This steel sheet has a predetermined chemical composition, in which the area ratio of plate martensite is 10% or more, the average grain size of prior austenite grains is 2.0 μm to 10.0 μm, the maximum diameter thereof is 20.0 μm or less, the amount of solid solution C in martensite is 0.20 mass % or less, the average carbide size is 0.25 μm or less, the crystal orientation difference between plate martensite and another martensite adjacent thereto in the same prior austenite grain is 10.0° or less, and the P concentration at grain boundaries of the prior austenite grains is 4.0 at % or less.

A hot press member includes excellent indentation peel strength which has a tensile strength of 1780 MPa or more. A plating layer has at a surface thereof a 10-point average roughness Rzjis of 25 μm or less, and a steel sheet contains, in mass %, not less than 0.25% but less than 0.50% of C, 1.5% or less of Si, 1.1-2.4% of Mn, 0.05% or less of P, 0.005% or less of S, 0.01-0.50% of Al, 0.010% or less of N, 0.001-0.020% of Sb, 0.005-0.15% of Nb, and 0.005-0.15% of Ti, the balance being Fe and incidental impurities. The average crystal grain size of prior austenite is 7 μm or less and the volume proportion of martensite is 90% or more, within 50 μm in the thickness direction from the surface of the steel sheet excluding the plating layer.

A cold- or hot-rolled steel strip with a metallic coating, the steel strip having iron as the main constituent and, in addition to carbon, an Mn content of 8.1 to 25.0 wt. % and optionally one or more of the alloying elements Al, Si, Cr, B, Ti, V, Nb and/or Mo. The uncoated steel strip is first cleaned, a layer of pure iron is applied to the cleaned surface, an oxygen-containing, iron-based layer containing more than five mass percent of oxygen is applied to the layer of pure iron. The steel strip is then annealed and is reduction-treated in a reducing furnace atmosphere during the annealing treatment to obtain a surface consisting mainly of metallic iron. The steel strip is then hot-dip coated with the metallic coating. This creates uniform and reproducible bonding conditions for the coating on the steel strip surface.

Provided are a high-strength steel sheet having a yield strength of 550 MPa or more and excellent fatigue-strength of a weld and a method for manufacturing the steel sheet. A high-strength steel sheet has a specified chemical composition, a steel microstructure observed in a cross section in a thickness direction parallel to a rolling direction including 40% to 75% of a martensite phase in terms of volume fraction, in which a total volume fraction of martensite grains whose average grain diameter ratios with respect to adjacent ferrite grains are ¼ or more and 1 or less is 60% or more with respect to an entire martensite phase, and a yield strength (YP) of 550 MPa or more.

Provided is a high-strength steel sheet including, in % weight, carbon (C): 0.04 to 0.15%, silicon (Si): 0.01 to 1.0%, manganese (Mn): 1.8 to 2.5%, molybdenum (Mo): 0.15% or less (excluding 0%), chromium (Cr): 1.0% or less (excluding 0%), phosphorus (P): 0.1% or less, sulfur (S): 0.01% or less, aluminum (Al): 0.01 to 0.5%, nitrogen (N): 0.01% or less, boron (B): 0.01% or less (excluding 0%), antimony (Sb): 0.05% or less (excluding 0%), one or more of titanium (Ti): 0.003 to 0.06% and niobium (Nb): 0.003 to 0.06%, a balance of Fe and other unavoidable impurities, and contents of the C, the Si, the Al, the Mo and the Cr satisfy the following Expression 1: Expression 1: {(2×(Si+Al))+Mo+Cr}/C≥15. The high-strength steel sheet comprises: a ferrite phase, a bainite phase, a martensite phase, and a residual austenite phase, the ferrite phase being less than 40% of area fraction in the microstructure.

One aspect of the present invention relates to a plated steel sheet for hot press forming, having an excellent impact property.

A hot-dip coated steel substrate coated with a layer of Sn directly topped by a zinc or an aluminum based coating is provided, the steel substrate having the following chemical composition in weight percent: 0.10≤C≤0.4%, 1.2≤Mn≤6.0%, 0.3≤Si≤2.5%, Al<2.0%, and on a purely optional basis, one or more elements such as P<0.1%, Nb ≤0.5%, B≤ 0.005%, Cr≤1.0%, Mo≤0.50%, Ni≤1.0%, Ti≤0.5%,
the remainder of the composition making up of iron and inevitable impurities resulting from the elaboration, the steel substrate further having between 0.0001 and 0.01% by weight of Sn in the region extending from the steel substrate surface up to 10 μm.

A method for controlling surface reactions on a steel strip running through a furnace includes, successively, a first section, a second section, and a third section separated by a sealing element, an atmosphere in the second and third sections being oxidizing and reducing respectively, the method including: heating, in the first section, the steel strip to between 600 and 750° C., while causing the atmosphere therein to be slightly oxidizing and to include: an H.sub.2 content inferior to 2%; an O.sub.2 content inferior to 0.1%; an H.sub.2O or CO.sub.2, or H.sub.2O+CO.sub.2 content superior to 0.03%; a controlled dew point ranging from −50 to −15° C.; and a controlled concentration of CO+CO.sub.2 maintained below 2%. All percentages are expressed in terms of volume.

Cooling section for a steel strip continuous annealing or galvanizing line arranged to handle a metal strip (1), said section comprising at least one area (2) for dry cooling set up to project gas on said steel strip and at least one wet cooling area (5) set up to project a liquid or a mixture of gas and liquid on said steel strip.

Provided is a plated steel sheet having both strength and workability. In a hot-dip Zn—Al—Mg-based plated steel sheet, the steel substrate contains C in an amount of 0.050 to 0.180% by mass, Si in an amount of 0.001 to 0.50% by mass, Mn in an amount of 1.00 to 2.80% by mass, Ti in an amount of 0.01 to 0.10% by mass, and B in an amount of 0.0005 to 0.0100% by mass, an average grain size of cementite after winding in a hot rolling step is not greater than 2 μm, a metal structure after a continuous hot-dip galvanizing step includes a ferrite phase and not less than 15 and less than 45% by area of a second phase, and the second phase is composed of martensite or composed of martensite and bainite and has an average crystal grain size of not greater than 8 μm.