Sathasivam with your 70-80 marks in JEE Advanced 2018 exam under the SC category according to last year marks vs rank distribution you can get a rank around 2500 in the SC category. Although at this time according to coaching's expert paper was a little difficult and there have been no errors like last time when 18 marks were given to each student so it is expected that cut-off will go down and your rank can be a little better but still with the 2000-2500 range of SC category rank your chance of getting admission in any of the IIT will be very difficult. Even if you get the chance then it might be in the ultra new IITs like IIT Jammu, IIT Goa, IIT Jodhpur etc.

With this range of marks you can apply for IIST, there you will have a good chance of getting admission.

Feel free to ask if you have any more queries. All the best!!