IronCAD's default CAD format (.ics) is not that well supported in other CAD programs, so your best bet for conversion of ISO STEP product data files (.step, .stp) to ICS format would be the original IronCAD. Other than that, you will have hard time finding a viable alternative for stp to ics conversion.
Convert ISO-10303 STEP product data to IronCAD 3D CAD file.
We have found 2 software records in our database eligible for .stp to .ics file format conversion.
Microsoft Windows software - convert stp to ics on Windows
A 3D modelling application
Convert from stp file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned stp to ics. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
A 3D desktop CAD software