Process of manufacturing a cold rolled steel sheet having excellent press formability

Based on the investigation as to the correlative relationship between the composition of a steel material, particularly the content of carbon and a soaking temperature for the hot rolling, the improvements of the stretch formability, deep-drawability and aging resistance of the cold rolled steel sheet and the peculiar behaviors of the effective additive ingredients under the above correlative relationship, a cold rolled steel sheet having excellent press-formability is obtained by soaking at 800.degree.-1,100.degree. C., a steel slab consisting of not more than 1.2% by weight of Si, 0.05-1.00% by weight of Mn, not more than 0.150% by weight of P, at least one of elements selected from the group consisting of Nb, Cr, Ti, Al, B and W in a total amount of 0.002-0.150% by weight in an extremely low range of not more than 0.005% by weight of C, followed by ordinary hot rolling, cold rolling and recrystallization annealing.

TECHNICAL FIELD 
This invention belongs to the technical field concerned with a process of 
manufacturing a cold rolled steel sheet having excellent 
press-formability. 
BACKGROUND ART 
In general, the cold rolled steel sheets for press forming which are used 
for outer plates of automobiles, gasoline tank and the like are required 
to be excellent in stretch formability, deep-drawability and aging 
resistance. 
The lower the yield strength (YS) and the higher the elongation (EL) and 
the work hardening exponent (n value) as the material properties, the more 
excellent the stretch formability of the steel sheets. 
The deep-drawability in the material properties is almost dominated by the 
Lankford value (r value). The higher the r value, the higher the limit of 
the deep-drawability. 
On the other hand, it has been known that when there remain C,N in a solid 
solution state in the steel sheet, the trouble called "stretcher strain" 
occurs upon working of press forming due to aging at room temperature. 
This necessitates the aging resistance, which is ordinarily evaluated by 
using the aging index (AI). This index is represented by the difference 
between the yield strength at 7.5% preliminary strain and the yield 
strength after heat treatment of 100.degree. C..times.30 minutes. The 
steel sheets for use in press working are required to have the AI value of 
not more than 3 kg/mm.sup.2. 
There have been heretofore proposed many processes of manufacturing the 
cold rolled steel sheets which are excellent in the above described 
stretch formability, deep-drawability, and aging resistance. For instance, 
there is a method of box annealing a low-carbon aluminum-killed steel 
having a carbon content of about 0.04% by weight (the amounts of the steel 
ingredients being hereinafter referred to briefly as "%"); and a method of 
box annealing or continuously annealing a steel sheet in which a 
carbonitride-forming element such as Ti, Nb or the like is added into the 
extremely low-carbon steel having a carbon content of not more than 0.1%. 
However, these conventional processes have the common feature that the 
temperature (hereinafter referred to as "soaking temperature") at which 
steel slabs are uniformly heated prior to the hot rolling is extremely 
high near 1,200.degree. C. 
The reason why the soaking temperature is so high is as follows: In the 
case of the low-carbon aluminum-killed steels, it is necessary to 
completely solid-solve AlN when soaking the steel slabs in order to obtain 
a high r value by the action of AlN precipitated upon box annealing after 
the cold rolling. Meanwhile, in the case of the extremely low-carbon steel 
to which are added Ti or Nb, since the Ar.sub.3 transformation point at 
which the austenite phase is transformed into the ferrite phase is 
extremely high near 900.degree. C., the hot roll-finishing temperature 
(FDT) must be high so as to avoid deterioration of the material properties 
due to the hot rolling at a temperature lower than the Ar.sub.3 
transformation temperature. 
But, for heating the steel slab at a high temperature of about 
1,200.degree. C., a huge energy is not only required but also the higher 
soaking temperature decreases the yield of the steel slab (due to the 
surface oxidation), and further promotes the interior oxidization in the 
vicinity of the surface of the steel slab, so that such a method has the 
drawback that the trouble such as the surface defect, the surface 
hardening and the like frequently occur. 
As mentioned above, the heating of the steel slab at high temperature leads 
to not only the consumption of much energy but also the surface defect and 
therefore there is strongly desired to establish the process of 
manufacturing the cold rolled steel sheets which lowers the soaking 
temperature of the steel slab and gives the excellent press-formability. 
There have been proposed several processes for manufacturing the cold 
rolled steel in which the soaking is carried out at a low temperature of 
not higher than 1,200.degree. C., followed by hot rolling, for instance, 
Japanese Patent Laid Open Application No. Sho 49-129,622 (Japanese Patent 
Application No. Sho 48-43,856), Japanese Patent Laid Open Application No. 
Sho 51-59,008 (Japanese Patent Application No. Sho 49-132,622) and 
Japanese Patent Laid Open Application No. Sho 55-58,333 (Japanese Patent 
Application No. Sho 53-129,071). However, in any case, in order to make 
the hot roll-finishing temperature to be not lower than the Ar.sub.3 
transformation point, the soaking temperature must be actually not lower 
than 1,100.degree. C. and in the very recent Japanese Patent Laid Open 
Application No. Sho 57-13,123 (Japanese Patent Application No. Sho 
55-84,696), the soaking temperature of the steel slab is 
1,100.degree.-1,250.degree. C. 
To the contrary, in the low-temperature soaking in which the lower limit is 
1,100.degree. C., the above described effects for saving energy and 
avoiding the decrease in the yield are suppressed to an extremely small 
degree and the material properties of the cold rolled steel sheets are not 
sufficiently improved as described hereinafter. 
In addition to the above, Japanese Patent Laid Open Application No. Sho 
53-64,616 (Japanese Patent Application No. Sho 51-140,532) discloses a 
process of manufacturing a steel sheet having an r value of 1.17-1.20 in 
which a rimmed steel slab having C of 0.05-0.11% is soaked at 
980.degree.-1,050.degree. C., and finished at a temperature of 
710.degree.-750.degree. C. Japanese Patent Laid Open Application No. Sho 
56-15,882 (Japanese Patent Application No. Sho 55-60,713) discloses a 
process of manufacturing a steel sheet having an r value of 1.1 in which a 
steel slab having C of 0.03% and Al of 0.05% is soaked at 950.degree. C. 
and finished at a temperature of 750.degree. C. However, they both relate 
to the manufacture of the steel sheets having an r value being as low as 
not more than 1.2 and essentially differ from the deep-drawing steel sheet 
aimed at by the invention. 
DISCLOSURE OF INVENTION 
An object of the invention is to provide a process of manufacturing a cold 
rolled steel sheet having excellent press-formability which overcomes the 
above described drawbacks in the prior art in the production of the cold 
rolled steel sheet for press working, and enables the treatment at a 
temperature of 800.degree.-1,100.degree. C., which is far lower than that 
of the prior art. 
The principal constitution of the invention is as follows: That is, the 
invention relates to a process of manufacturing a cold rolled steel sheet 
having an excellent press-formability in which a steel slab comprising not 
more than 0.005% of C, not more than 1.20% of Si, 0.05-1.00% of Mn, not 
more than 0.15% of P, a total amount of at least one element selected from 
the group consisting of Nb, Cr, Ti, Al, B and W being 0.002-0.15%, and the 
balance being Fe and incidental impurities is subjected to a soaking 
treatment at a temperature range of 800.degree.-1,100.degree. C., and hot 
rolled and succeedingly cold rolled and annealed for recrystallization.

BEST MODE OF CARRYING OUT INVENTION 
First, the fundamental experiments made by the inventors will be explained. 
Two kinds of steel slabs having the compositions as shown in Table 1 were 
prepared by continuously casting molten iron obtained through a 
bottom-blown converter and an RH degassing furnace. 
TABLE 1 
______________________________________ 
Steel 
sample Chemical composition (weight %) 
No. C Si Mn P S N Al Nb 
______________________________________ 
1 0.0022 0.012 0.12 0.012 
0.006 
0.0025 
0.032 
0.008 
2 0.0061 0.010 0.13 0.010 
0.005 
0.0033 
0.021 
0.011 
______________________________________ 
After the above two kinds of the steel slabs were once left to be cooled to 
room temperature, and then soaked in a soaking pit. 
The soaking temperature was varied over a range of 
750.degree.-1,250.degree. C., and the soaked steel slabs were hot rolled 
by means of a rougher consisting of 4 row rolls, and passed to a hot 
finisher consisting of 7 row rolls at two hot roll-finishing temperatures 
(FDT) of about 900.degree. C. and about 710.degree. C., and coiled as 
steel strips having a thickness of 3.2 mm at a constant temperature of 
about 500.degree. C. 
The hot rolled steel strips were pickled and cold rolled into cold rolled 
sheets having a thickness of 0.8 mm and then maintained at a temperature 
of 800.degree. C. through continuous annealing and skin-pass rolled 
finally at a reduction rate of 0.6% to obtain test samples. 
The influences upon the material properties of the test samples due to the 
differences in the soaking temperatures of the steel slabs are shown in 
FIGS. 1 (A), (B), (C), and (D). In the measurements of the material 
properties of the test samples, the tensile strength and the aging index 
(AI) were determined respectively using a tensile test piece of JIS Z 
22015 and a test piece taken in a rolling direction, and the r value, the 
elongation and the yield strength were expressed by the average value of 
three directions, i.e., a rolling direction, and 45.degree. and 90.degree. 
directions to the rolling. 
As seen from the measured results in FIG. 1, there is substantially no 
correlation in the test steel sample No. 2 having the carbon content of 
0.0061% shown in Table 1 between the soaking temperature within the 
temperature range of 1,000.degree.-1,250.degree. C. and the material 
properties of the cold rolled-annealed sheet, and the r value of the low 
FDT steel is low. On the other hand, it has been found that the properties 
of the test steel sample No. 1 having C of 0.0022% strongly depends upon 
the soaking temperature of the steel slab. More specifically, when the 
results in the case of a hot roll-finishing temperature (FDT) of 
900.degree. C. represented by the mark "o" are noticed, as the soaking 
temperature lowers as from 1,250.degree. C. to 1,100.degree. C., and 
1,000.degree. C., the elongation and the r value increased and the aging 
index (AI) and the yield strength (YS) lower and this indicates that the 
press-formability is conspicously improved. 
Meanwhile, when the measured results at the hot roll-finishing temperature 
(FDT) being 710.degree. C. expressed by the mark ".cndot.", are noticed, 
the material properties in the case of the soaking temperature being 
higher than 1,100.degree. C., are fairly inferior to those in the case of 
the soaking temperature being 900.degree. C. However, when the soaking 
temperature of the steel slab is not higher than 1,100.degree. C., the 
material properties become very excellent as in those when the hot 
roll-finishing temperature is 900.degree. C. However, when the soaking 
temperature is as low as lower than 800.degree. C., it is apparent that 
the material properties are rapidly deteriorated. 
This is an extremely important discovery. In the conventional process of 
manufacturing the cold rolled steel sheets for press forming, it has been 
common knowledge that the hot roll-finishing should not be effected at a 
temperature of lower than the Ar.sub.3 transformation point at which the 
steel is transformed from the .gamma.-phase to .alpha.-phase, because such 
heat treatment causes the remarkable deterioration of the material 
properties. However, the Ar.sub.3 transformation point of the test steel 
No. 1 used in the above test by the inventors is about 830.degree. C., and 
therefore the above test results completely break the conventional common 
knowledge. 
The phenomenon observed in the test steel No. 1 in the experimental results 
shown in FIG. 1 is caused by setting the soaking temperature of the steel 
slab to a far lower range of 800.degree.-1,100.degree. C. than that of the 
conventional processes. For this reason, according to the invention, the 
soaking temperature of the steel slab for the hot rolling is limited to a 
range of 800.degree.-1,100.degree. C. Based on the results of this 
fundamental experiment, the inventors have repeated the same experiment 
for confirming the effect of soaking of the steel slab at the low 
temperature with respect to a variety of steel slabs having different 
compositions from the test steel No. 1 and confirmed that the effect of 
the low temperature soaking is more improved by limiting the steel 
components as follows and that cold rolled steel sheets having excellent 
formability can be obtained. 
C: not more than 0.005% 
As seen from the properties of the test steel No. 2 having C of 0.0061% 
shown in FIG. 1, the effect in the low-temperature soaking disappears if 
the carbon content exceeds 0.005%. Thus, the carbon content is limited to 
not more than 0.005%, preferably not more than 0.004%. 
Si: not more than 1.20% 
Si is an element effective for strengthening the steel. However, if it 
exceeds 1.2%, the hardness is conspicuously increased and the elongation 
lowers and the yield strength is raised. Thus, it is limited to not more 
than 1.20%. 
Mn: 0.05-1.00% 
At least 0.05% of Mn is required to prevent the red shortness due to S, but 
if it exceeds 1.00%, it damages the ductility of the steel similarly to 
Si. Thus, the content of Mn is limited to a range of 0.05-1.00%. 
P: not more than 0.150% 
P is high in the ability for strengthening the steel due to formation of 
solid solution and is an element having activity for increasing the 
strength but if it exceeds 0.150%, it brings about conspicuous 
deterioration of the spot weldability. Thus, the content of P is limited 
to not more than 0.150%. Nb, Cr, Ti, Al, B and W: Total amount of at least 
one of these elements being 0.002-0.150%. 
These elements are important in the invention. The function and effects of 
these elements are considered as follows: 
(1) Any of these elements is a carbide, nitride or sulfide-forming element 
and when the steel slab is soaked at 800.degree.-1,100.degree. C. 
according to the invention, the forms of these precipitates extremely 
effectively influence the press-formability of the final product. 
(2) Apart from the effect based on the formation of the above precipitates, 
these elements behave similarly in view of the extremely great influence 
upon formation of micro-crystal grains and improvement of the texture when 
soaking the steel slab in the solid solution state. 
These additive elements have been heretofore widely used for the 
improvement of the properties of the iron steel materials, but it has been 
considered that the addition effects varies depending upon their addition 
amounts and the combined addition with other elements, and also depend 
greatly upon the chemical compositions of the base steels. However, it has 
been found that these additive elements serve very effectively to improve 
the formability of the cold rolled steel sheets which have been subjected 
to the soaking treatment at a low temperature of 800.degree.-1,100.degree. 
C. only in the case of an extremely low-carbon steel having a carbon 
content of not more than 0.005%, and that the functional effect is 
substantially equivalent in any elements. Therefore, these elements may be 
added alone or in a combination of two or more elements. If the total 
addition amount is less than 0.002%, no effect is observed, while if it 
exceeds 0.150%, the effect is not increased in proportion to the increased 
amount and the ductibity is adversely affected due to the hardening of the 
solid solution, so that the total addition amount is limited to a range of 
0.002-0.150%. The optimum addition amount and combination of these 
elements slightly differ depending upon the elements. Particularly, in the 
case of Nb or W, Al is within a range of 0.005-0.08%, and in a combination 
of Nb and W, any of the total amount or the single element amount is 
preferred to be a range of 0.002-0.020%. When at least two elements of Cr, 
Ti, B and Al are selected, the total amount thereof is optimum in the 
range of 0.002-0.090%. 
The reason of the limitation on the elements in the steel according to the 
invention has been explained but the balance consists of iron and 
incidental impurities besides the above elements. 
Explanation will be made with respect to the steps for producing the cold 
rolled steel sheets having the above described composition wherein the 
present invention is applied. 
The steel making process is not particularly limited but the combination of 
a converter and a degassing furnace is more effective in order to suppress 
the content of carbon to not more than 0.005%. 
The process of manufacturing the steel slab may be the conventional 
slabbing, that is ingot making-blooming method or a continuous casting 
method. 
With respect to the heating of the steel slab, it is important to soak it 
at a temperature range of 800.degree.-1,100.degree. C. If the soaking can 
be carried out within this temperature range, the method and apparatus for 
heating the slab are not limited and the temperature of the steel slab 
prior to the soaking is arbitrary. Accordingly, the steel slab may be one 
completely cooled to room temperature or one having a temperature higher 
than room temperature and it is merely necessary to soak the slab at a 
temperature range of 800.degree.-1,100.degree. C. by reheating. The 
soaking time is not particularly limited and if the entire steel slab is 
heated to the soaking temperature of 800.degree.-1,100.degree. C., the 
object can be attained but the soaking time is preferred to be from 10 
minutes to one hour. 
Therefore, with respect to the steel slab manufactured by the continuous 
casting, when the temperature of the steel slab is not lower than 
800.degree. C., it is unnecessary to once cool and reheat, but it is 
merely necessary to keep the temperature at a temperature range of 
800.degree.-1,100.degree. C. or to gradually cool the slab to this 
temperature range. Therefore, no particularly heating furnace is necessary 
in the case of the steel slab obtained by the continuous casting, and it 
is possible to attain the satisfactory effects only by regulating the 
cooling speed. 
In the hot rolling of the thus soaked steel slab, no adverse effect takes 
place on the material properties of the final cold rolled steel sheet so 
long as the rolling conditions such as rolling speed, rolling reduction, 
distribution of reduction in rolling, roll-finishing temperature and 
coiling temperature and the like are within the usual ranges. 
However, if the finishing temperature in the hot rolling is too low, the 
deformation resistance becomes high to make the rolling difficult, so that 
it is preferable to be higher than 550.degree. C. Further, since the 
surface oxidized layer of the hot rolled steel strip formed until coiling 
after the finish rolling highly influences the surface profile of the 
final cold rolled steel sheet, the finishing temperature is preferred to 
be as low as possible. Therefor, the finishing temperature is preferably 
550.degree.-850.degree. C. Since the steel containing an element or 
elements other than Nb and W is very low in the deformation resistance in 
the ferrite region, the finishing temperature may be lower than that of 
the steel to which Nb or W is added, and the temperature is preferred to 
be 550.degree.-680.degree. C. 
On the other hand, the temperature for coiling the hot rolled steel sheet 
is preferred to be a range of 400.degree.-600.degree. C., because as said 
temperature is lower, the pickling ability is improved and the pickling 
cost is reduced and the good surface profile can be ensured, so that the 
temperature is preferred to be 400.degree.-600.degree. C. 
The reduction in the cold rolling is preferred to be 50-95%. 
The recrystallization annealing may be carried out by any process of a box 
annealing using a bell furnace and a continuous annealing of a rapid 
heating type, but the continuous annealing is more preferable in view of 
the productivity and the uniformity of the material quality. The annealing 
temperature is preferably in a range of 650.degree.-850.degree. C. 
Meanwhile, the cooling speed after the soaking, or presence or absence of 
the over aging treatment in the case of continuous annealing has no 
substantial influence upon the present invention. 
In order to correct the profile of the cold rolled steel sheet after 
annealing, a tempering rolling may be additionally conducted under a 
reduction rate of not more than 1.5% through a skinpass. 
EXAMPLE 
With respect to the compositions shown in Table 2 satisfying the 
requirements of the invention, molten iron was produced by means of a 
bottom-blown converter and an RH degassing furnace and then continuously 
cast or ingot-made and then bloomed to produce a steel slab. 
The steel slabs thus obtained were subjected to soaking treatments at a 
temperature range of 850.degree.-1,080.degree. C. as shown in Table 3. The 
temperature of the steel slab prior to the soaking was different and 
20.degree.-1,100.degree. C. as shown in this Table. 
The thus soaked steel slabs were hot rolled at a hot roll-finishing 
temperature of 620.degree.-850.degree. C., and a hot roll-coiling 
temperature of 320.degree.-550.degree. C. to obtain hot rolled sheets each 
having a thickness of 2.8-3.2 mm. Then, the hot rolled sheets were cold 
rolled to cold rolled sheets each having a thickness of 0.8 mm, and as 
indicated in Table 3, they were subjected to the recrystallization 
annealing in a continuous annealing furnace at a uniform temperature of 
760.degree.-800.degree. C. All the annealed test sample sheets were 
treated by a skinpass to obtain the final products. 
The average properties of the final product in the rolling direction, and 
in the directions of 45.degree. and 90.degree. to the rolling are shown in 
Table 4. 
TABLE 2 
__________________________________________________________________________ 
Test Sample 
Chemical Composition (wt %) 
Steel No. 
C Si Mn P S N Nb Cr Ti Al B W 
__________________________________________________________________________ 
A 0.0013 
0.02 
0.15 
0.01 
0.008 
0.004 
0.006 
-- -- 0.041 
-- -- 
B 0.0024 
0.02 
0.15 
0.08 
0.006 
0.003 
0.004 
-- -- 0.025 
-- -- 
C 0.0032 
0.02 
0.15 
0.01 
0.008 
0.004 
-- 0.046 
-- 0.035 
-- -- 
D 0.0027 
0.02 
0.13 
0.01 
0.008 
0.002 
-- 0.070 
-- 0.063 
-- -- 
E 0.0032 
0.02 
0.15 
0.01 
0.005 
0.002 
-- -- 0.036 
-- -- -- 
F 0.0012 
0.02 
0.18 
0.01 
0.011 
0.005 
-- -- -- -- 0.0052 
-- 
G 0.0025 
0.90 
0.15 
0.01 
0.009 
0.003 
-- -- -- 0.024 
-- 0.004 
H 0.0010 
0.02 
0.15 
0.01 
0.007 
0.001 
-- -- -- 0.056 
-- -- 
I 0.0008 
0.02 
0.15 
0.09 
0.008 
0.003 
-- -- -- 0.028 
0.0035 
-- 
J 0.0027 
0.02 
0.15 
0.01 
0.008 
0.004 
0.002 
-- 0.020 
-- -- -- 
K 0.0046 
0.02 
0.70 
0.07 
0.008 
0.004 
-- 0.052 
0.025 
-- -- -- 
L 0.0023 
0.03 
0.20 
0.05 
0.007 
0.003 
-- -- 0.052 
0.016 
-- -- 
__________________________________________________________________________ 
TABLE 3 
__________________________________________________________________________ 
Steel slab temper- Hot rolling 
Test Process for 
ature prior to Finishing 
Coiling 
Annealing conditions 
sample 
manufacturing 
being charged into 
Soaking temperature 
temperature 
temperature Temperature 
steel 
steel slab 
soaking pit (.degree.C.) 
of steel slab (.degree.C.) 
(.degree.C.) 
(.degree.C.) 
Method 
(.degree.C.) 
__________________________________________________________________________ 
A continuous 
20 1080 780 550 continuous 
800 
casting 
B continuous 
1100 1100.about.1060 
880 500 " 760 
casting 
C continuous 
22 850 630 420 " 800 
casting 
D ingot making- 
23 1020 780 550 " 760 
blooming 
E continuous 
450 1030 660 520 " 800 
casting 
F continuous 
850 950 650 380 " 770 
casting 
G continuous 
380 1020 650 400 " 800 
casting 
H continuous 
25 1000 670 510 " 800 
casting 
I continuous 
25 950 620 470 " 800 
casting 
J continuous 
870 860 650 320 " 770 
casting 
K continuous 
520 1020 850 550 " 800 
casting 
L continuous 
20 1050 660 530 " 780 
casting 
__________________________________________________________________________ 
TABLE 4 
__________________________________________________________________________ 
Test 
Yield Tensile Aging Index 
sample 
strength 
strength 
Elongation 
AI value 
steel 
(kg/mm.sup.2) 
(kg/mm.sup.2) 
(%) -r value 
(kg/mm.sup.2) 
n value* 
__________________________________________________________________________ 
A 13 29 54 2.0 1.2 0.31 
B 21 37 44 1.9 1.6 0.26 
C 14 28 53 1.8 2.4 0.29 
D 14 29 52 2.0 1.5 0.28 
E 15 29 51 2.1 0.4 0.31 
F 13 28 54 1.7 2.0 0.30 
G 22 36 43 1.8 0.3 0.25 
H 14 27 51 1.8 1.0 0.28 
I 19 36 43 1.8 0.8 0.27 
J 14 29 53 2.2 1.0 0.29 
K 25 42 38 1.7 1.5 0.25 
L 18 32 47 1.8 0.5 0.27 
__________________________________________________________________________ 
Note: *n values at 10% and 20% of deformations. 
As seen from the property values of the materials shown in Table 4, the 
tensile strengths depend upon the compositions of the test sample steels 
and the test sample steels B, G, I, and K show the values of not less than 
35 kg/mm.sup.2 and the other samples are not more than 32 kg/mm.sup.2, but 
any sample steels are low in the yield strength and high in both the 
elongation, r value and n value, and show the aging index (AI) of not more 
than 3 kg/mm.sup.2. This indicates that all samples A-L are cold rolled 
steel sheets having excellent stretch formability, deep-drawability and 
aging resistance. 
The steel slabs shown in the above Example are ones having a thickness of 
about 100-250 mm which are produced by the ingot making-blooming method or 
a continuous casting method but the invention is obviously applicable to a 
sheet bar having a thickness of 20-60 mm produced directly from the molten 
steel through a sheet bar caster. 
That is, when the sheet bar is subjected to the hot rolling, it is merely 
necessary to uniformly heat the bar within a temperature range of 
800.degree.-1,100.degree. C. or to keep the temperature at said 
temperature range. Further, the cold rolled steel sheets according to the 
invention are used effectively as raw materials for manufacturing all 
sorts of the surface treating steel sheets such as continuous hot-dip 
galvanizing steel sheets by the in-line annealing system. 
According to the invention, a cold rolled steel sheet having excellent 
stretch formability, deep-drawability and aging resistance can be 
manufactured only by effecting the soaking treatment at a temperature 
range of 800.degree.-1,100.degree. C. when hot rolling a steel slab in 
which at least one of Nb, Cr, Ti, Al, B, and W is added in a total amount 
of 0.002-0.15% to an extremely low carbon steel having a carbon content of 
0.005% or less without being influenced by the subsequent hot rolling and 
cold rolling conditions and the annealing conditions. 
INDUSTRIAL APPLICABILITY 
As mentioned above, the temperature range for the soaking treatment 
according to the invention is a low temperature range which is contrary to 
the conventionally common knowledge, and therefore not only a huge amount 
of energy consumption can be saved to a large extent, but also due to the 
reduction in the amount of surface oxidation, the yield and the properties 
of the surface and interior of the product can be largely improved.