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 cross-rolling mill for rolling a block over a mandrel forms a hollow block. It includes a plurality of working rollers, each of which exerts a substantially radially aligned rolling force onto the block. The working rollers are supported in a roll stand, and the gap between the working rollers and preferably also the alignment of the rolling axis of at least one of the working rollers relative to the block can be modified. Hydraulic actuators, preferably hydraulic capsules, are provided in order to modify the rolling gap and preferably also the alignment of the rolling axis of at least one of the working rollers relative to the block.

A cross-rolling mill for rolling a block over a mandrel forms a hollow block. It includes a plurality of working rollers, each of which exerts a substantially radially aligned rolling force onto the block. The working rollers are supported in a roll stand, and the gap between the working rollers and preferably also the alignment of the rolling axis of at least one of the working rollers relative to the block can be modified. Hydraulic actuators, preferably hydraulic capsules, are provided in order to modify the rolling gap and preferably also the alignment of the rolling axis of at least one of the working rollers relative to the block.

A method disclosed herewith is a method for producing a first seamless metal tube with a first wall thickness and a second seamless metal tube with a second wall thickness by using a three-roll-type inclined rolling mill, and the method includes a first inclination rolling step (#5), a setting changing step (#10), and a second inclination rolling step (#15). At the first inclination rolling step, a first workpiece is rolled by the inclined rolling mill. At the setting changing step, a setup condition of the inclined rolling mill is changed in a manner (a) or (b) as described below. At the second inclined rolling step, a second workpiece is rolled by the inclined rolling mill under the changed condition. (a) When the second wall thickness is smaller than the first wall thickness, the cross angle of each of the inclined rolls is made greater than the cross angle set for the first inclination rolling step. (b) When the second wall thickness is larger than the first wall thickness, the cross angle of each of the inclined rolls is made smaller than the cross angle set for the first inclination rolling step.

The seamless pipe in which a thin-walled portion in a pipe circumferential direction is formed in a pipe axial direction, in which a line segment formed by connecting one end and the other end of the thin-walled portion along a pipe surface with a shortest distance in a formation direction of the thin-walled portion is inclined at an angle α of 5.0° or more with respect to the pipe axial direction. It is preferable that one end and the other end of the thin-walled portion are set from a region in a pipe selected with a shorter length between a length of 1.0 m in the pipe axial direction and 90% of a length in the pipe axial direction where the thin-walled portion turns once in the pipe circumferential direction.

The seamless pipe in which a thin-walled portion in a pipe circumferential direction is formed in a pipe axial direction, in which a line segment formed by connecting one end and the other end of the thin-walled portion along a pipe surface with a shortest distance in a formation direction of the thin-walled portion is inclined at an angle α of 5.0° or more with respect to the pipe axial direction. It is preferable that one end and the other end of the thin-walled portion are set from a region in a pipe selected with a shorter length between a length of 1.0 m in the pipe axial direction and 90% of a length in the pipe axial direction where the thin-walled portion turns once in the pipe circumferential direction.

Metallic tubular products having improved collapse resistance are disclosed. The metallic tubular products are produced by compressive forming processes. The method comprises identifying the types of stress that can be applied in order to change the residual stress profile of metallic tubular products, such as those that have completed a straightening process, and results in a residual stress profile that improves collapse resistance. The metallic tubular products are subjected to radial compression processing to control the residual stress profile and to enhance collapse resistance. The radial compression process may be used after the tubular product has been subjected to a straightening process.

Metallic tubular products having improved collapse resistance are disclosed. The metallic tubular products are produced by compressive forming processes. The method comprises identifying the types of stress that can be applied in order to change the residual stress profile of metallic tubular products, such as those that have completed a straightening process, and results in a residual stress profile that improves collapse resistance. The metallic tubular products are subjected to radial compression processing to control the residual stress profile and to enhance collapse resistance. The radial compression process may be used after the tubular product has been subjected to a straightening process.

A non-destructive method for determination of twist angle of an outlet product during rolling of an inlet product into said outlet product, comprising the steps of measuring a rotational inlet speed of the inlet product during said rolling, measuring a rotational outlet speed of the corresponding outlet product during said rolling in order to determine a delta rotation, measuring a longitudinal speed and determining a twist angle from said delta rotation and said longitudinal outlet and/or inlet speed.

A non-destructive method for determination of twist angle of an outlet product during rolling of an inlet product into said outlet product, comprising the steps of measuring a rotational inlet speed of the inlet product during said rolling, measuring a rotational outlet speed of the corresponding outlet product during said rolling in order to determine a delta rotation, measuring a longitudinal speed and determining a twist angle from said delta rotation and said longitudinal outlet and/or inlet speed.