US2254156A - Method of casting lead-bearing steel - Google Patents

Description

Patented Aug. 26, 1941 METHOD OF CASTING LEAD-BEARING STEEL Wilbur A. Saylor, Bellevue, Pa., and Laurin D. Woodworth, Youngstown, Ohio No Drawing.

Application July20, 1939,

Serial No. 285,628

1 Claim.

The present invention relates to the preparation of steel ingot molds used in the casting of steel ingots to which lead or lead alloys are t be added.

It is a recognized fact that lead added to steel, in a manner to obtain thorough mixing and distribution of the lead in the steel, is beneficial in that it develops exceptionally good machining properties inthe steel so treated. Ihe addition of lead differs from elements generally added to steel in that it neither combines nor alloys with iron, so that its beneficial efiects depend upon diffusing or distributing the lead in the steel. To effect this diffusion of the lead, it is added in the form of fine shot to the steel as it is being teemed into the mold, because the lead, having a density nearly one and one-half times that of molten steel, tends to sink rapidly to the bottom of the mold or ladle of steel to which it is added if it is added in the form of larger pieces. The lead shot used is preferably .02 inch in diameter, although it may be used in slightly larger or smaller particles.

In the usual practice of casting ingots of ordinary grades of steel, the metal is teemed through an opening in the bottom of the ladle, the stream of molten metal entering the top of the mold as near the center line of the latter as possible. When lead additions are to be made to such steels, the lead in the form of fine shot is added through a small, inclined trough or chute so that the stream of lead shot descends adjacent to or with the stream of metal.

Since lead has a boiling point slightly lower than the temperature of the liquid steel, a certain amount is lost through volatilization. When lead is added to steel cast in the usual cast iron molds, a certain proportion of the lead added is lost in other ways also. When the ingots are stripped, a portion of the lead that has been added is found on the outside walls of the ingot and upon the stool at the bottom of the ingot, the stool being a flat base upon which the ingot is cast and upon which it stands after solidification. These observations have proved that part' of the lead added is deposited at the outer surface of the ingot or between the ingot moldand the steel. Also, some of the lead is deflected as it is dropped into the mold and when it strikes the bare mold wall, it 'has a tendency to adhere or freeze to the mold, sincefthe latter is at a temperature somewhat below the melting point of the lead. The lead particles as they strike the mold appear to be in a molten conditionand are certainly at a temperature near their melting 55 point, which is sufficient to render the lead very soft and almost fluid-like. In either condition, the particles of lead are not deflected by the cold mold wall, but adhere to it. Also, the turbulence at the surface of the molten steel in the mold, produced by the flow of the stream from the ladle, tends to deflect some of the lead towards the mold wall as it strikes the surface of the molten steel. The lead thus deflected towards the mold melts as the steel rises above it, and when the latter contracts ,and cools to form a gap between it and the mold, the lead upon the mold, as well as part of that mingled with the steel at the surface, melts and tends to flow downward between the mold and the ingot surface.

It will thus be apparent that the lead concentrated at the surface of the ingot has the effect of producing a deleterious surface condition on the ingot. When the ingot is stripped, that is, when the mold is removed, the lead appears to adhere to the ingot where some of it is volatilized, hence contaminating the air with lead fumes. Some is also oxidized by the oxygen of the air, forming a finely divided lead oxide powder which tends to further contaminate the air, while the lead which does not escape into the air trickles down over the surface of the hot ingot. The lead volatilized or oxidized contaminates the air increasing the hazard to the workmen in the vicinity.

Under the present invention, the aforementioned defects are eliminated and it is possible to prevent the accumulation of lead on the surface of steel ingots to which lead is added without changing the usual practice in adding the lead.

An object of the present invention is to prevent the accumulation of lead on the surfaces of the ingot and reduce the amount volatilized by coating the interior of the mold with a viscous, semi-fluid, carbonaceous mixture.

Another object of the present invention is to obtain a greater proportion and a more uniform I distribution of lead from the top to the bottom of the ingot.

A further object is to treat the mold interior so as to produce ingots having clean sides and free from flaky lead-iron oxide compounds and thereby reduce contamination of the atmosphere by the lead fumes and dust.

Other objects and advantages will become apparent as the description proceeds.

As a specific example only, of the method of the present invention, there is subsequently described in detail the procedure of applying coal tar to the interior of the mold.

The coal tar preferred for use is .the virgin product obtained in the coking of coal to metallurgical coke by the by-product process, which is the process now almost universally employed for the production of coke for the blast furnaces. Under some conditions of production and handling this tar contains pitch-like bodies, which, if present, are removed by heating and straining the tar through a screen .or sieve. Whether these :bodies are present or not, the tar is maintained at a temperature between 90 degrees and 140 degrees Fahrenheit to render it sufficiently fluid to facilitate the next operation, which consists of applying it to the interior of the mold.

In the use of virgin tar, it has been found necessary to control the temperature of the molds, so that the heat of the mold will distill some of the more volatile constituents of the tar, leaving a somewhat gummy residue that will adhere firmly to the mold and not be rendered too fluid from the heat of the stream of liquid steel during the teeming. The most desirable temperature of the molds for application of the tar is between 150 degrees and 300 degrees Fahrenheit. In regular operation, this temperature range can be obtained by permitting the molds to cool and applying the tar promptly when they have cooled to within this temperature range.

The most desirable manner of coating the interior of the molds is to apply the tar by spraying. For this operation, a number of known devices are available for use, the chief requirement being that the fluid tar be introduced into the spray nozzle or nozzles under suificient head vices, the tar may be applied to the mold while, it is in any position, but it is usually most conveniently applied with the mold in an upright position. After applying the tar, the mold should be permitted to stand until it has cooled somewhat before .it is used for making a leaded steel ingot.

The tar may .be applied, if desired, with long handled brushes, butif applied in this manner, it is desirable to have the temperature of the tar and of the molds on the low side of the ranges, hereinabove mentioned, and the molds supported in an inclined position. Also, the molds should stand for a longer time before they are used for casting ingots.

Other fluid or semi-fluid mixtures of carbonaceous materials, such as asphaltum pitches andpetroleum residues may besuccessfully employed and applied in the manner similar to that described for tar. In the use of any of these mixtures, a requirement is that they be applied under conditions that will give a soft paste-like coating at the higher mold temperatures mentioned above and will not be converted into a coke-like mass containing little or no volatile matter when the steel is teemed into the mold.

As steel is teemed into a mold having its interior coated as hereinabove described, certain effects produced at the border line described by the liquid steel in contact with the mold are most helpful in accomplishing the desired result. Thus, as the liquid steel surface rises in the mold, the heat therefrom causes a rapid distillation of the volatile constituents of the coating which fills the mold with a reducing atmosphere so that the lead is not oxidized and any particles of this metal that may be adhering to the coating are projected back into the liquid steel. Also, the absorption of carbon :by the steel along this line causes an evolution of gas and a certain agitation of the metal which prevents any small particles of lead momentarily in motion on the surface of the liquid steel from coming into contact with the relatively cold mold.

The results obtained by the present invention may be by various modifications in the coating mixtures used, the methods of applying the coatings to the molds, and the method of introducing and mixing the two metals in the mold. For example, the steel may be bottom-poured; that is, the liquid steel may be introduced through a runner entering the mold at the bottom, while the lead is scattered upon the rising surface of liquid steel in the mold.

While we have described a specific embodiment of the present invention, it will be seen that we do not wish to be limited exactly thereto, since various modifications may be made without departing from the scope of the invention, as defined by the following claim.

We claim:

The method of manufacturing lead-bearing steel which consists in preparing the ingot molds by applying a coating of virgin tar to their inner surfaces while said molds are at a temperature of from about 150 F. to 300 F., permitting the molds to stand to expel those constituents of the coating volatilizable at said mold temperature, teeming molten steel into the molds and simultaneously adding lead thereto, said coating increasing the proportion of lead retained in the