JP4962294B2 - Steel material for brake disc and brake disc - Google Patents

Description

  The present invention relates to a steel material for a brake disc and a brake disc. Specifically, for brake discs that are excellent in crack resistance and wear resistance that can be used in disc brake devices such as automobiles and railway vehicles, and for hardened and tempered brake discs that are suitable for use as materials for the brake discs More specifically, steel materials are excellent in crack resistance and wear resistance, and are suitable for speed control at high speeds of 250 km / h or higher, and hardened and tempered steel materials for brake discs used as materials for the brake discs. About.

  A disk brake device such as an automobile or a railway vehicle is a device that performs rotational braking of a wheel by pressing a friction material called a pad or a lining on the brake disk.

  As a material of a brake disk constituting the disk brake device, cast iron such as FC250 defined in JIS G 5501 (1995) has been conventionally used for automobiles.

  However, cast iron brake discs have excellent wear resistance due to the lubricating action of graphite contained in cast iron, while the graphite has a poor crack resistance and a short life because the above-mentioned graphite is the starting point for the occurrence of thermal cracks.

  For this reason, Patent Document 1 discloses “brake disc steel excellent in breakage resistance”, Patent Document 2 and Patent Document 3 disclose “disc brake rotor steel”, and Patent Document 4 discloses “brake disc steel”. Steel and disc brake devices have been proposed.

Japanese Patent Laid-Open No. 56-44756 JP-A-60-52561 JP 60-52562 A JP-A-2005-36312

  An object of the present invention is to provide a brake disc that can be used in a disc brake device because it has excellent crack resistance and wear resistance even when the vehicle speed increases, particularly when traveling at a high speed of 250 km / h or higher. It is another object of the present invention to provide a hardened and tempered steel material for a brake disc suitable for use as a material for the brake disc.

The specific target of wear resistance in the present invention is that the amount of wear of the disk in a so-called “pin-on-disk wear test” using a copper-based sintered material for the pin test piece shown in the examples described later is 250 mm ^3. Is below.

  Further, a specific target of crack resistance in the present invention is a copper-based sintered material as a friction material after finishing into a shape of a brake disc for a Shinkansen with a diameter of 800 mm and a thickness of 20 mm, which will be described later in the examples. In combination, the crack does not occur when the brake is applied 1000 times from 300 km / h.

  The steels proposed in the above-mentioned patent documents 1 to 3 are applied to a brake disc of a vehicle. Of course, in the case of speed control at a high speed of more than 250 km / h recently, at the time of 200 km / h Also in the case of speed control at, it is assumed that the life may be reduced.

  The steel proposed in Patent Document 4 is extremely effective for speed control in the case of high speed running at 250 km / h. However, the speed control at the time of recent high speed running of 250 km / h or more, particularly, super high speed of 300 km / h. In the case of speed control during high-speed running, it is assumed that there is a possibility that a sufficiently long life cannot be secured.

  Therefore, in order to improve the life, the industry has demanded the development of steel that can further improve wear resistance and crack resistance, in particular, crack resistance.

  In order to meet such a demand, the present inventors made various studies on the chemical composition of steel used for the brake disc of the disc brake device, and studied using a copper-based sintered material as the friction material. As a result, first, the following knowledge (a) was obtained.

  (A) In the case of speed control at a high speed of 250 km / h or more, particularly in the case of speed control at a super-high speed of 300 km / h, the brake disc is slid by sliding between the copper-based sintered material and the brake disc. The surface temperature reaches as high as 900 ° C. or higher. And when martensite is easy to produce | generate by cooling from this high temperature of 900 degreeC or more, it becomes easy to produce a crack.

  Therefore, further studies were made, and the following findings (b) to (f) were obtained.

(B) If the Ac ₃ transformation point of the steel that is the material of the brake disc is increased, the formation of martensite during cooling can be suppressed even when the temperature of the brake disc surface reaches a high temperature of 900 ° C. or higher. In particular, if the Ac ₃ transformation point of steel exceeds 850 ° C., cracks are unlikely to occur.

(C) Increasing the content of Si is effective in order to suppress the increase in component cost and to make the Ac ₃ transformation point of steel exceed 850 ° C.

  (D) Toughness generally decreases with increasing Si content. However, if the quenching is performed by increasing the cooling rate in the temperature range of 350 to 250 ° C. during quenching, good toughness can be ensured after tempering.

(E) With respect to the occurrence of cracks in the brake disk, the austenite grain number and fracture toughness value of the brake disk in an unused state, in other words, after hot forging into a predetermined brake disk rough shape, The austenite grain size number and fracture toughness value in the state where the heat treatment of “tempering” has been performed have an influence. And, if the Ac ₃ transformation point of the steel exceeds 850 ° C., and is in the quenched and tempered state, the austenite grain size number is 6.0 or more and the fracture toughness value is 130 MPa · m ^1/2 or more, Even when the temperature of the surface of the brake disk reaches a high temperature of 900 ° C. or higher, the above-described specific target of crack resistance in the present invention can be achieved.

  (F) When the value of fn1 represented by the formula “fn1 = 15C + 10Cr + 6Mo + 200V−35” satisfies 0 or more, excellent wear resistance at high temperatures can be secured.

  The present invention has been completed based on the above findings.

  The gist of the present invention resides in the hardened and tempered steel material for brake discs shown in the following (1) and (2), and the brake disc shown in (3).

(1) By mass%, C: 0.1 to 0.6%, Si: more than 1.2% and 2.0% or less, Mn: 0.2 to 2.0%, Ni: 0.8 to 3.0%, Cr: 1.0-5.0%, Mo: 0.5-2.0%, V: 0.05-0.5%, Al: 0.001-0.10% and N : 0.0015 to 0.020%, the balance is made of a chemical composition of Fe and impurities, the Ac ₃ transformation point exceeds 850 ° C., the austenite grain size number is 6.0 or more, and 130 MPa · m Quenched and tempered steel material for brake disks, characterized by having a fracture toughness value of ^1/2 or more.

(2) The steel material for a brake disc that has been quenched and tempered according to (1) above, having a chemical composition in which a value of fn1 represented by the following formula (1) is 0 or more.
fn1 = 15C + 10Cr + 6Mo + 200V−35 (1).
However, the element symbol in the formula (1) represents the content in steel in mass% of the element.

  (3) A brake disc characterized by using the steel material for a brake disc as described in (1) or (2) above.

  Hereinafter, the invention relating to the steel material for a brake disc which has been quenched and tempered (1) and (2) and the invention relating to the brake disc of (3) are referred to as “present invention (1)” to “present invention (3), respectively. ) ". Also, it may be collectively referred to as “the present invention”.

  The “quenched and tempered steel material for brake discs” is a material for “brake discs”, and after hot forging into a predetermined brake disc rough shape, heat treatment of “quenching-tempering” Refers to those marked with

  The austenite grain size number refers to a value measured by “a steel-crystal grain size microscopic test method” described in JIS G 0551 (2005).

  The fracture toughness value refers to a value measured by a fracture toughness test performed at room temperature by a method described in JIS G 0564 (1999) using a so-called “CT test piece”.

  Since the brake disc of the present invention is excellent in wear resistance and crack resistance, it can be used in a disc brake device when the vehicle speed increases and the vehicle runs at a high speed of 250 km / h or higher. Further, the steel material for a brake disc that has been quenched and tempered according to the present invention is suitable for use as an inexpensive material for the brake disc.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, "%" display of the content of each element means "mass%".

(A) Chemical composition of quenched and tempered steel for brake disc C: 0.1 to 0.6%
C enhances the hardenability of the steel to improve toughness, and has the effect of improving the high temperature strength by secondary precipitation as carbonitride during tempering. However, if the content is less than 0.1%, the effect of addition is poor. On the other hand, if it exceeds 0.6%, it causes a decrease in toughness. Therefore, the content of C is set to 0.1 to 0.6%. In addition, the range of desirable C content is 0.15-0.3%.

Si: More than 1.2% and not more than 2.0% Si has an effect of increasing the Ac ₃ transformation point of steel. In particular, when the Si content exceeds 1.2%, the Ac _{3 of the} steel is increased. Since the transformation point exceeds 850 ° C. and the temperature of the brake disc surface reaches a high temperature of 900 ° C. or higher, the formation of martensite during cooling can be suppressed, so cracks occur in the brake disc. Can be prevented. In addition, the suppression of martensite generation is also effective in preventing the occurrence of tempering cracks when quenching the brake disc steel, which is the material of the brake disc, and reducing the tensile stress after tempering. However, if the Si content exceeds 2.0%, the toughness and high-temperature strength are lowered, so the crack resistance of the brake disk is lowered. Therefore, the Si content is more than 1.2% and not more than 2.0%. In addition, the range of desirable Si content is 1.3 to 1.7%.

Mn: 0.2 to 2.0%
Mn is an element effective in improving the hardenability of steel and increasing toughness. However, if the content is less than 0.2%, the effect of addition cannot be obtained, and if it exceeds 2.0%, segregation occurs and the toughness and strength are reduced. Therefore, the Mn content is set to 0.2 to 2.0%. In addition, the range of desirable Mn content is 0.5 to 1.0%.

Ni: 0.8-3.0%
Ni, like Mn, is an element effective for improving the hardenability of steel and improving toughness. However, if the Ni content is less than 0.8%, the effect of addition is poor. On the other hand, when the content exceeds 3.0%, the transformation point is lowered and the high temperature strength is lowered. Therefore, the Ni content is set to 0.8 to 3.0%. In addition, the range of desirable Ni content is 1.0 to 1.5%.

Cr: 1.0-5.0%,
Cr is an element effective for improving toughness and wear resistance. However, if the content is less than 1.0%, a sufficient effect cannot be obtained. Conversely, if the content is too much and exceeds 5.0%, the toughness is decreased, and the high temperature strength is also decreased. Will be invited. Therefore, the Cr content is set to 1.0 to 5.0%. In addition, the range of desirable Cr content is 1.3 to 1.7%.

Mo: 0.5-2.0%
Mo has the effect of increasing the toughness and high temperature strength by increasing the hardenability of the steel and increasing the temper softening resistance. Furthermore, the carbide of Mo is effective in improving the wear resistance. However, if the Mo content is less than 0.5%, the above effect cannot be obtained. On the other hand, if the Mo content exceeds 2.0%, the toughness is lowered. Therefore, the Mo content is set to 0.5 to 2.0%. In addition, the range of preferable Mo content is 0.5 to 1.0%.

V: 0.05-0.5%
V is an element effective for improving high-temperature strength and wear resistance. V also has a function of forming a nitride in combination with N and preventing coarsening of crystal grains during quenching. However, when the V content is less than 0.05%, these effects cannot be obtained. On the other hand, if the content of V increases, the toughness decreases, and if it exceeds 0.5%, the toughness decreases remarkably. Therefore, the content of V is set to 0.05 to 0.5%. In addition, the range of preferable V content is 0.1 to 0.3%.

Al: 0.001 to 0.10%
Al is an effective element for stabilizing and homogenizing deoxidation of steel. Al also has an effect of forming AlN in combination with N and preventing coarsening of crystal grains during quenching. However, when the Al content is less than 0.001%, these effects cannot be obtained. On the other hand, if the Al content exceeds 0.10%, the toughness is lowered, and it becomes a cause of steel flaws. Therefore, the content of Al is set to 0.001 to 0.10%. In addition, the range of desirable Al content is 0.01 to 0.08%.

N: 0.0015 to 0.020%
N combines with Al and V to form a nitride, and has an effect of preventing coarsening of crystal grains during quenching. However, if the N content is less than 0.0015%, the above-described effects cannot be obtained. On the other hand, if the content of N increases and exceeds 0.020%, the amount of nitride increases too much, causing a decrease in toughness. Therefore, the N content is set to 0.0015 to 0.020%. In addition, the range of desirable N content is 0.0020 to 0.010%.

  For the reasons described above, the steel material for a brake disc that has been quenched and tempered according to the present invention (1) contains elements from C to N within the above-mentioned range, and the balance is made up of a chemical composition of Fe and impurities. did.

  In addition, the steel material for brake discs quenched and tempered according to the present invention (1) is obtained when the value of fn1 represented by the above formula (1), that is, the value of [15C + 10Cr + 6Mo + 200V−35] is 0 or more. The abundance ratio of carbide, Mo carbide and V carbide is optimized, and good wear resistance at higher temperatures is ensured.

  The maximum value of fn1 is 136 when the contents of C, Cr, Mo, and V are 0.6%, 5.0%, 2.0%, and 0.5%, which are the above-described upper limit values, respectively. It may be. However, in order to ensure good toughness, the value of fn1 is preferably 10 or less.

  For the above reason, the steel material for brake discs according to the present invention (2) is the steel material for brake discs according to the present invention (1), and is represented by the formula (1). The value of fn1 is defined to be 0 or more.

  In addition, it is preferable that the contents of P and S in the impurities in the quenched and tempered steel for brake disc according to the present invention are as follows.

P: 0.03% or less P as an impurity increases segregation when the content is large, resulting in a decrease in toughness and crack resistance. In particular, when the content exceeds 0.03%, toughness and crack resistance are increased. The fall of the property becomes remarkable. Therefore, the P content is preferably 0.03% or less, and the lower the better.

S: 0.03% or less S as an impurity reduces toughness, and particularly when its content exceeds 0.03%, the toughness is significantly reduced. Therefore, the S content is preferably 0.03% or less, and the lower the better.

(B) Ac ₃ transformation point hardening and tempered Ac ₃ transformation point of the steel material for brake discs of steel for quenching and tempered brake disk is required to exceed 850 ° C.. This is because when the Ac ₃ transformation point is 850 ° C. or lower, when the temperature of the brake disk surface reaches 900 ° C. or higher due to sliding with the friction material, martensite is easily generated in the subsequent cooling process. This is because it is inevitable that the brake disc will crack.

Therefore, the Ac ₃ transformation point of the quenched and tempered steel material for brake disks according to the present invention is defined to exceed 850 ° C.

(C) Austenite grain number of quenched and tempered steel for brake discs Austenite grain number of quenched and tempered steel for brake discs, in other words, the austenite grain number of brake discs in an unused state is 6.0 or more Must. This is because when the austenite grain size number is less than 6.0, when the temperature of the brake disc surface becomes a high temperature of 900 ° C. or higher due to sliding with the friction material, the target of crack resistance in the present invention, that is, described later. After finishing into a brake disc shape for Shinkansen with a diameter of 800 mm and a thickness of 20 mm shown in the examples, in combination with a copper-based sintered material as a friction material, when the brake was applied 1000 times per hour from 300 km, This is because the goal of not generating cracks cannot be achieved.

  Therefore, the austenite grain size number of the steel for brake discs quenched and tempered according to the present invention is defined to be 6.0 or more. The austenite grain size number is preferably 7.0 or more.

  As described above, the austenite grain size number refers to a value measured by the “steel-crystal grain size microscopic test method” described in JIS G 0551 (2005).

  In the case of steel having the chemical composition described in the item (A), the austenite grain size number can be made 6.0 or more by, for example, heating to 870 to 930 ° C. and then quenching.

(D) Fracture toughness value of quenched and tempered steel for brake discs Fracture toughness value of quenched and tempered steel for brake discs, in other words, fracture toughness value of brake discs in an unused state is 130 MPa · m ¹ Must be greater than ^{/ 2} . This is because when the fracture toughness value is less than 130 MPa · m ^1/2 , when the surface of the brake disk reaches a high temperature of 900 ° C. or higher due to sliding with the friction material, the target of crack resistance in the present invention, After finishing to a shape of a brake disc for Shinkansen with a diameter of 800 mm and a thickness of 20 mm as shown in the examples described later, the brake was applied 1000 times from 300 km / h in combination with a copper-based sintered material as a friction material. This is because the goal of not generating cracks cannot be achieved.

Therefore, it was specified that the fracture toughness value of the steel material for brake discs quenched and tempered according to the present invention was 130 MPa · m ^1/2 or more. The fracture toughness value is preferably 140 MPa · m ^1/2 or more.

  In addition, as already stated, the above-mentioned fracture toughness value refers to a value measured by a fracture toughness test performed at room temperature by a method described in JIS G 0564 (1999) using a so-called “CT test piece”.

In the case of steel having the chemical composition described in the section (A), for example, if it is quenched so that the cooling rate in the temperature range of 350 to 250 ° C. during quenching is 0.8 ° C./s or more, the fracture The toughness value can be 130 MPa · m ^1/2 or higher, and the higher the cooling rate in the temperature range, the higher the fracture toughness value.

  The cooling rate of 0.8 ° C./s or higher in the temperature range of 350 to 250 ° C. during quenching is, for example, an oil temperature of 85 after heating to 870 to 930 ° C. described in the section (C). It can be obtained by performing oil quenching or water quenching in an oil having a temperature of ℃ or less.

(E) Configuration of Brake Disc The brake disc according to the present invention (3) has the chemical composition described in the item (A), and has the Ac ₃ transformation point and the austenite grain number described in the items (B) to (D). The steel material for brake discs according to the present invention (1) or the present invention (2) having a fracture toughness value is used as a raw material.

  This is a case where the brake disc using the steel material for a brake disc according to the present invention (1) or the present invention (2) as a raw material increases in vehicle speed and travels at a high speed of 250 km / h or more. However, it is because it is excellent in abrasion resistance and crack resistance.

  Therefore, the brake disc according to the present invention (3) is defined as being made of the steel material for a brake disc according to the present invention (1) or the present invention (2).

  Hereinafter, the present invention will be described in more detail with reference to examples.

Example 1
Steels 1 to 9 having chemical compositions shown in Table 1 were melted using an electric furnace. In Table 1, Steel 1, Steel 7, and Steel 8 are steels of the present invention examples in which the content of each component element is within the range defined by the present invention. On the other hand, Steels 2 to 6 and Steel 9 are steels of comparative examples in which the content of any of the constituent elements is out of the definition of the present invention. Table 1 also shows the value of fn1 represented by the above equation (1).

From each of the steel ingots of steels 1 to 9, a test piece for measuring the transformation point having a diameter of 3 mm and a length of 10 mm is prepared by machining, and the test piece is heated by a Formaster tester, The Ac ₃ transformation point of was measured.

Table 1 also shows the Ac ₃ transformation point of each steel. FIG. 1 shows the relationship between the Ac ₃ transformation point and the Si content for Steel 1 and Steels 5 to 9 having the same C, Mn, Ni, Cr, Mo, and V contents.

From FIG. 1, it is clear that when the content of other constituent elements is the same level, the Ac ₃ transformation point of the steel can be increased by increasing the Si content.

(Example 2)
For each of the steels 1 to 9 melted in Example 1, a part of the steel ingot is rolled into pieces, then heated to 1260 ° C. and hot forged to obtain a steel plate having a thickness of 45 to 100 mm. Produced.

  The steel plate thus obtained was heated to 870 to 1000 ° C., and then put into oil having an oil temperature of 85 ° C. or lower and stirred for oil quenching. In addition, the thermocouple was embedded in the plate | board thickness center of each steel plate, and the cooling rate in the 350-250 degreeC temperature range at the time of quenching was measured.

  A small sample was cut out from each steel plate obtained as described above, heated and held at 550 ° C., and then allowed to cool in the air and tempered. Next, after mirror polishing, the steel was corroded with a saturated aqueous solution of picric acid, and the austenite grain size number was measured based on the “steel-crystal grain size microscopic test method” described in JIS G 0551 (2005).

  Further, the remaining portion of the steel sheet from which the sample was cut was heated and held at 550 to 650 ° C., then allowed to cool in the atmosphere and tempered, and the hardness was adjusted to 280 to 310 in terms of Brinell hardness (HB hardness). It was adjusted.

  The above HB hardness of 280 to 310 is a level corresponding to the hardness of the brake disc used in the Shinkansen and the like.

  A so-called “CT test piece (compact test piece)” was cut out from each steel plate after quenching and tempering obtained as described above, and a fracture toughness test was performed.

  That is, a fracture toughness test was performed at room temperature by a method described in JIS G 0564 (1999) using a “CT test piece” having a thickness of 20 mm in the T-ST direction.

  Further, in order to perform a so-called “pin-on-disk wear test”, a disk specimen having a diameter of 175 mm and a thickness of 15 mm was cut out from a part of the quenched and tempered steel sheets.

  The pin-on-disk abrasion test is performed by pressing a copper-based sintered material having a diameter of 10 mm and a length of 25 mm against a disk test piece having a diameter of 175 mm and a thickness of 15 mm. The amount of wear was determined from the mass change of the disk before and after the test.

・ Pressing pressure: 0.9 MPa,
-Disk rotation speed: 500 rpm,
・ Sliding distance: 10 ⁵ m
・ Lubrication: None.

  Table 2 summarizes the investigation results of the thickness of each steel sheet, the heating temperature during quenching and the cooling rate in the temperature range of 350 to 250 ° C., the austenite grain number, the fracture toughness value, and the wear amount by the pin-on-disk wear test. Show.

  Note that “−” in the “Abrasion amount” column in Table 2 means that the pin-on-disk wear test was not performed.

  Moreover, in FIG. 2, about the test numbers 1-4 using the steel 1, the relationship between the cooling rate in the temperature range of 350-250 degreeC and a fracture toughness value is arranged and shown. Further, FIG. 3 shows the relationship between the Si content and the fracture toughness value for Test No. 2 and Test Nos. 8 to 12 where the cooling rate in the temperature range of 350 to 250 ° C. is 0.85 ° C./s. .

  2 and 3, the above test numbers are indicated as “No. 2”, “No. 12”, and the like.

  From Table 2 and FIGS. 2 and 3, when the austenite grain size number is the same level, if the cooling rate in the temperature range of 350 to 250 ° C. is increased during quenching, a large fracture toughness value can be obtained after tempering. When the cooling rate is 0.85 ° C./s, it is clear that the fracture toughness value does not decrease until the Si content is 2.0%.

Also, from Table 2, the wear amount of the disc made of the steel material for brake disc that has been quenched and tempered satisfying the conditions specified in the present invention in Test No. 1 and Test No. 2 of the present invention is less than 250 mm ^3. Therefore, it is clear that the wear resistance is excellent.

(Example 3)
For each of the steels 1 to 9 melted in Example 1, the other part of the steel ingot was divided and rolled into a round steel having a diameter of 360 mm, allowed to cool in the air, and cooled to room temperature.

  Next, the above-described round steel having a diameter of 360 mm is cut into lengths of 230 mm, heated to 1260 ° C., and then heated to a rough shape of a Shinkansen brake disk having a thickness of 25 to 100 mm. Forged between.

  After the hot-forged rough shaped material is heated to 900 ° C. or 1000 ° C., oil quenching or water quenching is performed in an actual machine heat treatment line, and the oil temperature is poured into oil having an oil temperature of 85 ° C. or lower. A quenching treatment was performed. That is, in the actual machine heat treatment line, each brake disc rough shaped material having a thin part thickness of 25 to 100 mm was subjected to quenching treatment in a state where five sheets were stacked.

  A small sample was cut out from each of the coarse-shaped materials quenched as described above, heated and held at 550 ° C., and then allowed to cool in the atmosphere to be tempered. Next, after mirror polishing, the steel was corroded with a saturated aqueous solution of picric acid, and the austenite grain size number was measured based on the “steel-crystal grain size microscopic test method” described in JIS G 0551 (2005).

  Moreover, about each other hardened | cured rough shape material, after heating and hold | maintaining to 550-650 degreeC, it stood to cool in air | atmosphere and gave a tempering process, and hardness was 280-310 by Brinell hardness (HB hardness). It was adjusted.

  As described above, the HB hardness of 280 to 310 is a level corresponding to the hardness of the brake disc used in the Shinkansen and the like.

  Next, each tempered rough shaped material is finished into a Shinkansen brake disk shape with a diameter of 800 mm and a thickness of 20 mm, and then in combination with a copper-based sintered material as a friction material, from 300 km / h A brake test in which the brake was applied 1000 times was performed, and the presence or absence of cracks on the sliding surface of the brake disc was visually examined after the test.

  In addition, after conducting the above test, a cross-sectional specimen for microstructural observation was cut out, mirror-polished, then corroded with nital, and the depth of the white layer (martensitic phase) generated in the brake disc disk by optical microscopic observation Was measured.

  Table 3 summarizes the results of the investigation of the heating temperature, the austenite grain size number, the fracture toughness value, the presence or absence of cracks on the sliding surface of the brake disc after the brake test, and the depth of the white layer.

  From Table 3, the brake discs made of steel materials for brake discs that are quenched and tempered satisfying the conditions specified in the present invention in Test No. 14, Test No. 15, Test No. 23 and Test No. 24 of the present invention. It is clear that there is no white layer and that the sliding surface has no cracks and is excellent in crack resistance.

  On the other hand, any of the sliding surfaces of the brake disc made of a steel material for a brake disc that has been quenched and tempered and deviates from the conditions specified in the present invention has cracks, which may be inferior in crack resistance. it is obvious.

That is, for test number 16 and test number 17, the steel 1 of the present invention example in which the Ac ₃ transformation point exceeds 850 ° C. and the content of each component element is within the range defined by the present invention is used. However, the fracture toughness value of quenched and tempered steel for brake discs is low and less than 130 MPa · m ^1/2 , so cracks occur on the sliding surfaces of brake discs made from the above steel materials for brake discs. It is inferior in crack resistance.

Similarly, with regard to test number 26, although the Ac ₃ transformation point exceeds 850 ° C. and the content of each component element is within the range specified by the present invention, the steel 1 of the present invention example is used, but it is quenched.・ Since the austenite grain size number of the tempered brake disc steel is small and less than 6.0, all of the sliding surfaces of the brake disc made of the above-mentioned brake disc steel have cracks, resulting in improved crack resistance. Inferior.

For test No. 18, steel 2 for which the Cr and Mo contents deviate from the provisions of the present invention and the Ac ₃ transformation point is as low as 816 ° C. is used. Since the value is low and less than 130 MPa · m ^1/2 , the sliding surface of the brake disk made of the above-mentioned steel material for brake disk is cracked and inferior in crack resistance. A white layer having a depth of 1.0 mm was observed on the brake disk of test number 18.

For test No. 19, steel 3 is used in which the contents of Si and Mo are not within the scope of the present invention, and the Ac ₃ transformation point is as low as 787 ° C., and the steel for brake discs is quenched and tempered. Since the value is low and less than 130 MPa · m ^1/2 , the sliding surface of the brake disk made of the above-mentioned steel material for brake disk is cracked and inferior in crack resistance. Note that a white layer having a depth of 2.1 mm was observed on the brake disk of Test No. 19.

Test No. 20 uses steel 4 in which the contents of Si and Ni deviate from the provisions of the present invention, and the Ac ₃ transformation point is as low as 840 ° C., and the fracture toughness of the steel for brake disks that has been quenched and tempered Since the value is low and less than 130 MPa · m ^1/2 , the sliding surface of the brake disk made of the above-mentioned steel material for brake disk is cracked and inferior in crack resistance. A white layer having a depth of 0.2 mm was observed on the brake disk of test number 20.

For Test No. 21 and Test No. 22, since the content of Si deviates from the definition of the present invention and the Ac ₃ transformation points are as low as 799 ° C. and 840 ° C., Steel 5 and Steel 6 are used. All of the sliding surfaces of the brake disk made of steel are cracked and inferior in crack resistance. Note that white layers having a depth of 1.9 mm and a depth of 0.1 mm were observed on the brake disks of these test numbers, respectively.

For test number 25, steel 9 whose Si content is not within the scope of the present invention is used, and the austenite grain size number of the quenched and tempered steel material for the brake disc is smaller than 6.0, Since its fracture toughness value is low and below 130 MPa · m ^1/2 , cracks are generated on the sliding surface of the brake disc made of the steel material for brake discs, and the crack resistance is poor.

  Since the brake disc of the present invention is excellent in wear resistance and crack resistance, it can be used in a disc brake device when the vehicle speed increases and the vehicle runs at a high speed of 250 km / h or higher. Further, the steel material for a brake disc that has been quenched and tempered according to the present invention is suitable for use as an inexpensive material for the brake disc.

C used in Example, Mn, Ni, Cr, the content of Mo and V for steel 1 and steel 5-9 in the same level is a diagram showing the relationship of Ac ₃ transformation point and the Si content. It is a figure which rearranges and shows the relationship between the cooling rate in the temperature range of 350-250 degreeC, and the fracture toughness value about the test numbers 1-4 of the Example using the steel 1. FIG. In the figure, the test number is represented as “No. 1”. It is a figure which arranges and shows the relation between Si content and fracture toughness value about test number 2 and test numbers 8-12 of an example whose cooling rate in the temperature range of 350-250 ° C is 0.85 ° C / s. . In the figure, the test number is indicated as “No. 2”.

Claims (3)

In mass%, C: 0.1 to 0.6%, Si: more than 1.2% and 2.0% or less, Mn: 0.2 to 2.0%, Ni: 0.8 to 3.0 %, Cr: 1.0 to 5.0%, Mo: 0.5 to 2.0%, V: 0.05 to 0.5%, Al: 0.001 to 0.10% and N: 0.00. 0015-0.020%, the balance is composed of the chemical composition of Fe and impurities, the Ac ₃ transformation point exceeds 850 ° C., the austenite grain size number is 6.0 or more, and 130 MPa · m ^1/2 Quenched and tempered steel material for brake discs having the above fracture toughness value. The steel material for a brake disc that has been quenched and tempered according to claim 1, characterized in that it has a chemical composition in which the value of fn1 represented by the following formula (1) is 0 or more.
fn1 = 15C + 10Cr + 6Mo + 200V−35 (1)
However, the element symbol in the formula (1) represents the content in steel in mass% of the element.   A brake disc comprising the steel material for a brake disc according to claim 1 or 2 as a raw material.

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