An integrated transverse rolling mill for rolling seamless tubes with skewed axes comprising a piercing stand (2) with skewed axis rolling rolls for piercing a first tube (7) having a first length which is shorter than the space which separates it from the subsequent finishing stand (3) in cooperation with the stake (6), which has the first piercing tip (20) at its end and the second finishing tip (21) at the finishing stand (3). The finishing stand (3) is arranged downstream of the piercing stand (2) so that during the rolling operations the front end of the first tube (7) may only be gripped by the finishing stand (3) once the rear end of the first tube (7) is no longer gripped by the rolling stand (2).

An integrated transverse rolling mill for rolling seamless tubes with skewed axes comprising a piercing stand (2) with skewed axis rolling rolls for piercing a first tube (7) having a first length which is shorter than the space which separates it from the subsequent finishing stand (3) in cooperation with the stake (6), which has the first piercing tip (20) at its end and the second finishing tip (21) at the finishing stand (3). The finishing stand (3) is arranged downstream of the piercing stand (2) so that during the rolling operations the front end of the first tube (7) may only be gripped by the finishing stand (3) once the rear end of the first tube (7) is no longer gripped by the rolling stand (2).

Low-alloy oil-well steel pipe includes a composition consisting, in mass %, of C: 0.40 to 0.65%, Si: 0.05 to 0.50%, Mn: 0.10 to 1.00%, P: 0.020% or less, S: 0.0020% or less, Cu: 0.15% or less, Cr: 0.40 to 1.50%, Mo: 0.50 to 2.50%, V: 0.05 to 0.25%, Ti: 0 to less than 0.01%, Nb: 0.01 to 0.2%, sol. Al: 0.010 to 0.100%, N: 0.006% or less, B: 0 to 0.0015%, and Ca: 0 to 0.003%, the balance being Fe and impurities. The structure has tempered martensite and 0 to less than 2% volume ratio of retained austenite. A grain size number of prior-austenite grain in the structure is 9.0 or more. An equivalent circular diameter of a sub-structure surrounded by a boundary having a crystal orientation difference of 15 or more from a packet boundary, a block boundary and a lath boundary is 3 m or less for the tempered martensite.

A rolling mill includes three or more rolling rolls aligned along a circumferential direction and arranged so each rotary shaft is skewed with respect to a pass line of a material to be rolled, wherein the material to be rolled made of a pipe or bar material passes between the rolling rolls while being rotated to undergo diameter reducing rolling. At least one rolling roll selected from the three or more rolling rolls is smaller in roll diameter than at least one other rolling roll. When at least one rolling roll having a relatively maximum roll diameter defined as a maximum diameter rolling roll and at least one rolling roll smaller in roll diameter than the maximum is defined as a small diameter rolling roll, the small diameter rolling roll has a roll diameter equal to or less than 90% of the roll diameter of the maximum diameter rolling roll.

A rolling mill includes three or more rolling rolls aligned along a circumferential direction and arranged so each rotary shaft is skewed with respect to a pass line of a material to be rolled, wherein the material to be rolled made of a pipe or bar material passes between the rolling rolls while being rotated to undergo diameter reducing rolling. At least one rolling roll selected from the three or more rolling rolls is smaller in roll diameter than at least one other rolling roll. When at least one rolling roll having a relatively maximum roll diameter defined as a maximum diameter rolling roll and at least one rolling roll smaller in roll diameter than the maximum is defined as a small diameter rolling roll, the small diameter rolling roll has a roll diameter equal to or less than 90% of the roll diameter of the maximum diameter rolling roll.

A rolling machine has pairs of rolls in a horizontal line of roll pair locations, with each pair including upper and lower rolls opposed across a vertical gap. A sliding frame is mounted on the rolling machine for movement alongside the rolling machine in a direction parallel to the line of roll pair locations. A lifting frame is mounted on the sliding frame for movement vertically relative to the sliding frame beside each of the roll pair locations. Other parts include a roll cassette and a load tray. The load tray is configured to carry the roll cassette, and is mounted on the lifting frame for movement relative to the lifting frame transversely into the vertical gap between the rolls at each roll pair location.