Provided is a high-strength steel sheet having excellent burring workability in a low temperature range, which is mainly used for automobile chassis parts such as members, lower arms, wheel disks, etc.; and a manufacturing method therefor.

A cold-rolled steel sheet has a predetermined chemical composition; a structure of the told-rolled steel sheet at a position which is ¼ of a sheet thickness from a surface contains, in a volume percentage, 80.0% or more of tempered martensite, more than 2.5% and less than 10.0% of residual austenite, 0% or more and 15.0% or less of ferrite and bainite in total, 0% or more and 3.0% or less of martensite, and the remainder structure. With respect to the above-described structure, a maximum value of a random specific intensity Iq is 4.0 or less, an average diameter of a region Rq which has an orientation within 10° from a crystal orientation in which the random specific intensity Iq is maximum is 10.0 μm or less, a surface density of the region Rq is 1000 pieces/mm.sup.2 or more, a tensile strength is 1310 MPa or more, a uniform elongation is 5.0% or more, and TS×λ is 35000 MPa.Math.% or more.

The present invention has as its object the provision of a coated steel member and coated steel sheet excellent in hydrogen embrittlement resistance in a corrosive environment and methods for manufacturing the same. The coated steel member of the present invention is provided on its surface with an Al—Fe-based coating containing Cu and one or more of Mo, Ni, Mn, and Cr in a total by mass % of 0.12% or more by heating, cooling, and manufacturing a coated steel sheet having a layer containing Cu on its surface under predetermined conditions.

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 μm 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×α), α 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 t.sub.D 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.

A method for producing a high-strength hot-dip galvannealed steel sheet, in which a high-strength steel sheet is used as a base material, includes a rolling step (x) of rolling a hot-dip galvannealed steel sheet with a coating layer having an Fe concentration of 8% to 17% by mass, and a heat treatment step (y) of heating the coated steel sheet which has been subjected to the rolling step (x) under the conditions satisfying the following formulae (1) and (2):
(273+T)×(20+2×log.sub.10(t))≥8000  (1)
40≤T≤160  (2) where T: heating temperature (° C.) of the coated steel sheet, and t: holding time (hr) at the heating temperature T.

A high-strength steel includes a steel structure with: in area fraction, 60.0% to less than 90.0% of ferrite, 0% to less than 5.0% of unrecrystallized ferrite, 2.0% to 25.0% of martensite, 0% to 5.0% of carbide, and 0% to 3.0% of bainite; in volume fraction, more than 7.0% of retained austenite; in a cross-sectional view of 100 μm×100 μm, a value obtained by dividing number of retained austenite that are not adjacent to retained austenite whose crystal orientations are different by a total number of retained austenite being less than 0.80, an average crystal grain size of the ferrite being 6.0 μm or less, an average crystal grain size of the retained austenite being 3.0 μm or less, and a value obtained by dividing, by mass %, an average content of Mn in the retained austenite by an average content of Mn in steel being 1.50 or more.

A steel sheet for the manufacture of a press hardened part is provided, having a composition of: 0.15%≤C≤0.22%, 3.5%≤Mn<4.2%, 0.001%≤Si≤1.5%, 0.020%≤Al≤0.9%, 0.001%≤Cr≤1%, 0.001%≤Mo≤0.3%, 0.001%≤Ti≤0.040%, 0.0003%≤B≤0.004%, 0.001%≤Nb≤0.060%, 0.001%≤N≤0.009%, 0.0005%≤S≤0.003%, 0.001%≤P≤0.020%. A microstructure has less than 50% ferrite, 1% to 20% retained austenite, cementite, such that the surface density of cementite particles larger than 60 nm is lower than 10{circumflex over ( )}7/mm.sup.2, and a complement of bainite and/or martensite, the retained austenite having an average Mn content of at least 1.1*Mn %. Press-hardened steel part obtained by hot forming the steel sheet, and manufacturing methods thereof.

The present invention relates to an ultra-high strength, hot-dip galvanized steel sheet having excellent surface quality and coating adherence and to a method for manufacturing thereof, the ultra-high strength, hot-dip galvanized steel sheet comprising: 0.1-0.3% by weight carbon (C); 0.1-2.0% by weight silicon (Si); 0.005-1.5% by weight aluminum (Al); 1.5-3.5% by weight manganese (Mn); 0.04% by weight or less phosphorus (P) (excluding 0% by weight); 0.015% by weight or less sulphur (S) (excluding 0% by weight); 0.02% by weight or less nitrogen (N) (excluding 0% by weight); the balance being Fe; and other inevitable impurities, and further comprising 0.01 wt.% to 0.07 wt.% of at least one kind of element selected from the group consisting of bismuth (Bi), tin (Sn) and antimony (Sb).

A high-strength plated steel sheet has a plated layer on the surface of a base steel sheet and contains predetermined steel components. The steel sheet includes, in the order from the interface of the base steel sheet and the plated layer towards the base steel sheet: a soft layer having a Vickers hardness that is 90% or less of the Vickers hardness at a portion t/4 of the base steel sheet, where t is a sheet thickness of the base steel sheet; and a hard layer containing martensite and bainite, and ferrite in predetermined ranges. The average depth D of the soft layer is 20 μm or greater, and the average depth d of an internal oxide layer is 4 μm or greater and smaller than D.

A galvannealed steel sheet includes: a scale-removed rolled steel sheet; and a galvannealed layer arranged on the scale-removed rolled steel sheet. When ten measurement points of the galvannealed steel sheet are set in a transverse direction by equally dividing a line-segment having a reference length of 50 mm by 10, a minimum P content of the galvannealed layer in the ten measurement points is 50% or more as compared with a maximum P content therein.