Onix doesn't evolve into Steelix by leveling up. You have to trade Onix while holding Metal Coat. Steelix can also be found in the wild.
It doesn't evolve by level. You need to trade an Onix holding Metal Coat to make it evolve into Steelix.
How to change your username
How to add a user avatar
Why was my post hidden?
Flags vs Downvotes
Official PokemonDb League
Type a Pokemon name, type, move or ability to see instant results.