Flow stress characterization of carbon steel S25C in the temperature range of cold forming with an emphasis on dynamic strain aging

Abstract

In this study, a new material model for carbon steel S25C, which exhibits typical dynamic strain aging (DSA), is presented. We detail the development of a closed-form function model. The total flow stress is divided into state variable-effective and DSA-effective flow stresses. The two flow stresses are first formulated as functions of temperature, with unknown material constants, at various strains and strain rates. They are then calculated using an optimization technique to minimize error. The material constants calculated at the sample points are fitted as functions of state variables and thus become material parameters; these are eventually used to describe the flow stress of S25C (with DSA) as a closed-form function. The proposed material model with fitted material parameters and constants was compared to the experimental flow stresses; the average and maximum error rates were 2.9 % and 10.3 %, respectively. Finally, we optimized the material constants and parameters to minimize the maximum errors of the sample state variables. We posed the problem as a min-max problem, and obtained average and maximum error rates of 1.4 % and 6.1 %, respectively. The importance of the closed-form function is emphasized in terms of its application to non-isothermal analyses, especially automatic multi-stage cold forging.