Method and apparatus for manufacturing hollow steel bars

Abstract

A method of manufacturing hollow steel bars comprising the steps of preparing a hollow billet with the dimensions meeting a condition expressed by the following formula (1) by piercing a steel billet with a piercer after heating, inserting a mandrel as an inner surface sizing tool into a hollow billet, and then rolling the hollow billet on a cross-rolling mill having three rolls arranged around a pass line to provide plastic working for reduction of the outside diameter and adjustment of the wall thickness of the hollow billet so as to meet a condition expressed by the following formula (2), and a manufacturing apparatus comprising an electric resistance heating unit, the piercer, and the cross-rolling mill, wherein t.sub.0 /d.sub.0 >0.1 (1) Rt<0.55Rd (2) where t.sub.0 =wall thickness of hollow billet before cross rolling d.sub.0 =outside diameter of hollow billet before cross rolling Rt=wall thickness reduction (%), Rt=(t.sub.0 -t.sub.1)/t0.times.100 Rd=outside diameter reduction (%), Rd=(d.sub.0 -d.sub.1)/d.sub.0 .times.100 t.sub.1 =wall thickness of hollow steel bar after cross rolling d1=outside diameter of hollow steel bar after cross rolling By means of such a method and system as stated above, long and thick-walled hollow steel bars of small diameter, approximately, 20-70 mm in the outside diameter, 0.25-0.40 in the wall thickness to outside diameter ratio (t.sub.1 /d.sub.1), and 2-6 m in length, can be produced with high dimensional accuracy and at low cost.

Dependencies are inferred from the claim text.