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Influence of Finish Rolling Temperature and Molybdenum Addition on Strengthening of Low Carbon Niobium Steels—A Computational and Experimental Study

Low C microalloyed steels with a ferritic matrix are attractive for thin sheet applications, because they offer superior stretch formability. Previous approaches to overcome their poor yield strength apply warm deformation in the two‐phase region (γ + α), achieving ultrafine grain sizes. Single‐hit deformation studies of various microalloyed steels highlight the beneficial role of small Mo additions on enhancing ferritic yield strength via interphase nanoprecipitation. However, the detailed role of Mo on grain refinement and precipitation strengthening in industrial‐like schedules for low C microalloyed steels remains unclear. A computational approach for alloy and process design aims at improving ferrite yield strength through lowering finish rolling temperature, and Mo addition is attempted here. Based on modeling results, two compositions (Nb and NbMo) are cast and deformed under plane strain compression with two finish deformation conditions, in the single‐ and two‐phase regions, at 950 and 750 °C. Warm finishing at 750 °C results in an ultrafine polygonal ferrite grain sizes averaging of 1.6 μm (NbMo) and 2.1 μm (Nb). An ultimate tensile strength of >700 MPa is found for the NbMo steel under both finishing conditions, with a maximum yield strength of 484 MPa. The role of Mo on the ferrite morphology and precipitation strengthening is assessed using electron microscopy.