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The constitutive equations for the flow behavior of a commercial 0.34 pct C-1.5 pct Mn-0.7 pct Si-0.083 pct V-0.018 pct Ti microalloyed steel were determined. For this purpose, uniaxial hot compression tests were carried out over a wide range of strain rates (10 Ϫ4 to 10 s Ϫ1 ) and temperatures (1123 to 1423 K). In combination with models developed in the literature, the experimental results permit the flow stress of the present steel to be predicted within ‫5ע‬ pct. It is shown that the classical constitutive equations must be modified to take the grain size into account, particularly when the latter is below 30 m.

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A study of the elastic response before and after tensile plastic strain was undertaken for two commercial low-alloyed TRIP steels. These steels, TRIP 700 (C-Mn-Al alloy) and TRIP 800 (C-Mn-Si) are commercial alloys used in sheet metal stamping. The behaviour of the instantaneous tangent modulus (E T ) versus stress during loading and unloading was measured for each degree of prestrain. Loading curves show a decrease in the E T of the deformed samples as compared with the undeformed state. Though at low stresses a highly linear response was measured for both steels, a decrease was obtained for TRIP 700 as strain increased, whereas TRIP 800 remained unchanged. During unloading, a progressive decrease in E T was obtained in all deformed states, with lower chord modulus values as the tensile plastic prestrain increased. The inelastic response observed is attributed mainly to microplastic strain caused by the displacement of mobile dislocations. Thus, the differences between the two TRIP steels studied are related to the microstructure and the different dislocation structures observed in them. A notable consequence of this study is a better accuracy in the prediction of springback passes due to a better understanding of these inelastic effects that stems from going beyond mere use of traditional Young's modulus values.KEY WORDS: TRIP steels; springback; Young's modulus; dislocation structure. This microplastic deformation results firstly from the short range of motion of mobile dislocations, 19) and secondly from the bowing of the dislocation line between pinning points following models proposed by Mott and Friedel,20,21) and by Granato and Lücke. 22) This extra deformation, which occurs below the internal strength level of the material, is recoverable, and is affected by the dislocation density and the total length of dislocation line that is able to move or bow out. Thus, when e mp grows, a decrease in E is expected. The following Eq. (2), proposed by Granato and Lücke,22) summarizes this idea:where b is the magnitude of the Burgers vector, r is the density of the mobile dislocations and x is the average displacement of a dislocation line of length l.As springback appears after tools have been removed, it is the behaviour of Young's modulus 'during unloading' that should be investigated to obtain a more accurate prediction of the recovery from strain after forming. Ghosh et al. 13,23) used uniaxial tensile tests to study strain recovery after plastic prestrain for a type of steel used in sheet metal stamping, and demonstrated the nonlinearity of the unloading part of the stress-strain curve. This "inelastic behaviour" results in an extra deformation that is not predicted by the usual values of Young's modulus, and again is resumed as microplastic strain. This deformation must be accounted for in FEM simulations in order to better predict the final shape of the pieces stamped. The nature of microplastic strain during unloading is mainly associated with the backward movement of the dislocations that pile up during...

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