A method for manufacturing a thermally treated steel sheet having 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 heating section, a soaking section and a cooling section including a cooling system, wherein a thermal path TP target is performed, such method comprising: A. preparation step comprising: 1) a selection sub step wherein: a. m target and the chemical composition are compared to a list of predefined products, whose 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 TP standard, comprising at least a heating, a soaking and a cooling steps, to obtain m standard, b. a heating path, a soaking path including a soaking temperature T soaking, the power cooling of the cooling system and a cooling temperature T cooling are selected based on TP standard and 2) a calculation sub step wherein through variation of the cooling power, new cooling paths CP x are calculated based on the selected product in step A.1.a) and TP standard, the initial microstructure mi of the steel sheet to reach m target, the heating path, the soaking path comprising T soaking and T cooling, the cooling step of TP standard being recalculated using said CP x in order to obtain new thermal paths TP x, each TP x corresponding to a microstructure m x, 3) a selection step wherein one TP target to reach m target is selected, TP target being chosen among the calculated thermal paths TP x and being selected such that m x is the closest to m target and B. a thermal treatment step wherein TP target is performed on the steel sheet. A. preparation step comprising: 1) a selection sub step wherein: a. m target and the chemical composition are compared to a list of predefined products, whose 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 TP standard, comprising at least a heating, a soaking and a cooling steps, to obtain m standard, b. a heating path, a soaking path including a soaking temperature T soaking, the power cooling of the cooling system and a cooling temperature T cooling are selected based on TP standard and 2) a calculation sub step wherein through variation of the cooling power, new cooling paths CP x are calculated based on the selected product in step A.1.a) and TP standard, the initial microstructure mi of the steel sheet to reach m target, the heating path, the soaking path comprising T soaking and T cooling, the cooling step of TP standard being recalculated using said CP x in order to obtain new thermal paths TP x, each TP x corresponding to a microstructure m x, 3) a selection step wherein one TP target to reach m target is selected, TP target being chosen among the calculated thermal paths TP x and being selected such that m x is the closest to m target and 1) a selection sub step wherein: a. m target and the chemical composition are compared to a list of predefined products, whose 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 TP standard, comprising at least a heating, a soaking and a cooling steps, to obtain m standard, b. a heating path, a soaking path including a soaking temperature T soaking, the power cooling of the cooling system and a cooling temperature T cooling are selected based on TP standard and a. m target and the chemical composition are compared to a list of predefined products, whose 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 TP standard, comprising at least a heating, a soaking and a cooling steps, to obtain m standard, b. a heating path, a soaking path including a soaking temperature T soaking, the power cooling of the cooling system and a cooling temperature T cooling are selected based on TP standard and 2) a calculation sub step wherein through variation of the cooling power, new cooling paths CP x are calculated based on the selected product in step A.1.a) and TP standard, the initial microstructure mi of the steel sheet to reach m target, the heating path, the soaking path comprising T soaking and T cooling, the cooling step of TP standard being recalculated using said CP x in order to obtain new thermal paths TP x, each TP x corresponding to a microstructure m x, 3) a selection step wherein one TP target to reach m target is selected, TP target being chosen among the calculated thermal paths TP x and being selected such that m x is the closest to m target and B. a thermal treatment step wherein TP target is performed on the steel sheet.