US2527186A - Process for blocking open hearth heats - Google Patents

Description

Patented Oct. 24, 1950 2,527,186 PROCESS FOR BLOCKING OPEN HEARTH Frederick J. Griifiths, Massillon, Ohio, assignor to Chromium Mining and smelting Corporation, Limited, Sault Ste. Marie, Ontario, Quebec, Canada, a corporation of Canada No Drawing. Application April 26, 1946, Serial No. 665,349

. 6 Claims.

This invention relates to metallurgy and has for an object the provision of an improvedmetallurgical method or process. More particularly, the invention contemplates the provision of an improved method or process for the manufacture of steel in the open hearth furnace. A further object of the invention is to provide an improved method or process for blocking open hearth furnace heats to retard or inhibit carbon loss.

Steel products are produced to a large extent by means of open hearth furnace processes. An open hearth process comprises a refining treatment of a batch or bath of molten metal which may be preceded by a melting treatment involving the melting of scrap iron or steel or both. Each batch or bath of metal treated is called a heat. The refining treatment usually involves carbon elimination and may involve the elimination of such other elements as silicon, phosphorus and sulphur. Elimination of these elements is accomplished by subjecting the metal to the action of basic material like lime and oxidizing material like iron oxide. The lime and iron oxide and substances like silica, which'may be present in the furnace charge or which may be formed by oxidation of silicon present in thecharge or both, form a slag Which ultimately provides a molten covering layer for the metal undergoing refining.

Iron oxide is maintained in the slag in substantial concentration throughout the refining period and some iron oxide dissolves in the metal. The iron oxide contained in the slag and in'the metal reacts with carbon contained in the metal and effects its elimination as a gaseous oxide of carbon.

After refining has been completed, it'iscustomary to add deoxidizing agents or alloying' ele-' ments or both. Deoxidizing agents and alloying elements usually are added when the carbon content of the metal has been reduced to about the amount which it is desired to retain in the finished product, and, therefore, steps must be taken to prevent further carbon elimination. Carbon elimination is retarded or prevented by preventing further reaction of iron oxide contained in the metal and slag with carbon contained in the metal. This procedure is called blocking the heat, and it is accomplished, usually, by providing silicon for reaction with the iron oxide in place of carbon, since silicon reacts preferentially with iron oxide. 1

According to some heretofore customary practices, silicon has been provided by adding lumps of low-silicon ferro-silicon to the molten metal baths. Ferrosilicon added to a bath is melted and dispersed therein, and the dispersed silicon-of the ferrosilicon reacts preferentially With'iron oxide dissolved in the bath, preventing reaction of the iron oxide with carbon contained in the under the trade name SIL-X ;:1ooo I7 Example III n Parts Sodium nitrate 217 'Fe-rrosilicon (57%"Si) (sold in commerce.

2 bath. Silicon dispersed in the bath reacts with iron oxide at the slag-metal interface to some extent, also, further reducing the possibility of iron oxide reacting with carbon of the bath.

Some disadvantages are attached to the practice of incorporating silicon in a bath of molten' steel by adding a silicon alloythereto. Thus, for,

example, considerable time may berequire'd to eifect melting of the alloy and dispersion of the silicon, and some silicates formed by reaction of the silicon With iron oxide may be retained and form undesirable inclusions in the steeL' The present invention is based on my discovery that the addition to a basic slag in an open hearth steel furnace'at the conclusion of the carbon elimination stage of an exothermic reaction mix! ture comprising a non-carbonaceous reducing agent, such as silicon or aluminum, and an oxidizing agent, like an alkali metal nitrate (sodiumnitrate) or an alkali metal chlorate (sodium chlorate), results in modification of the characteristics of the slag and. inhibition of the reaction 'betweencarbon contained in the steel and iron oxide contained in the steel and slag.

According to the preferred method of them-' vention, the non-carbonaceous reducing agent and the oxidizing agent are employed in" the form of an intimate admixture capable of reaction exothermically upon ignition to generatea substantial quantity of heat. A preferred exothermic reaction mixture employed in carrying out a method or process of the invention is one comprising, or even consisting essentially of, ferrosilicon and sodium nitrate in'which the ferrosilicon is present in such amount and proportion I as to provide silicon for reaction with all of the sodium nitrate and with iron oxide and calcium oxide of the slag with the stantial quantity of heat.

The"f0110wing examples illustrate types of re action mixtures which have been employed effectively in carrying out a method or process of the invention (proportions are given in parts by weight) Example I V Parts Sodium nitrate .14'5 Ferrosilicon (57% Si) (sold in commerce under the trade name SIL-X 1000 Example II e V Parts Sodium nitrate '75 Ferrosilicon (57% Si) (sold in commerce under the trade name S IL-X-217) "1000 production 'ofa subi Reaction mixtures of the types illustrated above preferably are employed in the form of small agglomerates in which the ferrosilicon is present in the form of small particles (preferably minus 65-mesh) which are intimately associated with andbonded together by means of the sodium nitrate. Agglomeration is effected by mixing the particles of ferrosilicon and finely divided sodium nitrate intimately while moistened with water in amount equal in Weight to about two to three percent of the weight of the mixture, molding the resulting plastic mixture into masses of suitable sizes, heating the masses carefully to drive ofi water and melt the sodium nitrate in place, and cooling to efiect solidification and crystallization of the sodium nitrate.

At the conclusion of the refining stage, when the carbon content has been reduced to the desired concentration and, the steel is ready for the addition of deoxidizers or alloying elements orboth, the slag-forming components have been well digested or reacted and there exists a well- ,shaped, creamy, mature slag which is characterized by a moderate, uniform boiling action over the entire surface. At this time, iron oxide of the slag may be passing slowly from the slag to the metal, as the result of reaction of the iron oxide dissolved inthe metal with the carbon of the metal, in order constantly to re-establish equilbrium between iron oxide in the metal and iron oxide in the slag in accordance with the relative solubilities of iron oxide in the metal and slag phases.

The addition to a well-shaped slag of a quantity of exothermic reaction'mixture of the type I. illustrated above efiectively retards oxidation of the carbon of the steel and permits the addition Of deoxidizers and alloying elements to be made and tapping of the heat to be carried out in a shorter period of time and with the production of a smaller amount of scrap than when oxidation of carbon is retarded by adding a low-silicon alloy to the bath.

The following example illustrates results obtained in employing processes of the invention using exothermic reaction mixtures of the types illustrated above:

Heat No. g'rigg Scrap Minutes Pounds Total 27 Heats 031 144, 030 Average per Heat 23. 4 5, 331 2. 3%

In the processes of the invention for which the data are given above, the average time from of scrap produced was 6.3% of the weight of the metal.

When added to molten slag 0n the surface of a molten bath of metal, an exothermic reaction mixture of the type illustrated above becomes ignited, the sodium nitrate reacts with a portion of the silicon of the ferrosilicon generating heat which efiects a local temperature rise in the slag and melts the remainder of the ferrosilicon. The silicon of the molten ferrosilicon reacts rapidly with the iron oxide of the slag, as the result of the heat generated and the increased temperature, disturbing the equilibrium between the iron oxide dissolved in the metal and the iron oxide dissolved in the slag and preventing theflow of iron oxide from the slag to the metal. Another result of reaction of the components of the exothermic reaction mixture is the production of an alkali metal oxide (sodium oxide) which tends to combine chemically with iron oxide of the slag, or, in other words, increases the capacity of the slag to retain iron oxide and prevent its transfer to the metal.

Exothermic reaction mixtures are employed in processes of the invention in relatively small amounts. The non-carbonaceous reducing agent, such as silicon, in the added reaction mixture preferably is present in amount sufficient to react with all of the oxidizing material of the mixture and with iron oxide of the slag and generate a substantial quantity of heat but insufficient to penetratethe slag and enter the molten steel in substantial amount. Normally, amounts of exothermic reaction mixtures equal in weight to about one to three percent (1.0 to 3.0%) of the weight of the slag may be employed satisfac torily in carrying out a method or process of the invention. Any suitable amounts of exothermic reaction mixtures may be employed in carrying out a method of the invention.

I claim:

1. In a method of producing steel in an open hearth furnace in which a molten bath of metal is subjected to oxidation during a refining period beneath and in contact with a basic slag containing calcium oxide and iron oxide to efiect carbon oxidation and the production of a molten metal product containing carbon in predetermined amount and oxidation of carbon thereafter is retarded, the improvement which comprises igniting in contact with the slag after completion of the refining period an exothermic reaction mixture comprising oxidizing material selected from the group consisting of alkali metal nitrates and alkali metal chlorates, and noncarbonaceous reducing material comprising an element of the group consisting of silicon and aluminum in amount sufficient to react with all of the oxidizing material of the mixture and with iron oxide of the slag and generate a substantial quantity of heat but not sufiicient to penetrate the slag and enter the molten metal in substantial amount, thereby to increase the temperature of the slag, promote reaction between the iron oxide of the slag and the non-carbonaceous reducing material and retard reaction between iron oxide contained in the slag and carbon contained in the metal.

2. In a method of producing steel in an open nitrates and alkali metal chlorates, and material containing elemental silicon in amount sufficient to react with all of the oxidizing material of the mixture and with iron oxide contained in the slag and generate a substantial quantity of heat but not suificient to penetrate the slag and enter the molten metal in substantial amount, thereby to increase the temperature of the slag, promote reaction between the iron oxide of the slag and the silicon-containing material and retard reaction between iron oxide of the slag and carbon contained in the metal.

3. In a method of producing steel in an open hearth furnace in which a molten bath of metal is subjected to oxidation during a refining period beneath and in contact with a basic slag containing calcium oxide and iron oxide to eiiect carbon oxidation and the production of a molten metal product containing carbon in predetermined amount and oxidation of carbon thereafter is retarded, the improvement which comprises igniting in contact with the slag after completion of the refining period an exothermic reaction mixture comprising sodium nitrate and material containing elemental silicon in amount suificient to react with all of the sodium nitrate of the mixture and with iron oxide contained in the slag and generate a substantial quantity of heat but not suflicient to penetrate the slag and enter the molten metal in substantial amount, thereby to increase the temperature of the slag, promote reaction between the iron oxide of the slag and the silicon-containing material and retard reaction between iron oxide of the slag and carbon contained in the metal.

4. In a method of producing steel in an open hearth furnace in which a molten bath of metal is subjected to oxidation during a refining period beneath and in contact with a basic slag containing calcium oxide and iron oxide to efiect carbon oxidation and the production of a molten metal product containing carbon in predetermined amount and oxidation of carbon thereafter is retarded, the improvement which comprises igniting in contact with the slag after completion of the refining period an exothermic reaction mixture comprising sodium nitrate and ferrosilicon in amount sufiicient 'to react with all of the sodium nitrate of the mixture and with iron oxide contained in the slag and generate a substantial quantity of heat but not sufficient to penetrate the slag and enter the molten metal in substantial amount, thereby to increase the temperature of the slag, promote reaction between the ironoxide of the slag and the ferrosilicon and retard reaction between iron oxide of the slag and carbon contained in the metal.

5. In the production of steel in a basic open hearth furnace process, the improvement which comprises igniting in contact with the slag after completion of the refinin period an exothermic reaction mixture comprising sodium nitrate and material containing elemental silicon in amount sufficient to react with all of the sodium nitrate of the mixture and with iron oxide contained in the slag and generate a substantial quantity of heat but not suflicient to penetrate the slag and enter the molten metal in substantial amount, thereby to increase the temperature of the slag,

promote reaction between the iron oxide of theslag and the silicon-containing material andretard reaction between iron oxide of the Slag and carbon contained in the metal.

6. In the production of steel in a basic open 7 hearth furnace process, the improvement which comprises igniting in contact with the slag after completion of the refining period an exothermic reaction mixture comprising, sodium nitrate and ferrosilicon in amount sufficient to react with all of the sodium nitrate of the mixture and with iron oxide contained in the slag and generate a substantial quantity of heat but not sufiicient to penetrate the slag and enter the molten metal in substantial amount, thereby to increase the temperature of the slag, promote reaction between the iron oxide and the ferrosilicon and retard reaction between iron oxide of the slag and carbon contained in the metal.

FREDERICK J. GRIFFITHS.

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

UNITED STATES PATENTS

Claims (1)

1. 5. IN THE PRODUCTION OF STEEL IN A BASIC OPEN HEARTH FURNACE PROCESS, THE IMPROVEMENT WHICH COMPRISES IGNITING IN CONTACT WITH THE SLAG AFTER COMPLETION OF THE REFINING PERIOD AN EXOTHERMIC REACTION MIXTURE COMPRISING SODIUM NITRATE AND MATERIAL CONTAINING ELEMENTAL SILICON IN AMOUNT SUFFICIENT TO REACT WITH ALL OF THE SODIUM NITRATE OF THE MIXTURE AND WITH IRON OXIDE CONTAINED IN THE SLAG AND GENERATE A SUBSTANTIAL QUANTITY OF HEAT BUT NOT SUFFICIENT TO PENETRATE THE SLAG AND ENTER THE MOLTEN METAL IN SUBSTANTIAL AMOUNT, THEREBY TO INCREASE THE TEMPERATURE OF THE SLAG, PROMOTE REACTION BETWEEN THE IRON OXIDE OF THE SLAG AND THE SILICON-CONTAINING MATERIAL AND RETARD REACTION BETWEEN IRON OXIDE OF THE SLAG AND CARBON CONTAINED IN THE METAL.