US3769004A

US3769004A - Method of producing a killed steel

Info

Publication number: US3769004A
Application number: US00141932A
Authority: US
Inventors: L Barnhardt; B Whitmore
Original assignee: Individual
Current assignee: Individual
Priority date: 1971-05-10
Filing date: 1971-05-10
Publication date: 1973-10-30
Anticipated expiration: 1990-10-30

Abstract

A method of producing a fully killed steel of low aluminum content and substantially free of aluminum oxides, without the need for the addition of other metallic or carbidic deoxidizers such as silicon, titanium, silicon carbide, carbon, etc. The steel is produced by the addition of aluminum to molten steel in an amount sufficient to kill the steel and provide a dissolved aluminum content in the order of between 0.03 to 0.06 percent. The steel is then stirred at a controlled rate in contact with an oxidizing slag until the amount of dissolved aluminum remaining in the steel is between 0.005 percent and 0.020 percent. The steel thus produced retains the mode of solidification of a killed steel, is low in dissolved oxygen and aluminum rich oxides, and is especially suitable for continuous casting in that it can be poured through relatively small diameter nozzles without plugging.

Description

W] METHOD or PRODUCING A KILLED STEEL [75] Inventors: Lyall F. Earnhardt, Center Valley;

Bruce C. Whitmore, Bethlehem, both of Pa.

[73] Assignee: John I. Iverson, Bethlehem, Pa.

[22] Filed: May 10, 1971 21 Appl. No.: 141,932

[52] US. Cl 75/129, 75/53, 75/58 [51] Int. Cl. C22c l/06 [58] Field of Search 75/58, 129

[56] References Cited UNITED STATES PATENTS 3,634,075 1/1972 Hoff 75/135 2,233,726 3/1941 Belding.. 75/58 2,705,673 4/1955 Jordan 75/58 X 3,426,833 2/1969 Randak et a1 75/58 X 3,598,383 8/1971 Moore 75/130 X [451 Oct. 30, 1973 Primary ExaminerL. Dewayne Rutledge Assistant ExaminerJ. E. Legru Attorney-John l. lverson [57] ABSTRACT A method of producing a fully killed steel of low aluminum content and substantially free of aluminum oxides, without the need for the addition of other metallic or carbidic deoxidizers such as silicon, titanium, silicon carbide, carbon, etc. The steel is produced by the addition of aluminum to molten steel in an amount sufficient to kill the steel and provide a dissolved aluminum content in the order of between 0.03 to 0.06 percent. The steel is then stirred at a controlled rate in contact with an oxidizing slag until the amount of dissolved aluminum remaining in the steel is between 0.005 percent and 0.020 percent. The steel thus produced retains the mode of solidification of a killed steel, is low in dissolved oxygen and aluminum rich oxides, and is especially suitable for continuous casting in that it can be poured through relatively small diameter nozzles without plugging.

4 Claims, No Drawings 1 METIEOD F PRODUCING, A KILLED STEEL BACKGROUND OF THE INVENTION tice to reduce the amount of dissolved oxygen in the molten steel by adding metallic or carbidic deoxidizers to the molten steel. These deoxidizers have a greater affinity for oxygen than does the molten steel and will chemically combine with oxygen and remove it from solution in the molten steel. One of the most popular deoxidizers used by steelmakers is aluminum, because of its low cost and powerful effect as a deoxidizer. I

The oxides resulting from the use of metallic deoxidizers, such as alumina A120 are relatively insoluble in molten steel, and some of the oxide particles remain suspended in the molten steel. When the steel is cast some of these particles become entrapped in the steel as it solidifies and appear in the finished steel as small non-metallic inclusions. Such inclusions can be a source of surface and internal defects in the finished steel and can also have a detrimental effect on the physical and mechanical properties of the steel.

Over the years other deoxidation practices, such as vacuum-carbon deoxidation, have been tried with the aim of minimizing the use of metallic deoxidizers to try to avoid introducing non-metallic inclusions into the steel. Such practices add to the manufacturing cost of the steel and frequently have not been fully effective in eliminating the use of metallic deoxidizers, such as aluminum. Thus, at the present time many steelmakers prefer to use a powerfuldeoxidizer, such as aluminum, in spite of the danger of introducing nonmetallic inclusions in the finished steel. 7

The use of aluminum asa deoxidizer has caused a serious problem whenever the aluminum killed molten steel is teemed through a relatively small internal diameter nozzle, such as in continuous casting. When the steel has been deoxidized with aluminum, deposits of aluminum oxide form in the bore and/or ports of the nozzle and disrupt the control of the rate of delivery of the molten steel to the continuous casting machine.

It is therefore an object of this invention to provide a deoxidation practice for steels which will produce a fully killed steel thatis measurably lower in oxygen content than conventionally non-aluminum killed steels.

It is a further object of this invention to provide a deoxidation practice using aluminum as a deoxidizer for producing fully killed steel that results in a low aluminum content, a low dissolved oxygen content and a low content of aluminum-rich oxide inclusions.

It is a further object of this invention to provide a deoxidation practice for continuous cast steels using aluminum as the deoxidizer which will produce a steel that can be teemed through small diameter nozzles without the formation of deleterious or restrictive deposits in the bore or ports of the nozzles.

It is a still further object of this invention to provide a fullykilled steel which when processed into rolled productswill have excellent surface and internal quallty.

Other and further objects of the invention will become apparent from the following description and claims.

SUMMARY OF THE INVENTION According to the present invention the foregoing objects are attained by (a) introducing molten steel containing dissolved oxygen into a vessel; (b) adding aluminum to the steel in an amount greater than that requir ed to chemically combine with allthe oxygen dissolved in the steel and provide a dissolved aluminum content of between 0.03 and 0.06 percent; (c) providing on the surface of the molten steel a layer of fluid slag containing iron oxide in an amount greater than that required to chemically combine with the amount of dissolved aluminum retained in the steel; (d) mechanically stirring the steel at a controlled rate and under conditions to cause transport of the aluminum and aluminum oxides in the steel to the slag-metal interface and cause a reaction of the dissolved aluminum with the iron oxide in the slag; (e) and then discontinuing the mechanical stirring when the amount of .dissolved aluminum in the steel is not more than 0.020 percent and not less than that which would allow the dissolved oxygen concentration to increase to a point where it would combine with the carbon in the steel to form carbon monoxide gas upon solidificationof the steel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The process-according to this invention involves initially lowering the dissolved oxygen content of a heat of molten steel by the addition of aluminum as a deoxidizer to the steel while it is being poured into or in contained in a vessel, such as a ladle. The aluminum is added in an amount greater than that required to chemically combine with all of the oxygen dissolved in the steel, so that after such combinations occur there will remain in the steel in the vessel dissolved aluminum in the order of between 0.03 and 0.06 percent.

The surface of the molten steel in the vessel is covered with a fluid slag layer preferably at least 6 inches thick. This maybe a portion of the slag from the steelmaking operation or a specially prepared slag layer. The slag should contain iron oxide in an amount greater than required to chemically combine with the amount of aluminum dissolved in the steel. Slags which contain between 12 to 25 percent iron as iron oxide will fit this requirement especially when the slag layer is at least 6 inches thick.

The molten steel in the vessel is then stirred by any convenient means at a controlled rate for a predetermined period of time. One low cost and convenient method of stirring the molten steel is by bubbling a small amount of non-reactive gas such as argon, helium or nitrogen, up through the steel in the vessel.

The circulation so produced substantially increases the contact between the molten steel and the oxidizing slag layer and promotes a slag-metal reaction that may be characterized by the following formula 2A1 (in steel) 3 P60 (in slag) 3 Fe (in steel) A1203 (solid particles) The A1 solid particles so formed are readily absorbed and held by the slag.

in addition, the circulation of the molten steel tends to promote the float-out of the aluminum oxide particles that are formed initially in the molten steel as the product of the deoxidation reaction of the aluminum addition with the dissolved oxygen in the molten steel.

The stirring of the metal can also be produced by other means such as by induction stirring or with heat resistant paddles, or by continuous circulation degassers.

The amount and duration of the circulation must be carefully controlled. The stirring is continued until the amount of dissolved aluminum has been reduced to 0.020 percent or less as determined by standard analytical techniques and terminated before the amount of the dissolved aluminum approaches zero. It is important that the stirring be terminated when the amount of dissolved aluminum is present in the steel in an amount not more than 0.020 percent and not less than 0.005 percent. It has been found that a residual amount of dissolved aluminum in the molten steel in this range will maintain the dissolved oxygen content in the steel at a value low enough to prohibit a reaction between the carbon and oxygen in the steel from occurring on solidification of the steel.

Under these conditions, we have been able to produce fully killed steels with total oxygen levels below 50 ppmsand residual dissolved aluminum levels of less than 0.020 percent, and which have retarded aging or non-aging properties. Such steels can be continuously cast successfully since they can easily be poured through the relatively small diameter nozzles (less than 1 inch diameter) without plugging the bore or ports of the nozzle or otherwise disrupting the control of the rate at which the metal is fed to the continuous casting machine. The reason why killed steels made in accordance with this invention do not plug the fused silica or other refractory nozzles commercially used in the continuous casting of steel is not fully known. However, it has been observed that as such steels pass through the nozzle, the rate of erosion of the nozzle bore appears to be approximately the same as the rate of accumulation of aluminum oxide in the bore of the nozzle.

During the stirring treatment in the ladle the quantity of aluminum that combines with the FeO in the slag is dependent upon the amount of aluminum in the steel before stirring and the amount of FeO in the slag and the rate and duration of the stirring. The initial aluminum content in the steel and the FeO content of the slag therefore provides the means by which one can predict duration of stirring required to produce the desired amount of uncombined aluminum in the molten steel in accordance with the following formulas:

l. T= Al -0.035/0.006 when slag FeO, is between 15-21 percent 2. T= Al10.035/0.008

grfiater t 2 ..P 2 1.. and where T stirring time 'in minutes using argon as the stirring medium injected at a rate of SCFM and pressure of 80 P816. Steel depth in ladle approximately 7 feet; heat size 100 tons; slag thickness approximately 12 inches. Percent A1, analysis of ladle sample for total aluminum before stirring.

when slag FeQ is The slag-metal reaction that is employed in this invention may be characterized by the following formula:

2 Al (in steel) 3 FeO (in slag) 3 Fe (in steel) A1 0 (solid particles) The A1 0,. solid particles so formed are readily absorbed and held by the slag.

The hypothesis described in the technical literature (T. B. King, Electric Furnace Steelmaking, Volume II, Chapter 20, Interscience Publishers, 1963), as boundary layer theory provides an understanding of the mechanism involved in the process.

For two fluids in contact (liquid slag and liquid steel) the theory involves the transport of matter, assuming the existence of boundary layers adjoining the slagmetal or reaction interface. For the process in which we are interested, the pertinent steps involved in the slag-metal reaction may be listed:

1. Transport of aluminum atoms in the metal to the slag-metal phase boundary.

2. Transport of FeO in the slag to the reaction interface.

3. Reaction between aluminum and FeO at the phase boundary.

In considering the above for definition of the step that controls the process rate, the reaction between aluminum and Fe() can be eliminated since this is very rapid in comparision to the transport steps. Under the conditions of stirring, the transport of aluminum in the metal would present no problem and aluminum concentration at the interface may be considered to be essentially the same as that of the bulk metal. The transport of FeO in the slag to the slag-metal interface is suggested to be the rate controlling step for the process.

The rate of the diffusional transport in the slag may be expressed in terms of oxygen loss from the slag:

This of course is proportional to the dealuminization rate of the bulk metal and therefore the rate of aluminum loss from the metal may be expressed:

where k k D,,"/V

The rate of the dealuminization process in influenced by the imposed agitation since agitation causes the reaction interfacial area (A) to bejncreased and the diffusion boundary layer thickness fiig") to be; diminished. The rate of transport of oxygen to the slag-metal interface is influenced by slag FeO concentration (more importantly by FeO activity in the slag).

The controlled dealuminization reaction occurs without the deleterioustransfer of oxygen from the slag into the bulk metal due to the very low solubility of oxygen in iron while finite amounts of soluble aluminum are present. Thermodynamic equilibrium solubility of oxygen in iron at steelmaking temperature (1600 C) is expressed by the reaction: 2 Al (.in steel) 3 0 (in steel) A1 0 (solid) A F =-292,800 93.7 T Percent Soluble Equilibrium Percent Oxygen Aluminum Dissolved in Fe (Max. Solubility at 1600 C) 0.040 0.00023 0.030 0.0003 0.020 0.00037 0.010 0.0006 0.005 0.0010

The maximum oxygen solubility values shown on the above table taken from J. Chipman and J. F. Elliott, Electric Furnace Steelmaking, Volume II, Chapter 16, lnterscience Publishers, 1963, demonstrate that the change in oxygen dissolved in the bulk metal during and as a result of the dealuminzation treatment is insignificant and beyond the limits of standard analytical methods to detect. The key to the process is the discontinuance of the dealuminization reaction while a finite concentration of dissolved aluminum remains in the bulk metal.

As an example of the process according to this invention, a 100 ton heat of steel was made in a basic open hearth furnace and tapped into a ladle. Analysis of the steel in the furnace before tapping was Carbon 0.05%

Manganese 0.35%

Silicon 0.01%

Oxygen 0.035%

Aluminum 0% Upon tapping, 3.3 pounds of aluminum per ton of steel'were added to the steel in the ladle as the steel flowed from the furnace into the ladle. The steel in the ladle was at a temperature of approximately 2900 F. A one foot thick layer of slag from the furnace was allowed to accumulate on the surface of the steel. The slag, at tap, had a FeO, content of approximately 16 percent.

Argon gas was injected into the steel near the bottom of the vessel from a hollow refractory covered tube of one inch internal diameter at a flow rate of approximately standard cubic feet per minute and pressure of 80 psig. for a period of 3.8 minutes. The steel was then continuously cast. The cast steel had a composition of Carbon 0.05%

' Manganese 0.35%

Silicon 0.01%

vOxygen (Total) 0.004%

Aluminum(dissolved in steel) 0.005%

No problems were encountered with plugging in a submerged-entry tundish nozzle. Port diameters of 56 inch 'in the fused silica tundish nozzle not only remained free of obstruction from alumina plugging but were slightly eroded to larger diameter during the casting run. The cold-rolled sheet processed from the continuous cast slabs had excellent surface quality, less than 2 percent rejected for surface defects arising from non-metallic inclusions rich in aluminum oxide.

The example cited above is for a typical l00-ton open hearth heat of low carbon killed steel. In the course of our studies, 26 such heats (2600 tons) were produced using the practice described. All heats contained dissolved aluminum of 0.005 percent to 0.020 .percent, were completely killed having total oxygen content of less than 0.005 percent, produced'excellent quality sheet, and were continuously cast by flowing through submerged-entry tundish nozzles (port diameter of /2 inch and 13/16 inch) without the occurrence of flow restriction due to nozzle plugging.

We claim: 1

1. Method of producing a killed steel especially suit able for continuous casting comprising:

a. adding aluminum to molten steel which contains dissolved oxygen in an amount greater than that required to combine chemically with all of the oxygen dissolved in the steel, and produce a dissolved aluminum content of between 0.03 and 0.06 percent,

prior to, during or after the steel is introduced into a vessel, providing on the surface of said steel a layer of fluid slag containing at least 12 percent iron oxide and in an amount substantially greater than that required to combine chemically with the dissolved aluminum retained in the steel,

stirring the molten metal in the vessel to expedite transport of dissolved aluminum'in the steel to the slag-metal interface and to cause reaction of said aluminum with the iron oxide in the slag,

d. discontinuing said circulation when the amount of uncombined aluminum in the steel is not more than 0.020 percent and not less than that which would permit reaction between carbon and oxygen in the steel to occur on solidification of the steel.

2. The method of claim 1 in which the amount of stirring is dependent on the initial FeO content of the slag.

3. The method of claim lin which the stirring is produced by bubbling a non-reactive gas through the steel.

4. The method of claim 1 in which the fluid slag layer is at least 6 inches thick.

. UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, Yog 00% Dated CGODGI 30, 1973 Inventor) L all F. Barnlmrdt, et a1.

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 6, "Assignee: John I. Iverson should read Assignee: Bethlehem Steel Corporation Column LL, line 27, "comparision" should read comparison Signed and sealed this 30th day of April 19%..

(SEAL) At'test:

EDWARD MJ LETGHER R. 1G. ARSHALL 13mm Attesting Officer Commissioner "of Patents FORM po'wso (10439) USCOMM-DC c0370 P69 ".5, GOVERNMENT PRINTING OFFICE: I," 0-36-33

Claims

2. The method of claim 1 in which the amount of stirring is dependent on the initial FeO content of the slag.
3. The method of claim 1 in which the stirring is produced by bubbling a non-reactive gas through the steel.
4. The method of claim 1 in which the fluid slag layer is at least 6 inches thick.