For high-speed continuous rolling, such as in wire rod mill and hot strip mill, behaviors of various carbon steels under high strain-rate hot deformation were investigated. For the purpose, a compression type hot deformation simulator of the following performance was developed; maximum strain rate, 300sec^-1; minimum time interval between each deformation, 15msec; and maximum strain, over 2. A high speed rolling mill was also used, and a strain rate of 800sec^-1 was obtained by this mill. A technique of two-pass rolling was also developed.
Using these experimental devices, steels of 0.05 to 0.81%C were tested in the temperature region between 800°C and 1 100°C. The main results obtained were as follows:
(1) The features of stress-strain curves did not change essentially with the increase in strain rate. It was shown that the dynamic recrystallization of austenite determined the form of the curves.
(2) In the simulator experiments, the maximum stress σ_m, the steady state stress σ_s, and the mean resistance to deformation k_fm, were all found describable satisfactorily well as a function of the Zener-Hollomon Parameter, Z, with an activation energy of 63 800kcal/mol and with an exponent of Z lying between 0.09-0.17. The grain diameter of recrystallized austenite also seemed to be described as a function of Z.
(3) In the rolling experiments, k_fm was also shown to be well described as a function of Z, as k_fm=CZ^r provided that the calculated finishing temperature be taken as the temperature of deformation, k_fm and r both showing a fair agreement with those obtained from the simulator experiment.
(4) The effects of successive deformation on the stress, with a short interval of 0.5sec and below, are nearly the same as those of single deformation, provided the overall strain is the same. Accordingly, the above conclusions (1) and (2) should also apply to the cases of successive deformation as a rough approximation.