We propose a flow-stress characteristic of SUS430F steel that takes in account the effects of temperature, strain rate and their deformation history. In the framework of this characteristic, the history effects of strain rate and temperature are estimated through the plastic strain energy stored. The formulated characteristic (σ) may be shown as a function of temperature (θ), strain rate ((έ), and the stored energy W or the reference stress (σst). The energy is stored through plastic deformation and released during annealing process. The energy is also referred by the yield flow stress (σst), which is measured under the reference condition. The discussion on the characteristic is extended to the material at high temperature (1073–1473 K) with the σ + gamma phase of a given phase ratio. The equilibrium ratio of (α)- or (γ)-phase at a given temperature can be estimated on the basis of an equilibrium phase diagram. In order to introduce the flow-stress characteristic with phase transformation using the proposed formulation, we also analyze the phase transformation rate from (α) to α + γ with temperature elevation, and from (α) to >γ + α in the cooling process on the basis of the “time–temperature–transformation” diagram that includes the quenching process as well. The flow stress characteristic and phase ratio are estimated simultaneously for a hot forging process.