JP2005248262A - Electrogalvanized steel sheet, manufacturing method and manufacturing apparatus thereof - Google Patents

Abstract

An object of the present invention is to stably produce a hot-dip galvanized steel sheet having a low sliding resistance during press forming and exhibiting stable and excellent press formability.
An electrogalvanized steel sheet comprising a Zn-based oxide layer comprising fine irregularities having an average thickness of 10 nm or more on a plated surface.
[Selection figure] None

Description

  The present invention relates to an electrogalvanized steel sheet excellent in slidability at the time of press forming, a manufacturing method thereof, and a manufacturing apparatus.

  In recent years, from the viewpoint of improving rust prevention, the proportion of galvanized steel sheets used in automotive panel parts has increased. There are two types of galvanized steel sheets: electrogalvanized steel sheets and hot dip galvanized steel sheets. In particular, an electrogalvanized steel sheet is mainly used for an outer panel of an automotive panel because the surface finish is good.

  In the electrogalvanized steel sheet, the zinc forming the plating layer is soft and has a low melting point. Accordingly, the sliding resistance with the press die is large and adhesion is likely to occur. For this reason, material inflow at the bead portion is reduced, and press cracks are often caused.

  In order to improve this, a technique is usually adopted in which highly viscous oil is applied and an oil film is formed at the interface between the zinc and the mold to improve lubricity and press formability. However, when the surface pressure level is high due to severe processing sites or high-strength steel forming, oil shortage occurs and the effect cannot be sufficiently obtained.

Therefore, as a method of further suppressing the contact between the mold and zinc, Patent Document 1 discloses a phosphate treatment that improves the bead sliding characteristics under high surface pressure of a zinc phosphate-based plated steel sheet and is further excellent in workability. A technique for providing a galvanized steel sheet is disclosed. Further, Patent Document 2 includes a coating made of zinc oxide, zinc carbonate and zinc hydroxide as a total of 50 mg / m ² or more as zinc, so that there is no damage to the plating layer at the time of press working, and formability, chemical conversion treatment, etc. Discloses a comprehensively excellent galvanized steel sheet and a method for producing the same.

  In the zinc phosphate-based plated steel sheet represented by Patent Document 1, a hard phosphate film is detached during pressing, which adheres to the mold, and a defect such as a scratch is generated on the pressed product. There is a problem that the quality is significantly lowered. In addition, the steel sheet of Patent Document 1 often has a phenomenon of galling due to the deposits.

On the other hand, the film made of zinc oxide, zinc carbonate, and zinc hydroxide disclosed in Patent Document 2 has a low slidability at a site such as a double bead, and therefore cannot always provide excellent press formability. Moreover, in patent document 2, the installation which sprays the gas containing carbonic acid is required, and there existed a problem that manufacturing cost became high.
JP 2003-119568 A JP 2000-328220 A

  In consideration of the above-mentioned problems, the present invention provides an electrogalvanized steel sheet having a low sliding resistance during press molding, stable weldability, no galling, etc., and stably exhibiting excellent press formability, and its An object is to provide a manufacturing method.

  In order to solve these problems, the present inventors have made extensive studies such as the surface shape of the zinc-based plated steel sheet, and as a result, formed a zinc-based oxide on the surface layer of the electrogalvanized steel sheet, thereby significantly improving the press formability. It has been found that an excellent steel sheet can be obtained. The present invention is based on this finding. Means of the present invention for solving the above problems are as follows.

(1) An electrogalvanized steel sheet having a Zn-based oxide layer composed of fine irregularities having an average thickness of 10 nm or more on a plated surface.
(2) After electrogalvanizing the steel sheet, the steel sheet is brought into contact with an acidic solution having a pH buffering action, and an oxidation treatment is performed in which the holding time until washing is 1 to 30 seconds. Manufacturing method of plated steel sheet.

(3) Production of an electrogalvanized steel sheet characterized by subjecting the steel sheet to electrogalvanizing and then contacting with an acidic electrogalvanizing bath and performing an oxidation treatment with a retention time of 1 to 30 seconds until washing with water. Method.
(4) The method for producing an electrogalvanized steel sheet as described in (2) above, wherein the acidic galvanized steel sheet is brought into contact with an acidic solution having a pH buffering action under a condition where the relative flow rate is 0.5 m / second or more.

(5) The method for producing an electrogalvanized steel sheet according to (3) above, wherein the acidic galvanizing bath is brought into contact with an acidic electrogalvanizing bath under a condition where the relative flow rate is 0.5 m / sec or more.
(6) A device for producing an electrogalvanized steel sheet having a plurality of electrolytic cells, a first electrolytic cell in which only a relative flow rate is applied to the electrogalvanized steel sheet and energization is turned off, and the first electrolytic cell And a second electrolytic cell that allows the steel plate to pass through without being immersed in a plating solution.

  General roughness parameters are defined as irregularities on the order of microns (μm) or higher, which are measured for roughness curves whose length is on the order of millimeters or more. However, the fine unevenness in the present invention is 100 nm or less in terms of the average roughness Ra (hereinafter simply referred to as “Ra”) of the roughness curve, and the average interval S (hereinafter simply referred to as “S”) of the local unevenness. )) Is a surface roughness of 1000 nm or less, and is calculated from a roughness curve having a length of several microns.

  Note that the thickness of the oxide layer in the present invention can be obtained by Auger electron spectroscopy (AES) combined with Ar ion sputtering. In this method, after sputtering to a predetermined thickness, the composition at that depth can be obtained by correcting the correlation sensitivity factor from the spectral intensity of each element to be measured. The oxygen content caused by oxides or hydroxides decreases and becomes constant after reaching a maximum value at a certain depth (this may be the outermost layer). Here, the depth at which the oxygen content is a half of the sum of the maximum value and the constant value at a position deeper than the maximum value is defined as the oxide thickness.

  According to the present invention, an electrogalvanized steel sheet having a small sliding resistance during press forming and stably exhibiting excellent press formability can be stably produced.

The present invention will be described in detail below.
The plating film of the electrogalvanized steel sheet is usually characterized by having unevenness due to the lamination of hexagonal plate crystals. Further, the form of the film changes depending on the ferrite structure of the steel sheet before electroplating and electroplating conditions. Lubricating oil is usually used for press molding. When the surface pressure is low, unevenness due to crystals becomes a micropool, and the amount of oil retained increases, resulting in a low coefficient of friction and deterioration of sliding characteristics. It may not be seen. However, when the sliding distance is increased, the unevenness due to the soft zinc crystal is easily flattened, and the oil holding force is reduced, so that the friction coefficient is increased.

  Furthermore, the metallic zinc forming the plating layer of the electrogalvanized steel sheet is a low melting point metal and is likely to adhere to the press die. Therefore, galling is likely to occur in the metallic zinc due to the digging action caused by the accumulated deposits. In particular, in press molding of difficult-to-mold parts, there are cases in which the bead passage portion has a high surface pressure and a die having double bead portions called double beads. Here, in a double bead die press, the same part is slid twice during press molding. Therefore, the unevenness resulting from the crystal form in the first pass of the bead is flattened, the oil retaining effect is lost, and the new surface is exposed. Furthermore, it is considered that zinc adhesion due to direct contact between the mold and zinc occurs during the second bead passage, and galling and press cracking are likely to occur.

In order to suppress such adhesion between the electrogalvanized steel sheet and the mold, it is effective to uniformly coat a thick oxide layer on the surface. This is because the oxide is harder and has a higher melting point than the metal. By forming the Zn-based oxide layer by oxidizing metal zinc on the surface of the electrogalvanized steel sheet, it is possible to suppress adhesion between the electrogalvanized steel sheet and the press mold. It is effective for improving the characteristics. As will be described later, a more preferred form is a state in which a zinc-based zinc-based oxide having fine unevenness obtained by the production method of the present invention covers almost the entire plating surface.

  About the oxide layer in this plating surface layer, favorable slidability is obtained by making the average thickness uniformly 10 nm or more, and the above-mentioned adhesion phenomenon can be suppressed. Furthermore, it is more effective when the average thickness of the oxide layer is 20 nm or more. If the oxide layer is sufficiently thick and is 10 nm or more, it remains even in the case where the oxide layer on the surface layer is worn in the press molding process in which the contact area between the mold and the workpiece becomes large, and is slidable. This is because it does not cause a decrease. Further, if the oxide layer is not uniform, adhesion between the mold and the steel plate occurs at a portion where the oxide layer is thin, and press formability deteriorates. Therefore, it is necessary to form the oxide layer uniformly. On the other hand, there is no upper limit to the average thickness of the oxide layer from the viewpoint of slidability. However, when a thick oxide layer is formed, the surface reactivity is extremely lowered, and it becomes difficult to form a chemical conversion film. Therefore, the thickness of the oxide layer is desirably 200 nm or less, more preferably 100 nm or less.

  In the present invention, the “Zn-based oxide” present on the plating surface may include not only a Zn-based oxide but also a Zn-based hydroxide, and all of them are Zn-based hydroxides. It may be a thing. Further, since impurities are included during the oxidation treatment, for example, a small amount of P, S, N, B, Cl, Na, Mn, Ca, Mg, Ba, Sr, Si, Al, etc. is taken into the oxide layer. However, the effect of the present invention is not impaired.

  As a method for forming an oxide layer, it is effective to contact an electrogalvanized steel sheet with an acidic solution having a pH buffering action described later, adjust the amount of the liquid film, leave it for 1 to 30 seconds, and then wash and dry it. It is. Here, the acidic solution indicates an aqueous solution having a pH of 1 to 4. If the pH exceeds 4, the dissolution rate of zinc is slow. On the other hand, if the pH is less than 1, the dissolution of zinc becomes excessive, and not only does it take time to form an oxide, which will be described later, but the plating layer is severely damaged, and the original function as a rust-proof steel sheet is reduced. .

  Although the oxide layer formation mechanism is not clear, it can be considered as follows. When the electrogalvanized steel sheet is brought into contact with an acidic solution, dissolution of zinc occurs from the steel sheet side. This dissolution of zinc simultaneously causes a hydrogen evolution reaction. Therefore, it is considered that as the dissolution of zinc proceeds, the hydrogen ion concentration in the solution decreases, and as a result, the pH of the solution increases and a Zn-based oxide layer is formed on the surface of the electrogalvanized steel sheet. Thus, in order to form a Zn-based oxide, it is necessary to increase the pH of the solution in contact with the steel sheet as the zinc is dissolved. Therefore, it is effective to adjust the holding time until water washing after contacting the steel sheet with the acidic solution. At this time, if the holding time is less than 1 second, the solution is washed out before the pH of the solution in contact with the steel plate rises, so that an oxide cannot be formed. Moreover, even if it is left for 30 seconds or more, no change is observed in the oxide formation. Therefore, after the steel sheet is brought into contact with an acidic solution having a pH buffering action and the amount of the liquid film is adjusted, it is left for 1 to 30 seconds.

  In the present invention, the holding time until water washing is important for oxide formation. In this holding process, an oxide (or hydroxide) having a special fine uneven structure grows. A more preferable holding time is 2 to 10 seconds. The acidic solution used in the present invention needs to have a pH buffering action.

  For this purpose, it is effective to add a component having a pH buffering effect to the acidic solution. The addition of the above components makes the pH of the treatment solution stable during actual production. In addition, the addition of the component prevents a local increase in pH and imparts an appropriate reaction time in the Zn-based oxide formation process due to the increase in pH accompanying the dissolution of Zn described above. For this reason, the oxide growth time can be secured, and this acts on the formation of an oxide having fine irregularities, which is a feature of the present invention. The anionic species of the acidic solution is not particularly defined, and examples include chlorine ion, nitrate ion, sulfate ion and the like. More preferably, it is a sulfate ion.

There is no restriction | limiting in the component seed | species as long as it has pH buffer property in an acidic region as such a component which has pH buffer property. For example, acetate such as sodium acetate (CH ₃ COONa), sodium phthalate citrate (Na ₃ C ₆ H ₅ O ₇ ) such as potassium hydrogen phthalate ((KOOC) ₂ C ₆ H ₄ ), and dicitrate Succinates such as sodium citrate succinate (Na ₂ C ₄ H ₄ O ₄ ) such as potassium hydrogen (KH ₂ C ₆ H ₅ O ₇ ), lactates such as sodium lactate (NaCH ₃ CHOHCO ₂ ), tartaric acid One or more of tartrate and borate phosphate such as sodium (Na ₂ C ₄ H ₄ O ₆ ) can be used.

  Moreover, as the density | concentration, it is desirable that it is the range of 5-50 g / I, respectively. This is because when the concentration is less than 5 g / I, the pH buffering effect is insufficient, and a predetermined oxide layer cannot be formed. This is because it takes a long time to form an oxide. When the plated steel sheet is brought into contact with the acidic solution, Zn is eluted and mixed from the plating, but this does not significantly disturb the formation of the Zn-based oxide. Therefore, the Zn concentration in the acidic solution is not particularly defined. A more preferable pH buffer and its concentration is a solution in which sodium acetate trihydrate is in the range of 10 to 50 g / I, more preferably in the range of 20 to 50 g / I. The oxide of the present invention can be obtained.

  Further, the oxide of the present invention can be obtained in any manner even when an electrogalvanizing process for applying electrogalvanizing to a steel sheet is used as an acidic solution. A large amount of zinc is incorporated in the acidic electrogalvanizing bath, and by providing a pH increase accompanying the above-mentioned dissolution of zinc, the oxide growth time can be ensured. It acts on the formation of oxides having a certain fine irregular shape. Here, there is no particular limitation on the acidic electrogalvanizing bath, and a general-purpose plating bath may be used.

  As a method for forming the oxide having the fine unevenness shape of the present invention using the above electrogalvanizing bath, after plating a predetermined plating amount in an electroplating facility having a plurality of electrolytic cells, By turning off the electrolytic cell and providing a section for giving a relative flow rate of 0.5 gal / second or more to the steel sheet, followed by a section for allowing air to flow without being immersed in the plating solution. The oxide can be obtained.

  FIG. 4 is a schematic view showing an example of an apparatus for producing the steel plate. Reference numeral 11 in the figure denotes a plurality of electrolytic cells for performing electrogalvanization on the steel plate 12. On the downstream side of these electrolytic cells 11, a first electrolytic cell 13 in which only a relative flow rate is given to the steel sheet subjected to electrogalvanization and the energization is turned off is arranged. On the downstream side of the first electrolytic bath 13, a second electrolytic bath 14 and a washing bath 15 are disposed in order so that the steel plate can be emptied without being immersed in the plating solution. In such a manufacturing apparatus, after electrogalvanizing the steel plate 12 in a plurality of electrolytic cells 11, the energization is turned off in the first electrolytic cell 13, and a relative flow velocity of 0.5 m / sec or more is applied to the steel plate in the electrogalvanizing bath. Then, the electrogalvanized steel sheet of the present invention is manufactured by allowing the steel sheet to pass through without being immersed in the plating solution in the second electrolytic cell 14 and then rinsing the steel sheet in the water rinsing tank 15.

  In the above manufacturing apparatus, an alkaline treatment tank (not shown) for bringing the steel sheet into contact with the alkaline treatment liquid as described later is provided between the large number of electrolytic tanks 11 and the downstream first electrolytic tank 13. You may arrange.

  As described above, in order to improve the press formability of the electrogalvanized steel sheet, it is necessary to uniformly form an oxide having a thickness of 10 nm or more (more preferably 20 nm or more) on the surface of the electrogalvanized steel sheet. The electrogalvanized steel sheet is active because the surface layer is covered with pure Zn. For this reason, the amount of reaction of zinc changes due to the change of the conditions in contact with the acidic solution depending on the site, and a uniform oxide film cannot be formed. The present inventors have found that in order to form a uniform oxide film, it is effective that the relative flow rate between the electrogalvanized steel sheet and the acidic solution is 0.5 m / second or more.

  The reason is estimated as follows. As described above, the oxide is formed in the drying process after being immersed in an acidic solution having a pH buffering action, and the starting point of oxide formation is considered to be an etch pit when immersed in the acidic solution. Therefore, the solid-liquid reaction in the acidic solution needs to be uniform on the steel plate.

  When the electrogalvanized steel sheet comes into contact with the acidic solution, a very small amount of zinc dissolves in an extremely short time, and the generated hydrogen is adsorbed on the steel sheet surface. The amount of reaction between zinc and the steel sheet varies depending on the amount of zinc ions at the interface between the steel sheet and the solution and the inhibition of the solid-liquid reaction by hydrogen adsorbed on the steel sheet. In particular, in the case of electrogalvanized steel sheet, since there are irregularities due to crystals, hydrogen adsorbed on the steel sheet is difficult to desorb, and the difference in the amount of zinc ions at the local interface tends to occur, resulting in a non-uniform reaction. It is difficult to form a uniform oxide film. At this time, if there is a relative flow velocity of 0.5 m / sec or more between the solution and the steel plate, after the solid-liquid reaction, zinc ions and hydrogen move offshore, and the reaction amount between the steel plate and the solution becomes constant. When the relative flow rate of the steel plate and the solution is slower than 0.5 m / sec, the moving speed becomes slow and the uniform reaction is inhibited. The relative flow rate of the steel plate and the solution is more preferably 1.0 m / second or more.

  In addition, the relative flow velocity in this invention has shown the average flow velocity of the solution in the steel plate advancing direction seen from the steel plate. For example, the steel plate may be passed through a solution having no flow rate in a certain direction at a speed of 0.5 m / second or more. Further, in the case of dipping in a solution having a flow velocity in the direction opposite to the traveling speed of the steel sheet and the traveling direction of the steel sheet, the sum of the speeds may be 0.5 m / second or more. Further, the acidic solution may be sprayed on the steel plate traveling at 0.5 m / second or more in a direction perpendicular to the traveling direction of the steel plate.

The method of contacting the acidic solution is not particularly limited as long as the relative flow rate between the plated steel sheet and the acidic solution can be 0.5 m / second or more. Specifically, there are a method of immersing a plated steel plate in an acidic solution, a method of spraying an acidic solution on the plated steel plate, and the like, but it is desirable that the steel plate is finally present in the form of a thin liquid film. This is because if 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 liquid film is desirably adjusted to 3 g / m ² or less. The liquid film amount can be adjusted by a squeeze roll, air wiping or the like.

  In addition, in order to form a uniform oxide layer by bringing the steel sheet into contact with the acidic solution as described above, it is also effective to bring the surface into contact with an alkaline solution before homogenizing the surface with the acidic treatment liquid. . The electrogalvanized steel sheet generally uses an acidic plating bath, and the reactivity of the steel sheet may become uneven due to the remaining plating bath. From this point of view, the purpose of contacting with the alkaline solution before contact with the acidic treatment solution is to neutralize the remaining plating bath on the steel sheet surface, suppress local excessive etching, and reduce the surface of the electrogalvanized steel sheet. By making it uniform, the reaction at the time of contact with the acidic solution is made uniform.

  There is no restriction | limiting in particular in the method to contact an alkaline solution, An effect is acquired by processing by immersion or spray. The alkaline solution desirably has a pH of 10 or more, and more preferably has a pH of 11 or more. This is because when the pH is lower than 10, it takes a long time to neutralize the plating bath. If it is said pH range, there will be no restriction | limiting in particular in the kind of solution, Sodium hydroxide etc. can be used.

  By applying fine irregularities to the Zn-based oxide, further reduction in sliding resistance can be realized. Here, the fine irregularities mean that the roughness curve has an average roughness (Ra) of 100 nm or less and an average interval (S) of local irregularities of 1000 nm or less.

  It is thought that the reason why the sliding resistance is lowered by the fine unevenness is that the concave portions of the fine unevenness function as a group of fine oil pits and can effectively hold the lubricating oil therein. That is, in addition to the sliding resistance reduction effect by suppressing the adhesion with the press die as an oxide, the sliding resistance is further reduced by the fine oil sump effect that can effectively hold the lubricating oil in the sliding portion. We think that effect is expressed. The sliding condition is particularly effective under a sliding condition with a low contact surface pressure.

  For example, the structure of the fine unevenness is, for example, that the surface of the Zn-based oxide layer has fine unevenness, or is formed directly on the plating surface or on the layered oxide layer and / or hydroxide layer. Fine unevenness may be formed by the distribution of a Zn-based oxide having a shape such as a plate shape or a flake shape. As for fine unevenness | corrugation, Ra is 100 nm or less, and S is 800 nm or less. Even if Ra and S are further increased, no significant improvement in the sump effect is observed, and it is necessary to thicken the oxide, making it difficult to manufacture. Although the lower limit of these parameters is not particularly specified, it has been confirmed that Ra is 3 nm or more and S is 50 nm or more and has a sliding resistance reducing effect. Ra is more preferably 4 nm or more. Further, even if Ra is 3 nm or more, if the fine unevenness is too small, it will approach a smooth surface and the effect as a sump of viscous oil will be reduced, which is considered undesirable.

  One effective method of controlling Ra and S is to include Fe in a Zn-based oxide as will be described later. When Fe is contained in the Zn-based oxide, the Zn oxide becomes finer and increases in number depending on the content. By controlling the Fe content and growth time, the size and distribution of the Zn oxide can be adjusted, and Ra and S can be adjusted. The shape of the fine irregularities is not limited to this.

  The surface roughness parameters of Ra and S are obtained by quantifying and extracting the surface shape of the Zn-based oxide using a scanning electron microscope or a scanning probe microscope (atomic force microscope, etc.) having a three-dimensional shape measurement function. From a roughness curve of several μm, it can be calculated according to the mathematical formula described in “Japanese Surface Roughness Term” B-0660-1998 etc. Further, the shape of the fine irregularities can be observed using a high-resolution scanning electron microscope. Since the oxide is as thin as several tens of nanometers, it is effective to observe using a low acceleration voltage, for example, 1 kV or less. In particular, when the secondary electron image is observed except for low-energy secondary electrons centered on several eV as the electron energy, the contrast caused by oxide charging can be reduced. (Non-patent document 1: Masayasu Nagoshi, two others, “Actual surface of material viewed with ultra-low acceleration scanning electron microscope”, Surface technology, 2003, Vol. 54, No. 1, p. .31-34).

  A method for imparting fine unevenness to the Zn-based oxide is not particularly limited, but one effective method is to make the Zn-based oxide an oxide containing Zn and Fe. By including Fe in the Zn-based oxide, the size of the Zn-based oxide can be reduced. Fine irregularities can be formed as a collection of fine oxides. The reason why the oxide containing Zn and Fe becomes an oxide having fine irregularities is not clear, but it is estimated that the growth of Zn oxide is suppressed by Fe or the oxide of Fe. Although the suitable ratio (percentage) of Fe with respect to the sum of Zn and Fe is not clear, the inventors have confirmed that Fe is effective at least in the range of 1 at% to 50 at%. A more preferable range is 5 to 25 at%.

  Such an oxide containing Zn and Fe can be formed by adding Fe to the acidic solution in the Zn-based oxide forming method in which the oxide solution is brought into contact with the acidic solution having a pH buffering action described above. A preferred concentration range is 1 to 200 g / l as divalent or trivalent Fe ions. An even more preferable range is 1 to 80 g / l. The method for adding Fe ions is not particularly limited. For example, if the Fe ion concentration is 1 to 80 g / l, ferrous sulfate (7 hydrate) can be added in the range of 5 to 400 g / l. It is.

  Regarding the production of the electrogalvanized steel sheet according to the present invention, the additive element component in the plating film is not particularly limited. That is, for example, even if Al, Pb, Sb, Si, Sn, Mg, Mn, Ni, Ti, Li, Cu, In, or the like is contained in the adhesive film, the effect of the present invention is not impaired.

The present invention will be described in more detail by the following examples, but the scope of the present invention is not particularly limited by these examples.
(Example 1)
1. Substrate
First, an electrogalvanized film was formed on a cold-rolled steel plate having a thickness of 0.8 mm so as to have a plating amount of 50 g / m ² . Next, it was immersed in an acidic treatment liquid (50 ° C.) described in Table 1 below, followed by roll drawing, and adjusted so that the liquid volume was 3 g / m ² or less (partially 10 g / m ² ). Then, it was left standing at room temperature in the atmosphere for a predetermined time, washed with water and dried. The standing time was changed depending on the sample.

Further, as a homogenization treatment prior to partial contact with the pickling solution, a sodium hydroxide degreasing agent, a product of Nippon Parkerizing Co., Ltd., product name: FC-4370 with a concentration of 20 g / l (50 ° C., pH 13 : Glass electrode) for 5 seconds.
For the immersion in the acidic treatment liquid in this experiment, a liquid circulation type cell capable of changing the flow rate was used. The relative flow rate was calculated as follows. Flow velocity (m / sec) = flow rate (m ³ / sec) / cross-sectional area (m ² )

  In addition, the cross-sectional area of the liquid circulation type cell used for this experiment is height 0.01m x width 0.25m. For the specimens prepared by the above method, evaluation of sliding characteristics and chemical conversion treatment were performed as a press formability test. Moreover, the thickness of the oxide layer was measured about the sample. Hereinafter, a characteristic evaluation method and a film analysis method will be described.

(1) Press formability (sliding property) evaluation 1 (Friction coefficient measurement-flat plate sliding 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 measurement 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 5 having a roller 4 in contact therewith and capable of moving up and down. The first load cell 7 for measuring the pressing load N applied to the friction coefficient measuring sample 1 by the bead 6 is attached to the slide table support 5 by pushing up the 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. As a lubricating oil, a test cleaning oil (trade name: Preton R352L) manufactured by Sugimura Chemical Co., Ltd. was applied to the surface of Sample 1 and tested.

FIG. 2 is a schematic perspective view of the used bead. In Figure 2, the length _{L 1} of the sliding direction of the bead top 59 mm, the length _{L 2} of the sliding direction of the bead lower the sample 1 is pressed is 50 mm, the width W is 10 mm. Further, the lower portions at both ends in the sliding direction of the bead are configured by curved surfaces having a curvature of 4.5 mmR. The bead slides with its lower surface pressed against the surface of the sample 1. When this bead is used, the coefficient of friction under a long sliding distance can be evaluated. In the friction coefficient measurement test, the pressing load N was 400 kgf, and the sample drawing speed (horizontal moving speed of the slide table 3) was 20 cm / min. The pressing surface pressure under this condition is 7.8 MPa, which is a relatively low surface pressure condition. The friction coefficient μ between the test material and the bead was calculated by the formula: μ = F / N. The results are shown in Table 2 below.

(2) Evaluation of press formability (sliding characteristics) 2
The press formability was evaluated by conducting a test under the following conditions, which is a test simulating press forming with high surface thickness bending back deformation.

(2-1) Draw bead test
Applying 1.5 g / m ² of press cleaning oil (trade name: Preton R352L) manufactured by Sugimura Chemical Co., Ltd. as a lubricant to a 30 × 250 (mm) test piece, a bead tip radius of 5 mm, a bead height of 3 mm, and pressing force A draw bead test was performed at 1000 kgf and a drawing speed of 200 mm / min to obtain a drawing resistance (R). The results are shown in Table 2 above.

(2-2) Sliding after processing
The test was performed under the same conditions as the draw bead test, and the coefficient of friction was measured using the flat plate sliding tester shown in (1), with the surface of the obtained test piece subjected to sliding by the bead as the target surface. . The bead shown in FIG. 3 was used for the friction coefficient measurement test conditions. In FIG. 3, the length L ₁ in the sliding direction of the upper part of the bead is 12 mm, the length L ₂ in the sliding direction of the lower part of the bead to which the sample 1 is pressed is 3 mm, and the lateral width W is 10 mm. Moreover, the lower part of the both ends of the bead sliding direction is comprised by the curved surface of curvature 4.5mmR. The pressing load N was 400 kgf, and the sample drawing speed (the horizontal moving speed of the slide table 3) was 100 cm / mm. The pressing surface pressure under this condition is 130.4 MPa, which is a relatively high surface pressure condition. The results are shown in Table 2 above.

(3) Chemical conversion processability
The chemical conversion property was evaluated by the following method. The sample was coated with about 1 g / m ² of rust-preventing oil (trade name: NOXLAST 550HN, manufactured by Parker Kosan), followed by alkaline degreasing (trade name: FC-E2001, spray treatment, spray pressure 1 kgf, manufactured by Nihon Parkerizing Co., Ltd.). / Cm ² ), water washing, surface treatment (trade name: PL-Z, manufactured by Nihon Parkerizing Co., Ltd.), chemical conversion treatment (trade name: PB-L3080, manufactured by Nihon Parkerizing Co., Ltd.) Formed. At this time, the chemical conversion treatment time was constant (2 minutes), but in alkaline degreasing, the concentration of the degreasing solution was 1/2 and the degreasing time was 30 seconds, which was milder than the standard conditions. Evaluation of chemical conversion treatment was evaluated by ○ and × by the appearance after chemical conversion treatment.
○: There is no scale and the entire surface is covered finely with phosphate crystals.
X: There is a region where phosphate crystals are not formed in a wide range.
The results are shown in Table 2 above.

(4) Measurement of oxide layer thickness
Using Auger Electron Spectroscopy (AES), the Ar sputtering and AES spectrum measurements were repeated to measure the depth distribution of the composition of the plating film surface portion. The conversion from the sputtering time to the depth was calculated from the sputtering rate obtained by measuring a SiO ₂ film having a known film thickness. The composition (at%) was determined from the Auger peak intensity of each element by correcting the relative sensitivity factor, but C was not taken into account in order to eliminate the influence of contamination. The depth distribution of the oxygen concentration caused by oxides and hydroxides is high near the surface and decreases and becomes constant as it goes inside. The depth which is ½ of the sum of the maximum value and the constant value was defined as the oxide thickness. The area of about 2 μm × 2 μm of the flat portion was the object of analysis, and the average value of the results of measurement at any two to three points was taken as the average oxide layer thickness. The results are shown in Table 1 above.

(5) Measurement of fine unevenness and roughness parameters of oxide
The formation of fine unevenness of the Zn-based oxide is due to the use of a scanning electron microscope (trade name: LE01530 manufactured by LEO Co., Ltd.) and an Everhart-Thomaly type secondary electron installed in the sample chamber at an acceleration voltage of 0.5 kV. This was confirmed by observing a high-magnification secondary electron image using a detector.

  The surface roughness of the Zn-based oxide was measured using an electron beam three-dimensional roughness analyzer (trade name: ERA-8800FE manufactured by Elionix). The measurement was performed at an acceleration voltage of 5 kV and a working distance (working distance) of 15 mm, and the sampling interval in the in-plane direction during the measurement was 5 nm or less (observation magnification was 40000 times or more). In order to avoid charging by electron beam irradiation, gold deposition was performed. 450 or more roughness curves having a length of about 3 μm were cut out from the scanning direction of the electron beam per region where the Zn-based oxide was present. Three or more locations were measured per sample.

  From the above roughness curve, the average roughness (Ra) of the roughness curve and the average interval (S) of local irregularities of the roughness curve were calculated using the analysis software attached to the apparatus. Here, Ra and S are parameters for evaluating the roughness and period of the fine irregularities, respectively. These general definitions are described in Japanese Industrial Standard “Surface Roughness Term” B-0660-1998 and the like. The example of the present invention is a roughness parameter for a roughness curve having a length of several μm, and Ra and S thereof are calculated according to mathematical formulas defined in the above document.

When an electron beam is irradiated on the sample surface, carbon-based contamination grows and may appear in the measurement data. This effect is likely to be noticeable when the measurement area is small as in this case. Therefore, in the data analysis, a Spline hyperfilter having a cut-off wavelength half of the length in the measurement direction (about 3 μm) was applied to remove this influence. For the calibration of this apparatus, SHS thin film step standard (steps 18 nm, 88 nm, 450 nm) of VLSI Standard, traceable to the National Research Institute NIST in the United States, was used.
The case where the average roughness Ra of the roughness curve that is the scope of the present invention is 100 nm or less and the average interval S of the local irregularities is the surface roughness of 1000 nm or less is designated as “◯”. The results are shown in Table 2 above.

The schematic front view which shows a friction coefficient measuring apparatus. The schematic perspective view of the bead in FIG. FIG. 3 is a schematic perspective view of a bead having a shape and size different from those of FIG. 2. Schematic of the manufacturing apparatus of the steel plate which concerns on this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Sample for friction coefficient measurement, 2 ... Sample stand, 3 ... Slide table, 4 ... Roller, 5 ... Slide table support stand, 6 ... Bead, 7 ... 1st load cell, 8 ... 2nd load cell, 9 ... Rail, 11 ... Electrolyzer, 12 ... Steel plate, 13 ... First electrolyzer, 14 ... Second electrolyzer, 15 ... Washing tank.

Claims (6)

An electrogalvanized steel sheet comprising a Zn-based oxide layer comprising fine irregularities having an average thickness of 10 nm or more on a plated surface. An electrogalvanized steel sheet, which is obtained by subjecting the steel sheet to an acidic solution having a pH buffering action after performing electrogalvanization on the steel sheet, and performing an oxidation treatment with a holding time until washing with water of 1 to 30 seconds. Production method. A method for producing an electrogalvanized steel sheet, comprising subjecting the steel sheet to electrogalvanization, then contacting with an acidic electrogalvanizing bath, and performing an oxidation treatment with a retention time of 1 to 30 seconds until washing with water. The method for producing an electrogalvanized steel sheet according to claim 2, wherein the acidic galvanized steel sheet is brought into contact with an acidic solution having a pH buffering action under a condition where the relative flow rate is 0.5 m / sec or more. The method for producing an electrogalvanized steel sheet according to claim 3, wherein the electrogalvanized steel sheet is brought into contact with an acidic electrogalvanizing bath under a condition where the relative flow velocity is 0.5 m / sec or more. A device for producing an electrogalvanized steel sheet having a plurality of electrolytic cells, a first electrolytic cell in which only a relative flow rate is applied to the electrogalvanized steel sheet and energization is turned off, and a downstream side of the first electrolytic cell And a second electrolytic cell for allowing the steel sheet to pass through without being immersed in a plating solution.

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