Abstract: A steel pipe is produced by a method including performing diameter-reducing rolling on a steel pipe in a temperature range of from 600° C. to Ac3 with a reduction in diameter of not less than 30%, preferably after heating the steel pipe to temperatures of not lower than Ac1, the steel pipe being produced by seam-welding strip steel, or a method further including the step of performing heat treatment of holding the rolled steel pipe in a temperature range of from 600° C. to 900° C. for a time of 1 second or longer during cooling subsequent to the diameter-reducing rolling or by reheating the rolled steel pipe after the cooling.

Abstract: There are provided a high dimensional accuracy pipe and a manufacturing method thereof, the pipe being in response to wide requirements of sizes, being manufactured at inexpensive cost, and having a sufficient fatigue strength. The detailed structure thereof is as follows. A push-to-pass process is performed in which, while a plug 1 is being charged in a metal pipe 5, the metal pipe is pushed in a hole provided in a die 2 and is then allowed to pass therethrough. As a result, a high dimensional accuracy pipe can be obtained in which at least one of the deviation of the outer diameter, the deviation of the inner diameter, and the deviation of the thickness in the circumferential direction is 3.0% or less as processed.

Abstract: A steel tube having a composition which contains: 0.05 to 0.30% of C; 1.8 to 4.0% of Mn; Si; and Al is subjected to a diameter-reducing rolling process in which the total diameter-reduction rate is no less than 20% and the temperature at which the diameter-reducing rolling process is finished is no higher than 800° C., whereby a structure constituted of martensite and/or bainite or further of ferrite is obtained as a transformation product from the deformed ?. As a result, a steel tube having tensile strength of 1000 MPa or more and excellent three-point-bending property can be obtained. The composition of the steel tube of the present invention may further include at least one type of element selected from the group consisting of Cu, Ni, Cr and Mo, or at least one type of element selected from the group consisting of Nb, V, Ti and B, or at least of one type selected from the group consisting of REM and Ca.

Abstract: A stainless steel tube having excellent formability for secondary operation comprises: a chemical composition including not more than 0.20 mass % of C; not more than 1.5 mass % of Si; not more than 2.0 mass % of Mn; 10-18 mass % of Cr; not more than 0.03 mass % of N; or further at least one type of element selected from the group of: not more than 0.6 mass % of Cu; not more than 0.6 mass % of Ni; not more than 2.5 mass % of Mo; not more than 1.0 mass % of Nb; not more than 1.0 mass % of Ti; and not more than 1.0 mass % of V; Fe as the remainder and the inevitable impurities; and a structure constituted of ferrite or ferrite and martensite, wherein the TE value defined by the following formula exceeds 25,000 Mpa·%, TE=TS×(El+21.9) TS represents the tensile strength in the tube axial direction, and El represents the elongation in such direction.

Abstract: A welded steel pipe is formed by heating or soaking an untreated welded steel pipe having a steel composition containing, on the basis of mass percent: about 0.03% to about 0.2% C, about 2.0% or less of Si, not less than about 1.0% to about 1.5% Mn, about 0.1% or less of P, about 0.01% or less of S, about 1.0% or less of Cr, about 0.1% or less of Al, about 0.1% or less of Nb, about 0.1% or less of Ti, about 0.1% or less of V, and about 0.01% or less of N; and by reduction-rolling the treated steel pipe at a cumulative reduction rate of at least about 35% and a final rolling temperature of about 500° C. to about 900° C. The welded steel pipe exhibits excellent hydroformability, i.e., has a tensile strength of at least about 590 MPa and an n×r product of at least about 0.22. The treated steel pipe is preferably reduction-rolled at a cumulative reduction rate of at least about 20% below the Ar3 transformation point.

Abstract: The invention provides a high-carbon steel pipe having superior cold workability and induction hardenability, and a method of producing the steel pipe. The method comprises the steps of heating or soaking a base steel pipe having a composition containing C: 0.3 to 0.8%, Si: not more than 2%, and Mn: not more than 3%, and then carrying out reducing rolling on the base steel pipe at least in the temperature range of (Ac1 transformation point—50° C.) to Ac1 transformation point with an accumulated reduction in diameter of not less than 30%. A structure in which the grain size of cementite is not greater than 1.0 &mgr;m is obtained, thus resulting in improved cold workability and induction hardenability.

Abstract: The invention provides a high-carbon steel pipe having superior cold workability and induction hardenability, and a method of producing the steel pipe. The method comprises the steps of heating or soaking a base steel pipe having a composition containing C: 0.3 to 0.8%, Si: not more than 2%, and Mn: not more than 3%, and then carrying out reducing rolling on the base steel pipe at least in the temperature range of (Ac1, transformation point −50° C.) to Ac1, transformation point with an accumulated reduction in diameter of not less than 30%. A structure in which the grain size of cementite is not greater than 1.0 &mgr;m is obtained, thus resulting in improved cold workability and induction hardenability.

Abstract: A welded steel pipe is formed by heating or soaking an untreated welded steel pipe having a steel composition containing, on the basis of mass percent: about 0.05% to about 0.2% C; about 0.2% or less of Si; about 1.5% or less of Mn; about 0.1% or less of P; about 0.01% or less of S; about 0.1% or less of Al; and about 0.01% or less of N; and by reduction-rolling the treated steel pipe at a cumulative reduction rate of at least about 35% and a final rolling temperature of about 500° C. to about 900° C. The welded steel pipe exhibits excellent hydroformability, i.e., has a tensile strength of at least about 400 MPa and an n×r product of at least about 0.22. The treated steel pipe is preferably reduction-rolled at a cumulative reduction rate of at least about 20% below the Ar3 transformation point. The welded steel pipe is suitable for forming structural components.

Abstract: A welded steel pipe is formed by heating or soaking an untreated welded steel pipe having a steel composition comprising, on the basis of mass percent: about 0.05% to about 0.3% C; about 2.0% or less of Si; more than about 1.5% to about 5.0% Mn; about 0.1% or less of P; about 0.01% or less of S; about 0.1% or less of Cr; about 0.1% or less of Al; about 0.1% or less of Nb; about 0.3% or less of Ti; and about 0.01% or less of N; and by diameter-reduction-rolling the treated steel pipe at a accumulated diameter reduction rate of at least about 35% and a finish rolling temperature of about 500° C. to about 900° C. The welded steel pipe exhibits excellent hydroformability, i.e., has a tensile strength of about 780 MPa or more and a n×r product of at least about 0.15. The treated steel pipe is preferably diameter-reduction-rolled at a accumulated diameter reduction rate of at least about 20% below the Ar3 transformation point.

Abstract: A steel pipe containing fine ferrite crystal grains, which has excellent toughness and ductility and good ductility-strength balance as well as superior collision impact resistance, and a method for producing the same are provided. A steel pipe containing super-fine crystal grains can be produced by heating a base steel pipe having ferrite grains with an average crystal diameter of di (&mgr;m), in which C, Si, Mn and Al are limited within proper ranges, and if necessary, Cu, Ni, Cr and Mo, or Nb, Ti, V, B, etc. are further added, at not higher than the Ac3 transformation point, and applying reducing at an average rolling temperature of &thgr;m (° C.) and a total reduction ratio Tred (%) within s temperature range of from 400 to Ac3 transformation point, with di, &thgr;m and Tred being in a relation satisfying a prescribed equation.

Abstract: The invention provides a steel pipe being superior in workability, particularly in bending workability, in which an r-value in the axial direction of the pipe in a portion where melting or transformation of a steel material has occurred during seam welding is as high as comparable to that in a portion where melting or transformation of the steel material has not occurred, and a method of producing the steel pipe. In the high-workability steel pipe, an r-value in the longitudinal direction is not less than 1.2, more preferably not less than 1.6, over an entire area in the circumferential direction, including a seamed portion. The steel pipe is produced by a method comprising the step of performing diameter-reducing rolling on a steel pipe in a temperature range of from 600° C.

Abstract: A stainless steel tube for a automobile structure member having excellent formability for secondary operation comprises: a chemical composition including not more than 0.20 mass % of C; not more than 1.5 mass % of Si; not more than 2.0 mass % of Mn; 10-18 mass % of Cr; not more than 0.03 mass % of N; or further at least one type of element selected from the group of: not more than 0.6 mass % of Cu; not more than 0.6 mass % of Ni; not more than 2.5 mass % of Mo; not more than 1.0 mass % of Nb; not more than 1.0 mass % of Ti; and not more than 1.0 mass % of V; Fe as the remainder and the inevitable impurities; and a structure which is constituted of ferrite or ferrite and martensite, wherein the TE value defined by the following formula (1) exceeds 25,000 MPa·%.

Abstract: A steel tube having a composition which contains: 0.05 to 0.30% of C; 1.8 to 4.0% of Mn; Si; and Al is subjected to a diameter-reducing rolling process in which the total diameter-reduction rate is no less than 20% and the temperature at which the diameter-reducing rolling process is finished is no higher than 800 ° C., whereby a structure constituted of martensite and/or bainite or further of ferrite is obtained as a transformation product from the deformed &ggr;. As a result, a steel tube having tensile strength of 1000 MPa or more and excellent three-point-bending property can be obtained. The composition of the steel tube of the present invention may further include at least one type of element selected from the group consisting of Cu, Ni, Cr and Mo, or at least one type of element selected from the group consisting of Nb, V, Ti and B, or at least of one type selected from the group consisting of REM and Ca.

Abstract: A welded steel pipe is formed by heating or soaking an untreated welded steel pipe having a steel composition comprising, on the basis of mass percent: about 0.05% to about 0.3% C; about 2.0% or less of Si; more than about 1.5% to about 5.0% Mn; about 0.1% or less of P; about 0.01% or less of S; about 0.1% or less of Cr; about 0.1% or less of Al; about 0.1% or less of Nb; about 0.3% or less of Ti; and about 0.01% or less of N; and by diameter-reduction-rolling the treated steel pipe at a accumulated diameter reduction rate of at least about 35% and a finish rolling temperature of about 500° C. to about 900° C. The welded steel pipe exhibits excellent hydroformability, i.e., has a tensile strength of about 780 MPa or more and a n×r product of at least about 0.15. The treated steel pipe is preferably diameter-reduction-rolled at a accumulated diameter reduction rate of at least about 20% below the Ar3 transformation point.

Abstract: This invention provides a rolling mill and a rolling-mill train which are compact. Reducers to be coupled with them can be made compact. The inner surfaces of pipes to be rolled with the rolling mill and the rolling-mill train can be prevented from becoming squarish. The rolling mill comprises a plurality of rolls disposed symmetrically around the pathline of the rolling mill, an annular driving bevel gear of a large diameter for driving and rotating the rolls, an input-shaft mechanism for rotating the driving bevel gear, a plurality of transmission mechanisms disposed at regular intervals along the driving bevel gear, and a housing for holding them. The input-shaft mechanism (i) has an input shaft inserted in the housing from its outside and an input bevel gear of a small diameter mounted on the input shaft and engaging with the driving bevel gear and (ii) is disposed between two adjacent transmission mechanisms.

Abstract: A welded steel pipe is formed by heating or soaking an untreated welded steel pipe having a steel composition containing, on the basis of mass percent: about 0.03% to about 0.2% C, about 2.0% or less of Si, not less than about 1.0% to about 1.5% Mn, about 0.1% or less of P, about 0.01% or less of S, about 1.0% or less of Cr, about 0.1% or less of Al, about 0.1% or less of Nb, about 0.1% or less of Ti, about 0.1% or less of V, and about 0.01% or less of N; and by reduction-rolling the treated steel pipe at a cumulative reduction rate of at least about 35% and a final rolling temperature of about 500° C. to about 900° C. The welded steel pipe exhibits excellent hydroformability, i.e., has a tensile strength of at least about 590 MPa and an n×r product of at least about 0.22. The treated steel pipe is preferably reduction-rolled at a cumulative reduction rate of at least about 20% below the Ar3 transformation point.

Abstract: A welded steel pipe is formed by heating or soaking an untreated welded steel pipe having a steel composition containing, on the basis of mass percent: about 0.05 % to about 0.2% C; about 0.2% or less of Si; about 1.5% or less of Mn; about 0.1% or less of P; about 0.01% or less of S; about 0.1% or less of Al; and about 0.01 % or less of N; and by reduction-rolling the treated steel pipe at a cumulative reduction rate of at least about 35% and a final rolling temperature of about 500° C. to about 900° C. The welded steel pipe exhibits excellent hydroformability, i.e., has a tensile strength of at least about 400 MPa and an n×r product of at least about 0.22. The treated steel pipe is preferably reduction-rolled at a cumulative reduction rate of at least about 20% below the Ar3 transformation point. The welded steel pipe is suitable for forming structural components.

Abstract: The invention provides a high-carbon steel pipe having superior cold workability and induction hardenability, and a method of producing the steel pipe. The method comprises the steps of heating or soaking a base steel pipe having a composition containing C: 0.3 to 0.8%, Si: not more than 2%, and Mn: not more than 3%, and then carrying out reducing rolling on the base steel pipe at least in the temperature range of (Ac1, transformation point −50° C.) to Ac1, transformation point with an accumulated reduction in diameter of not less than 30%. A structure in which the grain size of cementite is not greater than 1.0 &mgr;m is obtained, thus resulting in improved cold workability and induction hardenability.

Abstract: A method and apparatus for smoothing a thick walled portion of a steel pipe produced by pressure-welding two opposite longitudinal edges of an open pipe with a squeeze roll after being subjected to induction heating. Outer and inner reduction rollers pressure sandwiches the thick walled portion from the outer and inner surfaces of the pipe, a support supports the inner reduction roller to be rotatable and containing a water passage for cooling water, a connecting rod connects the support device to a coupler and contains a further water passage for feeding cooling water to the water passage, and an anchor holds the connecting rod. In the method of producing steel pipes two opposite longitudinal edges of the open pipe are preformed before being subjected to the induction heating and thereafter a thick walled portion is smoothed by the above-described.

Abstract: The steel pipe has a structure composed mainly of ferrite or ferrite plus pearlite or ferrite plus cementite. The steel pipe is characterized by grain size not greater than 3 &mgr;m, preferably not greater than 1 &mgr;m, elongation greater than 20%, tensile strength (TS:MPa) and elongation (E1:%) whose product is greater than 10000, and percent ductile fracture greater than 95%, preferably 100%, measured by Charpy impact test on an actual pipe at −100° C. The structure is characterized by C: 0.005-0.03%, Si: 0.01-3.0%, Mn: 0.01-2.0%, and Al: 0.001-0.10% on a weight basis, and is composed of ferrite or ferrite and a secondary phase, with ferrite grains being not greater than 3 &mgr;m and the secondary phase having an areal ratio not more than 30%. A steel pipe stock having the above-mentioned composition is heated at a temperature of (Ac1+50° C.) to 400° C. and subsequently reduced at a rolling temperature of (Ac1+50° C.) to 400° C.