Whether they are porcelain-coated cast iron, or plain old cast iron, these grates on a hob are simple to clean. Some grates are even dishwasher safe (including cast iron grates) so before you begin cleaning the harder way, you should check with the manufacturer to see if, indeed, you can simply plop them in a dishwasher. Otherwise, even if the grates have burnt-on food, you can clean them up with no more effort than cleaning a burnt pan.
Put on your vinyl or rubber dishwashing gloves, unless you don’t mind getting your hands in the dish washing detergent. Fill your kitchen sink with warm water and add about 1 tbsp (more if you desire) of a liquid dish-washing detergent.
Let the grates cool down if you’ve recently been cooking--although the dish water is warm, the temperature change from very hot to cooler may damage grates. Place them gently in the soapy water.
Allow the grates to soak for 15 minutes, if there is grease (burnt on or not) or stuck-on food. Normally you wouldn’t want to soak cast iron (especially pans) due to the possibility of rust, but as long as you limit the soaking time and wash and dry them quickly, it should not be a problem.
Place a few drops of dish detergent directly on a nylon scrubbing pad or scrub brush (or use steel wool, pre-embedded with soap if cast iron grates (not porcelain coated) have burnt on food or grease. Even though there is already detergent in the water, this gives a little extra cleaning power--which is especially important if the grates are very dirty.
Scrub each grate separately, using the scrub brush or pad. Rinse it in warm water immediately after cleaning. Dry it well with a towel and lay it on another towel as you continue this process with the other grates.