JP7589007B2 - Stainless steel plate for brake disc rotor, brake disc rotor, and method for manufacturing stainless steel plate for brake disc rotor - Google Patents

Description

The present invention relates to a stainless steel sheet for brake disc rotors, which has excellent lubricity, workability and high-temperature strength, a brake disc rotor and a manufacturing method for the stainless steel sheet for brake disc rotors, and relates to a stainless steel sheet suitable for use in disc rotors and the like that require a stable friction coefficient and thin, lightweight construction.

Disc brakes are widely used as one type of braking system. They work by squeezing a disk-shaped structure called a disc rotor, which is connected to the tire, with brake pads, converting kinetic energy into thermal energy through friction, slowing down the speed of a car or motorcycle. In automobiles, flake graphite cast iron (hereafter referred to as cast iron) is used as the material for the disc rotor due to its thermal conductivity, cost, etc.

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. Previously, this red rust was not very noticeable because the disc was positioned below 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 no longer negligible, and there has been a demand for improved corrosion resistance.

Stainless steel is a material with excellent corrosion resistance, and martensitic SUS410-based materials are widely used for motorcycles and other two-wheeled vehicles. This is because the disc rotors of motorcycles are exposed and highly visible, so corrosion resistance is important. However, stainless steel has the problem of being inferior in thermal conductivity to cast iron. The brake system of motorcycles is exposed, and stainless steel has excellent cooling properties, so it is used without problems in normal use. However, even for motorcycles, there is the problem that the disc rotor heats up excessively in harsh braking conditions such as races, making it difficult to stabilize the friction coefficient. In the case of automobiles, the brake system including the tires is housed inside the tire housing, so the disc rotor is difficult to cool, and low thermal conductivity is one of the issues, so stainless steel has not been used.

However, in recent years, the use of "regenerative braking," which converts kinetic energy during driving into electrical energy and recovers it, has been growing rapidly in EVs, FCVs, and HVs. This application reduces the frictional heat generated by the friction between the disc rotor and pads, expanding the possibility of its application to stainless steel, which has a lower thermal conductivity than cast iron.

Another issue that has prevented stainless steel from being used in automobile disc brakes is formability. Motorcycle disc rotors are ring-shaped, disc-shaped, and are produced by stamping out stainless steel plate, so no major processing is required. On the other hand, current automobile disc rotors are hat-shaped, like a disc with the center narrowed, and are produced by casting. Deep drawing is required to process and form such a shape using stainless steel plate as the material. However, the stainless steel used in motorcycles is martensitic stainless steel, which is extremely hard and difficult to process. One way to solve this problem is hot stamping, which involves pressing at high temperatures, which has become widespread in recent years. This has made it possible to precisely form hat shapes into stainless steel as well.

Furthermore, with the recent strengthening of environmental regulations, there is a strong demand for improved fuel efficiency in automobiles, which requires thinner and lighter disc rotors. However, cast iron has low strength, and because it is made by casting, there is a limit to how thin it can be. In addition, it is said that the maximum temperature that can be reached when braking in an automobile is close to 700°C. In driving conditions where braking is used frequently, such as on mountain roads, the temperature that can be reached may be as high as 300°C. Cast iron has low high-temperature strength, and when it is thinned, it is not possible to ensure the strength required for a disc rotor at high temperatures, so there was an issue that it was not possible to make it thinner and lighter.

Against this background, in order to meet the recent demands for aesthetics, formability, and thin-walled, lightweight automobiles, it is necessary to make disc rotors out of stainless steel. Similarly, in motorcycles, there is a need to meet the demand for a stable coefficient of friction in harsh braking conditions such as during races. However, unlike cast iron, stainless steel does not have the lubricating effect of dispersed graphite flakes, and there is an issue that the coefficient of friction can become excessively high under certain conditions. An excessively high coefficient of friction can lead to abnormal wear of the disc rotor and brake pads, causing unstable braking performance and a shortened lifespan.

Patent Document 1 is available regarding stainless steel disc rotors, but it focuses mainly on workability and corrosion resistance, not lubricity. Patent Document 2 describes an iron-based sintered alloy that utilizes Cu and MnS to improve wear resistance, but it is not an invention related to stainless steel and does not mention stainless steel disc rotors.

Patent No. 6537659 JP 2009-155696 A

The present invention relates to a stainless steel sheet for brake disc rotors, which has excellent lubricity, processability and high-temperature strength. The parts that are the subject of the problem to be solved by the present invention are braking system parts, particularly disc rotors.
Disk rotors are constantly rubbing when braking, so lubrication is required to maintain a stable friction coefficient. Cast iron, the current material used in automobile brakes, is made up of pearlite and graphite, and the graphite acts as a solid lubricant, allowing for stable braking performance. However, adding C to stainless steel causes Cr carbides to precipitate, so C cannot exist as graphite.
Furthermore, because automobile disc rotors are hat-shaped, they require formability. Furthermore, the temperatures they reach are around 100°C when driving in normal city areas, around 300°C when driving on mountain roads, and can reach a maximum of nearly 700°C, so strength in the medium to high temperature range is required in order to make them thinner. Since cast iron is formed by casting, making the disc rotor thinner can cause poor melt flow, making it impossible to form. In addition, because of its low strength, there is a problem that making the rotor thinner does not ensure sufficient strength for the disc rotor.
The present invention provides a stainless steel sheet for brake disc rotors, a brake disc rotor, and a method for producing a stainless steel sheet for brake disc rotors, which have excellent lubricity, workability and high-temperature strength.

In order to solve the above problems, the inventors of the present invention conducted a detailed investigation focusing on inclusions in stainless steel sheets. In the steel sheets targeted by the present invention, inclusions crystallize out during the steelmaking stage. By appropriately controlling the shape and dispersion state of the inclusions, the inclusions become solid lubricants and improve lubricity when used as parts. However, if the shape of the inclusions is spherical, they may be prone to cracking during hot stamping and may reduce high-temperature strength. Therefore, the inventors thought that by increasing the reduction rate of hot rolling, it would be possible to suppress cracking during hot stamping, ensure lubricity when used as a part, and suppress the reduction in high-temperature strength. And, as a result of various investigations to achieve this objective, the following findings were obtained.

By appropriately controlling the amount of S added and setting the reduction rate during hot rolling to 85% or more, the sulfides are elongated. The elongation of the sulfides effectively exerts a lubricating effect, ensuring lubrication while suppressing cracking during hot stamping and further suppressing a decrease in high-temperature strength. As a result, we have succeeded in providing a stainless steel sheet with excellent lubricity, workability, and high-temperature strength that can be used for disc rotors.

The gist of the present invention to solve the above problems is to
(1) A stainless steel sheet for brake disc rotors, in which the area ratio of sulfides present in a matrix having a particle size of 0.1 μm or more and 100 μm or less is 1% or more and 50% or less, and among these, the proportion of sulfides having an aspect ratio of 1.1 or more relative to the length in the longitudinal direction and the length perpendicular thereto is 50% or more.
(2) In mass%,
C: 0.001-0.5%,
N: 0.001-0.5%,
Si: 0.01 to 5.0%,
Mn: 0.01-12%,
P: 0.001-0.1%,
S: 0.0001-1.0%,
Cr: 10.0-35.0%,
Ni: 0.01 to 5.0%,
Cu: 0.001 to 3.0%,
Mo: 0.001 to 3.0%;
The stainless steel sheet for brake disc rotors according to (1), characterized in that the balance is Fe and impurities and the stainless steel sheet is martensitic.
(3) In mass%,
C: 0.001-0.5%,
N: 0.001-0.5%,
Si: 0.01 to 5.0%,
Mn: 0.01-12%,
P: 0.001-0.1%,
S: 0.0001-1.0%,
Cr: 10.0-35.0%,
Ni: 0.01 to 5.0%,
Cu: 0.001 to 3.0%,
Mo: 0.001 to 3.0%;
The stainless steel sheet for brake disc rotors according to (1), characterized in that the balance is Fe and impurities and the stainless steel sheet is ferritic.
(4) In mass%,
C: 0.001-0.5%,
N: 0.001-0.5%,
Si: 0.01 to 5.0%,
Mn: 0.01-12%,
P: 0.001-0.1%,
S: 0.0001-1.0%,
Cr: 10.0-35.0%,
Ni: 2.0 to 35.0%,
Cu: 0.001 to 5.0%,
Mo: 0.001 to 5.0%;
The stainless steel sheet for brake disc rotors according to (1), characterized in that the balance is Fe and impurities and the stainless steel sheet is austenitic.
(5) In mass%,
C: 0.001-0.5%,
N: 0.001-0.5%,
Si: 0.01 to 5.0%,
Mn: 0.01 to 12.0%,
P: 0.001-0.1%,
S: 0.0001-1.0%,
Cr: 10.0-35.0%,
Ni: 0.01 to 8.0%,
Cu: 0.001 to 5.0%,
Mo: 0.001 to 5.0%;
The stainless steel sheet for brake disc rotors according to (1), characterized in that the balance is Fe and impurities, and the stainless steel sheet is a two-phase system.
(6) In place of a part of the Fe, further, in mass%,
Nb: 0.01-1.0%,
Ti: 0.001 to 1.0%,
B: 0.0001 to 0.0100%,
Al: 0.001-4.0%,
W: 0.001-3.0%,
V: 0.001-1.0%,
Sn: 0.01 to 1.0%,
Mg: 0.0001-0.01%,
Sb: 0.001 to 0.5%,
Zr: 0.001 to 1.0%,
Ta: 0.001 to 1.0%,
Hf: 0.001-1.0%,
Co: 0.001 to 1.0%,
Ca: 0.0001-0.02%,
REM: 0.001-0.5%,
Ga: 0.0001-0.5%
The stainless steel sheet for a brake disc rotor according to any one of (2) to (5), characterized in that it contains one or more of the following:
(7) A stainless steel sheet for a brake disc rotor according to any one of (1) to (6), characterized in that when the stainless steel sheet is 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 water-cooled, the average coefficient of friction is measured in a pin-on-disc test at room temperature, with a sliding speed of 3.9 m/s, the pressing pressure being increased from 0.4 to 0.8 to 1.6 to 2.1 to 2.5 MPa, and the sliding distance being 3.5 km for each pressing pressure, and the difference between the maximum and minimum values of the average coefficient of friction for each pressing pressure is 0.3 or less.
(8) A stainless steel sheet for a brake disc rotor according to any one of (1) to (7), characterized in that when the stainless steel sheet for a brake disc rotor is subjected to a hot stamping simulated heat treatment (hereinafter simply referred to as "simulated heat treatment") in which the stainless steel is heated to 1,050°C, held there for 5 seconds or more, and then water-cooled, the 0.2% yield strength of the material at 700°C is 50 MPa or more.
(9) A brake disc rotor comprising the stainless steel plate for brake disc rotors according to any one of (1) to (8).
(10) A method for producing a stainless steel sheet for brake disc rotors according to any one of (1) to (8), characterized in that the reduction ratio during hot rolling is 85% or more.

As is clear from the above explanation, the present invention improves the lubricity, workability, and high-temperature strength of stainless steel sheets, providing a material suitable for disc rotors of automobiles and motorcycles, and achieving significant benefits such as improved appearance and safe braking in a variety of environments.

Stainless steels for brake disc rotors are not particularly limited, but examples include martensitic, ferritic, austenitic and two-phase. The reasons for specifying the content of each component of stainless steel are described below. Note that two-phase systems include ferrite-austenite two-phase systems and ferrite-martensite two-phase systems.

Here, martensitic means that the martensite phase is 80% or more by area in the steel sheet. In the case of a hot-rolled sheet, the martensite phase accounts for the majority, and in the case of a hot-rolled annealed sheet, the ferrite phase accounts for the majority. After quenching treatment by hot stamping, the martensite phase or the martensite phase + ferrite phase structure is obtained. In addition, a small amount of austenite phase may remain.
The term "ferritic" means that the ferrite phase accounts for 80% or more by area of the steel sheet, with the remainder including retained austenite phase, martensite phase, and precipitates.
The term "austenitic" means that the austenite phase accounts for 80% or more by area of the steel sheet, with the remainder including the ferrite phase, the martensite phase, and precipitates.
A two-phase steel sheet has two crystal phases, one of which occupies 20 to 80% by area, with the remainder including the other crystal phase, precipitates, and inclusions.

A preferred composition (mass %) of the stainless steel sheet of the present invention will be described below.
C deteriorates formability and corrosion resistance, and reduces high-temperature strength. The lower the content, the better, so the content of (A) is set for each stainless steel. However, an excessive reduction in C content leads to an increase in refining costs, so the content of (B) is preferable. The content of (C) is even more preferable.
In martensitic systems, (A) = 0.001 to 0.500%,
(B) = 0.003 to 0.100%,
(C)=0.005-0.080%.
In the ferrite system,
(A)=0.001-0.500%,
(B) = 0.002 to 0.100%,
(C)=0.003-0.050%.
In the austenitic system,
(A)=0.001-0.500%,
(B) = 0.008 to 0.280%,
(C)=0.010-0.150%.
In a two-phase system,
(A)=0.001-0.500%,
(B) = 0.005 to 0.280%,
(C)=0.010-0.100%.

Like C, N deteriorates formability and corrosion resistance, and reduces high-temperature strength. The lower the content, the better, so the content of (A) is set for each stainless steel. However, an excessive reduction in N content leads to increased refining costs, so the content of (B) is preferable. The content of (C) is even more preferable.
In martensitic systems, (A) = 0.001 to 0.500%,
(B) = 0.005 to 0.1%,
(C)=0.01-0.05%.
In the ferrite system,
(A)=0.001-0.500%,
(B) = 0.003 to 0.100%,
(C)=0.005-0.050%.
In the austenitic system,
(A)=0.001-0.500%,
(B) = 0.005 to 0.200%,
(C)=0.010-0.080%.
In a two-phase system,
(A)=0.001-0.500%,
(B) = 0.10 to 0.25%,
(C)=0.15-0.20%.

Silicon 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, in consideration of pickling properties and toughness, the content of (B) is preferable. Furthermore, in consideration of manufacturability, the content of (C) is preferable.
For martensitic systems, (A) = 0.01 to 5.0%,
(B) = 0.13 to 3.0%,
(C)=0.15-1.0%.
In the ferrite system,
(A)=0.01-5.0%,
(B) = 0.13 to 3.0%,
(C)=0.15-1.0%.
In the austenitic system,
(A)=0.01-5.0%,
(B) = 0.13 to 4.0%,
(C)=0.15-3.5%.
In a two-phase system,
(A)=0.01-5.0%,
(B) = 0.13 to 4.0%,
(C)=0.15-3.5%.

Mn is an element added as a deoxidizer and contributes to increasing high-temperature strength in the medium temperature range. In addition, when added in combination with S, it forms MnS and acts as a solid lubricant. However, excessive addition forms Mn-based oxides on the surface at high temperatures, which makes it easy for poor scale adhesion and abnormal oxidation to occur. In particular, when added in combination with Mo or W, there is a tendency for abnormal oxidation to occur more easily relative to the amount of Mn, so the content is set to (A). Furthermore, in consideration of pickling properties and room temperature ductility in steel sheet manufacturing, the content of (B) is preferable. More preferably, the content of (C) is used.
For martensitic steels, (A) = 0.01 to 12.0%,
(B)=0.30~3.0%,
(C)=0.50-1.5%.
In the ferrite system,
(A)=0.01-12.0%,
(B)=0.30~3.0%,
(C)=0.50-1.5%.
In the austenitic system,
(A)=0.01-12.0%,
(B) = 0.20 to 5.0%,
(C)=0.30-3.5%.
In a two-phase system,
(A)=0.01-12.0%,
(B) = 0.20 to 5.0%,
(C)=0.30-3.5%.

P is an impurity that is mainly mixed in from raw materials during steelmaking and refining, and if the content is high, toughness and weldability decrease. For this reason, it is desirable to reduce it as much as possible, but to reduce it to less than 0.001%, costs will increase due to the use of low-P raw materials, so in this invention, it is set to 0.001% or more. On the other hand, a content of more than 0.1% will significantly harden the steel, and will also deteriorate its corrosion resistance, toughness, and pickling properties, so the upper limit is set to 0.1%. Considering raw material costs, 0.008 to 0.08% is desirable, and 0.01 to 0.05% is even more desirable. The component content in martensitic, ferritic, austenitic, and two-phase steels is the same.

Although S deteriorates corrosion resistance and oxidation resistance, it is a very important element that not only improves workability by combining with Ti and C, but also forms sulfides with Cr, Mn, etc., and exhibits lubricity. This effect is apparent from 0.0001%, so the lower limit is set at 0.0001%. On the other hand, excessive addition of S reduces the amount of dissolved Ti by combining with Ti and C and causes coarsening of precipitates, which reduces high-temperature strength, so the upper limit is set at 1.0%. Furthermore, taking into account refining costs and high-temperature oxidation properties, 0.0005 to 0.8% is preferable. 0.01 to 0.6% is more preferable. 0.03 to 0.5% is even more preferable. The component content in martensitic, ferritic, austenitic, and two-phase steels is the same.

In the present invention, Cr is an essential element for ensuring oxidation resistance and corrosion resistance. When the content is small, oxidation resistance 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 property, the content is preferably (B). More preferably, the content is (C).
For martensitic steels, (A) = 10.0 to 35.0%,
(B)=10.5-15.0%,
(C)=11.0-13.0%.
In the ferrite system,
(A)=10.0-35.0%,
(B)=10.5-25.0%,
(C)=11.0-18.0%.
In the austenitic system,
(A)=10.0-35.0%,
(B) = 16.0 to 25.0%,
(C)=17.0-22.0%.
In a two-phase system,
(A)=10.0-35.0%,
(B) = 19.0 to 25.0%,
(C)=20.0-23.0%.

Ni is an element that improves acid resistance, toughness, and high-temperature strength, and is added as necessary. However, excessive addition increases costs, so the content is set to (A). Considering manufacturability, the content of (B) is preferable, and the content of (C) is even more preferable.
For martensitic systems, (A) = 0.01 to 5.0%,
(B) = 0.03 to 2.0%,
(C)=0.05-1.0%.
In the ferrite system,
(A)=0.01-5.0%,
(B) = 0.03 to 2.0%,
(C)=0.05-1.0%.
In the austenitic system,
(A) = 2.0 to 35.0%,
(B) = 3.0 to 20.0%,
(C)=5.0-15.0%.
In a two-phase system,
(A)=0.01-8.0%,
(B) = 0.5 to 5.0%,
(C)=1.0-3.0%.

Cu is an element effective in improving corrosion resistance. Although it improves high-temperature strength by precipitation strengthening due to ε-Cu precipitation, excessive addition reduces hot workability, so the content is set to (A). Furthermore, in consideration of thermal fatigue properties, manufacturability, and weldability, the content of (B) is preferable. The content of (C) is even more preferable.
For martensitic systems, (A) = 0.001 to 3.0%,
(B) = 0.03 to 2.0%,
(C)=0.05-1.6%.
In the ferrite system,
(A)=0.001-3.0%,
(B) = 0.03 to 2.0%,
(C)=0.05-1.6%.
In the austenitic system,
(A)=0.001-5.0%,
(B) = 0.1 to 2.0%,
(C)=0.5-1.5%.
In a two-phase system,
(A)=0.001-5.0%,
(B)=0.05-2.0%,
(C)=0.1-1.5%.

Mo is an element effective for solid solution strengthening at high temperatures, and is added to improve corrosion resistance and high-temperature salt damage resistance. Excessive addition significantly deteriorates room temperature ductility and oxidation resistance, so the content is set to (A). Furthermore, in consideration of thermal fatigue properties and manufacturability, the content is preferably (B). More preferably, the content is (C).
For martensitic systems, (A) = 0.001 to 3.0%,
(B) = 0.03 to 1.5%,
(C)=0.05-0.9%.
In the ferrite system,
(A)=0.001-3.0%,
(B) = 0.03 to 1.5%,
(C)=0.05-0.9%.
In the austenitic system,
(A)=0.001-5.0%,
(B) = 0.1 to 4.0%,
(C)=0.2-2.0%.
In a two-phase system,
(A)=0.001-5.0%,
(B) = 0.1 to 4.0%,
(C)=0.2-2.0%.

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. The component contents in martensitic, ferritic, austenitic, and two-phase steels are the same.

Nb is an element that is effective in improving high-temperature strength through solid solution strengthening and precipitation strengthening of fine precipitates. It also fixes C and N as carbonitrides, and contributes to the development of recrystallized texture, which affects the corrosion resistance and r-value of the finished sheet. These effects are apparent from 0.01% and above, so the lower limit is set at 0.01%. On the other hand, adding more than 1.0% significantly hardens the steel and also deteriorates manufacturability, so the upper limit is set at 1.0%. Also, taking into account raw material costs and toughness, 0.01 to 0.6% is desirable.

Ti is an element that combines with C, N, and S to improve corrosion resistance, intergranular corrosion resistance, room temperature ductility, and deep drawability. In addition, when added in combination with Nb and Mo, adding an appropriate amount increases the amount of Nb and Mo dissolved during hot rolling and annealing, improves high-temperature strength, and improves thermal fatigue properties. This effect is apparent from 0.001% or more, so the lower limit is set at 0.001%. On the other hand, adding more than 1.0% increases the amount of dissolved Ti, reducing room temperature ductility, and forms coarse Ti-based precipitates that become the starting point of cracks during hole expansion processing, degrading press workability. In addition, oxidation resistance is also degraded, so the amount of Ti added is set to 1.0% or less. Furthermore, considering the occurrence of surface defects and toughness, 0.05 to 0.2% is preferable.

B is an element that improves the secondary workability during press processing of products, high-temperature strength, and thermal fatigue properties. B causes fine precipitation of Laves phases and the like, and exerts the long-term stability of these precipitation strengthening, suppressing strength decline and improving thermal fatigue life. This effect is exerted at 0.0001% or more. On the other hand, excessive addition causes hardening, degrading intergranular corrosion and oxidation resistance, and also causing weld cracks, so the content is set to 0.0100% or less. Furthermore, taking into consideration corrosion resistance and manufacturing costs, 0.0001 to 0.0010% is preferable. Even more preferable is 0.0001 to 0.0005%.

Aluminum 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. This effect is stably manifested from 0.001%. On the other hand, excessive addition hardens the steel and significantly reduces uniform elongation as well as toughness, so the upper limit is set at 4.0%. Furthermore, taking into consideration the occurrence of surface defects, weldability, and manufacturability, 0.01 to 2.2% is desirable.

Like Mo, W is an effective element for solid solution strengthening at high temperatures, and also produces a Laves phase (Fe ₂ W) to bring about the effect of precipitation strengthening. In particular, when added in combination with Nb or Mo, a Laves phase of Fe ₂ (Nb, Mo, W) precipitates, but adding W suppresses the coarsening of this Laves phase and improves the precipitation strengthening ability. This effect occurs when added in an amount of 0.001% or more. On the other hand, adding more than 3.0% increases the cost and reduces room temperature ductility, so the upper limit is set to 3.0%. Furthermore, considering manufacturability, low temperature toughness, and oxidation resistance, the amount of W added is preferably 0.001 to 1.5%.

V is an element that improves corrosion resistance and is added as needed. This effect is stably achieved when added at 0.001% or more. On the other hand, when added at more than 1.0%, precipitates become coarse and high-temperature strength decreases, as well as oxidation resistance deteriorates, so the upper limit is set at 1.0%. Furthermore, when considering manufacturing costs and manufacturability, a content of 0.08 to 0.5% is desirable.

Sn is an element that improves corrosion resistance and is added as necessary to improve high-temperature strength in the medium temperature range. These effects are apparent at 0.01% or more. On the other hand, adding more than 1.0% significantly reduces manufacturability and toughness, so the content is set at 1.0% or less. Furthermore, taking into consideration oxidation resistance and manufacturing costs, 0.01 to 0.1% is preferable.

Mg is sometimes added as a deoxidizing element, and also refines the structure of the slab, contributing to improved formability. Mg oxides also become precipitation sites for carbonitrides such as Ti(C,N) and Nb(C,N), and have the effect of finely dispersing and precipitating these. This effect is apparent at 0.0001% or more, and contributes to improved toughness. However, excessive addition leads to deterioration of weldability, corrosion resistance, and surface quality, so the upper limit is set at 0.01%. Considering refining costs, 0.0003 to 0.0010% is desirable.

Sb contributes to improving corrosion resistance and high-temperature strength, so 0.001% or more is added as necessary. The upper limit is set at 0.5% because the addition of more than 0.5% can cause excessive slab cracking and reduced ductility during steel plate production. Furthermore, taking into account refining costs and manufacturability, 0.01 to 0.3% is preferable.

Like Ti and Nb, Zr is a carbonitride forming element that improves corrosion resistance and deep drawability, and is added as necessary. These effects are manifested at 0.001% or more. On the other hand, adding more than 1.0% significantly deteriorates manufacturability, so the content is set at 1.0% or less. Furthermore, when considering cost and surface quality, 0.001 to 0.2% is preferable.

Ta and Hf combine with C and N to improve toughness, so 0.001% or more is added as necessary. However, adding more than 1.0% increases costs and significantly deteriorates manufacturability, so the upper limit is set at 1.0%. Furthermore, taking into account refining costs and manufacturability, 0.01 to 0.08% is preferable.

Co contributes to improving high-temperature strength, so 0.001% or more is added as necessary. Addition of more than 1.0% leads to a deterioration in toughness, so the upper limit is set at 1.0%. Furthermore, taking into consideration refining costs and manufacturability, 0.01 to 0.1% is preferable.

Ca is sometimes added for desulfurization, and this effect is apparent at a content of 0.0001% or more. However, adding more than 0.02% produces coarse CaS, which deteriorates toughness and corrosion resistance, so the upper limit is set at 0.02%. Furthermore, taking into account refining costs and manufacturability, a content of 0.0003 to 0.0020% is desirable.

REM may be added as necessary to improve toughness and oxidation resistance by refining various precipitates, and this effect is manifested at 0.001% or more. However, the addition of more than 0.5% significantly deteriorates castability and reduces ductility, so the upper limit is set at 0.5%. Furthermore, taking into account refining costs and manufacturability, 0.001 to 0.05% is preferable. According to the general definition, REM (rare earth elements) refers to two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). They may be added alone or as a mixture.

Ga may be added in an amount of 0.5% 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 and cost, as well as ductility and toughness, 0.0020% or less is preferable.

Other components are not specified in the present invention, but in the present invention, Bi, etc. may be added at 0.001 to 0.1% as necessary. It is preferable to reduce the amount of general harmful elements and impurity elements such as As and Pb as much as possible.

In the present invention, from the viewpoint of lubricity during use, it is important that the sulfides are expanded and present in a large amount, and for this purpose, it is necessary to appropriately control the amount of S added and set the hot rolling reduction to 85% or more. It has been found that from the viewpoint of lubricity in the product sheet, it is necessary for the sulfides to be present with a specific size, aspect ratio, and area ratio.
The sulfides are inclusions such as CrS and MnS, and in the present invention, the inclusions are inclusions with an S content of 20% by mass or more. The sulfides can be identified by observation with a transmission electron microscope (for example, a 200 kV field emission transmission electron microscope JEM2100F manufactured by JEOL) and analysis with an attached EDS device (for example, a 200 kV field emission transmission electron microscope JEM2100F manufactured by JEOL). The samples were collected by a double-sided jet electrolytic polishing method so that t/4 could be observed, and 10 arbitrary points were observed and analyzed at 50,000 times magnification. At this magnification, it is possible to observe the state of the sulfides almost uniformly. In addition, Fe, Cr, Mn, and S were quantified at the same observation points using an EDS device, and if the S content was 20% by mass or more, it was determined to be a sulfide.

Specifically, after hot-rolled sheet annealing, the area ratio of sulfides having a particle size of 0.1 μm to 100 μm is 1% to 50%, and among them, the ratio of sulfides having an aspect ratio of 1.1 or more in the longitudinal direction and the length perpendicular to it is 50% or more. The precipitation state before hot stamping is the same as the precipitation state after hot stamping. By having the area ratio of sulfides having a particle size of 0.1 μm to 100 μm be 1% to 50%, and among them, the ratio of sulfides having an aspect ratio of 1.1 or more in the longitudinal direction and the length perpendicular to it is 50% or more, sufficient lubricity and strength can be obtained. If the aspect ratio of the longitudinal length and the length perpendicular to it is less than 1.1, localized accumulation of strain is likely to occur in MnS and around MnS, which is likely to become the starting point of cracks during hot stamping. If the particle size exceeds 100 μm, it is likely to become the starting point of cracks during manufacturing and use. If the area ratio of sulfides with a particle size of 0.1 μm to 100 μm is less than 1%, the amount present is so small that lubricity cannot be obtained. If it exceeds 50%, the excessive MnS present will become the starting point for cracks during manufacturing.

From the above, MnS is defined as a sulfide having an area ratio of 1% to 50% and a particle size of 0.1 μm to 100 μm in terms of circle equivalent diameter, and among these, the ratio of sulfides having an aspect ratio of 1.1 or more in the longitudinal direction to the length perpendicular thereto is 50% or more.
The particle size of the sulfide is preferably 0.5 μm or more and 95 μm or less, and more preferably 1 μm or more and 90 μm or less.
The area ratio of sulfides is preferably 5% or more and 45% or less, and more preferably 10% or more and 40% or less.
The proportion of sulfides having an aspect ratio of 1.1 or more is preferably 60% or more, and more preferably 70% or more.
The particle size, aspect ratio and area ratio of the sulfides were measured by observing the t/4 part of the hot-rolled annealed material using an optical microscope. The magnification was 500 times, and the number of fields of view was 5. After observing these areas, only the precipitates were colored and the images were processed. The particle size of each particle was calculated as a circle equivalent diameter using the image analysis software "ImageJ" manufactured by NIH, and the average particle size, average aspect ratio and average area ratio of the five fields of view were calculated.
This resulted in the successful provision of a stainless steel sheet suitable for use in disc rotors.

Next, the manufacturing method will be described. The manufacturing method of the steel sheet of the present invention comprises the steps of steelmaking, hot rolling, annealing, and pickling. In steelmaking, a method in which steel containing the above-mentioned essential components and components added as necessary is melted in a converter and then subjected to secondary refining is suitable. The melted molten steel is made into a slab according to a known casting method (continuous casting). The slab is heated to a predetermined temperature and hot rolled to a predetermined plate thickness by continuous rolling. In hot rolling, the slab is rolled to a plate thickness of 1 mm to 20 mm in a hot rolling mill consisting of multiple stands, and then coiled. The reduction rate of the hot rolling is 85% or more, preferably 88%, and more preferably 90% or more. The coiled hot-rolled coil is annealed at a predetermined temperature using an annealing furnace and then pickled. As for the pickling method, an existing pickling method may be applied. Note that annealing after the hot rolling process may be omitted.

Steels with the chemical compositions shown in Tables 1-1 to 1-4 and 2 were melted and cast into ingots, which were then hot-rolled to a thickness of 6 mm. The resulting hot-rolled sheets were held at 850°C for 4 hours and cooled to room temperature to produce hot-rolled annealed sheets. In the descriptions of "phase" in Tables 1-1 to 1-4 and 2, α', α, γ, and 2 phases mean "martensite", "ferritic", "austenitic", and "two-phase", respectively. Nos. A1 to A84 in Tables 1-1 to 1-4 are steels according to the present invention, and Nos. B1 to B44 in Table 2 are comparative steels. Values outside the scope of the present invention are underlined.

The hot-rolled annealed sheets were subjected to hot stamping simulated heat treatment (hereinafter simply referred to as "simulated heat treatment") in which the sheets were heated to 1,050°C, held there for at least 5 seconds, and then cooled in water. After the simulated heat treatment, the steel sheets were pickled.

A disk-shaped test piece measuring φ175 mm x 3 mmt was made from the pseudo-heat-treated material, and a pin-on-disk test was performed at room temperature with a sliding speed of 3.9 m/s, in which the pressing pressure was increased stepwise from 0.4 → 0.8 → 1.6 → 2.1 → 2.5 MPa, and the average friction coefficient was measured at each pressing pressure. The sliding distance was 3.5 km at each pressing pressure, for a total of 17.5 km. If the difference between the maximum and minimum average friction coefficients at each pressing pressure is 0.3 or less, it is considered possible to apply the material to a general disc rotor, so those with a difference between the maximum and minimum average friction coefficients of 0.3 or less were deemed to pass (marked with a circle in Tables 3-1, 3-2, and 4).

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 a tensile test was conducted at 700°C to measure the 0.2% yield strength (based on JIS G 0567, with values rounded off to the nearest whole number). If the 0.2% yield strength at 700°C is 50 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 50 MPa or more at 700°C were deemed to have passed (marked with a circle in Tables 3-1, 3-2, and 4).

In order to evaluate the toughness of hot-rolled sheets, Charpy test pieces (notched in the C direction) were prepared from the hot-rolled sheets (before hot-rolling annealing) and Charpy impact tests were performed at room temperature. If the average impact value of three tests is 10 J/cm2 or ^more , the hot-rolled sheets can be stably manufactured, so those having an average impact value of 10 J/cm2 or ^more were deemed to have passed (marked with a circle in Tables 3-1, 3-2, and 4).

To evaluate the press formability at high temperatures of the hot-rolled annealed sheet before hot stamping, high-temperature tensile test pieces were taken from the hot-rolled annealed sheet so that the rolling direction was the tensile direction, and a tensile test was conducted at 1050°C to measure the breaking elongation (based on JIS G 0567, with the values rounded off to the nearest whole number). Here, if the breaking elongation at 1050°C is 50% or more, it can be processed into a hat shape, so those with a breaking elongation of 50% or more at 1050°C were deemed to pass (marked with a circle in Tables 3-1, 3-2, and 4).

As is clear from Tables 3-1 to 3-2 and Table 4, the stability of the friction coefficient after the simulated heat treatment and the 0.2% yield strength at 700°C are superior to the comparative examples. If either the stability of the friction coefficient after the simulated heat treatment or the 0.2% yield strength at 700°C was unsatisfactory, or if the hot-rolled sheet toughness and press formability were poor, it was determined that the steel was unsuitable for use as a disc rotor. This shows that the steel defined in the present invention has excellent lubricity, high-temperature strength, and formability.

In Comparative Examples B1, B2, B11, B12, B21, B22, B31, and B32, the C and N concentrations were outside the upper limits, and large amounts of coarse carbonitrides precipitated, so that the 0.2% proof stress at 700° C. was insufficient. In addition, the press formability was poor due to hardening.
In Comparative Examples B3, B13, B23, and B33, the Si concentration was outside the upper limit, and large amounts of coarse carbonitrides were precipitated, so that the 0.2% proof stress at 700° C. was insufficient. In addition, the press formability was poor due to hardening.
In Comparative Examples B4, B14, B24, and B34, the Mn concentration was below the lower limit, and sufficient sulfides were not formed, resulting in insufficient stability of the friction coefficient.
In Comparative Examples B5, B15, B25, and B35, the P concentration was outside the upper limit, and large amounts of coarse phosphides were precipitated, so that the 0.2% proof stress at 700° C. was insufficient. In addition, the hot-rolled sheet toughness and press formability were poor due to hardening.
In Comparative Examples B6, B16, B26, and B36, the S concentration was outside the upper limit, the amount of dissolved Ti was reduced, and the precipitates were coarsened, so that the 0.2% proof stress at 700° C. was insufficient. In addition, the press formability was poor due to hardening.
In Comparative Examples B7, B17, B27, and B37, the Cr concentration was outside the upper limit, and a large amount of coarse carbonitrides precipitated, so that the 0.2% proof stress at 700° C. was insufficient. In addition, the hot-rolled sheet toughness and press formability were poor due to hardening.
In Comparative Examples B8, B18, B28, and B38, the Ni concentration was below the lower limit, and the amount of solute Ni was reduced, so that the 0.2% proof stress at 700° C. was insufficient. In addition, the hot-rolled sheet toughness was poor due to hardening.
Comparative Examples B9, B19, B29, and B39 were outside the lower limit of the Cu concentration, and therefore Cu precipitation was insufficient, resulting in insufficient 0.2% yield strength at 700° C. Furthermore, the hot-rolled sheet toughness and press formability were poor due to hardening.
Comparative Examples B10, B20, B30, and B40 were outside the lower limit of the Mo concentration, and the amount of dissolved Mo was reduced, resulting in insufficient 0.2% yield strength at 700° C. Furthermore, press formability was poor due to hardening.

Comparative examples B41, B42, B43, and B44 had hot rolling reduction rates below the lower limit, the sulfides were not sufficiently expanded, and the friction coefficient was not stable enough.

Claims (9)

In mass%,
C: 0.001-0.5%,
N: 0.001-0.5%,
Si: 0.01 to 5.0%,
Mn: 0.01-12%,
P: 0.001-0.1%,
S: 0.03 to 1.0%,
Cr: 10.0-35.0%,
Ni: 0.01 to 5.0%,
Cu: 0.001 to 3.0%,
Mo: 0.001 to 3.0%;
The balance is Fe and impurities, the steel sheet is martensitic, and after the steel sheet is quenched, the martensite phase accounts for 80 area % or more;
A stainless steel plate for brake disc rotors, characterized in that the area ratio of sulfides having a particle size of 0.1 μm or more and 100 μm or less present in a parent phase is 1% or more and 50% or less, and among these, the proportion of sulfides having an aspect ratio of 1.1 or more relative to the length in the longitudinal direction and the length perpendicular thereto is 50% or more in terms of area ratio. In mass%,
C: 0.001-0.5%,
N: 0.001-0.5%,
Si: 0.01 to 5.0%,
Mn: 0.01-12%,
P: 0.001-0.1%,
S: 0.03 to 1.0%,
Cr: 10.0-35.0%,
Ni: 0.01 to 5.0%,
Cu: 0.001 to 3.0%,
Mo: 0.001 to 3.0%;
The balance is Fe and impurities, the alloy is ferritic , and the ferritic phase accounts for 80 area % or more;
A stainless steel plate for brake disc rotors, characterized in that the area ratio of sulfides present in a parent phase and having a particle size of 0.1 μm or more and 100 μm or less is 1% or more and 50% or less, and among these, the proportion of sulfides having an aspect ratio of 1.1 or more relative to the length in the longitudinal direction and the length perpendicular thereto is 50% or more in terms of area ratio. In mass%,
C: 0.001-0.5%,
N: 0.001-0.5%,
Si: 0.01 to 5.0%,
Mn: 0.01-12%,
P: 0.001-0.1%,
S: 0.03 to 1.0%,
Cr: 10.0-35.0%,
Ni: 2.0 to 35.0%,
Cu: 0.001 to 5.0%,
Mo: 0.001 to 5.0%;
The balance is Fe and impurities, the alloy is austenitic , and the austenite phase accounts for 80 area % or more;
A stainless steel plate for brake disc rotors, characterized in that the area ratio of sulfides having a particle size of 0.1 μm or more and 100 μm or less present in a parent phase is 1% or more and 50% or less, and among these, the proportion of sulfides having an aspect ratio of 1.1 or more relative to the length in the longitudinal direction and the length perpendicular thereto is 50% or more in terms of area ratio. In mass%,
C: 0.001-0.5%,
N: 0.001-0.5%,
Si: 0.01 to 5.0%,
Mn: 0.01 to 12.0%,
P: 0.001-0.1%,
S: 0.03 to 1.0%,
Cr: 10.0-35.0%,
Ni: 0.01 to 8.0%,
Cu: 0.001 to 5.0%,
Mo: 0.001 to 5.0%;
The balance is Fe and impurities.
A ferrite-austenite two-phase system or a ferrite-martensite two-phase system , one of the two crystal phases of the two-phase system being 20 to 80 area %;
A stainless steel plate for brake disc rotors, characterized in that the area ratio of sulfides having a particle size of 0.1 μm or more and 100 μm or less present in a parent phase is 1% or more and 50% or less, and among these, the proportion of sulfides having an aspect ratio of 1.1 or more relative to the length in the longitudinal direction and the length perpendicular thereto is 50% or more in terms of area ratio. Further, in mass %,
Nb: 0.01-1.0%,
Ti: 0.001 to 1.0%,
B: 0.0001 to 0.0100%,
Al: 0.001-4.0%,
W: 0.001-3.0%,
V: 0.001-1.0%,
Sn: 0.01 to 1.0%,
Mg: 0.0001-0.01%,
Sb: 0.001 to 0.5%,
Zr: 0.001 to 1.0%,
Ta: 0.001 to 1.0%,
Hf: 0.001-1.0%,
Co: 0.001 to 1.0%,
Ca: 0.0001-0.02%,
REM: 0.001-0.5%,
Ga: 0.0001-0.5%
The stainless steel sheet for a brake disc rotor according to any one of claims 1 to 4 , characterized in that it contains one or more of the following: 6. The stainless steel sheet for brake disc rotors according to any one of claims 1 to 5, characterized in that when the stainless steel sheet for brake disc rotors is subjected to a simulated hot stamp heat treatment in which the sheet is heated to 1050°C, allowed to remain there for 5 seconds or more, and then water-cooled, the average coefficient of friction is measured in a pin-on-disk test at room temperature, with a sliding speed of 3.9 m/s, the pressing pressure being increased from 0.4 to 0.8 to 1.6 to 2.1 to 2.5 MPa, and the sliding distance being 3.5 km for each pressing pressure, and the difference between the maximum and minimum values of the average coefficient of friction for each pressing pressure is 0.3 or less. 6. A stainless steel sheet for brake disc rotors according to any one of claims 1 to 5, characterized in that when subjected to a simulated hot stamp heat treatment in which the steel is heated to 1,050°C, held there for 5 seconds or more, and then water-cooled, the 0.2% yield strength of the material at 700°C is 50 MPa or more. A brake disc rotor comprising the stainless steel sheet for brake disc rotors according to any one of claims 1 to 7 . The method for producing a stainless steel sheet for brake disc rotors according to any one of claims 1 to 7, characterized in that the reduction rate during hot rolling is 85% or more.