Disclosed is a ferritic stainless steel with improved high temperature creep resistance. The disclosed ferritic stainless steel comprises by weight %: 0.005 to 0.03% of C, 0.005 to 0.03% of N, 0.05 to 0.9% of Si, 0.05 to 0.9% of Mn, 14.0 to 19.0% of Cr, 0.1 to 0.6% of Ti, 0.1 to 0.6% of Nb, 0.1 to 0.6% of Cu, 0.01 to 0.04% of P, 0.01% or less (excluding 0) of S, and the balance of iron (Fe) and inevitable impurities, and is characterized by satisfying Expressions (1) and (2) below.

0.5≤Nb/Cu≤3  Expression (1)

20≤[2Nb+Ti]/[C+N]  Expression (2)

wherein Nb, Cu, Ti, C, and N denote contents (wt %) of each element.

A method is for producing a high strength coated steel sheet having a yield stress YS>800 MPa, a tensile strength TS>1180 MPa, and improved formability and ductility. The steel contains: 15%≤C≤0.25%, 1.2%≤Si≤1.8%, 2%≤Mn≤2.4%, 0.1%≤Cr≤0.25%, Al≤0.5%, the remainder being Fe and unavoidable impurities. The sheet is annealed at a temperature higher than Ac3 and lower than 1000° C. for a time of more than 30 s, then quenched by cooling it to a quenching temperature QT between 250° C. and 350° C., to obtain a structure consisting of at least 60% of martensite and a sufficient austenite content such that the final structure contains 3% to 15% of residual austenite and 85% to 97% of martensite and bainite without ferrite, then heated to a partitioning temperature PT between 430° C. and 480° C. and maintained at this temperature for a partitioning time Pt between 10 s and 90 s, then hot dip coated and cooled to the room temperature.

A tailor-welded blank is made of two steels, one steel of Alloy A and one steel of Alloy B. Alloy A comprises 0.10-0.50 wt % C, 0.1-0.5 wt % Si, 2.0-8.0 wt % Mn, 0.0-6.0 wt % Cr, 0.0-2.0 wt % Mo, 0.0-0.15 wt % Ti, and 0.0-0.005 wt % B and wherein Alloy B comprises 0.06-0.12 wt % C, 0.1-0.25 wt % Si, 1.65-2.42 wt % Mn, 0.0-0.70 wt % Cr, 0.08-0.40 wt % Mo, 0.0-0.05 wt % V, and 0.01-0.05 wt % Ti.

A tailor-welded blank is made of two steels, one steel of Alloy A and one steel of Alloy B. Alloy A comprises 0.10-0.50 wt % C, 0.1-0.5 wt % Si, 2.0-8.0 wt % Mn, 0.0-6.0 wt % Cr, 0.0-2.0 wt % Mo, 0.0-0.15 wt % Ti, and 0.0-0.005 wt % B and wherein Alloy B comprises 0.06-0.12 wt % C, 0.1-0.25 wt % Si, 1.65-2.42 wt % Mn, 0.0-0.70 wt % Cr, 0.08-0.40 wt % Mo, 0.0-0.05 wt % V, and 0.01-0.05 wt % Ti.

A steel sheet has a predetermined chemical composition and a metal structure represented by, in area fraction, polygonal ferrite: 40% or less, martensite: 20% or less, bainitic ferrite: 50% to 95%, and retained austenite: 5% to 50%. In area fraction, 80% or more of the bainitic ferrite is composed of bainitic ferrite grains that have an aspect ratio of 0.1 to 1.0 and have a dislocation density of 8×10.sup.2 (cm/cm.sup.3) or less in a region surrounded by a grain boundary with a misorientation angle of 15° or more. In area fraction, 80% or more of the retained austenite is composed of retained austenite grains that have an aspect ratio of 0.1 to 1.0, have a major axis length of 1.0 μm to 28.0 μm, and have a minor axis length of 0.1 μm to 2.8 μm.

This hot-rolled steel sheet has a predetermined chemical composition, in which in a case where the thickness is denoted by t, a metallographic structure at a t/4 position from the surface includes, by area fraction, 77.0% to 97.0% of bainite or tempered martensite, 0% to 5.0% of ferrite, 0% to 5.0% of pearlite, 3.0% or more of residual austenite, and 0% to 10.0% of martensite, in the metallographic structure, the average grain size excluding the residual austenite is 7.0 μm or less, the average number density of iron-based carbides having a diameter of 20 nm or more is 1.0×10.sup.6 carbides/mm.sup.2 or more, a tensile strength is 980 MPa or more, and an average Ni concentration on the surface is 7.0% or more.

Disclosed is a hot-rolled steel strip having a tensile strength greater than 875 MPa and containing in mass-%: C 0.06-0.12, Si 0-0.5, Mn 0.70-2.20, Nb 0.005-0.100, Ti 0.01-0.10, V 0.11-0.40, whereby the total amount of V+Nb+Ti is 0.20-0.40 Al 0.005-0.150, B 0-0.0008, Cr 0-1.0, whereby the total amount of Mn+Cr is 0.9-2.5, Mo 0-0.5, Cu 0-0.5, Ni 0-1.0, P 0-0.05, S 0-0.01, Zr 0-0.1 Co 0-0.1 W 0-0.1 Ca 0-0.005, N 0-0.01, balance Fe and unavoidable impurities, and having a microstructure at ¼ thickness that is: at least 90% martensite and bainite with island-shaped martensite-austenite (MA) constituents, the remainder being: less than 5% polygonal ferrite and quasi-polygonal ferrite, less than 5% pearlite, less than 5% austenite, so that the total area percentage is 100%.

A steel sheet has a tensile strength of 1310 MPa or higher, a specified chemical composition, and a steel microstructure containing martensite at an area ratio of 70% or more, bainite at an area ratio of 30% or less, and ferrite and retained austenite at a total area ratio of 10% or less, in which, at a ¼ thickness position of the steel sheet, a number density of carbides having long axes of 0.5 μm or more is 60000 carbides/mm.sup.2 or less, in a ¼-to-¾ thickness region of the steel sheet, a number density of inclusion grains having equivalent circle diameters of 4.0 μm or more is 10 grains/mm.sup.2 or more and 30 grains/mm.sup.2 or less, and, in a surface-to-¼ thickness region of the steel sheet, a number density of inclusion grains having equivalent circle diameters of 4.0 μm or more is 27 grains/mm.sup.2 or less.

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

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.