Note: Descriptions are shown in the official language in which they were submitted.
~2~
The present -invention re~Lates to a method of
producing cold rolled steeL sheets used for a-utomotive
exterior plate and the like and adapted for deep drawing.
Co:Ld rolled steel sheets used for the produc-
tion of shaped articles through deep drawing, such as
cold rolled steel sheets for deep drawing and the like,
05
are required to be low in the yield strength (YS) and
high in the elongation (EQ), that is, to be excellent
in the ductility, and further to have a high Lankford
value (r-value) as important mechanical properties.
Moreover, shaped articles produced through deep drawing
are often used in the outer surface of mechanical
products, such as automotive exterior plate and the
like, and therefore it is an impor-tant property for the
cold rolled steel sheet to have an excellent surface
property.
Cold rolled steel sheets for deep drawing
have hi-ther-to been produced from a low-carbon aluminum
killed steel through a box annealing. However, recently
a continwous annealing method is widely used for the
production of the cold rolled steel sheets for deep
drawing from the low-carbon aluminum killed steel in
view of the improvement of produc-tivity and the energy
saving. However, the use of commonly used low-carbon
aluminum killed steel as a starting material can not
give satisfactorily excellent mechanical properties to
the resulting cold rolled steel sheets for deep drawing.
Accordingly, it was proposed to use ultra-low carbon
- 2 -
steel having a C content o~` as low as 0.020% or less as
a starting material for the production of cold rolled
steel sheet for deep drawing. However, it was difficult
to secu-re, in the conventional method, satisfactorily
high r-value and ductility enough to bear the deep
~rawing even in the use of such ultra-low carbon s-teel.
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Under these circumferences, there have been
proposed vario-us methods, wherein carbide- and nitride-
forming elements of Nb, Ti, ~r and the like are added
to ultra-low carbon steel. Among the prior arts
disclosing these methods, Japanese Patent Application
Publication No. 18,066/69 and Japanese Patent Laid-open
Specification No. 137,021/78 disclose cold rolled steel
sheets containing Ti and having deep drawability, and
methods of prod-ucing the steel sheets.
However, in -these methods, it is necessary -to
carry o-ut a hot rolling at a high finishing temperature,
and a high temperature heating of slab and a hi~h
temperature hot rolling rrlust 'be carried C)Ut:. However,
the high temperature heating of slab has such draw'backs
that cost for heating energy is high, yield is low due
to the oxidation of slab surface, quali~y of the result-
ing cold rolled s-teel sheet is poor d-ue to t'he increase
of interna:Lly oxidized procluct, and trou'bles occur
during the cold rolling. While, the high temperature
hot rolling is apt to cause breakage and other troubles
of roll and to deteriorate the quality of the s-urface
of the res-ulting cold rolled steel sheet.
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Further, .Japanese Patent Lai.d-open Specifica-
tion No. 1.3,123/82 discloses a method of p-roducing a
cold rolled steel sheet for deep drawing from a steel
containing C : 0.002-0.05% and Ti : 0.070-0.210% through
a low-temperature hot rolling. However, in this method,
a large amount of Ti is used, and therefore the resulting
cold rolled steel sheet is very e~pensive, and further
the cold rolled steel sheet is poor in the surface
property and in the treatable property of surface due
to the increase of Ti series inclusions.
The object of the present invention is to
provide a method o:E producing cold rolled steel sheets
adapted for deep drawing and having excellent ductility
and surface property from a Ti-containing steel through
a low-temperature hot rolling.
The feature of the present invention lies in
a method of producing cold rolled steel. sheets fo-r deep
drawing, comprising soaking at a temperat-ure lower than
1,100C a steel slab having a composition consisting
of, in % by we.ight, not more than 0. 015% of C, not more
than 0.40% of Mn~ not more than 0.03% of P, 0.005-0.100%
of sol. AQ, not more than 0.010% N, Ti in an amount
within the range satisfying the following formula
-O.020% < Ti - (~S -t ~TN) < O.004%
provided that the Ti in the formula does no-t include Ti
present in the slab in the form of oxide, and the
remainder being Fe and inciden-tal impuri.ties; finishing
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a hot rolling of the soaked slab at a temperat-lre of
600-780C; cold rolling the hot rollecl sheet, and
annealing the cold rolled sheet.
For a better understanding of the invention,
reference is taken to -the accompanying drawings, wherein:
Fig. 1 is a graph illustra-ting the influences
of -the M-value and soaking temperature of a steel slab
upon the properties of the resulting cold rolled steel
shee-t; and
Fig. 2 is a graph il:Lustrating the influence
of the finishing temperature of hot rolling of a steel
slab upon the properties of -the resulting cold rolled
s-teel sheet.
The inventors have made vario~ls basic experi-
ments and ascertained that, when an ultra-low carbon
steel having an M-value within a specifically limited
range, which M-value is def'ined by -the for~lula
M = Ti ~ S -~ T~N~
with respect to the amount of Ti present in the steel
in the form other than oxide, is soaked at a temperature
lower than the ordinary soaking temperature, a cold
rolled steel sheet having excellent deep drawability can
be obtained. This fact will be explained hereinafter.
In a basic e~periment, molten steels of
ultra-low carbon steels containing carbon in two
different levels and having a widely ranging M-value
(M=Ti-(38S+41-N)) as shown in the following 'I'a'ble 1 were
~2~
produced through a converter and an RH degassing
apparatus. In the steels substantially all of the
oxide was aluminum series oxide, and therefore total
amoun~ of Ti was used as the Ti content of the steels
in the calculation of the M-value.
Table 1 Chemical composition of ~teels used
in the basic e~periment (% by weight)
_
C Si Mn P S N 0 AQ Ti M
_ __ _
0.003 0.0013 0.002 0.03 0.008 -0.034
A ~ ~ ~ ~, ~ ~ ~ ~ ~
_ 0.004 0.0l 0.15 0.01 0.020 0.0086 0.004 0.05 0.065 0.~30
0.004 0.0015 0.002 ~.02 0.012 -0.038
B ~ ~ ~ ~ ~ ~ ~ ~ ~
_ 0.009 0.01 0 15 0.01 0 021 0.0072 0.005 0 06 0 055 L__
Each of molten steels shown in I'able 1 was
cast into a steel slab by means oE a continwous casting
apparatus, and the slab was cooled to about room temper-
ature. The slab was soaked at 1,260C, which is a
commonly used soaking temperature, or at l,080C or
940C, which is lower tha-n the commonly used soaking
temperature, and then subjected to hot rolling. The hot
rolling was carried o-ut by means of a hot strip mill
comprising 4 stands of roughing mills and 7 stands of
finishing mills to produce a hot rolled s-teel sheet
having a thickness of 3.2 mm. The finishing tempera-ture
-- 6 -
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in the hot rolling was about 730C, ancl the coil.ing
temperature was a'bout 580C in all steel samples.
To hot rolled steel sheet was picklecl, cold rolled
(final gauge : 0.7 mm), and then subjected to a
cont:inuous annealing at a constan-t temperature of 810C
for 30 seconds and to temper-rolling at a recluction rate
05
of 0.5%. The elongation EQ and r-value of the above
treated cold rolled steel sheet were platted in Fig. 1.
It can be seen from Table 1 that
(1) the properties of the resulting cold rolled
steel shee-t can be determined 'by -the M-value
lC
independen-tly of C content, and when the M-value
is within -the range of from 0.020% to less -t'han
0.00~%, a cold rolled s-teel sheet having excellent
deep drawability can be obtained; and
(2) When the soaking temperatwre of slab is hi~h,
the properties of col.d rolled s-teel sheet are very
poor independently of the :M-val-le.
The deep drawa'bi.lity of a Ti-containing ul.tra-
low carbon steel has hitherto been determine~ 'by the
ratio of the Ti con-tent to the C content. The reason has
hi.therto been metallographically explained as follows.
C is bonded with Ti to form a carbide TiC and -to decrease
the amoun-t of free state C or solute C, ~-hereby a (lll)
recrystallization texture, which acts favorably on the
improvement of deep drawability~ is developed in a
large amount duri.ng the recrystal]iza-tion anneali.ng.
However, the inventors have found o-ut a novel fact as
described above that, w'hen a Ti-containing wltra-low
carbon steel slab :is soaked at low temperat-ure a-nd then
hot rolled, the deep drawabili-ty of the resulting cold
roll.ed steel sheet is not determined by the ra-tio of
the Ti con-tent to the C content, but is determined by
the ratio of the Ti content to the (S+N) content.
Based on the above described basic experiments,
-the inventors have repeated experiments by changing hot
rolling condition and other conditions with respect to
steels having a chemical composition different from
that shown in Table 1, and ascertained -that a cold
rolled steel sheet having excellent cold drawability
can be obtai.ned by limi-ting the chemical composition of
the steel and -the production condi-tion of the cold
rolled steel sheet.
The present invention is based on the above
described cliscovery, ancl provides a methocl of producing
a cold rolled steel sheet for deep drawing, comprising
soaking at a tempe-rature lower than ]~100C a steel
slab havi.ng a composition consisting of, in % 'by weight,
not more than 0.015% of C, not more than 0.40% of Mn,
not more than 0.03% of P, 0.005-0.100% of sol. AQ, not
mo:re than 0.010% of N, Ti in an amount within -the range
satisfying the ~ollowing formula
-0.020% < Ti (3~8S + ~N) < O.004%
provided that the Ti in the formula does not include Ti
present in the sla'b in -the form of oxide, and the
6~
remainder being Fe ancl incidenta'L imT)urities; finishing
a hot rolling of the soaked sla'b at a temperatwre of
600-700C; cold rolling -the hot rollecl sheet~ and
annealing the cold rolled sheet.
An explanation will be made wi-th respect to
the reason for the limitation of the chemical composition
05
of the steel to be used in the presen-~ invention.
When the C content is high, the resulting
cold rolled steel sheet is high in the yield strength
and is poor in the elongation EQ, and fur-ther is un-
satisfactory in the r-value. Accordingly, -the C content
is limited to -not higher than 0.015%.
P acts to embrittle a cold rol]ed steel
sheet, particularly causes troubles, such as crack in
the secondary working and the like, after deep drawing.
Therefore, the P content is limited to not higher than
0.03%-
AQ is effec-tive for decreasing the oxygen
con-tent in a steel, and must 'be addecl to a steel sheet
in an amount of at least 0.005% in an acicl-soluble
form. However, when the amount of sol. AQ exceeds
0.100%, the surface property of the resulting cold
rolled steel sheet is poor. Therefore, the conten-t of
sol. AQ is limited to not higher than 0.100%.
When the N content is higher than 0.010%~
satisfactorily high ductility and ageing resistance can
not 'be obtained in the resulting cold rolled steel
sheet. Therefore, the N conten-t is limited to not
~L2~6~
higher than 0.010%.
Ti is an important element in -the present
invention. As already e~plained in the above described
basic experiments, it is necessary -to ad~ Ti to a
star-ting steel such that the M-value (=Ti-(~S+I~N)) is
within the range of from -0.020% to less than C).00~%,
05
preferably from 0.015% -to less than 0.00~%. However,
Ti may be bonded with oxygen depending upon the produc-
tion condition of the starting steel. Accordingly, in
the definition formula for the M-value, the amount of
Ti present in -the steel in the form of oxide is excluded.
Then, an explanation will be made with respect
to the production steps of a cold rolled steel shee-t
according to the present invention. The steel making
method is not particularly limited. However, in order
to decrease the C content to not higher than 0.015%, a
combination system of a converter and a degassing
apparatus is effective. A steel sla'b can be produced
by an optional method. ~[owever, the continuous casting
method or an ingot making-sla'b'bi.ng method is advan-
tageously used.
In the present invention, a step for producinga hot rolled steel strip from a steel sla'b is particu-
larly important. ~hen a steel slab after cooled to
about room temperature is soaked or a steel slab still
having a high temperat-ure is directly soaked~ a low
temperature soaking is necessary, wherein the steel
slab is soaked at an average temperature of less than
- 10 -
6~9
1,100C, preferably less than l,000C, as clear~ly
understovd from the basic experiments illustrated in
Fig~ 1.
Then, in the hot rol.ling, the ho-t rolling
:finishing temperature must be within the range of
600-7~0C, and is preferably within the range :Erom
600C to less than 700C, in order to obtain excellent
deep drawability as illustrated in Fig. 2.
The coiling -temperature after hot rolling is
not particularly limited. However, in order to improve
the pickling efficiency, the coiling temperature is
preferably not higher than 600C. The cold rolling
step is not particularly limited as well. However, in
order to obtain high r-value and to obtain low planer
anisotropicity, the cold rolling reduction rate is
preferably 50-95%.
The final annealing can be carried out by
either a box annealing by means of a bell furnace or a
continuous anneali.ng through a rapicl heating-short time
heating cycle. However, the continuous annealing is
superior to the box annealing in view of the productivity.
The annealing temperature is preferably within the
range of 650-900C. As to the heat cycle in the
continuous annealing, the cooling rate after constant-
temperature heating or the addition of overageing
treatment and the overageing condition have no-t an
~5
essential infl-uence upon the properties of the resulti.ng
cold rolled steel sheet. However, a grad-ual cooling at
~6~
a rate of 10C/sec or less, or an overageing treatrnent
at a temperature of a'bout 3S0C is effective for t'he
improvement of the properties, particularly the
ductility, of the prodwct.
After completion of the annealing, the cold
rolled steel sheet may be subjected to a temper-rolling
Q5
at a reduction rate of not higher than 1.5% in order to
correct its shape and for other purposes.
The following example is given for the purpose
of illustration of this invention and is not intended
as a limitation thereof.
Example
Molten steels having a chemical composition
shown in the following Table 2 were produced. In -the
steel shown in Table 2, Steels 1-~ are those of the
present inven-tion, and Steels 5-7 are conventional
steels. Each of the molten s-teels was rnacle into a sla'b
through a converter-degassing-continuous casting method.
The slab was cooled to room temperatwre ancl-then heated
to a soaking temperature shown in Table 2 in a heating
furnace. However, only in Steel 2, the cooling of the
2~
slab was stopped at about 500C, and the slab was
changed into the heating -furnace and heated to a soaking
temperature shown in Table 2.
The soaked slab was hot rolled into a 'ho-t
rolled shee-t of 3.2-3.8 mm thickness under a hot rolling
condition shown in Table 2, pickled, and then cold
rolled into a cold rolled sheet having a final ga-uge of
6~
0.7-0.8 m~ thickness. The cold rollecl s'heet of Steel 4
was subjected to a continuous annealing (constant
temperature : 800C) and successively to a ho-t-dip zinc
plating in a con-tinuous hot-clip zinc plating line.
Cold rolled sheets other than S-teel ~ were annealed at
a constant temperature of 820C in a con-tinuous annealing
05
line. After the annealing, Steels 3 and 5 were rapidly
cooled at a rate of not less than ~0C/sec, and subjected
to an overageing treatment at 350-~00C for 150 seconds.
~ll the above trea-ted steel sheets were subjected to a
temper-rolling at a reduc-tion rate of 0.3-0.8%, and the
surface properties and mechanical proper-ties of -the
resulting products were tested. The obtained results
are shown in the following Table 3.
It can be seen from Ta'ble 3 that the cold
rolled steel sheet accordi-ng to the present invention
is high in the ductility and r-value and has excellent
deep drawa'bility. Part;c~llarly, the hot-dip zinc
plated steel sheet (Steel ~l) is excellent in the throwing
power and adhesion, and the s-lrface properties of the
all resulting cold rolled steel sheets are excellent.
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InC~O j_ ~D ~ ~ ~O `J ~
~o . ~ ~9 ~ _ ~ r~ ~ r~
d d s~ ,~. o o o o o o o
o ~ .,~ o o~ ~ oo o ~ U)
.~ X ~ ~ ~o, ~ o ~ oo ~ o o
. U~ __ __ ~ ~
o , ~ r~ o ~ c~ u~ r~
~X 0 0 O O O C`l C`J
a O o O o o O O
~ . _ ~ -o O ~ _' .
o ~, C~l ~ ~ ~ ~ O U~
s~ E~ o o o o o o o
I _ o_ _O o O _O O o
O ~ ~ O ~ ~
d ~ o o o u~ o o o
.......... - o o o o o o o
o~ ~ C`l C`l ~I U~ o
~1 o o o o o o o o
o o o o o o o
~n _, ~ _ O o o o o o o
4~ d o ~ ~ c~l ~:t ~ un
o o~ z; o o ~ o o ~o ~
g ~ o o o o o o o
.~ o ___ o o o __ _'_ o o
., ~ ~ co ~ ~ ~ ~1 c~l o
o o~ u~ o ~ o o o o o ~
o~ u . . _ o o o ZIJ,
U r~l P~ O O O C`J O O O ~
___ - o--- o o o o o o u~
~,~ A ~ un un oo n o ~ ~o
~ ~ ~ ,~ o ~ ~ ~ ,, ~ ~
_ _ _o_ o_ _ _ o o o o
~1 `- ~ -------- - o E~
,~ r~ ~ In
o o ~1 o ~ o o ~E
E~ ~ o o o o o o o ,_
o o o o o o o I ~
__ __ _ __ _ _
, ,, u~ ~ c~l c~ _ u) ~ __
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c
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.. _._.__ _ V~ O~rl . ._
Ta'ble 3 Mechanical. p-roperties of
cold ro'lled steel sheets
Steel ~ ~ ~ EQ r
1 15 29 51 1.9
Steel of 2 14 29 52 2.1.
this invention 3 18 48 1.8
4 - 13 ~8 - - 53 - 2.1
...... _ . _
3~ 43 1.4
Comparati.ve 6 18 30 45 1.4
steel 7 19 31 44 1.5
_. _ _
(Note~ Test specimen : JIS No. 5
Pulling direction :
Directions at inclination angles of 0,
45~ and 90 with respect to the rolling
direc-tion. The property is a mean value
of the values in these directions~
As described above, according to the present
invention~ a cold rolled s-teel sheet having high r-value
and deep drawa'bility and f'wrther having low yield
strength and high elongation, t'hat is, having excellent
ductility, can be produced. Therefore, the present
invention can be applied to the production of a cold
rolled steel sheet which will be formed into mechanical
parts through deep drawing. Particularly, the res-ulting
cold rold steel sheet has high throwing power in the
plating, and is excellent in the adhesion and surface
property. Therefore, the present inven-tion is suitable
for the production of a cold rolled steel sheet to be
- :L5 -
used for the production of automotive exterior p~ate
through deep drawing.
05
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