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In constant strain rate tests, the occurrence of dynamic recrystallization (DRX) is traditionally identified from the presence of stress peaks in flow curves. However, not all materials display well-defined peaks when tested under these conditions. Using plain carbon, Nb-bearing and 321 austenitic stainless steels, it is shown that the onset of DRX can also be detected from inflections in plots of the strain hardening rate q against stress s or, equivalently, from inflections in ln q-ln s and ln q-e plots regardless the presence of stress peaks in the flow curves. These observations are verified by means of metallography. A unified description of the flow curve is introduced based on normalization of the stress and strain by the respective peak or steady state values. This approach reveals that, in a given material, the ratio of DRX critical stress to the peak or steady state stress is constant, as is that of the critical strain to the corresponding strain values. Furthermore, it is shown that the present technique can be used to establish the occurrence of DRX when this cannot be determined unambiguously from the shape of the flow curve.KEY WORDS: hot deformation; austenite; dynamic recrystallization. Fig. 1. Constant strain rate stress-strain curve typical of DRX.

In rolling, the strain rate in a rolling pass is not constant but depends on pass reduction r p , decreasing or increasing along the arc of contact. In the present work, high temperature compression tests were performed with the rate varying according to strain rate profiles pertaining to various flat rolling pass reductions. Due to the high rate sensitivity of the stress at elevated temperatures, the stress follows such variations in strain rate. This can lead to peaks in the flow curves without regard to dynamic recrystallization (DRX). Nevertheless, critical strains for the onset of DRX can still be defined if the stresses and strains in variable strain rate deformation are normalized by the peak stresses and strains that would be observed if the deformation were being performed at a series of constant strain rates equal to that of successive points along the roll bite. Using plain carbon and Nb-bearing steels, it is demonstrated that the DRX critical strains are lower when r p Ͻ30 % and higher when r p Ͼ30% than in constant e˙deformation at the same initial strain rate. The present method permits the more accurate extrapolation of laboratory test results to industrial conditions and enables rolling loads to be analyzed with greater precision.

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Using double-hit hot compression tests, the softening behavior of 304 Hs tainless steel was studied during unloading. The prestrains used were associated with the initiation of dynamic recrystallization (DRX) ( e c ), the peak strain(e p ), ½(e c 1 e p ), the strain at maximum softening rate ( e i ), and the onset of steady state flow(e s ). The following conditions of deformation were used: T 5 1000°C, 1050°C, and 1100°C, _e ¼ 0 : 01 and 0.1 s ÿ 1 ,and delay timesof0.3 to 1000 seconds. To define the aboveimportant strains, single-hit hot compression tests were performed over awider range of deformation conditions than the double-hit ones-i.e., 900°Ct o1 100°Ca nd _e ¼ 0 : 01 to 1s ÿ 1 .T he results showt hat a transition strain ( e *) separates the strain-dependentr ange of postdynamic softening from the strainindependent range.A ts trains between e c and e *, both metadynamic and static recrystallization contributetointerhitsoftening.The valueof e *obtained in this work was e * 5 4/3 e p .Itwas also found that the strainh ardening rate was identical at all thec ritical strains ( e *) and took the value ÿ 22 MPa.

Efficient operation of the computational models employed in process control systems require periodical assessment of the accuracy of their predictions. Linear regression is proposed as a tool which allows separate systematic and random prediction errors from those related to measurements. A quantitative characteristic of the model predictive ability is introduced in addition to standard statistical tests for model adequacy. Rolling force calculations are considered as an example for the application. However, the outlined approach can be used to a~sess the performance of any computational model.

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...