Abstract: Provided is a steel for nitrocarburizing that ensures mechanical workability before nitrocarburizing treatment. A steel for nitrocarburizing comprises: a chemical composition containing, in mass %, C: ?0.02% and <0.15%, Si: ?0.30%, Mn: 1.5-2.5%, P: ?0.025%, S: ?0.06%, Cr: 0.5-2.0%, Mo: 0.005-0.2%, V: 0.02-0.20%, Nb: 0.003-0.20%, Al: >0.020% and ?1.0%, Ti: >0.0050% and ?0.015%, N: ?0.0200%, Sb: 0.0030-0.010%, with a balance being Fe and inevitable impurities, satisfying Expression (1) or (2); and a steel microstructure where an area ratio of bainite phase is >50%, a prior austenite grain size is ?100 ?m, ?300/?m2 Ti precipitates with an equivalent circular particle size of ?30 nm disperse, Sb segregates to prior austenite grain boundaries.

Abstract: Provided is a steel component with excellent surface fatigue strength. The steel component has a nitride compound layer with a thickness of 5.0 ?m to 30.0 ?m and a hardened layer in an order from a component surface to a component inside, where a thickness of a porous layer on an outermost surface of the nitride compound layer is 3.0 ?m or less and 40.0% or less of a thickness of the nitride compound layer, and the hardened layer has a hardness of HV600 or more at a position of 50 ?m inward from the component surface, a hardness of HV400 or more at a position from the component surface to the component inside of 400 ?m, and a hardness of HV250 or more at a position from the component surface to the component inside of 600 ?m.

Abstract: Free-cutting steel that has the same or better machinability compared to low-carbon sulfur-lead composite free-cutting steel, despite of no-addition of Pb, is provided. Free-cutting steel contains, in mass %, C: 0.08% or less, Mn: 0.50 to 1.50%, P: 0.100% or less, S: 0.250 to 0.500%, N: 0.0050 to 0.0150%, O: more than 0.0100% and 0.0500% or less, Cr: 0.50 to 1.50%, at least one of Si, Al, or Ti: 0.050 to 0.500% in total, with the balance being Fe and inevitable impurities, with an A value defined by formula (1) satisfying 0.40 to 2.00, and with a B value defined by formula (2) satisfying 1.10×10?3 to 1.50×10?2; and a steel microstructure with distributed 3000 or more sulfide particles with an equivalent circular diameter of 5 ?m or less per mm2.

Abstract: Provided is steel for nitrocarburizing with excellent surface fatigue strength. The steel has a nitride compound layer with a thickness of 5.0 ?m to 30.0 ?m and a hardened layer in an order from a steel surface to steel inside, where a thickness of a porous layer on an outermost surface of the compound layer is 3.0 ?m or less and 40.0% or less of a compound layer's thickness, the hardened layer has hardness of HV600 or more, HV400 or more and HV250 or more at 50 ?m inward from the steel surface, from the steel surface to the steel inside of 400 ?m, and from the steel surface to the steel inside of 600 ?m, respectively, an unhardened portion excluding the compound and hardened layers has a predetermined chemical composition, and the hardened layer has a chemical composition with a higher N content than the unhardened portion.

Abstract: Provided is a free-cutting steel that, despites not containing Pb, has machinability by cutting higher than or equal to that of a low carbon sulfur-lead composite free-cutting steel. A free-cutting steel comprises: a chemical composition that contains, in mass %, C: less than 0.09%, Mn: 0.50% to 1.50%, S: 0.250% to 0.600%, O: more than 0.010% and 0.050% or less, and Cr: 0.50% to 1.50%, with a balance consisting of Fe and inevitable impurities, and in which a A value defined by the following formula (1) is 6.0 to 18.0, and a steel microstructure in which at least 500 particles/mm2 of sulfide of less than 1 ?m in equivalent circle diameter and at least 2000 particles/mm2 of sulfide of 1 ?m to 5 ?m in equivalent circle diameter are distributed.

Abstract: Free-cutting steel includes a specific composition, with an A value defined by a following formula (1) satisfying 4.0 or more and 20.0 or less, and remainder having Fe and an unavoidable impurity; and texture with 1000 or more sulfide particles with an equivalent circle diameter of less than 1 ?m per mm2, 500 or more sulfide particles with an equivalent circle diameter of 1 ?m or more and 5 ?m or less per mm2, and 1000 or more Pb particles with an equivalent circle diameter of 1 ?m or less per mm2, wherein A value=(Mn+5Cr)/S ??(1) here, symbols of elements in the formula indicate contents (% by mass) of the elements.

Abstract: Provided is a wire rod that has superior machinability by cutting regardless of the type of tool material and the type of lubricant and even in the case where no lubricant is used. A wire rod for cutting work comprises: a specific chemical composition; and Vickers hardness that satisfies the following expressions (1) and (2) in the case where an average aspect ratio of ferrite grains at a position of ¼ of a diameter from a surface of the wire rod for cutting work is more than 2.8, and satisfies the following expressions (3) and (4) in the case where the average aspect ratio is 2.8 or less, Have?350??(1) H??30??(2) Have?250??(3) H??20??(4).

Abstract: Disclosed are a case hardened steel which is suitable as a material for producing mechanical structural parts having high rotating bending fatigue strength and impact fatigue strength at a relatively low cost, and a method of producing the same. The case hardening steel has a chemical composition containing, by mass %, C, Si, Mn, P, S, Cr, Mo, B, Ti, N, and O within a range satisfying a predetermined relationship, and Al in at least a predetermined amount in relation to the B, N, and Ti contents, with the balance being Fe and inevitable impurities, wherein ?I?80 is satisfied, where I represents an area in ?m2 of an oxide-based inclusion located at the center of a fish-eye on a fracture surface of the case hardening steel after being subjected to carburizing-quenching and tempering and subsequently to a rotating bending fatigue test.

Abstract: Provided is a steel for nitrocarburizing that can ensure hardened case depth by suppressing precipitation of Cr, V, and Nb in a part of the surface layer very close to the surface. The provided steel comprises: a specific chemical composition satisfying 9.5?([Cr]/52+[V]/50.9+[Nb]/92.9+M)×103?18.5, with the balance being Fe and inevitable impurities; and a steel microstructure in which an area ratio of bainite phase with respect to the entire microstructure is more than 50%.

Abstract: Provided is a steel for nitrocarburizing that ensures mechanical workability before nitrocarburizing treatment. A steel for nitrocarburizing comprises: a chemical composition containing, in mass %, C: ?0.02% and <0.15%, Si: ?0.30%, Mn: 1.5-2.5%, P: ?0.025%, S: ?0.06%, Cr: 0.5-2.0%, Mo: 0.005-0.2%, V: 0.02-0.20%, Nb: 0.003-0.20%, Al: >0.020% and ?1.0%, Ti: >0.0050% and ?0.015%, N: ?0.0200%, Sb: 0.0030-0.010%, with a balance being Fe and inevitable impurities, satisfying Expression (1) or (2); and a steel microstructure where an area ratio of bainite phase is >50%, a prior austenite grain size is ?100 ?m, ?300/?m2 Ti precipitates with an equivalent circular particle size of ?30 nm disperse, Sb segregates to prior austenite grain boundaries.

Abstract: A case hardening steel suitable as raw material for producing a mechanical structural part having high rotating bending fatigue strength and pitting fatigue strength at relatively low cost and a method of producing the case hardening steel are provided. A case hardening steel comprises a chemical composition containing, in mass %, C, Si, Mn, P, S, Cr, Mo, Al, N, and O in a predetermined relationship, with a balance being Fe and inevitable impurities, wherein ?I?80 (where I denotes an area (?m2) of an oxide-based inclusion located in a fish eye central portion at a fracture surface after subjecting the case hardening steel to carburizing-quenching and tempering and then performing a rotating bending fatigue test).

Abstract: Provided is a wire rod that has superior machinability by cutting regardless of the type of tool material and the type of lubricant and even in the case where no lubricant is used. A wire rod for cutting work comprises: a specific chemical composition; and Vickers hardness that satisfies the following expressions (1) and (2) in the case where an average aspect ratio of ferrite grains at a position of 1/4 of a diameter from a surface of the wire rod for cutting work is more than 2.8, and satisfies the following expressions (3) and (4) in the case where the average aspect ratio is 2.8 or less, Have?350 ??(1) H??30 ??(2) Have?250 ??(3) H??20 ??(4).

Abstract: Provided is a steel for nitrocarburizing that can ensure hardened case depth by suppressing precipitation of Cr, V, and Nb in a part of the surface layer very close to the surface. The provided steel comprises: a specific chemical composition satisfying 9.5?([Cr]/52+[V]/50.9+[Nb]/92.9+M)×103?18.5, with the balance being Fe and inevitable impurities; and a steel microstructure in which an area ratio of bainite phase with respect to the entire microstructure is more than 50%.

Abstract: A case hardening steel suitable as raw material for producing a mechanical structural part having high rotating bending fatigue strength and pitting fatigue strength at relatively low cost and a method of producing the case hardening steel are provided. A case hardening steel comprises a chemical composition containing, in mass %, C, Si, Mn, P, S, Cr, Mo, Al, N, and O in a predetermined relationship, with a balance being Fe and inevitable impurities, wherein ?I?80 (where I denotes an area (?m2) of an oxide-based inclusion located in a fish eye central portion at a fracture surface after subjecting the case hardening steel to carburizing-quenching and tempering and then performing a rotating bending fatigue test).

Abstract: A steel plate has a chemical composition containing, by mass %, C: 0.03% or more and 0.08% or less, Si: 0.01% or more and 1.0% or less, Mn: 1.2% or more and 3.0% or less, P: 0.015% or less, S: 0.005% or less, Al: 0.08% or less, Nb: 0.005% or more and 0.07% or less, Ti: 0.005% or more and 0.025% or less, N: 0.010% or less, O: 0.005% or less and the balance being Fe and inevitable impurities, a structure being a dual-phase structure consisting of a bainite phase and island martensite, wherein the area fraction of the island martensite is 3% to 15%, the equivalent circle diameter of the island martensite is 3.0 ?m or less, and the remainder of the structure is a bainite phase.

Abstract: Disclosed are a case hardened steel which is suitable as a material for producing mechanical structural parts having high rotating bending fatigue strength and impact fatigue strength at a relatively low cost, and a method of producing the same. The case hardening steel has a chemical composition containing, by mass %, C, Si, Mn, P, S, Cr, Mo, B, Ti, N, and O within a range satisfying a predetermined relationship, and Al in at least a predetermined amount in relation to the B, N, and Ti contents, with the balance being Fe and inevitable impurities, wherein ?I?80 is satisfied, where I represents an area in ?m2 of an oxide-based inclusion located at the center of a fish-eye on a fracture surface of the case hardening steel after being subjected to carburizing-quenching and tempering and subsequently to a rotating bending fatigue test.

Abstract: A steel plate has a chemical composition containing, by mass %, C: 0.03% or more and 0.08% or less, Si: 0.01% or more and 1.0% or less, Mn: 1.2% or more and 3.0% or less, P: 0.015% or less, S: 0.005% or less, Al: 0.08% or less, Nb: 0.005% or more and 0.07% or less, Ti: 0.005% or more and 0.025% or less, N: 0.010% or less, O: 0.005% or less and the balance being Fe and inevitable impurities, a metallographic structure including a bainite phase and island martensite, and a polygonal ferrite in surface portions within 5 mm from the upper and lower surfaces, wherein the area fraction of the island martensite is 3% to 15%, the equivalent circle diameter of the island martensite is 3.0 ?m or less, the area fraction of the polygonal ferrite in the surface portions is 10% to less than 80%.

Abstract: Provided are a steel pipe for a linepipe with high compressive strength and a heavy wall thickness, and a method of manufacturing the same. Lowering of yield stress caused by a Bauschinger effect can be suppressed by optimizing the metal microstructure of a steel plate without requiring particular forming conditions in forming the steel pipe, and without requiring heat treatment after pipe making. A steel pipe has the composition which contains by mass % 0.03 to 0.10% C, 0.30% or less Si, 1.00 to 2.00% Mn, 0.015% or less P, 0.003% or less S, 0.080% or less Al, 0.005 to 0.035% Nb, 0.005 to 0.020% Ti, and Fe and unavoidable impurities as a balance. The steel pipe has metal microstructure where a fraction of bainite is 60% or more, a fraction of rolled ferrite is 5% or less, and a fraction of M-A constituent (MA) is 3% or less.

Abstract: A base metal for a high-toughness clad plate has a shear area of 85% or more in a ?20° C. DWTT test and includes, in terms of % by mass, C: 0.030% to 0.10%, Si: 0.10% to 0.30%, Mn: 1.30% to 1.80%, P: 0.015% or less, S: 0.003% or less, Mo: 0.05% to 0.50%, V: less than 0.010%, Nb: 0.010% to 0.060%, Ti: 0.005% to 0.020%, Al: 0.040% or less, Ca: 0.0010% to 0.0040%, N: 0.0030% to 0.0060%, and the balance being Fe and unavoidable impurities.

Abstract: A steel plate has a chemical composition containing, by mass %, C: 0.03% or more and 0.08% or less, Si: 0.01% or more and 1.0% or less, Mn: 1.2% or more and 3.0% or less, P: 0.015% or less, S: 0.005% or less, Al: 0.08% or less, Nb: 0.005% or more and 0.07% or less, Ti: 0.005% or more and 0.025% or less, N: 0.010% or less, O: 0.005% or less and the balance being Fe and inevitable impurities, a structure being a dual-phase structure consisting of a bainite phase and island martensite, wherein the area fraction of the island martensite is 3% to 15%, the equivalent circle diameter of the island martensite is 3.0 ?m or less, and the remainder of the structure is a bainite phase.