It doesn't hurt. It can't really considering Steve's warped pain tolerance and the fact that it's essentially a child's version of strength compared to an adult's. It does make Steve uncomfortable and yes, even a little angry despite knowing he's being tested right now. Such a deliberate move brings up bad memories of back alleys in Brooklyn. The meaner ones would always twist before Bucky reached him.
Steve knocks off his hand before he can let cool logic remind him that he's being played here. He doesn't know why Stark is provoking him, but that is clearly what is happening. "Tony, stop it."