Ferritic / austenitic duplex stainless steel sheet, method for manufacturing same, and brake disk rotor

Abstract

Provided is a ferritic / austenitic duplex stainless steel sheet having the composition comprising, by mass, 0.005-0.050% of C, 0.050-0.400% of N, 0.10-1.00% of Si, 0.50-5.00% of Mn, 0.001-0.080% of P, 0.0001-0.0050% of S, 18.0-27.0% of Cr, 1.00-9.00% of Ni, 0.10-5.00% of Mo, and 0.001-2.000% of Cu, with the balance being Fe and impurities. This ferritic / austenitic duplex stainless steel sheet has a ferrite phase of 1.0-70.0 vol%, an austenite phase of 30.0-99.0 vol%, and the average crystal grain size of the ferrite phase and the austenite phase being 50 μm or less.

Description

Ferrite-austenitic duplex stainless steel sheet, its manufacturing method, and brake disc rotor

[0001] The present invention relates to a ferritic-austenitic duplex stainless steel sheet, a manufacturing method thereof, and a brake disc rotor, and more particularly to a ferritic-austenitic duplex stainless steel sheet used for brake disc rotors, a manufacturing method thereof, and a brake disc rotor.

[0002] Disc brakes are widely used as one type of braking system. Disc brakes use brake pads to press against a disk-shaped structure called a brake disc rotor (hereinafter sometimes abbreviated as "disc rotor"), which is connected to the tire, converting kinetic energy into thermal energy through friction and slowing down the speed of automobiles and motorcycles. In automobiles, flake graphite cast iron (hereinafter referred to as "cast iron") is used as the material for disc rotors due to its thermal conductivity, manufacturability for complex structures, and cost.

[0003] Cast iron does not contain elements that improve corrosion resistance, making it less resistant to corrosion. If left unattended, red rust will quickly form. Traditionally, this red rust was not very noticeable due to the disc rotor's lower position than the eye, and the unique shape of the wheel. However, in recent years, with the shift to aluminum wheels due to demands for improved fuel efficiency, and the resulting thinner spokes, the red rust on disc rotors has become a significant issue. Therefore, there is a need to improve the corrosion resistance of disc rotors.

[0004] Furthermore, with the recent tightening of environmental regulations, there is a strong demand for improved automobile fuel efficiency, making it necessary to reduce the thickness and weight of disc rotors. However, cast iron has low strength and is produced by casting, which limits how thin the rotor can be. In addition, when braking, the disc rotor can reach temperatures of up to nearly 700°C. Even under driving conditions that require heavy braking, such as on mountain roads, the disc rotor can reach temperatures of 300°C. Because cast iron has low high-temperature strength, thinning the rotor makes it difficult to maintain the necessary strength at high temperatures, making it difficult to reduce the thickness and weight. Furthermore, because cast iron is formed by casting, thinning the disc rotor can sometimes result in poor melt flow, making it impossible to form.

[0005] Stainless steel is a material with excellent corrosion resistance, and martensitic SUS410 materials are widely used for motorcycles and other two-wheeled vehicles. This is because motorcycle disc rotors are exposed and highly visible, making corrosion resistance a priority. However, stainless steel has poorer thermal conductivity than cast iron. Because the brake system is exposed on motorcycles and has excellent cooling capabilities, stainless steel can be used without problems in normal use. On the other hand, in automobiles, the brake system, including the tires, is housed within the wheel wells, making it difficult to cool the disc rotor. This low thermal conductivity has been a challenge, and stainless steel has not been used in these applications. However, in recent years, the adoption of "regenerative braking," which converts kinetic energy during driving into electrical energy and recovers it, has rapidly increased in electric vehicles, fuel cell vehicles, and hybrid vehicles. This reduces the frictional heat generated by friction between the disc rotor and pads, expanding the possibility of using stainless steel, which has poorer thermal conductivity than cast iron.

[0006] Another issue that has hindered the application of stainless steel to automotive disc brakes is its formability. Motorcycle disc rotors are ring-shaped and are manufactured by punching out stainless steel sheet and then performing induction hardening, eliminating the need for extensive machining. Current automobile disc rotors, however, are hat-shaped, resembling a disk with a narrowed center, and are therefore manufactured by casting. Forming such a shape from stainless steel sheet requires press working (particularly deep drawing). However, the stainless steel sheet used in motorcycles is martensitic, and its extremely high hardness makes deep drawing difficult. One solution to this problem has been the widespread use of hot stamping, a method of pressing at high temperatures. This method has enabled stainless steel sheet to be precisely formed into a hat shape.

[0007] As mentioned above, the processing of stainless steel sheets into disc rotors for motorcycles can be done by induction hardening, since there is no significant processing required, while for automobiles, it is done by hot stamping, which is a press process performed at high temperatures. However, without hardening, the hardness cannot be ensured, resulting in deformation during braking, and the stable friction coefficient and wear resistance essential for safe braking cannot be ensured. In other words, with conventional martensitic stainless steel sheets, high-temperature heating (hardening) is required to form them into a hat shape and ensure hardness, making it difficult to save energy by omitting high-temperature heating.

[0008] As mentioned above, SUS410 stainless steel sheets may be applicable to automobile and motorcycle disc brakes. However, martensitic stainless steel sheets, such as SUS410 stainless steel sheets, contain large amounts of carbon and nitrogen (C) to achieve a hard martensite structure with excellent wear resistance. Adding large amounts of carbon and nitrogen (N) facilitates the precipitation of Cr carbonitrides, which can lead to a decrease in Cr in the matrix around the Cr carbonitrides, resulting in a deterioration of corrosion resistance. While this is unlikely to cause problems in normal use, in coastal areas and environments with deicing salt, corrosion resistance is insufficient, resulting in not only a deterioration in appearance but also increased wear on the disc rotor and brake pads and an unstable friction coefficient due to corrosion. While reducing C and N and adding Cr, Ni, and Mo are effective in improving corrosion resistance, reducing C and N reduces the hardness required to ensure wear resistance. However, because Cr, Ni, and Mo are expensive elements, increasing the amount of added elements directly increases costs. Furthermore, if quenching is not performed, it is not possible to ensure formability and hardness, making it difficult to save energy by omitting high-temperature heating.

[0009] On the other hand, SUS304, an austenitic stainless steel, offers excellent corrosion resistance. SUS304 is a versatile stainless steel, and its excellent corrosion resistance and weldability make it suitable for a wide range of applications, including tableware and chemical tankers. However, because it contains approximately 18% Cr and 8% Ni, its price fluctuates significantly with fluctuations in raw material prices. While it offers excellent workability, it lacks sufficient hardness. Therefore, in applications requiring wear resistance, its lifespan is ensured by thickening the material or by surface coating. However, thickening the material or surface coating increases the weight of the part, increases production costs, and reduces workability.

[0010] Against this background, stainless steel disc rotors are required to meet recent demands for improved appearance, thin and lightweight construction, wear resistance, low pad aggressiveness, stable friction coefficients, and energy-saving performance in automobiles. Furthermore, in coastal areas and environments where snow-melting salt is present, it is difficult to apply martensitic SUS (e.g., SUS410 and SUS420) or austenitic SUS (e.g., SUS304 and SUS316) from the perspective of achieving both the above-mentioned properties and cost.

[0011] Patent Document 1, which describes a technique for stainless steel plates used in brake disc rotors, describes the use of martensitic, ferritic, austenitic, or duplex stainless steel plates. In Patent Document 1, the friction coefficient is stabilized by dispersing sulfides. However, Patent Document 1 does not describe the amount of wear of the disc rotor and brake pads or their use in harsh environments such as coastal areas or where snow-melting salt is present. In particular, sulfides reduce corrosion resistance, so the stainless steel plates described in Patent Document 1 are considered unsuitable for use in harsh environments.

[0012] Japanese Patent Application Laid-Open No. 2022-68891

[0013] As mentioned above, SUS410-series stainless steel sheets may be applicable to automobile and motorcycle disc brakes. However, martensitic stainless steel sheets, such as SUS410-series stainless steel sheets, contain large amounts of carbon and nitrogen (C) to achieve a hard martensite structure with excellent wear resistance. Furthermore, omitting high-temperature heating makes it difficult to ensure forming and hardness, making energy conservation difficult. Adding large amounts of carbon and nitrogen (N) facilitates the precipitation of chromium carbonitrides, which can lead to a decrease in Cr in the matrix around the Cr carbonitrides, resulting in a deterioration of corrosion resistance. While this is unlikely to cause problems in normal use, in coastal areas and environments with deicing salt, insufficient corrosion resistance not only leads to deterioration of appearance, but also increases wear on disc rotors and brake pads and destabilizes the friction coefficient due to corrosion. While reducing C and N and adding Cr, Ni, and Mo are effective in improving corrosion resistance, reducing C and N content reduces the hardness required to ensure wear resistance. On the other hand, Cr, Ni, and Mo are expensive elements, and therefore, an increase in the amount of these elements added directly leads to an increase in costs.

[0014] Disc rotors, both on motorcycles and automobiles, are highly visible when installed, so they require corrosion resistance. High corrosion resistance is especially important in coastal areas near the sea or in environments with deicing salt, where rust is likely to occur.

[0015] When stainless steel plate is used as a brake disc rotor, it must have excellent wear properties. Brakes slow down automobiles and motorcycles by squeezing the disc rotor with brake pads, converting kinetic energy into thermal energy through friction. Therefore, extending the life of brakes requires reducing the amount of wear on the disc rotor and brake pads. The wear properties of disc rotors include wear resistance, which indicates the amount of wear on the disc, and pad aggressiveness, which indicates the amount of wear on the brake pads. When rust occurs on a disc rotor, the rust increases the amount of wear on the disc rotor. Furthermore, the rust increases surface roughness, which also increases the amount of wear on the brake pads. For this reason, disc rotors are required to have both wear resistance and low pad aggressiveness.

[0016] When stainless steel plate is used as a brake disc rotor, it is necessary to have excellent friction coefficient stability. As mentioned above, brake discs reach various temperatures from low to high under various driving conditions, so they need to be able to brake stably even when the brake disc temperature rises due to braking. Therefore, to ensure safe braking, it is necessary for the friction coefficient to change little even when exposed to high temperatures.

[0017] When stainless steel sheets are processed into brake disc rotors, they require excellent elongation. As mentioned above, current automotive disc rotors are hat-shaped, and they cannot be heated to high temperatures to omit quenching. Therefore, excellent elongation at room temperature is required for workability and energy-saving performance.

[0018] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a ferritic-austenitic duplex stainless steel sheet that can be used to manufacture brake disc rotors that have excellent corrosion resistance, wear resistance, low pad attack, and a stable friction coefficient, and that also has good processability into brake disc rotors, and a method for manufacturing the same. Another object of the present invention is to provide a brake disc rotor that has excellent corrosion resistance, wear resistance, low pad attack, and a stable friction coefficient.

[0019] To solve the above problems, the present inventors conducted detailed investigations focusing on the grain size of ferritic-austenitic duplex stainless steel sheets (hereinafter sometimes abbreviated as "duplex stainless steel sheets"). The duplex stainless steel sheets used in brake disc rotors, which are the subject of the present invention, are manufactured through hot rolling and annealing processes. Strain due to hot rolling is introduced into the duplex stainless steel sheet during the hot rolling process, and recrystallization occurs during the hot rolling and annealing processes, resulting in the formation of fine grains. To form fine grains, it is necessary to sufficiently introduce strain that serves as a nucleus for recrystallization. Since strain disappears more easily through recovery at higher temperatures, it is necessary to appropriately control the heating temperature before hot rolling. Furthermore, to sufficiently introduce strain, it is necessary to increase the rolling reduction ratio during hot rolling. Furthermore, to generate fine grains, it is necessary to appropriately control the finishing temperature and coiling temperature during hot rolling. Therefore, by appropriately controlling the chemical composition of the duplex stainless steel sheet, the heating conditions before hot rolling, the rolling reduction during hot rolling, the finishing temperature, and the coiling temperature, it is possible to introduce sufficient strain into the duplex stainless steel sheet, which serves as a nucleus for recrystallization, thereby refining the crystal grains of the duplex stainless steel sheet. This is thought to ensure not only the workability of the duplex stainless steel sheet, but also the corrosion resistance, wear resistance, low pad attack, and stable friction coefficient when used as a brake disc rotor. As a result of extensive investigations aimed at achieving this objective, the following findings were obtained.

[0020] In addition to the steel composition, by controlling the heating temperature before hot rolling to 1000 to 1250°C, the reduction during hot rolling to more than 40% but not more than 99%, the finishing temperature during hot rolling to 500 to 1100°C, and the coiling temperature during hot rolling to 450 to 1000°C, sufficient strain is introduced into the duplex stainless steel sheet, which serves as a nucleus for recrystallization and refines the crystal grains. Refining the crystal grains promotes work hardening during use and improves wear resistance. Furthermore, the fine crystal grains improve workability during punching and pressing. This has resulted in the successful provision of a duplex stainless steel sheet that is excellent in workability and provides good corrosion resistance, wear resistance, low pad attack, and a stable friction coefficient when applied to disc rotors. The present invention was completed based on the above findings and is exemplified as follows.

[0021] [1] A ferritic-austenitic duplex stainless steel sheet having a composition containing, on a mass basis, C: 0.005 to 0.050%, N: 0.050 to 0.400%, Si: 0.10 to 1.00%, Mn: 0.50 to 5.00%, P: 0.001 to 0.080%, S: 0.0001 to 0.0050%, Cr: 18.0 to 27.0%, Ni: 1.00 to 9.00%, Mo: 0.10 to 5.00%, Cu: 0.001 to 2.000%, with the balance being Fe and impurities, wherein the ferrite phase is 1.0 to 70.0 vol% and the austenite phase is 30.0 to 99.0 vol%, and the average crystal grain size of the ferrite phase and the austenite phase is 50 μm or less.

[0022] [2] By mass, V: 0.001 to 0.200%, B: 0.0001 to 0.0100%, Nb: 0.001 to 0.100%, Ti: 0.001 to 0.200%, Al: 0.005 to 0.100%, W: 0.001 to 1.500%, Sn: 0.001 to 0.100%, Mg: 0.0001 to 0.0100%, Sb: 0.010 to 0.300%, Zr: 0.001 to 0.200% , Ta: 0.010 to 0.080%, Hf: 0.010 to 0.080%, Co: 0.001 to 0.200%, Ca: 0.0001 to 0.0100%, REM: 0.001 to 0.500%, Ga: 0.0001 to 0.0200%, Bi: 0.001 to 0.080%. The ferritic-austenitic duplex stainless steel sheet according to [1] further contains one or more selected from the group consisting of 0.010 to 0.080%, ...%.

[0023] [3] Average grain boundary length is 0.10 μm / μm 2 The ferritic-austenitic duplex stainless steel sheet according to [1] or [2].

[0024] [4] The ferritic-austenitic duplex stainless steel sheet according to any one of [1] to [3], having a pitting potential of 200 mV vs. SSE or more.

[0025] [5] In a friction and wear test in accordance with JASO C406:2000, the average friction coefficient at a disk temperature of 60 ° C to 300 ° C is 0.25 to 0.65. Ferrite-austenitic duplex stainless steel plate according to any one of [1] to [4].

[0026] [6] In a friction and wear test in accordance with JASO C406:2000, the disc wear amount is 0.50 mm or less and the pad wear amount is 4.0 mm or less. [7] A ferritic-austenitic duplex stainless steel plate according to any one of [1] to [5].

[0027] [7] A ferritic-austenitic duplex stainless steel sheet according to any one of [1] to [6], having a fracture elongation of 20.0% or more.

[0028] [8] The ferritic-austenitic duplex stainless steel sheet according to any one of [1] to [7], which is for a brake disc rotor.

[0029] [9] A brake disc rotor comprising a processed part made of the ferritic-austenitic duplex stainless steel plate according to any one of [1] to [8].

[0030]

[10] A hot rolling process in which an ingot having a composition containing, by mass, C: 0.005 to 0.050%, N: 0.050 to 0.400%, Si: 0.10 to 1.00%, Mn: 0.50 to 5.00%, P: 0.001 to 0.080%, S: 0.0001 to 0.0050%, Cr: 18.0 to 27.0%, Ni: 1.00 to 9.00%, Mo: 0.10 to 5.00%, Cu: 0.001 to 2.000%, with the balance being Fe and impurities, is heated to 1000 ° C. to 1250 ° C., and then hot rolled at a rolling ratio of more than 40% to 99% or less and a finishing temperature of 500 ° C. to 1100 ° C. to obtain a hot-rolled sheet, and then coiled at a coiling temperature of 450 to 1000 ° C.; A method for producing a ferritic-austenitic duplex stainless steel sheet, comprising:

[0031]

[11] The method for producing a ferritic-austenitic duplex stainless steel sheet according to

[10] , further comprising a cold rolling step of cold rolling the hot rolled sheet at a rolling ratio of more than 40% but not more than 99% to obtain a cold rolled sheet.

[0032]

[12] The ingot contains, by mass, V: 0.001 to 0.200%, B: 0.0001 to 0.0100%, Nb: 0.001 to 0.100%, Ti: 0.001 to 0.200%, Al: 0.005 to 0.100%, W: 0.001 to 1.500%, Sn: 0.001 to 0.100%, Mg: 0.0001 to 0.0100%, Sb: 0.010 to 0.300%, Zr: 0.001 to 0.200%, Ta : 0.010 to 0.080%, Hf: 0.010 to 0.080%, Co: 0.001 to 0.200%, Ca: 0.0001 to 0.0100%, REM: 0.001 to 0.500%, Ga: 0.0001 to 0.0200%, and Bi: 0.001 to 0.080%.

[10] The method for producing a ferritic-austenitic duplex stainless steel sheet according to

[11] or

[12] , further containing one or more selected from the group consisting of Cr, Mn, Fe, Hf, Co, Ca, REM, Ga, and Bi.

[0033]

[13] A method for producing a ferritic-austenitic duplex stainless steel sheet according to any one of

[10] to

[12] , which is for use in a brake disc rotor.

[0034] According to the present invention, it is possible to provide a ferritic-austenitic duplex stainless steel sheet that is capable of producing a brake disc rotor that is excellent in corrosion resistance, wear resistance, low pad attack, and friction coefficient stability, and that is also easy to process into brake disc rotors, and a method for producing the same. Furthermore, according to the present invention, it is possible to provide a brake disc rotor that is excellent in corrosion resistance, wear resistance, low pad attack, and friction coefficient stability.

[0035] Hereinafter, embodiments of the present invention will be described in detail. The present invention is not limited to the following embodiments, and it should be understood that modifications and improvements to the following embodiments, as appropriate, based on the ordinary knowledge of those skilled in the art, without departing from the spirit of the present invention, also fall within the scope of the present invention. In this specification, the "%" designation for components means "% by mass" unless otherwise specified.

[0036] (1) Ferrite-austenitic duplex stainless steel sheet A duplex stainless steel sheet according to an embodiment of the present invention has a composition containing C: 0.005 to 0.050%, N: 0.050 to 0.400%, Si: 0.10 to 1.00%, Mn: 0.50 to 5.00%, P: 0.001 to 0.080%, S: 0.0001 to 0.0050%, Cr: 18.0 to 27.0%, Ni: 1.00 to 9.00%, Mo: 0.10 to 5.00%, Cu: 0.001 to 2.000%, with the balance being Fe and impurities.

[0037] Here, in this specification, the term "duplex stainless steel sheet" refers to a plate-shaped material formed from duplex stainless steel, and is a concept that includes strip-shaped materials. Furthermore, in this specification, "ferritic-austenitic" refers to a material whose metal structure at room temperature is primarily two-phase, consisting of ferrite and austenite. Therefore, "ferritic-austenitic" also includes materials containing small amounts of phases other than ferrite and austenite (e.g., martensite). Furthermore, in this specification, the term "impurities" refers to components that are mixed in during industrial production of duplex stainless steel sheets due to raw materials such as ores and scrap, and various factors in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention. For example, impurities include unavoidable impurities. Examples of impurities include O, As, and Pb.

[0038] Furthermore, the duplex stainless steel sheet according to the embodiment of the present invention may contain, as necessary, V: 0.001 to 0.200%, B: 0.0001 to 0.0100%, Nb: 0.001 to 0.100%, Ti: 0.001 to 0.200%, Al: 0.005 to 0.100%, W: 0.001 to 1.500%, Sn: 0.001 to 0.100%, Mg: 0.0001 to 0.0100%, Sb: 0.0 It may further contain one or more selected from: Cr: 0.10 to 0.300%, Zr: 0.001 to 0.200%, Ta: 0.010 to 0.080%, Hf: 0.010 to 0.080%, Co: 0.001 to 0.200%, Ca: 0.0001 to 0.0100%, REM: 0.001 to 0.500%, Ga: 0.0001 to 0.0200%, and Bi: 0.001 to 0.080%. Each component will be described in detail below.

[0039] <C: 0.005 to 0.050%> C is an element that dissolves in the matrix and has a significant effect on hardness. C forms carbides depending on the heat treatment, which reduces formability and corrosion resistance and leads to a decrease in high-temperature strength, so the content is set to (A). Furthermore, an excessive reduction in the C content leads to an increase in refining costs, so the content of (B) is preferred, and the content of (C) is more preferred. (A) = 0.005 to 0.050% (B) = 0.010 to 0.050% (C) = 0.010 to 0.040%

[0040] <N: 0.050 to 0.400%> Like C, N is an element that dissolves in the matrix and has a significant effect on hardness. Depending on the heat treatment, it can produce nitrides, which reduce formability and corrosion resistance and cause a decrease in high-temperature strength, so the content of (A) is set. Furthermore, an excessive reduction in the N content leads to an increase in refining costs, so the content of (B) is preferred, and the content of (C) is more preferred. (A) = 0.050 to 0.400% (B) = 0.100 to 0.300% (C) = 0.120 to 0.200%

[0041] <Si: 0.10 to 1.00%> Si is a useful element as a deoxidizer, and also an element that improves oxidation resistance and high-temperature salt damage resistance. However, an excessive increase in the Si content reduces room-temperature ductility, so the content of (A) was set. Furthermore, in consideration of pickling properties and toughness, the content of (B) is preferred, and in consideration of manufacturability, the content of (C) is even more preferred. (A) = 0.10 to 1.00% (B) = 0.10 to 0.80% (C) = 0.10 to 0.60%

[0042] <Mn: 0.50 to 5.00%> Mn is an element added as a deoxidizer and contributes to improving high-temperature strength in the medium temperature range. However, excessive addition of Mn causes Mn-based oxides to form on the surface at high temperatures, making it more likely to cause poor scale adhesion and abnormal oxidation. In particular, when Mn is added in combination with Mo or W, abnormal oxidation tends to occur more easily relative to the Mn content, so the content of (A) was set. Furthermore, considering the pickling properties and room-temperature ductility in steel sheet production, the content of (B) is preferred, and the content of (C) is more preferred. (A) = 0.50 to 5.00% (B) = 0.50 to 4.00% (C) = 3.00 to 4.00%

[0043] <P: 0.001 to 0.080%> P is an impurity that is mainly mixed in from raw materials during steelmaking refining, and a high content reduces toughness and weldability. For this reason, it is desirable to reduce P as much as possible, but using raw materials with low P content increases costs. On the other hand, excessive addition of P not only significantly hardens the steel, but also reduces corrosion resistance, toughness, and pickling properties, so the content of (A) was set. Considering raw material costs, the content of (B) is preferred, and the content of (C) is more preferred. (A) = 0.001 to 0.080% (B) = 0.001 to 0.060% (C) = 0.001 to 0.050%

[0044] <S: 0.0001 to 0.0050%> S is an element that reduces corrosion resistance and oxidation resistance. However, it not only improves workability by combining with Ti and C, but also forms sulfides by combining with Cr, Mn, etc., to provide lubricity. On the other hand, excessive addition of S causes S to segregate at grain boundaries, embrittling the steel sheet. Furthermore, S combines with Ti and C to reduce the amount of dissolved Ti and cause coarsening of precipitates, resulting in reduced high-temperature strength. Therefore, the content of (A) was set. Furthermore, considering refining costs and high-temperature oxidation properties, the content of (B) is preferred, and the content of (C) is more preferred. (A) = 0.0001 to 0.0050% (B) = 0.0001 to 0.0030% (C) = 0.0001 to 0.0020%

[0045] <Cr: 18.0-27.0%> Cr is an essential element for ensuring oxidation resistance and corrosion resistance. If the Cr content is low, oxidation resistance in particular cannot be ensured, and excessive addition leads to a decrease in workability, toughness, and brazeability, so the content of (A) was set. Furthermore, considering manufacturability and scale removal properties, the content of (B) is preferred, and the content of (C) is more preferred. (A) = 18.0-27.0% (B) = 19.0-26.0% (C) = 20.0-22.0%

[0046] <Ni: 1.00 to 9.00%> Ni is an element that improves oxidation resistance, toughness, and high-temperature strength. Excessive addition of Ni leads to increased costs, so the content of (A) was set. Furthermore, considering manufacturability, the content of (B) is preferred, and the content of (C) is more preferred. (A) = 1.00 to 9.00% (B) = 1.00 to 8.00% (C) = 2.00 to 3.00%

[0047] <Mo: 0.10 to 5.00%> Mo is an element that is effective for solid solution strengthening at high temperatures and also improves corrosion resistance and high-temperature salt damage resistance. Excessive addition of Mo significantly reduces room-temperature ductility and oxidation resistance, so the content of (A) was set. Furthermore, considering thermal fatigue properties and manufacturability, the content of (B) is preferred, and the content of (C) is more preferred. (A) = 0.10 to 5.00% (B) = 0.30 to 4.00% (C) = 0.30 to 0.70%

[0048] <Cu: 0.001 to 2.000%> Cu is an element effective in improving corrosion resistance. Cu improves high-temperature strength through precipitation strengthening due to ε-Cu precipitation, but excessive addition reduces hot workability, so the content is set at (A). Furthermore, taking into consideration thermal fatigue properties, manufacturability, and weldability, the content of (B) is preferred, and the content of (C) is more preferred. (A) = 0.001 to 2.000% (B) = 0.001 to 1.800% (C) = 0.800 to 1.500%

[0049] <V: 0.001 to 0.200%> V is an element that improves corrosion resistance, but if added in excess, precipitates become coarse, reducing high-temperature strength and oxidation resistance, so the content of (A) was set. Also, considering production costs and manufacturability, the content of (B) is preferred, and the content of (C) is more preferred. (A) = 0.001 to 0.200% (B) = 0.001 to 0.180% (C) = 0.040 to 0.150%

[0050] <B: 0.0001 to 0.0100%> B is an element that improves the secondary workability, high-temperature strength, and thermal fatigue properties of duplex stainless steel sheets during press forming. B induces fine precipitation of Laves phases and the like, and exhibits long-term stability of these precipitation strengthening, contributing to suppressing strength degradation and improving thermal fatigue life. On the other hand, excessive addition of B induces hardening, reducing intergranular corrosion resistance and oxidation resistance, as well as causing weld cracking, so the content of (A) was set. Furthermore, considering corrosion resistance and production costs, the content of (B) is preferred, and the content of (C) is more preferred. (A) = 0.0001 to 0.0100% (B) = 0.0001 to 0.0080% (C) = 0.0001 to 0.0050%

[0051] <Nb: 0.001 to 0.100%> Nb is an element effective in improving high-temperature strength through solid solution strengthening and precipitation strengthening of fine precipitates. Nb also fixes C and N as carbonitrides, and contributes to the development of recrystallization texture, which affects the corrosion resistance and r-value of duplex stainless steel sheets. Excessive addition of Nb not only significantly hardens the steel but also reduces manufacturability, so the content of (A) was set. Furthermore, considering raw material costs and toughness, the content of (B) is preferred, and the content of (C) is more preferred. (A) = 0.001 to 0.100% (B) = 0.001 to 0.080% (C) = 0.020 to 0.070%

[0052] <Ti: 0.001 to 0.200%> Ti is an element that combines with C, N, and S to improve corrosion resistance, intergranular corrosion resistance, room-temperature ductility, and deep drawability. Furthermore, when added in combination with Nb and Mo, adding an appropriate amount increases the amount of Nb and Mo dissolved in solid solution during hot rolling annealing, improves high-temperature strength, and improves thermal fatigue properties. On the other hand, excessive addition of Ti increases the amount of dissolved Ti, reducing room-temperature ductility, and also forms coarse Ti-based precipitates, which become the starting point for cracks during hole expansion processing and deteriorate press formability. Furthermore, since oxidation resistance and brazing ability are also deteriorated, the content of (A) was set. Furthermore, considering the occurrence of surface defects and toughness, the content of (B) is preferred, and the content of (C) is more preferred. (A)=0.001~0.200% (B)=0.001~0.150% (C)=0.001~0.030%

[0053] <Al: 0.005-0.100%> Al is added as a deoxidizing element and also improves oxidation resistance. Furthermore, Al is useful as a solid solution strengthening element for improving high-temperature strength. However, excessive addition of Al hardens the steel, significantly reducing uniform elongation and toughness, so the content of (A) was set. Furthermore, taking into consideration the occurrence of surface defects, weldability, and manufacturability, the content of (B) is preferred, and the content of (C) is more preferred. (A) = 0.005-0.100% (B) = 0.005-0.080% (C) = 0.005-0.040%

[0054] <W: 0.001 to 1.500%> Like Mo, W is an effective element for solid solution strengthening at high temperatures, and also produces a Laves phase (Fe2W), resulting in precipitation strengthening. In particular, when added in combination with Nb and Mo, a Laves phase of Fe2 (Nb, Mo, W) precipitates, but adding W suppresses the coarsening of this Laves phase, improving precipitation strengthening. On the other hand, excessive addition of W leads to increased costs and reduces room-temperature ductility, so the content of (A) was set. Furthermore, considering manufacturability, low-temperature toughness, and oxidation resistance, the content of (B) is preferred. (A) = 0.001 to 1.500% (B) = 0.001 to 1.000%

[0055] <Sn: 0.001 to 0.100%> Sn is an element that improves corrosion resistance and high-temperature strength in the medium temperature range. However, excessive addition of Sn significantly reduces manufacturability and toughness, so the content is set to (A). Furthermore, taking into consideration oxidation resistance and manufacturing costs, the content of (B) is preferred, and the content of (C) is more preferred. (A) = 0.001 to 0.100% (B) = 0.0015 to 0.080% (C) = 0.002 to 0.050%

[0056] <Mg: 0.0001 to 0.0100%> Mg not only functions as a deoxidizing element, but also refines the structure of the slab, contributing to improved formability. Furthermore, Mg oxides become precipitation sites for carbonitrides such as Ti(C,N) and Nb(C,N), and have the effect of finely dispersing and precipitating these, contributing to improved toughness. However, excessive addition of Mg leads to deterioration of weldability, corrosion resistance, and surface quality, so the content of (A) was set. Considering refining costs, the content of (B) is preferred. (A) = 0.0001 to 0.0100% (B) = 0.0001 to 0.0080%

[0057] <Sb: 0.010 to 0.300%> Sb is an element that contributes to improving corrosion resistance and high-temperature strength. However, excessive addition of Sb can cause slab cracking and excessive reduction in ductility during steel plate production, so the content is set at (A). Furthermore, taking into consideration refining costs and manufacturability, the content of (B) is preferred. (A) = 0.010 to 0.300% (B) = 0.010 to 0.200%

[0058] <Zr: 0.001 to 0.200%> Zr, like Ti and Nb, is a carbonitride-forming element and improves corrosion resistance and deep drawability. However, excessive addition of Zr significantly reduces manufacturability, so the content is set to (A). Furthermore, taking into consideration cost and surface quality, the content of (B) is preferable. (A) = 0.001 to 0.200% (B) = 0.001 to 0.180%

[0059] <Ta: 0.010-0.080%, Hf: 0.010-0.080%> Ta and Hf are elements that combine with C and N to contribute to improving toughness. However, excessive addition of Ta and Hf not only leads to increased costs but also significantly reduces manufacturability, so the contents are set at (A). Furthermore, taking into consideration refining costs and manufacturability, the contents of (B) are preferred. (A) = 0.010-0.080% (B) = 0.010-0.060%

[0060] <Co: 0.001 to 0.200%> Co is an element that contributes to improving high-temperature strength. However, excessive addition of Co leads to a decrease in toughness, so the content is set at (A). Furthermore, taking into consideration refining costs and manufacturability, the content of (B) is preferred, and the content of (C) is more preferred. (A) = 0.001 to 0.200% (B) = 0.001 to 0.180% (C) = 0.005 to 0.160%

[0061] <Ca: 0.0001 to 0.0100%> Ca is an element added for desulfurization. However, excessive addition of Ca generates coarse CaS, reducing toughness and corrosion resistance, so the content of (A) is set. Furthermore, taking into consideration refining costs and manufacturability, the content of (B) is preferred, and the content of (C) is more preferred. (A) = 0.0001 to 0.0100% (B) = 0.0001 to 0.0080% (C) = 0.0001 to 0.0070%

[0062] <REM: 0.001-0.500%> REM (rare earth element) is an element that contributes to improving toughness and oxidation resistance by refining various precipitates. However, excessive addition of REM significantly reduces castability and ductility, so the content of (A) was set. Furthermore, considering refining costs and manufacturability, the content of (B) is preferable. (A) = 0.001-0.500% (B) = 0.001-0.400%. Note that, as generally defined, REM refers collectively to two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) ranging from lanthanum (La) to lutetium (Lu). A single type of REM may be used, or multiple types may be used as a mixture.

[0063] <Ga: 0.0001 to 0.0200%> Ga is an element effective in improving corrosion resistance and suppressing hydrogen embrittlement. The Ga content is set to (A) from the viewpoint of forming sulfides and hydrides. Furthermore, the Ga content is preferably (B) from the viewpoints of manufacturability, cost, ductility, and toughness. (A) = 0.0001 to 0.0200% (B) = 0.0001 to 0.0100%

[0064] <Bi: 0.001 to 0.080%> Bi is an element that suppresses roping that occurs during cold rolling and improves manufacturability. However, an excessively high Bi content leads to a decrease in hot workability, so the content of (A) is set. Furthermore, from the viewpoint of stably obtaining the above effects, the content of (B) is preferred. (A) = 0.001 to 0.080% (B) = 0.001 to 0.070%

[0065] The metallographic structure of the duplex stainless steel sheet according to an embodiment of the present invention is a two-phase structure of a ferrite phase and an austenite phase, with the ferrite phase being 1.0 to 70.0 vol% and the austenite phase being 30.0 to 99.0 vol%. The metallographic structure may contain inclusions and precipitates. The proportions of the ferrite phase and the austenite phase in the metallographic structure of the duplex stainless steel sheet are related to corrosion resistance, toughness, and stability of the friction coefficient. In particular, by setting the ferrite phase proportion to 1.0 to 70.0 vol%, SCC (stress corrosion cracking) resistance is improved, enabling use as a brake disc rotor in a corrosive environment. On the other hand, if the ferrite phase proportion exceeds 70.0 vol%, the duplex stainless steel sheet becomes excessively soft, resulting in deformation when used as a brake disc rotor and a loss of stability of the friction coefficient. Furthermore, if the ferrite phase proportion is less than 1.0 vol%, SCC resistance is insufficient, making use as a brake disc rotor in a corrosive environment difficult. From the viewpoint of ensuring SCC resistance, the ferrite phase ratio is preferably 5.0 to 60.0 vol%, and more preferably 10.0 to 50.0 vol%. Furthermore, by setting the austenite phase ratio to 30.0 to 99.0 vol%, it is possible to ensure the toughness required for use as a brake disc rotor. On the other hand, if the austenite phase ratio is less than 30.0 vol%, the toughness of the duplex stainless steel plate is excessively reduced, and when used as a brake disc rotor, cracks and deformation occur, and the stability of the friction coefficient is impaired. Furthermore, if the austenite phase ratio exceeds 99.0 vol%, the SCC resistance is insufficient, making it difficult to use as a brake disc rotor in a corrosive environment. From the viewpoint of improving toughness when used as a brake disc rotor, the austenite phase ratio is preferably 40.0 to 95.0 vol%, and more preferably 50.0 to 90.0 vol%.

[0066] Here, in this specification, the proportions of ferrite and austenite phases are determined by the following method. A cross section perpendicular to the rolling direction of a duplex stainless steel plate is mirror-polished, and EBSD measurement is performed on the t / 4 portion of this cross section (a portion that is 1 / 4 of the thickness t of the duplex stainless steel plate). This measurement is performed using a JEOL Ltd. JSM-7200F scanning electron microscope, and images are observed in 1.00 μm measurement steps over a 170 × 510 μm area (magnification 500x), and analysis is performed using the analysis software "OIM" manufactured by TSL Solutions Co., Ltd. The area ratios of the ferrite and austenite phases are calculated by the analysis, and the results are used to determine the proportions of each phase.

[0067] In the duplex stainless steel sheet according to the embodiment of the present invention, the average grain size of the ferrite phase and the austenite phase is 50 μm or less. Here, the average grain size of the ferrite phase and the austenite phase refers to the average value of the grain size of the ferrite phase and the austenite phase. In applications where friction occurs during use, such as brake disc rotors, plastic deformation due to friction occurs directly below the sliding surface. When plastic deformation occurs, the surface undergoes work hardening, improving wear resistance. Since work hardening tends to occur more easily with smaller grain sizes, reducing the grain size leads to improved wear resistance. Furthermore, during punching and other processes, a smaller grain size suppresses the generation of burrs, so reducing the grain size is also effective in improving processability. In the duplex stainless steel sheet according to the embodiment of the present invention, controlling the average grain size of the ferrite phase and the austenite phase to 50 μm or less facilitates work hardening during use, improving wear resistance, and suppressing the generation of burrs during processing. If the average grain size of the ferrite phase and the austenite phase exceeds 50 μm, work hardening during use is insufficient, resulting in reduced wear resistance. Furthermore, burrs are more likely to occur during processing such as punching, resulting in reduced workability. From the viewpoint of stably ensuring the above effects, the average grain size of the ferrite phase and the austenite phase is preferably 40 μm or less, more preferably 30 μm or less. The lower limit of the average grain size of the ferrite phase and the austenite phase is not particularly limited, but is typically 1 μm, 2 μm, or 3 μm.

[0068] Here, in this specification, the average grain size of the ferrite phase and the austenite phase is determined by the following method. A cross section perpendicular to the rolling direction of a duplex stainless steel sheet was mirror-polished, and EBSD measurements were performed at the t / 4 portion of this cross section. This measurement was performed using a JEOL Ltd. JSM-7200F scanning electron microscope, and images were observed in 1.00 μm measurement steps over a 170 × 510 μm area (magnification: 500x), and analysis was performed using the analysis software "OIM" manufactured by TSL Solutions Co., Ltd. The grain size was measured using measurement points where the crystal orientation difference between adjacent measurement points was 15 to 65° (15≦X≦65) as grain boundaries. The average grain size was calculated as the average of the measured grain sizes of at least 500 grains.

[0069] The duplex stainless steel sheet according to the embodiment of the present invention has an average grain boundary length of 0.10 μm / μm 2 As described above, in applications where friction occurs during use, such as brake disc rotors, plastic deformation occurs due to friction directly below the sliding surface. When plastic deformation occurs, the surface undergoes work hardening, improving wear resistance. Work hardening tends to occur more easily when the crystal grain size is small and the grain boundary length is long, so increasing the grain boundary length leads to improved wear resistance. As described above, the average grain boundary length is set to 0.10 μm / μm 2 By controlling the grain boundary length to the above, work hardening during use is facilitated, and wear resistance is improved. From the viewpoint of stably ensuring this effect, the average grain boundary length is set to 0.15 μm / μm 2 More preferably, 0.20 μm / μm or more 2 The upper limit of the average grain boundary length is not particularly limited, but is typically 1.00 μm / μm 2 , 0.90 μm / μm 2 or 0.80 μm / μm 2 is.

[0070] Here, in this specification, the average grain boundary length is determined by the following method. A cross section perpendicular to the rolling direction of a duplex stainless steel sheet is mirror-polished, and EBSD measurements are performed at the t / 4 portion of this cross section. This measurement is performed using a JEOL Ltd. JSM-7200F scanning electron microscope, and images are observed in 1.00 μm measurement steps over a 170 × 510 μm area (magnification: 500x), and analysis is performed using the "OIM" analysis software from TSL Solutions Co., Ltd. Measurement points where the crystal orientation difference between adjacent measurement points is 15 to 65° (15≦X≦65) are considered to be grain boundaries, and the average grain boundary length is calculated.

[0071] The duplex stainless steel sheet according to the embodiment of the present invention preferably has a pitting potential of 200 mV vs. SSE or more. By controlling the pitting potential within this range, excellent corrosion resistance can be achieved. Here, in this specification, "pitting potential" refers to the pitting potential measured in accordance with JIS G0577:2014. The sodium chloride aqueous solution used in this measurement has a sodium chloride concentration of 3.5% and a temperature of 30°C. When a test piece is taken from a duplex stainless steel sheet after processing into a part such as a brake disc, if it is not possible to take a test piece in accordance with JIS G0577:2014, the test may be performed on a test piece of any size. From the viewpoint of maintaining the aesthetic appearance of the duplex stainless steel sheet, the pitting potential is more preferably 225 mV vs. SSE or more, and even more preferably 250 mV vs. SSE or more. The upper limit of the pitting potential is not particularly limited, but is typically 1100 mV vs. SSE, 1000 mV vs. SSE, or 900 mV vs. SSE.

[0072] In the friction and wear test according to JASO C406:2000, the duplex stainless steel sheet according to the embodiment of the present invention preferably has an average friction coefficient of 0.25 to 0.65 at disc temperatures of 60°C to 300°C. By controlling the average friction coefficient within this range, excellent friction coefficient stability can be achieved as a brake disc rotor. Here, in the friction and wear test, the average friction coefficient is determined when braking from 130 km / h in the room temperature effectiveness test of the second effectiveness test. Examples of pads (mating materials) used in the friction and wear test include non-asbestos organic (NAO) materials (non-steel (NS), low-steel (LS), semi-metallic (SM)), Cu sintered materials, etc. When taking test specimens from the duplex stainless steel sheet after processing into parts such as brake discs, the disc (disk-shaped test specimen) and the mating material may have shapes and dimensions that allow testing according to JASO C406:2000. The moment of inertia is 0.25 kg m 2 In order to be adaptable to various usage environments, the average friction coefficient of the duplex stainless steel plate is more preferably 0.26 to 0.64, and even more preferably 0.27 to 0.63.

[0073] In the friction and wear test according to JASO C406:2000, the duplex stainless steel sheet according to the embodiment of the present invention preferably exhibits a disc wear of 0.50 mm or less and a pad wear of 4.0 mm or less. By controlling the disc wear and pad wear within these ranges, excellent wear resistance and low pad aggressiveness can be achieved as a brake disc rotor. Here, the same pads (mating materials) as those described above can be used for the friction and wear test. Furthermore, when test pieces are taken from duplex stainless steel sheets after processing into parts such as brake discs, the disc-shaped test pieces and the mating materials may have shapes and dimensions that allow testing according to JASO C406:2000. In this test, the vehicle classification is P1, and the brake temperature before braking is a rear condition. Optional items and water immersion in the water recovery test are not performed. Furthermore, the moment of inertia is 0.25 kg m 2From the viewpoint of the lifespan of the disk and pad, it is more preferable that the disk wear amount is 0.40 mm or less and the pad wear amount is 3.0 mm or less, and it is even more preferable that the disk wear amount is 0.30 mm or less and the pad wear amount is 2.0 mm or less.

[0074] The duplex stainless steel sheet according to the embodiment of the present invention preferably has a fracture elongation of 20.0% or more. Controlling the fracture elongation within this range can be said to provide excellent workability at room temperature (25°C) (particularly, to enable hat-shaped formation). In particular, since conventional martensitic stainless steel sheets are processed into hat-shaped brake disc rotors by hot stamping at high temperatures, energy conservation by omitting heating during processing (i.e., workability at room temperature (25°C)) is an issue. In contrast, the duplex stainless steel sheet according to the present invention has excellent workability at room temperature (25°C), allowing for energy conservation by omitting heating during processing. Here, the fracture elongation in this specification refers to the fracture elongation at room temperature (25°C). The fracture elongation at room temperature (25°C) can be determined by the following method. A tensile test specimen is prepared from the duplex stainless steel sheet so that the rolling direction is the tensile direction, and a tensile test is performed at room temperature (25°C) in accordance with JIS Z2241:2011 to measure the fracture elongation. When a test specimen is taken from a duplex stainless steel sheet after processing into a part such as a brake disc, the rolling direction does not necessarily have to coincide with the tensile direction. Furthermore, if a test specimen conforming to JIS Z2241:2011 cannot be taken, the test specimen may be extended using a backing plate or the like. From the viewpoint of stably improving formability at room temperature (25°C), the breaking elongation of the duplex stainless steel sheet is more preferably 25.0% or more, and even more preferably 30.0% or more. The upper limit of the breaking elongation is not particularly limited, but is typically 35.0%.

[0075] The duplex stainless steel sheet according to the embodiment of the present invention is not particularly limited, but is preferably a hot-rolled annealed sheet or a cold-rolled annealed sheet, which makes it easier to obtain the above-mentioned characteristics.

[0076] In the duplex stainless steel sheet according to the embodiment of the present invention, in addition to the composition, the ratio of ferrite and austenite phases and the average crystal grain size are controlled within predetermined ranges, making it possible to manufacture brake disc rotors that are excellent in corrosion resistance, wear resistance, low pad attack, and friction coefficient stability, and the sheet also has good processability into brake disc rotors. Furthermore, this duplex stainless steel sheet improves the appearance (aesthetics) and energy-saving performance of the brake disc rotor, and also enables thinner and lighter brake disc rotors. The sheet can be used for brake disc rotors not only for automobiles but also for various other vehicles, such as motorcycles, tricycles, and snowmobiles, but is particularly suitable for use in automobile brake disc rotors.

[0077] (2) Manufacturing Method of Ferrite-Austenitic Duplex Stainless Steel Sheet The manufacturing method of a duplex stainless steel sheet according to an embodiment of the present invention is not particularly limited as long as it is a method capable of manufacturing a duplex stainless steel sheet having the above-described characteristics. An example of a manufacturing method of a duplex stainless steel sheet according to an embodiment of the present invention will be described below. When the duplex stainless steel sheet according to an embodiment of the present invention is a hot-rolled annealed sheet, the manufacturing method is carried out in the order of a steelmaking process, a hot-rolling process, an annealing process, and a pickling process. When the duplex stainless steel sheet according to an embodiment of the present invention is a cold-rolled annealed sheet, the manufacturing method is carried out in the order of a steelmaking process, a hot-rolling process, an annealing process, a pickling process, a cold-rolling process, an annealing process, and a pickling process. Note that when the duplex stainless steel sheet is a cold-rolled annealed sheet, the annealing process after the hot-rolling process may be omitted.

[0078] In the steelmaking process, a method is preferably used in which steel whose composition has been adjusted to the above composition is melted in a converter, followed by secondary refining. The melted steel is formed into ingots (ingots, slabs, etc.) using a known casting method (continuous casting). The ingots are heated to a predetermined temperature and hot-rolled to a predetermined thickness by continuous rolling (hot rolling process). In the hot rolling process, the ingots are rolled in a hot rolling mill consisting of multiple stands and then wound into a coil. The wound hot-rolled coil is annealed under predetermined conditions in an annealing furnace and then pickled to obtain a hot-rolled annealed sheet (annealing process and pickling process). The pickling process can be performed using an existing pickling method.

[0079] In order to obtain a duplex stainless steel sheet having the above-described properties, it is important to set the average grain size to 50 μm or less. To achieve this, in addition to the composition of the ingot, the heating temperature before hot rolling is lowered and the rolling reduction ratio during hot rolling is increased to introduce sufficient strain into the hot-rolled sheet during hot rolling and suppress strain recovery. To suppress strain recovery during hot rolling, the heating temperature before hot rolling is set to 1000°C to 1250°C. Furthermore, to introduce sufficient strain during hot rolling, the rolling reduction ratio during hot rolling is set to more than 40% and 99% or less. Furthermore, to refine the grains, the finishing temperature during hot rolling is set to 500°C to 1100°C, and the coiling temperature is set to 450°C to 1000°C. Details of these conditions are explained below.

[0080] <Heating temperature before hot rolling: 1000°C to 1250°C> The heating temperature before hot rolling is 1000°C to 1250°C. By controlling the heating temperature before hot rolling within this range, the average crystal grain size (and average grain boundary length) of the duplex stainless steel sheet can be adjusted to a predetermined range. From the viewpoint of suppressing coarsening of the average crystal grain size, the heating temperature before hot rolling is preferably 1010°C to 1240°C, more preferably 1020°C to 1230°C. Note that when the duplex stainless steel sheet is a cold-rolled annealed sheet, the heating temperature before hot rolling does not have to be within the above range.

[0081] <Rolling ratio during hot rolling: more than 40% and not more than 99%> The rolling ratio during hot rolling is more than 40% and not more than 99%. By controlling the rolling ratio during hot rolling within this range, the average crystal grain size (and average grain boundary length) of the duplex stainless steel sheet can be adjusted to a predetermined range. From the viewpoint of suppressing coarsening of the average crystal grain size, the rolling ratio is preferably more than 70% and not more than 99%, and more preferably more than 90% and not more than 99%. Note that when the duplex stainless steel sheet is a cold-rolled annealed sheet, the rolling ratio during hot rolling does not have to be within the above range.

[0082] <Finishing temperature during hot rolling: 500°C to 1100°C> The finishing temperature during hot rolling is 500°C to 1100°C. Here, the finishing temperature during hot rolling refers to the temperature at the time when hot rolling is completed. By controlling the finishing temperature during hot rolling within this range, the average crystal grain size (and average grain boundary length) of the duplex stainless steel sheet can be adjusted to a predetermined range. From the viewpoint of suppressing coarsening of the average crystal grain size, the finishing temperature during hot rolling is preferably 550°C to 1050°C, and more preferably 600°C to 1000°C.

[0083] <Coiling temperature during hot rolling: 450 to 1000°C> The coiling temperature during hot rolling is 450 to 1000°C. By controlling the coiling temperature during hot rolling within this range, the average crystal grain size (and average grain boundary length) of the duplex stainless steel sheet can be adjusted within a predetermined range. From the viewpoint of suppressing coarsening of the average crystal grain size, the coiling temperature during hot rolling is preferably 450 to 950°C, more preferably 450 to 900°C, and even more preferably 800 to 900°C.

[0084] The conditions for the annealing process after the hot rolling process are not particularly limited, and can be performed according to a known method. For example, the annealing temperature may be 1000°C to 1150°C, and the holding time may be 0 to 30 minutes. The annealing temperature is preferably 1000°C to 1125°C, and more preferably 1000°C to 1100°C. The holding time is preferably 0 to 25 minutes, and more preferably 0 to 20 minutes.

[0085] When the duplex stainless steel sheet is a cold-rolled annealed sheet, the hot-rolled sheet or the hot-rolled annealed sheet is cold-rolled (cold-rolling process). Cold-rolling can be performed using either a tandem rolling mill or a Sendzimir rolling mill. After rolling, the wound cold-rolled coil is annealed under specified conditions in an annealing furnace, followed by pickling to obtain a cold-rolled annealed sheet (annealing process and pickling process). The pickling process can be performed using an existing pickling method.

[0086] The rolling ratio during cold rolling is more than 40% and not more than 99%. By controlling the rolling ratio during cold rolling within this range, the average crystal grain size (and average grain boundary length) of the duplex stainless steel sheet can be adjusted to a predetermined range. From the viewpoint of suppressing coarsening of the average crystal grain size, the rolling ratio is preferably more than 70% and not more than 99%, more preferably more than 90% and not more than 99%. Furthermore, the conditions for the annealing temperature after cold rolling are not particularly limited and can be performed according to known methods. For example, the annealing temperature and holding time may be the same as those after the above-mentioned hot rolling process.

[0087] (3) Brake disc rotor The brake disc rotor according to the embodiment of the present invention includes a processed part made of the above-described duplex stainless steel sheet. Here, the processed part refers to various parts processed into the shape of a part used in the brake disc rotor. Therefore, for example, if the brake disc rotor is a brake disc rotor for an automobile, it refers to various parts processed into a hat shape from the duplex stainless steel sheet. Note that the shape of the brake disc rotor is not limited to a hat shape and may be determined appropriately depending on the type of vehicle in which the brake disc rotor is used.

[0088] The processed part of the duplex stainless steel sheet can be produced by processing the above-mentioned duplex stainless steel sheet into the shape of a brake disc rotor. The processing method is not particularly limited, and known methods such as press processing can be used.

[0089] The brake disc rotor according to the embodiment of the present invention includes a processed part made of the above-mentioned duplex stainless steel plate, and therefore has excellent corrosion resistance, wear resistance, low pad attack, and stable friction coefficient. In addition, this brake disc rotor has good appearance (aesthetics) and energy-saving performance, and can be made thin and lightweight.

[0090] The present invention will be described in detail below with reference to examples, but the present invention should not be construed as being limited to these examples.

[0091] Hot-rolled and annealed sheets or cold-rolled and annealed sheets were prepared as duplex stainless steel sheets as follows. Steels having the compositions shown in Table 1 (the balance being Fe and impurities) were melted and cast into ingots. The ingots were heated to the temperatures shown in Table 2 (heating temperatures before hot rolling), and then hot-rolled at the rolling reduction and finishing temperatures shown in Table 2 to obtain hot-rolled sheets with a thickness of 6 mm. These hot-rolled sheets were then coiled at the coiling temperatures shown in Table 2. The obtained hot-rolled sheets were then annealed at 1050°C for 5 minutes and cooled to room temperature (25°C) to obtain hot-rolled and annealed sheets. For Test No. 14 using steel type No. A14, the hot-rolled and annealed sheets were cold-rolled to a thickness of 5 mm, annealed at 1050°C for 60 seconds, and cooled to room temperature (25°C) to obtain cold-rolled and annealed sheets. Furthermore, for Test No. 15 using steel type No. A16, the hot-rolled and annealed sheets were cold-rolled to a thickness of 5 mm, annealed at 1050°C for 60 seconds, and cooled to room temperature (25°C) to obtain cold-rolled and annealed sheets. For No. 15, annealing after hot rolling was not performed, and the hot-rolled sheet was cold-rolled to a thickness of 5 mm. The cold-rolled sheet was then annealed at 1050°C for 60 seconds and cooled to room temperature (25°C) to obtain a cold-rolled annealed sheet.

[0092]

[0093]

[0094] The duplex stainless steel sheets (hot-rolled and annealed sheets or cold-rolled and annealed sheets) obtained above were evaluated as follows.

[0095] <Ratio of Ferrite Phase (α Phase) and Austenite Phase (γ Phase), Average Grain Size, and Average Grain Boundary Length> A cross section perpendicular to the rolling direction of the duplex stainless steel sheet obtained above was mirror-polished, and EBSD measurements were performed on the t / 4 portion of this cross section (a portion that is ¼ of the thickness t of the duplex stainless steel sheet). This measurement was performed using a JEOL Ltd. JSM-7200F scanning electron microscope, and images were observed in 1.00 μm measurement steps over a 170 × 510 μm area (magnification: 500x). Analysis was performed using the "OIM" analysis software from TSL Solutions Co., Ltd. The area ratios of the ferrite phase and the austenite phase were calculated by the analysis, and the results were used to represent the proportions of each phase. Furthermore, measurement points where the crystal orientation difference between adjacent measurement points was 15 to 65° (15≦X≦65) were used as grain boundaries to measure grain size. The average grain size was calculated as the average of the measured grain sizes of 500 crystals. Furthermore, measurement points where the crystal orientation difference between adjacent measurement points was 15 to 65° (15≦X≦65) were regarded as grain boundaries, and the average grain boundary length was calculated.

[0096] <Pitting potential> In order to evaluate the corrosion resistance during use, the hot-rolled annealed sheet or cold-rolled annealed sheet obtained above was pickled and then the pitting potential was measured. The pitting potential was measured in accordance with JIS G0577:2014, and the sodium chloride aqueous solution used in this measurement had a sodium chloride concentration of 3.5% and a temperature of 30°C. In this evaluation, if the pitting potential is 200 mV vs. SSE or more, it can be determined that the sheet has sufficient corrosion resistance to maintain its appearance.

[0097] <Average Friction Coefficient> After pickling the hot-rolled annealed sheet or cold-rolled annealed sheet obtained above, disks with an outer diameter of 90 mm (disk-shaped test pieces: thickness 6 mm for the hot-rolled annealed sheet, thickness 5 mm for the cold-rolled annealed sheet) were cut out and subjected to a friction and wear test in accordance with JASO C406: 2000. In the friction and wear test, braking from 130 km / h was performed as in the room temperature effectiveness test of the second effectiveness test, and the braking deceleration rate was 1.0 m / s 2 ~10.0 m / s 2The average friction coefficient during one braking stroke at each deceleration was calculated. The disc temperature during braking was set to 60°C to 300°C. The disc temperature was measured with a thermocouple at a position 1 mm below the sliding surface. The mating material (pad) used was a non-asbestos organic (NAO) material measuring 33 mm x 12 mm x 15 mm thick. The moment of inertia was 0.25 kg m 2 In this evaluation, if the average friction coefficient is between 0.25 and 0.65, it can be determined that the material is applicable to general disc rotors.

[0098] <Disc wear amount and pad wear amount> After the hot-rolled annealed sheet or cold-rolled annealed sheet obtained above was pickled, a disc with an outer diameter of 90 mm (disc-shaped test piece: 6 mm thick for the hot-rolled annealed sheet, 5 mm thick for the cold-rolled annealed sheet) was cut out and subjected to a friction and wear test in accordance with JASO C406:2000. In this test, the vehicle classification was P1, and the brake temperature before braking was rear condition. In addition, optional items and water immersion for the water recovery test were not performed. Furthermore, the moment of inertia was 0.25 kg m 2 The mating material (pad) was a non-asbestos organic (NAO) material measuring 33 mm x 12 mm x 15 mm thick. The difference in thickness between the disc and pad before and after the friction and wear test was taken as the disc wear and pad wear. The disc thickness was measured at the center of the sliding track width using a point micrometer at positions 0°, 120°, and 240° (0° was selected at an arbitrary position). The pad thickness was measured with a vernier caliper at the top, center, and bottom of the 33 mm x 12 mm surface. In this evaluation, if the disc wear was 0.50 mm or less and the pad wear was 4.0 mm or less, it was determined that the product could be used with a general disc rotor.

[0099] <Fracture elongation> After pickling the hot-rolled annealed sheet or cold-rolled annealed sheet obtained above, tensile test pieces were taken so that the rolling direction was the tensile direction, and a tensile test was performed at room temperature (25°C) in accordance with JIS Z2241: 2011 to measure the fracture elongation. In this evaluation, if the fracture elongation is 20.0% or more, it can be determined that the sheet can be processed into a hat shape, and therefore has good processability for general disc rotors.

[0100] The results of the above evaluation are shown in Table 3.

[0101]

[0102] As shown in Table 3, the duplex stainless steel sheets (examples of the present invention) of Test Nos. 1 to 23 had compositions, ferrite and austenite phase ratios, and average crystal grain sizes within appropriate ranges, resulting in good results for pitting potential, average friction coefficient, disk wear, pad wear, and fracture elongation. In contrast, the duplex stainless steel sheet (comparative example) of Test No. 24 had high C and N contents, and the ferrite and austenite phase ratios were not within the appropriate ranges. As a result, a large amount of carbonitrides precipitated, resulting in a decrease in pitting potential (corrosion resistance). Furthermore, this duplex stainless steel sheet was hardened, resulting in low fracture elongation. The duplex stainless steel sheet (comparative example) of Test No. 25 had a high Si content and was hardened, resulting in low fracture elongation. The duplex stainless steel sheet (comparative example) of Test No. 26 had a high Mn content, resulting in average crystal grain size and average grain boundary length outside the ranges, resulting in a low average friction coefficient. The duplex stainless steel sheet No. 27 (comparative example) had a high P content, which resulted in the precipitation of a large amount of coarse phosphides, resulting in a decreased pitting potential. Furthermore, due to the embrittlement caused by P, this duplex stainless steel sheet also had insufficient average friction coefficient, disc wear amount, pad wear amount, and fracture elongation. The duplex stainless steel sheet No. 28 (comparative example) had a high S content, which resulted in the precipitation of a large amount of sulfides, resulting in a decreased pitting potential. Furthermore, due to the embrittlement caused by S, this duplex stainless steel sheet also had insufficient average friction coefficient, disc wear amount, pad wear amount, and fracture elongation.

[0103] The duplex stainless steel sheet of Test No. 29 (Comparative Example) had a high Cr content, and the ratio of ferrite and austenite phases was not within the appropriate range. Furthermore, this duplex stainless steel sheet was excessively hardened, and therefore the elongation at break was insufficient. The duplex stainless steel sheet of Test No. 30 (Comparative Example) had a high Ni content, and the ratio of ferrite and austenite phases was not within the appropriate range, and therefore the average friction coefficient was reduced. The duplex stainless steel sheet of Test No. 31 (Comparative Example) had a high Mo content, and the ratio of ferrite and austenite phases was not within the appropriate range. Furthermore, this duplex stainless steel sheet was excessively hardened, and therefore the elongation at break was insufficient. The duplex stainless steel sheet of Test No. 32 (Comparative Example) had a high Cu content, and therefore the average crystal grain size and average grain boundary length were outside the range, and therefore it was excessively hardened, and therefore the elongation at break was insufficient. In the duplex stainless steel sheet No. 33 (comparative example), the manufacturing conditions were inappropriate, so recrystallization was not completed and the average crystal grain size and average grain boundary length could not be measured. In addition, the disc wear amount, pad wear amount, and fracture elongation of this duplex stainless steel sheet were also insufficient.

[0104] As can be seen from the above results, the present invention makes it possible to manufacture brake disc rotors that are excellent in corrosion resistance, wear resistance, low pad attack, and friction coefficient stability, and it is possible to provide a ferritic-austenitic duplex stainless steel sheet that is also excellent in processability into brake disc rotors, and a method for manufacturing the same. Furthermore, the present invention can provide brake disc rotors that are excellent in corrosion resistance, wear resistance, low pad attack, and friction coefficient stability.

Claims

1. A ferritic-austenitic duplex stainless steel sheet having a composition, on a mass basis, of C: 0.005-0.050%, N: 0.050-0.400%, Si: 0.10-1.00%, Mn: 0.50-5.00%, P: 0.001-0.080%, S: 0.0001-0.0050%, Cr: 18.0-27.0%, Ni: 1.00-9.00%, Mo: 0.10-5.00%, Cu: 0.001-2.000%, with the balance being Fe and impurities, wherein the ferrite phase is 1.0-70.0 vol. % and the austenite phase is 30.0-99.0 vol. %, and the average crystal grain size of the ferrite phase and the austenite phase is 50 μm or less.

2. By mass, V: 0.001 to 0.200%, B: 0.0001 to 0.0100%, Nb: 0.001 to 0.100%, Ti: 0.001 to 0.200%, Al: 0.005 to 0.100%, W: 0.001 to 1.500%, Sn: 0.001 to 0.100%, Mg: 0.0001 to 0.0100%, Sb: 0.010 to 0.300%, Zr: 0.001 to 0.200%, The ferritic-austenitic duplex stainless steel sheet according to claim 1, further comprising one or more selected from Ta: 0.010 to 0.080%, Hf: 0.010 to 0.080%, Co: 0.001 to 0.200%, Ca: 0.0001 to 0.0100%, REM: 0.001 to 0.500%, Ga: 0.0001 to 0.0200%, and Bi: 0.001 to 0.080%.

3. Average grain boundary length is 0.10 μm / μm 2 The ferritic-austenitic duplex stainless steel sheet according to claim 1 or 2.

4. The ferritic-austenitic duplex stainless steel sheet according to any one of claims 1 to 3, having a pitting potential of 200 mV vs. SSE or more.

5. A ferritic-austenitic duplex stainless steel sheet according to any one of claims 1 to 4, which has an average friction coefficient of 0.25 to 0.65 at disk temperatures of 60°C to 300°C in a friction and wear test in accordance with JASO C406:2000.

6. A ferritic-austenitic duplex stainless steel sheet according to any one of claims 1 to 5, in which the disc wear amount is 0.50 mm or less and the pad wear amount is 4.0 mm or less in a friction and wear test in accordance with JASO C406:2000.

7. A ferritic-austenitic duplex stainless steel sheet according to any one of claims 1 to 6, having a fracture elongation of 20.0% or more.

8. The ferritic-austenitic duplex stainless steel sheet according to any one of claims 1 to 7, which is used for a brake disc rotor.

9. A brake disc rotor comprising a processed part made of the ferritic-austenitic duplex stainless steel plate according to any one of claims 1 to 8.

10. A hot rolling process in which an ingot containing, by mass, C: 0.005-0.050%, N: 0.050-0.400%, Si: 0.10-1.00%, Mn: 0.50-5.00%, P: 0.001-0.080%, S: 0.0001-0.0050%, Cr: 18.0-27.0%, Ni: 1.00-9.00%, Mo: 0.10-5.00%, Cu: 0.001-2.000%, with the balance being Fe and impurities, is heated to 1000°C to 1250°C, and then hot rolled at a rolling ratio of more than 40% to 99% or less and a finishing temperature of 500°C to 1100°C to obtain a hot-rolled sheet, and then coiled at a coiling temperature of 450°C to 1000°C. A method for producing a ferritic-austenitic duplex stainless steel sheet, comprising the steps of:

11. The method for producing a ferritic-austenitic duplex stainless steel sheet according to claim 10, further comprising a cold rolling step of cold rolling the hot rolled sheet at a rolling reduction ratio of more than 40% and not more than 99% to obtain a cold rolled sheet.

12. The ingot contains, by mass, V: 0.001 to 0.200%, B: 0.0001 to 0.0100%, Nb: 0.001 to 0.100%, Ti: 0.001 to 0.200%, Al: 0.005 to 0.100%, W: 0.001 to 1.500%, Sn: 0.001 to 0.100%, Mg: 0.0001 to 0.0100%, Sb: 0.010 to 0.300%, Zr: 0.001 to 0.200%, Ta: : 0.010 to 0.080%, Hf: 0.010 to 0.080%, Co: 0.001 to 0.200%, Ca: 0.0001 to 0.0100%, REM: 0.001 to 0.500%, Ga: 0.0001 to 0.0200%, and Bi: 0.001 to 0.080%. The method for producing a ferritic-austenitic duplex stainless steel sheet according to claim 10 or 11, further comprising one or more selected from the group consisting of 0.010 to 0.080%, ...01 to 0.0100%, 0.001 to 0.500%, 0.0001 to 0.0200%, and 0.001 to 0.080%.

13. A method for producing a ferritic-austenitic duplex stainless steel sheet according to any one of claims 10 to 12, which is for use in brake disc rotors.

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