JP5590661B2 - Aluminum alloy blank for magnetic disk excellent in machinability and manufacturing method thereof - Google Patents

Description

  The present invention relates to an aluminum alloy blank used for a magnetic disk such as a hard disk mounted on a personal computer or a home appliance, and a method for manufacturing the same.

  As a substrate for a magnetic disk, a substrate obtained by punching an aluminum alloy plate into a disk shape and applying Ni-P plating is used. As the type of aluminum alloy, a high-strength Al—Mg alloy is used to suppress vibration during disk rotation. Moreover, high surface flatness is required for the disk-shaped substrate so that the distance between the rotating disk and the head is stabilized. The flatness of the magnetic disk substrate is finally obtained by performing double-side polishing after Ni-P plating, but the flatness of the aluminum alloy substrate itself is also required. In order to satisfy the flatness, the punched disc-shaped substrate is annealed. In order to chamfer the end of the disk-shaped substrate (hereinafter referred to as “blank”) after the annealing, the end of the blank is cut with a lathe and a cutting tool.

  A hard and thick oxide film containing Mg is formed on the surface and edges of the annealed blank. After that, the blank edge is cut, but when many pieces are repeatedly cut with the same cutting tool, the cutting tool gradually wears and eventually becomes unusable due to a defective shape, and is replaced with a new cutting tool at that time. The number of processed sheets until the tool can no longer be used is the tool life, and the longer the tool life, the better the productivity and the lower the cost.

  The tool life correlates with the oxide film on the blank surface and the edge where the tool contacts, and the harder and thicker the oxide film, the shorter the tool life. As a measure for extending the life of the tool, a method of chemically or mechanically removing the oxide film on the blank surface and the edge after annealing can be considered. These methods are known methods as described in Non-Patent Document 1. However, these methods have poor productivity and high cost.

    Patent Document 1 describes a clad aluminum alloy substrate for a magnetic disk, which is annealed by cladding a skin material on both surfaces of a core material. In this aluminum alloy substrate, it is described that the surface grindability is improved by decreasing the Mg content from the central part in the thickness direction of the core toward the surface layer. However, there is no description or suggestion about the machinability of the blank edge.

The Japan Institute of Light Metals “Aluminum Products and Manufacturing Technology” p154 (2001)

JP 2004-332068 A

  An object of the present invention is to provide an aluminum alloy blank for a magnetic disk and a method for producing the same that do not require the oxide film to be removed chemically or mechanically and can increase the tool life as compared with the prior art. is there.

  As a result of intensive studies, the present inventors have found an aluminum alloy blank for a magnetic disk and a method for manufacturing the same, which can make the tool life longer than before.

That is, the present invention provides an annealed aluminum alloy blank for a magnetic disk according to claim 1, which contains Mg: 3.5 to 10 mass% , Cr: 0.01 to 0.1 mass% , and the balance Al and unavoidable An Al—Mg alloy substrate made of a general impurity and an oxide film formed on the surface of the substrate, the average film thickness of the oxide film being 15 nm or less, and the average composition of the oxide film being Al, O and Mg aluminum alloy for the 0.05 ≦ Mg / (Al + O + Mg) ≦ 0.25 der at atoms is, magnetic disk maximum value of the flatness of the surface and excellent machinability, characterized in der Rukoto less than 5μm Blank.

The invention in claim 2, Mg: 3.5~10mass%, Cr : contains 0.01～0.1Mass%, a substrate of Al-Mg alloy and the balance Al and inevitable impurities, start heating From 30 to 60 ° C. in an atmosphere with a dew point of −20 to 20 ° C. for 30 minutes to 6 hours, and an aluminum alloy blank for a magnetic disk excellent in machinability It was set as the manufacturing method of this.

The invention in claim 3, was assumed to be -20 to 20 ° C. by humidifying the dew point in the baked blunt atmosphere. The invention furthermore relates to a fourth aspect, the outer diameter cutting component and the inner diameter side cutting portion of the blank after the annealing, and shall further comprising the step of cutting in bytes.

  According to the present invention, a conventional aluminum alloy plate for a magnetic disk can be applied as it is, and by reducing the hardness of an oxide film formed by annealing, the tool life can be extended compared to the conventional one, and the productivity and cost can be reduced. It becomes possible.

In the aluminum alloy blank which concerns on this invention, it is a perspective view which shows the inner diameter side cutting part and outer diameter side cutting part of the blank after annealing.

Hereinafter, embodiments of the present invention will be described in detail.
An aluminum alloy blank for a magnetic disk according to the present invention is obtained by punching an aluminum alloy having a predetermined alloy composition into a disk of a predetermined size, annealing it, and forming an oxide film on the inner diameter side and outer diameter side of the blank formed thereby. It is made by cutting a part with a cutting tool. Details will be described below.

A. Aluminum alloy substrate before annealing As an aluminum alloy before annealing used for an aluminum alloy blank for a magnetic disk according to the present invention (hereinafter simply referred to as “aluminum alloy blank”), a conventionally used JIS 5086 alloy or the like is used. Al-Mg based alloys for magnetic disks have high strength and are preferably used. Mg content is 3.5-10 mass%. If it is less than 3.5 mass%, the material strength is insufficient, which is inappropriate. On the other hand, if it exceeds 10 mass%, the Mg concentration of the oxide film described later exceeds 0.25 and the tool life is shortened, which is inappropriate.

  The aluminum alloy before annealing may further contain 0.01 to 0.1 mass% of Cr. When Cr is added, Mg element is taken in during annealing, and an Al—Mg—Cr intermetallic compound is formed in the aluminum alloy. The formation of such an intermetallic compound can inhibit the movement of Mg element from the aluminum alloy to the oxide film. Thereby, there exists an effect | action which reduces the Mg density | concentration in the above-mentioned oxide film. If the Cr content is less than 0.01 mass%, the Mg concentration in the oxide film cannot be sufficiently reduced. On the other hand, if it exceeds 0.1 mass%, the Al—Mg—Cr intermetallic compound increases, resulting in defects on the aluminum alloy surface.

  Although not an element that directly affects the tool life, one or both of Cu and Zn may be added from the viewpoint of smoothness on the plated surface after Ni-P plating. For Cu, it is 0.01 to 0.1 mass%, and for Zn, it is 0.1 to 0.5 mass%.

  Inevitable impurities include Fe, Si, Mn and the like. In particular, Fe and Si form intermetallic compounds to form defects on the surface of the aluminum alloy. Therefore, Fe and Si are preferably as small as possible, and it is preferable that both elements be 0.1 mass% or less.

  As the form of the substrate of the aluminum alloy before annealing used in the present invention, a rolled material or a plate material produced by a continuous casting method is suitably used. In the case of a rolled material, an aluminum alloy slab is cast and subjected to homogenization, hot rolling, and cold rolling. In the case of the continuous casting method, a plate-like aluminum alloy is cast and cold-rolled. For casting, a semi-continuous casting method (DC casting method), an electromagnetic field casting method, a horizontal continuous casting method, or the like can be suitably applied. After casting, in order to remove the coarse cell layer on the surface of the ingot, chamfering of about 0 to 25 mm may be performed. The homogenization treatment is performed for the purpose of eliminating micro segregation generated during casting. It is preferable to hold at 500 to 570 ° C. for about 1 to 24 hours. If it is less than 500 degreeC, cancellation | release of microsegregation may become inadequate, and when it exceeds 570 degreeC, there exists a danger that an aluminum alloy will melt. A plate having a desired thickness is obtained by hot rolling and cold rolling. Note that intermediate annealing may be performed for the purpose of softening the aluminum alloy work-hardened by rolling before cold rolling or during cold rolling. Hold at 300 to 570 ° C. for about 1 second to 3 hours. If the temperature is lower than 300 ° C, the softening of the aluminum alloy may be insufficient. If the temperature exceeds 570 ° C, the aluminum alloy may be melted. The thickness of the substrate is preferably about 0.7 to 2.0 mm, but recently there is a demand for further thinning.

B. Annealing method The aluminum alloy substrate prepared as described above is punched into a disk shape. In order to ensure flatness, the aluminum alloy substrate punched into a disk shape is annealed. Annealing is performed at a temperature of 270 ° C. to 320 ° C. for 30 minutes to 6 hours in an atmosphere having a dew point of −20 ° C. or higher from the start of temperature rise to the end of cooling. The above conditions are necessary to obtain the oxide film of the present invention.

  The higher the atmospheric temperature in annealing, the longer the holding time, the thicker the oxide film becomes, and the Mg contained in the aluminum alloy tends to move into the oxide film and concentrate. On the other hand, when the ambient temperature is low and the holding time is short, the formation of the aluminum alloy plate is not completed and appropriate flatness cannot be ensured. Accordingly, upper and lower limits are required for each of the atmospheric temperature and the holding time, the atmospheric temperature is 270 ° C. to 320 ° C., and the holding time needs to be 30 minutes to 6 hours.

However, a desired oxide film cannot be obtained only by specifying the temperature and time. It is necessary to set the dew point of the annealing atmosphere to −20 ° C. or higher from the start of temperature rise to the end of cooling. Preferably it is 0 degreeC or more. If the dew point of the annealing atmosphere is −20 ° C. or higher, the H ₂ O component in the atmosphere increases, and the Mg concentration in the oxide film can be kept low. In this case, the atmosphere may be air. When the dew point of the annealing atmosphere is less than −20 ° C., the H ₂ O component in the atmosphere decreases and the Mg concentration in the oxide film increases.

The reason why the Mg concentration in the oxide film can be kept low when the H ₂ O component is increased by setting the dew point of the annealing atmosphere to −20 ° C. or higher is considered as follows. When an aluminum alloy containing Mg is heat-annealed, Mg atoms move in the aluminum alloy and collect on the alloy surface and move into the oxide film, depriving oxygen from the Al oxide in the oxide film, or in the atmosphere. Oxidized in combination with oxygen. When the H ₂ O component in the annealing atmosphere is small, the amount of Al hydrate present in the oxide film is also small. Thereby, Mg atoms can move smoothly in the oxide film without being disturbed by the Al hydrate, and are successively oxidized and concentrated in the oxide film. On the other hand, when the amount of H ₂ O in the annealing atmosphere is large, the amount of Al hydrate present in the oxide film is also large. Since Mg atoms hinder the movement in the oxide film by the Al hydrate, concentration due to oxidation in the oxide film is suppressed. As a result, it is considered that the Mg concentration in the oxide film is lowered.

  In order to set the dew point of the annealing atmosphere to −20 ° C. or higher, the atmosphere in the furnace may be humidified. The humidification method is not particularly limited. For example, water vapor may be introduced into the furnace, or water may be sprayed. The upper limit of the dew point is not particularly limited. When the atmosphere in the furnace is humidified, it is preferable to immediately start raising the temperature when the dew point reaches −20 ° C. or higher, preferably 0 ° C. or higher. This is to shorten the time required for annealing and increase the production capacity. For annealing, a method of stacking a plurality of blanks via a highly flat spacer and performing pressure annealing for the purpose of ensuring flatness is suitably used.

  In the blank annealed as described above, Al and Mg, which are alloy components, move from the aluminum alloy into the oxide film. Therefore, strictly speaking, in the aluminum alloy after annealing, Al and Mg are reduced by the amount of movement compared to the aluminum alloy before annealing. However, since the oxide film thickness is sufficiently smaller than the aluminum alloy thickness (as shown in the examples and comparative examples, the former is less than 20 nm, whereas the latter is 1.62 mm), before and after annealing. The alloy composition in the aluminum alloy can be the same.

C. Oxide Film When the aluminum alloy substrate is annealed, a hard and thick oxide film containing Mg is formed on the blank surface and at the end. The oxide film thickness and the Mg concentration in the film depend on the annealing conditions. In the present invention, the oxide film thickness and the Mg concentration in the film are specified.

C-1. Mg concentration As described above, Mg contained in the aluminum alloy moves and concentrates in the oxide film. In the present invention, the Mg concentration in the oxide film is defined as Mg / (Al + O + Mg) represented by the number of atoms of Al, O, and Mg as an average composition in the oxide film. What is defined in this way is hereinafter referred to as “oxide film Mg concentration”. The oxide film Mg concentration is regulated to 0.05 ≦ Mg / (Al + O + Mg) ≦ 0.25. When the Mg concentration of the oxide film exceeds 0.25, the hardness of the oxide film is too large and the bite is significantly consumed. The lower the oxide film Mg concentration, the lower the hardness of the oxide film, and the tool is less likely to wear and the life is prolonged. However, since it is extremely difficult to make the concentration of Mg oxide film formed by annealing less than 0.05, the lower limit is set to 0.05.

  The oxide film Mg concentration is determined by measuring the blank edge by X-ray photoelectron spectroscopy (XPS) to determine the number of atoms of Al, O, and Mg. The measurement area at this time is usually about Φ1 mm. XPS measurement is performed in the depth direction from the outermost surface to the entire thickness of the oxide film while sputtering with Ar ions. The interval of the sputter depth is an interval at which 10 or more locations in the oxide film can be measured. In each measurement part, the atomic ratio of each element in the oxide film was obtained, Mg / (Al + O + Mg) was calculated, and the average of all measurement parts was calculated to obtain the oxide film Mg concentration. The oxide film thickness was defined as the depth from the outermost surface to the half value of the maximum value of the O concentration.

C-2. Film thickness The average film thickness of the oxide film is regulated to 15 nm or less. If the thickness exceeds 15 nm, the oxide film thickness is too thick, and the wear of the tool is remarkable. The thinner the oxide film, the harder it is to wear out. However, since an oxide film of at least 7 nm is formed by annealing, the lower limit of the oxide film thickness is 7 nm. The film thickness of the oxide film is measured by cutting a blank end portion in contact with a cutting tool with a microtome or the like to produce a cross-sectional thin piece and observing it with a transmission electron microscope (TEM).

D. Flatness The maximum flatness on the surface of the aluminum alloy blank is preferably less than 5 μm. When the maximum value of the flatness is 5 μm or more, there is a problem that, when the blank edge is cut with a cutting tool, the cutting tool blade is broken, which is inappropriate. The flatness of the blank is measured with a flatness measuring instrument or the like.

  As shown in FIG. 1, the aluminum alloy blank produced as described above is made into a product by cutting the inner diameter side cutting portion 2 and the outer diameter side cutting portion 3 of the blank 1 after annealing with a cutting tool. . In FIG. 1, 4 is the surface of the blank sample 1, 5 is the inner diameter side end surface of the blank sample 1, and 6 is the outer diameter side end surface of the blank sample.

  Hereinafter, preferred embodiments of the present invention will be described in detail based on examples and comparative examples.

Invention Examples 1-12 and Comparative Examples 13-19
After DC casting of the aluminum alloys (alloys A to G) having seven kinds of compositions shown in Table 1, the ingot surface is 20 mm chamfered, homogenized at 540 ° C. for 4 hours, and subjected to hot rolling and cold rolling. An aluminum alloy substrate with a plate thickness of 1.62 mm was produced. Subsequently, this aluminum alloy substrate was annealed at 300 ° C. for 1 hour to form an O material, thereby obtaining an O material sample.

First, the tensile strength and proof stress of the O material sample were measured as follows.
<Tensile strength>
The tensile strength of the O materialized JIS No. 5 test piece cut out in the rolling direction was measured using an Instron type tensile tester AG-50kNG manufactured by Shimadzu Corporation. The measurement conditions were a gauge distance of 50 mm and a crosshead speed of 10 mm / min.
<Strength>
For the O material sample cut in the rolling direction, the tensile strength was measured in the same manner as described above, and the 0.2% yield strength was obtained from the stress / strain curve.

Is a material strength of JIS 5086 alloy as a reference, a tensile strength of 245 N / mm ² or more, and passed thereby satisfying the yield strength 100 N / mm ² or more (○) and then, the tensile strength of 245 N / mm ² and less than / Alternatively, a proof stress of less than 100 N / mm ² was determined as a failure (x). As shown in Table 1, Alloys A to E and Alloy G passed and Alloy F failed.

For reference, a Cr-based intermetallic compound was investigated. The surface of the O material sample was mirror-polished, and crystal precipitates on the surface were observed with EPMA. A total area of 1 mm ² was observed at an apparatus magnification of 1000 times, and the number of Cr-based intermetallic compounds having a length of 1 μm or more was measured. From the alloys A, B, and D to G, 1 piece / mm ^{2 was} observed from the alloy C. Since this Cr intermetallic compound causes a defect of the aluminum alloy substrate, it is preferable that the amount is as small as possible.

  Next, six types of alloys that passed the material strength excluding the alloy F were subjected to performance evaluation of blank samples. A cold rolled material having a plate thickness of 1.62 mm was punched into a disk shape having an inner diameter of 24 mm and an outer diameter of 71 mm to obtain a disk-shaped aluminum alloy sample. After this sample was washed with hot water and dried, the samples of Example 1 to Comparative Example 18 were annealed under the conditions shown in Table 2. As the annealing furnace, a continuous furnace in which the sample proceeds in each of the temperature raising, holding, and cooling zones was used. The dew point of the furnace atmosphere was measured in each zone. In Example 1, the inside of the furnace was humidified, and the annealing was started after the dew point was adjusted to 20 ° C. In Example 2 to Comparative Example 18, annealing was started by introducing an atmospheric atmosphere into the furnace without humidification. In Comparative Example 19, the disc-shaped aluminum alloy sample punched out without annealing was used. In this way, the blank sample of Example 1 to Comparative Example 18 and the sample of Comparative Example 19 were produced. Hereinafter, the sample of Comparative Example 19 is also referred to as a blank sample.

  About the blank sample produced by the said procedure, the film thickness of the oxide film was measured. An arbitrary portion of the end of each sample is cut with a microtome to produce a cross-sectional slice, which is observed with a transmission electron microscope (TEM), and an average value of film thicknesses at five locations in one field of view of the TEM image is taken. The film thickness.

  The oxide film Mg concentration was determined by measuring the number of atoms of Al, O, and Mg at the blank edge by X-ray photoelectron spectroscopy (XPS). The measurement area was about Φ1 mm. XPS measurement was performed in the depth direction from the outermost surface to the entire oxide film thickness while sputtering with Ar ions. Here, the thickness of the oxide film was from the outermost surface to a depth that is a half value of the maximum value of the O concentration. The sputter depth interval was about 0.4 nm. In each measurement part, the atomic ratio of each element in the oxide film was obtained to calculate Mg / (Al + O + Mg), and the average of all measurement parts was calculated to obtain the oxide film Mg concentration.

  Next, the flatness was evaluated. Measurement was performed on 100 blank samples, and a maximum value of less than 5 μm was determined to be acceptable (◯), and 5 μm or more was determined to be unacceptable (×).

  Further, the machinability was evaluated. As shown in FIG. 1, the inner diameter side cutting portion 2 and the outer diameter side cutting portion 3 of the blank sample 1 were cut with a cutting tool, and the number of processed sheets (biting tool life) until the tool could not be used due to a defective shape was examined. When the tool life reached 150,000 sheets, it was determined to be acceptable (）), more than 100,000 sheets to less than 150,000 sheets were also acceptable (◯), and less than 100,000 sheets were determined to be unacceptable (×). Conventional tool life is around 50,000 sheets. The evaluation was completed when the tool life reached 150,000 sheets.

  Table 2 shows the alloy symbols, annealing conditions, oxide film thickness and Mg concentration, and evaluation results of flatness and machinability of the invention examples and comparative examples.

In Invention Examples 1 to 12, the film thickness and Mg concentration of the oxide film were within the desired ranges, and both flatness and machinability were acceptable.
In Invention Examples 1, 2, 4, and 5, the Cr addition amount of the alloy was in the range of 0.01 to 0.1 mass%, and the dew point in the annealing atmosphere was 0 ° C. or higher, so the machinability was Especially excellent.

In Comparative Example 13, since the dew point in the annealing atmosphere was less than −20 ° C., the Mg oxide film concentration was outside the range of the present invention, and the machinability was unacceptable.
In Comparative Example 14, since the annealing time exceeded 6 hours, the oxide film thickness was too thick, the Mg concentration of the oxide film was outside the range of the present invention, and the machinability was unacceptable.
In Comparative Example 15, since the annealing temperature exceeded 320 ° C., the oxide film thickness was too thick, and the Mg concentration of the oxide film was out of the range of the present invention, and the machinability was unacceptable.
In Comparative Example 16, since the annealing temperature was less than 270 ° C., the flatness was unacceptable. Therefore, it was judged to be defective and was not used for machinability evaluation.
In Comparative Example 17, the flatness was unacceptable because the annealing time was less than 30 minutes. Therefore, it was judged to be defective and was not used for machinability evaluation.
In Comparative Example 18, since the Mg content of the aluminum alloy G was too large, the Mg concentration of the oxide film was out of the range of the present invention, and the machinability was unacceptable.
In Comparative Example 19, since the annealing was not performed, the flatness was inferior. Therefore, it was judged as defective and was not used for machinability evaluation.

  By applying the method for manufacturing an aluminum alloy plate for magnetic disks of the present invention, the life of the cutting tool used for cutting the blank edge becomes longer than before, productivity is improved, and low cost is possible. Become.

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Claims (4)

An annealed aluminum alloy blank for a magnetic disk, which contains Mg: 3.5 to 10 mass% , Cr: 0.01 to 0.1 mass% , and is an Al—Mg alloy substrate comprising the balance Al and inevitable impurities And an oxide film formed on the surface of the substrate, the average film thickness of the oxide film is 15 nm or less, and the average composition of the oxide film is 0.05 ≦ Mg / (Al + O + Mg) ≦ 0.25 der is, an aluminum alloy blank for a magnetic disk having excellent machinability in which the maximum value of flatness on the surface, characterized in der Rukoto less than 5 [mu] m. Mg: 3.5-10 mass% , Cr: 0.01-0.1 mass% , Al-Mg alloy substrate consisting of the balance Al and unavoidable impurities, from the start of temperature rise to the end of cooling, A method for producing an aluminum alloy blank for a magnetic disk excellent in machinability, characterized by annealing at a temperature of 270 to 320 ° C for 30 minutes to 6 hours in an atmosphere having a dew point of -20 to 20 ° C. The manufacturing method of the aluminum alloy blank for magnetic discs excellent in the machinability of Claim 2 which makes the dew point in annealing atmosphere -20-20 degreeC by humidification. The method for producing an aluminum alloy blank for a magnetic disk excellent in machinability according to claim 2 or 3 , further comprising a step of cutting the inner diameter side cut portion and the outer diameter side cut portion of the blank after annealing with a cutting tool. .

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