High-silicon (1.5–2.5 wt%) steels were designed to achieve carbide-free bainitic matrices with retained austenite through industrial hot rolling, with coiling temperatures of 310 °C and 350 °C. The resulting ultrahigh strength steels (1409–1644 MPa) were characterized through tensile testing, Charpy impact toughness, and Kahn tear tests, while microstructural analysis was performed using scanning electron microscopy and X-ray diffraction. Both yield and tensile strengths correlated strongly with bainitic matrix characteristics, including phase fraction, dislocation density, carbon content, and plate thickness. Ductility showed dependence on film-type austenite content and Md temperature, mechanical stability. The steels exhibited exceptional impact toughness meeting industrial requirements, with ductile fracture behaviour observed down to −100 °C, challenging previous findings. Crack resistance values matched or exceeded those of comparable ultrahigh strength steels. The lower coiling temperature (310 °C) produced retained austenite with higher mechanical stability, benefiting tensile properties and crack resistance, while impact toughness remained largely unaffected by austenite stability due to high strain rates.

In this work, we provide with an overview of the main theories, metallurgical and microstructural concepts used for the design of carbide free bainitic steels adapted to industrial hot rolling and coiling practices. The selected alloys were cast and tested at pilot plant scale and the obtained good combinations of properties tend to validate the thorough and careful approach undertaken to predict the microstructures during the alloy design process.

The mechanisms driving variant selection phenomena in ausformed bainitic microstructures are not fully controlled yet. Previous results are not congruent among them, as they cannot be explained by the same rule. In this work, we aimed to understand the mechanisms governing variant selection in a medium carbon-high silicon steel subjected to ausforming treatments. To do so, the microstructural characterization in different regions of barreled samples has been combined with the simulation of the stress and strain state at the macro and micro level, by finite elements simulations and crystal plasticity simulations, respectively. The main conclusion is that the variant selection shown in these microstructures can be explained by understanding the distribution of the plastic strains at the macro and micro level. The selection of crystallographic variants is more pronounced in the regions which have been more deformed. Also, the deformation distributes along deformation microbands, where the most promoted variants seem to grow, in good agreement with previous results in ausformed lath martensite.