Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
2001-06-22
2002-02-26
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
C075S315000
Reexamination Certificate
active
06350295
ABSTRACT:
TECHNICAL FIELD
The invention relates to a method and composition for densifying aluminum additives, by compaction of aluminum (Al) and iron (Fe) fines into a physical form such as a briquette, thereby reducing aluminum losses to the tap slag and improving residual aluminum control during primary deoxidation of liquid steel in the ladle.
BACKGROUND OF THE INVENTION
Generally, molten steel resulting from converting a combination of molten iron and scrap steel in a basic oxygen furnace or electric arc furnace has to be deoxidized prior to its solidification in ingots or in continuously cast shapes. Such deoxidation occurs mainly in the steel ladle during tapping from the furnace or somewhat later at the ladle metallurgy station. Deoxidation is for the purpose of reducing the dissolved oxygen content of the molten steel to a predetermined and measurable narrow range required by the ultimate quality of the steel product. This implies the addition to the molten steel of specified amounts of deoxidizing agents such as carbon, manganese, silicon and aluminum generally used in combination. In addition to their deoxidizing function, these same elements may be added also for the purpose of forming an alloy with the steel to thereby alter the physical and mechanical properties of the latter. Several other, more expensive elements added to molten steel for alloying purposes, such as boron, vanadium, niobium, titanium and calcium, may also offer deoxidizing power but they are generally protected from being wasted to that function by introduction only after the dissolved oxygen has been reduced by at least two (2) of the elements cited, C, Mn, Si and Al.
Until the introduction of continuous casting and the almost compete conversion from ingot casting to “strand” casting around 1985, oxygen was one of the allies of the steelmaker. By controlling the carbon/oxygen balance, using little deoxidation except the specified carbon and manganese and occasionally some silicon and very little aluminum during ingot teeming, steelmakers produced well over 75% of all steel world-wide as good quality “rimming” steel ingots. The carbon monoxide (CO) gas developed by the carbon/oxygen reaction during solidification was “rimmed out” leaving no porosity in the clean, virtually inclusion-free surface layers of these ingots, while most internal porosities trapped by the final solidification were sealed without trace by the hot rolling process.
Unfortunately, rimming is not applicable to continuous casting because the high speed of solidification overtakes the upward flow of CO bubbles, entrapping large amounts of porosity in subsurface layers of as-cast steel. Today, over 95% of all steel is continuously cast, and it is “killed” as opposed to “live” or rimming. As the terms are used in this application, “killed” or deoxidized state means that the amount of “free” or dissolved oxygen contents have dropped from 150-400 ppm oxygen typical of rimmed steels down to 20-50 ppm oxygen in silicon and manganese killed (SiK) “long” steel products representing some 40% of total steel production. Free oxygen is further reduced down to the 0.1-5 ppm range in aluminum (Al) killed or AK “flat-rolled” products, about 60% of all steels. From the one-to-two order of magnitude difference in free oxygen resulting from aluminum and manganese (Al/Mn) as compared to silicon and manganese (Si/Mn) deoxidation, it is clear that Al/Mn vastly outperforms Si/Mn for full deoxidation and total absence of CO porosity.
However, even with superior deoxidation power, aluminum is not used on all strand cast steels because the products of aluminum deoxidation are alumina and aluminate spinel inclusions, which are solid at the steelmaking temperature of 3000° F. It is known that solid inclusions passing through the narrow tundish-to-mold metering nozzles tend to clog these refractory nozzles and to shut down the whole casting operation. This is particularly prevalent with the smaller nozzles used for billet, bloom and “dogbone” casting sections of the long products group of which only a small fraction, the Special Bar Quality (SBQ) subgroup may use a little aluminum “sacrificially,” typically about 1 lb/ton to prevent porosity while avoiding nozzle blockage. With the very large, oversized nozzles used for the large sections of slab casting of flat steel products, the alumina build-up problem is not sufficiently severe to prevent full aluminum deoxidation. Thus, flat product steels are virtually all aluminum deoxidized or Aluminum Killed (AK) using between 2 and 7 lb aluminum per ton of steel. The minimum amount of aluminum needed to deoxidize molten steel tapped from the furnace can be estimated as follows: the dissolved oxygen content of the steel before deoxidation varies typically from 600 to 1200 ppm. The deoxidation reaction is 2 Al+3/2 O
2
=Al
2
O
3
. Converting this into approximate weights: 2×27+3×16=102. This means that, for the reaction to be completed, 48 weight units of oxygen require 54 weight units of aluminum. Thus, 600 ppm O require 675 ppm Al, and 1200 ppm O require 1350 ppm Al. As 500 ppm equal one pound per ton, the minimum amount of aluminum required to deoxidize steel is 1.35 to 2.7 lb per ton. On the one hand, manganese and carbon take some share of the deoxidation work which reduces the need for Al by 0.5 to 1 lb per ton. On the other hand, most steels require a retained or residual Al content of 0.025 to 0.050% corresponding to an additional ½ to 1 lb per ton. In short, the minimum amount of aluminum required for the large group of low carbon AK steels is about 2 lbs per ton if no parasitic losses are encountered.
When fully deoxidizing with aluminum at tap, the average usage of aluminum is approximately 5 lb/ton, of which it is believed, without being held to any one particular theory or mode of operation, about 3 lb/ton are lost to slag (90%) and air (10%). Thus 2.7-2.9 lb aluminum per ton are lost to parasitic reactions with the slag involuntarily transferred with the molten steel tapping from the furnace.
Both Basic Oxygen Furnace (BOF) and Electric Arc Furnace (EAF) slags result from oxidizing carbon (C), Silicon (Si), manganese (Mn), and even some iron (Fe) to purify hot metal (BOF) and scrap (EAF), to achieve steel specifications and to raise the temperature to 3,000° F. required at tap. Only carbon leaves the melt as carbon monoxide (CO) gas. Some of it may stay in the slag but only as entrapped bubbles. The other elements form oxides, e.g., silicon dioxide (SiO
2
), manganese oxide (MnO), iron oxide (FeO), etc., which are fluxed to a liquid phase by the proper amount of burnt lime/dololime (CaO+MgO) and form a fluid slag which is poured in a slag pot after the steel is tapped out.
Typical chemical compositions of this furnace slag from a normal BOF or EAF melting and oxidizing process are as follows (in weight percent).
Cpd. GaO SiO
2
MgO Al
2
O
3
FeO MnO P
2
O
5
Cr
2
O
3
TiO
2
Na
2
O K
2
O
Wt.% 45 15 12 1.5 18 6 1.5 0.3 0.4 0.2 0.1
Typical aluminum consumption for such process is 4 lb/ton (loss 2 lb/ton).
Typical chemical compositions of this furnace slag from a highly energized EAF feeding a modern thin slab casting plant, are as follows (in weight percent).
Cpd. CaO SiO
2
MgO Al
2
O
3
FeO MnO P
2
O
5
Cr
2
O
3
TiO
2
Na
2
O K
2
O
Wt.% 27 11 9 1.0 45 5 1.0 0.6 0.3 0.1
Typical aluminum consumption for such process is 6.5 lb/ton (loss 4.5 lb/ton).
The percentage of FeO is the major difference between the two slags, directly causing the difference in aluminum losses. The parasitic reaction, reducing the FeO of the slag by aluminum is strongly exothermic, thus resulting in the following:
3 FeO (slag)+2 Al (deox)→Al
2
O
3
(new slag component)+3 Fe (new steel)
Similar, but less exothermic reactions occur with MnO, SiO
2
, P
2
O
5
, Cr
2
O
3
, TiO
2
and the alkalis and all contribute to aluminum losses. However, the reaction between Al and FeO represents 60-90% of all of the aluminum losses during tap, depending on the relative FeO content as illust
Bulan Clayton A.
Luyckx Leon A.
Andrews Melvyn
Buckingham Doolittle & Burroughs LLP
Bulan, Jr. Clayton A.
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