To add to AZ's post, the steelers are currently OVER the cap, somewhere between 15-20 million. Even if Watt was going to take a HUGE pay cut (probably still worth 15-20 million a year), it would be incredibly difficult for the steelers to get him.
Ill be honest I would love him on the browns, but i think its a lot more likely they go try and make a splash in the secondary, than the defensive line.
Edit: the Steelers most likely cut Joe Haden, which will save them a ton in cap space (12 million), they probably dont resign Vance McDonald or they cut eric ebron (either option saves 5), and they probably cut Steve nelson (8 million saved). So that gets them under the cap by 5-10 million. However that also means they got rid of their 2 best CBs.
The only other options they really have to get under the cap enough to get Watt are; restructure Big Ben, cut DeCastro. But not sure Big Ben is willing to give up massive amounts of money, while the rest of the team is stripped down, just so the steelers can give Watt big money.