JP6526765B2 - Martensitic stainless cold-rolled steel sheet for bicycle disc brake rotor excellent in hardenability and corrosion resistance, steel strip and manufacturing method thereof - Google Patents

Description

  The present invention relates mainly to a martensitic stainless cold-rolled steel plate for a bicycle disk brake rotor having excellent hardenability and corrosion resistance, used as a rotor of a disk brake of a bicycle, a steel strip, and a method of manufacturing the steel strip.

  Disc brakes for bicycles are mainly adopted as braking devices for high-class bicycles, and their rotors are required to have properties such as wear resistance, weather resistance, and lightness. Therefore, a composite of martensitic stainless steel and aluminum is used for the highest grade, and martensitic stainless steel is used for the lower class. In martensitic stainless steel, SUS420J1 and SUS420J2 were generally used as a steel type. The stainless steel of the bicycle disc brake rotor is adjusted to the shape and hardness by hot rolling-annealing-cold rolling and then processed into a predetermined shape by press molding, and adjusted to the desired hardness by heat treatment of quenching and tempering After being subjected to polishing and painting processes, it becomes a disc brake rotor.

  On the other hand, there is also a disc brake rotor manufactured by adjusting the hardness only by the quenching process without performing the tempering process, and a SUS410-based martensitic stainless steel is used for this. This reduced the manufacturing cost and expanded the range of use. Patent Document 1 discloses a SUS410-based martensitic stainless steel in which C + N is set to 0.04 to 0.10% and Mn is added to 1.0 to 2.5%. Further, Patent Document 2 discloses a SUS410-based martensitic stainless steel in which Ni is added at 0.60% or less instead of Mn of less than 0.5%. However, these steels were mainly developed for motorcycles and were not necessarily optimal for bicycles.

  Recently, in Patent Document 3, a steel material satisfying 38-43 HRC harder than that for motorcycles is disclosed for bicycles.

  However, when disc brakes are adopted for general purpose bicycles for the purpose of further expanding the range of use, the braking force required from the actual conditions of use is lower than ever, so martensitic stainless steels for disc brake rotors are adapted accordingly. It became necessary.

JP-A-57-198249 Japanese Patent Application Laid-Open No. 60-106951 International Publication No. 2012/157680

In the martensitic stainless steel sheet described in the background art, the hardness after quenching is hard, many alloy elements are added to obtain the required hardness, and it takes a long time for manufacturing parts, which is expensive. The
SUMMARY OF THE INVENTION The object of the present invention is to use a martensitic stainless steel cold rolling for a bicycle disc brake rotor which is inexpensive and sufficiently excellent in quality and has excellent hardenability and corrosion resistance as a rotor material of an inexpensive disc brake applicable to general purpose bicycles. It is providing a steel plate, a steel strip, and its manufacturing method.

  Until now, rotor materials for bicycle disc brakes have been adjusted to 38 to 44 by HRC in view of braking force, wear resistance and the like. This is because the bicycle in which the disc brake is used is a high-class bicycle such as a mountain bike, and excellent braking characteristics are required. Moreover, since the condition examination of the manufacturing process after cold-rolling is inadequate, the dispersion | variation in the hardness of every lot also used the lot, and the subject that the hard material was used more than necessary also had the subject.

  On the other hand, when a disc brake is used for a general purpose bicycle, it is important that the braking force is not so high and, rather, be inexpensive. Therefore, the characteristics of the required material also have to be changed.

  As a result of studies by the present inventors, it has become clear that a sufficient braking force can be maintained if the hardness is about 32 to 38 in HRC for a general purpose bicycle.

  In the case of HRC 32 to 38, disc brake materials for motorcycles can also be used, but disc brake materials for motorcycles usually use hot-rolled steel sheets and sufficient characteristics can be obtained, in particular, simply by cold-rolling-annealing this. And press processability could not be obtained. In addition, since the hardening conditions are also largely different, it was necessary to change the idea of hardening hardness.

  Therefore, the present inventors have optimized the component design as a bicycle disc brake rotor material and optimized the manufacturing process, and completed the invention of a martensitic stainless steel plate as a disc brake rotor material for general purpose bicycles.

The present inventors examined in detail the necessary characteristics as a disc brake rotor material for general purpose bicycles, and found out that it is as follows.
(A1) Cold-rolled steel plate is desirable because of plate thickness accuracy.
(A2) After cold rolling annealing, hardening by oil cooling gives a hardness of 32-38 HRC (A3) Hardening in a heat treatment furnace is desirable at low temperature for a short time (excellent low temperature hardenability)
(A4) Even if the heat treatment conditions change, it is better for the hardness change to be small (superior quench stability)

Furthermore, the present inventors worked on the development of a steel plate having these necessary characteristics and obtained the following findings.
(B1) The cold-rolled and annealed sheet is different from the hot-rolled and annealed sheet in hardenability, and the cold-rolled and annealed sheet is more excellent in hardenability.
(B2) The amount of precipitates having low temperature hardenability and excellent in quench stability is more important when N is larger than C (B3) The amount of precipitates centering on carbonitrides is important, and the hardenability improves as the amount of precipitates decreases.

Moreover, the present inventors newly discovered the subject of the cold-rolled steel plate which is not in the heat-rolled steel plate as follows.
Specifically, it was newly discovered that cold-rolled steel sheets before quenching are easy to develop.
The reason is that a cold-rolled steel sheet before quenching does not have a cooling rate that can suppress the precipitation of carbonitrides, so a Cr-depleted layer tends to form around the carbonitrides during cooling in the final annealing, which causes Cr deficiency. It is thought that it is easy to develop due to carbonitride around the bed.
If quenching is performed, carbonitrides can be solid-solved, so it is possible to suppress generation of heat, but since cold-rolled steel sheets before quenching have more carbonitrides than after quenching, after quenching Corrosion resistance is lower than that.
If the corrosion resistance of the cold rolled steel sheet before quenching is low, there is a possibility that the cold rolled steel sheet before quenching may develop during the manufacture of the disc brake rotor. If it is slight, it can be removed by polishing, but if the number is too large, the burden of polishing will increase. Therefore, it is preferable that the cold rolled steel plate is excellent in corrosion resistance before quenching.
In order to solve this problem, the steel of the present invention may contain N more than C, and by adding a small amount of V, V nitride is easily formed instead of Cr nitride which lowers the corrosion resistance, and generation of heat is suppressed. Found out that

  The present invention has been made based on these findings, and means for solving the problems of the present invention, that is, the martensitic stainless steel sheet of the present invention is as follows.

(1) mass%,
C: 0.020 to 0.060%,
N: 0.020 to 0.070%,
Si: 0.1 to 1.0%,
Mn: 1.0 to 1.5%,
P: 0.040% or less,
S: 0.015% or less,
Ni: 0.3% or less Cr: 10.5 to 13.5%,
Cu: 0.1% or less
V: more than 0.08% to 0.3%,
Al: 0.001 to 0.010%
Contains
And C and N satisfy Formula 1;
Also, γp, which is a phase balance index during hot rolling expressed by Equation 2, is 90 to 120,
The balance consists of Fe and unavoidable impurities,
The amount of precipitates in the steel is 0.2% or more and 2% or less in mass% ,
A martensitic stainless cold-rolled steel plate for a bicycle disk brake rotor having a plate thickness of 0.5 mm or more and 2.5 mm or less and excellent in hardenability and corrosion resistance.
0.03% ≦ C + 0.5 × N ≦ 0.09% ··· Formula 1
However, N C C
γp = 420C + 470N + 23Ni + 9Cu + 7Mn-11.5Cr-11.5Si-52Al-12Mo-47Nb-7Sn-49Ti-48Zr-49V + 189 ・ ・ ・ Formula 2
In addition, the element name in Formula 1 and Formula 2 means content (mass%) of each element.
(2) mass%,
Mo: 0.01 to 0.5%,
Sn: 0.003 to 0.1%,
Nb: 0.001 to 0.3%,
Ti: 0.05% or less,
Zr: 0.05% or less,
B: 0.0002 to 0.0050%
A martensitic stainless cold rolled steel plate for a bicycle disk brake rotor excellent in hardenability and corrosion resistance as described in (1), characterized in that it contains one or more elements, and the balance is composed of Fe and unavoidable impurities.
(3) The martensitic stainless cold rolled steel sheet for a bicycle disk brake rotor having excellent hardenability and corrosion resistance according to (1) or (2), characterized in that the steel sheet is a steel strip.
(4) The manufacturing process includes melting, casting, hot rolling, hot rolled sheet annealing, pickling, cold rolling, cold rolled sheet annealing, pickling, and the annealing temperature of the cold rolled sheet annealing is 700 to 800 ° C. A method for producing a martensitic stainless cold-rolled steel sheet for a bicycle disk brake rotor having excellent hardenability and corrosion resistance according to any one of (1) to (3), characterized in that

The martensitic stainless steel of the present invention makes it possible to manufacture a low-cost, two-wheeled disc brake rotor excellent in hardenability and corrosion resistance.
That is, according to the present invention, martensitic stainless steel for a bicycle disk brake rotor excellent in hardenability and corrosion resistance which is inexpensive and sufficiently excellent in quality as a rotor material of an inexpensive disk brake applicable to a general purpose bicycle A cold rolled steel sheet can be provided.

Hereinafter, embodiments of the present invention will be described.

<Chemical composition>
First, the reason for limiting the steel composition of the stainless steel plate of the present embodiment will be described. In addition, the notation of% about a composition means mass%, unless there is particular notice.

C: 0.020 to 0.060%
C enhances the hardness at the time of quenching, increases the austenite phase fraction at the time of quenching heating, and increases the amount of martensite after quenching. In order to provide the necessary hardness and braking force to the brake rotor of a general-purpose bicycle, 0.020% or more is required. On the other hand, if it exceeds 0.060%, the hardness becomes such that HRC exceeds 38, and problems such as brake noise occur. Considering the balance of hardness and toughness, it is desirable to make it 0.025 to 0.050%.

N: 0.0020 to 0.070%
N, like C, increases the hardness at the time of quenching, increases the austenitic phase fraction at the time of quenching and heating, and increases the amount of martensite after quenching. In order to provide the necessary hardness and braking force to the brake rotor of a general-purpose bicycle, 0.020% or more is required. On the other hand, if it exceeds 0.070%, the hardness becomes so that the HRC exceeds 38, and problems such as brake noise occur. Considering the balance of hardness and toughness, it is desirable to make it 0.030 to 0.050%.

0.03 ≦ C + 0.5 × N ≦ 0.09% Formula 1
Basically, the hardness of the steel sheet after quenching is influenced by the C + 0.5 × N content, although it is also affected by the influence of other elements and the structure. In order to satisfy the hardness range of 32 to 38 HRC, the lower limit is set to 0.03% and the upper limit is set to 0.09%.

N ≧ C
In order to realize excellent hardenability, carbides and nitrides need to be dissolved quickly during hardening. Generally, carbides tend to be coarser than nitrides, so in the present invention, N ≧ C. As a result, coarse carbides are less likely to be formed, and solid solution can be sufficiently achieved even by heating at a low temperature and for a short time, and as a result, the quench hardness is improved. That is, excellent low temperature hardenability can be obtained. Moreover, the stainless steel plate of the present embodiment generates V nitride by the addition of a small amount of V to improve the corrosion resistance. Therefore, the larger the amount of N, the greater the corrosion resistance improvement effect, and it is necessary to set N ≧ C.

Si: 0.1 to 1.0%
Si is an element that improves high-temperature strength and oxidation resistance, and is an element useful as a deoxidizing agent, and its effect is generated by addition of 0.1% or more. However, it is an element that reduces the martensitic phase at the time of quenching and lowers the toughness and increases the hardness, so the upper limit is made 1.0%.
Preferably, it is 0.1 to 0.5%.

Mn: 1.0 to 1.5%
Mn is an element useful as a deoxidizing agent and, like Ni and Cu, is an austenite-forming element, and increases the amount of martensite at the time of quenching. In addition, as an effect unique to Mn, a non-metallic inclusion (MnS) is formed to have an effect of improving hot workability. Furthermore, it has the effect of increasing the solubility of nitrogen in molten steel, and has the effect of suppressing the formation of bubble-based defects when adding a large amount of nitrogen. In order to obtain these effects, the content of Mn is at least 1.0% or more. However, when a large amount of Mn is contained, oxidation at the time of quenching and heating proceeds, and removal of the oxide film becomes difficult, and the surface quality of the material is degraded due to the coarsening of MnS. Furthermore, when a large amount of Mn is contained, it becomes difficult to lower the hardness of the noise generation at the time of braking. From these, the content of Mn is made 1.5% or less.

P: 0.040% or less P is an element having a large solid solution strengthening ability and is a ferrite forming element. Since it is an element harmful to corrosion resistance, it is preferable that the amount be as small as possible, and the upper limit is made 0.040%. When better corrosion resistance is required, 0.020% or less is preferable. However, since excessive reduction increases the dephosphorization load and increases the manufacturing cost, it is preferable to set the lower limit to 0.005%.

S: 0.015% or less S forms sulfide-based inclusions and degrades the general corrosion resistance (general corrosion and pitting corrosion) of the steel sheet, so the upper limit of the content is preferably as small as possible, 0.015 And%. The smaller the content of S, the better the corrosion resistance, but the lower the sulfur content, the desulfurization load increases and the manufacturing cost increases, so the lower limit is preferably made 0.0001%. In addition, Preferably it is 0.0005 to 0.0050%.

Ni: 0.3% or less Ni, like Mn and Cu, is an austenite-forming element and increases the amount of martensite at the time of quenching. However, since Ni is expensive, it is not positively added in the present invention, and is limited to the level of unavoidable impurities mixed from scrap, and the allowable upper limit is 0.3%. However, it is an element effective for suppressing the progress of pitting corrosion, and its effect is stably exhibited by the addition of 0.05% or more. In addition, it is effective to improve the toughness of the hot rolled sheet. Therefore, the content of 0.05% or more is preferable.

Cr: 10.5 to 13.5%
Cr is an essential element for ensuring corrosion resistance as a disc brake rotor. In order to form a passive film in the environment assumed, 10.5% or more is required, and this is made into the minimum. On the other hand, since Cr is a ferrite forming element, if the content of Cr exceeds 13.5%, the austenite fraction at the time of quenching and heating decreases, the amount of martensitic phase after quenching decreases, and the hardness becomes hard. There is a risk of shortage. In this case, it is necessary to add an austenite-forming element (Ni, Cu, Mn) according to the amount of Cr to secure the austenite phase fraction during quenching and heating. On the other hand, securing the phase fraction by the addition of the austenite-forming element is limited by the addition of each element due to various reasons, which also results in high cost, so the upper limit of the Cr content is 13.5%. .

Cu: 0.1% or less Cu, like Mn and Ni, is an austenite-forming element, and increases the amount of martensite during quenching. Moreover, it is an element which improves corrosion resistance. However, since there is a problem that the oxide film formed by heat generation at the time of sliding is changed to lower the strength of noise generation of the disc brake, the content thereof is 0.1% or less.

V: more than 0.05% to 0.3% or less In the present invention, V is a useful element. In the steel of the present invention, Cr carbonitride is generated during cooling of the final annealing, and a Cr deficient layer is formed around it. In the case of a heat-rolled steel sheet, the cooling rate is slow, so that the Cr-deficient layer can be eliminated by the healing effect. In the case of a cold-rolled steel sheet, the cooling rate is usually faster than that of a hot-rolled steel sheet, but not so fast as to suppress the formation of carbonitrides, and not so slow as to expect a healing effect. Therefore, a Cr-depleted layer remains around the carbonitrides, which tends to be the starting point of the blasting.
However, when V is added, its development is suppressed. This effect is manifested at a content of more than 0.05%. Although the reason is not clear, it is presumed that the addition of V forms a V nitride instead of a Cr nitride, so that it is difficult to form a Cr-deficient layer. V nitride is more stable at a higher temperature than Cr nitride, but can be dissolved by appropriately selecting the quenching conditions. Therefore, it is possible to prevent the generation of iron without significantly affecting the hardenability.
V is contained in a small amount in the raw material, and depending on the grade of the raw material, V is easily mixed, and the reduction thereof is accompanied by an increase in cost, so the inclusion of more than 0.08% is preferable.
On the other hand, excessive V addition leads to the formation of a large amount of V nitride, making it difficult to form a solid solution sufficiently even under appropriate quenching conditions. As a result, a decrease in hardenability, specifically, a decrease in martensitic hardness is caused. Therefore, V makes content of 0.3% an upper limit.

Al: 0.001 to 0.010%
Al is useful as a deoxidizing element, and its effect is exhibited at 0.001% or more. However, excessive addition affects the corrosion resistance and the like, so the upper limit is made 0.010%. When deoxidation is possible with other elements such as Si, 0.003% to 0.0008% is desirable in consideration of cost.

In addition to the limitation of these elements, the present invention is a martensitic stainless steel, and in order to form a martensitic phase (hereinafter, M phase), it is necessary to form an austenitic phase (hereinafter, γ phase) at high temperature Since each element is determined by the additive components, the respective elements need to be mutually adjusted to achieve phase balance. The phase balance index is expressed by Equation 2, and it is sufficient that this γp be 90 to 120. If it is less than 90, the γ phase generated at high temperature decreases, the M phase after quenching decreases, and the required hardness can not be obtained. Further, when γp exceeds 120, the stable γ phase, which does not cause M phase transformation even if the γ phase is quenched, is increased, the M phase is also reduced, and the required hardness can not be obtained. The most preferred γp is 90-110.

γp = 420C + 470N + 23Ni + 9Cu + 7Mn-11.5Cr-11.5Si-52Al-12Mo-47Nb-7Sn-49Ti-48Zr-49V + 189 ・ ・ ・ Formula 2

In addition, the element name in Formula 2 means content (mass%) of each element.
Moreover, Formula 2 is also a parameter | index which shows the maximum value of the austenite amount produced | generated at the time of 1100 degreeC heating. Specifically, "Metal Treatment" 1964, p. This is an improvement of Castro's equation introduced in the literature of 230-245, and is a known equation as an empirical equation for estimating the maximum phase fraction of the γ phase.

  Furthermore, in order to improve corrosion resistance, one or more of the following elements may be contained.

Mo: 0.01 to 0.5%
Mo may be added as needed to improve the corrosion resistance, and in order to exert these effects, it is preferable to set the lower limit to 0.01%. On the other hand, excessive addition inhibits the formation of M phase, so the upper limit is made 0.5%.

Sn: 0.003 to 0.1%
Sn is an element effective for improving the corrosion resistance after quenching, preferably 0.003% or more, and it is preferable to add 0.02% or more as needed. However, excessive addition is 0.1% as the upper limit in order to promote ear cracking during hot rolling.

Nb: 0.001 to 0.3%
Nb is an element which suppresses the sensitization and the drop in corrosion resistance due to the precipitation of chromium carbonitrides in stainless steel by forming carbonitrides. 0.001% or more is preferable. Furthermore, it is an element that greatly improves the heat resistance after quenching. Here, heat resistance means how hard it is to soften when it receives heat after quenching, and is also called temper softening resistance.
However, when Nb is excessively added, forming NbN in the disc brake rotor causes a decrease in toughness and noise, so this is not preferable, and the upper limit is made 0.3%.

Ti: 0.05% or less Ti is an element which suppresses deterioration of sensitization and corrosion resistance due to precipitation of chromium carbonitride in stainless steel by forming carbonitride. However, Ti carbonitride tends to be coarse, does not contribute to strengthening, and is only for fixing C and N, so the upper limit is 0.05%.

Zr: 0.05% or less Zr is also an element that suppresses deterioration of sensitization and corrosion resistance due to precipitation of chromium carbonitrides in stainless steel by forming carbonitrides. However, Zr carbonitride tends to be coarse, does not contribute to strengthening, and is only for fixing C and N, so the upper limit is 0.05%.

B: 0.0002 to 0.0050%
B is an element effective for improving the hot workability, and its effect is manifested at 0.0002% or more, so 0.0002% or more may be added. In order to improve the hot workability in a wider temperature range, it is desirable to be 0.0010% or more. On the other hand, excessive addition impairs the hardenability by complex precipitation of borides and carbides, so the upper limit is 0.0050%. When corrosion resistance is also taken into consideration, 0.0025% or less is desirable.

In addition to each element described above, it can be contained in the range which does not impair the effect of the present invention. It is preferable to reduce as much as possible Zn, Bi, Pb, Se, Sb, H, Ga, Ta, Ca, Mg, Zr, B, etc. including the above-mentioned P and S which are general impurity elements. On the other hand, the content ratio of these elements is controlled within the limit to solve the problems of the present invention, and Zn ≦ 100 ppm, Bi ≦ 100 ppm, Pb ≦ 100 ppm, Se ≦ 100 ppm, Sb ≦ 500 ppm, H as necessary. It contains one or more of ≦ 100 ppm, Ga ≦ 500 ppm, Ta ≦ 500 ppm, Ca ≦ 120 ppm, Mg ≦ 120 ppm, and Zr ≦ 120 ppm. Note that "ppm" is on a mass basis.
Further, the martensitic stainless cold rolled steel plate for a bicycle disk brake rotor of the present embodiment contains the above-described components, and the remaining portion is formed of Fe and unavoidable impurities.

<Precipitate>
The amount of precipitates in the steel of the cold rolled steel sheet of the present invention is 0.2% or more and 2% or less. The amount of precipitates is evaluated by the extraction residue method, and is defined as the ratio of the amount of residual residue to the amount of dissolution of the base material in mass%. The precipitates are mainly carbonitrides, and they act as pinning sites, prevent grain growth of the γ phase, and work to prevent a reduction in hardness until they are dissolved at the time of temperature rise during quenching. The effect is exhibited when it is 0.2% or more. However, if it exceeds 2%, it not only coarsens and loses its effect but also reduces solid solution C and N, so the hardness of the M phase is insufficient and the hardness of the steel sheet as a whole is insufficient. Corrosion resistance also deteriorates. More preferably, it is 1 to 2%.

In the present application, the amount of precipitates can be controlled to 0.2% or more and 2% or less by controlling the C content, the N content, and the heat history at the time of production.
Among them, the C content and the N content can be controlled at the melting stage, and when the amount is too small, the amount of precipitates becomes less than the lower limit, and when it is too large, the precipitates tend to exceed the lower limit. And it is sufficient.
However, the amount of precipitates is not determined only by the C content and the N content, and can be adjusted also by the heat history until the final product. Specifically, the temperature can be adjusted by the slab heating temperature, the hot rolling completion temperature, the cooling method, and the hot rolled sheet annealing method. This is because the precipitates repeat melting, precipitation, and growth due to the heat history.
On the other hand, the most important step to determine the amount of precipitate in the final product is the final annealing (cold rolled sheet annealing). In particular, it is possible to control the amount of precipitates by the difference in temperature, and the higher the temperature, the smaller the precipitates, and the lower the amount. Therefore, as described later, the cold-rolled sheet annealing conditions of the present invention may be set.
However, if the amount of precipitates can not be controlled to a certain range before final annealing, the final annealing alone can not control to a proper range.

  In addition, as the extraction residue method, here, a method is used in which a fixed amount of steel is electrolyzed and dissolved in an electrolytic solution, which is filtered with a filter, and the residue remaining without being filtered is evaluated as a precipitate. The amount of precipitates can be determined from the comparison of the mass of the electrolyzed steel and the residue. The filter used in the extraction residue method is usually not considered here because it can not capture precipitates of 0.2 μm or less.

<Steel plate thickness>
In addition, since a thin disc material is used as it is for a bicycle disc brake rotor, a cold-rolled steel sheet and a steel band having a thickness of 0.5 to 2.5 mm are suitable. If it is a heat-rolled steel plate, the plate thickness accuracy may be poor, and the braking performance may not be stable.

<Manufacturing method>
The martensitic stainless steel plate of the present invention is a cold rolled steel plate, and its manufacturing process is a process including melting, casting, hot rolling, hot rolled sheet annealing, pickling, cold rolling, cold rolled sheet annealing, and pickling is there. There are no particular limitations on the production equipment, and known production equipment can be used. Cold-rolled steel sheets are usually manufactured in the form of so-called steel strips, which are usually very long in the rolling direction, and are wound and stored and moved in a coiled form.

  The conditions for hot rolling are not particularly limited, but the slab heating temperature is preferably 1100 ° C. to 1250 ° C. The hot rolling finish temperature is preferably 800 ° C. or more. Furthermore, after hot rolling, it is cooled by air-water cooling or the like and wound into a coil.

  The annealing method is not particularly defined, but a method called box annealing is preferred. The annealing temperature is preferably 800 to 900 ° C. If it is less than 800 ° C., it is not sufficiently softened and hard to cold-roll. When the temperature exceeds 900 ° C., the γ grains become coarse and the toughness decreases, which is not preferable. Moreover, although it is a cooling rate after annealing, the cooling rate from 800 degreeC to 450 degreeC is 10 degrees C / min or less is preferable. When the cooling rate is as high as 10 ° C./min, the M phase is likely to be generated, which makes it difficult to perform cold rolling, which is not preferable.

  There is no particular limitation on the pickling, and the scale is removed with sulfuric acid or hydrofluoric acid. In order to remove the scale of the steel sheet or introduce a crack, it may be subjected to shot blasting, coil bending, etc. before pickling.

  In cold rolling, the hot-rolled steel sheet that has been annealed and pickled is cold-rolled to a thickness of 0.5 to 2.5 mm. The cold rolling method is also not specified. If it is less than 0.5 mm, it is easily deformed, and if it is more than 2.5 mm, it is too heavy to be suitable as a disc brake rotor for a bicycle.

Cold rolled sheet annealing is an important point in the present invention.
As for the annealing temperature of cold-rolled sheet annealing, it is desirable to set it as 700-800 ° C. If it is less than 700 ° C., recrystallization is insufficient and not preferable. When the temperature exceeds 800 ° C., the γ phase is formed, so that it becomes the M phase after cooling, which may cause cracking, which is not preferable. Moreover, the amount of precipitates can be 0.2%-2% as it is this temperature range. It is more preferably 750 to 780 ° C. from the viewpoint of achieving both recrystallization and the amount of precipitates. The cooling rate is preferably 5 ° C./s to 50 ° C./s. If the temperature is less than 10 ° C./s, the product tends to be deformed, which is not preferable. If the temperature is more than 100 ° C./s, the equipment becomes too expensive.

  The pickling after annealing can be pickled using a known method to make a cold rolled steel sheet.

  Since the martensitic stainless cold-rolled steel sheet of the present invention has excellent hardenability, it can be suitably used particularly as a disc brake rotor material for bicycles.

  Hereinafter, the effects of the present invention will be described by way of examples, but the present invention is not limited to the conditions used in the following examples.

Example 1
In this example, the slab obtained by melting and casting the steels of the components of Tables 1-1 and 1-2 is heated to 1150-1250 ° C., and the finishing temperature is in the range of 850-950 ° C. It hot-rolled to plate thickness of about 5 mm, and was set as the hot rolled sheet steel. The hot rolled steel sheet was cooled to 400 to 450 ° C. by air-water cooling. Then, it annealed at 1000-1100 ° C, and cooled to normal temperature. At this time, the average cooling rate in the range of 800 to 450 ° C. was 10 ° C./s or more. Subsequently, the hot rolled annealed sheet was pickled and cold rolled to obtain a 0.5 to 2.5 mm cold rolled sheet. Furthermore, after annealing at 700 to 800 ° C. for 1 minute, pickling was performed to obtain a cold rolled steel sheet. Various tests were carried out using this as a test steel.
In Table 1-1 and Table 1-2, in addition to the components, evaluation of N 例 C (examples satisfying N C C are 、, examples not satisfying X are), the results of Equation 1 and Equation 2, plate thickness Described. Further, in the following table, underlined items in each item indicate that it is out of the appropriate range of the present invention.

(Precipitate investigation)
The amount of precipitates of the test steel was measured by the extraction residue method. That is, a certain amount of steel was electrolyzed and dissolved in an electrolytic solution, which was filtered with a filter having a maximum diameter of 0.2 μm, and the residue remaining without filtration was evaluated as a precipitate.

(Hardenability test)
Using an electric furnace, the temperature rising rate was kept at 10 ° C./s or less, held at 900 ° C., 1000 ° C. and 1050 ° C. for 10 minutes, and after oil cooling, hardness (HRC) measurement was performed.
As evaluation of hardenability, first, those whose hardness deviated from 32-38 HRC at any of 900 ° C., 1000 ° C. and 1050 ° C. were rejected.
Moreover, the thing which shows 32-38 HRC at 900 degreeC was made into the low temperature hardenability pass.
Next, among those for which the hardness and low temperature hardenability were acceptable, those for which the difference in hardness between 1000 ° C. and 1050 ° C. is substantially the same (within 2 HRC) are regarded as the hardening stability pass (○), and 900 The thing whose difference in hardness of ° C-1050 ° C is less than 2 HRC was considered as the quenching stability excellent (◎).

(Corrosion resistance test)
Corrosion resistance was evaluated by a salt spray test. Specifically, as described below, two tests, that is, a test with a cold-rolled steel sheet and a test after quenching, were performed.

Testing with cold rolled steel sheet:
Salt-water spray testing (SST, Salt water Spray Testing) according to JIS Z 2371 is performed on the material as cold-rolled steel sheet, and those with a development point of 5 points or less after 4 h pass or fail more And

Post-burn testing:
First, the cold rolled steel plate was quenched by oil cooling after being held at 1000 ° C. for 10 minutes. Next, the material after quenching was subjected to # 600 polishing, and a salt spray test was conducted in accordance with JIS Z 2371. A material which did not develop in 4 h was regarded as a pass.
The results are shown in Table 2.

As is clear from Table 2, the inventive examples (A1 to A3 and A5 to A27) exhibited excellent low temperature hardenability and hardenability , and no corrosion resistance. Therefore, it turned out that it becomes an excellent bicycle disc brake rotor material.

  On the other hand, in the comparative examples (B1 to B23), any of low temperature hardenability, quench stability and corrosion resistance (cold rolled steel sheet as quenched, after quenching) is not satisfactory as follows, and can not be satisfied as steel for brake disc It was shown that the steel was a high cost steel, although its performance was satisfactory but the alloy addition amount was out of the proper range.

Specifically, the C content of B1 deviates from the upper limit of the appropriate range, N C C and Formula 1 are not satisfied, the amount of precipitates deviates from the upper limit, and the quench hardness becomes too high.
In B2, the C content was out of the lower limit of the appropriate range, and the quench hardness became too low.
In B3, the Si content was at the upper limit of the appropriate range, and Formula 2 was out of the lower limit, and the quench hardness was too low.

In B4, the Si content was out of the lower limit of the appropriate range, and the corrosion resistance was rejected due to insufficient deoxidation.
In the case of B5, the Mn content was out of the lower limit of the appropriate range, and the corrosion resistance was rejected.
In B6, the Mn content is out of the upper limit of the appropriate range, the quench hardness becomes too low, and the corrosion resistance is also rejected.

In B7 and B8, the P and S contents were outside the upper limit of the appropriate range, respectively, and the corrosion resistance was rejected.
In B9, the Cr content was out of the lower limit of the appropriate range, and the corrosion resistance was rejected.
In B10, the Cr content is outside the upper limit of the appropriate range, and the lower limit of Formula 2 is also outside, and the quench hardness is too low and the cost is high.

In B11, the Ni content was out of the upper limit of the appropriate range, and the cost became high.
In B12, the Cu content was out of the upper limit of the appropriate range, and a noise occurred.
In B13, the V content was out of the lower limit of the appropriate range, and the corrosion resistance of the cold-rolled steel sheet as it was did not reach the acceptance criteria of the present invention.
In B14, the V content exceeded the appropriate range, and the low temperature hardenability was a failure.
In B15, the V content was out of the upper limit of the appropriate range, and the lower limit of Formula 2 was also out, and the low temperature quench hardness was a rejection.
In B16, the N content was out of the lower limit of the appropriate range, did not satisfy N ≧ C, and the low temperature quenched hardness and the corrosion resistance were rejected.

In B17, the N content and the amount of precipitates were out of the upper limit of the appropriate range, and the quench hardness became too high.
In B18, the Al content was out of the upper limit of the appropriate range, and the corrosion resistance was rejected.
As for B19, equation 2 is out of the upper limit of the appropriate range, and the hardening hardness is too low.
As for B20, Formula 2 was out of the lower limit of the appropriate range, and low temperature hardenability was rejected.

B21 did not satisfy N ≧ C, the low temperature hardenability failed, and the corrosion resistance of the cold rolled steel sheet did not reach the acceptance criteria of the present invention.
As for B22, the expression 1 deviates from the upper limit of the appropriate range, and the hardening hardness becomes too low.
In B23, the C and N contents were out of the lower limit of the appropriate range, and the lower limit of Formula 1 was also out, the quench hardness was too low, and the corrosion resistance was also rejected.

Example 2
Among the cold-rolled steel sheets manufactured in <Example 1>, the annealing temperature is set to 700 to 800 ° C. in <Example 1> with respect to a sheet having the same composition and thickness as A1 steel and A27 steel. In <Example 2>, the annealing temperature was changed to 670 to 830 ° C. to perform cold-rolled sheet annealing, and other conditions were obtained as the sample steel under the same conditions as in <Example 1>. Thereafter, the same evaluation as in <Example 1> was performed. The results are shown in Table 3.

  When the cold-rolled sheet annealing temperature is in the range of 700 to 800 ° C., the steel of the present invention exhibits excellent hardenability and there is no problem in corrosion resistance (A1-2, A1-3, A1-4, A27-2, A27 -3, A27-4). However, if the annealing temperature of the cold-rolled sheet is lower than 700 ° C., recrystallization is insufficient, the precipitates are not sufficiently dissolved, and the corrosion resistance is not suitable (A1-1, A27-1). In addition, when the annealing temperature is higher than 800 ° C., the martensite (M) phase remains after cold rolling annealing, and the number of precipitates decreases, and the hardness decreases, which is not preferable (A1-5, A27-5) ).

  As is apparent from the above description, the martensitic stainless steel plate of the present invention has excellent hardenability and corrosion resistance, and is therefore most suitable for a bicycle disc brake rotor. By using this steel plate, an excellent bicycle disc brake rotor can be supplied, and the degree of social contribution can be enhanced. That is, the present invention has sufficient industrial applicability.

Claims (4)

In mass%,
C: 0.020 to 0.060%,
N: 0.020 to 0.070%,
Si: 0.1 to 1.0%,
Mn: 1.0 to 1.5%,
P: 0.040% or less,
S: 0.015% or less,
Ni: 0.3% or less Cr: 10.5 to 13.5%,
Cu: 0.1% or less
V: more than 0.08% to 0.3%,
Al: 0.001 to 0.010%
Contains
And C and N satisfy Formula 1;
Also, γp, which is a phase balance index during hot rolling expressed by Equation 2, is 90 to 120,
The balance consists of Fe and unavoidable impurities,
The amount of precipitates in the steel is 0.2% or more and 2% or less in mass% ,
A martensitic stainless cold-rolled steel plate for a bicycle disk brake rotor having a plate thickness of 0.5 mm or more and 2.5 mm or less and excellent in hardenability and corrosion resistance.
0.03% ≦ C + 0.5 × N ≦ 0.09% ··· Formula 1
However, N C C
γp = 420C + 470N + 23Ni + 9Cu + 7Mn-11.5Cr-11.5Si-52Al-12Mo-47Nb-7Sn-49Ti-48Zr-49V + 189 ・ ・ ・ Formula 2
In addition, the element name in Formula 1 and Formula 2 means content (mass%) of each element. In mass%,
Mo: 0.01 to 0.5%,
Sn: 0.003 to 0.1%,
Nb: 0.001 to 0.3%,
Ti: 0.05% or less,
Zr: 0.05% or less,
B: 0.0002 to 0.0050%
A martensitic stainless cold rolled steel plate for a bicycle disk brake rotor excellent in hardenability and corrosion resistance according to claim 1, characterized in that it contains one or more of them, and the balance consists of Fe and unavoidable impurities.   The martensitic stainless cold-rolled steel plate for a bicycle disk brake rotor having excellent hardenability and corrosion resistance according to claim 1 or 2, wherein the steel plate is a steel strip.   The manufacturing process includes melting, casting, hot rolling, hot rolled sheet annealing, pickling, cold rolling, cold rolled sheet annealing, and pickling, and the annealing temperature of the cold rolled sheet annealing is 700 to 800 ° C. The manufacturing method of the martensitic stainless cold-rolled steel plate for bicycle disk brake rotors excellent in hardenability and corrosion resistance as described in any one of Claim 1 to 3 made into any one of Claims 1-3.