Identification of the effective grain size responsible for the ductile to brittle transition temperature for steel with an ultrafine grain size ferrite/cementite microstructure with a bimodal ferrite grain size distribution

► The ductile to brittle transition temperature (DBTT) was elucidated for steels with a bimodal, ultrafine grain size, ferrite/cementite microstructure. ► The responsible effective grain size corresponded to the grain size of the large grain size regions. ► These grain boundaries determine the DBTT because these are high angle grain boundaries.