Is it true that to burn fat i should maintain my heart rate at 60 to 70% of it?

Probably: Many sources have long recommended a medium level of long duration exercise for fat loss. That should be effective. More recently, some have reported that a medium level exercise interspersed with short bursts of high intensity exercise is helpful to boost fat loss.

Yes: You are talking about aerobic exercise heart rate target zones. Calculate your maximal heart rate ( roughly 220-age). And exercise your hr to 70% of it.