Steel does ×½ to Fire

Generations VI to IX, single types. The other way round, Fire against Steel, is ×2.

65 Steel and 81 Fire species among the 1025 this site covers.

Attacker, Grass and Steel Kartana sprite

Kartana #0798

damage×½Generations VI to IX

Defender, Fire and Flying Ho-Oh sprite

Ho-Oh #0250

Steel did not exist in Generation I, so Steel against Fire has no value there.

Strongest Steel attackers

Kartana leads Steel at Atk 181

Sturdiest Fire defenders

Dual types that change the answer