CAD models are generally a pain to work with and this kind of artifact is actually quite common.
Some things that might help:
- Messing around with the phong tag (for instance disabling angle limit)
- Messing around with the normals tag (for instance deleting it)
- Importing with a higher resolution (so shorter max length).
That being said, usually it can be fixed with the above but some CAD models are simply broken and beyond repair. If you can, ask the person who exported the original STEP mesh for you to export it again with different settings.