ACOS E LIGAS ESPECIAIS COSTA E SILVA PDF
Sorry, this document isn't available for viewing at this time. In the meantime, you can download the document by clicking the 'Download' button above. Copia de Acos e Ligas Especiais Costa e Silva. March 12, | Author: Priscila An error occurred while loading the PDF. More Information. Metalografia dos Produtos Siderúrgicos Comuns (4a edição). H Colpaert, A Costa e Silva. Editora Blücher, *, Aços e ligas especiais. ALCE Silva.
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PDF | Aiming to meet new market demands for flat steel, steel companies have been developing new Below is a chart from the book “Aços e Ligas Especiais – ” (Steels and Special Alloys) that Source: Costa e Silva – Mei . Documents Similar To Meyers-Chawla-Princípios de Metalurgia Mecânica. Acos e Ligas Especiais- Costa e Silva Isbned myavr.info Uploaded by. Acos-e-Ligas-Especiais-Costa-e-Silva-Isbned-livro- myavr.info bab 2 KERANGKA ACUAN PROGRAM myavr.info myavr.info
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Agarwal, A. Hansen, A. Goldemberg, J. Brummett, C. Sandres, G. Gentil, V. Klokova, I. Fuels Oils , 41, Wolynec, S.
Gaylarde, C. Takeshita, E. Santos, A. Pereira, E. Nova , 29, Delgado, R. Rovai, F. Deshmukh, G. World , 38, Foulkes, F. Paramonov, V. Cho, S. Duarte, L. The funnel in Fig. The flow rate is controlled by restricting the opening with either a stopper rod or a slide gate. The stopper rod system is illustrated schematically in Fig.
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Flow control with a stopper rod is slightly more difficult than with slide gates because the stopper must be manipulated through the entire depth of the molten steel in the tundish, and the area of the annular opening that controls the flow is more sensitive to displacement. In addition, a continuous nozzle does not allow fast exchange of SEN tubes and requires some other means for emergency flow stoppage.
However, the stopper rod offers several significant advantages over slide gates: 1. In the three-plate slide gate, pictured in this figure, the central plate is moved hydraulically to adjust the opening between the upper and lower stationary plates by misaligning the hole in the sliding plate relative to the nozzle bore. Alternatively, the two-plate slide gate is missing the lowest plate, so the SEN is attached to the moving plate and travels as the opening is adjusted.
This has the disadvantage of continuous variation in the alignment of the nozzle relative to the strand centerline. In both systems, the joints are all flooded with low-pressure inert gas argon to protect against air entrainment in the case of leaks.
Flow through the slide gate is governed by the size of the overlapped openings of the plates, as illustrated in Fig. This opening size may be quantified in several different ways. Two popular measures are area opening fraction, fA, defined by the ratios of the shaded area to the total bore area, and linear opening fraction, fL, defined as the ratio of the distances S to T. For equal sized openings, these different measures of opening fraction are related by: 7 Fig. The linear opening fraction, fL in Eq.
The steel flow rate depends mainly on the height of molten steel in the tundish driving the flow and the pressure drop across the slide gate. Flow rate naturally increases with increasing slide gate opening position and with increasing tundish depth, as quantified in Fig. The flow rate produced under ideal conditions, such as presented in Fig. Flow is governed primarily by the condition of the jet entering the mold cavity, but is then affected by the amount of gas injection, the section size, the casting speed and electromagnetic forces.
In casting square sections with open-stream pouring, stream penetration is generally shallow. The surface is turbulent with high velocity flow toward the meniscus, so entrainment of mold slag is likely.
With a straight-through nozzle, stream penetration is very deep, and recirculating flow travels a long distance before flowing upward to the meniscus corners. With a quiet surface, slag entrainment is unlikely with this condition, but the meniscus might become too cold and stagnant. Adding side ports to the straight-down nozzle produces an intermediate condition. With shallow submergence, high casting speed and excessively large side ports, surface turbulence and mold slag entrainment are a danger in small-section billet casting with this nozzle.
In general, the relative size of the side and bottom ports can be adjusted to optimize the flow condition and thereby avoid defects. In slab casting, the mold flow pattern varies between the two extremes shown in Fig. With an upward-directed jet exiting the nozzle or a large amount of Fig. Fluid Flow in the Mold argon gas injection, the flow will quickly reach the top surface and travel away from the nozzle toward the narrow faces before being turned downward.
This single-roll flow pattern is more likely with multi-port nozzles, or a bifurcated nozzle with small, upward-directed ports. It is also encouraged by shallow nozzle submergence, low casting speed or large mold widths. Surface velocities and level fluctuations are high, so mold slag entrainment and surface defects are likely.
With the other typical mold flow pattern, the steel jet enters the mold cavity from a more deeply submerged nozzle with larger or downward-angled entry ports of a bifurcated nozzle. The submerged jet then travels across the width of the mold to impinge on the narrow faces. The jet then splits. Some of the flow travels upward toward the meniscus and back across the top surface toward the nozzle.
The Fig. Two large recirculating regions are formed in each symmetric half of the caster, so this flow pattern is termed double roll. Often, the flow pattern alternates between the single- and double-roll archetypes or it may attain some intermediate condition. Traditionally, understanding has been deduced from physical models constructed to scale from transparent plastic using water to simulate the molten steel.
Casting Volume accurate for singlephase flows regardless of the model scale factor, so long as the flow is fully turbulent.
Obtaining accurate flow patterns is very difficult when gas injection is significant, and some phenomena, such as slag layer behavior, cannot be modeled quantitatively, owing to the inherent differences in fluid properties, such as density and surface tension.
Mathematical models can yield added insight into flow. Computational models based on finite-difference or finite-element solution of the Navier-Stokes equations can include phenomena such as heat transfer, multiphase flow, and solidification in steel casting without the inaccuracies inherent in a water model.
Accurate calcu- Fig. Flow can also be measured directly in the actual steel caster. Surface velocities can be measured by monitoring the vibrations of a rod inserted into the flow through the top surface. The velocity component is then calculated from the time taken for signal disturbances to move between a pair of probes.
This method is accurate only in regions of unidirectional flow between the probes, such as found near the surface. Fluid Flow in the Mold methods require significant effort and expense to calibrate and operate. Alternatively, a crude estimate of steel flow direction across the top surface can be obtained using the same measurement method used to monitor slag layer thickness. The steel flow direction can be crudely estimated from the angle plowed up by the liquid steel as it flows around an inserted nail, as illustrated in Fig.
This angle can be captured as a frozen lump on the bottom of the nail, if care is taken. Because it is the last liquid processing step, poor control of flow in the mold can cause many defects that cannot be corrected. Problem sources include: the entrapment of Fig. Each of these problems related to fluid flow will be discussed in turn. This problem is worst at the final stage of flow in the mold, because there is little opportunity to prevent the reoxidation products from becoming entrapped in the final product as catastrophic large inclusions.
Open-stream pouring produces the worst air entrainment problems, as previously discussed. This can be minimized by controlling both flow in the tundish and the metering nozzle design and operation in order to produce the smooth stream shown in Fig. Specifically, high-speed flow in the tundish across the exit nozzles should be avoided by proper choice of flow modifiers and shape of the tundish near the nozzle exit. Castellated metered nozzles, for example, have grooves that introduce controlled roughness into the stream in order to avoid inconsistent severe roughness.
Entrainment is still possible, however, if there are leaks, cracks, inadequate sealing between the nozzle joints or if the nozzle material becomes porous.
If the internal pressure in the nozzle drops below atmospheric pressure, air will aspirate through any of these pathways into the nozzle. This can be identified by nitrogen pickup, but the oxygen reacts to form dendritic inclusion particles. Pressure in the nozzle is lowest just below the flow-control device, due to the venturi effect of the metal stream. For a given steel flow rate, both the pressure drop and the corresponding tundish height increase as the opening area is restricted, as shown in Fig.
Adding argon gas can raise the minimum pressure in the nozzle above ambient, as shown in Fig. Less gas is needed at low casting speed and at low tundish level, when the pressure drops are lower. Maintaining a high gas flow rate during these times may disrupt flow in the mold and be detrimental to steel quality.
Design of the nozzle and flow control geometry should promote smooth flow with minimal recirculation, in order to both minimize the pressure drops that allow reoxidation and to discourage 12 Copyright , The AISE Steel Foundation, Pittsburgh, PA. Each line on the top of the figure indicates the slide gate position corresponding to a different tundish level. A sufficiently thick slag layer over the surface of the steel in the mold is also important.
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Finally, flow in the mold should be controlled to avoid surface turbulence that could entrain air. The most obvious source of surface defects is the capture of foreign particles into the solidifying shell at the meniscus. Particles come from many sources, including argon bubbles and oxide inclusions generated by prior processes that are carried in with the steel entering the mold cavity.
If the meniscus is unstable, stagnant, or has a solidified lip or hook, bubbles may be captured, forming pinholes just beneath the surface of the slab, as shown in Fig. This is illustrated in Fig.
Even gas bubbles traveling with the incoming jet occasionally are carried into the lower recirculation zone. In a curved caster, large particles in the lower recirculation zone will spiral toward the inner radius, where they may become trapped in the solidifying shell.
This is illustrated by one of the trajectories in Fig.
Entrapped solid oxide particles eventually lead to surface slivers or internal defects, which act as stress concentration sites to reduce fatigue and toughness properties of the final product. Fluid Flow in the Mold Most particles are captured 13 m below the meniscus, independent of casting speed.
When the slab is rolled, the subsurface bubbles elongate and the layer of metal separating them from the surface becomes thinner.
Later during annealing, they can expand to raise the surface of the sheet locally, especially if the steel is weak such as ultra-low-carbon grades, or if hydrogen is present.
The capture of large inclusions into the solidifying shell then leads to obvious line defects or slivers in the final product. Vortexing most often occurs during conditions of asymmetrical flow, where steel flows rapidly through the narrow passage between the SEN and the mold. This creates swirling just beside the SEN, as shown in Fig. If it is then entrained with the jets exiting the into the liquid pool, showing one large particle spiraling lower recirculation zone toward solidifying shell on nozzle ports, this slag will be dispersed everywhere in inside radius wideface.
In addition to the vortex, slag may also be drawn downward by the recirculation pattern that accompanies flow from the nozzle ports. Thus, slag entrainment is most likely with shallow nozzle submergence and high casting speed. The entrainment of mold slag also occurs when the velocity across the top surface becomes high enough to shear mold slag fingers down into the flow, where they can be entrained.
To avoid shearing slag in this manner, the surface velocity must be kept below a critical value. This critical velocity has been measured in wateroil models as a function of viscosity and other parameters. The critical velocity may also be exceeded when the standing wave becomes too severe and the interface emulsifies, as sketched in Fig. This will be discussed in the next section. These variations take two forms: steady variation across the mold width known as a standing wave, and level fluctuations, where the local level changes with time.
While the standing wave can cause chronic problems with liquid slag feeding see the next section , the time-varying level fluctuations cause b the most serious surface defects. To avoid these prob- Fig.
Sudden jumps or dips in liquid level are much more serious, however. A sudden jump in local level can cause molten steel to overflow the meniscus. In the worst case, the steel can stick to the mold wall and start a sticker breakout. Alternatively, a jump in level can cause an irregular extended frozen meniscus shape, or hook. This extended meniscus can capture mold powder or possibly bubbles or inclusions, such as shown in Fig.
A sudden severe drop in liquid level exposes the inside of the solidifying shell to the mold slag and also leads to surface depressions. Relaxing the temperature gradient causes cooling and bending of the top of the shell toward the liquid steel. When the liquid level rises back, the solidification of new hot solid against this cool solid surface layer leads to even more bending and stresses when the surface layer reheats. The microstructural changes and surface depressions associated with level variations are serious because they initiate other quality problems in the final product.
These problems include surface cracks and segregation.
Surface cracks allow air to penetrate beneath the steel surface, where it forms iron oxide, leading to line defects in the final product.
These defects are difficult to distinguish from inclusion-related defects, other than by the simpler composition of their oxides. Mold level can be measured directly in several different ways, which include the popular NKK eddy-current sensor,31 radioactive source detection,32 electromagnetic methods such as the EMLI detector,33 and other methods.
Control of the total flow rate requires measurement of the average liquid level. This is best accomplished by measuring where the liquid steel surface level is the most stable typically midway between the SEN and the narrow face and by filtering time-averaging the signal. The objective of this sensor signal is to remove the influence of local fluctuations, which are not directly related to the average level needed by flow control device.
However, these local transient fluctuations are very important to surface quality. Thus, quality detection systems should always monitor the unfiltered signal from the level sensor. Even better is to monitor the unfiltered signal from a second sensor, positioned where the level fluctuations are greatest, usually near the narrow face. The temperature profile down the row of thermocouples can be used to indicate where the meniscus is located.
The quality of liquid level control can also be gathered by observing the oscillation marks on the exterior surface of the slab, which should be straight and regular with a spacing or pitch defined by the casting speed and oscillation frequency. Overlapping or wiggly oscillation marks indicate a serious flow problem in the mold.
The variable pitch of the deep, severely-distorted oscillation marks in Fig. For example, the curved oscillation marks in Fig.
High-frequency fluctuations due to the turbulent nature of the flow increase with increasing velocity across the top surface, which depends on the jet velocity and the mold flow pattern. Lubrication problems in both oil and powder casting lead to level variation defects.
Electromagnetic stirring too close to the meniscus can generate severe standing waves. Lower frequency level fluctuations can be caused by synchronized bulging and squeezing of the strand below the mold, which acts like an accordian bellows to alternately raise and lower the level in the mold. This problem is most likely to occur just after casting speed changes, due to bulging variations moving through the rolls. This was likely the root cause of periodic defects similar to those in Fig.
Complete strand stoppage is particularly detrimental because roll bending will then contribute to the bulging and the associated level variations. To do this requires an optimum delivery of lubricant to the meniscus perimeter.
Indeed, problems with the oil lubrication system are believed to be responsible in part for the defects associated with the meniscus overflow in Fig. The many severe consequences of inadequate lubricant include breakouts, cracks and surface depressions and are discussed in detail elsewhere. The 1- to 5-second dipping time is critical and should be optimized experimentally.
The entire profile of the slag layer can be found by inserting a board containing many nails, or by using sheets of steel and aluminum instead of nails.Excessive flow directed across the top surface can produce turbulence and lead to reoxidation and slag entrainment. There are five elements in speaking: Loading Preview.
Flow through the nozzle and its ports depends on the cross-sectional area of the opening at the flow control and on the sharpness of the edges there. Good decision through understanding people, cultures and markets. Transactional Speaking Classs at the second semester, the first year. Assessing Speaking. Author: Jeffrey Souza. It is about the ease and speed of the flow of Nommensen Pematangsiantar in transactional speaking speech.
Some students got stuck to say what they want to say.
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