Directly in the flame, only stainless steel keeps some limited mechanical resistance. Normal steel oxidises quickly up to breaking.
At a more reasonable place, like +300°C, don't worry too much, provided the stress is reasonable. A banal screw will corrode over time, but you see it on time.
If Bauhaus exists where you live, they sell stainless screws and nuts, including of guaranteed quality, and in small amount.