These are roll contact cooling, radiation and air convection from the bare strand surface just in the roll bite just above the spray region, cooling due to spray water impingement, and water convection cooling just below the spray region, where water runs down the strand and collects in the roll bite. Bulging of the steel shell caused by ferrostatic pressure can affect these heat-transfer subzones, especially near the roll bite and if the support rolls are spaced too widely apart. Secondary cooling mechanisms provided by water spray for steel and water film for aluminum have distinctly different characteristics. In spray cooling, water droplets impinge on to the very hot steel surface and vaporize instantaneously to create a boundary layer, which prevents the water from wetting the surface. Heat extraction is higher toward the center of the impingement region, where more of the high-speed droplets have enough momentum to penetrate the vapor layer. Extremely irregular flow conditions develop within the vapor boundary layer, and it eventually becomes wavy and is thinned out. The short contact times between the spray droplets and the strand surface increase with water velocity, owing to increased water momentum. Thus, the secondary cooling rate increases greatly with spray water flow rate, although it is almost independent of strand surface temperature. In contrast, under film cooling conditions, water flows along the surface at a uniform velocity. As a result, the boundary layer of vapor between the water film and the metal surface tends to be thicker and unperturbed. However, as the metal surface cools, the vapor layer breaks down and the water film starts to contact the strand surface. The area of contact increases with decreasing strand surface temperature and is accompanied by a sudden increase in heat transfer. The cooling process is transient and is difficult to control.
In the continuous casting of steel, the purpose of secondary cooling is to maintain the heat extraction and solidification initiated in the mold with minimal change in surface temperature in order to avoid generating tensile stresses large enough to cause cracking. Only about 50 to 60 pct of the total heat content (including superheat, latent heat, and sensible heat) is removed by secondary cooling. However, this heat-transfer process is critical in DC casting as the chill water extracts about 80 pct of the total heat content during the steady-state regime below the mold.
The extraction of heat by cooling water is quite complex for both water spray and film cooling conditions because it is governed by the water boiling water phenomena, which depends greatly on temperature. Four mechanisms of heat transfer can be distinguished when cooling water comes in contact with a hot metal surface. In order of increasing surface temperature, they are as follows.
1. Convective cooling at temperatures lower than 100 °C
2. Nucleate boiling between 100 °C and burnout temperature
3. Transition boiling between burnout and Leidenfrost temperatures
4. Film boiling at high temperatures (Leidenfrost temperature)
QUALITY PROBLEMS RELATED TO SECONDARY COOLING.
One of the most important considerations during the continuous casting process is the capability of attaining a defectfree slab or ingot. Two such quality issues are hot tearing and cold cracking and dimensional control. These problems are directly attributed to tensile mechanical and thermal stresses/strains generated during the casting process. Mechanically generated tensile strains, such as caused by inadequate mold lubrication or bending/straightening of the strand, usually act in the longitudinal direction and cause transverse cracking. During the casting process, rapid cooling can result in steep temperature gradients in the solidifying shell that can generate thermal strains as the shell expands and contracts. Sudden localized cooling can introduce tensile strains at the surface, whereas reheating can generate tensile strains at the solidification front. Thermal strains act predominantly in the transverse direction and are responsible for causing longitudinal cracks. Cracks can form if the generated tensile strain locally exceeds the strain to fracture of the metal. In steel, different regions of low ductility have been reported. The most important one lies within _50 °C of the solidus temperature and is responsible for “hot tear” cracks.
Most cracks in steel slabs and billets are hot tears, due to the zone of low ductility near the liquid front. Internal cracks are often seen near the corners, at the centerline or diagonally between opposite corners. Surface cracks can appear near both midface and corner regions. Some cracks that form below 900 °C during the straightening of the shell have been attributed to the embrittlement caused by precipitation of AlN near the grain boundaries.
Brimacombe et al. have summarized the causes of cracking problems in continuous cast steel. Improper secondary cooling practices contribute to many of these. Excessive spray cooling or insufficient spray length led to surface reheating, which induces tensile stresses beneath the surface, including the solidification front. This can cause internal cracks such as midway cracks in billet casting. Unsymmetrical cooling at the billet corners induces distortion and diagonal cracks. Excessive spraying of water can lead to rapid cooling and large tensile strains at the surface of slab castings, which can open small cracks formed in the mold. However, insufficient spray cooling below the mold can allow the slab to bulge out if the surface becomes too hot. This can lead to several defects, such as triple point cracks, midface cracks, midway cracks, centerline cracks and center segregation. Transverse surface and corner cracks begin in the mold, but can be opened by axial tensile stresses induced by spray cooling in slab casting, when the surface temperature is within the low-ductility range of 700 °C to 900 °C. Secondary cooling practices that lead to excessive surface temperature fluctuations also aggravate these cracks, especially in this critical temperature range.
The thermal stresses and strains generated in the ingot during the transient start-up phase of the DC casting process can initiate hot tears and cold cracks, especially in highstrength aluminum alloys. Hot tears generally form between the quarter points of a rectangular ingot beneath the ingot surface. cold cracks also originate at the ingot base and are located in the center half of the ingot width. High casting speeds tend to cause hot tears and low casting speeds increase the risk of cold cracks. The formation of hot tears has also been linked with the frictional forces between the ingot and mold, and the variability in cooling conditions during the transient start-up phase. In addition to cracks, thermal stresses related to secondary cooling also generate macrodeformation of the ingot base or butt curl especially during startup. Тhe production problems related to butt curl include the following: runouts of the melt, cold shuts, reduced rigid standing of the ingot on the bottom block, and low recovery rates. Ultimately, if the magnitude of butt curl is excessive, the ingot bottom may have to be sawed off.