Convert STEP to OBJ

What OBJ keeps that STL does not

Both formats hold triangles and nothing but triangles, so the geometry you get is identical. The difference is everything around the geometry.

An OBJ can name its objects, and a STEP assembly is full of names — "Housing", "Drive shaft", "M6 bolt". Those names survive, so the model arrives in Blender as a proper outliner tree rather than one unnamed mesh you then have to separate by hand.

An OBJ can also refer to materials. STEP stores a colour for each part, and those colours are written into a .mtl file that travels with the model. Open the .obj and the parts come in already coloured, which on a forty-part assembly is the difference between a model you can work with and a grey blob.

You will get a zip, and why

An OBJ with colours is never one file. The .obj holds the geometry and names a .mtl file beside it; the .mtl holds the colours. Handing over the .obj on its own would give you a grey model and no explanation, so both are packed into a zip.

Unzip the whole thing into one folder and open the .obj from there. The .mtl has to sit next to it — that is the only rule. If the STEP file had no colours at all, there is nothing for a .mtl to hold and you get a plain .obj instead.

Triangle count matters more here than you might think

A printer does not care how many triangles it is given. A viewport does. A STEP assembly converted at the finest setting can arrive as several million triangles, and that is enough to make Blender’s viewport unpleasant and a game engine refuse outright.

For anything that will be looked at rather than manufactured, start at Draft and move up only if the curves look wrong from the distance you will actually see them. The count is shown beside the control. It is much easier to convert again at a higher setting than to decimate a mesh afterwards without wrecking the flat faces.

Where the assembly tree goes

STEP stores parts in a tree: an assembly holds sub-assemblies, and those hold parts. OBJ has one flat list of named objects and no way to nest them.

So the tree is flattened into the names. A bolt inside a bracket assembly arrives called "Bracket assembly / M6 bolt", which keeps the one thing the tree was for — knowing which of fourteen identical bolts you are looking at — in a form OBJ can carry. Every name is unique, so nothing in the list is ambiguous even when the CAD program named forty bodies the same thing.