Easton's Bible Dictionary
Steel
The "bow of steel" in (A.V.) 2 Samuel 22:35; Job 20:24; Psalms 18:34 is in the Revised Version "bow of brass" (Heb. kesheth-nehushah). In Jeremiah 15:12 the same word is used, and is also rendered in the Revised Version "brass." But more correctly it is copper (q.v.), as brass in the ordinary sense of the word (an alloy of copper and zinc) was not known to the ancients.
In which case, surely it would have to be bronze (which is what most translations have - another alloy of copper). Copper on its own is far to soft for either an arrow or a bow. Some translations seem to have arrow rather than bow, presumably because a bow made out of copper, bronze or brass would be no use whatsoever. A copper one would just bend, while a bronze or brass one would have no spring. A bronze arrowhead is fine though.