A gradient steel material with a high plastic surface layer and a high strength inner layer, and a manufacturing method are provided. Weight percentages of the components of the gradient steel material are: C≤0.15%, Si≤1%, Mn≤1.5%, and the balance of Fe and inevitable impurities, the surface layer of the steel material being ferrite, and the inner layer being ferrite+bainite. The manufacturing method therefor comprises: smelting, casting, rolling, and a heat treatment, wherein in the heat treatment step, a steel material is heated to an austenite temperature Ac3 or more and kept at said temperature for more than 3 min; thereafter, the material is cooled to a temperature range between Ar3 and Ar1 in a two-phase zone at a cooling rate of less than 0.5° C./s, and is then cooled to room temperature at a cooling rate of greater than 5° C./s. The present steel material does not need to be obtained by means of the compound preparation of different materials as only a single material is processed. At the same time, the composition of the steel material is simple. Although the internal and external microstructures are different, the difference is a gradual process, and the strength at the interface is good.

The steel sheet has a steel microstructure containing ferrite: 6% to 90% by area, a microstructure composed of one or more of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained γ: 10% to 94% by area in total, and retained γ: 3% to 20% by volume, a ratio (S.sub.UB/S.sub.2nd)×100(%) of an area ratio S.sub.UB of an upper bainite with a width in the range of 0.8 to 7 μm, a length in the range of 2 to 15 μm, and an aspect ratio of 2.2 or more in contact with retained γ.sub.UB with a grain width in the range of 0.17 to 0.80 μm and an aspect ratio in the range of 4 to 25 to an area ratio S.sub.2nd of the microstructure composed of one or more of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained γ ranges from 2.0% to 15%.

A nanostructured bainitic steel of low carbon comprising of the following components in percentage by mass: Carbon—(0.24-0.28%), Manganese—(1.8-2%), Silicon—(2-2.5%), Nickel—(1.5-1.8%), Molybdenum—(0.2-0.25%), Chromium—(0.2-0.25%), Aluminium—(0.2-0.25%), and Cobalt—(0.45-0.5%) and the balance being Iron and unavoidable impurities and a method for preparation thereof.

A method and arrangement for manufacturing hot dip galvanized rolled high strength steel product is presented. The method comprises providing a rolled steel product, heating and annealing the rolled steel product for creating a layer of iron oxide on the surface of the rolled steel product, cooling the rolled steel product, having the iron oxide layer, in a first cooling step to a temperature in a temperature range of 560-600° C. and holding for 3-10 seconds, quenching said rolled steel product, covered with the layer of iron oxide, in a second cooling step by immersing it into a zinc bath comprising aluminium and having a temperature between 440-450° C. for 1-5 seconds and cooling the rolled steel product in a third cooling step to room temperature. An arrangement for implementing the method is also presented.

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.

This hot-rolled steel sheet has a predetermined chemical composition. The metallographic structure at a sheet thickness ¼ depth from a surface and at a center position in a sheet width direction in a sheet width cross section parallel to a rolling direction contains, by area %, 77.0% to 97.0% of bainite and tempered martensite in total, 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. The average grain size of the metallographic structure excluding the residual austenite is 7.0 μm or less. The C concentration in the residual austenite is 0.5 mass % or more. The 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 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.

A high-strength steel sheet comprises: a chemical composition containing C, Si, Mn, P, S, Al, N, Mo, Cr, Ca, and Sb with a balance consisting of Fe and inevitable impurities, wherein [% Si], [% Mn], [% P], [% Mo], and [% Cr] satisfy a predetermined relationship; a steel microstructure that contains ferrite, hard phase, and retained austenite and in which a carbon concentration in the retained austenite is 0.55% or more and 1.10% or less, an amount of diffusible hydrogen in the steel sheet is 0.80 mass ppm or less, a surface layer softening thickness is 5 μm or more and 150 μm or less, and a corresponding grain boundary frequency in a surface layer of the steel sheet after a high-temperature tensile test is 0.45 or less; and a tensile strength of 980 MPa or more.