Provided is a steel sheet and a method for manufacturing same, the steel sheet which can be used for automobile parts and the like, having excellent bending workability, and excellent balance of strength and ductility and of strength and hole expansibility.

Provided is a steel sheet which can be used for automobile parts and the like, and relates to a steel sheet having a superior balance of strength and ductility and strength and hole expansion ratio and superior bending formability, and a method for manufacturing same.

A steel sheet has a predetermined chemical composition containing, in mass %, C: more than 0.18% and less than 0.30%, Mn: more than 2.50% and 4.00% or less, and other elements. The steel micro-structure at a position at a depth of ¼ of the sheet thickness from the surface in an L cross section of the steel sheet is, in area %, tempered martensite: 25 to 90%, ferrite: 5% or less, retained austenite: 10 to 50%, and bainite: 5% or less. At a position at a depth of ¼ of the sheet thickness from the surface of the L cross section, the proportion of a total area of retained austenite grains having an area of 1 μm.sup.2 or more and having a grain shape circularity of 0.1 or more is less than 50% with respect to the entire area of retained austenite. The steel sheet satisfies the formula C.sub.Mnγ/C.sub.Mnα≥1.2.

Disclosed in the present disclosure is an ultrahigh-strength dual-phase steel. The matrix structure of the ultrahigh-strength dual-phase steel is ferrite and martensite, wherein the ferrite and the martensite are evenly distributed in an island shape. The ultrahigh-strength dual-phase steel contains the following chemical elements in percentage by mass: 0.12-0.2% of C, 0.5-1.0% of Si, 2.5-3.0% of Mn, 0.02-0.05% of Al, 0.02-0.05% of Nb, 0.02-0.05% of Ti, and 0.001-0.003% of B. Further disclosed in the present disclosure is a manufacturing method for the ultrahigh-strength dual-phase steel, comprising the steps of smelting and continuous casting, hot rolling, cold rolling, annealing, tempering, and leveling. The ultrahigh-strength dual-phase steel in the present disclosure has not only good mechanical properties but also excellent delayed cracking resistance and low initial hydrogen content, and can be suitable for manufacturing of vehicle safety structural parts.

Disclosed in the present disclosure is an ultrahigh-strength dual-phase steel. The matrix structure of the ultrahigh-strength dual-phase steel is ferrite and martensite, wherein the ferrite and the martensite are evenly distributed in an island shape. The ultrahigh-strength dual-phase steel contains the following chemical elements in percentage by mass: 0.12-0.2% of C, 0.5-1.0% of Si, 2.5-3.0% of Mn, 0.02-0.05% of Al, 0.02-0.05% of Nb, 0.02-0.05% of Ti, and 0.001-0.003% of B. Further disclosed in the present disclosure is a manufacturing method for the ultrahigh-strength dual-phase steel, comprising the steps of smelting and continuous casting, hot rolling, cold rolling, annealing, tempering, and leveling. The ultrahigh-strength dual-phase steel in the present disclosure has not only good mechanical properties but also excellent delayed cracking resistance and low initial hydrogen content, and can be suitable for manufacturing of vehicle safety structural parts.

A high-strength galvanized steel sheet includes a steel sheet having a chemical composition containing a predetermined component element, a mass ratio of a content of Si to a content of Mn in the steel (Si/Mn) being 0.1 or more and less than 0.2, and the balance: Fe and incidental impurities, and a steel structure in which an average grain size of inclusions containing at least one of Al, Si, Mg, and Ca and existing in an area extending from a surface to a position of ⅓ of a sheet thickness is 50 μm or less, and an average nearest distance between ones of the inclusions is 20 μm or more; and a galvanized layer provided on a surface of the steel sheet, in which an amount of diffusible hydrogen contained in the steel is less than 0.25 mass ppm, and a tensile strength is 1100 MPa or more.

A method of forming a shaped steel object is provided. The method includes cutting a blank from an alloy composition including 0.05-0.5 wt. % carbon, 4-12 wt. % manganese, 1-8 wt. % aluminum, 0-0.4 wt. % vanadium, and a remainder balance of iron. The method also includes heating the blank until the blank is austenitized to form a heated blank, transferring the heated blank to a press, forming the heating blank into a predetermined shape to form a stamped object, and decreasing the temperature of the stamped object to a temperature between a martensite start (Ms) temperature of the alloy composition and a martensite final (Mf) temperature of the alloy composition to form a shaped steel object comprising martensite and retained austenite.

A method of forming a shaped steel object is provided. The method includes cutting a blank from an alloy composition including 0.05-0.5 wt. % carbon, 4-12 wt. % manganese, 1-8 wt. % aluminum, 0-0.4 wt. % vanadium, and a remainder balance of iron. The method also includes heating the blank until the blank is austenitized to form a heated blank, transferring the heated blank to a press, forming the heating blank into a predetermined shape to form a stamped object, and decreasing the temperature of the stamped object to a temperature between a martensite start (Ms) temperature of the alloy composition and a martensite final (Mf) temperature of the alloy composition to form a shaped steel object comprising martensite and retained austenite.

The present invention relates to a zinc plated steel sheet having excellent surface quality and spot weldability, and a manufacturing method therefore. A zinc plated steel sheet according to one aspect of the present invention comprises a base steel sheet and a zinc-based plating layer formed on the surface of the base steel sheet, wherein the GDOES profile of oxygen, which is measured in the depth direction from the surface of the base steel sheet, has a form in which a local minimum point and a local maximum point alternately appear in the depth direction from the surface, and the difference (a local maximum value—a local minimum value) between the oxygen concentration (a local minimum value) at the local minimum point and the oxygen concentration (a local maximum value) at the local maximum point can be 0.1 wt % or more.

A non-oriented electrical steel sheet according to an embodiment of the present invention includes, in wt %, Si: 1.5 to 4.0%, Al: 0.1 to 1.5%, Mn: 0.05 to 1.5%, Sn: 0.015 to 0.1%, P: 0.005 to 0.05%, Ga: 0.001 to 0.004%, and Bi: 0.0005 to 0.003%, and the balance of Fe and inevitable impurities. An area fraction of texture in a {118}//ND orientation is higher than that of texture in a {111}///ND orientation.