METHOD FOR PROCESSING GALVANIZED COMPONENT

Abstract

The present invention suppresses deterioration in the corrosion resistance of a worked portion resulting from working cracks in a Zn-based plated layer (3) in a workpiece (2) formed into a predetermined shape by performing plastic working on a Zn-based plated steel sheet (1) coated with a Zn-containing metal as a raw material. That is, plastic working is performed on a raw material that is a Zn-based plated steel sheet (1) to obtain a workpiece (2) having a predetermined shape, and thereafter, pressurization processing is performed on a worked portion in a sheet thickness direction to deform the plated metal, thus decreasing the width of working cracks in the plated metal. Accordingly, it is possible to reduce the deterioration in the corrosion resistance of the worked portion of the Zn-based plated workpiece.

Claims

1. A processing method of a Zn-based plated workpiece for performing plastic working on a raw material that is a Zn-based plated steel sheet (1) to produce a workpiece (2) having a predetermined shape, comprising further performing pressurization processing by applying reduction to a worked portion of the workpiece (2) in a sheet thickness direction by using a processing punch (8) and a processing die (9) having a shape following a product shape of the worked portion such that the plated layer (3) is rolled in an in-plane direction of an underlying steel sheet (7) to reduce an interval between working cracks (4) in the plated layer (3).

2. The processing method of a Zn-based plated workpiece according to claim 1, wherein a steel sheet coated with a plated metal containing Zn, Al, and Mg is used as the Zn-based plated steel sheet (1).

Description

BRIEF DESCRIPTION OF DRAWINGS

[0016] FIG. 1 shows schematic cross-sectional views showing exemplary processing steps used by a processing method according to the present invention, wherein (a) shows a raw material before processing, (b) shows plastic working into a predetermined shape, and (c) shows pressurization processing on a worked portion in a sheet thickness direction.

[0017] FIG. 2 shows images as substitutes for drawings wherein “pre-pressurization” shows states of working cracks in a plated layer that have occurred in a worked portion, as observed from the surface of the worked portion, and “post-pressurization” shows states of the working cracks after pressurization processing in which reduction has been applied to the worked portion in the sheet thickness direction.

[0018] FIG. 3 is a graph showing a relationship between the pressurization force exerted on the worked portion and the surface exposure percentage (i.e., the underlying steel sheet exposure percentage) of the underlying steel sheet after pressurization.

[0019] FIG. 4 is a flowchart showing the conditions for a neutral salt spray cycle test.

BEST MODE FOR CARRYING OUT THE INVENTION

[0020] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

[0021] In FIG. 1, (a) is a diagram schematically showing a cross section of a Zn-based plated steel sheet 1 before processing. Since the Zn-based plated steel sheet 1 is in a state before plastic working, a plated layer 3 has not yet undergone working cracks, and, as shown in this drawing, the surface of an underlying steel sheet 7 is covered with the plated layer 3.

[0022] In FIG. 1, (b) shows a step of performing plastic working on a Zn-based plated steel sheet 1 by using a punch 5, a die 6, and a blank holder 12 to produce a workpiece 2 having a predetermined shape. At this time, irregular working cracks 4 occur in the plated layer 3. Since plastic working tends to exert a stronger tensile stress on the plated layer 3 in bulging processing than in drawing processing, the working cracks 4 in the plated layer 3 tend to be prominent. Also, the depth or width of the working cracks 4 increases with an increase in the degree of processing of plastic working, for example, an increase in the bulging height. Then, when the interval between adjacent working cracks 4 is widened to increase the exposure of the underlying steel sheet 7 from the surface, red rust occurs from the underlying steel sheet 7, resulting in deterioration in the corrosion resistance of the workpiece 2. The reason for this is that the interval between the working cracks 4 is widened beyond the extent of the sacrificial protection effect of the plated metal.

[0023] To make the interval between the working cracks 4 small, in the present invention, reduction is applied to the worked portion in the sheet thickness direction by using a pressurization punch 8 and a pressurization die 9, as shown in (c) of FIG. 1 as an example. This causes the plated layer 3 to undergo plastic deformation so as to be rolled in the in-plane direction of the underlying steel sheet 7. As a result, the interval between the working cracks 4 in the plated layer 3 is narrowed, so that the occurrence of red rust is suppressed by the sacrificial protection effect of the plated metal around the working cracks 4.

[0024] Regarding the pressurization using the pressurization punch 8 and the pressurization die 9, pressurization for simply deforming the plated layer 3 may be performed when the workpiece 2 has been finished into a predetermined shape, and the shape of the workpiece 2 itself will not be changed. In the case of restriking the workpiece 2 so as to be finished into a predetermined shape, pressurization processing on the plated layer 3 can also be performed simultaneously with the restriking.

[0025] The use of a Zn—Al—Mg-based plated steel sheet, which is a plated steel sheet coated with a plated metal containing Zn, Al and Mg, as the Zn-based plated steel sheet 1 can further enhance the sacrificial protection effect. With the Zn—Al—Mg-based plated steel sheet, when the underlying steel sheet 7 is exposed by the working cracks 4, the plated metal around the working cracks 4 is eluted, and the eluted components cause a dense Zn corrosion product containing Mg to cover the underlying steel sheet 7 around the working cracks 4, thereby suppressing corrosion. The Mg-containing Zn corrosion product has a higher protective performance than the Zn corrosion product of the Zn-plated steel sheet, and thus can achieve a stronger sacrificial protection effect.

EXAMPLES

[0026] Hereinafter, the present invention will be described more specifically by way of examples. However, the present invention is not limited to the examples.

[0027] Using a Zn—Al (6 wt %)—Mg (3 wt %) alloy-plated steel sheet having a sheet thickness of 1.2 mm and a plating deposition amount per side of 140 g/m.sup.2 as a raw material, bulging processing and pressurization on the worked portion were performed by the steps shown in FIG. 1.

[0028] The punch 5 used for the bulging processing has a columnar shape having a diameter of 200 mm and a shoulder portion having a radius of curvature of 10 mm. Meanwhile, the die 6 has an inner diameter of 203 mm and a shoulder portion having a radius of curvature of 10 mm. The blank holder 12 has an inner diameter of 202 mm. Then, as shown in (b) of FIG. 1, a bulged workpiece 2 having an inner diameter of 200 mm and a height of 40 mm was made by using the punch 5, the die 6, and the blank holder 12.

[0029] Then, pressurization processing was performed on a worked portion of the workpiece 2. As shown in (c) of FIG. 1, this pressurization processing was performed by using the pressurization punch 8, the pressurization die 9, and the blank holder 12. The shapes of the pressurization punch 8 and the pressurization die 9 were the same as the shapes of a head portion 10 and a vertical wall portion 11 of the workpiece 2.

[0030] Then, the pressurization force of the pressurization processing was set to three levels, namely, 30 kN, 40 kN, and 60 kN, and the pressurization direction was set to a direction from up to down on the paper plane relative to the head portion 10 of the workpiece 2, as indicated by the outlined arrow shown in (c) of FIG. 1.

[0031] Here, in (c) of FIG. 1, the head portion 10 is perpendicular to the direction of the outlined arrow, so that the pressurization force itself acts as “force of applying reduction in the sheet thickness direction”. However, at the vertical wall portion 11, which is slightly inclined relative to the direction of the outlined arrow, the pressurization force indicated by the outlined arrow is decomposed into “component force perpendicular to the wall surface of the vertical wall portion 11” and “component force parallel to the wall surface of the vertical wall portion 11”. Accordingly, at the vertical wall portion 11, “force of applying reduction in the sheet thickness direction” is slightly lowered than that acting on the head portion 10. However, the shapes of the pressurization punch 8 and the pressurization die 9 are the same as the shape of the vertical wall portion 11 of the workpiece 2, so that “component force parallel to the wall surface of the vertical wall portion 11” acts such that the plated layer 3 on the surface of the vertical wall portion 11 is expanded in the in-plane direction. As a result, the interval between the working cracks 4 in the plated layer 3 at the vertical wall portion 11 also can be narrowed to substantially the same level as that at the head portion 10.

[0032] The states of the working cracks in the plated layer 3 in the above-described pressurization processing before and after pressurization are shown in FIG. 2. FIG. 2 shows photographs obtained by photographing, at a magnification of 200× by an optical microscope, the state of the head portion 10 of the workpiece 2 before pressurization and the states of working cracks in the plated layer 3 on the same portion after being pressurized with the respective pressurization forces. Although no reference numeral is provided in FIG. 2, the white portions in the drawing show the plated layer 3, and the black portions in the drawing show portions where the underlying steel sheet 7 is exposed by the working cracks 4.

[0033] It can be seen in the drawing that, as a result of performing pressurization processing, the interval between adjacent working cracks 4 in the plated layer 3 has been narrowed.

[0034] In addition, before and after performing pressurization of the worked portion, the state of the working cracks 4 in the plated layer 3 at the head portion 10 of the workpiece 2 was observed at a magnification of 200× by an optical microscope, and the area ratio (=the underlying steel sheet exposure percentage) of the area in which the underlying steel sheet 7 was exposed by the working cracks 4 in the plated layer 3 relative to an observed area of 5 mm.sup.2 was evaluated.

[0035] The changes in the underlying steel sheet exposure percentage caused by pressurization are shown in FIG. 3. As indicated by this drawing, it can be inferred that as a result of performing pressurization, the exposure percentage of the underlying steel sheet 7 is decreased, and that the higher the pressurization force, the smaller the exposure percentage becomes and the greater the achieved effect of suppressing the occurrence of red rust is.

[0036] Further, the workpiece 2 before pressurization and the workpiece 2 pressurized at 30 kN were subjected to a neutral salt spray cycle test, and were evaluated for the corrosion resistance. The conditions for the neutral salt spray cycle test are those shown in FIG. 4. The number of cycles was set to 100.

[0037] As a result of the above-described 100-cycle test, red rust occurred from the head portion in the workpiece 2 that had not undergone pressurization. However, no red rust occurred from the head portion of the workpiece 2 in which the head portion was pressurized at 30 kN, so that it was confirmed that the processing method according to the present invention can suppress deterioration in the corrosion resistance of the Zn-based plated workpiece 2.

INDUSTRIAL APPLICABILITY

[0038] The processing method of a Zn-based plated workpiece according to the present invention is useful to suppress deterioration in the corrosion resistance, attributed to working cracks in a plated layer caused by plastic working, of a workpiece using a Zn-based plated steel sheet as a raw material, and to maintain good corrosion resistance.

REFERENCE SIGNS LIST

[0039] 1 Zn-based plated steel sheet [0040] 2 workpiece [0041] 3 plated layer [0042] 4 working cracks (in plated layer) [0043] 5 punch [0044] 6 die [0045] 7 underlying steel sheet [0046] 8 pressurization punch [0047] 9 pressurization die [0048] 10 head portion (of workpiece) [0049] 11 vertical wall portion (of workpiece) [0050] 12 blank holder