JP5045120B2 - Alloy hot-dip galvanized steel sheet - Google Patents

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 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 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.

As a technique for forming a metal oxide film on the upper layer of the zinc-based plating layer, Patent Document 7 discloses a metal oxide film of Mo, V, Cu, Sn, and Patent Document 8 describes Ni, Mn, Ti, Co, A technique for forming a metal oxide film of Ca, V, W, Sn, and Fe is disclosed. Patent Document 9 discloses a technique for distributing a metal oxide film formed on a zinc-based plating layer in an island shape or a mosaic shape.
JP-A-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 JP-A-5-214558 Japanese Unexamined Patent Publication No. 7-18400 JP-A-6-116746

  However, Patent Documents 1 to 9 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.

  Moreover, the technique of forming the oxide film of Sn on the surface layer of the zinc-based plated steel sheet disclosed in Patent Documents 7 to 9 utilizes the fact that the Sn oxide is harder than the plated layer. Thus, although the improvement effect was observed as compared with the case of using only the Zn-based oxide, the improvement effect was insufficient.

  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. However, when using high-strength steel as the base steel sheet for plating, the forming load is higher than that of soft steel, and galling and cracking are likely to occur, and the effect of the Zn-based oxide layer described in Patent Document 6 is insufficient. I found out. Moreover, when the Sn oxide layer was made to exist as described in Patent Documents 7 to 9, it was found that the improvement effect was seen but insufficient.

  As a result of further research, the present inventors have a limit to exhibit high lubricity even when the molding load is high only with the Zn-based oxide layer, and in addition to the Zn-based oxide layer, the metal state It has been found that remarkably high slidability can be obtained by mixing particles containing Sn as a main component. The reason for this is not clear, but the effect of reducing the shear resistance with the mold is due to the presence of a soft metal (Sn) on the surface compared to the Zn-Fe alloy and Zn-based oxide that are plating components. I think that it was because it was granted. Further, in the present invention, it is presumed that there is a concerted effect with the adhesion suppression effect by the Zn-based oxide by mixing the Zn-based oxide with particles containing Sn as a main component mainly in the metal state. .

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 the steel plate, and particles having Sn as an essential component mainly in a metal state and Zn are essential on the surface of the Fe-Zn alloy plating phase An alloyed hot-dip galvanized steel sheet characterized by the presence of an oxide as a component.
[2] The alloyed molten zinc according to [1], wherein an adhesion amount of Sn is 0.05 g / m ² or more, and an average film thickness of an oxide containing Zn as an essential component is 10 nm or more. Plated steel sheet.

  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 surface of the alloyed hot-dip galvanized steel sheet is composed of an Fe—Zn alloy. In order to stably improve the press formability, it is important to reduce the sliding resistance with the mold on the surface of the galvannealed steel sheet composed of the Fe-Zn alloy as described above. .
As a method for reducing the sliding resistance on the surface of the alloyed hot-dip galvanized steel sheet, a Zn-based oxide is present on the plated surface. This Zn-based oxide on the plating surface prevents adhesion to the mold and is effective in improving the sliding characteristics. However, under more severe conditions, such as when the load during press molding is high, the plating surface comes into contact with the mold at a high surface pressure and slides at a high surface pressure. Even if it exists, the plating surface and the mold come into direct contact and cause adhesion. In that case, the shearing stress between the plating alloy and the mold becomes a large sliding resistance. Here, when particles containing Sn as an essential component in a metal state as a main component are mixed, this sliding resistance is reduced. The reason for this is thought to be due to the presence of soft metal Sn, which is extended during sliding and spreads between the plating surface and the mold to prevent direct contact between them. Since metal Sn has a very small shear stress, the contact resistance between the mold and the plating surface is small. However, the metal Sn needs to be present at the same time as the Zn-based oxide. For example, even if metal Sn alone is applied to the alloyed hot dip galvanized surface, it has the effect of reducing contact resistance, but the Sn layer is easily deformed, so it is easily cut off at the apex of the plating unevenness and the unevenness of the mold. The effect disappears in a short time. Therefore, the effect is insufficient. In the present invention, the suppression of adhesion of a hard Zn-based oxide having a relatively high melting point is also utilized by mixing metal Sn with the Zn-based oxide. Moreover, the effect can be exhibited in the place crushed by making metal Sn into a particle form instead of a layer form. It is also estimated that the Zn-based oxide has an effect of holding the metal Sn particles on the plating surface.

  As described above, the galvannealed steel sheet of the present invention has an Fe-Zn alloy plating phase on at least one surface of the steel sheet, and the surface of the Fe-Zn alloy plating phase is Sn mainly composed of a metal state. It is assumed that there are particles having an essential component of Zn and an oxide having Zn as an essential component. This is the most important requirement in the present invention.

Sn in the particles is mainly in a metal state, but a part of the surface or the like may be an oxide. As the Sn coating amount and the average film thickness of the oxide containing Zn as an essential component increase, a higher slidability improvement effect can be obtained, but the Sn coating amount is 0.05 g / m ² or more and the oxide containing Zn as an essential component The film thickness is preferably 10 nm or more. When a flat convex portion is provided on the plating surface by temper rolling or the like, it is preferable that the oxide film thickness containing at least Zn in that portion as an essential component is 10 nm or more.

There is no particular upper limit on the Sn deposition amount and the oxide film thickness with Zn as an essential component, but the improvement effect is small when the Sn deposition amount exceeds 5 g / m ² and the average film thickness of the oxide with Zn as an essential component exceeds 80 nm. It is uneconomical.

In addition, the shape of the metal Sn particles is not particularly defined, and is not limited to a circle but may be an irregular shape. The size of the particles is not particularly specified, and it is sufficient that there is no problem in peeling. For example, assuming that the average value of the longest part and the shortest part of the particle is the average particle diameter, fine particles having an average particle diameter of 2 μm or less exhibit a sufficient effect.
Further, the distribution state of the metal Sn particles is not particularly defined and may be segregated, but it is preferable that the number of metal Sn particles is one or more per 20 μm × 20 μm area. When flat protrusions are provided on the plating surface by temper rolling, it is more effective to preferentially apply metal Sn to the plating surface because there is a high probability that the flat protrusions are in direct contact with the mold. It is.

FIG. 1 is a transmission electron micrograph showing one embodiment of the galvannealed steel sheet of the present invention. Specifically, FIG. 1 is a transmission electron microscope (TEM) image in a state where deposits are peeled off from the surface of the alloyed hot-dip galvanized steel sheet according to the present invention using an acetylcellulose film. In FIG. 1, the particulate matter that appears dark in the image is metallic Sn particles. FIG. 1 shows that metal Sn having a particle diameter of about 1 μm or less is dispersed.
As a result of analyzing the particles in FIG. 1 using an energy dispersive X-ray spectrometer (EDS) attached to the TEM, the main component was Sn, but Zn and O were also detected. Thus, although the metal Sn particles may contain some Zn or the surface layer or the like may be partially oxidized, the main component is the metal Sn particles.
The oxide layer containing Zn as an essential component only needs to contain at least Zn and O, and includes a case where other elements such as hydroxide are bonded.

  In the present invention, since particles having Sn as an essential component in a metal state and an oxide having Zn as an essential component are present on the surface of the steel sheet, the slidability is excellent. Even if an inorganic compound or the like is contained in the metal particles or oxide layer as impurities or intentionally, the effect of the present invention is not impaired.

  The surface of the steel plate is only required to have particles containing Sn as an essential component in the metal state and oxides containing Zn as an essential component, and the formation method is not particularly specified. The method of making it contact with a solution is mentioned. An example of the production method is shown below. Specifically, hot dip galvanizing is applied to the steel sheet, further alloyed by heat treatment, and if necessary, subjected to temper rolling to form a flat part, and then contacted with an acidic solution containing Sn ions. After leaving for 120 seconds, washing with water can simultaneously form particles containing Sn as an essential component and a Zn-based oxide on the plating surface. The point is that the dissolution of Zn, precipitation of Zn hydroxide, and substitutional precipitation of metal Sn occur simultaneously with the acid. The Sn concentration and the Sn source in the acidic solution may be appropriately selected and adjusted so that metal Sn particles are present on the surface.For example, Sn sulfate, nitrate, chloride, phosphate, etc. The ion concentration is preferably in the range of 0.1 to 50 g / l. If the ion concentration is less than 0.1 g / l, the precipitation of metal Sn is insufficient and the effect of improving the slidability may be insufficient. On the other hand, if it exceeds 50 g / l, the formation of the Zn-based oxide may be 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. During this time, Sn is deposited on the surface as metallic Sn. 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 preferably in the range of 0.5 to 2.0. When the pH of the acidic solution is higher than 0.5 to 2.0, it is preferable to adjust the pH 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.
Examples of the method of bringing the alloyed hot-dip galvanized steel sheet into contact with the acidic solution include a method of immersing the plated steel sheet in an acidic solution, a method of spraying the acidic solution onto the plated steel sheet, and applying the acidic solution to the plated steel sheet via a coating roll It is desirable that the thin film is finally present 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, the amount of the solution film formed on the steel sheet surface is preferably adjusted to 100 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 out before the pH of the solution rises and the metal Sn particles and the Zn-based oxide layer are formed, so that the effect of improving the slidability is obtained. I can't. If the amount of the solution film formed on the surface of the steel sheet is small even if it exceeds 120 seconds, there will be no significant change in the amount of metal Sn and the amount of Zn-based oxide, and productivity will be reduced. Processing times of seconds or longer are not preferred.
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.) In many cases, a small amount of a component other than the main component is added to a molten metal.

  Further, the additive element component to the galvannealed steel sheet according to the present invention is not particularly limited, and besides Al that is usually added, for example, Pb, Sb, Si, Sn, Mg, Mn, Ni, Even if Ti, Li, Cu or the like is contained or added, the effect of the present invention is not impaired.

  Furthermore, even if impurities are included in the treatment liquid used for oxidation treatment, etc., S, N, Pb, Cl, Na, Mn, Ca, Mg, Ba, Sr, Si, etc. may be taken into the oxide layer. 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 temper rolled. Subsequently, as an oxide formation treatment, after using SnSO ₄ as an Sn ion source in an acidic aqueous solution of sodium acetate 40 g / l and immersing in an acidic solution in which the addition amount was appropriately changed, roll squeezing and adjusting the liquid amount Then, it was allowed to stand at room temperature in the atmosphere for a predetermined time, sufficiently washed with water, and then dried. As a comparative material, a material that was subjected to the same treatment as described above without adding Sn, and a sample that was not subjected to the acidic solution treatment itself were produced.

  For the steel sheet produced as described above, the thickness of the oxide layer on the plating surface and the presence form of Sn were evaluated, and the friction coefficient was measured as a method for simply evaluating the press formability. The measuring method is as follows.

(1) Measurement of oxide layer thickness The surface of the plating surface was analyzed by Auger electron spectroscopy (AES) using Ar ion sputtering, 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. The thickness of the oxide layer was measured for each of two portions of the flat part (convex part) and the concave part during temper rolling, and the average value was defined as the thickness of the oxide layer.

(2) Presence of metal Sn on the plating surface The metal Sn in the flat part of the form was evaluated by the replica method. The 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 subjected to the treatment, and peeled off. After vapor deposition of carbon on the peeled surface, acetylcellulose was dissolved to obtain a thin sample for a transmission electron microscope (TEM). A thin-field sample was observed with a TEM (Philips CM30) at an acceleration voltage of 200 kV, and the particle diameter and number of metallic Sn were confirmed. Moreover, it confirmed that metal Sn was a main component using the energy dispersive X-ray spectrometer (EDS). The dispersion state of the metal Sn can also be performed with a scanning electron microscope in which the acceleration voltage is low, for example, 1 kV or less.

(3) 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. 2 is a schematic front view showing the 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 with the slide table 3 and capable of moving up and down. A first load cell 7 for measurement is attached to the slide table support 5. A second load cell 8 for measuring a sliding resistance force F for moving the slide table 3 in the horizontal direction in a state where the pressing force is applied is attached to one end of the slide table 3. 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. 3 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. 3 is 10 mm wide, 12 mm long in the sliding direction of the sample, and the lower part of both ends of the sliding direction is a curved surface with a curvature of 4.5 mmR. 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.
In Comparative Example 1 of No14, the oxidation treatment was not performed. The oxide thickness of the flat portion is about 8 nm, and the friction coefficient is higher in both conditions 1 and 2 than in the present invention.
Comparative Examples 2 and 3 of Nos. 15 and 16 were treated with an acidic solution, but were prepared without adding Sn, and did not contain Sn on the plating surface. In this case, an oxide layer mainly composed of Zn is formed on the flat portion of the plating surface. Therefore, although it is lower than the friction coefficient of the galvannealed steel sheet not subjected to the acid solution treatment (Comparative Example 1), it is higher than the friction coefficient of the present invention example. ing.
Nos. 1 to 13 are examples of the present invention including metal Sn and a Zn-based oxide layer. The friction coefficient of the example of the present invention is lower than that of the comparative example. In particular, the coefficient of friction under condition 2 where the surface pressure is high is stable at a low level.
Further, the present invention examples containing metal Sn and the oxide layer having the same thickness and comparative examples not containing metal Sn (e.g., No. 1 invention example 1 and No. 15 and comparative example 2, No. 2 In comparison with Example 2 of the present invention and Comparative Example 3) of No. 16, the invention example containing metal Sn has a lower coefficient of friction than the comparative example, and the effect of adding metal Sn is clear.

  The alloyed hot-dip galvanized steel sheet of the present invention has superior press formability compared to conventional products even when it is used when the forming load is high and it tends to cause mold galling or cracking, or when severe forming is required and cracking is likely to occur. Therefore, it can be applied in a wide range of fields mainly for automobile body applications.

The transmission electron micrograph which shows one Embodiment of the galvannealed steel plate of this invention. The schematic front view which shows a friction coefficient measuring apparatus. FIG. 2 is a schematic perspective view showing bead shapes and dimensions in FIG.

Explanation of symbols

1 Friction coefficient measurement sample
2 Sample stage
3 slide table
4 rollers
5 Slide table support
6 beads
7 First load cell
8 Second load cell
N Push load
F Sliding resistance force

Claims (1)

Fe-Zn alloy plating phase at least on one side of the steel plate,
Furthermore, on the surface of the Fe—Zn alloy plating phase, there are particles containing Sn as an essential component mainly composed of a metal state and an oxide containing Zn as an essential component, and the amount of Sn deposited is 0.05 g / An alloyed hot-dip galvanized steel sheet having an average film thickness of m ² or more and an oxide containing Zn as an essential component is 10 nm or more .