Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
Title of Invention
STEEL SHEET FOR CAN HAVING HIGH STRENGTH AND HIGH
FORMABILITY, AND METHOD FOR MANUFACTURING THE SAME
Technical Field
[0001]
The present invention relates to a steel sheet for a
can having high strength and high formability and a method
for manufacturing the same.
Background Art
[0002]
Among steel sheets used for beverage cans and food cans,
steel sheets referred to as DR (Double Reduce) materials may
be used for ends, bottoms, bodies of three-piece cans, drawn
cans and the like. Regarding the DR material, cold rolling
is performed again (secondary cold rolling) after annealing
and, therefore, the sheet thickness can be reduced easily as
compared with a SR (Single Reduce) material, where only
temper rolling with a small reduction ratio is performed,
and it is possible to reduce the can production cost by
using a thin steel sheet. The DR material is a thin hard
steel sheet because work hardening occurs by applying cold
rolling after annealing. However, the DR material has low
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with the SR material.
[0003]
Meanwhile, EOE (Easy Open End) has been used widely as
ends for beverage cans and food cans. In production of EOE,
it is necessary that a rivet to attach a tab is formed
through punch stretch forming and draw forming, and the
elongation of the material required for this forming
corresponds to the ductility of about 10% in a tensile test.
[0004]
Furthermore, regarding the body material for a three-
piece beverage can, after forming into the shape of a tube,
flange forming is applied to both ends to perform seaming of
a end and a bottom. Therefore, can body end portions are
required to have the ductility of about 10% likewise.
[0005]
Meanwhile, a steel sheet serving as a raw material for
producing a can is required to have strength in accordance
with the sheet thickness. In the case of the DR material, a
tensile strength of about 500 MPa or more is required in
order to ensure the can strength because the thickness is
reduced.
[0006]
It is difficult for the previously used DR material to
ensure both the above-described elongation and the strength
and, therefore, the SR material has been used as the body
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material for EOE and the beverage can. However, at present,
from the viewpoint of cost reduction, demands for
application of the DR material to the body material for EOE
and the beverage can have also been intensified.
Furthermore, this material can also be used as a raw
material of the steel sheet for cans, e.g., two-piece can
bodies, DI (Drawn and Ironed) cans, DRD (Draw-redraw) cans,
aerosol cans, and bottom ends.
[0007]
In consideration of them, Patent Literature 1 discloses
a method for manufacturing a steel sheet having a high r
value and excellent flange formability by producing a DR
material from low-carbon steel with a primary cold reduction
ratio of 85% or less.
[0008]
Patent Literature 2 discloses a method for
manufacturing a DR material, wherein the compatibility
between the hardness and the formability is ensured by
applying a nitriding treatment in a low-carbon steel
annealing step.
[0009]
Patent Literature 3 discloses a method for
manufacturing a end for an easy open can, wherein scoring is
performed in such a way that the ratio of score residual
thickness/steel sheet thickness becomes 0.4 or less through
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the use of a thin steel sheet which has a sheet thickness of
less than 0.21 mm and which is produced by subjecting a
steel slab containing C: 0.01% to 0.08%, Mn: 0.05% to 0.50%,
and Al: 0.01% to 0.15% to hot finish rolling at a
temperature lower than or equal to the Ar3 transformation
temperature, then performing cold rolling, subsequently
effecting recrystallization annealing through continuous
annealing, and thereafter, performing skin pass rolling with
a reduction ratio of about 5% to 10%.
[0010]
Patent Literature 4 discloses a continuous annealing DR
steel sheet for a welded can and a manufacturing method,
wherein the steel sheet contains C: 0.04% to 0.08%, Si:
0.03% or less, Mn: 0.05% to 0.50%, P: 0.02% or less, S:
0.02% or less, Al: 0.02% to 0.10%, and N: 0.008% to 0.015%,
the amount of (N total - N as AIN) in the steel sheet is
0.007% or more, and in the case where the relationship of X
?_ 10% and Y -0.05X + 1.4 are satisfied, where a total
ductility value in the rolling direction is represented by X
and the average value is represented by Y, the steel sheet
has excellent flange formability better than or equal to
that of a batch annealing DR steel sheet.
Citation List
Patent Literature
[0011]
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PTL 1: Japanese Unexamined Patent Application
Publication No. 63-7336
PTL 2: Japanese Unexamined Patent Application
Publication No. 2004-323905
PTL 3: Japanese Unexamined Patent Application
Publication No. 62-96618
PTL 4: Japanese Unexamined Patent Application
Publication No. 2007-177315
Summary of Invention
Technical Problem
[0012]
However, all the above-described conventional
technologies have problems as described below.
[0013]
Regarding the manufacturing method described in Patent
Literature 1, it is necessary to reduce the primary cold
reduction ratio and, therefore, very thin steel sheet cannot
be produced because of the limited finish thickness of hot
rolling. If the finish thickness of hot rolling is reduced,
the finish rolling temperature is lowered, so that it is
difficult to keep at a predetermined temperature.
[0014]
Regarding the manufacturing method described in Patent
Literature 2, it is necessary to apply the nitriding
treatment after recrystallization is completed. Therefore,
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even in the case where the nitriding treatment is applied in the continuous
annealing step, cost increases, e.g., a reduction in line speed and an
increase in
furnace length, are not avoided.
[0015]
Regarding the manufacturing methods described in Patent Literature 3 and
Patent Literature 4, the amount of Mn is controlled to a low level of 0.05 to
0.50
percent by weight. Consequently, it is not possible to respond to an increase
in
strength to ensure pressure-resistant strength against thickness reduction.
[0016]
The present invention has been made in consideration of such
circumstances, and it is an object to provide a high-strength high-formability
steel
sheet for a can and a method for manufacturing the same, wherein the steel
sheet can be applied to ends, bottoms, three-piece can bodies, two-piece can
bodies, DI cans, DRD cans, aerosol cans, and bottom ends and, in particular,
is
suitable for a material of EOE.
Solution to Problem
[0017]
The gist of the present invention is as described below.
[0018]
A first invention is a steel sheet for a can, characterized by comprising C:
0.070% or more and less than 0.080%, Si: 0.003% or more and 0.10% or less, Mn:
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0.51% or more and 0.60% or less, P: 0.001% or more and 0.100% or less,
S: 0.001% or more and 0.020% or less, Al: 0.005% or more and 0.100% or less,
N: 0.010% or less, and the remainder composed of Fe and incidental impurities,
on
a percent by mass basis, wherein in a cross-section in the rolling direction,
the
average grain size is 5 pm or more, the grain elongation rate is 2.0 or less,
the
difference in hardness determined by subtracting an average Vickers hardness
of a
cross-section between the surface and a depth of one-eighth of the sheet
thickness
from an average Vickers hardness of a cross-section between a depth of three-
eighths of the sheet thickness and a depth of four-eighths of the sheet
thickness is
points or more and/or the difference in hardness determined by subtracting a
maximum Vickers hardness of a cross-section between the surface and a depth of
one-eighth of the sheet thickness from a maximum Vickers hardness of a cross-
section between a depth of three-eighths of the sheet thickness and a depth of
four-eighths of the sheet thickness is 20 points or more, the tensile strength
is
500 MPa or more, and the elongation after fracture is 10% or more.
[0019]
A second invention is the steel sheet for a can, according to the first
invention, characterized in that regarding the above-described grain size, the
difference in average grain size determined by subtracting an average grain
size
between a depth of three-eighths of the sheet thickness and a depth of four-
eighths of the sheet thickness from an average grain size between the surface
and
a depth of one-eighth of the sheet thickness is 1 pm or more.
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[0020]
A third invention is the steel sheet for a can, according to the first
invention
or the second invention, characterized in that regarding the above-described
amount of nitrogen, the difference in average amount of N determined by
subtracting an average amount of N between the surface and a depth of one-
eighth of the sheet thickness from an average amount of N between a depth of
three-eighths of the sheet thickness and a depth of four-eighths of the sheet
thickness is 10 ppm or more.
[0021]
A fourth invention is the steel sheet for a can, according to any one of the
first invention to the third invention, characterized in that regarding
nitrides
having a diameter of 1 pm or less and 0.02 pm or more, the average nitride
number density between the surface and a depth of one-quarter of the sheet
thickness is larger than the average nitride number density between the
surface
and a depth of one-eighth of the sheet thickness.
[0022]
A fifth invention is the steel sheet for a can, according to any one of the
first
invention to the fourth invention, characterized in that regarding the above-
described nitrides having a diameter of 1 pm or less and 0.02 pm or more, the
value obtained by dividing the average nitride number density between the
surface and a depth of one-twentieth of the sheet thickness by the average
nitride
number density between the surface and a depth of one-quarter of the sheet
thickness is less than 1.5,
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[0023]
A sixth invention is the steel sheet for a can, according to any one of the
first invention to the fifth invention, characterized in that regarding the
above-
described amount of carbon, the amount of solid solution C in the steel is 51
ppm
or more.
[0024]
A seventh invention is a method for manufacturing a steel sheet for a
can, characterized by comprising the steps of slabbing a steel containing C:
0.070% or more and less than 0.080%, Si: 0.003% or more and 0.10% or less,
Mn: 0.51% or more and 0.60% or less, P: 0.001% or more and 0.100% or less,
S: 0.001% or more and 0.020% or less, Al: 0.005% or more and 0.100% or
less, N: 0.010% or less, and the remainder composed of Fe and incidental
impurities, on a percent by mass basis, through continuous casting, performing
hot rolling so as to coil at a temperature lower than 620 C, performing
rolling
with a primary cold reduction ratio of 86% or more in total and a cold
reduction
ratio of a final stand in primary cold rolling of 30% or more, performing
annealing in an atmosphere containing less than 0.020 percent by volume of
ammonia gas, and performing secondary cold rolling with a reduction ratio of
20% or less.
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Advantageous Effects of Invention
[0025]
According to the present invention, the high-strength high-formability steel
sheet for a can having a tensile strength of 500 MPa or more and an elongation
after fracture of 10% or more can be obtained. As a result, the formability of
the
steel sheet is improved and, thereby, cracking does not occur during rivet
forming
of EOE and flange formability of the three-piece can, it becomes possible to
produce a can from the DR material having a small thickness, and significant
thickness reduction of the steel sheet for a can is achieved.
Description of Embodiments
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[0026]
In order to solve the above-described problems, the
inventors conducted intensive research and obtained the
following findings.
[0027]
In order to ensure the elongation with respect to a
high-strength material, it is possible to ensure
compatibility between the strength and the elongation by
adding an appropriate amount of C so as to give strength,
introducing a strain into a surface layer while the
reduction ratio of the final stand in primary cold rolling
is improved, allowing ferrite grains in the surface layer to
become coarse through annealing while an ammonia gas in the
annealing atmosphere is limited to less than 0.020 percent
by volume to suppress nitridation of the surface layer, and
allowing the surface of the steel sheet to become mild while
the secondary cold reduction ratio is limited in an
appropriate range.
[0028]
Furthermore, if the coiling temperature after hot
rolling is high, precipitated cementite becomes coarse and a
local ductility is reduced. Therefore, it is also necessary
to limit the coiling temperature to an appropriate
temperature range.
[0029]
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In this regard, in the present specification, all
components of steel indicated in % are on a percent by mass
basis.
Meanwhile, a depth of three-eighths of the sheet thickness
refers to a position at a distance of three-eighths of the
sheet thickness from the surface in the direction of the
center of the film thickness. The same goes for the others,
that is, a depth of four-eighths of the sheet thickness, a
depth of one-eighth of the sheet thickness, a depth of one-
quarter of the sheet thickness, and a depth of one-twentieth
of the sheet thickness.
[0030]
The present invention will be described below in detail.
The steel sheet for a can according to the present invention
is a high-strength high-formability steel sheet for a can
having a tensile strength of 500 MPa or more and an
elongation after fracture of 10% or more. Then, such a
steel sheet can be produced by using a steel containing
0.070% or more and less than 0.080% of C and specifying the
coiling temperature after hot rolling and the secondary cold
reduction ratio to be appropriate conditions.
[0031]
The component composition of the steel sheet for a can
according to the present invention will be described.
[0032]
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C: 0.070% or more and less than 0.080%
Regarding the steel sheet for a can according to the
present invention, the ductility is ensured by reducing the
secondary cold reduction ratio, while high strength is
exerted by specifying the amount of C to be somewhat high.
If the amount of C is less than 0.070%, the tensile strength
of 500 MPa required for obtaining significant economic
effect based on a thickness reduction of the steel sheet is
not obtained. Therefore, the amount of C is specified to be
0.070% or more. Meanwhile, if the amount of C is 0.080% or
more, the steel sheet becomes too hard, and it becomes
impossible to produce a thin steel sheet through secondary
cold rolling while the formability is held ensured.
Therefore, the upper limit of the amount of C is specified
to be less than 0.080%.
[0033]
Si: 0.003% or more and 0.10% or less
If the amount of Si exceeds 0.10%, problems, e.g.,
reduction in surface-treatability and degradation of
corrosion resistance, are brought about. Therefore, the
upper limit is specified to be 0.10%. Meanwhile, a smelting
cost becomes too high to achieve less than 0.003%.
Therefore, the lower limit is specified to be 0.003%.
[0034]
Mn: 0.51% or more and 0.60% or less
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Manganese has functions of preventing hot shortness due
to S during hot rolling and making crystal grains finer and
is an element necessary for ensuring desirable material
properties. Furthermore, in order to satisfy the can
strength with a material having a reduced thickness, it is
necessary to increase the strength of the material. In
order to respond to this increase in strength, a required
amount of addition of Mn is 0.51% or more. On the other
hand, if Mn is added too much, the corrosion resistance is
degraded and the steel sheet becomes hard excessively.
Therefore, the upper limit is specified to be 0.60%.
[0035]
P: 0.001% or more and 0.100% or less
Phosphorus is a harmful element which makes the steel
hard, which degrades the formability and, at the same time,
which degrades even the corrosion resistance. Therefore,
the upper limit is specified to be 0.100%. Meanwhile, if P
is specified to be less than 0.001%, a dephosphorization
cost is too large. Therefore, the lower limit is specified
to be 0.001%.
[0036]
S: 0.001% or more and 0.020% or less
Sulfur is a harmful element which is present as an
inclusion in the steel and which causes reduction in the
elongation and degradation of the corrosion resistance.
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Therefore, the upper limit is specified to be 0.020%.
Meanwhile, if S is specified to be less than 0.001%, a
desulfurization cost is too large. Therefore, the lower
limit is specified to be 0.001%.
[0037]
Al: 0.005% or more and 0.100% or less
Aluminum is an element necessary as a deoxidizer in
steel making. If the amount of addition is small,
deoxidation becomes insufficient, inclusions increase, and
the formability is degraded. In the case where the content
is 0.005% or more, it is assumed that deoxidation is
performed sufficiently. Meanwhile, if the content exceeds
0.100%, the frequency of occurrence of surface defects
resulting from alumina clusters and the like increases.
Therefore, the amount of Al is specified to be 0.005% or
more and 0.100% or less.
[0038]
N: 0.010% or less
If large amounts of N is added, hot elongation is
degraded and cracking of slab occurs in continuous casting.
Therefore, the upper limit is specified to be 0.010%.
Meanwhile, if the amount of N is specified to be less than
0.001%, a smelting cost is too large. Therefore, it is
preferable that the amount of N is specified to be 0.001% or
more.
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In this regard, the remainder is specified to be Fe and
incidental impurities.
[0039]
Next, the mechanical properties of the steel sheet for
a can according to the present invention will be described.
[0040]
The tensile strength is specified to be 500 MPa or more.
If the tensile strength is less than 500 MPa, the thickness
of the steel sheet cannot be reduced to the extent that a
significant economic effect is obtained because the strength
of the steel sheet serving as a raw material for can
production is ensured. Therefore, the tensile strength is
specified to be 500 MPa or more.
[0041]
The elongation after fracture is specified to be 10% or
more. If the elongation after fracture is less than 10%,
cracking occurs in rivet forming in the case of application
to EOE. Moreover, in the case of application to a three-
piece can body as well, cracking occurs in flange forming.
Therefore, the elongation after fracture is specified to be
10% or more.
[0042]
In this regard, the above-described tensile strength
and the above-described elongation after fracture can be
measured by the method of tensile test for metallic
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materials shown in "JIS Z 2241".
[0043]
Next, crystal grains of the steel sheet for a can
according to the present invention will be described.
[0044]
The average grain size in a cross-section in the
rolling direction is specified to be 5 rn or more. The
state of crystal grains has a large influence on the final
mechanical properties of the steel sheet for a can according
to the present invention. If the average grain size in a
cross-section in the rolling direction is less than 5 m,
the ductility of the steel sheet is insufficient and the
formability is impaired.
[0045]
Furthermore, the grain elongation rate in a cross-
section in the rolling direction is specified to be 2.0 or
less. The elongation rate refers to a value representing
the degree of elongation of ferrite grain due to forming, as
shown in "JIS G 0202". If the grain elongation rate in a
cross-section in the rolling direction is more than 2.0, the
ductility in the direction perpendicular to the rolling
direction, which is important for the flange formability and
the neck formability, is insufficient. Although the
elongation rate increases along with secondary cold
reduction ratio, it is necessary that the steel contains
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0.070% or more of C in order to control to the above-
described elongation rate while the secondary cold reduction
ratio is up to about 20%. That is, if the C is less than
0.070%, the number of cementite grains precipitated after
hot rolling is reduced and, as a result, much of solid
solution C remains. The solid solution C suppresses growth
of grains during annealing and, thereby, the shapes of
crystal grains flattened through primary cold rolling remain
and the elongation rate increases.
In this regard, the above-described average grain size
in a cross-section in the rolling direction and the above-
described grain elongation rate in a cross-section in the
rolling direction can be measured by micrographic
determination of the apparent grain size shown in "JIS G
0551".
[0046]
By the way, in the case where there is no remark, the
surface and the back of the steel sheet are not specifically
distinguished.
[0047]
The Vickers hardness can be measured by the test method
for hardness shown in "JIS Z 2244". The Vickers hardness
test with a load of 10 gf is performed in such a way that
the hardness distribution in a steel sheet cross-section in
the sheet thickness direction can be evaluated appropriately.
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The measurement is performed at 10 places each, and an
average value of the measured values is specified to be the
average hardness of each cross-section. In this regard, a
maximum of the Vickers hardness measurement is specified to
be a cross-section Vickers maximum hardness.
[0048]
Regarding difference in hardness: 10 points or more and
20 points or more
In the case where a surface layer is made hard, the
strength increases. However, a mild center layer is
sandwiched between hard surface layers and, thereby, the
whole sheet is constrained, the ductility is reduced,
constriction occurs easily, and the formability is degraded.
In the case where the surface layer is mild and the center
layer is hard, only the center layer of the sheet is
constrained and, therefore, a high-strength high-formability
steel sheet is obtained, wherein the strength is high, and
reduction of ductility and constriction do not occur. If a
difference in cross-section average hardness is less than 10
points and/or the cross-section maximum hardness is less
than 20 points, the whole sheet has a uniform hardness and,
therefore, there is no difference from a current material
and a high-strength high-formability steel sheet cannot be
obtained. The tensile strength of 500 MPa or more and the
elongation after fracture of 10% can be achieved by
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specifying the difference in cross-section average hardness
to be 10 points or more and/or the cross-section maximum
hardness to be 20% or more.
[0049]
Difference in average grain size: 1 m or more
Regarding the above-described grain size of the high-
strength high-formability steel sheet for a can according to
the present invention, it is preferable that a difference in
average grain size determined by subtracting an average
grain size between a depth of three-eighths of the sheet
thickness and a depth of four-eighths of the sheet thickness
from an average grain size between the surface and a depth
of one-eighth of the sheet thickness is 1 m or more. This
is because in the case where the difference in average grain
size is 1 m or more, a steel sheet having excellent
characteristics ensuring the compatibility between the
strength and the elongation can be obtained. In this regard,
the ductility is improved because of enhance of mildness due
to the large grain size at a depth of one-eighth of the
sheet thickness, and the small grain size between a depth of
three-eighths of the sheet thickness and a depth of four-
eighths of the sheet thickness contributes to hardness and
high strength. Consequently, the compatibility between the
high strength and the elongation is ensured, and the tensile
strength of 500 MPa or more and the elongation after
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fracture of 10% can be achieved easily.
[0050]
Regarding the average amount of N between a depth of
three-eighths of the sheet thickness and a depth of four-
eighths of the sheet thickness, the amount of N of a sample
which had been subjected to electrolytic polishing up to the
depth of three-eighths of the sheet thickness was measured
by using a combustion method. Regarding the average amount
of N between the surface and a depth of one-eighth of the
sheet thickness, after one surface of a sample was sealed
with a tape, chemical polishing was performed from the
surface up to the depth of one-eighth of the sheet thickness
with oxalic acid, and the amount of N of the remaining
sample was measured by using a combustion method.
[0051]
Regarding difference in average amount of N: 10 ppm or
more
If the difference in average amount of N is less than
ppm, the whole sheet has a uniform amount of N and,
therefore, a significant enhancement of mildness due to
reduction in the amount of N of the surface layer cannot be
expected. However, in the case where the difference in
average amount of N is specified to be 10 ppm or more, the
mildness of the surface layer is enhanced because the amount
of N is small and, thereby, solid solution N contributing to
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solution strengthening is small, whereas the center layer
has a large amount of N and is hard, so that the
compatibility between the high strength and the elongation
is ensured and the tensile strength of 500 MPa or more and
the elongation after fracture of 10% or more can be achieved
easily. Consequently, a high-strength high-formability
steel sheet is obtained easily.
[0052]
Regarding the nitride number density, after chemical
polishing with oxalic acid or the like was performed up to a
predetermined position, electrolysis of 10 m was performed
by using a SPEED method to produce an extraction replica,
and the number of nitrides per unit field of view 1 m
square was measured with TEM. The nitrides were analyzed
with EDX, so as to be identified.
[0053]
Furthermore, regarding nitrides having a diameter of 1
m or less and 0.02 m or more, it is preferable that the
average nitride number density between the surface and a
depth of one-quarter of the sheet thickness is larger than
the average nitride number density between the surface and a
depth of one-eighth of the sheet thickness. This is because
if the the average nitride number density between the
surface and a depth of one-eighth of the sheet thickness is
small, fine precipitates are low in number, so as to enhance
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the mildness, and if the average nitride number density
between the surface and a depth of one-quarter of the sheet
thickness is large, fine precipitates are high in number, so
as to become hard on the basis of precipitation
strengthening, so that the compatibility between the high
strength and the elongation is ensured and the tensile
strength of 500 MPa or more and the elongation after
fracture of 10% or more can be achieved easily.
[0054]
Regarding average nitride number density ratio: 1.5 or
less
If the average nitride number density ratio is 1.5 or
more, the nitride number density of the surface layer
increases and precipitation strengthening due to nitrides
occurs, so that significant enhancement of mildness is not
expected. However, the tensile strength of 500 MPa or more
and the elongation after fracture of 10% or more can be
achieved easily by specifying the average nitride number
density ratio to be less than 1.5. Consequently, a high-
strength high-formability steel sheet is obtained easily.
[0055]
The amount of solid solution C was calculated from a
peak of internal friction. The internal friction was
measured with a torsional vibration type internal friction
measuring apparatus produced by Vibran, where the shape of a
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test piece was 1 mm x 1 mm x 80 mm, the measurement
frequency was 0.001 to 10 Hz, and the temperature was 0 C,
background of the measured data was removed and, thereafter,
Q-1 of the peak value was read. Calculation was performed
from Q-1 and a calibration curve. If the amount of solid
solution C in the steel is large, the strength increases on
the basis of strengthening due to solid solution C, and the
ductility increases because the amount of carbide serving as
a start point of fracture is reduced.
[0056] =
Next, a method for manufacturing a steel sheet for a
can according to the present invention will be described.
The high-strength high-formability steel sheet for a can,
according to the present invention, is produced by using a
steel slab which is produced through continuous casting and
which has the above-described composition, performing hot
rolling so as to coil at a temperature lower than 620 C,
performing rolling with a primary cold reduction ratio of
86% or more and a cold reduction ratio of a final stand in
primary cold rolling of 30% or more, performing annealing in
an atmosphere containing less than 0.020 percent by volume
of ammonia gas, and performing secondary cold rolling with a
reduction ratio of 20% or less.
[0057]
It is usually difficult to obtain a small sheet
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thickness, which exerts a significant economic effect, by
only one time of cold rolling. That is, to obtain a small
sheet thickness by one time of cold rolling applies an
excessive load to a rolling mill, and is impossible
depending on the equipment capacity. For example, in the
case where the final sheet thickness is specified to be 0.15
mm, if the sheet thickness after hot rolling is specified to
be 2.0 mm, a large primary cold reduction ratio of 92.5% is
required.
[0058]
Meanwhile, in order to reduce the sheet thickness after
the cold rolling, it is considered to perform rolling at the
stage of hot rolling in such a way as to have a thickness
smaller than usual. However, if the reduction ratio in the
hot rolling increases, the temperature of the steel sheet
during the rolling decreases significantly, so that a
predetermined finish rolling temperature is not obtained.
Furthermore, if the sheet thickness before the annealing is
reduced, in the case where continuous annealing is applied,
the possibility of an occurrence of a trouble, e.g.,
fracture or deformation, of a steel sheet during the
annealing increases. For these reasons, in the present
invention, a secondary cold rolling is applied after the
annealing, so as to obtain a very thin steel sheet.
[0059]
CA 02818682 2013-05-21
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Coiling temperature after hot rolling: lower than 620 C
If the coiling temperature after the hot rolling is
620 C or higher, formed pearlite microstructures become
coarse, and these serve as start points of brittle fracture.
Therefore, local ductility is reduced and the elongation
after fracture of 10% or more is not obtained. Therefore,
the coiling temperature after the hot rolling is specified
to be lower than 620 C. More preferably, the coiling
temperature is 560 C to 620 C.
[0060]
Primary cold reduction ratio: 86% or more
In the case where the primary cold reduction ratio is
small, in order to obtain a very thin steel sheet finally,
it is necessary to increase the reduction ratios in hot
rolling and cold rolling. It is not preferable to increase
the hot reduction ratio because of the above-described
reason, and it is necessary that the secondary cold
reduction ratio is limited for the reason described below.
For the above-described reasons, if the primary cold
reduction ratio is specified to be less than 86%, production
is difficult. Therefore, the primary cold reduction ratio
is specified to be 86% or more. More preferably, 90% to 92%
is employed.
[0061]
Reduction ratio of final stand in primary cold rolling:
CA 02818682 2013-05-21
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30% or more
In order to make the surface layer of the steel sheet
coarse grains so as to enhance mildness, it is necessary to
increase the reduction ratio of a final stand to introduce a
strain into the steel sheet surface layer and, thereby,
facilitate growth of ferrite grains during annealing. In
order to make the grain size of the surface layer coarse by
1 'Am as compared with that of the center layer, it is
necessary that the reduction ratio of a final stand in
primary cold rolling is specified to be 30% or more.
[0062]
Annealing
Regarding annealing, in order to suppress nitridation
of the surface layer, it is necessary to specify the ammonia
gas concentration in the atmosphere to be less than 0.020
percent by volume. Preferably, 0.018 percent by volume or
less is employed, and more preferably 0.016 percent by
volume or less is employed. Furthermore, it is necessary
that recrystallization is completed through annealing. It
is preferable that the soaking temperature is specified to
be 600 C to 750 C from the viewpoint of the operation
efficiency and prevention of fracture of the thin steel
sheet during annealing.
[0063]
Secondary cold reduction ratio: 20% or less
CA 02818682 2013-05-21
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The secondary cold reduction ratio is specified to be
20% or less. If the secondary cold reduction ratio is more
than 20%, work hardening due to secondary cold rolling
becomes excessive, and the elongation after fracture of 10%
or more is not obtained. Therefore, the secondary cold
reduction ratio is specified to be 20% or less. Preferably,
15% or less is employed, and more preferably, 10% or less is
employed.
[0064]
After the secondary cold rolling, steps of coating and
the like are performed in the usual manner, so as to
complete a steel sheet for a can.
EXAMPLES
[0065]
A steel having the component composition as shown in
Table 1, where the remainder was Fe and incidental
impurities, was prototyped, and a steel slab was obtained
through casting. The resulting steel slab was reheated at
1,250 C and, thereafter, hot rolling and primary cold
rolling were applied under the condition shown in Table 2.
The finish rolling temperature of the hot rolling was
specified to be 890 C, and pickling was applied after the
rolling. Subsequently, after the primary cold rolling,
continuous annealing at a soaking temperature of 630 C and a
soaking time of 25 seconds and secondary cold rolling under
CA 02818682 2013-05-21
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the condition shown in Table 2 were applied.
Both surfaces of the steel sheet obtained as described above
was subjected to Sn coating continuously, so as to obtain a
tin plate with an amount of adhesion of Sn of 2.8 g/m2 per
one surface. The test results are shown in Table 2 and
Table 3. In this regard, in Table 3, the grain size, the
amount of N, and the nitride number density refers to the
average grain size, the average amount of N, and the average
nitride number density, respectively.
[0066]
Table 1
Component composition (percent by mass)
No
Si Mn P S Al NRemarks
A 0.069 0.01 0.51 0.010 0.010 0.040 0.0070 Conparative steel
B 0.080 0.01 0.51 0.010 0.010 0.040 0.0070 Conparative
steel
C 0.070 0.01 0.50 0.010 0.010 0.040 0.0070 Conparative steel
D 0.070 0.01 0.61 0.010 0.010 0.040 0.0070 Conparative
steel
E 0.070 0.01 0.51 0.010
0.010 0.040 0.011 Conparative steel
F 0.070 0.01 0.51 0.010 0.010 0.040
0.0070 Invention steel
G 0.070 0.01 0.51 0.010 0.010
0.040 0.0095 Invention steel
Note:Underlined data indicate data which are out of the scope of the
invention.
..
. ,
- 30 -
[0067]
Table 2
Sheet Primary Reduction ratio Secondary
F
A
St verage
eel
Coiling thickness cold of final stand in cold
inal Ammonia gas Tensile Total Grain
shee t
No temperature after hot reduction primary cold reduction
concentration strength ductility grainelongation
typethickness
size
rolling ratio rolling ratio
rate
C mm % % % mm vol% MPa % Pm
1 A 610 2.6 90 30 18 0.213
0.018 495 11 5.5 1.80
2 B 610 2.6 90 30 18 0.213
0.018 501 9 5.7 1.80
3 C 610 2.6 90 30 18 0.213
0.018 496 11 5.5 1.80 n
4 D 610 2.6 90 30 18 0.213
0.018 502 9 5.7 1.80 0
I.,
m
E 610 2.6 90 30 18 0.213 0.018
505 9 5.8 1.80 H
CO
al
6 F 610 2.6 90 30 18 0.213
0.018 502 11 5.9 1.80 m
I.,
7 F 610 2.6 90 30 18 0.213
0.018 502 11 5.7 1.80
0
H
8 F 610 2.6 90 30 19 0.211
0.018 502 12 5.7 1.80 UJ
I
0
Ui
9 F 610 2.6 90 30 18 0.213
0.018 502 12 5.7 1.80 1
I.,
10 F 610 2.6 90 30
18 0.213 0.018 502 12 5.7 1.80 H
11 F 610 2.6 , 90 30 18 0.213 0.018
502 12 5.7 1.80
12 F 610 2.6 90 30
18 0.213 0.018 504 11 5.7 1.80
13 F 640 2.6 90 30
18 0.213 0.018 490 13 6.5 1.80
14 F 610 2.6 90 27
18 0.213 0.018 495 12 6.2 1.70
F 610 2.6 90 30 21 0.205 0.018 503
9 4.9 , 2.10
16 F 610 2.6 90 30
18 0.213 0.020 503 9 5.9 1.80
17 F 610 2.6 90 30
18 0.213 0.021 503 8 6.1 1.80
18 G 610 2.6 90 30 18 0.213 0.018
514 11 5.5 1.80
'
- 31 -
[0068]
Table 3
Average Vickers Maximum Vickers
Solid Pressure-
Form-
Grain size Amount of N hardness of cross- hardness of cross-
Nitride number density solution resitant Remarks
section section
C strength ability
1/20 1/8 1/4
No. Layer 2 Layer Layer Layer 1
- la yer
Layer Layer Layer Layer surface surface
surface (1/20)/
1 - Layer Layer 1 - Layer Layer - l
1* 2 ** 1* 2 ** 1" 2 ** 1* 2 ** ayer layer
layer layer (1/4)
1 layer 2 layer 2 2
=== .=...
Frri PPm Hv Hv /pri3
ppm
1 5.5 6.4 0.9 70 60 10 165 145 20 170 150
20 9 , 0.1 11 0.8 53 x 0 Comparative
example n
2 5.7 6.6 0.9 70 60 10 167 147 20 172 152
20 9 0.1 11 0.8 52 0 x ,Comparative
example 0
I.)
3 5.5 6.4 0.9 70 60 10 165 145 20 170 150
20 9 0.1 11 0.8 51 x 0 Comparative
example CO
H
CO
4 5.7 6.6 0.9 70 60 10 167 147 20 172 152
20 9 0.1 11 0.8 53 0 x Comparative
example 0,
co
I.)
5.8 6.7 0.9 70 60 10 168 148 20 173 153 20
9 0.1 11 0.8 50 0 x Comparative example
I.)
6 5.9 6.9 1.0 70 60 10 167 147
20 172 152 20 _ 9 0.1 11 0.8 51 0 0
Invention example 0
H
LO
7 5.7 6.6 0.9 72 63 9 167 147 20 172
152 20 20 6 5 4.0 46 0 0 Invention
example 1
_
0
u-,
8 5.9 6.9 1.0 72 63 9 167 147 20 172
152 20 20 6 5 4.0 46 0 0 Invention
example 1
I.)
9 5.7 6.6 0.9 72 62 10 168 147 21 173
152 21 20 6 5 4.0 46 0 , 0
Invention example H
5.7 6.6 0.9 72 63 9 169 147 22 172 152 20
18 0.5 11 1.6 46 0 0 Invention example
11 5.8 6.7 0.9 72 63 , 9 167 147 20 172 152 20
1 3.0 2 0.5 46 0 0 Invention example
12 5.7 6.6 0.9 72 63 , 9 167 147 20 172 152 20
20 6 5 4.0 53 0 0 Invention example
13 6.5 7.4 0.9 70 60 10 163 144 19 168 149
19 9 , 0.1 11 0.8 52 x 0 Comparative example
14 6.0 6.2 0.2 70 60 10 165 160 5 175 172 3 9 0.1
11 0.8 51 x 0 Comparative example
4.8 5.5 0.7 70 60 10 168 _ 149 19 173 _
154 19 9 0.1 11 0.8 53 0 x Comparative example
16 6.0 6.3 0.3 70 60 10 168 160 8 174 166 8 10 ,
2.0 8 1.3 51 0 x _Comparative example
17 , 6.0 6.1 0.1 70 60 10 168 190 -22 178
198 -20 20 3.0 5 4.0 51 0 x Comparative example
18 6.0 6.1 0.1 100 90 10 168 190 -22 178
198 -20 20 3.0 5 4.0 51 0 0 Invention example
Layer 1* :from depth of three-eighths to depth of four-eighths of sheet
thickness
Layer 2** :from surface to depth of one-eighth of sheet thickness
1/20 surface layer*** :from surface to depth of one-twentieth of sheet
thickness
1/8 surface layer**** :from surface to depth of one-eighth of sheet thickness
1/4 surface layer"' :from surface to depth of one-quarter of sheet thickness
CA 02818682 2013-05-21
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. [0069]
The coated steel sheet (tin plate) obtained as
described above was subjected to a heat treatment
corresponding to painting and baking at 21000 for 10 minutes
and, thereafter, a tensile test was performed. Regarding
the tensile test, a tensile test piece of JIS No. 5 size was
used and the tensile strength (strength after fracture) and
the elongation after fracture was measured at a cross head
speed of 10 mm/min.
Furthermore, a sample of the coated steel sheet was
taken, and the average grain size and the grain elongation
rate in a cross-section in the rolling direction were
measured. The average grain size and the grain elongation
rate in a cross-section in the rolling direction were
measured by a cutting method with a straight test line
described in "JIS G 0551" after the vertical cross-section
of the steel sheet was polished and grain boundaries were
revealed through nital etching.
[0070]
Regarding measurement of the pressure-resistant
strength, a sample having a sheet thickness of 0.21 mm was
formed into a lid of 63 mmcp and was attached to a welded can
body of 63 mmcD through seaming. Compressed air was
introduced into the can, and a pressure at which the can lid
was deformed was measured. The case where the can lid was
= CA 02818682 2013-05-21
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not deformed even when the pressure of the inside was 0.20
MPa was indicated by a symbol ED, the case where the can lid
was not deformed when the pressure of the inside was
increased up to 0.19 MPa and the can lid was deformed when
the pressure of the inside was 0.20 MPa was indicated by a
symbol (), and the case where the can lid was deformed at
0.19 MPa or less was indicated by a symbol x.
[0071]
Regarding the formability, the tester specified in JIS
B 7729 was used, and a test was performed by a method
specified in JIS Z 2247.
The case where the Erichsen value (forming height when
penetration cracking occurred) was 6.5 mm or more was
indicated by a symbol CD, the case of less than 6.5 mm and
6.0 mm or more was indicated by a symbol (), and the case of
less than 6.0 mm was indicated by a symbol x.
[0072]
As shown in Table 1 to Table 3, No. 6 to No. 12 and No.
18, which are invention examples, have excellent strength
and achieve the tensile strength of 500 MPa or more required
of a very thin steel sheet for a can. Furthermore,
excellent formability Is exhibited and the ductility of 10%
or more required for forming a lid and a three-piece can
body is provided.
[0073]
CA 02818682 2013-05-21
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Meanwhile, regarding No. 1, which is a comparative
example, the C content is too small, so that the tensile
strength is insufficient. Moreover, regarding No. 2, which
is a comparative example, the C content is too large, so
that the elongation is impaired through secondary cold
rolling and the elongation after fracture (in Table 2,
expressed as "total ductility") is insufficient. Regarding
No. 3, which is a comparative example, the Mn content is too
small, so that the tensile strength is insufficient.
Regarding No. 4, which is a comparative= example, the Mn
content is too large, so that the elongation is impaired
through secondary cold rolling and the elongation after
fracture is insufficient. Meanwhile, regarding No. 5, which
is a comparative example, the N content is too large, so
that the elongation is impaired through secondary cold
rolling and the elongation after fracture is insufficient.
[0074]
Regarding No. 13, which is a comparative example, the
coiling temperature is too high, so that crystal grains
become coarse and the strength is insufficient. Regarding
No. 14, which is a comparative example, the secondary cold
reduction ratio of a final stand is too small, so that the
average grain size is large, the average grain size of the
center layer is large, and the strength is insufficient.
Regarding No. 15, which is a comparative example, the
CA 02818682 2013-05-21
=
- 35 -
secondary cold reduction ratio is too large, so that the
elongation is impaired through secondary cold rolling and
the elongation after fracture is insufficient. Regarding No.
16 and No. 17, which are comparative examples, the ammonia
gas concentration in the annealing atmosphere is too high,
so that the elongation is impaired because of the surface
layer becoming hard and the elongation after fracture is
insufficient.
Industrial Applicability
[0075]
According to the present invention, a steel sheet for a
can having high strength and high formability in combination,
that is, a tensile strength of 500 MPa or more and an
elongation after fracture of 10% or more, can be obtained.
As a result, the formability of the steel sheet is improved
and, thereby, cracking does not occur even during rivet
forming of the EOE and during flange forming of the three-
piece can. Consequently, can production from the DR
material having a small sheet thickness becomes possible, so
that it is possible to contribute to development of the
industry to a great extent, for example, significant
thickness reduction of the steel sheet for a can is achieved.
