Abstract: The present invention provides a high-strength steel sheet, which can be used in various applications including automobile parts and exhibits excellent collision safety and excellent moldability, and a method for manufacturing the high-strength steel sheet. The high-strength steel sheet according to an aspect of the present invention satisfies a predetermined chemical composition and has a metallographic microstructure having ferrite fraction: 0% to 10%, MA fraction: 0% to 30%, hard phase other than ferrite and MA: 70% to 100% in terms of area proportion and retained austenite fraction: 5% to 30% in terms of volume proportion, and in the high-strength steel sheet, the skewness of IQ as analyzed by the EBRD method is ?1.2 to ?0.3 when the skewness is expressed by a predetermined relational expression in a case where crystal grains having a bcc structure and a bct structure are regarded as an aggregation of regions having an area of 0.05 ?m2.

Abstract: The present invention provides a high-strength steel sheet, which can be used in various applications including automobile parts and exhibits excellent collision safety and excellent moldability, and a method for manufacturing the high-strength steel sheet. The high-strength steel sheet according to an aspect of the present invention satisfies a predetermined chemical composition and has a metallographic microstructure having ferrite fraction: 0% to 10%, MA fraction: 0% to 30%, hard phase other than ferrite and MA: 70% to 100% in terms of area proportion and retained austenite fraction: 5% to 30% in terms of volume proportion, and in the high-strength steel sheet, the skewness of IQ as analyzed by the EBRD method is ?1.2 to ?0.3 when the skewness is expressed by a predetermined relational expression in a case where crystal grains having a bcc structure and a bct structure are regarded as an aggregation of regions having an area of 0.05 ?m2.

Abstract: In one aspect of the present invention, a hot-dip galvannealed steel sheet includes a steel sheet and a hot-dip galvannealed layer on the surface of the steel sheet. The steel sheet has a predetermined composition and has an average oxygen concentration of 0.10 mass % or less in the region of 1 ?m from the interface between the steel sheet and the hot-dip galvannealed layer toward the steel sheet. The metal microstructure of the steel sheet at a position of t/4 where t represents the sheet thickness of the hot-dip galvannealed steel sheet includes 50 to 85 area % of martensite, 15 to 50 area % of bainite, and 5 area % or less of ferrite.

Abstract: The present invention relates to a high-strength steel sheet which satisfies a specific chemical component composition, and which is configured such that: when the metal structure thereof is observed with a scanning electron microscope, polygonal ferrite is 30 to 60% by area; when the metal structure thereof is observed with an optical microscope, MA mixed structure is 21% by area or less; when the metal structure thereof is measured by an X-ray diffraction method, retained austenite is 8% by volume or more; an area ratio of polygonal ferrite having a circle equivalent diameter of 35.0 ?m or more is 0% and an area ratio of polygonal ferrite having a circle equivalent diameter of 10.0 ?m or more and less than 35.0 ?m is 10% or less in the polygonal ferrite; and connectivity as defined by a specific relational expression is 10% or more.

Abstract: In one aspect of the present invention, a hot-dip galvannealed steel sheet includes a steel sheet and a hot-dip galvannealed layer on the surface of the steel sheet. The steel sheet has a predetermined composition and has an average oxygen concentration of 0.10 mass % or less in the region of 1 ?m from the interface between the steel sheet and the hot-dip galvannealed layer toward the steel sheet. The metal microstructure of the steel sheet at a position of t/4 where t represents the sheet thickness of the hot-dip galvannealed steel sheet includes 50 to 85 area % of martensite, 15 to 50 area % of bainite, and 5 area % or less of ferrite.

Abstract: The present invention relates to a high-strength steel sheet which satisfies a specific chemical component composition, and which is configured such that: when the metal structure thereof is observed with a scanning electron microscope, polygonal ferrite is 30 to 60% by area; when the metal structure thereof is observed with an optical microscope, MA mixed structure is 21% by area or less; when the metal structure thereof is measured by an X-ray diffraction method, retained austenite is 8% by volume or more; an area ratio of polygonal ferrite having a circle equivalent diameter of 35.0 ?m or more is 0% and an area ratio of polygonal ferrite having a circle equivalent diameter of 10.0 ?m or more and less than 35.0 ?m is 10% or less in the polygonal ferrite; and connectivity as defined by a specific relational expression is 10% or more.

Abstract: 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 ?m or less. The average depth D of the soft layer is 20 ?m or greater, and the average depth d of an internal oxide layer is 4 ?m or greater and smaller than D.

Abstract: One aspect of the present invention is a high strength steel sheet having a specific component composition, wherein a metal structure of the steel sheet comprises polygonal ferrite, bainite, tempered martensite, and retained austenite; when the metal structure is observed with a scanning electron microscope, the metal structure satisfies polygonal ferrite: 10 to 50 area %, bainite: 10 to 50 area %, and tempered martensite: 10 to 80 area % with respect to the metal structure overall; and when the metal structure is measured by X-ray diffractometry, the metal structure satisfies retained austenite: 5.0 volume % or more, retained austenite with a carbon concentration of 1.0 mass % or less: 3.5 volume % or more, and retained austenite with a carbon concentration of 0.8 mass % or less: 2.4 volume % or less with respect to the metal structure overall.

Abstract: A high-strength hot-dip galvanized steel sheet with improved bending workability and reduced strength difference between a center part and end parts in the sheet width direction is provided. A method for manufacturing the high-strength hot-dip galvanized steel sheet is also provided. The hot-dip galvanized steel sheet has a hot-dip galvanizing layer on a surface of a base steel sheet containing: C, Mn, P, S, and Al; Ti and B in amounts satisfying expression (1): 0.005×[Mn]+0.02×[B]1/2+0.025?[Ti]?0.15; N; and Si as needed; the remainder being iron and unavoidable impurities. The base steel sheet has 50 area % or more of the martensite, 15-50 area % of the bainite, and 5 area % or less of the ferrite, with respect to the overall metallographic structure.

Abstract: Disclosed herein is a high-strength galvannealed steel sheet having a galvannealed layer on a surface of a base steel sheet and containing predetermined steel components. The steel sheet sequentially has, from the interface of the base steel sheet and the galvannealed layer, towards the base steel sheet: an internal oxide layer and containing at least one oxide selected from the group consisting of Si and Mn; a soft layer including the internal oxide layer, and satisfying a predetermined Vickers hardness; and a hard layer made up of a structure mainly composed of martensite. The average depth D of the soft layer is 20 ?m or greater, and the average depth d of the internal oxide layer is 4 ?m or greater and smaller than D. A coefficient of variation of KAM of the base steel sheet at the portion t/4 is 0.66 or less.

Abstract: The present disclosure relates to a high-strength hot-dip galvannealed steel sheet excellent in bake hardenability and bendability and having a component composition containing, in mass %, C: from 0.05 to 0.30%, Si: from 0.5 to 3.0%, Mn: from 0.2 to 3.0%, P: from 0 to 0.10%, S: from 0 to 0.010%, N: form 0 to 0.010%, and Al: from 0.001 to 0.10%, with the remainder being iron and unavoidable impurities. The steel sheet has a steel structure containing, in terms of area percentage, martensite: form 50 to 85% and ferrite: 0% or more and less than 5%, with the remainder being bainite. The steel sheet has a dislocation density of 5.0×1015 m?2 or more, a solute carbon amount of 0.08 mass % or more and a tensile strength of 1180 MPa or more.

Abstract: 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 ?m or greater, and the average depth d of an internal oxide layer is 4 ?m or greater and smaller than D.

Abstract: 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 ?m or less. The average depth D of the soft layer is 20 ?m or greater, and the average depth d of an internal oxide layer is 4 ?m or greater and smaller than D.

Abstract: 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 ?m or more, the average depth d of the internal oxidized layer is 4 ?m or more and less than D, and a tensile strength of 980 MPa or more.

Abstract: 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 ?m or more, the average depth d of the internal oxidized layer being 4 ?m or more and less than D, and a tensile strength being 980 MPa or more.

Abstract: A high-strength plated steel sheet 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 and bainite, and ferrite in predetermined ranges. The average depth D of the soft layer is 20 ?m or greater, and the average depth d of an internal oxide layer is 4 ?m or greater and smaller than D.

Abstract: Disclosed is an ultra high strength steel plate with at least 1100 MPa of tensile strength that has both an excellent strength-stretch balance and excellent bending workability, and a method for producing the same. The metal structure of the steel plate has martensite, and the soft phases of bainitic ferrite and polygonal ferrite. The area of the aforementioned martensite constitutes 50% or more, the area of the aforementioned bainitic ferrite constitutes 15% or more, and the area of the aforementioned polygonal ferrite constitutes 5% or less (including 0%). When the circle-equivalent diameter of the aforementioned soft phases is measured, the coefficient of variation (standard deviation/mean value) is less or equal to 1.0. The ultra high strength steel plate has at least 1100 MPa of tensile strength.

Abstract: In a steel sheet having a specific chemical composition and having a microstructure including ferrite that is a soft first phase by 20-50% in terms of the area ratio, the remainder being tempered martensite and/or tempered bainite that is a hard second phase, the microstructure of steel of a surface layer section of the steel sheet from the surface to the depth of 100 ?m and a center section of t/4-3t/4 (t is the sheet thickness) is controlled.

Abstract: Disclosed herein is a high-strength galvannealed steel sheet having a galvannealed layer on a surface of a base steel sheet and containing predetermined steel components. The steel sheet sequentially has, from the interface of the base steel sheet and the galvannealed layer, towards the base steel sheet: an internal oxide layer and containing at least one oxide selected from the group consisting of Si and Mn; a soft layer including the internal oxide layer, and satisfying a predetermined Vickers hardness; and a hard layer made up of a structure mainly composed of martensite. The average depth D of the soft layer is 20 ?m or greater, and the average depth d of the internal oxide layer is 4 ?m or greater and smaller than D. A coefficient of variation of KAM of the base steel sheet at the portion t/4 is 0.66 or less.

Abstract: In a high strength cold-rolled steel plate having a specific chemical composition, a soft first phase (ferrite) has an area ratio of 20-50%, the remainder being a hard second phase (tempered martensite and/or tempered bainite), among all the ferrite grains, ferrite grains that have an average grain diameter of 10-25 ?m account for a total area ratio of 80% or more, the number of the cementite grains that have an equivalent circle diameter of 0.3 ?m or more is more than 0.15 piece and 1.0 piece or less per 1 ?m2 of ferrite, and the tensile strength is 980 MPa or more.