There are lots of bar stock, hot or cold rolled, that is available. pay attention to carbon equivalent of the steel and follow AWS D1.1 recomendations for preheat, interpass temperature, joint design and inpection.
Use of smooth bars, I know of no regulation prohibiting this.
1910.27(b)(1)(iv)
Rungs, cleats, and steps shall be free of splinters, sharp edges, burrs, or projections which may be a hazard.