CA1070082A - Method for casting steel ingots - Google Patents

Abstract

METHOD FOR CASTING STEEL INGOTS

Abstract of the Disclosure A method for casting steel ingots. The ingot molds are coated on their inside walls with a finely divided slag forming material comprising a blend of various oxides and fluorides. The mold coating reduces the number of cracks and inclusions occurring at or near the surface of the steel ingot as it solidifies.

Description

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Background of he Invention This invention relates to a method of' casting steel ingots. It relates especially to the casting of' steel ingots into ingot molds whose internal surfaces have been coated with a coating containing a mixture of' finely divided selected oxides and fluorides.
The use of mold coatings on the internal surfaces of ingot molds to either improve the surface quality of the solidified ingot or to prevent the ingot from adhering to ingot mold is well known and a large variety of materials and techniques for applying such materials have been tried by steelmakers in the past. Heretof'ore such coatings have been expensive, difficult to apply and of' questionable value in improving the quality of the ingot, especially in the critical subsurface region immediately below the surface of the ingot.

Summary of the Invention It is therefore an object of this invention to provide a method of casting steel ingots which will produce steel ingots having superior surface and subsurface quality.
It is a further object of this invention to provide a method of casting steel ingots using an ingot mold coating which is relatively inexpensive.
It is a still further object of this invention to provide a method of casting steel ingots using a mold coating that is easy to apply and does not require special equipment or techniques.

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Other and further objects of this invention will become apparent from the following deseription and claims.
According to the present invention the foregoing objects are attained by applying to the internal surfaces of an ingot mold a coating of a liquid dispersion containing a mixture of finely divided oxides and fluorides, drying the coating and ~hen teeming molten steel into the coated mold.

Description of Preferred Embodiment The invention is espeeially suited for use with cast iron open ended ingot molds of the type used for easting earbon and alloy steel ingots at most steel pro-dueing plants. In practice, a liquid dispersion or slurry is prepared using a mixture of finely divided oxides and fluorides, the amounts of whieh are selected to provide eertain desirable physieal and chemical properties to the eoating when melted by the molten steel.
A satisfaetory slurry has been used eomprised of a mixture of aluminum oxides, ealeium oxides, silica and ealeium fluoride in substantially the ~ollowing proportions by weight:
A123 22%
CaO - 22.8%

CaF2 _ 15.6%
to whieh has been added suf~ieient water to enable the slurry to be sprayed or brushed on the interior sur~aees of the ingot mold. Other liquids su¢h as alcohol could be used instead o~ water as the liquid vehicle if desired. It is V~32 not necessary or even desirable to add any binder materials to the slurry since it has been observed that the above mixture tightly adheres to the surface of the ingot mold after the liquid which has evaporated.
The exact proportions of aluminum oxide, calcium oxide, silica and calcium fluoride can be varied for the proportions given so long as the resulting mixture has a plastic deformation point between 600C and 1200C, and a flowidity between 5 and 31 centimeters.
The term "plastic deformation point" and "flowidity"
of the mixture as used in this specification and claims are defined in United States Patent No. 3,649,249.
The apparatus used to determine the "flowidity" of the compositions of the present invention is illustrated in United States Patent No. 3,649,249 in Figure 2. This apparatus consists of a split mold comprising half mold sections, casters machines of low carbon steel. The mold sections when operatively assembled, as by clamping together the two half mold sections form a vertically disposed funnel or conical portions having an internal diameter of about three inches and an external diameter of 3 5/8 inches at the upper end and an internal diameter of 5/8 inches at the lower or discharge end thereof. The internal surface of the conical porti3n forms an angle of 39.5 with the vertical axis of the conical portion. A conical recess formed by the conical portion is adapked to receive the test sample of molten meterial. A rectangular base section connects with the lower end of the conical portion and defines a , .

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cylindrical well having a diameter of 5/8 inch and a depth of 3/4 inch. The base section has a rectangular section extending horizontally from one side thereof with an axial bore or conduit extending therethrough and having a circular cross-section 1/4 inch in diameter and a length of about 20 inches. The conduit intersects the lateral cylindrical well adjacent the lower end of the well and provides an outlet passage for the test material introduced into the conical recess which serves as a funnel to feed the test material into the conduit until the sample solidifies in the conduit.
The term "flowidity" as used in the specification is the distance in inches which the test material flows through the conduit before solidifying when 200 grams of the test material at a temperature of 2,600F is rapidly poured into the conical recess. The pouring technique and the timing of the test procedure should be standardized to obtain consistent results. The temperature of the mold, within normal operating limits has little effect on the ; measured flowidity.
The flat "plastic deformation point" of the slag as the term is used in the specification is determined by heating about a 20 gram piece of the solidified test material which is obtained from the above flowidity test at a rate of 50F per minute and pressing a graphite rod (or an aluminua or other suitable rod) against the piece of test material at frequent intervals as the temperature rises and the temperature at which the pieGe of slag plastically deforms under a slight pressure is the "plastic deformation point".

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' ' . . : ' It is preferred to use an unfused mixture of the oxides because of its lower cost. A prefused mixture could be used equally well in producing this invention but at greater expense due to the cost of preparing.
It is prelerred to apply the slurry mixture to a warm ingot mold so that the heat of the mold will assist in drying the coating. The slurry is applied so that the resulting dried coating is at least 0.3 millimeters thick.
Based on the mixture set forth above this would equal about 1 kilogram o~ the mixture per square meter of surface area coated.
While the exact mechanism of this invention is not known it is believed this mixture as it melts acts as a lubricant between the ingot and mold allowing contraction o~
the thin ingot skin without stressing the skin which would cause cracks. The molten mixture also apparently has the _Llb-.

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ability to pick-up surface and subsurface impurities and inclusion forming material from the molten steel and retain them while the ingot solidifies.
The use of this practice has pro~ided a 911%
reduction in the length of surface cracks and an 83% reduction in the sum of inclusion diameters at or :near the surface of the ingot.

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Claims (8)

1. Claims I claim:
   l. A method of casting steel ingots which com-prises applying to the internal surfaces of the ingot mold a coating of a liquid dispersion containing a mixture of finely divided oxides and fluorides, drying said coating and teeming molten steel into said coated mold.
2. 2. The method of Claim l in which the coating when dried is at least 0.3 mm. thick.
3. 3. The method of Claim 1 in which the oxides and fluorides are selected so as to provide a coating which has a plastic deformation point between 600° C and 1200° C.
4. 4. The method of Claim 1 in which the oxides and fluorides are reheated so as to provide a coating which has a flowidity between 5 and 41 centimeters.
5. 5. The method of Claim l in which the oxides and fluorides are unfused prior to teeming.
6. 6. The method of Claim l in which the dried coating contains not less than l kilogram of said mixture of oxides and fluorides per square meter of coated ingot mold surface.
7. 7. The method of Claim l in which the mixture of oxides and fluorides is selected from the group consisting of aluminum oxide, calcium oxide, silica and calcium fluoride.
8. 8. The method of Claim 1 in which the liquid is water.