JP4931673B2 - Painted steel sheet with excellent corrosion resistance - Google Patents

Description

  The present invention relates to a coated steel sheet coated with a highly corrosion-resistant zinc-based alloy plated steel sheet as a base material.

  Galvanized steel sheets have been used as the base material for coated steel sheets, but recently, many coated steel sheets coated with zinc-based alloy plated steel sheets with higher corrosion resistance have been used. . For example, as a high corrosion resistance zinc-based alloy plated steel sheet, the Zn-Al-Mg-based plated steel sheet is hot-plated by mass%, Al: 3 to 22%, Mg: 1 to 10%, and the balance substantially consisting of Zn. It has been known. This type of plated steel sheet has been considered difficult to stably produce products with good surface appearance, but recently, its production technology has also been established and has been put to practical use in a wide range of applications. Typical products include Zn-4 to 12% Al-1 to 3% Mg plated steel sheet. In addition, the Zn-Al-based plated steel sheet that has been hot-plated with Al: 50-60% by mass and the balance being substantially made of Zn has good corrosion resistance and heat resistance, and in particular, Zn-55% Al-plated steel sheet. Is called Galvalume steel plate and is widely used in the field of building materials. These plated steel sheets may be used uncoated, but there are many applications that are used after being subjected to clear coating or color coating in order to improve durability and designability.

  However, these highly corrosion-resistant zinc-based alloy-plated steel sheets are usually hard at a plating layer of 80 HV or more, and therefore, when severe bending is performed, the plated layer is likely to crack in the processed part. Even if some cracks occur in the plating layer, the sacrificial anti-corrosion action will work, so there will be no corrosion that immediately opens a hole in the steel sheet, but red rust will occur and the appearance will deteriorate, and long-term durability will also deteriorate. . Even when the coating is applied, there is a problem that the coating film is easily cracked in response to the cracking of the plating layer, and the corrosion resistance cannot be prevented from being lowered.

  Patent Documents 1 and 2 disclose a method of forming a coating film having a high elongation at break of 100% or more on the surface of the above zinc-based alloy plated steel sheet.

JP 2003-277903 A JP 2004-181860 A JP-A-2-14867 JP 7-126826 A JP-A-10-220187

  As shown in Patent Documents 1 and 2, if a coating film having high elongation at break is formed on a hard zinc-based alloy plating layer, cracking of the plating layer is reduced during bending, and the coating film is not torn. As a result, corrosion resistance at the processed portion can be improved as a result. However, according to detailed investigations by the inventors, it has been found that even when the techniques of Patent Documents 1 and 2 are applied, the corrosion resistance of the processed part may not be sufficiently improved in actual construction. When the cause was examined, when the forming process was performed in a low temperature environment (for example, 5 ° C.), cracking of the plating layer could not be sufficiently suppressed, and a crack may occur in the coating film. It was found that the corrosion progresses starting from the crack occurrence site. That is, simply forming a flexible coating film having an elongation of 100% or more cannot sufficiently improve the corrosion resistance of the processed part when molding is performed in winter.

  In view of such a current situation, the present invention has a marked improvement in the corrosion resistance reduction in the processed part even when it is formed in a low temperature environment in a coated steel sheet based on a hard high corrosion resistance zinc-based alloy plated steel sheet. An object is to provide a coated steel sheet.

  As a result of detailed investigations, the inventors of the present invention, as a property of the coating film formed on the plating layer, in addition to good elongation, a high Young's modulus is a hard plating in a molding process in a low temperature environment. It was found to be extremely effective in reducing the cracking of the layer. Moreover, it discovered that it was important to make the value of Young's modulus x coating film thickness sufficiently large according to the plating layer thickness. The present invention has been completed based on such findings.

That is, the above-mentioned purpose is a molten Zn-Al-Mg having a cross-sectional hardness of 80 HV or more, consisting of Al: 3 to 22%, Mg: 1 to 10%, the balance consisting of Zn and inevitable impurities. And having at least one coating layer satisfying the following condition A above the plating layer, and totaling the X values determined by the following formula (1) for each coating layer satisfying the condition A When the value is expressed as X _TOTAL , this is achieved by a coated steel sheet having excellent processed portion corrosion resistance, in which the following formula (2) is established.
Condition A: Elongation at break of the coating layer is 50% or more and Young's modulus is 5 N / mm ² or more (1) Formula: X = [Young's modulus of coating film (N / mm ² )] × [Coating thickness ( μm)]
(2) Formula: X _TOTAL ≧ 3 × [plating layer thickness (μm)]

Here, the coating thickness is the average thickness of the dried coating. When the number of coating layers satisfying the condition A is one, the X _TOTAL value is equal to the X value for the coating layer. The plating layer thickness is an average thickness after solidification. 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 , in the case of a Zn—Al—Mg-based alloy plating layer, the tip of the cone may be positioned at the α-Al portion of the plating layer cross section. For the cross-sectional hardness of the plating layer, an average value when the vicinity of the central portion of the plating layer is measured at n = 5 is adopted.

Generally as elements contained in the molten Zn-Al-Mg plated layer, Ti, B, Si, although Fe and the like, they are Ti: 0.1 wt% or less, B: 0.05 wt% Hereinafter, Si may be contained in a range of 2% by mass or less and Fe: 2% by mass or less .

Moreover, the plating layer thickness can be 15-50 micrometers.

  According to the present invention, even when a severe bending process is performed in a low-temperature environment of 5 ° C. or lower in a zinc-based alloy plated steel sheet having a hard, highly corrosion-resistant plating layer, the cracking of the plating layer in the bent portion is greatly increased. Therefore, the corrosion resistance in the portion is remarkably improved. Therefore, the desired high corrosion resistance is maintained even in the product molded and processed in winter, and the stabilization of the product quality that has conventionally fluctuated depending on the processing time is realized.

  High-corrosion-resistant zinc-based alloy-plated steel sheet with a hard plating layer, when bent, tends to cause large “cracking” in the plating layer on the outer surface of the bent portion (the surface far from the bending axis) . This is because tensile stress is generated on the outer surface of the bent part during bending, and when cracks enter the plating layer, the stress concentrates on the plated layer and becomes locally elongated, cracks open widely and cracks expand. It is thought to have invited. Conventionally, the corrosion resistance at the bent portion has been improved by covering the hard plating layer with a coating film that stretches well (Patent Documents 1 and 2). In this case, even if the plating layer is cracked during bending, as long as the coating does not break, the plating layer acts as a hydrostatic restraint force, which causes cracks in fresh places as the degree of processing increases. It is presumed that the cracks generated in the early stage are prevented from growing into large cracks locally. Moreover, since the coating film grows well, unless the plating layer cracks grow locally into large cracks, the coating film itself is prevented from cracking, and as a result, high corrosion resistance is maintained.

  However, when bending is performed under a low temperature environment in winter, it has been found that the conventional method of covering with a well-stretched coating is not always effective for ensuring corrosion resistance in the processed portion. The detailed cause is not necessarily clear, but under severe conditions at low temperatures (for example, 5 ° C. or less), the coating may provide a binding force sufficient to prevent the local cracks of the plating layer from expanding. The area of the exposed steel substrate under the paint film increases with the expansion of cracks, and defects in the paint film itself are likely to occur, resulting in a decrease in corrosion resistance. It is done.

According to the inventors' detailed research, as a coating film covering the hard plating layer, in addition to moderately high elongation, by forming a coating film with a high Young's modulus, severe conditions under low temperature environment It was also clarified that the expansion of cracks in the plating layer was remarkably suppressed and the corrosion resistance at the bent portion was improved. A high Young's modulus of the coating film means that the initial resistance of the coating film elongation is large. FIG. 1 schematically shows a stress-strain curve when a tensile test is performed on a free coating film obtained as described below. In the present invention, in this stress-strain curve, a straight line passing through the origin and a point on the stress-strain curve when the strain (elongation rate) of the coating film is 0.2% is assumed. The inclination is adopted as the Young's modulus (N / mm ² ) of the coating film. When the plating layer is covered with a coating film having a high Young's modulus, the restraining force from the coating film against deformation of the plating layer increases, and this restraining force has a large counteracting force against the expansion of cracks generated in the plating layer at the initial stage of bending. Thus, it is presumed that the stress concentration of the plating layer accompanying the increase in the workability is remarkably relieved as compared with the prior art even in the bending work in a low temperature environment. As a result, the plating layer is deformed more uniformly as a whole, and the area of the steel substrate exposed portion under the coating film is greatly reduced. Moreover, since a coating film exhibits moderately good elongation, the formation of coating film defects is also suppressed.

In the present invention, based on such knowledge, at least one coating layer that satisfies the following condition A is provided above the plating layer.
Condition A: The coating film layer may have a coating layer that has a breaking elongation of 50% or more and a Young's modulus of 5 N / mm ² or more but does not satisfy the condition A. The above is constituted by a coating layer satisfying the condition A.

A coating layer having a Young's modulus of less than 5 N / mm ² cannot sufficiently bear a restraining force against deformation of a hard zinc-based alloy plating layer in a severe processing environment in winter (for example, 5 ° C. or less). Accordingly, unless at least one coating layer having a Young's modulus of 5 N / mm ² or more is provided, it is difficult to sufficiently improve the corrosion resistance in the processed portion. More preferred characteristics, the conditions in place of the standard of "Young's modulus 5N / mm ² or more" of A can be provided with the provisions of "Young's modulus 10 N / mm ² or more", "Young's modulus 15N / mm ² The above definition may be provided.

  In addition to such a high Young's modulus, a coating layer having a high elongation at break of 50% or more may cause defects in the coating film itself when the deformation of the coating follows the progress of cracks in the plating layer during bending. Depending on the type, it has the effect of preventing the formation of tears.

  The Young's modulus and elongation at break of the coating film can be changed by known methods such as selection of the skeleton structure of the resin, adjustment of the type and amount of the curing agent, and blending of resins having different elongation rates. In general, Young's modulus tends to increase when (i) the molecular weight of the resin is low, (ii) the glass transition temperature (Tg) is high, (iii) the crystallinity is high, and (iv) there are many curing agents. There is. In the opposite case, the elongation at break tends to improve. Further, when melamine is used instead of isocyanate as a curing agent, Young's modulus generally increases.

On the other hand, in the bending process of the plated steel sheet, the crack generated in the plated layer tends to increase as the thickness of the plated layer increases. Therefore, a greater restraining force is required for the coating film as the plating layer thickness increases. The inventors consider that the binding force of the coating film against deformation of the plating layer depends on the product of “the Young's modulus of the coating film” and “the thickness of the coating film”, and zinc-based alloy plating having various plating layer thicknesses. Using steel plates, the relationship between the value of “Young's modulus of coating film” × “film thickness” and the effect of suppressing cracking of the plating layer was investigated in detail. As a result, in the coating layer satisfying the above condition A, when the X value is defined as in the following formula (1), the coating value of the coating layer is such that the X value is at least three times the plating layer thickness (μm) It has been found that by controlling the Young's modulus and the coating thickness, the effect of suppressing cracking of the plating layer (the plating layer cracking reduction rate described later) is rapidly improved.
(1) Formula; X = [Young's modulus of coating film (N / mm ² )] × [Coating thickness (μm)]
When two or more coating layers satisfying the above condition A are formed, the X value is calculated by the formula (1) for each coating layer, and the value obtained by totaling the X values in each coating layer, That is, it is sufficient that X _TOTAL is 3 times or more of the plating layer thickness (μm). That is, it is necessary to satisfy the following formula (2).
(2) Formula: X _TOTAL ≧ 3 × [plating layer thickness (μm)]
If this formula (2) is not satisfied, it is difficult to sufficiently improve the corrosion resistance at the bent portion even when a coating film having a high Young's modulus and elongation at break is formed.

In the present invention, the target zinc-based alloy plating has a plating layer with a cross-sectional hardness of 80 HV or more. In the case of a plating layer having a lower cross-sectional hardness than this, even if a plating layer crack occurs in the bent portion, the corrosion resistance as a coated steel plate is hardly a problem. On the other hand, when the cross-sectional hardness of the plating layer is 80 HV or more, cracking of the plating layer due to bending (particularly bending under a low temperature environment) tends to be manifested as a decrease in corrosion resistance of the bent portion. As described above, the zinc-based alloy plating contains Al: 3 to 22%, Mg: 1 to 10% , or Ti: 0.1% by mass or less, B: 0.05% or less, Si: The target is molten Zn—Al—Mg-based plating containing one or more of 2% or less and Fe: 2% or less, with the balance being Zn and inevitable impurities .

  Various steel plates that have been conventionally used depending on the application are applicable to the plating base plate. A typical example is an Al killed steel cold-rolled steel sheet having a thickness of about 0.3 to 1.5 mm. Examples of the steel composition include C: 0.01 to 0.10% by mass, Si: 0.3% by mass or less, Mn: 0.5% by mass or less, and P: 0.05 as necessary. Examples include steel containing not more than% by mass, S: not more than 0.02% by mass, and Al: not more than 0.04% by mass, with the balance being Fe and inevitable impurities. However, this is an example, and the steel type is not particularly limited in the present invention.

Conventional methods can be applied to the method for producing a coated steel sheet of the present invention.
Typically, a zinc-based alloy-plated steel sheet is manufactured in a continuous hot dipping line, and this is used as a base material for a coated steel sheet. Although the thickness of the plating layer can be about 5 to 200 μm, good results are obtained for many uses of coated steel sheets in the range of about 5 to 50 μm. Prior to coating, the plated steel sheet is subjected to known coating pretreatment as required. For example, pickling, surface adjustment treatment (acid type), chemical conversion treatment (chromate treatment, chromium-free treatment, etc.) can be employed. A method of performing only alkaline degreasing may be used. These coating pretreatments improve the coating film adhesion. Then, if necessary, one or more undercoats are applied, and then a topcoat is applied. In at least one layer of the undercoating and the overcoating, a paint whose components are adjusted so that the elongation at break and Young's modulus of the dried coating film satisfy the above-mentioned regulations is applied. At that time, the coating amount is adjusted so that the dry thickness of the coating film using the coating material satisfies the formula (1). The paint is selected from polyester, epoxy, acrylic, fluororesin, etc. As a coating method, a known method such as a roll coater method or a bar coater method can be used, and a continuous coating line can be used.

As a zinc-based alloy-plated steel sheet, an Al killed steel cold-rolled steel sheet having a thickness of 0.5 mm (nominal composition; C: 0.04 mass%, Si: 0.03 mass%, Mn: 0.20 mass%, P: 0) A plated steel sheet was prepared by applying molten Zn-6 mass% Al-3 mass% Mg plating on the surface of 0.01 mass%, S: 0.01 mass%, Al: 0.01 mass%, balance Fe). The plating layer thickness per side was set to three types of 7, 15, and 30 μm. The composition (mass%) of the plating bath is as follows.
Al: 6%, Mg: 3%, Ti: 0.002%, B: 0.0005%, Si: 0.01%, Fe: 0.1%, Zn: remainder On the obtained plated steel sheet, before coating As the treatment, pickling, surface conditioning (Nippon Paint, NPC700), washing (hot water washing), drying, and chemical conversion treatment (Si-based chromium-free treatment solution (Nippon Paint, EC2000) are applied with a roll coater. And then dried at 100 ° C.).

Various undercoat paints and topcoat paints having the characteristics shown in Table 1 were prepared as paints. The Young's modulus and elongation at break of the coating film were determined by conducting a tensile test with a free coating film as follows. First, a paint is applied to a fluorine film laminate plate to form a dry coating film. This coating film can be easily peeled from the fluorine film laminated plate, and a free coating film is obtained. The coating thickness is approximately the same as the coating layer to be evaluated. A strip-shaped sample having a width of 5 mm and a length of 70 mm is prepared from the free coating film. This sample is subjected to a tensile test using a tensile tester (manufactured by Shimadzu Corp., AGS-100B type autograph) at a distance between chucks of 50 mm and a tensile speed of 3 mm / min until the sample breaks. In the stress-strain curve, the slope of a straight line passing through the origin and a point on the stress-strain curve when the strain (elongation rate) of the coating film becomes 0.2% is obtained, and this is determined as the Young of the coating film. The rate (N / mm ² ). Further, the elongation percentage is calculated by dividing the distance between the chucks (displacement amount) at the time when the sample is broken by the initial length (50 mm).

  Each coated steel sheet sample shown in Table 1 was produced by applying each paint to the plated steel sheet. In addition to the two-layer type coated steel sheet in which the top coat film is formed on the surface of the undercoat film, the one-layer type coated steel sheet sample using only the paint prepared for the undercoat film, and the paint prepared for the top coat film A single-layer type coated steel sheet sample using only the same was prepared. Both the undercoat and topcoat were applied by the roll coater method, and the baking temperature was 210 ° C for the undercoat and 230 ° C for the topcoat. A sample embedded in a resin was prepared so that the cross-section of the obtained coated steel sheet sample could be observed, and the cross-sectional hardness of the plating layer was measured by the method described above using the sample. As a result, the cross-sectional hardness of the Zn-6% Al-3% Mg plating layer was in the range of 150 to 170 HV.

[Bending test in low temperature environment]
A specimen of 50 mm × 50 mm was taken from each coated steel sheet, and a bending test was performed as follows in a low temperature environment (5 ° C.) in consideration of the forming process performed in winter. The specimen was folded 180 ° around a 2 mm diameter rod for about 1 second so that the surface (test surface) having the plating layer and coating to be evaluated was outside the bend. At that time, the bending axis (the above-mentioned bar) was set to be perpendicular to the rolling direction of the test piece. Thereafter, two sheets of the same thickness as the test piece were sandwiched inside the bent portion, and fastened by applying force with a vise. Since there are two sandwiched plates, this bending test corresponds to “2T bending”.

The cross section perpendicular to the bending axis of the test piece after 2T bending is observed with a cross-sectional microscope, and the cracked portion of the plating layer existing in the measurement range (the steel substrate is a certain area) The total length of the portion exposed under the coating film) was measured, and this was defined as the “total crack length” of the coated steel sheet. Specifically, the measurement was performed as shown in FIG. That is, FIG. 2 schematically shows a cross section after the bending test. A plating layer 2 is present on the surface of the steel substrate 1, and cracks 3 are generated on the outside of the bend. In this figure, the coating film and the inner plating layer are omitted, and the outer plating thickness is exaggerated with respect to the plate thickness. Using the surface of the steel substrate as the reference line 4, the crack size 3 of the plating layer existing in the measurement range length (centered at the apex P of the bent portion) defined by the following equation (3) is based on the size of each crack. The value measured by the length of the line 4 was defined as the “crack length” of the crack, and the total value of the “crack length” for each crack 3 was defined as the “total crack length”.
(3) Formula; [Measurement range length] = {(n × T) + 2T} × π / 2
However, n: number of sheets sandwiched, T: sheet thickness In the example of FIG. 2, there are five cracks 3 in the measurement range where the steel substrate 1 is exposed under the coating film, and L ₁ + L ₂ + L ₃ + L ₄ + L ₅ The value corresponds to the “total crack length”.

On the other hand, the same bending test was performed on a plated steel plate (referred to as a base material) before coating, and the “total crack length” was determined by the above method. And "the plating layer crack reduction rate" was calculated by the following formula (4).
(4) Formula; [Plating layer crack reduction rate (%)] = ([total crack length of base material] − [total crack length of coated steel sheet]) / [total crack length of base material] × 100
As a result of various studies, in a conventional coated steel sheet using a Zn-6% Al-3% Mg plated steel sheet as a base material, the plating layer crack reduction rate in the test under this low temperature environment is at most 15%. Here, the plating layer crack reduction rate is 30% or more ○ (good), less than 10-30% is △ (slightly bad), less than 10% is × (bad), the plating layer cracking suppression effect Evaluated. The results are shown in Table 1.

[Corrosion resistance test of processed parts]
A test piece of 50 mm perpendicular to the rolling direction and 30 mm parallel to the rolling direction was cut out from each coated steel sheet sample, subjected to 2T bending at 5 ° C. in the same manner as in the bending test described above, and then the cut end face and back face Was coated with silicone resin. As shown in FIG. 3, the test piece after processing is placed on a resin plate standing upright with a width of 65 mm and a height of 150 mm with a silicone adhesive perpendicular to the surface of the resin plate and parallel to the width direction. The test sample was pasted. The test sample was subjected to a 500 hour salt spray test (JIS K-5600-7-1) to evaluate the occurrence of rust and blistering. The white rust generation area ratio and blister generation area ratio were both 10% or less in the processed part (good), while the white rust generation area ratio and the blister generation area ratio were both 30% or less. , At least one of which exceeded 10 to 30% was evaluated as △ (slightly defective), and at least one of white rust generation area ratio and blister generation area ratio was evaluated as x (defect). The ○ evaluation was determined to be acceptable. The results are shown in Table 1.
In the column of “Equation (2)” in Table 1, “○” is given to those satisfying Equation (2), and “X” is attached to those not satisfying (the same applies to Table 2 described later).

  As can be seen from Table 1, in the coated steel sheet of the present invention example, the crack reduction rate of the plating layer was stably 30% or more, and the processed part corrosion resistance was also excellent. That is, according to the present invention, in a coated steel sheet having a hard molten Zn-Al-Mg-based plating layer, an excellent plating layer cracking suppression effect is obtained even when a forming process is performed in a low-temperature environment. It was confirmed that the corrosion resistance of the processed part was stably improved remarkably. Moreover, in what formed the multilayer coating film, it turns out that the said improvement is implement | achieved if it has at least 1 layer or more of coating films prescribed | regulated by this invention, as shown by No. 12-14. Furthermore, as shown in No. 15, both the undercoat film and the topcoat film alone are (2) in total, even if the X value of the formula (1) is not more than 3 times the plating layer thickness. If the expression is satisfied, the above improvement effect can be obtained.

On the other hand, No. 21, which is a comparative example, did not satisfy the expression (1), so that the restraining force against deformation of the plating layer was not sufficiently exhibited, and the plating layer cracking suppression effect was small. As a result, the processed part was inferior in corrosion resistance. Nos. 23 and 25 do not have a coating layer having a Young's modulus of 5% or more, so that the bending force in a low temperature environment is insufficient in restraining force against deformation of the plating layer, and the elongation at break of the coating film is reduced. Even if it was good, the plating layer cracking suppression effect was small. As a result, the processed part was inferior in corrosion resistance. In Nos. 22 and 24, the fracture elongation of the coating layer was less than 50%, so it was considered that the coating film was cracked, the plating layer cracking suppression effect was not sufficiently exhibited, and the processed part corrosion resistance was inferior. Nos. 26 to 29 had a multilayer coating film, but both the undercoating film and the top coating film deviated from the provisions of the present invention, so the effect of suppressing cracking of the plating layer was small, and the processed part corrosion resistance was inferior .

The figure which showed the stress-distortion curve of the free coating film typically. The figure which showed typically the cross section perpendicular | vertical to the bending axis of the test piece after a bending process test. The figure which showed typically the installation condition of the test piece used for the corrosion resistance test of the process part.

1 Steel substrate 2 Plating layer 3 Crack 4 Reference line

Claims (3)

On the surface of the steel sheet , there is a molten Zn—Al—Mg-based plating layer consisting of Al: 3 to 22%, Mg: 1 to 10%, the balance being Zn and inevitable impurities, and having a cross-sectional hardness of 80 HV or more. The total of the X values determined by the following formula (1) for each coating layer having at least one coating layer satisfying the following condition A above the plating layer and satisfying the condition A is X _TOTAL When expressed, a coated steel sheet excellent in processed part corrosion resistance, which satisfies the following formula (2).
Condition A: Elongation at break of the coating layer is 50% or more and Young's modulus is 5 N / mm ² or more (1) Formula: X = [Young's modulus of coating film (N / mm ² )] × [Coating thickness ( μm)]
(2) Formula: X _TOTAL ≧ 3 × [plating layer thickness (μm)] The molten Zn—Al—Mg-based plating layer includes, by mass%, Al: 3 to 22%, Mg: 1 to 10%, Ti: 0.1% by mass or less, B: 0.05% or less, The coated steel sheet according to claim 1, comprising at least one of Si: 2% or less and Fe: 2% or less, with the balance being made of Zn and inevitable impurities. The coated steel sheet according to claim 1 or 2, wherein the plating layer thickness is 15 to 50 µm.

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