Modeling of the hot flow behavior of advanced ultra-high strength steels (A-UHSS) microalloyed with boron

In this research work, modeling of the hot flow behavior was carried out in a low carbon advanced ultra-high strength steels (A-UHSS) microalloyed with different amounts of boron (14, 33, 82, 126 and 214 ppm). For this purpose, experimental stress–strain data of uniaxial hot-compression tests over a wide range of temperatures (1223, 1273, 1323 and 1373 K (950, 1000, 1050 and 1100 °C)) and strain rates (10⁻³, 10⁻² and 10⁻¹ s⁻¹) were used. The stress–strain relationships as a function of temperature and strain rate were successfully described on the basis of the approach proposed by Estrin, Mecking, and Bergström, together with the classical Avrami equation and the conventional hyperbolic sine function. The analysis of the modeling parameters of the hot flow curves shows that boron additions to A-UHSS play a major role in softening mechanisms rather than on hardening. The peak stress (σ_p) and steady-state stress (σ_ss) values show a decreasing trend with increasing boron content, which indicates that boron additions promote a solid solution softening effect additional to that produced by DRX. The time for 50% recrystallization (t_50%) tends to increase with boron additions, revealing that boron additions cause a delay of the DRX kinetics during hot deformation. Similarly, the presence of boron in the steel decreases the apparent activation energy for recrystallization (Q_t), indicating that boron additions accelerate the onset of DRX. The constitutive equations developed in this way provided an excellent description of the experimental hot flow curves