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

Abstract

A martensitic stainless steel plate for a brake disc rotor, characterized by containing prescribed components, and having precipitates having a particle size of 2 μm or less present in a matrix at a density of 0.01 to 20 pieces / μm 2 By making the precipitates present finely during hot rolling and in annealing of the hot-rolled plate, productivity at the time of quenching can be improved, the temper softening resistance during use as a member can be improved, cracking during hot press forming can be suppressed, and further reduction in high-temperature strength can be suppressed. Thus, a martensitic stainless steel plate having excellent temper softening resistance, hardenability, formability, and high-temperature strength, which can be applied to a disc rotor, can be obtained.

Description

Technical Field

[0001] This invention relates to martensitic stainless steel sheets for brake disc rotors with excellent hardenability, formability, tempering softening resistance, and high-temperature strength, as well as brake disc rotors and methods for manufacturing martensitic stainless steel sheets for brake disc rotors. It relates to stainless steel sheets with excellent productivity, reduced liner (also called gasket) wear, and stable hardness, suitable for use in disc rotors and the like that require thin walls and lightweight construction. Background Technology

[0002] Disc brakes are widely used as a braking system. A disc brake has a disc-shaped structure called a disc rotor that is attached to the tire. By clamping the disc rotor with brake pads, kinetic energy is converted into heat energy through friction, thus reducing the speed of cars and two-wheeled vehicles. The disc rotor used in disc brakes will be referred to below as a "brake disc rotor".

[0003] In automobiles, flake graphite cast iron (hereinafter referred to as cast iron) is used for disc rotors due to considerations such as thermal conductivity and cost. Cast iron, lacking elements to improve corrosion resistance, has poor corrosion resistance and quickly develops red rust if left exposed. Previously, this red rust was not noticeable due to the disc rotor's low position below eye level and the shape of the wheel. However, in recent years, the demand for improved fuel efficiency has led to the use of aluminum in wheel materials. Furthermore, the thinning of wheel spokes has made the disc rotor more visible, and its rust has become undeniable. Therefore, there is a desire to improve the corrosion resistance of disc rotors.

[0004] Furthermore, with increasingly stringent environmental regulations in recent years, there is a strong desire to improve the fuel efficiency of automobiles, thus creating a need for thinner-walled, lightweight disc rotors. However, cast iron has limitations in thinning due to its low strength and the fact that it is manufactured through casting. In addition, it is said that the operating temperature of automotive brakes reaches approximately 700°C. Moreover, the temperature can reach 300°C under driving conditions where braking is frequently used, such as on mountain roads. Because cast iron has low high-temperature strength, it cannot guarantee the strength required for a disc rotor at high temperatures when thinning the wall, thus posing a challenge to achieving thinner, lighter weights. Furthermore, since cast iron is formed through casting, thinning the disc rotor's wall may impair molten metal flow, making it impossible to form.

[0005] Stainless steel is a material with excellent corrosion resistance, and martensitic stainless steel, specifically the SUS410 series, is widely used in motorcycles and other two-wheeled vehicles. This is because the disc rotor of a two-wheeled vehicle is exposed and easily noticeable, thus corrosion resistance is a key consideration. On the other hand, stainless steel has the disadvantage of lower thermal conductivity than cast iron. In two-wheeled vehicles, the braking system is exposed, resulting in excellent cooling, so even stainless steel can be used without problems in normal use. However, even in two-wheeled vehicles, the following issue exists: under harsh braking conditions such as racing, the disc rotor can overheat, leading to increased wear on the brake pads.

[0006] On the other hand, in the case of automobiles, the braking system, including the tires, is housed within the wheel arches, making it difficult to cool the disc rotor. Therefore, the low thermal conductivity of stainless steel is a challenge, making it unsuitable for automotive disc rotors. However, in recent years, the adoption of "regenerative brakes," which convert kinetic energy during driving into electrical energy and recover it, is rapidly expanding in EVs, FCVs, and HVs. Through its application, the frictional heat generated by the friction between the disc rotor and the lining is reduced, thus expanding the application possibilities for stainless steel, which has a lower thermal conductivity than cast iron.

[0007] Another issue hindering the application of stainless steel in automotive disc brakes is formability. Two-wheeled vehicle disc rotors are annular discs, manufactured by stamping sheet stainless steel and then high-frequency hardening, thus requiring minimal machining. On the other hand, current automotive disc rotors are cap-shaped, with the center of the disc deeply drawn, and are manufactured through casting. To form such a shape using stainless steel sheet as raw material, deep drawing is necessary. However, the stainless steel used in two-wheeled vehicles is martensitic stainless steel, which is extremely hard and difficult to deep draw. As a solution to this problem, hot stamping, which involves pressing and forming at high temperatures, has been promoted in recent years. This allows stainless steel to be precisely formed into a cap shape.

[0008] Against this backdrop, in order to meet the recent demands for aesthetics, formability, thin walls and lightweight in automobiles, the stainless steel version of disc rotors has become necessary.

[0009] As mentioned above, in the case of automobiles, the braking system, including the tires, is housed within the wheel arches, making it difficult to cool the disc rotor, which has low thermal conductivity. Furthermore, even in two-wheeled vehicles under harsh braking conditions such as racing, the disc rotor is overheated. However, martensitic stainless steel undergoes tempering softening due to the precipitation of C and N and the recovery of dislocations at high temperatures. If tempering softening occurs, there is a problem of excessive wear on the brake linings. In addition, abnormal wear on the disc rotor and brake linings can lead to unstable brake performance and shortened brake life. Therefore, in order to apply martensitic stainless steel to automotive disc rotors, it is necessary to address the requirement of reducing brake lining wear.

[0010] Regarding stainless steel disc rotors, there are patent documents 1 and 2. These documents describe steels whose tempering softening resistance is improved by specifying the original austenite grain size and the precipitation of Nb. This document relates to an invention that improves tempering softening resistance at 600°C. Furthermore, patent documents 3 and 4 describe steels whose tempering softening resistance is improved by specifying the original austenite grain size and the precipitation of Nb and Cu. This document relates to an invention that improves tempering softening resistance at 650°C. Moreover, each invention is used as a component that utilizes precipitates formed when exposed to high temperatures through braking. If the exposure time to high temperatures is short, the required precipitation time may not be achieved.

[0011] Existing technical documents

[0012] Patent documents

[0013] Patent Document 1: Japanese Patent No. 4569360

[0014] Patent Document 2: International Publication No. WO2008 / 044299

[0015] Patent Document 3: International Publication No. WO2007 / 122754

[0016] Patent Document 4: Japanese Patent No. 5200332 Summary of the Invention

[0017] The problem that the invention aims to solve

[0018] This invention relates to stainless steel plates for brake disc rotors that exhibit excellent hardenability, formability, temper softening resistance, and high-temperature strength. The components to be addressed by this invention are braking system components, particularly disc rotors.

[0019] As mentioned above, the machining of stainless steel sheets into disc rotors is not a major process in two-wheeled vehicles, so it is manufactured by high-frequency quenching. For automotive applications, it is done by hot stamping under high temperature. The stainless steel sheets used for hot stamping are manufactured by hot rolling and hot-rolled sheet annealing. The high-temperature treatment of hot stamping also serves as a quenching treatment. From a productivity point of view, low temperature and short time are preferred for quenching heat treatment. However, with conventional martensitic stainless steels, coarse Cr carbonitrides precipitate during the hot-rolled sheet annealing process when manufacturing stainless steel sheets. To obtain sufficient hardness for disc rotors, it is necessary to ensure solid solution of C and N. Therefore, it is necessary to dissolve the coarse Cr carbonitrides precipitated in the stainless steel sheet by heating at high temperature during quenching heat treatment. That is, conventional martensitic stainless steels require high temperature heating during forming, and improved hardenability is required for increased productivity. As a productivity issue of conventional martensitic stainless steels, it is possible to ensure excellent hardenability, which allows Cr carbonitrides to dissolve even with low temperature and short time heat treatment.

[0020] Because the disc rotor of an automobile is cap-shaped, it requires formability of the steel sheet. Specifically, it needs to be able to be pressed at high temperatures during hot stamping to form the cap shape.

[0021] When using steel plates as brake disc rotors, excellent temper softening resistance is required. Since the maximum temperature reached by a typical two-wheeler is around 500°C, martensitic stainless steel can be used. However, the brake discs of four-wheelers and racing two-wheelers experience significant temper softening due to their higher temperatures, making their application difficult. While patent documents 1-4 disclose temper softening resistance at 600°C and 650°C, the temper softening resistance at 700°C required in this invention is not described in any document.

[0022] Furthermore, when using steel plates as brake disc rotors, excellent high-temperature strength is required. The temperature reaches around 100°C in typical urban driving, around 300°C in mountain driving, and up to around 700°C. Therefore, in order to achieve thinner walls, strength in the medium to high temperature range is required.

[0023] The present invention provides a martensitic stainless steel sheet for brake disc rotors that has excellent hardenability and formability when the steel sheet is processed into a brake disc rotor, and excellent tempering softening resistance and high temperature strength when used as a brake disc rotor, a brake disc rotor using the same, and a method for manufacturing the martensitic stainless steel sheet for brake disc rotors.

[0024] Methods for solving problems

[0025] To address the aforementioned issues, the inventors of this invention focused on precipitates in stainless steel sheets and conducted a detailed investigation. The steel sheet, the subject of this invention, used as a brake disc rotor, is manufactured via hot rolling and hot-rolled sheet annealing. Precipitates precipitate in the steel sheet during the hot rolling and hot-rolled sheet annealing stages. These precipitates include Cr carbonitrides and other precipitates. Among these precipitates, Cr carbonitride precipitates, by appropriately controlling their size and dispersion, can dissolve at low temperatures and in a short time during heating for forming, thereby improving hardenability and increasing productivity. Furthermore, precipitates other than Cr carbonitrides do not dissolve during heating for forming; their fine presence in the product hinders dislocation recovery when used as a component, increasing tempering softening resistance. However, if the Cr carbonitride precipitates are large, dissolution requires high temperatures and long durations, reducing productivity. Furthermore, if the precipitates other than Cr carbonitrides are coarse, they may become more prone to cracking during hot stamping and use, or the tempering softening resistance may not improve, resulting in reduced high-temperature strength. Therefore, it is believed that by appropriately controlling the steel composition and hot rolling conditions to refine these precipitates in the steel sheet, the following results can be achieved: increased hardenability leading to improved productivity, suppression of cracking during hot stamping, ensuring tempering softening resistance when used as components, and suppression of high-temperature strength reduction. Various studies have been repeatedly conducted to achieve these objectives, resulting in the following findings.

[0026] By appropriately controlling the steel composition, setting the preheating temperature to 1000–1200°C, the finishing temperature to below 800°C, the cooling rate to above 10°C / second, and the coiling temperature to below 550°C, dislocation recovery during hot rolling is suppressed, and precipitates formed during hot rolling and annealing of the hot-rolled plate are refined. This refinement of precipitates during hot rolling and annealing primarily improves hardenability by refining Cr carbonitride precipitates. Even with low-temperature, short-duration heating during quenching, the precipitates dissolve, ensuring sufficient hardening hardness for the disc rotor. Furthermore, the refinement of precipitates other than Cr carbonitrides increases the resistance to tempering softening during component use, suppressing cracking during hot stamping and thus preventing high-temperature strength reduction. Since precipitates are present before use as finished products or components, high strength is maintained even in temperature ranges where tempering softening does not occur. It should be noted that the precipitates during hot rolling and annealing of hot-rolled plates are mainly carbonitrides, intermetallic compounds, and metallic Cu of Fe, Ti, Nb, V, Cu, Mo, W, Zr, Ta, and Hf. This results in the successful provision of stainless steel plates with excellent tempering softening resistance, hardenability, formability, and high-temperature strength, suitable for use in disc rotors.

[0027] The main idea of ​​the present invention, which solves the above-mentioned problems, is as follows.

[0028] (1) A martensitic stainless steel plate for brake disc rotors, characterized in that it contains, by mass percent:

[0029] C: 0.001~0.500%

[0030] N: 0.001~0.500%

[0031] Si: 0.01~5.00%

[0032] Mn: 0.010~12.000%

[0033] P: 0.001~0.100%

[0034] S: 0.0001~1.0000%

[0035] Cr: 10.0–35.0%

[0036] Ni: 0.010~5.000%

[0037] Cu: 0.0010~3.0000%

[0038] Mo: 0.0010~3.0000%

[0039] Nb: 0.0010~1.0000%

[0040] V: 0.0010~1.0000%,

[0041] The remainder consists of Fe and impurities. The average particle size of the precipitates present in the parent phase is less than 2 μm, with precipitates ranging from 0.01 to 20 particles / μm. 2 The density exists, and the quenching hardness index A, expressed by the following formula, is 200 to 800.

[0042] A=2566[%C]+1282[%N]-12[%Si]+4[%Cu]-6[%Mo]-184[%Nb]-125[%V]+239

[0043] (2) The martensitic stainless steel plate for brake disc rotor according to (1), characterized in that it further contains, by mass percent, one or more of the following elements in place of a portion of the above-mentioned Fe:

[0044] Ti: 0.001~1.00%

[0045] B: 0.0001~0.0100%

[0046] Al: 0.001–4.0%

[0047] W: 0.001~3.0%

[0048] Sn: 0.001~1.00%

[0049] Mg: 0.0001~0.0100%

[0050] Sb: 0.001~0.50%

[0051] Zr: 0.001~1.000%

[0052] Ta: 0.001~1.00%

[0053] Hf: 0.001~1.000%

[0054] Co: 0.001~1.00%

[0055] Ca: 0.0001~0.0200%

[0056] REM: 0.001~0.50%

[0057] Ga: 0.0001~0.5000%.

[0058] (3) The martensitic stainless steel plate for brake disc rotor according to (1) or (2) is characterized in that the elongation at break at 1050°C is 50% or more.

[0059] (4) A martensitic stainless steel plate for brake disc rotor, characterized in that it is a martensitic stainless steel plate for brake disc rotor as described in any one of (1) to (3), wherein, relative to the hardness when subjected to a simulated heat treatment of hot stamping (hereinafter referred to as "virtual heat treatment") after being heated to 1050°C and held for more than 5 seconds and then water-cooled, the decrease in hardness after further tempering at 700°C for 10 minutes after the virtual heat treatment is less than 150 in terms of Hv.

[0060] (5) A martensitic stainless steel plate for brake disc rotor, characterized in that it is a martensitic stainless steel plate for brake disc rotor as described in any one of (1) to (4), wherein when a simulated heat treatment of hot stamping at 700°C is performed after heating to 1050°C and holding for more than 5 seconds and then water-cooled (hereinafter referred to as "virtual heat treatment"), the yield strength of the material at 700°C is more than 50 MPa.

[0061] (6) A brake disc rotor, wherein the brake disc rotor used in any one of (1) to (5) is made of martensitic stainless steel sheet.

[0062] The method for manufacturing a martensitic stainless steel plate for a brake disc rotor according to any one of (7)(1) to (5) is characterized in that the finishing rolling temperature during hot rolling is set to 800°C or less, and the coiling temperature is set to 550°C or less.

[0063] According to the present invention, a material is provided that improves the hardenability and formability of stainless steel sheets, increases the tempering softening resistance and high-temperature strength of stainless steel sheets after virtual heat treatment, and is suitable for disc rotors of automobiles and two-wheeled vehicles, which can significantly improve the appearance and improve safe braking in various environments. Detailed Implementation

[0064] The basis for specifying the component content in steel is described below.

[0065] The term "martensitic stainless steel plate" here refers to stainless steel plates in which the martensitic phase constitutes more than 80% of the steel plate area during quenching. In hot-rolled plates (stainless steel plates before hot rolling and annealing), the martensitic phase constitutes the majority; in hot-rolled and annealed plates (the stainless steel plate of this invention), the ferrite phase constitutes the majority; and after quenching using hot stamping (the brake disc rotor of this invention), it becomes a martensitic phase or a mixture of martensitic and ferrite phases. Additionally, a slight amount of austenitic phase may remain.

[0066] The preferred composition (mass%) of the stainless steel sheet of the present invention will be described below.

[0067] C is an element that is dissolved in the parent phase and has a significant impact on hardness. Heat treatment generates carbides, which deteriorates formability and corrosion resistance, leading to a decrease in high-temperature strength; therefore, the content of (A) is set at a certain level. Furthermore, excessive reduction would increase refining costs; therefore, the content of (B) is preferred. A content of (C) is even more preferred.

[0068] (A) = 0.001~0.500%

[0069] (B) = 0.010~0.300%

[0070] (C) = 0.030–0.070%.

[0071] Both nitrogen (N) and carbon (C) are elements that are dissolved in the matrix phase and have a significant impact on hardness. Heat treatment generates nitrides, which deteriorates formability and corrosion resistance, leading to a decrease in high-temperature strength; therefore, the content of (A) is set at a certain level. Furthermore, excessive reduction would increase refining costs; therefore, the content of (B) is preferred. The content of (C) is even more preferred.

[0072] (A) = 0.001~0.500%

[0073] (B) = 0.010~0.100%

[0074] (C) = 0.020–0.050%.

[0075] Si is also a useful element as a deoxidizer and improves oxidation resistance and resistance to high-temperature salt damage. However, excessive addition will reduce room temperature ductility, so the content of (A) is set. However, if pickling properties and toughness are considered, the content of (B) is preferred. Furthermore, if manufacturability is considered, the content of (C) is preferred.

[0076] (A) = 0.01–5.00%

[0077] (B) = 0.10–1.00%

[0078] (C) = 0.20–0.40%.

[0079] Mn is added as a deoxidizer and contributes to increased high-temperature strength in the mid-temperature range. However, excessive addition leads to the formation of Mn-based oxides on the surface at high temperatures, making it prone to poor oxide scale adhesion and abnormal oxidation. In particular, when added in combination with Mo and W, there is a tendency for abnormal oxidation of the Mn content, therefore the content is set to (A). Furthermore, considering pickling properties and room-temperature ductility in steel sheet manufacturing, the content of (B) is preferred. The content of (C) is even more preferred.

[0080] (A) = 0.010~12.000%

[0081] (B) = 0.400–2.000%

[0082] (C) = 1.000–1.500%.

[0083] P is an impurity that mainly gets mixed into the raw materials during steelmaking and refining. If the content is high, the toughness and weldability will decrease. Therefore, it is preferable to minimize it as much as possible. However, setting it below 0.001% would increase costs due to the use of low-P raw materials, so it is set to 0.001% or more in this invention. On the other hand, since a content exceeding 0.100% leads to significant hardening, and corrosion resistance, toughness, and pickling properties also deteriorate, 0.100% is set as the upper limit. Considering the cost of raw materials, 0.008% to 0.080% is preferred, and 0.010% to 0.050% is even more preferred.

[0084] S is an element that degrades corrosion resistance and oxidation resistance, but it is one that not only improves processability by combining with Ti and C, but also provides lubricity by forming sulfides with Cr, Mn, etc. Its effect is noticeable starting from 0.0001%, therefore the lower limit is set at 0.0001%. On the other hand, excessive addition leads to combination with Ti and C, reducing the amount of dissolved Ti and causing coarser precipitates, resulting in lower high-temperature strength; therefore, the upper limit is set at 1.0000%. Furthermore, considering refining costs and high-temperature oxidation characteristics, 0.0005 to 0.0500% is preferred. Even more preferably, 0.0010 to 0.0100% is set.

[0085] Cr is an essential element in this invention to ensure oxidation resistance and corrosion resistance. In cases of low content, oxidation resistance cannot be guaranteed, and excessive addition leads to reduced processability and deterioration of toughness; therefore, the content is set to (A). Furthermore, considering manufacturability and oxide scale peeling resistance, the content of (B) is preferred. The content of (C) is even more preferred.

[0086] (A) = 10.0% to 35.0%

[0087] (B) = 10.5–15.0%

[0088] (C) = 11.0% to 13.0%.

[0089] Ni is an element that improves oxidation resistance, toughness, and high-temperature strength, and is added as needed. However, excessive addition will increase costs, so the content is set to (A). Considering manufacturability, the content of (B) is preferred. The content of (C) is even more preferred.

[0090] (A) = 0.010–5.000%

[0091] (B) = 0.030~0.600%

[0092] (C) = 0.050–0.080%.

[0093] Cu is an effective element for improving corrosion resistance. Precipitation strengthening, which utilizes ε-Cu precipitation, improves tempering softening resistance and high-temperature strength. However, excessive addition reduces hot workability, so the content is set to (A). Furthermore, considering thermal fatigue characteristics, manufacturability, and weldability, the content of (B) is preferred. The content of (C) is even more preferred.

[0094] (A) = 0.0010~3.0000%

[0095] (B) = 0.0100~2.0000%

[0096] (C) = 0.2000–1.6000%.

[0097] Mo is an effective element for solid solution strengthening at high temperatures and improves tempering softening resistance, corrosion resistance, and resistance to high-temperature salt damage; therefore, it is added. Excessive addition will significantly degrade room-temperature ductility and oxidation resistance; therefore, the content is set to (A). Furthermore, considering thermal fatigue characteristics and manufacturability, the content of (B) is preferred. Even more preferred is the content of (C).

[0098] (A) = 0.0010~3.0000%

[0099] (B) = 0.0100~1.0000%

[0100] (C) = 0.0300~0.5000%.

[0101] Nitrogen (Nb) is an effective element for improving tempering softening resistance and high-temperature strength through solid solution strengthening and precipitation strengthening of fine precipitates. Furthermore, it also has the following effects: fixing C and N as carbonitrides contributes to the corrosion resistance of the finished sheet (hot-rolled annealed sheet) and promotes the development of recrystallization texture that affects the r-value. Excessive addition leads to significant hardening and deterioration of manufacturability; therefore, the content is set at (A). Furthermore, considering raw material cost and toughness, the content of (B) is preferred. The content of (C) is even more preferred.

[0102] (A) = 0.0010~1.0000%

[0103] (B) = 0.0100~0.7000%

[0104] (C) = 0.1000~0.5000%.

[0105] V is an element that improves corrosion resistance, but if added in excess, the precipitates become coarser, resulting in reduced tempering softening resistance, lower high-temperature strength, and deteriorated oxidation resistance. Therefore, the content is set to (A). Furthermore, considering manufacturing costs and manufacturability, the content of (B) is preferred. Even more preferred is the content of (C).

[0106] (A) = 0.0010~1.0000%

[0107] (B) = 0.0030~0.5000%

[0108] (C) = 0.1000~0.4000%.

[0109] The steel plate of the present invention is further characterized in that the quenching hardness index A, expressed by the following formula, is 200 to 800. In the following formula, [% element symbol] refers to the content (mass %) of the element. By having a quenching hardness index A of 200 or more, sufficient hardness can be obtained for use as a brake disc rotor. If the quenching hardness index A exceeds 800, the quenching hardness becomes excessively high, and the toughness becomes insufficient during use.

[0110] A=2566[%C]+1282[%N]-12[%Si]+4[%Cu]-6[%Mo]-184[%Nb]-125[%V]+239

[0111] The remainder of the invention consists of Fe and impurities. It may also, if necessary, contain the following components to replace a portion of the Fe.

[0112] Ti, when combined with C, N, and S, enhances corrosion resistance, resistance to intergranular corrosion, room-temperature ductility, and deep-drawing capability. Furthermore, when added in combination with Nb and Mo, appropriate amounts increase the solid solution content of Nb and Mo during hot rolling annealing, improve high-temperature strength, and enhance tempering softening resistance and thermal fatigue characteristics. This effect begins to appear at levels above 0.001%, therefore the lower limit is set at 0.001%. On the other hand, additions exceeding 1.00% lead to an increase in the amount of solid-solid Ti, resulting in decreased room-temperature ductility and the formation of coarse Ti-based precipitates, which become the initiation point for cracking during pore-expanding processing, thus deteriorating press formability. Additionally, oxidation resistance also deteriorates; therefore, the Ti addition amount is set to 1.00% or less. Furthermore, considering the generation of surface defects and toughness, 0.001% to 0.20% is preferred.

[0113] Bo (B) is an element that improves the secondary processability, high-temperature strength, and thermal fatigue characteristics of the product during compression molding. Bo introduces fine precipitation of Laves phases and other phases, resulting in long-term stability of precipitation-strengthened components, which helps suppress strength reduction and improve thermal fatigue life. This effect is observed at concentrations of 0.0001% or higher. On the other hand, excessive addition can lead to hardening, deterioration of grain boundary corrosion resistance and oxidation resistance, and weld cracking; therefore, the concentration is set to 0.0100% or lower. Furthermore, considering corrosion resistance and manufacturing costs, a concentration of 0.0001 to 0.0050% is preferred. A concentration of 0.0001 to 0.0020% is even more preferred.

[0114] Al is added as a deoxidizing element and also improves oxidation resistance. Furthermore, as a solid solution strengthening element, Al is useful for improving high-temperature strength and tempering softening resistance. This effect is consistently observed starting from 0.001%. On the other hand, excessive addition leads to hardening, significantly reducing uniform elongation and toughness; therefore, an upper limit is set at 4.0%. Moreover, considering the generation of surface defects, weldability, and manufacturability, a concentration of 0.003 to 2.0% is preferred.

[0115] Like Mo, W is an effective element for solid solution strengthening at high temperatures, and it forms a Laffers phase (Fe₂W) to provide precipitation strengthening. In particular, when added in combination with Nb and Mo, the Laffers phase of Fe₂(Nb, Mo, W) precipitates. However, adding W suppresses the coarsening of this Laffers phase, thus improving precipitation strengthening and tempering softening resistance. This is effective at additions of 0.001% or higher. On the other hand, additions exceeding 3.0% lead to increased costs and reduced room-temperature ductility; therefore, the upper limit is set at 3.0%. Furthermore, considering manufacturability, low-temperature toughness, and oxidation resistance, the preferred W addition amount is 0.001% to 1.5%.

[0116] Sn is an element that improves corrosion resistance and enhances high-temperature strength in the mid-temperature range, and is therefore added as needed. These effects are observed at concentrations of 0.001% or higher. On the other hand, if the addition exceeds 1.00%, manufacturability and toughness decrease significantly, so it is set to 1.00% or lower. Furthermore, considering oxidation resistance and manufacturing costs, a concentration of 0.01% to 0.10% is preferred.

[0117] Mg is sometimes added as a deoxidizing element and also contributes to refining the microstructure of the slab and improving formability. Furthermore, Mg oxides serve as precipitation sites for carbonitrides such as Ti(C,N) and Nb(C,N), resulting in their fine dispersion and precipitation. This effect is observed at concentrations above 0.0001%, contributing to improved toughness. However, excessive addition can degrade weldability, corrosion resistance, and surface quality; therefore, the upper limit is set at 0.0100%. Considering refining costs, a concentration of 0.0003–0.0010% is preferred.

[0118] Sb contributes to improved corrosion resistance and high-temperature strength, therefore, it is added at least 0.001% as needed. Since additions exceeding 0.50% may lead to excessive cracking of the slab during steel sheet manufacturing and reduced ductility, the upper limit is set at 0.50%. Furthermore, considering refining costs and manufacturability, 0.01% to 0.30% is preferred.

[0119] Zr, like Ti and Nb, is a carbonitride forming element, which improves corrosion resistance and deep drawing ability, and is added as needed. These effects are observed at concentrations of 0.001% or higher. On the other hand, additions exceeding 1.000% lead to significant deterioration in manufacturability, so the concentration is set to 1.000% or lower. Furthermore, considering cost and surface finish, a concentration of 0.001% to 0.200% is preferred.

[0120] Ta and Hf combine with C and N to improve toughness, therefore, they are added at least 0.001% as needed. However, since adding more than 1.00% increases costs and significantly degrades manufacturability, the upper limit is set at 1.00%. Furthermore, considering refining costs and manufacturability, 0.01% to 0.08% is preferred.

[0121] Co contributes to improved high-temperature strength, therefore, 0.001% or more is added as needed. Since additions exceeding 1.00% lead to decreased toughness, the upper limit is set at 1.00%. Furthermore, considering refining costs and manufacturability, 0.01% to 0.10% is preferred. More preferably, it is set to 0.01% to 0.03%.

[0122] Ca is sometimes added for desulfurization, and this effect is observed at concentrations above 0.0001%. However, additions exceeding 0.0200% result in the formation of coarse CaS, which deteriorates toughness and corrosion resistance; therefore, the upper limit is set at 0.0200%. Furthermore, considering refining costs and manufacturability, a concentration of 0.0003 to 0.0020% is preferred.

[0123] Rare earth elements (REMs) are sometimes added as needed to improve toughness and oxidation resistance by utilizing the refinement of various precipitates, with this effect observed at concentrations above 0.001%. However, because additions exceeding 0.50% significantly worsen castability and reduce ductility, the upper limit is set at 0.50%. Furthermore, considering refining costs and manufacturability, a concentration of 0.001% to 0.05% is preferred. REMs (rare earth elements), by general definition, refer to scandium (Sc) and yttrium (Y), and 15 other elements from lanthanum (La) to lutetium (Lu) (lanthanide elements). They can be added individually or in mixtures.

[0124] Ga can also be added at 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 set to 0.0001%. Furthermore, from the viewpoints of manufacturability, cost, ductility, and toughness, it is preferably 0.0020% or less.

[0125] Regarding other components, no specific provisions are made in this invention, but 0.001 to 0.1% of Bi may be added as needed. It should be noted that common harmful elements and impurities such as As and Pb are preferably minimized as much as possible.

[0126] In this invention, from the viewpoints of productivity (hardenability), formability, and tempering softening resistance and high-temperature strength during use, the presence of fine precipitates in the finished product sheet (hot-rolled annealed sheet) is important. Therefore, it is necessary to appropriately control the composition of each element and make dislocation recovery difficult during hot rolling, thus using dislocations as nucleation sites. To suppress dislocation recovery during hot rolling, the finishing temperature is set below 800°C, the cooling rate is set above 10°C / second, and the coiling temperature is set below 550°C. Furthermore, it is recognized that precipitates in the finished product sheet (hot-rolled annealed sheet) need to exist at specific sizes and densities. It should be noted that precipitates are classified into Cr carbonitride precipitates and other precipitates. Other precipitates mainly include carbonitrides of Fe, Ti, Nb, V, Cu, Mo, W, Zr, Ta, Hf, intermetallic compounds, and metallic Cu.

[0127] Specifically, the following is specified: in the stainless steel plate (after hot rolling and annealing) for brake disc rotors, the average particle size of precipitates present in the parent phase is less than 2 μm, and the precipitate concentration is 0.01 to 20 particles / μm. 2 The density exists. It should be noted that some precipitates dissolve at the quenching heat treatment temperature and others do not. Cr carbonitrides dissolve, while other precipitates are almost insoluble.

[0128] The machining of the disc rotor is carried out through hot stamping and high-frequency quenching. The heating time for quenching heat treatment is generally very short for the sake of productivity. To obtain sufficient hardness for a disc rotor, it is necessary that even with short heating, the Cr carbonitrides precipitated during hot rolling or annealing of hot-rolled plates dissolve, ensuring the solid solution of C and N. The Cr carbonitrides are easily dissolved due to their fine presence, which helps ensure the solid solution of C and N and improves hardenability. The average particle size of the precipitates present in the parent phase is less than 2 μm, with precipitates ranging from 0.01 to 20 per μm. 2 The density of Cr carbonitrides is extremely fine, causing them to dissolve even with short heating during quenching heat treatment. If the average particle size exceeds 2 μm, Cr carbonitrides will not completely dissolve during short heating in quenching heat treatment, failing to ensure sufficient solid solution of C and N, resulting in insufficient quenching hardness. Extending the heating time further hinders productivity.

[0129] Precipitates other than Cr carbonitrides are almost insoluble during heating for quenching heat treatment. By existing in minute quantities in the finished and quenched product, they hinder dislocation movement, thus contributing to increased resistance to tempering softening and suppression of high-temperature strength reduction. Furthermore, the miniaturization of the precipitates makes them less likely to become crack initiation points during processing, thereby improving formability.

[0130] That is, the average particle size of the precipitates present in the parent phase is less than 2 μm, and the precipitates are distributed at a rate of 0.01 to 20 particles / μm. 2 The density is extremely fine, so the precipitates effectively hinder the movement of dislocations, which helps to improve the tempering softening resistance and high-temperature strength.

[0131] If the average particle size of the precipitates exceeds 2 μm, they are less likely to hinder dislocation movement, thus reducing their contribution to tempering softening resistance and high-temperature strength improvement. Furthermore, they are prone to becoming crack initiation points during hot stamping and use, hindering formability. If the density of the precipitates is less than 0.01 precipitates / μm... 2 The pinning intervals of dislocations widen, thus reducing their resistance to movement. Furthermore, if the density of the precipitate exceeds 20 precipitates / μm... 2 If the strength is excessively increased, it becomes prone to cracking. Based on the above, the precipitates are defined as follows: after annealing of hot-rolled steel, the average particle size of the precipitates present in the parent phase is less than 2 μm, and the precipitate concentration is 0.01–20 precipitates / μm. 2 The density exists.

[0132] The average particle size of the precipitates is preferably 5 nm to 1.5 μm, more preferably 5 nm to 1.0 μm. The density of the precipitates is preferably 0.1 particles / μm. 2 ~20 cells / μm 2 Further preferred is 1 per μm. 2 ~20 cells / μm 2 .

[0133] This successfully provides a stainless steel plate that can be applied to disc rotors.

[0134] As a method for identifying precipitates, observation can be performed using a transmission electron microscope (e.g., the JEM2100F 200kV field emission transmission electron microscope manufactured by NEC), and analysis using an attached EDS device (e.g., the JEM2100F 200kV field emission transmission electron microscope manufactured by NEC). Samples are collected by ion milling at a depth of t / 4 (where t is the thickness of the steel plate) along the thickness direction of the observable steel plate, and ten arbitrary locations are observed and analyzed at a magnification of 50,000x. This magnification allows for a relatively uniform observation of the precipitate's state. Furthermore, at these observation locations, the composition of Fe, Cr, Si, Mn, Ti, Nb, V, Cu, Mo, W, Zr, Ta, and Hf is quantified in mass percent using the EDS device. Values ​​exceeding the detected amounts of these steel plate components are considered precipitates. Regarding the calculation of the particle size and density of the precipitates, the same method was used to observe the sample. After observing these parts, only the precipitates were colored and image processed. Then, the NIH-manufactured image analysis software "ImageJ" was used to calculate the particle size of each particle with the equivalent circle diameter, and the average particle size and average density of the five fields of view were calculated.

[0135] The martensitic stainless steel sheet for brake disc rotors of the present invention is characterized by having an elongation at break of 50% or more at 1050°C. This results in excellent formability as a steel sheet.

[0136] The martensitic stainless steel sheet for brake disc rotors of the present invention is characterized in that, relative to the hardness after undergoing a simulated heat treatment (virtual heat treatment) involving heating to 1050°C and holding for more than 5 seconds followed by water cooling, the decrease in hardness after further tempering at 700°C for 10 minutes following the virtual heat treatment is less than 150 in Hv. Therefore, excellent tempering softening resistance can be achieved as a brake disc rotor.

[0137] The martensitic stainless steel sheet for brake disc rotors of the present invention is characterized in that, during the aforementioned virtual heat treatment, the 0.2% yield strength of the material at 700°C becomes 50 MPa or higher. Therefore, excellent high-temperature strength can be achieved as a brake disc rotor.

[0138] The brake disc rotor of the present invention is made using the martensitic stainless steel sheet for brake disc rotors described above. Specifically, it is formed into the shape of a brake disc rotor by hot stamping using the martensitic stainless steel sheet for brake disc rotors of the present invention, and is quenched by heat treatment during hot stamping. It exhibits excellent tempering softening resistance and excellent high-temperature strength.

[0139] The manufacturing method will be explained next.

[0140] The method for manufacturing the stainless steel plate for the brake disc rotor of the present invention includes the steps of steelmaking, hot rolling, annealing, and pickling. In steelmaking, the following method is suitable: steel containing the above-mentioned necessary components and components added as needed is smelted in a converter, followed by two refining processes. The molten steel is then used to form slabs according to a known casting method (continuous casting).

[0141] The slab is heated to a specified temperature and hot-rolled to a specified thickness through continuous rolling. After hot rolling, the slab is coiled through a hot rolling mill consisting of multiple stands. By finely precipitating the carbonitrides that precipitate during the annealing after hot rolling, even the short heating during hot stamping allows the carbonitrides to dissolve in the parent phase. To ensure fine carbonitride precipitation, dislocation recovery is difficult during hot rolling, thus using dislocations as nucleation sites. To suppress dislocation recovery during hot rolling, the finishing temperature is set to 800°C or below, and the coiling temperature is set to 550°C or below. Preferably, from a productivity point of view, the finishing temperature is set to 750°C or below, and the coiling temperature is set to 500°C or below. More preferably, the finishing temperature is 700°C or below, and the coiling temperature is 450°C or below; more preferably, the finishing temperature is below 700°C. It should be noted that the cooling rate between the finishing mill and the coiling should preferably be above 10°C / second and below 25°C / second.

[0142] The hot-rolled coils are annealed in an annealing furnace at a specified temperature, followed by pickling. The annealing temperature is 820℃~900℃ and the time is set to 3 hours~5 hours. As for the pickling method, any existing pickling method can be used.

[0143] The martensitic stainless steel sheet used for brake disc rotors manufactured in this manner can be used with a sheet thickness of 2.0 mm to 15.0 mm. Considering the rigidity and weight of the brake disc rotor, a thickness of 3.0 mm to 13.0 mm is preferred, and 4.1 mm to 9.0 mm is more preferred.

[0144] Example

[0145] The steels with the compositions shown in Tables 1 and 2 were melted and cast into ingots. These ingots were then hot-rolled to produce 6mm thick hot-rolled plates. Using the hot-rolling finishing and coiling temperatures shown in Tables 3 and 4, the cooling rate between finishing and coiling was set to 12°C / second. The resulting hot-rolled plates were held at 850°C for 4 hours and then cooled to room temperature to produce annealed hot-rolled plates. Nos. A1 to A34 in Tables 1 and 2 are the steels of this invention, and Nos. B1 to B13 in Table 2 are comparative steels. Values ​​deviating from the scope of this invention are underlined.

[0146] For the hot-rolled annealed sheet before hot stamping, in order to evaluate the press formability at high temperatures, a high-temperature tensile test piece is collected from the hot-rolled annealed sheet in such a way that the rolling direction becomes the tensile direction, and a tensile test is carried out at 1050°C, and the elongation at break is measured (in accordance with JIS G 0567, the value is rounded off to the nearest whole number after the decimal point). Here, if the elongation at break at 1050°C is 50% or more, it can be formed into a cap shape, so those with an elongation at break at 1050°C of 50% or more are set as qualified (recorded as "A" mark in "Press formability" in Table 3 and Table 4). For those that are unqualified, they are recorded as "X" mark in Table 3 and Table 4. The same applies to the evaluation of the hardenability, tempering softening resistance after hot stamping, and high-temperature strength of the following steel plates.

[0147] The hot-rolled annealed sheet is subjected to a hot stamping simulation heat treatment (hereinafter simply referred to as "virtual heat treatment") of heating to 1050°C and holding for 5 seconds or more, and then water cooling. After the virtual heat treatment, pickling is carried out on the steel plate. Through the evaluation of the steel plate after the virtual heat treatment, the hardenability, tempering softening resistance after hot stamping, and high-temperature strength of the steel plate are evaluated.

[0148] In order to evaluate the hardenability, test pieces subjected to heat treatment of holding at 900°C for 1 second and then water cooling and holding at 1100°C for 1 second and then water cooling are made (hereinafter referred to as "900°C quenched heat treatment material" and "1100°C quenched heat treatment material"), and the Vickers hardness is collected (in accordance with JIS Z 2244, the average value of the t / 2 part, a load of 5 kg, and n = 5 is used as the hardness. The value is rounded off to the nearest whole number after the decimal point). Here, if the difference in hardness between the 900°C quenched heat treatment material and the 1100°C quenched heat treatment material is 50 or less in terms of Hv, it can be applied to a general disc-shaped rotor, so those with a difference in hardness between the 900°C quenched heat treatment material and the 1100°C quenched heat treatment material of 50 or less in terms of Hv are set as qualified (recorded as "A" mark in "Hardenability" in Table 3 and Table 4).

[0149] In order to evaluate the tempering softening resistance, test pieces of the virtual heat treatment material and test pieces of the virtual heat treatment material subjected to a tempering treatment at 700°C for 10 minutes (hereinafter referred to as "tempering softening treatment material") are made, and the Vickers hardness is collected (in accordance with JIS Z2244, the average value of the t / 2 part, a load of 5 kg, and n = 5 is used as the hardness. The value is rounded off to the nearest whole number after the decimal point). Here, if the difference in hardness between the virtual heat treatment material and the tempering softening treatment material is 150 or less in terms of Hv, it can be applied to a general disc-shaped rotor, so those with a difference in hardness between the virtual heat treatment material and the tempering softening treatment material of 150 or less in terms of Hv are set as qualified (recorded as "A" mark in "Tempering softening resistance" in Table 3 and Table 4).

[0150] In order to evaluate the strength during use, high-temperature tensile test pieces were collected from the virtual heat-treated material in such a way that the rolling direction became the tensile direction, and a tensile test was carried out at 700 °C to measure the 0.2% yield strength (in accordance with JIS G 0567, and the value is rounded off to the nearest whole number below the decimal point). Here, if the 0.2% yield strength at 700 °C is 50 MPa or more, it can be applied to general disk-shaped rotors and the wall thickness can be thinned. Therefore, those with a 0.2% yield strength of 50 MPa or more at 700 °C were set as qualified (recorded as "A" mark in "High-temperature strength" in Table 3 and Table 4).

[0151] [Table 1]

[0152]

[0153] [Table 2]

[0154]

[0155] [Table 3]

[0156]

[0157] [Table 4]

[0158]

[0159] As shown in Table 3 and Table 4, in terms of the hardenability, press formability, temper softening resistance after virtual heat treatment, and 0.2% yield strength at 700 °C of the steel plate, the inventive examples are superior to the comparative examples. In the case where any one of the following is unqualified: the difference between the quenching hardness at 900 °C and the quenching hardness at 1100 °C, the difference in hardness before and after tempering, the elongation at break at 1050 °C, and the 0.2% yield strength at 700 °C, it is judged as not suitable for application as a disk-shaped rotor. From this, it can be seen that the hardenability, temper softening resistance, formability, and high-temperature strength of the steel specified in the present invention are excellent.

[0160] In Comparative Examples B1 and B2, since the C and N concentrations deviated from the upper limit, a large amount of coarse carbides and nitrides were precipitated. Therefore, the Cr carbides were not sufficiently dissolved by virtual heat treatment, and the temper softening resistance was poor. In addition, the coarse carbides and nitrides did not contribute to precipitation strengthening and also became the starting points of cracking. Therefore, the 0.2% yield strength at 700 °C and the press formability were poor.

[0161] In Comparative Example B3, the Si concentration deviated from the upper limit. Since Si increases the activity of C, coarse carbides were precipitated, and the temper softening resistance, 0.2% yield strength at 700 °C, and press formability were insufficient.

[0162] In Comparative Example B4, the Mn concentration deviated from the lower limit, and the 0.2% yield strength at 700 °C was insufficient.

[0163] Comparative Example B5 exhibits insufficient 0.2% yield strength at 700°C due to the excessive phosphorus concentration exceeding the upper limit, resulting in the precipitation of coarse phosphides. Furthermore, it suffers from insufficient compressibility due to hardening.

[0164] The S concentration in Comparative Example B6 exceeded the upper limit, resulting in coarser Ti-based precipitates and insufficient 0.2% yield strength at 700°C.

[0165] Comparative Example B7, due to its Cr concentration exceeding the upper limit, precipitated a large amount of coarse Cr carbonitrides, resulting in insufficient hardenability, tempering softening resistance, and 0.2% yield strength at 700°C. Furthermore, it exhibited poor compressibility due to hardening.

[0166] In Comparative Example B8, the Cu concentration was below the lower limit, resulting in insufficient Cu precipitation. This led to inadequate precipitation strengthening, insufficient tempering softening resistance, and insufficient 0.2% yield strength at 700°C.

[0167] Comparative Examples B9, 10, and 11, due to the concentrations of Mo, Nb, and V exceeding the lower limit, did not fully precipitate containing each element, resulting in insufficient precipitation strengthening, and inadequate tempering softening resistance and 0.2% yield strength at 700°C.

[0168] Comparative Example B12 exhibits excessively large Cr carbonitrides and precipitates due to its hot-rolled finishing temperature and hot-rolled coiling temperature exceeding the upper limit, resulting in poor tempering softening resistance, a yield strength of 0.2% at 700°C, and poor pressing formability.

[0169] Comparative Example B13 had a Ni concentration that exceeded the lower limit, and its yield strength of 0.2% at 700°C was insufficient.

Claims

1. A martensitic stainless steel plate for a brake disc rotor, characterized in that, It contains, by mass%: C：0.001～0.500％、 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% Nb: 0.0010~1.0000% V：0.0010～1.0000％， The remainder consists of Fe and impurities. The average particle size of the precipitates present in the parent phase is less than 2 μm, with precipitates ranging from 0.01 to 20 particles / μm. 2 The density exists, and the quenching hardness index A, expressed by the following formula, is 200–800. The elongation at break at 1050℃ is over 50%. When a simulated heat treatment, known as "virtual heat treatment," is performed, involving heating to 1050°C, holding for more than 5 seconds, and then water-cooling, the material's 0.2% yield strength at 700°C becomes above 50 MPa. The decrease in hardness (in Hv) after further tempering at 700°C for 10 minutes following the virtual heat treatment, relative to the hardness after the virtual heat treatment, is less than 150. A=2566[%C]+1282[%N]-12[%Si]+4[%Cu]-6[%Mo]-184[%Nb]-125[%V]+239.

2. The martensitic stainless steel plate for brake disc rotor according to claim 1, characterized in that, It further contains, by mass percent, one or more of the following elements in place of a portion of the Fe: Ti: 0.001~1.00% B：0.0001～0.0100％、 Al：0.001～4.0％、 W：0.001～3.0％、 Sn: 0.001~1.00% Mg: 0.0001~0.0100% Sb: 0.001~0.50% Zr：0.001～1.000％、 Ta: 0.001~1.00% Hf: 0.001~1.000% Co: 0.001~1.00% Ca: 0.0001~0.0200% REM: 0.001~0.50% Ga: 0.0001~0.5000%.

3. A brake disc rotor made of martensitic stainless steel sheet as described in claim 1 or claim 2.

4. The method for manufacturing the martensitic stainless steel plate for the brake disc rotor as described in claim 1 or claim 2, characterized in that, Set the finishing temperature during hot rolling to below 800℃ and the coiling temperature to below 550℃.

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