JP6509160B2 - Molten Al-Zn based plated steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a molten Al—Zn-based plated steel sheet which has excellent corrosion resistance of a flat plate portion and is also excellent in corrosion resistance of a processed portion due to good workability, and a method of manufacturing the same.

  The molten Al-Zn-based plated steel sheet exhibits excellent corrosion resistance as compared with other hot-dip galvanized steel sheets, because the sacrificial corrosion resistance of Zn and the high corrosion resistance of Al are compatible. For example, Patent Document 1 discloses a molten Al-Zn-based plated steel sheet containing 25 to 75% by mass of Al in a plating film. And, due to its excellent corrosion resistance, demand for molten Al-Zn plated steel sheets has increased in recent years mainly in the fields of building materials such as roofs and walls exposed to the long term outdoors, civil engineering and construction fields such as guard rails, wiring piping and sound barriers. ing.

  The plated film formed on the molten Al-Zn-based plated steel sheet is composed of an alloy layer present at the interface with the base steel sheet and an upper layer present thereon, the upper layer mainly containing Zn in supersaturation and Al being dendrite It comprises a solidified part (Al rich phase) and a remaining part of dendrite gaps (Zn rich phase), and has a structure in which a plurality of Al rich phases are laminated in the thickness direction of the plating film. Such a characteristic film structure complicates the corrosion propagation path from the surface, so that the corrosion is difficult to reach the substrate steel sheet easily, and the molten Al-Zn-based plated steel sheet has the same thickness as that of the plated film. Corrosion resistance superior to that of plated steel can be realized.

  In addition, a spangle pattern derived from solidification of the plating exists on the surface of the molten Al-Zn-based plated steel sheet. The spangles have fine irregularities corresponding to the Al-rich phase and the Zn-rich phase, and this diffusely reflects light, so the surface of the molten Al-Zn-based plated steel sheet has a beautiful appearance that shines silvery white. .

For such hot-dip Al-Zn-based plated steel sheets, using a hot-rolled steel sheet that has been subjected to pickling scale or a cold-rolled steel sheet obtained by cold rolling the hot-rolled steel sheet as a base steel sheet It is common to be manufactured.
Specifically, first, the base steel plate is heated to a specific temperature in an annealing furnace held in a reducing atmosphere, and along with annealing, removal of rolling oil and the like adhering to the steel plate surface and reduction removal of an oxide film are performed. Next, the base steel plate is immersed in the molten Al-Zn-based plating bath by passing the inside of the snout immersed in the plating bath at the lower end. Thereafter, the steel plate immersed in the plating bath is pulled up to above the plating bath via a sink roll, and a gas wiping nozzle disposed on the plating bath sprays a pressurized gas toward the surface of the steel plate. By adjusting the amount of plating adhesion and cooling with a cooling device, a molten Al-Zn-based plated steel sheet on which a desired plating film is formed is obtained.

  And, in order to secure the desired plating quality and material, adjustment and control of operation conditions such as heat treatment condition and atmosphere condition of annealing furnace in continuous hot-dip plating equipment mentioned above, composition of plating bath and cooling rate after plating Is done.

  In general, if the thickness of the plating film is the same, the upper layer having the effect of improving the corrosion resistance becomes thicker as the alloy layer becomes thinner. Therefore, suppressing the growth of the alloy layer contributes to the improvement of the corrosion resistance. In addition, since the alloy layer is harder than the upper layer and serves as a crack starting point during processing, the growth suppression of the alloy layer reduces the occurrence of cracks and also has an effect of improving bending workability. And since the underlying steel plate is exposed and inferior in corrosion resistance to the crack part which generate | occur | produced, reducing generation | occurrence | production of a crack will improve the corrosion resistance of a bending process part.

Japanese Patent Publication No. 46-7161

As described above, the hot-dip Al-Zn-based plated steel sheet is often used in the field of building materials such as roofs and walls which are exposed to the outside for a long time because of its excellent corrosion resistance.
And from the request | requirement about resource saving and energy saving in recent years, in order to attain lifetime improvement of a product, development of the fusion | melting Al-Zn type | system | group plated steel plate which was more excellent in corrosion resistance was desired.

  In addition, in the case of a hot-dip Al-Zn-based plated steel sheet manufactured in a continuous hot-dip plating facility, when the plating layer is solidified non-equilibrium by rapid cooling, the plating upper layer is hardened, so when bending is performed. The plated film may be cracked to form a crack, resulting in poor corrosion resistance at the machined part. Therefore, there has been a demand for improvement in the corrosion resistance of the machined portion by the improvement of the workability.

  In view of such circumstances, the present invention has a hot-dip Al-Zn-based plated steel sheet which has excellent corrosion resistance of a flat plate portion and is also excellent in corrosion resistance of a processed portion due to good workability. It is an object of the present invention to provide a method which can be manufactured by a continuous hot dip plating facility.

  As a result of intensive studies to solve the above problems, the present inventors have found that the corrosion resistance can be improved by containing at least one of Ca and Mg in a plating film. Furthermore, by limiting the size of the spangles formed on the plating film, it is possible to ensure good uniformity in appearance and to keep the Vickers hardness of the plating film in a specific range for the plated steel plate after cooling. It has been found that the corrosion resistance of the processed portion can be improved by softening and good processability.

The present invention has been made based on the above findings, and the summary thereof is as follows.
1. A molten Al-Zn-based plated steel sheet having a plated film on the surface of the steel sheet,
The plating film comprises an interface alloy layer present at the interface with the substrate steel plate and an upper layer present on the alloy layer, the upper layer comprising 20 to 95% by mass of Al, and 10% or less of the Al content, And having a composition containing 0.5 to 4.5% by mass of Mg , with the balance being Zn and unavoidable impurities,
The Vickers hardness of the said plating film is 50-100 Hv on average, The hot dip Al-Zn type | system | group plated steel plate characterized by the above-mentioned.

2 . The upper layer is characterized in that it further contains 0.01 to 10% by mass in total of one or more selected from among Mn, V, Cr, Mo, Ti, Sr, Ni, Co, Sb and B. The molten Al-Zn-based plated steel sheet as described in 1 above.

3 . The molten Al-Zn-based plated steel sheet as described in 1 or 2 above, wherein the average size of the spangles of the plating film is 0.5 mm or less.

4 . In a continuous hot-dip plating facility, a plating bath containing 20 to 95% by mass of Al, 10% or less of Si of Al content, and 0.5 to 4.5% by mass of Mg , with the balance being Zn and unavoidable impurities The base steel plate is immersed in the metal plate and subjected to hot-dip plating, and then the plated steel plate is held at a temperature of 250 to 375 ° C. for 5 to 60 seconds.

5 . The hot-dip galvanized steel sheet according to 4 above is cooled within 5 seconds from the bath temperature of the plating bath of -20 ° C to the bath temperature of the plating bath of -80 ° C within 5 seconds. Production method.

6 . Containing 0.01 to 10% by mass in total of one or two or more selected from among Mn, V, Cr, Mo, Ti, Sr, Ni, Co, Sb and B in the plating bath The manufacturing method of the fusion | melting Al-Zn type | system | group plated steel plate in any one of said 4 or 5 characterized by the above-mentioned.

7 . The method for producing a molten Al-Zn-based plated steel sheet according to any one of the above 4 to 6 , wherein the cooling time of the plated steel sheet is set to 3 seconds or less.

8 . The method for producing a molten Al-Zn-based plated steel sheet according to any one of 4 to 7 , wherein the holding temperature of the plated steel sheet after the cooling is 300 to 375 ° C.

9 . The holding time of the plated steel sheet after the cooling is 5 to 30 seconds, The method for producing a molten Al-Zn-based plated steel sheet according to any one of 4 to 8 above.

10 . After cooling the plated steel sheet, the plated steel sheet is further cooled to 375 ° C. or less before coming into contact with the top roll, and the method for producing a molten Al—Zn-based plated steel sheet according to any one of 4 to 9 above.

  ADVANTAGE OF THE INVENTION By this invention, while having the corrosion resistance of a favorable flat plate part, the hot-dip Al-Zn type | system | group plated steel plate excellent also in the corrosion resistance of a process part by favorable workability can be manufactured by continuous hot dip plating installation.

It is process drawing which showed one Embodiment of the manufacturing method of the fusion | melting Al-Zn type | system | group plated steel plate by this invention.

(Fused Al-Zn based plated steel sheet)
The molten Al-Zn-based plated steel sheet targeted by the present invention has a plated film on the surface of the steel plate, and the plated film is an interface alloy layer present at the interface with the base steel plate and an upper layer present on the alloy layer. It consists of The upper layer contains 20 to 95% by mass of Al, 10% or less of Si in Al content, and at least one of Ca and Mg, and the total content of Ca and Mg is 0.01 to 10%. %, The balance being composed of Zn and unavoidable impurities, and the Vickers hardness of the plating film is 50 to 100 Hv on average. Moreover, it is preferable that the average size of the spangles of the said plating film is 0.5 mm or less.

  The Al content in the plating film is 20 to 95% by mass, preferably 45 to 85% by mass, from the balance between corrosion resistance and operation surface. When the Al content in the upper layer of the plating is 20% by mass or more, dendrite solidification of Al occurs. Thus, the upper layer mainly contains Zn in a supersaturated state, and consists of a portion where Al is solidified with dendrite and a portion of the remaining dendrite gap, and the dendrite solidified portion has a structure excellent in corrosion resistance laminated in the film thickness direction of the plating film. . Further, the more the Al dendrites are laminated, the more complicated the corrosion progress path becomes, and the corrosion becomes difficult to reach the underlying steel sheet easily, so the corrosion resistance is improved. In order to obtain extremely high corrosion resistance, it is more preferable to make the Al content of the upper layer 45% by mass or more. On the other hand, when the Al content in the upper layer exceeds 95% by mass, the content of Zn having a sacrificial anticorrosive effect on Fe decreases, and the corrosion resistance is deteriorated. Therefore, the Al content of the upper layer is 95% by mass or less. In addition, if the Al content in the upper layer is 85% by mass or less, the amount of adhesion of plating decreases, and even when the steel base is easily exposed, it has a sacrificial anticorrosion effect on Fe and sufficient corrosion resistance. can get. Therefore, it is preferable to make Al content of a plating upper layer into 85 mass% or less.

  Further, Si suppresses the growth of the interface alloy layer formed at the interface with the substrate steel plate, is added to the plating bath for the purpose of improving the corrosion resistance and the workability, and is inevitably contained in the plating upper layer. Specifically, in the case of an Al-Zn-based plated steel sheet, when Si is contained in a plating bath and plating is performed, the steel sheet is immersed in the plating bath and at the same time Fe in the steel sheet surface and Al or Si in the bath. Alloying reaction to form a compound of Fe-Al system and / or Fe-Al-Si system. The formation of the Fe-Al-Si interface alloy layer suppresses the growth of the interface alloy layer. If the Si content of the plating bath is 3% or more of the Al content, it is possible to sufficiently suppress the growth of the interfacial alloy layer, which is preferable. On the other hand, when the Si content of the plating bath exceeds 10% of the Al content of the plating bath, a Si propagation phase is likely to occur in the upper layer of the formed plating film, causing a crack propagation path and reducing machinability. For this reason, the Si content in the plating bath is 10% or less of the Al content in the plating bath. Therefore, as described above, the composition of the plating upper layer is almost equal to the composition of the plating bath, so the Si content of the plating upper layer is 10% or less of the Al content of the plating upper layer.

  In the present invention, it is important that the upper layer of the plating film contains at least one of Ca and Mg, and the total content of Ca and Mg be 0.01 to 10% by mass. When the upper layer of the plating film is corroded, Ca and / or Mg are contained in the corrosion product, the stability of the corrosion product is improved, and as a result, the progress of the corrosion is delayed, the corrosion resistance is improved. There is. Here, the reason why the total content of Ca and / or Mg is 0.01 to 10% by mass is that the content is 0.01% by mass or more, and a sufficient corrosion delaying effect can be obtained, while the content is By setting the content to 10% by mass or less, an increase in manufacturing cost can be suppressed without the effect being saturated, and composition control of the plating bath can be easily performed.

  Further, from the viewpoint of obtaining better corrosion resistance, the upper layer of the plating film contains both of Ca and Mg, and the Ca content is 0.01 to 5% by mass, and the Mg content is 0.01 to 5% by mass. preferable. When the content of each of Ca and Mg is 0.01% by mass or more, a sufficient corrosion delaying effect can be obtained. On the other hand, when the content is 5% by mass or less, the manufacturing cost increases without the effect being saturated. This is because the composition control of the plating bath can be easily performed.

  Furthermore, similar to Ca and Mg, since the effect of improving the stability of the corrosion product and delaying the progress of the corrosion is exhibited, the upper layer further contains Mn, V, Cr, Mo, Ti, Sr, Ni. It is preferable to contain 0.01 to 10% by mass in total of one or more selected from Co, Sb and B.

  The interface alloy layer is present at the interface with the base steel plate, and as described above, Fe-Al based on the surface of the steel plate and Al and Si in the bath inevitably form an alloying reaction. And / or a compound of the Fe-Al-Si system. Since this interface alloy layer is hard and brittle, it is preferable that it be as thin as possible because it becomes a starting point of crack generation at the time of growth when thick.

Here, the interface alloy layer and the upper layer can be observed by using a scanning electron microscope or the like for the cross section of the plated film which has been polished and / or etched. Although there are several types of polishing methods and etching methods for cross sections, there is no particular limitation as long as they are generally used to observe the cross section of a plated film. If the observation condition with a scanning electron microscope is, for example, an acceleration voltage of 15 kV and the magnification of 1000 times or more in a backscattered electron image, it is possible to clearly observe the alloy layer and the upper layer.
In addition, as to whether or not one or more selected from Ca, Mg, Mn, V, Cr, Mo, Ti, Sr, Ni, Co, Sb and B exist in the upper layer, for example, glow It can confirm by carrying out the penetration analysis of a plating film with a discharge light emission analyzer. However, the use of a glow discharge optical emission analyzer is merely an example, and the presence / absence and distribution of Ca, Mg, Mn, V, Cr, Mo, Ti, Sr, Ni, Co, Sb, and B in the upper plating layer is examined. It is also possible to use other methods as long as

  In addition, one or two or more selected from among Ca, Mg, Mn, V, Cr, Mo, Ti, Sr, Ni, Co, Sb and B mentioned above can be selected from Zn, Al and It is preferable to form an intermetallic compound with one or more selected from Si. Since the Al-rich phase solidifies before the Zn-rich phase in the process of forming the plating film, the intermetallic compounds are discharged from the Al-rich phase in the process of solidification and collected in the Zn-rich phase in the plating upper layer. Since the Zn-rich phase corrodes prior to the Al-rich phase, it is a kind or one selected from Ca, Mg, Mn, V, Cr, Mo, Ti, Sr, Ni, Co, Sb and B in the corrosion product. Two or more species will be incorporated. As a result, the corrosion products can be stabilized more effectively in the early stages of corrosion. In addition, when the intermetallic compound contains Si, the intermetallic compound absorbs Si in the plating film and the excess Si in the plating upper layer decreases, so that the non-solid solution Si (Si phase) becomes the plating upper layer. It is further preferable because it is possible to prevent a decrease in bending workability due to the formation thereof.

  One or two or more selected from Ca, Mg, Mn, V, Cr, Mo, Ti, Sr, Ni, Co, Sb and B are selected from Zn, Al and Si. The following method is available as a method of confirming whether or not an intermetallic compound is formed with two or more types. A method of detecting these intermetallic compounds from the surface of the plated steel sheet by wide angle X-ray diffraction, or a method of detecting the cross section of the plated film by electron beam diffraction in a transmission electron microscope, or the like is used. Moreover, as long as methods other than these can detect the said intermetallic compound, you may use any method.

Moreover, as for the fusion | melting Al-Zn type | system | group plated steel plate of this invention, it is preferable that the average size of the spangles formed in the plating film shall be 0.5 mm or less. This is to reduce the visibility of the spangles and improve the appearance uniformity by reducing the spangle size. In particular, in the case of forming a coating film having a high gloss on a plated steel sheet, the effect of suppressing the popping out of the spangle can be brought about.
Here, with respect to the average size of the spangles, the plating surface of the sample is photographed using an optical microscope or the like, an arbitrary straight line is drawn on the photographed picture (image), and the number of spangles crossing the straight line is counted. The average spangle size can be obtained by dividing the length of the straight line by the number of spangles (length of straight line / number of spangles).

In addition, in the molten Al-Zn-based plated steel sheet of the present invention, the Vickers hardness of the plated film is 50 to 100 Hv on average. Here, the Vickers hardness of a plating film is the Vickers hardness of a plating upper layer.
By making the Vickers hardness of the plating film soft at 100 Hv or less on average, when the processing such as bending is performed, the plating film can follow the underlying steel plate and the occurrence of cracks can be suppressed, and as a result, the bending portion The same corrosion resistance as the flat plate portion can be secured also in Further, the reason why the lower limit of the Vickers hardness is set to 50 Hv is to prevent the plating film from adhering to the mold or the like during the forming process.
Furthermore, it is preferable that the adhesion amount of the plating film of the fusion | melting Al-Zn type | system | group plated steel plate of this invention is 35-150 g / m < ² > per single side | surface. If it is 35 g / m ² or more, excellent corrosion resistance can be obtained, and if it is 150 g / m ² or less, excellent workability can be obtained.

  Furthermore, the said plated steel plate can be used as the surface treatment steel plate which further has a chemical conversion treatment film and / or a coating film on the surface.

(Manufacturing method of molten Al-Zn based plated steel sheet)
The method for producing a melt A l-Zn-based plated steel sheet of the present invention will be described with reference to the drawings.
FIG. 1: is the figure which showed the rough flow about a part of manufacturing method of the fusion | melting Al-Zn type | system | group plated steel plate of this invention.

The method for producing a hot-dip Al-Zn-based plated steel sheet according to the present invention is a method for producing in a continuous hot-dip plating facility. By performing the production using the continuous hot-dip plating facility, it is possible to produce an Al—Zn-based plated steel sheet more efficiently than when combining the hot-dip plating facility with a batch-type heating facility.
And this invention performs processing (pre-processing process), such as a degreasing and an acid wash, and an annealing process (annealing process) with respect to a to-be-processed steel plate (base steel plate) as needed, as shown in FIG. Containing 20 to 95% by mass of Al, Si of 10% or less of the Al content, and at least one of Ca and Mg, and the total content of Ca and Mg is 0.01 to 10% by mass; The base steel plate is dipped in a plating bath consisting of Zn and unavoidable impurities, and subjected to hot-dip plating (plating step), preferably the hot-dip steel plate is subjected to a plating bath temperature of -20 ° C. After cooling (rapid cooling step) to a bath temperature of −80 ° C. within 5 seconds, the plated steel sheet is held at a temperature of 250 to 375 ° C. for 5 to 60 seconds (temperature holding step).

There is no particular limitation on the type of the base steel sheet used for the molten Al-Zn-based plated steel sheet of the present invention. For example, it is possible to use a pickled-descaled hot-rolled steel plate or strip, or a cold-rolled steel plate or strip obtained by cold rolling them.
Further, the conditions of the pretreatment step and the annealing step are not particularly limited, and any method can be adopted.

  About the conditions of the said hot-dip plating, if an Al-Zn type plating film can be formed in the said base steel plate, limitation in particular will not be carried out, It can carry out according to a conventional method. For example, after the base steel plate is reduced and annealed, it is cooled to near the temperature of the plating bath, and dipped in a plating bath, and then, a plating film having a desired thickness can be formed by wiping.

The hot-dip plating bath contains at least one of 20 to 95% by mass of Al, 10% or less of the Al content, and at least one of Ca and Mg, and the total content of Ca and Mg is 0.01 to 0.01. It is 10% by mass, and the balance consists of Zn and unavoidable impurities.
Further, for the purpose of obtaining a higher corrosion resistance effect, the plating bath contains both of Ca and Mg, and the Ca content is 0.01 to 5% by mass, and the Mg content is 0.01 to 5% by mass. preferable.

  Furthermore, the plating bath contains 0.01 to 10% by mass in total of one or more selected from Mn, V, Cr, Mo, Ti, Sr, Ni, Co, Sb, and B. Is preferred. By using a plating bath having such a composition, it is possible to form the plating film.

  As described above, the plating film formed by the Al—Zn-based plating bath is composed of an interface alloy layer present at the interface with the base steel plate and an upper layer present on the interface alloy layer. The composition of the upper layer is substantially the same as the composition of the plating bath as a whole, although Al and Si slightly lower on the interface alloy layer side. Therefore, control of the composition of the plating upper layer can be accurately performed by controlling the composition of the plating bath.

And, as shown in FIG. 1, in the production method of the present invention, the steel plate after the hot-dip plating is cooled within 5 seconds from the bath temperature -20 ° C of the plating bath to the bath temperature -80 ° C of the plating bath ( Rapid cooling step) is preferred. The rapid cooling step can suppress the formation of the spangles, and in particular, can achieve excellent appearance uniformity when forming a coating film. Specifically, the average size of the spangles can be suppressed to 0.5 mm or less.
In addition, from the viewpoint of obtaining a higher spangle suppressing effect, the cooling time of the rapid cooling is preferably within 3 seconds, and more preferably within 1 second. In addition, when the cooling time to the said metal-plating bath temperature -80 degreeC exceeds 5 seconds, there is no suppression effect of sufficient spangling, and an average size can not be made into 0.5 mm or less.
However, when the production is performed for an application in which the formation of spangles is not a problem or requires the formation of spangles, the rapid cooling step may not necessarily be performed, and the production method in that case is not limited.

Further, as shown in FIG. 1, it is preferable to further cool the plated steel sheet having been plated after immersion in the plating bath to 375 ° C. or less (cooling immediately before top roll) before contacting with the top roll. When the temperature of the plated steel plate plated prior to contact with the top roll exceeds 375 ° C., the plated film adheres to the top roll when it contacts the top roll, and part of the plated film peels off (metal pick-up) It is because there is a fear.
Here, the term "top roll" means a roll on which the plated steel plate first comes in contact after the base steel plate is subjected to hot-dip plating.

Next, a method of improving the processability of the plated film, which is most important in the present invention, will be described. In the manufacturing method of the present invention, it is important to hold the hot-dip galvanized steel sheet at a temperature of 250 to 375 ° C. for 5 to 60 seconds (temperature holding step). In this temperature holding step, the strain introduced in the plating film is released by the non-equilibrium solidification causing the above-described hardening of the plating film, and the Al-rich phase and the Zn-rich phase in Al-Zn based plating are released. Since the two-phase separation is promoted, the plating film can be softened. As a result, the processability can be improved. In addition, the plated film obtained by the manufacturing method of the present invention can improve the corrosion resistance of the processed portion by reducing the number and width of cracks generated during processing as compared to the plated film obtained by the conventional manufacturing method. It becomes.
When the holding temperature is less than 250 ° C. or when the holding time is less than 5 seconds, the hot-dip plating film cures quickly, and sufficient release of strain or separation of an Al-rich phase and a Zn-rich phase occurs. Since the process does not proceed, desired processability can not be obtained. On the other hand, it is not preferable that the holding temperature exceeds 375 ° C. because of the relationship with the metal pickup described above, and if the holding time exceeds 60 seconds, the holding time is too long. Not suitable for production.

In addition, in order to realize more excellent processability, the holding temperature of the plated steel sheet in the temperature holding step is preferably 300 to 375 ° C, and more preferably 350 to 375 ° C.
Similarly, the holding time of the hot-dip galvanized steel sheet is preferably 5 to 30 seconds, and more preferably 5 to 20 seconds.

  In the production method of the present invention, the temperature holding step is not particularly limited, and a molten Al-Zn-based plated steel sheet can be produced according to a conventional method. For example, as shown in FIG. 1, a chemical conversion treatment film is formed on the surface of a molten Al-Zn-based plated steel plate after the temperature holding step (chemical conversion treatment step), or a coating is formed separately in a coating facility (coating Film formation process).

The chemical conversion film is formed, for example, by applying a chromate treatment solution or a chromium free chemical conversion solution, and performing a drying treatment to obtain a steel plate temperature of 80 to 300 ° C. without washing with water, or a chromium free chemical conversion treatment. It is possible. These chemical conversion coatings may be a single layer or multiple layers, and in the case of multiple layers, a plurality of chemical conversion treatments may be sequentially performed.
Moreover, roll coater coating, curtain flow coating, spray coating etc. are mentioned as a formation method of the said coating film. After a paint containing an organic resin is applied, it is possible to form a coating film by heat drying by means of hot air drying, infrared heating, induction heating or the like.

Next, an embodiment of the present invention will be described.
Using a cold-rolled steel plate having a thickness of 0.35 mm manufactured by a conventional method as a base steel plate, a molten Al-Zn-based plated steel plate as a sample was manufactured in a continuous hot-dip plating facility. The composition of the plating bath, the cooling time of the steel plate after plating, the conditions of the holding temperature and time of the plated steel plate after passing through the top roll, and the composition of the plating upper layer are shown in Table 1.
Incidentally, the production of all of the molten Al-Zn alloy coated steel sheet as a sample, and bath temperature 600 ° C. of the plating bath, coating weight per one side 75 g / m ^2, ie a 150 g / m ² on both sides.

(Average size of plated film spangles)
For each sample of a molten Al-Zn-based plated steel sheet, photograph the plating surface with an optical microscope, draw 10 straight lines of 10 mm arbitrarily on the photograph, count the number of spangles crossing the straight line, and per spanle Calculate length = spangle size. The calculation results are shown in Table 1.
(Vickers hardness of plating film)
After polishing the cross section of the plated film for each sample of the molten Al-Zn-based plated steel sheet, using a micro Vickers hardness tester, any point on the upper layer side of the plated film is 20 points each with a load of 5 g from the cross sectional direction Was measured. The average value of the measured 20 points was calculated as the hardness of the plating film. The calculation results are shown in Table 1.

(Evaluation of corrosion resistance)
(1) Flat-plate part corrosion resistance evaluation The salt spray test based on JISZ2371-2000 was done about each sample of a fusion | melting Al-Zn type | system | group plated steel plate. The time until red rusting of each sample occurred was measured and evaluated according to the following criteria.
:: time of occurrence of red rust 4000 4000 hours ×: time of occurrence of red rust <4000 hours (2) corrosion resistance evaluation of bent portions For each sample of a molten Al-Zn-based plated steel sheet, 180 ° across 4 sheets of the same thickness After bending (4T bending), a salt spray test according to JIS Z2371-2000 was performed on the outside of the bending. The time until red rusting of each sample occurred was measured and evaluated according to the following criteria.
○: Red rusting time 4000 4000 hours ×: Red rusting time <4000 hours

(Evaluation of appearance uniformity)
The average size of the spangles of each sample of the molten Al-Zn-based plated steel sheet was evaluated according to the following criteria.
○: Average size of spangle ≦ 0.5 mm
:: Spangle average size> 0.5 mm

From Table 1, it can be seen that each sample of the inventive example is excellent in corrosion resistance as compared with each sample of the comparative example, and further has a Vickers hardness of 100 Hv or less and is soft.
Specifically, each sample of the example of the present invention is superior in corrosion resistance to samples 1 to 3 which do not contain Ca and Mg in the plating film. Moreover, each sample of this invention example has small Vickers hardness compared with sample 7, 10, 13, 35, 38, and 41 of the comparative example in which the holding temperature of the steel plate after top roll passing is low, and corrosion resistance of a bending part Excellent. Further, it is understood that among the samples of the invention example, the samples 4 and 32 which are not subjected to rapid cooling after plating are larger in spangle size than the other invention example samples.

  According to the present invention, it is possible to obtain a hot-dip Al-Zn-based plated steel sheet which has excellent flat plate portion corrosion resistance and excellent processability due to good processability, and is applied to a wide range of fields mainly in the building materials field can do.

Claims (6)

Continuous hot-dip plating equipment containing 45 to 95% by mass of Al, Si of more than 0% by mass and 10% or less of the Al content, and 0.5 to 4.5% by mass of Mg, with the balance being Zn and unavoidable The base steel plate is immersed in a plating bath containing chemical impurities and subjected to hot-dip plating, and thereafter the steel plate after plating is held at a holding temperature of 250 to 375 ° C. for 5 to 20 seconds. Method of producing plated steel sheet. Containing 0.01 to 10% by mass in total of one or two or more selected from among Mn, V, Cr, Mo, Ti, Sr, Ni, Co, Sb and B in the plating bath method for producing a molten Al-Zn-based plated steel sheet according to claim 1, wherein. The steel sheet after the hot-dip plating is characterized in that it is cooled from the temperature obtained by subtracting 20 ° C. from the bath temperature of the plating bath to the temperature obtained by subtracting 80 ° C. from the bath temperature of the plating bath within a cooling time of 5 seconds. The manufacturing method of the fusion | melting Al-Zn type | system | group plated steel plate of Claim 1 or 2 . Method for producing a molten Al-Zn-based plated steel sheet according to claim 3, characterized in that within 3 seconds between the cold却時. Method for producing a molten Al-Zn-based plated steel sheet according to claim 3 or 4 wherein the holding temperature of the steel sheet after pre-Symbol cooling characterized in that it is a three hundred to three hundred and seventy-five ° C.. The molten Al-Zn-based plated steel sheet according to any one of claims 3 to 5, further cooling to 375 ° C or less after cooling the steel sheet and before bringing the steel sheet into contact with the top roll. Manufacturing method.

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