Hot-dip Zn-based coated steel sheet

Abstract

The hot-dip Zn-based coated steel sheet has a steel sheet and a coating layer, the coating layer being formed on at least a part of the surface of the steel sheet. The coating layer has a chemical composition consisting of the following substances by mass%: Al: 6.00 to 35.00%, Mg: 2.00 to 12.00%, Ca: 0.005 to 2.00%, Si: 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%, Mn: 0 to 1.00%, Cr: 0 to 1.00%, and the balance: Zn and impurities. In the cross-section in the thickness direction of the coating layer, the area ratio of the MgZn2 phase is 15 to 60%, and the MgZn2 phase contains a Ca-based intermetallic compound having an equivalent circle diameter of 0.10 μm or less.

Description

Technical Field

[0001] The present invention relates to a hot-dip Zn-based coated steel sheet.

[0002] This application claims priority based on Japanese Patent Application No. 2020-174453 filed on October 16, 2020, and incorporates its content herein. Background Art

[0003] In recent years, in order to protect the global environment, it has been necessary to improve the fuel economy of automobiles. Regarding the improvement of the fuel economy of automobiles, for steel sheets (automobile steel sheets) used for automobile parts, in order to ensure collision resistance performance and reduce the weight of the vehicle body, the application of high-strength steel sheets has been carried out.

[0004] However, in the case where there are concerns about a reduction in plate thickness or the formation of holes due to corrosion, sometimes, even if the strength is increased, it is not possible to reduce the thickness below a certain fixed plate thickness. One of the purposes of increasing the strength of steel sheets is weight reduction through thinning. Therefore, even if high-strength steel sheets are developed, when the corrosion resistance is low, the application parts will be limited.

[0005] Even under the current situation, from the viewpoint of improving corrosion resistance, for automobile steel sheets, coated steel sheets, especially hot-dip galvanized steel sheets, are often used. However, for high-strength steel sheets that are developing high strength, in order to avoid concerns about a reduction in plate thickness due to corrosion, the requirement for improving corrosion resistance has become even higher.

[0006] In particular, in automobile steel sheets, for coated steel sheets, chemical conversion treatment for automobiles and electrodeposition coating are also carried out. Although an improvement in corrosion resistance is sought, there are the following problems: at the cut end faces and the like, after the chemical conversion treatment and electrodeposition coating are carried out, the corrosion resistance (end face corrosion resistance) is also reduced compared to the flat part.

[0007] For example, in Patent Document 1, 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%.

[0008] However, in Patent Document 1, although it has a certain effect on improving the corrosion resistance after coating, it does not target the improvement of the end face corrosion resistance. Among them, the corrosion resistance after coating is evaluated based on the maximum film swelling width from the cross-cut wound part after a corrosion acceleration test under 60 cycles or 120 cycles. From the research results of the present inventors, it can be said that the technology of Patent Document 1 is not effective for improving the end face corrosion resistance.

[0009] In addition, Patent Document 2 discloses a chemical conversion-treated steel sheet having excellent corrosion resistance, film adhesion, and adhesiveness. It is characterized in that a hot-dip Zn-Al-Mg alloy-plated steel sheet with a ternary eutectic structure of [Al / Zn / Zn2Mg] accounting for 60% or more of the area of the outermost surface of the plating layer is used as the base material, and the plating layer surface is covered with a precipitation layer, a phosphate coating film, and a chemical conversion coating film. The precipitation layer contains at least one selected from Ni, Co, Fe, and Mn, and the total adhesion amount of Ni, Co, and Fe is in the range of 0.05 to 5.0 mg / m 2 and the adhesion amount of Mn is in the range of 0.05 to 30 mg / m 2 . The phosphate coating film is composed of phosphate crystals with an average particle size of 0.5 to 5.0 μm. In this chemical conversion coating film, oxides or hydroxides of valve metals coexist with fluorides of valve metals. The base of the phosphate crystals bites into the plating layer and stands up from the plating layer. The chemical conversion coating film is an organic resin coating film sandwiching the plating layer exposed between the phosphate crystals or an interfacial reaction layer generated at the interface with the precipitation layer. Patent Document 2 also discloses that the chemical conversion-treated steel sheet has excellent corrosion resistance after painting.

[0010] However, regarding the chemical conversion-treated steel sheet of Patent Document 2, it is a prerequisite to have a chemical conversion coating film. In a plated steel sheet without a chemical conversion coating film, it cannot be said that sufficient corrosion resistance will be obtained. In addition, a special chemical conversion coating film is required, and it is difficult to apply it to steel sheets for automotive chemical conversion treatment.

[0011] Prior Art Documents

[0012] Patent Documents

[0013] Patent Document 1: Japanese Patent Application Laid-Open No. 2015-214747

[0014] Patent Document 2: Japanese Patent No. 4579715 Summary of the Invention

[0015] Technical Problem to be Solved by the Invention

[0016] The present invention has been completed in view of the above problems. However, the steel sheet used for automotive components is processed when made into components, so sufficient workability is also required. Therefore, even if the corrosion resistance is excellent, when the workability is insufficient, the application will become difficult.

[0017] Therefore, the object of the present invention is to provide a hot-dip Zn-based plated steel sheet having workability equal to or better than that of existing automotive plated steel sheets and more excellent end face corrosion resistance than existing automotive plated steel sheets.

[0018] Technical Means for Solving the Technical Problem

[0019] The inventors of the present invention have conducted research on improving the end face corrosion resistance after electrodeposition coating in a hot-dip Zn-based coated steel sheet. As a result, it has been found that when the coating layer contains Al, Mg, and Ca and has a predetermined structure, the end face corrosion resistance after electrodeposition coating is improved.

[0020] The present invention has been completed based on the above recognition. The gist of the present invention is as follows.

[0021] [1] A hot-dip Zn-based coated steel sheet according to one aspect of the present invention has a steel sheet and a coating layer formed on at least a part of the surface of the steel sheet. The coating layer has the following chemical composition, which is composed of, in mass%, Al: 6.00 to 35.00%, Mg: 2.00 to 12.00%, Ca: 0.005 to 2.00%, Si: 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%, Mn: 0 to 1.00%, Cr: 0 to 1.00%, and the balance: Zn and impurities. And in the coating layer, in the cross-section in the thickness direction, the area ratio of the MgZn2 phase is 15 to 60%, and the MgZn2 phase contains a Ca-based intermetallic compound having an equivalent circle diameter of 0.10 μm or less.

[0022] [2] Alternatively, in the hot-dip Zn-based coated steel sheet as described in [1] above, the chemical composition of the coating layer contains, in mass%, one or more selected from the group consisting of Al: 11.00 to 30.00%, Mg: 5.00 to 10.00%, and Ca: 0.10 to 1.00%.

[0023] [3] Alternatively, in the hot-dip Zn-based coated steel sheet as described in [1] or [2] above, the number density of the Ca-based intermetallic compound contained in the MgZn2 phase is 10 pieces / μm 2 or more.

[0024] [4] Alternatively, in the hot-dip Zn-based coated steel sheet as described in any one of [1] to [3] above, an alloy layer composed of an Al-Fe-based intermetallic compound having an average thickness of 0.05 to 3.0 μm is provided between the coating layer and the steel sheet.

[0025] [5] Alternatively, in the hot-dip Zn-based coated steel sheet as described in any one of [1] to [3] above, the steel sheet has an internal oxide layer in the surface layer portion on the coating layer side.

[0026] [6] It may also be that in the hot-dip Zn-based plated steel sheet as described in [4], the surface layer portion of the steel sheet on the alloy layer side has an internal oxide layer.

[0027] Advantages of the Invention

[0028] According to the above solution of the present invention, it is possible to provide a hot-dip Zn-based plated steel sheet having sufficient workability and excellent end face corrosion resistance. Detailed Embodiments

[0029] The hot-dip Zn-based plated steel sheet of an embodiment of the present invention (hereinafter, referred to as the plated steel sheet of the present embodiment) has a steel sheet and a plating layer having a predetermined chemical composition, and the plating layer is formed on at least a part of the surface of the steel sheet.

[0030] In addition, in this plating layer, in the cross section in the thickness direction, the area ratio of the MgZn2 phase is 15 to 60%, and the MgZn2 phase contains a Ca-based intermetallic compound having an equivalent circle diameter of 0.10 μm or less.

[0031] It may also be that the plated steel sheet of the present embodiment has an alloy layer between the steel sheet and the plating layer, and the alloy layer is composed of an intermetallic compound containing Fe and Al.

[0032] Hereinafter, a detailed description will be given.

[0033] <Steel Sheet>

[0034] The plated steel sheet of the present embodiment focuses on the plating layer, and the type of the steel sheet is not particularly limited. It can be determined according to the applied product, required strength, plate thickness, etc. For example, a hot-rolled steel sheet described in JIS G3193:2008 or a cold-rolled steel sheet described in JIS G3141:2017 can be used.

[0035] Preferably, the steel sheet has an internal oxide layer in the surface layer portion on the plating layer side (the interface side between the steel sheet and the plating layer) (in the case where an alloy layer described later is formed between the steel sheet and the plating layer, in the surface layer portion on the alloy layer side (the interface side between the steel sheet and the alloy layer)).

[0036] The internal oxide layer is formed by annealing the steel sheet before plating in a predetermined atmosphere. Since there is an internal oxide layer in the steel sheet, an effect of promoting the formation of the MgZn2 phase in which the Ca-based intermetallic compound is dispersed will be obtained. In the case where this effect is to be obtained, the thickness of the internal oxide layer is preferably 0.1 to 8.0 μm.

[0037] [Alloy Layer]

[0038] Alternatively, an alloy layer may be formed between the steel sheet and the coating layer in the present embodiment. By forming the alloy layer, the adhesion between the steel sheet and the coating layer is improved, so it is preferred. In order to obtain the above effects, preferably, the average thickness of the alloy layer is 0.05 to 3.0 μm.

[0039] The alloy layer is composed of an Al-Fe-based intermetallic compound (for example, an Al-Fe alloy, or an Al-Fe-Si alloy when Si is included in the coating layer). The alloy layer is mainly an intermetallic compound mainly composed of Al and Fe formed by the reaction of Al and Fe in a coating layer through heat treatment. In the present embodiment, a layer located between the steel sheet and the coating layer and containing 30% or more of Al and 30% or more of Fe is defined as the alloy layer. Sometimes, in this alloy layer, Si and Zn contained in the coating layer are each contained in an amount of 20% or less.

[0040] The presence or absence and thickness of the alloy layer are obtained by the following method: According to the element distribution image obtained by EDS measurement, the thickness of the Al-Fe-based intermetallic compound is measured.

[0041] <Coating layer>

[0042] In the plated steel sheet of the present embodiment, a coating layer is provided on at least a part of the surface of the steel sheet. The coating layer may 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 the present 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 thereof as the upper and lower limits.

[0046] Al: 6.00 to 35.00%

[0047] Al is an element effective for ensuring the end face corrosion resistance in a coating layer containing aluminum (Al), zinc (Zn), and magnesium (Mg). In addition, Al is also an element that helps to form an alloy layer (Al-Fe alloy layer) and is effective for improving the 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 in the MgZn2 phase, Ca-based intermetallic compounds are not formed, and the end face corrosion resistance of the coating layer decreases. Therefore, the Al content is set to 35.00% or less. The Al content is preferably 30.00% or less.

[0049] Mg: 2.00 - 12.00%

[0050] Mg is an element that has the effect of improving the end face corrosion resistance of the coating layer. In order to fully obtain the above effect, the Mg content is set to 2.00% or more.

[0051] On the other hand, when the Mg content exceeds 12.00%, not only does the end face corrosion resistance decrease, but also the workability of the coating layer decreases. In addition, manufacturing problems such as an increase in the amount of scum generated in the plating bath occur. Therefore, the Mg content is set to 12.00% or less. The Mg content is preferably 11.00% or less, and more preferably 10.00% or less.

[0052] Ca: 0.005 - 2.00%

[0053] Ca is an element required to form Ca-based intermetallic compounds. In the plated steel sheet of the present embodiment, in order to form Ca-based intermetallic compounds in the MgZn2 phase, the Ca content is set to 0.005% or more. The Ca content is preferably 0.01% or more, and more preferably 0.10% or more. In addition, Ca is also an element that, as the Mg content increases, reduces the amount of scum that is easily formed during the plating operation and helps improve the plating manufacturability.

[0054] On the other hand, when the Ca content exceeds 2.00%, Ca crystallizes as coarse intermetallic compounds, and the workability decreases. Therefore, the Ca content is set to 2.00% or less. The Ca content is preferably 1.00% or less.

[0055] Si: 0 - 2.00%

[0056] Si is an element that forms a compound with Mg, thereby helping to improve the end face corrosion resistance. In addition, Si is also an element that has the following effect: when forming the coating layer on the steel sheet, it inhibits the case where the alloy layer formed between the steel sheet and the coating layer becomes too thick and improves the adhesion between the steel sheet and the coating layer. Therefore, it may also be contained. In order to obtain the above effect, it is preferable to set the Si content to 0.10% or more. Regarding the Si content, more preferably, it is 0.20% or more.

[0057] On the other hand, when the Si content exceeds 2.00%, in the coating, the excess Si will crystallize, and the corrosion resistance of the end face will decrease or the processability of the coating will decrease. Therefore, the Si content is set to 2.00% or less. The Si content is more preferably 1.50% or less. It is not necessarily required to contain Si, and the lower limit is 0%.

[0058] Fe: 0 - 2.00%

[0059] When manufacturing the coating, Fe will be mixed into the coating as an impurity. Sometimes it will be contained up to about 2.00%, but when in this range, the adverse effect on the properties of the plated steel sheet of the present embodiment is small. Therefore, it is preferable to set the Fe content to 2.00% or less. Regarding the Fe content, more preferably, it is 1.50% or less, and further preferably, it is 1.00% or less.

[0060] The Fe content can also be 0%, but it is not easy to set the Fe content to 0%, so the Fe content can also be set to 0.10% or more.

[0061] 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, and more preferably, it is 3.0% or less.

[0062] However, in the coating of the plated steel sheet of the present embodiment, a part of Zn can be further replaced, for example, Sb, Sr, Pb, Sn, Cu, Ti, Ni, Mn, Cr are included within the following ranges. Since it is not necessarily required to contain these elements, the lower limit of the content is 0%. In addition, even if these elements are contained at the impurity level, they will not have a substantial impact on the properties of the coating.

[0063] Sb: 0 - 0.50%

[0064] Sr: 0 - 0.50%

[0065] Pb: 0 - 0.50%

[0066] When Sr, Sb, Pb are contained in the coating, the appearance of the coating will change, and zinc flowers will be formed, so that an increase in metallic luster can be confirmed. Therefore, it can also be made to contain one or more of Sr, Sb, Pb. When the above effect is to be obtained, it is preferable to set the content of one or more of Sr, Sb, Pb to 0.001% or more, or 0.01% or more.

[0067] On the other hand, when the content of these elements exceeds 0.50%, various intermetallic compounds are formed, and workability and corrosion resistance are deteriorated. In addition, when the content of these elements is excessive, the viscosity of the plating bath increases, and the bath building itself becomes difficult in many cases, and a plated steel sheet with good plating properties cannot be produced. Therefore, even if they are contained, the Sr content is set to 0.50% or less, the Sb content is set to 0.50% or less, and / or the Pb content is set to 0.50% or less.

[0068] Sn: 0~1.00%

[0069] Sn is an element that increases the Mg dissolution rate in a plating layer containing Zn, Al, and Mg. When the Mg dissolution rate increases, sacrificial corrosion resistance is improved and corrosion resistance is improved. Therefore, Sn may also be contained.

[0070] On the other hand, when the Mg dissolution rate is excessive, the corrosion resistance is reduced. Therefore, even when Sn is contained, the Sn content is set to 1.00% or less.

[0071] Cu: 0~1.00%

[0072] Ti: 0~1.00%

[0073] Ni: 0~1.00%

[0074] Mn: 0~1.00%

[0075] Cr: 0~1.00%

[0076] These elements contribute to improving corrosion resistance. Therefore, they may be contained. In order to obtain the above-mentioned effects, it is preferable that the content of one or more of these elements is set to 0.001% or more or 0.01% or more.

[0077] On the other hand, when the content of these elements becomes excessive, the viscosity of the coating bath increases, and the preparation of the coating bath itself often becomes difficult, and a plated steel sheet with good coating properties cannot be produced. Therefore, even when these elements are contained, the content of each element is set to 1.00% or less.

[0078] The chemical composition of the coating was measured by the following method.

[0079] First, an acid solution is obtained by stripping and dissolving the coating with an acid containing an inhibitor, where the inhibitor inhibits the corrosion of the steel substrate (steel). Then, the chemical composition of the coating can be obtained by measuring the resulting acid solution through ICP analysis (in the case where an alloy layer is formed between the coating and the steel plate, it will be the combined chemical composition of the coating and the alloy layer, but since the alloy layer is thin, the influence is small). Regarding the type of acid, there is no particular limitation as long as it can dissolve the coating. Regarding the chemical composition, it is measured as the average chemical composition.

[0080] [Structure]

[0081] In the cross-section in the thickness direction, the area ratio of the MgZn2 phase is 15 - 60%, and the MgZn2 phase contains Ca-based intermetallic compounds with an equivalent circle diameter of 0.10 μm or less.

[0082] As described above, the inventors of the present invention have studied the improvement of the end face corrosion resistance after electrodeposition coating in a hot-dip Zn-based coated steel sheet having a coating containing Al, Mg, and Ca. As a result, it has been found that when the coating has a predetermined structure, the end face corrosion resistance after electrodeposition coating is improved.

[0083] Specifically, it has been found that when the MgZn2 phase in which Ca-based intermetallic compounds are dispersed has an area ratio of 15 - 60% in the cross-section in the thickness direction of the coating, the end face corrosion resistance is improved.

[0084] Therefore, in the coated steel sheet of the present embodiment, in the cross-section of the coating, the area ratio of the MgZn2 phase is set to 15 - 60%, and the MgZn2 phase contains Ca-based intermetallic compounds with an equivalent circle diameter of 0.10 μm or less.

[0085] Here, in the present embodiment, the MgZn2 phase containing Ca-based intermetallic compounds with an equivalent circle diameter of 0.10 μm or less means that in the MgZn2 phase, there is 1 Ca-based intermetallic compound per μm 2 Above.

[0086] In addition, the Ca-based intermetallic compound refers to a compound containing 8 - 15 at% of Al, 8 - 15 at% of Ca, 70 - 84 at% of Zn, and 0 - 5 at% of Si and having a trigonal crystal structure.

[0087] The reason for improving the end face corrosion resistance by the MgZn2 phase containing Ca-based intermetallic compounds is unclear, but it is considered that the reason is that through the fine Ca-based intermetallic compounds, the dissolution of Mg from the MgZn2 phase as the matrix phase is promoted, and the sacrificial corrosion resistance is improved.

[0088] Even when the area ratio of the MgZn2 phase is less than 15% or the area ratio of the MgZn2 phase is 15% or more, if the Ca-based intermetallic compound with a size of 0.10 μm or less is not included in the MgZn2 phase, sufficient effects cannot be obtained.

[0089] On the other hand, when the area ratio of the MgZn2 phase containing the Ca-based intermetallic compound with an equivalent circle diameter of 0.10 μm or less exceeds 60%, the workability will be reduced, so it is not preferred.

[0090] The number density of the Ca-based intermetallic compound contained in the MgZn2 phase is preferably 5 per μm 2 or more, and more preferably, it is 10 per μm 2 or more.

[0091] Since the Ca-based intermetallic compound is preferably fine, it is preferred that the MgZn2 phase contains 1 per μm 2 or more of the Ca-based intermetallic compound with an equivalent circle diameter of 0.07 μm or less.

[0092] In the coating layer of the coated steel sheet of the present embodiment, it is not limited to other than the MgZn2 phase, but for example, it may also be a (Al-Zn) phase composed of Al and Zn, a Zn / Al / MgZn2 ternary eutectic structure, an Mg2Si phase, and / or other intermetallic compounds.

[0093] From the viewpoint of workability, it is preferred that, in terms of area ratio, the (Al-Zn) phase is 30 to 70%, the Mg2Si phase is 8.0% or less, and other intermetallic compounds are 10.0% or less. In addition, it is preferred that the equivalent circle diameter of other intermetallic compounds is 5 μm or less.

[0094] The area ratio of each phase of the cross-section of the coating layer is obtained by the following method.

[0095] From the coated steel sheet, a sample with a size of 25 mm in the direction perpendicular to the rolling direction and 15 mm in the rolling direction is extracted, and it is embedded in resin so that the thickness direction of the coating layer of the sample becomes the observation surface, and then polished to obtain a cross-section SEM image of the coating layer and an element distribution image based on EDS. Based on this SEM image and element distribution image, the area ratios of the MgZn2 phase, Zn / Al / MgZn2 ternary eutectic structure, (Al-Zn) dendrite, and other intermetallic compounds in the coating layer are measured. In the present embodiment, one field of view (180 μm × 150 μm) is photographed from each of 5 different samples, and a total of 5 fields of view (magnification 1500 times) of the cross-section EDS mapping image of the coating layer are photographed, and for each phase, the value obtained by averaging the area ratios obtained from the 5 fields of view is used as the area ratio of each phase.

[0096] In addition, the number density of the Ca-based intermetallic compound is determined by the following method.

[0097] A thin specimen for TEM observation is prepared from the coating layer of the plated steel sheet. For the MgZn2 phase contained in the coating cross-section, a TEM-EDS mapping image is obtained. The positions where Ca exists are judged as Ca-based intermetallic compounds, and the number of Ca-based intermetallic compounds with an equivalent circle diameter of 0.10 μm or less or 0.07 μm or less contained in the visual field is counted, and the number density of Ca-based intermetallic compounds of each size is calculated based on the measurement area. However, considering the measurement accuracy, intermetallic compounds of 0.001 μm or more are taken as the objects to be counted.

[0098] <Manufacturing method>

[0099] Next, 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.

[0100] The steel sheet of the present embodiment can be manufactured by a manufacturing method including the following steps (I) to (III).

[0101] (I) Plating step, in which the steel sheet is immersed in a plating bath containing Al, Mg, and Zn,

[0102] (II) Cooling step, in which the steel sheet (plated steel sheet) immersed in the plating bath is cooled at an average cooling rate of 15 °C / second or more until the temperature reaches the plating bath temperature to 20 °C.

[0103] (III) Post-heat treatment step, in which the plated steel sheet after the cooling step is heated to a temperature range of 100 to 220 °C.

[0104] The steel sheet provided to the plating step is not particularly limited, and it can be a steel sheet (hot-rolled steel sheet or cold-rolled steel sheet) obtained by a known method.

[0105] Alternatively, before the plating process, the steel sheet may be annealed. In the case of annealing, the annealing conditions may be well-known conditions. Examples include 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. When forming an internal oxide layer on the steel sheet, preferably, in the above atmosphere, the annealing temperature is set to 800 - 870°C and the annealing time is set to 60 - 130 seconds. When the annealing temperature is less than 800°C, the internal oxide layer will not be formed sufficiently. When it exceeds 870°C, it will be difficult to control the internal oxide layer to the desired thickness. When the annealing time is less than 60 seconds, there is a concern that the thickness of the internal oxide layer cannot be maintained sufficiently. When it exceeds 130 seconds, there is a concern that the internal oxide layer will become too thick, exceeding 8.0μm.

[0106] [Plating process]

[0107] In the plating process, the steel sheet is immersed in a plating bath to form a plating layer. Alternatively, in the case of annealing before the plating process, during the cooling process after annealing, the steel sheet is immersed in the plating bath.

[0108] The composition of the plating bath will be roughly the same as the composition of the plating layer to be formed. Therefore, the plating bath can be adjusted according to the composition of the plating layer to be formed.

[0109] [Cooling process]

[0110] In the cooling process, for the steel sheet (steel sheet with a plating layer on the surface) immersed in the plating bath, after adjusting the plating adhesion amount with a purge gas such as N2, it is cooled at an average cooling rate of 15°C / second or more from the plating bath temperature to 20°C.

[0111] Through this cooling, Ca is dissolved into the MgZn2 phase that crystallizes during solidification. Here, the dissolved Ca will precipitate during the post-heat treatment described later.

[0112] When the average cooling rate from the plating bath temperature to 20°C is less than 15°C / second, the dissolution of Ca into the MgZn2 phase is insufficient, and even if post-heat treatment is performed, the predetermined Ca-based intermetallic compound will not be obtained.

[0113] There is no need to limit the upper limit of the average cooling rate, but it can also be set to 60°C / second or less.

[0114] In addition, when precipitating finer Ca-based intermetallic compounds, specifically those with an equivalent circle diameter of 0.07 μm or less, it is preferable that, on the basis of setting the average cooling rate from the plating bath temperature to 20 °C to 15 °C / second or more, the average cooling rate from 270 °C to 20 °C is set to 30 °C / second or more. By setting the average cooling rate from 270 °C to 20 °C to 30 °C / second or more, the MgZn2 phase becomes finer, and the solid solution of Ca into the MgZn2 phase becomes sufficient. Therefore, the Ca-based intermetallic compounds precipitated during the subsequent post-heat treatment become finer.

[0115] In the plating structure, when the area ratio of the (Al-Zn) phase is 30 to 70%, it is preferable that the average cooling rate from the bath temperature to 300 °C is set to 20 to 40 °C / second.

[0116] [Post-heat treatment process]

[0117] In the post-heat treatment process, the plated steel sheet after the cooling process is heated (post-heat treatment) to a temperature range of 100 to 220 °C. Due to this post-heat treatment, Ca dissolved in the MgZn2 phase precipitates as intermetallic compounds finely into the MgZn2 phase. As a result, the MgZn2 phase contains Ca-based intermetallic compounds with an equivalent circle diameter of 0.10 μm or less.

[0118] When post-heat treatment is not performed, or the post-heat treatment temperature (heating temperature) is less than 100 °C, Ca-based intermetallic compounds do not precipitate. On the other hand, when the post-heat treatment temperature exceeds 220 °C, the temperature is too high, so there is not enough driving force for nucleation, and Ca-based intermetallic compounds do not precipitate.

[0119] When effectively precipitating Ca-based intermetallic compounds to further improve the end face corrosion resistance, the post-heat treatment temperature is preferably 150 °C or less. The reason for this is not clear in detail, but it is considered that when it is 150 °C or less, there is enough driving force for the precipitation of Ca-based intermetallic compounds, and the precipitated Ca-based intermetallic compounds are refined.

[0120] After heating to 100 to 220 °C, the holding time in this temperature range is not limited, but when the Ca-based intermetallic compounds are sufficiently precipitated, it is preferably 30 seconds or more. In addition, when the holding time exceeds 10 minutes, the productivity decreases, so it is preferable that the holding time is set to 10 minutes or less.

[0121] The above post-heat treatment process is carried out within 48 hours after the cooling process is completed. The reason is that when the time from the cooling process to the post-heat treatment is too long, the Ca in the coating layer becomes stabilized, and it becomes difficult to precipitate into the MgZn2 phase through post-heat treatment.

[0122] According to the above manufacturing method, the plated steel sheet of the present embodiment is obtained.

[0123] Example

[0124] As the steel sheet for plating, a cold-rolled steel sheet (0.2% C - 2.0% Si - 2.3% Mn) with a thickness of 0.8 mm was prepared.

[0125] 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.

[0126] 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 composed of N2 gas, with a dew point of 0°C, annealing was carried out at 860°C for 120 seconds.

[0127] 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. The plating bath composition and the composition of the formed coating are shown in Table 1.

[0128] For the plated base plate formed with a coating, it was cooled to 20°C or less under the conditions shown in Table 1, and post-heat treatment was carried out to obtain a plated steel sheet (hot-dip Zn-based plated steel sheet). The holding time of the post-heat treatment was set to 100 seconds. The time from the completion of the cooling process to the start of the post-heat treatment is shown in Table 1.

[0129] [Table 1]

[0130]

[0131] For the obtained plated steel sheet, by the above method, the area ratio of each phase in the plating phase, the number density of Ca-based intermetallic compounds with an equivalent circle diameter of 0.10 μm or less and Ca-based intermetallic compounds with an equivalent circle diameter of 0.07 μm or less in the MgZn2 phase, the thickness of the alloy layer, and the thickness of the internal oxide layer were measured.

[0132] In addition, for the obtained plated steel sheet, the end face corrosion resistance and workability were evaluated by the following method.

[0133] [End face corrosion resistance]

[0134] A 50×100 mm sample was extracted from the plated steel sheet and processed according to zinc phosphate treatment (SD5350 system: standard of Nippon Paint Industrial Coding Co., Ltd.). Then, electrodeposition coating (PN110 Powernix Grey: standard of Nippon Paint Industrial Coding Co., Ltd.) was applied to a thickness of 20 μm, and sintering was carried out at a sintering temperature of 150 °C for 20 minutes. The coated plated steel sheet (the plated steel sheet subjected to electrodeposition coating) was subjected to a combined cyclic corrosion test in accordance with JASO (M609-91). The maximum expansion widths at three locations from the end face of the sample were measured, and the average value was obtained to evaluate the corrosion resistance after coating.

[0135] In the case where the number of cycles of the above JASO (M609-91) is 150 cycles and the film expansion width from the end face is less than 1.0 mm, it is denoted as "AAA". In the case where it is less than 1.5 mm, it is denoted as "AA". In the case where it is 1.5 - 2.5 mm, it is denoted as "A". In the case where the film expansion width exceeds 2.5 mm, it is denoted as "B".

[0136] [Workability]

[0137] Regarding the workability of the coating, evaluation was carried out in terms of chalking resistance.

[0138] The plated steel sheet was cut into 40 mm (C) × 100 mm (L) × 0.8 mm (t). After bending it by 60° with a 5R bend using a V-bending testing machine manufactured by Discharge Precision Machining Research Institute Co., Ltd. with the C direction as the bending axis direction, evaluation was carried out based on the average value of five points of the peeling width of the coating due to tape peeling.

[0139] Specifically, the case where no peeling occurs is denoted as "AA", the case where the average peeling width is 0.1 - 0.5 mm is denoted as "A", and the case where the average peeling width exceeds 0.5 mm is denoted as "B".

[0140] The results are shown in Table 2.

[0141] [Table 2]

[0142]

[0143] From the results of Table 1 and Table 2, it can be seen that in Examples Nos. 3, 4, 6 - 11, 13 - 16, 21, 22, 25 - 33 of the present invention, the chemical composition and the area ratio of the MgZn2 phase in the cross-section in the thickness direction are within the scope of the present invention, and the MgZn2 phase contains a Ca-based intermetallic compound. Therefore, the end face corrosion resistance and workability are excellent.

[0144] On the other hand, in Nos. 1, 2, 5, 12, 17 to 20, 23, 24, and 34 as comparative examples, one or more of the chemical composition, the area ratio of the MgZn2 phase in the cross-section in the thickness direction, and the number density of the Ca-based intermetallic compound in the MgZn2 phase are outside the scope of the present invention. As a result, both the end face corrosion resistance and the workability are poor.

Claims

1. A hot-dip Zn-based coated steel sheet having: a steel sheet, and a coating layer formed on at least a part of the surface of the steel sheet; the coating layer has the following chemical composition, which is by mass%, Al：6.00～35.00％、 Mg: 2.00 to 10.00%, Ca: 0.03 to 0.50%, Si: 0 to 1.00%, Fe: 0 to 0.30%, 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%, Mn: 0 to 1.00%, Cr: 0 to 1.00%, and the balance: composed of Zn and impurities; and in the coating layer, in the cross-section in the thickness direction, the area ratio of the MgZn2 phase is 15 to 60%; the MgZn2 phase contains a Ca-based intermetallic compound having an equivalent circle diameter of 0.10 μm or less, The so-called MgZn2 phase contains Ca-based intermetallic compounds with an equivalent circle diameter of 0.10 μm or less, indicating that in the MgZn2 phase, there is 1 Ca-based intermetallic compound per μm 2 Above, the so-called Ca-based intermetallic compound refers to a compound containing 8 - 15 at% of Al, 8 - 15 at% of Ca, 70 - 84 at% of Zn, 0 - 5 at% of Si, and having a trigonal crystal structure.

2. The hot-dip Zn-based coated steel sheet according to claim 1, wherein the chemical composition of the coating layer contains, by mass%, Al：11.00～30.00％、 Mg: 5.00 to 10.00%, Ca: 0.10 to 0.50%.

3. The hot-dip Zn-based coated steel sheet according to claim 1 or 2, wherein The number density of the Ca-based intermetallic compound contained in the MgZn2 phase is 10 / μm 2 or more.

4. The hot-dip Zn-based coated steel sheet according to claim 1 or 2, wherein between the coating layer and the steel sheet, there is an alloy layer composed of an Al-Fe-based intermetallic compound having an average thickness of 0.05 to 3.0 μm.

5. The hot-dip Zn-based coated steel sheet according to claim 1 or 2, wherein the surface layer portion of the steel sheet on the coating layer side has an internal oxide layer.

6. The hot-dip Zn-based coated steel sheet according to claim 4, wherein the surface layer portion of the steel sheet on the alloy layer side has an internal oxide layer.

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