TWI787119B - Molten Al-Zn system coated steel sheet and its manufacturing method - Google Patents

Abstract

An object of the present invention is to provide a molten Al—Zn-based plated steel sheet which is stable and excellent in bending workability and corrosion resistance of a bent portion, and a method for producing the same. To achieve the object, the present invention is a molten Al-Zn-based plated steel sheet including a plated film containing Al: 45% by mass to 65% by mass and Si: 1.0% by mass to 3.0% by mass. , the remainder of which contains Zn, Fe, and unavoidable impurities. The molten Al-Zn-based plated steel sheet is characterized in that the plated film has dendrites mainly containing Al primary crystals, and Al-Zn-containing In the eutectic dendrite gap, the Al primary crystal includes a matrix of an α-Al phase and Zn precipitates, and the Zn content in the matrix is 30% by mass or less.

Description

Molten Al-Zn system coated steel sheet and its manufacturing method

The present invention relates to a molten Al—Zn-based plated steel sheet excellent in bending workability and corrosion resistance of a bent portion, and a method for producing the same.

It is known that molten Al—Zn-based plated steel sheets can achieve both the sacrificial corrosion resistance of Zn and the high corrosion resistance of Al, and thus exhibit high corrosion resistance even among hot-dip galvanized steel sheets. Therefore, molten Al—Zn-based plated steel sheets are often used in the field of building materials such as roofs and walls exposed to the outdoors for a long time, and in the field of civil engineering construction such as guardrails, wiring and piping, and soundproof walls. In particular, there is an increasing demand for materials with excellent corrosion resistance and maintenance-free materials in severe usage environments such as acid rain caused by air pollution, spraying of deicing agents for roads in snow-covered areas, and development of coastal areas. Therefore, in recent years, the demand for molten Al—Zn-based plated steel sheets has increased.

Here, the coating film of the molten Al-Zn-based plated steel sheet includes a main layer and an interface alloy layer existing at the interface between the base steel sheet and the main layer, and the main layer mainly includes: supersaturated Zn and Al dendrite ( dendrite) solidified part (the dendrite part of the α-Al phase), and the remaining part of the dendrite gap (between dendrites), and the α-Al phase is laminated in the film thickness direction of the plating film. a formed structure. Due to this characteristic film structure, the path of corrosion progress from the surface becomes complicated, so corrosion does not easily reach the base steel plate, molten Al-Zn-based plated steel plate and plated skin. Compared with the hot-dip galvanized steel sheet with the same film thickness, it can realize more excellent corrosion resistance.

However, although the hot-dip Al-Zn-based plated steel sheet has excellent corrosion resistance, there is a problem that the plated film is harder than the hot-dip galvanized steel sheet and the bending workability is inferior. Therefore, when bending the steel sheet, cracks (cracks) tend to occur in the plating film at the tip of the bent portion. Of course, this crack will damage the appearance, and the crack will reach the middle of the plating film, thereby causing the thickness of the plating protection layer to become thinner, or cracks penetrating through the plating film to expose the base steel plate, thereby becoming molten Al- The original excellent corrosion resistance of the Zn-based plated steel sheet is the reason why the bent portion is significantly reduced.

Therefore, various attempts have been made to improve the bending workability of molten Al—Zn-based plated steel sheets.

For example, techniques for improving bending workability by applying a predetermined heat history to a molten Al—Zn-based plated steel sheet after plating can be cited (for example, refer to Patent Document 1 and Patent Document 2).

[Prior art literature] [Patent Document]

Patent Document 1: Japanese Patent No. 3654521

Patent Document 2: Japanese Patent Laid-Open No. 2013-245355

In the techniques of applying heat history to the molten Al-Zn-based plated steel sheet as in Patent Document 1 and Patent Document 2, the plated film can be softened, and the bending workability can be improved to some extent.

However, the bending workability improved by the techniques of Patent Document 1 and Patent Document 2 is not sufficient for more severe bending work. Considering application to various building components, bending workability and processed parts are desired. Further improvement of corrosion resistance. Furthermore, development of a technology capable of improving bending workability and corrosion resistance of a processed portion more reliably (stable) is also desired.

In view of the circumstances, an object of the present invention is to provide a molten Al—Zn-based plated steel sheet which is stable and excellent in bending workability and corrosion resistance of a bent portion, and a method for producing the same.

In order to solve the above-mentioned problems, the inventors of the present invention conducted research on a molten Al-Zn-based plated steel sheet including a plated film containing Al: 45% by mass to 65% by mass and Si: 1.0% by mass. %~3.0% by mass, and the remainder contains Zn, Fe, and unavoidable impurities. As a result, it was found that the plating film has: dendrites mainly containing Al primary crystals, and dendrite gaps containing Al-Zn eutectics, wherein , in the Al primary crystal, when the Zn precipitates dispersed in the matrix of the α-Al phase are finer than 100nm, focusing on the influence on the hardening of dendrites, by suppressing the Zn content in the matrix When the temperature is low, dendrites can be softened while suppressing the above-mentioned miniaturization and increase of Zn precipitates, and stable and excellent bending workability and corrosion resistance of processed parts can be realized.

In addition, it was found that the above-mentioned Zn precipitates in the Al primary crystals are closely related to the conditions of the thermal history after the formation of the plating film, and that the thermal history after the formation of the plating film is determined by maximizing the temperature, heating time and cooling time By controlling the Zn content in the matrix to a low level by optimizing the space between them, a molten Al-Zn-based plated steel sheet having excellent bending workability and corrosion resistance of the processed portion can be obtained.

Furthermore, it was also found that in the Al-Zn eutectic, there is a structure in which Al parts and Zn parts are alternately arranged in stripes (hereinafter referred to as "stripe structure"), and when the period is 2 μm or less, attention is paid to reducing the The bending workability of the molten Al-Zn-based plated steel sheet can realize excellent bending workability and corrosion resistance of the processed part by eliminating the striated structure.

Furthermore, the so-called excellent bending workability described in the present invention means practically sufficient bending workability, and when evaluating with "T bending", at least "6T no cracks" is required, and "4T no cracks" is preferred. "degree. In addition, the so-called "T bending" is a 180° bending test performed with steel plates sandwiching the thickness of the steel plate. For example, in the case of "6T bending", six plates of the same thickness are sandwiched inside the target material. to make a 180° bend. In this case, "no cracks" means, for example, a state in which cracks are not observed when the outer tip of the bent portion is observed with a magnifying glass at 10 times. In addition, the bending test was a bending test in accordance with the adhesion test of plating described in Japanese Industrial Standard (Japanese Industrial Standard, JIS) G 3321 (2019).

By the way, the bending workability of ordinary molten Al-Zn-based plated steel sheets also depends on the conditions of the plating film, but most of them are above "12T without cracks", and even "10T bends" will not become "crack-free". ".

The present invention is based on the above findings, and its gist is as follows.

1. A molten Al-Zn-based coated steel sheet, comprising a coated film, the coated film The film has a composition containing Al: 45% by mass to 65% by mass, Si: 1.0% by mass to 3.0% by mass, and the remainder contains Zn, Fe, and unavoidable impurities. The molten Al-Zn-based plated steel sheet is characterized in that , the coating film has: dendrites mainly comprising Al primary crystals and dendrite gaps comprising Al-Zn eutectics, the Al primary crystals comprising a matrix of α-Al phase and Zn precipitates, in the matrix The Zn content is 30% by mass or less.

2. The molten Al—Zn-based plated steel sheet according to the above 1, wherein the average maximum diameter of the Zn precipitates in the Al primary crystals is 100 nm or more.

3. The molten Al-Zn-based plated steel sheet according to the above-mentioned 1 or the above-mentioned 2, wherein the Al-Zn eutectic in the dendrite gap does not have a stripe structure with a period of 2 μm or less.

4. A method for manufacturing a molten Al-Zn-based coated steel sheet, which is a method for manufacturing a molten Al-Zn-based coated steel sheet as described in any one of the above-mentioned 1 to the above-mentioned 3, characterized in that, comprising: A step of forming a plated film comprising Al: 45% by mass to 65% by mass and Si: 1.0% by mass to 3.0% by mass on the base steel sheet, with the remainder including Zn, Fe, and unavoidable impurities; and forming the After the coating is applied, the step of giving the steel sheet a heat history with a maximum attainable temperature of 150°C or more and 277°C or less. The heating time to 150° C. was set to be 3 hours or more, and the cooling time to 150° C. from the above-mentioned maximum attained temperature was set to be less than 2 hours.

According to the present invention, there can be provided a molten Al—Zn-based plated steel sheet which is stable and excellent in bending workability and corrosion resistance of a bent portion, and a method for producing the same.

Figure 1 is a diagram of the equilibrium state of the Al-Zn binary system.

Fig. 2 shows the Zn content in the matrix of the α-Al phase and the maximum diameter of the Zn precipitates in the Al primary crystals for samples of molten Al-Zn-based plated steel sheets of Comparative Example 1 and Example 14 of the present invention, respectively. The graph of the average value of , and the photograph obtained by observing the cross section of the Al primary crystal.

3 is photographs obtained by observing cross-sections of plated coatings of samples of molten Al—Zn-based plated steel sheets of Comparative Example 1, Invention Example 14, and Comparative Example 22. FIG.

4 is a graph showing the evaluation results of the corrosion resistance of the bent portion and a photograph obtained by observing the 1T bent portion for samples of molten Al—Zn-based plated steel sheets of Comparative Example 1 and Example 14 of the present invention.

(Molten Al-Zn-based plated steel sheet)

The molten Al-Zn-based plated steel sheet of the present invention has a plated film on the surface of the steel plate.

In addition, the plating film contains Al: 45% by mass to 65% by mass and Si: 1.0% by mass to 3.0% by mass, and the remainder substantially contains Zn, Fe, and Composition of impurities to avoid. When the coating film of the hot-dip-coated steel sheet has the above-mentioned composition, good corrosion resistance can be realized.

In addition, the plated film includes an interface alloy layer present on the interface side with the base steel sheet and a main layer present on the interface alloy layer.

In terms of the balance between the corrosion resistance and the work surface, the Al content in the plating film is set at 45% by mass to 65% by mass, preferably 50% by mass to 60% by mass.

When the Al content in the plating film is at least 45% by mass, dendrite solidification of Al primary crystals occurs, and a structure in which the dendrite solidified structure is laminated in the film thickness direction of the plating film can be obtained. By adopting the structure in which the dendrite solidified structure is laminated in the film thickness direction of the plating film, the corrosion progress path of the plating film becomes complicated, and the corrosion resistance can be improved. In addition, the more the dendrites are deposited, the more complicated the path of corrosion progresses, the less likely it is for corrosion to reach the base steel plate, and the corrosion resistance improves.

On the other hand, if the Al content in the plating film exceeds 65% by mass, almost all of the Zn present in the dendrites will be absorbed into the structure of Al primary crystals in solid solution, and the Al primary crystals cannot be suppressed when corrosion progresses. Dissolution reaction, corrosion resistance deterioration.

Si in the plated film is added for the purpose of suppressing the growth of the interface alloy layer formed at the interface with the base steel sheet and not deteriorating the adhesion between the plated film and the base steel plate.

In the case of the molten Al-Zn-based coated steel sheet of the present invention, when the steel sheet is immersed in an Al-Zn-based coating bath containing Si, Fe on the surface of the steel sheet is alloyed with Al or Si in the coating bath reaction, the intermetallic compound of Fe-Al system and/or Fe-Al-Si system is generated into a layer at the base steel plate/coating film interface (forming an interface alloy layer), which , the growth rate of Fe-Al-Si alloys is slower than that of Fe-Al alloys, so the higher the ratio of Fe-Al-Si alloys, the more growth of the overall alloy phase can be suppressed. Therefore, the Si content in the plating film needs to be 1.0% by mass or more.

On the other hand, the remaining Si is not consumed in the formation of the interface alloy layer and is precipitated as a Si phase in the plating film, but Si is electrochemically more expensive than Al primary crystals or Al-Zn eutectics. , and functions as a cathode, so it has the effect of promoting the corrosion of the plated film and lowering the corrosion resistance. Specifically, if the Si content in the plating film exceeds 3.0% by mass, not only the growth inhibitory effect of the alloy phase is saturated, but also the amount of the Si phase increases to promote corrosion. Therefore, the Si content is made 3.0% by mass or less. .

From the same viewpoint, the Si content in the plating film is more preferably 2.5% by mass or less.

In addition, the said plating film contains Zn, Fe, and unavoidable impurities.

Among these components, Fe is a component that is inevitably contained due to the elution of the steel plate or equipment in the bath in the coating bath, or a component that is supplied by diffusion from the base steel plate when the interface alloy layer is formed. A component unavoidably contained in the above-mentioned plating film. Fe in the plating film cannot be quantified by distinguishing between Fe taken in from the base steel sheet and Fe eluted from the plating bath. The Fe content in the plating film is generally about 0.3% by mass to 2.0% by mass.

Moreover, Cr, Ni, Cu, etc. are mentioned as an unavoidable impurity other than the said Fe.

There is no particular limitation on the total content of the Fe and the unavoidable impurities However, when contained in excess, various properties of the plated steel sheet may be affected, so the total is preferably 5.0% by mass or less, more preferably 3.0% by mass or less.

In addition, in the molten Al-Zn-based plated steel sheet of the present invention, the plated film may further contain a total of 0.01 mass %~10 mass%, one or two or more selected from Mg, Cr, Mn, V, Mo, Ti, Ca, Ni, Co, Sb and B. The reason for setting the total content of the above-mentioned components to 0.01% by mass to 10% by mass is that a sufficient corrosion retarding effect can be obtained without saturating the effect.

Furthermore, from the viewpoint of satisfying various characteristics, the adhesion amount of the plating film is preferably 45 g/m ² to 120 g/m ² per side. The reason is that, when the adhesion amount of the plating film is 45 ^g /m or more, sufficient corrosion resistance can be obtained for applications requiring long-term corrosion resistance such as building materials, and the adhesion of the plating film When the amount is 120 g/m ² or less, excellent corrosion resistance can be realized while suppressing the occurrence of plating cracks and the like during processing.

From the same viewpoint, the adhesion amount of the plating film is more preferably 45 g/m ² to 100 g/m ² .

Here, the deposition amount of the plating film can be derived, for example, by dissolving and peeling off a specific area of the plating film using a mixed solution of hydrochloric acid and hexamethylenetetramine described in JIS H 0401:2013. Coated film, and calculated from the difference in weight of the steel plate before and after peeling. In order to obtain the amount of plating adhesion on each side by this method, tape sealing is used to prevent the plating surface of the non-target side from being exposed, and then the dissolution is performed.

In addition, regarding the composition of the plating film, for example, the plating film can be dissolved by immersing in hydrochloric acid or the like, and analyzed by inductively coupled plasma (Inductively Coupled Plasma, ICP) emission spectroscopic analysis or atomic absorption spectroscopic analysis. Confirm the solution. This method is merely an example, and any method may be used as long as it can accurately quantify the component composition of the plating film, and is not particularly limited.

In addition, the coating film of the molten Al-Zn system plated steel sheet obtained by this invention is substantially the same as the composition of a coating bath as a whole. Therefore, control of the composition of the plating film can be performed with high precision by controlling the composition of the plating bath.

In addition, the interface alloy layer in the plating film is a layer present at the interface with the base steel sheet in the plating film, and is a layer containing Fe, Al, Si, Zn and unavoidable impurities. shaped interfacial alloy layer. As mentioned above, the interfacial alloy layer is necessarily formed by the alloying reaction between Fe on the surface of the base steel plate and Al or Si in the coating bath.

Since this interface alloy layer is hard and brittle, it becomes the starting point of cracks during processing when it grows thicker, so in the present invention, it is preferable to make it as thin as possible.

Here, in the molten Al-Zn-based plated steel sheet of the present invention, the plated film has dendrites mainly containing Al primary crystals and dendrite gaps containing Al-Zn eutectics.

Furthermore, in the molten Al-Zn-based plated steel sheet of the present invention, the Al primary crystals include a matrix of an α-Al phase and Zn precipitates, and the Zn content in the matrix is 30% by mass or less.

In the matrix of the α-Al phase, if Zn is supersaturated (at the Zn content In the state of exceeding 30% by mass) solidified in a solid solution state, the hardness increases due to the solid solution strengthening of Zn, so the elongation decreases and the bending workability decreases. Therefore, in the present invention, by limiting the Zn content in the matrix to 30% by mass or less, the solid solution strengthening of the Al primary crystals is suppressed, and the bending workability of the molten Al—Zn-based plated steel sheet is improved, and further Improve the corrosion resistance of the processed part. In addition, the finer the Zn precipitates, the more significant the decrease in bending workability due to precipitation strengthening tends to be. Therefore, by setting the Zn content in the matrix of the α-Al phase to 30% by mass or less, Zn precipitation can also be promoted. growth of things. It should be noted that the Zn content in the matrix refers to the content of Zn contained in the matrix, and does not include the content of Zn (Zn precipitates) that has separated out.

From the same viewpoint, the Zn content in the matrix is preferably 25% by mass or less, more preferably 20% by mass or less.

Furthermore, the so-called Zn precipitates are granular precipitates mainly composed of Zn. In the present invention, the Ultra Low Accelerating Voltage Scanning Electron Microscope (Ultra Low Accelerating Voltage Scanning Electron Microscope, hereinafter referred to as The spatial resolution of "extremely low accelerated SEM") is about 30 nm, and smaller Zn precipitates cannot be observed, so precipitates having a diameter of 30 nm or more are regarded as Zn precipitates.

Regarding the Al primary crystals, when Zn precipitates are dispersed in the matrix of the α-Al phase, as described above, there is a tendency for bending workability to decrease due to precipitation strengthening, and the finer the precipitates, the more prominent this tendency is. Therefore, it is more favorable for bending workability to greatly grow the Zn precipitates. Specifically, the average maximum diameter of the Zn precipitates in the Al primary crystals is preferably 100 nm or more.

Furthermore, the average of the maximum diameters of the Zn precipitates is, for example, in accordance with the maximum diameter when the Al primary crystals are observed with three or more fields of view by an ultra-low accelerated SEM (acceleration voltage 3 kV, magnification of 20,000 times or more). The major axis of the Zn-based precipitates present in each field of view was measured at 10 points in descending order, and the average value of these measured values was taken.

In addition, the plating film has dendrite gaps containing Al—Zn eutectics. The interdendritic interstitial space sometimes includes a simple Si phase in addition to the Al—Zn eutectic.

The Al—Zn eutectic constituting the dendrite gap includes Al parts and Zn parts. When this Al—Zn eutectic is heated to 277° C. or higher, the solid solubility of Zn in the Al portion increases, and the Zn portion becomes almost a solid solution, thereby becoming an Al portion containing Zn more supersaturated. Then, when the Al-Zn eutectic is cooled, it changes into an Al-Zn eutectic again below 277°C. At this time, the Al-Zn eutectic has a striped structure in which Al parts and Zn parts are alternately arranged in stripes .

In addition, the inventors of the present invention conducted research and found that although the mechanism is not clear, the stripe structure of the Al-Zn eutectic reduces the bending workability of the molten Al-Zn-based plated steel sheet. When the period of the stripes of the above-mentioned stripe structure is as small as 2 μm or less, the reduction in bending workability becomes remarkable.

Therefore, from the viewpoint of further improving the bending workability of the molten Al-Zn-based plated steel sheet of the present invention and the corrosion resistance of the processed portion, it is preferable that no period exists in the Al-Zn eutectic in the dendrite gap. It is a striped structure of 2 μm or less. Furthermore, there is no particular limitation on the lower limit value of the stripe period of the stripe structure. However, due to the performance of the measuring device described later, it is difficult to confirm the existence of the stripe-like structure when the period is less than 30 nm. Therefore, in the present invention, the stripe period is set to 30 nm. The above are regarded as striated organizations.

The striped structure of the above-mentioned Al-Zn eutectic can be measured by ultra-low acceleration SEM (acceleration voltage 3 kV) in the same way as Zn precipitates in Al primary crystals. In an SEM with a high accelerating voltage, for example, an accelerating voltage of 15kV or more, the stripe-like structure of the Al-Zn eutectic whose stripe period is as small as 2 μm or less cannot be detected, but in the present invention, by using an extremely low acceleration The presence or absence of the presence can be confirmed by observing with SEM. Furthermore, the Zn precipitates and the stripe-like structure of the Al-Zn eutectic are all finer than those generated when the heat history is applied, so for example, in the observation using an accelerating voltage of 15kV, this is not taken into account. The presence or absence of some.

Furthermore, there is no particular limitation on the method for controlling the Zn content in the above-mentioned matrix, the maximum diameter of Zn precipitates, and the presence or absence of stripe-like structures with a period of 2 μm or less. appropriate control.

For example, as described later, by determining the composition of the plating bath and optimizing the conditions of the thermal history after the formation of the plating film, the Zn content in the matrix, the maximum diameter and period of Zn precipitates can be controlled The presence or absence of a streak-like structure of 2 μm or less.

In addition, in the molten Al—Zn-based plated steel sheet of the present invention, a coating film can be formed directly on the above-mentioned plated film or a coating film can be formed via an intermediate layer.

In addition, the kind of said coating film and the method of forming a coating film are not specifically limited, It can select suitably according to the required performance. For example, formation methods, such as roll coater coating, curtain coating, and spray coating, are mentioned. After coating the paint containing organic resin, it can be heated and dried by hot air drying, infrared heating, induction heating and other methods to form a coating film.

In addition, the intermediate layer is not particularly limited as long as it is a layer formed between the coating film of the hot-dip coated steel sheet and the coating film. For example, primers such as chemical conversion treatment film and adhesive layer are mentioned. Regarding the above-mentioned chemical conversion treatment film, for example, it is possible to apply a chromate treatment solution or a chromium-free chemical conversion treatment solution, and perform a chromate treatment or a chromate treatment in which a steel sheet is dried at a temperature of 80° C. to 300° C. without washing with water. Chromium is formed by chemical conversion treatment. These chemical conversion treatment films can be single-layer or multi-layer, and in the case of multiple layers, a plurality of chemical conversion treatments can be performed sequentially.

(Manufacturing method of molten Al-Zn-based plated steel sheet)

The method for producing a molten Al—Zn-based plated steel sheet of the present invention includes a step of forming a plated film on a base steel plate, and a step of providing a heat history to the steel plate after forming the plated film.

In addition, the method of forming the plating film on the base steel sheet is not particularly limited. For example, it can be manufactured by washing, heating, and immersing the base steel sheet in a coating bath using continuous hot-dip coating equipment.

In the heating step of the base steel sheet, recrystallization annealing and the like are performed in order to control the structure of the base steel sheet itself, and to prevent oxidation of the steel sheet and reduce a small amount of oxide film existing on the surface, therefore, in a reducing atmosphere such as a nitrogen-hydrogen atmosphere, The heating is effective.

Furthermore, there is no particular limitation on the type of the base steel sheet or the components in the steel, and cold-rolled steel sheets or hot-rolled steel sheets can be used as appropriate according to the required performance or specifications. As the components in the steel, for example, C: 0.01 mass %~0.10% by mass of steel components, etc. However, C: Steel sheets having less than 0.01% by mass are not excluded in the present invention. In addition, in addition to C, Al, Si, Mn, and P as constituent elements, N, S, O, B, V, Nb, Ti, Cu, Mo, Cr, Co, Ni, Ca, Sr, In, A steel sheet in which Sn, Sb, etc. are added as trace elements also falls within the scope of the present invention.

In addition, there is no particular limitation on the method of obtaining the base steel plate. For example, in the case of the above-mentioned hot-rolled steel sheet, those that have undergone a hot-rolling step and a pickling step can be used, and in the case of the above-mentioned cold-rolled steel sheet, a cold-rolling step can be further added to manufacture. Furthermore, in order to obtain the characteristics of the steel sheet, a recrystallization annealing step or the like may be performed before the hot-dip coating step.

Regarding the plating bath used when forming the plating film, as described above, since the composition of the plating film as a whole is substantially the same as that of the plating bath, a bath containing Al: 45% by mass to 65% by mass is used. and Si: 1.0% by mass to 3.0% by mass, and the remainder substantially contains Zn, Fe, and unavoidable impurities.

Moreover, although the bath temperature of the said coating bath is not specifically limited, It is preferable to set it as the temperature range of (melting point+20 degreeC)-650 degreeC.

The reason for setting the lower limit of the bath temperature of the coating bath to the melting point + 20°C is that the bath temperature needs to be at least the freezing point in order to perform the hot-dip coating process. By setting the lower limit of the melting point + 20°C, the above Coagulation caused by a local drop in bath temperature of the coating bath. On the other hand, the reason for setting the upper limit of the bath temperature to 650°C is that, if it exceeds 650°C, rapid cooling of the plated film becomes difficult, and the temperature at the interface between the plated film and the base steel sheet becomes difficult. The interfacial alloy layer formed there may become thicker.

Furthermore, there is no particular limitation on the temperature of the base steel sheet immersed in the coating bath (entry sheet temperature). For example, to ensure that the continuous hot-dip From the viewpoint of plating characteristics during the plating operation or preventing changes in bath temperature, it is preferable to control within ±20° C. with respect to the temperature of the plating bath.

Furthermore, the time for immersing the base steel sheet in the coating bath is preferably 0.5 seconds or more. This is because, when the immersion time is less than 0.5 second, there is a possibility that a sufficient plating film cannot be formed on the surface of the base steel sheet. Furthermore, the upper limit of the immersion time is not particularly limited, but if the immersion time is prolonged, the interface alloy layer formed between the plated film and the steel sheet may become thicker, so it is preferably within 8 seconds. .

In addition, in the method for producing molten Al-Zn-based plated steel sheet of the present invention, in the step of imparting a heat history, the maximum attained temperature is 150° C. to 277° C. The temperature raising time to the said highest attainment temperature was 3 hours or more, and the cooling time from the said highest attainment temperature to 150 degreeC was made into less than 2 hours.

By providing such a heat history, a molten Al—Zn-based plated steel sheet that is stable and excellent in bending workability and corrosion resistance of bent portions can be obtained.

The reason for setting the maximum attainable temperature at the time of imparting the heat history to 150°C to 277°C is that the diffusion of Zn slows down when the maximum attainable temperature is less than 150°C, and the Al primary crystal cannot be fully realized. In addition, the stripped structure in the Al-Zn eutectic also remains, so that the bending workability of the molten Al-Zn-based plated steel sheet cannot be obtained sufficiently. On the other hand, if the maximum attained temperature exceeds 277°C, the solid solution strengthening and precipitation strengthening in the Al primary crystals are eliminated, and the stripe-like structure in the Al-Zn eutectic is also eliminated. However, after cooling to 277°C, the striped structure is regenerated in the Al-Zn eutectic, resulting in deterioration of the bending workability of the molten Al-Zn-based plated steel sheet.

From the same viewpoint, the maximum attained temperature at the time of providing the heat history is preferably 170°C to 250°C, more preferably 190°C to 230°C.

In addition, in the step of providing the thermal history, the reason for setting the temperature rise time from 100° C. to the maximum attained temperature to 3 hours or more is that by ensuring the temperature and time for Zn diffusion, the above-mentioned The Zn content in the matrix is controlled to be 30% by mass or less, and the maximum diameter of the Zn precipitates is controlled to be 100 nm or more on average. Thereby, the solid-solution strengthening and precipitation strengthening in the Al primary crystal can be sufficiently eliminated, and the striated structure can also be eliminated in the Al-Zn eutectic, so the bending of the molten Al-Zn-based plated steel sheet can be improved. Machinability, and then improve the corrosion resistance of the processed part.

From the same viewpoint, the temperature increase time from the above-mentioned 100° C. to the above-mentioned maximum attained temperature is preferably 5 hours or more. In addition, from the viewpoint of production efficiency, it is preferable to set the heating time from the above-mentioned 100° C. to the above-mentioned maximum attained temperature within 10 hours.

Furthermore, in the step of providing the heat history, the reason why the cooling time from the maximum attained temperature to 150° C. is set to less than 2 hours is to suppress as much as possible the plating achieved in the heating-up step. The structure of the film changes during the cooling stage, thereby maintaining the above-mentioned elimination of solid solution strengthening and precipitation strengthening, and suppressing the generation of striae structure.

From the same viewpoint, the cooling time from the maximum attained temperature to 150° C. is preferably 1.5 hours or less, more preferably 1 hour or less.

Here, FIG. 1 shows a diagram of the equilibrium state of the Al-Zn binary system.

In the usual hot-dip plating process, the cooling after plating is rapid cooling, so there is no time to discharge Zn from the dendrites during solidification, and Zn is solidified in a supersaturated (more than 30% by mass) solid solution state in the matrix. Therefore, Zn supersaturated in a solid solution in the α-Al phase (matrix) of the Al primary crystal causes solid solution strengthening and increases the hardness, resulting in a decrease in elongation and a decrease in bending workability.

And, when heating is performed after the formation of the above-mentioned plating film, the supersaturated Zn of the α-Al phase is precipitated, and the solid solubility of Zn is lowered. solidified state. In this case, it can be seen that by controlling the Zn content in the matrix to be 30% by mass or less, the solid solution strengthening of the Al primary crystal is eliminated.

In addition, the Al-Zn eutectic contains Al parts and Zn parts, and when it is heated to 277° C. or higher, the Zn solid solubility of the Al part increases, and the Zn part is almost solid-solved, thereby becoming an Al part containing Zn more supersaturated. Furthermore, it can be seen that by cooling after heating, it changes again to Al-Zn eutectic at 277° C. or lower, and this Al-Zn eutectic becomes a striped structure in which Al parts and Zn parts are alternately arranged in stripes.

In addition, in the manufacturing method of the molten Al-Zn-based plated steel sheet of the present invention, steps other than the step of forming a plated film and the step of providing a heat history described above are not particularly limited, and the Any steps are suitably carried out for the required properties of the molten Al-Zn-based plated steel sheet.

In addition, it may further include: in the molten Al-Zn system of the present invention described above A step of forming a coating film directly or via an intermediate layer on the obtained molten Al-Zn-based plated steel sheet.

In addition, the method of forming the said coating film is not specifically limited, It can select suitably according to the required performance. For example, formation methods, such as roll coater coating, curtain coating, and spray coating, are mentioned. After coating the paint containing organic resin, it can be heated and dried by hot air drying, infrared heating, induction heating and other methods to form a coating film.

In addition, the intermediate layer is not particularly limited as long as it is a layer formed between the coating film of the hot-dip coated steel sheet and the coating film. The type and formation method of the intermediate layer are the same as those described in the molten Al—Zn-based plated steel sheet of the present invention.

[Example]

<Sample 1~Sample 30>

(1) Manufacture of molten Al-Zn-based plated steel sheet

A cold-rolled steel sheet with a thickness of 0.35 mm produced by a conventional method was used as a base steel sheet, and annealing and plating were performed in a continuous hot-melt coating equipment to produce the coating composition shown in Table 1. and the amount of molten Al-Zn-based coated steel sheet A to molten Al-Zn-based coated steel sheet C.

In addition, regarding the composition of the coating bath used in the manufacture of hot-dip-coated steel sheets, Al: 55% by mass, Si: 1.6% by mass, Fe: 0.4% by mass, and the remainder substantially Zn and unavoidable impurities were used. Based on the composition (plating A) of , the composition (plating B, plating C) of the content of each component was changed.

In addition, the bath temperatures of the coating baths were all set to 590° C., and the coating entry plate temperature of the base steel sheet was controlled to be the same temperature as the coating bath temperature. Furthermore, the deposition amount of the plating film was controlled to be 85±10 g/m ² per side.

(2) Assignment of thermal history

A heat history was given to the obtained molten Al-Zn-based plated steel sheet under the conditions shown in Table 2 to obtain molten Al-Zn-based plated steel sheet of each sample.

(3) Confirmation of the deposition amount and composition of the plating film

Punch out 100mmΦ from the molten Al-Zn-based plated steel sheet of each sample, seal the non-measurement surface with tape, and seal it with a mixed solution of hydrochloric acid and hexamethylenetetramine as shown in JIS H 0401 (2013). The plating film was dissolved and peeled off, and the adhesion amount of the plating film was calculated from the difference in mass of the sample before and after peeling.

Then, the stripping solution was filtered, and the filtrate and solid content were analyzed separately. Specifically, components other than insoluble Si were quantified by performing ICP emission spectroscopic analysis on the filtrate.

In addition, solid content was melt|dissolved by adding sodium carbonate and sodium tetraborate after drying and ashing in the heating furnace of 650 degreeC. In addition, the melt was dissolved with hydrochloric acid, and insoluble Si was quantified by performing ICP emission spectroscopic analysis on the melt. The Si concentration in the plating film was obtained by adding the insoluble Si concentration obtained by the solid content analysis to the soluble Si concentration obtained by the filtrate analysis.

Table 1 shows the composition and adhesion amount of the obtained plating film A to plating film C.

[Table 1]

<Evaluation>

For each sample of the molten Al—Zn-based plated steel sheet obtained as described above, the following evaluations were performed. The evaluation results are shown in Table 1.

(1) Conditions for plating film

For the molten Al-Zn-based plated steel sheet of each sample, the cross-section of the plated film was observed by a very low-acceleration SEM, and the energy dispersive X-ray spectroscopy (hereinafter referred to as "EDX (Energy Dispersive X-Ray analysis) ”) for analysis.

The conditions for the observation and analysis of the coating film are set to use ULTRA55 (ultra-low acceleration SEM) manufactured by Zeiss (Zeiss) and Ultim Extreme (EDX) manufactured by Oxford Instruments (Oxford Instruments) to The acceleration voltage is 3kV, the observation magnification is 3000 times and 20000 times, and the point analysis is carried out on the specified parts.

Furthermore, the average maximum diameter of the Zn-based precipitates present in the Al primary crystals was observed by three viewing fields at a magnification of 20,000, and 10 were extracted from the Al primary crystals in each viewing field in descending order. Precipitates mainly composed of Zn were spotted, their major diameters were measured, and the average was calculated. In addition, with regard to the minimum period of the striae-like structure, three visual field observations were performed at 20000 magnifications, and the fringe periods of the existing striae-like structures were measured, and the smallest of these was taken as the minimum period.

Table 2 shows the conditions of the obtained plating film (the average Zn concentration in the matrix and the maximum diameter of Zn precipitates, and the presence or absence of Al—Zn eutectic stripe structure and the minimum period).

In addition, for the molten Al—Zn-based plated steel sheets of Sample 1 and Sample 14, photographs obtained by observing precipitates mainly composed of Zn existing in Al primary crystals are shown in FIG. 2 .

Furthermore, photographs obtained by observing the stripe-like structure of the Al-Zn eutectic on the molten Al-Zn-based plated steel sheets of Sample 1, Sample 14, and Sample 22 are shown in FIG. 3 .

(2) Bending workability

For the molten Al-Zn-based plated steel sheet of each sample, in the range of 10T to 0T, a "T-bend" bending test was performed while reducing the 2T each (according to the plating method described in JIS G 3321 (2019). Bending test (bending test of adhesion test), when observed with a magnifying glass at 10 times, the limit of bending T to become "no crack" was confirmed. The results are shown in Table 2.

In addition, the so-called "T bending" is a 180-degree bending test performed in the state which sandwiched the plate|board thickness of a steel plate. In addition, "no cracks" in observation means a state in which no cracks are observed at all when the outer tip of the bent portion is observed with a magnifying glass of 10 times. Furthermore, the "limit of bending T" is the smallest T in the T-bending without cracks. For example, when no cracks are observed in 6T bending and cracks are observed in 4T bending, the limit of bending T is "6T".

(3) Corrosion resistance of bent parts

The molten Al-Zn-based plated steel sheets of Sample 1 and Sample 14 were subjected to outdoor exposure in Chuo-ku, Chiba City in the state of T-bending in the range of 0T to 9T. dew test. The bent portion after the exposure test for 4 years and 8 months was visually observed and evaluated according to the following criteria. The evaluation results are shown in FIG. 4 .

(evaluation criteria)

1 point: Clear red rust

2 points: slightly red rust

3 points: no red rust

From the results in Table 2 and FIG. 4 , it can be seen that the samples of the examples of the present invention are well-balanced and excellent in both the bending workability and the corrosion resistance of the processed portion compared to the samples of the comparative examples.

[industrial availability]

According to the present invention, there can be provided a molten Al—Zn-based plated steel sheet which is stable and excellent in bending workability and corrosion resistance of a bent portion, and a method for producing the same.

Claims (4)

A molten Al-Zn-based plated steel sheet, characterized in that it includes a plated film containing Al: 45% by mass to 65% by mass and Si: 1.0% by mass to 3.0% by mass, and the remainder contains Zn , Fe and the composition of unavoidable impurities, in the molten Al-Zn-based plated steel sheet, The plating film has: dendrites mainly containing Al primary crystals, and dendrite gaps containing Al-Zn eutectics, The Al primary crystals include an α-Al phase matrix and Zn precipitates, and the Zn content in the matrix is 30% by mass or less. The molten Al—Zn-based plated steel sheet according to claim 1, wherein the average maximum diameter of the Zn precipitates in the Al primary crystals is 100 nm or more. The molten Al-Zn-based plated steel sheet according to Claim 1 or Claim 2, wherein the Al-Zn eutectic in the interdendritic spaces does not have a striated structure with a period of 2 μm or less. A method of manufacturing a molten Al-Zn-based coated steel sheet, characterized in that it is a method of manufacturing a molten Al-Zn-based coated steel sheet as described in Claim 1 or Claim 2, wherein the molten Al-Zn-based coated steel sheet Manufacturing methods for clad steel include: A step of forming a plated film on the base steel sheet, wherein the plated film contains Al: 45% by mass to 65% by mass and Si: 1.0% by mass to 3.0% by mass, and the remainder contains Zn, Fe, and unavoidable impurities. composition; and A step of providing the steel sheet with a heat history whose maximum attained temperature is 150°C or more and 277°C or less after the above-mentioned plating film is formed, In the step of providing the heat history, the heating time from 100°C to the maximum attained temperature is set to 3 hours or more, and the cooling time from the maximum attained temperature to 150°C is set to less than 2 hours .

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