Steel is an alloy, which is a combination of alloy and other elements. The alloy used is iron and other elements primarily carbon. Steel is widely used in construction and other applications because of its mechanical properties like tensile strength and ductility.
In a simple understanding, iron is the base metal of steel. Iron can have two crystalline forms i.e. allotropic forms, body-centered cubic (BCC) and face-centered cubic (FCC), depending on its temperature. It is the interaction of those allotropes with the alloying elements, primarily carbon, that gives steel and cast iron their range of unique properties.
The carbon in typical steel alloys may contribute up to 2.1% of its weight. Variation in the quantity of alloy ( in the steel either as solute elements or as precipitated phases) retards the movement of those dislocations that make iron comparatively ductile and weak. This controls its qualities such as the hardness, ductility, and tensile strength. Steel's strength compared to pure iron is only possible at the expense of iron's ductility, of which iron has an excess.
Carbon content increases the strength of steel alloy. Increasing the strength (hindering the dislocation motion) results in decreasing the elongation (which is normally based on dislocation motion). The high amount of hard and brittle cementite make the steel product to break under brittle failure. This would make them break during bending when directed for construction requirements.