PURPOSE: To offer a means for restraining the generation of wrinkle-like defects on the outer surface when a solid billet is pierced by hot piercing rolling.
CONSTITUTION: The solid billet 4 which is heated in a rotary hearth furnace 1 is worked to reduce the circumferential length over the full circumference and entire length of that billet with a rolling device 2 for reducing billet diameter. By this work, the grain size of crystal in the surface part is made at least ≥No.3 of ASTM grain size number. After that, piercing rolling is executed with a inclined roll type piercing mill 3. Because the grain size in the surface of the solid billet is refined by the work just before piercing and recrystallization, a hollow shell without wrinkle on the outer surface is obtained. The seamless steel tube with the high grade outside is obtained. This method can be easily applied to an existing process for manufacturing seamless steel tube.
AKIYAMA MASAYOSHI
NAKANISHI TETSUYA
Next Patent: MANUFACTURE OF DULL ROLL AND DULL-FINISHED STEEL SHEET