Steel chainrings will rust where the chain wears through the plating, but it's of no concern, except cosmetically.
If you ride regularly you'll not see the rust because the chain is constantly polishing the chainring the same way passing trains keeps the tracks polished (you only see rust where the wheels don't touch). If you don't ride for a while the rust can become more pronounced, but here too it's of no concern because the chain will polish it within the first few miles.
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