How to cool a steel roof during summer
Paint the roof white or silver to reflect as much sunlight as possible. This is a passive method once it is done.

Water removes heat better than air so if you have a source of water, or collected and stored rainwater, it could be sprayed on the roof during times of high heat.

This isn't "natural" but if you have an attic, you could install an exhaust fan with a thermostat switch that comes on when the attic gets too hot.

Good luck!