Aluminum alloy substrate for magnetic disk and magnetic disk using said aluminum alloy substrate for magnetic disk

Abstract

To provide an aluminum alloy substrate for magnetic disks having excellent smoothness by reduction of undulation, and a magnetic disk including the same.SOLUTION: An aluminum alloy substrate for magnetic disks contains an aluminum alloy containing Mg: 3.50-4.50 mass% and Cr: 0.04-0.20 mass%, with the balance being Al and inevitable impurities. In the aluminum alloy, the distribution density of Al-Cr intermetallic compounds with a longest diameter of 0.01-1.00 μm is 1 / 0.001 mm2 or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an aluminum alloy substrate for a magnetic disk having improved smoothness due to reduced waviness, and a magnetic disk using the aluminum alloy substrate for a magnetic disk. [Background technology]

[0002] Hard disk drives (hereinafter referred to as "HDDs") are widely used as storage devices in electronic devices such as computers and video recorders. HDDs incorporate magnetic disks for recording data. The magnetic disks consist of a circular aluminum alloy substrate for magnetic disks made of an aluminum alloy, a Ni-P plating layer covering the surface, and a magnetic layer laminated on the Ni-P plating layer.

[0003] In recent years, the amount of information stored on HDDs has been increasing, both in commercial applications such as servers and data centers, and in home applications such as personal computers and video recorders. To meet this demand, there is a need to increase the recording density of the magnetic disks incorporated into HDDs in order to increase the HDD capacity. To achieve this, it is necessary to form a smooth Ni-P plating layer on an aluminum alloy substrate.

[0004] Magnetic disks are typically manufactured by the following method. First, a disk blank is produced by punching a rolled aluminum alloy plate into a circular shape. Next, the disk blank is heated while being pressed from both sides in the thickness direction to reduce warpage of the disk blank. The disk blank is then cut and ground to form a desired shape, thereby obtaining an aluminum alloy substrate. The aluminum alloy substrate thus obtained can be sequentially subjected to pretreatment for forming a Ni-P plating layer, electroless Ni-P plating, and sputtering of a magnetic layer, thereby producing a magnetic disk.

[0005] Here, as the aluminum alloy used for the aluminum alloy substrate, JIS A5086 alloy is often used.

[0006] Magnetic disks are required to have larger capacities and higher densities to meet the needs of multimedia and other applications. To achieve even greater capacity, there is a trend toward reducing the flying height of the head during reading and writing. However, if there are undulations on the surface of the magnetic disk, they can collide with the magnetic head, causing recording errors. Therefore, there is a need to reduce such undulations. Furthermore, aluminum alloy substrates for magnetic disks have traditionally been required to have high yield strength to ensure stable processing and use.

[0007] In order to further improve the smoothness of the magnetic disk surface by reducing waviness, techniques for reducing the average crystal grain size on the surface of an aluminum alloy plate have been studied. For example, Patent Document 1 describes a method for reducing the average crystal grain size in an aluminum alloy plate for a magnetic disk substrate, which contains 4.5% by mass to 6.0% by mass of Mg, 0.10% by mass to 0.55% by mass of Mn, 0.025% by mass or less of Si, 0.025% by mass or less of Fe, and the remainder being Al and unavoidable impurities. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 2017-014584

[0009] According to the method of Patent Document 1, adding Mn or the like makes it possible to refine the crystal grain size and reduce waviness. However, the method of Patent Document 1 has the problem that the refinement of the crystal grain size is insufficient, making it difficult to adequately deal with the reduction in head flying height. Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention has been made in view of the above problems, and aims to provide an aluminum alloy substrate for a magnetic disk in which waviness has been reduced to improve smoothness, and a magnetic disk using the aluminum alloy substrate for a magnetic disk. [Means for solving the problem]

[0011] The present inventors have discovered that by controlling the Mg content and Cr content of the aluminum alloy and the dispersion density of the Al-Cr-based intermetallic compound, it is possible to obtain an aluminum alloy substrate for magnetic disks that has reduced surface waviness, improved smoothness, and high yield strength, and have thereby completed the present invention.

[0012] The present invention provides, in claim 1, an aluminum alloy comprising 3.50 to 4.50 mass% of Mg and 0.04 to 0.20 mass% of Cr, with the remainder being Al and unavoidable impurities. The dispersion density of Al-Cr intermetallic compounds with a maximum diameter of 0.01 to 1.00 μm in the aluminum alloy is 1 particle / 0.001 mm 2 That's all. The aluminum alloy substrate for magnetic disks is characterized by the above.

[0013] According to a second aspect of the present invention, in the first aspect, the aluminum alloy contains 0.07 to 0.20 mass% of Cr.

[0014] In claim 3 of the present invention, in claim 1 or 2, the aluminum alloy further contains one or more elements selected from the group consisting of Cu: 0.30 mass% or less, Zn: 0.60 mass% or less, Fe: 0.35 mass% or less, Si: 0.35 mass% or less, Mn: 0.60 mass% or less, and Be: 0.0020 mass% or less.

[0016] The present invention is 4 Now, claims 1 to 3 In any one of the above, the crystal grain size on the surface of the aluminum alloy substrate for magnetic disk is 13.0 μm or less.

[0017] The present invention is 5In the above, claims 1 to 4 The magnetic disk is characterized by having an electroless Ni-P plating layer on the surface of the aluminum alloy substrate for magnetic disk described in any one of the above, and a magnetic layer on the electroless Ni-P plating layer. [Effects of the Invention]

[0018] According to the present invention, by controlling the Mg content and Cr content, as well as the dispersion density of the Al-Cr-based intermetallic compound in the aluminum alloy, it is possible to provide an aluminum alloy substrate for magnetic disks that reduces waviness, improves smoothness, and has high yield strength, and a magnetic disk using this aluminum alloy substrate for magnetic disks. DETAILED DESCRIPTION OF THE INVENTION

[0019] A. Aluminum alloy substrates for magnetic disks An aluminum alloy substrate for magnetic disks (hereinafter sometimes referred to as "aluminum alloy substrate" or "substrate") according to the present invention will be described. The aluminum alloy substrate is prepared by preparing an aluminum alloy plate using an aluminum alloy of a predetermined alloy composition, and then punching this into an annular shape to obtain an aluminum alloy disk blank for magnetic disks (hereinafter sometimes referred to as "aluminum alloy disk blank" or "disk blank"). The disk blank is then subjected to pressure annealing, followed by cutting and grinding, and further subjected to strain relief heat treatment as necessary to obtain an aluminum alloy substrate.

[0020] The aluminum alloy of the aluminum alloy substrate contains, as essential elements, 3.50 to 4.50 mass% Mg (hereinafter simply referred to as "%") and 0.04 to 0.20% Cr, preferably 0.07 to 0.20%. It also contains, as optional elements, one or more selected from the group consisting of 0.30% or less Cu, 0.60% or less Zn, 0.35% or less Fe, 0.35% or less Si, 0.60% or less Mn, and 0.0020% or less Be. The aluminum alloy consists of the above essential and optional elements, with the balance being Al and unavoidable impurities.

[0021] Furthermore, the dispersion density of Al-Cr intermetallic compounds with a maximum diameter of 0.01 to 1.00 μm in the aluminum alloy of the aluminum alloy substrate is 1 particle / 0.001 mm 2 As a result, the growth of recrystallized grains during pressure annealing can be suppressed to make the crystal grain size fine, and as a result, the effect of reducing waviness can be obtained. By setting the dispersion density of the Al-Cr-based intermetallic compounds as described above, the crystal grain size on the surface of the aluminum alloy substrate can be made 13.0 μm or less, preferably 10.0 μm or less.

[0022] Furthermore, by setting the Mg content of the aluminum alloy to 3.50 to 4.50%, high yield strength can be achieved. This is because Mg exists mainly as solid solution Mg and has the effect of improving strength, and by setting the Mg content to 3.50 to 4.50%, the amount of solid solution Mg increases, improving yield strength.

[0023] As a result, it is possible to achieve both improved smoothness due to reduced waviness and high yield strength.

[0024] A-1. Aluminum alloy composition The composition of the aluminum alloy used in the aluminum alloy substrate and the reasons for limiting it will be explained in detail below.

[0025] Mg: 3.50-4.50% Mg is an essential element in aluminum alloys, and exists mainly as solid solution Mg, which has the effect of improving the strength of aluminum alloy substrates. Furthermore, Mg helps to form a uniform, thin, and dense zincate film during zincate treatment of aluminum alloy substrates, thereby improving the smoothness of the electroless Ni-P plating surface in the plating process that follows the zincate treatment process.

[0026] However, if the Mg content is less than 3.50%, the strength of the aluminum alloy substrate is insufficient, and it may deform during cutting, grinding, or other processes. Furthermore, the zincate coating formed by zincate treatment may become non-uniform, resulting in reduced plating adhesion and smoothness. On the other hand, if the Mg content exceeds 4.50%, the strength may become too high, causing cracks during rolling. Therefore, in the aluminum alloy substrate of the present invention, the Mg content of the aluminum alloy is specified to be 3.50 to 4.50%. In addition, the Mg content is preferably 3.70 to 4.40%, more preferably 3.80 to 4.30%, in view of the balance between strength and manufacturability.

[0027] Cr: 0.04 to 0.20% Cr is an essential element in aluminum alloys, existing as an Al-Cr intermetallic compound or dissolved Cr, and has the effect of further refining the recrystallized structure of aluminum alloy substrates, thereby reducing waviness on the substrate surface and improving its smoothness.

[0028] If the Cr content is less than 0.04%, the crystal grain size is not sufficiently refined, resulting in waviness. On the other hand, if the Cr content exceeds 0.20%, coarse Al-Cr intermetallic compounds are likely to be formed in the aluminum alloy substrate. If such coarse Al-Cr intermetallic compounds fall off the surface of the aluminum alloy substrate, plating pits are likely to be formed in the subsequent electroless Ni-P plating process. Furthermore, Al-Cr intermetallic compounds with a maximum diameter of approximately 1000 μm, known as coarse crystallized particles, may be formed, which can cause cracks to originate during rolling, etc. Therefore, in the aluminum alloy substrate of the present invention, the Cr content of the aluminum alloy is specified to be 0.04 to 0.20%. The Cr content is preferably 0.07 to 0.20%, more preferably 0.07 to 0.10%, in terms of the balance between crystal grain size and manufacturability.

[0029] In addition to Mg and Cr, the aluminum alloy may further contain one or more optional elements selected from the group consisting of Cu, Zn, Fe, Si, Mn, and Be. Each optional element will be described in detail below.

[0030] Cu:0.30% or less The aluminum alloy may contain 0.30% or less Cu as an optional element. Cu has the effect of suppressing Al elution from the aluminum alloy during zincate treatment in the manufacturing process of the magnetic disk. By setting the Cu content to 0.30% or less, a dense, thin Zn coating with little thickness variation can be adhered to the surface of the aluminum alloy substrate during the zincate treatment. Furthermore, by forming such a Zn coating, a smooth electroless Ni-P plating layer can be formed in the subsequent electroless Ni-P plating process.

[0031] However, if the Cu content exceeds 0.30%, the corrosion resistance of the aluminum alloy substrate decreases, and localized regions where Al is easily dissolved are formed. Therefore, when zincating is performed in the manufacturing process of a magnetic disk, the amount of Al dissolved on the surface of the aluminum alloy substrate tends to be uneven, and the thickness of the Zn coating tends to vary greatly. As a result, the adhesion between the electroless Ni-P plating layer and the aluminum alloy substrate decreases, and the smoothness of the electroless Ni-P plating layer decreases.

[0032] Thus, by setting the Cu content in the aluminum alloy to 0.30% or less, preferably 0.15% or less, the formation of plating pits can be suppressed and the smoothness of the electroless Ni-P plating layer can be further improved. The lower limit of the Cu content is preferably 0.005%, more preferably 0.010%. The Cu content may be 0% (0.000%).

[0033] Zn: 0.60% or less The aluminum alloy may contain 0.60% or less Zn as an optional element. Zn, like Cu, has the effect of suppressing the elution of Al from the aluminum alloy during zincate treatment. By setting the Zn content to 0.60% or less, a dense, thin Zn coating with little thickness variation can be adhered to the surface of the aluminum alloy substrate during zincate treatment. Furthermore, by forming such a Zn coating, a smooth electroless Ni-P plating layer can be formed in the subsequent electroless Ni-P plating process.

[0034] However, if the Zn content exceeds 0.60%, the corrosion resistance of the aluminum alloy substrate decreases, and localized regions where Al is easily dissolved are formed. Therefore, when zincating is performed, the amount of Al dissolved on the surface of the aluminum alloy substrate tends to be uneven, and the thickness of the Zn coating tends to vary greatly. As a result, the adhesion between the electroless Ni-P plating layer and the aluminum alloy substrate decreases, and the smoothness of the electroless Ni-P plating layer decreases.

[0035] Thus, by setting the Zn content in the aluminum alloy to 0.60% or less, preferably 0.50% or less, it is possible to suppress the formation of plating pits and further improve the smoothness of the electroless Ni-P plating layer. The lower limit of the Zn content is preferably 0.10%, more preferably 0.25%. The Zn content may be 0% (0.00%).

[0036] Fe, Si: 0.35% or less The aluminum alloy may contain 0.35% or less of Fe and Si as optional elements. Fe is hardly dissolved in the Al matrix and is dispersed in the aluminum alloy substrate as an Al-Fe intermetallic compound. When the aluminum alloy contains Mg, Si forms an Mg-Si intermetallic compound with Mg.

[0037] If such Al-Fe intermetallic compounds or Mg-Si intermetallic compounds are detached from the surface of the aluminum alloy substrate, plating pits are likely to form in the subsequent electroless Ni-P plating process. By setting the Fe and Si contents in the aluminum alloy to 0.35% or less, preferably 0.05% or less, and more preferably 0.01% or less, respectively, the amounts of the above-mentioned Al-Fe intermetallic compounds and Mg-Si intermetallic compounds present in the aluminum alloy substrate can be further reduced. As a result, the formation of plating pits can be suppressed and the smoothness of the Ni-P plating layer can be further improved. Note that the Fe and Si contents may each be 0% (0.000%).

[0038] In order to suppress the occurrence of plating pits due to the intermetallic compounds, it is preferable to reduce the contents of Fe and Si. However, these elements are contained not only in general purity metals but also in high-purity metals with an Al purity of 99.9% or more. Therefore, if an aluminum alloy substrate containing almost no Fe or Si is to be produced, a special treatment to remove these elements during casting is required, which increases the production cost of the aluminum alloy substrate.

[0039] By using an aluminum alloy containing 0.01% or less Fe and Si, respectively, an aluminum alloy substrate can be produced without special treatment for removing these elements. As a result, the smoothness of the aluminum alloy substrate can be further improved while avoiding an increase in the production cost of the aluminum alloy substrate. Furthermore, even if the Fe and Si contents in the aluminum alloy exceed 0.01%, as long as each is 0.35% or less, the aluminum alloy substrate can be produced using a lower purity base metal. This allows the production of the above-mentioned Al-Fe intermetallic compounds and Mg-Si intermetallic compounds to be suppressed, while reducing the material cost of the aluminum alloy substrate.

[0040] Mn: 0.60% or less The aluminum alloy may contain 0.60% or less of Mn as an optional element. Some of the Mn is dispersed in the aluminum alloy substrate as fine intermetallic compounds formed during casting. Mn that does not form intermetallic compounds during casting dissolves in the Al matrix, improving the strength of the aluminum alloy substrate through solid solution strengthening.

[0041] However, if the Mn content in the aluminum alloy exceeds 0.60%, coarse Al-Mn intermetallic compounds are likely to be formed in the aluminum alloy substrate, and if such coarse Al-Mn intermetallic compounds fall off from the surface of the aluminum alloy substrate, plating pits are likely to be formed in the subsequent electroless Ni-P plating process.

[0042] Thus, by setting the Mn content in the aluminum alloy to 0.60% or less, preferably 0.50 mass% or less, it is possible to suppress the formation of plating pits, form a smooth Ni-P plating layer, and further improve the strength of the aluminum alloy substrate. The lower limit of the Mn content is preferably 0.001%, more preferably 0.005%. The Mn content may be 0% (0.000%).

[0043] Be: 0.0020% or less Be is an element added to the molten metal when casting an aluminum alloy containing Mg to suppress oxidation of Mg. Furthermore, by limiting the Be content in the aluminum alloy to 0.0020% or less, the Zn coating formed on the surface of the aluminum alloy substrate during the magnetic disk manufacturing process can be made denser and the thickness variation can be reduced. As a result, the smoothness of the electroless Ni-P treatment layer formed on the aluminum alloy substrate can be further improved.

[0044] However, if the Be content in the aluminum alloy exceeds 0.0020%, Be-based oxides are likely to form on the surface of the aluminum alloy substrate when the aluminum alloy substrate is heated during its manufacturing process. Furthermore, if the aluminum alloy further contains Mg, Al-Mg-Be-based oxides are likely to form on the surface of the aluminum alloy substrate when the aluminum alloy substrate is heated. If the amount of these Be-based oxides or Al-Mg-Be-based oxides increases, the thickness of the Zn coating may vary significantly, potentially resulting in the formation of plating pits.

[0045] By setting the Be content in the aluminum alloy to 0.0020% or less, preferably 0.0010% or less, the amount of Be-based oxides and Al-Mg-Be-based oxides can be reduced, and the smoothness of the electroless Ni-P plating layer can be further improved. The lower limit of the Be content may be 0% (0.0000%), but is preferably 0.0002%.

[0046] Other elements The aluminum alloy may contain elements other than the essential elements and optional elements described above that are unavoidable impurities, such as Zr, Ti, B, and Ga. The effects of the present invention are not impaired as long as the content of each element is 0.10% or less and the total content is 0.30% or less.

[0047] As described above, in the present invention, Fe and Si can be actively added as optional components, but they may also be contained as inevitable impurities without being actively added. Si and Fe are contained as inevitable impurities not only in ingots of ordinary purity but also in high-purity ingots with an Al purity of 99.9% or more. Even when they are contained as inevitable impurities, as in the case of optional components, the effects of the present invention are not impaired as long as the Fe content and Si content are 0.35% or less, preferably 0.05% or less, and more preferably 0.01% or less, respectively.

[0048] A-2. The dispersion density of Al-Cr intermetallic compounds with a maximum diameter of 0.01 to 1.00 μm is 1 particle / 0.001 mm 2 End In the aluminum alloy substrate of the present invention, the dispersion density as an index of the dispersion state of Al-Cr intermetallic compounds with a maximum diameter of 0.01 to 1.00 μm present in the aluminum alloy is 1 particle / 0.001 mm 2 By doing so, the effect of further refining the recrystallized structure of the aluminum alloy substrate is exerted. As a result, the waviness of the aluminum alloy substrate surface can be reduced and the smoothness can be improved. Here, the Al-Cr intermetallic compounds are precipitates containing Al and Cr, and the dispersion state can be confirmed by EDX (energy dispersive X-ray analysis) analysis using a transmission electron microscope. Note that some compounds contain Fe and the like in addition to Al and Cr, and the dispersion state of these compounds can also be confirmed by EDX analysis using a transmission electron microscope.

[0049] The dispersion density of Al-Cr intermetallic compounds with a maximum diameter of 0.01 to 1.00 μm is 1 particle / 0.001 mm 2 If the maximum diameter of the intermetallic compound is less than 0.01 μm, the grain size will be insufficient, resulting in increased waviness and reduced smoothness. If the maximum diameter of the intermetallic compound is less than 0.01 μm or exceeds 1.00 μm, the grain size will not be sufficiently refined. Therefore, the dispersion density of the Al-Cr intermetallic compound having a maximum diameter of 0.01 to 1.00 μm is set to 1 grain / 0.001 mm. 2 The dispersion density is preferably 2 particles / 0.001 mm or more. 2 More preferably, 10 pieces / 0.001 mm or more. 2 Although there is no particular upper limit to the dispersion density, it is 50 particles / 0.001 mm according to the alloy composition and manufacturing method of the present invention. 2 In the present invention, the longest diameter is defined as the distance between the two most distant points on the contour line of the Al-Cr intermetallic compound in a surface image of the aluminum alloy substrate taken by TEM.

[0050] A-3.Crystal grain size The crystal grain size on the surface of the aluminum alloy substrate according to the present invention is preferably 13.0 μm or less, more preferably 10.0 μm or less, and even more preferably 8.0 μm or less, when recrystallized by pressure annealing. By reducing the crystal grain size in this way, the waviness of the substrate surface can be reduced and the smoothness can be improved. Such a crystal grain size is determined by dividing the above-mentioned Al-Cr-based compound having a maximum diameter of 0.01 to 1.00 μm by 1 particle / 0.001 mm. 2 By the above, the growth of recrystallized grains during pressure annealing is suppressed, and this is achieved. The crystal grain size on the surface of the aluminum alloy substrate is measured using a glow discharge optical emission spectrometer and a backscattered electron diffraction measuring device.

[0051] A-4 Manufacturing method of aluminum alloy plate (1) Casting process Aluminum raw materials with a specified alloy composition are melted to produce molten metal, which is then cast to produce ingots. Casting methods include semi-continuous casting (DC casting), permanent mold casting, and continuous casting (CC casting). In DC casting, molten metal poured through a spout loses heat through the bottom block, the water-cooled mold walls, and cooling water directly sprayed onto the outer periphery of the ingot, solidifying and being drawn downward as an ingot. In permanent mold casting, molten metal poured into a hollow mold made of cast iron or other material loses heat through the mold walls, solidifying and producing an ingot. In CC casting, molten metal is supplied through a casting nozzle between a pair of rolls (or belt casters or block casters), and thin plates are directly cast by removing heat from the rolls.

[0052] In such a casting process, it is preferable to carry out in-line a degassing treatment to reduce the amount of gas dissolved in the molten metal and a filtration treatment to remove solids from the molten metal.

[0053] For example, the degassing process can be performed using a process known as the SNIF (Spinning Nozzle Inert Flotation) process or the Alpur process. In these processes, a process gas such as argon gas or a mixture of argon and chlorine is blown into the molten metal while stirring the molten metal at high speed using a rotor with blades, forming fine bubbles of the process gas into the molten metal. This allows hydrogen gas and inclusions dissolved in the molten metal to be removed in a short period of time. An in-line degassing device can be used for the degassing process.

[0054] For example, the filtration process can be performed using a cake filtration method, a filter medium filtration method, etc. For the filtration process, a filter such as a ceramic tube filter, a ceramic foam filter, or an alumina ball filter can be used.

[0055] (2) Homogenization process After the ingot is produced, it may be subjected to facing and homogenization treatment as needed before hot rolling. The holding temperature in the homogenization treatment is preferably in the range of 500 to 570°C. The holding time in the homogenization treatment can be appropriately set, for example, in the range of 1 to 60 hours.

[0056] After the homogenization treatment and before the hot rolling, the ingot is preferably subjected to a heat treatment in which the ingot is held at a temperature in the range of 350 to 400°C, preferably 360 to 390°C, for at least 1 hour, preferably 2 to 48 hours. This is because precipitation of fine Al-Cr intermetallic compounds is most likely to occur in the temperature range of 350 to 400°C after the homogenization treatment. If the temperature is below 350°C, fine Al-Cr intermetallic compounds do not precipitate, while if the temperature exceeds 400°C, the Al-Cr intermetallic compounds become coarse. Furthermore, if the holding time is less than 1 hour, the precipitation effect of fine Al-Cr intermetallic compounds is not sufficiently obtained. Thus, by holding the ingot at a temperature in the range of 350 to 400°C for at least 1 hour after the homogenization treatment, Al-Cr intermetallic compounds having the desired longest diameter can be precipitated in a predetermined dispersed state.

[0057] (3) Hot rolling process Next, the ingot is hot-rolled to produce a hot-rolled sheet. The rolling conditions for the hot rolling are not particularly limited, but for example, the hot rolling can be performed at a starting temperature in the range of 300 to 550°C and a finishing temperature in the range of 260 to 380°C.

[0058] (4) Cold rolling process After hot rolling, the resulting hot-rolled sheet can be subjected to one or more passes of cold rolling to obtain a cold-rolled sheet. The rolling conditions for cold rolling are not particularly limited and can be appropriately set depending on the desired thickness and strength of the aluminum alloy sheet. For example, the total reduction rate in cold rolling can be 20 to 95%. The thickness of the cold-rolled sheet can be appropriately set, for example, within the range of 0.2 to 1.9 mm.

[0059] (5) Annealing process In the manufacturing method of the above embodiment, annealing may be performed, if necessary, at least either before the first pass of cold rolling or between passes. The annealing may be performed using a batch-type heat treatment furnace or a continuous heat treatment furnace. When a batch-type heat treatment furnace is used, the holding temperature during annealing is preferably set to 250 to 430°C and the holding time is preferably set to 0.1 to 10 hours. When a continuous heat treatment furnace is used, the residence time in the furnace is preferably set to 60 seconds or less and the temperature in the furnace is preferably set to 400 to 500°C. By performing annealing under these conditions, it is possible to restore the workability during cold rolling. Through the above steps, an aluminum alloy plate is produced.

[0060] A-3. Manufacturing method of aluminum alloy substrate When producing an aluminum alloy substrate from the above-mentioned aluminum alloy sheet, for example, the following method can be adopted. First, a disk blank having a circular ring shape is produced by punching the aluminum alloy sheet. Then, the disk blank is subjected to pressure annealing by heating while being pressurized from both sides in the thickness direction. This achieves the desired crystal grain size in the Al-Cr-based intermetallic compounds in a desired dispersed state obtained by the above-mentioned homogenization treatment, and further reduces distortion in the disk blank, thereby improving the flatness of the surface of the aluminum alloy substrate.

[0061] The holding temperature and pressure in pressure annealing can be appropriately selected, for example, from the ranges of 250 to 430°C, preferably 290 to 350°C, and 1.0 to 3.0 MPa. If the temperature is less than 250°C, the effect of inhibiting the growth of recrystallized grains cannot be sufficiently obtained. If the temperature exceeds 430°C, the recrystallized grains become coarse. The holding time in pressure annealing can be, for example, 30 minutes or more.

[0062] After pressure annealing, the disk blank is sequentially cut and ground, and then subjected to a stress relief heat treatment at 150 to 350°C for 0.1 to 10.0 hours as needed to remove stress generated during processing. Through these processing steps, an aluminum alloy substrate having a desired shape is obtained.

[0063] B. Magnetic disk B-1. Magnetic Disk Configuration The magnetic disk having the aluminum alloy substrate has, for example, the following configuration: the magnetic disk has an aluminum alloy substrate, an electroless Ni-P plating layer covering the surface of the aluminum alloy substrate, and a magnetic layer laminated on the electroless Ni-P plating layer. The electroless Ni-P plating layer is preferably an electroless Ni-P plating layer formed by electroless plating.

[0064] The magnetic disk may further have a protective layer made of a carbonaceous material such as diamond-like carbon and laminated on the magnetic layer, and a lubricating layer made of lubricating oil and applied on the protective layer.

[0065] B-2. Magnetic Disk Manufacturing Method When manufacturing a magnetic disk from an aluminum alloy substrate, for example, the following method can be used. First, the aluminum alloy substrate is degreased and cleaned to remove oils such as processing oils adhering to the surface of the aluminum alloy substrate. After degreased and cleaned, the aluminum alloy substrate may be etched using an acid, if necessary. If etching is performed, it is preferable to perform a desmutting treatment after etching to remove smut generated by etching from the aluminum alloy substrate. The treatment conditions for these treatments can be appropriately set depending on the type of treatment solution.

[0066] After these pre-plating treatments, a zincate treatment is performed to form a Zn coating on the surface of the aluminum alloy substrate. In the zincate treatment, a zinc immersion plating is performed to replace Al with Zn, thereby forming a Zn coating. A preferred zincate treatment is the so-called double zincate method, in which the Zn coating formed on the surface of the aluminum alloy substrate after the first zinc immersion plating is once stripped off and then another zinc immersion plating is performed to form a Zn coating. The double zincate method allows a denser Zn coating to be formed on the surface of the aluminum alloy substrate compared to the Zn coating formed by only the first zinc immersion plating. As a result, defects in the electroless Ni-P plating layer can be reduced in the subsequent electroless Ni-P plating process.

[0067] After forming a Zn coating on the surface of an aluminum alloy substrate by zincate treatment, the Zn coating can be replaced with a Ni-P plating layer by performing electroless Ni-P plating at around 90° C. Then, by replacing such a Zn coating with an electroless Ni-P plating layer in the electroless Ni-P plating process, a smooth electroless Ni-P plating layer with few plating pits can be formed.

[0068] Increasing the thickness of the electroless Ni-P plating layer tends to reduce plating pits, allowing for the formation of a smooth electroless Ni-P plating layer. Therefore, the plating thickness is preferably 7 μm or more, more preferably 18 μm or more, and even more preferably 25 μm or more. In practice, the upper limit of the plating thickness is approximately 40 μm.

[0069] By polishing the electroless Ni-P plated layer after the electroless Ni-P plating treatment, the smoothness of the surface of the electroless Ni-P plated layer can be further improved.

[0070] After electroless Ni-P plating (including polishing), a magnetic material is deposited on the electroless Ni-P plating layer by sputtering to form a magnetic layer. The magnetic layer may be composed of a single layer or multiple layers with different compositions. After sputtering, a protective layer made of a carbon-based material is formed on the magnetic layer by CVD. Next, a lubricating oil is applied to the protective layer to form a lubricating layer. This completes the magnetic disk. [Example]

[0071] An example of an aluminum alloy plate, a method for producing the same, and an aluminum alloy substrate produced from the aluminum alloy plate will be described below.

[0072] Specific aspects of these aluminum alloy plates and their manufacturing methods, as well as aluminum alloy substrates produced from these aluminum alloy plates and their manufacturing methods, are not limited to the aspects of the examples shown below, and the configurations can be appropriately changed from the examples within the scope that does not detract from the spirit of the present invention.

[0073] (1) Preparation of aluminum alloy plate The aluminum alloy sheets used for evaluation in the present examples were produced by the following method: First, a molten metal having the alloy composition shown in Table 1 was prepared in a melting furnace.

[0074] [Table 1]

[0075] Next, the molten metal in the melting furnace was transferred, and an ingot was produced by DC casting as shown in Table 2. Next, the surface of the ingot was chamfered to remove the segregation layer present on the ingot surface. After chamfering, the ingot was subjected to homogenization treatment under the conditions shown in Table 2. Furthermore, the ingot after the homogenization treatment was subjected to heat treatment at 350 to 400°C for the time shown in Table 2. Next, hot rolling was carried out under the conditions shown in Table 2 to obtain a hot-rolled sheet having a thickness shown in Table 2. Furthermore, cold rolling was carried out to obtain a cold-rolled sheet having a thickness shown in Table 2.

[0076] [Table 2]

[0077] (2) Preparation of aluminum alloy substrate The aluminum alloy plate was punched to obtain an annular aluminum alloy disk blank with an outer diameter of 98 mm and an inner diameter of 24 mm. Next, the obtained aluminum alloy disk blank was subjected to pressure annealing by holding it at the temperature shown in Table 2 for 3 hours while pressurizing it from both sides in the thickness direction. Furthermore, the outer peripheral end surface and inner peripheral end surface of each aluminum alloy disk blank after pressure annealing were cut to process it into an aluminum alloy disk blank with an outer diameter of 97 mm and an inner diameter of 25 mm. Thereafter, the plate surface of each aluminum alloy disk blank was ground so that the grinding amount was 10 μm. Thus, an aluminum alloy substrate was produced.

[0078] When the Cr content exceeded 0.20%, the thermodynamic calculation system (Thermocalc) revealed that coarse crystals might be formed, so no samples were prepared. Also, when the Mg content exceeded 4.50%, large cracks occurred during hot rolling, so no further processing beyond cold rolling was performed.

[0079] Measurement of the distribution density of Al-Cr intermetallic compounds with a maximum diameter of 0.01 to 1.00 μm: The surface of the aluminum alloy substrate prepared as described above was processed into a thin film of about 10 μm by mechanical polishing, and then electrolytic polishing was performed to prepare a sample for observation with a transmission electron microscope (TEM). The number distribution of the Al-Cr intermetallic compounds in the prepared thin film sample was measured at a dispersion density (number / 0.001 mm). 2 ) was measured as follows:

[0080] This measurement involved observation of a random surface of the plate material using a scanning transmission electron microscope (STEM). Al-Cr intermetallic compounds were identified using STEM-EDS analysis. For each sample, STEM images were taken at 20,000x magnification. If the intermetallic compounds were present, they were analyzed, and the dispersion density of Al-Cr intermetallic compounds with a longest diameter of 0.01 to 1.00 μm in the sample was investigated using EDX analysis. The thickness of the thin-section samples could be measured using an electron energy loss spectroscopy (EELS) detector attached to the transmission electron microscope (TEM), and measurements were performed in a thickness range of approximately 50 to 300 nm.

[0081] Grain size measurement: To measure the grain size, the rolled surface of the aluminum alloy substrate for microstructure observation was sputtered using a glow discharge spectroscopy (GDS, JY5000RF, HORIBA) at a gas pressure of 400 Pa, a power of 30 W, and 60 seconds. The sputtered surface was then measured using a scanning electron microscope (SEM-EBSD) with an attached electron backscatter diffraction (EBSD) analyzer to obtain texture orientation information. The measurement area of ​​the sample was 750 μm × 1000 μm, and the measurement step interval was 3 μm for grain sizes 10 μm or greater, 2.0 μm for grain sizes 8.0 μm or greater but less than 10.0 μm, and 1.0 μm for grain sizes less than 8.0 μm.

[0082] Next, the grain size was calculated from the obtained orientation data using EBSD analysis software (TSL's "OIM Analysis"), where grain boundaries with a misorientation of 5° or more were considered to be grain boundaries, and the diameter calculated as an equivalent circle was taken as the grain size.

[0083] Measurement of yield strength: The yield strength was measured in accordance with JIS Z2241 by subjecting cold-rolled aluminum alloy sheets to pressure annealing for 3 hours at the temperatures shown in Table 2 (pressure annealing simulation heating), and then measuring n=1 JIS No. 5 test pieces taken along the rolling direction. Strength was evaluated as pass (A) when the yield strength was 90 MPa or more, and fail (B) when it was less than 90 MPa. Note that, as with the measurements of the dispersion density and crystal grain size of the intermetallic compounds described above, an aluminum alloy substrate should be used to measure the yield strength, but it has been confirmed that the cold-rolled sheet after pressure annealing and the aluminum alloy substrate show the same numerical values ​​for the yield strength.

[0084] Table 2 shows the measurement results of the distribution density, grain size and yield strength of Al-Cr based intermetallic compounds with a maximum diameter of 0.01 to 1.00 μm, together with the evaluation of the yield strength.

[0085] As shown in Tables 1 and 2, Example 1 、2 In Examples 1 and 2, the specific alloy composition specified in the present invention was such that the crystal grain size was fine, waviness was reduced, and good smoothness was obtained. High strength was also achieved. Furthermore, in Examples 1 and 2, the distribution density of Al-Cr intermetallic compounds with a maximum diameter of 0.01 to 1.00 μm was 1 particle / 0.001 mm. 2 As a result, the crystal grain size could be further refined.

[0086] On the other hand, in Comparative Example 2, the alloy composition was not within the range specified in the present invention, so the crystal grain size became coarse and good smoothness could not be obtained. In Comparative Examples 1 and 3, the distribution density of Al-Cr based intermetallic compounds was not evaluated because the aluminum alloy did not contain Cr. In Comparative Example 3, the yield strength was low and high strength could not be achieved. Note that the yield strength was too low and the aluminum alloy substrate was deformed during peeling after pressure annealing, so the crystal grain size was not evaluated. In Comparative Example 4, the Mg content was too high, which caused large cracks during hot rolling, so no further processing was carried out after cold rolling. In Comparative Example 5, the distribution density of Al-Cr based intermetallic compounds having a maximum diameter of 0.01 to 1.00 μm was 0 pieces / 0.001 mm 2 Therefore, the crystal grain size was inferior to that of Examples 1 and 2. [Industrial Applicability]

[0087] According to the present invention, by controlling the Mg content, the Cr content, and the Al-Cr-based intermetallic compound, it is possible to provide an aluminum alloy substrate for magnetic disks that can reduce waviness, increase smoothness, and have high yield strength, and a magnetic disk using this aluminum alloy substrate for magnetic disks.

Claims

1. An aluminum alloy substrate for magnetic disks, comprising an aluminum alloy containing 3.50 to 4.50 mass% of Mg and 0.04 to 0.20 mass% of Cr, the balance being Al and unavoidable impurities, characterized in that the dispersion density of Al-Cr based intermetallic compounds having a longest diameter of 0.01 to 1.00 μm in the aluminum alloy is 1 particle / 0.001 mm 2 or more.

2. 2. The aluminum alloy substrate for magnetic disks according to claim 1, wherein the aluminum alloy contains 0.07 to 0.20 mass % of Cr.

3. 3. The aluminum alloy substrate for magnetic disks according to claim 1, further comprising one or more elements selected from the group consisting of Cu: 0.30 mass% or less, Zn: 0.60 mass% or less, Fe: 0.35 mass% or less, Si: 0.35 mass% or less, Mn: 0.60 mass% or less, and Be: 0.0020 mass% or less.

4. 4. The aluminum alloy substrate for magnetic disks according to claim 1, wherein the crystal grain size on the surface is 13.0 μm or less.

5. A magnetic disk comprising an aluminum alloy substrate for a magnetic disk according to any one of claims 1 to 4, an electroless Ni-P plating layer on the surface thereof, and a magnetic layer on the electroless Ni-P plating layer.

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