Abstract: A method for manufacturing a high-strength sheet having improved formability segregation accord which the chemical composition of the steel contains, in percent by weight: 0.1%?C?0.4% 4.2%?Mn?8.0% 1%?Si?3% 0.2%?Mo?0.5% the remainder being Fc and unavoidable impurities, the method comprising the steps of annealing a rolled sheet made of said steel by soaking it at an annealing temperature AT higher than the Ac3 transformation point of the steel, quenching the sheet by cooling it down to a quenching temperature QT between the Ms and Mf transformation points in order to obtain a final structure containing at least 65% of martensite and at least 20% of residual austenite, the sum of the ferrite and bainite contents being less than 10%, heating the sheet up to an overaging temperature PT between 300° C. and 500° C. and maintaining it at said temperature for a time Pt greater than 10s and cooling the sheet down to the ambient temperature. Sheet obtained.

Abstract: A method for manufacturing a high-strength steel sheet having a tensile strength of more than 1100 MPa and a yield strength of more than 700 MPa, a uniform elongation UE of at least 8.0% and a total elongation of at least 10%, made of a steel containing in percent by weight: 0.1%?C?0.25%, 4.5%?Mn?10%, 1%?Si?3%, 0.03%?Al?2.5%, the remainder being Fe and impurities resulting from the smelting, the composition being such that CMn Index=Cx(1+Mn/3.5)?0.6, the method comprising the steps of annealing a rolled sheet made of said steel by soaking it at an annealing temperature TA higher than the Ac transformation point of the steel but less than 1000° C., cooling the annealed sheet to a quenching temperature QT between 190° C. and 80° C. at a cooling speed sufficient to obtain a structure just after cooling containing martensite and retained austenite, maintaining the steel sheet at an overaging temperature PT between 350° C. and 500° C.

Abstract: A method for producing a high strength steel piece having desired mechanical properties, obtainable by a reference heat treatment comprising a first reference treatment and a final reference treatment comprising at least an overaging, The method comprising a step of heat treating on an equipment comprising at least an overaging means for which it is possible to set at least one operating point, the final treatment comprising an overaging for which it is possible to calculate two final treatment parameters OAP1 and OAP2 depending on an operating point of the overaging means. A minimum OAP1 min and a maximum OAP2 max final treatment parameters are determined in order to obtain the desired properties, at least one operating points of the overaging section means is determined such that OAP1?OAP1 min and OAP2?OAP2 max. The piece is heat treated accordingly.

Abstract: The invention relates to a double-annealed steel sheet, the composition of which includes, expressed in per cent by weight, 0.20% ?C ?0.40%, 0.8% ?Mn ?1.4%, 1.60% ?Si ?3.00%, 0.015 ?Nb ?0.150%, Al ?0.1%, Cr ?1.0%, S ?0.006%, P ?0.030%, Ti ?0.05%, V ?0.05%, B ?0.003%, N ?0.01%, the remainder of the composition being constituted of iron and unavoidable impurities resulting from processing, the microstructure being constituted, in area percentages, of 10 to 30% residual austenite, 30 to 60% annealed martensite, 5 to 30% bainite, 10 to 30% fresh martensite and less than 10% ferrite. The invention further relates to its fabrication method and the use of such sheet.

Abstract: The present invention provides a spot welded joint of at least two steel sheets. 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 ?m2 and containing more than the steel nominal phosphorus content.