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.

A structural steel having excellent brittle fracture resistance according to an aspect of the present invention comprises, by weight %, 0.02-0.12% of C, 0.01-0.8% of Si, 1.5-2.5% of Mn, 0.005-0.5% of Al, 0.02% or less of P, 0.01% or less of S, 0.0015-0.015% of N, and the remainder of Fe and unavoidable impurities, wherein an outer surface layer portion and an inner central portion are microstructurally divided along the thickness direction, the surface layer portion comprises tempered bainite as a matrix structure, the central portion comprises bainitic ferrite as a matrix structure, and the NDT temperature by the NRL drop-weight test may be −70° C. or lower.

A thin steel sheet has a specific chemical composition. The thin steel sheet has a microstructure in which ferrite is present in an area fraction of 5% or more and 60% or less, as-quenched martensite is present in an area fraction of 10% or less (including 0%), retained austenite is present in an area fraction of 5% or more and 20% or less, and upper bainite, lower bainite, and tempered martensite are present in a total area fraction of more than 15% and less than 85%; BCC iron that has a misorientation of 1° or less and surrounds retained austenite having an aspect ratio of 2.5 or higher is present in an area fraction of 5% or more and 70% or less; and a top 10% of retained austenite in terms of an equivalent circular diameter has an average aspect ratio of 2.5 or higher.

A thin steel sheet has a specific chemical composition. The thin steel sheet has a microstructure in which ferrite is present in an area fraction of 4% or less (including 0%), as-quenched martensite is present in an area fraction of 10% or less (including 0%), retained austenite is present in an amount of 7% or more and 20% or less, and upper bainite, lower bainite, and tempered martensite are present in a total area fraction of more than 71% and less than 93%; and BCC iron that has a misorientation of 1° or less and surrounds retained austenite having an equivalent circular diameter of 1 μm or less is present in an area fraction of 4% or more and 50% or less, and BCC iron that has a misorientation of more than 1° is present in an area fraction of 25% or more and 85% or less.

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.

The invention relates to steel with a specific composition of the following elements: 0.25%≤C≤0.35%, 0.80%≤Cr≤1.50%, 0.50%≤Ni≤1.50%, 0.0010%≤B≤0.0050%, 0.010%≤Ti≤0.060%, 0.40%≤Mn≤1.00% and 0.003%≤N≤0.0150%, whereby after being subjected to a quench and tempering treatment, the characteristics required by the ISO 898-1 standard for class 10.9 and class 12.9 bolts, screws and studs are achieved, while at the same time achieving high fatigue strength values. The use of this steel allows achieving the mentioned characteristics in large-sized parts or elements (up to a diameter of 75 mm), considerably reducing the alloying costs compared with the steels used today.

The invention relates to steel with a specific composition of the following elements: 0.25%≤C≤0.35%, 0.80%≤Cr≤1.50%, 0.50%≤Ni≤1.50%, 0.0010%≤B≤0.0050%, 0.010%≤Ti≤0.060%, 0.40%≤Mn≤1.00% and 0.003%≤N≤0.0150%, whereby after being subjected to a quench and tempering treatment, the characteristics required by the ISO 898-1 standard for class 10.9 and class 12.9 bolts, screws and studs are achieved, while at the same time achieving high fatigue strength values. The use of this steel allows achieving the mentioned characteristics in large-sized parts or elements (up to a diameter of 75 mm), considerably reducing the alloying costs compared with the steels used today.

A high strength steel product and a process for producing a high strength steel product, the high strength steel product being useful for producing frame components for vehicles and automobiles.

A hot-rolled flat steel product including (in wt %) C: 0.1-0.3%, Mn: 1.5-3.0%, Si: 0.5-1.8%, Al: ≤1.5%, P: ≤0.1%, S: ≤0.03%, N: ≤0.008%, optionally one or more of Cr: 0.1-0.3%, Mo: 0.05-0.25%, Ni: 0.05-2.0%, Nb: 0.01-0.06%, Ti: 0.02-0.07%, V: 0.1-0.3%, and B: 0.0008-0.0020%, the balance being iron and unavoidable impurities. This flat steel product possesses a tensile strength of 800-1500 MPa, a yield strength of >700 MPa, an elongation at break of 7-25%, and a hole expansion of more than 20%. The structure is at least 85 area % martensite, of which at least half is tempered martensite, with the remainder being ≤15 vol % residual austenite, ≤15 area % bainite, ≤15 area % polygonal ferrite, ≤5 area % cementite and/or ≤5 area % nonpolygonal ferrite, and has a kernel average misorientation of at least 1.50°. Also, a method for producing the flat steel product, wherein the microstructure of the flat steel product is set by the heat treatment.

A wear-resistant steel having excellent hardness and impact toughness and a method for producing same can include: 0.29-0.37 wt % of carbon, 0.1-0.7 wt % of silicon, 0.6-1.6 wt % of manganese, 0.05 wt % or less of phosphorus, 0.02 wt % or less of sulfur, 0.07 wt % or less of aluminum, 0.1-1.5 wt % of chromium, 0.01-0.8 wt % of molybdenum, 0.01-0.08 wt % of vanadium, 50 ppm or less of boron, and 0.02 wt % or less of cobalt; and optionally one or more of 0.5 wt % or less of nickel, 0.5 wt % or less of copper, 0.02 wt % or less of titanium, 0.05 wt % or less of niobium, and 2-100 ppm of calcium; with the remainder of Fe and other inevitable impurities.