JPH0610359B2 - High corrosion resistant Zn-based multi-layer electric steel sheet - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a Zn-based multi-layer plated steel sheet for automobile parts, which exhibits excellent corrosion resistance not only in the painted portion but also in the unpainted portion.

(Prior Art) Since a zinc-based plated steel sheet has an excellent sacrificial anticorrosive ability in a plated layer, it has been widely used in various fields such as automobiles, home appliances, and building materials, which are conventionally required to have corrosion resistance. In particular, when this zinc-based plated steel sheet is used for automobiles in recent years, in cold regions such as North America and Canada, anti-freezing agents such as rock salt are sprayed on roads in winter, and the corrosive environment becomes severe. It is required to have excellent resistance to blistering and puncture.

As galvanized steel sheets for automobiles, hot-dip galvanized steel sheets, galvanized galvanized steel sheets, or galvannealed steel sheets have been mainly used in the past. It is too much to withstand the above environment. Further, in the case of a galvannealed steel sheet, the workability of the plating layer is inferior.
Since it is difficult to make it 0 g / m ² or less, there is a problem in terms of processing. Therefore, in recent years, the plating layers have been Zn-Ni based and Zn-Fe based.
Electroplated steel sheets that are made of an alloy system such as a system or have multiple layers have been used.

(Problems to be Solved by the Invention) However, even if such an electroplated steel sheet is used, an unpainted portion such as a plate-matching portion or a bag structure portion or a thin coating portion is inevitably formed on the vehicle body. However, there is a problem that the corrosion resistance of such a portion is lowered and the perforation resistance is insufficient.

Therefore, the present invention provides a zinc-based multi-layer plated steel sheet which is excellent in corrosion resistance and puncture resistance even when there is an unpainted portion or a thin coating portion in an automobile body or the like.

(Means for Solving Problems) The present inventors have paid attention to an electric Zn-Fe alloy plated steel sheet,
As a result of various studies to improve its corrosion resistance and puncture resistance, silica, alumina, titanium oxide, Zn-Fe system plating,
They have found that the inclusion of oxide particles such as magnesia, chromium oxide, and zirconium oxide can improve corrosion resistance and puncture resistance.

However, if such a chemically stable oxide particle is contained in the plating layer, the phosphate treatment in the pretreatment of coating will be deteriorated, and there will be problems in coating and if the oxide particle is protruding, electrical resistance welding will occur. It was found that the welding machine tip did not contact uniformly during the process, and the weldability was poor, and the tip was worn during welding or press working, or the die was galled. Particularly, the phosphate treatability was remarkably reduced when the content of oxide particles was 0.05 wt% or more. Therefore, in the present invention, further Zn-Fe alloy plating excellent in phosphating property is further applied on the plating layer so as not to impair the coatability, and the protruding oxide particles are coated to improve weldability. The press workability was improved.

That is, in the present invention, the Fe content in the lower layer is 10 to 30 wt%,
One or more of silica, alumina, titanium oxide, magnesia, chromium oxide, zirconium oxide is added to 0.005 to
High corrosion resistant Zn-based multi-layer electroplated steel sheet having a Zn-Fe-based alloy plating layer containing 5 wt% and a Zn-Fe-based alloy plating layer having an Fe content of 10 to 30 wt% in the upper layer. Is provided.

In the present invention, when the eutectoid particles are dispersed in the lower layer as described above, the corrosion resistance and the puncture resistance are improved because when the corrosion reaches the lower layer, the oxide is a non-growth type corrosion product. It is considered to promote the formation and prevent the progress of corrosion to the inside.

The content of oxide particles such as silica, alumina, titanium oxide, magnesia, chromium oxide and zirconium oxide contained in the lower Zn-Fe system plating layer is 0.005 to 5 wt% of the steel plate.
The reason for this is that if it is less than 0.005 wt%, the effect of addition is hardly recognized due to corrosion resistance and puncture resistance, and even if it is contained in an amount exceeding 5 wt%, it is more In addition, there is no significant effect of improving the puncture resistance, and since a large amount of oxide particles must be added to the plating bath in order to co-deposit an amount of more than 5 wt% during electroplating, the particles aggregate. However, the particles are filtered by a filter device that removes impurities such as sludge in the plating solution, resulting in wasted oxide particles and increased production costs. It is difficult to manage.

Further, the Fe content is set to 10 to 30 wt% because when the content is less than 10 wt%, the alloy phase is mainly composed of η phase and its sacrificial anticorrosion ability is almost the same as that of zinc plating, so that the corrosion rate is too high. %, The alloy phase is mainly composed of a hard and brittle .GAMMA. Phase, and the plating layer is peeled off when the alloy layer is processed into a vehicle body member or the like. On the other hand, when the content is 10 to 30 wt%, δ ₁ is the main component of the alloy layer, and pure zinc or η
It becomes electrochemically more precious than the Zn-Fe alloy plating layer containing phases as the main component, and baser than the steel base. For this reason, the corrosion rate becomes small, and the steel substrate is protected for a long period of time.

The coating amount of the lower layer is preferably 10 to 50 g / m ² per side. This is because if it is less than 10 g / m ² , the base material is corroded before a stable corrosion product is formed when the plating layer is corroded, and improvement in corrosion resistance and puncture resistance due to the lower layer cannot be expected. . Also, if the amount of plating exceeds 50 g / m ² , powdering of the plated layer is likely to occur during the forming process.

Unlike the lower layer, the upper layer does not contain oxide particles,
This is because oxides such as silica, alumina, titanium oxide, magnesia, chromium oxide and zirconium oxide are chemically stable, and thus the phosphate treatment property is deteriorated. As is well known, the phosphate treatment is about 0.5-1 g /
Although m ^{2 is} dissolved and phosphate crystals are precipitated based on the dissolution reaction, if a chemically stable oxide is contained, the dissolution reaction becomes slow and the processing time becomes long.
In addition, since the Zn-Fe alloy is mainly dissolved in the surface layer of the plating layer, the oxide surface is in a concentrated state after the dissolution, and the phosphate crystals are not dense. Furthermore, the upper layer is coated with the protruding oxide particles of the lower layer to ensure that the welder tip makes uniform contact with the plating layer during electrical resistance welding, or to prevent wear of the welder tip and galling of the press die. This is because there is a problem when the oxide particles are contained in.

The reason for setting the Fe content of the upper layer to 10 to 30 wt% is the same as in the case of the lower layer described above.

The coating amount of the upper layer is preferably 0.5 to 10 g / m ² per side. This is because if the amount is less than 0.5 g / m ² , the surface of the lower layer cannot be completely covered and the phosphating property is deteriorated. On the other hand, even if it exceeds 10 g / m ² , in terms of quality characteristics,
Although there is no problem, the improvement in corrosion resistance due to the increase in the amount of adhesion is smaller than that in the lower layer, and the increase in manufacturing cost is greater.

Both the lower layer and the upper layer of the present invention can be obtained by performing pH 1 to 3 in a sulfuric acid type plating bath. In this case, an oxide must be added to the plating bath during the lower layer plating, and the oxide may be in the form of fine particles or colloidal sol. When the oxide particles are plated in a low pH plating bath, they are likely to be positively charged, so that they easily co-deposit in the plating layer.

In the present invention, one or two or more kinds of corrosion resistance improving elements such as Co, Cr, Ti, Ni, Mo and Mn may be added to the lower layer or the upper layer or both layers in order to further improve the corrosion resistance.

Next, the present invention will be described with reference to examples.

(Example) After cold-rolled steel sheet was subjected to pretreatment such as degreasing and pickling by a conventional method, lower layer plating and upper layer plating were performed under the following conditions.

(1) Plating bath (common to lower and upper layers except oxide particles) Ferrous sulfate 300 g / zinc sulfate 50 to 100 g / sodium sulfate 70 g / citric acid 1 g / pH 1.5 to 2.5 However, in the case of lower layer plating Oxide particles were added to the above plating bath as follows.

Average particle size addition amount Silica (SiO ₂ ) 16mμ 10-50g / Alumina (Al ₂ O ₃ ) 20mμ 10-30g / Titanium oxide (TiO ₂ ) 30mμ 15-40g / Magnesia (MgO) 50mμ 10-50g / Chromium oxide ( Cr ₂ O ₃ ) 20 mμ 10 to 30 g / zirconium oxide 30 mμ 15 to 40 g / (ZrO ₂ ) (2) Bath temperature (common to lower layer and upper layer) 40 to 60 ° C. (3) Current density (common to lower layer and upper layer) 20 to 80 A / dm ² The Fe content in the lower and upper co-plating layers is adjusted by changing the combination of zinc concentration, pH and current density in the bath, and the oxide particle content in the lower layer is oxide particle addition. This was done by changing the combination of amount, pH and current density.

Next, the steel sheet plated under the above conditions and the comparative material were treated with phosphate (Nippon Parker Rising Bonderite #
3030) was applied and cationic electrodeposition coating (Power Top U-30 manufactured by Nippon Paint Co., Ltd., coating thickness 20 μm) was carried out, and then the following test was conducted.

(1) Adhesion of coating film A cross cut until reaching a steel substrate is put in a coated steel sheet, and 50 cycles of the following combined cycle test for 1 cycle (24 hours) are performed, and the maximum coating film from the cross cut part after the test is performed. The blister width was measured and evaluated according to the following criteria.

(A) Combined cycle test Salt spray test (JIS Z 2371) 12 hr → 60 ° C dry 6 hr → Wet test (50 ° C, RH 95% or more) 6 hr (B) Evaluation criteria less than 3 mm ○ 3-10 mm △ over 10 mm × (2) Corrosion resistance against perforation After 50 cycles of the same composite cycle test as the above-mentioned coating film adhesion, the maximum erosion depth of the steel substrate near the cross cut portion was measured and evaluated according to the following criteria.

Less than 0.1mm ○ 0.1 to 0.2mm △ more than 0.2mm × (3) Powdering property Unpainted plated steel sheet is deep-drawn, pasted with cellophane tape on the processed part, and peeled off. It was evaluated by.

A small amount of adherence to the tape ○ A small amount of adherence to the tape △ A large amount of adherence to the tape × Tables 1 to 6 show the results of these tests.
Since the upper layer of the steel sheet of the present invention is Zn-Fe alloy plating containing no oxide particles, the coating film adhesion is similar to that of the conventional Zn-Fe alloy plating steel sheet. Further, it can be seen that the corrosion resistance and puncture resistance are excellent even with the same amount of adhesion.

(Effects of the Invention) As described above, the steel sheet of the present invention has the same coating film adhesion as that of the conventional Zn-Fe alloy plated steel sheet, and thus is suitable for use as an automobile member. In addition, since it is superior to conventional Zn-Fe alloy plated steel sheets in terms of puncture resistance, it has excellent corrosion resistance even when used in applications where unpainted or light paint is applied such as plate-bonded parts of automobile bodies and bag structure parts. To demonstrate
It is possible to prevent the vehicle body from being corroded.

Claims (2)

[Claims] 1. A lower layer having a Fe content of 10 to 30 wt%, silica, alumina, titanium oxide, magnesia, chromium oxide,
0.005 to 5 wt% of one or more zirconium oxide
% Zn-Fe alloy plating layer is included, and Fe is the upper layer.
A highly corrosion-resistant Zn-based multi-layer electroplated steel sheet having a Zn-Fe-based alloy plating layer with a content of 10 to 30 wt%. 2. The coating amount of the lower layer is 10 to 50 per side.
g / m ² , the coating weight of the upper layer is 0.5-10 per side
High corrosion resistant Zn-based multi-layer electroplated steel sheet according to claim 1, characterized in that it is g / m ² .