US3518079A - Production of rimmed steels - Google Patents

Description

United States Patent 3,518,079 PRODUCTION OF RIMMED STEELS Raymond C. Oswald, Poland, Ohio, assignor to Jennifer Corporation, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing, Filed Oct. 30, 1967, Ser. No. 679,179 Int. Cl. C21c 7/00 US. Cl. 75--53 7 Claims ABSTRACT OF THE DISCLOSURE The production of rimmed steels, particularly medium and high carbon rimmed steels, by the addition of a rimming agent of iron carbonate to a molten steel bath.

This invention relates to the production of rimmed steels. More particularly, it relates to a novel rimming agent, iron carbonate (FeCO for the production of medium and high carbon rimmed steels.

The production of rimmed steels of high quality requires the evolution of large quantities of gas, principally carbon monoxide, during solidification of the ingot. The gas evolution occurs principally along the side walls of the ingot since the walls produce good nucleating sites and since the metal near the side walls cool first and the solubility of a gas in liquid steel decreases as the temperature decreases. The metal adjacent the walls of the mold solidify first and the gas evolution in this area reduces the carbon content of the steel adjacent the wall thereby forming an outside ferritic zone known as the rim which has a minimum of oxide inclusions.

Ingots which have had poor rimming action (weak carbon monoxide evolution) contain many gas bubbles near the ingot surface which are entrapped by the freezing metal interface during solidification. When an ingot is subsequently rolled out, many of these gas bubbles, called blowholes, which have been entrapped near the ingot surface open up to the atmosphere, oxidize, and cause seam and sliver type surface defects.

It is well known that medium and high carbon steels, i.e., steels having carbon contents of approximately .12 percent and higher are hard to rim. The poor rimming action in such steels is caused by an insuflicient amount of oxygen available in the steel to combine with carbon in the steel to evolve carbon monoxide gas which is necessary for good rimming action. The product of the dissolved carbon and the dissolved oxygen in a steel bath is a constant for any given temperature. This relationship is expressed by the formula [C] [O]=K. The value of the constant K increases with temperature but the relationship between the dissolved carbon and the dissolved 0xygen holds true. From this relationship it can be seen that as the carbon content of the steel increases, the dissolved oxygen content decreases. Thus, in medium and high carbon steels, the oxygen content is low and there is insufiicient oxygen to produce the necessary rimming action.

The problem of obtaining good rimming action in medium and high carbon steels has become more acute in recent years with the advent of very tall molds which are used to provide exceptionally large plates or large weld free coils now required by steel users. The increase in the pour height of an ingot increases the ferrostatic head in the mold and thus increases the suppression of the evolution of the carbon monoxide gas.

Many expedients have been proposed and many utilized in an attempt to improve rimming action in high and medium carbon steels. It has been suggested that the steel be refined to a low carbon content to enable the oxygen content of the steel to increase and then recarburize by adding carbon to the steel as it is tapped into a ladle.

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This is a dangerous practice since it is likely to result in an improper carbon analysis. Furthermore, the addition of excessive amounts of carbon to liquid steel causes heavy foaming; and since the reaction is highly endothermic, frequently too much temperature is lost in the liquid steel before teeming.

The addition of normal fluoride-bearing rimming promoters such as sodium fluoride is generally ineffective in promoting rimming in medium and high carbon steels. They expel fluoride gas which helps nucleate other gas evolution, but there is no oxygen present in the promoter to help form the necessary carbon monoxide.

Nitrate-bearing rimming promoters provide excess oxygen but the nitrogen which also dissolves in the steel is harmful to the physical properties of the finished product because of the rapid increase in aging associated with elevated nitrogen contents.

Iron oxide, generally in the form of mill scale, has been added to the steel during teeming as a rimming agent. However, additions of this type contain enough other impurities to create excessive undesirable nonmetallic inclusions in the steel. In addition, the iron oxides decompose relatively fast so that the rimming action does not have a sufiicient duration.

Sodium carbonate has also been tried but was found to be unsatisfactory as a rimming agent. The following reaction results from the addition of sodium carbonate to a steel bath:

The sodium oxide (N320) is stable and, therefore, the oxygen tied up by the sodium is not available to react with the carbon and promote rimming. In addition, the rapid decomposition of the sodium carbonate requires excessive amounts to be added in order to maintain the rimming. The stable sodium oxide floats to the top of the ingot causing a heavy slag formation which can suppress gas evolution. Furthermore, excessive slagging must be done on the molten ingot tops to allow for proper capping of the ingot.

I have found that a vigorous rimming action can be produced in medium and high carbon steels by the addition of iron carbonate (FeCO to the molten steel while it is teemed. Furthermore, the defects referred to above are eliminated by the use of iron carbonate as a rimming agent.

Iron carbonate reacts with steel as shown by the following equation:

For each mole of iron carbonate added, three moles of carbon monoxide are formed. Thus iron carbonate is 50 percent more efficient than sodium carbonate which forms only two moles of carbon monoxide per mole of carbonate. Furthermore, iron carbonate does not decompose as fast as sodium carbonate, nor does it form stable compounds which must be removed as slag. Finally, iron carbonate is free of nonmetallics so there is no increase in the inclusion content of the steel.

' Iron carbonate while quite efiicient itself can also be used in conjunction with other rimming promoters. An example would be using it with a sodium fluoride promoter. The iron carbonate would provide the dissolved oxygen and the fluoride would provide a nucleating gas for better carbon monoxide removal. The iron carbonate should comprise at least 40 percent of the total combined rimming agent and generally a percent iron carbonate addition should be employed.

The amount of iron carbonate to be used will depend upon factors well known to those skilled in the art, such as tap or turn down carbons, the amount and type of ladle alloy additions, the ingot pour height, etc. In general, the amount of iron carbonate will vary from 0.5

to 3.0 pounds per ton of steel. Where only a small amount is necessary, it can all be added just after cushioning. As more iron carbonate is needed, it can be added throughout teeming. Generally, it is desirable to add more to the bottom portions of the ingot because of the greater ferrostatic head which tends to suppress proper gas evolution.

While my invention is particularly directed to the production of medium and high carbon rimmed steels, there are circumstances Where iron carbonate can be very useful as a rimming agent in the production of low carbon steels. For example, if an excessive scrap charge is utilized in an open hearth or a very low silicon, high phosphorus hot metal is used, the steel produced frequently has a very low oxygen content. In a basic oxygen steelmaking process, the oxygen content in the steel for a given carbon level is generally lower than the oxygen content is for a comparable carbon level in steel produced by other methods since the kinetics of the reaction in the basic oxygen steelmaking process are so rapid that equilibrium conditions are never even approached. In these instances iron carbonate can be successfully used as a rimming agent to provide the oxygen necessary for proper rimming action even in low carbon steels.

While I have described preferred embodiments of my invention, it may be otherwise embodied within the scope of the appended claims.

I claim:

1. A method for the production of rimmed steel comprising progressively adding to liquid steel while teeming the steel into an ingot mold a rimming agent consisting essentially of iron carbonate.

2. The method as set forth in claim 1 in which the rimming agent contains at least 40 percent iron carbonate.

3. The method as set forth in claim 1 in which 0.5 to 3.0 pounds of iron carbonate are added per ton of steel.

4. A rimming agent for addition to molten steel as the steel is teemed into an ingot mold consisting essentially of at least forty percent iron carbonate and at least one additional rimming promoter selected from the group consisting of iron oxide, a fluoride rimming promoter and sodium carbonate, the balance being incidental impurities.

5. A rimming agent for addition to molten steel in amounts of 0.5 to 3.0 pounds per ton of steel as the steel is teemed into an ingot mold consisting essentially of iron carbonate and at least one rimming promoter selected from the group consisting of iron oxide, a fluoride rimming promoter and sodium carbonate.

6. The rimming agent of claim 4 wherein the iron carbonate comprises about percent of the rimming agent.

7. A method for the production of rimmed steel comprising progressively adding to the liquid steel while teeming into an ingot mold a rimming agent consisting essentially of iron carbonate and at least one rimming promoter selected from the group consisting of iron oxide, a fluoride rimming promoter and sodium carbonate.

References Cited UNITED STATES PATENTS 1,672,446 6/1928 Cape -53 2,025,425 12/1935 Smith 7553 2,854,329 9/1958 Rossborough 75-53 L. DEWAYNE RUTLEDGE, Primary Examiner I. LEGRU, Assistant Examiner US. Cl. X.R.