A steel sheet has a chemical composition containing, by mass %, C: 0.20% or more and 0.40% or less, Si: more than 1.0% and 3.0% or less, Mn: 1.5% or more and 3.5% or less, P: 0.002% or more and 0.010% or less, S: 0.0002% or more and 0.0020% or less, sol. Al: 0.40% or less (not including 0%), and N: 0.0100% or less, with the balance being Fe and incidental impurities; and a steel microstructure containing, in area fraction, tempered martensite: 45% or more and 83% or less, bainite: 15% or more and 53% or less, and retained austenite: 2% or more. Carbides in the tempered martensite have an average particle size of 0.40 ?m or less, an average amount of C in the retained austenite is 0.5% by mass or more, and a tensile strength is 1,470 MPa or more.

Provided is an alloyed hot-dip galvanized steel sheet including a base steel sheet, the base steel sheet containing a given amount of C, Si, Mn, and other elements. The alloyed hot-dip galvanized steel sheet is provided with an alloyed hot-dip galvanized layer on a surface of the base steel sheet, the alloyed hot-dip galvanized layer containing, in mass %, Fe: more than or equal to 5% and less than or equal to 15%, and having a thickness of more than or equal to 3 m and less than or equal to 30 m. The alloyed hot-dip galvanized steel sheet includes an A layer immediately under the surface of the base steel sheet, the A layer being formed in the base steel sheet and having a thickness of more than or equal to 2 m and less than or equal to 20 m from the surface of the base steel sheet, containing more than or equal to 50 vol % of a ferrite structure, and containing more than or equal to 90 mass % of unoxidized Fe, less than or equal to 10 mass % of a total of contents of oxides of Fe, Si, Mn, P, S, and Al, and less than 0.05 mass % of C.

A high-strength galvanized steel sheet having improved post-work impact resistance, a method for producing the high-strength galvanized steel sheet, and a high-strength member produced using the steel sheet. The high-strength galvanized steel sheet includes a steel sheet having a microstructure including ferrite and carbide-free bainite, martensite and carbide-containing bainite, and retained austenite, the total area fraction of ferrite and carbide-free bainite being 0% to 55%, the total area fraction of martensite and carbide-containing bainite being 45% to 100%, and the area fraction of retained austenite being 0% to 5%. Additionally, a galvanizing layer is disposed on the steel sheet. The density of gaps that cut across the entire thickness of the galvanizing layer in a cross section of the galvanizing layer, which is taken in the thickness direction so as to be perpendicular to the rolling direction, is 10 gaps/mm or more.

The present method for manufacturing a high strength steel sheet having a tensile strength of 780 MPa or higher includes continuous annealing by heating a steel sheet having a predetermined chemical composition to 750 C. to 900 C. and holding the steel sheet in the temperature range for 0 seconds to 300 seconds, in which, during heating and holding, a hydrogen concentration in a furnace atmosphere is less than 10 volume %, when a temperature of the steel sheet is 700 C. or lower, a furnace body average value is higher than 3.01 and lower than 0.07, when the temperature is higher than 700 C. and 800 C. or lower, the value is higher than 1.36 and lower than 0.07, when the temperature is higher than 800 C., the value is higher than 3.01 and 0.53 or lower, and a dew point is lower than 10 C.

High-strength galvannealed steel sheet including any of a) an oxide containing Fe and Mn, b) an oxide containing Fe and Mn and an Fe oxide, c) an oxide containing Fe and Mn and a Mn oxide, d) an oxide containing Fe and Mn, an Fe oxide, and a Mn oxide, and e) an Fe oxide and a Mn oxide is present in a zinc coated layer. The total amount of oxide is 0.01 to 0.100 g/m.sup.2; the ratio by mass % of Mn to Fe, e.g., Mn/Fe, contained in the oxide is 0.10 to 10.00; an oxide of at least one selected from Fe and Mn is present in an amount of 60% or more; and an oxide of at least one selected from Fe and Mn is present in a surface layer portion of a steel sheet in an amount of 0.040 g/m.sup.2 or less (not including zero).

Provided is an alloyed hot-dip galvanized steel sheet including a base steel sheet, the base steel sheet containing a given amount of C, Si, Mn, and other elements. The alloyed hot-dip galvanized steel sheet is provided with an alloyed hot-dip galvanized layer on a surface of the base steel sheet, the alloyed hot-dip galvanized layer containing, in mass %, Fe: more than or equal to 5% and less than or equal to 15%, and having a thickness of more than or equal to 3 m and less than or equal to 30 m. The alloyed hot-dip galvanized steel sheet includes an A layer immediately under the surface of the base steel sheet, the A layer being formed in the base steel sheet and having a thickness of more than or equal to 2 m and less than or equal to 20 m from the surface of the base steel sheet, containing more than or equal to 50 vol % of a ferrite structure, and containing more than or equal to 90 mass % of unoxidized Fe, less than or equal to 10 mass % of a total of contents of oxides of Fe, Si, Mn, P, S, and Al, and less than 0.05 mass % of C.

Provided are a hot-dip zinc plated steel material and a method for preparing same, the hot-dip zinc plated steel material comprising: base iron comprising 0.01-1.6 wt % of Si and 1.2-3.1 wt % of Mn; a ZnAlMg alloy plating layer; and an Al-rich layer formed on the interface of the base iron and ZnAlMg alloy plating layer, wherein the rate of occupied surface area of the Al-rich layer is 70% or higher (including 100%).

Provided is a hot-dip galvanized steel sheet including a base steel sheet and a hot-dip zinc-based plating layer formed on the base steel sheet. The hot-dip zinc-based plating layer includes a Zn single phase having an average equivalent circular diameter of 120 m or less as a microstructure. In the Zn single phase, a Zn single phase having a crystal structure of which a {0001} plane is parallel to a surface of the steel sheet, is provided in an area fraction of 70% or less.

Disclosed herein is a high-strength galvannealed steel sheet having a galvannealed layer on a surface of a base steel sheet and containing predetermined steel components. The steel sheet sequentially has, from the interface of the base steel sheet and the galvannealed layer, towards the base steel sheet: an internal oxide layer and containing at least one oxide selected from the group consisting of Si and Mn; a soft layer including the internal oxide layer, and satisfying a predetermined Vickers hardness; and a hard layer made up of a structure mainly composed of martensite. The average depth D of the soft layer is 20 m or greater, and the average depth d of the internal oxide layer is 4 m or greater and smaller than D. A coefficient of variation of KAM of the base steel sheet at the portion t/4 is 0.66 or less.

A method for producing a zinc or zinc-alloy coated steel sheet with a tensile strength higher than 900 MPa, for the fabrication of resistance spot welds containing in average not more than two Liquid Metal Embrittlement cracks per weld having a depth of 100 ?m or more, with steps of providing a cold-rolled steel sheet, heating cold-rolled steel sheet up to a temperature T1 between 550? C. and Ac1+50? C. in a furnace zone with an atmosphere (A1) containing from 2 to 15% hydrogen by volume, so that the iron is not oxidized, then adding in the furnace atmosphere, water steam or oxygen with an injection flow rate Q higher than (0.07%/h??), ? being equal to 1 if said element is water steam or equal to 0.52 if said element is oxygen, at a temperature T?T1, so to obtain an atmosphere (A2) with a dew point DP2 between ?15? C. and the temperature Te of the iron/iron oxide equilibrium dew point, then heating the sheet from temperature T.sub.1 up to a temperature T.sub.2 between 720? C. and 1000? C. in a furnace zone under an atmosphere (A2) of nitrogen containing from 2 to 15% hydrogen and more than 0.1% CO by volume, with an oxygen partial pressure higher than 10.sup.?21 atm., wherein the duration to of heating of the sheet from temperature T.sub.1 up to the end of soaking at temperature T.sub.2 is between 100 and 500 s, soaking the sheet at T.sub.2, then cooling the sheet at a rate between 10 and 400? C./s, then coating the sheet with zinc or zinc-alloy coating.