A spot welded joint of at least two steel sheets is provided. At least one of the steel sheets presents yield strength above or equal to 600 MPa, an ultimate tensile strength above or equal to 1000 MPa, uniform elongation above or equal to 15%. The base metal chemical composition includes 0.05≤C≤0.21%, 4.0≤Mn≤7.0%, 0.5≤Al≤3.5%, Si≤2.0%, Ti≤0.2%, V≤0.2%, Nb≤0.2%, P≤0.025%, B≤0.0035%, and the spot welded joint contains a molten zone microstructure containing more than 0.5% of Al and containing a surface fraction of segregated areas lower than 1%, said segregated areas being zones larger than 20 μm.sup.2 and containing more than the steel nominal phosphorus content.

The present invention relates to a method for the manufacture of a hot-dip coated steel sheet coated with a zinc or an aluminum based coating including the provision of a specific steel sheet, a recrystallization annealing with specific heating, soaking and cooling sub-steps using an inert gas and a hot-dip coating; the hot dip coated steel sheet and the use of the hot-dip coated steel sheet.

The present invention relates to a method for the manufacture of a hot-dip coated steel sheet coated with a zinc or an aluminum based coating including the provision of a specific steel sheet, a recrystallization annealing with specific heating, soaking and cooling sub-steps using an inert gas and a hot-dip coating; the hot dip coated steel sheet and the use of the hot-dip coated steel sheet.

A cold-rolled and heat treated steel sheet, has a composition comprising 0.1%≤C≤0.4%, 3.5%≤Mn≤8.0%, 0.1%≤Si≤1.5%, Al≤3%, Mo≤0.5%, Cr≤1%, Nb≤0.1%, Ti≤0.1%, V≤0.2%, B≤0.004%, 0.002%≤N≤0.013%, S≤0.003%, P≤0.015%. The structure consists of, in surface fraction: between 8 and 50% of retained austenite, at most 80% of intercritical ferrite, the ferrite grains, if any, having an average size of at most 1.5 μm, and at most 1% of cementite, the cementite particles having an average size lower than 50 nm, martensite and/or bainite.

A cold-rolled and heat treated steel sheet, has a composition comprising 0.1%≤C≤0.4%, 3.5%≤Mn≤8.0%, 0.1%≤Si≤1.5%, Al≤3%, Mo≤0.5%, Cr≤1%, Nb≤0.1%, Ti≤0.1%, V≤0.2%, B≤0.004%, 0.002%≤N≤0.013%, S≤0.003%, P≤0.015%. The structure consists of, in surface fraction: between 8 and 50% of retained austenite, at most 80% of intercritical ferrite, the ferrite grains, if any, having an average size of at most 1.5 μm, and at most 1% of cementite, the cementite particles having an average size lower than 50 nm, martensite and/or bainite.

A hot-rolled coated steel sheet including: in wt %, C: 0.05-0.14%, Si: 0.1-1.0%, Mn: 1.0-2.0%, P: 0.001-0.05%, S: 0.001-0.01%, AI: 0.01-0.1%, Cr: 0.005-1.0%, Ti: 0.005-0.13%, Nb: 0.005-0.03%, N: 0.001-0.01%, Fe residues, and other inevitable impurities; a mixed structure of ferrite and bainite as a main phase; and as a remaining structure, one or more selected from the group consisting of martensite, austenite, and phase martensite (MA), wherein a fraction of the ferrite and bainite is 95-99 area % and Equation 1 is satisfied. [Equation 1] FCO.sub.{110}<112>+FCO.sub.{112}<111>≥10 where, FCO.sub.{110}<112> and FCO.sub.{112}<111>, each representing an area fraction occupied by a structure having ac crystal orientation of {110}<112> and {112}<111>.

A method for producing a press-mold-hardened part includes providing a steel strip having an aluminium-based coating; applying an inorganic, iron-containing conversion layer to the aluminium-based coating with a layer weight in relation to iron of 3-30 mg/m2; cold-rolling the steel strip to form a flexibly rolled strip with strip sections of different sheet thickness; cutting an initial sheet metal blank out of the flexibly rolled strip, with the blank having different sheet thicknesses with thinnest and thickest sheet sections; press-mold-hardening the initial sheet metal blank to form a part. Alternatively, the cold-rolling can take place before the cutting, and the application of the conversion layer can take place before or after the cutting, or, instead of the cold-rolling, at least two steel strip sections having an aluminium-based coating and different sheet thicknesses can be welded together, where the application of the conversion layer can take place before or after welding.

A method for producing a press-mold-hardened part includes providing a steel strip having an aluminium-based coating; applying an inorganic, iron-containing conversion layer to the aluminium-based coating with a layer weight in relation to iron of 3-30 mg/m2; cold-rolling the steel strip to form a flexibly rolled strip with strip sections of different sheet thickness; cutting an initial sheet metal blank out of the flexibly rolled strip, with the blank having different sheet thicknesses with thinnest and thickest sheet sections; press-mold-hardening the initial sheet metal blank to form a part. Alternatively, the cold-rolling can take place before the cutting, and the application of the conversion layer can take place before or after the cutting, or, instead of the cold-rolling, at least two steel strip sections having an aluminium-based coating and different sheet thicknesses can be welded together, where the application of the conversion layer can take place before or after welding.

Provided is a steel sheet having excellent image clarity after painting, including: carbon (C): 0.001% to 0.03%, silicon (Si): 0.001% to 0.35%, manganese (Mn): 0.05% to 2.2%, phosphorus (P): 0.003% to 0.1%, sulfur (S): 0.001% or 0.025%, aluminum (Al): 0.01% to 0.1%, nitrogen (N): 0.001% to 0.007%, and a remainder of iron (Fe) and inevitable impurities. The microstructure of the steel sheet mainly is ferrite phases. An R-cube texture of a surface layer of the steel sheet is 5% or less by area %.

Provided is a steel sheet having excellent image clarity after painting, including: carbon (C): 0.001% to 0.03%, silicon (Si): 0.001% to 0.35%, manganese (Mn): 0.05% to 2.2%, phosphorus (P): 0.003% to 0.1%, sulfur (S): 0.001% or 0.025%, aluminum (Al): 0.01% to 0.1%, nitrogen (N): 0.001% to 0.007%, and a remainder of iron (Fe) and inevitable impurities. The microstructure of the steel sheet mainly is ferrite phases. An R-cube texture of a surface layer of the steel sheet is 5% or less by area %.