Aluminum alloy disc blank for magnetic disc and magnetic disc

Abstract

An aluminum alloy disc blank for a magnetic disc made of an aluminum alloy containing Fe: 0.005 to 1.800 mass% with the balance being Al and inevitable impurities, characterized in that a flatness change of the aluminum alloy disc blank for a magnetic disc when the aluminum alloy disc blank for a magnetic disc is held in the atmosphere at 50°C or lower for 336 hours is 2.0 µm or less. Fig. 1

Description

Aluminum alloy disk blank for magnetic disk and magnetic disk

[0001] The present invention relates to an aluminum alloy disk blank for a magnetic disk and a magnetic disk.

[0002] Hard disk drives (hereinafter abbreviated 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 disk has an annular aluminum alloy substrate made of an aluminum alloy, a Ni-P plating layer covering the surface of the aluminum alloy substrate, and a magnetic layer laminated on the Ni-P plating layer.

[0003] In recent years, the amount of information recorded on HDDs has been increasing, both in business applications such as servers and data centers, and in home applications such as personal computers and video recorders. To increase the capacity of HDDs in response to this trend, there is a demand for increasing the recording density of the magnetic disks incorporated into HDDs. To increase the recording density of the magnetic disks, a smooth Ni-P plating layer is formed 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 an annular 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 into a desired shape, thereby obtaining an aluminum alloy substrate. The aluminum alloy substrate thus obtained can be sequentially subjected to a pretreatment for forming a Ni—P plating layer, an electroless Ni—P plating treatment, and sputtering of a magnetic layer, thereby producing a magnetic disk.

[0005] As the aluminum alloy used for the aluminum alloy substrate, JIS A5086 alloy is widely 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, a method has been adopted to reduce the thickness of magnetic disks and increase the number of disks mounted. However, this method reduces rigidity as the disks become thinner, making disk flutter more likely. This occurs because, when magnetic disks are rotated at high speeds, unstable airflows are generated between the disks, causing vibrations (fluttering) of the magnetic disks. Fluttering increases positioning errors of the head, which is the reading unit. To combat this fluttering, a method has been adopted in which helium is filled into the hard disk to reduce fluid forces. This is because helium's viscosity is approximately one-eighth that of air. This method reduces fluttering, which occurs due to the gas flow caused by the hard disk's rotation, by reducing the fluid forces of the gas. However, the need to fill the hard disk with helium increases costs. On the other hand, when the inside of a hard disk is an air atmosphere, there is a tendency to reduce the flying height of the head during reading and writing in order to further increase capacity. However, if there are convex defects on the surface of the magnetic disk, they will collide with the magnetic head and cause recording errors, so there is a demand to reduce the convex defects.

[0007] Therefore, various techniques for reducing foreign matter such as intermetallic compounds in an aluminum alloy substrate have been investigated, with the aim of further improving smoothness by reducing convex defects. For example, Patent Document 1 discloses an aluminum alloy plate for magnetic disk substrates, which contains 2.0 to 6.0 wt% Mg, 0.05 to 0.15 wt% Cu, 0.10 to 0.30 wt% Zn, 0.05 to 0.12 wt% Zr, and 0.2 wt% or less (including 0 wt%) Sn, and the contents of Cu, Zn, Zr, and Sn satisfy the relational expression 0.15 wt%≦2Cu+6Zr−3Zn−0.1Sn≦0.32 wt% (wherein Cu, Zr, Zn, and Sn are each in wt%), and further contains one or two of more than 0.01 wt% and less than 0.05 wt% Mn, and more than 0.01 wt% and less than 0.05 wt% Cr, with the remainder being unavoidable impurity elements and Al. ２ A method for reducing Si compounds and Al--Fe compounds is described.

[0008] Japanese Patent Application Publication No. 10-008177

[0009] According to the method of Patent Document 1, twin-roll continuous casting can increase the cooling rate when the molten metal solidifies, thereby making the intermetallic compounds finer. However, the method of Patent Document 1 has a problem in that it is difficult to reduce convex defects in the disk blank caused by factors other than the intermetallic compounds.

[0010] The present invention has been made in view of the above problems, and the inventors have discovered that by controlling the change in flatness of an aluminum alloy disk blank for a magnetic disk, an aluminum alloy disk blank for a magnetic disk with excellent smoothness can be obtained, and have thus completed the present invention.

[0011] One aspect of the present invention is an aluminum alloy disk blank for a magnetic disk, which comprises an aluminum alloy containing 0.005 to 1.800 mass% Fe, the balance being Al and unavoidable impurities, characterized in that when the aluminum alloy disk blank for a magnetic disk is held in the air at 50°C or less for 336 hours, the change in flatness of the aluminum alloy disk blank for a magnetic disk is 2.0 μm or less. Another aspect of the present invention is an aluminum alloy disk blank for a magnetic disk, wherein the aluminum alloy further contains one or more elements selected from the group consisting of 1.80 mass% or less Mn, 2.50 mass% or less Ni, 1.00 mass% or less Cu, 0.70 mass% or less Zn, 4.50 mass% or less Mg, 0.30 mass% or less Cr, 0.15 mass% or less Zr, 14.00 mass% or less Si, 0.0015 mass% or less Be, 0.10 mass% or less Sr, 0.10 mass% or less Na, and 0.10 mass% or less P. Another aspect of the present invention is an aluminum alloy disk blank for a magnetic disk, wherein the crystal grain size on the surface of the aluminum alloy disk blank for a magnetic disk is 18 μm or less. Another aspect of the present invention is an aluminum alloy disk blank for a magnetic disk, characterized in that the crystal grain size on the surface of the aluminum alloy disk blank for a magnetic disk is 15 μm or less. Another aspect of the present invention is an aluminum alloy disk blank for a magnetic disk, characterized in that the crystal grain size on the surface of the aluminum alloy disk blank for a magnetic disk is 12 μm or less. Another aspect of the present invention is an aluminum alloy disk blank for a magnetic disk, characterized in that the crystal grain size on the surface of the aluminum alloy disk blank for a magnetic disk is 11 μm or less. Another aspect of the present invention is an aluminum alloy disk blank for a magnetic disk, characterized in that the crystal grain size on the surface of the aluminum alloy disk blank for a magnetic disk is 10 μm or less.Another aspect of the present invention is a magnetic disk characterized by having a Ni-P plating layer on the surface of an aluminum alloy substrate made of an aluminum alloy disk blank for a magnetic disk, and a magnetic layer on the Ni-P plating layer.

[0012] The aluminum alloy disk blank for a magnetic disk according to the present invention can control the change in flatness and improve the smoothness.

[0013] FIG. 1 is a diagram showing the measurement range of the flatness of an aluminum alloy disk blank.

[0014] A. Aluminum Alloy Disk Blank for Magnetic Disk The following describes an aluminum alloy disk blank for magnetic disks (hereinafter sometimes referred to as "aluminum alloy disk blank") according to the present invention. The aluminum alloy disk blank is obtained by preparing an aluminum alloy plate using an aluminum alloy of a predetermined alloy composition and punching it into a disk shape. The aluminum alloy disk blank is made of an aluminum alloy containing 0.005 to 1.800 mass% Fe, with the remainder being Al and unavoidable impurities. When the aluminum alloy disk blank is held in air at 50°C or below for 336 hours, the change in flatness of the aluminum alloy disk blank is 2.0 μm or less. The change in flatness is calculated as the absolute value of {(flatness of the aluminum alloy disk blank before holding at 50°C or below for 336 hours) - (flatness of the aluminum alloy disk blank after holding at 50°C or below for 336 hours)}. By keeping this change in flatness at 2.0 μm or less, residual stress is less likely to occur during surface cutting or grinding of the aluminum alloy disk blank, and convex defects are less likely to be generated during annealing performed after processing. As a result, the smoothness of the aluminum alloy disk blank is improved, and the occurrence of convex defects on the magnetic disk surface can be prevented.

[0015] A-1. Alloy Composition of Aluminum Alloy The composition of the aluminum alloy used in the aluminum alloy disk blank and the reasons for limiting it will be explained in detail below.

[0016] Fe: 0.005 to 1.800 mass% Fe exists mainly as second-phase particles (e.g., Al-Fe intermetallic compounds), with some present as solid solution in the matrix. By generating second-phase particles and dissolving in the matrix, Fe exerts the effect of improving the strength and rigidity of the aluminum alloy disk blank. However, if the Fe content is less than 0.005 mass%, the strength of the aluminum alloy disk blank is too low, resulting in deformation of the aluminum alloy disk blank when peeling the disk blank after pressure annealing. On the other hand, if the Fe content exceeds 1.800 mass%, coarse intermetallic compounds are generated, and during etching, zincating, cutting, or grinding of the aluminum alloy disk blank, the intermetallic compounds fall off, causing large dents and reducing surface smoothness. Furthermore, a high Fe content increases the strength of the aluminum alloy disk blank, resulting in cracks during rolling. Therefore, the Fe content in the aluminum alloy is set to 0.005 to 1.800 mass%, and preferably 0.010 to 1.500 mass% in view of the balance between the strength, rigidity, and manufacturability of the aluminum alloy disk blank.

[0017] In one embodiment, the aluminum alloy disk blank may further contain, in addition to Fe, one or more of the following elements as optional components: Mn (manganese), Ni (nickel), Cu (copper), Zn (zinc), Si (silicon), Be (beryllium), Cr (chromium), Zr (zirconium), Mg (magnesium), Sr (strontium), Na (sodium), and P (phosphorus). In this case, it contains Fe: 0.005 to 1.800 mass%, Mn: 1.80 mass% or less, Ni: 2.50 mass% or less, Cu: 1.00 mass% or less, Zn: 0.70 mass% or less, Mg: 4.50 mass% or less, C. r: 0.30 mass% or less, Zr: 0.15 mass% or less, Si: 14.00 mass% or less, Be: 0.0015 mass% or less, Sr: 0.10 mass% or less, Na: 0.10 mass% or less, and P: 0.10 mass% or less. The present invention provides an aluminum alloy disk blank for a magnetic disk, which comprises an aluminum alloy containing one or more elements selected from the group consisting of the following, with the balance being Al and unavoidable impurities, wherein the change in flatness of the aluminum alloy disk blank for a magnetic disk is 2.0 μm or less when the aluminum alloy disk blank for a magnetic disk is maintained in the atmosphere at 50° C. or less for 336 hours. Each optional component will be described in detail below.

[0018] Mn: 1.80 mass% or less The aluminum alloy disk blank may contain 1.80 mass% or less of Mn as an optional component. Mn exists primarily as second-phase particles (e.g., Al-Mn intermetallic compounds), with some present as solid solution in the matrix. By generating second-phase particles and dissolving in the matrix, Mn effectively improves the strength and rigidity of the aluminum alloy disk blank. However, if the Mn content is high, coarse intermetallic compounds are generated, which may cause large pits and reduce surface smoothness during etching, zincating, cutting, or grinding of the aluminum alloy disk blank. Furthermore, if the Mn content is high, the strength of the aluminum alloy disk blank increases, which may lead to cracking during rolling. Therefore, the Mn content is preferably 1.80 mass% or less. The Mn content is preferably 0.01 to 1.50 mass % in view of the balance between strength, rigidity and manufacturability of the aluminum alloy disk blank.

[0019] Ni: 2.50 mass% or less The aluminum alloy may contain 2.50 mass% or less of Ni as an optional component. Ni exists primarily as second-phase particles (e.g., Al-Ni intermetallic compounds), with some Ni present as solid solution in the matrix. Through the generation of second-phase particles and solid solution in the matrix, Ni exerts the effect of improving the strength and rigidity of the aluminum alloy disk blank. However, if the Ni content is high, coarse intermetallic compounds are generated, and during etching, zincating, cutting, or grinding of the aluminum alloy disk blank, the intermetallic compounds may fall off, causing large pits and reducing surface smoothness. Furthermore, if the Ni content is high, the strength of the aluminum alloy disk blank increases, which may lead to cracking during rolling. Therefore, the Ni content is preferably 2.50 mass% or less. The Ni content is preferably 0.01 to 2.00 mass % in view of the balance between the strength, rigidity and manufacturability of the aluminum alloy disk blank.

[0020] Cu: 1.00 mass% or less The aluminum alloy may contain 1.00 mass% or less of Cu as an optional component. Cu exists mainly as second-phase particles (e.g., Al-Cu intermetallic compounds) and has the effect of improving the strength and Young's modulus of the aluminum alloy disk blank. It also reduces the amount of Al dissolved during zincate treatment of the aluminum alloy disk blank. Furthermore, it adheres a zincate coating uniformly, thinly, and densely during zincate treatment, thereby improving smoothness in the subsequent plating process.

[0021] However, if the Cu content is too high, the corrosion resistance of the aluminum alloy disk blank decreases, and regions where Al is easily dissolved are formed. Therefore, when zincating is performed in the manufacturing process of the magnetic disk, unevenness in the amount of Al dissolved on the surface of the aluminum alloy substrate tends to occur, and the thickness of the Zn film tends to vary greatly. As a result, there is a risk of a decrease in adhesion between the Ni-P plating layer and the aluminum alloy substrate, and a decrease in the smoothness of the Ni-P plating layer.

[0022] By setting the Cu content in the aluminum alloy to preferably 1.00 mass% or less, more preferably 0.50 mass% or less, the rigidity and strength of the aluminum alloy disk blank can be further increased, the formation of plating pits can be suppressed, and the smoothness of the Ni-P plating layer can be further improved. The lower limit of the Cu content is preferably 0.005 mass%.

[0023] Zn: 0.70 mass% or less The aluminum alloy may contain 0.70 mass% or less Zn as an optional component. Zn reduces the amount of Al dissolved during zincate treatment of an aluminum alloy disk blank, and also helps the zincate coating formed during zincate treatment to adhere uniformly, thinly, and densely, thereby improving smoothness and adhesion in the subsequent plating process. Furthermore, Zn forms second-phase particles with other additive elements, thereby improving Young's modulus and strength.

[0024] However, if the Zn content is high, the corrosion resistance of the aluminum alloy disk blank decreases, and regions where Al is easily dissolved are formed in the aluminum alloy disk blank. Therefore, when zincating is performed in the manufacturing process of the magnetic disk, unevenness in the amount of Al dissolved on the surface of the aluminum alloy substrate occurs, and the thickness of the Zn film tends to vary greatly. As a result, there is a risk of a decrease in adhesion between the Ni-P plating layer and the aluminum alloy substrate and a decrease in the smoothness of the Ni-P plating layer.

[0025] By setting the Zn content in the aluminum alloy to preferably 0.70 mass% or less, more preferably 0.50 mass% or less, the rigidity and strength of the aluminum alloy disk blank can be further increased, the formation of plating pits can be suppressed, and the smoothness of the Ni—P plating layer can be further improved. The lower limit of the Zn content is preferably 0.10 mass%.

[0026] Mg: 4.50 mass% or less The aluminum alloy may contain 4.50 mass% or less of Mg as an optional component. Mg is mainly present as a solid solution in the matrix, with a portion present as second-phase particles (e.g., Mg-Si intermetallic compounds). This enhances the strength and rigidity of the aluminum alloy disk blank.

[0027] However, if the Mg content is high, coarse Al-Mg intermetallic compounds are generated, and when the aluminum alloy disk blank is etched, zincated, cut, or ground, the intermetallic compounds fall off, causing large dents and reducing the surface smoothness. In addition, a high Mg content increases the strength, which can lead to cracks during rolling.

[0028] By setting the Mg content in the aluminum alloy to preferably 4.50 mass% or less, more preferably 3.50 mass% or less, and even more preferably 3.00 mass% or less, the strength and rigidity of the aluminum alloy disk blank can be further increased. The lower limit of the Mg content is preferably 1.00 mass%.

[0029] Cr: 0.30 mass% or less The aluminum alloy may contain 0.30 mass% or less of Cr as an optional component. A portion of the Cr disperses in the aluminum alloy as fine intermetallic compounds formed during casting, improving the rigidity of the aluminum alloy disk blank. Cr that does not form intermetallic compounds during casting of the aluminum alloy dissolves in the Al matrix, improving the strength of the aluminum alloy disk blank through solid solution strengthening.

[0030] Furthermore, Cr can further improve the machinability and grindability of the aluminum alloy disk blank and refine the recrystallized structure, thereby further improving the adhesion between the aluminum alloy substrate and the Ni—P plating layer and suppressing the occurrence of plating pits.

[0031] However, if the Cr content in the aluminum alloy is excessively high, coarse Al-Cr intermetallic compounds are likely to be formed in the aluminum alloy disk blank, and if such coarse Al-Cr intermetallic compounds fall off from the surface of the aluminum alloy disk blank, plating pits are likely to be formed in the subsequent electroless Ni-P plating treatment.

[0032] By setting the Cr content in the aluminum alloy to 0.30 mass% or less, the rigidity and strength of the aluminum alloy disk blank can be further improved. Furthermore, the occurrence of plating pits can be more effectively suppressed, and the smoothness of the Ni—P plating layer can be further improved. The lower limit of the Cr content is preferably 0.030 mass%.

[0033] Zr: 0.15 mass% or less The aluminum alloy may contain 0.15 mass% or less Zr as an optional component. A portion of the Zr disperses in the aluminum alloy disk blank as fine intermetallic compounds formed during casting of the aluminum alloy, improving rigidity. Zr that does not form intermetallic compounds during casting of the aluminum alloy dissolves in the Al matrix, improving the strength of the aluminum alloy disk blank through solid solution strengthening.

[0034] Furthermore, Zr can further improve the machinability and grindability of the aluminum alloy disk blank and refine the recrystallized structure, thereby further improving the adhesion between the aluminum alloy substrate and the Ni—P plating layer and suppressing the occurrence of plating pits.

[0035] However, if the Zr content in the aluminum alloy is excessively high, coarse Al-Zr intermetallic compounds are likely to be formed in the aluminum alloy disk blank, and if such coarse Al-Zr intermetallic compounds fall off from the surface of the aluminum alloy disk blank, plating pits may be likely to be formed in the subsequent electroless Ni-P plating process.

[0036] By setting the Zr content in the aluminum alloy to 0.15 mass% or less, it is possible to suppress the formation of plating pits, form a smooth Ni—P plating layer, and further improve the rigidity and strength of the aluminum alloy disk blank. The lower limit of the Zr content is preferably 0.050 mass%.

[0037] The aluminum alloy may contain 14.00 mass% or less of Si as an optional component. Si exists mainly as second-phase particles (Si particles, Al-Fe-Si intermetallic compounds, etc.) and has the effect of improving the rigidity and strength of the aluminum alloy disk blank.

[0038] However, if the Si content in the aluminum alloy is excessively high, coarse particles and intermetallic compounds are likely to be formed in the aluminum alloy disk blank, and if such coarse particles and intermetallic compounds fall off from the surface of the aluminum alloy disk blank, plating pits may be likely to be formed in the subsequent electroless Ni—P plating process.

[0039] By setting the Si content in the aluminum alloy to 14.00 mass% or less, the rigidity and strength of the aluminum alloy disk blank can be further improved. The lower limit of the Si content is preferably 0.10 mass%.

[0040] Be: 0.0015 mass% or less Be is an element added to the molten metal when casting an aluminum alloy containing Mg in order to suppress oxidation of Mg. Furthermore, by including 0.0015 mass% or less of Be in the aluminum alloy, 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 Ni—P treatment layer formed on the aluminum alloy substrate can be further improved.

[0041] However, if the Be content in the aluminum alloy is excessively high, Be-based oxides are likely to form on the surface of the disk blank when the disk blank is heated during the manufacturing process of the aluminum alloy disk blank. Furthermore, if the aluminum alloy further contains Mg, Al-Mg-Be-based oxides are likely to form on the surface of the disk blank when the aluminum alloy disk blank is heated. If the amount of these oxides is large, the thickness of the Zn coating will vary greatly, which may lead to the occurrence of plating pits.

[0042] By setting the Be content in the aluminum alloy to preferably 0.0015 mass% or less, more preferably 0.0010 mass% or less, the amount of Al-Mg-Be oxides can be reduced and the smoothness of the Ni-P plating layer can be further improved.

[0043] Sr, Na, and P: 0.10 mass% or less. The inclusion of Sr, Na, and P in the aluminum alloy reduces the size of second-phase particles (mainly Si particles) in the aluminum alloy disk blank, thereby improving plating properties. It also reduces the non-uniformity of the size of second-phase particles in the aluminum alloy disk blank, thereby reducing the variation in impact resistance properties of the aluminum alloy disk blank. Therefore, one or more elements selected from the group consisting of preferably 0.10 mass% or less Sr, preferably 0.10 mass% or less Na, and preferably 0.10 mass% or less P may be selectively added to the aluminum alloy. However, even if each of Sr, Na, and P exceeds 0.10 mass%, the effect saturates and no further significant improvement is obtained. To achieve the above effect, it is more preferable that each of Sr, Na, and P is 0.001 mass% or more.

[0044] Other Elements The aluminum alloy may contain elements that are unavoidable impurities other than the essential and optional elements described above. Examples of these elements include Ti, B, Si, and Ga. The effects of the present invention are not impaired as long as the content of each element is 0.05 mass% or less, and the total content is 0.15 mass% or less. As described above, in the present invention, Si can be actively added as an optional element, but it may also be contained as an unavoidable impurity without being actively added. Si is contained as an unavoidable impurity not only in general purity ingots but also in high-purity ingots with an Al purity of 99.9% or more. Even when Si is contained as an unavoidable impurity, the effects of the present invention are not impaired as long as the content is 0.100 mass% or less. When Si is actively added as an optional element, the Si content in the aluminum alloy is preferably 14.00 mass% or less, in order to further improve the rigidity and strength of the aluminum alloy disk blank as described above.

[0045] Flatness Change of Aluminum Alloy Disk Blank The aluminum alloy disk blank according to the present invention exhibits a flatness change (the absolute value of the difference between the flatness before and after holding) of 2.0 μm or less when held in air at 50° C. or less for 336 hours. By minimizing the flatness change to 2.0 μm or less, convex defects can be reduced and the surface smoothness can be improved. Aluminum alloy disk blanks exhibiting a flatness change of more than 2.0 μm are prone to generating residual stress during surface cutting or grinding, and the residual stress does not completely disappear even after annealing after processing. When sputtering is performed, the residual stress is released, generating convex defects and reducing the surface smoothness. Therefore, the aluminum alloy disk blank according to the present invention exhibits a flatness change of 2.0 μm or less when held in air at 50° C. or less for 336 hours. The flatness change of the aluminum alloy disk blank is preferably 1.0 μm or less, more preferably 0.9 μm or less, and even more preferably 0.8 μm or less.

[0046] In the present invention, flatness is expressed as the difference between the maximum peak height and the maximum valley depth over the entire surface of an aluminum alloy disk blank. Here, the maximum peak height is the difference between the average line of the profile curve in the measurement range and the highest value within the measurement range, and the maximum valley depth is the difference between the average line and the lowest value within the measurement range. FIG. 1 is a diagram showing the measurement range for the flatness of an aluminum alloy disk blank 2. As shown in FIG. 1, when the radius from the center 1 of the aluminum alloy disk blank 2 to the inner diameter of the aluminum alloy disk blank 2 is Rx (mm), and the radius from the center 1 of the aluminum alloy disk blank 2 to the outer diameter of the aluminum alloy disk blank 2 is Ry (mm), the measurement range is the area enclosed by a circle of Rx+1 (mm) and a circle of Ry-1 (mm). The aluminum alloy disk blanks used for measuring flatness were stored at 0 to 50°C within one hour after peeling each aluminum alloy disk blank after pressure annealing. In addition, preliminary tests have confirmed that if the disk blank is separated after pressure annealing and then stored at 0 to 50°C within one hour, the change in flatness when the same aluminum alloy disk blank is held at 50°C or less for 336 hours is not affected by temperature and shows the same value.

[0047] Crystal Grain Size The crystal grain size on the surface of the aluminum alloy disk blank according to the present invention is preferably 18 μm or less, more preferably 15 μm or less, even more preferably 12 μm or less, particularly preferably 11 μm or less, and even more particularly preferably 10 μm or less. By reducing the crystal grain size in this way, convex defects can be reduced. More specifically, an aluminum alloy disk blank with a small crystal grain size as described above has an increased number of crystal grain boundaries, which increases deformation resistance and suppresses changes in flatness. As a result, residual stress is less likely to occur during surface cutting or grinding, and residual stress is effectively eliminated even during annealing performed after processing. This reduces the occurrence of convex defects when sputtering is performed, improving smoothness. Therefore, the crystal grain size is preferably 18 μm or less, more preferably 15 μm or less, even more preferably 12 μm or less, particularly preferably 11 μm or less, and even more particularly preferably 10 μm or less.

[0048] A-2. Manufacturing Method of Aluminum Alloy Sheet (1) Casting Process Aluminum raw material with a predetermined alloy composition is melted to produce a molten metal, which is then cast to produce an ingot. 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 from the bottom block, the water-cooled mold wall, and cooling water directly ejected onto the outer periphery of the ingot, solidifies, and is drawn downward as an ingot. In permanent mold casting, molten metal poured into a hollow mold made of cast iron or the like loses heat from the mold wall, solidifies, and produces an ingot. In CC casting, molten metal is supplied through a casting nozzle between a pair of rolls (or belt casters, block casters), and a thin plate is directly cast by extracting heat from the rolls.

[0049] 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.

[0050] For example, the degassing process may 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 in 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 may be used for the degassing process.

[0051] The filtration treatment can be performed using, for example, a cake filtration method or a filter medium filtration method, etc. Furthermore, the filtration treatment can be performed using, for example, a ceramic tube filter, a ceramic foam filter, an alumina ball filter, or the like.

[0052] (2) Homogenization Treatment Step After the ingot is produced, the ingot may be subjected to facing and homogenization treatment as needed before hot rolling. The holding temperature in the homogenization treatment can be appropriately set within the range of, for example, 500 to 570°C. The holding time in the homogenization treatment can be appropriately set within the range of, for example, 1 to 60 hours.

[0053] (3) Hot Rolling Step 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 with a starting temperature in the range of 400 to 550°C and a finishing temperature in the range of 260 to 380°C.

[0054] (4) Cold Rolling Step 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 higher the total reduction in cold rolling, the smaller the grain size, so the total reduction in cold rolling is preferably 70% or more, more preferably 80% or more. There is no upper limit to the total reduction in cold rolling, but if it is too high, the cold rolling time will be longer and costs will increase, so it is set to about 95%. The thickness of the cold-rolled sheet can be appropriately set, for example, within the range of 0.2 to 1.9 mm.

[0055] (5) Annealing Step 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 250 to 430°C and the holding time is preferably in the range of 0.1 to 10 hours. Furthermore, when a continuous heat treatment furnace is used, the residence time in the furnace is preferably 60 seconds or less and the temperature in the furnace is preferably 400 to 500°C. By performing the annealing treatment under these conditions, the workability during cold rolling can be restored. An aluminum alloy sheet is produced by the above steps.

[0056] A-3. Manufacturing Method of Aluminum Alloy Substrate: The aluminum alloy plate is punched to produce an annular aluminum alloy disk blank. The aluminum alloy disk blank is then pressure-annealed by heating while being pressurized from both sides in the thickness direction, thereby reducing distortion of the aluminum alloy disk blank and improving its flatness. The holding temperature and pressure in the pressure annealing can be appropriately selected, for example, from the ranges of 250 to 430°C and 1.0 to 3.0 MPa. The holding time in the pressure annealing can be, for example, 30 minutes or more.

[0057] After pressure annealing and peeling of each aluminum alloy disk blank, it is preferable to store it in the air at a temperature range of −10°C or lower. By storing it at −10°C or lower before cutting and grinding, it is possible to suppress changes in the flatness of the disk blank. By reducing the change in flatness in this way, it is possible to reduce convex defects in the aluminum alloy disk blank and improve surface smoothness. Aluminum alloy disk blanks with a change in flatness of more than 2.0 μm are prone to generating residual stress during surface cutting and grinding, and the residual stress does not completely disappear even during annealing performed after processing. When sputtering is performed, the residual stress is released, causing convex defects and reducing smoothness. Therefore, after pressure annealing and peeling of the aluminum alloy disk blank, it is preferable to store it in the air at a temperature range of −10°C or lower, more preferably at −20°C or lower, and even more preferably at −25°C or lower. Although there is no particular lower limit for the storage temperature, if the temperature is too low, costs increase, so the temperature is preferably −40° C. or higher, more preferably −38° C. or higher, and even more preferably −35° C. or higher. Note that the above storage step is different from the storage of aluminum alloy disk blanks at 0 to 50° C. for one hour or less, which is performed when measuring flatness.

[0058] Next, the aluminum alloy disk blank is subjected to cutting and grinding processes in order to produce an aluminum alloy substrate having a desired shape. After these processes, a stress relief heat treatment is performed at 150 to 350°C for 0.1 to 10.0 hours to remove stress generated during processing. Through these steps, an aluminum alloy substrate is produced.

[0059] B. Magnetic Disk B-1. Configuration of Magnetic Disk The magnetic disk equipped with the aluminum alloy substrate has, for example, the following configuration. That is, the magnetic disk has an aluminum alloy substrate made of an aluminum alloy disk blank, a Ni—P plating layer covering the surface of this aluminum alloy substrate, and a magnetic layer laminated on this Ni—P plating layer. It is preferable that the Ni—P plating layer is an electroless Ni—P plating layer formed by electroless plating.

[0060] 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.

[0061] B-2. Method for Manufacturing Magnetic Disk When manufacturing a magnetic disk from an aluminum alloy substrate, for example, the following method can be adopted. First, the aluminum alloy substrate is degreased and cleaned to remove oils such as processing oil 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 set appropriately depending on the type of treatment solution.

[0062] After these pre-plating treatments, a zincate treatment is performed to form a Zn film 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 film. A so-called double zincate method is preferably employed as the zincate treatment, in which the Zn film formed on the surface of the aluminum alloy substrate after the first zinc immersion plating is performed is once stripped, and then a Zn film is formed by performing another zinc immersion plating. The double zincate method allows a denser Zn film to be formed on the surface of the aluminum alloy substrate compared to a Zn film formed by only the first zinc immersion plating. As a result, defects in the Ni—P plating layer can be reduced in the subsequent electroless Ni—P plating treatment.

[0063] After forming a Zn film on the surface of an aluminum alloy substrate by zincate treatment, electroless Ni—P plating treatment can be performed to replace the Zn film with a Ni—P plating layer. By replacing such a Zn film with a Ni—P plating layer in the electroless Ni—P plating treatment, a smooth Ni—P plating layer with few plating pits can be formed.

[0064] Increasing the thickness of the Ni—P plating layer tends to reduce plating pits, allowing for the formation of a smooth Ni—P plating layer. Therefore, the thickness of the Ni—P plating layer 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 thickness of the Ni—P plating layer is approximately 40 μm.

[0065] By polishing the Ni--P plated layer after the electroless Ni--P plating process, the smoothness of the surface of the Ni--P plated layer can be further improved.

[0066] After electroless Ni-P plating (including polishing), a magnetic material is deposited on the Ni-P plating layer by sputtering to form a magnetic layer. The magnetic layer may be composed of a single layer, or may be composed of multiple layers with different compositions. After sputtering, a protective layer made of a carbon-based material is formed on the magnetic layer by CVD, if necessary. Next, a lubricating oil is applied to the protective layer to form a lubricating layer. This completes the magnetic disk.

[0067] Examples of an aluminum alloy plate, a manufacturing method thereof, and an aluminum alloy disk blank manufactured from the aluminum alloy plate will be described below. Note that the specific aspects of the aluminum alloy plate, the manufacturing method thereof, the aluminum alloy disk blank, and the manufacturing method thereof according to the present invention are not limited to the aspects of the examples shown below, and the configurations can be appropriately changed from the examples within the scope of the present invention.

[0068] (1) Preparation of Aluminum Alloy Sheets Aluminum alloy sheets used for evaluation in each example were prepared by the following method: First, a molten metal having the chemical compositions shown in Table 1 was prepared in a melting furnace.

[0069] Next, the molten metal in the melting furnace was transferred, and an ingot was produced by the casting method 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 a heat treatment under the conditions shown in Table 2, thereby carrying out a homogenization treatment. Next, hot rolling was carried out under the conditions shown in Table 2 to obtain a hot-rolled sheet. Furthermore, cold rolling was carried out under the conditions shown in Table 2 to obtain an aluminum alloy sheet.

[0070] (2) Preparation of Aluminum Alloy Disk Blanks 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. The obtained aluminum alloy disk blank was then subjected to pressure annealing by holding it at the temperature shown in Table 2 for 3 hours while applying pressure from both sides in the thickness direction. Thereafter, the aluminum alloy disk blank was stored under the conditions shown in Table 2. In this manner, test materials of the aluminum alloy disk blanks of each example were prepared.

[0071] The evaluation method for each test material is explained below. Note that B3 had a small amount of Fe and was too low in strength, so it was prone to deformation during peeling after pressure annealing and could not be used as an aluminum alloy disk blank, so it was not evaluated. Also, B4 had a large amount of Fe and was too high in strength, so it cracked during rolling and could not be used as an aluminum alloy disk blank, so it was not evaluated.

[0072] -Method of measuring crystal grain size First, the crystal grain size was measured on the rolled surface of a test material of an aluminum alloy disk blank for microstructure observation using a glow discharge optical emission spectrometer (Glow Discharge Spectroscopy: GDS, JY5000RF, manufactured by HORIBA) at a gas pressure of 400 Pa and an output of 30 W for 60 seconds. The texture orientation information was obtained by measuring the sputtered surface with an electron backscatter diffraction (SEM-EBSD) attached to a scanning electron microscope. The measurement area of ​​the sample was 1000 μm × 1000 μm, and the measurement step interval was 3 μm when the crystal grain size was 12 μm or more, 2 μm when the crystal grain size was 8 μm or more but less than 12 μm, and about 1 / 4 when the crystal grain size was less than 8 μm.

[0073] From the obtained orientation data, the grain size was measured using EBSD analysis software ("OIM Analysis" manufactured by TSL). At this time, a grain boundary with a misorientation of 5° or more was regarded as a grain boundary, and the diameter calculated as an equivalent circle was taken as the grain size.

[0074] - Investigation of flatness change First, the flatness of the aluminum alloy disk blank that had been pressure annealed and, if necessary, stored as described above was measured within 1 hour after peeling. Thereafter, the aluminum alloy disk blank was placed flat on a surface plate or the like and left at the temperature and time shown in Table 2. The flatness was measured again, and the absolute value of the difference in flatness before and after leaving {(flatness before leaving at 50°C or less for 336 hours) - (flatness after leaving at 50°C or less for 336 hours)} was calculated as the flatness change. The meaning of flatness is as described above. The flatness was measured using a ZyGO flatness measuring instrument (MESA). When the flatness change was 2 μm or less, the fluttering property was rated A (excellent), and when it exceeded 2 μm, the fluttering property was rated D (poor). The results are shown in Table 2. In Table 2, "-" indicates that no treatment or evaluation was performed.

[0075]

[0076]

[0077] As shown in Tables 1 and 2, test materials A1 to A12 have the chemical composition of the present invention, and when the disk blanks are held in the air at 50°C or less for 336 hours, the change in flatness of the disk blanks is 2.0 μm or less. Therefore, these test materials can improve the surface smoothness.

[0078] For test materials B1 and B2, the change in flatness of the disk blank exceeded 2.0 μm when the disk blank was held in the air at 50° C. or less for 336 hours. As a result, convex defects were likely to occur, and the smoothness was reduced.

[0079] 1. Center of aluminum alloy disc blank 2. Aluminum alloy disc blank

Claims

1. An aluminum alloy disk blank for a magnetic disk, comprising an aluminum alloy containing 0.005 to 1.800 mass% Fe, the remainder being Al and unavoidable impurities, characterized in that when the aluminum alloy disk blank for a magnetic disk is held in the atmosphere at 50°C or less for 336 hours, the change in flatness of the aluminum alloy disk blank for a magnetic disk is 2.0 μm or less.

2. The aluminum alloy disk blank for magnetic disks according to claim 1, wherein the aluminum alloy further contains one or more elements selected from the group consisting of Mn: 1.80 mass% or less, Ni: 2.50 mass% or less, Cu: 1.00 mass% or less, Zn: 0.70 mass% or less, Mg: 4.50 mass% or less, Cr: 0.30 mass% or less, Zr: 0.15 mass% or less, Si: 14.00 mass% or less, Be: 0.0015 mass% or less, Sr: 0.10 mass% or less, Na: 0.10 mass% or less, and P: 0.10 mass% or less.

3. An aluminum alloy disk blank for a magnetic disk according to claim 1 or 2, characterized in that the crystal grain size on the surface of said aluminum alloy disk blank for a magnetic disk is 18 μm or less.

4. An aluminum alloy disk blank for a magnetic disk according to claim 1 or 2, characterized in that the crystal grain size on the surface of said aluminum alloy disk blank for a magnetic disk is 15 μm or less.

5. An aluminum alloy disk blank for a magnetic disk according to claim 1 or 2, characterized in that the crystal grain size on the surface of said aluminum alloy disk blank for a magnetic disk is 12 μm or less.

6. An aluminum alloy disk blank for a magnetic disk according to claim 1 or 2, characterized in that the crystal grain size on the surface of said aluminum alloy disk blank for a magnetic disk is 11 μm or less.

7. An aluminum alloy disk blank for a magnetic disk according to claim 1 or 2, characterized in that the crystal grain size on the surface of said aluminum alloy disk blank for a magnetic disk is 10 μm or less.

8. A magnetic disk comprising an aluminum alloy substrate made of an aluminum alloy disk blank for magnetic disks according to any one of claims 1 to 7, and a Ni-P plating layer on the surface of the aluminum alloy substrate, and a magnetic layer on the Ni-P plating layer.