A steel volume is modeled in a computer by means of a plurality of volume elements. The state of the steel volume at a given time comprises, for each volume element, characteristic quantities of an enthalpy existing at said time in the respective volume element and percentages, in which the steel is available in the respective volume element at the time in austenite, ferrite and cementite phases. For at least one volume element, the computer determines the time gradient of the characteristic quantities by resolving thermal conductivity and phase transition equations. One of the characteristic quantities is a locally invariable mean interstitial element concentration within the volume element in the austenite phase thereof.