AU2010267413B2

AU2010267413B2 - Zn-Al-Mg coated steel sheet and producing method thereof

Info

Publication number: AU2010267413B2
Application number: AU2010267413A
Authority: AU
Inventors: Kazuhiko Honda; Yoshio Kimata; Tooru Oohashi; Satoshi Uchida
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2009-06-30
Filing date: 2010-06-29
Publication date: 2015-05-21
Anticipated expiration: 2030-06-29
Also published as: WO2011001662A1; AU2010267413A1; JP5043234B2; TW201114946A; JPWO2011001662A1; TWI406967B; MX2011013944A

Abstract

Disclosed is a Zn-Al-Mg hot-dip coated steel sheet comprising: a steel sheet; and a hot-dip coated layer comprising 4 to 22 mass% inclusive of Al and 1 to 5 mass% inclusive of Mg, with the remainder being Zn and unavoidable impurities. In the hot-dip coated steel, the ratio of the X-ray diffraction intensity [I(200)] of face (200) of an Al phase to the X-ray diffraction intensity [I(111)] of face (111) of the Al phase on a cross-section surface of the hot-dip coated layer which is taken in parallel to the surface of the hot-dip coated layer [i.e., a diffraction intensity ratio I(200)/ [I(111)] is 0.8 or more.

Description

I SPECIFICATION TITLE OF INVENTION Zn-Al-Mg COATED STEEL SHEET AND PRODUCING METHOD THEREOF 5 Field of the Invention [0001] The present invention relates to a highly corrosion-resistant Zn-Al-Mg coated steel sheet that can be applied to a variety of uses as a steel sheet for, for example, home electronics, automobiles, and construction materials. Particularly, the present invention 10 relates to a Zn-Al-Mg coated steel sheet having an excellent appearance and a producing method thereof. Priority is claimed on Japanese Patent Application No. 2009-156018, filed June 30, 2009, the content of which is incorporated herein by reference. 15 Description of Related Art [0002] Since a Zn-Al-Mg coated steel sheet has an excellent corrosion resistance, an increasing amount of the Zn-Al-Mg coated steel sheet has been used in recent years. However, since the solidification mechanism in hot-dip coating by a multiternary alloy is 20 complicated, a variety of poor appearances are easily caused in Zn-Al-Mg coating. In the past, a variety of techniques were suggested to improve the appearance of the Zn-Al-Mg coated steel sheet. [0003] For example, when a coating bath composition in the vicinity of a ternary 25 eutectic point is employed, there is a problem in that a Zn 11 Mg 2 phase that is liable to be 2 discolored and deteriorates the surface appearance, is locally crystallized in the microstructure of a coating layer. In order to solve the problem, Patent Citation I discloses a technique that suppresses the local crystallization of the Zn 1 Mg 2 phase that deteriorates the surface 5 appearance, which is liable to be discolored, by controlling the bath temperature to 470*C or more and the cooling rate after coating to 0.5 *C/s or more so as to form a coating layer having a microstructure in which [a primary Al phase] or [a primary Al phase] and [a Zn single phase] are mixed in the matrix having [a ternary eutectic microstructure of Al/Zn/Zn 2 Mg]. 10 [0004] Patent Citation 2 discloses that, when 60% or more of crystals in the ternary eutectic microstructure of Al/Zn/MgZn alloy in the coating layer have an equivalent circular diameter of 100 im or more, a microstructure that is not easily discolored can be obtained. 15 In addition, Patent Citation 2 discloses that fine and uniform equiaxial crystals can be obtained as Al phase crystals by including an intermetallic compound having a specific lattice plane in a coating layer so as to grow a lot of primary dendrite arms of an Al phase in the <110> orientation. Patent Citation 2 also discloses that, consequently, coating recesses and protrusions by the uneven growth of the dendrite of the Al phase are 20 removed, and a flat appearance can be obtained. [0005] Separately from the poor appearance, the Zn-Al-Mg coated steel sheet has another problem of orange peel. As shown in FIG. 1, the surface of the Zn-Al-Mg coated steel sheet exhibits a surface appearance like orange peel on which a combination 25 of irregular white portions and circular luster portions is finely scattered. In more detail, 3 as shown in FIG 2, the white portion is an Al-phase dendrite that is exposed on the coating surface, and the luster portion is a ternary eutectic microstructure. Since the orange peel is, in general, not particularly attractive, it is necessary to improve the state of the orange peel even when there is no poor appearance (recesses and 5 protrusions of coating) on a coating surface, but a more attractive appearance is required. [0006] A more preferable orange peel has a so-called fine-textured appearance in which the white portions and the luster portions are finely dispersed, and has a lot of flat luster portions. The luster portion in the orange peel is composed of ternary eutectic crystals, 10 and has a flat surface state in comparison to the white portion. However, the state of the orange peel significantly varies in products. Particularly, when the amount of coating is large, the area of each of the white portions and the area of each of the luster portions increases, and the orange peel in an ugly state tends to increase. 15 With the conventional techniques, it was difficult to stably form a coating surface having a preferable orange peel which has a fine texture. Patent Citation [0007] 20 [Patent Citation 1] Japanese Patent Publication No. 3179401 [Patent Citation 2] Japanese Unexamined Patent Application, First Publication No. 2006-283155 Non-Patent Citation [0008] 25 [Non-Patent Citation 1] Tetsu-to-Hagane, Vol. 81(1995), No. 6, p. 643 4 SUMMARY OF THE INVENTION Problems to be Solved by the Invention 5 [0009] Therefore, an object of the present invention is to obtain a fine-textured orange peel having a lot of flat luster portions by clarifying the conditions for a necessary coating layer in the Zn-Al-Mg coated steel sheet, and to stably produce a coated steel sheet having the orange peel. 10 Methods for Solving the Problem [0010] The inventors manufactured Zn-Al-Mg coated steel sheets having different orange peel states, and investigated the characteristics. As a result, it was found that 15 favorable orange peel was obtained when the number of the white portions per unit area was large, and the fraction of the area of the luster portions was large. Particularly, it was found that the state of the orange peel became favorable when the number of the white portions increased. [0011] 20 Therefore, the inventors investigated the orientation of an Al phase composing the white portions on the surface of a steel sheet and the structures of dendritic crystals. Firstly, as a result of investigating the orientation of the Al phase by X-ray diffraction (XRD), it was found that there were a lot of crystal grains whose (200) plane was parallel to the surface of the steel sheet and a few crystal grains whose (111) plane was parallel to 25 the surface of the steel sheet in a steel sheet exhibiting a favorable orange peel in 5 comparison to steel sheets not exhibiting a favorable orange peel. In addition, as a result of analyzing the structures of the Al-phase dendritic crystals by EBSD method or scanning electron microscopy (SEM), it was also found that the plane in parallel with the surface of the coating layer in the steel sheet exhibiting a 5 favorable orange peel has a lot of cruciform-looking dendritic crystals and a few hexagon-looking dendritic crystals. [0012] The inventors carried out studies in the above manners, and completed the present invention. The summery of the present invention is as follows. 10 (1) A Zn-Al-Mg coated steel sheet according to an aspect of the present invention includes a steel sheet; and a coating layer including 4 to 22 mass% of Al, I to 5 mass% of Mg, and the balance including Zn and inevitable impurities, in which the diffraction intensity ratio 1 (200) / I (111), which is a ratio of the X-ray diffraction intensity of the (200) plane of an Al phase 1 (200) to the X-ray diffraction intensity of the 15 (111) plane of the Al phase I (111) in the cross-sectional surface of the coating layer parallel to the surface of the coating layer, is 0.8 or more. (2) In the Zn-Al-Mg coated steel sheet according to the above (1), the coating layer may also include 0.0001 to 2.0 mass% of Si. (3) In the Zn-Al-Mg coated steel sheet according to the above (1) or (2), the 20 coating layer may further include 0.0001 to 0.5 mass% of one or a combination of Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, a Group III element, REM, Hf, and inevitable impurities. (4) In the Zn-Al-Mg coated steel sheet according to the above (1) or (2), the coating layer may further include 0.0001 to 0.5 mass% of one or a combination of Ni, Ti, 25 Zr, and Sr.

6 (5) In the Zn-Al-Mg coated steel sheet according to the above (1) or (2), the area percentage of cruciform-looking dendritic crystals of the Al-phase in the cross-sectional surface of the coating layer parallel to the surface of the coating layer may be 5% or more of the total cross-sectional area of the coating layer. 5 (6) In the Zn-Al-Mg coated steel sheet according to the above (1) or (2), the number of white portions on the surface of the coating layer may be 100 -/cm2 or more, and the area percentage of luster portions on the surface of the coating layer may be 94% or more of the total surface area of the coating layer. (7) In the Zn-Al-Mg coated steel sheet according to the above (1) or (2), the 10 amount of coating per one face in the coating layer may be 50 to 300 g/m 2 . (8) In a producing method of the Zn-Al-Mg coated steel sheet according to the above (1) or (2), the temperature of the coating layer immediately before wiping is a temperature exceeding the solidification start temperature of a Zn-Al-Mg coating metal, and the temperature of the coating layer immediately after the wiping is a temperature 15 that is I 0 0 C or more lower than the solidification start temperature of the Zn-Al-Mg coating metal. (9) In the producing method of the Zn-Al-Mg coated steel sheet according to the above (8), after the wiping, the Zn-Al-Mg coated steel sheet may be cooled at a cooling rate of 10 *C/s or less until the solidification completion temperature of the Al phase in 20 the Zn-Al-Mg coating metal. (10) In the producing method of the Zn-Al-Mg coated steel sheet according to the above (8) or (9), the amount of coating per one face in the coating layer may be controlled to 50 to 300 g/m 2 . 25 Effects of the Invention 7 [0013] According to the present invention, it is possible to stably provide a Zn-Al-Mg coated steel sheet which is excellent in terms of the appearance that includes an orange peel having a fine texture and a lot of flat portions with no variation of the quality with 5 products. BRIEF DESCRIPTION OF THE DRAWINGS [0014] FIG 1 is an explanatory view of an orange peel on the surface of the Zn-Al-Mg 10 coated steel sheet using a photograph. FIG 2 is an explanatory view of the orange peel using an electron microscope photograph. FIG 3 is an explanatory view of the orange peel using an electron microscope photograph. 15 FIG 4 is a diagram showing the relationship among the number of the white portions on the surface of the orange peel, the area of the luster portions on the surface of the orange peel, and the grades of the orange peel. FIG 5 is a diagram showing an example of the diffraction patterns obtained by an XRD measurement of a face parallel to the surface of the coated steel sheet. 20 FIG 6 is a diagram showing the relationship between the diffraction intensity ratio 1 (200) / I (111) of the coating layer and the grades of the orange peel on the surface of the coated steel sheet. FIG 7A is an example of the images obtained as a result of measuring the coating layer by EBSD method. 25 FIG 7B is an example of the images shown in FIG. 7A which is binarized so that 8 the (200) plane orientation portion becomes white. FIG 8 is a diagram showing the relationship between the area ratio (percentage) of the cruciform-looking dendritic crystals on the surface of the coating layer and the grades of the orange peel on the surface of the coated steel sheet. 5 FIG 9 is a diagram showing the relationship among the diffraction intensity ratio 1 (200) / I (111) of the coating layer, the area ratio of cruciform-looking dendritic crystals, and the grades of the orange peel. DETAILED DESCRIPTION OF THE INVENTION 10 [0015] A coating bath in an embodiment of the present invention is a bath prepared by adding Al to a molten Zn bath, and, furthermore, adding one or both of Si and Mg according to necessity. [0016] 15 The reason why the amount of Al is limited to 4 to 22 mass% in the embodiment is that, when the amount of Al is less than 4 mass%, the effect of improving the corrosion resistance is not sufficient, and, when the amount is 22 mass% or more, the effect of improving the corrosion resistance is saturated. In addition, when the amount of Al is less than 4 mass%, an Al phase is not crystallized as a primary crystal, and therefore a 20 problem regarding of orange peel, which is caused by the primary-Al crystal, does not occur. [0017] As such, when the amount of Al is less than 4 mass%, the effect of improving the corrosion resistance is not sufficient. In addition, when the amount of Al exceeds 22 25 mass%, the effect of improving the corrosion resistance is saturated. Therefore, the 9 amount of Al is 4 to 22 mass%. In order to further improve the corrosion resistance, the amount of Al is preferably 5 mass% or more, and more preferably 10 mass% or more. In addition, in order to lower the melting point of the coating bath or increase the coating adhesiveness, the amount of Al is preferably 20 mass% or less, and more preferably 15 5 mass% or less. [0018] When the amount of Mg is less than 1 mass%, the effect of improving the corrosion resistance is not sufficient. In addition, when the amount of Mg exceeds 5 mass%, the coating layer becomes brittle, and the adhesiveness is degraded. Therefore, 10 the amount of Mg is I to 5 mass%. In order to further improve the corrosion resistance, the amount of Mg is preferably 2 mass% or more, and more preferably 3 mass% or more. Furthermore, in order to further increase the adhesiveness of the coating layer, the amount of Mg is preferably 4.5 mass% or less, and more preferably 4 mass% or less. When elements other than Al and Mg are not added, the balance is composed of 15 Zn and inevitable impurities. [0019] Furthermore, it is also possible to form a coating layer by adding 0.0001 to 2 mass% of Si to the above basic composition. Si is included to improve the corrosion resistance, but the effect of improving the corrosion resistance is saturated even when 2 20 mass% or more of Si is added. The reason why the lower limit of Si concentration is set to 0.0001 mass% is that it is difficult to control the concentration industrially when the concentration of Si is 0.0001 mass% or less. [0020] The coating bath may contain 0.0001 to 0.5 mass% of one or a combination of 25 Fe, Sb, Pb, Sn, and inevitable impurities in addition to the above chemical elements (Zn, 10 Al, Mg, and Si). In addition, even when the coating bath contains 0.0001 to 0.5 mass% of one or a combination of Ca, Co, Mn, P, B, Bi, Cr, a Group III element (one or more of Group III elements), REM, and Hf, there may be a preferable case in which the corrosion resistance is further improved according to the amount without impairing the effects 5 according to the present invention. The reason why the lower limit of concentration of the above chemical elements (Fe, Sb, Pb, Sn, inevitable impurities, Ca, Co, Mn, P, B, Bi, Cr, Group III elements, REM, and Hf) is set to 0.0001 mass% is that it is difficult to control the concentration industrially when the concentration of the chemical elements is 0.0001 mass% or less. 10 [0021] Furthermore, the coating bath may contain 0.0001 to 0.5 mass% of one or a combination of Ni, Ti, Zr, and Sr. All of these elements crystallize intermetallic compounds with Al, and thus have an effect of improving the surface flatness. However, when the amount of each of the elements (Ni, Ti, Zr, and Sr) exceeds the upper limit, 15 there is a case in which the appearance after coating becomes crude, and a poor appearance is caused. The reason why the lower limit of concentration of Ni, Ti, Zr, and Sr is set to 0.0001 mass% is that it is difficult to control the concentration industrially when the concentration of the chemical elements is 0.0001 mass% or less. [0022] 20 In the present invention, the Al phase is an island or dendrite-shaped phase having clear boundaries in the coating layer. The Al phase corresponds to, for example, the "Al phase" at a high temperature in the ternary equilibrium diagram of Zn-Al-Mg (a solid solution that dissolves Zn in Al of a solid state). The amount of Zn dissolved in a solid state of the Al phase at a high temperature varies with the concentration of Al in the 25 coating bath. The Al phase at a high temperature is divided into a fine Al phase and a 11 fine Zn phase at room temperature, but the island or dendrite shape observed at room temperature may be considered to keep the shape and boundary of the Al phase at a high temperature. A phase that is derived from the Al phase at a high temperature (called the "primary-Al crystal") and maintains the shape and boundary of the Al phase will be 5 called the "Al phase" in the present specification. [0023] The Al phase has different amounts of elements that are dissolved therein according to the concentration of the alloy in the coating bath and different phase morphologies at room temperature in a ternary system of Al-Zn-Mg and in a quaternary 10 system of Al-Zn-Mg-Si. In any case, since the Al phase keeps the shape and boundary derived from the primary Al crystal, and can be clearly differentiated in microscopic observation, this will be called the "Al phase" in the specification. [0024] Here, a non-preferable state of the orange peel is a state in which a surface 15 appearance like an orange peel on which a combination of irregular white portions and circular luster portions are scattered is exhibited, as shown in FIG 1. In this state, the white portions are large, and the area percentage of the luster portions is low. Therefore, the degree of the orange peel appearance can be evaluated by the size of the white portions and the ratio of the white portions to the coating surface. In the embodiment, it 20 is preferable that the surface of the coating layer contain the luster portions in an area percentage of 94% or more, and the number of the white portions be 100 -/cm2 (100 portions per 1 cm 2) or more. Thereby, it is possible to provide a Zn-Al-Mg (Zn-Al-Mg-based) coated steel sheet having an excellent appearance. In more detail, on an arbitrary steel sheet, the white portions are portions in which the dendrites of the Al 25 phase exposed on the coating surface are gathered, and the luster portions are portions in 12 which the coating surface is covered with a ternary eutectic microstructure as shown in FIGS. 2 and 3. The portions observed by the inventors in which the dendrites of the Al phase were exposed on the coating surface had a size (length) of about 10 to 100 pm in the longitudinal direction, and the size (width) of the dendrite arm was about 5 to 50 pm. 5 The white portion of the orange peel as mentioned in the specification refers to a portion in which the dendrites of the Al phase exposed on the coating surface are gathered. In addition, when the number of the white portions in the orange peel is evaluated, the white portions in the orange peel, which visually appear to be integrated, are evaluated as one white portion. 10 [0025] Here, the ternary eutectic crystal refers to a solidification microstructure that is generated when a coating layer within the above element concentration ranges is during final solidification in the coated steel sheet of the ternary system of Zn-Al-Mg or the quaternary system of Zn-Al-Mg-Si. 15 [0026] The inventors prepared a lot of Zn-Al-Mg coated steel sheets by changing conditions, such as the coating composition, the cooling rate after coating, and the like, sampled specimens having different states of the orange peels from the above, and investigated the characteristics. 20 A grade of I to 6 (orange peel grade) was given to each of the specimens according to the state of the orange peel in each of the sampled specimens, in which the grades I to 6 were in a preferable order. Meanwhile, an orange peel on a boundary from which the orange peel could be evaluated as "good" was set to the standard grade 3. The relationship between the area ratio (percentage) of the luster portions and the number 25 of the white portions in each of the specimens was investigated. The results are shown 13 in FIG 4. [0027] As clear from FIG 4, when the number of the white portions (portions including the Al phase) was 100 -/cm2 or more, and the area percentage of the luster portions 5 (portions including the ternary eutectic crystals) was 94% or more (100% is not included), the Zn-Al-Mg coated steel sheets tended to exhibit good orange peels whose grades were 3 or less. When the white portions are finely dispersed, the number of the white portions increases. In addition, when the area percentage of the luster portions increases, the area percentage of the white portions decreases. Therefore, an orange peel having 10 fine white portions and a lot of flat luster portions can be expressed by the number of the white portions per unit area and the area percentage of the luster portions. [0028] The white portion in the orange peel is known as a primary Al phase generated in a dendrite shape as described above. The inventors wondered that the crystal 15 structure of the Al phase varies with the dispersion state of the white portions. Therefore, the inventors investigated the X-ray diffraction patterns of the surfaces of Zn-Al-Mg coated steel sheets by XRD. As a result, it was found that the steel sheets exhibiting good orange peels whose grades were 3 or less had a lot of crystal grains in which the (200) plane of the Al crystal (the primary Al phase) was parallel to 20 the surface of the steel sheet and a few crystal grains in which the (111) plane of the Al crystal was parallel to the surface of the steel sheet. [0029] FIG 5 shows an example of the diffraction pattern obtained by an XRD measurement of a face parallel to the surface of a steel sheet in the Zn-Al-Mg 25 (Zn-Al-Mg-based) coating layer.

14 When a CuKox 1 ray is used, peaks are generated at a 20 position of 38.4* and a 20 position of 44.80 in the diffraction pattern from the Al crystal. The 20 position of 38.40 (the interplanar spacing d is 2.34 A) corresponds to the (11l) plane of the Al crystal. The 20 position of 44.80 (the interplanar spacing d is 2.02 A) corresponds to the (200) 5 plane of the Al crystal. Therefore, when the value of the diffraction intensity of the peak at 38.40 is high, there are a lot of crystal grains in which the (Il1) plane is parallel to the surface of the steel sheet. In addition, when the value of the diffraction intensity of the peak at 44.80 is high, there are a lot of crystal grains in which the (200) plane is parallel to the surface of the steel sheet. 10 [0030] Furthermore, the inventors carried out XRD measurements on the cross-sectional surfaces of coating layers parallel to the surface of the coating layers of Zn-Al-Mg coated steel sheets having different orange peel grades, obtained the ratios of the X-ray diffraction intensities 1 (200) of the (200) planes of Al phases to the X-ray 15 diffraction intensities I (111) of the (11l) planes of the Al phases, that is, the diffraction intensity ratio 1 (200) / I (111) from the obtained X-ray diffraction patterns, and investigated the relationships between the diffraction intensity ratios and the orange peel grades of the steel sheets. Here, the X-ray diffraction intensity 1 (200) and the X-ray diffraction intensity I (111) are obtained from the area of each of the corresponding 20 peaks. [0031] Meanwhile, the amount of coating is adjusted so as to be 100 to 350 g/m 2 on both faces of the steel sheet. The relationship between the diffraction intensity ratio I (200) / I (111) and the orange peel grades of the steel sheet is shown in FIG. 6. It is 15 found from the results of FIG 6 that, when the value of the diffraction intensity ratio I (200) / I (111) is 0.8 or more, good Zn-Al-Mg coated steel sheets, whose values of the orange peel grades of the steel sheets is 3 or less, can be obtained. Here, the upper limit of the diffraction intensity ratio 1 (200) / I (111) is not particularly limited. Therefore, 5 the upper limit of the diffraction intensity ratio 1 (200) / 1 (111) is infinite. [0032] In addition, the number of the white portions was measured in the steel sheets which had a value of the diffraction intensity ratio 1 (200) / I (111) of 0.8 or more, and a value of the orange peel grade of the steel sheet of 3 or less, the number of the white 10 portions was 100 -/cm2 or more in all of the steel sheets. Meanwhile, the number of the white portions on the surface of the coating layer was counted visually. Here, the upper limit of the number of the white portion that can be counted visually may also be 10000 2 -/cm2 [0033] 15 Next, similarly to Patent Citation 2, the microstructure of the face parallel to the coating surface of the dendritic crystal of the Al phase was investigated by EBSD method. As a result, it was found that the Zn-Al-Mg coated steel sheets having a large X-ray diffraction intensity ratio 1 (200) / I (111) and exhibiting a good orange peel whose grade was 3 or less had a lot of cruciform-looking dendritic crystals (cruciform dendritic 20 crystals) and a few hexagon-looking dendritic crystals. [0034] Meanwhile, the electron back scattering diffraction (EBSD) method is a method in which convergent electron beams are irradiated at certain intervals (a certain distance between irradiation points) on a specimen set in a lens tube of a scanning electron 25 microscope (SEM), and the crystal structure and crystal orientation of each of the crystals 16 that compose a polycrystalline material are analyzed from electron back scattering diffraction images (EBSD images) generated from the irradiation points. Meanwhile, in the EBSD method, a specimen is used in which the surface of the coating layer is mirror-polished. That is, the crystal structures or crystal orientations on the 5 cross-sectional surface of a coating layer parallel to the surface of the coating layer are analyzed. [0035] For the Zn-Al-Mg coated steel sheets having different orange peel grades, only the portions of the cruciform-looking dendritic crystals were extracted through an image 10 processing from the EBSD images obtained from the results of measurements by the EBSD method, and the area percentages of the cruciform-looking dendritic crystals were evaluated. FIG 7A shows an example of the image obtained by the EBSD method. In addition, FIG 7B shows an example of the image of FIG 7A that is binarized so that the portion of the (200) plane orientation becomes white. 15 [0036] The relationship between the area percentage of the cruciform-looking dendritic crystals obtained by the image analysis of the EBSD images and the orange peel grade of the steel sheet is shown in FIG 8. It is found from the results of FIG. 8 that, when the area percentage of the cruciform-looking dendritic crystals of the Al phase is 5% or more 20 of the total cross-sectional area of the coating layer, a favorable Zn-Al-Mg coated steel sheet whose value of the orange peel grade of the steel sheet is 3 or less can be obtained. Meanwhile, when the alloy composition of the Zn-Al-Mg coating layer is taken into consideration, the upper limit of the area percentage of the cruciform-looking dendritic crystals may be 50% or less. 25 In addition, when the number of the white portions was measured in the steel 17 sheets in which the area percentage of the cruciform-looking dendritic crystals of the Al phase was 5% or more, the number of the white portions was 100 -/cm 2 or more in all of the steel sheets. [0037] 5 Next, the inventors investigated the relationship between the X-ray diffraction intensity ratio 1 (200) / I (111) and the area percentage of the cruciform-looking dendritic crystals. FIG. 9 shows the relationship among the obtained diffraction intensity ratio I (200) / 1 (111), the area percentage of cruciform-looking dendritic crystals, and the orange peel grades. It could be confirmed from FIG 9 that the value of the X-ray 10 diffraction intensity ratio 1 (200) / I (111) was in a range of 0.8 or more, and the area percentage of the cruciform-looking dendritic crystals was in a range of 5% or more in favorable steel sheets whose orange peel grades were 3 or less. [0038] As such, the number of the fine white portions increases so that the 15 improvement of the orange peel, in which the white portions decreases as a whole, corresponds to an increase in crystal grains in which the (200) plane of the Al crystal is parallel to the coating surface (that is, an increase in the area percentage of the cruciform-looking dendritic crystals). The reason is considered to be as follows. [0039] 20 As described in Patent Citation 2, it is known that the plane on which the dendrites of the Al phase easily grow in the solidification process of the ternary system of Zn-Al-Mg is the (110) plane. Therefore, when the dendrites of the Al phase grow in parallel on the surface of the steel sheet, Al crystals seen from the surface of the steel sheet become strongly oriented toward the (nOO) planes ((100) plane, (200) plane, and the 25 like) or (mmm) plane ((111) plane, (222) plane, and the like).

18 When the Al crystals are strongly oriented toward the (mmm) planes, since the (mmm) planes of the dendrites of the Al phase are parallel to the coating surface, the dendrites of the Al phase look like hexagonal dendritic crystals whose arms are extended in six directions from the crystal nuclei when the dendrites of the Al phase are observed 5 from the direction of the coating surface. In addition, when the Al crystals are strongly oriented toward the (nOO) planes, since the (nOO) planes of the dendrites of the Al phase are parallel to the coating surface, the dendrites of the Al phase look like cruciform dendritic crystals whose arms are extended in four directions from the crystal nuclei when the dendrites of the Al phase are observed from the direction of the coating surface. 10 [0040] As described above, under conditions in which the orientation toward the (nOO) plane is relatively high, the dendrites of the Al phase are oriented so as to appear to be a fine cruciform when seen from the perpendicular direction to the coating surface. The arms of the dendrite grow while branching into a primary arm, secondary 15 arms, and ternary arms, but they are a single crystal, and the growth orientation of the crystal is the (110) orientation, which is not changed. Therefore, when the coating layer is cut at a face parallel to the coating surface, all arms appear to have the same shape, and all the arms of the crystal oriented toward the (nOO) plane appear to have a cruciform shape. 20 [0041] It is considered that the flow passages of a melt, which penetrate in the perpendicular direction to the coating surface, are easily formed since the angle between arms is wider in the cruciform crystal than in the hexagonal crystal. Therefore, since the dendrites of the Al phase are oriented toward the (200) plane in the solidification 25 process of coating, and the crystals formed into a cruciform shape when seen from the 19 coating surface have a favorable fluidity of a coating melt in the perpendicular direction to the coating surface, the dendrites of the Al phase on the coating surface are easily covered with the coating melt. In summary, it is presumed that, since the Al phase on the coating surface is easily covered with the ternary eutectic crystal that is solidified at 5 the end, ultimately, the white portions on the coating surface decreases, and the orange peel becomes favorable. [0042] The mechanism in which the dendrites of the Al phase are oriented toward the (200) plane is not clear. However, considering the fact that the dendrites of the Al phase 10 are oriented toward the (200) plane by setting the temperature of the coating layer immediately before wiping to a temperature exceeding the solidification start temperature of the Zn-Al-Mg (Zn-Al-Mg-based) coating metal, and, consequently, cooling the coating layer so as for the average steel sheet temperature one second after the passage of the wiping to be a temperature that is I 0C or more lower than the solidification start 15 temperature of the Zn-Al-Mg coating metal, it is considered that crystallization of a lot of crystal nuclei of the fine Al phase having a certain grain diameter in an initial stage of solidification or less influences the orientation of the Al phase. It is considered that, since the fine Al phase can easily rotate in a coating melt in comparison to coarse crystals, the Al phase is oriented in a direction in which the fluidity of the coating melt is easily 20 secured, that is, the fluidity of a coating melt in the perpendicular direction to the coating surface that becomes important in the solidification shrinkage stage of the coating layer is easily secured, and the Al phase that appears to be cruciform when seen from the coating surface becomes easily generated. [0043] 25 In general, it is important to increase the degree of undercooling at the 20 solidification start point in order to generate a lot of fine crystal nuclei. Therefore, it is presumed that it is possible to maximize the degree of undercooling at the solidification start point by making solidification start at the wiping position at which the amount of coating is controlled by a colliding force of gas jet, a lot of the crystal nuclei of the Al 5 phase are generated, and an Al phase which appears to be cruciform when seen from the coating surface is easily generated. [0044] In the present invention, the thickness of the coating layer is not particularly limited. However, in a thin coated steel sheet, the surface appearance is often favorable 10 even when the cooling after wiping is not particularly controlled. In addition, there are cases in which the state of the orange peel is degraded when the amount of coating provided to a single face is in a range of 50 to 300 g/m2. Therefore, the amount of coating provided to a single face of the Zn-Al-Mg coated steel sheet in which the white portions on the coating layer are controlled is preferably 50 to 300 g/m 2 . When the 15 amount of coating is controlled to be thick, it is considered that the state of the orange peel is not favorable since the temperature of the steel sheet (the temperature of the coating layer) after wiping is not easily decreased. [0045] In order to manufacture a Zn-Al-Mg coated steel sheet having a good orange 20 peel as above, basically, the solidification process of the coating layer is controlled. The fraction of Al crystals increases which are dendritic crystals oriented in the orientation of the (200) plane, and appearing to be fine cruciform when seen from the perpendicular direction to the coating surface, by the control of the solidification process. [0046] 25 That is, the solidification process of the coating layer is controlled by the 21 following method. Firstly, gas is ejected to a steel sheet pulled up from a coating bath so as to carry out wiping through which excess coating metal (Zn-Al-Mg coating metal) is wiped away. Cooling during the wiping is controlled so that the temperature of the steel sheet (the temperature of the coating layer) immediately after the wiping is made to 5 be a temperature that is I 0C or more lower than the solidification start temperature of the Zn-Al-Mg coating metal (the solidification start temperature - I 0 0 C or lower). In order to control the temperature of the steel sheet immediately after the wiping, for example, conditions, such as the temperature of the coating bath and the temperature and amount of the wiping gas, may be controlled. Furthermore, the coated steel sheet may 10 be cooled in air or at 10 *C/s (*C/second) or less after the wiping. Meanwhile, when the temperature of the steel sheet immediately after the wiping was a temperature that was I 0 0 C or less lower than the solidification start temperature, it was not possible to obtain a coated steel sheet having a favorable orange peel. In the past, generally, the cooling of a coating layer was not controlled by 15 wiping. That is, the coating layer is often in a completely molten state so that the amount of coating can be easily controlled during the wiping and immediately after the wiping. Particularly, the coating temperature is set to a temperature that is sufficiently higher than the solidification start temperature during the wiping in order to decrease the viscosity of coating. For example, as described in Non-Patent Citation 1, the model of 20 laminar flow is used for the analysis of the amount of coating during the wiping. Therefore, in the past, there were cases in which the state of the orange peel was degraded when the amount of coating was controlled to be thick. [0047] When the steel sheet (coated steel sheet) is cooled immediately after the wiping 22 to a temperature that is I 0C or more lower than the solidification start temperature, dendritic crystals which appear to be fine cruciform when seen from the perpendicular direction to the coating surface are generated in accordance to the fine and homogeneous nucleation. 5 (0048] On the other hand, when the temperature of the steel sheet (the temperature of the coating layer) immediately after the wiping is made to a temperature that is I 0C or less lower than the solidification start temperature, in the solidification process in accordance with the subsequent decrease in temperature, large dendrites of the Al phase 10 are easily grown on the coating surface, and dendritic crystals which appear to be hexagonal when seen from the perpendicular direction to the coating surface are easily generated. [0049] In addition, after the wiping, if the coating metal is cooled in air until the coating 15 metal becomes the final solidification temperature, there is a tendency that the area of flat portions increases, and the orange peel is further improved. It is presumed that, when the cooling rate decreases, the area of flat portions increases since a duration in which a coating melt is discharged from spacing of the dendrites that are dendritic crystals appearing to be fine cruciform when seen from the 20 perpendicular direction to the coating surface to the surface of the coating layer can be sufficiently secured. Practically, a certain degree of the cooling rate is required in order to secure productivity. However, in order to obtain a Zn-Al-Mg coated steel sheet having a more favorable orange peel, it is preferable to cool the Zn-Al-Mg coated steel sheet at a 25 cooling rate of 10 *C/s or less. In the past, it was common to manufacture a Zn-Al-Mg 23 coated steel sheet by carrying out cooling at a cooling rate of 10 *C/s or more after wiping in order to secure productivity. In this case, it was difficult to obtain a coated steel sheet having a favorable orange peel. Meanwhile, when the growth of the dendrites of the Al phase (nucleation) is taken into consideration, cooling at a cooling 5 rate of 10 *C/s or less may be carried out until the solidification start temperature of the Al phase in the Zn-Al-Mg coating metal. [0050] Meanwhile, it is necessary to increase the start temperature of the wiping (the temperature of the steel sheet immediately before the wiping) to higher than the 10 solidification start temperature of the coating metal. That is, the temperature of the steel sheet is controlled so that the temperature of the steel sheet immediately before the wiping exceeds the solidification start temperature of the Zn-Al-Mg coating metal. When the start temperature of the wiping is lower than the solidification start temperature of the coating metal, the distribution of the Al phase becomes uneven, and 15 appearance defects other than poor orange peel occur. It is preferable to start the wiping in a temperature range of higher than the solidification start temperature to I 0C or less higher than the solidification start temperature (the solidification start temperature + I 0 0 C or lower) in order to easily carry out temperature control immediately after the wiping. 20 Here, the temperature of the steel sheet immediately before the wiping is an average temperature of the coated steel sheet at the cooling start point by a wiping gas. In addition, the temperature of the steel sheet immediately after the wiping is an average temperature of the coated steel sheet I second after the start of the cooling by a wiping gas. The temperature of the steel sheet may be indirectly measured using a radiation 25 thermometer, or directly measured using a contact type thermometer.

24 Meanwhile, the above method of manufacturing the Zn-Al-Mg coated steel sheet can be used for both a thin coated steel sheet (more than 0 g/m2 to less than 50 g/m 2 ) and a thick coated steel sheet (50 to 300 g/m 2 ). Particularly, the above method of manufacturing the Zn-Al-Mg coated steel sheet can be preferably used for a thick coated 5 steel sheet. [0051] Hereinafter, the embodiment of the present invention will be described, and the conditions employed in the examples are an example of the conditions for confirming the applicability and effects of the present invention. Therefore, the present invention is not 10 limited to these example. [Example 1] [0052] A 0.8 mm-thick cold-rolled steel sheet was prepared, and heating, annealing, and Zn-Al-Mg coating were carried out using a non-oxidizing furnace-type continuous 15 hot-dip coating line, thereby producing a coated steel sheet. The annealing atmosphere was set to an atmosphere of a mixed gas of 10 vol.% (volume%) of hydrogen and 90 vol.% of nitrogen, the annealing temperature was set to 750'C, and the annealing time was set to 3 minutes. In a hot-dip coating, the steel sheet was immersed in a coating bath of 430*C having the composition shown in Table 1 for 3 minutes, and the amount of 20 coating was adjusted by N 2 gas wiping. After the hot-dip coating, cooling in air (air cooling) or gas cooling at 10 *C/s or less was carried out. The other test conditions were the conditions shown in Table 1. [0053] Regarding the appearance of coating, a sample was cut out from the coated steel 25 sheet, and an orange peel finish (orange peel) on the sample surface was evaluated using 25 grades. An orange peel on a boundary from which the sample could be evaluated as "good" was set to the standard grade 3, a grade of 1 to 6 was given to each sample according to the state of the orange peel on the taken sample, in which the grades 1 to 6 were in a preferable order, and each sample was evaluated. 5 In addition, the number of the white portions was counted visually. [0054] An XRD sample and an EBSD sample were prepared from each of the samples, and XRD measurement and EBSD measurement were carried out, respectively. [0055] 10 The evaluation results are shown in Table 1. The orange peels were good in the coated steel sheets of the examples. [0056] [Table 1] 26 IC) tn ) IC) I) IC) C: co 0 . CN < O4 c -5~0 ~ O c u CN , ' w 6 , 2

C

4)t 0 0 t 'r.~ t e .0 C> 0 e 00 00 cn 00 z CC" to 0 C 0 0 0.0 o 2 0 u.

27 [0057] 0.4 to 2.0 mm-thick cold-rolled steel sheets were prepared, and heating, annealing, and Zn-Al-Mg coating were carried out using a non-oxidizing furnace-type continuous hot-dip coating line, thereby producing coated steel sheets. The annealing 5 atmosphere was set to an atmosphere of a mixed gas of 10 vol.% of hydrogen and 90 vol.% of nitrogen, the annealing temperature was set to 750'C, and the annealing time was set to 3 minutes. In a hot-dip coating, the steel sheets were immersed in coating baths having the compositions shown in Table I for 3 minutes, and the amount of coating was adjusted by N 2 gas wiping. Cooling after the coating was air cooling. The other 10 test conditions were shown in Table 2. Meanwhile, in the case of Table 2, the solidification start temperature of the coating metal was 428*C. [Example 2] [0058] The appearance of the coating was evaluated using grades in the same manner as 15 in Example 1. [0059] An XRD sample was prepared from each sample by mirror-polishing the coating surface, and an XRD measurement was carried out. In addition, the temperature of the steel sheet one second after the wiping was 20 measured using a contact type thermometer. The measured temperature of the steel sheet was evaluated as the temperature of the steel sheet immediately after the wiping. [0060] The evaluation results are shown in Table 2. The orange peels were good in the coated steel sheets of the examples in Nos. 3 to 5. In the coated steel sheets of the 25 comparative examples in Nos. 6 to 9, the states of the orange peels deteriorated since the 28 temperature of the steel sheet immediately after the wiping was high (higher than 418 0 C). In addition, in the coated steel sheets of the reference examples in Nos. 1 and 2, the orange peels were good since the amount of coating was small. [0061] 5 [Table 2] 29 CU m 0.1 0. tm CO 0 U U U U CO r'q C) m (N 0 10 W 7 cc ICT O N CS W t t C 0 0 UC 4)0 0 en 11 n e E25) CZ0UC C> 000C0C000C U C) C 0 0 000 0 0 oo o(N ( 0 - 1

--

3

-

( C. CU CA 0 0 M0 z ~1- -o IN r- 00 Industrial Applicability 30 [0062] It is possible to stably manufacture a Zn-Al-Mg coated steel sheet that is excellent in terms of the appearance including an orange peel which has a fine texture and a lot of flat portions. 5

Claims

1. A Zn-Al-Mg coated steel sheet, including: a steel sheet; and 5 a coating layer including 4 to 22 mass% of Al, I to 5 mass% of Mg, and a balance including Zn and inevitable impurities, wherein a diffraction intensity ratio 1 (200) / I (111), which is a ratio of an X-ray diffraction intensity of a (200) plane of an Al phase I (200) to an X-ray diffraction intensity of a (111) plane of the Al phase I (111) in a cross-sectional surface of the 10 coating layer parallel to a surface of the coating layer, is 0.8 or more.

2. The Zn-Al-Mg coated steel sheet according to claim 1, wherein the coating layer includes 0.0001 to 2.0 mass% of Si. 15

3. The Zn-Al-Mg coated steel sheet according to claim I or 2, wherein the coating layer further includes 0.0001 to 0.5 mass% of one or a combination of Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, a Group III element, REM, Hf, and inevitable impurities. 20

4. The Zn-Al-Mg coated steel sheet according to claim I or 2, wherein the coating layer further includes 0.0001 to 0.5 mass% of one or a combination of Ni, Ti, Zr, and Sr.

5. The Zn-Al-Mg coated steel sheet according to claim 1 or 2, 25 wherein an area percentage of cruciform-looking dendritic crystals of the Al 32 phase in the cross-sectional surface of the coating layer parallel to the surface of the coating layer is 5% or more of a total cross-sectional area of the coating layer.

6. The Zn-Al-Mg coated steel sheet according to claim I or 2, 5 wherein the number of white portions on the surface of the coating layer is 100 -/cm2 or more, and an area percentage of luster portions on the surface of the coating layer is 94% or more of a total surface area of the coating layer.

7. The Zn-Al-Mg coated steel sheet according to claim I or 2, 10 wherein the amount of coating per one face in the coating layer is 50 to 300 g/m2.

8. A producing method of the Zn-Al-Mg coated steel sheet according to claim 1 or 2, wherein a temperature of the coating layer immediately before wiping is a 15 temperature exceeding the solidification start temperature of a Zn-Al-Mg coating metal, and a temperature of the coating layer immediately after the wiping is a temperature that is I 0C or more lower than the solidification start temperature of the Zn-Al-Mg coating metal. 20

9. The producing method of the Zn-Al-Mg coated steel sheet according to claim 8, wherein, after the wiping, the Zn-Al-Mg coated steel sheet is cooled at a cooling rate of

10 *C/s or less until a solidification completion temperature of the Al phase in the Zn-Al-Mg coating metal. 25 10. The producing method of the Zn-Al-Mg coated steel sheet according to claim 8 or 33 9, wherein the amount of coating per one face in the coating layer is controlled to 2 50 to 300 g/mn