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Today's steelmaking often reaches demand with the mass production of standard alloys through continuous casting of billets, blooms, and slabs. Many customers desire a mid-grade steel, ranging from 0.9 to 0.53 percent carbon by weight for its balance of toughness and strength; however, these steel grades pass through a peritectic phase transition between delta-ferrite and the liquid steel to produce austenite. The peritectic phase transition causes a heterogeneous shrinkage, adding significant stresses to the already weak solid shell, which tend to result in facial defects, such as cracking and oscillation marks. Consequently, these grades of steel are known as “peritectic steels,” to represent the controlling aspect of the difficulty in continuously casting them.
Peritectic steels are sub-classified into hypo-peritectic and hyper-peritectic steels, depending on the side of the peritectic phase transition they fall on. Hyper-peritectic steels are less prone to surface cracks as compared to hypo-peritectic steels; this results from the relatively close liquidus temperatures of d-ferrite and austenite at these compositions. It is possible to nucleate d-ferrite and austenite simultaneously, or even directly nucleate austenite in hyper-peritectic steels (1). Hypo-peritectic steels were unable to be continuously cast until after 1995 (2).
The peritectic phase transition is divided into two stages: the peritectic reaction and the peritectic transformation. The reaction occurs first at the d-ferrite-liquid interface, creating a thin film of austenite (3). Phelan et al. discovered that the reaction is controlled by the temporary re-melting of the d-ferrite, which mixes with the remaining liquid to create a localized pool of austenitic-range carbon composition. The pool quickly freezes to create a thin film of austenite dividing the d-ferrite and liquid; a schematic diagram is shown in Figure 1. At this point, the peritectic transformation takes place: carbon diffuses across the austenite film to the d-ferrite, equilibrating carbon composition to allow d-ferrite to austenite and liquid steel to austenite transformation. This process is illustrated in Figure 2.
Because the d-ferrite solidifies first, the shell is initially composed entirely of d-ferrite, with additional d-ferrite dendrites growing inward. Upon reaching the peritectic temperature, the austenite forms some distance away from the outside of the shell, creating a tensile force on the outside of the continuously cast object. The tensile forces are caused by the shrinkage in transforming the body-centered cubic d-ferrite to the denser face-centered cubic austenite. This transformation causes peritectic steels to undergo double the shrinkage normally encountered by cooling and solidifying, as compared to other grades of steel (5). In uncontrolled castings, the forces reach equilibrium by creating bulges and valleys in the shell; this is schematically shown in Figure 3. The shell typically bulges in thicker sections, and dips in thinner sections, both of which arise due to imperfections in the mold wall and non-uniform flow of mold powder (5). Due to the geometry of the defect, the bulges contact the mold wall, while the valleys do not, resulting in greater thermal transfer at the bulges which increases the solidification rate at the bulges; the valleys exhibit the opposite. Consequently, the solidification rate is now non-uniform.
While the bulges are effectively prohibited from further shrinkage, the valleys are not. Thermal stresses cause the valleys to shrink further, ripping the dendrites apart; the remaining liquid, rich in alloy additions, fills the void. Solidification is further delayed at the valleys due to the composition. As the shell passes out of the mold, the intensive cooling generated by water spray creates thermal stresses in excess of the strength of the valley; small longitudinal facial cracks proceed to appear (5). These cracks often go below the de-scaling depth and can be difficult to heal, due to oxidation and decarburization (6).
The simplest solution to casting of peritectic steels is a slower cooling rate. The lower cooling rate reduces the rate of solidification (4), allowing time for the buildup of a uniform d-ferrite shell before the peritectic phase transition (5). In turn, this prevents the rise of bulges and valleys, which prevents the formation of small longitudinal facial cracks; a 30% slower cooling rate lowers the occurrence of facial cracks by 50% (5). In addition to simply decreasing flow rate, the thermal transfer can be slowed by usage of a proper mold flux. It has been found that a slightly basic mold flux with a high crystallizing temperature provides the optimum cooling rate; this provides the correct viscosity while creating micro-size air gaps due to the crystalline nature of the flux (5). However, the slower cooling rate increases the necessary mold residence time to form a shell of adequate thickness, resulting in a 20% slower production rate in a continuous casting machine (1). Many plants still refuse to cast peritectic steels, unwilling to cast at the slower rate necessary to avoid the inherent problems with the composition (7).
Surface cracks resulting from the peritectic phase transition can also be mitigated by good casting practice. Reducing the turbulence of liquid metal leaving the submerged entry nozzle contributes to the formation of a uniform shell, as does maintaining a consistent meniscus level. These methods decrease the occurrence of hotspots and sustain a regular starting height for shell solidification (5). De-alignment of the rolls can cause uneven stresses on the weak shell; consequently, a reduction in cracking can be achieved by consistent roll gaps and well-maintained rolls (5). Ensuring adequately low levels of dissolved sulfur and an adequately high ratio of dissolved manganese to sulfur also reduce the occurrence of cracking (5).
If specialization in casting of peritectic steels is considered, further improvements can be made. Molds made of heat-resistant, low thermal conductivity materials have shown promise in decreasing the heat transfer rate and increasing uniformity in the solidified shell. Molds should be further designed to have the correct taper, especially on the short side of the mold (5). Mold oscillations should take into account the relative weakness of peritectic steels and the properties resultant of their dual-phase shell structure by increasing frequency and decreasing travel; this minimizes oscillation mark defects (2). Air-mist systems replacing the typical water sprays attain a more uniform cooling rate on the solidified shell below the mold; the typical water spray systems are prone to generating uneven thermal stresses on peritectic steels (5).
Numerous difficulties exist in continuously casting peritectic steels, especially at the high speeds necessary to fill demand in the modern competitive steel production market. Fortunately, much research is being conducted with the end goal of casting at the same high speed regardless of composition.
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