A high-strength galvanized steel sheet and method for producing the same. The high-strength galvanized steel sheet including a specified chemical composition and a microstructure comprising, in terms of area fraction, a tempered martensite phase: 30% or more and 73% or less, a ferrite phase: 25% or more and 68% or less, a retained austenite phase: 2% or more and 15% or less, and other phases: 10% or less (including 0%), the other phases comprising a martensite phase: 3% or less (including 0%) and a bainitic ferrite phase: less than 5% (including 0%), the tempered martensite phase having an average grain size of 8 m or less, the ferrite phase having an average grain size of 5 m or less, and the retained austenite phase having a C content less than 0.7% by mass.

A hot-dip galvanized steel sheet having a good appearance and good adhesion to a coating, the hot-dip galvanized steel sheet having a composition including, on a mass basis: C: 0.20% to 0.50%, Si: 0.1% to 3.0%, Mn: 0.5% to 3.0%, P: 0.001% to 0.10%, Al: 0.01% to 3.00%, and S: 0.200% or less, a remainder being Fe and incidental impurities, wherein the hot-dip galvanized steel sheet includes an internal oxidation layer and a decarburized layer, the internal oxidation layer having a thickness of 4 m or less, the decarburized layer having a thickness of 16 m or less, and 50% or more by area of the internal oxidation layer is composed of a Si oxide containing Fe and/or Mn represented by Fe.sub.2XMn.sub.2-2XSiO.sub.Y, wherein X ranges from 0 to 1, and Y is 3 or 4.

This cold-rolled steel sheet has a predetermined chemical composition, in which a metallographic structure of a t/4 portion, which is at a position of a sheet thickness t from a surface of the cold-rolled steel sheet in a sheet thickness direction, includes, by volume percentage, retained austenite: 2.5% or more and 10.0% or less, tempered martensite: 80.0% or more and 97.5% or less, ferrite and bainite: 0.0% or more and 15.0% or less in total, and martensite: 0.0% or more and 3.0% or less, and in a surface layer area at a position 25 m away from the surface in the sheet thickness direction, an amount of solute Si is 0.30% or more and 1.50% or less by mass %, a volume percentage of ferrite in the metallographic structure is 0.0% or more and 20.0% or less, and a density of ferrite grains having a grain size of 15 m or more is 0 grains/mm.sup.2 or more and 3,000 grains/mm.sup.2 or less.

There is provided a hot-dip galvanized steel sheet and a hot-dip galvannealed steel sheet, which have excellent elongation properties, and methods for manufacturing the hot-dip galvanized steel sheet and the hot-dip galvannealed steel sheet. The present disclosure relates to a hot-dip galvanized steel sheet in which a hot-dip galvanized layer is formed on a surface of abase steel sheet, the hot-dip galvanized steel sheet having excellent elongation properties and being characterized by the composition and the microstructure thereof.

There is provided a hot-dip galvanized steel sheet and a hot-dip galvannealed steel sheet, which have excellent elongation properties, and methods for manufacturing the hot-dip galvanized steel sheet and the hot-dip galvannealed steel sheet. The present disclosure relates to a hot-dip galvanized steel sheet in which a hot-dip galvanized layer is formed on a surface of a base steel sheet, the hot-dip galvanized steel sheet having excellent elongation properties and being characterized by the composition and the microstructure thereof.

A multipurpose continuous processing line able for heat treating and hot dip coating a steel strip comprising: an annealing section (1) for heating the steel strip to a predetermined annealing temperature and for maintaining the steel strip at said annealing temperature, a first transfer section (2), an overaging section (3) able to maintain the temperature of the steel strip between 300 C. and 700 C., a second transfer section (4) able to adjust the temperature of the steel strip to allow the hot dip coating of the strip and, a hot dip coating section (5), wherein the first transfer section (2) comprises, in sequence, cooling means (21) and heating means (22).

Hot-stamped steel includes: a base metal that is steel including a tempered portion having hardness corresponding to 85% or less of the highest quenching hardness, the highest quenching hardness being defined as a Vickers hardness at a depth position spaced away from a surface by times a sheet thickness in a case of performing water quenching after heating at a temperature equal to or higher than an A.sub.c3 point and retention for 30 minutes; and a Zn coating layer that is formed on the tempered portion of the base metal. The Zn coating layer includes a solid-solution layer including a solid-solution phase that contains Fe and Zn that is solid-soluted in Fe, and a lamella layer that includes the solid-solution phase and a capital gamma phase. An area ratio of the lamella layer in the Zn coating layer is 20% or less.

A method for producing a high strength coated steel sheet having an improved ductility and an improved formability, the chemical composition of the steel containing: 0.13%C0.22%, 1.9%Si2.3%, 2.4%Mn3%, Al0.5%, Ti<0.05%, Nb<0.05%, the remainder being Fe and unavoidable impurities. The sheet is annealed at temperature TA higher than Ac3 but less than 1000 C. for a time of more than 30 s, quenched by cooling to a quenching temperature QT between 200 C. and 280 C. in order to obtain a structure consisting of austenite and at least 50% of martensite, the austenite content being such that the final structure can contain between 3% and 15% of residual austenite and between 85% and 97% of the sum of martensite and bainite, without ferrite, heated up to a partitioning temperature PT between 430 C. and 490 C. and maintained at this temperature for a time Pt between 10 s and 100 s, hot dip coated and cooled to the room temperature.

A low-carbon, low-alloy and high-formability dual-phase steel having a tensile strength of greater than or equal to 590 MPa, a hot-dip galvanized dual-phase steel, and a manufacturing method therefor by means of rapid heat treatment. The steel comprises the following components, in percentage by mass: 0.04-0.12% of C, 0.1-0.5% of Si, 1.0-2.0% of Mn, P0.02%, S0.015%, 0.02-0.06% of Al, and can further comprise one or two of Cr, Mo, Ti, Nb and V, wherein Cr+Mo+Ti+Nb+V0.5%, and the balance is Fe and other unavoidable impurities. The manufacturing method therefor includes smelting, casting, hot rolling, cold rolling, and rapid heat treatment processes. In the present invention, by controlling rapid heating, short-term thermal insulation and rapid cooling processes during the process of rapid heat treatment, the recovery of a deformed structure, recrystallization and austenite transformation processes are changed, the nucleation rate is increased, the grain growth time is shortened, grains are refined, the strength and n value of the material are improved, and the interval range of the material performance is expanded.

A heat-rolled steel plate has a chemical composition that contains, in mass %, C, Si, Mn, P, S, Al, N and Ti, and that satisfies Formula (1); and a microstructure containing bainite and ferrite. In the interior of the steel plate an average value of pole densities of an orientation group {100}<011> to {223}<110> is 4 or less, and a pole density of a {332}<113> crystal orientation is 4.8 or less. In an outer layer of the steel plate, a pole density of a {110}<001> crystal orientation is 2.5 or more. Furthermore, among Ti carbo-nitrides in the steel plate, the number density of fine Ti carbo-nitrides having a particle diameter of 10 nm or less is 1.010.sup.17 per cm.sup.3 or less, and a bake hardening amount is 15 MPa or more.

[Ti]48/14[N]48/32[S]0(1)