I think that this research can help you "Fereshteh-Saniee F., Fatehi-Sichani F., “An investigation on determination of flow

curves at room temperature and under forming conditions”, Journal of Materials

Processing Technology, Vol. 177, pp. 478-482, 2006."

the most influence in a forging process is the

friction. When the compression load is applied, the workpiece begins to deform.

Once above the elastic behavior of the material, the workpiece is deformed

uniformly. In this portion of the deformation, when the height is reduced, the area

increases uniformly along the same. In this case, the stress and strain are calculated

directly.  In this moment is not so easy to calculate the stress and

strains. To solve this problem, it is used a correction factor of the curve (FereshtehSaniee

& Fatehi-Sichani, 2006).

Do you can find the answer, for example, in "Metal forming practise" from Heinz Tschaetsch.