The production method of a seamless steel pipe includes a heating step of heating an Nb-containing steel material to 800 to 1030° C., a pipe-making step of producing a hollow shell by performing piercing-rolling or elongation-rolling on the Nb-containing steel material, by using a piercing mill including a plurality of skewed rolls, a plug disposed between the plurality of skewed rolls, and a mandrel bar, and a cooling step immediately after rolling, of carrying out cooling using a cooling liquid on a hollow shell portion that passes between rear ends of the plurality of skewed rolls, in the hollow shell, so as to reduce an outer surface temperature of the hollow shell portion to 700 to 1000° C. within 15.0 seconds after the hollow shell portion passes between the rear ends of the plurality of skewed rolls.

A flat rolled metal product (1) is first hot-rolled in at least one rolling stand (2), then fed to a cooling zone (5) arranged downstream of the rolling stand (2) and finally cooled in the cooling zone (5). During the rolling in the rolling stand (2), the flat rolled product (1) is oriented horizontally. Before running into the cooling zone (5) and/or when running into the cooling zone (5), the flat rolled product (1) is turned by a first acute angle (a) about an axis running in the transporting direction (x), so that after completion of the turning about the axis the flat rolled product (1) is oriented obliquely. The flat rolled product (1) is cooled in the cooling zone (5) while it is oriented obliquely. The product (1) is then returned to horizontal orientation.

A hot and cold composite formed square and rectangular steel tube and a production method for the same are provided. The radius of an outer corner of the square and rectangular steel tube meets the following conditions: when t is less than or equal to 6 mm, R is greater than 0 and less than 2.0 t; when t is greater than 6 mm and less than or equal to 10 mm, R is greater than 0 and less than 2.5 t; when t is greater than 10 mm, R is greater than 0 and less than 3.0 t, wherein t is the wall thickness of a straight tube part of the square and rectangular steel tube; R is the radius of each of the outer corners of the four corners of the square and rectangular steel tube; and the wall thickness of each corner of the square and rectangular steel tube is between 1.0 t and 1.8 t.

A hot and cold composite formed square and rectangular steel tube and a production method for the same are provided. The radius of an outer corner of the square and rectangular steel tube meets the following conditions: when t is less than or equal to 6 mm, R is greater than 0 and less than 2.0 t; when t is greater than 6 mm and less than or equal to 10 mm, R is greater than 0 and less than 2.5 t; when t is greater than 10 mm, R is greater than 0 and less than 3.0 t, wherein t is the wall thickness of a straight tube part of the square and rectangular steel tube; R is the radius of each of the outer corners of the four corners of the square and rectangular steel tube; and the wall thickness of each corner of the square and rectangular steel tube is between 1.0 t and 1.8 t.

A method for controlling or regulating the temperature of a steel strip in hot forming in a hot strip mill. A superordinate open-loop or closed-loop controller has a process model that predetermines the temperature development of the hot strip. The target values of the individual units are adjusted based on this predetermined temperature development.

A method for controlling or regulating the temperature of a steel strip in hot forming in a hot strip mill. A superordinate open-loop or closed-loop controller has a process model that predetermines the temperature development of the hot strip. The target values of the individual units are adjusted based on this predetermined temperature development.

A liquid coolant (6) is fed into a header line (4) by means of a pump assembly (5). Branch lines (9a to 9d), in which control valves (11a to 11d) are arranged, branch off from the header line (4) to application units (10a to 10d). The coolant (6) is applied to a hot rolled material (2) made of metal by means of the application units (10a to 10d), and the rolled material (2) is thus cooled. For limit modulation values (kLim) of the control valves (11a to 11d), a control unit (12) of the cooling unit (3) uses setpoint flows (Ka* to Kd) of the application units (10a to 10d) to determine individual working pressures (pAa to pAd) which must prevail in the header line (4) for the setpoint flows(Ka* to Kd*) to flow in the branch lines (9a to 9d).

A liquid coolant (6) is fed into a header line (4) by means of a pump assembly (5). Branch lines (9a to 9d), in which control valves (11a to 11d) are arranged, branch off from the header line (4) to application units (10a to 10d). The coolant (6) is applied to a hot rolled material (2) made of metal by means of the application units (10a to 10d), and the rolled material (2) is thus cooled. For limit modulation values (kLim) of the control valves (11a to 11d), a control unit (12) of the cooling unit (3) uses setpoint flows (Ka* to Kd) of the application units (10a to 10d) to determine individual working pressures (pAa to pAd) which must prevail in the header line (4) for the setpoint flows(Ka* to Kd*) to flow in the branch lines (9a to 9d).

A method for producing a steel tube including the step of the manufacturing a steel tube having an inner tube wall, an outer tube wall, and a free tube cross-section enclosed by the inner tube wall, wherein after the manufacturing, the steel tube includes at least one contaminant on the inner tube wall. After the manufacturing of the steel tube, the inner tube wall is cleaned by introducing liquid or solid CO.sub.2 into the free tube cross-section, and of applying the liquid or solid CO.sub.2 onto the inner tube wall to remove the contaminant therefrom.

A method for producing a steel tube including the step of the manufacturing a steel tube having an inner tube wall, an outer tube wall, and a free tube cross-section enclosed by the inner tube wall, wherein after the manufacturing, the steel tube includes at least one contaminant on the inner tube wall. After the manufacturing of the steel tube, the inner tube wall is cleaned by introducing liquid or solid CO.sub.2 into the free tube cross-section, and of applying the liquid or solid CO.sub.2 onto the inner tube wall to remove the contaminant therefrom.