Your best option for exporting of .CATpart files to ISO STEP format (.stp, .step) would be to use CATIA PLM/CAD/CAM/CAE software suite that was actually used to make these files in first place.
If you do not have access to CATIA, your best option for catpart to stp conversion is to look for some 3rd party alternative with CATIA support. There exist also the option of using a dedicated 3D model / CAD conversion software that should be capable of this conversion.