JPH1161481A - Zinc-nickel alloy-plated steel sheet excellent in powdering resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance adhesion between a plated layer and a steel sheet after plating and to improve the powdering resistance of the alloy-plated steel sheet by specifying the size and number of etch pits within crystal grains in the surface of a steel sheet to be plated and controlling occupancy area of the crystal grains. SOLUTION: In this plating process, as the steel sheet to be electroplated, a steel sheet having in the surface such crystal grains that the ratio of occupancy area of crystal grains which have within them a >=1×10<4> /mm<2> (number of etch pits)/(unit occupancy area) of etch pits each having a >=2×10<-5> μm<2> size in terms of area, is >=80%, is used. The production of such a steel sheet that meets the above conditions, comprises: pickling a steel sheet for 5 sec with a pickling solution that has a 50 g/l sulfuric acid concn. and is maintained at 50 deg.C; and thereafter, subjecting the pickled steel sheet to pre-dip for >=1 sec in a plating solution that contains 140 g/l of ZnSO4 .7 H2 O, 260 g/l of NiSO4 .6H2 O, 20 g/l of Na2 SO4 and 20 g/l of K2 SO4 and has a pH of <2.0 and also is maintained at 50 deg.C and at a 1.2 m/sec flow velocity; wherein the pickling process and the pre-dip process are not particularly specified and, as these processes, any respective processes can be used provided that the objective etch pits can be formed by the processes.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Zn-Ni alloy plated steel sheet having excellent powdering resistance.

[0002]

2. Description of the Related Art Zn-Ni alloy-plated steel sheets have excellent corrosion resistance and are therefore widely used mainly for applications such as automotive materials. In addition, by performing chromate treatment and organic resin coating on the upper layer of the Zn-Ni alloy plating layer,
In recent years, a so-called thin-film type organically coated steel sheet having improved perforation resistance has been actively used in recent years.

[0003] However, since the Zn-Ni alloy plating film is originally higher in hardness than a pure Zn plating film or the like, so-called powdering in which the plating film peels off in a powder form during press working. It has been pointed out that the phenomenon is likely to occur. Therefore, as a steel sheet with improved powdering resistance of Zn-Ni alloy plated steel sheet, for example,
As disclosed in Japanese Patent Application Laid-Open No. 62-5239, a steel sheet which has been subjected to double-layer plating in which the Ni content of the upper layer is higher than the Ni content of the lower layer has been proposed.

[0004] However, in this steel sheet, the method of pretreatment for plating is not limited at all. Therefore, the amount of peeling of the plating film starting from the interface between the steel sheet as the base and the Zn-Ni alloy plating layer is large, and the powder resistance is low. The ring properties were extremely insufficient.

[0005]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a Zn-Ni alloy plated steel sheet which is excellent in powdering resistance and solves the problems of the prior art.

[0006]

Means for Solving the Problems The present inventors have conducted intensive studies on the mechanism of causing a powdering phenomenon in a Zn-Ni alloy plated steel sheet during press working. As a result, the powdering of the Zn-Ni alloy plated steel sheet
It has been found that, starting from the interface with the Ni alloy plating layer, it is generated in such a way that it propagates from there to the entire plating layer.

Further, the powdering resistance of a Zn—Ni alloy-plated steel sheet is determined by pickling, which is performed as a pretreatment for plating, or in a non-current-carrying portion immediately after the base plate enters the plating cell, usually for a short time of less than 1 second. It was also found that it greatly depends on the conditions of the plating solution immersion treatment (pre-dip into the plating solution) which is inevitably performed. Then, the present inventors, Zn
-Immediately before the Ni alloy plating was performed, as a result of intensive studies focusing on the etching state of the steel sheet surface, when pre-dipping into the plating solution described above, the pH of the plating solution and the pre-dip time were set to predetermined ranges, and It has been found that a Zn-Ni alloy-plated steel sheet having excellent powdering resistance can be obtained by controlling the state of formation of the etch pits, and the present invention has been accomplished.

That is, according to the present invention based on the above findings, the occupied area ratio of crystal grains in which etch pits having an area of 2 × 10 ⁻⁵ μm ² or more exist in an amount of 1 × 10 ⁴ / mm ² or more is reduced.
A Zn-Ni alloy-plated steel sheet having excellent powdering resistance, wherein a Zn-Ni alloy-plated layer is formed on a steel sheet surface of 80% or more. In the present invention described above, the Zn—Ni alloy plating layer is preferably applied to a Zn—Ni alloy plated steel sheet that is a Zn—Ni alloy electroplating layer.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail. The Zn—Ni alloy plated steel sheet of the present invention is a Zn—Ni alloy plated steel sheet having a Zn—Ni alloy plated layer formed on the surface of the steel sheet. In the present invention, the Ni content in the Zn—Ni alloy plating layer is not particularly limited. However, in a region where the Ni content is less than 5 wt% or more than 25 wt%, the corrosion resistance is deteriorated. The Ni content of
It is preferably in the range of 25 wt%.

[0010] The Ni content at this time represents the average Ni content of the Zn-Ni alloy plating layer, and two or more Zn-Ni alloy plating layers having different Ni contents are sequentially formed. Or the Ni content may temporarily change from the steel plate side to the surface side. Further, the Zn-Ni alloy plating layer in the present invention, other than Zn and Ni, as unavoidable impurities, Fe, Pb, Na, K, Ca, Sr,
It may contain Al, Ti, Ir, Ta, Cd, Cu and the like.

Further, the surface of the Zn—Ni alloy plating layer may be subjected to a pretreatment for improving the chemical conversion property, slidability and the like, and the surface of the Zn—Ni alloy plating layer may be Chromate treatment and organic resin coating for improving the puncture resistance may be applied. Zn-Ni of the present invention
The alloy plated steel sheet is characterized in that the etching state of the surface of the steel sheet, which is the base of the Zn—Ni alloy plated layer, is controlled.

That is, the Zn—Ni alloy plated steel sheet of the present invention has one etch pit having an area of 2 × 10 ⁻⁵ μm ² or more.
× 10 ⁴ grains / mm ² or more occupied area ratio of crystal grains of 80% or more
A Zn-Ni alloy-plated steel sheet having excellent powdering resistance in which a Zn-Ni alloy-plated layer is formed on the surface of the steel sheet as described above. The etch pit is formed by pre-dip into a plating solution.

The surface of the steel sheet where there is no etch pit is inactive because it has not been etched, and the adhesion between the Zn-Ni alloy plating deposited thereon and the steel sheet is weak.
Therefore, when the Zn—Ni alloy-plated steel sheet is pressed, a portion where no etch pit is present becomes a starting point of the powdering phenomenon, and powdering is generated in such a manner that the portion propagates to the entire plating layer.

On the other hand, since the inside of the etch pit is an extremely active surface which has been etched, Zn
-When the Ni alloy plating is deposited, the adhesion becomes extremely strong. Therefore, the portion where the etch pit exists does not become the starting point of the powdering phenomenon, and the presence of a large number of etch pits significantly improves the powdering resistance of the Zn—Ni alloy plated steel sheet.

In particular, by performing a pre-dip for 1 second or more in a plating solution having a pH of less than 2.0, the etch pit density is significantly increased. On the other hand, if the area of the etch pit is 2 × 10 ⁻⁵ μm ² or more, strong plating adhesion can be obtained, but if it is less than 2 × 10 ⁻⁵ μm ² , the plating adhesion is weak and powdering is difficult. Therefore, the etch pit in the present invention is defined by an etch pit having an area of 2 × 10 ⁻⁵ μm ² or more.

If the area of the etch pit is 25 μm ² or less, there is no adverse effect on the steel sheet of the present invention. That is, the etch pits to be counted by the present invention are etch pits in which the area of one etch pit is 2 × 10 ⁻⁵ μm ² or more, and more preferably, the area of one etch pit is 2 × It is an etch pit of 10 ^{-5 to} 25 μm ² .

Hereinafter, the area of the etch pit and the occupied area ratio in the present invention will be described. [Area of etch pit:] The area of the etch pit in the present invention means a projected area on the steel sheet surface. Also,
In the present invention, “an etch pit having an area of 2 × 10 ⁻⁵ μm ² or more” means that one etch pit has an area of 2 × 10 ^−5.
μm ² or more, the number of the etch pits in one crystal grain is counted, and the area of one crystal grain is measured. / Mm ² ).

If the etch pit density of one obtained crystal grain is not less than 1 × 10 ⁴ / mm ² , the precipitation of Zn—Ni alloy plating having a strong adhesion force will occur in the crystal grain. In many cases, the powdering is remarkably suppressed, and the crystal grains are “passed”, and if the crystal grains are less than 1 × 10 ⁴ / mm ² , the precipitation of Zn—Ni alloy plating having strong adhesion is small in the crystal grains. , The gap becomes the starting point of powdering,
The determination of “fail” is made for each crystal grain.

In the present invention, the above observations are performed at least 10 times, preferably 50 times or more, more preferably 100 times or more.
This is performed for at least one crystal grain. [Occupied area ratio:] Next, the occupied area ratio of crystal grains having the above-described etch pit density in the present invention will be described.

The occupied area ratio in the present invention indicates the ratio (percentage) of the total area of the “accepted” crystal grains to the total area of the crystal grains observed as a result of the above observation. For example,
If the total area of the eight crystal grains of “Pass” is 800 μm ² and the total area of ^two crystal grains of “Fail” is 400 μm ² ,
The occupied area ratio is (800/1200) × 100 = 67%, which is outside the scope of the present invention.

In the present invention, the area occupied by the crystal grains having etch pits having an area of 2 × 10 ⁻⁵ μm ² or more at 1 × 10 ⁴ / mm ² or more on the steel sheet surface is defined as 80% or more. Usually, the density of etch pits varies greatly from crystal grain to crystal grain, and the state of the steel sheet surface is a mixture of crystal grains having a large number of etch pits and small crystal grains.

The density of etch pits is 1 × 10 ⁴ / mm
^{In two} or more crystal grains, Zn-
Due to the large amount of Ni plating, powdering is significantly suppressed. On the other hand, in the crystal grains having an etch pit density of less than 1 × 10 ⁴ / mm ² , the precipitation of Zn—Ni plating having a strong adhesion is small, and the gap becomes a starting point of powdering. Therefore, the amount of powdering generated increases.

Furthermore, the existence density of the etch pit is 1 × 10
^If the occupied area ratio of crystal grains of ⁴ grains / mm ² or more is 80% or more, even if local powdering occurs, it is difficult to propagate to the surroundings, and as a result, the powdering resistance becomes good. If it is less than 80%, the propagation of powdering occurs easily, so that the powdering resistance deteriorates. For the above reasons, in the present invention, the existence density of the etch pits is 1 × 10 ⁴ pits / mm ²
The occupied area ratio of the above crystal grains is set to 80% or more.

Next, a method for observing the etching state of the steel sheet surface in the present invention will be described. Since the etching state of the steel sheet surface cannot be observed while the Zn-Ni alloy plating layer is formed, the Zn-Ni alloy plating layer is first dissolved and removed. At this time, in order to avoid melting the Zn—Ni alloy and dissolving the surface of the steel sheet at the same time as changing the etching state, special attention is required for the melting method.

A preferred dissolution method is, for example, 20% Na
OH aqueous solution 80cc, 10% triethanolamine aqueous solution 40cc,
A method of immersing a Zn-Ni alloy-plated steel sheet in a solution of 25 cc mixed with 75 cc of water and 7 cc of 35% hydrogen peroxide water at 25 ° C. is used. The plating layer can be completely dissolved and removed.

As a method for dissolving the Zn—Ni alloy plating layer, any method capable of completely dissolving and removing the Zn—Ni alloy plating layer without changing the state of the steel sheet surface can be used. However, the present invention is not limited to this. The observation of the etching state of the steel sheet surface after dissolving and removing the Zn—Ni alloy plating layer is preferably performed using a scanning electron microscope.

In this case, the acceleration voltage can be observed in the range of 1 to 30 kV, but it is easier to observe the etching state of the steel sheet surface if the acceleration voltage is set to a relatively low acceleration voltage of about 1 to 10 kV. The magnification may be appropriately selected depending on the size of the crystal grains and the size of the etch pit, and it is preferable to perform observation while using a magnification of 1000 to 300,000 in combination.

The shape of the etch pit varies depending on the crystal orientation of the crystal grains on the surface of the steel sheet, conditions for pickling and pre-dip to a plating solution, and various shapes such as a square, a rectangle, a diamond, a triangle, a hexagon, an octagon, and a circle. Shapes are observed.
FIG. 1 shows an example of the results obtained by dissolving and removing the plating film of the Zn—Ni alloy-plated steel sheet of the present invention by the above-described method, and observing the etch pits on the steel sheet surface with a scanning electron microscope.

According to the present invention, 2 ×
The number of etch pits having an area of 10 ⁻⁵ μm ² or more is counted for each crystal grain, and the density of etch pits having an area of 2 × 10 ⁻⁵ μm ² or more in the crystal grains is 1 × 10 ⁴ / mm. ^Two
It is determined whether or not this is the case. Further, based on the area of the crystal grains and the determination result, the steel sheet indicates that the occupied area ratio of the crystal grains in which the etch pit having the area of 2 × 10 ⁻⁵ μm ² or more is 1 × 10 ⁴ / mm ² or more is 80% % Is determined.

In the present invention, when counting the number of etch pits for each crystal grain, the number of etch pits present at the crystal grain boundary is calculated by dividing the value obtained by dividing the number of adjacent crystal grains by the number of each crystal grain. Should be distributed to In addition, when the generation of etch pits is active, adjacent etch pits are connected to each other, or the entire crystal grain is etched without retaining the shape of the etch pit, resulting in a rough surface like a file of a file. There is.

In such a case as well, nucleation of zinc-plated crystals occurs at the vertices and sides of the etch pits. Therefore, the number of connected etch pits is not counted as one, but the etch pits before the connection are formed. If the shape of the pit can be recognized, the number of the pit may be counted, and if the shape of the pit looks like a tooth or a step, the number of teeth and the number of steps may be counted.

Next, a method for producing the Zn—Ni alloy plated steel sheet of the present invention will be described. The Zn—Ni alloy-plated steel sheet of the present invention can be manufactured by a normal electric zinc-based plating line. For example, Zn-Ni alloy plating is performed in an electric zinc-based plating line using a steel sheet that has undergone a hot rolling step, a pickling step, a cold rolling step, an annealing step, and the like as a material.

In the electro-zinc plating line, prior to plating, degreasing and washing with water for cleaning the surface of the steel sheet and pickling and washing with water to activate the surface of the steel sheet are performed. The pickling may be performed by a usual method, for example, a method of immersing in sulfuric acid or hydrochloric acid having a concentration of 5 to 500 g / l and a bath temperature of 20 to 80 ° C. for 1 to 20 seconds.

Subsequent to these treatments, the steel sheet invades the plating cell, and in a portion where no current is applied immediately after the entry, the surface of the steel plate is etched by pre-dip into the plating solution. Plating is performed. In order to produce the steel sheet of the present invention, since it is necessary to control the state of the formation of etch pits on the steel sheet surface, the pre-dip to the plating solution performed immediately before plating in the electro-zinc plating line under appropriate conditions There is a need to do.

That is, in the present invention, for example, after pickling with sulfuric acid having a concentration of 50 g / l and a liquid temperature of 50 ° C. for 5 seconds, the concentrations of ZnSO ₄ .7H ₂ O are 140 g / l and NiSO ₄・ 6H ₂ O
Containing 260 g / l, 20 g / l Na ₂ SO ₄ and 20 g / l K ₂ SO ₄ , pH
Is less than 2.0, the bath temperature is 50 ° C., and the solution flow rate is 1.2 m / s. Is possible.

However, in the present invention, the types of the pickling solution and the plating solution are not limited to the above-mentioned specific conditions. The conditions are not limited to the pickling conditions and the pre-dip conditions for the plating solution. The processing conditions for the Zn-Ni alloy plating performed after the pre-dip are as follows:
There is no limitation, and any Zn-Ni alloy plating method including a known Zn-Ni alloy plating method can be applied.

In addition, there are no particular limitations on various treatments on the surface of the Zn—Ni alloy plating layer, or chromate treatment on the upper layer of the Zn—Ni alloy plating layer, or a treatment method for coating with an organic resin. Instead, all processing methods including known processing methods can be applied.

[0038]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below based on embodiments. (Example 1) A very low carbon steel sheet having a thickness of 0.8 mm was used as a plating base plate, and Zn was processed in the order of alkaline electrolytic degreasing → water washing → pickling → water washing → pre-dip into a plating solution → Zn-Ni alloy electroplating. -Ni alloy plated steel sheets were produced.

Pickling, pre-dip into plating solution and Zn
-Ni alloy electroplating is performed under the following conditions, the pH of the pre-dip plating solution and the pre-dip time are changed to change the etching state of the steel sheet surface,
By changing the addition amount of the nSO ₄ · 7H ₂ O and NiSO ₄ · 6H ₂ O in the plating solution, samples were prepared with varying Ni content of Zn-Ni alloy plating layer (Invention Example 1 To 12, Comparative Examples 1 to 5).

Table 1 shows the pH of the pre-dip plating solution, the pre-dip time, and the Ni content in the plating layer. [Pickling] - bath _{composition: H 2 SO 4 50g / l} · bath temperature: 50 ° C. - pickling time: ## pre-dip and Zn-Ni alloy plating to the plating solution] 5 seconds ・ PH: 1.0 to 2.6 ・ Bath temperature: 50 ° C ・ Flow rate: 1.2m / s ・ Pre-dip time: 0.5 to 30 seconds ・ Plating current density: 100A / dm ²・ Amount of plating: 20g / m ² The powdering resistance of the produced Zn—Ni alloy plated steel sheet was evaluated by a reverse 0T bending test.

That is, an adhesive tape (manufactured by Nitto) having a width of 30 mm was previously applied to the center of the 30 mm × 100 mm sample on the plating surface side, and the sample was placed without any gaps with the adhesive portion of the plating surface tape attached to the inner center. Folded. Subsequently, the sample was opened, bent back to its original shape, and the tape was peeled off.

The count number of Zn attached to the tape was measured by X-ray fluorescence, and the powdering resistance was evaluated according to the following criteria. Table 1 shows the evaluation results. ◎: Zn count number ≦ 20cps ○: 20cps <Zn count number ≦ 50cps △: 50cps <Zn count number ≦ 100cps ×: 100cps <Zn count number Further, the Zn-Ni alloy plating film of each sample was melted by the following method. It was removed and the etched state of the steel sheet surface was observed with a scanning electron microscope.

That is, for each sample,
By observing 100 crystal grains on the steel sheet surface, 2 ×
The occupied area ratio of crystal grains in which etch pits having an area of 10 ⁻⁵ μm ² or more were present at 1 × 10 ⁴ / mm ² or more was calculated. Table 1 shows the calculation results. [Method of dissolving and removing the Zn-Ni alloy plating film] ・ Bath composition: 20% NaOH aqueous solution 800cc 10% triethanolamine aqueous solution 400cc Water 750cc 35% hydrogen peroxide aqueous solution 70cc ・ Bath temperature: 25 ° C ・ Immersion time: 60 minutes FIG. 2 shows the relationship between the pH of the plating solution during pre-dip, the pre-dip time, and the occupied area ratio.

The numerical values, symbols, signs, etc. in the graph of FIG. 2 indicate the following contents. Numerical value in parentheses in the graph: Pre-dip time (seconds) ｐＨ: pH vs. occupied area ratio of pre-dip plating solution-○-○-: Same as above (pre-dip time = 3 seconds) Occupied area ratio (%): 2 × Occupied area ratio of crystal grains having 1 × 10 ⁴ etch pits / mm ² or more having an area of 10 ⁻⁵ μm ² or more A: Powdering resistance is in the area of ◎ or ○ in the above evaluation criteria α _pH : The pH range of the pre-dip plating solution from which a Zn-Ni alloy-plated steel sheet having excellent powdering resistance can be obtained. As is clear from Table 1, the Zn-Ni alloy-plated steel sheets of the present invention all have excellent powdering resistance. It was excellent.

Further, as shown in FIG. 2, the pH of the plating solution at the time of pre-dipping is less than 2.0, more preferably 1.
It can be seen that the Zn—Ni alloy-plated steel sheet of the present invention having excellent powdering resistance can be manufactured by setting the pre-dip time to 8 seconds or less and the pre-dip time to 1 second or more.

[0046]

[Table 1]

[0047]

As described above, the Zn-Ni alloy-plated steel sheet of the present invention has excellent powdering resistance and is extremely valuable industrially.

[Brief description of the drawings]

FIG. 1 is a scanning electron microscope photograph showing an example of the result of observing etch pits on the surface of a steel sheet by dissolving and removing a plating film of a Zn—Ni alloy-plated steel sheet of the present invention.

FIG. 2 is a graph showing a relationship between a plating solution pH during pre-dip, a pre-dip time, and an occupied area ratio.

Claims (1)

[Claims] The present invention relates to a Zn-based steel sheet having an etch pit having an area of 2 × 10 ⁻⁵ μm ² or more in an area of 1 × 10 ⁴ / mm ² or more. A Zn-Ni alloy plated steel sheet having excellent powdering resistance, characterized in that a Ni alloy plated layer is formed.