JP2785672B2 - Zn-based alloy-plated steel sheet with excellent perforation resistance and plating adhesion - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Zn-based alloy-coated steel sheet having excellent puncture resistance and plating adhesion.

[0002]

2. Description of the Related Art Surface-treated steel sheets for automobiles are often used in harsh corrosive environments exposed to various chemical components in the soil at high humidity and in the air. Especially in cold regions,
Since a large amount of salt is sprayed on the road as a snow melting agent, perforated corrosion of the automobile floor surface due to the snow melting agent has become a problem.

[0003] For this reason, surface-treated steel sheets having high corrosion resistance that can withstand such severe corrosive environments have been required.
Increasing the coating weight of the Zn-based alloy electroplated steel sheet is being studied, but increasing the coating weight further increases the cost burden and is not preferable. In addition, there is a disadvantage that the increase in the amount of plating adhesion deteriorates the weldability and the powdering resistance (the phenomenon that the plating film peels into a powder during press working due to poor plating adhesion). Especially Zn with high plating film hardness
-The powdering resistance of Ni-plated electroplated steel sheet rapidly deteriorates with the increase in the amount of adhesion. Therefore, increasing the amount of adhesion to increase the corrosion resistance has reached its limit.

[0004] Therefore, an organic composite coated steel sheet in which a chromate film and an organic resin film (coating film) containing silicate are applied to the surface of a Zn-based plated steel sheet has been proposed (JP-A-58-2241).
Nos. 74 and 60-174879, etc.). However, this organic coated steel sheet also has a weak adhesion between the base material and the plating, so it is difficult to say that the powdering resistance has been sufficiently improved, and it has sufficient corrosion resistance in the processed part where the organic coating layer and the chromate film are easily scratched. It did not improve.

[0005] Thereafter, it has been shown that plating adhesion can be improved by adding a heavy metal component such as Cu, Ni, Cr, Mo or the like to the base material or adding P to the base material and defining the composition of the plating film within a specific range. (JP-A-61-284561, 61-28)
No. 4594). These publications contain Cu, N
It has been shown that plating adhesion can be improved by having a plating layer containing at least 50% Zn as the lowermost layer on the surface of a steel sheet to which one or more of i, Cr, and Mo and P are added. According to our findings, the addition of heavy metal components such as Cu, Ni, Cr, and Mo, when these elements are locally concentrated in steel, cause Zn and alloy elements other than Zn in the film to Is easily formed, and the puncture resistance is deteriorated instead of being unfavorable.

[0006] Japanese Patent Application Laid-Open Nos. 5-65595 and 5-65675
Nos., 5-65676, C, N, Si, Mn,
Using a base material with P, S, Al, Nb, Ti, Cu, Ni, and B within a predetermined range, applying a Zn-based alloy plating thereon, and further covering with a chromate film and an organic film to improve corrosion resistance and It has been reported that a surface-treated steel sheet with good formability can be manufactured, but this is also merely improving the formability of conventional corrosion-resistant steel from the viewpoint of the base metal, and from the viewpoint of powdering resistance, etc. Has no advantage over the case of using a normal ultra-low carbon steel base material.

[0007]

SUMMARY OF THE INVENTION Thus, an object of the present invention is to provide, under such circumstances, excellent puncture resistance and
An object of the present invention is to provide a surface-treated steel sheet having extremely good plating adhesion. Specifically, without increasing the plating weight
As a surface-treated steel sheet that can achieve plating resistance that satisfies perforation resistance that satisfies use in severe corrosive environments caused by snow melting agents and powdering resistance under severe processing conditions
An object of the present invention is to provide a Zn-based alloy plated steel sheet.

[0008]

Means for Solving the Problems Here, the present inventors have
In order to achieve this purpose, as a result of various studies, the active addition of 0.045% or more of P to the cold-rolled steel sheet, and in particular, the total amount of Cr, Cu, and Mo as impurities were suppressed to 0.10% or less, Preferably, by setting the grain size of the surface to 0.0003 mm ² or less in average cross-sectional area, it is unexpectedly found that the adhesion and perforation resistance of the plating film are significantly improved,
The present invention has been completed.

Therefore, the gist of the present invention is that P contains 0.045 to 0.120% by weight and contains P as an impurity.
At least one side of a cold rolled steel sheet having a total of 0.10% or less of Cr, Cu, and Mo contains an element that is more noble in potential than Zn in a total content of 0.5 to
Includes 30% by weight, and coating weight is 5-50 in total plating weight
It is a Zn-based alloy-plated steel sheet having a g / m ² Zn-based alloy electroplating film and excellent in puncture resistance and plating adhesion.

In another aspect, the present invention provides
And contains P: 0.045 to 0.120%, Cr as an impurity,
A cold-rolled steel sheet having a total grain size of 0.0003 mm ² or less in terms of the average cross-sectional area of the grains when the total of Cu and Mo is 0.10% or less and the grain size of the surface when observed from a direction perpendicular to the steel sheet surface is Zn
0.5 to 30% by weight in total content of noble elements more potential than
Includes Zn with a coating weight of 5 to 50 g / m ^{2 in} total plating weight
The present invention is a Zn-based alloy-plated steel sheet characterized by being provided with a system-based electroplating film and having excellent puncture resistance and plating adhesion.

According to a preferred embodiment of the present invention, a chromate film is further provided as a second layer on the Zn-based alloy-plated steel sheet obtained as described above, and an organic film is further provided as a third layer on the chromate film. You may.

[0012]

The operation of the present invention will be described below in detail.
The surface-treated steel sheet of the present invention, that is, a Zn-based alloy-plated steel sheet,
It is obtained by subjecting a Zn-based alloy to electroplating on a cold-rolled steel sheet containing a relatively large amount of P and having a small content of Cr, Cu, and Mo as impurities, and containing P and containing Cr, Cu, and Mo. Zn on cold rolled steel sheet with low content and small surface grain size
It is obtained by subjecting a base alloy to electroplating.

Next, each component in the cold-rolled steel sheet, the reason for limiting the crystal size of the surface of the cold-rolled steel sheet, and the reason for limiting the plating will be described. In addition, "%" of each component of the cold-rolled steel sheet and "%" of each component constituting the plating film mean "% by weight".

P: P is the most characteristic element in the present invention. P has the effect of densifying rust in the early stage of corrosion, forming a stable rust layer with low oxygen permeability, and improving the puncture resistance. This action covered the steel sheet surface with a Zn-based alloy electroplated steel sheet. Especially notable in some cases. This is because, in the case of a plated steel sheet, the corrosion of the plating film occurs first, and the corrosion of the steel occurs when the plating film is partially thinned or peeled off. In the case of alloy electroplated steel sheets, the rate at which the initial corrosion of steel progresses is slow, and there is sufficient time to form a stable rust layer.If the steel sheet surface is covered with a Zn-based alloy electroplated steel sheet, The preferential elution of potential-potential Zn occurs in the plating film, enriching the potential noble non-Zn alloy element, and forming a stable rust layer similar to the heavy metal added to steel to this non-Zn alloy element This is probably because there is a promoting effect.

[0015] The effect of adding P to steel, which is unique to such a Zn-based alloy electroplated steel sheet, appears from 0.045%. Therefore, the lower limit of P is set to 0.045%. On the other hand, if it exceeds 0.120%, the degree of the effect reaches a plateau, and the weldability and the formability deteriorate. Therefore, the upper limit is 0.120%. Preferably, it is 0.045 to 0.080%.

Cu, Mo, Cr: The addition of Cu, Cr and Mo has the effect of forming a stable oxide on the steel surface and promoting the formation of a stable rust layer in the case of a cold-rolled steel sheet. In the case of a system alloy electroplated steel sheet, preferential elution of potential-negative Zn occurs in the process of corrosion of the plating film, Zn is preferentially corroded, and non-Zn alloy elements concentrate on the steel sheet surface In particular, there is no need to add these elements. In addition, Cu, Cr, and Mo added to the steel are locally concentrated in the steel, or locally concentrated in the steel during the corrosion process. This facilitates the formation of a local battery with an alloy element, thereby accelerating the corrosion of the plating film, causing local corrosion of the steel before a sufficient stable rust layer is formed, and deteriorating the perforation resistance. When Cu, Cr, and Mo are locally concentrated in the steel, the adhesion of plating is also reduced. Therefore, it is preferable not to add Cu, Cr and Mo to the steel as much as possible, and the total of Cu, Cr and Mo should be 0.10% or less, preferably 0.06% or less of the impurity level in total.

In the Zn-based alloy electroplated steel sheet according to the present invention, sufficient corrosion resistance improvement hardening can be obtained by itself, but in order to further improve the puncture resistance and plating adhesion, the following is required. desirable.

In the cold-rolled steel sheet used for the plated steel sheet according to the present invention, a stable rust layer is formed at the initial stage of corrosion by adding P in an amount of 0.045 to 0.120%. To suppress this, it is important how thick and dense this stable rust layer is formed.

In a preferred embodiment of the present invention, these points are realized by defining the surface crystal size of the cold-rolled steel sheet. Thus, according to the present invention, by improving the plating adhesion, the puncture resistance is improved. Successfully formed a sufficient stable rust layer. Hereinafter, preferred embodiments of the present invention will be described in detail.

In a cold-rolled steel sheet, P is concentrated at the grain boundaries. This P-enriched portion is easily etched at the time of pickling in plating pretreatment. In addition, since the portion where P is not concentrated is relatively difficult to be etched, minute irregularities are formed, and when plating is performed thereon, plating adhesion is improved by an anchor effect with plating. In particular, in the case of a Zn-based alloy electroplated steel sheet, if the plating is hard and the plating adhesion is poor, a phenomenon occurs in which the plating peels off (powder) during processing, but according to a preferred embodiment of the present invention, this phenomenon occurs. Is improved.

Further, since the plating is less likely to be peeled off in the processed portion, the effect of retaining rust at the initial stage of corrosion is increased, and a dense stable rust layer is easily formed, so that the corrosion resistance is significantly improved.

Such an effect occurs when the crystal grain size on the surface of the cold-rolled steel sheet is 0.0003 mm ² or less in the average sectional area of the crystal grains when observed from a direction perpendicular to the steel sheet surface. Normally, the size of the crystal grains is observed in the cross section, and the average particle size of the surface portion and the central portion is determined.

That is, as the crystal grains become finer and the number of crystal grain boundaries increases, the above-mentioned minute irregularities are formed more due to the difference in etching. When plating is performed thereon, the plating effect occurs due to the anchor effect with the plating. The adhesion is improved.

For this reason, in a preferred embodiment of the present invention, the crystal grain size on the surface of the cold-rolled steel sheet is 0.0003 mm ² or less as the average cross-sectional area of the crystal grains when observed from a direction perpendicular to the steel sheet surface. The smaller crystal grain size is not particularly specified, but the production limit is about 0.00003 mm ² .

As a method of reducing the crystal grain size in this way, it is most desirable to lower the finishing temperature of hot rolling to just above the Ar ₃ point. By this method, usually, the average cross-sectional area of the crystal grains when observed from the perpendicular direction to the steel sheet surface
It is possible to reduce the crystal grain size of the cold-rolled steel sheet is 0.002 ~0.0005mm ² about the 0.0003 mm ² or less. As another means, there can be mentioned a method in which the annealing temperature is set within the recrystallization temperature + 20 ° C.

The plating applied to the surface of such a cold-rolled steel sheet has a high plating film hardness, and has a remarkable effect of improving the corrosion resistance and plating adhesion when the Zn-based alloy electroplated film is apt to cause powdering.

Further, the total content of the elements which are more noble in potential than Zn in the Zn-based alloy plating film is 0.5 to 30% by weight. Zn
If the total potential content of noble elements no more than 0.5% is less than 0.5%, the elution of Zn in the early stage of corrosion does not cause concentration of noble elements nobleer than Zn, so elution will occur until Zn is completely eliminated. Subsequently, the perforation resistance deteriorates. On the other hand, when the total content of elements that are more noble in potential than Zn exceeds 30% by weight, the plating becomes too hard, and the plating peels because the addition of P to the cold-rolled steel sheet exceeds the effect of improving the adhesion of the plating. Easier to do. Preferably, for example, 4-20% for Zn-Ni system, Zn-Ni
0.5 to 20% for Co-based, 1 to 20% for Zn-Cr-based and other elements which are more noble than Zn, more preferably Zn-Ni
The content is 4 to 15% in the system, 0.5 to 10% in the Zn-Co system, and 1 to 15% in the Zn-Cr system and other elements which are more noble than Zn.

Here, as the element which is more noble in potential than Zn, for example, Ni, Co, Fe, Cr, Sn, Pb, Cu and the like can be mentioned, but preferably Ni, Co, Cr, Fe
Particularly preferred are Ni, Co and Cr. Further, the plating film structure may be a multilayer structure. At this time, the element which is more noble in potential than Zn is 0.5 to 30% by weight as the average composition of plating.
Should be fine.

The coating weight is 5% by the total weight of the plating.
g / m, sufficient Taiana Aki property is obtained is less than ^2, the cost is increased in the 50 g / m ^2, greater than The coating weight since deterioration also occurs in the powdering property is the total weight of the plating 5 to 50 g / and m ^2. Preferably from 10 to 40 g / m ^2.

In order to further increase the corrosion resistance, it is desirable to apply a chromate film as the second layer and an organic film as the third layer on the Zn-based alloy plating layer. In that case, the chromate film has the effect of increasing the corrosion resistance of the plating film and the function of adhering the first Zn-based alloy plating layer and the third organic film. By making the Zn-based alloy plating layer and the organic film adhere to each other, powdering is more difficult to occur, so that the initial corrosion rate of the cold-rolled steel sheet is suppressed, and the formation of a stable rust layer is further promoted.

The method of forming the chromate film can be any of a coating type, a reaction type and an electrolytic type. The attached amount of the chromate film is preferably 10 to 200 mg / m ² as the attached amount of Cr. Ten
can not be expected corrosion resistance improvement is less than ^{mg / m 2, 200 mg /} m
^If it exceeds ² , workability and weldability deteriorate. More preferably 30
100100 mg / m ² . Hexavalent Cr is used for the chromate film.
Is preferably present. Cr ⁶⁺ , if the steel plate is damaged,
It has the effect of suppressing corrosion from there.

The coating-type and reaction-type chromate treatment solutions mainly contain a partially reduced chromic acid solution, and if necessary, an aqueous resin or a water-soluble organic resin such as an acrylic resin and / or colloidal silica. Is contained. In this case, the ratio of Cr ³⁺ : Cr ⁶⁺ is preferably 50: 1 to 1: 3. The ratio of Cr ³⁺ : Cr ⁶⁺ is adjusted to a predetermined ratio using a general reducing agent. At this time, the coating type chromate is applied with this solution using a roll coater, a spray or the like, and dried without washing with water to obtain a film. Reactive chromate is made of insoluble Cr on the surface by immersing a plated steel sheet in this solution.
₂ O ₃ is generated to obtain a film.

The electrolytic chromate is chromic anhydride and sulfuric acid,
1 of anions such as phosphoric acid fluoride or halogen oxyacid
Cathodic electrolytic treatment in a bath containing one or more species,
Rinse and dry to form a film.

The coating type and the reaction type chromate have the advantage of excellent corrosion resistance, and therefore are particularly preferable for a surface-treated steel sheet for automobiles in which corrosion resistance is important. Among them, the coating type is particularly preferable because one-side processing is possible.

The organic film has a function of shielding the plating film and the steel sheet from corrosive substances such as water and chlorine. In addition to this, in the stage of forming a stable rust layer in the early stage of corrosion, this environmental shielding effect suppresses rapid corrosion of the rust layer and contributes to the formation of a stable rust layer.

The thickness of the organic film is preferably from 0.1 to 2 μm. If it is less than 0.1 μm, the effect of improving corrosion resistance is not seen, and if it exceeds 2 μm, the weldability and electrodeposition coating properties are significantly reduced.

This organic film can be formed using various resin-based coating compositions conventionally used in the production of painted steel sheets. As the organic resin film, a thermosetting film which is simple in equipment and has a fast film formation is industrially preferable, but an ultraviolet or electron beam curable film or a room temperature drying film can also be used. Among them, a particularly preferred resin type is an epoxy resin which is a thermosetting film, and an epoxy resin and silica as main components are most suitable.

This resin liquid is applied on the chromate film by a suitable application means such as a roll coater, and the coating is cured by heating to form an organic resin film. The heating temperature is 80 to 250 ° C in the case of the epoxy type.

Since the effect of these coatings can significantly improve the corrosion resistance especially in the processed part, the Zn-alloy-coated steel sheet, which is the surface-treated steel sheet according to the present invention, is used as a surface-treated steel sheet for automobiles which is severely processed and corroded. It has particularly excellent performance. Next, the operation and effect of the present invention will be described more specifically with reference to examples.

[0040]

【Example】

(Example 1) In this example, a 0.8 mm-thick cold-rolled steel sheet having the chemical composition shown in Tables 1 and 2 was subjected to alkaline electrolytic degreasing and pickling, and then Zn was subjected to the following method. A system alloy was electroplated.

Plating method: The plating bath was a zinc sulfate bath (pH =
By using 2), the alloy component in the film was changed by adding a sulfate of the metal and changing the alloy metal ion concentration / Zn ion concentration to change the alloy component ratio in the film. The total concentration of Zn and alloy component ions in the bath was 100 g / l. In addition, as a conductive electrolyte,
100 g / l Na ₂ SO ₄ was added.

Using the above plating bath, a current density of 40 to 60 A /
Plating was performed using a direct current of dm ^{2 at} a bath temperature of 40 to 60 ° C. The adhesion amount of the plating film was changed by controlling the amount of electricity.

For the samples shown in Tables 3 and 4, a chromate layer was formed on the plating by a coating type chromate treatment and an organic resin (epoxy resin) layer was formed by a roll coater coating. Each of the plated steel sheets thus obtained was evaluated for its properties in the following manner.

Evaluation of corrosion resistance: The surface-treated steel sheet prepared as described above was cylindrically drawn into a shape having a diameter of 50 mm and a height of 25 mm.
The inside and the edge of the cylinder were subjected to waterproof processing, and as a test sample, a perforated corrosion test was performed to evaluate the perforation resistance. SST (6hr) → Dry (50 ℃ × 2hr)
→ If nothing is applied on the plating with one cycle of wet (relative humidity 95%, 50 ° C x 16hr), the maximum erosion depth after performing 40 cycles of repetitive tests is measured with a point micrometer, Good if less than 0.2 mm (○), 0.
If it was 1 mm or less, it was marked as (◎). When applying a coating consisting of chromate and organic resin on the plating, measure the maximum pit depth after 200 cycles.
(×), good (○) if less than 0.2 mm, excellent (優) if less than 0.1 mm.

The corrosion resistance equivalent to “◎” in this test corresponds to the corrosion resistance that satisfies the use in a severe corrosive environment caused by a snow melting agent or the like without increasing the amount of plating. Are excellent characteristics that could not be realized in practice.

Plating adhesion (powdering resistance) test:
The surface-treated steel sheet prepared as described above was subjected to cylindrical drawing to a shape of 50 mm in diameter and 25 mm in height, and the plating adhesion was examined by the peeling amount of the plating film peeled off by the tape peeling test on the cylindrical side surface. Those with less than 10 mg / tape are excellent (◎), those with 10-30 mg / tape are good (○),
Samples with 30 mg / tape or more were evaluated as defective (x).

The test results are shown in Tables 1 to 4. As is evident from the results, both the corrosion resistance and the plating adhesion are excellent in the examples of the present invention.

On the other hand, when the chemical composition of the base material, the plating amount or the plating film composition is out of the range specified in the present invention, either or both of the corrosion resistance and the plating adhesion are poor. This tendency does not change even if there is a chromate layer and an organic resin layer on the plating film.

[0049]

[Table 1]

[0050]

[Table 2]

[0051]

[Table 3]

[0052]

[Table 4]

Example 2 In this example, Example 1 was repeated using a similar test steel in which the average crystal size on the surface of the cold-rolled steel sheet was changed. Tables 5 to 8 collectively show the steel composition, crystal grain size, and the like of the test steels and the results of the tests. The plating adhesion equivalent to “に” in the tape peeling test is a characteristic that could not be achieved at the same time as the evaluation of “◎” or more in the above-mentioned corrosion resistance test.

[0054]

[Table 5]

[0055]

[Table 6]

[0056]

[Table 7]

[0057]

[Table 8]

[0058]

According to the present invention, as described above, by adding a specific alloying element having a corrosion inhibiting action to a cold-rolled steel sheet, it is possible not only to increase the corrosiveness of the entire plated steel sheet but also to intentionally increase the corrosion resistance. By reducing the crystal grain size on the surface of the cold-rolled steel sheet, the adhesion between the plating film and the steel sheet can be improved and the corrosion resistance can be further increased. For this reason, the Zn-based alloy-plated steel sheet according to the present invention can be used as a rust-preventive steel sheet for automobiles that emphasizes high corrosion resistance of a processed portion, and has extremely high industrial value.

Continuation of front page (56) References JP-A-1-108392 (JP, A) JP-A-6-228772 (JP, A) JP-A-5-65676 (JP, A) JP-A-5-65675 (JP) JP-A-5-65595 (JP, A) JP-A-61-284594 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C25D 5/00-5/52 C23C 30/00

Claims (3)

(57) [Claims] 1. A cold-rolled steel sheet containing, by weight%, P: 0.045 to 0.120% and having a total of Cr, Cu, and Mo as impurities of 0.10% or less, and at least one surface of which is more noble than Zn. And a plating resistance of 0.5 to 30 g / m ² in terms of the total weight of the plating. Zn alloy plated steel sheet with excellent adhesion. 2. The composition contains P: 0.045 to 0.120% by weight, and the total of Cr, Cu and Mo as impurities is 0.10% or less,
The crystal grain size of the surface when observed from the direction perpendicular to the steel sheet surface is a cold-rolled steel sheet with an average cross-sectional area of the crystal grains of 0.0003 mm ² or less. comprises 0.5 to 30 wt%, the amount of plating deposition is excellent in coating adhesion and pitting corrosion resistance properties, characterized in that the Zn-based alloy electroplated film of 5 to 50 g / m ² by total weight of the plating is applied Zn-based alloy plated steel sheet. 3. A Zn-based alloy comprising a Zn-based alloy plated steel sheet according to claim 1 and a chromate film as a second layer and an organic film as a third layer on the chromate film. Plated steel sheet.