JP2020158879A - Manufacturing method of hot-dip galvanized steel sheet - Google Patents

Abstract

To provide a manufacturing method of a hot-dip galvanized steel sheet having excellent press formability, without requiring industrially special environmental measures or facility specifications.SOLUTION: A hot-dip galvanized steel sheet, to which temper rolling is applied, is brought into contact, as long as one second or more, with zinc sulfate aqueous solution having pH of 4.2-7.0, and containing one or more kinds of fluorine compounds selected from between NaHF2, KHF2, HF, NaF, KF as much as 0.10 g/L or more and 5.0 g/L or less in total, and then washed with water.SELECTED DRAWING: Figure 2

Description

The present invention relates to a method for producing a hot-dip galvanized steel sheet having low sliding resistance during press forming and having excellent press formability.

Galvanized steel sheets are widely used in a wide range of fields, mainly for automobile body applications, and in such applications, they are press-formed and used. However, the galvanized steel sheet has a drawback that the press formability is inferior to that of the cold-rolled steel sheet. This is because the sliding resistance of the galvanized steel sheet in the press die is larger than that of the cold-rolled steel sheet. The zinc-based plated steel sheet is less likely to flow into the press die at the portion where the sliding resistance between the die and the bead is large, and the steel sheet is likely to break.

In particular, in hot-dip galvanized steel sheets (hereinafter, also referred to as GI), there is a phenomenon that the sliding resistance further increases due to the plating adhering to the mold (mold galvanizing), and cracks occur during continuous press forming. It has an adverse effect on automobile productivity, such as occurring.

Furthermore, from the viewpoint of tightening CO ₂ emission regulations in recent years, the ratio of high-strength steel sheets used tends to increase for the purpose of reducing the weight of the vehicle body. When a high-strength steel sheet is used, the surface pressure during press forming increases, and the adhesion of plating to the die becomes a more serious problem.

As a method for solving the above problems, in Patent Document 1 and Patent Document 2, an alloyed hot-dip galvanized steel sheet (hereinafter, also referred to as GA) to be alloyed has a pH buffering action after temper rolling. It discloses a technique for forming a zinc-based oxide on the surface layer of GA and improving press moldability by contacting it with an acidic solution, leaving it to stand for 1 to 30 seconds after the contact is completed, and then washing and drying it with water.

Here, the GI not subjected to the alloying treatment has a particularly low surface activity. This is because a small amount of Al is added to the hot-dip galvanized bath to adjust the alloying reaction between the base iron and zinc, and the Al oxide derived from Al in the bath is added to the surface of the hot-dip galvanized steel plate. This is because it exists and the Al oxide concentration on the surface is higher than that of GA.

In contrast to such a GI having low surface activity, in Patent Document 3, as a method for forming a zinc-based oxide described in Patent Documents 1 and 2, the surface is contacted with an alkaline solution before contact with an acidic solution. Discloses a method of removing Al oxides to activate the surface and promote the formation of oxides.

Patent Document 4 describes an oxide containing a crystal structure represented by Zn ₄ (SO ₄ ) _1-X (CO ₃ ) _X (OH) ₆ · nH ₂ O with respect to a GI having a similarly low activity. As a method for forming a layer, a method for removing Al oxide on the surface by contacting with an alkaline solution before contact with an acidic solution to activate the surface and promote the formation of the oxide is disclosed.

In Patent Document 5, a molten Zn-Al-based galvanized steel sheet containing 20-95 mass% of Al is brought into contact with an alkaline solution, and HF is further added to the acidic treatment liquid to form an oxide layer. It discloses how to promote it.

Patent Document _6, by dipping the GI in aqueous zinc sulfate solution containing an oxidizing agent such as _{H 2 O 2, CuSO 4 ·} 5H 2 O and KMnO _4, discloses a method of forming the oxide layer.

Similar to Patent Documents 1 and 2, Patent Document 7 is a method for forming an oxide layer on the surface of a galvanized steel sheet by holding it in contact with an acidic treatment liquid for 1 to 60 seconds and then washing and drying it with water. Discloses a method for promoting the formation of an oxide layer by containing NaHF ₂ and / or KHF ₂ in a total amount of 0.01 g / L or more and 5.0 g / L or less in an acidic treatment liquid.

JP-A-2002-256448 Japanese Unexamined Patent Publication No. 2003-306781 Japanese Unexamined Patent Publication No. 2004-3004 International Publication No. 2015/129283 Japanese Unexamined Patent Publication No. 2010-90401 US6,528,182 B1 Japanese Unexamined Patent Publication No. 2017-160506

Zairyo-to-Kankyo, 42, 761-769 (1993)

When an attempt is made to form a zinc-based oxide on the surface of a GI having a low surface activity, an alkali such as contacting with an alkaline solution is used as described in Patent Document 3 in order to remove the Al oxide on the surface. Pretreatment is required. It is indispensable to install a new alkaline pretreatment facility in a manufacturing facility that does not have an alkaline pretreatment facility, and due to the line layout, a GI in which a zinc-based oxide is formed on the surface of a line where an alkaline pretreatment facility cannot be installed. Cannot be manufactured.

Further, in both GI and GA, from the viewpoint of improving the sliding characteristics during press molding, it is preferable to increase the thickness of the zinc-based oxide layer on the surface and increase the formed area ratio, but the alkali pretreatment is not performed. In that case, the thickness of the oxide is thin and the formed area ratio is also low.

Further, the addition of HF to the acidic treatment liquid disclosed in Patent Document 5 can generate a zinc-based oxide on the surface without performing an alkali pretreatment, but it is industrialized from the viewpoint of corrosiveness to equipment. Not realistic.

Patent Document 6 is effective for GA and galvanized steel sheets (hereinafter, also referred to as EG) having a small amount of Al-based oxides, but has a function of removing Al-based oxides on the GI surface layer in the treatment liquid. Therefore, it cannot be applied to GI with a large amount of Al-based oxides.

In Patent Document 7, as in Patent Document 5, the added NaHF ₂ and / or KHF ₂ is changed to HF having high corrosiveness to equipment, and equipment specifications having corrosion resistance to HF are required at the time of industrial manufacturing. There is concern that it will become. Further, Patent Document 7 requires a step of holding for 1 to 60 seconds after contacting with an acidic solution containing NaHF ₂ and / or KHF ₂ , and the production speed in the case of industrial production is usually 100 m / min. Therefore, there is a problem that the step of holding for 1 to 60 seconds requires a new equipment length of 100 m or more at the maximum.

Here, in Non-Patent Document 1, the influence of the HF concentration on the corrosion resistance of the stainless steel sheet is investigated, and it is clarified that pitting corrosion does not occur in the stainless steel sheet when the HF concentration is 0.01 M or less. ing. In the pH range of the examples disclosed in Patent Document 7, it is assumed that this concentration will be exceeded.

The present invention has been made in view of such circumstances, and provides a method for producing a hot-dip galvanized steel sheet having excellent press formability without industrially requiring equipment specifications having corrosion resistance to HF. The purpose is to do.

In order to solve the above problems, the inventors have conducted various studies on the surface treatment of hot-dip galvanized steel sheets. As a result, the following was found and the present invention was completed.

After hot-dip zinc plating is applied to the steel plate and temper rolling is performed, the total amount of one or more fluorine compounds selected from NaHF ₂ , KHF ₂ , HF, NaF, and KF is 0.10 g / L or more 5 The concentration of the component present as HF in the zinc sulfate-containing solution is adjusted by contacting the solution containing zinc sulfate having a pH of 4.2 to 7.0, which contains 0.0 g / L or less, for 1 second or longer and washing with water. , The zinc-based oxide layer can be formed on the plating surface in a short time by reducing the equipment specifications to those having corrosion resistance to HF.

Here, the pH of the solution containing the above-mentioned fluorine compound is limited to 4.2 to 7.0 for the following reasons. Usually, the corrosion resistant material used in industrial manufacturing equipment is stainless steel. In the case of stainless steel, as shown in Non-Patent Document 1, it is known that pitting corrosion does not occur when the HF concentration is 0.01 M or less. On the contrary, when the HF concentration exceeds 0.01 M, it is necessary to use an expensive material having better corrosion resistance than stainless steel, and the equipment cost becomes a big problem. As shown in FIGS. 1 to 3, the pH of the solution containing the fluorine compound is 4.2 or more, the ratio of HF to the total amount of fluorine is 0.05 or less, and among NaHF ₂ , KHF ₂ , NaF, and KF. The HF concentration in the solution containing one or more selected fluorine compounds in a total amount of 0.10 g / L or more and 5.0 g / L or less is 0.0097 M or less. Therefore, even if stainless steel is used, the problem of pitting corrosion does not occur, and there is no need to use a material that is more expensive than stainless steel.

On the other hand, when the pH of the solution containing the fluorine compound exceeds 7.0, there is a problem that Zn ions in the aqueous solution containing zinc sulfate form a precipitate as Zn hydroxide.

The reason for using a solution containing zinc sulfate is that the zinc-based oxide layer formed on the plating surface contains zinc sulfate, and the zinc ions and sulfate ions, which are the main components of the zinc-based oxide, are present in the aqueous solution. This is because the zinc oxide formation reaction is promoted.

The concentration of zinc sulfate heptahydrate in the aqueous solution containing zinc sulfate is 3 to 300 g / L. If it is less than 3 g / L, the concentration of sulfate ions and zinc ions is low, so that the oxide film formation rate becomes slow, and if it exceeds 300 g / L, zinc sulfate is likely to be precipitated.

Further, the step of holding for 1 to 60 seconds as in Patent Document 7 after contacting with an aqueous solution containing zinc sulfate is unnecessary, and the manufacturing equipment for this step can be omitted.

Here, the reason why the step of holding for 1 to 60 seconds is unnecessary is as follows. In the present invention, the Zn of the plating layer is dissolved when the zinc plating layer comes into contact with the treatment liquid. When Zn is dissolved, hydrogen ions are consumed, and the pH of the polar surface layer rises, so that zinc in the treatment liquid is precipitated as a zinc-based oxide. Although the pH increase here is slight, it differs from Patent Document 7 in that it has a higher pH range than Patent Document 7 and that zinc ions are contained in the treatment liquid, and therefore, for 1 to 60 seconds. It is considered that the holding process becomes unnecessary.

As a result, a hot-dip galvanized steel sheet having excellent press formability can be produced without activation treatment with alkali.

The present invention has been made based on the above findings, and the gist thereof is as follows.

[1] A method for producing a hot-dip galvanized steel sheet having a zinc-based oxide layer on its surface. The hot-dip galvanized steel sheet is temper-rolled and then NaHF ₂ , KHF ₂ , HF, NaF, KF. One or more fluorine compounds selected from the above are brought into contact with an aqueous solution containing zinc sulfate having a pH of 4.2 to 7.0 containing 0.10 g / L or more and 5.0 g / L or less in total for 1 second or more. A method for producing a hot-dip galvanized steel sheet having a zinc-based oxide layer on the surface, which is then washed with water.

[2] The hot-dip galvanized steel sheet is brought into contact with an aqueous solution containing zinc sulfate and then washed with water, and then the surface of the zinc-based oxide layer is brought into contact with an alkaline aqueous solution for 0.5 seconds or longer, and then washed with water. The method for producing a hot-dip galvanized steel sheet according to [1], which comprises drying.

[3] The method according to [1] or [2], wherein the time from contacting the hot-dip galvanized steel sheet with the aqueous solution containing zinc sulfate to washing with water is less than 1 second. A method for producing a hot-dip galvanized steel sheet having a zinc-based oxide layer on its surface.

[4] The hot-dip galvanized steel sheet having a zinc-based oxide layer on the surface according to any one of [1] to [3], which is characterized in that the hot-dip galvanized steel sheet is not alloyed. Manufacturing method.

In the present invention, the hot-dip galvanized steel sheet is a galvanized steel sheet manufactured by immersion in a hot-dip galvanized bath. The plating layer is required to contain Zn in an amount of 50% by mass or more, but other compositions are not specified. For example, in addition to Zn, B, Mg, Al, Si, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Nb, Mo, Pb, In, Sn, Ba, W and the like can be used. It may be included.

According to the present invention, even if a fluorine compound is used, it is not necessary to have equipment specifications having corrosion resistance against HF, and an excellent press is used in a manufacturing method that does not require holding for a long time after contact with a treatment liquid. A hot-dip galvanized steel sheet having moldability can be obtained.

According to the present invention, since the friction coefficient during press forming is lowered, the sliding resistance at the cracking risk portion is small and the overhangability is improved, and when a high-strength hot-dip galvanized steel sheet is press-formed or relatively low in strength. When a hot-dip galvanized steel sheet is press-formed into a complicated shape, it can have excellent press-formability.

A hot-dip galvanized steel sheet that can stably form a zinc-based oxide film with excellent sliding characteristics and is industrially feasible for GI with low surface activity without alkali pretreatment. A manufacturing method can be provided.

This is the result of calculating the ionization state due to the change in pH in the aqueous solution of HF. This is the result of calculating the ionization state due to the pH change in the aqueous solution of NaHF ₂ . This is the result of calculating the ionization state due to the pH change in the aqueous solution of NaF. It is a schematic front view which shows the friction coefficient measuring apparatus. It is a schematic perspective view which shows the bead shape and dimension in FIG. It is a schematic perspective view which shows the bead shape and dimension in FIG. It is a schematic diagram which showed the evaluation criteria for evaluating the appearance unevenness.

The present invention will be described in detail below.

When producing a hot-dip galvanized steel sheet, after hot-dip galvanizing the steel sheet, temper rolling is usually performed to secure the material. In the GI that is processed by pressing or the like, temper rolling is performed using a dull roll. This is because GI, which is not alloyed after plating, has a smooth plating surface, poor retention of lubricating oil during pressing, and poor moldability. Therefore, the surface is formed uneven with dull rolls to retain the lubricating oil. This is to improve the sex.

In this temper rolling, uneven shape is imparted to the smooth plated surface of GI by contact with dull roll. The portion in contact with the tempered rolling roll becomes a recess on the plated surface.

GA that is alloyed after plating is also subjected to temper rolling by dull roll after alloying treatment, but the alloying treatment causes unevenness with a depth of several μm on the surface, and it comes into contact with dull roll. Is mainly a convex part. Since the convex portion on the surface of the hot-dip galvanized steel sheet is a portion that the die comes into direct contact with during press forming, the presence of a hard and high melting point substance that prevents adhesion to the die is a sliding characteristic. It is important for improvement. In this respect, the presence of the oxide layer on the plating surface layer is effective in improving the sliding characteristics because the oxide layer prevents adhesion to the mold.

Further, during actual press molding, the oxide on the plating surface layer is worn and scraped off. Therefore, when the contact area between the mold and the work material is large, a sufficiently thick oxide layer exists on the plating surface with a high coverage. It is necessary to do.

Normally, a thin continuous Al-based oxide layer is formed on the plated surface layer of a galvanized steel sheet, but this thin Al-based oxide is not sufficient to obtain good slidability and is even thicker. An oxide layer must be formed.

In contrast to the above, in the present invention, the steel sheet is hot-dip galvanized, tempered and rolled, and then contacted with an aqueous solution containing zinc sulfate for 1 second or longer, and then washed with water to make the plated surface zinc-based. Form an oxide layer.

However, the Al-based oxide layer on the plating surface of the hot-dip galvanized steel sheet is relatively stable in an acidic solution and inhibits the zinc dissolution reaction during the treatment of contact with an aqueous solution containing zinc sulfate. It is difficult to generate oxides in the part where an object exists. In GI, this problem becomes more remarkable because the concentration of Al oxide on the plating surface layer is high. Therefore, in order to generate an oxide, it is necessary to remove the Al-based oxide layer before contacting it with an acidic solution, or to remove the Al-based oxide by contacting it with an aqueous solution containing zinc sulfate.

Temperary rolling is performed when the hot-dip galvanized steel sheet is manufactured, and at that time, the Al-based oxide layer on the plated surface of the portion in contact with the rolling roll (dull roll) is physically removed. So far, temper rolling using dull rolls has been performed, and since dull rolls have irregularities with a surface roughness of several μm in Ra, the convex portions on the roll surface mainly come into contact with the steel sheet surface. As a result, the surface of the hot-dip galvanized steel sheet is activated only at the contact portion with the dull roll, and the surface is not activated except at the contact portion.

In the case of GI, the portion where the convex portion of the dull roll surface is in contact exists as a concave portion as compared with the surroundings, and the portion where the convex portion on the dull roll surface is not in contact exists as a convex portion as compared with the surroundings. Therefore, in the conventional temper rolling using dull roll, zinc-based oxide is generated only in the concave portion where the surface is activated when it is brought into contact with the acidic solution, and the convex portion where the surface is not activated is zinc. The formation of system oxides will be suppressed. During press forming, the actual contact with the press die is mainly the convex part of the plated steel sheet and not the concave part where the zinc-based oxide layer is formed, so the effect of improving the press formability is small and insufficient. It was.

In the case of GA, unlike the η layer of GI, the plating film is mainly δ ₁ , so the plating film is hard, and even in the conventional temper rolling using dull roll, the convex part of the roll surface is the plating surface. Since the Al-based oxide present in the convex portion, which has a large proportion of contact with the convex portion and easily comes into contact with the press die during press molding, is removed and activated, a relatively large sliding property improving effect can be obtained. Was there. However, especially under conditions where the surface pressure rises, the press die may come into contact with the recesses that are not in contact with the ultra-high quality rolling roll, and it is necessary to form zinc oxide in such areas as well. there were.

As a result of examination based on the above findings, in the present invention, the total amount of one or more fluorine compounds selected from NaHF ₂ , KHF ₂ , HF, NaF, and KF in an aqueous solution containing zinc sulfate. It is assumed that the content is 0.10 g / L or more and 5.0 g / L or less. Etching property of an aqueous solution containing zinc sulfate with respect to an Al-based oxide by containing one or more fluorine compounds selected from NaHF ₂ , KHF ₂ , HF, NaF, and KF in an aqueous solution containing zinc sulfate. The step of removing the Al-based oxide that inhibits the reaction becomes unnecessary before the contact with the acidic treatment liquid.

When the Al-based oxide is present on the surface layer of the hot-dip galvanized steel sheet as described above, the dissolution of Zn by the aqueous solution containing zinc sulfate is inhibited, so that the reactivity is significantly lowered. On the other hand, in an aqueous solution containing zinc sulfate, one or more fluorine compounds selected from NaHF ₂ , KHF ₂ , HF, NaF, and KF are added in a total amount of 0.10 g / L or more and 5.0 g / L or less. By containing it, the Al-based oxide is removed at the same time as the contact with the aqueous solution containing zinc sulfate, so that the Zn dissolution reaction is not hindered. If it is less than 0.10 g / L, the time required for removing the Al-based oxide becomes long, and the productivity decreases. On the other hand, if it exceeds 5.0 g / L, the precipitation reaction of the zinc-based oxide is lowered, so that the productivity is lowered. Based on the above, the total amount of one or more fluorine compounds selected from NaHF ₂ , KHF ₂ , HF, NaF, and KF contained in the aqueous solution containing zinc sulfate is 0.10 g / L or more and 5.0 g / L or less. And. By setting the content to 0.10 g / L or more, Al-based oxides can be removed in a short time. By setting the content to 5.0 g / L or less, it is possible to prevent precipitation or the like from occurring.

The pH of the aqueous solution containing zinc sulfate is 4.2 to 7.0. FIGS. 1 to 3 thermodynamically theoreticalize the ionization state of HF, NaHF ₂ , and NaF (0.5 mol / l in F amount) by using OLI Fluoride: Stream Analyzer Version 9.5 manufactured by OLI. The calculated result is shown. The pH of the aqueous solution was changed with HCl or NaOH. The presence or absence of zinc sulfate heptahydrate did not affect the ionized state of F.

As shown in FIGS. 1 to 3, when the pH of the aqueous solution containing zinc sulfate is less than 4.2, the ratio of HF to the total amount of F exceeds 0.05. For _{example, NaHF 2, KHF 2, NaF} , since F amount F content is higher NaHF ₂ is at a concentration of 5 g / L of the most in the molecule of the _KF is 0.16 mol / l, the HF concentration in the aqueous solution 0 Since HF having a large corrosion resistance load on the equipment increases to 0.01 mol / l or more, expensive equipment specifications having higher corrosion resistance than stainless steel for HF are required at the time of industrial manufacturing. On the other hand, when the pH is 4.2 or more, the HF concentration in the aqueous solution is less than 0.01 mol / l, and the load on the equipment is light.

Further, according to the above, if the pH is adjusted to 4.2 or more, the same ionized state is obtained even with NaF, KF, etc., and it is shown that these can be used. NaHF ₂ and / or KHF ₂ are superior in that no pH adjustment is particularly required. Common acids and bases can be used for pH adjustment. For example, sulfuric acid and sodium hydroxide are desirable from the viewpoint of cost and contamination reduction.

On the other hand, when the pH exceeds 7.0, Zn ions in the aqueous solution containing zinc sulfate form a precipitate as Zn hydroxide, which causes dents and scratches on the surface of the steel plate. Therefore, the pH needs to be 7.0 or less.

Treatment liquid containing zinc sulfate, in order to promote the formation of zinc oxide, may also contain an oxidizing agent such as _{NaNO 3, NaNO 2, H 2} O 2, CuSO 4 · 5H 2 O and KMnO ₄ .. In particular, NaNO ₃ is suitable for mass production because it has relatively little deterioration due to an increase in the amount of treatment.

The method of contacting the hot-dip galvanized steel sheet with an aqueous solution containing zinc sulfate is not particularly limited, and there are a method of immersing the plated steel sheet, a method of spraying on the plated steel sheet, a method of applying via a coating roll, and the like.

The contact time between the hot-dip galvanized steel sheet and the aqueous solution containing zinc sulfate shall be 1 second or longer. If the contact time is less than 1 second, a sufficient Zn-based oxide cannot be obtained, and the effect of improving slidability cannot be obtained. In order to obtain a further effect, it is preferably 3 seconds or more. On the other hand, if it exceeds 30 seconds, the amount of the zinc-based oxide layer increases too much, which may lead to deterioration of weldability and adhesiveness. Therefore, 30 seconds or less is preferable. More preferably, it is 15 seconds or less.

It is preferable that the surface of the zinc-based oxide layer formed in the above step is held in contact with an alkaline aqueous solution for 0.5 seconds or longer, and then washed with water and dried (neutralization treatment). Similarly, the method of bringing the surface of the zinc-based oxide layer into contact with the alkaline aqueous solution is not limited, and there are a method of immersing the plated steel sheet, a method of spraying on the plated steel sheet, a method of applying via a coating roll, and the like.

If the aqueous solution containing zinc sulfate remains on the surface of the steel sheet after washing with water and drying, rust is likely to occur when the steel sheet coil is stored for a long period of time. From the viewpoint of preventing the occurrence of such rust, a treatment is performed to neutralize the sulfate ions remaining on the surface of the steel sheet by contacting the steel sheet with the alkaline solution by dipping it in an alkaline solution or spraying the alkaline solution. The alkaline solution preferably has a pH of 12 or less in order to prevent dissolution of the zinc-based oxide formed on the surface. There is no limitation on the solution to be used, and sodium hydroxide, sodium carbonate, sodium phosphate, sodium pyrophosphate, sodium tripolyphosphate and potassium salts thereof can be used. Also, it contains molten zinc-based plated steel plate and zinc sulfate. It is preferable to wash with water within 1 second after contacting with the aqueous solution. If the time required for washing with water is 1 second or longer, uneven appearance may occur.

The zinc-based oxide in the present invention is an oxide or hydroxide mainly composed of zinc as a metal component, and when the total amount of metal components such as iron and Al is less than that of zinc, or sulfuric acid. , Nitrate, chlorine and other anions are also included in the zinc oxide of the present invention when they are contained in a total amount smaller than the number of moles of oxygen and hydroxyl groups.

Further, the zinc-based oxide layer may contain an anion component such as sulfate ion used for adjusting the pH of an acidic solution, but the zinc-based oxide layer has an anion component such as sulfate ion and a pH buffering action. Impures such as S, N, P, B, Cl, Na, Mn, Ca, Mg, Ba, Sr, Si contained in the acidic solution, S, N, P, B, Cl, Na, Mn, Ca, Mg , Ba, Sr, Si, O, and C are incorporated into the zinc-based oxide layer without impairing the effects of the present invention.

The present invention will be described in more detail by way of examples.
A hot-dip galvanized steel sheet having a thickness of 0.7 mm, which was annealed after cold rolling, was subjected to hot-dip galvanizing by a conventional method. Next, temper rolling was performed. The amount of zinc plating was adjusted to 45 g / m ² per side. After temper rolling, it was immersed in the aqueous solution shown in Table 1 for a predetermined time. The concentrations of NaHF ₂ and KHF _{2 in} the aqueous solution containing zinc sulfate were 0 to 10.0 g / L, and the liquid temperature was 60 ° C. After the immersion treatment, a part is sprayed with an alkaline solution (sodium pyrophosphate aqueous solution) having a pH of 10.54 and a temperature of 50 ° C. for a predetermined time to neutralize the sulfate ions remaining on the surface of the steel sheet, and then at 50 ° C. Hot water was sprayed on the steel plate.

The press formability was evaluated with respect to the GI obtained as described above. The press formability (sliding characteristics during press forming) was evaluated by the friction coefficient.

The method for measuring the thickness of the zinc-based oxide layer, the method for measuring the zinc-based oxide formation area ratio, and the method for evaluating the sliding characteristics are as follows.

[1] Measurement of thickness of zinc-based oxide layer A fluorescent X-ray analyzer was used to measure the thickness of the zinc-based oxide layer. The voltage and current of the tube at the time of measurement were set to 30 kV and 100 mA, and the spectroscopic crystal was set to TAP to detect OKα rays. When measuring the OKα ray, the intensity at the background position was measured in addition to the peak position so that the net intensity of the OKα ray could be calculated. The integration time at the peak position and the background position was set to 20 seconds, respectively. In addition, silicon wafers having silicon oxide films of 96 nm, 54 nm, and 24 nm formed by cleavage to appropriate sizes were also measured at the same time, and the zinc-based oxide layer was determined from the measured OKα ray intensity and silicon oxide film thickness. The thickness of was calculated.

[2] Measurement of Zinc Oxide Generation Area Ratio Using an extremely low acceleration SEM, observe any 10 points in a 35 μm × 45 μm field on the surface of the hot-dip galvanized steel plate, and obtain a zinc-based SEM image. The area ratio of the zinc-based oxide-generated portion was measured from the difference in brightness between the oxide-generated portion and the non-oxide-generated portion, and the average value was taken as the zinc-based oxide-generated area ratio.

[3] Method for measuring friction coefficient In order to evaluate press formability, the friction coefficient of each test material was measured as follows.
FIG. 4 is a schematic front view showing the friction coefficient measuring device. As shown in the figure, the sample 1 for measuring the friction coefficient collected from the test material is fixed to the sample table 2, and the sample table 2 is fixed to the upper surface of the horizontally movable slide table 3. A vertically movable slide table support 5 having a roller 4 in contact with the slide table 3 is provided on the lower surface of the slide table 3, and by pushing up the slide table support 5, the bead 6 presses the load N on the sample 1 for measuring the friction coefficient. The first load cell 7 for measuring the above is attached to the slide table support base 5. A second load cell 8 for measuring the sliding resistance force F for moving the slide table 3 in the horizontal direction while applying the pressing force is attached to one end of the slide table 3. As a lubricating oil, a rust preventive cleaning oil (Preton R352L, Preton is a registered trademark) manufactured by Sugimura Chemical Industrial Co., Ltd. was applied to the surface of Sample 1 for the test.

5 and 6 are schematic perspective views showing the shape and dimensions of the beads used. The lower surface of the bead 6 slides while being pressed against the surface of the sample 1. The shape of the bead 6 shown in FIG. 5 is a curved surface having a width of 10 mm, a length of the sample in the sliding direction of 5 mm, and lower ends of both ends in the sliding direction having a radius of curvature of 1.0 mm R. It has a plane with a length of 3 mm in the direction of motion. The shape of the bead 6 shown in FIG. 6 is a curved surface having a width of 10 mm, a length of 59 mm in the sliding direction of the sample, and a curvature of 4.5 mm R at both ends in the sliding direction, and the lower surface of the bead on which the sample is pressed has a width of 10 mm and slides. It has a flat surface with a directional length of 50 mm.

The coefficient of friction was measured under the following two conditions.
[Condition 1]
Using the bead shown in FIG. 5, the pressing load N: 400 kgf (3922N) and the sample drawing speed (horizontal moving speed of the slide table 3) were set to 100 cm / min.
[Condition 2]
Using the bead shown in FIG. 6, the pressing load N: 400 kgf (3922N) and the sample drawing speed (horizontal moving speed of the slide table 3) were set to 20 cm / min.
The coefficient of friction μ between the test material and the bead was calculated by the formula: μ = F / N.

[4] Evaluation method of degreasing property The degreasing property was evaluated by the water wetting rate after degreasing. The prepared test piece was coated with Preton R352L, a cleaning oil for pressing manufactured by Sugimura Chemical Industrial Co., Ltd., at 2.0 g / m ² on one side, and then using an alkaline degreasing solution of FC-L4460 manufactured by Nippon Parkering Co., Ltd. The sample was degreased. Deterioration of the alkaline degreasing liquid in the automobile production line was simulated by adding 10 g / L of Preton R352L, a cleaning oil for pressing manufactured by Sugimura Chemical Industrial Co., Ltd., to the degreasing liquid in advance. Here, the degreasing time was 60 seconds and the temperature was 37 ° C. At the time of degreasing, the degreasing liquid was stirred with a propeller having a diameter of 10 cm at a speed of 150 rpm. The degreasing property was evaluated by measuring the water wetting rate of the test piece 20 seconds after the completion of degreasing.

[5] Investigation of Impact on Equipment For the purpose of investigating the effect of promoting corrosiveness on equipment, the pitting potential of SUS304 was measured in each treatment liquid. The sweep range of the potential was 11500 mV (vs. SHE, the sweep speed was 2 mV / min from the natural immersion potential. Those with pitting potential were marked with x, and those without pitting potential were marked with ◯.

[6] Evaluation of appearance unevenness The appearance unevenness was visually evaluated. Based on the appearance sample shown in FIG. 7, a score of 1 to 5 was given and evaluated. It should be noted that 4 points or more indicate that it is good, and 5 points indicate that it is even better.

The following items were clarified from Table 1.

In the example of the present invention in which an aqueous solution containing zinc sulfate containing NaHF ₂ and / or KHF ₂ in an appropriate range and GI were brought into contact with each other for oxide formation treatment, a sufficient film thickness was obtained as compared with the comparative example. Therefore, excellent press formability is obtained. Further, when the oxidizing agent is added, the film thickness increases at the same holding time, and the press formability (sliding characteristics) is more stable.

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

1 Sample for coefficient of friction measurement 2 Sample table 3 Slide table 4 Roller 5 Slide table support 6 Bead 7 1st load cell 8 2nd load cell 9 Rail N Pressing load F Sliding resistance

Claims (4)

A method for producing a hot-dip galvanized steel sheet having a zinc-based oxide layer on its surface. The hot-dip galvanized steel sheet is temper-rolled and then selected from NaHF ₂ , KHF ₂ , HF, NaF, and KF. One or more selected fluorine compounds are brought into contact with an aqueous solution containing zinc sulfate having a pH of 4.2 to 7.0 containing 0.10 g / L or more and 5.0 g / L or less in total for 1 second or more, and then washed with water. A method for producing a hot-dip galvanized steel sheet having a zinc-based oxide layer on its surface. The hot-dip galvanized steel sheet is brought into contact with an aqueous solution containing zinc sulfate and then washed with water, and then the surface of the zinc-based oxide layer is brought into contact with an alkaline aqueous solution for 0.5 seconds or longer, and then washed with water and dried. The method for producing a hot-dip galvanized steel sheet having a zinc-based oxide layer on the surface according to claim 1, wherein the method is performed. The zinc-based surface according to claim 1 or 2, wherein the time from contacting the hot-dip galvanized steel sheet with the aqueous solution containing zinc sulfate to washing with water is less than 1 second. A method for producing a hot-dip galvanized steel sheet having an oxide layer. The method for producing a hot-dip galvanized steel sheet having a zinc-based oxide layer on the surface according to any one of claims 1 to 3, wherein the hot-dip galvanized steel sheet is not alloyed.