US2510154A - Process for treatment of molten stainless steel - Google Patents

Description

Patented June 6, 1950 PROCESS FOR TREATMENT OF MOLTEN STAINLESS STEEL Harry Tanczyn, Baltimore, Md., assignor, by mesne assignments, to Armco Steel Corpora tion, a corporation of Ohio No Drawing. Application July 11, 1945, Serial No. 604,541

7 Claims. (Cl. 22-215) 1 This invention is a companion to that of my copending application, Serial No. 604,542, filed July 11, 1945, and entitled Stainless steel process and product and relates to stainless steel, more particularly to a method of casting the steel and to a composition for combatting stainless steel teeming and casting scums.

An object of my invention is the provision of a simple, reliable and highly satisfactory method of producing stainless steel ingots or other cast products involving the teeming of metal into a mold or the like.

It is a further object to provide a, rapid, emcient and dependable method of producing stainless steel ingots of improved clean quality in the cast condition.

A further object of my invention is the pro- .vision of an efficacious addition agent for combatting stainless steel teeming scums, which agent is thoroughly satisfactory to the result of rapidly fluxing the scum from the teemed steel.

A still further object of my invention is the provision of stainless steel ingots or the like which in the cast condition are of good surface quality and substantially free of teeming scum inclusions.

Other objects in part will be obvious and in part pointed out hereinafter.

The invention accordingly consists in the combination of elements, composition of ingredients and mixture of materials, and in the several operational steps and in the relation of each of the same to one or more of the others as described herein, the scope of the application of which is indicated in the following claims.

As conducive to a clearer understanding of certain features of my invention, it may be noted at this point that stainless steels are defined as steels which comprise 10% to 35% chromium, with or without nickel, and with or without supplemental additions of columbium, titanium, aluminum, molybdenum, tungsten, vanadium, cobalt, copper, manganese, silicon, sulphur, and the like, for special purposes, and a remainder which is substantially all iron. The carbon content usually is low, this being on the order of 0.03% to 0.20% although it may be higher for certain purpose steels.

A preponderant tonnage of stainless steel manufactured today is subjected at one time or another to teeming and casting operations such as in the production of ingots from an initial melt or subsequent melt of the steel. In teeming the stainless steel into molds, a problem of considerable magnitude arises which has to do with the prevention or elimination of undesirable scums in the cast metal. These scums usually are rich in oxides both of iron and chromium and on occasions include in quantity oxides of special purpose constituents of the steel. Likewise, nitrides of certain elements of the alloy steel usually contribute as a further scum component along with the oxides. At times, the scums assume a refractory or semi-refractory quality which is thick, viscous, frothy, gummy or even semi-solid, and do not readily melt in contact with the molten stainless steel in the mold.

Such stainless steels as higher chromium steels, and stainless steels comprising one or more of aluminum, titanium, columbium, and the like, give particularly refractory scums which are even more troublesome in the mold than those of certain other types of corrosion resistant steel. In some instances of pouring stainless steel under conditions of exposure to the atmosphere, the scum mounts up to several inches deep atop the cast metal. V

The fluidification or alleviation of the deleterious scums of teeming and casting stainless steel heretofore has been attempted with various fluxing agents only with quite limited success and with marked ineffectiveness in the instances of high chromium steels and steels including any such elements as titanium, columbium, aluminum, or the like.

The stainless steel teeming scums usually are driven into the newly cast steel by the force of the pouring stream or are turned under at the edges of the mold or are otherwise forced to inclusion in the cast metal during the course of the teeming and casting operation. As a result the castings after solidification require an extensive amount of surface conditioning such as grinding to remove non-metallics and metal of inferior quality, or are rejected, which in either event is costly. In some cases the included scums offer an obstacle to successful forging or rolling, or render the metal unsuitable for those uses where high quality Or strength is important. The cast stainless steels which contain segregations of scum therefore are susceptible to a host of metallurgical failures. They are not successfully polished to uniform brightness and luster. In local surface areas the metal is subject to undue corrosion.

An object of my invention accordingly is the provision of a simple, direct and practical method of teeming and casting stainless steel, which method is highly eflicient in preventing or eliminating deleterious scums of teeming with respect to any of a wide variety of stainless steels, even the higher chromium stainless steels and those steels including one or more such elements as titanium, columbium, aluminum, or the like.

Referring now more particularly to the practice of my invention, I achieve in teeming and casting molten stainless steel, illustratively into ingots, a highly effective prevention or elimination of deleterious scums in the mold by use of one or more lithium salts of the group consisting of lithium fluoride, carbonate, oxalate, nitrate and chloride, in contact with the steel. In accordance with my invention, I add to the molten stainless steel a scum-fluxing agent comprising in predominating amount or preferably consisting in substantial entirety of one or more of the above noted salts of lithium, for example, to a ladle of the steel before lip pouring into a mold, or, preferably to the steel in teeming the same to the mold, or at the mold as by shifting the lithium salt inside the mold over the rising steel.

Of the salts just identified I find that the fluoride, carbonate and oxalate of lithium are particularly eflicient, alone, or together with one or more of each other, even in the casting of high temperature stainless steels such as those comprising 20% or more chromium, and stainless steels including at least one of such elements as titanium, aluminum, columbium, or some other element or elements having a tendency to produce a highly refractory non-metallic scum in and on the metal during the casting operations whether the chromium content of the steel be high or low. The fluoride of lithium is in general the most active of all the lithium salts for defeating the deleterious scums of teeming and casting the stainless steels and, therefore, I prefer to use this salt either in whole or in part.

By treating the molten stainless steel with one or more lithium salts the resulting stainless steel ingots or other cast products which I achieve are of a. highly desirable quality substantially free of teeming scum inclusions. Likewise, the stainless steel castings have a much superior surface and are remarkably devoid of scabs, coldshuts, wrinkles, rolls, splash marks, and like defects. The stainless steel ingots which result from my pouring and casting method are satisfactory for reduction as by rolling with a minimum of preliminary conditioning of the surface thereof. The ingots give a high not working yield with little splitting or checking, as along forged or rolled edges, of the type ordinarily casued by scum or other surface flaws, and give products which otherwise are substantially free of the efiects of teeming scum.

As illustrative of the practice of my invention I provide, as in a suitable electric furnace, a bath of 18-8 chromium-nickel stainless steel of 0.5% titanium grade. From the furnace I tap the metal into a ladle preparatory to teeming the same into ingot molds, illustratvely of inch by 10 inch average dimension inside on the horizontal. In preparing the molds themselves, I put inside on the bottom of each a small quantity, say for example approximately 2 ounces to 4 ounces, of a combatting scum agent consisting substantially all of lithium fluoride, the salt preferably being enclosed in small paper bags or the like. Thereafter I teem the stainless steel from the ladle into the molds and over the lithium salt therein.

During the teeming and casting operation the lithium fluoride in the ingot molds thoroughly mixes with the molten steel and sets up a highly effective fluxing action in preventing and eliminating thick scums in the mold. Should it be desired. additions of lithium fluoride are made to the mold as by sprinkling to the cast metal as the teeming continues, or by mixing into the cast molten metal even after teeming, to supplement or serve in lieu of previous additions of the salt to the mold. I leave the scum-fluxed ingots to cool and solidify, and thereafter strip off the molds. The resulting chromium-nickel-titanium stainless steel ingots are of good surface quality and are substantially free of scum inclusions such as those including refractory nitrides of titanium or oxides of chromium. As stripped, the ingots illustratively are ready for storage or sale, or are easily made ready for reduction and subsequently are reduced to such products as those rolled or forged of the steel.

Thus it will be seen that there has been provided in this invention a method and teeming scum fluxing agent, and products resulting, of casting stainless steel in which the various objects hereinbefore set forth together with many thoroughly practical advantages are successfully achieved. It will be seen that the method is simple, effective and suitable for use in the production of castings from any of a wide variety of stainless steels, and is industrially practicable with known and tried casting apparatus.

As many possible embodiments may be made of my invention and as many changes may be made in the embodiments hereinbefore set forth, it is to be understood that all matter described herein is to be interpreted as illustrative, and not as a limitation.

I claim as my invention:

1. In the production in an iron metal mold of cast stainless steel products substantially devoid of scabs, cold shuts, wrinkles, rolls and splash marks, the art which includes contacting molten steel during teeming, and the scum formed by contact of the molten steel with the atmosphere, with a teeming scum flux essentially consisting of lithium salt of the group consisting of fluoride, carbonate, oxalate, chloride, and nitrate of lithium.

2. In the production in a mold of cast stainless steel products substantially devoid of scabs, cold shuts, Wrinkles, rolls and splash marks in which the stainless steel contains at least one of titanium, columbium and aluminum, the art which includes adding to the molten steel during the teeming thereof, and to the scum there formed, a teeming scum flux consisting of at least one lithium salt of the group consisting of the fiuo ride, carbonate, oxalate, chloride and nitrate of lithium.

3. In the production in a mold of cast stainless steel products substantially devoid of scabs, cold shuts, wrinkles, rolls and splash marks in which the steel contains at least 20% chromium, the art which includes adding to the molten stream of metal during teeming, and to the scum there formed, a teeming scum flux consisting of at least one lithium salt of the group consisting of the fluoride, carbonate, oxalate, chloride and nitrate of lithium.

4. In the production of stainless steel ingots substantially devoid of scabs, cold shuts, wrinkles, rolls and splash marks, the art which includes teeming metal into an ingot mold, and during said teeming sprinkling on the surface of, the rising metal a teeming scum flux essentially consisting of lithium salt of the group consisting of the fluoride, carbonate, oxalate, chloride and nitrate of lithium.

5. In the production of stainless steel ingots substantially devoid of scabs, cold shuts, wrinkles, rolls and splash marks, the art which includes placing in an ingot mold a teeming scum flux essentially consisting of lithium salt of the group consisting of the fluoride, carbonate, oxalate, chloride and nitrate of lithium, and then teeming metal into the mold.

6. In the teeming of stainless steel in an iron metal mold, the art which includes contacting the molten steel in the mold and the scum there formed with a teeming scum flux consisting of at least one lithium salt of the group consisting of fluoride, carbonate, oxalate, chloride and nitrate of lithium.

7. In the teeming of stainless steel in an iron metal mold, the art which includes adding to the mold a teeming scum flux essentially consisting of lithium fluoride, and then teeming the metal thereinto.

HARRY TANCZYN.

REFERENCES crrEn The following references are or record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,672,446 Cape June 5, 1928 1,748,217 Grossman Feb. 25, 1930 2,265,985 Allen Dec. 16, 1941 FOREIGN PATENTS Number Country Date 110,022 Austria Mar. 28, 1940 423,489 Great Britain Feb. 1, 1935 476,115 Great Britain Dec. 2, 1937 509,442 Great Britain July 17, 1939 OTHER REFERENCES Chemical Rubber Publishing 00., Cleveland, Ohio.

Refining Metals Electrically, Page 140; edited by Barton. Published in 1926 by the Penton Pub- 20 lishing 00., Cleveland, Ohio.

Claims (1)

1. IN THE PRODUCTION IN AN IRON METAL MOLD OF CAST STAINLESS STEEL PRODUCTS SUBSTANTIALLY DEVOID OF SCABS, COLD SHUTS, WRINKLES, ROLLS AND SPLASH MARKS, THE ART WHICH INCLUDES CONTACTING MOLTEN STEEL DURING TEEMING, AND THE SCUM FORMED BY CONTACT OF THE MOLTEN STEEL WITH THE ATMOSPHERE, WITH A TEEMING SCUM FLUX ESSENTIALLY CONSISTING OF LITHIUM SALT OF THE GROUP CONSISTING OF FLUORIDE, CARBONATE, OXALATE, CHLORIDE, AND NITRATE OF LITHIUM.