{私/俺/僕}の心は{鋼鉄/鉄/鋼}{で/から}できている
{Watashi,Watakushi/Ore/Boku} no kokoro ha {koutetsu/tetsu/hagane} {de/kara} dekite iru.
be made of = ……で/から できて いる ...{de/kara} dekite iru
"できる dekiru" is a verb that basically means "be possible to", "be able to", but when it's used in a passive voice it can also mean "be composed of". The phrase sounds a bit unnatural to me, but I can't come up with a better translation. Hope this would help.