Unless your running flood coolant ditch the carbide.Use just a quality two-flute HSS endmill.
Drill a start hole at the end of the slot with a drill bit,7/16 is a good size for a 1/2" slot.Run the mill at 1/2 the speed you would for mild steel,about 250 rpm.
Plunge all the way trough and mill it in one pass running about .002" chipload per flute.Brush on thread cutting oil forget all the floozy perfume.
I regularly cut 13/16 wide x 3" long slots in 3/8 thick 304ss on a 2hp b-port clone single pass.It takes a lot more time than the same cut in MS,just be patient.
I just need one more tool,just one!