TW201326459A - Process for manufacturing cold rolled steel sheets excellent in terms of phosphatability and corrosion resistance under paint films - Google Patents

Abstract

Electrolytic Zn plating is carried out on the surface of a cold-rolled steel sheet so that the adhesion amount of Zn is 100-5,000 mg/m<SP>2</SP>, and the resulting cold-rolled steel sheet is washed with water and then brought into contact with an aqueous solution containing P. In this connection, the P concentration of the aqueous solution containing P is 0.001-2 g/L and the temperature of the aqueous solution containing P is within the range of 30-60 DEG C.

Description

Method for producing cold-rolled steel sheet excellent in phosphating treatment property and corrosion resistance after coating

The present invention relates to a method for producing a cold-rolled steel sheet having a sufficient phosphatability film and excellent corrosion resistance after coating.

In recent years, in order to reduce the amount of CO ₂ emissions from automobiles, it is a problem for automakers to reduce the amount of CO ₂ emissions from automobiles. For the weight reduction of the vehicle body, the thinning of the steel sheet used is the most effective, but if the strength of the steel sheet is the same, only the thickness of the steel sheet is reduced, the rigidity of the steel sheet is reduced, and the vehicle cannot be secured when a collision occurs. The safety of the personnel inside. Therefore, the thickness of the steel plate is reduced, and the high-strength steel sheet which is reduced in rigidity due to the thinning of the steel sheet is compensated by the high strength of the steel, and the movement of the material for the vehicle body is gradually enhanced, and the high-strength steel sheet having a tensile strength of 1180 MPa as of now. There are also active developments in the use of the car body.

When the steel sheet is subjected to high strength, for example, a method of adding solid-solution strengthening by adding an alloying element such as Si or Mn; a method of miniaturizing the crystal grain; and adding a precipitate forming element such as Nb, Ti, or V to perform precipitation strengthening. The method; and the method of strengthening the formation of the hardened metamorphic tissue of the granulated iron are effective.

In general, the addition of an alloying element increases the strength, and on the other hand, the ductility is lowered, so that the press forming of the shape of the part is not easily performed. However, even in solid solution strengthening, Si is compared to other elements. In the meantime, since the influence of the decrease in ductility is small, it is an effective element in achieving high strength in the case of ensuring ductility. Therefore, it is almost necessary to add Si in a steel sheet which combines both workability and high strength.

However, the equilibrium oxygen partial pressure of the Si system in the oxide is very low. Generally, in the reducing environment in the continuous annealing furnace used in the manufacture of the cold rolled steel sheet, it is more likely to be oxidized. Therefore, if the steel sheet containing Si is passed through the continuous annealing furnace, Si is selectively oxidized on the surface of the steel sheet to form SiO ₂ . According to this, if the steel sheet which has formed SiO ₂ on the surface is subjected to phosphating treatment before coating, since SiO ₂ suppresses the reaction between the phosphating treatment liquid and the steel sheet, there is a so-called "phosphorization crystal". The presence of the "rust" part. In such a steel sheet which has been subjected to phosphating after the phosphating treatment, rust may occur in the water washing stage after the phosphating treatment, and it is assumed that the corrosion resistance of the steel sheet after electrodeposition coating is very poor even if the rusting stage is not reached. . Therefore, it is quite difficult to use a high-strength cold-rolled steel sheet containing Si for use in a vehicle body.

There has been a lot of proposals for improving the phosphating treatment property of such a high-strength cold-rolled steel sheet containing Si. For example, Patent Document 1 proposes a cold-rolled steel sheet having an oxide having an atomic ratio [Si/Mn] of 1 or less formed on its surface, and a method for producing the same, which is a (Si/Mn) ratio of the steel sheet component, an annealing temperature, The partial pressure ratio of hydrogen to moisture in the environment is specified as a parameter. However, since the method requires the annealing temperature to decrease as the amount of Si of the steel sheet component increases, it is necessary to increase the moisture ratio of the environment in order to obtain the required strength and to extend the high-temperature annealing. However, on the contrary, since Fe-based oxide is formed on the surface of the steel sheet, it cannot be a product. That is, for the current high-strength steel plate mainstream There is no applicable technology for steel sheets containing about 1.0% Si.

The high-strength cold-rolled steel sheet proposed in Patent Document 2 is a steel sheet having a Si: 0.05 to 2% and [Si]/[Mn] ≦ 0.4, and the size of the Si-Mn composite oxide on the surface of the steel sheet and the area per unit area are defined. The surface coverage of the steel sheet with the number of oxides and Si as the main component.

The high-strength cold-rolled steel sheet proposed in the patent document 3 specifies the (Mn/Si) of the Mn-Si composite oxide on the surface of the steel sheet for a steel sheet of Si: 0.1 to 1% and [Si]/[Mn]≦0.4. The ratio of the ratio, the size, and the number of units per unit area, and the surface coverage of the steel sheet mainly composed of Si.

The high-strength cold-rolled steel sheet proposed in the patent document 4 specifies the (Mn/Si) of the Mn-Si composite oxide on the surface of the steel sheet for a steel sheet of Si: 0.1 to 2% and [Si]/[Mn]≦0.4. The ratio of the ratio, the size, and the number of units per unit area, and the surface coverage of the steel sheet mainly composed of Si.

The techniques of Patent Documents 2 to 4 can be applied to steel sheets containing up to 2% of Si, and the production method is an example in which the pickling conditions after hot rolling and the dew point during continuous annealing are suppressed to -40 ° C or lower. However, it is necessary to satisfy a steel sheet having a specific Si/Mn ratio, and there is a disadvantage that the degree of freedom of the steel sheet component is small. Moreover, the dew point at the time of continuous annealing is set to -40 ° C or less, and it is difficult to control considering the dew point fluctuation of a actual manufacturing line, and it is not a technique suitable for mass production.

Patent Document 5 proposes a steel sheet having a surface coverage of Si-based oxide on the surface of a steel sheet having a Si: 0.4% or more and [Si]/[Mn] ≧ 0.4, and a pickling after annealing. Manufacturing method.

Patent Document 6 proposes a technique of performing a grinding of 2.0 g/m ² or more on a steel sheet containing Si in an amount of 0.5% by mass or more after annealing.

Patent Document 7 proposes: 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 are utilized. The ~100 ° C alkaline solution is applied for 2 to 50 seconds.

In the techniques of Patent Documents 5 to 7, the oxide layer formed on the surface after annealing is removed. In the example of Patent Document 5, it is necessary to use a high concentration of acid to remove the Si-based oxide. Promoting the formation of a passive film on a steel substrate may not necessarily have the disadvantage of enhancing the phosphating effect. In Patent Document 6 or 7, it is necessary to provide a grinding stage or an acidic solution treatment → alkali solution treatment in the production line, which results in an increase in equipment and an increase in cost, which is unrealistic.

Patent Document 8 proposes a technique in which a galvanized coating having an adhesion amount of 10 to 2000 mg/m ² is provided on the surface of the steel sheet, and a mold having scratch resistance and phosphating treatment properties is obtained by having a predetermined crystal orientation. This technique is mainly proposed to improve the scratch resistance of the mold. The related phosphating treatment property suggests that even if a slight amount of micro-Zn adheres, a microcell is formed between the Zn adhesion portion and the exposed portion of the steel sheet to cause phosphating. The treatment reaction was active. However, when the Si concentration of the steel sheet is high, a considerable portion of the surface of the steel sheet is covered with SiO ₂ oxide. When the portion is exposed to the steel sheet, it is difficult to form a microcell. Further, it was found that when a plating bath was used in a plating bath and a galvanized coating film was formed under the same conditions as those exemplified in the examples, the type of the alkali degreasing liquid before the phosphating treatment was not sufficiently degreased.

[Advanced technical literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open No. Hei 4-276060

[Patent Document 2] Japanese Patent No. 3934604

[Patent Document 3] Japanese Patent Laid-Open Publication No. 2005-290440

[Patent Document 4] Japanese Patent No. 3889768

[Patent Document 5] Japanese Patent Laid-Open Publication No. 2004-323969

[Patent Document 6] Japanese Patent Laid-Open Publication No. 2003-226920

[Patent Document 7] Japanese Patent Laid-Open Publication No. 2007-009269

[Patent Document 8] Japanese Patent Laid-Open Publication No. 2006-299351

Accordingly, in the case of a cold-rolled steel sheet in which Si is added without lowering the ductility and achieving high strength, it is difficult to sufficiently satisfy the phosphating treatment property, and the current situation is to suppress the application of the high-strength steel sheet to the automobile body.

An object of the present invention is to provide a method for producing a cold-rolled steel sheet having excellent phosphating treatment properties and corrosion resistance after application to a steel sheet containing Si as a reinforcing element.

The present inventors have paid attention to the fact that if SiO ₂ is formed on the surface of the steel sheet, the formed portion does not dissolve the Fe of the main component of the steel sheet, and thus the phosphating crystal formation reaction does not occur. Further, the inventors of the present invention have thought that the dissolution reaction on the surface of the steel sheet by a certain method may be related to the formation reaction of the phosphating crystal. Further, the inventors of the present invention thought that the metal Zn was formed into a zinc phosphate film of a phosphate film by a reaction with a phosphating solution. As a result of the review, it was confirmed that a zinc phosphate film was formed, and only a sufficiently thin amount of a thin Zn layer was applied to the surface of the cold-rolled steel sheet, and even a cold-rolled steel sheet containing Si was subjected to a phosphating crystal formation reaction. As a result, the inventors of the present invention It was confirmed that a zinc phosphate film was formed after the phosphating treatment. Further, the above "Zn layer" means not only a zinc layer uniformly adhered to one surface of the surface of the cold-rolled steel sheet but also a discontinuous zinc layer which is unevenly adhered to the surface of the cold-rolled steel sheet and partially formed on the surface of the cold-rolled steel sheet.

However, the phosphating treatment is carried out in the order of alkali degreasing→surface adjustment→phosphate treatment, which is a general process in which the oil is gradually mixed in the alkali degreasing step, resulting in poor degreasing ability in the actual production line. The present inventors have found that it is impossible to sufficiently remove the rust preventive oil or the like imparted to the steel sheet by impregnating a steel sheet which is subjected to electroplating and is only subjected to water washing in the degreasing liquid of such an actual production line. water. Such a water-repellent steel sheet may have poor wettability with the phosphating treatment liquid in the original state, and surface unevenness may occur, so that it is important to completely remove the oil on the surface of the steel sheet after alkali degreasing. From this point of view, it is known that after the electroplating of zinc is applied and water washing is performed, the contact with the aqueous solution containing P is further removed, and even if the degreasing liquid assumed to be used in the actual production line is used, the oil of the steel sheet can be removed, and the oil can be obtained. A sufficient water wetting rate is obtained.

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

[1] A method for producing a cold-rolled steel sheet having excellent phosphating treatment property and corrosion resistance after application, wherein zinc plating is applied to a surface of a cold-rolled steel sheet so that a Zn adhesion amount is 100 to 5000 mg/m ² , and after washing with water, Further, the cold-rolled steel sheet is brought into contact with an aqueous solution containing P and having a P concentration of 0.001 to 2 g/L and a temperature of 30 to 60 °C.

[2] A method for producing a cold-rolled steel sheet having excellent phosphating treatment property and corrosion resistance after application, wherein the surface of the cold-rolled steel sheet is subjected to electroplating zinc so that the amount of Zn adhered is 100 to 1000 mg/m ² , and after washing with water, Further, the cold-rolled steel sheet is brought into contact with an aqueous solution containing P and having a P concentration of 0.001 to 2 g/L and a temperature of 30 to 60 °C.

According to the present invention, a cold-rolled steel sheet excellent in phosphating treatment property and corrosion resistance after coating can be obtained. Since the surface-concentrating element is formed as an oxide of Si and Mn, it is possible to form a sufficient phosphating film for a steel sheet which is hard to form a phosphating film even in a coating step of automobile production. Good corrosion resistance after painting.

Generally, a cold-rolled steel sheet is produced by subjecting a cold-rolled steel sheet to heat treatment in a hydrogen-containing reducing atmosphere at a temperature in the range of 700 to 900 °C. However, when heating is performed in a reducing environment, there is a phenomenon in which an easily oxidizable element in the steel sheet component is concentrated on the surface of the steel sheet by an oxide form (hereinafter referred to as "surface concentration"). The representative oxide is SiO ₂ , MnO or a Si—Mn composite oxide. These oxides are present in the surface of the steel sheet, and the reaction of etching the steel sheet by the phosphating treatment liquid to precipitate the phosphating crystal is suppressed, and a portion where the phosphating crystal is not formed on the surface of the steel sheet is generated, and the so-called "rust" is generated. Skin, resulting in poor phosphating treatment.

In view of this, when galvanizing the surface of the steel sheet, since Zn covers the oxide concentrated on the surface, a reaction between Zn and the phosphating treatment liquid occurs even if the surface of the steel sheet having the oxide is present. Further, even if it is impossible to completely cover the oxide thickened on the surface, the Zn present in the periphery reacts with the phosphating solution, so that the phosphating film can be easily formed.

Here, in the case of a cold-rolled steel sheet in general, Fe which is an element-based steel sheet component which reacts with the phosphating treatment liquid. However, when the surface is subjected to galvanization, the Zn becomes an element which reacts with the phosphating solution. Further, the formed phosphate crystals also form phosphorite (Zn ₂ Fe(PO ₄ ) ₂ ‧4H ₂ O) in a conventional cold-rolled steel sheet. However, in the present invention, a considerable amount of phosphate crystals become zinc phosphate (Zn ₃ (PO ₄ ) ₂ ‧4H ₂ O).

In order to obtain such an effect of improving the phosphating treatment property, the amount of Zn adhered to the surface of the steel sheet must be 100 mg/m ² or more. As described above, in the present technology, in order to allow the Zn imparted to the surface to function as a phosphating crystal, it is necessary to sufficiently cover the surface of the steel sheet. That is, if the adhesion amount of 10 () mg/m ² is not exceeded, Zn cannot cover the surface of the steel sheet, and the improvement of the phosphating treatment cannot be obtained. On the other hand, even if the amount of Zn adhesion increases, there is no problem in terms of phosphating property. However, if only the phosphating property of the cold-rolled steel sheet itself is improved, since the increase in the amount of Zn adhesion is implicated in cost increase, the upper limit is imposed. Set to 5000 mg/m ² . From the viewpoint of further suppressing cost increase, it is preferably 1000 mg/m ² or less.

There are various methods for attaching Zn to the surface of a steel sheet. However, the present invention is preferably a method of performing electroplating. In the present invention, the appropriate adhesion amount of Zn which achieves an effect is 5000 mg/m ² or less. For example, the hot plating method cannot cope with such thin plating.

Usually, the phosphating treatment is carried out in the order of: alkali degreasing → surface conditioning → phosphate treatment. In the first alkali degreasing step, it is necessary to remove the rust preventive oil applied to the steel sheet or the press washing oil frequently used in press forming the automobile body outer panel. However, even if the steel sheet which has been subjected to thin electrogalvanization is directly immersed in the alkali degreasing liquid, the oil may not be removed. In particular, when alkali degreasing is performed on a plurality of vehicle bodies which are successively flown in a coating line of an automobile manufacturer, it is judged that oil may be mixed or the alkali degreasing liquid may be deteriorated. Therefore, depending on the situation, it may be transferred to the next surface adjustment step in a state where the degreasing is not sufficiently performed to generate water. At such a water-repellent portion, the surface conditioning liquid cannot be surely imparted, and in the next phosphate treatment step, the phosphate crystals are coarsened or there is a portion where no crystals are formed, which causes adverse effects on the phosphate treatment. .

Here, the present invention is immersed in an aqueous solution containing P (phosphorus) after performing electrogalvanization. By immersing in an aqueous solution containing P, the surface will adhere to a small amount. P, whereby the alkali degreasing liquid can be sufficiently degreased even if it is judged to be deteriorated or the like. The mechanism is related to the presumption. It is considered that if the electroplating zinc bath uses a general zinc sulfate bath, the sulfate will be introduced into the galvanized coating because the sulfate will increase the affinity with the oil, so the degreasing will be converted to difficult. On the other hand, when the aqueous solution containing P is brought into contact with the steel sheet, the sulfate present on the surface is washed away, and P is added in a small amount, whereby the affinity with the oil can be lowered. Therefore, the judgment will improve the degreasing property.

The P concentration in the P-containing aqueous solution which is brought into contact with the steel sheet is preferably in the range of 0.001 to 2 g/L. The reason is that if it is less than 0.001 g/L, the washing effect of the sulfate is small, and the adhesion of P to the surface may be insufficient. On the other hand, even if it exceeds 2 g/L, there is still no large difference in effect.

The temperature of the aqueous solution containing P is preferably in the range of 30 to 60 °C. The reason is that if the temperature is less than 30 ° C, it takes a long time to wash the sulfate and P, and the continuous annealing equipment requires a large amount of equipment. On the other hand, if it exceeds 60 ° C, although the effect is sufficient, there is an additional need for equipment for heating, etc., which is economically unsuitable.

The method of bringing the steel sheet into contact with the aqueous solution containing P is not particularly limited. For example, a dipping method, a spraying method, or the like can be employed The spraying pressure, the nozzle path, the distance from the nozzle to the steel sheet, and the like in the spraying method are not particularly limited as long as sufficient conditions for bringing the aqueous solution into contact with the steel sheet are satisfied.

In addition, one of the objects of the present invention is to promote the formation of a film on a steel sheet which does not form a phosphate film by the presence of, for example, SiO ₂ on the surface of the cold rolled steel sheet after annealing, and is therefore suitable for containing Si, for example, 0.5% or more. High-strength cold-rolled steel sheets, etc. However, since Zn adheres to the surface of the steel sheet in accordance with the Zn adhesion amount of 100 to 5000 mg/m ² , that is, only a slight amount of Zn is present on the surface of the steel sheet, the corrosion resistance after coating is improved, and thus the viewpoint of corrosion resistance after coating is obtained. It can also be applied to general cold rolled steel sheets. Therefore, the present invention is a technique for ensuring phosphating treatment property and corrosion resistance after coating for all cold-rolled steel sheets.

[Examples]

According to the conventional steelmaking process, the A~H steel having the composition shown in Table 1 is melted, and continuous casting is performed to form a steel embryo. Then, the steel embryo is further heated to 1,250 ° C, and the finish rolling temperature is 850 ° C. Hot rolling at a coiling temperature of 600 ° C forms a hot rolled sheet having a thickness of 3.0 mm. After the hot-rolled sheet was pickled, it was cold-rolled to a thickness of 1.5 mm to form a test material. The test material is subjected to a laboratory reduction heating simulator, and is heated in a nitrogen atmosphere containing 10 vol% of hydrogen at a temperature of 800 to 850 ° C for a long period of 2 minutes to obtain an annealed sheet (cold-rolled steel sheet). .

For the annealed sheet (cold-rolled steel sheet) obtained as described above, an aqueous solution containing zinc sulfate heptahydrate: 1 mol/L and adjusted to pH 2.0 with sulfuric acid is used, and the anode is plated with a cerium oxide plate, and electroplating is performed. Zn is attached to the surface. The amount of Zn adhesion is varied by changing the current density and the energization time. The plated sample was immersed in an aqueous solution of sodium diphosphate (Na ₄ P ₂ O ₇ ‧10H ₂ O) for 3 seconds after being subjected to water washing. The solution was based on a P concentration of 0.5 g/L and a temperature of 50 ° C, and a part of the solution was changed in concentration and temperature and evaluated. Further, for comparison, an annealed sheet (cold-rolled steel sheet) which was not subjected to electroplating and had no Zn adhered on its surface was prepared.

Next, the phosphating treatment shown below was carried out on the cold-rolled steel sheet obtained as described above.

First, the commercially available alkali degreasing liquid (Nihon Parkerizing Co., Ltd., FINECLEANER FC-E2001) was used to build a bath at a predetermined concentration, and the diluted bath was diluted to a predetermined concentration twice as long as the deterioration was assumed. The water-wet rate of the steel sheet after washing with water was evaluated for 2 minutes. Those who have a water wetting rate of 80% or more are rated as "○", those who are less than 80% are rated as "△", and those who are less than 50% are rated as "X", and are regarded as indicators of degreasing.

Next, a cold-rolled steel sheet which has been degreased by using the above-mentioned degreased solution diluted to a predetermined concentration twice is immersed in a surface conditioning liquid (manufactured by Nihon Parkerizing Co., Ltd., PL-ZTH), and in a phosphate treatment liquid (Nihon Parkerizing) In the (stock) system, PALBOND PB-L3080), phosphating treatment was carried out by immersion under the conditions of a bath temperature of 43 ° C and a treatment time of 120 seconds.

The SEM was used to observe the surface of the cold-rolled steel sheet after phosphating, and the field of view was observed at a magnification of 300 times, according to the presence or absence of the region (rust scale) in which the phosphating crystal was not formed, and the degree of unevenness of the crystal state, and The phosphating treatment score was evaluated in the following five stages.

5 points: no scale was found and the crystals were uniform.

4 points: Although some crystals were found to be uneven, no rust was found.

3 points: Found small scales.

2 points: Large scales were found.

1 point: Most of the large scale was found.

The phosphatized steel sheet was further coated with a commercially available ED (Kansai Paint Co., Ltd., GT-10) to have a coating thickness of 20 μm, and then cut with a NT cutter (registered trademark) on the coated surface. After the incision, it was immersed in warm saline (5% NaCl, 50 ° C) for 10 days. After the immersed sample was covered with a polyester tape and the peeling operation was performed, the maximum peeling width on one side from the cut mark was measured. The combined conditions of the results obtained above are shown in Tables 2 to 5.

From Tables 2 to 5, in the steels A, B, and C, since Si is not contained in a large amount in the steel sheet component, good phosphating can be obtained even if the electroless zinc is not applied to the surface (Comparative Examples 1 to 3). Handling property, but the peeling width is large and the corrosion resistance after coating is poor.

Comparative Examples 9 and 10 and Inventive Examples 1 to 8 (Steel A), Comparative Examples 11 and 12, and Inventive Examples 9 to 16 (Steel B), Comparative Examples 13 and 14 and Inventive Examples 17 to 24 (Steel C) It is found that when zinc plating is applied to the surface, the peeling width after the warm salt water immersion test is reduced, and in particular, when the adhesion amount is 100 mg/m ² or more, the peeling width is low, and it is stable and after painting. Excellent corrosion resistance.

Steel D~H is an example in which zinc is not present on the surface (Comparative Examples 4 to 8), and rust is formed in phosphating crystals, especially G, H, and phosphorus with a Si content of 1.5% or more. The crystallization was almost unformed and the score was 1 point.

In the case of electroplating, if the Zn adhesion amount is less than 100 mg/m ² (Comparative Examples 15 to 24), it is still a phosphating crystallization level in which scales are present, which is still insufficient.

When the Zn adhesion amount is 100 mg/m ² or more (invention examples 25 to 64), phosphating crystals having no scale are obtained, and the phosphating treatment property is excellent. At the same time, it is known that the peeling width after the warm salt water immersion test is also low, and it is stable and hardly affected by the steel component.

Further, after electrogalvanizing, there was no example in which the aqueous solution containing P was contacted (Comparative Example 25), and even if the aqueous solution containing P was contacted, the P concentration was low (Comparative Examples 26 and 27), and Examples of low temperatures (Comparative Examples 28, 29), Although it is possible to obtain sufficient degreasing property by using a dewatering liquid having a predetermined concentration in the built-up bath, if the degreasing liquid diluted to a predetermined concentration twice is degreased by simulating the deterioration state in the actual coating production line, water repelling occurs after degreasing. situation.

On the other hand, in the examples in which the P concentration and the treatment liquid temperature are within the range of the present invention (invention examples 65 to 69 and 70), sufficient degreasing property can be obtained even if the degreasing liquid is diluted.

(industrial availability)

Even if a high-strength cold-rolled steel sheet containing a large amount of reinforcing elements such as Si has a good phosphating property before coating and good corrosion resistance after coating, it is most suitable for use in, for example, an automobile body.

Claims (2)

A method for producing a cold-rolled steel sheet having excellent phosphatability and corrosion resistance after application, wherein zinc plating is applied to a surface of a cold-rolled steel sheet so that a Zn adhesion amount is 100 to 5000 mg/m ² , and after washing with water, Further, the cold-rolled steel sheet is brought into contact with an aqueous solution containing P and having a P concentration of 0.001 to 2 g/L and a temperature of 30 to 60 °C. A method for producing a cold-rolled steel sheet having excellent phosphating treatment property and corrosion resistance after coating, wherein the surface of the cold-rolled steel sheet is subjected to electroplating zinc so that the Zn adhesion amount is 100 to 1000 mg/m ² , and after washing with water, The cold-rolled steel sheet is in contact with an aqueous solution containing P and having a P concentration of 0.001 to 2 g/L and a temperature of 30 to 60 °C.