The investment should be in HP and Sp.Def, and some in Def for a defensive Steelix.
252 HP / 240 Sp.Def / 16 Def for a more rounded tank.
Offensive variants can forgo Sp.Def and go for Atk investment with Rock Head.
252 HP / 252 Atk / 4 Def (0 speed IVs for maximum Gyro Ball damage) and Head Smash.
Hope I helped!