Disclosed is a hot-pressed member that can exhibit very high tensile strength after hot pressing as high as TS: 1780 MPa or more, and excellent indentation peeling strength at projection welds by properly adjusting its chemical composition and its microstructure such that a prior austenite average grain size is 7 μm or less within a range of 50 μm or less in a thickness direction from a surface of the member, a volume fraction of martensite is 90% or more, and an average intergrain distance of Nb and Ti carbonitrides having a grain size of less than 0.10 μm within a depth range of 20 μm to 100 μm in the thickness direction from the surface of the member is 5 μm or less.

A high-strength cold-rolled steel sheet having a specified chemical composition and a microstructure including ferrite having an average crystal grain diameter of 2 μm or less in an amount of 10% to 25% in terms of volume fraction, retained austenite in an amount of 5% to 20% in terms of volume fraction, martensite having an average crystal grain diameter of 2 μm or less in an amount of 5% to 15% in terms of volume fraction, and the balance being a multi-phase structure including bainite and tempered martensite having an average crystal grain diameter of 5 μm or less, in which a relational expression, 0.35≦V2/V1≦0.75 (1), is satisfied, where V1 is a volume fraction of phases which are different from ferrite and V2 is a volume fraction of tempered martensite.

A steel sheet suitable as a starting material for a vehicle impact absorbing member with high absorption of impact energy and resistance to cracking contains, by mass %, C: 0.08-0.30%, Mn: 1.5-3.5%; Si+Al: 0.50-3.0%, P: 0.10% or less, S: at most 0.010%, and N: at most 0.010%, and optionally, one or more types selected from Cr: at most 0.5%, Mo: at most 0.5% , B: at most 0.010%, Ti: less than 0.04%, Nb: less than 0.030%, V: less than 0.5%, Ca: at most 0.010%, Mg: at most 0.010%, REM: at most 0.050%, and Bi: at most 0.050%. The microstructure contains, by area %, bainite: more than 50%, martensite: 3-30%, and retained austenite: 3-15%, the remainder comprising ferrite having an average grain diameter of less than 5 mm. The product of uniform elongation and hole expansion ratio is at least 300%.sup.2 and 5% effective flow stress is at least 900 MPa.

A nitrided packaging steel in the form of a flat steel product and method for producing a nitrided packaging steel with a carbon content of 10-1000 ppm and uncombined nitrogen, dissolved in the steel, of more than 100 ppm. The nitriding is performed in two stages: a first stage, in which a molten steel is nitrided to a nitrogen content of at most 160 ppm by introducing a nitrogen-containing gas and/or a nitrogen-containing solid into the molten steel, and a second stage, in which a flat steel product produced from the nitrided molten steel by cold rolling is treated with a nitrogen-containing gas in order to increase further the amount of uncombined nitrogen in the flat steel product. The second stage is performed in an annealing furnace, in which the flat steel product is at the same time annealed in a recrystallizing manner. The packaging steels produced are distinguished by great strength, in excess of 600 MPa, and good elongation to fracture, regularly in excess of 5%, as well as by good forming properties.

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

Provided is a low-cost cold rolled steel sheet for zirconium-based chemical conversion treatment that contains Si and Mn and has excellent zirconium-based chemical convertibility even when a relatively large amount of Si-based oxide and Si—Mn-based oxide exists on the steel sheet surface. A cold rolled steel sheet for zirconium-based chemical conversion treatment comprises a chemical composition containing, in mass %, C: 0.05% to 0.30%, Si: 0.01% to 1.4%, Mn: 0.14% to 3.2%, P: 0.10% or less, and S: 0.01% or less, with a balance consisting of Fe and inevitable impurities, wherein a Si/Mn mass ratio in steel is 0.10 to 0.7, and a surface coating ratio of steel sheet surface layer oxides having Si content of 10 mass % or more is 40% or less.

The present invention has as its object the provision of a steel member and steel sheet having high tensile strength and toughness and excellent in hydrogen embrittlement resistance in a corrosive environment and methods for manufacturing the same. The steel member of the present invention has predetermined chemical constituents and has a maximum value of content of Cu in a range from the surface to a depth of 0 to 30 μm of 1.4 times the content of Cu at a depth of 200 μm.

A high-yield-ratio high-strength electrogalvanized steel sheet having an electrogalvanized coating layer formed on a surface of a base steel sheet, in which the base steel sheet has a certain chemical composition, and a steel microstructure, in which a total area fraction of one or both of bainite containing carbides having an average grain diameter of 50 nm or less and tempered martensite containing carbides having an average grain diameter of 50 nm or less is 90% or more in the whole of the steel microstructure, and in which a total area fraction of one or both of the bainite containing and the tempered martensite containing carbides is 80% or more in a region from the surface of the base steel sheet to a position located at ⅛ of a thickness of the base steel sheet, and diffusible hydrogen in steel in an amount of 0.20 mass ppm or less.

A steel sheet including a steel micro-structure containing, in volume fraction, tempered martensite: 85% or more, retained austenite: 5% or more to less than 15%, and ferrite, pearlite, bainite, and as-quenched martensite being less than 10% in total, when contents of Mn and C in the retained austenite are denoted by Mn.sub.A and C.sub.A, and when contents of Mn and C in a matrix are denoted by Mn.sub.M and C.sub.M, respectively, following Formulas (1) to (3) are satisfied, and the number of carbides having an equivalent circle radius of 0.1 μm or more is 100 or less in a region measuring 20000 μm.sup.2, and the steel sheet has a tensile strength of 1100 MPa or more. The steel sheet is excellent in crash resistance and formability.

Mn.sub.A/Mn.sub.M≥1.2  (1)

C.sub.A/C.sub.M≤5.0  (2)

C.sub.A≤1.0  (3)

A cold rolled and heat treated steel sheet having a composition with the following elements, expressed in percentage by weight 0.10%≤Carbon≤0.5%, 1%≤Manganese≤3.4%, 0.5%≤Silicon≤2.5%, 0.03%≤Aluminum≤1.5%, Sulfur≤0.003%, 0.002%≤Phosphorus≤0.02%, Nitrogen≤0.01% and can contain one or more of the following optional elements 0.05%≤Chromium≤1%, 0.001%≤Molybdenum≤0.5%, 0.001%≤Niobium≤0.1%, 0.001%≤Titanium≤0.1%, 0.01%≤Copper≤2%, 0.01%≤Nickel≤3%, 0.0001%≤Calcium≤0.005%, Vanadium≤0.1%, Boron≤0.003%, Ceriums≤0.1%, Magnesiums≤0.010%, Zirconiums≤0.010% the remainder composition being composed of iron and the unavoidable impurities caused by processing, and a microstructure of the said rolled steel sheet having by area fraction, 10 to 30% Residual Austenite, 5 to 50% Annealed Bainite, 10 to 40% of Bainite, 1% to 20% Quenched Martensite, and less than 30% Tempered Martensite where the combined presence of Bainite and Residual Austenite shall be 30% or more.