JP4650128B2 - Method for producing alloyed hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet - Google Patents

Description

  The present invention relates to a method for stably producing an alloyed hot-dip galvanized steel sheet having excellent press formability and exhibiting good degreasing properties in an alkaline degreasing treatment performed before chemical conversion treatment, and alloyed hot-dip zinc. The present invention relates to a plated steel sheet.

  Alloyed hot-dip galvanized steel sheets are widely used in a wide range of fields, especially for automobile bodies, because they are superior in weldability and paintability compared to galvanized steel sheets. The alloyed hot-dip galvanized steel sheet for such applications is subjected to press forming and used. However, the alloyed hot-dip galvanized steel sheet has a disadvantage that its press formability is inferior to that of a cold-rolled steel sheet. This is because the sliding resistance of the alloyed hot-dip steel sheet in the press die is larger than that of the cold-rolled steel sheet. That is, the alloyed hot-dip galvanized steel sheet is less likely to flow into the press mold at the portion where the sliding resistance between the mold and the bead is large, and the steel sheet tends to break.

An alloyed hot-dip galvanized steel sheet is formed by galvanizing the steel sheet and then heat-treating to form an Fe-Zn alloy phase by diffusion of Fe in the steel sheet and Zn in the plating layer to cause an alloying reaction. It has been made. This Fe-Zn alloy phase is usually a film composed of a Γ phase, a δ ₁ phase, and a ζ phase, and as the Fe concentration decreases, that is, in the order of Γ phase → δ ₁ phase → ζ phase, hardness and melting point Tends to decrease. For this reason, from the viewpoint of slidability, a coating with high hardness, high melting point and high Fe concentration is effective, and alloyed hot-dip galvanized steel sheet, which emphasizes press formability, Manufactured with high average Fe concentration.

  However, a high Fe concentration film tends to form a hard and brittle Γ phase at the plating-steel sheet interface, and has a problem that a phenomenon of peeling from the interface during processing, that is, so-called powdering is likely to occur. Therefore, as shown in Patent Document 1, in order to achieve both slidability and powdering resistance, there is a method of applying a hard Fe-based alloy as a second layer to the upper layer by a technique such as electroplating. It has been taken.

  In addition to this, as a method for improving the press formability when using a zinc-based plated steel sheet, a method of applying a high-viscosity lubricating oil is widely used. However, this method has problems such as a coating defect due to poor degreasing in the painting process due to the high viscosity of the lubricating oil, and press performance becoming unstable due to oil shortage during pressing. Therefore, there is a strong demand for improving the press formability of the galvannealed steel sheet itself.

  As a method for solving the above problems, Patent Document 2 and Patent Document 3 describe that the surface of a zinc-based plated steel sheet is subjected to electrolytic treatment, dipping treatment, coating oxidation treatment, or heat treatment to oxidize mainly ZnO. A technique for improving weldability or workability by forming a film is disclosed.

  Patent Document 4 discloses that a plated steel sheet is immersed in an aqueous solution containing 5 to 60 g / l of sodium phosphate and having a pH of 2 to 6, or subjected to electrolytic treatment, or the above aqueous solution is applied to the surface of a zinc-based plated steel sheet. Discloses a technique for improving the press formability and chemical conversion treatment by forming an oxide film mainly composed of P oxide.

  Patent Document 5 describes a technique for improving press formability and chemical conversion treatment by generating Ni oxide by electrolytic treatment, immersion treatment, coating treatment, coating oxidation treatment, or heat treatment on the surface of a galvanized steel sheet. Is disclosed.

  However, when the above prior art is applied to an alloyed hot-dip galvanized steel sheet, the effect of improving press formability cannot be stably obtained. As a result of conducting detailed studies on the causes of the present invention, the alloyed hot-dip galvanized steel sheet is caused by the presence of Al oxide, the surface reactivity is inferior, and the surface unevenness is large. I found out. That is, when the prior art is applied to an alloyed hot-dip steel sheet, the surface reactivity is low, so that a predetermined film can be formed on the surface even when electrolytic treatment, immersion treatment, coating oxidation treatment, heat treatment, etc. are performed. Is difficult, and the film thickness becomes thin in a portion with low reactivity, that is, a portion with a large amount of Al oxide. Also, since the surface irregularities are large, it is the surface protrusions that come into direct contact with the press die during press molding, but the sliding resistance at the contact portion between the thin part of the protrusions and the mold As a result, the effect of improving press formability cannot be sufficiently obtained.

  Therefore, as a result of studies conducted by the present inventors to improve the above problems, the following knowledge was obtained and a patent application was filed (Patent Document 6).

  In general, galvannealed steel sheets are subjected to temper rolling after hot dip galvanizing → alloying treatment. By comparison, it exists as a convex portion. Since the flat part is the main component that actually contacts the press mold during press molding, the press formability can be stably improved by reducing the sliding resistance at the flat part. In order to reduce the sliding resistance in this flat part, it is effective to prevent adhesion between the plating layer and the mold. To that end, a hard and high melting point film should be formed on the surface of the plating layer. It was found that the method of forming an oxide layer on the plating surface layer by contacting with an acidic solution is effective for forming such an oxide layer.

And based on the above knowledge, after the invention which concerns on patent document 6 is hot-dip galvanized to a steel plate, it alloyed by heat processing, and also gave temper rolling, and formed the flat part on the iron-zinc alloy plating surface. Then, it was brought into contact with an acidic solution, held for 1 to 30 seconds, and washed with water to complete an oxide hot-dip galvanized steel sheet manufacturing method characterized by forming an oxide layer on the plating surface layer.
Japanese Patent Laid-Open No. 1-319661 Japanese Unexamined Patent Publication No. 53-60332 Japanese Patent Laid-Open No. 2-190483 Japanese Patent Laid-Open No. 4-88196 Japanese Patent Laid-Open No. 3-19093 Japanese Patent Application No. 2002-116026

  In the above-mentioned Patent Document 6, the following has been found out while conducting a more detailed study on properties other than press formability. In the coating line applied after pressing, a degreasing treatment is performed using an alkaline degreasing liquid, and then a chemical conversion treatment → electrodeposition coating is performed. Here, it was found that although the degreasing portion on the outer surface of the vehicle body can be completely degreased, poor degreasing occurs in a portion where there is almost no flow of degreasing liquid such as the inner surface portion of the vehicle body. Therefore, a detailed investigation was made on the sample in which the degreasing failure occurred, and it was found that it was caused by the hydroxyl group (—OH) present in the outermost layer.

  In view of such circumstances, the present invention improves the above-mentioned problems, has excellent slidability during press molding, and exhibits good degreasing properties in an alkaline degreasing treatment performed before chemical conversion treatment. It aims at providing the method of manufacturing a galvanized steel plate stably, and its alloying hot-dip galvanized steel plate.

  The inventors of the present invention made further studies to solve the above problems. As a result, it was found that after forming a Zn-based oxide layer on the surface of the plated steel sheet, P was adsorbed on the oxide surface by contacting with an aqueous solution containing P, and good degreasing properties were obtained. .

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] Hot-dip galvanizing is applied to the steel sheet, further alloyed by heat treatment, temper rolling, contact with an acidic solution, left for 1 to 30 seconds after contact is completed, and then washed with water, In the method for producing an alloyed hot-dip galvanized steel sheet in which a Zn-based oxide layer of 10 nm or more is formed on the surface of the plated steel sheet, the water washing is performed using a P-containing aqueous solution having a P concentration of 0.5 to 5000 ppm. Manufacturing method of hot dip galvanized steel sheet.
[2] A plated steel sheet produced by the method for producing an alloyed hot-dip galvanized steel sheet according to [1] above, wherein the oxide layer containing Zn and Al as essential components in the flat surface layer of the plated steel sheet is 10 nm or more An alloyed hot-dip galvanized steel sheet characterized by

  According to the present invention, the alloyed molten zinc has low sliding resistance during press molding, exhibits stable and excellent press formability, and exhibits good degreasing properties in an alkaline degreasing treatment performed before chemical conversion treatment. A plated steel sheet can be manufactured stably.

  When producing an alloyed hot-dip galvanized steel sheet, the steel sheet is hot-dip galvanized and then further heated and alloyed. The difference in reactivity between the steel sheet and the plating interface during the alloying process. Thus, irregularities exist on the surface of the galvannealed steel sheet. However, after the alloying treatment, temper rolling is usually performed for securing the material, and the plating surface is smoothed and unevenness is alleviated by contact with the roll during temper rolling. Therefore, at the time of press molding, the force required for the mold to crush the convex portion on the plating surface is reduced, and the sliding characteristics can be improved.

  Such a galvannealed steel sheet surface is flattened and flattened by temper rolling or the like (hereinafter referred to as a flat part) because the mold is in direct contact with the mold during press molding. The presence of a hard and high-melting substance that prevents the adhesion of the resin is important for improving the slidability. In this respect, the presence of the oxide layer on the surface layer is effective in improving the sliding characteristics because the oxide layer prevents adhesion with the mold.

  During actual press molding, the oxide on the surface layer is worn away and scraped off. Therefore, when the contact area between the mold and the workpiece is large, a sufficiently thick oxide layer must be present. Although an oxide layer is formed on the plating surface by heating during alloying treatment, most of it is destroyed by contact with the roll during temper rolling, and the new surface is exposed. In order to obtain this, a thick oxide layer must be formed before temper rolling. Taking this into consideration, even if a thick oxide layer is formed before temper rolling, it is impossible to avoid the destruction of the oxide layer that occurs during temper rolling. It exists unevenly and good slidability cannot be obtained stably.

  For this reason, good slidability can be stably obtained by subjecting the alloyed hot-dip galvanized steel sheet that has been subjected to temper rolling, in particular to a flat surface of the plated surface, to a process that uniformly forms an oxide layer.

  The alloyed hot-dip galvanized steel sheet is brought into contact with an acidic solution, and then left for 1 to 30 seconds in a state where a liquid film of the acidic solution is formed on the surface of the steel sheet, followed by washing with water and drying to form an oxide layer on the plating surface layer. Can be formed. However, when rust preventive oil is applied thereafter, a degreasing defect may occur in the alkaline degreasing step before the chemical conversion treatment.

  The occurrence mechanism of such degreasing failure is not clear, but can be considered as follows. The formation of the oxide layer on the plating surface described above utilizes a hydroxide precipitation reaction due to a pH increase accompanying the dissolution of Zn in the acid solution liquid film, and a part of the formed oxide layer. Or all are considered to be hydroxides of Zn. And it is thought that a hydroxyl group (-OH) exists in the outermost layer of an oxide, and the lipophilicity of the steel plate surface rises by increasing the adsorptivity with the additive in antirust oil. On the other hand, the alkaline degreasing liquid is mainly composed of an alkali builder that saponifies the rust preventive oil and emulsifies and disperses it in the liquid, and a surfactant that improves the permeability of the degreasing liquid. Therefore, when the surface is highly lipophilic, it takes a long time to emulsify and disperse. Even in this case, if the convection of the liquid is sufficient, it can be physically dispersed, but in the so-called defatted liquid near the stationary state where the convection of the liquid is not sufficient, the rust preventive oil component remains for a considerable time. For this reason, chemical conversion unevenness occurs in the subsequent chemical conversion treatment.

  Therefore, in order to obtain good degreasing properties, it is important to block the adsorption of the hydroxyl group on the surface of the plated steel sheet and the additive of the rust preventive oil before the application of the rust preventive oil. When the aqueous solution and the steel sheet are brought into contact with each other, P is adsorbed on the oxide surface layer, the lipophilicity of the steel sheet surface can be suppressed, and the degreasing failure can be improved. That is, it is considered that the oxide surface is covered with a small amount of P, thereby preventing the hydroxyl group and the additive in the rust preventive oil from being bonded.

  The P concentration of the aqueous solution containing P is 0.5 to 5000 ppm. If the concentration is less than 0.5 ppm, the effect of adsorbing P on the surface of the steel sheet is not sufficient, and if it exceeds 5000 ppm, the oxide formed on the surface due to the etching action of P decreases, and satisfactory press formability is obtained. Absent. In addition, there is no particular problem as long as it is within the above-mentioned concentration range, but when the time until oiling after treatment is long, uneven cleaning may be noticeable, so normal washing with water after washing with a P-containing aqueous solution may occur. It is desirable to do it again.

  The pH of the aqueous solution containing P is preferably in the range of 4.0 to 12.0. If it is less than 4.0, the aqueous solution itself dissolves the plated steel sheet. On the other hand, if it exceeds 12.0, it will be in the pH region where the Zn-based oxide dissolves. there's a possibility that. When the pH is not within this range, the pH can be adjusted to the above range by adding a slight alkali such as sodium hydroxide or diluting with water.

  The temperature of the aqueous solution containing P is preferably in the range of 20 to 70 ° C. If it is less than 20 ° C., it will be difficult to complete P adsorption in a short time. On the other hand, if it exceeds 70 ° C., not only the P adsorption effect is saturated, but also cleaning unevenness and the like are likely to occur. The washing time is preferably in the range of 0.5 to 10.0 seconds. If it is less than 0.5 seconds, the adsorption of P may not be completed sufficiently. On the other hand, the treatment exceeding 10 seconds not only lengthens the production line but also causes etching of the oxide layer and the plating surface by the P-containing aqueous solution, and may not be able to ensure sufficient slidability.

  There is no restriction | limiting in particular in the processing method by such P containing aqueous solution, The method of apply | coating through the method of immersing a plating steel plate, the method of spraying, an application | coating roll, etc. are mentioned. Among them, the method of spraying on the surface of the steel sheet is the most desirable method because it requires a small amount of processing liquid and at the same time completes processing in a relatively short time due to a synergistic effect with the fluid flow effect.

  The oxide layer in the present invention is a layer made of an oxide and / or hydroxide containing Zn and Fe as essential elements.

  Moreover, regarding the production of the galvannealed steel sheet according to the present invention, Al must be added to the plating bath, but the additive element components other than Al are not particularly limited. That is, the effect of the present invention is not impaired even if Pb, Sb, Si, Sn, Mg, Mn, Ni, Ti, Li, Cu or the like is contained or added in addition to Al.

Next, the present invention will be described in more detail with reference to examples.
A conventional alloyed hot-dip galvanized film was formed on a cold-rolled steel sheet having a thickness of 0.8 mm, and further subjected to temper rolling. Subsequently, an oxide layer was formed using a processing facility having the configuration shown in FIG.

First, after activating the activation treatment tank 1 and immersing in an acidic solution tank 2 in an acidic solution of pH 1.5 containing sodium acetate 40 g / l and ferrous sulfate 5 g / l at a liquid temperature of 30 ° C. Then, a liquid film was formed on the surface of the steel sheet with the squeezing roll 3. At this time, the pressure of the squeeze roll was adjusted so that the liquid film amount was about 1 g / m ² . Next, after passing through the water washing tank 5, in the washing tank 6, spray treatment of the aqueous solution containing P, spraying and washing 50 ° C hot water on the steel plate in the hot water washing tank 7, drying in the dryer 8, plating An oxide layer was formed on the surface. For comparison, an untreated one in which all the treatment facilities having the configuration shown in FIG.

P-containing aqueous solutions that are sprayed in the washing tank 6 include sodium diphosphate (Na ₄ P ₂ O ₇ · 10H ₂ O), disodium hydrogen phosphate (Na ₂ HPO ₄ · 12H ₂ O), and triphosphate Using an aqueous solution of sodium (Na ₃ PO ₄ .12H ₂ O) and sodium dihydrogen phosphate (NaH ₂ PO ₄ .2H ₂ O), the P concentration and pH were partially changed. For comparison, an aqueous solution of sodium hydroxide (NaOH) having a pH of 12.0 was also used. The temperature of the treatment liquid is 50 ° C, and the liquid film formation time (the time from passing through the squeeze roll to the entry of the cleaning tank 6) and the time of passing through the cleaning tank 6 are 5 seconds and 2 seconds, respectively. Adjusted the speed. For comparison, after washing in the washing tank 5, the washing tank 6 was evacuated, and thereafter, the same treatment as described above was performed.

Next, the steel plate produced as described above was subjected to measurement of a friction coefficient and evaluation of alkali degreasing property before chemical conversion as a method for simply evaluating press formability. In addition, measurement of a friction coefficient and evaluation of alkali degreasing property before chemical conversion treatment were performed as follows.
(1) Press formability evaluation test (Friction coefficient measurement test)
In order to evaluate the press formability, the friction coefficient of each test material was measured as follows.

  FIG. 2 is a schematic front view showing the friction coefficient measuring apparatus. As shown in the figure, a friction coefficient measurement sample 11 collected from a test material is fixed to a sample table 12, and the sample table 12 is fixed to the upper surface of a slide table 13 that can move horizontally. On the lower surface of the slide table 13, there is provided a slide table support base 15 having a roller 14 in contact therewith and capable of moving up and down, and by pushing it up, a pressing load N on the friction coefficient measurement sample 11 by the bead 16 is applied. A first load cell 17 is attached to the slide table support base 15. A second load cell 18 for measuring a sliding resistance force F for moving the slide table 13 in the horizontal direction in a state where the pressing force is applied is attached to one end of the slide table 13. In addition, the cleaning oil Preton R352L for press made by Sugimura Chemical Co., Ltd. was applied to the surface of the sample 11 as a lubricating oil, and the test was performed.

  3 and 4 are schematic perspective views showing the shape and dimensions of the beads used. The bead 16 slides with its lower surface pressed against the surface of the sample 11. The bead 16 shown in FIG. 3 has a width of 10 mm, a length of 12 mm in the sliding direction of the sample, and a lower portion at both ends of the sliding direction is configured with a curved surface having a curvature of 4.5 mmR. It has a plane with a direction length of 3 mm. The shape of the bead 16 shown in FIG. 4 is 10 mm wide, 6.9 mm long in the sliding direction of the sample, and the lower part at both ends of the sliding direction is a curved surface with a curvature of 4.5 mmR. It has a plane with a direction length of 60 mm.

The friction coefficient measurement test was conducted under the following two conditions.
[Condition 1]
The bead shown in FIG. 3 was used, the pressing load N was 400 kgf, and the sample drawing speed (horizontal moving speed of the slide table 13) was 100 cm / min.
[Condition 2]
The bead shown in FIG. 4 was used, the pressing load N was 400 kgf, and the sample drawing speed (horizontal moving speed of the slide table 13) was 20 cm / min.

The coefficient of friction μ between the test material and the bead was calculated by the formula: μ = F / N.
(2) Alkaline degreasing evaluation before chemical conversion treatment Rust prevention oil was applied to each specimen, and the oil coating amount was kept constant at about 2 g / m ² by holding it vertically for 24 hours. After dipping in an alkaline degreasing solution (Nippon Parkerizing FC-L4460) at 45 ° C. for 2 minutes, it was washed with water at a spray pressure of 1 kg / cm ² for 30 seconds. Thereafter, the specimen was held vertically for 30 seconds, and the water wetting rate at that time (the ratio of the area where water repelling did not occur relative to the total area of the specimen) was visually determined. Here, when the degreasing is completely completed, the water wetting rate is 100%, and the water wetting rate decreases as the degreasing failure occurs.

  Note that the alkaline degreasing solution is used by adjusting the pH to about 11.0 by blowing carbon dioxide in consideration of deterioration of the treatment solution over time. The test was carried out in a completely stationary state.

  The test results obtained from the above are shown in Table 1.

From the test results shown in Table 1, the following matters were clarified.
(1) No. Since 1 is not treated with an acidic solution, an oxide film sufficient to improve the slidability is not formed on the flat portion, and the coefficient of friction is high.
(2 No. 2 is a comparative example in which the oxide formation treatment was performed but the water washing tank 5 was washed only with water. Compared with No1, the friction coefficient was lower and the slidability was improved. However, the water wetting rate after alkaline degreasing is very low, and the alkaline degreasing property is poor.
(3) No. 3 is a comparative example in which washing was performed with a diluted solution of an aqueous sodium hydroxide solution for comparison. No. cleaned only with water. Compared with 2, the water wetting rate after alkaline degreasing is slightly improved, but complete degreasing has not been achieved, and the water wettability is not good even though the pH is in the weak alkaline region due to dilution. This is because in the case of a solution obtained by diluting a strong alkali such as sodium hydroxide, the pH is drastically lowered due to the reaction with a slight acid, and it is considered that it is difficult to keep the steel plate surface in a weak alkali state. .
(3) No. Examples 4 to 10 are examples in which washing treatment was performed with a sodium diphosphate aqueous solution. In the present invention examples (Nos. 5 to 9) in which the P concentration is within the range defined by the present invention, improvement in slidability is observed, and the water wetting rate after alkaline degreasing is 100%. Degreases. On the other hand, the comparative example (No. 4) in which the P concentration deviates to the low concentration side is insufficient, although the water wettability is slightly improved. Further, in the comparative example (No. 10) in which the P concentration deviates to the high concentration side, the water wettability is 100%, but the oxide film thickness is reduced and the slidability is not sufficiently improved.
(4) No. 11 to 16 are examples of the present invention in which a cleaning treatment was performed with a P-containing aqueous solution other than sodium diphosphate. It shows improved slidability and good alkaline degreasing properties.

  Since the alloyed hot-dip galvanized steel sheet of the present invention is excellent in weldability and paintability, it can be applied in a wide range of fields mainly for automobile body applications.

The figure which shows the principal part of the oxide layer formation processing equipment used in the Example. The schematic front view which shows a friction coefficient measuring apparatus. FIG. 3 is a schematic perspective view showing bead shapes and dimensions in FIG. Schematic perspective view showing bead shape and dimensions in FIG.

Explanation of symbols

1 Activation treatment tank
2 Acidic solution tank
3 Drawing roll
4 Shower washing machine
5 Flush tank
6 Washing tank
7 Hot water bath
8 Dryer S Steel plate
11 Friction coefficient measurement sample
12 Sample stage
13 Slide table
14 Laura
15 Slide table support
16 beads
17 First load cell
18 Second load cell
19 Rail N Pressing load F Sliding resistance

Claims (2)

  The surface of the galvanized steel sheet is subjected to hot dip galvanizing on the steel sheet, alloyed by heat treatment, temper rolled, contacted with an acidic solution, left for 1 to 30 seconds after contact is completed, and then washed with water. In the method for producing a galvannealed steel sheet in which a Zn-based oxide layer having a thickness of 10 nm or more is formed, the rinsing is performed with a P-containing aqueous solution having a P concentration of 0.5 to 5000 ppm. A method of manufacturing a steel sheet.   A plated steel sheet produced by the method for producing an galvannealed steel sheet according to claim 1, wherein an oxide layer containing Zn and Al as essential components in a flat surface layer of the plated steel sheet is 10 nm or more. An alloyed hot-dip galvanized steel sheet.

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