JP2014240505A - Cold rolled sheet steel excellent in chemical convertibility - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cold rolled sheet steel having excellent chemical convertibility, especially even in an extra low carbon system, without being restricted by the composition the steel sheet.SOLUTION: In a cold rolled sheet steel in which oxides adhere onto the surface discontinuously so as to be distributed, when the ratio of the oxide area to a unit area of the cold rolled sheet steel surface in a plane view is defined as an oxide area ratio M(%), and the mean diameter of the oxides in a plane view is defined as an oxide mean diameter D(μm), M and D satisfy following formulas (1) and (2): 5≤M≤50 (1), 0.005×M≤D≤0.35 (2). The oxides may be, for example, oxides containing Si.

Description

  The present invention relates to a cold-rolled steel sheet excellent in chemical conversion treatment, and particularly relates to a cold-rolled steel sheet excellent in chemical conversion treatment and made of ultra-low carbon steel.

  Cold-rolled steel sheets are used for various purposes in a wide range of fields such as automobiles, home appliances, and building materials. In many cases, a cold-rolled steel sheet is used by coating its surface. In this case, the surface of the cold-rolled steel sheet is usually subjected to a chemical conversion treatment (typically, a phosphate treatment) that is a base treatment for coating. By the chemical conversion treatment, a chemical conversion crystal film is formed, and this chemical conversion crystal film provides corrosion resistance and adhesion to the coating film formed thereon. For this reason, a cold-rolled steel sheet is required to have good chemical conversion treatment (phosphate treatment), that is, to form a good chemical conversion crystal film in a short time.

  By the way, the application range of galvanized steel sheets has been expanded, and accordingly, there is a problem that the formation rate of the chemical conversion film is inferior in some cold-rolled steel sheets compared to the chemical conversion rate for galvanized steel sheets. It has become. When such a cold-rolled steel sheet is used in comparison with a galvanized steel sheet and a cold-rolled steel sheet, if an attempt is made to grow a good conversion crystal film, the productivity is lowered and the conversion crystal film is grown in a short time. If it tries to do so, a chemical conversion crystal film having a desired quality cannot be formed, and the corrosion resistance and the adhesion to the coating film are lowered.

  Although the chemical conversion treatment conditions such as the composition of the chemical conversion treatment liquid influence the chemical conversion treatment properties of the steel sheet, the properties of the steel sheet itself often have a great influence on the chemical conversion treatment performance. In particular, in the case of a cold-rolled steel sheet made of an ultra-low carbon steel having a C content reduced to 0.01% by mass or less, a slow growth rate of chemical crystals often becomes a problem.

Examples of conventional techniques related to chemical conversion properties of cold-rolled steel sheets include the following.
For example, Patent Document 1 describes that a cold-rolled steel sheet having residual strain on the surface layer of the steel sheet and having a predetermined amount of cementite precipitated in the steel has good chemical conversion properties. However, the steel plate composition to which this technology can be applied is limited, and it is difficult to apply this technology to extremely low carbon steel plates.

  Patent Document 2 discloses, as a method for producing an ultra-low carbon cold-rolled steel sheet, at least one selected from the group consisting of a surface-active organic compound having an acid group and a metal salt and an esterified derivative thereof before annealing. It is described that an extremely low carbon cold-rolled steel sheet excellent in chemical conversion property can be produced by immersing the steel sheet with a solution containing the organic compound. However, this method is not necessarily effective enough to meet the recent demands of automobile manufacturers for ensuring chemical conversion properties equivalent to those of galvanized steel sheets.

  On the other hand, Patent Document 3 proposes a technique for improving chemical conversion property by thinly plating Ni on the surface of a cold-rolled steel sheet after annealing, and this technique is widely adopted. However, in an ultra-low carbon steel plate, even if Ni is plated, sufficient chemical conversion property cannot always be secured. In addition, when Ni is plated, the surface of the steel sheet is activated and the affinity with the oily agent contained in the rust-preventing oil is increased, so that the surface of the steel sheet repels water in the degreasing and cleaning process before chemical conversion treatment. There has also been pointed out the problem of poor cleaning. Furthermore, in the case of box annealing, annealing and Ni plating cannot be performed in-line.

  In the case of targeting high-tensile steel plates, Patent Documents 4 and 5 describe techniques for ensuring chemical conversion properties by controlling the surface concentration state of Si and Mn contained in steel. These techniques are based on the technical idea of suppressing deterioration of chemical conversion treatment due to the concentration of oxides by suppressing or preventing the concentration of Si and Mn in the steel as oxides on the surface. However, the techniques of Patent Documents 4 and 5 have limitations on applicable steel sheet compositions. For example, the C content of steel is limited to more than 0.1 mass% and 0.01 to 0.3 mass% in Patent Documents 4 and 5, respectively.

JP-A-8-134536 JP-A-9-3534 JP-A-57-2889 JP 2004-323969 A JP 2008-121045 A

  The present invention has been made in view of the above problems, and is not limited by the composition of the steel sheet, and even in the case of an extremely low carbon steel, in particular, good chemical conversion treatment property equivalent to that of a zinc-based plated steel sheet. It aims at providing the cold-rolled steel plate which shows.

In order to solve the above problems, the present inventors have studied in detail the process of forming a film by chemical conversion treatment. In zinc phosphate treatment, which is a typical chemical conversion treatment, it is considered that a zinc phosphate coating is deposited by the following reaction.
[Anode reaction]
Fe → Fe ²⁺ + 2e ^-
[Cathode reaction]
10H ⁺ + NO ₃ ⁻ + 8e ⁻ → NH ₄ ⁺ + 3H ₂ O
_{^{2H + + 2e - → H 2}} ↑
[Film deposition reaction]
2H ₂ PO ₄ ⁻ + 2Zn ²⁺ + Fe ²⁺ + 4H ₂ O
_{→ Zn 2 Fe (PO 4)} 2 · 4H 2 O ( phosphophyllite) + 4H ⁺

In these reactions, the etching reaction occurs in the portion where the metal Fe is exposed on the surface of the steel sheet, and as a result, H ⁺ is consumed as a cathode reaction, and the portion near the interface with the steel plate in the chemical conversion treatment liquid. PH increases. As a result, the phosphate becomes insoluble in the chemical conversion treatment liquid of the portion, and phosphate crystals (phosphophyllite) precipitate as a solid phase and grow.

  The ultra-low carbon steel sheet is close to pure iron in that the amount of additive elements is small, and the exposure rate of metal Fe on the steel sheet surface is considered to be large compared to low-carbon steel sheets and high-tensile steel sheets containing more additive elements. It is done. For example, in the case of a high-strength steel plate, Si oxide and Mn oxide are generated on the surface layer of the steel plate. Therefore, the exposure ratio of metal Fe is reduced accordingly, and etching of Fe is hindered. For this reason, if "the higher the exposure ratio of metallic Fe on the steel sheet surface, the higher the rate of Fe etching reaction (anode reaction) and the higher the growth rate of chemical conversion crystals", It is considered that the growth rate of chemical crystals increases with an ultra-low carbon steel plate as compared with a tensile steel plate.

  However, in reality, the growth rate of chemical crystals is slower in ultra-low carbon steel plates than in low-carbon steel plates and high-tensile steel plates. As an example, if the chemical conversion treatment is performed under standard chemical conversion treatment conditions and the chemical conversion treatment reaction is completed when the increase in the amount of coating adhered to the processing time is saturated, the chemical conversion treatment is performed in about 60 seconds for a galvanized steel sheet. Whereas the reaction is completed, the chemical conversion treatment reaction is completed in about 120 seconds for the ultra-low carbon steel sheet.

For this reason, the present inventors have stated that, in the case of an ultra-low carbon steel sheet, since it is close to pure iron, there are few cathode reaction sites, and H ⁺ is not easily consumed. It takes time for the pH in the vicinity to increase, which slows the growth reaction of the chemical crystals. " Thus, according to the model of the present inventors, in the ultra-low carbon steel plate, the growth rate of the zinc phosphate crystal is not slow because the exposure rate of metal Fe on the steel plate surface is small.

  Therefore, the present inventors proceeded with investigations so that the surface of the ultra-low carbon steel sheet is in a state where the cathode reaction is likely to occur. The present invention has been completed based on the knowledge obtained by such studies, and the gist thereof is the following cold-rolled steel sheet.

A cold-rolled steel sheet with oxides dispersed and adhered to the surface,
The area ratio of the oxide per unit area when the steel sheet surface is observed by SEM is M (%), the average diameter of the oxide is D (μm), the oxide area ratio M and the oxide average A cold-rolled steel sheet, characterized in that the diameter D satisfies the relationship of the following formulas (1) and (2).
5 ≦ M ≦ 50 (1)
0.005 × M ≦ D ≦ 0.35 (2)

The oxide may be an oxide containing Si.
The amount of Si contained in the cold-rolled steel sheet may be% by mass and Si: 0.1% or less.
The amount of C contained in the cold-rolled steel sheet may be% by mass, and C: 0.01% or less. The amount of Mn contained in the cold-rolled steel sheet is% by mass, and Mn: 0.5 It may be the following.

The cold-rolled steel sheet of the present invention has good chemical conversion properties.
When this cold-rolled steel sheet is manufactured, a product containing Si as the oxide is obtained by degreasing and cleaning using a cleaning liquid containing a silicate alkali degreasing agent. Such a manufacturing method can be implemented at low cost. In other words, the cold-rolled steel sheet according to the present invention, in which the oxide is an oxide containing Si, can be reduced in cost.

  If the Si content of the cold-rolled steel sheet is 0.1% by mass or less, it is easy to obtain a cold-rolled steel sheet whose surface satisfies the relationship of the above formulas (1) and (2).

  In general, it is difficult to perform a chemical conversion treatment satisfactorily on a cold-rolled steel sheet having a mass%, a C content of 0.01% or less, and an Mn content of 0.5 or less. In the present invention, a chemical conversion treatment can be satisfactorily performed even on a cold-rolled steel sheet having such a composition.

FIG. 1 (a) is an AsB image, and FIG. 1 (b) is obtained based on the AsB image of FIG. 1 (a). It is a binarized image. FIG. 2 (a) is an AsB image, and FIG. 2 (b) is based on the AsB image in FIG. 2 (a). It is the obtained binarized image. No. in Table 2 2 is an AsB image of the surface of a cold-rolled steel sheet annealed under condition 1. No. in Table 2 2 is an AsB image of the surface of a cold-rolled steel sheet annealed under the condition 2; No. in Table 2 3 is an AsB image of the surface of a cold-rolled steel sheet annealed under condition 3. No. in Table 2 4 is an AsB image of the surface of a cold-rolled steel sheet annealed under condition 4. No. in Table 2 1 is an SEM image of a cold-rolled steel sheet that has been annealed and subjected to chemical conversion treatment under condition 1. No. in Table 2 It is a SEM image of the cold-rolled steel plate which annealed on the conditions of 2 and gave the chemical conversion treatment. No. in Table 2 3 is an SEM image of a cold-rolled steel sheet that has been annealed and subjected to chemical conversion treatment under the condition 3; No. in Table 2 4 is an SEM image of a cold-rolled steel sheet annealed and subjected to chemical conversion treatment under the condition of No. 4. It is a SEM image of the galvanized steel plate which carried out the chemical conversion treatment in the same time as the chemical conversion treatment with respect to a cold-rolled steel plate. It is a figure which shows the relationship between an oxide area ratio, an oxide average diameter, and the quality of chemical conversion property. It is a figure which shows the relationship between soaking time and soaking temperature, and the quality of chemical conversion treatment.

1. Cold-rolled steel sheet of the present invention As described above, the cold-rolled steel sheet of the present invention is a cold-rolled steel sheet in which oxides are dispersed and adhered to the surface,
The area ratio of the oxide per unit area when the steel sheet surface is observed by SEM is M (%), the average diameter of the oxide is D (μm), the oxide area ratio M and the oxide average The cold rolled steel sheet is characterized in that the diameter D satisfies the relationship of the following formulas (1) and (2).
5 ≦ M ≦ 50 (1)
0.005 × M ≦ D ≦ 0.35 (2)

  When the oxide area ratio M and the average oxide diameter D satisfy the relations of the above formulas (1) and (2), the surface of the cold-rolled steel sheet is in a state in which the anode reaction and the cathode reaction are likely to occur and is favorable. It can be converted into a chemical.

1-1. About the oxide adhering to the surface of the cold-rolled steel sheet The oxide area ratio M is the ratio of the oxide adhering to the surface of the cold-rolled steel sheet covering the surface of the cold-rolled steel sheet as a base. It becomes an indicator to show. This oxide provides the cathode site, and the cold-rolled steel sheet surface mainly provides the anode site. When the oxide area ratio M decreases, the cathode sites decrease and the rate of the cathode reaction described above decreases. On the other hand, when the oxide area ratio M increases, the anode sites decrease and the rate of the anode reaction described above decreases. Becomes slower. In order for the above-described film deposition reaction to proceed rapidly, it is necessary that neither the anodic reaction nor the cathodic reaction be slower than a certain level. For this purpose, it is necessary for the oxide area ratio M to fall within the range defined by the above equation (1).

  Even when the oxide area ratio M satisfies the relationship of the above formula (1), if the oxide average diameter D is outside the range defined by the above formula (2), the formation of the cold-rolled steel sheet is performed. The processability is inferior. When the average oxide diameter D is too large (0.35 <D), the oxide locally covers the steel sheet surface over a wide range. On the other hand, when the average oxide diameter D is too small (D <0.005 × M), the ratio of fine oxides that cannot be recognized to oxides that can be recognized as particles increases, and such fine oxides It is considered that the anode site is widely covered by. In any case, it is presumed that the efficient film deposition reaction is hindered and the chemical conversion property is lowered.

  When satisfy | filling the relationship of the said (1) Formula and (2) Formula, in many cases, most oxides are granular.

  As shown in the examples described later, it was confirmed that the above-described effects of the present invention can be obtained when the oxide contains Si. This Si-containing oxide is not limited to the one consisting essentially of Si and O. For example, Mn and Al, which are steel components, or an alkali component resulting from a degreasing agent may be included. .

  The present invention is based on the assumption that Si-containing oxides provide cathode sites on the steel sheet surface. Even if the oxide on the surface of the steel plate does not contain Si, it is considered that the cathode site is provided. Therefore, in the present invention, the oxide on the surface of the steel plate is not limited to that containing Si.

  Next, the procedure for calculating the oxide area ratio M and the average oxide diameter D will be described by taking as an example the case where the oxide contains Si. The oxide area ratio M can be obtained, for example, by the following procedures 1) to 3).

  1) Using an FE-SEM (Field Emission Scanning Electron Microscope) with an AsB (Angle Selective Backscattered Electron) detector, an arbitrary region on the surface of the steel sheet is, for example, 5000 Shoot with an AsB detector at double magnification.

  You may confirm that the deposit | attachment on the steel plate surface contains Si as needed. For example, if the Fe-SEM is equipped with an EDX (Energy-Dispersive X-ray) analyzer, the deposit on the steel plate is an oxide containing Si by the EDX analyzer. It can be confirmed whether or not.

2) Select any five regions in the AsB image obtained in 1) above. For each region, the brightness of each point constituting the AsB image is made to correspond to either the oxide containing Si or the cold-rolled steel sheet surface (hereinafter referred to as “iron”) that forms the base of the oxide. Thus, an image represented by binarization (hereinafter referred to as “binarized image”) is obtained. Each region is, for example, a rectangular region having an area of 28 μm ² (7 μm × 4 μm).

  In the AsB image, since the atomic number of the atoms constituting the observation object greatly affects the luminance of the image, the luminance of Si-containing oxide and Fe are greatly different from each other. Can be identified. Therefore, by binarization based on the luminance of each point of the AsB image, an oxide region containing Si and a region where the steel plate is exposed can be identified.

  However, in the AsB image, the luminance may be greatly different in a continuous region corresponding to the oxide containing Si, or the luminance of the grain boundary of the iron base may be comparable to the luminance of the oxide containing Si. . In these cases, even if binarization is performed using a specific luminance as a threshold value, the oxide containing Si and the iron ground cannot be accurately identified. In order to perform accurate identification, the region occupied by the oxide containing Si may be specified by visual judgment. For example, a transparent sheet may be overlaid on the AsB image, and a binarized image may be obtained by painting a region that is determined to correspond to an oxide containing Si on the sheet with an oil-based marking pen or the like.

  1 and 2 show examples of AsB images and binarized images. FIG. 1 is an image obtained on the surface of a cold-rolled steel sheet having a good chemical conversion treatment. FIG. 1 (a) is an AsB image, and FIG. 1 (b) is an AsB image of FIG. 1 (a). It is the binarized image obtained based on. FIG. 2 is an image obtained with respect to the surface of a cold-rolled steel sheet with poor chemical conversion treatment, FIG. 2 (a) is an AsB image, and FIG. 2 (b) is an AsB of FIG. 2 (a). It is the binarized image obtained based on the image. In the binarized images of FIG. 1B and FIG. 2B, the region corresponding to the iron ground and the region corresponding to the oxide containing Si are represented in black and white, respectively.

  3) From the binarized image, the area of each region corresponding to the oxide containing Si and separated from each other is calculated. Further, based on the calculated area of each region, a diameter corresponding to a circle of each region is calculated.

  The calculation of the area and diameter is performed for each of the five regions selected from the above AsB image. For each region, the area of the region corresponding to the oxide containing Si is summed, and further, a value obtained by summing the total area for the five regions is obtained, and the total value with respect to the total area of the five regions is calculated. The ratio is an oxide area ratio M (%). Moreover, let the average value of the equivalent circle diameter of all the particles in the said 5 area | region be oxide average diameter D (micrometer).

  Calculation of the oxide area ratio M and the average oxide diameter D from the binarized image can be performed using commercially available image analysis software.

Here, even if it is determined that the micro area having an area of less than 0.001 μm ² corresponds to the area containing Si due to the luminance, such a micro area is a foreign object or a fine edge of the iron ground. This is likely due to halation in Japan. Further, even if the minute region is an oxide containing Si, the minute region may not substantially provide a cathode site (contribute as a starting point of the degassing reaction). For this reason, a micro region having an area of less than 0.001 μm ² is excluded from the calculation of the total area of the region corresponding to the oxide containing Si and the average oxide diameter D.

  In the cold rolled steel sheet of FIG. 1, the oxide area ratio M is 28.87%, and the average oxide diameter D is 0.164 μm. In the cold rolled steel sheet of FIG. 2, the oxide area ratio M is 25.24%, and the average oxide diameter D is 0.113 μm. Each of the cold-rolled steel sheets in FIGS. 1 and 2 satisfies the relationship of the expression (1). On the other hand, the relationship of the formula (2) satisfies the cold-rolled steel sheet of FIG. 1, but does not satisfy the cold-rolled steel sheet of FIG.

1-2. Composition of Cold Rolled Steel Sheet In the present invention, the composition of the cold rolled steel sheet is not particularly limited. However, when the Si content of the cold-rolled steel sheet is 0.1% by mass or less, a cold-rolled steel sheet whose surface satisfies the relationship of the above formulas (1) and (2) is easily obtained by the manufacturing method described later.

  Further, when the conventional cold-rolled steel sheet is mass%, the C content is 0.01% or less, and the Mn content is 0.5% or less, it was not possible to obtain good chemical conversion treatment properties. However, even if the cold-rolled steel sheet of the present invention has such a composition (when it is an extremely low carbon steel), good chemical conversion treatment can be obtained. The influence of C and Mn in the cold-rolled steel sheet on chemical conversion treatment is as follows.

  When the C content of the cold-rolled steel sheet exceeds 0.01%, cementite precipitates and tends to appear on the surface of the cold-rolled steel sheet. Cold-rolled steel sheets with cementite appearing on the surface often have good chemical conversion properties even when not according to the present invention.

  Mn is likely to be concentrated on the surface of the cold-rolled steel sheet during annealing. In this case, while preventing the etching reaction of Fe, MnS is formed in the cold-rolled steel sheet, and this is considered to act similarly to the above-mentioned cementite. Actually, when the content ratio of Mn is 0.5% by mass or more, Mn can contribute to the improvement of chemical conversion treatment. Thus, when the content rate of Mn is high, even if it is not based on this invention, favorable chemical conversion treatment property may be obtained.

  Therefore, if it is difficult to obtain good chemical conversion treatment properties according to the present invention, the contents of C and Mn in the cold-rolled steel sheet are 0.01% or less and 0.5% or less, respectively, in mass%. In this case, the present invention is particularly effective.

2. The manufacturing method of the cold-rolled steel plate of this invention Although the manufacturing method of the cold-rolled steel plate of this invention is not specifically limited, Hereinafter, a suitable manufacturing method is demonstrated. In the manufacturing method described below, a cold-rolled steel sheet obtained by cold rolling a steel sheet is degreased and cleaned using a degreasing agent, and then batch-annealed (box annealing). Thereby, granular Si oxide is formed on the surface of the cold rolled steel sheet.

2-1. Degreasing process Generally, in the manufacturing process of a cold-rolled steel sheet, the surface of the steel sheet is degreased and cleaned using a cleaning liquid containing a degreasing agent after cold rolling and before annealing. Examples of the degreasing method include supplying the cleaning liquid to the surface of the cold rolled steel sheet by spraying, immersing the cold rolled steel sheet in the cleaning liquid, and electrolytic degreasing of the cold rolled steel sheet in the cleaning liquid. Such a degreasing cleaning is also performed in a preferred method for producing the cold-rolled steel sheet of the present invention. After the degreasing cleaning, the cold-rolled steel sheet may be dried as it is, or may be dried after being washed with water.

An aqueous solution of a silicic acid-based alkaline degreasing agent is used as the cleaning liquid used in the degreasing step. As will be described later, when such a degreasing component remains on the surface of the cold-rolled steel sheet, a cold-rolled steel sheet having an Si-containing oxide adhered to the surface is obtained. The degreasing agent, in particular, sodium orthosilicate _{(2Na 2 O · SiO 2 ·} xH 2 O) is preferred. Sodium orthosilicate is softened, fluidized, and granulated at the temperature of the annealing step described below, and easily forms an oxide that satisfies the relationship of the above formula (2), while low temperature (for example, 60 ° C. or lower). Due to its high viscosity, it easily remains on the steel sheet surface and provides a silicate component. In the case of using a sodium orthosilicate aqueous solution as the cleaning liquid, it has been confirmed that the components resulting from the sodium orthosilicate remain to some extent on the steel sheet surface even after degreasing and washing with a normal electrolytic cleaning line.

2-2. Annealing process The cold-rolled steel sheet that has undergone the above degreasing process is set so that the soaking temperature H (° C.) and the soaking time T (hr) satisfy the relationship of the following formula (3) in an annealing furnace in a reducing atmosphere. , Anneal.
(H-650) × (T-10) ≧ 200 (3)

  The reducing atmosphere gas in the annealing furnace is a mixed gas having a hydrogen concentration of 40% or less and the balance being substantially inert to the cold-rolled steel sheet.

Here, the presumed mechanism in which the oxide containing granulated Si is formed in the surface of a cold-rolled steel plate is demonstrated. Hereinafter, regarding the composition of the atmospheric gas during annealing, only the hydrogen concentration is described, and the remainder of hydrogen is assumed to be an inert gas with respect to the cold-rolled steel sheet. An example of such an inert gas is nitrogen (N ₂ ).

  The silicate alkali degreasing agent becomes a glassy fluid at a temperature of 600 to 650 ° C. or higher. The soaking temperature H varies depending on the composition of the cold-rolled steel sheet and the mechanical properties required for the cold-rolled steel sheet, but in most cases, it is set to a temperature higher than 650 ° C. (for example, 700 ° C. to 1000 ° C.). Therefore, it is thought that the component resulting from sodium orthosilicate adhering to the steel plate surface as described above is in a fluid state.

  The component (fluid) in a fluid state is considered to be granulated when the wettability with respect to the steel sheet surface is high when the wettability with respect to the steel sheet surface is high while the wettability with respect to the steel sheet surface is low. When annealing is completed in a state where the fluid is wet and spread, the oxide containing Si becomes a film, that is, a state in which the steel sheet surface is continuously covered over a wide area, thereby inhibiting the anode reaction during the chemical conversion treatment. As a result, the chemical conversion processability decreases. When the oxide containing Si becomes granular, as described above, good chemical conversion processability is obtained.

  When a cold-rolled steel sheet is annealed in a highly reducing atmosphere (for example, an atmosphere having a hydrogen concentration of 100%), the cold-rolled steel sheet becomes one in which oxide is continuously deposited over a wide area, and a good chemical conversion treatment is performed. Does not have sex. On the other hand, if a cold-rolled steel sheet is annealed in an atmosphere with a low reducibility, that is, an atmosphere that is reducible with respect to Fe but slightly oxidizing with respect to Si and Mn, The wettability of the fluid with respect to the surface of the rolled steel sheet is lowered. In such an atmosphere, it can suppress that Si and Mn concentrate on the steel plate surface. If Si or Mn is concentrated on the surface of the steel sheet and an oxide film of Si or Mn is formed on the surface of the steel sheet, the wettability of the fluid with respect to the surface of the steel sheet increases, and the fluid is considered not to be granulated. .

  Therefore, the annealing atmosphere is weakly reducing, specifically, the hydrogen concentration in the atmosphere is 40% or less. Thereby, surface concentration of Si and Mn can be suppressed. Regarding the lower limit of the hydrogen concentration, it is sufficient that the oxidation of Fe does not proceed, so the hydrogen concentration is set to 1% or more.

In order for the granulation of the fluid to proceed, the above-mentioned requirements, ie,
(i) a soaking temperature sufficiently high (above 650 ° C.) that the components of the degreasing agent are fluidized, and
(ii) Annealing atmosphere (reducing property) that provides a steel sheet surface that does not wet and spread the fluid excessively
In addition, it is necessary that the soaking time is sufficiently long, that is, longer than 10 hours as shown in the above equation (3). As shown in an experimental example to be described later, when the soaking temperature is high, an oxide containing granular Si can be obtained even if the soaking time is short. Because of this relationship, if the soaking temperature H and the soaking time T satisfy the relationship of the above equation (3), an oxide containing Si that satisfies the relationship of the above equations (1) and (2) is obtained. can get.

  As described above, according to the presumed mechanism of the present inventors, in the present invention, the oxide attached to the surface of the cold-rolled steel sheet does not necessarily contain Si. However, from the viewpoint of manufacturing convenience and cost reduction, it is preferable to degrease and clean using a cleaning liquid containing a silicate-based alkaline degreasing agent. In this case, as an oxide attached to the surface of the cold-rolled steel sheet, Si Is obtained.

Example 1
As a test material, a cold-rolled steel sheet having the composition shown in Table 1 and the balance consisting of Fe and inevitable impurities and degreased by an electrolytic cleaning line after cold rolling was used. Degreasing in the electrolytic cleaning line was performed by immersing the cold-rolled steel sheet in a 3% sodium orthosilicate aqueous solution (temperature: 90 ° C.) for 2 minutes.

  This cold-rolled steel sheet is cut into a size that allows box annealing by a prototype machine, and the two steel sheets that have been cut are overlapped, with the conditions shown in Table 2 (each condition of No. 1 to 4), Box annealing was performed.

  FE-SEM observation was performed on the surface of the steel sheet after annealing (the surface overlapped during annealing) by the method described above. As an apparatus for FE-SEM observation, SUPRA55VP manufactured by Carl Zeiss was used. This apparatus was equipped with an AsB detector, and an AsB image was taken with this AsB detector at an acceleration voltage of 5 kV and a working distance (WD) of 4.5 mm.

  3 to 6 are respectively No. 1 in Table 2. The AsB image of the cold-rolled steel plate surface annealed on the conditions of 1-4 is shown. Based on this AsB image, the oxide area ratio M and the oxide average diameter D were determined by the above-described method.

  Next, chemical conversion treatment was performed on the annealed steel sheet using a commercial chemical conversion solution manufactured by Nippon Paint. This chemical conversion treatment liquid was for carrying out the zinc phosphate treatment. The total acidity was 22 pt, the free acidity was 0.7 pt, and the toner value was 3.0 pt. This chemical conversion treatment liquid was adjusted to 43 ° C., and the chemical conversion treatment was performed by immersing the steel sheet in this chemical conversion treatment solution for 90 seconds.

  After performing such a chemical conversion treatment, the appearance of the surface of the cold-rolled steel sheet was visually observed, and was observed with a SEM at a magnification of 1000 times, and the quality of the chemical conversion film was judged.

  7 to 10, No. 1 in Table 2 is shown. The SEM image of the cold-rolled steel plate which annealed on the conditions of 1-4 and gave the chemical conversion treatment is shown. In FIG. 11, the SEM image of the galvanized steel plate which carried out the chemical conversion treatment in the same time as the chemical conversion treatment with respect to these cold-rolled steel plates is shown. Table 3 shows the oxide area ratio M, the oxide average diameter D, and the quality of the chemical conversion film for each annealing condition.

  No. The annealing conditions of 2 and 3 satisfy the requirement that the hydrogen concentration in the annealing atmosphere is 40% or less, and the soaking temperature H and soaking time T satisfy the relationship of the above formula (3). On the surface of the cold-rolled steel sheet annealed under such conditions, granular oxide is adhered (see FIGS. 4 and 5), the oxide area ratio M, and the oxide average diameter D (see Table 3). Satisfies the relationship of the equations (1) and (2). That is, no. Cold-rolled steel sheets obtained by annealing under conditions 2 and 3 are examples of the present invention. In these cold-rolled steel sheets, after the chemical conversion treatment, the chemical-crystallized film almost completely covered the underlying cold-rolled steel sheet (see FIGS. 8 and 9), and the chemical conversion treatment property was good. These conversion crystal films had the same coverage as the conversion crystal film (see FIG. 11) obtained when the galvanized steel sheet was subjected to conversion treatment under the same conditions.

  On the other hand, no. The annealing conditions of 1 satisfy the requirement that the hydrogen concentration in the annealing atmosphere is 40% or less, but the soaking temperature T and the soaking time T satisfy the relationship of the above formula (3) because the soaking time T is short. Did not meet. No. In the annealing conditions of No. 4, the soaking temperature H and the soaking time T satisfy the relationship of the above formula (3), but the requirement that the hydrogen concentration in the annealing atmosphere is 40% or less was not satisfied.

  Compared with the granular oxides of FIGS. 4 and 5, a film-like oxide having a large area and a small oxide adhered to the surface of the cold-rolled steel sheet annealed under these conditions (FIG. 3). And FIG. 6). In any of the cold-rolled steel sheets, the oxide area ratio M satisfied the relationship of the formula (1), but the average oxide diameter D did not satisfy the relationship of the formula (2). That is, no. Cold-rolled steel sheets obtained by annealing under conditions 1 and 4 are not examples of the present invention. None of these cold-rolled steel sheets were chemically treated after the chemical conversion treatment. Specifically, in these cold-rolled steel sheets, the appearance by visual observation is not good, and as a result of SEM observation, generation of scale, that is, a portion where the chemical-crystal film does not cover the underlying cold-rolled steel sheet is seen. (See FIGS. 7 and 10).

(Example 2)
A hot-rolled coil having the composition shown in Table 4 was prepared, and this coil was degreased by cold rolling and electrolytic cleaning with an actual machine, and then box annealing was performed. The degreasing conditions in the electrolytic cleaning were the same as those in Example 1, and the annealing was performed under a plurality of conditions within the range shown in Table 5. About the steel plate after annealing, surface observation and calculation of oxide area ratio M and oxide average diameter D are performed by the same method as in Example 1. Further, chemical conversion treatment is performed by the same method as in Example 1. , And its evaluation.

  FIG. 12 shows the relationship between the oxide area ratio, the average oxide diameter, and the chemical conversion treatment quality. FIG. 13 shows the relationship between the soaking time and soaking temperature and the chemical conversion treatment quality.

  In FIGS. 12 and 13, “O” indicates that no clear scale is observed in the observation field by SEM observation, and “×” indicates that some of the clear scale is observed. Is shown. What was shown by "(circle)" had the covering property equivalent to the chemical conversion crystal film obtained when a galvanized steel plate was subjected to chemical conversion treatment in the same time.

  As shown in FIG. 12, when the oxide area ratio M is 5% or more and 50% or less and the oxide average diameter D is 0.005 × M μm or more and 0.35 μm or less, that is, ( When the relationship of the formulas (1) and (2) was satisfied, the chemical conversion treatment was good.

  From the aspect of the manufacturing method, as shown in FIG. 13, the soaking temperature is 650 ° C. or higher, the soaking time is 10 hours or more, and the conditions satisfying the relationship of the above expression (3) are good. Chemical conversion processability was obtained.

  The present invention can be applied to cold-rolled steel sheets used in the fields of automobiles, home electric appliances, building materials, etc., which are subjected to chemical conversion treatment. Even when the cold-rolled steel sheet is made of an ultra-low carbon steel, good chemical conversion properties can be obtained.

Claims (4)

A cold-rolled steel sheet with oxides dispersed and adhered to the surface,
The area ratio of the oxide per unit area when the steel sheet surface is observed by SEM is M (%), the average diameter of the oxide is D (μm), the oxide area ratio M and the oxide average A cold-rolled steel sheet, characterized in that the diameter D satisfies the relationship of the following formulas (1) and (2).
5 ≦ M ≦ 50 (1)
0.005 × M ≦ D ≦ 0.35 (2)   The cold-rolled steel sheet according to claim 1, wherein the oxide is an oxide containing Si.   The cold-rolled steel sheet according to claim 1 or 2, wherein the amount of Si contained in the cold-rolled steel sheet is, by mass%, Si: 0.1% or less.   The amount of C and Mn contained in the cold-rolled steel sheet is, in mass%, C: 0.01% or less and Mn: 0.5% or less, respectively. Cold rolled steel sheet.

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