JP7179966B2 - Ferritic stainless steel sheets for automobile brake disc rotors, automobile brake disc rotors and hot stamped products for automobile brake disc rotors - Google Patents

Description

TECHNICAL FIELD The present invention relates to a ferritic stainless steel sheet for automobile brake disc rotors, automobile brake disc rotors, and hot-stamped products for automobile brake disc rotors, which are excellent in heat resistance and formability, and particularly to automobile brakes that require high-temperature strength. The present invention relates to a ferritic stainless steel sheet suitable for use in disk rotors and the like.

Disc brakes are widely used as one of the braking systems of automobiles. By pressing a disc-shaped structure called a disc rotor, which is connected to the tire, between the brake pads, friction converts kinetic energy into thermal energy, slowing down the car. Flaky graphite cast iron (hereinafter referred to as cast iron) is used as the material of the disk rotor due to its thermal conductivity, cost, and the like.

Since cast iron does not contain any elements that improve corrosion resistance, it is inferior in corrosion resistance, and if left unattended, it quickly develops red rust. Conventionally, this red rust was not very noticeable due to the position of the disc below the line of sight and the shape of the wheel. However, due to the recent demand for improved fuel efficiency, wheels are being made of aluminum and spokes are getting thinner, so rust on discs can no longer be ignored, and improvements in corrosion resistance have been desired.

Stainless steel is a material with excellent corrosion resistance, and martensitic SUS410 series materials are widely used for motorcycles and other two-wheeled vehicles. This is because the disc rotor of a two-wheeled vehicle is exposed and easily visible, and corrosion resistance is emphasized. On the other hand, stainless steel has a problem that its thermal conductivity is inferior to that of cast iron. In motorcycles, the brake system is exposed, and stainless steel is used without problems because of its excellent cooling performance. In the case of automobiles, the brake system, including the tires, is housed in the tire house, so the disc rotor is difficult to cool, and one of the issues is low thermal conductivity, which is why stainless steel has not been applied.

However, in recent years, EVs, FCVs, HVs, etc. have rapidly adopted "regenerative braking" that converts and recovers kinetic energy during running into electrical energy. Since this application reduces the frictional heat generated by the friction between the disc rotor and the pad, the possibility of application to stainless steel, which is inferior to cast iron in thermal conductivity, is expanding.

Another issue that has hampered the application of stainless steels in automotive disc brakes is formability. A disc rotor for a motorcycle is a ring-shaped disc, and is manufactured by punching from a plate-shaped stainless steel, so there is no need for large processing. On the other hand, the disk rotors of current automobiles have a shape called a hat shape, which looks like a disk whose center is squeezed, and are manufactured by casting. Deep drawing is required to process stainless steel into such a shape. However, the stainless steel that has been used for motorcycles is martensitic stainless steel, which is extremely hard and difficult to process. As one method for solving this problem, hot stamping, which is a method of pressing at a high temperature, has been widely used in recent years. This made it possible to mold stainless steel into a hat shape with high accuracy.

Against this background, it is necessary to reduce the thickness and weight of disc rotors in order to meet the recent demand for improved fuel efficiency. However, cast iron has low strength and is manufactured by casting, so there is a limit to how thin it can be made. In addition, it is said that the maximum temperature reached during braking of an automobile reaches around 700°C, and it may be difficult to apply it to martensitic stainless steel, which has a heat resistance temperature of around 500°C. In some cases, the temperature reaches 300° C. under driving conditions such as mountain roads where brakes are frequently used.

There is Patent Document 1 regarding stainless steel disk rotors for automobiles, but it focuses mainly on formability, not on high-temperature strength. Further, in Patent Document 2, strength is improved by a martensite phase utilizing highly saturated solute C and N, but strength around 700° C. is not mentioned. Moreover, all of the patent documents utilize the martensite structure, and none of them can ensure heat resistance in the vicinity of 700°C.

Japanese Patent No. 5700172 JP 2016-117925 A

TECHNICAL FIELD The present invention relates to a ferritic stainless steel sheet for automobile brake disc rotors, which is excellent in heat resistance and formability. The parts to be solved by the present invention are braking system parts of automobiles, particularly disc rotors.
Since the disk rotor of an automobile is shaped like a hat, formability is required. In addition, the temperature reached is about 100°C in general urban driving, about 300°C in mountain road driving, and reaches about 700°C at maximum, so strength in the medium to high temperature range is required for thinning. Since cast iron is formed by casting, if the disk rotor is made thin, the flow of molten metal will be poor and it may not be possible to form it. In addition, since the strength is low, if the thickness is reduced, there is a problem that sufficient strength cannot be secured as a disc rotor. Ferritic stainless steel can be formed into a hat shape with high accuracy by hot stamping. However, stainless steel, which has low strength, cannot be thinned. On the other hand, stainless steel, which has high strength, requires an excessive load during hot stamping, and it may not be possible to form it into a hat shape with high accuracy, or cracks may occur. Also, martensitic stainless steel is excellent in formability by hot stamping, but its heat resistance temperature is about 500° C., and both formability and heat resistance cannot be achieved.

The present invention provides a ferritic stainless steel sheet for automobile brake disc rotors, an automobile brake disc rotor, and a hot-stamped product for automobile brake disc rotors, which have excellent high-temperature strength and excellent formability.

In order to solve the above problems, the present inventors paid attention to the precipitates on the ferritic stainless steel sheet and investigated them in detail. In the temperature range in which the parts targeted by the present invention are formed by hot stamping, precipitates may occur in the steel. Finely dispersed precipitates can improve the strength of the material. However, the presence of precipitates prior to forming can lead to excessive strength and reduced elongation of the steel, which can lead to cracking during forming. Therefore, it was thought that the formability and the strength after forming could be ensured by forming fine precipitates during hot stamping. As a result of various investigations aimed at achieving this object, the following findings were obtained.

The amount of Si added is appropriately controlled, the finishing temperature after hot rolling is 900 to 1100 ° C., and the winding temperature is 650 ° C. or less, so that the crystal grain size is increased during heating during hot stamping, Precipitates are deposited during hot stamping. Furthermore, by setting the finishing temperature to over 950° C., the crystal grain size is effectively increased, and the strength in the intermediate temperature range is also improved. Since the precipitates are finely precipitated within the crystal grains of the steel, excellent high-temperature strength can be obtained during use as a disc rotor. Precipitates that precipitate at grain boundaries tend to grow and coarsen. On the other hand, it has been found that the precipitates are mainly precipitated in the crystal grains by appropriately controlling the crystal grain size during heating during hot stamping. Precipitates within grains are less likely to grow and coarsen during use than precipitates at grain boundaries. Precipitation strengthening is effectively achieved by the fine precipitation of precipitates within grains during hot stamping. As a result, we have succeeded in providing a heat-resistant ferritic stainless steel sheet that can be applied to disk rotors.

The gist of the present invention for solving the above problems is as follows.
[1] In mass%, C: 0.001 to 0.05%, N: 0.001 to 0.05%, Si: 0.3 to 4.0%, Mn: 0.01 to 2.0 %, P: 0.01 to 0.05%, S: 0.0001 to 0.02%, Cr: 10 to 20%, Ti: 0.001 to 0.5%, Nb: 0 .01 to 0.8% containing one or two types, the balance being Fe and impurities,
Heating to 1000 ° C., and then heat treatment (hereinafter referred to as “hot stamping pseudo heat treatment”) in which cooling is performed by staying at 890 to 700 ° C. for 1 minute or more and 10 minutes or less. A ferritic stainless steel sheet for automobile brake disc rotors, characterized by having a density of 0.01 to 20/μm ² of precipitates with a diameter of 500 nm or less, which is for hot stamping.

[2] In mass%, C: 0.001 to 0.05%, N: 0.001 to 0.05%, Si: 0.3 to 4.0%, Mn: 0.01 to 2.0 %, P: 0.01 to 0.05%, S: 0.0001 to 0.02%, Cr: 10 to 20%, Ti: 0.001 to 0.5%, Nb: 0 .01 to 0.8% containing one or two types, the balance being Fe and impurities,
A ferritic stainless steel sheet for automobile brake disc rotors constituting a hot-stamped product having a grain size of 100 to 200 μm and precipitates having a grain size of 500 nm or less at a density of 0.01 to 20/μm ² .

[3] In mass%, C: 0.001 to 0.05%, N: 0.001 to 0.05%, Si: 0.3 to 4.0%, Mn: 0.01 to 2.0 %, P: 0.01-0.05%, S: 0.0001-0.02%, Cr: 10-20%, Ti: 0.001-0.5%, Nb: 0.001-0.5%. A ferritic stainless steel sheet for automobile brake disc rotors containing 01 to 0.8% of 1 or 2 and the balance being Fe and impurities.

[4] A ferritic stainless steel sheet for automobile brake disc rotors according to the present invention, which is for hot stamping.
[5] The ferrite for automobile brake disc rotors of the present invention, which has an elongation at break of 50% or more at 1000°C and a 0.2% proof stress at 700°C of 80 MPa or more after the hot stamping pseudo heat treatment. system stainless steel plate.
[6] A ferritic stainless steel sheet for automobile brake disc rotors according to the present invention, which has a 0.2% proof stress of 80 MPa or more at 700°C.
[7] The ferritic stainless steel sheet for automobile brake disc rotors according to the present invention, wherein the crystal grain size is 130 to 200 μm.
[8] A ferritic stainless steel sheet for automobile brake disc rotors according to the present invention, which has an elongation at break of 50% or more at 1000°C.
[9] A ferritic stainless steel sheet for automobile brake disc rotors according to the present invention, which has a 0.2% proof stress of 170 MPa or more at 300°C.

[10] In place of part of the Fe, B: 0.0001 to 0.0030%, Al: 0.001 to 4.0%, Cu: 0.01 to 3.0% in mass% , Mo: 0.01 to 3.0%, W: 0.001 to 2.0%, V: 0.001 to 1.0%, Sn: 0.01 to 0.5%, Ni: 0.01 ~1.0%, Mg: 0.0001-0.01%, Sb: 0.005-0.5%, Zr: 0.001-0.3%, Ta: 0.001-0.3%, Hf: 0.001-0.3%, Co: 0.001-0.3%, Ca: 0.0001-0.01%, REM: 0.001-0.2%, Ga: 0.0002- 0.3% of one or more ferritic stainless steel sheets for automobile brake disc rotors according to the present invention.

[11] An automobile brake disc rotor using the stainless steel sheet of the present invention.
[12] A hot-stamped article for automobile brake disc rotors, which uses the stainless steel sheet of the present invention.

According to the present invention, it is possible to improve the heat resistance and formability of ferritic stainless steel sheets, provide materials suitable for automobile brake disc rotors, and achieve significant effects such as weight reduction and aesthetic improvement.

When manufacturing an automobile brake disc rotor by hot stamping using a ferritic stainless steel sheet, the steel sheet is heated to around 1000° C. and hot stamped. A steel sheet before hot stamping is required to have sufficient ductility in hot stamping performed at around 1000°C. On the other hand, for automobile brake disc rotors after hot stamping, it is necessary to achieve sufficient high-temperature strength.

As described above, precipitates may form in the hot stamping temperature range. Finely dispersed precipitates in steel can improve the strength of the material. However, the presence of precipitates prior to forming may result in excessive strength and reduced elongation, which may lead to cracking during hot stamping. Therefore, the present invention secures hot stamping moldability and strength after molding by finely depositing precipitates during hot stamping.

We focus on the grain size of steel during hot stamping. When the crystal grain size is small, the ratio of crystal grain boundaries in the steel is high, so precipitation at the crystal grain boundaries increases during hot stamping. Precipitates that precipitate at grain boundaries tend to grow and coarsen, making it difficult to obtain fine precipitates. In the present invention, it has been found that the precipitates are mainly precipitated in the grains by growing the crystal grain size and appropriately controlling it during heating during hot stamping. Intragranular precipitates grow more slowly than grain boundary precipitates and are less likely to coarsen during use. Precipitation strengthening occurs effectively after hot stamping and secures strength after forming by fine precipitation of precipitates in grains during hot stamping.

As described above, in the present invention, it is important from the viewpoint of high-temperature strength after hot stamping that the precipitates are finely precipitated in the grains. I found that I needed to. Specifically, it has been found that by setting the crystal grain size to 100 to 200 μm after hot stamping, the refinement of precipitates can be realized. It is known that the crystal grain size during hot stamping is the same as the crystal grain size after hot stamping. In such a crystal grain size range, the deposited precipitates are finely precipitated in the grains and are difficult to grow, and it is presumed that there is a corresponding relationship between them.
Therefore, in the present invention, the metal structure is defined by the crystal grain size after hot stamping. By controlling the crystal grain size after hot stamping to 100 to 200 μm, precipitates are finely precipitated during hot stamping and are difficult to grow, so that precipitation strengthening is effectively exhibited. When the crystal grain size after hot stamping was 100 μm or more, fine precipitates were formed, and sufficient yield strength up to around 700° C. was obtained. Furthermore, when the crystal grain size after hot stamping is 130 μm or more, a sufficient yield strength was obtained even in a medium temperature range around 300°C.

Crystal grains in steel grow by heating with a hot stamp, increasing the grain size. There is a tendency that the larger the crystal grain size before hot stamping, the larger the crystal grain size during and after hot stamping. If the crystal grain size after hot stamping exceeds 200 μm, the crystal grain size of the steel sheet before hot stamping is also large, and as a result, the toughness of the steel sheet is remarkably lowered. Therefore, the upper limit of the crystal grain size after hot stamping was set to 200 μm.

In addition, in order to effectively develop precipitation strengthening after hot stamping, it is specified that precipitates with a grain size of 500 nm or less exist in the steel after hot stamping at a density of 0.01 to 20 / μm ² . . Precipitates having a grain size of 500 nm or less are present at a density of 0.01 to 20/μm ² , so that a sufficient yield strength can be obtained up to around 700°C. If the grain size exceeds 500 nm, precipitation strengthening becomes difficult to act. If the density of precipitates is less than 0.01/μm ² , the amount of precipitates is so small that precipitation strengthening is difficult to act. If it exceeds 20 pieces/μm ² , the strength increases excessively and cracks are likely to occur. From the above, it is desirable that precipitates having a particle size of 500 nm or less exist in the grains at a density of 0.01 to 20/μm ² .

If the product to be evaluated is a hot-stamped product or an automobile brake disc rotor, which is the final product, the crystal grain size and precipitate density in steel can be evaluated. On the other hand, when the product to be evaluated is a steel plate before hot stamping, the steel plate is subjected to hot stamping pseudo heat treatment, and then the grain size and precipitate density in the steel are evaluated. As the hot stamping pseudo heat treatment, heat treatment may be performed by heating to 1000° C. and then cooling at 890 to 700° C. for 1 minute or more and 10 minutes or less, for example, 2 minutes.

The grounds for specifying the content of ingredients in steel will be described below.
C degrades formability and corrosion resistance, lowers the high-temperature elongation and high-temperature strength of the steel sheet, and causes excessive precipitate density due to the precipitation of Cr carbonitrides and Nb carbonitrides after hot stamping. The less the amount, the better. Therefore, it is set to 0.05% or less. 0.020% or less is desirable. More desirably, the content is 0.0015% or less. However, excessive reduction leads to an increase in refining cost, so it is preferable to make it 0.001% or more.

Like C, N degrades formability and corrosion resistance, lowers the high-temperature elongation and high-temperature strength of the steel sheet, and precipitates Cr carbonitrides and Nb carbonitrides after hot stamping, resulting in an excessive precipitate density. , the lower the content, the better. Therefore, it is set to 0.05% or less. 0.020% or less is desirable. More desirably, the content is 0.015% or less. However, excessive reduction leads to an increase in refining cost, so it is preferable to make it 0.001% or more.

Si is an element that is also useful as a deoxidizing agent, and is an element that improves high-temperature strength, oxidation resistance, and high-temperature salt damage resistance. High-temperature strength, oxidation resistance, and high-temperature salt damage resistance improve as the amount of Si increases. Precipitation control is important for improving high-temperature strength, and the effect can be obtained by precipitating a large amount of fine precipitates. Si has the effect of finely precipitating precipitates during aging, and the effect is stably exhibited from 0.3%. However, excessive addition of Si reduces the ductility of the steel sheet at room temperature and high temperature, hardens the hot-rolled sheet and reduces toughness, refines the grain size, and excessively generates precipitates during hot stamping. Therefore, the upper limit is set to 4.0%. Also, considering the pickling property and toughness, it is preferably 0.3% or more, and preferably 3.5% or less. Furthermore, considering manufacturability, 3.0% or less is desirable.

Mn is an element added as a deoxidizing agent and contributes to an increase in high-temperature strength in the medium temperature range. At the same time, Mn-based oxides are formed on the surface layer at high temperatures, resulting in poor scale adhesion and abnormal oxidation. In particular, when Mo and W are added together, abnormal oxidation tends to occur more easily with respect to the amount of Mn. Therefore, the upper limit was made 2.0%. Furthermore, considering the pickling property and room temperature ductility in steel plate production, the content is preferably 0.01% or more and 1.5% or less. More desirably, the content is 1.0% or less.

P is an impurity mainly mixed from raw materials during steelmaking refining, and when the content is high, the toughness and weldability of the steel sheet are lowered. For this reason, it is desirable to reduce the P content as much as possible. However, in order to reduce the P content to less than 0.01%, the cost increases due to the use of low-P raw materials. More desirably, it should be 0.02% or more. On the other hand, if the content exceeds 0.05%, the hardness is significantly increased, and the corrosion resistance, toughness, and pickling properties are deteriorated. More desirably, the content is 0.04% or less.

S is an element that deteriorates corrosion resistance and oxidation resistance, but the effect of improving workability by combining with Ti and C appears from 0.0001%, so the lower limit was made 0.0001%. Furthermore, considering refining cost, 0.0010% or more is desirable. On the other hand, excessive addition combines with Ti and C to reduce the amount of solid solution Ti and cause precipitates to coarsen, which lowers the toughness and high-temperature strength of the steel sheet, so the upper limit was made 0.02%. Furthermore, considering high-temperature oxidation characteristics, 0.0090% or less is desirable.

Cr is an essential element for securing oxidation resistance and corrosion resistance in the present invention. If it is less than 10%, the oxidation resistance cannot be ensured, the 700° C. proof stress after hot stamping is lowered, and the crystal grain size is increased. On the other hand, if it exceeds 20%, workability and toughness are deteriorated, and the number of precipitates after hot stamping becomes excessive. Furthermore, considering manufacturability and scale removability, the content is preferably 12% or more, and preferably 18% or less. More desirably, it is 15% or less.

One or two of Ti: 0.001 to 0.5% and Nb: 0.01 to 0.8% are contained.
Ti is an element that combines with C, N and S to improve corrosion resistance, intergranular corrosion resistance, room temperature ductility and deep drawability. Moreover, it adds as needed. Further, in the combined addition of Nb and Mo, by adding an appropriate amount, the solid solution amount of Nb and Mo during hot rolling annealing is increased, the high-temperature strength is improved, and the thermal fatigue characteristics are improved. Since the effect is exhibited from 0.001% or more, the lower limit was made 0.001%. On the other hand, if the addition exceeds 0.5%, the amount of solid solution Ti increases and the ductility of the steel sheet at room temperature and high temperature decreases, and the number of precipitates after hot stamping becomes excessive, and further coarse Ti-based precipitates. It becomes the starting point of cracks during hole expansion and deteriorates press workability. In addition, the amount of Ti added is set to 0.5% or less because the oxidation resistance is also deteriorated. Furthermore, considering the occurrence of surface flaws and toughness, the content is preferably 0.05% or more, and is preferably 0.2% or less.

Nb is an element effective in improving high-temperature strength through solid-solution strengthening and precipitation strengthening of fine precipitates. It also has the role of fixing C and N as carbonitrides and contributing to the development of the recrystallized texture that affects the corrosion resistance and r-value of the product sheet. Since these effects appear from 0.01%, the lower limit was made 0.01%. On the other hand, addition of more than 0.8% lowers the high-temperature ductility of the steel sheet, and the number of precipitates after hot stamping becomes excessive, resulting in significant hardening and deterioration of manufacturability. 8%. Moreover, considering raw material cost and toughness, 0.3% or more is desirable, and 0.6% or less is desirable.

As components in the steel, the balance is Fe and impurities in addition to the above composition. The present invention may further contain the following components instead of part of the Fe, if necessary.

B is an element that improves secondary workability, high-temperature strength, and thermal fatigue properties during press working of the product. B brings about fine precipitations such as Laves phases, develops long-term stability of these precipitation strengthening, and contributes to suppression of strength reduction and improvement of thermal fatigue life. This effect is expressed at 0.0001% or more. On the other hand, excessive addition causes hardening, degrades intergranular corrosion resistance and oxidation resistance, and causes weld cracking. Furthermore, considering corrosion resistance and manufacturing cost, 0.0010% or less is desirable. More desirably, the content is 0.0005% or less.

Al is an element that is added as a deoxidizing element and also improves oxidation resistance. In addition, it is useful as a solid-solution strengthening element for improving high-temperature strength. Its action is stably expressed from 0.001%. On the other hand, excessive addition hardens the steel and significantly lowers the uniform elongation and toughness, so the upper limit was made 4.0%. Furthermore, considering the occurrence of surface flaws, weldability, and manufacturability, the content is desirably 0.01% or more, and desirably 2.2% or less.

Cu is an element effective in improving corrosion resistance. Its action is stably expressed from 0.01%. In addition, high-temperature strength is improved by precipitation strengthening due to precipitation of ε-Cu, but excessive addition lowers hot workability, so the upper limit was made 3.0%. Furthermore, considering thermal fatigue properties, manufacturability and weldability, 1.6% or less is desirable.

Mo is an element effective for solid-solution strengthening at high temperatures, and is added in an amount of 0.01% or more as necessary in order to improve corrosion resistance and high-temperature salt damage resistance. Addition of 3.0% or more significantly deteriorates room-temperature ductility and oxidation resistance, so the content was made 3.0% or less. Furthermore, in consideration of thermal fatigue characteristics and manufacturability, it is desirable to be 0.3% or more and 0.9% or less.

Like Mo, W is an element effective for solid-solution strengthening at high temperatures, and produces a Laves phase (Fe ₂ W) to bring about precipitation strengthening. In particular, when W is added in combination with Nb or Mo, the Laves phase of Fe ₂ (Nb, Mo, W) precipitates, but the addition of W suppresses the coarsening of the Laves phase and improves the precipitation strengthening ability. It works with additions above 0.001%. On the other hand, addition of more than 2.0% increases the cost and reduces room-temperature ductility, so the upper limit was made 2.0%. Furthermore, considering manufacturability, low-temperature toughness and oxidation resistance, the W addition amount is preferably 1.5% or less.

V is an element that improves corrosion resistance and is added as necessary. This effect is stably exhibited at addition of 0.001% or more. On the other hand, if it is added in excess of 1%, the precipitates become coarse and the high-temperature strength is lowered, and the oxidation resistance is deteriorated, so the upper limit was made 1%. Furthermore, considering the production cost and manufacturability, it is preferably 0.08% or more and 0.5% or less.

Sn is an element that improves corrosion resistance, and is added as necessary in order to improve high-temperature strength in the intermediate temperature range. These effects are expressed at 0.01% or more. On the other hand, if added in excess of 0.5%, the manufacturability and toughness are remarkably lowered, so the content was made 0.5% or less. Furthermore, in consideration of oxidation resistance and manufacturing cost, 0.1% or more is desirable.

Ni is an element that improves acid resistance, toughness, and high-temperature strength, and is added as necessary. These effects are expressed at 0.01% or more. On the other hand, if it is added in excess of 1.0%, the cost increases, so the content was made 1.0% or less. Furthermore, considering manufacturability, 0.08% or more is desirable, and 0.5% or less is desirable.

Mg is sometimes added as a deoxidizing element, and is an element that refines the structure of the slab and contributes to the improvement of formability. Moreover, Mg oxide serves as precipitation sites for carbonitrides such as Ti(C,N) and Nb(C,N), and has the effect of finely dispersing and precipitating them. This action appears at 0.0001% or more and contributes to the improvement of toughness. However, excessive addition leads to deterioration of weldability, corrosion resistance and surface quality, so the upper limit was made 0.01%. Considering refining cost, 0.0003% or more is desirable, and 0.0010% or less is desirable.

Sb contributes to the improvement of corrosion resistance and high-temperature strength, so 0.005% or more is added as necessary. Addition of more than 0.5% may excessively cause slab cracking and ductility deterioration during steel sheet production, so the upper limit is made 0.5%. Furthermore, considering the refining cost and manufacturability, it is desirable to be 0.01% or more and 0.3% or less.

Zr is a carbonitride-forming element like Ti and Nb, and is an element that improves corrosion resistance and deep drawability, and is added as necessary. These effects are expressed at 0.001% or more. On the other hand, addition of more than 0.3% significantly deteriorates the manufacturability, so the content was made 0.3% or less. Furthermore, considering cost and surface quality, 0.1% or more is desirable, and 0.2% or less is desirable.

Zr, Ta and Hf combine with C and N to contribute to the improvement of toughness, so 0.001% or more is added as necessary. However, addition of more than 0.3% increases the cost and significantly deteriorates manufacturability, so the upper limit is made 0.3%. Furthermore, considering refining cost and manufacturability, 0.01% or more is desirable, and 0.08% or less is desirable.

Co contributes to improvement of high-temperature strength, so 0.001% or more is added as necessary. Addition of more than 0.3% leads to deterioration of toughness, so the upper limit is made 0.3%. Furthermore, in consideration of refining cost and manufacturability, 0.01% or more is desirable, and 0.1% or less is desirable.

Ca is sometimes added for desulfurization, and this effect is exhibited at 0.0001% or more. However, addition of more than 0.01% generates coarse CaS, degrading toughness and corrosion resistance, so the upper limit was made 0.01%. Furthermore, considering the refining cost and manufacturability, it is desirable to be 0.0003% or more and 0.0020% or less.

REM may be added as necessary from the viewpoint of improving toughness and oxidation resistance by refining various precipitates, and this effect is exhibited at 0.001% or more. However, addition of more than 0.2% significantly deteriorates castability and lowers ductility, so the upper limit was made 0.2%. Furthermore, considering refining cost and manufacturability, 0.05% or less is desirable. REM (rare earth element) is a generic term for two elements, scandium (Sc) and yttrium (Y), and fifteen elements (lanthanoids) from lanthanum (La) to lutetium (Lu), according to a general definition. They may be added singly or as a mixture.

Ga may be added at 0.3% or less in order to improve corrosion resistance and suppress hydrogen embrittlement. From the viewpoint of formation of sulfides and hydrides, the lower limit is preferably 0.0002%. Furthermore, 0.0020% or less is preferable from the viewpoints of manufacturability and cost as well as ductility and toughness.

Other components are not particularly defined in the present invention, but in the present invention, Bi or the like may be added in an amount of 0.001 to 0.1%, if necessary. In addition, it is preferable to reduce general harmful elements such as As and Pb and impurity elements as much as possible.

Next, a manufacturing method will be described.
The steel sheet manufacturing method of the present invention comprises the steps of steelmaking-hot rolling-annealing-pickling. In steelmaking, a method of smelting steel containing the above-mentioned essential components and optionally added components in a converter and then 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. Hot rolling is performed by a hot rolling mill consisting of multiple stands and then coiled.
Annealing after the hot rolling process may be omitted.

In order to obtain a crystal grain size of 100 to 200 μm during hot stamping, the finishing temperature after hot rolling is preferably 900 to 1100°C. If the finishing temperature is less than 900° C., the crystal grain size of the steel sheet does not grow sufficiently, and as a result, the crystal grain size after hot stamping does not grow to 100 μm or more. On the other hand, if the finishing temperature exceeds 1100° C., the crystal grain size of the steel sheet will grow excessively, and the crystal grain size after hot stamping will exceed 200 μm. More preferably, the finishing temperature after hot rolling is over 950°C. By setting the finishing temperature to over 950° C., the crystal grain size grows to 130 μm or more, and the effect of improving the strength in the intermediate temperature range is exhibited.
Further, if the coiling temperature exceeds 650°C, the toughness of the hot-rolled sheet is lowered, so it is preferable to set the coiling temperature to 650°C or lower.

Next, the molding method will be explained. Forming of the steel sheet of the present invention is a hot stamping method in which the steel sheet is heated to a predetermined temperature, formed into a hat shape at a high temperature, and then cooled. The heating temperature is 900 to 1000° C., and cooling is performed after molding. In order to precipitate a large amount of fine precipitates, cooling is performed at 890 to 700° C. so that the mixture is retained for 1 minute or more and 10 minutes or less. If the residence time is less than 1 minute, precipitation will not occur sufficiently and the amount of precipitation strengthening will be small, so the lower limit is made 1 minute. If this time is excessively long, finely precipitated precipitates grow and coarsen, and the amount of precipitation strengthening decreases. Moreover, the upper limit is set to 10 minutes because the productivity drops significantly. Furthermore, considering the stability of precipitates, 1.5 to 5 minutes is desirable.

In the present invention, the "ferritic stainless steel sheet for automobile brake disc rotors constituting a hot-stamped product" means a steel sheet after hot-stamping. That is, it means a hot-stamped product for automobile brake disc rotors using a stainless steel plate.
A hot-stamped product for an automobile brake disc rotor using a stainless steel plate means a hot-stamped product for an automobile brake disc rotor obtained by subjecting a stainless steel plate to hot stamping.
An automobile brake disc rotor made of stainless steel plate means an automobile brake disc rotor obtained by subjecting a stainless steel plate to hot stamping and further processing.

The steels having the compositions shown in Tables 1 and 2 are melted and cast into slabs, the slabs are hot-rolled under the hot-rolling conditions shown in Tables 3 and 4 to form hot-rolled coils with a thickness of 6 mm, and pickled. provided. No. in Table 1. A1 to A34 are steels of the present invention; B1 to B14 are comparative steels; B15 is an unheat treated steel. Numerical values outside the scope of the present invention are underlined.

A hot stamping simulated heat treatment (hereinafter simply referred to as “pseudo heat treatment”) is performed by heating the hot-rolled sheet (excluding B15) thus obtained to 1000° C., holding it at 890 to 700° C. for 2 minutes, and then cooling it with water. provided. When cracks occurred in the steel plate after the simulated heat treatment, "crack" was described in the column of "quality after simulated heat treatment/remarks" in Table 4.

The crystal grain size of t/4 part of the hot stamp simulated heat treated material was measured (according to JIS G 0551, the numerical value is rounded off to the nearest whole number). The photographing magnification was 50 times, the number of photographing fields was 5, and the average grain size of the 5 fields was calculated. In addition, using a 200 kV field emission transmission electron microscope (EM-2100F) manufactured by JEOL Ltd., the simulated heat-treated material was observed in 5 fields of bright field observation at a magnification of 12,500 times to evaluate precipitates. For the grain size of precipitates, the circle-equivalent diameter of the precipitates contained in the bright-field observation image was measured. The average precipitate density in 5 fields of view was calculated for precipitates having a grain size of 500 nm or less.

In addition, a high-temperature tensile test piece was taken from the hot stamping simulated heat treatment material so that the rolling direction was the tensile direction, and a tensile test was performed at 300 ° C. and 700 ° C. to measure the 0.2% proof stress (JIS G 0567 compliant, numbers are rounded to the nearest whole number). Here, if the 0.2% proof stress at 300 ° C. is 150 MPa or more and the 0.2% proof stress at 700 ° C. is 80 MPa or more, it can be applied to a general disc rotor and thinned. A sample having a 2% yield strength of 150 MPa or more and a 0.2% yield strength of 80 MPa or more at 700° C. was regarded as acceptable, and marked A in Tables 3 and 4. Furthermore, those having a 0.2% yield strength of 170 MPa or more at 300° C. and a 0.2% yield strength of 100 MPa or more at 700° C. are marked as particularly excellent. Other than the above, X mark was described as failure.

In addition, in order to evaluate the press formability at high temperature of the hot-rolled sheet before hot stamping, a high-temperature tensile test piece was taken from the hot-rolled sheet so that the rolling direction was the tensile direction, and a tensile test was performed at 1000 ° C. , and breaking elongation were measured (according to JIS G 0567, numerical values are rounded off to the nearest whole number). Here, if the breaking elongation at 1000 ° C. is 50% or more, it can be processed into a hat shape, so those having a breaking elongation at 1000 ° C. of 50% or more are accepted. . Furthermore, those having an elongation at break of 65% or more at 1000°C are marked as particularly excellent. Other than the above, X mark was described as failure.

In addition, in order to evaluate the toughness of the hot-rolled sheet, a Charpy test piece (C-direction notch) was produced from the hot-rolled sheet and subjected to a Charpy impact test at room temperature. When the average impact value of the three tests was 10 J/cm ² or less, it was displayed as "poor toughness" in the "remarks" column of the steel plate quality.

As is clear from Tables 1 to 4, the 0.2% proof stress at 700° C. after the simulated hot stamping heat treatment is superior to the comparative examples in the inventive examples. In addition, it can be seen that the invention examples in which the finishing temperature of the hot-rolled sheet is over 950° C. have a crystal grain size of 130 μm or more, and the 300° C. yield strength is all “S”, which is particularly excellent. If either one of the 0.2% yield strength at 300 ° C. and 700 ° C. after the above simulated heat treatment and the breaking elongation at 1000 ° C. of the hot-rolled sheet fails, and if the toughness of the hot-rolled sheet is poor, the disc rotor It was determined that the application as From this, it can be seen that the steel specified in the present invention is excellent in heat resistance and formability.

In Comparative Examples B1 and B2, the concentrations of C and N deviated from the upper limits, respectively, and the 1000° C. breaking elongation of the steel sheets was poor.
In Comparative Example B3, the Si concentration was below the lower limit, the number of precipitates after the simulated heat treatment was insufficient, and the yield strength at 300°C and 700°C was low. In Comparative Example B4, the Si concentration exceeded the upper limit, the 1000° C. elongation of the steel sheet was poor, the crystal grain size after the simulated heat treatment was too small, the number of precipitates was excessive, and cracks occurred.
In Comparative Example B5, the Mn concentration exceeded the upper limit, and the yield strength at 300°C and 700°C was insufficient.
In Comparative Examples B6 and B7, the concentrations of P and S deviated from the upper limits, respectively, resulting in poor toughness of the steel sheets.
In Comparative Example B8, the Cr concentration was below the lower limit, the high-temperature strength decreased, and the 300° C. and 700° C. yield strengths after the simulated heat treatment were poor. Moreover, as is clear from the fact that the crystal grain size after the simulated heat treatment was excessively large, the crystal grain size in the steel sheet was also excessively large, resulting in poor toughness of the steel sheet.
In Comparative Examples B9, B10, and B11, the Cr concentration, Ti concentration, and Nb concentration deviated from the upper limits, respectively, and the 1000 ° C. breaking elongation of the steel sheet was poor, and the number of precipitates in the pseudo heat treatment became excessive, resulting in cracking. rice field.

In Comparative Example B12, the finishing temperature of hot rolling was above the upper limit, and the grain size of the steel plate after the quasi-heat treatment was too large.
In Comparative Example B13, the finishing temperature of hot rolling was below the lower limit, the crystal grain size after quasi-heat treatment was too small, and the number of precipitates was too small.
In Comparative Example B14, the coiling temperature for hot rolling was above the upper limit, and the toughness of the steel sheet was poor.

For B15, the left end of Table 2 indicates "Not", that is, the crystal grain size, the number of precipitates, and the yield strength at 300°C and 700°C were evaluated without hot stamping pseudo heat treatment. Precipitation did not proceed and the number of precipitates was too small, resulting in poor yield strength at 300°C and 700°C.

Claims (9)

in % by mass,
C: 0.001 to 0.05%,
N: 0.001 to 0.05%,
Si: 0.3 to 4.0%,
Mn: 0.01 to 2.0%,
P: 0.01 to 0.05%,
S: 0.0001 to 0.02%,
Cr: 10 to 20%, Ti: 0.001 to 0.5%, Nb: 0.01 to 0.8%
contains one or two types, the balance being Fe and impurities,
When heat treatment (hereinafter referred to as "hot stamp pseudo heat treatment") is performed by heating to 1000 ° C. and then cooling at 890 to 700 ° C. for 1 minute or more and 10 minutes or less,
The crystal grain size is 100 to 200 μm,
Precipitates with a particle size of 500 nm or less have a density of 0.01 to 20 / μm ² ,
A ferritic stainless steel sheet for automobile brake disc rotors, characterized by being for hot stamping. in % by mass,
C: 0.001 to 0.05%,
N: 0.001 to 0.05%,
Si: 0.3 to 4.0%,
Mn: 0.01 to 2.0%,
P: 0.01 to 0.05%,
S: 0.0001 to 0.02%,
Cr: 10 to 20%, Ti: 0.001 to 0.5%, Nb: 0.01 to 0.8%
contains one or two types, the balance being Fe and impurities,
A ferritic stainless steel sheet for automobile brake disc rotors constituting a hot-stamped product having a grain size of 100 to 200 μm and precipitates having a grain size of 500 nm or less at a density of 0.01 to 20/μm ² . 2. The ferrite for an automobile brake disc rotor according to claim 1, wherein the breaking elongation at 1000° C. is 50% or more, and the 0.2% proof stress at 700° C. is 80 MPa or more after the hot stamping pseudo heat treatment. system stainless steel plate. 3. The ferritic stainless steel sheet for automobile brake disc rotors according to claim 2, wherein the 0.2% proof stress at 700[deg.] C. is 80 MPa or more. The ferritic stainless steel sheet for automobile brake disc rotors according to any one of claims 1 to 4, wherein the grain size is 130 to 200 µm. 5. The ferritic stainless steel sheet for automobile brake disc rotors according to claim 2 , wherein the 0.2% proof stress at 300[deg.] C. is 170 MPa or more. In place of part of the Fe, in mass% further,
B: 0.0001 to 0.0030%,
Al: 0.001 to 4.0%,
Cu: 0.01 to 3.0%,
Mo: 0.01 to 3.0%,
W: 0.001 to 2.0%,
V: 0.001 to 1.0%,
Sn: 0.01 to 0.5%,
Ni: 0.01 to 1.0%,
Mg: 0.0001-0.01%,
Sb: 0.005 to 0.5%,
Zr: 0.001 to 0.3%,
Ta: 0.001 to 0.3%,
Hf: 0.001 to 0.3%,
Co: 0.001 to 0.3%,
Ca: 0.0001 to 0.01%,
REM: 0.001 to 0.2%,
Ga: 0.0002-0.3%
The ferritic stainless steel sheet for automobile brake disc rotors according to any one of claims 1 to 6, characterized in that it contains one or more of: An automobile brake disc rotor using the stainless steel plate according to any one of claims 1 to 7. A hot-stamped product for automobile brake disc rotors using the stainless steel sheet according to any one of claims 1 to 7.