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Not quite.
With a good radiator cap, steam should not escape, the cap is there to prevent steam. As the coolant heats up and expands, the caps holds it in until the set pressure. As it continues to expand, it escapes to the overflow tank as a liquid (because it's at the radiator, definitely not the hottest part of the engine, where the heat from the heads and all the cast interior passages provide lots of nucleation sites for bubbles to form). As the engine cools it gets sucked back in.
That is why the fluid level in the overflow tank varies between a hold and cold engine.
With a failed rad cap, the engine will produce a steady stream of steam bubbles and that is what will escape into the overflow tank. As there is now a constant flow of steam into it heating it up, the coolant will eventually evaporate from that because it's not pressurized.
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