US2190116A - Method of producing ingots - Google Patents

Description

Feb. 13, 1940. E. GATHMANN METHOD OF PRODUCING INGOTS Original Filed June l, 1938 maentor Fig.5.2l

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X i Q Patented Feb. 13, 1941) PATENT OFFICE METHOD or raonUclNG moors Emil Gathmann, Catonsville, Md., assignor to Gathmann Research Incorporated, Catonsville, Md., a corporation of Maryland Original application June 1, 1938, Serial' No.

211,285. Divided and this application December 5, 1938, Serial No. 244,105'

5 Claims.

This invention relates to methods of producing tion if the outer structure is not to be torn ory ingots and particularly to methods of producing big-end-up ingots.

This application is a continuation in part of my application, Serial No. 159,405, led August 16, 1937, and a division of my application Serial No. 211,285, filed June 1, 1938.

As is now well known to those familiar with the art, ingots generally designated as big-endup ingots are the only type in which interior soundness can be obtained commercially.

'I'here are. of course, many factors iniiuencing mold and ingot design; and the perfect ingot,

adjudged from all practical standpoints, never has been and probably neverV will be produced commercially because of the incompatibility of design factors. For example, in theory a very large degree of upward and outward ingot taper is metallurgically desirable, as a continued increase in cross section from the bottom towards the top of the ingot is necessary to effect progressive upward solidiiiation of the liquid ingot metal, an essential to soundness. A large degree of taper also shortens the timeperiod of solidiflcation due to the closer contact of the mold matrix with the forming ingot; and, as segregation of carbon and other elements in the steel is directly proportional to the time of solidiiication, it heretofore quite naturally has been the belief that a large continuous taper materially reduces the degree of segregation. I have found, however, that an excess of continuous taper will defeat its own purpose by preventing the ingot from settling into the mold as the metal shrinks and solidles. When cast in a mold having a large continuous taper from bottom to top, the partially solidified ingot is caught by the Walls of the mold at the upper portion of the chamber and is hung there, forcing the ingot to shrink upwardly uponitself and causing an air gap to form between the ingot surface and the mold chamber walls vin the bottom and intermediate portions. The formation of such an air gap prevents the progressively upward extraction of heat from the ingot by the mold, retarding solidii'lcation and increasing segregation. The weight of the ingotis usually more than the strength of the initially-solidified surfaces of the ingot can bear, and the skin and sub-skin structure are ruptured horizontally, producing the Well-known hanging tears in the ingot below the shrinkhead portion. Furthermore, a large continuoustaper is objectionable in mill practice because the ingot must be given an excessive number of initial blooming passes in reducing it to the required uniform cross seclapped by the rolls.

It therefore is apparent that in order to improve upon prior and existing mold and ingot practice, an ingot must be formed with regard not to one factor, but to various considerations, such as interior and, surface Hingot soundness, amount of top and bottom crop loss, rolling properties as affecting both the way the ingot metal is worked in the rolls and the cost per ton of product of reducing the ingot in the rolls to the required section, mold life, and so forth.

An object of this invention is to provide an improved method of casting or producing bigend-up ingots in which the shape of the ingot and relative rates of cooling of portions of the ingot are so related and balanced with respect to various factors, including the factors referred to above, as to promote soundness and reduced segregation throughout the interior of the ingot, to reduce surface defects, to reduce crop-loss and to facilitate initial reduction of the ingot to the desired shape in the mill. l

In the drawing:

Figure 1 is a central vertical sectional view of a mold adapted for forming ingots in accordance with my invention, the section being taken on the line I-I of Figure 2;

Figure 2 is a 'top'horizontal plan view of the mold shown in Figure 1 with the shrinkhead casing removed;

Figure 3 is a vertical side elevation of a steel vingot produced in accordance with the invention; and

Figure 4 is a cross section on line 4--4 of Figure 3 `showing graphically the types of crystallization in the ingot shown in Figure 3.

My invention may be practiced to particular advantage in producing ingots having the corrugated contours disclosed in my United States ,4o

Patent No. 2,092,551, issued September 7, I1937, but also may be practiced in producing ingots having various other cross sectional contours including, for example, slab type ingots as well as ingots of the kind disclosed herein. An at present preferred cross sectional contour is shown in Figure 4of the drawing.

A number of theories regarding the formation of an ingot are subscribed to by metallurglsts. I

have found that usually an ingot consists of three 50 b. A zone or layer of dendritic or columnar crystals;

c. A central zone of larger equiaxed crystals.

The physical properties of the chill crystals a as well as the equiaxed crystals c usually are much better than those of the dendritic or columnar crystals b. The initial or chill crystals a form a tough protective sheathing for the ingot, tendingn to resist the formation of surface cracks during solidiiication and reduction, and it therefore is desirable that the ingot be cooled in such a manner as to produce a relatively thick layer of chill crystals. The extreme outer or surface portion of the vchilled crystal layer forms immediately upon contact of the molten metal with the mold Walls; and, in order to cause the chill effect to extend inwardly from the extreme surface of the ingot, it is necessary to maintain the bottom and side surfaces of the ingot in contact -with the mold walls for a substantial period. Soon after 1 the pouring of an ingot has been completed, the mold begins to expand, increasing in chamber cross section and length, and the ingot begins to contract, decreasing in cross section and length. Hence, if a mold has substantially vertical walls, the walls and ingot will become separated soon after pouring, causing the formation of an insulating air space between the mold walls and ingot and removing the chilling effect of the mold walls. The formation of this air space may be delayed materially, as is now known, by providing mold walls which taper upwardly and outwardly (big-end-up) such an arrangement resulting in substantially longer contact of the mold walls and ingot surface metal during successive stages of the cooling of the ingot. Thus, by providing walls tapered as stated above, it is possible to obtain a decidedly thicker initial chill crystallization a than when using blg-end-down or parallel mold walls. I have found that, generally stated, the thickness of the chill crystallization zone is greater when the degree of big-end-uptaper is greater; that is, a mold matrix having a high degree of such taper will produce a chill crystal zone thicker than a mold matrix having a less degree of taper. Hence, insofar as the formation of the desired chill crystals a is alone concerned, it is desirable to employ a rather marked degree of big-end-up taper in the mold matrix walls.

On the other hand, if the mold chamber walls are highly tapered uniformly upwardly and outwardly from bottom to top, certain diiculties are encountered. One of such difllculties, as previously stated, results from a tendency of the ingots to hang in the mold walls in the top of the mold chamber just below the shrinkhead, so that the ingot is not free to descend during solidication to maintain contact of the body and lower portion of the ingot surface with the body and bottom, mold chamber walls, which contact is essential for progressive, upward sclidication. In my United States Patents 1,643,241 of September 20, 1927, and 1,819,705 of August 18, 1931, I have disclosed ingots which are tapered upwardly and outwardly to a greater degree in their lower portions than in their extreme upper portions. This construction eliminates some of the difficulty previously encountered because of hanging ofthe upper part of the ingot surfaces in the mold matrix, and has been found advantageous in the making of millions of tons of commercial ingots. The. ingots disclosed in the patents referred to furthermore have the advanta-ge that the total difference in cross sectional dimension between the small end of the ingot and the large end of the ingot required to insure progressive solidification may be reduced somewhat, and in addition uniform reduction of the ingot surface in the rolling mills is considerably facilitated.

I now have found, through the production and examination of many ingots, that a metallurgically more sound and a more easily rollable ingot can be produced by Yso shaping the ingot during cooling that the extreme lower portions of opposed side surfaces are tapered to a considerably greater degree than the intermediate body side surface portions adjoining and extending above the extreme lower side surface portions. Results observed in commercial production have established that the lower side surface portions, comprising from about to 50%, preferably about one-third, of the total height of the ingot body, should be tapered from two to twenty times as much as the taper of the upper and adjoining tapered surfaces. The amount of taper per inch of vertical height in the lower side surface portions should be increased as the cross section of the ingot increases and should extend at least 2 but not more than 20 outwardly and upwardly in relation to the vertical axis of the ingot, but should not exceed about twenty times the taper per inch of height of the upper adjoining tapered side surfaces, regardless of the cross section.

I have found that in ingots of about twentyinch maximum cross section, the bottom side surface portions should -be tapered about four times as much per inch of height as the main or intermediate side surface portions. The result of this differential taper is to produce an initial chill zone a which is thicker in the bottom side surface portions of the ingot than in the upper` are tapered to substantially the same extent per inch as the adjoining upper portions. The effect is to produce a more uniform progressive solidiilcation from bottom to top of the ingot than has heretofore been possible. To further promote the desired progressive solidication, the chamber wallsvof molds used for casting ingots in accordance with the invention preferably should diminish in thickness progressively from bottom to top. y

Figure l shows one type of mold M which may be used in producing ingots in accordance with the invention. The mold M comprises opposed side Walls as a whole designated I and a bottom or bottom wall generally designated 2. Lifting and handling trunnions L of a known kind may be provided. The mold is shown as being provided with a hot top or shrinkhead casing H, which may be of known construction. Both the lower portions 3 of the mold matrix side walls from A to B), and the main or body portions 4 from B to C) are substantially straight and are tapered upwardly and outwardly from the longitudinal axis Y o'f the mold chamber; but the lower portions 3 are tapered upwardly and outwardly to a far greater degree per foot of height than are the main or body portions 4. In a mold .chamber of twenty-inch maximum cross sec'- -tional dimension, the taper of the body portion 4 should preferably be about half an inch per foot.

It will be understood, however, that the relative amount of ta per should vary in dependence upon the size of the mold chamber and ,the specification of the steel forming the ingot. Likewise, the height of the more greatly tapered lower wall portions 3 varies in dependence upon the size of the mold chamber. In a mold of twenty-inch chamber cross section, the lower portion 3 preferably is about one-third of the total height of the mold chamber.

tapered portion 3 may also vary considerably, in dependence upon the horizontal cross section of the mold chamberemployed, but the lower tapered portion 3 should preferably in any case be between about 20% and 50% of the total chamber length of the metallic mold exclusive of the shrinkhead casing H, and it should be tapered from about two to twenty times as much per inch l lof height as the adjoining upper side walls 4'.`

The dotted lines X and the dotted lines W show clearly the great diierence in the amount of taper of the wall portions 3 and the wall portions 4.

It is usually advantageous to employ an extreme upper wall portion 5 of substantially parallel contour, shown extended at Z. i Depending upon the size and cross section ofthe ingot, this section 5 may be of varying length, but should not exceed in length the inside diameter of the hot top or shrinkhead casing H,and in vertical height should be from about 5% to about 20% of the total mold chamber height.

It is desirable to provide smoothly rounded fillets F between the straight mold wall portions 3 and 4, and 4 and 5 of the mold chamber. The provision of these smooth llets prevents the formation of horizontal surface cracks in the mold chamber, and, furthermore, prevents hangthe rtop of the mold chamber portion 5 to the top of the mold chamber portion 4, andr from the top of the mold chamber portion 4 to the upper part of the more highly taperedlower mold chamber portion 3. The corrugations being of a plane vertical contour will thus merge orV vanish in the upper third or half of the highly tapered section 3 of the mold chamber, substantially as shown in the drawing at V in Figure 1.

'Ihis type of corrugation provides a lower mold chamber of substantially rectangular contour withu four preferably rounded corners e.

the lower part 3 of the mold chamber walls is subject to the washing and erosive action of the molten ingot metal as it is teemed into the mold chamber, and the corrugations, if extended to the bottom 2 of the chamber, are vfrequently burned and washed away to such an extent that The relative amounts of x taper and the relative height of the more greatly Such con struction increases the mold life materially, as

after relatively few ingots have been produced in the mold, stripping of the ingots from the mold is at times rendered quite diiilcult, thus delaying stripping operations as well as shortening the mold life. Since there are no projecting corrugations R in the lower side Walls of the mold chamber 3, there is less danger of cutting the walls of the chamber due to this surging action of the steel as it is being teemed and danger of the ingots sticking to the lower part of the mold chamber walls 3 is greatly reduced and the mold life lengthened. l

`The contour of the bottom of the ingot and consequently the contour of the mold bottom wall 2 is important. I have found that by far the best results are obtained when the main or central part of the bottom wall 2a is of substantially flattish or slightly dished contour in vertical cross section and the marginal concave portion 2b is struck on a short radius. he concave portion 2b intervenes between and merges with the main bottom wall portion 2a and the lower side wall portions 3. In the mold disclosed in Figure l for the purposes of illustration, the bottom wall 2-2a-2b is integral with the vertically extending walls 3, 4, and 5, and is apertured for the reception of a removable plug P. The plug may be a closure plug of any suitable kind or, in cases in which the practice of any particular ingot casting plant requires bottom pouring, the mold bottom may be apertured to form a gate for introducing molten steel through the bottom of the mold in the usual manner of bottom teeming.

The main bottom wall portion 2a, when dished, t should be struck on a relatively long radius K,

and the marginal wall portion 2b should be struck on a relatively short radius r, the length of which latter radius should be not less than about 5% and not more than about 20% of the maximum cross section measurd between opposite side i walls having the differentially tapered portions or sections, The use of relatively small radii for the marginal portion 2b is important because it has .been found that large radii are more apt to produce subcutaneous cleavage planes and surface cracks in the ingot bottom than are the relatively smaller radii. The use of a relatively long radius K for the bottom wall proper`2a produces a relatively shallow dished contour, rather than a deeply rounded contour. The relatively shallow dished contour 2a is much to be preferred to a deeply hallowed-out bottom wall because it re-l sults in less butt crop when the ingot is reduced to a bloom or slab.

As is apparent, an ingot cast in a mold embodying my invention will have a contour corresponding to the mold chamber contour already de- `scribed. Briefly, however, it may be stated that the ingot I shown in Figure 3'has side surfaces I' formed with corrugations R'S' corresponding to the corrugations R-S of the mold chamber walls, and a bottom surface 2'2a-2b'. The lower portions 3 of the side surfaces are tapered to a greater degree than are the intermediate side surfaces 4' above and adjoining the lowerl surfaces 3'. Preferably the vertical length of the lower surface portions 3 comprises from about 20% to about 50% of the total vertical twenty times more than the intermediate side surface portions 4' above and adjoining the surface portions 3. The extreme upper surface portions 4 or x'nay be' substantially straight or parallel with the longitudinal axis Y of the ingot. The lines X', W and Z indicate graphically the difference between the upward and outward tapers of the lower side surface portions 3 and the surface portions 4' and 5 thereabove.

The shrinkhead portion H of the ingot consists primarily of dendritic crystals similar to the crystals in the zone b of the ingot proper, although the crystals in the shrinkhead are usually somewhat larger than those in the zone b, as shown in Figure 4, because the metal has cooled and solidified more slowly than in the body of the ingot.

Because of the greatly increased upward and outward taper in the bottom portions 3 of the side walls of the mold chamber, these portions of the mold chamber walls are rendered somewhat more heavy than the upper portions and as at least the lowermost portion of said side walls preferably is not corrugated, some cutting of these lower side walls by the erosive action of the stream of molten metal poured into the mold may take place without deleteriousresults. Thus,

' even though some of the lower chamber side wall portions 3 are cut away by the molten stream, the increased taper is such that even considerable cutting will not result in serious sticking of the ingot within the mold chamber portion 3, and thus will not interfere with the stripping of the ingot after it has solidified.

In the rolling of an ingot I cast in a mold having the contour described, there will be less unworked section and hence less bottom crop than when rolling ingots produced in molds previously known. I have found that the bottom crop loss with ingots cast in my improved mold chambers will be less than bottom crops from ingots heretofore produced, by as much as 25% to 50%. Furthermore, the improved new bottom contour, particularly as regards the flattish or slightly dished main bottom portion and the concave marginal chamber portion, will result in the production of an ingot base which will more readily stand in vertical position in the soaking pit and which will sink less deeply into the layer of coke forming the floor of the soaking or reheating pit. Thus, a smaller part of the side walls of the ingot will be decarburized or cut by the action of the heated coke on the ingot metal.

As is now well known to those familiar with the art, the highest yield of sound homogeneous steel is obtained from ingots cast of fully dioxi- .dized or degasified steel in molds so designed that from 80% to 95% of the ingot is formed and completely solidified i a heat-absorbing metallic mold having a big-end-up chamber surmounted by a heat insulating hot top or shrinkhead casing designed to contain from 5% to 20% of the total molten ingot volume. In using molds having the differential chamber tapers according to my present invention, the best results are usually obtained with the following practice. In topteerning, I prefer to use a method that includes relatively slow pouring until a pool extending to the mold walls ceases after the pool has been formed, providing, of course, that the ladle nozzle has been reasonablywell centered. 'Ihe full pouring rate should be reduced as soon as the metal has risen to the top of the mold chamber proper and has commenced to ll the shrinkhead casing. The casing chamber itself should preferably be teemed at from one-tenth to one-half the rate of a fully opened stopper.

The methodof producing an ingot of welldeoxidized steel in accordance with my present invention comprises confining and chilling the ingot in different vertical zones at different rates. Thus, the bottom 2' and the lower side surface portions 2b' and 3', comprising from about 20% to about 50%'of the ingot length are chill-cooled at a relatively very rapid mean rate, the intermediate side surface portions 4' are chill-cooled at a relatively slower mean rate, the extreme upper side surfaceportions 5', comprising from about 5% to about 20% of the ingot length, are chill-cooled at a relatively still lower rate and the shrinkhead portion H', if used, may be cooled at the lowest rate. side portions 3' decreasesprogressively throughout these portions from their bottoms to their tops, and the rate of cooling the intermediate side portions 4' decreases progressively throughout these portions 4 from their bottoms to their tops less rapidly than the decrease of cooling rate from bottom to top of the lower side surface portions 3'. The ingot is so shaped during cooling that the volume of ingot metal per unit of height increases at a predetermined rate upwardly throughout the lower portion 3 of the ingot comprising from 20% to 50% of the ingot height, and the volume of ingot metal per unit of height in the ingot portion 4' above and adjoining the lower portion 3 increases at a lesser predetermined rate. Furthermore, the diminishing thickness of the mold walls causes heat to be abstracted from the ingot at rates per unit or surface area which decrease progressively and relatively rapidly from bottom to top of the lower ingot portion 3', and at rates per unit of surface area which decrease progressively but relatively less rapidly from the bottom of the ingot portion 4" upwardly. 'I'he rate of cooling is determined on the basis of the lapse of time between the completion of teeming and the solidification of the metal.

By virtue of my novel method of producing ingots it is possible to obtain ingots of materially improved interior and surface soundness, to prolong mold life, to reduce crop loss. and to facilitate reduction in rolling or forging. The total taper or difference in cross section at the top and bottom of an ingot of twenty-inch maximum cross section would 4be approximately only four to five inches, thereby making for good rolling properties. Nevertheless, the lower part of the ingot at 3' has four times as much taper per inch of vertical height as the adjoining' upper tapered portion, so that the initial chill solidification extends into the ingot more deeply at the bottom and the time period of solidification of the entire ingot is decreased. The quicker cooling of the lower portion of the ingot both on its sides and its bottom enhances the progressive cooling and solidification from bottom to top, thereby producing an The rate of cooling the lower The novel method disclosed herein may be employed to advantage in producing. non-ferrous ingots as well as steel ingots.

The practicing of the invention has been illustrated and described as now preferred, but it will be understood that various changes may be made without departing from the invention as dened in the claims.

I claim:

l. In a method of producing big-end-up ingots,

chill-cooling the bottom and lower portions of at least two opposed ingot sides for a distance of from 20% to 50% of the total length of the ingot at a re/latively rapid mean rate, the rate of cooling said opposed lower side portions progressively decreasing relatively rapidly and substantially un'iformly throughout said opposed lower side rportions from bottom to top thereof; chill-cooling vthe opposed intermediate side portions above and adjoining said opposedlower side portions at a relatively slower mean rate, the rate of cooling said opposed intermediate side portions decreasing progressively throughout said opposed intermediate side portions from bottom to top thereof relatively less rapidly than the decrease of cooling rate from bottom to top of said opposed lowerside portions; and chill-cooling opposed extreme upper side portions at a rate still lower than the mean rate of cooling the opposed intermediate side portions.

2. In a method f producing, big-end-up ingots, chill-cooling the bottom and lower portions of at least two opposed ingot sides for a distance of from 20% to 50% of the total length of the 85 "ingot at a relatively rapid mean rate, the rate of cooling said opposed lower side portions progressively decreasing relatively rapidly and substantially uniformly throughout said opposed lower side portions from bottom to top thereof; and chill-cooling the opposed side portions above and adjoining said opposed lower side portions at a relatively slower mean rate, the rate of cooling said last-named opposed side portions decreasing progressively throughout their lengths from bottom to top thereof relatively less rapidly than the decrease of cooling rate from bottom to top of said opposed lower side portions.

3. In a method ofproducing' big-end-up ingots, conning molten ingot metal to so shape the resultant ingot Vthat the volume of ingot metal per unit'of height increases at a predetermlned substantially uniform rate upwardly throughout the lower portion of the ingot comprislng from. 20% to 50% of the ingot height, and the volume of ingot metal per unit of height in the ingot portionabove'and adjoining said lower portion increases at a lesser predetermined rate; abstracting heat at rates per unit of surface area which decrease progressively and relatively rapidly from bottom to top of said lower portion; and abstracting heat from the ingot portion above and adjoining said lower portion at rates per unit of surface area which decrease progressively but relatively less rapidly from bottom upwardly.

4. In a method of producing big-and-up ingots, chill-cooling the bottom and lower portions of at least two opposed ingot sides for a distance of approximately one-third of the total length o1' the ingot at a relatively rapid mean rate, the rate of cooling said opposed lower side portions progressively decreasing relatively rapidly and substantially uniformly throughout said opposed lower side portions from bottom to top thereof; and chill-cooling the opposed side portions above and adjoining said opposed lower side portions at a relatively slower mean rate, the rate of cooling said last-named opposed side portions decreasing progressively throughout their lengths from bottom to top-thereof relatively less rapidly than the decrease of cooling rate from bottom to top of said opposed lower side portions.

`5. In a method of producing big-endeup ingots of generally rectangular cross section, chillcooling the bottom and lower` portions of the four ingot sides for-a distance of from 20% to 50% of the total length of the ingot at a relatively rapid mean rate, the rate of cooling said lower side portions progressively decreasing relatlvely rapidly and substantially uniformly throughutsaid lower side portions from bottom to top thereof; and chill-cooling the side portions above and adjoining said llower side portions at a relatively slower mean rate, the rate of cooling said last-named side portions decreasing progressively throughout their lengths from bottom to `top thereof relatively less rapidly than the decrease of cooling rate from bottom to top 1 of said lower side portions.

EMIL GATHMANN;

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