  Fossil-free ironmaking is indispensable for reducing massive anthropogenic
CO2 emissions in the steel industry. Hydrogen-based direct reduction (HyDR) is
among the most attractive solutions for green ironmaking, with high technology
readiness. The underlying mechanisms governing this process are characterized
by a complex interaction of several chemical (phase transformations), physical
(transport), and mechanical (stresses) phenomena. Their interplay leads to rich
microstructures, characterized by a hierarchy of defects ranging across several
orders of magnitude in length, including vacancies, dislocations, internal
interfaces, and free surfaces in the form of cracks and pores. These defects
can all act as reaction, nucleation, and diffusion sites, shaping the overall
reduction kinetics. A clear understanding of the roles and interactions of
these dynamically-evolving nano-/microstructure features is missing. Gaining
better insights in these effects could enable improved access to the
microstructure-based design of more efficient HyDR methods, with potentially
high impact on the urgently needed decarbonization in the steel industry.
