It reverts to its draconic form.
In AD&D 2e, D&D 3.5, and D&D 4th edition, a steel dragon reverts to its true dragon form when slain.
In AD&D 2nd edition, according to the Monstrous Compendium: Greyhawk Adventures Appendix, the steel dragon's shapechanging ability comes from the ability to use polymorph self five times per day with unlimited duration. According to the AD&D 2e (revised) Player's Handbook, p. 206, polymorph self ends when the caster is slain.
In D&D 3.5's stats for the steel dragon on the Wizards of the Coast website (also Dragon #339), the steel dragon's shapechanging relies on the spell-like ability to polymorph five times per day. This spell also ends when the creature is slain (D&D 3.5 Player's Handbook, p.263).
In D&D 4e, the steel dragon's ability to assume human form (Draconomicon: Metallic Dragons, p.172) references the Change Shape ability (Monster Manual 2, p.216), which specifically ends when the creature dies.