Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
HIGH TENSILE COLD-ROLLED STEEL SHEET AND METHOD FOR
MANUFACTURING SAME
TECHNICAL FIELD
The present invention relates to a high tensile cold-rolled
steel sheet having 590 MPa or higher tensile strength suitable
for the reinforcing members of pillar and dashboard of automobile,
and the like, specifically to a high tensile cold-rolled steel
sheet having a good strength-elongation balance, and showing
excellent crashworthiness at about 10 s-1 of strain rate, and to
a method for manufacturing thereof.
BACKGROUND ART
Conventional high tensile cold-rolled steel sheets having
590 MPa or higher tensile strength (TS) were limited in their
use places in the car body because of their poor press-forming
property.
For the car body, weight reduction or safety assurance
relating to the regulation of gas emissions in view of pollution
control has become a recent critical issue. To this point, there
has appeared an encouraging movement to adopt the high tensile
cold-rolled steel sheets as reinforcing members of pillar and
dashboard, and the like. The movement strongly requests to
provide the high tensile cold-rolled steel sheets with higher
press-forming property and crashworthiness than ever.
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In the related art of high tensile cold-rolled steel sheets
for automobile, having excellent press-forming property or
excellent crashworthiness, JP-A-61-217529, (the term "JP-A"
referred to herein signifies the "Japanese Patent Laid-Open
Publication") , for example, discloses a high tensile cold-rolled
steel sheet having significantly improved elongation by adopting
a microstructure containing 10% or more of retained austenite.
This high tensile cold-rolled steel sheet, however, is not studied
in terms of crashworthiness.
JP-A-11-61327 discloses a high tensile cold-rolled steel
sheet having a microstructure which is controlled to have 3 to
30% of area percentage of martensite and 5}Im or smaller average
regionsize of martensite, andhaving0.13orlarger work-hardening
exponent (n value), 75% or smaller yield ratio, 18000 MPa=% or
larger strength-elongation balance, and 1.2 or larger
hole-expansion ratio. The crashworthiness of the high tensile
cold-rolled steel sheet is evaluated by the n value.
The n value observed in the disclosed patent, however, is
determined by a static tensile test (the strain rate per JIS is
approximately in a range from 10-3 to 10-2 s-1) . Since a car-crash
generates 10 to 103 s-1 of strain rate in a reinforcing member,
the n value derived from the static tensile test cannot fully
evaluate the crashworthiness. To this point, the high tensile
cold-rolled steel sheet was re-evaluated taking into account the
strain rate on crashing, which is described later, and there was
confirmed that satisfactory crashworthiness cannot be attained.
Japanese Patent No. 3253880 discloses a method for
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manufacturing high tensile cold-rolled steel sheet having a
microstructure structured by ferrite and martensite, and having
excellent press-forming property and crashworthiness. The
crashworthiness of the high tensile cold-rolled steel sheet is
evaluated by the absorbed energy at 2000 s-1 of strain rate. The
absorbed energy which is determined by that strain rate level
is the energy necessary to absorb actually the energy on car-crash
by the deformation of the reinforcing member.
JP-A-10-147838 discloses a high tensile cold-rolled steel
sheet which improves the crashworthiness by controlling the area
percentage of martensite and the ratio of the hardness of martensite
to the hardness of ferrite. The hardness of martensite and of
ferrite is determined by a Vickers hardness gauge. However, as
described in Table 4 on page 189 of "Proceedings of the
International Workshop on the Innovative Structural Materials
for Infrastructure in 21st Century" [T. Ohmura et al.;"ULTRA-STEEL
2000 ", National Research Institute for Metals(2000)],thecorrect
hardness of martensite cannot be evaluated by Vickers hardness
gauge because the hardness of martensite has a dependency on the
size of indentation. According to an investigation given by the
inventors of the present invention, no correlation was found
between the crashworthiness and the Vickers hardness. The
disclosed patent evaluates the crashworthiness by the absorbed
energy at 800 s-1 of strain rate.
As of the reinforcingmembers, amember for energy absorption
is largely deformed within a very short time on crashing, and
the strain rate at that moment reaches to levels from 102 to 103
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s-1 . Accordingly, in the related art, the crashworthiness of high
tensile cold-rolled steel sheets was evaluated by the absorbed
energy and static-dynamic ratio at 102 to 103 s-1 of strain rate,
as described in Japanese Patent No. 3253880 and JP-A-10-147838.
The term "static-dynamic ratio" referred to herein is the
ratio of the strength determined by a dynamic tensile test at
strain rates from 102 to 103 s-1 to the strength determined by
a static tensile test at strain rates from 10-3 to 10-2 s-1. Larger
ratio means larger strength and larger absorbed energy on crash.
To improve the crashworthiness of car body, it is also
important to protect cabin without deforming the parts to secure
life-space of occupants. For the reinforcing members used to
those positions, about 10 s-1 level of absorbed energy becomes
important because the reinforcing members at those positions give
smaller deformation than that of the reinforcing members for simply
absorbing impact energy, thus giving smaller strain rate even
within the same crashing time.
Nevertheless, the related art studied very little the means
to improve the absorbed energy at strain rates of about 10 s-1.
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide a high
tensile cold-rolled steel sheet having a good strength-elongation
balance (TS*EL) and attaining excellent crashworthiness at about
s-1 of strain rate, and to provide a method for manufacturing
thereof.
The characteristics targeted in the present invention are
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the following.
(1) Tensile strength: TS ? 590 MPa
(2) Strength-elongation balance: TS*El - 16000 MPa=%
(3) Crashworthiness: at 10 s-1 of strain rate,
(a) Absorbed energy up to 10% strain: 59 MJ=m3 or larger
(b) Absorbed energy up to 10% strain per 1 MPa of tensile
strength: 0.100 MJ=m3/MPa or larger
The above object is attained by a high tensile cold-rolled
steel sheet: consisting essentially of 0.04 to 0.13% C, 0.3 to
1.2 % Si, 1. 0 to 3. 5% Mn, 0. 04 % or less P, 0. 01 % or less S, 0. 07 %
or less Al, by mass, and balance of Fe and inevitable impurities;
having a microstructure containing 50% or larger area percentage
of ferrite and 10% or larger area percentage of martensite, and
having 0.85 to 1.5 of ratio of intervals of the martensite in
the rolling direction to those in the sheet thickness direction;
and having 8 GPa or larger nano strength of the martensite.
The high tensile cold-rolled steel sheet can be manufactured
by a method having the steps of: hot-rolling a steel slab having
the above composition, into a steel sheet, followed by coiling
the steel sheet at coiling temperatures ranging from 450 C to
650 C; cold-rolling the coiled steel sheet at cold-rolling
reductions ranging from 30 to 70%; annealing the cold-rolled steel
sheet by heating to a temperature region of [the coiling temperature
+ the cold-rolling reduction percentage x 4.5] - [the coiling
temperature + the cold-rolling reduction percentage x 5.5] ( C)
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and cooling the annealed steel sheet to temperatures of 340 C
or below at average cooling rates of 10 C/s or more.
In a broad aspect, moreover, the present invention relates
to a high tensile cold-rolled steel sheet: consisting
essentially of 0.04 to 0. 13% C, 0.3 to 1.2% Si, 1.0 to 3.5% Mn,
0.04% or less P, 0.01% or less S, 0.07% or less Al, by mass,
optionally further containing at least one element selected
from the group consisting of 0.5% or less Cr, 0.3% or less Mo,
0.5% or less Ni, 0.002% or less B, by mass, 0.05% or less Ti
and 0.05% or less Nb, by mass and balance of Fe and inevitable
impurities; having a microstructure containing 50% or larger
area percentage of ferrite and 10% or larger area percentage
of martensite, and having 0.85 to 1.5 of ratio of intervals of
the martensite in the rolling direction to those in the sheet
thickness direction; and having 8 GPa or larger nano strength
of the martensite.
In another broad aspect, the present invention relates to
a method for manufacturing high tensile cold-rolled steel
sheet, comprising the steps of: hot-rolling a steel slab
consisting essentially of 0.04 to 0. 13% C, 0.3 to 1. 2% Si, 1.0
to 3.5% Mn, 0.04% or less P, 0.01% or less S, 0. 07 0 or less Al,
by mass optionally further containing at least one element
selected from the group consisting of 0.5% of less Cr, 0.3% or
less Mo, 0.5% or less Ni, 0.002% or less B, 0.05% or less Ti,
0. 05% Nb, by mass, and balance of Fe and inevitable impurities,
into a steel sheet, followed by coiling at coiling temperatures
ranging from 450 C to 650 C; cold-rolling the coiled steel
sheet at cold-rolling reductions ranging from 30 to 70%;
annealing the cold-rolled steel sheet by heating to a
temperature range of [the coiling temperature + the
cold-rolling reduction percentage x 4.5] - [the coiling
temperature + the cold-rolling reduction percentage x 5.5] ( C)
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; and cooling the annealed steel sheet to temperatures of 340 C
or below at average cooling rates of 10 Cls or higher thereby
manufacturing a high tensile cold-rolled steel sheet having a
microstructure containing 50% or larger area percentage of
ferrite and 10% or larger area percentage of martensite, and
having 0.85 to 1.5 of ratio of intervals of the martensite in
the rolling direction to those in the sheet thickness
direction; and having 8 GPa or larger nano strength of the
martensite.
BRIEF DESCRIPTION OF THE DRAWING
Figure 1 is a sketch illustrating the method for
determining the method of intervals of
martensite in the rolling direction to that in the sheet
thickness direction.
EMEODIMENTS FOR CARRYING OUT THE INVENTION
Although precise measurement of stress-strain relation at
strain rates around 10 S-1 was very difficult, a recently
developed sensing block type impac=t tensile tester has allowed
the precise measurement thereof.
The inventors of the present invention applied the sensing
block type impact tensile tester to investigate the absorbed
energy of high tensile cold-rolled steel sheet at strain rates
around 10 S-1, and derived the following findings.
1) To increase the absorbed energy, it is important to
control the microstructure so as the area percentage of ferrite
to become 50% or larger, the area percentage of martensite to
become 10% or larger, and the ratio of intervals of the
martensite in the rolling direction to that in the sheet
thickness direction to become a range from 0.85 to 1.5, and to
bring the nano hardness of the martensite to 8 GPa or larger.
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2) To attain that microstructure, it is important to
adjust the balance of ingredients mainly of C, Mn, and Si, and
to control appropriately the coiling temperature, the
cold-rolling reduction,
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the annealing temperature, and the cooling rate after annealing.
In particular, the strength-elongation balance and the
crashworthiness are improved by setting higher annealing
temperature when the coiling temperature and the cold-rolling
reduction are at high level, thereby forming the martensite while
minimizing the formation of a banded structure.
3) With the actions of 1) and 2), the high tensile
cold-rolled steel sheet attains higher absorbed energy than that
of conventional high tensile cold-rolled steel sheet having the
same tensile strength therewith.
The present invention has been perfected based on these
findings. The detail of the present invention is described in
the following.
1. Ingredients
C:
The C content is required to be 0.04% by mass or more to
control the tensile strength appropriately and to assure the area
percentage of martensite to 10% or larger. If, however, the C
content exceeds 0.13% by mass, the weldability significantly
deteriorates. Accordingly, the C content is specified to a range
from 0.04 to 0.13% by mass, and preferably from 0.07 to 0.12%
by mass.
Si:
Silicon is an important element to control the dispersed
state of martensite and to control the nano hardness of the
martensite. To prevent the softening of the martensite during
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cooling after annealing, the Si content is required to be 0.3%
by mass or more. If, however, the Si content exceeds 1.2% by
mass, the effect saturates, and the chemical conversion treatment
performance significantly deteriorates. Consequently, the Si
content is specified to a range from 0.3 to 1.2% by mass, and
preferably from 0.4 to 0.7% by mass.
Mn:
The Mn content is required to be 1.0% by mass or more to
assure 590 MPa or higher tensile strength. Manganese is extremely
effective to increase the nano hardness of martensite. If,
however, the Mn content exceeds 3.5% by mass, the strength
significantly increases, and the elongation largely decreases.
Therefore, the Mn content is specified to a range from 1.0 to
3.5% by mass, and preferably from 2.3 to 2.8% by mass.
P:
Phosphorus segregates in prior-austenite grain boundary
to deteriorate thelow temperature toughness, and also segregates
in steel to increase the anisotropy of steel sheet, thus
deteriorating the workability. Accordingly, the P content is
specified to 0. 04 % by mass or less, and preferably 0. 02 % by mass
or less. Smaller P content is more preferable.
S:
When S segregates in prior-austenite grain boundary or when
large amount of S precipitates as MnS , the lowtemperature toughness
decreases, and hydrogen crack tends to occur. Consequently, the
S content is specified to 0.01% by mass or less, and preferably
0.006% by mass or less. Smaller S content is more preferable.
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Al:
Aluminum is added as an effective element to deoxidizing
steel to improve the cleanliness thereof. To attain the effect,
the Al content is preferably adjusted to 0.001% by mass or more.
If, however, the Al content exceeds 0.07 s by mass, a large amount
of inclusions appears to cause flaws on the cold-rolled steel
sheet. Therefore, the Al content is specified to 0.07% by mass
or less, and preferably 0.05% by mass or less.
Balance includes Fe and inevitable impurities. The
inevitable impurities are N, 0, Cu, and the like. Since N enhances
aging and deteriorates elongation properties, the N content is
preferably limited to 0.005% by mass or less.
Other than the above basic elements, addition of at least
one element selected from the group consisting of 0.5% or less
Cr, 0.3% or less Mo, 0.5% or less Ni, and 0.002% or less B, by
mass is effective to improve the quenchability and to control
the amount of martensite.
Cr:
Chromium is preferably added by an amount of 0. 02% by mass
or more to improve the quenchability and to control the amount
of martensite. However, the Cr content exceeding 0.5% by mass
deteriorates the performance of electrodeposition coating which
is given to the press-formed parts. Accordingly, the Cr content
is specified to 0. 5% by mass or less, and preferably 0. 2% by mass
or less.
Mo:
Molybdenum is preferably added by an amount of 0.05% by
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mass or more to improve the quenchability and to control the amount
of martensite. If, however, the Mo content exceeds 0.3% by mass,
the cold-rolling performance deteriorates. Consequently, the
Mo content is specified to 0.3% by mass or less, and preferably
0.2% by mass or less.
Ni:
Nickel is preferably added by an amount of 0.05% by mass
or more to improve the quenchability and to control the amount
of martensite. If, however, the Ni content exceeds 0. 5% by mass,
the cold-rolling performance deteriorates. Consequently, the
Ni content is specified to 0.5% by mass or less, and preferably
0.3% by mass or less.
B:
Boron is preferably added by the amount of 0.0005% by mass
or more to improve the quenchability and to control the amount
of martensite. If, however, the B content exceeds 0 . 002 o by mass ,
the cold-rolling performance deteriorates. Consequently, the
B content is specified to 0. 002% by mass or less, and preferably
0.001% by mass or less.
Other than the previously described basic ingredients, or
other than the basic ingredients with the addition of above elements,
the addition of at least one element selected from the group
consisting of 0.05% or less Ti and 0.05% or less Nb, by mass,
is more effective in improving the quenchability, refining the
ferrite, and controlling the dispersion of martensite.
Ti:
Titanium is preferably added by an amount of 0. 005% by mass
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or more to refine the ferrite grains and thus to control the
dispersion of martensite. If, however, the Ti content exceeds
0.05% by mass, the effect saturates. Therefore, the Ti content
is specified to 0.05% by mass or less, and preferably from 0.005
to 0.02% by mass or less.
Nb:
With the similar reason to that of Ti, the Nb content is
specified to 0.05% by mass or less, and preferably from 0.005
to 0.02% by mass.
2. Structure
2-1. Area percentage of ferrite
To attain16000MPa=%orlargerstrength-elongation balance
(TS*El) , the area percentage of ferrite is required to be adjusted
to 50% or larger. If the area percentage of the ferrite is smaller
than 50%, the amount of hard phase other than the ferrite becomes
large, which results in excess strength to deteriorate the
strength-elongation balance. At strain rates around 10 s-1, since
the increase in the stress during deformation of ferrite is large,
if the area percentage of ferrite is small, the absorbed energy
cannot be increased. Accordingly, the area percentage of ferrite
is preferably in a range from 60 to 80%.
2-2. Area percentage of martensite
To attain16000MPa=%orlargerstrength-elongation balance
(TS*El) and to improve the crashworthiness, the area percentage
of martensite is required to be adjusted to 10% or more. If the
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area percentage of martensite is smaller than 10%, satisfactory
crashworthiness cannot be attained. The area percentage of
martensite is preferably in a range from 20 to 40%.
Other than the ferrite and the martensite, the presence
of austenite, bainite, cementite, pearlite, and the like is
acceptable. These additional phases, however,are preferably
less as far as possible, and 10% or smaller area percentage thereof
is preferred. In particular, since the austenite deteriorates
the crashworthiness, the area percentage of austenite is
preferably adjusted to smaller than 3%.
The determination of area percentage of ferrite, martensite,
and other phases was conducted by: mirror-polishing the
sheet-thickness cross section in the rolling direction of the
steel sheet; etching the polished surface using a 1.5% nital;
observing the etched surfaceusing a scanning electron microscope
(SEM) at a position of 1/4 sheet thickness to prepare photographs
(at xl000 magnification); and then processing the photographs
by an image-analyzer.
2-3. Ratio of intervals of martensite
To attain16000MPa= sorlargerstrength-elongation balance
(TS*E1) , and to attain 59 MJ=m-3 or higher absorbed energy up to
10% strain at 10 s-1 or larger strain rate, and 0.100 MJ=m 3/MPa
or higher absorbed energy up to 10% strain per 1 MPa of tensile
strength, the ratio of intervals of the martensite in the rolling
direction to that in the sheet thickness direction, (the ratio
of intervals of martensite) , is required to be adjusted to a range
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from 0. 85 to 1.5. If the ratio becomes smaller than 0. 85 or larger
than 1.5, sufficient elongation and crashworthiness cannot be
attained.
Since martensite is harder than ferrite, and thus becomes
a hindrance to the migration of dislocation (strain), the
dislocation preferentially moves through a region free from
martensite. As a result, when the ratio of intervals of martensite
exceeds 1. 5, that is, when the intervals of phases in the rolling
direction widens larger than the intervals of phases in the sheet
thickness direction, or when the ratio of intervals of martensite
becomes smaller than 0. 85, that is, when the intervals of phases
in the sheet thickness direction becomes wider than those in the
rolling direction, the dislocation moves through a region of wide
intervals of phases, or through a region without the martensite.
As a result, sufficient elongation and crashworthiness cannot
be attained.
To the contrary, when the ratio of intervals of martensite
is between 0.85 and 1.5, and is close to 1, that is, when there
is not much difference between the intervals of phases in the
sheet thickness direction and those in the rolling direction,
the migration of dislocation is suppressed by the martensite,
which increases the amount of accumulated dislocation to increase
the deformation stress, thereby improving the crashworthiness.
In addition, the elongation also increases because the
distribution of martensite becomes relatively uniform.
The ratio of intervals of martensite is preferably in a
range from 1.0 to 1.3.
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According to the cold-rolled steel sheet of the present
invention, the ratio of intervals of martensite in the sheet width
direction to those in the sheet thickness direction tends to become
close to 1 compared with the ratio of intervals of the phases
in the rolling direction to those in the sheet thickness direction.
According to the present invention, theref ore, the direction which
maximizes the intervals of martensite is represented by the rolling
direction, and the degree of dispersion of martensite is evaluated
by the ratio of intervals of phases in the rolling direction to
those in the sheet thickness direction.
The ratio of intervals of martensite was determined as
follows.
Steel sheet cross section in the rolling direction was
observed by SEM. On thus prepared photograph of the section,
taken at x1000 magnification, five lines having 50 ~m in width
were drawn at spacing of 20 um or more each in the rolling direction
and in the sheet thickness direction. The intervals of martensite
existing on each of the lines were measured, and the average
intervals thereof in each of the rolling direction and the sheet
thickness direction were derived, and then the ratio of the
respective average intervals was adopted as the ratio of intervals
of phases.
The procedure to determine the ratio of intervals of
martensite is described below referring to Fig. 1, where a single
line is drawn in each of the rolling direction and the sheet
thickness direction.
The average intervals of martensite in the rolling direction
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are (al + a2 + a3 + a4 + a5) /5, while those in the sheet thickness
direction are (bl + b2 + b3) /3.
Therefore, the ratio of intervals of martensite is expressed
by
{ (al + a2 + a3 + a4 + a5) /5 } / { (bl + b2 + b3) /3 } .
3. Nano hardness of martensite
To attain 59 MJ=m-3/MPa or higher absorbed energy up to 10%
strain, and to attain 0.100 MJ=m 3/MPa or higher absorbed energy
up to 10% strain per 1 MPa of tensile strength, at 10 s-1 of strain
rate, the nano hardness of martensite is further requested to
be adjusted to 8 GPa or more.
If the nano hardness is smaller than 8 GPa, the
strength-elongation balance and thecrashworthinessdeteriorate.
A presumable reason of the deterioration is that, when the nano
hardness of martensite is small and when the deformation stress
of martensite is small, the effect of the martensite to suppress
the migration of dislocation becomes weak. Larger nano hardness
of martensite is more preferable, and 10 GPa or larger nano hardness
thereof is preferable.
The nano hardness of martensite is the hardness determined
by the following procedure.
Surface of a steel sheet is ground to a position of 1/4
sheet thickness, and the surface is treated by electropolishing
to remove the grinding strain. The hardness of martensite on
the polished surface is determined at 15 points using TRIBOSCOPE
(Hysitron, Inc.), and the average value of the 15 point values
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is adopted as the nano hardness. The measurement was given on
almost equal indentation sizes. That is, the determination of
hardness was given by adjusting the load so as the contact depth
which is proportional to the size of indentation to become 50 20
nm. One side of the indentation was about 350 100 nm.
4. Manufacturing method
After preparing a molten steel adjusted to above composition
by a known method such as the one applying converter, a steel
slab was prepared by casting the molten steel by a known method
such as continuous casting process. Then, the steel slab was
heated, followed by hot-rolling by a known method to obtain a
steel sheet.
4-1. Coiling temperature
The hot-rolled steel sheet is required to be coiled at coiling
temperatures ranging from 450 C to 650 C. If the coiling
temperature is below 450 C, the strength of steel sheet increases
to increase the possibility of fracture thereof during
cold-rolling. If the coiling temperature exceeds 650 C, the
banded structure significantly develops and remains even after
cold-rolling and annealing, which fails to control the ratio of
intervals of martensite within a desired range. The coiling
temperature is preferably in a range from 500 C to 650 C.
4-2. Cold-rolling reduction
The coiled steel sheet is required to be cold-rolled at
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cold-rolling reductions ranging from 30 to 70%. If the
cold-rolling reduction is smaller than 30%, the structure becomes
coarse, and the target ratio of intervals of martensite becomes
smaller than 0.85, thereby deteriorating both the elongation and
the crashworthiness. If the cold-rolling reduction exceeds 70%,
banded structure is formed after annealing, and the ratio of
intervals of martensite exceeds 1.5.
4-3. Heating temperature during annealing
Since, even within the range of the present invention, high
coiling temperature and high cold-rolling reduction likely
generate the banded structure, the annealing needs to be given
at elevated temperatures to avoid the formation of band structure.
To do this, the heating temperature during annealing is required
to be varied depending on the coiling temperature and the
cold-rolling reduction, or to be required to enter a temperature
region of [the coiling temperature + the cold-rolling reduction
percentage x 4.5] - [the coiling temperature + the cold-rolling
reduction percentage x 5.5] ( C). If the heating temperature
is below [the coiling temperature + the cold-rolling reduction
percentage x 4.5], the banded structure cannot be diminished,
the desired ratio of intervals of martensite cannot be attained,
and further the diffusion of substitution elements such as Si
and Mn becomes insufficient, thereby failing in attaining 8 GPa
or larger nano hardness of martensite. If the heating temperature
exceeds [the coiling temperature + the cold-rolling reduction
percentage x 5. 5] ( C) , the austenite diffuses nonuniformly during
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heating, which fails to attain the desired ratio of intervals
of martensite. Furthermore, the nano hardness of martensite
cannot be increased to 8 GPa or larger, thus deteriorating the
elongation and the crashworthiness presumably because the
austenite become coarse and the martensitic block size after
annealing becomes coarse.
To bring the ratio of intervals of martensite to further
preferable range frorn 1. 0 to 1. 3, it is preferred to conduct heating
within an austenite single phase region above the Ac3
transformation point, while not exceeding the above upper limit
temperature. Particularly when the cold-rolling reduction is
60% or larger, heating is preferably done in the austenite single
phase region.
The holding time during heating is preferably 30 seconds
or more because less than 30 seconds of heating may formmartensite
at 10% or larger area percentages after annealing and may raise
difficulty in attaining stable characteristics over the whole
length of the coil. If, however, the holding time exceeds 60
seconds, the effect saturates, and the manufacturing cost
increases. Therefore, the holding time is preferably not more
than 60 seconds.
4-4. Cooling condition after annealing
The annealed steel sheet is required to be cooled to 340 C
or below at cooling rates of 10 C/sec or higher. If the cooling
rate is lower than 10 C/sec, or if the cooling-stop temperature
exceeds 340 C, the desired nano hardness of martensite cannot
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be attained. The cooling rate referred to herein is the average
cooling rate between the lower limit temperature of the above
heating temperatures, or [the coiling temperature + the
cold-rolling reduction percentage x4.5] ( C),and the temperature
to cool at cooling rates of 10 C/sec or higher.
If the cooling rate exceeds 50 C/sec, the cooling likely
becomes nonuniform, and the desired characteristics in the width
direction of the steel sheet may not be attained. Accordingly,
the cooling rate is preferably adjusted to 50 C/sec or smaller.
The temperature for cooling at that cooling rate is
preferably adjusted to 300 C or below, and 270 C or below is more
preferable.
The treatment after the cooling at that cooling rate is
not specifically limited. For example, cooling to room
temperature may be given by a known method such as air-cooling
(allowing standing) and slow-cooling. The reheating after the
cooling, however, should be avoided because the reheating tempers
to soften the martensite.
As described above, since the annealed steel sheet is
required to be rapidly cooled at cooling rates of 10 C/sec or
higher, the annealing is advantageously conducted in a continuous
annealing furnace. The 30 seconds or longer holding time in the
continuous annealing process can be attained by selecting the
annealing temperature (ultimate highest temperature in the
continuous annealing) to a temperature in the above heating
temperature region, and by holding the steel within the temperature
region for 30 seconds or more. For example, the soaking time
CA 02522607 2005-10-17
- 20 -
(or called the "annealing time") at the annealing temperature
may be selected to 30 seconds or more, or, after reaching the
annealing temperature, the steel may be slowly cooled to the lower
limit of the above heating temperature region, while adjusting
the retention time in the heating temperature region to 30 seconds
or more.
Example 1
Steel Nos. A to ZZ having the respective compositions given
in Table 1-1 and Table 1-2 were ingoted by a converter, and then
they were treated by continuous casting to prepare the respective
slabs. These slabs were heated to temperatures ranging from
1100 C to 1250 C, followed by hot-rolling, thus prepared the
respective steel sheets having thicknesses given in Table 2-1
and Table 2-2. These steel sheets were coiled at the respective
coiling temperatures given in Table 2-1 and Table 2-2. Then,
cold-rolling, continuous annealing, and controlled cooling were
given to these steel sheets under the conditions given in Table
2-1 and Table 2-2, thus obtained the respective high tensile
cold-rolled steel sheet Nos. 1 to 39.
The Ac3 transformation point given in Table 1-1 and Table
1-2 was determined by preparing samplesfrom the respective sheet
bars after hot-rough-rolling, using Thermec Master Z of Fuji
Electronics Industrial Co., Ltd.
Thus prepared respective high tensile cold-rolled steel
sheets were subjected to structural observation, ordinary static
tensile test, sensing block type high speed tensile test at 10
CA 02522607 2005-10-17
- 21 -
s-1 of strain rate, and nano hardness test.
The structural observation and the nano hardness test were
given by the above-described methods, thereby determining the
area percentage of ferrite and martensite, the ratio of intervals
of martensite, and the nano hardness of martensite.
The ordinary static tensile test and the sensing block type
high speed tensile test at 10 s-1 of strain rate were given by
the following methods.
i) Static tensile test: With a JIS No.5 specimen defining
the direction lateral to the rolling direction as the longitudinal
direction, the tensile strength TS and the elongation El were
determined in accordance with JIS Z2241.
ii) Sensing block type high speed tensile test: The tensile
test was given in the lateral direction to the rolling direction
at 10 s-1 of strain rate, using a Sensing block type impact tensile
tester (TS-2000, Saginomiya Seisakusho, Inc.) The absorbed
energy up to 10% strain and the absorbed energy up to 10% strain
per 1 MPa of tensile strength were determined.
The results are given in Table 3-1 and Table 3-2.
The high tensile cold-rolled steel sheet Nos. 1, 3, 5, 7,
8, 10, 12, 14 to 19, 21 to 23, 29 to 34, and 37 to 39, which were
the Examples of the present invention, showed 590 MPa or higher
tensile strength and 16000 MPa=o or higher excellent
strength-elongation balance, further gave 59 MJ=m3 or higher
absorbed energy up to 10% strain at 10 s-1 of strain rate, and
0.100 MJ=m 3/MPa or higher absorbed energy up to 10% strain per
1 MPa of tensile strength, which proves their excellent
CA 02522607 2005-10-17
- 22 -
crashworthiness.
CA 02522607 2005-10-17
- 23 -
N M Lf) N M m N O O co m
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CA 02522607 2005-10-17
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CA 02522607 2005-10-17
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CA 02522607 2005-10-17
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CA 02522607 2005-10-17
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