Can you hot iron your hair after keratin treatment at home opn the second day?

Yes, you absolutely can hot iron or flat iron your hair straight. In fact, after you get a keratin treatment done, your hairdresser should have told you that in case you get any crimps or waves in your hair (from sleeping, for instance), you shouldflat iron the crimp out.

As long as you don't wash your hair on the second day, you're good to go.