Glass plate, disc-shaped glass, glass substrate for magnetic disk, and method for manufacturing glass plate

Abstract

The glass plate of one embodiment is a rectangular plate having a thickness of less than 0.68 mm. In this glass plate, the flatness of a square measurement area of 100 mm per side, said square measurement area being cut out from the center area of the glass plate excluding the edge areas on both of the long side direction and the short side direction of the glass plate, is 30 μm or less. When the measurement area is subjected to a first heat treatment in which the measurement area is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a speed of 50°C / hour, the heat shrinkage of the measurement area is 130 ppm or less. When the measurement area is subjected to a second heat treatment in which the measurement area is maintained at Tg-160°C [wherein Tg (°C) stands for the glass transition temperature of the glass plate] for 60 seconds and then cooled to room temperature in air, the resulted amount of change in the flatness of the measurement area is 10 μm or less.

Description

[Technical Field]

[0001] The present invention relates to a glass substrate for a magnetic disk used in a hard disk drive device, a circular glass, a glass plate, and a method for manufacturing the glass plate. [Background technology]

[0002] With the recent rise of cloud computing, many hard disk drives (HDDs) are being used in cloud data centers to increase storage capacity. HDDs use magnetic disks, which are circular non-magnetic glass substrates with a magnetic layer, as their storage medium. In order to increase the storage capacity of HDDs, it is desirable to increase the number of thin magnetic disks installed in addition to increasing the recording density of the magnetic disks.

[0003] To achieve higher recording densities, magnetic disk recording methods such as thermally assisted magnetic recording (HAMR) and microwave-assisted magnetic recording (MAMR) have been investigated in addition to conventional perpendicular magnetic recording. In recent years, heat treatment of magnetic films has been performed to form magnetic recording layers suitable for these recording methods. Heat treatment is performed, for example, by heating a glass substrate at high temperatures after magnetic film formation or by forming a magnetic film while heating the glass substrate at high temperatures. In this case, the temperature of the magnetic film can reach 700°C or higher, far exceeding 600°C. Because this heat treatment heats the glass substrate along with the magnetic film, glass substrates for magnetic disks must have high heat resistance, i.e., a high glass transition temperature (Tg), to prevent thermal deformation.

[0004] Methods that can be used to manufacture glass plates that serve as the base material for magnetic disk glass substrates include the press method, float method, Furcol method, Pittsburgh method, downdraw method, Colburn method, redraw method, etc. Among these, the float method, Furcol method, Pittsburgh method, downdraw method, Colburn method, and redraw method are suitable for manufacturing glass substrates for flat panel displays (FPDs) such as liquid crystal displays because they make it easier to manufacture large-sized glass plates than the press method.

[0005] FPDs use glass substrates on which electronic elements such as thin film transistors (TFTs) are mounted. In the TFT manufacturing process, the glass substrate is heated to high temperatures, which causes the glass substrate to thermally shrink and easily change in size. For this reason, when producing glass plates using the above-mentioned methods, such as the float method, the glass plate is slowly cooled while being formed, and the conditions for the slow cooling are adjusted to reduce the residual stress in the glass plate and the thermal shrinkage rate. Offline annealing, in which a glass plate cut to a predetermined size from a formed long glass sheet, is heat-treated, is known as a method for further reducing the thermal shrinkage rate of the glass plate (Patent Document 1). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-178711 Summary of the Invention [Problem to be solved by the invention]

[0007] It has been found that when a magnetic disk glass substrate is fabricated from a glass plate with a high glass transition temperature (Tg) produced by the float method or other methods described above and a magnetic disk is fabricated using this glass substrate, the glass substrate thermally shrinks and deforms, causing distortion, due to heat treatment of the magnetic film, resulting in a deterioration in the flatness of the magnetic disk. It has also been found that this deterioration in flatness occurs significantly when a thin magnetic disk glass substrate is used. Poor flatness of the magnetic disk makes it more likely to flutter in a HDD device, making stable reading impossible.

[0008] Therefore, an object of the present invention is to provide a glass substrate for a magnetic disk that can suppress deterioration of flatness due to heat treatment for forming a magnetic recording layer of the magnetic disk, and a circular glass, glass plate, and method for manufacturing the glass plate that are used for such a glass substrate for a magnetic disk. [Means for solving the problem]

[0009] One aspect of the present invention is a glass plate. The glass plate is a rectangular glass plate having a plate thickness of less than 0.68 mm, the flatness of a square measurement region having a side of 100 mm cut out from a central region of the glass plate excluding end regions each having a length of 5 to 20% of the short side of the glass plate on the inner side of the glass plate from each end in a short side direction of the glass plate and end regions each having a length of 5 to 20% of the long side of the glass plate on the inner side of the glass plate from each end in a long side direction of the glass plate is 30 μm or less, the thermal shrinkage of the measurement area is 130 ppm or less when the measurement area is subjected to a first heat treatment in which the measurement area is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour; When the glass transition temperature of the glass plate is expressed as Tg (°C), the change in flatness of the measurement area due to a second heating treatment in which the measurement area is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere is 10 μm or less.

[0010] It is preferable that the short side length exceeds 900 mm.

[0011] The glass plate is preferably a portion cut out from a long glass sheet formed using any one of the float process, the Fourcol process, the Pittsburgh process, the downdraw process, the Colburn process, and the redraw process.

[0012] The difference between the amount of thermal shrinkage S1 of the measurement area in the direction in which the thermal shrinkage rate is smallest among the in-plane directions of the measurement area and the amount of thermal shrinkage S2 of the measurement area in the direction in which the thermal shrinkage rate is largest is 1.0 μm. below It is preferable that:

[0013] the glass plate has been subjected to an annealing treatment to reduce thermal shrinkage; The glass plate before the annealing treatment has anisotropy in thermal shrinkage, in which the magnitude of the thermal shrinkage varies depending on the in-plane direction of the region of the glass plate corresponding to the measurement region, The difference between the amount of thermal shrinkage S1 of the region in the in-plane direction where the thermal shrinkage rate is the smallest and the amount of thermal shrinkage S2 of the region in the in-plane direction where the thermal shrinkage rate is the largest may be greater than 1.0 μm.

[0014] Another aspect of the present invention is a glass plate. The glass plate is a rectangular glass plate having a thickness of less than 0.68 mm, a flatness of 30 μm or less, and two orthogonal sides each having a length of 95 to 120 mm, The thermal shrinkage rate when subjected to a first heat treatment in which the temperature is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour is 130 ppm or less; When the glass transition temperature of the glass plate is expressed as Tg (°C), the change in flatness due to a second heat treatment in which the glass plate is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere is 10 μm or less.

[0015] The rectangular glass plate is a raw plate that will be used to produce a circular glass plate having a circular outer periphery, The area of ​​the main surface of the rectangular glass plate is preferably 1.6 times or less the area of ​​the inside of the outer periphery of the disk-shaped glass.

[0016] Another aspect of the present invention is a glass disk. The disk-shaped glass has a plate thickness of less than 0.68 mm, a flatness of 30 μm or less, and a circular outer periphery with a diameter of 95 to 100 mm, The thermal shrinkage rate when subjected to a first heat treatment in which the temperature is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour is 130 ppm or less; When the glass transition temperature of the disk-shaped glass is expressed as Tg (°C), the change in flatness due to a second heat treatment in which the glass is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere is 10 μm or less.

[0017] Another aspect of the present invention is a glass substrate for a magnetic disk. The magnetic disk glass substrate has a thickness of less than 0.68 mm, a flatness of 30 μm or less, and a diameter of 95 to 100 mm, The thermal shrinkage rate when subjected to a first heat treatment in which the temperature is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour is 130 ppm or less; When the glass transition temperature of the magnetic disk glass substrate is expressed as Tg (°C), the change in flatness due to a second heat treatment in which the substrate is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere is 10 μm or less.

[0018] It is preferable that the amount of change in circularity due to the first heat treatment is 0.5 μm or less.

[0019] Another aspect of the present invention is a method for producing a glass sheet. The method for producing the glass plate includes: The glass plate material from which the glass plate is made is subjected to an annealing heat treatment, The glass plate is A rectangular plate having a thickness of less than 0.68 mm, the flatness of a square measurement region having a side of 100 mm cut out from a central region of the glass plate excluding end regions each having a length of 5 to 20% of the short side of the glass plate on the inner side of the glass plate from each end in a short side direction of the glass plate and end regions each having a length of 5 to 20% of the long side of the glass plate on the inner side of the glass plate from each end in a long side direction of the glass plate is 30 μm or less, the thermal shrinkage of the measurement area is 130 ppm or less when the measurement area is subjected to a first heat treatment in which the measurement area is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour; When the glass transition temperature of the glass plate is expressed as Tg (°C), the change in flatness of the measurement area due to a second heating treatment in which the measurement area is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere is 10 μm or less.

[0020] Another aspect of the present invention is a method for producing a glass sheet. The method for producing the glass plate includes: Annealing a glass plate material that is the base of the glass plate; and removing the glass plate from the glass plate material after the annealing treatment. The glass plate is A rectangular plate having a thickness of less than 0.68 mm, a flatness of 30 μm or less, and two orthogonal sides each having a length of 95 to 120 mm, The thermal shrinkage rate when subjected to a first heat treatment in which the temperature is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour is 130 ppm or less; When the glass transition temperature of the glass plate is expressed as Tg (°C), the change in flatness due to a second heat treatment in which the glass plate is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere is 10 μm or less.

[0021] The rectangular glass plate is a square plate that serves as a base for a circular glass plate having a circular outer periphery, The area of ​​the main surface of the rectangular glass plate is preferably 1.6 times or less the area of ​​the inside of the outer periphery of the disk-shaped glass.

[0022] Another aspect of the present invention is a method for producing a glass sheet. The method for producing the glass plate includes: A method for manufacturing a disk-shaped glass, Annealing a glass plate material that is the basis for the disk-shaped glass; and removing the glass disc from the glass plate material after the annealing treatment. The glass plate material is a rectangular glass plate, The disk-shaped glass is The plate thickness is less than 0.68 mm, the flatness is 30 μm or less, and the diameter is 95 to 100 mm; The thermal shrinkage rate when subjected to a first heat treatment in which the temperature is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour is 130 ppm or less; When the glass transition temperature of the disk-shaped glass is expressed as Tg (°C), the change in flatness due to a second heat treatment in which the glass is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere is 10 μm or less.

[0023] The rectangular glass plate is square, The area of ​​the main surface of the rectangular glass plate is preferably 1.6 times or less the area of ​​the inside of the outer periphery of the disk-shaped glass. [Effects of the Invention]

[0024] The above-mentioned magnetic disk glass substrate can suppress deterioration of flatness due to heat treatment for forming a magnetic recording layer of a magnetic disk. Furthermore, the above-mentioned glass plate and disk-shaped glass can provide such a magnetic disk glass substrate. Furthermore, the above-mentioned glass plate manufacturing method can provide the above-mentioned glass plate. [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 1(a) is an external view of a glass plate (large glass plate) according to one embodiment, and FIG. 1(b) is a plan view illustrating a measurement area of ​​the glass plate. [Figure 2] FIG. 1(a) is an external view of a glass plate (segmented glass) according to one embodiment, and FIG. 1(b) is a plan view of the glass plate showing the portion that will become the disk-shaped glass. [Figure 3] 1 is an external view of a disk-shaped glass according to an embodiment. FIG. [Figure 4] 1 is an external view of a magnetic disk glass substrate according to an embodiment of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0026] Hereinafter, a glass plate, a method for manufacturing a glass plate, a glass disk, and a glass substrate for a magnetic disk according to one embodiment will be described in detail.

[0027] (large sheet glass) Fig. 1(a) shows an external view of a glass plate 10 according to one embodiment, and Fig. 1(b) shows a plan view illustrating a measurement region 13 of the glass plate 10, which will be described later.

[0028] The glass plate 10 is a rectangular plate with a thickness of less than 0.68 mm.

[0029] When the thickness of the glass plate 10 is less than 0.68 mm, the thickness of a magnetic disk glass substrate (hereinafter also referred to as a glass substrate) made from the glass plate 10 can be reduced when the magnetic disk is made into a magnetic disk, thereby increasing the number of magnetic disks that can be mounted in an HDD device. The thickness of the glass plate 10 is preferably less than 0.61 mm, more preferably less than 0.58 mm. The lower limit of the thickness of the glass plate 10 is not particularly limited, but is, for example, 0.2 mm.

[0030] The short side length of the glass plate 10 is preferably greater than 900 mm. This allows for the production of many magnetic disk glass substrates from the glass plate 10, thereby reducing the manufacturing cost of magnetic disk glass substrates. The long side length of the glass plate 10 may be longer than the short side length, as in the example shown in FIG. 1, or may be equal to the short side length. That is, the glass plate 10 is rectangular or square. When the glass plate 10 has a short side and a long side, the ratio of the long side length to the short side length (long side length / short side length) is preferably 1.2 or less. By subjecting a glass plate material from which the glass plate 10 is made, having the ratio of 1.2 or less, to the precision annealing process described below, it becomes easier to reduce the thermal shrinkage rate and the anisotropy of the thermal shrinkage rate (described below). This is thought to be because a workpiece that is closer to a square is less likely to experience differences in thermal history due to differences in position within the workpiece surface during the annealing process. In this specification, such a glass plate having a short side length of greater than 900 mm may be referred to as a "large glass plate." The long side length of the glass plate 10 is preferably 2000 mm or less. If the long side length exceeds 2000 mm, it may be difficult to maintain a uniform temperature inside the furnace during precision annealing, which will be described later.

[0031] The flatness of the measurement area 13 cut out from the central region of the glass plate 10 is 30 μm or less. The flatness of the measurement area 13 is 30 μm or less, which reduces the amount of grinding or polishing required when producing a magnetic disk glass substrate from the glass plate 10, allowing for high yields of magnetic disk glass substrates to be produced. Furthermore, the flatness of the measurement area 13 is 30 μm or less, which reduces the likelihood of fluttering when a magnetic disk produced using the glass plate 10 as the base material is rotated at high speed, allowing for stable reading by the head of the reading unit of the HDD device. In particular, when the magnetic disk is thin, the rigidity of the glass substrate is low, which can cause deflection that leads to fluttering. However, the low flatness of the glass plate 10 makes it possible to suppress fluttering even when the disk is thin. In this specification, flatness refers to flatness in accordance with JIS B0621-1984. Flatness measurement can be performed, for example, using an interferometric flatness measuring device and phase measurement interferometry (phase shift method) at a predetermined measurement wavelength (e.g., 680 nm). The flatness of the measurement area 13 is preferably 20 μm or less, more preferably 10 μm or less. The above flatness refers to the flatness of the measurement area 13 before the first or second heat treatment described below is performed. The same applies hereinafter unless otherwise specified.

[0032] The central region 12 of the glass plate 10 refers to a region of the glass plate 10 excluding edge regions 11a, each extending from each end in the short side direction of the glass plate 10 to the inside of the glass plate 10, having a length Le of 5 to 20% of the length L of the short side 10a of the glass plate 10, or edge regions 11b, each extending from each end in the long side direction of the glass plate 10 to the inside of the glass plate 10, having a length We of 5 to 20% of the length W of the long side 10b of the glass plate. The length of the central region 12 in the short side direction is Lc, and the length in the long side direction is Wc.

[0033] The measurement area 13 is a square area with sides of 100 mm cut out from the central area 12 of the glass plate 10. The measurement area 13 does not have to be cut out as shown in FIG. 1(b) and can be cut out arbitrarily from the central area 12. The size and shape of the measurement area 13 depend on the size of the glass plate ("segmented glass" described later) that will be used to make the magnetic disk glass substrate. and shape Close to.

[0034] According to the inventor's investigations, it has been found that, when the glass transition temperature of glass plate 10 is expressed as Tg (°C), the thermal shrinkage of measurement area 13 is 130 ppm or less when a first heating treatment is performed in which measurement area 13 is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour, and the change in flatness of measurement area 13 is 10 μm or less when a second heating treatment is performed in which measurement area 13 is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere, thereby achieving the following effects: That is, when a magnetic film is heat-treated in a magnetic disk glass substrate made from glass plate 10 having a thickness of less than 0.68 mm and a flatness of measurement area 13 of 30 μm or less, thermal shrinkage of the glass substrate is suppressed, thereby suppressing deformation such as bending of the glass substrate due to thermal shrinkage, and as a result, deterioration of the flatness of the glass substrate is suppressed. By achieving this effect, deterioration of the flatness of the glass plate 10 of 30 μm or less is suppressed on the glass substrate after heat treatment of the magnetic film, and fluttering is suppressed when the glass plate 10 is made into a magnetic disk and rotated at high speed.

[0035] For these reasons, the glass plate 10 of this embodiment has a thermal shrinkage of 130 ppm or less in the measurement area 13 after a first heat treatment in which the measurement area 13 is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour. The change in flatness of the measurement area 13 after a second heat treatment in which the measurement area 13 is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere is 10 μm or less. If the thermal shrinkage of the measurement area 13 after the first heat treatment exceeds 130 ppm and the change in flatness of the measurement area 13 after the second heat treatment exceeds 10 μm, deformation of the glass substrate, such as bending during thermal shrinkage, cannot be suppressed, resulting in a deterioration in the flatness of the glass plate. Even slight thermal shrinkage can significantly impair a high flatness of 30 μm or less. The conditions for the first and second heat treatments are determined with reference to the conditions for heat treatment of the magnetic film. The conditions for the first heat treatment are determined from the viewpoint of temperature conditions that enable evaluation of the thermal shrinkage rate during high-temperature, long-term heating, which is presumed to be related to the deterioration of the flatness of the glass substrate during heat treatment of the magnetic film at 600°C or higher. The conditions for the second heat treatment are determined from the viewpoint of temperature conditions that enable direct evaluation of the degree of deterioration of the flatness of the glass substrate during heat treatment of the magnetic film at 600°C or higher. This is because the temperatures required to form a magnetic film with an L10 structure, which is considered optimal for energy-assisted magnetic recording (EAMR), such as thermally assisted magnetic recording (HAMR) and microwave-assisted magnetic recording (MAMR), can reach 700°C or higher, far exceeding 600°C. In this specification, unless otherwise specified, the heat shrinkage rate refers to the heat shrinkage rate before and after the first heat treatment, and means the maximum value of the heat shrinkage rates measured in 25 directions parallel to the main surface of the object, passing through the center of the object, and changing in circumferential directions by 7.2 degrees each. Note that by measuring the heat shrinkage rate using the above method, it is possible to measure the heat shrinkage rate in all directions (360 degrees), and therefore it is possible to evaluate the heat shrinkage rate more accurately than conventional methods.

[0036] The heating (heating), temperature maintenance, and cooling (temperature decreasing) in the first heat treatment are preferably performed continuously in an atmosphere (e.g., the atmosphere in a single annealing furnace) under the same conditions except for the temperature conditions. The temperature increase in the first heat treatment is preferably performed from room temperature (normal temperature) over 2.5 hours. In other words, the substrate to be treated is preferably heated from room temperature to 700°C at a rate of 270°C / hour. Furthermore, the temperature decrease is preferably performed from 700°C to room temperature at a rate of 50°C / hour. In addition, when performing the first heat treatment, in order to avoid significant deterioration in the flatness of the substrate to be treated (which may be the measurement area 13 of the glass plate 10, or the glass plate 20, the glass disc 30, or the magnetic disk glass substrate 40 described below), it is preferable to perform the first heat treatment by sandwiching the substrate between two setters (described below) from above and below. Avoiding significant deterioration in flatness enables accurate measurement of the thermal shrinkage rate. In this case, the size of the setter should be equal to or larger than that of the substrate to be treated. The thickness of the setter placed on top of the substrate to be treated should be set so as not to interfere with the thermal shrinkage of the substrate to be treated and to maintain its flatness (for example, maintaining a flatness of 30 μm or less). Needless to say, a setter with a weight that reduces the thickness of the substrate to be treated is inappropriate. Since the flatness of the substrate to be treated is maintained, the same substrate to be treated can be used to evaluate the thermal shrinkage rate due to the first heat treatment and then to evaluate the change in flatness due to the second heat treatment (described below). The evaluation of the thermal shrinkage rate due to the first heat treatment and the evaluation of the amount of change in flatness due to the second heat treatment described below may be performed using separate substrates to be treated.

[0037] With respect to the second heat treatment, cooling in the atmosphere means allowing the substrate to cool in an atmosphere at room temperature without temperature adjustment to control the cooling rate. The room temperature is, for example, 25°C. The heating and temperature maintenance of the substrate in the second heat treatment are performed, for example, in an atmosphere between two heaters in a heating device equipped with two panel-shaped heaters, with the substrate held by a substrate holder. This heating device is modeled after a substrate heating chamber installed in a known single-wafer vacuum film deposition device used for depositing magnetic films on magnetic disks, etc. The substrate is set vertically relative to the ground in a known substrate holder (also called a carrier) for film deposition. Three or four L-shaped leaf spring support members are fixed to the substrate holder (e.g., the substrate holder described in paragraph 0045 and Figure 4 of JP 2011-117019 A). The tips of the support members are pressed against the outer peripheral edge of the substrate, thereby securing the substrate to the substrate holder by the elasticity of the leaf springs. It can hold not only circular but also rectangular and other shaped substrates in the same way. By adjusting the specifications of the substrate holder, substrates of various shapes can be heated in the same way. However, since the elastic force of the leaf spring from the support member is always applied to the substrate in a direction that bends the substrate, it is important to consider that the substrate is more likely to bend than when heated without a support member (for example, when placed flat).

[0038] The thermal shrinkage rate can be determined, for example, by measuring the change in length in the measurement area 13 before and after the heat treatment and calculating it according to the following formula. C (thermal shrinkage rate) = (L0-L) / L0 Here, L0 is the length before heat treatment, and L is the length after heat treatment. The sign of C is positive if the material shrinks due to heat treatment, and negative if it expands. L0 and L can be determined, for example, by making two markings on the surface of the cut-out measurement area 13 and measuring the distance between the two markings before and after heat treatment. Alternatively, L0 and L may be the lengths of the measurement area 13 before and after heat treatment. These lengths are preferably lengths passing through the center of the measurement area 13. When the measurement object is a disk-shaped glass or a glass substrate for a magnetic disk, the diameter may be used. Furthermore, when evaluating the anisotropy of the thermal shrinkage, for example, lengths (e.g., diameters) in 25 directions, which are changed in 7.2 degree increments in the circumferential direction from the center of the object to be measured, can be used. The thermal shrinkage in 25 directions is measured, and the absolute value of the difference between the thermal shrinkages obtained by subtracting the minimum value from the maximum value can be used as an index of the anisotropy of the thermal shrinkage. Note that the amount of thermal shrinkage (S) may be used instead of the thermal shrinkage (C), and the absolute value of the difference between the maximum and minimum values ​​of the amount of thermal shrinkage in the above 25 directions may be calculated and used as an index of the anisotropy. S (amount of thermal shrinkage) is S = (L0 - L).

[0039] The thermal shrinkage of the measurement area 13 after the first heat treatment is preferably 90 ppm or less, more preferably 50 ppm or less. The change in flatness of the measurement area 13 after the second heat treatment is preferably 7.5 μm or less, more preferably 5 μm or less.

[0040] The glass plate 10 is preferably a portion cut from a long glass sheet formed using any one of the float process, the Fourcol process, the Pittsburgh process, the downdraw process, the Colburn process, and the redraw process. Large-sized glass plates 10 can be obtained from glass sheets formed using these methods, allowing a large amount of individualized glass, which serves as the basis for magnetic disk glass substrates, to be obtained from the glass plate 10, thereby reducing the manufacturing cost of magnetic disk glass substrates. Furthermore, these methods are advantageous for forming glass sheets with high glass transition temperatures (Tg), thereby reducing the manufacturing cost of glass plates 10 with high glass transition temperatures (Tg). Specific examples of downdraw processes include the slot downdraw process and the overflow downdraw process. It is also preferable that at least one main surface of the glass plate 10 is a fire-polished surface. This allows for the omission of some of the grinding and polishing processes of the main surfaces of the substrate, which are generally required when manufacturing magnetic disk glass substrates, and for the reduction of the machining allowance. In other words, it is preferable that at least one of the main surfaces of the glass plate 10 is an unground surface and / or an unpolished surface.

[0041] In a glass sheet obtained by the above-mentioned methods such as the float method, the thickness of both ends in the width direction of the glass sheet, which is orthogonal to the longitudinal direction of the glass sheet (the direction in which the glass flows out of the melting furnace), is usually thicker than that of the central part in the width direction, so the glass plate material that will become the glass plate 10 is cut out from the remaining part of the glass sheet after both ends in the width direction of the glass sheet are cut off. Usually, the glass plate material that will become the glass plate 10 is cut out so that the width direction of the glass sheet coincides with the short side direction or long side direction of the glass plate 10.

[0042] The glass plate 10 may have anisotropy in thermal shrinkage. Thermal shrinkage anisotropy refers to the characteristic that the magnitude of the thermal shrinkage varies depending on various directions in the plane of the main surface of the measurement area 13. The inventors' investigations have revealed that if the glass plate 10 has anisotropy in thermal shrinkage, the circularity (JIS B0621-1984) of the glass substrate for a magnetic disk obtained from the glass plate 10 may be deteriorated when the magnetic film is heat-treated. In particular, when the glass plate material from which the glass plate 10 is made is a portion cut from a glass sheet formed using the above-mentioned method, such as the float process, the thermal shrinkage is likely to vary depending on the in-plane direction of the glass sheet, resulting in anisotropy in the thermal shrinkage. Furthermore, the inventors have found that the directions in the in-plane directions of the main surface where the thermal shrinkage is maximum and minimum are not necessarily perpendicular to each other by 90 degrees. In other words, in the past, when evaluating the anisotropy of thermal shrinkage, the thermal shrinkage rates were measured in two directions: the longitudinal direction of the glass sheet and the width direction of the glass sheet, which is perpendicular to the longitudinal direction. The difference between these directions was used as an index of anisotropy. However, it has been found that this method sometimes fails to accurately evaluate the maximum and minimum thermal shrinkage values ​​and their difference. Although the reason for this is not entirely clear, it is generally believed that in methods for continuously producing long glass sheets, such as the float process and downdraw process, glass flowing out of a melting furnace or softening furnace is stretched in the width direction while being pulled in the flow direction, and thus is formed into a glass sheet. This causes the glass to be pulled in an oblique direction that combines the two perpendicular directions. Furthermore, the pulling direction varies depending on the forming conditions and the position within the glass sheet, and also changes over time. In addition, the thermal history varies depending on the position. Therefore, it is thought that the direction of maximum and minimum thermal shrinkage, as well as the magnitude of the thermal shrinkage, varies in various ways. Therefore, to accurately evaluate anisotropy, it is necessary to cut the desired glass from a long glass sheet and examine it in all directions.

[0043] As described above, if the circularity of the outer periphery of a magnetic disk glass substrate deteriorates, the magnetic disk will wobble when rotated at high speed, making fluttering more likely. Therefore, it is extremely important to accurately determine the thermal shrinkage values ​​in the directions where the thermal shrinkage is maximum and minimum, as well as the difference between them, for the magnetic disk glass substrate, the disk-shaped glass substrate from which the disk-shaped glass is formed, and the glass plate from which the disk-shaped glass is formed. To prevent such deterioration in the circularity of the glass substrate, it is preferable that the difference (absolute value) between the thermal shrinkage C1 in the direction where the thermal shrinkage is minimum and the thermal shrinkage C2 in the direction where the thermal shrinkage is maximum, among the in-plane directions of the measurement region 13, be 10 ppm or less. Furthermore, it is preferable that the difference (absolute value) between the thermal shrinkage S1 of the measurement region in the direction where the thermal shrinkage is minimum and the thermal shrinkage S2 of the measurement region in the direction where the thermal shrinkage is maximum be 1.0 μm or less.

[0044] The glass plate 10 is preferably subjected to an annealing treatment (e.g., "precision annealing" described below) to reduce the thermal shrinkage. The glass plate (the glass plate material from which the glass plate 10 is made) before the annealing treatment of the present invention has anisotropy in thermal shrinkage, i.e., the magnitude of the thermal shrinkage varies depending on the in-plane direction of the region of the glass plate corresponding to the measurement region 13. The difference (absolute value) between the thermal shrinkage C1 in the in-plane direction where the thermal shrinkage is smallest and the thermal shrinkage C2 in the in-plane direction where the thermal shrinkage is largest may be greater than 10 ppm. Furthermore, the difference (absolute value) between the thermal shrinkage S1 of the region in the direction where the thermal shrinkage is smallest and the thermal shrinkage S2 of the region in the direction where the thermal shrinkage is largest may be greater than 1.0 μm. Even when such anisotropy in thermal shrinkage exists, the annealed glass plate 10 of the present invention satisfies the above-described ranges of change in thermal shrinkage and flatness. Therefore, when a magnetic film is heat-treated in a magnetic-disk glass substrate obtained from the glass plate 10, deterioration in flatness and roundness is suppressed. The change (deterioration) in the circularity is preferably 0.5 μm or less, and more preferably 0.2 μm or less.

[0045] According to one embodiment, a square measurement region having sides of 100 mm cut out from the entire glass plate 10 including the edge regions 11a and 11b of the glass plate 10 preferably has flatness, thermal shrinkage, and change in flatness that satisfy the above-mentioned ranges, similar to the measurement region 13. More magnetic-disk glass substrates can be produced from such a glass plate 10 than when magnetic-disk glass substrates are produced from the central region 12.

[0046] The glass plate 10 is preferably made of a material such as aluminosilicate glass, soda-lime glass, soda aluminosilicate glass, aluminoborosilicate glass, or borosilicate glass.

[0047] The glass transition temperature (Tg) of the glass plate 10 is preferably 750°C or higher, and more preferably 770°C or higher. A magnetic disk glass substrate made from a glass plate 10 with such a high glass transition temperature (Tg) is less likely to deform at high temperatures, and is therefore highly effective in preventing deterioration of flatness when the magnetic film is heat-treated at, for example, 700°C. The upper limit of the glass transition temperature (Tg) of the glass plate 10 does not need to be particularly set, but is preferably 850°C or lower. If the glass transition temperature (Tg) exceeds 850°C, it may be difficult to form a thin sheet glass. The Young's modulus of the glass plate 10 is preferably 80 GPa or more. If the Young's modulus is less than 80 GPa, warping due to elastic stress from the support member for holding the substrate occurs when the magnetic film is heat-treated, for example, at 700°C. This warping, combined with the warping due to the heat treatment, can significantly deteriorate the flatness. If the flatness deteriorates too much, problems such as the substrate falling off the substrate holder during film formation can occur. The average linear expansion coefficient of the glass plate 10 at 100 to 300°C is 45 × 10 -7 / °C or less. -7 If the temperature exceeds 1000 K / °C, the risk of cracking the substrate may increase when the substrate is rapidly heated or cooled to improve productivity. The density of the glass plate 10 is 2.65 g / cm 3 Preferably, it is 2.60 g / cm or less. 3 If the density is too high, the weight increases when the glass substrate is used for a magnetic disk, and the power consumption of the HDD tends to increase.

[0048] (Glass plate manufacturing method) The glass plate 10 described above can be manufactured by a glass plate manufacturing method that includes an annealing process in which a glass plate, which is the base material for the glass plate 10, is heated under predetermined conditions. In the following description, this annealing process of the glass plate performed under the predetermined conditions will be referred to as "precision annealing." The precision annealing is performed on the glass plate so that the measurement area 13 of the glass plate 10 satisfies the above-mentioned ranges of thermal shrinkage and flatness change.

[0049] According to the inventor's research, as described above, when a small piece of glass is extracted from a conventional glass plate manufactured by the float method or other methods to prepare a glass substrate for a magnetic disk and then heat-treated to form a magnetic film serving as a magnetic recording layer, the glass substrate thermally shrinks and deforms, resulting in a deterioration in the flatness of the magnetic disk. Because glass sheets formed by the float method or other methods are stretched in various directions and rapidly cooled while maintaining a large area, it is difficult to maintain constant stress, temperature, and thermal history across the entire glass sheet, making it difficult to uniformly reduce the thermal shrinkage across the entire glass sheet. This is thought to result in variations in the thermal shrinkage rate due to differences in the in-plane position of the glass sheet. Therefore, when a portion is extracted from the glass sheet and subsequently heated, the amount of thermal shrinkage may be large. Specifically, when a glass sheet is used as a glass substrate for an FPD, it is sufficient that a single thermal shrinkage value measured across the entire glass sheet falls within an acceptable range, eliminating the need to consider in-plane variations in thermal shrinkage. However, because magnetic disk glass substrates are significantly smaller than FPD glass substrates, it has been found that in-plane variations in thermal shrinkage can result in magnetic disk glass substrates with large thermal shrinkage. Furthermore, the heat treatment temperatures for magnetic films formed on magnetic disk glass substrates have been increasing in recent years, sometimes reaching temperatures above 700°C. This temperature is close to the glass transition temperature (Tg) of high-heat-resistant glass substrates. The heat treatment conditions for such magnetic films are much more severe than the heating conditions (e.g., 350–600°C) used for forming TFTs on FPD glass substrates. Therefore, even slight thermal shrinkage that is not a problem for FPD glass substrates due to slow cooling during molding has been found to have a significant adverse effect on the heat treatment of the magnetic film. That is, it has become clear that the heat treatment of the magnetic film causes the glass substrate to undergo large thermal shrinkage and deformation such that the glass substrate bends during the thermal shrinkage.The inventors have discovered that by performing the above-mentioned precision annealing on the glass plate material that is the basis for the glass plate 10, it is possible to precisely remove the glass plate 10 while maintaining the flatness of the glass plate 10 at a predetermined value or less, so as to prevent in-plane variation in the thermal shrinkage rate; in other words, it is possible to obtain a glass plate 10 in which the above-mentioned changes in the thermal shrinkage rate and flatness of the measurement area 13 are within a predetermined range.

[0050] Furthermore, as the inventors continued their research, they found that, in order to solve the above-mentioned problem of the glass substrate deforming so as to bend while thermally shrinking when the magnetic film is heat-treated, even if a conventionally known annealing treatment such as offline annealing is performed, the following problem occurs: That is, when a conventional annealing treatment is performed on a glass plate material that is the source of a large glass sheet, the effect of the annealing (such as the effect of reducing the thermal shrinkage rate) is not uniformly distributed throughout the entire surface of the glass plate material. As a result, even if the thermal shrinkage rate of the large glass sheet as a whole when subjected to the first heat treatment described above is equal to or less than a predetermined value, when a plurality of rectangular glass sheets, each measuring 95 to 120 mm on a side, are cut out (single-pieced) from the large glass sheet, some of the individual glass sheets will not have a thermal shrinkage rate equal to or less than the predetermined value, or will warp when the magnetic film is heat-treated as a glass substrate for a magnetic disk, resulting in variations in the properties of the individual glass sheets. In particular, when the glass plate material from which the glass plate 10 is made is a portion cut out from a glass sheet formed using the above-mentioned methods such as the float method or the down-draw method, it has been found that the above-mentioned variations may occur in a magnetic-disk glass substrate made by cutting out from the central region of the glass sheet excluding the edge regions near each side. Since it is not possible to directly measure the annealing effect of a narrower region within the central region in the central region of a large glass plate, even if there is a portion with a low annealing effect, it is not possible to notice this. The inventors investigated the causes of such variations even with conventional annealing and inferred that the main influence was a slight difference in thermal history between the peripheral portion and the central portion of the glass plate during the annealing process. They also found that the thermal shrinkage rate of the central portion (central region) of a large glass plate cannot be accurately determined unless the target portion is cut out, because, as long as it is connected to the surrounding peripheral portion (edge ​​region), the peripheral portion constrains the movement of the central portion, preventing thermal shrinkage of the central portion, or the central portion shrinks excessively due to thermal shrinkage of the peripheral portion. The inventors discovered that glass plate 10 obtained by the precision annealing described above eliminates variations in the annealing effect, and that even in the individualized glass (singulated glass) extracted from the central region 12, such variations in properties between individualized glass pieces are suppressed. Therefore, the measurement region 13 is cut out from the central region 12 of glass plate 10, as described above. The sizes of the end regions 11a, 11b, which determine the size of the central region 12, are determined from the viewpoint of suppressing the above-mentioned variations in the thermal shrinkage rate and the amount of change in flatness among the plurality of individual glass pieces.

[0051] For the above reasons, precision annealing is performed on the glass plate 10 so that the measurement area 13 of the glass plate 10 satisfies the above-mentioned ranges of thermal shrinkage and flatness change. Precision annealing is preferably performed by heating at Tg-110°C or higher for 4 hours or more, more preferably at Tg-80°C or higher for 4 hours or more. The conditions for such heating are determined with reference to the conditions for heat treatment of the magnetic film. The heating (heating), temperature maintenance, and cooling (temperature reduction) in precision annealing are preferably performed continuously in an atmosphere with the same conditions except for the temperature conditions (e.g., the atmosphere in a single annealing furnace). The temperature increase in precision annealing is preferably performed from room temperature (normal temperature) over 2.5 hours. In other words, the glass plate is preferably heated from room temperature to a temperature of preferably Tg-110°C or higher, more preferably Tg-80°C or higher, at a rate of 270°C / hour. The temperature is preferably lowered from a temperature of Tg-110°C or higher, more preferably from a temperature of Tg-80°C or higher, to room temperature at a rate of 50°C / hour.

[0052] The precision annealing is preferably performed using a plate material for annealing treatment (hereinafter referred to as a setter) described below, which allows for efficient production of a glass plate 10 that satisfies the above-mentioned ranges of flatness, thermal shrinkage, and flatness change.

[0053] The setter has a plate-like shape with a pair of main surfaces, at least one of which is configured to contact a main surface of the glass plate material that will become the glass sheet 10. The main surface of the setter is wider than the main surface of the glass plate material that will become the glass sheet 10, and extends beyond the entire periphery of the glass plate material. The length of extension is, for example, 5 centimeters or more in the direction away from the center of the glass plate material.

[0054] In order to efficiently obtain a glass plate 10 having a flatness of 30 μm or less, the flatness of the setter is preferably less than 30 μm, more preferably 20 μm or less, and even more preferably 10 μm or less.

[0055] The thermal conductivity of the setter is, for example, 1 to 200 W / (m·K) at 20° C. If the thermal conductivity of the setter is within this range, when precision annealing is performed on the glass plate material that is the basis for the glass plate 10, the glass plate material is likely to be heated and cooled evenly, and variation in the magnitude of the thermal shrinkage rate after precision annealing depending on the in-plane position of the glass plate 10 can be effectively suppressed.

[0056] Examples of materials for the setter include alumina (Al2O3), silicon carbide (SiC), silicon nitride (Si3N4), zirconia (ZrO2), sialon (Si3N4·Al2O3), steatite, spinel, cordierite, etc. Among these, alumina (Al2O3) and silicon carbide (SiC) are preferably used.

[0057] An example of precision annealing using a setter is a method using two setters and a heat insulating material. According to one embodiment, precision annealing is preferably performed in a state in which a glass plate is sandwiched between two setters having main surfaces larger than the glass plate and surrounded by a heat insulating material disposed in the gap between the setters. The heat insulating material is preferably made of a fibrous material having high heat resistance. As the fibrous material, in addition to rock wool, which will be described below, inorganic fibers such as ceramic fiber and glass fiber are preferably used. In this example, precision annealing is performed in a state in which the setters and the glass plate are stacked so that one glass plate is sandwiched between two setters having an area larger than the glass plate, and the gap between the two setters adjacent to the side (edge) of the glass plate is further filled with highly heat-resistant rock wool. The two setters have the same shape and are arranged to overlap each other almost exactly in the thickness direction while sandwiching the glass plate (i.e., without any in-plane misalignment), and the outer periphery of the setter extends out from the glass plate almost evenly all around, leaving a gap between the two setters near the entire edge of the glass plate. By lightly packing the gap with highly heat-resistant rock wool, the entire glass plate is covered with the setter and rock wool, and the setter load can be applied appropriately and evenly to the main surface of the glass plate. In this example, precision annealing places a lighter weight on the glass plate than precision annealing a laminate formed by alternately stacking multiple setters and multiple glass plate materials. This prevents the weight of the setter and glass plate materials from hindering the in-plane expansion and contraction of the glass plate materials located below, thereby contributing to reducing the thermal shrinkage rate regardless of the in-plane position of the glass plate 10. Rock wool is both insulating and breathable, so it can effectively seal gaps between setters, making it easier to heat or cool the entire glass sheet evenly (uniform heating), reducing the thermal shrinkage rate regardless of the position within the glass sheet. The rock wool is used to such an extent that it does not interfere with the load applied to the glass sheet by the setter placed on the glass sheet, and therefore the load of the setter is applied appropriately and evenly to the main surfaces of the glass sheet, which can prevent the flatness of the glass sheet from deteriorating during precision annealing, and in some cases can reduce the flatness. In other words, the above method can reduce the effect of the weight of the setter on the glass plate material, thereby reducing the thermal shrinkage rate of the glass plate 10 regardless of the position within the surface, while enhancing the effect of reducing the flatness of the glass plate material. The precision annealing described above is not limited to being performed on the original plate material of the glass plate 10, but can also be performed on the individualized glass plate material that will be the original glass plate 20, as will be described later.

[0058] The glass plate 10 can be manufactured by the glass plate manufacturing method including the above-described precision annealing.

[0059] (segmented glass) Fig. 2(a) shows an external view of a glass plate 20 according to one embodiment, and Fig. 2(b) shows a plan view of the glass plate 20, with the portion that will become the disk-shaped glass indicated by a broken line.

[0060] Glass plate 20 is a rectangular plate having a thickness of less than 0.68 mm, a flatness of 30 μm or less, and two orthogonal sides each having a length of 95 to 120 mm. This glass plate is smaller in size than glass plate (large glass plate) 10 described above, and may be referred to as "segmented glass" in this specification.

[0061] The glass plate 20 is a rectangular plate measuring 95 to 120 mm, and has a size suitable for producing a disk-shaped glass (described below) that serves as the raw material for a magnetic disk glass substrate, and thus for producing a magnetic disk glass substrate, with little machining allowance required. The glass plate 20 is preferably a square plate. By subjecting the glass plate material that is the basis for the square glass plate 20 to precision annealing, it becomes easier to reduce the thermal shrinkage rate and the anisotropy of the thermal shrinkage rate, for the same reasons as above. Note that even when the ratio of the length of the vertical and horizontal sides is slightly different (for example, when the ratio is 0.95 to 1.05), it is still considered within the above-mentioned range of square. The thickness of the glass plate 20 is preferably less than 0.61 mm, more preferably less than 0.58 mm. The lower limit of the thickness is not particularly limited, but is, for example, 0.2 mm.

[0062] The thermal shrinkage of the glass sheet 20 after a first heat treatment in which the glass sheet is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour is 130 ppm or less. The thermal shrinkage is preferably 90 ppm or less, more preferably 50 ppm or less. Furthermore, when the glass transition temperature of the glass sheet 20 is expressed as Tg (°C), the glass sheet 20 undergoes a second heat treatment in which the glass sheet is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere. The change in flatness of the glass sheet 20 is preferably 10 μm or less. The change in flatness is preferably 7.5 μm or less, more preferably 5 μm or less.

[0063] The glass plate 20 is a raw plate from which a disk-shaped glass is made. The area of ​​the main surfaces of the glass plate 20 is preferably 1.6 times or less, more preferably 1.5 times or less, the area of ​​the inside of the outer periphery of the disk-shaped glass. In this way, since the area of ​​the main surfaces of the glass plate 20 is close to the area of ​​the inside of the outer periphery of the disk-shaped glass (the influence of the internal hole is slight and can be ignored), the disk-shaped glass is likely to have the two characteristics of the glass plate 20 described above, namely, the thermal shrinkage rate and the amount of change in flatness. In other words, the above characteristics of the disk-shaped glass do not deviate significantly from the above characteristics of the glass plate 20 before cutting. This effect is particularly effective in a glass plate 20 produced by performing precision annealing (precision annealing after singulation) on a singulated glass plate material from which the glass plate 20 is made. That is, because the area of ​​the individualized glass plate is small, the effect of precision annealing easily reaches every corner of the outer periphery of the individualized glass plate, and the annealing effect varies little depending on the in-plane position. Therefore, the effect of suppressing deterioration of flatness when heat-treating the magnetic film in the disk-shaped glass cut from the glass plate 20 is greater. Thus, the closer the shape of the glass plate to be precision annealed is to the shape of the magnetic disk glass substrate, the greater the effect of precision annealing. Here, "segmentation" means obtaining segmented glass 20 from large glass sheet 10, or obtaining glass sheet material of the same dimensions as segmented glass 20 from the glass sheet material that is the source of large glass sheet 10.

[0064] The glass plate 20 is obtained, for example, by cutting out the glass plate 10 (precision annealed), or by cutting out a glass plate material that is the basis of the glass plate 10 and then performing precision annealing on the individual pieces.

[0065] The glass plate 20 is preferably subjected to an annealing treatment (for example, the precision annealing described above) to reduce the thermal shrinkage. The glass plate (the glass plate material from which the glass plate 20 is made) before the annealing treatment has anisotropy in the thermal shrinkage, i.e., the magnitude of the thermal shrinkage varies depending on the in-plane direction of the glass plate. The difference (absolute value) between the thermal shrinkage C1 in the in-plane direction where the thermal shrinkage is smallest and the thermal shrinkage C2 in the in-plane direction where the thermal shrinkage is largest may be greater than 10 ppm. Furthermore, the difference (absolute value) between the thermal shrinkage S1 of the glass plate in the direction where the thermal shrinkage is smallest and the thermal shrinkage S2 of the glass plate in the direction where the thermal shrinkage is largest may be greater than 1.0 μm.

[0066] The glass plate 20 preferably has a difference (absolute value) of 10 ppm or less between the thermal shrinkage C1 in the direction in which the thermal shrinkage is smallest and the thermal shrinkage C2 in the direction in which the thermal shrinkage is largest among the in-plane directions of the glass plate 20. Furthermore, the difference (absolute value) between the amount of thermal shrinkage S1 in the direction in which the thermal shrinkage is smallest and the amount of thermal shrinkage S2 in the direction in which the thermal shrinkage is largest is preferably 1.0 μm or less.

[0067] The glass plate 20 described above is produced, for example, by cutting it from a large glass plate 10 and dividing it into individual pieces. The cutting method may involve forming cut lines and breaking the glass plate using a known scriber (cutter), or by irradiating the large glass plate 10 with laser light to form defects at regular intervals and then joining the defects to separate the glass plate 20 from the large glass plate 10. The glass plate 20 is preferably cut from the central region 12 of the large glass plate 10. Therefore, it is preferable that at least one of the main surfaces of the glass plate 20 be a fire-polished surface. This can omit some of the grinding and polishing processes typically required for manufacturing glass substrates for magnetic disks, or reduce the machining allowance. In other words, it is preferable that at least one of the main surfaces of the glass plate 20 be an unground and / or unpolished surface.

[0068] The glass plate 20 can be produced, for example, by dicing a glass plate material before precision annealing that will become the large glass plate 10, and then performing precision annealing. That is, the glass plate 20 can be produced by a glass plate manufacturing method that includes precision annealing the diced glass plate material that will become the glass plate 20. In this method, precision annealing is performed by heating the individualized glass plate material that will become the glass plate 20, in the same manner as the precision annealing described above for the large glass sheet 10. The individualized glass plate material that will become the glass plate 20 used in this method is, for example, a plate material that is cut out from the glass plate material that will become the large glass sheet 10 without precision annealing, and has approximately the same dimensions and shape as the glass plate 20. According to the study of the present inventors, it has been found that by performing precision annealing on the singulated glass plate material that is the basis for the glass plate 20, a glass plate 20 can be obtained that has an even smaller amount of change in the thermal shrinkage rate and flatness, as described above, compared to the glass plate 20 cut out from the precision-annealed large glass plate 10. Therefore, by performing precision annealing on the singulated glass plate material that is the basis for the glass plate 20, a glass plate 20 can be obtained that has an even smaller amount of change in the thermal shrinkage rate and flatness, as described above.

[0069] (disk glass) FIG. 3 shows an external view of a disk-shaped glass 30 according to one embodiment.

[0070] The disk-shaped glass 30 is, for example, a raw plate that will become a glass substrate for a magnetic disk. The disk-shaped glass 30 has a circular outer periphery. A hole (inner hole) penetrating through the thickness direction is provided at the center of the disk-shaped glass, and the disk-shaped glass may have an annular shape, but may not have an inner hole, as in the disk-shaped glass 30 of the example shown in FIG.

[0071] The glass disk 30 has a thickness of less than 0.68 mm and a flatness of 30 μm or less. The glass disk 30 has a thermal shrinkage of 130 ppm or less when subjected to a first heat treatment in which the glass disk 30 is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour. The thermal shrinkage is preferably 90 ppm or less, more preferably 60 ppm or less, and even more preferably 50 ppm or less. When the glass disk 30 has a glass transition temperature Tg (°C), the glass disk 30 has a change in flatness of 10 μm or less when subjected to a second heat treatment in which the glass disk 30 is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere. The change in flatness is preferably 7.5 μm or less, more preferably 6 μm or less, and even more preferably 5 μm or less. The thickness of the disk-shaped glass 30 is preferably less than 0.61 mm, more preferably less than 0.58 mm. The lower limit of the thickness is not particularly limited, but is, for example, 0.2 mm.

[0072] The disc-shaped glass 30 preferably has a difference (absolute value) of 10 ppm or less between the thermal shrinkage C1 in the in-plane direction of the disc-shaped glass 30 where the thermal shrinkage is smallest and the thermal shrinkage C2 in the direction where the thermal shrinkage is largest. The difference (C2 - C1) is more preferably 7 ppm or less, and even more preferably 5 ppm or less. The difference (absolute value) of the thermal shrinkage S1 in the direction where the thermal shrinkage is smallest and the thermal shrinkage S2 in the direction where the thermal shrinkage is largest is preferably 1.0 μm or less. The difference (S2 - S1) is more preferably 0.7 μm or less, and even more preferably 0.5 μm or less.

[0073] The glass disk 30 is obtained by cutting out the glass plate 20, for example. The glass disk 30 can be cut out from the glass plate 20 by, for example, a known scribing method or coring method. The scribing method can be performed using, for example, a diamond scriber, a scribing wheel, a laser, or the like. Therefore, it is preferable that at least one main surface of the glass disk 30 is a fire-polished surface. This makes it possible to omit part of the grinding and polishing of the main surface of the substrate, which is generally required when manufacturing a glass substrate for a magnetic disk, and to reduce the machining allowance. In other words, it is preferable that at least one main surface of the glass disk 30 is an unground and / or unpolished surface. When the glass disk 30 is to be used as a raw material (intermediate body) for a magnetic disk glass substrate, the diameter of the glass disk 30 is preferably adjusted according to the size of the magnetic disk glass substrate to be finally manufactured. The numerical values ​​and numerical ranges shown below are all examples. When the glass disk is to be used as a raw material for a magnetic disk glass substrate having a nominal diameter of 3.5 inches, the outer diameter (diameter) can be 95 to 100 mm. When an inner hole is provided, the inner diameter (diameter) can be 23 to 25 mm. On the other hand, when the glass disk is to be used as a raw material for a magnetic disk glass substrate having a nominal diameter of 2.5 inches, the outer diameter (diameter) can be 65 to 70 mm. When an inner hole is provided, the inner diameter (diameter) can be 18 to 20 mm.

[0074] (Magnetic disk glass substrates) Fig. 4 shows an external view of a magnetic-disk glass substrate 40 according to one embodiment. The magnetic-disk glass substrate 40 shown in Fig. 4 has an inner hole in the center.

[0075] The size of the glass substrate 40 is not limited, but may be, for example, the size of a magnetic disk glass substrate having a nominal diameter of 3.5 inches or 2.5 inches. In the case of a magnetic disk glass substrate having a nominal diameter of 3.5 inches, the outer diameter (diameter) may be, for example, 95 to 100 mm, and the inner hole diameter (diameter) may be, for example, 24 to 26 mm. Specifically, for example, the outer diameter (diameter) may be 95 mm or 97 mm, and the inner hole diameter (diameter) may be, for example, 25 mm. On the other hand, in the case of a magnetic disk glass substrate having a nominal diameter of 2.5 inches, the outer diameter (diameter) may be, for example, 65 to 70 mm, and the inner hole diameter (diameter) may be, for example, 19 to 21 mm. Specifically, for example, the outer diameter (diameter) may be 65 mm or 67 mm, and the inner hole diameter (diameter) may be, for example, 20 mm.

[0076] The magnetic disk glass substrate 40 has a thickness of less than 0.68 mm and a flatness of 30 μm or less. The magnetic-disk glass substrate 40 has a thermal shrinkage of 130 ppm or less when subjected to a first heat treatment in which the substrate is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour. The thermal shrinkage is preferably 90 ppm or less, more preferably 60 ppm or less, and even more preferably 50 ppm or less. When the glass transition temperature of the magnetic-disk glass substrate 40 is expressed as Tg (°C), the change in flatness caused by the second heat treatment in which the substrate is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere is 10 μm or less. The change in flatness is preferably 7.5 μm or less, more preferably 6 μm or less, and even more preferably 5 μm or less. The thickness of the magnetic-disk glass substrate 40 is preferably less than 0.61 mm, more preferably less than 0.58 mm. The lower limit of the thickness is not particularly limited, but is, for example, 0.2 mm.

[0077] The magnetic-disk glass substrate 40 preferably has a difference (absolute value) of 10 ppm or less between the thermal shrinkage C1 in the in-plane direction of the magnetic-disk glass substrate 40 in which the thermal shrinkage is smallest and the thermal shrinkage C2 in the direction in which the thermal shrinkage is largest. The difference (C2 - C1) is more preferably 7 ppm or less, and even more preferably 5 ppm or less. The difference (absolute value) between the amount of thermal shrinkage S1 in the direction in which the thermal shrinkage is smallest and the amount of thermal shrinkage S2 in the direction in which the thermal shrinkage is largest is preferably 1.0 μm or less. The difference (S2 - S1) is more preferably 0.7 μm or less, and even more preferably 0.5 μm or less.

[0078] The magnetic disk glass substrate 40 is obtained by a manufacturing method for a magnetic disk glass substrate, which includes, for example, grinding and / or polishing of the main surfaces of the glass disk 30. The manufacturing method for a magnetic disk glass substrate may include, in addition to grinding and / or polishing of the main surfaces of the glass disk 30, processing such as forming a chamfered surface, grinding and / or polishing of the edge surfaces, chemical strengthening, cleaning, etc. Furthermore, when the glass substrate for a magnetic disk 40 is produced from a glass disk 30 that does not have an inner hole, the manufacturing method for a magnetic disk glass substrate may include processing for forming an inner hole in the glass disk 30 (for example, processing by the above-mentioned scribing method or coring method using a core drill).

[0079] According to one example, a method for manufacturing a glass substrate for a magnetic disk is carried out as follows. Specifically, chamfered surfaces are formed on the inner and outer peripheral end surfaces of a circular ring-shaped glass disk 30. Next, the main surfaces of the glass disk with the chamfered surfaces formed thereon are ground. In the grinding process, the main surfaces of the glass disk are ground using a grinding member having fixed abrasive grains formed in a sheet form, or a slurry containing free abrasive grains. Next, the main surfaces of the ground glass disk are polished. In the polishing process, polishing is carried out using a polishing pad and a slurry containing free abrasive grains with a grain size smaller than the free abrasive grains used in the grinding process. The polishing process can be divided into multiple processes and carried out using abrasive grains with different grain sizes or polishing pads with different hardnesses.

[0080] When chemical strengthening is performed, it is preferable to perform it before or after the final polishing treatment. The chemical strengthening treatment is performed, for example, by immersing the disk-shaped glass in a molten solution of a mixed salt of multiple types of nitrates. The cleaning treatment involves cleaning the disk-shaped glass with a cleaning solution after chemical strengthening or after the final polishing treatment. Note that a cleaning treatment may be added between the above treatments as needed.

[0081] (Experimental Example 1) To investigate the effects of the present invention, various magnetic disk glass substrates (Conventional Example 1 and Examples 1 to 5) shown in the table below were prepared, and the thermal shrinkage and change in roundness upon the first heat treatment were evaluated, as well as the degree of deterioration in flatness upon the second heat treatment. Regarding the thermal shrinkage, the difference in thermal shrinkage (C2-C1), the difference in thermal shrinkage (S2-S1), and the angle between the maximum and minimum directions of the thermal shrinkage were also evaluated to evaluate the anisotropy of the thermal shrinkage. However, when the difference in thermal shrinkage (S2-S1) was 0.5 μm or less, the anisotropy was deemed to be extremely small, and therefore the angle was not measured.

[0082] [Table 1]

[0083] [Table 2]

[0084] The magnetic disk glass substrates of Conventional Example 1 and Examples 1 to 5 all had the following specifications. Aluminosilicate glass with a glass transition temperature (Tg) of 810°C Outer diameter 97mm, inner diameter 25mm, plate thickness 0.5mm, flatness of main surface 5μm. The average retardation on the main surfaces of all the magnetic disk glass substrates prepared in the examples was 0.5 nm or less. In other words, the residual stress of the magnetic disk glass substrates prepared in the examples is sufficiently small, so it is considered that the residual stress has almost no effect on various evaluations.

[0085] The glass substrates of Conventional Example 1 and Examples 1 to 5 were each produced in the following manner. (Conventional Example 1) A glass sheet was formed using the overflow downdraw method while being slowly cooled, and both ends, which were thicker than the central portion in the width direction, were cut off. A predetermined region of the glass sheet was cut out from the remaining glass sheet to obtain a rectangular glass plate with a short side of 1000 mm, a long side of 1200 mm, and a thickness of 0.6 mm. The long side direction of the glass plate corresponded to the width direction of the glass sheet. A glass plate (segmented glass) was cut into a square with a side of 109 mm from the central region of the obtained glass plate, excluding end regions 200 mm inward from both ends in the short and long sides. Then, a disk-shaped glass having a diameter of 99 mm was cut from the above-mentioned individualized glass by a scribing method. At this time, the area ratio of the individualized glass to the disk-shaped glass was about 1.54. Then, using known methods, circular hole formation, chamfering, adjustment of the outer diameter and inner diameter, edge polishing, grinding and polishing of the main surfaces, cleaning, etc. were performed to obtain a magnetic disk glass substrate having the above specifications. Example 1 A magnetic disk glass substrate was obtained in the same manner as in Conventional Example 1, except that the precision annealing was performed on a rectangular glass plate (large plate) having short sides of 1000 mm, long sides of 1200 mm, and a thickness of 0.6 mm. Example 2 A magnetic disk glass substrate was obtained in the same manner as in Conventional Example 1, except that the individual glass plates were subjected to precision annealing. Example 3 A magnetic disk glass substrate was obtained in the same manner as in Example 2, except that the size of the segmented glass was 106 mm×106 mm and the area ratio of the segmented glass to the disk-shaped glass was about 1.46. Example 4 A magnetic disk glass substrate was obtained in the same manner as in Example 2, except that the size of the segmented glass was 112 mm×112 mm and the area ratio of the segmented glass to the disk-shaped glass was about 1.63. Example 5 A magnetic disk glass substrate was obtained in the same manner as in Example 4, except that the segmented glass was formed into a rectangular shape measuring 118.3 mm x 106 mm.

[0086] The precision annealing was performed by placing the glass plate in an annealing furnace with the atmospheric temperature adjusted to 700°C (Tg-110°C) and holding it there for 4 hours. More specifically, the temperature was raised to 700°C over 2.5 hours, held at 700°C for 4 hours, and then lowered at a rate of 50°C / hour. The glass plate was sandwiched between two setters whose main surfaces were wider than the glass plate and whose dimensions extended beyond the entire periphery of the glass plate, with the outer peripheries of the setters extending 5 cm beyond the entire periphery of the glass plate. Rock wool was then lightly packed into the gap between the two setters that contacted the sides of the glass plate, so that the entire glass plate was covered with the setters and rock wool.

[0087] <Measurement of heat shrinkage rate and circularity change> The glass substrate to be measured was subjected to the following first heat treatment. (First Heat Treatment) The glass substrate is placed in an annealing furnace at room temperature and heated to 700°C. The glass substrate is maintained at 700°C for 4 hours, and then cooled from 700°C to 400°C at a rate of 50°C / h.

[0088] Regarding the thermal shrinkage rate, glass base The thermal shrinkage rate was calculated from the change in diameter before and after the first heat treatment in a total of 25 directions passing through the center of the plate and spaced at central angle intervals of 7.2 degrees around this center, and the maximum value was taken as the thermal shrinkage rate of the glass substrate. The difference in thermal shrinkage (C2-C1) is the difference (absolute value) between the thermal shrinkage C1 in the direction with the smallest thermal shrinkage among the 25 directions and the thermal shrinkage C2 in the direction with the largest thermal shrinkage. The difference in thermal shrinkage (S2-S1) is the difference (absolute value) between the thermal shrinkage S1 in the direction with the smallest thermal shrinkage among the 25 directions and the thermal shrinkage S2 in the direction with the largest thermal shrinkage.

[0089] The change in circularity was calculated by subtracting the circularity before the first heat treatment from the circularity after the first heat treatment. All values ​​of the change in circularity were absolute values. The circularity was measured using a circularity measuring instrument. In principle, the circularity after the first heat treatment was greater than the circularity before the first heat treatment. The degree of deterioration in circularity was evaluated as follows: if the difference in circularity of the outer periphery of the glass substrate measured before and after the first heat treatment was 0.2 μm or less, it was evaluated as A; if it was more than 0.2 μm and 0.5 μm or less, it was evaluated as B; and if it was more than 0.5 μm, it was evaluated as C. Grades A and B were evaluated as indicating that the deterioration in circularity was suppressed.

[0090] <Measurement of flatness change> The glass substrate to be measured was subjected to the second heat treatment described below. (Second Heat Treatment) The glass substrate is placed in a heating apparatus at room temperature and heated to 650°C (corresponding to Tg -160°C) in 50 seconds. After maintaining the glass substrate at 650°C for 60 seconds, it is removed from the apparatus and allowed to cool naturally to room temperature in the atmosphere. As described above, the heating apparatus used is equipped with two panel heaters arranged parallel to each other at a distance. The glass substrate is attached to a holder (substrate holder) and can be placed upright in the gap between the panel heaters. The holder (substrate holder) with the glass substrate attached can be moved back and forth between the inside and outside of the heating apparatus, which is in the atmosphere.

[0091] The change in flatness was calculated by subtracting the flatness before the second heat treatment from the flatness after the second heat treatment. Each flatness value and the change in flatness were expressed as absolute values. The degree of deterioration in flatness was evaluated as follows: if the difference in flatness measured before and after the second heat treatment was 7 μm or less, it was rated as A; if it was more than 7 μm but less than 10 μm, it was rated as B; and if it was more than 10 μm, it was rated as C. Of these, A and B were evaluated as indicating that the deterioration in flatness had been suppressed.

[0092] A comparison between Conventional Example 1 and Example 1 reveals that precision annealing of the large plate before singulation reduces the thermal shrinkage rate during the first heat treatment to 130 ppm or less and suppresses the change in flatness during the second heat treatment to 10 μm or less. It also reveals that the difference in thermal shrinkage rate (C2-C1) is 10 ppm or less, suppressing the deterioration of the roundness of the glass substrate. A comparison between Example 1 and Example 2 reveals that precision annealing of the glass plate material after it has been singulated is more effective in suppressing deterioration in the flatness of the glass substrate than precision annealing of the large plate before it is singulated, and also more effective in suppressing deterioration in the roundness of the glass substrate. Comparison of Examples 2 to 4 reveals that by setting the area ratio of the segmented glass to the disk-shaped glass to be 1.6 or less, preferably 1.5 or less, the thermal shrinkage rate during the first heat treatment is reduced, and the effect of suppressing the amount of change in flatness accompanying the second heat treatment is improved. It is also clear that the difference (C2-C1) in the anisotropy of the thermal shrinkage rate is reduced, and the effect of reducing the amount of change in the roundness of the glass substrate is improved. A comparison between Example 4 and Example 5 shows that, compared to when the shape of the segmented glass is not square, when the shape of the segmented glass is square, the thermal shrinkage rate during the first heat treatment is reduced and the effect of suppressing the amount of change in flatness accompanying the second heat treatment is improved. It is also clear that, with regard to the anisotropy of the thermal shrinkage rate, the difference in the thermal shrinkage rates (C2-C1) is reduced, improving the effect of reducing the amount of change in the roundness of the glass substrate.

[0093] In addition, when the conditions for the first heat treatment for measuring the thermal shrinkage of other magnetic disk glass substrates (multiple) manufactured by the method of Conventional Example 1 were changed to increase the temperature from room temperature to 600°C at a rate of 100°C / hour, hold at 600°C for 80 minutes, and then decrease the temperature from 600°C to room temperature at a rate of 100°C / hour, the thermal shrinkage was measured and found to be 20 to 40 ppm, which was significantly smaller than that of Conventional Example 1 when the first heat treatment was performed. This is thought to be mainly due to the fact that both the heating temperature and the maintenance time in the thermal shrinkage measurement conditions were relaxed. This shows that the thermal shrinkage value is significantly affected by the heat treatment conditions in the measurement conditions.

[0094] (Experimental Example 2) Twenty glass substrates for magnetic disks were manufactured under the conditions of Conventional Example 1, and the thermal shrinkage rates (maximum values ​​of the thermal shrinkage rates in the above 25 directions) were measured. The difference (variation) between the maximum and minimum values ​​among the 20 substrates was calculated to be 188 ppm. Magnetic disk glass substrates were prepared under the conditions of Example 1 in the same manner as above, and the difference (variation) between the maximum and minimum values ​​of the thermal shrinkage among 20 substrates was calculated to be 37 ppm. Magnetic disk glass substrates were prepared under the conditions of Example 2 in the same manner as above, and the difference (variation) between the maximum and minimum values ​​of the thermal shrinkage among 20 substrates was calculated to be 9 ppm.

[0095] In addition, the same procedure as above (Experimental Example 2) was used except that instead of a glass substrate for a magnetic disk, segmented glass immediately before being cut out into a disk-shaped glass was used, and the variations in thermal shrinkage rates (difference between maximum and minimum values) of Conventional Example 1, Example 1, and Example 2 were compared, and results roughly similar to those of above (Experimental Example 2) were obtained. The above results show that (1) precision annealing reduces the variation in thermal shrinkage between individual pieces of glass, and (2) precision annealing reduces the variation in thermal shrinkage when performed on individual pieces of glass rather than on large sheets of glass.

[0096] The magnetic disk glass substrate, the circular glass plate, the glass plate, and the method for manufacturing the glass plate of the present invention have been described in detail above. However, the present invention is not limited to the above-described embodiments and examples, and various improvements and modifications may be made without departing from the spirit and scope of the present invention. [Explanation of symbols]

[0097] 10 Glass Plate (Large Glass Plate) 10a Short side 10b Long side 11a,11b End area 12 Central area 13 Measured area 20 Glass plate (segmented glass) 30 Glass disc 40 Glass substrate for magnetic disks

Claims

1. A rectangular glass plate having a plate thickness of less than 0.68 mm, the flatness of a square measurement region having a side of 100 mm cut out from a central region of the glass plate excluding end regions each having a length of 5 to 20% of the short side of the glass plate on the inner side of the glass plate from each end in a short side direction of the glass plate, and end regions each having a length of 5 to 20% of the long side of the glass plate on the inner side of the glass plate from each end in a long side direction of the glass plate, is 30 μm or less; the thermal shrinkage of the measurement area is 130 ppm or less when the measurement area is subjected to a first heat treatment in which the measurement area is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour; When the glass plate has a glass transition temperature Tg (°C), the change in flatness of the measurement area resulting from a second heat treatment in which the measurement area is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere is 10 μm or less; the thermal shrinkage of the measurement area is the maximum value of the thermal shrinkage measured in 25 directions that are parallel to the main surface of the measurement area, pass through the center of the measurement area, and are changed in circumferential directions by 7.2 degrees.

2. 2. The glass plate according to claim 1, wherein the short side length exceeds 900 mm.

3. 2. The glass plate according to claim 1, wherein the glass plate is a portion cut from a long glass sheet formed using any one of a float process, a Furcol process, a Pittsburgh process, a downdraw process, a Colburn process, and a redraw process.

4. 4. The glass plate according to claim 1, wherein a difference between a thermal shrinkage amount S1 of the measurement region in a direction in which the thermal shrinkage rate is smallest among the in-plane directions of the measurement region and a thermal shrinkage amount S2 of the measurement region in a direction in which the thermal shrinkage rate is largest is 1.0 μm or less.

5. A rectangular glass plate having a thickness of less than 0.68 mm, a flatness of 30 μm or less, and two orthogonal sides each having a length of 95 to 120 mm, a thermal shrinkage rate of 130 ppm or less when subjected to a first heat treatment in which the temperature is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour; When the glass transition temperature of the glass plate is expressed as Tg (°C), the change in flatness due to a second heat treatment in which the glass plate is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere is 10 μm or less; the thermal shrinkage percentage when subjected to the first heat treatment is a maximum value of thermal shrinkage percentages measured in 25 directions that are parallel to a main surface of the glass plate, pass through a center of the glass plate, and are changed in circumferential directions by 7.2 degrees.

6. The rectangular glass plate is a raw plate that will be used to produce a circular glass plate having a circular outer periphery, The glass plate according to claim 5 , wherein the area of ​​the main surface of the rectangular glass plate is 1.6 times or less the area of ​​the inside of the outer periphery of the disk-shaped glass.

7. A circular glass plate having a thickness of less than 0.68 mm, a flatness of 30 μm or less, and a circular outer periphery with a diameter of 95 to 100 mm, a thermal shrinkage rate of 130 ppm or less when subjected to a first heat treatment in which the temperature is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour; When the glass transition temperature of the disk-shaped glass is expressed as Tg (°C), the amount of change in flatness resulting from a second heat treatment in which the disk-shaped glass is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere is 10 μm or less; the thermal shrinkage rate when the first heat treatment is performed is the maximum value of the thermal shrinkage rates measured in 25 directions that are parallel to the main surfaces of the disk-shaped glass, pass through the center of the disk-shaped glass, and are changed by 7.2 degrees in the circumferential direction.

8. 8. The disk-shaped glass according to claim 7, wherein a difference between a thermal shrinkage amount S1 in the direction in which the thermal shrinkage rate is smallest among the 25 directions and a thermal shrinkage amount S2 in the direction in which the thermal shrinkage rate is largest is 1.0 μm or less.

9. A glass substrate for a magnetic disk having a thickness of less than 0.68 mm, a flatness of 30 μm or less, and a diameter of 95 to 100 mm, a thermal shrinkage rate of 130 ppm or less when subjected to a first heat treatment in which the temperature is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour; When the glass transition temperature of the magnetic disk glass substrate is expressed as Tg (°C), the change in flatness caused by a second heat treatment in which the substrate is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere is 10 μm or less; the thermal shrinkage rate when the first heat treatment is performed is the maximum value of the thermal shrinkage rates measured in 25 directions that are parallel to the main surface of the magnetic-disk glass substrate, pass through the center of the magnetic-disk glass substrate, and are changed in circumferential directions by 7.2 degrees.

10. 10. The magnetic disk glass substrate according to claim 9, wherein a difference between a thermal shrinkage amount S1 in the direction in which the thermal shrinkage rate is smallest among the 25 directions and a thermal shrinkage amount S2 in the direction in which the thermal shrinkage rate is largest is 1.0 μm or less.

11. 11. The magnetic disk glass substrate according to claim 9, wherein the change in circularity caused by the first heat treatment is 0.5 [mu]m or less.

12. A method for manufacturing a glass sheet, comprising: The method includes a step of annealing a glass plate material that is the base of the glass plate, The glass plate is A rectangular plate having a plate thickness of less than 0.68 mm, the flatness of a square measurement region having a side of 100 mm cut out from a central region of the glass plate excluding end regions each having a length of 5 to 20% of the short side of the glass plate on the inner side of the glass plate from each end in a short side direction of the glass plate, and end regions each having a length of 5 to 20% of the long side of the glass plate on the inner side of the glass plate from each end in a long side direction of the glass plate, is 30 μm or less; the thermal shrinkage of the measurement area is 130 ppm or less when the measurement area is subjected to a first heat treatment in which the measurement area is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour; When the glass plate has a glass transition temperature Tg (°C), the change in flatness of the measurement area resulting from a second heat treatment in which the measurement area is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere is 10 μm or less; the thermal shrinkage of the measurement region is the maximum value of the thermal shrinkage measured in 25 directions that are parallel to a main surface of the measurement region, pass through the center of the measurement region, and are changed in circumferential directions by 7.2 degrees.

13. A method for manufacturing a glass sheet, comprising: Annealing a glass plate material that is the base of the glass plate; and removing the glass plate from the glass plate material after the annealing treatment. The glass plate is A rectangular plate having a plate thickness of less than 0.68 mm, a flatness of 30 μm or less, and two orthogonal sides each having a length of 95 to 120 mm, a thermal shrinkage rate of 130 ppm or less when subjected to a first heat treatment in which the temperature is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour; When the glass transition temperature of the glass plate is expressed as Tg (°C), the change in flatness due to a second heat treatment in which the glass plate is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere is 10 μm or less; the thermal shrinkage rate when the first heat treatment is performed is a maximum value of thermal shrinkage rates measured in 25 directions that are parallel to a main surface of the glass plate, pass through a center of the glass plate, and are changed in circumferential directions by 7.2 degrees.

14. The rectangular glass plate is a square plate that serves as a base for a circular glass plate having a circular outer periphery, The method for manufacturing a glass plate according to claim 12 or 13, wherein an area of ​​a main surface of the rectangular glass plate is 1.6 times or less an area of ​​an inner periphery of the disk-shaped glass.

15. A method for manufacturing a disk-shaped glass, Annealing a glass plate material that is the basis for the disk-shaped glass; and removing the glass disc from the glass plate material after the annealing treatment. The glass plate material is a rectangular glass plate, The disk-shaped glass is The plate thickness is less than 0.68 mm, the flatness is 30 μm or less, and the diameter is 95 to 100 mm; a thermal shrinkage rate of 130 ppm or less when subjected to a first heat treatment in which the temperature is maintained at 700°C for 4 hours and then cooled from 700°C to 400°C at a rate of 50°C / hour; When the glass transition temperature of the disk-shaped glass is expressed as Tg (°C), the amount of change in flatness resulting from a second heat treatment in which the disk-shaped glass is maintained at Tg-160°C for 60 seconds and then cooled to room temperature in the atmosphere is 10 μm or less; the thermal shrinkage rate when the first heat treatment is performed is the maximum value of the thermal shrinkage rates measured in 25 directions that are parallel to the main surfaces of the disk-shaped glass, pass through the center of the disk-shaped glass, and are changed in circumferential directions by 7.2 degrees.

16. The rectangular glass plate is square, The method for manufacturing a disk-shaped glass according to claim 15, wherein an area of ​​a main surface of the rectangular glass plate is 1.6 times or less an area of ​​an inner periphery of the disk-shaped glass.

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