There is currently a gap in our understanding of mechanisms that contribute to high strength and high plasticity in high strength UFG ferritic steel with nano-size Fe3C carbides in situations that involve com-bination of various strain rates and high temperature.In this regard,we describe the mechanistic basis of obtaining high strength-high plasticity combination in an ultrafine-grained(UFG)(～500±30 nm)ferritic steel with nano-size carbides,which sustained large plastic deformation,exceeding 100％elon-gation at a temperature significantly below 0.5 of the absolute melting point(Tm).To address the missing gap in our knowledge,we conducted a series of experiments involving combination of strain rate and temperature effects in conjunction with electron microscopy and atom probe tomography(APT).Strain rate studies were carried out at strain rates in the range of 0.0017-0.17 s-1 and at different temperatures from 25℃to 600℃.Dynamic recrystallization occurred at 600℃,resulting in a significant decrease in yield and tensile strength.Nevertheless,the UFG ferritic steels had an advantage in tensile strength(σUTS)and elongation-to-failure(εf)at 600℃,especially at strain rate of 0.0017 s-1,with high σUTS of 510 MPa and excellent low temperature(＜0.42Tm)superplasticity(εf=110％).These mechanical properties are significantly superior compared to similar type of steels at identical temperature.A mechanistic under-standing of mechanical behavior of UFG ferritic steels is presented by combining the effect of strain rate,temperature,and nano-size carbides.