Method for producing welded steel pipe and welded steel pipe

1. A method for producing a welded steel pipe, comprising the steps of:

preparing a welded pipe; and

expanding the welded pipe over an entire length thereof by using a pipe expanding head to form a welded steel pipe such that an undulation wavelength ratio D defined by Formula (1) is not more than 0.8, or not less than 1.8:

D=p/λ   (1)

where p is an undulation wavelength in an axial direction of the welded steel pipe, and λ is a Timoshenko's buckling wavelength as defined by the following Formula (2):

λ=3.44×( r×t ) 1/2   (2)

where r is an inner radius of the welded steel pipe, and t is a wall thickness of the welded steel pipe.

2. The method for producing a welded steel pipe, according to claim 1 , wherein

the step of expanding the welded pipe over the entire length thereof comprises the substeps of:

setting a relative movement pitch of the pipe expanding head with respect to the welded pipe such that the undulation wavelength ratio D is not more than 0.8, or not less than 1.8; and

expanding the welded pipe over the entire length thereof at the set movement pitch.

3. A welded steel pipe produced by being expanded over an entire length thereof, and having an undulation in an axial direction thereof, wherein

an undulation wavelength ratio D determined by the following Formula (1) is not more than 0.8, or not less than 1.8:

D=p/λ   (1)

where p is an undulation wavelength of the welded steel pipe, and λ is a Timoshenko's buckling wavelength as determined by the following Formula (2):

λ=3.44×( r×t ) 1/2   (2)

where r is an inner radius of the welded steel pipe, and t is a wall thickness of the welded steel pipe.