Yes, it is possible with an ultrasonic flaw detector. I read from your description that you have access to the steel, with the rubber in the inside surface. I would suggest a ringdown technique as described in this application note:
For a 5 mm steel wall, I would suggest trying a 5 MHz delay line transducer. Delay line transducers usually work better than contact transducers in this test because you see more multiples of the steel backwall echo due to impedance relationships.