I am going to say thats cast iron sewage pipe. Smack it with a sledgehammer, and see what color it is when broken. If it breaks sort of like glass(e.g with sharp edges, seems brittle) with a gray or whiteish inside it is probably cast. To me, that looks like cast iron because it is resistant to corrosion and it doesn't show flaky rust. The liner is probably plastic, to prevent the sewage or water flowing through it from damaging it. new cast iron pipe often has it in my experiance.
BTW, you can't forge it easily. It does melt decently though. if it as thick as you say, what else would it be?
I just realized you probably won't be able to break it.
it has a lot of impurities in it, and a LOT of carbon.