JP2008156729A - Zn-Al-BASED PLATING-COATED STEEL SHEET EXCELLENT IN UNBENDING RESISTANCE, AND ITS PRODUCTION METHOD - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive coated steel sheet using a hot dip Zn-50 to 60% Al alloy plated steel sheet having excellent corrosion resistance as a base, whose unbending resistance is remarkably improved, and which is suitable for a roof. <P>SOLUTION: In the Zn-Al-based plating-coated steel sheet excellent in unbending resistance, regarding a coated steel sheet in which the surface of a steel sheet is provided with a plating layer containing, by mass, 50 to 60% Al, and the balance substantially Zn, and the layer upper than the plating layer is provided with a coating film, the cross-sectional hardness H<SB>M</SB>(HV) of the base metal and the cross-sectional hardness H<SB>P</SB>(HV) of the plating layer satisfy inequality; H<SB>M</SB>>H<SB>P</SB>and inequality; H<SB>P</SB>≥90. and, preferably, it is further controlled in such a manner that inequality; H<SB>M</SB>≤145 is satisfied as well. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention is a coated steel sheet coated on a highly corrosion-resistant Zn-Al-based plating layer, and is particularly suitable for exterior building materials such as a roof, which has improved anti-bending resistance at a low cost, and It relates to the manufacturing method. In this specification, the process of reversely deforming and returning the bent shape of the bent portion to a shape close to a flat shape before being bent is called “bending back”.

  A coated steel sheet based on a Zn-50 to 60% Al alloy-plated steel sheet is widely used for exterior building materials such as a roof because it is superior in corrosion resistance and durability to those based on a Zn-plated steel sheet. When roofing with such a coated steel sheet, bending is usually performed by roll forming or the like, but depending on the construction method, a part of the bent portion of the processed part is repeatedly bent and then bent again. Bending operations may be added.

  FIG. 1 schematically illustrates a cross-sectional shape of a bent processed portion constructed on a roof. This is an example of a process called “ant hook process” which is mainly performed on a roof in a cold region. The base material 10 is a portion made of “steel” derived from a portion of a steel plate material that is a plating original plate. A coating layer 11 made of “plating layer + coating film” is formed on the surface of the base material 10. A bending process is performed so that a part of the steel sheet is almost adhered by roll forming, and the coating layer 11 undergoes severe compressive deformation at the inside of the bend such as A1, A2, and A3, and the outside of the bend such as B1 and B2. Subjected to severe tensile deformation at locations.

  In FIG. 2, the external appearance of the roof member before and after the operation which returns bending about a part of this dovetail processing part is added is illustrated typically. FIG. 2A shows a state before returning the bending, and FIG. 2B shows a state after returning the bending. When processing the edge part of a roof, for example, as shown in Drawing 2 (a), a part of dovetail processing part 20 and flat part 22 is cut in cutting position 21 shown with a broken line. Next, bending of the dovetail processing portion 20 is returned at the end portion from the cutting position 21 and is flattened in the direction of the arrow to once form the shape as shown in FIG. Thereafter, the end is processed by further applying a desired bending process or the like.

  When the plating layer is Zn-50 to 60% Al alloy plating, in the state where the bending is returned as shown in FIG. 2B, the portion where the plating layer is subjected to compressive deformation when bent (see FIG. 1 of A1, A2, A3, etc.), the plating layer is likely to crack. Zn-50-60% Al alloy plating may have a high anticorrosion effect, and such a crack in the plating layer is not usually a big problem. However, necking (thickness reduction) is likely to occur in the base steel at the location where the plating layer is cracked, which is a problem. This necking causes cracks in the base material itself in subsequent processing steps. In this specification, such a property that necking is unlikely to occur at the time of bending back, and the base material at the bending back portion is not easily cracked by subsequent processing is called “bending resistance”.

  FIG. 3 shows a bent portion (FIG. 3 (a)) of a sample subjected to 2T bending (T is the plate thickness) of a conventional coated steel plate based on a molten Zn-55% Al alloy-plated steel plate, and thereafter The cross-sectional shape of the part (FIG.3 (b)) which returned the bending of the sample which bent back is illustrated. This coated steel sheet is formed by temper rolling with an elongation of 1% after hot dipping, applying a paint by a roll coater method, raising the temperature to a material temperature of 210 ° C., and then cooling with water to form a baked coating film. . The thickness of the base material 10 is about 0.35 mm, and the cross-sectional hardness of the plating layer 12 ′ before bending is about 115 HV. The coating film is present on the surface of the plating layers 12 and 12 ', but is omitted in the figure. As shown in FIG. 3B, when bending back is performed, cracks occur in the plating layer 12 ′ on the side subjected to compressive deformation inside the bending, and the cracked portion (that is, the plating layer on the surface of the base material 10). In a portion where 12 'does not exist, necking (thickness reduction) occurs in the base material 10 (for example, an arrow portion). This is probably because the restraint force by the plating layer 12 'does not act on the surface of the base material 10 where the plating layer 12' is cracked, and the tensile stress concentrates on the portion at the time of bending back.

  In order to suppress such necking, it is extremely effective that the plated layer 12 'is not cracked or even if it is cracked when bent back. For that purpose, it is important that the plating layer is sufficiently softened so that it can follow the deformation of the steel sheet by bending.

  Therefore, when applying a coated steel sheet based on a molten Zn-50 to 60% Al alloy-plated steel sheet to a roof or the like subjected to severe processing, the hot-dip plated steel sheet is heat-treated between the plating process and the coating process. In general, the plating layer is softened (Patent Document 1). The heat treatment is performed by annealing for a long time, for example, by holding at a temperature of about 200 ° C. for about 5 to 20 hours.

  FIG. 4 shows a conventional coated steel sheet that has been put into practical use as a roofing material, which has been subjected to baking coating based on a hot-dip Zn-55% Al alloy-plated steel sheet that has been subjected to heat treatment at 220 ° C. × 8 h after hot dipping. The cross-sectional shape at the time of investigating similarly to is typically illustrated. The coating film is omitted in the figure. In this case, the cross-sectional hardness of the plating layer 12 ′ before bending is softened to about 70 HV. As shown in FIG. 4B, only a very fine crack is generated in the plating layer 12 ′ subjected to compressive deformation inside the bend, and necking (thickness reduction) does not occur in the base material 10.

JP 2002-249862 A

  As described above, a coated steel sheet that has been subjected to a heat treatment for softening the plating layer on a molten Zn-50 to 60% Al alloy-plated steel sheet, and then subjected to baking coating, has a plating layer even when bent back. It is difficult to break, and as a result, the base material is less likely to be necked. Therefore, it can be said that such a coated steel sheet has excellent corrosion resistance and excellent resistance to bending back, and is suitable as an exterior member such as a roof.

  However, the heat treatment for softening the plating layer requires long-time heating, for example, 5 to 20 hours. For this reason, there is a problem that productivity is lowered and cost is increased.

  Further, the heat treatment for softening the plating layer gives high yield elongation (around 10%) to the steel, so that remarkable stretcher strain (see FIG. 7) is generated at the time of work forming, and good formability is hindered.

  From the above viewpoint, the present invention is a coated steel sheet based on a molten Zn-50-60% Al alloy-plated steel sheet excellent in corrosion resistance. An object is to provide a low-cost coated steel sheet.

  As a result of detailed studies, the inventors have found that the bending back resistance can be remarkably improved by controlling the base material to be harder than the plating layer without performing the treatment for softening the plating layer. The present invention has been completed.

That is, in the present invention, in the coated steel sheet having a coating layer of Al: 50 to 60% by mass, the balance substantially consisting of Zn on the surface of the steel sheet, and the upper layer (that is, the surface layer side) above the plating layer, The cross-sectional hardness H _M (HV) and the cross-sectional hardness H _P (HV) of the plating layer satisfy the following formulas (1) and (2), preferably adjusted to satisfy the following formula (3). There is provided a Zn-Al-based plated steel sheet suitable for a roof having excellent bend back resistance.
H _M > H _P (1)
H _P ≧ 90 ...... (2)
H _M ≦ 145 (3)

  Here, in the plating layer composition, “consisting essentially of Zn” means that mixing of elements other than Al and Zn is allowed within a range that does not impair the effects of the present invention. For example, as elements that are often contained in a molten Zn-Al alloy plating bath, Si: 0.2 to 3.0 mass%, Fe: 1.5 mass% or less, Mg: 0.5 mass% The following may be mentioned, and other impurities such as Cu, Pb, Sn, Ca, Ni, Mn, Cr, Ti, Na, B, and Sr are allowed to be mixed.

The “base material” is a portion of “steel” derived from a steel plate substrate used as a plating base plate. The cross-sectional hardness of the base material and the plating layer is a hardness measured for a steel plate cross section (hereinafter referred to as “L cross section”) parallel to the rolling direction and the plate thickness direction. The cross-sectional hardness of the plating layer can be measured with a load of 5 g using, for example, a micro Vickers hardness tester. At that time, the tip of the cone may be positioned at the α-Al portion of the plating layer cross section. Sectional hardness H _P of the cross-section hardness H _M and the plating layer of base material has an average value in the case of measuring the near thickness center of the base metal and the plating layer at n = 5, respectively, are employed. “Plating-coated steel sheet” means a steel sheet having a coating film on a plating layer.

Al killed steel containing C: 0.04 to 0.20% by mass can be used as the base material of the coated steel sheet.
The average crystal grain size of the base material is a value measured by the intercept method according to JIS G0511 for the L cross section, and is in the range of, for example, 7 to 50 μm.

  Moreover, in this invention, as a manufacturing method of the said coated steel plate, the cold-rolled steel plate of the property whose hardness after recrystallization annealing becomes 100-145HV is used as a plating original plate, Al: 50-60 mass% after heating with recrystallization, remainder “Hot-plating process” for passing through a hot-dip plating bath substantially composed of Zn, “temper rolling process” for rolling the obtained hot-dip plated steel sheet with an elongation of 0.2 to 4%, and coating After that, a manufacturing method having a “painting / baking process” in which the material temperature (final plate temperature) is heated to 190 to 250 ° C. within 5 minutes and cooled with water is provided. After the hot dipping process, it is desirable not to give a thermal history for holding the steel plate for 1 h or more in a temperature range of 130 ° C. or higher.

  According to the present invention, in a coated steel sheet based on a molten Zn-50 to 60% Al alloy-plated steel sheet that is inherently excellent in corrosion resistance, the anti-bending resistance is remarkably improved. If this coated steel sheet is used for exterior materials such as roofs, cracking of the steel material at the time of construction is prevented, and stretcher strain is less likely to occur compared to a coated steel sheet that has been annealed and softened. Rich. Further, since long-time heating for softening the plating layer is not required, productivity can be improved and costs can be reduced.

  The inventors have investigated in detail the occurrence of necking of the base material at the time of bending back using a coated steel plate based on a molten Zn-50-60% Al alloy-plated steel plate. As a result, in order to prevent necking, it has been confirmed that it is effective to soften the plating layer as in the conventional material. In this case, as shown in FIG. 4, the plating layer 12 ′ that has undergone compressive deformation inside the bend is sufficiently deformed when bent back due to its softness, and cracks are remarkably suppressed. If the plating layer does not crack, the underlying base material is unlikely to deform in a non-uniform manner, and therefore necking hardly occurs.

However, even when the plating layer is not softened, it has been clarified that necking of the base material is remarkably suppressed when the base material is bent back when the base material is controlled to be harder than the plating layer. That is, in the plated steel sheet of the present invention, the cross-sectional hardness H _M (HV) of the base material and the cross-sectional hardness H _P (HV) of the plating layer are controlled so as to satisfy the following formula (1).
H _M > H _P (1)

  FIG. 5 illustrates a cross-sectional shape in the case where the same investigation as in FIG. 3 is performed on the coated steel sheet of the present invention in Example 2 described later. In this case, the cross-sectional hardness of the plating layer 12 ′ before processing is 105 HV, and the base material hardness is 125 HV. As shown in FIG. 5B, the plating layer 12 'is slightly cracked after being bent back. However, necking of the base material is significantly suppressed. There are many unexplained parts about the mechanism at the present time. However, in the surface of the base material where the plating layer 12 ′ is present, the base material is harder than the plating layer. The restraining force from 12 ′ hardly acts on the surface of the base material, and as a result, the difference in stress applied to the base material at the time of bending back between the location where the plating layer 12 ′ exists and the location where the plating layer 12 ′ does not exist becomes very small. Conceivable. The plated layer 12 side is a “guaranteed surface” as a product.

  The coated steel sheet shown in FIG. 5 has been confirmed to have good workability without cracking of the base material at the bent back position in the roof construction using dovetail processing. Further, it has been confirmed that the cracks generated in the plated layer by processing do not impair the appearance of the roof, and the corrosion resistance of the steel sheet is sufficiently maintained.

The cross-sectional hardness H _P (HV) of the plating layer in the coated steel sheet based on the hot-dip Zn-50 to 60% Al alloy-plated steel sheet is controlled by performing temper rolling after the hot-dip plating or heat treatment. be able to. If temper rolling is performed, it becomes slightly harder than the hardness of hot-dip plating. It can be softened by heat treatment.

The heat treatment for controlling the hardness of the plating layer can be carried out in a temperature range of 130 to 250 ° C., but care must be taken not to deviate from the following formula (2). For example, the holding time in the above temperature range should not be 1 h or longer.
H _P ≧ 90 ...... (2)
When the plating layer is softened by heat treatment until the expression (2) is not satisfied, the yield elongation of steel increases, stretcher strain is generated during processing, and the workability is remarkably deteriorated. Moreover, the heat treatment time for softening becomes long, and the cost reduction effect with respect to the conventional material is small.

However, if the molten Zn-50-60% Al alloy coated steel sheet, if within 4 percent elongation that is temper rolling performed after hot dip plating, sectional hardness H _P plating layer is usually 100 The range is 120HV. If the formula (1) is satisfied within this range, sufficient anti-bending resistance can be obtained. Therefore, it is not necessary to perform a treatment that softens the plating layer after hot dipping.

Section of the base metal hardness H _M (HV) can be controlled by the composition of the steel sheet employed mainly as be plated (in particular C content). It can also be hardened slightly by temper rolling after hot dipping. When heat treatment at about 130 to 250 ° C. is performed to control the hardness of the plating layer, the base material may be hardened or slightly softened depending on the C content. In any case, what is necessary is just to control so that said (1) Formula may be satisfy | filled. However, if the base material becomes too hard, the workability of bending and bending back deteriorates, so it is desirable that the range satisfy the following expression (3).
H _M ≦ 145 (3)

  The average crystal grain size in the L cross section of the base material is preferably adjusted to 7 to 50 μm in order to suppress necking of the base material at the time of bending back.

  The composition of the plating layer is Al: 50-60% by mass, and the balance is substantially Zn. Within this range, the sacrificial anticorrosion effect of Zn and the durability improvement effect of Al are exhibited in a well-balanced manner, and a color coated steel sheet having high corrosion resistance suitable for roofing can be constructed. In particular, the Al content is more preferably set in the range of 55 ± 2.5% by mass.

It is desirable that the plating base plate as a base material is Al killed steel containing C: 0.04 to 0.20 mass%. Within this range, it becomes easy to control the hardness of the base material after painting and baking so as to satisfy the expressions (1) and (3).
More specifically, in terms of mass%, C: 0.04 to 0.20 mass%, preferably 0.07 to 0.11 mass%, Si: 0.30 mass% or less, Mn: 0.60 mass% or less. , P: 0.050 mass% or less, S: 0.025 mass% or less, the balance Fe and steel made of inevitable impurities. As an impurity element, the Sn content is desirably 0.15% by mass or less.

〔Manufacturing process〕
The coated steel sheet of the present invention can be produced, for example, as follows.
First, a cold-rolled steel sheet having a property that the hardness after recrystallization annealing is 120 to 145 HV is prepared as a plating original sheet. Whether or not it has such properties may be confirmed in advance by experiments. The plate thickness of the plating original plate is, for example, about 0.35 to 0.40 mm in the case of roofing.

  Next, this plating original plate is heated with recrystallization in a reducing atmosphere, and is subjected to hot dipping by passing it through a hot dipping bath made of Al: 50 to 60% by mass and the balance substantially consisting of Zn. This hot dipping process can be carried out using an existing continuous hot dipping line equipped with a reduction furnace, a hot dipping bath, a plating adhesion amount adjusting means, and a plating layer cooling zone. The plate passing conditions can follow the work standard of conventional hot-dip Zn-Al alloy plating. The plating layer thickness may be about 10 to 27 μm per side.

  After hot dipping, temper rolling with an elongation of 0.2 to 4% is performed. As a result, the plated surface is smoothed, and a plated steel sheet suitable as a coating original sheet is obtained. If the temper rolling rate is too high, the steel is hardened, which is not preferable. This temper rolling is efficient when performed using an in-line mill attached to the continuous hot dipping plating line.

  In the case where the plating layer is softened within a range satisfying the formulas (1) and (2), a heat treatment for holding the steel plate in a range of 130 to 250 ° C. can be performed at this stage. However, it is possible to satisfy the expression (1) by controlling the hardness of the base material as described above, and it is possible to satisfy the expression (2) without performing a heat treatment. Therefore, in consideration of productivity and cost, it is preferable not to give a heat history for holding for 1 h or more in a temperature range of 130 ° C. or higher after hot dipping.

  Next, painting is performed. As the paint, general color steel sheet paints such as polyester and epoxy can be used. By applying the paint on one or both sides of the plating surface by the roll coater method, etc., and then heating to 190-250 ° C. within 5 minutes in the furnace time (herein means the ultimate plate temperature) and cooling with water Bake. The coating thickness after baking may be about 10 to 25 μm. Usually, when the coating thickness is within this range, good results can be obtained in the in-furnace time of about 20 to 60 sec.

Example 1
Using an Al killed steel cold-rolled steel sheet (thickness 0.35 mm) of steel type A shown in Table 1, a Zn-55 mass% Al alloy plating was performed using a continuous hot dipping line. In this continuous hot dipping line, heat treatment was performed under conditions where the material temperature was raised to about 700 ° C. in a reducing atmosphere of hydrogen and nitrogen gas, and then immersed in the plating bath when the material temperature reached about 560 ° C. . The plating original plate is recrystallized by the heat treatment. The temperature of the plating bath was about 600 ° C., and as a result of analysis, the Zn-55 mass% Al plating bath contained 1.5 mass% of Si and 0.5 mass% of Fe. The steel sheet exited from the plating bath was adjusted in the amount of plating by a gas wiping nozzle in the process of being pulled up in a substantially vertical direction, and then cooled by air to solidify the plating layer. The plating layer thickness was about 22 μm on both sides. The plated steel sheet was subjected to temper rolling with an elongation of 1% using an in-line mill.

  The plated steel sheet was coated without being subjected to heat treatment for softening the plating layer. A paint based on polyester was prepared and applied to both sides of the plated steel sheet by a roll coater method. Normal washing and degreasing were performed before coating. Baking was performed under the condition that the temperature in the furnace was increased to about 210 ° C. in about 30 seconds and then cooled with water. The thickness of the coating after baking is about 17 μm on the product as a guarantee surface. The following investigation was conducted using the plated steel sheet thus obtained as a test material.

(Cross section hardness measurement)
A sample embedded in a resin was prepared so that the L cross section of the test material (cross section parallel to the rolling direction and the plate thickness direction) could be observed. The hardness of the plating layer cross section was measured at the center in the thickness direction of the plating layer with a load of 5 g using a micro Vickers hardness tester. The hardness of the base material cross section was measured at the center of the base material in the thickness direction with a load of 5 kg using a Vickers hardness tester. Both the rolling direction was measured at random 5 points were the average of n = 5 and sectional hardness H _P plating layer (HV) and a cross section of the base metal hardness H _M (HV).

[Measurement of crystal grain size of base material]
Using the sample embedded in the resin, the average crystal grain size (μm) of the base material was determined by a section method in accordance with JIS G0551.

[180 ° repeat bending test]
A strip-shaped sample having a width of 50 mm whose longitudinal direction is the rolling direction was cut out from the test material, and the bending resistance was examined by repeatedly bending the sample according to the following procedure. The bending axis is perpendicular to the rolling direction.
[1] Perform 180 ° 2T bending (T is the plate thickness).
[2] Bend back by hand and return to a flat shape by tightening with a vise.
[3] Perform 180 ° 2T bending in the same direction as [1] so that the bend line does not change from [1].
[4] Bend back by hand and return to a flat shape by tightening with a vise.
Thereafter, [3] and [4] are repeated.
[2] The number of cycles is counted once or twice when bent back as in [4].
When the bending back of each cycle is completed, the bending portion of the sample is observed with a microscope, and the above operation is repeated until cracking of the base material is observed.
Such a test was performed at room temperature and in an air atmosphere at 0 ° C. The evaluation of the anti-bending resistance was compared with the result of Conventional Example 1 described later, which is generally recognized to have good anti-bending resistance. Those with “number of times” or more were judged as having no problem in practical use, and ○ (good) and less than that were judged as x (bad).

[Pipe pressing test]
The coated steel plate (thickness 0.35 mm) was manually pressed against the surface of a 50 mmφ pipe, and the surface properties of the processed steel plate were observed. A case where a broken line was not conspicuous on the surface and a beautiful appearance was maintained was evaluated as ◯ (good), and a case where a non-uniform broken line which clearly deteriorated the appearance was generated was evaluated as x (defect).

Example 2
An experiment was performed in the same manner as in Example 1 except that an Al-killed cold-rolled steel plate (thickness 0.35 mm) of steel type B shown in Table 1 was used as the plating base plate.

Example 3
Experiments were performed in the same manner as in Example 1 except that an Al-killed steel cold-rolled steel plate (thickness 0.35 mm) of steel type C shown in Table 1 was used as the plating base plate.

<< Comparative Example 1 >>
Experiments were conducted in the same manner as in Example 1 except that an Al-killed cold-rolled steel sheet (thickness 0.35 mm) of steel type D shown in Table 1 was used as the plating base sheet.

<< Conventional Example 1 >>
For the steel sheet C shown in Table 1 cold-rolled steel sheet (0.35 mm thick) used as the plating base, and for the hot-dip galvanized steel sheet subjected to temper rolling with an elongation of 1%, before coating In addition, an experiment was performed in the same manner as in Example 1 except that a heat treatment for softening the plating layer was performed under the conditions of 220 ° C. × 8 h holding and furnace cooling.
In addition, this coated steel plate is used for a roof material in a cold region, and has characteristics similar to those of products that are evaluated as being good in anti-bending resistance.

[Results]
The plated coated steel sheet of each example corresponding to the present invention example is subjected to heat treatment for softening the plating layer by controlling the base material to be harder than the plating layer and satisfying the above formula (1). Although it was not done, it exhibited excellent bend-back resistance similar to that of Conventional Example 1 in which the heat treatment for softening the plating layer was performed. Good results were also obtained in the pipe pressing test. In FIG. 6, the external appearance photograph of the steel plate surface after the pipe pressing test in Example 2 is illustrated. In other examples, a beautiful appearance was maintained as in FIG.

  On the other hand, the thing of the comparative example 1 was inferior in the bending-proof property by the hardness of a base material being low and not satisfy | filling (1) Formula. This is because the necking of the base metal occurs at the place where the plating layer is cracked during the bending back of the repeated bending test, resulting in stress concentration at the necking part of the base material at a relatively early stage of repeated bending. It is thought that it has led to cracking. In Conventional Example 1, since the treatment for softening the plating layer is performed, the anti-bending resistance is improved. However, since high yield elongation is imparted by the heat treatment, there is a concern that remarkable stretcher strain is generated at the time of processing and the workability is inferior. In fact, in the pipe pressing test, a non-uniform broken line as shown in FIG. 7 was generated on the surface. Moreover, the cost increase by inserting the process which softens a plating layer cannot be denied.

The figure which showed typically the cross-sectional shape of the ant hook process part which may be formed with the roof which uses a coated steel plate. The figure which illustrated typically the appearance of the roof member before and after the operation which returns bending to a part of dovetail processing part is added. A conventional coated steel sheet (without the softening treatment of the plating layer) based on a molten Zn-55% Al alloy-plated steel sheet was subjected to 2T bending (T is the plate thickness), and then bent back. The figure which illustrated the cross-sectional shape of the part. A conventional coated steel sheet (with a coating layer softening treatment) based on a hot-dip Zn-55% Al alloy-plated steel sheet was subjected to 2T bending (T is the plate thickness), and then bent back. The figure which illustrated the cross-sectional shape of the part. The coated steel sheet of the present invention based on a hot-dip Zn-55% Al alloy-plated steel sheet (without the softening treatment of the plating layer) was bent at 2T (T is the plate thickness), and then bent back. The figure which illustrated the cross-sectional shape of the part. The drawing substitute photograph which shows the surface appearance at the time of giving the process pressed against a 50 mm diameter pipe about the coated steel plate of this invention based on the hot-dip Zn-55% Al alloy plating steel plate (without the softening process of a plating layer). The drawing substitute photograph which shows the surface appearance at the time of giving the process pressed against a 50 mm diameter pipe about the conventional coated steel plate (with the softening process of a plating layer) based on the hot-dip Zn-55% Al alloy plating steel plate.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Base material 11 Coating layer 12 Plating layer 12 'Plating layer which received compression deformation inside bending side 20 Dovetail processing part 21 Cutting position 22 Flat part

Claims (6)

The surface of the steel sheet, Al: 50-60% by weight, and the plating layer and the balance substantially Zn, in painted steel sheet having a coating film on an upper layer than the plating layer, the cross section of the base material hardness H _M (HV), A Zn—Al-based plated steel sheet having excellent bending back resistance in which the cross-sectional hardness H _P (HV) of the plating layer satisfies the following formulas (1) and (2).
H _M > H _P (1)
H _P ≧ 90 ...... (2)   2. The Zn—Al-based plated steel sheet according to claim 1, wherein the base material is Al killed steel containing C: 0.04 to 0.20 mass%. Furthermore, the Zn-Al system plating coating steel plate of Claim 1 or 2 which satisfy | fills following (3) Formula.
H _M ≦ 145 (3)   The Zn—Al-based plated steel sheet according to claim 1, wherein the base material has an average crystal grain size of 7 to 50 μm.   A cold-rolled steel sheet having a property of 100 to 145 HV after recrystallization annealing is used as a plating base plate, and after passing through recrystallization, Al: 50 to 60% by mass, and the remainder is passed through a hot dipping bath substantially consisting of Zn. The “hot-plating process”, the “temper rolling process” in which the obtained hot-dip plated steel sheet is rolled at an elongation of 0.2 to 4%, and the coating material is applied, and the material temperature is 190 minutes within 5 minutes in the furnace. The manufacturing method of the Zn-Al system plating coating steel plate in any one of Claims 1-4 which have a "painting / baking process" which heats to -250 degreeC and cools.   The manufacturing method of the Zn-Al system plating coating steel plate of Claim 5 which does not provide the heat history which hold | maintains for 1 h or more in the temperature range of 130 degreeC or more after a hot dipping process.