A method for manufacturing a thermally treated steel sheet having a chemical steel composition and a microstructure m target comprising from 0 to 100% of at least one phase chosen among: ferrite, martensite, bainite, pearlite, cementite and austenite, in a heat treatment line comprising: A. a preparation step comprising: 1) a selection sub step wherein the chemical composition and m target are compared to a list of predefined products, which microstructure includes predefined phases and predefined proportion of phases, in order to select a product having a microstructure m standard closest to m target and a predefined thermal path TP standard to obtain m standard, 2) a calculation sub step wherein at least two thermal path TP x, each TP x corresponding to a microstructure m x obtained at the end of TP x, are calculated based on the selected product of step A.1) and TP standard and the initial microstructure m i of the steel sheet to reach m target, the thermal enthalpy H released or consumed between m i and m target being calculated such that: H x = X ferrite * H ferrite + X martensite * H martensite + X bainite * H bainite + X pearlite * H pearlite + H cementite * X cementite + H austenite ∗ X austenite X being a phase fraction, all thermal cycle TP x being calculated such that T t + Δt = T t + φ Convection + φ radiance ρ ⋅ Ep ⋅ C pe Δt ± H x C pe with Cpe: the specific heat of the phase (J·kg -1 ·K -1), ρ: the density of the steel (g.m -3), Ep: thickness of the steel (m), φ: the heat flux (convective + radiative in W), H x (J.Kg -1), T: temperature (°C) and t: time (s), 3) an selection sub step wherein one thermal path TP target to reach m target is selected, TP target being chosen from TP x and being selected such that m x is the closest to m target, B. a thermal treatment step wherein TP target is performed on the steel sheet. A. a preparation step comprising: 1) a selection sub step wherein the chemical composition and m target are compared to a list of predefined products, which microstructure includes predefined phases and predefined proportion of phases, in order to select a product having a microstructure m standard closest to m target and a predefined thermal path TP standard to obtain m standard, 2) a calculation sub step wherein at least two thermal path TP x, each TP x corresponding to a microstructure m x obtained at the end of TP x, are calculated based on the selected product of step A.1) and TP standard and the initial microstructure m i of the steel sheet to reach m target, the thermal enthalpy H released or consumed between m i and m target being calculated such that: H x = X ferrite * H ferrite + X martensite * H martensite + X bainite * H bainite + X pearlite * H pearlite + H cementite * X cementite + H austenite ∗ X austenite X being a phase fraction, all thermal cycle TP x being calculated such that T t + Δt = T t + φ Convection + φ radiance ρ ⋅ Ep ⋅ C pe Δt ± H x C pe with Cpe: the specific heat of the phase (J·kg -1 ·K -1), ρ: the density of the steel (g.m -3), Ep: thickness of the steel (m), φ: the heat flux (convective + radiative in W), H x (J.Kg -1), T: temperature (°C) and t: time (s), 3) an selection sub step wherein one thermal path TP target to reach m target is selected, TP target being chosen from TP x and being selected such that m x is the closest to m target, 1) a selection sub step wherein the chemical composition and m target are compared to a list of predefined products, which microstructure includes predefined phases and predefined proportion of phases, in order to select a product having a microstructure m standard closest to m target and a predefined thermal path TP standard to obtain m standard, 2) a calculation sub step wherein at least two thermal path TP x, each TP x corresponding to a microstructure m x obtained at the end of TP x, are calculated based on the selected product of step A.1) and TP standard and the initial microstructure m i of the steel sheet to reach m target, the thermal enthalpy H released or consumed between m i and m target being calculated such that: H x = X ferrite * H ferrite + X martensite * H martensite + X bainite * H bainite + X pearlite * H pearlite + H cementite * X cementite + H austenite ∗ X austenite X being a phase fraction, all thermal cycle TP x being calculated such that T t + Δt = T t + φ Convection + φ radiance ρ ⋅ Ep ⋅ C pe Δt ± H x C pe with Cpe: the specific heat of the phase (J·kg -1 ·K -1), ρ: the density of the steel (g.m -3), Ep: thickness of the steel (m), φ: the heat flux (convective + radiative in W), H x (J.Kg -1), T: temperature (°C) and t: time (s), 3) an selection sub step wherein one thermal path TP target to reach m target is selected, TP target being chosen from TP x and being selected such that m x is the closest to m target, B. a thermal treatment step wherein TP target is performed on the steel sheet.