Over time, stove grates take on a greasy, cruddy film. If you want to make them look like new again, some really hot or boiling water can help cut through the build-up and make the cleaning chore easier.
The Kitchn offers instructions for cleaning enamelled cast iron grates. Before soaking them with hot water and soap, they pour boiling water over the (perfectly cool) grates in the sink to cut the grease. After the boiling water trick and the soap soak, a scrubbing with Bon Ami (a mild abrasive cleanser) finishes the job.
If your cast iron grates aren't enameled, you shouldn't do the soaking part, but the pour of boiling water could still help. For a no-scrub solution, you could also use ammonia.
Check out The Kitchn's post for the full tutorial.
How to Clean Enameled Cast Iron Stove Grates [The Kitchn]