There was minor factory defect on screen, which spread 1cm after a month of purchase. when we visited store they said we need to identify defect with in 15days and request for exchange, which was not noticeable.
I called customer care, raised case. but no status found. Please advise the approach
What you were told is correct. You have 15 days to return it. After that, it is entirely up to Apple's discretion how to handle any problems with your device. They have no way to know with any certainty if it was defective out of the box or if something happened since you opened it.
See iPhone Repair & Service - Apple Support
Iphone factory defect
