Disulfide bond formation and dimerization are not necessarily interconnected. Both can be done in vitro after expression - in principle... It always depends on your protein. You might also want to try soluble expression with a tag (such as intein), which gives you a more gentle treatment of your protein.

Try including 500 mM L-Arginine in your refolding buffer. It worked for me.

Just to add to what my colleagues have stated; Try to include DDT in your refolding buffer but you will need to remove it through dialysis before purification or you could consider refolding with it after purification.