What chemical characteristics give types of steel their properties?
(Source: Roy Beardmore, http://www.roymech.co.uk/ (defunct, via the Internet Archive)

As carbon content increases, the ability to resist a sudden impact decreases, as measured by the Charpy Impact Test.

As carbon content increases, ultimate tensile strength and Brinell Hardness increases. 

As far as a phase diagram:



increasing $\ce{}$ (cementite) content makes the steel more brittle and hard.

Near 0.76% carbon, the steel will be pearlite which is ductile (good for wires).  Below 0.76% carbon, there will be increased alpha-iron (ferrite). Above 0.76% there can be Ledeburite.  Austenite only exists at high temperature.  Depending upon the cooling rate of Austenite, Bainite (slow cooling) or Martensite (rapid cooling) can form.