Plated steel sheet

Abstract

The plated steel sheet has a steel sheet and a plating layer formed on at least a part of the surface of the steel sheet. The chemical composition of the plating layer contains, by mass%, Al: 6.00 to 35.00%, Mg: 3.00 to 15.00%, La + Ce: a total of 0.0001 to 0.5000%, and Zn. Regarding the plating layer, in the surface, the area ratio of the lamellar structure in which the (Al - Zn) phase and the MgZn2 phase are arranged in layers is 10 to 95%, the lamellar spacing of the lamellar structure is 2.5 μm or less, and the area ratio of the (Al - Zn) dendrites is 10% or less.

Description

Technical Field

[0001] The present invention relates to a plated steel sheet.

[0002] This application claims priority based on Japanese Patent Application No. 2020-175786 filed in Japan on October 20, 2020, the content of which is incorporated herein by reference. Background Art

[0003] In recent years, for automotive structural members, plated steel sheets have been used from the viewpoint of rust prevention. Mainly in the domestic Japanese market, hot-dip galvanized steel sheets such as alloyed hot-dip galvanized steel sheets have been applied. An alloyed hot-dip galvanized steel sheet is a plated steel sheet as follows: after hot-dip galvanizing is performed on a steel sheet, an alloying heat treatment is carried out to cause Fe to diffuse from the steel sheet (base steel sheet) into the coating layer, thereby improving weldability and corrosion resistance after painting. However, for hot-dip galvanized steel sheets, further improvement in corrosion resistance such as corrosion resistance after painting and red rust resistance is required.

[0004] As a method for improving the corrosion resistance of hot-dip galvanized steel sheets, adding Al to the Zn-based coating layer can be cited. For example, in the building materials field, as a highly corrosion-resistant plated steel sheet, hot-dip Al-Zn-based plated steel sheets have been widely put into practical use. Such a hot-dip Al-Zn-based coating layer is formed of dendritic α-(Zn, Al) phase (Al primary crystal part: in the Al-Zn binary phase diagram, etc., the α-(Zn, Al) phase that crystallizes as a primary crystal. Not necessarily a rich Al phase, and crystallizes as a solid solution of Zn and Al) that crystallizes from the molten state initially, and a structure (Zn / Al mixed phase structure) composed of Zn phase and Al phase formed in the gaps of the dendritic Al primary crystal part. Since the Al primary crystal part is passivated and the Zn concentration in the Zn / Al mixed phase structure is higher than that in the Al primary crystal part, corrosion concentrates on the Zn / Al mixed phase structure. As a result, corrosion progresses in a wormhole-like manner in the Zn / Al mixed phase structure, and the corrosion progress path becomes complex, so it is difficult for corrosion to easily reach the base steel sheet. Therefore, compared with a hot-dip galvanized steel sheet having the same coating layer thickness, a hot-dip Al-Zn-based plated steel sheet has more excellent corrosion resistance.

[0005] When using this hot-dip Al-Zn-based coated steel sheet as an outer panel of an automobile, the coated steel sheet is supplied to an automobile manufacturer or the like in a state where coating is performed by a continuous melting coating apparatus. Therefore, generally, after being processed into the shape of a panel member, chemical conversion treatment is performed, and then electro-deposition coating, intermediate coating, and top coating, which are comprehensive coatings for automobiles, are performed. However, regarding the outer panel using the hot-dip Al-Zn-based coated steel sheet, when damage occurs in the coating film, due to the unique coating phase structure composed of the above-mentioned two phases of the Al primary crystal part and the Zn / Al mixed phase structure, preferential dissolution of Zn (selective corrosion of the Zn / Al mixed phase structure) occurs at the damaged part at the coating film / coating interface. It progresses deep into the intact part of the coating, causing significant swelling of the coating film. As a result, there is a problem that sufficient corrosion resistance (corrosion resistance after coating) cannot be ensured.

[0006] For the purpose of improving corrosion resistance, addition of elements such as Mg to the Al-Zn-based coating is also being investigated. However, since it is speculated that even if Mg is added, the Al primary crystal part with a passivation film will still be formed in the hot-dip Al-Zn-based coated steel sheet, the problem of corrosion resistance (corrosion resistance after coating) when damage occurs in the coating film after coating has not been solved.

[0007] In view of such problems, Patent Document 1 discloses a hot-dip Zn-based coated steel sheet with excellent corrosion resistance after coating. In Patent Document 1, it is disclosed that when the coating layer contains Zn, Al, Mg, and Si, and the coating layer contains a laminated structure in which laminated Zn phases and laminated Al phases are alternately arranged with a total area ratio of 5% or more, swelling of the coating film in the coated state is suppressed.

[0008] In Patent Document 2, a hot-dip Al-Zn-based coated steel sheet is disclosed, which is characterized in that it has a coating layer containing, by mass%, Al: 25 to 90% and Sn: 0.01 to 10%, and also contains one or more selected from the group consisting of Mg, Ca, and Sr in a total amount of 0.01 to 10%. In Patent Document 2, it is disclosed that due to Sn, the Al oxide film formed around the above-mentioned α-Al phase is destroyed, and the solubility of the α-Al phase increases. Therefore, uniform corrosion of the coating layer in which both the α-Al phase and the Zn-rich phase dissolve occurs, thereby suppressing selective corrosion of the Zn-rich phase and improving corrosion resistance after coating.

[0009] In addition, Patent Document 3 discloses a chemically converted treated steel sheet, which

[0010] uses a molten Zn-Al-Mg alloy coated steel sheet in which the proportion of [Al / Zn / Zn2Mg ternary eutectic structure] in the outermost surface of the coating layer is 60 area% or more as a base material, and the surface of the coating layer is covered with a chemical conversion film.

[0011] In addition, in Patent Document 4, a zinc-based alloy plated steel sheet is disclosed, which has a Zn-Al-based alloy coating layer. The Zn-Al-based alloy coating layer contains Al: 0.18 to 5%, and also contains any one or two or more of Mg: 0.01 to 0.5%, La: 0.001 to 0.5%, and Ce: 0.001 to 0.5%. The balance is composed of Zn. In Patent Document 4, it is disclosed that zinc-based alloy coating layers such as Zn-Al-Mg-based and Zn-Al-Mg-Si-based have higher corrosion resistance than the conventional Zn-based coating layers.

[0012] Automobile parts are sometimes used in an environment where water accumulates, and sufficient corrosion resistance is required even in such a harsh environment. The inventors of the present invention conducted research and found that a Zn-Al-Mg-based coating layer containing a certain amount or more of Al has excellent sacrificial anticorrosion properties. On the other hand, in an environment where water accumulates, film peeling is likely to occur, and there are problems with the corrosion resistance after painting. However, in Patent Documents 1 to 4, the corrosion resistance after painting in such a harsh environment is not considered.

[0013] In addition, in the technology of Patent Document 1, a complex heat treatment process needs to be carried out for grain refinement control, and there is also a problem of increased manufacturing cost.

[0014] In addition, in the plated steel sheet of Patent Document 2, the adhesion to the electrodeposition coating film for automobiles deteriorates. In addition, in Patent Document 2, Sn addition is necessary, so there are also problems of increased alloy cost and difficulty in bath management.

[0015] In addition, in Patent Document 3, the corrosion resistance is improved by controlling the composition of the chemical conversion film. In addition, in order to improve the reactivity with the chemical conversion film, the coating layer has a ternary eutectic structure of Al / Zn / Zn2Mg as the main phase. Therefore, it is considered that although the chemical conversion processability is improved in the case of performing a normal chemical conversion treatment, the corrosion resistance after painting cannot be sufficiently obtained.

[0016] Therefore, in the past, a hot-dip galvanized steel sheet that can ensure sufficient corrosion resistance after painting, which is required for recent automotive structural components, has not been proposed.

[0017] Prior Art Documents

[0018] Patent Documents

[0019] Patent Document 1: Japanese Patent No. 6350780

[0020] Patent Document 2: Japanese Unexamined Patent Application Publication No. 2015-214747

[0021] Patent Document 3: Japanese Patent Publication No. 4579715

[0022] Patent Document 4: Japanese Unexamined Patent Application Publication No. 2006-249521 Summary of the Invention

[0023] Technical Problem to be Solved by the Invention

[0024] The present invention has been completed in view of the above problems. An object of the present invention is to provide a plated steel sheet having excellent corrosion resistance after coating, on the premise of a hot-dip galvanized steel sheet.

[0025] Technical Means for Solving the Technical Problem

[0026] The inventors of the present invention conducted research on improving the corrosion resistance after coating in a steel sheet having a coating layer containing Al, Mg, and Zn (Zn-Al-Mg alloy plated steel sheet).

[0027] As a result, it was newly found that the corrosion resistance after coating is improved by making the coating layer contain La and / or Ce and controlling the coating layer structure on the surface of the coating layer. Specifically, it was found that the corrosion resistance after coating is improved by increasing the area ratio of the lamellar structure of the (Al-Zn) phase and the MgZn2 phase and decreasing the area ratio of the (Al-Zn) dendrites on the surface of the coating layer.

[0028] The present invention has been completed based on the above recognition, and its gist is as follows.

[0029] [1] A plated steel sheet according to one aspect of the present invention is a plated steel sheet having a steel sheet and a coating layer formed on at least a part of the surface of the steel sheet. The chemical composition of the coating layer contains, by mass%, Al: 6.00 to 35.00%, Mg: 3.00 to 15.00%, La + Ce: a total of 0.0001 to 0.5000%, Si: 0 to 2.00%, Ca: 0 to 2.00%, Fe: 0 to 2.00%, Sb: 0 to 0.50%, Sr: 0 to 0.50%, Pb: 0 to 0.50%, Sn: 0 to 1.00%, Cu: 0 to 1.00%, Ti: 0 to 1.00%, Ni: 0 to 1.00%, and Mn: 0 to 1.00%, and the balance is composed of Zn and impurities. Regarding the coating layer, on the surface, the area ratio of the lamellar structure in which the (Al-Zn) phase and the MgZn2 phase are arranged in layers is 10 to 95%, the layer spacing of the lamellar structure is 2.5 μm or less, and the area ratio of the (Al-Zn) dendrites is 10% or less.

[0030] [2] Alternatively, in the plated steel sheet as described in [1] above, the chemical composition of the coating layer contains, in mass %, one or more of Al: 11.00 to 30.00%, Mg: 5.00 to 11.00%, and La + Ce: a total of 0.0010 to 0.1000%.

[0031] [3] Alternatively, in the plated steel sheet as described in [1] or [2] above, regarding the coating layer, on the surface, the area ratio of the lamellar structure is 60 to 95%.

[0032] [4] Alternatively, in the plated steel sheet as described in [3] above, regarding the coating layer, on the surface, the area ratio of the lamellar structure is 80 to 95%.

[0033] Advantages of the Invention

[0034] According to the above solution of the present invention, a plated steel sheet with excellent corrosion resistance after painting can be provided. Brief Description of the Drawings

[0035] Figure 1 is a micrograph of the surface of the coating layer of No. 23 as a comparative example.

[0036] Figure 2 is a micrograph of the surface of the coating layer of No. 19 as an inventive example.

[0037] Figure 3 is a micrograph of the surface of the coating layer of No. 19 as an inventive example. Detailed Description of the Invention

[0038] The plated steel sheet of an embodiment of the present invention (the plated steel sheet of this embodiment) has a steel sheet and a coating layer, and the coating layer is formed on at least a part of the surface of the steel sheet. In addition, in the plated steel sheet of this embodiment, the chemical composition of the coating layer contains, in mass %, Al: 6.00 to 35.00%, Mg: 3.00 to 15.00%, La + Ce: a total of 0.0001 to 0.5000%, Si: 0 to 2.00%, Ca: 0 to 2.00%, Fe: 0 to 2.00%, Sb: 0 to 0.50%, Sr: 0 to 0.50%, Pb: 0 to 0.50%, Sn: 0 to 1.00%, Cu: 0 to 1.00%, Ti: 0 to 1.00%, Ni: 0 to 1.00%, and Mn: 0 to 1.00%, and the balance consists of Zn and impurities. In addition, regarding the coating layer of the plated steel sheet of this embodiment, on the surface, the area ratio of the lamellar structure in which (Al - Zn) phases and MgZn2 phases are arranged in layers is 10 to 95%, the layer spacing of the lamellar structure is 2.5 μm or less, and the area ratio of (Al - Zn) dendrites is 10% or less.

[0039] <Steel sheet>

[0040] The key point of the plated steel sheet of this embodiment lies in the coating layer, and it is not particularly limited with respect to the type of steel sheet. It can be determined according to the products to be applied, the required strength, plate thickness, etc. For example, hot-rolled steel sheets described in JIS G3193:2008 or cold-rolled steel sheets described in JIS G3141:2017 can be used.

[0041] <Coating layer>

[0042] In the plated steel sheet of this embodiment, a coating layer is provided on at least a part of the surface of the steel sheet. The coating layer can be formed on one side of the steel sheet or on both sides.

[0043] The coating amount of the coating layer is preferably 15 to 250 g / m 2 .

[0044] [Chemical composition]

[0045] Hereinafter, the chemical composition of the coating layer of the plated steel sheet of this embodiment will be described. The % of the content of each element means mass %. In addition, the numerical range shown with "~" includes the values at both ends as the upper and lower limits.

[0046] Al: 6.00 to 35.00%

[0047] Al is an element effective for ensuring the corrosion resistance after painting in a coating layer containing aluminum (Al), zinc (Zn), and magnesium (Mg). Moreover, it is an element required for forming a lamellar structure in the coating layer of the plated steel sheet of this embodiment. In addition, Al also contributes to the formation of an alloy layer (Al-Fe alloy layer) and is effective for ensuring plating adhesion. In order to fully obtain the above effects, the Al content is set to 6.00% or more. The Al content is preferably 11.00% or more.

[0048] On the other hand, when the Al content exceeds 35.00%, the area ratio of (Al-Zn) dendrites becomes high, and the corrosion resistance after painting and the corrosion resistance of the cut end face of the coating layer decrease. Therefore, the Al content is set to 35.00% or less. Preferably, it is 30.00% or less.

[0049] Mg: 3.00 to 15.00%

[0050] Mg is an element having the effect of improving the corrosion resistance after painting of the coating layer. Moreover, it is an element required for forming a lamellar structure in the coating layer of the plated steel sheet of this embodiment. In order to fully obtain the above effects, the Mg content is set to 3.00% or more. The Mg content is preferably 5.00% or more.

[0051] On the other hand, when the Mg content exceeds 15.00%, the layered structure cannot be formed sufficiently, resulting in not only a decrease in corrosion resistance after painting but also a decrease in the workability of the coating. In addition, manufacturing problems such as an increase in the amount of sludge generated in the plating bath will occur. Therefore, the Mg content is set to 15.00% or less. The Mg content is preferably 11.00% or less.

[0052] La + Ce: a total of 0.0001 to 0.5000%

[0053] La and Ce are elements effective for stabilizing the layered structure in the coating. Even if the coating does not contain La and Ce, in the interior of the coating, in regions other than the surface layer, a layered structure may sometimes be formed. However, when the total content of La and Ce is less than 0.0001%, a layered structure will not be formed in the surface layer. Therefore, the area ratio of the layered structure on the surface of the coating cannot be ensured sufficiently.

[0054] On the other hand, when the total content of La and Ce exceeds 0.5000%, the viscosity of the plating bath increases, and it becomes difficult to prepare the plating bath itself, making it impossible to manufacture plated steel sheets with good plating properties. Therefore, it is preferable to set the total content of La and Ce to 0.5000% or less.

[0055] Si: 0 to 2.00%

[0056] Si is an element that forms a compound with Mg, thereby contributing to an improvement in corrosion resistance after painting. In addition, Si is also an element that has the following effect: when the coating is formed on a steel sheet, it inhibits the alloy layer formed between the steel sheet and the coating from being formed too thick, thereby improving the adhesion between the steel sheet and the coating. Therefore, Si can also be contained. In order to obtain the above effects, it is preferable to set the Si content to 0.10% or more. More preferably, it is 0.20% or more.

[0057] On the other hand, when the Si content exceeds 2.00%, in the coating, excess Si will crystallize, and in addition, the layered structure cannot be formed sufficiently, resulting in a decrease in corrosion resistance after painting. In addition, the workability of the coating will also decrease. Therefore, the Si content is set to 2.00% or less. Regarding the Si content, more preferably, it is 1.50% or less. It is not necessarily required to contain Si, and the lower limit of the content is 0%.

[0058] Ca: 0 to 2.00%

[0059] When Ca is contained in the coating, with the increase in the Mg content, the amount of scum formed during the plating operation tends to decrease, and the plating productivity improves. Therefore, Ca can also be contained. It is not necessarily required to contain Ca, and the lower limit is 0%, but in order to obtain the above effects, it is preferably set the Ca content to 0.03% or more, more preferably, to 0.10% or more.

[0060] On the other hand, when the Ca content is relatively high, the lamellar structure cannot be fully formed. Furthermore, Ca-based intermetallic compounds typified by the CaZn 11 phase are generated as other intermetallic compound phases at an area ratio of 10% or more, and the corrosion resistance after painting decreases. In addition, the corrosion resistance after painting of the flat part of the coating itself tends to deteriorate, and the corrosion resistance around the welded part sometimes also deteriorates. Therefore, the Ca content is set to 2.00% or less. The Ca content is preferably 1.00% or less.

[0061] Fe: 0 - 2.00%

[0062] When manufacturing the coating, Fe may be mixed into the coating as an impurity. Fe may sometimes be contained up to about 2.00%, but in this range, the adverse effects on the properties of the plated steel sheet of the present embodiment are small. Therefore, it is preferably set the Fe content to 2.00% or less. More preferably, it is 1.50% or less, and further preferably, it is 1.00% or less.

[0063] On the other hand, as described above, Fe may be mixed into the coating as an impurity. In order to completely prevent the mixing of Fe, the cost will be significantly increased, so the Fe content can also be set to 0.10% or more.

[0064] The basis of the chemical composition of the coating of the plated steel sheet of the present embodiment is that it has the above chemical composition, and the remaining part is Zn and impurities. The content of the impurities is preferably 5.0% or less, more preferably, 3.0% or less.

[0065] However, it is also possible to further replace a part of Zn. For example, the coating of the plated steel sheet of the present embodiment contains Sb, Pb, Cu, Sn, Ti, Sr, Ni, Mn (whether intentionally added or contained as an impurity) within the following ranges. Since it is not necessary to contain these elements, the lower limit of the content is 0%.

[0066] Sb: 0 - 0.50%

[0067] Sr: 0 - 0.50%

[0068] Pb: 0 - 0.50%

[0069] When Sr, Sb, and Pb are contained in the coating, the appearance of the coating changes to form spangles, thereby confirming an increase in metallic luster. Therefore, Sr, Sb, and Pb may also be contained. In order to obtain the above effects, it is preferable to contain 0.01% or more of one or more of Sb, Sr, and Pb. On the other hand, when the content of these elements exceeds 0.50%, various intermetallic compound phases are formed, and the workability and corrosion resistance deteriorate. In addition, when the content of these elements is excessive, the viscosity of the plating bath increases, and it often becomes difficult to form the plating bath itself, and it is impossible to manufacture a plated steel sheet with good plating properties. Therefore, it is preferable that the Sr content is 0.50% or less, the Sb content is 0.50% or less, and the Pb content is 0.50% or less.

[0070] Sn: 0 to 1.00%

[0071] Sn is an element that increases the dissolution rate of Mg in a coating containing Zn, Al, and Mg. When the dissolution rate of Mg increases, the corrosion resistance of the flat part deteriorates. Therefore, it is preferable that the Sn content is 1.00% or less.

[0072] Cu: 0 to 1.00%

[0073] Ti: 0 to 1.00%

[0074] Ni: 0 to 1.00%

[0075] Mn: 0 to 1.00%

[0076] These elements are elements that contribute to the improvement of corrosion resistance. Therefore, they may also be contained. In order to obtain the above effects, it is preferable that the content of one or more of Cu, Ni, Ti, and Mn is 0.01% or more. On the other hand, when the content of these elements is excessive, the viscosity of the plating bath increases, and it often becomes difficult to form the plating bath itself, and it is impossible to manufacture a plated steel sheet with good plating properties. Therefore, it is preferable that the content of each element is 1.00% or less.

[0077] The chemical composition of the coating is determined by the following method.

[0078] First, an acid solution is obtained by stripping and dissolving the coating with an acid containing an inhibitor, where the inhibitor is used to inhibit the corrosion of the steel base (steel). Then, by measuring the obtained acid solution by ICP analysis, the chemical composition of the coating can be obtained. Regarding the type of acid, there is no particular limitation as long as it is an acid that can dissolve the coating. Regarding the chemical composition, it is measured as the average chemical composition.

[0079] [The structure (phase) contained in the coating]

[0080] The coating of the plated steel sheet of the present embodiment is, for example, asFigure 2 As shown, in the surface, there is a laminated structure in which (Al-Zn) phase and MgZn2 phase are laminated, and the area ratio is 10 to 95%. In addition, in the coating layer of the coated steel sheet of the present embodiment, in the surface, the area ratio of (Al-Zn) dendrites is 10% or less. (For comparison, in Figure 1 the micrograph of the surface of the coating layer of a conventional coated steel sheet is shown)

[0081] Generally, when a steel sheet immersed in a plating bath containing Zn, Mg, and Al is cooled, in the coating layer, (Al-Zn) dendrites as primary crystals and a ternary eutectic structure of Zn / Al / MgZn2 are formed. The corrosion resistance of this (Al-Zn) dendrite is low. Therefore, even when the steel sheet is painted, when the coating film is damaged, etc., corrosion progresses inside the coating layer, and coating film swelling occurs. In contrast, if the corrosion resistance of the surface of the coating layer is high, even when the coating film is damaged, the progress of corrosion can be suppressed on the surface of the coating layer. From the research results of the present inventors, it is known that the laminated structure in which (Al-Zn) phase and MgZn2 phase are laminated has high corrosion resistance, and by forming the laminated structure in which (Al-Zn) phase and MgZn2 phase are laminated on the surface at a certain area ratio or more, the corrosion resistance is improved.

[0082] Therefore, in the coating layer of the coated steel sheet of the present embodiment, in the surface of the coating layer, the area ratio of the laminated structure in which (Al-Zn) phase and MgZn2 phase, which contribute to the improvement of corrosion resistance after painting, are laminated is set to 10% or more. The area ratio of the laminated structure in the surface is preferably 60% or more, more preferably 80% or more. The laminated structure not only has corrosion resistance after painting, but also has the effect of improving the LME resistance.

[0083] On the other hand, on the premise of the above chemical composition, it is not easy to make the area ratio of the laminated structure exceed 95% industrially. Therefore, the area ratio of the laminated structure is set to 95% or less.

[0084] In addition, in the coating layer of the coated steel sheet of the present embodiment, in the surface, the area ratio of (Al-Zn) dendrites that reduce the corrosion resistance after painting is set to 10% or less. Preferably, the area ratio of (Al-Zn) dendrites is small, and it can also be 0%.

[0085] The laminated structure is, for example, as Figure 3 shown, a structure in which (Al-Zn) phase and MgZn2 phase are laminated, and the finer the laminated interval, the greater the improvement effect of corrosion resistance and LME resistance after painting. The laminated interval that can obtain sufficient performance is 2.5 μm or less (2500 nm or less), preferably 500 nm or less. This kind of laminated structure is also called a feather-like structure.

[0086] The ternary eutectic structure of Zn / Al / MgZn2 is composed of, by area percentage, Zn phase: 45 - 60%, MgZn2 phase: 35 - 45%, and Al phase: 3 - 10%. In contrast, the lamellar structure is as follows: the fraction of each phase, by area percentage, is Zn phase: 0 - 10%, MgZn2 phase: 40 - 65%, and Al phase: 30 - 45%.

[0087] Alternatively, in the coating layer of the coated steel sheet of the present embodiment, on the surface, as phases other than the above-mentioned lamellar structure and (Al - Zn) dendrites, massive MgZn2 phase, ternary eutectic structure of Zn / Al / MgZn2, and other intermetallic compounds may also be included. The total of the remaining part is preferably 50% or less, and more preferably 30% or less.

[0088] The massive MgZn2 phase contributes to the improvement of corrosion resistance after coating. When sufficient effects are to be obtained, it is preferable to set the area ratio of the massive MgZn2 phase to 5% or more. On the other hand, from the viewpoint of workability, the area ratio of the MgZn2 phase is preferably 40% or less.

[0089] The area ratio of the ternary eutectic structure of Zn / Al / MgZn2 is preferably set to 45% or less. When the area ratio of the ternary eutectic structure of Zn / Al / MgZn2 exceeds 45%, a decrease in corrosion resistance after coating may be a concern.

[0090] In addition, since the MgSi2 phase and other intermetallic compounds reduce the corrosion resistance after coating, it is preferable to set them to 10.0% or less by area ratio, respectively. More preferably, the total area ratio is 10.0% or less. Examples of other intermetallic compound phases include CaZn 11 phase, Al2CaSi2 phase, Al2CaZn2 phase, etc.

[0091] For the structure of the coating layer (area ratio of each phase, lamellar spacing), the following method is used for measurement.

[0092] From the coated steel sheet of the present embodiment, a sample with a size of 25 mm in the direction perpendicular to the rolling direction × 15 mm in the rolling direction is extracted, embedded in resin, and polished so that the surface of the coating layer of the sample becomes the observation surface. Thereafter, an SEM image and an element distribution image based on EDS are obtained. Regarding the area ratios of the lamellar structure, massive MgZn2 phase, ternary eutectic structure of Zn / Al / MgZn2, (Al - Zn) dendrites, and other intermetallic compounds in the coating layer, one field of view is taken for each of the cross-sectional EDS mapping images of the coating layer from 5 different samples, and a total of 5 fields of view are taken (magnification 1500 times: 60 μm × 50 μm / 1 field of view), and calculated based on the images.

[0093] At this time, the (Al-Zn) phase and the MgZn2 phase composed of Al and Zn are arranged in layers. If the layer spacing is 4 μm or less, it is judged as a layered structure. Even if the (Al-Zn) phase and the MgZn2 phase are adjacent, as long as the short diameter of the (Al-Zn) phase or the MgZn2 phase exceeds 4 μm, they are respectively judged as (Al-Zn) dendrites and massive MgZn2 phases. If it is a layered structure of Zn phase, α phase, and MgZn2 with a layer spacing of 4 μm or less, it is judged as a Zn / Al / MgZn2 ternary eutectic structure. When the phase contains more than 10% of a metal other than (Zn, Al, Mg, Si), it is judged as other intermetallic compounds.

[0094] In addition, the layer spacing of the layered structure is obtained as follows: According to SEM observation, for the phase with the lowest area ratio within the phase forming the layered structure, the spacing to the adjacent phase is measured, and the average value of 10 measurements is calculated.

[0095] In addition, the area ratios of the layered structure, the Zn phase, the MgZn2 phase, and the Al phase constituting the Zn / Al / MgZn2 ternary eutectic structure can be obtained by the following method: Using image processing software or the like, the region where the tissue exists on the cross-sectional SEM image is surrounded by a line, and the area of the region surrounded by the line is calculated.

[0096] <Manufacturing method>

[0097] Bonding, a preferred manufacturing method for the plated steel sheet of the present embodiment will be described. Regarding the plated steel sheet of the present embodiment, regardless of the manufacturing method, as long as it has the above characteristics, this effect will be obtained. However, according to the following method, it can be stably manufactured, so it is preferred.

[0098] Specifically, the steel sheet of the present embodiment can be manufactured by a manufacturing method including the following steps (I) to (IV).

[0099] (I) Annealing step, which performs reduction annealing on the steel sheet

[0100] (II) Plating step, which immerses the steel sheet in a plating bath containing one or two of Al, Mg, Zn, and La and Ce to form a plated base plate

[0101] (III) Controlled cooling step, which cools the plated base plate to a cooling stop temperature of (Al-Zn / MgZn2 binary eutectic temperature - 30) °C to (Al-Zn / MgZn2 binary eutectic temperature - 10) °C at an average cooling rate of 15 °C / second or more

[0102] (Iv) Slow cooling process, which, after the controlled cooling process, cools to below 335°C at an average cooling rate of 5°C / second or less until reaching 335°C

[0103] [Annealing process]

[0104] In the annealing process, before the plating process, annealing (reducing annealing) is performed on a steel sheet (hot-rolled steel sheet or cold-rolled steel sheet) obtained by a known method. For the annealing conditions, known conditions can be used, for example, heating to 750 - 900°C in a 5% H2 - N2 gas atmosphere with a dew point of -10°C or higher and holding for 30 - 240 seconds.

[0105] [Plating process]

[0106] In the plating process, during the temperature drop after annealing, the steel sheet is immersed in a plating bath to form a coating, and it is made into a plated base plate.

[0107] Regarding the plating bath, preferably, by mass%, it contains Al: 6.00 - 35.00%, Mg: 3.00 - 15.00%, La + Ce: 0.0001 - 0.5000%, Si: 0 - 2.00%, Ca: 0 - 2.00%, and the balance is composed of Zn and impurities. Furthermore, it may also contain Fe, Sb, Sr, Pb, Sn, Cu, Ti, Ni, Mn as required. The composition of the plating bath is roughly the same as the composition of the formed coating, so it can be adjusted according to the chemical composition of the desired coating.

[0108] [Controlled cooling process]

[0109] In the controlled cooling process, after the plated base plate after the plating process (lifted from the plating bath) has its plating adhesion amount adjusted with a purge gas such as N2, it is cooled. During cooling, it is cooled to a cooling stop temperature of (Al - Zn / MgZn2 eutectic temperature - 30)°C to (Al - Zn / MgZn2 eutectic temperature - 10)°C at an average cooling rate of 15°C / second or more.

[0110] By cooling under the above conditions, the formation of (Al - Zn) dendrites is suppressed, and nuclei for the formation of a lamellar structure are formed. In the subsequent slow cooling process, a lamellar structure will be generated.

[0111] When the average cooling rate is less than 15°C / second, the (Al - Zn) phase and the MgZn2 phase will not form a lamellar structure, and a large amount of (Al - Zn) dendrites will be generated, resulting in a decrease in corrosion resistance after painting.

[0112] In addition, when the cooling stop temperature is lower than (Al - Zn / MgZn2 eutectic temperature - 30)°C

[0113] At this time, it is difficult to generate a sufficient amount of lamellar structure in the subsequent slow cooling process. In addition, when it is higher than (Al-Zn / MgZn2 binary eutectic temperature - 10) °C, the eutectic solidification conditions of the α-phase and the MgZn2 phase cannot be satisfied. As a result, it will be the reason for the relatively large formation of (Al-Mg) dendrites.

[0114] The upper limit of the average cooling rate does not need to be limited, but it can also be that due to restrictions such as equipment, the average cooling rate is set to 40 °C / second or less.

[0115] The Al-Zn / MgZn2 binary eutectic temperature can be obtained, for example, from the liquidus projection diagram of the Zn-Al-Mg ternary system.

[0116] [Slow cooling process]

[0117] In the slow cooling process, the plated base plate after the controlled cooling is stopped is cooled to 335 °C or lower in such a way that the average cooling rate up to 335 °C is 5 °C / second or less.

[0118] Through this slow cooling process, the generation nuclei of the lamellar structure formed in the controlled cooling process grow, and in the surface, a predetermined area ratio of the lamellar structure is obtained.

[0119] When the average cooling rate up to 335 °C exceeds 5 °C / second, the nucleus growth becomes insufficient, and the area ratio of the lamellar structure is not sufficient.

[0120] According to the above manufacturing method, the plated steel sheet of the present embodiment is obtained.

[0121] Examples

[0122] As the steel sheet for annealing and plating, a cold-rolled steel sheet (0.2% C - 2.0% Si - 2.3% Mn) with a thickness of 1.6 mm was prepared.

[0123] After cutting the steel sheet into 100 mm × 200 mm, annealing and hot-dip plating were continuously carried out using a batch-type hot-dip plating test apparatus.

[0124] During annealing, in a furnace with an oxygen concentration of 20 ppm or less, in an atmosphere composed of a gas containing 5% H2 gas and the remaining part being N2 gas and having a dew point of 0 °C, annealing was carried out at 860 °C for 120 seconds.

[0125] After annealing, the steel sheet was air-cooled with N2 gas. After the steel sheet temperature reached the bath temperature + 20 °C, it was immersed in a plating bath at the bath temperature shown in Table 1 for about 3 seconds.

[0126] For the plated base plate formed with a coating, the coating adhesion amount was adjusted to 40 - 80 g / m with N2 gas 2Thereafter, controlled cooling and slow cooling were carried out under the conditions shown in Table 2, and the temperature was cooled to room temperature. The controlled cooling stop temperature was in the range of (Al-Zn / MgZn2 eutectic temperature - 30)°C to (Al-Zn / MgZn2 eutectic temperature - 10)°C.

[0127] The temperature of the steel sheet was measured with a thermocouple spot-welded to the center of the plating base plate.

[0128] The composition of the formed coating is shown in Table 1. The remainder of Table 1 is Zn and impurities of 5.0% or less.

[0129] For the obtained plated steel sheet, the area ratio of each phase contained in the coating and the layer spacing of the laminated structure were measured by the following method.

[0130] From the obtained plated steel sheet, a sample with a size of 25 mm in the direction perpendicular to the rolling direction × 15 mm in the rolling direction was extracted, embedded in resin and polished so that the surface of the coating of the sample became the observation surface. Thereafter, an SEM image and an element distribution image based on EDS were obtained. Regarding the area ratio of the laminated structure, massive MgZn2 phase, Zn / Al / MgZn2 ternary eutectic structure, (Al-Zn) dendrite, and other intermetallic compounds in the coating, one field of view was taken for each of the cross-sectional EDS mapping images of the coating from 5 different samples, and a total of 5 fields of view were taken (magnification 1500 times: 60 μm × 50 μm / 1 field of view), and calculated based on the images.

[0131] In addition, the layer spacing of the laminated structure was obtained as follows: According to SEM observation, for the phase with the lowest area ratio in the phase forming the laminated structure, the interval to the adjacent phase was measured, and the average value of 10 points was calculated.

[0132] Regarding the laminated structure, it was a structure composed of, by area%, Zn phase: 0 - 10%, MgZn2 phase: 40 - 65%, and Al phase: 30 - 45%.

[0133] In addition, for the obtained plated steel sheet, the corrosion resistance after coating was evaluated.

[0134] Specifically, a 50×100 mm sample was extracted from the plated steel sheet, and phosphating Zn treatment (SD5350 system: standard made by Nippon Paint Industrial Coding Co., Ltd.) was carried out. Then, electrodeposition coating (PN110 Powernix (registered trademark) grey: standard made by Nippon Paint Industrial Coding Co., Ltd.) was carried out to make the thickness 20 μm, and sintering was carried out at a sintering temperature of 150 °C for 20 minutes. Then, the steel sheet was given a V-bend using a mold with a 60° angle and a radius of curvature of 10 mm. After the bend was restored, it was immersed in a 5% NaAl aqueous solution at 50 °C for 500 h. An adhesive tape was only pasted on the surface where the V-bending process was carried out, and a tape peeling test of instantaneously peeling it was carried out. For the area where the adhesive tape was pasted, the proportion of the area where the coating peeled off was obtained, and evaluation was carried out in the following manner.

[0135] (Evaluation)

[0136] B: Peeling area ratio is 25% or more

[0137] A: Peeling area ratio is 15% to less than 25%

[0138] AA: Peeling area ratio is 10% to less than 15%

[0139] AAA: Peeling area ratio is less than 10%

[0140] In addition, spot welding was carried out on the obtained steel sheet under the following conditions, the cross-section of the welded part was observed, and evaluation was carried out according to the length of the crack (LME crack).

[0141] That is, two plated steel sheets No. 1 to 30 recorded in the table were overlapped, the pressing electrode was pressed in such a way that the hitting angle was 7° and the load was 400 kgf, the current mode was set so that the weld diameter was 3.5×√t to 5.5×√t (t: plate thickness), and spot welding was carried out. For the pressing electrode, a type Cu-Cr electrode under JIS standard was used.

[0142] After spot welding, cutting was carried out along the plate thickness direction of the steel sheet parallel to the direction where the hitting angle was set. After cutting, the cross-section of the welded part, which was finished to a mirror polish by mechanical grinding and chemical grinding, was observed with an optical microscope, and the length of the LME crack in front of and outside the plastic metal ring area (Japanese: コロナボンド) was measured.

[0143] Judgment was made in the following manner according to the presence or absence of cracks.

[0144] (Evaluation)

[0145] A: There are cracks of 0.3 mm or less

[0146] AA: No cracks

[0147] The results are shown in Table 2.

[0148] [Table 1]

[0149]

[0150] [Table 2]

[0151]

[0152] As can be seen from Tables 1 to 2, for Nos. 2, 3, 5, 7 to 10, 12 to 17, 19 to 22, 24, 26 to 29 as inventive examples, excellent post - coating corrosion resistance was obtained.

[0153] On the other hand, in Nos. 1, 4, 6, 11, 18, 23, 25, 30 as comparative examples, the chemical composition of the coating and the structure of the tissue on the surface of the coating are outside the scope of the present invention, so the post - coating corrosion resistance is poor.

[0154] Industrial applicability

[0155] According to the present invention, it is possible to provide a plated steel material with post - coating corrosion resistance superior to that of conventional automotive plated steel sheets, and contribute to the development of the industry by extending the service life of automotive plated steel sheets.

[0156] Explanation of reference numerals

[0157] 1 Lamellar structure of (Al - Zn) phase and MgZn2 phase

[0158] 2 (Al - Zn) dendrites

[0159] 3 Ternary eutectic structure of Zn / Al / MgZn2

Claims

1. A plated steel sheet, characterized in that, having: a steel sheet, and a plating layer formed on at least a part of the surface of the steel sheet; the chemical composition of the plating layer contains, by mass%, Al：6.00～35.00％、 Mg: 3.00 to 15.00%, La + Ce: a total of 0.0001 to 0.5000%, Si: 0.10 to 1.50%, Ca: 0 to 1.00%, Fe: 0.10 to 0.70%, Sb: 0 to 0.50%, Sr：0～0.50％、 Pb: 0 to 0.50%, Sn: 0 to 1.00%, Cu: 0 to 1.00%, Ti: 0 to 1.00%, Ni: 0 to 1.00%, and Mn: 0 to 1.00%, with the balance consisting of Zn and impurities; with respect to the plating layer, on the surface, the area ratio of the lamellar structure in which (Al - Zn) phase and MgZn2 phase are arranged in layers is 10 to 95%, the lamellar spacing of the lamellar structure is 2.5 μm or less, the area ratio of (Al - Zn) dendrites is 10% or less, the area ratio of the lamellar structure in which (Al - Zn) phase and MgZn2 phase are arranged in layers is calculated as follows: obtain an SEM image of the cross-section of the plating layer and an elemental distribution image based on EDS. The (Al - Zn) phase composed of Al and Zn and the MgZn2 phase are arranged in layers. If the lamellar spacing is 4 μm or less, it is judged as a lamellar structure. Use image processing software to surround the area where the structure exists on the cross-section SEM image with a line, calculate the area of the area surrounded by the line, and calculate the area ratio of the lamellar structure.

2. The plated steel sheet according to claim 1, wherein the chemical composition of the plating layer contains, by mass%, Al：11.00～30.00％、 Mg: 5.00 to 11.00%, and La + Ce: a total of 0.0010 to 0.1000%.

3. The plated steel sheet according to claim 1 or 2, wherein with respect to the plating layer, on the surface, the area ratio of the lamellar structure is 60 to 95%.

4. The plated steel sheet according to claim 3, wherein with respect to the plating layer, on the surface, the area ratio of the lamellar structure is 80 to 95%.

Images