A steel sheet has a predetermined chemical composition containing, in mass %, C: more than 0.18% and less than 0.30%, Mn: more than 2.50% and 4.00% or less, and other elements. The steel micro-structure at a position at a depth of ¼ of the sheet thickness from the surface in an L cross section of the steel sheet is, in area %, tempered martensite: 25 to 90%, ferrite: 5% or less, retained austenite: 10 to 50%, and bainite: 5% or less. At a position at a depth of ¼ of the sheet thickness from the surface of the L cross section, the proportion of a total area of retained austenite grains having an area of 1 μm.sup.2 or more and having a grain shape circularity of 0.1 or more is less than 50% with respect to the entire area of retained austenite. The steel sheet satisfies the formula C.sub.Mnγ/C.sub.Mnα≥1.2.

Disclosed in the present disclosure is an ultrahigh-strength dual-phase steel. The matrix structure of the ultrahigh-strength dual-phase steel is ferrite and martensite, wherein the ferrite and the martensite are evenly distributed in an island shape. The ultrahigh-strength dual-phase steel contains the following chemical elements in percentage by mass: 0.12-0.2% of C, 0.5-1.0% of Si, 2.5-3.0% of Mn, 0.02-0.05% of Al, 0.02-0.05% of Nb, 0.02-0.05% of Ti, and 0.001-0.003% of B. Further disclosed in the present disclosure is a manufacturing method for the ultrahigh-strength dual-phase steel, comprising the steps of smelting and continuous casting, hot rolling, cold rolling, annealing, tempering, and leveling. The ultrahigh-strength dual-phase steel in the present disclosure has not only good mechanical properties but also excellent delayed cracking resistance and low initial hydrogen content, and can be suitable for manufacturing of vehicle safety structural parts.

Provided are a material for hot stamping, wherein the material includes: a steel sheet including carbon (C) in an amount of 0.19 wt % to 0.25 wt %, silicon (Si) in an amount of 0.1 wt % to 0.6 wt %, manganese (Mn) in an amount of 0.8 wt % to 1.6 wt %, phosphorus (P) in an amount less than or equal to 0.03 wt %, sulfur (S) in an amount less than or equal to 0.015 wt %, chromium (Cr) in an amount of 0.1 wt % to 0.6 wt %, boron (B) in an amount of 0.001 wt % to 0.005 wt %, an additive in an amount less than or equal to 0.1 wt %, balance iron (Fe), and other inevitable impurities; and fine precipitates distributed within the steel sheet. The additive includes at least one of titanium (Ti), niobium (Nb), and vanadium (V), and the fine precipitates include nitride or carbide of at least one of titanium (Ti), niobium (Nb), and vanadium (V) and trap hydrogen.

The present invention relates to steel used for a sash component and the like of a vehicle and, more specifically, to a hot-rolled steel sheet for a high-strength electric resistance welded steel pipe having excellent expandability and a method for manufacturing same, the hot-rolled steel sheet having a smaller decrease in the strength of a welding heat-affected zone (HAZ) formed during electric resistance welding, in comparison with a base material.

A high-strength galvanized steel sheet includes a steel sheet having a chemical composition containing a predetermined component element, a mass ratio of a content of Si to a content of Mn in the steel (Si/Mn) being 0.1 or more and less than 0.2, and the balance: Fe and incidental impurities, and a steel structure in which an average grain size of inclusions containing at least one of Al, Si, Mg, and Ca and existing in an area extending from a surface to a position of ⅓ of a sheet thickness is 50 μm or less, and an average nearest distance between ones of the inclusions is 20 μm or more; and a galvanized layer provided on a surface of the steel sheet, in which an amount of diffusible hydrogen contained in the steel is less than 0.25 mass ppm, and a tensile strength is 1100 MPa or more.

Methods of producing high strength continuously cast hot rolled steel sheet products are disclosed. The methods include continuously casting a steel slab and then hot rolling with finish rolling on a hot strip mill, quenching on the hot strip mill to form a predominantly matrensitic microstructure, and performing a thermal cycling step including soaking at an intercritical temperature followed by holding at a lower temperature. The resultant hot rolled steel sheet products have a microstructure comprising ferrite and retained austenite. Steels processed in accordance with the present invention exhibit favorable combined ultimate tensile strength and total elongation (UTS.Math.TE) properties, and may fall into the category of Generation 3 advanced high strength steels, desirable in various industries including automobile manufacturers.

The disclosed martensitic stainless steel is defined in its composition is by specified ranges of weight percentages of C; Mn; Si; ≤Mn+Si; ≤S; 10,000×Mn×S; P; Cr, with [Cr−10.3−80*(C+N).sup.2]≤(Mn+Ni); Ni; Mo; Mo+2W; Cu; Ti; V; Zr; Al; O; Ta; Nb; (Nb+Ta)/(C+N); Nb; N; Co; Cu+Co; Cu+Co+Ni; B; rare earths+Y; Ca; the remainder being iron and impurities resulting from processing. Its microstructure includes at least 75% martensite, at most 20% ferrite and at most 0.5% carbides, the size of the ferrite grains being between 4 and 80 μm, preferably between 5 and 40 μm. Also disclosed is a method of manufacturing such steel.

The present cooling device includes: when cooling regions obtained by dividing an entire cooling region into a plurality of portions in a steel sheet conveyance direction and three or more portions in a width direction are set as divided cooling surfaces, a cooling water nozzle 23 and a switching device that switches between collision and non-collision of cooling water jetted from the cooling water nozzle 23 with the divided cooling surface, the cooling water nozzle 23 and the switching device provided for each of the divided cooling surfaces; and a control device that controls operation of the switching device based on a width-direction temperature distribution. The cooling water nozzle 23 has a jet axis P inclined with respect to a vertical line to the entire cooling region when viewed in the steel sheet conveyance direction, and the cooling water goes to the side opposite to the cooling water nozzle 23 in the width direction after colliding with the divided cooling surface.

Disclosed is a follow-up process-omitting type high-strength hot-rolled steel sheet having an excellent yield ratio and a method for manufacturing the same. The hot-rolled steel sheet includes, in percent by weight (wt %), 0.12% or more and less than 0.3% of C, 0.5% or less of Si (excluding 0), 0.1 to 2.5% of Mn, 0.0005 to 0.005% of B, 0.02% or less of P, 0.01% or less of S, and the balance of Fe and inevitable impurities, has a microstructure including at least 95 vol % of martensite, has a yield ratio (yield strength/tensile strength) of 0.75 or more, is manufactured by continuous hot rolling, and is manufactured without performing a follow-up process such as cold rolling and heat treatment.

A cold rolled and annealed steel sheet includes by weight: 0.6≤C≤1.3%, 15.0≤Mn≤35%, 5≤Al≤15%, Si≤2.40%, S≤0.03%, P≤0.1%, N≤0.1%, possibly one or more optional elements chosen among Ni, Cr and Cu in an respective amount of up to 4.0%, up to 3.0% and up to 3.0% and possibly one or more elements chosen among B, Ta, Zr, Nb, V, Ti, Mo, and W in a cumulated amount of up to 2.0%, the remainder of the composition making up of iron and inevitable impurities resulting from the elaboration, the microstructure of the sheet including optionally up to 3% of kappa carbides, optionally up to 10.0% of granular ferrite, the remainder being made of austenite, the average grain size and average aspect ratio of the austenite being respectively below 6 μm and comprised between 1.5 and 6 and the average grain size and average aspect ratio of the ferrite, when present, being respectively below 5 μm and below 3.0.