This is the nature of the steam, water heating can only be evaporated in a saturated state, and the steam generated is also saturated steam. For example, in 1 atmospheric pressure, water boiling at 100 ℃, which is saturated water at saturation temperature. When steam evaporates at 1 atmosphere of pressure, its temperature must also be 100°C, the same temperature as saturated water.

To get superheated steam, the saturated steam must be heated further.