WO2014208112A1 - Glass that can be chemically strengthened - Google Patents

Abstract

A glass that can be chemically strengthened contains, in mol%, 55 to 70% of SiO₂, 2 to 5% of Al₂O₃, 0 to 5% (wherein 5% is excluded) of B₂O₃, 10 to 20% of Na₂O, 0 to 2% of K₂O, 1 to 5% of MgO, 0 to 6% of CaO, 1 to 8% of MgO + CaO, 0 to 3% of SrO, 0 to 3% of BaO, 0 to 3% of ZnO, 0.5 to 5% of TiO₂, 0.5 to 7% of ZrO₂, 0 to 0.5% of Sb₂O₃ and 0 to 0.5% of CeO₂, and does not contain Li₂O.

Description

Glass for chemical strengthening

The present invention relates to a glass for chemical strengthening. More specifically, the present invention relates to a glass for chemical strengthening, a glass plate for chemical strengthening made of the glass, and a cover glass formed by subjecting the glass plate to a chemical strengthening treatment.

Since glass is excellent in surface smoothness and surface strength, it is suitable for a display substrate (cover glass, touch panel, etc.), an information recording medium substrate (magnetic disk substrate, etc.) and the like. However, glass has a drawback that it is easily cracked or cracked. As a countermeasure, application of compressive stress to the surface by rapid cooling or ion exchange, so-called strengthening treatment is known. Among the strengthening treatments, the chemical strengthening treatment by ion exchange is suitable for the strengthening treatment of a thin glass plate.

Examples of the chemically strengthened glass include those proposed in Patent Documents 1 to 5 and the like. For example, Patent Document 1 proposes a glass composition for the purpose of imparting a large degree of strengthening by a chemical strengthening process involving ion exchange. Patent Document 2 proposes a glass composition range aimed at obtaining sufficient strength by chemical strengthening at a relatively low temperature for a short time.

JP 2005-15328 A JP 2012-20921 A WO2011-1114821A1 JP 2009-57271 A JP 2011-201711 A

When the glass plate having the composition disclosed in Patent Document 1 was immersed in a molten salt of potassium nitrate at 400 ° C. for 3 hours and chemically strengthened, the depth of the surface compressive stress layer was only 14 μm. The surface compressive stress was as small as 316 MPa. Therefore, with the glass composition disclosed in Patent Document 1, it is difficult to obtain sufficient strength. In addition, the thing described in Patent Document 4 has the same problem.

In the glass composition / chemical strengthening treatment conditions disclosed in Patent Document 2, the surface compressive stress layer depth is developed to exceed 20 μm, and the surface compressive stress is less than 600 MPa. When the thickness of the compressive stress layer is larger than 20 μm, the glass plate is liable to be damaged at the time of processing such as cutting, drilling and etching after chemical strengthening. Moreover, since the surface compressive stress is only less than 600 MPa, it is difficult to obtain sufficient strength. In addition, the thing described in Patent Document 3 has the same problem.

Under the glass composition and chemical strengthening treatment conditions disclosed in Patent Document 5, a surface compressive stress layer depth of about 150 μm is realized by chemical strengthening in a short time of 4 hours, but a glass of 1 mm or less like a cover glass. In the case of a plate, since the depth of the surface compressive stress layer is deep, fragments are shattered when the glass plate breaks, which is extremely dangerous. Moreover, in order to implement | achieve the surface compressive-stress layer depth below about 20 micrometers, it is necessary to shorten processing time, and it can be estimated that sufficient surface compressive stress cannot be obtained.

The present invention has been made in view of such circumstances, and an object thereof is to obtain a high surface compressive stress by a relatively thin surface compressive stress layer formed by chemical strengthening at a relatively low temperature for a short time. The object is to provide a glass having a composition, a glass plate made of the glass, and a cover glass formed by subjecting the glass plate to a chemical strengthening treatment.

In order to achieve the above object, the glass for chemical strengthening of the present invention is expressed in terms of mol%, SiO ₂ : 55 to 70%, Al ₂ O ₃ : 2 to 5%, B ₂ O ₃ : 0 to 5% ( However, 5% is not included.), Na ₂ O: 10 to 20%, K ₂ O: 0 to 2%, MgO: 1 to 5%, CaO: 0 to 6%, MgO + CaO: 1 to 8%, SrO : 0 ~ 3%, BaO: 0 ~ 3%, ZnO: 0 ~ 3%, TiO 2: 0.5 ~ 5%, ZrO 2: 0.5 ~ 7%, Sb 2 O 3: 0 ~ 0.5 %, CeO ₂ : 0 to 0.5%, and Li ₂ O is not contained. Further, the content of Na ₂ O is more preferably 15 to 20 mol%. Hereinafter, unless otherwise specified, “%” means “mol%”.

The glass plate for chemical strengthening according to the present invention is characterized by comprising the above glass for chemical strengthening. It is preferable that the glass transition point Tg of the glass plate for chemical strengthening is 500 degreeC or more. Further, the glass plate for chemical strengthening is preferably formed by melting the glass raw material and then molded by a pressing method. The molded body obtained by the molding by the pressing method is gradually cooled to room temperature. It is more preferable that the substrate is cooled and then cut and polished while leaving a desired portion.

The chemically strengthened cover glass of the present invention is a cover glass that has been chemically strengthened by immersing the glass plate in a molten salt containing potassium nitrate at a temperature of 400 ° C. or less for 3 hours or less, and the thickness of the compression stress layer on the glass surface. Is 5 to 15 μm, and the surface compressive stress is 600 MPa or more. Preferably, the thickness of the compressive stress layer on the glass surface is 5 to 10 μm, and the surface compressive stress is 800 MPa or more.

According to the present invention, a relatively thin surface compressive stress layer formed by a relatively low temperature and short time chemical strengthening treatment can provide a high surface compressive stress. Can be achieved.

Hereinafter, a glass for chemical strengthening according to the present invention, a glass plate for chemical strengthening made of the glass, and a cover glass formed by subjecting the glass plate to chemical strengthening will be described. In order to realize a glass having a composition capable of obtaining a high surface compressive stress by a relatively thin surface compressive stress layer formed by chemical strengthening at a relatively low temperature for a short time, a strict balance of components is required. The glass for chemical strengthening according to the present invention is expressed in mol%, SiO ₂ : 55 to 70%, Al ₂ O ₃ : 2 to 5%, B ₂ O ₃ : 0 to 5% (however, 5% is not included) ), Na ₂ O: 10 to 20%, K ₂ O: 0 to 2%, MgO: 1 to 5%, CaO: 0 to 6%, MgO + CaO: 1 to 8%, SrO: 0 to 3%, BaO : 0 to 3%, ZnO: 0 to 3%, TiO ₂ : 0.5 to 5%, ZrO ₂ : 0.5 to 7%, Sb ₂ O ₃ : 0 to 0.5%, CeO ₂ : 0 to 0.5% is contained, and Li ₂ O is not contained. When a chemically strengthened glass plate for chemical strengthening is subjected to a chemical strengthening treatment, a chemically strengthened glass plate (for example, a chemically strengthened cover glass) is obtained. In the following description, “%” means “mol%” unless otherwise specified.

In the present invention, attention has been paid to the action of non-bridging oxygen in the glass in order to obtain a higher surface compressive stress in a relatively thin surface compressive stress layer under the target chemical strengthening conditions. Normally, when SiO ₂ that is a glass-forming oxide is replaced with Al ₂ O ₃ that is an intermediate oxide, Al ₂ O ₃ functions as a network-forming oxide for glass, and non-crosslinked oxygen is reduced. Non-bridging oxygen has a higher negative charge density than bridging oxygen and has the effect of attracting alkali ions more strongly. Therefore, alkali ions are easier to move in glass that does not contain non-bridging oxygen. That is, ion exchange is easy to proceed. On the other hand, the smaller the amount of non-crosslinked oxygen, the stronger the network skeleton of the glass, the less the stress relaxation, and the greater the compressive stress. Such a phenomenon is also known in the case of MgO which is an intermediate oxide.

The present inventor dares to limit the use of Al ₂ O ₃ that is an intermediate oxide to a small amount, and also uses MgO, TiO ₂ , and ZrO ₂ that are also intermediate oxides in a certain range at the same time to facilitate ion exchange. By restricting the use of K ₂ O, CaO, SrO, BaO, ZnO, and B ₂ O ₃ that are known to inhibit ion exchange to a certain range, the ion exchange becomes dense. It was found that there was a region where it happened and led to the invention. Further, SiO ₂ , Al ₂ O ₃ and Na ₂ O are limited to predetermined amounts, and MgO, ZrO ₂ and TiO ₂ are simultaneously present in a certain amount, and K ₂ O, CaO, SrO, BaO, ZnO, B _By limiting ₂ O ₃ to a certain amount or less, the present inventors have found that the thickness of the compressive stress layer can be suppressed thin by chemical strengthening at a relatively low temperature and in a short time, and sufficient strength can be obtained.

In order to manufacture the glass sheet for chemical strengthening according to the present invention, the glass raw material is melted and then molded into a desired shape by a pressing method, or one or more surfaces are molded into a desired shape by a pressing method, and then the rest It is preferable to form five or less surfaces by grinding / polishing. Alternatively, the molded body obtained by molding by the press method may be gradually cooled and cooled to room temperature, and then cut and polished while leaving a desired portion.

When the ion exchange treatment is performed as the chemical strengthening treatment after the desired shape is obtained by the above method, a chemically strengthened glass plate (for example, a chemically strengthened cover glass) is obtained. The ion exchange treatment can be performed, for example, by immersing a glass plate in molten potassium nitrate at 400 ° C. or lower for 1 to 3 hours. As the immersion liquid, a part of potassium nitrate may be replaced with sodium nitrate. Moreover, you may perform a cutting | disconnection, grinding, an etching, etc. to a part of glass plate after ion exchange.

The thickness of the glass plate for chemical strengthening according to the present invention is preferably 1.2 mm or less. Since the thinner the glass plate is, the lighter it can be, the thickness is more preferably 1.0 mm or less, still more preferably 0.8 mm or less, and most preferably 0.5 mm or less.

The glass plate for chemical strengthening according to the present invention preferably has a glass transition point Tg of 500 ° C. or higher. When the glass transition point Tg is high, stress relaxation is less likely to occur during ion exchange, so that it is easy to obtain a high surface compressive stress.

In the glass sheet for chemical strengthening according to the present invention, the linear expansion coefficient α at 100 to 300 ° C. is preferably 80 to 120 × 10 ⁻⁷ / ° C., and preferably 85 to 110 × 10 ⁻⁷ / ° C. More preferably, it is 90 to 105 × 10 ⁻⁷ / ° C. If the linear expansion coefficient α of the glass is in the above range, the glass shrinkage during press molding can be corrected by the mold shape, and a desired shape can be easily obtained. Moreover, even if a mold having a larger linear expansion coefficient α than that of glass is used, it is possible to suppress breakage, breakage, and the like of the glass due to a difference in material expansion due to a temperature change during cooling after pressing.

Next, the reason for limiting the composition range (notation of mol%) of each component in the chemically strengthened glass according to the present invention as described above will be described.

SiO ₂ is a main component constituting glass and an essential component. If the amount is less than 55%, the chemical durability of the glass deteriorates. In order to maintain the glass transition point Tg at 500 ° C. or higher, the SiO ₂ content is preferably 55% or higher. On the other hand, when SiO ₂ exceeds 70%, the viscosity of the glass at a high temperature at the time of melting becomes high and defoaming becomes difficult. Therefore, the SiO ₂ content is in the range of 55% to 70%, preferably in the range of 58% to 65%, more preferably in the range of 58% to 64%, and still more preferably in the range of 60% to 64%. .

Na ₂ O is an essential component that improves the strength of the glass by being replaced with other cations such as K ions in the molten salt. If the amount is less than 10%, ion exchange is insufficient and a sufficient effect cannot be obtained. On the other hand, if it exceeds 20%, the linear expansion coefficient α of the glass becomes too large and the heat resistance is lowered. Therefore, the content of Na ₂ O is in the range of 10% to 20%. In particular, if the amount of Na ₂ O used is as large as 15% to 20%, it is advantageous to complete the ion exchange from Na ions to K ions with high density near the surface layer and to keep the shallow compressive stress layer. . Therefore, the Na ₂ O content is more preferably 15 to 20%.

Al ₂ O ₃ , which is an intermediate oxide, is a component that increases the glass transition temperature Tg and greatly affects chemical strengthening. It also acts as a network forming oxide and has the effect of promoting ion exchange. If it is less than 2%, the effect is not sufficient, and if it exceeds 5%, other intermediate oxides MgO, TiO ₂ and ZrO ₂ become difficult to function as network-forming oxides. Therefore, the content of Al ₂ O ₃ is in the range of 2 to 5%, and more preferably in the range of 3 to 5%.

ZrO ₂ has the effect of increasing the glass transition temperature Tg and mainly increasing the surface compressive stress. It also acts as a network-forming oxide and has the effect of promoting ion exchange. If it is less than 0.5%, the effect is not sufficient, and if it exceeds 7%, it tends to remain undissolved in the glass at the time of melting. Therefore, it is in the range of 0.5% to 7%, more preferably in the range of 1.5% to 6%, and still more preferably in the range of 2.5% to 5%.

TiO ₂ has the effect of increasing the glass transition temperature Tg and mainly increasing the compressive stress on the surface. It also acts as a network-forming oxide and has the effect of promoting ion exchange. If it is less than 0.5%, the effect is not sufficient, and if it exceeds 5%, the glass tends to be colored. Therefore, it is in the range of 0.5 to 5%, more preferably in the range of 1 to 4%, and still more preferably in the range of 2 to 4%.

The total content of ZrO ₂ and TiO ₂ is preferably in the range of 2.5 to 10%, and more preferably 4.0 to 10%. Further, when Na ₂ O is 15 to 20%, the total content of ZrO ₂ and TiO ₂ is preferably 2.5 to 6.6%. This facilitates the glass transition temperature Tg to be less than 590 ° C. When the glass product is formed by press molding, the mold temperature is set in the vicinity of the glass transition temperature Tg. Therefore, when the glass transition temperature Tg is 590 ° C. or higher, the mold life is shortened.

MgO improves the meltability by lowering the high temperature viscosity of the glass. It also acts as a network-forming oxide and has the effect of promoting ion exchange. If it is less than 1%, the effect is not sufficient, and if it exceeds 5%, it tends to be a network-modified oxide, which may inhibit ion exchange. Therefore, it is in the range of 1 to 5%, and more preferably in the range of 2 to 4%.

By simultaneously using Al ₂ O ₃ , ZrO ₂ , TiO ₂ , and MgO, which are intermediate oxides, an environment that facilitates ion exchange at a relatively low temperature and short time for chemical strengthening at a high density is prepared. By allowing Al ₂ O ₃ , ZrO ₂ , TiO _2, and MgO to coexist as intermediate oxides, it is considered that the network formation bias is reduced and a network with less variation is formed. Therefore, it is considered that the environment in which non-bridging oxygen is reduced more efficiently and ion exchange is easy is prepared. Furthermore, since a network with little variation is formed, it is considered that ion exchange is prevented from proceeding deeply. In order to accelerate ion exchange in a short time and increase the compressive stress value, and to suppress deep ion exchange, the total content of MgO, TiO ₂ and ZrO ₂ in the glass component The ratio of the amount and the content of Al ₂ O ₃ (MgO + TiO ₂ + ZrO ₂ ) / Al ₂ O ₃ is preferably 0.9 to 7, more preferably 1.4 to 4.5. preferable.

CaO lowers the high-temperature viscosity of the glass and improves its meltability, but has the effect of inhibiting ion exchange and reducing the compressive stress on the surface. If it exceeds 6%, the glass tends to devitrify. Therefore, it is in the range of 0 to 6%, more preferably in the range of 1 to 5%, and even more preferably in the range of 1.5 to 4%.

If the total amount of MgO and CaO is less than 1%, the meltability of the glass deteriorates, and if it exceeds 8%, the inhibition of ion exchange increases, and the surface compression stress is increased to 600 Mpa or more in ion exchange at a low temperature and in a short time. It becomes difficult. Therefore, it is in the range of 1 to 8%, and more preferably in the range of 2 to 7%.

SrO and BaO have the effect of making glass difficult to devitrify, but BaO is a deleterious substance, and SrO has properties similar to BaO. It is more preferably 1% or less, still more preferably 0.5% or less, and most preferably not contained.

B ₂ O ₃ acts to lower the liquidus temperature and high temperature viscosity of the glass and improve the meltability. Moreover, although there exists an effect | action which makes the thickness of a compressive-stress layer thin, it also has the effect | action which reduces the compressive stress of a surface. From such a viewpoint, it is in the range of 0 to 5% (however, 5% is not included), more preferably 2 to 5%, still more preferably 2 to 3%.

K ₂ O is a component that improves the meltability of the glass, but has the effect of increasing the thickness of the compressive stress layer on the surface and the effect of reducing the compressive stress on the surface. If it exceeds 2%, it becomes difficult to keep the surface compressive stress layer thickness in a shallow state of less than 20 μm. Therefore, the use is 2% or less. 1.5% or less is more preferable, and 1.0% or less is still more preferable.

ZnO improves the meltability of the glass at high temperatures, but when introduced into glass containing ZrO ₂ or MgO, the glass tends to devitrify, so it is 0 to 3%, more preferably 0 to 1%. Most preferably not.

From the above viewpoint, in order to improve the meltability of the glass and realize a large compressive stress at a relatively shallow compressive stress layer depth, K ₂ O, CaO, SrO, BaO, ZnO, B ₂ O ₃ and The total content of is preferably less than 10%.

In order to realize the preferred linear expansion coefficient α is, B and content ₂ O _3, Na ₂ O and K ₂ ratio of the total content of _{O B 2 O 3 / (Na} 2 O + K 2 O) The value of is preferably 0.11 to 0.27.

Li ₂ O dissolves into the molten salt as lithium ions during chemical strengthening and inhibits the necessary ion exchange reaction between sodium ions and potassium ions. For this reason, stable chemical strengthening becomes difficult. Moreover, since the effect | action which enlarges the thickness of a surface compressive-stress layer more than necessary is large, it is not contained in this invention.

Sb ₂ O ₃ and CeO ₂ can be used as a defoaming agent, but the effect is sufficient at 0.5% or less. Therefore, each is 0 to 0.5%. Preferably it is 0.05 to 0.5%. As the other defoaming agent, one or more kinds selected from the group of known SO ₃ , chloride and SnO ₂ may be selected and used in the range of 0.001 to 3%. Of these defoamers, Sb ₂ O ₃ is most preferred. CeO ₂ tends to be colored yellow, and Sb ₂ O ₃ is preferably used when colorless glass is required. In addition, SnO ₂ has a remarkable effect as a defoaming agent at a high temperature of 1550 ° C. or higher, but the alkali component tends to volatilize at a high temperature and the composition may change. Chloride has a high reactivity with platinum, and when platinum is used for the melting tank, the melting tank is easily damaged. When SO ₃ is used as a defoaming agent, it dissolves as SO _{2 in} the glass. Therefore, when the glass flows out, the platinum nozzle is energized, and foaming easily occurs.

According to the glass plate for chemical strengthening according to the present invention, a high surface compressive stress can be obtained by a relatively thin surface compressive stress layer formed by chemical strengthening treatment at a relatively low temperature and in a short time. Therefore, a chemically strengthened glass plate having high strength and capable of efficient production can be realized. The chemically strengthened glass plate according to the present invention can be suitably used as a display substrate (cover glass, touch panel, etc.), an information recording medium substrate (magnetic disk substrate, etc.) and the like.

Hereinafter, the structure and the like of the chemically strengthened glass according to the present invention will be described more specifically with reference to Examples 1 to 42 and Comparative Examples 1 to 4. Comparative Example 1 has a composition obtained by converting Example 1 (weight% notation) described in Patent Document 1 into mol% notation, and Comparative Example 2 shows Example D1 (mol% notation) described in Patent Document 2. ) Composition. Comparative Example 3 has the composition of Trial Example 61 (mol% notation) described in Patent Document 3, and Comparative Example 4 uses Example 8 (wt% notation) described in Patent Document 4 in mol% notation. The composition is converted.

Examples 1 to 42 shown in Tables 1 to 8 and Comparative Examples 1 to 4 shown in Table 9 were produced as follows. First, commonly used glass materials such as oxides, hydroxides, carbonates and nitrates are selected so that the glass compositions (mol%) shown in Tables 1 to 9 are obtained, so that the glass becomes 1 kg. Weighed and mixed. Next, it is put into a platinum crucible, put into an electric furnace at 1400-1600 ° C., melted for 3-5 hours, homogenized by defoaming and stirring, poured into a mold, and gradually cooled at a temperature near the glass transition temperature. And a glass block was obtained. After this glass block was cut and ground, both surfaces were polished to a mirror surface to obtain a glass plate having a thickness of 0.90 mm, a width of 40 mm, and a length of 40 mm. This glass plate is dipped in KNO ₃ salt at 360 to 400 ° C. for 2 to 3 hours, washed and dried, and the surface compressive stress (unit: MPa) and the thickness of the compressive stress layer (unit: μm) are determined according to Orihara Seisakusho. Measured with FSM-6000LE manufactured The glass transition temperature Tg, the glass yield point At, and the linear expansion coefficient α between 100 and 300 ° C. were measured under a temperature rising condition of 10 ° C. per minute using a thermomechanical analyzer “TMA / SS6000” manufactured by Seiko Instruments Inc. It was measured. Tables 1 to 9 also show the measurement results. In Examples 28 to 30, the thickness of the compressive stress layer was 15 μm to 20 μm, and in Examples 31 to 42, the strengthening conditions were changed.

As shown in Tables 1 to 6, Examples 1 to 30 show that the thickness of the compressive stress layer is less than 20 μm, but the surface compressive stress exceeds 600 MPa and exhibits high strength. On the other hand, the glass of Comparative Example 1 shown in Table 9 has a compressive stress layer thickness of 14 μm and is relatively shallow, but the surface compressive stress is a little as small as 316 MPa, and a large strength cannot be obtained. Since the amount of the intermediate oxide Al ₂ O ₃ is large, the intermediate oxide of MgO and ZrO ₂ is less likely to act as a network-forming oxide, and the amount of CaO that inhibits ion exchange is slightly high at 7%, so that ion exchange is easy. It seems that the environment was not reached. In Examples 31 to 42, as shown in Tables 7 to 8, the thickness of the compressive stress layer does not advance even when the strengthening conditions are changed, and the surface compressive stress is 600 MPa while the thickness of the compressive stress layer is kept below 20 μm. It can be seen that the strength exceeds the maximum.

In the glass of Comparative Example 2, the thickness of the compressive stress layer proceeds to 90 μm, the surface compressive stress is as small as 500 MPa, and a large strength cannot be obtained. It is presumed that the compressive stress layer has become thicker than necessary because it contains a Li ₂ O component. The glass of Comparative Example 3 has a small amount of Al ₂ O _{3 in the} intermediate oxide of 1% and does not contain the TiO ₂ and ZrO ₂ components in the intermediate oxide, so that it does not lead to an environment in which ion exchange is easy, and surface compression. It is estimated that the thickness of the stress layer was as deep as 29.0 μm and the compressive stress was 262 MP. Since the glass of Comparative Example 4 has a large amount of Al ₂ O ₃ of 8.5%, it does not work well as a network-forming oxide of MgO, TiO ₂ , and ZrO ₂ contained therein, and changes from Na ions to K ions. It is estimated that the thickness of the compressive stress layer was slightly shallow at 4.3 and the surface compressive stress was as small as 544 MPa under the chemical strengthening conditions of 400 ° C. × 3 hours.

If the thickness of the compressive stress layer exceeds 20 μm, cracks are likely to occur from the surface when a part of the cover glass is cut or drilled after chemical strengthening, so the thickness of the compressive stress layer is preferably less than 20 μm. 15 μm or less is more preferable, and 10 μm or less is more preferable. If the thickness of the surface compressive stress layer is shallower than 5 μm, if the glass surface is scratched, the glass tends to break and it becomes difficult to obtain a sufficient surface compressive stress value. Further, when the surface compressive stress is less than 600 MPa, sufficient strength of the cover glass cannot be obtained, so 600 MPa or more is preferable, 800 MPa or more is more preferable, and 1000 MPa or more is even more preferable.

Claims (13)

In mol%
SiO ₂ : 55 to 70%,
Al ₂ O ₃ : 2 to 5%,
B ₂ O ₃ : 0 to 5% (excluding 5%),
Na ₂ O: 10-20%,
K ₂ O: 0 to 2%,
MgO: 1-5%
CaO: 0-6%,
MgO + CaO: 1-8%,
SrO: 0 to 3%,
BaO: 0 to 3%,
ZnO: 0 to 3%,
TiO ₂ : 0.5 to 5%,
ZrO ₂ : 0.5-7%
Sb ₂ O ₃ : 0 to 0.5%
CeO ₂ : 0 to 0.5%
Chemical strengthening glass containing no Li ₂ O. The ratio of the total content of MgO, TiO ₂ and ZrO ₂ to the content of Al ₂ O ₃ (MgO + TiO ₂ + ZrO ₂ ) / Al ₂ O ₃ is 0.9-7. Glass for chemical strengthening. The glass for chemical strengthening according to claim 1 or 2, wherein the content of Na ₂ O is 15 to 20%. The glass for chemical strengthening according to any one of claims 1 to 3, wherein the total content of ZrO ₂ and TiO ₂ is 2.5 to 10%. The glass for chemical strengthening according to any one of claims 1 to 4, wherein the total content of K ₂ O, CaO, SrO, BaO, ZnO and B ₂ O ₃ is less than 10%. And content of B ₂ O _3, claim value of the ratio _{B 2 O 3 / (Na 2} O + K 2 O) between the total content of Na ₂ O and K ₂ O is 0.11 to 0.27 The glass for chemical strengthening according to any one of 1 to 5. The Na ₂ O content is 15 to 20%, the total content of ZrO ₂ and TiO ₂ is 2.5 to 6.6%, and the glass transition point (Tg) is less than 590 ° C. The glass for chemical strengthening according to any one of 1 to 6. A glass sheet for chemical strengthening comprising the glass according to any one of claims 1 to 7. The glass plate for chemical strengthening according to claim 8, wherein the glass transition point Tg is 500 ° C or higher. The glass sheet for chemical strengthening according to claim 8 or 9, wherein the glass raw material is melted and then molded by a pressing method. The glass plate for chemical strengthening according to claim 10, wherein the molded body obtained by molding by the pressing method is gradually cooled and cooled to room temperature, and then cut and polished while leaving a desired portion. A cover glass that has been chemically strengthened by immersing the glass plate according to any one of claims 8 to 11 in a molten salt containing potassium nitrate at a temperature of 400 ° C or lower for 3 hours or less, wherein the compressive stress on the glass surface A chemically strengthened cover glass having a layer thickness of 5 to 20 μm and a surface compressive stress of 600 MPa or more. 13. The chemically strengthened cover glass according to claim 12, wherein the thickness of the compressive stress layer on the glass surface is 5 to 10 μm and the surface compressive stress is 800 MPa or more.