Research on electromagnetic properties of electromagnetic stirring roller

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1)Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201999,China; 2)Steelmaking Plant,Baoshan Iron & Steel Co.,Ltd.,Shanghai 200941,China

Abstract: Using computer simulation and test measurements,the relationship between the electromagnetic properties of an electromagnetic stirring roller and the roller structure parameters was analyzed and compared.The distribution of the magnetic field,magnitude of the electromagnetic force,and matched-load characteristics were systematically investigated at different parameter values for the induction coils,iron core,shield,and roller sleeve.The results indicate that the change in the electromagnetic force with stirring frequency is non-linear and has a maximum value higher than 10 Hz;the electromagnetic force largely depends on the diameter of the core;and there is a mutual restriction relationship between the two-phase imbalance and the electromagnetic force.

Key words: electromagnetic stirring; silicon steel; continuous casting of slab

1 Introduction

To increase the equiaxed grain ratio of silicon steel,the application of electromagnetic stirring in the secondary cooling zone of the continuous casting process is indispensable.Around the world,there are three types of electromagnetic stirring devices typi-cally used in the secondary cooling zone of a slab,namely ABB’s box-style stirrer,Rotelec’s windings roller stirrer,and Nippon Steel’s DKS plug-in stirrer.The coil windings of a roller stirrer are installed in a hollow roll sleeve of the non-magnetic stainless steel,and this roller sleeve rotates with the movement of the slab as it is being cast.As it is installed directly on the surface of the slab,the roller stirrer can simultaneously support the slab and stir the molten steel inside the slab.Compared with the other two types of stirrers,the roller stirrer can produce the maximum electromagnetic stirring force with the minimum power output.It also has the advantages of simple installation,requiring only a slight change in the continuous casting sector,a small investment,and short manufacturing and maintenance cycles.The roller stirrer has been widely used by Baosteel and other major steel plants in China.

However,during the actual use of some processes,the equiaxed grain ratio of silicon steel often fluctuates and in some cases does not even reach the standard value[1-2].In addition,the roller stirrer is easily damaged,which results in a relatively short service life,especially when a phase imbalance occurs.A phase imbalance is harmful to the electrical equipment and induction coil,which affects their service lives as well as the distribution of the electromagnetic field and the generated electromagnetic force.In addition to the instability of the continuous casting process parameters,another important reason for a poor equiaxed grain ratio is an inappropriate magnetic structure of the roller stirrer and the stirring parameters used[3- 4].In this study,a systematic investigation was performed on the key electromagnetic parameters of the roller stirrer,based on which the relationships between the electromagnetic characteristics of the roller stirrer and its magnetic structure and electrical parameters were obtained,which enable optimization of the magnetic structure and technical process parameters of the roller stirrer.

2 Computer simulation

To clarify the relationship between the key equipment parameters of the roller stirrer and its electromagnetic performance,a three-dimensional computer simulation was conducted,which focused on the influence of the core parameters,coil para-meters,and other key parameters on the magnetic field distribution,size of the electromagnetic force,and coil voltage values[5-7].Since the roller stirrer has an axial-symmetric cylindrical shape,a 1/2 model was established,as shown in Fig.1,which includes the slab,roller sleeve,shield,coils,core,and air region.During the application,alternative currents were supplied to the five coils with phase degrees of 0°,90°,180°,270°,and 360°,respectively,that is,adjacent coils had a phase difference of 90°.In this way,a linear moving magnetic field is generated parallel to the slab width,which results in a stirring effect to improve the equiaxed grain ratio.

The main assumptions of this model are as follows:

(1) The slab and sleeve are non-magnetic;

(2) The temperature of all parts remains con-stant;

(3) The coil winding structure is homogeneous.

Table 1 lists the main physical properties of the components of the electromagnetic stirring roller.

Table1Physicalpropertiesofthecomponentsoftheelectromagneticstirringroller

ComponentRelative permeabilityResistivity/(Ω·m-1)Shield11.72 × 10-8Sleeve1.058.5 × 10-7Core1 000 (saturation 2.0 T)0.2

3 Model verification

To verify the accuracy of the model,the simula-tion results were compared with the measured values.Table 2 shows the key structural parameters of the roller stirrer,among which the coil length (L) was designed based on the total length of the roller stirrer,which is usually about 1/4 the total length of the roller.The core radius (R) and shield thickness (D) were determined based on the space in the roller stirrer cavity.Taking the roller stirrer with a diameter of 240 mm as an example,the cavity space is usually about 180 mm,and the relationship between the radius of the iron core,the thickness of the shield,and other necessary space (H,constant value) is as follows:2R+D+H=180 mm (50 mm

Table 2 Main structural parameters of the roller stirrer

To optimize the magnetic structure of the roller stirrer,the magnetic field intensities at 10-mm intervals from the front of the roller sleeve on the working side of the roller stirrer were calculated and measured,respectively,the results of which are shown in Fig.2.It can be seen from the figure that the calculated magnetic field intensity values have a very high coincidence with the measured values,having nearly the same distribution pattern.The magnetic field produced by the five coils has five peaks and six troughs,with a high distribution of the magnetic field in the middle region and a low distribution on both sides.The maximum magnetic field intensity in the middle exceeds 0.1 T.This distribution pattern is determined by the structure of the coils themselves,which is an especially effective characteristic that can be modified to improve the equiaxed grain ratio in the middle area of the slab being cast.

The abovementioned comparison of the calculated and measured values confirms the accuracy of the simulation model.Therefore,it is considered that the optimized calculation model has high reliability and accuracy.

Fig.2Comparisonofcalculatedandmeasuredvaluesoftheelectromagneticfluxdensitydistributionsat10-mmintervalsfromtherollerworkingsurface

4 Influence of different factors on electro-magnetic properties of roller stirrer

4.1 Influence of coil parameters on magnetic field,electromagnetic force,and phase imbalance

Generally,a roller stirrer uses a two-phase power supply,and the load-match characteristics of this two-phase power supply have a great impact on both the electromagnetic performance and the service life of the roller stirrer.As mentioned above,to improve the equiaxed grain ratio of the slab,the magnetic field distribution generated by the roller stirrer must be such that there are high values in the middle region and low values on either side.The corresponding coil parameters are shown in Table 3.To further optimize the structure param-eters of the coils,the effect of different magnetic field,electromagnetic force,and phase imbalance values are compared in coils with different struc-ture parameters.

Table 3 Optimization of coil structure parameters

In Table 3,P1,P2,andP3are the parameter values used,with the physical length of the coils remaining constant while the turns of each coil were adjusted to obtain a higher magnetic field intensity in the middle region of the roller stirrer.These adjustments were made to improve the stirring effect and reduce the phase imbalance between the two phases of the roller stirrer,as well as to improve its service life.P4is the parameter value when the physical length of the coils was adjusted in proportion to the turns of each coil.Fig.3 shows the calculated magnetic field distributions for each parameter value.

As can be seen in Fig.3,when the numbers of the turns of coils 1#and 5#were reduced (5 and 10 turns,respectively) and the number of turns of coil 3#was increased correspondingly,while the physical length of the coils remained constant,the magnetic field distribution changed only a bit,with a slight increase in the middle and a slight decrease on both sides.The average magnetic field intensities also slightly decreased by 1.05% and 2.63%,respec-tively.The reasons for this result are as follows:① The magnetic field intensity of the iron core in the middle area was almost saturated,which means it was difficult to increase the magnetic field intensity by increasing the number of the coil’s ampere turns;② magnetic leakage occurred on both sides of the roller stirrer;and ③ the magnetic field produced by coils 2#and 4#also played a significant role in the magnetic field distribution.The above three factors indicate that adjusting the turns of coils 1#,3#,and 5#only will not have a fundamental impact on the magnetic-field distribution.However,when the physical length of the coil is adjusted propor-tionally,corresponding to parameterP4,it can be seen that the peak and valley positions of the magnetic field distribution also shift correspond-ingly.At the same time,the middle area of the magnetic field is greatly reduced,and the average magnetic field intensity is reduced by 4.28%.

Fig.3Electromagneticfluxdensitydistributionat10-mmintervalsfromtherollerworkingsurfaceunderdifferentcoilparametervalues

Using parametersP1,P2,P3,andP4,the electromagnetic force on the slab is calculated to be 20.50,20.10,19.32,and 17.65 N,respectively,which indicates a gradual decreasing trend.The electromagnetic force at parameterP4exhibits the greatest decrease of 14%.

In Table 4,it can be seen that a phase imbalance between the two phases of the roller stirrer is unavoid-able for all calculation parameters.By increasing the number of turns of coil 3#and reducing the number of turns of the coils on both sides of the roller stirrer,this phase imbalance can be improved.According to the calculated results for parametersP1,P2,andP3in the table,it is considered that when the coil parameters are further optimized,phase balance is achievable.However,the physical length of the coils must be considered in the roller manufacture process of coil winding.Although the calculated results for parameterP4indicate that a change in the physical length of the coil has little effect on the phase imbalance,the electromagnetic force generated at this parameter value will inevitably be reduced,as mentioned above.

Table 4 Coil voltages under different conditions

In conclusion,with improvement in the phase balance,the electromagnetic force will inevitably decrease at the same parameter values,for which there is an optimal matching relationship.There-fore,this research focused on optimizing the magnetic structure of the roller stirrer via the phase imbalance and electromagnetic force,and a reason-able choice of the structural parameters should be made by weighing the advantages and disadvantages of each.

4.2 Influence of core parameters on the magnetic field and electromagnetic force

The core parameters also have a critical effect on the magnetic field and electromagnetic force.As shown in Fig.4,there is an optimal core diameter in the limited space of the roller stirrer cavity.

Fig.4Relationshipbetweendiameterofthecoreandelectro-magneticforce

The reasons for this phenomenon are considered to be as follows:when the core size is small,magnetic saturation is easily achieved under certain current intensity and frequency.With increase in the core diameter,the electromagnetic force can be gradually increased.At the same time,due to the space limitation of the roller stirrer,the thickness of the shield is reduced correspondingly.In this process,the size of the iron core plays a key role.As the core diameter continues to increase,the magnetic shielding effect of the shield gradually weakens.When the core is no longer saturated,increasing the core size will not further increase the electromagnetic force,but will further weaken the magnetic shielding effect of the shield.Therefore,under certain electric parameters,there is an optimal core diameter.

Under the condition of parameterP1mentioned above,the core diameter is adjusted to be relatively smaller (the relative reduction is 11.2%),which reduces the average magnetic field intensity by about 8.81%,and reduces the generated electro-magnetic force to about 16.46 N,i.e.,as much as 20%.As such,it can be seen that the core diameter has a critical effect on the electromagnetic properties of the roller stirrer,which is an important consideration when designing roller parameters.

4.3 Influence of stirring frequency on electro-magnetic force

With an increase in the stirring frequency,the input voltage on the coils will gradually increase under the same input current,and the relationship between them is almost linear,as shown in Fig.5.

Fig.5Relationshipbetweenfrequencyandvoltageofcoils

At the same time,with increases in the stirring frequency,the electromagnetic force first increases and then decreases slightly,which means there is an optimal stirring frequency,as shown in Fig.6.It is also evident in Fig.6 that there is a difference between the calculated and measured values.The frequency corresponding to the maximum measured electromagnetic force is about 11 Hz,whereas the calculated value is about 17 Hz.In addition to the calculation error,another important reason for this result is that a copper plate swing angle is actually measured,whereas in the computer simulation,the overall electromagnetic force on the slab at the front of the stirring roller is calculated.Therefore,although some differences arise between the two evaluation methods,both methods indicate that the optimal frequency corresponding to the maximum electromagnetic force is greater than 10 Hz,which is far larger than the actual frequency used.This is especially worth considering when performing parameter design and in the application of the stirring roller.

Fig.6Relationshipbetweenfrequencyandelectromagneticforce

5 Conclusions

The electromagnetic stirring roller is a key equip-ment used to produce high value-added products such as stainless steel and silicon steel.Devising ways to improve the performance and service life of this equipment has been an ongoing process and an important issue for local technicians and engineers.This study was focused on the influence of key equipment parameters of the roller stirrer on its electromagnetic properties,for which the following conclusions were obtained:

(1) The electromagnetic force first increases and then decreases with increases in the stirring frequency,and the optimal frequency is higher than 10 Hz.

(2) The core diameter has a decisive effect on the generated electromagnetic force.

(3) The relationship between the magnitude of the electromagnetic force and the phase balance is mutually restricted.

By optimizing the magnetic parameters and stirring frequency,the properties and application effect of the roller stirrer can be improved.