In this research work, the double differentiation mathematical method was used to identify more accurately the criti- cal stress ሺ ߪ ௖ ) and critical strain ሺ ߝ ௖ ) associated with the onset of dynamic recrystallization (DRX), which is based on changes of the strain hardening rate )ߝ߲/ߪ߲ ൌ ߠሺ as a function of the flow stress (Poliak and Jonas method, simplified by Najafizadeh and Jonas). For this purpose, a low carbon advanced ultra-high strength steel (A-UHSS) microalloyed with different amounts of boron (14, 33, 82, 126 and 214 ppm) was deformed by uniaxial hot-compression tests at high tempe- ratures 1100 ݀݊ܽ ሺ950,1000,1050 ° )ܥ and constant true strain rates ሺ 10 ିଷ ,10 ିଶ 10 ݀݊ܽ ିଵ ݏ ିଵ ) . Results indicate that both ߪ ௖ and ߝ ௖ increase with decreasing deformation temperature and in creasing strain rate. On the other hand, these criti- cal parameters tend to decrease as boron content increases. 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.