Method for tempering glass sheet, and apparatus therefor

A method for tempering a glass sheet, comprising:

• a heating step of preheating the glass sheet to a temperature close to the softening point of the glass sheet;
• a quenching step of blowing a cooling medium to both surfaces in the thickness direction of the heated glass sheet;
• a pre-quenching step performed between the heating step and the quenching step when a vicinity of a center portion in the thickness direction of the glass sheet has a temperature different from surfaces of the glass sheet;
• and
• an internal heating substep of selectively heating the vicinity of the center portion in the thickness direction of the glass sheet, relative to the surfaces, at least in the quenching step, wherein in or subsequent to the pre-quenching step, when the temperature at the center portion in the thickness direction of the glass sheet is designated as x° C., and the temperature at either one of both surfaces in the thickness direction of the glass sheet is designated as y° C., and when the temperature at the center portion in the thickness direction of the glass sheet is at least 620° C. and at most 700° C., x and y are in a relation to satisfy a primary expression of y=ax+b (where a and b are constants), and the constant a in the primary expression is at least 0.5 and at most 0.65, and the constant b in the primary expression is at least 60 and at most 180.