Boron in the range of a few ppm plays a vital role in microalloyed steel, mainly as a cheaper hardenability promoter than, e.g., chromium, molybdenum, or nickel. Boron microalloyed steel with boron contents around 30 mass ppm shows a more distinct decarburization zone close to the surface in comparison to B-free steel. It is assumed that boron oxide formation at the surface facilitates decarburization, associated with C loss to the atmosphere as CO or CO2, due to a synergistic effect of B and C [1]. Understanding of the coupling effects of involved, relatively light and volatile species C, B and O is of importance, since affected carbon concentration in steel influences the hardness of martensite and thus determines the surface-near mechanical properties [2]. In this paper, we discuss the microstructure and hardness dependence of the decarburized near-surface region on varied oxygen partial pressure by heat treatment experiments under a controlled gas atmosphere. Moreover, we analyze the resulting carbon distributions by EPMA measurements. Picoindentation measurements are employed in order to clarify the role of decarburization for the hardness decrease of the surface-near zone. References [1] D. Wipp, Boron influence on microstructural evolution and mechanical properties in micro-alloyed carbon steels Precipitation and segregation behavior, doctoral thesis, Wien, 2021. [2] B. Hutchinson, J. Hagström, O. Karlsson, D. Lindell, M. Tornberg, F. Lindberg, M. Thuvander, Acta Materialia 59 (2011) 5845–5858.