JP2014005489A - Method of manufacturing cold rolled steel sheet which has superior in press formation, chemical conversion processing, and after-coating anticorrosive properties - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a cold rolled steel sheet which has superior in press formation, chemical conversion processing, and after-coating anticorrosive properties.SOLUTION: There is provided a method of manufacturing a cold rolled steel sheet which has superior in press formation, chemical conversion processing, and after-coating anticorrosive properties, the method being characterized in carrying out: water washing after a holding process of holding for 1 to 30 seconds while sticking an acidic aqueous solution containing Zn ions on a surface of a cold rolled steel sheet after electric Zn plating of 100 to 5,000 mg/min an amount of Zn on the surface of the cold rolled steel sheet using the acidic aqueous solution containing the Zn ions; and forming an oxide and/or a hydroxide of Zn at the part where the Zn plating layer is formed to a mean thickness of 10 nm or larger.

Description

  The present invention relates to a method for producing a cold-rolled steel sheet excellent in press formability, chemical conversion property and post-coating corrosion resistance.

In recent years, as a measure against global warming, how to reduce the weight of the vehicle body in order to reduce CO ₂ emissions from automobiles has become a challenge for automobile manufacturers. Thinning the steel plate used is most effective for reducing the weight of the vehicle body, but reducing the thickness of the steel plate while keeping the strength of the steel plate the same will reduce the rigidity of the steel plate and improve occupant safety in the event of a collision. It becomes impossible to secure the sex. For this reason, the movement to adopt high-strength steel sheets as vehicle body materials, in which the plate thickness is reduced and the reduced rigidity is compensated by increasing the strength of the steel, is gradually increasing. Recently, high-strength steel sheets with a tensile strength of 1180 MPa class. The movement to use for body use is also becoming active.

  In order to increase the strength of steel sheets, alloying elements such as Si and Mn are added to strengthen the solution or refine crystal grains, and precipitate forming elements such as Nb, Ti, and V are added. Effective methods include precipitation strengthening and a method of strengthening by generating a hard transformation structure such as a martensite phase.

  In general, increasing the strength by adding alloying elements, on the other hand, causes a decrease in ductility, so there is a drawback that it is difficult to press-mold to create the shape of the part, but Si has an effect of decreasing ductility compared to other elements. Because of its small size, it is an effective element for increasing strength while ensuring ductility. For this reason, it can be said that the addition of Si is almost essential for a steel sheet having both workability and high strength.

However, since Si has a very low equilibrium oxygen partial pressure of oxide and is easily oxidized even in a reducing atmosphere in a continuous annealing furnace used in the production of general cold-rolled steel sheets, Is passed through a continuous annealing furnace, Si is selectively oxidized on the surface of the steel sheet to form SiO ₂ . When a steel sheet with SiO ₂ formed on the surface is subjected to a chemical conversion treatment before coating, this SiO ₂ inhibits the reaction between the chemical conversion solution and the steel sheet, so that there is a so-called part that does not form a chemical conversion crystal. Become. Steel sheets with scale after chemical conversion treatment may already have rust in the water washing stage after chemical conversion treatment, and even if they do not reach rust, the corrosion resistance of the steel sheet after electrodeposition coating is very poor. It was very difficult to use high strength cold-rolled steel sheets containing Si for body applications.

  Furthermore, since it is always subjected to press molding when actually used as a material for an automobile body, it is necessary to have excellent slidability during pressing. That is, for high-strength cold-rolled steel sheets containing Si, press formability is required simultaneously with chemical conversion treatment.

  As a method for improving the chemical conversion property and press formability of a high-strength cold-rolled steel sheet containing Si, there are the following proposals.

For example, Patent Document 1 has a zinc plating film with an adhesion amount of 10 to 2000 mg / m ^{2 on} the surface of the steel sheet and has a predetermined crystal orientation to achieve both mold galling resistance and chemical conversion treatment. Techniques to do this have been proposed. This technology was made mainly to improve mold galling resistance. Regarding chemical conversion treatment, even with a small amount of zinc, microcells were formed between the zinc adhesion part and the exposed steel sheet part. This suggests that the chemical conversion reaction becomes active. However, when the Si concentration of the steel plate is high, a considerable steel plate surface is covered with SiO ₂ oxide, and when this portion is a steel plate exposed portion, it cannot be said that microcells are necessarily formed. In addition, the friction coefficient measured by the inventors of the present invention was found to be the same as or slightly higher than that of cold-rolled steel sheets only by Zn electroplating, and sufficient slidability could not be obtained depending on the pressing conditions. Furthermore, a sulfuric acid bath is used as a plating bath, and when a galvanized film is formed under the same conditions as those presented in the examples, depending on the type of alkaline degreasing solution in the alkaline degreasing step performed before chemical conversion treatment Also found that degreasing was inadequate.

Patent Document 2 discloses that 60% of Zn oxide and / or Zn hydroxide having a Zn content of 70 to 500 mg / m ² is electrolyzed on a steel sheet surface in an aqueous solution containing Zn ions as a cathode. It is disclosed to obtain a cold-rolled steel sheet having the above coverage. In Patent Document 2, the film formed on the surface is Zn oxide and / or Zn hydroxide, and the slidability at the time of pressing is superior to that of metal Zn. It is described that it is easy. However, since the Zn oxide and / or hydroxide here is formed by electrolysis using a cold-rolled steel sheet as a cathode, the adhesion with the steel sheet is not always good and falls off during press forming. In some cases, especially when the contact length with the tool during sliding is long, the slipping film increases the friction coefficient, and on the contrary, it has been found that the moldability may be deteriorated. Furthermore, since oxides are formed by utilizing the reduction reaction by nitrate ions present in the solution, nitrate ions are consumed when the film is formed, and the production line requires frequent replenishment. There is a drawback in that the manufacturing process is complicated because the nitrate ion concentration must be strictly controlled.

  In Patent Document 3, a zero-valent Zn film is formed in the lower layer, one or more amorphous oxides of divalent Zn, P, S, and Si in the upper layer, or in addition, Mn, Ni as a third element Cold rolled steel sheets having an oxide containing at least one of Co, Mg, and Ca have been proposed. However, the upper layer amorphous oxide does not necessarily improve the chemical conversion treatment property. In particular, in recent years, the temperature of chemical conversion treatment has been actively reduced from the viewpoint of rationalization of production by automobile manufacturers, and it has been found that the chemical conversion treatment performance deteriorates with such low temperature of chemical conversion treatment.

Patent Document 4 discloses an electric Zn-plated steel sheet having a Zn-based oxide layer on the plating surface, and the amount of plating of the electric Zn-plated steel sheet described in the examples is 50 g / m ² . That is, in Patent Document 4, in an electro-zinc-plated steel sheet having a thick plating amount as described in the examples, by forming a Zn-based oxide layer on the plating surface, good press formability and chemical conversion processability It is only described that can be obtained. In Cited Document 4, the chemical conversion property of a high-strength steel sheet containing a large amount of Si is not studied.

JP 2006-299351 A Japanese Patent Laid-Open No. 2008-081808 JP-A-10-158858 JP 2005-248262 A

  As described above, in the case of a cold-rolled steel sheet to which Si is added for the purpose of achieving high strength without reducing ductility, it cannot be said that the technology that satisfies both the chemical conversion treatment property and press formability at the same time is sufficient. The current situation is obstructing the application to automobile bodies.

  The present invention solves the above-mentioned problems, and even for steel sheets containing 0.5 mass% or more of Si in the steel as a strengthening element, cold rolling excellent in press formability, chemical conversion treatment properties and corrosion resistance after coating. It aims at providing the manufacturing method of a steel plate.

The present inventors have found that when SiO ₂ is formed on the surface of the steel sheet, the portion where SiO ₂ is formed, for Fe which is the main component of the steel sheet does not dissolve, by noting that does not cause chemical crystal formation reaction. Then, it was considered that the formation of a dissolution reaction on the surface of the steel sheet by some method leads to a chemical conversion crystal formation reaction. In addition, metal Zn is considered to form a zinc phosphate film by reaction with the chemical conversion treatment solution, and as a result of examination, a small amount of Zn necessary for forming the zinc phosphate film is applied to the surface of the cold rolled steel sheet. Thus, it was confirmed that a chemical conversion crystal formation reaction progressed with respect to the Si-containing cold-rolled steel sheet, and as a result, a zinc phosphate coating without so-called scale could be formed after the chemical conversion treatment.

  However, by simply attaching Zn, the friction coefficient of the steel sheet, which is an index of slidability, is increased, which leads to deterioration of press formability. On the other hand, if it is held for a predetermined time without immediately washing with water after electro Zn plating, the adhered outermost layer of Zn can be oxidized to form Zn oxide and / or hydroxide. Since the Zn oxide and / or hydroxide is excellent in adhesion, the coefficient of friction can be kept low, a cold-rolled steel sheet with excellent press formability and excellent chemical conversion properties can be obtained. I found.

  The chemical conversion treatment is a general process that proceeds in the order of alkaline degreasing → surface conditioning → phosphate treatment. Among them, in the alkaline degreasing step, the oil applied to the steel sheet is mixed into the alkaline degreasing solution one after another. Therefore, the degreasing ability is considerably inferior in the actual line. And, if the anti-rust oil is applied to the degreasing liquid assuming such a real line and immersed in the steel plate immediately washed with water after electroplating, the anti-rust oil cannot be removed sufficiently and water repelling occurs. I found. Steel sheets with such water repellency have poor wettability with the chemical conversion treatment solution and surface unevenness. Therefore, it is important to completely remove oil on the steel sheet surface after alkaline degreasing. Examined from this point of view, when using a degreasing solution that assumes a real line by washing with water after holding for a predetermined time without immediately washing with water after contact with electro-zinc plating, and then contacting with an aqueous solution containing P However, it was found that the oil content of the steel sheet can be removed and a sufficient water wetting rate can be obtained.

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

[1] Using an acidic aqueous solution containing Zn ions on the surface of the cold-rolled steel sheet, after applying an electric Zn plating of 100 to 5000 mg / m ² as the amount of Zn, the acidic aqueous solution containing Zn ions on the surface After holding the film for 1 to 30 seconds with the film attached, it was washed with water to form Zn oxide and / or hydroxide with an average thickness of 10 nm or more on the part where the Zn plating layer was formed. A method for producing a cold-rolled steel sheet having excellent press formability, chemical conversion property and post-coating corrosion resistance.

  [2] The acidic aqueous solution containing Zn ion is a chemical having a pH buffering action, such as acetate, phthalate, citrate, succinate, lactate, tartrate, borate and phosphate. It is a solution containing 5 to 50 g / L of one or more of them and adjusting the pH to 1.0 to 5.0 with sulfuric acid, and in the holding step, the average film thickness of the acidic aqueous solution is 1 μm or more and 20 μm or less. The method for producing a cold-rolled steel sheet having excellent press formability, chemical conversion property and post-coating corrosion resistance as described in [1].

  [3] The press-molding according to [1] or [2], wherein after the water washing, the P-concentration is 0.001 to 2 g / L, and contacted with a P-containing aqueous solution having a temperature of 30 to 60 ° C. Method for cold-rolled steel sheet with excellent properties, chemical conversion properties and post-coating corrosion resistance.

  According to the present invention, a cold-rolled steel sheet excellent in press formability, chemical conversion property and post-coating corrosion resistance can be obtained. Even for high-strength steel sheets that contain Si, Mn, etc. in the steel, and because the oxides of Si and Mn are formed on the surface, it is difficult to form a chemical conversion coating in the coating process in automobile manufacturing. A sufficient chemical conversion film can be formed and good post-coating corrosion resistance can be obtained. Moreover, it is excellent in the slidability between the steel plate and the mold at the time of press forming, and can be suitably used as a steel plate for automobiles.

The schematic front view which shows a friction coefficient measuring apparatus. FIG. 2 is a schematic perspective view showing bead shapes and dimensions in FIG. FIG. 2 is a schematic perspective view showing bead shapes and dimensions in FIG.

Generally, a cold-rolled steel sheet (hereinafter sometimes abbreviated as a steel sheet) is manufactured by subjecting a cold-rolled steel sheet to a heat treatment in a range of 700 to 900 ° C. in a reducing atmosphere containing hydrogen. . However, heating in this reducing atmosphere causes a phenomenon in which easily oxidizable elements of the steel plate components are concentrated as oxides on the surface of the steel plate (hereinafter sometimes referred to as surface concentration). Typical oxides include SiO ₂ , MnO, and Si—Mn composite oxide. In the portion where these oxides are present on the steel sheet surface, the reaction of etching the steel sheet with the chemical conversion solution to precipitate the chemical crystals is hindered, and the part where the chemical crystals are not partially formed on the steel sheet surface, so-called skeins are generated, It will be inferior to chemical conversion processability.

On the other hand, when Zn plating is performed on the surface of the steel plate, Zn covers the oxide with which the surface has been concentrated, so that the reaction between Zn and the chemical conversion treatment solution occurs even on the steel plate surface where the oxide was present. Further, even if it is not possible to cover all of the oxide that has been surface-enriched by the Zn plating layer, Zn present around the oxide reacts with the chemical conversion treatment solution to cover the oxide such as SiO ₂ , that is, It is possible to easily form a chemical conversion film without any scale.

  However, simply applying Zn plating increases the friction coefficient of the steel sheet, and cracks are likely to occur during press forming. This is because the presence of Zn lowers the melting point of the surface and makes it easier for the tool and the steel sheet to adhere.

  On the other hand, in the present invention, an oxide layer (hereinafter sometimes abbreviated as an oxide layer) made of an oxide and / or hydroxide of Zn is formed on the outermost layer where the Zn plating layer is formed. To exist. If the average thickness of the oxide layer is less than 10 nm, the effect of improving the slidability cannot be obtained sufficiently, so the average thickness of the oxide layer is set to 10 nm or more. Thereby, the oxide layer can prevent the occurrence of adhesion due to direct contact between the mold and the steel sheet surface, and can improve the slidability. On the other hand, when the average thickness of the oxide layer is increased, the etching reaction by the chemical conversion solution is inhibited in the portion where the oxide is present, and therefore it is preferably 200 nm or less.

  Although the Fe-based oxide obtained by oxidizing the surface of the cold-rolled steel sheet deteriorates the slidability, in the present invention, the outermost layer in contact with the mold is a Zn oxide layer, and the slidability It is effective for. Further, the Zn oxide layer exhibits the same dissolution behavior as that of metal Zn when brought into contact with a chemical conversion solution having a pH in the range of 2 to 4, which is usually employed, and therefore does not interfere with the surface etching reaction.

In order to obtain the effect of improving the chemical conversion property, the Zn amount needs to be 100 mg / m ² or more. This is because when the amount of adhesion is less than 100 mg / m ² , Zn cannot cover the steel sheet surface, and no improvement in chemical conversion treatment is recognized. On the other hand, even if the Zn deposition amount increases, there is no problem from the viewpoint of chemical conversion, but since the increase in the Zn deposition amount leads to an increase in cost, the upper limit is set to 5000 mg / m ² .

  Moreover, in this invention, the method to make zinc adhere must be the method of electroplating by using the target cold-rolled steel plate as a cathode in the acidic aqueous solution containing Zn ion. This is because, in the electroplating of zinc, an acidic aqueous solution is generally used, and as described later, an acidic aqueous solution is required to oxidize only the surface layer of the deposited zinc layer.

  When the steel plate is taken out from the acidic aqueous solution after the electro-Zn plating, the acidic aqueous solution containing Zn ions is attached to the steel plate surface. If it hold | maintains for a predetermined time in this state (holding process), the outermost layer of the adhered galvanized layer can be oxidized. This is because Zn adhering to the steel sheet surface is dissolved (anodic reaction) by the adhering acidic aqueous solution, and hydrogen generation (cathode reaction) occurs at the same time, so the hydrogen ions in the adhering aqueous solution are consumed, and hydroxide ions Since the concentration increases, a reaction in which an oxide is formed is used. If the predetermined time is less than 1 second, an oxide layer having an average thickness of 10 nm or more cannot be stably formed.If it exceeds 30 seconds, a long space for holding is required. Since it is not realistic, the predetermined time needs to be 1 to 30 seconds.

  When the state of the acidic aqueous solution on the surface of the steel sheet is changed to a film-like state formed after taking out from the acidic aqueous solution, not only the concentration of hydroxide ions described above easily increases due to the small amount of liquid, but also hydroxylation. Since physical ions are also supplied by oxygen in the atmosphere, it becomes easy to oxidize only the surface layer. In this sense, reducing the liquid film thickness of the acidic aqueous solution is effective from the viewpoint of facilitating the supply of oxygen in the atmosphere, and the liquid film thickness (average film thickness) is preferably 20 μm or less. The lower limit can be considered from the limit of physically forming a liquid film, but on the other hand, when the liquid film is extremely thin, the above-described Zn dissolution reaction and hydrogen generation reaction do not occur, and oxide formation in the first place is not possible. Since it becomes impossible, it is preferably 1 μm or more.

  In addition, it is effective to use a pH buffering agent in the acidic aqueous solution from the viewpoint of using it for electroplating and using it for oxidation of only the surface layer in a state where the steel sheet is taken out from the acidic aqueous solution. This is because when using an acidic aqueous solution that does not have pH buffering action, while the steel sheet after electro-Zn plating is held in a state of being taken out, the amount of hydroxide ions increases while Zn is not sufficiently dissolved. This is because the rate of formation of the oxide layer of zinc formed is reduced. Examples of such a pH buffering agent include acetate, phthalate, citrate, succinate, lactate, tartrate, borate, and phosphate, and one of these It is preferable to contain 5 to 50 g / L of the above in total. This is insufficient to cause sufficient dissolution of Zn if it is less than 5 g / L, while dissolution of Zn is sufficient if it exceeds 50 g / L, but it interferes with the supply of hydroxide ions. Because. In addition, since it is premised on dissolution of Zn, it is necessary to bring the pH of the aqueous solution to the acidic region. In this case, it is important to adjust the pH with sulfuric acid. This is because the slidability is further improved by incorporating sulfate ions into the oxide layer. In addition, since zinc will melt | dissolve excessively when pH is too low, and it will become insufficient dissolution of zinc when pH is too high, it is preferable to adjust pH to the range of 1.0-5.0.

  These pH buffering agents only suppress the pH variation of the solution, and the effect of suppressing the variation only leads to promotion of oxide formation, and does not affect the electroplating process at all. .

  A normal chemical conversion treatment is performed in the order of alkali degreasing → surface conditioning → 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, if the surface oxide formation treatment as described above is performed and washed with water, the oil cannot always be stably removed. In particular, when performing alkaline degreasing 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 be deteriorated. There is a case where it is not applied and sent to the next surface adjustment step in a state where water repelling occurs. In such a water repellent part, the surface conditioning solution is not properly applied, and in the next phosphate treatment step, there is a part where the phosphate crystal is coarsened or a crystal is not formed. There is a risk.

  Therefore, in the present invention, preferably, electroplating is performed in an acidic aqueous solution containing Zn ions, and further an oxidation treatment is performed with an acidic aqueous solution attached to the surface of the steel sheet in the form of a film, followed by washing with water, and the P concentration is 0.001. It is preferable that the cold-rolled steel sheet is brought into contact with a P-containing aqueous solution having a temperature of 30 to 60 ° C. of ˜2 g / L. By contacting with the P-containing aqueous solution, a small amount of P 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, sulfate ions contained for adjusting the pH of the electric Zn plating bath are incorporated into the Zn plating film, and these sulfate ions are present in the oxide layer, particularly the outermost layer, and have an affinity for oil. It is considered that degreasing becomes difficult to improve the property. On the other hand, when an aqueous solution containing P is brought into contact with the steel sheet, sulfate ions present on the surface are washed away, and a small amount of P adheres to lower the affinity with oil, thus improving the degreasing property. I think that.

  The P concentration of the aqueous solution containing P brought into contact with the steel sheet is preferably in the range of 0.001 to 2 g / L. If it is less than 0.001 g / L, the washing effect of sulfate ions is small, and the adhesion of P to the surface may not be sufficient. On the other hand, even if it exceeds 2 g / L, there is no significant difference in effect.

  The temperature of the aqueous solution containing P is preferably in the range of 30 to 60 ° C. If the temperature is lower than 30 ° C, it takes time for washing of sulfate ions and adhesion of P, and the continuous annealing equipment requires long equipment. On the other hand, if the temperature exceeds 60 ° C., the effect is sufficient, but it is not economically appropriate because extra equipment for heating is required.

  The method for bringing the steel sheet into contact with the aqueous solution containing P is not particularly limited. For example, an immersion method or a spray method can be employed. The spray pressure, the nozzle diameter, the distance from the nozzle to the steel plate, and the like when the spray method is adopted only have to satisfy sufficient conditions for the aqueous solution to come into contact with the steel plate, and the conditions are not particularly limited.

The present invention aims to promote the formation of a film and to improve the slidability of a steel sheet on which a chemical conversion film is not formed due to the presence of SiO ₂ or the like on the surface of a cold-rolled steel sheet after annealing. Since it is one, it is suitably used for a high-strength cold-rolled steel sheet containing 0.5 mass% or more of Si, for example. However, Zn is adhered to the surface of the steel sheet so that the amount of Zn deposited is 100 to 5000 mg / m ² , that is, the presence of a slight amount of Zn on the surface of the steel sheet is recognized as an improvement in corrosion resistance after coating. It can be applied to cold-rolled steel sheets from the viewpoint of post-coating corrosion resistance. Therefore, the present invention is a technique that ensures chemical conversion and post-coating corrosion resistance for all cold-rolled steel sheets.

  Steels A to H having the composition shown in Table 1 are melted by a conventional steelmaking process, continuously cast into a slab, and then the slab is reheated to 1250 ° C., and then the finish rolling finish temperature is set. Hot rolling was performed at 850 ° C. and a coiling temperature of 600 ° C. to obtain a hot rolled sheet having a thickness of 3.0 mm. The hot-rolled sheet was pickled and cold-rolled to a thickness of 1.5 mm to obtain a test material. This test material was subjected to heat treatment at a temperature of 800 to 850 ° C. for 2 minutes at maximum in a nitrogen atmosphere containing 10 vol% of hydrogen using a laboratory reduction heating simulator to produce an annealed plate.

  Part of the heat-treated steel sheet contains zinc sulfate heptahydrate: 288 g / L, sodium acetate 20 g / L, sodium citrate 20 g / L, and an acidic aqueous solution adjusted to pH 2.0 with sulfuric acid Was applied to the anode using an iridium oxide plate to deposit Zn on the surface. A part of the steel plate was electroplated using an acidic aqueous solution adjusted to pH 2.0 by adding sulfuric acid to zinc sulfate heptahydrate: 288 g / L. The amount of deposited Zn was adjusted by changing the current density and energization time. The steel plate after electroplating was set in a squeezing device consisting of a rubber roll, the film thickness of the liquid was adjusted to 3 μm, kept as it was for a predetermined time, and then washed with water. A part was extracted from the acidic aqueous solution after electroplating and immediately washed with water. A part of the washed sample was brought into contact with a P-containing aqueous solution, and washed and dried again.

  The thickness of the surface zinc oxide layer was measured by a method as described below using an X-ray fluorescence spectrometer.

  The tube voltage and current at the time of measurement were set to 30 kV and 100 mA, and the intensity of the O-Kα ray was detected. When measuring the O-Kα ray, in addition to the peak position, the intensity at the background position was also measured so that the net intensity of the O-Kα ray could be calculated. The integration time at the peak position and the background position was 20 seconds each. In addition, a silicon wafer on which silicon oxide films with a thickness of 96 nm, 54 nm, and 24 nm processed to the same size as the sample are formed is set on the sample stage, and the intensity of O-Kα rays is also measured from these silicon oxide films By making it possible, a calibration curve of the oxide film thickness and the O-Kα line intensity was created, and the oxide film thickness of the test material was calculated as an oxide film thickness value in terms of silicon oxide film.

  The cold-rolled steel sheet thus produced was evaluated in the following three types.

(1) Slidability test The coefficient of friction was measured to evaluate the slidability. FIG. 1 is a schematic front view showing a friction coefficient measuring apparatus. As shown in the figure, a friction coefficient measuring sample 1 collected from a test material is fixed to a sample table 2, and the sample table 2 is fixed to the upper surface of a slide table 3 that can move horizontally. On the lower surface of the slide table 3, there is provided a slide table support base 5 having a roller 4 in contact therewith and capable of moving up and down, and by pushing it up, a pressing load N on the friction coefficient measurement sample 1 by the bead 6 is applied. A first load cell 7 for measuring 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 with the pressing force 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. (Preton is a registered trademark) was applied to the surface of Sample 1 as a lubricant, and the test was performed.

  2 and 3 are schematic perspective views showing the shape and dimensions of the beads used. 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, the lower part of both ends of the sliding direction is a curved surface with a curvature of 4.5 mmR, and the bottom surface of the bead to which the sample is pressed is 10 mm wide and in the sliding direction It has a 3mm long plane. The bead 6 shown in FIG. 3 has a width of 10 mm, a length of 69 mm in the sliding direction of the sample, and a lower portion at both ends of the sliding direction is formed by a curved surface having a curvature of 4.5 mm, and the bottom surface of the bead to which the sample is pressed is 10 mm in width and the sliding direction It has a flat surface with a length of 60 mm.

The friction coefficient measurement test was conducted under the following two conditions.
<Condition 1>
The bead shown in FIG. 2 was used, the pressing load N was 400 kgf, and the sample drawing speed (horizontal moving speed of the slide table 3) was 100 cm / min.
<Condition 2>
The bead shown in FIG. 3 was used, the pressing load N was 400 kgf, and the sample drawing speed (horizontal moving speed of the slide table 3) was 20 cm / min.
The coefficient of friction μ between the specimen and the bead was calculated by the formula: μ = F / N.

(2) Chemical conversion treatment evaluation In the test, a rust preventive oil (manufactured by Parker Kosan: NOXLAST 550HN (NOXLAST is a registered trademark)) was applied to the sample surface in advance.
Assuming the case where a commercially available alkaline degreasing liquid (manufactured by Nihon Parkerizing Co., Ltd., Fine Cleaner FC-E2001 (Fine Cleaner is a registered trademark)) is built at a predetermined concentration (standard conditions) and deteriorated to the same liquid, The deterioration liquid (deterioration conditions) which added the rust preventive oil in the ratio of 5g / L was produced, the steel plate was immersed for 2 minutes, and the water wet rate of the steel plate after water washing was evaluated. The water wetting rate was 80% or more, ○, 50% or more and less than 80%, and 50% or less ×.

Next, it is immersed in a surface conditioning solution (PL-X, manufactured by Nihon Parkerizing Co., Ltd.) after degreasing with a degreasing solution (no added rust preventive oil) that has been erected at a predetermined concentration, and low-temperature phosphate treatment (Japan Palbonding PB-L3065 (Palbond is a registered trademark) manufactured by Parkerizing Co., Ltd.) was immersed in a bath temperature of 35 ° C. and a processing time of 120 seconds for chemical conversion treatment. The surface of the steel sheet after chemical conversion treatment is observed using a SEM at 10 magnifications at a magnification of 300, and the following five steps are performed depending on the presence and size of the area (skee) where no chemical conversion crystals are formed and the non-uniformity of the crystalline state. It was evaluated with.
5 points: No scale is observed and the crystals are uniform.
4 points: Slight inhomogeneity of the crystal is observed, but no scaling is observed.
3 points: Smoke is observed.
2 points: A relatively small number of relatively large scales are observed.
1 point: Many relatively large scales are recognized.

(3) Corrosion resistance test after painting
For the sample that has been subjected to chemical conversion treatment in (2), with a commercially available ED coating (manufactured by Kansai Paint, GT-10) at a coating thickness of 20 μm, after cross-cutting with an NT cutter, It was immersed in warm salt water (5% NaCl, 50 ° C.) for 10 days. The sample after the immersion was covered with a polyester tape to cover the cross-cut portion and peeled off, and then the maximum peel width on one side from the cut portion was measured.

  Tables 2 to 4 show the test results.

From Tables 2-4, in the case of the original plate which does not provide Zn to a steel plate (Comparative Examples 1-8), the chemical conversion deteriorates (decrease in the score) with an increase in the amount of Si in the steel. On the other hand, when Zn of 100 mg / m ² or more was adhered, although it showed good chemical conversion even in the low-temperature type chemical conversion treatment used in this example, it was immediately washed with water. In the case (Comparative Examples 9 and 10), the friction coefficient is higher in both conditions than the original plate (Comparative Example 7). This is thought to be due to the fact that adhesion occurs when the adhered Zn is slid by the mold. On the other hand, in the case where the time until washing with water is increased (Invention Examples 1 to 3), the coefficient of friction is reduced in both conditions as the chemical conversion is improved. In addition, when increasing the time until washing with water, when the amount of the liquid film present on the surface is controlled (Invention Examples 4 to 6), the friction coefficient is further reduced. On the other hand, if the amount of Zn deposited is not enough even after increasing the time until water washing after controlling the amount of liquid film present on the surface and depositing Zn in this way, the improvement of chemical conversion is also a coefficient of friction. There was also no decrease in

  In the examples where the Zn deposition amount is within the scope of the present invention and the oxide or hydroxide of Zn of 10 nm or more is formed by the surface oxidation treatment (Invention Examples 7 to 38), all of A to H Good chemical conversion (score 5) is obtained for the steel grade of Si level, and the friction coefficient under any condition is smaller than that of the original plate (Comparative Examples 1 to 8) that does not give Zn to the steel plate, especially for tools. The fact that the friction coefficient is reduced even under condition 2 where the sliding distance is long does not cause a phenomenon in which the outermost Zn oxide peels off during sliding, and works effectively to reduce the friction coefficient. It can be seen that this shows good press formability.

  Furthermore, regarding the one-side peel width after the hot salt water immersion test, the original width of the steel sheet not imparting Zn (Comparative Examples 1 to 8) shows a tendency for the peel width to increase with an increase in the amount of Si in the steel. In an example in which the amount of deposited Zn is within the range of the present invention, the peel width is small in any steel sheet. That is, even in the case of A and B steel types having good chemical properties even in the state of the original plate, the one-side peel width after the hot salt water immersion test can be reduced in the present invention.

  However, in all of these invention examples (Invention Examples 1 to 38), when an alkaline degreasing solution simulating deterioration is used, the water wetting rate is 50% or less, and depending on the state of the alkaline degreasing solution, good degreasing It turns out that it cannot be said that it shows sex.

  On the other hand, when the process which contacts the process liquid containing P is performed, the degreasing improvement effect is recognized. For example, in the case of contact with an aqueous solution with a low P concentration (Examples 39 and 40 of the present invention), the water wetting rate after degreasing using an alkaline degreasing solution simulating deterioration has been improved to a level of less than 80%. Furthermore, in the examples using the aqueous solution having a higher P concentration (Invention Examples 41 to 45), the water wetting rate after degreasing is at a satisfactory level of 80% or more. In addition, the effect differs depending on the temperature of the treatment liquid containing P, and a good water-wetting rate after degreasing was obtained at a treatment liquid temperature of 30 ° C. or higher.

  The cold-rolled steel sheet produced by the method of the present invention has good press formability and chemical conversion treatment before coating, and high corrosion resistance after coating, even in a high-tensile cold-rolled steel sheet containing a large amount of reinforcing elements such as Si. It is good. The cold-rolled steel sheet produced by the method of the present invention is optimal for automobile body use, for example.

1 Sample for friction coefficient measurement
2 Sample stage
3 Slide table
4 Roller
5 Slide table support
6 beads
7 First load cell
8 Second load cell
9 rails
N Push load
F Sliding resistance force
P Tensile load

Claims (3)

Using an acidic aqueous solution containing Zn ions on the surface of the cold-rolled steel sheet, after applying Zn electroplating of 100 to 5000 mg / m ² as the amount of Zn, the acidic aqueous solution containing Zn ions on the surface is formed into a film shape After carrying out a holding step of holding for 1 to 30 seconds in the attached state, washing with water, forming an oxide and / or hydroxide of Zn with an average thickness of 10 nm or more on the portion where the Zn plating layer is formed A method for producing a cold-rolled steel sheet having excellent press formability, chemical conversion processability, and post-coating corrosion resistance.   The acidic aqueous solution containing Zn ion is one of acetate, phthalate, citrate, succinate, lactate, tartrate, borate and phosphate as a chemical having a pH buffering action. It is a solution containing 5 to 50 g / L of more than one kind and adjusted to pH 1.0 to 5.0 with sulfuric acid, and in the holding step, the average film thickness of the acidic aqueous solution is 1 μm or more and 20 μm or less. Item 2. A method for producing a cold-rolled steel sheet having excellent press formability, chemical conversion property and post-coating corrosion resistance according to Item 1.   3. The press formability and chemical conversion treatment according to claim 1 or 2, wherein after the water washing, the P concentration is 0.001 to 2 g / L, and the P-containing aqueous solution having a temperature of 30 to 60 ° C. is contacted. And a method for producing a cold-rolled steel sheet having excellent corrosion resistance after painting.

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