Grass feed rust stains on concrete
CLR is what you're looking for. I had a similar problem as you (I was cutting metal on my driveway and did not sweep up the fines that night...Rusted the next day!) and it worked like a charm. It comes in a grey bottle with a multicoloured label. 

Dilute using the measurements on the bottle, and scrub hard with a stiff bristled broom. Rinse well.  This stuff is awesome elsewhere in the house too - in particular to clean up around taps.