# Patent

## Title
Float glass for chemical strengthening

## Abstract

The present invention relates to a float glass for chemical strengthening, containing a bottom surface coming into contact with a molten metal at the time of forming and a top surface opposing the bottom surface, in which a difference Δ(N—Na2O2) determined by subtracting a square of a normalized Na2O surface concentration of the bottom surface which is a value obtained by dividing an Na2O concentration in the bottom surface by an Na2O concentration at a depth position of 100 μm therefrom, from a square of a normalized Na2O surface concentration of the top surface which is a value obtained by dividing an Na2O concentration in the top surface by an Na2O concentration at a depth position of 100 μm therefrom, is 0.040 or less.

## Background
TECHNICAL FIELD
The present invention relates to a float glass for chemical strengthening.
BACKGROUND ART
Recently, in a flat panel display device such as mobile phone or personal digital assistant (PDA), a thin sheet-like cover glass is disposed on a front surface of a display to extend in a wider region than the image display area with an aim to enhance the protection and beauty of the display.
Such the flat panel display device is required to be lightweight and thin and in order to meet this requirement, the thickness of a cover glass used for display protection is also required to be reduced.
However, decreasing the thickness of the cover glass causes the problems that the strength is reduced and the cover glass itself may be broken by dropping, etc. during use or carrying and therefore its primary role of protecting the display device cannot be fulfilled.
Accordingly, in the conventional cover glass, with an aim to improve the scratch resistance, a compressive stress layer is formed on the surface by chemically strengthening a float glass produced by a float method, and the scratch resistance of the cover glass is thereby enhanced.
It has been reported that warpage occurs in a float glass after chemical strengthening and impairs the flatness (Patent Document 1). The warpage is thought to occur due to the difference in the chemical strengthening behavior between a glass surface (hereinafter, also referred to as top surface) that is out of contact with molten tin at the time of float forming, and a glass surface (hereinafter, also referred to as bottom surface) that comes into contact with molten tin.
Heretofore, the reason for the difference in the chemical strengthening behaviors between the top surface of the float glass and the bottom surface has been considered to be the invasion of a molten metal into the glass surface in contact with the molten metal at the time of float forming (Patent Document 1).
In Patent Document 1, it is disclosed that the warpage is improved when a sheet-like material produced by a float method and processed is, without applying surface polishing, dipped in or put into contact with Li ion, Na ion or a mixed inorganic salt thereof and then chemically strengthened.
Furthermore, in order to reduce the warpage, conventionally employed is a coping method of decreasing the strengthening stress produced by chemical strengthening or performing chemical strengthening after subjecting the top surface and the bottom surface of a float glass to a grinding treatment, a polishing treatment, etc. so as to remove a surface heterogeneous layer.
PRIOR ART DOCUMENT
Patent Document
Patent Document 1: Japanese Patent No. 2,033,034
SUMMARY OF THE INVENTION
Problems that the Invention is to Solve
However, in the method described in Patent Document 1, the float glass needs to be dipped in a mixed inorganic salt before chemical strengthening, and this is cumbersome. In addition, the method of decreasing the strengthening stress may afford insufficient strength to the float glass after chemical strengthening.
Furthermore, the method of subjecting the top surface and the bottom surface of a float glass to a grinding treatment, a polishing treatment, etc. before chemical strengthening has a problem from the standpoint of enhancing the productivity, and it is preferable to omit such the grinding treatment, the polishing treatment, etc.
Accordingly, an object of the present invention is to provide a float glass for chemical strengthening, which can effectively suppress the warpage after chemical strengthening.
Means for Solving the Problems
The present inventors have found that the main cause of warpage occurring due to the difference in the chemical strengthening behaviors between the bottom surface and the top surface when chemically strengthening a soda-lime glass produced by a float process is not necessarily a metal invading a glass surface in contact with the molten metal at the time of float forming but is the difference in the weathering degree between the top surface and the bottom surface, namely, the difference in the degree of hydration-dealkalization.
Furthermore, they found that by suppressing the effect thereof, the strengthening degree in chemical strengthening can be equalized between the top surface and the bottom surface and therefore the warpage of a float glass after chemical strengthening can be reduced. In addition, they found that the weathering has a profound effect particularly in a low DOL region where the depth of compressive stress layer (DOL) is typically 20 μm or less, 15 μm or less or 10 μm or less, and by diminishing the effect of weathering degree in this region, the warpage of a float glass after chemical strengthening can be effectively reduced. They have accomplished the present invention based on these findings.
The present invention is as the following.
1. A float glass for chemical strengthening, containing a bottom surface coming into contact with a molten metal at the time of forming and a top surface opposing the bottom surface, in which a difference Δ(N—Na2O2) determined by subtracting a square of a normalized Na2O surface concentration of the bottom surface which is a value obtained by dividing an Na2O concentration in the bottom surface by an Na2O concentration at a depth position of 100 μm therefrom, from a square of a normalized Na2O surface concentration of the top surface which is a value obtained by dividing an Na2O concentration in the top surface by an Na2O concentration at a depth position of 100 μm therefrom, is 0.040 or less,
where each Na2O concentration is a value measured by a fluorescent X-ray analysis using an Na-Kα ray.
A float glass for chemical strengthening, containing a bottom surface coming into contact with a molten metal at the time of forming and a top surface opposing the bottom surface, in which a Δion exchange amount 1 which is a value obtained by subtracting an ion exchange amount 1 in the bottom surface from an ion exchange amount 1 in the top surface is 0.32 or less,
where the ion exchange amount 1 is a value determined according to the following formula (2-1):

Ion exchange amount 1=5.51×(normalized Na2O surface concentration)−0.038×(Sn concentration)  formula (2-1)

in formula (2-1),

the normalized Na2O surface concentration is a value obtained by dividing an Na2O concentration at the surface by an Na2O concentration at a depth position of 100 μm, where the Na2O concentration is a value measured by a fluorescent X-ray analysis using an Na-Kα ray; and
the Sn concentration is an Sn deposition amount (unit: as SnO2μg/cm2) per unit area of the top surface and the bottom surface. In the present specification, the unit of the Sn deposition amount per unit area is indicated as “as SnO2μg/cm2” in order to specify that the Sn deposition amount per unit area is expressed by a deposition mass in terms of SnO2 per 1 cm2 when Sn is assumed to exist in the form of SnO2. In the present specification, the Sn deposition amount (unit: μg/cm2) per unit area possesses the same meaning as the Sn deposition amount (unit: as SnO2μg/cm2) per unit area.
3. A float glass for chemical strengthening, containing a bottom surface coming into contact with a molten metal at the time of forming and a top surface opposing the bottom surface, in which W1 determined according to the following formula (3-1) is 56 or less:

W1=−16×(ΔH/Si)−6.47×(Sn concentration difference)−43.8×(Δion exchange amount 1)  formula (3-1)

in formula (3-1),

the ΔH/Si is a value obtained by subtracting a normalized hydrogen concentration in the bottom surface from a normalized hydrogen concentration in the top surface, where the normalized hydrogen concentration is a value obtained by dividing an average hydrogen concentration at a depth of 0 to 10 μm by an average hydrogen concentration at a depth of 105 to 110 μm, in which the average hydrogen concentration at a depth of 0 to 10 μm and the average hydrogen concentration at a depth of 105 to 110 μm are values measured under the following analysis conditions:
(Analysis Conditions)
Measurement apparatus: secondary ion mass spectrometry apparatus with a quadrupole mass spectrometer
Primary ion species: Cs+
Primary accelerating voltage: 5.0 kV
Primary ion current: 1 μA
Primary ion incident angle (angle from direction perpendicular to sample surface): 60°
Luster size: 200×200 μm2 
Detection region: 40×40 μm2 
Secondary ion polarity: minus
Electron gun for neutralization: used;
in formula (3-1),
the Sn concentration difference is a difference obtained by subtracting an Sn deposition amount (unit: μg/cm2) per unit area of the top surface from an Sn deposition amount (unit: as SnO2μg/cm2) per unit area of the bottom surface, and in the case where the glass does not contain SnO2, this is equivalent to the Sn deposition amount per unit area of the bottom surface; and
in formula (3-1),
the Δion exchange amount 1 is a value obtained by subtracting an ion exchange amount 1 in the bottom surface from an ion exchange amount 1 in the top surface, where the ion exchange amount 1 is determined according to the following formula:

Ion exchange amount 1=5.51×(normalized Na2O surface concentration)−0.038×(Sn concentration)

in which
the normalized Na2O surface concentration is a value obtained by dividing an Na2O concentration at the surface by an Na2O concentration at a depth position of 100 where the Na2O concentration is a value measured by a fluorescent X-ray analysis using an Na-Kα ray.
4. A float glass for chemical strengthening, containing a bottom surface coming into contact with a molten metal at the time of forming and a top surface opposing the bottom surface, in which an absolute value of W2 determined according to the following formula (4-1) is 56 or less:

W2=9.18×Δ[(ion exchange amount)/(H/Si)]+49  formula (4-1)

in formula (4-1),

the Δ[(ion exchange amount)/(H/Si)] is a value determined by subtracting a value obtained by dividing an ion exchange amount 1 in the bottom surface by a normalized hydrogen concentration H/Si in the same surface, from a value obtained by dividing an ion exchange amount 1 in the top surface by a normalized hydrogen concentration H/Si in the same surface,
where the ion exchange amount 1 is determined according to the following formula:

Ion exchange amount 1=5.51×(normalized Na2O surface concentration)−0.038×(Sn concentration)

in the formula, the normalized Na2O surface concentration is a value obtained by dividing an Na2O concentration at the surface by an Na2O concentration at a depth position of 100 μm, where the Na2O concentration is a value measured by a fluorescent X-ray analysis using an Na-Kα ray, and the Sn concentration is an Sn deposition amount (unit: as SnO2μg/cm2) per unit area of the top surface and the bottom surface;
and
where the normalized hydrogen concentration is a value obtained by dividing an average hydrogen concentration at a depth of 0 to 10 μm by an average hydrogen concentration at a depth of 105 to 110 μm, in which the average hydrogen concentration at a depth of 0 to 10 μm and the average hydrogen concentration at a depth of 105 to 110 μm are values measured under the following analysis conditions:
(Analysis Conditions)
Measurement apparatus: secondary ion mass spectrometry apparatus with a quadrupole mass spectrometer
Primary ion species: Cs+
Primary accelerating voltage: 5.0 kV
Primary ion current: 1 μA
Primary ion incident angle (angle from direction perpendicular to sample surface): 60°
Luster size: 200×200 μm2 
Detection region: 40×40 μm2 
Secondary ion polarity: minus
Electron gun for neutralization: used.
5. The float glass for chemical strengthening according to any one of the above 1 to 4, containing a bottom surface coming into contact with a molten metal at the time of forming and a top surface opposing the bottom surface, in which W3 determined according to the following formula (5-1) is 58 or less:

W3=744×[(ΔN—Na2O)+0.01×(Sn concentration difference)]  formula (5-1)

in formula (5-1),

the ΔN—Na2O is a value determined by subtracting a normalized Na2O surface concentration of the bottom surface which is a value obtained by dividing an Na2O concentration at the surface in the bottom surface by an Na2O concentration at a depth position of 100 μm therefrom, from the normalized Na2O surface concentration of the top surface which is a value obtained by dividing an Na2O concentration at the surface in the top surface by an Na2O concentration at a depth position of 100 μm therefrom, where each Na2O concentration is a value measured by a fluorescent X-ray analysis using an Na-Kα ray;
and
in formula (5-1),
the Sn concentration difference is a difference obtained by subtracting an Sn deposition amount (unit: as SnO2μg/cm2) per unit area of the top surface from an Sn deposition amount (unit: as SnO2μg/cm2) per unit area of the bottom surface, and in the case where the glass does not contain SnO2, this is equivalent to the Sn deposition amount per unit area of the bottom surface.
6. The float glass for chemical strengthening according to any one of the above 1 to 4, containing a bottom surface coming into contact with a molten metal at the time of forming and a top surface opposing the bottom surface, in which Δion exchange amount 2 which is a value obtained by subtracting an ion exchange amount 2 in the bottom surface from an ion exchange amount 2 in the top surface is 0.33 or less,
where the ion exchange amount 2 is a value determined according to the following formula (6-1):

Ion exchange amount 2=−0.02×(H/Si)+5.54×(N—Na2O concentration)−0.037×(Sn concentration)  formula (6-1)

in formula (6-1),

the H/Si is a normalized hydrogen concentration, where the normalized hydrogen concentration is a value obtained by dividing an average hydrogen concentration at a depth of 0 to 10 μm by an average hydrogen concentration at a depth of 105 to 110 μm, in which the average hydrogen concentration at a depth of 0 to 10 μm and the average hydrogen concentration at a depth of 105 to 110 μm are values measured under the following analysis conditions:
(Analysis Conditions)
Measurement apparatus: secondary ion mass spectrometry apparatus with a quadrupole mass spectrometer
Primary ion species: Cs+
Primary accelerating voltage: 5.0 kV
Primary ion current: 1 μA
Primary ion incident angle (angle from direction perpendicular to sample surface): 60°
Luster size: 200×200 μm2 
Detection region: 40×40 μm2 
Secondary ion polarity: minus
Electron gun for neutralization: used;
and
in formula (6-1),
the N—Na2O concentration is a normalized Na2O surface concentration which is a value obtained by dividing a surface Na2O concentration by an Na2O concentration at a depth position of 100 μm, where the Na2O concentration is a value measured by a fluorescent X-ray analysis using an Na-Kα ray, and
the Sn concentration is an Sn deposition amount (unit: as SnO2μg/cm2) per unit area.
7. The float glass for chemical strengthening according to the above 1, containing a bottom surface coming into contact with a molten metal at the time of forming and a top surface opposing the bottom surface, in which a square (ΔN—Na2O)2 of a difference ΔN—Na2O determined by subtracting a normalized Na2O surface concentration of the bottom surface which is a value obtained by dividing an Na2O concentration in the bottom surface by an Na2O concentration at a depth position of 100 μm therefrom, from a normalized Na2O surface concentration of the top surface which is a value obtained by dividing an Na2O concentration in the top surface by an Na2O concentration at a depth position of 100 μm therefrom, is 5.0×10−4 or less, where each Na2O concentration is a value measured by a fluorescent X-ray analysis using an Na-Kα ray.

8. The float glass for chemical strengthening according to the above 1, which is used for chemical strengthening in which a chemical strengthening temperature is T (unit: K) and a chemical strengthening time is t (unit: hours) and contains SiO2, in which a dol determined according to the following formula by using respective contents in mass % of SiO2, Al2O3, MgO, CaO, SrO, BaO, ZrO2, Na2O and K2O is 20 or less:

dol=−0.13×Al2O3−1.88×MgO−2.41×CaO−1.85×SrO−1.35×BaO−1.59×ZrO2+1.50×Na2O+2.42×K2O−129359/T+9.28×t0.5+182.88.

Al2O3, MgO, CaO, SrO, BaO, ZrO2, Na2O and K2O are not essential components. A salt used for chemical strengthening typically contains KNO3 in a concentration of from 95 to 100 mass %.
9. The float glass for chemical strengthening according to the above 1, which contains, in mass %, from 60 to 80% of SiO2, from 0 to 8% of Al2O3, from 8 to 22% of Na2O, from 0 to 7% of K2O, from 0 to 17% of MgO, from 0 to 22% of CaO, from 0 to 8% of SrO, from 0 to 8% of BaO, and from 0 to 5% of ZrO2. Here, for example, containing “from 0 to 7% of K2O” means that K2O is not essential but it may be contained up to 7%.
Preferable composition range is from 64 to 77% of SiO2, from 0.01 to 7% of Al2O3, from 10 to 18% of Na2O, from 0 to 5% of K2O, from 1 to 10% of MgO, from 1 to 12% of CaO, from 0 to 5% of SrO, from 0 to 5% of BaO, and from 0 to 3% of ZrO2.
10. The float glass for chemical strengthening according to the above 1, containing, in mass %, from 60 to 80% of SiO2, from 0.01 to 8% of Al2O3, from 8 to 22% of Na2O, from 0 to 7% of K2O and from 0 to 5% of ZrO2, in which in the case of containing MgO, CaO, SrO or BaO, the total of the MgO, CaO, SrO and BaO contents is from 5 to 25%, and a ratio Na2O/Al2O3 of Na2O and Al2O3 contents is 1.5 or more.

11. The float glass for chemical strengthening according to the above 10, in which the Na2O/Al2O3 is 6 or less.

12. The float glass for chemical strengthening according to the above 9, containing CaO, SrO or BaO, in which the total of the CaO, SrO and BaO contents is from 1 to 7%.

13. A method for producing a chemically strengthened glass having a depth of compressive stress layer of 20 μm or less, including chemically strengthening the glass for chemical strengthening described in the above 1.

Advantage of the Invention
In the float glass for chemical strengthening of the present invention, the difference in the weathering degrees between the top surface and the bottom surface is small, so that even when the stress produced by chemical strengthening is not decreased or a polishing treatment, etc. before chemical strengthening is simplified or omitted, the warpage of the float glass after chemical strengthening can be reduced and an excellent flatness can be obtained.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view showing the correlation between the normalized hydrogen concentration [obtained by dividing the average H/Si at (H/Si) 0 to 10 μm in SIMS analysis by the average H/Si at 105 to 110 μm] and the normalized Na2O surface concentration (obtained by dividing the surface Na2O concentration in fluorescent X-ray analysis by the Na2O concentration at a depth position of 100 μm) in the surface layer of a soda-lime glass sheet (raw sheet) before chemical strengthening.
FIG. 2 illustrates the mechanism in a glass before chemical strengthening, where Na+ in glass is ion-exchanged with H+ in the atmosphere.
FIG. 3 is a view showing the correlation between the ion exchange amount (K2O, wt %) (fluorescent X-ray analysis) of a soda-lime glass sheet after chemical strengthening and the normalized Na2O surface concentration (obtained by dividing the surface Na2O concentration in fluorescent X-ray analysis by the Na2O concentration at a depth position of 100 μm) before chemical strengthening (raw sheet); where wt % is mass %.
FIG. 4 is a schematic view illustrating the method for calculating the ion exchange amount in fluorescent X-ray analysis.
FIGS. 5A, 5B, 5C and 5D are schematic views illustrating the mechanism where when a soda-lime glass sheet in which Na+ and H+ are ion-exchanged is chemically strengthened by dipping it in a KNO3 molten salt, the ion exchange amount decreases. FIG. 5A is a graph showing an amount of K2O in a raw sheet. FIG. 5B is a graph showing an amount of K2O in a chemical strengthened article. FIG. 5C is a graph showing the ion exchange amount. FIG. 5D illustrates how the ion exchange amount is calculated.
FIG. 6 is a graph plotting, on the abscissa, the difference Δ(N—Na2O2) (Top-Bottom) between squared normalized Na2O surface concentrations in top surface and bottom surface of glass to be subjected to chemical strengthening, and on the ordinate, Δwarpage amount.
FIG. 7 is a graph plotting, on the abscissa, the difference between ion exchange amounts in top surface and bottom surface, and on the ordinate, Δwarpage amount.
FIG. 8 is a graph plotting, on the abscissa, the difference between values obtained by dividing the ion exchange amounts in top surface and bottom surface by the hydrogen concentration, and on the ordinate, Δwarpage amount.
FIG. 9 is a graph showing the results of multiple regression analysis using, as factors, the normalized Na2O surface concentration difference (ΔNa2O) and Sn concentration difference (deposition amount per unit area) in the top surface and bottom surface before chemical strengthening, and the Δwarpage amount.
FIG. 10 is a longitudinal cross-sectional view of the apparatus for producing the float glass for chemical strengthening of the present invention.
FIG. 11 is a cross-sectional view of a flat panel display in which the float glass for chemical strengthening of the present invention is chemically strengthened and then used as a cover glass for a flat panel display.
FIG. 12 shows a graph plotting W1 on the abscissa and Δwarpage amount on the ordinate.
FIG. 13 shows a graph plotting W2 on the abscissa and Δwarpage amount on the ordinate.
FIG. 14 shows a graph plotting W3 on the abscissa and Δwarpage amount on the ordinate.
FIG. 15 shows a graph plotting, on the abscissa, the difference (Δion exchange amount 2) between ion exchange amounts 2 in top surface and bottom surface, and on the ordinate, Δwarpage amount.
FIG. 16 shows a graph plotting, on the abscissa, a square [ΔN—Na2O (Top-Bottom)]2 of the difference obtained by subtracting the normalized Na2O surface concentration of the bottom surface from the normalized Na2O surface concentration of the top surface, and on the ordinate, Δwarpage amount.
FIG. 17 shows a graph plotting [(ΔN—Na2O)+0.01×(Sn concentration difference)] on the abscissa and W3 on the ordinate.



MODE FOR CARRYING OUT THE INVENTION
1. Weathering of Glass
The H profile (1H−/30Si−) on the surface of a soda-lime glass produced by a float method was analyzed by a secondary ion mass spectrometry apparatus (SIMS), and as a result, the depth of weathered (hydrated and dealkalized) layer was about 3 μm. This implies that at the time of chemical strengthening to an ion-exchange depth of 20 μm or less, as a cause of warpage occurring due to the difference in the chemical strengthening behaviors between the bottom surface and the top surface, the difference between weathering degrees in the top surface and the bottom surface is important.
The “weathering” as used in the present invention indicates a phenomenon where the glass surface is degraded due to corrosion of the glass surface by the atmosphere, usually, by the effect of humidity, and in the present invention, this indicates a phenomenon where an alkali metal component, typically, Na2O, in the surface layer of glass is desorbed. The weathering degree of glass can be analyzed by measuring the Na2O concentration by fluorescent X-ray analysis.
FIG. 1 illustrates the correlation between the normalized hydrogen concentration (SIMS analysis) and the normalized Na2O surface concentration (obtained by dividing the surface Na2O concentration in fluorescent X-ray analysis by the Na2O concentration at a depth position of 100 μm) in the surface layer of a soda-lime glass sheet (raw sheet) before chemical strengthening. As shown in FIG. 1, in the surface layer of a soda-lime glass sheet before chemical strengthening, the normalized hydrogen concentration is in an inverse relationship with the normalized Na2O surface concentration.
The graph of FIG. 1 indicates that as explained in FIG. 2, in the glass before chemical strengthening, Si—O—Na constituting the glass reacts with H2O in the atmosphere and Na+ and H+ are ion-exchanged. Accordingly, the normalized hydrogen concentration in the glass surface layer is considered to increase as the weathering degree of glass is larger.
FIG. 3 illustrates the correlation between the ion exchange amount (wt %) (fluorescent X-ray analysis) of a soda-lime glass sheet after chemical strengthening and the normalized Na2O surface concentration of a raw sheet. Here, with regard to the ion exchange amount, as shown in FIG. 4, the value obtained by subtracting the K2O analysis value before chemical strengthening (raw sheet) from the K2O analysis value after chemical strengthening is defined as the ion exchange amount.
It is seen from the graph of FIG. 3 that as the Na2O concentration in glass before chemical strengthening is higher, namely, as the weathering degree of glass is smaller, the ion exchange amount after chemical strengthening increases.
The graph of FIG. 3 also implies the following. That is, as shown in FIGS. 5A, 5B, 5C and 5D, when a soda-lime glass sheet in which Na+ and H+ are ion-exchanged as shown in FIG. 2 is chemically strengthened by dipping it in a KNO3 molten salt, the ion exchange of Na+ in glass with K+ is governed by entropy but in the case of exchange of H+ with K+, the ion exchange is considered to be not driven enthalpically, because H in glass is present as SiOH (weak acid) and if H+ and K+ are ion-exchanged, HNO3 (strong acid) is produced.
Accordingly, it is thought that the weathering degree of a soda-lime glass before chemical strengthening affects the ion exchange amount and the difference in the ion exchange amount between the top surface and the bottom surface causes the generation of warpage after chemical strengthening. This leads to an understanding that in order to suppress warpage of a soda-lime glass after chemical strengthening, it is important to control the difference (the Na2O concentration difference between the top surface and the bottom surface) in the weathering degrees of the glass surface layer between top surface and bottom surface of glass before chemical strengthening.
2. Sn Concentration
The Sn (tin) profile (120Sn−/30Si−) in the bottom surface of a soda-lime glass produced by a float method was analyzed by a secondary ion mass spectrometry apparatus (SIMS), and as a result, the depth of ion-exchanged layer and the Sn invasion depth were about 7 μm. This implies that at the time of chemical strengthening of low DOL to an ion-exchange depth, i.e., DOL, of 20 μm or less, as a cause of warpage occurring due to the difference in the chemical strengthening behaviors between the bottom surface and the top surface, the Sn concentration needs to be taken into account.
In this connection, both Δ(N—Na2O2) and (ΔN—Na2O)2 depend on the difference of weathering degrees but are not dependent directly on the Sn concentration. However, invasion of Sn into the bottom surface in a float bath is considered to occur due to ion-exchange with Na of the glass surface layer. Therefore, in glass having a large Sn deposition amount, the Na concentration of the surface layer is assumed to be low. Accordingly, the normalized Na2O surface concentration has a relationship with the Sn concentration. In other words, although not explicit, both Δ(N—Na2O2) and (ΔN—Na2O)2 can be said to depend on the Sn concentration.
At the time of float forming, when glass is densified by the invasion of Sn into the glass, the pathway for ion-exchange of Na ion with K ion is narrowed and, as a result, the ion exchange reaction is disturbed and the chemical strengthening is inhibited in the Sn invaded surface (bottom surface). This is considered to cause difference in the chemical strengthening behaviors between the top surface and the bottom surface, leading to warpage of the glass.
The Sn concentration of glass can be determined by measuring the Sn deposition amount per unit area. Specifically, it can be determined, for example, by etching the glass with a hydrofluoric acid solution and quantitatively determining the Sn concentration in the solution by ICP emission spectrometry.
3. Hydrogen Concentration
The float glass for chemical strengthening of the present invention is formed by a float method and has a bottom surface coming into contact with a molten metal at the time of forming and a top surface opposing the bottom surface. As described below, the hydrogen concentration difference between the top surface and the bottom surface is considered to sometimes work out to one of causes of warpage that is caused by chemical strengthening of the float glass.
In the production of glass by a float method, a glass plate is produced by continuously feeding onto a surface of a molten metal retained in a float bath, a molten glass from an upstream side to form a glass ribbon, concurrently drawing the formed glass ribbon from a downstream side end of the float bath, and annealing it in a lehr.
In the production of glass by a float method, an apparatus usually used in a type where a glass tank furnace and a float bath are connected through a canal and a spout and the flow passage is narrowed down. In this case, since the glass must be spread in the float bath, a molten glass at a higher temperature than in the case of another type of apparatus described later is poured onto the molten metal surface and formed.
At this time, when the hydrogen concentration in glass is high, hydrogen enters as the form of SiOH into the Si—O—Si bond network of glass, and the Si—O—Si bond is broken. When the hydrogen concentration in glass is high, the Si—O—Si bond is broken in many portions and since this leads to deterioration of thermal characteristics such as glass transition temperature, stress relaxation occurs at the time of chemical strengthening in which the glass is heated at a high temperature, resulting in decrease of the stress.
For this reason, the degree of stress produced at the time of chemical strengthening is low in a glass surface having a higher hydrogen concentration, out of the top surface and bottom surface in a float glass, and a stress is likely to be produced at the time of chemical strengthening in a glass surface having a lower hydrogen concentration.
In other words, it is considered that when a float glass having a lower hydrogen concentration in the top surface than in the bottom surface is chemically strengthened, a higher stress is produced in the top surface having a low hydrogen concentration than in the bottom surface having a high hydrogen concentration, and the glass is warped to project toward the top surface side, resulting in occurrence of warpage.
Therefore, as the hydrogen concentration in the top surface of a float glass is closer to that in the bottom surface, i.e., as the absolute value of the hydrogen concentration difference between the top surface and the bottom surface is smaller, the degree of stress produced comes close to an equilibrium state between the top surface and the bottom surface after chemical strengthening, and therefore the warpage is decreased.
In the present invention, since the average hydrogen concentration itself and the average hydrogen concentration difference itself can hardly be measured with good precision, the [1H−/30Si−] (also referred to as H/Si) proportional to the average hydrogen concentration is used as a direct indicator of the average hydrogen concentration, and the “difference in the normalized hydrogen concentrations between the top surface and the bottom surface” and the “difference in the normalized intensities between the top surface and the bottom surface”, which are proportional to the average hydrogen concentration difference above, are used as a direct indicator of the average hydrogen concentration difference.
In the present specification, the [1H−/30Si−] is a value measured under the following analysis conditions.
(Analysis Conditions)
Measurement apparatus: secondary ion mass spectrometry apparatus with a quadrupole mass spectrometer
Primary ion species: Cs+
Primary accelerating voltage: 5.0 kV
Primary ion current: 1 μA
Primary ion incident angle (angle from direction perpendicular to sample surface): 60°
Luster size: 200×200 μm2 
Detection region: 40×40 μm2 
Secondary ion polarity: minus
Electron gun for neutralization: used
The [1H−/30Si−], normalized intensity and normalized hydrogen concentration are described below. The secondary ion intensity IM1 of isotope M1 of element M in secondary ion mass spectrometry is proportional to the primary ion intensity IP, sputter ratio Y of matrix, concentration CM (ratio to the total concentration) of element M, existence probability α1 of isotope M1, secondary ionization ratio βM of element M, and transmission efficiency η (including detection efficiency of detector) of mass spectrometer.

IM1=A·IP·Y·CM·α1·βM·η  (formula 1)

In the formula, A is the ratio of detection area of secondary ion to scanning range of primary ion beam. In general, η of an apparatus can be hardly determined and in turn, the absolute value of βM cannot be obtained. Therefore, η is eliminated by using the main component element or the like in the same sample as a reference element and employing the ratio to (formula 1).
Assuming that the reference element is R and its isotope is RJ, (formula 2) is established:

IM1/IRj=(CM·α1·βM)/(CR·αj·βR)=CM/K  (formula 2)

in which K is a relative sensitivity factor of element M to element R.

K=(CR·αj·βR)/(α1·βM)  (formula 3)

In this case, the concentration of element M is determined according to (formula 4):

CM=K·IM1/IRj  (formula 4)

In the present invention, 1H− and 30Si− correspond to M1 and Rj, respectively. Therefore, from the (formula 2), the intensity ratio [1H−/30Si−] of those two is equivalent to a value obtained by dividing the average hydrogen concentration CH by K. That is, the [1H−/30Si−] is a direct indicator of the average hydrogen concentration.
The normalized intensity is a value obtained by dividing [1H−/30Si−] at a certain depth x by [1H−/30Si−] at a depth of 105 to 110 μm, that is, a value obtained by dividing CH/K at a certain depth x by CH/K at a depth of 105 to 110 μm. Since K is eliminated, the normalized intensity is eventually the same as a value obtained by dividing CH at a depth x by CH at a depth of 105 to 110 μm and, namely, is the normalized hydrogen concentration at a depth x.
The reason why the average hydrogen concentration at a depth of 105 to 110 μm is employed as the basis when calculating the normalized hydrogen concentration is because the region at a depth of 105 to 110 μm is considered to be an inner region where the average hydrogen concentration does not vary.
The absolute value of the normalized intensity difference between the top surface and the bottom surface in a float glass is obtained by secondary ion mass spectrometry (SIMS analysis), for example, through the following procedures (i) to (iii). Here, the analysis conditions described below are for exemplification and should be appropriately changed according to the measurement apparatus, sample, etc.
(i) In each of the top surface and the bottom surface, the secondary ion mass spectrometry is performed to a depth of 20 μm from the surface layer under the following analysis conditions.
(Analysis Conditions)
Measurement apparatus: secondary ion mass spectrometry apparatus with a quadrupole mass spectrometer
Primary ion species: Cs+
Primary accelerating voltage: 5.0 kV
Primary ion current: 1 μA
Primary ion incident angle (angle from direction perpendicular to sample surface): 60°
Luster size: 200×200 μm2 
Detection region: 40×40 μm2 
Secondary ion polarity: minus
Electron gun for neutralization: used
In the case where the intensity of 30Si− at a depth of 55 μm is smaller than the intensity of 30Si− at a depth of 5 μm by more than 3%, the analysis is preferably performed on a sample in which the surface of a glass substrate is previously etched by about 45 μm.
More specific analysis conditions are, for example, as follows.
(Analysis Conditions)
Measurement apparatus: secondary ion mass spectrometry apparatus with a quadrupole mass spectrometer
Primary ion species: Cs+
Primary accelerating voltage: 5.0 kV
Primary ion current: 1 μA
Primary ion incident angle (angle from direction perpendicular to sample surface): 60°
Luster size: 200×200 μm2 
Detection region: 40×40 μm2 
Sputter rate: 14 nm/sec
Secondary ion polarity: minus
Electron gun for neutralization: used
The secondary ion mass spectrometry apparatus with a quadrupole mass spectrometer includes, for example, ADEPT 1010, manufactured by Ulvac-Phi, Inc.
(ii) A value obtained by dividing [1H−/30Si−] at a depth of 0 to 10 μm in the [1H−/30Si−] profile obtained by secondary ion mass spectrometry by [1H−/30Si−] at a depth of 105 to 110 μm is defined as the normalized intensity at a depth of 0 to 10 μm in secondary ion mass spectrometry.
(iii) With regard to the normalized intensity at a depth of 0 to 10 μm obtained by secondary ion mass spectrometry, the absolute value of the difference between the top surface and the bottom surface is calculated.
4. Ion Exchange Amount
The ion exchange amount is a stress generation factor and is in a proportional relationship with the K2O concentration in glass after chemical strengthening. Therefore, the difference between ion exchange amounts in the top surface and the bottom surface can be analyzed by the K2O concentration difference. The K2O concentration can be analyzed by fluorescent X-ray analysis.
5. Warpage Amount
The float glass for chemical strengthening of the present invention is a float glass having a small amount of warpage after chemical strengthening. The warpage amount of the float glass can be measured by a contact-type surface profile analyzer [for example, SURFCOM (trade name), manufactured by Tokyo Seimitsu Co., Ltd.].
The warpage amount is measured as the difference between the highest point and the lowest point when measured with a contact-type surface profile analyzer, after correcting the base line so that the measurement start point and the measurement end point can be aligned at the same level. The warpage amount is indicated by a positive value when the float glass is warped in the convex direction to the top surface, and by a negative value when warped in the convex direction to the bottom surface.
The change in the warpage amount of the float glass before and after chemical strengthening can be measured according to the following formula.

ΔWarpage amount=(warpage amount after chemical strengthening)−(warpage amount before chemical strengthening)  (formula)

In the present invention, the measurement is performed on a central 9 cm-square region of a 10 cm-square float glass, and the absolute value of Δwarpage amount in terms of sheet thickness of 07 mm is preferably 58 μm or less, 56 μm or less, 54 μm or less, or 52 μm or less. When the absolute value of Δwarpage amount is not more than the upper limit above, the warpage after chemical strengthening can be decreased.
The CS (surface compressive stress) and DOL (depth of compressive stress layer) can be measured by a surface stress meter. In regard to the float glass for chemical strengthening of the present invention, the surface compressive stress of the chemically strengthened glass is preferably 650 MPa or more, and it is preferably used when the depth of compressive stress layer is 20 μm or less. When the depth of compressive stress layer is 20 μm or less, the product after chemical strengthening can be cut, and this is preferred. In this viewpoint, the depth of compressive stress layer is more preferably 15 μm or less.
6. Parameters
The discussion above implies the following parameters.
(1) Na2O Concentration Difference Between Top Surface and Bottom Surface Before Chemical Strengthening, and ΔWarpage Amount
It is considered that for controlling the warpage after chemical strengthening of a soda-lime glass, it is important to control the weathering degree, hydrogen concentration and Sn concentration in the glass surface layer before chemical strengthening.
Here, as shown in FIG. 1, in the surface layer in glass before chemical strengthening, the normalized hydrogen concentration is in an inverse relationship with the normalized Na2O surface concentration. In addition, as shown in FIG. 3, as the normalized Na2O surface concentration in glass before chemical strengthening is higher, the ion exchange amount after chemical strengthening is increased, and thus, the normalized Na2O surface concentration in glass before chemical strengthening is in a proportional relationship with the ion exchange amount.
Furthermore, when the increase in the hydrogen concentration of the glass surface layer is large, the Si—O—Si bond is broken in many portions and since this leads to deterioration of thermal characteristics such as glass transition temperature, stress relaxation occurs at the time of chemical strengthening in which the glass is heated at a high temperature, resulting in decrease of the stress. Therefore, it can be considered that stress generation resulting from chemical strengthening is attributable to the ion exchange amount and the relaxation degree. For this reason, the normalized Na2O surface concentration difference between top surface and bottom surface in glass before chemical strengthening is considered to have a correlation with the Δwarpage amount.
(1A) Difference Between Squared Normalized Na2O Surface Concentrations in Top Surface and Bottom Surface, and ΔWarpage Amount
FIG. 6 shows a graph plotting, on the abscissa, the difference Δ(N—Na2O2) (Top-Bottom) obtained by subtracting the squared normalized Na2O surface concentration of the bottom surface from the squared normalized Na2O surface concentration of the top surface, and on the ordinate, Δwarpage amount.
It is seen from the graph shown in FIG. 6 that the difference Δ(N—Na2O2) between squared normalized Na2O surface concentrations in the top surface and the bottom surface of a soda-lime glass sheet before chemical strengthening and the Δwarpage amount have a correlation represented by the following formula (1-1).

ΔWarpage amount=370×Δ(N—Na2O2)+45  formula (1-1)

In formula (1-1), the Δ(N—Na2O2) is the difference between squares of the values obtained by measuring, by fluorescent X-ray analysis, the normalized Na2O surface concentrations in the top surface and the bottom surface of glass to be subjected to chemical strengthening and is determined according to the following formula (1-2).

Δ(N—Na2O2)=(normalized Na2O surface concentration in top surface before chemical strengthening)2−(normalized Na2O surface concentration in bottom surface before chemical strengthening)2  formula (1-2)

Here, the normalized Na2O surface concentration is a value obtained by dividing the surface Na2O concentration by the Na2O concentration at a depth position of 100 μm. Each Na2O concentration is a value calculated from the relative intensity ratio to a standard sample by measuring the intensity of Na-Kα ray by fluorescent X-ray analysis. Incidentally, the Na2O concentration at a depth position of 100 μm is an Na2O concentration obtained by measuring the surface with a fluorescent X-ray after grinding off the glass to a depth of 100 μm from the surface. In addition, the analysis depth of the value measured by fluorescent X-ray analysis using an Na-Kα ray is typically 3 μm.
The difference between squared normalized Na2O surface concentrations in the top surface and the bottom surface of glass to be subjected to chemical strengthening is 0.040 or less, preferably 0.035 or less, 0.030 or less, or 0.025 or less. When the difference between squared normalized Na2O surface concentrations in the top surface and the bottom surface of glass to be subjected to chemical strengthening is 0.040 or less, even if a polishing treatment, etc. before chemical strengthening is simplified or omitted, warpage of the float glass after chemical strengthening can be reduced and excellent flatness can be obtained.
(1B) Square of Difference Between Normalized Na2O Surface Concentrations in Top Surface and Bottom Surface, and ΔWarpage Amount
FIG. 16 shows a graph plotting, on the abscissa, a square [ΔN—Na2O (Top-Bottom)]2 of the difference determined by subtracting the normalized Na2O surface concentration of the bottom surface which is a value obtained by dividing the Na2O concentration in bottom surface by the Na2O concentration at a depth position of 100 μm, from the normalized Na2O surface concentration of the top surface which is a value obtained by dividing the Na2O concentration in the top surface by the Na2O concentration at a depth position of 100 μm, and on the ordinate, Δwarpage amount.
It is seen from the graph shown in FIG. 16 that the square [ΔN—Na2O (Top-Bottom)]2 of the difference determined by subtracting the normalized Na2O surface concentration of the bottom surface from the normalized Na2O surface concentration of the top surface and the Δwarpage amount have a correlation represented by the following formula (7-1).

ΔWarpage amount=18000×(ΔN—Na2O)2+51  formula (7-1)

In formula (7-1), the (ΔN—Na2O)2 is a square of the difference between the values obtained by measuring, by fluorescent X-ray analysis, the normalized Na2O surface concentrations in the top surface and the bottom surface of glass to be subjected to chemical strengthening and is determined according to the following formula (7-2).

(ΔN—Na2O)2=[(normalized Na2O surface concentration in top surface before chemical strengthening)−(normalized Na2O surface concentration in bottom surface before chemical strengthening)]2  formula (7-2)

The square of the difference between normalized Na2O surface concentrations in the top surface and the bottom surface of glass to be subjected to chemical strengthening is 5.0×10−4 or less, preferably 4.5×10−4 or less, 4.0×10−4 or less, or 3.5×10−4 or less. When the square of the difference between normalized Na2O surface concentrations in the top surface and the bottom surface of glass to be subjected to chemical strengthening is 5.0×10−4 or less, even if a polishing treatment, etc. before chemical strengthening is simplified or omitted, warpage of the float glass after chemical strengthening can be reduced and excellent flatness can be obtained.
The Na2O concentration is adjusted by the method described later in (A) of “7. Production Method of Glass”, whereby the normalized Na2O surface concentrations in the top surface and the bottom surface of glass to be subjected to chemical strengthening can be adjusted, and Δ(N—Na2O2) or (ΔN—Na2O)2 can be adjusted. Specifically, it is preferable, for example, to decrease the Na2O concentration in the top surface by spraying water vapor or SO2 gas onto the top surface at the time of annealing the glass or increase the Na2O concentration in the bottom surface by lowering the flow rate of SO2 gas sprayed onto the bottom surface for the purpose of scratch prevention.
(2) Ion Exchange Amount Difference Between Top Surface and Bottom Surface after Chemical Strengthening and ΔWarpage Amount
The Δion exchange amount 1 that is the difference between ion exchange amounts in the top surface and the bottom surface after chemical strengthening is considered to have a correlation with the Δwarpage amount.
The ion exchange amount is proportional to the Na2O concentration before chemical strengthening and inhibited by Sn and therefore, can be determined according to the following formula (2-1).

Ion exchange amount 1=5.51×(normalized Na2O surface concentration)−0.038×(Sn concentration)  formula (2-1)

Hereinafter, the term “ion exchange amount” is sometimes used for indicating the ion exchange amount 1.
In formula (2-1), the normalized Na2O surface concentration is a value obtained by dividing the surface Na2O concentration by the Na2O concentration at a depth position of 100 μm. Here, each Na2O concentration is a value measured by fluorescent X-ray analysis using an Na-Kα ray. In addition, the Sn concentration is an Sn deposition amount (unit: as SnO2μg/cm2) per unit area of the top surface and the bottom surface.
The difference between ion exchange amounts 1 in the top surface and the bottom surface can be determined according to the following formula (2-2).

Ion exchange amount 1 difference=(ion exchange amount 1 in top surface)−(ion exchange amount 1 in bottom surface)  formula (2-2)

FIG. 7 shows a graph plotting, on the abscissa, Δion exchange amount 1 that is the difference between ion exchange amounts in the top surface and the bottom surface, and on the ordinate, Δwarpage amount. It is seen from the graph shown in FIG. 7, the Δion exchange amount 1 and the Δwarpage amount have a correlation represented by the following formula (2-3).

ΔWarpage amount=103×(Δion exchange amount 1)+24  formula (2-3)

The Δion exchange amount 1 is 0.32 or less, preferably 0.30 or less, 0.28 or less, 0.26 or less, or 0.24 or less.
When the difference between ion exchange amounts 1 in the top surface and the bottom surface after chemical strengthening, determined according to formulae (2-1) and (2-2), is 0.32 or less, even if a polishing treatment, etc. before chemical strengthening is simplified or omitted, warpage of the float glass after chemical strengthening can be reduced and excellent flatness can be obtained.
The Na2O concentration and the Sn concentration in the bottom surface are adjusted by the method described later in (A) of “7. Production Method of Glass”, whereby the Δion exchange amount 1 that is the difference between ion exchange amounts 1 in the top surface and the bottom surface after chemical strengthening can be adjusted. Specifically, it is preferable, for example, to decrease the Na2O concentration in the top surface by spraying water vapor or SO2 gas onto the top surface at the time of annealing the glass, increase the Na2O concentration in the bottom surface by lowering the flow rate of SO2 gas sprayed onto the bottom surface for the purpose of scratch prevention, or decrease the invasion amount of Sn into the bottom surface by lowering the temperature upstream of the float bath or increasing the hydrogen concentration in the atmosphere.
(3) Correlation of Hydrogen Concentration Difference and Sn Concentration Difference Between Top Surface and Bottom Surface Before Chemical Strengthening and Ion Exchange Amount Difference with ΔWarpage Amount
When multiple regression analysis is performed using, as factors, the hydrogen concentration difference, Sn concentration difference and ion exchange amount difference between the top surface and the bottom surface before chemical strengthening, and the Δwarpage amount, the following formula (3-1) is obtained.

W1=−16×(ΔH/Si)−6.47×(Sn concentration difference)−43.8×(Δion exchange amount 1)  formula (3-1)

In formula (3-1), the ΔH/Si is the difference (difference between normalized hydrogen concentrations) between values obtained by measuring, by SIMS analysis, the hydrogen concentration difference between the top surface and the bottom surface before chemical strengthening and is determined according to the following formula (3-2).

ΔH/Si=(normalized hydrogen concentration in top surface before chemical strengthening)−(normalized hydrogen concentration in bottom surface before chemical strengthening)  formula (3-2)

In formula (3-1), the Sn concentration difference is the difference obtained by subtracting the Sn deposition amount (unit: as SnO2μg/cm2) per unit area of the top surface from the Sn deposition amount (unit: as SnO2μg/cm2) per unit area of the bottom surface, and in the case where the glass does not contain SnO2, this is equivalent to the Sn deposition amount per unit area of the bottom surface.
The Δion exchange amount 1 is a value obtained by subtracting the ion exchange amount in the bottom surface from the ion exchange amount 1 in the top surface. The ion exchange amount is determined according to the above-mentioned formula (2-1).
FIG. 12 shows a graph plotting W1 on the abscissa and Δwarpage amount on the ordinate. It is seen from the graph shown in FIG. 12 that W1 has a correlation with the Δwarpage amount.
In formula (3-1), W1 is 56 or less, preferably 54 or less, 52 or less, or 50 or less. When W1 is 56 or less, even if a polishing treatment, etc. before chemical strengthening is simplified or omitted, warpage of the float glass after chemical strengthening can be reduced and excellent flatness can be obtained.
The hydrogen concentration and the Sn concentration in the bottom surface are adjusted by the method described later in (A) of “7. Production Method of Glass”, whereby W1 in formula (3-1) can be adjusted. Specifically, it is preferable, for example, to decrease the Na2O concentration in the top surface by spraying water vapor or SO2 gas onto the top surface at the time of annealing the glass, increase the Na2O concentration in the bottom surface by lowering the flow rate of SO2 gas sprayed onto the bottom surface for the purpose of scratch prevention, or decrease the invasion amount of Sn into the bottom surface by lowering the temperature upstream of the float bath or increasing the hydrogen concentration in the atmosphere.
(4) Correlation of Ion Exchange Amount Difference Between Top Surface and Bottom Surface and Hydrogen Concentration Difference Between Top Surface and Bottom Surface Before Chemical Strengthening with ΔWarpage Amount
The ion exchange amount is thought to be a stress generation factor, and the hydrogen concentration in the glass surface layer is thought to be a stress relaxation factor.
Namely, it is considered that as the hydrogen concentration in the glass surface layer is increased, the density of glass decreases. Since H in glass is present in the state of SiOH and the SiOH is produced resulting from breakage of a continuous crosslinked structure Si—O—Si in the glass, an increase in the hydrogen concentration in the glass surface layer is considered to cause a decrease in the density of glass, leading to stress relaxation.
The warpage of glass after chemical strengthening is thought to be attributable to the unbalance of stress difference between the top surface and the bottom surface and therefore, the value obtained by dividing the ion exchange amount by the hydrogen concentration is thought to have a correlation with the warpage amount.
FIG. 8 shows a graph plotting, on the abscissa, the difference between values obtained by dividing the ion exchange amounts in the top surface and the bottom surface by the normalized hydrogen concentrations in the top surface and the bottom surface before chemical strengthening, respectively, and on the ordinate, Δwarpage amount.
It is seen from the graph shown in FIG. 8 that the difference in the values obtained by dividing the ion exchange amounts (ion exchange amount) in the top surface and the bottom surface by the normalized hydrogen concentrations (H/Si) in the top surface and the bottom surface before chemical strengthening, respectively, and the Δwarpage amount have a correlation represented by the following formula (4-1).
In addition, when multiple regression analysis is performed using, as factors, the normalized hydrogen concentration, ion exchange amount difference and Δwarpage amount, the following formula (4-1) is obtained.

W2=9.18×Δ[(ion exchange amount)/(H/Si)]+49  formula (4-1)

In formula (4-1), the Δ[(ion exchange amount)/(H/Si)] is a value determined by subtracting a value that is obtained by dividing the ion exchange amount in the bottom surface by the normalized hydrogen concentration H/Si thereof, from a value that is obtained by dividing the ion exchange amount in the top surface by the normalized hydrogen concentration H/Si thereof.
In formula (4-1), the ion exchange amount is obtained according to the above-mentioned formula (2-1).
FIG. 13 shows a graph plotting W2 on the abscissa and Δwarpage amount on the ordinate. It is seen from the graph shown in FIG. 13 that W2 has a correlation with the Δwarpage amount.
In formula (4-1), the absolute value of W2 is 56 or less, preferably 54 or less, 52 or less, or 50 or less. When W2 is 56 or less, even if a polishing treatment, etc. before chemical strengthening is simplified or omitted, warpage of the float glass after chemical strengthening can be reduced and excellent flatness can be obtained.
The hydrogen concentration is adjusted by the method described later in (A) of “7. Production Method of Glass”, whereby W2 in formula (4-1) can be adjusted. Specifically, it is preferable, for example, to decrease the Na2O concentration in the top surface by spraying water vapor or SO2 gas onto the top surface at the time of annealing the glass, increase the Na2O concentration in the bottom surface by lowering the flow rate of SO2 gas sprayed onto the bottom surface for the purpose of scratch prevention, or decrease the invasion amount of Sn into the bottom surface by lowering the temperature upstream of the float bath or increasing the hydrogen concentration in the atmosphere.
(5) Na2O Concentration Difference and Sn Concentration Difference Between Top Surface and Bottom Surface Before Chemical Strengthening and ΔWarpage Amount
In order to control warpage of a soda-lime glass after chemical strengthening, it is important to control the weathering degree of the glass surface layer and Sn concentration before chemical strengthening, and therefore, the Na2O concentration difference and Sn concentration difference in the top surface and the bottom surface before chemical strengthening are considered to have a correlation with the Δwarpage amount.
When multiple regression analysis is performed using, as factors, the normalized Na2O surface concentration difference (ΔN—Na2O) and Sn concentration difference between the top surface and the bottom surface before chemical strengthening and the Δwarpage amount, as shown in FIG. 9, the normalized Na2O surface concentration difference and Sn concentration difference between the top surface and the bottom surface before chemical strengthening have a correlation represented by the following formula (5-1).

W3=744×[(ΔN—Na2O)+0.01×(Sn concentration difference)]  formula (5-1)

In formula (5-1), the ΔN—Na2O is the difference in the normalized Na2O surface concentrations that are values obtained by dividing the Na2O concentrations at the surface in the top surface and the bottom surface of glass to be subjected to chemical strengthening by the Na2O concentrations at a depth position of 100 respectively, and is determined according to the following formula (5-2). Here, each Na2O concentration is a value measured by fluorescent X-ray analysis using an Na-Kα ray.

ΔN—Na2O=(normalized Na2O surface concentration in top surface)−(normalized Na2O surface concentration in bottom surface)  formula (5-2)

In addition, the Sn concentration difference is an Sn concentration difference between the top surface and the bottom surface before chemical strengthening and is the difference obtained by subtracting the Sn deposition amount (unit: as SnO2μg/cm2) per unit area of the top surface from the Sn deposition amount (unit: as SnO2μg/cm2) per unit area of the bottom surface, and in the case where the glass does not contain SnO2, this is equivalent to the Sn deposition amount per unit area of the bottom surface.
FIG. 14 shows a graph plotting W3 on the abscissa and Δwarpage amount on the ordinate. It is seen from the graph shown in FIG. 14 that W3 has a correlation with the Δwarpage amount.
In formula (5-1), W3 is 58 or less, preferably 56 or less, 54 or less, or 52 or less. When W3 is 58 or less, even if a polishing treatment, etc. before chemical strengthening is simplified or omitted, warpage of the float glass after chemical strengthening can be reduced and excellent flatness can be obtained.
The Na2O concentration difference and the Sn concentration difference are adjusted by the method described later in (A) of “7. Production Method of Glass”, whereby W3 in formula (5-1) can be adjusted. Specifically, it is preferable, for example, to decrease the Na2O concentration in the top surface by spraying water vapor or SO2 gas onto the top surface at the time of annealing the glass, increase the Na2O concentration in the bottom surface by lowering the flow rate of SO2 gas sprayed onto the bottom surface for the purpose of scratch prevention, or decrease the invasion amount of Sn into the bottom surface by lowering the temperature upstream of the float bath or increasing the hydrogen concentration in the atmosphere.
(6) Ion Exchange Amount (Hydrogen Concentration, Na2O Concentration and Sn Concentration) Difference and ΔWarpage Amount
The ion exchange amount difference between the top surface and the bottom surface affects the warpage of glass after chemical strengthening, and the hydrogen concentration, Na2O concentration and Sn concentration are considered to be responsible for the ion exchange amount. Accordingly, with the normalized hydrogen concentration, normalized Na2O surface concentration and Sn concentration, the ion exchange amount shows a correlation represented by the following formula (6-1)

Ion exchange amount 2=−0.02×(H/Si)+5.54×(N—Na2O concentration)−0.037×(Sn concentration)  formula (6-1)

FIG. 15 shows a graph plotting, on the abscissa, the difference (Δion exchange amount 2) between ion exchange amounts 2 in top surface and bottom surface, and on the ordinate, Δwarpage amount. It is seen from the graph shown in FIG. 15 that the Δion exchange amount 2 and the Δwarpage amount have a correlation.
The Δion exchange amount 2 is a value determined according to the following formula (6-2)

ΔIon exchange amount 2=(Δion exchange amount 2 in top surface)−(Δion exchange amount 2 in bottom surface)  formula (6-2)

In formula (6-1), the Δion exchange amount 2 is 0.33 or less, preferably 0.31 or less, 0.29 or less, or 0.27 or less. When the Δion exchange amount 2 is 0.33 or less, even if a polishing treatment, etc. before chemical strengthening is simplified or omitted, warpage of the float glass after chemical strengthening can be reduced and excellent flatness can be obtained.
The hydrogen concentration, Na2O concentration and Sn concentration are adjusted by the method described later in (A) of “7. Production Method of Glass”, whereby the ion exchange amount 2 in formula (6-1) can be adjusted. Specifically, it is preferable, for example, to decrease the Na2O concentration in the top surface by spraying water vapor or SO2 gas onto the top surface at the time of annealing the glass, increase the Na2O concentration in the bottom surface by lowering the flow rate of SO2 gas sprayed onto the bottom surface for the purpose of scratch prevention, or decrease the invasion amount of Sn into the bottom surface by lowering the temperature upstream of the float bath or increasing the hydrogen concentration in the atmosphere.
7. Production Method of Glass
The method for reducing the Δwarpage amount by achieving a small difference in the weathering degrees between the top surface and the bottom surface of a float glass and a small difference in the amounts of metal invading the glass surface put into contact with a molten metal at the time of float forming includes, for example, methods described in the following (A) to (D). These methods may be used individually or in combination.
(A) At the time of annealing a glass coming out of a float bath, by spraying SO2 gas onto the glass, the alkali component Na2O of the glass is taken out from the glass as Na2SO4. By adjusting the amount of SO2 gas sprayed onto the glass, the amounts of alkali in the top surface and the bottom surface can be adjusted to the same level, and the difference in the weathering degree of glass can be reduced.
(B) Water vapor is sprayed on the top surface side of glass in a lehr.
(C) The residence time of molten glass in a float bath is shortened.
(D) The temperature in the upstream region of a float bath is lowered.
The present invention is described below based on the drawings, but the present invention is not limited thereto. FIG. 10 is a longitudinal cross-sectional view of a production apparatus for a float glass of the present invention. In FIG. 10, 12 is a tweel, 22 is a fixed refractory located below the tweel, and 23 is a spout lip.
Although omitted in the drawing, a raw material is continuously fed to a glass tank furnace to melt the raw material in a high temperature region inside the glass tank furnace, and the molten glass obtained is guided to a cooling region to control the temperature. The molten glass 1 at the controlled temperature passes through a connection groove 11, passes through a space 2 formed by the tweel 12 and the fixed refractory 22 located therebelow, then fed to a molten metal bath 5 through the spout lip 23, and formed into a glass ribbon 4.
The sheet thickness of the float glass is preferably 1.5 mm or less, more preferably 1.1 mm or less. In addition, although it is typically 0.7 mm or more, a glass thinner than that is also used, if desired.
The float glass for chemical strengthening of the present invention can reduce the warpage after chemical strengthening irrespective of the composition. The composition of the float glass for chemical strengthening includes, for example, the following glass compositions.
(i) A glass having a composition containing, in mass %, from 60 to 80% of SiO2, from 0.01 to 8% of Al2O3, from 8 to 22% of Na2O, from 0 to 7% of K2O, from 5 to 25% in total of RO (R=Mg, Ca, Sr, Ba), and from 0 to 5% of ZrO2.
(ii) A glass having a composition containing, in mass %, from 64 to 77% of SiO2, from 0.01 to 7% of Al2O3, from 10 to 18% of Na2O, from 0 to 5% of K2O, from 1 to 10% of MgO, from 1 to 12% of CaO, from 0 to 5% of SrO, from 0 to 5% of BaO, and from 0 to 3% of ZrO2.
(iii) A glass having a composition containing, in mass %, from 60 to 80% of SiO2, from 0.01 to 8% of Al2O3, from 8 to 22% of Na2O, from 0 to 7% of K2O, and from 0 to 5% of ZrO2, where in the case of containing MgO, CaO, SrO or BaO, the total of MgO, CaO, SrO and BaO contents is from 5 to 25% and the ratio Na2O/Al2O3 of Na2O and Al2O3 contents is 1.5 or more.
(iv) A glass having a composition containing, in mass %, from 60 to 80% of SiO2, from 0.01 to 8% of Al2O3, from 8 to 22% of Na2O, from 0 to 7% of K2O, and from 0 to 5% of ZrO2, where in the case of containing MgO, CaO, SrO or BaO, the total of MgO, CaO, SrO and BaO contents is from 5 to 25% and the ratio Na2O/Al2O3 of Na2O and Al2O3 contents is from 1.5 to 6.
(v) A glass having a composition containing, in mass %, from 60 to 80% of SiO2, from 0.01 to 8% of Al2O3, from 8 to 22% of Na2O, from 0 to 7% of K2O, and from 0 to 5% of ZrO2, where in the case of containing MgO, CaO, SrO or BaO, the total of MgO, CaO, SrO and BaO contents is from 5 to 25%, the total of CaO, SrO and BaO contents is from 1 to 7%, and the ratio Na2O/Al2O3 of Na2O and Al2O3 contents is 1.5 or more.
The float glass formed is cut into a predetermined size by a cutter not shown and then chemically strengthened, whereby a chemically strengthened float glass can be obtained.
The chemical strengthening is a treatment of forming a compressive stress layer on a glass surface by ion exchange at a temperature not more than a glass transition temperature, where an alkali metal ion having a small ion radius (typically, Li ion or Na ion) in a glass surface is exchanged with an alkali ion having a larger ion radius (typically, K ion). The chemical strengthening treatment can be performed by a conventionally known method.
An example where the float glass of the present invention is chemically strengthened and then used as a cover glass for a flat panel display is described below. FIG. 11 is a cross-sectional view of a display device where a cover glass is disposed. In the following description, front/back and right/left are based on the direction of the arrow in the drawings.
As illustrated in FIG. 11, a display device 10 generally includes a display panel 20 provided in a housing 15, and a cover glass 30 provided to cover the entire surface of the display panel 20 and surround the front of the housing 15.
The cover glass 30 is disposed mainly for the purpose of improving the beauty and strength of the display device 10, preventing the impact damage and the like and is formed from one sheet-shaped glass having a whole shape of nearly planer shape. As illustrated in FIG. 11, the cover glass 30 may be disposed to be spaced (to have an air layer) from the display side (front side) of the display panel 20 or may be attached to the display side of the display panel 20 with an adhesive film (not shown) having translucency.
On the front surface of the cover glass 30, which emits light from the display panel 20, a functional film 41 is provided, and on the back surface where light from the display panel 20 enters, a functional film 42 is provided at a position corresponding to the display panel 20. In FIG. 11, the functional films 41 and 42 are provided on both surfaces, but not limited thereto, and they may be provided on the front surface or back surface or may be omitted.
The functional films 41 and 42 have a function, for example, of preventing reflection of surrounding light, preventing impact damage, shielding electromagnetic wave, blocking near infrared ray, correcting color tone, and/or enhancing scratch resistance, and the thickness, shape, etc. are appropriately selected according to usage. The functional films 41 and 42 are formed, for example, by attaching a resin-made film to the cover glass 30. Alternatively, they may be formed by a thin film formation method such as deposition method, sputtering method or CVD method.
The reference numeral 44 is a black layer and is, for example, a coating film formed by coating an ink containing a pigment particle on the cover glass 30 and subjecting it to ultraviolet irradiation or heating/firing and then cooling. The display panel, etc. is made invisible from the outside of the housing 15 and thereby the aesthetics of appearance is enhanced. The reference numeral 44 is not limited to a black layer and may be, for example, a white layer.
PRACTICAL EXAMPLES
Practical Examples of the present invention are specifically described below, but the present invention is not limited thereto.
Practical Example 1
[Production of Float Glass]
Glass having the following composition was produced by a float method to have a sheet thickness of 0.7 mm and cut into a size of 10 cm×10 cm to produce float glass plates of Examples 1 to 4.
Composition A:
in mass %, SiO2: 71.5%, Al2O3: 1.8%, Na2O: 13.5%, K2O: 0.26%, MgO: 4.64%, CaO: 7.83%, ZrO2: 0.03%
Composition B:
SiO2: 71.5%, Al2O3: 1.8%, Na2O: 13.5%, K2O: 0.26%, MgO: 4.64%, CaO: 7.83%, ZrO2: 0.03%
Composition C:
SiO2: 71.5%, Al2O3: 1.8%, Na2O: 13.5%, K2O: 0.26%, MgO: 4.64%, CaO: 7.83%, ZrO2: 0.03%
[Evaluation Methods]
(1) Measurement of Hydrogen Concentration in Glass Surface Layer
The hydrogen concentration of each float glass of Practical Examples 1 and 2 and Comparative Examples 1 to 3 was analyzed down to a depth of 20 μm by secondary ion mass spectrometry. The [1H−/30Si−] profile of the float glass by secondary ion mass spectrometry is shown, and this profile can be regarded as equivalent to the hydrogen concentration profile.
Analysis conditions of the secondary ion mass spectrometry were as follows.
Measurement apparatus: ADEPT 1010, manufactured by Ulvac-Phi, Inc.
Primary ion species: Cs+
Primary accelerating voltage: 5.0 kV
Primary ion current: 1 μA
Primary ion incident angle (angle from direction perpendicular to sample surface): 60°
Luster size: 200×200 μm2 
Detection region: 40×40 μm2 
Sputter rate: 14 nm/sec
Secondary ion polarity: minus
Electron gun for neutralization: used
The [1H−/30Si−]'s at a depth of 0 to 10 μm and at a depth of 105 to 110 μm were measured, and the difference in the normalized intensities at a depth of 0 to 10 μm between the bottom surface (surface B) and the top surface (surface T) was calculated. In Table 1, “H/Si” indicates a value obtained by dividing the average value at a depth of 0 to 10 μm by the average value at a depth of 105 to 110 μm.
Here, field aperture of detector was 1, and ESA input lens of detector was 550.
(2) Measurement of Warpage Amount
After measuring the warpage amount by a contact-type surface profile analyzer (SURFCOM 1400D (trade name)) manufactured by Tokyo Seimitsu Co., Ltd. before chemical strengthening, each float glass was chemically strengthened with potassium nitrate molten salt at 425° C. for 150 minutes. The warpage amount after chemical strengthening was measured in the same manner, and the Δwarpage amount represented by (formula) Δwarpage amount=warpage amount after chemical strengthening−warpage amount before chemical strengthening was calculated. Here, the Δwarpage amount was the Δwarpage amount measured in a 9 cm-square float glass.
(3) Measurements of Na2O Concentration and K2O Concentration
As for the Na2O concentration and K2O concentration in the glass surface layer, the intensities of Na-Kα ray and K-Kα ray were measured respectively by fluorescent X-ray analysis by using ZSX Primus II manufactured by Rigaku Corporation, and the concentration was determined from the relative intensity ratio to the standard sample.
In Table 1, “N—Na2O” is the normalized Na2O surface concentration that is a value obtained by dividing the Na2O concentration by the Na2O concentration at a depth position of 100 μm. Here, the Na2O concentration is a value calculated from the relative intensity ratio to a standard sample by measuring the intensity of Na-Kα ray by fluorescent X-ray analysis. The ion exchange amount (K2O concentration) after chemical strengthening is shown in the column of K2O Ion Exchange Amount in Table 1.
(4) Measurement of Sn Concentration
Regarding the Sn concentration in the glass surface, the glass surface was etched with a hydrofluoric acid solution and the Sn concentration in the solution was quantitatively determined by ICP emission spectrometry. As for the ICP emission spectrometry, SPS3100 manufactured by SII NanoTechnology Inc. was used.
In Table 1, the glass of Compositions A to C does not contain SnO2, and the SnO2 concentration in the top surface was evidently 0 and therefore, was not measured. The same applies to Tables 2 to 6 below.
With respect to the glass of Examples 1 to 4, the normalized hydrogen concentration [0-10 μm average (H/Si)/105-110 μm (average H/Si)], N—Na2O concentration (surface concentration/concentration at depth position of 100 μm; hereinafter, also referred to as normalized Na2O surface concentration), K2O concentration and Sn concentration (deposition amount per unit area), before chemical strengthening, and the ion exchange amount (K2O concentration) and Δwarpage amount, after chemical strengthening, were determined, and the results are shown in Table 1.
The reason why the N—Na2O concentration in the top surface is less than 1 is considered because the SO2 gas sprayed onto the bottom surface flowed around the top surface side. In addition, the reason why the N—Na2O concentration in the top surface differs, for example, between Example 1 and Example 4 is considered because the SO2 gas spraying state fluctuated.











TABLE 1











Measurement Results After 






Chemical Strengthening















Measurement Results Before 
K2O Ion 



Com-
Ex-

Chemical Strengthening
Exchange 
ΔWarpage 















posi-
am-



K2O 
SnO2
Amount
Amount


tion
ple
Surface
H/Si
N—Na2O
(wt %)
(μg/cm2)
(wt %)
(μm)





A
1
Top
1.02
0.94
0.327
—
5.01
58




Bottom
1.35
0.91
0.323
6.18
4.71



B
2
Top
1.53
0.93
0.310
—
4.93
52




Bottom
1.60
0.91
0.305
5.46
4.58




3
Top
1.54
0.93
0.305
—
4.89
60




Bottom
1.91
0.90
0.300
5.65
4.53




4
Top
1.23
0.92
0.269
—
5.39
57




Bottom
1.54
0.90
0.265
6.06
5.04




5
Top
1.27
0.92
0.268
—
5.35
69




Bottom
2.02
0.89
0.265
6.13
4.87




6
Top
1.24
0.92
0.282
—
5.35
56




Bottom
1.64
0.91
0.279
5.57
5.06



C
7
Top
1.10
0.94
0.275
—
5.03
61




Bottom
1.38
0.91
0.272
6.05
4.74









Practical Example 2
Na2O Concentration Difference in Top Surface and Bottom Surface Before Chemical Strengthening and ΔWarpage Amount
Since the Na2O concentration difference between the top surface and the bottom surface of a soda-lime glass sheet before chemical strengthening is considered to have a correlation with the Δwarpage amount, the difference Δ(N—Na2O2) between squared normalized Na2O surface concentrations in the top surface and the bottom surface was determined from the data shown in Table 1, and the correlation with the Δwarpage amount was studied.
The results are shown in Table 2 and FIG. 6. FIG. 6 is a graph plotting, on the abscissa, the difference Δ(N—Na2O2) (Top-Bottom) between squared normalized Na2O surface concentrations in the top surface and the bottom surface of glass to be subjected to chemical strengthening, and on the ordinate, Δwarpage amount.
From the graph shown in FIG. 6, it was found that the difference Δ(N—Na2O2) between squared normalized Na2O surface concentrations in the top surface and the bottom surface of a soda-lime glass sheet before chemical strengthening and the Δwarpage amount have a correlation represented by the following formula (1-1).

ΔWarpage amount=370×Δ(N—Na2O2)+45  formula (1-1)

In formula (1-1), the Δ(N—Na2O2) is the difference between squares of the values obtained by measuring, by fluorescent X-ray analysis, the normalized Na2O surface concentration in the top surface and the bottom surface of glass to be subjected to chemical strengthening and is determined according to the following formula (1-2)

ΔNa2O=(normalized Na2O surface concentration in top surface before chemical strengthening)2−(normalized Na2O surface concentration in bottom surface before chemical strengthening)2  formula (1-2)

It was understood from the results shown in Table 2 and FIG. 6 that when Δ(N—Na2O2) in formula (1-1) is 0.040 or less, the Δwarpage amount can be 58 μm or less.












TABLE 2









Δ(N—Na2O{circumflex over ( )}2)
ΔWarpage


Ex-



(Top −
Amount


ample
Surface
N—Na2O
N—Na2O{circumflex over ( )}2
Bottom)
(μm)







1
Top
0.94
0.881
0.045
58



Bottom
0.91
0.836


2
Top
0.93
0.865
0.028
52



Bottom
0.91
0.837


3
Top
0.93
0.859
0.042
60



Bottom
0.90
0.817


4
Top
0.92
0.847
0.028
57



Bottom
0.90
0.819


5
Top
0.92
0.838
0.046
69



Bottom
0.89
0.791


6
Top
0.92
0.852
0.024
56



Bottom
0.91
0.828


7
Top
0.94
0.877
0.049
61



Bottom
0.91
0.828









Practical Example 3
Ion Exchange Amount Difference Between Top Surface and Bottom Surface after Chemical Strengthening and ΔWarpage Amount
Since the ion exchange amount difference between the top surface and the bottom surface is considered to have a correlation with the Δwarpage amount, the ion exchange amount difference (Δion exchange amount 1) was determined from the data shown in Table 1, and the correlation with the Δwarpage amount was studied.
The Δion exchange amount 1 is a value obtained by subtracting the ion exchange amount 1 in the bottom surface from the ion exchange amount 1 in the top surface. The ion exchange amount 1 was determined according to the following formula (2-1).

Ion exchange amount 1=5.51×(normalized Na2O surface concentration)−0.038×(Sn concentration)  formula (2-1)

Ion exchange amount 1 difference=(ion exchange amount in top surface)−(ion exchange amount in bottom surface)  formula (2-2)

The results are shown in Table 3 and FIG. 7. FIG. 7 is a graph plotting, on the abscissa, the Δion exchange amount 1 that is the difference in the ion exchange amount 1 between the top surface and the bottom surface, and on the ordinate, the Δwarpage amount.
From the graph shown in FIG. 7, it was found that the Δion exchange amount 1 and the Δwarpage amount have a correlation represented by the following formula (2-3).

ΔWarpage amount=103×(Δion exchange amount 1)+24  formula (2-3)

It was understood from the results shown in Table 3 and FIG. 7 that when Δion exchange amount 1 is 0.32 or less, the Δwarpage amount can be 58 μm or less.













TABLE 3





Ex-



Ion
ΔIon
ΔWarpage 


am-


SnO2
Exchange
Exchange
Amount


ple
Surface
N—Na2O
(μg/cm2)
Amount
Amount
(μm)







1
Top
0.94
—
5.18
0.37
58



Bottom
0.91
6.18
4.81




2
Top
0.93
—
5.13
0.29
52



Bottom
0.91
5.46
4.83




3
Top
0.93
—
5.11
0.34
60



Bottom
0.90
5.65
4.77




4
Top
0.92
—
5.07
0.32
57



Bottom
0.90
6.06
4.76




5
Top
0.92
—
5.05
0.38
69



Bottom
0.89
6.13
4.67




6
Top
0.92
—
5.09
0.28
56



Bottom
0.91
5.57
4.80




7
Top
0.94
—
5.16
0.38
61



Bottom
0.91
6.05
4.79









Practical Example 4
Hydrogen Concentration Difference and Sn Concentration Difference Between Top Surface and Bottom Surface Before Chemical Strengthening, Ion Exchange Amount Difference, and ΔWarpage Amount
Since a correlation is considered to exist among the hydrogen concentration difference and Sn concentration difference between the top surface and the bottom surface before chemical strengthening, the ion exchange amount difference and the Δwarpage amount, multiple regression analysis was performed based on the data shown in Table 1 by using these values as factors and, as a result, the following formula (3-1) was obtained. The data obtained from the data shown in Table 1 is shown in Table 4.

W1=−16×(ΔH/Si)−6.47×(Sn concentration difference)−43.8×(Δion exchange amount 1)  formula (3-1)

In formula (3-1), ΔH/Si is the difference (difference in the normalized hydrogen concentration) between values obtained by measuring, by SIMS analysis, the hydrogen concentration difference between the top surface and the bottom surface before chemical strengthening and is determined according to the following formula (3-2).

ΔH/Si=(normalized hydrogen concentration in top surface before chemical strengthening)−(normalized hydrogen concentration in bottom surface before chemical strengthening)  formula (3-2)

The Δion exchange amount 1 is a value obtained by subtracting the ion exchange amount in the bottom surface from the ion exchange amount in the top surface. The ion exchange amount is determined according to the above-mentioned formula (2-1).
FIG. 12 shows a graph plotting W1 on the abscissa and Δwarpage amount on the ordinate. The graph shown in FIG. 12 revealed that W1 has a correlation with the Δwarpage amount. It was understood from the results shown in Table 4 and FIG. 12 that when W1 is 56 or less, the Δwarpage amount can be 58 μm or less.
















TABLE 4





Ex-





Ion 
ΔIon

ΔWarpage 


am-




SnO2
Exchange 
Exchange
W1
Amount


ple
Surface
H/Si
Δ(H/Si) 
N—Na2O
(μg/cm2)
Amount
Amount
(μm)
(μm)
























1
Top
1.02
−0.33
0.94
—
5.18
0.37
61
58



Bottom
1.35

0.91
6.18
4.81





2
Top
1.53
−0.08
0.93
—
5.13
0.29
49
52



Bottom
1.60

0.91
5.46
4.83





3
Top
1.54
−0.37
0.93
—
5.11
0.34
57
60



Bottom
1.91

0.90
5.65
4.77





4
Top
1.23
−0.31
0.92
—
5.07
0.32
58
57



Bottom
1.54

0.90
6.06
4.76





5
Top
1.27
−0.75
0.92
—
5.05
0.38
68
69



Bottom
2.02

0.89
6.13
4.67





6
Top
1.24
−0.40
0.92
—
5.09
0.28
55
56



Bottom
1.64

0.91
5.57
4.80





7
Top
1.10
−0.28
0.94
—
5.16
0.38
60
61



Bottom
1.38

0.91
6.05
4.79












Practical Example 5
Ion Exchange Amount Difference Between Top Surface and Bottom Surface, Hydrogen Concentration Difference Between Top Surface and Bottom Surface Before Chemical Strengthening and ΔWarpage Amount
Since a correlation is considered to exist among the ion exchange amount difference between the top surface and the bottom surface, the hydrogen concentration difference between the top surface and the bottom surface before chemical strengthening and the Δwarpage amount, the correlation among the ion exchange amount difference between the top surface and the bottom surface, the hydrogen concentration difference between the top surface and the bottom surface before chemical strengthening, and the Δwarpage amount was studied from the data shown in Table 1.
The results are shown in Table 5 and FIG. 8. FIG. 8 is a graph plotting, on the abscissa, a value obtained by dividing the ion exchange amount difference between the top surface and the bottom surface by the hydrogen concentration difference between the top surface and the bottom surface before chemical strengthening, and on the ordinate, the Δwarpage amount.
In addition, since a correlation is considered to exist among the hydrogen concentration, the ion exchange amount and the Δwarpage amount, multiple regression analysis was performed based on the data shown in Table 1 by using these values as factors and, as a result, the following formula (4-1) was obtained.

W2=9.18×Δ[(ion exchange amount)/(H/Si)]+49  formula (4-1)

In formula (4-1), the Δ[(ion exchange amount)/(H/Si)] is a value determined by subtracting a value obtained by dividing the ion exchange amount in the bottom surface by the normalized hydrogen concentration H/Si, from a value obtained by dividing the ion exchange amount in the top surface by the normalized hydrogen concentration H/Si. The ion exchange amount was determined according to the above formula (2-1).
FIG. 13 shows a graph plotting W2 on the abscissa and Δwarpage amount on the ordinate. The graph shown in FIG. 13 revealed that W2 has a correlation with the Δwarpage amount. It was understood from the results shown in Table 5 and FIG. 13 that when W2 is 56 or less, the Δwarpage amount can be 58 μm or less.














TABLE 5








Ion 
Ion 
ΔIon




Ex-


Ex-
Exchange
Exchange

ΔWarpage 


am-


change
Amount/
Amount
W2
Amount


ple
Surface
H/Si
Amount
(H/Si)
(H/Si)
(μm)
(μm)







1
Top
1.02
5.18
5.08
1.51
63
58



Bottom
1.35
4.81
3.56





2
Top
1.53
5.13
3.36
0.35
52
52



Bottom
1.60
4.83
3.01





3
Top
1.54
5.11
3.32
0.82
57
60



Bottom
1.91
4.77
2.50





4
Top
1.23
5.07
4.11
1.03
58
57



Bottom
1.54
4.76
3.09





5
Top
1.27
5.05
3.98
1.67
64
69



Bottom
2.02
4.67
2.31





6
Top
1.24
5.09
4.10
1.17
60
56



Bottom
1.64
4.80
2.92





7
Top
1.10
5.16
4.69
1.21
60
61



Bottom
1.38
4.79
3.48









Practical Example 6
Na2O Concentration Difference and Sn Concentration Difference Between Top Surface and Bottom Surface Before Chemical Strengthening, and ΔWarpage Amount
Since the normalized Na2O surface concentration difference and Sn concentration difference between the top surface and the bottom surface before chemical strengthening are considered to have a correlation with the Δwarpage amount, multiple regression analysis was performed based on the data shown in Table 1 by using these values as factors. The results are shown in Table 6, FIG. 9 and FIG. 17.
As shown in FIG. 17, it was found that the normalized Na2O surface concentration difference and Sn concentration difference between the top surface and the bottom surface before chemical strengthening have a correlation represented by the following formula (5-1).

W3=744×[(ΔN—Na2O)+0.01×(Sn concentration difference)]  formula (5-1)

In formula (5-1), the ΔN—Na2O is a value obtained by subtracting the normalized Na2O surface concentration in the bottom surface from the normalized Na2O surface concentration in the top surface and is obtained according to the following formula (5-2).

ΔN—Na2O=(normalized Na2O surface concentration in top surface before chemical strengthening)−(normalized Na2O surface concentration in bottom surface before chemical strengthening)  formula (5-2)

FIG. 14 shows a graph plotting W3 on the abscissa and Δwarpage amount on the ordinate. The graph shown in FIG. 14 revealed that W3 has a correlation with the Δwarpage amount. It was understood from the results shown in Table 6 and FIG. 14 that when W3 is 58 or less, the Δwarpage amount can be 58 μm or less.














TABLE 6












ΔWarpage 






SnO2
ΔN—Na2O + 

Amount


Example
Surface
N—Na2O
ΔN—Na2O
(μg/cm2)
0.01*SnO2
W3 (μm)
(μm)







1
Top
0.94
0.024
6.18
0.086
64
58



Bottom
0.91







2
Top
0.93
0.015
5.46
0.070
52
52



Bottom
0.91







3
Top
0.93
0.023
5.65
0.080
59
60



Bottom
0.90







4
Top
0.92
0.015
6.06
0.076
57
57



Bottom
0.90







5
Top
0.92
0.026
6.13
0.087
65
69



Bottom
0.89







6
Top
0.92
0.013
5.57
0.069
51
56



Bottom
0.91







7
Top
0.94
0.026
6.05
0.087
65
61



Bottom
0.91









Practical Example 7
Ion Exchange Amount (Hydrogen Concentration, Na2O Concentration and Sn Concentration) Difference and ΔWarpage Amount
Since the hydrogen concentration, Na2O concentration and Sn concentration are considered to be responsible for the ion exchange amount, a correlation formula was determined from the data shown in Table 7 and, as a result, the following formula (6-1) was obtained.

Ion exchange amount 2=−0.02×(H/Si)+5.54×(N—Na2O concentration)−0.037×(Sn concentration)  formula (6-1)

Since the ion exchange amount difference between the top surface and the bottom surface is considered to affect the Δion exchange amount 2 determined according to formula (6-1), i.e., the difference in the ion exchange amounts 2 between the top surface and the bottom surface, and the warpage of glass after chemical strengthening, the correlation of the Δion exchange amount 2 determined according to formula (6-2), i.e., the difference in the ion exchange amounts 2 between the top surface and the bottom surface, with the Δwarpage amount was examined.

ΔIon exchange amount 2=(ion exchange amount 2 in top surface)−(ion exchange amount 2 in bottom surface)  formula (6-2)

FIG. 15 shows a graph plotting, on the abscissa, the difference (Δion exchange amount 2) between ion exchange amounts 2 in top surface and bottom surface, and on the ordinate, Δwarpage amount. The graph shown in FIG. 15 revealed that the Δion exchange amount 2 and the Δwarpage amount have a correlation.
It was understood from the results shown in Table 7 and FIG. 15 that when Δion exchange amount 2 in formula (6-1) is 0.33 or less, the Δwarpage amount can be 58 μm or less.















TABLE 7





Com-





Ion 
ΔIon
ΔWarpage 


posi-
Ex-



SnO2
Exchange
Exchange
Amount


tion
ample
Surface
H/Si
N—Na2O
(μg/cm2)
Amount 2
Amount 2
(μm)























A
1
Top
1.02
0.94
0
5.18
0.37
58




Bottom
1.35
0.91
6.18
4.81




B
2
Top
1.53
0.93
0
5.12
0.29
52




Bottom
1.60
0.91
5.46
4.83





3
Top
1.54
0.93
0
5.10
0.34
60




Bottom
1.91
0.90
5.65
4.76





4
Top
1.23
0.92
0
5.07
0.32
57




Bottom
1.54
0.90
6.06
4.76





5
Top
1.27
0.92
0
5.05
0.38
69




Bottom
2.02
0.89
6.13
4.66





6
Top
1.24
0.92
0
5.09
0.29
56




Bottom
1.64
0.91
5.57
4.80




C
7
Top
1.10
0.94
0
5.17
0.38
61




Bottom
1.38
0.91
6.05
4.79









Example 7
Na2O Concentration Difference Between Top Surface and Bottom Surface Before Chemical Strengthening and ΔWarpage Amount
Since the Na2O concentration difference between the top surface and the bottom surface of a soda-lime glass sheet before chemical strengthening is considered to have a correlation with the Δwarpage amount, a square [ΔN—Na2O (Top-Bottom)]2 of the difference obtained by subtracting the normalized Na2O surface concentration of the bottom surface from the normalized Na2O surface concentration of the top surface was determined from the data shown in Table 1, and the correlation with the Δwarpage amount was studied.
The results are shown in Table 8 and FIG. 16. In Table 8, for example, “5.9.E-04” means 5.9×10−4. FIG. 16 is a graph plotting, on the abscissa, a square [ΔN—Na2O (Top-Bottom)]2 of the difference obtained by subtracting the normalized Na2O surface concentration of the bottom surface from the normalized Na2O surface concentration of the top surface to be subjected to chemical strengthening, and on the ordinate, Δwarpage amount.
The graph shown in FIG. 16 revealed that the square (ΔN—Na2O)2 of the difference between normalized Na2O surface concentrations in the top surface and the bottom surface of a soda-lime glass sheet before chemical strengthening and the Δwarpage amount have a correlation represented by the following formula (7-1).

ΔWarpage amount=18000×(ΔN—Na2O)2+51  formula (7-1)

In formula (7-1), the (ΔN—Na2O)2 is a square of the difference between the values obtained by measuring, by fluorescent X-ray analysis, the normalized Na2O surface concentration in the top surface and the bottom surface of glass to be subjected to chemical strengthening and is determined according to the following formula (7-2):

(ΔN—Na2O)2=[(normalized Na2O surface concentration in top surface before chemical strengthening)−(normalized Na2O surface concentration in bottom surface before chemical strengthening)]2  formula (7-2)

It was understood from the results shown in Table 8 and FIG. 16 that when (ΔN—Na2O)2 in formula (7-1) is 5.0×10−4 or less, the Δwarpage amount can be 58 μm or less.











TABLE 8









ΔWarpage


Example
Surface
N—Na2O
(ΔN—Na2O){circumflex over ( )}2
Amount (μm)







1
Top
0.94
5.9.E−04
58



Bottom
0.91


2
Top
0.93
2.3.E−04
52



Bottom
0.91


3
Top
0.93
5.4.E−04
60



Bottom
0.90


4
Top
0.92
2.4.E−04
57



Bottom
0.90


5
Top
0.92
6.6.E−04
69



Bottom
0.89


6
Top
0.92
1.7.E−04
56



Bottom
0.91


7
Top
0.94
6.9.E−04
61



Bottom
0.91









[Glass Composition Example]

Composition Examples G1 to G16 in mass % of the float glass for chemical strengthening of the present invention, and the compressive stress CS (unit: MPa) and depth of compressive stress layer DOL (unit: μm) when the float glass was chemically strengthened, are shown in Tables 9 and 10.
In the Tables, Na2O/Al2O3 is the ratio of Na2O and Al2O3 contents, RO is the total of MgO, CaO, SrO and BaO contents, CaO+SrO+BaO is the total of CaO, SrO and BaO contents, strengthening temperature (unit: ° C.) and strengthening time (unit: h) are those in the chemical strengthening above, KNO3 is the concentration (unit: mass %) of KNO3 in a molten salt used for chemical strengthening, and dol is the dol described above. Here, when the concentration of KNO3 in a molten salt is not 100%, the remaining component is NaNO3.
















TABLE 9






G1
G2
G3
G4
G5
G6
G7
G8
G9
























SiO2
72
72
72
67
62
60
68
65.5
69


Al2O3
2
2
2
1
1
1
5
3
2


MgO
4.5
4.5
4.5
6
8
10
5
6
4


CaO
8
8
8
9
10
13
7
8
10


SrO
0
0
0
0
0
0
1
0
0


BaO
0
0
0
0
0
0
0
0
0


ZrO2
0
0
0
0
0
0
0
2.5
1


Na2O
13.2
13.2
13.2
15
14
10
13
9
12


K2O
0.3
0.3
0.3
2
5
6
1
6
2


Na2O/
6.6
6.6
6.6
15.0
14.0 
10.0
2.6
3.0
6.0


Al2O3











RO
12.5
12.5
12.5
15.0
18.0 
23.0
13.0
14.0 
14.0


CaO + 
8.0
8.0
8.0
9.0
10.0
13.0
8.0
8.0
10.0


SrO +











BaO











Strength-
425
425
425
425
425
450
425
425
425


ening











temper-











ature











Strength-
2.5
2.5
2.5
2.5
2.5
5
2.5
2.5
4


ening











time t











KNO3
100
97.5
95
100
100
100
100
100
100


dol
5
5
5
7
6
4
5
5
6


CS
680
580
480
690
730
810
760
770
710


DOL
5
5
5
7
6
4
5
5
6

























TABLE 10






G10
G11
G12
G13
G14
G15
G16
G17
G18
























SiO2
65
62.5
65
76.5
77.9
71
74
72
69


Al2O3
5
7.5
7
0.5
0.1
4
4
6
5


MgO
6
3
4
3
2
4
3.5
4
3


CaO
5
6
2
5
4
7
1.5
1
1


SrO
2
0
1
0
0
0
0
2
0


BaO
0
0
1
0
0
0
0
0
2


ZrO2
0.2
1
2
0
4
0
0
0
0


Na2O
14.3
20
16.5
14
12
10
15
11
17


K2O
2.5
0
1.5
1
0
4
2
4
3


Na2O/
2.9
2.7
2.4
28.0
120
2.5
3.8
1.8
3.4


Al2O3











RO
13.0
9.0
8.0
8.0
6.0
11.0
5.0
7.0
6.0


CaO + 
7.0
6.0
4.0
5.0
4.0
7.0
1.5
3.0
3.0


SrO + 











BaO











Strength-
425
425
425
425
425
425
400
400
400


ening











temper-











ature











Strength-
2.5
2.5
1
2.5
2.5
2.5
1
1
0.5


ening











time t











KNO3
100
100
100
100
100
100
100
100
100


dol
12
20
16
18
10
12
17
12
19


CS
720
720
730
470
590
630
550
630
560


DOL
12
20
16
18
10
12
17
12
19









While the present invention has been described in detail with reference to specific embodiments, it is apparent to one skilled in the art that various changes and modifications can be made therein without departing from the intension and scope of the present invention. This application is based on Japanese Patent Application (No. 2012-285511) filed on Dec. 27, 2012, and the whole of which are incorporated by reference.
DESCRIPTION OF REFERENCE NUMERALS AND SIGNS
1 Molten glass
5 Molten metal bath
10 Display device
15 Housing
20 Display panel
30 Cover glass

## Claims

1. A float glass for chemical strengthening, comprising a bottom surface coming into contact with a molten metal at the time of forming of the float glass and a top surface opposing the bottom surface, wherein a difference Δ(N—Na2O2) determined by subtracting a square of a normalized Na2O surface concentration of the bottom surface from a square of a normalized Na2O surface concentration of the top surface is 0.040 or less, where the normalized Na2O surface concentration is obtained by dividing an Na2O concentration in a measured surface, which is the bottom surface or the top surface, by an Na2O concentration at a depth of 100 μm from the measured surface, where the Na2O concentration is a value measured by a fluorescent X-ray analysis using an Na—Kα ray.

2. The float glass as claimed in claim 1, wherein W3 determined according to the following formula (5-1) is 58 or less:


W3=744×[(ΔN—Na2O)+0.01×(Sn concentration difference)]  formula (5-1),


wherein, in formula (5-1):
the ΔN—Na2O is a value determined by subtracting the normalized Na2O surface concentration in the bottom surface from the normalized Na2O surface concentration in the top surface; and
the Sn concentration difference is a difference obtained by subtracting an Sn deposition amount per unit area (unit: μg/cm2) in the top surface from an Sn deposition amount per unit area (unit: μg/cm2) in the bottom surface, where the Sn deposition amount per unit area is a deposition mass in terms of SnO2 per 1 cm2 when Sn is assumed to exist in the form of SnO2.

3. The float glass as claimed in claim 1, wherein Δion exchange amount 2 which is a value obtained by subtracting an ion exchange amount 2 in the bottom surface from an ion exchange amount 2 in the top surface is 0.33 or less,
where the ion exchange amount 2 is a value determined according to the following formula (6-1):


Ion exchange amount 2=−0.02×(H/Si)+5.54×(N—Na2O concentration)−0.037×(Sn concentration)  formula (6-1),



wherein, in formula (6-1):
the H/Si is a normalized hydrogen concentration, where the normalized hydrogen concentration is a value obtained by dividing an average hydrogen concentration at a depth of 0 to 10 μm by an average hydrogen concentration at a depth of 105 to 110 μm, and the average hydrogen concentration at a depth of 0 to 10 μm and the average hydrogen concentration at a depth of 105 to 110 μm are values measured under the following analysis conditions (a) to (i):
(a) Measurement apparatus: secondary ion mass spectrometry apparatus with a quadrupole mass spectrometer,
(b) Primary ion species: Cs+,
(c) Primary accelerating voltage: 5.0 kV,
(d) Primary ion current: 1 μA,
(e) Primary ion incident angle (angle from direction perpendicular to sample surface): 60°,
(f) Luster size: 200×200 μm2,
(g) Detection region: 40×40 μm2,
(h) Secondary ion polarity: minus, and
(i) Electron gun for neutralization: used;
the N—Na2O concentration is the normalized Na2O surface concentration; and
the Sn concentration is an Sn deposition amount per unit area(unit: μg/cm2), where the Sn deposition amount is a deposition mass in terms of SnO2 when Sn is assumed to exist in the form of SnO2.

4. The float glass as claimed in claim 1, wherein the float glass is used for chemical strengthening in which a chemical strengthening temperature is T (unit: K) and a chemical strengthening time is t (unit: hours) and contains SiO2, and a dol determined according to the following formula by using respective contents in mass % of SiO2, Al2O3, MgO, CaO, SrO, BaO, ZrO2, Na2O and K2O is 20 or less:


dol=−0.13×Al2O3−1.88×MgO−2.41×CaO−1.85×SrO−1.35×BaO−1.59×ZrO2+1.50×Na2O+2.42×K2O−129359/T+9.28×t0.5+182.88.

5. The float glass as claimed in claim 1, comprising: in mass %, from 60 to 80% of SiO2, from 0 to 8% of Al2O3, from 8 to 22% of Na2O, from 0 to 7% of K2O, from 0 to 17% of MgO, from 0 to 22% of CaO, from 0 to 8% of SrO, from 0 to 8% of BaO, and from 0 to 5% of ZrO2.

6. The float glass as claimed in claim 5, comprising at least one of CaO, SrO and BaO, wherein the total of the CaO, SrO and BaO contents is from 1 to 7%.

7. The float glass as claimed in claim 1, comprising: in mass %, from 60 to 80% of SiO2, from 0.01 to 8% of Al2O3, from 8 to 22% of Na2O, from 0 to 7% of K2O and from 0 to 5% of ZrO2, wherein in the case where at least one of MgO, CaO, SrO and BaO is contained in the float glass, the total of the MgO, CaO, SrO and BaO contents is from 5 to 25%, and a ratio of the Na2O content to the Al2O3 content (Na2O/Al2O3 is 1.5 or more.

8. The float glass as claimed in claim 7, wherein the ratio Na2O/Al2O3 is 6 or less.

9. A method for producing a chemically strengthened glass having a depth of compressive stress layer of 20 μm or less, comprising chemically strengthening the float glass as claimed in claim 1.

10. A float glass for chemical strengthening, comprising a bottom surface coming into contact with a molten metal at the time of forming of the float glass and a top surface opposing the bottom surface, wherein a Δion exchange amount 1 which is a value obtained by subtracting an ion exchange amount 1 in the bottom surface from an ion exchange amount 1 in the top surface is 0.32 or less,
where the ion exchange amount 1 is a value determined according to the following formula (2-1):


Ion exchange amount 1=5.51×(normalized Na2O surface concentration)−0.038×(Sn concentration)  formula (2-1),



wherein, in formula (2-1):
the normalized Na2O surface concentration is a value obtained by dividing an Na2O concentration at a measured surface, which is the bottom surface or the top surface, by an Na2O concentration at a depth position of 100 μm from the measured surface, where the Na2O concentration is a value measured by a fluorescent X-ray analysis using an Na—Kα ray; and
the Sn concentration is an Sn deposition amount per unit area (unit: μg/cm2) in the measured surface, where the Sn deposition amount per unit area is a deposition mass in terms of SnO2 per 1 cm2 when Sn is assumed to exist in the form of SnO2.

11. The float glass as claimed in claim 10, wherein W3 determined according to the following formula (5-1) is 58 or less:


W3=744×[(ΔN—Na2O)+0.01×(Sn concentration difference)]  formula (5-1),


wherein, in formula (5-1):
the ΔN—Na2O is a value determined by subtracting the normalized Na2O surface concentration in the bottom surface from the normalized Na2O surface concentration in the top surface; and
the Sn concentration difference is a difference obtained by subtracting an Sn deposition amount per unit area (unit: μg/cm2) in the top surface from an Sn deposition amount per unit area (unit: μg/cm2) in the bottom surface.

12. The float glass as claimed in claim 10, wherein Δion exchange amount 2 which is a value obtained by subtracting an ion exchange amount 2 in the bottom surface from an ion exchange amount 2 in the top surface is 0.33 or less,
where the ion exchange amount 2 is a value determined according to the following formula (6-1):


Ion exchange amount 2=−0.02×(H/Si)+5.54×(N—Na2O concentration)−0.037×(Sn concentration)  formula (6-1),



wherein, in formula (6-1):
the H/Si is a normalized hydrogen concentration, where the normalized hydrogen concentration is a value obtained by dividing an average hydrogen concentration at a depth of 0 to 10 μm by an average hydrogen concentration at a depth of 105 to 110 μm, and the average hydrogen concentration at a depth of 0 to 10 μm and the average hydrogen concentration at a depth of 105 to 110 μm are values measured under the following analysis conditions (a) to (i):
(a) Measurement apparatus: secondary ion mass spectrometry apparatus with a quadrupole mass spectrometer,
(b) Primary ion species: Cs+,
(c) Primary accelerating voltage: 5.0 kV,
(d) Primary ion current: 1 μA,
(e) Primary ion incident angle (angle from direction perpendicular to sample surface): 60°,
(f) Luster size: 200×200 μm2,
(g) Detection region: 40×40 μm2,
(h) Secondary ion polarity: minus, and
(i) Electron gun for neutralization: used;
the N—Na2O concentration is the normalized Na2O surface concentration; and
the Sn concentration is as defined in formula (2-1).

13. A float glass for chemical strengthening, comprising a bottom surface coming into contact with a molten metal at the time of forming of the float glass and a top surface opposing the bottom surface, wherein W1 determined according to the following formula (3-1) is 56 or less:


W1=−16×(ΔH/Si)−6.47×(Sn concentration difference)−43.8×(Δion exchange amount 1)  formula (3-1),


wherein, in formula (3-1):
the ΔH/Si is a value obtained by subtracting a normalized hydrogen concentration in the bottom surface from a normalized hydrogen concentration in the top surface, where the normalized hydrogen concentration is a value obtained by dividing an average hydrogen concentration at a depth of 0 to 10 μm by an average hydrogen concentration at a depth of 105 to 110 μm, and the average hydrogen concentration at a depth of 0 to 10 μm and the average hydrogen concentration at a depth of 105 to 110 μm are values measured under the following analysis conditions (a) to (i):
(a) Measurement apparatus: secondary ion mass spectrometry apparatus with a quadrupole mass spectrometer,
(b) Primary ion species: Cs+,
(c) Primary accelerating voltage: 5.0 kV,
(d) Primary ion current: 1 μA,
(e) Primary ion incident angle (angle from direction perpendicular to sample surface): 60°,
(f) Luster size: 200×200 μm2,
(g) Detection region: 40×40 μm2,
(h) Secondary ion polarity: minus, and
(i) Electron gun for neutralization: used;
the Sn concentration difference is a difference obtained by subtracting an Sn deposition amount per unit area (unit: μg/cm2) in the top surface from an Sn deposition amount per unit area (unit: μg/cm2) in the bottom surface, where the Sn deposition amount per unit area is a deposition mass in terms of SnO2 per 1 cm2 when Sn is assumed to exist in the form of SnO2; and
the Δion exchange amount 1 is a value obtained by subtracting an ion exchange amount 1 in the bottom surface from an ion exchange amount 1 in the top surface, where the ion exchange amount 1 is determined according to the following formula:


Ion exchange amount 1=5.51×(normalized Na2O surface concentration)−0.038×(Sn concentration),


wherein the normalized Na2O surface concentration is a value obtained by dividing an Na2O concentration at a measured surface, which is the bottom surface or the top surface, by an Na2O concentration at a depth position of 100 μm from the measured surface, where the Na2O concentration is a value measured by a fluorescent X-ray analysis using an Na—Kα ray.

14. The float glass as claimed in claim 13, wherein W3 determined according to the following formula (5-1) is 58 or less:


W3=744×[(ΔN—Na2O)+0.01×(Sn concentration difference)]  formula (5-1),


wherein, in formula (5-1):
the ΔN—Na2O is a value determined by subtracting the normalized Na2O surface concentration in the bottom surface from the normalized Na2O surface concentration in the top surface; and
the Sn concentration difference is as defined in formula (3-1).

15. The float glass as claimed in claim 13, wherein Δion exchange amount 2 which is a value obtained by subtracting an ion exchange amount 2 in the bottom surface from an ion exchange amount 2 in the top surface is 0.33 or less,
where the ion exchange amount 2 is a value determined according to the following formula (6-1):


Ion exchange amount 2=−0.02×(H/Si)+5.54×(N—Na2O concentration)−0.037×(Sn concentration)  formula (6-1),



wherein, in formula (6-1):
the H/Si is the normalized hydrogen concentration;
the N—Na2O concentration is the normalized Na2O surface concentration; and
the Sn concentration is the Sn deposition amount per unit area.

16. A float glass for chemical strengthening, comprising a bottom surface coming into contact with a molten metal at the time of forming of the float glass and a top surface opposing the bottom surface, wherein an absolute value of W2 determined according to the following formula (4-1) is 56 or less:


W2=9.18×Δ[(ion exchange amount 1)/(H/Si)]+49  formula (4-1),


wherein, in formula (4-1):
the Δ[(ion exchange amount 1)/(H/Si)] is a value determined by subtracting a value obtained by dividing an ion exchange amount 1 in the bottom surface by a normalized hydrogen concentration H/Si in the bottom surface, from a value obtained by dividing an ion exchange amount 1 in the top surface by a normalized hydrogen concentration H/Si in the top surface,
where the ion exchange amount 1 is determined according to the following formula:


Ion exchange amount 1=5.51×(normalized Na2O surface concentration)−0.038×(Sn concentration),


in the formula, the normalized Na2O surface concentration is a value obtained by dividing an Na2O concentration at a measured surface, which is the bottom surface or the top surface, by an Na2O concentration at a depth position of 100 μm from the measured surface, where the Na2O concentration is a value measured by a fluorescent X-ray analysis using an Na—Kα ray, and the Sn concentration is an Sn deposition amount per unit area (unit: μg/cm2) of the measured surface, where the Sn deposition amount per unit area is a deposition mass in terms of SnO2 per 1 cm2 when Sn is assumed to exist in the form of SnO2; and
where the normalized hydrogen concentration is a value obtained by dividing an average hydrogen concentration at a depth of 0 to 10 μm by an average hydrogen concentration at a depth of 105 to 110 μm, and the average hydrogen concentration at a depth of 0 to 10 μm and the average hydrogen concentration at a depth of 105 to 110 μm are values measured under the following analysis conditions (a) to (i):
(a) Measurement apparatus: secondary ion mass spectrometry apparatus with a quadrupole mass spectrometer,
(b) Primary ion species: Cs+,
(c) Primary accelerating voltage: 5.0 kV,
(d) Primary ion current: 1 μA,
(e) Primary ion incident angle (angle from direction perpendicular to sample surface): 60°,
(f) Luster size: 200×200 μm2,
(g) Detection region: 40×40 μm2,
(h) Secondary ion polarity: minus, and
(i) Electron gun for neutralization: used.

17. The float glass as claimed in claim 16, wherein W3 determined according to the following formula (5-1) is 58 or less:


W3=744×[(ΔN—Na2O)+0.01×(Sn concentration difference)]  formula (5-1),


wherein, in formula (5-1):
the ΔN—Na2O is a value determined by subtracting the normalized Na2O surface concentration in the bottom surface from the normalized Na2O surface concentration in the top surface; and
the Sn concentration difference is a difference obtained by subtracting an Sn deposition amount per unit area (unit: μg/cm2) in the top surface from an Sn deposition amount per unit area (unit: μg/cm2) in the bottom surface.

18. The float glass as claimed in claim 16, wherein Δion exchange amount 2 which is a value obtained by subtracting an ion exchange amount 2 in the bottom surface from an ion exchange amount 2 in the top surface is 0.33 or less,
where the ion exchange amount 2 is a value determined according to the following formula (6-1):


Ion exchange amount 2=−0.02×(H/Si)+5.54×(N—Na2O concentration)−0.037×(Sn concentration)  formula (6-1),



wherein, in formula (6-1):
the H/Si is the normalized hydrogen concentration;
the N—Na2O concentration is the normalized Na2O surface concentration; and
the Sn concentration is as defined in the formula of Ion exchange amount 1.