This investigation considers the size effect on the deformation behavior of simple tension in microforming and thus proposes a simple model of the tensile flow stress of sheet metal. Experimental results reveal that the measure of the flow stress can be represented as a hyperbolic function $tanh(T/D)$, which is a function of $T/D$ (sheet thickness/grain size). The predicted flow stress agrees very well with the published experiment. Notably, a specimen with smaller grains has lower normalized flow stress for a given $T/D$. Since the material properties of the macroscale specimen do not pertain to the microscale, a critical condition $(T/D)c$ that distinguishes the macroscale from the microscale in the tensile flow stress is subsequently proposed, based on the “affected zone” model, the pile-up theory of dislocations, and the Hall–Petch relation. The distribution of the predicted $(T/D)c$ is similar to the experimental finding that the $(T/D)c$ decreases as the grain size increases. However, the orientation-dependent factor $β$ is sensitive to $(T/D)c$. Hence, further study of the orientation-dependent factor $β$ is necessary to obtain a more accurate $(T/D)c$ and, thus, to evaluate and understand better the tensile flow stress of sheet metal in microforming.

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