Molten steel, normally exposed to an atmosphere of air, is protected against pick-up of impurities by placing carbon dioxide gas in such quantities and in such proximity to the surface to cause dissociation of the carbon dioxide at a rate which furnishes an atmosphere of carbon monoxide and gives off a negligible amount of oxygen to the steel, thus providing a gas barrier or shroud isolating the steel from the surrounding atmosphere and preventing pick-up therefrom of oxygen, nitrogen or hydrogen. This method may be applied to protecting steel being transferred from a ladle to a mold, or from a ladle to a tundish and from the tundish to a mold in continuous casting. In a method where a number of shrouding operations are carried out in series, carbon dioxide vapor, under pressure, is bled, in increments, from a storage vessel containing a body of liquid carbon dioxide in overlying ullage space containing vapor. Each increment is superheated, after it leaves the vessel, and is ultimately expanded and dispersed at ambient temperature to form the shroud. As each increment of vapor is removed from the vessel, it is replaced by withdrawing liquid carbon dioxide, vaporizing it and returning it to the ullage space.