JP2570937B2 - Manganese-chromium alloy electroplated steel sheet with excellent corrosion resistance and weldability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to an automobile, a building material,
Suitable as a rust-proof steel plate for all applications, such as for home appliances,
The present invention relates to a manganese-chromium alloy electroplated steel sheet having excellent corrosion resistance and weldability.

[0002]

2. Description of the Related Art Electroplated steel sheets have been widely used as rust-preventive steel sheets for automobiles, building materials, household appliances, and the like. Electroplated steel sheets used for such various applications are required to have higher quality year by year.
For example, automobile bodies are exposed to severe corrosive environments due to rock salt or calcium chloride sprayed on roads to prevent freezing of ice and snow in winter in cold regions. Excellent corrosion resistance is required to withstand severe harsh corrosive environments.

[0003] Of the electroplated steel sheets, electrogalvanized steel sheets have been used widely in various applications because of their excellent corrosion resistance due to the sacrificial corrosion protection of the galvanized layer. The amount is increasing every year. In order to improve the corrosion resistance of the electrogalvanized steel sheet, from the viewpoint of maintaining the sacrificial corrosion protection action of zinc, a zinc-iron alloy electroplated steel sheet having a zinc-iron alloy electroplated layer formed on at least one surface of the steel sheet, A zinc-nickel alloy electroplated steel sheet having a zinc-nickel alloy electroplating layer formed thereon,
Further, an organic composite electroplated steel sheet having an organic composite coating formed on a zinc-nickel alloy electroplated layer has been developed and has already been put to practical use.

[0004]

However, in recent years,
The demand for improving the corrosion resistance of steel sheets is much higher than the corrosion resistance level of the above-mentioned conventional zinc-based electroplated steel sheets, and there is a demand for the development of electroplated steel sheets having more excellent corrosion resistance. Furthermore, the conventional zinc-based electroplated steel sheet has the following problems in weldability. That is, when two zinc-based electroplated steel sheets overlapped with each other are spot-welded, a plating layer made of zinc having a low melting point in the welded portion is melted over a wide range by welding heat, so that a welding current is diffused, and welding is performed. The current density decreases. Therefore, in order to properly weld a zinc-based electroplated steel sheet, the welding current must be increased.

[0005] In addition, when a zinc-based electroplated steel sheet is spot-welded, the molten zinc in the plating layer of the welded portion reacts with copper contained in the electrode tip to form a hard and brittle zinc-copper alloy on the electrode tip. Layers form. As a result, since the electrode tip is severely worn, the so-called continuous hitting property is poor, and the life of the electrode tip is short.

[0006] When two zinc-based electroplated steel sheets are arc-welded, a bubble-like defect called a blowhole is generated in a weld bead. These blowholes are caused by the zinc in the plating film being vaporized by welding heat and remaining in the weld bead, and it is extremely difficult to prevent the occurrence of such blowholes. The presence of blowholes in the weld bead reduces the strength of the weld.

In recent years, from the viewpoint of the global environment, in order to improve the fuel efficiency of automobiles, the weight of automobile bodies has been actively reduced, and accordingly, the thickness of automobile steel sheets has been reduced. Is required. Therefore, the use of a zinc-based plated steel sheet whose thickness can be reduced even in the area around the foot of an automobile where a steel sheet without a plated layer has been used has been studied.
However, as described above, when the galvanized steel sheet is arc-welded, blowholes are generated in the weld bead, and the strength of the welded portion is reduced. Difficult to use.

Accordingly, it is an object of the present invention to provide an electroplated steel sheet which solves the above-mentioned problems, exhibits excellent corrosion resistance in any corrosive environment, and is excellent in spot weldability and arc weldability. is there.

[0009]

Means for Solving the Problems The present inventors have made intensive studies to solve the above-mentioned problems. As a result, a predetermined amount of the manganese-chromium alloy electroplating consisting essentially of chromium in the range of 5 to 90 wt.% And manganese in the range of 10 to 95 wt.% On at least one surface of the steel sheet. It has been found that the formation of a plating layer can provide an electroplated steel sheet excellent in both corrosion resistance and weldability.

[0010] The present invention has been made based on the above-mentioned findings, and an electroplated steel sheet of the present invention comprises: a steel sheet; and a manganese-chromium alloy electroplated layer formed on at least one surface of the steel sheet. And the manganese-chromium alloy electroplated layer substantially comprises 5 to 90 watts.
% of chromium and 10 to 95 wt.% of manganese, and the manganese-chromium alloy electroplating layer has a plating amount of 2 to 50 g / m ² per side of the steel sheet.
Is within the range.

[0011]

The reason why the manganese-chromium alloy electroplated layer has excellent corrosion resistance is presumed as follows. That is, manganese reacts with oxygen in a corrosive environment having a relatively high pH value to form a very stable corrosion product composed of Mn ₂ O ₃ . The protective action of the corrosion products imparts excellent corrosion resistance to the steel sheet.

On the other hand, in a low-pH, low-oxygen corrosive environment in which manganese corrosion products cannot exist, manganese, which is more active than chromium, elutes first and chromium concentrates on the surface of the steel sheet. An oxide layer is formed. Chromium forms a stable oxide film over a wide pH range, and this oxide film imparts excellent corrosion resistance to the steel sheet. Furthermore, even if a defect occurs in the oxide film formed as described above, active manganese is eluted at the defect, resulting in the formation of a chromium-enriched layer. During this time, the corrosion resistance of the steel sheet is maintained by the so-called sacrificial corrosion protection ability. Therefore, if a manganese-chromium alloy electroplated layer is formed on the surface of a steel sheet,
Excellent corrosion resistance can be imparted under any corrosive environment.

The melting point of the manganese-chromium alloy electroplated layer is much higher than that of zinc. There is no possibility that the welding current is diffused and the welding current is diffused to reduce the welding current density.
Therefore, appropriate welding can be performed with a current equivalent to that of the cold-rolled steel sheet. Further, unlike the zinc-based electroplated steel sheet, the molten metal in the plating layer of the welded portion reacts with the electrode tip, and no alloy layer is formed on the electrode tip. Therefore, wear of the electrode tip is small, continuous weldability is significantly improved, and the life of the electrode tip is long.

Furthermore, since it does not contain zinc, blow holes do not occur in the weld bead during arc welding unlike the zinc-based electroplated steel sheet. Therefore, since the strength of the welded portion does not decrease, this manganese-chromium alloy electroplated steel sheet can be used, for example, also around the foot of an automobile.

[0015] The chromium content in the manganese-chromium alloy electroplating layer should be limited to the range of 5 to 90 wt.%, And the manganese content should be limited to the range of 10 to 95 wt.%. is there. If the chromium content in the manganese-chromium alloy electroplating layer is less than 5 wt.% And the manganese content exceeds 95 wt.%, The corrosion resistance of chromium in a low pH, low oxygen corrosive environment does not occur, Since only manganese dissolves, the corrosion resistance decreases. On the other hand, when the chromium content in the manganese-chromium alloy electroplating layer is more than 90 wt.% And the manganese content is less than 10 wt.%, The stable Mn ₂ O ₃ The effect of forming a protective film made of the above cannot be obtained, and in particular, the corrosion resistance in an environment having a relatively high pH value is reduced.

The amount of plating of the manganese-chromium alloy electroplated layer should be limited to the range of 2 to 50 g / m ² per one side of the steel sheet. If the plating amount of the manganese-chromium alloy electroplated layer is less than 2 g / m ² per one side of the steel sheet, the effect of improving corrosion resistance cannot be obtained. On the other hand, if the plating amount exceeds 50 g / m ² per one side of the steel sheet, press formability and spot weldability deteriorate.

The manganese-chromium alloy electroplated steel sheet of the present invention is manufactured as follows. That is, 50-200
Using an electroplating solution containing g / l of manganese and 0.1 to 50 g / l of chromium, form a manganese-chromium alloy electroplating layer on at least one surface of the steel sheet by ordinary electroplating. .

A cation such as Na ⁺ , K ⁺ , NH ₄ ⁺ for imparting conductivity to the plating solution may be added to the electroplating solution. The electroplating solution may contain any other additives. In particular, it is effective to add polymers and complexing agents as manganese or chromium precipitation promoters. Also, as the anion, SO ₄ ^2- , Cl ^- and
BF ₄ ^- may also be any use. Accordingly, although the anion is not particularly limited, a fluorinated bath is relatively desirable from the viewpoint of good electrolytic efficiency.

The plating solution preferably has a pH value in the range of 0.5 to 3.0. The plating current density is not particularly specified because the composition of the plating layer can be changed thereby, but is preferably 5 to 200 A / dm ² . The flow rate of the plating solution is a relative speed to the steel sheet passing through the solution,
0.5 to 5 m / s is preferable, and the temperature of the plating solution is 30
~ 70 ° C is preferred. The steel sheet on which the plating layer is to be formed is not particularly limited, and either a cold-rolled steel sheet or a hot-rolled steel sheet can be used depending on the intended use.

Next, the present invention will be described in more detail with reference to examples and comparative examples.

EXAMPLE A 0.7 mm-thick cold-rolled steel sheet was electroplated under the conditions shown in A below to form a manganese-free steel sheet on the surface of the steel sheet.
A chromium alloy electroplating layer was formed. In this way,
Specimens of the present invention (hereinafter, referred to as the present invention) Nos. 1 to 12 shown in Table 1 having a plating layer within the scope of the present invention were prepared. Further, a hot-rolled steel sheet having a thickness of 2.6 mm was subjected to electroplating under the same conditions as shown in the following A to prepare specimens Nos. 13 to 16 of the present invention also shown in Table 1.

For comparison, a 0.7 mm-thick cold-rolled steel sheet was electroplated under the following condition A, and a manganese-chromium alloy electroplating layer outside the scope of the present invention was formed on the surface of the steel sheet. Specimens for comparison (hereinafter referred to as comparative specimens) Nos. 1 to 4 shown in Table 2 were prepared. Further, the steel plate was subjected to electroplating under the conditions shown in B, C, and D below to have a conventional plating layer.
Comparative Sample Nos. 5 to 8 shown in Table 1 were prepared. Furthermore, plate thickness
A 2.6 mm hot-rolled steel sheet is electroplated under the same conditions shown in B, C, and D, and has a conventional plating layer.
Comparative specimens Nos. 9 to 11 also shown in Table 2 were prepared.

A. Manganese - Composition chrome electroplating solution plating _{solution: Mn (BF 4) 2:} 100~600 g / l, Cr (BF 4) 3: 10~100 g / l, H 3 BO 3: 60 g / l, PEG: 2.5 g / l, temperature of plating solution: 50 ° C, pH value of plating solution: 2.0, relative flow rate of plating solution: 2.0 m / s, plating current density: 10-100 A / dm ² , plating amount: 2 It varied in the range of 5050 g / m ² .

B. The composition of the zinc electroplating solution plating _{solution: ZnSO 4 · 7H 2 O:} 500g / l, Na 2 SO 4: 60g / l, the plating solution temperature: 50 ° C., the plating solution of pH values: 1.7, plating current density: 50 ~ 80A / dm ² , Plating amount: 40g / m ² .

C. Zinc - Composition of nickel alloy electroplating solution plating _{solution: ZnSO 4 · 7H 2 O:} 150g / l, NiSO 4 · 6H 2 O: 350g / l, Na 2 SO 4: 60g / l, the plating solution temperature: 50 ° C. , pH value of the plating solution: 1.3, plating current density: 50 ~100A / dm ^2, plating amount: 30 g / m ^2.

D. Zinc - Composition of iron alloy electroplating solution plating _{solution: ZnSO 4 · 7H 2 O:} 200g / l, FeSO 4 · 7H 2 O: 300g / l, Na 2 SO 4: 30g / l, CH 3 COONa: 20g / l , Plating solution temperature: 50 ° C, Plating solution pH value: 1.2, Plating current density: 50 to 70 A / dm ² , Plating amount: 40 g / m ² .

[0026]

[Table 1]

[0027]

[Table 2]

The corrosion resistance and weldability of each of the thus-prepared specimens of the present invention and the comparative specimens were examined by performance tests described below, and the examination results are shown in Tables 1 and 2. .

(1) Corrosion resistance test: Each specimen was subjected to 50 cycles of a corrosion promotion test in which salt water spraying, drying and wetting were performed once a day for 50 cycles, and then the maximum hole depth generated in the specimen was examined. Was evaluated as follows. ◎: 0 to 0.1mm, ○: Over 0.1mm to 0.3mm, △: Over 0.3mm to 0.5mm, ×: Over 0.5mm.

(2) Weldability test: Spot Weldability Two test pieces each made of a cold-rolled steel sheet having a thickness of 0.7 mm and superposed with their plated surfaces facing outward (specimen No. 1 of the present invention)
-12 and comparative specimen Nos. 1-8) were spot-welded under the following conditions, and the number of spots at which continuous welding was possible when the nugget diameter became a predetermined size or less was determined. Was evaluated as follows. Welding conditions Welding current value: 11KA, pressurization: 250kgf, conduction time: 10 cycles. Number of continuous dots ◎: 5,000 or more, ○: 3,000 to less than 5,000, △: 1,000 to less than 3,000, ×: Less than 1,000.

B. Arc weldability As shown in Fig. 1, width (a) 50mm, length (b) 500mm, thickness
(c) Two test pieces 1 and 2 each consisting of a 2.6 mm hot-rolled steel sheet (samples Nos. 13 to 16 of the present invention and sample Nos. 9 to 11 for comparison)
Were partially overlapped with the respective plated surfaces 1a and 2a inside, and a joint (d) having a width of 0.2 mm was provided along the welding line at the joint portion. The test pieces 1 and 2 having such a joint shape were subjected to arc welding using a normal welding wire under the following conditions. An X-ray photograph of the bead portion of each of the test pieces welded in this manner was taken, and a 30 cm
The number of blow holes per unit was checked and evaluated according to the following.

Welding conditions: Welding: high-speed rotating arc welding, torch angle: 60 degrees, welding current value: 300A. Number of blowholes: ○: 0, △: 1 to 10, ×: More than 10

As is clear from Table 2, comparative specimens in which the chromium content in the plating layer is small outside the range of the present invention.
No. 1, Comparative Specimen No. 2 having a large chromium content outside the range of the present invention, and Comparative Specimen No. 3 having a small plating amount outside the range of the present invention all have corrosion resistance. It was bad. Comparative specimen No. 4 having a large plating amount outside the range of the present invention had poor spot weldability.

A comparative sample No. 5 in which a conventional zinc electroplating layer was formed on the surface of a steel sheet,
Specimen No. 6 for comparison with iron alloy electroplated layer,
No. 7 was poor in corrosion resistance and spot weldability.
Comparative specimen No. 8 in which a conventional zinc-nickel alloy electroplated layer was formed on the surface of a steel sheet had poor corrosion resistance.

A comparative sample No. 9 having a conventional zinc electroplated layer formed on the surface of a steel sheet and a comparative sample N having a conventional zinc-iron alloy electroplated layer formed thereon
In o.10, corrosion resistance and arc weldability were poor. The comparative sample No. 11 in which the conventional zinc-nickel alloy electroplated layer was formed on the surface of the steel sheet had poor arc weldability.

On the other hand, as is clear from Table 1, all of the specimens Nos. 1 to 16 of the present invention were excellent in corrosion resistance, spot weldability and arc weldability.

[0037]

As described above, according to the present invention,
An industrially useful effect is obtained in which an electroplated steel sheet exhibiting excellent corrosion resistance in any corrosive environment and having excellent spot weldability and arc weldability can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic perspective view of a test piece for testing arc weldability.

[Explanation of symbols]

 1 specimen, 1a plating layer, 2 specimen, 2a plating layer.

Claims (1)

(57) [Claims] 1. A steel sheet, comprising a manganese-chromium alloy electroplating layer formed on at least one surface of the steel sheet, wherein the manganese-chromium alloy electroplating layer comprises:
Consisting essentially of chromium in an amount of 5-90 wt.% And manganese in an amount of 10-95 wt.%, And
A manganese-chromium alloy electroplated steel sheet excellent in corrosion resistance and weldability, characterized in that the plating amount of the chromium alloy electroplated layer is in the range of 2 to 50 g / m ² per one side of the steel sheet.