1

Electromagnetic Strip Stabilizer for Hot Dip Galvanizing Lines

Peter Lofgren Mats Molander

ABB Corporate Research, Vasteras, Sweden

Jan-Erik Eriksson Olof Sjodén

ABB Automation Technology AB, Metallurgy Department, Vasteras, Sweden

Hans Sollander SSAB Tunnplåt AB, Borlange, Sweden

Patrick J Hanley

ABB Inc, Brewster, New York,USA

Presented at the Galvanizers Association 97th Meeting

Lexington, KY October 16-19, 2005

2

Electromagnetic Strip Stabilizer for Hot Dip Galvanizing Lines Peter Löfgren Mats Molander ABB Corporate Research, Vasteras, Sweden Jan-Erik Eriksson Olof Sjodén ABB Automation Technology AB, Metallurgy Department, Vasteras, Sweden Hans Sollander SSAB Tunnplåt AB, Borlänge, Sweden Patrick J Hanley ABB Inc, Brewster, New York, USA _____________________________________________________________________ Abstract ABB has developed an electromagnetic strip stabilizer that, without touching the strip, can reduce vibrations and oscillations at the air-knife, thereby giving a potential for:

• Improved coating layer quality • Increased line speed • Improved work environment by reducing the noise level from the air-knife • Cost savings by reducing over coating of zinc

The EM Stabilizer has been installed and tested at SSAB Tunnplåt AB. First results show that damping at the air-knife is achieved and that variations in zinc coating thickness are reduced.

3

Electromagnetic Strip Stabilizer for Hot Dip Galvanizing Lines Peter Löfgren Mats Molander ABB Corporate Research, Vasteras, Sweden Jan-Erik Eriksson Olof Sjodén ABB Automation Technology AB, Metallurgy Department, Vasteras, Sweden Hans Sollander SSAB Tunnplåt AB, Borlänge, Sweden Patrick J Hanley ABB Inc, Brewster, New York, USA ______________________________________________________________________________ Background ABB has developed an electromagnetic strip stabilizer (EM Stabilizer) that, without touching the strip, can reduce vibrations and oscillations as the strip passes the air-knife, thereby allowing for better coating control and giving a potential for higher line speed. Following successful development work by ABB and the steel producer SSAB, the first installation has been installed and tested at SSAB Tunnplåt AB in Sweden, Figures 1a-d.

Fig. 1a) EM Stabilizer installation Fig. 1b) Side view of magnets with support beam

Fig. 1c) Front view of magnets Fig. 1d) Control panel

4

Introduction Reducing strip vibration will improve the control of the air-knife action and make the coating thickness more uniform. This gives an improved quality and allows the coating to be made thinner, thus meeting an important requirement of steel users to optimize their products with respect to cost, weight and quality. Vibrations in the galvanizing line originate mainly from imperfections in the line’s mechanical components or the shape and properties of the strip itself. The impact of the line can be controlled to some extent by the monitoring and regular maintenance of critical components and parameters such as roll bearings and alignment of end rollers. Vibrations however, cannot be totally eliminated and are generally accentuated at high line speeds and on longer unsupported or free strip paths. Measures such as increased strip tension or installation of damping devices, e.g. air cushions, are reported as being successful, but there is still a considerable potential for alternative solutions. Damping vibrations without touching the strip i.e. with electromagnetic forces, seem to be the ideal solution and therefore ABB started a project to develop the EM Stabilizer. An extensive development work including theoretical studies, simulations and pilot plant trials has been carried out before the first installation was made in one of the galvanizing lines at SSAB Tunnplat AB in Sweden. Potential and target with the EM Stabilizer ABB, together with several customers, has evaluated the potential of the EM Stabilizer, summarized as follows:

• Improved quality resulting from a more even coating layer. • Increased line speed with maintained or improved coating quality. • Improved work environment due to noise reduction: The reduction of strip vibrations

allows the knife to be placed closer to the strip, thus requiring a lower air pressure and hence reducing the noise level.

• Cost savings: The excess zinc coating is in the range of 5-15 g/m2 which accounts for 5-10 percent of the process zinc consumption. Since zinc is expensive, only a slight reduction is needed to give a short pay back of the EM Stabilizer.

• The drive towards thinner coatings makes vibration control even more important in order to optimize productivity and quality.

One important feature to be evaluated in the SSAB installation is its potential to reduce the over coating of zinc. Today, an average margin of 5-7 percent of excess zinc is typical. The example in Figure 2 illustrates how this saving can be achieved. The left-hand part of the figure shows the process without stabilization and the right-hand part with EM stabilization. Without EM stabilization, a zinc margin of 5% is used, i.e. the average coating weight is 105% of the specification. This margin is chosen so that the zinc thickness variations, (here 10% (+/- 5%)), should not cause the zinc layer to be less than what is specified (100%) at any time. The EM Stabilizer will reduce the zinc coating variations; here a reduction of 1.25% is chosen, as this is the actual reduction in first trials noted at SSAB. Due to smaller variation, the set-point can be reduced to 103.75% without risk of having too thin coating (100%). Hence, a zinc saving of 1.25% is reached.

5

Fig. 2) Reduction of the zinc variation from 10% to 7.5 % gives a potential to reduce the zinc consumption by 1.25 %. Function and equipment The main electrical components of the EM Stabilizer are a cubicle with three frequency converters for independent control of the three pairs of electromagnets, a control cubicle including a PLC (programmable logic controller), the six electromagnets and a cooling water station. In addition, the EM Stabilizer is equipped with several air-cooled position sensors for detecting the strip position as a function of both time and space. Operation of the EM Stabilizer takes place from a PLC panel with alarm handling and operation control, Figure 3. The EM Stabilizer has three pairs of electro-magnets. Each pair has one electro-magnet on the front and one on the back of the strip. The top two pairs are arranged so that they cover the left and the right-hand sides of the strip. Together they effectively remove left-right vibrations (twisting) and first mode oscillations (i.e. the string mode). The third pair is located below the top pairs around the symmetry line of the strip. Their main task is to compensate for static deformations of the strip; typically cross bows, but also to remove the flapping mode of oscillations. The EM Stabilizer functions by applying three “semi”-static magnetic fields to the moving strip. The position sensors measure the discrepancy between the strip path and the optimum path line and feed these data to the PLC. Typical strip vibrations are in the range of 1-10 Hz; however, the control algorithm needs to be much faster to achieve damping.

6

Fig. 3) The EM Stabilizer system

Each magnet consists of an iron core with electric windings. The windings are series-connected and cooled by water. The magnet sections are enclosed in a stainless steel casing. The position sensors are mounted on a guide in between the two magnet levels. The EM Stabilizer works with ferromagnetism, which means that only ferromagnetic steel can be stabilized. Installation The mechanical mounting of the magnets is tailored to the line in question. The closer they are to the air-knife, the better will be the strip stabilization at the air-knife. At SSAB the stabilizer magnets were installed on a rig consisting of two pillars and an overhead beam for the mounting of the magnets, Figure 4a. From a practical point of view, this can be the most feasible way to install the EM Stabilizer on existing lines. It is advantageous to install the magnets as close as possible to the air-knife and for this reason, installation on the air-knife itself can be considered, Figure 4b. For lines equipped with a platform for the hot gauge, installation of the magnets suspended from the platform can be considered.

7

Fig. 4a-b) Principal ways to install the EM Stabilizer Results from pilot plant trials Experiments with a test rig in the ABB workshop have demonstrated that the EM Stabilizer can considerably reduce the vibration amplitude. Figure 5a shows a typical test run, where the vibration amplitude is damped by a factor of 4 to 5. Moreover, the decay time of an induced disturbance is greatly reduced and, since the EM Stabilizer acts very fast, even the first peak is dramatically reduced, Figure 5b.

0 1 2 3 4 5 6 7 8 9 10

-5

0

5

Stri

p de

flect

ion

[mm

]

Time [s]

0 2 4 6 8 10 12 14 16 18 20-20

-10

0

10

Time [s]

Stri

p de

flect

ion

[mm

]

Fig. 5a-b) Vibrations of the strip with (red) and without (blue) stabilization. a) continuous operation b) sudden induced disturbances. Tests performed with pilot equipment in the workshop. Strip width: 250 mm. Strip thickness 0.5 mm Results from full scale installation at SSAB After the first trials at SSAB, it was shown that vibrations at the magnets decrease. Figure 6 shows how the vibration amplitude is reduced by between 2 and 3 times when the EM Stabilizer

8

is switched on. This is a good result, especially as it was recorded before the tuning of the system.

0 5 10 15 20 25 30 35 40

Stri

p de

flect

ion

[mm

]Left side

0 5 10 15 20 25 30 35 40Time [s]

Strip

def

lect

ion

[mm

]

Right side

Fig. 6) Strip vibrations with (red) and without (blue) stabilization. Experiments performed at SSAB just after the installation in the line.

Up to now (September 2005), the EM Stabilizer has been run on approximately 100 km of strip and its performance is steadily improving. Recently, it was shown that the good damping at the three magnets accounts for a 20% reduction of the vibrations at the air-knife. Figure 7a-d. Bearing in mind that the EM Stabilizer is located some distance away from the air-knife, it is considered as a satisfactory result. Furthermore, comparing Figures 7a, b and c shows that the EM Stabilizer reduces uneven vibrations. Without stabilization the vibration amplitude is lower on the left-hand side compared to the right-hand side and middle position. With EM Stabilization the difference has disappeared or is very small. A deeper analysis of the measured data shows that the crossbow is reduced by around 40 percent with the EM Stabilizer. An important concern has been the effect on the coating layer, whether in the form of patterns or bare spots, at those locations where the magnetic forces arrive at the strip while the zinc is still liquid. From a theoretical point of view, this should not occur and the first trials have confirmed this, no negative impact on the coating layer was observed.

9

0 20 40 60 80 100 120 140 160 180 200

Right magnet

Stri

p de

flect

ion

0 20 40 60 80 100 120 140 160 180 200

Left magnet

Stri

p de

flect

ion

0 20 40 60 80 100 120 140 160 180 200

Middle magnet

Stri

p de

flect

ion

0 20 40 60 80 100 120 140 160 180 200

Air-knife

Time [sec]

Stri

p de

flect

ion

Fig. 7a-d) Vibrations at a) the right-hand magnet b) the left-hand magnet c) the middle magnet and d) the air-knife. Line speed 125m/min. Strip thickness 0.54 mm. Strip width 1255 mm.

10

Figures 8a-b, shows the zinc variations along the strip, recorded by the zinc gauge sensor installed directly in the line, for the same cases as presented in Figures 7a-c. With the EM Stabilizer active, the variations decrease by almost 25%. In principal, a reduction of 25% implies that the excess zinc can be reduced by the same amount, i.e. the typical over coating margin of 5% can be reduced to 3.75%. However, only a few strips have so far been evaluated with the zinc gauge sensor and a few measurements shows somewhat lower zinc variation reductions. More experience must therefore be gained before the excess set-point margin can be decreased.

0 500 1000 1500 2000 2500 3000 3500 4000 4500

Zn-w

eigh

t [g/

mm

2 ]

Front side

0 500 1000 1500 2000 2500 3000 3500 4000 4500

Zn-w

eigh

t [g/

mm

2 ]

Strip position [m]

Back side

Fig. 8a-b) Variations in zinc coating weight measured by the zinc gauge sensor for the a) front side and b) back side of the strip. (Same strip as shown in Figure 7)

Modeling and Simulation Modeling and simulation have been important for the understanding of phenomena related to vibrations and propagation of waves in the strip and thereby on the design and configuration of the EM Stabilizer, both concerning the hardware type issues (size, number and positions of magnets) and different control schemes. ABB works with a model that simulates the dynamics of a steel strip rolling. The model has been verified by comparing it with a FEM model and with data logged at the line at SSAB. The model has been used to analyze the effect of different configurations of the stabilizer, Figures 9 a-c, show results from a simulation of a 15 m long and 2 m wide strip. At time zero, an impulse force, slightly uneven in the cross direction, was applied at the middle of the strip, and the figures show the resulting wave propagation. The strip was rolled at a speed of 10 m/s in the positive direction (this unrealistically high speed was chosen for demonstration purposes only), and it can be noted how the wave front moves faster in the forward direction due to the rolling.

11

01

20

2

4

6

8

10

12

14

16-4-2024

x 10-3

m

m

t = 0.1 sec.

Strip

def

lect

ion

[m]

01

20

2

4

6

8

10

12

14

16-4-2024

x 10-3

m

m

t = 0.2 sec.

Str

ip d

efle

ctio

n [m

]

01

20

2

4

6

8

10

12

14

16-4-2024

x 10-3

m

m

t = 0.3 sec.

Strip

def

lect

ion

[m]

Fig. 9a-c) Simulated strip rolling at 10 m/s in the positive direction, 0.1, 0.2 and 0.3 seconds after an impulse force was applied at the middle of the strip. The model has also been verified against the galvanizing line at SSAB. Figure 10 shows frequency spectra obtained from logged data from two different strips, having different dominant frequencies.

0 2 4 60

2000

4000

6000Strip #1

Sim

ulat

ed s

pect

rum

0 2 4 60

1

2

3x 104 Strip #2

0 2 4 60

1

2

3

4x 1014

Spec

trum

of l

ogge

d si

gnal

Frequency [Hz]0 2 4 6

0

0.5

1

1.5

2

2.5x 1014

Frequency [Hz]

Fig. 10) Comparison between frequency spectra obtained from logged and simulated data.

12

The strips were simulated with disturbance forces applied in the form of band-pass filtered white noise at the bottom of the strip in an attempt to model the disturbances caused by the zinc pot equipment. The shift of the dominant frequency shown in the logged data is well covered by the simulations. In conclusion, the described model is a powerful tool for use in the designing of stabilizers for different conditions and it has proved to be very valuable for the current development works. Further development Existing installation at SSAB will be further tested and evaluated. Next full scale installation will be completed by the end of 2005. EM Stabilizer is expected to be ready for its market launch in the second half of 2006 This technology is not limited to reduce vibrations at the air-knife. Potentially, EM stabilization can be applied in other parts of the galvanizing line and other types of strip lines. An interesting concept is to use it on painting lines: the EM stabilizer can guide the strip through slots into a drying chamber, an application where the strip cannot be supported on rollers and where unintended contacts on the entrance or exit slot can degrade the quality. Conclusions The EM Stabilizer has been installed and tested at SSAB Tunnplåt AB. First results show that damping at the air-knife is achieved and that the variation in zinc coating thickness is reduced. It is foreseen that a lower position of the magnets gives a potential for improved damping of vibrations at the air-knife. The EM Stabilizer does not have any negative impact on the coating quality. ABB’s EM Stabilizer is the result of substantial research efforts that included thousands of hours of simulations, in-house tests and finally verification at SSAB. The technology offers solutions to long-standing problems in what is largely a traditionally minded industry. As a leading niche product, it has considerable market potential. More than 100 modern galvanizing lines merely in Europe can benefit from the EM Stabilizer. Around the world there are twice as many and in China many new lines are being built every year.