Provided herein is a low-alloy high-strength seamless steel pipe. The steel pipe of the present invention has a composition that contains, in mass %, C: 0.20 to 0.50%, Si: 0.01 to 0.35%, Mn: 0.45 to 1.5%, P: 0.020% or less, S: 0.002% or less, 0: 0.003% or less, Al: 0.01 to 0.08%, Cu: 0.02 to 0.09%, Cr: 0.35 to 1.1%, Mo: 0.05 to 0.35%, B: 0.0010 to 0.0030%, Ca: 0.0010 to 0.0030%, Mg: 0.001% or less, and N: 0.005% or less, and in which the balance is Fe and incidental impurities. The steel pipe has a microstructure in which the number of oxide-base nonmetallic inclusions satisfying the composition ratios represented by predefined formulae is 20 or less per 100 mm.sup.2, and in which the number of oxide-base nonmetallic inclusions satisfying the composition ratios represented by other predefined formulae is 50 or less per 100 mm.sup.2.

Provided herein is a low-alloy high-strength seamless steel pipe. The steel pipe of the present invention has a composition that contains, in mass %, C: 0.20 to 0.50%, Si: 0.01 to 0.35%, Mn: 0.45 to 1.5%, P: 0.020% or less, S: 0.002% or less, 0: 0.003% or less, Al: 0.01 to 0.08%, Cu: 0.02 to 0.09%, Cr: 0.35 to 1.1%, Mo: 0.05 to 0.35%, B: 0.0010 to 0.0030%, Ca: 0.0010 to 0.0030%, Mg: 0.001% or less, and N: 0.005% or less, and in which the balance is Fe and incidental impurities. The steel pipe has a microstructure in which the number of oxide-base nonmetallic inclusions satisfying the composition ratios represented by predefined formulae is 20 or less per 100 mm.sup.2, and in which the number of oxide-base nonmetallic inclusions satisfying the composition ratios represented by other predefined formulae is 50 or less per 100 mm.sup.2.

What is provided is a high-strength steel sheet including, by mass %: C: 0.05% to 0.15%; Si: 1.5% or less; Mn: 2.00% to 5.00%; P: 0.100% or less; S: 0.010% or less; Al: 0.001% to 2.000%; N: 0.010% or less; and a remainder of Fe and impurities, in which Ceq defined by Ceq=C+Si/90+Mn/100+1.5P+3S is less than 0.21, the high-strength steel sheet contains martensite in an area ratio of 98% or more, and a residual structure is in an area ratio of 2% or less, a two-dimensional homogeneous dispersion ratio S defined by S=Sy.sup.2/Sx.sup.2 (Sx.sup.2 is a dispersion value of Mn concentration profile data in a sheet width direction, and Sy.sup.2 is a dispersion value of the Mn concentration profile data in a sheet thickness direction) is 0.85 or more and 1.20 or less, and a tensile strength is 1200 MPa or more.

What is provided is a high-strength steel sheet including, by mass %: C: 0.05% to 0.15%; Si: 1.5% or less; Mn: 2.00% to 5.00%; P: 0.100% or less; S: 0.010% or less; Al: 0.001% to 2.000%; N: 0.010% or less; and a remainder of Fe and impurities, in which Ceq defined by Ceq=C+Si/90+Mn/100+1.5P+3S is less than 0.21, the high-strength steel sheet contains martensite in an area ratio of 98% or more, and a residual structure is in an area ratio of 2% or less, a two-dimensional homogeneous dispersion ratio S defined by S=Sy.sup.2/Sx.sup.2 (Sx.sup.2 is a dispersion value of Mn concentration profile data in a sheet width direction, and Sy.sup.2 is a dispersion value of the Mn concentration profile data in a sheet thickness direction) is 0.85 or more and 1.20 or less, and a tensile strength is 1200 MPa or more.

A hot rolled steel sheet having a composition including the following elements, 0.18%≤Carbon≤0.3%, 1.8%≤Manganese≤4.5%. 0.8%≤Silicon≤2%,0 001%≤Aluminum ≤0.2%, 0.1%≤Molybdenum≤1%, 0.001%≤Titanium≤0.2%, 0.0001%≤Boron≤0.01%, 0%≤Phosphorus≤0.09%, 0%≤Sulfur≤0.09%.,0%≤Nitrogen≤0.09%, 0%≤Chromium≤0.5%, 0%≤Niobium≤0.1%, 0%≤Vanadium≤0.5%, 0%≤Nickel≤1%, 0%≤Copper≤1%, 0%≤Calcium≤0.005%, 0%≤Magnesium≤0.0010% the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel sheet including in area fraction, of at least 70% martensite, 8% to 25% residual austenite, wherein the shape factor of the residual austenite is between 4 and 12.

A hot rolled steel sheet having a composition including the following elements, 0.18%≤Carbon≤0.3%, 1.8%≤Manganese≤4.5%. 0.8%≤Silicon≤2%,0 001%≤Aluminum ≤0.2%, 0.1%≤Molybdenum≤1%, 0.001%≤Titanium≤0.2%, 0.0001%≤Boron≤0.01%, 0%≤Phosphorus≤0.09%, 0%≤Sulfur≤0.09%.,0%≤Nitrogen≤0.09%, 0%≤Chromium≤0.5%, 0%≤Niobium≤0.1%, 0%≤Vanadium≤0.5%, 0%≤Nickel≤1%, 0%≤Copper≤1%, 0%≤Calcium≤0.005%, 0%≤Magnesium≤0.0010% the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel sheet including in area fraction, of at least 70% martensite, 8% to 25% residual austenite, wherein the shape factor of the residual austenite is between 4 and 12.

A metal powder for additive manufacturing having a composition including the following elements, expressed in content by weight: 0.01%≤C≤0.2%, 4.6%≤Ti≤10%, (0.45×Ti)−0.22%≤B≤(0.45×Ti)+0.70%, S≤0.03%, P≤0.04%, N≤0.05%, O≤0.05% and optionally containing: Si≤1.5%, Mn≤3%, Al≤1.5%, Ni≤1%, Mo≤1%, Cr≤3%, Cu≤1%, Nb≤0.1%, V≤0.5% and including eutectic precipitates of TiB.sub.2 and Fe.sub.2B, the balance being Fe and unavoidable impurities resulting from the elaboration, the volume percentage of TiB.sub.2 being equal or more than 10% and the mean bulk density of the powder being 7.50 g/cm.sup.3 or less. A manufacturing method by atomization is also provided.

This hot-rolled steel sheet has a predetermined chemical composition, in which a metallographic structure contains, by area %, less than 3.0% of residual austenite, 15.0% or more and less than 60.0% of ferrite, and less than 5.0% of pearlite, has a ratio L.sub.60/L.sub.7 of a length L.sub.60 of a grain boundary having a crystal misorientation of 60° to a length L.sub.7 of a grain boundary having a crystal misorientation of 7° about a <110> direction of 0.60 or more, has a standard deviation of a Mn concentration of 0.60 mass % or less, and has a tensile strength of 980 MPa or more.

Provided is a high strength steel sheet that has a predetermined chemical composition and is manufactured under optimum conditions, the high strength steel sheet having a steel microstructure including, by area, ferrite: 30% or more and 80% or less, tempered martensite: 3.0% or more and 35% or less, and retained austenite: 8% or more, wherein the quotient of the area fraction of grains of the retained austenite, the grains having an aspect ratio of 2.0 or more and a minor axis length of 1 μm or less, divided by the total area fraction of the retained austenite is 0.3 or more, wherein the quotient of the average Mn content (mass %) in the retained austenite divided by the average Mn content (mass %) in the ferrite is 1.5 or more.

The steel sheet of the present invention has a steel microstructure containing, in area fraction, martensite: 20% to 100%, ferrite: 0% to 80%, and another metal phase: 5% or less, in which, on a surface of the steel sheet, a ratio of dislocation density in metal phases at a widthwise edge of the steel sheet to dislocation density in the metal phases at a widthwise center of the steel sheet is 100% to 140%, and, at a thicknesswise center of the steel sheet, a ratio of dislocation density in the metal phases at the widthwise edge of the steel sheet to dislocation density in the metal phases at the widthwise center of the steel sheet is 100% to 140%. The maximum amount of warpage of the steel sheet when the steel sheet is sheared to a length of 1 m in a rolling direction is 15 mm or less.