Note: Descriptions are shown in the official language in which they were submitted.
2~41403
BACKGROUND OF THE INVENTION
Field of the Invention:
The present invention relates to a method of producing
a high-strength cold-rolled steel sheet suitable for various
05 types of working such as press working, bulging and so forth
and adaptable also to deep drawing.
In recent years, there is an increasing demand for
high-strength steel sheets in the field of automobile
production, in order to meet current requirements for
reduction in the weight of automobiles to attain a higher
fuel economy and for ensuring safety of drivers and
passengers.
In modern automobile production, high-strength cold-
rolled steel sheets are used not only for the inner panels
but also for outer panels such as engine hoods, trunk lid
and fenders. As a consequence, high-strength cold-rolled
steel sheet is required to have an excellent workability.
Description of the Related Art:
Hitherto, an art has been proposed in which, in order
to improve workability of cold-rolled steel sheet, the
carbon content of the steel is reduced and a carbonitride
formers are added to the steel. For instance, Japanese
Patent Laid-Open Publication No. 63-317648 discloses a cold-
rolled steel sheet in which Ti, Nb and B are added to a low-
carbon steel for the purpose of improving press-workability
and spot-weldability. It has also been proposed to add
strengthening elements such as P and Mn to the above-
2~11403
mentioned steel system. For instance, Japanese PatentPublication No. 61-11294 discloses a method of producing a
high-strength steel sheet having a superior workability in
which a steel enriched with P is continuously annealed after
05 a cold rolling. Similarly, Japanese patent Publication No.
1-28817 discloses a method in which a steel enriched with P
and Mn is continuously annealed to form a high-strength
cold-rolled steel sheet.
These known methods exhibit disadvantages. The method
disclosed in Japanese Patent Laid-Open No. 63-317648
cannot provide required strength, while the methods
disclosed in Japanese Patent Publication Nos. 61-11294 and
1-28817 inevitably reduce workability although they exhibit
improved strength. Under these circumstances, steel sheets
superior both in strength and workability are strongly
demanded.
SUMMARY 0~ THE INVENTION
An object of the present invention is to provide a
method of producing, from a low-carbon steel having an
extremely small carbon content, a high-strength cold-rolled
steel sheet suitable for working, and more particularly a
steel sheet having a superior workability, specifically a
Lankford value (r) of 1.8 or greater, a tensile strength
T.S.) of 40 kgf/mm2 or greater, an elongation (E~) of 40 %
or greater, and a truncated-cone height of 40 mm or greater
in the conical cup test.
To this end, according to the present invention, there
is provided a method of producing a high-strength cold-
2041403
rolled steel sheet suitable for working, comprising the stepsof:
preparing a steel consisting essentially of not
more than 0.02 wt% of C, not more than 1.0 wt% of Si, not
more than 2.0 wt% of Mn, and not less than 0.01 wt% but not
more than 0.10 wt% of Ti, the Ti, C and N contents being
determined to meet the condition of Ti ~ ~48/12) C wt% +
(48/14) N wt%, not less than 0.0010 wt% but not more than
0.0100 wt% of Nb, not less than 0.0002 wt% but not more than
0.0020 wt% of B, not less than 0.03 wt% but not more than
0.20 wt% of P, not more than 0.03 wt% of S, not less than
0.010 wt% but not more than 0.100 wt% of AQ, not more than
0.008 wt% of N, not more than 0.0045 wt% of 0, and the
balance substantially Fe and incidental inclusions;
subjecting the steel to an ordinary casting,
reheating the steel to a temperature not lower than 1,100C.
not higher than 1,250C., and a subsequent conventional hot-
rolling;
subjecting the hot-rolled steel to a cold rolling
conducted at a sheet temperature not higher than 300C under
such a condition that the sum of the rolling reductions of
passes in which the sheet temperature (TC) and a strain rate
(S 1) meet the following condition:
T X ~ ~ 50,000C S
is 50% or greater; and
subjecting the cold-rolled steel to a continuous
annealing.
The sheet temperature T (C) is the temperature of
73461-24
2041403
-
the steel sheet at positions immediately downstream from the
cold-rolling stands as measured by an infrared pyrometer,
while the strain rate is calculated in accordance with the
following formula:
= (2 n n/60 ~ ) X ~ ~ ) X ~n[1/(1 - r)]
where, n represents the roll peripheral speed
(rpm), Ho represents the sheet thickness at inlet side, r
represents the rolling reduction and R represents the radius
of the roll.
DESCRIPTION OF THE DRAWING
Fig. 1 is a graph which shows the relationship
between rolling reductions and various characteristics of the
steel sheet.
DETAILED DESCRIPTION OF THE INVENTION
Through an intense study on improvement in
workability of high-strength cold-rolled steel sheet, the
inventors have found that a high-strength cold-rolled steel
sheet having a superior workabilityr specifically a Lankford
value (r) of 1.8 or greater, preferably 2.1 or greaterr
20 especially preferably 2.1 to 2.3r a tensile strength (T.S.)
of 40 kgf/mm2 or greaterr preferably 40 to 54.3 kgf/mm2, an
elongation (E~) of 40 % or greater, preferably 40 to 50.3 %
and a truncated-cone height of 40 mm or greater, preferably
40 to 55 mm, can be obtained by selecting the strain-
imparting condition in the cold
B-
73461-24
2041~03
rolling of a very-low-carbon steel which is rich in P and
small in oxygen content.
The present invention is based upon the above-described
discovery. A description will be given first of the reason
05 why the condition is posed that the sum of the rolling
reductions of passes which meet the condition of T X _
50,000OC S-l between the sheet temperature (T C) and the
strain rate ~ (S-l) is 50 % or greater.
Three types of continuous-cast steel slabs A,B and C
having the compositions shown in Table 1 were prepared by a
converter.
TA~LE 1
Steel Contents (wt%)
type
Symbols C Si Mn P S A~ N Ti Nb 13 0 Ti*
A 0.0025 0.010.25 0.0750.008 0.0550.00220.0320.0040.00120.0031 0.014
}3 0.0025 0.010.22 0.0150.007 0.0480.00280.0330.0040.00110.0033 0.013
C O .0024 0.010.25 0.0770. OOû 0.0~70.00230.0330.0040.00110.007~ 0.016
Ti*=Ti-(48/12) C-(48/14) N
o
20~140~
-
Each slab was heated to 12500C and rough-rolled at a
rolling reduction of 88 %, followed by a hot finish-rolling
at a rolling reduction of 88 % (hot-rolling finish
temperature : 880C, coiling temperature: 500C) so as to be
formed into a hot coil of 4.0 mm thick. Then, an ordinary
cold rolling was effected at a rolling reduction of 82.5 %
so that the steel was formed into a sheet 0.7 mm thick.
Subsequently, a continuous annealing was conducted at 810C
followed by a temper rolling at a rolling reduction of 0.8
%, thereby producing a rolled steel sheet.
The cold rolling was conducted~while varying the sheet
temperature within the range of 30C to 3000C, while varying
the reduction rate, i.e,, the strain rate ~ within the
range between 10 S-l to 2,000 S-l. The sheet temperature
was controlled by varying the initial sheet temperature for
the cold rolling and the flow rate of the cooling water.
The Lankford value (r), elongation, tensile strength
and truncated-cone height were measured for each of the
sample steel sheets. The truncated-cone height, which is an
index indicative of the workability approximating that in
actual working was measured by a conical cup test conducted
under the following conditions:
punch diameter: 80 mm
die diameter : 140 mm ~
wrinkle pressing force: 10 t
Fig. 1 shows the relationship between these measured
values and the sum of the rolling reductions of the passes
- 2041A03
which meet the condition of the product of the cold rolling
sheet temperature and the strain rate being not smaller than
50,000OC S-l.
As will be clearly understood from Fig. 1, the low-
05 oxygen steel material A rich in P exhibited a tensilestrength (T.S.) which is smaller than that of the steel B
which has a small P content. In addition, when the sum of
the rolling reductions of the passes having the product of
the sheet temperature and the strain rate being 50,000 oc S-
1 or greater is 50% or above, the truncated-cone height
indicative of the workability approximating that of actual
working is remarkably improved to a value approximating that
of the steel B which has a large tensile strength, while the
elongation(E~) and the Lankford value (r) increase only
slightly.
The steel C which is rich both in P and C does not show
remarkable improvement in the properties indicative of the
workability such as the Lankford value (r), elongation (E~)
and the truncated-cone height.
In order to produce a high-strength cold-rolled steel
sheet having superior workability, therefore, it is
necessary to use a low-oxygen material ha-ving a large P
content and that the cold rolling is conducted under a
condition which meets the condition of the sum of the
rolling reductions of the passes having the product of the
sheet temperature and the strain rate being 50,000 C S-l or
greater is 50% or greater.
- 2041403
In conventional cold rolling of steel sheets, the sum
of the rolling reductions of passes which meet the condition
of the product of the sheet temperature and the strain rate
being 50,000 oc S-l or greater is generally around 30 %. In
05 order to raise the value of the sum of the rolling
reductions, it is necessary to take suitable measures such
as an increase in the rolling speed, control of flow rate of
cooling water, or elevation of the initial cold rolling
temperature through a continuous change from the preceding
step, which is usually pickling.
According to the invention,it is possible to obtain a
high-strength cold-rolled steel having high workability by
using a low-oxygen steel rich in P as the material and by
conducting the cold rolling under the specific condition
mentioned above. The reason why such superior workability
is obtained has not been clarified yet.
The reason, however, is considered to reside in the
following fact. In general, a microscopic observation of
structure of a steel sheet rich in P exhibits a segregation
zone in the thicknesswise central region of the sheet. In
contrast, the steel produced by the method of the present
invention does not exhibit such a degradation zone. This
suggests that a certain effect which could not be produced
by the conventional methods is caused on the segregation
zone by the cold rolling condition peculiar to the
invention. Although the reason is still unknown, it is
considered that the cold rolling condition peculiar to the
invention produces a uniform working effect in the
20~1403
thicknesswise direction so that a greater rolling effect is
produced on the segregation zone as compared to known
methods.
The segregation zone does not produce any substantial
OS unfavorable effect on the elongation Lankford value (r)
which is measured in tensile test. In the actual use of the
material, however, the segregation zone reduces the
uniformity of the steel sheet in the thicknesswise direction
and, hence, is considered to cause a reduction in the
workability.
According to the method of the present invention,
however, the cold rolling conducted under the specified
condition produces a working effect which serves to break
the segregation zone, so that the uniformity of the
structure in the thicknesswise direction of the steel sheet
is improved so as to improve the workability as confirmed
through the conical cup test which simulates the actual
condition of use. When the oxygen content in the steel is
large, however, the large quantity of the inclusions impedes
the cold-rolling straining of P in the segregation zone so
as to reduce the effect of improving the workability.
A description will now be given of the reason for
limitation of the chemical composition of the steel.
C: C serves, when added to the steel material together with
Ti, to strengthens the steel without impairing workability.
When the C content exceeds 0.02 wt%, however, effect of
improving the workability is saturated even when the content
of Ti is increased. The upper limit of C content,therefore,
2041403
-
is determined to be 0.02 wt%. In order to obtain an
excellent workability,therefore, the C content is preferably
below 0.006 wt%.
Si: The upper limit of Si content is set to be 1.0 wt%,
05 since the drawing characteristic of the steel is impaired
when the Si content exceeds 1.0 wt%.
Mn: This element is effective in raising the strength~
without impairing the drawing characteristic. Addition of
this element in an excessive amount reduces the drawing
characteristic so that the Mn content is limited to be not
more than 2.0 wt%.
Ti: This element serves to fix C and N in the steel so as to
prevent deterioration of the material caused by solid
solution of C. In addition, this element impedes formation
of BN so as to prevent reduction in the amount of solid
solution of B. In order to obtain an appreciable effect,
therefore, this element should be added in an amount
exceeding the sum of the C equivalent [(48/12) C wt%] and N
equivalent [(48/14) N wt%]. However, Ti content below 0.01
wt% is too low to enable Ti to produce any appreciable
effect. On the other hand, addition of Ti in excess of 0.10
wt% reduces the strength. Therefore, the Ti content should
be not less than 0.01 wt% and not more that 0.10 wt% and be
determined to exceed the value of [(48/12) C wt% + (48/14) N
wt%].
Nb: This element is essential since it improves the Lankford
value (r) and strengthens the steel when added together with
B. Nb content below 0.0010 wt%, however,does not produce
12
2041~03
any remarkable effect. On the other hand,addition of Nb in
excess of 0.0100 wt% reduces the workability so as to impair
the balance between strength and workability. The Nb
content,therefore,is determined to be not less than 0.0010
05 wt% but not more than 0.0100 wt%. When the steel is bound
to be a deep drawing, however, the Nb content is preferably
not less than 0.0075 wt%.
B: This element is indispensable since it improves the
strength when added together with Nb. B content below
0.0002 wt% does not produce any remarkable effect, while
addition of B in excess of 0.002 wt% seriously degrades the
material. The B content, therefore, is determined to be not
less than 0.0002 wt% but not more than 0.002 wt%.
Preferably, B content is determined to be not more than
0.0012 wt%.
P: This element is an important strengthening element. The
effect of this element is remarkable particularly when the
content is 0.03 wt% or more. However, addition of P in
excess of 0.20 wt% deteriorates the balance between strength
and workability and, in addition, causes an undesirable
effect on the brittleness of the steel. The content of P,
therefore, is determined to be not less than 0.03 wt% but
not more than 0.20 wt%, more preferably not less than 0.04
wt% but not more than 0.15 wt%.
S: A reduction in S content in the steel is necessary for
improving deep drawability. However, the undesirable effect
on the workability produced by S is not so serious when the
2041403
-
S content is reduced down below 0.03 wt%. The upper limit
of the S content is therefore set to be 0.03 wt%.
Ae: This element is necessary for improving yield of
carbonitride formers through deoxidation and for eliminating
05 generation of surface defects caused by formation of TiO2.
The effect of addition of this element, however, is not
appreciable when the content is below 0.010 wt%. In
addition, the deoxidation effect is saturated when the Ae
content is increased beyond 0.10 wt%. In addition,
increase in the Ae content tends to cause surface defect due
to generation of Ae2O3. The Ae content, therefore, is
determined to be not less than 0.01 wt% but not more than
0.10 wt%.
N: This element degrades deep drawability of the steel and,
in addition, reduces anti-secondary working embrittlement
due to bonding with B, unless it is fixed by Ti. Thus, a
greater N content uneconomically requires greater amount of
Ti. The N content,therefore, should be not more than 0.008
wt%, preferably not more than 0.006 wt%.
O: In order to improve workability which is the critical
requirement in the present invention, it is necessary to
reduce O concentration. When the O content exceeds 0.0045
wt%, the cold-rolling straining to the segregation zone is
impeded by a large amount of inclusions as explained before.
As a consequence, the effect of improving workability
produced by the cold straining is impaired and, in addition,
an effect which is not negligible is caused on the
brittleness. For this reason, the upper limit of O content
14
2041403
is set to be 0.0045 wt%, preferably to 0.004 wt%.
Reduction in the oxygen content in the steel is effected by
controlling the length of time of killed treatment in
degassing step in ordinary steel making process.
05 A description will now be given of the preferred
condition for the preparation of the starting steel material
having the above-described composition and preferred
condition for the production of a steel sheet from the
starting steel material.
The steel making process and a subsequent hot rolling
can be carried out in the same manner as the known process,
except that the oxygen content is reduced by the method
described above.
A material having satisfactory properties can be
obtained when the coiling temperature of the steel after the
hot rolling falls within the range of ordinary process,
e.g., between 4000C and 7000C. Thus, it is not necessary
to employ a specifically high coiling temperature. Rather,
it is preferred that the coiling temperature is
comparatively low, e.g., 5500C or less, in order to avoid
any deterioration in pickling property caused by the
thickening of scale and to prevent excessive softening of
the product.
The co - rolling may be conducted by using an ordinary
cold rolling mill, provided that the aforementioned cold
rolling condition is met. Namely, it is necessary that the
sum of the rolling reductions of passes which meets the
condition of the product of the sheet temperature and the
- 2041403
strain rate being not smaller than 50,000 oc S-l is 50 % or
greater. There is no restriction in the total rolling
reduction, i.e.,the sum of the reductions of all passes
employed, provided that the above-described condition is
05 met.
As stated before, the cold rolling sheet temperature
has to be not higher than 300OC because a cold rolling at
higher temperature causes concentration of shear deformation
to the surface region of the steel sheet, making it
difficult to work the central segregation zone.
When the steel having the described composition is
annealed by batch-type box annealing method, the steel tends
to become brittle due to grain boundary segregation of P due
to high P content, particularly when the cooling rate is
small. In order to obviate this problem, according to the
present invention, a continuous annealing method which
enables rapid heating and cooling. The annealing
temperature, however, may be not lower than
recrystallization temperature but not higher than A3
transformation temperature, as in the case of ordinary steel
annealing process.
The temper rolling subsequent to the annealing may be
effected under ordinary steel tempering condition with a
rolling reduction corresponding to the sheet thickness (mm),
for the purpose of, for example, obtaining optimum shape of
the sheet.
Example
2041403
Ten types of steels, including 7 types meeting the
composition condition of the invention and 3 types as
reference examples, were prepared in a converter and were
continuously cast into slabs. Each slab was hot-rolled to
os form a hot coil of 3,0 mm thick and cold-rolled to a
thickness of 0.72 mm. Subsequently, a continuous annealing
was conducted under ordinary condition. Then, the steel
sheets other than the type No. 3 were subjected to a temper
rolling with a rolling reduction of 0.7 ~, whereby 10 types
of steel sheets including one which has not been subjected
to temper rolling were prepared.
The roll used in the cold rolling had a diameter of 600
mm. The cold rolling speed was 1500 to 2500 m/min at the
outlet side of the cold rolling stand.
Among ten types of steel, each of type Nos. 1 and 2
were subjected to three different production conditions with
different cold-rolling and continuous annealing
conditions,so that three samples were produced for each of
the steel type Nos. 1 and 2. Similarly, two samples were
prepared from the steel type No. 1 through different
production conditions. Only one sample was prepared for
each of the remainder steel types.
Table 3 shows the hot-rolling and continuous annealing
conditions, Table 4 shows the cold rolling conditions and
Table 5 shows the result of examination of the properties of
the cold-rolled sample steel sheets.
TABLE 2
Steel Contents (wt%)
type Class
No. C Si Mn P S Ae N Ti Nb B O Ti*
1 Invention 0.00210.01 0.11 0.0550.0080.0400.00250.0320.00340.00080.0025 0.015
2 Invention 0.00260.02 0.45 0.0730.0120.0390.00270.0420.00240.00070.0019 0.022
3 Invention 0.00200.03 0.09 0.1300.0060.0810.00310.0720.00440.00100.0037 0.053
4 Invention 0.00290.02 0.33 0.0840.0050.0360.00150.0360.00700.00090.0033 0.019
Invention 0.00560.25 0.29 0.0850;0180.0240.00430.0510.00200.00060.0028 0.014
6 Invention 0.00800.02 0.34 0.0620.0270.0650.00510.0570.00990.00160.0036 0.008
C 7 Invention 0.00350.76 1.54 0.0420.0170.0350.00210.0610.00480.00110.0030 0.040
8 Comp. Ex. 0.00340.01 0.34 0.0600.0150.0500.00220.0450.00320.00120.0054 0.024
9 Comp. $x. 0.00300.02 0.24 0.0880.0100.0600.00190.0150.00250.00100.0034 -0.004
Comp. Ex. 0.00210.05 0.33 0.0680.0220.0610.00340.0380.02500.00050.0037 0.018
Comp. Ex.=Comparative Example Ti*=Ti-(48/12) C-(48/14) N
o
o
TABLE 3
Continuous annealing condition
Steel Slab Hot-roll , ,
Sample t e Classheating finishing Colllng CR* Re-Max heating
No. No.temp- (C) temp. (C) temp- ( C) crystallization temp. (C)
1 1 Invention 1200 920 480 77 770 790
2 1 Comp. Ex. 1200 920 480 34 770 790
3 1 Invention 1200 920 480 68 770*1 790
4 2 Invention 1150 910 500 61 780 810
2 Comp. Ex. 1150 910 500 40 780 810
6 2 Comp. Ex. 1150 910 500 *2 118 780 810
7 3 Invention 1100 900 550 62 800 850
8 4 Invention 1250 900 550 62 770 780
9 4 Comp. Ex. 1250 900 550 26 770 780
Invention 1200 880 600 55 750 880
11 6 Invention 1200 850 650 65 730 850
12 7 Invention 1250 890 550 51 760 850
13 8 Comp. Ex. 1200 900 550 63 770 800
14 9 Comp. Ex. 1200 900 550 65 770 800 O
Comp. Ex. 1200 900 550 63 770 800 ~_~
Comp. Ex.=Comparative Example
CR*: Sum of rolling reductions of paths which meets condition of sheet temp.(T)xstrain
rate(~)_50,000Cs~1 C~3
*1: Continuous hot-dip galvanizing line used
*2: Sheet temp. in cold-rolling exceeded 300C
20~1403
TABLE 4 ( 1 )
Sam- Steel Stand No. CR*
ple type Class Items
No.No. 1 2 3 4 5 6 (%
Rolling re- 37 47 24 5 - _ 76
duction (%)
1 1 Inven T (C) 50 100 130 140
-tion(s-l) 4001,1701,280 650
Tx(Cs 1) 20,000 117,000 166,000 91,000
Rolliny re- 57 19 19 15 - - 34
2 1Comp.T (C) 45 75 100 120
Ex.~ (s-l) 750 620 850 980
TX(Cs~l) 34,00047,000 85,000 117,000
duction (%) 42 18 8 - _ 68
3 1Inven T (C) 55 90 115 130
-tion (S-l) 430 960 850 630
TX(Cs 1) 24,000 87,00098,00082,000 - - -
Rolliny re- 17 40 40 17 4 - 61
duction (%)
4 2Inven T (C) 50 80 100 120 130
-tion ~ (5-l) 160 5201,120 960 500
Tx(Cs ) 8,000 42,000 112,000 115,000 65,000
dRolling (r%e) 48 29 14 14 12 - 40
T (C) 30 60 90 120 140
2Comp.
Ex. ( -1~ 610 810 690 860 990 - -
Tx~tCs 1)18,00048,000 62,000 103,000 139,000
Rolling re-43 35 18 11 10 - 117*
ductlon (%)
T (C) 350 350 350 360 360
6 2Comp.
Ex. ~ (s 1) 310 530 520 480 540
TX~(Cs 1) 109,000 186,000 181,000 174,000 194,000
duction (%) 47 44 18 3 - - 62
7 3 Inven T (C)55 90 110 120
-tion ~480 1,110 960 390
Tx~(Cs 1) 27,000 100,000 106,000 47,000
Comp. Ex.=Comparative Example *Sheet temp. 300C or above.
2041403
TALLE 4 (2)
Sam- Steel Stand No.
ple type Class Items CR*
No. No. 1 2 3 4 5 6 (%)
Rolling re- 33 28 28 19 12 4 63
Inven T (C) 40 70 70 120 130 140
8 4
-t1on (5-l) 350 520 840 960 910 570
T X ~ ( Cs 1)14,000 36,00059,000 116,000 119,000 79,000
Rolling re-33 25 23 17 17 9 26
duction (%)
9 4 Comp. T (C) 30 50 70 80 90 100
Ex. (s~l) 250 340 490 560 730 610
Tx~(Cs 1)8,00017,00034,00045,00066,00061,000
Rolling re-33 33 30 18 8 - 56
duction (~)
Inven T (C) 40 80 120 140 150
-tion (s~l)340600 970 1020 760
TX~(Cs 1)13,000 48,000 117,000 143,000 113,000
duction (%) 48 39 13 8 5 - 65
11 6Inven T (C) 35 70 100 110 120
-tlon ~ (s 1) 490 920 650 610 520
TX~(Cs 1) 17,00065,00065,00067,00062,000
Rolling re- 33 35 35 9 6 - 50
12 7Inven T (C) 40 70 100 130 140
-tion (S-l)350 690 1290 790 720
Tx~(Cs 1)14,000 48,000 129,000 102,000 101,000
Rolling re-33 28 2421 14 4 63
duction ( ~ )
13 8 Comp. T (C) 40 60 80100 120 130
Ex. ~ (s~l) 310 460 650860 870 500
TX~(Cs 1)12,00027,00052,00086,000 104,000 65,000
Rolling re-48 39 13 8 5 - 65
14 9Comp. T (C) 35 70 100 110 120
Ex. ~ (s ) 490 920 650 610 520
Tx~(Cs 1)17,00065,00065,00067,000 62,000
duction (~) 33 28 24 21 14 4 63
10 Comp. T ( C) 40 60 80 100 120 130
Ex. (5-l)310 460 650 860 870 500
TX~(Cs 1)12,00027,000 52,000 86,000 104,000 65,000
CR* : Sum of rolling reductions of paths vhich meets condition of sheet
temp.(T)Xstrain rate(~j~50,000Cs 1 Comp. Ex.=Comparative Example
TABLE 5 Comp. Ex.=Comparative Example
N P type Class (kgf/mls2) (kg~/mm2) k-~; T.S.~E~. r value crne t~ei~ht
1 1 Invention 20.0 35.4 50.3 85.7 2.2 55
2 1 Comp. Ex. 20.4 35.4 50.5 85.9 2.2 30
3 1 Invention 20.6 36.2 49.6 85.8 2.1 51
4 2 Invention 21.2 38.6 47.5 86.1 2.2 55
2 Comp. Ex. 22.5 38.5 47.5 86.0 2.2 25
6 2 Comp. Ex. 22.7 38.8 45.5 84.3 2.0 20
7 3 Invention 25.8 45.2 41.2 86.4 2.1 55
8 4 Invention 20.7 36.5 49.2 85.7 2.3 50
9 4 Comp. Ex. 20.9 36.1 49.1 85.2 2.2 33
Invention 23.3 40.5 45.3 85.8 2.1 52
11 6 Invention 28.1 48.5 36.4 85.1 2.0 45
12 7 Invention 24.9 54.3 33.4 87.7 2.0 53
13 8 Comp. Ex. 21.5 35.4 49.4 84.8 2.0 35 O
14 9 Comp. Ex. 26.4 34.8 42.1 76.9 1.6 20 ~-~
10 Comp. Ex. 22.0 36.1 43.1 79.2 2.0 30 O
2041403
From Table 5, it will be understood that the Sample
Nos. 2, 5, 6, 9, 13, 14 and 15 as reference examples showed
comparatively small values of truncated-cone height ranging
from 20 mm to 35 mm. In contrast, other samples which meet
the condition of the invention showed large values of
truncated-cone height ranging from 45 mm to 55 mm, thus
proving superior workability.
Sample No. 3 was subjected to a galvannealing
instead of the continuous annealing. This galvannealed steel
sheet also showed excellent workability as in the cases of
other samples meeting the conditions of the invention.
Sample No. 6 was cold-rolled at a cold-rolling
sheet temperature exceeding 300C, although the sum of the
rolling reductions of the passes having the product of the
sheet temperature and the strain rate exceeding 50,000~C S 1
was greater than 50 %. Consequently, this sample showed a
too small workability which was 20 mm in terms of truncated-
cone height.
The truncated-cone height was obtained by a
conventional conical cup test as follows. A truncated cone-
shaped test sample was first prepared by punching a sheet
with a punch having a diameter of 80 mm for a die with a
diameter of 140 mm. In this case, when a Y value was low or
a YS value was high, a wrinkle was produced in a tapered
portion of the test sample and when an n value was low, a
crack was produced in a top corner portion of the test
sample.
A critical height at which molding could be carried
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out without producing a wrinkle or crack in the test sample
was defined as truncated-cone height.
As will be understood from the foregoing
description, a method has been established by the present
invention which enables production of a high-strength cold-
rolled steel sheet having superior workability by processing
a low-oxygen low-carbon steel rich in P under specific cold-
rolling conditions. The cold-rolled steel sheet produced by
the method of the invention is suitable for use as a material
of products which are produced through press-forming,
bulging, deep-drawing and other plastic works.
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