JPH11256243A - Production of thick cold rolled steel sheet excellent in deep drawability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thick cold rolled steel sheet of >=1.0 mm sheet thickness excellent in deep drawability without the restriction of the draft at the finish rolling in hot rolling. SOLUTION: A steel slab contg., by mass, <=0.005% C, <=1.0% Si, <=2.0% Mn, <=0.15% P, <=0.02% S, 0.01 to 0.10% Al, <=0.005% N, Ti: (3.43*N+1.4*S+4*C) to 0.10, and the balance Fe with inevitable impurities is heated at 950 to 1,250 deg.C, thereafter, hot rolling is finished at >=850 deg.C, within 2.0 sec, cooling is started and is executed to make the average cooling rate till coiling to >=10 deg.C/s, it is coiled at >600 to 780 deg.C, is subjected to pickling and cold rolling and is subsequently annealed at the recrystallization temp. to less than the Ac3 transformation point to obtain a cold rolled steel sheet of >=1 mm sheet thickness. In this case, the hot rolling and cold rolling are executed to satisfy 1.75*t2+0.90<=t1<=1.75*t2+2.95 in the case the sheet thickness of the hot rolled sheet is defined as t1 (mm), and the sheet thickness of the cold rolled sheet as t2 (mm).

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 thick cold-rolled steel sheet having excellent deep drawability, which is used as a material steel sheet for automobile parts and the like.

[0002]

2. Description of the Related Art In automobile parts, the integral forming of a steel sheet has been advanced in order to reduce the weight and omit the steps. 1.0
In the case of structural members with a thickness of more than 1 mm, bending parts and drawing parts were conventionally manufactured by joining and integrating them, but for the same reason, these parts are processed using a single mold. Forming is progressing, and even a cold rolled thick steel sheet is required to have more workability.

[0003] As a method for producing a cold rolled thick steel sheet using ultra-low carbon steel excellent in deep drawability, Japanese Patent Application Laid-Open No. 63-10013 / 1988 is disclosed.
As disclosed in Japanese Patent Publication No. 4 (1993), the total reduction of hot finish rolling is 85% or more, and the final pass reduction is 10% or more and 50%.
Hereinafter, a technique has been proposed in which the winding temperature is limited to 600 ° C. or lower. This technology has a product thickness of 1
In the cold rolling process of a cold rolled steel sheet with a thickness of at least mm, it is difficult to perform high pressure (reduction rate of 80% or more), which is considered to be preferable for deep drawing, due to equipment or energy costs. This is a technique in which hot rolling conditions are defined so that a deep drawability as high as that of a high cold rolling rate can be obtained even at a low cold rolling rate.

[0004]

However, in this technique, the rolling reduction in the hot rolling stage is greatly restricted, and the operability is inferior. If the final pass of the hot rolling finish is performed at such a high rolling reduction, the plate thickness accuracy cannot be improved. Deterioration is a concern. In addition, since the winding temperature is as low as 600 ° C. or less, there is also a problem that carbide precipitation and aggregation do not sufficiently occur, and it is difficult to obtain good deep drawability.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and is intended to produce a thick cold-rolled steel sheet having a thickness of 1.0 mm or more, which is excellent in deep drawability without being restricted by a rolling reduction in hot rolling finish rolling. It provides a method.

[0006]

SUMMARY OF THE INVENTION According to the present invention, the relationship between the thickness of a hot-rolled sheet and the thickness of a cold-rolled sheet (product) is optimized, thereby making it possible to set a reduction ratio of hot finish rolling without a large reduction. In addition, it enables the production of cold rolled thick steel sheets with excellent deep drawability without setting the winding temperature particularly low.
And C ≦ 0.005%, Si ≦ 1.0%, Mn ≦ 2.0
%, P ≦ 0.15%, S ≦ 0.02%, Al: 0.01
０．１0.10%, N ≦ 0.005%, Ti: (3.43 * N + 1.
5 * S + 4 * C) -0.10%, the balance being Fe and unavoidable impurities.
After hot-rolling at 850 ° C. or higher,
Start cooling within seconds, and the average cooling rate until winding is 10
After cooling at a temperature of at least 600 ° C./s, winding at more than 600 to 780 ° C., performing pickling and cold rolling,
In order to obtain a cold-rolled steel sheet of 1 mm or more by performing annealing at a temperature lower than the Ac ₃ transformation point, when the thickness of the hot-rolled steel sheet is t1 (mm) and the thickness of the cold-rolled steel sheet is t2 (mm), 1.75 * t2 10.0 ≦ t1 ≦ 1.75 * t2
Hot rolling and cold rolling are performed to satisfy +2.95.

In the above steel components, T
The lower limit of i amount is (3.43 * N + 1.5 * S)%, and Nb:
0.005-0.04% and ((Ti- (3.43 * N + 1.5
* S)) / 4) + (Nb / 7.75) ≧ (1.0 * C). Or B: 0.0001 to 0.0
020%. Note that Ti and Nb
The symbol of an element in the formula for defining the content of the above means the content (mass%) of the element.

First, the limited range of the thickness of the hot rolled sheet and the cold rolled sheet (product sheet), which are important in the present invention, will be described based on experiments on which they are based. After heating the steel of the following components at 1150 ° C., the total rolling reduction in finish rolling is set to 80 to 90% (final pass rolling reduction 8%), and hot rolling is completed at 890 ° C.
A hot-rolled sheet having a thickness of 2.5 to 6.5 mm was obtained. After hot rolling, 0.5
After 2 seconds, it is cooled at a cooling rate of 70 ° C./s, wound up at 680 ° C., pickled, cold rolled at various rolling reductions to a thickness of 1.0 to 2.0 mm, and then continuously annealed at a soaking temperature of 850 ° C. Was done. A test piece was collected from the obtained product (cold rolled sheet), and the r value was determined. FIG. 1 is a graph showing the effect of the hot-rolled sheet thickness (t1) and the product (cold-rolled sheet) sheet thickness (t2) on the r value. In the figure, ○
Numbers in the middle indicate r values. Sample components (unit: mass%, balance substantially Fe) C: 0.0016%, Si: 0.01%, Mn: 0.1
4%, P: 0.005%, S: 0.004%, Al:
0.04%, N: 0.0023%, Ti: 0.04%

As is clear from the figure, a high r-value is obtained when the combination of the hot-rolled sheet thickness and the product (cold-rolled sheet) thickness is within a certain range, and the cold-rolling ratio at which the r-value reaches a peak is: It can be seen that it decreases as the product plate thickness increases. For example, at a product thickness of 1.0 mm, the r-value peaks at a cold rolling reduction of 73%,
Under a high pressure of 80% or more, the deep penetration is rather deteriorated.
This is a different finding from the prior art. The reason for this is not necessarily clear, but is considered as follows. That is, when the cold-rolling rate increases, the (200) texture develops during processing, and after the recrystallization, the unfavorable (200) orientation in the r-value increases to reflect this. When the hot-rolled sheet thickness is large, even if the cold-rolling rate is suitable for the case where the sheet thickness is small, if the cold-rolling rate is reduced, the absolute amount of strain received during processing increases. It is considered that the processed texture easily develops, and as a result, the r value decreases.

FIG. 1 shows the allowable range (1.75 * t2) of the hot rolled sheet thickness (t1) with respect to the product (cold rolled sheet) thickness (t2) of the present invention.
(0.90 ≦ t1 ≦ 1.75 * t2 + 2.95 ... (1) Formula) is also illustrated.
As is clear from the above results, the hot-rolled sheet thickness (t1)
If the value is smaller than the value on the left side of the equation (1), the rolling reduction in cold rolling becomes insufficient and good deep drawability cannot be obtained. On the other hand, even when t1 is larger than the right side of the equation (1), the reduction becomes excessive, and the deep drawability deteriorates. In addition, the cold rolling reduction is naturally determined by determining these sheet thicknesses,
In the present invention, it is less than 80%.

Next, the reasons for limiting the steel components of the present invention will be described. C ≦ 0.005% To obtain high ductility, the lower the C content, the better. 0.00
If it exceeds 5%, the amount of carbides increases, which adversely affects the deep drawability. Therefore, the upper limit is made 0.005%.

Si ≦ 1.0% If the content of Si is large, the strength is increased and the workability is reduced.
1.0% or less.

Mn ≦ 2.0% Mn is added in accordance with the desired strength, but if it exceeds 2.0%, the workability is reduced, so the upper limit is made 2.0%.

P ≦ 0.15% P is also added according to the desired strength, but is an element that greatly affects ductility. If it exceeds 0.15%, high ductility cannot be obtained. The upper limit is set to 0.15%.

S ≦ 0.02% Since S precipitates as Ti-based sulfide, if the amount of S is large, the amount of the precipitate increases, and the deep drawability decreases. Therefore, the upper limit is made 0.02%.

Al: 0.01 to 0.10% Al is required to be 0.01% for deoxidation.
If it exceeds 0%, inclusions increase and workability is impaired. Preferably it is 0.05% or less.

N ≦ 0.005% N precipitates as a nitride, but if the amount is large, the workability is impaired. The upper limit is set to 0.005% as a range having no problem in steelmaking technology.

Ti: (3.43 * N + 1.5 * S + 4 *
C) -0.10% Ti is added to fix C, N and S as precipitates. For this purpose, (3.43 * N + 1.5
* S + 4 * C) or more. Where N
When b is added, C may be fixed at Nb, so the lower limit of the Ti amount is (3.43 * N + 1.5 * S)%. However, if added in excess of 0.10%, solid solution Ti
Increases, and the workability decreases, so the upper limit is made 0.10%.

The steel used in the present invention contains the above components as essential components, and the balance consists of Fe and unavoidable impurities, but can contain one or more of the following components to improve the material.

Nb: 0.005 to 0.04% and ((Ti− (3.43 * N + 1.5 * S)) / 4) + (Nb / 7.7
5) ≧ (1.0 * C) Nb has the effect of refining crystal grains, improving in-plane anisotropy, and improving powdering resistance. Since the amount of C added is fixed together with Ti as a precipitate, ((Ti− (3.43 * N
+ 1.5 * S)) / 4) It is necessary that + (Nb / 7.75) is not less than (1.0 * C). On the other hand, if Nb is added unnecessarily, solute Nb increases and ductility is lowered, which leads to an increase in recrystallization temperature. Therefore, the upper limit is made 0.04%.

B: 0.0001 to 0.0020% B has an effect of improving secondary workability. If the amount is less than 0.0001%, the effect is too small.
When the content exceeds 020%, the deep drawability is reduced.
01% or more and 0.0020% or less. Preferably it is 0.0003-0.0010%.

Next, the manufacturing conditions will be described. First,
During hot rolling, a steel slab produced by ingot casting or continuous casting is heated at 950 to 1250 ° C. Heating temperature is 1
If the temperature exceeds 250 ° C., the amount of sulfide re-dissolved increases, so that fine precipitates increase after hot rolling, which is not preferable in ductility and deep drawability. On the other hand, if it is lower than 950 ° C., it is difficult to complete the subsequent hot rolling at 850 ° C. or higher. Preferably it is 1000 ° C. or higher and 1150 ° C. or lower.

Hot rolling is completed at 850 ° C. or higher. 85
If the temperature is lower than 0 ° C., the temperature may be lower than the Ar ₃ transformation point during hot rolling, and the plate shape may be deteriorated due to a discontinuous change in deformation resistance. This is because there is a possibility of adversely affecting the material after the cold rolling annealing. In the present invention, there is no restriction on the total rolling reduction and the final pass rolling reduction in finish rolling, and the product (cold rolled sheet)
The rolling reduction in the hot rolling stage may be appropriately set so that the thickness of the hot-rolled sheet satisfies the above-mentioned expression (1) with respect to the sheet thickness.

Cooling after hot rolling is carried out 2.0 hours after the completion of hot rolling in order to reduce the crystal grain size of the hot rolled sheet and obtain good deep drawability.
Start cooling within seconds and set the average cooling rate
Cooling is performed at a temperature of at least C / s. More preferably, the cooling start time is within 1.0 second, and the average cooling rate is 50 ° C./s or more.

The winding is performed at a high temperature exceeding 600 ° C. for precipitation of carbides. However, if the temperature exceeds 780 ° C., the ferrite grains may be abnormally coarsened, so the upper limit is set to 780 ° C. Preferably it is 650-750 degreeC.

As described above, the hot-rolled finished plate thickness is set in a range determined by the equation (1) using the product (cold-rolled plate) plate thickness as a variable. Annealing after cold rolling, the recrystallization temperature or more Ac ₃
The annealing may be performed at a temperature lower than the transformation point, and the method may be any of wire annealing and continuous annealing (including annealing of a hot dip coating line).

The cold-rolled steel sheet according to the present invention can also be used as a base sheet of a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet and an electroplated steel sheet.

[0028]

EXAMPLE Using a continuously cast slab of steel having the chemical composition shown in Table 1, the total reduction in the hot rolling stage was set to 80 to 95% (the reduction in the final pass of final rolling was 5 to 20%). A cold-rolled steel sheet was manufactured under the manufacturing conditions shown in Table 3, and the material properties were evaluated. In addition, the JIS No. 5 tensile test piece was used for the tensile test. The evaluation results are shown in the same table.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

[Table 3]

From Tables 2 and 3, it can be seen from the invention examples that there is no restriction on the rolling reduction in the hot rolling stage, and that a cold-rolled steel sheet having good elongation and r-value within a normal rolling level range can be obtained. I understand. In contrast, sample N whose components are outside the scope of the present invention
o. Sample Nos. 20 to 23 whose cooling start time or average cooling rate or winding temperature after hot rolling is out of the range of the present invention.
In No. 6, elongation and r-value are reduced. Further, it can be seen that the elongation and / or r value of Sample Nos. 27 to 30 in which the sheet thickness conditions are out of the range of the present invention are deteriorated as compared with the invention examples having the same level of tensile strength. Note that the (lower limit value, upper limit value) of t1 obtained from the equation (1) is (3.35, 5.40) in No. 27, and
(3.70, 5.75), No. 29 (2.65, 4.70), No. 3
At 0, it is (3.00,5.05).

[0033]

According to the present invention, in the hot rolling step, hot rolling may be performed such that the hot rolled sheet thickness is within a predetermined range with respect to the product (cold rolled sheet) sheet thickness. There is no restriction on the rolling reduction at the rolling stage, and the winding temperature can be high, so that it is excellent in operability and excellent in deep drawability. It is suitable.

[Brief description of the drawings]

[Fig. 1] Effect of hot-rolled sheet thickness (t1) on r value (number in ○)
7 is a graph showing the effect of the thickness of a product (cold rolled sheet) (t2).

Claims (3)

[Claims] 1. mass%, C ≦ 0.005%, Si ≦
1.0%, Mn ≦ 2.0%, P ≦ 0.15%, S ≦ 0.
02%, Al: 0.01 to 0.10%, N ≦ 0.005
%, Ti: (3.43 * N + 1.5 * S + 4 * C) -0.10%, the balance being Fe and unavoidable impurities. After finishing the inter-rolling, the cooling is started within 2.0 seconds, the cooling is performed so that the average cooling rate until winding is 10 ° C./s or more. After performing cold rolling, annealing at a temperature not lower than the recrystallization temperature and lower than the Ac ₃ transformation point to obtain a cold-rolled steel sheet of 1 mm or more requires a hot-rolled sheet thickness of t1 (mm) and a cold-rolled sheet thickness. When hot rolling and cold rolling are performed so that 1.75 * t2 0.90 ≦ t1 ≦ 1.75 * t2 + 2.95, where t2 (mm) is the thickness, cold-rolled steel sheet with excellent deep drawability characterized by excellent drawability Manufacturing method. 2. The method according to claim 1, wherein:
The lower limit of the Ti content is (3.43 * N + 1.5 * S)%, and N
b: 0.005 to 0.04% and ((Ti− (3.43 * N + 1.5 * S)) / 4) + (Nb / 7.7
5) A method for producing a cold rolled thick steel sheet excellent in deep drawability containing ≧ (1.0 * C). 3. The method for producing a cold rolled thick steel sheet according to claim 1, further comprising B: 0.0001 to 0.0020% and having excellent deep drawability.