JP2003201552A - Lubrication-treated steel sheet with excellent peeling resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lubricated steel sheet in which the peeling resistance in a blank cleaning step is effectively improved without lowering the film removal property in a coating pretreatment step. SOLUTION: A solid lubricating film mainly composed of zinc phosphate fine particles is formed on the surface of an alloyed hot-dip galvanized steel sheet in which the outermost surface of a plating layer is mainly composed of columnar crystals.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lubricated steel sheet excellent in peeling resistance suitable for use as a steel sheet for automobiles and the like. This is an advantageous improvement.

[0002]

2. Description of the Related Art Galvanized steel sheets such as hot-dip galvanized steel sheets and electrogalvanized steel sheets have excellent corrosion resistance, but are inferior in press formability as compared with cold-rolled steel sheets. Therefore, various proposals have conventionally been made on a method for improving the press formability of a galvanized steel sheet.

[0003] For example, by applying an iron-based hard plating to the upper layer of a galvanized layer to increase the surface hardness,
A method for preventing galling of plating and dies has been proposed (for example, Patent Document 1). In addition, a method has been proposed in which a film made of an oxyacid salt of P or B and a metal oxide is formed on the surface of a plating layer to improve slidability (for example, Patent Document 2). However, in the above-described conventional technology, it is necessary to separately install a dedicated post-processing facility after the normal galvanizing line, and thus there is a problem that the manufacturing cost of the steel sheet increases.

[0004] In addition, as a lubricated steel sheet having excellent lubricity and easy to press work, a zinc-based plated steel sheet coated with a zinc phosphate film is known. According to the method using a zinc phosphate film (prefos treatment), the film can be formed by sharing general-purpose equipment such as a dipping method and a coating method, and the effect of improving slidability is great. However, such a zinc phosphate film is inferior in the film-removing property (removability) in an alkali degreasing step, which is a pre-treatment in the painting step, and as a result, the chemical conversion treatment film becomes non-uniform, and as a result, after coating, There is a problem that appearance quality is deteriorated.

[0005] To improve this point, the present inventors have proposed a non-reactive lubricating material having excellent film-removing properties, comprising fixing zinc phosphate fine particles prepared in advance on a plated steel sheet with a water-soluble polymer binder. We have been focusing on the development of treated steel sheets. However, in forming the solid lubricating film, since the binder component is adjusted so that the adhesive force on the substrate is not so strong from the viewpoint of securing the film-removing property, it is required in a pre-process of pressing the automotive steel sheet. In a certain blank washing step, a part of the coating was peeled off, and sufficient lubrication performance could not be exhibited during press working.

[0006]

[Patent Document 1] JP-A-62-192597 (Claims)

[Patent Document 2] Japanese Patent Application Laid-Open No. 4-176878 (Claims)

[0007]

SUMMARY OF THE INVENTION The present invention has been developed in view of the above circumstances, and is directed to a non-reactive solid lubrication treatment in which zinc phosphate fine particles are fixed on a plated steel sheet with a water-soluble polymer binder. By controlling the crystal form of the base plating layer to a specific form, without reducing the film removal property in the coating pretreatment step, a lubricated steel sheet that has effectively improved the peel resistance in the blank cleaning step is provided. The purpose is to propose.

[0008]

That is, the present invention relates to a lubricated steel sheet obtained by forming a solid lubricating film on the surface of a galvannealed steel sheet, wherein the solid lubricating film comprises zinc phosphate fine particles. A lubricated steel sheet having excellent peeling resistance, characterized in that the crystal form of the outermost surface of the galvannealed layer is mainly composed of columnar crystals.

In the present invention, the crystal form of the outermost surface of the alloyed hot-dip galvanized layer is a diffraction line pattern measured by an X-ray diffraction method, and a peak intensity I at a lattice spacing d = 1.26 ° and a lattice spacing d ₀ = 1.28 ° peak intensity I ₀ ratio I
It is preferable that / I ₀ be 0.25 or more.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described specifically. It is known that when a zinc-based plated steel sheet is pressed, adhesion to a mold is likely to occur due to the soft nature of zinc, and sliding resistance is high. Usually, the use of press oils improves the formability considerably. However, when processing large-sized parts or difficult-to-mold parts, there is a case where an oil film is partially cut and a press crack occurs.

A solid lubricating film such as a zinc phosphate film is
Originally, it was considered effective for reducing the sliding resistance partially due to the oil film breakage and the like, and it was thought that the moldability could be improved by using it in combination with an appropriate press oil. However, the conventional zinc phosphate treatment involves mixing a soluble zinc compound and a reaction accelerator in an acidic solution mainly composed of phosphoric acid, and dissolving a part of the underlying plating layer on the surface of the plating layer, thereby forming a phosphoric acid. Since a zinc film is formed, a reaction layer necessarily exists at the interface between the formed film and the plating layer.

Although not all of the form of the reaction layer has been completely clarified, in many cases,
X-ray diffraction analysis shows the presence of Hopeite (hobbite: zinc phosphate tetrahydrate). In the case of a film formed by an immersion method, a 5 to 10 μm scale-like zinc phosphate crystal is often observed. These crystalline particles and the reaction layer are dissolved to some extent in the alkaline degreasing solution and partially removed, but not completely removed. If the chemical conversion treatment is performed in this state, the chemical conversion film becomes non-uniform, and the subsequent appearance of the coating deteriorates.

Therefore, the present inventors have attempted to solve the above-mentioned problem by applying a solid lubricating film mainly composed of zinc phosphate fine particles. That is, when forming a zinc phosphate film, an aqueous treatment solution containing preliminarily reacted and formed zinc phosphate fine particles and a water-soluble polymer binder is applied and dried on a steel sheet by a roll coater or the like to form a film. The coating liquid did not contain components that cause a chemical reaction with the plating layer, such as phosphoric acid. According to the above treatment, no crystalline particles or a reaction layer are formed at the interface between the film and the plating layer, so that the film is easily removed by dissolving the water-soluble polymer binder.

However, the non-reactive solid lubricating film that facilitates film removal has a weak adhesive strength to a substrate by itself.
As described above, in the blank cleaning step, which is a step before press processing of an automobile plate, a part of the coating was peeled off, and sufficient lubrication performance could not be exhibited in the pressing step.

The present inventors have conducted intensive studies to solve this problem, and as a result, have found that the above-mentioned problem can be solved by making the crystal form of the outermost surface of the base plating layer mainly composed of columnar crystals. Obtained.

In other words, the present inventors focused on the surface morphology of the underlying plating layer and examined the relationship with the peeling resistance in order to improve the adhesion to the substrate and improve the peeling resistance of the film. As a result, it has been found that by making the crystal morphology of the base plating plate surface mainly columnar, it is possible to advantageously improve the peeling resistance in the blank cleaning step without deteriorating the film removal property in the coating pretreatment step. It was. The undercoat layer was limited to a galvannealed layer from the viewpoint of forming clear crystals.

Further, the inventors have further studied a particularly preferable form as the crystal form of the outermost surface of the galvannealed layer. As a result, as shown in FIG. 1, in the diffraction line pattern measured by the X-ray diffraction method, the peak intensity I of the lattice spacing d = 1.26 ° (corresponding to columnar crystals) and the lattice spacing d ₀ = 1.28 ° (granularity) the ratio I / I ₀ of the peak intensity I ₀ of the equivalent) in crystal was found that crystalline form which satisfies 0.25 or more is particularly good.

Usually, as a crystal phase constituting the alloyed hot-dip galvanizing used as a steel sheet for automobiles, a Γ phase (Fe
₃ Zn ₁₀ ), Γ ₁ phase (Fe ₅ Zn ₂₁ ), δ ₁ phase (FeZn ₇ ), ζ phase (Fe
Zn ₁₃ ) is known _. These Fe—Zn alloy crystals develop in the order of Γ → Γ ₁ → δ ₁ → ζ from the steel sheet interface toward the plating surface due to the diffusion of Fe from the base steel sheet. Depending on the plating bath composition and alloying conditions during production,
The composition ratio of these crystal phases changes, and therefore, the crystal phases constituting the outermost layer of the plating are ζ phase and δ ₁ phase. The morphology of the plating surface observed by SEM or the like differs greatly due to the difference in the structure of the crystal phase in these surface layers.

That is, when the ratio of the δ ₁ phase is high,
The surface morphology is mainly composed of granular crystals, while when the ζ phase ratio is high, columnar crystals are mainly composed. When the X-ray diffraction patterns corresponding to these surface morphologies are analyzed, a peak near the lattice spacing d ₀ = 1.28 ° belonging to the δ ₁ phase (peak intensity I
₀ ) and the peak ratio (peak intensity I) near the lattice spacing d = 1.26Å belonging to the ζ phase, and I / I ₀ ≧ 0.
In the case of 25, the surface morphology is mainly composed of columnar crystals, and in the case of I / I ₀ <0.25, the surface morphology is mainly composed of granular crystals.

Accordingly, in the diffraction line pattern measured by the X-ray diffraction method, the ratio I / I ₀ of the peak intensity I at the lattice spacing d = 1.26 ° and the peak intensity I _{0 at} the lattice spacing d ₀ = 1.28 ° is obtained.
Particularly good results can be obtained by making the crystal form mainly composed of columnar crystals such that it becomes 0.25 or more.

In the present invention, the size of the zinc phosphate fine particles, which are the main component of the solid lubricating film, is preferably about 0.3 to 4.0 μm in average particle diameter. And
Such zinc phosphate fine particles are mixed with a water-soluble polymer binder to prepare a treatment liquid. The mixing ratio of the zinc phosphate fine particles and the water-soluble polymer binder is preferably about 3 to 50 parts by weight based on 100 parts by weight of the zinc phosphate fine particles.

Further, as the water-soluble polymer binder,
For example, water-soluble polymers such as methylcellulose, carboxymethylcellulose, hydroxyethylcellulose, polyvinyl alcohol, polyethylene glycol, xanthan gum and guar gum, and derivatives thereof, and salts thereof are advantageously suitable. In addition, if necessary,
A surfactant or the like as a dispersion stabilizer of the zinc phosphate fine particles may be contained.

The amount of the treatment liquid to be applied is as follows.
The amount of the solid lubricating film after drying is preferably about 0.05 to 2.0 g / m ^2. After applying an appropriate amount of the treatment liquid, drying is performed at a temperature of about 60 to 120 ° C. The main component of the solid lubricating film thus obtained is zinc phosphate fine particles, and the other components are water-soluble polymer binders.
After the film formation, these ratios are as follows: zinc phosphate fine particles: 65 to 97 mass%, water-soluble polymer binder: 3
It is preferably about 35 mass%.

[0024]

[Example] Hot dip galvanizing (plating bath composition Fe: 8 to 14% by mass, Al: 0.1 to 0.1%) on the surface of ordinary steel having a thickness of 0.8 mm.
After 2 mass%, the balance: zinc), an alloying treatment was performed to produce an alloyed hot-dip galvanized steel sheet. On this occasion,
The infiltration plate temperature, bath temperature and alloying treatment temperature were variously changed, and the crystal morphology of the outermost surface of the alloyed hot-dip galvanized layer was variously changed as shown in Table 1. Then, the average particle size
1.0 μm zinc phosphate particles: 5 to 20 mass% and a water-soluble polymer binder (carboxymethylcellulose sodium salt [degree of polymerization 700]): 0.5 to 5.0 mass%: An aqueous solution containing various ratios is applied as a treatment liquid.・ After applying the amount that the adhesion amount after drying becomes 1.0 g / m ² (per one side),
Dried at ° C. Table 1 also shows the results of examining the peeling resistance of the alloyed hot-dip galvanized steel sheet with the solid lubricating film thus obtained.

The crystal structure of the plating layer is analyzed by an X-ray diffraction method (Cu tube), the amount of coating film is analyzed by a fluorescent X-ray analysis (calibration method), and the plating surface morphology is determined by scanning. Observed with an electron microscope (SEM). The peel resistance was evaluated by applying a cleaning oil (Nippon Mitsubishi P1600) to the test piece, rubbing it 20 times with a polypropylene brush, degreased with petroleum benzene, and evaluating the change in the amount of adhesion before and after that. . If the amount of peeling in this test exceeds 20%, there is a great concern that slidability will be poor.

[0026]

[Table 1]

As is clear from the table, the galvanized steel sheet with a solid lubricating film obtained according to the present invention has excellent peel resistance.

FIGS. 2A to 2D show the results of SEM observation of the crystal morphology of the outermost surface of the plating layer. Same figure
It can be seen that in the invention examples (a) to (c), the plating crystals are mainly composed of columnar crystals, whereas in the comparative example of FIG. (d), the plating crystals are mainly composed of granular crystals.

[0029]

As described above, according to the present invention, the crystal form of the outermost surface of the alloyed hot-dip galvanized layer, which is the base plating layer, is mainly composed of columnar crystals, so that the film removal property in the coating pretreatment step is not reduced. In addition, a lubricated steel sheet having significantly improved peel resistance in the blank cleaning step can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an X-ray diffraction pattern of a galvannealed layer.

FIG. 2 is a scanning electron microscope (SEM) photograph showing the crystal form of the outermost surface of an alloyed hot-dip galvanized layer.

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Yoichi Tobiyama             1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba             Iron Research Institute (72) Inventor Chiaki Kato             1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba             Iron Research Institute F term (reference) 4K027 AA02 AA05 AA22 AB02 AB09                       AB28 AB36 AB44 AC73 AC82                       AD25 AE27                 4K044 AA02 BA10 BA17 BB03 BC07                       CA15 CA16 CA53

Claims (2)

[Claims] 1. A lubricated steel sheet having a surface of a galvannealed steel sheet coated with a solid lubricating film, wherein the solid lubricating film mainly comprises zinc phosphate fine particles,
Further, a lubricated steel sheet having excellent peeling resistance, characterized in that the crystal form of the outermost surface of the galvannealed layer is mainly composed of columnar crystals. 2. The crystal form of the outermost surface of the alloyed hot-dip galvanized layer is a diffraction line pattern measured by an X-ray diffraction method, and a peak intensity I at a lattice spacing d = 1.26 ° and a lattice spacing d ₀ = lubricated steel sheet according to claim 1, wherein the ratio I / I ₀ of the peak intensity I ₀ of 1.28Å is equal to or greater than or equal to 0.25.