JP4923920B2 - Steel sheet with excellent chemical conversion and mold galling resistance - Google Patents

Description

  The present invention relates to a steel sheet excellent in chemical conversion treatment and mold galling resistance, and for example, relates to a steel sheet excellent in chemical conversion treatment and mold galling resistance that is suitably used as a material for automobiles.

  For example, since a cold-rolled steel sheet is an inexpensive metal material, it is widely used in fields such as automobiles, home appliances, and building materials. Particularly in the automotive field, cold rolled steel sheets are still the mainstream of automotive materials because they have superior press formability and chemical conversion properties compared to other metal materials. In recent years, in the automobile industry, the weight of automobiles has been reduced from the viewpoint of improving fuel efficiency and reducing exhaust gas, and the use of high-strength cold-rolled steel sheets has been rapidly increasing, coupled with the need for improved collision safety. .

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 are formed as oxides on the steel plate surface. Here, the chemical conversion treatment property and the mold galling resistance depend on the material constituting the surface layer, and in particular, greatly depend on the amount of Si. Therefore, for example, when the amount of Si on the surface layer is defined as the ratio of the integrated value of Si and Fe on the surface layer of 40 nm (Si / Fe) measured using a glow discharge optical emission spectrometer, the amount of Si / Fe on the surface layer is large. In some cases, a portion called a scale is generated in which a chemical conversion treatment crystal is not formed in order to inhibit the reactivity with the chemical conversion solution. In addition, when press forming a high-altitude steel sheet, not only the forming load increases, but also there is a problem that mold galling occurs due to the formation of a local high surface pressure part, and when the surface layer has a small amount of Si / Fe Is particularly susceptible to mold galling.
From such a point of view, development of a high-strength steel sheet excellent in chemical conversion treatment property and mold galling resistance is eagerly desired.
In order to solve the above problems, Patent Literature 1, Patent Literature 2, Patent Literature 3 and the like disclose techniques for improving both the chemical conversion property and the resistance to galling of cold-rolled steel sheets.

Patent Document 1 discloses a technique for discontinuously depositing one or more metals of Ni, Mn, Co, Mo, and Cu on the surface of a cold-rolled steel sheet.
However, even if the technique 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. It is bad. Further, since elements such as Mo and Cu adversely affect the chemical conversion treatment property, there is also a problem that the chemical conversion treatment property deteriorates due to elution from the steel sheet to the chemical treatment solution during the chemical conversion treatment.

Patent Document 2 discloses an oxidation of a second element group consisting of a zero-thin zinc-based ultrathin film on the surface of a cold-rolled steel sheet, and an upper layer of one or more of divalent zinc, P, B, and Si. A technique for forming a multi-layered amorphous film made of a material is disclosed.
However, in the technique described in Patent Document 2, since the forming load and the local surface pressure increase when pressing a high-strength steel sheet, one or two kinds of divalent zinc and P, B, Si applied to the upper layer are used. When the oxide layer of the second element group consisting of the above is destroyed, the press formability is hindered by the adhesion between the lower-valent zinc and the die, and the mold galling resistance is deteriorated. 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, a powder of one or more metal oxides of Zn, Ni, Mn, Ti, Co, Mo, and Al is sprinkled on the surface of a cold-rolled steel sheet, and then subjected to temper rolling. A technique for forming a metal layer of 1000 mg / m ² or less on the steel sheet surface in terms of conversion is disclosed.
However, in the case of the technique described in Patent Document 3, it is considered that the powder and the steel plate are brought into close contact with each other by physical contact force 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. Further, Patent Document 3 describes that an effect can be obtained by uniformly dispersing the metal oxide on the surface of a steel sheet in forming a uniform chemical conversion film. 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.

Patent Document 4 discloses a technique for coating the surface of a zinc-based plated steel sheet with 1 to 1000 mg / m ² of zinc hydroxide.
The technology described in Patent Document 4 is a technology that uses a zinc-based plated steel sheet as a base steel sheet, and zinc-based plated steel sheets generally have zinc that is easily dissolved during chemical conversion treatment in the surface layer. It is not bad, and it is considered that there is almost no improvement effect of the description technique on the chemical conversion processability. Moreover, although the method of apply | coating and drying the dispersion of zinc hydroxide as the manufacturing method is disclosed, it is thought that the effect which improves press moldability for the same reason as patent document 3 is small. Furthermore, although a method by anodization is also disclosed, the present invention is impossible because the base is intended for cold-rolled steel sheets and the like.
JP-A-3-236491 JP-A-10-158858 JP-A-3-083062 JP-A-9-256169

  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 steel sheet excellent in chemical conversion treatment 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 steel sheet surface by electrolysis using a steel sheet as a cathode in an aqueous solution containing zinc ions. It has been found that 70 to 500 mg / m ² , the coverage is 60% or more, and that these are effective for improving chemical conversion treatment and mold galling resistance.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] A steel plate having zinc oxide and / or zinc hydroxide on the surface of the steel plate, wherein the zinc oxide and / or the zinc hydroxide is electrolyzed in an aqueous solution containing zinc ions using the steel plate as a cathode. It is formed on the steel sheet surface by processing, and the coating amount of the zinc oxide and / or zinc hydroxide on the steel sheet is 70 to 500 mg / m ^{2 in} terms of metallic zinc, and the coverage is 60% or more. A steel sheet excellent in chemical conversion treatment and mold galling resistance.
[2] In the above [1], the steel sheet contains Si in an amount of 0.1% by mass or more, and is a steel sheet excellent in chemical conversion property and mold galling resistance.
In the present specification, zinc oxide and / or zinc hydroxide is referred to as zinc-based oxide.

  According to the present invention, a steel sheet excellent in chemical conversion treatment property and mold galling resistance can be obtained. In particular, 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 for achieving both chemical conversion treatment and mold galling resistance of high-strength cold-rolled steel sheets. Industrially extremely valuable.

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 sheets with various elements added to steel to improve various properties such as mechanical properties (for example, high-strength steel sheets) are phosphate crystals at the time of chemical conversion treatment due to the influence of additional elements present in the steel. May become non-uniform. However, 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 various elements are added, and a stable chemical conversion coating can be obtained by the present invention.
The component of the steel plate of the present invention is not particularly limited. For example, a cold-rolled 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 and greatly impairs the chemical conversion processability, so the value of applying the treatment of the present invention is great. It is. Further, even during pressing, in the case of a steel sheet having an Si content of 0.1% by mass or more, mold galling is likely to occur because the strength of the steel sheet is increased, but by applying the treatment of the present invention, mold galling is greatly increased. To be suppressed.
In particular, in the case of a steel sheet containing 0.3% by mass or more of Si and having a Si content / Mn content ≧ 0.4, the chemical conversion processability has been remarkably deteriorated in the past. Since the processability is remarkably improved, it is particularly preferably used.

  In the present invention, the steel sheet is electrolyzed in an aqueous solution containing zinc ions using the steel sheet as a cathode to form a zinc-based oxide on the surface of the steel sheet. Zinc-based oxides formed by the cathodic electrolysis method have a very fine network shape with one side of 1 μm or less. By forming these zinc-based oxides on the steel sheet surface, 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, since zinc oxide is a high melting point metal and suppresses adhesion between the mold and the steel sheet, mold galling is less likely to occur. At the same time, it is considered that oil such as press oil applied at the time of pressing can be secured, and adhesion due to running out of oil due to sliding can be remarkably suppressed. Further, the cathodic electrolysis treatment is also effective from the viewpoint of controlling the coating amount.

Furthermore, in the present invention, in forming a zinc oxide on the surface of the steel sheet, the coating amount of a 70mg / m ² ~500mg / m ² of metal zinc terms. This is the most important requirement in the present invention. Thus, by defining the amount of zinc-based oxide coating on the surface of the steel sheet to an optimal amount, the effect of zinc-based oxide formation can be sufficiently exerted, and chemical conversion can be achieved. It becomes possible to obtain a steel sheet excellent in processability 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 applying a zinc-based oxide, as described above, the zinc-based oxide formed between the mold during pressing and the steel plate has a high melting point, and thus has an effect of suppressing adhesion. As a result, mold galling resistance is improved.
From such a point of view, when the coating amount is less than 70 mg / m ² , the nucleation site at the time of the chemical conversion treatment cannot be sufficiently supplied, and the effect of improving the chemical conversion treatment performance is small. On the other hand, when the coating amount exceeds 500 mg / m ² , the adhesion between the mold and the steel plate during pressing is suppressed, but the coating itself undergoes deformation, so that the amount of the film detaching increases and the detachment occurs. Since the separated film becomes a sliding resistance, the mold galling resistance deteriorates.
From the above, the coating amount of zinc oxide for stably improved as the chemical conversion treatability and耐型galling resistance ^{70mg / m 2 ~500mg / m 2} , preferably between 100 to 300 mg / m ^2.
The coating amount was calculated by measuring the intensity of Zn using fluorescent X-rays and comparing it with the known intensity of Zn.

The coverage of zinc-based oxide is 60% or more. This is an important requirement in the present invention, as is the amount of coating. Similarly to the coating amount, when the coverage is 60% or more, it is possible to improve the chemical conversion treatment 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 indicates 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.
The present invention is characterized in that a steel plate is electrolyzed in an aqueous solution containing zinc ions as a cathode to form a zinc-based oxide on the surface of the steel plate. Usually, the steel plate is used in an aqueous solution containing zinc ions. When electrolysis is performed as a cathode, metallic zinc is deposited on the surface of the steel sheet. Therefore, in the present invention, a zinc-based oxide can be formed on the surface of the steel sheet by using an electrolytic solution in which an appropriate amount of nitrate ions is further added to an aqueous solution containing zinc ions.
The amount of zinc ion in the aqueous solution is 25 to 200 g / l for zinc sulfate heptahydrate, 5 to 100 g / l for nitrate ion, 3 to 30 A / dm ² , current temperature is 30 to 70 ° C., plating The optimum range of the relative flow rate of the liquid is 0.5 to 2.0 m / sec. By performing the electrolytic treatment within these ranges, the zinc-based oxide of the present invention is formed.
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 a peak near 494 eV. There is no peak, and a peak is recognized only in the vicinity of 499 and 500 eV. From this peak, it becomes clear that it is a zinc-based oxide. Further, as a result of analysis in the depth direction (from the surface layer to the coating / underlying steel plate interface) by ion etching, in the present invention, the zinc-based oxide has no peak in the vicinity of 494 eV at any depth, and 499, 500 eV. Since the peak was recognized only in the vicinity, it is clear that the entire film is a zinc-based oxide.

The following examples further illustrate the present invention.
Using steel plates A to G composed of the components shown in Table 1 (the plate thicknesses were all 1.2 mm), these steel plates were first subjected to solvent ultrasonic degreasing with toluene to remove oil on the steel plate surfaces. Next, the surface layer Si / Fe is changed by performing a pickling treatment by immersing in a sulfuric acid solution adjusted in pH and hydrofluoric acid. Subsequently, the electrolytic treatment is performed using the steel plate as a cathode with the electrolytic bath composition and electrolysis conditions shown in Table 2. A zinc-based oxide film was formed on the steel sheet surface. As a part of the comparative example, a plating process was performed using a bath not containing nitrate ions to deposit metallic zinc.

About the steel plate obtained from the above, depth analysis is performed by sputter etching using an X-ray photoelectron spectrometer, and it is investigated whether the coating type is zinc-based oxide or metallic zinc. In the region, it was confirmed to be zinc-based oxide or metallic zinc. Further, the coating amounts of zinc-based oxide and metallic zinc were measured using fluorescent X-rays as metallic zinc. Furthermore, the coverage of the zinc-based oxide was subjected to 100 μm square zinc element mapping using an electron beam microanalyzer, and the ratio (coverage) of the existing area of zinc was calculated from the measurement area. In addition, for the surface Si / Fe measurement, a sample that was not subjected to plating after pickling was prepared and subjected to measurement.
Furthermore, the mold galling resistance and the chemical conversion treatment 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 at the time of actual pressing, a flat plate repeated sliding test using the friction coefficient measuring device of FIG. went. 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 measuring 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. A test was conducted by applying Daphne Oil Coat SK as a lubricant on the surface of Sample 1. 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 shape of the bead 6 shown in FIG. 2 is 10 mm wide, 12 mm long in the sliding direction of the sample, and the lower part of both ends of the sliding direction is a curved surface with a curvature of 4.5 mmR. It has a flat surface with a 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 test conditions for mold galling resistance are: Before the test, the cleaning oil Preton R352L manufactured by Sugimura Chemical Co., Ltd. was applied to the surface of Sample 1 and the same part was repeatedly slidably tested up to 20 times. The index of mold galling resistance was determined by the size of micro-adhesion between the steel plate and the mold during sliding. It should be noted that, when die galling occurred, the friction coefficient measuring device was set to automatically stop because the pulling force in the friction coefficient measuring device increased. Further, micro adhesion is evaluated based on the variation range of the pulling load during the sliding test as shown in FIG. 3, and the micro adhesion is good when the variation range of the pulling load is 10 kgf or less (FIG. 3-A). The case of larger than 10 kgf was regarded as poor micro adhesion (FIG. 3-B), and the mold galling resistance was evaluated and shown as follows.
X: 20 times of sliding is impossible (the friction coefficient measuring device stops due to generation of galling).
○: 20 times of sliding is possible.
(Double-circle): 20 times of sliding is possible and the adhesion at the time of the 5th sliding is suppressed.

(2) Chemical conversion treatment evaluation Chemical conversion treatment evaluation is performed using a commercially available chemical conversion chemical (Palbond PB-L3020 system, manufactured by Nihon Parkerizing Co., Ltd.) under conditions of a bath temperature of 43 ° C and a chemical conversion treatment time of 120 seconds. The uniformity of the chemical conversion treatment crystal was evaluated by performing subsequent surface SEM observation. The uniformity evaluation of the chemical conversion treatment crystal was judged according to the following criteria.
(Double-circle): There is no scale and nonuniformity in a chemical conversion treatment crystal.
○: The chemical conversion crystal does not have any scale but has some unevenness.
X: The chemical conversion treatment crystal has a scale.
Table 3 shows the test results obtained from the above.

From Table 3, test no. In all of Nos. 1 to 20, zinc-based oxides were formed on the surface of the steel sheet, and no formation of metallic zinc was observed. On the other hand, test no. In any of 21 to 24, only the outermost layer of the coating was a zinc-based oxide, and it was confirmed that metallic zinc was formed inside the coating. Since the outermost layer is thought to have been formed by oxidation by the atmosphere after deposition of metallic zinc, metallic zinc is the coating type.

From the test results shown in Table 3, the following matters became clear.
(1) No. Nos. 1 to 7 are the results of comparison by changing the surface layer Si / Fe by changing the conditions of the pickling treatment. 1, no. In the comparative example 2, since the surface layer Si / Fe is not suitable, almost no zinc oxide is formed (both the coating amount and the coverage are low outside the scope of the present invention), and as a result, the chemical conversion property is poor. I understand. On the other hand, no. In the case of 3 to 6 of the present invention examples, zinc oxide is formed, and since the coating amount is also within the scope of the present invention, the result that both mold galling resistance and chemical conversion treatment properties are excellent is obtained. I understand.
(2) No. 5 and no. 7 to 14 show the results when the coating amount of the zinc oxide is changed. No. In the case of Comparative Example No. 7, since the coating amount and the coverage are low outside the scope of the present invention, no effect is observed in both mold galling resistance and chemical conversion treatment. No. In the case of Comparative Example 8, since the amount of coating is larger than that of No7, a slight improvement in mold galling resistance is recognized, but the coating amount and coverage are low outside the scope of the present invention, and an effect of improving chemical conversion treatment is recognized. Absent. No. with more film than the scope of the present invention. In the case of 13 comparative examples, it can be seen that the resistance to mold galling is deteriorated. Further, the amount of film is larger than No. 13, and No. 1 outside the scope of the present invention. In the case of 14 comparative examples, it can be seen that not only the mold galling resistance but also the chemical conversion treatment performance deteriorates. No. whose film amount is within the scope of the present invention. 5 and no. In the case of Examples 9 to 12 of the present invention, both mold galling resistance and chemical conversion treatment are good. 5 and no. It turns out that the case of 10 is more suitable.
(3) No. 5 and no. 15-20 have shown the result at the time of changing the kind of steel plate. It can be seen that both steel sheet types are excellent in both resistance to mold galling and chemical conversion treatment.
(4) Nos. 21 to 24 are the results when electrolysis is performed under the same conditions as the others except that bath 2 in which sodium nitrate is removed from the electrolysis bath of bath 1 in Table 2 is used as the electrolysis bath. It can be seen that metal zinc is generated on the surface of the steel sheet, and the mold galling resistance is poor.

  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.

It is a schematic front view which shows a friction coefficient measuring apparatus. Example 1 It is a schematic perspective view which shows the bead shape and dimension in FIG. Example 1 It is a figure which shows the pulling load change width | variety at the time of a sliding test. Example 1

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Sample for friction coefficient measurement 2 Sample stand 3 Slide table 4 Roller 5 Slide table support stand 6 Bead 7 1st load cell 8 2nd load cell 9 Rail N Pushing load F Sliding resistance force

Claims (1)

A steel plate containing 0.1% by mass or more of Si and having zinc oxide and / or zinc hydroxide on the steel plate surface,
The zinc oxide and / or the zinc hydroxide is formed on the surface of the steel sheet by electrolytically treating the steel sheet as a cathode in an aqueous solution containing zinc ions,
Furthermore, the amount of coating of the zinc oxide and / or zinc hydroxide on the steel sheet is 70 to 500 mg / m ^{2 in} terms of metallic zinc, and the coverage is 60% or more. Steel plate with excellent galling properties.

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