600 degrees F is not bad. Most any valve with metallurgy better than brass or iron can handle that. Steel would be fine. Even the gaskets will work as long as the packing box is either extended to keep the TFE packing cool, or if the seals are Graphite.
If the valve is operating at 600C, then special metallurgy is necessary. Stainless (316) minimum, probably inconel stem and seat, and graphite seals.
Like everything else, it depends upon what material is flowing through the valve and it is necessary to find a material that is compatible with the lading at elevated temps.