How can I indicate that a required field is only required for final Submission, but not for just being able to Save the form?

I have a form where there are several fields; some are required and some not. This form can be saved at any time so the user can come back to it later to complete it, but they won't be able to actually Submit it unless all the required fields are complete.

Crude mockup below:

^{download bmml source – Wireframes created with Balsamiq Mockups}

Currently I have gone with the standard Asterisk * and a message at the top of the form that fields marked with * are required fields, but I'm a bit concerned that this is misleading, because they're only required for final submission and not for just saving the form for later.

Possibly I'm overthinking it, but is there a convention for 'some fields are required for submission but you can still save the record for later without completing everything'?