316L stainless steel is the most common cited material.
It has to be polished and passivated however.
Need help about polishing stainless plate - Cruisers & Sailing Forums
They are called chain plates not keel plates.
Mild steel is a really bad idea unless you are shooting a movie and the boat will be sunk in a couple weeks.
You wouldn't go wrong with ordering them from Garhauer Marine Hardware
They are known for good pricing and they know exactly what you need which some local machine shop may not.
Be careful of the size hole you use for the toggle.
The business end of the shroud (the wire that holds up the mast)
There are lots of wrong ways to install them.
Re-Bedding Deck Hardware Photo Gallery by Compass Marine at pbase.com
Also make sure that what you are bolting them to is solid.
Also make sure the core is solid where the chain plate penetrates the deck.
For not a lot of extra money you can get a another piece of ss steel fabricated to go on the opposite side of the knee, bulkhead or other attachment point.
That way instead of relying on just a washer you have a full length backing plate.
The job is either pretty easy or very hard depending on how hard it is to get to each plate.
If you want to know why they are important read this:
S/V Triumph lost in the atlantic
It is a long read but worth it to answer you particular question about if chain plates should be replaced if you plan on using your boat in heavy weather.
If you are young and plan on keeping the boat a long time and they are hard to get to and have a lot of extra money consider Titanium