What happens if the percentage of carbon used to make steel is increased?

Steels with higher carbon content display higher strength and hardness values. Carbon, in the form of carbides, prevents slips and dislocations through dispersion strengthening. Consequently, increasing carbon also tends to decrease melting temperature as the iron content is decreased. As evidenced by the phase diagram, iron (Fe) has a higher melting temperature than carbide (Fe3C) [you can look up an Fe-Fe3C phase diagram for reference].

Dispersion strengthening: Small particles (or carbides, in this case) located on grain boundaries and inside grains effectively block dislocation movement and grain boundary sliding. Dispersion-hardened alloys typically display rapid strain hardening and good creep resistance.