DESCRIPTION
TITLE OF INVENTION: HOT-ROLLED STEEL SHEET
TECHNICAL FIELD
[ 0 0 0 1 ] The present invention relates to a hot-rolled
steel sheet excellent in workability, in particular,
to a hot-rolled steel sheet excellent in stretchflangeabillty.
BACKGROUND ART
[ 0 0 0 2 ] In recent years, with respect to a demand for
reduction in weight of various steel sheets having
the purpose of improvement in fuel efficiency of an
automobile, thinning through high strengthening of a
steel sheet of an iron alloy or the like, application
of light metal such as A1 alloy or others have been
promoted. However, the light metal such as A1 alloy
has an advantage that specific strength is higher
than that of heavy metal such as steel but has a
disadvantage that it is remarkably expensive, so that
the application thereof is limited to special use.
Accordingly, the high strengthening of the steel
sheet is required to promote the reduction in weight
of various members more inexpensively and
extensively.
[ 0 0 0 3 ] The high strengthening of the steel sheet is
accompanied by a deterioration in a material property
such as formability (workability) in yeneral.
Therefore, it is important to achieve the high
strengthening without degrading material property in
development of a high-strength steel sheet. In
particular, stretch-flanging workability, burring
workability, ductility, fatigue endurance, corrosion
resistance, and the like are required of a steel
sheet used for automobile members such as an inner
sheet member, a structural member, and an underbody
member, and it is important how these material
properties and strength are exhibited at a high level
in a well-balanced manner. For example, tough hole
expandability ( h value) is required of a steel sheet
used for automobile members such as a s t r u c t . i l r a 1
member and an underbody member, which occupy about
20% of body weight. This is because press forming
mainly typified by stretch-flanging and burring is
performed after blanking, opening and the like by
shearing, punching and the like.
[ 0 0 0 4 ] In the steel sheet used for such members, it
is concerned that a flaw, a micro-crack, and others
occur in an edge formed by the shearing or the
punching, and a crack grows due to the generated flaw
or micro-crack to cause fatigue fracture. Therefore,
in the edge of the steel sheet, it is needed not to
cause the flaw, the micro-crack, and the like in
order to improve the fatigue endurance. As the flaw,
micro-crack, and the like which occur in the edge, a
crack is exemplified which occurs in parallel with
the sheet surface. The crack is sometimes referred
to as peeling. Conventionally, the peeling occurs
with a probability of about 80% in a 540 MPa class
steel sheet in particular, and occurs with a
probability of about 100% in a 780 MPa class steel
sheet. Further, the peeling occurs without
correlating with a hole expansion ratio. For
example, the peeling occurs when the hole expansion
ratio is even 50% or even 100%.
[0005] For example, as a steel sheet excellent in
the hole expandability (h value), a steel sheet in
which the main phase is ferrite and and which is
precipitation-strengthened by fine precipitates of
Ti, Nb, or the like and a manufacturing method
thereof are reported.
[0006] A hot-rolled steel sheet having a purpose of
high strength and improvement in stretchflangeability
is disclosed in Patent Literature 1.
Hot-rolled steel sheets having a purpose of
improvement in a stretch and stretch-flangeability
are disclosed in Patent Literatures 2 and 3 .
[0007] However, even by using the hot-rolled steel
sheets disclosed in the cited literatures 1 to 3, it
is difficult to sufficiently suppress the flaw and
the micro-crack on the edge formed by the shearing,
punching or the like. For example, the peeling
occurs after punching in the hot-rolled steel sheets
disclosed in Patent Literatures 2 and 3. A winding
condition for manufacturing the hot-rolled steel
sheet disclosed in the cited literature 1 is very
tough. Moreover, because the hot-rolled steel sheets
disclosed in Patent Literatures 2 and 3 contain Mo of
0.07% or more, which is an expensive alloying
element, a manufacturing cost is high.
CITATION LIST
PATENT LITERATURE
[0008] Patent Literature 1: Japanese Laid-open
Patent Publication No. 2002-105595
Patent Literature 2: Japanese Laid-open Patent
Publication No. 2002-322540
Patent Literature 3: Japanese Laid-open Patent
Publication No. 2002-322541
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0009] An object of the present invention is to
provide a hot-rolled steel sheet capable ot obtaining
excellent peeling resistance and excellent hole
expandability .
SOLUTION TO PROBLEM
[0010] The present inventors have obtained the
following findings as a result of keen examination in
order to achieve the above-described object.
[OOll] 1) Containing a specific amount of grains
having an intragranuiar misorientation of 5" to 14O in
all grains makes it possible to greatly improve hole
expandability.
[0012] 2) Containing Cr makes it possible to
suppress precipitation of coarse and large-aspect
ratio cementite, which makes the hole expandability
deteriorate, and secure solid-solution C so as to
balance excellent peeling resistance and excellent
hole expandability with each other.
[0013] 3) Containing Cr makes Cr solid-dissolve in
carbide containing Ti and increases an amount of
precipitation of a fine composite carbide, and allows
precipitation strengthening.
[0014] 4) Decreasing a Si content decreases a
transformation temperature and allows precipitation
of carbide containing Ti in a high-temperature region
which causes a variation in strength of a steel sheet
to be suppressed.
[0015] The invent~on is made based on such findings
and the following hot-rolled steel sheet is regarded
as a gist thereof.
[00161 (1)
A hot-rolled steel sheet comprising
a chemical composition represented by, in mass%,
C: 0.010% to 0.100%,
Si: 0.30% or less,
Mn: 0.40% to 3.00%,
P: 0.100% or less,
S: 0.030% or less,
Al: 0.010% to 0.500%,
N: 0.0100% or less,
Cr: 0.05% to 1.00%,
Nb: 0.003% to 0.050%,
Ti: 0.003% to 0.200%,
cu: 0.0% to 1.2%,
Ni: 0.0% to 0.6%,
Mo: 0.00% to 1.00%,
V: 0.00% to 0.20%,
Ca: 0.0000% to 0.0050%,
REM: 0.0000% to 0.0200%,
B: 0.0000% to 0.00208, and
the balance: Fe and impurities,
wherein
relationships represented by Expression 1 and
Expression 2 are satisfied,
0.005 5 [Si]/[Cr] I 2.000 ... Expression 1
0.5 5 [Mn]/[Cr] 5 20.0 . . . Expression 2
([Sil, [Crl, and lMn1 in the Expressions each
mean a content (mass%) of each of the elements), and
a proportion of grains having an intragranular
misorientation of 5" to 14O in all grains is 20% or
more by area ratio, the grain being defined as an
area which is surrounded by a boundary having a
misorientation of 15" or more and has a circleequivalent
diameter of 0.3 pm or more.
[00171 (2)
The hot-rolled steel sheet according to (I),
comprising a microstructure represented by
a volume ratio of cementite: 1.0% or less,
an average grain diameter of cementite: 2.00 pm
or less,
a concentration of Cr contained in cementite: 0.5
mass% to 40.0 mass%,
a proportion of cementite having a grain diameter
of 0.5 pm or less and an aspect ratio of 5 or less in
all cementite: 60 vol% or more,
an average grain diameter of a composite carbide
of Ti and Cr: 10.0 nm or less, and
a number density of the composite carbide of Ti
and Cr: 1.0 x 10~~/rnrno~r more.
[00181 ( 3 )
The hot-rolled steel sheet according to (1) or
(2), wherein, in the chemical composition,
Cu: 0.2% to 1.2%,
Ni: 0.1% to 0.6%,
Mo: 0.05% to 1.00%, or
V: 0.02% to 0.20%, or
any combination thereof is satisfled.
[00191 (4)
The hot-rolled steel sheet according to any one
of claims 1 to 3, wherein, in the chemical
composition,
Ca: 0.0005% to 0.0050%, or
REM: 0.0005% to 0.0200%, or
a combination thereof is satisfied.
[00201 ( 5 )
The hot-rolled steel sheet according to any one
of (1) to (4), wherein, in the chemical composition,
B: 0.0002% to 0.0020% is satisfied.
[00211 ( 6 )
The hot-rolled steel sheet according to any one
of (1) to (5), comprising a galvanized fllm on a
surface.
ADVANTAGEOUS EFFECTS OF INVENTION
[0022] According to the present invention, since a
proportion of gralns havlng an intragranular
misorientation of 5" to 14O, a Cr content, a volume
ratio of cementite, and others are appropriate,
excellent peeling resistance and excellent hole
expandability can be obtained.
DESCRIPTION OF EMBODIMENTS
LO0231 Hereinafter, an embodiment of the present
invention will be described.
[00241 First, a chemical composition of a hot-rolled
steel sheet according to the embodiment of the
present invention and a steel ingot or steel hi 1 let
to be used for manufacture the same will be
described. Though details will be described later,
the hot-rolled steel sheet according to the
embodiment of the present invention is manufactured
through rough rolling of the ingot or slab, finish
rolling, cooling, winding and others. Accordingly,
the chemical composition of the hot-rolled steel
sheet and the steel ingot or steel billet is one in
consideration of not only characteristics of the hotrolled
steel sheet but also the above-stated
processing. In the following description, " % " being
a unit of a content of each element contained in the
hot-rolled steel sheet means "massa" unless otherwise
stated. The hot-rolled steel sheet according to the
present embodiment and the steel ingot or steel
billet to be used for manufacture the same include a
chemical composition represented by: C: 0.010% to
0.100%, Si: 0.30% or less, Mn: 0.40% to 3.00%, P:
0.100% or less, S: 0.030% or less, Al: 0.010% to
0.500%, N: 0.0100% or less, Cr: 0.05% to 1.00%, Nb:
0.003% to 0.050%, Ti: 0.003% to 0.200%, Cu: 0.0% to
1.2%, Ni: 0.0% to 0.6%, Mo: 0.00% to 1.00%, V: 0.00%
to 0.20%, Ca: 0.0000% to 0.0050%, REM (rare earth
metal): 0.0000% to 0.0200%, B: 0.0000% to 0.0020%,
and the balance: Fe and impurities. As the
impurities, ones included in a raw materials, such as
ore and scrap, and ones included in a manufacturing
process are exemplified.
[0025] (C: 0.010% to 0.100%)
C combines with Nb, Ti or others to form
precipitates in a steel sheet and contributes to
improvement of the strength by precipitation
strengthening. C strengthens a grain boundary by
existing on the grain boundary as solid-solution C
and contributes to improvement in peeling resistance.
When a C content is less than 0.010%, the effects by
the above-described action cannot be sufficiently
obtained. Therefore, the C content is 0.010% or
more, and preferably 0.030% or more, and more
preferably 0.040% or more. When the C content is
more than 0.100%, an iron-based carbide, which
becomes an origin of a crack in a hole expansion
process, increases and a hole expansion ratio
deteriorates. Therefore, the C content is 0.100% or
less, and preferably 0.080% or less, and more
preferably C.070% or less.
[0026] (Si: 0.30% or less)
Si has an effect of suppressing precipitation of
an iron-based carbide such as cementite in a material
structure and contributing to improvement in
ductility and hole expandability, but when a content
thereof is excessive, a Ierrite transformation easily
occurs in a high-temperature region, and accordinyly
a carbide containing Ti easily precipitates in the
high-temperature region. The precipitation of Lhe
carbide in the hlgh-temperature region easlly causes
variations in an amount of precipitation, resulting
in causing a material variation in strength, hole
expandability, and the like. Further, the
precipitation of the carbide in the high-temperature
region decreases an amount of solid-solution C on the
grain boundary and makes the peeling resistance
deteriorate. Such a phenomenon is remarkable when a
Si content is more than 0.30%. Therefore, the Si
content is 0.30% or less, and preferably 0.10% or
less, and more preferably 0.08% or less. A lower
limit of the Si content is not particularly
specified, but from the viewpoint of suppression of
occurrence of scale-based defects such as a scale and
a spindle-shaped scale, the Si content is preferably
0.01% or more, and more preferably 0.03% or more.
[0027] (Mn: 0.40% to 3.00%)
Mn contributes to improvement of the strength by
solid-solution strengthening and quench
strengthening. Also, Mn promotes a transformation in
a para-equilibrium state at relatively iow
temperatures so as to make it easy to generate grains
having an intragranular misorientation of 5 O to 14'.
When a Mn content is less than 0.40%, the effects by
the above-described action cannot be sufficiently
obtained. Therefore, the Mn content is 0.40% or
more, preferably 0.50% or more, and more preferably
0.60% or more. When the Mn content is more than
3.00%, not only the effects by the above-described
action are saturated but also hardenability increases
excessively and formation of a continuous cooling
transformation structure excellent in the hole
expandability is difficult. Therefore, the Mn
content is 3.00% or less, and preferably 2.40% or
less, more preferably 2.00% or less.
[002B] (P: 0.100% or less)
P is not an essential element and is contained as
an impurity in the steel, for example. P segregates
to a grain boundary, and the higher a P content is,
the lower toughness is. Therefore, the P content as
low as possible is preferable. In particular, when
the P content is more than 0.100%, the decreases in
workability and weldabllity are remarkable.
Accordingly, the P content is 0.100% or less. From
the viewpoint of improvement in the hole
expandability and the weldability, the P content is
preferably 0.050% or less, and more preferably 0.030%
or less. A time and a cost are spent in reducing the
P content, and when reduction to less than 0.005% is
intended, the time and the cost increase remarkably.
Therefore, the P content may be 0.005% or more.
[0029] ( S : 0.030% or less)
S is not an essential element and is contained as
an impurity in the steel sheet, for example. S
causes a crack in hot rolling and generates an A type
inclusion leading to decrease in the hole
expandability. Therefore, a S content as low as
possible is preferable. In particular, when the S
content is more than 0.030%, the adverse effects are
remarkable. Accordingly, the S content is 0.030% or
less. From the viewpoint of improvement in the hole
expandability, the S content is preferably 0.010% or
less, and more preferably 0.005% or less. A time and
a cost are spent. in reducing the S content, and when
reduction to less than 0.001% is intended, the time
and the cost increase remarkably. Therefore, the S
content may be 0.001% or more.
[0030] (Al: 0.010% to 0.500%)
A1 acts as a deoxidizer at a steelmaking stage.
When an A1 content is less than 0.010%, the above
effect cannot be sufficiently obtained. Therefore,
the A1 conieiik is 0.010% or more, and preferably
0.020% or more, and more preferably 0.025% or more.
When the A 1 content is more than 0.500%, the effects
by the above-described action are saturated, and a
cost needlessly is high. Therefore, the A1 content
is 0.500% or less. When the A1 content is more than
0.100%, a non-metal inclusion increases and the
ductility and the toughness sometimes deteriorate.
Therefore, the A1 content is preferably 0.100% or
less, and more preferably 0.050% or less.
[0031] (N: 0.0100% or less)
N is not an essential element and is contained as
an impurity in the steel sheet, for example. N
combines with Ti, Nb or others to form nitride. The
nitride precipitates at a relatively high temperature
and easily becomes coarse, and has a possibility of
becoming an origin of a crack in the hole expansion
process. Further, the nitride is preferably fewer in
order to precipitate Nb and Ti as a carbide as
described later. Therefore, a N content is 0.0100%
or less. The N content is preferably 0.0060% or
less, and more preferably 0.0040% or less. A time
and a cost are spent in reducing the N content, and
when reduction to less than 0.0010% is intended, the
time and the cost increase remarkably. Therefore,
the N content may be 0.0010% or more.
[0032] (Cr: 0.05% to 1.00%)
Cr suppresses a pearlite transformation and
controls a size and a form of cementite by soliddissolving
in the cementite so as to make it possible
to improve the hole expandability. Cr also soliddissolves
in a carbide containing Ti and increases a
number density of a precipitate so as to increase
precipitation strengthening. When a Cr content is
less than 0.05%, the effects by the above-described
action cannot be sufficiently obtained. Therefore,
the Cr content is 0.05% or more, preferably 0.20% or
more, and more preferably 0.40% or more. When the Cr
content is more than 1.00%, the effects by the abovedescribed
action are saturated and not only a cost
needlessly is high but also a decrease in chemical
conversion treatability is remarkable. Therefore,
the Cr content is 1.00% or less.
LO0331 (Nb: 0.003% to 0.050%)
Nb finely precipiLaLes as a carbide in the
cooling after the rolling completion or after the
winding and improves strength by precipitation
strengthening. Moreover, Nb forms carbide so as to
fix C, thereby suppressing generation of cementite,
which is harmful to the hole expandability. When a
Nb content is less than 0.003%, the effects by the
above-described action cannot be sufficiently
obtained. Therefore, the Nb content is 0.003% or
more, and preferably 0.005% or more, and more
preferably 0.008% or more. When the Nb content is
more than 0,050%, the effects by the above-described
action are saturated and not only a cost needlessly
is high but also an increase in the precipitated
carbide decreases an amount of solid-solution C on
the grain boundary and sometimes makes the peeling
resistance deteriorate. Therefore, the Nb content is
0.050% or less, and preferably 0.040% or less, and
more preferably 0.020% or less.
100341 (Ti: 0.003% to 0.200%)
Ti finely precipitates as a carbide in the
cooling after the rolling completion or after the
winding and improves strength by precipitation
strengthening similarly to Nb. Moreover, Ti forms
carbide so as to fix C, thereby suppressing
generation of cementite, which is harmful to the hole
expandability. When a Ti content is less than
0.003%, the effects by the above-described action
cannot be sufficiently obtained. Therefore, the Ti
content is 0.003% or more, and preferably 0.010% or
more, and more preferably 0.050% or more. When the
Ti content is more than 0,200%, the effects by the
above-described action are saturated and not only a
cost needlessly is high but also an increase in the
precipitated carbide decreases an amount of solidsolution
C on the grain boundary and sometimes makes
the peeling resistance deteriorate. Therefore, the
Ti content is 0.200% or less, and preferably 0.170%
or less, and more preferably 0.150% or less.
[0035] Cu, Ni, Mo, V, Ca, REM, and B are not
essential elements but are optional elements which
may be appropriately contained up to specific amounts
in the hot-rolled steel sheet and the steel ingot or
steel billet.
LO0361 (Cu: 0.0% to 1.2%, Ni: 0.0% to 0.6%, Mo:
0.00% to 1.00%, V: 0.00% to 0.20%)
Cu, Ni, Mo, and V have an effect of improving
strength of the hot-rolled steel sheet by
precipitation strengthening or solid-solution
strengthening. Accordingly, Cu, Ni, Mo, or V, or any
combination thereof may be contained. Ic order to
obtain the eifects sufficiently, a Cu content is
p r e f e r a b l y 0.2% o r more, a N i c o n t e n t i s p r e f e r a b l y
0.1% o r more, a Mo c o n t e n t is p r e f e r a b l y 0.05% o r
m o r e , a n d a V c o n t e n t is p r e f e r a b l y 0.02% o r more.
However, when t h e Cu c o n t e n t i s more t h a n 1 . 2 % , t h e
N i c o n t e n t i s more t h a n 0 . 6 % , t h e Mo c o n t e n t i s more
t h a n l . 0 0 % , o r t h e V c o n t e n t is moro t h a n 0 . 2 0 % , t h e
e f f e c t s by t h e a b o v e - d e s c r i b e d a c t i o n a r e s a t u r a t e d
and a c o s t n e e d l e s s l y is h i g h . T h e r e f o r e , t h e Cu
c o n t e n t is 1.2% o r l e s s , t h e N i c o n t e n t i s 0.6% o r
l e s s , t h e Mo c o n t e n t is 1.00% o r less, and t h e V
c o n t e n t is 0.20% or less. T h u s , Cu, N i , Mo, and V
a r e t h e o p t i o n a l e l e m e n t s , and "Cu: 0.2% t o 1.2%",
nNi. . 0.1% t o 0.6%", "Mo: U.OS% t o 1.008", o r "V:
0.02% t o 0.20%", or a n y c o m b i n a t i o n t h e r e o f is
p r e f e r a b l y s a t i s f i e d .
100371 ( C a : 0.0000% t o 0 . 0 0 5 0 % , REM: 0.0000% t o
0 . 0 2 0 0 % )
Ca and REM are e l e m e n t s which c o n t r o l a form o f
t h e non-metal i n c l u s i o n , w h i c h b e c o m e s a n o r i g i n of
f r a c t u r e and c a u s e s a d e t e r i o r a t i o n of t h e
w o r k a b i l i t y , and improve t h e w o r k a b i l i t y .
A c c o r d i n g l y , C a o r REM, o r b o t h of them may be
c o n t a i n e d . I n o r d e r t o o b t a i n t h e e f f e c t s
s u f f i c i e n t l y , a C a c o n t e n t i s p r e f e r a b l y 0.0005% o r
m o r e , a n d a REM c o n t e n t i s p r e f e r a b l y 0.0005% o r
more. However, when t h e Ca c o n t e n t is more t h a n
0.0050% o r t h e REM c o n t e n t is more t h a n 0 . 0 2 0 0 % , t h e
e f f e c t s by t h e a b o v e - d e s c r i b e d a c t i o n a r e s a t u r a t e d
and a c o s t n e e d l e s s l y i s h i g h . T h e r e f o r e , t h e C a
content is 0.0050% or less, and the REM content is
0.0200% or less. Thus, Ca and REM are optional
elements, and "Ca: 0.0005% to 0.0050%" or "REM:
0.0005% to 0.0200%", or both of them is preferably
satisfled. REM represents elements of 17 kinds in
total of Sc, Y, and lanthanoid, and the "REM content"
means a content of a total of these 17 kinds of
elements.
[0038] (B: 0.0000% to 0.0020%)
B segregates to a grain boundary, and when B
exists with solid-solution C on the grain boundary, B
has an effect of increasing strength of grain
boundary. B also has an effect of improving the
hardenability and making the formation of the
continuous cooling transformation structure, which is
a microstructure desirable for the hole
expandability, easy- Accordingly, B may be
contained. In order to obtain the effects
sufficiently, a B content is preferably 0.0002% or
more, and more preferably 0.0010% or more. However,
when the B content is more than 0.0020%, slab
cracking occurs. Therefore, the B content is 0.0020%
or less. Thus, B is an optional element, and "B:
0.0002% to 0.0020%" is preferably satisfied.
[0039] In the present embodiment, relationships
represented by Expression 1 and Expression 2 are
satisfied.
0.005 5 [Si]/[Cr] 5 2.000 ... Expression 1
0.5 5 [Mn] / [Cr] < 20.0 . . . Expression 2
([Si], [Crl, and [Mnl in the Expressions each mean a
content (mass%) of each of the elements.)
[00401 In the present embodiment, it is very
important to control a proportion of grains having an
intragranular misorientation of 5' to 1 4 " , a size and
an amount of precipitation of a compositc carbide of
Ti and Cr, and a size arid a form of cementite.
Precipitation behavior of the composite carbide of Ti
and Cr and the cementite depends on a balance of a
content between Si and Cr. When a ratio of the
contents ( [Sil / [Crl ) is less than 0.005, the
hardenability increases excessively, and the
proportion of the grains having an intragranular
misorientation of to 14O decreases and the
composite carbide of Ti and Cr does not easily
precipitate in a low-temperature region. Therefore,
[Si]/[Cr] is 0.005 or more, preferably 0.010 or more,
and more preferably 0.030 or more. When the ratio of
the contents ([Sil/[Crl) is more than 2.000, the
proportion of the grains having an intragranular
misorientation of 5' to 14' decreases and the
precipitation of the composite carbide of Ti and Cr
in a high-temperature range causes the material.
variation, and an amount of solid-solution C
decreases and the peeling resistance deteriorates.
Moreover, when the ratio of the contents ([Si]/[Cr])
is more than 2.000, coarse cementite precipitates and
the hole expandability deteriorates. Therefore,
[Si]/[Cr] is 2.000 or less, preferably 1.000 or less,
and more preferably 0.800 or less.
[0041] Mn and Cr enhance the hardenability and
suppress the ferrite transformation at high
temperatures, thereby making it easy to generate
grains having an intragranular misorientation of 5' to
14' and suppressing the precipitation of the composite
carbide of Ti and Cr, resultinq in contribution to
stabilization of material. Meanwhile, effects of
precipitation control of cementite and enhancement of
the hardenability are different between Mn and Cr.
When a ratio of the contents ([Mnl/[Cr] 1 is less than
0.5, the hardenability increases excessively, the
proportion of the grains having an intragranular
misorientation of 5 O to 14O decreases, and the
precipitation of the composite carbide of Ti and Cr
does not easily occur in a low-temperature region.
Therefore, [Mn]/[Cr] is 0.5 or more, preferably 1.0
or more, and more preferably 3.0 or more. When the
ratio of the contents (IMn]/[Cr]) is more than 20.0,
control in specific size and form of cementite is
difficult. Therefore, [Mn]/[Cr] is 20.0 or less,
preferably 10.0 or less, and more preferably 8.0 or
less.
[0042] Next, characteristics of a grain i n the hotrolled
steel sheet according to the present
embodiment will be described. In the hot-rolled
steel sheet according to this embodiment, a
proportion of grains having an intragranular
misorientation of 5" to 14" in all grains is 20% or
more by area ratio, when the grain is defined as an
area which is surrounded by a boundary having a
misorientation of 15" or more and has a circleequivalent
diameter of 0.3 pm or more.
:0043: The proportion of the grains having an
intragranular misorientation of 5" to 14' in all the
grains can be measured by the following method.
First, a crystal orientation of a rectangular region
having a length in a rolling direction (RD) of 200 pm
and a length in a normal direction (ND) of 100 pm
around a 1/4 depth position (1/4t portion) of a sheet
thickness t from the surface of the steel sheet
within a cross section parallel to the rolling
direction, is analyzed by an electron back scattering
diffraction (EBSD) method at intervals of 0.2 pm, and
crystal orientation information on this rectangular
region is acquired. In the EASD method, irradiating
a sample inclined at a high angle in a scanning
electron microscope (SEM) with an electron beam,
photographing a Kikuchi pattern formed by
backscattering with a high-sensitive camera, and
performing computer image processing allow a
quantitative analysis of a microstructure and a
crystal orientation on a surface of a bulk sample.
This EBSD analysis is performed at a speed of 200
points/sec to 300 points/sec using, for example, a
thermal electric field emission scanning electron
microscope (JSM-7001F manufactured by JOEL Ltd.) and
an EBSD analyzer equipped with an EBSD detector
(HIKARI detector manufacture by TSL Co., Ltd.).
Then, a grain is defined as a region surrounded by a
boundary having a misorientation of 15' or more and
having a circle-equivalent diameter of 0.3 pm or more
from the acquired crystal orientation information,
the intragranular misorientation is calculated, and
the proportion of grains having an intragranular
misorientation of 5" to 14" in all grains is
obtained. The thus-obtained proportion is an area
fraction, and is equivalent also to a volume
fraction. The "intragranular misorientation" means
"Grain Orientation Spread (GOS)" being an orientation
spread in a grain. The intragranular misorientation
is obtained as an average value of misorientation
between the crystal orientation being a base and
crystal orientations at all measurement points in the
grain as described also in a document "KIMURA
Hidehiko, WANG Yun, AKINIWA Yoshiaki, TANAKA Keisuke
"Misorientation Analysis of Plastic Deformation of
Stainless Steel by EBSD and X-ray Diffraction
Methods", Transactions of the Japan Society of
Mechanical Engineers. A, Vol. 71, No. 712, 2005, pp.
1722-1728." Besides, an orientation obtained by
averaging the crystal orientations at all of the
measurement points in the grain is used as "the
crystal orientation being a base". The intragranular
misorientation can be calculated, for example, by
using software "OIM AnalysisTM Version 7.0.1." attached
to the EBSD analyzer.
- 21 -
[0044] The crystal orientation in a grain is
considered to have a correlation with a dislocation
density included in the grain. Generally, an
increase in dislocation density in a graln brings
about improvement in strength while decreasing
workability. However, the grains having an
intragranular misorientation of 5" to 14" can improve
the strength without decreasing workability.
Therefore, in the hot-rolled steel sheet according to
the present embodiment, the proportion of the grains
having an intragranular misorientation of 5' to 14O
is 20% or more. A grain having an intragranular
misorientation of less than 5" is difficult to
increase the strength though excellent in
workability, and a grain having an average
misorientation in the grain of more than 14" does not
contribute to improvement of stretch-flangeability
because it is different in deformability in the
grain. When the proportion of the grains having an
intragranular misorientation of 5' to 14' is less than
20% by area ratio, the stretch-flangeability and the
strength decrease, and excellent stretchflangeability
and strength cannot be obtained.
Accordingly, the proportion is 20% or more. Since
the grains having an intragranular misorientation of
5" to 14' are effective in the improvement in the
stretch-flangeability in particular, an upper limit
of the proportion is not particularly specified.
LO0451 Next, a desirable microstructure of rhe hotrolled
steel sheet according to the present
embodiment will be described. The hot-rolled steel
sheet according to the present embodiment preferably
has a microstructure represented by, a volume ratio
of cementite: 1.0% or less, an average grain diameter
of cementite: 2.00 ym or less, a concentratlon of Cr
contained in cementite: 0.5 mass% to 40.0 mass%, a
proportion of cementite having a grain diameter of
0.5 pm or less and an aspect ratio of 5 or less in
all cementite: 60 vol% or more, an average grain
diameter of a composite carbide of Ti and Cr: 10.0 nm
or less, and a number density of a composite carbide
of Ti and Cr: 1.0 x 1013/mm3 or more.
100461 [Volume ratio of cementite: 1.0% or less,
average grain diameter of cementite: 2.00 pm or less)
Stretch-flanging workability and burring
workability represented by a hole expansion ratio are
affected by a void, which becomes an origin of a
crack occurring in a punching process or a shearing
process. The void easily occurs in a position where
a hardness difference is large in a metal structure,
and when cementite is included in particular, a
matrix graln is subjected to excessive stress
concentratlon at an interface between the cementite
and the matrix and the void occurs there. When the
volume ratio of cementite is more than 1.0%, the hole
expandability easily deteriorates. Also when the
average grain diameter of cementite is more than 2.00
pm, the hole expandability easlly deteriorates.
Therefore, the volume ratio of cementite is
preferably 1.0% or less, and the average grain
diameter of cementite is preferably 2.00 pm or less.
Lower limits of the volume ratio and the average
grain diameter of cementite are not particularly
specified.
I00471 (Concentration of Cr contained in cementite:
0.5 mass% to 40.0 mass%)
Cr solid-dissolves in cementite and controls the
size and the form of cementite. When the
concentration of Cr contained in the cementite is 0.5
mass% or more, the cementite becomes rel-atively small
to a matrix grain, and anisotropy with respect to
deformation is small. Accordingly, since stress does
not easily concentrate dynamically and the void
accompanying the stress concentration does not easily
occur, the hole expandability improves. Therefore,
the concentration of Cr contained in cementite is
preferably 0.5 mass% or more. When the concentration
of Cr contained in cementite is more than 40.0 mass%,
the hole expandability and the peeling resistance are
sometimes made to deteriorate. Therefore, the
concentration of Cr contained in cementite is
preferably 40.0 mass% or less.
[0048] (Proportion of cementite having a grain
diameter of 0.5 pm or less and aspect ratio of 5 or
less in all cementite: 60 vol% or more)
When the proportion of cementite having the grain
diameter of 0.5 pm or less and the aspect ratio of 5
or less in all cementite is 60 vol% or more,
cementite becomes relatively small to the matrix
grain, and the anlsotropy with respect to the
deformation is small. Accordingly, since the stress
does not easily concentrate dynamically and the void
accompanying the stress concentration does not easily
occur, the hole expandability improves. Therefore,
the proportion is preferably 60 vol% or more. The
proportion may also be regarded as a proportion of
total volume of cementite having the grain diameter
of 0.5 ym or less and the aspect ratio of 5 or less
with respect to total volume of all cementite.
[0049] Here, a measuring method of the volume ratio,
the grain diameter, and the aspect ratio of
cementite, and the concentration of Cr contained in
cementite will be described. First, a sample for
transmission electron microscope is prepared from a
l/4 depth position (1/4 t portion) of a sheet
thickness t from a steel sheet surface of a test
piece cut out from a 1/4 W position or a 3/4 W
position of a sheet width of a steel sheet of a
sample material. Next, the sample for transmission
electron microscope is observed at an acceleration
voltage of 200 kV using a transmission electron
microscope, and cementite is specified from a
diffraction pattern thereof. Thereafter, a
concentration of Cr contained in the cementite is
measured using an energy dispersive X-ray
s2ectrometry atcached to the transmlsslon electron
microscope. Further, an observation of arbitrary ten
fields of view is made at a magnification of 5000
times to obtain an image thereof. Then, a volume
ratio, a grain diameter, and an aspect ratio of each
cementite is obtained from this image using image
analyzing software: and further a proportion of the
cementite having the grain diameter of 0.5 pm or less
and the aspect ratio of 5 or less in all the
cementite is obtained. The proportion obtained by
this method is a proportion (area fraction) of an
area in an observation surface, and the proportion of
the area is equivalent to a proportion of volume.
When the volume ratio and the grain diameter of
cementite are measured by this method, a measuring
limit of the volume ratio is about 0.01%, and a
measuring limit of the graln diameter is about 0,02
pm. As image-processing software, for example,
"Image-Pro" made by Media Cybernetics Inc. Unlted
States of America may he used.
[0050] (Average grain diameter of a composite
carbide of Ti and Cr: 10.0 nm or less, Number density
of a composite carbide of Ti and Cr: 1.0 x 1013/mm3 or
more)
The composite carbide of Ti and Cr contributes to
precipitation strengthening. However, when the
average grain diameter of this composite carbide is
more than 10.0 nm, an effect of the precipitation
strengthening cannot be sometimes sufficiently
obtained. Therefore, the average grain diameter of
this composite carbide is preferably 10.0 nm or less,
and more preferably 7.0 nm or less. A lower limit of
the average grain diameter of this composite carbide
is not particularly specified, but when the average
grain diameter is less than 0 . 5 nm, a mechanism of
the precipitation strengthening changes from an
Orowan mechanism to a Cutting mechanism, and there is
a possibility that an effect of desirable
precipitation strengthening cannot be obtained.
Therefore, the average grain diameter of this
composite carbide is preferably 0,5 nm or more.
Further, when the number density of this composite
carbide is less than 1.0 x 10'~/mm~, a sufficient
effect of precipitation strengthening cannot be
obtained and desired tensile strength (TS) cannot be
sometimes obtained while securing the ductility, the
hole expandability, and the peeling resistance.
Therefore, the number density of this composite
carbide is preferably 1.0 x 1013/mm3 or more, and more
preferably 5.0 x 10~~/mrn' or more.
[0051] Cr solid-dissolves in Tic and has an effect
of controlling a form of the composite carbide and
increasing the number density. When a solid solution
amount of Cr in the composite carbide is less than
2.0 mass%, this effect cannot be sometimes
sufficiently obtained. Therefore, the solid solution
amount is preferably 2.0 mass% or more. When the
sol-id solution is more than 30.0 mass%, a
coarse composite carbide is generated and the
sufficient precipitation strengthening cannot
sometimes obtained. Therefore, this solid solution
amount is preferably 30.0 mass% or less.
[0052] Here, a measuring method of a grain diameter
and the number density of the composite carbide, and
a concentration (solid solution amount) of Cr
contained in the composite carbide will be described.
First, a needle-shaped sample is prepared from a
sample material by cutting and electropolishing. At
this time, focused ion beam milling may be utilized
with the electropolishing as necessary. Then, a
three-dimensional distribution image of a composite
carbide is obtained from thc nccdlc-shaped samplc by
a three-dimensional atom probe measurement method.
The three-dimensional atom probe measurement method
allows integrated data to be restored and obtained as
the three-dimensional distribution image of real
atoms in real space. In a measurement of a grain
diameter of the composite carbide, a diameter when
the composite carbide is regarded as a spherical body
is found from the number oi constituent atoms and a
lattice constant of the composite carbide of an
observational object, and this is regarded as the
grain diameter of the composite carbide. Then, only
a composite carbide having the grain diameter of 0.5
nm or more is regarded as an object for measuring the
average grain diameter and the number density. Next,
the number density of the composite carbide is
obtained from volume of the three-dimensional
distribution image of the composite carbide and the
number of composite carbides. Diameters of arbitrary
30 or more composite carbides are measured, and an
average value thereof is regarded as an average grain
diameter of the composite carbide. The number of
atoms of each of Ti and Cr in the composite carbide
is measured to obtain a concentration of Cr contained
in the composite carbide from a ratio between both
the numbers. In obtaining the concentration of Cr,
the average value of arbitrary 30 or more composite
carbides may be found.
[0053] A microstructure of the matrix of the hotrolled
steel sheet according to the present
embodiment is not particularly limited, but is
preferably a continuous cooling transformation
structure (Zw) in order to obtain more excellent hole
expandability. The microstructure of the matrix may
include polygonal ferrite (PF) having a volume ratio
of 20% or less. When the polygonal ferrite having
the volume ratio of 20% or less is included, the
workability such as the hole expandability and the
ductility represented by uniform elongation can be
balanced more securely with each other. The volume
ratio of the microstructure is equivalent to an area
fraction in a measurement field of view.
LO0541 Here, the continuous cooling transformation
structure (Zw) means a transformation structure which
is at an intermediate stage between a microstructure
including polygonal ferrite or pearlite generated by
a diffusive mechanism and martensite generated by a
shearing mechanism without diffusing as mentioned in
Bainite Research Committee, Society on Basic
Research, the Iron and Steel Institute of
Japan/series; Recent Research on the Bainite
Structure of Low Carbon Steel. and its Transformation
Behavior-Final Report of the Bainite Research
committee-(1994 the Iron and Steel Institute of
Japan) (hereinafter, which is sometimes referred to
as a reference.). The continuous cooling
transformation structure (Zw) is mainly constituted
of bainitic ferrite (aoB), granular baj-nitic ferrite
(aBi, and quasi-polygonal ferrite (aq) and further
includes a small amount of retained austenite (yr) and
martensite-austenite (MA) as mentioned in page 125 to
page 127 in the reference as an optical microscope
observation structure. Quasi-polygonal ferrite,
whose internal structure does not appear by etching
similarly to polygonal ferrite but whose shape is
acicular, is a structure which is clearly
distinguished from polygonal ferrite. When lq
denotes a circumferential length of a targeted grain
and dq denotes a circle-equivalent diameter thereof,
a grain in which a ratio (lq/dq) therebetween is 3.5
or more can be regarded as the quasi-polygonal
ferrite. The continuous cooling transformation
structure (Zw) includes one or more of bainitic
ferrite, granular bainitic ferrite, quasi-polygonal
ferrite, retained austenite, and martensiteaustenite.
A total amount of the retained austenite
and the martensite-austenite is preferably 3 vol% or
less.
[0055] Here, a discriminating method of the
continuous cooling transformation structure (ZW) will
be described. In general, the continuous cooling
transformation structure (Zw) can be discriminated by
optical microscope observation by etching using a
nital reagent. When the discrimination by the
optical microscope observation is difficult, the
discrimination may be performed by the EBSD method.
In the discrimination of the continuous cooling
transformation structure (Zw), the one capable of
being discriminated by an image subjected to mapping
with a misorientation of each packet thereof being 15"
may be defined as the continuous cooling
transformation structure (Zw).as a matter of
convenience.
[0056] The hot-rolled steel sheet according to the
present embodiment can be obtained by a manufacturing
method including such a hot-rolling step and a
cooling step as described below, for example.
[0057] A steel ingot or steel billet may be prepared
by any method. For example, melting using a blast
furnace, a converter, an electric furnace, or the
like is performed, and adjustment of components is
performed so that ihe above-described chemical
composition can be obtained in various secondary
reiining, to preform casting. As the casting,
besides normal continuous casting or casting by an
ingot method, thin slab casting or the like may be
performed. Scrap may be used in material. Further,
when a slab is obtained by the continuous casting, a
high-temperature cast slab may be directly sent as it
is to a hot rolling mill or may be reheated in a
heating furnace after cooling to room temperature and
subjected to hot rolling.
[0058]
In the hot-rolling step, a hot-rolled steel sheet
is produced by heating a steel ingot or steel billet
having the above-described chemical components and
performing hot rolling. A heating temperature of the
steel ingot or steel billet (slab heating
temperature) is preferably a temperature SRT,in0C
represented by the Expression 3 or more to 1260°C or
less.
SRT,i, = 7000/{2.'15 - log ( [Ti] x [ C ] ) ) - 273 . . .
Expression 3
Here, [Ti] and [C] in the Expression 3 each
denotes a content of each of the elements by mass%.
[0059] The hot-rolled steel sheet according to the
present embodiment contains Ti. When the slab
heating temperature is less than SRT,in0C, Ti is not
sufficiently put into solution. When Ti is not put
into solution in heating a slab, it becomes difficult
to finely precipitate Ti as a carbide and improve
strength of steel by precipitation strengthening.
Furtner, it becomes difficult to obtain an effect of
suppressing generation of cementite harmful to the
hole expandability by fixing C with generation of a
Ti carbide. On the other hand, when the heating
temperature in a slab heating step is more than
1260°C, a yield is reduced by scale off. Therefore,
the heating temperature is preferably SRT,inOC or more
to 1260°C or less.
[0060] After heating the slab from SRTmInoC or more to
1260°C or less, rough rolling is performed without
particular waiting. When a finish temperature of the
rough rolling is less than 1050°C, a Nb carbide and a
composite carbide of Ti and Cr precipitate in
austenite coarsely, thereby making the workability of
the steel sheet deteriorate. Further, hot
deformation resistance in the rough rolling
increases, and there is a possibility of hindering
operation of the rough rolling. Therefore, the
finish temperature of the rough rolling is 1050°C or
more. An upper limit of the finish temperature is
not particularly specified but is preferably 1150°C.
That is because when the finish temperature is more
than 1150°C, a secondary scale generated in the rough
rnlling grows too much and it sometimes becomes
difficult to remove the scale in descaling or finish
rolling to be performed later. When a cumulative
reduction ratio of the rough rolling is less than
40%, it is impossible to sufficiently destroy a
solidification structure in casting and make a
crystal structure equiaxial, which inhibits the
workability of the steel sheet. Therefore, the
cumulative reduction ratio of the rough rolling is
40% or more.
[00611 A plurality of rough bars obtained by the
rough rolling may be joined to each other before the
finish rolling so as to continuously perform endless
rolling in the finish rolling. In this case, the
rough bars may be wound into a coil shape once,
stored in a cover having a heat insulating function
as necessary, and rewound again, thereafter
performing joining.
[0062] Between a roughing mill used for the rough
rolling and a finishing mill used for the finish
rolling, or among the respective stands of the
finishing mill, the rough bar may be heated using a
heating apparatus capable of controlling variations
in temperature in a rolling direction, a sheet width
direction, and a sheet thickness direction of the
rough bar. As a manner of the heating apparatus, gas
heating, energization heating, induction heating, and
the like can be variously cited. Performing such
heating makes it possible to control the temperature
in the rolling direction, the sheet width direction,
and the sheet thickness direction of the rough bar in
small variations in the hot rolling.
[0063] In order to control the proportion of grains
having an intragranular misorientation of 5' to 1 4 ' in
20% or more, cumulative strain in the last three
stages of the finish rolling is preferably 0.5 to 0.6
and on that basis, cooling is performed preferably
under the later-described condition. This is
because, the grains having an intragranular
misorientation of 5O to 14' are generated by
transformation in a para-equilibrium state at
relatively low temperatures, so that controlling a
dislocation density of austenite before
transformation to a certain range and controlling a
cooling rate thereafter to a certain range allow a
promotion of the generation of these grains. That
is, because controlling the cumulative strain in the
last three stages of the finish rolling and the
cooling thereafter allows control of a nucleation
frequency of the grains having an intragranular
misorientation of 5O to 14O and a growth rate
thereafter, it is also possible to control the
proportion of these grains as a result. More
specifically, the dislocation density of austenite
introduced by the finish rolling relates to the
nucleation frequency, and the cooling rate after the
rolling relates to the growth rate.
[0064] When the cumulative strain in the last three
stages of the finish rolling is less than 0.5, the
dislocation density of austenite to be introduced is
not sufficient, and the proportion of the grains
having an intragranular misorientation of 5" to 14'
becomes less than 20%. Accordingly, this cumulative
strain is preferably 0.5 or more. On the other hand,
when the cumulative strain in the last three stzges
of the finish rolling is more than 0.6,
recrystallization of austenite occurs during the
finish rolling and a stored dislocation density in
the transformation decreases. Also in this case, the
proportion of the grains having an intragranular
misorientation of 5 O to 14' becomes less than 20%.
Accordingly, this cumulative strain 1s preferably 0.6
or less.
100651 The here-described cumulative strain ( E ~ i~n ~ )
the last three stages of the finish rolling can be
found by the Expression 4.
Eeff = CE, (t, T ) . . . Expression 4
here is,
EL It, T) = slo/ex~l( t / ~ ~2i' 3 f ,
ZR = 70.exp (Q/RT) ,
zo = 8.46 x
Q = 1832005,
R = 8.314J/K.m01,
E ~ Ode notes logarithmic strain in reduction, t
denotes cumulative time until just before cooling in
the stages, and T denotes a rolling temperature in
the stages.
[0066] A finish temperature (rolling finish
temperature) of the finish rolling is preferably Ar3
point or more. When the rolling finish temperature
is less than Ar3 point, the dislocation density of
austenite before the transformation increases
excessively, and it becomes difficult to control. the
proportion of the grains having an intragranular
misorientation of 5 O to 14' in 20% or more.
[0067] The finish rolling is preferably performed
using a tandem mill, in which a plurality of rolling
mills are disposed linearly and which performs
continuous rolling in one direction to obtain a
predetermined thickness. When the finish rolling is
performed using the tandem mill, a steel sheet
temperature in the finish rolling is preferably
controlled so as to be in a range of Ar3 or more to
Ar3 + 150°C or less by performing the cooling (interstand
cooling) between a rolling mill and a rolling
mill. A temperature of the steel sheet in the finish
rolling exceeding Ar3 + 150°C causes too large grain
diameters, so than it is concerned that the toughness
deteriorates. Performing the inter-stand cooling
under such a condition as described above makes it
easy to limit the dislocation density range of
austenite before the transformation and control the
proportion of the grains having an intragranular
misorientation of 5 O to 1 4 " in 20% or more.
[0068] The Ar3 point is calculated by the Expression
5 considering an effect on a transformation point due
to the reduction based on the chemlcal components of
the steel sheet.
Ar3 point ("C) = 970 - 325 x [C] + 33 x [Si] +
287 x [PI + 40 x [All - 92 x ([Mn] 4- [Mo] + [Cu]) - 46
x ([Cr] 4- [Nil) . . . Expression 5
Yere, [Cl, [Sil, [PI, [All, [Mnl, [Mol, [Cu!,
[Cr], and [Nil denotes a content (inass%) of C, Si, P,
Al, Mn, Mo, Cu, Cr, and Ni, respectively. An element
which is not contained is calculated as 0%.
100691 Further, in the finish rolling, the
Expression 6 is preferably satisfied.
[0070]
... Expression 6
100711 Here, [Nb] and [Ti] denotes a content of Nb
and Ti by mass%, respectively, and t denotes a time
(sec) from a rolling completion in a stage one before
the last stage to a rolling start in the last stage,
and T denotes a rolling completion temperature (Co) in
the stage one before the last stage.
[0072] When the above-described Expression is
satisfied, from the rolling completion in the stage
one before the last stage to the rolling start in the
last stage, the recrystallization of austenite is
promoted and grain growth of austenite is inhibited.
Therefore, miniaturization of recrystallized
austenite grains during the rolling is performed, and
this makes i ~ tea sier to obtain a microstructure
suitable for the ductility and the hole
expandability.
[0073]
The cooling is performed to the hot-rolled steel
sheet after the hot rolling. It is desirable to, in
the cooling step, perform the cooling of the hotrolled
steel sheet in which the hot rolling is
completed (first cooling) at an average cooling rate
of more than 15'C/sec to a temperature zone of 500°C
to 650°C, and next perrorm the cooling of the abovedescribed
steel sheet (second cooling) on condition
that the average cooling rate to 450°C is 0.008"C/sec
to 1. OOO°C/sec.
[0074] (First cooling)
In a first cooling, a phase transformation from
austenite and a conflict between precipitation
nucleation of cementite and precipitation nucleation
of a Nb carbide and a composite carbide of Ti and Cr
occur. Then, when the average cooling rate in the
first cooling is 1S0C/sec or less, it becomes
difficult to control the proportion of the grains
having an intragranular misorientation of 5" to 14" in
20% or more, and because generation of a
precipitation nucleus of cementite has priority, the
cementite grows in the subsequent second cooling and
the hole expandability deteriorates. Therefore, the
average cooling rate is more than 15°C/sec. An upper
limit of the average cooling rate is not particularly
specified, but from the viewpoint of suppressing a
sheet warp due to thermal strain, the average cooling
rate is preferably 300°C/sec or less. Further, when
the cooling at more than 15OC/sec is stopped at more
than 650°C, it becomes difficult to control the
proportion of the grains having an lntragranular
misorientation of 5" 10 14" in 20% or more and
c e m e n t i t e e a s i l y o c c u r s due t o a s h o r t a g e of c o o l i n g ,
s o t h a t a d e s i r e d m i c r o s t r u c t u r e c a n n o t be o b t a i n e d .
T h e r e f o r e , t h i s c o o l i n g is performed t o 650°C o r less.
When t h e c o o l i n g a t more t h a n 15'C/sec is p e r f o r m e d t o
less t h a n 500cC, s u f f i c i e n t p r e c i p i t a t i o n does n o t
o c c u r i n t h e second c o o l i n g t h e r e a f t e r , so t h a t it
becomes d i f f i c u l t t o o b t a i n t h e e f f e c t of t h e
p r e c i p i t a t i o n s t r e n g t h e n i n g . T h e r e f o r e , t h i s c o o l i n g
is s t o p p e d a t a t e m p e r a t u r e of 500°C o r more.
[0075] (Second c o o l i n g )
A f t e r t h e f i r s t c o o l i n g , t h e steel s h e e t is
c o o l e d on c o n d i t i o n t h a t t h e a v e r a g e c o o l i n g r a t e t o
450°C is 0.008"C/sec t o 1.00O0C/sec. A t e m p e r a t u r e o f
t h e steel s h e e t d e c r e a s e s d u r i n g t h e second c o o l i n g ,
and w h i l e t h e t e m p e r a t u r e r e a c h e s 45OoC, t h e
g e n e r a t i o n of t h e g r a i n s h a v i n g a n i n t r a g r a n u l a r
m i s o r i e n t a t i o n of 5" to 14" is promoted and c e m e n t i t e ,
t h e Nb c a r b i d e , and t h e c o m p o s i t e c a r b i d e of T i and
C r p r e c i p i t a t e and grow. When t h e a v e r a g e ' c o o l i n g
r a t e t o 450°C is l e s s t h a n O . O O B 0 C / s e c , t h e p r o p o r t i o n
of t h e g r a i r ~ s l i a v i n y an i n t r a g r a n u l a r m i s o r i e n t a t i o n
of 5' t o 1 4 ' d e c r e a s e s and t h e Nb c a r b i d e and t h e
c o m p o s i t e c a r b i d e of T i and C r grow e x c e s s i v e l y , s o
t h a t it becomes d i f f i c u l t t o o b t a i n t h e e f f e c t of t h e
p r e c i p i t a t i o n s t r e n g t h e n i n g . T h e r e f o r e , t h i s a v e r a g e
c o o l i n g r a t e i s 0.008°C/sec o r more. When t h i s
a v e r a g e c o o l i n g r a t e is more t h a n l.OOO°C/sec, t h e
p r o p o r t i o n of t h e g r a i n s having an i n t r a g r a n u l a r
m i s o r i e n t a t i o n of 5' t o 14O d e c r e a s e s and a s h o r t a g e
of precipitation of the Nb carbide and the composite
carbide of Ti and Cr is caused, so that it becomes
difficult to obtain the effect of the precipitation
strengthening. Therefore, this average cooling rate
is l.OOO°C/sec or less. After the second cooling,
cooling may be freely performed. That is, as long as
it is possible to have specific microstructure and
chemical composition, after the second cooling,
cooling may be performed to room temperature by water
cooling or air cooling, or cooling may be performed
to room temperature after performing surface
treatment such as galvanization.
[0076] The hot-rolled steel sheet according to the
present embodiment can be obtained as described
above.
[0077] It is preferable to perform skin pass rolling
of the obtained hot-rolled steel sheet at a reduction
ratio of 0.1% to 2.0%. This is because the skin pass
rolling allows the ductility to be improved by a
correction of a shape of the hot-rolled steel sheet
and an introduction of mobile dislocation. Further,
it is preferable to perform pickling of the obtained
hot-rolled steel sheet. This is because the pickling
allows removal of scales attaching to a surface of
the hot-rolled steel sheet. After the pickling, the
skin pass rolling at a reduction ratio of 10.0% or
less may be performed and cold rolling at a reduction
ratio to about 40% may be performed. The above skin
pass rolling or cold rolling can be performed inline
or offline.
[0078] In the hot-rolled steel sheet according to
the present embodiment, moreover, heat treatment may
be performed on a hot dipping line after the hot
rolling or after the cooling, moreover additional
surface treatment of the hot-rolled steel sheet may
be performed. Giving plating on the hot dipping line
improves corrosion resistance of the hot-rolled steel
sheet.
100791 When the hot-rolled steel sheet after the
pickling is given the galvanization, the obtained
hot-rolled steel sheet may be immersed i n a
galvanizing bath, and alloying treatment may be
performed. Performing the alloying treatment
improves welding resistance to various welding such
as spot welding in addition to improvement in the
corrosion resistance in the hot-rolled steel sheet.
[0080] A thickness of the hot-rolled steel sheet is
12 mm or less, for example. The hot-rolled steel
sheet preferably has tensile strength of 500 MPa or
more, and more preferably has tensile strength of 780
MPa or more. Regarding the hole expandability, in a
hole expansion test method mentioned in the Japan
Iron and Steel Federation Standard JFS T 1001-1996, a
hole expansion ratio of 150% or more can be
preferably obtained for a 500 MPa class steel sheet,
and a hole expansion ratio of 80% or more can be
preferably obtained for a steel sheet with the
tensile strength of 780 MPa or more.
[0081] According to the present embodiment, since
the proportion of the grains having an intragranular
misorientation of 5' to 14O, the Cr content, the
volume ratio of cementite, and others are
appropriate, excellent peeling resistance and
excellent hole expandability can be obtained.
[0082] It should be noted that the above embodiments
merely iilustrate concrete examples of implementing
the present invention, and the technical scope of the
present invention is not to be construed in a
restrictive manner by these embodiments. That is,
the present invention may be implemented in various
forms without departing from the technical spirit or
main features thereof- For example, even a hotrolled
steel sheet produced by another method is
considered to be in a range of the embodiments as
long as the hot-rolled steel sheet has grains and a
chemical composition that satisfy the above
conditions.
EXAMPLE
[0083] Next, examples of the present invention will
be described. 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 various conditions as
long as the object of the present invention is
achieved without depazting from the spirit of the
present invention.
[ 0 0 8 4 ] (First experiment)
In a first experiment, first, steel ingots with a
mass of 3 0 0 kg which have chemical compositions
presented in Table 1 were melted in a high-frequency
vacuum melting furnace, to obtain steel billets with
a thickness of 7 0 mm by a rolling mill for test. The
balances of the steel ingots are Fe and impurities.
Then, these steel billets were heated to a
predetermined temperature and hot-rolled by a smallsized
tandem mill for test to obtain steel sheets
with a thickness of 2 . 0 mm to 3 . 6 mm. After a
completion of the hot rolling, the steel sheets were
cooled to a predetermined temperature imitating a
coiling temperature, charged in a furnace set to this
temperature, and cooled to 450°C at a predetermined
cooling rate. Thereafter, furnace cooling was
performed to obtain hot-rolled steel sheets. Table 2
presents these conditions. Further, regarding some
of the hot-rolled steel sheets, thereafter, pickling
was performed, plating bath immersion was performed,
and further alloying treatment was performed. Table
2 also presents presence/absence of the plating bath
immersion and presence/absence of the allying
treatment. In the plating bath immersion, immersion
in a Zn bath of 430°C to 460°C was performed and a
temperature for the alloying treatment was set to
500°C to 600°C. Blank columns in Table 1 each
indicate that a content of the element is below a
detection limit, and the balances are Fe and
impurities. Underlines in Table 1 or in Table 2
indicate that numeric values thereof deviate from a
range of the present invention or a preferable range.
Table 2 indicates that "rolling temperature before
the last one pass" is a rolling completion
temperature in a stage one before the last stage,
"interpass time" is a time period from a rolling
completion in the stage one before the last stage to
a rolling start in the last stage, and "finish
temperature" is a rolling completion temperature in
the last stage.

LO0861 [Table 21
LOO871 Thereafter, regarding each of the hot-rolled
steel sheets, measurement of a proportion of grains
- 47 -
having an intragranular misorientation of 5' to 14O,
observation of a microstructure, measurement of a
mechanical property, and confirmation of
presence/absence of fracture surface cracking were
performed by an EBSD analysis. Table 3 presents
these results. Underlines in Table 3 indicate that
numeric values thereof deviate from a range of the
present invention or a preferable range.
[0088] In the observation of the microstructure, an
area ratio (Zw) of a continuous cooling
transformation structure (Zw) and an area ratio of
polygonal ferrite ( P F ) in a 1/4 sheet thickness of
the hot-rolled steel sheets were measured. In the
observation of the microstructure, measurements of an
area ratio and an average grain diameter of
cementite, a proportion r of cementite having a grain
diameter of 0.5 pm or less and an aspect ratio of 5
or less in all cementite, and a concentration of Cr
contained in cementite were also performed. In the
observation of the microstructure, measurements of an
average grain diameter of a composite carbide of Ti
and Cr, a concentration of Cr in the composite
carbide of Ti and Cr, and a number density of thc
composite carbide of Ti and Cr were also performed.
These measuring methods are as described above.
[0089] In the measurement of the mechanical
property, a tensile test using a sheet thickness
direction (C direction) JIS 5 test piece and a hole
expansion test mentioned in JFS T 100:-1996 were
performed to find tenslle strength ( T S ) , elongation
(EL), and a hole expansion ratio ( h ) . The
confirmation of the presence/absence of the fracture
surface cracking was performed by visual observation.
[0091] As presented in Table 3, sincc test numbers 1
to 25 were in the range of the present invention,
high tensile strength could be obtained, an ex cell en^^
strength-ductility balance (TS x EL) and an excellent
strength-hole expansion balance (TS x h) could be
obtained, and excellent peeling resistance could be
obtained.
[ 0 0 9 2 ] On the other hand, since test numbers 26 to
43 deviated from the range of the present invention,
any of tensile strength, a strength-ductility
balance, a strength-hole expansion balance, and
peeling resistance was inferior.
INDUSTRIAL APPLICABILITY
[ 0 0 9 3 ] The present invention may be used for a
manufacturing industry and a utilization industry of
a hot-rolled steel sheet used for various steel
manufactures such as an inner sheet member, a
structural member, and an underbody member of an
automobile, for example.

CLAIMS
[Claim 11 A hot-rolled steel sheet comprising
a chemical composition represented by, in mass%,
C: O.UiO% to O.iOO%,
Si: 0.30% or less,
Mn: 0.40% to 3.00%,
P: 0.100% or less,
S: 0.030% or less,
Al: 0.010% to 0.500%,
N: 0.0100% or less,
Cr: 0.05% to 1.00%,
Nb: 0.003% to 0.050%,
Ti: 0.003% to 0.200%,
Cu: 0.0% to 1.2%,
Ni: 0.0% to 0.6%,
Mo: 0.00% to 1.00%'
V: 0.00% to 0.20%,
Ca: 0.0000% to 0.0050%,
REM: 0.0000% to 0.0200%,
B: 0.0000% to 0.0020%, and
the balance: Fe and impurities,
wherein
relationships represented by Expression 1 and
Expression 2 are satisfied,
0.005 5 [Sil /[Cr] < 2.000 . . . Expression 1
0.5 I [Mn] / [Cr] I 20.0 . . . Expression 2
([Si], [Cr], and [Mn] in the Expressions each
mean a content (mass%) of each of the elements), and
a proportion of grains having an intragranular
misorientation of 5' to 14" in all grains is 20% or
more by area ratio, the grain being defined as an
area which is surrounded by a boundary having a
misorientation of 15" or more and has a circleequivalent
diameter of 0.3 pm or more.
[Claim 21 The hot-rolled steel sheet according to
claim 1, comprising a microstructure represented by
a volume ratio of cementite: 1.0% or less,
an average grain diameter of cementite: 2.00 pm
or less,
a concentration of Cr contained in cementite: 0.5
mass% to 40.0 mass%,
a proportion of cementite having a grain diameter
of 0.5 yrn or less and an aspect ratio of 5 or less in
all cementite: 60 vol% or more,
an average grain diameter of a composite carbide
of Ti and Cr: 10.0 nm or less, and
a number density of the composite carbide of Ti
and Cr: 1.0 x 1013/mm3 or more.
[Claim 31 The hot-rolled steel sheet according to
claim 1 or 2, wherein, in the chemical composition,
Cu: 0.2% to 1.2%,
Ni: 0.1% to 0.6%,
Mo: 0.05% to 1.00%, or
V: 0.02% to 0.20%, or
any combination thereof is satisfied.
[Claim 41 The hot-rolled steel sheet according to
any one of claims 1 to 3, wherein, in the chemical
compositton,
Ca: 0.0005% to 0.0050%, or
REM: 0.0005% to 0.0200%, or
a combination thereof is satisfied.
[Claim 51 The hot-rolled steel sheet according to
any one of claims 1 to 4, wherein, in the chemical
composition, A: 0.0002% to 0.0020% is satisfied.
[Claim 61 The hot-rolled steel sheet according to
any one of claims 1 to 5, comprising a galvanized
film on a surface