The present invention relates to a highstrength
hot-rolled steel sheet having a maximum
tensile strength of 980 MPa or more and excellent low
temperature toughness and a manufacturing method
thereof, and in particular, relates to a hot-rolled
steel sheet including low temperature toughness for
enabling the use in a cryogenic region and a
manufacturing method thereof.
BACKGROUND ART
[ 0002] Reduction in weight of an automobile body has
been promoted by using a high-strength steel sheet in
order to suppress emission of carbon dioxide gas from
an automobile. Further, in addition to mild steel
sheets, a lot of high-strength steel sheets having a
maximum tensile strength of 980 MPa or more have been
used for the automobile body in order to ensure
safety of passengers.
[0003] Further, there is a requirement that such a
steel sheet used for a member is formed to then be
attached to an automobile as a part and then even if
the member receives a shock due to collision or the
like, the member is not easily destroyed, and further
t.here i~ also a requirement to improve also low
temperature toughness in order to ensure shock
- 1 -
resistance at a cold district in particular. This
low temperature toughness is prescribed by vTrs
(Charpy fracture appearance transition temperature)
or the like. Therefore, it is also necessary to
consider the above-described shock resistance itself
of a steel product. In addition, when the steel
sheet is increased in strength, plastic deformation
of the steel sheet becomes difficult, while a concern
for destruction becomes higher, and therefore
toughness is required as an important property.
[0004] As for a method of improving toughness in a
high-strength steel sheet, for e,x:ample, Patent
Literature 1 discloses a manufacturing method, and
there is known a method of setting a martensite phase
with an adjusted aspect ratio to a main phase.
Generally, it is known that the aspect ratio of
martensite relies on an aspect ratio of austenite
grains before transformation. That is, martensite
with a large aspect ratio means martensite
transformed from non-recrystallized austenite
(austenite extended by rolling), and martensite with
a small aspect ratio means martensite transformed
from recrystall~zed austenite.
[0005] It is necessary to increase a finish rolling
te.mperature to enable recrystallizat:ion of austenite,
and there is a tendency that a grain diameter of
austenite and furthermore a grain diameter of
martens\te become large. Gene.rally, it is known that
refining of a grain diameter creates an effect of
- 2 -
improving toughness, and therefore, when the aspect
ratio decreases, it is possible to decrease a factor
of toughness deterioration resulting from a shape,
but toughness deterioration resulting from coarsening
of crystal grains is caused, so that there is a limit
in improvement in low temperature toughness.
[0006] Patent Literature 1 refers to a method for
producing a thick steel sheet for structural member
of a large-sized industrial construction machine and
the like that includes both high strength and high
toughness by obtaining 3 to 18 of an aspect ratio of
prior austenite grains, but the steel sheet for
automobile is required to have further excellent low
temperature toughness. Further, the steel sheet
having grains with such an aspect ratio has
anisotropy of mechanical properties, to thus have
difficulty being formed into a general automobile
member, resulting in that there exists a problem that
the use is limited.
[0007] Patent Literature 2 discloses that ferrite
grains with an aspect ratio of 2 or less are set to a
main phase to thereby fabricate a high-toughness
steel sheet~ However, the main phase of thi& steel
sheet is ferrite, so that it is difficult to ensure
., .. the tensile strength of 980 MPa.•cor more.
[0008] Patent Literature 3 discloses that carbides
are made to finely precipitate in ferrite having an
average \grain diameter set to 5 to 10 fJ m, to thereby
improve strength and low temperature toughness of a
- 3 -
steel sheet. According to the method described in
Patent Literature 3, solid-solution Ti and/or the
like in steel are/is made to precipitate as carbide,
to thereby increase strength of the steel sheet.
However, in order to ensure a tensile strength of 980
MPa or more, finer precipitation and denser
dispersion are needed, and detailed setting of
cooling conditions after finish rolling is required.
Therefore, it is conceivable that the steel sheet
manufactured by this method has difficulty ensuring a
tensile strength of 980 MPa or more stably.
[0009] Patent Literature 4 discloses that the
structure of a steel sheet is set to a single phase
made of bainite phase or bainitic ferrite phase and
the amount of cementite at grain boundaries is
suppressed, to thereby improve low temperature
toughness of the steel sheet. However, the steel
sheet described in Patent Literature 4 has a tensile
strength of 604 to 764 MPa, and therefore it is
conceivably difficult to ensure a tensile strength of
980 MPa or more. Additionally, manufacture of a
thick hot-rolled steel sheet having a sheet thickness
of 8. 7 .mm or more is described, but no me•nt·ion" is
made regarding a manufacturing method of a thin hotrolled
steel sheet used for ·.an -automobile steel sheet.
[0010] Patent Literature 5 discloses that when
manufacturing a high-strength steel sheet having a
tensile,strength of 980 MPa or more, generation of a
MA (martensite-austenite mixed structure) phase to be
- 4 -
a starting point of destruction is suppressed, to
thereby improve low temperature toughness. Generally,
the mechanism in which the MA phase appears results
from the fact that C is concentrated in austenite by
some kind o£ cause. Thus, the steel type described
in Patent Literature 5 contains fixed amounts of Ti,
Nb, V, and Mo, which are carbide forming elements, to
thereby capture C to suppress concentration into
austenite, and thereby the generation of the MA phase
is suppressed.
However, these carbide forming elements are
expensive and are required to be added in large
amounts, so that the steel sheet described in Patent
Literature 5 is poor in economic efficiency.
Additionally, in Patent Literature 5, low temperature
toughness of a welding joint portion is mentioned,
but no mention is made regarding low temperature
toughness of a parent metal, which is important for
the steel sheet for an automobile body.
[0011] As above, a high-strength steel sheet that
exceeds 980 MPa has difficulty including excellent
low temperature toughness simultaneously.
CITATlDN LlST
PATENT LITERATURE
[0012] Patent Literature 1: Japanese Laid-open
Patent Publication No. 2011-52321
Patent Literature 2: Japanese Laid-open Patent
Publication No. 2008-231474
'
Patent Literature 3: Japanese Laid-open Patent
- 5 -
Publication No. 2011-17044
Patent Literature 4: Japanese Laid-open Patent
Publication No. 2013-014844
Patent Literature 5: Japanese Laid-open Patent
Publication No. 2012-077340
Patent Literature 6: Japanese Laid-open Patent
Publication No. 2000-109951
Patent Literature 7: Japanese Laid-open Patent
Publication No. 2010-138421
Patent Literature 8: Japanese Laid-open Patent
Publication No. 2009-052106
Patent Literature 9: Japanese Laid-open Patent
Publication No. 2008-266695
Patent Literature 10: Japanese Laid-open Patent
Publication No. 2006-161139
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0013] The present invention has been made in
consideration of the above-described problems, and an
object thereof is to provide a hot-rolled steel sheet
having both a maximum tensile strength of 980 MPa or
more and excellent low temperature toughness and a
mahufacturing method thereof.
SOLUTION TO PROBLEM
[0014] The present inventors were successful in
manufacturing a steel sheet having a maximum tensile
strength of 980 MPa or more and excellent low
temperat\ure tou_ghness by optimizing components and
manufacturing conditions of a hot-rolled steel sheet
- 6 -
and controlling structure of the steel sheet. The
gist thereof is as follows.
[0015] (1) A hot-rolled steel sheet, includes:
a structure that is a c6mposition containing, in
mass%,
C: 0.01 to 0.2%;
Si: 2.5% or less (not including "0" (zero))
Mn: 4.0% or less (not including "0" (zero))
P: 0.10% or less;
S: 0.03% or less;
Al: 0.001% to 2.0%;
N: 0.01% or less (not including "On (zero))
0: 0.01% or less (not including "OfT (zero))
Ti: 0.03 to 0.30%;
Nb: "0" (zero) to 0.30%;
Cu: "0" (zero) to 2.0%;
Ni: "0" (zero) to 2.0%;
Mo: "0" (zero) to 1 • 0% ;
V: "0" (zero) to 0.3%;
Cr: "0" (zero) to 2.0%;
Mg: "0" (zero) to 0.01%;
Ca: "0" (zero) to 0.01%;
EEM: "0" (zero) to 0.1%;
B: "0" (zero) to 0.01%; and,.
;
the balance b.eing composed of iron and impurities
and in which by volume fraction, 90% or more of
grain-shaped tempered martensite, or by volume
fractio~,. 90,% or more in total of both grain7shaped
tempered martensite and lower bainite is contained
- 7 -
and an average aspect ratio of the tempered
martensite and the lower bainite is 2 or less.
[0016] (2) The hot-rolled steel sheet according to
(1) described above, in which an effective crystal
grain diameter of the tempered martensite and the
lower bainite is 10 !LID or less.
( 3) The hot-rolled steel sheet according to ( 1) or
(2) described above, in which 1 X 10 6 (pieces/rnrn2
) or
more of iron-based carbides exist in the tempered
martensite and the lower bainite.
[0017] (4) The high-strength hot-rolled steel sheet
according to any one of ( 1) to ( 3) , further includes:
in mass%,
Nb: 0.01 to 0.30%.
(5) The hot-rolled steel sheet according to any one
of (1) to (4) described above, further includes:
in mass%,
one type or two or more types selected from the
group consisting of
Cu: 0.01 to 2. 0%;
Ni: 0.01 to 2.0%;
Mo: 0.01 to 1.0%;
V: 0.01 to 0.3%; and
Cr: 0.01 to 2. 0%.
( 6) The hot-Lolled steel sheet according to any one
of (1) to (5) described above, further includes:
in mass%,
one\,type or two or more types selectce.d from the
group consisting of
- 8 -
Mg: 0.0005 to 0.01%;
Ca: 0.0005 to 0.01%; and
REM: 0.0005 to 0.1%.
(7) The hot-rolled steel sheet according to any one
of (1) to (6) described above, further includes:
in mass%,
B: 0.0002 to 0.01%.
(8) The hot-rolled steel sheet according to any one
of (1) to (7) described above, in which a galvanized
layer or an alloyed galvanized layer is included on a
surface of the hot-rolled steel sheet.
[0018] (9) A manufacturing method of a hot-rolled
steel sheet, includes:
smelting a steel containing:
in mass%,
c: 0.01 to 0.2%;
Si: 2.5% or less (not including "0" (zero)) ;
Mn: 4.0% or less (not including " 0 ,, (zero) ) ;
P: 0.10% or less;
S: 0.03% or less;
Al: 0. 0 01 to 2.0%;
N: 0.01% or less (not including "0" (zero)) ;
. 0: 0. 0.1% or less (not including -" 0 N (zero) ) ;
Ti: 0.03 to 0.30%;
Nb: ".-G.:·n (zero) to 0.30%;
Cu: "0" (zero) to 2 . 0%;
Ni: "Orr (zero) to 2.0%;
Mo:\"0" (zero) to 1. 0%;
V: " 0 il (zero) to 0.3%;
- 9 -
Cr: "0" (zero) to 2.0%;
Mg: "0" (zero) to 0.01%;
Ca: "0" (zero) to 0.01%;
REM: "0" (zero) to 0.1%;
B: "0" (zero) to 0.01%; and
the balance being composed of
and casting the steel into a slab,
heating the cast slab to 1200°C or
iron and impurities
and then directly
higher or once
cooling the cast slab and then heating the cast slab
to 1200t or higher; performing hot rolling in which a
reduction ratio of rolling at the final stage of
rough rolling is set to 25% or more and a rolling
temperature is set to lower than ll00°C and the rough
rolling is completed, an obtained rough-rolled piece
is heated by lOt or higher before finish rolling, and
a finishing temperature of finish rolling to be
performed subsequently is set to 900t or higher; and
performing cooling at an average cooling rate of sot
/sec or more from the finishing temperature of the
finish rolling to 400t and performing coiling at
100°C or higher to lower than 400°C.
[0019] (10) The manufacturing method of the hotrolled.,
st·eel sheet according to (9) described above,
further includes: performing a galvanizing treatment
or a galvanealed treatment after the cuiling.
ADVANTAGEOUS EFFECTS OF INVENTION
[0020] According to the present invention, it is
possibl~ to provide a high-strength hot-rolled steel
sheet that has a maximum tensile strength of 980 MPa
- 10 -
or more and excellent low temperature toughness.
DESCRIPTION OF EMBODIMENTS
[0021] Hereinafter, the contents of the present
invention will be explained in detail.
As a result of earnest examination by the present
inventors, it was found that by volume fraction, 90%
or more of grain-shaped tempered martensite having an
average aspect ratio of effective crystal grains, (in
which the average aspect ratio will be simply
referred to as an aspect ratio, hereinafter), being 2
or less, or 90% or more in total of grain-shaped
tempered martensite and lower bainite having an
aspect ratio of effective crystal grains being 2 or
less is contained, and further an effective crystal
grain diameter of the tempered martensite and the
lower bainite is preferably set to 10 11m or less, and
additionally 1 X 10 6 (pieces/mm2
) or more of ironbased
carbides existing in the tempered martensite
and the lower bainite are contained, thereby making
it possible to ensure a high strength of 980 MPa or
more and high low temperature toughness.
Here, the effective crystal grain is an area
sur rounded by a .·g,rain boundary w i th ... a · misorient at ion
of 15° or more and can be measured by using EBSD or
theb. like. Its detail will be descrbbed later.
[0022] [Microstructure of a steel sheet]
First, there will be explained a microstructure
of a hot\- rolled steel sheet. oL.the present invention.
In the hot-rolled steel sheet a£ the present
- 11 -
invention, tempered martensite, or a mixed structure
of tempered martensite and lower bainite is set to
the main phase and the total volume fraction thereof
is set to 90% or more, and thereby a maximum tensile
strength of 980 MPa or more is ensured. Therefore,
it is necessary to obtain the main phase being
tempered martensite or being a mixed structure of
tempered martensite and lower bainite.
[0023] In the present invention, the tempered
martensite is the most important microstructure for
including strength, high bake hardenability, and low
temperature toughness. The tempered martensite is an
aggregation of lath-shaped crystal grains and
contains iron-based carbides having a major axis of 5
nm or more inside thereof, and further the iron-based
carbides are plural variants, namely belong to a
plurality of iron-based carbide groups extended in
different directions.
Like the present invention, when a coiling
temperature is set to 100~ or higher to lower than
400~, or when a structure of martensite is once
obtained and then is tempered at 100 to 600°C, the
,s.tructure ofu tempered martensite. a an be obtained.
[0024] The lo~er bainite is also an aggregation of
~lath-shaped crystal grains and contains iron-based
carbides having a major axis of 5 nm or more inside
thereof, and further the iron-based carbides are
single variants, namel~ belong to an iron-based
'
carbide group extended in one direction. By
- 12 -
observing extension directions of carbides, the
tempered martensite and the lower bainite can be
distinguished easily. Here, the iron-based carbide
group extended in one direction means an iron-based
carbide group where the difference of the extension
direction is within 5°
The lower bainite can be obtained by setting the
coiling temperature between 400° and the martensitic
transformation point, and as the coiling temperature
is higher in this temperature range, or as a cooling
rate thereafter is slower, the ratio of lower bainite
becomes larger.
[0025] When the volume fraction of one or the total
of the tempered martensite and the lower bainite is
less than 90%, the maximum high tensile strength of
980 MPa or more cannot be ensured, resulting in that
the maximum tensile strength of 980 MPa or more,
which is the requirement of the present invention,
cannot be ensured. Therefore, the lower limit of the
volume fraction is 90%. However, even if the volume
fraction is set to 100%, strength and excellent low
temperature toughness, which are the effects of the
present invention, are exhibited.
[0026] As 'other structures, by volume fraction, 10%
or less in total of one typ8 or~two or more types of
ferrite, fresh martensite, upper bainite, pearlite,
and retained austenite may also be contained in the
steel s~eet structure.
[0027] Here, the fresh martensite is defined as
- 13 -
martensite not containing carbide. The fresh
martensite is highly strong, but is extremely hard,
and thus deformation concentrates at an interface
with a different structure to be likely to become a
starting point of destruction, resulting in that the
fresh martensite is poor in low temperature toughness.
Further, even if the fresh martensite is set to the
main phase, hardness greatly varies even in the same
fresh martensite phases, and thus an interface is
likely to become a starting point of destruction.
Therefore, it is necessary to limit the volume
fraction of fresh martensite to 10% or less.
[0028] The retained austenite is, when a steel
product is plastically deformed at the time of pressmoldihg
or an automobile part is plastically de£ormed
at the time of collision, transformed into fresh
martensite, therefore causing the above-described
adverse effect similar to that of the fresh
martensite. Therefore, it is necessary to limit the
volume fraction to 10% or less.
[0029] The upper bainite is an aggregation of lathshaped
crystal grains and is an aggregation of laths
conta,ini.ng, carbides therebetween. The ·,carbides
contain~d between laths become a starting point of
destruction, to thus decrease low temperature
toughness. Further,
the upper bainite is
as compared to the lower bainite,
formed at high temperature to
thus be,low in strength, and when the upper bainite
is formed excessively, it becomes difficult to ensure
- 14 -
the maximum tensile strength of 980 MPa or more.
Such a tendency becomes prominent when the volume
fraction of upper bainite becomes greater than 10%,
so that it is necessary to limit the volume fraction
to 10% or less ..
[0030] The ferrite is a mass of crystal grains and
means a structure not containing a substructure such
as lath inside thereof. The ferrite is the softest
structure and causes a decrease in strength, and
therefore, in order to ensure the maximum tensile
strength of 980 MPa or more, the ferrite needs to be
limited to 10% or less. Further, it is extremely
soft as compared to the tempered martensite or the
lower bainite being the main phase, and thus
defor~ation concentrates at an interface between both
the structures to be likely to become a starting
point of destruction, resulting in that low
temperature toughness is decreased. Such a tendency
becomes prominent when the volume fraction becomes
greater than 10%, so that it is necessary to limit
the volume fraction to 10% or less.
The pearlite, similarly to the ferrite, causes a
decc.rease in strength ·and deteriora·t·io·n of low
temperature toughness, so that it is necessary to
limit the volume frac.tion to 10% or less.
[0031] As for the tempered martensite, fresh
martensite, bainite, ferrite, pearlite, austenite,
and the,remainin~ .structure that constitute the steel
sheet structure of the present invention as above,
- 15 -
identification of these structures, confirmation of
existing positions, measurement of area ratios can be
performed by the following methods. That is, with a
nital reagent and a reagent disclosed in Japanese
Laid-open Patent Publication No. 59-219473, of the
steel sheet, a rolling direction cross-section or a
cross section in a direction perpendicular to the
rolling direction is corroded to be observed by a
scanning electron microscope and a transmission
electron microscope at 1000 to 100000-fold
magnification, and thereby identification of these
structures, confirmation of existing positions,
measurement of area ratios can be performed.
[0032] Further, the structures can be distinguished
also by crysta~ orientation analysis using an FESEMEBSD
method [crystal orientation analysis method
using EBSD: Electron Back-Scatter Diffraction
belonging to a field emission scanning electron
microscope (FE-SEM: Field Emission Scanning Electron
Microscope)], or micro-region hardness measurement
such as micro Vickers hardness measurement. Since,
for example, the tempered martensite, upper bainite,
and. lower bainite,are differen~ •. in formation site ·Of
~ron-based carbide and crystal orientation related
matter (extension.;direction) as described above, the.
bainite and the tempered martensite can be easily
distinguished by observing iron-based carbides inside
lath-sha\ped c.rystal grains using a FE-SEM. to .examine
their extension directions.
- 16 -
[0033] In the present invention, the volume fraction
of each of ferrite, pearlite, bainite, tempered
martensite, and fresh martensite is calculated by the
following method. First, a sample is taken from an
observation surface that is a thickness-wise cross
section in parallel with the rolling direction of the
steel sheet, and the observation surface is polished
and nital etched. Then, the range of 1/8 thickness
to 3/8 thickness with 1/4 of the sheet thickness
being the center is observed by a FE-SEM, and area
fractions are measured, which are set as the volume
fractions. Further, 10 visual fields are each
measured at 5000-fold magnification, and an average
value of the measurements is set as the area ratio.
[0034] The fresh martensite and the retained
austenite are not sufficiently corroded by nital
etching, so that they can be clearly distinguished
from the above-described structures (ferrite,
bainitic ferrite, bainite, and tempered martensite)
during the observation by the FE-SEM. Therefore, as
the volume fraction of the £resh martensite, the
difference between the area fraction of a noncorroded
area ~bserved by the FE-SEM and the area
fraction of the retained austenite measured by an Xray
can be obtained.
[0035] Next, there will be described an average
crystal grain diameter and an identification
techniq'-\e. of the structures. In the pres.ent
invention, the average crystal grain diameter, the
- 17 -
ferrite, and further the retained austenite are
defined by using an EBSP-OIM (Electron Back Scatter
Diffraction Pattern-Orientation Image Microscopy)
method. In the EBSP-OIM method, a device and
software in which a highly inclined sample is
irradiated with electron beams in a scanning electron
microscope (SEM), a Kikuchi pattern formed by
backscattering is photographed by a high-sensitive
camera and is image processed by a computer, and
thereby a crystal orientation at an irradiation point
is measured for a short time period are constituted.
In the EBSP-OIM method, a microstructure and a
crystal orientation of a bulk sample surface can be
quantitatively analyzed. Further, an analysis area
of th~ EBSP-OIM method is an area capable of be~ng
observed by the SEM, and the EBSP-OIM method makes it
possible to analyze an area with a minimum resolution
of 20 nm, which varies depending on the resolution of
the SEM. In the present invention, by an image
mapped with a misorientation of crystal grains
defined as 15° being a threshold value of a highangle
grain boundary generally recognized as a
crystal grain boundary,· grains are visualiz.ed, from
which the average crystal grain diameter is obtained.
[0036] When the aspect ratio of effective crystal
grains, (each of which means an area surrounded by a
grain boundary with 15° or more, here), of the grainshapecd
t,empered martensite and lower .haini te exceeds
2, .. excellent toughness cannot be obtained. Therefore,
- 18 -
the aspect ratio of effective crystal grains of the
tempered martensite and bainite needs to be set to 2
or less.
Grains made flat in a specific direction have
large anisotropy and a crack propagates along a grain
boundary during a Charpy test, so that a toughness
value often becomes low. Thus, the effective crystal
grains need to be grains that are equiaxial as much
as possible. In the present invention, the rolling
direction cross section of the steel sheet is
observed, and a ratio of a length in the rolling
direction (L) to a length in the sheet thickness
direction (T) (~ L/T) is defined as the aspect ratio.
In the present invention, a sample is taken from
an observation surface that is a thickness-wise cross
section in parallel with the rolling direction of the
steel sheet, the observation surface is
electropolished, the range of 1/8 thickness to 3/8
thickness with 1/4 of the sheet thickness being the
center is analyzed by the EBSP-OIM method, aspect
ratios of all the effective crystal grains within a
visual field are measured, and an average value of
the aspect ratios i·s brought. ,,
[0037] In order to further improve low temperature
tough.ne.ss,". the effective crystal graiw diameter is
desirably set to 10 pm or less, in addition to the
condition that the tempered martensite or the lower
bainite \is set to the main phase .. ·~~ .. The effect of
improving low temperature toughness becomes prominent
- 19 -
when the effective crystal grain diameter is set to
10 /LID or less, so that the effective crystal grain
diameter is set to 10 fL m or less. It is more
desirably 8 fL m or less. The effective crystal grain
diameter to be described here means an area
surrounded by a grain boundary with a crystal
misorientation of 15° or more to be described by the
following technique, and corresponds to a block grain
diameter in terms of the martensite and the lower
bainite. The lower limit of lower bainite is
desirably set to 5%, further desirably set to 9 Slc
0 ' and
further desirably set to 12%. In this case, the
value of a fracture appearance transition temperature
(vTrs) of a Charpy test to be performed in conformity
with JIS z 2242 is likely to be -50°C or lower.
[0038] Further, the tempered martensite and the
lower bainite in the present invention desirably
contain 1 X 10 6 (pieces/mm2
) or more of iron-based
carbides. This is to increase low temperature
toughness of a parent phase to obtain an excellent
balance of strength and low temperature toughness.
That is, quenched martensite is excellent in strength
but .. plOOr in toughness, so that an improvement in·'
foughness is needsd. Thus, by making 1 X 10 6
(p.bece·s/mm2
) or more of iron-based e,arl;rides
precipitate, the toughness of the main phase can be
further improved.
[0039] \As a result that the pxesent inventors
examined the relationship between low temperature
- 20 -
toughness and a number density of iron-based carbide,
it became clear that setting the number density of
iron-based carbide inside the tempered martensite and
the lower bainite to 1 X 10 6 (pieces/mm2
) or more
makes it possible to ensure excellent low temperature
toughness. Therefore, the iron-based carbide is
desirably set to 1 X 10 6 (pieces/mm2
) or more. It is
more desirably 5 X 10 6 (pieces/mm2
) or more, and is
further desirably 1 X 10 7 (pieces /mm2
) or more.
Further, the iron-based carbides that have
precipitated by treatments of the present invention
have a size of 300 nm or less, which is small, and
most of them have precipitated in laths of the
martensite and the bainite, from which it is inferred
that they do not deteriorate low temperature
toughness.
[0040] As a method of measuring the number density
of iron-based carbide, a sample is first taken from
an observation surface that is a thickness-wise cross
section in parallel with the rolling direction of the
steel sheet. Then, the observation surface of the
sample is polished and nital etched, and the range of
1/8 thicknes~• to 3/8 thickness with 1/4 of th~ sheet
thickness bein~ the center is ob~erved by a FE~SEM,
.,, to- _thereby measure the number de,nsi·ty of iron-based
carbides. At this time, 10 visual fields are each
measured at 5000-fold magnification, to measure the
number ~ensity of iron-based carbides.
[0041] [Chemical composition of the steel sheet]
- 21 -
Next, there will be explained reasons for
limiting a chemical composition of the hot-rolled
steel sheet of the present invention.
Incidentally, % of each content means mass%.
(C: 0.01% to 0.2%)
C is an element contributing to a strength
increase of a base metal and an improvement in bake
hardenability, but is also an element generating
iron-based carbide such as cementite (Fe 3C) to be a
starting point of cracking during hole expansion.
When the content of C is less than 0.01%, an effect
of improving strength due to structure strengthening
by a low-temperature transformation generating phase
cannot be obtained. Further, when the content of C
exceeds 0.2%, ductility of the steel sheet decreases,
the iron-based carbide such as cementite (Fe 3C) to be
a starting point of cracking of a secondary shear
surface during punching increases, and formability
such as hole expandability deteriorates. Therefore,
the content of C is set to be in a range of 0.01% to
0.2%.
[0042] (Si: 2.5% or less (not including "0" (zero))
Si i·s an element contributing to a .st-rength
increase of a base metal and can be used also as a
.,, deoxidizing material of a multen steel, to thus be
preferably contained in a range of 0.001% or more
according to need. However, even when greater than
2. 5% of \Si is contained., the effect contributing to a
strength increase is saturated, so that the content
- 22 -
of Si is set to be in a range of 2.5% or less.
Further, containing 0.1% 6r more of Si suppresses
precipitation of iron-based carbide such as cementite
in the material structure and contributes to
improvements in strength and hole expandability due
to an increase in its content. Further, when the
content of Si exceeds 2.5%, the effect of suppressing
precipitation of iron-based carbide is saturated.
Thus, the desirable range of the content of Si is 0.1
to 2.5%.
[0043] (Mn: 4.0% or less (not including "0" (zero))
Mn is contained in order that the tempered
martensite or the lower bainite should become the
main phase in the steel sheet structure by solidsolution
strengthening and further quench
strengthening. Even when the content of Mn is set to
greater than 4%, this effect is saturated. On the
other hand, when the content of Mn is less than 1%,
an effect of suppressing ferrite transformation and
bainite transformation during cooling is not easily
exhibited, so that 1% or more is desirably contained.
It is desirably 1.4 to 3.0%.
[0044l (Ti: 0.03 to 0.3.0%) ...
Ti is the most important element for enabling
excellent low temperature toughness and a high
strength of 980 MPa or more both to be achieved.
Carbonitride of Ti or solid-solution Ti delays grain
growth clyring hot r.o.lling, which makes it possible .t.o
make the grain diameter of the hot-rolled steel sheet
- 23 -
fine and contributes to an improvement in low
temperature toughness. Besides, Ti, in addition to a
grain growth property by solid-solution N, exists as
TiN, to thereby contribute to an improvement in low
temperature toughness while making the crystal grain
diameter fine at the time of slab heating. Further,
Ti precipitates as carbonitride during rough rolling,
to thereby make crystal grains fine and contribute to
an improvement in low temperature toughness, so that
Ti is particularly important.
It is necessary to contain 0.03% or more of Ti in
order to obtain 10 Mm or less of the grain diameter
of the hot-rolled steel sheet. Further, even when
the content of Ti exceeds 0.30%, the above-described
effect is saturated and economic efficiency
deteriorates. The desirable range of the content of
Ti is 0.04 to 0.25%, and it is further desirably 0.05
to 0.20%.
[0045] (P: 0.10% or less)
P is an element contained in a molten iron,
segregates at grain boundaries, and decreases low
temperature toughness as its content increases.
-T·herefore, it is more. desirable as· the .content of P
is lower, and when greater than 0.10~ is contained,
workability and weldability are adversely affected,
so that the content of Pis set to 0.10% or less. In
view of weldability in particular, the content of P
is desi'\ably 0 _03% or less.
[0046] (S: 0.03% or less)
- 24 -
S is an element contained in a molten iron, and
when the content of S is too much, cracking during
hot rolling is caused and further inclusions such as
MnS to deteriorate hole expandability are generated.
Therefore, the content of Si should be decreased as
much as possible, but 0.03% or less is in an
allowable range, so that the content of S is set to
0.03% or less. However, when a certain degree of
hole expandability is required, the content of S is
desirably 0.01% or less, and more desirably 0.005% or
less.
[0047] (Al: 0.001 to 2.0%)
Al suppresses formation of coarse cementite and
improves low temperature toughness. Further, Al can
be used also as a deoxidizing material. However,
containing Al excessively increases the number of
coarse Al-based inclusions to cause deterioration of
hole expandability and surface flaws. Therefore, the
upper limit of the content of Al is set to 2.0%. The
upper limit of the content of Al is desirably 1.5%.
Incidentally, setting the content of Al to less than
0.001% is difficult, and thus this is the substantial
lower limit.
'[0048] (N: 0.01% or
N exists as .. T~i N .,
less (not including "0" (zero))
to thereby contribute to an ,
improvement in low temperature toughness while making
the crystal grain diameter fine at the time of slab
heating, However, there is a concern that N.,forms a
blowhole during welding to decrease strength of a
- 25 -
joint of a weld zone, so that it is
the content of N to 0.01% or less.
necessary to set
On the other hand,
setting the content of N to less than 0.0005% is not
desirable economically, so that the content of N is
desirably set to 0.0005% or more.
[0049] (0: 0.01% or less (not including "0" (zero))
0 forms oxides to deteriorate formability, so
that its content needs to be suppressed. When the
content of 0 exceeds 0.01% in particular, this
tendency becomes prominent, so that it is necessary
to set the content of 0 to 0.01% or less. On the
other hand, setting the content of 0 to less than
0.001% is not preferable economically, so that the
content of 0 is desirably set to 0.001% or more.
[0050] The basic chemical composition of the hotrolled
steel sheet of the present invention is
described above, and further the following components
can be contained.
[0051] (Nb: 0.01 to 0.30%)
Nb may also be contained because carbonitride of
Nb or solid-solution Nb delays grain growth during
hot rolling to thereby be able to make the grain
diameter of the hot-rolled• steel sheet fine and
improve low temperature toughness. However, when the
content of Nb•-is ·Jess than 0. 01%, the above-described
effect cannot be obtained sufficiently. Further,
when the content of Nb exceeds 0. 30%, a
recryste);l.lization temperature drops sign.i..f.icantly,
obtaining 2 or less of the aspect ratiu of tempered
- 26 -
martensite or lower bainite grains becomes difficult,
and low temperature toughness deteriorates.
Therefore, when Nb is contained according to need,
the content of Nb is desirably set to 0.01% to 0.30%.
[0052] (One type or two or more types selected from
the group consisting of Cu, Ni, Mo, V, and Cr)
Cu, Ni, Mo, V, and Cr suppress ferrite
transformation during cooling and make the steel
sheet structure become a tempered martensite or lower
bainite structure, so that one type or two or more
types selected from the group consisting of these
elements may also be contained. Further, they are
elements each having an effect of improving strength
of the hot-rolled steel sheet by precipitation
strengthening or solid-solution strengthening, and
one type or two or more types of them may also be
contained. However, when the content of each of Cu,
Ni, Mo, V, and Cu is less than 0.01%, the abovedescribed
effects cannot be obtained sufficiently.
Further, even when the content of Cu is greater than
2. 0%, the content of Ni is greater than 2. 0%, the
content of Mo is greater than 1.0%, the content of V
is greater, than 0. 3%, and ·th-e content of Ccr- is
greater than 2.0%, the above-described effects are
saturated ... .and economic efficiency deterio.nat.es. Thus,
when Cu~ Ni, Mo, V, and Cr are contained according to
need, it is desirable that the content of Cu is 0.01%
to L D% ,\ the content of N i is 0 . 0 1% to. 2 . 0 % , the
content of Mo is 0.01% to 1.0%, the content of V is
- 27 -
0.01% to 0.3%, and the content of Cr is 0.01% to 2.0%.
[0053] (One type or two or more types selected from
the group consisting of Mg, Ca, and REM)
Mg, Ca, and REM (rare-earth element) are elements
controlling form of non-metal inclusions to be a
starting point of destruction to cause deterioration
of workability and improving workability, so that one
type or two or more types of them may also be
contained. When the content of each of Mg, Ca, and
REM is 0.0005% or more, the effect becomes prominent,
so that 0.0005% or more of each of Mg, Ca, and REM is
designed to be contained. Further, even when the
content of Mg is set to greater than 0.01%, the
content of Ca is set to greater than 0.01%, and the
content of REM is set to greater than 0.1%, the
above-described effect is saturated and economic
efficiency deteriorates. Thus, it is desirable that
the content of Mg is set to 0.0005% to 0.01%, the
content of Ca is set to 0.0005% to 0.01%, and the
content of REM is set to 0.0005% to 0.1%.
[0054] (B: 0.0002 to 0.01%)
B delays ferrite transformation, to thereby
contribute to makin-g· the steel sheet st.ructure become
the tempered martensite or lower bainite structure.
Additionally, similarly to C, B segrega.tes at grain
boundaries and increases grain boundary strength, to
thereby improve low temperature toughness. Therefore,
.R. •. may a~so be contained in the h.o..t-.rolled steel sheet.
Howev&r, setting the content of B to 0:0002% or more
- 28 -
makes this effect become prominent, so that the lower
limit is desirably set to 0.0002%. On the other hand,
when the content of B exceeds 0.01%, the effect is
saturated and further economic efficiency
deteriorates, so that the upper limit is desirably
0.01%. It is more desirably 0.0005 to 0.005%, and
further desirably 0.0007 to 0.0030%.
[0055] The above elements are contained in the hotrolled
steel sheet and the balance is iron and
impurities. Here, as the impurities, ones contained
in raw materials such as ore and scrap, and ones to
be contained during a manufacturing process are cited
as an example.
[0056] Incidentally, regarding other elements, it is
confirmed that the effects of the present invention
are not impaired even when 1% or less in total of one
type or two or more types selected from the group
consisting of Zr, Sn, Co, Zn, and W is contained.
Among these elements, Sn has a risk that a flaw
occurs during hot rolling, so that the content of Sn
is more desirably 0.05% or less.
[0057] On the surface of the hot-rolled steel sheet
exp-lained above-1 ··a hot-dip galvaniz-ed layer is· .,,
provided by a hot~dip galvanizing treatment, and
fu:r;,ther an alloyed galvanized layeP.,is provided by an
alloying treatment after the galvanizing, and thereby
corrosion resistance can be improved in the hotrolled
s,teel sheet of the present invention having
the above-described structures and composition.
- 29 -
Further, the galvanized layer is not limited to pure
zinc, and may also contain elements of Si, Mg, Zn, Al,
Fe, Mn, Ca, Zr, and the like to achieve a further
improvement in corrosion resistance. Even when such
a galvanized layer is provided, excellent bake
hardenability and low temperature toughness of the
present invention are not impaired.
Further, even when any one of surface-treated
layers made by organic coating film forming, film
laminating, organic salts/inorganic salts treatment,
non-chromium treatment, and so on is provided, the
effects of the present invention can be obtained.
[ 0 0 5 8 l [Manufacturing method of the steel sheet]
Next, there will be explained a manufacturing
method of the hot-rolled steel sheet of the present
invention.
A tempered martensite single phase having an
aspect ratio of effective crystal grains being 2 or
less, or the total of both tempered martensite and
lower bainite is set to 90% or more for achieving
excellent low temperature toughness. Further, it is
desirable that the tempered martensite (and lower
bainite) have~ an effective cryst~al grain diameter ,of
10 11m or less, and contain 1 X 10 6 (pieces/mm2
) or
,''"more of iron-based carbides, and details of
manufacturing conditions for satisfying these
conditions will be described below.
[0059] 0 manufacturing method prior to hot rolling
is not particularly limited. That is, what is
- 30 -
necessary is to adjust a composition to be the abovedescribed
composition by performing smelting in a
blast furnace, an electric furnace, and so on, and
then performing various secondary refinings and next,
to perform casting by a method such as normal
continuous casting or thin slab casting. During
these times, a scrap may also be used for a raw
material as long as it can be controlled within a
component range of the present invention.
A cast slab is heated to a predetermined
temperature when performing hot rolling. In the case
of continuous casting, hot rolling may be performed
after the cast slab is once cooled to low temperature,
and then is reheated, or hot rolling may also be
perfoimed by heating the cast slab without cool~ng in
particular subsequently to continuous casting.
[0060] A slab heating temperature of the hot rolling
needs to be set to 1200t or higher. In the hotrolled
steel sheet of the present invention,
coarsening of austenite grains is suppressed by using
solid-solution Ti (and further Nb desirably), and
therefore, it is necessary to remelt TiC (and further
NbC) tha~~have precipitated at the time of casting.
When the slab heating temperature is lower than 1200t,
a long tim~ is required in order for carbides of Nb
and Ti to melt, and therefore, refining of the
crystal grain diameter thereafter and the effect of
improvir1;9 low-temperature toughness obtained by this
do not occur. Therefore, it is necessary to set the
- 31 -
slab heating temperature to 1200~ or higher. Further,
even if the upper limit of the slab heating
temperature is not particularly determined, the
effects of the present invention are exhibited, but
it is not economically preferable to set the heating
temperature to high temperature excessively.
Therefore, the slab heating temperature is desirably
set to lower than 1300°C.
[0061] Rough rolling needs to be performed in which
at the final stage, reduction is performed at a
reduction ratio of 25% or more, a rolling temperature
at the final stage is set to lower than 1100~, and
the rough rolling is completed. When the temperature
at the final stage of the rough rolling is 1100°C or
higher, a growth rate of the austenite grains
increases from the rough rolling to finish rolling
and the grain diameter becomes coarse, resulting in
that it becomes difficult to ensure excellent low
temperature toughness. Further, when the rolling
temperature at the final stage is lower than 1100°C
and the reduction ratio at the final stage is set to
25% or more, more excellent low temperature toughness
can. IDe ensured. - .• ·'\l'
[0062] This mechanism is unclear, but it is
conceivable that due to generation of carbonitride of
Ti caused by strain-induced precipitation, the growth
of the austenite grains from the rough rolling until
the fin\sh rolling . .can be suppressed, and therefore
excellent low temperature toughness can be obtained
- 32 -
by the effect of making the grain diameter fine.
Further, this effect becofues prominent as the
reduction ratio is larger, but when the reduction
ratio is 40% or more, there is a possibility that a
scale indentation flaw occurs in the steel sheet
surface. Accordingly, the reduction ratio at the
final stage of the rough rolling is desirably set to
less than 40%.
Therefore, the rolling at the final stage needs
to be performed at a reduction ratio of 25% or more
and at a rolling temperature of lower than 1100G
during the rought rolling. It is desirable that the
reduction ratio is 25% or more to less than 40% and
the rolling temperature is 1000°C or higher to lower
than 1100°C.
[0063] It is necessary to perform heating by a
heating device so that the temperature increases by
l0°C or higher from the temperature immediately before
heating by the time the finish rolling starts after
completion of the rough rolling. Performing heating
by l0°C or higher enables the aspect ratio of the
tempered martensite or both the tempered martensite
and the lower bainite to become 2~6r less. This
heating may be performed by an indudtion heating
device, for example, ~ut is not limited to this, and ~
even when the heating is performed by using a heat
retaining furnace, an energization heating device,
and/or t;he l.ike,. this effect can be exhibited.
Further, as the time until the finish rolling··starts
- 33 -
after completion of the rough rolling is longer, the
heating temperature needs to be increased, so that
the time until the finish rolling starts after
completion of the rough rolling is desirably set to
60 seconds or shorter. Further, the time until the
finish rolling starts after completion of the rough
rolling is desirably 30 seconds or shorter.
[0064] The mechanism why this heating makes the
aspect ratio become 2 or less is unclear, but it is
conceivable that recrystallization progresses by the
heating and recrystallization is completed before the
finish rolling and therefore the aspect ratio of
austenite decreases and the aspect ratio of the
tempered martensite or the lower bainite becomes 2 or
less.·
[0065] A finish rolling temperature of the finish
rolling (finishing temperature of the finish rolling)
following the rough rolling is set to 900~ or higher.
In the hot-rolled steel sheet of the present
invention, Ti (and further Nb desirably) in large
amounts are contained in order to refine the grain
diameter of austenite. As a result, when the finish
·•· rolling is finished in a tempe~ature zone of lower
'than 900~, austenite is not easfly recrystallized and
grains are extend.ed .-in the rolling direction to be·
likely to cause toughness deterioration. Thus, the.
finish rolling temperature is set to 900~ or higher.
It is d"'sirably not lower than 920°C nor higher than
1040°C.
- 34 -
_,
[0066] After the finish rolling, cooling is
performed at an average cooling rate of 50°C/sec or
more from the finish rolling temperature to 400~ and
coiling is performed. When this average cooling rate
is less than 50°C/sec, ferrite is formed during the
cooling, resulting in that it becomes difficult to
obtain, by volume fraction, 90% or more of the
tempered martensite single phase or the total of the
tempered martensite and the lower bainite as the main
phase. Therefore, it is necessary to set the average
cooling rate to 50°C/sec or more. However, as long as
ferrite is not formed in a cooling process, air
cooling may also be performed in a temperature zone
during the cooling process.
[0067] However, the average cooling rate from Bs to
a lower bainite generation temperature is desirably
set to 50°C/sec or more. This is to avoid formation
of the upper bainite. When the average cooling rate
from Bs to the lower bainite generation temperature
is less than 50°C/sec, the upper bainite is formed,
and there is a case that between laths of bainite,
fresh martensite (martensite whose dislocation
density is high·) is to be formed, or retained
austenite (to be the martensite whose dislocation
density is h.i g,h during working) to exist , and, ,,
therefore bake hardenability and low temperature
toughness deteriorate. Note that the Bs point is an
upper ba,inite generation start temperatur,.e, which is
determined by the components, and it i~ set to 550~
- 35 -
as a matter of convenience. Further, the lower
bainite generation temperature is also determined by
the components, and it is set to 400°C as a matter of
convenience. The average cooling rate is set to sot
/sec or more from the finish rolling temperature to
400°C, particularly from SSO to 400°C, and the average
cooling rate from the finish rolling temperature to
400°C is
[0068]
set to S0°C/sec or more.
Note that setting the average cooling rate
from the finish rolling temperature to 400t to sot
/sec or more also includes the condition that, for
example, the average cooling rate from the finish
rolling temperature to SSOoC is set to S0°C/sec or
more and the average cooling rate from SSO to 400°C is
set to less than S0°C/sec. However, there is
sometimes a case that the upper bainite is likely to
be generated under this condition and greater than
10% of upper bainite is generated partially.
Therefore, the average cooling rate from SSO to 400°C
is desirably set to S0°C/sec or more.
[0069] The maximum cooling rate at lower than 400t
is desirably set to less than 50°C/sec. This is to
enable tha structure in which the tempered-.martensite
or the lower bainite with the dislocation density and
the numbe.r.· density of iron-based carbide being within
the above-described ranges is the main phase. When
the maximum cooling rate is S0°C/sec or· more, it is
not .p.oss,ible to make the iron-based C3.rbide and the
dislocation density fall within the above-described
- 36 -
ranges, resulting in that high bake hardenability and
low temperature toughness cannot be obtained.
Therefore, the maximum cooling rate is desirably set
to less than 50°C/sec. Here, the cooling at the
maximum cooling rate of less than SOt/sec at lower
than 400°C is achieved by air cooling, for example.
Further, the above cooling not only means cooling but
also includes isothermal holding, namely coiling at
lower than 400°C. Further, the cooling rate control
in this temperature zone aims to control the
dislocation density and the number density of ironbased
carbide in the steel sheet structure, so that
even when cooling is once performed to the
martensitic transformation start temperature (Ms
point) or lower and thereafter the temperature is
increased to perform reheating, the maximum tensile
strength of 980 MPa or more, high bake hardenability,
and low temperature toughness, which are the effects
of the present invention, can be obtained.
[0070] The coiling temperature is set to 100°C or
higher to lower than 400t. This is to enable the
structure in which the tempered martensite single
phase ~r·the tempered martensite and the lower
bainite with the number density of iron-based carbide
being .. ••Hi thin the above-described range.· •• is/are the
main phase. When the coiling temperature is 400°C or
higher, it is not possible to make the tempered
w.artensi\te single phase or the. tempered martensite
and the lower bainite become the main phase. Further,
- 37 -
when the coiling temperature is lower than lOOt, it
is not possible to make the iron-based carbide fall
within the above-described range, resulting in that
excellent toughness cannot be obtained. Therefore,
it is necessary to set the coiling temperature to
100°C or higher to lower than 400°C.
[0071] Here, coiling is performed at the coiling
temperature being in a range of 400°C to the
martensitic transformation point, to thereby generate
the lower bainite, and as the temperature is higher
in this temperature range, or the rate of cooling
thereafter is slower, the ratio of the lower bainite
becomes larger. On the other hand, when coiling is
performed at the coiling temperature being in a
tempe~ature zone of the martensitic transformation
point to 100°C, the tempered martensite single phase
is obtained.
Incidentally, the coiling in this temperature
zone aims to control the number density of iron-based
carbides in the steel sheet structure, so that even
when cooling is once performed to lower than lOOt and
thereafter the temperature is increased to perform
reheating, the maximum tensile streng.th of 980 MPa or·
more and excellent toughness, which are the effects
oif_,.,the· present invention, can be oh·tained.
·[0072] In general, in order to_ ·obtain the martensite,
the ferrite transformation needs to be suppressed,
and the \cooling at 50°C/sec or .. more is needed.
Add~tionally, at low temperature, a temperature zone
- 38 -
transits from a temperature zone whose heat transfer
coefficient is relatively low and where it is not
easily cooled, called a film boiling region to a
temperature zone whose heat transfer coefficient is
large and where it is cooled easily, called a
nucleate boiling temperature region. Therefore, when
the temperature zone of lower than 400°C is set to a
cooling stop temperature, the coiling temperature
fluctuates easily, and with the fluctuation, the
quality of material also changes. Therefore, there
was often a case that the normal coiling temperature
is set to higher than 400°C or coiling is performed at
room temperature.
As a result, it is inferred that it was
convehtionally difficult to find that the maximum
tensile strength of 980 MPa or more and excellent low
temperature toughness can be ensured simultaneously
by the coiling from 100 to lower than 400t as the
present invention.
[0073] Incidentally, it is desirable to perform skin
pass rolling at a reduction ratio of 0.1% to 2% after
all the processes are finished for the purpose of
.... correcting t·h>e steel sheet shape_. and achievin-g an
improvement in ductility by introduction of mobile
.,_dislocation. Further, after a-lL th-e processes are
finished, pickling may be performed on the obtained
hot-rolled steel sheet according to need for the
purpose\of removing scales attached to the surface of _,_
the obtained hot-rolled steel sheet. Furthermore, it
- 39 -
is also possible to perform skin pass or cold rolling
at a reduction ratio of 10% or less inline or offline
on the obtained hot-rolled steel sheet after the
pickling.
[ 0 0 7 4 l The steel sheet of the present invention is
manufactured by undergoing continuous casting, rough
rolling, and finish rolling that are the normal hotrolling
process, and as long as the manufacturing
conditions prescribed above are satisfied, the other
manufacturing conditions are performed under normal
conditions, thereby making it possible to ensure the
maximum tensile strength of 980 MPa or more and low
temperature toughness, which are the effects of the
present invention.
Further, even if a heat treatment is performed in
a temperature range of 100 to 600t online or offline
for the purpose of precipitation of carbides after
the hot-rolled steel sheet is once manufactured, it
is possible to ensure excellent low temperature
toughness and the maximum tensile strength of 980 MPa
or more, which are the effects of the present
invention.
[0075] ·ln~identally, the stsel sheet wi~~ bhe·
maximum tensile strength of 980 MPa or mor~ in the
present invention indicates a steel sheet whose
maximum tensile stress measured by a tensile test
performed in conformity with JIS Z 2241 by using a
JIS No. ~ test piece cut out in a vertical direction
relative to the rolling direction of hot rolling is
- 40 -
·.~
980 MPa or more.
Further, the steel sheet excellent in toughness
at low temperature indicates a steel sheet whose
fracture appearance transition temperature (vTrs) of
a Charpy test performed in conformity with JIS Z 2242
is -40°C, desirably -50°C or lower, and further
desirably -60°C or lower. In the present invention,
the target steel sheet is used for automobiles mainly,
so that its sheet thickness becomes 3 mm or so in
many cases. Thus, when these evaluations are
performed, the surface of the hot-rolled steel sheet
is ground, and the steel sheet is worked into a 2.5
mm subsize test piece and the evaluations are
performed.
[Examples]
[0076] The technical contents of the present
invention will be explained while citing examples of
the present invention. Note that conditions in
examples are condition examples employed for
confirming the applicability and effects of the
present invention and the present invention is not
limited to these examples. The present invention can
employ .var-ious conditions a·s long as ·the object of
the pr~sent invention is achieved without departing
from the spirit of the present invention.
[0077] Hereinafter, there will be explained results
of examinations using steels A to S satisfying the
conditio\n of the cnmposition in the present invention
and steels a to k not satisfying the condition of the
- 41 -
composition in the present invention illustrated in
Table 1 as examples. Note that specifically La and
Ce are used as REM.
[0078] After these steels were cast, they were
directly heated to a temperature range of 1030°C to
1300°C, or they were once cooled to room temperature
and then reheated to the temperature range, and
thereafter hot rolling was performed under each of
conditions in Table 2-1 and Table 2-2, finish rolling
was performed at 760 to 1030t, cooling and coiling
were performed under each of the conditions
illustrated in Table 2-1 and Table 2-2, and hotrolled
steel sheets each having a sheet thickness of
3.2 mm were obtained. Thereafter, pickling was
performed, and then skin pass rolling at a reduction
ratio of 0.5% was performed.
[0079] Various test pieces were cut out of the
obtained hot-rolled steel sheets, and a material test,
a structure observation, and so on were performed.
As the tensile test, a JIS No. 5 test piece was
cut out in a direction vertical to the rolling
direction, and the test was performed in conformity
wci-th JIS Z 2242.
As measurement of a bake hardening amount, a JIS
No. 5 test piece was~ut out in a direction vertical
to the rolling direction, and the measurement was
performed in conformity with the method of a paint
bake hardening. t.est described in an appendix. of ... JIS G
'
3135. A pre-strain amount was set to 2%, and a heat
- 42 -
treatment condition was set to 170°C X 20 minutes.
The Charpy test was ~erformed in conformity with
JIS Z 2242, and the fracture appearance transition
temperature was measured. The sheet thickness of the
steel sheet of the present invention was less than 10
mm, so that front and rear surfaces of the obtained
hot-rolled steel sheets were ground to a thickness of
2.5 mm, and thereafter, the Charpy test was performed.
As for some of the steel sheets, the hot-rolled
steel sheets were heated to 660 to 720°C and were
subjected to a hot-dip galvanizing treatment or an
alloying heat treatment at 540 to 580t after the
galvanizing treatment, and hot-dip galvanized steel
sheets (GI) or alloyed hot-dip galvanized steel
sheet~ (GA) were obtained, and thereafter, a material
test was performed.
The microstructure observation was performed by
the above-described method, and the volume fraction
of each structure, the number density of iron-based
carbide, the effective crystal grain diameter, and
the aspect ratio were measured.
[0080] Results are illustrated in Table 3-1 and
Table 3-2.
It is found that only ones satisfying the
conditions of the,present invention have a maximum
tensile strength of 980 MPa or more and excellent
low-temperature toughness.
On ~he ·other hand, as for Steels A-3, Bc~A, E-4,
J-4, M-4, and S-~, where the slab heating temperature
- 43 -
was lower than 1200t, carbides of Ti and Nb that
precipitated during casting did not easily melt, so
that even if the other hot-rolling conditions were
set to be in the ranges of the present invention, it
was not possible to make the structural fraction and
the effective crystal grain diameter fall within the
ranges of the present invention and the strength and
low temperature toughness deteriorated.
As for Steels A-4, E-5, J-5, and M-5, the rough
rolling temperature was ll00°C or higher, the grain
diameter of austenite became too coarse, and the
crystal grain diameter of the tempered martensite
after transformation or the lower bainite also became
coarse, and therefore the low temperature toughness
deteriorated.
[0081] As for Steels B-5, E-5, J-5, and S-5, the
reduction ratio at the final stage of the rough
rolling was less than 25%, it was not possible to
make carbonitride of Ti caused by strain-induced
precipitation appear and to suppress coarsening of
the grain diameter of austenite, and therefore the
low temperature toughness deteriorated.
As for Steels A-5, B-6,, J--6, M-6, and S-6,
heating was not performed before the finish rolling
after the rough rolling was completed, and therefore
the recrystallization of austenite was not able to
progress and the aspect ratio of the effective
crystal;orains of the tempered martensita~fter
,~
transformation or the lower bainite became greater
- 44 -
than 2, and thereby the low temperature toughness
deteriorated.
As for Steels A-6, B-7, J-7, M-7, and S-7, the
finish rolling temperature was too low and rolling
was performed in a non-recrystallized austenite
region, and thereby grains were extended in the
rolling direction, resulting in that the aspect ratio
was large and the low temperature toughness
deteriorated.
[0082] As for St"eels A-7, B-8, J-8, M-8, and S-8,
the average cooling rate from the finish rolling
temperature to 400°C was less than 50°C/sec, ferrite
in large amounts was formed during cooling to make it
difficult to ensure strength, and an interface
betwe~n the ferrite and martensite became a starting
point of destruction, and therefore the low
temperature toughness deteriorated.
As for Steel A-8, the coiling temperature was
480°C, which was high, and the steel sheet structure
became an upper bainite structure, and therefore it
became difficult to ensure the maximum tensile
strength of 980 MPa or more, and coarse iron-based
carbides that precipitated between laths existing in
the upper bainite structu~e became a starting point
of destrwc;,tion, resulting in that the low."temperature
toughness deteriorated.
As for Steels B-9 and J-9, the coiling
tempe•rature was 580 to 620°C, which wB,s high, and the
'
steel sheet structure became a mixed structure of
- 45 -
ferrite and pearlite. As a result, it was difficult
to ensure the maximum tensile strength of 980 MPa or
more, and an interface between the ferrite in the
pearlite and iron-based carbide became a starting
point of destruction, and therefore the low
temperature toughness deteriorated.
[0083] As for Steel M-9, the coiling temperature was
room temperature, which was low, and the steel sheet
structure became tempered martensite and fresh
martensite, and therefore the fresh martensite became
a starting point of destruction, resulting in that
the low temperature toughness deteriorated.
Further, as illustrated in Steels A-9 and 10, B-
10 and 11, E-6 and 7, J-10 and 11, M-10 and 11, and
S-9 and 10, it was possible to confirm that the
quality of material of the present invention was able
to be ensured even when the alloyed hot-dip
galvanizing treatment or the alloyed hot-dip
galvanizing treatment was performed.
On the other hand, as for Steels a to k, where
the steel sheet components did not satisfy the ranges
of the present invention, it was not possible to
obtaincthe maximum tensile strength of ~80 MPa or
more and excellent low temperature toughness that
were determined in the present inventio,n .. -
[0084] Incidentally, when a sample was fabricated
under the same condition as that of Steel A-3 except
.that the. cooling rate from 550 to,400°C was set to
45°C/s; the average cooling rate from the finish
- 46 -
rolling temperature t.o 400°C was 73°C/s, and therefore
the average cooling rate satisfied 50°C/s or more.
However, the upper bainite became 10% or more and
variations were caused also in the quality of
material. Further, when a sample was tried to be
fabricated under the same condition as that of Steel
A-1 except that the content of 0 exceeded 0.01 mass%,
there was a problem in workability, and it was
possible to confirm that it cannot be treated as a
product.
[0085] [Table 1]
- 47 -
:::
::1
i:--:
I
'
I
"'" Cf)
s
a
b
c
f
....[.
h
Table 1 CHEMICAL CO:tvfPOSITION n. ,r "- C'C
::l
~
gj
ozno
;;;~
>O --;en
Vi~
.,~ -o '"z
0~
~
!>
:_. l:
"""'
ROUGH
ROLLING
FINAL STAGE
TEMPERATURE
ROUGH ROLLING
FlNAL STAGE
REDUCTION
RATIO
FIN!S!l
ROLLING
TEMPERA TUliE
AVEilAGE COOLl~G
RATE FROM FltliSH
TO 400' G
EACH UNOERllllE MEt.NS BEING OUT OF THE RANGE OF THE PRESENT lll'IENIION.
COOLING RATE FROM
5GO" C TO 400~ C
COILING
TEMPERATURE REMARK
0
0
CD
m
8
PJ
ty
f-"
(1)
N
I
f-'
'
' ,,
:·,
i·:,
:'"·.''
:·::
:·
ln
0
N--- ••••---
FINISH
!WILING
TEMPERATURE
AVERAGE COOLING
HATE FROM FINISH
TO 400' C
EACH UNDERLINE MEANS BEIIIG OUT OF THE RANGE OF THE PRESENT IIIVEilliOit
COOLING RATE FnOM
550" C TO 400" C
GOJL!NG
TEMPERATURE REMARK
>-l
PJ
()
>-'
([)
N
I
N
i
!
(J;
f-'
Table 3-1 MICROSTRUCTURE AND MECHANICAL PROPERTIES
STEEL ! TEMPERED
.. • NUMBER DENS 1TY EffECTIVE El
STEEL ;toWER REMAINING OTHER STRUCTURES
OF !RON-BASEO CRYSTAL GRAIN
TYPE * MARTENSITE BAINITE STRUCTURE CARB!DE DIAMETER ASPECT RAT! YP CMPa) TS (MPa) (%) vTrs Cc) REMARK
x1ot O/mm2) - (,u m)
A 1 HR 100 0 0 3. 4 7. 8 1.2 782 1023 12 GO INVENTION EXAMPLE
A-"2 HR 71 29 . 0 - 6. 3 8.3 1.3 934 1007 13 70 INVENT ION EXAMPLE
A-3 HR 69 0 3J fERRITE 5. 2 12.9 1.1 692 892 13 50 COMPARATIVE EXAMPLE
A-4 HR 81 19 0 - G. 8 11.3 1 746 1057 9 c20 COMPARATIVF EXAMPLE
A 5 HR 100 0 0 7.1 7. 3 n. 989 1046 10 10 COMPARATIVE_EX6MPLE
A-6 HR 100 0 0 4. 8 5.5 2. 3 957 1093 9 0 COMPARA VE EXAMELE
A 7 HR 66 0 34 FERRITE 5.9 7.2 1.4 705 9 14 30 COMPARATIVE EXAMPLE
AcS HR 0 0 100 UPPER BAINITE 0 9.2 0.8 576 824 15 10 CO PARATIVE __ EXAMfLE
A 9 G I 100 0 0 - 4.5 ). 7 1 852 998 14 50 I NYENTI ON EXAMPLE
A-10 GA 100 0 0 5.8 6. 6 1.1 880 983 14 -50 INVENTION EXAMPLE
8 I HR 90 0 10 FERRITE 3. 7 6.5 1.1 769 1021 12 50 INVENTION EXAMPLE
B-2 HR 25 - 15 0 3.9 7. 2 1.3 882 1019 13 -60 INVENT I ON EXAMPLE
B 3 HR 88 •. n,: 0 - 6.9 6. 5 I 949 1004 13 70 INVENTION EXA~PLE
B 4 HR 66 0 34 FERRITE 4. 2 I . 7 1.2 612 867 14 Jj) COMPARATIVE EXAMPLE
B 5 HR 74 25 0 - 5. 7 5. I 0.9 752 1093 9 0 COMPARATIVE EXAMPLE
8 6 HR 100 0 0 4. 9 8. I 2. ~ 934 1095 12 20 COMPARATIVE EXAMPLE
B 7 HR 100 0 0 4.8 4. 8 2 5 912 1055 10 -?0 C MPARATJVE EXAMPLE
8 8 HR ?7 0 73 FERRITE 4.3 6.4 1. 1 558 792 18 -30 COMPARATIVE 66b~PLE
B 9 HR 0 0 100 FERRITE. PEARLITE 0 7. 4 1.2 736 842 15 '0 COMPARATIVE EXAMPLE
E 10 G I 100 0 0 3.5 6. 7 I 899 1002 14 50 INVENTION EXAMPLE
B 11 GA 100 0 0 3.4 6. 7 1.1 948 984 14 50 INVENTION EXAMPLE
G-1 HR 100 0 0 4.9 6. 3 1 173 1035 13 50 INVENTION EXAMPLE
D 1 HR 100 0 0 3. 7 6. 5 1.3 781 1042 12 40 INVENTION EXAMPLE
E I HR 100 0 • 0 5.3 5. 9 0. 9 762 1026 12 50 INVENTION EXAMPLE
E 2 HR 71 29 ·o 0 4.5 7.3 0. 9 934 989 14 -50 INVENTION EXAMPLE
E 3 HR 91 9 0 7.6 5. 8 I 862 1007 13 60 INVENTION EXAMPLE
E 4 HR 80 ' 0 20 FERRITE 4.6 1.6 1.8 816 !W 13 0 COMPARATIV X MPLE
E 5 HR 83 17 0 5.8 14 2 1.9 843 1092 11 20 COMPARATIVE EXA~PLE
E 6 G r 100 0 0 5.5 5. I I 879 1021 13 50 INVENTION EXAMPLE
E 7 GA 100 0 0 5.8 6 1. 1 924 991 13 50 INVENTION EXAMPLE
F r HR 100 0 0 5. I 5. 7 1.3 749 1042 12 40 INVENT I ON EXAMPLE
B-1 HR 100 0 0 - 4 7. 3' I. 1 761 1006 13 -50 INVENTION EXAMPLE
H 1 HR 100 0 0 - 4.5 7.9 1.5 782 1124 13 50 INVENTION EXAMPLE
I 1 HR 100 0 0 5.3 7. 1 1 781 1019 14 40 INVENTION EXAMPLE
J-1 HR 100 0 0 - 4. 2 6 1.1 746 1047 12 60 INVENTION EXAMPLE
J 2 HR 53 47. 0 3.4 7.5 0.9 3'73 1007 14 50 INVENT I ON EXAMPLE
J 3 HR 91 9 0 5.9 6. 4 1.1 972 1026 13 70 INVENTION EXAMPLE
J 4 HR 67 0 33 FERRITE 3.9 11.9 0. 9 624 842 15 30 COMPARATIVE EXAMP E
J 5 HR 76 24 0 4.6 13. 5 1.7 806 1112 8 -10 COMPARATIVE EXAMPLE
J-6 HR 100 0 0 - 5. 1 7.4 2.3 933 1078 12 30 COMPARATIVE EXAMPL!;
J 7 HR 100 0 0 4. 3 3.8 2. I 924 1072 9 30 .COMPARATIV EXAMPL
EACH UNDERLINE MEANS BEING OUT OF THE RANGE OF THE PRESENT INVENTION.
* HR DENOTES HOT-ROLLED STEEL SHEET .. Gl CENOTES HOT-DiP GALVANIZED STEEL SHEET, GA DENOTES ALLOYED HOT-DIP GALVANIZED STEEL SHEET ON HOT-ROLLED STEEL SHEET.
0
0
(X)
-.]
eC]
!lJ
1:)
f-'
(I)
w
I
f-'
ln
"'
(]) OJ
X ::J
rt il
1-j
(]) rt s ::J'
(]) (])
~ -~
r-n
1-j 0
(]) . 1-j s (D
OJ
1-j
"' f-'-
OJ ' ::,::1
tJ il
f-' >='
(]) [j)
rt
1-j
f-'OJ
f-'
0
0
::J
rt
1-j
f-'-
1:)
>='
rt
f-'-
0
::J
rt
::J'
(])
1-j
:CD
0
f-n
f-'[
j)
tJ ~ H
(]) 0 z
0 0 t:J
0 co. c
:::J CD CJl
(]) ~ >-'1
[j) 7J
H
'0 ~ ~
0 0 1:"
[j) 0
[j) 0 ~
f-'- 1-j hj
tJ il hj
f-' . f-'- 1:"
(]) ::J H
lQ (l
rt ~
0 rt IJj
0 H
tJ 1:"
(]) rt H
::J' >-'1
>=' (]) >-<
[j)
(]) '0
il 1-j
(])
OJ [j)
rt (])
::J
OJ rt
<1 f-'-
(1) ::J
1-j <1
'< (])
::J
0 rt
0 f-'·
f-' 0
il ::J
il
f-J· f-'·
[j) rt
rt
1-j
f-'-
0
rt
·r:
Table
J-8 HR FERRITE 5 5.3 .J · T 643 T u;
J-9 HR fERRiTE. PEARLITE Q 8.1 4 887 I 114
J:-10 G r - 5.1 6.8 (\ nl.~
- l6 l9
---
N ~ v 4.Z t5.t5 1 iUI~ i£11.! IV -vv
,11-4 !iK 72 0 2S FERRITE 7. 2 12.2 0. 9 aM nc~ ' ~~ 1. U ~ Ut> 0'\ ;; ; l"t " ~ ; 1
H 30
00
-
~
FiiEsFiiE RR-IT-E-
----
----
6
--0. 6_
8. 3 I
3.1
116
1088
I 128
_960_
951 .. II:
-60
-oo
-50
--
' .E '
r'
' '"'""",.;
1 NV~NliON EXAMPLE
iNVENiiON EX'~PLE
IIIVEIITION EXAIIPI
r
- 16.8 5.9 L2 1285 lOOJ 9 -bU ' ls=n Bi I 73 i o 1 n 1 FERRITE 1s. 6 10. s 11 ss2 13i1 3 -2o cc.
'51"' M ' 'I - 17.5 118 !.3 1032 1638 6-30'
• T,! " ~ r " " 1'1~"' 1l!:"'\ 1·"1 ;/- l"t.
LOQ_ __::_ 16 l9_
83 7 I FERRITE I 1 8 __
I o 13. 9
-IU GA 0~ JZ 0 -
a-1 HR 0 0 100 FERRITE v. 1u.L I ;.-.
b-1 HR 91 0 9 RETAINED AUSTENITE ' ' ' ' ' '
c-1 HR 84 0 1.8 FERRITE
I d-1 I HR I iOO I o I o I
I f-1 I HR I 100 I I o I
-
FERRITE
----
\CH UNDERL E MEANS BEING OUT OF THE RAIIGE OF JHE PRESENT IIIVEIITIGN.
LJ..J..1.l .lrL==:
-10
0
! co
JBV~JJ!ljl_~
_1111[NJJON EXAMPLE
~
E SiEEL
-_STEEL
e-rt:t:l
* HR DENOTES HOi-ROLLED STEEL SHEET. Gl DENOTES HOT-DIP GALVANIZED STEEL SHEET. GA DENOTES AlLOYED HOT-0\P.GALVANIZED STEEL SHEET ON HOT-ROLLED STEEL SHEET.

CLAIMS
[Claim 1] A hot-rolled steel sheet, comprising:
a structure that is a composition containing, in
mass%,
C: 0.01 to 0.2%;
Si: 2.5% or less (not including "0" (zero)
Mn: 4.0% or less (not including "0" (zero)
P: 0.10% or less;
S: 0.03% or less;
Al: 0.001 to 2.0%;
N: 0.01% or less (not including "0" (zero)) ;
0: 0.01% or less (not including "0" (zero)) ;
Ti: 0. 0 3 to 0.30%;
Nb: "0" (zero) to 0.30%;
Cu: "0" (zero) to 2. 0%;
Ni: "0" (zero) to 2 . 0 % ;
Mo: "0" (zero) to 1 . 0 % ;
V: "0" (zero) to 0.3%;
Cr: "OfT (zero) to 2.0%;
Mg: "0" (zero) to 0.01%;
Ca: "0" (zero) to 0.01%;
REM: "0" (zero) to 0.1%;
B : " 0 " ( z,e r o ) to 0 . 0 1 %,,; and ;._;_
the balance being composed of iron and
impurities; and in which by volume fraction, 9D% or
more of grain-shaped tempered martensite, or by
volume fraction, 90% or more in total of both grainshaped
,\~empered martensite and lower baiJiti te is
contained and an average aspect ratio of the ~empered
- 53 -
martensite and the lower bainite is 2 or less.
[Claim 2] The hot-rolled steel sheet according to
claim 1, wherein
an effective crystal grain diameter of the
tempered martensite and the lower bainite is 10 MID or
less.
[Claim 3] The hot-rolled steel sheet according to
claim 1 or 2, wherein
1 X 10 6 (pieces/rnm2
) or more of iron-based
carbides exist in the tempered martensite and the
lower bainite.
[Claim 4] The hot-rolled steel sheet according to
any one of claims 1 to 3, further comprising:
in mass%,
Nb: 0.01 to 0.30%.
[Claim 5] The hot-rolled steel sheet according to
any one of claims 1 to 4, further comprising:
in mass%,
one type or two or more types selected from the
group consisting of
Cu: 0.01 to 2. 0%;
Ni: 0.01 to 2.0%;
Mo: o.. . 0·1 to 1.0%;
V: 0.01 to 0.3%; and
Cr: 0"01 to 2.0%.
.',,-
., .!
[Claim 6] The hot-rolled steel sheet according to
any one of claims 1 to 5, further comprising:
.in -znass%,
one type or two or more types selected from the
- 54 -
group consisting of
Mg: 0.0005 to 0.01%;
Ca: 0.0005 to 0.01%; and
REM: 0.0005 to 0.1%.
[Claim 7] The hot-rolled steel sheet according to
any one of claims 1 to 6, further comprising:
in mass%,
B: 0.0002 to 0.01%.
[Claim 8] The hot-rolled steel sheet according to
any one of claims 1 to 7, wherein
a galvanized layer or an alloyed galvanized layer
is included on a surface of the hot-rolled steel
sheet.
[Claim 9] A manufacturing method of a hot-rolled
steel sheet, comprising:
smelting a steel having a composition containing:
in mass%,
C: 0.01 to 0.2%;
Si: 2.5% or less (not including "0" (zero) ;
Mn: 4. 0% or less (not including "0" (zero) ;
P: 0.10% or less;
S: 0.03% or less;
Al.: ·. 0.001 to .2 ·• 0 % ;
N: 0.01% or less (not including
Q: .• 0. 01% or less (not including
Ti: 0.03 to 0.30%;
Nb: "0" (zero) to 0.30%;
Cu: "0" (zero) to 2.0%; Ni: "0" (zero) to 2.0%;
- 55 -
"0"- (zero) ) ;
\\ 0 ·{~ (zero) ) ;
Mo: "0" (zero) to 1.0%; V: "0" (zero) to 0. 3%·;
Cr: "0" (zero) to 2 . 0% i
j1"
Mg: "0" (zero) to 0.01%;
Ca: "0" . (zero) to 0.01%;
REM: "0" (zero) to 0.1%;
B: "0" (zero) to 0.01%; and
the balance being composed of iron and impurities
and casting the steel into a slab, and then directly
heating the cast slab to 1200t or higher or once
cooling the cast slab and then heating the cast slab
to 1200°C or higher;
performing hot rolling in which a reduction ratio
of rolling at the final stage of rough rolling is set
to 25% or fuore and a rolling temperature is set to
~
lower than 1100°C and the rough rolling is completed,
an obtained rough-rolled piece is heated by lOt or
higher before finish rolling, and a finishing
temperature of finish rolling to be performed
subsequently is set to 900t or higher; and
performing cooling at an average cooling rate of
sot/sec or more from the finishing temperature of the
fiaish rolling to 400°C and performcing coiling a't'
1 0 0 oC or higher to l·o we r than 4 0 0 oC . -
[Claim 10] The manufacturing method.of-the hot-rolled
steel sheet according to claim 9, further comprising:
performing a galvanizing treatment or a
galvanealed treatment after the coiling.