US3205067A - Removal of deleterious gases from molten metal - Google Patents

Description

United States Patent 3,205,067 REMOVAL OF DELETERIOUS GASES FROM MOLTEN METAL Charles Henry King, London, England, assignor to The British Oxygen Company Limited, a British company No Drawing. Filed Mar. 13, 1963, Ser. No. 264,767 Claims priority, application Great Britain, Mar. 22, 1962, 11,095/ 62 2 Claims. (CI. 75-59) In the manufacture of steel, it sometimes happens that trouble is experienced in the finished products due to the presence of excessive amounts of hydrogen gas which has become dissolved in the metal during the melting operation. When hydrogen is present in amounts above the safe limit, porosity occurs in the ingots :or castings, and in the case of the former, flaking, hair line cracks and other forms of unsoundness may develop in subsequent processing even though the porosity was not obvious in the ingot form. Porosity also arises from this cause in the preparation of ingots or castings made from non-ferrous metals, and in iron castings.

Methods which have so far been employed for reducing excessive amounts of hydrogen in metal include bubbling an inert or relatively inert gas through the molten metal, or blowing the surface of the metal with such a gas. Examples of gases used in these methods are argon, helium, nitrogen, carbon monoxide and carbon dioxide, selection from this range depending on the type of metal treated. Alternative methods for reducing excessive hydrogen content are vacuum casting and prolonged heat treatment of the solidified metal. All these methods have the disadvantage of requiring special apparatus, which in the case of vacuum casting and of heat treatment, is particularly costly. In the case of treatment with inert gases, the main disadvantages are the additional time required during which the metal must be maintained in the molten state, the consequent loss of temperature, and the splashing which may result from the treatment.

A further method which has been employed, particularly with ferrous metals, is the use of a solid compound which, when placed on, or plunged below, the metal surface, decomposes to produce a gas which is substantially insoluble in the metal but into which the hydrogen can diffuse and thus be removed. Normally the type .of material used for this purpose in the steel industry is a carbonate, such as limestone, which on heating decomposes to give carbon dioxide gas and an oxide, such as calcium oxide (lime). In steel degassing operations it is desirable that slag, particularly lime based slags, should be absent, and it is a disadvantage of the method just described that a solid material such as lime is a product of the reaction. When limestone is added to the surface, a layer of lime or slag forms in the vicinity of the added limestone ice creating a barrier between the metal surface and the carbon dioxide gas, thereby reducing the efficiency of trans fer of hydrogen into the gas. Also there are operational disadvantages associated with plunging solids below the surface of molten metal.

According to the present invention a process for reducing the hydrogen content of molten steel comprises introducin g to the molten metal surface an inert or relatively inert gas in the liquefied state. It has been found by analysis that when gas is added in this form, a substantial amount of hydrogen is removed; this is accomplished without excessive loss of heat and without the production of a slag. This is true even though the temperature of the gases in these states are very low, e.g. liquid nitrogen, l98 C. and liquid argon, 184 C.

The liquefied gas may be added to the molten metal in any convenient manner which insures that the bulk of the material reaches the surface in the liquid state.

A feature of the process is that, on the liquefied gas reaching the surface, the instantaneous change from the liquid state provides a blanket of gas through which the transfer of heat is sufiiciently retarded to enable the liquefied gas to move to other areas and eventually provide a thorough scavenging of the metal surface.

The invention is illustrated by the following experiment:

100 lbs. of scrap was melted in an electric furnace, under conditions conducive to obtaining a molten metal with a high hydrogen content. The bulk of the slag was removed from the metal surface, and a sample of the molten metal removed for analysis. About 1 /2 lbs. of liquid nitrogen was carefully added to the molten metal, and, after two minutes, a further sample of metal was taken for analysis. During this time the furnace currrent was passing, no ill effects being observed. It was found that the first sample taken contained 3.8 cc. of hydrogen per 100 grams of metal, and the second sample contained 1.7 cc. of hydrogen per 100 grams of metal, indicating a substantial degree .of degassing.

I claim:

1. In the process for reducing the hydrogen content of molten steel by the action of nitrogen thereon, the step of introducing the nitrogen in its liquid state to the surface of the molten steel.

2. The process as claimed in claim 1, wherein the proportion of liquid nitrogen to molten steel is of the order of one and a half percent by weight.

References Cited by the Examiner UNITED STATES PATENTS 2,068,785 1/37 Bain et al -59 2,395,458 2/46 Cape 7559 3,042,484 7/62 Mating 75 59 BENJAMIN HENKIN, Primary Examiner.

Claims (1)

1. IN THE PROCESS FOR REDUCING THE HYDROGEN CONTENT OF MOLTEN STEEL BY THE ACTION OF NITROGEN THEREON, THE STEP OF INTRODUCING THE NITROGEN IN ITS LIQUID STATE TO THE SURFACE OF THE MOLTEN STEEL.