Method for manufacturing glass blank for magnetic disk, method for manufacturing glass substrate for magnetic disk

A method for manufacturing a glass substrate for magnetic disk, the method comprising:

• a forming process of forming a disk-shaped glass blank by direct-pressing a molten glass by a pair of dies, wherein press forming is performed while the temperature of the pair of dies, over a period of time until a molten glass is separated from the die after coming into contact with the die, is set at a temperature lower than a glass transition point (Tg) and a mold release material is not attached to the surfaces of the pair of dies;
• and
• a shape processing process including:
• scribing an outside concentric circle and an inside concentric circle on the principal face of the disk-shaped glass blank to form two-concentric-circle shape cutting lines, and
• growing the circular cutting lines by partially heating the disk-shaped glass blank so that a portion outside the outside concentric circle and a portion inside the inside concentric circle are removed due to a difference in thermal expansion of the disk-shaped glass blank to form a ring-shaped glass substrate, wherein a pressing force applied to a cutting line for the outside concentric circle is higher than a pressing force applied to a cutting line for the inside concentric circle.