The parts manual I went through when we rebuilt my brother's 18RG only had chrome rings listed for 18RGEU engines.
Normal steel rings were listed for 18RC in the same book.
I have been told that normal steel rings can warp an distort in high revving engines.

I actually have a set of pistons that have totally screwed ringlands because of ring issues.
The rings actually overlapped each other in one piston by over 1mm an jammed into the ringland and then broke on either side, leaving just the jammed in bits behind.

UGLY.

Others no longer have nice square edges on their ringlands, but badly angled and worn ring lands.
These same pistons have very little wear from the engine bores, so I am guessing that the rings were not suitable.

Just for information purposes, Hastings Premium chrome rings cost $65.43 retail price when we built my brother's engine about three years ago.
I have the part number for .5mm OS rings, but it can be easily deciphered and translated into 1mm OS rings.

PM me if you want the part number.