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GLASS SHEET FOR CHEMICAL STRENGTHENING, MANUFACTURING METHOD OF STRENGTHENED GLASS SHEET, AND GLASS SHEET

20240132399 ยท 2024-04-25

    Inventors

    Cpc classification

    International classification

    Abstract

    In a glass sheet 12 for chemical strengthening having a thickness of 0.1 mm or less, the glass sheet 12 has a warped shape as a whole or includes a warped part in a portion thereof. When the glass sheet 12 is placed on a horizontal surface with one main surface 12a facing upward, a first peak position D1 is present in a part inside a peripheral edge portion 12e of the glass sheet 12, where the first peak position D1 is a position having the highest height from the horizontal surface in the glass sheet 12, and the peripheral edge portion 12e is a part having a width of 10 mm along a peripheral edge of the glass sheet 12.

    Claims

    1. A glass sheet for chemical strengthening having a thickness of 0.1 mm or less, wherein the glass sheet has a warped shape as a whole or includes a warped part in a portion thereof, the thickness of the glass sheet is t [mm], and a first peak position is present in a part inside a peripheral edge portion of the glass sheet, where the first peak position is, when a first placement form is adopted in which the glass sheet is placed on a horizontal surface with one main surface facing upward, a position having a highest height from the horizontal surface in the glass sheet, and the peripheral edge portion is a part having a width of 10 mm along a peripheral edge of the glass sheet.

    2. The glass sheet for chemical strengthening according to claim 1, wherein when the first placement form is adopted, a relationship of t.sup.2/W.sub.1OUT>0.005 is satisfied, where W.sub.1OUT [mm] is a height at a position having a highest height from the horizontal surface in the peripheral edge portion.

    3. The glass sheet for chemical strengthening according to claim 1, wherein when the first placement form is adopted, a relationship of W.sub.1OUT?0.20 mm is satisfied, where W.sub.1OUT [mm] is a height at a position having a highest height from the horizontal surface in the peripheral edge portion.

    4. The glass sheet for chemical strengthening according to claim 1, wherein when the first placement form is adopted, a relationship of W.sub.1OUT/t<5 is satisfied, where W.sub.1OUT [mm] is a height at a position having a highest height from the horizontal surface in the peripheral edge portion.

    5. The glass sheet for chemical strengthening according to claim 1, wherein a relationship of W.sub.1MAX/t<15 is satisfied, where W.sub.1MAX [mm] is the height of the glass sheet from the horizontal surface at the first peak position.

    6. The glass sheet for chemical strengthening according to claim 1, wherein the one main surface and another main surface on a back side of the one main surface are fire-polished surfaces.

    7. The glass sheet for chemical strengthening according to claim 1, wherein the thickness of the glass sheet is 0.05 mm or less.

    8. The glass sheet for chemical strengthening according to claim 1, wherein the glass sheet as a whole has a substantially uniform thickness.

    9. The glass sheet for chemical strengthening according to claim 1, wherein when a second placement form is adopted in which the glass sheet is placed on the horizontal surface with another main surface on a back side of the one main surface facing upward, a second peak position is present in a part inside the peripheral edge portion of the glass sheet, where the second peak position is a position having a highest height from the horizontal surface in the glass sheet.

    10. The glass sheet for chemical strengthening according to claim 9, wherein when the second placement form is adopted, a relationship of t.sup.2/W.sub.2OUT>0.005 is satisfied, where W.sub.2OUT [mm] is a height at a position having a highest height from the horizontal surface in the peripheral edge portion.

    11. The glass sheet for chemical strengthening according to claim 9, wherein when the second placement form is adopted, a relationship of W.sub.2OUT?0.20 mm is satisfied, where W.sub.2OUT [mm] is a height at a position having a highest height from the horizontal surface in the peripheral edge portion.

    12. The glass sheet for chemical strengthening according to claim 9, wherein when the second placement form is adopted, a relationship of W.sub.2OUT/t<5 is satisfied, where W.sub.2OUT [mm] is a height at a position having a highest height from the horizontal surface in the peripheral edge portion.

    13. The glass sheet for chemical strengthening according to claim 9, wherein a relationship of W.sub.2MAX/t<15 is satisfied, where W.sub.2MAX [mm] is the height of the glass sheet from the horizontal surface at the second peak position.

    14. The glass sheet for chemical strengthening according to claim 1, wherein the glass sheet is aluminosilicate glass, and the glass sheet includes as a glass composition, in mass %, from 50% to 80% of SiO.sub.2, from 5% to 25% of Al.sub.2O.sub.3, from 0% to 15% of B.sub.2O.sub.3, from 1% to 20% of Na.sub.2O, and from 0% to 10% of K.sub.2O.

    15. The glass sheet for chemical strengthening according to claim 14, wherein the glass sheet is aluminosilicate glass, and includes as the glass composition, in mass %, from 60%/to 80% of SiO.sub.2, from 8% to 20% of Al.sub.2O.sub.3, from 0% to 5% of B.sub.2O.sub.3, from 4% to 16% of Na.sub.2O, and from 0.01% to 10% of K.sub.2O.

    16. The glass sheet for chemical strengthening according to claim 1, wherein the glass sheet has a rectangular shape, and the glass sheet has a length of from 150 mm to 1100 mm and a width of from 150 mm to 1300 mm.

    17. A manufacturing method of a strengthened glass sheet, the manufacturing method comprising: preparing the glass sheet for chemical strengthening according to claim 1; cutting out a product-sized glass sheet from the glass sheet; and chemically strengthening the product-sized glass sheet to obtain a strengthened glass sheet.

    18. A glass sheet having a thickness of 0.1 mm or less, wherein the glass sheet has a warped shape as a whole or includes a warped part in a portion thereof, and when a first placement form is adopted in which the glass sheet is placed on a horizontal surface with one main surface facing upward, a first peak position is present in a part inside a peripheral edge portion of the glass sheet, where the first peak position is a position having a highest height from the horizontal surface in the glass sheet, and the peripheral edge portion is a part having a width of 10 mm along a peripheral edge of the glass sheet.

    19. The glass sheet according to claim 18, wherein when a second placement form is adopted in which the glass sheet is placed on the horizontal surface with another main surface on a back side of the one main surface facing upward, a second peak position is present in a part inside the peripheral edge portion of the glass sheet, where the second peak position is a position having a highest height from the horizontal surface in the glass sheet.

    Description

    BRIEF DESCRIPTION OF DRAWINGS

    [0045] FIG. 1 is a cross-sectional view illustrating a preparation step in a manufacturing method of a strengthened glass sheet.

    [0046] FIG. 2 is a cross-sectional view illustrating the preparation step in the manufacturing method of the strengthened glass sheet.

    [0047] FIG. 3 is a plan view illustrating a glass sheet for chemical strengthening.

    [0048] FIG. 4 is a plan view illustrating the glass sheet for chemical strengthening.

    [0049] FIG. 5 is a diagram illustrating a warpage distribution at Y=105 when a first placement form is adopted in Example 5.

    [0050] FIG. 6 is a diagram illustrating a warpage distribution when the first placement form is adopted in Example 14.

    [0051] FIG. 7 is a diagram illustrating a warpage distribution at Y=55 when the first placement form is adopted in Example 14.

    [0052] FIG. 8 is a diagram illustrating a warpage distribution when a second placement form is adopted in Example 14.

    [0053] FIG. 9 is a diagram illustrating a warpage distribution when the first placement form is adopted in Example 15.

    [0054] FIG. 10 is a diagram illustrating a warpage distribution when the second placement form is adopted in Example 15.

    [0055] FIG. 11 is a diagram illustrating a warpage distribution when the first placement form is adopted in Example 16.

    [0056] FIG. 12 is a diagram illustrating a warpage distribution when the second placement form is adopted in Example 16.

    [0057] FIG. 13 is a diagram illustrating a warpage distribution when the first placement form is adopted in Example 17.

    [0058] FIG. 14 is a diagram illustrating a warpage distribution when the second placement form is adopted in Example 17.

    [0059] FIG. 15 is a diagram illustrating a warpage distribution when the first placement form is adopted in Example 18.

    [0060] FIG. 16 is a diagram illustrating a warpage distribution when the second placement form is adopted in Example 18.

    [0061] FIG. 17 is a diagram illustrating a warpage distribution when the first placement form is adopted in Comparative Example 4.

    [0062] FIG. 18 is a diagram illustrating a warpage distribution when the second placement form is adopted in Comparative Example 4.

    [0063] FIG. 19 is a diagram illustrating a warpage distribution when the first placement form is adopted in Example 19.

    [0064] FIG. 20 is a diagram illustrating a warpage distribution when the second placement form is adopted in Example 19.

    DESCRIPTION OF EMBODIMENTS

    [0065] Hereafter, a glass sheet for chemical strengthening and a manufacturing method of a strengthened glass sheet according to an embodiment will be described with reference to the accompanying drawings.

    [0066] The manufacturing method of a strengthened glass sheet includes a preparation step of preparing a glass sheet for chemical strengthening, a cutting out step of cutting out a product-sized glass sheet from the prepared glass sheet, and a strengthening step of chemically strengthening the cut-out product-sized glass sheet to obtain a strengthened glass sheet.

    Preparation Step

    [0067] The glass sheet for chemical strengthening according to the present embodiment is manufactured by, for example, a known forming method involving sheet drawing, such as a down-draw method including an overflow down-draw method, a slot down-draw method, and a redraw method, or a float method. According to the overflow down-draw method, since both the front and back surfaces of a formed glass ribbon are formed without coming into contact with any part of a forming body in a forming process, there is an advantage that a fire-polished surface having a very smooth and very flat surface property is obtained, the very flat surface property being due to appropriate temperature control. In particular, in the case of an ultra-thin strengthened glass sheet, a scratch present on a glass surface at the time of bending serves as a starting point of breakage. Therefore, the overflow down-draw method is most suitable as a non-contact forming method that is unlikely to cause a scratch. In the present embodiment, a glass ribbon is formed by the overflow down-draw method, and a rectangular glass sheet having a predetermined size is obtained by cutting out the glass ribbon. The preparation step includes a forming step P1, an annealing step P2, and a cooling step P3 illustrated in FIGS. 1 and 2, as well as a cutting step (not illustrated).

    [0068] In the preparation step, first, the forming step P1 to the cooling step P3 are performed by using a manufacturing apparatus 1 illustrated in FIGS. 1 and 2 to obtain a band-shaped glass film 2 that serves as the base of the glass sheet for chemical strengthening.

    [0069] The forming step P1 is performed in a forming zone ZN1. In the forming step P1, a glass ribbon 5 is continuously formed from a molten glass 4 by a forming body 3 for the overflow down-draw method. The forming body 3 is accommodated in a forming furnace 6. A heating device (for example, a panel heater) or the like (not illustrated) that heats the forming body 3 is installed at the forming furnace 6.

    [0070] The forming body 3 includes a groove 3a that causes the molten glass 4 to flow in, a pair of side surfaces 3b, 3b that cause the molten glass 4 overflowing from both sides of the groove 3a to flow downward, and a lower end portion 3c that causes the molten glass 4 flowing downward along the respective side surfaces 3b to be fused (to converge). The molten glass 4 fused at the lower end portion 3c is formed into the glass ribbon 5 by the forming body 3.

    [0071] The glass ribbon 5 includes an effective portion 5a located at the center in a width direction (left-right direction in FIG. 1, and a direction perpendicular to the paper surface in FIG. 2) of the glass ribbon 5, and non-effective portions 5b each located at a respective one of both ends in the width direction, with the effective portion 5a interposed therebetween. The effective portion 5a is a part including a portion that is later turned into a product. The non-effective portions 5b are parts that are not turned into a product and are later discarded. Glass pool portions (also referred to as selvage portions) that are thicker than other parts are formed in parts corresponding to widthwise end edges (edges) of the glass ribbon 5 of the non-effective portions 5b forming the widthwise end portions of the glass ribbon 5.

    [0072] For the glass ribbon 5 immediately after being formed, shrinkage in the width direction is suppressed by using an edge roller 7 (a cooling roller) disposed immediately below the forming body 3.

    [0073] A pair of the edge rollers 7 are disposed sandwiching the glass ribbon 5 in a thickness direction. Each of the pair of edge rollers 7, 7 includes a shaft 7a extending in the width direction of the glass ribbon 5, and a first roll 7b and a second roll 7c coupled to each other via the shaft 7a. Both of the rolls 7b, 7c are made of a heat-resistant material (such as platinum or a platinum alloy), and are each brought into contact with a respective one of the non-effective portions 5b of the glass ribbon 5. The glass ribbon 5 is fed downward while being sandwiched in the thickness direction between the first rolls 7b, 7b of the pair of edge rollers 7, 7 and between the second rolls 7c, 7c of the pair of edge rollers 7, 7 to suppress shrinkage in the width direction of the glass ribbon 5.

    [0074] The annealing step P2 is performed in an annealing zone ZN2. In the annealing step P2, the glass ribbon 5 descending from the forming zone ZN1 is annealed to a temperature of a strain point or below while being guided downward. To perform the annealing step P2, an annealing furnace 8, and annealer rollers 9 disposed in a plurality of vertical stages (vertical five stages in the illustrated example) are used.

    [0075] The annealing furnace 8 is disposed below the forming furnace 6. In the annealing furnace 8, a heating device (for example, a panel heater) or the like (not illustrated) that adjusts the atmosphere temperature in the annealing furnace 8 is installed. For example, in the case of a material of a glass sheet for chemical strengthening, a heating device, a heat insulating member, or the like is preferably provided that can adjust the temperature in a region from a temperature lower than a softening point (for example, 860? C.) to the strain point (for example, 560? C.), with a temperature gradient formed in a width direction of the glass sheet in the temperature range. In the annealing step P2, in particular in the initial stage of the annealing step P2 where the temperature of the glass ribbon 5 is relatively high, the annealing rate is relatively increased toward the parts in the widthwise end portions (toward the glass pool portions), and the annealing rate is relatively decreased toward the part in the widthwise central portion. Specifically, the annealing step P2 is performed at a temperature decreasing rate of, for example, from 1? C. to 10? C./sec at the central portion and from 3? C. to 20? C./sec at the end portions. Accordingly, in the glass sheet (see FIGS. 3 and 4) obtained by the cutting step performed later, warpage at the peripheral edge portion of the glass sheet can be suppressed.

    [0076] In each stage of the plurality of vertical stages, a pair of the annealer rollers 9 are disposed sandwiching the glass ribbon 5 in the thickness direction. Each of the pair of annealer rollers 9, 9 includes a shaft 9a extending in the width direction of the glass ribbon 5, and a first roll 9b and a second roll 9c coupled to each other via the shaft 9a. Both of the rolls 9b, 9c are made of ceramic as an example, and can each come into contact with a respective one of the non-effective portions 5b of the glass ribbon 5. The glass ribbon 5 is guided downward between the first rolls 9b, 9b of the pair of annealer rollers 9, 9 and between the second rolls 9c, 9c of the pair of annealer rollers 9, 9.

    [0077] Here, the first rolls 9b, 9b and the second rolls 9c, 9c only restrict the glass ribbon 5 from shaking along the thickness direction without sandwiching the glass ribbon 5 from the front and back surfaces thereof. In other words, gaps are formed between the first roll 9b and the glass ribbon 5 and between the second roll 9c and the glass ribbon 5.

    [0078] Note that as another method for suppressing warpage at the peripheral edge portion of the glass sheet obtained by the cutting step, there is also a method in which the non-effective portions 5b of the glass ribbon 5 are sandwiched by the annealer rollers 9 from both the front and back sides thereof in the annealing furnace 8 to change a tensile force in the width direction acting on the glass ribbon 5. For example, increasing the tensile force in the width direction acting on the glass ribbon 5 can suppress the warpage in the glass ribbon 5, in particular at both ends in the width direction of the effective portion 5a. As a result, warpage at the peripheral edge portion of the glass sheet can be suppressed.

    [0079] The cooling step P3 is performed in a cooling zone ZN3. In the cooling step P3, the glass ribbon 5 that passed through the annealing zone ZN2 is cooled while being pulled downward by a support roller 10. The support roller 10 is disposed in a cooling chamber 11 disposed below the annealing furnace 8.

    [0080] A pair of the support rollers 10 are disposed sandwiching the glass ribbon 5 in the thickness direction. Each of the pair of support rollers 10, 10 includes a shaft 10a extending in the width direction of the glass ribbon 5, and a first roll 10b and a second roll 10c coupled to each other via the shaft 10a. Both of the rolls 10b, 10c are made of rubber as an example, and are each brought into contact with a respective one of the non-effective portions 5b of the glass ribbon 5. A conveyance speed V1 (sheet pulling speed) of the glass ribbon 5 is determined by pulling the glass ribbon 5 with the glass ribbon 5 sandwiched in the thickness direction between the first rolls 10b, 10b of the pair of support rollers 10, 10 and between the second rolls 10c, 10c of the pair of support rollers 10, 10. The glass ribbon 5 that passed through the cooling zone ZN3 along with the conveyance is obtained as the band-shaped glass film 2.

    [0081] When the cooling step P3 is completed, the cutting step is performed next.

    [0082] In the cutting step, a first cutting for cutting out a glass film original sheet from the band-shaped glass film 2, and a second cutting for cutting out a glass sheet for chemical strengthening from the glass film original sheet are performed.

    [0083] In the first cutting, the band-shaped glass film 2 is repeatedly cut (cut in a width direction) by a predetermined length to continuously cut out the glass film original sheets from the band-shaped glass film 2. Note that each of the cut-out glass film original sheets includes the effective portion 5a, and the non-effective portions 5b each located at a respective one of both ends, with the effective portion 5a interposed therebetween. In the second cutting, the non-effective portions 5b are separated and removed from each of the glass film original sheets to cut out the effective portion 5a as a glass sheet for chemical strengthening. Since the first and second cuttings can be performed by a known method, a detailed description thereof will be omitted.

    [0084] Note that in the present embodiment, the non-effective portions 5b of the band-shaped glass film 2 are separated and removed after cutting out of the glass film original sheet. However, the present invention is not limited thereto. As another method, the glass sheet for chemical strengthening may be continuously cut out from the band-shaped glass film 2 by first continuously separating and removing the non-effective portions 5b of the band-shaped glass film 2 and then repeatedly cutting (cutting in the width direction) the band-shaped glass film 2 from which the non-effective portions 5b have been removed by a predetermined length.

    [0085] In addition, a method may be employed in which the non-effective portions of the band-shaped glass film 2 are continuously separated and removed, and then the band-shaped glass film 2 is continuously wound in a roll shape with a band-shaped cushion material (such as a band-shaped protective sheet made of resin) interposed. In this case, it is sufficient that the band-shaped glass film 2 is unwound from the roll by a length required in a later step, and the glass sheet for chemical strengthening is cut out from the unwound band-shaped glass film 2. Therefore, an effect of improving collection efficiency of the glass sheet for chemical strengthening, that is, a cost reduction effect is achieved. Note that when this method is employed, the treatment of an end surface, such as polishing, heat treatment, and etching, which will be described later, may be performed after the glass sheet for chemical strengthening is cut out from the band-shaped glass film 2.

    [0086] When the first and second cuttings are performed, the cutting step is completed, whereby the preparation step is completed. Note that although the glass sheet for chemical strengthening is obtained by using the overflow down-draw method in the present embodiment, besides that, the glass sheet for chemical strengthening may be obtained by using the slot down-draw method, the float method, the redraw method, or the like.

    [0087] Here, the end surface of the cut-out glass sheet for chemical strengthening is preferably chamfered or treated to improve strength by polishing, heat treatment, etching, or the like. On the other hand, the front and back surfaces of the glass sheet for chemical strengthening are not subjected to a polishing treatment or the like (for example, a chemical polishing treatment, such as a slimming treatment for reducing the thickness) after forming. Therefore, the front and back surfaces of the glass sheet for chemical strengthening are fire-polished surfaces.

    [0088] When the preparation step is completed as described above, a glass sheet for chemical strengthening 12 (hereinafter simply referred to as the glass sheet 12) as illustrated in FIGS. 3 and 4 is prepared. The glass sheet 12 illustrated in FIG. 3 and the glass sheet 12 illustrated in FIG. 4 are the same glass sheet. FIG. 3 illustrates a case where a first placement form is adopted in which the glass sheet 12 is placed on a horizontal surface with one main surface 12a of the front and back surfaces of the glass sheet 12 facing upward. On the other hand, FIG. 4 illustrates a case where a second placement form is adopted in which the glass sheet 12 is placed on the horizontal surface with another main surface 12b on the back side of the one main surface 12a facing upward. Note that the horizontal surface in the present embodiment is a horizontal support surface provided on a surface plate (not illustrated).

    [0089] Note that the present embodiment illustrates as an example a case in which, of the front and back surfaces of the glass sheet 12, the one main surface 12a is a guaranteed surface, which is a surface suitable for being subjected to a film forming step or the like and whose surface property is to be guaranteed, and the another main surface 12b is a non-guaranteed surface, which is not required to have a surface property the same as or similar to that of the one main surface 12a. As a way of determining the guaranteed surface and the non-guaranteed surface, for example, of the front and back surfaces of the glass sheet 12, the one main surface 12a on the side on which the number of times of contact with conveyance rollers or the like until the point of time when the above-mentioned preparation step is completed is relatively small is used as the guaranteed surface, and the another main surface 12b on the side on which the number of times of contact is relatively large is used as the non-guaranteed surface. In this case, comparison between the one main surface 12a and the another main surface 12b reveals that defects, such as scratches and contamination, occur less on the one main surface 12a which is the guaranteed surface than on the another main surface 12b.

    [0090] While there is no intention to limit the type of the glass sheet 12, the glass sheet 12 in the present embodiment is aluminosilicate glass. The glass sheet 12 contains, as one example of the glass composition, in mass %, from 50% to 80% of SiO.sub.2, from 5% to 25% of Al.sub.2O.sub.3, from 0% to 15% of B.sub.2O.sub.3, from 1% to 20% of Na.sub.2O, and from 0% to 10% of K.sub.2O.

    [0091] More preferably, the glass sheet 12 according to the present embodiment contains as the glass composition, in mass %, from 60% to 80% of SiO.sub.2, from 8% to 18% of Al.sub.2O.sub.3, from 0% to 5% of B.sub.2O.sub.3, from 0.01% to 10% of Li.sub.2O, from 4% to 16% of Na.sub.2O, and from 0.01% to 10% of K.sub.2O.

    [0092] Note that increasing the content of Al.sub.2O.sub.3 greatly improves ion exchange performance of the glass for chemical strengthening, but with too high a content thereof, the devitrification deteriorates. That is, a liquidus temperature becomes too high or a liquidus viscosity becomes too low, making forming by the overflow down-draw method impossible.

    [0093] Na.sub.2O is an ion-exchange component and has effects of lowering high-temperature viscosity of the glass to enhance meltability and formability, reducing incidence of cracks, and lowering the strain point. In addition, Na.sub.2O is also a component that improves devitrification. However, when the content of Na.sub.2O is increased, thermal expansion coefficient may become too high, decreasing thermal shock resistance of the glass or making it difficult to match the thermal expansion coefficient of peripheral materials. Also, with too large a content thereof, the devitrification tends to deteriorate instead.

    [0094] B.sub.2O.sub.3 has the effect of lowering the liquidus temperature, high-temperature viscosity, and density of the glass. However, when the content of B.sub.2O.sub.3 is high, there is a possibility that burning occurs on a surface due to ion exchange. In addition, the strain point may be excessively lowered, causing stress relaxation to easily proceed during the ion exchange and a desired compressive stress may not be obtained.

    [0095] The type of the glass sheet 12 in the present embodiment is not limited to the glass for chemical strengthening and the present invention may be applied to other ultra-thin glass sheets, such as low-alkali glass substrates. In an ultra-thin glass sheet, regardless of the material, the problem of occurrence of breakage due to warpage is similarly induced in handling steps, such as cutting, cleaning, packing, and chemical strengthening, and thus shape management of the glass sheet is important.

    [0096] The glass sheet 12 in the present embodiment desirably has a thermal expansion coefficient of from 70 to 100?10.sup.?7/? C. at from 30? C. to 380? C. In order to suppress warpage after forming or at the time of chemical strengthening, it is preferable to decrease the thermal expansion coefficient. However, when the thermal expansion coefficient is not compatible with the peripheral materials, a problem, such as coming off of the glass substrate, may occur. For example, when the glass substrate is used as a cover glass for a foldable display, there is a metal or an organic material, such as an adhesive, around the glass substrate. If the thermal expansion coefficient of the glass substrate does not match that of the metal or the organic material, the glass substrate comes off when the glass substrate is bonded to the peripheral materials using an organic adhesive. With an object to make the thermal expansion coefficient of the glass substrate more easily match that of the peripheral materials, to increase the thermal expansion coefficient of the glass in the present invention, it is sufficient that the content of alkali metal oxide components or alkaline earth metal oxide components is increased, or the content of SiO.sub.2 or Al.sub.2O.sub.3 is decreased. To decrease the thermal expansion coefficient, it is sufficient that the content of alkali metal oxide components or alkaline earth metal oxide components is decreased, or the content of SiO.sub.2 or Al.sub.2O.sub.3 is increased.

    [0097] It is more preferable that the glass sheet 12 in the present embodiment desirably has a thermal expansion coefficient of from 75 to 92?10.sup.?7/? C. at from 30? C. to 380? C. When the thermal expansion is relatively small, thermal deformation in a high-temperature step of strengthening the glass sheet for chemical strengthening is small. Therefore, it is possible to suppress contact with a jig or the like for holding the glass in the chemical strengthening step or an increase in local warpage after strengthening due to partial remaining of a molten salt, such as high-temperature potassium nitrate.

    [0098] While the shape of the glass sheet 12 is not particularly limited, the glass sheet 12 has a rectangular shape in the present embodiment. An example of the size of the glass sheet 12 is 150 mm?150 mm to 1100 mm?1300 mm. The glass sheet 12 in the present embodiment has a long side 12x and a short side 12y, the length of the long side 12x being 400 mm or 500 mm, and the length of the short side 12y being 300 mm or 400 mm. Note that in the present glass sheet 12, the direction in which the long side 12x extends coincides with a sheet drawing direction (longitudinal direction of the glass ribbon 5) in the forming step P1 to the cooling step P3 described above. Also, as described above, even in the case where the band-shaped glass film 2 from which the non-effective portions 5b have been removed is wound into a roll shape and then the band-shaped glass film 2 unwound from the roll is cut to obtain the glass sheet 12, the direction in which the long side 12x of the glass sheet 12 extends coincides with the sheet drawing direction.

    [0099] Note that when the width of the glass ribbon 5 (band-shaped glass film 2) to be formed is sufficiently wide, the glass sheet 12 may be collected from the band-shaped glass film 2 such that the direction in which the short side 12y of the glass sheet 12 extends coincides with the sheet drawing direction.

    [0100] The thickness of the glass sheet 12 is 0.1 mm or less, preferably from 0.01 mm to 0.095 mm, more preferably from 0.02 mm to 0.085 mm, and still more preferably from 0.025 mm to 0.075 mm. For further thinning, the thickness of the glass sheet 12 can be 0.065 mm or less, 0.055 mm or less, or 0.05 mm or less. On the other hand, the lower limit of the thickness of the glass sheet 12 is 0.025 mm or more, and more preferably 0.03 mm or more. If the glass sheet 12 is too thin, the deflection of the glass sheet 12 becomes too large, making it difficult to secure strength. Moreover, if the glass sheet 12 is excessively too thin, the difference between the thickness of the glass pool portions at the end portions of the glass sheet 12 and the thickness of the central portion of the glass sheet 12 to be turned into a product becomes large at the time of forming, making it more difficult to improve the sheet thickness distribution of the glass sheet 12 after forming and suppress the warpage.

    [0101] In particular, when the glass sheet 12 is thin, the difference in sheet thickness between the non-effective portions 5b at both widthwise end portions of the band-shaped glass film 2 and the effective portion 5a at the widthwise central portion thereof, which is generated at the time of overflow forming, becomes more prominent. Therefore, warpage is more likely to be induced at or near the peripheral edge portion of the glass sheet 12, in particular at or near the corner portions. As it becomes difficult to remove the warpage and thus the effective portion decreases, the warpage shape degrades in the chemical strengthening treatment at a high temperature of, for example, 360? C. or more, or breakage occurs in the treatment step, which greatly reduce the efficiency of product collection as the glass for chemical strengthening. Moreover, it becomes very difficult to maintain the glass shape when the glass sheet for chemical strengthening 12 is subjected to the chemical strengthening step, further inducing an increase in the glass deformation.

    [0102] In the present embodiment, the glass sheet 12 as a whole has a substantially uniform thickness. The substantially uniform thickness herein means deviation in the thickness of the glass sheet 12 is ?20% or less. Note that the deviation in the thickness of the glass sheet 12 is preferably ?10%/9 or less, more preferably ?5% or less.

    [0103] The glass sheet 12 includes warpage, and unevenness is formed on the one main surface 12a and the another main surface 12b due to warpage. Thus, regardless of which of the first placement form and the second placement form is adopted, the glass sheet 12 includes a part at which the glass sheet 12 is lifted up from the horizontal surface mentioned above. Here, XY coordinates are adopted as illustrated in FIGS. 3 and 4 under each of the first and second placement forms. That is, one of the four corners of the glass sheet 12 is defined as an origin S, an X-axis [mm] is set in a direction in which the long side 12x extends, and a Y-axis [mm] is set in a direction in which the short side 12y extends. Accordingly, in consideration of the lengths of the long side 12x and the short side 12y (400 mm?300 mm or 500 mm?400 mm) described above in the present embodiment, the respective coordinates of the points A, B, and C illustrated in both figures are A (400, 0), B (400, 300), and C (0, 300), or A (500, 0), B (500, 400), and C (0, 400).

    [0104] Furthermore, the glass sheet 12 is defined as in the following (1) to (8). [0105] (1) The thickness of the glass sheet 12 is defined as t [mm]. [0106] (2) When the first placement form (FIG. 3) is adopted, a position having the highest height from the horizontal surface in the glass sheet 12 is defined as a first peak position D1. [0107] (3) A height of the glass sheet 12 from the horizontal surface at the first peak position D1 is defined as W.sub.1MAX [mm]. [0108] (4) When the second placement form (FIG. 4) is adopted, a position having the highest height from the horizontal surface in the glass sheet 12 is defined as a second peak position D2. [0109] (5) A height of the glass sheet 12 from the horizontal surface at the second peak position D2 is defined as W.sub.2MAX [mm]. [0110] (6) A part (the cross-hatched part having a width L1 illustrated in FIGS. 3 and 4) having a width of 10 mm along a peripheral edge (edge) of the glass sheet 12 is defined as a peripheral edge portion 12e. [0111] (7) When the first placement form (FIG. 3) is adopted, a height at a position D3 (hereinafter referred to as the first peripheral edge peak position D3) having the highest height from the horizontal surface in the peripheral edge portion 12e is defined as W.sub.1OUT [mm]. [0112] (8) When the second placement form (FIG. 4) is adopted, a height at a position D4 (hereinafter referred to as the second peripheral edge peak position D4) having the highest height from the horizontal surface in the peripheral edge portion 12e is defined as W.sub.2OUT [mm].

    [0113] In the present embodiment, the height at respective positions, including the first peak position D1 and the second peak position D2 mentioned above, of an upper surface of the glass sheet 12 in a state where the glass sheet 12 is horizontally placed is measured using a 1313SK type glass substrate warpage measuring device manufactured by Apollo Precision Inc. as a measuring device. Note that the first peak position D1, the second peak position D2, the first peripheral edge peak position D3, and the second peripheral edge peak position D4 illustrated in FIGS. 3 and 4 are merely examples of these positions.

    [0114] Here, a large number of measurement positions at which height measurement is performed by the above-mentioned measuring device are scattered on the XY coordinates. In other words, the height is not measured for each and every position on the glass sheet 12. Therefore, the first peak position D1 or the second peak position D2 merely means the position having the highest height of the measured positions. In other words, there is a high probability that there is a positional deviation between the position having the truly highest height (the position protruding upward the most of all the positions on the glass sheet 12) and the first peak position D1 or the second peak position D2. Therefore, there is a high probability that there is a difference in value between the height at the position having the truly highest height and the value of W.sub.1MAX or the value of W.sub.2MAX mentioned above. Therefore, it is necessary to narrow the interval between adjacent measurement positions to such an extent that this difference in value is negligible. The interval between adjacent measurement positions in each of the X-axis direction and the Y-axis direction is preferably 100 mm or less, more preferably 50 mm or less, more preferably 30 mm or less, still more preferably 20 mm or less, and most preferably 10 mm or less or 5 mm or less. However, from the viewpoint of shortening the time required for measurement, the interval between adjacent measurement positions may be widened as much as possible as long as necessary accuracy is achieved.

    [0115] Under the above-mentioned definitions (1) to (8), in the present glass sheet 12, as illustrated in FIG. 3, the first peak position D1 is present in a part inside the peripheral edge portion 12e of the glass sheet 12 (that is, a region closer to the center than the peripheral edge portion 12e in plan view). That is, in a case where the coordinates of the first peak position D1 are D1 (X.sub.1, Y.sub.1), 10<X.sub.1<390 is satisfied and 10<Y.sub.1<290 is satisfied when the long side 12x is 400 mm and the short side 12y is 300 mm. Also, 10<X.sub.1<490 is satisfied and 10<Y.sub.1<390 is satisfied when the long side 12x is 500 mm and the short side 12y is 400 mm. Note that when a part (the hatched part having a width L2 illustrated in FIGS. 3 and 4) having a width of 50 mm along the peripheral edge of the glass sheet 12 is defined as a second peripheral edge portion 12f, the first peak position D1 is preferably present in a part inside the second peripheral edge portion 12f of the glass sheet 12. That is, it is preferable that 50<X.sub.1<350 is satisfied and 50<Y.sub.1<250 is satisfied when the long side 12x is 400 mm and the short side 12y is 300 mm. It is also preferable that 50<X.sub.1<450 is satisfied and 50<Y.sub.1<350 is satisfied when the long side 12x is 500 mm and the short side 12y is 400 mm. Accordingly, the top portion of the warpage having the highest height is present further inside the glass sheet. Therefore, when the glass sheet is cut into individual small substrates in a subsequent step using diamond scribing, bending stress, laser, or the like in order to use the glass sheet for, for example, a smaller-sized foldable device, defects, such as breakage occurring at or near the end surface serving as a cutting start point and cutting error due to laser focus abnormality, are less likely to be induced.

    [0116] Also, in the present glass sheet 12, it is preferable that respective relationships of t.sup.2/W.sub.1OUT>0.005, W.sub.1OUT?0.20 mm (preferably W.sub.1OUT?0.10 mm), W.sub.1OUT/t<5, and W.sub.1MAX/t<15 are satisfied.

    [0117] Furthermore, in the present glass sheet 12, as illustrated in FIG. 4, the second peak position D2 is present in the part inside the peripheral edge portion 12e of the glass sheet 12. That is, in a case where the coordinates of the second peak position D2 are D2 (X.sub.2, Y.sub.2), 10<X.sub.2<390 is satisfied and 10<Y.sub.2<290 is satisfied when the long side 12x is 400 mm and the short side 12y is 300 mm. Also, 10<X.sub.2<490 is satisfied and 10<Y.sub.2<390 is satisfied when the long side 12x is 500 mm and the short side 12y is 400 mm. Note that the second peak position D2 is preferably present in the part inside the second peripheral edge portion 12f of the glass sheet 12. That is, when the long side 12x is 400 mm and the short side 12y is 300 mm, it is preferable that 50<X.sub.2<350 is satisfied and 50<Y.sub.2<250 is satisfied. It is preferable that 50<X.sub.2<450 is satisfied and 50<Y.sub.2<350 is satisfied when the long side 12x is 500 mm and the short side 12y is 400 mm.

    [0118] In addition, in the present glass sheet 12, it is preferable that respective relationships of t.sup.2/W.sub.2OUT>0.005, W.sub.2OUT?0.20 mm (preferably W.sub.2OUT?0.10 mm), W.sub.2OUT/t<5, and W.sub.2MAX/t<15 are satisfied.

    [0119] With the present glass sheet 12, it is possible to avoid breakage due to warpage as much as possible in the manufacturing steps that the glass sheet 12 undergoes before becoming a strengthened glass sheet.

    Cutting Out Step

    [0120] The cutting out step is performed on the glass sheet 12 prepared in the preparation step. Since the cutting out step can be performed by a known method, detailed description thereof will be omitted. When the cutting out step is completed, a product-sized glass sheet (for example, a glass sheet having a size suitable for a screen of a smartphone or a tablet PC) is obtained. Note that one product-sized glass sheet may be cut out from one glass sheet 12, or a plurality of the product-sized glass sheets may be cut out from one glass sheet 12.

    Strengthening Step

    [0121] In the strengthening step, the product-sized glass sheet obtained in the cutting out step is chemically strengthened to obtain a strengthened glass sheet with a compressive stress layer (a layer on which compressive stress acts) formed on each of the front surface side and the back surface side. Since specific modes of chemical strengthening are known, detailed description thereof will be omitted. Thus, the strengthening step is completed and a strengthened glass sheet is manufactured.

    Example 1

    [0122] As a first example, glass sheets for chemical strengthening (Examples 1 to 18 and Comparative Examples 1 to 4) having the parameters shown in [Table 1] to [Table 4] below were respectively prepared. The rates of occurrence of breakage (breakage rate) in the glass sheets due to warpage in manufacturing steps that the glass sheets undergo before becoming strengthened glass sheets were determined. As the manufacturing steps, specifically, a cutting out step of cutting out a glass sheet for chemical strengthening from a glass film original sheet, a cleaning step after the cutting, and an appearance inspection step were performed. Note that the glass sheet for chemical strengthening in the example is a glass sheet containing, in mol %, 61.6% of SiO.sub.2, 18.0% of Al.sub.2O.sub.3, 0.5% of B.sub.2O.sub.3, 3.0% of MgO, 14.5% of Na.sub.2O, 2.0% of K.sub.2O and 0.4% of SnO.sub.2, having a strain point of 564? C. and a thermal expansion coefficient of 91 10.sup.?7/? C. at from 30? C. to 380? C., and formed by an overflow down-draw method. In the glass sheets for chemical strengthening in the examples, the thermal history at the time of annealing, the cutting out position from the glass film original sheet, and the like are each adjusted.

    [0123] The parameters shown in [Table 1] to [Table 4] are all the same indexes as the parameters described in the above-mentioned embodiment. Also, the first surface in [Table 1] to [Table 4] indicates a case where the glass sheet is placed on a horizontal surface with the guaranteed surface (one main surface) facing upward (a case where the first placement form is adopted). On the other hand, the second surface indicates a case where the glass sheet is placed on the horizontal surface with the non-guaranteed surface (the another main surface) facing upward (a case where the second placement form is adopted). Furthermore, regarding the item Maximum value is inside, good means that the first peak position (second peak position) is present in the part inside the peripheral edge portion of the glass sheet, and poor means that the first peak position (second peak position) is present in the peripheral edge portion of the glass sheet. Here, the height from the horizontal surface of the glass sheet (long side?short side: 400 mm?300 mm) was measured at respective coordinates at which straight lines parallel to the Y-axis and straight lines parallel to the X-axis intersect each other, the straight lines being set at intervals of 5 mm on the XY coordinates mentioned above. In this first example, the case where the peripheral edge portion of the glass sheet had a width of 10 mm was considered. Accordingly, when at least one of the condition that the X-coordinate of the first peak position is 10 mm or lower or 390 mm or higher or the condition that the Y-coordinate of the first peak position is 10 mm or lower or 290 mm or higher is satisfied, the item of Maximum value is inside for the first peak position is poor. When neither condition is satisfied, the item of Maximum value is inside for the first peak position is good. Similarly, when at least one of the condition that the X-coordinate of the second peak position is 10 mm or lower or 390 mm or higher or the condition that the Y-coordinate of the second peak position is 10 mm or lower or 290 mm or higher is satisfied, the item of Maximum value is inside for the second peak position is poor. When neither condition is satisfied, the item of Maximum value is inside for the second peak position is good.

    [0124] A specific method for determining the breakage rate will be described with reference to Example 1 as an example. First, a large number of glass sheets for chemical strengthening manufactured under the same conditions as those of the glass sheets of Example 1 were prepared. Then, for each glass sheet, the presence or absence of breakage at the point of time when the cleaning step among the above-mentioned manufacturing steps was completed was grasped by the appearance inspection step. The proportion of the number of broken glass sheets among the large number of glass sheets was calculated to determine the breakage rate. By a similar method, the breakage rate was determined for Examples 2 to 18 and Comparative Examples 1 to 4 as well. Note that the number of glass sheets for chemical strengthening manufactured under the same conditions as in each Example and each Comparative Example was 200 to 500 (the number of glass sheets varies depending on the Examples and the Comparative Examples).

    TABLE-US-00001 TABLE 1 Example Example Example Example Example 1 2 3 4 5 Glass size Long side 400 400 400 400 400 X (mm) Short side 300 300 300 300 300 Y (mm) Sheet thickness t (mm) 0.10 0.07 0.05 0.05 0.05 First Maximum X.sub.1 (mm) 55 185 105 255 105 surface position Y.sub.1 (mm) 255 145 255 255 105 height Maximum value W.sub.1MAX 0.182 0.144 0.198 0.193 0.179 (mm) Maximum Good Good Good Good Good value is inside Peripheral edge W.sub.1OUT 0.081 0.093 0.065 0.092 0.144 portion (mm) maximum value First t.sup.2/W.sub.1OUT 0.123 0.053 0.038 0.027 0.017 surface W.sub.1OUT/t 1.8 1.3 1.3 1.8 2.9 W.sub.1MAX/t 0.8 2.1 4.0 3.9 3.6 Second Maximum X.sub.2 (mm) 55 165 105 255 305 surface position Y.sub.2 (mm) 205 145 205 255 55 height Maximum value W.sub.2MAX 0.119 0.111 0.096 0.200 0.169 (mm) Maximum Good Good Good Good Good value is inside Peripheral edge W.sub.2OUT 0.079 0.088 0.065 0.119 0.060 portion (mm) maximum value Second t.sup.2/W.sub.2OUT 0.127 0.056 0.038 0.021 0.042 surface W.sub.2OUT/t 0.8 1.3 1.3 2.4 1.2 W.sub.2MAX/t 1.2 1.6 1.9 4.0 3.4 Breakage rate (%) 0.0 0.0 0.0 0.0 0.0

    [0125] From the breakage rates shown in [Table 1], it can be seen that in Examples 1 to 5 in which the first peak position was present in the part inside the peripheral edge portion of the glass sheet, breakage of the glass sheet did not occur. Here FIG. 5 illustrates warpage distribution (distribution of unevenness on the guaranteed surface) at Y=105 including the first peak position when the first placement form was adopted in Example 5. Note that, in FIG. 5, heights measured (at X=5, 55, 105, 155, 205, 255, 305, 355, and 395 are extracted and illustrated.

    TABLE-US-00002 TABLE 2 Example Example Example Example Comparative Comparative 6 7 8 9 Example 1 Example 2 Glass size Long side X 400 400 400 400 400 400 (mm) Short side Y 300 300 300 300 300 300 (mm) Sheet thickness t (mm) 0.04 0.04 0.04 0.04 0.04 0.04 First Maximum X.sub.1 (mm) 105 155 105 155 105 105 surface position Y.sub.1 (mm) 55 255 205 205 5 295 height Maximum W.sub.1MAX (mm) 0.177 0.241 0.144 0.215 0.225 0.208 value Maximum Good Good Good Good Poor Poor value is inside Peripheral edge W.sub.1OUT (mm) 0.043 0.117 0.093 0.178 0.225 0.208 portion maximum value First t.sup.2/W.sub.1OUT 0.037 0.014 0.017 0.009 0.007 0.008 surface W.sub.1OUT/t 1.1 2.9 2.3 4.5 5.6 5.2 W.sub.1MAX/t 4.4 6.0 3.6 5.4 5.6 5.2 Second Maximum X.sub.2 (mm) 205 255 255 205 205 105 surface position Y.sub.2 (mm) 205 155 5 255 5 295 height Maximum W.sub.2MAX (mm) 0.154 0.152 0.175 0.118 0.347 0.187 value Maximum Good Good Poor Good Poor Poor value is inside Peripheral edge W.sub.2OUT (mm) 0.107 0.064 0.175 0.053 0.347 0.187 portion maximum value Second t.sup.2/W.sub.2OUT 0.015 0.025 0.009 0.030 0.005 0.009 surface W.sub.2OUT/t 2.7 1.6 4.4 1.3 8.7 4.7 W.sub.2MAX/t 3.9 3.8 4.4 3.0 8.7 4.7 Breakage rate (%) 0.0 0.0 0.8 1.5 11.8 12.0

    [0126] From the breakage rates shown in [Table 2], it can be seen that, of Examples 6 to 9 in which the first peak position was present in the part inside the peripheral edge portion of the glass sheet, breakage of the glass sheet did not occur at all in Examples 6 and 7. In addition, it can be seen that even in Examples 8 and 9 in which breakage occurred, the breakage rates of the glass sheet were greatly suppressed compared to those of Comparative Examples 1 and 2 in which the first peak position was present in the peripheral edge portion of the glass sheet.

    TABLE-US-00003 TABLE 3 Example Example Example Example Comparative 10 11 12 13 Example 3 Glass size Long side X 400 400 400 400 400 (mm) Short side Y 300 300 300 300 300 (mm) Sheet thickness t (mm) 0.035 0.035 0.035 0.035 0.035 First Maximum X.sub.1 (mm) 205 305 155 205 255 surface position Y.sub.1 (mm) 205 205 205 205 295 height Maximum value W.sub.1MAX 0.205 0.273 0.319 0.332 0.156 (mm) Maximum Good Good Good Good Poor value is inside Peripheral edge W.sub.1OUT (mm) 0.076 0.170 0.179 0.175 0.156 portion maximum value First t.sup.2/W.sub.1OUT 0.016 0.007 0.007 0.007 0.008 surface W.sub.1OUT/t 2.2 4.9 5.1 5.0 4.5 W.sub.1MAX/t 5.9 7.8 9.1 9.5 4.5 Second Maximum X.sub.2 (mm) 205 255 255 105 205 surface position Y.sub.2 (mm) 205 205 55 155 295 height Maximum value W.sub.2MAX 0.225 0.286 0.218 0.274 0.283 (mm) Maximum Good Good Good Good Poor value is inside Peripheral edge W.sub.2OUT (mm) 0.155 0.074 0.128 0.152 0.283 portion maximum value Second t.sup.2/W.sub.2OUT 0.008 0.017 0.010 0.008 0.004 surface W.sub.2OUT/t 4.4 2.1 3.7 4.3 8.1 W.sub.2MAX/t 6.4 8.2 6.2 7.8 8.1 Breakage rate (%) 0.0 0.0 0.0 0.0 3.0

    [0127] From the breakage rates shown in [Table 3], it can be seen that in Examples 10 to 13 in which the first peak position was present in the part inside the peripheral edge portion of the glass sheet, breakage of the glass sheet did not occur at all. On the other hand, it can be seen that in Comparative Example 3 in which the first peak position was present in the peripheral edge portion of the glass sheet, breakage of the glass sheet occurred even though the sheet thickness was the same as that in Examples 10 to 13.

    TABLE-US-00004 TABLE 4 Example Example Example Example Example Comparative 14 15 16 17 18 Example 4 Glass size Long side X 400 400 400 400 400 400 (mm) Short side Y 300 300 300 300 300 300 (mm) Sheet thickness t (mm) 0.03 0.03 0.03 0.03 0.03 0.03 First Maximum X.sub.1 (mm) 105 55 155 155 105 205 surface position Y.sub.1 (mm) 55 205 205 255 255 295 height Maximum W.sub.1MAX 0.220 0.309 0.216 0.321 0.315 0.464 value (mm) Maximum Good Good Good Good Good Poor value is inside Peripheral W.sub.1OUT (mm) 0.079 0.146 0.161 0.094 0.111 0.464 edge portion maximum value First t.sup.2/W.sub.1OUT 0.011 0.006 0.006 0.010 0.008 0.002 surface W.sub.1OUT/t 2.6 4.9 5.4 3.1 3.7 15.5 W.sub.1MAX/t 7.3 10.3 7.2 10.7 10.5 15.5 Second Maximum X.sub.2 (mm) 155 205 305 105 5 55 surface position Y.sub.2 (mm) 255 55 205 55 295 295 height Maximum W.sub.2MAX 0.241 0.331 0.186 0.406 0.479 0.316 value (mm) Maximum Good Good Good Good Poor Poor value is inside Peripheral W.sub.2OUT (mm) 0.127 0.052 0.098 0.233 0.479 0.316 edge portion maximum value Second t.sup.2/W.sub.2OUT 0.007 0.017 0.009 0.004 0.002 0.003 surface W.sub.2OUT/t 4.2 1.7 3.3 7.8 16.0 10.5 W.sub.2MAX/t 8.0 11.0 6.2 13.5 16.0 10.5 Breakage rate (%) 0.8 1.5 1.0 2.0 2.5 31.8

    [0128] From the breakage rates shown in [Table 4], it can be seen that although it was difficult to completely suppress breakage in the steps when the sheet thickness was as very thin as 0.03 mm, the breakage rate of the glass sheet was greatly suppressed in Examples 14 to 18 in which the first peak position was present in the part inside the peripheral edge portion of the glass sheet as compared with Comparative Example 4 in which the first peak position was present in the peripheral edge portion of the glass sheet.

    [0129] Here, in Examples 14 and 15, the condition that the first peak position is present in the part inside the peripheral edge portion of the glass sheet was satisfied and, in addition, all of the relationships listed below were satisfied: W.sub.1OUT<0.20 mm, t.sup.2/W.sub.1OUT>0.005, W.sub.1OUT/t<5, W.sub.1MAX/t<15, W.sub.2OUT?0.20 mm, t.sup.2/W.sub.2OUT>0.005, W.sub.2OUT/t<5, and W.sub.2MAX/t<15. On the other hand, in Example 16, among the relationships listed above, only the relationship of W.sub.1OUT/t<5 was not satisfied. In Example 17, the three relationships of W.sub.2OUT<0.20 mm, t.sup.2/W.sub.2OUT>0.005, and W.sub.2OUT/t<5 were not satisfied. In Example 18, the four relationships of W.sub.2OUT 0.20 mm, t.sup.2/W.sub.2OUT>0.005, W.sub.2OUT/t<5, and W.sub.2MAX/t<15 were not satisfied. Note that in Example 18, the second peak position was present in the peripheral edge portion of the glass sheet. Based on the above, it can be seen that, of the glass sheets whose thickness has been thinned to 0.03 mm, the breakage rate tends to be low in the glass sheets satisfying more of the relationships listed above.

    [0130] [Table 5] below shows the height [?m] of the glass sheet from the horizontal surface measured at respective coordinates shown in the same table when the first placement form was adopted in Example 14. FIG. 6 illustrates a warpage distribution (distribution of unevenness on the guaranteed surface) in the same case. Note that in [Table 5] and [Table 6] to [Table 16] described later, and in FIG. 6 and FIGS. 8 to 18 referred to later, heights measured at some coordinates are extracted and illustrated. Specifically, heights measured at respective coordinates at which nine straight lines parallel to the Y-axis represented by X=5, 55, 105, 155, 205, 255, 305, 355, and 395, respectively, and seven straight lines parallel to the X-axis represented by Y=5, 55, 105, 155, 205, 255, and 295, respectively, intersect each other are extracted and illustrated. Furthermore, FIG. 7 illustrates a warpage distribution (distribution of unevenness on the guaranteed surface) at Y=55 including the first peak position. Also in FIG. 7, heights measured at X=5, 55, 105, 155, 205, 255, 305, 355, and 395 are extracted and illustrated. In the same case, a mean value of the heights was 45 ?m (0.045 mm). A standard deviation of the heights was 0.050.

    TABLE-US-00005 TABLE 5 Example 14 Y-Coordinate First surface Y = 5 Y = 55 Y = 105 Y = 155 Y = 205 Y = 255 Y = 295 X-Coordinate X = 5 16 12 38 0 17 19 11 X = 55 11 30 26 130 85 43 52 X = 105 25 W.sub.1MAX 220 43 15 22 137 0 X = 155 W.sub.1OUT 79 188 25 15 61 16 37 X = 205 10 63 39 42 82 44 17 X = 255 26 9 24 2 113 12 47 X = 305 10 46 22 173 178 1 32 X = 355 21 150 17 11 67 5 44 X = 395 7 28 17 12 35 7 32

    [0131] [Table 6] below shows the height [?m] of the glass sheet from the horizontal surface measured at respective coordinates shown in the same table when the second placement form was adopted in Example 14. FIG. 8 illustrates a warpage distribution (distribution of unevenness on the non-guaranteed surface) in the same case. In the same case, a mean value of the heights was 69 ?m (0.069 mm). A standard deviation of the heights was 0.058.

    TABLE-US-00006 TABLE 6 Example 14 Y-Coordinate Second surface Y = 5 Y = 55 Y = 105 Y = 155 Y = 205 Y = 255 Y = 295 X-Coordinate X = 5 67 14 25 41 17 55 6 X = 55 32 150 54 46 19 217 28 X = 105 50 40 19 159 57 170 108 X = 155 58 140 38 130 109 W.sub.2MAX 241 55 X = 205 93 4 27 21 10 106 116 X = 255 W.sub.2OUT 127 194 38 212 176 127 46 X = 305 90 51 30 3 57 38 93 X = 355 51 61 27 66 0 60 29 X = 395 55 48 28 26 31 15 52

    [0132] [Table 7] below shows the height [?m] of the glass sheet from the horizontal surface measured at respective coordinates shown in the same table when the first placement form was adopted in Example 15. FIG. 9 illustrates a warpage distribution (distribution of unevenness on the guaranteed surface) in the same case. In the same case, a mean value of the heights was 63 ?m (0.063 mm). A standard deviation of the heights was 0.055.

    TABLE-US-00007 TABLE 7 Example 15 Y-Coordinate First surface Y = 5 Y = 55 Y = 105 Y = 155 Y = 205 Y = 255 Y = 295 X-Coordinate X = 5 63 11 23 25 6 20 40 X = 55 82 23 147 70 W.sub.1MAX 309 82 16 X = 105 89 32 82 91 133 72 106 X = 155 55 51 6 88 12 7 82 X = 205 94 27 112 44 1 12 W.sub.1OUT 146 X = 255 78 73 19 110 156 16 135 X = 305 91 47 19 0 143 190 80 X = 355 31 37 37 84 7 22 58 X = 395 111 16 16 40 21 36 57

    [0133] [Table 8] below shows the height [?m] of the glass sheet from the horizontal surface measured at respective coordinates shown in the same table when the second placement form was adopted in Example 15. FIG. 10 illustrates a warpage distribution (distribution of unevenness on the non-guaranteed surface) in the same case. In the same case, a mean value of the heights was 64 ?m (0.064 mm). A standard deviation of the heights was 0.083.

    TABLE-US-00008 TABLE 8 Example 15 Y-Coordinate Second surface Y = 5 Y = 55 Y = 105 Y = 155 Y = 205 Y = 255 Y = 295 X- X = 5 29 21 15 16 20 28 10 Coordinate X = 55 31 238 30 5 165 15 W.sub.2OUT 52 X = 105 0 290 18 107 57 54 7 X = 155 27 299 17 16 72 135 19 X = 205 49 W.sub.2MAX 331 37 120 4 5 10 X = 255 23 242 11 31 208 128 18 X = 305 8 155 20 31 0 101 8 X = 355 6 24 179 45 231 29 37 X = 395 3 32 24 18 9 37 14

    [0134] [Table 9] below shows the height [?m] of the glass sheet from the horizontal surface measured at respective coordinates shown in the same table when the first placement form was adopted in Example 16. FIG. 11 illustrates a warpage distribution (distribution of unevenness on the guaranteed surface) in the same case. In the same case, a mean value of the heights was 46 ?m (0.046 mm). A standard deviation of the heights was 0.048.

    TABLE-US-00009 TABLE 9 Example 16 Y-Coordinate First surface Y = 5 Y = 55 Y = 105 Y = 155 Y = 205 Y = 255 Y = 295 X- X = 5 87 14 12 12 11 30 19 Coordinate X = 55 105 164 19 64 69 30 25 X = 105 58 30 31 18 31 22 36 X = 155 57 10 87 158 W.sub.1MAX 216 81 18 X = 205 27 29 11 8 19 11 58 X = 255 W.sub.1OUT 161 25 9 2 11 19 17 X = 305 103 12 131 85 111 29 6 X = 355 5 17 37 51 13 85 26 X = 395 149 15 13 0 5 29 25

    [0135] [Table 10] below shows the height [?m] of the glass sheet from the horizontal surface measured at respective coordinates shown in the same table when the second placement form was adopted in Example 16. FIG. 12 illustrates a warpage, distribution (distribution of unevenness on the non-guaranteed surface) in the same case. In the same case, a mean value of the heights was 48 ?m (0.048 mm). A standard deviation of the heights was 0.037.

    TABLE-US-00010 TABLE 10 Example 16 Y-Coordinate Second surface Y = 5 Y = 55 Y = 105 Y = 155 Y = 205 Y = 255 Y = 295 X-Coordinate X = 5 54 39 20 23 22 14 30 X = 55 0 43 61 149 103 49 38 X = 105 72 87 37 41 17 22 17 X = 155 6 45 25 35 73 53 35 X = 205 W.sub.2OUT 98 103 134 59 73 72 13 X = 255 26 47 29 41 21 11 28 X = 305 10 36 59 96 W.sub.2MAX 186 69 11 X = 355 72 133 23 37 101 31 22 X = 395 66 27 23 19 13 28 19

    [0136] [Table 11] below shows the height [?m] of the glass sheet from the horizontal surface measured at respective coordinates shown in the same table when the first placement form was adopted in Example 17. FIG. 13 illustrates a warpage distribution (distribution of unevenness on the guaranteed surface) in the same case. In the same case, a mean value of the heights was 84 ?m (0.084 mm). A standard deviation of the heights was 0.076.

    TABLE-US-00011 TABLE 11 Example 17 Y-Coordinate First surface Y = 5 Y = 55 Y = 105 Y = 155 Y = 205 Y = 255 Y = 295 X-Coordinate X = 5 54 30 23 50 28 65 68 X = 55 57 82 100 137 163 219 37 X = 105 W.sub.1OUT 94 2 119 92 25 191 90 X = 155 53 16 33 127 9 W.sub.1MAX 321 39 X = 205 72 137 190 227 76 298 85 X = 255 63 102 7 51 14 319 19 X = 305 49 7 177 143 164 115 36 X = 355 52 56 24 25 150 134 39 X = 395 29 39 33 9 7 45 0

    [0137] [Table 12] below shows the height [?m] of the glass sheet from the horizontal surface measured at respective coordinates shown in the same table when the second placement form was adopted in Example 17. FIG. 14 illustrates a warpage distribution (distribution of unevenness on the non-guaranteed surface) in the same case. In the same case, a mean value of the heights was 90 ?m (0.09 mm). A standard deviation of the heights was 0.098.

    TABLE-US-00012 TABLE 12 Example 17 Y-Coordinate Second surface Y = 5 Y = 55 Y = 105 Y = 155 Y = 205 Y = 255 Y = 295 X-Coordinate X = 5 20 57 13 32 31 17 W.sub.2OUT 233 X = 55 41 314 33 122 238 73 97 X = 105 23 W.sub.2MAX 406 29 67 160 0 58 X = 155 81 359 169 254 182 58 90 X = 205 28 245 31 12 335 1 51 X = 255 39 115 56 71 287 30 71 X = 305 9 45 30 13 210 37 44 X = 355 23 103 33 62 189 21 25 X = 395 10 47 33 35 43 12 19

    [0138] [Table 13] below shows the height [?m] of the glass sheet from the horizontal surface measured at respective coordinates shown in the same table when the first placement form was adopted in Example 18. FIG. 15 illustrates a warpage distribution (distribution of unevenness on the guaranteed surface) in the same case. In the same case, a mean value of the heights was 70 ?m (0.07 mm). A standard deviation of the heights was 0.078.

    TABLE-US-00013 TABLE 13 Example 18 Y-Coordinate First surface Y = 5 Y = 55 Y = 105 Y = 155 Y = 205 Y = 255 Y = 295 X-Coordinate X = 5 43 14 18 27 34 36 8 X = 55 18 65 12 49 115 53 22 X = 105 65 71 36 233 8 W.sub.1MAX 315 15 X = 155 34 28 118 254 147 289 30 X = 205 49 72 3 238 79 84 0 X = 255 27 21 18 162 14 135 37 X = 305 23 30 92 165 19 276 23 X = 355 15 34 172 29 134 11 W.sub.1OUT 111 X = 395 67 20 17 17 14 29 16

    [0139] [Table 14] below shows the height [?m] of the glass sheet from the horizontal surface measured at respective coordinates shown in the same table when the second placement form was adopted in Example 18. FIG. 16 illustrates a warpage distribution (distribution of unevenness on the non-guaranteed surface) in the same case. As illustrated in the same figure in the same case, the second peak position D2 and the second peripheral edge peak position D coincide with each other (the value of W.sub.2MAX and the value of W.sub.2OUT are the same). In the same case, a mean value of the heights was 72 ?m (0.072 mm). A standard deviation of the heights was 0.094.

    TABLE-US-00014 TABLE 14 Example 18 Y-Coordinate Second surface Y = 5 Y = 55 Y = 105 Y = 155 Y = 205 Y = 255 Y = 295 X-Coordinate X = 5 3 15 15 0 7 59 W.sub.2MAX 479 X = 55 39 19 45 46 31 5 322 X = 105 6 69 208 162 49 15 191 X = 155 15 29 69 5 61 277 259 X = 205 12 17 3 73 87 29 171 X = 255 15 193 73 14 18 48 247 X = 305 6 0 6 8 86 15 144 X = 355 0 30 162 10 38 28 187 X = 395 20 31 38 22 40 11 178

    [0140] [Table 15] below shows the height [?m] of the glass sheet from the horizontal surface measured at respective coordinates shown in the same table when the first placement form was adopted in Comparative Example 4. FIG. 17 illustrates a warpage distribution (distribution of unevenness on the guaranteed surface) in the same case. As illustrated in the same figure in the same case, the first peak position D1 and the first peripheral edge peak position D3 coincide with each other (the value of W.sub.1MAX and the value of W.sub.1OUT are the same). In the same case, a mean value of the heights was 101 ?m (0.101 mm). A standard deviation of the heights was 0.128.

    TABLE-US-00015 TABLE 15 Comparative Example 4 Y-Coordinate First surface Y = 5 Y = 55 Y = 105 Y = 155 Y = 205 Y = 255 Y = 295 X-Coordinate X = 5 10 31 28 31 52 16 309 X = 55 31 52 143 50 212 35 337 X = 105 14 99 8 16 9 6 456 X = 155 20 58 200 251 112 25 425 X = 205 8 58 17 17 90 5 W.sub.1MAX 464 X = 255 4 30 51 179 176 42 379 X = 305 25 74 17 36 0 9 428 X = 355 126 19 12 80 21 203 277 X = 395 22 33 9 20 1 109 290

    [0141] [Table 16] below shows the height [?m] of the glass sheet from the horizontal surface measured at respective coordinates shown in the same table when the second placement form was adopted in Comparative Example 4. FIG. 18 illustrates a warpage distribution (distribution of unevenness on the non-guaranteed surface) in the same case. As illustrated in the same figure in the same case, the second peak position D2 and the second peripheral edge peak-position D4 coincide with each other (the value of W.sub.2MAX and the value of W.sub.2OUT are the same). In the same case, a mean value of the heights was 73 ?m (0.073 mm). A standard deviation of the heights was 0.077.

    TABLE-US-00016 TABLE 16 Comparative Example 4 Y-Coordinate Second surface Y = 5 Y = 55 Y = 105 Y = 155 Y = 205 Y = 255 Y = 295 X-Coordinate X = 5 90 38 24 22 11 33 135 X = 55 7 56 11 0 6 19 W.sub.2MAX 316 X = 105 69 21 21 80 217 122 71 X = 155 116 14 1 12 30 47 202 X = 205 88 16 217 259 248 252 22 X = 255 33 34 45 15 3 158 146 X = 305 74 8 108 53 22 161 18 X = 355 41 27 100 30 207 69 169 X = 395 59 15 16 7 12 21 50

    [0142] As described above, it can be seen that of Examples 14 to 18, even in Examples 17 and 18 in which the mean value of the heights and the standard deviation of the heights were relatively large, it was possible to suppress breakage of the glass sheet (see also [Table 4] mentioned above). That is, it was possible to suppress breakage even in a glass sheet including a portion where the height from the horizontal surface was increased due to warpage or a glass sheet in which the unevenness of the guaranteed surface and the non-guaranteed surface greatly changes due to warpage. It is presumed that such results were obtained because in each of the examples, the first peak position is present in the part inside the peripheral edge portion of the glass sheet.

    [0143] Next, as a second example, other glass sheets for chemical strengthening (Example 19) having the parameters shown in [Table 17] below were prepared. The rates of occurrence of breakage (breakage rate) in the glass sheets due to warpage in manufacturing steps that the glass sheets undergo before becoming strengthened glass sheets were determined. As the manufacturing steps, specifically, a cutting out step of cutting out a glass sheet for chemical strengthening from a glass film original sheet, a cleaning step after the cutting out, and an appearance inspection step were performed. Note that the glass sheet for chemical strengthening in the second example is a glass sheet containing, in mol %, 66.1% of SiO.sub.2, 14.0% of Al.sub.2O.sub.3, 2.5% of B.sub.2O.sub.3, 3.0% of MgO, 13.4% of Na.sub.2O, and 0.6% of K.sub.2O, having a strain point of 551? C. and a thermal expansion coefficient of 79?10.sup.?7/? C. at from 30? C. to 380? C., and formed by the overflow down-draw method.

    [0144] The height from the horizontal surface of the glass sheet (long side?short side: 500 mm?400 mm) of the second example was measured at respective coordinates at which straight lines parallel to the Y-axis and straight lines parallel to the X-axis intersect each other, the straight lines being set at intervals of 5 mm on the XY coordinates mentioned above. In this second example, the case where the peripheral edge portion of the glass sheet had a width of 50 mm was considered. Accordingly, when at least one of the condition that the X-coordinate of the first peak position is 50 mm or lower or 450 mm or higher or the condition that the Y-coordinate of the first peak position is 50 mm or lower or 350 mm or higher is satisfied, the item of Maximum value is inside for the first peak position is poor. When neither condition is satisfied, the item of Maximum value is inside for the first peak position is good. Similarly, when at least one of the condition that the X-coordinate of the second peak position is 50 mm or lower or 450 mm or higher or the condition that the Y-coordinate of the second peak position is 50 mm or lower or 350 mm or higher is satisfied, the item of Maximum value is inside for the second peak position is poor. When neither condition is satisfied, the item of Maximum value is inside for the second peak position is good.

    TABLE-US-00017 TABLE 17 Example 19 Glass size Long side X (mm) 500 Short side Y (mm) 400 Sheet thickness t (mm) 0.035 First Maximum position X.sub.1 (mm) 365 surface Y.sub.1 (mm) 265 height In-plane maximum value W.sub.1MAX (mm) 0.318 Maximum value is Good inside Peripheral edge portion maximum value W.sub.1OUT 0.154 (mm) First t.sup.2/W.sub.1OUT 0.008 surface W.sub.1OUT/t 4.4 W.sub.1MAX/t 9.1 Second Maximum position X.sub.2 (mm) 365 surface Y.sub.2 (mm) 265 height Maximum value W.sub.2MAX (mm) 0.377 Maximum value is Good inside Peripheral edge portion maximum value W.sub.2OUT 0.158 (mm) Second t.sup.2/W.sub.2OUT 0.008 surface W.sub.2OUT/t 4.5 W.sub.2MAX/t 10.8 Breakage rate (%) 0.0

    [0145] From the breakage rate shown in [Table 17], it can be seen that in Example 19 in which the first peak position was present in the part inside the peripheral edge portion (having a width of 50 mm) of the glass sheet, breakage of the glass sheet did not occur at all.

    [0146] [Table 18] below shows the height [?m] of the glass sheet from the horizontal surface measured at respective coordinates shown in the same table when the first placement form was adopted in Example 19. FIG. 19 illustrates a warpage distribution (distribution of unevenness on the non-guaranteed surface) in the same case. Specifically, heights measured at respective coordinates at which eleven straight lines parallel to the Y-axis represented by X=15, 65, 115, 165, 215, 265, 315, 365, 415, 465, and 485, respectively, and nine straight lines parallel to the X-axis represented by Y=15, 65, 115, 165, 215, 265, 315, 365, and 385, respectively, intersect each other are extracted and illustrated. In the same case, a mean value of the heights was 51 ?m (0.051 mm). A standard deviation of the heights was 0.062.

    TABLE-US-00018 TABLE 18 Example 19 Y-Coordinate First surface Y = 15 Y = 65 Y = 115 Y = 165 Y = 215 Y = 265 Y = 315 Y = 365 Y = 385 X-Coordinate X = 15 21 90 9 36 34 39 78 86 9 X = 65 11 108 71 47 39 35 55 35 16 X = 115 15 31 26 33 21 25 12 92 W.sub.1OUT 154 X = 165 28 35 9 30 23 24 27 31 26 X = 215 22 17 24 13 25 20 83 40 45 X = 265 39 20 37 18 11 24 23 28 15 X = 315 61 24 61 215 44 286 43 165 30 X = 365 43 18 67 280 33 W.sub.1MAX 318 7 0 86 X = 415 32 33 45 222 11 278 21 46 21 X = 465 30 28 38 30 52 49 55 18 24 X = 485 28 30 30 22 17 17 24 34 22

    [0147] [Table 19] below shows the height [?m] of the glass sheet from the horizontal surface measured at respective coordinates shown in the same table when the second placement form was adopted in Example 19. FIG. 20 illustrates a warpage distribution (distribution of unevenness on the non-guaranteed surface) in the same case. In the same case, a mean value of the heights was 67 ?m (0.067 mm). A standard deviation of the heights was 0.077.

    TABLE-US-00019 TABLE 19 Example 19 Y-Coordinate Second surface Y = 15 Y = 65 Y = 115 Y = 165 Y = 215 Y = 265 Y = 315 Y = 365 Y = 385 X-Coordinate X = 15 2 17 68 18 22 105 33 33 36 X = 65 23 19 135 22 37 62 71 23 54 X = 115 27 35 34 37 28 36 46 34 39 X = 165 33 130 49 54 50 53 55 26 66 X = 215 81 25 20 19 47 29 28 21 20 X = 265 26 20 33 63 61 138 53 132 98 X = 315 33 235 14 9 35 281 22 11 25 X = 365 W.sub.2OUT 158 266 275 318 55 W.sub.2MAX 377 175 221 136 X = 415 36 29 20 201 24 300 35 193 84 X = 465 29 38 28 10 22 26 27 25 29 X = 485 26 19 28 22 45 23 49 19 33

    REFERENCE SIGNS LIST

    [0148] D1 First peak position [0149] D2 Second peak position [0150] D3 First peripheral edge peak position [0151] D4 Second peripheral edge peak position [0152] 12 Glass sheet for chemical strengthening [0153] 12a One main surface [0154] 12b Another main surface [0155] 12e Peripheral edge portion