JP6435937B2 - Chemically strengthened glass plate and chemically strengthened glass - Google Patents

Description

  The present invention relates to a glass plate that can be easily manufactured by a float process and that can easily obtain high-strength chemically strengthened glass.

In order to obtain high-strength chemically tempered glass, the glass of the glass plate that is the base plate is compositionally, soda-lime silicate glass that is widely used as a float glass plate (the composition of the glass is In this specification, it is necessary to contain a large amount of Al ₂ O ₃ and ZrO ₂ in comparison with “glass for general purpose float glass”. (For such glass, see, for example, Patent Documents 1 and 2). However, glass containing a large amount of Al ₂ O ₃ and ZrO ₂ has a high melting temperature. If the glass is to have the same melting characteristics as general-purpose float glass, the alkali content is increased and the chemical durability is sacrificed, or the increased amounts of Al ₂ O ₃ and ZrO ₂ are increased. The glass composition was inevitably designed to keep the slight increase and to keep the strength improvement effect after chemical strengthening by these components to a slight increase.

On the other hand, for the purpose of obtaining meltability similar to that of general-purpose float glass and good chemical durability, Patent Document 3 studies a soda-lime glass containing ZrO ₂ . ZrO ₂ has the effect of facilitating chemical strengthening like Al ₂ O ₃ . However, ZrO ₂ tends to increase the melting temperature of glass than Al ₂ O ₃ . As is clear from the respective phase diagrams of the two-component system of SiO ₂ —Al ₂ O _{3 and} the two-component system of SiO ₂ —ZrO ₂ (Non-patent Document 1), in the high content system of SiO ₂ , ZrO ₂ It is foreseen that the melting temperature will be higher when it is contained.

JP 2005-015328 A JP-A-10-182182 JP 2000-143280 A

“PHASE DIAGRAMS FOR CERAMISTS 1969 SUPPLEMENT (FIGURES 2067-4149) THE AMERICA CERAMIC SOCIETY, INC. 1969. pp. 97, 110”

It is an object of the present invention to provide a glass plate that has a meltability equivalent to that of a general-purpose float glass, can easily obtain a chemically strengthened glass having a higher strength than that of a general-purpose float glass, and further has good chemical durability. Yes. In Patent Document 3, a soda-lime glass containing ZrO ₂ has been studied for the purpose of obtaining the same meltability as general-purpose float glass and good chemical durability, but was introduced in this document. It should be noted that even in the glass composition, there is a melting temperature (region where the viscosity is 10 ² dPa · s) close to 1600 ° C.
Patent Document 3 has not been studied to obtain a high-strength chemically strengthened glass. Therefore, if a glass plate is provided that meets this purpose based on the technique disclosed in Patent Document 3, the usual idea is Since it is necessary to increase the appropriate amount of Al ₂ O _{3 and the} amount of ZrO _2, it is necessary to increase the alkali content and to make a glass composition that sacrifices chemical durability.

  In view of the above, the present invention provides a glass plate that has a meltability equivalent to that of a general-purpose float glass, can be easily made into a chemically strengthened glass having a higher strength than that of a general-purpose flow and glass, and further has good chemical durability. This is the issue.

Chemically strengthened glass plate of the present invention, the glass composition, in _{mass%, SiO 2 58~72, Al 2} O 3 1.5~6, Na 2 O 12~18, K 2 O 0~5, MgO 1 to 8, CaO _{2 to 8,} a ZrO 2 1.5 to 10, _and, Na ₂ O + _K 2 O is 12 to 22, MgO + CaO is _{3~14, Al 2 O 3 + ZrO} 2 3-15,
In the mass ratio of Al ₂ O ₃ and ZrO ₂ , Al ₂ O ₃ / ZrO ₂ Is 0.15 to 2.7,
In the mass ratio of the total of Na ₂ O and K ₂ O and the total of Al ₂ O ₃ and ZrO ₂ , (Na ₂ O + K ₂ O) / (Al ₂ O ₃ + ZrO ₂ ) is 1.1-3. .0,
The temperature at which the viscosity is 10 ² dPa · s is 1520 ° C. or lower.

In the present invention, the numerical value range and the numerical value range defining the mass ratio are regarded as effective digits unless otherwise specified, and the maximum value is used for digits other than those shown (smaller side digits). Are rounded off to determine whether they are within the numerical range of the present invention. For example, in SiO ₂ 65-70, the minimum side can be defined as 64.5 and the maximum side can be defined as 70.4.
By setting it as the glass plate which has the said glass composition, it can be set as the glass plate which is excellent in chemical durability and is easy to manufacture with the float glass process. In addition, it is excellent in chemical durability in this invention. The haze value (Haze) of the glass after a pressure cooker test (The glass plate is hold | maintained for 24 hours in an environment whose temperature is 121 degreeC and humidity is 99.8% RH). Is less than 1.5%, preferably less than 1.0%, more preferably less than 0.8%. A high haze value after the test means that the glass surface is deteriorated by water vapor. The temperature at which the viscosity is 10 ² dPa · s is called a melting temperature, and indicates a temperature at which a glass raw material or glass is melted to enable glass melting, and if this value is 1520 ° C. or less. The glass raw material and glass can be easily melted with the same amount of energy as general purpose float glass, and defoaming and clarification are promoted. The temperature at which the viscosity is 10 ² dPa · s is preferably 1510 ° C. or lower, more preferably 1500 ° C. or lower. There is no particular lower limit for this temperature, but if it is less than 1400 ° C., the physical strength and chemical durability of the resulting glass plate tend to decrease, so this lower limit may be 1400 ° C.

  Further, the present invention provides the above-described glass plate for chemical strengthening by immersing the glass plate for chemical strengthening in a molten salt containing a monovalent cation having an ionic radius larger than the ionic radius of Na ions. It is a chemically strengthened glass plate obtained by ion exchange with the monovalent cation. In the chemically strengthened glass of the present invention, the surface compressive stress (CS) is 500 to 1300 MPa, and the compressed layer depth (DOL) is 5 to 50 μm, which can be relatively high strength.

  The glass sheet for chemical strengthening of the present invention can be made into a high-strength chemically strengthened glass while having a meltability equivalent to that of a general-purpose float glass, and is further effective in improving chemical durability.

  The glass plate of the present invention is flat and has a thickness of 0.1 mm to 4 mm, preferably 0.2 to 3.5 mm, more preferably 0.3 to 3.0 mm, and is manufactured by a float process. It is preferable. Hereafter, each component of the glass composition which forms the glass plate of this invention, and its introduction range are demonstrated.

1. About each component of glass composition <SiO ₂ >
SiO ₂ is a main component of glass and is an indispensable component for forming a glass network structure, and its content is set to 58 to 72% by mass. If it is less than 58% by mass, the glass structure becomes unstable, and if it exceeds 72% by mass, the melting temperature becomes high and it becomes difficult to melt. Preferably, it is 60-71 mass%, More preferably, it is 65-70 mass%.

<Al ₂ O ₃ >
Al ₂ O ₃ is a component that facilitates chemical strengthening of the glass plate, and its content is 1.5 to 6% by mass. If the content is less than 1.5% by mass, the glass plate tends to fail to exhibit the performance obtained by chemical strengthening. If the content exceeds 6% by mass, the melting temperature tends to be high and it is difficult to melt. . Considering these, the content of Al ₂ O ₃ is preferably _{2 to} 5.5% by mass, and more preferably 2.5 to 5% by mass.

<ZrO ₂ >
ZrO ₂ is a component that facilitates chemical strengthening of the glass plate, and its content is 1.5 to 10% by mass. ZrO ₂ has the effect of facilitating chemical strengthening like Al ₂ O ₃ . However, ZrO ₂ tends to increase the melting temperature of glass than Al ₂ O ₃ . As is clear from the respective phase diagrams of the two-component system of SiO ₂ —Al ₂ O _{3 and} the two-component system of SiO ₂ —ZrO ₂ (Non-patent Document 1), in the high content system of SiO ₂ , ZrO ₂ It is foreseen that the melting temperature will be higher when it is contained. In the present invention, by optimizing other components, even if the content is 1.5 to 10% by mass, the melting temperature of the glass is not increased, and the glass can be easily melted. ZrO ₂ not only facilitates chemical strengthening of the glass plate, but also has an effect of improving the chemical durability of the glass plate. If the content of ZrO ₂ is less than 1.5% by mass, the chemical durability tends to deteriorate, and if it exceeds 10% by mass, the density of the glass increases and the weight as a glass plate is heavy. Become. Considering these, the content of ZrO ₂ is preferably 1.7 to 8% by mass, and more preferably 2 to 6% by mass.

<Mass ratio of Al ₂ O ₃ and ZrO ₂ >
In the present invention, there is provided a glass plate that can be made of high-strength chemically tempered glass and has good chemical durability while having meltability equivalent to that of general-purpose float glass. In the glass composition system found in the present invention, the amount of ZrO ₂ can be relatively high. Then, the mass ratio of _Al 2 _{O 3} and _{ZrO 2, Al 2 O 3 /} ZrO 2 Is set to 0.15 to 2.7. If this mass ratio exceeds 2.7, the chemical durability tends to deteriorate. Considering this, the upper limit can be set to 2.5, and further to 1.8.

<Na ₂ O, K ₂ O>
Na ₂ O is a component that improves the solubility of glass, and has an effect of reducing the melting temperature of glass. Further, when the glass plate is immersed in the molten salt, the surface of the glass plate is exchanged with a monovalent cation having an ionic radius larger than that of Na ions such as K ions in the molten salt. It is also a component that improves compressive stress. In the present invention, the content of Na ₂ O is a 12 to 18 mass%. If the content is less than 12% by mass, the surface compressive stress obtained by chemical strengthening tends to be low, and if it exceeds 18% by mass, chemical durability tends to deteriorate. Considering these, the content of Na ₂ O is preferably 13 to 17% by mass, and more preferably 13.5 to 16% by mass.

K ₂ O, like Na ₂ O, is a component that improves the solubility of glass and has an effect on reducing the melting temperature of glass. Further, when the glass plate is immersed in the molten salt, the glass plate is compressed by ion exchange with a monovalent cation having an ionic radius larger than that of Na ions such as K ions in the molten salt. It is also a component that increases the layer depth. K ₂ O is not necessarily essential, but can be contained in the glass plate within a range of 0 to 5% by mass. If the content of K ₂ O exceeds 5% by mass, there is a tendency that the ion exchange is difficult to be suppressed by suppressing the movement of Na ions due to the mixed alkali effect with Na ₂ O. Considering this, the content of K ₂ O is preferably 4% by mass or less, and more preferably 3% by mass or less. On the other hand, when Na ₂ O and K ₂ O coexist in the glass plate, the water resistance of the glass plate is improved by the mixed alkali effect, so the lower limit of the content of K ₂ O is 0.3% by mass, You may set with 0.5 mass%.

The total content of Na ₂ O and _{K 2} O content is set to 12 to 22 mass%. The total amount of these components greatly affects the meltability of the glass and the chemical durability of the glass plate. If the content is less than 12% by mass, the chemical strengthening performance cannot be exhibited, the glass viscosity tends to increase and the productivity tends to deteriorate, and if it exceeds 22% by mass, the chemical durability tends to deteriorate. is there. Considering these, the total of the Na ₂ O content and the K ₂ O content is preferably 13 to 20% by mass, and more preferably 14 to 18% by mass.

<Mass ratio of the sum of Na ₂ O and K ₂ O to the sum of Al ₂ O ₃ and ZrO ₂ >
(Na ₂ O + K ₂ O) / (Al ₂ O ₃ + ZrO ₂ ) is a numerical value that affects the ion exchange capacity when the glass plate is immersed in the molten salt, that is, the ease of chemical strengthening of the glass plate. The range is 1.1 to 3.0. If it is less than 1.1, the chemical strengthening performance tends not to be exhibited, and if it exceeds 3.0%, the chemical durability tends to deteriorate. Considering these, the numerical range is preferably set to 1.2 to 2.9, more preferably 1.2 to 2.5.

<CaO, MgO>
CaO is a component having an action of lowering the viscosity of the molten glass when the glass is melted, and has an effect of improving productivity. It can be made to contain in a glass plate within the range of 2-8 mass%. When the content of CaO is more than 8% by mass, ion exchange is suppressed, and a desired chemical strengthening performance tends not to be obtained. Considering this, the content of CaO is preferably 7% by mass or less, more preferably 6% by mass or less.

MgO has the advantage that ion exchange in chemical strengthening is not hindered compared to CaO, but the effect of lowering the viscosity of molten glass at the time of glass melting is small compared to CaO. Therefore, it adjusts in the range of 1-8 mass%. If it exceeds 8 mass%, the glass viscosity tends to be high, and the productivity tends to deteriorate. Considering these, the content of MgO is preferably 1.5 to 7% by mass, and more preferably 2 to 6% by mass.
Moreover, the sum total of MgO content and CaO content shall be 3-14 mass%. The total amount of these components greatly affects the meltability of the glass and the chemical durability of the glass plate. If the content is less than 3% by mass, the glass viscosity tends to increase, and if it exceeds 14% by mass, the chemical durability tends to deteriorate. Considering these, the total of the MgO content and the CaO content is preferably 5 to 12% by mass, and more preferably 6 to 11% by mass.

<Other ingredients>
In the glass plate of the present invention, in addition to the above components, the glass composition includes, for example, Na ₃ SO ₄ -derived SO ₃ , Sb ₂ O ₃ as a glass refining agent for the purpose of defoaming at the time of dissolution, Various components such as SnO ₂ , transition metal compounds such as Fe ₂ O ₃ , TiO ₂ , CoO, NiO for the purpose of coloring the glass, and impurities derived from industrial glass raw materials are used to change the essence of the characteristics of the glass plate of the present invention. To the extent that, for example, their total amount is 0. It may be contained to the extent that it does not exceed 5% by mass. Further, _{fluorine, ZnO, B 2 O 3,} Li 2 O, the P 2 _{O 5,} etc., as the quantity of impurities inevitably be mixed from glass raw materials contained by not more than 0.1 weight% Also good.

2. About manufacture of the glass plate for chemical strengthening The glass plate of this invention is manufactured suitably by the float glass process. In addition to the float method, various molding methods such as a fusion method (including an overflow downdraw method), a downdraw method, a redraw method, a rollout method, and a press method can be employed.
In addition, the surface of the glass plate may be as it is molded by the above-described molding method, chemically roughened with hydrofluoric acid, etc., or physically roughened by polishing or blasting, or by the combination thereof. The surface may be roughened and functionality such as antiglare property may be imparted. The shape of the glass is not particularly limited, but is preferably a plate-like body. Moreover, when the shape of glass is plate shape, what was drilled by the glass surface etc., or the bending plate in a flat plate may be sufficient, and various shapes are included. Further, in the flat plate shape, a short shape or a disk shape is also within the scope of the present invention.

3. Production of Chemically Tempered Glass The chemically tempered glass of the present invention can be applied with the technology disclosed in the prior art. The surface layer of the chemically strengthened glass plate is the alkali metal ion A most contained in the glass plate. It is produced by ion exchange replacing alkali metal ions B having a larger ion radius than metal ions A.

  For example, when the alkali metal ion A is a sodium ion (Na ion), at least one of potassium ion (K ion), rubidium (Rb ion) and cesium ion (Cs ion) is used as the alkali metal ion B. be able to. When the alkali metal ion A is a sodium ion, it is preferable to use a potassium ion as the alkali metal ion B.

  In addition, one or more of nitrates, sulfates, carbonates, hydroxides and phosphates containing at least alkali metal ions B can be used for ion exchange. And when the alkali metal ion A is a sodium ion, it is preferable to use a nitrate containing at least a potassium ion.

  The chemically strengthened glass is produced by the step of bringing the chemically strengthening glass plate into contact with the salt containing the alkali metal ions B. “Contacting a chemically strengthened glass plate with salt” means contacting or immersing the chemically strengthened glass plate in a salt bath. Thus, in this specification, “contact” is a concept including “immersion”.

  In addition, as the contact form of the salt, a form in which the paste-like salt is brought into direct contact or a form in which the salt is immersed in a molten salt heated to a melting point or higher is also possible. It is desirable to immerse in

(Preheating process)
The temperature of the glass sheet for chemical strengthening brought into contact with the salt is not particularly limited. That is, it may be at room temperature and may be preheated, but preferably in a heated state. However, it is preferable that preheating temperature is below the glass transition point of a glass plate. When the glass transition point is exceeded, the shape of the glass plate is deformed, and a desired shape or size after chemical strengthening cannot be obtained. Note that the preheating temperature may be equal to or higher than the temperature of the salt to be contacted, which will be described later, or the same temperature or lower. The preheating time is not particularly limited.

(Ion exchange process)
The temperature of the salt to be contacted is not particularly limited. However, when the glass plate is immersed in the molten salt, the temperature is preferably equal to or higher than the melting point of the salt to be contacted from the strain point temperature or lower of the glass plate. If it is above the strain point, the compressive stress caused by ion exchange is easily relaxed, and a desired surface compressive stress cannot be obtained. When the salt to be contacted uses potassium ions as the alkali metal ions B, since the melting point of potassium nitrate is 333 ° C., it is immersed from 333 ° C. to a temperature below the strain point temperature of the glass plate. In this case, it is preferably 350 ° C. to (strain point temperature −10 ° C.), more preferably 370 ° C. to (strain point temperature −20 ° C.).

  Although the time which makes this glass plate contact with salt is not specifically limited, When this glass plate is immersed in molten salt, it is preferable that it is 0.5 to 8 hours. If it is less than 0.5 hour, ion exchange between the alkali metal ion A and the alkali metal ion B does not proceed sufficiently, and the desired surface compression stress and compression layer depth cannot be obtained. On the other hand, when it is 8 hours or more, the surface compressive stress caused by ion exchange is easily relaxed. Preferably, it is 0.5 to 6 hours, more preferably 1 to 5 hours.

(Cooling process)
The glass plate brought into contact with the salt for a predetermined time is cooled to room temperature through a cooling process. The cooling process is a case where either a glass plate brought into contact with salt is placed in a furnace previously maintained at a temperature and the cooling rate is controlled (slow cooling) or rapid cooling (cooling) is performed directly at room temperature. including. However, since the glass plate may be broken by rapid cooling, it is preferable to use a slow cooling step. The cooling rate is appropriately adjusted depending on the glass plate dimensions. In addition, the state which salt adhered to the glass plate at the time of a cooling process may be sufficient, and the atmosphere of a slow cooling process is not specifically limited, either. The glass plate after cooling removes the salt adhering with warm water, cold water, etc., and a chemically strengthened glass is obtained.

  The chemically tempered glass is produced by the preheating step, the ion exchange step, and the cooling step, but it does not have to be one step. That is, the process of manufacturing the chemically strengthened glass may be performed once or more, and the temperature and time of the preheating process, the ion exchange process, and the cooling process are not necessarily equal. Moreover, the structure of the salt in the ion exchange process is not necessarily the same. Furthermore, when using a manufacturing process twice or more, you may abbreviate | omit either the preheating process between ion exchange processes, a slow cooling process, or both.

  The chemically strengthened glass manufactured through the manufacturing process can measure surface compressive stress (CS) and compressed layer depth (DOL) using a surface stress meter based on the optical waveguide effect as an observation principle. Can be adjusted within a range of 500 to 1300 MPa and a compressed layer depth (DOL) within a range of 5 to 50 μm. If the surface compressive stress is less than 500 MPa, there is a concern that the surface compressive stress may break due to a contact impact with a high-hardness member or due to a drop impact. On the other hand, if the surface compressive stress is 1300 MPa or more, the internal tensile stress (CT) necessary for maintaining a balance with the integrated value of the compressive stress in the compressive stress layer becomes high. For example, when cutting the chemically strengthened glass In addition, chipping is likely to occur, and there is a concern that the yield may decrease. Therefore, the surface compressive stress is more preferably 550 to 1200 MPa, and further preferably 600 to 1000 MPa. If the compressed layer depth is less than 5 μm, scratches exceeding the compressive stress layer are caused by contact with the high hardness member, and there is a concern that the strength may be reduced. On the other hand, if it is 50 μm or more, the internal tensile stress becomes high, and the processing becomes difficult for cutting or the like. Therefore, the compressed layer depth is preferably 7 to 40 μm, and more preferably 10 to 30 μm.

<Preparation of glass plate>
Glass materials shown in Examples 1 to 8 and Comparative Examples 1 to 5 shown in Table 1 using raw materials such as silica sand, aluminum oxide, soda ash, sodium sulfate, potassium carbonate, calcium carbonate, magnesium oxide and zircon flour as raw materials A batch corresponding to 600 g of glass prepared so as to have a ratio of 1 was filled in a platinum crucible and melted at 1450 to 1650 ° C. for about 8 hours to obtain a clear glass melt. Thereafter, the glass melt was flown out and cast onto a heat-resistant and inert carbon plate to form a plate glass, and then kept at a temperature exceeding the glass transition point in an electric furnace and then slowly cooled, and a glass block was obtained after cooling. . The glass block was cut and polished to evaluate the characteristics.

<Evaluation of the obtained glass>
1) Evaluation of melting temperature The temperature at which the viscosity was 10 ² dPa · s was measured as the melting temperature. The temperature was measured by a ball pulling method using a ball pulling viscometer (manufactured by Opto Corporation).

2) Evaluation of thermal expansion coefficient (30-300 ° C.), glass transition point, annealing point, strain point Thermal expansion coefficient and glass transition point were measured using a thermomechanical analyzer TMA8310 (manufactured by Rigaku), respectively. It measured based on the prescription | regulation of R3102, JISR3103-3. In addition, a thermal expansion coefficient is an average linear expansion coefficient in 30-300 degreeC. The annealing point and strain point were measured by a beam bending method based on the provisions of JIS R3103-2 using a beam bending viscometer (manufactured by Opto Corporation).

3) Evaluation of density The density was measured by the Archimedes method based on the provisions of JIS Z 8807.

4) Evaluation of chemical durability Using a PC-422R2 (manufactured by Hirayama Seisakusho Co., Ltd.) a glass plate having a thickness of 2 to 3 mm, a pressure cooker test (PCT) at a temperature of 121 ° C. and a humidity of 99.8% for 24 hours. ). Thereafter, a haze value was measured with a D65 light source using a haze meter NDH2000 (manufactured by Nippon Denshoku Industries Co., Ltd.). This result is expressed as “Haze after PCT” in Table 2.

6) Chemical strengthening treatment Chemical immersion strengthening treatment is performed by immersing an optically polished glass plate having a thickness of 2 to 3 mm in a potassium nitrate molten salt bath maintained at a temperature 0.8 times the strain point temperature of the glass for 4 hours. Went.

7) Evaluation of surface compression stress (CS) and compression layer depth (DOL) of chemically strengthened glass About chemically strengthened glass plate, using a surface stress meter (manufactured by Toshiba Glass, FSM-60V), The surface compressive stress (CS) and the depth (DOL) of the compressive stress layer formed on the glass surface were measured. In the measurement using a surface stress meter, a refractive index of 1.52 and a photoelastic constant of 26.8 ((nm / cm) / MPa) were used. In addition, CS of the chemically strengthened glass obtained by performing the same process as the process of 6) on the 1.1 mm-thick general-purpose float glass was 679 MPa, and the DOL was 8 μm.

8) Evaluation of Vickers Hardness Vickers hardness was measured based on the provisions of JIS R 1610 and JIS Z 2244 using a microhardness tester HM-124 (manufactured by Akashi).

  Table 2 shows the evaluations obtained in the above 1) to 8). The glass plates obtained in Examples 1 to 8 were easily melted and excellent in chemical durability, and became high-strength glass plates by chemical strengthening treatment. The chemically tempered glass obtained in the examples had a compressive layer depth of 10 to 20 μm, a surface compressive stress of 900 to 1300 MPa, and higher strength than the chemically tempered glass obtained from general-purpose float glass. . Further, the Vickers hardness of the glass obtained in the present invention was excellent in hardness from 4.8 GPa to 6 GPa before chemical strengthening and from 5.0 GPa to 6.5 GPa after chemical strengthening.

Claims (3)

Glass composition, in _{mass%, SiO 2 58~72, Al 2} O 3 1.5~6, Na 2 O 12~18, K 2 O 0~5, MgO 1~8, CaO 2~8, ZrO 2 1.5 to 10 and Na ₂ O + K ₂ O is 12 to 22, MgO + CaO is _{3 to} 14, Al ₂ O ₃ + ZrO ₂ is ₃ to 15,
In the mass ratio of Al ₂ O ₃ and ZrO ₂ , Al ₂ O ₃ / ZrO ₂ Is 0.15 to 2.7,
In the mass ratio of the total of Na ₂ O and K ₂ O and the total of Al ₂ O ₃ and ZrO ₂ , (Na ₂ O + K ₂ O) / (Al ₂ O ₃ + ZrO ₂ ) is 1.1-3. .0,
The glass plate for chemical strengthening, wherein the temperature at which the viscosity is 10 ² dPa · s is 1520 ° C. or lower. In the mass ratio of Al ₂ O ₃ and ZrO ₂ , Al ₂ O ₃ / ZrO ₂ The glass plate for chemical strengthening according to claim 1, wherein is 0.2 to 1.8. Na ion contained in the glass plate by immersing the glass plate for chemical strengthening according to claim 1 or 2 in a molten salt containing a monovalent cation having an ionic radius larger than that of Na ions. And chemically tempered glass obtained by ion exchange of the monovalent cation.