JP4843969B2 - Stainless steel plate for disc brakes with excellent heat resistance and corrosion resistance - Google Patents

Description

  The present invention relates to a stainless steel plate suitable for use as a disc (rotary disc) for disc brakes of motorcycles, automobiles, bicycles, etc., and particularly suitable quenching hardness is obtained and tempered at high temperature by frictional heat during braking. Further, the present invention relates to a stainless steel plate for disc brakes that is excellent in heat resistance and corrosion resistance, that can maintain an appropriate hardness with little softening and that has a small decrease in corrosion resistance. Note that the steel sheet referred to in the present invention includes a steel strip.

  The function of disc brakes for motorcycles, automobiles, bicycles, etc. is to brake the vehicle by suppressing the rotation of the wheels by friction between the brake discs and the brake pads. For this reason, the brake disc is desired to have appropriate hardness. If the hardness is soft, the brake is weak and wears quickly due to friction with the brake pad. On the other hand, if it is too hard, there is a problem that brake noise is likely to occur. As the appropriate hardness of the brake disc, a hardness range of HRC 32 to 38 is recommended. Here, HRC is Rockwell hardness (C scale) defined in JIS Z 2245.

  Conventionally, martensitic stainless steel sheets have been used as brake disk materials from the viewpoints of hardness and corrosion resistance. Temporarily, martensitic stainless steel sheets with a high carbon content, such as SUS 420J2, were sometimes used after being quenched and tempered. As shown in Fig. 2, low carbon martensitic stainless steel sheets that can be used as-quenched are often used as brake disk materials.

In recent years, there has been a demand for improvement in fuel efficiency of motorcycles and automobiles from the viewpoint of global environmental conservation. In order to improve fuel efficiency, it is effective to reduce the weight of the vehicle body, and it is aimed to reduce the weight of the vehicle. Disc brakes, which are braking devices, are no exception, and in order to further reduce the weight of the vehicle, the brake discs are reduced in size and reduced in thickness (thinned).
However, the reduction in size and thickness of the brake disc causes a reduction in heat capacity, and the temperature increase of the brake disc due to frictional heat during braking becomes larger. For this reason, it is conceivable that the brake disc temperature at the time of braking rises to about 600 ° C. in accordance with the trend toward such miniaturization and thinning. For this reason, in the conventional material, there is a concern that the brake disc is tempered and softened, the durability is lowered and the corrosion resistance is also lowered, and a brake disc material excellent in heat resistance and corrosion resistance is desired.

In response to such a request, for example, Patent Document 3 contains an appropriate amount of one or more of Ti, Nb, V, and Zr, and is effective in softening due to temperature rise during use of the disc brake. A low-carbon martensitic stainless steel sheet that can suppress the decrease in hardness and suppress the hardness has been proposed.
Further, Patent Document 4 proposes a stainless steel for a disc brake that can effectively suppress temper softening by adding Nb or Nb in addition to Ti, V, and B and adding an appropriate amount. Yes.

In Patent Document 5, the GP value, which is a relational expression of the contents of C, N, Ni, Cu, Mn, Cr, Si, Mo, V, Ti, and Al in steel, is adjusted to 50 (%) or more. In addition, by setting Nb and V to appropriate amounts, an inexpensive steel for a disc brake rotor has been proposed that hardly causes material deterioration due to temperature rise during use.
Further, Patent Document 6 limits the amount of C + N to a specific range, contains an appropriate amount of austenite forming elements Mn, Ni, and Cu, and further contains an appropriate amount of Nb, and one or two of Zr, Ti, and Ta. A martensitic stainless steel for disc brakes having a high braking heat softening resistance and a high resistance to tempering softening that has the above-described properties and can ensure a desired hardness as it is quenched has been proposed.
JP-A-57-198249 JP-A-60-106951 JP 2002-146489 A Japanese Patent No. 3315974 JP 2002-121656 A JP 2001-192779

However, in the techniques described in Patent Document 3, Patent Document 4, Patent Document 5, and Patent Document 6, it is necessary to add a relatively large amount of an expensive alloy element, and the manufacturing cost of the disc brake increases, and 600 When held at C for a long time (about 2 hours), there is a problem that the hardness is drastically lowered and the corrosion resistance is also lowered at the same time.
The present invention advantageously solves the problems of the prior art and can ensure proper quenching hardness, and the hardness after holding at 600 ° C. for 2 hours is HRC (Rockwell hardness C defined by JIS Z 2245). The objective is to propose a stainless steel plate for disc brakes that has a heat resistance and corrosion resistance that can secure 32 or more on the scale) and that has little deterioration in corrosion resistance.

  In order to achieve the above-described problems, the present inventors diligently studied the influence of alloy elements on temper softening resistance and corrosion resistance, which are indicators of heat resistance of a martensitic stainless steel sheet. As a result, Nb and Ni are contained in proper amounts, V is further added, and the N is increased to a relatively low C, so that even after tempering at 600 ° C. for about 2 h, the HRC is higher than 32 It has been found that the hardness can be maintained and that the corrosion resistance is hardly lowered.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) In mass%, C: 0.02% or more and less than 0.10%, Si: 1.0% or less, Mn: 1.0 to 2.5%, P: 0.04% or less, S: 0.01% or less, Cr: more than 11.5%, 15.0% or less, Ni: 0.1 to 1.0%, Al: 0.10% or less, Nb: more than 0.08% to 0.6% or less, V: 0.02 to 0.3%, N: more than 0.03% to 0.10% or less, the following formulas (1) to (3)
0.03 ≦ {C + N− (13/93) Nb} ≦ 0.10 (1)
(5Cr + 10Si + 15Mo + 30Nb + 50V-9Ni-5Mn-3Cu-225N-270C) ≦ 45
……… (2)
{(14/50) V + (14/90) Nb} <N ......... (3)
(Here, Cr, Si, Mo, Nb, Ni, Mn, Cu, V, N, C: content of each element (mass%))
A stainless steel plate for a disc brake excellent in heat resistance and corrosion resistance, characterized in that it has a composition comprising the balance Fe and inevitable impurities .
(2 ) A disc brake disc obtained by heating and quenching the stainless steel plate for disc brake according to (1 ) to 1000 ° C. or less.

  According to the present invention, it has excellent corrosion resistance, can ensure an appropriate quenching hardness of HRC32 to 38, and can maintain a high hardness of HRC32 or higher even after tempering held at 600 ° C. for 2 hours, and has little decrease in corrosion resistance. In addition, a stainless steel plate for a brake disk having excellent heat resistance and corrosion resistance can be easily manufactured at low cost, and has a remarkable industrial effect. ADVANTAGE OF THE INVENTION According to this invention, there exists an effect that the disk for disc brakes (rotary disk) excellent in durability, such as a motorcycle, a motor vehicle, a bicycle, and a snowmobile, can be manufactured at low cost.

First, the reasons for limiting the composition of the stainless steel plate for brake disc of the present invention will be described. Hereinafter, mass% in the composition is simply referred to as%.
C: 0.02% or more and less than 0.10% C is an element that determines the hardness after quenching, and in the present invention, 0.02% or more may be contained in order to ensure proper quenching hardness in the range of HRC32 to 38. desirable. On the other hand, when the content is 0.10% or more, coarse Cr carbide is formed when tempered at a high temperature, so that it becomes a starting point of rusting and lowers corrosion resistance and toughness. For this reason, C was limited to less than 0.10%. From the viewpoint of corrosion resistance, it is preferably less than 0.05%.

Si: 1.0% or less
Si is an element that acts as a deoxidizing agent. In the present invention, it is desirable to contain 0.05% or more. However, Si is an element that stabilizes the ferrite phase, and an excessive content exceeding 1.0% decreases the quenching hardness. To lower, and further toughness. For this reason, Si was limited to 1.0% or less. From the viewpoint of toughness, it is preferably 0.3% or less.

Mn: 1.0-2.5%
Mn is an element useful for suppressing the formation of a ferrite phase at a high temperature, improving hardenability, and obtaining stable quenching hardness, and is contained in an amount of 1.0% or more. On the other hand, an excessive content exceeding 2.5% lowers the corrosion resistance. For this reason, Mn was limited to the range of 1.0 to 2.5%. In view of hardenability, it is preferably 1.5% or more.

P: 0.04% or less P is an element that lowers hot workability, and is desirably reduced as much as possible. However, excessive reduction causes an increase in manufacturing cost, so 0.04% was made the upper limit. From the viewpoint of manufacturability, it is preferably 0.02% or less.
S: 0.01% or less S, like P, is an element that reduces hot workability, and it is desirable to reduce it as much as possible. However, excessive reduction causes an increase in manufacturing cost, so 0.01% was made the upper limit. From the viewpoint of manufacturability, it is preferably 0.005% or less.

Cr: more than 11.5% and less than 15.0%
Cr is a useful element that improves the corrosion resistance characteristic of stainless steel, and in order to secure sufficient corrosion resistance, it needs to contain more than 11.5%. On the other hand, if the content exceeds 15.0%, workability and toughness deteriorate. For this reason, Cr was limited to more than 11.5% and not more than 15.0%. It is preferably 12.0% or more from the viewpoint of corrosion resistance and 13.5% or less from the viewpoint of toughness.

Ni: 0.1-1.0%
Ni is an element that improves the corrosion resistance and needs to be contained in an amount of 0.1% or more. On the other hand, if the content exceeds 1.0%, the diffusion rate of Cr decreases, and a long time is required for soft annealing of the hot-rolled sheet, resulting in a decrease in productivity. Therefore, in the present invention, Ni is limited to the range of 0.1 to 1.0%. Preferably, it is 0.3% or more from the viewpoint of corrosion resistance and 0.8% or less from the viewpoint of productivity.

Al: 0.10% or less
Al is an element that acts as a deoxidizing agent, and is added as a deoxidizing agent on steel making. However, if it is excessively left in steel, workability and toughness are lowered. For this reason, Al was limited to 0.10% or less. In addition, Preferably it is less than 0.01%.
Nb: more than 0.08% and less than 0.6%
Nb has strong bonding strength with C and N, forms Nb carbide and Nb nitride, and recovers strain when kept at a temperature around 600 ° C after quenching (strain recovery introduced during quenching) Is suppressed, temper softening resistance is increased, and heat resistance is improved. In order to acquire such an effect, it is necessary to contain over 0.08%, but when it contains over 0.6%, toughness will fall. For this reason, Nb was limited to the range of more than 0.08% and less than 0.6%. Preferably, it is 0.11% or more from the viewpoint of heat resistance and less than 0.3% from the viewpoint of toughness.

V: 0.02-0.3%
V is an element that forms fine carbides (VC) and nitrides (VN) at 600 to 700 ° C., increases temper softening resistance by precipitation hardening, and improves heat resistance. In the present invention, it contains 0.02% or more. It is necessary to let In particular, the heat resistance improvement effect of VN is remarkable, and when V is combined with high N, a greater effect is exhibited. On the other hand, the content exceeding 0.3% lowers toughness. For this reason, V was limited to the range of 0.02 to 0.3%. From the viewpoint of heat resistance, it is preferably 0.05% or more, and more preferably 0.10% or more.

N: more than 0.03% and 0.10% or less N, like C, is an element that determines the hardness after quenching. Further, solute N has an effect of improving the corrosion resistance. Further, N forms fine Cr nitride in the temperature range of 500 to 700 ° C., increases the temper softening resistance by the precipitation hardening action, and improves the heat resistance of the steel plate (disk). Further, the Cr carbide precipitated during tempering is coarse and becomes a starting point of rusting, but the Cr nitride is fine and does not become a starting point of rusting, and the corrosion resistance after tempering is hardly lowered. Therefore, even if the N is increased and the C is relatively lowered, the heat resistance and corrosion resistance are excellent even if the quenching hardness is the same. In order to acquire such an effect, it is necessary to contain N exceeding 0.03%. On the other hand, the content exceeding 0.10% causes a decrease in toughness. For this reason, in the present invention, N is limited to more than 0.03% and 0.10% or less. From the viewpoint of heat resistance and corrosion resistance, it is preferably 0.040% or more.

In the present invention, the above basic components are within the above range, and the following formulas (1) to (3)
0.03 ≦ {C + N− (13/93) Nb} ≦ 0.10 (1)
(5Cr + 10Si + 15Mo + 30Nb + 50V-9Ni-5Mn-3Cu-225N-270C) ≦ 45
……… (2)
{(14/50) V + (14/90) Nb} <N ......... (3)
(Here, Cr, Si, Mo, Nb, V, Ni, Mn, Cu, N, C: content of each alloy element (mass%))
Is contained so as to satisfy. In the calculation of the value on the left side of equation (2), when the Mo content is less than 0.01% and the Cu content is less than 0.05%, Mo and Cu are calculated as zero, respectively.

  Equation (1) is a condition for setting the quenching hardness to a hardness within a predetermined appropriate range. The quenching hardness has a strong correlation with the amount of dissolved C and N. When C and N combine with Nb to form Nb carbide and Nb nitride, they do not contribute to the hardness. Therefore, the hardness after quenching needs to be considered as a value obtained by subtracting the amount of C and N consumed as precipitates from the amount of C and N in the steel. In addition, since Cr carbonitride and V carbonitride are decomposed and solid-dissolved during quenching heating, Cr and V do not consume C and N that affect quenching hardness. When the intermediate term of the formula (1) is less than 0.03, the quenching hardness is less than the lower limit value (HRC32) of the predetermined appropriate range, while when it exceeds 0.10, it becomes higher than the upper limit value (HRC38). For this reason, the value of the intermediate term of the formula (1) is limited to a range of 0.03 to 0.10.

  Formula (2) is a condition for ensuring excellent quenching stability. The term “excellent in quenching stability” as used herein means that an austenite phase is generated by 80% by volume or more during quenching heating, transforms into a martensite phase during quenching by cooling over air cooling, and has a predetermined quenching hardness It means that it can be secured. When the left side of the formula (2) exceeds 45, the temperature range in which an austenite phase is generated by 80% by volume or more during quenching heating becomes narrow, and stable quenching hardness cannot be secured. For this reason, the value on the left side of equation (2) is limited to 45 or less.

  Equation (3) is a condition for ensuring the stability of solid solution N that improves hardness and corrosion resistance and for improving heat resistance by nitrides. When the N content does not satisfy Equation (3), N for generating Nb or V nitride is insufficient, and Nb nitride, V nitride and Cr nitride are not sufficiently precipitated, resulting in low heat resistance. Moreover, since the amount of solid solution N which remains without making a precipitate decreases, corrosion resistance also becomes low.

The balance other than the above components is Fe and inevitable impurities. Inevitable impurities include alkali metals such as Na, alkaline earth metals such as Mg and Ba, rare earth elements such as Y and La, and transition elements such as Hf, even if contained in an amount of 0.05% or less. It does not interfere with the effect.
The stainless steel plate of the present invention may be either a hot rolled steel plate or a cold rolled steel plate.
The manufacturing method of the stainless steel plate of the present invention is not particularly limited, and any of the generally used manufacturing methods of stainless steel plates can be applied. For example, the following manufacturing method is preferable.

The molten steel having the above composition is melted in a converter, electric furnace, etc., and further subjected to secondary refining such as VOD (Vacuum Oxygen Decarburization), AOD (Argon Oxygen Decarburization), etc. It is considered as a material. The casting method is preferably a continuous casting method from the viewpoint of productivity and quality.
Next, the steel material is preferably heated to 1100 to 1250 ° C. and is hot rolled into a hot-rolled steel plate having a predetermined thickness. For brake discs, the thickness is preferably about 3 to 8 mm. It is preferable that the hot-rolled steel sheet is further subjected to hot-rolled sheet annealing, and further descaled by shot blasting, pickling or the like as necessary to obtain a material for a brake disk. By this hot-rolled sheet annealing, the hardness of the hot-rolled steel sheet becomes 75 to 88 on the HRB (Rockwell hardness B scale) specified by JIS Z 2245, which is suitable as a material for brake discs. It can be used as a material for brake discs. In addition, after hot-rolled sheet annealing, you may give a leveler and a skin pass for shape correction. The hot-rolled sheet annealing is preferably more than 750 ° C. and 900 ° C. or less.

Since hot-rolled sheet annealing requires holding for several hours or more at the annealing temperature, it is preferable to use a batch type box furnace. After the holding at the annealing temperature is completed, it is gradually cooled in a box furnace. From the viewpoint of productivity, the cooling rate is preferably 20 ° C./h exceeding the annealing temperature to 500 ° C.
If the brake disc is thin (less than approximately 3 mm), the hot-rolled steel sheet is further cold-rolled, annealed as necessary, and further descaled, such as pickling as necessary. It can be used as a material for brake discs.

The manufacturing procedure of the brake disc is usually as follows.
Using the stainless steel plate described above as a raw material, a disk having a predetermined size is punched from the stainless steel plate to obtain a brake disc material. Next, after processing the brake disc material, such as making a hole to release the frictional heat generated during braking, a high frequency is applied to a predetermined area of the brake disc material, that is, the friction portion that is the part where the brake pad hits. A predetermined quenching process is performed by heating to a predetermined quenching heating temperature by induction heating or the like and then cooling to adjust the predetermined region (friction part) to a desired hardness. In addition, if it is the stainless steel plate of this invention, even if it sets the quenching heating temperature to the temperature of the range of 900-1000 degreeC which is a normal quenching heating temperature, it can ensure sufficient quenching hardness sufficiently and has the outstanding heat resistance and corrosion resistance. it can.

Then, if necessary, the oxide scale formed by quenching is removed by grinding, etc., or, if necessary, the area other than the friction part is coated and then the friction surface is polished to obtain a product (brake disc ).
Hereinafter, based on an Example, this invention is demonstrated in detail.

      Molten steel having the composition shown in Table 1 was melted in a high frequency furnace to form a 100 kgf steel ingot (steel material). Subsequently, these steel materials were formed into hot-rolled sheets (thickness: 5 mm) by ordinary hot rolling. Furthermore, these hot-rolled sheets were subjected to hot-rolled sheet annealing (reducing gas atmosphere, annealing after heating) at 800 ° C. for 8 hours. Subsequently, the hot-rolled sheets were subjected to pickling treatment to remove the scale on the surface to obtain a material for a brake disk.

Test materials (size: t × 30 × 30 mm) were collected from these raw materials, heated to the quenching heating temperature shown in Table 2 (holding: 10 min), and then subjected to quenching by air cooling. After the quenching treatment, a quenching stability test and a heat resistance test were performed. The test method was as follows.
(1) Quenching stability test After removing the surface scale by grinding the specimen after quenching (size: t x 30 x 30 mm), surface hardness with a Rockwell hardness tester in accordance with the provisions of JIS Z 2245 The HRC was measured at five points, and the average value was defined as the quenching hardness of the material. When the quenching hardness was 32 to 38 by HRC, the evaluation was ○, and when the hardness was other than 32 to 38 by HRC, the evaluation was x.
(2) Heat resistance test The specimen after quenching (size: t × 30 × 30 mm) was further subjected to tempering treatment (air cooling after treatment) at 600 ° C. × 2 h. After removing the surface scale by grinding on the tempered test piece, the surface hardness HRC was measured with a Rockwell hardness tester in accordance with JIS Z 2245, and the average value was obtained. Sex was evaluated.

In addition, two test materials were sampled from these raw materials, and the test materials were heated to 1000 ° C. (holding: 10 min), and then subjected to quenching treatment by air cooling. Then, one of them was subjected to a tempering treatment (air cooling after the treatment) that was held at 600 ° C. for 2 hours.
The corrosion resistance test was carried out on the test materials after quenching and after tempering. The test method was as follows.
(3) Corrosion resistance test A test piece (size: t x 70 x 150 mm) is taken from the test material, and the surface is wet-polished with # 320 emery abrasive paper, and then subjected to a combined cycle corrosion test (CCT: Cyclic Corrosion Test). Carried out. CCT conditions are: 0.5 mass% NaCl aqueous solution spray (room temperature 35 ° C.) 2.5 h, then dry (room temperature 60 ° C.) 1.0 h, then wet (room temperature 50 ° C., humidity 95%) 1.0 h 1 cycle. As a test, 4 cycles were used. After the test, the surface of the test piece was visually observed to measure the number of rusting points. Evaluation was made with no rusting point as ○, 1-4 as Δ, and 5 or more as ×. Δ and ○ were evaluated as having excellent corrosion resistance.

  The obtained results are shown in Table 2.

  All of the examples of the present invention have quenching hardness in the range of HRC32 to 38, excellent quenching stability, and further have excellent heat resistance of HRC32 or higher after tempering at 600 ° C. × 2 h, and further as quenched And excellent corrosion resistance after tempering. On the other hand, the comparative example outside the scope of the present invention has a quenching hardness outside the range of HRC32 to 38, or has a low heat resistance, or a low corrosion resistance after quenching or after tempering.

Claims (2)

mass%
C: 0.02% or more and less than 0.10%, Si: 1.0% or less,
Mn: 1.0 to 2.5%, P: 0.04% or less,
S: 0.01% or less, Cr: more than 11.5%, 15.0% or less,
Ni: 0.1 to 1.0%, Al: 0.10% or less,
Nb: more than 0.08% and 0.6% or less, V: 0.02 to 0.3%,
N: More than 0.03% and 0.10% or less is contained so as to satisfy the following formulas (1) to (3), and has a composition comprising the balance Fe and inevitable impurities, and has excellent heat resistance and corrosion resistance Stainless steel plate for disc brakes.
Record
0.03 ≦ {C + N− (13/93) Nb} ≦ 0.10 (1)
(5Cr + 10Si + 15Mo + 30Nb + 50V-9Ni-5Mn-3Cu-225N-270C) ≦ 45
……… (2)
{(14/50) V + (14/90) Nb} <N ......... (3)
Here, Cr, Si, Mo, Nb, Ni, Mn, Cu, V, N, C: Content of each element (mass%) A brake disc obtained by heating and quenching to 1000 ° C or lower using the stainless steel plate for disc brake according to claim 1 as a raw material.