JP2020158883A - High-strength cold-rolled steel sheet, method for manufacturing same, and water treatment agent - Google Patents

Abstract

To provide a method for producing a high-strength cold-rolled steel sheet to which Si and Mn are added as solid solution strengthening elements, in which selective oxidation of Si and Mn in heating annealing is effectively suppressed, and the chemical convertibility of the cold-rolled steel sheet is improved.SOLUTION: A surface of a steel sheet is subjected to a solution treatment in which an aqueous solution containing an Fe complex having acetylacetone or an acetylacetone derivative in an amount of 0.1 mass% or more, nitric acid in an amount of 0.5 mass% or more, and acetic acid in a concentration of 10 mass% or more is contacted to the surface of the steel sheet, and then annealing is performed in a reducing atmosphere under a predetermined condition. A liquid film of the aqueous solution contacting to the surface of the steel sheet by the solution treatment is formed into a film through a process of forming a gel film, and thus it is possible to form a uniform Fe oxide film having a uniform thickness. As a result, in the annealing, the selective oxidation of Si and Mn is appropriately suppressed over the entire surface of the steel sheet.SELECTED DRAWING: None

Description

The present invention is a method for producing a high-strength cold-rolled steel sheet used by undergoing chemical conversion treatment such as phosphate treatment or black dyeing treatment, and in particular, it is used by painting after performing chemical conversion treatment such as phosphate treatment. The present invention relates to a method for producing a high-strength cold-rolled steel sheet suitable for automobile member applications.

In recent years, there has been a strong demand for improving the fuel efficiency of automobiles in order to reduce CO ₂ emissions. Along with this, the movement to reduce the weight of the vehicle body by thinning the body parts has become active, and the need for increasing the strength of the steel plate, which is the material for the vehicle body parts, is increasing.
Addition of solid solution strengthening elements such as Si and Mn is effective for increasing the strength of the steel sheet. However, since these elements are more easily oxidized than Fe, the following problems occur in the production of high-strength steel sheets containing a large amount of these elements.

Usually, when a high-strength steel sheet is manufactured, the material is adjusted by heating and annealing the cold-rolled steel sheet in a non-oxidizing atmosphere or a reducing atmosphere at a temperature of about 600 to 900 ° C. In this heat annealing, easily oxidizing elements such as Si and Mn in steel are selectively oxidized even in a generally used non-oxidizing atmosphere or reducing atmosphere, and are concentrated on the surface of the steel sheet to form an oxide. .. Generally, steel sheets used for automobile parts are subjected to chemical conversion treatment such as phosphate treatment as a pre-coating treatment, but the above oxides inhibit the formation reaction of the chemical conversion treatment film by the chemical conversion treatment and significantly deteriorate the chemical conversion treatment property. Let me. Since the chemical conversion treatment of a cold-rolled steel sheet is one of the important treatments for ensuring corrosion resistance after painting, improving the chemical conversion treatment of a high-strength cold-rolled steel sheet is a heavy issue.

Conventionally, in order to improve the chemical conversion processability of high-strength steel sheets, for example, the following proposals have been made.
In Patent Document 1, a steel sheet is heated to 400 ° C. or higher in an Fe-oxidizing atmosphere to form an Fe oxide film on the surface of the steel sheet, and then reduced in an Fe-reducing atmosphere to form a high-strength cold-rolled steel sheet. A method for improving chemical conversion processability has been proposed.
Patent Document 2 describes a method in which a steel sheet is oxidized in an Fe-oxidizing atmosphere and then annealed in a predetermined reducing atmosphere, wherein the amount of Fe oxidation after the oxidation treatment and the coating of reduced Fe finally formed on the surface are provided. A method for improving the chemical conversion processability of a high-strength cold-rolled steel sheet by controlling the area ratio has been proposed.
Patent Document 3 proposes a technique for improving the chemical conversion processability of a high-strength cold-rolled steel sheet by forming a Si oxide inside the steel sheet and suppressing the formation of the Si oxide on the final surface of the steel sheet. Has been done.

Japanese Unexamined Patent Publication No. 2006-45615 International Publication No. 2011/078412 Japanese Patent No. 3386657

However, each of the above-mentioned conventional techniques has the following problems.
The methods of Patent Documents 1 and 2 require equipment for heating the steel sheet in an Fe-oxidizing atmosphere, and the construction of such equipment requires a great deal of cost. Further, when the Si content of the steel sheet is high (for example, when it is 0.5% by mass or more), Si is preferentially oxidized when heated in an Fe-oxidizing atmosphere, and the generated Si-based oxide oxidizes Fe. By inhibiting, the amount of Fe oxide produced decreases. As a result, the amount of reduced Fe produced during heating in an Fe-reducing atmosphere is reduced and surface Si-based oxides are produced, which may result in insufficient chemical conversion treatment.
Further, the technique of Patent Document 3 promotes the formation of Si oxide inside the steel sheet after winding by performing high-temperature winding during hot rolling, but on the inner and outer circumferences of the wound coil. Since there is a difference in the cooling rate, there is a problem that it is difficult to obtain uniform surface quality over the entire length of the coil.

Therefore, an object of the present invention is to solve the above-mentioned problems of the prior art, and to select Si and Mn in heat annealing in a method for producing a high-strength cold-rolled steel sheet to which Si and Mn are added as solid solution reinforcing elements. It is an object of the present invention to provide a manufacturing method capable of effectively suppressing oxidation and improving the chemical conversion processability of a cold-rolled steel sheet.

As a result of diligent studies and studies in order to solve the above-mentioned problems of the prior art, the present inventors have obtained an Fe complex, acetic acid and acetic acid having an acetylacetone or an acetylacetone derivative as a ligand for a steel sheet in advance. After performing a solution treatment to attach an aqueous solution containing nitric acid at a predetermined concentration, heat annealing is performed in an Fe-reducing atmosphere, whereby selective oxidation of Si and Mn in this heat annealing is effectively suppressed, and a chemical conversion treatment is performed. It has been found that a high-strength cold-rolled steel sheet having excellent properties can be obtained.

The present invention has been made based on such findings, and has the following gist.
[1] A method for producing a high-strength cold-rolled steel sheet containing Si or / and Mn.
An aqueous solution containing acetylacetone or an Fe complex having an acetylacetone derivative as a ligand at a concentration of 0.1% by mass or more, nitrate of 0.5% by mass or more, and acetic acid of 10% by mass or more in terms of Fe is applied to the surface of the steel plate. The solution treatment process to be attached and
The steel sheet after the solution treatment step, H ₂ concentration is 0.05 vol% or more, high-strength cold-rolled, characterized in that it comprises an annealing step of dew point heat treatment at 700 ° C. or higher in a reducing atmosphere of less than 10 ° C. Steel sheet manufacturing method.
[2] A method for producing a high-strength cold-rolled steel sheet according to the above-mentioned [1], wherein a high-strength cold-rolled steel sheet for chemical conversion treatment is produced.
[3] In the production method of the above [1] or [2], the steel sheet contains Si: 0.10% or more and / and Mn: 0.50% or more in mass%. Manufacturing method of cold-rolled steel sheet.

[4] In any of the above-mentioned production methods [1] to [3], the amount of Fe adhered to the surface of the steel sheet due to the aqueous solution adhered in the solution treatment step is 0.1 to 10.0 g / m ^2. A method for manufacturing a high-strength cold-rolled steel sheet.
[5] In any of the above-mentioned production methods [1] to [4], a high-strength cold-rolled steel sheet characterized in that a steel sheet having an aqueous solution adhered to its surface after a solution treatment step is directly introduced into an annealing step. Production method.
[6] In any of the above-mentioned production methods [1] to [4], a steel sheet having an aqueous solution adhered to its surface is heat-treated through a solution treatment step, and then introduced into an annealing step. Manufacturing method of rolled steel sheet.
[7] In any of the above-mentioned production methods [1] to [6], a method for producing a high-strength cold-rolled steel sheet, characterized in that a solution treatment step and an annealing step are continuously performed in a continuous annealing line.

[8] In any of the above-mentioned production methods [1] to [7], the steel sheet is in mass%, C: 0.040 to 0.500%, Si: 0.10 to 3.00%, Mn: Contains 0.50 to 5.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.100% or less, N: 0.0100% or less, and the balance is Fe and unavoidable. A method for producing a high-strength cold-rolled steel sheet, which comprises a component composition composed of impurities.
[9] In the production method of the above [8], the steel sheet is further made of Ti: 0.010 to 0.100%, Nb: 0.010 to 0.100%, B: 0.0001 to 0 in mass%. A method for producing a high-strength cold-rolled steel sheet, which comprises one or more selected from 0050%.
[10] In the production method of the above [8] or [9], the steel sheet is further increased in mass% by Mo: 0.01 to 0.50%, Cr: 1.00% or less, Ni: 0.50%. Below, Cu: 1.00% or less, V: 0.500% or less, Sb: 0.10% or less, Sn: 0.10% or less, Ca: 0.0100% or less, REM: 0.010% or less A method for producing a high-strength cold-rolled steel sheet, which comprises one or more selected from the above.

[11] A method for producing a high-strength cold-rolled steel sheet, which comprises any of the above-mentioned production methods [1] to [10], further comprising a chemical conversion treatment step for chemical conversion treatment of a steel sheet that has undergone an annealing step.
[12] In the production method of the above [11], a method for producing a high-strength cold-rolled steel sheet, characterized in that the steel sheet is phosphate-treated in the chemical conversion treatment step.
[13] In the production method of the above [11], a method for producing a high-strength cold-rolled steel sheet, which comprises black-dying a steel sheet in a chemical conversion treatment step.
[14] A high-strength cold-rolled steel sheet containing Si or / and Mn.
An aqueous solution containing an Fe complex having an acetylacetone or an acetylacetone derivative as a ligand at a concentration of 0.1% by mass or more, nitrate of 0.5% by mass or more, and acetic acid of 10% by mass or more in terms of Fe is attached to the surface of the steel sheet. a solution treatment step of, the steel plate of H ₂ concentration after the solution treatment step 0.05 vol% or more, the dew point was generated by passing through an annealing step of heat treatment at 700 ° C. or higher in a reducing atmosphere of less than 10 ° C. A high-strength cold-rolled steel sheet characterized in that the surface of the steel sheet is covered with reduced iron.

[15] In the high-strength cold-rolled steel sheet of the above [14], the steel sheet contains Si: 0.10% or more and / and Mn: 0.50% or more in mass%. Rolled steel plate.
[16] In the high-strength cold-rolled steel sheet of the above [14] or [15], the Fe adhesion amount of the reduced iron covering the surface of the steel sheet is 0.1 to 10.0 g / m ^2. Rolled steel plate.
[17] In the high-strength cold-rolled steel sheet according to any one of [14] to [16] above, the steel sheet is C: 0.040 to 0.500% and Si: 0.10 to 3.00% in mass%. , Mn: 0.50 to 5.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.100% or less, N: 0.0100% or less, and the balance is Fe. A high-strength cold-rolled steel sheet having a component composition composed of unavoidable impurities.
[18] In the high-strength cold-rolled steel sheet of the above [17], the steel sheet is further increased in mass% by Ti: 0.010 to 0.100%, Nb: 0.010 to 0.100%, B: 0. A high-strength cold-rolled steel sheet containing at least one selected from 0001 to 0.0050%.

[19] In the high-strength cold-rolled steel sheet of the above [17] or [18], the steel sheet is further increased in mass% by Mo: 0.01 to 0.50%, Cr: 1.00% or less, Ni: 0. .50% or less, Cu: 1.00% or less, V: 0.500% or less, Sb: 0.10% or less, Sn: 0.10% or less, Ca: 0.0100% or less, REM: 0.010 A high-strength cold-rolled steel sheet containing at least one selected from% or less.
[20] The high-strength cold-rolled steel sheet according to any one of [14] to [19] above, wherein a chemical conversion-treated film is provided on the upper portion of the reduced iron covering the surface of the steel sheet.
[21] The high-strength cold-rolled steel sheet according to the above [20], wherein the chemical conversion-treated film is a phosphate film.
[22] In the high-strength cold-rolled steel sheet of the above [20], the high-strength cold-rolled steel sheet is characterized in that the chemical conversion treatment film is an Fe ₃ O ₄ film obtained by black dyeing treatment.
[23] An aqueous treatment agent used for solution treatment for improving the chemical conversion processability of a steel sheet for chemical conversion treatment, wherein an Fe complex having acetylacetone or an acetylacetone derivative as a ligand is 0.1% by mass or more in terms of Fe, nitrate. An aqueous treatment agent for solution treatment, which comprises an aqueous solution containing 0.5% by mass or more of acetic acid and 10% by mass or more of acetic acid.

According to the present invention, in the production of a high-strength cold-rolled steel sheet containing Si and Mn, which are easily oxidizing elements, the selective oxidation of Si and Mn in heat annealing is effectively suppressed, and the steel sheet has excellent chemical conversion treatment properties. Can be manufactured. By applying the high-strength cold-rolled steel sheet produced by the present invention to, for example, an automobile structural member, it is possible to improve fuel efficiency by reducing the weight of the vehicle body.

Enlarged photograph of the surface appearance of the steel sheet subjected to solution treatment and heat treatment under the conditions (a) to (d) shown in Table 2.

The present invention is a method for producing a high-strength cold-rolled steel sheet containing Si and / and Mn, which are easily oxidizing elements, and a Fe complex having an acetylacetone or an acetylacetone derivative as a ligand and nitrate on the surface of the steel sheet. It has a solution treatment step (A) for adhering an aqueous solution (x) containing acetic acid at a predetermined concentration, and an annealing step (B) for heat-treating the steel sheet that has undergone this solution treatment step (A) in a reducing atmosphere.
The preferable composition of the high-strength cold-rolled steel sheet produced by the present invention will be described in detail later.

-Solution processing step (A)
This solution treatment step (A) is carried out after degreasing and cleaning the surface of the steel sheet by a known method, if necessary. The aqueous solution (x) attached to the surface of the steel sheet in this solution treatment step (A) is an Fe complex having acetylacetone or an acetylacetone derivative as a ligand (hereinafter, may be simply referred to as “Fe complex” for convenience of explanation). An aqueous solution containing 0.1% by mass or more of Fe, 0.5% by mass or more of nitrate, and 10% by mass or more of acetic acid, respectively, and the Fe content (x) contained in the aqueous solution (x) adhering to the surface of the steel sheet. The Fe complex) serves as the main iron source for the Fe oxide film formed on the surface of the steel sheet in the initial heating stage of the subsequent annealing step (B).

In the present invention, the aqueous solution (x) having a specific component and concentration as described above is used to form a film by a method called a sol-gel method, thereby forming a homogeneous and uniform thickness Fe oxide film. To do. That is, the solvent of the aqueous solution (x) liquid film (sol) adhering to the surface of the steel plate evaporates and decomposes with the passage of time and the temperature rise, but at that time, the solvent in the liquid film simply evaporates and the solute. Rather than precipitating, the entire liquid film gradually loses its fluidity due to aggregation and polymerization of the components in the liquid film, forming a film (gel) containing an Fe compound, and a film (Fe oxidation) through the process of forming this gel-like film. Since the film) is formed, a uniform and uniform thickness of Fe oxide film can be formed, and the amount of Fe adhered can be controlled relatively easily. In order to form the Fe oxide film through the process of forming such a gel-like film, the temperature at the initial stage of heating in the annealing step (B) can be used, but a part or all of the film formation is It may be done by the heat treatment performed before the annealing step (B). As a result of the above, in the annealing step (B), selective oxidation of Si and Mn is appropriately suppressed over the entire surface of the steel sheet, and the steel sheet is made of reduced iron produced by reduction of a Fe oxide film having a uniform and uniform thickness. By covering the surface, it is possible to improve the reactivity at the time of chemical conversion treatment such as phosphate treatment and black dyeing treatment.

When the series of film formation as described above is performed in the annealing step (B), first, in the initial heating of the annealing step (B), the liquid film (sol) of the aqueous solution (x) adhering to the surface of the steel sheet. ) Is formed into a gel-like film, and further decomposition of the gel-like film causes a uniform and uniform thickness of Fe oxide film to be formed over the entire surface of the steel sheet. In the subsequent annealing process, this Fe oxide film is formed. Reduction occurs. Since the formation of the Fe oxide film and its reduction occur continuously in this way, the selective oxidation of Si and Mn in the annealing step (B) is suppressed, and the reduced iron film having a uniform and uniform thickness covering the surface of the steel sheet is formed. The formation is more reliable, and the chemical conversion processability of the steel sheet can be improved.

Here, the components of the aqueous solution (x) for solution treatment will be described. First, the Fe complex having acetylacetone or an acetylacetone derivative as a ligand is the iron and oxygen source of Fe oxide produced in the annealing step (B). Become. This Fe complex has a structure in which three molecules of acetylacetone, which is a ligand, are coordinated with one Fe atom, and all acetylacetone is bonded to Fe by an O atom. Therefore, for example, considering the case where an Fe oxide film is formed in the annealing step (B), the hydrocarbon portion in the gel-like film is decomposed, but the Fe—O bond portion remains, so that the Fe reducing atmosphere It is considered that a Fe oxide film is formed even inside. In addition, both acetic acid and nitric acid are considered to contribute to the stable gelation of the liquid film during the film formation process. Further, nitric acid also acts as an oxidizing agent for Fe and dissolves the surface of the steel sheet, so that the surface of the steel sheet becomes active and contributes to stabilizing the adhesion of the gel-like film. Therefore, it is important that the aqueous solution (x) contains these three components at a predetermined concentration.
Since nitric acid dissolves the surface of the steel sheet as described above, Fe ions derived from the steel sheet may be contained in the film. That is, the main Fe source of the Fe oxide film formed on the surface of the steel sheet is the Fe complex in the aqueous solution (x), but Fe on the surface of the steel sheet is oxidized by the nitrate component in the aqueous solution (x), and this Fe oxide May be included as part of the coating.

The acetylacetone or acetylacetone derivative contained in the Fe complex contained in the aqueous solution (x) as a ligand is a compound having a 2,4-pentandion skeleton in the molecule and behaves as a bidentate ligand for a metal ion. , Form a very stable metal complex. The type of Fe complex using acetylacetone or an acetylacetone derivative as a ligand used in the present invention is not particularly limited as long as it can be industrially used, but from the viewpoint of water solubility and reduction of impurities in the subsequent annealing step. Those with a relatively low molecular weight are preferable, and for example, tris (2,4-pentandionato) iron (III), tris (2,4-hexanedionat) iron (III), tris (2,4-heptandionato) iron. (III), tris (3,5-heptandionato) iron (III) and the like are suitable, and one or more of these can be used. Among them, tris (2,4-pentanedionato) iron (III), which has the lowest molecular weight and is an inexpensive raw material, is particularly preferable.

In order to contain the Fe complex in the aqueous solution (x), the Fe complex may be dissolved directly in the aqueous solution, or the ligand and the Fe compound may be mixed in water to form the Fe complex.
In this aqueous solution (x), acetylacetone and its derivatives are bonded in the form of capping Fe atoms at the two oxygen atom sites in the molecule, and the formed Fe complex has a low-reactivity carbon skeleton on the outside. Therefore, the intermolecular polymerization reaction is unlikely to occur, and bath management is relatively easy from the viewpoint of sludge generation and the like.

As described above, in order to form a homogeneous and uniform film (Fe oxide film) from the liquid film adhering to the surface of the steel plate through the gel-like film, the added components and concentration of the aqueous solution are important, and therefore the present invention Then, an aqueous solution (x) containing an Fe complex having an acetylacetone or an acetylacetone derivative as a ligand at a concentration of 0.1% by mass or more, nitric acid of 0.5% by mass or more, and acetic acid of 10% by mass or more in terms of Fe. Is used. The mass% of the Fe complex in terms of Fe is the mass% of only Fe contained in the Fe complex.
If the concentration of the Fe complex having acetylacetone or an acetylacetone derivative as a ligand in terms of Fe is less than 0.1% by mass, the amount of Fe oxide formed in the heating process becomes insufficient, and good chemical conversion treatment cannot be obtained. .. From the above viewpoint, the more preferable concentration of the Fe complex (Fe conversion) is 1.0% by mass or more.
Here, the concentration (Fe conversion) of the Fe complex in the aqueous solution (x) is measured by ICP mass spectrometry.

Acetic acid is a part of the solvent in the aqueous solution (x) of the present invention, and if the concentration is less than 10% by mass, gelation of the above-mentioned liquid film does not stably occur, and an Fe oxide film formed in the heating process. May become non-uniform and good chemical conversion processability may not be obtained. From the above viewpoint, the more preferable concentration of acetic acid is 20% by mass or more.
Nitric acid is a component that plays a catalytic role in the polymerization reaction when the liquid film gels in the aqueous solution (x) of the present invention, and if the concentration is less than 0.5% by mass, gelation does not occur stably. , Good chemical conversion processability may not be obtained. From the above viewpoint, the more preferable concentration of nitric acid is 5.0% by mass or more.
On the other hand, there is no particular upper limit for each component, and the concentration at which the effect is saturated may be set as the upper limit so as not to cause a cost disadvantage, but in general, an Fe complex having acetylacetone or an acetylacetone derivative as a ligand is used. The concentration in terms of Fe is about 20% by mass, and the concentration of nitrate is about 20% by mass. Since the effects are saturated, these may be the upper limits. Further, since acetic acid mainly plays a role as a solvent, there is no upper limit by itself, and the upper limit is determined by the lower limit concentration of Fe complex and nitric acid.

Further, the amount of Fe adhered to the surface of the steel sheet by the aqueous solution (x) adhered in the solution treatment step (A) is preferably 0.1 to 10.0 g / m ² . If the amount of Fe adhered to the surface of the steel sheet by the aqueous solution (x) is less than 0.1 g / m ² , the chemical conversion processability may not be sufficiently improved. On the other hand, if the amount of Fe adhered to the surface of the steel sheet by the aqueous solution (x) exceeds 10.0 g / m ² , it is difficult to control the liquid film thickness during the solution treatment, and the Fe formed at the initial stage of heating in the annealing step (B). The amount of the oxide film adhered may become excessive, and unreduced Fe oxide may remain after the annealing step (B). From the above viewpoint, the more preferable Fe adhesion amount is 0.3 to 5.0 g / m ² .
The Fe adhesion amount can be adjusted by changing the Fe complex concentration in the aqueous solution (x) and the adhesion amount of the aqueous solution (x) on the surface of the steel sheet.

The effect of the presence or absence of gelation of the liquid film described above on the in-plane uniformity of the Fe oxide film formed on the surface of the steel sheet during the heat treatment was investigated. In this test, the steel sheet having the composition shown in Table 1 was subjected to solution treatment under the conditions (a) to (d) shown in Table 2. As for the composition of the aqueous solution used, the balance other than the components shown in Table 2 is water.
Of the conditions (a) to (d) shown in Table 2, (b) and (d) are test examples obtained by solution treatment with an aqueous solution satisfying the conditions of the present invention, and (a) and (c) contain nitric acid. This is a test example in which the solution was treated with an aqueous solution (an aqueous solution that does not satisfy the conditions of the present invention). In (a) and (c), since the aqueous solution does not contain nitric acid, gelation of the liquid film does not occur at the initial stage of heating. .. Of (a) to (b), (a) and (b) are those in which the heat treatment after the solution treatment is stopped and rapidly cooled when the temperature of the steel sheet reaches 400 ° C., and (c), ( In d), heating was stopped and rapidly cooled when the temperature of the steel sheet reached 800 ° C.

An enlarged photograph of the appearance of the steel sheet surface after the treatment according to (a) to (d) in Table 2 is shown in FIG. 1 ((a) to (d) in each photograph correspond to (a) to (d) in Table 2. ). As shown in FIG. 1, in (a) and (b) where the heat treatment after the solution treatment was performed at 400 ° C., a black Fe oxide film was formed on the surface of the steel sheet. On the other hand, in (c) and (d), in which the heat treatment after the solution treatment was performed at 800 ° C., the Fe oxide film formed by heating at 400 ° C. was reduced by being heated to a higher temperature and became white. There is. Here, since the aqueous solution for solution treatment does not contain nitric acid, gelation of the liquid film does not occur in the initial stage of heating (a), and the aqueous solution for solution treatment does not contain the liquid in the initial stage of heating in order to satisfy the conditions of the present invention. Comparing the appearance of (b) where the film gels, the latter has a more uniform appearance. This is because the Fe oxide film of (b) has a much more uniform in-plane distribution of the adhesion amount than the Fe oxide film of (a). Therefore, the appearances of (c) and (d) corresponding to the surface state immediately before the chemical conversion treatment in which the Fe oxide films of (a) and (b) are reduced correspond to (a) and (b). There is a superiority or inferiority relationship. In (c), where the appearance is inferior, black spot-like patterns are scattered, and because the amount of Fe oxide adhered is non-uniform in the plane, the locally high adhered portion remains without being reduced. It is considered that this is the case, and when the chemical conversion treatment is performed, it causes deterioration of the chemical conversion processability such as occurrence of scale.

As described above, the solution treatment is performed with an aqueous solution (x) satisfying the conditions of the present invention, and the Fe oxide film is formed through gelation of the liquid film at the initial stage of heating. It can be seen that the internal uniformity is improved and excellent chemical conversion processability can be obtained.
The method of adhering the aqueous solution (x) to the surface of the steel sheet is not particularly limited, and for example, a method such as a bar coater, a spray, a dip, a spin coat, or a roll coater can be used.

In the present invention, the liquid film of the aqueous solution (x) adhering to the surface of the steel sheet as described above becomes a film (Fe oxide film) via a gel-like film. Although such film formation occurs even at room temperature, it is possible to form a film in a short time by heating as necessary. In particular, in the present invention, in the annealing step (B) following the solution treatment step (A). Filming is possible using the temperature at the initial stage of heating. That is, by introducing the steel sheet to which the aqueous solution (x) (liquid film) is attached to the surface through the solution treatment step (A) as it is into the annealing step (B), it is relatively early in the heating in the annealing step (B). The liquid film can be formed into a film (Fe oxide film) via a gel-like film in a low temperature range (for example, about 400 ° C.). Therefore, it is not essential to perform the heat treatment for promoting the film formation of the liquid film before the annealing step (B), and it may be performed as needed. When performing such a heat treatment, the film formation may be completed before the annealing step (B), or the film may be heated to a state in the middle of filming (for example, a gel-like film state), and then the annealing step (for example, a gel-like film state) is performed. The film formation may be completed by heating in B). That is, in that case, the steel sheet to which the aqueous solution (x) is attached to the surface is heat-treated by an appropriate heating means through the solution treatment step (A), and then introduced into the annealing step (B).
Therefore, for example, when the solution treatment step (A) and the annealing step (B) are continuously performed in the continuous annealing line, the aqueous solution (x) (liquid film) adheres to the surface through the solution treatment step (A). The steel sheet may be introduced into the annealing step (B) as it is, or after the steel sheet having the aqueous solution (x) adhered to the surface through the solution treatment step (A) is heat-treated by an appropriate heating means, the annealing step ( It may be introduced in B).

・ Annealing process (B)
In the annealing step (B), the steel sheet after the solution treatment step (A), _{H 2} concentration is 0.05 vol% or more, the dew point is heated at 700 ° C. or higher in a reducing atmosphere of less than 10 ° C., then, predetermined Cool to the temperature of. In this annealing step (B), as described above, the liquid film of the aqueous solution (x) can be formed into a film in a relatively low temperature region (for example, about 400 ° C.) at the initial stage of heating to form an Fe oxide film on the surface of the steel sheet. However, the Fe oxide film is reduced to reduced iron by subsequent heating to the maximum temperature reached. In this annealing step (B), the Fe oxide film formed on the surface of the steel sheet with a uniform and uniform thickness is finally reduced, so that the entire surface of the steel sheet is covered without leaving unreduced Fe oxide. It will be uniformly covered with reduced iron.

The H ₂ concentration in the reducing atmosphere is set to 0.05 vol% or more, preferably 1.0 vol% or more in order to sufficiently reduce the Fe oxide. There is no particular upper limit of the H ₂ concentration, for example, 100 vol% may be used, but if the H ₂ concentration is higher than necessary, the cost will increase. Therefore, the upper limit is preferably about 40.0 vol%. The residual gas in the reducing atmosphere is usually N ₂ , H ₂ O and unavoidable impurities.
The dew point of the reducing atmosphere is less than 10 ° C., preferably 0 ° C. or lower. If the dew point is 10 ° C. or higher, there is a concern that Fe will be oxidized. Although there is no particular lower limit for the dew point, it is industrially difficult to carry out a dew point below -60 ° C, so about -60 ° C is a practical lower limit.

The annealing temperature (steel plate temperature) is 700 ° C. or higher, preferably 750 ° C. or higher. If the annealing temperature is less than 700 ° C., the reduction of the Fe oxide film is delayed, and it takes a long time to completely convert the reduced iron, which impairs productivity. In addition, the reduction of the Fe oxide film becomes insufficient, which tends to cause deterioration in chemical conversion treatment. There is no particular upper limit to the annealing temperature (steel plate temperature), but if it exceeds 950 ° C, the heating cost increases, so that it is preferably 950 ° C or lower, more preferably 900 ° C or lower. When the steel sheet is held at the annealing temperature in the annealing step (B), the steel sheet may be held at a constant temperature, or the temperature of the steel sheet is changed as long as the temperature does not deviate from the above temperature range. You may hold it while letting it. Further, the holding time is not particularly limited as long as the surface of the steel sheet is sufficiently reduced and the desired material can be obtained.
The cooling conditions of the steel sheet after the annealing step (B) are not particularly limited, and the cooling rate and the cooling stop temperature may be determined as necessary for material design and the like.

・ Chemical conversion process (C)
The production method of the present invention can further include a chemical conversion treatment step (C) for chemical conversion treatment of the steel sheet that has undergone the annealing step (B). In this chemical conversion treatment step (C), for the reason described above, the formation reaction of the chemical conversion treatment film is optimized, and excellent chemical conversion treatment property can be obtained.
The type of chemical conversion treatment is not particularly limited, but phosphate treatment such as zinc phosphate treatment is a typical example. In addition, for example, a black dyeing treatment for forming a Fe ₃ O ₄ film, which is a black film, on the surface of the steel sheet can also be applied.
In the manufacturing method of the present invention, as described above, all the steps may be carried out in continuous equipment (for example, continuous annealing line), each step may be carried out independently in separate and independent equipment, or Some steps may be carried out independently in an independent facility.

Next, the composition of the high-strength cold-rolled steel sheet produced according to the present invention will be described. In the following description, the unit of the content of each element is "mass%", but it is indicated by "%" for convenience.
The high-strength cold-rolled steel sheet to be manufactured by the present invention is a steel sheet containing Si or / and Mn as a solid solution reinforcing element, and is generally a high-strength steel sheet having a tensile strength TS of 340 MPa or more. Among them, steel sheets containing Si: 0.10% or more and / and Mn: 0.50% or more are likely to cause selective oxidation of Si and Mn during annealing. Therefore, steel sheets having such a component composition are targeted. Is preferable.

Further, as a more specific component composition of the steel plate, as basic components, C: 0.040 to 0.500%, Si: 0.10 to 3.00%, Mn: 0.50 to 5.00%, It is preferable to contain P: 0.100% or less, S: 0.0100% or less, Al: 0.100% or less, N: 0.0100% or less, and if necessary, Ti: 0.010 to One or more selected from 0.100%, Nb: 0.010 to 0.100%, B: 0.0001 to 0.0050%, Mo: 0.01 to 0.50%, Cr: 1. 00% or less, Ni: 0.50% or less, Cu: 1.00% or less, V: 0.500% or less, Sb: 0.10% or less, Sn: 0.10% or less, Ca: 0.0100% Hereinafter, one or more selected from REM: 0.010% or less can be contained. The reasons for these limitations will be described below.

・ C: 0.040 to 0.500%
C is an austenite stabilizing element, which is an effective element for improving strength and ductility. In order to obtain such an effect, the C content is preferably 0.040% or more. On the other hand, if the C content exceeds 0.500%, the weldability is significantly deteriorated, and an excellent strength-elongation balance may not be obtained due to the excessively hardened martensite phase. Therefore, the C content is preferably 0.500% or less.
-Si: 0.10 to 3.00%
Si is a ferrite stabilizing element, which is effective for solid solution strengthening of steel and improves the balance between strength and elongation. In order to obtain such an effect, the Si content is preferably 0.10% or more. On the other hand, when the Si content exceeds 3.00%, Si forms an oxide on the surface of the steel sheet during annealing even when the present invention is applied, and a local unreacted portion during chemical conversion treatment (hereinafter, this is referred to as "this". The occurrence of "scale") may deteriorate the chemical conversion processability. Therefore, the Si content is preferably 3.0% or less.

-Mn: 0.50 to 5.00%
Mn is an austenite stabilizing element and is an element effective for ensuring the strength of the annealed plate. In order to secure this strength, the Mn content is preferably 0.50% or more. However, if the Mn content exceeds 5.00%, even if the present invention is applied, a large amount of oxide is formed on the surface of the steel sheet during annealing, and scale is generated during the chemical conversion treatment, which deteriorates the chemical conversion processability. There is a risk. Therefore, the Mn content is preferably 5.00% or less.
-P: 0.100% or less P is an element effective for strengthening steel, but if the content is too large, embrittlement may occur due to grain boundary segregation and the impact resistance may be deteriorated. Therefore, the P content is preferably 0.100% or less. From the viewpoint of strengthening steel, the P content is preferably 0.001% or more.

-S: 0.0100% or less S becomes an inclusion such as MnS and causes deterioration of impact resistance and cracking along the metal flow of the welded portion. Therefore, the S content should be as low as possible, preferably 0.0100% or less.
-Al: 0.100% or less Excessive addition of Al causes deterioration of surface properties and moldability due to an increase in oxide-based inclusions, and also leads to high cost. Therefore, the Al content is preferably 0.100% or less, and more preferably 0.050% or less.
-N: 0.0100% or less N is an element that deteriorates the aging resistance of steel. Therefore, the smaller the N content, the more preferably 0.0100% or less.

-Ti: 0.010 to 0.100%
Ti is an element that contributes to improving the strength of the steel sheet by forming fine carbides and fine nitrides with C and N in the steel sheet. In order to obtain this effect, the Ti content is preferably 0.010% or more. On the other hand, if the Ti content exceeds 0.100%, this effect is saturated, so the Ti content is preferably 0.100% or less.
・ Nb: 0.010 to 0.100%
Nb is an element that contributes to strength improvement by strengthening solid solution and strengthening precipitation. In order to obtain this effect, the Nb content is preferably 0.010% or more. On the other hand, if the Nb content exceeds 0.100%, the ductility of the steel sheet may be lowered and the workability may be deteriorated. Therefore, the Nb content is preferably 0.100% or less.
-B: 0.0001 to 0.0050%
B is an element that enhances hardenability and contributes to improving the strength of the steel sheet. In order to obtain this effect, the B content is preferably 0.0001% or more. On the other hand, if B is excessively contained, the ductility may be lowered and the workability may be deteriorated. In addition, the excessive content of B also causes an increase in cost. Therefore, the B content is preferably 0.0050% or less.

・ Mo: 0.01 to 0.50%
Mo is an element effective for ensuring the strength of the annealed plate because it improves the hardenability of steel and promotes the formation of bainite and martensite. From the viewpoint of ensuring strength, the Mo content is preferably 0.01% or more. On the other hand, since Mo has a high alloy cost, a large amount of Mo added causes an increase in cost. Therefore, the Mo content is preferably 0.50% or less.
-Cr: 1.00% or less Cr is an effective element for ensuring the strength of the annealed plate because it improves the hardenability of steel and promotes the formation of bainite and martensite, but if the amount added is too large, it will be converted. It may deteriorate the processability. Therefore, the Cr content is preferably 1.00% or less.

-Ni: 0.50% or less, Cu: 1.00% or less, V: 0.500% or less Ni, Cu, V are effective elements for strengthening steel, but if added excessively, ductility due to a significant increase in strength In addition, excessive addition of these elements may cause an increase in cost. Therefore, when these elements are added, it is preferable that the Ni content is 0.50% or less, the Cu content is 1.00% or less, and the V content is 0.500% or less. In order to reinforce the steel, it is preferable that the Ni content is 0.05% or more, the Cu content is 0.05% or more, and the V content is 0.005% or more.
-Sb: 0.10% or less, Sn: 0.10% or less Sb and Sn have an action of suppressing nitriding near the surface of the steel sheet, but the effect is saturated even if they are added excessively. Therefore, when these elements are added, the Sb content is preferably 0.10% or less, and the Sn content is preferably 0.10% or less. In order to suppress nitriding, the Sb content is preferably 0.005% or more, and the Sn content is preferably 0.005% or more.

-Ca: 0.0100% or less Ca has the effect of improving ductility by controlling the shape of sulfides such as MnS, but the above effect is saturated even if it is added in excess, so the Ca content is 0.0100%. The following is preferable. In order to obtain the above effect, the Ca content is preferably 0.0010% or more.
-REM: 0.010% or less REM (rare earth metal: rare earth metal) controls the morphology of sulfide-based inclusions and contributes to the improvement of workability, but when added in excess, it causes an increase in inclusions and is processed. The REM content is preferably 0.010% or less because it may deteriorate the properties. In order to obtain the effect of improving workability, the REM content is preferably 0.001% or more.
The rest other than the basic component and the optional additive component described above are Fe and unavoidable impurities.

There are no particular restrictions on the manufacturing conditions of the cold-rolled steel sheet to which the solution treatment step of the present invention is applied. Usually, a steel slab having the above composition is roughly rolled and finished rolled in a hot rolling step, then the scale of the surface layer of a hot rolled plate is removed in a pickling step, and then cold rolling is performed. The hot rolling conditions, pickling conditions, and cold rolling conditions are not particularly limited.
The aqueous solution (x) used in the solution treatment step (A) of the present invention is an aqueous treatment agent used for solution treatment for improving the chemical conversion processability of the chemical conversion steel plate, and uses acetylacetone or an acetylacetone derivative as a ligand. It is an aqueous treatment agent consisting of an aqueous solution containing the Fe complex having a concentration of 0.1% by mass or more, nitric acid in an amount of 0.5% by mass or more, and acetic acid in an amount of 10% by mass or more in terms of Fe. The preferable composition of this aqueous treatment agent is as described above for the aqueous solution (x).

The high-strength cold-rolled steel sheet produced by the method of the present invention is a steel sheet containing Si or / and Mn, and contains an Fe complex having an acetylacetone or an acetylacetone derivative as a ligand in an Fe equivalent of 0.1% by mass or more. nitric acid 0.5 wt% or more, the solution treatment step of depositing an aqueous solution containing each at a concentration of at least 10 wt% of acetic acid on the steel sheet surface (a), the steel sheet and H ₂ concentration after the solution treatment step (a) The surface of the steel sheet is covered with reduced iron produced by undergoing the annealing step (B) of heat treatment at 700 ° C. or higher in a reducing atmosphere having a dew point of 0.05 vol% or more and a dew point of less than 10 ° C. It is a rolled steel plate. The preferable composition of the steel sheet is as described above. Further, the Fe adhesion amount of the reduced iron covering the surface of the steel sheet is preferably 0.1 to 10.0 g / m ² , and the reason is as described above. In addition, this high-strength cold-rolled steel sheet undergoes a chemical conversion treatment step (C), and a chemical conversion treatment film such as a phosphate film or a Fe ₃ O ₄ film by black dyeing treatment is applied to the upper part of the reduced iron covering the surface of the steel sheet. Can have.

[Example 1]
Steel having the component composition shown in Table 3 and having the balance of Fe and unavoidable impurities was melted to form a slab. The slab was heated to 1200 ° C., hot rolled, and wound. This hot-rolled plate was pickled and cold-rolled at a reduction ratio of 50%. The obtained cold-rolled steel sheet was subjected to a solution treatment step and an annealing step in sequence under the conditions shown in Tables 4 to 6.
In the solution treatment step, an aqueous solution was applied to the surface of the steel sheet by a roll coater. The amount of Fe adhered was controlled by adjusting the Fe concentration in the aqueous solution, the roll gap, and the peripheral peripheral speed of the roll to change the liquid film thickness. The amount of Fe adhered was quantified by fluorescent X-ray analysis of the dried steel sheet after applying the aqueous solution.
The annealing step was carried out in a furnace capable of adjusting the atmosphere, and the steel sheet coated with the aqueous solution in the solution treatment step was placed in the furnace as it was with a liquid film and annealed.

The cold-rolled steel sheet obtained as described above was subjected to chemical conversion treatment by the method shown below, and the chemical conversion processability was evaluated.
A 70 mm x 150 mm size test piece is collected from a steel plate, degreased with a degreasing agent (Nippon Paint Co., Ltd. "Surf Cleaner EC90" (trade name)), washed with water, and then surface-conditioned (Nippon Paint Co., Ltd.). The surface is adjusted with "5N-10" (trade name) for 30 seconds, and then immersed in a zinc phosphate treatment solution ("Surfdyne EC1000" (trade name) manufactured by Nippon Paint Co., Ltd.) at a temperature of 40 ° C. After 90 seconds of treatment, it was washed with water and dried.
Using a scanning electron microscope (SEM), the chemical conversion film was randomly observed in 5 fields at a magnification of 1000, and the area ratio (coverage ratio) covered with zinc phosphate crystals was evaluated. In addition, 10 zinc phosphate crystals in the field of view were randomly selected, and the average crystal size was determined. Based on the obtained results, the chemical conversion processability was evaluated in three stages according to the following criteria.
⊚: Especially good (coverage 100%, average crystal size 3 μm or less)
◯: Good (coverage 100%, average crystal size over 3 μm)
X: Insufficient (coverage less than 100%)

The above results are shown in Tables 4 to 6 together with the manufacturing conditions. As for the composition of the aqueous solution used, the balance other than the components shown in Tables 4 to 6 is water.
According to Tables 4 to 6, it can be seen that the high-strength cold-rolled steel sheets of the examples of the present invention are all excellent in chemical conversion treatment. On the other hand, the high-strength cold-rolled steel sheet of the comparative example is inferior in chemical conversion processability.

[Example 2]
A solution treatment step and an annealing step were sequentially carried out on the cold-rolled steel sheet obtained under the same conditions as in Example 1 under the conditions shown in Tables 7 and 8. The method for carrying out the solution treatment step and the annealing step, and the method for quantifying the amount of Fe adhered by the solution treatment were the same as in [Example 1].
The cold-rolled steel sheet obtained as described above was subjected to chemical conversion treatment by the method shown below, and the chemical conversion processability was evaluated.
A 70 mm x 150 mm size test piece is collected from a steel plate, degreased with a degreasing agent ("Surf Cleaner EC90" (trade name) manufactured by Nippon Paint Co., Ltd.), washed with water, and then iron black dyeing agent (Meltex Co., Ltd.). It was immersed in "Evonol S-34" (trade name) manufactured by Japan, treated at a liquid temperature of 140 ° C. for 1200 seconds, washed with water and dried.

Select randomly 10 (the position) from the plane of the steel sheet surface after the chemical conversion treatment, the L value was measured in the range of 6mmφ per point, the mean value L _ave and the maximum of the measured L value 10 It was determined difference L _max -L _min value L _max and the minimum value L _min. The black dyeing treatment, which is a chemical conversion treatment in this embodiment, blackens the surface of the steel sheet by forming an Fe ₃ O ₄ film on the surface of the steel sheet. Therefore, as in the case of zinc phosphate treatment, the smaller the unreacted portion of the steel sheet, the more uniform Fe ₃ O ₄ film is formed in the plane, and the lightness of the surface of the steel sheet becomes smaller. Based on the above results, the superiority or inferiority of chemical conversion processability was evaluated on a three-point scale according to the magnitude of the L value according to the following criteria. The L value of the surface of the steel sheet before the chemical conversion treatment was about 60.
⊚: Especially good (L _ave : less than 30, L _max −L _min : less than 10)
◯: Good (L _ave : less than 30, L _max −L _min : 10 or more)
×: Insufficient ( _Lave : 30 or more)

The above results are shown in Tables 7 and 8 together with the manufacturing conditions. As for the composition of the aqueous solution used, the balance other than the components shown in Tables 7 and 8 is water.
According to Tables 7 and 8, it can be seen that the high-strength cold-rolled steel sheets of the examples of the present invention are all excellent in chemical conversion treatment. On the other hand, the high-strength cold-rolled steel sheet of the comparative example is inferior in chemical conversion processability.

Claims (23)

A method for producing a high-strength cold-rolled steel sheet containing Si or / and Mn.
An aqueous solution containing acetylacetone or an Fe complex having an acetylacetone derivative as a ligand at a concentration of 0.1% by mass or more, nitrate of 0.5% by mass or more, and acetic acid of 10% by mass or more in terms of Fe is applied to the surface of the steel plate. The solution treatment process to be attached and
The steel sheet after the solution treatment step, H ₂ concentration is 0.05 vol% or more, high-strength cold-rolled, characterized in that it comprises an annealing step of dew point heat treatment at 700 ° C. or higher in a reducing atmosphere of less than 10 ° C. Steel sheet manufacturing method. The method for producing a high-strength cold-rolled steel sheet according to claim 1, wherein a high-strength cold-rolled steel sheet for chemical conversion treatment is produced. The method for producing a high-strength cold-rolled steel sheet according to claim 1 or 2, wherein the steel sheet contains Si: 0.10% or more and / and Mn: 0.50% or more in mass%. The production of a high-strength cold-rolled steel sheet according to any one of claims 1 to 3, wherein the amount of Fe adhered to the surface of the steel sheet due to the aqueous solution adhered in the solution treatment step is 0.1 to 10.0 g / m ^2. Method. The method for producing a high-strength cold-rolled steel sheet according to any one of claims 1 to 4, wherein the steel sheet to which the aqueous solution is attached to the surface through the solution treatment step is directly introduced into the annealing step. The method for producing a high-strength cold-rolled steel sheet according to any one of claims 1 to 4, wherein the steel sheet to which the aqueous solution is attached to the surface is heat-treated through the solution treatment step and then introduced into the annealing step. The method for producing a high-strength cold-rolled steel sheet according to any one of claims 1 to 6, wherein the solution treatment step and the annealing step are continuously performed in the continuous annealing line. The weight of the steel sheet is C: 0.040 to 0.500%, Si: 0.10 to 3.00%, Mn: 0.50 to 5.00%, P: 0.100% or less, S: Any of claims 1 to 7, which contains 0.0100% or less, Al: 0.100% or less, N: 0.0100% or less, and has a component composition in which the balance is composed of Fe and unavoidable impurities. A method for manufacturing a high-strength cold-rolled steel sheet described in Crab. Further, the steel sheet is one or more selected from Ti: 0.010 to 0.100%, Nb: 0.010 to 0.100%, and B: 0.0001 to 0.0050% in mass%. The method for producing a high-strength cold-rolled steel sheet according to claim 8, wherein the steel sheet is contained. Further, the steel sheet has Mo: 0.01 to 0.50%, Cr: 1.00% or less, Ni: 0.50% or less, Cu: 1.00% or less, V: 0.500% in mass%. Hereinafter, the claim is characterized by containing one or more selected from Sb: 0.10% or less, Sn: 0.10% or less, Ca: 0.0100% or less, and REM: 0.010% or less. Item 8. The method for producing a high-strength cold-rolled steel sheet according to Item 8 or 9. The method for producing a high-strength cold-rolled steel sheet according to any one of claims 1 to 10, further comprising a chemical conversion treatment step of forming a steel sheet that has undergone an annealing step. The method for producing a high-strength cold-rolled steel sheet according to claim 11, wherein in the chemical conversion treatment step, the steel sheet is phosphate-treated. The method for producing a high-strength cold-rolled steel sheet according to claim 11, wherein in the chemical conversion treatment step, the steel sheet is dyed black. A high-strength cold-rolled steel sheet containing Si or / and Mn.
An aqueous solution containing an Fe complex having an acetylacetone or an acetylacetone derivative as a ligand at a concentration of 0.1% by mass or more, nitrate of 0.5% by mass or more, and acetic acid of 10% by mass or more in terms of Fe is attached to the surface of the steel sheet. a solution treatment step of, the steel plate of H ₂ concentration after the solution treatment step 0.05 vol% or more, the dew point was generated by passing through an annealing step of heat treatment at 700 ° C. or higher in a reducing atmosphere of less than 10 ° C. A high-strength cold-rolled steel sheet characterized in that the surface of the steel sheet is covered with reduced iron. The high-strength cold-rolled steel sheet according to claim 14, wherein the steel sheet contains Si: 0.10% or more and / and Mn: 0.50% or more in mass%. The high-strength cold-rolled steel sheet according to claim 14 or 15, wherein the Fe adhesion amount of the reduced iron covering the surface of the steel sheet is 0.1 to 10.0 g / m ² . The weight of the steel sheet is C: 0.040 to 0.500%, Si: 0.10 to 3.00%, Mn: 0.50 to 5.00%, P: 0.100% or less, S: Any of claims 14 to 16, which contains 0.0100% or less, Al: 0.100% or less, N: 0.0100% or less, and has a component composition in which the balance is composed of Fe and unavoidable impurities. High-strength cold-rolled steel sheet described in Crab. Further, the steel sheet is one or more selected from Ti: 0.010 to 0.100%, Nb: 0.010 to 0.100%, and B: 0.0001 to 0.0050% in mass%. The high-strength cold-rolled steel sheet according to claim 17, wherein the steel sheet is contained. Further, the steel sheet has Mo: 0.01 to 0.50%, Cr: 1.00% or less, Ni: 0.50% or less, Cu: 1.00% or less, V: 0.500% in mass%. Hereinafter, the claim is characterized by containing one or more selected from Sb: 0.10% or less, Sn: 0.10% or less, Ca: 0.0100% or less, and REM: 0.010% or less. Item 17. The high-strength cold-rolled steel sheet according to Item 17 or 18. The high-strength cold-rolled steel sheet according to any one of claims 14 to 19, wherein a chemical conversion treatment film is provided on the upper portion of the reduced iron covering the surface of the steel sheet. The high-strength cold-rolled steel sheet according to claim 20, wherein the chemical conversion coating is a phosphate coating. The high-strength cold-rolled steel sheet according to claim 20, wherein the chemical conversion-treated film is a Fe ₃ O ₄ film obtained by black-dying treatment. An aqueous treatment agent used for solution treatment for improving chemical conversion treatment of steel sheets for chemical conversion treatment.
It is characterized by consisting of an aqueous solution containing an Fe complex having acetylacetone or an acetylacetone derivative as a ligand at a concentration of 0.1% by mass or more, nitric acid in an amount of 0.5% by mass or more, and acetic acid in an amount of 10% by mass or more in terms of Fe. Aqueous treatment agent for solution treatment.