it's not steam, it's water vapor. the difference is that steam is produced by raising the temperature of water above its boiling point, or lowering the pressure until the water boils. this steam is usually invisible. the "steam" that appears around liquid nitrogen (or anything else that is cold in a warm environment, i.e. when you open your freezer) is visible water-vapor, which is created because the air near the object is cooled. cooler air has a smaller capacity to hold gaseous water, so all the water that the now-cooler air used to contain condenses into water vapor. once the air warms back up, the water vapor is re-absorbed into the air and the "steam" magically disappears.