Chapter 5

Conventional ingot and continuous casting The previously discussed methods are used to manufacture a steel of a desired quality and composition. The refined molten steel must be cast into some useful shape for subsequent treatments and forming operations. In the conventional production of wrought steel products, the steel is cast into a large tapered cast iron vessel to form an ingot. The ingot is subsequently rolled into slabs or billets, which may be used for the production of standard product forms such as plate, sheet, pipe, rod, and wire. Alternatively, slabs or billets can be cast directly during the primary casting operation in process called continuous casting. Indeed, the development of economical continuous casting processes over the last 20 years has had a tremendous impact on the steel industry, as indicated in Table 5.1. While ingot casting and continuous casting both involve the solidification of the steel, the issues that affect the properties of the steel in the ingot and continuous processes are quite different. These will be discussed in this section.

Table 5.1. A 10-year summary of raw steel production by type and casting method.

Per Cent of Total Total by Casting Method

(thou. tons) Stain- Elec- Cont. Steel for

Year Total Carbon Alloy less BOP tric Ingots Cast Casting

2001 99,321 91.4 6.6 2.0 52.6 47.4 2,799 96,502 20

2000 112,242 90.5 7.4 2.1 53.0 47.0 4,044 108,175 23

1999 107,395 89.9 7.9 2.2 53.8 46.2 4,389 102,983 23

1998 108,752 89.2 8.7 2.0 54.9 45.1 4,840 103,883 29

1997 108,561 88.4 9.4 2.2 56.2 43.8 5,689 102,834 38

1996 105,309 88.9 9.1 2.0 57.4 42.6 7,141 98,131 37

1995 104,930 88.3 9.5 2.2 59.6 40.4 9,272 95,626 32

1994 100,579 89.0 9.0 2.0 60.7 39.3 10,527 90,026 26

1993 97,877 88.7 9.3 2.0 60.6 39.4 14,014 83,839 24

1992 92,949 88.7 9.2 2.1 62.0 38.0 19,207 73,718 24

Conventional ingots After the final ladle treatments are made and the chemistry of the steel is satisfactory, the ladle is tapped from the bottom by lifting the internal stopper-rod, permitting the flow of molten metal. The steel is poured or teemed (see Fig.5.1) into the ingot molds, where it begins to cool and solidify. Ideally, the ingot would cool uniformly, resulting in a chemically homogenous equiaxed structure, free from voids, cracks, and nonmetallic inclusions. However, the geometry of the ingot and the thermal properties of the steel promote an inward freezing process that is very slow. In fact, the center of the ingot typically is still molten when the ingot mold is removed or stripped from the ingot. Typical ingot solidification times are shown in Fig 5.2. After stripping, ingots are places in a furnace called a soaking pit, where the temperature of the ingot is controlled to promote homogenization of the steel. The nonuniform cooling that occurs in an ingot coupled with the many dissolved impurities and gasses, gives ride to various chemical segregation phenomena that generate defect structures in the ingot and ultimately affect the downstream properties or process ability. As in any other metal-casting process, the structure that develops during the solidification of an ingot is primarily controlled by heat transfer, fluid flow, and the solubility of the various dissolved species. Immediately upon pouring, a chill zone or skin is formed as the steel begins to solidify on the surface of the iron mold. As the skin thickens, stresses may be accommodated by the separation of the ingot from the mold wall, forming an air gap which significantly slows the subsequent cooling. Alternatively, solidification shrinkage may result in cracking of the skin. Cambered, corrugated, or fluted ingot molds, which increase the ingot surface area, may be used to promote uniform thick skin formation, preventing this type of surface cracking. Similar cracks and fissures, however, may also form in the interior of the ingot. If these are not open to the surface, they are generally welded shut during hot rolling and are inconsequential. Cracks that are open to the surface, however, will become oxidized and will not be readily fused. These defects give rise to undesirable seams in the final rolled product.

Fig.5.1 Conventional ingots being poured.

Fig. 5.2 Freezing time profiles for a typical steel ingot. Times given in minutes.

Many of the defects observed in an ingot are due to the growth morphologies and related segregation patterns that arise during solidification. The ingot skin is a chill zone of fine randomly oriented equiaxed grains. The growth rate of this equiaxed zone is initially high, but slows quickly as the mold-metal air gap is formed. As the skin thickens to 10-15mm, the equaixed structure gives way to inward columnar growth due to the selection of preferentially oriented grains. As the growth proceeds further, the thermal gradient continues to decrease until dendrite fragmentation and nucleation ahead of the columnar front give rise to an equiaxed zone in the center of the ingot. The columnar structure is not desirable in a steel ingot. An exaggerated columnar zone, termed ingotism, may lead to cracking during subsequent rolling operations. The most important factors controlling the “columnar to equiaxed transition” during dendritic solidification are the thermal gradient and the speed at which the columnar front is advancing. A low thermal gradient gives rise to a thick two-phase layer containing columnar dendrites. This layer, known as the mushy zone, has a thickness roughly equal to the freezing range of the steel, divided by the thermal gradient. As the thickness of the mushy zone increases, the likelihood of dendrite fragmentation increases, favoring the onset of equiaxed growth. The speed of the dendritic front is important because it is related to the dendrite tip undercooling. Undercooling increases with growth velocity so that a faster growing columnar front is at a lower temperature than a slower growing front. Thus, there is a greater volume of undercooled liquid ahead of the primary structure, where fragments can grow and where new nuclei may form. Accordingly, ingot casting practices that promote uniform cooling enhance the extent of the more desirable equiaxed structure. Chemical inhomogeneity may be significant in an ingot. Short-range inhomogeneity or microsegregation is due to the partitioning behavior of dissolved species, while long range nonuniformity or macrosegregation is typically due to the convective flow patterns in the molten steel. The solubility of gases in the molten steel generally decreases with decreasing temperature. More importantly, the solubility in the solid may be much lower than the solubility in the liquid. Upon solidification, this partitioning behavior results in the liberation of gases, depending on the amount originally present in the steel. The most important process is the reaction of oxygen, in the form of FeO, with carbon in the steel. Carbon monoxide gas is evolved, and various defect structures may be generated. The control of these structures is accomplished through melt deoxidation and casting practices, and four standard classifications have been established, based on the degree of oxygen removal. Leading to a discussing of the specific classifications, let’s consider a steel that undergoes on ladle deoxidation processing. When poured into the ingot mold, solidification of the skin ensues rapidly and a large amount of oxygen is immediately released near the mold surface. The rising bubbles of evolved gas result in a boiling or rimming action characterized by an upward flow near the mold walls. If the rimming action is sufficiently severe, gas bubbles are swept upward along the mold surfaces toward the top of the ingot. This flow serves to clean the steel in the outer regions of the ingot and facilitates the escape of a large portion of the evolved gas. A negative side effect of such violent flow in the mold is that macrosegregation may be significant. A rimmed steel, therefore, is characterized by excellent surface properties and significant chemical inhomogeneity.

Consider now a process identical to the one described above, with one modification. After the ingot is poured and the rimming action begins, a metal cap is placed over the ingot. This capping process inhibits the release of gas at the metal surface and suppresses the upward flow of the rimming action. Accordingly, a capped steel is characterized by significantly less macrosegregation than that exhibited by a rimmed steel, but also by a thinner rimmed zone. Furthermore, if the rimming is suppressed too early, blowholes may be generated along the ingot surfaces, ultimately resulting in the appearance of seams. It should be noted that there is a continuum of intermediate ingot structures between rimmed and capped steels. The two processes should be considered in combination, facilitating a desired balance between chemical

Killed Capped Rimmed

homogeneity and surface properties. In practice, capping is most effective for steels between 0.15 and 0.30% carbon, while rimming is best applied to steels with lower carbon content.

The capping process is a method for mechanical suppression of the deleterious effects of dissolved oxygen. A chemical alternative to theses methods involves removal of the oxygen from the molten steel prior to casting. This is generally done with silicon additions of ferrosilicon, high silicon pig iron, or silico-manganese. If the steel is deoxidized sufficiently, the evolution of gas is completely suppressed, killing the rimming action. The resulting ingot is relatively uniform in structure and the prevention of the boil results in substantially decreased macrosegregation. Because no gas bubbles are formed in the melt, solidification shrinkage is accommodated at the upper surface by the formation of a large shrinkage cavity or pipe. Killed steels are typically used when the essential quality is structural soundness. In general, all steels with a carbon content higher than 0.30 wt.% are killed. Semikilled steels are only partially deoxidized and typically contain 0.15-0.30% carbon. Some gas evolution is observed and internal blowholes are formed to an extent that accommodates much or all of the total solidification shrinkage. Therefore, the pipe observed in a killed ingot is not present. Typical ingot structures are shown in Fig. 5.3.

Continuous Casting The continuous casting process was developed so that the product form produced could be directly rolled on a finishing mill, thus bypassing the ingot casting and slabbing operations. In addition, the process has led to improvements in both yield and quality. Coupled with the general trend away from the large integrated steel mill toward the smaller specialized minimill, the development of the continuous casting process has significantly changed the way that a large portion of today’s steel is produced. Currently, over 50% of the world’s steel is produced with continuous casting processes. In this section, the features of the process that affect the metallurgical quality of the steel are briefly discussed.

The distinguishing characteristic of a continuous casting process is that the mold is open on both ends so that the solidified metal can be drawn out while the molten metal is being poured in the opposite end. The principal components of a continuous casting line, or strand, are shown in Fig.5.4. The primary tasks that must be accomplished by the strand are similar to those of ingot casting. The molten metal must first be delivered to the casting strand. This is done by pouring from the ladle into the vessel known as a tundish, which controls the flow and distributes the steel to one or more open-ended, water-cooled copper molds, where solidification begins. A shroud

Fig. 5.3 The effect of oxygen content and control on typical ingot structure.

Killed Capped Rimmed

protects the steel from oxidation during the transfer. Upon contact with the mold, an outer skin or shell is immediately formed. Sticking of the steel to the mold surface is prevented by mold oscillation. After exiting the mold, complete freezing of the molten core is achieved through secondary cooling, using direct water spray. The distance from the mold to the location of complete freezing is known as the metallurgical length. Finally, as the cast slab moves beyond the secondary cooling zone, it is cut to the desired length using a torch or shear mounted on a sliding frame. Early continuous casting strands were of the vertical design, where all of these components were simply aligned in an upright configuration. To reduce the overall height and the required tundish elevation, several different configurations were developed, as shown in Fig. 5.5. These modifications permitted installation of continuous casting strands in existing plants.

Fig. 5.4 (right) A schematic of a continuous casting strand showing the major components.

Fig. 5.5 (below) Principal types of continuous casting. V=vertical; VB=vertical with bending; VPB=vertical with progressive bending; CAS=circular arc with straight mold; CAC=circular arc with curved mold; PBC=progressive bending with curved mold; H=horizontal.