JP2008285705A - Alloy hot-dip galvanized steel sheet - Google Patents

Abstract

An alloyed hot-dip galvanized steel sheet having excellent press formability is provided.
A Fe-Zn alloy plating phase has at least one surface of a steel plate, the Fe-Zn alloy plating phase has a flat portion on the plating surface, and Zr is further formed on the surface of the flat portion. An oxide containing Zn as an essential component in an atomic ratio of Zr / Zn of 0.01 to 0.4 is formed with an average thickness of 10 nm to 200 nm. For example, an alloyed hot-dip galvanized steel sheet containing Zr with a Zr / Zn ratio of 0.05 and an oxide layer mainly composed of Zn formed at 27.6 nm has a pressing load of 400 kgf and a pressing load of 1500 kgf at a sample temperature of 25 ° C. (room temperature). In this case, the friction coefficients are 0.128 and 0.098, respectively.
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Description

  The present invention relates to an alloyed hot-dip galvanized steel sheet having excellent press formability even in a material having a high forming load, such as a high-strength alloyed hot-dip galvanized steel sheet, which tends to cause mold galling.

An alloyed hot-dip galvanized steel sheet is widely used in a wide range of fields, mainly for automobile body applications, because it is superior in weldability and paintability compared to a galvanized steel sheet that is not subjected to alloying treatment. 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 galvanized 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. 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 coating film having a high Fe concentration has a problem that a hard and brittle Γ phase is easily formed at the plating-steel plate interface, and a phenomenon of peeling from the interface during processing, that is, so-called powdering is likely to occur. For this reason, 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.

  In addition to this, as a method for improving the press formability when using a galvanized 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 coating 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 and workability by forming a film is disclosed.

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

  In Patent Document 5, the surface of a zinc-based plated steel sheet is improved in press formability and chemical conversion treatment by generating Ni oxide by electrolytic treatment, dipping treatment, coating treatment, coating oxidation treatment, or heat treatment. Technology is disclosed.

In Patent Document 6, an alloyed hot-dip galvanized steel sheet is brought into contact with an acidic solution to form an oxide mainly composed of Zn on the surface of the steel sheet, suppressing adhesion between the plating layer and the press mold, and slidability. A technique for improving the above is disclosed.
Japanese Patent No. 1-319661 JP-A-53-60332 Japanese Patent Laid-Open No. 2-190483 JP-A-4-88196 Japanese Patent Laid-Open No. 3-191093 JP2003-306781

  However, Patent Documents 1 to 6 are effective for a relatively low-strength alloyed hot-dip galvanized steel sheet that is often used for automobile outer plates, but because of the high load during press molding, In a high-strength galvannealed steel sheet with increased contact surface pressure, the effect of improving press formability cannot always be obtained stably.

  In view of such circumstances, an object of the present invention is to provide an alloyed hot-dip galvanized steel sheet having excellent press formability even in a material having a high forming load such as a high-strength alloyed hot-dip galvanized steel sheet, which is likely to cause die galling. And

The inventors of the present invention made further studies to solve the above problems. As a result, the following knowledge was obtained.
An oxide layer mainly composed of Zn is formed on the surface of the alloyed hot-dip galvanized steel sheet manufactured by the method of Patent Document 6, and this oxide layer mainly composed of Zn is condensed with the mold during pressing. Wear resistance is reduced and sliding resistance is reduced. This oxide mainly composed of Zn (hereinafter sometimes referred to as Zn-based oxide) is mainly formed in a flat portion formed by temper rolling or the like. In actual press molding, the surface that preferentially contacts the mold is this flat part. When the contact surface pressure is low, the Zn oxide on the surface of the flat part is in direct contact between the mold and the plating layer surface. By suppressing the above, an effect of improving press formability can be obtained. However, when high strength steel is used as the base steel plate for plating, the forming load is higher than that of soft steel, and galling and cracking are likely to occur. In such a case, the Zn-based oxide described in Patent Document 6 is used. The layer was found to be inefficient.

And, as a result of further earnest research to find out this cause, the inventors of the present invention have developed a high lubricity even when the molding load is high in the Zn-based oxide layer. I found it important. The reason for this is considered to be that when the adhesion of the oxide layer is low, the oxide layer is easily removed by contact with the mold, and the alloy phase of the plating and the mold are in direct contact and are liable to cause adhesion. Particularly when the forming load is high, the contact surface pressure between the steel plate and the mold immediately before pressing is high, and the effect of removing oxides during pressing is large, and this tendency is considered to be remarkable. The present inventors have found that inclusion of Zr in a Zn-based oxide is effective for the above solution.
The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] Fe-Zn alloy plating phase is provided on at least one surface of a steel plate, the surface of the Fe-Zn alloy plating phase has a flat portion, and Zr is added to the surface of the flat portion. An alloyed hot-dip galvanized steel sheet characterized in that an oxide containing 0.01 to 0.4 in terms of Zn and containing Zn as an essential component is formed with an average thickness of 10 nm to 200 nm.

  According to the present invention, an alloyed hot-dip galvanized steel sheet having a low sliding resistance during press forming and excellent press formability can be obtained.

  The alloyed hot-dip galvanized steel sheet has macro unevenness on the surface of the plating phase due to the difference in reactivity between the steel sheet and the plating interface during the alloying treatment and the angular shape of the Fe-Zn alloy. In such an alloyed hot-dip galvanized steel sheet, for example, by providing a flat portion on the surface of the plating phase by temper rolling, the unevenness on the surface of the plating phase is alleviated to smooth the surface, and at the same time, the convex portion on the surface of the plating phase is flattened. (Hereinafter, the flattened convex portion is referred to as a flat portion). Since the flat part of the surface of the galvannealed steel sheet formed in this way is the part where the mold comes into direct contact during press forming, reducing the sliding resistance of the flat part can improve the press formability. It is important to lead to stable improvement. As a method for reducing the sliding resistance of the flat portion, there is a method in which a hard and high-melting substance that prevents adhesion to the mold is present in the flat portion. In this respect, the presence of the Zn-based oxide layer on the surface of the flat portion is effective in improving the sliding characteristics because the oxide layer prevents adhesion with the mold.

When the load at the time of press molding is high, the plating phase surface and the mold come into contact with each other at a high surface pressure, and are slid at a high surface pressure. For this reason, if the adhesion between the Zn-based oxide layer and the plating phase surface is low, the oxide layer peels off and the surface of the plating alloy comes into contact with the mold, thereby reducing the adhesion suppressing effect. Here, when Zr is contained in the Zn-based oxide layer, the adhesion between the plating phase surface and the Zn-based oxide layer is improved, and the slidability is improved. The reason for this is not clear, but it is presumed that Zr changes the physical properties and film shape of the Zn-based oxide to improve the adhesion with the surface of the plating phase.
Furthermore, in the present invention, the amount of Zr contained in the flat portion surface is set to 0.01 to 0.4 in terms of the atomic ratio of Zr / Zn. If the atomic ratio of Zr / Zn is smaller than 0.01, the effect of containing Zr is insufficient. When the atomic ratio of Zr / Zn exceeds 0.4, although effective, it is difficult to produce and disadvantageous industrially.

In the present invention, the oxide layer containing Zr is a layer made of an oxide containing Zr in an atomic ratio of Zr / Zn of 0.01 to 0.4 and containing Zn as an essential component. The oxide layer containing Zn as an essential component here may contain at least Zn and O, and other elements such as hydroxide and components contained in the treatment liquid may be bonded. included.
And the average thickness of such an oxide layer shall be 10 nm or more and 200 nm or less in a flat part. When the average thickness of the oxide layer is less than 10 nm, the effect of lowering the sliding resistance is insufficient even if Zr is contained. On the other hand, when the average thickness of the oxide layer exceeds 200 nm, the coating is destroyed during press working, the sliding resistance increases, and the weldability tends to decrease. In the present invention, the thickness and composition of the oxide layer in the flat portion are defined, but there is no problem even if a similar oxide layer exists in a region other than the flat portion. From the viewpoint of slidability, the presence of the oxide layer in a region other than the flat portion is preferable because the slidability tends to be improved. The reason is considered that when the contact surface pressure is high, the region other than the flat portion comes into direct contact with the mold in addition to the flat portion.
As described above, in the present invention, an oxide containing Zr in an atomic ratio of Zr / Zn of 0.01 to 0.4 and having Zn as an essential component is formed on the surface of the flat portion with an average thickness of 10 nm to 200 nm.

  In the present invention, if Zr is contained in the Zn-based oxide, the slidability is excellent. Therefore, the effects of the present invention can be achieved even if other metal ions or inorganic compounds are contained as impurities or intentionally. Is not impaired.

  Here, as a method of forming a Zn-based oxide containing Zr on the flat portion surface, a method of contacting with an acidic solution containing Zr can be mentioned. Specifically, steel sheet is hot dip galvanized, alloyed by heat treatment, temper rolled to form a flat part, then contacted with an acidic solution containing Zr, and left for 1 to 120 seconds after completion of contact Then, a Zn-based oxide layer of 10 nm or more is formed on the surface of the galvanized steel sheet by washing with water. The acidic solution may contain Zn. The Zr and Zn concentrations in the acidic solution may be adjusted so that the content in the film is within the range defined by the present invention, an example of which is shown below.

  In order to contain Zr ions in the acidic solution, the acidic solution contains at least one of Zr sulfate, nitrate, chloride, and phosphate in the range of 0.1 to 50 g / l as the Zr ion concentration. It is preferable to do. If the ion concentration is less than 0.1 g / l, the Zr content is insufficient, and the effect of improving the slidability may be insufficient. On the other hand, if it exceeds 100 g / l, the formation of oxide becomes unstable.

The acidic solution used preferably has a pH buffering action in the pH range of 2.0 to 5.0. This is because when an acidic solution having a pH buffering action in the pH range of 2.0 to 5.0 is used, it is maintained for a predetermined time after contact with the acidic solution, so that the dissolution of Zn and the Zn-based oxidation are caused by the reaction between the acidic solution and the plating layer. This is because a product formation reaction occurs sufficiently, and an oxide layer can be stably obtained on the steel sheet surface. Zr is considered to be effectively incorporated into the Zn-based oxide during this period. Acidic solutions with such pH buffering properties include acetates such as sodium acetate (CH ₃ COONa), phthalates such as potassium hydrogen phthalate ((KOOC) ₂ C ₆ H ₄ ), sodium citrate (Na Citrates such as ₃ C ₆ H ₅ O ₇ ) and potassium dihydrogen citrate (KH ₂ C ₆ H ₅ O ₇ ), succinates such as sodium succinate (Na ₂ C ₄ H ₄ O ₄ ), and lactic acid Examples include lactate such as sodium (NaCH ₃ CHOHCO ₂ ), tartrate such as sodium tartrate (Na ₂ C ₄ H ₄ O ₆ ), borate, phosphate, etc., and at least one of these, It is preferable to use an aqueous solution containing each component content in the range of 5 to 50 g / l. When the concentration is less than 5 g / l, the pH of the solution rises relatively quickly with the dissolution of zinc, so that an oxide layer sufficient for improving the slidability cannot be formed. On the other hand, if it exceeds 50 g / l, dissolution of zinc is promoted, and not only does it take a long time to form an oxide layer, but the plating layer is also severely damaged, and it may lose its original role as a rust-proof steel sheet. Because it is.

  The pH of the acidic solution is desirably in the range of 0.5 to 2.0. This is because when the pH exceeds 2.0, precipitation of Zr ions (formation of hydroxide) occurs in the solution, and the Zr-based oxide is not effectively taken into the oxide layer. On the other hand, if the pH is too low, dissolution of zinc is promoted and not only the amount of plating is reduced, but also the plating film is cracked and easily peeled during processing. Therefore, the pH is preferably 0.5 or more. When the pH of the acidic solution is higher than the range of 0.5 to 2.0, the pH can be adjusted with an inorganic acid having no pH buffering property such as sulfuric acid.

The temperature of the acidic solution is preferably in the range of 20 to 70 ° C. If it is less than 20 ° C., the production reaction of the oxide layer takes a long time, and the productivity may be lowered. On the other hand, when the temperature is high, the reaction proceeds relatively quickly, but conversely, processing unevenness tends to occur on the surface of the steel sheet, so it is desirable to control the temperature to 70 ° C. or lower.
There is no particular limitation on the method of bringing the alloyed hot-dip galvanized steel sheet into contact with the acidic solution. For example, the method of immersing the plated steel sheet in the acidic solution, the method of spraying the acidic solution onto the plated steel sheet, the acidic solution via the coating roll There is a method of applying to the plated steel sheet, and it is desirable that it is finally formed in a thin liquid film form on the surface of the steel sheet. This is because when the amount of acidic solution present on the steel sheet surface is large, the pH of the solution does not increase even if zinc dissolution occurs, and only zinc dissolution occurs one after another. This is because it not only has a long time but also severely damages the plating layer, and it is considered that the original role as a rust-proof steel sheet is lost. From this viewpoint, it is preferable and effective that the amount of the solution film formed on the surface of the steel plate is adjusted to 10 g / m ² or less. The amount of the solution film can be adjusted by a squeeze roll, air wiping or the like.

Moreover, after contacting an acidic solution, the time to water washing (holding time to water washing) is preferably 1 to 120 seconds. If the time until washing with water is less than 1 second, the acidic solution is washed away before the pH of the solution rises and the Zr-based oxide layer and Zn-based oxide layer are formed. This is because the improvement effect cannot be obtained, and even if it exceeds 120 seconds, the amount of the oxide layer is not changed.
Moreover, when manufacturing these products, in order to suppress the growth of hard and brittle alloy layers at the interface between the molten metal and the base steel plate and to improve the plating adhesion, the main component (Zn, Al, etc.) A component other than the main component (for example, Al with respect to the main component Zn) is often added to a molten metal in a small amount.
Further, the additive element component to the galvannealed steel sheet according to the present invention is not particularly limited, and in addition to Al that is usually added, for example, Fe, Pb, Sb, Si, Sn, Mg, Mn, Even if Ni, Ti, Li, Cu or the like is contained or added, the effect of the present invention is not impaired.

  Furthermore, S, N, Pb, Cl, Na, Mn, Ca, Mg, Ba, Sr, Si, P, etc. are taken into the oxide layer due to impurities contained in the treatment solution used for oxidation treatment, etc. However, the effect of the present invention is not impaired.

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 to form a flat portion on the surface of the plated phase. Subsequently, as an oxide forming treatment, it was immersed for 3 seconds in an acidic solution obtained by adding Zr at a concentration shown in Table 1 to an acidic aqueous solution of sodium acetate 40 g / l. The temperature was appropriately changed as shown in Table 1. Then, after carrying out roll squeezing and adjusting the amount of liquid, it was allowed to stand at room temperature in the atmosphere for 1 to 30 seconds, sufficiently washed with water, and then dried. As a comparative material, a material which was processed in the same manner as described above without adding Zr was produced. Note that Zr (SO ₄ ) ₂ .4H ₂ O was used as the Zr ion source.

  As a technique for measuring the thickness of the oxide layer and the content of Zr in the flat part of the plating surface layer for the steel sheet produced as described above, and simply evaluating the adhesion and press formability of the oxide layer The coefficient of friction was measured. The measuring method is as follows.

(1) Measurement of the oxide layer thickness Using Auger electron spectroscopy (AES), the surface area of the plating layer is analyzed using Ar ion sputtering in the depth direction of the constituent elements, and the composition distribution of each element in the depth direction. Was measured. In the composition distribution, at a position where the O concentration is deeper than the maximum value, the depth at which the O value is 1/2 of the sum of the maximum value and the constant value is defined as the thickness of the oxide layer. And the thickness of the oxide layer was measured about two places of the flat part, and let the average value be the thickness of the oxide layer.

(2) Measurement of Zr content of oxide layer The replica method was used to evaluate the Zr content in the oxide layer in the flat portion. An acetylcellulose film was pressure-bonded to the surface of the sample for 30 seconds via acetone on the surface of the alloyed hot-dip galvanized steel sheet having the oxide layer, and peeled off. After carbon was deposited on the peeled surface, acetylcellulose was dissolved to prepare a sample for a transmission electron microscope (TEM). Standard-less quantification (thin film approximation method) using an energy dispersive X-ray spectrometer (EDS) for oxide layer flakes collected from a flat oxide layer using TEM (Philips CM30) at an acceleration voltage of 200 kV ) And the atomic ratio between Zr and Zn was calculated from the obtained results. The measurement was carried out at five or more locations per sample and averaged.

(3) Evaluation of adhesion of oxide layer A test was conducted in which an acetylcellulose film was pressure-bonded to the surface of a sample for 30 seconds via acetone on the surface of an alloyed hot-dip galvanized steel sheet on which the oxide layer was present. . Using the SEM (LEO1530), the sample surface was scanned at an acceleration voltage of 0.5 kV, and an image was captured as digital data. Image capture was performed under the same conditions for the same field of view before and after the peel test. Here, the SEM image was captured under the condition that the surface oxide distribution can be visualized (the region where the oxide layer is present is shown in dark contrast (see JP-A-2005-121467). The average brightness of the flat area was digitized, and each image data was binarized by brightness in areas where oxides were not present and areas where natural oxide film thickness would be present. The area where an object is present is obtained, and the decrease rate of the area before and after the peel test is used as an index of adhesion, that is, the area ratio of the film peeled by the peel test is obtained, and the smaller the value, the higher the adhesion. The observation magnification of SEM was 2000 times, and the average was obtained by measuring 5 different fields per sample.The brightness was digitized in 256 gradations using commercially available software Photoshop (manufactured by Adobe). Digitized.

(4) Slidability 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. 1 is a schematic front view showing a friction coefficient measuring apparatus. As shown in the figure, a friction coefficient measuring sample 1 collected from a test material is fixed to a sample table 2, and the sample table 2 is fixed to the upper surface of a slide table 3 that can move horizontally. On the lower surface of the slide table 3, there is provided a slide table support base 5 having a roller 4 in contact therewith and capable of moving up and down, and by pushing it up, a pressing load N applied to the friction coefficient measuring sample 1 by the bead 6 is applied. A first load cell 7 for measurement is attached to the slide table support 5. A second load cell 8 is attached to one end portion of the slide table 3 in order to measure the sliding resistance force F for moving the slide table 3 in the horizontal direction with the pressing force applied. In addition, the cleaning oil Preton R352L for press made by Sugimura Chemical Co., Ltd. was applied to the surface of the friction coefficient measurement sample 1 as a lubricant, and the test was performed.

  FIG. 2 is a schematic perspective view showing the shape and dimensions of the beads used. The bead 6 slides with its lower surface pressed against the surface of the friction coefficient measurement sample 1. The shape of the bead 6 shown in FIG. 2 is 10 mm wide, 12 mm long in the sliding direction of the sample, the lower part of both ends of the sliding direction is a curved surface with a curvature of 4.5 mmR, and the bottom surface of the bead against which the sample is pressed is 10 mm wide and in the sliding direction It has a 3mm long plane.

  For the measurement of the friction coefficient, assuming a severe press environment with high strength alloyed hot dip galvanized steel sheet with high forming load and high galling, press load N is 400kgf at room temperature (25 ° C). Changed to 1500 kgf. The sample drawing speed (the horizontal movement speed of the slide table 3) is 100 cm / min. Under these conditions, the pressing load N and the pulling load F were measured, and the coefficient of friction μ between the test material and the bead was calculated by the formula: μ = F / N.

  The test results obtained from the above are shown in Table 1. In Table 1, condition 1 indicates a pressing load of 400 kgf and a sample temperature of 25 ° C. (room temperature), and condition 2 indicates a pressing load of 1500 kgf and a sample temperature of 25 ° C. (room temperature).

From the test results shown in Table 1, the following matters were clarified.
Nos. 1 and 2 are comparative examples in which the treatment with an acidic solution was performed, but the oxide layer did not contain Zr. Compared to the coefficient of friction of the comparative example in which the acidic solution treatment is not performed, the oxide layer mainly composed of Zn is formed on the flat portion of the plating phase surface, which is lower, but compared to the example of the present invention. high. Further, in the oxide layer adhesion test, peeling occurred with an area ratio of more than 50% on average.
Nos. 3 to 12 are examples of the present invention in which an oxide layer containing Zr is applied to a flat portion. The area reduction rate is less than 20%, which is lower than that of the comparative example, and the adhesion is good.
Further, the coefficient of friction of the example of the present invention is lower than that of the comparative example. In particular, the friction coefficient is low and stable under condition 2 where the surface pressure is high.
It can be seen that the higher the Zr content, the better the adhesion of the oxide layer. The coefficient of friction is related to the oxide layer thickness, and the thicker the layer, the smaller the coefficient of friction tends to be. When the oxide layer thickness is comparable and the inventive example containing Zr and the comparative example not containing Zr, the inventive example clearly has higher adhesion and a lower friction coefficient. This characteristic expression is an effect due to the inclusion of Zr in the oxide layer.
In Comparative Example 3 of No. 13, no oxidation treatment was performed. The friction coefficient is higher in both conditions 1 and 2 than in the example of the present invention.

  Since it has excellent slidability, it has excellent press formability and can be applied in a wide range of fields, mainly for automobile body applications.

Schematic front view showing friction coefficient measuring device Schematic perspective view showing bead shape and dimensions in FIG.

Explanation of symbols

1 Sample for friction coefficient measurement
2 Sample stage
3 Slide table
4 Roller
5 Slide table support
6 beads
7 First load cell
8 Second load cell
N Push load
F Sliding resistance force

Claims (1)

  The Fe—Zn alloy plating phase has at least one surface of the steel sheet, and the surface of the Fe—Zn alloy plating phase has a flat portion, and the surface of the flat portion has Zr as Zr / Zn atoms. An alloyed hot-dip galvanized steel sheet characterized in that an oxide containing Zn as an essential component in a ratio of 0.01 to 0.4 is formed with an average thickness of 10 nm to 200 nm.

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