JPH07258884A - Surface treated steel sheet excellent in corrosion resistance and coating suitability, and its production - Google Patents

Abstract

PURPOSE:To impart excellent press formability, adhesive strength of a coating film and corrosion resistance to a steel sheet by subjecting a Zn-Cr composite plated layer to etching treatment and thereafter executing phosphating. CONSTITUTION:This steel sheet is a one in which the surface is applied with a Zn-Cr composite plated layer of 5 to 40g/m<2> coating weight contg. 1 to 25wt.% Cr, the surface of this plated layer is coated with phosphating coating of >0.5 to 4g/m and the boundary of the composite plated layer and phosphate film is made rich in Cr. Moreover, the Zn-Cr composite plated layer is preferably incorporated with total 0.1 to 3wt.% of one or more kinds among Fe, Ni and Co. Concretely, immediately after the formation of the Zn-Cr composite plating by an electroplating method, it is subjected to simple cleaning without executing oil coating and degreasing, and the face of the plated layer is etched by an acidic soln. of 1 to 3 OH. Thus, the plated face is made rich in Cr, its activity is uniformized, and phosphating is executed in a state in which chemical conversion treatment is stabilized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface-treated steel sheet excellent in corrosion resistance and paintability and a method for producing the same, and particularly to a surface-treated steel sheet suitable for an automobile body requiring long-term rust prevention and its production. Regarding the method.

[0002]

2. Description of the Related Art Conventionally, in order to improve the corrosion resistance of an automobile body, various measures have been taken in terms of painting and car body design. However, from the viewpoint of reliability for long-term rust prevention, the surface is rustproofed. Rusted steel sheets are commonly used for automobile bodies. Although there are galvanized steel sheets, galvannealed steel sheets, and the like as rustproof steel sheets, further development of thin and highly corrosion resistant steel sheets that can achieve both low cost and high corrosion resistance is awaited.

As a rust preventive plating for improving the corrosion resistance of a steel sheet, for example, Zn-disclosed in Japanese Patent Publication No. 2-51996, Japanese Patent Application Laid-Open No. 64-55397, Japanese Patent Application Laid-Open No. 3-240994, etc. Cr composite plating is known. Steel sheets coated with these Zn-Cr composite platings have very excellent corrosion resistance due to the corrosion inhibition effect of the corrosion products of Cr contained in the plating components. In addition, Zn-Cr composite plating does not passivate because it retains a relatively base potential, so that the so-called sacrificial anticorrosive action is effective for a long period of time, and it has excellent corrosion resistance even under conditions where the base metal is exposed. Have.

Further, Japanese Patent Application Laid-Open No. 1-177386 discloses a plated steel sheet in which a chromate film is formed on the mounting surface of a Zn-Cr plated layer. In this plated steel sheet, in addition to the sacrificial anticorrosion action and corrosion resistance of the Zn-Cr plating layer as the undercoat plating layer, the upper chromate film is Zn-Cr.
By effectively acting on the plating layer, the corrosion resistance of the steel sheet is further enhanced.

Further, in addition to the above-mentioned chromate coating, Z
1 to 500 mg / m of phosphorus oxide on the surface of n-plated steel sheet
A plated steel sheet coated with ² is disclosed in JP-A-4-88196. According to this plated steel sheet, the Zn-based alloy plating surface is coated with an oxide such as phosphorous oxide to improve the press formability and chemical conversion treatability of the steel sheet. The coating amount of this phosphorous oxide is adjusted to be relatively small so that the state of the coating becomes amorphous to improve the lubricity of the steel sheet and the press formability of the steel sheet.

[0006]

However, the Zn-Cr composite plated steel sheet described above is very important as a rust-preventive steel sheet for automobiles, although it has good corrosion resistance such as exposure environment in a normal flat plate. The characteristic corrosion resistance of the joint is insufficient. In other words, an automobile always has a mating portion due to its structure, and when observing the corrosion state of an actual vehicle, this mating portion is most corroded. The factors that cause severe corrosion in the mating part include insufficient formation of the phosphate coating after pressing and electrodeposition coating, relatively easy accumulation of moisture, and It is considered that the non-uniform drying makes the corrosive environment of the material more severe.

[0007] Further, the Zn-Cr composite plated steel sheet is oiled by the steel sheet manufacturer and then shipped to an automobile manufacturer.
The assembly process, the degreasing process, the phosphate process, and the painting process are passed through to manufacture the automobile body. Therefore, Cr oxides are deposited on the surface of the steel sheet due to oxidation over time including a distribution process after plating, and the Cr oxides inhibit the formation of a phosphate film in the phosphate treatment step. Further, a stable degreasing treatment before the phosphate treatment forms a stable oxide of Cr on the surface of the steel sheet, which inhibits the formation of the phosphate coating. Therefore, when such Cr oxide is deposited on the surface of the steel sheet, there is a problem that good coating film adhesion and corrosion resistance after coating (rust resistance in appearance) cannot be obtained.

When the plated steel sheet having a chromate film as described above is used as a steel sheet for automobiles, since the chromate film is inactive, it is necessary to form sound phosphatized crystals in the phosphating step. I can't. As a result, sufficient coating film adhesion cannot be obtained, and as a result, the corrosion resistance after coating tends to be inferior to that of ordinary rust-preventive steel sheets (automobiles caused by coating scratches caused by smashing stones etc. External rust, etc.)

Furthermore, the coated steel sheet having a phosphorous oxide coating has a coating amount of 1 to 500 mg / m ^{2 which} is a very small amount, and it cannot be said that the effect of improving the corrosion resistance is sufficient. The present invention has been made in view of the above points, and an object thereof is to provide a surface-treated steel sheet excellent in corrosion resistance and coating film adhesion, and a method for manufacturing the same.

[0010]

Means for Solving the Problems As a result of investigations aimed at improving the drawbacks of the conventional steel sheet, the present inventors have carried out an etching treatment with an acidic solution on the surface of the Zn--Cr composite plating to slightly reduce the surface. By making Cr-rich and further providing an appropriate amount and a dense phosphate film on the surface thereof, it has been possible to complete an anticorrosive steel sheet for automobiles having excellent corrosion resistance and paintability.

The constitution of the film of the surface-treated steel sheet of the present invention and the method for producing the same are as follows. According to the present invention, Z is formed on the surface of a steel sheet and contains 1 to 25% by weight of Cr and is applied at a plating amount of 5 to 40 g / m ^2.
n-Cr composite plating layer and 0.5 on this composite plating layer
And a phosphate coating formed with a coating amount of 4 g / m ² or less, wherein the composite plating layer has a Cr-rich interface at the interface with the phosphate coating. Provided is a surface-treated steel sheet having excellent properties.

According to a preferred embodiment of the present invention, the composite plating layer contains 0.01 to 3% by weight in total of at least one of Fe, Ni and Co. Further, according to the present invention, Zn-Cr is formed on the surface of the steel sheet.
A plating step for forming a composite plating layer, an etching step for etching the composite plating layer with an acidic solution having a pH of 1 to 3, and a phosphate treatment for the etched composite plating layer to form a phosphate coating film. The method for producing a surface-treated steel sheet having excellent corrosion resistance and coatability is provided. The composite plating layer can be formed on one side or both sides of the steel plate. When it is formed on both sides, a phosphate coating is formed on at least one side thereof.

[0013]

According to the surface-treated steel sheet of the present invention, first, a Zn-Cr composite plating layer is provided on the surface of the steel sheet, the steel sheet is subjected to an etching treatment with an acidic solution, and then a phosphate coating is formed on at least one surface. To provide. The functions of the Zn-Cr composite plating layer and the phosphate coating will be described below.

First, the Zn-Cr composite plating layer contains 1 to 25% by weight of Cr. If the Cr content is less than 1% by weight, the corrosion resistance at the seam portion of the steel sheet deteriorates, and particularly in combination with the phosphate coating described below, a sufficient synergistic effect cannot be obtained. In other words, since the phosphate film consists of scale-like or block-like crystals, the barrier effect of the film itself against corrosion factors (water, oxygen, chlorine, etc.) is not perfect, and even if there is a phosphate film, the underlying Although the plating layer is gentle, it is exposed to a corrosive environment. Therefore, when the Cr content in the composite plating layer is low, the corrosion resistance of the plating layer itself is low, so that corrosion is likely to occur, and the alkalizing of the corrosive environment (generation of OH ions) caused by the corrosion reaction results in the phosphate. Promotes dissolution of the film. As a result, the corrosion resistance effect of the phosphate film is rapidly lost, and sufficient corrosion resistance cannot be obtained when the Cr content is less than 1% by weight.

On the other hand, when the content of Cr exceeds 25% by weight, the plating layer itself becomes brittle, and when it is subjected to press molding, peeling of the plating and powdering (plating film falling off in powder form) may occur. become. Further, since the plating layer becomes inactive too much, it becomes difficult to form a dense phosphate film, and the coating film adhesion and the corrosion resistance after coating are adversely affected. For the above reasons, the Cr content is preferably 1 to 25% by weight, and particularly preferably 3 to 15% by weight.

Further, the Zn-Cr composite plating layer is made of Fe,
It is preferable to contain at least one of Ni and Co in a total amount of 0.01 to 3% by weight, and in this case, better corrosion resistance can be obtained. That is, Fe, Ni, and Co, which are noble metals as compared with Zn and Cr, serve as cathode points during the phosphate treatment to promote uniform dissolution of the plating layer,
It has an effect of improving the production density of phosphate crystal nuclei.
Further, Fe, Ni, and Co are considered to have an action of suppressing alkalization of the corrosive environment that occurs during the above-mentioned corrosion of the plated layer. If the content of Fe, Ni, Co is less than 0.01% by weight, the above effect is not remarkable, and if it exceeds 3% by weight, the plating layer itself becomes brittle, and the plating is peeled off or powdered during press molding. Will occur. Also, when forming the plating layer, it causes a decrease in electrolytic efficiency,
It also causes an increase in manufacturing cost. Therefore, Fe,
The Ni and Co content is preferably 0.01 to 3% by weight.

The amount of the Zn-Cr composite plating deposited is preferably 5 to 40 g / m ² . 5
If it is less than g / m ² , the corrosion resistance is insufficient and the adhesion amount is 40
If it exceeds g / m ² , weldability and plating peeling are likely to occur. The Zn-Cr composite plating layer may be formed by a normal method, and the manufacturing method is not particularly specified.

In the case of producing Zn-Cr system plating by electroplating, the surface condition of the plating film formed can be controlled by controlling the concentration of Zn ions and Cr ions in the plating bath. It is considered that this is because Zn and Cr are electrodeposited through the complex hydroxide, and thus the hydroxide remains on the surface of the deposited film, but the formation of this layer changes depending on the ion concentration. To be

Next, the function of the phosphate film will be described. The phosphate film effectively acts on a portion such as a joint portion of the steel sheet and gives the steel sheet excellent corrosion resistance. That is, in a structure such as a mating portion existing in an automobile, the coating part is not sufficiently covered, and thus the plated portion is likely to be exposed unlike other portions. In addition, moisture and the like are likely to accumulate at the joints of the steel sheets, which is an environment where corrosion is very likely to occur. Therefore, by forming a phosphate coating on the steel sheet before working, good corrosion resistance can be provided even in the joint portion.

The type of the phosphate coating is not particularly specified, and a commercially available phosphate treatment liquid can be used, but it is considered that it is used in a series of coating processes of current automobile manufacturers. The zinc phosphate-based treatment liquid is the most preferable because it is of the same type. Particularly, Ni and Mn are preferably added from the viewpoint of corrosion resistance of the phosphate coating itself.
If the amount of the phosphate coating adhered is 0.5 g / m ² or less, good corrosion resistance cannot be obtained, and if it exceeds 4 g / m ² , the weldability deteriorates. Further, if it exceeds 4 g / m ² , the process peeling of the phosphate coating itself cannot be ignored during press molding. Therefore, the coating amount of the phosphate coating is more than 0.5 and 4 g / m ² or less, and particularly preferably more than 0.5 and 2.5 g / m ² .

The method for forming the phosphate-treated film is not particularly specified, but it is most desirable in terms of manufacturing cost, efficiency, etc. to use chemical conversion treatment equipment attached to the electroplating line. The treatment method is not particularly limited to the immersion type or the spray type. Also for the pretreatment of the phosphate treatment, the surface conditioning treatment generally performed can be performed.

The steel sheet surface-treated as described above is Zn
Immediately after the formation of the Cr composite plating (simple cleaning without oiling or degreasing-contamination of the plating solution components and surface contamination that may occur during the plate passing process from the plating section to the phosphate processing section) Since the products (steel plates) that have been subjected to the phosphate treatment (to the extent that they are removed) can be provided to automobile manufacturers, oxidation over time, including the distribution process of the steel sheets, can be prevented, and it is excellent due to the synergistic effect of the plating layer and the phosphate film. It can be an unprecedented automobile steel sheet that has both corrosion resistance and excellent paintability.

Further, according to the method for producing a surface-treated steel sheet of the present invention, the pH of the plated surface is adjusted to pH 1 before forming the phosphate film.
By performing the etching treatment with the acid solution of Nos. 3 to 3, the corrosion resistance of the steel sheet can be further improved. The present inventors consider the reason for this as follows.

A distribution of Cr is recognized on the surface portion of the plating, and active and inactive portions are locally present depending on the presence or absence of Cr. When the plating surface is subjected to an etching treatment with an acidic solution in this state, Zn in the active portion of the plating surface is selectively eluted and the steel sheet surface becomes relatively Cr-rich, increasing the uniformity of the activity of the plating surface. In addition, the surface of the plating may be locally oxidized due to factors such as uneven washing with water when the plating is formed.In this case, the surface of the plating is etched to dissolve the oxidized portion and to improve the activity of the surface of the plating. It can be made uniform. As a result, the chemical conversion treatment property of the plating surface can be stabilized. The term "chromium rich" means a state in which the Cr content of the coating is higher than the average.

As described above, the etching treatment is intended to improve the activity uniformity of the plating surface before the chemical conversion treatment,
The degree of etching treatment is so slight as to promote inactivation of the steel sheet surface after chemical conversion treatment.

Further, a portion where Zn can be eluted still remains on the plated surface even after such an acidic liquid treatment, and the vicinity of this portion is a zinc phosphate crystal in the subsequent phosphate treatment step. And consequently the corrosion resistance of the steel sheet is enhanced. In addition, the gap portion of the phosphate crystal is further strengthened in the phosphate treatment step, and as a result, the entire surface becomes inactive.

The acidic solution used here is not particularly limited as long as it has a pH of 1.0 to 3.0, and an aqueous solution of sulfuric acid, oxalic acid, hydrochloric acid or the like may be used.
Alternatively, a plating solution may be used. Furthermore, metal ions that can be substitutionally deposited on the film during etching may coexist in the acidic solution. If the pH of the acidic solution is below 1,
Since the etching effect is very strong and the amount of plating solution is large, not only the corrosion resistance is deteriorated but also it is economically disadvantageous. When the pH of the acidic solution is higher than 3,
The effect of etching the surface is insufficient, and the effect of improving corrosion resistance is small. Further, as a method of etching treatment, dipping or spraying can be used.

As described above, Zn-- is applied to the surface of the steel sheet by electroplating using a line having electroplating equipment.
With Cr-based composite plating, without oiling or winding
A surface-treated steel sheet having excellent corrosion resistance and paintability can be stably and efficiently produced by etching the surface with an acidic solution to make it Cr-rich and further performing a phosphate treatment by a spray or immersion method. As the steel plate, any steel plate such as a corrosion resistant steel plate, a mild steel plate and a high tensile steel plate may be used.

[0029]

EXAMPLES Examples of the present invention will be described below together with comparative examples. First, the procedure for creating a sample of a surface-treated steel sheet will be described. The total amount of Zn ²⁺ ions and Cr ³⁺ ions was 0.1.
A plating solution prepared by adding an organic brightening agent to a sulfuric acid acidic plating bath having a pH of 2.0 containing 3 to 1.5 moles and containing 0.1 ⁺ to 0.5 moles of Na ⁺ ions is prepared. A main plating layer is formed on the cold-rolled steel sheet with a current density of 60 to 100 A / dm ² . At this time, the coating weight can be adjusted to a predetermined value by changing the plating time, and the composition of the coating can be changed by changing the current density and the ion concentration ratio of Zn and Cr in the plating bath. In addition, the Cr content of the obtained film is determined by the Zn, C of the solution in which the film is dissolved.
It is measured by analyzing the r concentration. A sample having a plating layer containing no Cr was also prepared as a comparative sample.

The steel sheet on which the plating film has been formed in this manner is immersed in a plating solution (pH 2.0) or an acidic solution such as a sulfuric acid aqueous solution or a hydrochloric acid aqueous solution having a pH of 1.0 to 3.0 for a predetermined time (1 second to 15 seconds). ) It is immersed and the plating layer on the surface of the steel sheet is etched.

Further, the plating film thus obtained is subjected to a dipping-type and spray-type phosphate treatment to form a phosphate film on the upper layer of the plating film. This phosphate treatment is performed as soon as possible after adjusting the plating film,
A commercially available phosphating solution was used, and pretreatments such as degreasing and surface conditioning were also performed under standard conditions. The thickness of the phosphate coating was controlled by the free acid concentration, total acid concentration, and treatment time. Furthermore, the film thickness of the produced phosphate film was determined by dissolving the film with a chromic acid or ammonium dichromate solution and measuring the weight difference before and after the dissolution. Further, the above-mentioned comparative sample was also subjected to the phosphate treatment.

The following tests were carried out on various steel plate samples prepared under the above conditions. (1) Corrosion resistance test As for the corrosion resistance, the corrosion resistance was evaluated by a sample that assumed a steel plate joining portion. The sample assuming the steel plate mating part is a steel plate of 150 × 70 mm which has been subjected to chemical conversion treatment after plating and 100 × 5.
A 0 mm steel plate was spot-welded with the phosphate-treated surfaces facing each other, and then subjected to chemical conversion treatment and electrodeposition coating. Then, the sample was supplied to the corrosion acceleration test shown below, taken out after a predetermined cycle, disassembled, and the maximum corrosion depth on the plated surface was measured.

Corrosion Acceleration Test [50 ° C. 85% RH → 50 ° C. 30% RH → Left at room temperature → 5
% NaCl water immersion] as one cycle 24
Time was set so that it would be time.

Corrosion resistance evaluation of mating part Maximum perforation depth after 150 cycles less than 0.1 mm ○ 0.1 or more and less than 0.2 mm △ 0.2 or more and less than 0.4 mm × 0.4 mm or more × × (2) Paintability and post-painting corrosion resistance test Regarding paintability, coating film adhesion was evaluated.

The coating of the steel sheet was formed by performing electrodeposition coating, intermediate coating and top coating on the steel sheet sample which was further subjected to the chemical conversion treatment in order to simulate the automobile manufacturing process.
The electrodeposition coating was 20 μm, and the total film thickness of the intermediate coating and the top coating was 90 μm in all samples.

The adhesion of the coating film was measured at 40 ° C. for the sample after coating.
After being kept in pure water for 240 hours and immediately taken out, a cutter was used to scratch 2 mm square × 100 squares, and then the tape was immediately peeled off, and the number of peeled squares was evaluated. .

No tape peeling ○ Less than 5 pieces △ More than 5 pieces × Corrosion resistance after coating is 150 × 70 mm, and a cross-cut is put into the above-mentioned sample after coating with a cutter. The width was measured.

Corrosion acceleration test [50 ° C. 85% RH → 50 ° C. 30% RH → room temperature standing → 5
% NaCl water spray] as one cycle and one cycle 24
Time was set so that it would be time.

Maximum blister width on one side after 120 cycles Less than 1 mm ○ ○ Less than 1 to 2 mm ○ Less than 2 to 5 mm △ 5 mm or more × (3) Pressability Pressability was evaluated by a draw bead test.

The draw bead test, as shown in FIG.
A sample piece 6 having a predetermined width is sandwiched between a male die 2 (bead 2) having a convex portion and a female die 4 facing this, and the bead 2 and the female die 4 are pressed against each other with a predetermined pressure. The sample piece 6 was pulled upward by 100 mm at a speed of 200 mm / min. After that, the sample piece 6 is removed from the die, the oil on the surface is removed with a solvent, and the weight of the sample piece 6 is measured. The difference between the weight of the sample piece 6 measured before the test and the bead 2
The amount of film peeling per unit area was calculated from the area that passed through and compared.

Evaluation of peeling amount 0 or more and less than 1.5 g / m ² ○ 1.5 g / m ² or more and less than 2.5 g / m ² Δ 2.5 g / m ² or more and less than 3.5 g / m ² × 3.5 g / M ² or more XX (4) Weldability Using a DR type electrode with a tip diameter of 6 mm, a pressure of 200 kg,
Spot welding was performed under the conditions of an energization time of 12 cycles and a welding current of 11 KA, and the evaluation was performed according to the following criteria.

No continuous welding of electrode tip at 500 points ○ Welding occurred at less than 500 points × The results of the above-mentioned tests are shown in Tables 1 to 4. As shown by the test results, the samples of the surface-treated steel sheet of the present invention all have excellent physical properties, but the samples of Comparative Examples that deviate from the scope of the present invention have coating properties, corrosion resistance, workability, etc. It is understood that in this respect, it is inferior to the surface-treated steel sheet of the present invention.

Further, according to the surface-treated steel sheet of the present invention, not only excellent corrosion resistance and paintability can be obtained, but also in the future, the phosphate treatment step in the body manufacturing process of an automobile manufacturer can be omitted. It becomes possible and the manufacturing process can be rationalized. The present invention is not limited to the above-described embodiments, but can be variously modified within the scope of the invention.

[0044]

[Table 1]

[0045]

[Table 2]

[0046]

[Table 3]

[0047]

[Table 4]

[0048]

As described above, according to the surface-treated steel sheet of the present invention, after the Zn-Cr composite plating layer is formed on the surface of the steel sheet, the plating surface is subjected to etching treatment with an acid solution to be Cr-rich, Then, a phosphate coating is provided on the surface of the plating coating.

Therefore, it is possible to improve the corrosion resistance in places where coating is difficult to reach, such as the joints of steel plates, and to suppress the formation of Cr oxides due to the time-dependent oxidation after plating.
A surface-treated steel sheet having good coating film adhesion and corrosion resistance can be provided.

[Brief description of drawings]

FIG. 1 is a schematic view showing a draw bead test apparatus.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toyofumi Watanabe 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd.

Claims (3)

[Claims] 1. A Zn-Cr composite plating layer formed on the surface of a steel sheet, containing 1 to 25% by weight of Cr, and applied at a plating amount of 5 to 40 g / m ² , and on this composite plating layer. A phosphate coating formed with a coating amount of more than 0.5 and 4 g / m ² or less, and the composite plating layer has an interface with the phosphate coating of Cr.
A surface-treated steel sheet excellent in corrosion resistance and paintability, which is characterized by being rich. 2. The composite plating layer is made of Fe, Ni, Co.
The surface-treated steel sheet according to claim 1, characterized in that at least one of them is contained in a total amount of 0.01 to 3% by weight. 3. A plating step of forming a Zn—Cr composite plating layer on the surface of a steel sheet, an etching step of etching the composite plating layer with an acid solution having a pH of 1 to 3, and a phosphorus step on the etched composite plating layer. A method of producing a surface-treated steel sheet excellent in corrosion resistance and coatability, comprising: a phosphate treatment step of performing a salt treatment to form a phosphate coating.