Superplastic properties of fine-grained ultrahigh carbon steels (UHCS) have been greatly improved through the addition of 3 wt% Si (UHCS-3Si) and through improved processing conditions. This material showed an elongation to failure of 1300% under optimum superplastic conditions. It is also superplastic at very high strain rates, i.e. 10-2 s-1, in the temperature range between 800 and 825°C. An analysis of the effect of silicon additions on the UHCS and the influence of the introduction of temperatures regions in the phase diagram on the superplastic properties is made.