JPH08225888A - Production of corrosion resistant steel sheet excellent in chemical convertibility and cold rolled steel sheet therefrom - Google Patents

Abstract

PURPOSE: To provide a method for producing a steel sheet excellent in both of chemical convertibility and corrosion resistance without executing special surface modifying and to provide a method for producing a cold rolled steel sheet therefrom. CONSTITUTION: This corrosion resistant steel sheet has a chemical componental compsn. contg., by weight, 0.0003 to 0.005% C, <=0.1% Si, 0.1 to 2.0% Mn, 0.002 to 0.03% P, 0.003 to 0.020% S, 0.01 to 0.1% sol.Al, 0.05 to 0.5% Cu, <=0.10% Cr (contg. the case of no addition) and 0.0002 to 0.0020% B and furthermore satisfying 24×S/32+2×P/31>=Cu/63>=2×S/32. The surface of this steel sheet is moreover provided with an oxidized film having 10 to 100Å thickness. In the production of the cold rooled steel sheet, annealing is executed at >=500 deg.C for <=10min.

Description

Detailed Description of the Invention

[0001]

This invention relates to the corrosion resistance of a steel sheet as it is,
In particular, the present invention relates to a steel sheet excellent in atmospheric corrosion resistance and also excellent in chemical conversion treatment as a coating base treatment, and a method for manufacturing the cold rolled steel sheet.

[0002]

2. Description of the Related Art Conventionally, plated steel sheets have been mainly used as means for improving the corrosion resistance of steel sheets. However, the plated steel sheet has problems that the plating layer peels off during press working to cause rust from this portion, and that zinc is on the surface of the steel sheet, which deteriorates the electrode during spot welding.

In recent years, Cu-P-added corrosion-resistant steel plates have appeared. The characteristic of this steel plate is that rust is generated only on the surface layer,
The corrosion resistance is improved by preventing the inward progress. However, since P is added to these steel sheets, problems such as embrittlement and workability deterioration of steel materials have become apparent.

On the other hand, for example, Japanese Patent Laid-Open No. 61-11
Japanese Patent No. 7249, Japanese Patent Laid-Open No. 63-203747, and Japanese Patent Laid-Open No. 2-50940 disclose a technique for a steel sheet to which Cr is added in order to improve corrosion resistance. However, such a steel material containing Cr has a potential problem of pitting corrosion due to chlorine ions.

Further, a steel sheet used in the fields of automobiles, electric equipment, building materials, etc., which is a main application of a thin steel sheet, is rarely used as it is, and its surface is usually coated and used. In the case of coating, a phosphate treatment, that is, a so-called chemical conversion treatment is performed as a base treatment to increase the adhesion between the paint and the base steel sheet. By making the above chemical conversion treatability of the steel sheet surface good, the corrosion resistance after coating becomes good. However, in the corrosion-resistant steel plate, Cu which significantly deteriorates the chemical conversion treatment property.
Is not only added, but the surface is relatively electrochemically stable, so the chemical conversion treatability is extremely poor.

[0006]

Therefore, in order to solve the above-mentioned problems, the object of the present invention is to grasp the behavior of each chemical component in steel sheets of various steel types, and to determine the contents of S, P and Cu. By controlling within a proper range, both the chemical conversion treatability and the corrosion resistance are good without special surface modification, the steel sheet having the originally difficult compatibility properties, and its cold rolled steel sheet. To provide a preferred manufacturing method of

[0007]

[Means for Solving the Problems] In order to solve the above problems, the inventors of the present invention have earnestly studied and obtained the following findings. Generally, the corrosion resistance of a corrosion-resistant steel sheet is improved by reducing the reactivity of the surface. Therefore, in the corrosion-resistant steel sheet, the formation of microcells effective for the formation of nuclei in the initial stage of chemical conversion treatment hardly occurs, and the chemical conversion treatment property is poor. Therefore, it is important to control the contents of S, P and Cu, which are elements that have a great influence on the electrochemical reactivity of the steel sheet surface, within an appropriate range in order to achieve both chemical conversion treatability and corrosion resistance. I came to the conclusion.

On the basis of the above conclusion, as a result of examining the proper range of the S, P and Cu contents for achieving the above object, the following matters have been clarified. That is, in the process of forming the chemical conversion treatment film, nuclei of zinc phosphate crystals are formed on the surface of the steel sheet at the initial stage. This nucleation site is near the anode where Fe ions in the steel sheet are eluted. Then, nuclei are also generated from each of a large number of other nucleation sites in the vicinity of the nucleation site, each nuclei grows, and these nuclei combine to form a zinc phosphate film.

S exists as a sulfide in the steel material, and this sulfide acts as a cathode point. However, Cu elutes from the anode point and precipitates in the vicinity of the cathode point.
It acts to stabilize the corrosion point and enhance corrosion resistance. In this way, Cu reduces the cathode point, so that the number of microcells produced is reduced. That is, the number of microcells generated is
The number of the anodic points decreases as the number of the anodic points decreases, and as a result, the anodic points serving as active points effective for the chemical conversion treatment decrease and the chemical conversion treatment property deteriorates.

On the other hand, as the P content increases, the amount of P existing on the surface of the steel sheet increases and the chemical conversion treatability becomes better. By the way, it is known that when a corrosion test is performed in the atmosphere, as the P content is increased, rust is generated in the surface layer to prevent the rust from progressing into the interior. We have found that weight loss increases.
This acts as an adverse factor from the viewpoint that P in the steel sheet has an adverse effect on the initial rusting property and that the corrosion weight loss should be suppressed until stable rust is formed. It is thought to be because it is.

From the above, it was found that limiting the contents of S, P and Cu in the steel sheet to within an appropriate range is important for achieving both corrosion resistance and chemical conversion treatability of the steel sheet. .

Next, it is generally considered that the surface oxide film of the steel sheet is harmful in performing the chemical conversion treatment, but in nature, the surface oxide film is usually present on the metal surface. Moreover, since the presence of a dense oxide film is effective for enhancing the corrosion resistance of the steel material itself, it is more desirable to obtain good chemical conversion treatability in the presence thereof, rather than removing it.

Therefore, the contents of Si, Mn and Al, which have an important influence on the formation of the oxide layer, are changed, and the contents of S, P and Cu are also changed to evaluate the corrosion resistance and the chemical conversion treatability. As a result, it was found that by controlling the contents of these elements within an appropriate range, it is possible to obtain an oxide film having both good corrosion resistance and good chemical conversion treatability.

First, the present inventors conducted a corrosion resistance test using test materials prepared from corrosion resistant steel plates having various chemical composition. As a result, in order to obtain good corrosion resistance, the Cu content is at least twice the S content in terms of atomic weight ratio between the Cu content and the S content, that is,
It was found that it is necessary to satisfy the relational expression of Cu (wt.%) / 63 ≧ 2 × S (wt.%) / 32.

Next, a chemical conversion treatability test was conducted using substantially the same test material as described above. The chemical conversion treatability was evaluated by P ratio after subjecting a predetermined test piece to a chemical conversion treatment for 30 seconds by PBL-3080 manufactured by Nippon Parkerizing Co., Ltd. However, the P ratio is expressed by the following formula (2): Represented by When the test piece was subjected to a chemical conversion treatment, it was hopite (Zn ₃ (PO ₄ ) ₂ .4H ₂ O) and phosphophyllite (Zn ₂ Fe (P).
_{O 4) 2 · 4H 2 O} ) is generated, phosphophyllite desirable as it is chemical convertibility is large the amount of light, therefore, P ratio is higher is preferable.

FIG. 1 is a graph showing the relationship between the S, P and Cu contents of the steel sheet and the chemical conversion treatability. S and P improve chemical conversion treatability by adding an appropriate amount and deteriorate corrosion resistance by adding excessively, whereas Cu improves corrosion resistance by adding an appropriate amount, contrary to these.
Excessive addition deteriorates chemical conversion treatability. That is, S
Considering that P and Cu have contradictory effects on the improvement of the chemical conversion treatability, the contribution of each of these elements to the improvement of the chemical conversion treatability was examined.

FIG. 1 summarizes the test results of this chemical conversion treatment evaluation from this point of view. As a result, as is clear from the figure, in order to obtain excellent chemical conversion treatability, the contents of S, P and Cu were {24 × S (wt.%) / 32 +
It was found that it is necessary to satisfy the relationship of 2 × P (wt.%) / 31} -Cu (wt.%) / 63 ≧ 0.

Furthermore, the chemical composition is constant (see Table 1 below).
Which is the chemical composition of the steel No. 16 of the present invention) in which the corrosion test is performed using a test material prepared from a corrosion-resistant steel plate in which only the thickness of the oxide film on the steel plate surface is changed,
As a result of studying the results, the oxide film effectively acts on the corrosion resistance of the steel sheet as it is (bare corrosion resistance), but when corrosion progresses from the paint peeling portion, the steel sheet with a thick oxide film has a very narrow bulge. I knew it was going on. That is, in the case where an extremely thick oxide film is present, the elution of the phosphate film from the cross-cut portion carved on the surface of the steel sheet and the oxidation caused by this are more than in the case where a thin oxide film is present. It is believed that this has been promoted and that the corrosion resistance after painting has been adversely affected. On the other hand, if the oxide film is extremely thin, the corrosion resistance of the steel sheet itself is significantly deteriorated.

FIG. 2 is a graph showing the relationship between the thickness of the oxide film on the surface of the corrosion-resistant steel sheet, the chemical conversion treatment property, and the corrosion resistance, summarizing the above test results. The thickness of the oxide film on the surface of the steel sheet was measured by the so-called reduction method. As is clear from the figure, in order to obtain a corrosion-resistant steel sheet excellent in both corrosion resistance and chemical conversion treatability, the thickness of the oxide film on the steel sheet surface is 1
It was found that it was achieved when it was in the range of 0 to 100Å.

The corrosion-resistant steel sheet and the method for manufacturing the cold-rolled steel sheet according to the present invention have been completed based on the above-mentioned findings, and have the following constitutions.

The corrosion-resistant steel sheet of the first invention is carbon (C): 0.
0003 to 0.005 wt.%, Silicon (Si): 0.1 wt.% Or less, manganese (Mn): 0.1 to 2.0 wt.%, Phosphorus (P):
0.002 to 0.03 wt.%, Sulfur (S): 0.003 to 0.020 wt.%
%, Acid-soluble aluminum (sol.Al): 0.01 to 0.1 w
t.%, copper (Cu): 0.05 to 0.5 wt.%, chromium (C
r): 0.10 wt.% or less (including no addition), and boron (B): 0.0002 to 0.0020 wt.% and containing sulfur (S), phosphorus (P) and copper (Cu) The content is
Formula (1) below: 24 × S (wt.%) / 32 + 2 × P (wt.%) / 31 ≧ Cu (wt.%) / 63 ≧ 2 × S (wt.%) / 32 ---- ---- It is characterized by having a chemical composition that satisfies (1).

In addition to having the chemical composition of the first aspect of the invention, the corrosion-resistant steel sheet of the second aspect further comprises an oxide film having a thickness of 10 to 100 Å on at least one surface of the steel sheet. It has characteristics.

The method for producing a corrosion-resistant cold-rolled steel sheet according to the third invention is
A method of cold-rolling a steel having the chemical composition according to the first aspect of the invention to prepare a cold-rolled steel sheet, and then annealing the cold-rolled steel sheet to produce a corrosion-resistant cold-rolled steel sheet,
The annealing is characterized in that the temperature at 500 ° C. or higher is 10 minutes or less.

[0024]

The reason for limiting the chemical composition of the corrosion-resistant steel sheet of the present invention will be described.

(1) C: The content of C is preferably as small as possible in order to increase the corrosion resistance. However, if the content is less than 0.0003 wt.%, A large cost is required. On the other hand, when the C content exceeds 0.005 wt.%, The corrosion resistance deteriorates. Therefore, the C content is 0.0003-
It should be limited to the range of 0.005 wt.%.

(2) Si: Si is easily oxidized into stable oxide SiO ₂ . This is non-conductive and hardly soluble, and significantly deteriorates the chemical conversion treatment property. Therefore,
It is desirable that the amount be small, and if the Si content exceeds 0.1 wt.%, The chemical conversion treatability deteriorates. Therefore, the Si content is set to 0.
It should be limited to 1 wt.% Or less.

(3) Mn: Mn is an element that easily precipitates on the surface of the steel sheet and becomes an oxide in the surface layer of the steel sheet. Since it has conductivity, it works effectively as a nucleus of initial adhesion during chemical conversion treatment. However, if the Mn content is less than 0.1 wt. On the other hand, the above oxide becomes a nucleus of corrosion and adversely affects the corrosion resistance, and if the Mn content exceeds 2.0 wt.%, The corrosion resistance is significantly deteriorated. Therefore, the Mn content is 0.1 to 2.0.
It should be limited to the range of wt.%.

(4) P: P is an element effective in improving chemical conversion treatability. However, its content is 0.002 w
If it is less than t.%, the effect is not sufficiently exerted. On the other hand, if the P content is excessive, the corrosion weight loss increases in atmospheric corrosion, and if it exceeds 0.03 wt.%, The corrosion weight loss is remarkable.
Therefore, the P content should be limited to the range of 0.002 to 0.03 wt.%.

(5) S: S hinders the formation of a dense surface oxide layer which is effective in improving corrosion resistance. Further, sulfides are formed in the steel, which act as cathode inclusions and activate the steel sheet surface. As a result, S deteriorates the corrosion resistance of the steel sheet as it is. However, S is an effective element for improving the chemical conversion treatability. However, the S content is 0.003 wt.
If it is less than 0.1%, the chemical conversion treatability is poor, while its content is 0.
If it exceeds 020 wt.%, The corrosion resistance is extremely deteriorated. Therefore,
The S content should be limited to the range of 0.003 to 0.020 wt.%.

(6) sol.Al: sol.Al is effective as a deoxidizing element for molten steel. However, if the sol.Al content is less than 0.01 wt.%, The effect is small, while if the content exceeds 0.1 wt.%, A large amount of alumina-based oxide is formed on the surface of the steel sheet, and The oxide film becomes too thick and chemical conversion treatability deteriorates. Therefore, the sol.Al content should be limited to the range of 0.01 to 0.1 wt.%.

(7) Cu: Cu is a micro cell anode.
The elution from the cathode portion and the precipitation near the sulfide, which is the cathode point, reduce the activation of the cathode point. Therefore, although it is an effective element for increasing the corrosion resistance of the steel sheet, it adversely affects the chemical conversion treatability. Cu content is 0.
If it is less than 05 wt.%, Effective corrosion resistance cannot be obtained. On the other hand, if the content exceeds 0.5 wt.%, Not only the effect of improving the corrosion resistance is saturated, but also the chemical conversion treatability is significantly deteriorated. Therefore, the Cu content should be limited to the range of 0.05 to 0.5 wt.%.

(8) Cr: Cr is an element effective for improving the corrosion resistance, but in the environment where chlorine ions are present such as spraying of snow melting salt, the problem of pitting corrosion occurs. Also, Cu
For the same reason as above, the chemical conversion treatability is deteriorated. And
Corrosion resistance can be ensured without necessarily adding Cr. On the other hand, if the Cr content exceeds 0.10 wt.%, The above-mentioned problems will occur. Therefore, the Cr content is 0.1
It should be limited to 0 wt.% Or less (including no addition).

(9) B: B is an element effective in suppressing corrosion from the grain boundary. However, if the content is less than 0.0002 wt.%, The above effect is not exhibited. On the other hand, if the B content exceeds 0.0020 wt.%, The hot deformation resistance increases, and excessive addition increases the manufacturing cost and poses a practical problem. Therefore, the B content is 0.0002.
It should be limited to the range of 0.0020 wt.%.

(10) Relationship between S, P and Cu: As described above, S and P improve chemical conversion treatability by adding an appropriate amount and deteriorate corrosion resistance by adding excessively, Contrary to these, Cu improves corrosion resistance by adding an appropriate amount, and deteriorates chemical conversion treatment property by adding excessively. By the way, in order to obtain good corrosion resistance, as described above, it is necessary that the Cu content is at least twice the S content in terms of atomic weight ratio between the Cu content and the S content. is there. On the other hand, in order to obtain a steel sheet having excellent chemical conversion treatability, as described above,
As can be seen from {24 x S (wt.%) / 32 + 2 x P (wt.%)
It is necessary to satisfy the relationship of /31}-Cu(wt.%)/63≧0. Therefore, in order to obtain a steel sheet having an oxide film in which good corrosion resistance and good chemical conversion treatment are compatible, S, P
And the Cu content is the following formula (1): 24 × S (wt.%) / 32 + 2 × P (wt.%) / 31 ≧ Cu (wt.%) / 63 ≧ 2 × S (wt.%) / 32 --------------- (1) should be satisfied.

(11) Other elements: Even if Ni is added for the purpose of improving the surface properties of steel slabs and steel sheets and ensuring workability, if the content is 1 wt.% Or less,
For these purposes, Ti, Nb, Zr, V, Hf
And at least one of Ta and the like, if the content of each of these elements is 0.1 wt.% Or less, there is no effect on the good chemical conversion treatment and corrosion resistance of the steel sheet of the present invention. Does not reach. In addition, even when a small amount of impurities such as Sn and Pb mixed in by the scrap input in the steelmaking process is contained in the steel sheet of the present invention, it does not have any influence on the good chemical conversion treatability and corrosion resistance. Absent. Therefore, the case where these elements are contained within the above range of content is also within the scope of the present invention.

Further, the steel sheet of the present invention may be any of cast steel sheet, hot rolled steel sheet and cold rolled steel sheet.

Next, the reasons for limiting the method of manufacturing the corrosion-resistant cold-rolled steel sheet according to the present invention will be described. A slab having the above-described chemical composition within the scope of the present invention (the same chemical composition as the steel No. 16 of the present invention in Table 1 described later) was prepared, and the slab heating temperature, hot rolling condition and annealing condition As a result of investigating the effect of the difference on the steel sheet, the following findings were obtained. That is, when a cold-rolled steel sheet is manufactured by a conventional method, the oxide scale on the surface of the steel sheet is pickled and removed after hot rolling, so that the influence of the previous steps on the surface properties of the steel sheet is small. On the other hand, the manufacturing conditions in the subsequent steps, especially the annealing conditions, have a great influence on the concentration of elements on the steel sheet surface and the thickness of the oxide film on the steel sheet surface. That is, when the steel sheet is at a temperature of 500 ° C. or higher during annealing, elements such as Si in the steel sheet that deteriorate the chemical conversion treatment tend to segregate on the surface of the steel sheet. Therefore, in annealing, it is advantageous to shorten the time for which the steel sheet is at a high temperature of 500 ° C. or higher in order to prevent the segregation, and if this time is 10 minutes or less, a particularly desirable surface oxide film is obtained. .

FIG. 3 is a graph showing the influence of the time during which the steel sheet was at a temperature of 500 ° C. or higher on the chemical conversion treatability of the steel sheet, which was obtained from the above-described test for examining the annealing conditions of the cold-rolled steel sheet. As is clear from the figure, particularly excellent chemical conversion treatability is exhibited when the temperature is 500 ° C. or higher for 10 minutes or less. Therefore, in order to manufacture a corrosion-resistant steel sheet having further excellent chemical conversion treatability, the time at which the temperature of 500 ° C. or higher in the annealing of the cold rolled steel sheet should be limited to 10 minutes or less.

It is to be noted that, in the manufacturing steps until obtaining the pickled steel sheet, the iron making steps such as the blast furnace method and the smelting reduction method, the steel making steps such as the electric furnace method and the converter method, the continuous casting method, the ingot making method and the strip. Casting or cast steel plate process such as casting method,
The hot rolling step, and the production conditions in each step of the pickling step are within the range of normal operating conditions, and have no influence on the good chemical conversion treatability and corrosion resistance that the steel sheet of the present invention should have. Does not reach. Further, the steel sheet of the present invention,
It can also be used as an original plate for surface treatment such as plating.

[0040]

Next, the present invention will be described in more detail with reference to Examples. Steels No. 1 to 16 of the present invention having chemical composition within the range of the present invention shown in Table 1 and steels of comparative steel No. 1 to 14 having chemical composition outside the range of the present invention were prepared. .

[0041]

[Table 1]

Then, each of the above steels was hot-rolled by a conventional method to prepare hot-rolled steel sheets. For each of the hot-rolled steel sheets thus obtained, a cold-rolled steel sheet was prepared by subjecting the steel to pickling and cold rolling by a conventional method, and then, to each of the cold-rolled steel sheets thus obtained, After annealing for 120 seconds at a temperature of 820 ° C, temper tempering is applied to obtain a plate thickness of 0.8 mm.
The cold-rolled annealed steel sheet of was prepared. As a result of an analytical test of the chemical composition of each cold-rolled annealed steel sheet thus obtained, it was confirmed that all the cold-rolled annealed steel sheets were in agreement with the chemical composition of each of the steel pieces shown in Table 1. confirmed.

Thus, the cold-rolled annealed steel sheet (hereinafter, referred to as "hereinafter referred to as" steel No. 1 to 16 of the present invention "having the chemical composition within the range of the present invention, obtained by the manufacturing conditions within the range of the present invention. The present invention specimen "No. 1 to 16", and
Comparative steel No. 3 having a chemical composition outside the scope of the present invention.
Cold rolled annealed steel sheets (hereinafter, referred to as “Comparative Specimen” No. 1
14), the thickness of the oxide layer on the surface of the steel sheet was measured, the corrosion resistance test was performed on the as-steel sheet and after coating, and the chemical conversion treatment test was performed. The test methods and evaluation methods are as follows.

A reduction method was used to measure the thickness of the oxide layer.
In this method, the surface oxide layer of the steel sheet is electrochemically reduced at a current density of 6.2 mA / cm ² in an aqueous solution of sodium tetraborate and dilute hydrochloric acid adjusted to pH 8.0, and oxidation is performed from the time required for the reduction. It is a measure of the layer thickness.

The corrosion resistance of the as-steel plate was tested by the salt water spraying atmospheric exposure test method. The evaluation of this corrosion resistance is
Spray 0.5% NaCl aqueous solution onto the steel sheet once / day, and
The corrosion weight loss value after 0 days was used.

The post-painting corrosion resistance test was carried out by the outdoor accelerated exposure test method. The corrosion resistance was evaluated by spraying the steel sheet with a 5.0 wt.% NaCl aqueous solution twice / week and evaluating the maximum swollen width generated in the cross-cut portion after 35 weeks.

The chemical conversion treatment test method and evaluation method are as follows:
According to the above-mentioned method, the P ratio shown in the above formula (2) and the crystal structure of the chemical conversion coating by an electron microscope were observed.

FIG. 4 shows an electron microscopic crystal structure of the phosphate film formed by the chemical conversion treatment. In the figure, a crystal structure whose entire surface is covered with a fine phosphate film is marked as "excellently good", and a crystal structure which is covered with a fine phosphate film but has a slightly large crystal grain size is shown. "Good"
A circle indicates that the crystal structure has a coarse phosphate crystal film partially present, and a circle indicates that the crystal structure is “somewhat poor”.
Then, the crystal structure, which is only a coarse phosphate crystal film, is represented by "X" as "poor". In the figure, ⊚ and ∘ are examples of the crystal structures of the samples of the present invention No. 16 and 6, and Δ and × are examples of the crystal structures of the comparative samples No. 3 and 7, respectively.

Table 2 shows the above-mentioned test results for each of the test specimen of the present invention and the test specimen for comparison, that is, the evaluation results of the thickness of the oxide film on the steel sheet surface and the corrosion resistance and the chemical conversion treatment property. The following items can be seen from Table 2.

[0050]

[Table 2]

For all the samples No. 1 to 16 of the present invention, the corrosion weight loss value was 300 g / m ² or less and the maximum bulge width was 3.5 mm or less, and the corrosion resistance was satisfied both in the as-steel plate and after coating. In addition, the crystal structure of the chemical conversion coating is ◎ or ○, and the P ratio is 9
It was 0% or more and was excellent in chemical conversion treatability. Moreover,
Among these, the sample No. 1, 3 to 5, 7 to 14 and 1 of the present invention in which the thickness of the oxide film is in the range of 10 to 100Å
For all of the 6 types, the P ratio was 94% or more, and the crystal structure of the chemical conversion treatment film was particularly good (marked with ⊚), the chemical conversion treatment was further excellent, and the maximum swelling width was 3.0 mm.
It was below, and the corrosion resistance after coating was further excellent.

On the other hand, in the comparative sample, at least one of the contents of each element of the chemical composition is outside the scope of the present invention, or the content of each element of the chemical composition is Is within the scope of the present invention, but since it does not satisfy the above (1), a steel sheet excellent in both corrosion resistance and chemical conversion treatability was not obtained. That is, the test results of the comparative specimen are as follows.

Comparative sample No. 1 was inferior in corrosion resistance because only the C content was outside the range of the present invention. Comparative sample No. 2 was inferior in chemical conversion treatability because only the Si content was high outside the range of the present invention. Comparative specimen N
O.3 was inferior in corrosion resistance because only Mn content was high outside the range of the present invention. Comparative sample No. 4 had a high P content only outside the range of the present invention, and therefore had good chemical conversion treatability but poor corrosion resistance. Comparative sample No. 5 was inferior in corrosion resistance because only the S content was high outside the range of the present invention.

Since the comparative sample No. 6 had a low Mn content only outside the range of the present invention, it was not excellent in chemical conversion treatability.
In addition, the surface oxide film was too thin, resulting in poor corrosion resistance. Comparative sample No. 7 was excellent in corrosion resistance but inferior in chemical conversion treatability because only the Cu content was high outside the range of the present invention. Comparative sample No. 8 is sol. Al
Since only the content was high outside the range of the present invention, the chemical conversion treatability was poor. Comparative sample No. 9 was excellent in chemical conversion treatability, but poor in corrosion resistance, since only the Cu content was low outside the range of the present invention. Comparative specimen N
In o.10, only the B content was low outside the range of the present invention, so the corrosion resistance was poor due to the progress of intergranular corrosion. Comparative sample No. 11 was inferior in chemical conversion treatability because only the S content was low outside the range of the present invention.

The comparative specimens Nos. 12 and 13 were within the range of the present invention for each chemical component composition, but did not satisfy the relationship on the right side of the above formula (1), and therefore, had no corrosion resistance. It was inferior. In the comparative sample No. 14, the content of each chemical component composition was within the range of the present invention, but the above (1)
Since the relationship on the left side of the formula was not satisfied, the chemical conversion treatability was poor.

Next, the billet of the steel No. 16 of the present invention having a chemical composition within the range of the present invention was subjected to a conventional hot rolling,
A hot rolled steel sheet was prepared. The hot-rolled steel sheet thus obtained is subjected to pickling and cold rolling by a conventional method to prepare a cold-rolled steel sheet, and then the cold-rolled steel sheet thus obtained is subjected to a predetermined 8 After annealing under various conditions, a predetermined temper rolling was performed to prepare a cold rolled annealed steel sheet having a sheet thickness of 0.8 mm. The cold-rolled annealed steel sheet thus prepared (hereinafter,
"Invention specimen" No. 30-A to 30-H), measurement of the thickness of the oxide layer on the surface of the steel sheet, corrosion resistance test for each of the steel sheet and after coating, and A chemical conversion treatment test was conducted. Each test method and evaluation method are the same as those described above.

Table 3 shows the samples No. 30-A to 30 of the present invention.
The above annealing conditions and test results for -H are shown.
In the present invention specimens No. 30-C, 30-E, 30-G, and 30-H, the time of annealing at a temperature of 500 ° C. or higher is 10 minutes or less, and the other invention specimens In the sample, the above time was more than 10 minutes.

[0058]

[Table 3]

The following matters can be seen from Table 3. That is, all the samples No. 30-A to 30-H of the present invention were excellent in both corrosion resistance and chemical conversion treatability. That is, the content of each element of the chemical composition is within the scope of the present invention,
In addition, when the S, P and Cu contents satisfy the above formula (1), the corrosion resistance and the corrosion resistance can be obtained both by the conventional manufacturing method and by the manufacturing method within the scope of the present invention. It was possible to obtain a steel sheet having both excellent chemical conversion treatability, and among these, by the production method within the scope of the present invention, it was possible to produce a steel sheet having further superior corrosion resistance and chemical conversion treatability.

[0060]

As described above, according to the present invention,
It is possible to provide a corrosion-resistant steel sheet excellent in chemical conversion treatability and a method for manufacturing a cold-rolled steel sheet thereof without adding special equipment. Therefore, in particular, when it is used for products requiring corrosion resistance such as automobiles, electric equipment and building material products, it is suitable to be used as it is as a steel plate, or it may be used by coating for corrosion protection. It is possible to provide a corrosion-resistant steel sheet having excellent chemical conversion treatability and a method for manufacturing the cold-rolled steel sheet, which is suitable, and brings an extremely useful effect industrially.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between S, P and Cu contents of a steel sheet and chemical conversion treatability.

FIG. 2 is a graph showing the relationship between the thickness of an oxide film on the surface of a corrosion-resistant steel sheet and chemical conversion treatability and corrosion resistance.

FIG. 3 is a graph showing the influence of the time during which the steel sheet was at a temperature of 500 ° C. or higher on the chemical conversion treatability of the steel sheet during annealing of the cold rolled steel sheet.

FIG. 4 shows an electron microscopic crystal structure of a phosphate film formed by chemical conversion treatment.

Claims (3)

[Claims] 1. Carbon (C): 0.0003 to 0.005 w
t.%, silicon (Si): 0.1 wt.% or less, manganese (Mn): 0.1 to 2.0 wt.%, phosphorus (P): 0.002 to 0.03 wt.%, sulfur (S): 0.003 to 0.020 wt.% , Acid soluble aluminum (sol.Al): 0.01-0.1 wt.%
, Copper (Cu): 0.05 to 0.5 wt.%, Chromium (Cr): 0.10 wt.% Or less (including no addition), and boron (B): 0.0002 to 0.0020 wt.%,
In addition, the content of sulfur (S), phosphorus (P) and copper (Cu) is expressed by the following formula (1): 24 × S (wt.%) / 32 + 2 × P (wt.%) / 31 ≧ Cu (wt) .%) / 63 ≧ 2 × S (wt.%) / 32 --------------- Chemical conversion composition characterized by having a chemical composition satisfying (1) Excellent corrosion resistance steel plate. 2. In addition to having the chemical composition as set forth in claim 1, further comprising an oxide film having a thickness of 10 to 100 Å on at least one surface of the steel sheet. Corrosion resistant steel plate with excellent processability. 3. A method for producing a corrosion-resistant steel sheet by cold rolling steel having the chemical composition according to claim 1 to prepare a cold-rolled steel sheet, and then annealing the cold-rolled steel sheet. In the method for producing a corrosion-resistant cold-rolled steel sheet having excellent chemical conversion treatability, the annealing is performed at a temperature of 500 ° C. or higher for 10 minutes or less.