Martensitic stainless steel sheet for brake disc rotor, brake disc rotor, and method for manufacturing martensitic stainless steel sheet for brake disc rotor

Abstract

To provide a stainless steel plate for a brake disk rotor, the stainless steel plate having excellent hardenability, formability, tempering softening resistance, high temperature strength, and friction coefficient stability.SOLUTION: A stainless steel plate for a brake disk rotor is provided in which a precipitate existing in a parent phase and has a particle diameter of 1.5 μm or less exists at the areal rate of 0.1-10.0%. The precipitate during hot rolling and annealing hot rolled steel plate finely exists, which improves productivity at the time of hardening, improves temper softening resistance during usage as a component, stabilizes structure, suppresses cracking during hot stamping, and can suppress lowering of high temperature strength. Thereby, a stainless steel plate is obtained which is applicable to a disk rotor and is excellent in hardenability, formability, tempering softening resistance, high temperature strength, and friction coefficient stability.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a martensitic stainless steel sheet for brake disc rotors, which has excellent hardenability, formability, temper softening resistance, high-temperature strength, and friction coefficient stability, and to a method for manufacturing a brake disc rotor and a martensitic stainless steel sheet for brake disc rotors.The present invention relates to a stainless steel sheet that is suitable for use in disc rotors and the like that require excellent productivity, reduced pad wear, stable hardness, and thin, lightweight construction. [Background technology]

[0002] Disc brakes are widely used as one type of braking system. They work by squeezing a disc-shaped structure called a disc rotor, which is connected to the tire, between brake pads, converting kinetic energy into thermal energy through friction, thereby slowing down the speed of automobiles and motorcycles. In automobiles, flake graphite cast iron (hereinafter referred to as cast iron) is used as the material for disc rotors due to its thermal conductivity and cost.

[0003] Cast iron has poor corrosion resistance because it does not contain elements that improve corrosion resistance, and if left unattended, red rust will quickly form. Traditionally, this red rust was not very noticeable because the disc was positioned lower than the line of sight and the 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 disc rust more noticeable, and there has been a demand for improved corrosion resistance.

[0004] Furthermore, with the recent tightening of environmental regulations, there is a strong demand for improved automobile fuel efficiency, which requires thinner and lighter disc rotors. However, cast iron has low strength, and because it is made by casting, there are limits to how thin it can be. In addition, it is said that the maximum temperature reached when braking in a car is close to 600°C. Furthermore, in driving conditions where braking is used frequently, such as on mountain roads, the temperature reached can be as high as 300°C. Cast iron has low high-temperature strength, so when thinned, it is unable to maintain the strength required for a disc rotor at high temperatures, making it impossible to make it thinner and lighter. Furthermore, because cast iron is formed by casting, thinning the disc rotor can sometimes result in poor melt flow and make it impossible to form.

[0005] Stainless steel is a material with excellent corrosion resistance, and martensitic SUS410 materials are widely used for motorcycles and other two-wheeled vehicles. This is because motorcycle disc rotors are exposed and highly visible, making corrosion resistance a priority. However, stainless steel has the drawback of poorer thermal conductivity than cast iron. Because the brake system on motorcycles is exposed and its excellent cooling properties allow stainless steel to be used without any 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 poor thermal conductivity has made 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 issue that has prevented the use of stainless steel in automotive disc brakes is formability. Motorcycle disc rotors are ring-shaped, disc-shaped, and are stamped from stainless steel plate and then induction-hardened, requiring no significant processing. Current automobile disc rotors, on the other hand, are hat-shaped, resembling a disc with a narrowed center, and are manufactured by casting. Deep drawing is required to form such a shape from stainless steel plate. However, the stainless steel used in motorcycles is martensitic stainless steel, which is extremely hard and difficult to process. One solution to this problem has been the widespread use of hot stamping, a press-forming process at high temperatures. This has made it possible to precisely form hat shapes from stainless steel. Against this background, in order to meet the recent demands for aesthetics, thin and lightweight components, and formability in automobiles, it has become necessary to use stainless steel for disc rotors.

[0007] As mentioned above, stainless steel sheets are processed into disc rotors for motorcycles, where the process is not extensive, by induction hardening. For automobiles, hot stamping is used, where the rotors are pressed at high temperatures. The high-temperature process of hot stamping also serves as the hardening process. From a productivity perspective, low-temperature and short-time quenching heat treatment is preferable. However, in conventional martensitic stainless steels, coarse chromium carbonitrides precipitate during hot-rolled sheet annealing. To achieve sufficient hardness for disc rotors, solute carbon and nitrogen must be contained, and high-temperature heating during quenching heat treatment is required to dissolve the chromium carbonitrides. Therefore, existing martensitic stainless steels require high-temperature heating during forming, and improving hardenability is essential to improve productivity. Achieving excellent hardenability, such as dissolving chromium carbonitrides even during low-temperature and short-time heat treatment, is a productivity challenge for existing martensitic stainless steels.

[0008] Patent Document 1 describes a stainless steel disc rotor with excellent hardenability, in which the composition N≧C is used to suppress coarsening of carbides, and the carbides are solid-dissolved by heating at a low temperature for a short time, ensuring hardenability. However, the heating conditions in this patent are 900-1050°C with a 10-minute hold, and there is no consideration of heating temperatures above 1050°C or short hold times of several minutes, which are required in the present invention. There is no mention of press formability at high temperatures or friction coefficient stability during use, which are features of the present invention. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Patent No. 6526765 Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention relates to a stainless steel sheet for brake disc rotors, which has excellent hardenability, formability, temper softening resistance, high-temperature strength, and friction coefficient stability. The problem to be solved by the present invention is directed to braking system components, particularly disc rotors.

[0011] As mentioned above, stainless steel sheets are processed into automotive disc rotors using hot stamping, a process that involves pressing at high temperatures. From a productivity perspective, low-temperature, short-time quenching is preferable. However, with conventional martensitic stainless steels, coarse chromium carbonitrides precipitate during hot-rolled sheet annealing. To achieve sufficient hardness for disc rotors, solute carbon and nitrogen must be ensured, and high-temperature heating during quenching dissolves the chromium carbonitrides. In other words, existing martensitic stainless steels require high-temperature heating during forming, and improving hardenability is a key challenge for improving productivity.

[0012] Because automobile disc rotors are hat-shaped, the steel sheet must have formability, specifically, press formability at high temperatures during hot stamping to form the hat shape.

[0013] As mentioned above, in automobiles, the brake system, including the tires, is housed within the wheel well, making it difficult for the disc rotor to cool and resulting in poor thermal conductivity. Even in motorcycles, disc rotors can become excessively heated under harsh braking conditions, such as during racing. However, when martensitic stainless steel is exposed to high temperatures, C and N precipitates and dislocations recover, resulting in temper softening. This temper softening leads to excessive pad wear. Furthermore, abnormal wear of the disc rotor and brake pads can lead to unstable braking performance and a shorter lifespan. Therefore, to meet the demand for reduced brake pad wear, steel plates used as brake disc rotors must have excellent temper softening resistance. While martensitic stainless steels can be used in general motorcycles, where temperatures reach a maximum of around 500°C, the higher temperatures of brake discs for automobiles and racing motorcycles result in significant temper softening, making their application difficult.

[0014] Furthermore, when steel plate is used for brake disc rotors, it needs to have excellent high-temperature strength. The temperature that it reaches is around 100°C when driving in normal urban areas, around 300°C when driving on mountain roads, and can reach a maximum of nearly 600°C, so strength in the medium to high temperature range is required to make the plate thinner.

[0015] As mentioned above, brake discs reach temperatures ranging from low to high under various driving conditions, and so they must be able to brake stably even when the brake disc temperature rises due to braking. Therefore, to ensure safe braking, they must maintain a constant coefficient of friction even when the temperature changes.

[0016] The present invention provides a martensitic stainless steel sheet for brake disc rotors, which has excellent hardenability and formability when the steel sheet is processed into brake disc rotors, and also has excellent temper softening resistance, high-temperature strength, and friction coefficient stability when used as a brake disc rotor; a brake disc rotor using the same; and a method for manufacturing a martensitic stainless steel sheet for brake disc rotors. [Means for solving the problem]

[0017] To address the above-mentioned issues, the inventors conducted detailed research focusing on precipitates in stainless steel sheets. The steel sheets used for brake disc rotors, which are the subject of this invention, are manufactured through hot rolling and hot-rolled sheet annealing. Precipitates form in the steel sheets during the hot-rolling and hot-rolled sheet annealing stages. These precipitates include Cr carbonitrides and other types of precipitates. By appropriately controlling the size and dispersion of Cr carbonitride precipitates, they dissolve at low temperatures and in a short time during heating for forming, improving hardenability and productivity. Furthermore, precipitates other than Cr carbonitrides do not dissolve during heating for forming, but their fine presence in the product prevents dislocation recovery during use, improving tempering softening resistance, and stabilizing the friction coefficient during braking by suppressing microstructural changes during use. Brakes are devices that decelerate a vehicle by converting its kinetic energy into thermal energy through friction between the brake disc and brake pad. Therefore, brake discs are exposed to high temperatures due to frictional heat during braking at high speeds and under high loads. Exposure to high temperatures reduces solid solution strengthening due to the precipitation of solute C and N dissolved in the steel sheet, and reduces dislocation strengthening due to the recovery of dislocations introduced during quenching. The reduction in solid solution strengthening and dislocation strengthening results in a decrease in hardness (temper softening), changing the contact conditions between the disc and pad during braking. Because changes in contact conditions also affect the friction coefficient, temper softening destabilizes the friction coefficient. By precipitating fine, abundant precipitates and improving temper softening resistance, the reduction in solid solution strengthening and dislocation hardening is suppressed, and changes in the friction coefficient are also suppressed. However, if Cr carbonitride precipitates are coarse, they require high temperatures and long times to dissolve, reducing productivity. Furthermore, if precipitates other than Cr carbonitrides are coarse, cracking is more likely to occur during hot stamping and use, temper softening resistance is not improved, high-temperature strength is reduced, and structural changes can lead to an unstable friction coefficient. Therefore, by appropriately controlling the steel composition and hot rolling conditions, it was thought that it would be possible to refine these precipitates in the steel sheet, improve hardenability and thereby increase productivity, suppress cracking during hot stamping, and ensure tempering softening resistance when used as a part, while suppressing the decrease in high-temperature strength and stabilizing the friction coefficient.As a result of extensive investigations to achieve this object, the following findings were obtained.

[0018] By appropriately controlling the steel composition, setting the pre-hot rolling heating temperature to between 1000°C and 1200°C, the hot rolling finishing temperature to below 800°C, a cooling rate of at least 10°C / sec, and a coiling temperature below 550°C, the carbonitrides formed during the steelmaking process are fully dissolved, dislocation recovery during hot rolling is suppressed, and precipitates formed during hot rolling and hot-rolled sheet annealing are refined. Refining precipitates during hot rolling and hot-rolled sheet annealing allows the precipitates to dissolve even at low temperatures and for short periods during quenching, improving hardenability and ensuring sufficient quenched hardness for disc rotors. Furthermore, refining precipitates other than Cr carbonitrides improves temper softening resistance during use as a component, suppressing microstructural changes and stabilizing the friction coefficient. Furthermore, cracking during hot stamping is suppressed, preventing a decrease in high-temperature strength. Because precipitates are present in the finished sheet before use as a component, high strength is achieved even in temperatures where temper softening does not occur. The precipitates formed during hot rolling and annealing of hot-rolled sheets are mainly carbonitrides and intermetallic compounds of elements such as Fe, Ti, V, Cu, Mo, W, Zr, Ta, and Hf, as well as metallic Cu. Ti, in particular, easily combines with C and N to form carbonitrides, and this effect is more pronounced when added in combination than when added alone. This has resulted in the successful provision of a stainless steel sheet with excellent hardenability, formability, temper softening resistance, high-temperature strength, and friction coefficient stability suitable for disc rotors.

[0019] The gist of the present invention to solve the above problems is (1) In mass%, C: 0.001 to 0.500%, N: 0.001 to 0.500%, Si: 0.01 to 5.00%, Mn: 0.010~12.000%, P: 0.001~0.100%, S: 0.0001 to 1.0000%, Cr: 10.0~35.0%, Ni: 0.010~5.000%, Cu: 0.0010~3.0000%, Mo: 0.0010 to 3.0000%, V: 0.0010~1.0000%, Ti: 0.0100 to 1.0000% and the balance being Fe and impurities, the average particle size of precipitates present in the matrix is ​​1.5 μm or less, the precipitates are present in an area ratio of 0.1 to 10.0%, and a quench hardness index A, expressed by the following formula, is 200 to 800. A=1319[%C]+1699[%N]+23[%Si]+2[%Cu] +35[%Mo]-245[%Ti]-22[%V]+329 (2) Part of the Fe is replaced with, in mass%, B: 0.0001~0.0100%, Al: 0.001 to 4.0%, W: 0.001 to 3.0%, Sn: 0.001 to 1.00%, Mg: 0.0001 to 0.0100%, Sb: 0.001 to 0.50% Zr: 0.001 to 1.000%, Ta: 0.001 to 1.00%, Hf: 0.001 to 1.000%, Co: 0.001 to 1.00%, Ca: 0.0001 to 0.0200%, REM: 0.001~0.50%, Ga: 0.0001 to 0.5000% The martensitic stainless steel sheet for a brake disc rotor according to (1), characterized in that it contains one or more of the following:

[0020] (3) A martensitic stainless steel sheet for a brake disc rotor according to (1) or (2), characterized in that the fracture elongation at 1050°C is 50% or more. (4) A martensitic stainless steel sheet for brake disc rotors according to any one of (1) to (3), characterized in that the hardness of the martensitic stainless steel sheet for brake disc rotors, when subjected to a hot stamping simulated heat treatment (hereinafter simply referred to as "simulated heat treatment") in which the sheet is heated to 1050°C, held there for 5 seconds or more, and then cooled at a cooling rate of 1°C / sec or more, is reduced by 150 Hv or less after further tempering at 600°C for 2 hours after the simulated heat treatment. (5) A martensitic stainless steel sheet for a brake disc rotor according to any one of (1) to (4), characterized in that when the martensitic stainless steel sheet for a brake disc rotor is subjected to a heat treatment in which the sheet is heated to 1050°C, held there for 5 seconds or more, and then cooled at a cooling rate of 1°C / sec or more, the 0.2% yield strength of the material at 725°C is 40 MPa or more. (6) A martensitic stainless steel sheet for a brake disc rotor according to any one of (1) to (5), characterized in that when the martensitic stainless steel sheet for a brake disc rotor is subjected to a heat treatment in which the sheet is heated to 1050°C, held there for 5 seconds or more, and then cooled at a cooling rate of 1°C / sec or more, the friction coefficient at a disc temperature of 60°C to 300°C is 0.25 to 0.65 in the JASO C 406 test (braking from 130 km / h in the room temperature effectiveness test of the second effectiveness test).

[0021] (7) A brake disc rotor made using the martensitic stainless steel sheet for a brake disc rotor according to any one of (1) to (6).

[0022] (8) A method for producing a martensitic stainless steel sheet for a brake disc rotor according to any one of (1) to (6), characterized in that the slab heating temperature is 1000°C or higher and 1200°C or lower, the finishing temperature during hot rolling is 800°C or lower, and the coiling temperature is 550°C or lower. [Effects of the Invention]

[0023] As is clear from the above explanation, the present invention improves the hardenability, formability, high-temperature strength, temper softening resistance, and friction coefficient stability of stainless steel sheet, providing a material suitable for disc rotors of automobiles and motorcycles, and is highly effective in improving appearance and providing safe braking in a variety of environments. DETAILED DESCRIPTION OF THE INVENTION

[0024] The grounds for specifying the content of elements in steel will be described below.

[0025] Here, martensitic steel sheet refers to a steel sheet in which the martensite phase accounts for 80% or more by area when the steel sheet is quenched. Martensitic steel sheets are mostly martensite phases in hot-rolled sheets (stainless steel sheets before hot-rolling and annealing), and mostly ferrite phases in hot-rolled and annealed sheets (stainless steel sheets of the present invention). After quenching by hot stamping (brake disc rotors of the present invention), the martensitic steel sheet has a martensite phase or a martensite phase + ferrite phase structure. A small amount of austenite phase may also remain.

[0026] A preferred composition (mass %) of the stainless steel sheet of the present invention will be described below. C is an element that dissolves in the matrix and has a significant effect on hardness. Depending on the heat treatment, it can produce carbides, which can degrade formability and corrosion resistance and reduce high-temperature strength, so the content is set at (A). Furthermore, excessive reduction in the content leads to increased refining costs, so the content of (B) is desirable. Even more desirable is the content of (C). (A) = 0.001 to 0.500%, (B) = 0.010 to 0.300%, (C) = 0.030 to 0.100%.

[0027] Like C, N is an element that dissolves in the matrix and has a significant effect on hardness. Depending on the heat treatment, it can form nitrides, which can degrade formability and corrosion resistance and reduce high-temperature strength, so the content is set at (A). Furthermore, excessive reductions can lead to increased refining costs, so the content of (B) is desirable. Even more desirable is the content of (C). (A) = 0.001 to 0.500%, (B) = 0.010 to 0.100%, (C) = 0.020 to 0.050%.

[0028] Si is a useful element as a deoxidizer and also improves oxidation resistance and resistance to high-temperature salt damage. However, excessive addition reduces room-temperature ductility, so the content is set at (A). However, when considering pickling properties and toughness, the content of (B) is desirable. Furthermore, when considering manufacturability, the content of (C) is desirable. (A)=0.01~5.00%, (B)=0.10~1.00%, (C) = 0.20 to 0.40%.

[0029] Mn is an element added as a deoxidizer and contributes to increasing high-temperature strength in the medium temperature range. However, excessive addition can cause Mn-based oxides to form on the surface at high temperatures, making scale adhesion poor and prone to abnormal oxidation. In particular, when added in combination with Mo or W, abnormal oxidation tends to occur more easily relative to the Mn content, so the content of (A) was set. Furthermore, considering the pickling properties and room-temperature ductility in steel sheet manufacturing, the content of (B) is desirable. The content of (C) is even more desirable. (A)=0.010~12.000%, (B) = 0.400 to 2.000%, (C) = 0.900 to 1.500%.

[0030] P is an impurity that is mainly introduced from raw materials during steelmaking and refining, and a high content reduces toughness and weldability. Therefore, it is desirable to reduce its content as much as possible. However, reducing it to less than 0.001% would increase costs by using low-P raw materials, so the present invention specifies a content of 0.001% or more. On the other hand, a content of more than 0.100% significantly hardens the steel and also deteriorates corrosion resistance, toughness, and pickling properties, so the upper limit is set at 0.100%. Considering raw material costs, a content of 0.008 to 0.080% is desirable, and a content of 0.010 to 0.050% is even more desirable.

[0031] S is an element that deteriorates corrosion resistance and oxidation resistance, but it not only improves workability by combining with Ti and C, but also forms sulfides with Cr, Mn, and other elements, providing lubricity. This effect is apparent from 0.0001% and therefore the lower limit was set at 0.0001%. However, 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, so the upper limit was set at 1.0000%. Furthermore, considering refining costs and high-temperature oxidation properties, a content of 0.0005 to 0.0500% is preferable. A content of 0.0010 to 0.0100% is even more preferable.

[0032] In the present invention, Cr is an essential element for ensuring oxidation resistance and corrosion resistance. If the content is low, oxidation resistance in particular cannot be ensured, and excessive addition leads to a decrease in workability and a deterioration in toughness, so the content is set to (A). Furthermore, in consideration of manufacturability and scale spalling properties, the content of (B) is preferable, and the content of (C) is even more preferable. (A)=10.0~35.0%, (B)=10.5~15.0%, (C) = 10.5-13.0%.

[0033] Ni is an element that improves acid resistance, toughness, and high-temperature strength, and is added as needed. However, excessive addition increases costs, so the content of (A) is set. Considering manufacturability, the content of (B) is preferable, and the content of (C) is even more preferable. (A)=0.010~5.000%, (B) = 0.030 to 0.600%, (C) = 0.050 to 0.080%.

[0034] Cu is an element effective in improving corrosion resistance. Precipitation strengthening due to ε-Cu precipitation improves temper softening resistance, high-temperature strength, and friction coefficient stability, and this effect is apparent from 0.0010%. Therefore, the lower limit was set at 0.0010%. Excessive addition reduces hot workability, so the content of (A) was set. Furthermore, considering thermal fatigue properties, manufacturability, and weldability, the content of (B) is desirable. The content of (C) is even more desirable. (A)=0.0010~3.0000%, (B) = 0.0100 to 2.0000%, (C) = 0.2000 to 1.6000%.

[0035] Mo is an element effective for solid solution strengthening at high temperatures, and is added to improve temper softening resistance, friction coefficient stability, corrosion resistance, and high-temperature salt damage resistance. This effect is apparent from 0.0010% and the lower limit is set at 0.0010%. Excessive addition significantly deteriorates room-temperature ductility and oxidation resistance, so the content is set at (A). Furthermore, considering thermal fatigue properties and manufacturability, the content of (B) is desirable. The content of (C) is even more desirable. (A)=0.0010~3.0000%, (B) = 0.0100 to 1.0000%, (C) = 0.0100 to 0.5000%.

[0036] V is an element that improves corrosion resistance, and this effect is apparent from 0.0010% onwards, so the lower limit is set at 0.0010%. Excessive addition causes precipitates to coarsen, reducing temper softening resistance, high-temperature strength, and friction coefficient stability, as well as deteriorating oxidation resistance, so the content is set at (A). Furthermore, considering production costs and manufacturability, the content of (B) is desirable, and the content of (C) is even more desirable. (A)=0.0010~1.0000%, (B) = 0.0030 to 0.7000%, (C) = 0.1000 to 0.5000%.

[0037] Ti is an element that combines with C, N, and S to improve corrosion resistance, intergranular corrosion resistance, room-temperature ductility, and deep drawability. When added in combination with Mo, adding an appropriate amount increases the amount of Mo dissolved during hot-rolling annealing, improving high-temperature strength, improving temper softening resistance and thermal fatigue properties, and stabilizing the friction coefficient by suppressing microstructural changes. However, excessive addition increases the amount of dissolved Ti, reducing room-temperature ductility, and also forms coarse Ti-based precipitates that become the starting point for cracks during hole expansion processing, degrading press formability. Since oxidation resistance also deteriorates, the content of (A) was set. Furthermore, considering the occurrence of surface defects and toughness, the content of (B) is preferable. The content of (C) is even more preferable. (A)=0.0100~1.0000%, (B)=0.050~0.5000%, (C) = 0.100 to 0.2000%.

[0038] The steel sheet of the present invention is further characterized in that the quench hardness index A, which is expressed by the following formula based on the component contents, is 200 to 800. In the formula below, [% element symbol] means the content (mass%) of the element. When the quench hardness index A is 200 or more, sufficient hardness for use as a brake disc can be obtained. If the quench hardness index A exceeds 800, the quench hardness becomes excessively large and the toughness during use becomes insufficient. In consideration of formability, the quench hardness index A is set to 250 to 750, more preferably 300 to 700. A=1319[%C]+1699[%N]+23[%Si]+2[%Cu] +35[%Mo]-245[%Ti]-22[%V]+329

[0039] In the present invention, the balance is Fe and impurities. Furthermore, if necessary, the following components may be contained in place of a portion of the Fe.

[0040] B is an element that improves secondary workability during press forming of products, high-temperature strength, and thermal fatigue properties. B induces fine precipitation of Laves phases and other phases, which enhances the long-term stability of these precipitation strengthening phases, contributing to the prevention of strength degradation and the improvement of thermal fatigue life. This effect is exhibited at 0.0001% or more. However, excessive addition of B causes hardening, degrading intergranular corrosion and oxidation resistance, and also causing weld cracking, so the content is set to 0.0100% or less. Furthermore, considering corrosion resistance and manufacturing costs, a content of 0.0001 to 0.0050% is preferable. A content of 0.0001 to 0.0020% is even more preferable.

[0041] Al is added as a deoxidizing element and also improves oxidation resistance. It is also useful as a solid-solution strengthening element for improving high-temperature strength and temper softening resistance. This effect is stably exhibited from 0.001%. However, excessive addition hardens the steel, significantly reducing uniform elongation and toughness, so the upper limit is set at 4.0%. Furthermore, considering the occurrence of surface defects, weldability, and manufacturability, a content of 0.003 to 2.0% is desirable.

[0042] Like Mo, W is an effective element for solid-solution strengthening at high temperatures, and also produces a Laves phase (Fe2W), which enhances precipitation strengthening. In particular, when added in combination with Mo, the Laves phase Fe2(Mo,W) precipitates. Adding W inhibits the coarsening of this Laves phase, improving precipitation strengthening and tempering softening resistance. This effect occurs when added at 0.001% or more. However, adding more than 3.0% increases costs and reduces room-temperature ductility, so the upper limit is set at 3.0%. Furthermore, considering manufacturability, low-temperature toughness, and oxidation resistance, a W content of 0.001 to 1.5% is desirable.

[0043] Sn is an element that improves corrosion resistance and improves high-temperature strength in the medium temperature range, so it is added as needed. These effects are manifested at 0.001% or more. On the other hand, adding more than 1.00% significantly reduces manufacturability and toughness, so the content is set at 1.00% or less. Furthermore, considering oxidation resistance and manufacturing costs, a content of 0.001 to 0.10% is preferable.

[0044] Mg is sometimes added as a deoxidizing element, and also refines the structure of slabs, contributing to improved formability. Furthermore, Mg oxides serve as precipitation sites for carbonitrides such as Ti(C,N), promoting their finely dispersed precipitation. This effect is apparent at 0.0001% or more, contributing to improved toughness. However, excessive addition can lead to deterioration of weldability, corrosion resistance, and surface quality, so the upper limit is set at 0.0100%. Considering refining costs, a content of 0.0003 to 0.0010% is desirable.

[0045] Sb contributes to improving corrosion resistance and high-temperature strength, so 0.001% or more is added as needed. Addition of more than 0.50% can cause slab cracking and excessive reduction in ductility during steel plate production, so the upper limit is set at 0.50%. Furthermore, considering refining costs and manufacturability, a content of 0.01 to 0.30% is preferable.

[0046] Zr, like Ti, is a carbonitride-forming element and improves corrosion resistance and deep drawability, and is added as needed. These effects are manifested at 0.001% or more. However, addition of more than 1.000% significantly deteriorates manufacturability, so the content is set at 1.000% or less. Furthermore, considering cost and surface quality, a range of 0.001 to 0.200% is desirable.

[0047] Ta and Hf combine with C and N to improve toughness, so they are added in an amount of 0.001% or more as needed. However, adding more than 1.00% increases costs and significantly deteriorates manufacturability, so the upper limit is set at 1.00%. Furthermore, considering refining costs and manufacturability, a content of 0.01 to 0.08% is desirable.

[0048] Co contributes to improving high-temperature strength, so 0.001% or more is added as needed. Addition of more than 1.00% leads to a deterioration in toughness, so the upper limit is set to 1.00%. Furthermore, considering refining costs and manufacturability, the content is preferably 0.01 to 0.10%, and even more preferably 0.01 to 0.03%.

[0049] Ca is sometimes added for desulfurization, and this effect is manifested at 0.0001% or more. However, addition of more than 0.0200% generates coarse CaS, which deteriorates toughness and corrosion resistance, so the upper limit is set at 0.0200%. Furthermore, considering refining costs and manufacturability, a Ca content of 0.0003 to 0.0020% is preferable.

[0050] REM may be added as needed to improve toughness and oxidation resistance by refining various precipitates, and this effect is manifested at 0.001% or more. However, addition of more than 0.50% significantly deteriorates castability and reduces ductility, so the upper limit is set at 0.50%. Furthermore, considering refining costs and manufacturability, a range of 0.001 to 0.05% is desirable. REM (rare earth elements), as generally defined, refers collectively to two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) ranging from lanthanum (La) to lutetium (Lu). They may be added alone or in mixtures.

[0051] Ga may be added in an amount of 0.5000% or less to improve corrosion resistance and suppress hydrogen embrittlement. From the viewpoint of sulfide and hydride formation, the lower limit is preferably 0.0001%. Furthermore, from the viewpoints of manufacturability, cost, ductility, and toughness, 0.0020% or less is preferable.

[0052] Although the present invention does not particularly specify other components, in the present invention, Bi etc. may be added in an amount of 0.001 to 0.1% as needed. It is preferable to reduce the amount of common harmful elements and impurity elements such as As and Pb as much as possible.

[0053] In the present invention, from the viewpoints of productivity (hardenability) during forming, formability, temper softening resistance during use, high-temperature strength, and friction coefficient stability, it is important that precipitates are present in a finely divided form in the product sheet (hot-rolled and annealed sheet). This can be achieved by appropriately controlling the composition of each element and thoroughly solid-solving the carbonitrides formed during the steelmaking process, thereby making dislocation recovery difficult during hot rolling and using dislocations as nucleation sites. To fully solid-solubilize the carbonitrides formed during the steelmaking process, the slab heating temperature is set to 1000°C or higher and 1200°C or lower. To suppress dislocation recovery during hot rolling, the hot-rolling finishing temperature is set to 800°C or lower, the cooling rate is set to 10°C / sec or higher, and the coiling temperature is set to 550°C or lower. It has also been discovered that precipitates must be present in the product sheet (hot-rolled and annealed sheet) with a specific size and area ratio. The precipitates are classified into Cr carbonitride precipitates and other precipitates. The other precipitates are mainly carbonitrides of Fe, Ti, V, Cu, Mo, W, Zr, Ta, Hf, etc., intermetallic compounds, and metallic Cu.

[0054] Specifically, the specification stipulates that in stainless steel sheets for brake disc rotors (after hot-rolled sheet annealing), the average grain size of precipitates present in the matrix must be 1.5 μm or less, and that the precipitates must be present at an area ratio of 0.1 to 10.0%. Note that some precipitates dissolve at the quenching heat treatment temperature, while others do not; Cr carbonitrides dissolve, while other precipitates are almost insoluble.

[0055] Disk rotors are processed using hot stamping or induction hardening, and the heating time for quenching is generally very short for productivity reasons. To achieve sufficient hardness for a disk rotor, even a short heating period must dissolve the chromium carbonitrides that precipitate during hot rolling or hot-rolled sheet annealing, ensuring solute carbon and nitrogen. The fine chromium carbonitrides dissolve easily, contributing to the preservation of solute carbon and nitrogen and improving hardenability. When the average particle size of the precipitates in the matrix is ​​1.5 μm or less and the precipitates are present at a fine area ratio of 0.1–10.0%, the chromium carbonitrides dissolve even during the short heating period of quenching. If the average particle size exceeds 1.5 μm, the chromium carbonitrides cannot be completely dissolved during the short heating period of quenching, resulting in insufficient solute carbon and nitrogen, resulting in insufficient hardness. Longer heating times impede productivity.

[0056] Precipitates other than Cr carbonitrides hardly dissolve during the heating process for quenching, and their fine presence in the formed and quenched product prevents dislocation movement, contributing to improved temper softening resistance, suppressed deterioration of high-temperature strength, and improved friction coefficient stability. Furthermore, finer precipitates make them less likely to become the starting point for cracks during processing, improving formability. That is, the average particle size of the precipitates present in the matrix is ​​1.5 μm or less, and the precipitates are present in a fine form with an area ratio of 0.1 to 10.0%, so that the precipitates effectively hinder the movement of dislocations, contribute to improving temper softening resistance and high-temperature strength, and stabilize the friction coefficient.

[0057] If the average particle size of the precipitates exceeds 1.5 μm, they do not provide resistance to dislocation movement, and their contribution to improving temper softening resistance, high-temperature strength, and friction coefficient stability is reduced. They also tend to become the starting point for cracks during hot stamping and use, impairing formability. If the area fraction of the precipitates is less than 0.1%, the dislocation pinning interval becomes wider, making them less resistant to dislocation movement. Furthermore, if the area fraction of the precipitates exceeds 10.0%, strength increases excessively, making cracks more likely to occur. Therefore, the definition of precipitates is that after hot-rolled sheet annealing, the average particle size of the precipitates present in the parent phase is 1.5 μm or less, and the precipitates are present in an area fraction of 0.1 to 10.0%.

[0058] The average grain size of the precipitates is preferably 5 nm or more and 1.25 μm or less, and more preferably 5 nm or more and 1.0 μm or less. The area ratio of the precipitates is preferably 0.2% or more and 9.0% or less, and more preferably 0.3% or more and 7.0% or less. This has enabled us to successfully provide a stainless steel sheet that can be used for disc rotors.

[0059] Precipitates can be identified by observation using a transmission electron microscope (e.g., a 200 kV field-emission transmission electron microscope, JEM2100F, manufactured by JEOL Ltd.) and analysis using an attached EDS device (e.g., a 200 kV field-emission transmission electron microscope, JEM2100F, manufactured by JEOL Ltd.). Samples were collected using ion milling to allow observation of t / 4, and 10 random locations were observed and analyzed at 50,000x magnification. This magnification allows for the observation of the precipitate state in a nearly uniform manner. In addition, the composition of Fe, Cr, Si, Mn, Ti, V, Cu, Mo, W, Zr, Ta, and Hf at the same observation locations was quantified by the EDS device in mass percent. Precipitations were identified when values ​​greater than the added amounts of the steel sheet components were detected. The particle size and area ratio of the precipitates were calculated by observing the sample in the same way, and after observing these areas, only the precipitates were colored and the images were processed.The particle size of each particle was then calculated as a circle-equivalent diameter using the image analysis software "ImageJ" manufactured by NIH, and the average particle size and average area ratio of the five fields of view were calculated.

[0060] The martensitic stainless steel sheet for brake disc rotors of the present invention is characterized by having a fracture elongation of 50% or more at 1050° C. This allows the steel sheet to achieve excellent formability.

[0061] The martensitic stainless steel sheet for brake disc rotors of the present invention is characterized in that, compared to the hardness obtained when the quasi-heat treatment is performed, the hardness loss after the quasi-heat treatment and subsequent tempering at 600°C for two hours is 150 Hv or less, thereby enabling the brake disc rotor to achieve excellent temper softening resistance.

[0062] The martensitic stainless steel sheet for brake disc rotors of the present invention is characterized in that, when subjected to the above-mentioned pseudo-heat treatment, the material has a 0.2% yield strength of 40 MPa or more at 725°C, thereby achieving excellent high-temperature strength for brake disc rotors.

[0063] The martensitic stainless steel sheet for brake disc rotors of the present invention is characterized in that, when subjected to the above-mentioned simulated heat treatment, the friction coefficient at disc temperatures of 60°C to 300°C is 0.25 to 0.65 in the JASO C 406 test (braking from 130 km / h in the room temperature braking test of the second braking test), thereby realizing excellent friction coefficient stability as a brake disc rotor.

[0064] The brake disc rotor of the present invention is made using the martensitic stainless steel sheet for brake disc rotors of the present invention. Specifically, the martensitic stainless steel sheet for brake disc rotors of the present invention is hot stamped to form the shape of the brake disc rotor, and then quenched by heat treatment during hot stamping. The martensitic stainless steel sheet for brake disc rotors of the present invention has excellent temper softening resistance, excellent high-temperature strength, and excellent friction coefficient stability.

[0065] Next, the manufacturing method will be described. The method for manufacturing a stainless steel sheet for brake disc rotors of the present invention comprises the steps of steelmaking, hot rolling, annealing, and pickling. In steelmaking, a steel containing the essential components and optional additives is preferably produced in a converter, followed by secondary refining. The produced molten steel is formed into slabs using a known casting method (continuous casting). The slabs are heated to a predetermined temperature and hot-rolled to a predetermined thickness using continuous rolling. The slab heating temperature is set to 1000°C or higher to fully dissolve the carbonitrides formed during the steelmaking process. Furthermore, since excessive heating increases manufacturing costs, the slab heating temperature is set to 1200°C or lower. From the perspective of manufacturability, the slab heating temperature is preferably set to 1050°C or higher but 1200°C or lower. More preferably, the temperature is set to 1100°C or higher but 1200°C or lower. The hot-rolled sheet is then coiled after being rolled in a multi-stand hot rolling mill. By finely precipitating carbonitrides during annealing after hot rolling, they can be solid-dissolved in the matrix even with short heating times during hot stamping. Fine precipitation of carbonitrides can be achieved by making dislocations less likely to recover during hot rolling and using the dislocations as nucleation sites. To suppress dislocation recovery during hot rolling, the hot rolling finishing temperature is set to 800°C or less and the coiling temperature to 550°C or less. From a productivity perspective, the finishing temperature is preferably set to 750°C or less and the coiling temperature to 500°C or less. More preferably, the finishing temperature is set to 700°C or less and the coiling temperature to 450°C or less. The cooling rate between finishing and coiling is set to 10°C / sec or more. The coiled hot-rolled coil is annealed at a specified temperature in an annealing furnace and then pickled. The annealing temperature is set to 820°C to 900°C for 3 to 5 hours. Existing pickling methods can be applied. [Example]

[0066] Steels having the chemical compositions shown in Tables 1 and 2 were melted and cast into ingots, which were then hot-rolled to a thickness of 6 mm. The slab heating temperature, hot-rolling finish temperature, and hot-rolling coiling temperature shown in Tables 3 and 4 were used, and the cooling rate between finish and coiling was 12°C / sec. The resulting hot-rolled sheets were held at 850°C for 4 hours and then cooled to room temperature to obtain hot-rolled annealed sheets. Nos. A1 to A33 in Tables 1 and 2 are steels according to the present invention, and Nos. B1 to B14 in Table 2 are comparative steels. Values ​​outside the scope of the present invention are underlined.

[0067] To evaluate the press formability at high temperatures of the hot-rolled and annealed sheets before hot stamping, high-temperature tensile test pieces were taken from the hot-rolled and annealed sheets so that the rolling direction was the tensile direction, and a tensile test was carried out at 1050°C to measure the breaking elongation (in accordance with JIS G 0567, with values ​​rounded off to the nearest whole number).Here, if the breaking elongation at 1050°C is 50% or more, it can be formed into a hat shape, so those with a breaking elongation at 1050°C of 50% or more were judged to be acceptable (marked with a circle in Tables 3 and 4).

[0068] The hot-rolled annealed sheets were subjected to a simulated hot stamping heat treatment (hereinafter simply referred to as "simulated heat treatment") in which they were heated to 1050°C, held there for 5 seconds or more, and then cooled at a cooling rate of 1°C / sec or more. After the simulated heat treatment, the steel sheets were pickled. The hardenability, tempering softening resistance after hot stamping, and high-temperature strength of the steel sheets were evaluated by evaluating the steel sheets after the simulated heat treatment.

[0069] To evaluate hardenability, test pieces were prepared by heat treatment, namely, holding at 900°C for 1 second followed by cooling at a rate of 1°C / sec or faster, and holding at 1100°C for 1 second followed by cooling at a rate of 1°C / sec or faster (hereafter referred to as "900°C-quenched material" and "1100°C-quenched material"). Vickers hardness measurements were taken (based on JIS Z 2244, t / 2 section, 5 kg load, n = 5, average hardness value; values ​​rounded to the nearest whole number). Since the difference in hardness between the 900°C-quenched and 1100°C-quenched materials is 50 Hv or less, the material is suitable for general disc rotor applications. Therefore, materials with a difference in hardness between the 900°C-quenched and 1100°C-quenched materials of 50 Hv or less were considered acceptable (marked with a circle in Tables 3 and 4).

[0070] To evaluate temper softening resistance, test pieces were prepared from the pseudo-heat-treated material and from the pseudo-heat-treated material that had been tempered at 600°C for two hours (hereafter referred to as "temper-softened material"), and Vickers hardness measurements were taken (based on JIS Z 2244, t / 2 part, 5 kg load, n = 5 average value taken as hardness. Values ​​were rounded to the nearest whole number). Here, if the difference in hardness between the pseudo-heat-treated material and the temper-softened material was 150 Hv or less, the material could be used in a general disc rotor, so a test piece with a difference in hardness between the pseudo-heat-treated material and the temper-softened material of 150 Hv or less was deemed to pass (marked with a circle in Tables 3 and 4).

[0071] To evaluate the strength during use, high-temperature tensile test pieces were taken from the pseudo-heat-treated material so that the rolling direction was the tensile direction, and tensile tests were conducted at 725°C to measure the 0.2% yield strength (in accordance with JIS G 0567, with values ​​rounded off to the nearest whole number).Here, if the 0.2% yield strength at 725°C is 40 MPa or more, it can be used in general disc rotors and can be made thinner, so those with a 0.2% yield strength of 40 MPa or more at 725°C were deemed to have passed (marked with a circle in Tables 3 and 4).

[0072] To evaluate the stability of the friction coefficient during use, a disk-shaped test piece with an outer diameter of 90 mm and a thickness of 6 mm was prepared from the simulated heat-treated material and subjected to the JASO C 406 test (braking from 130 km / h in the room temperature braking test of the second braking test) to measure the friction coefficient during braking. The deceleration was 1.0 m / s. 2 ~10.0m / s 2 The average friction coefficient during one braking cycle at each deceleration was calculated. If the friction coefficient is 0.25 to 0.65 at a disc temperature of 60°C to 300°C during braking, it is applicable to a general disc rotor, so those with a friction coefficient of 0.25 to 0.65 at a disc temperature of 60°C to 300°C were deemed to have passed (marked with a circle in Tables 3 and 4). The disc temperature was measured with a thermocouple at a position 1 mm from the sliding surface.

[0073] [Table 1]

[0074] [Table 2]

[0075] [Table 3]

[0076] [Table 4]

[0077] As is clear from Tables 3 and 4, the hardenability, press formability, tempering softening resistance after simulated heat treatment, 0.2% yield strength at 725°C, and friction coefficient at disk temperatures of 60°C to 300°C of the steel sheets of the present invention examples are superior to the comparative examples. If any one of the following failed: the difference between the hardness at 900°C and the hardness at 1100°C quenching, the difference in hardness before and after tempering, the fracture elongation at 1050°C, the 0.2% yield strength at 725°C, or the friction coefficient at disk temperatures of 60°C to 300°C, the steel was deemed unsuitable for use as a disk rotor. This demonstrates that the steels specified in the present invention are excellent in hardenability, formability, tempering softening resistance, high-temperature strength, and friction coefficient stability.

[0078] In Comparative Examples B1 and B2, the C and N concentrations were outside the upper limits, resulting in the precipitation of large amounts of coarse carbonitrides, which prevented Cr carbides from being fully dissolved by quasi-heat treatment, resulting in poor hardenability, temper softening resistance, and friction coefficient stability. Furthermore, the coarse carbonitrides did not contribute to precipitation strengthening and served as crack initiation sites, resulting in poor 0.2% proof stress and press formability at 725°C. In Comparative Example B3, the Si concentration was outside the upper limit. Si increases the activity of C, causing the precipitation of coarse carbides, resulting in insufficient hardenability, temper softening resistance, friction coefficient stability, 0.2% yield strength at 725°C, and press formability. In Comparative Example B4, the Mn concentration was below the lower limit, and the 0.2% proof stress at 725°C was insufficient. In Comparative Example B5, the P concentration was outside the upper limit, and a large amount of coarse phosphides precipitated, resulting in insufficient 0.2% proof stress and friction coefficient stability at 725° C. Furthermore, press formability was insufficient due to hardening. In Comparative Example B6, the S concentration was outside the upper limit, causing coarsening of Ti-based precipitates, resulting in insufficient 0.2% yield strength at 725°C and insufficient press formability. In Comparative Example B7, the Cr concentration was outside the upper limit, and a large amount of coarse Cr carbonitrides precipitated, resulting in insufficient hardenability, temper softening resistance, friction coefficient stability, and 0.2% yield strength at 725°C. Furthermore, the press formability was poor due to hardening. In Comparative Example B8, the Cu concentration was below the lower limit, resulting in insufficient Cu precipitation and insufficient precipitation strengthening, resulting in insufficient temper softening resistance, friction coefficient stability, and 0.2% yield strength at 725°C. In Comparative Examples B9 and B10, the Mo and V concentrations were below the lower limits, respectively, and precipitates containing each element were not sufficiently formed. As a result, precipitation strengthening was insufficient, resulting in insufficient temper softening resistance, friction coefficient stability, and 0.2% yield strength at 725°C.

[0079] In Comparative Example B11, the slab heating temperature was outside the lower limit, and the hot rolling finish temperature and hot rolling coiling temperature were outside the upper limit, so that the carbonitrides formed in the steelmaking stage were not sufficiently dissolved, and the Cr carbonitrides and precipitates were excessively coarsened, resulting in poor hardenability, temper softening resistance, friction coefficient stability, 0.2% proof stress at 725°C, and press formability. In Comparative Example B12, the Ni concentration was below the lower limit, and the 0.2% proof stress at 725°C was insufficient. Comparative Example B13 had a Ti concentration outside the lower limit, and was deficient in temper softening resistance, friction coefficient stability, and 0.2% yield strength at 725°C. In Comparative Example B14, the slab heating temperature was below the lower limit, and the Cr carbonitrides formed during the steelmaking stage did not dissolve sufficiently, resulting in poor hardenability, temper softening resistance, friction coefficient stability, 0.2% yield strength at 725°C, and press formability. In Comparative Example B15, the hot rolling finish temperature and hot rolling coiling temperature were outside the upper limits, causing excessive coarsening of Cr carbonitrides and precipitates, resulting in poor hardenability, temper softening resistance, friction coefficient stability, 0.2% yield strength at 725°C, and press formability.

Claims

1. In mass%, C: 0.001 to 0.500%, N: 0.001-0.500%, Si: 0.01-5.00%, Mn: 0.010-12.000%, P: 0.001-0.100%, S: 0.0001-1.0000%, Cr: 10.0-35.0%, Ni: 0.010-5.000%, Cu: 0.0010-3.0000%, Mo: 0.0010-3.0000%, V: 0.0010-1.0000%, Ti: 0.0100~1.0000% and the balance being Fe and impurities, the average particle size of precipitates present in the matrix is ​​1.5 μm or less, the precipitates are present in an area ratio of 0.1 to 10.0%, and a quench hardness index A, expressed by the following formula, is 200 to 800. A=1319[%C]+1699[%N]+23[%Si]+2[%Cu] +35[%Mo]-245[%Ti]-22[%V]+329 Here, the precipitates are defined as those whose composition of Fe, Cr, Si, Mn, Ti, V, Cu, Mo, W, Zr, Ta, and Hf is quantified at the observation point of the precipitates and whose value is equal to or greater than the amount of the steel sheet components added.

2. In place of a part of the Fe, B: 0.0001 to 0.0100%, Al: 0.001-4.0%, W: 0.001-3.0%, Sn: 0.001 to 1.00%, Mg: 0.0001-0.0100%, Sb: 0.001 to 0.50%, Zr: 0.001 to 1.000%, Ta: 0.001 to 1.00%, Hf: 0.001-1.000%, Co: 0.001 to 1.00%, Ca: 0.0001-0.0200%, REM: 0.001-0.50%, Ga: 0.0001-0.5000% 2. The martensitic stainless steel sheet for a brake disc rotor according to claim 1, further comprising at least one of the following:

3. 3. The martensitic stainless steel sheet for brake disc rotors according to claim 1, wherein the fracture elongation at 1050°C is 50% or more.

4. 4. A martensitic stainless steel sheet for brake disc rotors according to any one of claims 1 to 3, characterized in that, compared to the hardness when subjected to a hot stamping simulated heat treatment (hereinafter simply referred to as "simulated heat treatment") in which the sheet is heated to 1050°C, held there for 5 seconds or more, and then cooled at a cooling rate of 1°C / sec or more, the martensitic stainless steel sheet for brake disc rotors is further tempered at 600°C for 2 hours after the simulated heat treatment, the reduction in hardness is 150 Hv or less.

5. 5. A martensitic stainless steel sheet for a brake disc rotor according to any one of claims 1 to 4, characterized in that when the martensitic stainless steel sheet is subjected to a heat treatment in which the sheet is heated to 1050°C, held there for 5 seconds or more, and then cooled at a cooling rate of 1°C / sec or more, the 0.2% yield strength of the material at 725°C is 40 MPa or more.

6. 6. A martensitic stainless steel sheet for brake disc rotors according to any one of claims 1 to 5, characterized in that when the martensitic stainless steel sheet for brake disc rotors is subjected to a heat treatment in which the sheet is heated to 1050°C, held there for 5 seconds or more, and then cooled at a cooling rate of 1°C / sec or more, the sheet has a friction coefficient of 0.25 to 0.65 at disc temperatures of 60°C to 300°C in a JASO C 406 test (braking from 130 km / h in the room temperature braking effectiveness test of the second braking effectiveness test).

7. A brake disc rotor made using the martensitic stainless steel sheet for brake disc rotors according to any one of claims 1 to 6.

8. The method for producing a martensitic stainless steel sheet for brake disc rotors according to any one of claims 1 to 6, characterized in that the slab heating temperature is 1000°C or higher and 1200°C or lower, the finishing temperature during hot rolling is 800°C or lower, and the coiling temperature is 550°C or lower.

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