The wall is Sheetrock over styrofoam panel no clearance under Sheetrock. Is there a way to mount this without opening the wall. Reading has indicated steel studs do not hold screws well
Depending on your TV mount style, I would simply drill extra holes in it and run four to six sheet metal screws through into the studs. 8 to 12 sheet metal screws will easily carry that television. This assumes that it's tight to the wall and not an articulating arm style mount
Size the holes in the mount so that the screws slip through freely, and be sure not to overtighten so you don't strip the threads in the studs. Just snug them up.
As long as the wall is faced with quality, intact 1/2" standard sheet rock, 4-each, 1/4" standard toggle bolts will work just fine. A quarter inch bolt is rated at 200-Lbs, sheer strength. The 1/2" rock is also rater at 200-Lbs both sheer and tension with a standard 1/4" toggle bolt configuration.
I happen to disagree about the holding strength of a standard metal stud. With a standard #12 sheet metal screw, the pull-out strength would easily be over 75-Lbs per unit; of course no stripping. P.
How heavy is your mount and your TV ? That is the first question to answer in order to determine what methods of mounting will work for you.
For example if your mount and TV total weight is 30LBS .. consider using anchors that can hold up to 50lbs each such as these screw in anchors:
You can on the other hand build a mount strip across the 24 inch centers for a total of 48 inches width with 3 stud mounting points and you could even add drywall anchors if you were a little freaked and wanted to do over kill - your TV mount would then mount to the two strips (which can be simple flat bar or the shelving U Bar - both commonly found at HD or Lo's).