JP5853683B2 - Method for producing cold-rolled steel sheet with excellent chemical conversion and post-coating corrosion resistance - Google Patents

Description

  The present invention relates to a method for producing a cold-rolled steel sheet that forms a sufficient chemical conversion film and has good corrosion resistance after coating.

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. To reduce the weight of the car body, it is most effective to reduce the thickness of the steel sheet used.However, if the steel sheet strength is kept the same while the steel sheet strength remains the same, the rigidity of the steel sheet decreases, and this time the passenger is in a collision. It becomes impossible to secure safety. 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 steel, has gradually increased. The movement to use for body use is also becoming active.

  In order to increase the strength of the steel sheet, a method of solid solution strengthening by adding alloying elements such as Si and Mn, a method of refining crystal grains, and precipitation by adding precipitate forming elements such as Nb, Ti, V, etc. A method of strengthening and a method of strengthening by generating a hard transformation structure such as a martensite phase are effective.

  In general, the increase in strength by adding an alloying element, on the other hand, causes a decrease in ductility, and thus has a drawback that it is difficult to perform press forming to form the shape of a part. However, among solid solution strengthening, Si is an element effective in increasing strength while ensuring ductility because it has a smaller effect on ductility than other elements. 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, Si has a very low equilibrium oxygen partial pressure of oxide and is easily oxidized in a reducing atmosphere in a continuous annealing furnace used in the production of general cold-rolled steel sheets. When passing through the 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, the SiO ₂ inhibits the reaction between the chemical conversion solution and the steel sheet, so that there is a so-called part where no conversion crystals are formed. Will do. And the steel plate in which the scale is present after the chemical conversion treatment may already have rust in the water washing stage after the chemical conversion treatment, and even if it does not reach rust, the corrosion resistance of the steel plate after electrodeposition coating Therefore, it was very difficult to use a high-strength cold-rolled steel sheet containing Si for body use.

  There have been many proposals for improving the chemical conversion properties of such high-strength cold-rolled steel sheets containing Si. For example, Patent Document 1 includes a cold-rolled steel sheet on which an oxide having an atomic ratio [Si / Mn] of 1 or less is formed on the surface, and a manufacturing method thereof (Si / Mn) ratio of steel sheet components, annealing temperature, The thing which prescribed | regulated the partial pressure ratio of the hydrogen of the atmosphere and a water | moisture content as a parameter is proposed. However, this method requires that the annealing temperature be lowered as the Si content of the steel sheet component increases, so if high temperature annealing is required to obtain the desired strength and elongation, the moisture ratio of the atmosphere must be increased. I must. However, since a Fe-based oxide is formed on the surface of the steel sheet, it is not a product. In other words, this technique cannot be applied to steel sheets containing about 1.0% Si, which is the mainstream of current high-strength steel sheets.

  In Patent Document 2, for Si: 0.05-2% and [Si] / [Mn] ≦ 0.4, the size of Si-Mn composite oxide on the surface of the steel sheet, the number per unit area, and Si A high-strength cold-rolled steel sheet that defines the steel oxide surface coverage of the main oxide has been proposed.

  In Patent Document 3, Si: 0.1 to 1% and [Si] / [Mn] ≦ 0.4, the (Mn / Si) ratio, size, and unit area of the Mn-Si composite oxide on the steel plate surface A high-strength cold-rolled steel sheet has been proposed in which the number of the steel sheets and the surface coverage of the oxide-based steel sheet mainly composed of Si are defined.

  In Patent Document 4, Si: 0.1 to 2% and [Si] / [Mn] ≦ 0.4, the (Mn / Si) ratio, size, and unit area of the Mn-Si composite oxide on the steel plate surface A high-strength cold-rolled steel sheet has been proposed in which the number of the steel sheets and the surface coverage of the oxide-based steel sheet mainly composed of Si are defined.

  The techniques of Patent Documents 2 to 4 can be applied to steel sheets containing up to 2% Si, and examples of the production method include pickling conditions after hot rolling and dew point during continuous annealing. It is supposed to keep below -40 ℃. However, it is necessary for the steel sheet to satisfy a specific Si / Mn ratio, and there is a drawback that the degree of freedom of the steel sheet components is small. Also, setting the dew point during continuous annealing to -40 ° C or lower is a technology that is not suitable for mass production because it is quite difficult to control considering the dew point fluctuation of the actual production line.

  Patent Document 5 describes a cold-rolled steel sheet in which the surface coverage of the Si-based oxide on the steel sheet surface is defined for a steel sheet of Si: 0.4% or more and [Si] / [Mn] ≧ 0.4, and an acid after annealing. A manufacturing method for washing is proposed.

Patent Document 6 proposes a technique of grinding a steel plate surface by 2.0 g / m ² or more after annealing with respect to a steel plate containing 0.5 mass% or more of Si.

  In Patent Document 7, after annealing a steel sheet containing Si: 0.5 to 2.0%, it is treated with an acidic solution having a pH of 0 to 4 and a temperature of 10 to 100 ° C. for 5 to 150 seconds, and a pH of 10 to 14 and a temperature of 10 to 100 ° C. A technique of treating with an alkaline solution of 2 to 50 seconds has been proposed.

  The techniques of Patent Documents 5 to 7 all remove the oxide layer formed on the surface after annealing, but in the example of Patent Document 5, a high-concentration acid is used to remove the Si-based oxide. In this case, since the formation of a passive film on the iron base is promoted, there is a drawback that it does not necessarily improve the chemical conversion property. In Patent Documents 6 and 7, it is necessary to provide a grinding section or an acidic solution treatment → alkali solution treatment section in the line, which leads to an increase in equipment length and cost, which is not realistic.

Patent Document 8 discloses a technique that has a zinc plating film with an adhesion amount of 10 to 2000 mg / m ^{2 on} the surface of a steel sheet and has a predetermined crystal orientation so as to achieve both mold galling resistance and chemical conversion treatment. Has been proposed. This technology was made mainly to improve mold galling resistance. Regarding chemical conversion treatment, even with a small amount of Zn, microcells were formed between the Zn adhesion and the steel plate exposure. This suggests that the chemical conversion reaction becomes active. However, when the Si concentration of the steel sheet is high, a considerable part of the steel sheet surface is covered with SiO ₂ oxide, and if this part is an exposed part of the steel sheet, it cannot be said that microcells are necessarily formed. . In addition, a sulfuric acid bath is used as the electroplating bath. When a Zn plating film is formed under the same conditions as shown in the examples, sufficient degreasing cannot be performed depending on the type of alkaline degreasing solution before chemical conversion treatment. I understood that.

JP-A-4-76060 Japanese Patent No. 3934604 JP 2005-290440 JP Japanese Patent No. 3889768 Japanese Patent Laid-Open No. 2004-323969 JP 2003-226920 A JP 2007-009269 A JP 2006-299351 A

  In this way, in the case of cold-rolled steel sheet with Si added for the purpose of achieving high strength without reducing ductility, the technology that satisfies chemical conversion treatment is still not sufficient, and application of high-strength steel sheet to automobile bodies It is the present condition that is hindering.

  An object of the present invention is to solve the above-mentioned problems for a steel sheet containing Si as a strengthening element, and to provide a method for producing a cold-rolled steel sheet having excellent chemical conversion property and corrosion resistance after coating. .

The inventors of the present invention focused on the fact that when SiO ₂ is formed on the steel sheet surface, Fe, which is the main component of the steel sheet, does not dissolve in the formed portion, so that no chemical conversion crystal formation reaction occurs. 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 reacts with the chemical conversion treatment solution to form a zinc phosphate coating as a chemical conversion coating, and as a result of examination, a sufficient amount of thin Zn to form a zinc phosphate coating was cold-rolled steel sheet. By applying to the surface, it was confirmed that a chemical conversion crystal formation reaction also progressed for a cold-rolled steel sheet containing Si, and as a result, a zinc phosphate coating could be formed after the chemical conversion treatment.

  However, the chemical conversion treatment is a general process that proceeds by alkali degreasing → surface conditioning → phosphate treatment. Of these, oil is mixed one after another in the alkaline degreasing process, so the actual line is considerably degreased. Ability is inferior. And, if the steel sheet that has been subjected to electro-Zn plating and washed with water is immersed in the degreasing liquid assuming such a real line, the rust preventive oil etc. applied to the steel sheet cannot be sufficiently removed and water repelling occurs. I found it. Since the steel sheet with such water repellency has poor wettability with the chemical conversion solution as it is and surface unevenness occurs, it is important to completely remove oil on the surface of the steel sheet after alkaline degreasing. From this point of view, after applying electro-zinc plating and washing with water, it is possible to remove the oil content of the steel sheet even when using a degreasing solution assuming an actual line by contacting with an aqueous solution containing P. It was found that a water wetting rate was obtained.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] After electroplating Zn so that the amount of Zn deposited on the cold-rolled steel sheet surface is 100 to 5000 mg / m ² and washing with water, it contains P and the concentration of P is 0.001 to 2 g / L A method for producing a cold-rolled steel sheet having excellent chemical conversion properties and post-coating corrosion resistance, wherein the cold-rolled steel sheet is brought into contact with an aqueous solution having a temperature in the range of 30 to 60 ° C.
[2] After applying electric Zn plating so that the amount of Zn adhered to the cold-rolled steel sheet surface is 100 to 1000 mg / m ² and washing with water, it contains P, and the concentration of P is 0.001 to 2 g / L A method for producing a cold-rolled steel sheet having excellent chemical conversion properties and post-coating corrosion resistance, wherein the cold-rolled steel sheet is brought into contact with an aqueous solution having a temperature in the range of 30 to 60 ° C.

  According to the present invention, a cold-rolled steel sheet having excellent chemical conversion properties and post-coating corrosion resistance can be obtained. A sufficient amount of chemical conversion coating is applied even to steel sheets that are difficult to form in the coating process in automobile manufacturing because of the formation of oxides of Si and Mn, known as surface concentration elements, on the surface. It can be formed and good post-coating corrosion resistance can be obtained.

Generally, a cold-rolled steel sheet is manufactured by heat-treating a cold-rolled steel sheet in a reducing atmosphere containing hydrogen in a range of 700 to 900 ° C. 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. Moreover, even if it is not possible to completely cover the surface-enriched oxide, Zn present in the periphery reacts with the chemical conversion treatment solution, so that a chemical conversion film can be easily formed.

Here, in the case of a normal cold-rolled steel sheet, the element that reacts with the chemical conversion treatment liquid is steel sheet component Fe. However, when Zn plating is performed on the surface, this Zn becomes an element that reacts with the chemical conversion solution. Also, the phosphate crystals formed are phosphophyllite (Zn ₂ Fe (PO ₄ ) ₂ · 4H ₂ O) in ordinary cold-rolled steel sheets. Becomes Hopeite (Zn ₃ (PO ₄ ) ₂ · 4H ₂ O).

In order to obtain such an effect of improving the chemical conversion property, the amount of Zn deposited on the steel sheet surface needs to be 100 mg / m ² or more. As described above, in the present technology, Zn applied to the surface functions to form a chemical conversion treatment crystal, and thus the steel plate surface needs to be sufficiently covered. That is, 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 observed. On the other hand, even if the Zn deposition amount increases, there is no problem from the viewpoint of chemical conversion, but the increase in Zn deposition amount leads to cost increase only for the purpose of improving the chemical conversion properties of the cold-rolled steel sheet itself, so the upper limit is 5000 mg / m ² . Preferably, it is 1000 mg / m ² or less.

Various methods for attaching Zn to the surface of the steel plate are conceivable, but the method by electroplating is the best. In the present invention, since the appropriate Zn deposition amount having an effect is 5000 mg / m ² or less, for example, the hot dipping method cannot cope with such thin plating.

  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, even if a steel plate subjected to thin electric Zn plating is immersed in an alkaline degreasing solution as it is, oil cannot always be 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 is recirculated to the next surface adjustment step in a state where water repelling occurs. In such a water-repellent part, the surface conditioning liquid is not properly applied, and in the next phosphating process, there is an adverse effect on the phosphating process, such as a part where the phosphate crystals are coarsened or crystals are not formed. There is.

  Therefore, in the present invention, the electrode is immersed in a P-containing aqueous solution after being subjected to electro Zn plating. By dipping in an aqueous solution containing P, 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, when a general zinc sulfate bath is used as the electric Zn plating bath, sulfate radicals are incorporated into the Zn plating film, and this sulfate radical increases the affinity with oil. It will be difficult. On the other hand, when an aqueous solution containing P is brought into contact with the steel sheet, the sulfate radicals 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 this is less than 0.001 g / L, the washing effect of sulfate radicals is small, and 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 it is less than 30 ° C, it takes time to wash the sulfate radicals and adhere 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.

In the present invention, since one of the purposes is to promote the formation of a film on a steel sheet on which the chemical conversion film is not formed due to the presence of SiO ₂ or the like on the surface of the cold-rolled steel sheet after annealing, Si is used for example. It is suitably used for high-strength cold-rolled steel sheets containing 0.5% or more. However, the amount of Zn deposited on the steel sheet surface is 100-5000 mg / m ² and Zn is adhered, that is, the presence of a slight amount of Zn on the steel sheet surface improves the corrosion resistance after painting, so a general cold-rolled steel sheet Can also be applied from the viewpoint of corrosion resistance after painting. For this reason, this invention is a technique by which chemical conversion property and corrosion resistance after coating are ensured 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 heat-treated in a nitrogen atmosphere containing 10 vol% of hydrogen in a range of 800 to 850 ° C. for 2 minutes at maximum using a laboratory reduction heating simulator to produce an annealed plate (cold rolled steel plate).

An iridium oxide plate at the anode using an aqueous solution containing 1 mol / L of zinc sulfate heptahydrate and adjusted to pH 2.0 using sulfuric acid, with respect to the annealed plate (cold rolled steel plate) obtained above. Was used for electroplating to deposit Zn on the surface. The amount of deposited Zn was changed by changing the current density and the energization time. The sample after the electroplating was washed with water and then immersed in an aqueous solution of sodium diphosphate (Na ₄ P ₂ O ₇ · 10H ₂ O) for 3 seconds. The solution was evaluated based on a P concentration of 0.5 g / L and a temperature of 50 ° C. with some changes in concentration and temperature. For comparison, an annealed plate (cold rolled steel plate) that was not electroplated and did not adhere Zn to the surface was also prepared.

  Next, the following chemical conversion treatment was performed on the cold-rolled steel sheet obtained as described above.

  First, a commercially available alkaline degreasing solution (manufactured by Nihon Parkerizing Co., Ltd., Fine Cleaner FC-E2001) was built at a specified concentration and diluted to twice the specified concentration assuming a deterioration. In each case, the cold-rolled steel sheet was immersed for 2 minutes, and the water wettability of the steel sheet after washing with water was evaluated. A sample having a water wettability of 80% or more was evaluated as ◯, a sample having less than 80% was evaluated as Δ, and a sample having a water wettability rate of 50% or less was evaluated as ×.

  Next, the cold-rolled steel sheet degreased with the degreasing solution diluted to twice the specified concentration described above is immersed in a surface conditioning solution (manufactured by Nihon Parkerizing Co., Ltd., PL-ZTH), and a phosphate treatment solution (Nippon Parkerizing) Chemical conversion treatment was carried out by dipping in Palbond PB-L3080 (manufactured by Co., Ltd.) under conditions of bath temperature: 43 ° C. and treatment time: 120 seconds.

The surface of the cold-rolled steel sheet after chemical conversion treatment is observed using a SEM at 10 magnifications at a magnification of 300 times, and chemical conversion treatment is performed depending on the presence and size of areas (skees) in which chemical conversion crystals are not formed and the unevenness of the crystalline state. Evaluation was made according to the following five levels.
5 points: No skelton is observed and the crystals are uniform.
4 points: Slight non-uniformity of the crystal is observed, but no skein is observed.
3 points: Smoke is observed.
2 points: A relatively large scale is observed.
1 point: Many relatively large scales are recognized.

The steel plate after the chemical conversion treatment was further applied with a commercially available ED coating (GT-10, manufactured by Kansai Paint Co., Ltd.) with a coating thickness of 20 μm, and a cross cut was made with an NT cutter (registered trademark) on the painted surface. Then, 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 after peeling, the maximum peel width on one side from the cut was measured.
The results obtained as described above are shown in Tables 2 to 5 together with the conditions.

  From Tables 2 to 5, steels A, B and C do not contain much Si in the steel plate components, and therefore good chemical conversion treatment is possible even in the examples (Comparative Examples 1 to 3) where the surface is not subjected to electric Zn plating. Although obtained, the peel width is large and the corrosion resistance after coating is poor.

In Comparative Examples 9, 10 and Reference Examples 1-8 (Steel A), Comparative Examples 11, 12 and Reference Examples 9-16 (Steel B), Comparative Examples 13, 14 and Reference Examples 17-24 (Steel C), the surface When applying electroplating to the surface, the peel width after the hot salt water immersion test decreases, and it can be seen that the peel width level is low and stable and excellent in post-coating corrosion resistance, especially at an adhesion amount of 100 mg / m ² or more. .

  Since steels D to H contain a large amount of Si, in examples where the surface is not subjected to Zn plating (Comparative Examples 4 to 8), scales are observed in the conversion crystals, and particularly, the amount of Si that is 1.5% or more G, In H, it became the level of the grade 1 where the conversion crystal was hardly formed.

In the case where the Zn deposition amount is less than 100 mg / m ² (Comparative Examples 15 to 24), the level of chemical conversion crystals that can still be seen is not sufficient.

In the examples where the Zn deposition amount is 100 mg / m ² or more (Examples 25 to 64 of the present invention), any conversion crystals having no scale are obtained and the conversion treatment is excellent. At the same time, the peel width after the hot salt water immersion test is low and stable, indicating that the steel component is hardly affected.

  In addition, after electroplating with Zn, the P concentration is low (Examples 25 and 27), and the temperature is low even if it is contacted with an aqueous solution containing P (Comparative Examples 26 and 27). In low examples (Comparative Examples 28 and 29)), degreasing solution built to the specified concentration provides sufficient degreasing properties, but degreasing diluted to twice the specified concentration simulating the deterioration state in the actual coating line In the case of degreasing with liquid, water repelling occurred after degreasing.

  On the other hand, in the examples where the P concentration and the treatment liquid temperature are within the scope of the present invention (Invention Examples 65 to 69, 70), sufficient degreasing properties were obtained even in the diluted degreasing liquid.

  A high-tensile cold-rolled steel sheet containing a large amount of strengthening elements such as Si also has good chemical conversion properties before painting and good corrosion resistance after painting.

Claims (2)

After applying electric Zn plating so that the amount of Zn deposited is 100 to 5000 mg / m ^{2 on the} surface of a cold rolled steel sheet containing 0.5% or more of Si, and after washing with water,
Degreasing, chemical conversion treatment, and coating , characterized in that the cold-rolled steel sheet is brought into contact with an aqueous solution containing P, the concentration of P being 0.001 to 2 g / L, and the temperature being in the range of 30 to 60 ° C. A method for producing a cold-rolled steel sheet having excellent post-corrosion resistance. After applying electric Zn plating so that the amount of Zn deposited is 100 to 1000 mg / m ^{2 on the} surface of a cold rolled steel sheet containing 0.5% or more of Si, and after washing with water,
Degreasing, chemical conversion treatment, and coating , characterized in that the cold-rolled steel sheet is brought into contact with an aqueous solution containing P, the concentration of P being 0.001 to 2 g / L, and the temperature being in the range of 30 to 60 ° C. A method for producing a cold-rolled steel sheet having excellent post-corrosion resistance.