How are you testing to see if split lines are exported? SOLIDWORKS does export the geometry with the split lines. It may be that the software that is importing the geometry is merging the faces on import so the split lines disappear. For instance, if you are testing this by reimporting into SOLIDWORKS, then make sure the "Merge entities" option is turned off in the Import Options dialog (which you can get in the open dialog if you set the file type to STEP and then choose the options button). If Merge Entities is on, the split lines will likely go away since SOLIDWORKS merges connected faces with the same underlying geometry definition. If it is off, then the split lines will be preserved.
Other software that may be importing the STEP files could have similar options and would have to be set there to preserve the split lines.
I hope this helps,
Thanks Jim - that has fixed it! I had send stp files to a customer, the split lines are used to show trim lines on a component and I thought they were being removed when in reality they were not.
I have another query you may be able to help me with - I will post it on the forum, maybe others have the same question