Provided are a steel sheet with excellent weldability, and a production method therefor.

The steel sheet is characterized by having a specific composition and a metallographic structure containing, in terms of an area ratio, ferrite of 25% or more and 65% or less, martensite having iron-based carbides precipitated in the grains of 35% or more and 75% or less, and the balance structure other than the ferrite and the martensite of 20% or less (including 0%) in total, the average grain diameters of the ferrite and the martensite being respectively 5 m or lower, the total of concentration of Si and Mn at interface between the ferrite and the martensite being, in terms of an atomic concentration, 5% or more, and having a tensile strength of 900 MPa or higher.

The present disclosure relates to a method for producing a semifinished product with modified surface, comprising at least one method step of at least regionally modifying the surface of a coated, skin-pass-rolled, oiled, cleaned metallic steel substrate via low-pressure or atmospheric-pressure plasma treatment of said surface regions with oxygen, argon or a mixture of oxygen and argon as process gas. The present disclosure further relates to the accordingly produced semifinished products and/or flat steel products, optionally formed semifinished products and/or flat steel products, and also to their use.

The invention relates to a flat steel product for hot forming, consisting of a steel substrate which consists of a steel having 0.1-3 wt % of Mn and optionally up to 0.01 wt % of B, and of an Al-based protective coating applied to the steel substrate. The iron-free mass fraction in the protective coating of Mg as additional alloy constituent adds up to less than 2.50% Mg. In addition, the iron-free mass fraction in the protective coating of Mn as additional alloy constituent adds up to more than 0.30% Mn and the iron-free mass fraction in the protective coating of Si as additional alloy constituent adds up to less than 1.80%.

The hot dip galvannealed steel sheet of the present disclosure has a base steel sheet and a plating layer. The base steel sheet has a predetermined chemical composition by mass %. An existence ratio ER of internal oxides identified under predetermined conditions by observation of the cross-section of the base steel sheet is 40% or less and an interval I of internal oxides is 300 ?m or more. The hot dip galvannealed steel sheet of the present disclosure is excellent in appearance etc.

A high-frequency heating method for a hot stamping process includes: a first heating step of high-frequency heating a steel sheet, which has an aluminum (Al) coating layer formed on an iron (Fe)-based base material, to a first target temperature at a first heating rate; a second heating step of melting the coating layer by high-frequency heating the steel sheet, which has passed through the first heating step, to a second target temperature at a second heating rate, wherein the second heating rate is lower than the first heating rate; and a third heating step of high-frequency heating the steel sheet, which has passed through the second heating step, to a third target temperature at a third heating rate, wherein the third heating rate is lower than the second heating rate. A compound is formed by a reaction between a material of the coating layer and a material of the base material in the second heating step. Therefore, the coating layer is not shifted even when high-frequency heating is performed at a temperature equal to or higher than the melting point of the coating layer.

A method for producing a coated steel sheet having a tensile strength of at least 1450 MPa and a total elongation of at least 17% is provided. The method includes providing a cold rolled steel sheet having a chemical composition in weight %: 0.34%C0.45%, 1.50%Mn2.30%, 1.50%Si2.40%, 0%<Cr0.7%, 0%Mo0.3%, 0.10%Al0.7%, optionally 0%Nb0.05%, and a remainder of Fe and unavoidable impurities. The sheet is annealed at an annealing temperature higher than the Ac3 transformation point of the steel, quenched to a quenching temperature lower than the Ms transformation point of the steel and between 150 C. and 250 C., reheated to a partitioning temperature between 350 C. and 450 C., maintained at the partitioning temperature for at least 80 s, then coated by galvannealing, with an alloying temperature between 470 C. and 520 C. A steel sheet is also provided.

Provided are a plated steel sheet and a method for manufacturing same, the plated steel sheet comprising: a base steel sheet; a ZnMgAl plating layer provided on at least one surface of the base steel sheet; and an FeAl inhibition layer provided between the base steel sheet and the ZnMgAl plating layer. The plating layer comprises, by weight %, 4 to 10% of Mg and 5.1-25% of Al and the remainder being Zn and unavoidable impurities with respect to components not including iron (Fe) diffused from the base steel sheet. The plating layer comprises a 24-50% MgZn.sub.2 phase in phase fraction. In the MgZn.sub.2 phase, an Al single phase is present in the ratio of 1-30% relative to the cross-sectional area of the total MgZn.sub.2 phase.

A steel sheet for a container includes a steel sheet, a Sn coated layer which is provided as an upper layer of the steel sheet and contains Sn in an amount of 560 to 5600 mg/m.sup.2 in terms of Sn metal, and a chemical treatment layer which is provided as an upper layer of the Sn coated layer and contains a Zr compound in an amount of 3.0 to 30.0 mg/m.sup.2 in terms of Zr metal and a Mg compound in an amount of 0.50 to 5.00 mg/m.sup.2 in terms of Mg metal.

When a hot-dip galvanizing treatment is performed on a steel sheet containing Si in an amount of 0.2 mass % or more by using a continuous hot-dip galvanizing apparatus including an annealing furnace in which a heating zone, a soaking zone, and a cooling zone are arranged in this order, a snout adjacent to the cooling zone, and hot-dip galvanizing equipment, a humidified nitrogen-hydrogen gas mixture containing moisture in such a manner that a certain expression is satisfied is supplied into a region on the downstream side of the soaking zone, gas nozzles are arranged over the entire perimeter of an inner wall of the snout, nitrogen gas or a nitrogen-hydrogen gas mixture is supplied through the gas nozzles downward along the inner wall, and the dew point in the snout is controlled to be ?50? C. to ?35? C.

A steel sheet having a tensile strength (TS) of 1180 MPa or more, high LME resistance, and good weld fatigue properties. The steel sheet has a specific chemical composition and a specific steel microstructure. Crystal grains containing an oxide of Si and/or Mn in a region within 4.9 ?m in a thickness direction from a surface of the steel sheet have an average grain size in the range of 3 to 10 ?m, the lowest Si concentration L.sub.Si and the lowest Mn concentration L.sub.Mn in the region within 4.9 ?m in the thickness direction from the surface of the steel sheet and a Si concentration T.sub.Si and a Mn concentration T.sub.Mn at a quarter thickness position of the steel sheet satisfy a specified formula.