JPH0920996A - Production of electroplated steel sheet excellent in corrosion resistance after working - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing an electroplated steel sheet excellent in plating adhesion and corrosion resistance after working required for an automotive use or the like. SOLUTION: This is a method for producing an electroplated steel sheet excellent in corrosion resistance after working in which electroplating is applied at 50 to 150A/dm<2> current density by using a plating bath obtd. by adding a plating soln. contg. Zn by >=0.5mol/l and in which the molar concn. ratio of Co/(Zn+Co) is regulated to 0.2 to 0.6 with colloidal silica having 7 to 60nm average grain size by 10 to 50g/l as silica and whose pH is regulated to 1.8 to 3.5. In this way, the sedimentation of silica in the plating bath can be prevented.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a method of manufacturing a superior Zn-Co-SiO ₂ electroplated steel sheet corrosion resistance after processing.

[0002]

2. Description of the Related Art Automotive steel sheets are required to have good pressability, that is, excellent workability, and good adhesion between the steel sheet and paint, and high corrosion resistance. At present, Zn-based plated steel sheets such as Zn plating, Zn-Ni alloy plating, and Zn-Fe alloy plating are used as steel sheets for automobiles in order to satisfy these required performances. However, it cannot be said that the conventional plated steel sheet sufficiently satisfies the required corrosion resistance performance.

Therefore, a surface-treated steel sheet having higher corrosion resistance was developed, and Zn-Mn plating or Zn-Cr was developed.
A plated steel sheet in which the type of alloying element added to Zn plating such as plating is changed is being studied. Further, in addition to Zn alloy plating, Zn plating or composite plating in which oxide particles are dispersed and co-deposited on Zn alloy plating has been studied, and high corrosion resistance performance is expected.

For example, Japanese Examined Patent Publication No. 56-49999 describes that the Zn plating layer can have a corrosion resistance of about 1.5 to 3 times that of a pure Zn plated steel sheet by dispersion plating containing ₂ to 15% of SiO _2. ing. In Japanese Examined Patent Publication No. 62-6760, Fe ²⁺ , C is added to oxide particles having a particle size of 1 to 100 nm.
A method of performing dispersion plating by adsorbing o ²⁺ and Ni ²⁺ is disclosed, and in the publication, Zn-10% Ni-10% is disclosed.
It is stated that a steel sheet having a SiO ₂ plating composition exhibits about twice the corrosion resistance as a steel sheet having a Zn-13% Ni plating composition. Furthermore, in Japanese Patent Publication No. 5-5911, Zn-S
A method of using a plating bath in which silica particles of 100 nm or less and NO ₃ ⁻ ions are added for producing an iO ₂ plated steel sheet is disclosed, and by containing approximately 5% or more of SiO ₂ , Zn-13% Ni plating is disclosed. It has been shown to have about three times the corrosion resistance of steel plates.

As described above, the corrosion resistance is improved by co-depositing various oxides. However, as a method of co-depositing an oxide, Japanese Patent Publication No. 56-49999 discloses that in order to improve the corrosion resistance, SiO ₂ is added in the plating bath.
Is required to be added at a high concentration of 50 to 200 g / l. In a plating bath containing a high concentration of oxide, the solution viscosity increases and the plating solution flow rate decreases, causing insufficient diffusion of ions at the plating interface, causing plating burns, and agglomeration of oxides due to continuous plating production. However, there is a problem in that not only the production problems but also the expected corrosion resistance cannot be obtained because the precipitation state and the distribution state become non-uniform due to the occurrence of precipitation and the lack of stability of the bath.

The addition of a large amount of oxide to such a plating bath is due to the fact that the surface of the oxide is negatively charged, so that a small amount causes less eutectoid oxide and a smaller effect of improving corrosion resistance. . Therefore, in Japanese Patent Publication No. 62-6760, Z
A method has been developed in which an oxide that adsorbs only Fe ²⁺ , Co ²⁺ , and Ni ²⁺ without adsorbing n ²⁺ is added to facilitate co-deposition during plating. Certainly, in this case, the effect of improving the corrosion resistance appears from the low concentration of SiO ₂ of 10 g / l, but when the concentration of SiO ₂ in the bath is low, the improving effect is small and the corrosion resistance is Zn-13% Ni plated steel sheet. Is about 1.1 times. In order to secure double the corrosion resistance of Zn-13% Ni plated steel sheet, the SiO ₂ concentration must be as high as 150 g / l, and as in the above case, the stability of the bath and the precipitation form and distribution state There were problems such as unevenness.

Further, SiO_TwoNO as a co-deposition aid_Three ^-
In Japanese Patent Publication No. 5-5911, the addition of
SiO_TwoThe addition amount of 5g / l is effective from a very low concentration.
Yes, when 10 g / l is added, SiO_TwoTo 1.6%
Contains, and shows 20 times the corrosion resistance of pure Zn plated steel sheet
Is written. Like this_Three ^-Is SiO_TwoFor eutectoid
Plays a major role, but at the same time reduces plating adhesion
There was a problem in plating performance other than the corrosion resistance. Further
As can be seen from the examples, the applied current density is 50 A.
/ Dm^TwoBelow, used in electroplating operations
100 A / dm ^TwoThe above current density causes plating burns.
Problem.

[0008]

DISCLOSURE OF THE INVENTION The object of the present invention is to solve the difficulties in the manufacturing process in the conventional dispersion plating technique and the problems in the quality of the steel sheet to be obtained, and to achieve the plating adhesion required for the steel sheet for automobiles and the like. To provide a method for producing an electroplated steel sheet which is excellent in corrosion resistance and corrosion resistance after working.

[0009]

When used for automobiles or household appliances, plated steel sheets are often processed. In this case, the corrosion resistance of the flat plate is good, but the corrosion resistance may be poor in the processed state, and therefore the corrosion resistance in the processed state is important. Therefore, as the plating performance of the plated steel sheet produced in the present invention, the plating adhesion is good and the corrosion resistance after processing is excellent, and there is no problem such as aggregation, precipitation or gelation of the dispersant in the production process of the plated steel sheet. The manufacturing method of the plated steel sheet was examined.

That is, first, in the development of the present steel sheet, the type of base plating was investigated. As base plating, Zn plating, Zn-Ni plating, Zn-C
O plating and Zn-Fe plating were selected. Next, prototypes of plated steel sheets were made in plating baths combining plating bath concentrations, concentration ratios, and plating conditions such as base plating species and plating solution pH, and corrosion resistance after processing was investigated.
The present invention has been reached.

That is, according to the present invention, a plating bath containing Zn in an amount of 0.5 mol / l or more and a Co / (Zn + Co) molar concentration ratio of 0.2 to 0.6 has an average grain size of Colloidal silica having a diameter of 7 to 60 nm is used as silica 10
Electroplated steel sheet having excellent post-working corrosion resistance, characterized in that electroplating is performed at a current density of 50 to 150 A / dm ² using a plating bath having a pH of 1.8 to 3.5 added in an amount of -50 g / l. Is a manufacturing method.

[0012]

[Operation] The present invention will be described in detail below. The molar ratio of Co (Co / Zn + Co) in the plating solution is 0 to 0.6.
It was found that silica co-deposited in the plating layer in the range of. When the Co molar ratio is 0.65 or more, silica is hardly codeposited in the plating layer, and Zn-Co having the same Co content is obtained.
Corrosion resistance performance equivalent to that of plated steel sheets. When Co is not codeposited in the plating layer, the effect of improving the corrosion resistance is small, and in order to improve the corrosion resistance, the Co molar ratio is 0.2 to
It is preferably set to 0.6. More preferably 0.3 to
It is preferably 0.6, and more preferably 0.4 to 0.55. The reason for this is not clear, but in the plating layer produced in this Co concentration range, silica is easily co-deposited in the plating layer, and further Co is co-deposited. It is estimated to improve the corrosion resistance of.

Regarding the pH of the plating solution, 1.8 to 3.
It is preferably set to 5. More preferably 1.8-3.
2, and more preferably 2.0 to 3.0. When the pH is less than 1.8, the amount of silica co-deposited in the plating layer is small, and thus the effect of improving the corrosion resistance is small. Also pH
If it exceeds 3.5, silica may gelate when the plated steel sheet is continuously produced, which is not preferable in operation.

Regarding the current density, 50 to 150 A / d
It is better to manufacture with m ² . At a current density of more than 150 A / dm ² , plating burns occur and the plating adhesion deteriorates. The cause of this plating burn is not clear, but the silica present at the steel sheet interface during electrodeposition agglomerates due to the rapid pH rise due to the rapid generation of hydrogen due to the high current density, and prevents the diffusion of ions, causing the diffusion of ions to the steel sheet. Is the rate-determining factor, and it is presumed that galling occurred. Further, from the viewpoint of the operability of electroplating, operability at a current density of less than 50 A / dm ² is not preferable because it lowers the productivity.

The plating bath temperature is preferably 40 to 65 ° C. If it is less than 40 ° C, uneven appearance may occur,
If it exceeds 65 ° C, the evaporation of the liquid becomes severe and the amount of water replenished increases, which is not preferable. The Zn concentration in the plating bath is 0.5 mol / l or more. When the Zn concentration is less than 0.5 mol / l, the electrodeposition efficiency decreases, and when the current density changes even when the bath composition is set to an appropriate bath composition, the Co content in the plating layer changes greatly. However, it is not preferable because the corrosion resistance after processing may be poor.

The Zn concentration in the plating bath is preferably 2.0 mol / l or less, more preferably 1.3 mol / l or less. When it exceeds 2.0 mol / l, the solubility of Zn sulfate or Co sulfate decreases during ion supply, and not only the corrosion resistance after processing decreases but also Zn or Co is added as a component when the plating bath temperature decreases. This is because the crystals to be crystallized out.

The average particle size of silica is preferably 7 to 60 nm. If the average particle size is less than 7 nm, silica gels in the plating solution in the plated steel sheet manufacturing process, which is not preferable in operation. The reason for this is that as the average particle size becomes smaller, the stability against pH decreases, and when the gelling pH becomes lower than the pH at which zinc hydroxide is formed, which is 5.25, silica gels before reaching the cathode interface. It is presumed that the gelled silica returns to the plating bath more than the silica taken into the plating layer. Also, 60n
If it exceeds m, silica may precipitate in the plating solution, and if the plating solution is left for a long time, it may precipitate and aggregate at the bottom of the tank, and silica may not be dispersed in the plating solution at the start of operation. This is because.

As the method for measuring the average particle diameter of the silica, the specific surface area average diameter obtained from the measured value of the specific surface area by the BET method and the particle density can be used.
Further, the amount of silica added is preferably 10 to 50 g / l, more preferably 22 to 50 g / l. When it is less than 10 g / l, the amount of silica co-deposited is small and the effect of improving the corrosion resistance is small. On the other hand, when it exceeds 50 g / l, the viscosity of the plating solution becomes high and plating burns or partial non-uniform eutectoid may occur, which is not preferable because the original plating performance cannot be exhibited.

[0019]

[Example] Alkaline electrolytic degreasing of cold rolled steel plate (SPCC)
Then, after pickling and washing with water, the obtained steel sheet is used.
Zn-Co-S under the plating bath composition and plating conditions shown in
iO _TwoA plated steel plate was prototyped. In addition, sulfuric acid is used in the plating bath.
Conductivity aid using a sulfuric acid bath with zinc and cobalt sulfate added
As an additive, 0 to 100 g / l of sodium sulfate was added. Sa
In addition, the following A, B and C were used as silica.

A. Colloidal silica: manufactured by Nissan Chemical Industries, Ltd., ST-O (average particle diameter of silica is 12 nm) B.I. Colloidal silica: manufactured by Nissan Chemical Industries, Ltd., ST
-OL (average particle diameter of silica: 42 nm) C.I. Colloidal silica (comparative example): Nissan Chemical Industries, Ltd.
Co., Ltd., ST-OXS (average particle size of silica is 5 nm) Further, for comparison, Zn-N with Zn-Ni as the base plating is used.
i-SiO ₂ plated steel plate and base plating are Zn-F
Table 2 and Table 3 show the plating bath composition and plating conditions in the case of the Zn-Fe-SiO ₂ plated steel sheet of e.

In the case of a Zn-Ni-SiO ₂ plated steel sheet, zinc sulfate and nickel sulfate are used in the plating bath.
The plating bath in the case of n-Fe-SiO ₂ coated steel sheet were added zinc sulfate and ferrous sulfate. As a plating performance investigation, a plating adhesion test and a post-working corrosion resistance test were conducted on a steel sheet having a coating adhesion amount of 20 g / m ² by the following method.

[Plating adhesion test]
A so-called zero T bending adhesion test was conducted in which the processed portion was bent once and peeled off with a cellophane tape. As an evaluation standard, the amount of Zn peeled from the tape after peeling was visually determined and evaluated according to the following criteria. ◯: No plating peeling Δ: Slight plating peeling ×: Plating peeling [Corrosion resistance test] Fig. 3 shows a plated steel sheet sample for investigation of corrosion resistance after processing. 3A shows a perspective view of the sample, and FIG. 3B shows a cross-sectional view of the sample. Further, in FIGS. 3A and 3B, reference numeral 1 indicates a surveyed portion, 2 indicates an edge portion, 3 indicates a non-surveyed surface, and the hat height H is 25 mm in FIG. 3B.

In this sample, the obtained plated steel sheet was press-formed with a blank diameter of 90 mmΦ and a punch diameter of 50 mmΦ, and then the edge portion and the non-surveyed surface were brushed with a rust preventive pigment.
It was prepared by heating at 80 ° C. for 20 minutes. next,
The unpainted and processed sample shown in Fig. 3 is
By the combined cycle corrosion acceleration test method shown below, the state of red rust generation in the plated processed part without coating and after processing was investigated. As the evaluation criteria, the following 5 grades were evaluated.

The (Evaluation criteria) 5: Zn-Ni coating weight 40 g / m level ² of the plated steel sheet or 4: Zn-Ni is worse than plated steel coating weight 40 g / m ^2, and deposited Zn-Ni plated 30g / m ²
Above the level of galvanized steel sheet 3: Zn-Ni coating amount is poorer than galvanized steel sheet with 30 g / m ² and Zn-Ni coating amount is 20 g / m ²
Above the level of the galvanized steel sheet 2: more than the level of the cold-rolled steel sheet that is worse than the galvanized steel sheet with the Zn-Ni coating amount of 20 g / m ² and 1: the level of the cold-rolled steel sheet Conditions) Salt spray (35 ° C for 7 hours) → Dry (60 ° C for 3 hours) → Wet (50 ° C for 14 hours) Zn-Co-SiO ₂ plated steel sheet, Zn-Ni-SiO
_The performance test results of the _2- plated steel sheet and the Zn-Fe-SiO ₂ -plated steel sheet are shown in Tables 1, 2 and 3 together with the composition of the plating bath and the plating conditions.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

[Table 3]

In addition, Zn-Co without coating and after processing
-Situation of red rust on SiO ₂ plated steel sheet and Co in plating bath
The relationship with the molar ratio is shown in Fig. 1. Zn without coating and after processing
FIG. 2 shows the relationship between the occurrence of red rust on the —Co—SiO ₂ plated steel sheet and the pH of the plating bath. Further, FIGS. 1 and 2 also show the corrosion resistance in the case of Zn-Ni alloy electroplated steel sheets (Ni content rate 12 to 13%) having different adhesion amounts as comparative materials. In addition, in FIG. 1 and FIG.
Zn-Ni plating amount is 40, 30, 20 respectively
The level of the number of days until the occurrence of 50% red rust in the Zn-Ni alloy electroplated steel sheet in the case of g / m ² is shown.

Further, FIG. 4 and FIG. 5 show the fifth embodiment of Table 1.
Zn-Co-SiO ₂ plated steel sheet obtained under the conditions of
And Zn-Co-obtained under the conditions of Comparative Example 5 in Table 1.
The elemental analysis result of the depth direction of a steel plate by glow discharge emission spectrometry (GDS) of each SiO ₂ plated steel plate is shown.
From these results, according to the production method of the present invention, Co and silica are dispersed and co-deposited in the Zn plating layer, and it is possible to produce a plated steel sheet excellent in both corrosion resistance after processing and plating adhesion. .

[0030]

According to the present invention, Co is contained in the Zn plating layer.
Zn, which is co-deposited with silica and has excellent corrosion resistance after processing
-Co-SiO ₂ electroplated steel sheet is made possible to manufacture. Furthermore, according to the present invention, the precipitation of silica due to gelation in the plating bath can be prevented, and the problem in the manufacturing process can be solved.

[Brief description of drawings]

[Fig. 1] Zn-Co-SiO ₂ without coating and after processing
It is a graph which shows the red rust generation | occurrence | production state of a plated steel plate, and the relationship between Co molar ratio in a plating bath.

[Fig. 2] Zn-Co-SiO ₂ without coating and after processing
It is a graph which shows the red rust generation | occurrence | production state of a plated steel plate, and the relationship between plating bath pH.

3 (a) is a perspective view and FIG. 3 (b) is a sectional view showing a plated steel sheet sample for investigation of corrosion resistance after processing.

FIG. 4 Zn—Co—SiO obtained by the method of the present invention
₂ is a graph showing the results of elemental analysis in the depth direction of a plated steel sheet by glow discharge emission spectrometry of the ₂ plated steel sheet.

FIG. 5: Zn—C when the conditions of the method of the present invention are not satisfied
by glow discharge optical emission spectrometry o-SiO ₂ coated steel sheet is a graph showing the results of elemental analysis of the plated steel sheet depth direction.

[Explanation of symbols]

 1 Survey section 2 Edge section 3 Non-survey surface

Claims (1)

[Claims] 1. The plating bath contains Zn in an amount of 0.5 mol / l or more, and the molar concentration ratio of Co / (Zn + Co) is 0.2 to.
A plating bath having a mean particle diameter of 7 to 60 nm was added to a plating solution of 0.6 in an amount of 10 to 50 g / l as silica, and a plating bath having a pH of 1.8 to 3.5 was used to obtain a current density of 50 to A method for producing an electroplated steel sheet having excellent corrosion resistance after working, which comprises performing electroplating at 150 A / dm ² .