I don't know you are asking for. API K-55 is a casing grade, not a material. Sure, API makes some material requirements for that grade of casing, but it also requires dimensional, weight, and marking requirements to be met to be considered K-55.
If you want to know what steel grade would be equivalent, you're out of luck. The chemical analysis requirements for K-55 are very broad. While, generally most mills will use a low-carbon manganese steel, I've seen 4130 used.