Brass is an alloy of copper and zinc, not steel. (Bronze, by the way, is an alloy of copper and tin, sometimes with zinc, silicon and other metals added.)
All copper wire is formed by drawing it through successively smaller dies. The "hard-drawn" wire produced by this process has high tensile strength, but it's brittle and hard to bend.
For general-use electrical wire, the hard-drawn wire is annealed by heating then gradually cooling it to relieve crystalline stresses, producing so-called "soft-drawn" wire.
The irregular crystalline structure of hard-drawn or "work-hardened" copper wire has somewhat higher resistance than annealed wire.
Oxygen-free high-conductivity (OFHC) copper wire is at least 99.95% copper.
