It is shown experimentally that the kinetics of interpass softening, normally described in terms of the strain, strain rate and temperature, can be more conveniently specified as a function of the strain hardening rate, strain rate and temperature prior to unloading. This approach significantly reduces the number of experiments required to generate sufficient data for modeling purposes. It also eliminates the need to determine the retained strain to predict the softening kinetics in multi-hit deformation and simplifies the extrapolation of laboratory data to the conditions of industrial processing.KEY WORDS: hot deformation; interpass softening; strain hardening.puted from experimental values of X; it is then related to the experimental variables (e, ė, T def , T s , D 0 ) so that values of the coefficients in Eq. (2) can be derived. These can subsequently be employed in process control. Since the set of Eqs. (1) and (2) involves a large number of parameters that also vary with the conditions of prestraining, laboratory simulations require a considerable amount of experimental work.7) Moreover, further difficulties can be faced when the experimental results are applied to industrial processing, because the sensitivity of the various softening parameters to the prestraining conditions can be different in the simulations and in the mill.In the present work, the parameters of post-deformation softening are evaluated by utilizing the data from conventional flow curves. This permits a significant reduction in the experimental work necessary to establish the softening kinetics and simplifies the application of the results of laboratory simulations to industrial hot working conditions. As will be seen below, the method is based on the current value of the (normalized) rate of work hardening. In a subsequent work, this approach will be extended to deformation under conditions of varying strain rate, as in the roll bite of industrial mills.
ExperimentalThe present study was concerned with a plain low C-Mn steel (wt%: 0.04 % C, 0.22 % Mn, 0.012 % Si, 0.05 % Al) that was shown earlier 10) to undergo DRX during high-temperature, constant strain rate deformation. Therefore, various mechanisms of post-deformation softening can be expected.Laboratory melt ingots were hot rolled to 20 mm thick plates using a laboratory rolling mill. Standard cylindrical compression specimens with initial heights of 15 mm and initial diameters of 10 mm were machined from the plates with the specimen axes parallel to the plate transverse direction.Fractional softening was determined by means of interrupted uniaxial compression testing using a Gleeble ® 1 500 thermomechanical simulator. In the tests, specimens were reheated to 1 100°C for 30 s and then cooled to the test temperature (900-1 050°C) at 5°C/s. After 20 s of soaking at the test temperature, the specimens were compressed in a single hit to a true strain of 1.2 (70 % height reduction) and in the double-hit tests to first-hit strains of e f1 ϭ0.11Ϫ0.5 (10-40 %). In all the tests, constant...