Modeling the flow behavior of a medium carbon microalloyed steel under hot working conditions
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I.
INTRODUCTION
DURING the past decade, increasing proportions of hotforged parts are being manufactured from medium carbon microalloyed steel.[1] These steels are especially suitable for automobile components such as connecting rods, crankshafts, and wheel hubs. Their mechanical properties are generally adequate in most cases, although their toughnesses are consistently low. In order to improve the latter property, the parameters of the forming process must be optimized. This is especially important when the final microstructure is produced directly by the forming process, and no subsequent heat treatment is carried out. Appropriate understanding of the microstructural behavior of the steel under consideration is therefore required, together with the constitutive relation describing material flow. The latter can then be used to carry out computer simulations, by means of which the final microstructural state can be improved and the design of the forming process can be optimized. The constitutive equations of flow can be represented by the following set of relations: «z 5 f (s, T, Si , Pj )
[1]
dSi 5 g (s, T, Si , Pj ) dt
[2]
where εz is the strain rate, s is the flow stress, T is the temperature, Si is the set of state variables that describe the current microstructure (grain size, dislocation density, etc.), and Pj is characteristic properties such as the atomic volumes, bond energies, diffusion constants, etc. Eq. [1] is known as the ‘‘rate equation’’ and has received more attention in the literature than Eq. [2], the ‘‘structural evolution equation.’’ This is because the internal microstructure evolves in a complex manner with time, through
J.M. CABRERA, Associate Professor, A. AL OMAR, Postdoctoral Fellow, and J.M. PRADO, Professor, are with the Departamento de Ciencia de Materiales e Ingenierı´a Metalu´rgica, Universidad Polite´cnica de Catalun˜a, 08028-Barcelona, Spain. J.J. JONAS, CSIRA/NSERC Professor, is with the Department of Mining and Metallurgical Engineering, McGill University, Montreal, PQ, Canada H3A 2B2. Manuscript submitted December 12, 1996.
METALLURGICAL AND MATERIALS TRANSACTIONS A
the interaction of mechanisms such as work hardening, dynamic recovery, and dynamic recrystallization. The aim of the present work was the determination of simplified constitutive equations for the hot working of a medium carbon microalloyed steel. The points of departure were the models and equations proposed previously in the literature. II.
EXPERIMENTAL PROCEDURE
A commercial 0.34 pct C microalloyed steel containing V, Ti, and Al was selected for this study (its chemical composition is listed in Table I). In order to study the flow behavior, uniaxial hot compression tests were carried out at true strain rates εz ranging from 1024 to 10 s21 at a series of testing temperatures. Cylindrical specimens were prepared, which were 11.4 mm in height and 7.6 mm in diameter. Three different testing machines were employed, depending on the required true strain rate: an electromechanical device for εz ≤ 0.3 s21, a
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