When opening a STEP file you should get a dialog that allows you to choose what is translated to an Inventor file. You can turn off object filters, for example Surfaces (see image below) and then Inventor will only bring in Solid objects from the STEP file.
But it looks as if the source file that created your STEP file was all surfaces. In that case all you can do is either request to have the source file be made with a solid conversion or use the surface commands within Inventor to create a solid. Those commands are Stitch and Sculpt.
That looks like a surface model, I believe you’ll have to use the thicken command on the varying surfaces
Your biggest issue will be if all surfaces it could have gaps,I can make a complete model with surfaces in Solidworks that I can turn into a solid but if I bring it in as a surface in Inventor just to see there can be gaps and the other way around solidworks can find gaps in models that others wont.Anyway just sharing my experiences with you
In inventor you can use repair bodies command, https://knowledge.autodesk.com/support/inventor-products/learn-explore/caas/CloudHelp/cloudhelp/2014/ENU/Inventor/files/GUID-25947CE0-AD5A-4DCF-8B17-FD824575A36B-htm.html
And you can stitch surface edges, if you have "watertight " surface then you can sculpt it to solid, usually there can be hundreds errors and caps, It takes a quite long time. Sometimes you can have better result when you import your step file to inventor and make repair bodis and save as export it . X_B file, and import it again.
Well In my opinion the geometry is what I call "Broken". Normally if everything is ok as soon you load the step file it will be solid, however when this happened you have to repair the geometry, meaning surfaces, and that is hard-work to do and a time consuming task. The bad news: There is no other way.
However you could try this before click ok and as soon you open/import in the dialog box-Part Options rather than select Composit select Stitch and then do all the other things, but make sure Stitch is the current options before press ok.
Sometimes during the loading process Inventor will stitch together all that is possible to stitch.
If that doesnt bring good results so you can try different options but one of them is trying to check which of the edges are open or where is a gap between edges, that can help, be patient or use another software to repair the mesh/Nurb.
Hope this help
Please see link below hope this helps
Yes, That work when the geometry is not "broken", but from your images above it looks to me that certainly it is "broken"