Phosphorus-doped high-strength steels are typically brittle at room temperature. In contrast to the non-hardening embrittlement of body-centred cubic (bcc) steels which decreases toughness without increasing strength, we observed an increase in toughness of about 20% by adding a large amount (0.053 wt%) of phosphorus (P) to a high-strength bcc steel with an ultrafine elongated ferrite grain structure processed by warm calibre rolling at 500 °C which produced a 91% reduction in area. The enhanced toughness is attributed to P segregation, which causes grain boundaries to become feasible crack propagation paths, thereby enhancing delamination toughening. The 0.053% P steel showed a microstructure and tensile properties similar to those of 0.001% P steel (reference steel).
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