WO2014013623A1 - Method for producing steel sheet having excellent chemical conversion properties and galling resistance - Google Patents

Abstract

Provided is a method for producing a steel sheet having excellent chemical conversion properties and galling resistance. An electrolysis treatment is carried out in an aqueous solution containing a zinc ion and a nitrate ion using a steel sheet as a cathode to form a zinc oxide and/or a zinc hydroxide on the surface of the steel sheet in such a manner that the amount of the zinc oxide and/or the zinc hydroxide becomes 70 to 500 mg/m² in terms of metal zinc content and the covering rate of the surface of the steel sheet becomes 60% or more. Subsequently, the steel sheet is washed with an aqueous solution containing phosphorus.

Description

Method for producing steel sheet with excellent chemical conversion and mold galling resistance

The present invention relates to a method for producing a steel sheet excellent in chemical conversion treatment and mold galling resistance, which is suitably used as, for example, an automobile material.

Cold rolled steel sheets are widely used in fields such as automobiles, home appliances, and building materials. Particularly in the automotive field, cold-rolled steel sheets are used because of demands regarding sheet thickness accuracy and flatness. In recent years, the use of high-strength cold-rolled steel sheets as automobile steel sheets has increased rapidly from the viewpoint of reducing CO ₂ emissions and ensuring safety in automobiles.

A high-strength steel plate is a steel plate to which Si, Mn, or the like is added as an element in the steel, and these elements form an oxide typified by, for example, SiO ₂ or MnSiO _{4 on the} surface of the steel plate in the annealing process. Here, paying attention to the chemical conversion treatment property, when a steel sheet having SiO ₂ formed on the surface in this way is subjected to a chemical conversion treatment before coating, there is a so-called skein portion where no chemical conversion crystal is formed. If a site called a scale in which a chemical conversion treatment crystal is not formed is generated, when applied as a steel sheet for automobiles, it is not preferable because it may cause a decrease in coating adhesion and a decrease in corrosion resistance. And it is known that such a scale greatly depends on the amount of SiO _{2 in} the surface layer. This is because, among oxides formed in the annealing process, Si · Mn composite oxides such as MnSiO ₄ are easily soluble, but SiO ₂ is insoluble in acids other than hydrofluoric acid, so that the chemical conversion treatment liquid This is because it does not dissolve in the phosphoric acid constituting, and inhibits the reaction with the chemical conversion solution.

Next, paying attention to mold galling resistance, when press-molding a high-strength steel sheet, not only the molding load increases, but also a problem called mold galling occurs due to the occurrence of a local high surface pressure portion. Die squeezing causes local adhesion due to contact and sliding between the steel plate and the metal that constitutes the press die at high surface pressure, and scratches may occur due to the accumulation of adhesions. This is thought to occur due to an increase in sliding resistance.

From the above viewpoint, development of a high-strength steel sheet excellent in chemical conversion property and mold galling resistance is eagerly desired.

On the other hand, as a technique for improving both the chemical conversion processability and the galling resistance of a cold-rolled steel sheet, Patent Document 1 discloses that one or more metals of Ni, Mn, Co, Mo, and Cu are cooled. A technique for discontinuously depositing on the surface of a rolled steel sheet is disclosed.

Patent Document 2 discloses that a cold rolled steel sheet has a zero-valent zinc-based ultrathin coating on the lower layer, and an upper layer is a second element composed of one or more of divalent zinc, P, B, and Si. A technique for forming a multi-layered amorphous film composed of a group of oxides is disclosed.

Further, in Patent Document 3, temper rolling is performed after powders of one or more metal oxides of Zn, Ni, Mn, Ti, Co, Mo, and Al are dispersed on the surface of a cold-rolled steel sheet. A technique for forming a metal layer of 1000 mg / m ² or less on a steel sheet surface in terms of metal is disclosed.

However, even if the method described in Patent Document 1 is applied to a cold-rolled steel sheet containing Si, the Si oxide remains as it is on the surface of the steel sheet, so that the chemical conversion property is still poor. In addition, elements such as Mo and Cu have an adverse effect on the chemical conversion treatment property, and there is a problem that the chemical conversion treatment property deteriorates due to elution from the steel sheet to the chemical conversion treatment solution during chemical conversion treatment.

In Patent Document 2, since a forming load and a local surface pressure increase when pressing a high-strength steel sheet, a second element composed of one or more of divalent zinc and P, B, Si applied to the upper layer is used. When the oxide layer of the group is destroyed, the press formability is hindered by the adhesion between the lower-valent zinc and the die, and the mold galling resistance deteriorates. Moreover, in patent document 2, the steel plate surface distribution rate of the said film | membrane is described as 50% or less. When the surface coverage is low in this way, in the case of difficult-to-mold parts such as double beads, it is considered that the uncoated base steel plate and the mold come into contact with each other to cause adhesion and cause mold galling.

In Patent Document 3, it is considered that the powder and the steel sheet are in close contact with each other due to physical adhesion by temper rolling. In other words, the adhesion between the powder and the steel sheet is very low. During pressing, especially in the case of difficult-to-process materials such as high-strength steel sheets, the powder is severely detached from the steel sheet, and the pressed powder accumulates. Scratches are likely to occur. Patent Document 3 describes that a uniform chemical conversion film is formed by uniformly dispersing metal oxide on the surface of a steel sheet. However, although the definition of dot is not described, it is unclear, but it is generally considered that the coverage is 50% or less, and when the coverage is low, it is difficult to mold parts such as double beads. It is conceivable that the uncoated base steel sheet and the mold come into contact with each other to cause adhesion and cause mold galling.

Therefore, the present inventors have studied to improve the above problems, and have applied for Patent Document 4. That is, in Patent Document 4, zinc oxide and / or zinc hydroxide is formed on a steel sheet surface by electrolytic treatment using a steel sheet as a cathode in an aqueous solution containing zinc ions, and the coating amount is 70 in terms of metallic zinc. It has been found that it is effective to improve the chemical conversion treatment property and the mold galling resistance to have a coverage of 60% or more at ~ 500 mg / m ² .

However, in recent years, the temperature of the chemical conversion liquid has been lowered in the trend of reducing CO _{2 and} reducing the cost in the automobile manufacturing process, and good chemical conversion performance can be obtained only by the film formed in Patent Document 5. I know I can't. Specifically, it is considered that the ability to dissolve the film is reduced because the etching property of the steel sheet is lowered and the surface state becomes more sensitive due to the low temperature of the chemical conversion treatment liquid.

JP-A-3-236491 JP-A-10-158858 JP-A-3-083062 JP 2008-81808 A

The present invention has been made in order to advantageously solve the above problems, and an object of the present invention is to provide a method for producing a steel sheet excellent in chemical conversion treatment property and mold galling resistance.

The inventors of the present invention have made extensive studies to solve the above problems. As a result, zinc oxide and / or zinc hydroxide is formed on the surface of the steel sheet by electrolytic treatment using the steel sheet as a cathode in an aqueous solution containing zinc ions and nitrate ions. Conversion to 70 to 500 mg / m ² , a coverage of 60% or more, and a process of bringing the steel sheet into contact with the phosphorous-containing aqueous solution after the electrolytic treatment can be performed even when the chemical conversion treatment liquid is cooled. The inventors of the present invention have found that it is effective for improving processability and anti-scoring property.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] Electrolytic treatment with a steel plate as a cathode in an aqueous solution containing zinc ions and nitrate ions, and zinc oxide and / or zinc hydroxide on the surface of the steel plate is converted to a metal zinc content of 70 to 500 mg / m ² and steel sheet surface coverage: formed to be 60% or more, and then contacting the steel sheet with an aqueous solution containing phosphorus, to produce a steel sheet excellent in chemical conversion property and mold galling resistance Method.
[2] In the above [1], the steel sheet contains 0.1% by mass or more of Si, and the method for producing a steel sheet excellent in chemical conversion treatment property and mold galling resistance.
[3] A method for producing a steel sheet excellent in chemical conversion treatment property and mold galling resistance, characterized in that the phosphorus content of the aqueous solution containing phosphorus is 5 to 5000 mass ppm in the above [1] or [2] .
[4] In any one of [1] to [3], the aqueous solution containing phosphorus has a pH of 4 to 12, and a method for producing a steel sheet having excellent chemical conversion property and mold galling resistance .

According to the present invention, a steel sheet excellent in chemical conversion treatment and mold galling resistance can be obtained. In particular, even when the chemical conversion treatment liquid is lowered in temperature, good chemical conversion treatment and mold galling resistance can be obtained. In addition, the present invention has an effect on cold-rolled steel sheets, particularly high-strength cold-rolled steel sheets containing Si, and is an effective technique that achieves both chemical conversion processability and die-squeeze resistance of high-strength cold-rolled steel sheets. Industrially extremely valuable.

FIG. 1 is a schematic front view (Example) showing a dynamic friction coefficient measuring apparatus. FIG. 2 is a schematic perspective view showing the bead shape and dimensions in FIG.

Hereinafter, the present invention will be described in detail.
The steel plates targeted in the present invention are hot-rolled steel plates and cold-rolled steel plates. In particular, the present invention is optimal for cold-rolled steel sheets that are frequently used in the automotive field and the like. Steel plates with various elements added to steel to improve various properties such as mechanical properties (for example, high-strength steel plates) have non-uniform phosphate crystals during chemical conversion treatment due to the influence of additive elements present on the surface. May be. On the other hand, a uniform chemical conversion coating is always required for steel sheets. From such a viewpoint, it is valuable to apply the present invention to a steel sheet to which the above various elements are added, and a stable chemical conversion coating can be obtained by the present invention.

The components of the steel sheet targeted by the present invention are not particularly limited. For example, a steel sheet having a Si content of 0.1% by mass or more is preferably used. This is because when the Si content in the steel is 0.1% by mass or more, Si oxide is usually formed on the surface of the steel sheet, which greatly impairs the chemical conversion processability. Therefore, the value of applying the treatment of the present invention is great. Because. Even when pressing, steel sheets with a Si content of 0.1% by mass or more tend to cause mold galling due to the increased strength of the steel sheet. By applying the treatment of the present invention, mold galling is greatly suppressed. Is done.
In particular, in the case of a steel plate containing 0.3 mass% or more of Si and having a Si content / Mn content ≧ 0.4, the chemical conversion processability is significantly deteriorated by the conventional method. However, since the chemical conversion treatment property is remarkably improved by applying the present invention, the present invention is applied to a steel sheet containing Si by 0.3 mass% or more and having Si content / Mn content ≧ 0.4. Are preferably used.

In the present invention, electrolytic treatment is performed using a steel plate as a cathode in an aqueous solution containing zinc ions and nitrate ions to form zinc oxide and / or zinc hydroxide (hereinafter sometimes referred to as zinc-based oxide) on the surface of the steel plate. It is characterized by making it. The zinc-based oxide formed by the cathodic electrolysis method has a very fine network shape with one side of 1 μm or less. By forming these zinc-based oxides on the surface of the steel plate, this network The shape of the film contributes to improvement of mold galling resistance. Although the mechanism for improving the anti-galling property due to the zinc-based oxide having a network-like film form is not clear, it can be considered as follows. First, zinc oxide is a refractory metal and suppresses adhesion between the mold and the steel sheet, so that it is difficult for mold galling to occur, and at the same time, oil such as press oil applied during pressing can be secured. This is thought to be because it is possible to remarkably suppress the adhesion due to oil shortage due to movement.
Furthermore, cathodic electrolysis is also effective from the viewpoint of controlling the amount of zinc-based oxide formed.

In the present invention, electrolytic treatment is performed using a steel plate as a cathode in an aqueous solution containing zinc ions and nitrate ions to form a zinc-based oxide on the surface of the steel plate. Usually, when electrolytic treatment is performed using a steel plate as a cathode in an aqueous solution containing only zinc ions, zinc metal is deposited on the surface of the steel plate. Therefore, in the present invention, it is possible to form a zinc-based oxide on the surface of the steel sheet by further adding nitrate ions to the aqueous solution containing zinc ions.
The amount of zinc ion in the aqueous solution is 0.1-1 mol / L, the nitrate ion is 0.1-1 mol / L as nitric acid, the current density is 1-30 A / dm ² , the liquid temperature is 30-70 ° C., The relative flow rate is in the optimum range of 0.5 to 2.0 m / s. By performing the electrolytic treatment within these ranges, the zinc-based oxide of the present invention is easily formed. The compound to which zinc ions or nitrate ions are added is not particularly limited, and examples of the zinc ion supply compound include zinc sulfate, zinc chloride, and zinc nitrate, and examples of the nitrate ion supply compound include sodium nitrate and potassium nitrate.
Moreover, it can be confirmed with an X-ray photoelectron spectrometer that a zinc-based oxide is formed on the surface of the steel sheet. By investigating the binding energy of zinc, it is possible to distinguish between metallic zinc and zinc oxide / zinc hydroxide. Specifically, the binding energy of metallic zinc has a peak near 494 eV, and the binding energies of zinc oxide and zinc hydroxide have peaks near 499 and 500 eV, respectively. Therefore, the zinc-based oxide of the present invention has no peak in the vicinity of 494 eV, and a peak is recognized only in the vicinity of 499 and 500 eV. From this peak, it becomes clear that the zinc-based oxide is a zinc-based oxide. Furthermore, when analysis in the depth direction (from the surface layer to the base steel plate interface) is performed by ion etching, in the present invention, the zinc-based oxide has no peak in the vicinity of 494 eV at any depth, 499, A peak is recognized only in the vicinity of 500 eV, and the entire coating becomes a zinc-based oxide.

Furthermore, in the present invention, in forming a zinc oxide on the surface of the steel sheet, the amount to ^{70mg / m 2 ~ 500mg / m} 2 by metal zinc terms. This is one of the most important requirements in the present invention. Thus, by defining the formation amount of zinc-based oxide on the surface of the steel sheet to an optimum amount, the effect of zinc-based oxide formation is sufficiently obtained. It is possible to obtain a steel sheet that is exhibited and excellent in chemical conversion treatment and mold galling resistance. The mechanism of improving chemical conversion treatment by forming zinc-based oxides is not clear, but is thought to be due to the promotion of nucleation during chemical conversion treatment by forming zinc-based oxides on the steel sheet surface. . In addition, by forming zinc-based oxide on the surface of the steel sheet, as described above, the zinc-based oxide formed between the mold during pressing and the steel sheet has a high melting point, and therefore has the effect of suppressing adhesion. Have. As a result, mold galling resistance is improved.
When the amount of zinc-based oxide formed on the surface of the steel sheet is less than 70 mg / m ² , the nucleation sites during the chemical conversion treatment cannot be sufficiently supplied, and the chemical conversion treatment improvement effect is small. On the other hand, when the amount of zinc-based oxide formed on the surface of the steel sheet is more than 500 mg / m ² , the zinc-based oxide itself undergoes deformation, although the adhesion between the mold and the steel sheet during pressing is suppressed. The desorption amount of the system oxide increases, and the desorbed zinc-based oxide becomes the sliding resistance.
From the above, the zinc-based oxide for stably improving the chemical conversion treatment property and the mold galling resistance is 70 mg / m ² to 500 mg / m ² , preferably 100 to 300 mg / m ² .
The formation amount of the zinc-based oxide is calculated by measuring the intensity of Zn using fluorescent X-rays and comparing it with the intensity of the known Zn quantity.

Moreover, the coverage of the steel sheet surface of zinc-based oxide is 60% or more. This is also one of the important requirements in the present invention. By setting the coverage to 60% or more, it becomes possible to improve the chemical conversion property and the mold galling resistance. When the coverage is less than 60%, when the steel sheet is processed, the press mold and the base steel sheet are in direct contact with each other, so that micro adhesion occurs, the friction coefficient increases, and the press formability decreases. .
In the present invention, the coverage of the zinc-based oxide means the area ratio of the zinc-based oxide covering the surface of the steel sheet. Specifically, the coverage is 100 μm square using an electron beam microanalyzer. Zinc element mapping is performed, and it can be calculated from the measurement area (10000 μm ² ) by the ratio of the existing area of zinc.

After the electrolytic treatment using the steel plate as a cathode to form a zinc-based oxide on the steel plate surface, the steel plate is brought into contact with an aqueous solution containing phosphorus. This is also an important requirement. The normal chemical conversion treatment is performed in the order of alkali degreasing → surface adjustment → phosphate treatment. In the first alkaline degreasing step, it is necessary to remove rust preventive oil applied to the steel sheet, press washing oil frequently used during press molding of the automobile body outer plate, and the like. However, in particular, when alkaline degreasing is performed on a number of vehicles that flow one after another on a car manufacturer's painting line, etc., oil may be mixed in or the alkaline degreasing solution may deteriorate. Even if immersed in the liquid, the oil cannot always be removed. In some cases, the steel sheet may be turned to the next surface adjustment step in a state where the degreasing is not sufficiently performed and water repelling occurs. In such water repelling part, the surface conditioning liquid is not properly applied, and in the next phosphating process, there is a part that the phosphate crystal is coarsened or there is a part where no crystal is formed, which has an adverse effect on the phosphating process. There is.

Therefore, in the present invention, after a zinc-based oxide film is formed on the steel sheet, it is brought into contact with (for example, immersed in) a phosphorus-containing aqueous solution. By dipping in a phosphorus-containing aqueous solution, a small amount of phosphorus adheres to the surface, which makes it possible to sufficiently degrease even when considering the deterioration of the alkaline degreasing solution. Although this mechanism is presumed, OH groups are present in some of the zinc-based oxides, and it is considered that degreasing is difficult because the OH groups enhance the affinity with oil. In addition, when zinc-based oxide is applied by an electroplating method containing sulfuric acid, sulfate radicals are taken into the Zn plating film, and this sulfate radical increases the affinity with oil, so degreasing is considered difficult. . When an aqueous solution containing phosphorus is brought into contact with the steel sheet, the OH group is bonded to P before the bond between the oil and the OH group, so that the affinity with the oil is lowered, and the sulfate group is formed during the formation of the zinc-based oxide. If present, the sulfate radicals present on the surface are washed away, and a small amount of P adheres to lower the affinity with the oil, so that the degreasing property is considered to be improved.

The aqueous solution containing phosphorus is not particularly limited as long as it contains phosphorus. There is no particular limitation as long as it contains at least one phosphorus compound selected from phosphoric acid, condensed phosphoric acid, phosphorous acid, hypophosphorous acid, or salts thereof. Specific examples include orthophosphoric acid, pyrophosphoric acid, trimetaphosphoric acid, tetrametaphosphoric acid, hexametaphosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, triammonium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate , Trisodium phosphate, sodium pyrophosphate, aluminum phosphate, ammonium hypophosphite, ammonium phosphite, triammonium phosphate, and the like.

The phosphorus content in the aqueous solution containing phosphorus is preferably 5 to 5000 mass ppm. If it is less than 5 mass ppm, the effect of adsorbing phosphorus on the steel sheet surface may not be sufficient. Moreover, when it exceeds 5000 massppm, an effect will be saturated and the increase in chemical | medical solution cost and waste liquid processing cost may be caused. More preferably, it is 1000 ppm or less. Further, when the time until oiling after treatment is long, cleaning unevenness may be conspicuous. Therefore, it is desirable to perform normal water washing again after washing with a phosphorus-containing aqueous solution. In addition, phosphorus content rate shall be calculated | required in phosphorus metal amount conversion.

The pH of the aqueous solution containing phosphorus is preferably 4-12. If it is less than 4, the zinc-based oxide is easily dissolved, and the effect of improving mold galling resistance and chemical conversion treatment properties may be impaired. On the other hand, even when the pH exceeds 12, the amount of dissolution of the zinc-based oxide increases, so that the effect of improving mold galling resistance and chemical conversion treatment properties may be impaired. When the pH is not within the above range, it is possible to adjust the pH within the above range by slightly adding an acid such as phosphoric acid, hydrochloric acid or sulfuric acid, or an alkali such as sodium hydroxide, or diluting with water. it can.

The temperature of the aqueous solution containing phosphorus is preferably in the range of 20 to 70 ° C. If it is lower than 20 ° C., it may be difficult to complete phosphorus adsorption in a short time. On the other hand, when the temperature exceeds 70 ° C., not only the phosphorus adsorption effect is saturated, but also the aqueous solution containing phosphorus is likely to be dried and unevenness in appearance is likely to occur. The washing time is preferably in the range of 1 to 10 seconds. If it is less than 1 second, the adsorption of phosphorus may not be sufficiently completed. On the other hand, the treatment exceeding 10 seconds not only lengthens the production line, but also dissolution of zinc oxide by an aqueous solution containing phosphorus may occur, and a sufficient effect may not be ensured.
There is no restriction | limiting in particular in the method of the water washing process by the aqueous solution containing such phosphorus. The method of immersing a plated steel plate, the method of spraying, the method of apply | coating through an application | coating roll, etc. are mention | raise | lifted. Among them, the method of spraying on the surface of the steel sheet is the most desirable method because it requires a small amount of processing liquid and at the same time completes processing in a relatively short time due to a synergistic effect with the fluid flow effect.

Using steel plates A to F having the components shown in Table 1 (the plate thicknesses are both 1.2 mm), these steel plates were first subjected to solvent ultrasonic degreasing with toluene to remove oil on the steel plate surface. Next, electrolytic treatment was performed using the steel plate as a cathode under the electrolytic bath composition and electrolysis conditions shown in Table 2 to form a zinc-based oxide on the steel plate surface. In addition, as part of the comparative example, electroplating was performed using an electrolytic bath not containing nitrate ions to deposit metallic zinc.

About the steel plate obtained by the above, depth analysis was carried out by sputter etching using an X-ray photoelectron spectrometer, and it was investigated whether the product formed by electrolytic treatment was zinc-based oxide or metallic zinc. . In the comparative example treated using an electrolytic bath not containing nitrate ions, it is metal zinc in the entire region in the depth direction, and in other than the comparative example, it is a zinc-based oxide in the entire region in the depth direction. Each was confirmed.

Further, the amounts of zinc-based oxide and metallic zinc were measured using fluorescent X-rays as metallic zinc. Here, metal zinc plating was performed in advance to prepare a calibration plate for the Zn content by ICP and the Zn intensity by fluorescent X-rays, and the zinc content was measured. Further, the coverage of the zinc-based oxide was subjected to 100 μm square zinc element mapping using an electron beam microanalyzer, and the ratio of the existing area of zinc (coverage) was calculated from the measurement area.

Furthermore, mold galling resistance and chemical conversion property were evaluated by the following methods.

(1) Evaluation of mold galling resistance In order to evaluate galling resistance under conditions of high surface pressure assuming a bead passage part during actual pressing, the friction coefficient measuring device shown in FIG. A dynamic test was performed. As shown in FIG. 1, 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 horizontally movable slide table 3. A 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 when this is pushed up, a pressing load N applied to the friction coefficient measurement sample 1 by the bead 6. A first load cell 7 is attached to the slide table support 5. A second load cell 8 for measuring a sliding resistance force F for moving the slide table 3 in the horizontal direction in a state where the pressing force is applied is attached to one end of the slide table 3. In addition, the cleaning oil Preton R352L for press made by Sugimura Chemical Co., Ltd. was applied as a lubricating oil to the surface of the sample 1 and tested. The pressing load in the mold galling resistance test was N: 1200 kgf, and the sample drawing speed (horizontal moving speed of the slide table 3): 100 cm / min.
FIG. 2 is a schematic perspective view showing the bead shape and dimensions used in the evaluation of mold galling resistance. The bead 6 slides with its lower surface pressed against the surface of the sample 1. The bead 6 shown in FIG. 2 has a width of 10 mm, a length of 12 mm in the sliding direction of the sample, and a lower portion at both ends in the sliding direction is formed by a curved surface with a curvature of 4.5 mmR. It has a plane with a direction length of 3 mm. If this bead is used, the friction coefficient in the bead passage part at the time of press molding can be evaluated.
The mold galling resistance evaluation test condition is that the same part of the sample 1 coated with the cleaning oil Preton R352L for press manufactured by Sugimura Chemical Co., Ltd. on the surface before the test is repeatedly subjected to a sliding test of up to 40 times, and the number of sliding is possible. Therefore, it was used as an index of anti-mold galling resistance. Here, the number of slidable times is set so that the friction coefficient measuring device automatically stops when the steel sheet and the mold adhere to each other, that is, when die galling occurs. It was set to stop when F exceeded 500 kgf.
X: The slidable number of times is less than 17 times (the friction coefficient measuring device is stopped due to the occurrence of mold galling).
○: The slidable number of times is 17 to 29 times (the friction coefficient measuring device is stopped due to the occurrence of mold galling).
A: 30 times of sliding is possible. Here, the number of sliding times is 17 times. The above-mentioned mold galling resistance is applied to a 270 MPa class cold-rolled steel sheet that is used without generating galling with a pressing load of 400 kgf. This is the average number of times that mold galling occurs when evaluation is performed. If it is possible to slide 17 times or more, it is considered that mold galling will not occur in practice, so the number of sliding is based on 17 or more. .

(2) Chemical conversion treatment evaluation Using a commercially available chemical conversion treatment agent (Palbond PB-L3065 system manufactured by Nihon Parkerizing Co., Ltd.), chemical conversion treatment was performed under conditions of a bath temperature of 35 ° C. and a time of 120 seconds, and the surface SEM after chemical conversion treatment. The uniformity of the chemical conversion crystal was evaluated by observing. Here, the surface SEM observation is evaluated with a 300 × field of view, and the evaluation area is approximately 0.1 mm ² . The uniformity evaluation of the chemical conversion treatment crystal was judged according to the following criteria.
XX: No chemical conversion crystal is observed on almost the entire surface.
X: A chemical conversion treatment crystal is not recognized on about one side.
(Triangle | delta): Three or more micro scales are recognized.
◯: Two or less micro skeins and / or three or more coarse crystals are observed.
(Double-circle): There is no scale in a chemical conversion treatment crystal, and there are two or less coarse crystals.
In addition, the size of the above-described micro scale is 200 μm ² or less. Further, the coarse crystal is defined as a chemical conversion crystal having a long side of 15 μm or more.

The test results obtained above are shown in Table 2 together with the conditions.

From Table 2, test no. In all of Nos. 1 to 20 and 24 to 28, zinc-based oxides were formed on the surface of the steel sheet, and formation of metallic zinc was not observed. In addition, the following matters became clear.
(1) No. 1 to 6 are the results of comparison by changing the amount of zinc oxide by changing the energization time. No. 1, no. In Comparative Examples 1 and 2 of 2, since the amount of the zinc-based oxide is small, it can be seen that the chemical conversion property is poor. No. It can be seen that Comparative Example 3 of No. 6 has a low resistance to mold galling due to the large amount of zinc-based oxide. On the other hand, no. In the case of Examples 3 to 5 of the present invention of 3 to 5, it can be seen that both mold galling resistance and chemical conversion treatment are excellent.
(2) No. No. 7 4 is a comparative example 4 having an amount of zinc-based oxide substantially equal to 4, but having a low coverage. It can be seen that the mold galling resistance and the chemical conversion treatment are poor. No. No. 8 is No.8. This is Example 4 of the present invention in which the amount of zinc-based oxide and the coverage were within the scope of the present invention by extending the energization time under the same energization conditions as in No. 7. It can be seen that the mold galling resistance and the chemical conversion treatment property are good.
(3) No. 4 and no. Examples 9 to 14 are examples in the case where the water-washing treatment is not performed with the aqueous solution containing phosphorus or the phosphorus content is changed. No washing with an aqueous solution containing phosphorus. It can be seen that Comparative Examples 5 and 6 of 9 and 10 have poor chemical conversion properties. On the other hand, no. 4 and no. Invention Examples 2 and 5 to 8 of Nos. 11 to 14 are washed with an aqueous solution containing phosphorus under suitable conditions, and it can be seen that the chemical conversion treatment property and the mold galling resistance are excellent.
(4) Invention Examples 9 to 11 of Nos. 15 to 17 are examples in which the pH of the water washing treatment was changed with an aqueous solution containing phosphorus. It can be seen that both have good chemical conversion treatment properties and mold galling resistance.
(5) No. 4 and no. Invention Examples 2 to 18 to 20 and 12 to 14 are examples in which the ion supply compound to the electrolytic bath was changed. It can be seen that good mold galling resistance and chemical conversion properties are obtained when any compound is used.
(6) No. 4 and no. Invention Examples 2 and 15 to 19 of 24-28 are examples in which the type of steel sheet was changed. It can be seen that both have good chemical conversion treatment properties and mold galling resistance.

ADVANTAGE OF THE INVENTION According to the present invention, it is possible to suppress die galling during press molding, to provide a steel sheet that exhibits good chemical conversion treatment during subsequent chemical conversion treatment, and can be applied in a wide range of fields centering on automobile body applications. Become.

DESCRIPTION OF SYMBOLS 1 Friction coefficient measurement sample 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 N Pushing load F Sliding resistance force

Claims (4)

1. The steel plate is subjected to electrolytic treatment in an aqueous solution containing zinc ions and nitrate ions as a cathode, and zinc oxide and / or zinc hydroxide is converted to a metal zinc amount of 70 to 500 mg / m ^{2 on the} steel plate surface. Surface coverage: formed to be 60% or more,
   Then, the manufacturing method of the steel plate excellent in the chemical conversion treatment property and the mold galling resistance characterized by making the said steel plate contact the aqueous solution containing phosphorus.
2. 2. The method for producing a steel sheet having excellent chemical conversion property and anti-galling resistance according to claim 1, wherein the steel sheet contains 0.1% by mass or more of Si.
3. 3. The method for producing a steel sheet excellent in chemical conversion treatment and mold galling resistance according to claim 1 or 2, wherein the phosphorus content of the aqueous solution containing phosphorus is 5 to 5000 mass ppm.
4. 4. The method for producing a steel sheet excellent in chemical conversion treatment property and anti-galling property according to any one of claims 1 to 3, wherein the pH of the aqueous solution containing phosphorus is 4 to 12.

Images