CHAPTER IX MOLDS AND POURING PRACTICE

Transcription

1 CHAPTER IX MOLDS AND POURING PRACTICE HERE is probably no phase of basic open-hearth steelmaking that T is more of an art and less of a science than mold and pouring practice, It varies widely from plant to plant; consequently it is obviously impossible to set down in detail in this chapter the many ramifications of practice that are now used. The discussion that follows therefore attempts only to give an outline of the more common methods, with examples sufficient to indicate some of the variations in procedure, and to emphasize a few of the many underlying problems that face the open-hearth operator at the pouring platform. MOLD DESIGN Ingot molds are hollow castings of considerable wall thickness. Despite this, non-uniformity in wall thickness should be strictly avoided to prevent the setting up of unnecessary stresses. Molds are cast in various forms, sizes, and shapes depending on the type of steel, and on whether the ingot is to be withdrawn from the mold or the mold from the ingot. The selection of ingobmold material, which is usually cupola or blasbfurnace iron containing approximately 3.5 per cent carbon, 1 per cent silicon, 0.9 per cent manganese, 0.2 per cent phosphorus, and 0.07 per cent sulphur, is governed by numerous considerations that affect mold life.(" Factors in addition to wall thickness that should be given primary consideration at the foundry are casting skin, shrinkage stresses, and casting temperature. Fadon in Mold Design. In use, mold thickness is of greatest importance, for it must be heavy enough to freeze the molten metal rising in the mold and to reduce the temperature of the steel below that of the melting point of cast iron to prevent the inner surface of the mold from melting, which would result in the welding of the ingot to the mold. The skin of an ingot, which solidifies immediately upon contact with the relatively cold walls of the mold, must soon become thick enough to contract on the fluid mass and pull away slightly from the sides of the mold. The separation of the ingot from the mold depends entirely on the heat taken up by the mold from the steel. 939

2 940 BASIC OPEN HEARTH STEELMAKING The shape of the ingot mold has a marked effect on the solidifying metal owing to pressure exerted by the metal and the mold. Molds which permit the solidifying metal to accommodate itself to this pressure are, in the order of their preference, rectangular, square, polygonal with convex inner faces, and round. The octagonal and hexagonal forms, with the correct curvature of the inner faces, are said to be equal to round ingots of a periphery equaling the circle which circumscribes the former and are an attempt to preserve the advantages and reject the disadvantages of the round ingot. One of the 'advantages of a round ingot is the ease with which surface defects can be machined off. Mold Design for Rimmed Steels. In the production of rimmed steel, the general or overall dimensions of molds are of primary importance. Flat sided square Rippled orcormgo'kd Fluted or rectangular Flg. 67. Typical mold aon-aodlona for rimmed ateelr. Mold contour is not so important for rimmed steels as for other types that are more sensitive to cracking. Square molds at least 25 in. in cross-section lend themselves readily to the pouring of rimmed steel and seem to be of the right proportion so far as the solidification rate is concerned. Molds of 20 in. or less cross-sectional dimension are not so satisfactory as the rates of rise during pouring and of solidification are too great (see Chapter VIII). Where ingot cracks are not a problem, a flat-sided mold (Fig. 67A) is usually preferred because of less tendency to promote development of scabs and because there is a decided increase in the life of the mold when compared with rippled or corrugated molds (Fig. 67B). The latter are used to avoid excessive ingot cracks by increasing the initial thickness of the ingot skin through the promotion of faster freezing by increasing the inner mold-wall area by curvature. Mold height plays an important part in deteriing the internal structure of rimmed steel. Too much height produces a high ferrostatic pressure that retards gas formation in the lower part of the ingot which, unless compensated by very slow pouring, will result in a thin skin in this section. Such difficulties become apparent & ingot height is increased above 70 in. Comer ~alii of either rectangular or square rnolh are generally kept within the

3 MOLDS AND POURING PRACTICE 941 Fig. 68. Recbngular mold for rimmed ingob to be rolled into slabs. (Courtesy ShenangoPenn Mold Compeny.)

4 949 BASIC OPEN HEARTH STEELMAKING range of to 3 in. to prevent setting up strains in the ingot, which would produce an internal crack if the cornera were too well rounded, or an external crack if the corners were too sharp. Typical Mold Designs for Rimmed Steels. The cross-sections of three common types of ingot molds are sketched in Fig. 67. Flat-sided square molds (A) are commonly used for ingots that are to be rolled into small billets, flats, and sheet bar. Such molds are usually of relatively small dimensions, say 25 in. square or less, and about 70 in. high. The weight of the mold generally equals the weight of the ingot poured in it. Taper of the sides is from to 1 in. total for the mold length. Flatsided rectangular molds are used also for ingots that are to be rolled into slabs for plates, skelp, or other flat-rolled products. The sides or ends, or both, may be rippled or fluted (B- or C) as the case demands. Operating conditions usually determine the exact dimensions. A typical example of a rectangular mold for ingots to be rolled into slabs is shown in detail in Fig. 68. Note that some curvature is produced on the sides, a condition that reduces cracking during the early stages of solidification of the ingot. Corner radius is important in controlling vertical ingot cracks at the corner of the ingot. This particular mold shows a corner radius of 1.5 in. Some plants find that corner radii up to 3 in. suit their operating conditions. This slab-type mold offers good mold life compared to the ripple type (Fig. 67B) and also produces an ingot with a minimum of scabs when pouring and mold preparation are good. The size of this type of mold is governed by the finished slab size and the size of the slab mill or bloomer. However, the mold should not diverge too much from the 2 to 1 ratio of the cross-section. A drawing of a big-end-down rippled mold is shown in Fig. 69. This type is desirable for blooms and large billets where the avoidance of ingot cracking is an important factor. The deep ripples cause a rapid freezing of the skin of the ingot. The thick initial skin formed is able to withstand the pressure of the ferrostatic head created by very rapid pouring. The disadvantage of this mold lies in the fact that the projecting part of the ripple tends to burn off, particularly at the bob tom portion of the mold. This shortens the life of the mold and promotes scab formation on the ingot during the latter stages of the mold's life. Bottle-top Mold Design for Rimmed Steels. Bottle-top molds, used for producing mechanically capped rimmed steel, usually should conform to the same general characteristics as the rimmed-steel molds. As discussed previously (page 219), capped or bottle-topped steel I

5 MOLDS AND POURING PRACTICE Fig. 69. A big-end-down rippled mold for rimmed ingob to be rolled into blooms md large billeb. (Courtesy bmegie-lllinois Steel Corporation.)

6 244 BASIC OPEN HEARTH STEELMAKING develops a much thinner skin, and thereby has less segregation, than a full-rimmed steel owing to the early stoppage of the rimming action caused by the sealing effect of the steel contacting the cap. Molds with a cross-section, for example, of 26 by 53 in. lend themselves readily to bottle-top steelmaking, whereas molds of smaller sizes, in the neighborhood of 22 by 22 in., are likely to produce very thinskinned ingots unless pouring speeds are greatly reduced. The shoulder of this type of mold should be square to pe+t the trapping of any slag that may rise on the steel, and to prevent it from coming to

7 MOLDS AND POURING PRACTICE 245 rest under the cap. Slag beneath the cap may result in the development of defects through blowing of the ingot as the cap is removed. Exact pouring height is determined with more difficulty on the squareshouldered molds than on molds where the decrease in cross-section from the mold to the top opening is gradual. The cap of the mold should have a bulged bottom so that the solidifying steel is put under compression by the gas pressure of the ingot and hence materially helps the cap in the sealing operation. The size of the cap is usually determined by the equipment available for the placement and removal of the cap. Figure 70illustrates a typical bottle-top mold used in making capped steel. This particular mold is tapered somewhat from the mold wd to the top opening, a condition that facilitates control of pouring height but has the disadvantage of allowing slag to become trapped under the cap. Round caps and mold openings are aho frequently used in place of the rectangular form shown here. The opening in the top of the mold depends to a large extent upon what equipment is available on the pouring platform for placing the cap on the mold. Where cranes or mechanical means are available, a heavy cap of relatively large dimensions is desired. Where the cap must be placed on the mold by hand, a light cap is used. This usually restricts the size of the opening in the top of the mold and leads to difficulties in centering the stream properly during pouring. Molds for Semikilled Steel. Semikilled steels differ from the rimmed and mechanically capped grades in that a limited amount of deoxidizers is added to diminish gas evolution. This results in a quieter freezing, which tends to distribute the impurities more uniformly throughout the ingot. This type of steel probably offers greater freedom in mold design. In general, however, molds for semikilled steel are similar to the type used for open or rimmed steel, except that pouring height is not so much of a problem. Mold Design for Killed Steel. Killed steels, or steels more thoroughly deoxidized than semikilled steels, are poured in a wide variety of molds depending upon individual plant practices and equipment. In such molds two common factors need be considered: (1) the molds are usually a taper type with the big end up, and (2) a hot top is employed. An example of a mold for killed steel is sketched in Fig. 71. The big-end;up design prevents the ingot from bridging over during solidification; in other words, freezing occurs from the aides and bottom in such a manner that the last part of the ingot proper remdhing liquid is

8 Fig. 71. Mold used for killed steel, with stationary hot top. (Courtesy Valley Mould and Iron Corporation.)

9 MOLDS AND POURING PRACTICE 947 directly under the hot top. The mold shown in Fig. 71 has a stationary hot top that rests on a machined collar. The collar fits tightly on the mold to prevent steel fins from forming at this point and producing hanger cracks. Other types of hot tops permit the top to be moved up and down in the mold for adjustment of ingot weights. Most big-end-up molds have an opening in the bottom that is closed with a clay or steel plug before pouring. Other molds of the hottop type have a closed bottom. The closed bottom makes cleaning more difficult but does eliminate the use of a bottom plug. Polygonal molds are sometimes used for killed steels because of their favorable effect on ingot structure; however the decreased production in the blooming mill with this type of ingot is a distinct disadvantage at times of high operations. Also it is generally true that because fully killed steels are usually susceptible to ingot cracking, the molds are frequently heavily rippled, fluted, or corrugated (Fig. 71). HOT TOPS, STOOLS, AND COATINGS As killed-steel ingots invariably pipe deeply in solidiication, a sinkhead or hot top is usually employed in connection with the mold. There are two major types of hot tops in prevalent use; the first is a brick-lined steel shell and the second is a refractory made entirely of clay. Both types have a volume of approximately 15 per cent of the volume of the ingot, and trouble from ingot piping may be expected if this ratio is materially reduced. Hot Tops for Killed Steel. The steel-shell brick-lined hot top usually requires relining only after a hundred or more heats; however, the inner surfaces of this type of hot top are repaired between heats by coating the bricks with slurry so that a smooth and even contour is maintained. A rough surface or improper taper in the hot top will, of course, cause "hanging" of the ingot and will produce transverse cracks. In most cases, this type of hot top can be adjusted, by the use of wooden wedges, to various heights in the mold. A few moments after the pouring of the ingot, the wedges are removed so that the hot top is free to settle as the ingot contracts, and it is from this condition that the term "floating hot top" is derived. Hot tops with the shells resting directly on the mold do not come under this classification. The refractory or clay hot top is used only once, for it is necessary to break it off the ingot during the stripping operation when the ingot is withdrawn from the mold (Fig. 72). This type of hot top can also be adjusted for various pouring heights by the use of wooden wedges.

10 248 BASIC OPEN HEARTH STEELMAKING Mold Stools. Molds of the type that are removed from the ingot (big-end-down), which are usually employed in the making of rimmed, bottle-top, and semikilled steels, require a flat stool, generally made of cast iron. Copper stools have been used, but the cost is excessive, for the stool must be entirely of copper owing to the fact that inserts Fig. 72. Ingots being stripped. (Courtesy Inland Steel Company.) melt out because of improper and insufficient heat transfer from the copper insert to the surrounding cast-iron stool. There are two factors that should be observed in stool design: many operators believe that the stool should be as thick as operating conditions will permit, up to a maximum of 15 in., to allow the stool to be cut out to a considerable degree before scrapping is necessary.

11 MOLDS AND POURING PRACTICE 949 I It will also be found that as the stool cute out to form a concave depression, the cutting action of the steel on the stool decreases and hence the stool life is increased. The second factor in stool design is the economic one of minimum size still compatible with good operating condi tions. Mold Inspection and Life. Because an ingot mold is subjected to sudden and extreme temperature changes, sooner or later it becomes cracked or fire checked. This condition first becomes evident at the bottom, then progresses steadily upward, until finally the entire inner walls may become covered with a network of cracks, hence adequate means of inspection of, molds must be set up if constant control of surface quality and conditioning costs of the ingots is to be maintained. The length of the life of a mold is usually measured by the consumption of metal (by erosion) in pounds per ton of steel ingots produced. This is calculated aa the ratio of mold weight to ingot weight produced; this value, however, ignores the important requirements for surface condition that have to be met by the ingot. The true measure of the useful life of a mold should be the appearance of the ingot and not an increme in mold life at the expense of satisfactory ingot-surface condition; the losses incurred by surface imperfection may be much greater than the total cost of molds per ton of steel poured. However, the expenses entailed in the acquisition and maintenance of molds amply justify a careful supervision of all factors influencing their life. An ideal practice is to have molds purchased at the same time kept on the same drag. This permits the use of several drags of molds with varying degrees of service, and the more exacting grades of steel can be poured into the better sets. Mold life is greatly affected by the pouring cycle. When this cycle is too short to allow the mold to cool below 350 F. (175OC.), a distinct lowering of mold life will occur, and defective ingots will be produced. Molds should not be set too close to each other on the buggies aa radiation from the adjoining sides may cause premature failure in the walls at this point. Failure in general should be about evenly divided between cracked molds and molds discarded because of fire checking. If loss from cracking is less than 50 per cent, it would seem that the molds are too soft, and fire check too eaaily. To prevent carelessness and incorrect use of molds, it is essential that adequate records be kept, and the time at which a mold should be scrapped should depend upon the product being made. After this economic factor has been definitely determined by each plant, mold life or condition can be governed accordingly. For example, flat-sided molds of an average size of 21 by 40 in. should

12 250 BASIC OPEN HEARTH STEELMAKING produce in the neighborhood of 600 tons of ingots with an expected decrease in this figure if the mold walls are corrugated or rippled. Big-end-up molds will show a further decrease in life. Molds that have outlived their usefulness should be scrapped, and this should be determined at the mold-preparation station where molds should be inspected regularly. Mold Cortings. Mold preparation or coating is of vital importance to subsequent surface quality of the ingot, and realizing this fact, nearly all modern plants have extensive preparation yards and buildings. Yards should be under cover to protect molds from the weather, especially in plants that use a variety of molds and have numerous sets idle for extended periods of time. Unclean, dirty, and rusty molds induce local surface reactions resulting in ingot defects. If molds are not thoroughly dry, sufficient steam to cause considerable agitation of the metal will develop and will produce ingot defects. To cast in cold molds or hot molds not only produces defective ingots but also reduces mold life rapidly. The best temperature for molds is generally considered to be between 200 and 300 F. (95 and 150 C.). There is one point on which all investigators agree: the molds must be cooled sufficiently before re-use, because undesirable results are obtained when molds are used while still too hot. It is not only that the protective coating burns or refuses to adhere; more important, the formation of a network of deep cracks occurs at a much earlier stage and in a much more pronounced manner, ultimately leading to the peeling off of entire areas located on the inner sides of the molds. Coatings have one function to perform; i.e., preventing undesirable material from sticking to the sides of the molds ahead of the rising steel. The perfect coating should evolve a gas at the point of contact with the molten steel as it rises in the mold. This evolution of gas forces foreign material away from the wall and hence prevents its being entrapped in the skin or surface of the ingot. Several materials have been used for mold coatings, and those now in general use are liquid tar, powdered pitch, sugar, powdered aluminum, and graphite. When liquid tar is used, cold molds produce excessively heavy coatings which react with the rising steel to cause a boiling action; on the other hand, extremely hot molds will coke the tar and cause pitting of the ingot surface; the best range for inner mold-surface temperature is probably about 250 to 350 F. (120 to 175 C.). Liquid tar is applied to the molds by various methods; those more generally used are: (1) lowering the molds into a tank containing a fixed-height pressure spray,

13 MOLDS AND POURING PRACTICE 951 which atomizes heated tar and allows it to settle on the mold walls as a finely dispersed cloud; (2) lowering the molds into a tank over a device which permits a flow of heated tar to fill the mold; or (3) dipping the molds into a tank containing heated tar, which coats both the inside and the outside of the mold. Powdered pitch is also applied by various methods, the most common being the use of an air syphon to draw the pitch from a conveyed container; or by inserting a funnel in the air line so that the powder may be introduced to the stream of air. Various grades of sugar are used, and application to the molds generally conforms to the methods used in the coating with pitch. Powdered aluminum suspended in a vehicle is applied by means of a spray gun or a brush. VARIETIES OF POURING PRACTICE, Top Pouring. Probably 85 per cent or more of all steel made today is top poured, in which case the stream goes directly into the mold, and each mold is poured separately. A perfectly good heat of steel in the furnace can be ruined by any of several subsequent procwing details, one of the most important being pouring. Some of the details necessary to good pouring practice are the following: 1. When the ladle is brought to the pouring platform, the stopperrod assembly shown in Fig. 73 should be raised slowly, and preferably by one man. If it is raised too high-as might happen when two or more men are operating the leve~it will be lifted out of the well and cause a running stopper. 2. If a stopper is hard to open because of freezing of metal around the stopper head, a wooden pricker should be inserted in the nozzle and the stopper head forced upward by lowering the ladle on the pricker. 3. The stream of metal should be well centered over the mold at all times so that it does not strike the walls.!liquid steel hitting the mold walls causes a scabby ingot surface and shortens the life of the mold. 4. The ladleman should open the nozzle so that the initial stream runs slowly to form a pool of metal in the bottom of the mold before the nozzle is opened wide. This pool will "cushion" the full force of the stream and keep splashing to a minimum. 5. Care should be taken to avoid a fan-shaped stream as this causes scabs or a shell to form on the ingot surface. If heats are on the cool side, "tits " or "jiggers " frequently form around the nozzle which in turn cause a spray or fan-shaped stream. These tits can be easily knocked off while the ladle is being moved to the next mold.

14 959 BASIC OPEN HEARTH STEELMAKING 6. pressure should be kept on the stopper rod.while the ladle is moved from one mold to the next to maintain a,tight set of the stopper head.on the nozzle. Fig. 73. Sectlon through pouring ladle showing mtopper-rod assembly. (Court- Republic Steel Corporation.) 7. The nozzle should be opened up with oxygen if the steel is too cold and is freezing in the nozzle. The judicious use of oxygen prevents many a frozen stopper. 8. In pouring killed steel in big-end-up molds with hot tops, care must be taken to regulate the pouring at the junction of the mold and

15 MOLDS AND POURING PRACTICE 253 hot top to avoid fins; these cause hanger cracks when the ingot freezes. Tun-dish or Basket Pouring. AB an alternative to pouring the steel directly from the ladle into the molds, an intermediate pouring vessel Fig. 74. Pouring ingots with a tun dish. may be used. Such vessels are termed pouring baskets or tun dishes (see Fig. 74). The method usually involves pouring from the tapping ladle through a large nozzle into the vessel, and pouring two molds simultaneously from the vessel through small nozzles at either end of the dish. This method of pouring has three advantages: (1) greater opportunity.for separation of nonmetallic inclusions from the metal;

16 254 BASIC OPEN HEARTH STEELMAKING (2) less splashing in the mold because of the reduction in the ferrostatic head; and (3) the possibility of maintaining slower pouring rates, which produces better surface on the ingot. It is necessary, however, when this method is used, to tap the steel at a higher temperature than is standard practice. Bottom Pouting. Bottom pouring is used in special cases where an unusually good ingot surface is desired, and entails extra production costs over and above those for regular top-poured steel. In bottom pouring, molds are set up on special stools in groups of two or more; a Section A-A Fig. 75. A six-mold stool for bottom pouring. Company.) (Courtesy Shenango-Penn Mold six-mold stool is shown in Fig. 75. The stools have grooves in them in which the runner brick are placed and openings are set for the num-, ber of molds to be poured in one group. A mold is set over each opening in the group. A fountain or center-pouring device is placed in the center of each group which connects with the runner to each mold. In bottom pouring the metal rises steadily in the mold with very little agitation, as the main force of the stream coming from the ladle is absorbed in the down gate and runners. Under these conditions of pouring the stream of metal from the ladle is invariably maintained at a much more continuous rate than in top pouring. Bottom pouring offers the further advantage that the number of ingots poured simultaneously can be varied to control the rate of rise in the mold.

17 MOLDS AND POURIN,G PRACnCE 255 The brick lining of the runners may. be severely eroded by highly oxidized rimming steels, which constitutes a possible source of inclusions. Steels killed with aluminum do not attack the runner brick as do open or rimmed steels. POURING VARIOUS GRADES OF STEEL Numerous factors such as rate of rise, bottom splash, the stream striking the side of the mold, and others are common to all types of steel so far as pouring is concerned. It is convenient to think of an ingot on which pouring has just been completed as a paper bag filled with water. If too much water is put in the bag, the bag will break. The same condition is true of a steel ingot. Immediately after pouring, there is a solidified steel shell filled with liquid metal. If the shell cannot withstand the ferrostatic pressure, it will break, and an ingot crack is formed. Therefore to prevent ingot cracks, a skin must be produced on the ingot that is strong enough to withstand the ferrostatic pressure. Two factors are involved: - First, the skin must be continuous, i.e., it should have no weak spots such as are caused by steel splashing up ahead of the main body and solidifying against the sides of the mold. When this happens the red hot blob thus formed is oxidized on its surface, the main body of steel does not weld to it thoroughly, and a line of weakness is formed in the ingot skin by the oxide. Second, even if the skin is of good uniform quality, it must still be thick enough to withstand the ferrostatic pressure present all during the period when the mold is being filled. Each type of mold has a definite pouring rate so far as cracking is concerned. A rippled or fluted mold can be filled more rapidly than a straight-walled mold, because the larger exposed surface of these types will result in a thicker skin of solidified steel being formed in the same period. Other surface defects, such as scabs and slivers, are the result of bottom splash, the stream striking the side of the mold, and ragged pouring streams that cause excessive waves in the steel as it rises in the mold. Bottom splash means steel splashing off the stool as the nozzle is first opened. This steel sticks to the side of the mold, as high as 15 to 18 in. from the bottom, and the inner surface is immediately oxidized, which prevents the bottom-splash material from fusing to the main body of the steel, and a scab results when the ingot is rolled. This condition is intensified if the steel is slow in rising in the mold. Also the shell formed by the bottom splash contracts away from the mold at its upper part, and as the steel rises over this

18 956 BASIC OPEN HEARTH STEELMAKING it runs down between the shell and the mold wall, thus producing another discontinuity in the skin of the ingot. Quite often a crack develops through this line of weakness, which is usually termed a butt crack. Scabs are also formed by the stream striking the side of the mold. The splashed metal forms a layer of steel at the point of contact that does not weld to the main body of the ingot entirely and is too thick to be removed by the scaling action in the soaking pits. Ragged pouring stream or temporary changes in pouring rate resulting from manipulating the nozzle often cause excessive churning in the rising steel. If the waves thus produced are large enough, they remove the coating from the mold and the steel sticks to the mold wall which leads to conditions quite similar to those just described. Pouring Semikilled Steels. In pouring an ingot of semikilled steel the following practice is usually observed. The steel pourer opens the nozzle as easily as possible to minimize the bottom splash. As.soon as a cushion of steel has formed on the stool, say 3 in. deep, the nozzle is gradually opened to a full stream. If the interval between the initial opening and the time at which the bottom splash on the sides of the mold is covered is too great, the possibility of scab formation is increased. Splash pans made of thin steel sheets are sometimes used to prevent bottom splash. The stream is allowed to run full until the ingot is about 6 in. from the desired height. At this point the stream is reduced to about half volume. During this period of slow pouring, aluminum is usually added to complete the deoxidation of the ingot. Small-sized granular aluminum is generally preferred to shot aluminum as it can be added much later to the ingot. The usual practice is to add just enough to the top to prevent the ingot from breaking through the solidified top skin and bleeding. In other words the steel has been deoxidized to the point where formation of gas from the 0 + C reaction is not sufficient to rupture the top skin. If rupture occurs, steel is forced out onto the top of the ingot, and skin blowholes form directly below the rupture. If excmsive aluminum is used, the top of the ingot presents a sunken appearance, and excessive pipe may be experienced. The ideal condition is to have the top slightly bulged. This indicates that some gas is prment and the pipe is at a minimum. Pouring Capped Steel. As noted in Chapter VIII, capped steel is made by pouring a slightly rising rimming steel into a bottle-top mold and applying a cast-iron cap. This is illustrated in Fig. 76. As the steel rises against the cap, it freezes and the rimming action is mechanically stopped. Perhaps the best way to explain the pouring of capped

19 MOLDS AND POURING PRACTICE steel is to describe the action of an ingot from the time pouring is started to the time.it is stripped. The ladle, is centered over the mold, and the steel pourer opens the nozzle slowly to prevent bottom splash. As some splash is bound to occur, however, unless splash pans are used, the nozzle of the ladle is fully opened as soon as a cushion of steel some 3 in. deep is formed on the bottom of the mold. The full stream brings the molten steel up over the splash that is sticking to the side of the mold, some 6 or 8 in. above the stool, before C; ,JZ., Fig. 76. Ingot-mold cap. (Courtesy Republic Iron and Steel Company.) the frozen shell has sufficient time to oxidize excessively on its interior surface; if it does oxidize it will not fuse readily with the molten metal and a large scab results. The ingot is poured to within a foot of the top of the mold, and shot aluminum is added slowly to settle the metal so that the maximum amount of steel may be poured into the mold and also to make certain that the steel expands somewhat. If the amount of aluminum necessary at this point is excessive, some aluminum should be added to the bottom of the ingot immediately after pouring begins. Wild heats often have a tendency to bootleg on the bottom (see page 259)) owing to the excessive action caused by metal freezing at the bottom as well as the sides, and the addition

20 958 BASIC OPEN HEARTH STEELMAKING of aluminum to the bottom of the mold will prevent this as well as avoid the excessive use of shot aluminum at the top. To complete the pouring of the ingot, the stream is reduced and the 'mold is filled as full as possible. The ingot should be rimming uniformly by this time and rising slowly. The heavy cast-iron cap is placed over the top opening and the steel is allowed to rise against it. The steel freezes, sealing the top of the ingot, and rimming action stops as the gas pressure builds up inside the ingot. To obtain the ideal type of capped ingot it is necessary first to have a steel of the necessary rimming properties to form a rim zone of sufficient thickness during the interval between the beginning of pouring and sealing at the cap. Two factors can prevent the formation of a proper rim zone in this type of ingot. First, if the steel is too low in Q, it will not rim actively enough to cleanse the gas bubbles from the skin on the lower part of the ingot. This is the same condition that causes water-marked ingots on ordinary rimmed steel of poor rimming properties. Second, if the time interval between the beginning of pouring and sealing is too short, the ingot rim will not have time to grow to sufficient thickness even though the steel is of good rimming quality, hence it follows that it is difficult to make capped steel in small molds, say smaller than 20 by 20 in. in crosssection, unless the pouring rate is greatly reduced. Conversely, if the ingot is too large in cross-section or if the pouring rate is too low, the rim will be excessive, and the ingot structure approaches that of a full-rimmed ingot with the resulting segregation and spongy top. Ideal conditions must be worked out for each shop, so far as mold size, pouring speed, and capping time are concerned. Once these ideal conditions are established, it is possible to make capped'steel with up to 0.30 per cent carbon with good surface and with yields higher than possible with any other type of ingot structure. Pouring Rimmed Steel. Rimmed and capped steels are more difficult to pour properly than are the killed or semikilled grades. The same practice is used in opening the nozzle on rimmed steel as on semikilled steel. In addition to the bottom splash, however, there is another condition to contend with on rimmed steels. When a few inches of steel are in the bottom of the mold, the steel is giving off gas throughout its entire volume. This is caused by the chilling effect of the stool and mold walls; this may cause the steel to foam up in the mold as high as 18 in. above its normal level. As more steel-is poured into the mold and as the bottom of the ingot freezes to a greater thickness, the cooling effect of the stool is loat and the gas from this source

21 MOLDS AND POURING PRACTICE 959 ceases to form. The steel will then settle rapidly, leaving a shell or "bootleg" sticking up against the sides of the mold. As the main body of steel rises over this shell, the same conditions and results are obtained as in excessive bottom splashing. Bottom bootlegging occurs if a heat is excessively "wild" in the mold. A corrective measure is to add aluminum immediately upon opening the nozzle to prevent the release of excessive gas at this early stage. As the steel continues to rise in the mold, more bootlegging may be experienced if the heats are excessively wild. In pouring these types it may be necessary to add aluminum to the metal until the mold is full. When the metal in the mold is about 6 in. from the desired height, the stream is retarded and a final aluminum addition is made if necessary, and the mold is then filled slowly. The steel should be rimming well at the end of pouring, and most plants prefer to see the ingot rim in flat, i.e., with no appreciable expansion or contraction. If the ingot grows in the mold, it is an indication of excessive blowhole formation in the solidifying rimmed zone. This condition may be caused by insufficient oxygen content in the steel or by the use of too much deoxidizer. Growing ingots are shadowed or "water-marked" when stripped if the growing is excessive. The term water-marked is used because the ingots look as if they had been immersed in water since the bottom portion is much darker than the top. This is due to the blowholes near the surface at the bottom of the ingot that insulate the skin of the ingot from the - hot steel in the interior and hence allow a rapid cooling effect over the surface of the lower portions. The poorer the ingot has rimmed, the higher will be the dark portion of the ingots. Water-marked ingots must be heated carefully for if the skin is removed by scaling in the soaking pits, oxygen will penetrate into the blowholes, thus preventing complete welding of these holes during rolling. Pouring Killed Steel. Killed steel does not need the care on the platform that is necessary with a semikilled or rimmed steel. Bottom splash must be taken care of in the same manner, and the steel should, of course, rise as quietly in the molds as possible. When the steel rises to the hot top, the stream should be slackened so that a tight seal is made between the mold and the hot top. The hot top is then filled slowly, and after the stream has been shut off, an insulating compound is usually applied to the top of the ingot to increase further the.efficiency of the hot top. Killed or thoroughly deoxidized steel is generally poured in inverted or big-end-up moldy with suitable hot tops. The mold (see Fig. 77)

22 960 BASIC OPEN HEARTH STEELMAKING is designed to bring about progressive freedng from bottom to top and the hot top functions as a well or reservoir for the last portion of the metal to solidify, thereby centering the pipe cavity in that area., To avoid heavy top discard during rolling-that is, cropping below the sink-head-it is important that this last metal poured be kept fluid until all the lower part of the ingot has solidified. >To accomplish this purpose, two methods have received consideration; viz., (1) adding heat and (2) conserving heat already in the molten metal. Flg. 77. smooth ' Big-end-up mold used for killed <steel. Company.) (Courtesy Shenango-Penn Mold Heat may be imparted to the sinkhead by burning gas, or by electrical means. ' These methods, however, are costly and are not in wide use. A number of patented compounds are on the market that develop heat through exothermic reaction when applied to the top of the sink head. The materials that are commonly used for simple insulation of the sink-head top are sand, coke breese, and fire-clay. In adding any carbonaceous material, care must be exercised to avoid carbon absorption in areas adjacent to the sink head. Some plants spread straw over the ingot top before the carbonaceous insulating material is added. REFERENCES 1. IRON AND STEEL INBTITUTE): report[^] on the Heterogeneity of steel Ingots, J. Iron Steel Inst., v. 117, 1928, pp Special Report No. 2, 1932, 267 pp. Special Report No. 16, 1937, 238 pp. (section VI). Special Report No. 25, 1939,650 pp. (mtion VIII).