This article provides a detailed discussion on the effect of boron in heat-treated steel and thermomechanically-simulated steel. It describes the boron hardenability mechanism and the effect of composition and heat treatment parameters on boron hardenability. The article examines the hardening behavior of unalloyed boron steel and low-alloyed boron steel in heat treatment experiments by varying the austenitizing temperatures and cooling conditions. It also discusses the applications of boron steels.