welcome to BlenderExchange :)
At this time of writing, STEP files aren't able to to be imported natively into Blender. You'll need to save your files in a different format: .stl, .ply, even .obj and others are available.
@Duarte is correct: photorealism has little to do with filetypes. What's more, STEP files don't (yet) contain lighting, camera or volumetric data, so you can't import previously designed photorealistic scenes from CAD programs into Blender via STEP.
Arguably the biggest problems with imported files are:
- triangulation of mesh faces
- disconnected edges
- random origin points
- odd scaling, and
- no smoothing (perfectly smooth vector shapes are
often converted to meshes, which means polygonal shapes rather than
curves). This is a real headache with complex shapes.
I'm not sure if STEP files also store material data: color, gloss, emission, transparency and other properties - I don't think they do. No software is a magic bullet solution. The ability to achieve Photorealism will always be the responsibility of the artist, not the software.