Steel is usually treated with some sort of finish, bluing, browning, etc. this colours the engraving at the same time, so it does't show as nicely as on polished brass, with the engraving blackened. I use Super Blue, a cold blue liquid, to blacken engraving, and then polish off the stain on the brass. The black remains in the engraving, and highlights it. It lasts as long as the polished brass stays polished. Once the brass oxidizes, the blackened engraving is less dramatic.