The aim of this contribution is to provide an overview on results achieved by the application in the open continuous casting practice of a solid-liquid dispersion of a mold flux called PROIL™. The use of PROIL™ has delivered significant improvements in the process in terms of billet surface defects reduction, expanding casting grade capability, and sharp breakout depletion. Additional benefits have also been achieved in better billet square shape and reduced sparkling phenomena in the mold. These results have been confirmed in use by several customers, and hundreds of tons of this material have been consumed so far. Specific equipment has been engineered for delivering and dosing PROIL™.
PROIL™ is a trademark registered by Prosimet spa.
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1 Introduction
Since the beginning of the technology of steel continuous casting, oils of different nature have been used as mold lubricants. Later, the use of mold flux resulted in a sharp increase of semis surface quality, which attained the level of current standard of industrial applications. However, mold flux has seen implementation only in close casting technology, while almost no advancement has been made on lubricants for an open casting set up. Furthermore very few literature is available regarding this kind of lubrication.
This contribution will show the benefits of the application of PROIL™, a new product consisting of a dispersion of a specific casting powder in synthetic oil. An extensive use of PROIL has proved to be effective in achieving several improvements comparable with results typical of close casting environment These results have been confirmed in practice in several plants worldwide. This contribution will also highlight the value innovation added to some specific customers that results in an outstanding increase of casting process efficiency.
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In the final part, the specific equipment engineered for delivering and dosing PROIL into the mold will be described; the typical thixotropy of PROIL and its influence on handling will be considered under a field experience perspective.
2 Oil Lubrication, Main Principles
The process of continuous casting was invented in the 1950’s in order to increase the efficiency of steel production. Since then, continuous casting has been developed further to improve on yield, quality, and cost effectiveness. Traditionally oil has been used as a lubricant for casting small sized billets in open stream from the tundish, while slab and bloom molds have been lubricated by mold fluxes and steel is cast through a submerged entry nozzle (Fig. 1). A large amount of literature is nowadays available regarding mold flux application whereas the precise lubrication role of the oil, which pyrolyzes in contact with the liquid steel, is not completely clear [1].
Fig. 1
Schematic diagram of casting set-up for billets
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During the casting, the oil is pumped to an oiling plate at the top of the mold, from which it seeps down inside of the mold wall through a narrow slot; this may represent an issue if a clogging occurs in this area.
A variety of oils, such as vegetable, synthetic, and blends, can be used, the feed rate of lube oil is typically in the range of 0.014 to 0.17 (standard value 0.05) ml per min per mm of mold periphery for a 125 mm square billet [2]. A proper lubrication by oil is essential to ensure a good billet quality in terms of surface defects and cracks; additionally, the presence of oil at and below the meniscus is likely to affect heat transfer rates in the mold and is an important factor in billet shrinkage. On the other hand, an excessive oil flow is uneconomical and contributes to the formation of pinholes, dangerous flames, and smoke on the casting floor.
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Heat from the strand surface is transferred to the mold cooling water through a series of thermal resistance layers, such as the air gap between mold and strand, the mold wall, and the mold cooling water interface. Of these, the air gap constitutes the largest resistance to the heat flow and accounts for as much as 84% of the total resistance, and as a consequence the pattern of the heat removal in the mold is largely dependent on the dynamics formation of this gap.
3 PROIL, a Liquid Casting Powder
3.1 The Starting Point
By exploring new possibilities for mold flux technology applications, different powders to be used in open casting have been formulated. The most important feature of these products is a very fast melting with little consumption of heat. It has been observed that the use of such a product leads to several advantages in the final quality of the billets.
On the other hand, a major holding point of using this type of powder is the problematic mold feeding of the powder itself resulting in an erratic increase of NMI (non metallic inclusion).
Taking into account the promising results of this concept application in open casting, it has been decided to overcome the problem of mold feeding by mixing casting powders in a specific liquid medium; this formulation has been tested with a reported outstanding performance since the early beginning [3].
3.2 Product Features
The oil-powder mixture has been formulated to comply with two main properties closely interwin ed: viscosity and sedimentation. The key principle for designing this kind of material can be summarized like this:
“Viscosity must be low enough to ensure an acceptable flowability and at the same time high enough to ensure the lowest rate of Sedimentation”.
This result has been achieved by combining a proper liquid medium (a synthetic oil) with both specific viscosity and density and the use of additives able to stabilize this solid-liquid dispersion [4].
The final product shows the typical behavior of a thixotropic material with a shear thinning response (Fig. 2); thanks to that, PROIL has proved to be extraordinarily stable with no bottom settling after several months on standing. Due to this shear thinning property, it is a common practice to stir the product before using it in order to reduce its viscosity to a suitable level for being pumped and dosed into the mold (Fig. 3).
Fig. 2
PROIL appearance
Fig. 3
The thixotropy behavior and shear thinning response
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3.3 How PROIL Works
The best definition of this material is offered by its name: it is a powder, in this case a casting powder, with the characteristics of a liquid. More precisely this product is working as a liquid slag of a casting powder pumped intto the mold at room temperature. This is possible due to the fact that the solid component (a formulated casting powder) of this dispersion in synthetic oil is melting very fast at the expense of heat produced by burning off the liquid medium. This feature perfectly matches normal operations of continuous casting of long products in open steel stream, where thermal insulation of the liquid steel bath is not an issue. PROIL actually provides a liquid slag with all known properties of commercial casting powder for continuous casting in the close steel stream.
The product is fed through a pipe over the top of mold and dripped down directly on the liquid steel with no need to distribute it on all faces of the mold (Fig. 4); the pressure produced by the first impact wave of the steel stream pushes this slag towards the mold walls. Thanks to its balanced slag surface tension and viscosity, PROIL is able to provide a proper wettability and spreadability, avoiding any potential slag entrapment. From field experience it has been observed as a general consequence that a feeding rate fixed below a critical limit does not give any interference with the liquid steel stream.
Fig. 4
PROIL feeding into the mold
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Due to that, it is very easy to explain one of the first phenomena observed when liquid powder is used, which is the strong reduction of pyroclastic activity or sparkling, an event normally dangerous for operators This slag formed on top of liquid steel is dragged in the mold-strand gap and enhances the lubrication; due to its higher conductivity, the heat transfer during the solidification process results in a more homogeneous and balanced support of the billet withdrawal.
“By looking under a mold design perspective, it is thus possible assume that both length and taper are somewhat increased”
To sum up, by replacing oil with PROIL (Fig. 5), the following outcomes have to be considered:
1.
Enhanced lubrication and heat removal
2.
More uniform heat transmission
3.
Molten slag layer formation on liquid steel
Fig. 5
Lubrication comparison of oil vs PROIL
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All of these can influence the casting process with astonishing results.
4 Plant Experience and Main Achievements
Throughout the years, several improvements have been achieved depending on which effect has been more highlighted during the use of PROIL.
4.1 Strand Breakout and Casting Speed
By considering point 1 in the section above, an improved cooling in the primary cycle is usually reported during the PROIL application; this can be observed by a thermal scanning of the billets after the straightener or by checking DT water in the cockpit. Sometimes a T difference between oil and PROIL lines around 50–60 K has been reported. Since the cooling is increased, a thicker shell is formed at the same casting speed (Table 1) reducing the strand breakout occurrence. A RHIM customer (A) that decided to switch to PROIL reported a drop in B/O figures of around 80% (Fig. 6).
TABLE 1
Plant A
Casting condition
Square section (mm)
Casting speed (m/min)
Steel grades
Casting radius (m)
160
2.15
AISI 1006, 1008, 10B06, 10B26
9
Fig. 6
Strand B/O reduction plant (A)
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Of course, an increased cooling capacity in the first cycle may allow the increase of the casting speed and the final productivity. According to the plant trial experience, it has been possible to raise the velocity by about 10% and sometimes even by 20%.
4.2 Off Squareness
Rhomboidity or off squareness as shape defects in square billets has been well known since the earlier times of casting technology, especially in the open stream practice; the main consequences of this occurrence may affect the billet quality and the rolling operation. Rhomboidal billets usually present off corner cracks in the obtuse corner, while, in the rolling phase, off squareness may bring about problems in billet movement, in particular during reheating. This last issue may result in billets jumping at the furnace entrance causing a pile formation and at the end rolling downtime [5].
Rhomboidity is determined by the difference between the two diagonals (d1 and d2) expressed in mm or percentage (%):
In literature it is possible find several suggestions in order to reduce rhomboidity: those consider primary and secondary cooling, oscillation table modification, adjustment of foot rolls and so on.
By using PROIL, due to its more uniform heat transmission, a general rhomboidity reduction has been detected (Tables 2 and 3); the magnitude of this reduction may range depending on casting conditions. In general, the larger the casted section, the more noticeable the decrease; a similar behavior has been detected by casting higher alloyed grades (Fig. 7).
TABLE 2
Rhomboidity (%) reduction at plant B
Romb. (%)
Oil (N. of billets)
PROIL (N. of billets)
0–6%
15
33
6–12%
24
9
>12%
3
0
TABLE 3
Casting condition (about 40 billets compared)
Casting condition
Square section (mm)
Casting speed (m/min)
Steel grades
Casting radius (m)
150
2.25
MC
C 0.4%;
Mn 0.65%
10
Fig. 7
a,b Same strand casted first with oil and then with PROIL
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It is to be stressed that the higher the distortion, the more relevant and effective is the reduction (see rhomboidity data > 12% in Tables 2 and 3).
A different plant (C) has tested PROIL with the conditions shown in Table 4 (higher alloyed grades and larger sections):
TABLE 4
Test at plant (C)
Casting condition
Square section (mm)
Casting speed (m/min)
Steel grades
Casting radius (m)
160
2.05
C 0.4%;
Si 0.18%
Mn 0.65%
9
In this case the final results in terms of off squareness have been even more evident (Table 5).
TABLE 5
Rhomboidity results at plant (C)
Rhomboidity
mm
mm
mm
mm
mm
mm
mm
mm
mm
mm
mm
Average
Oil
13
15
11
8
5
8
6
12
10
–
–
9.8
PROIL
1
2
2
3
1
2
2
2
3
3
4
2.3
4.3 Scale Effect
Also scale effect can be positively influenced by a more uniform heat removal coming from the PROIL application (Fig. 8).
Fig. 8
Billet casted with oil and PROIL (arrow pointed)
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Usually scale oxide formation is visible at secondary cooling and causes a yield loss of about 1–2%, but sometimes under particular circumstances this may peak up to 3–5% and significantly disturb the process (impaired surface quality, pickling occurrence, and so on) [6]. From some PROIL field experience, a scale reduction of around 30% has been detected.
4.4 Reduced Sparkling and “Splashing” Occurrence
One of the most appreciated (in particular by the floor operators) improvement is the reduction of this tedious and at the same time dangerous phenomenon. As mentioned earlier, this is the result of the molten slag layer formation on the surface of liquid steel and PROIL may help not only by improving safety, but also by providing a longer cleaning cycle of some exposed parts of CCM as well as increasing the operational life of some components such as nozzles (plant A for instance reported a doubled working life of these) (Fig. 9).
Fig. 9
Mold in operation with oil (a) and PROIL (b)
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4.5 Possibility to Cast New Grades (B Grades)
Usually an open stream set up is used to cast “low value” grades such as reinforced steel bar grades. Indeed, it is quite common that, if a plant decides to move to “higher value” products, it has to change the casting set up and switch to close casting; this may represent a costly investment in money and time. It is worth noting that sometimes this change might not completely succeed.
An interesting topic is represented by the Boron steel grades, a range that has proved to be able to replace carbon steel and low-alloyed steel in several applications, such as automotive and mechanical industry. By using the Boron-added steel, it is possible produce cold heading quality (CHD) items (such as bolts, screws) by simple wire manufacturing process (Fig. 10; [7]).
Fig. 10
Some items produced by cold drawing process
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Actually most of the B grades are casted by a submerged entry nozzle and mold flux use. Plant (A) is now able to produce these qualities in open casting by applying PROIL and with N2 inert bellow. In the past they had to outsource these qualities to a plant producing these in a close casting environment.
We can state that it is possible to run an open stream CCM achieving the same results as in a close casting environment (Fig. 11).
Fig. 11
Open or close casting
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5 PROIL System—A Few Facts About Dosing Equipment
Most important for the application of PROIL, in terms of reliability, working life and cost, has been the selection of a suitable and accurate system for dosing it into the mold. Actually, the best way to feed PROIL is using a diaphragm pump electrically actuated and controlled by an inverter. By adjusting the stroke length and the frequency, it is possible to feed PROIL into the mold with the necessary flow accuracy. According to the field experience acquired, the expected flow rate has been set at about 100 g of PROIL/ton of steel casted.
It is crucial to reduce the operative viscosity before feeding the pump and to keep it constant during the application. This can be done by vigorously stirring PROIL before and during its use (Figs. 12 and 13).
Fig. 12
Multistrand dosing system. (Courtesy of Stopinc AG)
Fig. 13
An example of very effective drum stirrer
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Recently a new device based on a different concept has been tested, the development regarding this part is ongoing.
6 Conclusions
The practice in using PROIL during last few years and in different conditions has showed that replacing the standard lube oil with a liquid powder is technically possible. Moreover this new product has made it possible to get results comparable to the ones typical for the close casting set up by keeping the open stream casting environment.
The extensive application of PROIL in the field has proved to be effective to achieve the following results:
Reduce strand breakout and or increase casting speed
Reduce rhomboidity
Reduce scaling effect and improve surface quality
Reduce sparkling and other pyrophoric phenomena typical of open casting
Increase the potential steel grades portfolio casted in open stream
and maybe more to come …
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