I'm totally new to cooking with cast iron. When I got my first pan I seasoned it properly in the oven at about 350 with lard. However, I've been cooking hamburgers on it recently. I usually cook the burgers in the middle of the pan. After cooking they leave brown looking residue on the pan in the spot they were cooking and I usually scrape it off and sometimes scrub with a sponge or paper towel. Even after all that scrubbing (even scrubbing with salt and oil) you can still tell something was cooked right there because it seems to be a little brown. Well, I ignored it and just re-seasoned it in the oven a couple times. Now that spot and some area surrounding it is a dark reddish brown. Is this rust? It feels normal compared to other parts of the pan