An electric resistance welded steel pipe having excellent formability and torsional fatigue resistance and a method for manufacturing the same. The electric resistance welded steel pipe includes a seam region and a base metal region, the seam region having a range of ±10 degrees in a pipe circumferential direction with respect to an electric resistance welded seam formed in a pipe longitudinal direction, the base metal region being a region other than the seam region. The electric resistance welded steel pipe has an r-value in the pipe longitudinal direction of 1.0 or greater, H (mm) and W (mm) satisfy a specified formula, and Ts.sub.(MAX) (mm) and Tb.sub.(Ave) (mm) satisfy a specified formula.

An electric resistance welded steel pipe having excellent formability and torsional fatigue resistance and a method for manufacturing the same. The electric resistance welded steel pipe includes a seam region and a base metal region, the seam region having a range of ±10 degrees in a pipe circumferential direction with respect to an electric resistance welded seam formed in a pipe longitudinal direction, the base metal region being a region other than the seam region. The electric resistance welded steel pipe has an r-value in the pipe longitudinal direction of 1.0 or greater, H (mm) and W (mm) satisfy a specified formula, and Ts.sub.(MAX) (mm) and Tb.sub.(Ave) (mm) satisfy a specified formula.

In a method for manufacturing a metal pipe from a metal plate using a forming tool, the position of the tool is optimized simply and correctly by incorporating individuality of the raw material plate into setting of the tool position.

As a preparatory stage, a forming process is analyzed by simulation for each plate. Based on result of the analysis, correlation between a deformed shape value of a raw pipe and tool position information is acquired. Then, the forming process for each plate is stored as correlation between the deformed shape value of the raw pipe and the tool position information. During pipe manufacturing, a deformed shape value of the raw pipe is measured actually while a plate is passed. On the basis of the actually measured deformed shape value, a forming process for the raw pipe is expected and assumed (by using the correlation). Tool position information necessary for implementing the expected and assumed forming process is retrieved from the stored correlation. The retrieved tool position information is realized at a stand array.

In a method for manufacturing a metal pipe from a metal plate using a forming tool, the position of the tool is optimized simply and correctly by incorporating individuality of the raw material plate into setting of the tool position.

As a preparatory stage, a forming process is analyzed by simulation for each plate. Based on result of the analysis, correlation between a deformed shape value of a raw pipe and tool position information is acquired. Then, the forming process for each plate is stored as correlation between the deformed shape value of the raw pipe and the tool position information. During pipe manufacturing, a deformed shape value of the raw pipe is measured actually while a plate is passed. On the basis of the actually measured deformed shape value, a forming process for the raw pipe is expected and assumed (by using the correlation). Tool position information necessary for implementing the expected and assumed forming process is retrieved from the stored correlation. The retrieved tool position information is realized at a stand array.

A high-strength high-toughness electric-resistance-welded steel pipe having high resistance to post-weld heat treatment is provided. The steel pipe having a composition including C: 0.01% to 0.12%, Si: 0.05% to 0.50%, Mn: 1.0% to 2.2%, P: 0.03% or less, S: 0.005% or less, Al: 0.001% to 0.10%, N: 0.006% or less, Nb: 0.010% to 0.100%, and Ti: 0.001% to 0.050%. The steel pipe having a structure composed of 90% or more by volume of a bainitic ferrite phase and 10% or less (including 0%) by volume of a second phase. The bainitic ferrite phase having an average grain size of 10 μm or less, and the structure containing fine Nb precipitates having a particle size of less than 20 nm dispersed in a base material portion. The steel pipe having high strength and toughness that is maintained through post-weld heat treatment, including heating to a temperature of 600° C. or more.

A high-strength high-toughness electric-resistance-welded steel pipe having high resistance to post-weld heat treatment is provided. The steel pipe having a composition including C: 0.01% to 0.12%, Si: 0.05% to 0.50%, Mn: 1.0% to 2.2%, P: 0.03% or less, S: 0.005% or less, Al: 0.001% to 0.10%, N: 0.006% or less, Nb: 0.010% to 0.100%, and Ti: 0.001% to 0.050%. The steel pipe having a structure composed of 90% or more by volume of a bainitic ferrite phase and 10% or less (including 0%) by volume of a second phase. The bainitic ferrite phase having an average grain size of 10 μm or less, and the structure containing fine Nb precipitates having a particle size of less than 20 nm dispersed in a base material portion. The steel pipe having high strength and toughness that is maintained through post-weld heat treatment, including heating to a temperature of 600° C. or more.

A high-strength high-toughness electric-resistance-welded steel pipe having high resistance to post-weld heat treatment is provided. The steel pipe having a composition including C: 0.01% to 0.12%, Si: 0.05% to 0.50%, Mn: 1.0% to 2.2%, P: 0.03% or less, S: 0.005% or less, Al: 0.001% to 0.10%, N: 0.006% or less, Nb: 0.010% to 0.100%, and Ti: 0.001% to 0.050%. The steel pipe having a structure composed of a 90% or more by volume of a bainitic ferrite phase as a main phase and 10% or less (including 0%) by volume of a second phase. The bainitic ferrite phase having an average grain size of 10 μm or less, and the structure containing fine Nb precipitates having a particle size of less than 20 nm dispersed in a base material portion. The steel pipe having high strength, toughness, and high resistance that is maintained through post-weld heat treatment.

A high-strength high-toughness electric-resistance-welded steel pipe having high resistance to post-weld heat treatment is provided. The steel pipe having a composition including C: 0.01% to 0.12%, Si: 0.05% to 0.50%, Mn: 1.0% to 2.2%, P: 0.03% or less, S: 0.005% or less, Al: 0.001% to 0.10%, N: 0.006% or less, Nb: 0.010% to 0.100%, and Ti: 0.001% to 0.050%. The steel pipe having a structure composed of a 90% or more by volume of a bainitic ferrite phase as a main phase and 10% or less (including 0%) by volume of a second phase. The bainitic ferrite phase having an average grain size of 10 μm or less, and the structure containing fine Nb precipitates having a particle size of less than 20 nm dispersed in a base material portion. The steel pipe having high strength, toughness, and high resistance that is maintained through post-weld heat treatment.

A steel pipe collapse strength prediction model generation method, a steel pipe collapse strength prediction method, a steel pipe manufacturing characteristics determination method, and a steel pipe manufacturing method capable of highly accurately predicting the collapse strength of a steel pipe after forming or a coated steel pipe in consideration of the pipe-making strain during forming. Into a steel pipe collapse strength prediction model generated by the prediction model generation method, steel pipe manufacturing characteristics including the shape of a steel pipe to be predicted after forming, strength characteristics, and the pipe-making strain are input to predict the collapse strength after forming. Into a steel pipe collapse strength prediction model, steel pipe manufacturing characteristics including the shape of a coated steel pipe to be predicted after forming, strength characteristics, the pipe-making strain, and coating conditions are input to predict the collapse strength of the coated steel pipe.

A steel pipe collapse strength prediction model generation method, a steel pipe collapse strength prediction method, a steel pipe manufacturing characteristics determination method, and a steel pipe manufacturing method capable of highly accurately predicting the collapse strength of a steel pipe after forming or a coated steel pipe in consideration of the pipe-making strain during forming. Into a steel pipe collapse strength prediction model generated by the prediction model generation method, steel pipe manufacturing characteristics including the shape of a steel pipe to be predicted after forming, strength characteristics, and the pipe-making strain are input to predict the collapse strength after forming. Into a steel pipe collapse strength prediction model, steel pipe manufacturing characteristics including the shape of a coated steel pipe to be predicted after forming, strength characteristics, the pipe-making strain, and coating conditions are input to predict the collapse strength of the coated steel pipe.