Publish Time: 2021-08-19 Origin: Site
Abstract: We present a segmented coupling model for slab casting by roller electromagnetic stirring (In-roll EMS) of electromagnetic, flow, heat transfer, and solidification behavior based on magnetohydrodynamics and solidification theory. A three-dimensional (3-D) segmented coupling model that included electromagnetic, flow, and heat transfer elements was established using Ansoft Maxwell and ANSYS Fluent software. The effects of the roller sleeve, magnetic shielding ring, coil, core, molten steel, and air domain on the electromagnetic, thermal and flow fields were studied numerically. The accuracy of the model was verified by measuring the magnetic flux density at the centerline in a pair of rollers and the electromagnetic force of the copper plate. Based on the numerical results of the optimal technical parameters, the effect of the IN-ROLL EMS on the solidification of Fe–17 wt% Cr–0.6 wt% Ni stainless steel was explored. The results indicated that with each additional pair of electromagnetic rollers, the average electromagnetic force increased by 2969 N/m3 in the casting direction, and 5600 N/m3 in the central section of the rollers. With increasing number of pairs of rollers, the effective stirring region increased, and the velocity of molten steel at the solidification front first increased but then decreased. The strong electromagnetic swirling washing effect reduced the solidification rate of the slab shell and promoted the superheated dissipation of molten steel in the center of the strand. The center equiaxed crystal ratio of the slab was improved to 69% with two pairs of IN-ROLL EMS rollers and electromagnetic parameters of 400 A/7 Hz, which was beneficial for obtaining a uniform and dense solidified structure to improve the subsequent hot working performance and product quality.
Keywords: roller electromagnetic stirring (IN-ROLL EMS); Fe–17 wt% Cr–0.6 wt% Ni steel; solidification structure; number of pairs for rollers; washing effectAbstract: We present a segmented coupling model for slab casting by roller electromagnetic stirring (In-roll EMS) of electromagnetic, flow, heat transfer, and solidification behavior based on magnetohydrodynamics and solidification theory. A three-dimensional (3-D) segmented coupling model that included electromagnetic, flow, and heat transfer elements was established using Ansoft Maxwell and ANSYS Fluent software. The effects of the roller sleeve, magnetic shielding ring, coil, core, molten steel, and air domain on the electromagnetic, thermal and flow fields were studied numerically. The accuracy of the model was verified by measuring the magnetic flux density at the centerline in a pair of rollers and the electromagnetic force of the copper plate. Based on the numerical results of the optimal technical parameters, the effect of the IN-ROLL EMS on the solidification of Fe–17 wt% Cr–0.6 wt% Ni stainless steel was explored. The results indicated that with each additional pair of electromagnetic rollers, the average electromagnetic force increased by 2969 N/m3 in the casting direction, and 5600 N/m3 in the central section of the rollers. With increasing number of pairs of rollers, the effective stirring region increased, and the velocity of molten steel at the solidification front first increased but then decreased. The strong electromagnetic swirling washing effect reduced the solidification rate of the slab shell and promoted the superheated dissipation of molten steel in the center of the strand. The center equiaxed crystal ratio of the slab was improved to 69% with two pairs of IN-ROLL EMS rollers and electromagnetic parameters of 400 A/7 Hz, which was beneficial for obtaining a uniform and dense solidified structure to improve the subsequent hot working performance and product quality.
1. Introduction
Nickel-saving stainless steel alloys, such as Fe–17 wt% Cr–0.6wt% Ni, have a ferrite structure at room and high temperatures that causes them to develop columnar crystals easily during solidification with a very low ratio of equiaxed crystal in steel castings. The columnar crystal structure has obvious anisotropy in the subsequent plastic working process, which is likely to produce wrinkle-like defects in plate products due to poor deep drawability. Controlling the cast structure and obtaining a high ratio of equiaxed crystals during the continuous casting (CC) process is important to improve the product’s subsequent hot working performance and quality; this has always been a central issue for the steel industry.
IN-ROLL EMS can replace the normal pinch roller in the CC machine with different pairs of rollers. Different arrangements of the rollers in the segments will have varied magnetic field distributions, areas of EMF, and molten steel flow patterns. Considering the importance of metallurgical behavior in the SCZ of a slab casting with regard to controlling the internal quality of the strand, a coupled model of three-dimensional (3-D) electromagnetic, flow, heat transfer, and solidification behavior in the SCZ has been developed in this study.
The magnetic flux density was measured using a LakeShore Digital Signal Processing Mode Tesla Meter 475 (Zhongke Electric, Hunan, China). The EMF was tested using a thrustmeter manufactured in-house, as shown in Figure 3.
Figure 3. Copperplate measurement method: (a) Structure diagram; (b) physical device diagram.
The main chemical components of the Fe–17 wt% Cr–0.6 wt% Ni steel are listed in Table 2.
Figure 4. Sample cutting from slab casting for metallographic analysis.
3. Results and Discussion
3.1. Analysis of Electromagnetic Field
Figure 5a shows a comparison of the calculated and measured values of magnetic flux density, and Figure 5b shows the EMF on the centerline of the wide surface with a pair of rollers. The measured and calculated values of magnetic flux density on the centerline of the roller and the EMF of the copper plate are roughly in agreement, which verifies the reliability of the model to an acceptable level. Figure 5b shows that the EMF increased rapidly and then decreased slowly as the frequency increased, and the greatest EMF of a pair of roller stirrers was obtained at a frequency of 9 Hz.
Figure 5. Comparison of measured and calculated values on the centerline of the wide face of the pair of rollers: (a) magnetic flux density, (b) electromagnetic force.
Figure 6a–c presents the distribution of the magnetic flux density on the surface of the slab for a current intensity of 400 A and a frequency of 7 Hz under cases of one pair, two pairs and three pairs, respectively, in which the effective area of the magnetic flux density increased with an increasing number of rollers. The traveling wave magnetic field had a certain directionality which produced an end effect, resulting in a larger magnetic flux density on the thrust side (right side of the strand in Figure 6) than on the start side (left side of the strand in Figure 6).
Figure 6. Contour of magnetic flux density on the surface of the stand with (a) one pair, (b) two pairs, and (c) three pairs of rollers.
Figure 7a reveals the distribution of the EMF along the centerline in the casting direction under a different number of pairs of rollers for a current of 400 A and frequency of 7 Hz, and Figure 7b shows the distribution of the EMF along the centerline of the rollers in the wide direction. For one, two, and three pairs of rollers, the maximum EMF on the centerline of the slab along the casting direction was 12,090, 18,573, and 21,229 N/m3, respectively, and the average EMF was 2023, 5066, and 7962 N/m3, respectively. The maximum EMF on the centerline of the wide surface for each pair of rollers was 12,354, 18,084, and 22,874 N/m3, respectively, and the average EMF was 10,247, 15,730, and 21,336 N/m3, respectively. The maximum force was located on the thrust side of the slab, and the EMF of the molten steel increased with increasing number of pairs of rollers.
Figure 7. Distribution of internal electromagnetic force in the strand with different number of rollers (a) along the centerline in the casting direction, (b) along the centerline of the rollers in the wide direction.
Figure 8a shows the distribution of the EMF in the casting direction under two pairs of rollers at different frequencies, and Figure 8b reveals the distribution of the EMF in the casting direction under the two pairs of rollers at different currents. The distribution of the EMF indicates that it was small at both ends, large in the middle, and evenly distributed between the rollers. The maximum EMF at the center of the strand increased from 4750 to 19,000 N/m3 as the current intensity increased from 200 to 400 A. In addition, the maximum EMF at the center of the strand decreased from 20,838 to 17,995 N/m3 when the frequency increased from 4 to 8 Hz. The strand exhibited a certain magnetic conductivity when the magnetic induction lines from the air into the strand deviated, clustering in one location and forming a magnetic shield. The difference in magnetic flux between the interior and edges of the strand resulted in an uneven distribution of the induced current, which was mostly concentrated on the surface of the slab, a phenomenon known as the “skin effect”. This effect leads to a reduction in the penetration of the magnetic field at higher frequencies [17]. It shows that the solidified shell with a certain electrical conductivity has a certain shielding effect on the magnetic field, and therefore the central magnetic induction intensity decreases slightly as the current frequency increases.
Figure 8. Distribution of the electromagnetic force in the casting direction under the two pairs of rollers at (a) different frequencies and under (b) different currents.
3.2. Analysis of Flow and Solidification Behavior
Figure 9a demonstrates the velocity distribution along the centerline in the casting direction on the characteristic line of molten steel with a different number of pairs of rollers, and Figure 9b shows the velocity distribution along the centerline of the rollers in the wide direction. An increase in the number of rollers led to an increase in the local volume of the EMF on the strand, and the EMF was the driving force of the flow of molten steel to wash the solidification front in the SCZ. The effective washing velocity range—defined as the range over which the flow velocity is greater than the casting speed—of the solidification front along the casting direction was 4.0–4.35 m, 3.8–4.35 m, and 3.6–4.35 m for one, two, and three pairs of rollers, respectively, and the maximum washing velocity was 0.7, 0.8, and 0.76 m/s, respectively. Zhang et al. [18] found that the high-velocity jet flow from the side holes can lead a larger turbulence zone in the mold zone and part of the SCZ. Although the EMF of two pairs of rollers is lower than that of the three pairs, the washing region of the two pairs is further down, leaving lower turbulent kinetic energy intensity in the offset mold area. Thus, the strand has a greater maximum washing velocity with two pairs of rollers than with three pairs. Figure 8b shows that the maximum flow velocity under different numbers of pairs of rollers was distributed on one side of the strand. The flow velocity on the thrust side of the EMF was greater than on the start side, which is roughly in agreement with the motion characteristics of the traveling wave magnetic field.
Figure 9. Distribution of velocity with different numbers of rollers (a) along the centerline in the casting direction, and (b) along the centerline of the rollers in the wide direction.
Figure 10a–d shows the temperature distribution and molten steel streamline on the center surface of the narrow face in the slab with 0–3 pairs of rollers. The EMF caused the molten steel to move from one side of the narrow surface to the other, and the continuity of the flow to the narrow solidification front led to the formation of an upper and lower circulation of the molten steel, resulting in a uniform core temperature and mixing of the slab. With an increasing number of pairs of rollers, the area of molten steel flow at the cross-section expanded and the forced heat exchange between the central high-temperature molten steel and solidified shell led to a larger low-temperature zone in the center of the strand. According to solidification theory, a lower temperature of the central molten steel is more conducive to the formation of nucleation particles. Xu et al. pointed out [19] that the washing of molten steel against the solidification front may cause the “melting” of the dendrite arm to provide nucleation particles for the formation of equiaxed crystals, which ultimately increases the central equiaxed crystal ratio of the strand.
Figure 10. Temperature distribution and flow on the narrow center surface of the strand with (a) zero pairs, (b) one pair, (c) two pairs, and (d) three pairs of rollers.
Figure 11a illustrates the variation of the solidified shell at the start side along the casting direction at the center of the narrow face for the strand with different numbers of pairs of rollers, and Figure 11b shows the change in shell thickness at the thrust side along the casting direction at the center of the narrow face for the strand with different numbers of pairs of rollers. The solidification front is considered the location where the liquid phase fraction is 0.3. For zero, one, two, and three pairs of rollers, the thickness of the solidified shell at the outlet of the calculation domain was 42.37, 40.96, 40.14, and 38.43 mm on the start side of the EMF, respectively, and 42.37, 42.27, 37.62, and 37.60 mm on the thrust side of the EMF, respectively. The high-velocity flow of molten steel rushes to the solidification front and interrupts some of the columnar crystals, resulting in the slow growth of the solidified shell in the stirring region. The solidification rate on the electromagnetic thrust side was significantly lower than the start side, which roughly coincides with the characteristics of the traveling wave magnetic field.
Figure 11. Distribution of the shell thickness at the narrow center face of the strand on the (a) start side and (b) thrust side.
3.3. Experiments of Solidification Structure Obtained by IN-ROLL EMS
Two pairs of rollers were selected for Fe–17 wt% Cr–0.6 wt% Ni steel slab casting in the experiments of solidification structure control by IN-ROLL EMS. The semisolid zone in the center of the slab when two pairs of rollers were used was larger than when one pair of rollers was used. Although the EMF was smaller than when three pairs were used, the washing velocity of the solidification front was greater with two pairs than three pairs, which was beneficial for the formation of equiaxed crystals in the strand. In addition, the cost of instrumentation and power consumption are lower when two pairs of rollers are used. The solidification structures of the slab produced when the IN-ROLL EMS was turned off and on were compared during the experiment, as shown in Figure 12. When the IN-ROLL EMS was turned off, the macrostructure of the slab was more developed in the columnar crystal, which is related to the characteristics of Fe–17 wt% Cr–0.6 wt% Ni steel. Having a content of Cr in the steel greater than 16% led to a solidification process without the α→γ phase transition process, with the ferrite structure maintained. Pang et al. [20] found that there was no phase transition to hinder the development of columnar crystals during the process of grain growth; thus, the grain size was coarse, and the chemical elements were prone to segregation, which can seriously affect product quality. When the IN-ROLL EMS was turned on with electromagnetic parameters of 400 A and 7 Hz, the EMF generated by the traveling wave magnetic field caused the molten steel to flow violently and wash the columnar crystal front to reduce the temperature gradient at the solidification front, inhibiting the growth of columnar crystals. At the same time, the high-velocity flow of the molten steel can break up the columnar dendrite arm to form free nuclei in the central low-temperature area. Finally, the central equiaxed crystal ratio of the strand was increased to 69%.
Figure 12. Cross-section of the as-cast macrostructure of the strand (a) without IN-ROLL EMS and (b) with two pairs of rollers used in IN-ROLL EMS (at 400 A and 7 Hz).
4. Conclusions
Here, a 3-D segmented coupling model for electromagnetic, flow, and heat transfer behavior was established for the slab casting of stainless steel. The effects of IN-ROLL EMS on the magnetic field distribution and solidification behavior were revealed, and the optimal technical parameters to control the as-cast macrostructure of steel Fe–17 wt% Cr–0.6 wt% Ni have been presented. The main conclusions are as follows:
1. The characteristics of the traveling wave magnetic field of the IN-ROLL EMS in the SCZ will produce a maximum EMF being located on the starting side of the slab strand. For each additional pair of electromagnetic rollers, the average EMF in the casting direction increases by 2969 N/m3, and the average EMF in the center section of the rollers increases by 5600 N/m3.
2. With an increasing number of pairs of stirring rollers, the effective stirring area of the molten steel inside the strand is enlarged by the EMF, and the velocity of molten steel at the solidification front first increases and then decreases. The flow washing effect from the strong electromagnetic force will reduce the solidification rate of the local shell and accelerate the superheated dissipation of the molten steel center, which is beneficial for the formation of equiaxed crystal.
3. The use of two pairs of electromagnetic rollers at 400 A and 7 Hz can produce a center equiaxed crystal ratio of 69% in the slab strand of 200 mm × 1280 mm, which helps to improve its hot working behavior.
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