This article provides a model that regards the evolution behavior of the friction coefficient in the cylindrical compression test as a function of true strain on the basis of experimental results, allowing the effect of friction on the deformation curve at extremely high strain level to be evaluated and corrected for the first time. The compressive tests were carried out at a stroke rate of 1.2 mm/s on IHS38MSV hypoeutectoid steel with various lubricants at temperatures ranging from 800°C to 1200°C. The results showed that the friction coefficient for the compressive process was not constant and the variation could be approximated by an exponential equation along with the true strain. Microstructure observation showed that the stress increase in the later stages of process should be closely related to the large increase in the friction coefficient. The corrected curves were found to correlate well with the microstructure observation.
In order to analyze the evolution behavior of friction coefficients during large strain hot forging processes, compression tests were carried out on IHS38MSV hypoeutectoid steel samples at a stroke rate of 1.2 mm/s and temperatures ranging from 1073 to 1473 K. The simulation results of the geometrical change in the sample during the compression process obtained by using various friction coefficients were compared with experimental results in order to verify the validity of the ''instantaneous friction coefficient''. The results showed that the friction coefficient was a constant at lower strain level, and the instantaneous friction coefficient, m s , showed an approximately exponential relationship at higher strain level. The simulation results obtained by using m s showed better results as compared to those obtained by using average friction coefficients (m c and m a ). It was also found from the simulation results that at a certain strain level, the variation of friction coefficient did not have a strong effect on the shape of deformed sample; however, the radius of the top surface of the sample, R t , changed greatly due to friction.
With the objective of evaluating the accuracy of the upper-bound theory for calculating the average Tresca friction coefficient m in the hot forging process, we performed simulations using different values of m in each compression process to high strain levels. It was found that the upper-bound theory is not applicable at high strain levels, because the contact surface of the cylindrical sample is composed of an originally flat end surface and the annular portion formed by the contact of the lateral surface with the anvil surface. The relation among P ¼ R m H R t H 0 ; true strain, and m could be expressed by a 0 þ a 00 e þ a 000 e 2 À P À Á þ b 0 þ b 00 e þ b 000 e 2 À Á m þ c 00 e þ c 000 e 2 À Á m 2 ¼ 0: Here, the m values obtained were in good agreement with the actual ones used in the simulations. The value of m of the arbitrary geometry cylindrical sample could also be directly read from a contour map with a relationship among nominal strain, parameter B of the corresponding standard sample, and m.
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