Sorry, I must have confused you with another poster that said that cast iron was not used to make piston rings.
You must mean me. I never said they are not made of cast iron, just not plain ole grey cast iron. Other posts and my own research shows that most are made from grey cast iron, but most are also heat treated and this makes them more flexible. Plus we have found that there is a fine grain cast iron which is also more flexible than the usual grey iron used for foundry castings.