A preferable aspect of the present invention provides a hot-rolled steel sheet with excellent low-temperature toughness, a steel pipe using the same, and a manufacturing method therefor, wherein the hot-rolled steel sheet contains, by weight, 0.35-0.65% C, 0.01-0.4% Si, 13-26% Mn, 0.01-0.3% Ti, 0.01% or less B, 4% or less Al, 1-6% Cr, 0.05% or less P, 0.02% or less S, 0.01% or less N, 0.01-2% Cu, 0.001-0.015% Nb, and the balance Fe and other unavoidable impurities, the alloy elements satisfying the following relational formulas—[Relational formula 1] 70<[10*(C/12)+(Mn/55)+(Al/27)]*100<95 and [Relational formula 2] 4<100*(Cr/52+100*(Nb/93))<9; wherein a microstructure comprises, by area fraction, 97% or more (including 100%) of austenite and 3% or less (including 0%) of a carbide, the crystal grain size of the austenite being 18-30 μm or less; and wherein the size of the carbide is 0.5 μm or less.

The present disclosure provides a low temperature resistant oil casing having high strength and high toughness, and the manufacturing method thereof, the chemical composition of the oil casing by mass of: C: 0.08-0.14%, Si: 0.1-0.4%, Mn: 0.6-1.3%, Cr: 0.5-1.5%, Mo: 0.2-0.5%, Ni: 0.2-0.5%, Nb: 0.02-0.05%, V: 0-0.1%, Al: 0.01-0.05%, Ca: 0.0005-0.005%, and the balance being Fe and unavoidable impurities. The method of manufacturing the oil casing includes: (1) smelting and continuous casting; (2) perforating and continuous rolling; (3) heat treatment, wherein an austenitizing temperature is controlled in the range of 900-930° C., and held for 30-60 min, followed by quenching, subsequently, tempering at temperature of 480-600° C., holding the temperature for 50-80 min; (4) hot sizing.

Provided is an alloy having a high strength and a low thermal expansion coefficient. The alloy according to the present disclosure includes a chemical composition containing, in mass %: C: 0.10% or less, Si: 0.50% or less, Mn: 0.15 to 0.60%, P: 0.015% or less, 5: 0.0030% or less, Ni: 30.0 to 40.0%, Cr: 0.50% or less, Mo: 0.50% or less, Co: 0.250% or less, Al: 0.0150% or less, Ca: 0.0050% or less, Mg: 0.0300% or less, N: 0.0100% or less, O: 0.0300% or less, Pb: 0.0040% or less, and Zn: 0.020% or less, one or more elements selected from the group consisting of Nb: 0 to less than 0.145%, Ti: 0 to less than 0.145%, and. V: 0 to less than 0.145%: 0.015 to less than 0.145% in total, with the balance being Fe and impurities, and satisfying Formula (1).

(Nb+3×Ti+V)/(C+N)≤6.00   (1)

Provided is a forming system that expands and deforms a metal material to perform forming, the forming system including: a forming device that performs the forming; and at least one transport device that transports the metal material, in which the transport device transports the metal material to the forming device in a held state, in a case where a pretreatment device that performs pretreatment for the forming is present, the transport device transports the metal material to the pretreatment device in a held state, and during a period from a transport process in which the metal material is transported to any of devices by the transport device to a treatment process performed by any of the devices, at least one of the transport device and any of the devices continues holding the metal material.

A piercing machine includes a plurality of skewed rolls, a plug, a mandrel bar and an outer surface cooling mechanism. The outer surface cooling mechanism is disposed around the mandrel bar at a position that is rearward of the plug, and with respect to an outer surface of a hollow shell advancing through a cooling zone which has a specific length in an axial direction of the mandrel bar and which is located rearward of the plug, as seen from an advancing direction of the hollow shell, the outer surface cooling mechanism ejects a cooling fluid toward an upper part of the outer surface, a lower part of the outer surface, a left part of the outer surface and a right part of the outer surface of the hollow shell to cool the hollow shell inside the cooling zone.

An electric resistance welded steel pipe includes a base metal zone and an electric resistance welded zone. The base metal zone has a predetermined chemical composition and a microstructure including, by volume, ferrite: more than 30%, and bainite: 10% or more. The total volume fraction of the ferrite and the bainite is 70% or more and 95% or less. The balance being one or two or more phases selected from pearlite, martensite, and austenite. Further, when regions surrounded by boundaries between adjacent crystals having a misorientation of 15° or more are defined as crystal grains, the average size of the crystal grains is less than 7.0 μm, and the volume fraction of crystal grains having a size of 40.0 μm or more is 30% or less. A compressive residual stress generated in the inner and outer surfaces of the steel pipe in the axial direction is 250 MPa or less.

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 high-strength seamless steel pipe, and a method for manufacturing same. A high-strength seamless steel pipe of the present invention has a yield strength of 758 MPa or more, and a K.sub.ILIMIT value of 23.0 MPa√m or more as an evaluation index of sulfide stress corrosion cracking resistance.

A seamless steel pipe of the present invention is a seamless steel pipe having a composition including, in mass %, C: 0.01 to 0.12%, Si: 0.01 to 0.8%, Mn: 0.10 to 2.00%, P: 0.050% or less, S: 0.040% or less, Al: 0.010 to 0.100%, Cu: 0.03 to 0.80%, Ni: 0.01 to 0.50%, Mo: 0.01 to 0.20%, Sb: 0.002 to 0.50%, Cr: 0.004% or less, W: 0.002% or less, and the balance Fe and incidental impurities, and a structure including a ferrite phase having an area percentage of 50 to 65%, a pearlite phase having an area percentage of 2% or less, and one or both of a bainite phase and a martensitic phase representing the remainder, the seamless steel pipe having a yield strength of 230 MPa or more, and a tensile strength of 380 MPa or more.

A stainless steel seamless pipe having high strength and excellent corrosion resistance. The stainless steel seamless pipe has a specified composition in which C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy a predetermined formula, a microstructure containing at least 25% martensitic phase, at most 65% ferrite phase, and at most 40% retained austenite phase by volume, and a yield strength of 758 MPa or more.