Note: Descriptions are shown in the official language in which they were submitted.
20811~
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a method of
manufacturing a cold rolled steel sheet that exhibits
excellent resistance to cold-work embrittlement and a
small planar anisotropy by the continuous annealing
method which is suitable as a pressed steel sheet for use
in automobiles.
Description of the Related Art
When a cold rolled steel sheet is manufactured, a
continuous annealing, including heating and cooling which
last for a short period of time, is generally conducted
subsequent to the cold rolling. In this continuous
annealing process, the material quality of the product is
greatly affected by the chemical composition of the
material. Hence, to obtain a steel sheet exhibiting
excellent deep drawing property and stretchability, it
has been the practice to add a carbide/nitride producing
component, such as Ti or Nb, to the extra low carbon
steel.
However, the steel sheet in which Ti or Nb is
present is characterized in that Ti is readily combined
with C, S, N or O in the steel to form a precipitate.
Consequently, the grain boundary is cleaned and the grain
boundary strength is thus greatly reduced, increasing the
2~81496
possibility that a brittle fracture (the fracture due to
cold-work embrittlement) will occur after deep drawing.
Also, it has been a practice to obtain a high-strength
steel sheet by adding Mn, Si or P to the steel material.
In that case, however, since Si and P readily embrittle
the steel sheet, the resistance to cold-work
embrittlement greatly deteriorates. To improve such a
drawback, B has been added to the steel in the form of a
solid solution to increase the grain boundary strength,
like C.
However, it is well known that adding B deteriorates
the formability. Therefore, the proportion of B to be
added is restricted to such a small value that sufficient
resistance to cold-work embrittlement cannot be obtained.
Various other methods of improving the deep drawing
property and stretchability of the steel sheet by
controlling the conditions of hot rolling, cold rolling
or annealing during the manufacturing process of the
steel sheet have also been suggested. Generally, the hot
rolling finishing temperature is set to an Ar3
transformation point or above from the viewpoint of
improving the deep drawing property. The coiling
temperature is between 650 and 800C from the viewpoint
of improving the formability, especially deep drawing
properties. The annealing temperature is set to a
208 ~ 496
relatively low temperature which is equal to or higher than
the recrystallization temperature and which is effective in
terms of the energy.
Unexamined Japanese Patent Publication No.
62-278232 (published December 3, 1987) discloses a method of
manufacturing a cold rolled steel sheet of the aforementioned
type for use in non-aging deep drawing by the direct hot-
rolling method. Unexamined Japanese Patent Publication No.
1-177321 (published July 13, 1989) discloses a method of
manufacturing a cold rolled steel sheet of the aforementioned
type which exhibits an excellent deep drawing property.
Unexamined Japanese Patent Publication No. 2-200730
(published August 9, 1990) discloses a method of
manufacturing a cold rolled steel sheet of the aforementioned
type which exhibits an excellent press formability. In any
of these methods, although B is added to improve the
resistance to cold-work embrittlement, there is no concrete
disclosure to exhibit brittle transition temperature. Also,
coiling is performed at a high temperature of 640~C or above
which impairs descaling ability in a pickling process.
Therefore, in any of these methods, a sufficient improvement
in the resistance of cold-work embrittlement cannot be
expected.
Unexamined Japanese Patent Publication No.
63-241122 ~published October 6, 1988) discloses a method of
manufacturing a continuously galvanized steel sheet for use
in a super deep drawing. In this method, the proportion of B
contained is 0.0010 % or below, which is too small to improve
~A 7
3461-39
2081 4~6
the resistance to cold-work embrittlement.
Unexamined Japanese Patent Publication No. 62-40318
(published February 21, 1987) discloses a method of
manufacturing a cold rolled steel sheet exhibiting an
excellent deep drawing property. Unexamined Japanese Patent
Publication No. 1-188630 (published July 27, 1989) discloses
a method of manufacturing a cold rolled steel sheet
exhibiting an excellent press formability. However, in any
of these methods, there is no concrete description of the
resistance to cold-work embrittlement, and annealing is
conducted at a temperature ranging between the
recrystallization temperature and 800C. Therefore, a
sufficient improvement of the resistance to cold-work
embrittlement cannot be expected.
Unexamined Japanese Patent Publication No.
61-133323 (published June 20, 1986) discloses a method of
manufacturing a steel sheet exhibiting an excellent
formability. Unexamined Japanese Patent Publication No.
62-205231 (published September 9, 1987) discloses a method of
manufacturing a high-strength steel sheet. Both of these
methods are disclosed to the manufacture of a slab thinner
than a normal one and to alleviation or simplification of the
rolling process of steel sheet using such a thin slab.
However, in the former method, there is no concrete
description on the conditions of the annealing which is
conducted subsequent to the cold rolling process. Although
there is a concrete disclosure of the resistance to cold-work
embrittlement, the effect thereof is insufficient. In the
.;~ ~ 73461-39
i A
2081 496
latter method, there is a concrete disclosure of the
annealing which is conducted at a temperature of 775~C or
below. However, sufficient improvement in the resistance to
cold-work embrittlement cannot to be expected under such
conditions.
In any of the aforementioned conventional methods,
it is thus difficult to readily obtain a cold rolled steel
sheet exhibiting an excellent deep drawing property and an
excellent resistance to cold-work embrittlement.
Planar anisotropy, known as one of barometers of
the press formability, is generally evaluated by ~r. The
closer to zero the planar anisotropy value is, the more
uniform characteristics in each direction can be obtained,
which is desirable in terms of press formability. Unexamined
Japanese Patent Publication No. 61-64852 (published April 3,
1986) discloses a method of improving this planar anisotropy
by adding a relatively large amount of Nb in an extra low
carbon steel. Although this method is effective in improving
the planar anisotropy, it deteriorates elongation (El) or r
value. No method of improving the resistance to cold-work
embrittlement as well as the planar anisotropy has been
disclosed.
73461-39
2081496
SUMMARY OF THE INVENTION
It is an object of the present invention to provide
a method of manufacturing a cold rolled steel sheet for
use in deep drawing which exhibits an excellent
resistance to cold-work embrittlement and a small planar
anisotropy while maintaining an excellent deep drawing
property without the need for finely controlling the
manufacturing conditions even when a continuous annealing
process is employed.
The present inventors have made intensive studies on
the composition to be added and the manufacturing method
and discovered that it is possible to manufacture a cold
rolled steel sheet for use in deep drawing which exhibits
an excellent resistance to cold-work embrittlement and a
slnall planar anisotropy from an extra low carbon steel in
which Ti, Nb, B and Al are present each in an adequate
amount by adequately setting the hot rolling and
annealing conditions in the manufacturing process.
That is, the present invention provides a method of
manufacturing a cold rolled steel sheet which exhibits an
excellent resistance to cold-work embrittlement and a
small planar anisotropy which comprises the steps of
preparing, as a material, a steel whose composition
consists of:
C : 0.004 wt% or less
2~81~96
Si : 0.10 wt% or less
Mn : 0.50 wt% or less
Ti : between 0.01 wt% and 0.10 wt%
Nb : between 0.003 wt% and 0.03 wt%
B : between 0.001 wt% and 0.004 wt%
Al : between 0.03 wt% and 0.10 wt%
P : 0.025 wt% or less
S : 0.01 wt% or less
N : 0.006 wt% or less
Ti and C satisfying the following equation:
3 ~ Ti*/C ~ 12
where Ti* = Ti - (48/14)N - (48/32)S
balance : iron and unavoidable impurities,
performing a hot rolling on the material steel under the
conditions of a finish temperature between 800C and
900C, coiling the material at a temperature lower than
650C, performing a cold rolling, performing a continuous
annealing at a temperature between 830C and an Ac3
transformation point, and performing skin pass rolling.
Other features and variations of the present
invention will be apparent from the following description
taken in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. l(a) is a graph showing the relationship
between the coiling temperature and the brittle
2081 496
transition temperature;
Fig. l(b) is a graph showing the coiling temperature
and the planar anisotropy (~r);
Fig. 2(a) is a graph showing the relation between
the annealing temperature and the brittle transition
temperature;
Fig. 2(b) is a graph showing the relation between
the annealing temperature and the planar anisotropy (~r);
and
Fig. 3 is a graph showing the relation between the
thickness of the steel and the brittle transition
temperature regarding steels in which different amounts
of B are present.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention will be described below
concretely.
First, the reason for the restrictions placed on the
compositions will be explained.
C : 0.004 wt% or less
A smaller possible proportion of C is advantageous
to improve the material quality. An increase in the
amount of C contained increases the amount of Ti required
to fix C, thus increasing the amount of precipitates
produced and thereby deteriorating the material quality.
More than 0.004 wt% of C greatly deteriorates the
2081 496
material quality. Therefore, up to 0.004 wt% of C is
preferred.
Si : 0.10 wt% or less
Although the presence of Si is advantageous to
obtain adequate steel strength, it promotes the cold-work
embrittlement and degrades the phosphatability. Thus,
the upper limit of the proportion of Si is set to 0.10
Wt%.
Mn : 0.50 wt% or less
Although the inclusion of Mn is effective to obtain
an adequate strength of the steel, as in the case of Si,
it increases the tendency for a solid solution to be
produced and hence deteriorates the drawing property.
The presence of Mn also increases the production cost.
Hence, the upper limit of the proportion of Mn is set to
0.50 wt%.
Ti : 0.01 to 0.1 wt%
3 ~ Ti*/C ~ 12
where Ti* = Ti - t48/14)N - (48/32)S
The presence of Ti promotes precipitation of N and
S and hence improves the deep drawing property. That is,
in a cold rolled steel sheet on which the continuous
annealing has been conducted, a reduction in the amounts
of C, N and S contained alone is not enough to provide
the press formability which is as good as that of a steel
208 1 496
sheet which has been subjected to the box annealing
process. In this invention, Ti promotes precipitation of
N and S in the hot rolling process. Precipitation of C
is promoted by a combination of Ti and Nb which will be
described below. Precipitation of N by Ti enables B to
be present in a solid solution which is effective to
improve the resistance to cold-work embrittlement.
To stabilize C, N and S, at least 0.01 wt% of Ti
must be added. More than 0.1 wt% of Ti does not increase
the effect thereof.
Furthermore, it is necessary for Ti and C to be
added in a range which satisfies the following equation
(1)
3 S Ti*/C S 12
where Ti* = Ti - (48/14)N - (48/32)S.
The amount of Ti obtained by the above equation is
the effective amount of Ti other than the amount which is
consumed as nitride or sulfide. When Ti*/C < 3, if
coiling is performed at a low temperature of 650C or
less during the hot rolling process, as in the case of
the present invention, part of C remains in the form of
a solid solution, deteriorating the deep drawing
property. When Ti*/C > 12, although the deep drawing
property does not deteriorate, the phosphatability
deteriorates. As a result, 3 S Ti*/C S 12.
2o8lq96
Nb : 0.003 to 0.03 wt%
The presence of Nb, which is a carbide forming
component, improves the deep drawing property. The
addition of Nb together with Ti increases the average r
value and elongation. At least 0.003 wt% is required to
obtain the effect of Nb. However, more than 0.03 wt% of
Nb reduces the elongation. Thus, the desired proportion
of Nb is between 0.003 wt% and 0.03 wt%.
B : 0.001 to 0.004 wt%
As mentioned above, the addition of B intensifies
the grain boundary, like C, and hence improves the
resistance to cold-work embrittlement. However, an
excessive proportion of B increases the tendency for the
average r value and elongation to deteriorate, and thus
is not desirable in terms of the steel sheet for use in
deep drawing. A preferred proportion of B is between
0.001 wt% and 0.004 wt%.
Al : 0.03 to 0.1 wt%
Al is a nitride forming component. The addition of
Al together with Ti and Nb forms composite precipitates
which are inferred as (Ti, Nb)C and (Ti, Al)N and hence
promotes precipitation of C and N. It also improves the
formability, particularly, the deep drawing property and
reduces the planar anisotropy. At least 0.03 wt% of Al
is necessary for the above-mentioned effects. More than
208 1 49~
0.1 wt% of Al does not improve the effect of Al and
increases the production cost. Therefore, a desired
proportion of Al is between 0.03 wt% and 0.1 wt%.
P : 0.025 wt% or less
An excessive proportion of P increases the amount of
grain boundary which is segregated and hence promotes the
grain boundary embrittlement, and thus, deteriorates the
resistance to cold-work embrittlement. Hence, the
smaller the proportion of P, the better. 0.025 wt% or
less of P is allowable.
S : 0.01 wt% or less
An excessive proportion of S, which is a hazardous
component, readily promotes the grain boundary
embrittlement and thus deteriorates the resistance to
cold-work embrittlement. Thus, a smaller possible
proportion of S is desired. 0.01 wt% or less of S is
allowable.
N : 0.006 wt% or less
Like C, a smaller possible proportion of N is
desirable from the viewpoint of improvement in the
formability, particularly, deep drawing properties. The
presence of N also deteriorates the resistance to strain
aging. Thus, up to 0.006 wt% of N is allowable.
The reason for the restrictions placed on the
manufacturing process conditions in the present invention
will be described below. 2 08 1 4 9 6
Steel making process
Steel may be manufactured in a normal method which
employs, for example, a converter. There is no
restriction on the conditions of the steel making
process.
Steel may be manufactured in a normally employed
continuous casting or ingot casting method.
Hot rolled process
Finishing temperature : 800 to 900C
A finishing temperature lower than 800C
deteriorates the average r value and the elongation due
to residual strain. A finishing temperature higher than
900C increases the size of the grains and hence
deteriorates the average r value. Thus, a desired
finishing temperature range is from 800C and 900C.
Coiling temperature : lower than 650C
Conventionally, a high coiling temperature ranging
from 650C to 800C has been employed because it has been
considered that coiling conducted at such a high
temperature further increases the size of the TiC
precipitates and thus improves the elongation and average
r value. It has also been considered that nuclei of TiC
and (Ti, Al)N are not readily generated and the
precipitation speed is thus slowed down or precipitation
14
- 208 1 496
is made incomplete in the coiling conducted at a low
temperature, making precipitation of C and N insufficient
and deteriorating the elongation ar-d average r value.
The present inventors made various experiments in
which different coiling temperatures were employed, and
discovered that coiling conducted at a low temperature
provided a steel sheet which exhibited an excellent
resistance to cold-work embrittlement and a small planar
anisotropy.
The results of the experiments are shown in Fig. 1
(a) which is a graph showing the relation between the
coiling temperature and the brittle transition
temperature which is the index of the cold-work
embrittlement. Fig. 1 (b) is a graph showing the
relation between the coiling temperature and the planar
anisotropy ~r. As shown in these figures, a reduction in
the annealing temperature improves the resistance to
cold-work embrittlement and reduces the planar
anisotropy.
In the steel having the composition restricted by
the present invention, it is considered that the planar
anisotropy is reduced because precipitation of (Ti, Nb)C
and (Ti, ~l)N begins in the high-temperature range
obtained before the hot rolling is finished and is
promoted in the coiling conducted at a low temperature,
i.e., from about 300C to lower than 650C,
73461-39
208 1 496
precipitating C and N to a sufficient extent and reducing
the size of the grains which have been subjected to the
hot rolling process. It is also considered that the
formation of such precipitates promotes segregation of B
into the grain boundary, intensifies the grain boundary
and thus improves the resistance to cold-work
embrittlement.
Thus, the upper limit of the coiling temperature is
set to 650C from the viewpoint of an improvement in the
resistance to cold-work embrittlement and a reduction in
the planar anisotropy. Although there is no restriction
on the lower limit, a desirable lower limit is set to
300C with the cooling ability and cooling time or the
coil shape obtained taken into consideration.
The samples used in the aforementioned experiments
were manufactured under the following conditions using,
as a material, a steel which contained 0.003 wt% of C,
0.01 wt% of Si, 0.15 wt% of Mn, 0.03 wt% of Ti, 0.005 wt%
of Nb, 0.002 wt% of B, 0.06 wt% of Al, 0.015 wt% of P,
0.005 wt% of S and 0.004 wt% of N.
Hot rolling finishing temperature : 890C
Coiling temperature : 300 to 850C
Cold rolling reduction : 80%
Thickness of a cold rolled sheet : 0.7 mm
Continuous annealing conditions : 860C and 20
2o8l496
seconds
Skin pass reduction : 1%
The brittle transition temperature was measured by
measuring the highest temperature at which the brittle
fracture occurred in each of the conical cup samples each
having a blank diameter of 50 mm, a diameter of a dice of
24.4 mm and a punch diameter of 20.64 mm in the crash
tests by employing different testing temperatures.
The planar anisotropy ~r was calculated by the
following equation (2) using the value in the L direction
(the direction of rolling) rL, the value in the D
direction (the direction which is 45 degrees from the
direction of rolling) rD and the value in the C direction
(the direction which is 90 degrees from the direction of
rolling) rc which were measured using the sample to which
a tensile strain of 15~ was applied beforehand:
~r = (rc + rL - 2rD)/2 ... (2)
As is clear from Fig. 1 (b), a desirable range of
the planar anisotropy ~r is as follows:
0 ~ ~r ' 0.25
A planar anisotropy ~r of more than 0.25 increases
the inhomogeneous strain distribution and thus
deteriorates the formability.
Continuous annealing temperature : 830C to Ac3
transformation point
17
2081 4q6
Conventionally, no restriction has been placed on
the annealing temperature in the continuous annealing
process because it has been considered that the material
characteristics are determined by the hot rolling
conditions. However, the present inventors have
researched and found that the annealing temperature
greatly affected cold-work embrittlement (the brittle
transition temperature) and the planar anisotropy (~r),
as shown in Figs. 2 (a) and 2 (b).
Fig. 2 (a) shows the relation between the annealing
temperature and the brittle transition temperature. Fig.
2 (b) shows the relation between the annealing
temperature and the planar anisotropy (~r).
It is considered that the resistance to cold-work
embrittlement was not improved in the annealing conducted
at a temperature less than 830C because segregation of
B into the grain boundary was insufficient. It is also
considered that the planar anisotropy was not reduced in
the annealing conducted at a temperature less then 830C
because the recrystallized grain orientation was affected
by the cold-rolled grain orientation.
In an annealing conducted at a temperature higher
than the Ac3 transformation point, the size of the grains
will increase, deteriorating the resistance to cold-work
embrittlement and increasing the planar anisotropy due to
18
208 1 496
the transformation.
Thus, a preferred continuous annealing temperature
is from 830C and Ac3 transformation point from the
viewpoint of improvement in the resistance to cold-work
embrittlement and reduction in the planar anisotropy.
The samples employed in the experiments were
manufactured under the following conditions using, as a
material, a steel which contained 0.004 wt% of C, 0.02
wt% of Si, 0.19 wt% of Mn, 0.025 wt% of Ti, 0.01 wt% of
Nb, 0.0025 wt% of B, 0.08 wt% of Al, 0.02 wt% of P, 0.006
wt% of S and 0.003 wt% of N.
Hot rolling finishing temperature : 880C
Coiling temperature : 600C
Cold rolling reduction : 70%
Thickness of the cold rolled sheet : 1.2 mm
Continuous annealing conditions : 700 to 950C and
20 seconds
Skin pass reduction : 1%
The brittle transition temperature and ~r were
measured in the same manner as the aforementioned one.
As stated above, the resistance to cold-work
embrittlement is greatly affected by the chemical
composition of the material and the hot rolling and
continuous annealing temperatures. This resistance to
cold-work embrittlement is also affected by the thickness
19
208I ~96
of the steel sheet. In the case of the same material,
the thicker the steel sheet, the higher the brittle
transition temperature of the resistance to cold-work
embrittlement (see Fig. 3).
The advantages of the present invention can be most
readily obtained when the thickness is 1.0 mm or more at
which deterioration in the resistance to cold-work
embrittlement most readily occurs. The upper limit of
the thickness is set to 5.0 mm because it is difficult to
manufacture a cold rolled steel sheet having a thickness
of more than 5.0 mm.
The samples employed in the experiments were
manufactured under the following conditions using, as a
material, a steel which contained 0.003 wt% of C, 0.01
wt% of Si, 0.15 wt% of Mn, 0.026 wt% of Ti, 0.008 wt% of
Nb, 0.0026 wt% (26 ppm) or 0.0005 wt% (5 ppm) of B, 0.07
wi% of Al, 0.021 wt% of P, 0.005 wt% of S and 0.002 wt%
of N and which had a thickness ranging from 0.6 mm to 3.1
mm.
Hot rolling finishing temperature : 880C
Coiling temperature : 600C
Continuous annealing conditions : 840C and 40
seconds
Cold rolling reduction : 65 to 73 %
(The brittle transition temperature was measured in
2081~96
the same manner as the aforementioned one.)
Other conditions
Although regarding the cold rolling and skin pass
rolling processes, the normally employed conditions can
be used, a preferred cold rolling reduction is between 50
and 95 % while a preferred skin pass rolling is between
0.5 and 2 %.
Examples
Table 1 shows the chemical composition of each of
the slabs manufactured by the continuous casting method
from a molten steel manufactured by a normal
manufacturing process. After hot rolling was performed
on the steels having the compositions shown in Table l
under the conditions shown in Table 2 to obtain hot
rolled sheet coils having a thickness of 3.5 mm, cold
rolling was performed to obtain cold rolled sheets having
a thickness of 1.2 mm. Thereafter, continuous annealing
was conducted at various temperatures shown in Table 2,
and then skin pass rolling was performed at a reduction
of l %.
Table 1
C~ENICAL COMPOSITION t~TZ)
SYMBOL Ti*/C REMARKS
C SiMn TiNb BAl P S N
A 0.0025 0.01 0.170.028 0.0050.00150.0530.012 0.0050.0035 3.4 SUITABLE
EXAMPLE
B 0.0016 0.02 0.130.035 0.0040.00250.0710.011 0.0060.0021 11.8
C 0.0018 0.01 0.090.021 0.0090.00180.0580.009 0.0050.0022 3.7
D 0.0021 0.01 0.120.028 0.0050.00220.0650.009 0.0040.0018 7.5
E 0.0029 0.01 0.120.005 0.006 - 0.061 0.011 0.0060.0033 -5.2 COMPARATIVE
EXAMPLE
Y 0.0022 0.02 0.10 _ 0.0090.00270.0710.011 0.0060.0021 -7.4
G 0.0019 0.02 0.110.035 - 0.0005 0.053 0.012 0.0060.0025 9.2
~ 0.0024 0.01 0.160.012 0.0050.00160.0570.013 0.0050.0019 0.5
Underlined figure is out of the range restricted by the present invention
Table 2
N0. SYM30L OF STEEL MATERIAL CHARACTERISTICS HOT ROLLING CONDITIONS ANNEALING REMARRS
TEMPERATURE C
YS TS El AVERAGE ~r Tcr *1 FDT *2 CT *3
kgf/mm2kgf/=m2 Ir VALUE C C C
1 A 16.8 30.5 52 2.20.18 -160 880 540 850 SUITABLE
EXAMPLE
2 B 17.3 31.0 52 2.10.21 -150 890 580 850
3 C 15.2 29.8 53 2.40.25 -140 885 610 870
4 D 15.9 30.1 52 2.20.17 -155 890 510 880
A 16.2 29.9 53 2.30.75 -70 880 680 870 COMPARISON
EXAMPLE
6 B 18.3 32.3 48 1.70.69 -100 880 580 770
7 E 20.3 34.7 46 1.61.03 -55 885 540 860
8 F 19.6 33.5 47 1.70.85 -100 870 535 860
9 G 20.1 33.9 44 1.60.77 -75 870 520 865
E 18.9 33.0 48 1.80.58 -150 870 580 875
Underlined figure is out of the range restricted by this invention
*1 Tcr: Brittle transition temperature
*2 FDT: Final finishing temperature
*3 CT : Coiling temperture
~;~,
2081~g6
The tensile characteristics, the average r value,
the planar anisotropy (~r) and the cold-work
embrittlement (brittle transition temperature) of the
thus-obtained cold rolled steel sheets were examined.
The results of the examinations are shown in Fig. 2.
The tensile test was conducted in conformity with
JIS No. 5. The average r value was calculated from rL, rD
and rc by the following equation.
Average r value = (rL + 2rD + rc)/4
~r and the brittle transition temperature were
obtained in the same manner as the aforementioned ones.
As is clear from Table 2, in the examples (sample
Nos. 1 through 4) of the present invention, TS ~ 29.5
Kgf/mm , El 2 50% and the average r value ~ 2Ø Also,
the brittle transition temperature < -140C and ~r ~
0.25, that is, substantially no cold-work brittle
fracture occurred and the planar anisotropy was very
less.
In the comparative examples (sample Nos. 5 through
6) manufactured from the material having the composition
restricted by the present invention under the
manufacturing conditions which were out of the range
restricted by the present invention, the bri~tle
transition temperature was high and the planar anisotropy
~r ' 0.69. In the comparative examples (sample Nos. 7
24
208l 496
through 10) manufactured from the material having the
composition which was out of range restricted by the
present invention under the manufacturing conditions
restricted by the present invention, the brittle
transition temperature ' -100C, and ~r ' 0.58.
Thus, the cold rolled steel sheets alone which
satisfy both the composition and manufacturing conditions
restricted by the present invention have excellent
characteristics.
The present invention is directed to manufacture of
a cold rolled steel sheet for use in deep drawing which
exhibits an excellent resistance to cold-work
embrittlement and a very small planar anisotropy using,
as a material, an extra low carbon steel in which
adequate amounts of Ti, Nb, B and Al are present under
the appropriate hot rolling and continuous annealing
conditions even when the continuous annealing process is
used.
The cold rolled steel sheet obtained in this
invention is suitable for use in, for example,
automobiles, where excellent press formability is
required.
