Electric resistance welded steel pipe and method for manufacturing the same

An electric resistance welded steel pipe, wherein

• the electric resistance welded steel pipe is a thick-walled electric resistance welded steel pipe and comprises a welded portion comprising a heat-affected zone having a steel microstructure principally including a bainitic ferrite phase and/or a bainite phase,
• the steel microstructure at half the wall thickness of the heat-affected zone of the welded portion includes a bainitic ferrite phase and/or a bainite phase in a total area ratio of 90% or more relative to the entire microstructure at half the wall thickness of the heat-affected zone of the welded portion,
• in the steel microstructure located 1 mm in the wall thickness direction below the outer surface of the heat-affected zone of the welded portion, the bainitic ferrite phase and/or the bainite phase has an average grain size of 20 μm or less, and
• the average grain size of the bainitic ferrite phase and/or the bainite phase located 1 mm in the wall thickness direction below the outer surface of the heat-affected zone of the welded portion is 0.5 times or more and 2 times or less the average grain size of the bainitic ferrite phase and/or the bainite phase at half the wall thickness of the heat-affected zone of the welded portion.