A basement floor drain cover can prevent items to clog the drain from entering it. The cover keeps items from falling into the drain as well. Floor drain covers in the basement sometimes rust. The rust could be so severe that it would cause the drain cover to freeze that eventually makes it almost impossible to remove. Basement floor drain covers are commonly made of cast iron. This is because of its susceptibility to rust. The rust happens when a basement floods because of melting rain or snow. Well, it happens as well when the drain clogs and gray water backs up into the basement. A damp basement may also cause a drain cover to rust.
If the drain cover screws in place are rusted, then spray them with rust solvent. Or you can also apply penetrating oil but wait a few minutes and then try again to remove the screws. Use a flat head screwdriver if the entire cover is rusted to the drain to scrape around the edge of the cover and remove as much rust as you can. Then use a hammer to tap round the edge to try to loosen the cover. Insert a spade or crowbar under the edge of the cover. In extreme cases, jackhammer use may be necessary to remove the concrete around the drain cover which may also require replacing the drain.
Consider installing a plastic as a replacement. Plastic covers are made of polypropylene. They do not provide the protection required for a basement drain but also have the advantages in resisting rust and corrosion with less expensive than cast iron drain covers. The plastic is sturdy and will last for years.