JP6512889B2 - Float method and float plate glass for producing float plate glass - Google Patents

Description

  The present invention relates to a float process and a float sheet glass for producing a float sheet glass.

  Chemically hardened aluminosilicate glass panes have been used for many years, especially as glass covers for displays of portable terminals, such as laptops and smartphones, due to their high mechanical strength at small board thicknesses of less than 1 mm. It is used. For the production of aluminosilicate glass sheet, in addition to other drawing methods, in particular the float method familiar to the person skilled in the art is taken into account. However, after chemical tempering, it is observed that the aluminosilicate glass sheet has a curvature or warp, also known to the skilled person as warp, if the float method is used. This distortion is a withdrawal method of the asymmetry of the float method, as well as different sides of the float sheet glass known as the tin bath side and the opposite upper side of the float sheet glass, and different degrees of reinforcement under the same strengthening conditions, It is due to the fact that this leads to distortion.

  According to DE 3 607 404, the formation of strains can be avoided, for example, by grinding and polishing the surface of the glass sheet before chemical strengthening, but this is very complicated.

  There is also a technical effort to provide a floated glass sheet with a layer that is made to change ion exchange during chemical strengthening and not cause any distortion. This technical solution is also very complicated and is only possible in special cases where the applied layer does not adversely affect the subsequent process steps.

  WO 13146438 discloses a flat glass in which the sodium content of one side is purposely adjusted 0.2 to 1.2% by mass lower than the other side, which likewise makes the float flat glass resistant to distortion during chemical strengthening. ing. The disadvantage is that the two surfaces of the glass sheet are significantly different chemically from one another, which can lead to problems during further processing of the glass cover for the display. In particular, then the correct orientation of the glass sheet must also be ensured during the further processing process, ie with increased effort during this further processing.

DE3607404 WO13146438

  The object of the present invention is to provide a method based on the float method for producing highly chemically toughable float sheet glass, wherein the float sheet glass produced according to the present method is added after the chemical strengthening process In spite of omitting certain process steps, such as surface coating or grinding, it will have little distortion. Furthermore, the object of the present invention is to provide a corresponding highly chemically toughable glass sheet. Moreover, float glass will have little asymmetry with respect to the chemical composition on both sides.

  This problem is solved by the independent claims. Preferred embodiments are given in the dependent claims.

In the case of the float process according to the invention for producing float sheet glass in a draw-out zone with a float bath and annealing lehr, the molten glass is continuously pumped onto the molten metal and drawn in the draw-out direction to the molten metal A glass ribbon of thickness D is formed having a facing tin bath side and an upper side facing away from the molten metal, where the glass ribbon is cooled across the float bath, lifted from the molten metal, and Further transfer to the lehr, where the glass ribbon has a temperature T _top on the upper side and the temperature T _bottom on the tin bath side, where the glass ribbon has two along its longitudinal sides Have peripheral regions, which start from the longitudinal sides and extend towards the center of the ribbon, and between which a useful region is arranged; In this, the floor temperature of the annealing furnace is a T _floor, and the ceiling temperature of the annealing furnace is a T _ceiling. Float according to the invention, the upper temperature T _top is T _g + 20K from the drawer zone falls T _g -20K part A, the temperature difference at any point x in the effective area of the glass ribbon [Delta] T _Glass = T _top - It is characterized by T _bottom being higher than 0K.

The inventors have found that if the upper side of the glass ribbon has a temperature higher than the tin side of the glass ribbon in a temperature range near the glass transition temperature T _g (where the temperatures T _top and T _bottom are respectively the same points on the glass ribbon) We have found that asymmetric cooling of the glass ribbon leads to a large reduction in strain after chemical strengthening (with respect to x). Here, T _g is a known glass transition temperature to those skilled in the art that conforms to ISO 7884-8. Usually, the portion A of the glass ribbon is corresponding drawer zone below the glass transition temperature The T _g present in annealing furnace.

On the other hand, according to the prior art, when the glass ribbon is cooled in a lehr, it is sought to keep the glass ribbon to be cooled as constant as possible over the entire cross section at the location x of the drawing zone, When the temperature is lowered slowly towards the withdrawal direction. Therefore, it is attempted to adjust the heating in the lehr such that as little temperature non-uniformity as possible is present in the cross section of the glass ribbon perpendicular to the draw direction. In particular, according to the prior art, at the point of the _lehr it is desirable that the upper temperature T _top of the glass ribbon corresponds to the temperature T _bottom on the tin bath side of the glass ribbon. This is typically achieved by the same floor temperature T _floor and ceiling temperature T _ceiling in a _lehr .

However, according to the present invention, the cooling of the glass ribbon is carried out such that the temperature T _top at the portion A of the drawer zones upper temperature T _top falls T _g -20K from T _g + 20K is higher than T _bottom. At that time, it was found that the strain after chemical strengthening decreases as the temperature difference ΔT _glass = T _top −T _bottom increases. The upper side of the glass ribbon tends to form a convex upper side when the temperature difference is ΔT _glass = 0 after chemical strengthening, while the upper convex warpage after chemical strengthening has a temperature difference ΔT _glass It can be decreased with the increase and finally the upper side may even have a concave camber.

  Here, the exact mechanism of action of the asymmetric temperature control according to the present invention on the strain after chemical strengthening is not known. Upper and tin baths that affect the diffusivity of potassium and / or sodium ions such that less ions are exchanged on the upper side of the float sheet glass in a subsequent process of chemical strengthening without being bound by any particular theory It is speculated that side-by-side asymmetric temperature control results in regular differences in the glass structure.

  The float process is generally characterized by delivering molten glass onto the molten metal, withdrawing it to form a glass ribbon, and cooling. The float process according to the invention corresponds in basic point to the standard float process for producing soda lime-plate glass, however, on a smaller scale, typical volume throughput of 10 to 50 tonnes / day Accompanied by Molten tin is preferably used as the molten metal. A float sheet glass produced by the float method is a sheet glass produced by another drawing method, for example, a slight residue of tin on the float bath side of the sheet remains in a very thin surface layer, and thus floats It can be distinguished by the fact that the glazing always has a higher tin content on the float bath side than in the middle or above the plate.

The molten glass delivered onto the molten metal is generally a molten aluminosilicate glass. Aluminosilicate glass, commercially available soda lime glass and, for example, as compared with low Al ₂ O ₃ borosilicate glass Schott AG trademark Borofloat ^(R), accentuated by a very good chemical strengthening properties, preferably in accordance with the present invention therefore . The glass may have a composition or a specific composition, for example in the range of the following compositions listed in Table 1:

  The withdrawal zone beginning in the float bath comprises, downstream of the float bath in the withdrawal direction, usually at least one dross box, a lehr, and an area for separating and further processing the glass ribbon. At the end of the molten metal, the glass ribbon is removed from the melt and transferred further, for example by means of rollers. The area through which the glass ribbon passes the first roller is usually referred to as the dross box. The dross box is usually separated from the float bath by at least one partition and likewise from the subsequent lehr by at least one partition.

  The glass ribbon has two peripheral regions along its longitudinal side, which start from the longitudinal side and extend towards the center of the ribbon and between which an active area is arranged ing. In the peripheral area, the glass ribbon usually contacts the top roller used to transport the glass ribbon in the withdrawal direction. Furthermore, the top roller is used for the lateral adjustment of the width of the glass ribbon. The peripheral area of the glass ribbon, also called selvedges, generally has a larger thickness and at least a much larger thickness along the outer edge. The active area of the glass ribbon includes the area of the glass ribbon between the peripheral areas where the glass ribbon has a uniform thickness. According to the invention, a temperature difference between the upper side of the glass ribbon and the tin bath side is created. On the other hand, in the peripheral region of the glass ribbon where the upper side and the tin bath side meet, the temperature difference may be less or completely absent. Therefore, the temperature difference according to the invention is created at least in the active area of the glass ribbon or at any point in the glass ribbon which is the active area and not the peripheral area.

The temperatures T _top and T _{bottom of the} glass ribbon and the difference T _glass thereof depend on the position in the drawing direction. Furthermore, there is also a low dependence of the temperatures T _top and T _bottom of the glass ribbon transverse to the direction of withdrawal, but here the temperature difference between them is very small and can usually be ignored . The temperature difference ΔT _glass preferably measures T _top and T _bottom at a position of the glass ribbon defined by the x coordinate in the withdrawal direction and the z coordinate transverse to the withdrawal direction and takes a difference Should be sought by Advantageously, the temperatures T _top and T _bottom are the temperatures at the _top and _bottom in the middle of the glass ribbon.

In the dross box, the temperature T _top of the glass ribbon can fall below T _g as soon as possible. Therefore, first part or portion above the temperature T _top falls T _g -20K from T _g + 20K may be located dross box. However, usually, Xu temperature at the inlet of the cooling furnace is higher than T _g + 20K, therefore, at least one portion upper temperature T _top falls T _g -20K from T _g + 20K are arranged in the annealing furnace. Thus, it may be a plurality of portions in which the upper temperature T _top falls T _g -20K from T _g + 20K present in the pull-out direction. Temperature of upper, in accordance with the present invention, the portion A of the higher drawer zone than the temperature T _bottom of the tin bath side, advantageously, as viewed in the drawing direction, the T _g -20K upper temperature T _top from T _g + 20K is the last part down, therefore, the glass ribbon will not be further heated longer the T _g downstream of this part a.

In an advantageous embodiment of the float process, the temperature difference ΔT _glass = T _top −T _bottom at any point x of the active area of the glass ribbon in part A is at least 0.25 K, preferably at least 0.5 K, More preferably, it is at least 1 K, particularly preferably at least 1.5 K. The advantageous value of the temperature difference ΔT _glass is relatively low compared to the absolute level of the temperature. However, it should be taken into account that the glass ribbon has to be brought to a highly asymmetric temperature profile or a highly asymmetric heating in order to achieve such small temperature differences, since Is preferably only of small thickness D, so that the temperature difference ΔT _glass between the upper side and the tin bath side becomes very fast and uniform. The temperature difference [Delta] T _Glass is from direct measurements of T _top and T _bottom can not be measured reliably only with a lot of effort, also because it measures the surface temperature T _top and T _bottom ( These can be measured, for example, by means of a high-temperature thermometer), since there is always the possibility that measurement errors occur in the order of the temperature difference T _glass . Therefore, the temperature difference T _Glass is advantageously determined indirectly from the temperature T _ceiling and T _floor of the furnace.

Here, the temperature of the glass ribbon derived from the temperatures T _ceiling and T _floor of the furnace can be measured, for example, by two-dimensional FEM analysis. Here, the temperature field in the lehr is represented by the following equation:
In the formula, 密度 is density, c _p is specific heat capacity, 引 き_出し_z is drawing speed, λ is the thermal conductivity of glass, and x is the space coordinate in the drawing direction of the glass ribbon. Moreover, it can be inferred that heat exchange between the glass and its surroundings by thermal conduction and by surface radiation takes place, which gives rise to the boundary conditions of the following formula:

Here, T _U represents the ambient temperature, which corresponds to the temperature T _ceiling or T _floor in the _lehr , σ represents the Stefan-Boltzmann constant, α represents the heat transfer coefficient at the glass-air interface, ε Represents emissivity and y represents spatial coordinates perpendicular to the draw direction and perpendicular to the glass ribbon.

Here, according to FEM analysis as a rule of thumb, with respect to a plate glass having a thickness of about 0.55 _{mm, the} temperature difference ΔT _lehr = T _ceiling −T _floor in the slow cooling furnace is the temperature difference ΔT _glass = T _top −T _bottom 15 It turns out that it is -30 times. For the thickness of the glass ribbon of 0.5 mm and 0.62 mm, the following approximation can be determined from simulation calculations:

In an advantageous embodiment of the float process, the temperature difference ΔT _lehr = T _ceiling −T _floor at any point x in the active area of the glass ribbon of part A is at least ₁₅ K, preferably at least 30 K, more preferably It is at least 50 K, particularly preferably at least 75 K. Asymmetric temperature control in a lehr is an advantageous way to create an asymmetric temperature profile in the glass ribbon. Typically, a lehr for a glass ribbon has a plurality of continuously arranged segments, wherein in each segment heating segments are arranged above and below the glass ribbon. In each segment, typically, the upper and lower heating segments are separately controllable, so that the asymmetric temperature profile can be adjusted. The temperature profile which actually occurs in the lehr may, of course, also be dependent on further factors, which need not necessarily correspond to the predetermined temperature profile.

The temperature T _ceiling may be measured by means of a thermocouple which is arranged as far as possible from the glass ribbon and as far as possible from the heating segment. Therefore, T _ceiling is the temperature in the lehr at a position spaced about 10 to 100 mm above the glass ribbon, not the temperature of the lehr ceiling, for example. Naturally, the temperature _Tceiling depends on the location along the withdrawal zone. The temperature T _ceiling is preferably measured by means of a thermocouple located at a distance of 30 to 60 mm from the upper side of the glass ribbon along the drawing direction. The same applies correspondingly to the temperature _floor , so that this temperature is the temperature in the lehr at a distance of about 10 to 100 mm below the glass ribbon, and advantageously, the glass along the withdrawal direction. It is measured by a number of thermocouples located 30-60 mm away from the tin bath side of the ribbon.

Unlike the temperature difference ΔT _glass = T _top −T _bottom , the temperatures T _ceiling and T _{floor and} hence ΔT _lehr are properly obtained by measurement and according to the prior art so that the highest possible temperature uniformity can be adjusted in the _lehr. It is typically measured in a lehr.

In an advantageous embodiment of the float process, the ΔT _glass / D ratio is at least 0.5 K / mm, preferably at least 1 K / mm, more preferably at least 2 K / mm, particularly preferably at least 3 K / mm. is there. When cooling thick glass sheets having a thickness of more than 2 mm according to the prior art, it is generally not possible to eliminate small temperature differences between the upper and lower sides, since glass sheets of larger thickness are It is because it has a higher heat shielding effect and promotes the temperature difference between the upper side and the lower side. However, for small board thicknesses, the advantageous temperature difference is extremely large.

In a preferred embodiment of the float process, the ΔT _lehr / D ratio is at least 30 K / mm, preferably at least 60 K / mm, more preferably at least 100 K / mm, particularly preferably at least 150 K / mm. When cooling thick sheet glass having a thickness of more than 2 mm according to the prior art, temperature differences of more than 10 K can not generally be excluded, since sheet glass with a larger thickness has a higher heat shield This is because it has an effect and promotes a temperature difference between the ceiling temperature T _ceiling and the floor temperature T _floor . However, for small board thicknesses, the advantageous temperature difference is extremely large.

In an advantageous embodiment of the float process, the molten glass has a proportion of Al ₂ O ₃ of at least 5% by weight, preferably of at least 10% by weight. The chemical strengthening properties of such glasses are particularly high.

In an advantageous embodiment of the float process, the float glass sheet can be highly chemically strengthened to a surface compressive stress CS of at least 600 MPa and a reinforcement layer depth DoL of at least 30 μm. CS and DoL may be measured stress-optically, for example, using a Luceo apparatus FSM 6000. Particularly advantageously, the float sheet glass is strengthened in a KNO ₃ molten salt within 4 hours at a temperature of T _g -200 K, with a surface compressive stress CS of at least 600 MPa and a depth DoL of a strengthening layer of at least 30 μm. be able to.

  In a preferred embodiment of the float process, the glass ribbon has a thickness D of less than or equal to 2.0 mm, preferably less than or equal to 1.0 mm and particularly preferably between 0.3 mm and 0.7 mm in the effective area. . Here, the thickness D should mean the thickness of the glass ribbon in the effective area of the glass ribbon. This may, for example, be measured in the middle of the glass ribbon and corresponds approximately to the targeted thickness of the float glass sheet. It will be appreciated that the glass ribbon may, for example, have a greater thickness for the ear. Glass sheets of such small thickness can be produced particularly well by the float process, can be chemically strengthened and have a slight weight.

In an advantageous embodiment of the float method, in the portion B1 of the drawer zones upper temperature T _top falls to T _g + 20K from T _g + 50K, or the upper temperature T _top falls T _g -50K from T _g -20K In part B2 of the drawing zone, the temperature difference ΔT _glass at any point x in the active area of the glass ribbon is at least 0.25 K, preferably at least 0.5 K, more preferably at least 1.0 K, particularly preferably Is at least 1.5K. Thus, the asymmetric temperature control on the upper side and tin bath side of the glass sheet preferably advantageously not only extends just over the part A, but also starts as early as upstream of the part A or downstream of the part A as seen in the withdrawal direction. Continue. Thereby, the effect of reducing distortion can be further enhanced.

In a preferred embodiment of the float process, in the part B1 or B2, the temperature difference ΔT _lehr at any point x in the active area of the glass ribbon is at least ₁₅ K, preferably at least 30 K, particularly preferably at least 50 _K. is there. Thereby, the effect of reducing distortion can be further enhanced.

In an advantageous embodiment of the float process, the temperature difference ΔT _lehr = T _ceiling at any point x in the active area of the glass ribbon, in any part C of the _lehr , having a length of at least 2 m, preferably at least 5 m. -T _floor is at least 15 K, preferably at least 30 K, particularly preferably at least 50 K.

In an advantageous embodiment of the float process, the glass ribbon is _{subjected to} the influence of a heating power P _below and a heating power P _above from _{below in a lehr} , with a length of at least 2 m, preferably at least 5 m. In part D of the _lehr , the P _below / P _above ratio is less than 0.5, preferably less than 0.3, particularly preferably less than 0.1. In order to achieve a uniform temperature distribution of the glass ribbon in the lehr, in the conventional cooling process according to the prior art, the glass ribbon is likewise subjected to the same heating power from above and below. On the other hand, in the preferred mode of operation of the lehr, in part D, the glass ribbon is subjected to the influence of higher heating power from above. In a particularly advantageous manner, the lower heating element may even be completely switched off. In that case, the part D advantageously corresponds to the part A, but this may also include the parts B1 and / or B2 or may extend over the first half or the entire lehr.

In a further advantageous embodiment of the float process, the glass ribbon is heated from above in the lehr by a heat source and the heat released is preferably in the upper half of the glass ribbon, particularly preferably up to 50 μm thick. Absorbed in the surface layer of The temperature difference ΔT _glass = T _top −T _bottom in the glass ribbon can be increased particularly effectively if the thermal energy supplied from above is absorbed directly by the thin surface layer of the glass ribbon and is not transmitted. This is generally not the case for blackbody radiation with T ̃600 K and high 2 to 4 μm radiation components, since aluminosilicate glasses are not opaque in this wavelength range. On the other hand, if the heating from above is performed by convecting a stream of hot air or by infrared radiation of wavelengths> 4 μm, preferably> 10 μm, for example a CO ₂ laser, heat is introduced directly to the surface of the glass ribbon Be done. Preferably the heat source of the heat dissipated absorbed in the upper half of the glass ribbon, particularly preferably in the surface layer of up to 50 μm thick, may be the only heat source above the lehr, or else It may be present in addition to conventional heat sources.

In an advantageous embodiment of the float process, the glass ribbon is cooled from below in a lehr, so that the temperature difference ΔT _glass = T _top −T _bottom in the glass ribbon can likewise be increased particularly effectively. For example, the lower side may be cooled by a stream of air, or a water bath may be provided on the floor of the lehr. The air flow has to be optionally preheated so that the cooling output does not become too high. With the aid of a water bath, a large area of particularly uniform cooling can be achieved.

Furthermore, subject of the present invention may also be standardized strain W _s after the chemical strengthening, less than 300 [mu] m, advantageously less than 200 [mu] m, particularly preferably less than 100 [mu] m, and chemically strengthening the previous plates, + less than 20 [mu] m, Preference is given to float glass sheets having an initial strain (pre-warp) of less than 0 μm, particularly preferably of less than −20 μm.

Here, the float sheet glass is manufactured in the float method, and the tin bath side is a sheet which has not been subjected to any surface removal treatment such as grinding, polishing or etching. Advantageously, the float glass sheet has an Al ₂ O ₃ fraction of at least 5% by weight.

Advantageously, the float sheet glass can be highly chemically strengthened to a surface compressive stress CS of at least 600 MPa and a depth DoL of depth of layer of at least 30 μm, particularly preferably The float sheet glass has a surface compressive stress CS of at least 600 MPa and a reinforcement layer depth of Dol of at least 30 μm within 4 hours at a temperature of T _g- 200 K in KNO ₃ molten salt It can be highly chemically enhanced.

  The float glass panes according to the invention preferably have a typical thickness of less than 1.5 mm, preferably less than 1.0 mm and particularly preferably less than 0.6 mm.

"Chemical standardized strain W _s after strengthening" or short "standardized strain W _s" represents the specific characteristics of float glass which is not reinforced to form a distortion after chemical strengthening, where standardized strain W _s is predetermined plate dimension having a width b ₀ of length l ₀ and 130mm of 217 mm, float glass having a predetermined plate thickness D ₀ of 0.70mm is distorted to have after predetermined chemical tempering method It is. Unless explicitly stated otherwise, the strain values defined herein are in principle for the reinforced state and not for the non-hardened state. The strain value before strengthening is also referred to herein as initial strain. Therefore, "standardized strain W _s after chemical strengthening" is also referred to herein for short as "standardized strain W _s " and represents the unique properties of the non-reinforced plate.

For the determination of standardized strain W _s after the chemical strengthening, the advantageously float plate glass having a plate thickness D ₀ of 0.70mm is advantageous width advantageous length l ₀ and 130mm of 217 mm b ₀ Cut into a plate with the following, however, not subjected to any further processing such as cleaning, grinding or polishing processes. Not be removed sulfur-containing coatings, however, this was found to have only very little effect on the standardized strain W _s. Chemical strengthening of the plate is performed according to a standardized chemical strengthening method, where the plate is for 4 hours at a temperature of T _g- 200 K in potassium nitrate molten salt with> 99.9% KNO ₃ before strengthening Cure over time. As the upper and tin bath sides of the float sheet glass are thereby under the same temperature-time distribution, asymmetry in the chemical strengthening of the upper and tin bath sides can not be attributed to the strengthening process. After a standardized chemical strengthening process, the compressive stress CS of the surface is typically at least 800 MPa and the depth DoL of the strengthening layer is at least 30 μm. The plate is subsequently removed from the molten salt and washed, and the strain is subsequently determined in accordance with DIN 50441-5; 1998-05, which in principle has a positive sign. According to DIN 50441-5: 1998-05, the definition of the term "strain" in DIN 50441-5: 1998-05 corresponds to the strain defined in ASTM F 1390-92. In terms of value, the standardized strain W _s has a strain according to DIN 50441-5: 1998-05. However, furthermore, the standardized strain W _s is given a positive sign when the upper side of the float glass sheet is convex after chemical strengthening and a negative sign when the tin bath side of the float glass sheet is convex. . Therefore, in a standardized strain, the direction of warpage with respect to the orientation of the float glass sheet in the float process is taken into account differently from the strain according to DIN 50441-5: 1998-05.

In float glass of the other plate thicknesses, normalized strain W _s after the chemical strengthening, the strain W was measured according to the method described above, and the following formula:
W _s = W · (D / D ₀ ) ² , where D ₀ = 0.70 mm
And can be approximately measured by converting to a predetermined plate thickness D ₀ for a standardized strain W _s .

Similarly, standardized strain W _s after the chemical strengthening of float glass of the other plate dimensions, having a length l and width b also can be approximately measured according to the above-described method, and the following formula :
W _s = W · [(b ² + l ² ) / (b ₀ ² + l ₀ ² )] ^1/2
Based on, it can be converted to a predetermined thickness dimension for standardized strain W _s.

However, the dimensions of the plate should not be too far from the standardized dimensions. Because these are approximate formulas. The width b, length l and thickness D of the plate should be between 50% and 200% for the standardized dimensions b ₀ , l ₀ and D ₀ respectively.

The float glazing may also have a strain, referred to herein as initial strain, prior to chemical strengthening. The initial strain is measured on a plate of size 217 mm × 130 mm, corresponding to a diagonal of about 10 inches, according to DIN 50441-5: 1998-05 according to the method described above. In some cases, if the sample shape is different, it is converted to a standardized plate shape. Just as in the case of standardized strain W _s , the initial strain is marked with a positive symbol when the upper side of the float glass is convex and with a negative symbol when the tin bath side of the float glass is convex. Be done. Therefore, the direction of warpage relative to the orientation of the float glass sheet in the float process is taken into account in the initial strain, unlike the strain according to DIN 50441-5: 1998-05.

  The initial strain of float glass sheet produced according to the prior art is usually positive, ie the upper side of the float glass sheet has a slight convex camber and the float bath side has a slight concave camber. In the case of the float sheet glass according to the invention, the warpage is just reversed, ie the upper side has a concave warpage and the lower side has a convex warpage, whereby the initial strain has a negative sign Is attached.

  Preferably, the float glass sheet has an initial strain of less than 20 μm, preferably less than 0 μm, particularly preferably less than −20 μm.

In an advantageous embodiment of the float glass, the difference in Na ₂ O content at the upper side of Na ₂ O content Na ₂ O _top and tin bath side of the standardized float glass after chemical strengthening ΔNa ₂ O = Na ₂ O _top -na ₂ O _bottom is greater than -0.2% by weight and less than 0.2 wt%. According to the present invention, the present invention is characterized, inter alia, by the chemical composition of the upper side of the float sheet glass only slightly different from the chemical composition of the tin bath side, and at the same time very low standardized after chemical strengthening It has been realized that it is possible to provide a float sheet glass having a distortion W _s . Therefore, a very low value of normalized strain W _s after the chemical strengthening achieved, this time, Na ₂ O in the upper Na ₂ O content Na ₂ O _top and tin bath side of float glass after chemical strengthening The content difference ΔNa ₂ O = Na ₂ O _top −Na ₂ O _bottom is more than −0.2% by mass and less than 0.2% by mass. Therefore, advantageously, it is possible to provide a float glass sheet which is very unlikely to form strain after chemical strengthening and at the same time has very little asymmetry with respect to the chemical composition on both sides. A distinction between the float bath side and the upper side of the float glazing in the further processing process is not necessary, which means that the method has been greatly simplified.

For example, the Na ₂ O concentration can be measured by X-ray fluorescence with a Bruker S8 Tiger measuring device at an acceleration voltage of 20 kV and a current of 50 mA. Sulfur-containing coatings should be removed before performing the measurement.

  The person skilled in the art knows various ways in which the float glass pane itself can be clearly identified and the tin bath side and the upper side of the float glass pane can be distinguished. For example, the tin bath side can be identified based on fluorescence emission by irradiation of ultraviolet light, or the tin content of the surface may be measured. The tin bath side has in principle a higher tin content than the upper side of the float sheet glass.

In an advantageous embodiment of the float sheet glass, the difference between the upper Na ₂ O concentration Na ₂ O _top and the Na ₂ O concentration on the tin bath side of the non-chemically strengthened float sheet glass ΔNa ₂ O = Na ₂ O _top −Na ₂ O _bottom is more than −0.2% by mass and less than 0.2% by mass. The inventor has also noticed that the float glass sheet before chemical strengthening likewise has very little asymmetry in the chemical composition on the upper side and the float bath side. The non-reinforced state in this case means that the compressive stress of the surface is at most 300 MPa and the depth (DoL) of the reinforced layer is at most 15 μm. The difference in Na ₂ O concentration before chemical strengthening, ΔNa ₂ O, tends to be higher than the reference amount after strengthening.

  Advantageously, float glass panes are produced by the process according to the invention.

A schematic illustration of a portion of the withdrawal zone in cross section A schematic plan view of a portion of the withdrawal zone Figure showing a graphic illustration of the upper temperature over the withdrawal zone with parts A, B1 and B2 Fig. 1 schematically shows a float sheet glass according to the invention Figure showing the temperature difference ΔT _glass in a glass ribbon with a thickness D of 0.5 mm as a function of ceiling temperature and floor temperature in a lehr

  The invention will be described in the following on the basis of exemplary embodiments.

Composition of 61% by mass of SiO _2, 17% by mass of Al ₂ O ₃ , 12% of B ₂ O _3, 12% of Na ₂ O, 4% of K ₂ O, 4% of MgO, 1.3% of ZrO ₂ and a fining agent SnO ₂ The molten glass with H.sub.2 was fed onto a molten tin bath and drawn to form a glass ribbon of 0.57 mm thickness and approximately 2500 mm width. Glass has a T _g of 616 ° C.. The withdrawal speed was 200 to 250 m / h, and the throughput was approximately 25 tons per day. Moreover, to avoid scratching, the tin bath side of the glass ribbon at the beginning of the annealing belt is exposed to a stream of SO ₂ containing gas with 50 l / h (liters per hour) of SO ₂ and 250 l / h N ₂ . did.

The glass ribbon is cooled in a conventional rollered lehr, wherein the rollered lehr has a plurality of successive segments S _n where the glass ribbon is heated by the heating element from above each time P _{n, above} and from the bottom were under the influence of the heating power P _{n, below} . Here, the heating element is conventionally designed and emits thermal radiation which corresponds approximately to that of the black body at this temperature. Most of the radiant energy is emitted in the near infrared region at a wavelength of 1-4 μm. The aluminosilicate glass is at least partially transparent to this radiation, so that thermal radiation is not absorbed in the thin surface layer and a relatively slight temperature difference could be achieved in the glass ribbon . Conventional heating elements are not an advantageous heat source for the implementation of the method according to the invention.

At the beginning and end of each segment S _n , the temperature T _ceiling above the glass ribbon and the temperature T _floor below the glass ribbon were measured with a thermocouple in the middle region of the glass ribbon. Various temperature treatments of the glass ribbon were performed according to Table 2. Downstream of the lehr, the glass ribbon was cut into large float sheet glass and small float sheet glass of size 217 mm × 130 mm for performing strain measurements. The float glass sheet was not subjected to any further processing such as a cleaning, grinding or polishing process prior to the chemical strengthening process.

The float sheet glass had a slight strain, hereinafter called initial strain, prior to chemical strengthening. The initial strain was measured on a plate of size 217 mm × 130 mm corresponding to a diagonal of about 10 inches according to DIN 50441-5: 1998-05 according to the above method. Just as in the case of standardized strain W _s , the initial strain is marked with a positive symbol when the upper side of the float glass is convex and with a negative symbol when the tin bath side of the float glass is convex. Be done. Therefore, the direction of warpage relative to the orientation of the float glass sheet in the float process is taken into account in the initial strain, unlike the definition of strain according to DIN 50441-5: 1998-05.

Chemical strengthening of the plate was carried out continuously at 416 ° C. in 100% potassium nitrate, ie at a temperature 200 K below T _g for 4 hours. For all samples, the compressive stress CS of the surface after chemical strengthening was in the range of 850 MPa to 950 MPa, and the depth DoL of the strengthening layer was 30 μm to 45 μm. The CS and DoL were measured stress-optically in the usual way using a Luceo instrument FSM 6000.

The strain after chemical strengthening was measured according to DIN 50441-5: 1998-05 on a plate of size 217 mm × 130 mm, corresponding to a diagonal of about 10 inches. The standardized strain W _s after chemical strengthening was calculated from the measured strain and the plate thickness D if necessary based on the difference in plate thickness.

Table 2 tabulates the temperatures and heating power in the individual segments of the lehr, as well as the measured and initial strain values and the standardized strain W _s .

Example V1 is a comparative example according to the prior art with substantially symmetrical cooling. In segment 1 of the lehr, a 0.7 mm thick glass ribbon was placed under the influence of a heating power of 43 kW from above and below. The upper temperature T _ceiling of the glass ribbon is 618 ° C. at the beginning of segment 1 and 611 ° C. at the end, and the temperature T _floor below the glass ribbon is 613 ° C. and finish at the beginning of segment 1 It was 602 ° C in part. Therefore, the temperature difference ΔT _lehr was 5 K at the beginning of the segment and 9 K at the end. As a result, in the glass ribbon with a thickness of 0.7 mm, a temperature difference TΔ _glass of 0.18 K at the beginning of segment 1 and 0.17 K at the end was found. The glass sheet had a slight initial strain of 24 μm and a very high standardized strain of 853 μm.

Example V2 is likewise a comparative example with cooling somewhat asymmetric, but still not corresponding to the cooling according to the invention. In segment 1 of the lehr, a 0.57 mm thick glass ribbon was placed under the influence of a heating power of 24 kW each from above and below. The upper temperature T _ceiling of the glass ribbon is 652 ° C. at the beginning of segment 1 and 611 ° C. at the end, and the temperature T _floor below the glass ribbon is 648 ° C. and finished at the beginning of segment 1 It was 592 ° C in part. Therefore, the temperature difference T Δ _lehr was 5 K at the beginning of segment 1 and 19 K at the end. The cause of the temperature difference Δ _lehr which increases in segment 1 despite the symmetrical heating up and down is the strong heating of the asymmetry in segment 2 of the _lehr , which has a retroactive effect on segment 1. As a result, in the glass ribbon, a temperature difference TΔ _glass of 0.17 K at the beginning of segment 1 and 0.62 K at the end is found. The flat glass has an initial strain of 67 μm and is high, but with a clearly reduced 799 μm strain after chemical strengthening as compared to Example A, which reveals a standardized strain W _s after chemical strengthening of 443 μm. did. The difference in Na ₂ O concentration on the upper side and the float bath side is in the range of -0.1 ± 0.1% by mass before chemical strengthening by fluorescent X-ray analysis measurement, and it is further reduced after chemical strengthening At that time, it is in the range of -0.1% by mass to 0.0% by mass. Therefore, float glass has very good symmetry with respect to Na ₂ O concentration.

Example A is an example according to the invention with strong asymmetry cooling. In segment 1 of the lehr, a 0.57 mm thick glass ribbon is placed under the influence of the heating power of 144.4 kW from above, and the heating power of 0 kW from below (that is, the lower heating in segment 1 was turned off) Under the influence of The upper temperature T _ceiling of the glass ribbon is 659 ° C. at the beginning of segment 1 and 645 ° C. at the end, and the temperature T _floor below the glass ribbon is 635 ° C. and finish at the beginning of segment 1 It was 622 ° C in part. Therefore, the temperature difference T Δ _lehr was 24 K at the beginning of segment 1 and 23 K at the end. As a result, in the glass ribbon, a temperature difference TΔ _glass of 0.76 K at the beginning of segment 1 and 0.74 K at the end is found. The glass sheet had an initial strain of -50 μm and a clearly reduced strain of 419 μm after chemical strengthening, which revealed a standardized strain W _s after chemical strengthening of 231 μm. It should be noted that in the case of Example A, an initial strain of −50 μm with the opposite sign is obtained, which is the reverse of the strain after chemical strengthening.

  However, the reduction in strain compared to Example V2 can not be attributed to the formation of an initial strain with the opposite sign, since the initial strain has only changed 117 μm from 67 μm to -50 μm. The strain is reduced by 380 μm from 799 μm to 419 μm. The change in strain after chemical strengthening is more than three times the change in initial strain. Up to a third of the 380 μm strain drop can be attributed to the change in initial strain.

The difference in Na ₂ O concentration on the upper side and the float bath side is in the range of -0.1 ± 0.1% by weight before chemical strengthening and further decreases after chemical strengthening according to X-ray fluorescence analysis, At that time, it is in the range of -0.1% by mass to 0.0% by mass. Such low differences in Na ₂ O concentration are generally less detectable by X-ray fluorescence analysis, so that defined measurement errors already exist in the order of differences in Na ₂ O concentration. Therefore, float glass has very good symmetry with respect to Na ₂ O concentration. Thus, the glazing according to the invention is characterized in that the upper side and the tin bath side have a very similar chemical composition, where substantially the differences unavoidable by the float method, eg a tin-containing surface layer Exists. However, float sheet glass has approximately the same Na ₂ O concentration on both sides and can be further processed without regard to the float bath side and the tin bath side in the subsequent manufacturing process, eg coating. The slight Na ₂ O leaching on the tin bath side is likely to be attributed to the tin bath side SO ₂ coating to avoid scratching by the conveyor rollers. Float glass after chemical strengthening not chemically strengthened prior but also the difference between the concentration of Na ₂ O in concentration of Na ₂ O Na ₂ O _top and tin bath side of the upper float plate glass -0.2 mass percent 0 Less than 2% by mass.

FIG. 1 schematically shows the relevant part of the cross section of a float glass device suitable for carrying out the method according to the invention. The withdrawal zone (9) is the end of the float bath (10) with molten metal (13) in the withdrawal direction (8), the dross box (11), where the tin bath side (15) and the upper side (16) The glass ribbon (14) having is passed through the first conveyor roller (17) after being taken out of the molten metal (13), and the lehr (12), where the glass ribbon (14) is cooled to a low stress state Have. Annealing furnace (12) has a segment which is arranged in a plurality of rapid succession (12a, 12b), in each segment, the glass ribbon (14), above the heating element (31a, 31b) heating power from above by P _{n , above} , and under the influence of the heating power P _{n, below} from below with the lower heating elements (32a, 32b) respectively. Here, the heating elements (31, 32) were conventionally designed and emitted thermal radiation approximately equivalent to that of the black body at the temperature of the furnace. Here, most of the radiant energy is emitted at a wavelength of 1 to 4 μm in the near infrared region. Moreover, it is possible by the supply pipe (34), a protective film is exposed to SO ₂ containing gas stream tin bath side (15) with lehr (12).

  FIG. 2 schematically shows a plan view of part of the withdrawal zone shown in FIG. In the top view, it can be read that the heating elements (31, 32) respectively extend over the width of the glass ribbon (14) so that a uniform temperature is achieved across the width of the glass ribbon (14).

FIG. 3 schematically shows the temperature profile along the extraction zone, where the upper temperature T _top of the glass ribbon passes through a specific range with respect to the glass transition temperature T _{g according} to the area of the extraction zone , Parts A, B1 and B2 are defined. In fact, the temperature profile does not have to be uniform, but may have local or temporary maxima, in particular in the region of the dross box. Therefore, the temperature profiles shown are merely exemplary.

  FIG. 4 shows a sheet glass (1) according to the invention, having a tin bath side (15) which was in contact with the molten metal (13) during the float process, and an opposite upper side (16). The glass sheet preferably has a sulfur-containing coating (2) on the tin bath side (15).

FIG. 5 shows the calculated temperature difference ΔT _glass in a glass ribbon having a thickness D of 0.5 mm as a function of the ceiling temperature and the floor temperature in the lehr, as obtained from simulation calculations. With a floor temperature T _{floor of} 605 ° C., a ceiling temperature T _{ceiling of} 680 ° C. and a temperature difference ΔT _lehr of 75 K, for example, a temperature difference ΔT _{glass of} only _2.5 K is obtained.

Reference Signs List 1 float sheet glass 2 sulfur-containing coating 8 drawing direction 9 drawing zone 10 float bath 11 dross box 12 lehr 12a lehr, segment 1
12b Slow cooling furnace, segment 2
13 Molten metal 14 Glass ribbon 15 Glass ribbon / tin bath side of glass sheet 16 Glass ribbon / upper side of glass glass 17 Conveyor roller 30 Forming gas atmosphere 31a Upper annealing furnace heating, segment 1
31b Upper annealing furnace heating, segment 2
32a Lower annealing furnace heating, segment 1
32b Lower annealing furnace heating, segment 2
34 SO ₂ supply pipe

Claims (17)

A float process for producing float sheet glass (1) in a draw out zone (9) comprising a float bath (10) and a lehr (12), wherein molten glass is continuously applied onto molten metal (13) Thickness with a tin bath side (15) facing out the molten metal (13) and drawn out in the withdrawal direction (8) and an upper side (16) facing away from the molten metal (13) Form a glass ribbon (14) of D, where the glass ribbon (14) is cooled over the float bath (10), lifted from the molten metal (13) and transferred further to the lehr (12), where , The glass ribbon (14) has two peripheral regions along its longitudinal sides, which start from the longitudinal sides and extend towards the center of the ribbon and between the peripheral regions Effective area Are arranged, wherein the glass ribbon (14) has a temperature T _top at the upper (16) and having a temperature T _bottom at Suzuyoku side (15), wherein the bed temperature of the annealing furnace is a T _floor, and ceiling temperature of lehr in the float process is a T _ceiling, at a temperature T _top is T _g + 20K from the drawer zone falls T _g -20K portion a of the upper side (16), the glass ribbon (14 temperature difference [Delta] T _glass at any point x in the effective region of) = T _top -T _bottom is rather high than 0K, and the temperature difference at any point x in the effective area of the glass ribbon (14) in the portion a [Delta] T Said float method, characterized in that _lehr = T _ceiling -T _floor is at least 15K . The temperature difference ΔT _glass = T _top −T _bottom at any point x in the effective area of the glass ribbon (14) in part A is at least 0.25 K, preferably at least 0.5 K, more preferably at least 1 K, The float process according to claim 1, which is particularly preferably at least 1.5K. Temperature difference _ΔT lehr = T _ceiling -T _floor at any point x in the effective area of the glass ribbon (14) in the portion A is, even without least 30K, more preferably at least 50K, particularly preferably is at least 75K The float method according to claim 1 or 2. The ratio of Δ T _glass / D is at least 0.5 K / mm, preferably at least 1K / mm, further preferably at least 2K / mm, particularly preferably at least 3K / mm, Claims 1 to 3 The float method according to any one of the above. The ratio of _Δ T lehr / D is at least 30K / mm, preferably at least 60K / mm, further preferably at least 100K / mm, particularly preferably at least 150K / mm, any of claims 1 to 4 The float method described in paragraph 1 below. The molten glass is at least 5% by weight, preferably has a Al ₂ O ₃ ratio of at least 10 wt%, a float method according to any one of claims 1 to 5.   A method according to any of the preceding claims, wherein the float glass sheet (1) can be highly chemically strengthened to a surface compressive stress CS of at least 600 MPa and a depth DoL of a reinforcement layer of at least 30 μm. Float method.   The glass ribbon (14) has a thickness D in the effective area of less than or equal to 2.0 mm, preferably less than or equal to 1.0 mm, particularly preferably between 0.3 mm and 0.7 mm. The float method according to any one of the above. In part B1 of the drawer zone temperature T _top of the upper falls T _g + 20K from T _g + 50K, or in part B2 of the drawer zone, in which the upper temperature T _top falls T _g -50K from T _g -20K, wherein The temperature difference ΔT _glass at any point x in the active area of the glass ribbon (14) is at least 0.25 K, preferably at least 0.5 K, more preferably at least 1.0 K, particularly preferably at least 1. 9. A float process according to any one of the preceding claims, which is 5K. In said part B1 or B2, the temperature difference ΔT _lehr at any point x in the active area of the glass ribbon (14) is at least ₁₅ K, preferably at least 30 K, more preferably at least 50 _K. Float method described. In any part C of the _lehr (12) having a length of at least 2 m, preferably at least 5 m, the temperature difference ΔT _lehr = T _ceiling -T at any point x in the active area of the glass ribbon (14) _11. A float process according to any one of the preceding claims, wherein the _floor is at least 15 K, preferably at least 30 K, particularly preferably at least 50 K. The glass ribbon (14) is _subjected to heating power P _below from _below and heating power P _above from _{above in} the _lehr (12), where it has a length of at least 2 m, preferably at least 5 m. 12. The method according to claim 1, wherein in part D of the _lehr (12), the ratio P _below / P _above is less than 0.5, preferably less than 0.3, particularly preferably less than 0.1. The float method according to any one of the above.   The glass ribbon (14) is heated by a heat source (20) from above in the lehr (12) and the heat released (21) is preferably in the upper half of the glass ribbon (14), in particular 13. The float process as claimed in claim 1, wherein the surface layer is preferably absorbed at a thickness of up to 50 [mu] m.   The float process according to any of the preceding claims, wherein the glass ribbon (14) is cooled from below in the lehr (12). Obtained by the float process for the production of float glass (1) according to claim 1, less than standardized strain W _s after the chemical strengthening 300 [mu] m, advantageously less than 200 [mu] m, particularly preferably float below 100μm is A method of producing sheet glass (1) , wherein said sheet before chemical strengthening has an initial strain of less than +20 μm, preferably less than 0 μm, particularly preferably less than −20 μm . Manufacturing method . Upper concentration of Na ₂ O Na ₂ O _top and difference [Delta] Na ₂ O = Na ₂ the concentration of Na ₂ O in the tin bath side O of the float glass after chemical strengthening (1) _top -Na ₂ O _bottom -0. The method for producing a float sheet glass (1) according to claim 15, which is more than 2% by mass and less than 0.2% by mass. Difference concentration of Na ₂ O in the upper concentration of Na ₂ O Na ₂ O _top and tin bath side of the float glass that has not been chemically strengthened _{(1) ΔNa 2 O = Na} 2 O top -Na 2 O bottom -0 The method for producing a float sheet glass (1) according to claim 15 , which is larger than 2% by mass and smaller than 0.2% by mass.