Process for manufacturing a hot-dip galvanized or galvannealed steel sheet having a TRIP, transformation-induced plasticity, microstructure comprising ferrite, residual austenite and optionally martensite and/or bainite, said process comprising the steps consisting in: - providing a steel sheet whose composition comprises, by weight: 0.01 ≤ C ≤ 0.22% 0.50 ≤ Mn ≤ 2.0% 0.2 ≤ Si ≤ 2.0% 0.005 ≤ Al ≤ 2.0% Mo < 1.0% Cr ≤ 1.0% P < 0.02% Ti ≤ 0.20% V ≤ 0.40% Ni ≤ 1.0% Nb ≤ 0.20%, the balance of the composition being iron and unavoidable impurities resulting from the smelting, - oxidizing said steel sheet in a direct flame furnace where the atmosphere comprises air and fuel with an air-to-fuel ratio between 0.80 and 0.95, so that a layer of iron oxide having a thickness from 0.05 to 0.2 μm is formed on the surface of the steel sheet, and an internal oxide of at least one type of oxide selected from the group consisting of Si oxide, Mn oxide, Al oxide, complex oxide comprising Si and Mn, complex oxide of Si and Al, complex oxide comprising Mn and Al, and complex oxide comprising Si, Mn and Al is formed, wherein the oxidation of the steel sheet is performed by heating it from ambient temperature to a heating temperature T1, - reducing said oxidized steel sheet, at a reduction speed from 0.001 to 0.01 μm/s in order to make the internal oxide continue to grow in depth of steel sheet, and to achieve a complete reduction of the layer of iron oxide wherein the reduction of said oxidized steel sheet consists in a heat treatment performed in a furnace where the atmosphere comprises from 2 to less than 15% by volume of hydrogen, the balance of the composition being nitrogen and unavoidable impurities, wherein said heat treatment comprises a heating phase from the heating temperature T1 to a soaking temperature T2, a soaking phase at said soaking temperature T2 for a soaking time t2, and a cooling phase from said soaking temperature T2 to a cooling temperature T3, wherein said temperature T1 is between 680 and 800°C and wherein said soaking temperature T2 is between 770 and 850 °C, - hot-dip galvanising said reduced steel sheet to form a zinc-coated steel sheet, and - optionally, subjecting said hot-dip coated steel sheet to an alloying treatment to form a galvannealed steel sheet. - providing a steel sheet whose composition comprises, by weight: 0.01 ≤ C ≤ 0.22% 0.50 ≤ Mn ≤ 2.0% 0.2 ≤ Si ≤ 2.0% 0.005 ≤ Al ≤ 2.0% Mo < 1.0% Cr ≤ 1.0% P < 0.02% Ti ≤ 0.20% V ≤ 0.40% Ni ≤ 1.0% Nb ≤ 0.20%, the balance of the composition being iron and unavoidable impurities resulting from the smelting, 0.01 ≤ C ≤ 0.22% 0.50 ≤ Mn ≤ 2.0% 0.2 ≤ Si ≤ 2.0% 0.005 ≤ Al ≤ 2.0% Mo < 1.0% Cr ≤ 1.0% P < 0.02% Ti ≤ 0.20% V ≤ 0.40% Ni ≤ 1.0% Nb ≤ 0.20%, - oxidizing said steel sheet in a direct flame furnace where the atmosphere comprises air and fuel with an air-to-fuel ratio between 0.80 and 0.95, so that a layer of iron oxide having a thickness from 0.05 to 0.2 μm is formed on the surface of the steel sheet, and an internal oxide of at least one type of oxide selected from the group consisting of Si oxide, Mn oxide, Al oxide, complex oxide comprising Si and Mn, complex oxide of Si and Al, complex oxide comprising Mn and Al, and complex oxide comprising Si, Mn and Al is formed, wherein the oxidation of the steel sheet is performed by heating it from ambient temperature to a heating temperature T1, - reducing said oxidized steel sheet, at a reduction speed from 0.001 to 0.01 μm/s in order to make the internal oxide continue to grow in depth of steel sheet, and to achieve a complete reduction of the layer of iron oxide wherein the reduction of said oxidized steel sheet consists in a heat treatment performed in a furnace where the atmosphere comprises from 2 to less than 15% by volume of hydrogen, the balance of the composition being nitrogen and unavoidable impurities, wherein said heat treatment comprises a heating phase from the heating temperature T1 to a soaking temperature T2, a soaking phase at said soaking temperature T2 for a soaking time t2, and a cooling phase from said soaking temperature T2 to a cooling temperature T3, wherein said temperature T1 is between 680 and 800°C and wherein said soaking temperature T2 is between 770 and 850 °C, - hot-dip galvanising said reduced steel sheet to form a zinc-coated steel sheet, and - optionally, subjecting said hot-dip coated steel sheet to an alloying treatment to form a galvannealed steel sheet.