 2008 Carl Hanser Verlag, Munich, Germany www.ijmr.de Not for use in internet or intranetsites. Not for electronic distribution. Ignacio Mejía a , Edgar López-Chipres a , Cuauhtémoc Maldonado a , Arnoldo Bedolla-Jacuinde a , José María Cabrera b,c a Instituto de Investigaciones Metalúrgicas, Universidad Michoacana de San Nicolás de Hidalgo, Morelia Michoacán, México b Departament de Ciència dels Materials i Enginyeria Metal.lúrgica, ETSEIB-Universitat Politècnica de Catalunya, Barcelona, Spain c Centre Tecnològic de Manresa, Manresa, Spain Modeling of the hot deformation behavior of boron microalloyed steels under uniaxial hot-compression conditions The present study shows that the hot deformation behavior of boron microalloyed steels can be quantitatively de- scribed by constitutive equations. These equations take into account both dynamic recovery and recrystallization phe- nomena. They have been fitted using experimental data tak- en from hot compression tests of four boron microalloyed steels in order to determine their characteristic parameters. The tests were carried out over a wide range of tempera- tures (950, 1000, 1050 and 1100 8C) and strain rates (10 –3 , 10 –2 and 10 –1 s –1 ). The analysis of the characteristic parameters of the constitutive equations describing the hot flow behavior of these steels shows that boron additions play a major role in softening mechanisms rather than on hardening. A quantification of the boron effect is also pre- sented. The experimental data were compared with the pre- dictions of the proposed model and an excellent agreement between measured and predicted values for all boron micro- alloyed steels over a wide range of temperatures and strain rates was obtained. Keywords: Boron microalloyed steel; Hot deformation be- havior; Dynamic recrystallization (DRX); Softening; Con- stitutive equations 1. Introduction The study of boron in iron and steel spans a period of more than fifty years. Initially, boron was treated as a “trace im- purity”. Current manufacturing techniques, however, fre- quently involve the addition of small quantities of boron to steels to enhance specific properties, e. g. the hardenability of ferritic steel [1] and the creep resistance of an austenitic steel [2]. Some applications of steels have revealed that small amounts of boron are useful to improve the hot work- ability and prevent intergranular fracture at lower tempera- tures, since free boron is considered to segregate along grain boundaries and exhibit a strengthening effect [3]. In most cases it is assumed that boron is segregated on grain boundaries where it forms precipitates and alters the char- acter of the grain boundary/precipitates or matrix/precipi- tates interfaces in such a way that microcavity formation may be suppressed. It has also been reported that boron seg- regates towards grain boundaries in the very first stage of recrystallization and increases the cohesion of the inter- faces. Therefore, intergranular fracture does not occur easily when sufficient boron is added to the steels [4]. In hot working processes, the effect of boron on the harden- ability of austenitic steel is pronounced [5 – 12], particularly its interaction with grain boundaries, which is relevant to hot working and recrystallization of austenite. Segregated boron atoms on austenite grain boundaries will change their interfacial energy and this in turn exerts some effect on the softening behavior of austenite during hot working [13]. The effects of small additions on high temperature transfor- mation and deformation processes in low carbon steels are complex, sometimes contradictory, but particularly signifi- cant, considering the very small levels of boron required for these effects. Nevertheless, an understanding of the thermomechanical behavior of steels is essential in the simulation and control of the hot forming operations. Moreover, a precise predic- I. Mejía et al.: Modeling of the hot deformation behavior of boron microalloyed steels 1336 Int. J. Mat. Res. (formerly Z. Metallkd.) 99 (2008) 12 B Basic Fig. 1. Sketch of the plastic flow behavior of c-Fe under high tempera- ture deformation conditions. (The flow stress curve exhibits cyclic DRX at relatively low strain rates and high temperatures and single peak DRX at high strain rates and low temperatures).