The Solidification of Steel Ingots

Transcription

1 The Solidification of Steel Ingots Steel has been chosen as the metal whose solidification will be used to tie in the principles discussed in the previous papers. Although steel is the most important BY B. R. QUENEAU; ME~ER AIME tion of specific steel products, and it should be emphasized that no one type should be considered superior to the others. Fully deoxidized steels, known as killed 1 I I I l ~ T I I I I I I I I I I I I I.O1 a02.a.a or.m.oo.lo.ll.ls.l3.i4.i4.i6.ll el0.l#.m hr C.n< Cvbn In htb FIG I-RELATION BETWEEN CARBON AND OXYGEN IN LIQUID IRON (MAR- SHALL AND CHIPMAN). I atm, 2805'F. Open-hearth curve: Lime-silica ratio 2.5 to 3.5; 10 to 20 pct FeO in slag for carbon above 0.1 pct. practical example that could be chosen, its solidification is complicated by the presence of many elements added either intentionally or present as impurities. The liquid steel bath in an open-hearth furnace contains carbon, manganese, phosphorus, sulphur, and many residual alloying elements such as nickel, copper, and molybdenum. The bath also contains oxygen, the concentration of which is a function of the carbon content, as shown in Fig. I. Deoxidizers such as silicon and aluminum may be added either before or after tapping depending on the grade of steel being made. There are four types of steel ingots: Killed, semikilled, capped, and rimmed, and these differ from each other in their state of oxidation. Each type of steel has advantages in the produc- * Chief Metallurgist. Duquesne Works, U.S. Steel Co.. Duquesne. Pa. steels, have little or no gas evolution on solidification. When the steel solidifies in the mold, shrinkage occurs which causes a large void known as "pipe." TO minimize the amount of metal that has to be discarded on account of pipe, a big-end-up mold is used together with a refractory "hot top" which supplies molten steel to the main body of the ingot while solidification proceeds, Fig. 2. A section through a 32x32 in. ingot is shown in Fig. 3. The "hot top" volume is about 15 pct of the ingot, and the yield from killed steel in billet form is about 80 pct of the ingot weight. High quality machinery and tool steels are rolled from killed-steel ingots, but at the present time this represents less than 20 pct of the total steel production in the United States. In order to reduce cost and to increase

2 B. R. QUENEAU 53 the ingot yield, mild steel for structural purposes is not fully deoxidized and thus is known as semikilled steel. Some carbon monoxide gas is liberated on solidification, forming blowholes in the steel on solidification. The presence of these blowholes minimizes piping by distributing small voids throughout the ingot, Fig. 4. If not exposed to the air, these blowholes weld together during rolling, and the product yield is close to 90 pct of the ingot weight. Structural steel for the Empire State building, the girders of the George Washington bridge, and the plates of the U.S.S. America all were made with this grade of steel, and the production of semikilled steel amounts to over 30 pct of the total. With low carbon, highly oxidized steels, carbon monoxide gas is evolved rapidly during initial solidification and results in an outer rim of nearly pure iron. If the carbon-oxygen reaction is allowed to go substantially to completion, the product is called rimmed steel, Fig. 5. If the reaction is suppressed after a short time by preventing further evolution of gas from the top of the ingot, the steel is called capped steel. Capping is the process of closing the top of the ingot and can be accomplished either mechanically by covering a bottle-top ingot with an iron cover, Fig. 6, or by an addition of aluminum or ferrosilicon to the top of an open ingot. These methods of capping cause a solid layer of steel to be formed across the top of the ingot and thus prevent further gas evolution. In capped steels, the rim is thinner and there is less segregation or concentration of impurities than in rimmed steels, Fig. 7. The presence of the nearly pure iron rim on both of these grades results in an excellent surface finish on flat rolled products, and therefore they are used extensively in the production of sheet and strip. The product yield of these grades is slightly higher than that of semikilled steel. Approximately 50 pct of the total United States production is either capped or rimmed steel. Many steel defects are incident to pouring of steel into molds and solidification. In addition to segregation and nonmetallic inclusions which are present in all steel ingots, TOP PLUC L - - METALLIC PLUC FIG 2-KILLED INGOT IN BIG-END-UP MOLD. scabs are formed on the surface by steel splashing on the stool or the bottom of the mold and may remain on the billet after rolling, as in Fig. 8. An irregular mold surface will interfere with the normal contraction of the ingot and transverse cracks in the ingot skin may result, Fig. 9. Any crack or blowhole at the surface will roll out into a seam, sliver, or other surface defect as shown in Figs. 10 and 11. Pipe, segregation, and inclusions are defects in the steel which have to be controlled by correct steelmaking practice and ingot mold design and cannot be remedied once the steel is solidified. Surface defects such as scabs, seams, and cracks can be removed from the rolled product by scarfing with oxy-acetylene torches, grinding with abrasive wheels, or chipping with air hammers, or by machine. This short outline of defects in steel has been given to emphasize that the bottleneck to the production of high quality steel can be said to be the problem of solidification of liquid steel. The metallurgical dream of pouring molten steel into

3 54 SOLIDIFICATION OF STEEL INGOTS FIG. 3~(above) FIG. 4~(right) For analysis see page 56

4 FIG. s-(above) FIG. 7-(right) For analysis see page 56 B. R. QUENEAU 55

5 56 SOLIDIFICATION OF STEEL INGOTS FIG 6--TOPPING A STEEL INGOT. Shows height to which steel is poured, and cap ready for placing on mold. Solid black areas represent steel already solidified. FIG 3-SECTION OF 32X32 IN. INGOT WEIGHING 19,180 LB KILLED STEEL BIG END-UP MOLD. C 0.22 C 0.24 C 0.09 Locatio1l I II Ladle Analysis, Prt Mn P S Check Analyses, Pet Si 0.29 Location C Mn P S Si I ~ ~::~ ~:~~ ~:~~~ ~:~~~ ~:~~ ,28 II , ,043 0, , ,28 FIG 4-SECTION OF 29X66 IN. INGOT WEIGH ING 43,510 LB SEMIKILLED STEEL. Ladle Analysis, Pet Mn p Check Analyses, Pet S Location C M n P S I 0, , II FIG 5-SECTION OF 20X40 IN. INGOT WEIGH ING 12,600 LB RIMMED BESSEMER STEEL. Ladle Analysis, Pet,Mn P 0, Check Analyses, Pet C Mn P S S o 099 o 075 o o 056 o 019 o 023 o 021 FIG 7-SECTION OF 27X42 IN. INGOT WEIGH ING.22,000 LB CAPPED STEEL. Ladle Analysis, Pet C Mn P S Check Analyses, Pet Locotion C Mn P S , OIl OIl O.OIl ~~ g.~~ ~.~~ ~.~~~ ~:g~~ forming rolls or some other method of direct casting to semifinished product is a pleasant one not only from the viewpoint of quality, but also of cost. However, direct FIG 8-ScABS ON SURFACE OF 4 IN. SQ BILLET.

6 B. R. QUENEAU 57 casting of steel presents many difficulties even in forming small bars, while casting slabs for rollinginto sheet products appears to be insurmountable at the present time. 04 pct Mn. If the steel is ordered to the low side of this optimum, it is possible to add sufficient deoxidizer, usually aluminum, to the ladle to obtain the desired FIG 9-LoNGITUDINAL AND TRANSVERSE CRACKS IN INGOT. FIG IO--DEEP SEAMS ON SURFACE OF 4 IN. SQ BILLET. Thus for many years to come, molten steel will be teemed into ingot molds. RIMMED STEELS Production and Structure of Rimmed Steels In finishing a heat of rimmed steel in the open hearth, it is necessary to have the bath contain a large and controlled amount of oxygen. Since the analysis of the steel is the chief factor influencing the oxygen content, it follows that there is an optimum analysis for capped and rimmed steels which will vary somewhat from shop to shop but is normally about 0.08 pct C and amount of rimming action in the molds. However, in the case of steels whose ordered analysis is on the high side of the optimum, it is necessary to obtain as much FeO as possible in the steel bath. These heats must be worked on the cold side since the oxygen content is markedly increased with decrease in temperature, Fig. 12, and the slag must be kept high in FeO content by making late ore additions. Finally, the heat must be worked down low in carbon in the furnace to build up the oxygen in the bath before tap and then recarburized back to the desired carbon in the ladle. Unfortunately, the above conditions

7 58 SOLIDIFICATION OF STEEL INGOTS tend to decrease steel cleanliness which becomes progressively worse as the top limit in: carbon for rimmed steels is approached. In this respect, rimmed and capped steels FIG II-CLUSTERED SEAMS ON SURFACE OF 4 IN. SQ BILLET. are the direct opposites of semikilled and killed steels. In the latter steels, it is the lower carbon steels that give the most trouble from nonmetallic inclusions, the cleanliness of the steel increasing with increase in carbon content. On tapping a rimming heat, coal and ferromanganese are added to the ladle as 0..9 D.a 1 I lit _-+_---J,~-_ j I I il ~ D71j II------i--I I I I ~ D6 I ~ ~ I I -:S I I.~ 0.5 I " 1,g I \ ~.. -r-- -- \\-~ P., D.3c---r- -\-r- - - \. \ \ I 0.2 r--' I i\ r\. t \\',l ~s,",.,. I, t;t;,... r--... ::.. SO-,r) D.'", ~ " 1600C /... ~'~'" IZ, '2.,) I--.o-l ("=.: ~ ',0 l} lo I1'D 0. 0 o.s.. {J 4~ Percent FeD in Metal FIG 12-RELATION OF FEO CONTENT TO C CONTENT INBASIC OPEN-HEARTH STEEL. Finishing period, normal slag, low manganese and low phosphorus. Temperature 16oo C (2912 F) when tapped. Samples taken from beneath the interface. Dotted lines show corresponding curves converted from oxygen values as given by Herty and associates. required to meet the ordered analysis. About >4 lb Al per ton is added to the ladle on lower carbon grades, but the aluminum addition must be made sparingly so that the pourer can control the final deoxidation in the mold. On pouring rimmed steel into open top, big-end-down molds, a measured amount of aluminum shot is added uniformly throughout the filling of the mold. If the correct amount of aluminum is added, the characteristic rimming action will take place with the level

8 B. R. QUENEAU 59 of the ingot remamlng constant or rising slightly. As the top surface gradually freezes in from the mold walls, the liquid center becomes smaller in size. It is customary to cover the top of rimmed ingots with a piece of heavy steel plate after a specified time interval to stop the rimming action. Such an ingot is shown schematically in Fig. I3b. There is a row of elongated primary blowholes near the surface in the lower half of the ingot, another row of deep seated spherical blowholes the full length of the ingot (frequently called secondary blowholes), and also large blowholes scattered throughout the core of the ingot. If rimmed steel is overoxidized when poured, it will be wild in the mold and an excessive amount of gas evolution will occur in the early stages of solidification. Instead of rimming straight across, the level of the steel will drop forming a "bootleg," and additional aluminum must be added while pouring subsequent ingots. Such an ingot structure is shown in Fig. I3C. It will be seen that the rapid gas evolution on initial solidification was so vigorous as to prevent any gas from being trapped in the rim even under the high pressure existing in the lower half of the ingot. If the steel rises during rimming, insufficient gas is being liberated, resulting in primary blowholes forming higher up the ingot as shown in Fig. I3a. If no aluminum is being added to the mold and the steel coritinues to rise, sodium fluoride may be added on subsequent ingots to promote gas evolution. It will be seen by the above that, in practice, the quality of rimming steel is dependent to a large extent on the steel pourer. Since the oxygen content of the steel in the ladle at present cannot be controlled exactly, the steel is overoxidized slightly and correct rimming action is obtained by adding suitable amounts of aluminum during the pouring of the ingot. Since 1856 when Bessemer first made rimming ingots, a large number of investi- gators have published results of their studies, but there is no unanimity of opinion as to the mechanism of solidification in this grade of steel. However, A QVEROEOXIDIZEO,!,;' d r.~! B PROPERLY O O.xlOIZEO FIG I3-COMPARISON OF STRUCTURES OF RIMMED INGOTS. papers published in the late 1930'S by Washburn and Nead,1 Chipman and Fon Dersmith,2 McCutcheon and Chipman,3 Halley and Washburn,4 Hultgren and Phragmen,5 and Hayes and Chipman 6 greatly increased our knowledge of rimmed steels. This work has been summarized both. by Halley in the chapter on "Ingot Structure and Segregation" in Basic Open Hearth Steel Making 7 and by Larsen in a Symposium on Segregation held by the AIME in The following discussion is taken in large part from these two reviews. c UNOERD(OX 101 Zr 0 Blowhole Format-ion -in R-immed Steel When the steel is first poured into the mold, a chill zone or "skin" is formed first with small equiaxed crystals and with a composition similar to the ladle analysis. Gas evolution soon begins and the thickness of the solid metal, before the formation of the primary blowholes, is dependent on the relative rate of solidification and gas evolution. A schematic drawing showing the formation of these blowholes is given in Fig. 14. A bubble of gas is formed first at the liquid-solid interface as in Fig. I4a. If the gas evolution is fast and the liquid metal is moving rapidly, this bubble will be swept away. With slower gas evolution and little motion of the liquid, the bubble

9 60 SOLIDIFICATION OF STEEL INGOTS will grow as solidification proceeds, as shown in Fig. I4b and c. If the bubble is growing slightly faster than the wall is advancing, the protruding bubble will I4dz, ez, fz, and gz. This second type of bubble is the one usually formed and the blowholes have synchronized contractions and expansions as shown in Fig. 15..,. ~ "'. " " "" l ' " '. " '. FIG I4-DIA.GRAM SHOWING FORMATION OF RIM HOLE AND RIM CHANNEL (HULTGREN). FIG IS-RIM HOLES IN EARLY CLOSED INGOT. Axial section one-sixth from bottom. Unpolished, unetched, X 4, showing synchronized contractions and expansions, blunt inner ends.(hultgren). break off periodically as shown in Fig. 14dl' el, and g!. If the liquid metal is moving rapidly past the solid surface, the bubble will be swept away periodically carrying some of the gas already surrounded by solid metal, as shown in Fig. In the upper portion of the ingot, the rapid motion of the liquid sweeps the bubbles from the solid surface and no primary blowholes are formed. To prevent the formation of primary blowholes too close to the surface in the lower third of the

10 B. R. QUENEAU 6 I ingot, it is necessary to control the rate of pouring to permit thjs sweeping action to take place while the ingot is half full. Once primary blowholes start to form, they con- last metal to freeze. Hayes and Chipmane have determined these ratios, which they term the distribution constants, k, for a number of elements in steel. The tendency d! ; ; ; : a ; ;. ; OM, *"'a FIG 16-SEGREGATION OF COPPER, MANGANESE, AND SUL- PHUR FROM SURFACE TO CENTER OF AN 18x39 IN. INGOT OF RIMMED STEEL. SAMPLES THE HEIGHT OF THE INGOT. TAKEN AT )Q tinue to grow until the ingot is capped or freezes over. The pressure increases rapidly upon capping and gas evolution is temporarily stopped so that another zone of metal free of blowholes is formed. With further solidification an excess of carbon and oxygen at the interface results in further gas evolution forming the secondary blowholes. The core zone then solidifies without formation of pipe, since further gas evolution occurs as soon as shrinkage relieves the pressure. Thus, scattered blowholes will be found throughout the core. Segregation in Rintn~cd Steel The basic cause of segregation is the lower solubility of impurities in solid than in liquid iron. The metal that first solidifies is lower in impurities than the liquid, and the tendency of the different elements to segregate in liquid iron can be expressed as a ratio between the concentration of the element in the first metal to freeze and the concentration of the element in the to segregate is directly proportional to I - k. Segregation in rimmed steel is greatly affected by the gas evolution. As previously mentioned, the skin that is TABLE I-Segregation Distribution Constants (k) and Segregation Coescients (I - k) for a Number of Comnzoit Elements in Steel Element k I-k Sulphur Oxygen Carbon Phosphorus Silicon Manganese formed first has a composition similar ta the ladle analysis. As gas evolution begins, the liquid metal moves rapidly past the solid surface, removing the enriched film at the interface which produces a rim low in impurities. The percentage of impurities passes through a minimum at a distance of 2 to 4 in. away from the mold wall and reaches a maximum at the secondary blowholes, Fig. 16. Tlie composition does not

11 6 2 SOLIDIFICATION OF STEEL INGOTS vary much from the rim to the center, but there is considerable segregation longitudinall~ in the ingot. The amount of longitudinal segregation differed markedly less concentration of impurities in the core. Another major advantage of capped steels as compared to rimmed is that taller ingots can be poured. Whereas rimmed ingots r,,., ID YI.D a m rr o 1*001 "ICII. PTLI CC*l LWO'W,".,,,C,IO* *1 I0 =I u*, OT lm..lm* FIG 17-DISTRIBUTION OF SULPHUR IN A NORMAL R~MED INGOT (HALLEY AND WASH~URN).. I :. i. I ' :..',. 6 i......, I...? :,. r!..... : -. A B PROPCRLV OYERDLOIlDlZCD. DCOYlDlZrD in the ingots studied by Hayes and Chipman6 and those by Halley and Wa~hburn.~ This discrepancy probably can be accounted for by the difference in size of the ingots examined. The ingot split by Hayes and Chipman was an 18x39 in. ingot weighing 11,500 lb and had little longitudinal segregation, whereas marked longitudinal segregation was observed in the 29x43 in. ingot weighing 18,000 lb studied by Halley and Washburn, Fig. 17. In capped steel, the deoxidation practice is such that the steel will rise in the molds. The rimming action in the molds is stopped as soon as the steel makes a seal against the cap, and the ingots therefore have a thin rim. The structure of a typical capped ingot is shown in Fig. 18a. The primary blowholes are near the surface and extend two thirds of the way up the ingot. If the steel rises too rapidly and is capped in less than half a minute, the blowholes will be so close to the surface that on subsequent rolling they may open up and cause deep seams in the rolled product. Arapidly rising steel will have a continuous row of primary blowholes the full length of the ingot as shown in Fig. 18b. In general, the segregation in capped steels will be less than in rimmed steels because the short rimming time produces FIG 18-COMPARISON OF STRUCTURES OF CAPPED INGOTS. are usually limited in height to 75 in., capped ingots can be poured to roo in. with resulting increase in yield. The matter of product yield cannot be overemphasized in a discussion of these grades since they are sold in a highly competitive market and the price is markedly affected by the percentage of finished product obtained from a heat of steel. For instance, an increase in yield of only I pct in a mill making a million tons of ingots per year represents a gain of ~o,ooo tons of shipped product for little increase in cost. As stated in the introduction, the aim in producing semikilled steels is partially to deoxidize the heat so that the central pipe cavity will be kept to a minimum by the formation of blowholes throughout the ingot body. A common practice therefore is to make no deoxidizing additions to the furnace and to add manganese required to meet the melting specifications in the ladle together with a variable amount of ferrosilicon or aluminum, or both, depending on the grade of steel. Then, final control of deoxidation is obtained by adding aluminum shot during pouring in the mold. If suficient deoxidizer has not been added, the ingot top will bulge excessively from

12 B. R. QUENEAU 63 the internal gas pressure and may break open and bleed. Such a condition is shown in Fig. I9C. On the other hand, if the steel is overdeoxidized, the ingot top will sink necessary to form carbon monoxide. Hence, killed steels are usually considered to have no evolution of gas on solidification. This is not necessarily true, howeve.r, since both A OV(AOtO)(ID1Z(O B PROPCRLY DCOXIDIZED c UNOEROtOXIDIZEO FIG I9-COMPARISON OF STRUCTURES OF SEMIKILLED INGOTS. and assume a concave appearance. The concave surface indicates that considera'ble piping is present in the ingot as shown in Fig. I9a. The correct deoxidation practice will result in the ingot top being flat or slightly bulged as in Fig. I9b. The structure of semikilled ingots is similar to that of killed steels in that there is a well-defined chill zone of small equiaxed crystals, a columnar zone consisting of crystals preferentially oriented and an interior zone of randomly oriented equiaxed crystals. The segregation pattern is also similar, although it is somewhat modified by the gas evolution. In general, semikilled steel shows less marked segregation than killed steel, and the normal inverted "V" segregate found in killedsteel ingots may be absent. The excellent paper by Tenenbaum 9 on semikilled steels shows several examples of ingots without a well-defined inverted "V," Fig. 20. This question will be discussed more fully under killed steels. KILLED STEELS Production and Struct-ure of Killed Steels In the manufacture of killed steels sufficient quantities of deoxidizers are added to reduce the oxygen content below that FIG 2o-SnuKILLED JNGOT WITHOUT INVERTED V SEGREGATION (TENEl\'BAU~I). hydrogen and nitrogen are dissolved III steel in appreciable amounts and they may be evolved during solidification. The amount evolved will depend on the con-

13 64 SOLIDIFICATION OF STEEL INGOTS centration at the liquid-solid interface and on the pressure, and it is possible for them to form blowholes in a manner similar to carbon monoxide. It is necessary, there- tend to trap scum inclusions in the surface of the ingot, and surging of the steel frequently causes folds in the surface by liquid steel flowing down between the solidified skin and the mold wall. Fast pouring may cause excessive pressure within the ingot while the skin is thin and will produce skin ruptures and ingot cracks. On solidification, the structure of a killed-steel ingot consists of a zone of small equiaxed crystals followed by a zone of columnar crystals nearly perpendicular to the mold wall. In the remainder of the ingot, the crystals again are randomly oriented. These three zones are common to all killed steels, though their relative extent and position vary with composition of the steel and pouring conditions. In the chill zone, there is little time for segregation and the steel is nearly uniform in composition. Segregation oj Killed Steels FIG 21-TYPICAL SEGREGATION KILLED STEEL INGOT. A = Maximum positive segregation. B = V segregation. C = Center of ingot. D = Inner lines inverted V segregation. E = Outer lines inverted V segregation. F = Negative segregation. PATTERN OF fore, in,the production of killed steels to be sure to keep the gas content in the steel to a minimum. Final deoxidation and adjustment of the analysis of killed steels is accomplished in the ladle with no additions being made to the molds. Ferrosilicon is added to the ladle to produce a final silicon content of about 0.15 to 0.30 pct. Aluminum usually is added in amounts varying from fc lb per ton to approximately 3 lb per ton. The steel normally is poured into big-end-up molds with refractory hot tops. The rate of pouring should be controlled carefully for each mold size. Slow pouring speeds Microsegregation occurs in the columnar zone where the less pure liquid is trapped between the long dendrites. The tendency of the different elements to segregate has been covered under rimmed steel, but it should be noted that, without gas evolution, the average composition of the solid steel remains approximately the same as that of the melt. This is true, at least, for the first 5 or 6 in. of the ingot. Beyond this depth, the rate of growth of the dendrites is not sufficient to entrap the segregate, and the excess segregation appears to be rejected in advance of the face of solidification. This rejection of the less pure liquid causes macrosegregation in the ingot, which greatly affects its quality. A schematic drawing of the segregation pattern of a hot-topped ingot is shown in Fig. 21. The main points of this pattern are outlined below. Maximum positive segregation occurs in the top middle portion of the ingot at A. Although the most extreme segregation is restricted to the sink-head, considerable

14 segregation son~etimes extcnds do\vn into the ingot. \. segregation occurs in the midtilc partions of ingots as im1)erfectly formeti inverteti cones as shown from -1 to C. This segregation is generally more prominent in ingots of 18 to 25 in. width than in either sn~aller or larger ingots. Invcrted V segregation occurs in ingots as a series of truncated concs as shown at D and E. These slightly curved lines taper inwardly suggesting crude incomplete inverted V's whose apexes generally would he \\ell al~ovc thc top of the ingot. Negative segregation is present in the center of the lower half of the ingot as shown at F. The segregating elements are minimum in the lower part of the zone and increase further up the ingot to the center, C', where its composition approximates the average composition. Any theory on the segregation in killed steels during solidification must account for the complete segregation pattern, anti to datc none has been 1)resrnteti which is entircly satisfactory. The most authoritative work on the subject is probably that pul~lished by the su1)committce of the British Iron and Steel Institute in a serics of nine reports issued from 1926 to 193q.l~ But after this extensive research, thc subcommittee still \\-as unable to offer an explanation for the complrx phenomena occurring during solidification. It must be admitted frankly that the following hypothesis, which is held by many metallurgists today, is mainly conjecture. The zone of negative segregation is believed to be produced by the formation of free crystals within the body of the ingot which, on account of their higher density, settle to the bottom. The zone tends to be cone shaped since solidification from the sides is going on simultaneously with the accumulation of purer crystals on the bottom. The liquid displaced by the solid crystals moves upward in the ingot causing some of the less pure liquid at the liquid- solid interface to rise anti be trapped to form the inverted 1'. \Vhen solidification is nrarly com~)letc, the liquid from the hot top mov's do\vn into the body of the ingot forming the V segregate. The main arguments against this hypothesis are as follo\vs: I--If crystals are precipitated out of the liquid in the centcr of the ingot, the remaining liquid should drop in temperature before further solidification can occur. Ho\rever, experimental results indicate that the center of an ingot maintains a constant temperature during solidification. 2 The formation of the V segregate in the core of the ingot hardly can be accounted for by the flow of liquid metal from the hot top since the area immediately below the hot top is frequently sound and free of macrosegregation. 3 The formation of the inverted V by the cntra~)ment of impure liquid rising in the ingot \vould result in a streak substantially parallel to the mold walls. IIowever the inverted 1' actually slopes in the opposite tlirection. 1Vork on gases in steel at South \Vorks indicates that these invertrd V streaks are the result of gas evolution in the ingot. Some preliminary datalo were given in 1947 to show that hytlrogen markedly affected the segregation pattern of killed-sterl ingots. The introduction of hydrogen into the ingot resulted in the formation of many dark blotches in the macroetched sections of 12x12 in. blooms, Fig. 22. These rapidly etching areas were found to be highly alloyed and contained numerous sulphide inclusions. It is believed that it is possi1)le that thc concentration of hydrogen becomes sufficiently great at the solid-liquid interface to form a bubble. The hydrogen may then diffuse into the solid metal with time or escape upwards in thc ingot anti be replaced by liquid metal high in impurities. Studies of fractures through inverted V segregation show many fine dendrites present in the elon-

15 66 SOLIDIFICATION OF STEEL INGOTS FIG 22-STRUCTURE OF INGOT IN WHICH SEGREGATION IS INFLUEl'CED BY HYDROGEN. FIG 23-SECTION OF FRACTURED 34X6o IN. KILLED STEEL INGOT.

16 B. R. QUENEAU '. FIG 24-CROSS-SECTIONS OF SOUND KILLED STEEL INGOT FREE OF MARKED SEGREGATION.

17 68 SOLIDIFICATION OF STEEL INGOTS gated blowholes, Fig. 23. If care is taken in making a heat low in gas content, then the ingots will be sound and free of marked segregation, Fig. 24. As discussed previously, the segregation pattern of a semikilled ingot is modified by the formation of many blowholes during solidification. In the case of considerable gas evolution, the porosity may result in many small segregated areas with the complete elimination of the inverted V. In an attempt to cover the solidification of steel briefly, only some of the more imvortant characteristics of the different types of ingots can be mentioned. However, this lecture will have been successful if it has emphasized how little is known about the solidification of steel and the importance of continued research on the subject. It is especially necessary to make fundamental studies on the solidification of killed steels. Although no mold can improve the quality of steel poured into it, the necessary knowledge for improving mold design would minimize defects in steel at this stage in the production. I. T. S. washburn and T. H. Nead: Structure of Rimmed-Steel Ingots. Trans. AIME (1937) 125, P J. Chipman and C. R. FonDersmith: Rate of Solidification of Rimming Ingots. Trans. AIME (1937) 175, P K. C. McCutcheon and J. Chipman: Evolution of Gases from Rimming Steel Ingots. Trans. AIME (1938) 131, p J. W. Halley and T. S. Washburn: Dis- tribution of the Metalloids - in Rimmed - ~ Ingots. T&S. AIME (1938) 131, p., A. Hultgren and G. Phragmen: Solid~fication of Rimmine Steel Ingots. Trans. AIME (1939) 135;~ A. Hayes and J. Chipman: Mechanism of Solidification and Segregation in a Low- Carbon Rimming-Steel Ingot. Trans. AIME (1939) 13s P Basic Open Htarth Steelmaking. Committee on Physical Chemistry of Steelmaking. AIME, B. M. Larsen: Review of Factors Underlying Segregation in Steel Ingots. Trans. AIME (1945) 162, p M. Tenenbaum: Structure, Segregation and Solidification of Semikilled Steel Ingots. Trans. AIME (1948) 176, p. 108; Metals Technology (September 1947). I o. B. R. Queneau: Discussion on ref. g: Trans. AIME (1948) 176, p. 169; Metals Technology (June 1948). I I. Iron and Steel Institute (London) : Reports on Heterogeneity of Steel Ingots. First Report. Journal Iron and Steel Inst. (1926) 1x3, pp ; Second Report. Ibid. (1928) 117, pp ; Third Report Ibid. (1929) 1x9, pp ; Fourth Report. Iron and Steel Inst. Special Report NO. 2 (1932) 267 pp.; Fifth Report, Sprcial Report No. 4 (1933) 79 pp.; Sixth Report. Special Report No. g (1935) 236 pp.; Seventh Report, Special- Keporl No. ga (1936) 70 pp.; Eighth Report. Special Report No. 16 (1937) 238 pp.; Ninth Report, Special Report No. 27 (1939) 84 PP. DISCUSSION J. W. SPRETNAK*-I wish to discuss the probable role of vertical solidification in controlling ingot structures and segregation patterns. The data of L. H. Nelson12 on the transverse and vertical solidification in four killed-steel ingots were analyzed in terms of the kinetics of the two freezing processes. The plot of the vertical solidification is presented in Fig. 25. It is evident that two distinct types of curves are necessary to fit the data; the first of the form D = atn, and the second of the form D = a + bt2. There is evidence that the initial curve describes the columnar crystallization and that the second curve describes the equi-axed crystallization, just as the two sections of the transverse solidification curve describe columnar and equi-axed crystallization in killed-steel ingots. TABLE 11-Extent - of Vertical Solidification Vertical Hei ht Solidifi- of Height, Mold cation. Ingot. Ingot In. Ratio* L/W In. Pct 13x13 in. 17x17 In. ao in. 4 short 20 in. 4 long * Cross-sectional area of ingot divided by the cross-sectional area of the mold. The height along the axis of the ingot at which the vertical and transverse solidification would meet has been calculated, using the second section of the curves. The results of these calculations are presented in Table 11. Associate Professor of Metallurgy. Ohio State University. Columbus. l1 L. H. Nelson-Solidification of Steel in Ingot Molds. Trans. ASM (1934) 22, p. 193.

18 JENEAU 69 It 'is evident that the calculated height of the vertical solidification varies from 37 to 98 pct of the height of the ingot. A general trend to be noted is that as the ingot becomes "squattier," To have negative segregation occur in this zone, it would be necessary to have liquid of loner solute content feeding the interstices of the dendrites than that of the filler liquid FIG 25-PLOT VERTICAL SOLIDIFICATION IN KILLED-STEEL INGOTS. OF DATA BY NELSON ON the expected height of vertical solidification increases. The results of these calculations are presented graphically in Fig. 26. The solid-line cones indicate the height of vertical solidification based on the second sections of the curves. If the as~um~tion~isfmade that these ingots froze entirely as columnar crystals (actually they were columnar plus equi-axed), then the expected height of vertical solidification can be calculated by using only the initial parts of the transverse and vertical solidification curves; the results of these calculations are given as the broken-line cones. The latter calculation yields a cone which closely approximates the appearance of ingots that solidify entirely as columnar crystals. Thus, the equi-axed crystallization must play a vital role in the kinetics of freezing and in determining the height of vertical solidification. There are two questions I wish to ask the author: I. The negative segregation in the bottom half of the ingot is proposed to be the result of the solidification of purer liquid metal. What is the source of this purer liquid metal? The fundamental process of dendritic crystalliuation can lead only to enriched filler liquid. Unless experimental evidence can prove otherwise, the absence of undercooling in the center of the ingot in the 6rst period of solidification is strong evidence against the explanation based on settling dendrite skeletons. Likewise, the liquid ahead of the transverse liquid-solid interface can be only of average or enriched composition. FIG 26-PLOT TO SCALE OF CALCULATED EXTENT OF VERTICAL SOLIDIFICATION FOR ACTUAL CASE OF COLUMNAR PLUS EQUI-AXED FREEZING AND FOR IDEALIZED CASE OF ALL COLUMNAR FREEZING. composition as indicated by the liquidus curve. Possibly this could be explained by the filler liquid rising upward in the liquid core of the ingot and being displaced by liquid of average composition, by means of convection currents. The process of liquid of average composition feeding the interstices of the dendrites will then yield the negative segregation observed. 2. Would the author venture an explanation of the commonly observed phenomenon of "herring bone" structure along the axis of the ingot in the upper part of killed steel ingots? This structure consists,of a series of V-shaped cavities, which appear to indicate some periodicity in the freezing process. L. F. MONDOLFO~-I would like to preface my remarks by mentioning that most of my experience has been with aluminum ingots and that my experience with steel has been very meager. However, there are some observations that I think apply to all metals. In considering the inverse segregation pattern and the accelerated growth from the bottom, it seems to me that two very important factors should be considered: Shrinkage and hydrostatic pressure. At the start, the liquid is in direct contact with the mold and the heat transfer is from liquid to mold coating, to mold. After a while, when the outside shell t Associate Professor. Department of Metallurgical Engineering, Illinois Institute of Technology. Chicago.

19 '1O SOLIDIFICATION OF STEEL INGOTS of frozen metal has built up to a certain thickness, the shrinkage of this shell creates a gap between the metal and the mold. At this stage the heat from the inside melt has to go through solid shell, air gap, mold coating, mold, before it can be dispersed to the outside. The addition of the air gap at the sides and the rise of temperature of the mold tend to slow down the heat transfer and, therefore, to reduce the rate of crystallization from the sides toward the center. At the bottom, where the weight of the ingot prevents the formation of an air gap, the solidification is slowed down much less and that explains the more rapid growth from the bottom up. The formation of the air gap at the sides has another effect. The outside shell cannot lose heat to the mold as fast as it receives it from the molten part. Its temperature tends to rise and its strength to decrease. Thus, especially near the bottom, the remaining liquid, which is under hydrostatic pressure, can break through the solidified part. Since this liquid, due to normal segregation, is richer in alloying elements, it will cieate streaks of inverse segregation, which tend to reach further toward the outside at the bottom, where the pressure is higher. This infiltration is especially evident in aluminum alloys where the eutectics usually break through the frozen shell and form the well-known liquated zone at the outside of the ingots. In this way, the combined accelerated growth from the bottom and higher hydrostatic pressure at the bottom will produce the inverted V shape of the inverse segregation. Probably other factors such as convection currents in the liquid, fall of crystals from the top, etc., as pointed out by the author, may contribute to the inverse segregation pattern. I think, however, that the factors that I have mentioned play an important if not decisive part. C. E. SIMSS-The reasons advanced to account for negative segregation are very interesting, but the question probably cannot be answered satisfactorily without further experimental evidence. The following data may throw some light on the subject. In some experimental work, 600 Ib of molten SAE,1035 steel was cast into a fire-clay. brick mold to make an ingot 9x18~1~ in. tall. A Pt-Pt-Rh Assistant Director, Battelle Memorial Institute. Columbus. Ohio. thermocouple, protected by a fused silica tube, was laced on the vertical centerline but somewhat above the center of mass, where the last freezing would be expected to occur. ~lthough poured at a temperature of about 2&0 F (optical pyrometer). the center was at the liquidus temperature of z740 F within 2 min when the first reading was taken. The temperature then stayed constant at the liquidus until 19 min had passed, then dropped to the peritectic temperature of 2720 F by 30 min. At 40 min, freezing was complete. This was even more striking when a sand mold, which is a better insulator, was used. Under these conditions,. the center metal stayed constant at the liquidus temperature for the first 27 min then fell to the peritectic temperature by 60 min and was completely frozen in 68 min. The drop in temperature from the liquidus to the peritectic could readily be interpreted as a period during which free crystals of ferrite formed in a predominantly liquid area. There appears also to be sufficient time for these ferrite crystals to settle out and account for the negative segregation near the bottom center of an ingot. High positive segregation higher up in the ingot can probably be explained by the mechanism reported by ~isho~ and Fritz13 for certain types of small steel castings. They found that in castings, provided with neckeddown risers, areas with a carbon content of 0.45 pct may be obtained in a steel with a ladle analysis of 0.25 pct. Toward the end of the freezing in an ingot, there is a long,-narrow center core of metal ostensibly liquid but which is beginning to form skeleton,dendrites. In the process of feeding to compensate for solidification shrinkage, some areas can be.fed only by draining interdendritic liquid from higher up. This interdendritic liquid is high in all impurities, but particularly high in carbon, and will enrich the area into which it drains, resulting in positive segregation. The area from which it drains, however, may have its interdendritic spaces filled with liquid metal, from the hot top, :which has normal composition. This last area may then show 1s H. F. Bishop and K. E. Fritz: Segregation in Small Steel Castings. Trans. Amer. Foundrymen's Assn. (1947) 55, p. 412.

20 lower than normal average composition or negative segregation. P. D. FROST&-I have observed agglomerations of what appear to be nonmetallic inclusions in the bottom sections of killed-steel ingots. It has been my experience that they were associated with heats which were poured too cold. Tlie middle cuts of billets from a cold heat invariably have wide solidification pat; terns, as revealed by deeply etched cross-sections. These heats usually contain numerous nonmetallic inclusions. Whenever a middle-cut etch test indicated a cold heat there often would be agglomerations of nonmetallics in the bottom cuts. One of these "gobs" found in the deepetched cross-section of a bottom cut in a carbon steel ingot is shown in Fig. 27. These could also be detected at the blooming mill shears by the shear drags. Oftentimes several feet would have to be discarded from the bottom.end of a billet before these impurities were eliminated. R. L. KELLER and E. A. LORIAII-The examination of longitudinal etched sections of killed-steel ingots of average sizes indicates that the V and inverted V segregate zones are usually minimized and that the main defect is center porosity revealed as a series of shrinkage voids at the axis of the ingot and extending from within a foot of the hot-top junction to within a foot of the bottom. The condition is not peculiar to any one heat but is characteristic of the grade. Since it appears from observations of split ingots that axial porosity is inherent in the different solidifying tendencies of various grades, the assumption would be that mold design must be altered with change in grade; however the number of mold designs may be much less than the total number of grade variations. In order to achieve this by means of economical and practical changes in ingot (mold) design, it becomes apparent that the production of sound steel in alloy grades must depend necessarily upon: I-The treatment of the ingot during solidification to prevent the formation of the axial voids, or a-the reduction or elimi- O Research Engineer. Battelle Memorial Institute, Columbus, Ohio. 11 Mellon Institute of Industrial Research. Pittsburgh. nation of the porosity in the final product by proper working of the center during rolling or forging. Axial porosity and V segregation may go hand in hand, especially in ingots 18 to 25 in. wide. In narrower ingots and in certain alloy grades within these limits the tendency may be for the transverse solidification rate to be sufficiently rapid to cause axial porosity not of a "chevron" but of a haphazard appearance, resulting from bridging across the vertical axis of the ingot. Under such circumstances, it is clear that axial porosity may not always be associated with V segregation. However, for purposes of this discussion we are speaking only of axial porosity of the chevron shape. Just how an ingot might be treated to affect a completely sound center is somewhat difficult to perceive in the light of known solidification rates, but it is conceivable that ingots of good size for production purposes (say, 500 to 1000 sq in. cross-section in nearly square ingots) may be produced as sound as the 144 sq in. cross-section ingots illustrated in the reports of the Heterogeneity of Steel Ingots Committee, British Iron and Steel Institute. A more immediate and practical solution for the producer, until such information on ingot solidification is forthcoming, is to soak the ingots in the heating pits prior to rolling for as long a time as is profitable and to provide for the rolling of such ingots at slow speeds and with heavy drafts. Longitudinal sections of killed ingots reveal the more pronounced inclination of columnar crystals towards the top of the ingot as they grow inward from the mold wall. This inclination actually traces the increasing effect of the vertical component and indicates that this effect is not felt just as soon as the molten metal meets the mold wall, but that it is produced by the last component to take part in the freezing process. Observations on ingots wider than 18 in. indicate quite strongly that, during freezing, the vertical component of heat abstraction decreases at a slower rate than the transverse and thus progressively induces solidification in the face of transverse solidification until it actually predominates near the vertical axis of the ingot., The inducement of vertical solidification in the fact of transverse solidification is believed ' to entrap the lower melting point constituents

21 72 SOLIDIFICATION" OF STEEL INGOTS FIC 27-CROSS-SECTlON OF DOTTOM CUT IN A CARDON STEEL INCOT SHOWIN"C A "COD." -~-l,

22 B. R. QUENEAU 73 which precede transverse solidification in a band, or bands, inclined towards the longitudinal axis, and the effect becomes more noticeable at each higher level in the ingot. By increasing the width of the core, wherein V segregation and axial porosity normally reside, there is less tendency to entrap the lower melting point constituents or to do so (by reason of the faster cooling rate) as a more intimately distributed mix and thus effect a great reduction in segregation and large axial voids. Therefore it would seem possible to employ mold designs which would minimize both V and inverted V segregation and still produce ingots of reasonably good size for tonnage production. It has been indicated in Queneau's paper and by our observations that a sufficiently fast transverse cooling rate can be achieved to reduce the inverted V segregate in large ingots to invisibility on macroetching. This cooling rate may be increased further by the use of a thicker mold wall with a consequent greater volume of metal for heat abstraction. The V segregate and axial porosity may be decreased to a minimum (while keeping the transverse rate the same) by three variable factors: I-Increased vertical cooling rate, z- increased mold wall taper, and 3-decreased height of ingot with respect to cross-section. In each case we are considering that the transverse cooling rate is maintained fast enough to prevent the formation of inverted V segregation. Obviously the second and third factors involve decreased ingot yield, greater mold costs per ton of ingots produced, and awkward ingot handling (soaking pit hogs) which would tend to discourage their consideration. However, the first factor could be produced without any of these disadvantages. An increased vertical cooling rate could be achieved by using watercooled stools, better contact between mold and stool, increased stool size, and heavier bottoms on molds.