Display omitted ► Critical conditions for the DRX onset of boron microalloyed steels are determined. ► σ c and ɛ c tend to increase as strain rate increases and temperature decreases. ► σ c and ɛ c tend to decrease as boron content increases. ► Critical ratios σ c/ σ p and ɛ c/ ɛ p remain fairly constant (≈0.82 and ≈0.53). ► Boron additions promote solid solution softening and acceleration of the DRX onset. From the present research, the critical conditions associated with the onset of dynamic recrystallization (DRX) of hot deformed boron microalloyed steels were precisely determined based on changes in the strain hardening rate ( θ) as a function of the flow stress. For this purpose, a low carbon steel microalloyed with four different amounts of boron (29, 49, 62 and 105 ppm) was deformed by uniaxial hot-compression tests at high temperature (950, 1000, 1050 and 1100 °C) and constant true strain rate (10 −3, 10 −2 and 10 −1 s −1). Results indicate that the critical conditions for the initiation of dynamic recrystallization depend on the temperature and strain rate. In addition, both critical stress σ c, and critical strain ɛ c, were noticed to decrease as boron content increased. Such a behavior is attributed to a solute drag effect by boron atoms on the austenitic grain boundaries and also to a solid solution softening effect. The critical ratios σ c/ σ p and ɛ c/ ɛ p for all boron microalloyed steels remain fairly constant (≈0.82 and ≈0.53, respectively), such values are in agreement with those commonly reported for Al-killed, C–Mn, Nb, Nb–Ti, high carbon and stainless steels.