The steel type can be an ass load of different mons so I'l just go in-depth with the most common options and leave a special mentions section like I did with the ground section.
Skarmory @ Leftovers
Ability: Sturdy
EVs: 248 HP / 252 SpD / 8 Spe
Careful Nature
- Defog/Spikes/Stealth Rock
- Roost
- Whirlwind/Taunt
- Iron Head/Brave Bird/Counter
Skarm is probably the most common steel type used in this build and for good reason. Walling all the powerful fairies while providing either defog or spikes support is no easy feat. The spread is the standard spdef skarm spread that allows it to take on a majority if the spattackers in the tier. A physically defensive spread can be used if you wish but spdef is more effective in this meta due to strong spatk fairy types such as Mgarde and Mdiancie and skarm's natural def and typing is usually enough to wall a lot of physical attackers regardless. Whether you run spikes, Stealth Rock, or defog in the first slot is dependent on what the rest of the team needs and changes with all the different variations of the build. Roost is standard recovery to keep skarm healthy throughout the match which is incredibly important considering skarm typically plays an important role in the team's hazard game. Whirlwind is used to phaze set-up sweepers that try to use skarm as fodder and can also be used as an incredible weapon when paired with spikes support. Taunt can be used on a faster spread or just for general utility against opposing hazard setters or defensive mons. Iron head is the most common option for skarm's attacking move since it is being used as a fairy counter but brave bird can also be a viable option if you're weak to fighting or grass mons. Counter is also a nice option to beat Mgross or other strong physically offensive mons that try to muscle through skarm.
Ferrothorn @ Leftovers
Ability: Iron Barbs
EVs: 252 HP / 88 Def / 168 SpD
Relaxed Nature
IVs: 0 Spe
- Spikes/Stealth Rock/Filler
- Leech Seed
- Gyro Ball
- Power Whip/Thunder Wave/Protect
Ferro is a great steel to fit into the core as it is one of two water resists that fit into the build. Ferro also has a multitude of different moves that give it multiple ways of supporting the team. No other steel can provide spikes, water resist, and t-wave all in one slot making ferro a strong contender to use in the core. The spread is the standard ferrothorn spread that has been used for all of gen 6 not much to explain there. The hazard you choose to run is dependent on what the other members of the core contain and whether your hazard remover is a spinner or defogger. The "Filler" in the hazard slot represents the moves slashed over power whip since it's possible to build the core to where the ground type has rocks and you have a defogger so ferrothorn has the freedom of running power whip plus one of those other options. Leech seed helps give ferro some longevity as it lacks reliable recovery, it gets off chip damage on opposing switch-ins, and also makes it a bit easier for the next mon to come in and take a hit due to the leech seed recovery. Gyro ball is ferro's attack of choice due to it's low speed and that it is meant to check fairies. I guess iron head is an option to use over gyro if you are using t-wave but it is not too necessary of a change. Power whip allows ferro to check various waters in the tier such as azumaril, keldeo locked into anything but secret sword, gyarados, and manaphy. Thunder Wave cripples common switch-ins to ferro including heatran, talonflame, and the zards and is generally annoying as all hell. Protect can be used to stall leech seed recovery and gain leftovers since ferro lacks synthesis or some kind of reliable recovery. Protect also helps scout against choiced users that threaten ferro like scarf tran or specs keldeo.
Heatran @ Leftovers
Ability: Flash Fire
EVs: 248 HP / 192 SpD / 68 Spe
Calm Nature
IVs: 0 Atk
- Stealth Rock
- Taunt/Roar
- Toxic/Will-O-Wisp/Flash Cannon/Earth Power
- Lava Plume
Heatran @ Leftovers/Power Herb
Ability: Flash Fire
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Magma Storm
- Earth Power
- Taunt
- Toxic/Solar Beam
Heatran may not be the greatest stop to fairies, bar clefable, due to the weakness to fighting/ground coverage but it provides a massive amount of versatility in return. I actually had no idea how to slash the set due to multiple entirely different sets fitting in here so I just broke it up into a few different sets to make it easier to read although the spdef set still has a stupid number of possibilities anyways. The spread is the standard for both sets allowing tran to either take spatks or stallbreak effectively. The spdef set will be discussed first. Stealth Rock is only used if the ground type on the team lacks it otherwise this slot can be used for one of trans various support options. Lava Plume is the stab of choice and gives off a 30% chance to burn whatever is being switched into tran. Earth Power can be used to take out opposing heatran or get off damage on mons resistant to it's fire stab. Flash cannon is a reliable stab to take out fairy types or rock types trying to switch into lava plume. Taunt allows heatran to stop clefable from setting up and also provides utility against opposing defensive cores. Toxic lures in and cripples water types such as keldeo, manaphy, rotom-w, and slowbro that try to switch into heatran. Wow is more niche due to lava plume having a high burn rate but can be used to cripple physical attackers that threaten tran, namely azumarill. Roar can phaze set-up sweepers trying to set up on tran but is generally less useful than taunt due to not being able to handle clefable as reliably. The magmatran stallbreaker set's moves work the same way as stated above just with a few variations. Magma Storm is the bread and butter of the set that gets off nice damage along with trapping mons in against heatran allowing it to do what it wants with it's trapped prey. Solarbeam+Power Herb is a cool option to lure in and slaughter water types that try to switch into heatran. This set should be used on teams in need of a dedicated stallbreaker and where offensive fairies are not extremely threatening to the team.
Scizor @ Scizorite
Ability: Light Metal
EVs: 248 HP / 44 Atk / 16 Def / 200 SpD
Impish Nature
- Bullet Punch
- Bug Bite/Knock Off/Superpower
- Roost
- Swords Dance
Mzor is of course going to be in the conversation anywhere a defensive steel type is needed. The spread is the standard bulky Mzor spread capable of living two earthquakes from lando-t and avoiding the 2HKO from Mgarde's focus blast. Mzor checks a ton of mons that can threaten this core including gengar, Mgarde, Malt, and Mgross. Bullet punch is the standard stab move of choice and has priority to potentially sweep late game. Bug bite is a more powerful stab move that takes care of slowbro, bisharp, ferrothorn, and other mons resistant to bullet punch while having more power than knock off and avoiding the stat drops caused by superpower. Knock off is not as strong as bug bit but can be used to cripple scizor's checks early-mid game so they can be vulnerable to a late game scizor sweep. Superpower can be used to lure in heatran and take off a fat chunk of hp off of it which is nice with calm mind clefable. Roost and swords dance are the set's main moves and make Mzor an alternate win con as well as give it some general longevity.
Special Mentions:
Spdef Excadrill can again be used here as a spinner but it exaggerates the weakness to water way further than most people would like.
Bronzong is a very cool option that is able to wall Mgarde and set up rocks in one slot which no other mon is capable of doing.
Klefki is another great option that possesses priority spikes and thunder wave and is also a very solid weavile counter which has become a threat in the current meta.
Vanilla scizor can also be a nice option for this slot if your mega is already taken and you want that bug/steel typing along with defog support.
Jirachi is also a very good contender for this slot as it is another one of the few Mgarde counters available, plus it has a massive movepool to abuse for the core. There is honestly a shit load more steels that can go here that are pretty niche but usable but I think by now it's obvious as to what will and will not work here.