JP6777140B2 - Manufacturing method of high-strength galvanized steel sheet - Google Patents

Description

The present invention relates to a method for producing a high-strength galvanized steel sheet, including a high-strength hot-dip galvanized steel sheet, which is particularly suitable for use in automobile members.

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, in the production of hot-dip galvanized steel sheets (including alloyed hot-dip galvanized steel sheets) using a high-strength steel sheet containing a large amount of them as a base material, the following The problem arises.

Usually, when a hot-dip galvanized steel sheet is produced, the steel sheet is heat-annealed at a temperature of about 600 to 900 ° C. in a non-oxidizing atmosphere or a reducing atmosphere, and then hot-dip galvanized. In the heat annealing before the plating treatment, the easily oxidizing element in the steel is selectively oxidized even in a generally used non-oxidizing atmosphere or reducing atmosphere, and is concentrated on the surface of the steel sheet to form an oxide. Since this oxide lowers the wettability of hot-dip zinc during the hot-dip galvanizing process, the wettability of the hot-dip galvanizes sharply decreases as the concentration of easily oxidizable elements in the steel increases, causing frequent non-plating. Further, even when non-plating does not occur, the plating adhesion deteriorates because the oxide exists between the steel sheet and the plating layer. In particular, since Si significantly reduces the wettability of hot-dip zinc even when added in a small amount, Mn, which has a smaller effect on wettability, is often added to hot-dip galvanized steel sheets. However, since Mn oxide also reduces the wettability of hot-dip zinc, the above-mentioned non-plating problem becomes remarkable when a large amount is added.

In response to the above problems, Patent Document 1 describes a method in which a steel sheet is annealed and then pickled to dissolve and remove oxides formed on the surface, and then annealed again to perform hot-dip galvanizing. Has been proposed.
Further, in Patent Document 2, first, the steel sheet is heated in an Fe-oxidizing atmosphere to rapidly generate Fe oxide on the surface at an oxidation rate equal to or higher than a predetermined value, thereby performing surface selective oxidation of the additive element on the surface of the steel sheet. A method has been proposed in which the wettability of the surface of a steel sheet with molten iron is improved by suppressing it and subsequently heating it in a Fe-reducing atmosphere to reduce the Fe oxide.
Further, in Patent Document 3, a water-based acidic liquid composition containing a hydroxy acid compound and, if necessary, a water-soluble Fe-containing substance and a nitric acid substance are brought into contact with a steel sheet to precipitate a thin film mainly composed of Fe oxide. A method has been proposed in which surface selective thickening of Si and Mn is suppressed and plating wettability is improved by baking in a Fe-reducing atmosphere.

Japanese Patent No. 39565550 Japanese Patent No. 2587724 Japanese Patent No. 5672127

However, each of the above-mentioned conventional techniques has the following problems.
In the method of Patent Document 1, when the amount of alloying elements added is large, oxides are formed again on the surface during reannealing, and the plating adhesion may deteriorate even if appearance defects such as non-plating do not occur. Further, in the case of steel containing high Si, since the formed oxide after annealing is inactive to acid, it may not be sufficiently removed, and the upper limit of the applicable amount of Si is relatively 0.80% by mass. Low. Further, there is a problem that the productivity is low because the annealing step is required twice.
Further, the method of Patent Document 2 cannot be applied to a CGL that does not have equipment capable of heating in a Fe-oxidizing atmosphere, and a great cost is required for new installation. Further, since the method is to obtain Fe oxide by oxidizing the base steel sheet itself, in the case of steel containing high Mn, Mn is dissolved in Fe oxide, and Mn oxide is easily formed on the surface of the steel sheet during reduction annealing. Therefore, a sufficient effect may not be obtained.

Further, the method of Patent Document 3 does not require equipment capable of heating in a Fe-oxidizing atmosphere, but since amorphous Fe oxide is rapidly precipitated when the liquid film on the surface of the steel sheet dries. There is a problem that the Fe oxide adhesion amount becomes non-uniform due to the difference in the precipitation rate in the steel sheet surface, and the plating appearance becomes uneven. In addition, since hydroxy acids have a carboxyl group and a hydroxy group in the molecule, a polymerization reaction due to an ester bond is likely to occur between the molecules depending on the bath temperature of the treatment liquid, and it is industrially difficult to control the bath composition. There is a risk of becoming.
Further, the deterioration of the plating adhesion due to the oxide on the surface of the high-strength steel sheet may occur in plating other than hot-dip galvanizing.

Therefore, an object of the present invention is to solve the above-mentioned problems of the prior art and to produce a plated steel sheet using a high-strength steel sheet to which Si and Mn are added as solid solution reinforcing elements as a base material. It is an object of the present invention to provide a manufacturing method capable of effectively suppressing selective oxidation of Si and Mn in heat annealing before a plating treatment such as, and improving the surface appearance and plating adhesion of a plated steel sheet. Another object of the present invention is to provide an aqueous treatment agent for solution treatment capable of effectively suppressing selective oxidation of Si and Mn in heat annealing before plating.

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 Fe complexes, acetic acid and acetic acid having an acetylacetone or an acetylacetone derivative as a ligand with respect to the steel sheet in advance. By subjecting to a solution treatment to attach an aqueous solution containing nitric acid at a predetermined concentration and then heating and annealing in a Fe-reducing atmosphere, selective oxidation of Si and Mn in this heating annealing is effectively suppressed, and the plated steel sheet is plated. It was found that the surface appearance and plating adhesion of the steel sheet were improved.

The present invention has been made based on such findings, and has the following gist.
[1] A method for producing a high-strength plated steel sheet using a steel sheet containing Si or / and Mn as a base material.
An aqueous solution containing acetylacetone or an Fe complex having an acetylacetone derivative as a ligand on the surface of a steel plate at a concentration of 0.10% 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. And the solution treatment process to attach
The steel sheet after the solution treatment step, H ₂ concentration is 0.05 vol% or more, and annealing steps which dew point is heated at 700 ° C. or higher in a less than 10 ° C. reducing atmosphere,
A method for producing a high-strength plated steel sheet, which comprises a plating process for plating a steel sheet that has undergone the annealing step.
[2] Production of a high-strength galvanized steel sheet according to the above-mentioned production method of [1], wherein the steel sheet contains Si: 0.10% or more and / and Mn: 0.50% or more in mass%. Method.

[3] In the production method of the above [1] or [2], the amount of Fe adhered to the surface of the steel sheet by the aqueous solution adhered in the solution treatment step is 0.1 to 10.0 g / m ^2. Manufacturing method of plated steel sheet.
[4] In any of the above-mentioned production methods [1] to [3], production of a high-strength galvanized steel sheet characterized by introducing a steel sheet having an aqueous solution adhered to its surface through a solution treatment step into an annealing step as it is. Method.
[5] In any of the above-mentioned production methods [1] to [3], high-strength plating is characterized in that 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. Steel sheet manufacturing method.
[6] In any of the above-mentioned manufacturing methods [1] to [5], the plating treatment step includes a step of hot-dip galvanizing a steel sheet that has undergone an annealing step (provided that it is alloyed after hot-dip galvanizing). .) A method for manufacturing a high-strength plated steel sheet.
[7] In the production method of the above [6], a method for producing a high-strength plated steel sheet, characterized in that a solution treatment step, an annealing step, and a plating treatment step are continuously performed in a continuous hot-dip galvanizing 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 plated steel sheet, which comprises a component composition composed of impurities.
[9] In the production method of the above [8], the steel sheet is further increased in mass% by Ti: 0.010 to 0.100%, Nb: 0.010 to 0.100%, B: 0.0001 to 0. A method for producing a high-strength galvanized 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 galvanized steel sheet, which comprises one or more selected from the above.
[11] An aqueous treatment agent for solution treatment for suppressing selective oxidation of easily oxidizing elements during annealing of steel sheets.
It is characterized by being composed of an aqueous solution containing an Fe complex having acetylacetone or an acetylacetone derivative as a ligand at a concentration of 0.10% 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.

According to the present invention, in the production of a plated steel sheet using a high-strength steel sheet containing Si and Mn, which are easily oxidizing elements, as a base material, selective oxidation of Si and Mn in heat annealing before plating is effectively suppressed. It is possible to produce a high-strength galvanized steel sheet having a good surface appearance and excellent plating adhesion. By applying a high-strength plated steel sheet such as a high-strength hot-dip galvanized steel sheet manufactured 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 plated steel sheet using a steel sheet containing Si and / and Mn, which are easily oxidizing elements, as a base material, and has an acetylacetone or an acetylacetone derivative as a ligand on the surface of the steel sheet. A solution treatment step (A) for adhering an aqueous solution (x) containing an Fe complex, nitric acid and acetic acid at a predetermined concentration, and an annealing step (a) in which a steel sheet that has undergone this solution treatment step (A) is heat-treated in a reducing atmosphere. It has a B) and a plating treatment step (C) for plating the steel sheet that has undergone the annealing step (B).
The preferable conditions of the steel sheet (high-strength steel sheet) used as the base material of the high-strength plated 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 0.10% 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, in the liquid film (sol) of the aqueous solution (x) adhering to the surface of the steel plate, the solvent 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 carried out 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, the wettability of plating can be improved.

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 plating wettability 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 with respect to a metal ion. , Forming 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, for example, tris (2,4-pentandionato) iron (III), tris (2,4-hexanedionat) iron (III), tris (2,4-heptandionat) 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.
Further, in the aqueous solution (x), acetylacetone and its derivative 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 therefore, bath management is relatively easy from the viewpoint of sludge generation and the like.
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.

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 0.10% 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 Fe complex having acetylacetone or an acetylacetone derivative as a ligand in terms of Fe is less than 0.10% by mass, the amount of Fe oxide formed in the heating process becomes insufficient, and a good surface appearance after plating can be obtained. I can't. 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, the above-mentioned liquid film gelation does not occur stably, and the Fe oxide film formed in the heating process is formed. May become non-uniform and a good surface appearance after plating 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. , A good surface appearance after plating 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 nitric acid is about 20% by mass, and the effects are saturated. Therefore, these may be the upper limits. Further, since acetic acid mainly serves as a solvent, there is no upper limit by itself, and the upper limit is determined by the lower limit concentration of the 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 . By setting such an Fe adhesion amount, especially when alloying treatment is performed after hot-dip galvanizing, even for a steel sheet having a large content of Si or the like, which is generally known as an alloying delay element, the plating film has an appropriate Fe concentration. It is possible to lower the alloying temperature required for this. 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 alloying temperature may not be sufficiently reduced. 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 heating 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 plating 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 to be a cause of appearance defects such as non-plating when plating is performed.

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 an excellent plated appearance 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. Such film formation occurs even at room temperature, but 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). In a low temperature range (for example, about 400 ° C.), the liquid film can be formed into a film (Fe oxide film) via a gel-like film. 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), the annealing step (B), and the plating treatment step (C) are continuously performed in the continuous hot-dip galvanizing line, the surface is subjected to the solution treatment step (A). The steel sheet to which the (x) (liquid film) is attached may be directly introduced into the annealing step (B), or the steel sheet to which the aqueous solution (x) is attached to the surface through the solution treatment step (A) may be appropriately heated. After the heat treatment in, it may be introduced into the annealing step (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 filmed in a relatively low temperature region (for example, about 400 ° C.) at the initial stage of heating to form a 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 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, but if the H ₂ concentration is higher than necessary, the cost will increase, so it is preferable to set the upper limit to 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 may 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 poor appearance after plating and deterioration of plating adhesion. 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 according to the material design and other needs. For example, hot dip galvanizing in the subsequent plating process (C). From the viewpoint of bath management and stabilization of plating wettability, it is preferable to control the plate temperature immediately before immersion in the plating bath so as to be close to the bath temperature (for example, about 470 to 500 ° C.).

-Plating process (C)
In this plating process (C), the steel sheet annealed in the annealing step (B) is plated. Since the steel sheet undergoes the solution treatment step (A) and the annealing step (B), selective oxidation of Si and Mn on the surface of the steel sheet is suppressed, and the surface of the steel sheet is uniformly covered with active reduced iron. Deterioration of plating adhesion due to oxide is suppressed.
The type of plating performed in the plating treatment step (C) is not particularly limited, and may be any of hot-dip plating, electroplating, electroless plating, vapor phase plating, etc., and zinc or zinc alloy plating, tin or tin alloy plating, The type of plating component (metal or alloy) such as aluminum or aluminum alloy plating does not matter.

Among the plating treatments, hot-dip galvanizing (including the case of alloying after hot-dip galvanizing) is widely applied to steel sheets for various purposes and is also suitable for steel sheets for automobile parts. , The case where hot-dip galvanizing is performed in the plating treatment step (C) will be described.
After annealing in the annealing step (B), the steel sheet cooled to a predetermined temperature is immersed in a hot-dip galvanizing bath to perform hot-dip galvanizing treatment. As described above, when the steel sheet undergoes the solution treatment step (A) and the annealing step (B), selective oxidation of Si and Mn on the surface of the steel sheet is suppressed, and the surface of the steel sheet is uniformly covered with active reduced iron. Therefore, good wettability of hot-dip zinc can be obtained, and deterioration of plating adhesion due to oxides can be suppressed.
Further, if necessary, alloying treatment may be performed after hot dip galvanizing.
The hot-dip galvanizing conditions are not particularly limited, but the bath temperature of the hot-dip galvanizing bath is preferably about 440 to 550 ° C. If the bath temperature of the hot-dip galvanizing bath is less than 440 ° C., Zn solidifies in the low temperature portion due to temperature fluctuations in the plating bath, which may make the hot-dip galvanizing bath unsuitable. On the other hand, if the bath temperature exceeds 550 ° C, the bath evaporates violently, vaporized Zn may adhere to the inside of the furnace, making operation difficult, and alloying may proceed during plating to result in overalloying. There is.

When the alloying treatment is not performed after the plating treatment, the Al concentration in the hot-dip galvanizing bath is preferably about 0.13 to 0.24% by mass. If the Al concentration in the bath is less than 0.13% by mass, Fe-Zn alloying may proceed and the plating adhesion may deteriorate, while if it exceeds 0.24% by mass, defects due to Al oxide may occur. There is.
When the alloying treatment is performed after the plating treatment, the Al concentration in the hot-dip galvanizing bath is preferably about 0.10 to 0.20% by mass. If the Al concentration in the bath is less than 0.10% by mass, a large amount of Γ phase may be generated due to alloying of the plating film, and the plating adhesion may deteriorate. On the other hand, if it exceeds 0.20% by mass, Fe—Zn alloying may not proceed.

When the alloying treatment is performed after hot-dip galvanizing, the alloying treatment conditions are not particularly limited, but the alloying treatment temperature is preferably more than 460 ° C and less than 600 ° C. When the alloying treatment temperature is 460 ° C. or lower, the progress of alloying is slow, and it takes a long time to sufficiently alloy, which is not efficient. On the other hand, at 600 ° C. or higher, alloying proceeds too much, and a hard and brittle Zn—Fe alloy layer may be excessively formed at the base iron interface to deteriorate the plating adhesion.
When the material of the base steel sheet deteriorates during the alloying treatment, the treatment temperature should be lowered as necessary, for example, more than 460 ° C and less than 560 ° C.
In the manufacturing method of the present invention, as described above, all the steps may be carried out in a continuous facility (for example, a continuous hot-dip galvanizing line), or each step may be carried out independently in a separate and independent facility. Alternatively, some steps may be carried out independently in an independent facility.

Next, the base steel sheet of the plated steel sheet manufactured by 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 base steel sheet to be plated (for example, hot dip galvanized) in 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 composition of the base material steel plate, as basic components, C: 0.040 to 0.500%, Si: 0.10 to 3.00%, 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 if necessary, Ti: 0. One or more selected from 010 to 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. It can contain one or more selected from 0100% or less and REM: 0.010% or less. 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%, even if the present invention is applied, Si forms an oxide on the surface of the steel sheet during annealing, which deteriorates the wettability of hot-dip zinc during plating, resulting in adhesion to plating. There is a risk of reduced sex. 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 the wettability of hot-dip zinc is deteriorated during plating. There is a risk of reduced sex. 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, it may cause embrittlement due to grain boundary segregation, which may deteriorate the impact resistance, and melt. When the alloying treatment is performed after galvanizing, the alloying reaction may be delayed. 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 austenite-forming element and is an element effective for ensuring the strength of the annealed plate. 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 austenite-forming element and is an element effective for ensuring the strength of the annealed sheet. However, if the amount added is too large, an oxide is formed on the surface of the steel sheet during annealing, and the plating appearance May deteriorate. 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 effect 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.

In addition, there are no special restrictions on the manufacturing conditions of the base steel sheet. 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 cold rolling is performed if necessary. The hot rolling conditions, pickling conditions, and cold rolling conditions are not particularly limited. That is, the base steel sheet may be either a hot-rolled steel sheet or a cold-rolled steel sheet.
The aqueous solution (x) used in the solution treatment step (A) of the present invention is an aqueous treatment agent for solution treatment for suppressing selective oxidation of easily oxidizing elements during quenching of a steel plate, and contains an acetylacetone derivative. An aqueous treatment agent for solution treatment, which comprises an aqueous solution containing an Fe complex contained in a position at a concentration of 0.10% 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).

A 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, an annealing step, and a plating treatment step in sequence under the conditions shown in Tables 4 to 7. Each step was carried out using an experimental facility in which each was carried out independently.
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 adjusted by changing the liquid film thickness (the liquid film thickness was adjusted by adjusting the roll gap and the roll peripheral speed of the roll coater) or the Fe concentration in the aqueous solution. 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.
In the plating process, the steel sheet was hot-dip galvanized or electrogalvanized. Hot-dip galvanizing was performed in a hot-dip galvanizing bath having an Al concentration of 0.132% by mass in the bath. Further, in the _{electro-galvanized,} and the aqueous solution of pH2.5 to a ZnSO 4 · _7H 2 O were dissolved 440 g / L the plating solution, current density 100A / dm ² in the cathodic electrolysis treatment 10 seconds.
In addition, a part of the hot-dip galvanized steel sheet was continuously alloyed. In this alloying treatment, the holding time was set to 20 seconds, and the temperature at which the Fe concentration in the plating film reached 10% by mass was defined as the alloying treatment temperature. The Fe concentration in the plating film was measured by inductively coupled plasma emission spectroscopy.

The surface appearance and plating adhesion of the hot-dip galvanized steel sheet (GI), alloyed hot-dip galvanized steel sheet (GA), and electrogalvanized steel sheet (EG) obtained as described above were evaluated by the following methods.
-Surface appearance The presence or absence of appearance defects such as non-plating, unevenness, pinholes, and cracks was visually judged, and evaluation was performed according to the following criteria, and ○ and Δ were passed.
〇: No non-plating, unevenness, pinholes, or cracks are observed.
Δ: No non-plating, pinholes, or cracks were observed, but slight unevenness was observed.
X: One or more of non-plating, clear unevenness, pinholes, and cracks are observed.

-Evaluation of galvanized steel sheet (GI) and electrogalvanized steel sheet (EG) plating adhesion (GI, EG adhesion): Ball impact test Plating adhesion of hot-dip galvanized steel sheet (GI) and electrogalvanized steel sheet (EG) To evaluate the properties, a ball impact test is used, and after peeling the cellophane tape (registered trademark; the same applies hereinafter), the processed part is evaluated according to the following criteria by visually determining the presence or absence of peeling of the plating layer, and ○ and △ are passed. did. In this test, the ball mass was 1.8 kg and the drop height was 100 cm.
◯: No peeling of the plating layer Δ: Slight peeling of the plating layer ×: Non-slight peeling of the plating layer

-Plating adhesion (GA plating adhesion) of alloyed hot-dip galvanized steel sheet (GA): powdering test The plating adhesion of alloyed hot-dip galvanized steel sheet (GA) was evaluated by the powdering resistance. Specifically, cellophane tape is attached to the alloyed hot-dip zinc-plated steel plate, the tape surface is bent and bent back 90 degrees, and the cellophane with a width of 24 mm is placed inside the processed portion (compression processed side) in parallel with the bent portion. The tape is pressed and pulled apart, the amount of zinc adhering to the 40 mm length part of the cellophane tape is measured as the Zn count number by fluorescent X-ray, and the amount obtained by converting the Zn count number per unit length (1 m) is as follows. Ranked according to the criteria. Ranks 1 and 2 were good (◯), rank 3 was generally good (Δ), rank 4 and above were bad (x), and ○ and Δ were passed.
Rank 1: X-ray fluorescence count is 0 or more and less than 2000 Rank 2: X-ray fluorescence count is 2000 or more and less than 5000 Rank 3: X-ray fluorescence count is 5000 or more and less than 8000 Rank 4: X-ray fluorescence count Is 8000 or more and less than 10000 Rank 5: Fluorescent X-ray count is 10000 or more

The above results are shown in Tables 4 to 7 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 7 is water.
According to Tables 4 to 7, it can be seen that the high-strength plated steel sheets of the examples of the present invention are all excellent in surface appearance and plating adhesion. On the other hand, the high-strength plated steel sheet of the comparative example is inferior in surface appearance, plating adhesion, or both.

Claims (11)

A method for producing a high-strength plated steel sheet using a steel sheet containing Si or / and Mn as a base material.
An aqueous solution containing acetylacetone or an Fe complex having an acetylacetone derivative as a ligand on the surface of a steel plate at a concentration of 0.10% 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. And the solution treatment process to attach
The steel sheet after the solution treatment step, H ₂ concentration is 0.05 vol% or more, and annealing steps which dew point is heated at 700 ° C. or higher in a less than 10 ° C. reducing atmosphere,
A method for producing a high-strength plated steel sheet, which comprises a plating process for plating a steel sheet that has undergone the annealing step. The method for producing a high-strength plated steel sheet according to claim 1, wherein the steel sheet contains Si: 0.10% or more and / and Mn: 0.50% or more in mass%. The method for producing a high-strength plated steel sheet according to claim 1 or 2, 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 . The method for producing a high-strength plated steel sheet according to any one of claims 1 to 3, wherein the steel sheet to which the aqueous solution is adhered to the surface through the solution treatment step is directly introduced into the annealing step. The method for producing a high-strength galvanized steel sheet according to any one of claims 1 to 3, wherein 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. The method according to any one of claims 1 to 5, wherein the plating treatment step is a step of hot-dip galvanizing a steel sheet that has undergone an annealing step (provided that the alloying treatment is performed after hot-dip galvanizing). High-strength plated steel sheet manufacturing method. The method for producing a high-strength plated steel sheet according to claim 6, wherein the solution treatment step, the annealing step, and the plating treatment step are continuously performed in the continuous hot-dip galvanizing 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. Method for manufacturing high-strength plated 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 plated steel sheet according to claim 8, wherein the high-strength plated steel sheet is contained. Further, the steel sheet is, in terms of mass%, Mo: 0.01 to 0.50%, Cr: 1.00% or less, Ni: 0.50% or less, Cu: 1.00% or less, V: 0.500%. 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 galvanized steel sheet according to Item 8. An aqueous treatment agent for solution treatment for suppressing selective oxidation of easily oxidizing elements during annealing of steel sheets.
It is characterized by being composed of an aqueous solution containing an Fe complex having acetylacetone or an acetylacetone derivative as a ligand at a concentration of 0.10% 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.