PURPOSE: To obtain inexpensively a steel pipe having a different-thickness section by disposing alternately drawing rolls of passes having respectively specific ellipticity and outside-diameter drawing rates.
CONSTITUTION: A stretch reducer 3 disposed alternately with the drawing rolls of the pass having 1.03W1.30 ellipticity and 3W10% outside diameter drawing rate and the drawing rolls of the pass having 0.99W1.01 ellipticity and 0W1% outside diameter drawing rate are used. The base steel pipe 2 heated in a heating furnace 1 is rolled alternately by the odd-numbered drawing rolls, i.e., the drawing rolls provided with the large ellipticity and outside diameter drawing rate and the even-numbered rolls, i.e., the drawing rolls providing with the small ellipticity and outside diameter drawing rate of the stretch reducer 3. The pipe is rolled by the even-numbered rolls under the tension of 0W0.8 times the deformation resistance of the pipe applied thereto. The steel pipe having the different- thickness section is inexpensively obtd. by repeating alternately the rolling to increase only the wall thickness of the flange parts.
SUMIMOTO DAIGO
ICHIHARA HIROHISA
