moi nguoi dich giup minh voi1. Effect of stringent to lenient grade order When two grades have the same relative grade specification, (e.g.. 10 : 90 or 40 : 60), then the model predicts that the order in which they are cast should not matter. This is because mixing is dominated by turbulent flow, so all elements should diffuse in a roughly equal manner (according to their concentration). When two grades which have very different tolerances for mixing with the other must be cast together... moi nguoi dich giup minh voi1. Effect of stringent to lenient grade order When two grades have the same relative grade specification, (e.g.. 10 : 90 or 40 : 60), then the model predicts that the order in which they are cast should not matter. This is because mixing is dominated by turbulent flow, so all elements should diffuse in a roughly equal manner (according to their concentration). When two grades which have very different tolerances for mixing with the other must be cast together in a sequence, however, the order in which they are cast can make a difference. This section considers the question of whether it is better to cast the stringent grade before the lenient grade (10 : 60) or vice versa (40 : 90). Previous work by Damle and Sahai8 has suggested that casting from “stringent to lenient”, (10 : 60) is better. The present model predictions can be evaluated by comparing the 10 : 60 and 40 : 90 columns in Tables III and IV. There is no consistent trend. For most of the runs, the stringent to lenient specification (10 : 60) indeed produces less intermixed tonnage than the lenient to stringent (40 : 90). This is particularly true for the large tundish, where this finding applies to all but 3 of the runs in Table IV. Mixing in the tundish is characterized by a long tail before the asymptote at C=1 is reached. This is seen in Figure 5. This means that much steel must exit the tundish before the old grade remaining in the tundish is diluted enough to satisfy a stringent new grade. The first steel to solidify is near the submerged entry nozzle, so the composition leaving the tundish controls the surface composition of the slab. Thus, there is less intermixed length along the surface (which controls the new grade), when the lenient grade is cast last. Because a large tundish contains more steel than the strand, tundish mixing dominates the intermixed length. If mixing along the center does not change much, it is better to cast the stringent grade first. Unfortunately, a stringent old grade is worse at the center by about the same extent that a stringent new grade is worse at the surface. This means that casting the stringent grade first 16 (10:60) may decrease intermixing at the surface at the expense of increasing intermixing at the center. Composition along the center is controlled by mixing in the strand, which determines the intermixed length of the old grade. When mixing in the strand dominates the total intermixed length, it is better to cast from lenient to stringent (40 : 90). This is the case for most of the runs with the small tundish, and all of the large slab sizes, including thicker molds (run 13) or wider molds (run 11). In addition, higher casting speed increases the metallurgical length, which extends mixing along the centerline. Decreasing the tundish volume at ladle open (run 3) again increases the importance of strand mixing, particularly for the small tundish. For these cases, the lenient grade should be cast first (40:90). These two balancing effects can be understood by comparing Figures 7 a) and b). Casting the stringent grade last (Fig. 7 b) decreases centerline mixing but increases surface intermixing. Although the amount of intermixed steel does not change much, the location of the intermixed slab is translated much further up the strand (towards the new grade) for this 40 : 90 case. This could have serious consequence if measures were not taken to track the new location of this intermixed steel. Often the effect of strand mixing (increasing intermixed length for the stringent to lenient case in the centerline of the old grade) almost exactly cancels the tundish effect (reducing intermixed length at the surface of the new grade). In these cases, the order of casting the stringent and lenient grades does not matter very much.2. Effect of tundish weight history The weight of steel left in the tundish at the time of ladle open greatly affects the intermixed length. In general, lower tundish weight tends to decrease intermixing. Factors include lowering the tundish weight at ladle open (compare runs 2 and 3), delaying the time before refilling the tundish (compare runs 1 and 4), and lowering the maximum tundish weight (compare small and large tundish results). These trends hold for all of the grade specifications investigated except the overlapping case. The importance of the lower tundish weight history depends greatly on the severity of the grade change: there is less benefit for a lenient new grade. The effect of varying tundish weight at ladle open (for the same steady weight of 45 tonnes) is illustrated for the large tundish in Figure 8. Increasing the tundish weight at ladle open from 17 15 to 45 tonnes is seen to extend the intermixing range considerably. In a real operation, the intermixing would be even greater, because the tundish plug flow fraction would likely decrease as well. This increase in intermixed length is the reason why many operations drain the tundish down before opening the new ladle, despite the quality problems such as reoxidation and slag entrainment that can be encountered. The magnitude of this effect depends on the size of the tundish. For the small tundish, the effect of tundish weight history is much less. In fact, draining the tundish to only 1 tonne (run 3) improves mixing by less than 4 tonnes (13%), relative to leaving the weight constant at its steady value of 11 tonnes throughout casting (run 2). This finding simply shows that mixing in the strand controls the intermixed length once the tundish weight is low enough. In addition to lowering the minimum and maximum tundish weights, delaying the refilling is also beneficial. Holding the weight low after opening the new ladle and / or a slower filling rate to steady state (run 4) both tend to shorten the intermixed length. The effect is relatively minor, however. Refilling gradually over 12 - 15 minutes, including a 3-4 minute hold after ladle open reduces the intermixed tonnage by only a few tonnes, relative to the 2-3 minute total refilling time for standard conditions. As expected, the effect is greater for the larger tundish. A slower filling rate makes little difference to intermixing after a long hold time. If the hold before refilling is long enough to drain old grade from the tundish, this indicates there is no added benefit from slow refilling. Deciding how long to wait before refilling the tundish should depend on the extent of the mix: stringent new grades benefit from holding a low tundish level longer before refilling; lenient new grades can refill quickly with no difference in intermixed length
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07/07/16 10:37:49moi nguoi dich giup minh voi1. Effect of stringent to lenient grade order When two grades have the same relative grade specification, (e.g.. 10 : 90 or 40 : 60), then the model predicts that the order in which they are cast should not matter. This is because mixing is dominated by turbulent flow, so all elements should diffuse in a roughly equal manner (according to their concentration). When two grades which have very different tolerances for mixing with the other must be cast together... Xem thêm.Xem thêm 5 bình luận
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