Title of invention: Hot-dip galvanized steel sheet with excellent corrosion resistance after painting
Technical field
[0001]
　The present invention relates to a hot-dip Zn-plated steel sheet having excellent corrosion resistance after painting.
Background technology
[0002]
　In recent years, plated steel sheets have been used for automobile structural members from the viewpoint of rust prevention, and alloyed hot dip galvanized steel sheets have been mainly applied in the domestic market. The galvannealed steel sheet is a plating with improved weldability and corrosion resistance after painting by diffusing Fe from the steel sheet (base steel sheet) in the plating layer after hot-dip galvanizing the steel sheet. It is a steel plate. For example, the plated steel sheet shown in Patent Document 1 is typically used in Japan as a plated steel sheet for automobiles.
[0003]
　Usually, plated steel sheets for automobiles are used in a state of being formed into a complicated shape from a plate shape, and therefore, in many cases, they are used for press forming. In the case of alloyed hot-dip galvanized steel sheet, the plating layer becomes hard due to the diffusion of Fe from the base steel sheet, so the plating layer is easily peeled off, which is not seen in hot-dip galvanized steel sheet where the plating layer is soft such as powdering and flaking. There are also specific problems that cannot be solved.
[0004]
　Further, in a plated steel sheet having a hard plated layer, the plated layer is easily damaged by external pressure, and once cracks propagate to the interface with the base steel sheet, the plated layer peels off from the interface and falls off. The problem is that it is easy. For example, when an alloyed zinc-plated steel sheet is used for the outer panel of an automobile, the coating and the plating layer are peeled off at the same time due to the collision (chipping) of pebbles caused by the hops of a traveling vehicle, and the base steel sheet is likely to be exposed. Corrosion may be more severe than that of a plated steel sheet having a soft plating layer. Furthermore, from the viewpoint of rust prevention, the alloyed hot-dip galvanized steel sheet contains Fe in the plating layer, so when such chipping occurs, corrosion of the plating layer readily causes reddish brown rust to occur immediately. It causes a problem in the appearance of the car.
[0005]
　As a solution to these problems, it is effective to apply a plated steel sheet whose plating layer has toughness and does not contain Fe. For example, hot-dip galvanized steel sheets are mainly used in North America, Europe, and the like as automobile-plated steel sheets that do not contain Fe in the plating layer. However, the hot-dip galvanized steel sheet that has not been alloyed does not cause chipping, and does not contain Fe in the plating layer like the alloyed hot-dip galvanized steel sheet, so that red rust at the initial stage of corrosion does not occur, but it is in the coated state. However, since the plated layer easily corrodes under the coating film and the coating film rises (swells), it is by no means suitable as a plated steel sheet for automobiles.
[0006]
　As a method of making the plating highly corrosion resistant, addition of Al to Zn can be mentioned. In the construction material field, a hot-dip Al-Zn based plated steel sheet has been widely put into practical use as a highly corrosion resistant plated steel sheet. Such a plated layer of the molten Al-Zn-based plating has a dendrite-like α-(Zn, Al) phase that is first crystallized from the molten state (Al primary crystal part: in the Al-Zn binary phase diagram, etc. Α-(Zn, Al) phase that does not necessarily crystallize as an Al-rich phase, but crystallizes as a solid solution of Zn and Al. (Zn/Al mixed phase structure). Since the Al primary crystal part is passivated and the Zn/Al mixed phase structure has a higher Zn concentration than the Al primary crystal part, the corrosion concentrates on the Zn/Al mixed phase structure. As a result, the corrosion progresses in a Zn/Al multiphase structure in a worm-eating manner, and the corrosion progression path becomes complicated, so that the corrosion easily reaches the base steel sheet. As a result, the hot dip Al-Zn system plated steel sheet has excellent corrosion resistance as compared with the hot dip galvanized steel sheet having the same plating layer thickness.
[0007]
　When such a hot-dip Al—Zn-based plated steel sheet is used as an automobile outer panel, the plated steel sheet is provided to an automobile manufacturer or the like in a state of being subjected to plating in a continuous hot-dip plating facility, and after being processed into a panel part shape there. In general, chemical conversion treatment, and electrodeposition coating, intermediate coating, and overall coating for automobiles are generally applied. However, when the coating film is damaged, the outer panel using the hot dip Al—Zn system plated steel sheet is caused by the unique plating phase structure composed of the two phases of the above-mentioned Al primary crystal part and Zn/Al mixed phase structure. Then, preferential dissolution of Zn (selective corrosion of Zn/Al mixed phase structure) occurs from the scratched portion at the coating/plating interface. It is known that there is a problem that sufficient corrosion resistance (corrosion resistance after coating) cannot be ensured as a result of a large swelling of the coating film as it progresses deep into the healthy part of the coating.
[0008]
　For the purpose of improving the corrosion resistance, addition of Mg to Al-Zn system plating is also under study. For example, in Patent Document 2 and Patent Document 3, Mg is added to the plating composition to form a Zn/Al/MgZn 2 ternary eutectic structure containing a Mg compound such as MgZn 2 in the plating layer to improve corrosion resistance. Dissolved hot-dip Zn-Al-Mg-based plated steel sheet is disclosed. However, it is presumed that the Al primary-crystal part having a passivation film is still formed on the hot-dip Al—Zn-based plated steel sheet disclosed in Patent Document 2, and after coating, the coating film was damaged. It is considered that the problem of corrosion resistance (corrosion resistance after painting) has not been solved.
[0009]
　Further, Patent Document 4 discloses a hot-dip Al—Zn-based plated steel sheet in which Bi is added to destroy the passivation of the primary crystal part of Al to improve the corrosion resistance after coating, but the specified production is required. It is estimated that the Al primary crystal part contained in the plating layer formed by the process still has a noble potential as compared with the surrounding Zn/Al/MgZn 2 ternary eutectic structure, and its corrosion resistance after coating has an It is considered that it is not satisfactory as a galvanized steel sheet for use. Furthermore, the addition of Bi may lead to deterioration in chemical conversion treatability and increase in manufacturing cost.
[0010]
　Therefore, conventionally, a hot-dip Zn-plated steel sheet excellent in corrosion resistance after coating has not been developed, and a hot-dip Zn-plated steel sheet particularly suitable for automobile applications has not been present.
Prior art documents
Patent literature
[0011]
Patent Document 1: Japanese Patent Laid-Open No. 2003-253416
Patent Document 2: International Publication No. 00/71773
Patent Document 3: Japanese Patent Laid-Open No. 2001-329383
Patent Document 4: Japanese Patent Laid-Open No. 2015-214749
Summary of the invention
Problems to be Solved by the Invention
[0012]
　The problem to be solved by the present invention is to provide a hot-dip Zn-plated steel sheet having excellent corrosion resistance after coating.
Means for solving the problem
[0013]
　The inventors of the present invention have studied the application of plated steel sheets to automobiles, and as a result of diligently examining a plated layer having excellent corrosion resistance after coating, a lamellar structure in which a layered Zn phase and a layered Al phase are alternately aligned in the plated layer (hereinafter, It was found that the swelling of the coating film in the coated state is suppressed when the total value of the area fraction is 5% or more.
[0014]
　The structure I is a structure that cannot be obtained by a normal method of hot dip plating, and the higher the area fraction of the structure I in the plated layer is, the more the corrosion resistance after coating of the plated layer is improved.
[0015]
　Based on the above findings, the inventors have found means capable of providing a hot-dip Zn-plated steel sheet in which undercoat corrosion in a coated state is suppressed, particularly a hot-dip Zn-plated steel sheet for automobiles.
　The features of the present invention are as follows.
　(1) 10-40% by mass of Al, 0.05-4% by mass of Si and 0.5-4% by mass of Mg are contained on at least a part of the surface of the steel sheet, and the balance is Zn and inevitable impurities. The plating layer has a plating layer, and the
　plating layer contains a lamella structure in which a layered Zn phase and a layered Al phase are alternately aligned in a plating layer cross section in an area fraction of 5% or more, and Fe, Mn, Ti, Sn, In, A hot-dip Zn-plated steel sheet, characterized in that the total proportion of intermetallic compounds containing any one or more of Bi, Pb and B is regulated to 3% or less in terms of area fraction.
　(2) The hot-dip Zn-based plating of the present invention, wherein the plating layer contains 10 to 30 mass% of Al, 0.05 to 2.5 mass% of Si, and 2 to 4 mass% of Mg. Steel plate.
　(3) The hot-dip Zn-plated steel sheet according to the present invention, wherein the plating layer contains the lamella structure in an area fraction of 20 to 80%.
　(4) The hot-dip Zn-plated steel sheet according to the present invention, wherein the plated layer contains the lamella structure in an area ratio of 40 to 50%.
　(5) The plating layer is characterized by containing a Zn/Al/MgZn 2 ternary eutectic structure composed of a Zn phase, an Al phase, and a MgZn 2 phase in an area fraction of 20 to 90%. The hot-dip Zn-plated steel sheet of the present invention.
　(6) The hot-dip Zn-plated steel sheet according to the present invention, which has an interface alloy layer made of an Al—Fe intermetallic compound having a thickness of 100 nm to 2 μm at the interface between the plating layer and the steel sheet.
[0016]
　Since the hot-dip Zn-plated steel sheet of the present invention is excellent in corrosion resistance after coating and is also excellent in chipping resistance, it is possible to contribute to industrial development by achieving a long life of the coated galvanized steel sheet.
Brief description of the drawings
[0017]
[FIG. 1] A BSE image obtained by photographing the plating layer of a Zn—Al—Mg-based plated steel sheet obtained by cooling the temperature range of 275 to 180° C. for 200 seconds after immersion in a plating bath at a magnification of 2000 (Example 20). Indicates.
FIG. 2 shows a BSE image (Example 20) obtained by photographing a region I in FIG. 1 at 10000 times.
FIG. 3 shows a BSE image (Example 20) obtained by photographing the tissue I in FIG. 2 at a magnification of 30,000.
FIG. 4 shows a BSE image (Comparative Example 19) of a plating layer of a Zn-based plated steel sheet obtained by immersing in a plating bath and then cooling to room temperature at a cooling rate of 10° C./second.
FIG. 5 shows a BSE image (Comparative Example 19) obtained by photographing Region II in FIG. 4 at 10000 times.
MODE FOR CARRYING OUT THE INVENTION
[0018]
　Hereinafter, the details of the hot-dip Zn-based plated steel sheet having excellent corrosion resistance after coating of the present invention will be described.
[0019]
　First, in the field of development of plated steel sheets, mass% representation is usually used to define the composition of the plated layer. In the present invention as well, in accordance with this rule, unless otherwise specified,% display means mass% display.
[0020]
　The hot-dip Zn-plated steel sheet of the present invention contains Zn, Al, Mg and Si as essential constituent elements of the plating layer.
[0021]
　Al is an essential element for improving the corrosion resistance after coating of the plating layer and further improving the chipping resistance. Although the details of the structure I will be described later, the higher the ratio of the structure I formed inside the Al primary crystal part, the higher the corrosion resistance after coating and the better the chipping resistance. Since the minimum Al concentration required to form the tissue I is 10%, the lower limit of the Al concentration is set to 10%.
　Further, if the Al concentration exceeds 40%, the structure I cannot be formed, so the upper limit of the Al concentration is set to 40%. Considering the formation of the tissue I, the more preferable Al concentration is 10 to 30%. Further, from the viewpoint of operation, it is desirable that the melting point of the plating layer is low and the plating bath temperature is low. The preferable plating bath temperature is lower than 480° C., and the Al concentration in this case is 10 to 20%. Further, when pressing a steel sheet for automobiles, if the melting point of the plating layer is low, the metal in the plating layer may be seized in the press die, but if the Al composition is 10% or more, it will be more difficult than hot-dip Zn plating. Also, since the melting point of the plating layer becomes high, the seizure resistance is improved. Since the melting point of the plating layer increases as the Al composition increases, the seizure resistance improves as the Al composition increases.
[0022]
　Mg is also an essential element for imparting corrosion resistance after coating to the plated layer. When Mg is added to the plating layer, it exists as MgZn 2 and Mg 2 Si which are intermetallic compounds . When it exists as MgZn 2 , most of it exists in the plating layer as a Zn/Al/MgZn 2 ternary eutectic structure.
　Under the corrosive environment, such Mg-based intermetallic compound is eluted as Mg ions in the corrosive environment. Mg ions form a Zn-based corrosion product as an insulating film and rust as a barrier film, thereby suppressing the entry of a corrosion factor into the plating layer or under the coating film, thereby contributing to the improvement of corrosion resistance. Since the minimum Mg concentration necessary for imparting excellent corrosion resistance after coating for plating is 0.5%, the lower limit of the Mg concentration is set to 0.5%. In order to obtain more excellent corrosion resistance after coating, the Mg concentration is preferably 2% or more. On the other hand, when the Mg concentration exceeds 4%, the formation of the structure I described later is hindered and the structure I having an area fraction of 5% or more cannot be formed. Therefore, the upper limit value is set to 4%.
[0023]
　Next, Si contained in the plating layer will be described. In the present invention, Si is an essential constituent element of the plating layer. When Si is contained in the plating bath, it suppresses the reactivity between Zn and Al contained in the plating bath and the Fe element in the plating original plate (base iron). That is, Si is an interfacial alloy layer (especially Al-Zn-Fe compound) made of an Al-Fe-based intermetallic compound that affects the adhesion and workability of the plating layer by controlling the reactivity between the plating layer and the base iron. ) Is an essential element for controlling the formation behavior.
　The minimum addition concentration required to suppress this interface alloy layer is 0.05%. If it is less than 0.05%, the interface alloy layer grows immediately after immersion, and ductility to the plating layer is no longer possible. Further, since the base iron and the plating layer are alloyed with each other, an Fe-Zn intermetallic compound or an Al-Fe intermetallic compound is formed in the plating layer, and the structure I is not sufficiently formed. It also causes deterioration of corrosion resistance. On the other hand, when the Si concentration exceeds 4%, a noble Si phase having a potential remains in the plating layer and acts as a cathode portion in corrosion, resulting in a decrease in corrosion resistance after coating. 4%. In addition, when the Si phase is excessively generated, chipping resistance and seizure resistance decrease. In order to secure excellent corrosion resistance after painting, the Si concentration is preferably 2.5% or less.
[0024]
　In addition to Al, Mg and Si, the essential constituent element of the plating layer according to the present invention is Zn. Further, inevitable impurities such as Fe, Mn, and Ti that diffuse from the steel sheet into the plating layer and Sn, In, Bi, Pb, and B that are inevitably mixed in the manufacturing process of the plating bath are Fe, Mn, Ti, and Sn. , In, Bi, Pb, B containing at least one kind (hereinafter, this intermetallic compound is referred to as “other intermetallic compound” in order to distinguish it from the intermetallic compound generated in the interface alloy layer). It may be contained in the plating layer. Zn is required to be contained in the plating layer at a certain concentration or more in order to secure the sacrificial corrosion resistance of the plating layer, the corrosion resistance, and the appropriateness as a coating base treatment for automobile plated steel sheet. Need to occupy most.
[0025]
　When the plating layer is composed of such constituent elements, the plating layer has a structural structure that is substantially composed of Zn phase and Al phase, and generally has a thickness of about 3 to 50 μm.
[0026]
　Next, the structure of the plating layer will be described.
　A typical plating structure of the plating layer according to the present invention is shown in FIG. The plating layer according to the present invention is mainly composed of the following structures (1) to (4).
　(1) Lamellar structure in which layered Zn phase and layered Al phase are alternately arranged (2 in FIG. 2, also referred to as “structure I”),
　(2) Granular Zn phase and granular Al formed so as to cover structure I Structure composed of phases (3 in FIG. 2, hereinafter also referred to as “structure II”),
　(3) Zn/Al/MgZn 2 ternary eutectic structure formed by Zn—Al—Mg ternary eutectic reaction (4 in FIG. 1, hereinafter also referred to as “eutectic structure”),
　(4) Mg 2 Si phase (5 in FIG. 1)
　,
　(5) Al—Fe at the interface between the plating layer and the base iron. An interfacial alloy layer (6 in FIG. 1)
　made of the intermetallic compound is formed.
[0027]
　The layered Zn phase and the layered Al phase in the structure I are not particularly limited, but generally have an aspect ratio (ratio of short side to long side of crystal grain size: short side/long side) of 0.1 or less. It may be layered. The thickness of the layered Zn phase and the layered Al phase is not particularly limited, but is generally about 20 to 500 nm, and particularly about 20 to 100 nm. Therefore, in the structure I, a striped pattern having a repeating unit of about 40 to 1000 nm composed of a layered Zn phase and a layered Al phase is formed as shown in FIG.
[0028]
　The granular Zn phase in Structure II is not particularly limited, but is generally granular with an aspect ratio (short side/long side) of more than 0.1 and 1 or less, and a crystal grain size of 80 to 800 nm. Good. Similarly, the granular Al phase in Structure II is not particularly limited, but is generally granular with an aspect ratio (short side/long side) of more than 0.1 and 1 or less, and a crystal grain size of 80 to 700 nm. May be
[0029]
　Here, the process of forming the texture in the plating layer will be described. In the process of cooling from the bath temperature, Al primary crystals (α-(Zn, Al) phase crystallized as primary crystals) first crystallize and grow in dendrite form. At this time, the solidification of the plating layer progresses in a non-equilibrium state due to the high cooling rate, so the solidification proceeds in a state where the average Al concentration in the primary Al crystal is higher than in the equilibrium diagram. When cooling further progresses and the temperature drops to the eutectic temperature, the liquid phase existing outside the Al primary crystal undergoes a Zn/Al/MgZn 2 ternary eutectic reaction or a Zn/Al binary eutectic reaction. This completes the solidification. When the cooling further proceeds and the temperature becomes equal to or lower than the eutectoid temperature of 275° C., solid phase transformation occurs inside the Al primary crystal (α-(Zn,Al) phase), and inside the Al primary crystal, the Zn phase and Al It has a structure composed of two phases. In the present invention, the solid phase transformation behavior is controlled to form the structure I inside the Al primary crystal.
[0030]
　According to the method for producing a hot-dip Zn-plated steel sheet of the present invention, which will be described in detail later, a structure I, which is a structure that cannot be obtained by a normal hot-dip plating method, is obtained. As described above, the structure I is a lamellar structure in which the layered Zn phase and the layered Al phase are alternately arranged, and the lamellar structure is formed inside the Al primary crystal part (1 in FIG. 1). .. The average composition of the entire tissue I is not particularly limited, but generally, the Al concentration is 15 to 55 mass %, and the balance is Zn and unavoidable impurities of less than about 2 mass %.
　As will be described in detail later, the structure I is a structure formed by a eutectoid reaction that occurs in the temperature range of 180 to 275° C., and the temperature range of 180 to 275° C. has an average cooling rate of 0.095 to 1.9° C./sec. The area fraction of the structure I in the cross-section of the plating layer becomes 5% or more only when it is cooled by. Under the cooling conditions disclosed in the present invention, the cooling rate is lower than that in the case of the normal process, so it is considered that the structure I is formed as a result of the diffusion of Zn atoms and Al atoms progressing during the eutectoid reaction. On the other hand, in the case of the normal process, since the cooling rate is as high as 10° C./s, the diffusion of Zn atoms and Al atoms cannot proceed sufficiently, and as a result, the structure I is not formed. The cooling conditions disclosed in the present invention are difficult to realize in the current production line including the continuous galvanizing line, and there have been no examples discovered so far. In the structure I, the lamella spacing is as small as 40 to 1000 nm, so the ratio of the different phase interface of the Zn phase/Al phase in the structure is very high, and the Zn phase/Al phase is higher than the characteristics of the Al phase itself contained in the structure. The properties of the heterophasic interfaces of the phases dominate. Since the interphase interface of Zn phase/Al phase has high interfacial energy, it easily corrodes in a corrosive environment, and as a result, the entire structure I can corrode in a corrosive environment.
　Therefore, the inclusion of the structure I suppresses the selective corrosion of the structure other than the Al primary crystal part, which occurs in the conventional hot-dip Al—Zn-based plating and hot-dip Zn—Al—Mg-based plating, and after coating, Corrosion resistance is improved. Further, since the structure I is mainly composed of a Zn phase and an Al phase capable of being plastically deformed, it has excellent ductility and consequently contributes to improvement of chipping resistance. The effect of improving the corrosion resistance after coating and the chipping resistance by the structure I becomes larger as the area fraction of the structure I contained in the plating layer becomes higher.
[0031]
　If the total value of the area fraction of the structure I is less than 5%, the effect of improving the corrosion resistance after coating cannot be obtained, so the lower limit value is made 5%. On the other hand, as described above, the higher the area fraction of the structure I, the greater the effect of improving the corrosion resistance after coating and the chipping resistance, so the upper limit may be 100%, and generally 90% or 80%. %. According to the method for producing a hot-dip Zn-plated steel sheet of the present invention, it is possible to reliably achieve an area fraction of the structure I of about 50% or more. From the viewpoint of reliably and remarkably improving both the corrosion resistance after coating and the chipping resistance, and further the seizure resistance, the area fraction of the structure I is preferably 15% or more, more preferably 20% or more. Yes, and most preferably 40% or more.
[0032]
　In the present invention, unless otherwise specified, the “area fraction” means the arithmetic average value of the area fractions of a desired structure in a cross section of a plating layer when calculating the area fraction of a desired structure for five or more randomly selected samples. Is to say. This area fraction actually represents the volume fraction in the plating layer.
[0033]
　The structure II is a structure composed of a granular Al phase and a granular Zn phase, and generally, the Al concentration contained in the structure is 20 to 55 mass% and the Zn concentration is 45 to 80 mass %. As will be described later in detail, the structure II is a structure that can be formed by the eutectoid reaction when cooled in the temperature range of 180 to 275°C. The structure II has a granular form of Zn phase and Al phase contained in the structure, which is different from the structure I, and is of the same quality as the structure (3 in FIG. 5) formed by a normal plating process. In the structure II, the area fraction of the heterophase interface between the Zn phase and the Al phase occupied in the structure is low, and the passive film is formed on the entire structure. As a result, Structure II has a noble potential due to the passivation film, which promotes corrosion of surrounding tissues and reduces post-paint corrosion resistance. Therefore, in order to secure the corrosion resistance after painting, the area fraction of the structure II is preferably low. As a result of examining the manufacturing process, the generation of the structure II can be completely suppressed, so the lower limit of the area fraction of the structure II is set to 0%. On the other hand, when the area fraction of the structure II is 40% or more, the corrosion resistance after coating is deteriorated regardless of any structure control. Therefore, the upper limit value is set to 40%. In order to impart excellent corrosion resistance after coating to the plated layer, it is desirable that the area fraction of the structure II be less than 30%, more preferably less than 20%.
[0034]
　On the other hand, since the structure II is mainly composed of the Zn phase and the Al phase capable of being plastically deformed like the structure I, it has excellent ductility, and as a result, can contribute to the improvement of chipping resistance. If the total area fraction of the structures I and II is less than 10%, it becomes difficult to obtain the effect of improving the chipping resistance, so the lower limit of the total value of the structures I and II is preferably 10%. Moreover, even if the area fraction of only the structure I is 10% or more, the chipping resistance is superior to that of the conventional hot-dip Zn-based plating and hot-dip galvannealing. As will be described later in detail, the area fractions of the texture I and the texture II in the plated layer can be obtained from the backscattered electron image (BSE image) of the SEM by using image processing.
[0035]
　The Zn/Al/MgZn 2 ternary eutectic structure means a Zn phase, an Al phase, and a MgZn 2 phase finally solidified outside the Al primary crystal part by a Zn-Al-Mg eutectic reaction at 335°C. It is a structured layered structure of a Zn layer, an Al layer, and a MgZn 2 layer, and can contribute to the improvement of corrosion resistance after coating. This is because Mg contained in the structure forms an insulating coating of a corrosion product generated by corrosion of the plating layer. By setting the area fraction of the Zn/Al/MgZn 2 ternary eutectic structure to 20% or more, the corrosion resistance after coating can be further improved, so the lower limit is preferably 20%. On the other hand, since the Zn/Al/MgZn 2 ternary eutectic structure contains MgZn 2 which is an intermetallic compound phase having poor toughness, it is inferior in ductility to the structures I and II. When the area fraction of the Zn/Al/MgZn 2 ternary eutectic structure having poor ductility in the plating layer exceeds 90%, the chipping resistance decreases, so the upper limit is preferably 90%. When the concentration of Mg contained in the plating layer is low, Zn/Al/MgZn 2In addition to the ternary eutectic structure, a Zn/Al binary eutectic structure may be formed in the plated layer. The Zn/Al binary eutectic structure is a structure composed of a Zn phase and an Al phase formed by a Zn/Al binary eutectic reaction after crystallization of an Al primary crystal part. This organization, since the Zn-5% Al is solidified eutectic composition includes Al in a low concentration of about an average 3-6% in the tissue, MgZn 2 because it does not contain phase, Zn / Al / MgZn 2 three The effect of improving corrosion resistance is smaller than that of the original eutectic structure. Therefore, from the viewpoint of corrosion resistance after coating, it is preferable that the area fraction of the Zn/Al binary eutectic structure is low.
[0036]
　Further, as a result of examining the corrosion resistance and the chipping resistance after coating of the plating layer by the present inventors, it was found that the structure I contributes to the improvement of both the corrosion resistance after coating and the chipping resistance.
[0037]
　For plated steel sheets for automobiles, the period from cut scratches to coating swelling and red rusting is important, but in the structure of the plating layer, the higher the area fraction of structure I, the higher the corrosion resistance of the plating layer after coating. Is improved. For example, it has been found that when the area fraction of the structure I is 5% or more, the corrosion resistance after coating is superior to that of a commercially available hot-dip galvanized steel sheet. This is because the structure I according to the present invention is a structure that contributes to the improvement of the corrosion resistance after coating. When the area fraction of the structure I in the plating layer is 20% or more and the area fraction of the structure II is less than 20%, the corrosion resistance after coating is further improved. When the total value of the area fraction of the structure I in the plated layer is 40% or more and the area fraction of the structure II is less than 10%, the corrosion resistance after coating is further improved. In the present invention, the structure II does not have a favorable effect on the corrosion resistance after coating, so that the area fraction thereof is preferably lower.
[0038]
　Further, as a result of examining the chipping resistance, it was found that when the structure I contained in the plated layer was 5% or more, the chipping resistance was also improved. When Mg is contained in the Zn-based plating layer, an intermetallic compound having poor workability such as MgZn 2 or Mg 2 Si is easily formed, but the content of Mg in the Zn-based plating layer is 4% by mass or less. In that case, MgZn 2 or Mg 2 Si in a form that hinders chipping resistance is not generated. Further, an Al—Fe based intermetallic compound is formed as an interface alloy layer at the interface between the base iron and the plating layer. The interface alloy layer preferably has a thickness of 100 nm or more in order to secure the adhesion between the base iron and the plating layer, but since it is a brittle intermetallic compound, if the thickness exceeds 2 μm, the chipping resistance decreases. .. When a large amount of these intermetallic compounds is present, the toughness of the plating layer is reduced and the chipping resistance is reduced.
[0039]
　Next, a characteristic manufacturing method of the hot-dip Zn-plated steel sheet of the present invention will be described.
　The material of the steel material as the base material of the hot-dip Zn-plated steel sheet of the present invention is not particularly limited, and Al killed steel, ultra-low carbon steel, high-carbon steel, various high-strength steels, Ni- and Cr-containing steels, etc. are used. It is possible. There is also no particular limitation on the steel making method, the strength of the steel, the hot rolling method, the pickling method, the cold rolling method and other pretreatment processes for the steel material.
[0040]
　The steel materials such as C and Si are not particularly limited. It has not been confirmed that elements such as Ni, Mn, Cr, Mo, Ti and B added to the steel material affect the Zn-based plating layer in the present invention.
[0041]
　For the method for producing the hot-dip galvanized steel sheet of the present invention, the Sendzimir method, the pre-plating method, etc. can be applied. When Ni is used as the type of pre-plating, Ni may be contained in the intermetallic compound mainly composed of Al and Fe when the plating layer is heated.
[0042]
　A Zn-based plating bath may be prepared by mixing Zn-Al-Mg-based and Al-Si alloys so that each component has a predetermined concentration and melting at 450 to 650°C. A Zn-based plating layer can be formed on the surface of the substrate by immersing the substrate whose surface has been sufficiently reduced in a plating bath at 350 to 600° C. and pulling it up. In order to control the adhesion amount of the plating layer, wiping with N 2 gas is carried out immediately after hot dip plating .
[0043]
　When a plating layer is produced in a plating bath having a composition according to the present invention by a normal hot dip plating process, a plating structure as disclosed in FIG. 4 is formed. That is, the plating layer is composed of a Zn/Al/MgZn 2 ternary eutectic structure and a Mg 2 Si phase (5 in FIG. 4). The structure I in the present invention is not formed by natural cooling, furnace cooling, solidification cooling rate in a normal hot dip plating process, or cooling from the melting point to room temperature at a cooling rate of 10° C./sec or more.
[0044]
　A method of forming the tissue I will be described. The structure I is formed by satisfying the following cooling conditions 1 and 2.
　{1} Cooling condition 1: In the present invention, it is necessary to control the cooling rate from the plating bath temperature to 275°C at 10°C/sec or more. By setting the cooling rate to 10° C./s or more, formation of the tissue I can be promoted. Further, in consideration of the slow cooling in the latter stage, the cooling rate from the plating bath temperature to 275° C. is preferably 40° C./s or less.
　{2} Cooling condition 2: A temperature range from 275°C to 180°C is cooled at an average cooling rate of 0.095 to 1.9°C/sec.
　The structure I is formed inside the Al primary crystal only by performing both the treatments {1} and {2}. When the cooling rate under the cooling condition 2 exceeds 1.9° C./sec, the structure I is not formed at all or is not sufficiently formed, and the entire Al primary crystal is composed of the structure II. Therefore, the upper limit thereof is 1. 9°C/sec. On the other hand, even when the cooling rate is less than 0.095° C./sec, the structure I is not formed at all or is not sufficiently formed, so that the corrosion resistance is not improved. When the cooling rate is less than 0.095°C/sec, the plating and the diffusion of base metal proceed excessively, and as a result, the interface alloy layer made of Al-Fe intermetallic compound grows to a thickness exceeding 2 μm. , Leading to a reduction in chipping resistance. Furthermore, if the cooling rate is less than 0.095° C./sec, other intermetallic compounds generated from the impurities derived from the plating bath and the impurities diffused from the base iron are likely to be generated in addition to the interface alloy layer. The chipping property tends to deteriorate. Therefore, the lower limit value is set to 0.095° C./second.
　{3} Cooling condition 3: Next to {1}{2}, the cooling condition from 180° C. to room temperature is not particularly limited, but the average cooling rate is 2° C./sec from the viewpoint of suppressing the growth of the interface alloy layer. The above is desirable.
[0045]
　FIG. 1 and FIG. 2 described above are examples of a plated structure formed by the method for producing a hot-dip Zn-plated steel sheet according to the present invention, in which structure I is formed. Since the plated steel sheet obtained by the present invention is a hot dip plated layer, an interface alloy layer of Al-Fe based intermetallic compound of less than 1 μm is usually formed at the interface with the plated layer. In addition, the total abundance of intermetallic compounds (other intermetallic compounds) containing any one or more of Fe, Mn, Ti, Sn, In, Bi, Pb, and B as unavoidable impurities in the plating layer is defined as the area. If the ratio is regulated to 3% or less, there is almost no effect on performance. On the other hand, when the area ratio of the other intermetallic compound exceeds 3%, the corrosion resistance and the chipping resistance decrease.
[0046]
　Hereinafter, a method for analyzing the structure of the plated steel sheet manufactured by the method for manufacturing a hot-dip Zn-based plated steel sheet of the present invention will be described.
[0047]
　Regarding the component composition of the plating layer, it is possible to grasp the component composition of the plating layer by immersing the plated steel sheet in 10% HCl to which an inhibitor is added and performing ICP analysis of the stripping solution.
[0048]
　The constituent phase of the plating layer is analyzed by X-ray diffraction using a Cu target from the surface of the plating layer. It can be confirmed that the constituent phase obtained in the present invention is a plating layer mainly composed of Zn phase, Al phase, and MgZn 2 phase. No other phases are observed. Since the amount of Mg 2 Si phase is very small, it cannot be observed as a main peak by X-ray diffraction.
[0049]
　The structure contained in the plated layer can also be analyzed by using a transmission electron microscope (TEM). The texture morphology can be confirmed from a normal bright field image, and the crystal grain sizes of the Zn phase and the Al phase can be easily measured by using the dark field image. Further, the Zn phase, the Al phase, and the MgZn 2 phase can also be identified by identifying the crystal structure of the crystal phase existing in the phase from the diffraction pattern . The thicknesses of the layered Al phase and the layered Zn phase of the tissue I and the lamellar spacing of the tissue I can be easily measured by using the bright field image and the dark field image of the TEM. Further, the thicknesses of the layered Al phase and the layered Zn phase of the structure I and the lamellar spacing of the structure I can be measured from the SEM image taken at a magnification of about 30,000 times.
[0050]
　The structure contained in the plating layer can be analyzed by observing the cross section of the plating layer using a backscattered electron image of a scanning electron microscope (SEM). Usually, most of the crystal phase contained in the plating layer is composed of Al element and Zn element, and therefore, as shown in the backscattered electron image of FIG. 1, the contrast of light and shade varies depending on the composition of the element contained in the crystal phase. That is, the black portion has a high Al concentration and the white portion has a high Zn concentration. Therefore, it is possible to measure the area ratio of the black part and the white part in the plating layer by a simple image analysis, and determine the area fraction of the Al phase and the Zn phase contained in the plating layer.
[0051]
　The area fractions of the structure I and the structure II are composed of a structure I having a lamellar structure, a granular Zn phase and an Al phase, using a commercially available drawing software for an SEM image taken at a magnification of about 5000 times. It is possible to estimate the area ratio of each tissue II by drawing the boundary line of the tissue II and analyzing the image. The area fraction of Mg 2 Si contained in the plating layer can be grasped from the area proportion of Mg and Si present in the element mapping image produced by using EDS.
[0052]
　The performance evaluation of the plating layer will be described.
　Corrosion resistance after coating of the plated layer is determined by subjecting the plated steel sheet sample to Zn phosphoric acid treatment and electrodeposition coating to produce a cross-cut flaw that reaches the base iron, and subject the coated plated steel sheet to a combined cycle corrosion test. It can be evaluated by measuring the maximum swelling width around the generated cross-cut and obtaining the average value. A sample having a small swollen width is evaluated to have excellent corrosion resistance. Further, since the appearance of red rust significantly deteriorates the appearance of the coated steel sheet, it is generally evaluated that the longer the period until the occurrence of red rust, the better the corrosion resistance.
[0053]
　The evaluation of the chipping resistance of the plating layer is performed by applying the same electrodeposition coating as in the case of evaluating the corrosion resistance after coating described above to the plating layer, and then applying an intermediate coating, a top coating and a clear coating to obtain a four-layer structure. It can be evaluated by making a coating film, colliding the crushed stone with the coating film that is kept at a constant temperature, visually observing the degree of peeling (peeling), and observing the peeling condition visually and by image processing. it can.
Example
[0054]
　Tables 1-1 to 1-6 show examples disclosed in the present invention.
　As the plating bath, a plating bath having the components shown in Table 1-1 and Table 1-2 was prepared. The plating bath temperature was 455 to 585°C. A cold-rolled steel plate (carbon concentration: 0.2%) having a plate thickness of 0.8 mm was used as a plating original plate. The original plate was cut into 100 mm×200 mm, and then plated with an in-house manufactured batch-type hot dip tester. The plate temperature was monitored using a thermocouple spot-welded to the center of the plated original plate.
[0055]
　Before immersion in the plating bath, the surface of the original plating plate at 800° C. is reduced with N 2 -5% H 2 gas in a furnace with an oxygen concentration of 20 ppm or less, and air-cooled with N 2 gas to bring the immersion plate temperature to a bath temperature +20° C. After reaching, it was immersed in the plating bath for about 3 seconds. After immersion in the plating bath, the pulling rate was 100 mm/sec. At the time of pulling out, the amount of deposited plating was adjusted with N 2 wiping gas.
[0056]
　After pulling out the steel sheet from the plating bath, the plating layer was cooled from the plating bath temperature to room temperature under the cooling conditions (cooling conditions 1 to 3) shown in Table 1-1 and Table 1-2.
[0057]
[Table 1-1]

[0058]
[Table 1-2]

[0059]
[Table 1-3]

[0060]
[Table 1-4]

[0061]
[Table 1-5]

[0062]
[Table 1-6]

[0063]
　The sample of the obtained hot-dip Zn-based plated steel sheet was cut into 25 (c)×15 (L) mm, embedded in a resin, and polished, and then a SEM image of the cross section of the plating layer and an element mapping image by EDS were prepared. The components and structure of the plated layer are shown in Tables 1-3 and 1-4. Here, Zn/Al/MgZn 2 ternary eutectic structure composed of structure I, structure II, Zn phase, Al phase, and MgZn 2 phase (in Tables 1-3 and 1-4, "Zn/ Al/MgZn 2 ternary eutectic structure”, Zn/Al binary eutectic structure, interface alloy layer composed of Al—Fe intermetallic compound, Mg 2 Si phase, Si phase and other intermetallic compounds The area fraction and the thickness of the interface alloy layer were measured from the SEM image and the element mapping image. The "interface alloy layer" is not included in the area fraction of the plating layer. Further, "other intermetallic compounds" in Tables 1-3 and 1-4 mean Fe-Zn and Fe-Zn derived from base iron in addition to Al-Fe-based intermetallic compounds in which Fe and Al derived from base iron are bonded. Is a general term for Fe—Zn-based intermetallic compounds bonded to each other and intermetallic compounds due to impurities contained in the plating bath, and included in the area ratio of the plating layer. The interfacial alloy layer and the “other intermetallic compound” are distinguished from each other by the Al—Fe metal compound, which is not contained in Zn and Mg but exists in the interfacial layer as a single Al—Fe intermetallic compound. Others were judged to be "other intermetallic compounds". 　1 and 2 show No. 1 in Table 1. 20 is an SEM image (BSE image) of Example 20 (Example 20). Structure I (2 in FIG. 2), structure II (3 in FIG. 2), Zn/Al/MgZn 2 in the plated layer
A ternary eutectic structure (4 in FIG. 1), a Mg 2 Si phase (5 in FIG. 1), and an interface alloy layer (6 in FIG. 1) are formed. Typical values ​​of the thickness and lamella spacing of the layered Al phase and the layered Zn layer formed in the structure I are shown in Table 2.
[0064]
[Table 2]

[0065]
　Of the constituent structures in the plating layer, namely, structure I, structure II, Zn/Al/MgZn 2 ternary eutectic structure, Zn/Al binary eutectic structure, Mg 2 Si phase, interfacial alloy layer and other intermetallic compounds The area fraction was calculated by image analysis by photographing 5 fields of view (plating layer: 50×200 μm) each from 5 different samples of the cross-section EDS mapping image of the plating layer. Further, the thickness of the interfacial alloy layer existing at the plating layer/steel plate interface was also estimated by measuring the thickness of the Al—Fe based intermetallic compound from the cross-sectional EDS mapping image. The SEM is JEOL/JSM-700F and the EDS detector is also JEOL. The accelerating voltage is 15 kV, and the cross-section plating structure of about 500 to 10,000 times is analyzed by element distribution mapping by EDS.
　The Zn/Al/MgZn 2 ternary eutectic structure and the Zn/Al binary eutectic structure can be distinguished by measuring the amount of Mg every 5 μm in the range of 3 μm×3 μm in the SEM-EDS element distribution image. The range where the amount of Mg was 2% or more was determined to be the Zn/Al/MgZn 2 ternary eutectic structure, and the range below was determined to be the Zn/Al binary eutectic structure.
[0066]
　Corrosion resistance after coating of the plated layer was carried out by subjecting a plated steel plate sample of 50×100 mm to Zn phosphoric acid treatment (SD5350 system: standard by Nippon Paint Industrial Coding Co., Ltd.), and then electrodeposition coating (PN110 Powernics Gray-Japan. Paint plating which carried out the paint industrial coding company standard) at 20 μm and baked it at a baking temperature of 150° C. for 20 minutes, and then made cross-cut scratches (40×√2 2 pieces) reaching the base metal. The steel sheet was subjected to a combined cycle corrosion test according to JASO (M609-91), and after 120 cycles, the maximum swelling width at eight locations around the crosscut was measured and evaluated by obtaining an average value. When the number of cycles of the above JASO (M609-91) is 60, 90 and 150 cycles, "A" when the swelling width from the crosscut scratch is 1 mm or less, "B" when it is 1 to 2 mm, and 2 When it was ˜4 mm, it was designated as “C”, and when red rust was generated, it was designated as “D”.
　The combined cycle corrosion test according to JASO (M609-91) was repeated with the following (1) to (3) as one cycle.
(1) Salt spray, 2 hours (5% NaCl, 35°C)
(2) Drying, 4 hours (60°C)
(3) Wetting, 2 hours (50°C, humidity 95% or more)
[0067]
　The powdering resistance of the plating layer was determined by cutting the plated steel plate into 40 mm (C) x 100 mm (L) x 0.8 mm (t), and using a V bending test, the C direction was the bending axis direction, and the bending direction was 60 at 5R. After bending, evaluation was made from a five-point average value of the peeling width of the plating layer generated by peeling the tape. Specifically, “A” indicates that no peeling occurs at all, “B” indicates that the average peeling width is 0.1 to 1 mm, “C” indicates that the average peeling width is 1 to 2 mm, and the average peeling width is The case of 2 mm or more was defined as "D".
[0068]
　The chipping resistance of the plating layer is the same as in the case of evaluating the above-mentioned corrosion resistance after coating on the plating layer, followed by intermediate coating, topcoat coating, and clear coating, and the overall film thickness is A coating film was prepared so as to have a thickness of 40 μm, and 100 g of No. 7 crushed stone was cooled to −20° C. from a distance of 30 cm with an air pressure of 3.0 kg/cm 2 using a gravure tester (manufactured by Suga Test Instruments Co., Ltd.) . The coating was impacted at a 90 degree angle. After that, the peeling part of the plating layer at the collision part is exposed using an adhesive tape, the diameter of the peeled part is measured, and 5 pieces are selected from the ones with a large peeling diameter, and the average value is taken as the peeling diameter of the test material. And The smaller the peeled diameter, the better the chipping resistance. "A" when the average peel diameter is less than 1.0 mm, "B" when the average peel diameter is 1.0 mm or more and less than 1.5 mm, and "B" when the average peel diameter is 1.5 mm or more and less than 3.0. The chipping resistance was evaluated as "C" and "D" when the average peel diameter was 3.0 mm or more.
[0069]
　The seizure resistance of the plating layer is obtained by collecting two primary test pieces each having a width of 80 mm and a length of 350 mm, subjecting them to draw bead processing using a jig imitating a die and a bead, and treating the surface of the steel sheet. A slide having a length of 150 mm or more was generated between the surface and the die shoulder and the bead portion. In addition, the local radii of the die shoulder and the bead portion of the jig used in the above test were 2 mmR and 5 mmR, the pressing pressure of the die was 60 kNm 2 , and the drawing speed of the draw beading was 2 mmin. In addition, at the time of the test, 10 mg/m 2 of lubricating oil (550S: manufactured by Nippon Parkerizing Co., Ltd.) was applied to both surfaces of the test piece .
[0070]
　In addition, as a comparison target of the examples, the plated steel sheet having a composition outside the scope of the claims, without Si and excessive, holding time insufficient or excessive, and holding temperature outside the range (Nos. 89 to 91 in Table 1). Comparative Examples other than), hot-dip galvanized steel sheet (No. 89 in Table 1), galvannealed steel sheet (No. 90 in Table 1), electrogalvanized steel sheet (No. 91 in Table 1). Was prepared and subjected to the above evaluation. The results will be described.
[0071]
　In Comparative Example 1, since the Al concentration in the plating layer was insufficient, the lamella structure (structure I) in which the layered Zn phase and the layered Al phase were alternately aligned was not sufficiently formed, resulting in insufficient chipping resistance and corrosion resistance. Met.
[0072]
　In Comparative Example 2, since the cooling rate under the cooling condition 2 was less than 0.095° C./sec, the interface alloy layer grew to a thickness exceeding 2 μm, and the chipping resistance was insufficient. In addition, as a result that the structure I was not formed, the corrosion resistance was insufficient.
[0073]
　In Comparative Example 5, since the plating layer does not contain Si, the reaction between Zn and Al contained in the plating bath and the Fe element in the plating original plate cannot be suppressed, and a large amount of impurities are contained in the plating layer. The element was mixed. As a result, an intermetallic compound (other intermetallic compound) containing any one or more of Fe, Mn, Ti, Sn, In, Bi, Pb and B is formed in the plating layer in an extremely large amount exceeding 3%. In addition, the interface alloy layer was formed thick, and the chipping resistance was insufficient. Further, the Al concentration in the plating layer was insufficient, and Fe-Zn intermetallic compounds and Al-Fe intermetallic compounds derived from impurity elements were formed in large amounts in the plating layer, and the structure I was not sufficiently formed. As a result, the corrosion resistance was insufficient.
[0074]
　In Comparative Example 10, since the Si concentration in the plating layer was excessive, a large amount of a potential-noble Si phase was generated in the plating layer, and seizure resistance, chipping resistance, and corrosion resistance were insufficient.
[0075]
　In Comparative Example 11, since the Mg concentration in the plated layer was insufficient, the effect of forming a Zn-based corrosion product as an insulating film and forming rust as a barrier film was low. As a result, the corrosion resistance was insufficient.
[0076]
　In Comparative Example 18, since the cooling rate under the cooling condition 1 was less than 10° C./sec, the structure I was not sufficiently formed, resulting in insufficient chipping resistance and corrosion resistance.
[0077]
　In Comparative Example 19, since the cooling rate under the cooling condition 2 was higher than 1.9° C./sec, the structure I was not formed at all, and as a result, the chipping resistance and the corrosion resistance were insufficient.
[0078]
　In Comparative Example 22, since the Si concentration in the plating layer was insufficient, the reaction between Zn and Al contained in the plating bath and the Fe element in the plating original plate could not be suppressed, and a large amount of impurity elements were present in the plating layer. Mixed. As a result, other intermetallic compounds were formed in a large amount in the plated layer in excess of 3%, and the interface alloy layer was formed thickly, resulting in insufficient chipping resistance. Further, a large amount of Fe—Zn based intermetallic compounds and Al—Fe based intermetallic compounds derived from impurity elements were formed in the plated layer, and the structure I was not sufficiently formed, resulting in insufficient corrosion resistance.
[0079]
　In Comparative Example 23, since the cooling rate under the cooling condition 2 is less than 0.095° C./sec, the interfacial alloy layer grows to a thickness exceeding 2 μm, and further other intermetallic compounds are generated exceeding 3%. The chipping resistance was insufficient. In addition, as a result that the structure I was not formed, the corrosion resistance was insufficient.
[0080]
　In Comparative Example 31, since the cooling rate under the cooling condition 2 is less than 0.095° C./sec, the interfacial alloy layer grows to a thickness exceeding 2 μm, and other intermetallic compounds are generated exceeding 3%. The chipping resistance was insufficient. In addition, as a result that the structure I was not formed, the corrosion resistance was insufficient.
[0081]
　In Comparative Example 32, since the Al concentration in the plating layer was excessive, the structure I was not formed, and as a result, the corrosion resistance was insufficient.
[0082]
　In Comparative Example 37, since the Mg concentration in the plating layer was insufficient, the effect of forming a Zn-based corrosion product as an insulating film and rust as a barrier film was low. As a result, the corrosion resistance was insufficient.
[0083]
　In Comparative Example 40, since the Si concentration in the plating layer was insufficient, the reaction between Zn and Al contained in the plating bath and the Fe element in the plating original plate could not be suppressed, and a large amount of impurity elements were present in the plating layer. Mixed. As a result, other intermetallic compounds were formed in a large amount in the plated layer in excess of 3%, and the interface alloy layer was formed thickly, resulting in insufficient chipping resistance. Further, a large amount of Fe—Zn based intermetallic compounds and Al—Fe based intermetallic compounds derived from impurity elements were formed in the plated layer, and the structure I was not sufficiently formed, resulting in insufficient corrosion resistance.
[0084]
　In Comparative Example 43, the cooling rate under the cooling condition 2 was higher than 1.9° C./second, so that the structure I was not formed, and as a result, the chipping resistance and the corrosion resistance were insufficient.
[0085]
　In Comparative Example 44, the cooling rate under the cooling condition 1 was less than 10° C./sec. Therefore, the structure I was not formed, and as a result, the chipping resistance and the corrosion resistance were insufficient.
[0086]
　In Comparative Example 45, since Mg was not contained in the plating layer, the effect of forming a Zn-based corrosion product as an insulation coating and rust as a barrier coating was low. As a result, the corrosion resistance was insufficient.
[0087]
　In Comparative Example 48, the cooling rate under the cooling condition 2 was less than 0.095° C./sec. Therefore, the interface alloy layer grew to a thickness exceeding 2 μm, and the chipping resistance was insufficient. In addition, as a result that the structure I was not formed, the corrosion resistance was insufficient.
[0088]
　In Comparative Example 50, since the Mg concentration in the plating layer was excessive, the structure I was not sufficiently formed, resulting in insufficient chipping resistance and corrosion resistance.
[0089]
　In Comparative Example 55, since the Al concentration in the plating layer was excessive, the structure I was not formed, and as a result, the corrosion resistance was insufficient.
[0090]
　In Comparative Example 56, the Al concentration in the plating layer was insufficient, and thus the structure I was not formed, resulting in insufficient chipping resistance and corrosion resistance.
[0091]
　In Comparative Example 58, since the plating layer does not contain Si, the reaction between Zn and Al contained in the plating bath and the Fe element in the plating original plate cannot be suppressed, and a large amount of impurities are contained in the plating layer. The element was mixed. As a result, other intermetallic compounds were formed in a large amount in the plated layer in excess of 3%, and the interface alloy layer was formed thickly, resulting in insufficient chipping resistance. Further, a large amount of Fe—Zn based intermetallic compounds and Al—Fe based intermetallic compounds derived from impurity elements were formed in the plated layer, and the structure I was not sufficiently formed, resulting in insufficient corrosion resistance.
[0092]
　In Comparative Example 64, the cooling rate under the cooling condition 2 was higher than 1.9° C./sec. Therefore, the structure I was not formed, and as a result, the chipping resistance and the corrosion resistance were insufficient.
[0093]
　In Comparative Example 65, the cooling rate under the cooling condition 2 was less than 0.095° C./sec. Therefore, the interface alloy layer grew to a thickness exceeding 2 μm, and the chipping resistance was insufficient. In addition, as a result that the structure I was not formed, the corrosion resistance was insufficient.
[0094]
　In Comparative Example 67, since the Si concentration in the plating layer was insufficient, the reaction between Zn and Al contained in the plating bath and the Fe element in the plating original plate could not be suppressed, and a large amount of impurity elements were present in the plating layer. Mixed. As a result, other intermetallic compounds were formed in a large amount in the plated layer in excess of 3%, and the interface alloy layer was formed thickly, resulting in insufficient chipping resistance. Further, a large amount of Fe—Zn based intermetallic compounds and Al—Fe based intermetallic compounds derived from impurity elements were formed in the plated layer, and the structure I was not sufficiently formed, resulting in insufficient corrosion resistance.
[0095]
　In Comparative Example 75, since the amount of impurities contained in the plating bath was large, the area fraction of the total abundance of other intermetallic compounds contained as inevitable impurities in the plating layer exceeded 3%. The corrosion resistance and chipping resistance were insufficient.
[0096]
　In Comparative Example 83, since the Al concentration in the plating layer was excessive, the structure I was not formed, and as a result, the corrosion resistance was insufficient.
[0097]
　In Comparative Example 87, since the Al concentration in the plating layer was excessive, the structure I was not formed, and as a result, the corrosion resistance was insufficient.
[0098]
　In Comparative Example 88, since the Si concentration in the plating layer was excessive, a large amount of an electrically noble Si phase was generated in the plating layer, and the corrosion resistance, chipping resistance, and seizure resistance were insufficient.
[0099]
　In Comparative Examples 89 to 91, the plating layer was a simple zinc plating layer that did not contain Al, Mg, and Si as in the present invention, and therefore the corrosion resistance and chipping resistance were insufficient. Furthermore, Comparative Examples 89 and 91 also had insufficient seizure resistance.
[0100]
　On the other hand, Examples 3, 4, 6 to 9, 12 to 17, 20, 21, 24 to 30, 33 to 36, 38, 39, 41, 42, 46, 47, 49, 51 to 54 of the present invention. , 57, 59 to 63, 66, 68 to 74, 76 to 82, 84 to 86 have good corrosion resistance, chipping resistance and seizure resistance.
Explanation of symbols
[0101]
　　　1...Al primary crystal part
　　　2...structure I
　　　3...structure II
　　　4...Zn/Al/MgZn 2 ternary eutectic structure
　　　5...Mg 2 Si phase
　　　6...interface alloy layer
The scope of the claims
[Claim 1]
　A plating layer containing 10 to 40% by mass of Al, 0.05 to 4% by mass of Si, 0.5 to 4% by mass of Mg, and the balance Zn and unavoidable impurities on at least a part of the surface of the steel sheet. The
　plating layer has a lamella structure in which a layered Zn phase and a layered Al phase are alternately aligned in a cross section of the layer, and contains 5% or more by area fraction of Fe, Mn, Ti, Sn, In, Bi, Pb. , B to regulate the total content of intermetallic compounds containing at least one of B and B to be 3% or less in terms of area fraction.
[Claim 2]
　The hot-dip Zn-based plating according to claim 1, wherein the plating layer contains 10 to 30 mass% of Al, 0.05 to 2.5 mass% of Si, and 2 to 4 mass% of Mg. Steel plate.
[Claim 3]
　The hot-dip Zn-plated steel sheet according to claim 1 or 2, wherein the plating layer contains the lamella structure in an area fraction of 20 to 80%.
[Claim 4]
　The hot-dip Zn-based plated steel sheet according to claim 3, wherein the plated layer contains the lamella structure in an area fraction of 40 to 50%.
[Claim 5]
　The plated layer contains a Zn /Al/MgZn 2 ternary eutectic structure composed of a Zn phase, an Al phase, and a MgZn 2 phase in an area fraction of 20 to 90%. 5. A hot-dip Zn-plated steel sheet according to any one of 4 to 4.
[Claim 6]
　The molten Zn according to any one of claims 1 to 5, wherein an interface alloy layer made of an Al-Fe intermetallic compound having a thickness of 100 nm to 2 µm is provided at an interface between the plating layer and the steel sheet. Series plated steel sheet.