| 摘要 |
Seamless quenched and tempered steel pipe having a wall thickness WT higher than or equal to 6 mm and lower than or equal to 35 mm, wherein the steel pipe has a chemical composition consisting of 0.050-0.090 wt% C, 0.80-1.65 wt% Mn, 0.10-0.45 wt% Si, 0.10-0.60 wt% Cr, 0.05-0.45 wt% Ni, 0.05-0.40 wt% Mo, 0.020-0.040 wt% Al, 0.0030-0.0090 wt% N, 0.0008-0.0050 wt% Ca, 0.000-0.040 wt% Nb, 0.000-0.020 wt% Ti, 0.000-0.070 wt% V, 0.000-0.030 wt% Zr, 0.000-0.030 wt% Ta based on the composition weight, the remaining being iron and impurities; wherein (V + Nb) content is lower than 0.07 wt%; wherein, defining a first parameter P1 = (60 x C) + Cr + [5 x e( 35 x Mo/WT) ] + 50 x (V+Nb), the chemical composition satisfies a first condition P1 ‰¤ 14.5; wherein the quenched and tempered steel pipe has, at mid thickness and possibly also at the inner surface of the pipe, a microstructure comprising, in volume percentage, tempered bainite greater than or equal to 40% and ferrite (V F ) lower than or equal to 60%, and has at a distance up to 2 mm, starting from both outer and inner surfaces of the pipe or from the outer surface only, a microstructure comprising, in volume percentage, tempered martensite lower than 70% and tempered bainite higher than 30%; and wherein the maximum hardness values, measured at 1.5-2.0 mm depth from the outer and inner surfaces, are below 235 HV 10 . |
