CN112399964A

CN112399964A - Chemically strengthened glass and method for producing same

Info

Publication number: CN112399964A
Application number: CN201980044625.9A
Authority: CN
Inventors: 马田拓实; 今北健二
Original assignee: Asahi Glass Co Ltd
Current assignee: AGC Inc
Priority date: 2018-07-03
Filing date: 2019-06-20
Publication date: 2021-02-23
Anticipated expiration: 2039-06-20
Also published as: JPWO2020008901A1; JP7255594B2; WO2020008901A1; CN112399964B; CN115716714B; CN115716714A

Abstract

A chemically strengthened glass having a compressive stress layer and a CS measured by using an optical waveguide surface stress meter₀Is more than 500MPa, is prepared fromD represents the depth from the glass surface at the position where the compressive stress value measured by the optical waveguide surface stress meter is 0, and CS represents the compressive stress value at the position where the depth from the glass surface is (D/2)₂The value of | x |/| y | obtained by the formula in the specification is 0 to 0.8, and the DOL measured by a scattered light photoelastic stress meter is 50 μm or more.

Description

Chemically strengthened glass and method for producing same

Technical Field

The present invention relates to a chemically strengthened glass and a method for producing the same.

Background

Chemically strengthened glass is used for cover glass of portable terminals and the like.

Chemically strengthened glass is glass in which a compressive stress layer is formed on the surface of the glass by bringing the glass into contact with a molten salt containing metal ions such as alkali metal ions and causing ion exchange between the metal ions in the glass and the metal ions in the molten salt. The strength of chemically strengthened glass depends to a large extent on the stress distribution expressed by the value of compressive stress with depth from the surface of the glass as a variable.

It is considered that the protective glass of a portable terminal or the like is less likely to be broken by bending by increasing the value of the compressive stress on the glass surface.

Further, it is considered that, by forming the compressive stress layer in a deep portion of the glass by increasing the depth of the compressive stress layer, cracking is made less likely even when a large impact is applied.

However, when a compressive stress layer is formed on the surface of glass, a tensile stress layer is inevitably formed inside the glass. When the value of the internal tensile stress is large, the chemically strengthened glass is broken violently at the time of breakage, and the fragments are easily scattered. Therefore, methods of increasing the surface compressive stress and the depth of the compressive stress while reducing the value of the internal tensile stress are being studied.

Patent document 1 shows a typical stress distribution in the case where two ion exchange treatments are performed. This distribution is composed of the following two straight line components (patent document 1, fig. 8): a straight line indicating a stress distribution from the glass surface to a point X located at a certain depth, and a straight line indicating a stress distribution from the point X to a point where the stress is zero. If such a stress distribution is used, the internal tensile stress value can be reduced while increasing the compressive stress of the surface and increasing the depth of the compressive stress.

Documents of the prior art

Patent document

Patent document 1: international publication No. 2015/127483

Disclosure of Invention

Problems to be solved by the invention

In a typical stress distribution in the case where the ion exchange treatment is performed twice, the stress changes greatly in the vicinity of the glass surface with only a slight change in depth from the glass surface.

However, in a process of actually manufacturing a cover glass of a portable terminal or the like, the surface of the chemically strengthened glass may be polished for the purpose of removing scratches or the like formed on the surface. However, in a chemically strengthened glass having a stress distribution in which a stress greatly changes even if the depth from the surface of the glass slightly changes, the surface compressive stress after polishing greatly differs due to a slight difference in the polishing amount. This results in a reduction in yield.

The invention provides a chemically strengthened glass which has high strength and little change of characteristics even when the surface is polished after chemical strengthening.

Means for solving the problems

The present invention provides a chemically strengthened glass having a compressive stress layer on a glass surface, wherein a value CS of the compressive stress on the glass surface of the chemically strengthened glass measured by an optical waveguide surface stress meter₀The depth from the glass surface at a position where the compressive stress value measured by an optical waveguide surface stress meter is 0 is set to be 500MPa or more as D [ unit: mum of]And CS is a value of compressive stress at a position having a depth of (D/2) from the glass surface measured by an optical waveguide surface stress meter₂The depth (D/2) from the glass surface is determined by the following equationThe ratio | x |/| y | of the absolute value of the slope x of the stress distribution up to the point of depth D to the absolute value of the slope y of the stress distribution up to the point of depth D from the point of depth D/2 is 0 to 0.8, and the depth DOL of the compressive stress layer of the chemically strengthened glass measured by a scattered light photoelastic stress meter is 50 μm or more,

x＝(CS₂-CS₀)/((D/2)-0)＝2×(CS₂-CS₀)/D

y＝(0-CS₂)/(D-(D/2))＝-2×CS₂/D。

in the chemically strengthened glass of the present invention, CS is represented by the following formula₁And the CS₀The slope z of the stress distribution from the glass surface to the position having a depth of 1 μm can be determined to be not less than-100 MPa/μm and less than 0, and the CS₁The value of compressive stress at a position having a depth of 1 μm from the glass surface measured using an optical waveguide surface stress meter was used.

z＝(CS₁-CS₀)/(1μm-0μm)＝(CS₁-CS₀)/1μm

The chemically strengthened glass may have a plate shape with a thickness of 2000 μm or less.

The glass composition of the central portion of the chemically strengthened glass in the thickness direction may contain 50% or more of SiO, expressed in mol% based on oxides₂5% or more of Al₂O₃And in the glass composition of the central portion in the thickness direction of the chemically strengthened glass, Li₂O、Na₂O and K₂The total content of O may be 5% or more, and Li₂Content of O and Li₂O、Na₂O and K₂The ratio of the total content of O may be 0.5 or more.

In the chemically strengthened glass of the present invention, the chemically strengthened glass may have a basic composition, expressed in mol% on an oxide basis, of: 50 to 80 percent of SiO₂5 to 25 percent of Al₂O₃0 to 10% of B₂O₃0 to 10% of P₂O₅2 to 20 percent of Li₂O, 0.5-10% of Na₂O, 0 to 5% of K₂O, 0-15% of MgO, ZnO, CaO, SrO and BaO, and 0-5% of ZrO₂+TiO₂。

The method for producing chemically strengthened glass of the present invention may be a method for producing chemically strengthened glass, the method comprising: comprises a step of immersing a glass plate in a potassium-containing strengthening salt at 400 to 500 ℃ for 1 to 8 hours; and then bringing the glass sheet to a temperature of 300 ℃ or lower, wherein the metal ions contained in the strengthening salt are contained in an amount of 100 mass%, and the potassium-containing strengthening salt may contain 70 mass% or more of potassium ions.

In the above-described method for producing a chemically strengthened glass of the present invention, the glass composition of the glass plate before chemical strengthening may contain 50% or more of SiO in mol% based on oxides₂5% or more of Al₂O₃And Li is contained in the glass composition before the chemical strengthening of the glass sheet₂O、Na₂O and K₂The total content of O may be 5% or more, and Li₂Content of O and Li₂O、Na₂O and K₂The ratio of the total content of O may be 0.5 or more.

Effects of the invention

According to the present invention, it is possible to obtain a chemically strengthened glass having high strength, suppressing scattering of fragments at the time of fracture, and having little change in characteristics even when the surface is polished after chemical strengthening.

Drawings

Fig. 1 is a graph showing the stress distribution in the vicinity of the surface measured on the chemically strengthened glass of example 1 using an optical waveguide surface stress meter.

Fig. 2 is a graph showing the stress distribution measured on the chemically strengthened glass of example 1 using a scattered light photoelastic stress meter.

Fig. 3 is a stress distribution diagram obtained by synthesizing data of the optical waveguide surface stress meter and data obtained by the scattered light photoelastic stress meter.

Detailed Description

In the present specification, unless otherwise specified, "to" indicating a numerical range is used in a meaning including numerical values described before and after the range as a lower limit value and an upper limit value.

In the present specification, the term "stress distribution" refers to a stress distribution in which the value of compressive stress from the glass surface to the central portion is expressed by using the depth from the glass surface as a variable. In addition, "depth of compressive stress (DOL)" is a depth at which the value of compressive stress is zero in the stress distribution.

"internal tensile stress (CT)" means the value of the tensile stress at the 1/2 depth of the sheet thickness t of the glass.

In the present specification, "chemically strengthened glass" refers to glass after being subjected to a chemical strengthening treatment, and "glass for chemical strengthening" refers to glass before being subjected to a chemical strengthening treatment.

In the present specification, the glass composition of the glass for chemical strengthening is sometimes referred to as "basic composition of chemically strengthened glass". Except for the case where the extreme ion exchange treatment is performed, the glass composition of the portion deeper than DOL of the chemically strengthened glass is the basic composition of the chemically strengthened glass.

In the present specification, unless otherwise specified, the molar composition is expressed in terms of molar percentage based on oxides, and the mol% is abbreviated as "%".

In addition, the phrase "substantially not contained" as to the glass composition means that the glass composition is not higher than the level of impurities contained in the raw materials or the like, that is, is not intentionally contained. Specifically, for example, less than 0.1%.

< fracture (break bad) >, of glass

In the case where the glass sheet is deflected by an impact, when the deflection amount thereof becomes large, a large tensile stress is applied to the glass surface so that the glass is broken. In the present specification, such breakage is referred to as "glass breakage caused by bending mode". The chemically strengthened glass of the present invention (hereinafter, may be referred to as "present strengthened glass") forms a large compressive stress on the glass surface, and therefore, glass breakage due to bending mode is suppressed.

On the other hand, in the case where a small and hard object collides with the glass plate, stress caused by impact is generated inside the glass plate, whereby the glass plate is broken from the inside. In the present specification, such breakage is referred to as "glass breakage caused by impact mode". The present tempered glass suppresses glass breakage due to impact mode because the compressive stress layer is formed all the way to the inside of the glass sheet.

If the value of the compressive stress of the glass surface is large and the compressive stress layer is formed all the way to the inside of the glass sheet, glass breakage due to bending mode and glass breakage due to impact mode are both suppressed, and therefore the glass should become very unlikely to break. However, when a compressive stress layer is formed inside a glass plate, an internal tensile stress that cancels out the compressive stress layer is generated. When the internal tensile stress is large, the glass is broken vigorously, and the fragments are scattered.

Since the stress distribution in the glass is appropriately adjusted, glass breakage due to bending mode and glass breakage due to impact mode are both suppressed, and scattering of fragments is small even at the time of breakage.

As a method for obtaining a chemically strengthened glass in which glass fracture due to bending mode and glass fracture due to impact mode are both suppressed and scattering of fragments is small even at the time of fracture, a method is known in which a lithium aluminosilicate glass is subjected to an ion exchange treatment to generate two stress distributions having different slopes.

In this case, the lithium aluminosilicate glass may be chemically strengthened by an ion exchange treatment using a strengthening salt containing sodium ions and potassium ions. Alternatively, chemical strengthening may be performed by ion exchange treatment using a strengthening salt containing potassium ions after ion exchange treatment using a strengthening salt containing sodium ions.

According to these methods, a large compressive stress is generated by ion exchange between sodium ions and potassium ions in a shallow region from the surface, and a small compressive stress is generated by ion exchange between lithium ions and sodium ions in a deeper region.

Sodium ions having a small ionic radius diffuse into the glass at a high rate, while potassium ions having a large ionic radius diffuse into the glass at a low rate. Therefore, even if ion exchange between lithium ions and sodium ions occurs in a deep region, ion exchange between sodium ions and potassium ions is less likely to occur.

In the region near the surface where ion exchange of sodium ions and potassium ions occurs, potassium ions having a large ionic radius enter the glass, thereby generating a large compressive stress. In the deeper region, a comparatively small compressive stress is generated up to the deep region by ion exchange of lithium ions with sodium ions. The chemically strengthened glass thus obtained has a stress distribution which is steep near the surface and relatively gentle in a deep portion and is bent in two stages. By forming such a stress distribution, it is possible to obtain chemically strengthened glass having a high surface strength, being less likely to be broken even when a strong impact is applied thereto, and being less likely to scatter fragments when broken.

However, in this stress distribution, since the value of the compressive stress of the outermost surface becomes very large, so-called "delayed chipping (チッピング)" easily occurs in the vicinity of the surface. The delayed fracture is a phenomenon in which the end portion is damaged by applying only a slight force to a portion to which an excessive stress is introduced by chemical strengthening. When the surface of the chemically strengthened glass having such a stress distribution is polished, even a slight difference in the polishing amount causes a large difference in the stress on the outermost surface.

The present strengthened glass regulates the stress distribution near the surface of the glass, and thus these problems are not easily generated.

< determination of compressive stress >

By using a combination of the optical waveguide surface stress meter and the scattered light photoelastic stress meter, the compressive stress in the chemically strengthened glass can be accurately measured.

The compressive stress was measured using two devices for the following reasons.

A method using an optical waveguide surface stress meter is widely known as a method capable of accurately measuring the stress of a glass sample in a short time. However, this method can only measure the stress in principle when the refractive index decreases from the surface of the sample to the inside.

In chemically strengthened glass, the refractive index of a layer in which sodium ions are replaced with potassium ions in the glass decreases from the surface to the inside, and therefore the stress can be measured using an optical waveguide surface stress meter. However, since such a refractive index distribution does not occur in a layer in which lithium ions in glass are replaced with sodium ions, the stress cannot be measured by an optical waveguide surface stress meter.

FIG. 1 shows an example of a stress distribution obtained by measuring stress of the present tempered glass using an optical waveguide surface stress meter. The dotted line portion of the graph actually generates stress caused by ion exchange of lithium ions and sodium ions, but the stress cannot be measured by an optical waveguide surface stress meter.

The stress measuring method using the scattered light photoelastic stress meter can measure stress in principle without depending on the refractive index distribution. However, since the scattered light photoelastic strain gauge is affected by light scattering at the glass surface, it is not possible to accurately measure the stress in the vicinity of the glass surface. FIG. 2 is an example of a stress distribution obtained by measuring stress on the present tempered glass using a scattered light photoelastic stress meter. The dashed part of the graph is not reliable because it is affected by light scattering on the glass surface.

Therefore, the stress can be analyzed by measuring the stress using two kinds of stressors and synthesizing the results. This method is described in detail in international publication No. 2018/056121. Fig. 3 is an example of the synthesized stress distribution diagram.

As the optical waveguide surface stress meter, FSM-6000 manufactured by flexography can be used, for example. When FSM-6000 and the attached software FsmV are used, stress can be measured with high precision.

The scattered light photoelastic strain gauge may be, for example, SLP-1000 manufactured by kindling. In the case of using FSM-6000 as the optical waveguide surface stress meter and SLP-1000 as the scattered light photoelastic stress meter, data can be easily synthesized using dedicated software.

< chemically strengthened glass >

The tempered glass is preferably plate-shaped, and usually has a flat plate shape, but may have a curved surface.

The thickness of the tempered glass is preferably 400 μm or more, more preferably 600 μm or more, and still more preferably 700 μm or more. This is because the strength of the glass is improved. The thickness of the present tempered glass is preferably as large as possible for improving strength, but is preferably 2000 μm or less, more preferably 1000 μm or less for weight reduction.

The present tempered glass preferably generates compressive stress by ion exchange of sodium ions with potassium ions in a region shallower from the surface, and generates compressive stress by ion exchange of lithium ions with sodium ions in a deeper region. This is because such chemically strengthened glass has a stress distribution which is steep near the surface and relatively gentle in a deep portion and is bent in two stages, and as a result, the surface has high strength and is not easily broken even when a strong impact is applied.

In the present tempered glass, the value CS of the compressive stress in the glass surface measured by an optical waveguide surface stress meter₀The strength is high because of 500MPa or more. For example, in FIG. 1, the CS value of the glass surface represented by the arrow "a" is CS₀。CS₀More preferably 900MPa or more, still more preferably 950MPa or more, and particularly preferably 1000MPa or more.

When the compressive stress value CS in the glass surface of the tempered glass₀When the pressure is 1200MPa or less, delayed disintegration is suppressed, and therefore, it is preferable. CS₀More preferably 1100MPa or less, and still more preferably 1050MPa or less.

Depth D [ unit: μ m ] is preferably 3 μm or more because the fracture due to the bending mode can be suppressed. In order to suppress the fracture due to the bending mode, D is more preferably 4 μm or more, and still more preferably 5 μm or more. In order to suppress the fracture caused by the bending mode, it is more preferable if D is about 10 μm. When D is too large, the tensile stress generated in the glass core layer increases. In order to prevent the severe fracture due to the tensile stress, D is preferably 20 μm or less, more preferably 15 μm or less, and still more preferably 10 μm or less.

For example, in fig. 1, the depth of "a point having a CS value of 0" indicated by an arrow D is D. As described above, when the reinforcing layer by ion exchange between lithium ions and sodium ions is formed inside the glass, the reinforcing layer by ion exchange between lithium ions and sodium ions is formed in the vicinity of the point at which the CS value is 0, and therefore, the stress cannot be accurately measured by the optical waveguide surface stress meter. D is considered to represent the depth to which potassium ions are diffused.

In the present tempered glass, the depth from the glass surface measured by an optical waveguide surface stress meter is (D/2) [ unit: μ m ] and the slope x [ unit: MPa/μm ] is preferably-200 MPa/μm or more and 200MPa/μm or less. The slope x is more preferably-100 MPa/μm or more and 100MPa/μm or less, and still more preferably-70 MPa/μm or more and 70MPa/μm or less. As the absolute value of x is smaller, the thickness of the layer having an appropriate compressive stress value is increased, and excellent characteristics can be obtained even when the surface is polished.

In addition, the slope x is preferably a negative value. The location where the highest compressive stress is required is the glass surface where high compressive stress is required in suppressing breakage caused by bending mode. When the slope of the stress distribution has a positive portion, a portion where more than necessary stress is introduced is generated in the glass, so that delayed fracture easily occurs, or the stress value of the outermost surface is insufficient, so that fracture due to the bending mode easily occurs.

For example, in fig. 1, the slope of the stress distribution from the glass surface indicated by arrow a to a position at which the depth from the glass surface indicated by arrow c is (D/2) is x.

D, the compressive stress value CS of the glass surface₀And a compressive stress value CS at a position of depth (D/2) from the glass surface₂And x is determined by the following equation.

x＝(CS₂-CS₀)/((D/2)-0)

＝2×(CS₂-CS₀)/D

In the tempered glass, when the slope y [ unit: when the MPa/μm ] is-125 MPa/μm or less, the high-pressure stress layer required for suppressing the fracture due to the bending mode can be preferably formed to be thin in the outermost layer. The smaller the slope y, the better, but it is usually-500 MPa/μm or more.

For example, in fig. 1, the slope of the stress distribution from a position where the depth from the glass surface indicated by the arrow c is D/2 to a position where the depth from the glass surface indicated by the arrow D is y.

Here, the above-mentioned D and CS are used₂And y is determined by the following equation.

y＝(0-CS₂)/(D-(D/2))

＝-2×CS₂/D

In the present tempered glass, when the ratio | x |/| y | of the absolute value of the stress distribution slope x to the absolute value of y is 0.8 or less, it is preferable to form a stress region suitable for suppressing both delayed fracture and fracture due to bending mode while keeping the high-pressure stress layer of the outermost glass layer thin. | x |/| y | is more preferably 0.6 or less. In addition, the minimum value of | x |/| y | is 0. That is, | x |/| y | is preferably 0 to 0.8, and more preferably 0 to 0.6.

In addition, x is preferably 0 or less, and y is preferably a negative value, so that x/y is preferably 0 to 0.8, and more preferably 0 to 0.6. If x/y is positive, the smaller is more preferable.

In the present tempered glass, it is preferable that the slope z of the stress distribution from the glass surface to the depth of 1 μm measured by an optical waveguide surface stress meter is-100 MPa/μm or more because it is suitable for suppressing an increase in the thickness of the stress region in both delayed fracture and fracture due to bending mode. More preferably, z is-80 MPa/μm or more. z is usually 100MPa/μm or less, preferably 0MPa/μm or less.

For example, in fig. 1, the slope of the stress distribution from the glass surface indicated by arrow a to a position at a depth of 1 μm from the glass surface indicated by arrow b is z.

CS is a value of compressive stress at a depth of 1 μm measured by an optical waveguide surface stress meter₁[ unit: MPa of]In the case, z is obtained by the following equation.

z＝(CS₁-CS₀)/(1μm-0μm)

＝(CS₁-CS₀)/1μm

In the present tempered glass, the depth of compressive stress DOL measured by a scattered light photoelastic stress meter is preferably 50 μm or more. For example, in fig. 2, the depth of a point indicated by an arrow e where the CS value is 0 is DOL. Since the projection of general asphalt is about 50 μm in some cases, it is preferable to form a compressive stress from the surface layer of glass to a depth of 50 μm in order to prevent cracking when the glass sheet collides with the asphalt. When the thickness is t μm, DOL/t is preferably 0.1 or more, more preferably 0.12 or more, and still more preferably 0.14 or more. On the other hand, if DOL is too large relative to t, too large tensile stress occurs in the sheet thickness center layer, and severe fracture occurs at the time of scratching. Therefore, DOL/t is preferably 0.25 or less, more preferably 0.22 or less, and particularly preferably 0.2 or less.

There is a substantially positive correlation between D and DOL, with the DOL tending to increase as D increases. This is because DOL is determined by the depth of ion exchange between lithium ions and sodium ions, and D is determined by the depth of ion exchange between sodium ions and potassium ions, but generally, the degree of difficulty in the progress of ion exchange depends on the composition of the glass.

< glass for chemical strengthening >

The chemically strengthened glass of the present invention is obtained by chemically strengthening a glass for chemical strengthening (hereinafter referred to as "the present glass for strengthening") which is a lithium aluminosilicate glass. Note that, except for the case where the extreme chemical strengthening treatment is performed, the chemical strengthening glass has the same composition as the glass composition of the central portion in the thickness direction of the chemical strengthening glass, that is, the following description of the composition of the chemical strengthening glass also applies to the composition of the central portion in the thickness direction of the chemical strengthening glass.

The lithium aluminosilicate glass contains, for example, SiO in an amount of 50% or more in mol% based on the oxide₂5% or more of Al₂O₃And Li₂O、Na₂O and K₂A total content of O of 5% or more, and Li₂Containing of OAmount and Li₂O、Na₂O and K₂Ratio of total content of O (Li)₂O/(Li₂O+Na₂O+K₂O)) 0.5 or more.

The glass for strengthening has a composition described below, more preferably expressed in mol% based on oxides.

50 to 80 percent of SiO₂5 to 25 percent of Al₂O₃0 to 10% of B₂O₃0 to 10% of P₂O₅2 to 20 percent of Li₂O, 0.5-10% of Na₂O, 0 to 5% of K₂O。

In addition, MgO + ZnO + CaO + SrO + BaO (the total content of MgO, ZnO, CaO, SrO and BaO) is preferably 0 to 15%, and ZrO is preferably ZrO₂+TiO₂(ZrO₂And TiO₂The total content of) is preferably 0 to 5%.

Such glasses are susceptible to forming a preferred stress distribution by chemical strengthening treatment. Hereinafter, the preferred glass composition will be described.

SiO₂Is a component constituting the glass skeleton. In addition, SiO₂Is a component for improving chemical durability, SiO₂Is a component for reducing the occurrence of cracks when a scratch is formed on the surface of glass. SiO 2₂The content of (b) is preferably 50% or more, more preferably 55% or more, and further preferably 58% or more.

In addition, SiO is used for improving the meltability of the glass₂The content of (b) is preferably 80% or less, more preferably 75% or less, and further preferably 70% or less.

Al₂O₃Is a component effective for improving the ion exchange property at the time of chemical strengthening and increasing the surface compressive stress after strengthening, and is also a component for increasing the glass transition temperature (Tg) and increasing the young's modulus. Al (Al)₂O₃The content of (b) is preferably 5% or more, more preferably 7% or more, and still more preferably 13% or more.

In addition, Al is added to improve the melting property₂O₃The content of (b) is preferably 25% or less, more preferably 23% or less, and further preferably 20% or less.

B₂O₃B is not essential, but may be contained for the purpose of improving the meltability during glass production or the like₂O₃. In order to reduce the slope of the stress distribution in the vicinity of the surface of the chemically strengthened glass, it is preferable to contain B₂O₃In this case B₂O₃The content of (b) is preferably 0.5% or more, more preferably 1% or more, and further preferably 2% or more.

B₂O₃Is a component which is likely to cause stress relaxation after chemical strengthening, and therefore, in order to prevent a decrease in surface compressive stress due to stress relaxation, B is₂O₃The content of (b) is preferably 10% or less, more preferably 8% or less, further preferably 5% or less, and particularly preferably 3% or less.

Li₂O is a component that generates surface compressive stress by ion exchange, and is an essential component of lithium aluminosilicate glass. By chemically strengthening the lithium aluminosilicate glass, a chemically strengthened glass having a preferred stress distribution is obtained. To increase the depth of compressive stress layer DOL, Li₂The content of O is preferably 2% or more, more preferably 3% or more, and further preferably 5% or more.

In addition, Li is used for suppressing devitrification in the production of glass or in bending₂The content of O is preferably 20% or less, more preferably 15% or less, and further preferably 10% or less.

Na₂O is a component that forms a surface compressive stress layer by ion exchange with a molten salt containing potassium, and is a component that improves the meltability of the glass. Na (Na)₂The content of O is preferably 0.5% or more, more preferably 1% or more, and further preferably 1.5% or more.

In addition, Na₂The content of O is preferably 10% or less, more preferably 8% or less, and further preferably 6% or less.

K₂O is not an essential component, but K may be contained to improve the meltability of the glass and to suppress devitrification₂O。K₂The content of O is preferably 0.1% or more, more preferably 0.5% or more, and further preferably 1% or more.

In addition, in order to increase the value of compressive stress generated by ion exchange, K₂The content of O is preferably 5% or less, more preferably 3% or less, and further preferably 1% or less.

Li₂O、Na₂O and K₂The alkali metal oxides such as O are all components for lowering the melting temperature of the glass, and are preferably contained in a total amount of 5% or more. Li₂O、Na₂O、K₂Total of O contents (Li)₂O+Na₂O+K₂O) is preferably 5% or more, more preferably 7% or more, and further preferably 8% or more.

To maintain the strength of the glass, (Li)₂O+Na₂O+K₂O) is preferably 20% or less, more preferably 18% or less.

Alkaline earth metal oxides such as MgO, CaO, SrO, BaO, and ZnO all improve the meltability of glass, but tend to lower the ion exchange performance.

The total content of MgO, CaO, SrO, BaO, and ZnO (MgO + CaO + SrO + BaO + ZnO) is preferably 15% or less, more preferably 10% or less, and still more preferably 5% or less.

When any one of MgO, CaO, SrO, BaO, and ZnO is contained, MgO is preferably contained in order to improve the strength of the chemically strengthened glass.

When MgO is contained, the content of MgO is preferably 0.1% or more, and more preferably 0.5% or more. In order to improve the ion exchange performance, the MgO content is preferably 10% or less, and more preferably 8% or less.

When CaO is contained, the content of CaO is preferably 0.5% or more, and more preferably 1% or more. The content of CaO is preferably 5% or less, more preferably 3% or less, for the purpose of improving ion exchange performance.

When SrO is contained, the SrO content is preferably 0.5% or more, and more preferably 1% or more. In order to improve the ion exchange performance, the SrO content is preferably 5% or less, more preferably 3% or less.

When BaO is contained, the content of BaO is preferably 0.5% or more, more preferably 1% or more. In order to improve the ion exchange performance, the content of BaO is preferably 5% or less, more preferably 1% or less, and further preferably substantially no BaO.

ZnO is a component for improving the meltability of the glass, and may contain ZnO. When ZnO is contained, the content of ZnO is preferably 0.2% or more, and more preferably 0.5% or more. In order to improve the weatherability of the glass, the content of ZnO is preferably 5% or less, more preferably 3% or less.

TiO₂The component for suppressing scattering of fragments at the time of breakage of the chemically strengthened glass may contain TiO₂. In the presence of TiO₂In the case of (2) TiO₂The content of (b) is preferably 0.1% or more. To inhibit devitrification during melting, TiO₂The content of (B) is preferably 5% or less, more preferably 1% or less, and further preferably substantially no TiO₂。

ZrO₂Is a component for increasing the surface compressive stress by ion exchange, and may contain ZrO₂. In the presence of ZrO₂In the case of (2)₂The content of (b) is preferably 0.5% or more, more preferably 1% or more. In addition, ZrO for suppressing devitrification at the time of melting₂The content of (b) is preferably 5% or less, more preferably 3% or less.

In addition, TiO₂And ZrO₂Content of (TiO)₂+ZrO₂) Preferably 5% or less, more preferably 3% or less.

Y₂O₃、La₂O₃And Nb₂O₅Is a component for suppressing the breakage of the chemically strengthened glass, and may contain Y₂O₃、La₂O₃And Nb₂O₅. When these components are contained, the content of each component is preferably 0.5% or more, more preferably 0.5% or more, further preferably 1% or more, particularly preferably 1.5% or more, and most preferably 2% or more.

In addition, Y₂O₃、La₂O₃And Nb₂O₅The total content of (b) is preferably 9% or less, more preferably 8% or less. Thus, the glass is less likely to devitrify during melting, and deterioration in quality of the chemically strengthened glass can be prevented. In addition, Y₂O₃、La₂O₃And Nb₂O₅The content of (b) is preferably 7% or less, more preferably 6% or less, still more preferably 5% or less, particularly preferably 4% or less, and most preferably 3% or less, respectively.

Ta may be contained in a small amount for suppressing the breakage of the chemically strengthened glass₂O₅、Gd₂O₃However, since the refractive index and the reflectance are high, the content thereof is preferably 1% or less, more preferably 0.5% or less, and even more preferably substantially not contained.

P may be contained for the purpose of improving ion exchange performance₂O₅. In the presence of P₂O₅In case of (2) P₂O₅The content of (b) is preferably 0.5% or more, more preferably 1% or more. For improving chemical durability, P₂O₅The content of (b) is preferably 10% or less, more preferably 5% or less, and further preferably 3% or less.

When the glass is colored, the coloring component may be added within a range that does not inhibit achievement of the desired chemical strengthening properties. The coloring component may be, for example, Co₃O₄、MnO₂、Fe₂O₃、NiO、CuO、Cr₂O₃、V₂O₅、Bi₂O₃、SeO₂、TiO₂、CeO₂、Er₂O₃、Nd₂O₃. They may be used alone or in combination.

The total content of the coloring components is preferably 7% or less. This can suppress devitrification of the glass. The content of the coloring component is more preferably 5% or less, still more preferably 3% or less, and particularly preferably 1% or less. When it is desired to improve the visible light transmittance of the glass, it is preferable that these components are not substantially contained.

In addition, SO may be appropriately contained₃Chlorides and fluorides as fining agents in glass melting. Preferably substantially no As₂O₃. In the presence of Sb₂O₃In the case of (2), it is preferably 0.3% or less, and more preferably0.1% or less, most preferably substantially no Sb₂O₃。

In order to suppress stress relaxation during chemical strengthening, the glass transition temperature (Tg) of the glass for strengthening is preferably 480 ℃ or higher, more preferably 500 ℃ or higher, and still more preferably 520 ℃ or higher.

In addition, in order to accelerate the ion diffusion rate at the time of chemical strengthening, Tg is preferably 700 ℃ or lower. In order to easily obtain a deep DOL, the Tg is more preferably 650 ℃ or less, and still more preferably 600 ℃ or less.

The Young's modulus of the present glass for reinforcement is preferably 70GPa or more. The higher the young's modulus, the more likely the reinforcing glass is to scatter fragments when broken. Therefore, the Young's modulus is more preferably 75GPa or more, and still more preferably 80GPa or more. On the other hand, when the young's modulus is too high, diffusion of ions during chemical strengthening is slow, and it tends to be difficult to obtain deep DOL. Therefore, the Young's modulus is preferably 110GPa or less, more preferably 100GPa or less, and still more preferably 90GPa or less.

The Vickers hardness of the glass for reinforcing is preferably 575 or more. The larger the vickers hardness of the glass for chemical strengthening is, the more likely the vickers hardness after chemical strengthening becomes larger, and the more likely the scratches are formed when the chemically strengthened glass falls. Therefore, the vickers hardness of the glass for chemical strengthening is more preferably 600 or more, and still more preferably 625 or more.

The vickers hardness after chemical strengthening is preferably 600 or more, more preferably 625 or more, and further preferably 650 or more.

The larger the vickers hardness, the more difficult the scratches to be formed, and therefore, the larger the vickers hardness, the vickers hardness of the present reinforcing glass is usually 850 or less. It tends to be difficult to obtain sufficient ion exchange properties in glasses having too high vickers hardness. Therefore, the vickers hardness is preferably 800 or less, and more preferably 750 or less.

The breaking toughness value (pre-research property value of Kao bad) of the glass for reinforcement is preferably 0.7MPa m^1/2The above. The larger the fracture toughness value is, the more the chemically strengthened glass tends to be inhibited from scattering of fragments at the time of fracture. The fracture toughness value is more preferably 0.75MPa m^1/2Above, more preferably 0.8MPa · m^1/2The above.

The value of fracture toughness is usually 1 MPa.m^1/2The following.

The average thermal expansion coefficient (alpha) of the glass for reinforcing in the range of 50 ℃ to 350 ℃ is preferably 100X 10^-7Below/° c. When the average expansion coefficient (α) is small, the glass is less likely to warp during glass molding or cooling after chemical strengthening. The average expansion coefficient (. alpha.) is more preferably 95X 10^-7Preferably 90X 10 or less/° C^-7Below/° c.

The smaller the average thermal expansion coefficient (α) is, the more preferable the suppression of the warpage of the chemically strengthened glass is, but the smaller the average thermal expansion coefficient is, usually 60X 10^-7Above/° c.

In the present glass for reinforcing, the viscosity is 10²Temperature at dPa · s (T)₂) Preferably 1750 ℃ or lower, more preferably 1700 ℃ or lower, and further preferably 1680 ℃ or lower. T is₂Usually above 1400 ℃.

In the present glass for reinforcing, the viscosity is 10⁴Temperature at dPa · s (T)₄) Preferably 1350 ℃ or lower, more preferably 1300 ℃ or lower, and still more preferably 1250 ℃ or lower. T is₄Usually above 1000 ℃.

< method for producing chemically strengthened glass >

The present tempered glass can be produced, for example, by subjecting a glass for chemical tempering having the above composition to a chemical tempering treatment.

The glass for chemical strengthening can be produced by a general glass production method as described below, for example. The following production method is an example of the case of producing a chemically strengthened glass having a plate shape, but the glass for chemical strengthening may have a shape other than a plate shape.

Glass raw materials are appropriately prepared so as to obtain glass having a preferable composition, and heated and melted in a glass melting furnace. Then, the glass is homogenized by bubbling, stirring, addition of a fining agent, or the like, formed into a glass plate having a predetermined thickness, and slowly cooled. Alternatively, the sheet may be formed into a plate shape by a method of forming the sheet into a block shape, gradually cooling the block, and then cutting the block.

Examples of the method of forming into a sheet include a float method, a press method, a fusion method, and a downdraw method. Particularly, in the case of manufacturing a large glass plate, the float method is preferable. In addition, a continuous forming method other than the float method, such as a fusion method and a downdraw method, is also preferable.

The glass ribbon obtained by the forming is subjected to grinding and polishing treatments as required, thereby forming a glass plate. In the case of cutting a glass plate into a predetermined shape and size or chamfering the glass plate, if the cutting and chamfering of the glass plate are performed before the chemical strengthening treatment described later, a compressive stress layer is also formed on the end face by the chemical strengthening treatment, which is preferable.

Then, chemically strengthened glass is obtained by subjecting the formed glass plate to a chemical strengthening treatment, followed by washing and drying.

< chemical strengthening treatment >

The chemical strengthening treatment is a treatment of replacing metal ions having a small ionic radius (typically, lithium ions or sodium ions) in the glass with metal ions having a large ionic radius (typically, sodium ions or potassium ions with respect to lithium ions and potassium ions with respect to sodium ions) in the metal salt by bringing the glass into contact with the metal salt by a method such as dipping in a melt of a metal salt (for example, potassium nitrate) containing metal ions having a large ionic radius (typically, sodium ions or potassium ions with respect to sodium ions).

The method of "Li — Na exchange" in which lithium ions and sodium ions in the glass are exchanged is preferably used because the chemical strengthening treatment is fast. In addition, in order to form a large compressive stress by ion exchange, it is preferable to use a method of "Na — K exchange" in which sodium ions and potassium ions in the glass are exchanged.

The combined use of the "Li-Na exchange" and the "Na-K exchange" is more preferable because a high surface compressive stress and a deep compressive stress layer can be formed in a relatively short processing time. In this case, it is more effective to perform "Na-K exchange" after performing "Li-Na exchange".

Examples of the molten salt used for the chemical strengthening treatment include nitrates, sulfates, carbonates, chlorides, and the like. Among them, examples of the nitrate include: lithium nitrate, sodium nitrate, potassium nitrate, cesium nitrate, silver nitrate, and the like. Examples of the sulfate include: lithium sulfate, sodium sulfate, potassium sulfate, cesium sulfate, silver sulfate, and the like. Examples of the carbonate include: lithium carbonate, sodium carbonate, potassium carbonate, and the like. Examples of chlorides include: lithium chloride, sodium chloride, potassium chloride, cesium chloride, silver chloride, and the like. These molten salts may be used alone or in combination of two or more.

More specifically, the present tempered glass can be produced by a tempering method (hereinafter referred to as "the present tempering method") described below.

The strengthening treatment method comprises a step of immersing the glass plate in a strengthening salt containing potassium. The potassium-containing reinforcing salt is preferably a reinforcing salt containing 70 mass% or more of potassium ions, and more preferably a reinforcing salt containing 90 mass% or more of potassium ions, where the mass of the metal ions contained in the reinforcing salt is 100 mass%. By using such a strengthening salt, a high-pressure stress layer can be formed in the surface layer. From the viewpoint of ease of handling such as boiling point and risk, a nitrate containing potassium is preferable as the potassium-containing strengthening salt.

The strengthening salt preferably contains potassium nitrate, and may contain, as components other than potassium nitrate, alkali metal or alkaline earth metal nitrates such as sodium nitrate and magnesium nitrate. In addition, impurity levels of lithium may be included.

In the present strengthening treatment method, the glass sheet is preferably immersed in a strengthening salt containing potassium at 400 to 500 ℃. When the temperature of the potassium-containing strengthening salt is 400 ℃ or higher, ion exchange is easily performed, and thus it is preferable. Further, by the occurrence of stress relaxation, the ratio of | x |/| y | in the stress distribution is easily made smaller. The temperature of the potassium-containing strengthening salt is more preferably 420 ℃ or higher. Further, it is preferable that the temperature of the potassium-containing strengthening salt is 500 ℃ or lower because excessive stress relaxation of the surface layer can be suppressed. The temperature of the potassium-containing strengthening salt is more preferably 480 ℃ or lower.

It is also preferable that the glass is immersed in the potassium-containing strengthening salt for 1 hour or more because the surface compressive stress is increased. Further, by the occurrence of stress relaxation, the ratio of | x |/| y | in the stress distribution is easily made smaller. The immersion time is more preferably 2 hours or more, and still more preferably 3 hours or more. When the immersion time is too long, not only productivity is lowered, but also compressive stress is sometimes lowered due to a relaxation phenomenon. In order to increase the compressive stress, the immersion time is preferably 8 hours or less, more preferably 6 hours or less, and further preferably 4 hours or less.

The glass sheet may also be immersed in another strengthening salt before being immersed in the potassium-containing strengthening salt. As the other enhancing salt, an enhancing salt containing sodium is preferable. From the viewpoint of easy handling similar to that of potassium nitrate, the sodium-containing strengthening salt is preferably a sodium nitrate strengthening salt. When the mass of the metal ions contained in the strengthening salt is taken as 100 mass%, it is preferable that the metal ions contain 70 mass% or more of sodium ions.

After the glass sheet is immersed in the potassium-containing strengthening salt, it is preferably kept at a temperature of 300 ℃ or lower. This is because, when a high temperature of more than 300 ℃ is reached, the compressive stress generated by the ion exchange treatment is reduced by the relaxation phenomenon. The holding temperature after immersing the glass sheet in the potassium-containing strengthening salt is more preferably 250 ℃ or less, and still more preferably 200 ℃ or less.

The treatment conditions for the chemical strengthening treatment may be appropriately selected in consideration of the characteristics, composition, type of molten salt, and the like of the glass, such as time and temperature.

The chemically strengthened glass of the present invention is particularly useful as a cover glass for use in mobile devices such as mobile phones and smart phones. Further, the present invention is useful for a cover glass of a display device such as a television, a personal computer, or a touch panel which is not intended to be carried, an elevator wall surface, and a wall surface (full-screen display) of a building such as a house or a building. Further, the glass composition is useful for applications such as building materials such as window glass, interior materials for table tops, automobiles, airplanes, and the like, protective glass for these, and housings having curved shapes.

Examples

The present invention will be described below with reference to examples, but the present invention is not limited thereto.

Glass raw materials were blended so as to have compositions of glasses a to E represented by mole percentages based on oxides in table 1, and weighed so as to be 400g in glass. Subsequently, the mixed raw materials were put into a platinum crucible, put into an electric furnace at 1500 to 1700 ℃, melted for about 3 hours, defoamed, and homogenized.

TABLE 1

	Glass A	Glass B	Glass C	Glass D	Glass E
						SiO₂	70.42	62.60	66.84	66.00	70.00
Al₂O₃	13.01	15.50	13.42	15.09	7.50
						Li₂O	8.37	7.06	8.63	6.87	8.00
Na₂O	2.45	4.16	2.52	4.05	5.30
						K₂O	0.03	0.05	0.03	0.05	1.00
MgO	2.82	0.92	2.91	0.90	7.00
						CaO	0.00	1.54	0.00	1.50	0.00
SrO	0.00	1.16	0.00	1.13	0.00
						ZnO	0.87	0.00	0.90	0.00	0.20
B₂O₃	1.84	6.95	4.54	4.28	0.00
						P₂O₅	0.02	0.02	0.02	0.02	0.00
TiO₂	0.00	0.00	0.00	0.00	0.04
						ZrO₂	0.00	0.00	0.00	0.00	1.00
Fe₂O₃	0.03	0.03	0.03	0.03	0.00
						SO₃	0.08	0.08	0.08	0.08	0.00
SnO₂	0.07	0.01	0.07	0.00	0.00

The resulting molten glass was poured into a metal mold, held at a temperature higher than the glass transition temperature by about 50 ℃ for 1 hour, and then cooled to room temperature at a rate of 0.5 ℃/min, to thereby obtain a glass block. The obtained glass block was cut and ground, and finally mirror-polished on both sides to obtain a glass plate having a thickness of 800 μm.

Using the glasses a to E, chemically strengthened glasses of the following examples 1 to 15 were produced and evaluated. Examples 1, 2, 5, 6, 8, 11, and 14 are examples, and examples 3, 4, 7, 9, 10, 12, 13, and 15 are comparative examples.

[ chemical strengthening treatment ]

(example 1)

The glass plate comprising glass a was immersed in 100% sodium nitrate strengthening salt (sodium-containing strengthening salt) at 450 ℃ for 1 hour, then immersed in 100% potassium nitrate (potassium-containing strengthening salt) at 450 ℃ for 1 hour, and washed with cold water.

(examples 2 to 15)

Chemically strengthened glasses of examples 2 to 15 were obtained in the same manner as in example 1, except that the glasses shown in the column of glass in Table 2 were used, and the time for immersion in the sodium-containing strengthening salt was set to the time (unit: time) shown in treatment time 1 in Table 2, and the time for immersion in the potassium-containing strengthening salt was set to the time (unit: time) shown in treatment time 2.

[ measurement of stress distribution ]

The stress values were measured using an optical waveguide surface stress meter FSM-6000 and a scattered light photoelastic stress meter SLP-1000 manufactured by TOYO CORPORATION. CS is shown in Table 2₀(unit: MPa), D (unit: μm), x (unit: MPa/μm), y (unit: MPa/μm), x/y, z (unit: MPa/μm), and DOL (unit: μm). FIG. 1 shows stress distribution obtained by FSM-6000 and FIG. 2 shows stress distribution obtained by SLP-1000 for the chemically strengthened glass of example 1. FIG. 3 shows the results of the synthesis.

TABLE 2

	Glass	Treatment time 1	Treatment time 2	CS₀	D	x	y	x/y	z	DOL
											Example 1	Glass A	1	1	976	4.8	-79	-332	0.24	-49	108
Example 2	Glass A	1	2	975	6.8	-50	-240	0.21	-20	180
											Example 3	Glass A	1	4	944	9.8	-46	-150	0.30	-18	27
Example 4	Glass B	1	1	1257	4.8	-262	-262	1.00	-262	170
											Example 5	Glass B	1	2	1055	5.2	-56	-292	0.19	-59	150
Example 6	Glass B	1	4	981	6.6	-42	-263	0.16	-8	53
											Example 7	Glass C	1	1	975	5.1	-191	-191	1.00	-191	134
Example 8	Glass C	1	2	925	5.2	-74	-229	0.32	-56	210
											Example 9	Glass C	1	4	911	7.4	-44	-206	0.21	-17	34
Example 10	Glass D	1	1	1203	4.6	-262	-262	1.00	-262	142
											Example 11	Glass D	1	2	1085	5.2	-79	-342	0.23	-56	190
Example 12	Glass D	1	4	1077	7.0	-40	-260	0.15	-5	37
											Example 13	Glass E	1	1	801	6.9	-116	-116	1.00	-116	100
Example 14	Glass E	1	2	746	8.8	-53	-113	0.47	-35	140
											Example 15	Glass E	1	4	701	12.0	-36	-80	0.45	-4	33

Example 4 having x/y of 1 is likely to cause delayed fracture, resulting in a decrease in yield during polishing. In example 5 in which the same glass B was strengthened, the above-mentioned problems did not occur. This is probably because the strengthening treatment time is appropriate, and x/y is as low as 0.19.

In both examples 2 and 3, the glass a was subjected to chemical strengthening treatment, and the compressive stress layer was pulled up to a position where DOL was as deep as 180 μm in example 2, whereas DOL was as shallow as 27 μm in example 3 in which the treatment time 2 was too long, and therefore, it was easy to break when dropped on sand.

It will be apparent to those skilled in the art that the present invention has been described in detail and with reference to specific embodiments thereof, but that various changes or modifications can be made therein without departing from the spirit and scope thereof. It is to be noted that the present application is based on japanese patent application published on 7/3 in 2018 (japanese patent application 2018-126894), the entire contents of which are incorporated by reference. In addition, all references cited herein are incorporated by reference in their entirety.

Claims

1. A chemically strengthened glass having a compressive stress layer on a surface thereof, wherein,

the value CS of the compressive stress of the glass surface of the chemically strengthened glass measured by an optical waveguide surface stress meter₀The pressure of the mixture is more than 500MPa,

d is the depth from the glass surface at the position where the compressive stress value measured by the optical waveguide surface stress meter is 0[ unit: mum of]And CS is a value of compressive stress at a position having a depth of (D/2) from the glass surface measured by an optical waveguide surface stress meter₂The ratio | x |/| y | of the absolute value of the slope x of the stress distribution from the glass surface to the position having the depth of (D/2) and the absolute value of the slope y of the stress distribution from the position having the depth of (D/2) to the position having the depth of D, which is obtained by the following formula, is 0 to 0.8, and

the chemically strengthened glass has a depth of compressive stress layer DOL of 50 [ mu ] m or more as measured by a scattered light photoelastic stress meter,

x＝(CS₂-CS₀)/((D/2)-0)＝2×(CS₂-CS₀)/D

y＝(0-CS₂)/(D-(D/2))＝-2×CS₂/D。

2. the chemically strengthened glass as claimed in claim 1, wherein the glass is represented by the formula CS₁And the CS₀The slope z of the stress distribution from the glass surface to the position having a depth of 1 μm is determined to be not less than-100 MPa/μm and less than 0,

the CS₁For the value of compressive stress at a position of a depth of 1 μm from the glass surface measured using an optical waveguide surface stress meter,

z＝(CS₁-CS₀)/(1μm-0μm)＝(CS₁-CS₀)/1μm。

3. the chemically strengthened glass according to claim 1 or 2, wherein the chemically strengthened glass has a plate shape having a thickness of 2000 μm or less.

4. The chemically strengthened glass according to any one of claims 1 to 3, wherein the glass is one which is obtained by forming a glass composition comprising, in mol% on an oxide basis,

the glass composition of the central part of the chemically strengthened glass in the thickness direction contains more than 50% of SiO₂5% or more of Al₂O₃And is and

in the glass composition of the central part of the chemically strengthened glass in the thickness direction, Li₂O、Na₂O and K₂A total content of O of 5% or more, and Li₂Content of O and Li₂O、Na₂O and K₂The ratio of the total content of O is 0.5 or more.

5. The chemically strengthened glass according to any one of claims 1 to 4, wherein the chemically strengthened glass has a basic composition, expressed in mol% on an oxide basis, of:

50 to 80 percent of SiO₂、

5 to 25 percent of Al₂O₃、

0 to 10% of B₂O₃、

0 to 10% of P₂O₅、

2% -20% of Li₂O、

0.5 to 10 percent of Na₂O、

0 to 5% of K₂O、

0 to 15% of MgO + ZnO + CaO + SrO + BaO, and

0 to 5% of ZrO₂+TiO₂。

6. A method for producing a chemically strengthened glass according to any one of claims 1 to 5, wherein the method for producing a chemically strengthened glass comprises:

soaking the glass plate in strengthening salt containing potassium at 400-500 deg.c for 1-8 hr; and

the glass sheet is then brought to a temperature below 300 ℃ and

the metal ions contained in the strengthening salt are 100 mass%, and the potassium-containing strengthening salt contains 70 mass% or more of potassium ions.

7. The method for producing chemically strengthened glass according to claim 6, wherein the glass plate has a glass composition before chemical strengthening which comprises the following components in mol% based on oxides

SiO 50% or more₂、5％Al above₂O₃And is and

in the glass composition of the glass sheet before chemical strengthening, Li₂O、Na₂O and K₂A total content of O of 5% or more, and Li₂Content of O and Li₂O、Na₂O and K₂The ratio of the total content of O is 0.5 or more.