US2626436A

US2626436A - Ingot mold

Info

Publication number: US2626436A
Application number: US247047A
Authority: US
Inventors: Marburg Edgar
Original assignee: United States Steel Corp
Current assignee: United States Steel Corp
Priority date: 1951-09-18
Filing date: 1951-09-18
Publication date: 1953-01-27
Anticipated expiration: 1970-01-27

Description

E. MARBURG Jan. '27, 11953 INGOT MOLD 2-S!-IEETS-SHEET 1 Filed Sept. 18, 1951 Patented Jan. 27, 1953 INGOT MOLD Edgar Mar-burg, Pittsburgh, Pa., assiznor to United States Steel Company, a corporation at New Jersey Application September E8, 1951, Serial No. 247,947

7 s on. (or. 2%139) This invention relates to improvements in ingot I olds.

Molten steel in an ingot mold solidifies inwardly from the mold wall at-an extremely rapid rate initially. The rate of solidification decreases sharply to a depth of about 1V2 inches; beyond this latter depth it is relatively low and decreases slowly. Mathematically solidification follows Feilds equation, d=7t /t, in which d is the solidifled depth in inches, 3 is the time in minutes, and

is is a constant. The value of it varies from about 0.75 to 1.00, increasing with increase in mold wall thickness and with decrease in ingot width. The devalue increases in this fashion because in ingot molds, where the wall thickness is at least 2 inches, a much larger proportion of the extracted heat is absorbed in heating the mold wallsihan is lost by radiation. The thicker the wall, the more heat extracted and the more rapid the rate of solidification. For the same reason solidification is more rapid from the corners of an ingot oi rectangular cross section than from the sides,

the corners being subject to heat extraction by two adjacent mold walls.

ing ingot shell is constantly enlarging with respect to the volume of the contained liquid metal. As a result the surface of the liquid metal drops continuously during solidification, and a void or pipe, whose shape approximates an inverted cone, is formed in the middle of the ingot. Pipe may extend downwardly to mid-height or even lower in an ingot. Metal may solidify completely across an ingot at one or several levels near the top; such solidification is referred to as bridging. Pipe cavity above bridging, or pipe that is free from bridging is referred to as primary or open pipe; pipe cavity below bridging is designated secondary pipe. As the latter type of pipe is protected against oxidation, its surfaces may .be welded together when the ingot is rolled. Because primary pipe is subject to oxidation by contact with the air. however, its surfaces cannot be welded eiiec- Mtively by rolling; hence primary pipe may result in so-called lamination in the rolled product. De-

pending upon the application, a greater or less extent oi the upper portion of the rolled bloom containing primary pipe is cropped, that is, it is sheared ofi and rejected as scra metal.

To minimize pipe in mined-steel ingots, hot tops or collars of refractory insulating material (generally fireclay), which extend variably 2 to 8 inches inside the mold (depending upon the weight of ingot desired), are placed on molds prior to casting ingots therein. ingots, liquid metal is teemed from a ladle to fill the mold and the superimposed hot top; the metal contained in the latter is referred to as the sinkhead. The hot top conserves heat so that the sinkhead metal does not freeze so rapidly as does the metal next to the mold wall. By maintaining a reservoir of liquid metal suificient to compensate for all of the contraction in the ingot body, the hot top causes all of the pipe cavity to be confined to the sinkhead. After the ingot is rolled to a bloom or slab, the sinkhead portion is cropped, and rejected as scrap. With use of a hot top, sound steel can be obtained below about it per cent top discard; without a hot top, the top discard necessary to eliminate pipe may be 35 per cent or. more of the total ingot weight.

It has long been recognized as desirable that heat be extracted more rapidly from the bottom portion of a solidifying ingot than from the top portion. Desirably the bottom portion solidifies completely while there is still a relatively wide column of liquid metal in the top portion. Final solidification of the liquid column in the middle of an ingot proceeds rapidly in a vertical direction. The face of vertical solidification approximates a modified V-shape, with a rounded lower portion in any vertical section. The angle of the V as it approaches the top of an ingot is a cri-- terion of the internal soundness of the ingot. If this V-angle is sufiiciently wide, segregated material diffuses effectively into the sinkhead and the interior of the ingot is sound; that is, it is substantially free from pipe and porosity. ,If on the other hand the V-angle is sharp, segregation and porosity are much more liable to develo in the upper middle portion of the ingot. If the V does not reach the top, that is, if bridging occurs, secondary pipe and axial porosity "invariably develop below the bridging.

To secure favorable solidification from the base upwardly, mold walls taper upwardly and commonly are 2 or 3 inches thicker at the base than at the top. For example, the wall of a 24 by 24 inch mold may be 5 inches thick at the top and 7 inches thick near the base. Numerous experiments in which ingots cast in molds with in the casting of Thus temperature gradients 'are established,

which cause heat to fiow downwardly in the mold walls. As a result of this heat fiow, the normally slower solidification rates adjacent to the thin upper walls are accelerated, whereas the normally faster solidification rates adjacent to the thick lower walls are retarded. Thus downward heat fiow from the thin to the thick end of a uniformly tapered mold wall equalizes solidification rates throughout the height of the mold.

An object of the present invention is to provide improved ingot molds which effectively produce a desirable solidification pattern in the ingot, that is, the ingot solidifies transversely more rapidly across the bottom portion than it does across the top portion, and the solidifying surface advances vertically as a wide-angled V, centered on the vertical axis.

A further object of the invention is to provide improved ingot molds of a wall configuration which accelerates transverse heat extraction from the bottom portion of a solidifying ingot and decelerates transverse heat extraction from the top portion, which procedure is favorable to the production of sound ingots.

In accomplishing these and other objects of the invention, I have provided improved details of structure, preferred forms of which are shown in the accompanying drawings, in which: a

Figure 1 is a top plan view of an ingot mold which embodies features of the present invention;

Figures 2 and 3 are vertical sectional views of the mold taken on lines 11-11 and III-III respectively of Figure l;

Figure 4 is a horizontal sectional view of the mold taken on line IV-IV of Figure 2;

Figure 5 is a top plan view of a modified ingot mold which also embodies features of the inven-' tion;

Figures 6 and 7 are vertical sectional views of the modified mold taken on lines VI-VI and VII-VII respectively of Figure 5;

Figure 8 is a horizontal sectional view of the modified mold taken on line VIIIV1II of Figure 6; and

Figures 9, l0 and 11 are schematic plan and horizontal sectional views to illustrate the manner in which the constant k changes with differ-- ent proportioning of the sides and corners in a a 2:1 by 24 inch mold having side walls 5 inches t ck.

In the embodiment of the invention shown in Figures 1 to 4 the mold is of substantially square cross section and has a bottom It, side walls 42 and comer walls It. The side walls are of substantially uniform thicknes throughout their height, although if desired, they can have the usual vertical corrugations. The corners taper upwardly in longitudinal section, so that at the base. they are much thicker than the sides, at the top somewhat thinner, and at an intermediate height of the same thickness.

verse solidification is much greater than it would be in a mold of uniform wall thickness. As the corner thickness decreases upwardly, the k-value for solidification also decreases. At the hell where the sides and corners are of the same thickness, the rate is similar to that in a conventional ingot mold, where the sides and corners are of equal thickness in any transverse section. Above this height the ratio of corner to side thickness decreases progressively below unity. Hence the efficacy of corners as chills decreases to practical disappearance at the top of the mold. Thus the k-value for solidification across the top of the ingot is less than that for an ingot cast in a mold of uniform wall thickness, and is considerably less than the k-value for solidification across the base of the same ingot.

Heat fiow throughout a, mold constructed as shown in Figures 1 to 4 may be considered in comparison with that in a mold of conventional design As discussed previously, undesirable downward heat fiow occurs in conventional molds with tapered walls. As the side walls of the mold shown in Figure 2 are of uniform thickness, no temperature gradients are established .therein; hence there is no tendency for heat to fiow down-' wardly. In a conventional mold, such as Figure 10 shows schematically, the comers are of the same thickness as the sides, or at least bear a constant ratio thereto; hence the effect of the corners on heat extraction at the middle plane does not vary greatly throughout the height of the mold. Because of the continually varying ratio of corner thickness to side thickness in the mold shown in Figure 3, however, the tendency for heat to fiow from the middle of a side to a corner is maximum near the base, minimum at the top, and varies uniformly between these sections. Thus this mold is advantageously designed to cause increased rates of heat extraction, hence faster solidification near the base than across the top of an ingot.

As the wall thickness at the corners of the mold, shown in Figure 3 increases downwardly, there is a tendency for heat to fiow downwardly in the mold corners, which downward heat fiow is detrimental to the eifectiveness of the mold. As the temperatures at mold corners are consider ably lower than those at the middles of the sides of molds, however, the longitudinal gradients in the corners are considerably smaller than the transverse gradients from the middles of the sides to the corners. Further the longitudinal gradients are much longer than the transverse gradients. Hence the unfavorable downward heat fiow in the comers is considerably smaller than the tinctive feature of the modified mold is in the-- shape of the outer surface of the corners. This surface does not taper continuously as in Figure 3, but the taper in the upper and lower portions, designated II and ll respectively. is considerably less or none at all, and that in an intermediate portion 20, considerably greater. Heat extraction from an ingot cast in a mold constructed as shown in Figures 5 to 8 is generally similar to that described for Figures 1 to 4. However, the volume of mold metal in the upper portion of the mold shown in Figures 5 to 8 is less and that in the lower portion greater than the volumes of corresponding portions of the mold in Figures 1 to 4; consequently the difierence in heat extraction rates between the top and the bottom is greater in the mold in Figures 5 to 8.

The present invention has been illustrated as applied to a big-end-up mold, but all the principles thus far described are applicable as well to big-end-down molds. However, in big-end-up molds the effect of the base It on heat extraction by the side walls requires consideration. The base of a big-end-up mold is normally thicker than the side walls; hence the temperature of the base during the solidification of an ingot is lower than that of the side walls. Thus heat flows from these walls downwardly into the base,

just as heat flows transversely from the hotter middles of the sides to the colder corners. It has been mentioned that downward heat fiow in tapered mold walls is disadvantageous. In walls of uniform thickness, such as the side walls l2, however, a temperature gradient develops first only at the extreme lower portion of the wall. As solidification continues, this gradient proceeds upwardly, but it is always stronger in lower portions of the mold. Thus the effect of a closed base in conjunction with side walls of uniform thickness is to cause solidification to proceed desirably from the base to the top of an ingot. The heavier the base, the more eil'ectively it performs this function. For this reason, the bottom It in both mold constructions illustrated has been widened to the same widths as the comers, as indicated at 22. The widened portion can be of any height up to height of the center of the hemispherical portion at the bottom of the mold cavity. Big-end-down molds may be similarly thickened in the lower portion of their sides up to about a third of their height.

From the foregoing description it is seen that the present invention affords an improved ingot mold that assures that the bottom portion of an ingot solidifies transversely at a faster rate than does the top portion. The mold not only accelerates cooling of the bottom portion of the ingot, but actually decelerates cooling of the top portion.

While I have shown and described only certain preferred embodiments of the invention, it is apparent that other modifications may arise. Therefore I do not wish to be limited to the disclosure set forth but only by the scope of the appended claims.

I claim:

1. An ingot mold of substantially rectangular cross section in which the side walls are of substantially uniform thickness from bottom to top and the corner walls are of greater thickness than the side walls adjacent the bottom to accelerate solidification of an ingot in this portion of the mold, and of less thickness than the side walls adjacent the top to decelerate solidification in this later portion, as compared with solidification rates of .an ingot cast in a mold of uniform wall thickness in transverse section equal to the thickness of the side walls of the described mold.

2. An ingot mold of substantially rectangular cross section in which the side walls are of substantially uniform thickness from bottom to top and the comer walls taper upwardly and are of greater thickness than the side walls adjacent the bottom so that they can extract heat from the side walls to accelerate solidification of an ingot in this portion of the mold and are of less thickness than the side walls adjacent the top to retard solidification in this later portion, as compared with solidification rates of an ingot cast in a mold of uniform wall thickness in transverse section equal to the thickness of the side walls of the described mold.

3. An ingot mold as defined in claim 2 in which the corner walls in longitudinal section taper substantially uniformly throughout their height.

4. An ingot mold as defined in claim 2 in which the corner walls in longitudinal section taper but slightly in their top and bottom portions and taper abruptly in their intermediate portions.

5. A big-end-up ingot mold of substantially rectangular cross section in which the side walls are of substantially uniform thickness from bottom to top, the corner walls are of greater thickness than the side walls adjacent the bottom to accelerate solidification of an ingot in this portion of the mold and of less thickness than the side walls adjacent the top to decelerate solidification in this latter region, and the bottom of the mold extends outwardly below the side walls to further acelerate solidification in the bottom portion, as compared with solidification rates of an ingot cast in a mold of uniform wall thickness in transverse section equal to the thickness of the side walls of the described mold.

EDGAR MARBURG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,753,823 B1086 May 13, 1930