sen design optimisation for high-speed thin slab...

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SEN design optimisation for high-speed thin slab casting The design of submerged entry nozzles (SENs), particularly for thin slab caster funnel-type moulds, is critical for controlling steel flow turbulence in order to avoid mould flux entrainment and meniscus instability, and promote even heat transfer and steady flow conditions. Use of water modelling and mathematical modelling techniques have helped optimise SEN design, resulting in successful plant trials.

The design of SENs is critical for controlling steel flow turbulence in continuous casting moulds. This is

most important for thin slab funnel-type moulds, which operate at high casting speeds in relatively confined spaces. Controlling turbulence is important in order to: avoid mould flux entrainment which result in slivers in the final product [1,2], avoid meniscus instability [3,4], evenly distribute heat transfer for uniform shell growth [5], and attain steady flow conditions.

In the present work the authors focused their effort in emphasising the role of SEN design on turbulence control and meniscus stability by proposing a new design which is able to attain highly symmetric flows with even liquid distribution in the mould corners and a vortex-free bath surface.

DESCRIPTION OF WATER MODELA full-scale water model of a thin slab caster was built, as shown in Figure 1. Full-scale water modelling simultaneously satisfies both Reynolds and Froude similarity numbers and is the ideal and most realistic setup for thin-slab modelling. The geometry of this mould is referred to as

Authors: Yong Tang, Gerald Nitzl, Christoph Eglsaeer, Mark Pachol and Alfons LuefteneggerRHI

a funnel-type mould due to the rounded enlargement of the thickness in the middle section. This design allows the positioning of the SEN in the mould. The vertical cross-section of the submerged nozzles, hereinafter referred to as SEN-1, SEN-2 and SEN-3, are shown in Figure 2. The first two SENs were selected as benchmark designs for studying their capability to cast steel at speeds of 5.5 and 7.5m/min. Both designs are characterised by an outlet geometry of two ports shaping the jet of steel during the transfer from tundish to mould. SEN-3 is an RHI-optimised design aimed to accomplish the goals set above. In each case two submersion depths were tested – 220mm (shallow position) and 350mm (deep position) – which were measured from the meniscus level to the bottom of each SEN. The geometry of SEN-3 is optimised by means r Fig 1 Water model setup

r Fig 2 SEN inner profiles used in water model

1 2 3

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STEELMAKING AND CASTING

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MATHEMATICAL SIMULATIONSOpenfoam software was used to simulate the unsteady single phase flow conditions in the casting mould. The 3D domain of the simulation included the whole SEN and the mould. The mesh size ranged from 5mm to 20mm and the total mesh number was approximately 0.6 million. Four combinations of casting speeds and immersion depth were investigated (see Table 1). The results for numerical and water modelling were then compared.

RESULTS AND DISCUSSIONFluid flow overview Figures 3a-c give an overview of the fluid flow visualised through dye injection experiments, corresponding to SEN-1, SEN-2 and SEN-3, at 220mm immersion depth and 7.5m/min casting speed. SEN-1 provides an upper roll flow without generating an active lower roll flow and creates a stagnant large zone below the nozzle bottom. The meniscus analysis shows heavy

of port geometry, size and angles to form stable jets of steel flowing into the mould. The two upper ports control sub-meniscus velocity and its fluctuation; the two bottom ports control energy dissipation of the jets of steel. Within the optimisation process the aim has been to achieve very stable performance under varying operational conditions of casting speed and immersion depth.

The experimental matrix for the water modelling trials is shown in Table 1.

Dye injections and Particle Image Velocimetry (PIV) were employed in order to characterise the flow pattern for each of the SENs. The dye injections simulated plug flow from the SEN into the mould and also gave a good representation of mixing behaviour. Image sequences were captured for both the initial dye injection and a subsequent period of steady-state flow. The images from the water modelling experiments were processed using image processing software.

r Fig 4 Averaged flow pattern in the mould for three SEN profiles at 350mm immersion depth and 7.5m/min casting speed

r Fig 3 Averaged flow pattern in the mould for three SEN profiles at 220mm immersion depth and 7.5m/min casting speed

A

B

C

A

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depressions at the position of the funnel, inducing the formation of vortices. SEN-2 also induces a single strong upper roll flow but without a stagnant zone below the nozzle bottom, however, the presence of severe meniscus depressions remains, indicating instability at the position of the funnel. SEN-3 induces an active double roll flow in the upper and lower parts of the mould, in spite of its shallow position. Moreover, it maintains a stable meniscus without the formation of strong vortices or depressions. The four-port design here indicates that it can provide more symmetric steel flow patterns and less bath oscillation than two-port design as the mould width and casting speed increase.

Flow patterns at 350mm immersion depth and 7.5m/min casting speed are shown in Figures 4a and 4b. For SEN-1 and SEN-2, comparing Figures 3 and 4, the two different immersion depths show distinctive differences in terms of the resulting flow pattern. In the case of SEN-1 the stagnant zone below the nozzle bottom is decreased, however, the meniscus depression remains, indicating the presence of surface vortices. At immersion depth 350mm the flow pattern for SEN-2 experiences more radical changes, since large stagnant zones exist in the mould corners due to non-symmetric dye mixing behaviour. In contrast with the other two SENs, SEN-3, shown in Figure 4c, maintains the double roll flow without leaving stagnant zones and without depressions on the meniscus surface.

CFD analysis of meniscus stability Meniscus stability was simulated through CFD unsteady state conditions using the criterion of the resultant 3-D fluctuating-velocity vector. This vector was located 10mm below the meniscus at the position of the mould funnel for the SENs. Figures 5a-c show the history of instantaneous velocities for the three SENs at a casting speed of 7.5m/min and 220mm immersion depth. Examination of Figure 5 results in the following observations:

` SEN-1 creates the highest instantaneous velocities, reaching magnitudes up to 0.90m/s. This conveys a high probability for flux entrainment under actual casting conditions. Low velocities, eg 0.03m/s, are also observed. Average amplitude of the velocity is 0.38m/s.

` SEN-2 is similar to SEN-1, with a low frequency of wide velocity variations. The average amplitude of the velocity is 0.39m/s.

` SEN-3, with a more stable meniscus, creates average velocity amplitude of 0.23m/s, which is considerably smaller than the other two designs.

At the lower casting speed SEN-3 also shows the most stable meniscus with only small velocity oscillations, followed by SEN-2 and SEN-1, respectively. SEN-3 is much more stable than SEN-2 and SEN-1 ensuring, eventually, a better slab quality.

Image analysis of meniscus stability In order to analyse mould surface fluctuations with different SENs, image processing was used to analyse the wave height. Figure 6 shows the wave amplitudes, representing a topographical history of the mould surface wave at 7.5m/min casting speed and 220mm immersion depth. These topographies express the surface wave fluctuations with respect to time across all three designs and are derived from captured images from the water model experiment.

r Fig 5 Velocity fluctuation history of monitor point near the meniscus of the mould for three SEN profiles (casting speed: 7.5m/min)

Casting Casting speed Casting speed conditions 5.5m/min 7.5m/minImmersion A C depth 220mm Immersion B D depth 350mm

r Table 1 Casting conditions for water model



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For comparison, only mould surface waves on one side of the nozzle are shown in the picture. The red colour indicates the peak of the meniscus and the blue colour indicates a valley or depression in the meniscus. Areas shown in green are those that maintain a relatively static level.

It is evident that the use of SEN-1 results in the highest wave heights and fluctuations, indicated by the more continuous presence of red peaks, particularly near the meniscus level close to the narrow face of the mould. Level fluctuations in the area close to the SEN also produce high magnitudes. In the actual caster such conditions would eventually lead to the entrainment of flux, as explained in the introduction and reported by Suzuki, et al [6].

The greater presence of green areas in SEN-3 indicates a more stable meniscus. This has been achieved by the optimised exit port geometry. It is also important to emphasise the different frequencies of meniscus oscillations using these SENs. For example, waves changing with time in the case of SEN-1, have the largest wave lengths and amplitudes, followed by SEN-2 and SEN-3.

Figure 7 shows the averaged wave amplitude on the mould surface (blue colour). The standard deviation (sd) of wave amplitude is shown (hatched), suggesting that SEN-3 has smallest averaged wave amplitude under the casting conditions of shallow immersion depth (A and C in table 1). The wave amplitude is reduced significantly for SEN-1 and SEN-2 when immersion depth moves from 220mm to 350mm (conditions A to B, C to D in Table 1). For SEN-3 the wave amplitude only decreases slightly when more deeply immersed. That means SEN-3 is less sensitive to the immersion depth changes. SEN-3 provides the most stable flow conditions during the casting ramping process with a casting sequence.

r Fig 6 Wave fluctuation topography history from water modelling

r Fig 7 Averaged wave amplitude and deviation of the three SENS under different casting conditions


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Piskorski from NUcor Hickman for their support during the SEN trials. Without their patience and full co-operation, the trails would not have been so successful. The authors also wish to thank Professor rodolfo Morales from the National Polytechnic Institute for his contribution. MS

Yong Tang is from RHI AG, Technology Center, Leoben, Austria, Gerald Nitzl and Christoph Eglsaeer are from RHI AG, Global Marketing ISO, Vienna, Austria and Mark Pachol and Alfons Lueftenegger are from RHI AG, Flow Control, Hammond,USA.

REFERENCES[1]S Feldbauer and A W cramb, 13th PTD Conference Proc., ISS, 410 commonwealth Drive, Warrendle PA, pp327-340, 1997[2]W H Emling and T Waugaman, Steelmaking Conf. Proc., ISS, 410 commonwealth Drive, Warrendle PA, pp371-379, 1994[3]S Kumar, B N Walker, I V Samarasekera and J K Brimacombe, 13th PTD Conference Proc., ISS, 410 commonwealth Drive, Warrendle PA, pp119-141, 1997[4]E Torres-Alonso, r D Morales, S Garcia-Hernandez, A Najera-Bastida and A Sandoval ramos, Metallurgical and Mater. Trans. B, 2008[5]r D Morales and P E ramirez-Lopez, Proc. Aistech Conf., p20, 1996[6]M Suzuki, M Nakada, ISIJ Int., 41, pp670-682, 2002

PLANT TRIALSThe plant trials of rHI’s novel four-port thin slab SEN (SEN-3) have been conducted at NUcor Hickman. The thin slab width varies from 36ins to 64.5ins (about 0.9m-1.6m). The casting speed ranges from 150 to 225ins/min (3.8-5.6m/min). A more stable meniscus level was observed during the trials with more than 200 pieces. According to customer feedback, this new optimised thin slab SEN showed excellent performance at high casting speed. NUcor Hickman has placed an order with a fixed market share for this SEN after the successful trial. Figure 8 shows the preheat process of a thin slab casting SEN used in the trials.

CONCLUSIONSWater modelling experiments and mathematical simulations of fluid dynamics on the meniscus stability of thin slab casting mould have been carried out. comparing three high-performance SENs a benchmark study was completed resulting the following conclusions:

1 The fluid flow pattern of liquid steel inside the mould created with SEN-3 is more symmetric than those provided by the other two SENs.

2 The amplitudes of the velocity fluctuations at the meniscus level are the smallest for SEN-3 followed by SEN-2 and SEN-1, respectively.

3 The fluid flow pattern of liquid steel provided by SEN-3 is relatively independent of the immersion depth. By comparison, the flow pattern in SEN-2 is highly dependent on the immersion depth.

4 Statistical analysis of wave amplitudes measured in the water model indicates that the smallest standard deviations and average waves amplitudes are found with SEN-3. Thus, SEN-3 induces small meniscus oscillations of a lower amplitude and higher frequency than the other two SENs.

5 SEN-3 is recommended as the best choice to cast steel in a modern thin slab caster at high casting speeds and varying immersion depths.

6 SEN-3 performance results on a thin slab caster are in line with the positive results from modelling work. A more stable meniscus has been observed in the full operational window during an industrial trial with more than 200 pieces.

ACKNOWLEDGEMENTSThe authors wish to thank Bobby Hanna and Terry

r Fig 8 Preheat of a TS SEN used in the trials


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