FIELD: metallurgy.
SUBSTANCE: present invention relates to a steel sheet having a tensile strength of more than 1,100 MPa, a yield strength of more than 700 MPa, a uniform elongation UE of at least 8.0 % and a total elongation TE of at least 10.0 %, while the sheet is made of steel having a chemical composition containing in weight percent: 0.1 % ≤ C ≤ 0.25 %, 4.5 % ≤ Mn ≤ 10 %, 1 % ≤ Si ≤ 3 %, 0.03 % ≤ Al ≤ 2.5 %, the rest is Fe and inevitable impurities, while the chemical composition is such that CMnIndex = Cx(1 + Mn/3.5) ≤ 0.6, while the steel sheet has a structure containing at least 20 % of residual austenite and at least 65 % of martensite, and the amount of ferrite and bainite is less than 10 %. Also a method for producing such a sheet is disclosed, comprising the following steps: heating a rolled sheet made of the specified steel to a temperature AT that is higher than the steel transformation temperature Ac3, but lower than 1,000 °C, and holding the rolled sheet at the temperature AT for 60–200 s to obtain a heated sheet, cooling the heated sheet to a temperature QT 190–80 °C with a cooling rate sufficient to produce a sheet having a structure immediately after cooling, containing martensite and residual austenite, wherein the temperature QT is such that the steel sheet obtained by the above method has a final structure containing at least 20 % of residual austenite and at least 65 % of martensite, and the sum of the contents of ferrite and bainite is less than 10 %, the exposure of the cooled sheet at the over-temperature PT that is 350–500 °C during the time of the overageing Pt 5–600 s and cooling the steel sheet to ambient temperature. Also disclosed is a variant of the above steel sheet and a variant of its production.
EFFECT: simplification of the method for manufacturing a high-strength steel sheet using a continuous heat treatment line.
32 cl, 4 tbl
