A steel pipe comprising: a base material portion composed of a tubular steel plate; and a welded part that is provided at a butt portion of the steel plate and extends in a longitudinal direction of the steel plate, wherein the steel plate has a chemical composition containing, by mass%, C: 0.030% to 0.070%, Si: 0.05% to 0.50%, Mn: 1.05% to 1.65%, Al: 0.010% to 0.070%, Ti: 0.005% to 0.020%, Nb: 0.005% to 0.045%, Ca: 0.0010% to 0.0050%, N: 0.0010% to 0.0050%, Ni: 0% to 0.50%, Mo: 0% to 0.50%, Cr: 0% to 0.50%, Cu: 0% to 0.50%, V: 0% to 0.100%, Mg: 0% to 0.0100%, REM: 0% to 0.0100%, P: limited to 0.015% or less, S: limited to 0.0015% or less, O: limited to 0.0040% or less, and 0.10% or less of Sb, Sn, Co, or 0.005% or less of Pb or Bi and 0.0005% or less of H, a remainder: Fe and impurities, the steel plate has a Ceq of 0.250 to 0.350, the Ceq being defined by the following Expression, an internal microstructure, which is a microstructure in a range from a position deep over 1.0 mm from a surface of the base material portion to a plate thickness center in a depth direction, includes 85% or more of one or both of granular bainite and bainite in terms of a total area ratio and includes 1.0% or less of a martensite-austenite constituent in terms of an area ratio, in the internal microstructure, a maximum hardness is 248 Hv or less and an average hardness is 170 to 220 Hv, the base material portion has a texture having an integration degree of <110> of 1.5 or more in a plane parallel to a plate surface at a 1/4 position of a plate thickness from the surface in a plate thickness direction, a surface layer area microstructure, which is a microstructure in a range of 1.0 mm from the surface of the base material portion in the depth direction, includes 95% or more of one or both of granular bainite and tempered bainite in terms of a total area ratio, in the surface layer area microstructure, a maximum hardness is 250 Hv or less; the base material portion has a tensile strength of 520 to 760 MPa; a plate thickness of the steel plate is 15 mm or less, wherein maximum hardness, average hardness and tensile strength are measured by the methods described in the description; Ceq = C + Mn / 6 + Ni + Cu / 15 + Cr + Mo + V / 15 where, [Mn], [Ni], [Cu], [Cr], [Mo], and in the Expression are respectively the amounts of C, Mn, Ni, Cu, Cr, Mo, and V in the steel plate in terms of mass%; the mechanical properties and the microstructure were determined according to the description. a base material portion composed of a tubular steel plate; and a welded part that is provided at a butt portion of the steel plate and extends in a longitudinal direction of the steel plate, wherein the steel plate has a chemical composition containing, by mass%, C: 0.030% to 0.070%, Si: 0.05% to 0.50%, Mn: 1.05% to 1.65%, Al: 0.010% to 0.070%, Ti: 0.005% to 0.020%, Nb: 0.005% to 0.045%, Ca: 0.0010% to 0.0050%, N: 0.0010% to 0.0050%, Ni: 0% to 0.50%, Mo: 0% to 0.50%, Cr: 0% to 0.50%, Cu: 0% to 0.50%, V: 0% to 0.100%, Mg: 0% to 0.0100%, REM: 0% to 0.0100%, P: limited to 0.015% or less, S: limited to 0.0015% or less, O: limited to 0.0040% or less, and 0.10% or less of Sb, Sn, Co, or 0.005% or less of Pb or Bi and 0.0005% or less of H, a remainder: Fe and impurities, the steel plate has a Ceq of 0.250 to 0.350, the Ceq being defined by the following Expression, an internal microstructure, which is a microstructure in a range from a position deep over 1.0 mm from a surface of the base material portion to a plate thickness center in a depth direction, includes 85% or more of one or both of granular bainite and bainite in terms of a total area ratio and includes 1.0% or less of a martensite-austenite constituent in terms of an area ratio, in the internal microstructure, a maximum hardness is 248 Hv or less and an average hardness is 170 to 220 Hv, the base material portion has a texture having an integration degree of <110> of 1.5 or more in a plane parallel to a plate surface at a 1/4 position of a plate thickness from the surface in a plate thickness direction, a surface layer area microstructure, which is a microstructure in a range of 1.0 mm from the surface of the base material portion in the depth direction, includes 95% or more of one or both of granular bainite and tempered bainite in terms of a total area ratio, in the surface layer area microstructure, a maximum hardness is 250 Hv or less; the base material portion has a tensile strength of 520 to 760 MPa; a plate thickness of the steel plate is 15 mm or less, wherein maximum hardness, average hardness and tensile strength are measured by the methods described in the description; Ceq = C + Mn / 6 + Ni + Cu / 15 + Cr + Mo + V / 15 where, [Mn], [Ni], [Cu], [Cr], [Mo], and in the Expression are respectively the amounts of C, Mn, Ni, Cu, Cr, Mo, and V in the steel plate in terms of mass%; the mechanical properties and the microstructure were determined according to the description.