61results about How to "Excellent SSC resistance" patented technology

There is provided a seamless steel pipe for line pipe, wherein a chemical composition consists, by mass percent, of C: 0.03-0.10%, Si: ≦0.50%, Mn: 1.0-2.0%, P: ≦0.050%, S: ≦0.005%, Cr: 0.05-1.0%, Mo: 0.01-0.30%, Al: 0.001-0.10%, N: ≦0.01%, Ni: 0.04-2.0%, Ca: 0.0005-0.0050%, Cu: 0-2.0%, Ti: 0-0.05%, Nb: 0-0.05%, V: 0-0.10%, the balance: Fe and impurities, and satisfies the conditions of Cu+Ni: ≧0.10%, and Mo+V: ≦0.30%, wherein in a scale formed on the surface of the steel pipe, metal particles consisting mainly of Ni or Cu having an average circle-equivalent diameter of 0.1-5 μm exist, and a distance from a boundary between the base metal of the steel pipe and the scale to a region in which the metal particles do not exist is 20 μm or longer.

A high-strength seamless steel pipe for oil country tubular goods comprising, by mass %, C: 0.20% to 0.50%, Si: 0.05% to 0.40%, Mn: more than 0.6% to 1.5% or less, P: 0.015% or less, S: 0.005% or less, Al: 0.005% to 0.1%, N: 0.006% or less, Mo: more than 1.0% to 3.0% or less, V: 0.05% to 0.3%, Nb: 0.001% to 0.020%, B: 0.0003% to 0.0030%, O: 0.0030% or less, and Ti: 0.003% to 0.025%, and wherein Ti / N: 2.0 to 5.0 is satisfied, a volume fraction of a tempered martensitic is 95% or more, prior austenite grains have a grain size number of 8.5 or more, and in a cross-section perpendicular to a rolling direction, the number of nitride-based inclusions having a grain size of 4 m or more is 100 or less per 100 mm2, the number of nitride-based inclusions having a grain size of less than 4 μm is 1000 or less per 100 mm2, the number of oxide-based inclusions having a grain size of 4 μm or more is 40 or less per 100 mm2, and the number of oxide-based inclusions having a grain size of less than 4 μm is 400 or less per 100 mm2, and methods of producing the same.

A steel for an oil well pipe, having high strength and excellent SSC resistance, consists of, by mass %, C: 0.30 to 0.60%, Si: 0.05 to 0.5%, Mn: 0.05 to 1.0%, Al: 0.005 to 0.10%, Cr+Mo: 1.5 to 3.0%, wherein Mo is 0.5% or more, V: 0.05 to 0.3%, Nb: 0 to 0.1%, Ti: 0 to 0.1%, Zr: 0 to 0.1%, N (nitrogen): 0 to 0.03%, Ca: 0 to 0.01%, and the balance Fe and impurities; P 0.025% or less, S 0.01% or less, B 0.0010% or less and O (oxygen) 0.01% or less. The method involves heating the steel at 1150° C. or more; producing a seamless steel pipe by hot working; water-cooling the pipe to a temperature in a range of 400 to 600° C. immediately after finishing the working; and subjecting the pipe to a heat treatment for bainite isothermal transformation in a range of 400 to 600° C.

Provided is a high-strength seamless steel pipe having the composition which contains, by mass %, 0.20 to 0.50% C, 0.05 to 0.40% Si, 0.3 to 0.9% Mn, 0.015% or less P, 0.005% or less S, 0.005 to 0.1% Al, 0.008% or less N, 0.6 to 1.7% Cr, 0.4 to 1.0% Mo, 0.01 to 0.30% V, 0.01 to 0.06% Nb, 0.0003 to 0.0030% B, and 0.0030% or less O (oxygen). The high-strength seamless steel pipe has the microstructure where a volume fraction of a tempered martensitic phase is 95% or more, and prior austenitic grains have a grain size number of 8.5 or more, and a segregation degree index Ps which is defined by a formula Ps=8.1 (XSi+XMn+XMo)+1.2XP relating to XM which is a ratio between a segregated portion content and an average content is set to less than 65.

The present invention provides a steel which simultaneously satisfies a plurality of characteristics, specifically, a steel for tubes with excellent sulfide stress cracking resistance, including, C: 0.2 to 0.7%; Si: 0.01 to 0.8%; Mn: 0.1 to 1.5%; S: not more than 0.005%; P: not more than 0.03%; Al: 0.0005 to 0.1%; Ti: 0.005 to 0.05%; Ca: 0.0004 to 0.005%; N: not more than 0.007%; Cr: 0. 1 to 1.5%; and Mo: 0.2 to 1.0%; the balance being Fe, Mg and impurities, being characterized in that: the content of Mg is not less than 1.0 ppm and not more than 5.0 ppm; and inclusions of not less than 50% of the total number of those in steel have such a morphology that Mg—Al—O-based oxides exist at the central part of the inclusion, Ca—Al-based oxides enclose the Mg—Al—O-based oxides, and Ti-containing-carbonitrides further exist on a periphery of the Ca—Al-based oxides.

The present invention provides a martensitic stainless steel for oil wells that has good SSC resistance and excellent low-temperature toughness under the coexistence of Cl-, wet CO2 and a small amount of H2S. The steel material contains, by mass %, C: 0.16-0.22%, Si: 0.1-0.8%, Mn: 0.25-1.00%, P: 0.025% or less, S: 0.010% or less, Cr: 12.0-13.5%, Al: 0.010% or less, Ni: 0-0.2%, Cu: 0-0.10%, Mo: 0-0.20%, Ti: 0-0.050%, N: 0.01-0.1%, and the balance is composed of Fe and impurities. It may also contain predetermined amounts of one or more of Nb and V and / or predetermined amounts of one or more of Ca, Mg, La, and Ce.

Low-alloy oil-well steel pipe includes a composition consisting, in mass %, of C: 0.40 to 0.65%, Si: 0.05 to 0.50%, Mn: 0.10 to 1.00%, P: 0.020% or less, S: 0.0020% or less, Cu: 0.15% or less, Cr: 0.40 to 1.50%, Mo: 0.50 to 2.50%, V: 0.05 to 0.25%, Ti: 0 to less than 0.01%, Nb: 0.01 to 0.2%, sol.Al: 0.010 to 0.100%, N: 0.006% or less, B: 0 to 0.0015%, and Ca: 0 to 0.003%, the balance being Fe and impurities. The structure has tempered martensite and 0 to less than 2% volume ratio of retained austenite. A grain size number of prior-austenite grain in the structure is 9.0 or more. An equivalent circular diameter of a sub-structure surrounded by a boundary having a crystal orientation difference of 15° or more from a packet boundary, a block boundary and a lath boundary is 3 μm or less for the tempered martensite.

An oil well steel pipe for embedding-expanding excellent in the SSC resistance after expanding. The pipe is made of a steel consisting of, by mass %, C: 0.05 to 0.45%, Si: 0.1 to 1.5%, Mn: 0.1 to 3.0%, P: 0.03% or less, S: 0.01% or less, sol.Al: 0.05% or less, and the balance being Fe and impurities, with a soluble N content of 40 ppm or less. The steel can comprise one or more selected from V, Ti, Nb, B, Cr, Mo, Ni, Cu and Ca.

The present invention provides a steel which simultaneously satisfies a plurality of characteristics, specifically, a steel for tubes with excellent sulfide stress cracking resistance, including, C: 0.2 to 0.7%; Si: 0.01 to 0.8%; Mn: 0.1 to 1.5%; S: not more than 0.005%; P: not more than 0.03%; Al: 0.0005 to 0.1%; Ti: 0.005 to 0.05%; Ca: 0.0004 to 0.005%; N: not more than 0.007%; Cr: 0.1 to 1.5%; and Mo: 0.2 to 1.0%; the balance being Fe, Mg and impurities, being characterized in that: the content of Mg is not less than 1.0 ppm and not more than 5.0 ppm; and inclusions of not less than 50% of the total number of those in steel have such a morphology that Mg—Al—O-based oxides exist at the central part of the inclusion, Ca—Al-based oxides enclose the Mg—Al—O-based oxides, and Ti-containing-carbonitrides further exist on a periphery of the Ca—Al-based oxides.

The invention provides a martensitic stainless steel seamless pipe for oil country tubular goods having high strength, and excellent sulfide stress corrosion cracking resistance and a method for manufacturing the same. The martensitic stainless steel seamless pipe for oil country tubular goods has a yield stress of 655 to 758 MPa, and has a composition containing, in mass %, C: 0.10% or less, Si: 0.5% or less, Mn: 0.05 to 2.0%, P: 0.030% or less, S: 0.005% or less, Ni: 4.0 to 8.0%, Cu: 0.02% or more and less than 1.0%, Cr: 10.0 to 14.0%, Mo: 1.0 to 3.5%, V: 0.003 to 0.2%, Co: 0.02% or more and less than 1.0%, Al: 0.1% or less, N: 0.1% or less, Ti: 0.50% or less, and the balance Fe and incidental impurities, wherein C, Mn, Cr, Cu, Co, Ni, Mo, W, Nb, N, and Ti satisfy the predetermined relations.

This high-strength steel material for oil wells has a chemical composition which includes, in mass%, 0.70-1.8% of C, 0.05-1.00% of Si, 12.0-25.0% of Mn, 0.003-0.06% of Al, not more than 0.03% of P, not more than 0.03% of S, not more than 0.10% of N, over 0.5% but not more than 2.0% of V, 0-2.0% of Cr, 0-3.0% of Mo, 0-1.5% of Cu, 0-1.5% of Ni, 0-0.5% of Nb, 0-0.5% of Ta, 0-0.5% of Ti, 0-0.5% of Zr, 0-0.005% of Ca, 0-0.005% of Mg, and 0-0.015% of B, the remainder comprising Fe and impurities, and which satisfies [0.6< / =C-0.18V-0.06Cr<1.44]. The metallographic structure substantially comprises an austenite single phase. V carbides having a circle-equivalent diameter of 5-100 nm are present in a number density of at least 20 / [mu]m2. The yield strength is at least 654 MPa.

An oil well steel pipe for embedding-expanding excellent in the SSC resistance after expanding. The pipe is made of a steel consisting of, by mass %, C: 0.05 to 0.45%, Si: 0.1 to 1.5%, Mn: 0.1 to 3.0%, P: 0.03% or less, S: 0.01% or less, sol.Al: 0.05% or less, and the balance being Fe and impurities, with a soluble N content of 40 ppm or less. The steel can comprise one or more selected from V, Ti, Nb, B, Cr, Mo, Ni, Cu and Ca.

Provided herein is a martensitic stainless steel seamless pipe, intended for oil country tubular goods, having high strength, and excellent sulfide stress corrosion cracking resistance. A method for producing such a martensitic stainless steel seamless pipe is also provided. The martensitic stainless steel seamless pipe for oil country tubular goods has a composition that contains, in mass %, C: 0.035% or less, Si: 0.5% or less, Mn: 0.05 to 0.5%, P: 0.03% or less, S: 0.005% or less, Cu: 2.6% or less, Ni: 5.3 to 7.3%, Cr: 11.8 to 14.5%, Al: 0.1% or less, Mo: 1.8 to 3.0%, V: 0.2% or less, N: 0.1% or less, and the balance Fe and unavoidable impurities, and in which C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti satisfy the predetermined relations.

Provided is a low alloy oil-well steel pipe having a yield strength of 793 MPa or more, and an excellent SSC resistance. A low alloy oil-well steel pipe according to the present invention includes a chemical composition consisting of: in mass%, C: 0.25 to 0.35%; Si: 0.05 to 0.50%; Mn: 0.10 to 1.50%; Cr: 0.40 to 1.50%; Mo: 0.40 to 2.00%; V: 0.05 to 0.25%; Nb: 0.010 to 0.040%; Ti: 0.002 to 0.050%; sol. Al: 0.005 to 0.10%; N: 0.007% or less; B: 0.0001 to 0.0035%; and Ca: 0 to 0.005%; and a balance being Fe and impurities. In a microstructure of the low alloy oil-well steel pipe, a number of cementite particles each of which has an equivalent circle diameter of 200 nm or more is 100 particles / 100 μm2 or more. The above low alloy oil-well steel pipe has a yield strength of 793 MPa or more.

A seamless steel pipe produced by heating a steel billet, which has a chemical composition C: 0.15 to 0.20%, Si: not less than 0.01% to less than 0.15%, Mn: 0.05 to 1.0%, Cr: 0.05 to 1.5%, Mo: 0.05 to 1.0%, Al≦0.10%, V: 0.01 to 0.2%, Ti: 0.002 to 0.03%, B: 0.0003 to 0.005% and N: 0.002 to 0.01%, further optionally one or more of Ca, Mg and REM in a specific amount, under the provision that the conditions “C+(Mn / 6)+(Cr / 5)+(Mo / 3)≧0.43” and “Ti×N<0.0002−0.0006×Si” are satisfied, with the balance being Fe and impurities, wherein P≦0.025%, S≦0.010% and Nb<0.005% among the impurities, to a temperature of 1000 to 1250° C. followed by pipe-making rolling at a final rolling temperature 900 to 1050° C., and then quenching the resulting steel pipe directly from a temperature not lower than the Ar3 transformation point followed by tempering at a temperature range from 600° C. to the Ac1 transformation point, or instead of the above after the said pipe-making rolling, complementarily heating the resulting steel pipe in a temperature range from the Ac3 transformation point to 1000° C. in-line, and then quenching it from a temperature not lower than the Ar3 transformation point followed by tempering at a temperature range from 600° C. to the Ac1 transformation point, has high strength and excellent toughness and at the same time has a high yield ratio and is excellent in SSC resistance as well.

A seamless steel pipe for line pipe has a chemical composition consisting, by mass percent, of C: 0.03-0.15%, Si: ≦0.50%, Mn: 1.0-2.0%, P: ≦0.050%, S: ≦0.005%, Cr: 0.1-1.0%, Al: 0.001-0.10%, N: ≦0.01%, Ni: 0.05-2.0%, B: 0.0003-0.0015%, Ca: 0.0002-0.0050%, Mo: 0.10-0.50%, Ti: 0.001-0.05%, Cu: 0-2.0%, Nb: 0-0.05%, V: 0-0.10%, the balance: Fe and impurities, and satisfying the conditions of 2Nb+4V+Mo≦0.50, wherein a metal micro-structure of the steel pipe contains 50% or more of bainite, in an area fraction, a wall thickness of the steel pipe is 25 mm or larger, and in a scale formed on the surface of the steel pipe, metal particles consisting mainly of Ni or Cu having an average circle-equivalent diameter of 0.1-5 mm exist. A distance from a boundary between the base metal of the steel pipe and the scale to a region in which the metal particles do not exist is 20 mm or longer.

Provided are a steel and an oil well steel pipe which have excellent SSC resistance, even in a high pressure H2S environment. This steel contains 0.15-0.45% of C, 0.10-1.0% of Si, 0.10-0.8% of Mn, atmost 0.050 of P, at most 0.010% of S, 0.01-0.1% of Al, at most 0.010% of N, 0.1-2.5% of Cr, 0.35-3.0% of Mo, 0.05-2.0% of Co, 0.003-0.040% of Ti, 0.003-0.050% of Nb, 0.01-0.50% of Cu, and 0.01-0.50% of Ni, and satisfies the equation below. In the microstructure, the grain size of prior-austenite is less than 5 microns, and the block size is less than 2 microns. The steel contains at least 90 vol%total of tempered martensite and tempered bainite. C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15-Co / 6+ alpha>=0.70 (1), (3C+Mo+3Co) / (3Mn+Cr)>=1.0 (2).

A steel material for oil country tubular goods that has a high strength and excellent SSC resistance is provided. The steel material according to this invention contains, in mass %, C: more than 0.45 to 0.65%, Si: 0.10 to 1.0%, Mn: 0.1 to 1.0%, P: 0.050% or less, S: 0.010% or less, Al: 0.01 to 0.1%, N: 0.01% or less, Cr: 0.1 to 2.5%, Mo: 0.25 to 5.0%, and Co: 0.05 to 5.0%, and satisfies expressions (1) and (2), and contains 90% or more of tempered martensite by volume ratio:C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15−Co / 6+α≥0.70  (1)(3C+Mo+3Co) / (3Mn+Cr)≥1.0  (2)Effective B=B−11(N−Ti / 3.4) / 14  (3)where, α in expression (1) is 0.250 when effective B (mass %) defined by expression (3) is 0.0003% or more, and is 0 when effective B is less than 0.0003%.

A seamless steel pipe contains, (mass %), C: 0.02 to 0.15%; Si: 0.05 to 0.5%; Mn: 0.30 to 2.5%; Al: 0.01 to 0.10%; Ti: 0.001 to 0.010%; N: up to 0.007%; Cr: 0.05 to 1.0%; Mo: not less than 0.02% and less than 0.5%; Ni: 0.03 to 1.0%; Cu: 0.02 to 1.0%; V: 0.020 to 0.20%; Ca: 0.0005 to 0.005%; and Nb: 0 to 0.05%, where carbon equivalent is not less than 0.430% and less than 0.500%, the microstructure main phase from the surface to an in-the-wall portion is tempered martensite or tempered bainite, prior austenite grain size is lower than 6.0, a portion between 1 mm from the inner surface and 1 mm from the outer surface has Vickers hardness of 250 Hv or lower, and yield strength is 555 MPa or higher.

Provided is a high-strength seamless steel pipe having the composition which contains, by mass %, 0.20 to 0.50% C, 0.05 to 0.40% Si, 0.3 to 0.9% Mn, 0.015% or less P, 0.005% or less S, 0.005 to 0.1% Al, 0.008% or less N, 0.6 to 1.7% Cr, 0.4 to 1.0% Mo, 0.01 to 0.30% V, 0.01 to 0.06% Nb, 0.0003 to 0.0030% B, and 0.0030% or less O (oxygen). The high-strength seamless steel pipe has the microstructure where a volume fraction of a tempered martensitic phase is 95% or more, and prior austenitic grains have a grain size number of 8.5 or more, and a segregation degree index Ps which is defined by a formula Ps=8.1 (XSi+XMn+XMo)+1.2XP relating to XM which is a ratio between a segregated portion content and an average content is set to less than 65.

The seamless steel pipe of the present embodiment consists of in mass %, C: 0.21 to 0.35%, Si: 0.10 to 0.50%, Mn: 0.05 to 1.00%, P: 0.025% or less, S: 0.010% or less, Al: 0.005 to 0.100%, N: 0.010% or less, Cr: 0.05 to 1.50%, Mo: 0.10 to 1.50%, Nb: 0.010 to 0.050%, B: 0.0003 to 0.0050%, and Ti: 0.002 to 0.050%, the balance being Fe and impurities. In a main body region of the seamless steel pipe, a grain size number of prior-austenite grain conforming to ASTM E112 is 7.0 or more, a difference between a maximum value and a minimum value of the grain size number is 1.0 or less, yield strength is 655 to less than 862 MPa, and a difference between a maximum value and a minimum value of tensile strength is 27.6 MPa or less.

Disclosed is a low alloy steel for oil well pipes which has excellent sulfide stress cracking resistance and is suitable for casing and tubing for oil wells or gas wells. Specifically disclosed is a low alloy steel for oil well pipes containing, in mass %, 0.2-0.35% of C, 0.05-0.5% of Si, 0.05-1.0% of Mn, not more than 0.025% of P, not more than 0.01% of S, 0.005-0.10% of Al, 0.1-1.0% of Cr, 0.5-1.0% of Mo, 0.002-0.05% of Ti, 0.05-0.3% of V, 0.0001-0.005% of B, not more than 0.01% of N, not more than 0.01% of O (oxygen), 0-0.1% of Nb, 0-0.01% of Ca, 0-0.01% of Mg and 0-0.1% of Zr, and having a half-value breadth (H) and a hydrogen diffusion coefficient (D) (10−6 cm2 / s) satisfying the following formula (1):30H+D≦19.5  (1).

One embodiment of the present invention provides an ultrahigh-strength steel having excellent cold workability and SSC resistance, comprising, by wt%, carbon (C) in an amount of more than 0.08% and equal to or less than 0.2%, 0.05-0.5% of silicon (Si), 0.5-2% of manganese (Mn), 0.005-0.1% of aluminum (Al), 0.01% or less of phosphorus (P), 0.0015% or less of sulfur (S), 0.001-0.03% of niobium (Nb), 0.001-0.03% of vanadium (V), 0.001-0.03% of titanium (Ti), 0.01-1% of chromium (Cr), 0.01-0.15% of molybdenum (Mo), 0.01-0.5% of copper (Cu), 0.05-4% of nickel (Ni), 0.0005-0.004% of calcium (Ca), and the balance of Fe and other inevitable impurities, wherein the microstructure of a surface layer part, which is the region from the surface to 10% of the total thickness, comprises 90 area% or more of polygonal ferrite, the microstructure of the region excluding the surface layer part comprises 90 area% or more of tempered martensite or 90 area% or more of a mixed structure of tempered martensite and tempered bainite, and the dislocation density of the surface layer part is 3*10 14 / m 2 or less.

A thick-wall oil-well steel pipe has a wall thickness of 40 mm or more, excellent SSC resistance and high strength. The thick-wall oil-well steel pipe has a composition containing, in mass %, C: 0.40 to 0.65%, Si: 0.05 to 0.50%, Mn: 0.10 to 1.0%, P: 0.020% or less, S: 0.0020% or less, sol. Al: 0.005 to 0.10%, Cr more than 0.40 to 2.0%, Mo: more than 1.15 to 5.0%, Cu: 0.50% or less, Ni: 0.50% or less, N: 0.007% or less, and O: 0.005% or less. The number of carbide which has a circle equivalent diameter of 100 nm or more and contains 20 mass % or more of Mo is 2 or less per 100 mm2. The thick-wall oil-well steel pipe has yield strength of 827 MPa or more. A difference between a maximum value and a minimum value of the yield strength in the wall-thickness direction is 45 MPa or less.

A high-strength seamless steel pipe for oil country tubular goods comprising, by mass %, C: 0.20% to 0.50%, Si: 0.05% to 0.40%, Mn: 0.3% to 0.9%, P: 0.015% or less, S: 0.005% or less, Al: 0.005% to 0.1%, N: 0.006% or less, Mo: more than 1.0% to 3.0% or less, V: 0.01% to less than 0.05%, Nb: 0.001% to less than 0.01%, B: 0.0003% to 0.0030%, O: 0.0030% or less, and Ti: 0.003% to 0.025%, and wherein Ti / N: 2.0 to 5.0 is satisfied, a volume fraction of a tempered martensitic is 95% or more, prior austenite grains have a grain size number of 8.5 or more, and in a cross-section perpendicular to a rolling direction, the number of nitride-based inclusions having a grain size of 4 μm or more is 100 or less per 100 mm2, the number of nitride-based inclusions having a grain size of less than 4 μm is 1000 or less per 100 mm2, the number of oxide-based inclusions having a grain size of 4 μm or more is 40 or less per 100 mm2, and the number of oxide-based inclusions having a grain size of less than 4 μm is 400 or less per 100 mm2.

Provided is a high-strength seamless steel pipe having the composition which contains, by mass %, 0.20 to 0.50% C, 0.05 to 0.40% Si, 0.3 to 0.9% Mn, 0.015% or less P, 0.005% or less S, 0.005 to 0.1% Al, 0.008% or less N, more than 0.6% and 1.7% or less Cr, more than 1.0% and 3.0% or less Mo, 0.01 to 0.30% V, 0.001% or more and less than 0.01% Nb, 0.0003 to 0.0030% B, and 0.0030% or less O (oxygen). The high-strength seamless steel pipe has the microstructure where a volume fraction of a tempered martensitic phase is 95% or more, and prior austenitic grains have a grain size number of 8.5 or more, and a segregation degree index Ps which is defined by a formula Ps=8.1 (XSi+XMn+XMo)+1.2XP is set to less than 65.

Provided is a high-strength seamless steel pipe having the composition which contains, by mass %, 0.20 to 0.50% C, 0.05 to 0.40% Si, 0.3 to 0.9% Mn, 0.015% or less P, 0.005% or less S, 0.005 to 0.1% Al, 0.008% or less N, more than 0.6% and 1.7% or less Cr, more than 1.0% and 3.0% or less Mo, 0.01 to 0.30% V, 0.001% or more and less than 0.01% Nb, 0.0003 to 0.0030% B, and 0.0030% or less O (oxygen). The high-strength seamless steel pipe has the microstructure where a volume fraction of a tempered martensitic phase is 95% or more, and prior austenitic grains have a grain size number of 8.5 or more, and a segregation degree index Ps which is defined by a formula Ps=8.1 (XSi+XMn+XMo)+1.2XP is set to less than 65.

A low-alloy steel pipe includes C: 0.15% to less than 0.30%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P: at most 0.030%, S: at most 0.0050%, Al: 0.005 to 0.100%, O: at most 0.005%, N: at most 0.007%, Cr: 0.10% to less than 1.00%, Mo: 1.0% to not more than 2.5%, V: 0.01 to 0.30%, Ti: 0.002 to 0.009%. Nb: 0 to 0.050%, B: 0 to 0.0050%, Ca: 0 to 0.0050%, Mo / Cr≥2.0, and the balance being Fe and impurities. The pipe has a crystal grain size number of 7.0 or more, 50 or more particles of cementite based on equivalent circle diameter and area of the matrix, M2C-based alloy carbide in a number density of not less than 25 / μm2, and a yield strength of 758 MPa or more.