JP7291860B1 - Hot-dip Al-Zn-based plated steel sheet and manufacturing method thereof - Google Patents

Abstract

An object of the present invention is to provide a hot-dip Al-Zn coated steel sheet and a method for producing the same, which can more reliably improve the bending workability and the corrosion resistance of the bent portion. SOLUTION: To achieve the above object, the present invention provides a plating film having a composition containing 45 to 65% by mass of Al, 1.0 to 3.0% by mass of Si, and the balance being Zn, Fe and unavoidable impurities. , wherein the plating film has dendrites mainly composed of Al primary crystals and dendrite gaps containing Al-Zn eutectic, and the Al primary crystals are α- A matrix of Al phase and precipitates of Zn are included, and the Zn content in the matrix is 18% by mass or more and 30% by mass or less. [Selection figure] None

Description

TECHNICAL FIELD The present invention relates to a hot-dip Al-Zn-coated steel sheet having excellent bending workability and corrosion resistance of bent portions, and a method for producing the same.

Hot-dip Al-Zn coated steel sheets are known to exhibit high corrosion resistance among hot-dip galvanized steel sheets because both the sacrificial corrosion resistance of Zn and the high corrosion resistance of Al can be achieved. Therefore, hot-dip Al-Zn coated steel sheets are widely used in the field of building materials such as roofs and walls that are exposed to the outdoors for a long period of time, and in the field of civil engineering and construction such as guardrails, wiring and piping, and soundproof walls. In particular, the demand for materials with excellent corrosion resistance and maintenance-free materials is increasing in harsher usage environments such as acid rain due to air pollution, spraying of snow-melting agents to prevent road freezing in snowy areas, and development of coastal areas. Therefore, the demand for hot-dip Al-Zn coated steel sheets is increasing in recent years.

Here, the plating film of the hot-dip Al-Zn coated steel sheet consists of the main layer and the interfacial alloy layer existing at the interface between the base steel sheet and the main layer. The α-Al phase has a structure in which multiple layers are laminated in the film thickness direction of the plating film. there is This characteristic film structure complicates the path of corrosion progression from the surface, making it difficult for corrosion to easily reach the base steel plate. Superior corrosion resistance can be achieved compared to hot-dip galvanized steel sheets.

However, although the hot-dip Al-Zn-coated steel sheet has excellent corrosion resistance, there is a problem that the coating film is harder than the hot-dip galvanized steel sheet and the bending workability is inferior. Therefore, when the steel plate is bent, cracks are likely to occur in the plating film at the tip of the bent portion. This crack, of course, damages the appearance, but when the crack reaches halfway through the plating film, the thickness of the coating in that part becomes thin, or the crack penetrates the plating film and exposes the base steel plate. , and the like, and the excellent corrosion resistance inherent in the hot-dip Al-Zn-coated steel sheet has been significantly reduced in the bent portion.

Therefore, various attempts have been made to improve the bending workability of hot-dip Al-Zn coated steel sheets.
For example, there is a technique for improving bending workability by applying a predetermined heat history to a molten Al-Zn plated steel sheet after plating (see, for example, Patent Documents 1 and 2).

Japanese Patent No. 3654521 Japanese Unexamined Patent Application Publication No. 2013-245355

The technique of subjecting a hot-dip Al-Zn-coated steel sheet to a heat history, such as those disclosed in Patent Documents 1 and 2, can soften the coating film and improve the bending workability to some extent.
However, the bending workability improved by the techniques of Patent Documents 1 and 2 is not sufficient when more severe bending is performed, and considering application to various building members, bending workability and A further improvement in the corrosion resistance of the processed part has been desired. Further, there has also been a demand for the development of a technique capable of more reliably (stablely) improving the bending workability and the corrosion resistance of the worked portion.

In view of such circumstances, it is an object of the present invention to provide a hot-dip Al-Zn plated steel sheet that is stably excellent in bending workability and corrosion resistance of a bent portion, and a method for producing the same.

The present inventors have found that a hot-dip Al-Zn-based plating having a composition containing Al: 45 to 65% by mass and Si: 1.0 to 3.0% by mass, with the balance being Zn, Fe and unavoidable impurities As a result of studying steel sheets to solve the above problems, the plating film has dendrites mainly composed of Al primary crystals and dendrite gaps containing Al-Zn eutectic. focused on the fact that fine Zn precipitates of 100 nm or less scattered in the matrix of the α-Al phase affect dendrite hardening. Furthermore, the inventors have found that dendrite softening can be achieved while suppressing the above-mentioned Zn precipitates from becoming finer and increasing, and that stable and excellent bending workability and corrosion resistance of the worked portion can be achieved.
In addition, focusing on the fact that the Zn precipitates in the Al primary crystal described above are closely related to the conditions of the thermal history after the formation of the plating film, the thermal history after the formation of the plating film In addition, by optimizing the heating time and cooling time, the Zn content in the matrix can be kept low, and a hot-dip Al-Zn coated steel sheet having excellent bending workability and corrosion resistance of the worked part can be obtained. I found what I can do.
Furthermore, the Al-Zn eutectic has a structure in which Al parts and Zn parts are alternately arranged in stripes (hereinafter referred to as "stripe structure"). Focusing on reducing the bending workability of the plated steel sheet, the present inventors also found that by eliminating the striped structure, it is possible to achieve excellent bending workability and corrosion resistance of the worked part.

The excellent bending workability referred to in the present invention means practically sufficient bending workability. A degree of "crack" is required. In addition, "T bending" is a 180 ° bending test that is performed with the plate thickness of the steel plate sandwiched. Make a 180° bend. At this time, "no crack" means a state in which no crack is observed when the outer tip of the bent portion is observed with a magnifying glass of 10 times, for example. The bending test is a bending test conforming to the plating adhesion test described in JIS G 3321 (2019).
Incidentally, the bending workability of ordinary hot-dip Al-Zn-coated steel sheets depends on the conditions of the plating film, but is "12T no cracks" or higher, and "10T bending" does not often result in "no cracks".

The present invention was made based on the above findings, and the gist thereof is as follows.
1. A hot-dip Al-Zn plated steel sheet having a coating film containing 45 to 65% by mass of Al and 1.0 to 3.0% by mass of Si, with the balance being Zn, Fe and unavoidable impurities,
The plating film has dendrites mainly composed of Al primary crystals and dendrite gaps containing Al-Zn eutectic,
The Al primary crystal contains an α-Al phase matrix and Zn precipitates, and the Zn content in the matrix is 18% by mass or more and 30% by mass or less ,
In a bending test based on the plating adhesion test described in JIS G 3321 (2019), the test piece is bent 180 ° at an inner interval nt (where t is the plate thickness of the plated steel plate, n: the number of plated steel plates). A molten Al-Zn system characterized in that the bending workability indicated by the minimum nt at which cracks are not observed when the outer surface of the bent portion is observed with a magnifying glass of 10 times is 6 T or less. Galvanized steel sheet.

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

3. The Al-Zn eutectic in the dendrite gaps is characterized by not having a striped structure in which Al portions and Zn portions are alternately arranged in stripes with a period of 2 μm or less when observed with an ultra-low accelerating voltage SEM. , the hot dip Al-Zn plated steel sheet according to 1 or 2 above.
4. In a bending test based on the plating adhesion test described in JIS G 3321 (2019), the test piece is bent 180 ° at an inner interval nt (where t is the plate thickness of the plated steel plate, n: the number of plated steel plates). The bending workability indicated by the minimum nt at which cracks are not observed when the outer surface of the bent portion is processed and observed with a 10x magnifier is 4T or less, according to 1 to 3 above. The hot dip Al-Zn plated steel sheet according to any one of the above.

5. The method for producing a hot-dip Al-Zn-coated steel sheet according to any one of 1 to 4 above,
A step of forming a plating film on a base steel sheet having a composition containing 45 to 65% by mass of Al and 1.0 to 3.0% by mass of Si, with the balance being Zn, Fe and unavoidable impurities;
After forming the plating film, the steel sheet is subjected to a heat history with a maximum temperature of 150 ° C or higher and 277 ° C or lower,
In the step of applying the thermal history, the temperature rising time from 100 ° C. to the maximum temperature is set to 3 hours or more, and the cooling time from the maximum temperature to 150 ° C. is less than 2 hours. A method for producing a hot-dip Al-Zn coated steel sheet.

According to the present invention, it is possible to stably provide a hot-dip Al-Zn-coated steel sheet that is excellent in bending workability and corrosion resistance of a bent portion, and a method for producing the same.

It is an Al-Zn binary system equilibrium diagram. For the hot-dip Al-Zn coated steel sheet samples of Comparative Example 1 and Inventive Example 14, the Zn content in the α-Al phase matrix, the average maximum diameter of Zn precipitates in the Al primary crystal, and , and a photograph of the cross section of the Al primary crystal. 1 is a photograph of a cross-section of a plated film of each of hot-dip Al-Zn plated steel sheet samples of Comparative Example 1, Inventive Example 14, and Comparative Example 22. FIG. 3 is a graph showing evaluation results of corrosion resistance of bent portions and photographs of 1T bent portions of samples of hot-dip Al-Zn plated steel sheets of Comparative Example 1 and Inventive Example 14, respectively.

(Hot-dip Al-Zn coated steel sheet)
The hot dip Al-Zn plated steel sheet of the present invention has a plated film on the surface of the steel sheet.
The plating film has a composition containing 45 to 65% by mass of Al and 1.0 to 3.0% by mass of Si, with the remainder being substantially Zn, Fe and unavoidable impurities. When the plating film of the hot-dip plated steel sheet has the composition described above, good corrosion resistance can be achieved.
The plated film consists of an interfacial alloy layer existing on the side of the interface with the base steel sheet and a main layer existing on the interfacial alloy layer.

The Al content in the plating film is 45 to 65% by mass, preferably 50 to 60% by mass, from the viewpoint of the balance between corrosion resistance and operational aspects.
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 dendrite solidification structures are laminated in the film thickness direction of the plating film can be obtained. By adopting a structure in which the dendrite solidification structure is laminated in the film thickness direction of the plating film, the corrosion progression path of the plating film becomes complicated, and corrosion resistance can be improved. In addition, the more dendrites are laminated, the more complicated the corrosion progression path becomes, making it more difficult for corrosion to easily reach the base steel plate, thereby improving the corrosion resistance.
On the other hand, when the Al content in the plating film exceeds 65% by mass, most of the Zn present in the dendrite is incorporated into the structure dissolved in the Al primary crystals, suppressing the dissolution reaction of the Al primary crystals during the progress of corrosion. corrosion resistance deteriorates.

Si in the plating film is added for the purpose of suppressing the growth of an interfacial alloy layer formed at the interface with the base steel plate and not deteriorating the adhesion between the plating film and the base steel plate.
In the case of the hot-dip Al-Zn-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 and Al and Si in the coating bath undergo an alloying reaction, resulting in Fe -Al-based and/or Fe-Al-Si-based intermetallic compounds are formed in layers at the substrate steel plate/plating film interface (interfacial alloy layer is formed). Since the growth rate is slower than that of the -Al alloy, the higher the ratio of the Fe-Al-Si alloy, the more the growth of the entire alloy phase can be suppressed. Therefore, the Si content in the plating film should be 1.0% by mass or more.
On the other hand, the surplus Si that is not consumed in the formation of the interfacial alloy layer precipitates as a Si phase in the plating film, but the Si phase is electrochemically more noble than the Al primary crystal and the Al-Zn eutectic. Since it acts as a cathode, it has the effect of accelerating corrosion of the plating film and lowering the corrosion resistance. Specifically, when the Si content in the plating film exceeds 3.0% by mass, not only does the effect of suppressing the growth of the alloy phase described above saturate, but the amount of the Si phase increases and corrosion is promoted. , the Si content is 3.0% by mass or less.
From the same point of view, the Si content in the plating film is more preferably 2.5% by mass or less.

The plated film contains Zn, Fe and unavoidable impurities.
Among these components, Fe is inevitably included when the steel sheet and equipment in the bath dissolve into the plating bath, and Fe is supplied by diffusion from the base steel sheet when the interfacial alloy layer is formed. It is a component inevitably contained in the plating film. Regarding Fe in the plating film, it is not possible to distinguish and quantify Fe taken in from the base steel sheet and Fe eluted from the plating bath. The Fe content in the plating film is usually about 0.3 to 2.0% by mass.
Moreover, Cr, Ni, Cu, etc. are mentioned as unavoidable impurities other than said Fe.
The total content of Fe and the inevitable impurities is not particularly limited, but if contained excessively, it may affect various properties of the plated steel sheet, so the total content should be 5.0% by mass or less. is preferred, and 3.0% by mass or less is more preferred.

In addition, in the hot-dip Al-Zn-coated steel sheet of the present invention, the plating film can improve the stability of corrosion products and retard the progress of corrosion. % of one or more selected from Mg, Cr, Mn, V, Mo, Ti, Ca, Ni, Co, Sb and B. The reason why the total content of the above components is set to 0.01 to 10% by mass is that a sufficient corrosion retarding effect can be obtained and the effect will not be saturated.

From the viewpoint of satisfying various characteristics, the coating weight of the plating film is preferably 45 to 120 g/m ² per side. When the coating weight of the plating film is 45 g/m ² or more, sufficient corrosion resistance can be obtained even for applications that require long-term corrosion resistance, such as building materials. This is because, when it is m ² or less, it is possible to achieve excellent corrosion resistance while suppressing the occurrence of plating cracks and the like during processing.
From the same point of view, it is more preferable that the coating amount of the plating film is 45 to 100 g/m ² .

Here, the amount of the plating film is calculated from the difference in weight of the steel sheet before and after the peeling by dissolving and peeling the plating film of a specific area with a mixed solution of hydrochloric acid and hexamethylenetetramine shown in JIS H 0401: 2013, for example. can be derived in a way that In order to obtain the amount of plating deposited per side by this method, the dissolution described above can be performed after sealing with tape so that the plating surface of the non-target side is not exposed.

Further, the component composition of the plating film can be confirmed by, for example, immersing the plating film in hydrochloric acid or the like to dissolve the solution, and then using ICP emission spectroscopic analysis, atomic absorption spectrometry, or the like. 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.

The plating film of the hot-dip Al-Zn-coated steel sheet obtained by the present invention has almost the same composition as the plating bath as a whole. Therefore, the composition of the plating film can be accurately controlled by controlling the composition of the plating bath.

In addition, the interfacial alloy layer in the plating film is a layer existing at the interface with the base steel plate in the plating film, and is a layered interfacial alloy layer containing Fe, Al, Si, Zn and inevitable impurities. be. As described above, the interfacial alloy layer is inevitably formed by an alloying reaction between Fe on the surface of the base steel sheet and Al and Si in the plating bath.
Since this interfacial alloy layer is hard and brittle, if it grows thick, it becomes a starting point for cracks during working.

Here, in the hot-dip Al-Zn plated steel sheet of the present invention, the plating film has dendrites mainly composed of Al primary crystals and dendrite gaps containing Al-Zn eutectic.
In the hot-dip Al-Zn plated steel sheet of the present invention, the Al primary crystal contains an α-Al phase matrix and Zn precipitates, and the Zn content in the matrix is 30% by mass or less. Characterized by

When solid solution of Zn supersaturated in the α-Al phase matrix (with a Zn content exceeding 30 mass%) solidifies, hardness increases due to solid solution strengthening of Zn, and elongation decreases. and bending workability is lowered. 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 crystal is suppressed, and the bending workability of the hot-dip Al-Zn coated steel sheet is improved. corrosion resistance. In addition, since the decrease in bending workability due to precipitation strengthening tends to become more pronounced as the Zn precipitates become finer, the Zn content in the α-Al phase matrix is reduced to 30% by mass or less. , can also promote the growth of Zn precipitates. The Zn content in the matrix is the content of Zn contained in the matrix, and does not include the content of precipitated/separated Zn (Zn deposits).
From the same point of view, the Zn content in the matrix is preferably 25% by mass or less, more preferably 20% by mass or less.

The Zn precipitates are granular precipitates containing Zn as a main component. (referred to as "ultra-low acceleration SEM") has a spatial resolution of about 30 nm, and Zn precipitates smaller than this cannot be observed.

Regarding the Al primary crystal, when Zn precipitates are scattered in the matrix of the α-Al phase, as described above, precipitation strengthening tends to reduce bending workability. becomes more pronounced as it becomes finer. Therefore, it is advantageous for bending workability to grow the Zn precipitates to a large size. Specifically, it is preferable that the average maximum diameter of the precipitates of Zn in the Al primary crystal is 100 nm or more.
The average maximum diameter of the Zn precipitates is, for example, when the Al primary crystal is observed in three or more fields with an ultra-low acceleration SEM (accelerating voltage of 3 kV, magnification of 20000 times or more), It is a value obtained by measuring the major axis of the precipitate mainly composed of Zn at 10 points in descending order and averaging the measured values.

Further, the plating film has dendrite gaps containing Al-Zn eutectic. The dendrite gaps may contain a single Si phase in addition to the Al-Zn eutectic.
The Al—Zn eutectic forming the dendrite gaps consists of an Al portion and a Zn portion. When the Al—Zn eutectic is heated to 277° C. or higher, the Zn solid solubility in the Al part increases and the Zn part becomes a solid solution, forming an Al part containing Zn in a more supersaturated state. After that, when the Al-Zn eutectic is cooled, it changes to the Al-Zn eutectic again at 277°C or lower. It will have a striped texture arranged in the

As a result of research by the present inventors, although the mechanism is not clear, this Al-Zn eutectic stripe-like structure reduces the bending workability of the hot-dip Al-Zn-coated steel sheet. It was found that when the period of the stripes in the morphology structure is as small as 2 μm or less, the bending workability is significantly deteriorated.
Therefore, from the viewpoint of further improving the bending workability and the corrosion resistance of the worked portion of the hot-dip Al-Zn-coated steel sheet of the present invention, the Al-Zn eutectic in the dendrite gaps does not have a striped structure with a period of 2 μm or less. is preferred. The lower limit of the stripe period of the stripe structure is not particularly limited. However, if the period of the stripe-shaped structure is less than 30 nm, it is difficult to confirm the presence of the stripe-shaped structure due to the performance of the measuring device described later. It is regarded as striped tissue.

The striped structure of the Al—Zn eutectic described above can be measured by an ultra-low acceleration SEM (accelerating voltage of 3 kV), like the precipitates of Zn in the Al primary crystal. With an SEM with a high acceleration voltage, for example, an acceleration voltage of 15 kV or more, the stripe-like structure of the Al—Zn eutectic with a small stripe period of 2 μm or less could not be detected. It is possible to confirm the presence or absence by observing with SEM. The Zn precipitates and the Al-Zn eutectic striped structure are both finer than those formed when thermal history is applied. No consideration was given to the presence or absence of

The above-described method for controlling the content of Zn in the matrix, the maximum diameter of Zn precipitates, and the presence or absence of a striped structure with a period of 2 μm or less is not particularly limited, and optimization of manufacturing conditions, etc. can be controlled as appropriate.
For example, as will be described later, by determining the composition of the plating bath and optimizing the thermal history conditions after forming the plating film, the Zn content in the matrix, the maximum diameter of Zn precipitates, and the period It is possible to control the presence or absence of a striped texture of 2 μm or less.

Further, in the hot dip Al-Zn plated steel sheet of the present invention, a coating film can be formed on the plating film directly or via an intermediate layer.
The type of the coating film and the method of forming the coating film are not particularly limited, and can be appropriately selected according to the required performance. For example, forming methods such as roll coater coating, curtain flow coating, and spray coating can be used. After applying the coating material containing the organic resin, it is possible to form a coating film by heating and drying by means of hot air drying, infrared heating, induction heating, or the like.

Further, the intermediate layer is not particularly limited as long as it is a layer formed between the plating film of the hot-dip plated steel sheet and the coating film. Examples thereof include chemical conversion coatings and primers such as adhesive layers. For the chemical conversion coating, for example, a chromate treatment solution or a chromium-free chemical conversion treatment solution is applied, and without washing with water, it is formed by chromate treatment or chromium-free chemical conversion treatment that performs drying treatment at a steel sheet temperature of 80 to 300 ° C. Is possible. These chemical conversion coatings may be single-layered or multi-layered, and in the case of multi-layered coatings, a plurality of chemical conversion treatments may be performed sequentially.

(Manufacturing method of hot-dip Al-Zn coated steel sheet)
A method for producing a hot-dip Al-Zn plated steel sheet according to the present invention comprises a step of forming a plating film on a base steel plate, and a step of imparting a heat history to the steel plate after forming the plating film.

A method for forming the plating film on the base steel sheet is not particularly limited. For example, it can be produced by washing, heating, and immersing the base steel sheet in a plating bath in a continuous hot-dip plating facility.
In the heating step of the base steel sheet, recrystallization annealing is performed to control the structure of the base steel sheet itself. Heating in a reducing atmosphere such as is effective.

Furthermore, the type of the base steel sheet and the ingredients in the steel are not particularly limited, and cold-rolled steel sheets, hot-rolled steel sheets, etc. can be used as appropriate according to the required performance and standards. For example, C: 0.01 to 0.10% by mass can be used. However, a steel sheet containing less than 0.01% C is not excluded in the present invention. In addition to C, Al, Si, Mn, and P as component elements, N, S, O, B, V, Nb, Ti, Cu, Mo, Cr, Co, Ni, Ca, Sr, In, and Steel sheets containing Sn, Sb, etc. also fall within the scope of the present invention.
In addition, the method of obtaining the base steel sheet is not particularly limited. For example, in the case of the hot-rolled steel sheet, a hot-rolled steel sheet and a pickling process may be used, and in the case of the cold-rolled steel sheet, a cold-rolling process may be further added. Furthermore, in order to obtain the characteristics of the steel sheet, it is also possible to undergo a recrystallization annealing process or the like before the hot dip plating process.

Regarding the plating bath used to form the plating film, as described above, the composition of the plating film as a whole is almost the same as the composition of the plating bath, so Al: 45 to 65% by mass and Si: A composition containing 1.0 to 3.0% by mass, with the balance substantially consisting of Zn, Fe and unavoidable impurities is used.

Also, the bath temperature of the plating bath is not particularly limited, but is preferably in the temperature range of (melting point +20°C) to 650°C.
The reason why the lower limit of the bath temperature of the plating bath is the melting point + 20 ° C. is that the bath temperature must be above the solidification point in order to perform the hot-dip plating treatment, and the melting point + 20 ° C. This is to prevent solidification due to a local temperature drop of the plating bath. On the other hand, the upper limit of the bath temperature is set to 650°C because if it exceeds 650°C, rapid cooling of the plating film becomes difficult, and the interfacial alloy layer formed at the interface between the plating film and the base steel sheet becomes thick. This is because there is a risk

Further, the temperature of the substrate steel sheet entering the plating bath (entry sheet temperature) is not particularly limited. For example, it is preferable to control the temperature of the plating bath within ±20° C. from the viewpoint of securing plating properties in continuous hot-dip plating operation and preventing changes in bath temperature.

Furthermore, the time for immersing the base steel sheet in the plating bath is preferably 0.5 seconds or longer. This is because if the immersion time is less than 0.5 seconds, a sufficient plating film may not be formed on the surface of the base steel sheet. The upper limit of the immersion time is not particularly limited, but if the immersion time is prolonged, the interfacial alloy layer formed between the plating film and the steel sheet may become thicker, so it is preferably within 8 seconds. .

In the method for producing a hot-dip Al-Zn-coated steel sheet of the present invention, in the step of imparting the thermal history, the maximum temperature reached is 150° C. or more and 277° C. or less, and the temperature is raised from 100° C. to the maximum temperature reached. The cooling time is set to 3 hours or more, and the cooling time from the maximum temperature to 150°C is set to less than 2 hours.
By imparting such a thermal history, it is possible to stably obtain a hot-dip Al-Zn plated steel sheet that is excellent in bending workability and corrosion resistance of the bent portion.

The reason why the maximum temperature reached when the thermal history is applied is 150° C. or more and 277° C. or less is that if the maximum temperature is less than 150° C., the diffusion of Zn slows down, and solid solution strengthening and precipitation strengthening in the Al primary crystals occur. cannot be sufficiently eliminated, and the striped structure in the Al-Zn eutectic remains, so that the bending workability of the hot-dip Al-Zn coated steel sheet cannot be sufficiently obtained. On the other hand, when the maximum temperature exceeds 277°C, the solid solution strengthening and precipitation strengthening in the Al primary crystal are eliminated, and the striped structure in the Al-Zn eutectic is also decomposed. A stripe-shaped structure is generated again in the Al-Zn eutectic when passing through, resulting in deterioration of the bending workability of the hot-dip Al-Zn coated steel sheet.
From the same point of view, the maximum temperature reached when the heat history is applied is preferably 170°C or higher and 250°C or lower, and more preferably 190°C or higher and 230°C or lower.

In addition, in the step of applying the thermal history, the heating time from 100° C. to the maximum temperature is set to 3 hours or more because the temperature and time for Zn diffusion are ensured, so that Zn in the matrix This is because the content is suppressed to 30% by mass or less, and the average maximum diameter of the precipitates of Zn is controlled to 100 nm or more. As a result, the solid solution strengthening and precipitation strengthening in the Al primary crystal can be sufficiently eliminated, and the striped structure can also be eliminated in the Al-Zn eutectic. It is possible to improve the workability and, in turn, the corrosion resistance of the processed portion.
From the same point of view, the temperature rising time from 100° C. to the highest temperature is preferably 5 hours or longer. From the viewpoint of production efficiency, it is preferable that the temperature rise time from 100° C. to the highest temperature be within 10 hours.

Furthermore, in the step of applying the thermal history, the cooling time from the maximum temperature to 150 ° C. is set to less than 2 hours because the structure of the plating film achieved in the heating stage changes in the cooling stage. This is for suppressing this as much as possible, maintaining elimination of the above-described solid solution strengthening and precipitation strengthening, and suppressing the occurrence of a stripe-like structure.
From the same point of view, the cooling time from the highest temperature to 150° C. is preferably 1.5 hours or less, more preferably 1 hour or less.

Here, FIG. 1 shows an Al-Zn binary system equilibrium diagram.
In a normal hot-dip plating process, cooling after plating is rapid, so Zn is not released from dendrites in time during solidification, and the matrix solidifies with Zn supersaturated (more than 30% by mass) as a solid solution. Therefore, solid-solution strengthening occurs due to supersaturated solid-soluted Zn in the α-Al phase (matrix) of the primary Al crystal, and as a result of the increase in hardness, the elongation decreases and the bending workability decreases.
When heating is applied after the plating film is formed, supersaturated Zn in the α-Al phase precipitates and the solid solubility of Zn decreases, and the subsequent cooling separates the α-Al phase matrix and Zn precipitates. It solidifies as it is. At this time, by controlling the Zn content in the matrix to be 30% by mass or less, the solid-solution strengthening of Al primary crystals can be eliminated.
The Al-Zn eutectic consists of an Al part and a Zn part, and when this is heated to 277°C or higher, the Zn solid solution in the Al part increases and the Zn part becomes almost a solid solution, containing Zn in a more supersaturated state. It becomes the Al part to do. Then, by cooling after heating, it changes to Al-Zn eutectic again at 277 ° C or less, but this Al-Zn eutectic may have a striped structure in which Al parts and Zn parts are alternately arranged in stripes. Recognize.

In addition, in the method for producing a hot-dip Al-Zn coated steel sheet of the present invention, the steps other than the step of forming the plating film and the step of imparting a thermal history are not particularly limited. Any step can be carried out as appropriate depending on the required performance.

The method further comprises a step of forming a coating film directly or via an intermediate layer on the hot-dip Al-Zn plated steel sheet obtained by the above-described method for producing a hot-dip Al-Zn plated steel sheet of the present invention. can also

The method for forming the coating film is not particularly limited, and can be appropriately selected according to the required performance. For example, forming methods such as roll coater coating, curtain flow coating, and spray coating can be used. After applying the coating material containing the organic resin, it is possible to form a coating film by heating and drying by means of hot air drying, infrared heating, induction heating, or the like.

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

<Samples 1 to 30>
(1) Production of hot-dip Al-Zn-coated steel sheet A cold-rolled steel sheet with a thickness of 0.35 mm manufactured by a conventional method was used as the base steel sheet. Hot-dip Al-Zn plated steel sheets A to C having the plating film composition and coating amount shown in are produced.
The composition of the plating bath used in the production of the hot dip plated steel sheet is Al: 55% by mass, Si: 1.6% by mass, Fe: 0.4% by mass, and the balance is substantially Zn and unavoidable impurities (plating A ) was used as a basis, and compositions with different contents of each component were used (Plating B, Plating C).
Moreover, the bath temperature of the plating bath was set to 590° C. in all cases, and the plate temperature of the substrate steel plate entering the plating bath was controlled to be the same temperature as the plating bath temperature. Furthermore, the coating weight of the plating film was controlled so as to be 85±10 g/m ² per side.

(2) Application of Thermal History The obtained hot-dip Al-Zn plated steel sheets were subjected to heat history under the conditions shown in Table 2 to obtain hot-dip Al-Zn plated steel sheets of each sample.

(3) Confirmation of adhesion amount and composition of plating film Each sample is hot-dip Al-Zn coated steel plate, punched out 100mmφ, after sealing the non-measurement surface with tape, as shown in JIS H 0401 (2013) The plating was dissolved and peeled off with a mixed solution of hydrochloric acid and hexamethylenetetramine, and the amount of the plating film was calculated from the difference in mass of the sample before and after the peeling.
After that, the stripping liquid was filtered, and the filtrate and the solid content were analyzed. Specifically, components other than insoluble Si were quantified by subjecting the filtrate to ICP emission spectroscopic analysis.
The solid content was dried and incinerated in a heating furnace at 650° C., and then melted by adding sodium carbonate and sodium tetraborate. In addition, insoluble Si was quantified by dissolving the melt with hydrochloric acid and subjecting the solution to ICP emission spectroscopic analysis. The Si concentration in the plating film was obtained by adding the insoluble Si concentration obtained by solid content analysis to the soluble Si concentration obtained by filtrate analysis.
Table 1 shows the compositions and deposits of the obtained plating films A to C.

<Evaluation>
Each sample of the hot-dip Al-Zn coated steel sheet obtained as described above was evaluated as follows. Table 1 shows the evaluation results.

(1) Coating film conditions For each sample of the hot-dip Al-Zn coated steel sheet, the cross section of the coating film was observed by ultra-low acceleration SEM and analyzed by energy dispersive X-ray spectroscopy (hereinafter referred to as "EDX"). gone.
The conditions for observation and analysis of the above plating film are as follows: Zeiss ULTRA55 (ultra-low acceleration SEM) and Oxford Instruments Ultim Extreme (EDX) are used, acceleration voltage is 3 kV, observation magnification is 3000 times and 20000 times. A point analysis was used.
The average maximum diameter of Zn-based precipitates present in the Al primary crystals was determined by observing 3 visual fields at 20,000 magnifications. It was obtained by extracting, measuring the major axis, and calculating the average. In addition, the minimum period of the stripe-shaped structure was determined by observing three fields of view at a magnification of 20,000, measuring the stripe period of the existing stripe-shaped structure, and taking the smallest one among them as the minimum period.
Table 2 shows the conditions of the resulting plating film (Zn concentration in the matrix, average maximum diameter of Zn precipitates, presence or absence of Al-Zn eutectic stripe-like structure, and minimum period).
FIG. 2 shows photographs of the precipitates mainly composed of Zn present in the Al primary crystals of the hot-dip Al-Zn coated steel sheets of Samples 1 and 14. As shown in FIG.
Further, FIG. 3 shows photographs of the Al-Zn eutectic striped structures of the hot-dip Al-Zn coated steel sheets of Samples 1, 14 and 22. As shown in FIG.

(2) Bending workability For the hot-dip Al-Zn-coated steel sheet of each sample, the bending test of "T bending" (JIS G 3321 (2019) of the coating described in JIS G 3321 (2019) while decreasing by 2T in the range of 10T to 0T A bending test conforming to the adhesion test) was performed, and when observed with a magnifying glass at a magnification of 10, the limit of bending T at which "no cracks" occurred was confirmed. Table 2 shows the results.
In addition, "T-bending" is a 180° bending test performed in a state in which the plate thickness of the steel plate is sandwiched. Also, "no cracks" when observed means a state in which no cracks are observed 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 among the T-bending that did not crack. For example, if 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 Portion The hot-dip Al-Zn coated steel sheets of Samples 1 and 14 were subjected to an outdoor exposure test in Chuo Ward, Chiba City, while being subjected to T bending in the range of 0T to 9T. After the exposure test for 4 years and 8 months, the bent portion was visually observed and evaluated according to the following criteria. The evaluation results are shown in FIG.
(Evaluation criteria)
1 point: Clear red rust 2 points: Slight red rust 3 points: No red rust

From the results shown in Table 2 and FIG. 4, it can be seen that the samples of the invention examples are superior to the samples of the comparative examples in both bending workability and corrosion resistance of the worked portion in a well-balanced manner.

According to the present invention, it is possible to stably provide a hot-dip Al-Zn-coated steel sheet that is excellent in bending workability and corrosion resistance of a bent portion, and a method for producing the same.

Claims (4)

A hot-dip Al-Zn plated steel sheet having a coating film containing 45 to 65% by mass of Al and 1.0 to 3.0% by mass of Si, with the balance being Zn, Fe and unavoidable impurities,
The plating film has dendrites mainly composed of Al primary crystals and dendrite gaps containing Al-Zn eutectic,
The Al primary crystal contains an α-Al phase matrix and Zn precipitates, and the Zn content in the matrix is 18% by mass or more and 30% by mass or less,
In a bending test based on the plating adhesion test described in JIS G 3321 (2019), the test piece is bent 180 ° at an inner interval nt (where t is the plate thickness of the plated steel plate, n: the number of plated steel plates). A molten Al-Zn having a bendability of 4T to 6T , which is the minimum nt at which cracks are not observed when the outer surface of the bent portion is observed with a magnifying glass of 10 times. system plated steel sheet. 2. The hot-dip Al-Zn plated steel sheet according to claim 1, wherein the average maximum diameter of said Zn precipitates in said Al primary crystal is 100 nm or more. The Al-Zn eutectic in the dendrite gaps is characterized by not having a striped structure in which Al portions and Zn portions are alternately arranged in stripes with a period of 2 μm or less when observed with an ultra-low accelerating voltage SEM. , The hot dip Al-Zn plated steel sheet according to claim 1 or 2. A method for producing a hot-dip Al-Zn-coated steel sheet according to claim 1 or 2,
A step of forming a plating film on a base steel sheet having a composition containing 45 to 65% by mass of Al and 1.0 to 3.0% by mass of Si, with the balance being Zn, Fe and unavoidable impurities;
After forming the plating film, the steel sheet is subjected to a heat history with a maximum temperature of 150 ° C or higher and 277 ° C or lower,
In the step of applying the thermal history, the temperature rising time from 100 ° C. to the maximum temperature is set to 3 hours or more, and the cooling time from the maximum temperature to 150 ° C. is less than 2 hours. A method for producing a hot-dip Al-Zn coated steel sheet.

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