Precipitation-strengthening is widely applied to high strength steel sheet for automotive use since several strength grades are easily achieved by controlling amount of microalloyed component. Recently, finer carbide dispersion has been required to obtain higher strength by smaller addition of carbide formers like titanium and niobium. Here, interface precipitation, one of the carbide formation phenomena during γ→α transformation, can be the efficient method to promote very fine carbides by lowering precipitation temperature. This study deals with relationship between transformation temperatures and hardness of ferritic steel strengthened by carbides generated by the interface precipitation. Two kinds of 0.04%C steels containing Ti and Nb of the same amount as carbon content in atomic were hot-rolled, followed by the soaking at various temperatures for 600s. The rapid-cooled samples before the soaking for 600s exhibited higher hardness than slow-cooled samples. Large carbides generated by interface precipitation were observed in slow-cooled Ti-bearing steel with a transmission electron microscope. In slow-cooled Nb-bearing steel, large NbC precipitated in austenite before γ→α transformation. The results are suggesting that lowering transformation temperature and suppressing carbides precipitation in austenite are important to obtain high strength by interface precipitation.