JP2767650B2 - Galvanized steel sheet with excellent weldability, pressability and chemical conversion treatment - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a galvanized steel sheet having excellent weldability, pressability, and chemical conversion properties.

(Prior Art) As a method of improving the weldability of a galvanized steel sheet, for example, as disclosed in JP-A-51-110783,
An oxide film such as Al ₂ O _{3 is} formed on the surface of the plated steel sheet,
It has been proposed to utilize the high melting point and high electric resistance of the oxide to improve the weldability, prevent contact between the electrode chip and the plated metal, prevent the chip from being melted, and extend the life. Further, as shown in JP-A-59-104463, the ZnO / Zn ratio is increased by heating the surface of the plated steel sheet.
It has been proposed to generate an oxide film having a thickness of 0.1 to 0.70 and to improve the weldability as well.

However, even with such a method, satisfactory results are still difficult to obtain on an industrial scale, and there is a strong demand for improved weldability of plated steel sheets.

Further, as a method for improving the pressability of a galvanized steel sheet, for example, as described in JP-A-62-185883,
Galvanized steel sheet, such as a method of applying an electrolytic chromate treatment to the surface of a plated steel sheet to generate an oxide film of Cr ₂ O ₃ or a method of applying an iron-zinc alloy plating described in JP-A-62-192597. A method has been proposed in which a hard film is formed on the upper surface to prevent galling of the plating and the dies at the time of pressing to improve the lubricity of the press.

Further, JP-A-1-136952 discloses that the pressability is improved by applying or coating an organic substance such as an organic lubricating film or a lubricating oil on the surface of a plated steel sheet.

(Problems to be Solved by the Invention) However, products manufactured by such a method have the following inadequate points for use by automobile users and the like.

The outline of the process used by automobile users consists of the steps of cleaning the steel sheet with oil, pressing, degreasing, chemical conversion, and painting.In the case of electrolytic chromate-treated steel, the chemical conversion in the chemical conversion In the case of a steel sheet to which a film is not formed and to which a lubricating oil or a lubricating film is applied, the oil drops during the cleaning process, so that sufficient lubricating performance is not exhibited. Further, a load is applied to the degreasing step before the chemical conversion treatment, and the cost is increased. On the other hand, zinc-plated steel sheets that have been subjected to iron-zinc alloy flash plating have problems such as higher steel sheet costs than electrolytic chromate treatment, and are low-cost, capable of chemical conversion treatment, degreasing, etc. No load on the process of
The development of a galvanized steel sheet having excellent pressability is desired.

Further, as described above, there is a strong demand for a galvanized steel sheet having excellent chemical conversion treatment properties in combination with weldability and pressability.

(Means for Solving the Problems) The gist of the present invention is to generate a ZnO content of 30 to 3000 mg / m ² on the surface of a galvanized steel sheet, and form a P oxide of P on the surface thereof with a P oxide of 1 to 500 mg / m ² . 1 to 500 mg / Zn as Zn oxide
A zinc-coated steel sheet excellent in weldability, pressability, and chemical conversion treatment characterized by being coated with a mixed oxide composed of m ² and other oxides.

The galvanized steel sheet, which is the subject of the present invention, is produced by various manufacturing methods such as hot-dip plating, electroplating, vapor deposition plating, and thermal spraying.As a plating composition, other than pure Zn, Zn and Fe, Zn And Ni, Zn and Al, Zn and Mn, Zn and Cr, Zn and P
Containing one or more alloying elements and impurity elements for improving various functions such as corrosion resistance.

In addition, there are those in which ceramic fine particles such as SiO ₂ and Al ₂ O ₃ , oxides such as TiO ₂ , and organic polymers are dispersed in the plating layer. Alternatively, there is a layer whose composition changes. Also, a steel sheet coated with an organic film in which particles containing Zn as a main component are dispersed on these platings is also a target. For example, an alloyed hot-dip galvanized steel sheet obtained by heating and alloying a plating layer of a hot-dip galvanized steel sheet and iron along the ground, zinc and its alloys (eg, iron, nickel,
Alloy with chrome, etc.)
A steel sheet heated to 0 to 550 ° C. and alloyed with the base iron, not only a single alloy layer, but also, for example, a multi-layer alloy plated by electroplating, and SiO ₂ , Al ₂ O ₃ What dispersed ceramic particles such as above, on these plating layers,
Alternatively, there is a steel sheet in which an organic film containing zinc particles is directly coated on a steel sheet. As the form of the rust-preventive steel sheet, there are a double-sided plating, a single-sided plating, and a differently-plated steel sheet in which different platings are applied vertically.

The present inventors, regardless of the type of galvanized steel sheet, as long as the plating is mainly Zn, by forming ZnO on the surface of the plated steel sheet, Fe, Zn at the tip of the electrode tip in spot welding. It has been found that the electrode protection metal as a main component is generated, thereby greatly improving the life of the electrode tip.

In the above-mentioned conventional plated steel sheet, the ZnO-based oxide film has a ZnO content of 30 to 3000 mg / m which is considered to be excellent in weldability.
² (per side) Formation was unstable. Here, the oxide film mainly composed of ZnO means ZnO in the oxide,
For example, it may contain a compound such as a component element or an oxide thereof contained in the plating layer. Also,
In an electrochemical treatment such as anodization, a component or a compound contained in the treatment liquid may be contained.

In order to form an oxide film mainly composed of ZnO on the surface of a galvanized layer, the present inventors contact a steel sheet with an acid-containing oxidizing agent aqueous solution as a first method. It has been found that it is easy to produce 30 to 3000 mg / m ² (per one side) in a ZnO amount, and it is possible to provide a galvanized steel sheet having excellent weldability. The function of the acid is to dissolve some of the plating layer surface, supply ions such as Zn from the plating layer, and raise the pH in the solution that comes into contact with the plating layer. In the bath
It functions to oxidize Zn and the like to form an oxide film mainly composed of ZnO on the surface of the plating layer.

By containing, for example, 10 to 100 g / HNO ₃ as an oxidizing agent, Zn or the like can be oxidized to form an oxide film mainly composed of ZnO on the plating layer surface. The reason why the lower limit of HNO ₃ is set to 10 g / is that if it is less than that, oxidation becomes difficult and an oxide film cannot be formed. Further, the upper limit of HNO ₃ was set to 100 g / even if the content exceeds that, the effect as an oxidizing agent is saturated, dissolving Zn and Fe on the alloy layer surface,
In particular, by dissolving Fe, the generation of Fe oxides increases, and the effect of improving the life of the spot welding tip is reduced.

Further, as oxidizing agents, KMnO ₄ , Ca (ClCO) ₂ , K ₂ Cr ₂ O ₇ , Na
By adding ClO ₃ , ClO ₂ , KNO ₃ , NaNO _3, etc., the formation of a surface film is promoted.

As a method for bringing the aqueous solution of Zn (NO ₃ ) ₂ and HNO ₃ into contact with the steel sheet, any method such as immersion or spraying may be used. After immersion or spraying, for example, dry heating gas is sprayed on the surface,
Heating at a temperature of not more than ° C. makes it possible to effectively treat even a thinner solution because it becomes a concentrated solution by evaporation of water and reacts at a high temperature.

Thus, the composition of the oxide film and the like formed by performing the oxide film formation treatment is mainly composed of ZnO, oxides of Fe, Zn and F
The hydroxides of e, which may be used alone or as a mixture, and may contain impurities such as Al. However, from the characteristics of the surface film, it is desirable to use an oxide film containing a large amount of ZnO component that uniformly covers the surface and lowers the film resistance.

In order to generate an oxide film mainly composed of ZnO, Zn (NO ₃ ) _{2 is} used as a replenisher for Zn ions at 100 to 600 g / m. Zn ions for contributing to activation and generating ZnO can be supplied.

The reason that the lower limit of Zn (NO ₃ ) ₂ is set to 100 g / is that if it is less than that, Zn ions on the surface of the alloy layer are insufficient and an oxide film cannot be formed. Also, the upper limit is 600g
The reason why the value of / is exceeded is that if it exceeds that, the film will be formed too much, the resistance will increase, and the resistance to heat generated by the weldable electrode tip will cause the weldability to deteriorate due to the enlargement of the electrode tip diameter.

In the treatment bath, Fe and Zn during plating, Mn, Al as impurities,
P, Si, etc. may elute. Of these, it is preferable to add Zn ions to the bath in advance because it is not necessary to dissolve and supply the Zn ions from the plating layer, and ZnO precipitates in a shorter time. It is desirable that elution of other impurities be suppressed to a minimum. In particular, Fe
If the content exceeds 1 g / g, oxides and hydroxides of Fe are formed on the surface and the surface turns yellow, deteriorating the product quality of the steel sheet surface, and Fe oxides and hydroxides become resistance films. And the life of the spot welding tip is reduced. Therefore, in the present invention, the Fe ion concentration is not specified, but it is desirable to make it as low as possible.

In order to form an oxide film mainly composed of ZnO, a steel sheet is put into an oxidizing agent aqueous solution containing Zn (NO ₃ ) ₂ 100-600 g / and HNO ₃ 10-100 g / at a bath temperature of 30-80 ° C. for 0.2- By contacting for 10 seconds, an oxide film forming process can be performed.

The reason for setting the treatment bath temperature to 30 to 80 ° C. and the lower limit to 30 ° C. is to facilitate oxidation of Zn ions on the plating surface,
If it is less than that, the reaction rate is low and it is difficult to obtain a surface film. The upper limit is set to 80 ° C. because the reaction proceeds excessively, an excessive oxide film is generated, and the weldability deteriorates. Although the contact time may be shortened even if the temperature exceeds 80 ° C., it is difficult to control the temperature when the time is shortened to a high temperature. Therefore, the temperature is desirably 80 ° C. or less.

For this reason, depending on the balance with the line speed, the contact treatment time such as immersion or spraying is set to 0.2 to 10 seconds because if less than 0.2 seconds, the oxide film generation treatment is insufficient and the weldability is not improved. This is because, even if the treatment is performed for more than 10 seconds, the generation of the oxide film becomes too large, and the weldability is deteriorated.

As a second method, for example, Zn (NO ₃ ) ₂ .6H ₂
O: 400 g /, HNO ₃ : In an aqueous solution of 1 g /, using a zinc-based plated steel sheet as a cathode, current density 1 to 20 A / dm ² , processing time 0.5 to 10
An oxide having excellent weldability can be generated in seconds.

Further, as a third method, an alloying treatment and an oxide film forming treatment are performed after hot-dip plating, electroplating or vapor deposition plating, whereby an oxide film mainly composed of ZnO can be surely formed. As a specific method, for example, an alloying furnace for producing galvannealed steel sheet with a sheet temperature of 40
Steam-water treatment in which the temperature is adjusted to 0 to 800 ° C., the alloy is passed through the furnace to the surface at a speed at which alloying is completed, and then water and air are injected with a steam-water nozzle to secure the dew point of the atmosphere. By doing so, the oxide film forming reaction can be effectively performed. Furthermore, by performing an alloying process and an oxide film generation process after hot-dip plating, electroplating or vapor deposition plating outside the line, an oxide film mainly composed of ZnO can be reliably generated. In this method, if a method similar to the above method is adopted, an oxide film forming reaction mainly composed of ZnO can be reliably and effectively performed.

In addition, in addition to the steam-water treatment described above, the oxide film generation treatment generates an oxide film mainly composed of ZnO by spraying steam on the plating surface, or outside the line, in a heating furnace in which the open air is adjusted to an oxidizing atmosphere. Either method may be adopted, for example, a heat treatment is performed to generate an oxide film mainly composed of ZnO.

Next, the upper layer of the magnetized material layer having excellent weldability is coated with an oxide having excellent pressability and chemical conversion treatment properties as described below.

In order to impart lubricity during press molding, a method of forming a hard film on the surface is effective. In this regard, electrolytic chromate treatment and iron-zinc alloy plating are effective, but the former cannot form a chemical conversion coating, and the latter requires a large amount of treatment and increases costs.

In order to solve these problems, the hard coating on the surface is an oxide coating, which can be dissolved in a chemical conversion treatment solution to form a chemical conversion treatment film. It is necessary to have no adverse effect on

From such a viewpoint, the present inventors have found that a mixed film of a P oxide and a Zn oxide may be formed on the surface of the zinc-based plated steel oxide. This oxide film becomes a glass-like film like the chromate film, suppresses galling of the plating during pressing, and improves the slidability. Furthermore, since it is dissolved in the chemical conversion treatment liquid, unlike a chromate film, a chemical conversion treatment film can be formed, and there is no adverse effect even if it is dissolved in the chemical conversion treatment liquid. It is difficult for Zn oxide alone to form a press slidability improving film by a wet method, but in a mixed crystal state with P and Zn oxide, press slidability can be significantly improved.
Zn oxide can also form a chemical conversion treatment film, and there is no adverse effect even if it is dissolved in a chemical conversion treatment solution.

Although the structure of Zn and P oxide films is not clear, Zn-O, P
The network mainly consists of -O bonds and partially -O
It is presumed that the macromolecule structure in the amorphous state in which H, CH ₃ groups, etc. are bonded, and the metal supplied from the plating is substituted.

Further, since this film is an oxide film, it does not dissolve even in a washing step using oil or a degreasing step, so that the lubrication performance is not reduced and no load is imposed on other steps.

Sulfuric acid,
It is effective to add an inorganic acid such as nitric acid or hydrochloric acid, or a salt composed thereof.

Further, the film may contain a substance contained in the treatment bath or in the plating as an impurity. These impurities include Zn, Al, Cr, Co, Mn, Pb, Sn, Cu, Ti, Si, B, N, S, P, Cl,
Examples include K, Na, Mg, Ca, Ba, In, C, Fe, V, W, and Ni.

 Next, the coating amount range of the coating will be described.

Regarding the amount of this film, the oxide film needs to be amorphous in order to obtain good press formability, and the amount of the film must be small in order for the P oxide film to be amorphous. For this reason, the amount of the film may be 1 mg / m ² or more of P oxide (as P).
If it exceeds 500 mg / m ² , the film becomes a crystalline film, and on the contrary, the lubricity deteriorates and the pressability decreases. Further, the formation of the chemical conversion coating becomes insufficient. Therefore, the appropriate amount of the P oxide film is 1 mg / m ² or more and 500 mg / m ² or less as P,
Preferably it is 200 mg / m ² or less.

Then the adhesion of such coatings, 1-500 mg in order to improve the film-forming properties or the like of Zn oxide as Zn / m ^2, 1000 mg in total of Zn oxide and phosphoric acid / m ² (Zn, as a P- ) Mix the following. Thus, it is recognized that the structure of the oxide film is made uniform, the film formability is improved, the lubricity is improved, and the press formability is further improved. A lower limit of 1 mg / m ² is sufficient.

Thus, by simultaneously forming a film mainly composed of Zn and P oxides on a galvanized steel sheet, press formability and chemical conversion treatment are improved.

As a method for forming such an oxide film, for example,
Zinc nitrate 100-800g /, sodium phosphate 5-60g /,
A method of immersing a ZnO-coated plated steel sheet in an aqueous solution having a pH of 2 to 6, electrolytic treatment using the plated steel sheet as a cathode in the aqueous solution, or spraying the aqueous solution on the plated steel sheet is adopted.

Also, one or more of sulfuric acid, nitric acid, perchloric acid and the like may be added to such an aqueous solution as an etching agent.
Addition of up to 10 g / preferably improves the adhesion of the oxide film.

When the oxide film is formed in this manner, the plating layer and a part of the alloy metal in the plating layer are mixed into the oxide film as other oxides.

(Examples) Next, examples of the present invention will be described together with comparative examples.

Note 1) Type of galvanized steel sheet: AS: galvannealed steel sheet (Fe: 10%, Al: 0.25%, residual Zn), EG: electrogalvanized steel sheet, GI: galvanized steel sheet (Al: 0.3%, Fe: 0.8%, Pb: 0.
1%, residual Zn), HA: Semi-alloyed hot-dip galvanized steel sheet (Fe: 5%,
Al: 0.3%, residual Zn), all steel sheets are 0.8mm in thickness.

Zn / Zn-Cr: Zinc alloy plating containing lower layer 10% Cr, upper layer zinc 2g
/ m ² Note 2) Formation of ZnO coating ・ Immersion was performed by immersing a zinc-based plated steel sheet in an aqueous solution of Zn (NO ₃ ) ₂ 400 g / and HNO ₃ 70 g / at 50 ° C. for 1 to 10 seconds.

・ Electrolysis is performed using a steel plate as a cathode
A / dm ² , produced by electrolysis for 1 to 7 seconds.

-Air-water spray was generated by spraying atomized water at a temperature of 500 ° C from 80 to 150 / min.

Note 3) The oxide film is zinc nitrate 100-800 g / phosphoric acid 5-
Electrolytic treatment (5 to 10 A / cm ² , 1.0 to 1.5 seconds) or immersion treatment was performed using a plated steel sheet as a cathode or an anode in an aqueous solution containing 60 g / to form an oxide film.

Note 4) Chemical conversion property SD5000 (manufactured by Nippon Paint Co., Ltd.) was used as the chemical conversion solution.
After degreasing and surface conditioning according to the prescription, a chemical conversion treatment was performed. The chemical conversion treatment film was judged by SEM (secondary electron beam image) as follows: ○ if the film was uniformly formed, △ if the film was partially formed, × if the film was not formed. It was determined.

Note 5) Press formability (friction coefficient) Sample size: 17mm x 300mm, Tensile speed: 500mm / min,
Corner bead shoulder R: 1.0 / 3.0 mm, sliding length: 200 mm, Oiling: Knoxville Last 530F40,1g / m ² condition, performs several points tested among the surface pressure of 100～600Kgf, measured withdrawal weighted Then, the friction coefficient was determined from the surface pressure and the gradient of the pulling load.

Note 6) Welding conditions Welding conditions are as follows.

1) Pressing force: 250kgf 2) Initial pressurizing time: 40Hr 3) Energizing time: 12Hr 4) Holding time: 5Hr 5) Welding current: 11kA 6) Tip tip diameter: 5.0φ (frustum cone type) 7) End of electrode life Judgment: Number of dots that can secure a 3.6 mm nugget diameter at 85% of welding current 8) Electrode material: Cu-Cr (commonly used) Welding is performed with one side of the plated steel sheet up and the other side down. , 2
Continuous hitting points were obtained by stacking the sheets.

Note 7) Oxide measurement is GDS (glow discharge spectroscopy) ICAP
(Ion plasma emission spectrometry).

Note 8) Measurement of ZnO film Only the plating layer was dissolved with a 5% iodine methyl alcohol solution, and the extraction residue was melted with a mixed solvent (sodium borate monocarbonate 3). Z
Converted to nO amount.

(Effect of the Invention) In this way, in spot welding, the number of continuous hit points is increased, welding can be performed for a long time without replacing the tip, and the durability of the tip can be improved. In addition, the productivity by welding can be improved,
In addition, the appropriate welding current range is the same level as the conventional material, and the weldability is good.

Further, it is possible to improve the slidability in a press to a level equal to or higher than that of a cold-rolled steel sheet and to form a chemical conversion coating.
As a result, excellent effects can be obtained, such as lower costs than in the past, a reduction in the load on the user's process, and an improvement in productivity by pressing.

──────────────────────────────────────────────────の Continuation of front page (72) Inventor Katsura Arai 5-3 Tokai-cho, Tokai-shi, Aichi Prefecture Nippon Steel Corporation Nagoya Works (56) References JP-A-3-126893 (JP, A) JP-A-3-287784 (JP, A) JP-A-3-287787 (JP, A) JP-A-2-47294 (JP, A) JP-A-2-4983 (JP, A) JP-A-64-25988 (JP) JP, A) JP-A-1-294897 (JP, A) JP-A-1-149974 (JP, A) JP-A-1-172578 (JP, A) JP-A-61-64884 (JP, A) (58) ) Fields investigated (Int.Cl. ⁶ , DB name) C23C 2/00-2/40 C23C 22/00-22/86 C23C 28/00-28/04 C23C 30/00 C25D 11/00-11/38 C23C 14/58 C23C 16/56 C23C 18/00-18/54

Claims (1)

(57) [Claims] (1) A ZnO content of 30 to 3 on the surface of a galvanized steel sheet.
000 mg / m ² was produced, and the upper layer was coated with a mixed oxide consisting of ¹ to 500 mg / m ² as P oxide as P and 1 to 500 mg / m ² as Zn oxide as Zn and other oxides. Galvanized steel sheet with excellent weldability, pressability and chemical conversion characteristics.