JPH06346149A - Production of high strength cold rolled steel sheet good in plane anisotropy and excellent in deep drawability - Google Patents

Abstract

PURPOSE:To provide the method for producing a high strength cold rolled steel sheet good in plane anisotropy and excellent in deep drawability. CONSTITUTION:Steel having a compsn. contg. <=0.015% C, <=0.7% Si, 0.40 to 1.90% Mn, <=0.090% P, 0.015 to 0.080% Al, 0.010 to 0.06% Ti and 0.0003 to 0.0025% B is subjected to hot rolling, is subjected to >=65% cold rolling and is subjected to continuous annealing at the recrystallization temp. to the Ac3 point. In this way, the cold rolling of a good high strength cold rolled steel sheet excellent in (r) value which is the index of deep drawability, small in the absolute value of DELTAr and excellent in deep drawability can be made possible with small consumption energy of the cold rolling.

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 steel plate for high strength deep drawing.

[0002]

2. Description of the Related Art In recent years, high-strength steel sheets have been used because of the demand for improving fuel efficiency by reducing vehicle safety and weight. Of these, deep drawability is required for the outer and inner panel.
High-strength steel sheets for deep drawing are manufactured by adding carbonitride forming elements such as Ti and Nb to ultra-low carbon steel to which solid solution strengthening elements such as Mn, P and Si are added. For example, as a technique of adding Ti, there is Japanese Patent Publication No. 59-42742. Also N
As a method of adding b and Ti in combination, JP-A-60-779
No. 57 and Japanese Patent Application Laid-Open No. 61-246327 disclose a technique for adding Ti and V in combination.

All of these have the drawbacks that the absolute value of the r-value is as low as about 2.0 and that the in-plane anisotropy is large, which delays the description of high-strength steel sheets for parts that require deep drawability. There is. As a technique for improving this in-plane anisotropy, Japanese Patent Laid-Open No.
Japanese Patent Laid-Open No. 107414 is disclosed. This is to add Ti in an amount that fixes N and to fix C in Nb to reduce the in-plane anisotropy. However, this technique also has a low absolute value of the r value, and cannot apply a high-strength steel plate to a part that requires deep drawability. Further, these steels have the drawbacks that when they are rolled at a high speed during hot rolling, the strength increases, the cold rolling load increases, and cold rolling productivity and cold rolled sheet thickness accuracy deteriorate.

[0004]

DISCLOSURE OF THE INVENTION The present invention overcomes the above-mentioned drawbacks and provides a technique for producing a high-strength cold-rolled steel sheet having excellent deep drawability and less in-plane anisotropy without lowering the cold-rolling productivity. For the purpose of completion.

[0005]

Means for Solving the Problems In carrying out a detailed investigation of the relationship between various components of ultra-low carbon steel and material properties, the inventors have found that ultra-low C and an appropriate amount of Ti and Nb added to overcome the above-mentioned drawbacks. The combination technique was found and the present invention was completed. The gist of the present invention is, by weight%, C: 0.0015% or less, Si: 0.70% or less, Mn: 0.40 to 1.90%, P: 0.090% or less, Al: 0.015 to 0. 0.080%, Ti: 0.01
0 to 0.060%, B: 0.0003 to 0.0025
%, And optionally, a steel having a composition containing 0.020% or less of Nb is 70 after hot rolling and 65% or more of cold rolling.
This is a method for producing a steel sheet having a small in-plane anisotropy and a good deep drawability, which is characterized by continuous annealing at a temperature of 0 ° C. or more and Ac ₃ transformation point or less.

The present invention will be specifically described below. First, the reason why the steel component composition is limited to the above range in the present invention will be described. It was known that the lower C is, the more advantageous the material is. However, as a result of detailed examination of the influence of the amount of C, the present inventors found that the workability was better than expected and the in-plane anisotropy was higher than expected. It was found that The experimental facts are described below.

Si: 0.010%, Mn: 1.50%,
P: 0.050%, N: 0.0020%, Al: 0.0
35%, B: 0.0005% based on Ti: 0.07
% -Nb: 0.040%, Ti: 0.04% -Nb:
0.01%, Ti: 0.005% -Nb: 0.005%
For steel changed to C: 0.0003 to 0.0050%
The steel with the composition changed in the range of
After heating and holding at 250 ° C. for 1 hour, hot rolling was performed at a finishing temperature of 930 ° C. to 3.2 mm. Cooling rate after hot rolling is 50 ℃
After cooling for 1 hour to 650 ° C./sec, the furnace was cooled. Measure the tensile properties of this hot-rolled sheet, and after pickling 0.80m
It was cold-rolled to a thickness of m, annealed in a thermal cycle of 800 ° C. × 1 min of continuous annealing, and subjected to a material property investigation. The tensile strength increases slightly as the C content increases, but it is 412 MPa.
There was no significant change in the range of ˜423 MPa. r-value,
The relationship between Δr, the amount of C, Ti, and Nb is shown in FIG. The numbers in the figure are the amounts of Ti and Nb added.

As can be seen clearly from the figure, when the added amounts of Ti and Nb are small (Ti 0.005%, Nb 0.005%),
The r-value only slightly increases with a decrease in the C content, whereas in the Ti: 0.04%, Nb: 0.01% additive material, the r-value rapidly increases when the C content becomes 15 ppm or less.
The absolute value of Δr is also close to zero. On the other hand, Ti: 0.
When the amount of addition is too large as 07% and Nb: 0.04%, the r value is hardly improved even if the C amount is 15 ppm or less, and Δr is negative and the absolute value remains large. In this way, by extremely reducing the C content and adding an appropriate amount of Ti and Nb, it is possible to manufacture an extremely excellent deep drawability and high strength steel sheet. It is presumed that this is because TiC and NbC having an appropriate size distribution can be produced by an appropriate combination of Ti and Nb and the amount of C, and this produces a texture that is advantageous for improving deep drawability.

FIG. 2 shows the effects of tensile strength and C content of the hot rolled sheet. As can be seen from the figure, in the case where Ti and Nb are not added, the strength hardly changes even if the C content changes. On the other hand, a material containing a large amount of Ti and Nb has a high strength and hardly becomes soft even if the amount of C is reduced. Ti: 0.04
%, Nb: 0.01% of the added component system has a C content of 15 pp
If it is less than m, the strength is rapidly reduced. Since the cold-rolled steel sheet material is cold-rolled and annealed before it is used as a product, the lower the strength of the hot-rolled sheet, the easier the cold-rolling. Therefore, it goes without saying that the soft hot rolled sheet is preferable as the cold rolled material. The amount of C was specified from the above experimental facts. That is, good r
Value, Δr, the amount of C is specified to be 15 ppm or less as a condition that the strength of the hot-rolled sheet is not increased, and is preferably 10 ppm or less in order to improve the r value and the in-plane anisotropy.

Since FIG. 1 suggests that the characteristics are deteriorated when the amount of addition of Ti and Nb is too large unlike the conventional knowledge, the effect of the amounts of Nb and Ti was examined when the amount of C was 10 ppm. The results are shown in Fig. 3. The component test conditions other than the amounts of C, Nb, and Ti are the same as in the case of FIG. The amount of C is 10
The target was ppm, but it varied in the range of 8 to 12 ppm. As can be seen from the figure, the r value increases with an increase in the Ti amount, reaches the maximum value at 0.050%, and decreases slightly with the addition of more. When Ti and Nb are added in combination, there is no great influence in the low Ti addition region, but the r value decreases in the high Ti addition region. Further, when the amount of Nb added increases, the r value becomes lower in most of the Ti addition region than in the Ti single addition material. Δ
r decreases with an increase in the Ti addition amount when Ti alone is added or when Nb: 0.02% is added in combination, but when the Ti addition amount increases, the value becomes negative and the absolute value increases. N
When 0.04% of b is added, it becomes negative Δr in the whole area of Ti addition, and its absolute value also becomes large. Good r value, Δ
The amount of Ti and Nb was specified as a condition that combines r. A preferable range is Ti: 0.020 to 0.045%, Nb:
It is 0.015% or less.

Next, components other than C, Nb and Ti will be described. It is well known that Si is an element that strengthens deep drawability without deteriorating it, but if it exceeds 0.70%, the surface properties of the cold rolled steel sheet deteriorate. For this reason, the amount of Si was specified as 0.70%. The lower limit is 0.001%, which is the lower limit industrially practicable because Si may not be added in some cases.
Is. Mn is also an element effective for strengthening without impairing the deep drawability. However, if it exceeds 2.0%, it becomes difficult to achieve an extremely low carbon, and the strength of the hot-rolled sheet becomes remarkably large, which deteriorates the cold rolling property. Therefore, the upper limit is specified to be 2.0%. The lower limit is 0.45%. The preferred range is 0.5
0 to 1.5%.

Similar to Si and Mn, P is also an element effective for high strength of the steel sheet without deteriorating the deep drawability. However, 0.
If it exceeds 10%, the ductility and deep drawability of the steel sheet will be deteriorated, and at the same time, even if B, which will be described later, is added, the secondary workability of the product after pressing cannot be secured. For these reasons, the upper limit was specified as 0.10%. Al is 0.01 because it is deoxidized.
5% must be added, but the addition of more than 0.080% adversely affects the surface properties, so the upper limit is 0.080.
Specified in%. Since S becomes an inclusion such as MnS or TiS or a precipitate and deteriorates the ductility, it is preferably made 0.02% or less.

N combines with Al and Ti to form AlN and TiN
Since it becomes a precipitate, it does not have a great influence on the material properties of the steel sheet, but if it is too much, it adversely affects the workability, so it is preferably made 0.0040% or less. B is an element that segregates at the grain boundaries and improves the secondary workability of the steel sheet, so B must be added. In order to secure the secondary workability, it is necessary to add at least 0.0003%. However, if it exceeds 0.0025%, the steel sheet is hardened during hot rolling. Further, since the in-plane anisotropy of the r value is increased, 0.00
The upper limit was 25%.

The steel having the above composition is melted in a conventional melting furnace such as a converter and an electric furnace-a vacuum degassing furnace. Then, continuous casting,
The billet is formed by ingot casting, and the thickness of the billet does not impair the features of the present invention. Even if the steel slab is subjected to hot rolling as it is at a high temperature, even if it is reheated after cooling and hot-rolled, and the hot-rolling heating temperature changes, the characteristics of the present invention are not impaired. I don't care. The hot rolling finishing temperature is preferably set to the Ac ₃ point temperature or higher from the viewpoint of ensuring deep drawability. In the hot rolling ROT cooling, it is not necessary to specify the cooling conditions because the steel sheet does not harden in the steel of the present invention even if it is rapidly cooled. The winding temperature does not have to be specified in particular, but in the present invention, it is mainly carried out in the range of 600 to 700 ° C. in consideration of pickling property and the like.

The hot-rolled sheet is cold-rolled after descaling.
The cold rolling rate must be 65% or more to obtain a good r value and Δr. 75% is preferable for the same reason
As described above, it is 90% or less. The steel of the present invention has better cold rolling energy and plate thickness accuracy than the prior art steels. Table 2 shows the cold energy consumption and the cold-rolled sheet thickness variation of the steel having the components shown in Table 1 at a cold reduction of 70%. The cold rolling energy consumption is shown as a ratio to the comparative steel, and the plate thickness fluctuation is shown as a value obtained by subtracting the minimum plate thickness from the maximum plate thickness over the entire length of the 20 ton coil.
As can be seen from Table 2, the energy consumption of cold rolling can be greatly reduced to 70% of the comparative steel produced by the conventional technique. Another feature is that it is possible to significantly reduce the plate thickness variation of the cold rolled coil. This effect is very useful industrially.

[0016]

[Table 1]

[0017]

[Table 2]

The annealing may be continuous annealing or box annealing, but the effect of the present invention is remarkable in the continuous annealing method. The annealing temperature must be higher than the recrystallization temperature to secure workability. In order to reveal the characteristics of the present invention, it is preferable to anneal at a high temperature of 775 ° C or higher. Cooling after recrystallization annealing may be rapid cooling, gradual cooling, or overaging treatment,
Even if it is not performed, the characteristics of the present invention are not impaired, and thus there is no particular limitation. The annealed steel sheet is temper-rolled as necessary and provided as a product. The method of the present invention is zinc other than cold rolled steel,
R even when applied to hot-dip aluminum or electroplated steel
It exhibits high values and small in-plane anisotropy.

[0019]

EXAMPLE Steels having the components shown in Table 3 were produced under the production conditions shown in Table 4. The material properties of the steel sheet are also shown in Table 4.
The tensile property is a value in the rolling direction, and the r value is an average value in a usual method. The energy consumption ratio of cold rolling is 1.
It was set to 0, and it was shown by the ratio with the closest final product strength. A-1 to H-2 are Examples within the scope of the present invention,
K-1 to P-1 are comparative examples. B-1 and C-1 are examples of the 343 MPa class. High C content 343MP
It can be seen that the r value is higher and the in-plane anisotropy is smaller than that of the a-class comparative example K-1. Also, it can be seen that the energy consumption of cold rolling can be significantly reduced. A-1, D-1, A-2
Is an example of 372 MPa class. A steel sheet having an r-value and in-plane anisotropy superior to L-1 of the comparative example is obtained. Especially A
-2 is an example of high temperature annealing, but an extremely high value of r value = 2.5 was obtained.

E-1, F-1, G-1, E-2. F-2
Is an example within the scope of the present invention in the 392 MPaKg class. Three
N- which is an example in which the C content in the 92 MPa class is outside the range of the present invention.
1 and Ti and Nb contents are excellent r values as compared with P-1 outside the range of the present invention, and the in-plane anisotropy is also small,
It can be seen that the cold rolling energy consumption is also small. H-
1, I-1, J-1, and H-2 are examples of the scope of the present invention in the 441 MPa class. M-1 is an example in which the amount of C is outside the range of the present invention, and 0-1 is an example in which the amounts of Ti and Nb are outside the range of the present invention. In each case, the r value is low and the absolute value of Δr, which is an index of in-plane anisotropy, is large as compared with the examples of the 441 MPa class within the scope of the present invention.

[0021]

[Table 3]

[0022]

[Table 4]

[0023]

According to the present invention, a steel sheet having a high r value and a small absolute value of Δr and having excellent deep drawability can be manufactured without using a large amount of energy consumed for cold rolling.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between C amount, r value, and Δr,

FIG. 2 is a graph showing the relationship between the C content and the tensile strength of the hot rolled sheet,

FIG. 3 is a graph showing the relationship between Ti amount, r value, and Δr.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kunio Nishimura 1-1, Toibata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka New Nippon Steel Corporation Yawata Works

Claims (2)

[Claims] 1. C: 0.0015% or less by weight%, Si: 0.70% or less, Mn: 0.40 to 1.90%, P: 0.090% or less, Al: 0.015 to 0 0.080%, Ti: 0.010 to 0.060%, B: 0.0003 to 0.0025%, hot rolled steel having a composition of 6 after descaling.
A method for producing a high-strength cold-rolled steel sheet having small in-plane anisotropy and excellent deep drawability, which is characterized by performing continuous annealing at a temperature of not less than recrystallization temperature and not more than Ac ₃ transformation point after cold rolling of 5% or more. . 2. By weight%, C: 0.0015% or less, Si: 0.70% or less, Mn: 0.40 to 1.90%, P: 0.090% or less, Al: 0.015 to 0. 0.080%, Ti: 0.010 to 0.060%, B: 0.0003 to 0.0025%, Nb: 0.020% or less, the steel containing the composition is hot-rolled, and after descaling. 6
A method for producing a high-strength cold-rolled steel sheet having small in-plane anisotropy and excellent deep drawability, which is characterized by performing continuous annealing at a temperature of not less than recrystallization temperature and not more than Ac ₃ transformation point after cold rolling of 5% or more. .