US3810753A - Process for casting molten aluminum killed steel continuously and the solidified steel shapes thus produced - Google Patents

Abstract

A MOLTEN ALUMINUM KILLED STEEL CONTAINING 0.005-0.30% OR CARBON, 0.10-1.50% OF MANGANESE, 0.01-0.03% OF SILICON, 0.04-10% OF ALUMINUM, AND THE REMAINDER IRON AND INCIDENTAL IMPURITIES IS INTRODUCED INTO A MOLD FOR CONTINUOUSLY CASTING STEEL AND CAST THEREIN TO PRODUCE A SOLIDIFIED STEEL SHAPE CONTINUOUSLY. THE MOLTEN STEEL IS INTRODUCED INTO THE MOLD THROUGH A TUNDISH NOZZLE HAVING AN INTERNAL DIAMETER SUFFICIENTLY LARGE TO PREVENT PLUGGING. IN A PREFERRED BARIANT, AN ALUMINUM ADDITION IS MADE IN THE LADLE IN AN AMOUNT TO LARGELY BUT NOT COMPLETELY DEOXIDIZE THE STEEL, THE MOLTEN STEEL IS AGITATED WITH R-H OR D-H VACUUM DEGASSING APPARATUS, AND THE DEOXIDIZATION IS COMPLETED BY MAKING AN ALUMINUM ADDITION IN THE VACUUM VESSEL OF THE DEGASSING APPARATUS. THE SOLIDIDIED STEEL SHAPES MAY BE CAST IN THE FORM OF SLABS, AND THE SLABS ROLLED TO PRODUCE FLAT ROOLED PRODUCTS SUCH AS STEEL STRIP OR PLATE. THE QUALITY OF THE SOLIDIFIED STEEL SHAPES IS IMPROVED BY PREVENTING OXIDATION OF THE ALUMINUM CONTENT OF THE MOLTEN KILLED STEEL PRIOR TO CASTING, AND/OR BY PROVIDING A FLUID SLAG OVER THE MOLTEN STEEL IN THE MOLD TO AID IN SEPARATING NONMETALLIC INCLUSIONS THEREFROM PRIOR TO SOLIDIFICATION.

Description

United States Patent Ofice ABSTRACT OF THE DISCLOSURE A molten aluminum killed steel containing ODDS-0.30%

of carbon, 0.10-l.50% of manganese, 0.01-0.03% of silicon, 0.04-l0% of aluminum, and the remainder iron and incidental impurities is introduced into a mold for continuously casting steel and cast therein to produce a solidified steel shape continuously. The molten steel is introduced into the mold through a tundish nozzle having an internal diameter sufiiciently large to prevent plugging. In a preferred variant, an aluminum addition is made in the ladle in an amount to largely but not completely deoxidize the steel, the molten steel is agitated with R-H or D-H vacuum degassing apparatus, and the deoxidization is completed by making an aluminum addition in the vacuum vessel of the degassing apparatus. The solidified steel shapes may be cast in the form of slabs, and the slabs rolled to produce flat rolled products such as steel strip or plate. The quality of the solidified steel shapes is improved by preventing oxidation of the aluminum content of the molten killed steel prior to casting, and/ or by providing a fluid slag over the molten steel in the mold to aid in separating nonmetallic inclusions therefrom prior to solidification.

BACKGROUND OF THE INVENTION This invention broadly relates to a process for casting molten killed steel continuously. In some of the more specific variants, the invention further relates to a novel process for casting aluminum killed steel continuously to produce solidified steel shapes which may be hot worked into products such as steel strip and plate.

One recent important innovation in steel making is the casting of molten steel continuously into semi-finished shapes such as blooms, slabs and billets. The successful continuous casting of shapes equivalent in section to conventional semi-finished shapes eliminates the ingot and primary mill stages of conventional prior art rolled steel production, and thus offers important economic advantages.

Although the continuous casting of steel seems to be simple in principle, there are many difficulties inherent in the process. This is due in part to the high melting point, high specific heat and low thermal conductivity of steel, and the necessity for close control of variables such as the temperature and oxygen content of the molten steel. The molten steel must be killed sufficiently to prevent pin holes from forming in the surface of the solidified shapes as they are cast. Any tendency of the molten steel to effervesce excessively and form pin holes or blowholes in the thin solidified skin that is formed initially around the molten steel interior of the shapes is very undesirable. This is true from the standpoint of weakening the skin and increasing the chance of escape of molten steel therefrom, as well as from the standpoint of surface imperfections. The initial temperature of the molten steel fed to the mold from the tundish via the tundish nozzle is of importance as the solidification rate must be predictable so as to form a skin of suflicient thickness and strength to support the casting. It is therefore apparent that a successful continuous casting operation depends to a large extent 3,810,753 Patented May 14, 1974 upon providing properly deoxidized molten steel which is at a uniform casting temperature.

The melt of steel to be deoxidized and continuously cast may be prepared by a number of prior art steel making practices, but the basic oxygen process results in substantially lower costs and is often preferred. In accordance with one prior art basic oxygen process, the furnace is charged with scrap, molten ferrous metal from a blast furnace, an other charge materials necessary to produce a low carbon steel upon blowing with oxygen. In instances where the melt is to be cast into conventional ingots, the final temperature need not be higher than about 2900 F. and it is possible to stop the blow after the manganese, silicon, phosphorus and sulfur have been reduced to desirable levels and sufiicient carbon is present to impart strength to the steel after vacuum degassing. In instances where the melt is to be cast continuously into semi-finished shapes, the melt temperature should be about 50-75 F. higher, i.e., about 2950-2975 F. and it is necessary to continue the oxygen blow for a longer period of time to assure that the higher melt temperature is attained. This usually lowers the carbon content to about 0.04-0.06% and the oxygen content is about 400-700 parts per million. It is necessary to deoxidize the steel before continuous casting and in theory deoxidation may be effected by making aluminum and/or silicon additions. However, it is not possible to continuously cast a low carbon steel which contains more than 0.02-0.03% of aluminum by the prior art processes, and steels to be continuously cast have been limited heretofore to silicon killed steels or steels deoxidized by vacuum degassing. When attempts were made to continuously cast aluminum killed steel having aluminum contents above 0.020.03% in accordance with the prior art practices, the nonmetallic phases in the steel deposited on the internal surface of the tundish nozzle choked off the flow of molten steel.

It is essential that the molten steel be fed to the continuous casting mold in a predictable manner and so as to maintain a controlled level of molten metal, and thus the deposition of refractory nonmetallic phases in the tundish nozzle cannot be tolerated in a continuous casting operation. As a result, aluminum killed steels containing more than 0.02-0.03% of aluminum were not thought to be capable of being continuously cast heretofore.

The presence of aluminum in the molten aluminum killed steel in amounts over 0.020.03% also tends to cause large quantities of nonmetallic inclusions to accumulate along the surfaces of continuously formed castings produced by prior art processes. As a result, castings of inferior quality are produced which require a large amount of surface conditioning before they are suitable for rolling into fiat rolled steel products. Aluminum oxide containing nonmetallic inclusions are especially undesirable when continuously casting slabsto be rolled into steel strip and plate as massive agglomerates form along the side walls of the mold as the partially solidified steel casting descends. The agglomerates are difiicult to remove and mar a substantial portion of the surface area of the fully solidified slabs.

As a result of the foregoing and other limitations and disadvantages of the prior art, an entirely satisfactory process for continuously casting aluminum killed steels having an aluminum content above 0.02-0.03% was not available prior to the present invention. Inasmuch as there is a large demand for flat rolled aluminum killed steel products for uses which require good weldability, uniformity in quality, fine grain size, toughness, drawability, formability, stability, and ductility, it would be very advantageous to provide a continuous casting process for aluminum killed steels as yields over conventional ingots would be markedly higher and production costs substantially lower.

It is an object of the present invention to provide a novel process for casting aluminum killed carbon steel continuously.

It is a further object to provide the solidified steel shapes produced from molten aluminum killed steel by the continuous casting process of the invention.

It is a further object to provide a novel process for preparing molten aluminum killed steel from a melt of unkilled steel produced by a prior art process, and then continuously casting the molten steel into solidified steel shapes.

It is still a further object to provide a novel process for continuously casting slabs from aluminum killed steel which may be rolled to produce products such as flat rolled steel strip or plate.

It is still a further object to provide a novel process for continuously casting aluminum killed steel wherein the quality of the solidified steel shapes is improved,

Still other objects and advantages of the invention Wlll be apparent from the following detailed description and the examples.

DETAILED DESCRIPTION OF THE INVENTION INCLUDING PREFERRED VARIANTS THEREOF In accordance with the present invention, a molten aluminum killed steel containing 0.005-0.30% of carbon, 0.10-1.50% of manganese, 0.0l0.03% of silicon, 0.04- 0.10% of aluminum and the remainder iron and incidental impurities is introduced into a mold for continuously casting steel and is cast therein to produce a solldified steel shape continuously. The phosphorus content of the steel should be less than 0.04%, the sulfur content should be less than 0.05%, the nitrogen content should be less than 0.03%, and the oxygen content should be less than 200 parts per million. Preferably, the steel contains about 0.005-015 of carbon, about 0.20-0.60% of manganese, about 0.0080.025% of silicon, about 0.040.06% of aluminum, about .005-0.02% of phosphorus, about 0.01- 0.035% of sulfur, about 0.0030.02% of nitrogen and less than 125 parts per million of oxygen. For best results, the steel should contain about 0.005-0.015% of phosphorus, about 0.01'8-0.026% of sulfur and less than 100 parts per million of oxygen. The nitrogen content is preferably about 0.0l2-0.'02% in instances where high physical strength is important. As will be described more fully hereinafter, there are certain preferred compositions, procedures and conditions which produce superior results.

A melt of steel for use in practicing the present invention may be obtained from any suitable prior art steel making furnace such as an open hearth furnace, an electric furnace, or a basic oxygen furnace. However, it is usually preferred to use molten steel produced by a basic oxygen process.

The prior art basic oxygen process for producing low carbon steel is satisfactory without modification. The low carbon steels produced by a basic oxygen process usually contain not more than 0.30% and often less than 0.15% of carbon, and in most instances about 0.06-0.l2% of carbon. The manganese content of the steel is less than 1.0% and is usually about 0.l-0.40%, the silicon content is less than 0.1% and is preferably less than 0.02% and often below 0.01%, the phosphorus content is less than 0.04% and is preferably below 0.02%, the sulfur content is less than 0.05% and is usually below 0.03%, and the remainder is iron and incidental impurities. Trace amounts of tramp elements such as copper, tin, lead, zinc and the like may be present but their concentrations are very low. The oxygen content of the steel is below 800 parts per million (p.p.m.) and is often about 400-700 ppm, and the nitrogen content is below 0.03% and is often about 0.003-0.004%. The temperature of the melt is usually about 2850-3000 F. and is preferably about 29 0-2970 F. fo continuous casting.

Basic oxygen steel making processes and apparatus, vacuum degassing processes and apparatus, and continuous casting processes and apparatus for use in preparing steel melts, vacuum degassing and continuously casting the same are disclosed in numerous patents and literature references, including the text The Making, Shaping and Treating of Steel, 8th edition, edited by Harold E. McGannon (1964), the disclosures of which are incorporated herein by reference. Pages 453-456, 552-556 and 664-666 of the text The Making, Shaping and Treating of Steel are especially pertinent.

It is not possible to produce a furnace melt having the desired composition for continuous casting as tapped. Accordingly, the melt of steel initially produced by a prior art steel making process is adjusted in composition to produce an aluminum killed steel as defined herein. This may be conveniently accomplished by charging the steel making furnace with ingredients and employing operating procedures and practices which will result in approximately the desired carbon, phosphorus and sulfur contents in the steel as tapped from the furnace, or which will result in the desired contents thereof after the aluminum and/or manganese containing addition agents are added. The aluminum content in the steel as tapped is markedly lower than desired, and the oxygen content is markedly higher. It is necessary to add aluminum in an amount to arrive at the desired aluminum and oxygen contents in the final aluminum killed steel to be continuously cast. A manganese addition should be made as it is not practical to charge enough manganese to a basic oxygen furnace to result in the desired manganese content in the steel as tapped due to losses of manganese to the furnace slag. A nitrogen addition may be made in instances where higher physicals are desired.

The aluminum and manganese additions, and the nitrogen addition when made, may be made in the ladle at the time of tapping the heat. The aluminum addition is preferably made in the form of metallic aluminum such as pig aluminum or aluminum shot. Manganese may be added in the form of high or medium carbon ferromanganese, electrolytic manganese, or other suitable alloys of manganese. The nitrogen addition may be made in the form of calcium cyanamide or other suitable nitrogen addition agents. In instances where it is desired to adjust the carbon content of the steel, this may be done by selecting a high carbon manganese addition agent, or by making a carbon addition in the ladle in accordance with prior art practices. While the entire aluminum addition may be made in the ladle at the time of tapping the heat, this practice does not allow close control of the final aluminum and oxygen contents in the melt, and the melt temperature may also vary substantially. This is due in part to the marked variations in the percent recovery of aluminum when added under the conditions of tapping the heat, and to oxidation of the metallic aluminum during tapping. As a result, it is desirable to provide an improved method of making the aluminum final addition.

In accordance with a preferred variant of the invention, the aluminum addition at the time of tapping the melt is not sufficient to completely deoxidize the steel. The initial addition of aluminum to the ladle may be at the rate of about 65-95% and preferably about 75-80% of the total amount of aluminum required to completely deoxidize the steel. For example, approximately 3-3.5 pounds of aluminum per ton of steel may be added during tapping and the aluminum content of the partially deoxidized steel may be below 0.04% and is usually about 0.020.03%. The oxygen content is substantially higher than desired for continuous casting and is above parts per million, and is usually above 200 parts per million. A sample of the partially deoxidized steel is taken from the ladle immediately after tapping, and the aluminum and oxygen contents are determined. Thereafter, a second metallic aluminum addition is made under nonoxidizing conditions and preferably while agitating the melt to assure a uniform and predictable composition. This may be conveniently accomplished by agitating the melt of steel and subjecting it to vacuum degassing employing apparatus which allows additions to be made Within the vacuum vessel. Dortmund-Herder (D-H) and Ruhrstahl-Heraeus (R-H) vacuum degassing apparatus is usually preferred as it meets the requirements set out above. When employing D-H or R-H apparatus, the usual prior art operating conditions may be used. R-H vacuum degassing apparatus is often preferred, and argon is the preferred inert gas for use therein. The R-H apparatus results in a pumping action whereby the molten metal in the ladle is vigorously agitated.

The final addition of aluminum is made to the vacuum vessel of the vacuum degassing apparatus. The amount of aluminum in the second addition is based upon the analysis of a steel sample taken immediately following tapping, and it is sufficient to produce the desired final aluminum and oxygen contents in the steel. The percent recovery of aluminum is predictable when made under these conditions, and it is possible to arrive at final aluminum and oxygen contents which closely approximate the desired levels. Also, the stirring action assures a uniform composition and temperature throughout the ladle.

The steel is usually subjected to vacuum degassing over a period of approximately 5-15 minutes, but it is understood that the vacuum degassing step is terminated when the temperature of the molten steel reaches the optimum level for continuous casting. In most instances, it is preferred to make the final aluminum addition in a plurality of portions, with agitation between addition of the portions and with at least two minutes of agitation after addition of the last portion. The amount of aluminum added during the vacuum degassing step is usually less than 1.5 pounds per ton of steel, and is preferably about 0.6-1.3 pounds per ton of steel. In some instances, the quantity of aluminum added at the time of tapping is sufficient for complete deoxidation and it is not necessary to add aluminum at the time of degassing.

The degassing step is primarily for agitating the molten steel in the ladle, and for providing optimum conditions for the final aluminum addition. There is very little, if any, vacuum deoxidation due to the steel being largely deoxidized by the aluminum addition to the ladle. However, elemental hydrogen may be removed to some extent.

After making the additions to the ladle to arrive at the desired molten aluminum killed steel composition, the slag covered ladle is transferred to a continuous caster. The steel is poured periodically into the slag covered tundish, and is then introduced into the open end of the continuous casting mold by means of the tundish nozzle. The continuous casting mold is water cooled, and solidification of the steel is initiated therein following prior art practices. A body of molten steel is present in the top of the mold, and a steel casting with a solidified skin surrounding a liquid steel core is with drawn from the bottom. solidification of the molten interior of the casting is accomplished by means of water sprays located below the mold. The solidified steel shapes, which may be in the form of slabs, may be cut into desired lengths, cooled, surface conditioned, reheated to a hot working temperature and hot worked such as by hot rolling following conventional practices to produce flat rolled steel products such asstrip and plate. The hot rolled strip may be further reduced in thickness by cold rolling, or it may be given other conventional treatments to arrive at a final steel product.

In accordance with a preferred variant of the invention, the quality of the solidified steel shapes is improved by preventing oxidation of the aluminum content of the molten aluminum killed steel prior to casting, and/or by providing a fluid slag over the molten steel in the mold to aid in separating nonmetallic inclusions therefrom prior to solidification. The oxidation of the aluminum content of the steel may be prevented by providing a fluid slag layer thereover at all times, such as in the tapping ladle,

the tundish of the continuous caster, and the open top of the continuous casting mold. The slags that are employed may be in accordance with prior art practices. A limesilica slag may be employed over the molten metal in the continuous casting mold as it is eflective in agglomerating aluminum oxide-containing nonmetallic inclusions and retaining the agglomerates as they float upward in the molten metal in the continuous casting mold due to their lower specific gravity. The mold usually is reciprocating vertically, and the aluminum oxide-containing agglomerates work upward in the molten metal and are retained in the slag, and are prevented from depositing within the solidified casting or on the surfaces thereof. The incoming molten metal flowing from the nozzle of the tundish is preferably introduced beneath the slag layer and so as to be in intimate contact therewith. The continuous reciprocation of the mold and the introduction of the molten metal therein beneath the slag layer are very effective in causing the aluminum oxide-containing nonmetallic inclusions to form massive agglomerates which become trapped in the slag.

It is essential that the tundish nozzle for introducing the molten killed steel into the continuous casting mold have an internal diameter which is sufficiently large to prevent plugging thereof by nonmetallic inclusions. The minimum internal diameter varies directly with the aluminum content in the steel within the ranges set out herein. For example, the minimum internal diameter for casting an aluminum killed steel having an aluminum content of 0.04% is about 1% inches, and in instances where the aluminum content is about 0.10%, then the minimum internal diameter is about 1% inches. These minimum internal diameters are substantially greater than those employed heretofore in continuous casting. For some reason which is not fully understood at the present time, the larger diameter tundish nozzles do not become plugged with the refractory aluminum oxide-containing inclusions which are present in the molten steel initially in finely divided form and thus should readily pass through the prior art tundish nozzles. As a result, it is possible to continuously cast the aluminum killed steels of the present invention without interruption over a normal continuous casting cycle. It is understood that the internal diameter of the tundish nozzle may be increased above 1% inches if desired, and the nozzle may be, for example, 2, 3, or 4 inches in diameter.

The composition of the aluminum killed steel employed in practicing the invention is based upon the results of spectographic analysis of samples. Thus, the total amounts of the various elements in the steel are determined and the steel composition is based thereon.

The foregoing detailed description and the following specific examples are for purposes of illustration only, and modifications may be made therein without departing from the invention.

EXAMPLE I A melt of low carbon steel is prepared by a basic oxygen process following prior art steel making practices, and the melt is tapped from the furnace into a ladle. The steel has a temperature of 2940 F. and the ladle contains 300 tons of molten steel. The steel contains 0.04% of carbon, 0.16% of manganese, less than 0.01% of silicon, less than 0.01% of phosphorus, less than 0.025% of sulfur, less than 0.004% of nitrogen, 500-600 parts per million of oxygen, and the remainder iron and trace amounts of incidental impurities.

The ladle additions are 1000 pounds of pig aluminum, and 2400 pounds of ferromanganese containing 78% of manganese, 6.5% of carbon and the remainder iron and incidental impurities. The resulting incompletely deoxidized steel contains 0.06% of carbon, 0.35% of manganese, 0.04% of aluminum, approximately 220 parts per million of oxygen, and the remaining constituents in the steel do not change appreciably.

The incompletely deoxidized steel is subjected to a vacuum treatment in Ruhrstahl-Heraens (R-H) vacuum degassing apparatus. The R-H apparatus is operated in accordance with prior art practice using argon as the inert gas for injection into one of the two legs which are submerged in the molten steel. Partially deoxidized molten steel is withdrawn from the ladle and introduced into the vacuum vessel where it is subjected to a sub-atmospheric pressure of approximately 12 mm. of mercury, and the molten steel then flows down the other leg and is returned to the ladle.

After vacuum treatment for approximately one minute, aluminum shot in an amount of 100 pounds is added to the molten steel in the vacuum vessel and the vacuum treatment is continued for an additional 2 minutes. This is followed by adding a second 100 pounds of aluminum shot, vacuum treatment for an additional 2 minutes, adding a third 100 pound portion of aluminum shot, and vacuum treatment for 2 minutes after the third addition of aluminum shot. The final temperature of the molten steel is 2910 F., the aluminum content is 0.05%, and the remaining constituents do not change appreciably. There is substantially no removal of carbon in the form of carbon monoxide due to the vacuum treatment. The R-H apparatus is used primarily for agitating the molten steel in the ladle and providing a convenient method of making the final aluminum addition under non-oxidizing conditions and with a high and predictable percentage recovery of aluminum. As a result, it is possible to closely control the final chemistry and temperature, and the agitation assures that the melt is homogeneous. The ladle is provided with a prior art slag layer at all times following tapping to prevent oxidation of the aluminum content of the killed steel.

The molten aluminum killed steel is poured periodically from the ladle into the slag covered tundish of the continuous caster as required to maintain the desired level of molten steel therein. The molten steel is withdrawn from the tundish through the tundish nozzle and is introduced into the upper end of an open-ended water cooled continuous casting mold for casting slabs at a rate to maintain the desired level of molten metal therein. A layer of a fluid lime-silica-fluorospar slag is maintained over the top of the molten metal in the mold to prevent oxidation and to aid in removing massive agglomerates of aluminum oxide-containing inclusions. The mold is reciprocated vertically and a casting in the form of a slab having a solidified outer shell and a molten interior is withdrawn from the mold continuously. The casting is sprayed with water as it descends below the mold until it is completely solidified, and it is then cut into predetermined lengths. The slabs are allowed to cool, inspected for surface imperfections, scarfed, reheated to a hot rolling temperature, hot rolled into steel strip having a thickness of approximately 0.1 inch, and then cold rolled to approximately 60% reduction following conventional practices.

The tundish nozzle has an internal diameter of 1% inches and no difiiculty is experienced with respect to plugging of the nozzle during the continuous casting cycle of approximately one hour. The slabs require only a routine amount of scarfing to condition them for rolling. Thus, the prior art problems of plugging of the tundish nozzle and poor surface which are characteristic of prior art attempts to continuously cast aluminum killed steels are overcome.

EXAMPLE II The general procedure of Example I is repeated in another continuous casting run with the exception of employing a tundish nozzle having an internal diameter of inch, which is characteristic of prior art practice. After a short period of time, the tundish nozzle is plugged with aluminum oxide-containing refractory material. Thus, it is not possible to continuously cast the aluminum killed steel of the invention under the conditions of this example.

8 EXAMPLE III The general procedure of Example I is repeated with the exception of employing a tundish nozzle having an internal diameter of 1% inches and adjusting the aluminum killed steel composition to an aluminum content of 0.04%. The tundish nozzle does not plug during the continuous casting cycle and the results are substantially the same as in Example I.

EXAMPLE IV The general procedure of Example I is followed with the exception of adjusting the aluminum content of the molten steel to 0.10% aluminum. No difiiculty is experienced in continuously casting the steel with the higher aluminum content of 0.10%, and the results are substantially the same as in Example I.

I claim:

1. In a process for casting a molten aluminum killed steel consisting essentially of 0.0050.30% of carbon, 0.l0l.50% of manganese, residual silicon in an amount of about 0.001-0.03%, about 0.04-0.10% of aluminum and the remainder iron and incidental impurities including nonmetallic inclusions wherein the steel is introduced into a casting mold through a nozzle and the steel is cast in the mold to produce a solidified steel shape, the improvement comprising introducing the said molten aluminum killed steel into a continuous casting mold through a nozzle having an internal diameter varying from a minimum of 1% inches when the said steel contains 0.04% of aluminum to a minimum of 1% inches when the said steel contains 0.10% of aluminum, the internal diameter of the said nozzle being sufliciently large to prevent plugging thereof by the nonmetallic inclusions and the deoxidizing agent for the said steel consisting essentially of aluminum, and continuously casting the said molten aluminum killed steel in the said con tinuous casting mold to produce a solidified steel shape continuously.

2. The process of claim 1 in combination with the further improvement 'wherein said molten aluminum killed steel introduced into said mold contains less than 0.04% of phosphorous, less than 0.05% of sulfur, less than 0.03 of nitrogen and less than 200 parts per million of oxygen.

3. The process of claim 1 in combination with the further improvement wherein said molten aluminum killed steel introduced into said mold contains about 0.04-0.06% of aluminum and less than 200 parts per million of oxygen.

4. The process of claim 1 in combination with the further improvement wherein said molten aluminum killed steel introduced into said mold contains about 0.04-0.06% of aluminum, about 0.0050.02% of phosphorus, about 0.01-0.35% of sulfur and less than parts per million of oxygen.

5. The process of claim 1 in combination with the further improvement wherein said molten aluminum killed steel introduced into said mold contains about .0l-0.15% of carbon, about 0.200.60% of manganese, about 0.0080.025% of silicon, about 0.04-0.06 of aluminum, about 0.005-0.02% of phosphorus, about Obi-0.035% of sulfur, about 0.0030.02% of nitrogen, and less than 125 parts per million of oxygen.

6. The process of claim 5 in combination with the further improvement wherein said molten aluminum killed steel introduced into said mold contains about 0.012-0.2% of nitrogen.

7. The process of claim 1 in combination with the further improvement wherein said molten aluminum killed steel introduced in said mold is cast into a solidified steel slab continuously, and the slab is rolled to produce a flat rolled steel product.

8. The process of claim 1 in combination with the further i p ovement wherein the aluminum content of said molten aluminum killed steel introduced into said mold is prevented from being oxidized by atmospheric oxygen prior to casting into the solidified steel shape.

9. The process of claim 1 in combination with the further improvement wherein a fluid layer of slag covers the molten steel in the mold and said molten aluminum killed steel is introduced into the mold beneath the slag layer.

10. The process of claim 1 in combination with the further improvement wherein a fluid layer of slag covers the molten steel in the mold and the molten steel is introduced into the mold beneath the slag layer, the aluminum content of the molten steel is prevented from being oxidized by atmospheric oxygen prior to casting into the solidified steel shape, the molten aluminum killed steel introduced in said mold contains about 0.010.15% of carbon, about 0.200.60% of manganese, about 0.008- 0.025% of silicon, about 0.04-0.06% of aluminum, about 0.0050.02% of phosphorus, about 0.010.035% of sulfur, about 0.003-0.02% of nitrogen and less than 125 parts per million of oxygen, the molten killed steel in said mold is cast into a solidified steel slab continuously, and the slab is rolled to produce steel strip.

11- The process of claim 1 in combination with the further improvement wherein said molten aluminum killed steel introduced into said mold is produced by steps comprising preparing a melt of carbon steel by a basic oxygen process and deoxidizing the melt by adding aluminum thereto.

12. The process of claim 11 in combination with the further improvement wherein said melt of steel prepared by the basic oxygen process contains initially not more than 0.30% of carbon, less than 1.0% of manganese, less than 0.1% of silicon, less than 0.04% of phosphorus, less than 0.05% of sulfur, less than 0.03% of nitrogen, less than 800 parts per million of oxygen and the remainder iron and incidental impurities.

13. The process of claim 11 in combination with the further improvement wherein said melt of steel is tapped into a laddle and an aluminum addition is made in the ladle in an amount to largely but not completely deoxidize the steel, partially deoxidized molten steel is withdrawn from the ladle and is introduced into the vacuum vessel of a vacuum degassing apparatus, an aluminum addition is made to the partially deoxidized steel in the vacuum vessel in an amount to complete the deoxidation and to reduce the oxygen content of the steel to less than 125 parts per million, and the withdrawn molten steel is returned to the ladle.

14. The process of claim 11 in combination with the further improvement wherein said melt of steel prepared by the basic oxygen process contains initially about 0.06- 0.08% of carbon, about 0.10-0.40% of manganese, less than 0.01% of silicon, less than 0.02% of phosphorus, less than 0.03% of sulfur, less than 0.02% of nitrogen, about 400-700 parts per million of oxygen, and the remainder iron and incidental impurities.

15. The process of claim 14 in combination with the further improvement wherein said molten aluminum killed steel introduced into said mold contains about 0.040.06% of aluminum.

16. The process of claim 15 in combination with the further improvement wherein said melt of steel is tapped into a ladle and an aluminum addition is made in the ldale in an amount to largely but not completely deoxidize the steel, partially deoxidized molten steel is withdrawn from the ladle and is introduced into the vacuum vessel of a vacuum degassing apparatus, an aluminum addition is made to the partially deoxidized steel in the vacuum vessel in an amount to complete the deoxidation and to reduce the oxygen content of the steel in the ladle to less than 125 parts per million and the withdrawn molten steel is returned to the ladle, the aluminum content of the molten steel is prevented from being oxidized by atmospheric oxygen prior to casting into the solidified shape, and a fluid layer of slag covers the molten steel in the mold and the molten steel is introduced into the mold beneath the slag layer.

17. The process of claim 16 in combination with the further improvement wherein the molten aluminum killed steel in said mold is cast into a solidified slab continuously, and the slab is hot rolled to produce steel strip.

18. The process of claim 11 in combination with the further improvement wherein the aluminum content of said molten aluminum killed steel is prevented from being oxidized by atmospheric oxygen prior to casting into the solidified steel shape, a fluid layer of slag covers the molten steel in the mold, and said molten aluminum killed steel is introduced into the mold beneath the slag layer.

19. The process of claim 12 in combination with the further improvement wherein the aluminum content of said molten aluminum killed steel is prevented from being oxidized by atmospheric oxygen prior to casting into the solidified steel shape, a fluid layer of slag covers the molten steel in the mold, said molten aluminum killed steel is introduced into the mold beneath the slag layer, and said nozzle has a minimum internal diameter of 1% inches.

References Cited UNITED STATES PATENTS 2,597,979 5/1952 Darmara 148-12 3,183,078 5/1965 Ohtake et a1 -49 3,188,246 6/1965 Olt et al. 148-16 3,206,301 9/1965 Daubersy 164-57 X 3,208,844 9/1965 Kato et al. 75-49 3,315,323 4/1967 Speith et al 164-82 3,392,009 7/1968 Holmes et al 164-66 X 3,412,781 11/1968 Richards 164-82 X 3,435,992 4/1969 Tisdale et al. 164-281X 3,459,537 8/ 1969 Hornak 75-58 X 3,465,811 9/1969 Castelet 164-281 3,467,167 9/1969 Mahin 164-56 3,512,957 5/1970 Brotzmann et al 75-49 3,517,726 6/1970 Mills et al. 164-82 3,519,059 7/ 1970 Voskoboinikov et al. 164-281 X 3,411,897 11/1968 Lindber-g 164-56 X 3,567,432 3/ 1971 Wardell 164-82 X 3,578,064 5/1971 Mills et al. 164-281 3,540,518 1 1/ 1970 Osterholtz 164-82 FOREIGN PATENTS 1,332,350 6/1963 France 164-82 OTHER REFERENCES Making, Shaping and Treating of Steel; US. Steel, 8th ed., 1964, pp. 454, 455.

Journal of Metals; August 1969, vol. 2, No. 8, pp. 2, 61, 68-73.

Continuous Casting; edited by McBride et al., Interscience PubL, New York, 1962, pp. 155, 172-176.

Continuous Casting of Steel: The Iron and Steel Institute, London, Special Report 89, 1965, pp. 88-95.

Duderstadt et'al.: article in Journal of Metals, April 1968, pp. 89, 92-94.

L. DEWAYNE RUTLEDGE, Primary Examiner P. D. ROSENBERG, Assistant Examiner US. Cl. X.R. 75-58; 16456 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,810, 753 Dated 4 May 14, 1974 Inventor(s) Robert S. Miltenberger It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line l5, change "0.04-lO%" to 0.04-0.l0%

Column}; line 9, change "an" to and Column 3, line 37, change "0.005-0l5%" to O. O050.l5%

Column 3, line 39, change LOGS-0.02%" to (LOGS-0.02% Column 5, line 55, change "with drawn" to withdrawn Column 9, line 40, change "laddle" to ladle Column 9, line 65, change "ldale" to lad le Signed and sealed this 24th day of September 1974.

(SEAL) Attest:

McCOY M. GIBSON C. MARSHALL DANN Attesting Officer Commissioner of Patents FORM po'wso (10459) USCOMM-DC 6O376-P69 ",5. GOVERNMENT PRINTING OFFICE 2 I569 35-334,