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.

A method for manufacturing a bainite high-strength seamless steel tube, comprising the following steps: smelting, manufacturing a billet, heating, perforating, rolling, stretch reducing or sizing to obtain tube, and cooling. In the cooling step, the quenching starting temperature is controlled to be at least 20° C. higher than the Ar3 temperature of the steel grade; the finish cooling temperature is controlled to be within a range between T1 and T2, where T1=519-423 C-30.4Mn, T2=780-270 C-90Mn, and the units of the T1 and the T2 are ° C.; in the formulas, C and Mn respectively represent the mass percents of element C and element Mn of the steel grade, the content of the element C is 0.06-0.2%, and the content of the element Mn is 1-2.5%; the cooling rate is controlled to be 15-80° C./s; and the finished product of the bainite high-strength seamless steel tube is directly obtained after the cooling step. The manufacturing of a bainite high-strength seamless steel tube using the method requires neither the addition of precious alloying elements nor the subsequent heat treatment. Therefore the production costs are low.

A method for manufacturing a bainite high-strength seamless steel tube, comprising the following steps: smelting, manufacturing a billet, heating, perforating, rolling, stretch reducing or sizing to obtain tube, and cooling. In the cooling step, the quenching starting temperature is controlled to be at least 20° C. higher than the Ar3 temperature of the steel grade; the finish cooling temperature is controlled to be within a range between T1 and T2, where T1=519-423 C-30.4Mn, T2=780-270 C-90Mn, and the units of the T1 and the T2 are ° C.; in the formulas, C and Mn respectively represent the mass percents of element C and element Mn of the steel grade, the content of the element C is 0.06-0.2%, and the content of the element Mn is 1-2.5%; the cooling rate is controlled to be 15-80° C./s; and the finished product of the bainite high-strength seamless steel tube is directly obtained after the cooling step. The manufacturing of a bainite high-strength seamless steel tube using the method requires neither the addition of precious alloying elements nor the subsequent heat treatment. Therefore the production costs are low.

A mandrel bar of a piercing machine includes: a bar body; a coolant channel formed inside the bar body and through which a coolant flows; an inner surface cooling mechanism which is disposed in a cooling zone and is connected to the coolant channel and which, during piercing-rolling or during elongating rolling, ejects the coolant to outside of the bar body to cool an inner surface portion of a hollow shell inside the cooling zone; and an inner surface damming mechanism which is disposed adjacent to the cooling zone on a rearward side of the cooling zone and which, during piercing-rolling or during elongating rolling, suppresses the coolant that is ejected to outside of the bar body from contacting the inner surface portion of the hollow shell that is positioned rearward of the cooling zone.

A mandrel bar of a piercing machine includes: a bar body; a coolant channel formed inside the bar body and through which a coolant flows; an inner surface cooling mechanism which is disposed in a cooling zone and is connected to the coolant channel and which, during piercing-rolling or during elongating rolling, ejects the coolant to outside of the bar body to cool an inner surface portion of a hollow shell inside the cooling zone; and an inner surface damming mechanism which is disposed adjacent to the cooling zone on a rearward side of the cooling zone and which, during piercing-rolling or during elongating rolling, suppresses the coolant that is ejected to outside of the bar body from contacting the inner surface portion of the hollow shell that is positioned rearward of the cooling zone.

A seamless steel pipe has a t/D.sub.out of 0.05 to 0.40, where t is a wall thickness (mm) of the seamless steel pipe, and D.sub.out is an outside diameter (mm) of the seamless steel pipe. The seamless steel pipe has a maximum depth d.sub.max (mm)≤0.350 mm for defects in an inner surface of the steel pipe on a cross section perpendicular to a pipe axis. The seamless steel pipe has an average defect depth d.sub.ave (mm)≤0.200 mm for defects having a depth of 0.050 mm or more in the inner surface of the steel pipe. The seamless steel pipe, per millimeter of an inner circumferential length of the pipe, has at most 30 defects having a depth of 0.050 mm or more in the inner surface of the steel pipe.

A seamless steel pipe has a t/D.sub.out of 0.05 to 0.40, where t is a wall thickness (mm) of the seamless steel pipe, and D.sub.out is an outside diameter (mm) of the seamless steel pipe. The seamless steel pipe has a maximum depth d.sub.max (mm)≤0.350 mm for defects in an inner surface of the steel pipe on a cross section perpendicular to a pipe axis. The seamless steel pipe has an average defect depth d.sub.ave (mm)≤0.200 mm for defects having a depth of 0.050 mm or more in the inner surface of the steel pipe. The seamless steel pipe, per millimeter of an inner circumferential length of the pipe, has at most 30 defects having a depth of 0.050 mm or more in the inner surface of the steel pipe.

A steel pipe for a fuel injection pipe has a chemical composition consisting of, by mass %: C: 0.17 to 0.27%, Si: 0.05 to 0.40%, Mn: 0.30 to 2.00%, P: 0.020% or less, S: 0.0100% or less, O: 0.0040% or less, Ca: 0.0010% or less, Al: 0.005 to 0.060%, N: 0.0020 to 0.0080%, Ti: 0.005 to 0.015%, Nb: 0.015 to 0.045%, Cr: 0 to 1.00%, Mo: 0 to 1.00%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, V: 0 to 0.15%, and the balance: Fe and impurities. The metal micro-structure consists substantially of tempered martensite, or tempered martensite and tempered bainite. The hardness is within the range of 350 to 460 HV1. A lattice spacing of a (211) diffraction plane measured by CoKα characteristic X-ray diffraction is 1.1716 Å or less, and a half-value width of the (211) diffraction plane is 1.200° or less. The number density of cementite having a diameter of 50 nm or more is 20/μm.sup.2 or less.

The seamless steel pipe of the present embodiment consists of in mass %, C: 0.21 to 0.35%, Si: 0.10 to 0.50%, Mn: 0.05 to 1.00%, P: 0.025% or less, S: 0.010% or less, Al: 0.005 to 0.100%, N: 0.010% or less, Cr: 0.05 to 1.50%, Mo: 0.10 to 1.50%, Nb: 0.010 to 0.050%, B: 0.0003 to 0.0050%, and Ti: 0.002 to 0.050%, the balance being Fe and impurities. In a main body region of the seamless steel pipe, a grain size number of prior-austenite grain conforming to ASTM E112 is 7.0 or more, a difference between a maximum value and a minimum value of the grain size number is 1.0 or less, yield strength is 655 to less than 862 MPa, and a difference between a maximum value and a minimum value of tensile strength is 27.6 MPa or less.