SECC is electrolytic zinc-plated steel as specified in the JIS spec, JIS G 3313. In a quick read through that spec, however, I did not find anything that would explain your "F20" callout. If there were an "E" followed by a number, that would refer to the minimum coating mass. Additionally, there are callouts for temper grade, chemical treatment, etc, but as near as I can tell, none of those use an "F." You might want to get your own copy of JIS G 3313 to see for yourself.
Is there any chance that the drawing you took this from is old and could have been writted per an outdated spec? That would be my only guess since this callout does not make sense per the 2005 spec that I have.