A method for manufacturing a high-strength steel sheet having a tensile strength of more than 1100 MPa, a yield strength of more than 700 MPa, a uniform elongation UE of at least 8.0% and a total elongation of at least 10.0%, 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 impurities including N, S, P and residual elements including Cr, Ni, Mo, Cu, and B, the N content being less than 0.01%, the S content less than 0.01%, the P content less than 0.02 %, the Cr content less than 0.1%, the Ni content less than 0.1%, the Mo content less than 0.05%, the Cu content less than 0.2% and the B content less than 0.0010%, the composition being such that: CMnIndex = Cx 1 + Mn / 3.5 < 0.6 C and Mn being the contents in C and Mn in weight %, 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 1 transformation point of the steel and higher than the Ac3 transformation point of the steel but less than 1000°C, and maintaining the sheet at the annealing temperature for 60 s to 200 s, - cooling the annealed sheet to a quenching temperature QT between 190° and 80°C, at a cooling speed sufficient to obtain a structure just after cooling containing martensite and retained austenite, the quenching temperature QT being such that the structure of the steel after heat treatment contains at least 20% of retained austenite and at least 65% of martensite, the sum of the ferrite and bainite contents being less than 10%, - maintaining the steel sheet at an overaging temperature PT between 350°C and 500°C for an overaging time Pt of more than 5 s and, - cooling the sheet down to the ambient temperature, wherein the contents of microstructural elements are given as a surface fraction based on optical and scanning microscope pictures. the remainder being Fe and impurities resulting from the smelting, the impurities including N, S, P and residual elements including Cr, Ni, Mo, Cu, and B, the N content being less than 0.01%, the S content less than 0.01%, the P content less than 0.02 %, the Cr content less than 0.1%, the Ni content less than 0.1%, the Mo content less than 0.05%, the Cu content less than 0.2% and the B content less than 0.0010%, the composition being such that: CMnIndex = Cx 1 + Mn / 3.5 < 0.6 C and Mn being the contents in C and Mn in weight %, 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 1 transformation point of the steel and higher than the Ac3 transformation point of the steel but less than 1000°C, and maintaining the sheet at the annealing temperature for 60 s to 200 s, - cooling the annealed sheet to a quenching temperature QT between 190° and 80°C, at a cooling speed sufficient to obtain a structure just after cooling containing martensite and retained austenite, the quenching temperature QT being such that the structure of the steel after heat treatment contains at least 20% of retained austenite and at least 65% of martensite, the sum of the ferrite and bainite contents being less than 10%, - maintaining the steel sheet at an overaging temperature PT between 350°C and 500°C for an overaging time Pt of more than 5 s and, - cooling the sheet down to the ambient temperature, - annealing a rolled sheet made of said steel by soaking it at an annealing temperature TA higher than the Ac 1 transformation point of the steel and higher than the Ac3 transformation point of the steel but less than 1000°C, and maintaining the sheet at the annealing temperature for 60 s to 200 s, - cooling the annealed sheet to a quenching temperature QT between 190° and 80°C, at a cooling speed sufficient to obtain a structure just after cooling containing martensite and retained austenite, the quenching temperature QT being such that the structure of the steel after heat treatment contains at least 20% of retained austenite and at least 65% of martensite, the sum of the ferrite and bainite contents being less than 10%, - maintaining the steel sheet at an overaging temperature PT between 350°C and 500°C for an overaging time Pt of more than 5 s and, - cooling the sheet down to the ambient temperature, wherein the contents of microstructural elements are given as a surface fraction based on optical and scanning microscope pictures.