For decades, Grain Boundary Engineering (GBE) has proven to be one of the most effective approaches to tailor the mechanical properties of metallic materials, although there are limits to the fineness and types of microstructures achievable, due to the grain size increasing rapidly once being exposed to thermal loads (low thermal stability of crystallographic boundaries). Here we deploy a novel Chemical Boundary Engineering (CBE) approach, augmenting the variety in available alloy design strategies, which enables us to create a material with an ultrafine hierarchically heterogeneous microstructure even after being heated to high temperatures. When applied to plain steels with carbon content of only up to 0.2 wt.% this approach yields ultimate strength levels beyond 2.0 GPa in combination with good ductility (>20%). Although demonstrated here for plain carbon steels, the CBE design approach is in principle applicable also to other alloys.