The paper presents a phenomenological model of the ductile-to-brittle transition in steels due to the action of low temperatures and stress raisers (SR)—notches, cracks, $etc.$ Regularities of the effect of strength and toughness of steel on the critical temperature of brittleness $T_{\textrm{c}}$ for specimens with an annular notch in tension or pre-cracked specimens in three-point bending are considered. As shown, the excessive (supercritical) low-temperature hardening of steel, unbalanced with the required margin of its toughness, when real strength of the steel, $\sigma_{0.2}$, exceeds critical strength for a given type of SR, $\sigma_{0.2\textrm{c}}$, is the reason for brittleness of specimens with SR. Physical essence of the quantity $\sigma_{0.2\textrm{c}}$ is explained in terms of specific characteristics of the steel resistance to break, $B_{\textrm{r}}$, reflecting its margin of toughness. Recommendations are formulated for optimising a set of mechanical properties for products such as fastening bolts with SR in the form of a screw thread, to prevent their cold brittleness during low-temperature operation.