Hi Anis & Ralf - is the STEP file you are referring to recently one created by a conversion from the IGES file?

Instead of that you want to get a STEP (and possibly an SAT) file created from the original application that exported this IGES file, exported from the original native model data.

That may help keep the data all connected together as a solid more easily rather than trying to deal with the individual surfaces in the IGES file.

- Michael