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

Abstract

The present invention provides a stainless steel sheet for brake disc rotors, the stainless steel sheet having a composition that contains, on a mass basis, not less than 0.001% but less than 0.080% of C, 0.0010-0.5000% of N, 0.01-5.00% of Si, 0.010-12.000% of Mn, 0.001-0.100% of P, 0.0001-1.0000% of S, 10.0-35.0% of Cr and 0.010-5.000% of Ni, and one or both of 0.0001-1.0000% of Nb and 0.0001-1.0000% of Ti, with the balance being made up of Fe and impurities, while having an A value represented by formula (1) of 65 to 150. With respect to this stainless steel sheet for brake disc rotors, the ratio of ferrite phase is 1.0-50.0%; the ratio of martensite phase is 50.0- 99.0%; and the Vickers hardness is 300 to 400 HV.　(1): (A value) = 420C + 470N + 23Ni + 7Mn - 11.5Cr - 11.5Si - 47Nb - 49Ti + 189 In the formula, the atomic symbols represent the contents (%) of the respective elements.

Description

Stainless steel plate for brake disc rotor and manufacturing method thereof, and brake disc rotor and manufacturing method thereof

[0001] The present invention relates to a stainless steel sheet for a brake disc rotor, a manufacturing method thereof, and a brake disc rotor and a manufacturing method thereof. More specifically, the present invention relates to a stainless steel sheet for a brake disc rotor that has excellent workability and a stable coefficient of friction, a manufacturing method thereof, a brake disc rotor that has excellent corrosion resistance and braking performance and can be made thin and lightweight, and a manufacturing method thereof.

[0002] Disc brakes are widely used as one type of braking system. Disc brakes work by squeezing a disc-shaped structure called a brake disc rotor (hereinafter sometimes abbreviated as "disc rotor") connected to the tire between brake pads, 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 generally used as the material for disc rotors due to its thermal conductivity 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 being positioned lower than the line of sight and the unique shape of the wheel. However, in recent years, with the demand for improved fuel efficiency, wheels have been made of aluminum, and the spokes have become thinner, making the red rust on disc rotors no longer negligible. 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, the temperature of a disc rotor can reach a maximum of nearly 600°C during braking, and even under driving conditions that require heavy braking, such as on mountain roads, the temperature of the disc rotor can reach 300°C. Because cast iron has low high-temperature strength, thinning the rotor's thickness makes it difficult to ensure the necessary strength for a disc rotor at high temperatures, making it difficult to achieve a thin, lightweight rotor. Furthermore, because cast iron is formed by casting, thinning the rotor's thickness can sometimes result in poor melt flow, making it impossible to form.

[0005] On the other hand, stainless steel offers 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 on a motorcycle is exposed and its excellent cooling capabilities allow stainless steel to be used without problems in normal use. However, even in motorcycles, disc rotors can overheat under harsh braking conditions, such as during races, resulting in increased brake pad wear. In automobiles, the brake system, including the tires, is housed within the wheel wells, making disc rotors difficult to cool. This low thermal conductivity poses a challenge, making stainless steel unsuitable for this purpose. 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 (EVs), fuel cell vehicles (FCVs), and hybrid vehicles (HVs). This reduces the frictional heat generated by friction between the disc rotor and pads, expanding the potential for stainless steel, which has poorer thermal conductivity than cast iron, to be used.

[0006] Another obstacle to the application of stainless steel to automotive disc brakes is its formability. Motorcycle disc rotors are ring-shaped discs, manufactured by punching stainless steel sheets and then performing induction hardening, eliminating the need for extensive machining. Current automotive disc rotors, however, are hat-shaped, resembling a disk with a narrowed center, and are therefore manufactured by casting. Forming such a shape using stainless steel sheets requires press working (particularly deep drawing). However, the stainless steel used in motorcycles is martensitic stainless steel, which is extremely hard and therefore difficult to deep-draw. One solution to this problem has been the widespread use of hot stamping, a press process performed at high temperatures. This has enabled stainless steel to be precisely formed into a hat shape. Against this backdrop, the shift to stainless steel disc rotors is necessary to meet the demands for aesthetics (corrosion resistance), thinness, lightness, and formability.

[0007] As mentioned above, stainless steel sheet can be processed into disc rotors for motorcycles, which do not require extensive processing, by induction hardening. Meanwhile, for automobiles, hot stamping is used, which involves pressing at high temperatures. Conventional martensitic stainless steel requires high-temperature heating to form the hat-shaped disc rotor. Ferritic stainless steel can be formed into the hat-shaped disc rotor without high-temperature heating. However, because ferritic stainless steel is soft, it deforms during braking, resulting in a lower coefficient of friction stability, which is essential for safe braking. Thus, with a single martensitic or ferritic phase, it is difficult to achieve both energy savings by eliminating high-temperature heating (quenching) and the ease of forming the hat-shaped disc rotor and a stable coefficient of friction.

[0008] Patent Document 1 describes a stainless steel disc rotor with excellent strength, wear resistance, toughness, and punching workability, and describes a stainless steel disc rotor with a composition of, in mass %, 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%. %, Al: 0-0.200%, B: 0-0.010%, N: 0.020% or less, the balance being Fe and unavoidable impurities, the matrix (metal base) being composed of ferrite and martensite phases, the ferrite phase being present in an amount of 5-22 volume %, and the Vickers hardness HV30 being 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 are hat-shaped, and require workability into this shape and a stable friction coefficient, whereas Patent Document 1 makes no particular mention of workability or a stable friction coefficient.

[0009] Japanese Patent Application Laid-Open No. 2018-135571

[0010] As mentioned above, stainless steel sheets used as materials for automobile disc rotors must be easy to process into hat shapes. Furthermore, disc rotors reach a wide range of temperatures under various driving conditions, so they must be able to brake stably even when the temperature rises due to braking. Therefore, from the perspective of ensuring safe braking, they must maintain a constant coefficient of friction even when the temperature changes.

[0011] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a stainless steel sheet for brake disc rotors that has excellent workability and a stable coefficient of friction, and a method for manufacturing the same. Another object of the present invention is to provide a stainless steel sheet for brake disc rotors that has excellent workability and can provide a stable coefficient of friction. A further object of the present invention is to provide a brake disc rotor that has excellent corrosion resistance and braking performance and can be made thin and lightweight, and a method for manufacturing the same.

[0012] As a result of intensive research aimed at solving the above problems, the inventors have discovered the following (a) to (c), and have completed the present invention. (a) A stainless steel sheet in which the composition, the proportion of ferrite and martensite phases, and the Vickers hardness are controlled within predetermined ranges has properties suitable for manufacturing brake disc rotors. (b) The above stainless steel sheet can be manufactured by heating a hot-rolled or cold-rolled sheet having a predetermined composition at a rate of 1.0°C / second or more, holding it at an annealing temperature of 1020°C to 1200°C for 0 seconds or more, and then cooling it at a rate of 1.0°C / second or more. (c) A hot-rolled or cold-rolled stainless steel sheet having a predetermined composition is suitable as a material for brake disc rotors manufactured by hot stamping. That is, the present invention comprises the following [1] to

[13] .

[0013] [1] On a mass basis, it contains 0.001% or more and less than 0.080% C, 0.0010 to 0.5000% N, 0.01 to 5.00% Si, 0.010 to 12.000% Mn, 0.001 to 0.100% P, 0.0001 to 1.0000% S, 10.0 to 35.0% Cr, 0.010 to 5.000% Ni, and one or two selected from 0.0001 to 1.0000% Nb and 0.0001 to 1.0000% Ti, with the balance being Fe and impurities, and has a composition in which the A value represented by the following formula (1) is 65 to 150, and the proportion of ferrite phase is 1.0 to 50.0%, A stainless steel sheet for brake disc rotors having a martensite phase ratio of 50.0 to 99.0% and a Vickers hardness of 300 to 400 HV. A value = 420C + 470N + 23Ni + 7Mn - 11.5Cr - 11.5Si - 47Nb - 49Ti + 189 ... (1) where each element symbol represents the content (%) of each element.

[0014] [2] By mass, 0.001 to 3.000% Cu, 0.001 to 3.000% Mo, 0.001 to 1.000% V, 0.0001 to 0.0300% B, 0.001 to 4.000% Al, 0.001 to 3.000% W, 0.0001 to 1.0000% Sn, 0.0001 to 0.0100% Mg, 0.001 to 0.500% Sb, 0.001 to 1.000% Zr , 0.001 to 1.000% Ta, 0.001 to 1.000% Hf, 0.0001 to 1.0000% Co, 0.0001 to 0.0200% Ca, 0.001 to 0.500% REM, 0.0001 to 0.5000% Ga, and 0.0010 to 0.1000% Bi. [1] The stainless steel sheet for brake disc rotors according to [1], further containing one or more selected from the group consisting of: 0.001 to 1.000% Ta, 0.001 to 1.000% Hf, 0.0001 to 1.0000% Co, 0.0001 to 0.0200% Ca, 0.001 to 0.500% REM, 0.0001 to 0.5000% Ga, and 0.0010 to 0.1000% Bi.

[0015] [3] The stainless steel sheet for a brake disc rotor according to [1] or [2], which satisfies the following condition when subjected to EBSD measurement: (IQ value of ferrite phase) - (IQ value of martensite phase) ≥ 10,000.

[0016] [4] The stainless steel sheet for a brake disc rotor according to any one of [1] to [3], which satisfies one or more of the following properties: (A) a breaking elongation at room temperature of 10.0% or more; and (B) an average friction coefficient at 60 to 300°C of 0.25 to 0.65.

[0017] [5] By mass, 0.001% or more and less than 0.080% C, 0.0010 to 0.5000% N, 0.01 to 5.00% Si, 0.010 to 12.000% Mn, 0.001 to 0.100% P, 0.0001 to 1.0000% S, 10.0 to 35.0% Cr, 0.010 to 5.000% Ni, 0.0001 to 1.0000% Nb, and 0.0001 to 1.0000% A method for manufacturing a stainless steel sheet for brake disc rotors, comprising heating a hot-rolled or cold-rolled sheet having a composition containing one or two elements selected from the group consisting of 1.0000% Ti and 1.0000% Mn, the balance being Fe and impurities, and having an A value of 65 to 150, as represented by the following formula (1), at a rate of 1.0°C / second or more, holding the sheet at an annealing temperature of 1020°C to 1200°C for 0 second or more, and then cooling the sheet at a rate of 1.0°C / second or more: A value = 420C + 470N + 23Ni + 7Mn-11.5Cr-11.5Si-47Nb-49Ti + 189 ... (1) where each element symbol represents the content (%) of each element.

[0018] [6] By mass, 0.001 to 3.000% Cu, 0.001 to 3.000% Mo, 0.001 to 1.000% V, 0.0001 to 0.0300% B, 0.001 to 4.000% Al, 0.001 to 3.000% W, 0.0001 to 1.0000% Sn, 0.0001 to 0.0100% Mg, 0.001 to 0.500% Sb, 0.001 to 1.000% Zr, 0. [5] The method for producing a stainless steel sheet for a brake disc rotor according to [5], further containing one or more selected from the group consisting of 0.001 to 1.000% Ta, 0.001 to 1.000% Hf, 0.0001 to 1.0000% Co, 0.0001 to 0.0200% Ca, 0.001 to 0.500% REM, 0.0001 to 0.5000% Ga, and 0.0010 to 0.1000% Bi.

[0019] [7] A brake disc rotor comprising a processed product of the stainless steel plate for a brake disc rotor according to any one of [1] to [4].

[0020] [8] A method for manufacturing a brake disc rotor, comprising processing the stainless steel plate for a brake disc rotor according to any one of [1] to [4] at room temperature.

[0021] [9] The method for manufacturing a brake disc rotor according to [8], in which no heat treatment is performed after the processing.

[0022]

[10] A stainless steel sheet for a brake disc rotor, which is a hot-rolled or cold-rolled sheet containing, by mass, 0.001% or more and less than 0.080% C, 0.0010 to 0.5000% N, 0.01 to 5.00% Si, 0.010 to 12.000% Mn, 0.001 to 0.100% P, 0.0001 to 1.0000% S, 10.0 to 35.0% Cr, 0.010 to 5.000% Ni, and one or two elements selected from 0.0001 to 1.0000% Nb and 0.0001 to 1.0000% Ti, the balance being Fe and impurities, and having a composition in which the A value represented by the following formula (1) is 65 to 150: A value=420C+470N+23Ni+7Mn−11.5Cr−11.5Si−47Nb−49Ti+189 (1) In the formula, each element symbol represents the content (%) of each element.

[0023]

[11] By mass, 0.001 to 3.000% Cu, 0.001 to 3.000% Mo, 0.001 to 1.000% V, 0.0001 to 0.0300% B, 0.001 to 4.000% Al, 0.001 to 3.000% W, 0.0001 to 1.0000% Sn, 0.0001 to 0.0100% Mg, 0.001 to 0.500% Sb, 0.001 to 1.000% Zr,

[11] The stainless steel sheet for brake disc rotors according to

[10] , further containing one or more selected from 0.001 to 1.000% Ta, 0.001 to 1.000% Hf, 0.0001 to 1.0000% Co, 0.0001 to 0.0200% Ca, 0.001 to 0.500% REM, 0.0001 to 0.5000% Ga, and 0.0010 to 0.1000% Bi.

[0024]

[12] A brake disc rotor comprising a processed product of the stainless steel plate for a brake disc rotor according to

[10] or

[11] .

[0025]

[13] A method for manufacturing a brake disc rotor, comprising processing the stainless steel plate for a brake disc rotor according to

[10] or

[11] by hot stamping.

[0026] According to the present invention, it is possible to provide a stainless steel sheet for brake disc rotors that has excellent workability and a stable coefficient of friction, and a method for manufacturing the same. Also, according to the present invention, it is possible to provide a stainless steel sheet for brake disc rotors that has excellent workability and can provide a stable coefficient of friction. Furthermore, according to the present invention, it is possible to provide a brake disc rotor that has excellent corrosion resistance and braking performance and can be made thin and lightweight, and a method for manufacturing the same.

[0027] 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.

[0028] [Embodiment 1] A stainless steel plate for brake disc rotors according to Embodiment 1 of the present invention (hereinafter abbreviated as "stainless steel plate") has a composition containing 0.001% or more and less than 0.080% C, 0.0010 to 0.5000% N, 0.01 to 5.00% Si, 0.010 to 12.000% Mn, 0.001 to 0.100% P, 0.0001 to 1.0000% S, 10.0 to 35.0% Cr, 0.010 to 5.000% Ni, and one or two elements selected from 0.0001 to 1.0000% Nb and 0.0001 to 1.0000% Ti, with the balance being Fe and impurities, and having an A value of 65 to 150.

[0029] In this specification, the term "impurities" refers to components that are mixed in during the industrial production of stainless steel sheets due to various factors in the manufacturing process, such as raw materials like ores and scrap, and are acceptable within a range that does not adversely affect the present invention. Examples of impurities include As and Pb.

[0030] Furthermore, the stainless steel sheet according to the first embodiment of the present invention contains 0.001 to 3.000% Cu, 0.001 to 3.000% Mo, 0.001 to 1.000% V, 0.0001 to 0.0300% B, 0.001 to 4.000% Al, 0.001 to 3.000% W, 0.0001 to 1.0000% Sn, 0.0001 to 0.0100% Mg, 0.001 to 0.500% % Sb, 0.001 to 1.000% Zr, 0.001 to 1.000% Ta, 0.001 to 1.000% Hf, 0.0001 to 1.0000% Co, 0.0001 to 0.0200% Ca, 0.001 to 0.500% REM, 0.0001 to 0.5000% Ga, and 0.0010 to 0.1000% Bi.

[0031] Each component will be described in detail below.

[0032] <C: 0.001% or more and less than 0.080%> C is an element that dissolves in the matrix and has a significant effect on hardness. Furthermore, C forms carbides depending on the heat treatment, which deteriorates workability and corrosion resistance and reduces high-temperature strength. Therefore, the C content is set to 0.001% or more and less than 0.080%. Furthermore, excessive reduction of C leads to increased refining costs, so the C content is preferably 0.030% or more and less than 0.080%, and more preferably 0.050 to 0.075%.

[0033] <N: 0.0010 to 0.5000%> Like C, N is an element that dissolves in the matrix and has a significant effect on hardness. Furthermore, N forms nitrides during heat treatment, which deteriorates workability and corrosion resistance and reduces high-temperature strength. Therefore, the N content is set to 0.0010 to 0.5000%. Furthermore, excessive reduction of N leads to increased refining costs, so the N content is preferably 0.0020 to 0.0400%, and more preferably 0.0040 to 0.0200%.

[0034] <Si: 0.01 to 5.00%> Si is an element useful as a deoxidizer, and also an element that improves oxidation resistance and high-temperature salt damage resistance. However, excessive addition of Si reduces room-temperature ductility, so the Si content is set to 0.01 to 5.00%. However, in consideration of pickling ability and toughness, the Si content is preferably 0.10 to 1.00%, and in consideration of manufacturability, the Si content is more preferably 0.30 to 0.70%.

[0035] <Mn: 0.010 to 12.000%> Mn is an element added as a deoxidizer and contributes to increasing 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 scale adhesion poor and abnormal oxidation more likely to occur. 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 Mn content is set to 0.010 to 12.000%. Furthermore, considering the pickling properties and room-temperature ductility in steel sheet production, the Mn content is preferably 0.100 to 2.000%, and more preferably 0.200 to 0.400%.

[0036] <P: 0.001 to 0.100%> 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 keeping it below 0.0001% would increase costs by using low-P raw materials. Therefore, the P content is set to 0.001% or more. On the other hand, if the P content exceeds 0.100%, the steel becomes significantly hard and its corrosion resistance, toughness, and pickling ability deteriorate, so the upper limit is set to 0.100%. Considering raw material costs, the P content is preferably 0.008 to 0.080%, and more preferably 0.010 to 0.050%.

[0037] <S: 0.0001 to 1.0000%> S is an element that deteriorates 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., thereby providing lubricity. This effect is apparent from 0.0001%; ​​therefore, the lower limit of the S content is set to 0.0001%. On the other hand, excessive addition of S combines with Ti and C to reduce the amount of solute Ti and cause coarsening of precipitates, resulting in reduced high-temperature strength. Therefore, the upper limit of the S content is set to 1.0000%. Furthermore, considering refining costs and high-temperature oxidation properties, the S content is preferably 0.0003 to 0.0100%, and more preferably 0.0005 to 0.0050%.

[0038] <Cr: 10.0 to 35.0%> 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 reduced workability and toughness. Therefore, the Cr content is set to 10.0 to 35.0%. Furthermore, in consideration of manufacturability and scale removal properties, the Cr content is preferably 11.0 to 19.0%, and more preferably 15.0 to 17.0%.

[0039] <Ni: 0.010 to 5.000%> Ni is an element that improves acid resistance, toughness, and high-temperature strength. Since excessive addition of Ni increases costs, the Ni content is set to 0.010 to 5.000%. In consideration of manufacturability, the Ni content is preferably 1.000 to 3.000%, and more preferably 1.800 to 2.200%.

[0040] <Nb: 0.0001 to 1.0000%> Nb is an element effective in improving temper softening resistance and high-temperature strength through solid solution strengthening and precipitation strengthening of fine precipitates, and stabilizes the friction coefficient by suppressing structural changes. 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 stainless steel sheets. However, excessive addition of Nb significantly hardens the steel and also deteriorates manufacturability, so the Nb content is set to 0.0001 to 1.0000%. Furthermore, considering raw material costs and toughness, the Nb content is preferably 0.0005 to 0.1000%, and more preferably 0.0010 to 0.0300%.

[0041] <Ti: 0.0001 to 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. Furthermore, when added in combination with Mo, adding an appropriate amount increases the amount of Mo dissolved during hot-rolling annealing, improves high-temperature strength, improves tempering softening resistance and thermal fatigue properties, and stabilizes the friction coefficient by suppressing microstructural changes. However, excessive addition of Ti increases the amount of dissolved Ti, reducing room-temperature ductility. It also forms coarse Ti-based precipitates, which act as crack initiation sites during hole expansion and degrade press workability. Excessive addition of Ti also reduces oxidation resistance. Therefore, the Ti content is set to 0.0001 to 1.0000%. In addition, in consideration of the occurrence of surface defects and toughness, the Ti content is preferably 0.0005 to 0.5000%, and more preferably 0.0010 to 0.0200%.

[0042] <A Value: 65 to 150> The A value is expressed by the following formula (1) and is an index of hardness and processability. A Value = 420C + 470N + 23Ni + 7Mn - 11.5Cr - 11.5Si - 47Nb - 49Ti + 189 ... (1) In the formula, each element symbol represents the content (%) of each element. By controlling the A value to 65 or more, sufficient hardness for use as a brake disc rotor can be obtained. By setting the A value to 150 or less, excessive hardness is suppressed, making it possible to form the rotor into a hat shape. In consideration of processability, the A value is preferably 65 to 125, and more preferably 65 to 100.

[0043] <Cu: 0.001 to 3.000%> Cu is an element effective in improving corrosion resistance. Furthermore, Cu improves temper softening resistance, high-temperature strength, and the stability of the friction coefficient through precipitation strengthening due to ε-Cu precipitation. Excessive addition of Cu reduces hot workability, so the Cu content is set to 0.001 to 3.000%. Furthermore, in consideration of thermal fatigue properties, manufacturability, and weldability, the Cu content is preferably 0.010 to 1.000%, and more preferably 0.030 to 0.200%.

[0044] <Mo: 0.001 to 3.000%> Mo is an element effective for solid-solution strengthening at high temperatures, and also improves temper softening resistance, friction coefficient stability, corrosion resistance, and high-temperature salt damage resistance. These effects are apparent from a content of 0.001%. Therefore, the lower limit of the Mo content is set to 0.001%. Furthermore, excessive addition of Mo significantly deteriorates room-temperature ductility and oxidation resistance, so the upper limit of the Mo content is set to 3.000%. Furthermore, in consideration of thermal fatigue properties and manufacturability, the Mo content is preferably 0.010 to 1.000%, and more preferably 0.030 to 0.200%.

[0045] <V: 0.001 to 1.000%> V is an element that improves corrosion resistance, and this effect is apparent from 0.001%. Therefore, the lower limit of the V content is set to 0.001%. Furthermore, excessive addition of V causes precipitates to coarsen, reducing tempering and softening resistance, high-temperature strength, and the stability of the friction coefficient, as well as deteriorating oxidation resistance. Therefore, the upper limit of the V content is set to 1.000%. Furthermore, taking into consideration production costs and manufacturability, the V content is preferably 0.010 to 0.500%, and more preferably 0.030 to 0.100%.

[0046] <B: 0.0001 to 0.0300%> B is an element that improves secondary workability during press forming, high-temperature strength, and thermal fatigue properties. B induces fine precipitation of Laves phases and the like, exhibiting long-term stability of these precipitation strengthening properties, contributing to suppressing strength degradation and improving thermal fatigue life. These effects are exhibited when the B content is 0.0001% or more. On the other hand, excessive addition of B causes hardening, degrading intergranular corrosion and oxidation resistance, and causing weld cracking, so the upper limit of the B content is set to 0.0300%. Furthermore, considering corrosion resistance and manufacturing costs, the B content is preferably 0.0005 to 0.0200%, and more preferably 0.0010 to 0.0100%.

[0047] <Al: 0.001 to 4.000%> Al is added as a deoxidizing element and also improves oxidation resistance. Furthermore, as a solid-solution strengthening element, Al is also useful for improving high-temperature strength and temper softening resistance. These effects are stably exhibited when the Al content is 0.001% or more. However, excessive addition of Al hardens the steel, significantly reducing uniform elongation and toughness, so the upper limit of Al is set to 4.000%. Furthermore, taking into consideration the occurrence of surface defects, weldability, and manufacturability, the Al content is preferably 0.0015 to 0.100%, and more preferably 0.002 to 0.040%.

[0048] <W: 0.001 to 3.000%> Like Mo, W is an effective element for solid-solution strengthening at high temperatures and also produces a Laves phase (FeW), resulting in precipitation strengthening. In particular, when added in combination with Mo, a Laves phase of Fe(Mo,W) precipitates. Adding W inhibits the coarsening of this Laves phase, improving precipitation strengthening and tempering softening resistance. These effects are exhibited when the W content is 0.001% or more. On the other hand, a W content exceeding 3.000% increases costs and reduces room-temperature ductility. Therefore, the upper limit of the W content is set to 3.000%. Considering manufacturability, low-temperature toughness, and oxidation resistance, the W content is preferably 0.001 to 1.500%.

[0049] <Sn: 0.0001 to 1.0000%> Sn is an element that improves corrosion resistance and improves high-temperature strength in the medium temperature range. These effects are manifested when the Sn content is 0.0001% or more. On the other hand, if the Sn content exceeds 1.0000%, manufacturability and toughness are significantly reduced, so the upper limit of the Sn content is set to 1.0000%. Furthermore, in consideration of oxidation resistance and manufacturing costs, the Sn content is preferably 0.0005 to 0.1000%, and more preferably 0.0010 to 0.0100%.

[0050] <Mg: 0.0001 to 0.0100%> Mg is sometimes added as a deoxidizing element, and also refines the structure of the slab, contributing to improved workability. Furthermore, Mg oxides formed by the addition of Mg serve as precipitation sites for carbonitrides such as Ti(C,N), resulting in the finely dispersed precipitation of these compounds. These effects are manifested when the Mg content is 0.0001% or more, contributing to improved toughness. However, because excessive addition of Mg leads to deterioration of weldability, corrosion resistance, and surface quality, the upper limit of the Mg content is set to 0.0100%. Considering refining costs, the Mg content is preferably 0.0003 to 0.0010%.

[0051] <Sb: 0.001 to 0.500%> Sb contributes to improving corrosion resistance and high-temperature strength. These effects are manifested when the Sb content is 0.001% or more. However, if the Sb content exceeds 0.500%, slab cracking and excessive reduction in ductility may occur during the production of stainless steel sheets, so the upper limit of the Sb content is set to 0.500%. Furthermore, in consideration of refining costs and manufacturability, the Sb content is preferably 0.010 to 0.300%.

[0052] <Zr: 0.001 to 1.000%> Like Ti, Zr is a carbonitride-forming element and an element that improves corrosion resistance and deep drawability. These effects are manifested when the Zr content is 0.001% or more. On the other hand, if the Zr content exceeds 1.000%, manufacturability deteriorates significantly, so the upper limit of the Zr content is set to 1.000%. Furthermore, in consideration of cost and surface quality, the Zr content is preferably 0.001 to 0.200%.

[0053] <Ta: 0.001 to 1.000% and Hf: 0.001 to 1.000%> Ta and Hf are elements that combine with C and N to contribute to improving toughness. This effect is exhibited when the respective contents of Ta and Hf are 0.001% or more. However, if the respective contents of Ta and Hf exceed 1.000%, not only will costs increase but manufacturability will also be significantly impaired. Therefore, the upper limit of each of the contents of Ta and Hf is set to 1.000%. Furthermore, in consideration of refining costs and manufacturability, the respective contents of Ta and Hf are preferably 0.010 to 0.080%.

[0054] <Co: 0.0001 to 1.0000%> Co is an element that contributes to improving high-temperature strength. This effect is manifested when the Co content is 0.0001% or more. However, a Co content of more than 1.0000% leads to a deterioration in toughness, so the upper limit of the Co content is set to 1.0000%. Furthermore, in consideration of refining costs and manufacturability, the Co content is preferably 0.0005 to 0.2000%, and more preferably 0.0010 to 0.1000%.

[0055] <Ca: 0.0001 to 0.0200%> Ca is an element added for desulfurization. This effect is exhibited when the Ca content is 0.0001% or more. However, if the Ca content exceeds 0.0200%, coarse CaS is generated, deteriorating toughness and corrosion resistance. Therefore, the upper limit of the Ca content is set to 0.0200%. Furthermore, in consideration of refining costs and manufacturability, the Ca content is preferably 0.0003 to 0.0020%.

[0056] <REM: 0.001 to 0.500%> REM (rare earth elements) are elements that contribute to improving toughness and oxidation resistance by refining various precipitates. These effects are realized when the REM content is 0.001% or more. However, if the REM content exceeds 0.500%, castability is significantly impaired and ductility is reduced. Therefore, the upper limit of the REM content is set to 0.500%. Furthermore, considering refining costs and manufacturability, the REM content is preferably 0.001 to 0.050%. 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). These elements may be added alone or in combination.

[0057] <Ga: 0.0001 to 0.5000%> Ga is an element that contributes to improving corrosion resistance and suppressing hydrogen embrittlement. These effects are manifested when the Ga content is 0.0001% or more. However, if the Ga content exceeds 0.5000%, manufacturability decreases and costs increase, so the upper limit of the Ga content is set to 0.5000%. Furthermore, in consideration of ductility and toughness, the upper limit of the Ga content is preferably 0.0020%.

[0058] <Bi: 0.0010 to 0.1000%> Bi is an element that suppresses roping that occurs during cold rolling and improves manufacturability. This effect is exhibited when the Bi content is 0.0010% or more. However, if the Bi content exceeds 0.1000%, hot workability deteriorates, so the upper limit of the Bi content is set to 0.1000%. Furthermore, from the viewpoint of stably obtaining the above effect, the Bi content is preferably 0.0012 to 0.0800%.

[0059] In addition to the above composition, the stainless steel sheet according to the first embodiment of the present invention has the following characteristics: a ferrite phase ratio of 1.0 to 50.0%, a martensite phase ratio of 50.0 to 99.0%, and a Vickers hardness of 300 to 400 HV. These characteristics will be described in detail.

[0060] <Ferrite Phase Ratio: 1.0 to 50.0% and Martensite Phase Ratio: 50.0 to 99.0%> The metallographic structure of a stainless steel sheet is a single-phase martensite structure or a dual-phase structure consisting of a ferrite phase and a martensite phase. The remainder of the metallographic structure of the stainless steel sheet is inclusions and precipitates. The ratio of the ferrite phase and the martensite phase in the metallographic structure of the stainless steel sheet is related to workability and the stability of the friction coefficient. In particular, by setting the ferrite phase ratio to 1.0 to 50.0%, workability is improved, particularly forming into a hat shape even at room temperature. On the other hand, if the ferrite phase ratio exceeds 50.0%, the stainless steel sheet becomes excessively soft, resulting in deformation when used as a brake disc rotor and a loss of friction coefficient stability. Furthermore, if the ferrite phase ratio is less than 1.0%, the stainless steel sheet becomes hard and workability is impaired. The ferrite phase ratio is preferably 5.0 to 50.0%, more preferably 10.0 to 50.0%, from the viewpoint of improving workability into complex shapes. Furthermore, by setting the martensite phase ratio to 50.0 to 99.0%, it is possible to ensure hardness sufficient for use as a brake disc rotor. On the other hand, if the martensite phase ratio is less than 50.0%, the stainless steel sheet becomes excessively soft, resulting in deformation when used as a brake disc rotor, and impairing the stability of the friction coefficient. Furthermore, if the martensite phase ratio exceeds 99.0%, the stainless steel sheet becomes hard, impairing workability. From the viewpoint of improving workability into complex shapes, the martensite phase ratio is preferably 50.0 to 95.0%, more preferably 50.0 to 90.0%.

[0061] Here, in this specification, the proportions of ferrite and martensite phases are determined by the following method. A cross section perpendicular to the rolling direction of a stainless steel sheet is mirror-polished, and the cross section is observed using an optical microscope. Specifically, the optical microscope image of the cross section is binarized using NIH's image analysis software "ImageJ" to identify inclusions and precipitates (the remainder other than the ferrite and martensite phases), and the area proportion of the inclusions and precipitates in the total observed area is calculated as the "proportion of inclusions and precipitates." The cross section is then etched with a mixture of hydrofluoric acid, nitric acid, and glycerin in a volume ratio of 1:2:3. Similarly to the above, the optical microscope image of the cross section is binarized using NIH's image analysis software "ImageJ" to identify the ferrite phase, and the area proportion of the ferrite phase in the total observed area is calculated as the "proportion of ferrite phase." The cross section is observed at the center (t / 2) in the thickness direction, and a total area of ​​200 μm × 200 μm or more is observed in multiple fields of view. The proportion of martensite phase is calculated using the following formula: Martensite phase proportion = 100 - (ferrite phase proportion + inclusion and precipitate proportion).

[0062] <Vickers hardness: 300 to 400 HV> The Vickers hardness of a stainless steel plate is related to its properties when used as a brake disc rotor. If the Vickers hardness is less than 300 HV, excessive deformation occurs when used as a brake disc rotor, reducing the stability of the friction coefficient and wear resistance. Furthermore, if the Vickers hardness exceeds 400 HV, excessive wear occurs on the mating brake pad. For these reasons, the Vickers hardness is set to 300 to 400 HV. From the viewpoint of reducing brake squeal, the Vickers hardness is preferably 300 to 380 HV, and more preferably 300 to 370 HV. Herein, the Vickers hardness is determined by the following method. A cross section of the stainless steel plate perpendicular to the rolling direction is polished to a mirror finish, and then the Vickers hardness is measured at the center in the thickness direction (t / 2) in accordance with JIS Z 2244 under conditions of a load of 5 kg and the number of measurement samples (n) = 5, and the average value of these measurements is taken as the Vickers hardness.

[0063] The stainless steel sheet according to the first embodiment of the present invention can satisfy the following condition when subjected to EBSD measurement: (IQ value of ferrite phase) - (IQ value of martensite phase) ≥ 10,000. By making the difference in IQ (Image Quality) values ​​10,000 or more, the balance between the hard martensite phase and the soft ferrite phase can be controlled within an appropriate range, making it easier to achieve both wear resistance and workability. From the viewpoint of stably ensuring this effect, the difference in IQ values ​​is preferably 12,500 or more, more preferably 15,000 or more. The upper limit of the difference in IQ values ​​is not particularly limited, but is, for example, 100,000. Measurement of the IQ value by EBSD can be performed using a scanning electron microscope (SEM) according to a known method.

[0064] The stainless steel sheet according to the first embodiment of the present invention can satisfy one or more of the following characteristics. <Fracture elongation at room temperature: 10.0% or more> If the fracture elongation of a stainless steel sheet at room temperature (25°C) is 10.0% or more, it can be said that the sheet has excellent workability at room temperature (particularly, it can be formed into a hat shape). In particular, since conventional martensitic stainless steel sheets are formed into hat-shaped brake disc rotors by hot stamping at high temperatures, energy saving by omitting heating during processing (i.e., workability at room temperature) is an issue. The stainless steel sheet according to the first embodiment of the present invention has excellent workability at room temperature, and therefore energy saving can be achieved by omitting heating during processing. From the viewpoint of consistently improving formability at room temperature, the fracture elongation at room temperature is preferably 10.3% or more, and more preferably 10.5% or more. The upper limit of the fracture elongation at room temperature is not particularly limited, but is, for example, 30.0%. Herein, the fracture elongation at room temperature is determined by the following method. A tensile test piece is taken so that the rolling direction of the stainless steel plate is the tensile direction, and a tensile test is carried out at room temperature (25°C) in accordance with JIS Z2241:2011 to measure the breaking elongation.

[0065] <Average Friction Coefficient at 60 to 300°C: 0.25 to 0.65> If the average friction coefficient of a stainless steel plate at 60 to 300°C is 0.25 to 0.65, excellent stability of the friction coefficient can be achieved when used as a brake disc rotor. This average friction coefficient is preferably 0.26 to 0.64. Herein, the friction coefficient of a stainless steel plate at 60 to 300°C is determined by the following method. A disc-shaped test piece with an outer diameter of 90 mm is prepared from the stainless steel plate, and a JASO C406 test (braking from 130 km / h in the room temperature braking test of the second braking effect test) is performed, and the friction coefficient during braking (when the disc-shaped test piece is at 60 to 300°C) is measured. At this time, the deceleration is 1.0 m / s 2 ~10.0 m / s 2 The friction coefficient during one braking operation at each deceleration is calculated, and the average value of these is taken as the average friction coefficient. The temperature of the disc-shaped test piece is measured using a thermocouple at a position 1 mm from the sliding surface toward the inside of the disc-shaped test piece.

[0066] The method for producing a stainless steel sheet according to the first embodiment of the present invention is not particularly limited as long as it is capable of producing a stainless steel sheet having the above-described characteristics. For example, the stainless steel sheet according to the first embodiment of the present invention can be produced by heating a hot-rolled or cold-rolled sheet having the above-described composition at a rate of 1.0°C / second or more, holding it at an annealing temperature of 1020°C to 1200°C for 0 seconds or more, and then cooling it at a rate of 1.0°C / second or more. Therefore, the stainless steel sheet obtained by this production method is a hot-rolled annealed sheet or a cold-rolled annealed sheet. The features of this production method will be described in detail.

[0067] When the stainless steel sheet is a hot-rolled annealed sheet, its manufacturing method is carried out in the order of steelmaking, hot rolling, annealing, and pickling. When the stainless steel sheet is a cold-rolled annealed sheet, its manufacturing method is carried out in the order of steelmaking, hot rolling, annealing, pickling, cold rolling, annealing, and pickling. In the steelmaking process, a method is preferably used in which steel containing the above-mentioned components is melted in a converter, followed by secondary refining. The melted molten steel is formed into a slab using a known casting method (continuous casting). The slab is heated to a predetermined temperature and hot-rolled to a predetermined thickness by continuous rolling (hot rolling process). In the hot rolling process, the slab is rolled in a hot rolling mill consisting of multiple stands and then coiled into a coil. The coiled hot-rolled coil is annealed in an annealing furnace under predetermined conditions and then pickled to obtain a hot-rolled annealed sheet (annealing process and pickling process). The pickling step can be carried out by an existing pickling method. The annealing step is carried out under the following conditions.

[0068] <Heating Rate: 1.0°C / sec or More> The heating rate is calculated by dividing the temperature change due to heating from room temperature to the annealing temperature by the time required for the heating. In other words, the heating rate in the annealing process refers to the heating rate in the temperature range from room temperature to the annealing temperature. When the thickness of a hot-rolled or cold-rolled sheet is large, the heating rate to the annealing temperature tends to be slow. If the heating rate is slow, the recovery of strain introduced during hot rolling is promoted during heating, the recrystallized grains of the ferrite phase become coarse, and the ferrite phase becomes soft. Therefore, it is not possible to control the Vickers hardness to 300 to 400 HV. Therefore, to suppress excessive recovery of strain, the heating rate to the annealing temperature is set to 1.0°C / sec or more. The heating rate can be controlled, for example, by adjusting the furnace temperature or the sheet passing speed, or by preheating. From the viewpoint of productivity, the heating rate is preferably 2.0°C / sec or more, and more preferably 3.0°C / sec or more. The upper limit of the heating rate is not particularly limited, but is, for example, 10.0° C. / sec, typically 5.0° C. / sec.

[0069] <Annealing Temperature: 1020°C to 1200°C and Holding Time 0 Seconds or More> By setting the annealing temperature of a hot-rolled or cold-rolled sheet to 1020°C to 1200°C and the holding time to 0 seconds or more, the proportion of ferrite phase in the parent phase of the stainless steel sheet can be controlled to 1.0 to 50.0%, and the proportion of martensite phase can be controlled to 50.0 to 99.0%. If the annealing temperature is less than 1020°C, carbides cannot be sufficiently dissolved during annealing, resulting in an excessively high proportion of ferrite phase. Furthermore, the amount of dissolved carbon in the martensite phase cannot be sufficiently secured, resulting in excessive softening. Similarly, if the annealing temperature exceeds 1200°C, the proportions of ferrite phase and martensite phase cannot be controlled within the specified range, and the desired properties cannot be obtained. Note that, in this specification, "holding time 0 seconds" means that heating is stopped when the target annealing temperature is reached. The upper limit of the holding time is not particularly limited, but is preferably 200 seconds, more preferably 150 seconds, and even more preferably 100 seconds. The above-mentioned annealing conditions refer to the annealing conditions after hot rolling when the stainless steel sheet is a hot-rolled annealed sheet. Furthermore, the above-mentioned 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.

[0070] <Cooling Rate: 1.0°C / Sec or More> The cooling rate is calculated by dividing the temperature change from the annealing temperature to 400°C by the time required for cooling. That is, the cooling rate is the cooling rate in the temperature range from the annealing temperature to 400°C. When the thickness of a hot-rolled sheet or cold-rolled sheet is large, the cooling rate after annealing tends to be slow. If the cooling rate is slow, self-tempering occurs in the martensite phase transformed from the austenite phase during cooling, softening the martensite phase, and the Vickers hardness cannot be controlled to 300 to 400 HV. In order to suppress self-tempering, the cooling rate after annealing is set to 1.0°C / Sec or more. The cooling rate can be controlled, for example, by adjusting the temperature in the furnace or by air cooling. From the viewpoint of productivity, the cooling rate is preferably 2.0°C / Sec or more, and more preferably 3.0°C / Sec or more. The upper limit of the cooling rate is not particularly limited, but is, for example, 20.0° C. / sec, typically 15.0° C. / sec.

[0071] When producing a cold-rolled annealed sheet, the hot-rolled annealed sheet obtained above is cold-rolled (cold rolling process). Either a tandem rolling mill or a Sendzimir rolling mill may be used for cold rolling. After rolling, the wound cold-rolled coil is annealed under predetermined conditions using an annealing furnace, and then pickled to obtain a cold-rolled annealed sheet (annealing process and pickling process). The pickling process can be performed using an existing pickling method. The annealing process is performed under the above conditions.

[0072] The stainless steel sheet according to the first embodiment of the present invention has excellent workability and a stable friction coefficient because the composition, the ratio of ferrite and martensite phases, and the Vickers hardness are controlled within predetermined ranges. In particular, this stainless steel sheet can be processed into a hat-shaped brake disc rotor at room temperature, which allows for energy savings compared to processing by hot stamping.

[0073] The stainless steel sheet according to the first embodiment of the present invention has excellent workability and a stable coefficient of friction, and can therefore be used in applications where these properties are required. In particular, this stainless steel sheet is ideal for use as a brake disc rotor. Brake disc rotors can be used for various vehicles, including not only automobiles but also motorcycles, tricycles, and snowmobiles, but are particularly suitable for use as brake disc rotors for automobiles.

[0074] The brake disc rotor according to the first embodiment of the present invention includes the above-described processed product of stainless steel plate. Here, the processed product of stainless steel plate refers to 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 refers to 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 determined appropriately depending on the type of vehicle in which the brake disc rotor is used.

[0075] The brake disc rotor according to the first embodiment of the present invention can be manufactured by processing (for example, press working such as deep drawing) the above-mentioned stainless steel plate at room temperature (for example, −10 to 50° C.) The processing conditions are not particularly limited and can be adjusted appropriately depending on the size of the brake disc rotor to be manufactured, the type of stainless steel plate to be used, and the like.

[0076] After processing, no heat treatment is required. This is because the stainless steel sheet already has optimal properties for use in brake disc rotors. Therefore, it can be used as a brake disc rotor as is after processing, which reduces the manufacturing cost of brake disc rotors.

[0077] The brake disc rotor according to the first embodiment of the present invention is made of stainless steel plate, which has good corrosion resistance and is resistant to red rust (i.e., has a good appearance), and can be made thin and lightweight. Furthermore, this brake disc rotor uses stainless steel plate, which has excellent workability and a stable coefficient of friction, and can be manufactured by pressing at room temperature. Furthermore, since heat treatment after pressing can be omitted, manufacturing costs can be reduced and braking performance is also good.

[0078] [Embodiment 2] A stainless steel sheet for brake disc rotors according to Embodiment 2 of the present invention (hereinafter simply referred to as "stainless steel sheet") is a hot-rolled or cold-rolled sheet having a composition containing 0.001% or more and less than 0.080% C, 0.0010 to 0.5000% N, 0.01 to 5.00% Si, 0.010 to 12.000% Mn, 0.001 to 0.100% P, 0.0001 to 1.0000% S, 10.0 to 35.0% Cr, 0.010 to 5.000% Ni, and one or two elements selected from 0.0001 to 1.0000% Nb and 0.0001 to 1.0000% Ti, the balance being Fe and impurities, and having an A value of 65 to 150.

[0079] The stainless steel sheet according to the second embodiment of the present invention contains 0.001 to 3.000% Cu, 0.001 to 3.000% Mo, 0.001 to 1.000% V, 0.0001 to 0.0300% B, 0.001 to 4.000% Al, 0.001 to 3.000% W, 0.0001 to 1.0000% Sn, 0.0001 to 0.0100% Mg, 0.001 to 0.500% % Sb, 0.001 to 1.000% Zr, 0.001 to 1.000% Ta, 0.001 to 1.000% Hf, 0.0001 to 1.0000% Co, 0.0001 to 0.0200% Ca, 0.001 to 0.500% REM, 0.0001 to 0.5000% Ga, and 0.0010 to 0.1000% Bi.

[0080] As described above, the stainless steel sheet according to the second embodiment of the present invention has the same composition as the stainless steel sheet according to the first embodiment of the present invention, so a description of the composition will be omitted and only the differences will be described.

[0081] The stainless steel sheet according to the second embodiment of the present invention is a hot-rolled sheet or a cold-rolled sheet having a predetermined composition, as described above. The hot-rolled sheet is produced by carrying out the steps of a steelmaking process and a hot-rolling process in this order. The cold-rolled sheet is produced by carrying out the steps of a steelmaking process, a hot-rolling process, an annealing process, a pickling process, and a cold-rolling process in this order. Each step can be the same as described above.

[0082] The stainless steel sheet according to the second embodiment of the present invention is not annealed and cooled under predetermined conditions after final rolling (i.e., after hot rolling in the case of a hot-rolled sheet, or after cold rolling in the case of a cold-rolled sheet). Therefore, unlike the stainless steel sheet according to the first embodiment of the present invention, the ferrite phase ratio, martensite phase ratio, and Vickers hardness are not controlled within predetermined ranges. Therefore, the stainless steel sheet according to the second embodiment of the present invention is difficult to process at room temperature, and the coefficient of friction is not sufficiently stable in its current state. However, the stainless steel sheet according to the second embodiment of the present invention can be easily processed into a predetermined shape (e.g., a hat shape) by press working at a high temperature. Furthermore, since the ferrite phase ratio, martensite phase ratio, and Vickers hardness can be controlled within predetermined ranges during press working at a high temperature (processing by hot stamping), the coefficient of friction is also improved in stability.

[0083] A brake disc rotor according to a second embodiment of the present invention includes the above-described processed stainless steel sheet. This brake disc rotor can be manufactured by hot stamping a stainless steel sheet, as described above. Hot stamping conditions may be set to control the ferrite phase ratio, martensite phase ratio, and Vickers hardness within predetermined ranges. For example, hot stamping conditions may include a heating rate of 1.0°C / sec or higher, a heating temperature of 1020 to 1200°C, a holding time of 0 seconds or longer, and a cooling rate of 1.0°C / sec or higher. The cooling rate is the cooling rate within the temperature range from the heating temperature to 400°C, and is calculated by dividing the temperature change from the annealing temperature to 400°C by the time required for cooling. Furthermore, in order to stably control the ferrite phase ratio, martensite phase ratio, and Vickers hardness within predetermined ranges, heat treatment may be performed as needed after hot stamping.

[0084] The brake disc rotor according to the second embodiment of the present invention is manufactured by hot stamping using a stainless steel plate as the material, which has excellent workability and can provide a stable coefficient of friction. Therefore, the brake disc rotor has excellent corrosion resistance and braking performance, and can also be made thin and lightweight.

[0085] 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.

[0086] Steels having the compositions shown in Tables 1-1 and 1-2 (the balance being Fe and impurities) were melted and cast into ingots. The ingots were heated to 1150 to 1250°C and hot-rolled to a thickness of 6 mm. The resulting hot-rolled sheets were heated, annealed, and cooled under the conditions (heating rate, annealing temperature, holding time, and cooling rate) shown in Tables 2-1 and 2-2 to obtain hot-rolled annealed sheets (Test Nos. 1 to 25 and 27 of the present invention, and Test Nos. 1 to 13 of the comparative examples). The hot-rolled annealed sheets using steel type No. A26 were further cold-rolled to a thickness of 5 mm, which was then heated at a rate of 3.5°C / sec, held at an annealing temperature of 1050°C for 0 seconds, and cooled at a cooling rate of 3.5°C / sec to obtain cold-rolled annealed sheets (Test No. 26 of the present invention). For A28, in order to evaluate workability during hot stamping, annealing was not performed after hot rolling, and the sheet was left in the form of a hot-rolled sheet.

[0087]

[0088]

[0089]

[0090]

[0091] The hot-rolled and annealed sheets and cold-rolled and annealed sheets obtained above were evaluated as follows. For the hot-rolled sheet of steel type No. A28, only the workability during hot stamping was evaluated.

[0092] <Proportions of Ferrite Phase, Martensite Phase, Inclusions, and Precipitates> Cross sections perpendicular to the rolling direction of the hot-rolled annealed sheet and the cold-rolled annealed sheet were mirror-polished, and optical microscope images of the cross sections were binarized using NIH's image analysis software "ImageJ" to identify inclusions and precipitates (the remainder other than the ferrite phase and martensite phase). The area ratio of the inclusions and precipitates to the total observed area was calculated as the "proportion of inclusions and precipitates." The cross sections were then etched with a mixture of hydrofluoric acid, nitric acid, and glycerin in a volume ratio of 1:2:3. Similarly to the above, optical microscope images of the cross sections were binarized using NIH's image analysis software "ImageJ" to identify the ferrite phase, and the area ratio of the ferrite phase to the total observed area was calculated as the "proportion of ferrite phase." The cross section was observed at the center (t / 2) in the thickness direction, and a total area of ​​200 μm × 200 μm or more was observed in multiple fields of view. The martensite phase ratio was calculated using the following formula: Martensite phase ratio = 100 - (ferrite phase ratio + inclusion and precipitate ratio).

[0093] <Vickers Hardness> After the cross sections perpendicular to the rolling direction of the hot-rolled annealed sheet and the cold-rolled annealed sheet were polished to a mirror finish, the Vickers hardness was measured at the center (t / 2) in the thickness direction in accordance with JIS Z 2244 under the conditions of a load of 5 kg and the number of measurement samples (n) = 5, and the average value thereof was taken as the Vickers hardness.

[0094] <Difference in IQ Values: (IQ Value of Ferrite Phase) - (IQ Value of Martensite Phase)> Cross sections perpendicular to the rolling direction of the hot-rolled annealed sheet and the cold-rolled annealed sheet were mirror-polished, and the IQ values ​​of these cross sections were measured by EBSD. The IQ values ​​were measured using a JEOL Ltd. JSM-7200F scanning electron microscope, and images of a 40 x 130 μm area (magnification 2000x) were observed in 0.05 μm measurement steps. Analysis was performed using the "OIM" analysis software from TSL Solutions Co., Ltd. In the analysis, the martensite phase and the ferrite phase were each extracted, and the average IQ values ​​were calculated, and the results were used as the IQ values ​​of each phase.

[0095] <Workability: Breaking Elongation> Tensile test specimens were taken so that the rolling direction of the hot-rolled annealed sheet and the cold-rolled annealed sheet was the tensile direction, and a tensile test was performed at room temperature (25°C) in accordance with JIS Z2241:2011 to measure the breaking elongation. In this evaluation, if the breaking elongation was 10% or more, it could be said that the sheet could be processed into a hat shape and was therefore rated as passing. In addition, tensile test specimens were taken so that the rolling direction of the hot-rolled sheet of steel type No. A28 was the tensile direction, and a tensile test was performed at a hot stamping simulated heat treatment temperature (1050°C) in accordance with JIS G0567:2020 to measure the breaking elongation. In this evaluation, if the breaking elongation was 50% or more, it could be said that the sheet could be processed into a hat shape and was therefore rated as passing.

[0096] <Average Friction Coefficient> Disc-shaped test specimens with an outer diameter of 90 mm were prepared from the hot-rolled annealed sheets and the cold-rolled annealed sheets, and subjected to the JASO C 406 test (braking from 130 km / h in the room temperature braking test of the second braking test), and the friction coefficient during braking (when the disc-shaped test specimens were at 60°C to 300°C) was measured. At this time, the deceleration rate was 1.0 m / s 2 ~10.0 m / s 2 The friction coefficient during one braking stroke at each deceleration was calculated, and the average value was taken as the average friction coefficient. The temperature of the disc-shaped test specimen was measured using a thermocouple at a position 1 mm away from the sliding surface toward the inside of the disc-shaped test specimen. In this evaluation, if the average friction coefficient was 0.25 to 0.65, it could be said that the specimen was applicable to a general brake disc rotor and was therefore considered to have passed. The results of each of the above evaluations are shown in Tables 3-1 and 3-2.

[0097]

[0098]

[0099] As shown in Tables 3-1 and 3-2, Tests No. 1 to 27 of the invention examples had a ferrite phase ratio of 1.0 to 50.0%, a martensite phase ratio of 50.0 to 99.0%, and a Vickers hardness of 300 to 400 HV. The evaluation results for elongation at break (workability at room temperature) and average friction coefficient also passed. Test No. 28 of the invention example also passed the evaluation result for elongation at break (workability by hot stamping). In contrast, Test No. 1 of the comparative example had excessive hardening due to an excessively high C content, resulting in reduced elongation at break (workability at room temperature). Test No. 2 of the comparative example had a high N content, a too high A value, and a too low ferrite phase ratio, resulting in excessive hardening and reduced elongation at break (workability at room temperature). Test No. 2 of the comparative example had a high N content, a too high A value, and a too low ferrite phase ratio, resulting in excessive hardening and reduced elongation at break (workability at room temperature). In Test No. 3, the Si content was high and the A value was too low, so the ratio of ferrite phase and martensite phase was outside the specified range, resulting in excessive softening and a large average friction coefficient. In Comparative Example Test No. 4, the Mn content was high, the A value was too high, and the ratio of ferrite phase and martensite phase was outside the specified range, resulting in excessive hardening and a decrease in fracture elongation (workability at room temperature). In Comparative Example Test No. 5, the P content was too high, so the P content was too high and the steel was excessively hardened and a decrease in fracture elongation (press workability).

[0100] Comparative Example Test No. 6 had an excessively high S content, which caused excessive sulfide precipitation and made fracture more likely to occur at sulfide-initiated points, resulting in a decrease in fracture elongation (workability at room temperature) and excessive softening. Comparative Example Test No. 7 had a high Cr content and a low A value, which caused the proportions of ferrite and martensite phases to fall outside the specified range, resulting in excessive softening and a high average friction coefficient. Comparative Example Test No. 8 had a low Ni content and a low A value, which caused the proportions of ferrite and martensite phases to fall outside the specified range, resulting in excessive softening and a high average friction coefficient. Comparative Example Test No. 9 had a high Nb content and a low A value, which caused the proportions of ferrite and martensite phases to fall outside the specified range, resulting in excessive softening and a high average friction coefficient. In Test No. 10, the Ti content was high and the A value was too low, so the ratio of ferrite and martensite phases was outside the specified range, resulting in excessive softening and a high average friction coefficient. In Comparative Example Test No. 11, the annealing temperature after hot rolling was too low, so carbonitrides were not sufficiently dissolved, resulting in the ratio of ferrite and martensite phases being outside the specified range. In addition, the amount of dissolved carbon in the martensite phase was not sufficiently secured, resulting in excessive softening and a high average friction coefficient. In Comparative Example Test No. 12, the heating rate when annealing the hot-rolled sheet was too slow, which promoted recovery of strain during hot rolling and caused coarsening of recrystallized grains of the ferrite phase, resulting in excessive softening and a high average friction coefficient. In Comparative Example Test No. 13, the cooling rate after annealing the hot-rolled sheet was too slow, resulting in excessive softening due to self-tempering, resulting in a high average friction coefficient.

[0101] As can be seen from the above results, the present invention can provide a stainless steel sheet for brake disc rotors that has excellent workability and a stable coefficient of friction, and a method for manufacturing the same. The present invention can also provide a stainless steel sheet for brake disc rotors that has excellent workability and can provide a stable coefficient of friction. Furthermore, the present invention can provide a brake disc rotor that has excellent corrosion resistance and braking performance and can be made thin and lightweight, and a method for manufacturing the same.

Claims

1. The composition contains, on a mass basis, 0.001% or more and less than 0.080% C, 0.0010 to 0.5000% N, 0.01 to 5.00% Si, 0.010 to 12.000% Mn, 0.001 to 0.100% P, 0.0001 to 1.0000% S, 10.0 to 35.0% Cr, 0.010 to 5.000% Ni, and one or two selected from 0.0001 to 1.0000% Nb and 0.0001 to 1.0000% Ti, with the balance being Fe and impurities, and the A value represented by the following formula (1) being 65 to 150; The ratio of the ferrite phase is 1.0 to 50.0%, The proportion of martensite phase is 50.0 to 99.0%, A stainless steel plate for brake disc rotors having a Vickers hardness of 300 to 400 HV. A value = 420C + 470N + 23Ni + 7Mn-11.5Cr-11.5Si-47Nb-49Ti + 189 ... (1) In the formula, each element symbol represents the content (%) of each element.

2. On a mass basis, 0.001 to 3.000% Cu, 0.001 to 3.000% Mo, 0.001 to 1.000% V, 0.0001 to 0.0300% B, 0.001 to 4.000% Al, 0.001 to 3.000% W, 0.0001 to 1.0000% Sn, 0.0001 to 0.0100% Mg, 0.001 to 0.500% Sb, 0.001 to 1.000% Zr, 2. The stainless steel plate for brake disc rotors according to claim 1, further comprising one or more selected from the group consisting of 0.001 to 1.000% Ta, 0.001 to 1.000% Hf, 0.0001 to 1.0000% Co, 0.0001 to 0.0200% Ca, 0.001 to 0.500% REM, 0.0001 to 0.5000% Ga, and 0.0010 to 0.1000% Bi.

3. When EBSD measurements were performed, the following conditions were met: (IQ value of ferrite phase)-(IQ value of martensite phase)≧10000 3. The stainless steel sheet for a brake disc rotor according to claim 1 or 2, which satisfies the above.

4. The following characteristics: (A) The breaking elongation at room temperature is 10.0% or more. (B) The average friction coefficient at 60 to 300°C is 0.25 to 0.

65.

3. The stainless steel sheet for a brake disc rotor according to claim 1 or 2, which satisfies one or more of the above requirements.

5. The steel contains, on a mass basis, 0.001% or more and less than 0.080% C, 0.0010 to 0.5000% N, 0.01 to 5.00% Si, 0.010 to 12.000% Mn, 0.001 to 0.100% P, 0.0001 to 1.0000% S, 10.0 to 35.0% Cr, 0.010 to 5.000% Ni, 0.0001 to 1.0000% Nb, and 0.0001 to 1.0000% Zr. A method for producing a stainless steel sheet for brake disc rotors, comprising heating a hot-rolled sheet or cold-rolled sheet having a composition containing one or two types selected from the group consisting of 1.0000% Ti and 1.0000% Ti, with the balance being Fe and impurities, and having an A value of 65 to 150, as represented by the following formula (1), at a rate of 1.0°C / second or more, holding the sheet at an annealing temperature of 1020°C to 1200°C for 0 second or more, and then cooling the sheet at a rate of 1.0°C / second or more. A value = 420C + 470N + 23Ni + 7Mn-11.5Cr-11.5Si-47Nb-49Ti + 189 ... (1) In the formula, each element symbol represents the content (%) of each element.

6. By mass, the composition contains: 0.001 to 3.000% Cu, 0.001 to 3.000% Mo, 0.001 to 1.000% V, 0.0001 to 0.0300% B, 0.001 to 4.000% Al, 0.001 to 3.000% W, 0.0001 to 1.0000% Sn, 0.0001 to 0.0100% Mg, 0.001 to 0.500% Sb, 0.001 to 1.000% Zr, 0.

6. The method for producing a stainless steel sheet for brake disc rotors according to claim 5, further comprising one or more selected from the group consisting of 0.001 to 1.000% Ta, 0.001 to 1.000% Hf, 0.0001 to 1.0000% Co, 0.0001 to 0.0200% Ca, 0.001 to 0.500% REM, 0.0001 to 0.5000% Ga, and 0.0010 to 0.1000% Bi.

7. A brake disc rotor comprising the processed stainless steel plate for brake disc rotors according to claim 1 or 2.

8. A method for manufacturing a brake disc rotor, comprising processing the stainless steel sheet for brake disc rotors according to claim 1 or 2 at room temperature.

9. The method for manufacturing a brake disc rotor according to claim 8, wherein no heat treatment is performed after said processing.

10. A stainless steel sheet for brake disc rotors, which is a hot-rolled or cold-rolled sheet containing, by mass, 0.001% or more and less than 0.080% C, 0.0010 to 0.5000% N, 0.01 to 5.00% Si, 0.010 to 12.000% Mn, 0.001 to 0.100% P, 0.0001 to 1.0000% S, 10.0 to 35.0% Cr, 0.010 to 5.000% Ni, and one or two elements selected from 0.0001 to 1.0000% Nb and 0.0001 to 1.0000% Ti, with the balance being Fe and impurities, and having a composition in which the A value, represented by the following formula (1), is 65 to 150. A value = 420C + 470N + 23Ni + 7Mn-11.5Cr-11.5Si-47Nb-49Ti + 189 ... (1) In the formula, each element symbol represents the content (%) of each element.

11. On a mass basis, 0.001 to 3.000% Cu, 0.001 to 3.000% Mo, 0.001 to 1.000% V, 0.0001 to 0.0300% B, 0.001 to 4.000% Al, 0.001 to 3.000% W, 0.0001 to 1.0000% Sn, 0.0001 to 0.0100% Mg, 0.001 to 0.500% Sb, 0.001 to 1.000% Zr, 11. The stainless steel plate for brake disc rotors according to claim 10, further comprising one or more selected from the group consisting of 0.001 to 1.000% Ta, 0.001 to 1.000% Hf, 0.0001 to 1.0000% Co, 0.0001 to 0.0200% Ca, 0.001 to 0.500% REM, 0.0001 to 0.5000% Ga, and 0.0010 to 0.1000% Bi.

12. A brake disc rotor comprising the processed stainless steel plate for brake disc rotors according to claim 10 or 11.

13. A method for manufacturing a brake disc rotor, comprising processing the stainless steel sheet for brake disc rotors according to claim 10 or 11 by hot stamping.