The shape and main features of the crystallizer of thin slab and medium thin slab continuous casting machines are introduced. The effects of the surface area of molten steel, molten steel flow, mold heat transfer, thin slab thickness, billet drawing speed, etc. on the continuous casting process and slab properties in different types of molds were comparatively analyzed. The advanced technology adopted by Angang ASP was discussed, and it was pointed out that the thickness of the molds of newly built and renovated thin slab continuous casters in recent years has shown an increasing trend, thereby solving some problems in equipment operation and improving the quality of the cast slabs.
Keywords: medium and thin slab; copper mould tube ; slab quality
The crystallizer is the slab forming device on the continuous casting machine and one of the core equipment of the continuous casting machine. Its function is to dissipate heat through forced cooling of the molten steel continuously injected into the inner cavity of the mold. The molten steel is gradually solidified into a cast slab with the required cross-sectional shape and a certain shell thickness, and the cast slab with the core still in the liquid phase is continuously pulled out from the lower opening of the mold. During the solidification process in which molten steel is injected into the crystallizer and gradually forms a billet shell of a certain thickness, the crystallizer is subject to the combined effects of mechanical stress and thermal stress at the same time. Its operating status is directly related to the forward flow of the continuous casting machine, the quality and output of the cast slab. The crystallizer technology of conventional large slab, bloom and billet continuous casters at home and abroad has been basically mature. This article only discusses the crystallizer technology of thin slab and medium-thin slab continuous casters.
According to incomplete statistics, as of the end of 2008, a total of 65 thin slab (including medium-thin slab) continuous casting and rolling production lines had been built in the world, with an annual production capacity of 110.08 million tons. Among them, the CSP production process accounts for more than 50% of the world’s total thin slab continuous casting and rolling production capacity; the FTSR process accounts for 18.35% of the total production capacity. Among them, China has built a total of 14 thin slab continuous casting and rolling production lines of different types, with an annual production capacity of 35.3 million tons.
Types and main features of crystallizers
The core of thin slab and medium thin slab continuous casting technology is the crystallizer. The design requires that there must be enough space in the meniscus area of the mold to insert an immersion nozzle, and that no solidification bridge is formed between the nozzle wall and the crystallizer wall. The temperature distribution of the molten steel must be uniform, which is conducive to the melting of the mold slag. The liquid steel flows smoothly in the meniscus area to prevent excessive turbulence and slag entrainment. The mold geometry should be such that the stress on the billet shell is minimized during billet drawing.
Figure 1 shows the first-generation vertically curved crystallizer of Demark’s ISP process. The upper part is a vertical section, the lower part is an arc section, the side plates are adjustable, the upper section is rectangular, and the size is (60~80)mm×( 650~1330)mm. After the Arvedi production line in Italy adopted this crystallizer, it was found that this shape of crystallizer could only use a lamellar immersed nozzle. Moreover, even if the long nozzle of this special shape is very thin, it can only maintain a gap of 10 to 15 mm between the nozzle and the crystallizer wall when inserted into the mold, resulting in poor melting of the mold slag on the wide side where the nozzle is inserted. And it is difficult to obtain a constantly controllable mold slag layer, and the surface quality of thin slabs is poor.
Figure 1 Schematic diagram of Demark’s ISP process vertical curved crystallizer
The Arvedi Factory began to improve the crystallizer in 1993 and redesigned its upper cross-sectional shape from the original parallel plate shape to a small funnel shape. The maximum thickness of the wide edge of the upper mouth of the crystallizer is 60+(10×2) mm. This shape is maintained until the lower mouth of the crystallizer still has a small bulge of (1.5×2) mm. In recent years, the bulge of its crystallizer has become larger and larger. The maximum thickness of the wide edge of the upper mouth now used has reached 60+(25×2) mm, and the lower mouth is 60+(5×2) mm. The submersible nozzle is still in the shape of a thin sheet. Although the wall thickness has increased, the gap from both sides of the wall has been greatly increased, which improves the melting condition of the mold slag layer and the surface quality of the thin slab has also been greatly improved.
Figure 2 shows the funnel-shaped crystallizer used in the CSP process of SMS company. There are parallel sections on both sides of the wide edge of the upper opening, which are connected to the arc sections. The upper opening has a larger cross-section. This funnel shape is maintained in the mold to a length of 700 mm, and the thickness of the slab at the exit of the mold is 50 to 70 mm. The total length of the crystallizer is 1120 mm. The funnel shape of the upper mouth is conducive to the immersion of the immersion nozzle, and a tapered space is formed in the vertical direction between the two wide panels of the crystallizer. The two side walls outside the funnel area are still parallel, and the distance between the two side walls is equivalent to the thickness of the slab. The shape of the funnel-shaped crystallizer meets the requirements of long nozzle insertion, mold slag melting and thin slab thickness. It has been used in multiple production lines and has achieved good results.
Figure 2 SMS CSP process funnel-shaped crystallizer
The innovative point of the funnel-shaped mold is that it breaks the limitation of the traditional slab continuous casting mold that the cross-section is the same. The shape and size of the solidification shell in the mold cavity changes according to the law of gradually shrinking the non-rectangular cross-section. However, the molten steel will deform when solidifying in this mold. In particular, the stress generated by mechanical deformation during billet drawing may lead to the occurrence of solid-liquid interface cracks and ultimately affect the quality of hot-rolled coils. Therefore, the ideal shape of the funnel-shaped crystallizer is to minimize the bending deformation rate of the two-phase area between the shells, so that the actual deformation rate of the solid-liquid interface of the shell during the deformation process is less than the critical strain rate for crack generation. Based on the above requirements, the funnel-shaped crystallizer must ensure that the curvature of the thickness transition zone is designed accurately and the pulling speed is as stable as possible.
Beijing Iron and Steel Research Institute and other units have conducted in-depth research on thin slab continuous casting mold technology and believe that funnel-shaped mold technology fundamentally solves the service life problem of immersed nozzles, making efficient continuous production of thin slabs a reality. . At the same time, due to the large surface area at the top of the funnel-shaped crystallizer, conditions are created for the melting of the mold slag.
The disadvantage of the funnel-shaped mold is that the deformation of the billet shell in the mold is prone to cracks, which limits the continuous casting of thin slabs of difficult-to-cast varieties such as peritectic steel. The transition section of the crystallizer must be designed in conjunction with the calculation of the shrinkage of the molten steel itself. Although the transition section from arc to straight line in this type of mold is only 100mm, and it only takes a few milliseconds for the slab to pass through this section at a pulling speed of 5 m/min, it still needs to be given great attention. The finite element method is generally used to optimize the design of the deformation section. In order to reduce or slow down the deformation of the billet in the funnel-shaped mold, the following measures can be taken:
(1) Minimize the opening of the funnel;
(2) Transition the billet from funnel shape to rectangular shape and lengthen the deformation section;
(3) Optimize the shape design of the inner cavity of the mold to make the metal deformation distribution in the mold more uniform and gentle, reduce the deformation rate, and reduce the possibility of cracks in the cast slab.
The maintenance of the funnel-shaped crystallizer is not much different from that of the conventional continuous casting crystallizer. Main attention should be paid to avoiding scratches on the surface of the copper plate and the adhesion of residual steel. Sufficient lubrication should be ensured during the casting process to prevent steel splashing, steel overflow and steel leakage accidents caused by nozzle rupture.
Considering that the funnel-shaped mold is not suitable for casting narrow medium-width thin slabs, the parallel plate-shaped straight mold has a flake-shaped immersed nozzle that is too costly and difficult to cast thin slabs. Therefore, two key improvements were made to the funnel-shaped crystallizer.
(1) The graphic scheme of the funnel-shaped crystallizer has an arc area in the middle area of the chord length, and the arc radius continuously increases from top to bottom. Both sides of the arc area are inclined triangular plane areas. This circular scheme is characterized by any cross-section parallel to the X-axis. The angles are equal and equal to the starting angle, so the middle arc on any section is not tangent to the straight lines on both sides but intersects.
(2) The funnel-shaped opening is an important parameter in designing this type of crystallizer. The opening of the crystallizer is large, and the slope of the billet shell is also large when it moves downward, resulting in an increase in the pulling resistance. By appropriately changing the cross-sectional shape of the immersed nozzle and reducing the opening from 170 mm to 120-140 mm, the casting effect is better.
The funnel-shaped crystallizer has two major shortcomings: short service life and the shape of the mold cannot ensure the best surface quality of the cast slab. After a long period of research and heat transfer calculations, SMS company has optimized the shape of its upper part to ensure that no vortex is generated after the molten steel enters, and greatly reduces surface defects of the cast slab. The automatically controlled water cooling system ensures reasonable heat transfer on the upper part of the copper plate and improves the cooling effect. The water flow rate in the cooling water tank outside the copper plate is controlled to be above 10 m/s, the water pressure is above 0.6 MPa, and the evaporation of water is reduced. The life of the crystallizer is greatly improved, generally between 200 and 500 furnaces, and can still be used after grinding. The optimized design of the funnel-shaped crystallizer is 1100mm long; the maximum bulge on one side of the copper plate of the crystallizer is 50-60mm; the height of the funnel area of the copper plate is 750-850mm, and the lower part of the crystallizer is flat. Use water tank cooling parallel to the copper plate working surface. The CSP thin slab continuous caster of Maanshan Iron and Steel Co., Ltd. in my country adopts this optimized design of funnel-shaped crystallizer. Practice has proved that for every 5mm increase in the bulge of the funnel-shaped crystallizer, the surface quality can be improved exponentially. Obviously, this is closely related to the increase in the upper opening area, which is beneficial to the addition, melting and adsorption of inclusions.
In addition to being related to the shape of the crystallizer itself, the flow characteristics of molten steel in the CSP funnel-type crystallizer are mainly affected by the immersed nozzle structure. During production, it was discovered that the funnel-shaped mold also had design flaws when pouring slabs of different steel types and specifications, which were mainly manifested as longitudinal cracks in the slab, adhesion during the continuous casting process, and more steel leakage accidents. In response to these problems, Beijing Iron and Steel Research Institute cooperated with Tangshan Iron and Steel Co., Ltd. to optimize the wide and narrow copper plates of the funnel-shaped mold to meet the high drawing speed requirements for the production of low carbon steel and reduce crack defects.
The H²-shaped crystallizer is the core equipment of Italian Danieli Company’s FTSC process (Flexible Thin Slab Casting) thin slab continuous casting machine. The shape of this crystallizer is shown in Figure 3. Its advantage is that the internal volume is large, the flow rate of the passing molten steel is also large, and it has a better natural deceleration effect of the molten steel. The length of the crystallizer is 1200 mm, the width is 1220~1620mm, and the thickness is 55, 60, 65, and 70 mm.
Figure 3 H²-shaped crystallizer of Danieli’s FTSC process
The main feature of this crystallizer is that its bulging shape runs through the entire copper plate from top to bottom and continues to the middle of the fan-shaped section 1. A set of rollers with holes are specially designed at the exit of the mold to level the bulging shape of the cast billet. The length of the equipment for leveling the bulging billet is twice as long as when only the crystallizer of a continuous casting machine is used. That is to say, the stress on the shell is greatly reduced compared with just using the crystallizer to flatten the bulge of the shell. Moreover, the internal volume of the H²-shaped crystallizer is increased and can hold more molten steel. At the same time, the enlarged size of the upper part of the mold can make the design of the nozzle shape more reasonable, ensure the stability of the liquid level in the mold, improve the lubrication effect of the mold slag, improve the heat exchange conditions, increase the drawing speed, and reduce the tendency of cracks.
The main difference between the H²-shaped crystallizer and the funnel-shaped crystallizer is that the deformation of the green shell in the crystallizer is not completed at a certain height, but the shape and size of the solidified shell along the entire height are gradually reduced according to the non-rectangular cross-section. Danieli believes that the H²-shaped mold can better control the formation process of the initial shell and produce high-quality slabs.
Parallel plate straight crystallizer
Figure 4 shows the parallel-plate straight crystallizer of Voestalpine’s CONROLL process. The immersed nozzle is also flat, and the molten steel flows out from both sides of the nozzle. The cross-sectional size of the crystallizer is (70~135) mm×1500mm. This type of thin slab is actually a medium-thin slab. From the perspective of energy conservation and consumption reduction, Voestalpine analyzed and concluded that the production of 70-90mm thick slabs consumes the least energy and has low processing costs. It also believes that there is no need to pursue too thin a slab thickness and tends to be medium-thick. From the perspective of the shape of the mold, Voestalpine emphasized that only when the molten steel does not deform when solidifying in it and the liquid level is kept stable can it be beneficial to eliminate cracks on the surface of the slab. To promote the floating of inclusions in the molten steel in the crystallizer and prevent slag entrainment, it is recommended to use a parallel plate-shaped straight crystallizer.
Figure 4 Voestalpine CONROLL process parallel plate straight crystallizer
The cross-section of the inner cavity of the parallel plate-shaped straight mold is congruent rectangular from top to bottom, which can avoid shell stress caused by deformation of the cast slab when the slab thickness is thin. Moreover, the crystallizer heat can be ensured to be evenly dissipated in the width and length directions. However, when the cast slab is thin, the immersed nozzle must be made into a thin sheet. From the perspective of the stress on the shell, the parallel-plate straight crystallizer is better than the funnel-shaped and H²-shaped crystallizer; from the perspective of space size, the funnel-shaped and H²-shaped crystallizer is better than the parallel-plate straight crystallizer. Both Demag and VAI companies have adopted flake-shaped immersed nozzles specially developed for thin slab continuous casting machines to improve the condition of mold slag. However, there is only a 10-15mm gap between the thin immersed nozzle inserted into the mold and the crystallizer wall, resulting in poor melting of the mold slag on the wide side where the nozzle is inserted. And it is difficult to obtain a constantly controlled protective slag layer, which affects the surface quality of the thin slab. In addition, in view of the special shape and harsh working conditions of the lamellar immersed nozzle, the service life of the nozzle is still low despite the use of advanced refractory materials such as BN and ZrO.
At present, most thin slab continuous casting machines with a thickness of 50 to 90 mm use funnel-shaped crystallizers.
Effect of mold performance on thin slab continuous casting
Surface area of molten steel in the crystallizer
It can be seen from calculations that the surface area of the molten steel in the funnel-shaped mold is approximately 1.4 times the cross-sectional area of the slab. The molten steel surface area of the H² mold is larger, while the molten steel surface area of the parallel-plate straight mold is equal to the cross-sectional area of the cast slab. The surface area of molten steel is large, the steel flow is easy to stabilize, and a large amount of molten steel is stored, which is conducive to controlling the stability of the liquid level and the floating of inclusions. At the same time, there is sufficient heat to ensure the melting of the mold slag.
Liquid steel flow in the crystallizer
During the billet drawing process, according to the principle of equal flow of molten steel along the cross-section, the downward flow rate of molten steel in the funnel-shaped mold varies greatly along various parts of the cross-section. Therefore, it is easy to cause turbulence and surface slag inclusion. The higher the pulling speed, the more obvious this phenomenon is. The funnel of the H² crystallizer tends to be flatter, which is much more stable than the funnel-shaped crystallizer. This problem does not exist in the parallel-plate straight crystallizer.
Compared with the parallel-plate straight crystallizer, the upper opening of the funnel-shaped crystallizer ensures sufficient insertion space for the immersed shroud and melting of the mold slag, and provides favorable conditions for the use of thicker-walled shrouds. However, the design and manufacture of the funnel-shaped crystallizer is complex, maintenance and processing are difficult, the friction resistance increases during billet drawing, and the operating cost is high. The parallel plate straight crystallizer is the same as the traditional slab continuous casting machine, with easy maintenance and low consumption.
Heat transfer in the crystallizer
The thermocouple test in the crystallizer shows that the parallel plate-shaped straight crystallizer has a certain heat transfer in the width direction (see Figure 5) and can obtain a uniform solidified shell. The two-dimensional temperature analysis results show that the funnel-shaped crystallizer has uneven heat transfer.
Figure 5 Temperature distribution of the crystallizer copper plate
The parallel plate-shaped straight crystallizer is conducive to the lateral flow of molten steel (caused by the molten steel sprayed from the nozzle) and the lateral convection caused by the temperature difference, so it is beneficial to reduce the temperature difference in various parts of the molten pool. In the funnel-shaped crystallizer, due to the enlargement of the funnel area, convection is affected, and the temperature in the area near the meniscus is low, which easily forms a “bridging” phenomenon. For parallel plate straight molds, the distance between the immersed nozzle and the mold wall in the thickness direction of the slab needs to be adjusted to ensure that the steel does not solidify.
Thickness of thin slab
The increased thickness of the cast slab is conducive to the floating of inclusions in the molten steel in the mold. The slab is thicker and the compression ratio is large, which is conducive to improving the coil quality. In addition, increasing the diameter of the immersed nozzle can extend the service life of the nozzle and increase the number of continuous casting furnaces, which is beneficial to improving the production capacity of the continuous casting machine. When the output is similar, a lower pulling speed can be used, which reduces the wear of the crystallizer and reduces the probability of leakage. Under the condition that the required roll weight is the same, the length of the slab should be short, and the length of the conveying roller and heating furnace should be short, which can save investment. The slab thickness and mold shape selection options for several thin slab continuous casting processes are shown in Table 1.
Table 1 Selection of slab thickness and mold shape for medium and thin slab continuous casting process
|Thin slab thickness/mm
|Parallel plate straight crystallizer
|Parallel plate straight crystallizer
|Parallel plate straight crystallizer
In recent years, with the development of thin slab continuous casting and rolling technology, especially the successful use of liquid core soft reduction technology. Taking into account the market’s requirements for coil quality and increasing the output of continuous casting machines, the area of the upper opening of the thin slab continuous casting mold has gradually increased. It is beneficial to uniform the composition and temperature of the molten steel, keeping the liquid level stable, preventing slag entrainment and reducing surface defects of the cast slab. From the perspective of energy saving and consumption reduction, VAI believes that 70~90mm thick slabs consume the least energy in production and have the lowest processing costs, tending to be of medium thickness. Sumitomo Corporation also analyzes market demand and strives to fully reach the level of large-scale steel conglomerates in terms of product variety and quality. It insists on developing medium plate continuous casting machines with slab thicknesses of 90 to 120 mm. The original Demag company redesigned the shape of the upper mouth surface from the original parallel plate shape to a small funnel shape. The maximum thickness of the wide edge of the upper opening has been increased to 60+ (2×25) mm after several improvements, and this bulge is extended to the exit of the crystallizer to remain at 60+ (2×50) mm. In this way, although the wall thickness of the flaky immersed nozzle has increased, the gap between it and both sides of the mold wall is still greatly increased, which improves the melting condition of the mold slag and also greatly improves the surface quality of the thin slab. The CSP has increased from 50mm to more than 70 mm, and the thickness of the FTSR billet has increased to more than 90 mm.
Regarding the width of the thin slab, the funnel-shaped mold is limited by the solidification shrinkage of the molten steel, and the minimum slab width is 600 mm, while the parallel-plate straight mold has no such restriction.
Liquid level fluctuation, vibration and lubrication
Since the continuous casting speed of thin slabs is higher than that of traditional slabs, the stability of the mold liquid level is even more important. It is necessary to maintain a sufficient liquid slag layer on the mold steel liquid surface, which can continuously leak into the gap between the billet shell and the copper plate, and provide good lubrication to effectively prevent bonded steel leakage and longitudinal cracks on the cast billet surface. However, the thin slab drawing speed is high and the crystallizer space is limited, making it difficult to obtain a constant liquid slag layer, and the slag consumption is significantly reduced compared with traditional slabs. To resolve this issue, the following measures were taken:
(1) Use mold powder with low melting point and low viscosity.
(2) The crystallizer adopts high frequency (more than 300 times/min) and small amplitude (3 mm) non-sinusoidal vibration to reduce the tensile stress on the primary green shell and reduce vibration marks, and improve the surface quality of the cast slab.
ASP continuous caster crystallizer
Anshan Iron and Steel’s second steelmaking 1700 mm production line has two ASP continuous casting machines (first generation ASP), with a design capacity of 2.4 million tons per year. It uses a parallel plate-shaped long crystallizer (see Figure 4), and the pulling speed is 2.5~3.0m. /min, the cross-sectional size of the crystallizer is (100~135) mm× (900~1550)mm, the height of the copper plate of the crystallizer is 1200mm, and the thickness of the copper plate of the crystallizer is 27 mm. Anshan Iron and Steel Co., Ltd. has two dual-flow ASP continuous casting machines (second-generation ASP) with a design capacity of 5 million tons per year. It also uses a parallel plate-shaped long crystallizer with a cross-sectional size of (135~170) mm × (900 ~2000)mm, pulling speed: 1.8~3.5m/min.
Advanced technology adopted
The main features of the ASP continuous casting machine crystallizer are basically the same as those of the VAI CON-ROLL process parallel plate straight crystallizer. The advanced technologies used in the two generations of ASP continuous casting machine crystallizers are:
(1) The advanced technologies adopted by Angang No. 2 Steelmaking 1700ASP continuous casting machine include online mold width adjustment technology, mold expert system, automatic mold liquid level control, steel breakage prediction technology, and mold hydraulic vibration technology. ①The vibration device uses two servo-controlled hydraulic cylinders to achieve sinusoidal and non-sinusoidal vibration, with a vibration frequency of 40~300cpm and an amplitude of 1~12 mm (±0.5~6 mm). The advantage is that the amplitude, frequency and non-sinusoidal coefficients can be adjusted online. This vibration mode has no wear, no maintenance, easy replacement, short replacement time, shallow vibration marks on the slab, and high surface quality of the slab. ② Steel breakage prediction technology, in order to prevent steel breakage caused by bonding between the molten steel and the crystallizer wall, two rows of 24 thermocouples are arranged on the wide and narrow side copper plates of the crystallizer. During casting, based on the measured changes in the temperature of the mold wall, the tendency of possible steel breakouts can be predicted in a timely manner so that measures can be taken to reduce or avoid the occurrence of steel breakouts. ③Quick replacement technology of crystallizer immersed nozzle. The crystallizer immersed nozzle is externally installed on the intermediate tank. In order to meet the needs of 16 continuous casting furnaces, the immersion nozzle needs to be replaced once during the casting process. This technology can ensure continuous pouring during the replacement of the nozzle, and the replacement time is only 3 seconds, creating conditions for continuous pouring of multiple furnaces to improve the operation rate of the casting machine. ④The cooling water tank is installed on the back plate of the crystallizer, which reduces the thickness of the copper plate and saves a large amount of copper plate material when the number of uses remains the same.
(2) The advanced technologies used in the parallel-plate long crystallizer of Anshan Iron and Steel Co., Ltd.’s 2150ASP continuous casting machine include ladle slag detection technology, crystallizer immersion nozzle quick replacement technology, crystallizer copper plate cooling technology, and crystallizer hydraulic online width adjustment technology. Hydraulic vibration technology and crystallizer expert system. Among them, the crystallizer expert system can perform steel breakout prediction, crystallizer heat flow monitoring, crystallizer vibration monitoring, and crystallizer friction monitoring.
From 2006 to 2008, Anshan Iron and Steel Co., Ltd. mass-produced more than 700,000 tons of X70-X80 hot-rolled coils at the 2150ASP production line. The coil structure is acicular ferrite mixed structure, with uniform and fine grains and few inclusions. The product meets the technical requirements of China’s Sichuan-East Gas Pipeline Project and China’s Second West-East Gas Pipeline Project.
Funnel-shaped crystallizers, H² crystallizers and parallel plate-shaped straight crystallizers each have their own characteristics. The surface area of the molten steel in different types of molds, the flow of molten steel, and the heat transfer of the crystallizer all have an impact on the continuous casting process and the performance of the cast slab. Relying on the support of a series of related technologies, all types of thin slab and medium-thin slab continuous casting machine molds can meet the needs of industrial mass production. At present, my country’s self-processed funnel-type crystallizer for thin slab continuous casting machines has been widely used in many thin slab continuous casting and rolling production lines.
Ansteel ASP also adopts a series of advanced technologies in the parallel plate straight crystallizer. The inclusions of IF steel cast by the ASP process are significantly reduced, and defects such as edge cracks and warping of pipeline steel are greatly reduced, enabling defect-free medium-thin slabs to be directly loaded into the furnace for rolling.
In recent years, with the widespread adoption of liquid core reduction technology, the crystallizer of thin slab continuous casting machines has tended to have a medium thickness, thereby solving many problems that plagued thin slab continuous casting. In addition, there are the development of thin-walled immersed nozzles, the development of high-speed continuous casting mold slag, the installation of electromagnetic braking devices on the mold, the adoption of hydraulic vibration, and the improvement of secondary cooling control systems. Promote the continuous improvement of product quality produced by funnel-shaped crystallizers and parallel plate-shaped straight crystallizers.