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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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High Al low C -Mn steels are attracting growing interest because of their unique mechanical properties. The major efforts in studying these steels are focused on the effects of heat treatment routes on the final properties. Hot rolling, although being decisively important receive much less attention. The practical aspects of hot strip rolling of Nb microalloyed high Al steels are addressed in the paper. The data from production hot mill are analyzed in conjunction with the results of laboratory high temperature mechanical simulations. Mill data indicate that Al suppresses the effects of microalloying on softening of austenite. Al additions raise the austenite-to-ferrite transformation temperature under industrial mill conditions resulting in uncontrollable formation of ferrite in the surface layers during last hot rolling passes. This, in turn, leads to uncontrollable variation in tension, friction, rolling forces and mass flow. In general, high Al steels are much more sensitive to the variations in bar temperature than that of conventional HSLA steels, so that small variations in processing conditions can induce marked differences in the mill performance and the quality of the hot band. The severity of the related adverse effects also depends on hot mill configuration, capabilities and equipment.