Primary columnar crystal growth characteristics affected by double stirrings in continuous casting square billet

The primary columnar crystal deviates due to forced flow induced by a mold electromagnetic stirrer (M-EMS). On this basis, the flow field can be qualitatively analyzed. As the growth characteristics of primary columnar crystal by double stirrings are still unknown, an electromagnetic swirling flow in the nozzle (EMSFN) approach was applied with M-EMS during a plant trial of continuous casting square billet. We first investigated the deflected angle of the primary columnar when using M-EMS or EMSFN solely. The deflected angle reduces gradually from the billet surface to the center when using only M-EMS. With increasing the EMSFN current, the deflected angles increase first, then reduce. When double stirrings with opposite directions are applied, the deflected angles fluctuate between positive and negative values near the billet surface. When the current is 600 A, the ratio of negative angles reaches 45%. This fluctuation of growth direction forms an interlaced dendrite structure, which restrains the solute migrating to the billet center.


Introduction
Forced flow caused by a mold electromagnetic stirrer (M-EMS) can accelerate the superheat dissipation and uniform the flow field, which is favorable to the internal quality of steel.Meanwhile, some researchers found the forced flow induced by the stirring affects primary columnar crystal (PCC) growth characteristics.Owing to forced convection in the vicinity of solid-liquid interface, the PCC deviates during the solidification process [3] .When both solute concentration and flow velocity are in certain ranges, the deflected angle raises with increasing the velocity [1] .The PCC is always parallel to heat transfer direction.Thus, the flow pattern and the heat transfer can be qualitatively and quickly evaluated by investigating PCC growth characteristics instead of numerical simulation method.
However, the present research results focus on those by one stirring on molten metal.The growth characteristics of PCC affected by double stirrings are more complex and still unknown.The flow pattern produced by double stirrings can be obtained by using some nozzle swirling flow approach [2] or an alternating current stirring modifier (AC-SM) [3] together with M-EMS.Without modifying the existing structure of continuous casting equipment, an electromagnetic swirling flow in the nozzle (EMSFN) can generate a second rotating flow in mold.Differing from M-EMS, EMSFN makes the molten steel rotate outwards from the nozzle outlet to the wall of the mold and can offer an opposite swirling flow in the tangential direction.In our previous work, unlike those by M-EMS, the number of finer equiaxed crystals was increased, and the segregation degree was reduced by using only EMSFN in the square steel billet [4] .When the EMSFN device with the opposite stirring direction combined with M-EMS was applied, the billet quality was further improved [4] .
In this paper, the PCC characteristics in square steel billet under the action of double stirrings were investigated after an industrial test of continuous casting.We first analyzed the PCC characteristics and tangential velocity by using M-EMS and EMSFN, respectively.Then, by using them together, the variation trend of PCC affected by double stirrings was determined.

Plant trial
The high-carbon steel were used as the test samples.The chemical composition (mass fraction, %) is C 0.70, Si 0.20, Mn 0.56, P 0.01, S 0.01, Cr 0.04, Ni 0.01, and Cu≤0.25.The parameters of casting are given in Table 1.In plant trial, the M-EMS was on or off in accordance with experimental condition.The current frequency and current intensity of M-EMS were, respectively, 4 Hz and 500 A. The EMSFN device was set between tundish and mold [4] .The stirring direction of EMSFN was opposite to the tangential flow by M-EMS.

Laboratory experiment
The dendrite morphology in the square billet was studied after macro etching.The deflected angle of PCC was calculated.Figure 1 shows the measurement area of the columnar crystal deflected angle.The deflected angles were measured in four rectangle areas with the size 2.5 mm×5 mm, located at 5 mm~25 mm from billet surface in triangular area on the cross-section.Compared with that in the mold corner, the fluid flow is more sufficient near the side wall.Therefore, the tangential velocity was studied according to the average value of the positive deflected angle in the red middle region located at 35 mm from the two side surfaces.Figure 2 depicts the deflected angle direction.The deflected angle is zero degrees when the growth direction of the PCC is perpendicular to the wall of the mold.The deflected angle influenced by forced flow with M-EMS or EMSFN is positive.It is negative when it deviates to the opposite side.When the double stirrings are applied, the deflected angle is positive influenced by M-EMS.
Referring to Wang et al.'s research [5] , under the action of one stirring, the relevance of the velocity of tangential flow to the deflected angle can be written as: V T =0.172-0.0226ln22.748-θ (1) where V T is tangential flow velocity, m/s, θ is PCC deflected angle, deg.It is noted that this formula cannot present the relationship when using double stirrings.
The carbon contents at four positions shown in Figure 1 were measured.To characterize macrosegregation, a carbon segregation index was calculated, which indicates whether the carbon is depleted or not.

Columnar crystal growth when using M-EMS or EMSFN
Figure 3 displays the dendrite morphology obtained by using M-EMS or EMSFN.When using M-EMS, the majority of columnar crystals are located within 5 mm～25 mm from the billet surface and deviate due to the forced flow.When using the EMSFN device at 200 A and 400 A, the solidification structure is dominated by the columnar crystal owing to the low nozzle swirling flow magnitude.When the current reaches 600 A, multitudinous equiaxed crystals show up in the billet center, and the morphology resembles that by using only M-EMS.
The deflected angles of columnar crystal located within 5 mm～25 mm from the billet surface were measured.Figure 4 shows the primary columnar crystal deflected angles at different distances when using M-EMS.In the billet corners, there are some columnar crystals with negative angles whose growth directions are opposite to those near the billet surface.From the two billet corners to the middle region (the red measurement area is located within 35 mm from the two side surfaces), the deflected angles gradually increase and are mainly positive.From the billet centerline, the deflected angles on the left and right sides distribute symmetrically, which is coincidental with Wang et al.'s work [5] .When the mold is rectangular, the flow is the most sufficient near the side wall of the mold, where the tangential velocity is larger than those at the corners.Furthermore, the impact of molten steel on the mold corner wall leads to a chaotic flow field and non-directional heat transfer.Thus, there are columnar crystals with negative angles in the billet corners and some with positive angles near the billet surface.Deflected angles of PCC when using only EMSFN are shown in Figure 5.When the current is 200 A, the growth directions of PCC are neither perpendicular to the billet surface nor deviate in one direction.However, they fluctuate in the positive and negative directions within a small range of angle values.With increasing the current to 400 A, the magnitude of EMSFN increases, leading to the reduction of the fluctuation.The growth of columnar crystals becomes unidirectional and deviates towards the upstream side of the forced flow.When the current is 600 A, due to the enhancement of the magnitude of EMSFN, the deflected angles increase significantly, and the distribution characteristics are similar to that with M-EMS from Figure 4.After calculation, all the average values of positive deflected angles from billet surface by using EMSFN are smaller than those by using M-EMS, as shown in Figure 6.The velocity of tangential flow is calculated according to the average value of positive deflected angle.Figure 7 depicts the tangential velocity at solid-liquid interface along radial direction.When using M-EMS, the average deflected angle at 10 mm from surface is 21.21 deg, indicating that the tangential velocity is 0.162 m/s as the solid-liquid interface is 10 mm from mold wall.With the solid-liquid interface moving forward, the tangential velocity gradually decreases.This is because under the action of M-EMS, the maximum tangential force is located near the mold wall, and the minimum one is located at the stirring center.
The tangential velocities induced by EMSFN are lower than those by M-EMS.When the current is 200 A, the velocity remains at a low level owing to the low magnitude of EMSFN.When the current reaches 400 A, unlike M-EMS, the maximum velocity is neither near mold wall nor at mold center but outside the nozzle outlet.When the current is 600 A, this unique variation trend of tangential velocity is represented perfectly, which firstly increases and then decreases.It proves that the flow direction of EMSFN is from nozzle outlet to mold wall.This flow pattern can raise the difficulty of solute transportation.

Columnar crystal growth when using M-EMS and EMSFN
The characteristics of PCC under the action of double stirrings were studied with M-EMS and EMSFN simultaneously.The opposite stirring direction provides more uniform flow field and a lower macrosegregation degree [4] .The current was increased from 200 A to 600 A. Figure 8 displays the solidification structure when the current of EMSFN is 0 A and 600 A. It is found there are some PCCs located from 5 mm to 10 mm whose deflected angles are opposite to those from 10 mm to 15 mm.We measured the deflected angle along the radial direction.Figure 9 shows the deflected angles within the different instances from the billet surface when using double stirrings.At the different current intensities, the variant trends and the distribution characteristics of deflected angles measured from 10 mm to 25 mm are similar to those under the single stirring.However, many deflected angles have negative values in the outermost measurement area.
After calculation, the average value of the positive deflected angle increases and then decreases from the billet surface to the center, which is similar to that of using only EMSFN.The deflected angles near the billet surface are smaller than those obtained by using M-EMS only.This is because the flow produced by M-EMS is greatly attenuated with the double stirrings.With raising the EMSFN current, the ratio of positive deflected angle decreases.This means that there are considerable PCCs with negative angles.Under the influence of the stirring induced by EMSFN in the opposite direction, the flow in the upper mold becomes complex and chaotic, resulting in an irregular direction of heat transfer at the mold wall.Therefore, those growth directions of PCC appear to fluctuate between positive and negative values.The number of negative angles in the outermost area reaches 45% with increasing the current of EMSFN to 600 A. To demonstrate the influence of this growth direction fluctuation on the billet quality, the angle difference between the first layer (5 mm~10 mm) and the second layer (10 mm~15 mm) is calculated and compared with those obtained using only M-EMS. Figure 10 gives two kinds of differences in deflected angle.It indicates the differences between the positive deflected angles, or those between the negative deflected angles in the first layer and the positive deflected angles in the second layer.Figure 11 shows the angle differences between the two outermost layers.With increasing the current of EMSFN from 200 A to 600 A, the number of angle differences with high value increases significantly and is more than that when using only M-EMS.It is noted here that the morphology of the interlaced dendrite structure is different from that in the columnar to equiaxed transition (CET) zone.The crystals in the CET zone originate from the side arm of the columnar crystal or the equiaxed crystal nucleus.This interlaced dendrite structure is formed by successive columnar crystals with different growth directions (positive or negative direction) or with a big angle difference in one growth direction (positive direction).When the nozzle rotating flow meets the tangential flow produced by M-EMS, due to the formation of multitudinous little vortices near mold wall [6] , the heat transfer direction is not affected by the sole action of the EMS.It results in some columnar crystals growing along the opposite direction during the initial stage of solidification.Afterwards, when the forced flow by the M-EMS plays a dominate role, the growth direction undergoes a further transformation.However, this structure can increase the distance of the solute transport path, which increases penetration difficulty of solute elements from the side wall to the stirring center.For reducing centerline segregation, the effect of this interlaced dendrite structure is the same as that of the CET zone [7] .
To verify the effect of the double stirrings on solute element distribution, carbon segregation on the cross-section of the square billet was investigated.Figure 12 shows that carbon segregation index when the current of EMSFN is 600 A. When using only M-EMS, the segregation indexes at 10 mm and 70 mm from the billet surface are 0.971 and 1.083, respectively, indicating the solute elements depleted at the billet surface and enriched at the billet center.After using double stirrings, due to the block effect of the interlaced dendrite structure, the carbon-negative segregation at the billet surface is alleviated, leading to less carbon transported to the billet center.For high-carbon steel, the reduction of centerline segregation can decrease the severity of porosity and shrinkage defects in the center of the billet.This is beneficial for improving mechanical properties during the finished product processing [8,9] .

Conclusions
The growth characteristics of primary columnar crystal by a single stirring and double stirrings are studied respectively using M-EMS and EMSFN.The conclusions are as follows: (1) Under the action of one stirring induced by M-EMS, the deflected angles are distributed symmetrically from the left and the right surfaces to the center.The maximum and minimum deflected angles appear near the billet surface and at the billet center, respectively, indicating that maximum tangential velocity appears near mold wall and decreases gradually to stirring center.That tangential velocity is 0.162 m/s at 10 mm from the mold wall.
(2) When the current is more than 400 A, the distribution of deflected angle by using EMSFN only resembles that when using M-EMS.However, the deflected angle first increases and then decreases from the billet surface to the center as the maximum tangential velocity appears at the nozzle outlet.
(3) When double stirrings are applied, and those directions are opposite, the deflected angles decrease and fluctuate between positive and negative values at 5 mm~10mm from the billet surface.The number of negative deflected angles is increased when increasing the magnitude of EMSFN.When the current is 600 A, the ratio of the negative angle reaches 45%.The fluctuation indicates that an interlaced dendrite structure forms in the vicinity of the billet surface, which can block the solute penetration to billet center during the solidification process.

Figure 1 .
Figure 1.Measurement area of primary columnar crystal deflected angle.

Figure 3 .
Figure 3. Morphology of solidification structure at (a) the selected position, when the currents of EMSFN device are (b) 200 A, (c) 400 A, and (d) 600 A, or (e) using only M-EMS.

Figure 5 .
Figure 5. Columnar crystal deflected angles in the different distances from the billet surface with EMSFN device at (a) 200 A, (b) 400 A, and (c) 600 A.

Figure 6 .
Figure 6.Average positive deflected angle of primary columnar crystal.

Figure 7 .
Figure 7. Velocity of tangential flow along the radial direction.

Figure 8 .
Figure 8. Solidification structure by single and double stirrings.

Figure 9 .
Figure 9. Deflected angles of primary columnar crystal under the action of double stirrings when the EMSFN device at (a) 200 A, (b) 400 A, and (c) 600 A.

Figure 10 .
Figure 10.Two kinds of deflected angle difference: (a) a negative deflected angle with positive deflected angle, (b) two positive deflected angles.

Figure 11 .
Figure 11.Angle differences between the two outermost layers after double stirrings.

Figure 12 .
Figure 12.Carbon segregation indexes under the joint action of double stirrings when the current of EMSFN is 600 A.

Table 1 .
Parameters of casting.