A hot-rolled steel sheet suitable for the production ofAPI5L-grade X80 steel pipes, the steel sheet having a tensile strength in accordance with JIS Z 2241, measured on a No. 5 test piece described in JIS Z 2201 from the R direction, of 710 MPa or more, the steel sheet consisting of, by mass%: C = 0.02% to 0.06%; Si = 0.05% to 0.5%; Mn = 1 % to 2%; Nb = 0.05% to 0.12%; Ti = 0.005% to 0.02%; P ≦ 0.03%; S ≦ 0.005%; O ≦ 0.003%; Al = 0.005% to 0.1 %; N = 0.0015% to 0.006%; Ca = 0.0005% to 0.003%; V ≦ 0.15 %, wherein "0", or zero, %, is not included Mo ≦ 0.3%, wherein "0", or zero, %, is not included N - 14/48 x Ti ≧ "0" (zero)% by mass%; 0 < S/Ca < 0.8; optionally further comprising one kind or two or more kinds from among: Cr = 0.05% to 0.3%; Cu = 0.05% to 0.3%; Ni = 0.05% to 0.3%; B = 0.0002% to 0.003% by mass%; and optionally further comprising: REM = 0.0005% to 0.02% by mass%; and the remaining portion made up of Fe and inevitable impurity elements, and the sheet thickness being 16 mm or more, wherein the polygonal pro-eutectoid ferrite fraction is 3% or more and 20% or less, and the rest of the phases are a low-temperature transformation phase comprising bainitic ferrite, granular bainitic ferrite, quasi-polygonal ferrite, retained austenite and martensite-austenite and 1% or less of pearlite in a microstructure at a depth of a half thickness from a steel sheet surface, the number average crystal grain size of a whole of the microstructure which is the average grain size (i.e. the sum of grain sizes/ the number of crystal grains) when a number distribution is found by each grain size of the crystal grain measured by an electron back scatter diffraction pattern method is 1 μm or more and 2.5 μm or less, the area average grain size which is the average grain size (i.e. the grain size corresponding to an average area) when a distribution is found in which the number distribution by each crystal grain size is multiplied by the average area of the grain size measured by the electron back scatter diffraction pattern method is 3 μm or more and 9 μm or less, the standard deviation of the area average grain size of the microstructure is 0.8 μm or more and 2.3 μm or less, and the reflected X-ray intensity ratio / in a direction and in a direction relative to a plane in parallel to the steel sheet surface at the depth of the half thickness from the steel sheet surface is 1.1 or more. C = 0.02% to 0.06%; Si = 0.05% to 0.5%; Mn = 1 % to 2%; Nb = 0.05% to 0.12%; Ti = 0.005% to 0.02%; P ≦ 0.03%; S ≦ 0.005%; O ≦ 0.003%; Al = 0.005% to 0.1 %; N = 0.0015% to 0.006%; Ca = 0.0005% to 0.003%; V ≦ 0.15 %, wherein "0", or zero, %, is not included Mo ≦ 0.3%, wherein "0", or zero, %, is not included N - 14/48 x Ti ≧ "0" (zero)% by mass%; 0 < S/Ca < 0.8; Cr = 0.05% to 0.3%; Cu = 0.05% to 0.3%; Ni = 0.05% to 0.3%; B = 0.0002% to 0.003% by mass%; and REM = 0.0005% to 0.02% by mass%; and the remaining portion made up of Fe and inevitable impurity elements, and the sheet thickness being 16 mm or more, wherein the polygonal pro-eutectoid ferrite fraction is 3% or more and 20% or less, and the rest of the phases are a low-temperature transformation phase comprising bainitic ferrite, granular bainitic ferrite, quasi-polygonal ferrite, retained austenite and martensite-austenite and 1% or less of pearlite in a microstructure at a depth of a half thickness from a steel sheet surface, the number average crystal grain size of a whole of the microstructure which is the average grain size (i.e. the sum of grain sizes/ the number of crystal grains) when a number distribution is found by each grain size of the crystal grain measured by an electron back scatter diffraction pattern method is 1 μm or more and 2.5 μm or less, the area average grain size which is the average grain size (i.e. the grain size corresponding to an average area) when a distribution is found in which the number distribution by each crystal grain size is multiplied by the average area of the grain size measured by the electron back scatter diffraction pattern method is 3 μm or more and 9 μm or less, the standard deviation of the area average grain size of the microstructure is 0.8 μm or more and 2.3 μm or less, and the reflected X-ray intensity ratio / in a direction and in a direction relative to a plane in parallel to the steel sheet surface at the depth of the half thickness from the steel sheet surface is 1.1 or more.