Hii Ajay

This is Shalini.

First of all congratulations.As I am form CSE so I have little bit knowledge about CE ,so I can answer your questions in short. 

As getting admission on PSU depends on some factors-

1.No.of candidates who appeared for exam.

2.No.of vacancies available.

3.Level of difficulty of GATE Paper. 

Generally for SC category one need to score more then 70 marks for getting interview call or be in first 1000 rank.And as you mentioned above that you get 521 in all over India so I think you will surely get a call from various PSU and probably from all IITS. 

Congratulations once again.

Hope my answer will help you.

Thank you and all the best.