Interphase precipitation is the phenomenon that when steels containing strong carbide-forming elements are transformed from austenite into ferrite,alloy carbides are precipitated in sheets as a result of periodic nucleation at migrating interphase boundary.Nano-sized carbides formed through interphase precipitation are used to strengthen low carbon steels due to its large amount of precipitation strengthening.The effects of transformation temperatures and alloy elements on the hardness of ferrite strengthened by VC interphase precipitation in low carbon steels were investigated in the present study.It is found that when the transformation temperature becomes lower,the transformed microstructure changes from equi-axed grain boundary ferrite into acicular Widmanstatten or bainitic ferrite.Such transition also corresponds to the decrease in hardness at lower transformation temperatures due to the absence of VC interphase precipitation in acicular ferrite as confirmed by three-dimensional atom probe analyses.On the other hand,by lowering the Mn content,the formation of acicular ferrite is promoted while Si addition does not have large influence on the ferrite morphology.In contrast,higher N content suppress the formation of acicular ferrite,especially at lower transformation temperature at which intra-granular ferrite is extensively formed instead.The hardness in different alloys can be well understood qualitatively by considering the morphology of ferrite,indicating that the control of ferrite morphology is extremely important to obtain better mechanical properties with interphase precipitation for industrial use.