U.S. patent application number 10/553898 was filed with the patent office on 2006-10-19 for high tensile strength cold-rolled steel sheet and method for production thereof.
Invention is credited to Naoki Nishiyama, Tetsuo Shimizu, Shusaku Takagi.
Application Number | 20060231176 10/553898 |
Document ID | / |
Family ID | 34220740 |
Filed Date | 2006-10-19 |
United States Patent
Application |
20060231176 |
Kind Code |
A1 |
Takagi; Shusaku ; et
al. |
October 19, 2006 |
High tensile strength cold-rolled steel sheet and method for
production thereof
Abstract
The high tensile cold-rolled steel sheet consists 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, has a microstructure containing 50% or
larger area percentage of ferrite and 10% or larger area percentage
of martensite, has 0.85 to 1.5 of ratio of intervals of the
martensite in the rolling direction to those in the sheet thickness
direction, and has 8 GPa or larger nano strength of the martensite.
The high tensile cold-rolled steel sheet has a good
strength-elongation balance, and shows excellent crashworthiness at
about 10 s.sup.-1 of strain rate. Therefore, the high tensile
cold-rolled steel sheet is suitable for reinforcing members for
pillar and dashboard of automobile.
Inventors: |
Takagi; Shusaku; (Tokyo,
JP) ; Shimizu; Tetsuo; (Tokyo, JP) ;
Nishiyama; Naoki; (Tokyo, JP) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN & CHICK, PC
220 Fifth Avenue
16TH Floor
NEW YORK
NY
10001-7708
US
|
Family ID: |
34220740 |
Appl. No.: |
10/553898 |
Filed: |
August 18, 2004 |
PCT Filed: |
August 18, 2004 |
PCT NO: |
PCT/JP04/12160 |
371 Date: |
October 21, 2005 |
Current U.S.
Class: |
148/603 |
Current CPC
Class: |
C21D 2211/005 20130101;
C22C 38/04 20130101; C21D 8/0436 20130101; C21D 8/0473 20130101;
C21D 2211/008 20130101; C21D 8/0236 20130101; C22C 38/02
20130101 |
Class at
Publication: |
148/603 |
International
Class: |
C21D 8/02 20060101
C21D008/02 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 26, 2003 |
JP |
2003301473 |
Jul 15, 2004 |
JP |
2004208834 |
Claims
1. 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.
2. The high tensile cold-rolled steel sheet as in claim 1 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, and 0.002% or
less B, by mass.
3. The high tensile cold-rolled steel sheet as in claim 1 further
containing at least one element selected from the group consisting
of 0.05% or less Ti and 0.05% or less Nb, by mass.
4. The high tensile cold-rolled steel sheet as in claim 2 further
containing at least one element selected from the group consisting
of 0.05% or less Ti and 0.05% or less Nb, by mass.
5. 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% or less Al, by mass, and
balance of Fe and inevitable impurities, into a steel sheet,
followed by coiling at coiling temperatures ranging from
450.degree. C. to 650.degree. 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.times.4.5]-[the coiling temperature+the cold-rolling
reduction percentage.times.5.5] (.degree. C.); and cooling the
annealed steel sheet to temperatures of 340.degree. C. or below at
average cooling rates of 10.degree. C./s or higher.
6. The method for manufacturing high tensile cold-rolled steel
sheet as in claim 5, wherein the steel slab further contains 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.
7. The method for manufacturing high tensile cold-rolled steel
sheet as in claim 5, wherein the steel slab further contains at
least one element selected from the group consisting of 0.05% or
less Ti and 0.05% or less Nb, by mass.
8. The method for manufacturing high tensile cold-rolled steel
sheet as in claim 6 wherein the steel slab further contains at
least one element selected from the group consisting of 0.05% or
less Ti and 0.05% or less Nb, by mass.
Description
TECHNICAL FIELD
[0001] 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.sup.-1 of strain rate, and to a
method for manufacturing thereof.
BACKGROUND ART
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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 .mu.m or smaller average
region size of martensite, and having 0.13 or larger 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.
[0006] 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.sup.-3 to 10.sup.-2 s.sup.-1).
Since a car-crash generates 10 to 10.sup.3 s.sup.-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.
[0007] Japanese Patent No. 3253880 discloses a method for
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.sup.-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.
[0008] 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 21.sup.st Century" [T. Ohmura et al.;
"ULTRA-STEEL 2000", National Research Institute for Metals (2000)],
the correct 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.sup.-1 of strain rate.
[0009] As of the reinforcing members, a member for energy
absorption is largely deformed within a very short time on
crashing, and the strain rate at that moment reaches to levels from
10.sup.2 to 10.sup.3 s.sup.-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
10.sup.2 to 10.sup.3 s.sup.-1 of strain rate, as described in
Japanese Patent No. 3253880 and JP-A-10-147838.
[0010] The term "static-dynamic ratio" referred to herein is the
ratio of the strength determined by a dynamic tensile test at
strain rates from 10.sup.2 to 10.sup.3 s.sup.-1 to the strength
determined by a static tensile test at strain rates from 10.sup.3
to 10.sup.-2 s.sup.-1. Larger ratio means larger strength and
larger absorbed energy on crash.
[0011] 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.sup.-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.
[0012] Nevertheless, the related art studied very little the means
to improve the absorbed energy at strain rates of about 10
s.sup.-1.
DISCLOSURE OF THE INVENTION
[0013] 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 10
s.sup.-1 of strain rate, and to provide a method for manufacturing
thereof.
[0014] The characteristics targeted in the present invention are
the following.
(1) Tensile strength: TS.gtoreq.590 MPa
(2) Strength-elongation balance: TS*El.gtoreq.16000 MPa%
(3) Crashworthiness: at 10 s.sup.-1 of strain rate,
[0015] (a) Absorbed energy up to 10% strain: 59 MJm.sup.-3 or
larger
[0016] (b) Absorbed energy up to 10% strain per 1 MPa of tensile
strength: 0.100 MJm.sup.-3/MPa or larger
[0017] 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.
[0018] 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.degree. C. to
650.degree. 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.times.4.5]-[the
coiling temperature+the cold-rolling reduction
percentage.times.5.5] (.degree. C.); and cooling the annealed steel
sheet to temperatures of 340.degree. C. or below at average cooling
rates of 10.degree. C./s or more.
BRIEF DESCRIPTION OF THE DRAWING
[0019] FIG. 1 is a sketch illustrating the method for determining
the ratio of intervals of the martensite in the rolling direction
to that in the sheet thickness direction.
EMBODIMENTS FOR CARRYING OUT THE INVENTION
[0020] Although precise measurement of stress-strain relation at
strain rates around 10 s.sup.-1 was very difficult, a recently
developed sensing block type impact tensile tester has allowed the
precise measurement thereof.
[0021] 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.sup.-1, and derived the following findings.
[0022] 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.
[0023] 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,
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.
[0024] 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.
[0025] The present invention has been perfected based on these
findings. The detail of the present invention is described in the
following.
1. Ingredients
C:
[0026] 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:
[0027] 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
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:
[0028] 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:
[0029] Phosphorus segregates in prior-austenite grain boundary to
deteriorate the low 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:
[0030] When S segregates in prior-austenite grain boundary or when
large amount of S precipitates as MnS, the low temperature
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.
Al:
[0031] 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% 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.
[0032] 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.
[0033] 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:
[0034] 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:
[0035] Molybdenum 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 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:
[0036] 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:
[0037] 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% 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.
[0038] 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:
[0039] Titanium is preferably added by an amount of 0.005% by mass
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:
[0040] 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
[0041] To attain 16000 MPa% or larger strength-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.sup.-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
[0042] To attain 16000 MPa% or larger strength-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 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%.
[0043] 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%.
[0044] 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 surface using a scanning electron microscope (SEM) at a
position of 1/4 sheet thickness to prepare photographs (at
.times.1000 magnification); and then processing the photographs by
an image-analyzer.
2-3. Ratio of Intervals of Martensite
[0045] To attain 16000 MPa% or larger strength-elongation balance
(TS*El), and to attain 59 MJm.sup.-3 or higher absorbed energy up
to 10% strain at 10 s.sup.-1 or larger strain rate, and 0.100
MJm.sup.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 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.
[0046] 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.
[0047] 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.
[0048] The ratio of intervals of martensite is preferably in a
range from 1.0 to 1.3.
[0049] 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, therefore, 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.
[0050] The ratio of intervals of martensite was determined as
follows.
[0051] Steel sheet cross section in the rolling direction was
observed by SEM. On thus prepared photograph of the section, taken
at .times.1000 magnification, five lines having 50 .mu.m in width
were drawn at spacing of 20 .mu.m 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.
[0052] 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.
[0053] The average intervals of martensite in the rolling direction
are (a.sub.1+a.sub.2+a.sub.3+a.sub.4+a.sub.5)/5, while those in the
sheet thickness direction are (b.sub.1+b.sub.2+b.sub.3)/3.
[0054] Therefore, the ratio of intervals of martensite is expressed
by
{(a.sub.1+a.sub.2+a.sub.3+a.sub.4+a.sub.5)/5}/{(b.sub.1+b.sub.2+b.sub.3)/-
3}. 3. Nano Hardness of Martensite
[0055] To attain 59 MJm.sup.-3/MPa or higher absorbed energy up to
10% strain, and to attain 0.100 MJm.sup.-3/MPa or higher absorbed
energy up to 10% strain per 1 MPa of tensile strength, at 10
s.sup.-1 of strain rate, the nano hardness of martensite is further
requested to be adjusted to 8 GPa or more.
[0056] If the nano hardness is smaller than 8 GPa, the
strength-elongation balance and the crashworthiness deteriorate. 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.
[0057] The nano hardness of martensite is the hardness determined
by the following procedure.
[0058] 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 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
[0059] 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
[0060] The hot-rolled steel sheet is required to be coiled at
coiling temperatures ranging from 450.degree. C. to 650.degree. C.
If the coiling temperature is below 450.degree. C., the strength of
steel sheet increases to increase the possibility of fracture
thereof during cold-rolling. If the coiling temperature exceeds
650.degree. 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.degree. C. to 650.degree. C.
4-2. Cold-Rolling Reduction
[0061] The coiled steel sheet is required to be cold-rolled at
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
[0062] 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.times.4.5]-[the coiling temperature+the cold-rolling
reduction percentage.times.5.5] (.degree. C.). If the heating
temperature is below [the coiling temperature+the cold-rolling
reduction percentage.times.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.times.5.5] (.degree. C.), the austenite
diffuses nonuniformly during 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.
[0063] To bring the ratio of intervals of martensite to further
preferable range from 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.
[0064] The holding time during heating is preferably 30 seconds or
more because less than 30 seconds of heating may form martensite 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
[0065] The annealed steel sheet is required to be cooled to
340.degree. C. or below at cooling rates of 10.degree. C./sec or
higher. If the cooling rate is lower than 10.degree. C./sec, or if
the cooling-stop temperature exceeds 340.degree. C., the desired
nano hardness of martensite cannot 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.times.4.5] (.degree. C.), and the temperature to cool at
cooling rates of 10.degree. C./sec or higher.
[0066] If the cooling rate exceeds 50.degree. 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.degree.
C./sec or smaller.
[0067] The temperature for cooling at that cooling rate is
preferably adjusted to 300.degree. C. or below, and 270.degree. C.
or below is more preferable.
[0068] 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.
[0069] As described above, since the annealed steel sheet is
required to be rapidly cooled at cooling rates of 10.degree. 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 (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
[0070] 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.degree. C. to 1250.degree. 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.
[0071] The Ac3 transformation point given in Table 1-1 and Table
1-2 was determined by preparing samples from the respective sheet
bars after hot-rough-rolling, using Thermec Master Z of Fuji
Electronics Industrial Co., Ltd.
[0072] 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
s.sup.-1 of strain rate, and nano hardness test.
[0073] 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.
[0074] The ordinary static tensile test and the sensing block type
high speed tensile test at 10 s.sup.-1 of strain rate were given by
the following methods.
[0075] 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.
[0076] ii) Sensing block type high speed tensile test: The tensile
test was given in the lateral direction to the rolling direction at
10 s.sup.-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.
[0077] The results are given in Table 3-1 and Table 3-2.
[0078] 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% or higher excellent strength-elongation
balance, further gave 59 MJm.sup.-3 or higher absorbed energy up to
10% strain at 10 s.sup.-1 of strain rate, and 0.100 MJm.sup.-3/MPa
or higher absorbed energy up to 10% strain per 1 MPa of tensile
strength, which proves their excellent crashworthiness.
TABLE-US-00001 TABLE 1-1 Steel Chemical composition (mass %)
Ac.sub.3 No. C Si Mn P S Al N Cr Mo Ni B Ti Nb (.degree. C.) A
0.061 0.41 2.78 0.013 0.005 0.035 0.004 -- -- -- -- -- -- 878 B
0.122 0.32 2.64 0.015 0.004 0.032 0.003 -- -- -- -- -- -- 853 C
0.090 0.57 2.58 0.012 0.005 0.037 0.004 -- -- -- -- -- -- 875 D
0.082 0.68 2.01 0.010 0.003 0.024 0.004 -- -- -- -- -- -- 882 E
0.055 0.46 1.54 0.012 0.005 0.034 0.003 -- -- -- -- -- -- 883 F
0.100 0.49 2.46 0.014 0.004 0.036 0.004 -- -- -- -- -- -- 868 G
0.085 0.69 2.72 0.013 0.003 0.021 0.005 -- -- -- -- -- -- 882 H
0.115 0.42 2.35 0.009 0.002 0.027 0.004 -- -- -- -- -- -- 860 I
0.095 0.92 2.50 0.015 0.004 0.034 0.003 -- -- -- -- -- -- 889 J
0.075 0.53 2.41 0.013 0.001 0.036 0.003 0.03 -- -- -- -- -- 878 K
0.079 0.51 2.37 0.014 0.003 0.029 0.004 -- 0.1 -- -- -- -- 879 L
0.092 0.35 2.13 0.012 0.002 0.040 0.002 -- -- 0.08 -- -- -- 864 M
0.086 0.55 2.31 0.010 0.004 0.033 0.003 -- -- -- 0.0005 -- -- 875 N
0.071 0.42 2.36 0.008 0.003 0.035 0.004 -- 0.06 -- 0.0008 -- --
877
[0079] TABLE-US-00002 TABLE 1-2 Steel Chemical composition (mass %)
Ac.sub.3 No. C Si Mn P S Al N Cr Mo Ni B Ti Nb (.degree. C.) O
0.087 0.64 2.69 0.014 0.005 0.026 0.003 -- -- -- -- 0.009 -- 879 P
0.092 0.59 2.53 0.011 0.004 0.032 0.004 -- 0.05 -- -- 0.006 -- 876
Q 0.033 0.68 2.53 0.012 0.003 0.025 0.004 -- -- -- -- -- -- 902 R
0.105 0.21 2.38 0.010 0.005 0.038 0.004 -- -- -- -- -- -- 854 S
0.098 0.43 0.82 0.013 0.002 0.033 0.003 -- -- -- -- -- -- 866 T
0.075 0.54 3.8 0.013 0.002 0.033 0.004 -- -- -- -- -- -- 879 U
0.152 0.72 2.69 0.013 0.002 0.033 0.003 -- -- -- -- -- -- 863 V
0.052 0.47 1.42 0.033 0.003 0.032 0.005 -- -- -- -- -- -- 885 W
0.125 0.39 1.58 0.025 0.003 0.037 0.004 -- -- -- -- -- -- 856 X
0.075 0.51 2.39 0.029 0.005 0.025 0.005 -- -- -- -- -- -- 877 Y
0.077 0.99 1.27 0.010 0.003 0.020 0.004 -- -- -- -- -- -- 898 Z
0.072 0.55 2.56 0.018 0.003 0.036 0.003 -- -- -- -- -- 0.012 880 ZZ
0.108 0.63 2.61 0.013 0.004 0.035 0.003 -- -- -- -- 0.018 0.014 871
* Underline designates outside the range of the invention.
[0080] TABLE-US-00003 TABLE 2-1 Coiling Hot-rolled Cold-rolling
Cold-rolled [Coiling temperature + [Coiling temperature + Steel
sheet temperature sheet thickness reduction sheet thickness
cold-rolling reduction cold-rolling reduction No. Steel No.
(.degree. C.) (mm) (%) (mm) percentage .times. 4.5] (.degree. C.)
percentage .times. 5.5] (.degree. C.) 1 A 600 3.2 50 1.6 825 875 2
A 600 2.2 27 1.6 722 749 3 B 600 3.2 44 1.8 798 842 4 B 600 3.2 44
1.8 798 842 5 C 550 3.0 53 1.4 789 842 6 C 550 4.8 71 1.4 870 941 7
D 600 2.4 50 1.2 825 875 8 E 450 2.4 58 1.0 711 769 9 E 450 2.4 58
1.0 711 769 10 F 650 3.0 40 1.8 830 870 11 F 650 3.0 40 1.8 830 870
12 G 600 2.0 50 1.0 825 875 13 G 600 2.0 50 1.0 825 875 14 H 500
3.2 50 1.6 725 775 15 I 550 3.2 44 1.8 748 792 16 J 600 2.6 46 1.4
807 853 17 K 650 3.6 36 2.3 812 848 18 L 600 3.2 50 1.6 825 875 19
M 550 3.6 36 2.3 712 748 20 M 700 3.6 36 2.3 862 898 Temp. of Steel
sheet Maximum heating Holding time in heating Average forcefully
stop No. temp. (.degree. C.) temp. region (s) cooling rate
(.degree. C./s) cooling (.degree. C.) Remark 1 830 30 20 250
Example 2 730 60 20 250 Comparative example 3 810 90 25 275 Example
4 760 90 25 275 Comparative example 5 820 80 20 250 Example 6 880
80 20 250 Comparative example 7 860 60 30 220 Example 8 850 110 40
200 Example 9 800 110 40 200 Comparative example 10 840 60 15 240
Example 11 840 60 5 240 Comparative example 12 840 90 40 250
Example 13 840 90 40 350 Comparative example 14 770 90 20 230
Example 15 780 80 20 260 Example 16 820 60 10 290 Example 17 840 60
20 270 Example 18 830 75 25 240 Example 19 730 70 15 280 Example 20
880 70 15 280 Comparative example * Underline designates outside
the range of the invention.
[0081] TABLE-US-00004 TABLE 2-2 Coiling Hot-rolled Cold-rolling
Cold-rolled [Coiling temperature + [Coiling temperature + Steel
sheet temperature sheet thickness reduction sheet thickness
cold-rolling reduction cold-rolling reduction No. Steel No.
(.degree. C.) (mm) (%) (mm) percentage .times. 4.5] (.degree. C.)
percentage .times. 5.5] (.degree. C.) 21 N 550 3.6 56 1.6 802 858
22 O 500 3.2 69 1.0 811 880 23 P 600 3.2 44 1.8 798 842 24 Q 600
3.6 36 2.3 762 798 25 R 550 3.2 38 2.0 721 759 26 S 550 3.2 44 1.8
748 792 27 T 600 3.2 50 1.6 825 875 28 U 500 3.2 50 1.6 725 775 29
G 600 2.0 50 1.0 825 875 30 V 600 2.0 50 1.0 825 875 31 W 480 2.3
57 1.0 737 794 32 H 500 3.2 50 1.6 725 775 33 X 580 2.6 62 1.0 859
921 34 P 600 3.2 63 1.2 884 947 35 P 600 3.2 44 1.8 798 842 36 Y
500 2.8 64 1.0 788 852 37 Y 610 2.8 61 1.1 885 946 38 Z 520 3.2 50
1.6 745 795 39 ZZ 620 2.4 33 1.6 769 802 Temp. of Steel sheet
Maximum heating Holding time in heating Average forcefully stop No.
temp. (.degree. C.) temp. region (s) cooling rate (.degree. C./s)
cooling (.degree. C.) Remark 21 840 60 20 250 Example 22 840 90 35
210 Example 23 820 90 15 240 Example 24 780 80 15 270 Comparative
example 25 740 60 20 260 Comparative example 26 760 60 15 250
Comparative example 27 830 75 25 240 Comparative example 28 750 70
15 260 Comparative example 29 840 90 40 340 Example 30 840 90 15
250 Example 31 780 90 10 300 Example 32 730 45 15 320 Example 33
885 300 15 250 Example 34 910 360 15 300 Example 35 910 300 15 300
Comparative example 36 800 60 50 350 Comparative example 37 902 60
20 280 Example 38 770 130 30 260 Example 39 790 90 15 250 Example *
Underline designates outside the range of the invention.
[0082] TABLE-US-00005 TABLE 3-1 Area Tensile Area Area percentage
Ratio of Steel sheet strength Elongation percentage percentage and
kind of other intervals of No. Steel No. (MPa) (%) of ferrite (%)
of martensite (%) phase (%) (kind) martensite 1 A 843 19.5 70 30 0
1.36 2 A 820 18.9 75 25 0 0.71 3 B 886 18.1 60 40 0 1.50 4 B 852
17.8 70 30 0 1.59 5 C 821 21.6 65 35 0 1.26 6 C 842 18.3 55 45 0
1.57 7 D 708 24.0 80 20 0 1.13 8 E 621 27.1 80 20 0 1.21 9 E 673
23.2 70 30 0 1.57 10 F 834 21.8 70 30 0 1.29 11 F 808 19.1 70 30 0
1.29 12 G 867 20.6 60 40 0 1.15 13 G 821 18.8 60 40 0 1.15 14 H 849
21.5 65 35 0 0.95 15 I 854 19.7 75 25 0 1.02 16 J 803 23.1 80 20 0
1.20 17 K 857 20.9 75 25 0 1.05 18 L 754 21.5 85 15 0 1.33 19 M 839
21.5 65 35 0 1.10 20 M 880 17.6 55 45 0 1.59 Absorbed energy* Steel
sheet Nano hardness of TS*EI balance Absorbed energy* per TS 1 MPa
No. martensite (GPa) (MPa %) (MJ m.sup.-3) (MJ m.sup.-3 MPa.sup.-1)
Remark 1 9.4 16439 88.8 0.105 Example 2 9.3 15498 77.9 0.095
Comparative example 3 8.0 16037 88.8 0.100 Example 4 7.2 15166 78.4
0.092 Comparative example 5 10.5 17734 94.4 0.115 Example 6 10.2
15409 79.1 0.094 Comparative example 7 9.3 16992 75.8 0.107 Example
8 9.5 16829 65.8 0.106 Example 9 7.4 15614 62.6 0.093 Comparative
example 10 11.6 18181 100.1 0.120 Example 11 7.2 15433 75.1 0.093
Comparative example 12 12.7 17860 104.9 0.121 Example 13 7.5 15435
78.8 0.096 Comparative example 14 12.1 18254 100.2 0.118 Example 15
9.6 16824 90.5 0.106 Example 16 10.6 18549 95.6 0.119 Example 17
12.7 17911 99.4 0.116 Example 18 9.7 16211 80.7 0.107 Example 19
11.2 18039 96.5 0.115 Example 20 10.2 15488 83.6 0.095 Comparative
example * Underline designates outside the range of the
invention.
[0083] TABLE-US-00006 TABLE 3-2 Area Tensile Area Area percentage
Ratio of Steel sheet strength Elongation percentage percentage and
kind of other intervals of No. Steel No. (MPa) (%) of ferrite (%)
of martensite (%) phase (%) (kind) martensite 21 N 892 21.1 60 40 0
1.00 22 O 822 22.7 70 30 0 1.27 23 P 849 21.0 65 35 0 1.25 24 Q 531
29.2 95 5 0 1.72 25 R 793 18.2 75 25 0 1.58 26 S 559 27.1 85 15 0
1.73 27 T 973 14.3 60 40 0 1.62 28 U 1054 13.9 45 55 0 1.67 29 G
825 20.1 60 40 0 1.15 30 V 639 26.3 70 25 5 (bainite) 1.14 31 W 789
21.2 73 18 9 (bainite) 1.18 32 H 783 21.5 63 30 7 (bainite) 0.95 33
X 877 21.5 56 44 0 1.00 34 P 881 21.4 62 38 0 1.02 35 P 910 18.1 43
55 2 (bainite) 1.05 36 Y 622 26.2 90 10 0 1.45 37 Y 701 27.2 74 24
2 (austenite) 1.05 38 Z 825 22.2 82 18 0 1.17 39 ZZ 873 21.3 72 28
0 1.22 Absorbed energy* Steel sheet Nano hardness of TS*EI balance
Absorbed energy* per TS 1 MPa No. martensite (GPa) (MPa %) (MJ
m.sup.-3) (MJ m.sup.-3 MPa.sup.-1) Remark 21 13.4 18821 107.9 0.121
Example 22 12.1 18659 97.0 0.118 Example 23 11.8 17829 97.6 0.115
Example 24 7.6 15505 52.0 0.098 Comparative example 25 7.4 14433
75.3 0.095 Comparative example 26 7.2 15149 52.5 0.094 Comparative
example 27 7.8 13914 89.5 0.092 Comparative example 28 9.5 14651
97.0 0.092 Comparative example 29 8.9 16583 89.1 0.108 Example 30
8.5 16806 66.5 0.104 Example 31 9.3 16727 86.0 0.109 Example 32
12.1 16835 82.2 0.105 Example 33 12.2 18856 108.7 0.124 Example 34
13.3 18853 107.5 0.122 Example 35 7.3 16471 82.8 0.091 Comparative
example 36 7.1 16296 57.8 0.093 Comparative example 37 10.3 19067
85.5 0.122 Example 38 10.7 18315 92.4 0.112 Example 39 10.8 18595
98.6 0.113 Example *The value up to 10% strain at 10 s.sup.-1 of
strain rate. * Underline designates outside the range of the
invention.
* * * * *