JP3297783B2 - Manufacturing method of cold-rolled steel sheet for drawing with excellent corrosion resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a cold-rolled steel sheet for drawing which is excellent in corrosion resistance and is mainly used for steel sheets for automobiles and the like.

[0002]

2. Description of the Related Art In recent years, plated steel sheets have been increasingly used as interior and exterior materials for automobiles because of their need for excellent corrosion resistance. However, plated steel sheets are expensive, and have drawbacks such as deterioration of formability, reduction in workability due to occurrence of blowholes at welded portions, and costly measures for blowholes. For this reason, there is a strong demand for a steel sheet that is relatively inexpensive and has excellent corrosion resistance in place of a plated steel sheet.

A cold-rolled steel sheet that can meet such demands has properties such as excellent corrosion resistance, particularly excellent puncture resistance, little coating swelling after coating, and excellent drawability. It is necessary to have both.

[0004] As a method for producing a cold-rolled steel sheet for deep drawing excellent in corrosion resistance, for example, Japanese Patent Application Laid-Open No. 4-246128 discloses a method in which C: less than 0.006%, Si: 1.0% or less, Mn: 1.5% by weight.
%, P: 0.03 to 0.15% or less, S: 0.03% or less, Al:
0.01 to 0.10%, N: 0.01% or less, Cu: 0.18 to 1.5% and
Nb: 0.001 to 0.2% (however, Nb / C ≧ 3 and 0.01 ≦ (Cu
/64+P/31)×(Nb/C)≦0.10) is hot-rolled and cold-rolled, followed by recrystallization annealing in a predetermined temperature range.

Japanese Patent Application Laid-Open No. 3-150315 discloses that, by weight%, C: 0.001 to 0.006%, Si: 0.20% or less, Mn: 0.
10-1.5%, P: 0.04-0.10%, S: 0.010% or less, A
l: 0.02-0.07%, Cu: 0.10-0.40%, Ni: 0.0020-0.0
A method has been proposed in which a slab containing 080% and N: 0.0020 to 0.0080% is subjected to box rolling after hot rolling and cold rolling under predetermined conditions.

[0006] The above method is intended for a material steel containing a large amount of P and Cu in order to improve the corrosion resistance of the steel sheet. However, in recent years, in order to recycle (recycle) the steel sheet, it is difficult to remove it from the molten steel, and when it is reused as a casting, it segregates in the casting and tends to cause casting defects. There is a demand for a small number of steel sheets, and it is necessary to reduce the content of Cu and P as much as possible in steel sheets for automobiles while improving corrosion resistance.

[0007]

SUMMARY OF THE INVENTION The present invention relates to a method for producing Cu
Production of cold-rolled steel sheets with low P and Cu contents, with excellent corrosion resistance, and with excellent drawability that can be used without plating, based on the demand from the viewpoint of recycling to reduce the content of P and P It has been made to provide a method for performing the above.

[0008]

The gist of the present invention resides in a method for producing a cold-rolled steel sheet for drawing excellent in corrosion resistance, characterized by processing a raw steel slab having the following chemical composition under the following conditions.

Chemical composition of raw steel slabs: C: 0.001 to 0.006%, Si: 0.30% or less, Mn: 0.10 to 0.50%, P: less than 0.03%, S:
0.010% or less, Cu: 0.15 to 0.40%, Cr: 0.1%
Below, N: 0.005% or less, Al: 0.005 to 0.10
%, Ti: 0.01-0.1%, Nb: 0-0.1%
B: 0 to 0.0010%, Fe and inevitable impurities: balance.

Processing conditions The raw steel slab is heated at 1050-1170 ° C. for 180-300 minutes, hot-rolled, cold-rolled, and then continuously annealed at 780 ° C. or more.

[0011]

First, the function and effect of each component contained in the base steel used in the method of the present invention and the reason for limiting the content will be described. “%” Of the alloy component content is “% by mass”.
Means

C: 0.001 to 0.006% C is an element that has a great effect on corrosion resistance and formability. If the content of C exceeds 0.006%, cementite, which is a starting point of corrosion, may be formed, and the moldability is also deteriorated. On the other hand, if the content is less than 0.001%, the cost for steel making increases and it is not economical. Therefore, the content of C is 0.001
To 0.006%.

Si: 0.30% or less Si has a small effect of deteriorating the formability of steel, and is an element suitable for increasing the strength of steel. However, if added in excess, a dense zinc phosphate film is formed at the time of painting. It is difficult to be produced, and the swelling resistance of the coated film after coating may be deteriorated. Therefore, the Si content is set to 0.30% or less. The Si content may be substantially zero.

Mn: 0.10 to 0.50% When the Mn content exceeds 0.50%, the elongation decreases, and sufficient formability as a cold-rolled steel sheet for drawing may not be obtained. On the other hand, if it is less than 0.10%, not only the required strength cannot be obtained, but also minute MnS is reduced, and surface flaws due to FeS may occur.

Therefore, the content of Mn is set to 0.10 to 0.50%.

P: less than 0.030% P is frequently added to conventional corrosion-resistant steel as an element for improving corrosion resistance as described above. However, in the method of the present invention, Cu can be concentrated on the surface by heat treatment to improve the corrosion resistance as described later. In such a case, the effect of adding P is small, so there is no need to actively add P. Further, containing P at 0.03% or more is contrary to the object of the present invention, in that the content of P is reduced to obtain a steel material suitable for recycling.
Therefore, the content of P is set to less than 0.03%. Although there are technical and economic limitations, it is difficult to reduce P in steel to 0, but a lower value is desirable.

S: 0.010% or less S is an element which has an adverse effect on the pitting corrosion resistance. Inclusions which become the starting point of pitting corrosion increase with an increase in the content.
If the S content is 0.010% or less, the corrosion resistance to perforation is improved by reducing the starting point. Therefore, the S content is set to 0.010% or less. S is also an impurity, and the lower the content, the better.

Cu: 0.15 to 0.40% Cu is an essential element for improving corrosion resistance. In order to obtain the effect, it is necessary to contain 0.15% or more. Generally, the corrosion resistance is improved up to about 0.7% depending on the addition amount, but steel materials with a high Cu content are unsuitable for recycling. Therefore, in the present invention, the content of Cu is 0.
40% or less, and the heat treatment described later
The corrosion resistance is ensured by increasing the concentration of.

Cr: 0.1% or less When a steel material containing Cr is used bare, the corrosion resistance may be improved by an oxide film formed on its surface. However, this oxide film deteriorates the chemical conversion treatment, and the deterioration leads to the deterioration of the corrosion protection of the coating film. Since the corrosion resistance of a sound coating film is superior to that of the oxide film described above, it is more important to obtain a sound coating film for steel materials to be used after painting. Cr-containing steel easily causes film swelling even at defective portions of the film. Therefore, it is desirable not to contain Cr, but if the content is 0.1% or less, the degree of deterioration of chemical conversion treatment is small.

N: 0.005% or less It is known that N improves the drawability of a steel sheet by precipitating as AIN. However, when it is present as solid solution N, the formability is degraded, and when it is precipitated as TiN, the precipitation of TiC which improves the drawability is suppressed. Therefore, the content of N is set to 0.005% or less. In addition, since N has the above-mentioned preferable effects, at least
It is desirable to contain 0.001%.

Al: 0.005 to 0.10% Al (aluminum) is necessary for deoxidizing steel. Further, there is also an effect of precipitating solid solution N which deteriorates the formability as AlN to improve the formability. To ensure these effects, a content of 0.005% or more is required. However, even if the content of Al exceeds 0.10%, the effect does not increase and only the cost increases, so the upper limit is set to 0.10%.

Ti: 0.01-0.1% Ti is a solid solution that degrades the formability C and N and Ti (C, N)
｛11 which is advantageous for improving drawability
A texture of 1 ° orientation is formed. However, if the content is less than 0.01%, there is no effect, and a stretcher strain due to the occurrence of YPE (yield point elongation) may be generated during press molding. On the other hand, since the contents of C and N are kept low, the excessive addition of Ti exceeding 0.1% only causes an increase in cost. Therefore, the content of Ti is set to 0.01 to 0.1%.

Nb: 0-0.1% Nb precipitates as NbC, which is advantageous for improving drawability.
A texture of 1 ° orientation is formed. Therefore, Ti can be added as needed to supplement the action of Ti. However, if the content is less than 0.01%, the effect is small. On the other hand, since the addition amount of C is kept low, even if Nb is added in excess of 0.1%, the production cost only increases. Therefore, when Nb is added, its content is preferably 0.01 to 0.1%.

B: 0 to 0.0010% B is also an element that can be added as needed. The ultra-low carbon and the precipitation of C as TiC and NbC improve the formability of the steel sheet, but may cause embrittlement after drawing. B is an effective element for preventing embrittlement after this processing. Therefore, it is preferable to add B particularly when embrittlement after processing is concerned. When the content of B is less than 0.0001%, the above effect is small. Therefore, when B is added, the content is preferably set to 0.0001% or more. On the other hand, if the content exceeds 0.0010%, the effect is saturated. Therefore, the desirable content of B is 0.0001 to 0.0010.
%.

Next, the reason why the manufacturing conditions to be performed using the material steel having the above composition are determined as described above will be described.

The raw steel is melted in a converter or the like, and made into a slab by continuous casting or the like. The slab is hot-rolled, and the heating temperature is 1050 to 1170 ° C. If the temperature is lower than 1050 ° C, an edge or the like partially cuts _three points of Ar during rolling, which may cause deterioration in workability or shape defects. Meanwhile, 1170
At high temperature heating exceeding ℃, the surface properties may be deteriorated due to the melting of the concentrated Cu. Therefore, the heating temperature is 1050 ~
Set to 1170 ° C.

The heating time in the above temperature range is 180 to 300 minutes. This is to sufficiently concentrate Cu on the slab surface. During this heating, the residual Cu
Is concentrated on the surface to improve corrosion resistance.
It is necessary to heat for 180 minutes or more in order to concentrate the iron to the extent that the effect of improving the corrosion resistance is sufficiently exhibited.

However, even if the heating time is lengthened unnecessarily, the corrosion resistance is not improved and the scale loss and the heating cost are lost. Therefore, the heating time should be suppressed to 300 minutes or less.

The slab after the heating is hot-rolled, but the conditions are not particularly limited. After hot rolling, cold rolling is performed by a conventional method.

The thus obtained cold rolled steel sheet is subjected to continuous annealing. If the annealing temperature is lower than 780 ° C, it will not recrystallize sufficiently,
In some cases, satisfactory moldability cannot be obtained. Therefore, the annealing temperature should be 780 ° C or higher. The upper limit of the annealing temperature is
In order to avoid surface roughness after molding due to coarsening of crystal grains, the temperature should be kept at around 880 ° C.

FIGS. 1 to 3 summarize the results of investigations on the corrosion resistance of steel sheets produced by the method of the present invention after coating.
As a test piece, a portion having no defect such as a flaw on a surface collected from a cold-rolled steel strip having a thickness of 0.8 mm obtained by the above-described production method was used. In the test, after a chemical conversion treatment, an electrodeposition coating, an intermediate coating, and a top coating were applied, a coating film subjected to a cross cut reaching the steel sheet surface was used.

The corrosion test was carried out under a corrosion condition of 0.5% NaCl salt spray → dry → wet as one cycle. FIGS. 1 to 3 show the test results after 60 cycles. The evaluation is shown in FIG.
The measurement was performed by measuring the maximum corrosion depth of the cross cut portion and the maximum blister width of the coating film shown in (b).

FIG. 1 shows the relationship between the heating holding time and the maximum corrosion depth in a steel sheet in which the Cu content and the heating holding time during hot rolling were changed (the temperature was kept constant at 1100 ° C.).

FIG. 1 shows that the maximum corrosion depth becomes shallower as the Cu content increases. However, the content of Cu
Even at 0.20%, by keeping the heating and holding time at 180 minutes or more, Cu is concentrated on the steel sheet surface, the maximum corrosion depth becomes shallow, and there is almost no difference in the maximum corrosion depth from that with 0.35% Cu content. I have.

FIG. 2 shows the effect on the swelling width of the coating film when the contents of Cr and Si in the steel sheet are changed. If the Cr content exceeds 0.1%, the swelling of the coating film increases, but this is because the chemical conversion property deteriorates. When the content of Si is as high as 0.50%, the maximum swelling width is large even when the content of Cr is 0.1% or less. This means that the effect of deteriorating the chemical conversion treatment properties of Si is large.

FIG. 3 shows that the content of C in the steel sheet is changed.
It is the result of investigating the effect of C on the maximum corrosion depth.
It can be seen that the maximum corrosion depth sharply increases when C exceeds 0.006%.

[0037]

EXAMPLES Steels having the chemical compositions shown in Tables 1 and 2 were melted by a conventional method, formed into slabs by continuous casting, heated at the heating temperature and heating time shown in the same table, and then subjected to hot rolling. The finishing temperature of hot rolling was 900 ° C, and a hot rolled steel sheet with a thickness of 3.2 mm was produced at a winding temperature of 550 ° C. After the hot rolling, the surface was observed to check for the occurrence of surface flaws.

After pickling the hot-rolled steel sheet with an HCI solution,
Cold rolling was performed to a sheet thickness of 0.8 mm, and continuous annealing was performed at the annealing temperatures shown in Tables 1 and 2 to produce a cold-rolled steel sheet.

[0039]

[Table 1]

[0040]

[Table 2]

Using these steel sheets, the puncture resistance (maximum corrosion depth) and the swelling resistance of the coating film were examined. As a test piece for corrosion resistance evaluation, use a part without defects such as scratches on the surface,
After chemical conversion treatment, electrodeposition coating, intermediate coating, and top coating, the coating film was subjected to a cross cut reaching the steel sheet surface. The corrosion test was carried out under the above-described corrosion conditions of 0.5% NaCl salt water spray → dry → wet cycle. The test results after 60 cycles are shown in Tables 3 and 4.

On the other hand, using a JIS No. 5 test piece, the base metal tensile properties (rolling direction) and the average Rankford value (hereinafter referred to as r
Value) was measured. The r value was measured by a three-point method after 15% tensile strain was applied. r = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4. Here, r ₀ is the r value in the rolling direction, r ₄₅ is the r value in the 45 ° direction with respect to the rolling direction, and r ₉₀ is the r value perpendicular to the rolling direction.

Further, in order to investigate the brittleness after processing, the sampled steel sheet was processed into a disk and drawn at a drawing ratio of 1.8%.
Was formed, cooled to various test temperatures, placed on the table 2, and the weight 3 was dropped to apply an impact to break it. The brittle transition temperature (longitudinal crack transition temperature) was determined from the difference between the brittle fracture surface and the ductile fracture surface. The above aperture ratio is
It is represented by [disk diameter / (punch diameter + plate thickness)] × 100 (%).

Tables 3 and 4 show the results of a series of these tests.

[0045]

[Table 3]

[0046]

[Table 4]

As apparent from Table 3, in the steel sheet manufactured by the method of the present invention, the maximum corrosion depth is as small as 0.41 mm or less, and the swollen width of the coating film is as small as 2.8 mm or less. Also, the r value is 1.9 or more, which satisfies the characteristics required for a cold-rolled steel sheet for drawing. It is also clear that the vertical crack transition temperature is low and the secondary work embrittlement is small.

On the other hand, in Comparative Examples (see Table 4) in which the composition of the raw steel and / or the manufacturing conditions are out of the range defined by the present invention, at least one of the above-mentioned properties is inferior.

[0049]

According to the method of the present invention, it is possible to produce a cold-rolled steel sheet for drawing having low P, low Cu and excellent in puncture resistance and coating film swelling resistance. Although the steel sheet by this method has a low P, the heating conditions of hot rolling are 1050-1170 ° C and the heating holding time is 180-300 minutes. are doing. Further, the C content of 0.006% or less also contributes to the improvement of corrosion resistance. Further, by using low Si and low Cr, the coating film swelling resistance is good. A steel sheet manufactured under these conditions can be used without plating, particularly as an interior and exterior steel sheet for automobiles.

[Brief description of the drawings]

FIG. 1 is a view showing the influence of the Cu content of a steel sheet and the heating and holding time during hot rolling on the maximum corrosion depth.

FIG. 2 is a diagram showing the results of an investigation on the influence of the Cr and Si contents of a steel sheet on coating film swelling.

FIG. 3 is a diagram showing an effect of C of a steel sheet on a maximum corrosion depth.

FIG. 4 is a schematic view showing a method of a brittle fracture test after drawing.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-7-62446 (JP, A) JP-A-5-311235 (JP, A) JP-A-5-195145 (JP, A) JP-A-4- 141554 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 9/46-9/48 C21D 8/00-8/04

Claims (1)

(57) [Claims] (1) In mass%, C: 0.001 to 0.006%, Si: 0.
30% or less, Mn: 0.10 to 0.50%, P: less than 0.03%, S:
0.010% or less, Cu: 0.15 to 0.40%, Cr: 0.1% or less, N:
0.005% or less, Al: 0.005 to 0.10%, Ti: 0.01 to 0.1
%, Nb: 0 to 0.1% and B: 0 to 0.0010%, the balance being 10%.
Production of cold-rolled steel sheets for drawing with excellent corrosion resistance, characterized by heating at 50 to 1170 ° C for 180 to 300 minutes, hot rolling, cold rolling, and continuous annealing at 780 ° C or higher. Method.