A harmonized method was developed to assess CO2 mitigation performance and avoidance costs for different steel production routes, including blast furnace with blast oxygen furnace (BOF), direct reduced iron (DRI) production with electric arc furnace (EAF), and Hisarna with BOF. Mass and energy balances were used to evaluate each route's cradle-to-gate CO2eq emissions. Results indicate that using either CO2 capture and storage (CCS) or biomass can reduce, but not eliminate, CO2eq emissions in the iron and steel sector. However, the combination of CCS and biomass (BECCS) can result in CO2-neutral or even CO2-negative steelmaking. The results show that implementation of BECCS is possible at an avoidance cost < 100 €/t CO2eq. BECCS combined with carbon neutral electricity has a CO2 mitigation potential of 107–139% with CO2 avoidance costs of 59–157 €/t CO2eq. Reducing biomass upstream emissions could further improve these results. The Biomass-DRI-EAF-CCS route has the highest CO2 mitigation potential (146%), while the Biomass-Hisarna-BOF-CCS route has the lowest CO2 avoidance cost (54 €/t CO2eq). This study indicates that the developed harmonized methodology can also be applied in other industrial sectors to screen different portfolios of mitigation options.