lab experiments on the innovative rapid thick strip casting process

International Journal of Minerals, Metallurgy and Materials Volume 19, Number 5, May 2012, Page 391 DOI: 10.1007/s12613-012-0569-3

Corresponding author: Dieter Senk E-mail: [email protected] © University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2012

Lab experiments on the innovative rapid thick strip casting process Richard Nagy and Dieter Senk

Department of Ferrous Metallurgy IEHK, RWTH Aachen University, Aachen 52056, Germany (Received: 10 January 2012; revised: 16 February 2012; accepted: 20 February 2012)

Abstract: Rapid thick strip casting (RTSC) by Anton Hulek, Inventmetall®, is an innovative concept for the production of hot strips with a final as-cast thickness of about 25 mm before rolling. The innovation of the mechanism consists in a vertical mould performing a caterpillar motion. This moving mould has an unconventional parallelogram-shaped cross-section. The conventional rectangular shape is formed in the shaping machine, which is placed straight below the mould. Further elements of the technology are state-of-the-art. For the investigation of this new casting system theoretical calculations were complemented with practical experiments. The investigation focused mainly on two key aspects: the characteristics of the mould and the shaping process. For the practical analysis a static mould with three pairs of elements in laboratory scale was developed and commissioned by the Dept. of Ferrous Metallurgy @ RWTH Aachen University. The shaping experi-ments were carried out in model scale with two different materials and in variable boundary conditions. The results of these experiments de-livered important mechanical as well as thermal informations about the casting system.

Keywords: continuous casting; rapid solidification; strip metal; moulds; near-net-shape casting

1. Introduction

If a company wants to keep its market position, it needs to develop innovative technologies for higher quality or increased productivity. Against this background near-net- shape casting (NNSC) processes have been developed and optimized for the production of hot strip steel. They should complement the conventional hot rolling technologies. Through developments in this field and with the direct coupling of a continuous caster with a hot strip mill it is possible to produce flat products more economically. The procedure-specific high productivity and lower investment costs make such technologies highly profitable.

The rapid thick strip casting (RTSC) technology repre-sents a development in this field. The particular innovation of the system is a caterpillar mould, which encloses an unconventional, parallelogram-shaped cross-section. Straight below the moving mould, a shaping machine is placed with the secondary cooling zone. Here, the conventional rectan-gular shape is formed by inline-shaping with liquid core. Subsequently, the solidification ends in a so-called calibrat-ing machine (Fig. 1). The final as-cast thickness is in a range

of 20 to 30 mm. The final hot strip thickness after direct inline-rolling depends on the number of sequential rolling stand and steps.

Fig. 1. Schematic layout of the RTSC technology and the as-cast cross-section development [5] (unit: mm).

392 Int. J. Miner. Metall. Mater., Vol.19, No.5, May 2012

The RTSC technology has been invented in Austria by A.J. Hulek at begin of the 1990s [1-2] as a process of high productivity slab casting. The first practical and theoretical studies were carried out at the University of Leoben/Austria in 2000 [3-4]. The technology was presented internationally at the Korean-German Symposium in 2005 [5]. Subse-quently, an extensive theoretical investigation occurred at the RWTH Aachen University in 2008 [6-7]. Finally, a static test facility in model scale with three pairs of mould modules was developed and commissioned in 2010.

2. Simulation experiments on the ‘vertical cat-erpillar mould’

2.1. Test facilities

The static test facilities consisted of two halves, each with three mould modules. These modules were made by cold forming of a copper-silver alloy (CuAg0.1P) by Saar-Me- tallwerke GmbH. They were 500 mm long, 80 mm high and

30 mm thick (Fig. 2). The entire mould height therefore amounted to 240 mm. The casting thickness was 70 mm and so the facility capacity accounted for 45 kg of steel melt. The angle of the plates amounted to 25° in the bend region. Hence the original mould shape proportion was reproduced realisticly. In the back of the elements cooling fins were milled with a depth of 18 mm. This was decisive from the point of cooling efficiency. The cooling process was achie-ved by full-cone spraying nozzles produced by Lechler GmbH.

The two mould halves were fixed on two separated steel structures. They assured the stability of the whole construc-tion. The appropriate contact pressure of the two mould halves was provided by horizontal threaded rods. The dead weight of the upper elements at real operation was simulated additionally with vertical adjusting screws. The force was adjusted with a dynamometric tool and it was important for controlling the mould tightness during the casting process.

Fig. 2. A mould module and preparation for the strain measurement: (a) dimensions and shape of a mould module; (b) strain gauges on the back side of the modules.

After the mould has been prepared, the system was set on a refractory basement with a centric bottom tapping, blocked by a ceramic stopper rod. Below the tap hole a box with sand was placed in order to pick-up the excess melt. On the box a rocker system was arranged as a help tool for re-leasing the stopper rod.

2.2. Instrumentation of the mould

To get the most possible information about the tempe-rature and deformation behaviour of the modules the mould was exploited. In doing that the symmetry of the mould was used to reduce the number of measurement points. For the temperature profile NiCr/Ni thermocouples were positioned in the middle and the lower modules in two different depths, 5 and 25 mm below the hot surface. They were placed at the characteristic points of the mould and additionally at the positions of the strain gauges. The temperature conditions in the melt were measured at four points as well. The sampling

rate was up to 10 Hz.

In order to follow the deformation strain gauges were put on the mould at the representative spots. A foil rosette con-sists of three strain gauges, which are arranged exactly at 45° to each other; with one rosette a two-dimensional de-formation state could be evaluated. To pursue the distortions of a certain spot in the space, two rosettes are necessary, which are mounted at the right angle surfaces. Fig. 2 shows such a prepared mould module with bonded foil strain gauges at the outside of the mould and in a cooling fin.

Finally, as the last link in the chain “Spider” amplifiers from the company HBM transferred the data of both the ther-mocouples and the strain gauge rosettes to a computer system.

2.3. Experiments

With the static test facilities six experiments were carried out. Four times a steel grade C50 and twice a peritectic steel

Richard Nagy et al., Lab experiments on the innovative rapid thick strip casting process 393

grade were prepared. In order to get the most information the boundary conditions like cooling efficiency or superheat were varied. The test matrix is shown in Table 1.

The experiments were carried out according to the following scheme.

(1) Preparation of the equipment. The mould had been assembled, lowered to the refractory bottom and at least the thermocouples of the melt were installed into their holders. The clamping screws for the vertical forces were set and the stopper system was installed. Finally, the entire facility was covered by refractory material. Fig. 3 shows the complete facility before casting.

(2) Preheating of the aggregates. Before the start of the experiment all necessary units had been brought up to the right temperature. The centre runner, the stopper and ladle were preheated. They reached a temperature of about

1250°C before casting. The mould was heated up by a gas burner from below, through the bottom tap hole.

Table 1. Test matrix

Parameter Trial 1 Trial 2 Trial 3 Trial 4 Trial 5 Trial 6Steel grade C50 C50 C50 C50 C10 C10

Tliq / °C 1497 1497 1497 1497 1529 1529Tcast / °C 1697 1713 1713 1717 1745 1743

ΔT (superheat) / K 200 216 216 220 216 214Cooling intensity /

(m−2⋅s−1) 4.0 1.2 1.5 2.5 2.3 2.3

Contact time in the mould / s ⎯ 15 20 18 ⎯ 22

Zirconium sand (at the stopper tip)

x x

Facing (in the range of the stopper)

x x x x

Note: “⎯” means no data; “x” means applies here.

Fig. 3. Basic construction with the mould and the complete facility: (a) basic construction with three pairs of mould modules; (b) facility before casting without refractory cover material.

(3) Melting and alloying. In an open 100 kg capacity induction furnace 88 kg of iron were melted and the steel composition prepared. The peritectic steel melting operation was carried out under inert Ar atmosphere. After reaching the tapping temperature, the steel was alloyed. The target analysis of the two steel grades is listed in Table 2.

Table 2. Compositions of the two steel grades wt%

Steel grade C Mn Si S P Al

I 0.5 0.80 0.300 0.020 0.020 0.060

II 0.1 0.45 0.015 0.007 0.015 0.044

(4) Casting, cooling and tapping. In the first step the melt was tapped from the furnace into a ladle. The temperature drop in the ladle including the tapping process was calculated in average with 35 K/min. Subsequently, the melt was cast at a predefined velocity through a center runner from the ladle (Fig. 4). The mould cooling was started simultaneously or just right before pouring. After the melt

had reached the defined residence time in the mould, the stopper was released and the residual melt was tapped into the box. The mould cooling was switched off at less than 300°C in the strand shell.

Fig. 4. Casting process.


2.4. Results

Fig. 5 shows the solidified strand shell in the mould for example in trial #3. In Fig. 5(b) the measured shell thickness at the top can be seen. Fig. 6 presents the background of the calculations. Figs. 7 and 8 show exemplary two results of temperature and deformation measurements in trial #4. The time axis is scaled for 60 s.

With the help of the temperature measurement in two different depths at the same spot important physical values could be calculated: surface temperatures, thermal gradients and heat fluxes. Additionally, in knowledge of the boundary conditions, like contact time in the mould and with the help of the strand shell´s thickness development, it was also possible to estimate the heat transfer coefficients (Eq. (1)) and average solidification coefficients (Eq. (2)):

Fig. 5. Strand shell at the end of the experiment and after the verification (example trial #3). Shell deformation after cooling by strong contraction of the Cu-modules: (a) strand shell after tapping in the mould; (b) verification of the strand shell at the top (unit: mm).

Fig. 6. Sketch for the thermal measurements and calculations: (a) calculation of the surface temperature; (b) model to the heat transfer coefficients (X1, X2, dCu, dCu2: distances of the thermocouples from the cooled surface; α: heat transfer coefficient).

Fig. 7. Measured temperature profile in the middle module in trial #4 (C50; V=3.0 L/min; t=18 s).

sol 1 co1

st co

1( )( ) ( )

( )

tT T t s t d

q t

κ

λ λ

=−

− − (1)

where ( )tκ is the heat transfer coefficient, solT the calcu-

Fig. 8. Measured strains at the midway of the long side, middle module in trial #4 (C50; V=3.0 L/min; t=18 s).

lated solidus temperature [8], 1( )T t the measured tem-perature at point 1, ( )q t the heat flux, ( )s t the thickness of the strand shell, stλ the thermal conductivity of steel,

coλ the thermal conductivity of copper, and co1d the distance of the thermocouple 1 from the hot surface.


s=K·t 0.5 (2)

where K is the solidification coefficient (mm/min0.5), s the measured shell thickness after the experiment, and t the residence time of the melt in the mould (contact time).

3. Simulation of the part ‘shaping machine’

The shaping machine with several rolls and with the secondary cooling zone is placed directly below the moving mould. The initial parallelogram strand cross-section is reduced for a conventional rectangular shape with a thickness of about 25 mm at this place. The step is quality-related in the casting process, because the critical strain rates may not be exceeded. Otherwise hot cracks would occur at the solidification front. For this reason par-ticular attention was paid to the behaviour of the corner and bend region of the cross-section.

For the investigation several experiments were arranged. The tests were divided into the following groups:

(1) Experiments with plasticine modelling material. The results of this test series have already been published in the proceedings of the Aachener Stahlkolloquium Conference in 2009 [9].

(2) Test series for the investigation of the shaping process with liquid core. In these experiments a mould in a model scale was used, at which the long side of the mould was movable. Therefore, with a help of a hydraulic cylinder soft reduction and liquid core reduction could be simulated [10]. In this work two different steel grades were investigated.

(3) Metal sheet experiments with the parallelogram cross-section. This analysis can be further divided into cold and hot tests. In the following section these experiments are described in more detail.

4. Metal sheet model experiments

To examine the effect of a strain hardening material behaviour during the shaping process experiments with material grade S255 were carried out. Thereby, it had to be cleared how far the concentrated forming effect of the bending points extends into the adjoining cross-section areas. If the deformations appear even through strong local plastic effects, the local strains could arise strongly. On the other hand, if the loads can be distributed on a wider area by strain hardening, the local strains stay small. Hence, the critical strain rates probably will not be exceeded.

For these analyses hot and cold tests were carried out. The models after the shaping process were investigated either with a photogrammetric measurement system [11] or with microstructure analysis.

4.1. The model

As the simulation of a 8 to 10 mm thick strand shell at the end of the mould filled with liquid or mushy steel metal sheets with a thickness of 1.5 mm were bent for the paralle-logram cross-section shape. The model scale amounted to ml=5.0. Fig. 9 shows the cross-section dimensions. In the experiments two different model lengths were used, indeed l=500 mm and l=80 mm. With these two different model types effects from the length could be determined.

Fig. 9. Metal sheet model: (a) dimensions of the model (unit: mm); (b) model without sand filling.

In order to control the resistance to deformation the core of the cross-sections was filled in three different ways: moulding and tamped sand with different grain sizes and humidity, and modeling plasticine.

The surfaces of the models were marked with a grid pattern squarely using an electrochemical process for the deformation measurement with the Vialux system [9]. The grid constant was 1 mm. The bending areas of the cross- sections were prepared from the inside of the sheets as well.

4.2. The rolling mill

The test models were rolled on a model mill at the Institute of Metal Forming (IBF) @ RWTH Aachen Univer-sity. The mill operates with a pinion gear and can be rebuilt from the duo (hot rolling) to a quarto (cold rolling) machine. The roll adjustment is done with the help of an electric motor with 3 kW power. The 110 kW Leonard drive operates with direct current. The maximum rolling force amounts to 1.4 MN and the maximum rolling speed is 1200


m/min for the duo and 500 m/min for the quarto machine. In the duo arrangement the roll diameter (d) is 300 mm, and in the four-high stand arrangement, d=130 mm. The roll width amounts to a maximum of 300 mm (Fig. 10).

4.3. Cold experiments

The aim of the cold tests was to simulate the deforma-tions in the shaping machine with a sandwich material, in the model scale at room temperature. The focus was on the following questions: how strongly localized are the defor-mations in the bending region and whether the neutral axis of the strand shell remains central during the bending process.

The test matrix contained six experiments, at which the

length of the models and the core of the cross-sections were varied. The height of the reductions per pass and the roll speeds were accommodated to the core materials. This meant that the plasticine models were rolled with four stitches (28-21-15-11-8 mm) and those with sand filling each with six stitches (28-21-17-14-12-10-8 mm). The deformation rate reached 80 mm/s at the plasticine core and 20 to 28 mm/s at the sand core models. The maximum deformation forces moved at the last stitch between 10 to 72 kN. After the models had been rolled, the strains on the sheet were measured using photogrammetric methods from the outside and partially from the inside of the sheet as well. Fig. 11 shows the example pictures of these experiments.

Fig. 10. Model mill and prepared model surface: (a) model mill from the company Sack; (b) measurement grid on the model surface.

Fig. 11. Cold shaping experiments: (a) carrying out the cold test; (b) model after the final rolling pass.

4.4. Hot experiments

To investigate the effect of the rolling process at high temperatures, hot experiments in addition to cold tests were carried out. In the hot tests the same temperature level was simulated, at which the cross-section enters in the reality in the shaping machine. A total of three experiments with different heat treatments combined with rolling processes were performed. The first intention was to determine, by means of micro-structure investigations, how far the for-

ming effect of the bending points extends in the adjoining areas. Secondly, the different heat treated and rolled samples had to be compared with each other and with a reference structure. As reference steel the grade S255 was used with its typical chemical composition.

Experimental procedure:

Firstly, the models were prepared as described above and then they all were filled with stamped sand. All the three models had the same length. The heating-up process before


the first stitch was identical in the series: the trials lasted for two hours in a muffle furnace and were heated up homo-geneously to 1150 °C. Subsequently, the rolling program ran as follows (Fig. 12).

Fig. 12. Hot experiments: (a) carrying out the hot test; (b) model after the final rolling pass (test 1).

#1) After the test specimen had been taken out of the furnace, it was rolled reversing with three stitches to the final thickness. The heights of the stitches were twice 7 mm and once 6 mm (28/21/14/8 mm).

#2) Two stitches of each 7 mm (28/21/14 mm) were done after the heating process. Subsequently, the model was replaced in the furnace. After 20 min of heating period two more stitches with a height of 3 mm (14/11/8 mm) were carried out.

#3) Experiment #3 ran similarly to test #2 with the ex-ception that the second heating phase was extended to double time in the oven. Thereby, the homogenization was longer.

In this test series the rolling speed was 45 mm/s during the first two stitches and 20 mm/s at the last two ones. The rolling forces grew naturally with the deformation degree and amounted in the last stitch from 600 till 900 kN.

After the rolling process had been completed, samples were taken from the edge regions and from the centres of each adjoined areas. Afterwards the samples were examined metallographically.

4.5. Results

The results of cold tests showed highly localized strains in the bend regions. They appeared in the extent of about 25 mm around the bending line. The average compressions on the outside of the sheets amounted to ε=10%-12%. At the same time on the inside of the cross-section an average expansion of approximately ε=5% was detected. The values show a slight scatter, but no dependence on the model length (Fig. 13).

5. Conclusion and summary

For the practical test of a RTSC-mould a static facility

Fig. 13. Exemplary results of strain measurements in the cold tests: (a) strains on the outer surface; (b) strains on the inner surface.

with 240 mm height was designed and commissioned by RWTH Aachen University. The processes in the shaping machine were modeled with both, the plasticine material and with the help of steel sheets in model scale. The last one happened both with cold and hot tests. The findings can be summarized as follows:

(1) The new mould design proved to be very resistant and stiff. Only a few light surface damages due to the static top pouring process could be observed.

(2) The temperature development in the mould was completely satisfying. The spray cooling of the mould and the cooling fins in the module’s back side worked reliably. There is still enough room for further improvements.

(3) The temperature-induced deformation of the modules were made primarily in the horizontal width direction and then delayed in the vertical direction upwards. The fixing conditions on the back plane steel structure are important.

(4) The strand shell development in the mould matched the expected one. The solidification was mainly homoge-neous in cross section. Exceptions were only the corner regions, where the solidification turned out faster in this model.

(5) An important finding was that the strand shell shrink-ed off from the mould wall during the solidification. For further insight, the adjustment of the ferrostatic pressure would be highly relevant. Thereby, the boundary conditions, such as the cooling intensity or the melt contact time, played more of a secondary role.

(6) Another finding of the experimental program was that some small ‘offset prints’ of module’s misalignment could be seen on the casting surface. They were caused by the expansion and contraction of the mould modules. Here again, the fixing conditions and the cooling of the modules play a major role.

(7) The analysis of the shaping process after several test series showed that the deformations, caused by bended regions of the cross-section, remained locally. Through this


local concentration of the strains, the critical strain rates could be exceeded theoretically. Because of that the shaping process should be adjusted to the grade of steel in any case. The mould radii have to be optimized, too.

The static lab experiments with the variation of several parameters offered important facts influencing the process and a plant design. For this reason, the next logical step should be to design and test a dynamic RTSC-system pilot plant with moving vertical mould and shaping machine at a capacity of several tons of liquid steel.

Acknowledgment

This work would not have been possible without the Saar-Metallwerke GmbH, Lechler GmbH, Dept. of Metal Forming @ RWTH Aachen University and the company Inventmetall with Mr. Anton J. Hulek. They supported this work gratefully.

References [1] A. Hulek, Method for the Vertical Continuous Casting of a

Steel Strip, United States Patent, Patent No.US 6945311 B2, 2009.

[2] A. Hulek, Plant for the Continuous Casting of Steel, United States Patent, Patent No.US 4953615, 1990.

[3] C. Bernhard, Bericht über Versuchsserie zur Untersuchung

des Wärmehaushalts einer Kupferplatte in Kontakt mit einer Stahlschmelze, Institut für Eisenhüttenkunde an der Montanuniversität Leoben, 2000.

[4] O. Harrer, Temperaturrechnung für die Kokillenplatten zum RTSC Verfahren, Institut für Verformungskunde und Hüttenmaschinen an der Montanuniversität Leoben, 2001.

[5] A.J. Hulek, Rapid thick strip casting: a new concept for strip casting, [in] Proceedings of the Korean-German New Steel Technology Symposium, Düsseldorf, 2005.

[6] D. Senk and R. Nagy, Theoretische Untersuchung des RTSC Verfahrens, Dept. of Ferrous Metallurgy, RWTH Aachen University, 2007.

[7] R. Nagy, A.J. Hulek, and D. Senk, Rapid thick strip casting: continuous casting with moving moulds, [in] Proceedings of the 6th European Conference on Continuous Casting, Ric-cione, 2008, p.21.

[8] M. Suzuki, R. Yamaguchi, K. Murakami, and M. Nakada, Inclusion particle growth during solidification of stainless steel, ISIJ Int., 41(2001), No.3, p.247.

[9] R. Nagy and D. Senk, RTSC (Rapid Thick Strip Casting), ein Verfahren mit mitlaufender Kokille, [in] Proceedings of the 24th ASK Aachener Stahlkolloquium, Aachen, 2009, p.245.

[10] H. Cremers, Einfluss der Soft Reduction auf die Mittensei-gerung [Diploma Thesis], RWTH Aachen University, Aachen, IEHK, 2009

[11] Homepage von VIALUX Messtechnik + Bildverarbeitung GmbH, http://www.vialux.de/HTML/autogr.htm (2011)

lab experiments on the innovative rapid thick strip casting process

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