JP7560738B2 - Support roll segment for continuous casting machine and continuous casting machine - Google Patents

Description

The present invention relates to a support roll segment for a continuous casting machine and a continuous casting machine.

Conventionally, blooms, billets, and other cast pieces cast by continuous casting machines are at risk of internal or surface cracks due to bulging, which tries to expand the solidified shell outward. To address this issue, support rolls are provided below the mold of the continuous casting machine to support the cast piece pulled out of the mold. The support rolls are placed in a relatively high-temperature, highly corrosive environment, and are subjected to loads from the cast piece. For this reason, the optimal materials and structures are selected and adopted, particularly for the roll shafts of the support rolls and the frames that support them.

For example, Patent Document 1 describes a foot roll for continuous casting, which is located at the initial solidification position directly below the mold in a steel continuous casting device, and which has a 2.3 to 3.3 mm thick welded overlay layer of cobalt-based alloy formed on the surface of the base material by overlay welding.

Patent Document 2 also describes a continuous casting machine in which a slab support roll and a slab support side roll are arranged below the casting mold, approximately 10 m from the meniscus, and each roll is made of SUS630, a 13Cr martensitic stainless steel.

JP 2002-1502 A JP 2003-275853 A

The foot rolls directly below the mold described in Patent Document 1 form an overlay welded layer made of a cobalt-based alloy, and are intended to primarily improve the durability of the roll surface. Similarly, the side rolls supporting the cast piece described in Patent Document 2 are intended to prevent uneven wear on the roll surface. In other words, neither of the technologies described in Patent Documents 1 and 2 takes into consideration the roll shaft portion or the frame supporting the roll shaft portion.

The present invention has been made in consideration of the above circumstances, and aims to provide a support roll segment and a continuous casting machine equipped with the support roll segment that can be used in high-temperature and highly corrosive environments and under conditions where a large load is applied.

In order to solve the above problems, the present invention employs the following configuration.
[1] A support roll segment arranged between the lower end of a mold of a continuous casting machine and a position 2800 mm away along a conveying path of a cast piece,
a frame; and a plurality of support rolls rotatably supported by the frame for supporting the strip drawn from the mold,
Each of the plurality of support rolls is provided with a roll shaft portion,
The frame and the roll shaft portion are made of ferritic-austenitic duplex stainless steel,
The ferritic-austenitic duplex stainless steel has a 0.2% yield strength of 345 MPa or more, a pitting corrosion index (PREN) represented by the following formula (2) of 20 or more, and when subjected to an aging treatment at 475°C for 500 hours and then a Charpy impact test specified in JIS Z 2242:2018 is performed and the Brinell hardness is measured, the fracture transition temperature does not increase, the upper shelf energy does not decrease, and the Brinell hardness does not increase, compared to before the aging treatment. A support roll segment for a continuous casting machine.
PREN=[Cr]+3.3[Mo]+16[N]...(1)
In the formula (1), [Cr], [Mo], and [N] respectively represent the contents (mass%) of Cr, Mo, and N in the ferritic-austenitic duplex stainless steel.
[2] The support roll segment for a continuous casting machine according to [1], characterized in that a surface of the ferritic-austenitic duplex stainless steel is subjected to a salt bath soft nitriding treatment.
[3] The support roll segment for a continuous casting machine according to [1] or [2], characterized in that the cast piece is a bloom or a billet.
[4] A mold;
a support roll segment positioned below the mold;
The support roll segment includes a frame and a plurality of support rolls rotatably supported by the frame to support a cast piece drawn from the mold,
Each of the plurality of support rolls is provided with a roll shaft portion,
The frame and the roll shaft portion, which are arranged between the lower end of the mold and a position 2800 mm away along the conveying path of the cast piece, are made of ferritic-austenitic duplex stainless steel,
The ferritic-austenitic duplex stainless steel has a 0.2% yield strength of 345 MPa or more, a pitting corrosion index (PREN) represented by the following formula (2) of 20 or more, and when aging treatment is performed at 475°C for 500 hours and then a Charpy impact test specified in JIS Z 2242:2018 is performed and the Brinell hardness is measured, the fracture transition temperature does not increase, the upper shelf energy does not decrease, and the Brinell hardness does not increase, compared to before the aging treatment.
PREN=[Cr]+3.3[Mo]+16[N]...(2)
In the formula (2), [Cr], [Mo], and [N] respectively represent the contents (mass%) of Cr, Mo, and N in the ferritic-austenitic duplex stainless steel.
[5] The continuous casting machine according to [4], characterized in that a surface of the ferritic-austenitic duplex stainless steel is subjected to a salt bath soft nitriding treatment.
[6] The continuous casting machine according to [4] or [5], wherein the cast piece is a bloom or a billet.

According to the support roll segment for a continuous casting machine of the present invention, the frame and roll shaft are made of ferritic-austenitic duplex stainless steel with a 0.2% yield strength of 345 MPa or more, a pitting corrosion index (PREN) of 20 or more, and no increase in fracture transition temperature, no decrease in upper shelf energy, or no increase in Brinell hardness before and after aging treatment at 475°C for 500 hours. Therefore, even when these are placed in a high-temperature and highly corrosive environment and used under conditions where a large load is applied, they can exhibit excellent corrosion resistance and, since the amount of plastic deformation is small, durability can be improved.

In addition, according to the continuous casting machine of the present invention, the frame and roll shaft parts arranged within a range of 2800 mm from the bottom end of the mold are made of ferritic-austenitic duplex stainless steel having a 0.2% yield strength of 345 MPa or more, a pitting corrosion index (PREN) of 20 or more, and no increase in fracture transition temperature, no decrease in upper shelf energy, or no increase in Brinell hardness before and after aging treatment at 475°C for 500 hours. Therefore, even when these are placed in a high-temperature and highly corrosive environment and used under conditions where a large load is applied, they can exhibit excellent corrosion resistance and the amount of plastic deformation is small, thereby improving durability. This reduces the frequency of maintenance work on the continuous casting machine and increases the productivity of cast pieces.

1 is a schematic explanatory diagram of a continuous casting machine according to an embodiment of the present invention. FIG. FIG. 2 is a front schematic view showing a support roll segment according to an embodiment of the present invention. FIG. 2 is an exploded front schematic view showing a support roll segment according to an embodiment of the present invention. FIG. 2 is a side schematic view showing a frame and a support roll that constitute a support roll segment according to an embodiment of the present invention. FIG. 2 is a side schematic view showing a frame and a support roll that constitute a support roll segment according to an embodiment of the present invention.

In the continuous casting of special steel bar and wire, the molten steel contains a large amount of additives, the casting speed is fast (casting speed: 1.0 m/s or more), and the rolls have a small cross section and dimensional restrictions, making it easy for internal cracks to occur in the slab. Therefore, in order to prevent bulging of the slab, it is necessary to support the slab by placing multiple support rolls directly below the mold.

Therefore, when placing multiple support rolls directly below the mold, a device called a support roll segment, in which multiple support rolls are attached to a frame, is generally placed directly below the mold.

However, the various machines that make up a continuous casting machine are used in an environment that requires heat resistance, corrosion resistance, and specific mechanical properties, and the materials that can be used are limited. Conventionally, 18Cr stainless steels such as "SUS304" and "SUS630" have been used for the roll shafts and frames of the support roll segments arranged in a range from the bottom end of the mold to a position of 2800 mm along the cast slab transport path. However, these stainless steels have insufficient corrosion resistance and mechanical properties, and both the roll shafts and the frame may undergo plastic deformation, resulting in a decrease in the ability to suppress bulging. As a countermeasure, it is possible to increase the load capacity by increasing the shaft diameter of the roll shafts or increasing the thickness of the frame, but since a large number of support rolls must be arranged in a small range in a continuous casting machine as mentioned above, it may be difficult to adjust the shaft diameter and thickness.

In addition, if the mold flux used during continuous casting adheres to the stainless steel roll shaft or frame, it can cause severe corrosion of these. Corrosion tends to be particularly severe when mold flux containing fluorine is used.

18Cr stainless steel is generally considered to have slightly inferior corrosion resistance to chlorine, and there has been no particular knowledge about its corrosion resistance to fluorine. However, the inventors have discovered that in environments where the steel is subjected to high temperatures and large loads, such as continuous casting equipment, the corrosion resistance of 18Cr stainless steel due to fluorine becomes a problem.

The above-mentioned circumstances regarding plastic deformation and corrosion in support roll segments are not limited to the continuous casting of special steel wire rods, but also generally apply to continuous casting machines that manufacture slabs for general steel wire rods, thick steel plates, thin steel plates, and other types of cast pieces, and improvements to the support roll segments are also required in these cases.

The inventors conducted extensive research and discovered that the problems of corrosion and plastic deformation due to load can be eliminated by using a specific ferritic-austenitic stainless steel as the material for the roll shaft and frame. The following describes an embodiment of the present invention.

The support roll segment of this embodiment is a support roll segment for a continuous casting machine, which is arranged between a lower end of a mold of the continuous casting machine and a position 2800 mm away from the lower end of the mold along a strip transport path, and which has a frame and a plurality of support rolls rotatably supported by the frame to support the strip pulled out of the mold, each of which is provided with a roll shaft portion, and the frame and the roll shaft portion are made of ferritic-austenitic duplex stainless steel, which has a 0.2% yield strength of 345 MPa or more and a pitting corrosion index (PREN) represented by the following formula (2) of 20 or more, and when a Charpy impact test specified in JIS Z 2242:2018 is performed and the Brinell hardness is measured after aging treatment at 475°C for 500 hours, the fracture transition temperature does not increase, the upper shelf energy does not decrease, and the Brinell hardness does not increase, compared to before the aging treatment.
PREN=[Cr]+3.3[Mo]+16[N]...(A)
In the formula (A), [Cr], [Mo], and [N] respectively represent the contents (mass%) of Cr, Mo, and N in the ferritic-austenitic duplex stainless steel.
The continuous casting machine of this embodiment is a continuous casting machine in which the above-mentioned frame and roll shaft portion are provided between the lower end of the mold and a position 2800 mm away along the transport path of the cast strip.

The support roll segment and continuous casting machine of this embodiment are described below. Figure 1 shows a schematic diagram of the continuous casting machine.

The continuous casting machine 10 shown in FIG. 1 includes a mold 11, a nozzle 12 for injecting molten steel into the mold 11, and a number of support rolls 13 located below the mold 11. Three molds 11 are installed in the continuous casting machine 10 shown in FIG. 1. The three molds 11 are arranged along the back side of the paper surface of FIG. 1, and only one mold 11 at the front side is shown in FIG. 1. The continuous casting machine 10 includes a curved band 15 for curving the continuous cast slab 20 produced from the mold 11, a straightening band 16 for bending back the curved continuous cast slab 20 (hereinafter referred to as the slab 20), and a horizontal band 17 for transporting the slab 20 in a horizontal direction. The support rolls 13 in FIG. 1 are arranged in the area of the curved band 15. The rolls arranged in the straightening band 16 and the horizontal band 17 are segment rolls. The support rolls 13 are incorporated in the support roll segment S described later. The continuous casting machine 10 is also provided with transport paths 32 for transporting the rectangular cast pieces 20 drawn from the three molds 11. Each transport path 32 directly below the molds 11 is partitioned by multiple support rolls 13.

The mold 11 is cylindrical with a rectangular hole, and produces a cast piece 20 with a cross section that matches the shape of this rectangular hole. The mold 11 is equipped with a primary cooling means (not shown) for cooling the molten steel in the rectangular hole. The location indicated by the reference symbol 11a is the lower end of the mold 11.

Next, FIG. 2 shows a schematic diagram of the support roll segment S, and FIG. 3 shows an exploded schematic diagram of the support roll segment S. Each support roll segment S is equipped with 7 to 10 stages of support rolls 13, and a plurality of such support roll segments S are arranged along the transport direction of the cast slab 20 to form the transport path 32 of the continuous casting machine shown in FIG. 1. In addition, spray nozzles (not shown) are arranged between the support rolls 13 as secondary cooling means, enabling the cast slab 20 to be cooled.

The support roll segments S shown in Figures 2 and 3 define transport paths 32 for transporting the slab 20 provided in the continuous casting machine 10. That is, in each transport path 32, a long side support roll 31A that contacts the long side of the slab and supports the long side, and a short side support roll 31B that contacts the short side of the slab and supports the short side are arranged adjacent to the transport path 32 so as to surround each transport path 32.

The support roll segment S is equipped with multiple frames 33A to 33F. The support roll segment S shown in Figures 2 and 3 is composed of a total of six frames 33A to 33F combined together. Each frame 33A to 33F is composed of a support 34 and multiple bearing bodies 35 provided on the support 34. The frames 33A to 33F that make up the support roll segment S shown in Figures 2 and 3 include frames 33A, 33B, 33C, and 33F in which the bearing body 35 is arranged only on one side of the support 34 as shown in Figures 2 to 4, and frames 33D and 33E in which the bearing body 35 is arranged on both sides of the support 34 as shown in Figures 2, 3, and 5.

A support roll 13 is attached to the bearing body 35 of each of the frames 33A to 33F. Each support roll 13 is composed of a roll body 31a and a roll shaft portion 31b inserted into the central axis of rotation of the roll body 31a. The roll shaft portion 31b is fixed to the bearing body 35. The roll body 31a is supported so as to be freely rotatable relative to the roll shaft portion 31b.

Next, the materials of the frames 33A to 33F and the roll shaft portion 31b will be described.
In the support roll segment S and the continuous casting machine 10 of this embodiment, at least the frame and the roll shaft portion arranged between the lower end 11a of the mold 11 and a position 2800 mm away along the conveying path 32 of the slab 20 must be made of ferritic-austenitic duplex stainless steel. The surface temperature of the slab 20 is relatively high between the lower end 11a of the mold 11 and a position 2800 mm away along the conveying path 32 of the slab 20. Mold flux is attached to the surface of the slab 20 immediately after it is pulled out of the mold 11. When the mold flux falls off the surface of the slab 20, it falls onto the frame and the roll shaft portion located in the range from the lower end 11a of the mold 11 to a position 2800 mm away. In the region more than 2800 mm away from the lower end 11a of the mold 11 along the conveying path 32 of the slab, the water-soluble component of the mold flux is dissolved and diluted in the cooling water that cools the slab 20, so that the frame and the roll shaft portion are less damaged. In this way, the frame and the roll shaft in the range from the lower end 11a of the mold 11 to a position 2800 mm away are in a high-temperature environment and are easily corroded by mold flux. In conventional continuous casting machines, the durability of the frame and the roll shaft parts was insufficient.

Therefore, in this embodiment, the frame and roll shaft portion arranged between the lower end 11a of the mold 11 and a position 2800 mm away along the conveying path 32 of the cast piece are made of ferritic-austenitic duplex stainless steel (hereinafter sometimes referred to as duplex stainless steel) that has a 0.2% yield strength of 345 MPa or more, a pitting corrosion index (PREN) expressed by the following formula (2) of 20 or more, and when a Charpy impact test specified in JIS Z 2242:2018 is performed and the Brinell hardness is measured after aging treatment at 475°C for 500 hours, the fracture transition temperature does not increase, the upper shelf energy does not decrease, or the Brinell hardness does not increase compared to before aging treatment. The frame includes a support body 34 and a bearing body 35, and both the support body 34 and the bearing body 35 are made of the ferritic-austenitic duplex stainless steel according to this embodiment.

PREN=[Cr]+3.3[Mo]+16[N]...(B)
In the formula (B), [Cr], [Mo], and [N] respectively represent the contents (mass%) of Cr, Mo, and N in the ferritic-austenitic duplex stainless steel.

Duplex stainless steels generally suffer from a phenomenon known as 475°C embrittlement. For example, embrittlement occurs when heated to around 300°C to 550°C for a certain period of time, causing an increase in hardness and a decrease in ductility and toughness. It is called 475°C embrittlement because it becomes embrittled particularly rapidly around 475°C. However, when the surface temperature of the frame and roll shaft parts arranged between the bottom end of the mold of the continuous casting machine according to this embodiment and a position 2800 mm away along the conveying path of the cast piece was measured, it was found to be less than 140°C at most. When the duplex stainless steel according to this embodiment is subjected to aging treatment at 475°C for 500 hours and then subjected to a Charpy impact test as specified in JIS Z 2242:2018 and the Brinell hardness is measured, the fracture transition temperature does not increase, the upper shelf energy does not decrease, and the Brinell hardness does not increase compared to before the aging treatment. In other words, high-temperature embrittlement at 475°C is unlikely to occur, so even if it is applied to the frame and roll shaft parts whose surface temperatures are at most less than 140°C, the possibility of embrittlement occurring is extremely low.

In this embodiment, the fracture transition temperature, upper shelf energy, and Brinell hardness are measured for duplex stainless steel after aging treatment at a soaking temperature of 475°C and a soaking time of 500 hours, and for duplex stainless steel before aging treatment. Then, it is confirmed whether the fracture transition temperature does not increase, the upper shelf energy does not decrease, or the Brinell hardness does not increase after aging treatment.

The fracture transition temperature is measured in accordance with JIS Z 2242:2018. The test piece is a standard test piece specified in JIS Z 2242:2018, and is a V-notch. The test temperature range is 20 to -100°C. The fracture transition temperature is determined by obtaining a transition curve in accordance with Appendix D of JIS Z 2242:2018, and the fracture transition temperature is the temperature at which the ductile fracture ratio is 50%.

The upper shelf energy is the absorbed energy in the upper shelf region on the transition curve created when measuring the fracture transition temperature.

The Brinell hardness is measured in accordance with JIS Z 223:2008, with an indenter diameter of 10 mm and a test force of 3000 kgf.

In addition, the duplex stainless steel according to this embodiment has a 0.2% yield strength of 345 MPa or more. By making the 0.2% yield strength 345 MPa or more, there is no risk of the frame and roll shaft undergoing plastic deformation even if a load from the slab is applied to the support roll due to the effect of bulging.

Furthermore, the duplex stainless steel according to this embodiment has a pitting corrosion index (PREN) of 20 or more. The frame and roll shaft according to this embodiment are placed in an environment where mold flux containing fluorine is likely to adhere and is susceptible to corrosion, but by using a material with a pitting corrosion index (PREN) of 20 or more, corrosion resistance can be ensured. The pitting corrosion index (PREN) is preferably 24 or more. There is no particular need to limit the upper limit of the pitting corrosion index (PREN), but it may be, for example, 50 or less.

Furthermore, the duplex stainless steel according to the present embodiment may have a linear expansion coefficient of less than 13×10 ⁻⁶ K ⁻¹ , which can reduce plastic deformation of the frame and the roll shaft due to heat.
Moreover, the duplex stainless steel according to the present embodiment preferably has a Brinell hardness HBW of at least 200. This makes it possible to prevent plastic deformation of the frame and the roll shaft portion.

Duplex stainless steels that satisfy the above conditions include, for example, lean duplex steels SUS821L1 and SUS323L, general-purpose duplex steels SUS329J1, SUS329J3L, and SUS329J4L, and super duplex steel SUS327L1.

The main chemical components and pitting corrosion index (PREN) of the above duplex stainless steels are as follows:
SUS821L1: 21Cr-2Ni-3Mn-1Cu-0.17N, PREN=24.
SUS323L: 23Cr-4Ni-0.15N, PREN=25.
SUS329J1L: 25Cr-4.5Ni-2Mo, PREN=32.
SUS329J3L: 25Cr-5Ni-3Mo-0.15N, PREN=37.
SUS329J4L: 25Cr-6Ni-3Mo-0.15N, PREN=37.
SUS327L1: 25Cr-7Ni-4Mo-0.28N, PREN=43.

Among these, SUS327L1 has a PREN of 43, a 0.2% yield strength of 550 MPa or more, a linear expansion coefficient of 12.7×10 ⁻⁶ K ⁻¹ , and a Brinell hardness HBW of 310, and is therefore the most preferable.

It is also preferable that the surface of the ferritic-austenitic duplex stainless steel is subjected to a salt bath nitrocarburizing treatment. This makes it possible to further increase the durability of the frame and roll shaft.

The support roll segment S and continuous casting machine 10 of this embodiment preferably produce blooms or billets as the cast pieces 20. However, the present invention is not limited to the production of blooms or billets, and can also be applied to continuous casting machines that produce cast pieces for general steel wire rods, cast pieces for thick steel plates, cast pieces for thin steel plates, and other cast pieces.

As described above, according to the support roll segment S for a continuous casting machine of this embodiment, the frame and roll shaft parts arranged in the range from the lower end 11a of the mold 11 to 2800 mm along the conveying path 32 are made of ferritic-austenitic duplex stainless steel, which has a 0.2% yield strength of 345 MPa or more and a pitting corrosion index (PREN) expressed by the following formula (2) of 20 or more. When a Charpy impact test specified in JIS Z 2242:2018 is performed and the Brinell hardness is measured after aging treatment at 475°C for 500 hours, the fracture transition temperature does not increase, the upper shelf energy does not decrease, or the Brinell hardness does not increase compared to before the aging treatment. Therefore, even when these are placed in a high-temperature and highly corrosive environment and used under conditions where a large load is applied, they can exhibit excellent corrosion resistance and can be made more durable because the amount of plastic deformation is small.

In addition, according to the continuous casting machine of this embodiment, the frame and roll shaft parts arranged in the range from the lower end of the mold to 2800 mm are made of ferritic-austenitic duplex stainless steel, which has a 0.2% yield strength of 345 MPa or more and a pitting corrosion index (PREN) expressed by the following formula (2) of 20 or more. When a Charpy impact test specified in JIS Z 2242:2018 is performed and the Brinell hardness is measured after aging treatment at 475°C for 500 hours, the fracture transition temperature does not increase, the upper shelf energy does not decrease, and the Brinell hardness does not increase compared to before aging treatment. Therefore, even when these are placed in a high-temperature and highly corrosive environment and used under conditions where a large load is applied, they can exhibit excellent corrosion resistance and the amount of plastic deformation is small, so durability can be improved. This reduces the frequency of maintenance work on the continuous casting machine and increases the productivity of the cast pieces.

In the continuous casting machine shown in FIG. 1 equipped with the support roll segments shown in FIG. 2 and FIG. 3, the number of charges that the support roll segments could be used continuously was measured when continuous casting was performed over a number of charges. In the present invention example, the frame (support body, bearing body) and roll shaft portion arranged between the bottom end of the mold and a position of 2800 mm were made of SUS327L1, and in the comparative example, SUS304. The surface of each stainless steel was subjected to a salt bath soft nitriding treatment. Table 1 shows the physical properties of SUS327L1 and SUS304. As a result of the measurement, the support roll segment of the comparative example could be used continuously from 768 to 797 charges without maintenance, while the support roll segment of the present invention example could be used continuously from 1441 to 1537 charges without maintenance, and the life of the support roll segment of the present invention example was extended by 1.9 times compared to the conventional example.

When SUS327L1 was subjected to aging treatment at 475°C for 500 hours and then subjected to a Charpy impact test specified in JIS Z 2242:2018 and its Brinell hardness was measured, the fracture transition temperature did not increase, the upper shelf energy did not decrease, and the Brinell hardness did not increase compared to before the aging treatment.

During maintenance, the corrosion status of the roll shaft located between the bottom end of the mold and the position 2800 mm was checked. The results are shown in Table 2. In the comparative example, the corrosion of the roll shaft was severe in the range from the bottom end of the mold to 400 mm, and the corrosion of the roll shaft was also significant in the range from the bottom end of the mold to 2700 mm, but in the invention example, there was almost no corrosion of the roll shaft and the roll body. The corrosion status of the frame was almost the same as the corrosion results of the roll shaft. In Table 2, ◎ indicates that the proportion of the corroded area on the entire surface of the roll shaft is 0%, ○ indicates that the proportion of the corroded area on the entire surface of the roll shaft is more than 0% and 50% or less, △ indicates that the proportion of the corroded area on the entire surface of the roll shaft is more than 50% and less than 100%, and × indicates that the proportion of the corroded area on the entire surface of the roll shaft is 100%.

The amount of plastic deformation of the roll shaft was also evaluated. For the roll shafts located between the bottom end of the mold and a position 2800 mm away, the amount of plastic deformation of the central axis of the roll shaft after use and the central axis of the roll shaft before use were measured. The results are shown in Table 3 below, and the average amount of plastic deformation of the inventive example was about 1/7 of that of the comparative example.

10...continuous casting machine, 11...mold, 11a...lower end of mold, S...support roll segment, 13...support roll, 31A...support roll for long side, 31B...support roll for short side, 31a, roll body, 31b...roll shaft, 32...transport path for cast piece, 33A-33F...frame.

Claims (6)

A support roll segment disposed between a lower end of a mold of a continuous casting machine and a position 2800 mm away along a conveying path of a cast piece,
a frame; and a plurality of support rolls rotatably supported by the frame for supporting the strip drawn from the mold,
Each of the plurality of support rolls is provided with a roll shaft portion,
The frame and the roll shaft portion are made of ferritic-austenitic duplex stainless steel,
The ferritic-austenitic duplex stainless steel has a 0.2% yield strength of 345 MPa or more, a pitting corrosion index (PREN) represented by the following formula (2) of 20 or more, and when subjected to an aging treatment at 475°C for 500 hours and then a Charpy impact test specified in JIS Z 2242:2018 is performed and the Brinell hardness is measured, the fracture transition temperature does not increase, the upper shelf energy does not decrease, and the Brinell hardness does not increase, compared to before the aging treatment. A support roll segment for a continuous casting machine.
PREN=[Cr]+3.3[Mo]+16[N]...(1)
In the formula (1), [Cr], [Mo], and [N] respectively represent the contents (mass%) of Cr, Mo, and N in the ferritic-austenitic duplex stainless steel. The support roll segment for a continuous casting machine according to claim 1, characterized in that the surface of the ferritic-austenitic duplex stainless steel is subjected to a salt bath soft nitriding treatment. The support roll segment for a continuous casting machine according to claim 1 or 2, characterized in that the cast piece is a bloom or a billet. A mold and
a support roll segment positioned below the mold;
The support roll segment includes a frame and a plurality of support rolls rotatably supported by the frame to support a cast piece drawn from the mold,
Each of the plurality of support rolls is provided with a roll shaft portion,
The frame and the roll shaft portion, which are arranged between the lower end of the mold and a position 2800 mm away along the conveying path of the cast piece, are made of ferritic-austenitic duplex stainless steel,
The ferritic-austenitic duplex stainless steel has a 0.2% yield strength of 345 MPa or more, a pitting corrosion index (PREN) represented by the following formula (2) of 20 or more, and when aging treatment is performed at 475°C for 500 hours and then a Charpy impact test specified in JIS Z 2242:2018 is performed and the Brinell hardness is measured, the fracture transition temperature does not increase, the upper shelf energy does not decrease, and the Brinell hardness does not increase, compared to before the aging treatment.
PREN=[Cr]+3.3[Mo]+16[N]...(2)
In the formula (1), [Cr], [Mo], and [N] respectively represent the contents (mass%) of Cr, Mo, and N in the ferritic-austenitic duplex stainless steel. The continuous casting machine according to claim 4, characterized in that the surface of the ferritic-austenitic duplex stainless steel is subjected to a salt bath nitrocarburizing treatment. The continuous casting machine according to claim 4 or 5, characterized in that the cast piece is a bloom or a billet.

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