A method for manufacturing a steel product, comprising the successive steps of: - providing a heated steel starting product at a temperature comprised between 380°C and 700°C, said heated steel starting product having a metastable fully austenitic structure, said heated steel starting product having a composition consisting of, in percent by weight: 0.15% ≤ C ≤ 0.40%, 1.5% ≤ Mn ≤ 4.0%, 0.5% ≤ Si ≤ 2.5%, 0.005% ≤ Al ≤ 1.5%, with 0.8% ≤ Si+Al ≤ 2.5%, S ≤ 0.05%, P ≤ 0.1%, at least one element chosen among Cr and Mo, such that: 0% ≤ Cr ≤ 4.0%, 0% ≤ Mo ≤ 0.5%, and 2.7% ≤ Mn + Cr + 3 Mo ≤ 5.7%, and optionally one or several elements chosen among: Nb ≤ 0.1%, Ti ≤ 0.1%, Ni ≤ 3.0%, 0.0005% ≤ B ≤ 0.005%, 0.0005% ≤ Ca ≤ 0.005%, the balance of the composition consisting of iron and unavoidable impurities resulting from the smelting, - subjecting said heated steel starting product to a hot forming step at a temperature comprised between 700°C and 380°C, with a cumulated strain ε b between 0.1 and 0.7, in at least one location of said heated steel starting product, to obtain a hot-formed steel product, the structure of the steel remaining fully austenitic, then - quenching the hot-formed steel product by cooling it down, at a cooling rate VR 2 superior to the critical martensitic cooling rate, to a quenching temperature QT lower than the martensite start temperature Ms of the steel in order to obtain a structure containing between 40% and 90% of martensite, the rest of the structure being austenite, then - maintaining at, or reheating the product up to a holding temperature PT between QT and 470°C and holding the product at said temperature PT for a duration Pt between 5 s and 600s. - providing a heated steel starting product at a temperature comprised between 380°C and 700°C, said heated steel starting product having a metastable fully austenitic structure, said heated steel starting product having a composition consisting of, in percent by weight: 0.15% ≤ C ≤ 0.40%, 1.5% ≤ Mn ≤ 4.0%, 0.5% ≤ Si ≤ 2.5%, 0.005% ≤ Al ≤ 1.5%, with 0.8% ≤ Si+Al ≤ 2.5%, S ≤ 0.05%, P ≤ 0.1%, at least one element chosen among Cr and Mo, such that: 0% ≤ Cr ≤ 4.0%, 0% ≤ Mo ≤ 0.5%, and 2.7% ≤ Mn + Cr + 3 Mo ≤ 5.7%, and optionally one or several elements chosen among: Nb ≤ 0.1%, Ti ≤ 0.1%, Ni ≤ 3.0%, 0.0005% ≤ B ≤ 0.005%, 0.0005% ≤ Ca ≤ 0.005%, the balance of the composition consisting of iron and unavoidable impurities resulting from the smelting, 0.15% ≤ C ≤ 0.40%, 1.5% ≤ Mn ≤ 4.0%, 0.5% ≤ Si ≤ 2.5%, 0.005% ≤ Al ≤ 1.5%, with 0.8% ≤ Si+Al ≤ 2.5%, S ≤ 0.05%, P ≤ 0.1%, 0% ≤ Cr ≤ 4.0%, 0% ≤ Mo ≤ 0.5%, and 2.7% ≤ Mn + Cr + 3 Mo ≤ 5.7%, Nb ≤ 0.1%, Ti ≤ 0.1%, Ni ≤ 3.0%, 0.0005% ≤ B ≤ 0.005%, 0.0005% ≤ Ca ≤ 0.005%, - subjecting said heated steel starting product to a hot forming step at a temperature comprised between 700°C and 380°C, with a cumulated strain ε b between 0.1 and 0.7, in at least one location of said heated steel starting product, to obtain a hot-formed steel product, the structure of the steel remaining fully austenitic, then - quenching the hot-formed steel product by cooling it down, at a cooling rate VR 2 superior to the critical martensitic cooling rate, to a quenching temperature QT lower than the martensite start temperature Ms of the steel in order to obtain a structure containing between 40% and 90% of martensite, the rest of the structure being austenite, then - maintaining at, or reheating the product up to a holding temperature PT between QT and 470°C and holding the product at said temperature PT for a duration Pt between 5 s and 600s.