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Texture control of 3.04%-Si electrical steel sheets by local laser melting and directional solidification
Materials Letters 185 (2016) 43–46
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Texture control of 3.04%-Si electrical steel sheets by local laser melting and directional solidification Yohan Yoon n,1, Jungryoul Yim 1, Eunho Choi, Junghan Kim, Kyuhwan Oh, Youngchang Joo nn Department of Materials Science and Engineering, Seoul National University, Seoul, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 11 August 2016 Accepted 20 August 2016 Available online 22 August 2016
〈100〉 axes of the rod-shaped grains of laser irradiated cold-rolled 3.04%-Si steel sheets are parallel to either solidification direction or normal direction of the steel sheet. The portion of desirable textures was enhanced by controlling of atmosphere and the direction of heat flow. The area fraction of the texture, which has 〈100〉 axis parallel to normal direction of the steel sheet, in the laser-affected steel sheets in air and argon atmosphere increased up to 23.4% and 26.3%, respectively, compared to a conventionally annealed steel sheet (8.2%). & 2016 Elsevier B.V. All rights reserved.
Keywords: Electrical steels Texture Directional solidification EBSD Laser processing
1. Introduction The texture (preferred crystallographic orientation) of polycrystalline materials plays a critical role in material properties [1,2]. Thus, texture control for the materials, especially electrical steel sheet, has been studied to obtain its more desirable properties [3]. When electrical steels have {hkl}〈100〉 textures, where {hkl} planes and 〈100〉 directions are aligned in the rolling plane and the rolling direction, respectively, the steel sheets have the highest possible magnetic induction and the lowest energy loss since 〈100〉 is the easiest direction of magnetization [1,4]. In non-oriented electrical steel sheets, magnetic isotropy and induction can be enhanced when 〈100〉 axes are randomly oriented but still parallel to the plane of the steel sheet. Thus, two textures should be aligned for superior magnetic isotropy, i.e. 〈100〉// ND textures where 〈100〉 axes are parallel to the sheet normal and 〈100〉⊥ND textures where 〈100〉 axes are radially-distributed on the sheet plane. The most effective way to align 〈100〉 axes in silicon steel is to use directional solidification since it is reported that 〈100〉 axes are parallel to the direction of solidification in steel [5,6]. In this study, we applied laser crystallization technique to n
Corresponding author. Co-corresponding author. E-mail addresses: [email protected] (Y. Yoon), [email protected] (J. Yim), [email protected] (E. Choi), [email protected] (J. Kim), [email protected] (K. Oh), [email protected] (Y. Joo). 1 Both the authors contributed equally to this study. nn
http://dx.doi.org/10.1016/j.matlet.2016.08.105 0167-577X/& 2016 Elsevier B.V. All rights reserved.
control the 〈100〉 axis in electrical steel sheets, which is meltmediated process involving very high rates of heating and cooling [7]. Silicon steel sheets were melted through the entire thickness and 〈100〉 axes were controlled by changing the solidification direction. Furthermore, it is also reported that atmospheres during the laser process are important for the control of the microstructures of the laser-annealed silicon steel sheets. 0.35 mm-thick cold-rolled silicon steel sheets (Si: 3.04, Al: 1.35, Mn: 0.21, P: 0.0046, C: 0.0022, and N: 0.0013 mass%) were used in this study. A 400 W Nd: YAG (1064 nm) welding laser was used for annealing the steel sheets. To control the texture of the Si steel sheets, a small area on the steel sheet should be melted through the entire thickness. At the same time, damage of the steel sheets caused by evaporation must be prevented during laser processing because this damage will make the efficiency of the electrical steel sheet low. For these purposes, the parameters of the laser processing were optimized. More details of optimized laser parameters, such as diameter of the laser beam, average beam power, defocusing value, were already reported [8]. The optimized laser conditions are as follows; beam diameter: 3 mm, output power: 400 W, pulse repetition rate: three pulses per second, laser emission duration time: 20 ms, defocusing value: 10 mm. The laser power as a function of time can be divided into two parts; the first part is laser irradiation on the steel sheet, and the second part is the switch-off of the laser. The time of laser irradiation during one period is called the laser-emission duration time. During the laser process, three shots of the laser beam irradiated the steel sheet with three pulses per second. The steel sheets were irradiated by the laser in ambient air and in flowing
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Fig. 1. Optical micrographs of the laser-irradiated (a) in-plane view and (b) crosssectional view of laser irradiated cold-rolled 3.04%-Si steel sheets. laser beam diameter: 3 mm, laser output power: 400 W, 3 pulses per a second, laser emission duration time (pulse width): 20 ms, defocusing value: 10 mm, and three lasershots. (c) The magnified cross-sectional image of the green box in Fig. 2(b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
argon atmosphere. The grain morphologies and crystal orientations with respect to the sample directions (the rolling direction, transverse direction, and sample normal) were analyzed using field-emission scanning electron microscopy (JEOL JSM-6500F) equipped with the electron back-scattered diffraction (EBSD) detector (Oxford instrument NORDLYS 2). To observe the grain morphologies using EBSD, band contrast images from the EBSD data were used. Fig. 1 shows the optical micrographs of the laser-irradiated 3.04%-Si steel sheets with optimized laser parameters. The top and cross-sectional view optical micrographs in Fig. 1a and b provide three types of grain morphologies in the laser-affected region: columnar grains, rod-shaped grains, and tiny grains. Note that the formation of the columnar grains and rod-shaped grains is due melting and solidification of the laser irradiated area of the sample. The solidification direction is the main factor to determine grain structures. The columnar grains are formed when the solidification direction is vertical (perpendicular to the sample surface) due to the contact of melted area with metal table where the laser annealing was performed. On the contrary, the lateral solidification direction (parallel to the sample surface) due to noncontact of the melted area induces rod-shaped grains. At the edge area of the laser-affected zone, tiny grains are created by recrystallization process since the laser power is not enough to melt
the sample. Interestingly, Fig. 1c shows the boundary between the tiny-grain region and rod-shaped-grain region indicated by yellow arrow, indicating different thermal history. Fig. 2a presents EBSD image (crystal directions are referenced to the rolling direction) of the laser annealed sample. The rodshaped grains parallel to the rolling direction are aligned to 〈100〉. However, it is not sufficient to confirm that all the rod-shaped grains have the 〈100〉 axis parallel to the solidification direction. Therefore, the crystal orientations in Fig. 2a were re-ordered with respect to the direction of solidification of each rod-shaped grain (see Fig. 2b). Interestingly, most of the rod-shaped grains have 〈100〉 axes parallel to the directions of solidification of the grains. The possible scenario of this directional solidification is that 〈100〉 crystal orientation is the most favorable direction in electrical steels since it is the most loosely packed direction. However, the boundary between columnar grains and rod-shaped grains is not perpendicular to the sample surface due to the viscosity of the melted area, so the rod-shaped grains are not perfectly parallel to the surface. If the melted region of the steel sheet could be prevented from touching the metal supporter, this undesired microstructure would be minimized. Fig. 3 shows EBSD images of the sample in which four corners were supported, so the melted region does not touch with the metal table (hereafter air-contact sample) (see inset in Fig. 3a). The air-contact laser annealed sample has larger rod-shaped grains and smaller columnar grains than the metal-contact laser annealed sample since the direction of solidification is parallel to the steel sheet. Crystal orientation maps in Fig. 3b and c show that the textures of the rod-shaped grains in the air-contact sample have similar aspect to those of metal-contact sample. However, one difference is that the air-contact sample has columnar grains with 〈100〉// ND fiber component (see Fig. 3c). This means that the columnar grains solidified normally to the steel sheet. Even though air has lower thermal conductivity compared to the steel, heat loss still occurs through the surface of the steel sheet. Note that 〈100〉// ND fiber textures are expected to enhance the magnetic isotropy of the electrical steel. Therefore, air-contact laser processing is efficient way to control magnetic isotropy of non-oriented electrical steel sheet. It is well known that atmosphere plays a role in microstructural evolution during annealing [9]. In this study, argon gas was continuously flowed over the lased surface during laser processing for the comparison of microstructures of laser-processed steel sheets to those of the laser annealed sample under air ambient. Fig. 4a shows the crystal orientation map of the top-view of the argonprocessed steel sheet. The most significant difference between airprocessed and argon-processed laser annealed samples is that the argon-processed sample has only rod-shaped grains and no columnar grains. The possible explanation for this observation is that the temperature of the center of the melted region of the argonprocessed sample does not reach the solidifying point before the rod-shaped grains solidify to the center of the melted region due to the lower thermal conductivity of argon gas (0.0177 W/m K) [10] than air (0.0235 W/m K) [11]. Fig. 4a shows a EBSD image with respect to the normal direction of the argon-processed steel sheet. Note that the area fraction of the 〈100〉// ND component in the argon-processed steel sheet is larger than that of the sample processed in the ambient air atmosphere (Fig. 3c), whereas the 〈110〉// ND fiber texture in the argon-processed steel sheet is smaller than that of the sample processed in the ambient air atmosphere. This indicates that 〈100〉// ND component is preferred for the rod-shaped grains under argon ambient since {100} grains preferentially grow at the expense of {110} grains during argon annealing [12]. Fig. 4b shows a quantitative analysis for the textures of the laser-processed steel sheets (air-contact and air-contact in argon ambient) and a reference steel sheet annealed
Y. Yoon et al. / Materials Letters 185 (2016) 43–46
45
Fig. 2. Crystal orientation maps of the top-view of the laser irradiated sample. laser beam diameter: 3 mm, laser output power: 400 W, 3 pulses per a second, laser emission duration time (pulse width): 20 ms, defocusing value: 10 mm, and three laser-shots. The crystal directions are referenced to (a) the rolling direction, (b) the solidification directions of the rod-shaped grains.
Fig. 3. (a) a Band-contrast image and (b), (c) crystal orientation maps from EBSD data of the cold-rolled, 3.04%-Si steel sheets that were lased under the same conditions as given in Figs. 2 and 3. The four corners of the sample were supported and the melted region was in contact with the air (air-contact) (see the inset of Fig. 4a). The sample was lased in an air atmosphere. The crystal orientation maps are with respect to (b) the rolling direction and (c) the normal direction of the steel sheet.
Fig. 4. (a) a crystal orientation map from EBSD data of the cold-rolled, 3.04%-Si steel sheets that were lased under the same conditions as given in Figs. 2 and 3 and this figure. The four corners of the sample were supported and the melted region was in contact with the air (air-contact). The sample was laser-irradiated while argon gas was flowed over the lased surface. The crystal orientation maps are with respect to the normal direction of the steel sheet. (b) Quantitative analysis of the area fractions of three types of textures produced by various annealing conditions.
conventionally. The area fractions of 〈100〉// ND fiber texture and 〈111〉//ND fiber texture in the non-oriented electrical steel sheets processed by conventional annealing are about 8.2% and 16.3%, respectively. The area fractions of the 〈100〉// ND fiber texture in the steel sheet laser-processed in the ambient air atmosphere and the argon-processed steel sheet increased up to 23.4%, and 26.3%, respectively. In addition, the fractions of 〈111〉// ND fiber texture of air- and argon-processed steel sheets decrease to 9.4%, and 10.4%, respectively. This indicates that local melting and directional solidification by laser processing under argon ambient is much more
efficient than the conventional annealing process to increase the 〈100〉// ND component and suppress 〈111〉// ND component in steel sheets and to enhance magnetic isotropy of electrical steel sheets since 〈111〉// ND fiber texture is not beneficial to the magnetic properties of the electrical steel sheets. The microstructures of steel sheets lased under various conditions were examined using optical microscopy and EBSD. To use the fact that the 〈100〉 axis is parallel to the solidification direction in silicon steels, the direction of heat flow out of the melted region was controlled. In addition, the atmosphere was changed to
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Y. Yoon et al. / Materials Letters 185 (2016) 43–46
achieve desirable microstructures. The fraction of 〈100〉// ND fiber textures of the argon-processed laser annealed sample increased up to 26.3%, which is more than three times that of the conventionally annealed sample. Furthermore, the fraction of 〈111〉// ND fiber textures of the argon-processed laser annealed sample dropped down to 10.4% from 16.3%. Since 〈100〉// ND fiber textures is beneficial and 〈111〉// ND fiber textures need to be suppressed to achieve desirable magnetic properties of the electrical steel sheets, laser annealing with air-contact in argon ambient is the most effective way.
References [1] J.-T. Park, J.A. Szpunar, Acta Mater. 51 (11) (2003) 3037–3051. [2] J.Y. Park, K.S. Han, J.S. Woo, S.K. Chang, N. Rajmohan, J.A. Szpunar, Acta Mater.
50 (7) (2002) 1825–1834. [3] Y.H. Sha, C. Sun, F. Zhang, D. Patel, X. Chen, S.R. Kalidindi, L. Zuo, Acta Mater. 76 (2014) 106–117. [4] B.D. Cullity, Introduction to Magnetic Materials, Addison-Wesley Pub. Co., Reading, Mass, 1972. [5] K.E. Easterling, Introduction to the physical metallurgy of welding, 2nd ed., Butterworth Heinemann, Oxford, Boston, 1992. [6] D.A. Porter, K.E. Easterling, M.Y. Sherif, Phase Transformations in Metals and Alloys, 3rd ed., CRC Press, Boca Raton, FL, 2009. [7] J.S. Im, M.A. Crowder, R.S. Sposili, J.P. Leonard, H.J. Kim, J.H. Yoon, V.V. Gupta, H. Jin Song, H.S. Cho, Phys. Status Solidi (a) 166 (2) (1998) 603–617. [8] Y.H. Yoon, S.-M. Yi, J.-R. Yim, J.-H. Lee, G. Rozgonyi, Y.-C. Joo, Microelectron. Eng. 87 (11) (2010) 2230–2233. [9] R. Khondker, A. Mertens, J.R. McDermid, Mater. Sci. Eng.: A 463 (1–2) (2007) 157–165. [10] C.Y. Ho, R.W. Powell, P.E. Liley, J. Phys. Chem. Ref. Data 1 (2) (1972) 279–421. [11] R.C. Weast, CRC handbook of chemistry and physics, 1st Student ed., CRC Press, Boca Raton, FL, 1988. [12] N.H. Heo, K.H. Chai, J.G. Na, Acta Mater. 48 (11) (2000) 2901–2910.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Texture control of 3.04%-Si electrical steel sheets by local laser melting and directional solidification Yohan Yoon n,1, Jungryoul Yim 1, Eunho Choi, Junghan Kim, Kyuhwan Oh, Youngchang Joo nn Department of Materials Science and Engineering, Seoul National University, Seoul, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 11 August 2016 Accepted 20 August 2016 Available online 22 August 2016
〈100〉 axes of the rod-shaped grains of laser irradiated cold-rolled 3.04%-Si steel sheets are parallel to either solidification direction or normal direction of the steel sheet. The portion of desirable textures was enhanced by controlling of atmosphere and the direction of heat flow. The area fraction of the texture, which has 〈100〉 axis parallel to normal direction of the steel sheet, in the laser-affected steel sheets in air and argon atmosphere increased up to 23.4% and 26.3%, respectively, compared to a conventionally annealed steel sheet (8.2%). & 2016 Elsevier B.V. All rights reserved.
Keywords: Electrical steels Texture Directional solidification EBSD Laser processing
1. Introduction The texture (preferred crystallographic orientation) of polycrystalline materials plays a critical role in material properties [1,2]. Thus, texture control for the materials, especially electrical steel sheet, has been studied to obtain its more desirable properties [3]. When electrical steels have {hkl}〈100〉 textures, where {hkl} planes and 〈100〉 directions are aligned in the rolling plane and the rolling direction, respectively, the steel sheets have the highest possible magnetic induction and the lowest energy loss since 〈100〉 is the easiest direction of magnetization [1,4]. In non-oriented electrical steel sheets, magnetic isotropy and induction can be enhanced when 〈100〉 axes are randomly oriented but still parallel to the plane of the steel sheet. Thus, two textures should be aligned for superior magnetic isotropy, i.e. 〈100〉// ND textures where 〈100〉 axes are parallel to the sheet normal and 〈100〉⊥ND textures where 〈100〉 axes are radially-distributed on the sheet plane. The most effective way to align 〈100〉 axes in silicon steel is to use directional solidification since it is reported that 〈100〉 axes are parallel to the direction of solidification in steel [5,6]. In this study, we applied laser crystallization technique to n
Corresponding author. Co-corresponding author. E-mail addresses: [email protected] (Y. Yoon), [email protected] (J. Yim), [email protected] (E. Choi), [email protected] (J. Kim), [email protected] (K. Oh), [email protected] (Y. Joo). 1 Both the authors contributed equally to this study. nn
http://dx.doi.org/10.1016/j.matlet.2016.08.105 0167-577X/& 2016 Elsevier B.V. All rights reserved.
control the 〈100〉 axis in electrical steel sheets, which is meltmediated process involving very high rates of heating and cooling [7]. Silicon steel sheets were melted through the entire thickness and 〈100〉 axes were controlled by changing the solidification direction. Furthermore, it is also reported that atmospheres during the laser process are important for the control of the microstructures of the laser-annealed silicon steel sheets. 0.35 mm-thick cold-rolled silicon steel sheets (Si: 3.04, Al: 1.35, Mn: 0.21, P: 0.0046, C: 0.0022, and N: 0.0013 mass%) were used in this study. A 400 W Nd: YAG (1064 nm) welding laser was used for annealing the steel sheets. To control the texture of the Si steel sheets, a small area on the steel sheet should be melted through the entire thickness. At the same time, damage of the steel sheets caused by evaporation must be prevented during laser processing because this damage will make the efficiency of the electrical steel sheet low. For these purposes, the parameters of the laser processing were optimized. More details of optimized laser parameters, such as diameter of the laser beam, average beam power, defocusing value, were already reported [8]. The optimized laser conditions are as follows; beam diameter: 3 mm, output power: 400 W, pulse repetition rate: three pulses per second, laser emission duration time: 20 ms, defocusing value: 10 mm. The laser power as a function of time can be divided into two parts; the first part is laser irradiation on the steel sheet, and the second part is the switch-off of the laser. The time of laser irradiation during one period is called the laser-emission duration time. During the laser process, three shots of the laser beam irradiated the steel sheet with three pulses per second. The steel sheets were irradiated by the laser in ambient air and in flowing
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Fig. 1. Optical micrographs of the laser-irradiated (a) in-plane view and (b) crosssectional view of laser irradiated cold-rolled 3.04%-Si steel sheets. laser beam diameter: 3 mm, laser output power: 400 W, 3 pulses per a second, laser emission duration time (pulse width): 20 ms, defocusing value: 10 mm, and three lasershots. (c) The magnified cross-sectional image of the green box in Fig. 2(b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
argon atmosphere. The grain morphologies and crystal orientations with respect to the sample directions (the rolling direction, transverse direction, and sample normal) were analyzed using field-emission scanning electron microscopy (JEOL JSM-6500F) equipped with the electron back-scattered diffraction (EBSD) detector (Oxford instrument NORDLYS 2). To observe the grain morphologies using EBSD, band contrast images from the EBSD data were used. Fig. 1 shows the optical micrographs of the laser-irradiated 3.04%-Si steel sheets with optimized laser parameters. The top and cross-sectional view optical micrographs in Fig. 1a and b provide three types of grain morphologies in the laser-affected region: columnar grains, rod-shaped grains, and tiny grains. Note that the formation of the columnar grains and rod-shaped grains is due melting and solidification of the laser irradiated area of the sample. The solidification direction is the main factor to determine grain structures. The columnar grains are formed when the solidification direction is vertical (perpendicular to the sample surface) due to the contact of melted area with metal table where the laser annealing was performed. On the contrary, the lateral solidification direction (parallel to the sample surface) due to noncontact of the melted area induces rod-shaped grains. At the edge area of the laser-affected zone, tiny grains are created by recrystallization process since the laser power is not enough to melt
the sample. Interestingly, Fig. 1c shows the boundary between the tiny-grain region and rod-shaped-grain region indicated by yellow arrow, indicating different thermal history. Fig. 2a presents EBSD image (crystal directions are referenced to the rolling direction) of the laser annealed sample. The rodshaped grains parallel to the rolling direction are aligned to 〈100〉. However, it is not sufficient to confirm that all the rod-shaped grains have the 〈100〉 axis parallel to the solidification direction. Therefore, the crystal orientations in Fig. 2a were re-ordered with respect to the direction of solidification of each rod-shaped grain (see Fig. 2b). Interestingly, most of the rod-shaped grains have 〈100〉 axes parallel to the directions of solidification of the grains. The possible scenario of this directional solidification is that 〈100〉 crystal orientation is the most favorable direction in electrical steels since it is the most loosely packed direction. However, the boundary between columnar grains and rod-shaped grains is not perpendicular to the sample surface due to the viscosity of the melted area, so the rod-shaped grains are not perfectly parallel to the surface. If the melted region of the steel sheet could be prevented from touching the metal supporter, this undesired microstructure would be minimized. Fig. 3 shows EBSD images of the sample in which four corners were supported, so the melted region does not touch with the metal table (hereafter air-contact sample) (see inset in Fig. 3a). The air-contact laser annealed sample has larger rod-shaped grains and smaller columnar grains than the metal-contact laser annealed sample since the direction of solidification is parallel to the steel sheet. Crystal orientation maps in Fig. 3b and c show that the textures of the rod-shaped grains in the air-contact sample have similar aspect to those of metal-contact sample. However, one difference is that the air-contact sample has columnar grains with 〈100〉// ND fiber component (see Fig. 3c). This means that the columnar grains solidified normally to the steel sheet. Even though air has lower thermal conductivity compared to the steel, heat loss still occurs through the surface of the steel sheet. Note that 〈100〉// ND fiber textures are expected to enhance the magnetic isotropy of the electrical steel. Therefore, air-contact laser processing is efficient way to control magnetic isotropy of non-oriented electrical steel sheet. It is well known that atmosphere plays a role in microstructural evolution during annealing [9]. In this study, argon gas was continuously flowed over the lased surface during laser processing for the comparison of microstructures of laser-processed steel sheets to those of the laser annealed sample under air ambient. Fig. 4a shows the crystal orientation map of the top-view of the argonprocessed steel sheet. The most significant difference between airprocessed and argon-processed laser annealed samples is that the argon-processed sample has only rod-shaped grains and no columnar grains. The possible explanation for this observation is that the temperature of the center of the melted region of the argonprocessed sample does not reach the solidifying point before the rod-shaped grains solidify to the center of the melted region due to the lower thermal conductivity of argon gas (0.0177 W/m K) [10] than air (0.0235 W/m K) [11]. Fig. 4a shows a EBSD image with respect to the normal direction of the argon-processed steel sheet. Note that the area fraction of the 〈100〉// ND component in the argon-processed steel sheet is larger than that of the sample processed in the ambient air atmosphere (Fig. 3c), whereas the 〈110〉// ND fiber texture in the argon-processed steel sheet is smaller than that of the sample processed in the ambient air atmosphere. This indicates that 〈100〉// ND component is preferred for the rod-shaped grains under argon ambient since {100} grains preferentially grow at the expense of {110} grains during argon annealing [12]. Fig. 4b shows a quantitative analysis for the textures of the laser-processed steel sheets (air-contact and air-contact in argon ambient) and a reference steel sheet annealed
Y. Yoon et al. / Materials Letters 185 (2016) 43–46
45
Fig. 2. Crystal orientation maps of the top-view of the laser irradiated sample. laser beam diameter: 3 mm, laser output power: 400 W, 3 pulses per a second, laser emission duration time (pulse width): 20 ms, defocusing value: 10 mm, and three laser-shots. The crystal directions are referenced to (a) the rolling direction, (b) the solidification directions of the rod-shaped grains.
Fig. 3. (a) a Band-contrast image and (b), (c) crystal orientation maps from EBSD data of the cold-rolled, 3.04%-Si steel sheets that were lased under the same conditions as given in Figs. 2 and 3. The four corners of the sample were supported and the melted region was in contact with the air (air-contact) (see the inset of Fig. 4a). The sample was lased in an air atmosphere. The crystal orientation maps are with respect to (b) the rolling direction and (c) the normal direction of the steel sheet.
Fig. 4. (a) a crystal orientation map from EBSD data of the cold-rolled, 3.04%-Si steel sheets that were lased under the same conditions as given in Figs. 2 and 3 and this figure. The four corners of the sample were supported and the melted region was in contact with the air (air-contact). The sample was laser-irradiated while argon gas was flowed over the lased surface. The crystal orientation maps are with respect to the normal direction of the steel sheet. (b) Quantitative analysis of the area fractions of three types of textures produced by various annealing conditions.
conventionally. The area fractions of 〈100〉// ND fiber texture and 〈111〉//ND fiber texture in the non-oriented electrical steel sheets processed by conventional annealing are about 8.2% and 16.3%, respectively. The area fractions of the 〈100〉// ND fiber texture in the steel sheet laser-processed in the ambient air atmosphere and the argon-processed steel sheet increased up to 23.4%, and 26.3%, respectively. In addition, the fractions of 〈111〉// ND fiber texture of air- and argon-processed steel sheets decrease to 9.4%, and 10.4%, respectively. This indicates that local melting and directional solidification by laser processing under argon ambient is much more
efficient than the conventional annealing process to increase the 〈100〉// ND component and suppress 〈111〉// ND component in steel sheets and to enhance magnetic isotropy of electrical steel sheets since 〈111〉// ND fiber texture is not beneficial to the magnetic properties of the electrical steel sheets. The microstructures of steel sheets lased under various conditions were examined using optical microscopy and EBSD. To use the fact that the 〈100〉 axis is parallel to the solidification direction in silicon steels, the direction of heat flow out of the melted region was controlled. In addition, the atmosphere was changed to
46
Y. Yoon et al. / Materials Letters 185 (2016) 43–46
achieve desirable microstructures. The fraction of 〈100〉// ND fiber textures of the argon-processed laser annealed sample increased up to 26.3%, which is more than three times that of the conventionally annealed sample. Furthermore, the fraction of 〈111〉// ND fiber textures of the argon-processed laser annealed sample dropped down to 10.4% from 16.3%. Since 〈100〉// ND fiber textures is beneficial and 〈111〉// ND fiber textures need to be suppressed to achieve desirable magnetic properties of the electrical steel sheets, laser annealing with air-contact in argon ambient is the most effective way.
References [1] J.-T. Park, J.A. Szpunar, Acta Mater. 51 (11) (2003) 3037–3051. [2] J.Y. Park, K.S. Han, J.S. Woo, S.K. Chang, N. Rajmohan, J.A. Szpunar, Acta Mater.
50 (7) (2002) 1825–1834. [3] Y.H. Sha, C. Sun, F. Zhang, D. Patel, X. Chen, S.R. Kalidindi, L. Zuo, Acta Mater. 76 (2014) 106–117. [4] B.D. Cullity, Introduction to Magnetic Materials, Addison-Wesley Pub. Co., Reading, Mass, 1972. [5] K.E. Easterling, Introduction to the physical metallurgy of welding, 2nd ed., Butterworth Heinemann, Oxford, Boston, 1992. [6] D.A. Porter, K.E. Easterling, M.Y. Sherif, Phase Transformations in Metals and Alloys, 3rd ed., CRC Press, Boca Raton, FL, 2009. [7] J.S. Im, M.A. Crowder, R.S. Sposili, J.P. Leonard, H.J. Kim, J.H. Yoon, V.V. Gupta, H. Jin Song, H.S. Cho, Phys. Status Solidi (a) 166 (2) (1998) 603–617. [8] Y.H. Yoon, S.-M. Yi, J.-R. Yim, J.-H. Lee, G. Rozgonyi, Y.-C. Joo, Microelectron. Eng. 87 (11) (2010) 2230–2233. [9] R. Khondker, A. Mertens, J.R. McDermid, Mater. Sci. Eng.: A 463 (1–2) (2007) 157–165. [10] C.Y. Ho, R.W. Powell, P.E. Liley, J. Phys. Chem. Ref. Data 1 (2) (1972) 279–421. [11] R.C. Weast, CRC handbook of chemistry and physics, 1st Student ed., CRC Press, Boca Raton, FL, 1988. [12] N.H. Heo, K.H. Chai, J.G. Na, Acta Mater. 48 (11) (2000) 2901–2910.