When it comes to “frozen” steel parts, nothing beats heat. Fire is your friend. When it gets red hot it will move.
If getting it red hot isn’t an option, getting it hot enough to turn beeswax to smoking liquid as thin as water is the next best option.
If heat isn’t an option, time and PB Blaster is the go to alternative.
WD40 isn’t worth the effort to take it off the home store shelf.
If you just want to get rust off and can’t use muriatic acid but can submerge the part, good old white vinegar works incredibly well.