- 1 - Published in Millenium Steel 2002, pages 153-160

Edge heating in hot strip mills

Induction heating of transfer bar edges compensates for strip edge cooling during rolling and improves quality and yield. Modern automatic compact designs are readily retrofitted within existing mill layouts.

Siebo Kunstreich, Danieli Rotelec, France

1. HISTORY

The basic principle of induction heating is shown on figure 1. Coils powered with AC current on a magnetic core generate a magnetic flux, which crosses a steel bar. In turn, the magnetic flux induces eddy currents in the bar, which circulate in a loop around the flux tube (electrical currents do not cross the magnetic flux that generates them). By moving the poles of the magnetic core toward the edge of the bar, the section of metal through which the eddy currents circulate is reduced and, consequently, the current density at the edge is increased. Consequently, the energy dissipated, which is proportional to the square of the current density (Joule's Law), will heat the edge much more than the rest of the bar.

gap

U-type inductor C-type inductor

Fig.1 : Induction edge heating with transverse flux. Magnetically, C-type inductors have the best efficiency, but initially were not used because the pole gap was thought not to be adjustable to bar shape. As a result, two separate U-type inductors were used for gap adjustment in spite of low efficiency.

A magnetic flux is always a closed circuit and so if a magnetic flux crosses a bar, there must be a return circuit for this flux. Two magnetic configurations can be used (Fig. 1): the "U-type", where the flux crosses the bar twice, and the "C-type", where the flux crosses the bar only once.

opening. The C-type inductor permits a considerable increase in the heating efficiency compared to the conventional design. Originally however, these advantages were not used, because it was believed that the gap in the C-type configuration could not readily be changed.

Initially, the U-type configuration was used, because the position of two independent inductors can easily be adjusted according to the thickness and shape of the bar. However, the drawback of this configuration is a rapid decrease in heating efficiency if the gap between the upper and lower inductor poles increases. As a consequence, the available heating power rapidly decreases with increasing gap, and the head and tail of the bar can generally not be heated enough, since they are not flat and require a large

Edge heating the transfer bars in hot strip mills was developed in Japan using U-type inductors, and Japan was the sole country until 1988 that used this process industrially.

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In cooperation with Irsid and Sollac/Fos, Danieli Rotelec developed a C-type inductor, the gap of which can be adjusted by independent movement of the upper and lower arms [1]. A patented design provides an articulation in the magnetic core that does not create magnetic losses, thus high heating efficiency is maintained at big gaps enabling the head and tail of the bars that are frequently deformed to be heated (Fig.2) [2].

2. Why edge heating? 2.1 Temperature drop at bar edges During hot rolling, the temperature of the transfer bar gradually decreases. Near the edges, the surface exposed to radiant cooling is larger than in the middle of the transfer bar, hence the edges cool faster than the bulk. Fig. 3 shows a typical temperature distribution of a transfer bar edge on the roller table before the finishing mill. (computed). The red curve indicates the average temperature across the thickness of the bar as a function of the distance from the edge. A typical temperature drop at the edge is 100C. This value depends on the rolling and cooling conditions and on the time that the bar has passed in the roughing mill after it has left the reheating furnace. Computer simulations taking into account the conditions of the roughing mill can calculate the temperature distribution and temperature drop at the edge.

EFFI

CIEN

CY [%

]

EFFICIENCY OF 'U-TYPE' AND 'C-TYPE' EDGE HEATER

ELECTRICAL GAP [ mm]

Conventional heater "U" New heater "C"

Fig.2 : Electrical efficiency of C- versus U-type edge heater. The Rotelec's C-type maintains high efficiency at big gaps and, hence, is more suitable for head and tail heating [2].

Half thickness

Average temperature In thickness direction [C]1025

1000

975

9500 50 100 150

Distance from edge [mm]

Bar edge temperature distribution

104010301020

1010 After a prototype installation was built in 1987 at Sollac/Fos [3], the Japanese hot strip mills accepted the C-type design and Danieli Rotelec delivered three of the four new edge heaters installed in Japan since 1989, namely, NKK-Keihin (1989), Sumitomo Wakayama (1990 and 1993) and KSC Chiba (1994). Outside Japan, Danieli Rotelec delivered edge heater installations at CSC Kaohsiung (1996), BHP Western Port (1998), TKS Beeckerwerth (1999) and TKS Bruckhausen (2000).

Fig.3 : Computer-simulated temperature distribution through bar thickness. Heat radiation at bar edges causes temperature drops that must be compensated by edge heating. Calculations determine amount and profile of the drops as a function of the rolling history of the bar, typically 100C on 50mm.

Today only, U-type inductors are no more built; C-type inductors have become state-of-the-art. Although third parties now offer C-type edge heaters with similar design, Danieli Rotelec is the sole supplier having experience and references with automatic gap adjustment and independent positioning of the upper and lower inductor arms.

2.2 Problems with cold edges The reduced edge temperature causes various problems during the rolling in the finishing mill:

In low carbon steels, if the temperature drops below the austenite transformation, rapid recrystallization at

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the edges will result in brittle coarse ferrite grains. In micro-alloyed steels, an early precipitation may occur at too low temperature, leaving fewer elements in solution for the final precipitation in the ferrite of the micro-alloying elements. In both cases, mechanical properties are reduced.

In stainless steels, the temperature drop in the edges that combines a lower ductility with higher rolling pressure, results in rolled-in scale, surface defects and cracks.

Cold edges are harder and, hence, make thickness control more difficult and increase wear of the work rolls.

These effects require the coil edges to be trimmed to ensure overall quality. An alternative would be to maintain the absolute temperature at the edge above a critical value by setting the furnace temperature at higher values, but that would decrease the furnace capacity and increase energy consumption and scale losses. 2.3 Benefits from heated edges The purpose of edge heating is to compensate the temperature drop in the edges of the transfer bar in order to stop or to decrease the above-described problems.

Fig.4 : Effect of edge heating on temperature and structure. Hot band thickness 2 mm, rolling temperature AR3 + 20C, samples taken 25 mm from edge. Edge heating reduces the 30-mm coarse grain band to 14 mm thus saving 16 mm trimming [3].

Fig. 4 shows an example of the surface temperature measured across the hot band width at the exit of the finishing mill [3]. The solid line refers to a hot band with edge heating; the dotted line refers to a

hot band without edge heating. The critical temperature point Ar3 of the austenite transformation is also shown. Without edge heating, a width of 30 mm from the edge was rolled below the critical temperature. With edge heating, this width was reduced to 14 mm. Theoretically, the microstructure in the 14-30 mm band should show coarse grain structure if the bar is rolled without edge heating, and fine grain structure if the bar edge has been heated. This is confirmed by the micrographs of samples taken at 25 mm from the edge. The left sample that was not heated shows the coarse grain structure, the right sample that was heated shows a fine grain structure same as in the middle of the bar [3].

Fig.5 : Instantaneous effect of induction heating. Time on x-axis is in 4-sec. increments and goes from right to left. The transition from blue (cold) to red (hot) edges occurs in less than one second [4]. Fig. 5 shows a thermal image of a hot band as measured with a scanning camera [4]. Time on the x-axis is in 4 second increments and goes from right to left. As soon as the edge heater is switched on, the transition from blue (cold) to red (hot) edges occurs in less than one second, demonstrating the instantaneous effect of induction heating.

STRIP MICROSTRUCTURE AT 25 mm FROM EDGE WITHOUT EDGE HEATER WITH EDGE HEATER

Distance from hot band edge [mm]

Strip

edg

e su

rface

tem

pera

ture

[C]

STRIP MICROSTRUCTURE AT 25 mm FROM EDGE WITHOUT EDGE HEATER WITH EDGE HEATER

Distance from hot band edge [mm]

Strip

edg

e su

rface

tem

pera

ture

[C]

Generally speaking, reheating of the edges improves quality and mechanical properties and thus reduces trimming. The following examples show some typical improvements:

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Fig.6 : Improvement of elongation on low carbon steels. The heated hot band has almost constant quality across the full width, whereas the non heated hot band shows considerable deterioration in a 50-mm band from edge [2]. Fig. 6 shows the degree of elongation across the width of the hot band for heated (solid line) and non-heated (dotted line) hot band. The non-heated hot band shows a considerable deterioration up to 50 mm from the edge, whereas the heated hot band has almost constant quality over the full width [2].

Fig.7 : Reduction of rolled-in scale on ferritic stainless steel. The x-axis represents the number of hot rolling sequences, the y-axis the percentage of defective coils in each sequence. Edge heating reduces the defect ratio by a factor of three.

Fig. 7 shows the reduction of surface defects related to rolled-in scale on ferritic stainless steel coils[3].. The x-axis represents the number of hot rolling sequences; the y-axis represents the percentage of defective coils of each sequence. Red squares represent coils rolled with edge heating; yellow squares coils rolled without edge heating. This figure illustrates qualitatively, that surface defects are substantially reduced by edge

heating. A detailed quantitative analysis carried out on more than 6,500 austenitic and ferritic stainless steel coils showed that, although defect ratio on austenitic grades is five to ten times higher than on ferritic grades, edge heating, in both cases, reduces defect ratio by at least a factor of three [5].

Distance across hot band width [ ]

squares: with edge h

crosses: without edge h

Distance across hot band width [ ]

squares: with edge h

crosses: without edge h

3 - Heating method 3.1 Temperature profile control The purpose of edge heating is to compensate partially or fully the temperature drop at the edges of the transfer bar. The temperature rise T generated in the edges can be controlled by the electrical power supplied to the inductor. Constant electrical power and constant transfer speed of the bar mean constant temperature rise T along the bar length.

NUMBER OF HOT ROLLING SEQUENCE

% O

F D

EF

EC

TIV

E C

OIL

S

In the thickness of the bar, the temperature rise is almost constant, since the heat energy generated by induction is almost constant through the bar thickness. This is due to the transverse flux configuration and is fairly correct within the usual bar thickness range of 25 to 50 mm, if the coil current frequency is chosen appropriately. Across bar width, however, heating must be the inverse of the temperature drop profile of the bar (Fig. 3) in order to compensate for it. Such a heating profile can easily be obtained if the inductor poles are located close to the edges and the eddy currents in the bar are compressed at the edges (Fig. 1). The temperature inside the bar can be computed. Figure 8 shows a typical temperature distribution and figure 9 shows that the temperature rise T decreases exponentially with increasing distance from the bar edge. The slope of this decrease can be made steeper or flatter, within a certain range, by moving the inductor poles outward or inward with respect to the bar edge.

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Fig.8 : Computer simulation of heating. Temperature distribution in the bar edge (half bar width). The bar runs through the inductor from right to left, y=0 represents the inductor position.

3.2 Power requirement The example of Fig. 9 has been calculated as follows: The thermal power PTH to be injected into the bar edge to obtain a given temperature rise and a given profile is directly proportional to the bar thickness t, bar speed v, and the integral of the temperature rise T over the distance x from the edge : PTH = cP v t T dx with cP = specific heat of steel(1)

= density of steel The edge temperature rise T can be well approximated by an equivalent exponential curve T = T0 exp(-x/b) (2) with b 30 mm, depending on the relative position between inductor poles and edge (wrap). Equations (1) and (2) combine into a simple equation for the thermal power: PTH = cP v t b T0 = k v t b T0 (3) Note: The temperature rise T considered in the above equations is the average across the bar thickness, not the surface temperature. A surface temperature obtained from measurements should therefore be transformed into average temperature before using the above equation. Within an error of 5-6% however, this transformation can be neglected. The edge temperature rise T0 at x=0 is used for convenience and computation purpose only

and should be considered as the extrapolation of the temperature profile to the edge. In case of comparison with measured data, it is more convenient to refer to T at some distance from the edge.

Temperature rise delta T versus

distance from edge

0

20

40

60

80

100

120

140

0 50 100 150distance from edge [mm]

tem

pera

ture

rise

del

ta T

[C

]

standard slope 30

steep slope 23

f lat slope 40

Computation data: specific heat 0,157 kcal/kgC specific weight 7,55 kg/cm3 Required thermal power PTH= 538 kW at T=100C, speed 1,25 m/s, thickness 30 mm

Fig.9 : Example of heating temperature profile. T decreases exponentially with increasing distance from bar edge. The slope of the decrease can be made steeper or flatter by moving the inductor poles outward or inward with respect to the edge. In this example, T=100C is obtained for a 30-mm thick bar running at 1,25 m/s through the inductor, if a thermal power PTH = 538 kW is generated by the inductor.

3.3 Limitation of heating capacity with gap setting The heating power of an inductor depends on the Amp-turns of the coil and on the pole gap. For a given coil design, the coil current is limited by the thermal rating of the coil (section, type and cooling of the conductor) therefore, there is an electrical limitation to the heating power of any inductor. It has been shown that heating efficiency drops with increasing gap (Fig. 2)., more importantly, the bigger the gap, the smaller will be the maximum thermal power PMAX that can be generated by one inductor. PMAX is shown for one inductor per edge as a function of the pole gap in Fig. 10, curve `coil 40`. The x-axis is called `electrical` gap, because the values refer to the real distance between the magnetic

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iii). Item i) above determines how much thermal power per edge PTH is required for given temperature rise/bar thickness/bar speed triplet. Item ii) above tells how much power PMAX is available as a function of gap opening, i.e. warp acceptance. The conclusion is then easy: If the capacity PMAX is greater than the requirement PTH, one inductor per edge is enough. If, however, PMAX is smaller than PTH, two inductors per edge have to be installed.

core of the poles, not to the apparent distance between pole covers. Note: The curve "coil 30" refers to the first C-type inductor manufactured in 1997. The curve named "coil 40" refers to the currently-used inductor of size 750mm. This inductor can inject a thermal power PTH = 780kW when operating at an electrical gap of 120 mm or PTH = 530 kW when operating at en electrical gap of 180mm. The curve named "coil 45" refers to inductors with increased size and/or increased magnetic flux. At present, we do not recommend to increase the width of the inductor, because this would imply to increase the free space between two adjacent rollers of the roller table to more than 800mm (roll pitch to more than 1200mm). We also do not recommend increasing the magnetic flux above the presently achieved design level, because a higher power density would increase sparking problems on the roller table. Therefore, the range at or above the curve named "coil 45" is not recommended although it is perfectly possible to build such inductors.

Maximum thermal pow er versus gap one inductor per edge

0

200

400

600

800

1000

1200

50 100 150 200 250 300 350e le ctrica l ga p [m m ]

ther

mal

pow

er P

th [k

W]

coil 40

coil 45

coil 30

3.4 One or two inductors per edge Generally, one or two inductors per edge are installed on the roller table. Investment cost and space availability obviously favour one inductor. The heating requirement may be determined as follows:

Fig.10 : Maximum available heating power versus pole gap. "Electrical" gap refers to the distance between poles without thermal shields. Coil-30 line refers to Danieli Rotelec's first edge heater; coil-40 to current 750-mm large inductors. Coil 45 extrapolates to even bigger inductors the industrial reliability of which has not yet been proven.

i). The three parameters, temperature rise (T0), bar thickness (t) and bar speed (v) determine, according to equation (3), the necessary thermal power PTH to be injected into the edge. These parameters must be chosen for the most demanding case, i.e. the biggest product of the three factors. The speed to be considered is not necessarily the speed of stand F1, but the speed at which the bar runs into the edge heater. It may be higher than the F1 speed and/or the crop shear speed, if the edge heater is installed before the crop shear. Generally, the problem is to specify T0.

3.5 Installation on the roller table From a process point of view, the installation of the edge heater as close as possible to the entry of the finishing mill is preferred, since the obtained temperature rise will be fully available and the bar edge will not cool down again. The closest positions are between descaler and F1 (Fig. 11) or between crop shear and descaler. Another apparent advantage of these positions is a smaller warp, since head and tail are already cut. Generally, however, no space is available at these locations for an efficient warp detector, and the gap opening cannot be efficiently reduced for safety reasons. Moreover, no space will be available for two inductors per edge, at least in most existing mills. Finally for maintenance these positions are less advisable, because the edge heater will suffer from water and scale.

ii). The operator must decide, up to which deformation (warp) the bar shall be heated, and is particularly relevant if the bar has to be heated over its full length, because head and tail warp determine the maximum gap opening. Once the maximum gap is chosen, PMAX results from Fig. 10. At this point of the analysis, the operator will notice the important advantage of independent upper and lower arm adjustment that permits to operate at smaller gaps and, hence, with bigger power availability, see the example described in chapter 4.3.

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4 - Mechanical design of edge heaters From a practical point of view, the best position is on the roller table before the crop shear (Fig. 12).

4.1 Modular edge heater cars Each C-type inductor consists of two coils, two magnetic poles and one articulated magnetic core with upper and lower arms (Fig. 1).

One or two inductors are installed on a movable frame called edge heater car. Figure 12 shows a double inductor car designed for ground installation. The overall dimensions of such car are approximately : Length: 5,100 mm Height: 2,600 mm Width: 1,200 mm for single inductor

car; 2,400 mm for a double inductor car

Fig.11 : Single inductor car installed between descaler and F1, BHP Western Port Width of inductor head: 720mm,

clearance for installation 750mm One car is installed on each side of the delay table. The electrically driven cars move on rails to adjust their position according to the bar width and a pulse generator enables a position accuracy of at 5mm by PLC control. The total stroke of the carrier mechanism depends on the range of bar widths to be covered. If enough backspace is available, the stroke can be increased by simple extension of the rails and of the supporting structure such that maintenance on the inductors is possible even without downtime of the rolling mill.

Fig.12: Double inductor car installed before crop shear , China Steel Corporation, Kaohsiung hot-strip mill n2 If the roller table environment does not

allow enough back space for the ground installation, a hanging version can be proposed. The car length is then reduced from approximately 5200 mm to 2500 mm, however the height is increased from 2600 mm to 5400 mm. This version requires a total lifting height below crane of approximately 10 m for installation/removal to/from the rails.

This area is more maintenance-friendly, because it is dryer than the other side of the crop shear, and the space for two inductors is available. The drawback of bigger warp acceptance can be compensated by optimized gap setting. Generally, the roll pitch of the existing roller table must be adapted such that the lower inductor arm can move into the free space between two rolls. Further, special electrical insulations must be provided between rolls, table frame and roll motors in order to avoid damage to motors, bearings and bar surface by electrical sparks. Therefore, it is recommended to replace the existing roller table within a distance of two rolls before and behind the edge heater by a new group, which can be pre-manufactured and installed during a mill scheduled outage.

4.2 Damage prevention The following have been designed to obtain maximum safety of operation with misshapen bars. A patented articulation/joint has been developed to permit the two poles to move separately without additional flux losses. An electrical motor with a gear and electrical clutch actuates each arm of the yoke separately. Adequate sensors measure the absolute position of the arms. The actual value of the gap is adjusted

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within 2 mm by the PLC with reference to the bar thickness and the bar shape. The gap can follow the slope of the bar from head to tail in real time provided the position of the edge heater on the table permits the installation of bar shape detectors (warp detectors, infrared cameras, see chapter 6.1). If not, the gap is set to preprogrammed values as a function of bar thickness and warp ranges. Typical values of minimum and maximum gap position are 100 and 520 mm, respectively. In case the bar is misshapen, the following means are provided in order to avoid damage to the inductors: Electrical opening: Under normal

operation, one warp detector installed upstream of the edge heater provides a signal, which is used to automatically set the upper and/or lower arm to the appropriate position.

Electrical emergency opening: In case the warp detector was not installed or does not operate correctly, a warp detection arm can be fixed to the upper inductor arm and will mechanically detect, if the upward warp comes too close to the position of the upper pole. This arm then will give a signal that is used to open the upper and lower arms. This emergency opening occurs in less than 1 second.

Mechanical protection of upper arm: To prevent damage of the upper arm, a mechanical roll protection being placed aside the pole will mechanically lift the upper arm if touched and pushed by a misshapen bar. A special design permits the upper arm to swing upwards from any position.

Mechanical protection of lower arm: To prevent damage of the lower arm, a ski in the middle of the delay table facing the inductors is installed. The width of this ski must be adapted to the width of the smallest bar that has to be heated.

Mechanical emergency opening: In case of breakdown of electrical power, the electrical clutches of the arm motorization open automatically. The weight of the lower arm is sufficient to open immediately. The upper arm is provided with a counterweight so that it also opens immediately. Two shock absorbers reduce the velocity at the end of the movement.

These means have been designed to obtain maximum safety of operation with misshapen bars. 4.3 Benefit of gap adjustment Independently of the safety aspect explained in the previous chapter, the design with adjustable gap and independent positioning of upper and lower arms gives a very important advantage as far as the available heating power PMAX is concerned. This is illustrated by the following example: Fig. 13 shows a 30 mm thick bar with an upward warp of 100 mm and a downward warp of 50 mm that must be heated. The left figure (fixed gap) shows that the electrical gap must be set at 230 mm to accept the warp of +100/-50 mm. (Warp 100+50 mm plus safety margin 20+20 mm plus thermal screen 20+20 mm = 230 mm). As already mentioned, the bigger the gap, the smaller becomes the maximum thermal power PMAX that can be generated by one inductor. For a gap of 230 mm, PMAX becomes 390 kW (Fig. 10). This is not enough for the example of figure 9 that requires a thermal power PTH =538 kW to produce a temperature rise T0 of 100C for a 30 mm thick bar traveling at 1.25 m/s. That means that two inductors must be installed per edge to satisfy heating requirement and warp acceptance simultaneously. If, however, the inductor is provided with independent positioning of upper and lower arms, the centre and right parts of figure 13 (bar head and bar tail) show, that a gap setting of 170 mm and 150 mm respectively, is enough to accept a bar head with +100 mm warp or a bar tail with -50 mm warp. The available power PMAX at 170 mm and 150 mm gap is respectively 565 kW and 640 kW (Fig. 10). This is more than the required PTH =538 kW , to produce a temperature rise T0 of 100C for a 30mm thick bar traveling at 1.25 m/sin this example. This means that with independent positioning of the upper and lower arms, heating requirement and warp acceptance can simultaneously, be satisfied with only one inductor per edge. Independent arm positioning is a necessary, but not sufficient condition to obtain that advantage. The warp of an incoming bar has to be known/measured,

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and the arm position must be regulated in real time. This is described in chapter 6.2.

The above example shows that indications of power values should clearly name the type of power that is concerned (thermal vs. electrical). Since there is more than a factor of four between kW of thermal power per edge and kVA of power rating of the main transformer, indications of power values without clear identification make the comparison of different installations hazardous and impossible. Although it is obvious for electrical engineers to make a clear distinction between such values, publications usually do not. With the same aim to promote transparency, we would like to underline that any performance indication, whether efficiency or thermal power or electrical power, must be linked to the corresponding gap value or it makes no sense.

50

100 30

20 20

20 20

FIXED GAP 230mm for warp up +100mm and down - 50mm

20 20

100 30

10 20

ADJUSTABLE GAP BAR HEAD 170mm for warp up +100mm

ADJUSTABLE GAP BAR TAIL 150 mm for warp down -50mm

20 10 30

20 20

50

Fig.13 : Fixed versus adjustable gap. Head and tail warp of +100/-50 mm requires an electrical gap setting of 230 mm in case of fixed gap, whereas individual adjustment of upper and lower arms permits to operate at 170 respectively 150 mm gap. At 230-mm gap the available heating power PMAX = 390 kW, whereas at 170/150 mm it is 565/640 kW!

THERMAL POWER INTO PTH = k1 t v T(x) dx [kW] 700

= k2 t v T0

ELECTRICAL PER EDGE : (=0.70 at gap 140mm) PEL = PTH / [kW] 1000

OUTPUT POWER FROM INVERTERS POUT = 2 PEL +LL [kW] 2100

BAR DATA: EDGE TEMP.LOSS T0 [C] 100,0 BAR THICKNESS t [mm] 32,5 BAR SPEED v [m/s] 1;5

POWER RATING OF TRANSFORMER : PTR [kVA] 2900 POWER RATING INVERTERS PINV = 2 PEL / 0.8 [kW] 2400

t

INVERTER

T 0

T

T 0

INVERTER

5 - Electrical design 5.1 Power ratings Fig. 14 shows how to estimate the power ratings of an edge heater installation. The primary input data is the

temperature drop to be compensated (or temperature rise to be generated), bar thickness and bar speed Example: Temperature rise 100C for a 32.5 mm thick bar running through the edge heater at a speed of 1.5 m/s.

This data determines according to equation (3) the required thermal power PTH that has to be generated in the bar edge. Assume PTH = 700 kW. Fig.14 :

Example of power ratings. Heating T=100C at bar thickness 32,5 mm and bar speed 1,5 m/s requires thermal power PTH= 700 kW per edge, electrical power input into the inductor PEL= 1000 kW, inverter power rating for both edges PINV 2400 kW and power transformer rating PTR 2900 kVA.

The thermal power PTH divided by the efficiency gives the electrical power per edge PEL that has to be supplied to the inductor (or indictors). Assuming operation at electrical gap 150 mm gives efficiency =70%, and electrical power PEL= 1,000 kW (input power to the edge heater).

5.2 Power regulation Electrically, an edge heater is a single-phase parallel resonance circuit powered by a thyristor inverter with thyristor phase commutation by load. The resonance frequency 0 of the oscillator is given by 0 = (LC)-1/2. L is the reactance of the coils that are installed around the inductor poles. C is the capacity of the capacitor bank that is installed on the edge heater car, not in the electrical room, to keep the distance

Considering approximately 90% efficiency for power supply and line losses as well as 10% spare power, one obtains the rating of the inverter(s) PINV for two edges. Example: PINV = 2,400 kW.

The rating of the power transformer PTR finally is determined by the inverter rating. Example: PTR = 2,900 kVA

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between capacitors and coils short, and hence to keep the line losses low. The frequency is in the range of 250 - 300 Hz. For a given design, i.e. given resistance of coils and coil-capacitor connections, the impedance of this circuit depends on the pole gap and whether a bar is present or not. Without bar in the gap, i.e. without load, the power input required to maintain the oscillation is low and the circuit consumes only the power that corresponds to the internal losses. As soon as the bar enters into the gap, the circuit load increases and the power input must be increased instantaneously, if not the circuit stops oscillating. Since starting the oscillator under load is not fast enough, the circuit must be started without load and must be kept under oscillation between two closely following bars. Oscillation without load means low power from the inverter which requires a special design of inverter to permit rapid change of operational mode. To obtain a given temperature rise T0 in the bar edge, a given thermal power PTH (equation 3) must be generated by the inductor in the edge, i.e. a given electrical output power PEL = PTH/ must be delivered by the inverter. Since depends on the gap setting that may change during heating, the setting value PEL must be calculated in real time as a function of gap position. Moreover, since PTH depends on the bar speed v that may also change during heating, PEL must also be calculated in real time as a function of bar speed. Consequently, the control method is as follows: The initial setting values for an incoming bar are bar thickness t and T0. The bar thickness remains constant during heating, but T0 may be changed during heating. Both t and T0 are stored in a PLC that calculates, in real time, the corresponding thermal power PTH (equation 3) as a function of the actual bar speed, as well as the related electrical power PEL, as a function of the actual efficiency. This is in turn determined by the actual gap position. The actual result PEL becomes the setting value of the inverter. The inverter is operated with output power control, not with current or voltage control, and thus imposes PEL independently of the impedance change that may occur on the oscillating circuit. Another regulation loop that operates independently of the output

power control monitors and limits the actual coil current to its nominal value. This function overrides the power control and constitutes the limitation of the maximum power PMAX as per Fig. 10. For reasons of investment cost saving, only one inverter is generally used for both motor side and operator side inductors. However, this is advisable only if the temperature profile on both sides is the same and if the bar is well centred. If individual temperature setting per side is required, or if the bars can move relative to the centre, then this requires individual inverters each side. There is another reason to use individual inverters per side; even individual temperature setting is not required. If the bar is not well centered with respect to the table center, the relative positions of pole and edge are different from one side to the other. Different relative positions imply different load coupling of the oscillating circuit; that in turn implies different impedances on both sides. Since both sides are connected in parallel to the same inverter, the power delivered to one side and to the other becomes out of control. 6 -Process automation 6.1 Data interface with mill levels 1 & 2The edge temperature drop could be compensated automatically by a regulation that uses temperature measurements before and after the edge heater. Such regulation, however, is not used, because scale, water and bar shifting make bar temperature measurements unreliable. The heating temperature T is given either by level 2 as a setting value that is stored in the edge heater PLC/PC system , or it can be replaced by a number that characterizes typical rolling schedules and hence typical temperature drops. In addition to T, level 2 must send for each incoming bar a set-up message (c.f. Fig.15) comprising: Compulsorily Bar number (for identification of bar), Bar width (for position setting of edge

heater cars), Bar thickness (for heating power

calculation), and optionally Bar length

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Heating mode (cold run, constant T heating, variation of T along bar length etc.)

Date, time, steel grade, power correction or any other mill-specific data.

In addition to the set-up message, the following signals from the mill must be provided to the PLC in real time, preferentially by hard-wired connection: Actual bar speed upstream and

downstream of edge heater Three or four actual bar positions (hot

metal detectors, crop shear cut or the like)

With this data, the edge heater PLC/PC system operates and controls the entire installation fully automatically, elaborates a detailed heating report for each bar, stores heating results and fault messages in a history file and communicates with level 2 and with the operator HMI. Maintenance HMI in the electrical room can be added optionally. The operator can switch from automatic to semi-automatic operation and choose the heating value T manually, see HMI screen in Fig.15. However, he cannot control the movements of edge heater cars and inductor arms that remain PLC-controlled to prevent damage. Fig.15: Example of HMI screen, set-up data. The set-up data from level 2 for the actually heated and the next incoming bar is displayed on the left side of the screen (date, time, bar data, heating mode and heating temperature). The right side shows the operator set-up data, bar shift and upper/lower arm offset in automatic and semiautomatic mode, operator/motor side setting of power, temperature and wrap position in the semiautomatic mode. 6.2 Bar tracking and warp detection Automatic operation of edge heaters is essential. The PLC/PC system must be able to track the bar for automatic heating switch on (and off) in due time and

recognize bar deformations for automatic gap setting. The first function is relatively simple. A hot metal detector identifies bar head and tail positions at a given time, the PLC calculates the actual position by integration of the bar speed signal over time and recalibrates the position with subsequent hot metal detectors. The second function is less easy. To detect upward warps, conventional installations use mechanical flags arranged on one or several levels above the pass line that give an electrical contact if touched by the bar. This contact is then used to set the upper arm to appropriate positions. If no space is available to install such warp detectors, the upper arm is set "blindly" according to preprogrammed gap values for bar-head, middle and tail. The downward warp is never detected and the lower arm is pre-set "blindly". This type of operation must include sufficient safety margin for the arm positions and, hence, cannot operate at very small gap. To optimize the operation at the smallest possible gap, Danieli Rotelec developed an optical warp detection system. Two infrared cameras located on the motor and operator sides determine the actual bar edge position at a given location upstream from the edge heater and generate an analog signal that indicates the distance of the upper and lower bar edge with respect to the pass line. The camera has a built-in microprocessor to eliminate noise signals due to water and scale on the bar. The signal is fed to the PLC that controls the upper and lower arm position with the appropriate time delay. 6.3 Quality control The comparison between setting and actual T value of the bar edge is the most important information for quality control of the hot coils. It may happen, that a local bar deformation (warp) requires a big gap setting for some seconds , hence the bar edge is under heated along the corresponding length. As surface temperature measurements in real time are not very reliable and do not permit accuracy better than 15%, Danieli Rotelec developed a sophisticated system that combines performance test measurements under steady state

- 12 - Published in Millenium Steel 2002, pages 153-160

conditions with real time measurements under operating conditions, and real time computation that determines the actual temperature rise T, with a reproducibility better than 2%. That data is generated as a function of bar length, stored for each bar and can be used and further processed by level 2 for any appropriate quality control system, Fig.16. Fig.16: Example of HMI screen, heating result. For each bar as identified by its set-up data (top left of the screen) the heating result is indicated in terms of actual power and temperature versus the bar length (bottom of the screen). Light and heavy faults that might have occurred are displayed in the middle of the screen. 7 -Conclusion C-type edge heaters were developed in the late 1980s by Danieli Rotelec and Irsid and are now very sophisticated machines

thanks to several exclusive/proprietary features. (table 1): Key electrical and mechanical components have been designed for high efficiency and long life. These are compact design integration into existing hot strip mills without major revamping, a rapid gap opening mechanism for safe operation in emergency situations, independent positioning of the upper and lower inductor arms, combined with an optical warp detector for high power availability for head and tail heating, and real time generation of actual edge temperature rise for effective hot band quality control versus bar length.

EXCLUSIVE FEATURES BENEFITS - C-type adjustable gap (patent) - Multi-conductor Roebel- type coil (patent) - Ceramic thermal screen (patent)

High efficiency Long life

- Quick gap opening

Safety in emergency situation

- Independent positioning of upper/lower arms - Optical warp detector

High power availability for head and tail heating

- Real time generation of actual edge T rise

Hot band quality control versus bar length

Table 1

Siebo Kunstreich is Managing Director of Danieli Rotelec, France References [1]. J.Hellegouarc'h, G.Prost, J.Ruer (Rotelec), Revue de Mtallurgie Juillet-Aot 1989,

p. 593-602 [2]. S.Fukushima, K.Okamura, T.Kase (Sumitomo Wakayama), G.Prost (Rotelec), IEEE

Transactions on Industr. Applic., Vol.29,No.5, Sept/Oct.1993,p. 854-858 [3]. F.Blanchet, G.Dantin, T.Prasse, G.Prats, G.Roux Sollac Fos), Revue de Mtallurgie

Sept. 1991, p. 1134-1141 [4]. C.Queens (Thyssen Krupp Stahl Beeckerwerth), private communication [5]. G.Roux (Sollac Fos), private communication [6] Picture from BHP Western Port [7]. Picture from China Steel Corporation, Kaohsiung, hot strip mill No.2