(D EDP Sciences, Les Ulis DOI: 10.1051/jp4:200306SO Study of high strain rate plastic deformation of low carbon microalloyed steels using experimental observation and computational modeling J. Majta, A. K. Zurek1, C. P. Trujillo1 and A. Bator Akademia Gomiczo-Hutnicza, Metallurgy and Materials Science Department, Mickiewicza 30, 30-059 Krakow, Poland Los Alamos National Laboratory, MST8, MS-G755, Los Alamos, NM, U. S. A. Abstract. This work presents validation of the integrated computer model to predict the impact of the microstructure evolution on the mechanical behavior of niobium-microalloyed steels under dynamic loading conditions. The microstructurally based constitutive equations describing the mechanical behavior of the mixed ex and y phases are proposed. It is shown that for a given finishing temperature and strain, the Nb steel exhibits strong influence of strain rate on the flow stress and final structure. This tendency is also observed in calculated results obtained using proposed modeling procedures. High strain rates influence the deformation mechanism and reduce the extent of recovery occulTing during and after deformation and, in turn, increase the driving force for transformation. On the other hand, the ratio of nucleation rate to growth rate increases for lower strain rates (due to the higher number of nuclei that can be produced during an extended loading time) leading to the refined ferrite structure. However, as it was expected such behavior produces higher inhomogeneity in the final product. Multistage quasistatic compression tests and test using the Hopkinson Pressure Bar under different temperature, strain, and strain rate conditions, are used for verification of the proposed models. 1. INTRODUCTION The need to understand and consider explicitly the microstructural and mechanical effects involved by differences between static and dynamic loading are well recognized in engineering. It is difficult to describe materials behavior under dynamic loading using general " universal " method of analysis. For example, mechanical twinning as a plastic deformation mechanism should be considered in addition to dislocation motion. The importance of the twinning depends on the lattice and temperature and increases when strain rate increases or temperature decreases. The proper description of correlations among high strain rate and mechanical and microstructural behavior of material are very important issues, not only because of final products implementations, but also during manufacturing process. In several metal forming processes (drawing, rolling of long products, forging), it bas been estimated that the strain rates in the deformation zones are very high (up to lu4 s ). It is, therefore, clear that the influence of strain rate in modeling of such metal forming processes should be taken into account. There are number of ideas that have been proposed and successfully employed to describe the mechanical behavior and microstructure evolution as a function of strain rate. In each case obtained empirical equations depend on the investigated material. In recent years, modern metal forming processes as intercritical rolling (i. e., deformation in the austenite-ferrite phase region) or warm rolling of steels are studied and implemented. Here, the idea is to increase workability as well as the as-hot rolled strength by increasing austenite pancaking, thereby refining and work hardening the ferrite. In such a situation, the hardening process of untransformed austenite increases, but additionally, the driving forces for potential ferrite recrystallization start to be accumulated in the aiready transformed ferrite structure. In consequence, the resulted structure is inhomogeneous, i. e. consists of very fine recrystallized grains together with transformed, hardened neighbors. Qualitative and, especially, quantitative evaluation of such structure is extremely difficult. Additionally, the déformation under dynamic loading makes this analysis very complex, however, necessary. Deformation processes in the two-phase c-y+a) region seem to be able to