We have traditionally used A193-B16 bolts for valves up to 1050F. Beyond 1050F we use INCONEL 625 or B446 bolts. This works fine but is really expensive.
EN 10269 permits use of the X22CrMoV12-1 (DIN 1.4923) bolts to 600 C (1100 F). This is a 12 chrome creep resistant steel, perhaps...