Method of manufacturing flange for wind towers using ring rolling method

A method of manufacturing a flange through a ring rolling process using a ring rolling machine which includes a main roll for pressing a circumferential outer surface of a blank, a pressure roll for pressing a circumferential inner surface of the blank, and a pair of axial rolls for pressing upper and lower surfaces of the blank, the method comprising:

• expanding by ring rolling an inner diameter and outer diameter of the blank using the ring rolling machine;
• transferring the pressure roll vertically so that a protrusion provided on a circumferential outer surface of the pressure roll comes into contact with the circumferential inner surface of the blank;
• and
• forming by ring rolling a depression in the circumferential inner surface of the blank using the protrusion of the pressure roll,
• wherein in said forming the depression, as a thickness of the blank changes from S1 to S2, a cross-sectional area A1 of the protrusion and a cross-sectional area A2 of the depression satisfy Equation (a), where Equation (a) is S1>S2(1+A1/A2).