Stainless steel sheet and method for producing same, and brake disc rotor and method for producing same

Abstract

Provided is a stainless steel sheet which contains, on a mass basis, 0.001-0.150% of C, 0.0010-0.3000% of N, 0.01-3.00% of Si, 0.010-5.000% of Mn, 0.001-0.100% of P, 0.0001-0.1000% of S, 15.0-25.0% of Cr, and 0.010-5.000% of Ni, with the balance being made up of Fe and impurities, wherein: the A value represented by formula (1) is 55 to 150; the area ratio of a structure having a KAM value of 0.00° to 0.65° is 20.0% to 75.0% and the area ratio of a structure having a KAM value of more than 0.65° but not more than 5.00° is 25.0% to 80.0%; and the Vickers hardness is 200 HV to 300 HV.　(1) A value = 420C + 470N + 23Ni + 7Mn - 11.5Cr - 11.5Si + 189 In the formula, each element symbol represents the content (%) of the element.

Description

Stainless steel plate and method for manufacturing the same, and brake disc rotor and method for manufacturing the same

[0001] The present invention relates to stainless steel sheets and methods for manufacturing the same, and to brake disc rotors and methods for manufacturing the same. More specifically, the present invention relates to stainless steel sheets and methods for manufacturing the same that are excellent in terms of processability and stability of the coefficient of friction, and to brake disc rotors and methods for manufacturing the same that are excellent in terms of stability of the coefficient of friction.

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

[0003] Cast iron lacks elements to improve corrosion resistance, resulting in poor corrosion resistance and rapid rust formation if left untreated. Traditionally, this rust was not very noticeable due to the low position of the disc rotor and the unique shape of the wheel. However, in recent years, the demand for improved fuel efficiency has led to the use of aluminum for wheels and thinner spokes, making the rust on disc rotors 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 fuel efficiency in automobiles, which necessitates the thinning and weight reduction of disc rotors. However, cast iron has low strength and is manufactured by casting, so there are limits to how thin it can be made. In addition, the temperature reached by disc rotors during braking can reach up to nearly 700°C. Moreover, under driving conditions that involve frequent braking, such as on mountain roads, the temperature reached by disc rotors can reach 300°C. Cast iron has low high-temperature strength, and when thinned, it cannot maintain the necessary strength for a disc rotor at high temperatures, making thinning and weight reduction difficult. Also, since cast iron is formed by casting, thinning the disc rotor can result in poor molten metal flow, sometimes making it impossible to form.

[0005] Stainless steel is a material with excellent corrosion resistance, and martensitic SUS410 series material is widely used in motorcycles and other two-wheeled vehicles. This is because the disc rotors of motorcycles are exposed and easily visible, making corrosion resistance important. However, stainless steel has lower thermal conductivity than cast iron. In motorcycles, the brake system is exposed and has excellent cooling properties, so stainless steel can be used without problems in normal use. However, even in motorcycles, under harsh braking conditions such as racing, the disc rotor can overheat, leading to increased brake pad wear. On the other hand, in automobiles, the brake system, including the tires, is housed in the wheel well, making it difficult to cool the disc rotor, and the low thermal conductivity has been one of the challenges, so stainless steel has not been applied. However, in recent years, the adoption of "regenerative braking," which converts kinetic energy during driving into electrical energy and recovers it, has been rapidly increasing in EVs, FCVs, and HV vehicles. As a result, the frictional heat generated by the friction between the disc rotor and pads is reduced, expanding the possibility of application to stainless steel, which has lower thermal conductivity than cast iron.

[0006] Another challenge hindering the application of stainless steel to automobile disc brakes has been its machinability (formability). Motorcycle disc rotors are ring-shaped and manufactured by punching out a sheet of stainless steel (stainless steel plate) and then high-frequency hardening, so no major processing is required. On the other hand, current automobile disc rotors have a hat shape, a shape where the center of the disc is narrowed, and are manufactured by casting. To process stainless steel plate into such a shape, press working (especially deep drawing) is required. However, the stainless steel plates used in motorcycles are martensitic stainless steel plates, which are extremely hard, making deep drawing difficult. One solution to this problem has become widespread in recent years: hot stamping, which involves press working at high temperatures. This has made it possible to process stainless steel plates into a hat shape with high precision. Against this backdrop, in order to meet the recent demands for aesthetics, thin-walled and lightweight designs, and machinability in automobiles, the use of stainless steel for disc rotors is becoming necessary.

[0007] As mentioned above, the processing of stainless steel sheets into disc rotors can be done by high-frequency induction hardening for motorcycles because there is no major processing involved, whereas for automobiles, it is done by hot stamping, which involves pressing at high temperatures. In other words, with conventional martensitic stainless steel, high-temperature heating was essential for processing into hat shapes and motorcycle disc rotors. While ferritic stainless steel can be processed into hat shapes without high-temperature heating, ferritic stainless steel is softer and deforms during braking, resulting in a less stable coefficient of friction than martensitic stainless steel, which is essential for safe braking. Thus, with single-phase martensitic or ferritic stainless steel, it is difficult to achieve both energy savings by omitting high-temperature heating (hardening) and the machinability of hat shapes and motorcycle disc rotors, as well as the stability of the coefficient of friction.

[0008] Patent Document 1 describes a technology for stainless steel disc rotors with excellent strength, wear resistance, toughness, and punching processability, with the following composition in mass percent: C: 0.080 to 0.120%, Cr: 16.0 to 18.0%, Si: 1.00% or less, Mn: 1.00% or less, Ni: 1.00 to 3.00%, Mo: 0 to 3.00%, Cu: 0 to 2.00%, Ti: 0 to 0.80%, Nb: 0 to 0.80%. A multiphase stainless steel sheet has been proposed having a chemical composition of %, Al: 0-0.200%, B: 0-0.010%, N: 0.020% or less, with the remainder being Fe and unavoidable impurities, a matrix (metal base material) consisting of a ferrite phase and a martensite phase, a metal structure with a ferrite phase abundance of 5-22 volume%, and a Vickers hardness HV30 of 350-450 HV. However, the stainless steel sheet in Patent Document 1 is intended for use in motorcycle disc rotors, not automobile disc rotors. In particular, as mentioned above, automobile disc rotors have a hat shape, and processability to this shape and stability of the coefficient of friction are required, whereas Patent Document 1 does not specifically mention processability or stability of the coefficient of friction.

[0009] Japanese Patent Publication No. 2018-135571

[0010] As described above, stainless steel sheets used as disc rotor material are required to have excellent machinability, and in particular, stainless steel sheets used as disc rotor material for automobiles are required to have excellent machinability into a hat shape. Furthermore, since disc rotors reach temperatures ranging from low to high under various driving conditions, it is necessary for them to be able to brake stably even when the temperature of the disc rotor rises due to braking. Therefore, from the perspective of safe braking, it is required that they exhibit a constant coefficient of friction even when the temperature changes.

[0011] This invention was made to solve the above-mentioned problems and aims to provide a stainless steel sheet with excellent machinability and friction coefficient stability, and a method for manufacturing the same. Furthermore, this invention aims to provide a brake disc rotor with excellent friction coefficient stability, and a method for manufacturing the same.

[0012] The inventors of the present invention have conducted diligent research to solve the above problems and have found the following (a) and (b), leading to the completion of the present invention. (a) A stainless steel sheet having a composition, a microstructure with a KAM value of 0.00 to 0.65°, an area ratio of a microstructure with a KAM value greater than 0.65° and 5.00° or less, and a Vickers hardness controlled within a predetermined range has properties suitable for brake disc rotors and their manufacture. (b) The above stainless steel sheet can be manufactured by heating a hot-rolled or cold-rolled material having a predetermined composition at a rate of 1,000°C / second or less, holding it at an annealing temperature of 700 to 900°C for 1.0 to 20.0 hours, and then cooling it at a rate of 1,000°C / second or less. That is, the present invention is as follows [1] to [9].

[0013] [1] Stainless steel sheet containing, by mass, C: 0.001 to 0.150%, N: 0.0010 to 0.3000%, Si: 0.01 to 3.00%, Mn: 0.010 to 5.000%, P: 0.001 to 0.100%, S: 0.0001 to 0.1000%, Cr: 15.0% to 25.0%, Ni: 0.010 to 5.000%, with the remainder being Fe and impurities, having an A value represented by the following formula (1) of 55 to 150, having a microstructure area ratio of 20.0 to 75.0% with a KAM value of 0.00 to 0.65°, a microstructure area ratio of 25.0 to 80.0% with a KAM value greater than 0.65° and 5.00° or less, and a Vickers hardness of 200 to 300 HV. A value = 420C + 470N + 23Ni + 7Mn - 11.5Cr - 11.5Si + 189 ... (1) In the formula, each element symbol represents the content (%) of each element.

[0014] [2] By mass, Nb: 0.0001 to 1.0000%, Ti: 0.0001 to 1.0000%, Cu: 0.001 to 2.000%, Mo: 0.001 to 2.000%, V: 0.001 to 1.000%, B: 0.0001 to 0.0300%, Al: 0.0010 to 4.0000%, W: 0.001 to 3.000%, Sn: 0.0001 to 1.0000%, Mg: 0.0001 to 0.0100%, Sb: 0. The stainless steel sheet according to [1], further containing one or more selected from 0.001 to 0.500%, Zr: 0.001 to 1.000%, Ta: 0.001 to 1.000%, Hf: 0.001 to 1.000%, Co: 0.0001 to 1.0000%, Ca: 0.0001 to 0.0200%, REM: 0.001 to 0.500%, Ga: 0.0001 to 0.5000%, and Bi: 0.0010 to 0.1000%.

[0015] [3] Large-angle grain boundary density is 0.10 μm / μm 2 The stainless steel plate described in [1] or [2] above.

[0016] [4] A stainless steel sheet according to any one of [1] to [3] that satisfies all of the following characteristics: (A) the elongation at break at room temperature is 13.0% or more, and (B) the average coefficient of friction at 60 to 300°C is 0.25 to 0.65.

[0017] [5] A stainless steel plate for use as a brake disc rotor, as described in any one of [1] to [4].

[0018] [6] A method for manufacturing stainless steel sheets, comprising heating a hot-rolled or cold-rolled sheet having a composition on a mass basis containing C: 0.001 to 0.150%, N: 0.0010 to 0.3000%, Si: 0.01 to 3.00%, Mn: 0.010 to 5.000%, P: 0.001 to 0.100%, S: 0.0001 to 0.1000%, Cr: 15.0% to 25.0%, Ni: 0.010 to 5.000%, with the remainder being Fe and impurities, and having an A value represented by the following formula (1) of 55 to 150, at a rate of 1,000°C / second or less, holding at an annealing temperature of 700 to 900°C for 1.0 to 20.0 hours, and then cooling at a rate of 1,000°C / second or less. A value = 420C + 470N + 23Ni + 7Mn - 11.5Cr - 11.5Si + 189 ... (1) In the formula, each element symbol represents the content (%) of each element.

[0019] [7] The hot-rolled or cold-rolled sheet is composed of, by mass, Nb: 0.0001 to 1.0000%, Ti: 0.0001 to 1.0000%, Cu: 0.001 to 2.000%, Mo: 0.001 to 2.000%, V: 0.001 to 1.000%, B: 0.0001 to 0.0300%, Al: 0.0010 to 4.0000%, W: 0.001 to 3.000%, Sn: 0.0001 to 1.0000%, Mg: 0.0001 to 0.0100%, S A method for manufacturing a stainless steel sheet according to [6], further comprising one or more selected from b: 0.001 to 0.500%, Zr: 0.001 to 1.000%, Ta: 0.001 to 1.000%, Hf: 0.001 to 1.000%, Co: 0.0001 to 1.0000%, Ca: 0.0001 to 0.0200%, REM: 0.001 to 0.500%, Ga: 0.0001 to 0.5000%, and Bi: 0.0010 to 0.1000%.

[0020] [8] A brake disc rotor comprising a processed stainless steel plate as described in any one of [1] to [5].

[0021] [9] A method for manufacturing a brake disc rotor, which involves processing a stainless steel plate as described in any one of [1] to [5].

[0022]

[10] The method for manufacturing a brake disc rotor according to [9], wherein the processing is carried out at room temperature.

[0023]

[11] A method for manufacturing a brake disc rotor according to [9] or

[10] , wherein no heat treatment is performed after the processing.

[0024] According to the present invention, it is possible to provide a stainless steel sheet with excellent machinability and stability of the coefficient of friction, and a method for manufacturing the same. Furthermore, according to the present invention, it is possible to provide a brake disc rotor with excellent stability of the coefficient of friction, and a method for manufacturing the same.

[0025] The embodiments of the present invention will be described in detail below. The present invention is not limited to the embodiments described below, and it should be understood that modifications, improvements, etc., made to the embodiments described below, based on the ordinary knowledge of those skilled in the art, without departing from the spirit of the invention, also fall within the scope of the present invention. In this specification, "%" in relation to components means "mass%" unless otherwise specified.

[0026] (1) Stainless steel sheet The stainless steel sheet according to the embodiment of the present invention contains C: 0.001 to 0.150%, N: 0.0010 to 0.3000%, Si: 0.01 to 3.00%, Mn: 0.010 to 5.000%, P: 0.001 to 0.100%, S: 0.0001 to 0.1000%, Cr: 15.0% to 25.0%, Ni: 0.010 to 5.000%, with the remainder being Fe and impurities, and has a composition in which the A value represented by the following formula (1) is 55 to 150. A value = 420C + 470N + 23Ni + 7Mn - 11.5Cr - 11.5Si + 189 ... (1) In the formula, each element symbol represents the content (%) of each element.

[0027] Herein, "stainless steel sheet" refers to a plate-shaped material formed from stainless steel, and the concept includes strip-shaped materials. Furthermore, "impurities" in this specification refers to components that are mixed in during the industrial manufacture of stainless steel sheets due to raw materials such as ore and scrap, and various factors in the manufacturing process, and which are acceptable as long as they do not adversely affect the present invention. For example, impurities include unavoidable impurities. Examples of impurities include O, As, and Pb. The impurities may include O at 0.020% or less, As at 0.1% or less, and Pb at 0.0100% or less.

[0028] Furthermore, the stainless steel sheet according to the embodiment of the present invention may optionally contain: Nb: 0.0001 to 1.0000%, Ti: 0.0001 to 1.0000%, Cu: 0.001 to 2.000%, Mo: 0.001 to 2.000%, V: 0.001 to 1.000%, B: 0.0001 to 0.0300%, Al: 0.0010 to 4.0000%, W: 0.001 to 3.000%, Sn: 0.0001 to 1.0000%, Mg: 0.00 It may further contain one or more elements selected from the following: 0.01-0.0100%, Sb: 0.001-0.500%, Zr: 0.001-1.000%, Ta: 0.001-1.000%, Hf: 0.001-1.000%, Co: 0.0001-1.0000%, Ca: 0.0001-0.0200%, REM: 0.001-0.500%, Ga: 0.0001-0.5000%, and Bi: 0.0010-0.1000%.

[0029] The following provides a detailed explanation of each component.

[0030] <C: 0.001-0.150%> C is an element that dissolves in the matrix and greatly affects hardness. Furthermore, C can form carbides during heat treatment, reducing workability and corrosion resistance, and leading to a decrease in high-temperature strength. Therefore, the C content should be within (A). Also, excessively reducing C leads to increased refining costs, so the C content should preferably be within (B), and the C content should be even more preferable. (A) = 0.001-0.150% (B) = 0.030-0.120% (C) = 0.030-0.090%

[0031] <N: 0.0010 to 0.3000%> Like C, N is an element that dissolves in the matrix and greatly affects hardness. Furthermore, N can form nitrides during heat treatment, reducing workability and corrosion resistance, and leading to a decrease in high-temperature strength. Therefore, the N content should be within (A). Also, excessively reducing N leads to increased refining costs, so the N content should preferably be within (B), and even more preferably within (C). (A) = 0.0010 to 0.3000% (B) = 0.0020 to 0.0400% (C) = 0.0040 to 0.0200%

[0032] <Si: 0.01-3.00%> Si is a useful element as a deoxidizing agent, and also improves oxidation resistance and resistance to high-temperature salt damage. However, excessive addition of Si reduces room-temperature ductility, so the Si content should be (A). However, considering pickling resistance and toughness, the Si content of (B) is preferable, and considering manufacturability, the Si content of (C) is even more preferable. (A) = 0.01-3.00% (B) = 0.10-1.00% (C) = 0.30-0.70%

[0033] <Mn: 0.010 to 5.000%> Mn is an element added as a deoxidizing agent and contributes to improving high-temperature strength in the medium temperature range. However, excessive addition of Mn can lead to the formation of Mn-based oxides on the surface at high temperatures, making it prone to 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 with respect to the Mn content, so the Mn content should be (A). Furthermore, considering pickling properties and room-temperature ductility in steel sheet manufacturing, the Mn content of (B) is preferable, and the content of (C) is more preferable. (A) = 0.010 to 5.000% (B) = 0.100 to 2.000% (C) = 0.200 to 0.500%

[0034] <P: 0.001-0.100%> P is an impurity that is mainly introduced from the raw materials during steelmaking and refining. When the P content is high, toughness and weldability decrease. For this reason, it is desirable to reduce P as much as possible, but excessive reduction of P will result in increased costs due to the use of raw materials with low P content. On the other hand, when the P content increases, it hardens significantly and corrosion resistance, toughness, and pickling properties deteriorate, so the P content should be (A). Considering the raw material cost, the P content of (B) is preferable, and the P content of (C) is more preferable. (A) = 0.001-0.100% (B) = 0.008-0.080% (C) = 0.010-0.050%

[0035] <S: 0.0001 - 0.1000%> S is an element that deteriorates corrosion resistance and oxidation resistance. However, it not only combines with Ti and C to improve workability, but also combines with Cr, Mn, etc. to form sulfides and exhibit lubricity. On the other hand, if S is added excessively, it combines with Ti and C to reduce the amount of dissolved Ti and cause coarsening of precipitates, resulting in a decrease in high-temperature strength. Therefore, S is set to the content of (A). Furthermore, considering refining cost and high-temperature oxidation characteristics, the content of S is preferably that of (B), and more preferably that of (C). (A) = 0.0001 - 0.1000% (B) = 0.0001 - 0.0200% (C) = 0.0001 - 0.0100%

[0036] <Cr: 15.0% - 25.0%> Cr is an essential element for ensuring oxidation resistance and corrosion resistance. When the content of Cr is low, oxidation resistance cannot be ensured especially. When the content of Cr is high, it causes a decrease in workability and deterioration of toughness. Therefore, Cr is set to the content of (A). Also, considering manufacturability and scale peelability, the content of Cr is preferably that of (B), and more preferably that of (C). (A) = 15.0% - 25.0% (B) = 15.0% - 19.0% (C) = 15.5 - 17.0%

[0037] <Ni: 0.010 - 5.000%> Ni is an element that improves oxidation resistance, toughness and high-temperature strength. Since excessive addition of Ni increases cost, Ni is set to the content of (A). Considering manufacturability, the content of Ni is preferably that of (B), and more preferably that of (C). (A) = 0.010 - 5.000% (B) = 1.000 - 3.000% (C) = 1.800 - 2.400%

[0038] <A value: 55 to 150> The A value is represented by the following formula (1) and serves as an index for hardness and workability.A value = 420C + 470N + 23Ni + 7Mn - 11.5Cr - 11.5Si + 189... (1) In the formula, each element symbol represents the content (%) of each element. By controlling the A value to 55 or more, sufficient hardness can be obtained for use as a brake disk rotor. By setting the A value to 150 or less, excessive hardness is suppressed, and it becomes possible to process a hat shape or a disk rotor for a two-wheeler. Considering workability, the A value is more preferably 60 to 100, and most preferably 60 to 95.

[0039] [Optional elements] The stainless steel sheet according to an embodiment of the present invention may further contain one or more selected from Nb, Ti, Cu, Mo, V, B, Al, W, Sn, Mg, Sb, Zr, Ta, Hf, Co, Ca, REM, Ga, Bi in place of a part of Fe. All of these elements are optional elements and may not be contained.

[0040] <Nb: 0 to 1.0000%> Nb is an element effective for improving the tempering softening resistance and high-temperature strength by solid solution strengthening and precipitation strengthening of fine precipitates, and stabilizes the friction coefficient by suppressing tissue changes, but is not a particularly essential element, and the Nb content may be 0%. In addition, Nb also has a role of fixing C and N as carbonitrides and contributing to the development of a recrystallized grain structure that affects the corrosion resistance and r value of the stainless steel sheet. However, excessive addition of Nb not only significantly hardens the stainless steel sheet but also reduces productivity, so the upper limit value of the Nb content is controlled to 1.0000%. That is, Nb can be the content of (A). Also, considering raw material costs and toughness, the content of Nb is preferably the content of (B), more preferably the content of (C), and still more preferably the content of (D).(A) = 0.0001 to 1.0000% (B) = 0.0001 to 0.5000% (C) = 0.0001 to 0.1000% (D) = 0.0001 to 0.0200%

[0041] <Ti: 0-1.0000%> Ti is an element that combines with C, N, and S to improve corrosion resistance, intergranular corrosion resistance, room temperature ductility, and deep drawability, but it is not an essential element, and the Ti content may be 0%. When added in combination with Mo, adding an appropriate amount increases the amount of Mo dissolved during hot rolling and annealing, improves high-temperature strength, improves tempering softening resistance and thermal fatigue characteristics, and stabilizes the coefficient of friction by suppressing structural changes. However, excessive addition of Ti increases the amount of dissolved Ti, which reduces room temperature ductility, and forms coarse Ti precipitates, which become the starting point for cracks during hole enlargement and reduce press workability. Excessive addition of Ti also reduces oxidation resistance. Therefore, the upper limit of the Ti content is controlled to 1.0000%. That is, the Ti content can be as shown in (A). Furthermore, considering the occurrence of surface defects and toughness, the Ti content of (B) is preferable, and the content of (C) is more preferable. (A) = 0.0001 to 1.0000% (B) = 0.0001 to 0.5000% (C) = 0.0001 to 0.0300%

[0042] <Cu: 0-2.000%> Cu is an effective element for improving corrosion resistance, but it is not an essential element, and the Cu content may be 0%. In addition, Cu improves tempering softening resistance, high-temperature strength, and friction coefficient stability through precipitation strengthening by ε-Cu precipitation. Since excessive addition of Cu reduces hot workability, the upper limit of the Cu content is controlled to 2.000%. That is, the Cu content can be (A). Furthermore, considering thermal fatigue characteristics, manufacturability, and weldability, the Cu content of (B) is preferable, and the content of (C) is more preferable. (A) = 0.001-2.000% (B) = 0.010-1.000% (C) = 0.010-0.450%

[0043] <Mo: 0-2.000%> Mo is an element effective for solid solution strengthening at high temperatures, improving tempering softening resistance, friction coefficient stability, corrosion resistance, and high-temperature salt damage resistance. However, it is not an essential element, and the Mo content may be 0%. Furthermore, excessive addition of Mo significantly reduces room-temperature ductility and oxidation resistance, so the upper limit of the Mo content is controlled to 2.000%. That is, the Mo content can be (A). In addition, considering thermal fatigue characteristics and manufacturability, the Mo content of (B) is preferable, and the content of (C) is more preferable. (A) = 0.001-2.000% (B) = 0.001-1.000% (C) = 0.001-0.250%

[0044] <V: 0-1.000%> V is an element that improves corrosion resistance, but it is not an essential element, and the V content may be 0%. Adding an excess of V causes the precipitates to coarseen, reducing the stability of tempering softening resistance, high-temperature strength, and friction coefficient, as well as oxidation resistance. Therefore, the upper limit of the V content is controlled to 1.000%. That is, the V content can be as shown in (A). Furthermore, considering manufacturing costs and manufacturability, the V content is preferably as shown in (B), and more preferably as shown in (C). (A) = 0.001-1.000% (B) = 0.001-0.500% (C) = 0.001-0.150%

[0045] <B: 0-0.0300%> B is an element that improves secondary workability during press forming, high-temperature strength, and thermal fatigue characteristics, but it is not an essential element, and the B content may be 0%. In addition, B brings about fine precipitation such as the Laves phase, and these precipitation strengthenings exhibit long-term stability, contributing to the suppression of strength reduction and improvement of thermal fatigue life. On the other hand, excessive addition of B leads to hardening, reduces intergranular corrosion resistance and oxidation resistance, and causes welding cracks, so the upper limit of the B content is controlled to 0.0300%. That is, the B content can be the same as (A). Also, considering corrosion resistance and manufacturing costs, the B content of (B) is preferable, and the B content of (C) is more preferable. (A) = 0.0001-0.0300% (B) = 0.0001-0.0200% (C) = 0.0001-0.0090%

[0046] <Al: 0-4.0000%> Al is added as a deoxidizing element and also improves oxidation resistance, but it is not an essential element, and the Al content may be 0%. Al is also useful as a solid solution strengthening element for improving high-temperature strength and temper softening resistance. On the other hand, excessive addition of Al hardens the material, significantly reducing uniform elongation and toughness; therefore, the upper limit of the Al content is controlled to 4.0000%. That is, the Al content can be (A). Furthermore, considering surface defect occurrence, weldability, and manufacturability, the content of (B) is preferable, and the content of (C) is more preferable. (A) = 0.0010-4.0000% (B) = 0.0010-0.1000% (C) = 0.0010-0.0500%

[0047] <W: 0-3.000%> Like Mo, W is an effective element for solid solution strengthening at high temperatures, forming a Laves phase (Fe2W) and providing precipitation strengthening. However, it is not an essential element, and the W content can be 0%. In particular, when added in combination with Mo, a Laves phase of Fe2(Mo,W) precipitates, but the addition of W suppresses the coarsening of this Laves phase, improving precipitation strengthening ability and tempering softening resistance. On the other hand, excessive addition of W increases costs and reduces room-temperature ductility, so the upper limit of the W content is controlled to 3.000%. That is, the W content can be (A). Furthermore, considering manufacturability, low-temperature toughness, and oxidation resistance, the W content (B) is preferable. (A) = 0.001-3.000% (B) = 0.001-1.500%

[0048] <Sn: 0-1.0000%> Sn is an element that improves corrosion resistance and enhances high-temperature strength in the medium temperature range, but it is not an essential element, and the Sn content may be 0%. On the other hand, if the Sn content is excessively high, manufacturability and toughness will decrease significantly, so the upper limit of the Sn content is controlled to 1.0000%. That is, the Sn content can be (A). Furthermore, considering oxidation resistance and manufacturing costs, the Sn content of (B) is preferable, and the content of (C) is more preferable. (A) = 0.0001-1.0000% (B) = 0.0001-0.1000% (C) = 0.0001-0.0300%

[0049] <Mg: 0-0.0100%> Mg functions as a deoxidizing element and also refines the slab structure, contributing to improved workability. However, it is not an essential element, and the Mg content may be 0%. In addition, the Mg oxide produced by the addition of Mg becomes a precipitation site for carbonitrides such as Ti(C,N), which finely disperse and precipitate these, contributing to improved toughness. However, excessive addition of Mg leads to a decrease in weldability, corrosion resistance, and surface quality, so the upper limit of the Mg content is controlled to 0.0100%. That is, the Mg content can be (A). Considering the refining cost, the Mg content (B) is preferable. (A) = 0.0001-0.0100% (B) = 0.0003-0.0010%

[0050] <Sb: 0-0.500%> Sb contributes to improved corrosion resistance and high-temperature strength, but it is not an essential element, and the Sb content may be 0%. However, excessive addition of Sb may cause slab cracking and reduced ductility during the manufacturing of stainless steel sheets, so the upper limit of the Sb content is controlled to 0.500%. That is, the Sb content can be (A). Also, considering refining costs and manufacturability, the Sb content (B) is preferable. (A) = 0.001-0.500% (B) = 0.010-0.300%

[0051] <Zr: 0-1.000%> Zr, like Ti, is a carbonitride-forming element and improves corrosion resistance and deep drawability. However, it is not an essential element, and the Zr content may be 0%. On the other hand, if the Zr content is excessively high, the manufacturability will decrease significantly, so the upper limit of the Zr content is controlled to 1.000%. That is, the Zr content can be (A). Also, considering cost and surface quality, the Zr content (B) is preferable. (A) = 0.001-1.000% (B) = 0.001-0.200%

[0052] <Ta: 0-1.000% and Hf: 0-1.000%> Ta and Hf are elements that contribute to improving toughness by bonding with C and N, but they are not particularly essential elements, and the Ta and Hf content may both be 0%. However, if the content of Ta and Hf becomes excessively high, it will increase costs and significantly reduce manufacturability, so the upper limits for the Ta and Hf content are controlled to 1.000% each. That is, the Ta and Hf content can be (A) each. Furthermore, considering refining costs and manufacturability, the Ta and Hf content (B) each is preferable. (A) = 0.001-1.000% (B) = 0.010-0.080%

[0053] <Co: 0-1.0000%> Co is an element that contributes to improving high-temperature strength, but it is not an essential element, and the Co content may be 0%. However, if the Co content is excessively high, it will lead to a decrease in toughness, so the upper limit of the Co content is controlled to 1.0000%. That is, the Co content can be (A). Furthermore, considering refining costs and manufacturability, the Co content of (B) is preferable, and the content of (C) is more preferable. (A) = 0.0001-1.0000% (B) = 0.0005-0.2000% (C) = 0.0010-0.1000%

[0054] <Ca: 0-0.0200%> Ca is an element added for desulfurization, but it is not an essential element, and the Ca content may be 0%. However, if the Ca content is excessively high, coarse CaS is formed, which reduces toughness and corrosion resistance, so the upper limit of the Ca content is controlled to 0.0200%. That is, the Ca content can be (A). Also, considering refining costs and manufacturability, the Ca content (B) is preferable. (A) = 0.0001-0.0200% (B) = 0.0003-0.0020%

[0055] <REM: 0-0.500%> REM (rare earth elements) are elements that contribute to improving toughness and oxidation resistance by refining various precipitates, but they are not particularly essential elements, and the REM content may be 0%. However, if the REM content is excessively high, castability will decrease significantly, as will ductility, so the upper limit of the REM content is controlled to 0.500%. That is, the REM content can be (A). Also, considering refining costs and manufacturability, the REM content (B) is preferable. Note that REM, according to the general definition, refers to the two elements scandium (Sc) and yttrium (Y), and the 15 elements from lanthanum (La) to lutetium (Lu) (lanthanoids). These elements may be added individually or as a mixture of two or more. (A) = 0.001-0.500% (B) = 0.001-0.050%

[0056] <Ga: 0 to 0.5000%> Ga is an element that contributes to improved corrosion resistance and suppression of hydrogen embrittlement, but it is not an essential element, and the Ga content may be 0%. However, if the Ga content is excessively high, manufacturability decreases and costs increase, so the upper limit of the Ga content is controlled to 0.5000%. That is, the Ga content can be (A). Also, considering ductility and toughness, the Ga content (B) is preferable. (A) = 0.0001 to 0.5000% (B) = 0.0001 to 0.0020%

[0057] <Bi: 0 to 0.1000%> Bi is an element that suppresses roping that occurs during cold rolling and improves manufacturability, but it is not an essential element, and the Bi content may be 0%. However, if the Bi content is excessively high, it will lead to a decrease in hot workability, so the upper limit of the Bi content is controlled to 0.1000%. That is, the Bi content can be (A). Furthermore, from the viewpoint of stably obtaining the above effect, the Bi content of (B) is preferable. (A) = 0.0010 to 0.1000% (B) = 0.0012 to 0.0800%

[0058] In addition to the above composition, the stainless steel sheet according to the embodiment of the present invention has the following characteristics: the area ratio of microstructures with a KAM value of 0.00 to 0.65° is 20.0 to 75.0%, the area ratio of microstructures with a KAM value greater than 0.65° and 5.00° or less is 25.0 to 80.0%, and the Vickers hardness is 200 to 300 HV. These characteristics will be described in detail.

[0059] <The area ratio of structures with a KAM value of 0.00 to 0.65° is 20.0 to 75.0%, and the area ratio of structures with a KAM value greater than 0.65° and less than or equal to 5.00° is 25.0 to 80.0%> The microstructure of stainless steel sheets is a two-phase structure consisting of a structure with a KAM value of 0.00 to 0.65° (ferrite phase) and a structure with a KAM value greater than 0.65° and less than or equal to 5.00° (martensite phase). In addition, the remainder of the microstructure of stainless steel sheets other than each phase consists of inclusions and precipitates. In the microstructure of stainless steel sheets, the area ratio of structures with a KAM value of 0.00 to 0.65° and structures with a KAM value greater than 0.65° and less than or equal to 5.00° is related to workability and the stability of the coefficient of friction. By setting the area ratio of the microstructure with a KAM value of 0.00 to 0.65° to 20.0 to 75.0%, processability is improved, and in particular, it becomes possible to form hat shapes and motorcycle disc rotors even at room temperature. On the other hand, if the area ratio of the microstructure with a KAM value of 0.00 to 0.65° is less than 20.0%, the stainless steel sheet becomes excessively hard, making it difficult to process into brake disc rotors without heating or heat treatment. Also, if the area ratio of the microstructure with a KAM value of 0.00 to 0.65° exceeds 75.0%, the stainless steel sheet becomes excessively soft, impairing the stability of the coefficient of friction. From the viewpoint of improving the stability of the coefficient of friction, the area ratio of the microstructure with a KAM value of 0.00 to 0.65° is preferably 22.0 to 73.0%, and more preferably 25.0 to 70.0%. Furthermore, by setting the area ratio of structures with a KAM value greater than 0.65° and less than or equal to 5.00° to 25.0% to 80.0%, the stability of the friction coefficient suitable for use as a brake disc rotor can be ensured. On the other hand, if the area ratio of structures with a KAM value greater than 0.65° and less than or equal to 5.00° exceeds 80.0%, the stainless steel sheet hardens, impairing its workability. Also, if the area ratio of structures with a KAM value greater than 0.65° and less than or equal to 5.00° falls below 25.0%, the stainless steel sheet becomes excessively soft, impairing the stability of the friction coefficient. From the viewpoint of improving workability for complex shapes, the area ratio of structures with a KAM value greater than 0.65° and less than or equal to 5.00° is preferably 27.0% to 78.0%, and more preferably 30.0% to 75.0%.

[0060] In this specification, the area ratios of microstructures with KAM values ​​between 0.00 and 0.65° and microstructures with KAM values ​​greater than 0.65° and less than or equal to 5.00° are determined by the following method. The thickness-direction cross section of a stainless steel sheet perpendicular to the rolling direction is polished to a mirror finish, and EBSD measurements are performed on the t / 4 portion of this cross section (1 / 4 of the thickness t of the stainless steel sheet). This measurement is performed using a scanning electron microscope JSM-7200F manufactured by JEOL Ltd., observing an image in a region of approximately 40 × 125 μm (magnification 2000x) in 0.25 μm measurement steps, and performing analysis using the analysis software "OIM" manufactured by TSL Solutions Co., Ltd. From the KAM values ​​at each measurement point, the area ratios of microstructures with KAM values ​​between 0.00 and 0.65° and microstructures with KAM values ​​greater than 0.65° and less than or equal to 5.00° are calculated. The area ratio of each microstructure is calculated based on the entire observation field, and the observation field is selected to include at least 50 crystal grains.

[0061] The sum of the area percentages of tissues with a KAM value of 0.00 to 0.65° and the area percentages of tissues with a KAM value greater than 0.65° and 5.00° or less is not particularly limited, but is preferably 98.0% or more, more preferably 98.5% or more. By controlling the sum of the area percentages of the tissues within this range, it becomes easier to ensure the stability of processability and friction coefficient. The upper limit of the sum of the area percentages of the tissues is not particularly limited and may be 100%.

[0062] <Vickers hardness: 200-300 HV> The Vickers hardness of a stainless steel sheet is related to its properties when used as a brake disc rotor. If the Vickers hardness is less than 200 HV, excessive deformation occurs when used as a brake disc rotor, reducing the stability of the coefficient of friction and lowering wear resistance. If the Vickers hardness exceeds 300 HV, it becomes difficult to straighten the stainless steel sheet at room temperature. For these reasons, the Vickers hardness should be between 200 and 300 HV. From the viewpoint of reducing brake noise, a Vickers hardness of 210-290 HV is preferred, and 220-280 HV is more preferred. In this specification, the Vickers hardness is determined by the following method. After polishing the thickness-direction cross section of a stainless steel sheet perpendicular to the rolling direction to a mirror finish, the Vickers hardness is measured at the center of the thickness direction (t / 2) under the conditions of a load of 5 kg and a number of measurement samples (n) = 5, in accordance with JIS Z2244-1:2024, and the average value of these measurements is taken as the Vickers hardness.

[0063] In addition to the above-mentioned features, the stainless steel sheet according to the embodiment of the present invention may have the following features.

[0064] <Large angle grain boundary density: 0.10μm / μm 2 The stainless steel sheet according to the embodiment of the present invention has a large-angle grain boundary density of 0.10 μm / μm 2 The above is preferable. A higher large-angle grain boundary density makes it easier for the structure directly beneath the sliding surface of the brake disc to be refined by the sliding motion between the brake disc and the brake pad, thereby improving wear resistance. Large-angle grain boundary density of 0.10 μm / μm 2 If the density is less than 0.15 μm / μm, excessive deformation will occur when used as a brake disc rotor, reducing the stability of the coefficient of friction and lowering wear resistance. From the viewpoint of wear resistance, the large-angle grain boundary density should be 0.15 μm / μm. 2 The above is preferable, and 0.20 μm / μm 2 The above is more preferable, 1.00 μm / μm 2 The above is even more preferable. The upper limit of the large-angle grain boundary density is not particularly limited, but for example, 3.00 μm / μm 2 That is the case.

[0065] In this specification, the large-angle grain boundary density is determined by the following method. The thickness-direction cross section of the stainless steel sheet perpendicular to the rolling direction is polished to a mirror finish, and EBSD measurement is performed on the t / 4 portion of this cross section (1 / 4 of the thickness t of the stainless steel sheet). This measurement is performed using a scanning electron microscope JSM-7200F manufactured by JEOL Ltd., observing an image in a region of approximately 40 × 125 μm (magnification 2000x) with a measurement step of 0.25 μm, and performing analysis using the analysis software "OIM" manufactured by TSL Solutions Co., Ltd. Measurement points where the crystal orientation difference between adjacent measurement points is 15 to 65° (15 ≤ X ≤ 65) are defined as large-angle grain boundaries, and the grain boundary length is measured. The large-angle grain boundary density is calculated by dividing the grain boundary length by the EBSD measurement area. The observation field is selected to include at least 50 crystal grains.

[0066] <Elongation at break at room temperature: 13.0% or more> In the embodiment of the present invention, the stainless steel sheet preferably has an elongation at break of 13.0% or more at room temperature (25°C). If the elongation at break of the stainless steel sheet at room temperature is 13.0% or more, it can be said that it has excellent workability at room temperature (in particular, it can be formed into hat shapes and motorcycle disc rotors). In particular, the processing of conventional martensitic stainless steel sheets into hat-shaped brake disc rotors is carried out by hot stamping, which is performed by pressing at high temperatures, and motorcycle disc rotors are also manufactured by high-frequency induction hardening, so energy saving by omitting heating during processing (i.e., workability at room temperature) is an issue. Since the stainless steel sheet according to the embodiment of the present invention has excellent workability at room temperature, energy saving can be achieved by omitting heating during processing. From the viewpoint of stably improving formability at room temperature, the elongation at break at room temperature is preferably 15.0% or more, and more preferably 17.0% or more. The upper limit of the elongation at break at room temperature is not particularly limited, but for example it is 35.0%. Hereinafter, in this specification, the elongation at break at room temperature is determined by the following method. Tensile test specimens are taken from stainless steel sheets so that the rolling direction is the tensile direction, and tensile tests are performed at room temperature (25°C) in accordance with JIS Z2241:2011, and the elongation at break is measured.

[0067] <Average coefficient of friction at 60 to 300°C: 0.25 to 0.65> The stainless steel sheet according to an embodiment of the present invention preferably has an average coefficient of friction of 0.25 to 0.65 at a disk temperature of 60 to 300°C in a friction and wear test conforming to JASO C406:2000. By controlling the average coefficient of friction within such a range, excellent coefficient of friction stability as a brake disk rotor can be achieved. Here, in the friction and wear test, the average coefficient of friction when braking from 130 km / h in the normal temperature effectiveness test of the second effectiveness test is determined. In the second effectiveness test, braking is performed at various deceleration rates, and thus the disk temperature changes depending on the magnitude of the deceleration. When braking is performed at various deceleration rates (specifically, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0 m / s 2 ), the disk temperature generally falls within the range of 60 to 300°C. Therefore, the coefficient of friction stability can be evaluated by calculating the average value of the coefficient of friction during one braking at each deceleration rate and then calculating those average coefficients of friction. Also, examples of the pad (counter material) in the friction and wear test include non-asbestos organic (NAO) materials (non-steel (NS), low steel (LS), semi-metallic (SM)), Cu sintered materials, and the like. When taking a test piece from the processed stainless steel sheet for parts such as a brake disk, the disk (disk-shaped test piece) and the counter material may have any shape and dimensions as long as tests conforming to JASO C406:2000 can be carried out. The moment of inertia is 0.25 kg·m 2 shall be used. From the perspective of coping with various usage environments, the average coefficient of friction of the stainless steel sheet at 60 to 300°C is more preferably 0.26 to 0.64, and even more preferably 0.27 to 0.63.

[0068] <Disc wear amount: 0.50 mm or less> In the stainless steel plate according to the embodiment of the present invention, it is preferable that the disc wear amount is 0.50 mm or less in a friction wear test in accordance with JASO C406:2000. By controlling the disc wear amount within this range, excellent wear resistance can be achieved as a brake disc rotor. Here, the same pad (mating material) as described above can be used for the friction wear test. When taking test pieces from a stainless steel plate after processing into parts such as brake discs, the disc (disc-shaped test piece) and mating material only need to have a shape and dimensions that allow for testing in accordance with JASO C406:2000. Moment of inertia is 0.25 kg・m 2 From the viewpoint of disk lifespan, the disk wear amount is more preferably 0.40 mm or less, and even more preferably 0.30 mm or less.

[0069] (2) Method for Manufacturing Stainless Steel Sheets The method for manufacturing stainless steel sheets according to the embodiments of the present invention is not particularly limited as long as it is possible to manufacture stainless steel sheets having the above-described characteristics. For example, stainless steel sheets according to the embodiments of the present invention can be manufactured by heating a hot-rolled or cold-rolled material having the above-described composition at a rate of 1,000°C / second or less, holding it at an annealing temperature of 700 to 900°C for 1.0 to 20.0 hours, and then cooling it at a rate of 1,000°C / second or less. Therefore, the stainless steel sheets obtained by this manufacturing method are hot-rolled and annealed sheets or cold-rolled and annealed sheets. The characteristics of this manufacturing method will be described in detail.

[0070] When the stainless steel sheet is a hot-rolled and annealed sheet, the manufacturing method is carried out in the following order: steelmaking process, hot rolling process, annealing process, and pickling process. When the stainless steel sheet is a cold-rolled and annealed sheet, the manufacturing method is carried out in the following order: steelmaking process, hot rolling process, annealing process, pickling process, cold rolling process, annealing process, and pickling process. In the steelmaking process, a method is preferably used in which steel containing the above components is melted in a converter, followed by secondary refining. The molten steel is formed into slabs according to a known casting method (continuous casting). The slabs 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 slabs are rolled in a hot rolling mill consisting of multiple stands and then wound into coils. The wound hot-rolled coils are annealed in an annealing furnace under predetermined conditions, and then pickled to obtain a hot-rolled and annealed sheet (annealing process and pickling process). The pickling process can be carried out using existing pickling methods. The pickling process may be omitted depending on the application. The annealing process is carried out under the following conditions.

[0071] <Heating rate: 1,000°C / second or less> The heating rate is calculated by measuring the temperature change of the hot-rolled or cold-rolled material from room temperature to the annealing temperature using a thermocouple and dividing it by the time required for the heating. In other words, the heating rate in the annealing process means the average heating rate in the temperature range from room temperature to the annealing temperature. If the heating rate is too fast, the recovery of the strain introduced during hot rolling is suppressed during heating, and the recrystallization of the microstructure with a KAM value of 0.00 to 0.65° does not occur sufficiently, resulting in hardening. Therefore, it is not possible to control the Vickers hardness to 200 to 300 HV. Accordingly, in order to promote the recovery of strain, the heating rate to the annealing temperature should be 1,000°C / second or less. The heating rate can be controlled by, for example, adjusting the furnace temperature and the sheet passing speed, or by preheating. The heating rate is preferably 0.500°C / second or less, more preferably 0.100°C / second or less, and even more preferably 0.020°C / second or less.

[0072] <Annealing temperature: 700-900°C, and holding time: 1.0-20.0 hours> By annealing hot-rolled or cold-rolled material at a temperature of 700-900°C and holding time of 1.0-20.0 hours, the area ratio of microstructures with a KAM value of 0.00-0.65° in the stainless steel sheet is 20.0-75.0%, the area ratio of microstructures with a KAM value greater than 0.65° and 5.00° or less is 25.0-80.0%, and the large-angle grain boundary density is 0.10 μm / μm 2 The above can be controlled. If the annealing temperature is less than 700°C and the annealing time is less than 1.0 hour, the austenite phase is not sufficiently formed during annealing, so the area ratio of the microstructure with a KAM value of 0.00 to 0.65° becomes too high. Also, since carbides are not sufficiently formed, recrystallization of the microstructure with a KAM value of 0.00 to 0.65° is difficult to occur, resulting in excessive hardening. If the annealing temperature is greater than 900°C and the annealing time is greater than 20.0 hours, the microstructure with a KAM value greater than 0.65° and less than or equal to 5.00° becomes excessively hardened, and the grain size becomes excessively coarse, reducing the large-angle grain boundary density, so the desired properties cannot be obtained. From the viewpoint of obtaining stable hardness, the annealing temperature is preferably 700 to 890°C, more preferably 705 to 880°C, and even more preferably 705 to 875°C. From the viewpoint of obtaining stable hardness, the annealing time is preferably 1.0 to 18.0 hours, more preferably 1.0 to 15.0 hours, and even more preferably 1.0 to 5.0 hours. Note that the above annealing conditions refer to the annealing conditions after hot rolling when the stainless steel sheet is a hot-rolled annealed sheet. Also, the above annealing conditions refer to the annealing conditions after cold rolling when the stainless steel sheet is a cold-rolled annealed sheet, and the annealing conditions after hot rolling are not particularly limited.

[0073] <Cooling rate: 1,000°C / second or less> The cooling rate is calculated by measuring the temperature change of the annealed material from the annealing temperature to 350°C using a thermocouple and dividing it by the time required for cooling. In other words, the cooling rate is the average cooling rate in the temperature range from the annealing temperature to 350°C. If the cooling rate is too fast, the self-tempering of the microstructure with a KAM value greater than 0.65° and less than or equal to 5.00° that has transformed from the austenite phase during cooling is suppressed and hardened, making it impossible to control the Vickers hardness to 200-300 HV. To promote self-tempering, the cooling rate after annealing should be 1,000°C / second or less. The cooling rate can be controlled, for example, by adjusting the furnace temperature or by air cooling. From the viewpoint of productivity, the cooling rate is preferably 0,500°C / second or less, more preferably 0,100°C / second or less, and even more preferably 0,020°C / second or less.

[0074] When manufacturing cold-rolled annealed sheets, hot-rolled annealed sheets are cold-rolled (cold rolling process). Cold rolling can be performed using either a tandem rolling mill or a Zenzimir rolling mill. After rolling, the wound cold-rolled coils are annealed in an annealing furnace under predetermined conditions, and then pickled to obtain cold-rolled annealed sheets (annealing process and pickling process). The pickling process can be carried out using existing pickling methods. The annealing process is performed under the conditions described above.

[0075] The stainless steel sheet according to the embodiment of the present invention has excellent machinability and friction coefficient stability because its composition, the area ratio of microstructures with a KAM value of 0.00 to 0.65° and microstructures with a KAM value greater than 0.65° and 5.00° or less, and its Vickers hardness are controlled within a predetermined range. In particular, this stainless steel sheet can be processed into hat shapes and brake disc rotors for motorcycles at room temperature, thus saving energy compared to processing by hot stamping or high-frequency induction hardening.

[0076] The stainless steel sheet according to the embodiment of the present invention has excellent machinability and friction coefficient stability, and can therefore be used in applications where these characteristics are required. In particular, this stainless steel sheet is ideal for use in brake disc rotors. As a brake disc rotor, it can be used in brake disc rotors of various vehicles such as automobiles, motorcycles, tricycles, and snowmobiles, but it is particularly suitable for use in brake disc rotors of automobiles and motorcycles.

[0077] (3) Brake disc rotor The brake disc rotor according to the embodiment of the present invention comprises the above-mentioned processed stainless steel plate. Here, the processed stainless steel plate means a part obtained by processing a stainless steel plate into the shape of a brake disc rotor. Therefore, for example, if the brake disc rotor is a brake disc rotor for an automobile, it means a part obtained by processing a stainless steel plate into a hat shape. The shape of the brake disc rotor is not limited to a hat shape and may be appropriately determined according to the type of vehicle in which the brake disc rotor is used.

[0078] The brake disc rotor according to the embodiment of the present invention is made of stainless steel sheet, and therefore has good corrosion resistance and is less prone to red rust (i.e., good aesthetics), and can be made thin and lightweight. Furthermore, this brake disc rotor uses stainless steel sheet, which has excellent workability and friction coefficient stability, as a material, and can be manufactured by press working at room temperature, and heat treatment after press working can be omitted, thus reducing manufacturing costs and providing good braking performance.

[0079] (4) Method for manufacturing a brake disc rotor The brake disc rotor according to the embodiment of the present invention can be manufactured by processing the stainless steel plate described above. The processing is not particularly limited and includes straightening, deep drawing, punching, and other press working.

[0080] The processing is preferably carried out at room temperature (for example, -10 to 50°C). Other processing conditions are not particularly limited and can be adjusted as appropriate depending on the size of the brake disc rotor to be manufactured and the type of stainless steel plate used.

[0081] Heat treatment is not required after processing. This is because the stainless steel sheet already possesses the optimal properties for use in brake disc rotors. Therefore, it can be used as a brake disc rotor immediately after processing, thus reducing the manufacturing cost of the brake disc rotor.

[0082] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[0083] As stainless steel sheets, hot-rolled and cold-rolled annealed sheets were produced as follows. Steel with the composition shown in Table 1 (the remainder being Fe and impurities) was melted and cast into an ingot. The ingot was heated to 1250°C and hot-rolled to obtain a 7 mm thick hot-rolled sheet. The obtained hot-rolled sheet was heated, annealed, and cooled under the conditions shown in Table 2 (heating rate, annealing temperature, holding time, and cooling rate) to obtain a hot-rolled annealed sheet (Test Nos. 1 to 18 of the present invention example, Test Nos. 1 to 8 of the comparative example). Furthermore, for Test No. 17 of the present invention example, the hot-rolled annealed sheet was further cold-rolled to obtain a 6 mm thick cold-rolled sheet, then heated at a heating rate of 0.940°C / second, held at an annealing temperature of 600°C for 1.0 hour, and cooled at a cooling rate of 0.940°C / second to obtain a cold-rolled annealed sheet.

[0084]

[0085]

[0086] The hot-rolled annealed sheets (Test Nos. 1 to 16 and 18 of the present invention example, and Test Nos. 1 to 8 of the comparative example) and cold-rolled annealed sheets (Test No. 17 of the present invention example) obtained above were evaluated as follows.

[0087] <Area ratio of microstructures with KAM values ​​of 0.00 to 0.65° and microstructures with KAM values ​​greater than 0.65° and less than or equal to 5.00°> The thickness-direction cross-sections perpendicular to the rolling direction of hot-rolled and cold-rolled annealed sheets were polished to a mirror finish, and EBSD measurements were performed on the t / 4 portion (1 / 4 of the thickness t of the stainless steel sheet) of these cross-sections. This measurement was performed using a scanning electron microscope JSM-7200F manufactured by JEOL Ltd., observing an image in a region of approximately 40 × 125 μm (magnification 2000x) with a measurement step of 0.25 μm, and analyzing it using the analysis software "OIM" manufactured by TSL Solutions Co., Ltd. From the KAM values ​​at each measurement point, the area ratios of microstructures with KAM values ​​of 0.00 to 0.65° and microstructures with KAM values ​​greater than 0.65° and less than or equal to 5.00° were calculated. The area ratio was based on the entire observation field, and the observation field was selected to include at least 50 crystal grains.

[0088] <Vickers Hardness> After polishing the thickness-direction cross-section perpendicular to the rolling direction of hot-rolled and cold-rolled annealed sheets to a mirror finish, the Vickers hardness was measured at the center of the thickness direction (t / 2) in accordance with JIS Z2244-1:2024, under the conditions of a load of 5 kg and a number of measurement samples (n) = 5. The average value of these measurements was taken as the Vickers hardness.

[0089] <Large-Angle Grain Boundary Density> The thickness-direction cross-sections perpendicular to the rolling direction of hot-rolled and cold-rolled annealed sheets were polished to a mirror finish, and EBSD measurements were performed on the t / 4 portion (1 / 4 of the thickness t of the stainless steel sheet) of these cross-sections. This measurement was performed using a scanning electron microscope JSM-7200F manufactured by JEOL Ltd., observing an image of an area of ​​approximately 40 × 125 μm (magnification 2000x) with a measurement step of 0.25 μm, and analyzing it using the analysis software "OIM" manufactured by TSL Solutions Co., Ltd. The grain boundary length was measured as a large-angle grain boundary at measurement points where the crystal orientation difference between adjacent measurement points was 15 to 65° (15 ≤ X ≤ 65). The large-angle grain boundary density was calculated by dividing the grain boundary length by the EBSD measurement area. The observation field was selected to include at least 50 crystal grains.

[0090] <Elongation at Break (Processability)> Tensile test specimens were taken from hot-rolled and cold-rolled annealed sheets so that the rolling direction was the tensile direction. Tensile tests were performed at room temperature (25°C) in accordance with JIS Z2241:2011, and the elongation at break was measured. In this evaluation, if the elongation at break is 13.0% or higher, it can be determined that the material can be processed into hat shapes or motorcycle disc rotors.

[0091] <Average coefficient of friction at 60-300°C> After pickling the hot-rolled and cold-rolled annealed plates, discs with an outer diameter of 90 mm (disk-shaped test pieces: 6 mm thick for hot-rolled annealed plates, 5 mm thick for cold-rolled annealed plates) were cut out, and friction and wear tests were conducted in accordance with JASO C406:2000. In the friction and wear test, braking was performed from 130 km / h, the same as in the room-temperature effectiveness test of the second effectiveness test, and the braking deceleration was 1.0 m / s. 2 ~10.0 m / s 2 The average friction coefficient during one braking phase at each deceleration was calculated. The disc temperature during braking was set to 60-300°C. The disc temperature was measured using a thermocouple at a position 1 mm below the sliding surface. A 33 mm x 12 mm x 15 mm thick non-asbestos organic (NAO) material was used as the mating material (pad). The moment of inertia was 0.25 kg·m 2 In this evaluation, it was determined that if the average friction coefficient is between 0.25 and 0.65, it can be applied to general disc rotors.

[0092] <Disk Wear Amount> After pickling hot-rolled and cold-rolled annealed plates, discs with an outer diameter of 90 mm (disk-shaped test specimens: 6 mm thick for hot-rolled annealed plates, 5 mm thick for cold-rolled annealed plates) were cut out, and friction wear tests were performed in accordance with JASO C406:2000. The difference in disc thickness before and after the friction wear test was defined as the disc wear amount. The disc thickness was measured at the center of the width of the sliding mark using a point micrometer at 0°, 120°, and 240° (0° was selected at any position), and the maximum value was defined as the disc wear amount. A 33 mm x 12 mm x 15 mm thick non-asbestos organic (NAO) material was used as the mating material (pad). The moment of inertia was 0.25 kg·m 2Based on this evaluation, it can be determined that if the diameter is 0.50 mm or less, it is applicable to general disc rotors. The vehicle category was set to P1.

[0093] The results of each of the above evaluations are shown in Table 3.

[0094]

[0095] As shown in Table 3, Test Nos. 1 to 18 of the present invention had an A value of 55 to 150, a tissue area ratio of 20.0 to 75.0% with a KAM value of 0.00 to 0.65°, a tissue area ratio of 25.0 to 80.0% with a KAM value greater than 0.65° and 5.00° or less, a Vickers hardness of 200 to 300 HV, and good evaluation results for elongation at break (processability at room temperature) and average friction coefficient. Furthermore, Test Nos. 1 to 18 of the present invention also had a large-angle grain boundary density of 0.10 μm / μm 2 In summary, the amount of disc wear was also minimal.

[0096] In contrast, in Comparative Example Test No. 1, the C and N content was too high, resulting in an excessively high A value and an excessively high annealing temperature. This caused the area ratios of microstructures with KAM values ​​between 0.00 and 0.65° and those with KAM values ​​between 0.65° and 5.00° to fall outside the specified range. As a result, the hot-rolled annealed sheet became excessively hard, reducing the elongation at break (workability at room temperature) and increasing the average coefficient of friction. In Comparative Example Test No. 2, the Si content was too high, resulting in an excessively low A value and an excessively low annealing temperature. This caused the area ratios of microstructures with KAM values ​​between 0.00 and 0.65° and those with KAM values ​​between 0.65° and 5.00°, as well as the large-angle grain boundary density, to fall outside the specified range. As a result, the hot-rolled annealed sheet became excessively soft, resulting in a low average coefficient of friction. Furthermore, the amount of disc wear was also high. In comparative example 3, the high Mn content and the heating rate and annealing temperature during annealing were outside the specified range, resulting in hardening of the microstructure with a KAM value greater than 0.65° and less than or equal to 5.00°. As a result, the hot-rolled annealed sheet became excessively hard, and the elongation at break (workability at room temperature) decreased. In comparative example No. 4, the excessive P content led to P segregation, causing embrittlement and a decrease in elongation at break (workability at room temperature). In comparative example No. 5, the excessive S content led to S segregation, causing embrittlement and a decrease in elongation at break (workability at room temperature). In comparative example No. 6, the high Cr content and low A value resulted in the area ratio of microstructures with a KAM value of 0.00 to 0.65° and microstructures with a KAM value greater than 0.65° and less than or equal to 5.00° being outside the specified range. As a result, the hot-rolled annealed sheet became excessively soft, and the average coefficient of friction decreased. Furthermore, the amount of disc wear was also high. In comparative example test No. 7, the Ni content was high, the A value was too high, and the annealing temperature and cooling rate after annealing were outside the specified range. As a result, the area ratio of microstructures with a KAM value of 0.00 to 0.65° and microstructures with a KAM value greater than 0.65° and less than or equal to 5.00° was outside the specified range. Consequently, the hot-rolled annealed sheet became excessively hard, and the elongation at break (workability at room temperature) decreased.In comparative example No. 8, the annealing temperature was low and the holding time was short, resulting in the area ratio of microstructures with KAM values ​​of 0.00 to 0.65° and microstructures with KAM values ​​greater than 0.65° and 5.00° or less, as well as the large-angle grain boundary density, being outside the specified range. As a result, the hot-rolled annealed sheet became excessively soft, and the average coefficient of friction decreased. Furthermore, the amount of disc wear was also high.

[0097] As can be seen from the above results, the present invention provides a stainless steel sheet with excellent machinability and friction coefficient stability, and a method for manufacturing the same. Furthermore, the present invention provides a brake disc rotor with excellent friction coefficient stability, easy processing, and the ability to reduce thickness and weight, and a method for manufacturing the same.

Claims

1. Stainless steel sheet containing, by mass, C: 0.001 to 0.150%, N: 0.0010 to 0.3000%, Si: 0.01 to 3.00%, Mn: 0.010 to 5.000%, P: 0.001 to 0.100%, S: 0.0001 to 0.1000%, Cr: 15.0% to 25.0%, Ni: 0.010 to 5.000%, with the remainder being Fe and impurities; having an A value represented by the following formula (1) of 55 to 150; having an area ratio of 20.0 to 75.0% of the structure with a KAM value of 0.00 to 0.65°; having an area ratio of 25.0 to 80.0% of the structure with a KAM value greater than 0.65° and 5.00° or less; and having a Vickers hardness of 200 to 300 HV. A value = 420C + 470N + 23Ni + 7Mn - 11.5Cr - 11.5Si + 189 ... (1) In the formula, each element symbol represents the content (%) of each element.

2. By mass, Nb: 0.0001-1.0000%, Ti: 0.0001-1.0000%, Cu: 0.001-2.000%, Mo: 0.001-2.000%, V: 0.001-1.000%, B: 0.0001-0.0300%, Al: 0.0010-4.0000%, W: 0.001-3.000%, Sn: 0.0001-1.0000%, Mg: 0.0001-0.0100%, Sb: 0.0 The stainless steel sheet according to claim 1, further containing one or more selected from 0.1 to 0.500%, Zr: 0.001 to 1.000%, Ta: 0.001 to 1.000%, Hf: 0.001 to 1.000%, Co: 0.0001 to 1.0000%, Ca: 0.0001 to 0.0200%, REM: 0.001 to 0.500%, Ga: 0.0001 to 0.5000%, and Bi: 0.0010 to 0.1000%.

3. Large angle grain boundary density is 0.10μm / μm 2 The stainless steel plate according to claim 1 or 2.

4. A stainless steel sheet according to any one of claims 1 to 3, satisfying all of the following characteristics: (A) having a fracture elongation of 13.0% or more at room temperature, and (B) having an average coefficient of friction of 0.25 to 0.65 at 60 to 300°C.

5. A stainless steel plate according to any one of claims 1 to 4, for use as a brake disc rotor.

6. A method for manufacturing stainless steel sheets, comprising heating a hot-rolled or cold-rolled sheet having a composition on a mass basis containing C: 0.001 to 0.150%, N: 0.0010 to 0.3000%, Si: 0.01 to 3.00%, Mn: 0.010 to 5.000%, P: 0.001 to 0.100%, S: 0.0001 to 0.1000%, Cr: 15.0% to 25.0%, and Ni: 0.010 to 5.000%, with the remainder being Fe and impurities, and having an A value represented by the following formula (1) of 55 to 150, at a rate of 1,000°C / second or less, holding at an annealing temperature of 700 to 900°C for 1.0 to 20.0 hours, and then cooling at a rate of 1,000°C / second or less. A value = 420C + 470N + 23Ni + 7Mn - 11.5Cr - 11.5Si + 189 ... (1) In the formula, each element symbol represents the content (%) of each element.

7. The hot-rolled or cold-rolled sheet has the following composition by mass: Nb: 0.0001 to 1.0000%, Ti: 0.0001 to 1.0000%, Cu: 0.001 to 2.000%, Mo: 0.001 to 2.000%, V: 0.001 to 1.000%, B: 0.0001 to 0.0300%, Al: 0.0010 to 4.0000%, W: 0.001 to 3.000%, Sn: 0.0001 to 1.0000%, Mg: 0.0001 to 0.0100%, S A method for manufacturing a stainless steel sheet according to claim 6, further comprising one or more elements selected from b: 0.001 to 0.500%, Zr: 0.001 to 1.000%, Ta: 0.001 to 1.000%, Hf: 0.001 to 1.000%, Co: 0.0001 to 1.0000%, Ca: 0.0001 to 0.0200%, REM: 0.001 to 0.500%, Ga: 0.0001 to 0.5000%, and Bi: 0.0010 to 0.1000%.

8. A brake disc rotor comprising a processed stainless steel plate according to any one of claims 1 to 5.

9. A method for manufacturing a brake disc rotor by processing a stainless steel plate according to any one of claims 1 to 5.

10. The method for manufacturing a brake disc rotor according to claim 9, wherein the processing is performed at room temperature.

11. A method for manufacturing a brake disc rotor according to claim 9 or 10, wherein no heat treatment is performed after the processing.

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