JP2001192798A - Aluminum plated steel sheet excellent in corrosion resistance in worked area, and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an aluminum plated steel sheet excellent in corrosion resistance in the worked area by press working, and its manufacturing method. SOLUTION: In the aluminum plated steel sheet excellent in corrosion resistance in a worked area, the total number of plating cracks penetrating through a plating layer, occurring when draw-bead working is performed at (100 to 300) mm/min drawing rate and 5-25% percentage of reduction in thickness by using a die of 3-5 m projection-side R, 1.5-2.5 mm recess-side R, and 1-6 mm projection length, is <=80 pieces for 10 mm in the drawing direction and also the sum of the length of cracks in a plating surface layer is <=1 mm for 10 mm in the drawing direction. The aluminum plated steel sheet can be manufactured by rolling a hot-dip aluminum coated steel sheet at 1-10% elongation percentage by using a rolling roll of <=80 mmϕdiameter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aluminum-plated steel sheet having excellent corrosion resistance after press working and a method for producing the same.

[0002]

2. Description of the Related Art In order to cope with the recent tightening of environmental regulations, a Pb-Sn alloy-coated steel sheet containing lead, which has been conventionally used as a material for fuel tanks, that is, a turn-coated steel sheet (Japanese Patent Publication No. 57-61833). ) Is being replaced with another plated steel sheet. Application of materials such as hot-dip galvanized steel sheets, Zn-Ni electroplated steel sheets, and Sn-Zn plated steel sheets has been studied. The fuel tank is generally provided externally at the lower part of the vehicle body, and is used in a severely corrosive environment particularly in an area where snow-melting salt is sprayed on a road. Aluminum-plated steel sheets have excellent corrosion resistance in this harsh environment and are widely used in the field of building materials because they are inexpensive plated steel sheets. In general, about 7 to 10% by weight of Si is added to aluminum plating for operational reasons, and this is continuously dispersed in the plating layer in a dendritic manner. The Si layer is very brittle and easily induces plating cracks during processing, and in some cases, cracks reaching the base iron may occur.

Further, an aluminum-containing plated steel sheet has a brittle alloy layer containing Fe, Al, and Si as a main component at a thickness of 1 to 5 μm at an interface between the plated layer and the steel sheet. Cause Defects in these plating layers and alloy layers cause corrosion factors such as moisture and chlorine to propagate therethrough, thereby deteriorating the corrosion resistance in a corrosive environment. In particular, if there is a plating crack penetrating to the base iron, it acts as a corrosion starting point, so that corrosion is easily induced. In order to solve these problems, an attempt may be made to suppress the occurrence of plating defects during processing by using a press oil having good lubricity, but since the lubrication is performed by a liquid, the sliding resistance at the time of pressing is reduced. , But has little effect on suppressing plating damage.

Further, a technique of applying an organic resin film having a high lubricity as disclosed in Japanese Patent Application Laid-Open No. 9-142466 has been studied. When the thickness of the film is 1 μm or more, it has a great effect on suppressing plating cracking, but on the other hand, it significantly deteriorates the weldability, so that it was difficult to apply it in actual operation. Further, for the purpose of avoiding deterioration of weldability while maintaining workability, a technique of applying an organic resin film which is released in a degreasing step after pressing has been studied. Applying this method is effective in suppressing plating cracking and maintaining weldability, but it is difficult to remove the film in the conventional degreasing process, so it is necessary to greatly extend the degreasing time, and Since the organic film deteriorates the degreasing solution, the cost of degreasing increases.

Further, as a means for improving the workability of plating itself, a method is disclosed in Japanese Patent Application Laid-Open No. Sho 60-77966 in which a plating layer is worked by using shot blasting. When this method is used, although it is effective in suppressing plating cracks, the unevenness of the surface becomes large and the appearance deteriorates, and in some places, a place where the plating thickness is thin is formed, so that it becomes a corrosion starting point, and the corrosion resistance is significantly reduced. . As described above, there has been no technique in the conventional study range that can suppress plating cracks during pressing, does not adversely affect the subsequent degreasing and welding steps, and maintain corrosion resistance.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems and to provide an aluminum-plated steel sheet having excellent corrosion resistance of a portion processed by press working and a method for producing the same.

[0007]

Means for Solving the Problems As a result of intensive studies to solve these problems, the inventors have been able to reproduce the plating properties after press working by draw bead processing and to specify the corrosion resistance after working. It has been found that the workability can be improved by rolling under the conditions and adding the working, and the present invention has been achieved. The points of the invention are as follows: (1) Convex side R: 3 to 5 mm, concave side R : 1.5-2.5m
m, using a mold having a protrusion length of 1 to 6 mm, a drawing speed of 100 to 300 mm / min, and a thickness reduction rate of 5 to 25%.
The total number of plating cracks that penetrate the plating layer when draw bead processing is performed is 80 or less per 10 mm in the drawing direction, and the total crack length in the plating surface layer is 1 mm or less per 10 mm in the drawing direction. Aluminum plated steel sheet with excellent workability and corrosion resistance.

(2) The elongation rate of the hot-dip aluminum-plated steel sheet is reduced to
The method for producing an aluminum-plated steel sheet having excellent workability and corrosion resistance according to the above (1), wherein the aluminum-plated steel sheet is rolled in a range of 10%. (3) The method for producing an aluminum-plated steel sheet excellent in workability and corrosion resistance according to (2), wherein the rolling in two directions perpendicular to each other is alternately performed one or more times.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION A coated steel sheet applied to the present invention is a hot-dip aluminum-coated steel sheet produced in an aluminum plating bath containing 7 to 10% by mass of Si and unavoidable impurities. The plating adhesion amount is not particularly limited as long as it is within a range in which it can be manufactured. In the processing used for processing parts such as ordinary fuel tanks, the reduction in the thickness of the steel sheet is 25% or less, but corrosion at a part where the thickness is 5% or more is a problem.
Here, the sheet thickness reduction allowance is a numerical value defined by (steel sheet initial thickness−steel sheet thickness after processing) × 100 / (steel sheet initial thickness). This is because plating damage occurs with the increase in the degree of processing and acts as a corrosion starting point. In particular, in the case of an aluminum-plated steel sheet manufactured under normal conditions, when the thickness reduction allowance exceeds 5%, it penetrates the plating layer. This is because cracks are generated and induce corrosion.

After an ordinary aluminum-plated steel sheet which has not been subjected to skin pass rolling is actually pressed, it is 20 mm from the side wall.
A corner sample was cut out. After measuring the thickness reduction with a micrometer, embed the resin and polish it to observe the cross section along the press extrusion direction.
Observed at 0x. As an example, FIG. 1 shows a schematic diagram of a cross section in the case of a sheet thickness reduction allowance of 10%. As shown in the figure, the following three types of cracks occur in the plated steel sheet.

Crack 1 penetrating the plating layer (reached from the plating surface to the steel plate) Crack 2 occurring only in the plating layer 2 Crack 3 in the alloy layer (easy to occur even with a relatively low degree of processing) The inventors have found that the crack 1 penetrating the plating layer has the greatest influence on the corrosion resistance, and that the corrosion resistance after processing can be evaluated by measuring the number and size of the cracks.

That is, FIG. 2 shows the thickness reduction allowance measured by the above method for a plurality of specimens cut out from various parts of the sample after the actual press molding, and the number of plating cracks penetrating the plating layer confirmed per 10 mm, In addition, the results of measuring the total crack length in the plating surface layer are shown. From FIG. 2, the number of plating cracks penetrating the plating layer and the crack length in the plating surface layer confirmed per 10 mm in the member processed by the actual press exist in a certain range with respect to the thickness reduction allowance. I understand.

FIG. 3 shows the relationship between the plating damage quantified by this method and the corrosion resistance. Corrosion resistance was determined by applying a melamine-based black coating of 20 μm to a 70 × 150 mm member cut out of an aluminum-plated steel tank after the actual pressing, applying a melamine-based black coating, and performing a JIS Z
After a salt spray test according to 2371 was performed for 40 days, the swelling from the crosscut portion was measured. Maximum swollen width is 10
mm or less, ○: 10 to 15 mm, ×: 15 mm or more
Indicated by

As is clear from FIG. 3, the corrosion resistance after coating of the processed aluminum-plated steel sheet is largely affected by the number of plating cracks penetrating the plating layer and the crack length in the plating surface layer, and penetrates the plating layer. When the number of plating cracks is 80 or less per 10 mm and the total crack length in the plating surface layer is 1 mm or less per 10 mm, good corrosion resistance is exhibited. As a result of earnestly studying a method for simply evaluating the corrosion resistance after processing, the plating damage situation after actual press responds very well to the plating damage generated by the draw bead processing with specified mold conditions and processing conditions. Corrosion resistance has a correspondence with the plating damage situation. Therefore, it has been found that it is possible to evaluate the corrosion resistance after processing by grasping the plating damage state of the sample subjected to the draw bead processing under specific conditions.

That is, FIG. 4 shows a cross-sectional shape of a die used in the draw bead processing. The present inventor processes the die while changing the shape of the die and the drawing speed of the draw bead variously to reduce the thickness reduction and the drawing. The number of plating cracks penetrating the plating layer per 10 mm in the direction and the total crack length in the plating surface layer were measured. As a result, the drawing speed is 100 to 300 mm / with a mold having a convex side R3 to 5 mm, a concave side R1.5 to 2.5 mm, and a convex part length of 1 to 6 mm.
It was clarified that by performing the draw bead processing in min, the relationship between the reduction in the sheet thickness and the plating damage in the actual press shown in FIG. 2 can be reproduced. Further, as a result of examining the corrosion resistance after painting of the material subjected to the draw bead processing under these conditions, the relationship between the plating damage and the corrosion resistance in the actual press shown in FIG. 3 can be reproduced.

Further, the present inventors have searched for a method of manufacturing an aluminum-plated steel sheet having excellent corrosion resistance after processing by the above-described evaluation method. As a result, the steel sheet is subjected to rolling with a specific elongation by a rolling roll having a specific diameter. It has been found that plating cracks generated during the subsequent processing can be greatly reduced and the corrosion resistance can be improved. As shown in the hatched area in FIG. 5, the elongation rate is 1 to 1 with a rolling roll having a diameter of 80 mmφ or less.
It has been clarified that by performing rolling in the range of 0%, plating defects in subsequent draw bead processing are significantly reduced.

That is, when draw bead processing is performed at a sheet thickness reduction rate of 5 to 25%, the number of plating cracks penetrating the plating layer is 80 or less per 10 mm in the drawing direction.
In addition, the total crack length in the plating surface layer can be reduced to 1 mm or less per 10 mm in the drawing direction. When a roll having a roll diameter of more than 80 mm is used for rolling, plating defects after draw bead processing hardly change. Also, if the elongation during rolling exceeds 10%,
Cracking of the alloy layer generated during rolling causes the plating damage in subsequent draw bead processing to be promoted.

It is not always clear why the rolling of a steel sheet at a specific elongation with a specific diameter of a rolling roll can significantly reduce plating cracks generated during subsequent processing. It is presumed that, by doing so, dendritic crystals (dendrites) of the Al layer existing in the plating layer were entangled to prevent the propagation of plating cracks.
As for the rolling direction, it is more effective to perform the rolling in two orthogonal directions (for example, the longitudinal direction and the width direction of the steel sheet). This is because, in press working, the steel sheet is deformed in both the longitudinal direction and the width direction of the steel sheet, so that it is necessary to perform rolling in both directions. However,
In the case of processing in only one direction, such as bending, the effect is sufficiently exhibited if rolling is performed in the direction in which the processing is performed. Further, when it is difficult to perform rolling from two orthogonal directions such as a long plate material, by performing cross rolling, an effect similar to that can be obtained.

[0019]

EXAMPLE A hot-dip aluminum-plated steel sheet (300 × 300 mm, thickness 0.8 t, plating composition Al-8% Si, coating weight 40 g / m ² , chromate coating 20 mg / m ²⁾ ), And rolling was performed while changing the roll system, elongation, and rolling direction. Thereafter, a sample of 40 × 300 mm was cut out for draw bead processing, and 1 to 2.5 g / corrosive oil containing mineral oil as a main component was used.
After applying m ² , a mold with convex side R4mm, concave side R2mm, convex part length 4mm (material SKD-11, Cr plating 20μ)
m) at a drawing speed of 200 mm / min, and a draw bead processing was performed so that the sheet thickness reduction rate was 5 to 25%. A part of the processed sample was cut out, polished after filling with a resin, and polished to observe plating damage by cross-sectional observation. The plating damage degree is the number of plating cracks penetrating the plating layer confirmed per 10 mm,
In addition, the total crack length in the plating surface layer was measured.

After that, the applied rust preventive oil was removed by alkali degreasing, and the corrosion resistance was evaluated by the following method. (1) Bare corrosion resistance JIS Z 237 for samples with dimensions of 40 x 150 mm
After performing a salt spray test according to No. 1 for 30 days and visually observing the appearance, the area ratio of white rust and red rust was measured. (Evaluation criteria) ：: White rust area ratio 10% or less, no red rust ○: White rust area ratio 50% or less, no red rust △: White rust area ratio more than 50%, red rust area ratio 10% or less ×: Red rust area ratio 20 %Super

(2) Corrosion resistance after coating A melamine black coating of 20 μm was applied to a sample having a size of 40 × 150 mm and baked at 140 ° C. for 20 minutes. Thereafter, a cross cut was made and subjected to a salt spray test. The appearance after 60 days was visually observed. (Evaluation criteria) ：: No red rust generated ○: No red rust generated except from cross cut △: Red rust area ratio 5% or less ×: Red rust area ratio more than 5%

Table 1 shows the results. When the rolling method according to the present invention was applied, the occurrence of plating cracks after draw bead processing was significantly reduced as compared with the case where rolling was not performed (Comparative Example 7). Both bare corrosion resistance and corrosion resistance after painting are good. On the other hand, when the degree of rolling is insufficient, as in Comparative Examples 8 and 9, or when rolling is excessive, as in Comparative Example 10, there is no effect of suppressing plating cracking and no effect of improving corrosion resistance is exhibited. Further, even when a roll having a large diameter is used as in Comparative Examples 11 and 12, the effect of suppressing the plating crack is not recognized.

[0023]

[Table 1]

[0024]

The present invention provides an aluminum-plated steel sheet which does not adversely affect the degreasing / welding process after press working, suppresses plating cracks during press working, and maintains corrosion resistance, and a method for producing the same. However, its industrial value is extremely high.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view showing a case where a draw bead is processed with a thickness reduction of 10%.

FIG. 2 is a graph showing the relationship between the sheet thickness reduction when a normal aluminum-plated steel sheet is formed by actual press forming, the number of plating cracks penetrating the plating layer, and the total crack length of the plating cracks in the plating surface layer, which were confirmed per 10 mm. It is a figure showing a relation.

FIG. 3 is a diagram showing the relationship between the number of through-plated cracks after press forming and corrosion resistance after painting, and the relationship between the total crack length of through-plated cracks and corrosion resistance after painting.

FIG. 4 is a diagram showing a cross-sectional shape of a mold used in draw bead processing.

FIG. 5 is a diagram showing a relationship between a roll diameter, an elongation rate, and a region where a plating crack suppression effect is exhibited.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Crack which penetrates a plating layer 2 Crack which generate | occur | produced only in a plating layer 3 Crack of an alloy layer 4 Steel plate 5 Alloy layer 6 Plating layer 7 Crack length in a surface layer 8 Convex side R 9 Concave side R 10 Convex part length 11 Pull-out direction

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Junichi Komon 20-1 Shintomi, Futtsu-shi, Chiba F-term in the Technology Development Division, Nippon Steel Corporation 4E002 AD05 AD12 BB16 BB18 BC02 BC10 CB03 CB09 4K027 AA02 AA05 AB02 AB05 AB48 AC87 AE21

Claims (3)

[Claims] 1. The convex side R: 3 to 5 mm, the concave side R: 1.5 to
2.5 mm, using a mold having a protrusion length of 1 to 6 mm, a drawing speed of 100 to 300 mm / min, and a thickness reduction rate of 5
２５25%, the total number of plating cracks that penetrate the plating layer caused by draw bead processing is 10% in the drawing direction.
80 or less per mm, and the total crack length in the plating surface layer is 1 m per 10 mm in the drawing direction.
m or less, wherein the aluminum-plated steel sheet has excellent corrosion resistance in the workable part. 2. A hot-dip aluminum-plated steel sheet,
Elongation rate of 1 to 10 with a rolling roll having a diameter of 80 mmφ or less
%. The method for producing an aluminum-plated steel sheet having excellent corrosion resistance in a processed part according to claim 1, wherein the steel sheet is rolled in the range of%. 3. The method for producing an aluminum-plated steel sheet having excellent corrosion resistance in a processed part according to claim 2, wherein, in the rolling, rolling in two directions perpendicular to each other is alternately performed one or more times.