JP2004323974A - HOT DIP Zn-Al BASED ALLOY PLATED STEEL SHEET, AND ITS PRODUCTION METHOD - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot dip Zn-Al based alloy plated steel sheet combining the three of bending workability, plating adhesion on press forming and edge face rusting resistance though their reconcilement has heretofore been difficult. <P>SOLUTION: The plated layer has a componential composition comprising, by mass, 10 to 40% Al and Si: (0.0005 to 0.15)×[%Al] (wherein, [%Al] is the content (mass%) of Al), and the balance Zn with inevitable impurities. Further, the abundance ratio of a granular structure with a major axis of ≤3 μm in the plated layer is controlled to ≥85%. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a hot-dip Zn-Al-based alloy-plated steel sheet widely used in the fields of building materials, home appliances, and the like, and particularly to improving its workability, adhesion, and end face rust resistance.

  Hot-dip galvanized steel sheets are lightweight and excellent in waterproofness, heat insulation, corrosion resistance, workability, and the like, and are therefore used in large quantities in the field of building materials such as roofs and sidings. In particular, recently, the Construction Recycling Law has been enacted, and products with higher corrosion resistance than before have been required to extend the life of houses.

  Examples of steel sheets having better corrosion resistance than hot-dip Zn-coated steel sheets include hot-dip 5% Al-Zn coated steel sheets with aluminum contained in the plating layer and hot-dip 55% Al- steel with higher aluminum content to improve corrosion resistance. A 1.6% Si-Zn plated steel sheet is known, and the demand for the latter is particularly remarkable.

However, the above-mentioned hot-dip 55% Al-1.6% Si-Zn-coated steel sheet has better bending workability, plating adhesion during press forming, and flaws than hot-dip Zn-coated steel sheet and hot-dip 5% Al-Zn coated steel sheet. And the corrosion resistance at the cut end face is inferior.
It is said that the greatest cause of poor bending workability is that the plating layer becomes hard due to the high Al concentration, and that a hard and brittle alloy layer is formed at the interface.
The cause of the peeling of the plating layer during press forming is not necessarily established, but when the inventors observed the peeling site, the peeling of the plating layer was observed between the steel plate and the alloy layer or between the alloy layer and the upper plating. Therefore, it is considered that the existence of the alloy layer is involved.
Further, the corrosion resistance at the end face is inferior because the ductility of the coating layer is low, so that the coating layer is not deformed enough to cover the end face of the steel sheet exposed by scratching or cutting, and the Zn content is hot-dip galvanized steel sheet or hot-dip 5% It is known that the sacrificial corrosion prevention performance of Zn is not sufficiently exhibited because it is lower than that of the Al-Zn plated steel sheet.

In order to improve the end face rust resistance described above, attempts have been made to subsequently contain Mg in the Zn-Al-based alloy plating layer, and various Zn-Al-Mg ternary alloy-plated steel sheets have been proposed (for example, Patent Document 1 and Patent Document 2).
The mechanism by which the addition of Mg improves the rust resistance of the end face has not yet been fully elucidated.However, Mg elutes with Zn in the early stage of corrosion and suppresses the corrosion of Zn. At present, the effect of stabilizing zinc hydroxide and basic zinc chloride known as rust is considered to be promising.
However, it is also known that Mg significantly deteriorates the workability of the plating layer by forming Zn and an intermetallic compound phase such as MgZn ₂ or Mg ₂ Zn ₁₁ . For example, even in the Zn-Al-Mg-based alloy plated steel sheet having the lowest Al content of 3 mass% or less disclosed in Patent Document 1, the bending workability is 55% Al-1.6% Si-Zn plated steel sheet level. There is only.

  As described above, the workability (bending workability), the plating adhesion during press forming, and the resistance of the Zn-Al-based alloy-coated steel sheet and Zn-Al-Mg-based alloy-coated steel sheet that have been conventionally developed for building materials There has not yet been any product that simultaneously satisfies the rust resistance of the end face, and its development has been demanded.

JP-A-56-96062 JP 2000-104154 A

  The present invention advantageously satisfies the above-mentioned requirements, and provides a hot-dip Zn-Al-based alloy-coated steel sheet having both excellent workability, plating adhesion and corrosion resistance, and particularly corrosion resistance at the cut end face (end face rust resistance). The aim is to propose with an advantageous manufacturing method.

By the way, the present inventors have conducted intensive studies to achieve the above object, and as a result, devised the structure of the Zn-Al-based alloy plating layer to make the structure finer and more uniform, and Si or Si. It has been found that by concentrating the oxide on the surface of the plating layer, the workability, plating adhesion, and edge rust resistance can be effectively improved.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
(1) A hot-dip coated steel sheet having a Zn-Al-based alloy plating layer on the surface, the plating layer comprising: Al: 10 to 40 mass% and Si: (0.0005 to 0.15) × [% Al] (where [% Al] (Al content (mass%)), the balance is composed of Zn and unavoidable impurities, and the proportion of a long-diameter: 3 μm or less granular structure in the plating layer is 85% or more. A hot-dip Zn-Al alloy plated steel sheet characterized by the following.

(2) The concentration of Si present from the surface in the thickness direction of the Zn-Al-based alloy plating layer to 1/100 depth from the surface in the thickness direction of the plating layer becomes 1/4 to 2/4 depth region from the surface in the thickness direction of the plating layer. The hot-dip Zn-Al-based alloy-plated steel sheet according to the above (1), wherein the concentration of the existing Si is 1.0 times or more.

(3) The steel sheet to be plated contains Al: 10 to 40 mass% and Si: (0.0005 to 0.15) × [% Al] (where [% Al] is the Al content (mass%)), and the remainder is Plating by immersion in a hot-dip bath with a composition of Zn and unavoidable impurities, and maintained in a temperature range of 280 ° C or higher and 430 ° C or lower for 10 seconds or more, or at a rate of 15 ° C / s or lower. A method for producing a hot-dip Zn-Al-based alloy-coated steel sheet, comprising gradually cooling and further cooling at a rate of 30 ° C./s or more in a temperature range of 280 ° C. or less.

(4) The steel plate to be plated contains Al: 10 to 40 mass% and Si: (0.0005 to 0.15) x [% Al] (where [% Al] is the Al content (mass%)), and the balance is Dipping and plating in a hot-dip bath that becomes the composition of Zn and unavoidable impurities, and after cooling to 50 ° C or lower, the temperature is raised again to a temperature range of 280 ° C or higher and 430 ° C or lower, and the temperature range is 10 seconds or longer. A method for producing a hot-dip Zn-Al-based alloy-plated steel sheet, comprising cooling at a rate of 30 ° C./s or more after holding.

  According to the present invention, it is possible to obtain a hot-dip Zn-Al-based alloy-plated steel sheet that combines the three features of bending workability, which has conventionally been considered difficult to stand up, plating adhesion during press forming, and end surface rust resistance. .

Hereinafter, the present invention will be described specifically.
First, FIG. 1 shows a plating bath containing Al: 22.0 mass% and Si: 0.53 mass%, the balance being Zn and unavoidable impurities (0.03 mass% or less), satisfying the composition range of the present invention. 4 shows a scanning electron microscope (SEM) photograph of a cross section of a plating layer of a sample.
As shown in the figure, the structure of the plating layer is as follows: Zn rich phase (white area), Zn-Al mixed crystal 1 (dark gray area), Zn-Al mixed crystal 2 (light gray area), Zn-Al It is composed of mixed crystal 3 (lamellar region) and very little Si (fine black particles).
According to the Zn-Al binary equilibrium phase diagram (for example, JLMurray: "Binary Alloy Phase Diagrams 1", Second Edit, ASM International (1990), p. 221), the plating bath of this composition exists at 277 ° C. It almost matches the composition of the eutectoid point.

Next, when the plated steel sheet shown in FIG. 1 was held at 375 ° C. for 3 minutes and cooled at a rate of about 100 ° C./s, the plated layer changed to an extremely fine and uniform structure as shown in FIG. did. In this enlarged photograph, energy distribution type X-ray spectroscopy (EDX) was performed on a black granular region and a white granular region having a size of several 100 nmφ, and Al was mainly detected from the former, and Zn was mainly detected from the latter. From this, it is estimated that eutectoid transformation occurred by holding at 375 ° C.
Therefore, it has been found that the use of this eutectoid transformation makes it possible to make the plating layer fine and uniform.

This plated steel sheet was subjected to a 2T bending test in which two test pieces were sandwiched and bent at 180 degrees in accordance with JIS Z 2248-1996. FIG. 3 shows a cross-sectional photograph of the plating layer outside the bend after the test.
As shown in the figure, no crack was observed in the plating layer, indicating that the adhesion to the steel sheet was good.

  Next, according to JIS Z 2248-1996, the plated steel sheet was subjected to a 0T bending test for performing close contact bending. The sample after the 0T bending (width: 50 mm, length: 100 mm) was subjected to the combined cycle corrosion test (300 cycles) shown in FIG. 4, and the area of red rust occurrence including the bent portion was 5% of the test piece. Below, it showed extremely good corrosion resistance. Also, little red rust was observed on the cut end face, and the end face rust resistance was also good.

Furthermore, the above-mentioned plated steel sheet is cut into a width: 20 mm and a length: 300 mm, and a draw bead test is performed under the following conditions: load: 200 kg, punch tip diameter: 0.5 mmR, drawing distance: 60 mm, drawing speed: 3.33 mm / s. However, no peeling of the plating layer was observed, indicating good plating adhesion.

  One of the reasons why the plated steel sheet of the present invention exhibits excellent bending workability and plating adhesion is that there is almost no hard and brittle alloy layer at the interface with the steel sheet, and two, the starting point of cracks in the plated layer. This is considered to be because there was no or propagation path and the ductility was significantly improved. The reason for the excellent end face rust resistance is considered to be that the area covered by the plating layer on the end face of the cut portion is significantly increased as compared with the conventional Zn-Al plated steel sheet due to the improvement in ductility of the plating layer.

Further, FIG. 5 shows the results of analysis of Si, Zn and Al in the thickness direction of the plating layer by a glow discharge optical emission spectrometer (GDS) of the plated steel sheet.
As shown in FIG. 5, in this plated steel sheet, Si is concentrated on the surface of the plated layer. This is considered to be Si or an oxide of Si, and this Si concentration is also considered to be one of the causes of exhibiting excellent end face rust resistance.

Next, the composition of the plating layer according to the present invention will be described.
Al: 10-40 mass%
Al is an important element that is added for the purpose of improving corrosion resistance firstly and secondly for forming eutectoid transformation.
In order to make the plating layer fine and uniform by eutectoid transformation, the Al content needs to be controlled in the range of 10 to 40 mass%. This is because if the Al content is less than 10 mass%, not only is the high corrosion resistance not obtained not only on the flat surface but also on the end face, but also in the fine eutectoid structure, the coarse β-Zn with a major axis of more than 3 μm. A large amount of a phase or lamellar Zn-Al eutectic structure (corresponding to Zn-Al mixed crystal 3 in Fig. 1) precipitates, whereas if it exceeds 40 mass%, a coarse α-Al phase with a major axis longer than 3 µm or a lamellar This is because a large amount of Zn-Al eutectic structure (Zn-Al mixed crystal 3) is precipitated in a large amount. More preferably, it is in the range of 20 to 25 mass%.

Si: (0.0005 to 0.15) x [% Al] (However, [% Al] is the content of Al (mass%))
Si is an element added for the first purpose of suppressing the alloying reaction between Al and a steel sheet, and secondly for the purpose of improving corrosion resistance by concentrating on the surface. The amount of addition must be (0.0005 to 0.15) times the amount of Al. This is because if it is less than this, coarse Al-Fe, Zn-Fe, Al-Fe-Si, Al-Fe-Zn, Al-Fe-Zn-Si intermetallic compounds are formed at the interface between the steel sheet and the plating layer. Is generated in a large amount, and the bending workability and the plating adhesion at the time of press molding are significantly deteriorated. On the other hand, if it is more than this, Si is likely to precipitate coarsely in the plating layer, and the bending workability is deteriorated again. More preferably, it is (0.001 to 0.1) times the amount of Al.

In the present invention, Mn can be added for the purpose of improving the ductility of the plating layer.
Mn: 0.01 to 2.0 mass%
Mn is an element useful for improving the ductility of the plating layer. However, if its content is less than 0.01 mass%, its addition effect is poor, while if it exceeds 2.0 mass%, it combines with Al and Si in the bath, In addition to the formation of dross, the formation of an interfacial alloy layer is promoted, and the bending workability and the plating adhesion during press forming deteriorate. Therefore, Mn can be contained as needed in the range of 0.01 to 2.0 mass%, more preferably in the range of 0.1 to 1 mass%.

  Further, in the present invention, a small amount of Mg and / or Cu can be added for the purpose of improving the surface appearance and preventing blackening. However, in order to prevent the ductility of the plating layer from deteriorating, the amount of addition should be 1.0 mass% or less in either case of single addition or composite addition.

  The addition of Mn: 0.01 to 2.0 mass% and / or the addition of a small amount of Mg or Cu are also within the scope of the present invention as long as they have the same effect as the present invention. Not excluded.

Next, the structure of the plating layer will be described.
In order to improve the ductility of the plating layer, each phase of the plating layer must be finely and uniformly dispersed.
Therefore, in the present invention, the existence ratio of the granular structure having a major axis of 3 μm or less in the plating layer is set to 85% or more. Here, the existence ratio of the granular structure is an area ratio in a plating layer cross section.

  Further, in order to further improve the plating adhesion at the time of press forming, Al-Fe, Zn-Fe, Al-Fe-Si, Al-Fe-Zn, Al-Fe- It is preferable to have no alloy layer made of a Zn-Si based intermetallic compound, or to make the length of the alloy layer particles less than 0.3 μm even if it is formed. If the particles of the alloy layer become coarser than this, the particles of the alloy layer cover the interface, and the plating layer is likely to peel off during press forming.

Furthermore, in order to improve the corrosion resistance of the plating layer, especially the corrosion resistance of the cut portion, it is important to improve the ductility of the plating layer first. It was found that the corrosion resistance was improved.
The vicinity of the plating surface in this case is from the surface in the thickness direction to 1/100 depth, and the Si concentration in this region is present in a region 1/4 to 2/4 depth from the surface in the thickness direction of the plating layer. Preferably, the concentration is not less than 1.0, that is, not less than 1.0. More preferably, it is 1.5 times or more.

Experiments and results which are the basis for limiting the Al and Si concentrations in the plating layer and the structure of the plating layer as described above are as follows.
A low-carbon cold-rolled steel sheet having a thickness of 1.2 mm is placed in an Al-Si-Zn bath in which the Al content is changed in the range of 2 to 40 mass% and the Si content is changed in the range of (0 to 0.2) times the Al content. The coated steel sheet was immersed for 2 seconds, the plating thickness was adjusted so that the plating thickness on one side became 20 μm, and the coated steel sheet was cooled to room temperature at a rate of 15 ° C./s. For plated steel sheets that have been held for ~ 300 seconds and air cooled (average cooling rate from holding temperature to 100 ° C: 15 ° C / s) or mist cooled (average cooling rate from holding temperature to 100 ° C: 75 ° C / s) We investigated bending workability, end face rust resistance, the existence ratio of granular structure, Si concentration near the surface, plating adhesion during press forming, and the type and size of the alloy layer at the interface between the steel sheet and the plating layer. .

・ Bending workability The sample was cut into three pieces with a width of 60 mm and a length in the rolling direction of 120 mm, and subjected to a 0T bending test in accordance with JIS Z 2248-1996. Cracks were visually observed for three samples (total of 12 visual fields) in four fields at 10 mm intervals in the width direction of one sample with a microscope, and evaluated by the following five steps, and an average value was obtained.
Rank 5: No crack is observed.
Rank 4: 1 to 10 hair cracks / field of view that do not reach the steel base.
Rank 3: 11 hair cracks / field of view that did not reach the steel base (however, no coarse cracks reaching the steel base).
Rank 2: 1 to 5 coarse cracks reaching the steel substrate / view.
Rank 1: 6 or more coarse cracks reaching the steel base per field.

・ End face rust resistance Four samples of 60 mm in width and 120 mm in length in the rolling direction are cut so that the cut portion in the width direction becomes a lower burr, and 300 cycles of the combined cycle corrosion test shown in FIG. 4 are performed. The red rust occurrence rate (area ratio) of a total of eight sides, that is, the right and left sides (two sides) per one sheet, was evaluated in the following five stages, and the average value was obtained.
Rank 5: Red rust occurrence rate less than 5% Rank 4: Red rust occurrence rate 5% or more, less than 10% Rank 3: Red rust occurrence rate 10% or more, less than 20% Rank 2: Red rust occurrence rate 20% or more, less than 30% Rank 1 : Red rust occurrence rate 30% or more

・ Presence ratio (area ratio) of granular structure
The sample was cut into three pieces of 10 mm in width and 15 mm in length in the rolling direction, embedded in carbon resin so that the cross section in the rolling direction became the observation surface, mirror-polished by buffing, and granulated with a long diameter of 3 μm or less by SEM observation. The area ratio of the tissue was measured. The area ratio was measured by tracing a single phase having a major axis of more than 3 μm, a lamellar Zn—Al mixed crystal structure having a longitudinal direction of more than 3 μm, a Si and an alloy layer, and measuring these area ratios with an image analyzer. , Determined by subtracting from 100%.
The SEM imaging conditions were as follows: an acceleration voltage: 25 kV, a 3000-fold reflected electron image, and randomly selected 30 fields of view for each sample, and the average value was determined.

-Si concentration near the surface Five samples were cut into 40 mm x 40 mm samples, and the Si concentration profile in the depth direction of the plating layer was measured by a glow discharge emission spectrometer under the conditions of a sputtering rate of Fe: 24 nm / s. In this profile, the integrated intensity in the thickness direction from the surface of the plating layer to the depth 1/4 to 2/4 from the surface of the plating layer to the depth 1/100 in the thickness direction is normalized to the unit thickness. The ratio of the strength Iav normalized to the unit thickness: Is / Iav was determined, and the average value at five locations was determined.

・ Plating adhesion during press molding A sample was punched into a circle with a diameter of 100 mm, punch diameter: 50 mm, die and punch shoulder R: 10 mm, wrinkle pressing load: 0.5 ton, and a cylindrical drawing test was performed at a drawing ratio of 2.0. The outer side surface of the test piece formed into a cylinder was observed, and the adhesion of the plating layer was evaluated on a three-point scale.
Rank 3: No peeling of the plating layer was observed.
Rank 2: No clear peeling of the plating layer is observed, but there is variation due to cracks in the plating layer.
Rank 1: Clear peeling of the plating layer is observed.

The presence / absence of the alloy layer and the maximum major axis The sample was subjected to constant voltage electrolysis of 0 mV with respect to the saturated calomel electrode in a 1 mass% salicylic acid-4 mass% maleic acid-2 mass% potassium iodide-methanol solution to remove the plating layer. Thereafter, the presence of an alloy layer at the interface between the plating layer and the base steel sheet was determined by measuring the X-ray diffraction pattern. When the alloy layer was present in this sample, the region was observed by SEM (observation of a backscattered electron image of 3000 times) to measure the maximum major axis in 30 visual fields.

  FIGS. 6 and 7 show the relationship between the area ratio of the granular structure having a major diameter of 3 μm or less, the bending workability, and the end face rust resistance in the hot-dip Zn—Al-based alloy-coated steel sheet thus obtained. FIG. 8 shows the results of a study on the relationship between Si concentration and end surface rust resistance, and FIG. 9 shows the results of a study on the relationship between the presence or absence of an alloy layer and the maximum major axis of the alloy layer and the plating adhesion during press forming. Shown respectively.

As shown in FIGS. 6 and 7, both the bending workability and the end face rust resistance are as good as 4 or more when the area ratio of the granular structure having a long diameter of 3 μm or less is 85% or more. .
In addition, as shown in FIG. 8, when the ratio of Is / Iav is 1.0 or more, the rating is remarkably improved to 4.5 or more on average.
Further, as shown in FIG. 9, it can be seen that the plating adhesion at the time of press forming is good when there is no alloy layer particle having a major axis of 0.3 μm or more at the interface between the steel sheet and the plating layer.

  As described above, in the present invention, the corrosion resistance including the cut portion was improved not by the conventional addition of a large amount of Mg but by the improvement of the structure of the plating layer and the surface concentration of Si. There is no deterioration of workability.

Next, a manufacturing method for obtaining the above-described plating layer structure will be described.
As the steel sheet to be plated in the present invention, any steel sheet manufactured by a usual method, for example, a low carbon aluminum killed steel sheet, an ultra low carbon steel sheet, or the like can be suitably used. In the present invention, these steel sheets are immersed in a hot-dip Zn-Al-based alloy plating bath, so-called hot dip plating, the steel sheets are pulled up from the plating bath, the amount of adhesion is adjusted by gas wiping or the like, then cooled, and the molten steel is cooled. A Zn-Al based alloy plating layer is formed.

Here, in the present invention, the bath composition of the hot-dip Zn—Al-based alloy plating bath contains Al at 10 to 40 mass% and Si at (0.0005 to 0.15) times the Al amount, and the balance is Zn and unavoidable impurities. The composition must be adjusted. Here, the unavoidable impurities are Fe, Pb, Sn, Cd, and the like, and these must be suppressed so that the total amount does not exceed 0.05 mass%.
Further, the plating bath temperature is set to be equal to or higher than the liquidus temperature and equal to or lower than (liquidus temperature + 100 ° C.). The lower the temperature, the more difficult it is to control the amount of deposition, while the higher the temperature, the more easily an alloy layer is formed at the interface between the steel sheet and the plating layer. Therefore, the preferred range is 30 ° C or higher and 70 ° C or lower from the liquidus line. .

After being pulled out of the plating bath, the cooling rate up to 430 ° C. is not particularly limited, but is preferably about 15 ° C./s or more because it is better not to coarsen the single-phase structure.
During the following 430 ° C to 280 ° C, this temperature range must be maintained for at least 10 seconds, or this temperature range must be cooled at a rate of 15 ° C / s or less.
The reason for staying in this temperature range is to dissolve the single phase and the Zn-Al eutectic structure. Therefore, if the holding time is less than 10 seconds or the cooling rate is higher than 15 ° C./s, these structures remain undissolved, and a fine and uniform plating structure cannot be obtained. The upper limit of the holding time is not particularly limited. However, if the holding time is 10 minutes, the solution is completely dissolved. Most preferably, the holding time is about 30 to 150 seconds, and the cooling rate is about 1 to 5 ° C./s.

Subsequent cooling from 280 ° C must be performed at a rate of 30 ° C / s or more.
The reason for this is that the plating layer has a granular structure in which fine α-Al phase and β-Zn phase are mixed by eutectoid transformation in the cooling process. Therefore, if the cooling rate is lower than this, the particles become coarse, and a fine and uniform granular structure cannot be obtained.

Further, in the present invention, after raising from the plating bath, adjusting the basis weight by gas wiping or the like, once cooling to 50 ° C or lower, the temperature is raised again to a temperature range of 280 ° C or higher and 430 ° C or lower, The same effect can be obtained by holding for 10 seconds or more and then cooling at a rate of 30 ° C./s or more.
As a reheat treatment method, after being wound in a coil and heated and held in a Box furnace, a large amount of gas of 50 ° C or less can be blown and air-cooled. A more preferable method is to raise the temperature, hold for a predetermined time, and then cool with gas cooling or mist spray.

  In addition, between the cooling process and coil winding, a tension leveler for shape correction and skin pass rolling for smoothing the plating surface can be performed as necessary. Further, after the structure refinement treatment of the plating layer, a chemical conversion treatment and one or two layers of color coating can be performed.

Example 1
C: 0.044 mass%, Si: 0.01 mass%, Mn: 0.18 mass%, S: 0.007 mass% and Al: 0.020 mass%, the balance being a composition of Fe and unavoidable impurities, sheet thickness: 1.0 mm The low-carbon aluminum-killed cold-rolled steel sheet was used as a steel sheet to be plated, and was subjected to hot-dip Zn-Al alloy plating by a continuous hot-dip plating apparatus. The composition of the hot-dip plating bath was adjusted using 99.9 mass% Zn ingot, 99.99 mass% Al particles, and 3 mass% Si-Al and 13 mass% Si-Al alloy ingots so as to have the composition shown in Table 1.
In the plating treatment, the immersion time was about 2 seconds, the target adhesion thickness was 25 μm (one side), and the heat pattern after plating was measured with a radiation thermometer. Table 1 shows the plating bath temperature, the average cooling rate between 430 and 280 ° C, and the average cooling rate between 280 and 50 ° C. Table 1 shows the plating bath composition, and it has been confirmed that the plating layer has the same composition as the plating bath composition.

From the hot-dip Zn-Al-based alloy-plated steel sheet thus obtained, a test piece was sampled as described above, and the major axis: the abundance ratio (area ratio) of a granular structure of 3 μm or less, Al-Fe at the interface between the steel sheet and the plating layer, Judgment of presence / absence of alloy layer consisting of Zn-Fe, Al-Fe-Si, Al-Fe-Zn, Al-Fe-Zn-Si based intermetallic compound, maximum major axis, concentration of Si up to 1/100 depth The amount (Is / Iav) was measured, and bending workability, end face rust resistance, and plating adhesion during press molding were investigated.
The results obtained are also shown in Table 1.
The existence ratio of the grain structure, the presence or absence and the maximum major axis of the alloy layer, the concentration of Si up to the depth of 1/100 of the surface layer (Is / Iav), bending workability, end face rust resistance, and plating adhesion during press forming Is the same as described above.

  As is clear from the table, according to the present invention, any of the plated layer structures having a fine and uniform structure in which the proportion of a granular structure having a major axis of 3 μm or less is 85% or more has excellent bending workability and resistance to bending. The end face rust and the plating adhesion at the time of press molding are obtained together. In particular, when the surface concentration of Si (Is / Iav) was set to 1.0 or more, more excellent end face rust resistance could be obtained. Furthermore, when the alloy layer was not present at the interface between the steel sheet and the plated layer, or even when the alloy layer had a major axis of 0.3 μm or less, the plating adhesion during press forming was particularly excellent.

Example 2
C: 0.0012 mass%, Si: 0.02 mass%, Mn: 0.05 mass%, S: 0.005 mass% and Al: 0.021 mass%, the balance being a composition of Fe and unavoidable impurities. The ultra-low carbon aluminum-killed cooled steel sheet was cut into a test piece having a width of 60 mm and a length of 200 mm, degreased, pickled, washed, and then subjected to hot-dip Zn-Al alloy plating by a hot-dip plating simulator. The composition of the hot-dip plating bath was adjusted using 99.9 mass% Zn ingot, 99.99 mass% Al particles, and 3 mass% Si-Al and 13 mass% Si-Al alloy ingots to obtain the composition shown in Table 2.
In the plating treatment, the immersion time was about 2 seconds, and the target adhesion amount was 25 μm (one side). Table 2 shows other plating conditions. Table 2 shows the plating bath composition, and it has been confirmed that the plating layer has the same composition as the plating bath composition.

Some of the prepared hot-dip coated steel sheets (Nos. 1, 3, 4, 8, and 10 to 12) were further reheated to various temperatures in an oven, then quenched in water or 90 ° C hot water, and air-cooled. Processing and cooling treatment were performed. The average cooling rate from the start of cooling in each method to 50 ° C is 200 ° C / s or more for underwater quenching, about 100 ° C / s for quenching in hot water, 23 ° C / s for air cooling, and Case: 12 ° C./s. Table 2 also shows each processing condition. The average cooling rate immediately after plating when reheating is shown in parentheses.
The major axis of the hot-dip Zn-Al-based alloy-coated steel sheet thus obtained: the existence ratio (area ratio) of a granular structure of 3 μm or less, the determination of the presence or absence of an alloy layer at the interface between the steel sheet and the plating layer, and the measurement of the maximum major axis. Of the surface layer to a depth of 1/100 (Is / Iav), bending workability, end face rust resistance and plating adhesion during press forming were investigated.
Table 3 shows the obtained results.

  As is clear from Table 3, according to the present invention, any of the plating layer structures having a fine and uniform structure in which the proportion of a granular structure having a major axis of 3 μm or less is 85% or more has excellent bending workability and resistance to bending. The end face rust and the plating adhesion at the time of press molding are obtained together. In particular, when the surface concentration of Si (Is / Iav) was set to 1.0 or more, more excellent end face rust resistance could be obtained. Furthermore, at the interface between the steel sheet and the plating layer, there is no Al-Fe, Zn-Fe, Al-Fe-Si, Al-Fe-Zn, Al-Fe-Zn-Si alloy layer made of an intermetallic compound, When the major axis was 0.3 μm or less, the plating adhesion during press molding was particularly excellent.

5 is an SEM photograph of a cross section of a hot-dip Zn—Al-based alloy plating layer plated according to a conventional method. 4 is an SEM photograph of a cross section of a hot-dip Zn—Al-based alloy plating layer plated according to the method of the present invention. It is a cross-sectional photograph of the plating layer after performing 0T bending test with respect to the plating steel plate of this invention. It is the figure which showed the processing condition per 1 cycle in a combined cycle corrosion test. 4 is a GDS measurement result showing a change in Si strength in a thickness direction of a plating layer of a plated steel sheet of the present invention. FIG. 3 is a diagram showing a relationship between an area ratio of a granular structure and bending workability. It is the figure which showed the relationship between the area ratio of granular structure and end surface rust resistance. FIG. 4 is a diagram showing a relationship between Si surface concentration (Is / Iav) and end face rust resistance. FIG. 3 is a diagram showing a relationship between the maximum major diameter of the alloy layer and plating adhesion during press forming.

Claims (4)

  A hot-dip coated steel sheet having a Zn-Al-based alloy plating layer on the surface, the plating layer comprising: Al: 10 to 40 mass% and Si: (0.0005 to 0.15) x [% Al] (where [% Al] is Al content (mass%)), the balance being Zn and unavoidable impurities, and the plating layer having a major axis of 3 μm or less having a granular structure of 85% or more. Hot-dip Zn-Al alloy plated steel sheet.   The concentration of Si present at a depth of 1/100 from the surface in the thickness direction of the Zn-Al-based alloy plating layer is 1/4 to 2/4 of the depth from the surface in the thickness direction of the plating layer. The hot-dip Zn-Al-based alloy-plated steel sheet according to claim 1, wherein the concentration is 1.0 or more times the concentration.   The steel plate to be plated contains Al: 10 to 40 mass% and Si: (0.0005 to 0.15) x [% Al] (where [% Al] is the Al content (mass%)), and the balance is Zn and inevitable Plating by immersion in a hot-dip bath with the composition of the target impurities, and holding at a temperature range of 280 ° C or higher and 430 ° C or lower for 10 seconds or more, or gradually cooling the temperature range at a rate of 15 ° C / s or lower. A method for producing a hot-dip Zn-Al-based alloy-plated steel sheet, further comprising cooling a temperature range of 280 ° C or lower at a rate of 30 ° C / s or higher. The steel plate to be plated contains Al: 10 to 40 mass% and Si: (0.0005 to 0.15) x [% Al] (where [% Al] is the Al content (mass%)), and the balance is Zn and inevitable After being immersed in a hot-dip plating bath that becomes the composition of the target impurities and plating, and cooled to 50 ° C or lower, the temperature is raised again to a temperature range of 280 ° C or higher and 430 ° C or lower, and maintained at the temperature range for 10 seconds or longer. A method for producing a hot-dip Zn—Al-based alloy-coated steel sheet, comprising cooling at a rate of 30 ° C./s or more.

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