Oneplus India - Warranty issue

The phone's screen started showing blue ink on the right upper corner of the screen and it gradually spread to the entire screen. When I approached to the service center ( same as mentioned above) they denied to cover it under warranty stating that there is a very small hairline internal defect which is usually caused only due to, as per the training they got from OnePlus, some physical damage and hence cannot be taken under warranty. I told them that neither any physical damage was caused to the phone, nor it showed any sign of some physical damage. But they still didn't agree to take it under warranty. They are charging Rs ****/- for the repair. As a student, this amount is too heavy to my pocket and I have no other option for online studies so I had to give it to the serive centre. I am looking forward for your help.