3 ust Have Steel Pokemon for your Steel Mono
By the Loved Finnath (Finnath?) I meant Cataquack64, I Mistyped ._.

 1.) FERROTHORN
Ferrothorn can be played both Offensive And Defensive. AT THE SAME TIME. Ferro (Imma call him Ferro) Can learn Curse and Gyro Ball. Curse raises Attack and Defense but Lowers Speed, and Gyro Ball is a physical Steel move that has more power the less Speed. It can also learn Power Whip (which is a glorified Vine Whip) For STAB and that Attack Boost From Curse. As you can tell, it has higher Attack than Special Attack, even tho its a grass type. Defense wise, ferro is AMAZEBALLZ. Its signature ability, Iron Barbs, causes physical attackers to lose 1/8 of their Health, so Its Fighting Weakness it rendered Useless.  Its two weaknesses are Fighting and a double weakness to Fire, so that means Macargo can OHKO one, but their weaknesses can be slowed by Thunder Wave (that is, if its not OHKO'd) it can learn Stealth Rock and Spikes, which can hurt pokemon that switchs in (Take THAT, Fake Out/U-Turn/Regenerator Meinshao!) Combining Rocky Helmet with Iron Barbs hurts pokemon even MORE. Wether its offensive or defensive, FERRO can be a THORN in your side ;).

 2.) SKARMORY
Just like Ferro, Skarmory can be played both ways. But either way it's a pain in the ass. A PAIN!!! Its unique typing, Steel and Flying, gives a huge movepool and only two weaknesses, Fire and Electric. Skarmory can be played well defensively.  It can set up traps with Spikes and Stealth Rock (better have Rapid Spin or Defog!), and if it feels threatened, it can Whirlwind or Roar (This bird can roar? Wow) its foe away. And if its hurt, it can Roost its way to full health. This thing is also a physical wall. It has a whopping base 140 defense (which is stupid because Fire and Electric have mainly special moves). This means its foe can have trouble taking it down, and you can't OHKO it cuase it has Sturdy, which fucks you in the ass. Offensive wise, Skarm is Ok. It can learn Swords Dance to double its 80 Attack, and Brave Bird to cuase massive STAB Damage. It can also learn Iron Head, which can flinch le foe, and its signature move Steel Wing, which can raise its already high Defense.