JP2003113441A - Steel sheet superior in phosphating property - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel sheet containing C, Si and Mn, which can be satis factorily phosphated even when containing a comparatively large amount of Si. SOLUTION: This steel sheet has 80% or less of a ratio of Si-containing oxide occupying in 10 μm length of the steel sheet surface in an average measured on arbitrarily selected five places, when a cross section in the transversal direction to the steel sheet surface is observed with an electron microscope at a magnification of 50,000 times or higher.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel sheet used for steel sheets for automobiles and the like, and particularly to Si sheet.
The present invention relates to a steel sheet capable of ensuring excellent phosphating property even if the content is relatively large.

[0002]

2. Description of the Related Art In recent years, CO for the purpose of preventing global warming
_{2 As a} measure to curb emissions, a new target for improving fuel efficiency of automobiles has been set, and a tax incentive system for low-fuel consumption vehicles has been introduced. Reducing the weight of automobiles is an effective means of improving fuel efficiency, and from the viewpoint of reducing the weight, higher tensile strength of materials is strongly required. On the other hand, since a steel sheet for automobiles is severely processed, it is necessary to add elements such as C, Si and Mn in order to achieve both high tensile strength and workability.

By the way, a steel sheet used as an automobile body material is formed into parts and then assembled, and thereafter, a phosphate treatment is performed as a chemical conversion treatment prior to coating. By performing such an phosphating treatment, it is possible to improve the adhesion of the coating film in the subsequent coating process.

However, the steel sheet to which Si or the like is added is
In an atmosphere of reduction annealing that is performed in a normal manufacturing process, Si, which is an easily oxidizable element, is preferentially oxidized and concentrated on the surface of the steel sheet, and a Si-containing oxide layer is formed on the surface. And
Even if the steel sheet with the oxide layer formed on the surface is subjected to the phosphate treatment in this way, it is not possible to form zinc phosphate crystals uniformly and finely, and the phosphate crystals are partially deficient. It will be the surface condition. Furthermore, even if a coating such as electrodeposition coating is applied to the surface of a steel sheet having such a poor phosphate treatment, a coating film with good adhesion cannot be obtained, or the corrosion resistance after coating will deteriorate. is there.

Various techniques have been proposed so far in order to solve such problems, for example, Japanese Patent Laid-Open No. 5-320.
No. 952 discloses a method of forming an iron coating layer of ^{20 to} 1500 mg / m ^{2 on} a steel plate by an electroplating method, and JP-A-2000-87257 forms a zinc plating of 4 g / m ² or less. It is disclosed that a chemical conversion treatment such as a phosphate treatment or a chromate treatment is subsequently performed. However, these methods have a problem in that the cost is increased due to the increase in the number of processes because the electroplating equipment is additionally required.

In Japanese Patent No. 19712227, Mn / S is used.
The i ratio is specified, and in Patent No. 2951480, the phosphating property is improved by adding Ni, but the effect depends on the Si content in the steel sheet, and the Si content is high. Further improvement is considered necessary for steel sheets.

Further, Japanese Patent No. 2937005 discloses a method of grinding the surface of an annealed steel plate with a brush roll or a plate brush. However, this method also requires additional grinding equipment, and there is a concern that the appearance may be poor such as surface scratches.

[0008]

SUMMARY OF THE INVENTION The present invention has been made under these circumstances, and an object thereof is to perform a favorable phosphating treatment even when it contains a relatively large amount of Si. It is to provide a steel sheet that can be manufactured.

[0009]

The steel sheet having excellent phosphating property according to the present invention is a steel sheet containing C, Si, and Mn, and a cross section in a direction orthogonal to the steel sheet surface is magnified by an electron microscope at a magnification of 50,000. When observed at twice or more, the ratio of Si-containing oxide to the steel plate surface length of 10 μm is 80% or less on the average of five arbitrarily selected steel plate surfaces. Steel plate surface length 10 μm
It is a preferable embodiment that the ratio of Si-containing oxides in all is 80% or less.

In the steel sheet of the invention, the Si-containing oxide is present within a depth of 1 μm from the surface of the steel sheet substrate in the electron microscope observation, and in the electron microscope observation, a depth of 1 μm from the steel sheet substrate surface. The amount of solid solution Si in the steel plate within the range is 0.8 times or less of the average Si amount of the steel plate.

The "steel sheet surface" means not only the state of the steel sheet substrate surface as described later but also the state in which a Si-containing oxide layer or the like is formed on the steel sheet substrate surface. On the other hand, the “steel plate substrate surface” is a surface of the steel plate substrate that does not contain the Si-containing oxide. In addition, the above "steel plate substrate surface is not included in" the depth within 1 μm from the steel plate substrate surface ".

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is configured as described above, and the operation of the steel sheet having such a configuration will be described along with the history of its completion. FIG. 1 is a diagram schematically showing a cross-sectional structure near the surface of a conventional steel sheet. (A) shows before annealing, (b) shows after annealing, and (c) shows after pickling.

Usually, the steel sheet is annealed for the purpose of strain relief and structure adjustment after hot rolling or cold rolling. Since the atmosphere of reduction annealing performed in a normal manufacturing process has an atmosphere gas composition in which Fe and Si, which are easily oxidizable elements, are oxidized, since these elements are present in the steel sheet surface, Diffusion / concentration, and oxide is formed on the surface of the steel sheet substrate. Fig. 1 shows such a state.
It is (b), and the steel plate surface has Si-Mn oxide (Si, M
Not only oxides of n alone but also complex oxides of Si and Mn are included). When pickling treatment is performed after annealing, Mn oxide is easily removed, but Si oxide is difficult to remove by normal pickling using hydrochloric acid or sulfuric acid.
As shown in (c), it remains on the surface of the steel sheet substrate.

By the way, in the phosphate treatment, when the steel sheet is immersed in a phosphoric acid acidic solution containing zinc ions and the like, the reaction proceeds as in the following formulas (1) to (3), and zinc ions are present. , Insoluble zinc phosphate (in the case of a steel plate, zinc iron phosphate) is deposited on the surface of the material. By uniformly precipitating such zinc phosphate crystals, a coated steel sheet having good adhesion to the coating can be obtained even when the coating is applied.

As described above, since the dissolution reaction of the steel sheet is a driving force in the precipitation reaction of the phosphate coating, it is necessary to contact the phosphoric acid acidic solution with Fe of the steel sheet. However, since the Si-containing oxide is difficult to dissolve in the acidic phosphoric acid solution, when it is present over a wide area on the surface of the steel sheet, the dissolution reaction of the base steel sheet is inhibited by the oxide layer and the phosphate salt It is difficult for the treatment reaction to proceed.

Fe → Fe ²⁺ ＋ 2e ⁻ (1) 2H ⁺ ＋ 2e ⁻ ＋ (O) → H ₂ O (2) 2Zn (H ₂ PO ₄ ) ₂ → 2ZnHPO ₄ + (Fe ²⁺ ) → Zn ₂ Fe ( _{PO 4) 2 · 4H 2 O} ↓ (3)

The present inventors have paid attention to the relationship between the structure near the surface of the steel sheet and such phosphating reaction, and as shown in FIG. 2 (a diagram schematically showing the cross-sectional structure near the surface). If it is possible to realize a state in which the Si-containing oxide is dispersed inside the base steel sheet after annealing, the amount of Si diffused to the steel sheet surface can be reduced and the generation of the Si-containing oxide on the steel sheet base surface can be suppressed. , Fe required for the above phosphate treatment reaction
It was considered that the above reaction proceeded favorably by ensuring a state in which many parts were exposed.

Therefore, the present inventors paid attention to the distribution of the Si-containing oxide inside the steel sheet, produced steel sheets having different distributions of the Si-containing oxide under various conditions, and then treated the surface state after the phosphate treatment. By observing the cross-sections of the steel sheets with good and non-good steel sheets with an electron microscope and performing elemental analysis, the relationship between the cross-sectional structure near the surface of the base steel sheet and the phosphatability was clarified as follows.

In the present invention, in order to determine the degree of precipitation of the above Si-containing oxide near the surface of the steel sheet, a scanning electron microscope (SEM) is used as an electron microscope for photographic observation.
Although a transmission electron microscope (TEM) was used, the photographic observation is not limited to these electron microscopes, and other electron microscopes that can determine the distribution state of the Si oxide are also included.

That is, according to the clarification by the present inventors, in order to obtain a good phosphatized surface, the Si-containing substance which inhibits the phosphating reaction is obtained on the surface of the steel sheet before phosphating. It is necessary to reduce the area of the oxide and secure the exposed area of the metallic Fe, and when the cross section in the direction orthogonal to the steel plate surface is observed with an electron microscope at a magnification of 50,000 times or more, the steel plate surface length is 10 μm. If the proportion of the Si-containing oxide in the above is set to 80% or less on an average at five arbitrarily selected points, the metallic Fe serving as the anodic point of the phosphating reaction (exposed point of the steel substrate) is It was found that the dispersed state can be secured and the phosphate treatment can be performed well. The length of the Si-containing oxide in 10 μm is preferably 70% or less, more preferably 50% or less.

In order to secure such a state, it is preferable to allow Si-containing oxide to exist in the steel plate base material within a depth of 1 μm from the surface of the steel plate base material in the electron microscope observation. Si-containing oxide is formed in the steel sheet, and solid solution S in the steel sheet within a depth of 1 μm from the steel sheet substrate surface is formed.
It is preferable that the i amount is 0.8 times or less the average Si amount of the steel sheet. The amount of solid solution Si in the steel sheet within a depth of 1 μm from the surface of the steel sheet substrate is preferably 0.7 times or less, more preferably 0.5 times or less, the average Si amount of the steel sheet.

The Si-containing oxide means SiO ₂ , Fe ₂
It refers to SiO _{4 and the} like. Further, the Si-containing oxide existing within a depth of 1 μm from the surface of the steel sheet substrate may be precipitated not only at the grain boundaries but also within the grains.

The cross-sectional structure as shown in FIG. 2 can be realized by controlling the conditions of reduction annealing, and it is preferable to increase the oxygen partial pressure in the annealing as compared with the normal annealing atmosphere. As a method of increasing the oxygen partial pressure, it is effective to raise the dew point during annealing. For example, in a 3% hydrogen atmosphere, the dew point is -25 ° C or higher, preferably -20 ° C or higher. By thus increasing the oxygen partial pressure and performing the annealing, the formation of Si-containing oxide on the surface of the steel sheet substrate can be suppressed, and the internal oxidation of Si can be achieved.
If the oxygen partial pressure is too high, that is, if the dew point is too high, iron is oxidized and the appearance is deteriorated. Therefore, it is preferable to perform annealing with the dew point at 0 ° C. or lower.

The steel sheet used in the present invention contains C, Si and Mn as basic components. In order to secure the strength or enhance the hardenability to improve the strength and simultaneously improve the workability, C is 0.03% or more, preferably 0.05% or more, and Si is 0.3% or more, preferably Is 0.
5% or more, and Mn 0.2% or more, preferably 1.
It is preferable to contain 0% or more. However, if these contents are excessive, the phosphating property and processability are deteriorated, so C content is 0.30% or less, preferably 0.2%.
0% or less, Si 2.5% or less, preferably 2.0% or less, more preferably 1.5% or less, Mn 3% or less, preferably 2.6% or less, more preferably 2.0%. The following is recommended.

As components other than the above C, Si and Mn, the steel sheet used in the present invention has basic components such as Al, P and S, and if necessary Ti, Nb, Mo, V, Zr, N,
Although various elements such as B are included, the content of these elements is not particularly limited as long as they are normally contained in a steel sheet. The steel sheet used in the present invention may contain, in addition to these elements, trace amounts of components that do not affect its characteristics, and such steel sheets are also included in the steel sheet used in the present invention.

[0026]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples, and may be appropriately applied within a range compatible with the gist of the preceding and the following. Modifications can be made and implemented, and all of them are included in the technical scope of the present invention.

Example 1 Using a cold-rolled steel sheet containing 1.2% by mass of Si and 2.0% by mass of Mn, degreasing, annealing and pickling were carried out under the following conditions to precipitate Si-containing oxides. Different steel plates were obtained.

(Invention Example 1) Degreasing: Immersion in 5% aqueous sodium hydroxide solution at 60 ° C. for 30 seconds → washing → dry annealing: 3% hydrogen-containing inert atmosphere, dew point -20 ° C., 87
Pickling at 0 ° C x 200s by heating; soaking in 3% hydrochloric acid aqueous solution at 60 ° C for 5 seconds → washing with water → drying (Invention Example 2) Degreasing: soaking in 5% aqueous sodium hydroxide solution at 60 ° C for 30 seconds → washing with water → drying annealing: Inert atmosphere containing 3% hydrogen, dew point -15 ° C, 87
Acid pickling at 0 ° C x 200s; immersion in 3% hydrochloric acid aqueous solution at 60 ° C for 5 seconds → water washing → drying (Comparative Example 1) Degreasing: immersion in 5% sodium hydroxide aqueous solution at 60 ° C for 30 seconds → water washing → dry annealing: 3 % Hydrogen-containing inert atmosphere, dew point -30 ° C, 87
0 ° C. × 200 s heat pickling; immersion in 60 ° C. 3% hydrochloric acid aqueous solution for 5 seconds → washing → drying Each steel plate thus obtained is cut in a direction orthogonal to the steel plate surface, and a cross section including the vicinity of the surface is cut. TEM observation and qualitative analysis were performed. In the TEM observation, after depositing a platinum protective film on the surface of the steel sheet, a cross section of about 10 μm × 5 μm including the vicinity of the surface was processed by a focused ion beam (FIB) to a thickness of about 0.1 μm. Using a field emission transmission electron microscope [HF2000: manufactured by Hitachi, Ltd.] equipped with a line spectroscope, a magnification of 15,000 and 6
The observation was performed at a magnification of 0000 times.

FIGS. 3 to 8 (all of which are micrographs as substitutes for drawings) show the results of transmission electron microscope observation of the steel sheet cross sections of Inventive Example 1, Inventive Example 2 and Comparative Example 1.
3, 5, and 7 were taken at a magnification of 15,000, and FIGS. 4, 6, and 8 were taken at a magnification of 60,000. The black-colored protective film on the surface of the steel sheet substrate in FIGS. 3 to 8 is the platinum protective film, and the white or gray part at the boundary between the protective film and the surface of the steel sheet substrate indicates the Si-containing oxide on the surface of the steel sheet substrate. . Further, the white background portion or the gray portion scattered in the steel sheet base material indicates the Si-containing oxide in the steel sheet.

In a low-magnification visual field in which the entire steel plate surface length of 10 μm can be observed in one visual field, the proportion of Si-containing oxide in the steel plate surface length of 10 μm cannot be accurately measured. Therefore, according to the present invention, the visual field is sequentially moved to occupy 10 μm in a continuous manner in the manner of measuring the ratio of the Si-containing oxide of approximately 1 μm in one visual field of 50,000 times or more.
The ratio of the i-containing oxide was determined. In this experiment, 6000
Observation was carried out at a magnification of 0 times to determine the ratio of the Si-containing oxide.

As a result of the measurement as described above, the proportion of the Si-containing oxide in the steel plate surface length of 10 μm was 20% in the present invention example 1 and 12% in the present invention example 2, while the comparative example 1 was used. Results in a high occupancy rate of Si-containing oxide of 90%. This means that, in FIGS. 3 to 6, the Si-containing oxide on the surface of the steel sheet base material is intermittently present in the present invention example 1 and the present invention example 2, whereas the steel sheet cross section of the comparative example 1 is In FIG. 7 and FIG. 8 shown above, it is consistent with the fact that a continuous Si-containing oxide phase is formed on the surface of the steel sheet substrate.

Further, the presence or absence of Si-containing oxide and the amount of solid solution Si in the base steel sheet within a depth of 1 μm from the surface of the steel sheet base are determined by an energy dispersive X-ray spectroscope ( energy dispersive spectrom
eter, hereinafter abbreviated as EDS), accelerating voltage: 200
The measurement was performed under the measurement conditions of kV and electron beam diameter: about 20 nm.

FIGS. 9, 12 and 15 (all micrographs as substitutes for drawings) show a steel sheet cross section including the vicinity of the surface of the base steel sheet of Inventive Example 1, Inventive Example 2 and Comparative Example 1 at a magnification of 500.
It is a scanning electron microscope observation photograph taken at 00 times, the protective film in the figure shows the platinum protective film, and the black or gray portion located at the boundary between the protective film and the steel substrate is Si on the surface of the steel sheet substrate. The concentrated phase of the contained oxide is shown. Moreover, the gray part scattered in a steel plate shows Si containing oxide in a steel plate. The ED for the analysis positions 1 to 3 in these observation photographs
Qualitative analysis was performed on S. The results of measuring the analysis positions 1 to 3 in FIGS. 9, 12 and 15 are shown in FIGS.
3 and FIG. Incidentally, FIG. 10, FIG. 13 and FIG.
The numbers at the upper left of the respective peak diagrams in 6 correspond to the analysis positions shown in FIGS. 9, 12 and 15, respectively.

From these results, in Inventive Example 1 and Inventive Example 2, the Si-containing oxide is present in the base steel sheet within a depth of 1 μm from the steel sheet base surface, whereas in Comparative Example 1, the steel sheet is used. Si-containing oxide is present only on the surface of the substrate,
It can be seen that it does not exist in the base steel sheet.

Further, regarding the amount of solid solution Si in the base steel sheet having a depth of 1 μm or less from the surface of the steel sheet base material, the composition is analyzed at intervals of 100 nm from the surface of the steel sheet base material in the depth direction (calculated by each metal ratio of Fe, Si and Mn). And asked. FIG. 9,
The vertical lines in FIGS. 12 and 15 indicate the places where the analysis was performed. The measured results are shown in Table 1. Incidentally, FIG.
1, FIG. 14 and FIG. 17 are based on the data of Inventive Example 1, Inventive Example 2 and Comparative Example 1 in Table 1, and Si and Mn
2 is a graph showing the relationship between the element concentration of and the depth from the surface of the steel sheet substrate.

[0036]

[Table 1]

From FIGS. 11, 14 and 17 and Table 1, in Example 1 of the present invention, a depth of 1 μm (1
The amount of solid solution Si within 000 nm is 0.1 to 0.3% by mass, and in Inventive Example 2 is 0.0 to 0.3% by mass (note that the depth from the steel sheet substrate surface in Inventive Example 2 is 100 nm). Of S
i: 1.1% by mass, depth of 300 nm from the surface of the steel sheet substrate
Si: 2.4 mass% is S in the steel plate matrix from FIG.
The amount of solid solution Si is considerably small and that of Comparative Example 1 shows that the amount of solid solution Si exceeds the regulation of the present invention in many places.

Next, these steel sheets were subjected to a phosphate treatment to evaluate their phosphate treatability.

The phosphate treatment was carried out by immersing the sample after the annealing and pickling in a commercially available immersion type zinc phosphate treatment solution at 40 ° C. for 2 minutes. After that, the precipitation morphology of phosphate crystals on the surface of the sample was observed with a scanning electron microscope. 18 to 20 show the steel sheet surfaces after subjecting the steel sheets of Inventive Example 1, Inventive Example 2 and Comparative Example 1 to the above-mentioned phosphate treatment at a magnification of 1000, respectively.
It was taken at double. The crystal in the photograph is a phosphate film.

Further, the sample after the phosphate treatment is at 75 ° C.
It was immersed in the 5% chromic anhydride aqueous solution for 15 minutes, and the amount of the phosphate coating adhered was calculated from the weight difference before and after the immersion. As a result, the amount of the phosphate film deposited was 2.57 g in Inventive Example 1
/ M ² and Example ² of the present invention was 2.63 g / m ² and both were 2 g.
/ M ² or more, while Comparative Example 1 was 1.02 g /
It was a small amount of m ² .

From these results, the steel sheet of the present invention in which the dew point was raised, that is, the oxygen partial pressure was raised to perform annealing to cause internal oxidation of Si to reduce the amount of Si-containing oxide on the surface of the steel sheet substrate, It can be seen that a uniform and dense phosphatized film is formed when the salt treatment is performed.

Example 2 A cold-rolled steel sheet containing 1.5% by mass of Si and 1.8% by mass of Mn was used.
After performing a degreasing treatment of immersing for 0 seconds, washing with water and drying are performed, and thereafter, as shown in Table 2, annealing is performed at 840 ° C for 120 seconds in an inert atmosphere containing 3% hydrogen having different dew points, and further 60 ° C. Was pickled with 5% aqueous solution of 3% hydrochloric acid, washed with water and dried to obtain a test material.

These test materials were cut in a direction orthogonal to the surface of the steel sheet, and a TEM observation of a cross section including the vicinity of the surface was carried out by the same method as in Example 1 above. That is, for each sample, 5 TEM observation samples containing a steel plate surface length of 10 μm or more were used.
Samples were prepared and observed at a magnification of 60,000 to determine the proportion of Si-containing oxide in the steel plate surface length of 10 μm in each sample.

The test material was immersed in a commercially available immersion type zinc phosphate treatment solution at 40 ° C. for 2 minutes, and then the phosphate crystal precipitation form on the surface of the sample was measured by a scanning electron microscope.
It was observed at 0 times. Furthermore, the test material after the phosphate treatment was dipped in a 5% chromic anhydride aqueous solution at 75 ° C. for 15 minutes, and the amount of the phosphate coating adhered was calculated from the weight difference before and after the immersion. The results are also shown in Table 2.

[0045]

[Table 2]

From Table 2, it can be seen that, in the steel sheet surface length of 10 μm, the ratio of the Si-containing oxide is 80% or less at the average of five positions. In 3 to 5, N at the average of 5 locations exceeds 80%
o. It can be seen that, as compared with Nos. 1 and 2, the precipitation form of the phosphate film is good and the phosphate film adhesion amount is large. In addition, the above No. Nos. 4 and 5 are Nos. Since the amount of the phosphate coating adhered is larger than that of No. 3, it is found that it is desirable that the ratio of the Si-containing oxide occupying the steel plate surface length of 10 μm is 80% or less.

[0047]

EFFECTS OF THE INVENTION The present invention is configured as described above, and in a steel sheet containing a relatively large amount of Si in order to achieve both high tensile strength and workability, the precipitation morphology of Si-containing oxide is appropriately controlled. By doing so, it has become possible to secure excellent phosphatability. With the realization of such a steel sheet, it has become possible to obtain a coated steel sheet with good coating adhesion even when coating such as electrodeposition coating is applied.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a cross-sectional structure in the vicinity of the surface of a steel sheet substrate in a conventional steel sheet.

FIG. 2 is a diagram schematically showing a cross-sectional structure in the vicinity of the surface of a steel sheet substrate in the steel sheet of the present invention.

FIG. 3 is a drawing-substituting photomicrograph showing the results of observation by a transmission electron microscope (magnification: 15,000 times) of the cross section of the steel sheet of Example 1 of the present invention.

FIG. 4 is a drawing-substituting micrograph showing a result of observing a cross section of a steel sheet of Inventive Example 1 with a transmission electron microscope (magnification: 60,000 times).

FIG. 5 is a drawing-substituting photomicrograph showing a result of observation with a transmission electron microscope of a cross section of a steel sheet of Inventive Example 2 (magnification: 15000 times).

FIG. 6 is a drawing-substituting micrograph showing a result of observation with a transmission electron microscope of a cross section of a steel plate of Inventive Example 2 (magnification: 60,000 times).

FIG. 7 is a drawing-substitute micrograph showing a result of observation with a transmission electron microscope of a cross section of a steel plate of Comparative Example 1 (magnification: 15,000 times).

FIG. 8 is a drawing-substituting micrograph showing a result of transmission electron microscope observation of a cross section of a steel plate of Comparative Example 1 (magnification: 60,000 times).

FIG. 9 is a drawing-substituting micrograph showing a scanning electron microscope observation result of a cross section of a steel sheet of Example 1 of the present invention.

10 is a peak diagram showing a result of qualitative analysis by EDS for analysis positions 1 to 3 in FIG. 9 according to Example 1 of the present invention.

11 is based on the data of Example 1 of the present invention in Table 1,
5 is a graph showing the relationship between the elemental concentrations of Si and Mn and the depth from the surface of the steel sheet substrate.

FIG. 12 is a drawing-substituting micrograph showing the results of scanning electron microscope observation in the cross section of the steel sheet of Inventive Example 2.

FIG. 13 is a peak diagram showing a result of qualitative analysis by EDS for analysis positions 1 to 3 in FIG. 12 according to Example 2 of the present invention.

14 is based on the data of Example 2 of the present invention in Table 1,
5 is a graph showing the relationship between the elemental concentrations of Si and Mn and the depth from the surface of the steel sheet substrate.

FIG. 15 is a drawing-substituting micrograph showing a scanning electron microscope observation result of a cross section of a steel plate of Comparative Example 1.

16 is a peak diagram showing a result of qualitative analysis by EDS for analysis positions 1 to 3 in FIG. 15 according to Comparative Example 1. FIG.

FIG. 17 is a graph showing S based on the data of Comparative Example 1 in Table 1.
It is the graph which showed the relationship between the element concentration of i and Mn, and the depth from the steel plate base material surface.

FIG. 18 is a drawing-substituting micrograph showing a result of observing a sample surface with a scanning electron microscope after subjecting a steel sheet according to Example 1 of the present invention to a phosphate treatment.

FIG. 19 is a drawing-substituting photomicrograph showing the results of scanning electron microscope observation of the sample surface after the phosphate treatment of the steel sheet according to Example 2 of the present invention.

FIG. 20 is a drawing-substituting micrograph showing a result of scanning electron microscope observation of a sample surface after the phosphate treatment of the steel sheet according to Comparative Example 1.

Continued front page    (72) Inventor Hidekazu Ido             1-5-5 Takatsukadai, Nishi-ku, Kobe City Stock Association             Company Kobe Steel Works, Kobe Research Institute

Claims (4)

[Claims] 1. In a steel sheet containing C, Si, Mn, a cross section in a direction orthogonal to the steel sheet surface is observed with an electron microscope at a magnification of 50.
Steel plate surface length 10 μm when observed at 000 times or more
A steel sheet excellent in phosphating property, characterized in that the proportion of Si-containing oxide in the above is 80% or less on the average of five arbitrarily selected steel sheet surfaces. 2. Si occupying a surface length of the steel sheet of 10 μm
The steel sheet according to claim 1, wherein the proportions of the contained oxides are all 80% or less. 3. The steel sheet according to claim 1, wherein the Si-containing oxide is present within a depth of 1 μm from the surface of the steel sheet substrate under the electron microscope observation. 4. The amount of solid solution Si in the steel plate within a depth of 1 μm from the surface of the steel plate substrate in the electron microscope observation is
4. The average Si content of the steel sheet is 0.8 times or less.
The steel plate according to any one of 1.