Glass substrate and its production process

1. A process for producing a glass substrate, which comprises forming molten glass obtained by melting raw materials into a glass ribbon in a float bath, annealing the glass ribbon by a cooling apparatus and cutting the glass ribbon to form the glass substrate,

wherein a hydrogen concentration in an atmosphere of the float bath exceeds 3%, and a glass retention time in the float bath is from 4 to 15 minutes,

wherein the glass substrate has a composition comprising, as represented by mass percentage based on oxides,

SiO 2

  55 to 65%,

Al 2 O 3

  4 to 8%,

MgO

  6 to 9%,

CaO

 0.1 to 5%,

SrO

 0.5 to 6%,

BaO

  0 to 2%,

MgO + CaO + SrO + BaO

 6.6 to 19%,

Na 2 O

  0 to 5%,

K 2 O

 9.5 to 21%,

Na 2 O + K 2 O

  10 to 22%,

ZrO 2

 0.5 to 5%,

Fe 2 O 3

0.06 to 0.15%,

where the composition has a specific gravity of at most 2.7, an average thermal expansion coefficient from 50 to 350° C. of from 80×10 −7 /° C. to 90×10 −7 /° C., and a glass transition temperature of at least 640° C., and

wherein

when a viscosity is η, the temperature at which log η=2 is satisfied is at most 1,550° C., and

a yellow coloring b* on a surface of the glass is at most 8 after a silver paste is applied on the glass surface, followed by firing, and then the silver paste is removed.

2. The process for producing the glass substrate according to claim 1 , wherein the temperature of the glass ribbon in the float bath is at most 1,200° C., and the temperature of the glass ribbon when the annealing is started is at least 640° C.

3. The process for producing the glass substrate according to claim 1 , wherein an amount of Na 2 O is from 0 to 4.5% an amount of K 2 O is from 10 to 14%.

4. The process for producing the glass substrate according to claim 1 , wherein SiO 2 is present in an amount of from 55 to 64%.

5. The process for producing the glass substrate according to claim 1 , wherein SiO 2 is present in an amount of from 55 to 63%.

6. The process for producing the glass substrate according to claim 1 , wherein SiO 2 is present in an amount of from 55 to 62%.

7. The process for producing the glass substrate according to claim 1 , wherein Al 2 O 3 is present in an amount of from 4 to 7.5%.

8. The process for producing the glass substrate according to claim 1 , wherein Al 2 O 3 is present in an amount of from 4 to 7%.

9. The process for producing the glass substrate according to claim 1 , wherein Al 2 O 3 is present in an amount of from 4 to 6.5%.

10. The process for producing the glass substrate according to claim 1 , wherein MgO is present in an amount of from 6.5 to 9%.

11. The process for producing the glass substrate according to claim 1 , wherein MgO is present in an amount of from 7 to 9%.

12. The process for producing the glass substrate according to claim 1 , wherein MgO is present in an amount of from 7.5 to 9%.

13. The process for producing the glass substrate according to claim 1 , wherein CaO is present in an amount of from 0.5 to 4.5%.

14. The process for producing the glass substrate according to claim 1 , wherein CaO is present in an amount of from 1 to 4.5%.

15. The process for producing the glass substrate according to claim 1 , wherein SrO is present in an amount of from 1.6 to 6%.

16. The process for producing the glass substrate according to claim 1 , wherein SrO is present in an amount of from 3 to 5%.

17. The process for producing the glass substrate according to claim 1 , wherein the composition further comprises at least one member selected from the group consisting of As 2 O 3 , Sb 2 O 3 , P 2 O 5 , F and Cl in an amount of at most 2%.

18. The process for producing the glass substrate according to claim 1 , wherein the composition further comprises at least one member selected from the group consisting of La 2 O 3 , TiO 2 , and SnO 2 in an amount of at most 5%.