I assume you mean the steel box is sealed so that when it is heated, no air can escape and the air pressure inside the box increases due to the thermal expansion of the air.  Are you familiar with the Ideal Gas Law?   PV = nRT

The problem is not very well stated because a box has stress concentrations due to the edges, so the stress also depends on the radius at the square edge. A better problem would have been to ask about a steel sphere as an ideal sphere has no stress concentration. The thickness of the steel will also effect the stress.

The problem also lacks another important piece of data, the temperature of the air and box when the box was sealed.