Hot-dip Zn-Al plated steel sheet and method for manufacturing the same

Abstract

To provide a hot dip Zn-Al-based alloy coated steel sheet which highly achieves both planar part corrosion resistance and post-coating corrosion resistance.SOLUTION: A hot dip Zn-Al-based alloy coated steel sheet has a steel sheet and a plating layer formed on at least one side of the steel sheet. The plating layer has a composition of Al:5.1 to 22.0%, Si:0.0 to 2.2%, Fe:0.0 to 2.0% in mass%, and the rest of Zn and an inevitable impurity. A cross-sectional area ratio of an old FCC phase in the plating layer is 10% or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a hot-dip Zn-Al plated steel sheet and a method for producing the same. [Background technology]

[0002] In recent years, plated steel sheets have been widely used in automotive structural components for their corrosion resistance. In the Japanese domestic market, alloyed hot-dip galvanized steel sheets (GA) and hot-dip galvanized steel sheets (GI) are the main types of zinc-plated steel sheets used in automobiles. Against the backdrop of the need for longer lifespans for automobiles and the diversification of usage environments due to climate change, there is a demand for even longer lifespans, i.e., higher corrosion resistance, for zinc-plated steel sheets.

[0003] It is known that increasing the Al content in the plating layer above the value for typical zinc-plated steel sheets (less than 1%) is an effective means of improving the corrosion resistance of the unprocessed flat surface (general surface) of zinc-plated steel sheets (hereinafter simply referred to as "flat surface corrosion resistance"). For example, molten zinc-5% by mass aluminum alloy plated steel sheets (Galfan) and molten 55% by mass aluminum-zinc alloy plated steel sheets (Galvalume) are known as zinc-plated steel sheets with improved flat surface corrosion resistance. These molten Zn-Al plated steel sheets consist of an interface alloy layer made of an Fe-Al intermetallic compound on a base steel sheet, and a plating layer present on this interface alloy layer. The plating layer mainly contains supersaturated Zn and consists of a portion where Al has solidified in dendrites (α-Al phase) and a portion of the remaining dendrite gaps (Zn-rich phase), with the α-Al phase being laminated in the thickness direction of the plating layer. The distinctive structure of this plating layer complicates the corrosion pathway from the surface of the plating layer, making it difficult for corrosion to easily reach the underlying steel sheet. As a result, hot-dip Zn-Al plated steel sheets have superior corrosion resistance to flat surfaces compared to hot-dip galvanized steel sheets with the same plating layer thickness.

[0004] Furthermore, Patent Document 1 describes a method for manufacturing a molten Zn-Al-Si alloy plated steel sheet having a plating composition consisting of Al: 30-70% by weight, Si: 0.05-2.0% by weight, the remainder being Zn and unavoidable impurities, in which the refinement of spangles on the plated surface can be suppressed by performing annealing before plating under predetermined conditions. Patent Document 2 describes that by using a plating bath consisting of Al: 3.0-10% by weight, Si: 0.01-1.0% by weight, Mg: 0.05-1.0% by weight, unavoidable impurities: less than 0.02% by weight, the remainder being Zn, and by setting the plating bath temperature and the cooling rate after plating within predetermined ranges, the occurrence of a tortoiseshell spangle pattern can be suppressed and corrosion resistance can be improved. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2000-219949 [Patent Document 2] Japanese Patent Application Publication No. 2-175852 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] However, while hot-dip Zn-Al plated steel sheets have superior corrosion resistance to flat surfaces compared to hot-dip galvanized steel sheets, they suffer from inferior corrosion resistance after painting. This is because, when the coating is damaged and the exposed area of ​​the underlying steel sheet is exposed to a corrosive environment, blistering of the coating progresses significantly faster than with hot-dip galvanized steel sheets. Hot-dip Zn-Al plated steel sheets with a high Al content, such as those described in Patent Document 1, suffer precisely from this problem. Even when the Al content is in the range of 3.0 to 10% by weight, as described in Patent Document 2, if a relatively high Al content is adopted to improve corrosion resistance to flat surfaces, corrosion resistance after painting is inferior. In other words, it has not been possible to achieve a high degree of compatibility between corrosion resistance to flat surfaces and corrosion resistance after painting.

[0007] In view of the above issues, the present invention aims to provide a hot-dip Zn-Al plated steel sheet and a method for manufacturing the same that achieves a high degree of compatibility between corrosion resistance of the flat portion and corrosion resistance after painting. [Means for solving the problem]

[0008] The inventors of the present invention have diligently studied to solve the above problems and have obtained the following findings. In order to ensure sufficient corrosion resistance of the flat plate portion, the Al content of the molten Zn-Al plated steel sheet must be 5.1% or more. Furthermore, if the cross-sectional integral ratio of the prior FCC phase in the plating layer of the molten Zn-Al plated steel sheet is 10% or less, excellent corrosion resistance after painting can be achieved. In order to make the cross-sectional integral ratio of the prior FCC phase 10% or less, it is necessary to use a plating bath with a component composition that contains a predetermined amount of Al, optionally contains predetermined amounts of Si and Fe, and the remainder consists of Zn and unavoidable impurities, and to make the cooling rate during the cooling process after gas wiping, from the solidification temperature of the plating bath until the temperature of the steel sheet reaches 300°C, a predetermined value or more. Furthermore, by making the amount of Zn solid solution in the Al-rich portion contained in the prior FCC phase 10% by mass or less, the corrosion resistance after painting can be further improved, which is preferable. The amount of Zn dissolved in the Al-rich portion can be adjusted by the temperature of the steel plate immersed in the plating bath during the hot-dip plating process (hereinafter also referred to as the "immersion plate temperature").

[0009] In other words, the gist of the present invention is as follows:

[0010] [1] comprising a steel plate and a plating layer formed on at least one side of the steel plate, The aforementioned plating layer has a composition consisting of, by mass%, Al: 5.1-22.0%, Si: 0.0-2.2%, and Fe: 0.0-2.0%, with the remainder being Zn and unavoidable impurities. A hot-dip Zn-Al plated steel sheet in which the cross-sectional integral ratio of the prior FCC phase in the plating layer is 10% or less.

[0011] [2] The prior FCC phase has a Zn-rich portion and an Al-rich portion, The hot-dip Zn-Al alloy coated steel sheet according to [1] above, wherein the Zn solid solution amount in the Al-rich portion is 10% by mass or less.

[0012] [3] The hot-dip Zn-Al alloy coated steel sheet according to [1] or [2] above, wherein the component composition further contains one or more selected from Ni, Cr, Ti, V, Mn, Co, Cu, Mo, W, B, Sn, Bi, Sb, Sr, and Ca in a total amount of 0.01 to 3.0% by mass.

[0013] [4] A hot-dip plating step of immersing a steel sheet in a plating bath having a component composition containing, in mass%, Al: 5.1 to 22.0%, Si: 0.0 to 2.2%, and Fe: 0.0 to 2.0%, with the balance being Zn and unavoidable impurities, and subjecting the steel sheet to hot-dip plating; Thereafter, a gas wiping step of performing gas wiping on the steel sheet to adjust the adhesion amount of the hot-dip plating on the steel sheet; Thereafter, a cooling step of cooling the steel sheet at a cooling rate of 30°C / second or more until the temperature of the steel sheet reaches 300°C from the solidification temperature of the plating bath, thereby solidifying the hot-dip plating on the steel sheet to form a plating layer; A method for manufacturing a hot-dip Zn-Al alloy coated steel sheet, comprising:

[0014] [5] After the gas wiping step and before the cooling step, a step of cooling the steel sheet to solidify the hot-dip plating on the steel sheet to form a primary plating layer, and thereafter, a heating step of heating the steel sheet to melt the primary plating layer to a hot-dip plating state; a method for manufacturing a hot-dip Zn-Al alloy coated steel sheet according to [4] above.

[0015] [6] In the hot-dip plating step, the temperature of the steel sheet immersed in the plating bath is set to be not less than (the temperature of the plating bath - 60)°C and not more than (the temperature of the plating bath - 40)°C; a method for manufacturing a hot-dip Zn-Al alloy coated steel sheet according to [4] or [5] above.

[0016] [7] The manufacturing method of the molten Zn-Al alloy plated steel sheet according to any one of [4] to [6] above, wherein the component composition further contains a total of 0.01 to 3.0% by mass of one or more selected from Ni, Cr, Ti, V, Mn, Co, Cu, Mo, W, B, Sn, Bi, Sb, Sr, and Ca.

Advantages of the Invention

[0017] The molten Zn-Al alloy plated steel sheet of the present invention can highly achieve both the corrosion resistance of the flat part and the corrosion resistance after painting. Further, according to the manufacturing method of the molten Zn-Al alloy plated steel sheet of the present invention, a molten Zn-Al alloy plated steel sheet with highly compatible corrosion resistance of the flat part and corrosion resistance after painting can be manufactured.

Embodiments for Carrying Out the Invention

[0018] Hereinafter, embodiments of the molten Zn-Al alloy plated steel sheet and its manufacturing method according to the present invention will be described. The embodiments described below are an example of embodying the present invention, and do not limit the configuration of the present invention with its specific examples.

[0019] [Molten Zn-Al alloy plated steel sheet] The molten Zn-Al alloy plated steel sheet according to an embodiment of the present invention has a steel sheet and a plating layer formed on at least one side of the steel sheet. The plating layer has a component composition containing, in mass%, Al: 5.1 to 22.0%, Si: 0.0 to 2.2%, and Fe: 0.0 to 2.0%, and the balance being composed of Zn and inevitable impurities, and the cross-sectional area fraction of the old FCC phase in the plating layer is 10% or less. In the molten Zn-Al alloy plated steel sheet, the old FCC phase has a Zn-rich part and an Al-rich part, and it is preferable that the Zn solid solution amount in the Al-rich part is 10% by mass or less.

[0020] (Steel sheet) The type of steel sheet (base steel sheet) used in this embodiment is not particularly limited. For example, a hot-rolled steel sheet or strip that has undergone pickling and descaling treatment, or a cold-rolled steel sheet or strip obtained by cold-rolling them, can be used as the base steel sheet. The hot rolling process can be carried out in the usual way, consisting of slab heating, rough rolling, and finish rolling followed by winding. Furthermore, the heating temperature and finish rolling temperature in the hot rolling process are not particularly limited and can be carried out at normal temperatures. The pickling process performed after hot rolling can also be carried out in the usual way, such as cleaning using hydrochloric acid or sulfuric acid. The cold rolling process performed after the pickling process is also not particularly limited, but can be carried out with a reduction ratio of, for example, 30-90%. If the reduction ratio is 30% or more, the mechanical properties of the base steel sheet will not deteriorate. On the other hand, if the reduction ratio is 90% or less, rolling costs can be reduced. When using cold-rolled steel sheets as a base sheet, it is more economical and preferable to use unannealed sheets, although annealed base sheets may also be used.

[0021] (Al:5.1~22.0% by mass) Sufficient corrosion resistance of flat plates can be obtained by setting the Al content of the plating layer of a hot-dip Zn-Al plated steel sheet to 5.1% by mass or more. This is because a thick oxide film mainly composed of Al is formed on the surface of the plating layer, reducing the oxygen reduction rate that constitutes the cathode reaction in an atmospheric corrosive environment, and as a result the corrosion rate decreases. Furthermore, the higher the Al content of the plating layer, the greater the effect of reducing the cathode reaction and the corrosion rate. Therefore, the Al content of the plating layer of a hot-dip Zn-Al plated steel sheet should be 5.1% by mass or more, preferably 5.5% by mass or more, and more preferably 6.0% by mass or more. On the other hand, if the Al content of the plating layer of a hot-dip Zn-Al plated steel sheet exceeds 22.0% by mass, the amount of precipitated prior FCC phase becomes excessive, and the corrosion resistance after painting deteriorates significantly. Therefore, the Al content of the plating layer of a hot-dip Zn-Al plated steel sheet should be 22.0% by mass or less, preferably 20.0% by mass or less, and more preferably 18.0% by mass or less.

[0022] (Si:0.0~2.2% by mass) The plating layer of a molten Zn-Al plated steel sheet may contain Si. The inclusion of Si delays the alloying reaction between Al in the plating layer and the underlying steel sheet, allowing for a thinner interfacial alloy layer. This suppresses the peeling of the plating layer starting from the interfacial alloy layer, especially during particularly severe processing such as unbending after bending. Therefore, the Si content of the plating layer of a molten Zn-Al plated steel sheet should be 0.0 mass% or more, preferably 0.1 mass% or more. On the other hand, if the Si content of the plating layer exceeds 10% relative to the Al content, the effect of Si saturates, and further addition of Si leads to increased costs and bath dross, as well as deposition of single-phase Si within the plating layer, degrading the corrosion resistance of the flat plate portion. In this embodiment, since the Al content of the plating layer is 22.0 mass% or less, the Si content of the plating layer should be 2.2 mass% or less, preferably 2.0 mass% or less, and more preferably 1.8 mass% or less.

[0023] (Fe:0.0~2.0% by mass) The plating layer of a molten Zn-Al plated steel sheet may contain Fe. The inclusion of Fe forms a high-melting-point AlFe intermetallic compound, which acts as a nucleus during solidification, resulting in a plating layer with a fine structure and excellent uniform solubility. Therefore, the Fe content of the plating layer of a molten Zn-Al plated steel sheet should be 0.0 mass% or more, preferably 0.1 mass% or more. The Fe content of the plating layer increases with the growth of the interfacial alloy layer formed at the interface between the substrate steel sheet and the plating layer, and the interfacial alloy layer grows more easily when the plating bath temperature and the plate entry temperature are increased. On the other hand, if the Fe content of the plating layer exceeds 2.0 mass%, the workability decreases significantly. Therefore, the Fe content of the plating layer should be 2.0 mass% or less, preferably 0.5 mass% or less.

[0024] (Optional addition element) The plating layer of the hot-dip Zn-Al plated steel sheet may further contain, as an optional additive, one or more elements selected from Ni, Cr, Ti, V, Mn, Co, Cu, Mo, W, B, Sn, Bi, Sb, Sr, and Ca, in a total amount of 0.01 to 3.0 mass%. These elements have the effect of stabilizing corrosion products and further improving corrosion resistance, as well as adjusting the spangle size and improving the surface appearance.

[0025] (Remainder: Zn and inevitable impurities) The remainder of the plating layer's composition consists of Zn and unavoidable impurities. Even if elements such as Be, P, Cd, and Pb are present in the plating layer within a range of 1.0 mass% or less each, this does not impair the effects of the present invention. On the other hand, if the plating layer of a molten Zn-Al plated steel sheet contains 0.04 mass% or more of Mg, the prior FCC phase is more likely to form, and excellent post-coating corrosion resistance cannot be obtained. Therefore, the Mg content of the plating layer of a molten Zn-Al plated steel sheet should be less than 0.04 mass%.

[0026] (Section integral ratio of the old FCC phase: 10% or less) When the Al content of a molten Zn-Al alloy exceeds 5.1% by mass, a microstructure with a face-centered cubic (FCC) structure crystallizes as the primary crystal during the solidification process, according to the phase diagram. This FCC structure further undergoes a segregation reaction at approximately 380°C, separating into a Zn-rich portion that contains almost no Al and an Al-rich portion mainly composed of Al with dissolved Zn. In this invention, the "prior FCC phase" refers to a phase that, during the solidification process, has an FCC structure and, after the segregation reaction, has become a microstructure in which the Zn-rich portion and the Al-rich portion are mixed. The prior FCC phase in this invention has a component composition in which the Al content is 10% by mass or more and 30% by mass or less, with the remainder being Zn and unavoidable impurities. Typically, the prior FCC phase can be observed in backscattered electron images using a scanning electron microscope as a microstructure in which a bright-contrast Zn-rich portion forms a matrix, with granular, dark-contrast Al-rich portions dispersed within it.

[0027] The prior FCC phase does not exhibit significantly inferior corrosion resistance compared to Zn under atmospheric conditions and has virtually no effect on the corrosion resistance of flat plates. However, in alkaline environments such as under a coating, and when the exposed underlying steel plate in defective areas becomes a large-area cathodesite, the corrosion rate becomes extremely high, resulting in selective corrosion. Therefore, the higher the cross-sectional integral ratio of the prior FCC phase, the significantly higher the coating blistering rate in areas under the coating and where the underlying steel plate is exposed, such as cut defects, leading to a deterioration in corrosion resistance after painting. In particular, if the cross-sectional integral ratio of the prior FCC phase in the plating layer exceeds 10%, corrosion resistance after painting deteriorates. Therefore, the cross-sectional integral ratio of the prior FCC phase in the plating layer should be 10% or less, preferably 5% or less, and more preferably 1% or less. There is no particular lower limit for the cross-sectional integral ratio of the prior FCC phase in the plating layer, but 0% is even more preferable.

[0028] The integral ratio of the prior FCC phase in the plating layer can be controlled by the composition of the plating bath, the temperature of the plating bath, the cooling rate after plating, and whether or not a heating process is performed after plating.

[0029] The cross-sectional integral ratio of the prior FCC phase in the plating layer can be measured as follows, for example. The cross-section of a molten Zn-Al plated steel sheet is embedded and polished, and a backscattered electron image is obtained using a scanning electron microscope (SEM). The SEM magnification is usually around 500x, and it is preferable to observe about 5 fields of view with the SEM. In the obtained backscattered electron image, each microstructure of the plating layer is classified by microstructural observation and EDX (energy-dispersive X-ray analysis), and the area of ​​each is measured. By dividing the area occupied by the prior FCC phase by the area occupied by the plating layer, the cross-sectional integral ratio of the prior FCC phase in that field of view can be determined. The arithmetic mean of the cross-sectional integral ratios of the prior FCC phase in all measured fields of view is taken as the cross-sectional integral ratio of the prior FCC phase in that sample.

[0030] (Zn solid solution content in the Al-rich portion of the old FCC phase: 10% by mass or less) The corrosion rate of the prior FCC phase depends on the amount of Zn dissolved in the Al-rich portion of the prior FCC phase. The greater the amount of Zn dissolved in the Al-rich portion, the more active the Al-rich portion becomes, and the higher the corrosion rate. Even when the plating layer has a prior FCC phase, the deterioration of corrosion resistance after coating can be reduced by keeping the amount of Zn dissolved in the Al-rich portion of the prior FCC phase to 10% by mass or less. Therefore, the amount of Zn dissolved in the Al-rich portion is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 1% by mass or less. There is no particular lower limit to the amount of Zn dissolved in the Al-rich portion, but it is most preferably 0% by mass.

[0031] The amount of Zn dissolved in the Al-rich portion of the prior FCC phase can be measured by the following method. A molten Zn-Al plated steel sheet is sheared to an appropriate size, and X-ray diffraction is performed using Cu-Kα as the X-ray source. The amount of Zn dissolved in the Al-rich portion can be determined from the shift in the peak position of the observed α-Al diffraction peak due to the solid solution of Zn. Since the atomic radius of Zn is smaller than that of Al, the lattice constant of α-Al decreases when Zn is dissolved. At this time, the diffraction peak of the (200) plane of α-Al, observed near 2θ = 44.7° in X-ray diffraction at a wavelength of 1.54178 Å using Cu-Kα as the X-ray source, shifts to a higher angle. In Al-Zn binary alloys, Vegard's law holds between the lattice constant and the amount of Zn dissolved, so the diffraction peak angle s of the (200) plane of α-Al Al Therefore, the amount of Zn solid solution c in the Al-rich region can be calculated using the following empirical formula. Zn It can be measured. c Zn (mass%)=271.65×s Al (degree)-12121

[0032] (η-Zn phase and Zn-Al eutectic structure) The plating layer mainly consists of an η-Zn phase and a Zn-Al eutectic structure, in addition to the prior FCC phase. The η-Zn phase has a component composition in which the Al content is 2% by mass or less, with the remainder being Zn and unavoidable impurities. The Zn-Al eutectic structure has a component composition in which the Al content is 3% by mass or more and 7% by mass or less, with the remainder being Zn and unavoidable impurities. The η-Zn phase and the Zn-Al eutectic structure have an excellent balance between corrosion resistance of the flat plate and corrosion resistance after coating. Therefore, the combined cross-sectional integral ratio of the η-Zn phase and the Zn-Al eutectic structure is preferably 85% or more, and more preferably 95% or more. There is no particular upper limit to the cross-sectional integral ratio of the η-Zn phase and the Zn-Al eutectic structure, but it is even more preferable for the combined ratio to be 100%. The cross-sectional integral ratio of the η-Zn phase and the Zn-Al eutectic structure can be measured in the same way as the prior FCC phase.

[0033] The plating layer can consist of MgZn2 phase and Mg2Zn as phases other than those mentioned above. 11 phase, Mg2Si phase, FeAl3 phase, Fe4Al 13 The material may contain phases such as Fe2Al5 phase, Fe2Al5Si phase, Fe2Al5Si phase, Si phase, etc., and as long as the total integral ratio of the sections is 5% or less, the effects of the present invention will not be impaired.

[0034] (Interfacial alloy layer) Hot-dip Zn-Al plated steel sheets have an interfacial alloy layer between the base steel sheet and the plating layer. The structure of the interfacial alloy layer is not particularly limited and generally consists of Fe-Al intermetallic compounds. The composition of the interfacial alloy layer is also not particularly limited and generally has a component composition consisting of Al: 10-60%, Si: 0-10%, Zn: 0-10% by mass, with the remainder being Fe and unavoidable impurities.

[0035] [Method for manufacturing hot-dip Zn-Al plated steel sheets] A method for manufacturing a molten Zn-Al plated steel sheet according to one embodiment of the present invention comprises: a molten plating step in which a steel sheet is immersed in a plating bath having a component composition of Al: 5.1 to 22.0%, Si: 0.0 to 2.2%, and Fe: 0.0 to 2.0% by mass, with the remainder being Zn and unavoidable impurities, and the steel sheet is subjected to a molten plating treatment; a gas wiping step in which the steel sheet is then gas-wiped to adjust the amount of molten plating adhering to the steel sheet; and a cooling step in which the steel sheet is then cooled at a cooling rate of 30°C / second or more until the temperature of the steel sheet reaches 300°C from the solidification temperature of the plating bath, thereby solidifying the molten plating on the steel sheet and forming a plating layer. In the molten plating step, it is preferable that the immersion plate temperature be (temperature of the plating bath - 60)°C or higher and (temperature of the plating bath - 40)°C or lower.

[0036] A method for manufacturing a molten Zn-Al plated steel sheet according to one embodiment of the present invention may include a step of cooling the steel sheet after performing the above-described molten plating step and gas wiping step to solidify the molten plating on the steel sheet and form a plating layer (hereinafter also referred to as the "primary plating layer"). In this case, after forming the primary plating layer, a heating step is performed in which the steel sheet is heated to melt the primary plating layer and bring it into a molten plated state, and then the above-described cooling step is performed.

[0037] (Hot-dip plating process) In the hot-dip plating process, a steel sheet is immersed in a plating bath to perform hot-dip plating. The composition of the plating layer of a hot-dip Zn-Al plated steel sheet is almost identical to the composition of the plating bath used in the hot-dip plating process. Therefore, the composition of the plating bath should be the same as the composition of the desired plating layer, and should consist of Al: 5.1-22.0%, Si: 0.0-2.2%, and Fe: 0.0-2.0% by mass, with the remainder being Zn and unavoidable impurities. The plating bath may also contain one or more elements selected from Ni, Cr, Ti, V, Mn, Co, Cu, Mo, W, B, Sn, Bi, Sb, Sr, and Ca as optional additives, in a total amount of 0.01-3.0% by mass. The same description as for the remaining Zn and unavoidable impurities applies to the composition of the plating layer described above.

[0038] In the molten plating process, the temperature of the plating bath is preferably in the range of (solidification temperature + 30)°C to (solidification temperature + 60)°C, relative to the solidification temperature of the Zn-Al alloy corresponding to the composition of the plating bath as read from a general Zn-Al binary equilibrium phase diagram. If the temperature of the plating bath is below (solidification temperature + 30)°C, the viscosity of the plating bath will be high, and it may be difficult to control the amount of plating deposited. Therefore, it is preferable to set the temperature of the plating bath to (solidification temperature + 30)°C or higher. On the other hand, if the temperature of the plating bath exceeds (solidification temperature + 60)°C, the development of the interfacial alloy layer when immersed in the plating bath will be significant, and the workability of the molten Zn-Al plated steel sheet may deteriorate. Therefore, it is preferable to set the temperature of the plating bath to (solidification temperature + 60)°C or lower.

[0039] (Intrusion plate temperature) Conventionally, the entry plate temperature was controlled to be approximately the same as the plating bath temperature in order to facilitate control of the plating bath temperature. The inventors discovered that, although the principle is not clear, the amount of Zn dissolved in the Al-rich portion tends to decrease as the entry plate temperature decreases. The inventors then found that when manufacturing molten Zn-Al alloy plated steel sheets, setting the entry plate temperature to 40°C or more lower than the plating bath temperature reduces the amount of Zn dissolved in the Al-rich portion, thereby reducing the deterioration of corrosion resistance after coating. Therefore, in the molten plating process, the entry plate temperature is preferably (plating bath temperature - 40)°C or lower, and more preferably (plating bath temperature - 45)°C or lower. On the other hand, if the entry plate temperature is excessively low, the reactivity between the steel sheet and the plating bath decreases, making it easier for defects such as unplated areas to occur. Therefore, it is preferable to lower the entry plate temperature to a range that does not cause defects. Therefore, in the molten plating process, the plate temperature is preferably (plating bath temperature - 60)°C or higher, and more preferably (plating bath temperature - 55)°C or higher.

[0040] (Gas wiping) After the hot-dip galvanizing process, the steel sheet is gas-wiped to adjust the amount of hot-dip galvanizing coating on the sheet. The gas wiping method is not particularly limited and may be performed by known methods. The amount of galvanizing coating is not particularly limited, but to ensure proper operational efficiency, it should be 30 to 250 g / m² per side of the base steel sheet. 2 It is preferable to keep it within the range of [specify range].

[0041] (cooling process) In the cooling process, it is important to cool the steel sheet at a rate of 30°C / second or higher until the temperature of the steel sheet reaches 300°C from the solidification temperature of the plating bath. The solidification temperature of the plating bath can be determined from the composition of the plating bath using a general Zn-Al binary equilibrium phase diagram. The temperature of the steel sheet and the plating layer after the gas wiping process is generally 400°C or higher, although this depends on the composition of the plating layer. By setting a high cooling rate from the solidification temperature of the plating bath to 300°C, i.e., the cooling rate from the molten state to 300°C of the plating layer, the amount of precipitated prior FCC phase can be reduced, and the corrosion resistance after painting can be improved. Therefore, the cooling rate from the solidification temperature of the plating bath to 300°C of the steel sheet should be 30°C / second or higher, preferably 40°C / second or higher, and more preferably 50°C / second or higher. On the other hand, if the cooling rate from the solidification temperature of the plating bath to 300°C exceeds 120°C / second, the post-coating corrosion resistance improvement effect saturates, and equipment costs increase. Therefore, it is preferable that the cooling rate from the solidification temperature of the plating bath to 300°C of the steel plate is 120°C / second or less.

[0042] (Heating process) Even if the cooling rate does not meet the above range during the cooling process after the gas wiping process, the effects of the present invention can be obtained by heating the steel plate after cooling to melt the plating layer and then cooling at the above cooling rate. In the heating process, it is important to return the primary plating layer, which contains the precipitated prior FCC phase, to a molten state by heating the primary plating layer. For this reason, the steel plate is heated to 400°C or higher in the heating process. After the heating process, the above cooling process is performed, that is, by cooling at a cooling rate of 30°C / second or higher until the temperature of the steel plate drops from 400°C to 300°C, the amount of precipitated prior FCC phase can be reduced and the corrosion resistance after painting can be improved. [Examples]

[0043] Using a 0.8 mm thick ultra-low carbon cold-rolled steel sheet manufactured by conventional methods as the base sheet, hot-dip Zn-Al plated steel sheets of the present invention and comparative examples were manufactured in a hot-dip galvanizing facility. The immersion time was 3 seconds, and the amount of plating deposited was approximately 70 g / m², adjusted by controlling the gas wiping flow rate. 2 The thickness was controlled to approximately 10 μm. Table 1 shows the composition of the plating bath, solidification temperature, plating bath temperature, plate temperature, and cooling rate in the cooling process for each example. In cases No. 20 and 21, after gas wiping, cooling was performed at a cooling rate of 15°C / second to form the primary plating layer. After that, the molten Zn-Al plated steel sheet was heated to 485°C to melt the plating layer, and then cooled at the cooling rates shown in Table 1.

[0044] For each of the obtained hot-dip Zn-Al coated steel sheets, the cross-sectional area fractions of the old FCC phase, η-Zn phase, Zn-Al eutectic structure, and other phases in the coating layer were measured by the following method. The cross-section of the hot-dip Zn-Al coated steel sheet was embedded and polished, and multiple locations were observed at a magnification of 500 times by SEM to obtain a backscattered electron image. From the obtained backscattered electron image, the structure in the coating layer was classified into the old FCC phase, η-Zn phase, Zn-Al eutectic structure, and other phases by tissue observation and semi-quantitative analysis by EDX. The area of each phase was measured and divided by the area occupied by the coating layer within the field of view to obtain the area fraction of each phase in that field of view. The weighted average of the area fractions of each structure for 5 fields of view was taken as the representative value of the area fraction of each phase in that sample. The measurement results are shown in Table 1. In each example, an interfacial alloy layer was formed between the base steel sheet and the coating layer.

[0045] Also, for each of the obtained hot-dip Zn-Al coated steel sheets, the Zn solid solution amount in the Al-rich part of the old FCC phase was measured by the following method. The hot-dip Zn-Al coated steel sheet was sheared into 20 mm × 20 mm, and X-ray diffraction with a wavelength of 1.54178 Å using Cu-Kα as the X-ray source was performed. The diffraction peak angle s Al of the (200) plane of α-Al observed near 2θ = 44.7° was read, and the Zn solid solution amount c Zn in the Al-rich part was evaluated using the following equation based on Vegard's law. The measurement results are shown in Table 1. c Zn (mass%) = 271.65 × s Al (degree) - 12121

[0046] [Corrosion Resistance Evaluation] For the hot-dip Zn-Al coated steel sheets of the present invention examples and comparative examples, the flat part corrosion resistance and the corrosion resistance after painting were evaluated respectively.

[0047] (Flat Part Corrosion Resistance) Evaluation samples were prepared by shearing molten Zn-Al plated steel sheets of the present invention example and comparative example to a size of 70 mm × 150 mm, and the ends of each evaluation sample were sealed. For each evaluation sample, the corrosion resistance of the flat portion of the molten Zn-Al plated steel sheet was evaluated by a combined cycle corrosion test consisting of adhesion of artificial seawater and repeated wetting and drying under a constant dew point, as specified in ISO 16539B. The amount of salt adhesion was 1000 mg / m². 2 The following procedure was followed. After a 28-day test, corrosion products were removed according to the method for removing corrosion products from zinc alloys specified in ISO 8407, and the amount of corrosion per unit area of ​​the plating layer was quantified from the weight change before and after the test. The corrosion resistance of the flat plate was evaluated using the following four-level evaluation criteria (score), and a score of 1 or 2 was considered a pass. The evaluation results are shown in Table 1.

[0048] 1: (Corrosion amount) ≤ 5g / m 2 2: 5g / m 2 <(Corrosion amount) ≤ 7.5g / m 2 3: 7.5g / m 2 <(Corrosion amount) ≤ 10g / m 2 4: 10g / m 2 <(Amount of corrosion)

[0049] (Corrosion resistance after painting) Evaluation samples were prepared by shearing hot-dip Zn-Al plated steel sheets of the present invention example and comparative example to a size of 70 mm × 150 mm. These samples were then subjected to zinc phosphate conversion treatment and electrodeposition coating. The zinc phosphate conversion treatment was performed under standard conditions using PB-SX35 manufactured by Nippon Parkerizing Co., Ltd., and the electrodeposition coating was performed using Electron GT100 cationic electrodeposition paint manufactured by Kansai Paint Co., Ltd. to a coating film thickness of 15 μm. Subsequently, a cross-cut defect reaching the base steel sheet was made in the center of the evaluation sample, with a length of 60 mm and an intersection angle of 60 degrees, to create a corrosion resistance test piece. A combined cycle corrosion test as specified in SAE J2334 was performed on each of the obtained corrosion resistance test pieces, and the corrosion status after 30 cycles was evaluated. The post-coating corrosion resistance was determined on a scale of 4 levels based on the area of ​​paint film blistering from the cut defect. An evaluation result of 1 or 2 was considered a pass. The evaluation results are shown in Table 1.

[0050] 1: (Area of ​​paint film blistering) < 20 mm 2 2:20mm 2 ≤ (Area of ​​paint film blistering) < 40 mm 2 3:40mm 2 ≤ (Area of ​​paint film blistering) < 80 mm 2 4:80mm 2 ≤ (Area of ​​paint film blistering)

[0051] [Table 1] TIFF0007841490000002.tif25595

[0052] As shown in Table 1, the hot-dip Zn-Al plated steel sheet of the present invention was confirmed to have excellent corrosion resistance in both the flat portion and the post-painting corrosion resistance. [Industrial applicability]

[0053] According to the present invention, it is possible to provide a hot-dip Zn-Al plated steel sheet that achieves a high degree of both corrosion resistance in the flat portion and corrosion resistance after painting, and this can be suitably used as a plated steel sheet for automobiles.

Claims

1. It comprises a steel plate and a plating layer formed on at least one side of the steel plate, The aforementioned plating layer has a composition comprising, by mass%, Al: 5.1 to 22.0%, Si: 0.0 to 2.2%, and Fe: 0.0 to 2.0%, with an Mg content of less than 0.04% by mass, and the remainder consisting of Zn and unavoidable impurities. A hot-dip Zn-Al plated steel sheet in which the cross-sectional integral ratio of the prior FCC phase in the plating layer is 10% or less.

2. The aforementioned prior FCC phase has a Zn-rich portion and an Al-rich portion. The hot-dip Zn-Al plated steel sheet according to claim 1, wherein the amount of Zn solid solution in the Al-rich portion is 10% by mass or less.

3. The hot-dip Zn-Al plated steel sheet according to claim 1 or 2, wherein the component composition further contains 0.01 to 3.0% by mass of one or more selected from Ni, Cr, Ti, V, Mn, Co, Cu, Mo, W, B, Sn, Bi, Sb, Sr, and Ca.

4. A hot-dip plating process is performed by immersing a steel sheet in a plating bath having a composition in mass percent of Al: 5.1 to 22.0%, Si: 0.0 to 2.2%, and Fe: 0.0 to 2.0%, with an Mg content of less than 0.04 mass percent, and the remainder consisting of Zn and unavoidable impurities, thereby subjecting the steel sheet to a hot-dip plating treatment. Subsequently, a gas wiping step is performed on the steel plate to adjust the amount of hot-dip plating adhering to the steel plate. Subsequently, the steel plate is cooled at a cooling rate of 30°C / second or more until the temperature of the steel plate reaches 300°C from the solidification temperature of the plating bath, thereby solidifying the molten plating on the steel plate and forming a plating layer; A method for manufacturing a hot-dip Zn-Al plated steel sheet having the following characteristics.

5. A method for manufacturing a hot-dip Zn-Al plated steel sheet according to claim 4, comprising: a step of cooling the steel sheet after the gas wiping step and before the cooling step to solidify the hot-dip plating on the steel sheet and form a primary plating layer; and a heating step of heating the steel sheet thereafter to melt the primary plating layer and bring it to the state of hot-dip plating.

6. The method for manufacturing a hot-dip Zn-Al plated steel sheet according to claim 4, wherein in the hot-dip plating step, the temperature of the steel sheet immersed in the plating bath is set to be (temperature of the plating bath - 60)°C or higher and (temperature of the plating bath - 40)°C or lower.

7. A method for producing a hot-dip Zn-Al plated steel sheet according to any one of claims 4 to 6, wherein the aforementioned component composition further contains 0.01 to 3.0% by mass of one or more selected from Ni, Cr, Ti, V, Mn, Co, Cu, Mo, W, B, Sn, Bi, Sb, Sr, and Ca.

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