A method for producing high-strength galvanized steel sheets having excellent coating adhesion, workability and appearance. The method comprises hot rolling a slab comprising, by mass %, C: 0.05 to 0.30%, Si: 0.1 to 2.0% and Mn: 1.0 to 4.0%, then coiling the steel sheet into a coil at a specific temperature T.sub.C, and pickling the steel sheet, cold rolling the hot-rolled steel sheet resulting from the hot rolling, annealing the cold-rolled steel sheet resulting from the cold rolling under specific conditions, and galvanizing the annealed sheet resulting from the annealing in a galvanizing bath containing 0.12 to 0.22 mass % Al.

A high-strength multi-phase steel having minimum tensile strengths of 750 MPa and preferably having a dual-phase microstructure for a cold- or hot-rolled steel strip, in particular for lightweight vehicle construction is disclosed. The high-strength multi-phase steel has improved forming properties and a ratio of yield point to tensile strength of at most 73%. The high-strength multi-phase steel includes in mass %: C0.075 to 0.105; Si0.600 to 0.800; Mn1.000 to 0.700; Cr0.100 to 0.480; Al0.010 to 0.060; N 0.00200.0120; S0.0030; Nb0.005 to 0.050; Ti0.0050 to 0.050; B0.0005 to 0.0040; Mo0.200; Cu0.040%; Ni0.040 % the remainder iron, including typical elements accompanying steel that are not mentioned above, which represent contamination resulting from smelting.

The invention deals with a cold rolled and hot dip steel sheet, with a tensile strength of at least 980 MPa, with yield strength above or equal to 500 MPa, with total elongation above or equal to 8%, the composition consisting by weight percent: 0.05<C<0.15%, 2<Mn3%, Al<0.1%, 0.3<Si<1.5%, 0.01%<Nb<0.05%, N<0.02%, 0.1<Cr+Mo<1%, 0.0001<B<0.0025, Ti<0.5%, V<0.01%, S<0.01%, P<0.05% the remainder of the composition being iron and unavoidable impurities resulting from the smelting and the microstructure contains, in surface fraction: between 50 and 95% of martensite and between 5 and 50% of the sum of ferrite and bainite, wherein the ferrite grain size is below 10.Math..Math., and wherein the aspect ratio of the ferrite grain size is between 1 and 3. The steel according to the invention is oxidized and subsequently reduced during heating, soaking and cooling steps of the annealing.

Provided is a technique in which a reduction annealing method is used to efficiently produce a high-strength hot-dip galvanizing substrate or high-strength hot-dip galvannealing substrate, which is useful as a raw material for producing a high-strength plated steel sheet suppressed in the occurrence of bare spot. The substrate for hot-dip galvanizing or hot-dip galvannealing of the present invention satisfies the following condition: when the mapping intensity of Fe, which is obtained by using an electron probe microanalyser in a measurement field of view of 33.6 m41.4 m on the surface after reduction annealing, of 0 to 240 is divided into 16 parts at an interval of 15, the area occupied by a mapping intensity of 195 or more has 70% or more.

A method for continuously annealing a steel strip includes annealing the steel strip in a vertical annealing furnace, an atmosphere in the furnace being separated by a partition disposed within a space spanning from the heating zone to the soaking zone. An atmosphere gas is supplied from an outside of the furnace into the furnace to form a furnace gas that is discharged from a steel strip entrance at a lower portion of the heating zone, a part of the furnace gas is sucked and discharged into a refiner including a deoxygenator and a dehumidifier such that oxygen and moisture in the part of the furnace gas are removed to form a gas having a lowered dew point, and the gas having a lowered dew point is returned into the furnace. The method further includes controlling a temperature of the steel strip passing through the partition to a range of 550 C. to 700 C.

A method of annealing of steel sheets is provided which includes a first step consisting in fully oxidizing the surface of such steel sheet thus creating a fully oxided surface layer, a second step consisting in selectively oxidizing elements other than iron of such steel, in an area extending under said fully oxided layer, thus creating a selectively oxided internal layer and a third step consisting in fully reducing said fully oxided surface layer.

There is provided a high-strength hot-dipped steel sheet having excellent coating adhesion and a method for manufacturing the steel sheet. The steel sheet has a chemical composition, and a coating layer is disposed on the steel sheet. The chemical composition includes, by mass %, C: 0.02% or greater and 0.30% or less, Si: 0.01% or greater and 2.0% or less, Mn: 0.2% or greater and 3.0% or less, P: 0.08% or less, S: 0.02% or less, and Al: 0.001% or greater and 0.40% or less, with the balance being Fe and incidental impurities. The coating layer has a coating weight per side of 30 to 90 g/m.sup.2 and contains exfoliated base steel in an amount of 0.3 to 1.5 g/m.sup.2.

Provided are a high-strength galvanized hot-rolled steel sheet and a method for manufacturing the steel sheet. A steel sheet has a chemical composition, and a galvanized layer is disposed on the steel sheet. The chemical composition includes, in mass %, C: 0.02% or greater and 0.30% or less, Si: 0.01% or greater and 2.5% or less, Mn: 0.3% or greater and 3.0% or less, P: 0.08% or less, S: 0.02% or less, and Al: 0.001% or greater and 0.20% or less. The galvanized layer has a coating weight per side of 20 to 120 g/m.sup.2. A surface of the steel sheet has a specific surface area ratio, r, of 2.5 or less, and an amount of Si present in the galvanized layer and an amount of Mn present in the galvanized layer respectively satisfy: amount of Sir0.06, and amount of Mnr0.10.

The hot-dip plated steel material includes a steel material and a hot-dip plated layer disposed on a surface of the steel material, the hot-dip plated layer has a certain chemical composition, and the hot-dip plated layer has a diffraction intensity obtained from a result of X-ray diffraction measurement, the diffraction intensity satisfying a certain relationship.

A zinc or zinc-alloy coated rolled steel strip or sheet includes 0.08-0.28 C, 1.4-4.5 Mn, 0.01-0.5 Cr, 0.01-2.5 Si, 0.01-0.6 Al. The steel has a tensile strength of 950-1550 MPa, yield strength of 550-1400 MPa, yield ratio 0.50, and Ri/t4. The microstructure has 70 tempered martensite+bainite, 10 fresh martensite, 2-20 retained austenite, and 20 polygonal ferrite. The steel has a decarburized zone where the C content at a depth of 20 m is not more than 75% of the carbon content in the middle of the steel strip and/or the microhardness at a depth of 20 m is not higher than 75% of the microhardness in the middle of the steel strip. The adhesion of the zinc or zinc alloy coating is 1 or 2 according to SEP 1931.