JPH07166366A - Production of zn-ni alloy plated steel sheet excellent in chemical convertibility and press formability - Google Patents

Abstract

PURPOSE:To produce a Zn-Ni alloy plated steel sheet excellent in chemical convertibility and press formability by plating at least either face of a steel sheet with a Zn-Ni alloy having a specified compsn.and thereafter bringing it into contact with an acidic soln. contg. phosphoric acid ions and citric acid ions. CONSTITUTION:At least either face of a cold rolled steel sheet is electroplated with a Zn-Ni alloy having a compsn. contg., by weight, 10 to 17% Ni, and the balance Zn. Next, the Zn-Ni alloy plated steel sheet is subjected to spraying treatment by an acidic soln. of pH 3 to 4 contg. sodium primary phosphate or potassium primary phosphate and sodium secondary phosphate or potassium secondary phosphate, contg. at least either H2PO4 ions and HPO<2->4 ions, furthermore added with sodium citrate or potassium citrate and contg. C6H5O<3->7 ions by >=10g/l. The Zn-Ni alloy plated steel sheet excellent in chemical convertibility and press formability can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a Zn-Ni alloy-plated steel sheet excellent in chemical conversion treatment property and press formability.

[0002]

2. Description of the Related Art Zn-Ni alloy plated steel sheets have the same adhesion amount.
Since it has 5 to 10 times more corrosion resistance than Zn-plated steel sheet,
As a steel plate for preventing early corrosion of automobile bodies by road anti-icing materials in winter, which has been a problem in recent years, its use amount has increased, and its use parts have been widely varied from trunk fenders to bonnets / trunk lid outers.
Also, the plated steel sheet used is a single-sided plated steel sheet for the rust-preventing effect (anti-perforation effect against piercing) in the interior of the car body when the plated steel sheet was in the early stages of use, such as pebbles. We are shifting to double-sided plated steel sheets aimed at improving corrosion resistance (preventing external rust) after the coating film is peeled off after bouncing.

However, the phosphate crystals formed when chemical conversion treatment is performed on the Zn-based electroplated steel sheet are hopite.
e, Zn ₃ (PO ₄ ) ₂・ 4H ₂ O) This hopite forms on the cold-rolled steel sheet (Phosphophylite, Zn)
_{To 3} Fe (PO ₄₎ as compared to ₃ · 4H ₂ O adhesion of the paint is inferior, its application area automotive inner surface has a problem in automotive applications the outer surface side mainly.

On the other hand, in the case of a single-sided plated steel sheet, the plated surface is often the inside of the punch during pressing, in other words, the cold-rolled surface is the overhanging surface during pressing, so the plated surface has an effect on press formability. However, in the case of double-sided plating, there was a problem in that the plated surface became an overhanging surface during press working, and the friction of the plated surface itself was large.

As a method for solving these problems, Zn-Ni
In order to improve the chemical conversion treatment of plated steel sheet, Zn-Ni plating is applied as disclosed in Japanese Patent Publication No. 2-56437, and then Zn in the plating layer surface is treated by treatment in a plating solution or an acid solution.
There is a method in which the chemical conversion reaction is made uniform by obtaining a Ni-etched surface by preferentially dissolving the components to obtain a dense chemical conversion film.

However, when it is immersed in the plating solution, the pH of the plating surface rises due to Zn elution on the plating surface. As a result, Zn hydroxide is generated on the plating surface and remains on the plating surface. This can be removed by washing with water or brushing after plating, but if washing with water or brushing is insufficient, it causes unevenness. In addition, when treated in an acid solution such as sulfuric acid, Zn dissolves in the plating surface layer and becomes slightly Ni-rich, but cracks occur in the plating and Zn in the plating layer
Since it also dissolves, the Ni content in the plating layer also increases and the powdering property deteriorates. Further, as disclosed in Japanese Patent Application Laid-Open No. 58-210194, after the plating, cathodic treatment, cathodic → anodic treatment, anodic → cathodic treatment, anodic treatment, and dipping treatment are performed to remove oxides on the surface of electroplating, thereby eliminating the interface. There is a method of improving chemical conversion treatability by removing fixed and semi-amorphous oxides and hydroxides by reduction or dissolution. By this method, the plating surface becomes a surface free of hydroxide and oxides.
In addition to the alloy phase, electrochemically base Zn remains, so that microcells may form during the chemical conversion treatment, resulting in unevenness.

On the other hand, as a method for improving the press formability of a Zn-Ni plated steel sheet, as disclosed in Japanese Patent Laid-Open No. 141894/1985, Zn-Ni alloy plating is applied in two layers to form an upper Ni layer. There is a method of making the content higher than that of the lower layer Ni and a method of adhering P to the plating surface layer as disclosed in JP-A-1-234592. In JP-A-60-141894, the press forming property is improved by utilizing the powdering of the upper layer of the plating, so that when a large number of samples are continuously formed, the peeled powder adheres to the press die. It is not practical because it causes star's eye. Further, in JP-A-1-234592, although the pressability is improved by the adhesion of P, η is formed on the surface of the plating film before the treatment.
When the processing time is short, there is a difference in the amount of P adhering, and the effect of improving pressability becomes uneven on the plated surface and the appearance is poor due to chemical conversion treatment etc. Becomes Further, if the pH of the solution is less than 4, the dissolution of Zn in the plating layer becomes violent, the amount of coating adhesion decreases greatly, and partial overetching occurs and the appearance of plating and chemical conversion treatment becomes poor.

[0008] Recently, good deep-drawing steel sheets made of ultra-low carbon steel have been widely used for automobile bodies, and press working on steel sheets has become complicated and strict. As a result, higher press formability is required for plating than ever before, and the above-mentioned technology has a limit in dealing with it.

[0009]

SUMMARY OF THE INVENTION The present invention is based on the conventional Zn-
The press formability is far better than that of Ni-plated steel sheets, and even and uniform chemical conversion treatment surface can be obtained, and the post-treatment time after plating can be shortened to improve productivity. It is an object of the present invention to propose a method for producing a Zn-Ni alloy plated steel sheet that can be used.

[0010]

Means for Solving the Problems The present inventors have focused on chemical conversion treatability and press formability as performance when using a Zn-Ni alloy plated steel sheet as an outer panel for automobiles, and the above-mentioned performance is compatible and production The present invention has been achieved as a result of intensive research and study on a short-time post-treatment method for a Zn-Ni plated steel sheet for improving the property.

That is, the present invention contains at least one of H ₂ PO ₄ ⁻ ions and HPO ₄ ²⁻ ions after applying a Zn—Ni alloy plating having a Ni content of 10 to 17 wt% to at least one surface of a steel sheet. , And contains 10g / l or more of C ₆ H ₅ O ₇ ³⁻ ion
Zn having excellent chemical conversion processability and press formability, characterized in that a solution having a pH of 3 or more and 4 or less is brought into contact with the plated surface
-Ni alloy plated steel sheet manufacturing method.

[0012]

The present invention will be described in detail below. The Zn-Ni plated steel sheet has a large amount of pure zinc, zinc oxide, zinc hydroxide, etc. on the plating surface, and the reactivity on the plating surface is different. If the chemical conversion treatment is performed in this state, the processing becomes uneven. Therefore, in the present invention, pure zinc, zinc oxide, and zinc hydroxide on the plating surface are removed, and the reactivity on the plating surface is made uniform, and at the same time, P, which is the core of the chemical conversion treatment, is uniformly attached to the plating surface. . As this method,
The contact with a solution containing at least one of H ₂ PO ₄ ⁻ ion and HPO ₄ ²⁻ ion and containing C ₆ H ₅ O ₇ ³⁻ ion was examined. However, when treated with the above-mentioned solution having a pH of more than 4, the effect was insufficient in a short time treatment and unevenness might occur after the chemical conversion treatment. On the other hand, when treated with the above-mentioned solution having a pH of 4 or less, the effect was confirmed to be stable even after a short time treatment.

The difference in the effect depending on the pH is considered as follows.
To be This means that most of the pure zinc and acid
For highly reactive parts such as zinc oxide and zinc hydroxide,
It is thought that it will be removed by reacting with hydrogen ions in the treatment liquid.
Efficient with a treatment liquid with a high hydrogen ion concentration of pH 4 or less
This is because the reaction proceeds. However, H₂PO_Four ^-
Ion and HPO_Four ^2-Contains at least one of the ions, C₆
H_FiveO₇ ^3-Low pH when treated with a solution containing no ions
Over-etching or plating that partially dissolves the plating
A severe crack occurred in the. On the other hand, H₂PO_Four ^-
Ion and HPO_Four ^2-Contains at least one of the ions C₆H_FiveO₇
^3-When treated with a solution containing more than 10 g / l of ions
The defects were eliminated and a good treated steel sheet could be manufactured.

The mechanism of the effect of this C ₆ H ₅ O ₇ ³⁻ ion was estimated as follows. That is, it is presumed that C ₆ H ₅ O ₇ ³⁻ ions combine with dissolved Zn at the plating interface to form zinc citrate. Zinc citrate is insoluble in water but soluble in dilute acids and alkalis. The pH at the steel sheet interface where hydrogen ions were consumed by the dissolution of zinc becomes a liquid in the neutral range, and the zinc citrate thus produced becomes insoluble and is adsorbed at the steel sheet interface.
Dissolution of Zn is suppressed in the adsorbed portion. After that, when a fresh treatment liquid having a pH of 4 or less arrives, zinc citrate, which is dissolved in the dilute acid, is dissolved, but since it is adsorbed, partial etching is suppressed and uniform etching is performed. In other words, it was presumed that excessive solubilization of Zn was suppressed by the formation of zinc citrate in the solubilized portion where the solubilization occurred vigorously in this reaction mechanism. And in order to maintain this effect stably, C ₆
H ₅ O ₇ ³⁻ ion needs to be 10 g / l or more.

On the other hand, when the contact time with the treatment liquid is shorter than 1 second, pure zinc, zinc oxide, and zinc hydroxide are not sufficiently removed, and the reactivity of the plating surface becomes uneven, resulting in uneven chemical conversion treatment. Is not preferable. Further, if it is longer than 10 seconds, the plating will be severely cracked, which will cause powdering and the operation will reduce the line speed, which is not economical. Further, by the contact treatment with the solution, P, which is the nucleus of the chemical conversion treatment reaction, is uniformly dispersed and attached. Therefore, the chemical conversion crystals formed by the chemical conversion treatment in the next step are uniform and fine. Further, in the plating surface layer, soft phases such as pure zinc, zinc oxide, zinc hydroxide and the like are reduced, and since P is attached, lubricity is improved and press formability is improved.

[0016] While the forms of the P is not known why the chemical conversion treatability and press formability are improved H ₂ PO ₄ ^-
PH of 4 including at least one of ion and HPO ₄ ^2- ion
The P compound formed on the surface due to the synergistic effect of Ni adhesion on the outermost plating surface and P adhesion by the contact with the following solution becomes a P compound mainly composed of Ni (for example, becomes a Ni-P alloy coating) It is considered that the hardness of the surface layer is increased. Any contact method such as dipping or spraying may be used. That is, the preferable treatment time for obtaining good chemical conversion treatment property and press moldability is 1 second or more and less than 10 seconds.

The temperature of the treatment liquid of the present invention is not limited, but at 40 ° C. or lower, its effect is not sufficiently exhibited in a short time treatment of less than 5 seconds. The temperature is preferably 40 ° C. or higher in order to improve productivity.

[0018]

[Example] Using a cold rolled steel plate equivalent to SPCC with a plate thickness of 0.7 mm,
Plated. The plating conditions were as follows. Zn-Ni plating bath composition _{ZnSO 4 · 7H 2 O: 130} g / l NiSO 4 · 6H 2 O: 250 g / l Na 2 SO 4 ·: 40 g / l pH: 1.5 Bath temperature: 60 ° C. Current density: 80A / Dm ² Plating time: 10 seconds Plating coverage: 30 g / m ² (Ni content 12.5%) Post-treatment liquid composition Sodium monophosphate or potassium phosphate 100 g / l
0% of the above and dibasic sodium phosphate or dibasic potassium phosphate
Mix in the range of ~ 60 g / l and add sodium citrate or potassium citrate in the range of 0-80 g / l, Table 1
The solution shown in was prepared.

Immediately after plating, a spray treatment was performed with the treatment liquid of Table 1 immediately after the plating. Then, it was washed with water and dried, and the chemical conversion treatability and press workability were investigated. The results are shown in Table 2. To determine the chemical conversion treatability, visual observation and electron microscopic observation were performed. In the crystal investigation, scanning electron microscope observation was performed randomly, and evaluation was made based on uniformity, crystal compactness, and the like. The evaluation criteria of the chemical conversion film at this time are as follows.

Visual evaluation of chemical conversion film 5: Appearance is uniform and no unevenness 4 ... Appearance is very uniform although there is very thin unevenness 3 ... Appearance is partially uneven 2 ... Appearance is uneven over the entire surface 1… Appearance has extremely severe unevenness Chemical conversion treatment crystal survey A… Uniform and dense at 10 locations B… Slightly non-uniform crystals were observed, but dense C… 1 to 1 mixed particles of fine and coarse particles D: Observed at 4 spots: Mixed particles of fine grains and coarse grains were observed at 5 spots or more. For press formability, the limit drawing ratio (LDR) in a cylindrical deep drawing tester was investigated. As the lubricating oil in the measurement, Daphne oil coat Z5 (trade name) manufactured by Idemitsu Kosan Co., Ltd., which is used as a rust preventive oil, was used in all cases.

For the slidability examination, the non-surveyed surfaces were stuck together as shown in FIG. 2, and the pull-out load was measured so that the examined surface became the sliding portion. The pressing pressure was 80 kg. OT bending survey (bending the plated steel plate to 180 degrees and then peeling the cellophane tape) as a survey of the powdering property of the plating
Then, the quality of the powdering property was judged by the plating powder on the tape.

○: No peeling △: Slight peeling × ... Large peeling

[0023]

[Table 1]

[0024]

[Table 2]

The effect of citrate ions on overetching is shown in FIG. The over-etching was evaluated by visually observing the presence or absence of dot-like defects after plating and by observing with a SEM the presence or absence of a partially dissolved portion. Treatment condition: After plating, treatment was performed for 5 seconds using solutions B, C, D, H and I in Table 1. ○… Both visual and SEM observations are good. △… Visual observation is good. ・ SEM observation shows dissolved parts.

[0026]

EFFECTS OF THE INVENTION According to the present invention, a Zn-Ni plated steel sheet having excellent chemical conversion treatability and press formability can be produced by a short post-treatment after plating.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the amount of citrate ion added and overetching.

FIG. 2 is an explanatory diagram of a slidability test device.

Claims (1)

[Claims] 1. A H after Ni content on at least one surface of the steel sheet was subjected to a Zn-Ni alloy plating of 10~17wt% ₂ PO ₄ ^-
Ion and HPO ₄ ²⁻ ion and at least one of C ₆ H ₅ O ₇ ³⁻ ion having a pH of 3 or more and 4 or less is brought into contact with the plating surface. A method for producing a Zn-Ni alloy plated steel sheet having excellent processability and press formability.