US3158466A - Product for refining effervescent, quiescent and semi-quiescent steel in the casting - Google Patents

Description

United States Patent France No Drawing. Filed Jan. 19, 1961, Ser. No. 83,619 Claims priority, application France Jan. 21, 1960 Claim. (Cl. 75-58) The present invention relates to a product for refining effervescent, quiescent and semi-quiescent steel in the casting.

It is a well-known fact that steel shows often various defects, of which some are due to their actual composition, while others are due to extraneous conditions, such as, for example, the condition of the refractory material in the casting ladle, of the varnishes or cements which are to protect the ingot mold and which may or may not be suitable, and the like.

As a matter of fact, traces of refractory material or of lime may be carried along by the molten steel, either into the middle or just underneath the skin of the ingot. Furthermore, bubbles may be produced at such points and form objectionable inclusions which often lead to the production of undesirable lines during subsequent rolling.

Lastly, during its casting into ingot molds, the steel is subjected to the action of energetically stirred air which saturates the steel \m'th oxygen which is associated with that of the oxides already contained in the metal.

These various impurities are detrimental to the grade of the steel which should be purified under the best operative conditions.

The conventional methods of purification resort to silicon as a purifying agent, in the form of a Fe-Si or Fe-Cr-Si alloy or the like silicon alloy with low carbon contents, or else, to aluminium, or a mixture of silicon and aluminium, or again, to manganese alloyed with iron or in the form of spiegeleisen.

Now, the amount of effervescence obtained with mild steels under such conditions is insufficient and the conventional means resorted to for increasing the time during which the heads of the ingots are open, have in practice no substantial action on the block or ingot.

It is one object of the present invention to provide a product for refining effervescent, quiescent and semiquiescent steel in the casting.

It is another object of the present invention to provide a product for refining effervescent, quiescent and semiquiescent steel in the casting, and in particular to provide pulverulent reducing products having a high dissolving capacity for metal oxides and forming, under normal operative conditions, particles showing the desired consistency for an easy agglutination.

These products have a double action, to wit:

(1) A chemical action adapted to reduce to a minimum the sulfur and oxygen contents of the metal;

(2) A physical action adapted to cut out the formation of fine and very fine inclusions by furthering their agglutination or caking.

As a matter of fact, and in accordance with Stokes law and for the same duration of refining, the formation of slags by the inclusions is more marked when the steel is more fluid, that is when it is hotter and when the inclusions are larger.

In accordance with the present invention, a number of alkaline or the like fluorinated derivatives are used, either in admixture with suitable derivatives of boron, or as chemical combinations with the latter, e.g. in the form of fluoborates, either alone or in association with alkaline or the like hydroborons.

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It is also possible to resort to borates or phosphates, whether anhydrous or hydrated, considered either singly or separately or in admixture with the above-mentioned compounds.

In order to further the solubility of oxides in the above products, there is incorporated with the latter a flux such as e.g. fluorides, chiefly sodium, lithium or potassium fluorides and a reducing agent such as carbon, silicon, manganese or aluminium.

These latter elements may be also obtained in statu nascendi during operation by resorting to the corresponding carbides or fluorides, the carbides showing, in fact, a large reducing capacity with reference to the metal oxides and sulfides.

The reduction is activated by the presence of alkaline carbonates, the basic character of which forms an essential factor for such a reduction.

A typical composition of such refining powders is as follows, by weight:

Percent Anhydrous borax Sodium fluoride 10 Sodium carbonate 1 Potassium carbonate 1 Graphite 5 Lampblack 3 The preparation of such a mixture is as follows:

The anhydrous alkaline borates are admixed with alka line fluorides by incorporating with the mixture alkaline carbonates and highly subdivided carbon in a stable condition, such as graphite, or else, boric acid is admixed with alkaline fluorides in the presence of alkaline fluxes, or again, alkaline fluorides or the like fluoborates are admixed with fluoboric acid or alkaline or the like fluorides, fluosilicates or fluotitanates.

The alkaline derivatives may be replaced entirely or partly by fluorides of rare earth elements, such as cerium fluoride, fluoborate or fluosilicate.

The carbon may be replaced in the above mixtures, either partly or entirely, by elements of the fourth group of Mendeleevs Periodic Table. Boron may be replaced similarly in the above mixtures, either totally or partly, by elements of the third group such as titanium.

The steels with which the mixture may be incorporated are chiefly effervescent steels, but it may also be incorporated, under certain conditions, with quiescent or semiquiescent steels.

The addition of said products leads to a substantial increase of the eifervescence of the blocks and it leads often, chiefly in the case of ingots weighing less than five metric tons, to a rising of the level of the boiling ingot, the lowering of said level down to normal level being obtained only at the end of the operation, a few minutes before lifting.

Thus, in the case of mild steels, the addition considered, executed in successive stages, maintains the effervescent conditions and produces a powerful and protracted boiling inside the ingot molds.

In the case of a hard quiescent steel having a resistance above 60 kg. i.e. a steel which is generally not treated by exothermic shrinkage-hole preventing powders, it is found that the upper level of the ingot is lowered by a few centimeters, which shows that the evolution of the gases is much more marked.

The powder tobe incorporated with the steel should, in its most active form, contain a proportion of the solvent and reducing product as high as and even and never underneath these values, so as to prevent the slag formed from showing a too great compacity.

The product obtained is introduced into the molten steel in amounts lower than or equal to 0.1 kg. per metric ton, but the conditions of operation may require lowering these amounts down to 0.01 kg, or else, one uses 1 or even 2 kg. per metric ton.

It should also be remarked that for effervescent, semiquiescent and, chiefly, quiescent steels, with large carbon contents, the addition of the improved refining product allows obtaining, either in the ingot mold or in the casting ladle or, generally speaking, at any stage of the production of steel, a considerable reduction in the nitrogen contents, nitrogen being, as well-known in the art, generally present in steel in the form of fine nitride needles which disturb the crystalline network of the steel. A large fraction of the nitrides formed is solubilized in the fluid slag produced under the action of the refining powder and the boronitrides, formed during operation and which are stable up to 2,000 C., modify substantially the crystalline structure and lead to a clear improvement in the grade of the steel, this being due, in particular, to a substantial increase in the mechanical resistance of the latter.

The incorporation of the refining product is performed as follows: the product may be added, either by a shovel, at the moment of the casting in a ladle orin an ingot mold, or else, it maybe introduced inside gauged cardboard boxes, each of which contains an accurate weight of the product adapted for the casting conditions and the grade of the steel. It may also be carried by blisterpreventing cardboards, generally positioned at the bottom of the ingot mold, before the casting, and this increases not only the protective action of these cardboards, but also forms a reliable safeguard, whereby one makes sure the product has been actually added at the proper moment during the casting. Lastly, it allows obtaining through adjustment of the length of the cardboard introduced into the ingot mold, the automatic measurement of the product in accordance with the size of the block.

The advantages obtained through the use of the pulverulent mixture for refining purposes in accordance with the present invention are chiefly as follows:

(a) A clean reduction of the recesses in the head of the ingot and the reduction of the sulfur contents which should be very low and less than 0.02%, if possible, so as to reduce the segregation of the sulfur;

(b) The reduction of the oxygen contents through a selective removal of the oxide; I

(c) A reduction of the amount of inclusions in the steel, whether their origin is internal or external;

(d) The uniform closing of the head of the ingot after a longer opening leading to a better evolution of the gases;

(e) The considerable reduction of the nitrogen contents which leads to a clear improvement in the grade of the steel;

(3) The addition of the product does not produce any unpleasant or troublesome smoke or smell at the moment of its incorporation.

A few of the numerous characteristic applications of the present invention will now be illustrated by the following examples:

Example 1 22 metric tons of mild steel are treated with 120 kg.

of a Fe-Mn alloy and 100 kg. of spiegeleisen and there is added at the moment of the casting 1,000 gr. of the typical composition of the improved product disclosed hereinabove into an ingot weighing 4.5 tons.

The results have been as follows:

Treated Nonsteel, treated percent steel,

percent Waste at the head 2. 5 4. 5 Waste at the end 2 2. 5

7 Example 2 50 metric tons of steel for strip iron are treated with 250 kg. of an alloy of Fe-Mn and 30 kg. of anthracite and there is added at the moment of the casting 1,000 gr. of the same refining powder into an ingot mold for ingets of 7 tons.

Total waste: treated steel 2.9%; non-treated steel 13%. As concerns the reduction of the nitrogen contents and by way of example, it may be mentioned that said contents are lowered from 0.015% down to 0.004% through a treatment of 25 tons of steel by 50 kg. of the refining powder.

Obviously, the present invention is by no means limited by the assumed theory of its operation or by any particular example and it may be modified in various manners within the scope of the accompanying claim.

The steels treated in accordance with the present invention may be considered as more highly refined from all points of view.

While I have disclosed several embodiments of the present invention, it is to be understood that these embodiments are given by example only and not in a limiting sense, the scope of the present invention being determined by the objects and the claim.

I claim:

An alkaline refining product for steel adapted to be incorporated into said steel during its casting at a ratio of 0.01 kg. to 2 kg. per metric ton,

said alkaline refining product consisting of a mixture about of anhydrous borax, about 10% of sodium fluoride,

about 1% of sodium carbonate,

about 1% of potassium carbonate,

about 5% of graphite and about 3% of lampblack.

References Cited in the file of this patent UNITED STATES PATENTS 1,975,084 Davies Oct. 3, 1934 2,173,205 Johannsen Sept. 19, 1939 2,196,853 Cinamon Apr. 9, 1940 2,357,014 Merlub-Sobel et a1 Aug. 29, 1944 2,829,078 Aull et -al Apr. 1, 1958 2,932,564 Evans Apr. 12, 1960