JPH02111839A - Aluminum alloy sheet for disk having superior plating suitability and its production - Google Patents

Abstract

PURPOSE:To obtain an Al alloy sheet having a smooth plated surface minimal in plating defects by specifying the composition of an Al alloy containing Cu and Zn and also specifying respective sizes and quantities of an intermetallic compound composed principally of Al-Fe and an intermetallic compound composed principally of Mg-Si. CONSTITUTION:An Al alloy having a composition which contains, as essential components, >3-6%, by weight, Mg, 0.03-<0.3% Cu, and 0.03-0.4% Zn and in which the amounts of Fe and Si among impurities are regulated to <=0.07% and <=0.06%, respectively, in the case of semicontinuous casting and also to <=0.1% and <=0.1%, respectively, in the case of continuous casting of steel sheet is cast at >=710 deg.C. The resulting ingot is subjected to soaking treatment at >=450 deg.C is then subjected to hot rolling at >=500 deg.C initial temp. and to cold rolling. By this method, the Al alloy sheet in which the size of an intermetallic compound composed principally of Al-Fe and the number of the above intermetallic compounds of <=5mu are regulated to <=10mu and <=5 pieces/2mm<2>, respectively, and also the size of an intermetallic compound composed principally of Mg-Si and the number of the above intermetallic compounds of >=5mu are regulated to <=8mu and <=5 pieces/2mm<2>, respectively, and which has superior surface accuracy after polishing minimal in nodules and micropits can be obtained.

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to an aluminum alloy plate for disks having excellent plating properties, and more specifically, a plating layer is formed on the surface of an aluminum alloy substrate for disks such as magnetic disks used as recording media for computers. When subjected to the process of forming nodules, it has excellent surface treatment properties and plating properties, and also has excellent nodule forming properties. The present invention relates to an aluminum alloy plate for disks having fewer micropits and excellent surface precision after polishing, and a method for manufacturing the same. (Prior Art) In general, a substrate for a disk such as a magnetic disk used as a recording medium for a computer must be lightweight, non-magnetic, and rigid enough to withstand high-speed rotation.
From the viewpoints that good surface precision can be obtained by precision cutting or polishing, and furthermore, it has a certain degree of corrosion resistance, etc.
Aluminum alloy is used. Conventional Example 1: Taking a magnetic disk substrate as an example, the main method for forming a magnetic film on such an aluminum alloy substrate for a magnetic disk was a coating method, but in recent years, with the increasing density of 1m disks, For this purpose, plating methods and sputtering methods have been developed and are being applied. In the case of magnetic disk substrates made by these plating methods or sputtering methods,
Plating such as N1-P is applied as a base treatment. Conventionally, AA5086 alloy (Al-
Mg type) is the most commonly used, and in some cases JI
S, 7075 alloy (Al-Zn-Mg based) is also used. (Problem to be solved by the invention) However, when these conventional aluminum alloys are used as base materials,
A crystallized product whose main component is A Q -Fe with a diameter exceeding 0 μm (
Because there are many intermetallic compounds such as Al-Fe, Aff-Fe-5i, Al-Fe-Mn, etc.) and crystallized substances whose main component is Mg-8L, it is necessary to During processing, these coarse intermetallic compounds fall off from the aluminum alloy base and form holes, which has the disadvantage that the surface after base plating tends to become rough. In particular, when JIS 7075 alloy (Al-Zn-Mg system) is used as a base material, since this aluminum alloy contains excessive amounts of Cu and Zn, these coarse intermetallic compounds also form on the surface after plating. Not only does this cause the surface to become rough, but since it is a heat treatable alloy, if the cooling rate is not properly adjusted during annealing to remove distortion from a disk punched or machined from a rolled plate, Another disadvantage was that internal stress was generated. As explained above, conventional aluminum alloys for disks have the drawback that the roughness of the disk surface tends to increase, or that plating defects such as pits (small holes) tend to occur due to this. In order to eliminate such defects, the plating film should be coated with a thickness of, for example, 30 to 50
The method used was to form a relatively thick layer of around μm and then finish it by polishing. However, there is a problem in that increasing the thickness of the plating film requires a large amount of cost for plating and polishing. Therefore, in order to reduce costs and improve productivity, it has become an important issue to reduce the thickness of the plating film. Furthermore, apart from the thickness of the plating film, it is also important to reduce the number of pits and to reduce the roughness in the plating pretreatment to improve the efficiency of the plating process. Therefore, for example, purity is 99.9% or 99.99%.
Attempts have also been made to improve the quality of the intermetallic compounds by using Aβ base metal, but simply increasing the purity of the Afl base metal used will not only increase the roughness of the plated surface. , a problem arises in that the adhesion of the plating layer also decreases. The inventor believes that these causes are due to the precipitation of zinc due to the reduced Fe content in the aluminum material due to the use of high-purity Al base metal during zinc replacement, which is generally used as a pre-plating method for aluminum alloys. It was found that this is due to the roughness and non-uniformity of the grains. Therefore, the inventor of the present invention investigated various problems with conventional aluminum materials as explained above, and as a result of extensive research to satisfy many demands for magnetic disks, the inventors added Cu and Zn as alloying elements. They discovered that plating properties could be improved by doing so, and previously proposed an aluminum alloy for disks (Japanese Patent Publication No. 62-2018). The aluminum alloy for discs according to this proposal has already been put into practical use and has received high praise. However, magnetic disks are increasingly required to have a cost reduction of 1-down, and therefore it is important to further reduce the thickness of the base plating and to reduce the polishing method during polishing. Therefore, how to obtain a smooth plated surface by suppressing the formation of hemispherical protrusions (so-called nodules) on the disk surface after base plating, and
A major development issue is how to reduce small pits (so-called micropits) in the extreme surface layer, which have not been a problem in the past. The object of the present invention is to satisfy the various requirements for disks such as magnetic disks and optical disks as well as aluminum materials for disks as described above, to have excellent surface treatment properties and plating adhesion, and to have micro-pits. An object of the present invention is to provide an aluminum alloy plate for a disk, which has fewer plating defects such as nodules and a smooth plated surface. Another object of the present invention is to provide a method for manufacturing an aluminum alloy plate for discs at low cost. (Means for Solving the Problems) In view of the above circumstances, the inventors of the present invention have conducted various studies and found that the generation of nodules and micro pits on the plated surface is caused by the formation of the substrate surface after pre-treatment for plating. There is a strong correlation with unevenness, and in order to achieve this, it is necessary to reduce intermetallic compounds (so-called crystallized substances and coarse compounds) that cause unevenness on the substrate surface after the plating process. Unlike the previous studies, where various intermetallic compounds were considered to be the subject of investigation all at once, it is now necessary to distinguish between intermetallic compounds whose main component is A-Fe and intermetallic compounds whose main component is Mg-Si. We have discovered that it is possible by separately specifying different sizes and amounts, and at the same time imparting plating properties by adjusting the composition, etc., and have hereby accomplished the present invention. That is, the present invention uses 3% <Mg56% as an essential element.
, 0.03%≦Cu<0.3% and 0.03%≦Zn≦
Fe50.07%, Si≦0.06% in the case of semi-continuous casting, Fe50.1%, Si≦0.1 in the case of continuous thin plate casting. % and has a regulated composition. The maximum size of the intermetallic compound containing A Q -Fe as the main component is 10 μm or less, and the number of the intermetallic compounds with a size of 5 μm or more is 5 pieces and 10.2 mm” or less,
Furthermore, the maximum size of the intermetallic compound containing Mg-Si as a main component is 8 μm or less, and the number of the intermetallic compounds with a size of 5 μm or more is 5 pieces and 10.2 mm2 or less. The object of the present invention is to provide an aluminum alloy plate for disks, which has excellent surface treatment properties and plating adhesion, and has excellent surface precision with few nodules and micropits. In addition, the manufacturing method requires 3% Mg56 as an essential element.
%, 0.03%≦Cu<0.3% and 0.03%≦Zn
≦0.4%, and Fe and Si among impurities are replaced with Fe.
50.07%, Si≦0.06%, is cast at 710°C or higher, and the resulting ingot is subjected to soaking treatment at 450°C or higher, followed by a hot rolling start temperature of 500''. It is characterized by performing hot rolling and cold rolling of C or higher.Furthermore, another manufacturing method includes 3% Mg56 as an essential element.
%, 0.03%≦Cu<0.3% and 0.03%≦Zn
≦0.4%, and Fe and Si among impurities are replaced with Fe.
An aluminum alloy having a composition controlled to 50.1% and Si≦0.1% was cast at a temperature of 690°C or higher to obtain a thin continuous cast coil with a thickness of 3mm or more. This is characterized by cold rolling. The present invention will be explained in more detail below. (Function) First, the reason for limiting the chemical components in the present invention will be explained. M: Mg is an element necessary to provide mechanical strength necessary for the disk base. However, if the content is less than 3%, this effect is insufficient, and if it exceeds 6%, edge cracking tends to occur during rolling, productivity decreases, and high-temperature oxidation during melting and casting causes MgO, etc. Non-metallic inclusions tend to form, which is not preferable. Therefore, 1M cost is 3%
<Mg≦6%. Cu, Zn: Cu and Zn provide uniform etching properties during pre-plating treatment, and have the effect of uniformly and finely depositing zinc on the substrate surface during zinc replacement treatment. This reduces the roughness of the base plating film surface and contributes to improving the adhesion of the film. However, if Cu and Zn are each less than 0.03%, the above effects cannot be obtained, and if Cu is more than 0.3% or Zn is more than 0.4%, nodules may be generated. This is not preferable because it may result in a large amount of etchant or excessive etching of grain boundaries, impairing the smoothness of the plated surface. Therefore, Cu m is 0.03%≦CI<0.3%
Cu: 0.05-0.2%, more preferably Cu: 0.05-0.2%
and ZrJl is in the range of 0°03≦Zn<0.4%, more preferably Zn:O, '06-0.3%. Note that even if Cu and Zn are within the above ranges, if each is added alone, the difference in chemical reactivity between crystal grains will be significant, resulting in a change in grain size). Cu and Zn are added at the same time because Jl causes a step difference, which is undesirable. Intermetallic compounds mainly composed of AQ-Fe such as Al-Fe, AM-Fe-Si, Al-Fe-Mn, and Mg
Intermetallic compounds mainly composed of -Si are caused by Fe and SL mixed in as base metal impurities, but they are processed as substrates for disks, and during cutting, polishing, and Both of these are undesirable because they cause protrusions or depressions due to falling off during processing steps such as grinding, or they fall off or melt during the plating pretreatment step, causing micro-pits 1- on the plated surface. In particular, in the plating pretreatment process, A
If there is an intermetallic compound mainly composed of l-Fe or an intermetallic compound mainly composed of Mg-5i exceeding 8 μm, the amount of etching during the plating pretreatment process is l Omg.
Even under mild etching conditions of /dm2 or less, this is not preferable because it causes micro bits on the surface of the plating film. Furthermore, even if the maximum value of the intermetallic compound whose main component is AQ-Fe is 10 μm or less, and the maximum value of the intermetallic compound whose main component is Mg-5i is 8 μm or less,
5 intermetallic compounds whose main component is Al-Fe with a diameter of 5 μm or more are present in an area exceeding 70.2 mm2, or 5 intermetallic compounds whose main component is Mg-Si with a diameter of 5 μm or more are 10.2
mm", it is not preferable because it may have the same adverse effect as one coarse intermetallic compound. Therefore, the maximum size of the intermetallic compound whose main component is Al-Fe. is 10 μm or less, and five of the intermetallic compounds each having a size of 5 μm or more are 10.2 mm or less, and furthermore, the maximum size of the intermetallic compound whose main component is Mg-Si is 8 μm or less. Moreover, 5μ
The number of intermetallic compounds having a diameter of 5 m or more is 70.2 mm 2 or less. However, the amount of etching during the plating pretreatment process was 11 to 2
6μm under strong etching conditions of 0mg/dm” or more
If an intermetallic compound whose main component is A Q -Fe exceeding 5 μm or an intermetallic compound whose main component is Mg-Si exceeds 5 μm is present, it is not preferable because it causes micro pits on the surface of the plating film. Furthermore, even if the maximum size of the intermetallic compound whose main component is Al-Fe is 6 μm or less and the maximum size of the intermetallic compound whose main component is 1Mg-5i is 5 μm or less, Al-
5 pieces of intermetallic compound mainly composed of Fe 10.2mm"
If five intermetallic compounds mainly composed of Mg-8L with a diameter of 4 μm or more are present in a size exceeding 10.2 mm, it may have substantially the same adverse effect as one coarse compound. Therefore, it is more preferable that the maximum size of the intermetallic compound containing Al-Fe as the main component is 6 μm or less.
m2 or less, and furthermore, the maximum value of the intermetallic compound containing Mg-8L as a main component is 5 μm or less, and 4 μm or less.
The number of the intermetallic compounds having a size of 5 m or more is 10.2 mm 2 or less. The size and amount of these intermetallic compounds (those whose main component is Al-Fe, those whose main component is Mg-Si) are greatly influenced by the content of Fe and Si in the material. However, the influence of casting conditions, soaking treatment conditions, etc. is also large. For example, casting conditions can be reduced by increasing the casting temperature during U-making, reducing the thickness of the cast plate as in so-called continuous thin plate casting, and increasing the cooling rate during solidification. Therefore, the content of impurities Fe and Si in the aluminum alloy in the present invention is not uniquely determined, but in the case of the semi-continuous casting method, Fe50.07
%, Si≦0.06% is desirable. More preferably, F
e≦0.04%, SiS2.04%. In addition, in the case of continuous thin plate casting method, Fe50.1%, Si≦0.1%,
More preferably Fe50.07%, Si≦0.06%
It is. There is no particular lower limit for the content of Fe and Si, but in the case of intermetallic compounds with a thickness of 2 μm or less, plating defects are unlikely to occur unless the plating film thickness is extremely thin (for example, 3 μm or less); Furthermore, in the case of Si, Fe>0°003% and Si>0.005% are desirable from an economical point of view, since it can be reduced by the soaking treatment conditions described later. In addition, in the aluminum alloy for disks according to the present invention, Mn, Cr, Ti, etc. other than these impurities are used to refine recrystallized grains, prevent grain gloss during high-temperature heat treatment, refine U structure, etc. Because it has the effect of
Although it may be included within the range allowed by the JI35086 alloy, it may cause coarsening of the above-mentioned intermetallic compounds. Since it may cause inclusion, Mn≦
Desirably, the content is 0.4%, Cr≦0.1%, and Ti≦0.1%. Further, although elements such as B and Be are often added during melting and casting, it is desirable that each element be at loop per million or less. Next, a method for manufacturing an aluminum alloy plate for a disk according to the present invention will be explained. First, as a casting method, for the aluminum alloy having the above-mentioned composition, either the normal semi-continuous casting method with an ingot thickness of 300 to 600 mm, or the thin plate continuous casting method with an ingot thickness of 30 mm or less are used. Good but less pure A
It is preferable to use a thin plate continuous casting method that allows the above-mentioned intermetallic compounds to be reduced and made finer even if a metal is used. In addition, in the case of the thin plate continuous casting method, the coil plate thickness during U manufacturing is 3mm+, since it is desirable to perform cold rolling of 50% or more from the viewpoint of accuracy of punching, cutting, polishing, etc. as a disk base. It should be more than that. In any casting method, it is preferable to raise the casting temperature to facilitate reduction of intermetallic compounds, and in particular, in the case of a normal semi-continuous casting method, a temperature of 710° C. or higher is desirable. Next, the aluminum alloy ingot (or continuous thin plate coil) is soaked and rolled by a conventional method. However, in the case of semi-continuous casting method, the soaking treatment should be 450°C or higher,
In order to reduce the intermetallic compound containing Mg-8L as a main component, it is more preferable to maintain the temperature at 500° C. or higher for 1 hour or more. Regarding rolling, large ingots are hot-rolled and cold-rolled from the viewpoint of productivity, but in order to reduce intermetallic compounds whose main components are Mg and -Si, the hot rolling start temperature is set at 500°C.
'C or higher is preferable. However, in the case of continuously cast thin coils, hot rolling may be omitted and only cold rolling may be performed, but if the coil plate thickness is relatively thick, hot rolling under the above conditions may be performed subsequent to casting. . In either case, in the cold rolling process, annealing (300 to 300 b After forming the rolled plate into a disk shape by punching or cutting, it is annealed to remove distortion if necessary.During this heat treatment, if a load is applied to the disk surface, the distortion will be corrected. It is desirable because the effect is large.Next, the roughness of a normal rolled plate is, for example, Ra=0.1~
Since it is 0.5 μm, which is large for a disk substrate, and it is necessary to further reduce distortion, the disk surface is removed by cutting or polishing. In this case, if the surface is removed less than 10 μm, the distortion removal will not be sufficient, and if the surface is removed more than 500 μm, the performance of the disk will be satisfactory.
Since this is wasteful from an economic point of view such as productivity and cost, it is preferable that the thickness of the surface removed be 10 to 500 μm for a disk base made of an aluminum alloy plate. In this processing step, annealing is performed if necessary to remove strain. Next, pretreatments such as degreasing, pickling, and Zn substitution are performed, and a non-magnetic plating layer such as N1-P is formed thereon. The etching in the pretreatment may be either mild etching (etching amount 3 to 10 mg/dm2) or strong etching (etching amount 11 oIg/d112 or more). According to the present invention, strong etching is possible, and even if the final polishing allowance is reduced, excellent post-polishing surface accuracy with few pits can be obtained. Etching is preferably carried out by pickling. In this respect, it is superior to conventional aluminum alloy plates for disks, in which only weak etching could be applied. If the plating film thickness is less than 3 μm, the surface roughness of the disk will be large due to the influence of the pretreatment, and micro pits will likely remain. Furthermore, the amount of finish polishing will inevitably be small, and it will be difficult to obtain a uniform plating film with small roughness, so it is better to form a plating film with a thickness of 3 μin or more, and from the viewpoint of surface strength of the disk base. The thickness is preferably 5 μm or more. Further, even if the thickness of the plated skin becomes thick, the performance will not particularly deteriorate, but it is economically disadvantageous to make it too thick, so it is not preferable to make it thicker than 30 μm. After the plated disk manufactured in this manner is finished polished, a magnetic film is further formed by plating or sputtering, and the disk is used as a magnetic disk. Finish polishing allowance can be reduced to 0.2-2μm.
is desirable. Even in this case, a smooth surface with few micro pits and excellent surface precision can be obtained after polishing.

[Left below]

Next, examples of the present invention will be shown. (Example) Alloys Nα1 to Nα7 (inventive examples) and Nα8 to Na13 (comparative examples) having the chemical components shown in Table 1 were melted by the casting method shown in Table 1, and after filtering, semi-continuous In the case of cast materials, after ingot making and cutting, the thickness is 400mm x
The first slab is loooomm width x 3500mm length.
After performing soaking treatment under the conditions shown in the table, hot rolling and cold rolling were performed to obtain a plate thickness of 2 mm. 6mm for thin plate continuous cast material
Cast into a coil shape with thickness x 800mm width (x length), 40
After annealing at a temperature of 0°C for 5 hours, cold rolling was performed to give a plate thickness of 2 mm. Next, after punching out these plate materials, they were subjected to strain relief annealing to obtain hollow discs with an outer diameter of 130 mm and an inner diameter of 40 mm. Furthermore, the surface of the disk was cut (surface removal 60μm) and R
After manufacturing an aluminum alloy substrate for a magnetic disk with a thickness of 1 μm and a plate thickness of 1.88 mm, the substrate was treated under the following conditions to evaluate the surface treatment properties, adhesion of plating, and the occurrence of surface nodules after plating. The plating surface was polished and the occurrence of micro pits was investigated. The results are shown in Tables 1 to 3. (Processing conditions) Degreasing (U-Cleaner UA-68, 5%, Uzai Kogyo Co., Ltd.
0'C 15 minutes, immersion) → Pickling (AD-101 manufactured by Uezai Kogyo)
, 10%) → Zinc replacement (Uezai Kogyo WAD-301, R,
T, 1 minute immersion) → Nitric acid peeling (50% nitric acid, R, T,
, 30 seconds, immersion) → Zinc replacement (AD-301 manufactured by Uezai Kogyo)
, room temperature, 30 seconds, immersion) → N1-P plating (Kamizai Kogyo Nimuden HDX, 90°C, immersion, plating thickness 10μl11
). In addition, pickling is performed at 65℃ in the case of mild etching.
For strong etching, it was immersed at 75°C for 5 minutes (etching amount 23 mH/dm2). Regarding the size and amount of the intermetallic compound, Regarding the evaluation of the aluminum alloy substrate for the disk, the open chain 1, Qmm'' was measured and evaluated using a scanning electron microscope at a magnification of 1000 times. Regarding the surface treatment properties, we observed the surface after the second zinc substitution, and found that those with uniform and even precipitates, those with coarse O1 precipitation and coarse grains, and those in between. Those in which the grain boundaries and grain boundaries were significantly etched were marked as △. Regarding the adhesion of the plating, those in which the plating did not peel off due to 90' bending were rated 0, and those in which even part of the plating peeled off were rated x. Regarding the evaluation of nodules, the plated surface was measured at 20 random locations, a total of 80 mm, using a roughness meter, and those with 0 to 2 protrusions of 0.2 μm or more were measured as 0, and those with 3 to 10 protrusions were evaluated as Δ.
, 11 items were marked as ×. Regarding the surface accuracy after final polishing, after polishing the plated surface to a mirror finish using alumina powder, we observed 100 points at 640x magnification and inspected those without any pits with a maximum diameter of 2 μm or more.
Those with 1 to 4 pits were rated Δ, and those with 5 or more pits or even one pit of 8 μm or more were rated ×. Note that the polishing method was 0°5 μm and 1.5 μm. From Table 1, in the example of the present invention, the size of the A Q -Fe-based crystallized product is 10 μm or less, and 5 of the crystallized products with a size of 5 μm or more are used.
0.2 mm2 or less, and the size of the Mg-5i-based product is 8 μm or less, and 5 of the crystallized products with a size of 5 μm or more 70
．． It is 2 mm2 or less. However, even among the comparative examples in which the aluminum alloy composition is outside the scope of the present invention, there are some that have the same quantity and size as the inventive examples, but as is clear from Tables 2 and 3,
In these comparative examples, the surface treatment properties and plating adhesion were poor, or there were many nodules and micro pits even though the surface treatment and plating adhesion were good. On the other hand, it can be seen that the examples of the present invention have excellent surface treatment properties and adhesion of plating, and have extremely few nodules and micropits, which are much more biased than the comparative examples. In particular, even with strong etching, there are very few nodules, and in addition, even with final polishing (less than 0.5 μm polishing), there are very few micro pits and excellent surface precision is achieved. [Margins below] (Effects of the invention) As detailed above, according to the present invention, Cu, Zn
In addition to containing two types of essential elements, Fe and Si among impurities are controlled, and A f among intermetallic compounds is contained.
The size and amount of l-Fe-based intermetallic compounds and Mg-Si-based intermetallic compounds are regulated, resulting in excellent surface treatment and plating adhesion, as well as a smooth plated surface with few plating defects such as micro pits and nodules. An aluminum alloy for discs can be obtained. Furthermore, it is possible to perform a strong etching base treatment, reduce the thickness of the plating, and reduce the need for polishing the plated surface, so products such as magnetic disks can be provided at low cost. It is particularly suitable as a material for magnetic disks, optical disks, etc. Patent applicant Hisashi Nakamura, patent attorney representing Kobe Steel, Ltd.

Claims (7)

[Claims] (1) In weight% (the same applies hereinafter), as essential elements 3%<
Mg≦6%, 0.03%≦Cu<0.3% and 0.03
%≦Zn≦0.4%, and Fe and S among impurities
In the case of semi-continuous casting, i is Fe≦0.07%, Si≦0.
06%, in the case of continuous thin plate casting, Fe≦0.1%, Si≦0.
It has a composition regulated to 1%, and the maximum size of intermetallic compounds mainly composed of Al-Fe is 10 μm or less, and the number of intermetallic compounds of 5 μm or more is 5 / 0 .2 mm^2 or less, and furthermore, the maximum size of the intermetallic compound mainly composed of Mg-Si is 8 μm or less, and the number of intermetallic compounds of 5 μm or more is 5/0 An aluminum alloy plate for disks, which has excellent surface treatment properties and plating adhesion characterized by a thickness of .2 mm^2 or less, and has excellent surface precision after polishing with few nodules and micro pits. (2) The aluminum alloy has the above composition, and A
The maximum size of the intermetallic compound containing l-Fe as a main component is 6 μm or less, and the number of the intermetallic compounds with a size of 5 μm or more is 5 pieces/0.2 mm^2 or less, and
Claim 1: The maximum size of the intermetallic compound whose main component is Mg-Si is 5 μm or less, and the number of intermetallic compounds with a size of 4 μm or more is 5 pieces/0.2 mm^2 or less.
Aluminum alloy plate for discs described in . (3) The aluminum alloy plate for a disk is subjected to mild etching at an etching amount of 3 to 10 mg/dm^2 during a plating pretreatment step.
Aluminum alloy plate for discs described in . (4) The aluminum alloy plate for a disk according to claim 2, wherein the aluminum alloy plate for a disk is subjected to strong etching with an etching amount of 11 mg/dm^2 or more during a plating pretreatment step. (5) 3%<Mg≦6%, 0.03%≦ as essential elements
Contains Cu<0.3% and 0.03%≦Zn≦0.4%, and contains Fe and Si among impurities such as Fe≦0.07% and S
An aluminum alloy having a composition regulated to i≦0.06% is cast at 710°C or higher, and the resulting ingot is soaked at 450°C or higher, followed by hot rolling at a hot rolling start temperature of 500°C or higher. A method for producing an aluminum alloy plate for a disk, which has excellent surface treatment properties and plating adhesion, and has excellent post-polishing surface precision with few nodules and micro pits, the method comprising cold rolling. (6) The method according to claim 5, wherein the soaking treatment is carried out under conditions of holding the temperature at 500°C or higher for 1 hour or more. (7) 3%<Mg≦6%, 0.03%≦ as essential elements
Contains Cu<0.3% and 0.03%≦Zn≦0.4%, and Fe and Si among impurities are Fe≦0.1% and Si
Excellent surface preparation and plating characterized by casting an aluminum alloy with a composition controlled to ≦0.1% at a temperature of 690°C or higher to obtain a thin continuous cast coil with a thickness of 3 mm or more, which is then cold rolled. A method for producing an aluminum alloy plate for a disk, which has adhesive properties and has excellent surface precision after polishing with few nodules and micro pits.