Provided are a high-strength steel sheet and a method for manufacturing the steel sheet. The high-strength steel sheet has a specified chemical composition with the balance being Fe and inevitable impurities, a microstructure including, in terms of area ratio, 25% or less of a ferrite phase, 75% or more of a bainite phase and/or a martensite phase, and 5% or less of cementite, in which, in a surface layer that is a region within 50 μm from the surface in the thickness direction, the area ratio of a ferrite phase is 5% to 20%, and a tensile strength is 1180 MPa or more.

A high-strength plated steel sheet sequentially includes an internal oxidized layer, a soft layer containing the internal oxidized layer, and a hard layer including a structure having metallic structure which contains a low-temperature-transformation produced phase in a proportion of 20 to 85% by area of the whole of the metallic structure, polygonal ferrite in a proportion more than 10% by area, and 70% or less by area of the whole of the metallic structure, and-retained austenite in a proportion of 5% or more by volume of the whole of the metallic structure. The high-strength plated steel sheet satisfies the average depth D of the soft layer being 20 μm or more, the average depth d of the internal oxidized layer being 4 μm or more and less than D, and a tensile strength being 980 MPa or more.

When a non-oriented electrical steel sheet is produced by hot rolling a slab containing, by mass %, C: not more than 0.0050%, Si: 1.5-5.0%, Mn: 0.20-3.0%, sol. Al: not more than 0.0050%, P: not more than 0.2%, S: not more than 0.0050% and N: not more than 0.0040% to form a hot rolled sheet, cold rolling the hot rolled sheet without conducting a hot band annealing and then subjecting to a finish annealing, a compositional ratio of CaO in oxide-based inclusions existing in the slab defined by CaO/(SiO.sub.2+Al.sub.2O.sub.3+CaO) is set to not less than 0.4 and/or a compositional ratio of Al.sub.2O.sub.3 defined by Al.sub.2O.sub.3/(SiO.sub.2+Al.sub.2O.sub.3+CaO) is set to not less than 0.3, and a coiling temperature in the hot rolling is set to not lower than 650° C.

Disclosed herein is a high-strength plated steel sheet having a plated layer on the surface of a base steel sheet and containing predetermined steel components. The steel sheet includes, in the order from the interface of the base steel sheet and the plated layer towards the base steel sheet: a soft layer having a Vickers hardness that is 90% or less of the Vickers hardness at a portion t/4 of the base steel sheet, where t is a sheet thickness of the base steel sheet; and a hard layer consisting of a structure which is mainly composed of martensite and bainite and in which the average grain size of prior austenite is 20 μm or less. The average depth D of the soft layer is 20 μm or greater, and the average depth d of an internal oxide layer is 4 μm or greater and smaller than D.

The high-strength plated steel sheet of the present invention has a plated layer on the surface of a base steel sheet and contains predetermined steel components. The steel sheet includes, in the order from the interface of the base steel sheet and the plated layer towards the base steel sheet: a soft layer having a Vickers hardness that is 90% or less of the Vickers hardness at a portion t/4 of the base steel sheet, where t is a sheet thickness of the base steel sheet: and a hard layer containing martensite, bainite, and ferrite in predetermined ranges. The average depth D of the soft layer is 20 μm or greater, and the average depth d of an internal oxide layer is 4 μm or greater and smaller than D.

Disclosed herein is a high-strength plated steel sheet containing an internal oxidized layer, a soft layer including the internal oxidized layer, and a hard layer including a structure having metallic structure containing a low-temperature-transformation produced phase in a proportion of 70% or more by area of the whole of the metallic structure, in which polygonal ferrite is in a proportion of 0% or more by area, and 10% or less by area of the same, and retained austenite is in a proportion of 5% or more by volume of the same. The high-strength plated steel sheet satisfies the average depth D of the soft layer is 20 μm or more, the average depth d of the internal oxidized layer is 4 μm or more and less than D, and a tensile strength of 980 MPa or more.

Disclosed herein is a high-strength plated steel sheet containing an internal oxidized layer, a soft layer including the internal oxidized layer, and a hard layer including a structure having metallic structure containing a low-temperature-transformation produced phase in a proportion of 70% or more by area of the whole of the metallic structure, in which polygonal ferrite is in a proportion of 0% or more by area, and 10% or less by area of the same, and retained austenite is in a proportion of 5% or more by volume of the same. The high-strength plated steel sheet satisfies the average depth D of the soft layer is 20 μm or more, the average depth d of the internal oxidized layer is 4 μm or more and less than D, and a tensile strength of 980 MPa or more.

A gear includes a sintered body, in which Fe is contained as a principal component, Ni is contained in a proportion of 2 mass % or more and 20 mass % or less, Si is contained in a proportion of 0.3 mass % or more and 5.0 mass % or less, C is contained in a proportion of 0.005 mass % or more and 0.3 mass % or less, and one element selected from the group consisting of Ti, V, Y, Zr, Nb, Hf, and Ta is defined as a first element, that is contained in a proportion of 0.01 mass % or more and 0.7 mass % or less.

A method of forming a cutting element comprises disposing diamond particles in a container and disposing a metal powder on a side of the diamond particles. The diamond particles and the metal powder are sintered so as to form a polycrystalline diamond material and a low-carbon steel material comprising less than 0.02 weight percent carbon and comprising an intermetallic precipitate on a side of the polycrystalline diamond material. Related cutting elements and earth-boring tools are also disclosed.

A hot-rolled strip steel product having a chemical composition consisting of, in terms of weight percentages (wt. %): 0.025%-0.070% C, 0%-1.10% Si, 0.50%-2.0% Mn, <0.020% P, <0.050% S, <0.010% N, 0%-0.60% Cr, 0%-0.20% Ni, 0%-0.25% Cu, 0%-0.20% Mo, 0%-0.15% Al, 0%-0.050% Nb, 0.020%-0.20-% V, 0.020%-0.15% Ti, 0%-0.0010% B, remainder Fe and inevitable impurities, wherein the strip steel product has a microstructure comprising of, in terms of volume percentages (vol. %), ferrite ≥90%, wherein the ferrite structure comprises 10%-50% quasi-polygonal ferrite and a reminder of ferrite structure is polygonal ferrite and/or bainite: and wherein the steel strip product has an average ferrite grain size of <10 μm, an average hole expansion ratio of ≥50%, a yield strength (Rp0.2%) longitudinal to rolling direction of ≥660 MPa and a tensile strength of ≥760 MPa.