If I was adopted but my parents got divorced and my father died last week, would I be his heir?

Please  take a look at the county court records to see if you were in fact adopted, or the state agency that has birth death records, or anyother agency that may have that information. Even though it appears your father may never have signed off on the adoption, that doesn't mean that adoption didn't take place. Yes if you were adopted, you would have been his heir, assuming there is an estate to inherit. If you are not adopted, and there was an estate to be probated, you should consullt a probate lawyer to file a petition for heirship, as there may have been some  reason for the adoption to have not been completed even though your step father intended to adopt you.