A cold-rolled annealed dual-phase steel is provided, having a microstructure of ferrite and martensite, and comprising the following chemical elements in mass percentage: 0.08% to 0.1% of C, 1.95% to 2.2% of Mn, 0.1% to 0.6% of Si, 0.020% to 0.050% of Nb, 0.020% to 0.050% of Ti, 0.015% to 0.045% of Als, 0.40% to 0.60% of Cr, 0.2% to 0.4% of Mo, 0.001% to 0.005% of Ca, and the balance being Fe and other inevitable impurities. A method for manufacturing the cold-rolled annealed dual-phase steel is also provided, comprising the steps of: (1) smelting and casting; (2) hot rolling; (3) cold rolling; (4) annealing; and (5) temper rolling.

A steel sheet includes a predetermined chemical composition and a metal structure represented by, in area fraction, ferrite: 50% to 95%, granular bainite: 5% to 48%, martensite: 2% to 30%, and upper bainite, lower bainite, tempered martensite, retained austenite, and pearlite: 5% or less in total.

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%, 0%≤Sulfur≤0.003% 0.002%≤Phosphorus≤0.02%, 0%≤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%, 0%≤Vanadium≤0.1%, 0%≤Boron≤0.003%, 0%≤Cerium≤0.1%, 0%≤Magnesium≤0.010%, 0%≤Zirconium≤0.010% the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of the steel sheet having in area fraction, 10 to 30% Residual Austenite, 10 to 40% Bainite, 5% to 50% Annealed Martensite, 1% to 20% Quenched Martensite and less than 30% Tempered Martensite, wherein the cumulated amounts of Bainite and Residual Austenite is more than or equal to 25%.

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 structural steel having excellent brittle crack propagation resistance, according to one aspect of the present invention, comprises, by wt %, 0.02-0.12% of C, 0.01-0.8% of Si, 1.7-2.5% of Mn, 0.005-0.5% of Al, and the balance of Fe and inevitable impurities, wherein an outer surface part and an inner center part thereof are microstructurally distinguished in the thickness direction, and the surface part comprises tempered bainite as a base structure, comprises fresh martensite as a second structure and can comprise austenite as a residual structure.

in a heat treatment method for a tubular shaft for a drive shaft having a ball spline structure for a plunging and an undercut region with a reduced diameter, a carburizing-austempering is performed such that a deep portion hardness of the undercut region is a value between HRC 35 to HRC 50.

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.

Disclosed are a steel with controlled steel ratio and a manufacturing method therefor. The steel comprises the following components in percentage by mass: C: 0.245-0.365%, Si: 0.10-0.80%, Mn: 0.20-2.00%, P:≤0.015%, S:≤0.003%, Cr: 0.20-2.50%, Mo: 0.10-0.90%, Nb: 0-0.08%, Ni: 2.30-4.20%, Cu: 0-0.30%, V: 0.01-0.13%, B: 0-0.0020%, Al: 0.01-0.06%, Ti: 0-0.05%, Ca:≤0.004%, H:≤0.0002%, N:≤0.013%, O:≤0.0020%, and the balance of Fe and inevitable impurities, wherein the components satisfy (8.57*C+1.12*Ni)≥4.8% and 1.2%≤(1.08*Mn+2.13*Cr)≤5.6%. The steel has excellent low-temperature impact toughness and aging impact toughness at −20° C. and −40° C., a rationally controlled yield ratio, and ultra-high strength, ultra-high toughness, and ultra-high plasticity, which can be used in applications such as offshore platform mooring chains, mechanical structures, and automobiles that require high strength and toughness of the steel.

Provided is a martensite-based stainless steel component that has a nitride layer on a surface of a martensite-based stainless steel with a constituent composition including, in percent by mass, 0.25 to 0.45% of C, 1.0% or less of Si, 0.1 to 1.5% of Mn, 12.0 to 15.0% of Cr, and 0.5 to 3.0% of Mo, with a remainder being Fe and impurities, wherein hardness at a position of a depth of 0.1 mm from a surface of the martensite-based stainless steel component is 650 HV or more, and a number density of carbide with an equivalent circle diameter of 1 μm or more is 100 particles/10000 μm.sup.2 in a sectional structure at the position of the depth of 0.1 mm from the surface of the martensite-based stainless steel component. Also, a method for manufacturing the martensite-based stainless steel component is provided.

Adopted are a hot-rolled steel sheet having a predetermined chemical composition, in which a region, where a rotation angle between a normal line of a surface and a (011) pole near the normal line becomes 5° or less, is 0.150 or less from the surface in terms of a sheet thickness direction position standardized by a sheet thickness, and a region, where the rotation angle becomes 20° or more, is 0.250 or more from the surface in terms of the sheet thickness direction position standardized by the sheet thickness and a manufacturing method thereof.