JP5338263B2 - Method for producing galvanized steel sheet - Google Patents

Description

  The present invention relates to a method for producing a zinc-based plated steel sheet, in particular, a zinc-based plated steel sheet having excellent press formability with high productivity.

  Zinc-based galvanized steel sheets are widely used in a wide range of fields centering on automobile body applications. In such applications, press forming is often performed, but a zinc-based plated steel sheet has a disadvantage that it is inferior in press formability as compared with a cold-rolled steel sheet. This is because the sliding resistance of the galvanized steel sheet against the press mold is larger than that of the cold-rolled steel sheet. This is because it is difficult to flow into the mold and breakage easily occurs.

  Among galvanized steel sheets, galvanized, heat-treated, alloyed hot dip zinc in which Fe in the steel sheet and Zn in the plating film are diffused to form a film consisting of an Fe-Zn alloy phase In the plated steel sheet, the press formability is improved by increasing the Fe concentration in the plated film, forming a galvanized film that is preferable for slidability with high hardness, high melting point, and low adhesion. However, when a zinc plating film with a high Fe concentration is formed, a Γ phase, which is a hard and brittle Fe-Zn alloy phase, is formed at the plating film-steel interface, so the phenomenon that the plating film peels from the interface during press forming, a so-called powder. A ring is likely to occur.

  For this reason, in Patent Document 1, in order to achieve both slidability and powdering resistance, a technique in which a hard Fe-based alloy layer is applied to the surface of the galvanized film on which no Γ phase is formed by an electroplating method or the like. Has been proposed. However, this technique causes a significant increase in manufacturing cost.

  In addition, in order to improve the press formability of the galvanized steel sheet, a method of applying a high-viscosity lubricating oil is also 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 unstable press performance due to oil shortage during press molding.

Several methods have also been proposed for improving the press formability by forming an oxide layer on the surface of the zinc-based plating film. For example, Patent Document 2 and Patent Document 3 disclose a method of forming an oxide layer mainly composed of ZnO by performing electrolytic treatment, immersion treatment, coating oxidation treatment, or heat treatment, and Patent Literature 4 discloses phosphoric acid. A method of forming an oxide layer mainly composed of P by immersing a plated steel sheet in an aqueous solution containing 5 to 60 g / L of sodium and having a pH of 2 to 6, or performing an electrolytic treatment in such an aqueous solution, or applying such an aqueous solution However, Patent Document 5 describes a method of forming a Ni oxide layer by electrolytic treatment, immersion treatment, coating treatment, coating oxidation treatment or heat treatment, and Patent Literature 6 describes a pH buffering action of an alloyed hot-dip galvanized film. A method is disclosed in which an oxide layer mainly composed of Zn is formed on the surface of a plating film by contacting with an acidic solution having the above. Among them, the method described in Patent Document 6 is advantageous in terms of production cost and environmental load because an oxide layer can be formed simply by leaving it in the air after contacting with an acidic solution having a pH buffering action. It is.
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

  However, in the method described in Patent Document 6, when an oxide layer having a desired thickness is to be formed, it is necessary to leave it in the atmosphere for a certain period of time after contact with the acidic solution, resulting in poor productivity. .

  An object of this invention is to provide the manufacturing method of the zinc-based plated steel plate which can form the oxide layer of desired thickness on the plating film surface by leaving for a short time after contacting an acidic solution.

  The present inventors diligently studied a method of forming an oxide layer on the surface of a zinc-coated steel sheet, and after contacting a pickling solution having a pH buffering action, in an atmosphere having a higher oxygen concentration than the atmosphere. It has been found that an oxide layer having a desired thickness can be formed in a shorter time if left untreated.

  The present invention was made on the basis of such knowledge, in the method for producing a zinc-based plated steel sheet in which a zinc-based plating is applied to a steel sheet, and then an oxide layer is formed on the surface of the plating film by contacting with an acidic solution. A zinc-based plated steel sheet characterized by using an acidic solution having a pH buffering action as the acidic solution, and leaving it in an atmosphere where the oxygen concentration exceeds the oxygen concentration in the atmosphere for 1 to 60 seconds after washing and washing with water. A manufacturing method is provided.

In the production method of the present invention, an atmosphere in which the oxygen concentration exceeds the atmospheric oxygen concentration is an atmosphere having an oxygen concentration of 40 vol.% Or more, or an acidic solution having a pH buffering action is used as an acetate, phthalate, citron. Acid solutions containing at least one of acid salts, succinates, lactates, tartrate, borates, phosphates, and having a pH in the range of 1.0 to 5.0, acetates, phthalates It is preferable to use an acidic solution containing at least one of acid salts, citrates, succinates, lactates, tartrate, borates, and phosphates in the range of 5 to 50 g / l. . In addition, after contacting with the acidic solution, neutralizing the acidic solution remaining on the surface by contacting with the alkaline solution, or after contacting the acidic solution, the adhesion amount of the liquid film formed on the steel sheet surface is 15 g / m ² The following is preferable.

  According to the present invention, after contact with the pickling solution, it is possible to form an oxide layer mainly composed of Zn having a desired thickness on the surface of the plating film by allowing it to stand for a shorter time, and has low sliding resistance and excellent press formability. It has become possible to produce a coated steel sheet with high productivity.

  In the present invention, after contacting with a pickling solution having a pH buffering action, it is left in an atmosphere having a higher oxygen concentration than the atmosphere, so that an oxide layer having a desired thickness can be formed in a shorter time than before. ing. Although this mechanism is not necessarily clear, it can be considered as follows. In other words, when the plating film surface of the zinc-based plated steel sheet is brought into contact with an acidic solution, the dissolution of zinc begins, but at the same time, hydrogen is generated, so the hydrogen ion concentration in the solution decreases, the pH of the solution increases, An oxide layer mainly composed of zinc is formed on the surface of the plating film. At this time, since the dissolution reaction of zinc with oxygen and the consumption reaction of hydrogen ions proceed simultaneously, the formation of an oxide layer is promoted when left in an atmosphere having a high oxygen concentration after contact with an acidic solution, and even when left for a short time. Thus, an oxide layer having a desired thickness is formed. In addition, by using an acidic solution having a pH buffering action, the pH of the solution can be gradually increased to sufficiently dissolve zinc, and as a result, an oxide layer having a desired thickness can be formed. .

  In the present invention, an oxide layer having a desired thickness is formed by standing for a shorter period of time than before, but the standing time is 1 to 60 seconds. This is because if the solution is left for less than 1 second, the pH of the solution rises and the acidic solution is washed out before the oxide layer mainly composed of Zn is formed. This is because no change is observed in the amount of the oxide layer.

  As the atmosphere having a high oxygen concentration, it is preferable to use an atmosphere having an oxygen concentration of 40 vol.% Or more in order to form an oxide layer having a desired thickness in a shorter time.

As the acidic solution having a pH buffering action, those having a buffering action in a pH range of 1.0 to 5.0 are preferable. This is because, when the pH is less than 1.0, the dissolution of zinc is promoted, but it becomes difficult to form an oxide layer, and when the pH exceeds 5.0, the dissolution rate of zinc becomes slow. Examples of acidic solutions having such a buffering action include acetates such as sodium acetate (CH ₃ COONa), phthalates such as potassium hydrogen phthalate ((KOOC) ₂ C ₆ H ₄ ), and sodium citrate ( Citrates such as Na 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 ), Use an aqueous solution containing at least one of lactate such as sodium lactate (NaCH ₃ CHOHCO ₂ ), tartrate such as sodium tartrate (Na ₂ C ₄ H ₄ O ₆ ), borate, and phosphate. it can. At this time, the concentration of each salt is preferably in the range of 5 to 50 g / l. This is because when the concentration of each salt is less than 5 g / l, the pH of the solution rises instantaneously, and an oxide layer with a desired thickness cannot be formed in a short time, and when it exceeds 50 g / l, dissolution of zinc occurs. This is because it is promoted, the plating film becomes severely damaged, and the function as a rust-proof steel sheet may be lost. Even if impurities such as S, N, P, B, Cl, Na, Mn, Ca, Mg, Ba, Sr, and Si contained in an acidic solution having a pH buffering action are incorporated into the oxide layer, The effect of the invention is not impaired.

There are no particular restrictions on the method of bringing the galvanized steel sheet into contact with the acidic solution. The method involves immersing the plated steel sheet in the acidic solution, spraying the acidic solution onto the plated steel sheet, and applying the acidic solution to the plated steel sheet via a coating roll. Although there is a method of coating, it is preferable to finally make it exist in a thin liquid film with an adhesion amount of 15 g / m ² or less on the surface of the steel sheet. This is because when the amount of the acidic solution present on the surface of the steel sheet exceeds 15 g / m ² , the pH does not increase even if zinc is dissolved, and only zinc is dissolved one after another. This is because it takes a long time and the plating film may be severely damaged. The amount of adhesion can be adjusted by a squeeze roll or air wiping.

  If the pickling solution cannot be completely removed at the time of washing with water after contacting the acidic solution, rust is likely to be generated when the steel sheet is stored for a long period of time. Therefore, after making it contact with an acidic solution, it is preferable to make it contact with an alkaline solution and neutralize the acidic solution which remains on the steel plate surface. At this time, in order to prevent dissolution of the oxide layer formed in contact with the acidic solution, it is preferable to use an alkaline solution having a pH of 12 or less, such as an aqueous sodium hydroxide solution or an aqueous sodium phosphate solution.

  In the present invention, the oxide layer formed on the surface of the plating film by being brought into contact with an acidic solution is composed of an oxide and / or hydroxide mainly composed of zinc and containing at least one metal such as Fe or Al. The thickness of this oxide layer needs to be 10 nm or more in order to reduce the sliding resistance during press molding.

Steel sheet GA in which an alloyed hot-dip galvanized film is formed by a conventional method on a cold-rolled steel sheet having a thickness of 0.8 mm, a steel sheet GI that has not been subjected to an alloying treatment after forming a hot-dip galvanized film by a conventional method, and Zn- The galvanium steel sheet GL on which a 55% Al plating film was formed was temper-rolled with an elongation of 1.0%. Then, using an oxide layer forming treatment facility as shown in FIG. 1, a portion of the activation tank 1 was sprayed with a pH 10 sodium hydroxide aqueous solution, washed with water, and acid solution tank 2 with sulfuric acid and pH buffer. After dipping in an acidic solution of 50 ° C and pH 2.0 consisting of a mixed solution of sodium dihydrogen phosphate 30 g / l and citric acid 20 g / l having an action, the adhesion amount 10 g / A liquid film of m ² was formed. Next, after changing the standing time in the atmosphere adjustment device 4 to 1 to 60 seconds and washing it with spray water of 50 ° C. in the # 1 washing tank 5, and pH 10 sodium hydroxide aqueous solution in the neutralization tank 6 Spray the water to neutralize the acidic solution remaining on the surface, spray hot water at 50 ° C in # 2 cleaning tank 7, clean it with dryer 8, and dry the oxide layer on the plating film surface. A sample in which was formed was prepared. At this time, the atmosphere in the atmosphere adjusting device 4 was changed to three types of air, 25 vol.% O ₂ +75 vol.% N ₂ and 40 vol.% O ₂ +60 vol.% N ₂ . Note that some samples were not activated or neutralized. For comparison, a sample not subjected to such a series of treatments was also prepared.

  About the produced sample, the friction coefficient for evaluating the thickness and slidability of the oxide film layer was measured as follows.

  Measurement of oxide layer thickness: Using a fluorescent X-ray analyzer, the tube voltage and current were set to 30 kV and 100 mA, respectively, the spectroscopic crystal was set to TAP, and the peak intensity of the O-Kα ray in the oxide layer was measured. It was measured. The thickness of the oxide layer of the sample is converted to the thickness of the silicon oxide film from a calibration curve of the thickness of the silicon oxide film prepared in advance using a silicon wafer of known thickness and the peak intensity of the O-Kα line. Asked.

Measurement of friction coefficient: A friction coefficient measuring apparatus shown in FIG. 2 was used. First, the sample 11 was fixed to a sample table 12 fixed to the upper surface of a slide table 13 that can move horizontally. Next, a vertically movable slide table support table 15 provided on the lower surface of the slide table 13 and having a roller 14 on the upper surface was pushed up, and the sample 11 was pressed against a bead 16 provided above the sample table 12. At this time, the pressing load N applied to the sample 11 was measured by the first load cell 17 provided on the lower surface of the slide table support base 15. Then, in a state where the sample 11 is pressed against the bead 16, the slide table 13 is moved in the horizontal direction, and the sliding resistance force F at that time is measured by the second load cell 18 provided at one end of the slide table 13, The friction coefficient μ (= F / N) of sample 11 was determined. At this time, the measurement was performed by applying the cleaning oil Preton R352L (registered trademark) for press, manufactured by Sugimura Chemical Co., to the surface of the sample 11. Further, two types of beads having the shapes and dimensions shown in FIGS. 3 and 4 were used as the beads 16, and the friction coefficient μ under the following conditions 1 and 2 was obtained.
Condition 1: Bead in Fig. 3, Pressing load N = 400kgf, Horizontal moving speed of slide table 13 = 100cm / min
Condition 2: Bead of Fig. 4, Pressing load N = 400kgf, Horizontal moving speed of slide table 13 = 20cm / min
The results are shown in Tables 1 and 2.

  In any steel sheet of GA, GI, and GL, use an acidic solution having a pH buffering action, and after contact with the acidic solution, leave it in the atmosphere or in an atmosphere where the oxygen concentration exceeds the atmospheric oxygen concentration for 1 to 60 seconds. An oxide layer having a thickness of 10 nm or more is formed, but in an atmosphere in which the oxygen concentration exceeds the oxygen concentration in the atmosphere, an oxide layer having a desired thickness can be left in a shorter time than in the atmosphere. It can be seen that it is formed. It can also be seen that the higher the oxygen concentration, the shorter the standing time until an oxide layer with a desired thickness is formed.

It is a figure which shows an example of the oxide layer formation processing equipment used for the manufacturing method of this invention. It is a figure which shows the friction coefficient measuring apparatus used in the Example. FIG. 3 is a diagram showing an example of a bead shape / dimension used in the friction coefficient measuring apparatus of FIG. FIG. 4 is a diagram showing another example of the bead shape and dimensions used in the friction coefficient measuring apparatus in FIG.

Explanation of symbols

1 Activation tank
2 Acidic solution tank
3 Drawing roll
4 Atmosphere adjusting device
5 # 1 washing tank
6 Neutralization tank
7 # 2 cleaning tank
8 Hair dryer
S steel plate
11 samples
12 Sample stage
13 Slide table
14 Laura
15 Slide table support
16 beads
17 First load cell
18 Second load cell
19 rails
N Push load
F Sliding resistance force

Claims (6)

  In the method of manufacturing a zinc-based plated steel sheet, in which an oxide layer is formed on the surface of the plating film by bringing the steel sheet into contact with an acidic solution after the zinc-based plating is performed, an acidic solution having a pH buffering action is used as the acidic solution. A method for producing a galvanized steel sheet, which is left in an atmosphere in which the oxygen concentration exceeds the oxygen concentration in the atmosphere for 1 to 60 seconds and washed with water after contacting the solution.   2. The method for producing a galvanized steel sheet according to claim 1, wherein an atmosphere having an oxygen concentration exceeding 40 vol.% Is used as the atmosphere in which the oxygen concentration exceeds the atmospheric oxygen concentration.   As an acidic solution having a pH buffering action, it contains at least one of acetate, phthalate, citrate, succinate, lactate, tartrate, borate and phosphate, and has a pH of 1.0. 3. The method for producing a galvanized steel sheet according to claim 1, wherein an acidic solution in the range of -5.0 is used.   An acidic solution containing at least one of acetate, phthalate, citrate, succinate, lactate, tartrate, borate and phosphate in the range of 5 to 50 g / l. 4. The method for producing a galvanized steel sheet according to claim 3, wherein the galvanized steel sheet is used.   5. The method for producing a galvanized steel sheet according to any one of claims 1 to 4, wherein after the contact with the acidic solution, the acidic solution remaining on the surface is brought into contact with the alkaline solution and neutralized. The method for producing a galvanized steel sheet according to any one of claims 1 to 5, wherein the amount of adhesion of a liquid film formed on the surface of the steel sheet is 15 g / m ² or less after contacting the acidic solution. .

Images