Comparative Examples 1 to 3 were obtained.

Various measurements regarding corrosion resistance and surface properties were performed using each sample material. Specifically, the area ratio of the recesses on the steel plate surface, the surface roughness Rz in the center roughness curve on the steel plate surface, and the maximum diameter of the recesses were measured, and the corrosion resistance was further evaluated.

Rz on the surface of the steel plate was measured by a method according to JIS B 0601:2013 after performing ultrasonic cleaning using acetone on a 50 mm square sample cut out from each test material. The arithmetic mean roughness Rz was measured three times in a direction perpendicular to the rolling direction, and the average value was calculated and evaluated.

The area ratio of the recesses and the maximum diameter of the recesses on the steel sheet surface were measured using a three-dimensional shape measuring machine (Keyence: The surface of the steel plate was observed using VR-3000), and the area ratio of the recesses was calculated. The concave portion was defined as a place having a height of 0 μm or less and having a circular depression based on the height of the evaluation surface of 0 μm measured by a three-dimensional shape measuring device. Areas other than the concave portions were defined as convex portions. Note that a reference plane is set when analyzing a three-dimensional profile obtained using a three-dimensional shape measuring machine. This reference plane is automatically determined by the accompanying analysis software. The reference plane corresponds to the median depth of the entire measurement area. In the present invention, the reference plane was set from the height information of the entire field of view measured at a magnification of 120 times. Furthermore, the maximum diameter of the recess was calculated from the measured three-dimensional profile. Note that the observation of the steel plate surface was evaluated using an observation magnification of 120 times. FIG. 1 shows an example of a three-dimensional profile.

In the evaluation of corrosion resistance, artificial seawater CCT was performed on samples of L150 mm x W70 mm cut out from each test material to obtain samples for measurement. Artificial seawater CCT is a repeated dry-wet test using Aquamarine (manufactured by Yasu Pharmaceutical Co., Ltd.) in a combined cycle testing machine (Suga Test Machine: CYP-90). Specifically, artificial seawater was sprayed on the test material at an angle of 70° to 75° for 4 hours as a spraying process, then maintained at RH 25% for 2 hours as a drying process, and held at RH 95% for 2 hours as a wet process. This is a test in which one cycle consists of This artificial seawater CCT was performed for 12 cycles. The degree of rust that occurred after the above 12 cycles of CCT was evaluated by evaluating the appearance of the sample using a rating number (RN) based on JIS G 0595:2004. An RN of 5 or more was considered a pass (○), and an RN of 4 or less was judged a fail (×). In addition, the adhesion between the precipitated salt and the material surface is evaluated by the presence or absence of traces of the precipitated salt on the rust generated by CCT. If there is no trace, it is evaluated as no adhesion (○), and if there is a trace, it is evaluated as ( x).

Table 1 shows the results of these various measurements.

As shown in Table 1, in the present invention examples that meet the requirements of the present invention, the area ratio of the recesses is within the range of 40% to 60%, the Rz is within the range of 3.0 to 20.0 μm, and the recesses are in the range of 40% to 60%. The maximum diameter was 400 μm or less. The examples of the present invention had excellent corrosion resistance compared to Comparative Examples 1 to 3, which did not meet the requirements of the present invention.

[Example 2]
First, SUS821L1 was melted as a duplex stainless steel using an electric furnace, a converter, and an AOD process, and was continuously cast into a slab.

The slab after continuous casting was then hot rolled into a steel plate using a conventional method. Then, using the hot-rolled steel plate as a starting material, annealing, pickling, final cold rolling, final annealing, and shot blasting are performed in this order, and for some test materials, pickling is performed after shot blasting. Ta. In this way, a steel plate with a thickness of 1.5 mm was obtained.

The final annealing was performed at 1050°C to 1180°C, and the soaking time was 0.05 to 1.00 minutes. For shot blasting after final annealing, a shot material with an average particle size of 0.4 mm was used. The material of the shot material was iron. Other shot blasting conditions were as listed in Table 2. Pickling after shot blasting was carried out at a liquid temperature of 40° C. using a mixed acid of 30 g/L hydrofluoric acid and 90 g/L nitric acid. In this way, test materials of Inventive Examples 2 to 5 and Comparative Examples 4 to 7 were obtained.

Using each sample material, in the same manner as in Example 1, the area ratio of the recesses on the steel plate surface, the surface roughness Rz in the center roughness curve on the steel plate surface, and the maximum diameter of the recesses were measured.

Furthermore, the average particle diameters of the ferrite phase and austenite phase were measured by the EBSD method. The measurement was performed at a measurement magnification of 2000 times and a step of 0.2 μm, and the obtained data was analyzed and calculated using TSL OIM analysis software. Austenite and ferrite were determined by referring to iron FCC and iron BCC from the software database. One grain boundary was set with a misorientation of 15° or more as a grain boundary, and the equivalent circle diameter was calculated. The value obtained by calculating the arithmetic mean of the obtained equivalent circle diameters was defined as the crystal grain size.

In the evaluation of corrosion resistance, artificial seawater CCT was performed on samples of L150 mm x W70 mm cut out from each test material to obtain samples for measurement. Artificial seawater CCT is a repeated dry-wet test using Aquamarine (manufactured by Yasu Pharmaceutical Co., Ltd.) in a combined cycle testing machine (Suga Test Machine: CYP-90). Specifically, artificial seawater was sprayed on the test material at an angle of 70° to 75° for 4 hours as a spraying process, then maintained at RH 25% for 2 hours as a drying process, and held at RH 95% for 2 hours as a wet process. This is a test in which one cycle consists of This artificial seawater CCT was performed for 12 cycles. The degree of rust generated after the above 12 cycles of CCT was evaluated by evaluating the appearance of the sample using a rating number (RN) based on JIS G 0595:2004. An RN of 5 or more was considered a pass (○), and an RN of 4 or less was judged a fail (×). Moreover, when RN was 6 or more, the corrosion resistance was particularly excellent (◎). The adhesion between the precipitated salt and the material surface is evaluated by the presence or absence of traces of the precipitated salt on the rust generated by CCT. If there is no trace, it is evaluated as no adhesion (○), and if there is a trace, it is evaluated as (×). did.

The results of these various measurements are shown in Table 2.

As shown in Table 2, in all of the examples of the present invention that meet the requirements of the present invention, the area ratio of the recesses is in the range of 40% to 60%, the Rz is in the range of 3.0 to 20.0 μm, and the recesses are in the range of 40% to 60%. The maximum diameter was in the range of 400 μm or less. Furthermore, by optimizing the annealing conditions after finish rolling, a duplex stainless steel in which the grain size difference between the ferrite phase and the austenite phase was 5.0 μm or less was obtained, and the corrosion resistance was further improved.

As described above, all of the invention examples that met the requirements of the present invention had better corrosion resistance than the comparative examples that did not meet the requirements of the present invention. In particular, the invention examples in which Rz was in the range of 3.0 to 15.0 μm had excellent corrosion resistance. Therefore, all of the examples of the present invention that meet the requirements of the present invention can be evaluated as steel plates having surface properties with excellent corrosion resistance suitable for use as building materials, for example.

Claims (2)

Concave and convex portions are provided on the surface of the steel plate,
The area ratio of the recess is 40 to 60%,
The surface roughness Rz in the center roughness curve of the surface is 3.0 to 20.0 μm,
A ferrite-austenite duplex stainless steel sheet for building materials, characterized in that the maximum diameter of the recess is 400.0 μm or less. The ferrite-austenite duplex stainless steel sheet for building materials according to claim 1, wherein the difference between the average grain size of the ferrite phase and the average grain size of the austenite phase contained in the steel is 5.0 μm or less. .