I've never had issues getting a Chinese bayonet to unscrew, but I do have a 55/56 Tula that is pretty well impossible to get the bayonet off of. Any more pressure and I'll start stripping things.
I've been meaning to buy some penetrating oil or something to attack it, but I don't really care all that much about it and I don't shoot that SKS as compared to my Chinese one much anyway. Good luck, and if you're worried about putting too much pressure on it, I'd suggest stopping before you strip or mar something, it's what I had to do.