Skew rolling apparatus, method for manufacturing seamless pipe shell, and method for manufacturing seamless steel pipe

A skew rolling apparatus comprising:

• a skew pierce rolling mill configured to pierce roll a steel material into a seamless pipe shell;
• a skew outside-diameter mill following the skew piercing mill in a pass direction of the seamless pipe shell in the skew rolling apparatus,
• the skew piercing mill comprising a plurality of pierce rolling rolls provided circumferentially around a pass line, a plug provided between the plurality of piercing rolls, and a bar configured to hold the plug,
• the skew outside-diameter mill comprising a plurality of outside-diameter rolling rolls provided circumferentially around the pass line,
• the skew rolling apparatus satisfying the following formulae (1) and (2),
• 3×(1.2PO+2G)≤LS≤0.8BL (1)
• 1.2PO+2G>DS≥DB+2G (2),
• where LS is a distance (mm) between a rolling zone of the pierce rolling rolls and a rolling zone of the outside-diameter rolling rolls, DS is a roll gap between the plurality of outside-diameter rolling rolls, PO is an outside diameter (mm) of the plug, G is a gap (mm) between the plug and the pierce rolling rolls, BL is a length (mm) of the bar, and DB is an outside diameter (mm) of the bar.