DESCRIPTION
TITLE OF INVENTION
Hot-Rolled Steel Sheet with Excellent Press
5 Formability and Production Method Thereof
TECHNICAL FIELD
[OOOl]
The present invention relates to a hot-rolled steel
10 sheet with excellent press formability suitable for an
automobile, and a production method thereof.
BACKGROUND ART
[0002]
15 Recently, due to growing worldwide awareness of the
environment, it has been strongly demanded in the
automotive field to reduce the carbon dioxide emission or
improve fuel consumption. For solving these tasks,
weight reduction of a vehicle body may be effective, and
20 application of a high-strength steel sheet may be being
promoted to achieve the weight reduction. At present, a
hot-rolled steel sheet with a tensile strength of a 440
MPa level may be often used for automotive underbody
components. Despite the demand for application of a
25 high-strength steel sheet so as to cope with the weight
reduction of a vehicle body, a hot-rolled steel sheet
having a tensile strength of 500 MPa or more may
currently settle for its application to a part of the
components. Main causes thereof may include
30 deterioration of press formability associated with an
increase in strength.
[0003]
Many underbody members of an automobile may have a
complicated shape to ensure high rigidity. In press
35 forming, various kinds of workings such as burring,
stretch flanging and stretching may be applied and
therefore, workability responding to these works may be
6 required of the hot-rolled steel sheet as a blank. In
general, the burring workability and the stretch flanging
workability may be considered to have a correlation with
a hole expanding ratio measured in a hole expanding test,
5 and development of a high-strength steel sheet improved
in the hole expandability has been heretofore advanced.
[0004]
As for the measure to enhance the hole
expandability, it is said that elimination of a second
10 phase or an inclusion in the structure of a hot-rolled
steel sheet may be effective. The plastic deformability
of such a second phase or an inclusion may significantly
differ from that of the main phase and therefore, when a
hot-rolled steel sheet is worked, stress concentration
15 may occur at the interface between the main phase and the
second phase or inclusion. In turn, a fine crack working
out to a starting point for fracture may be readily
generated at the boundary between the main phase and the
second phase or inclusion. Accordingly, it may greatly
20 contribute to enhancement of hole expandability to limit
the amount of a second phase or an inclusion and thereby
reduce the starting point for crack generation as much as
possible.
[OOOS]
For these reasons, a hot-rolled steel sheet with
excellent hole expandability may be ideally a singlephase
structure steel, and in a dual-phase structure
steel, the difference in the plastic deformability
between respective phases constituting the dual-phase
30 structure may be preferably small. That is, it is
preferable that the hardness difference between
respective phases is small. As the hot-rolled steel
sheet excellent in hole expandability in line with such a
way of thinking, a steel sheet having a structure mainly
35 composed of bainite or bainitic ferrite has been proposed
(for example, Patent Document 1) .
CITATION LIST
PATENT LITERATURE
[0006]
Patent Document 1: Japanese Patent Publication (A)
H09-170048
Patent Document 2: Japanese Patent Publication (A)
2010-090476
Patent Document 3: Japanese Patent Publication (A)
2007-009322
Patent Document 4: Japanese Patent Publication (A)
H11-080892
SUMMARY OF THE INVENTION
TECHNICAL PROBLEM
[0007]
However, even in a hot-rolled steel sheet with
improved hole expandability, a crack may be often
generated in the stretch flange forming area at the
actual press forming, giving rise to inhibition of
application of a high-strength steel sheet.
[OOOS]
The present inventors have made intensive studies
about the cause of crack generation at the actual press
forming in a conventional hot-rolled steel sheet, despite
excellent hole expandability. As a result, the present
inventors have found that forming in a hole expanding
test may greatly differ from forming in the actual
stretch flanging and even when the hole expandability is
excellent, the stretch flanging workability may not be
excellent.
[0009]
The hole expansion ratio indicating hole
expandability is an opening ratio when a bored hole is
expanded by a punch and a crack generated in the punched
end face penetrates the sheet thickness. On the other
hand, stretch flanging is a working to stretch the sheet
edge part cut by a shear or the like when forming a
flange. In this way, forming in a hole expansion test
may greatly differ from forming in the actual stretch
flanging. Such a difference may cause a difference in
the stress state and the strain state of a hot-rolled
steel sheet, and the deformation limit amount leading to
fracture may be varied. The deformation limit amount may
be considered to vary because the metallic structure
greatly affecting fracture is changed according to the
stress state and the strain state.
[ 0 0 10 ]
The present inventors have found that because of
these reasons, even when the hole expandability is
increased, the stretch flanging workability is not
necessarily high and fracture occurs in the stretch
flanging area at the actual press forming.
Conventionally, such a finding was not known, and even
when a technique aiming at increasing the hole expansion
ratio measured in a hole expansion test has been
proposed, the stretch flanging workability has not be
taken into consideration (for example, Patent Documents 2
and 3). In particular, as in Patent Document 3, the
stretch flange characteristics may be evaluated by the
hole expansion ratio, and the term "stretch flange
characteristics" has been used by performing an
evaluation having no connection with the actual stretch
flanging .
[ 0 0 11 ]
In addition, the workability of a high-strength
steel sheet has been heretofore evaluated also by the
"strength-elongation balance" using, as the indicator, a
product (TSxEL) of tensile strength (TS) and elongation
at break (EL) (for example, Patent Document 4) . However,
the workability is evaluated by the breaking strength and
elongation in a tensile test, which may be different from
side bend elongation as in the actual stretch flanging
and may not accurately evaluate the workability including
stretch flanging workability. Accordingly, in the
invention described in Patent Document 4 where the
workability is evaluated also by the "strength-elongation
balance", acicular ferrite is precipitated in place of
bainite to enhance the impact resistance and with respect
5 to the stretch flanging workability, conversely, a void
offering a starting point for a crack may be likely to be
formed. Furthermore, because of acicular ferrite
precipitation, reduction in the ductility may not be
avoided.
10 [0012]
The present invention pays attention to the actual
stretch flanging as well, and an object of the present
invention is to provide a hot-rolled steel sheet with
excellent press formability, which can be kept from
15 cracking at the stretch flanging and has good hole
expandability comparable to conventional techniques, and
a production method thereof.
SOLUTION TO PROBLEM
20 [ 0 0 13 1
The present inventors believe that, in order to
encourage application of a high-strength hot-rolled steel
sheet to an underbody member of an automobile, it is
important to understand factors governing the
25 characteristics of respective workings applied and
reflect them in designing the structure of a hot-rolled
steel sheet, and made a large number of intensive
studies.
[ 0 0 14 ]
30 In the hole expanding and stretch flanging, a crack
generated in the edge part of a steel sheet may grow due
to ductile fracture. That is, a plurality of voids may
be formed and grow at the interface between martensite or
a hard second phase and a soft phase upon application of
35 a strain, and voids may be connected to each other,
whereby a crack may develop. Accordingly, forming a
structure composed of phases where the strength
difference between adjacent phases is small may be an
important factor in enhancing the hole expandability as
well as the stretch flanging workability.
[OOlS]
On the other hand, the present inventors have made
investigations on a structure factor affecting the
stretch flanging workability by performing a side bend
test simulating stretch flanging. As a result, it has
been found that even a steel sheet increased in the hole
10 expandability by forming a structure composed of phases
having a small strength difference is sometimes low in
the side bend elongation. It has been also found that
the side bend elongation is governed by the dispersed
state of either one or both of martensite and retained
15 austenite (hereinafter, sometimes referred to as MA), a
hard second phase of cementite, and a hard second phase
particle such as inclusion.
[0016]
In general, the hole expanding may be a working to
20 expand a bored hole, and the stretch flanging may be a
working to stretch a steel sheet marginal part when
forming a flange by bending a steel sheet edge part. In
either working, a strain may decrease toward the inside
of the workpiece from the edge part. The decrease ratio
25 here may be called a stain gradient. However, the
stretch flanging may be a working establishing a small
strain gradient as compared with the hole expanding and
therefore, paying attention to the strain gradient, a
fine crack generated in the punching edge part may be
30 more likely to develop to the inside in the stretch
flanging than in the hole expanding.
[0017]
It has been thus found that even when the hole
expandability is excellent, a crack develops at the
35 stretch flanging to cause fracture depending on the
existing state (dispersed state) of a phase or particle
contributing to crack propagation, such as MA, cementite
and inclusion in the steel sheet. That is, MA, cementite
and an inclusion may work out to a starting point for
void formation and therefore, be preferably reduced as
much as possible. However, because of, for example,
addition of carbon so as to achieve high strength or
limitation of the refining technology, complete
elimination of such a phase or a particle may be
difficult.
Also, in the conventional techniques described
above, hole expandability may be equated with stretch
flanging workability and since relatively good hole
expandability may be obtained, elimination of MA,
cementite and an inclusion and existing condition thereof
had not been studied.
[ 0 0 18 ]
Accordingly, the present inventors have made further
intensive studies on the technique for improving the
existing state (dispersed condition) of MA, cementite and
an inclusion and the stretch flanging workability. As a
result, a void formation/connection index L (formula 1)
reflecting the dispersed state of MA, cementite and an
inclusion has been proposed, and it has been found that
this index exhibits a strong correlation with the side
bend elongation indicating stretch flangeability. That
is, the textural structure is controlled to satisfy the
strength and hole expandability and at the same time,
have a high numerical value as the void
formation/connection index L, whereby a hot-rolled steel
sheet having excellent press formability and good hole
expandability can be obtained.
[0019]
(formula 1)
no, ni and nNA: number densities (pieces/pm2) of a
cementite, an inclusion and MA, respectively,
Do, Di and Dm: average diameters (pm) of a cementite,
an inclusion and MA, respectively, and
Lo, Li and LMA: average intervals (pm) of a cementite,
an inclusion and MA, respectively.
[0020]
Also, the present inventors have ascertained, from
their verification of the relationship between the void
formation/connection index L and the side bend
elongation, that when the void formation/connection index
L becomes 11.5 (pm-l) or more, the side bend elongation
gradient is increased and more sensitively affects the
stretch flange workability. Accordingly, it has been
found that by controlling the structure to have a void
formation/connection index L of 11.5 (p-l)or more, voids
formed are less likely to be connected and higher stretch
flanging workability is obtained.
The present invention has been accomplished based on
these findings, and the gist of the present invention
resides in the followings.
[0021]
(1)
A hot-rolled steel sheet with excellent press
formability, comprising, in mass%,
C: 0.03 to 0.10%,
Si: 0.5 to 1.5%,
Mn: 0.5 to 2.0%, and
the balance of Fe and unavoidable impurities,
as impurities,
P: limited to 0.05% or less,
S: limited to 0.01% or less,
Al: limited to 0.30% or less,
N: limited to 0.01% or less,
wherein in the metallic structure of said steel sheet,
the area fraction of ferrite is 70% or more, the area
fraction of bainite is 30% or less, the area fraction of
either one or both of martensite and retained austenite
is 2% or less, and
with regard to respective average intervals, average
diameters and number densities of cementite, an inclusion
5 and either one or both of martensite and retained
austenite, a void formation/connection index L defined by
formula 1 is 11.5 or more:
(formula 1)
no, ni and nm: number densities of a cementite, an
inclusion and either one or both of martensite and
retained austenite, respectively, and the unit is
pieces/pm2;
15 Do, Di and Dm: average diameters of a cementite, an
inclusion and either one or both of martensite and
retained austenite, respectively, and the unit is pm; and
Lo, Li and Lm: average intervals of a cementite, an
inclusion and either one or both of martensite and
20 retained austenite, respectively, and the unit is pm.
(2)
The hot-rolled steel sheet with excellent press
formability as set force in (I), wherein said steel sheet
further comprises one or more of, in mass%,
Nb: 0.08% or less,
Ti: 0.2% or less,
V: 0.2% or less,
W: 0.5% or less,
Mo: 0.4% or less,
Cu: 1.2% or less,
Ni: 0.6% or less,
Cr: 1.0% or less,
B: 0.005% or less,
Ca: 0.01% or less, and
REM: 0.01% or less.
(3
The hot-rolled steel sheet with excellent press
5 formability as set force in (1) or (2), wherein in said
steel sheet, the X-ray random intensity ratios of {211)
plane parallel to a surface of the steel sheet at the 1/2
thickness position, the 1/4 thickness position and the
1/8 thickness position in the thickness direction from
10 the surface are 1.5 or less, 1.3 or less, and 1.1 or
less, respectively.
(4)
A method for producing a hot-rolled steel sheet with
15 excellent press formability, comprising:
a step of subjecting a slab made of a steel
comprising, in mass%,
C: 0.03 to 0.10%,
Si: 0.5 to 1.5%,
Mn: 0.5 to 2.0%, and
the balance of Fe and unavoidable impurities,
as impurities,
P: limited to 0.05% or less,
S: limited to 0.01% or less,
Al: limited to 0.30% or less,
N: limited to 0.01% or less,
reheating the slab to a temperature of 1,150°C or more and
holding the slab for 120 minutes or more, thereafter
performing rough rolling the slab,
30 a step of performing finish rolling such that the
end temperature becomes between Ae3-30°C and Ae3+300C,
a step for performing primary cooling to a
temperature between 510 and 700°C at a cooling rate of
50°C/s or more,
35 a step of performing air cooling for 2 to 5 seconds,
a step of performing secondary cooling at a cooling
rate of 30°C/s or more,
a step of performing coiling at a temperature of 500
to 600°C, and
a step of performing cooling to 200°C or less at an
5 average cooling rate of 30°C/h or more to obtain a steel
sheet, wherein:
Ae3 = 937-477C+56Si-20Mn-16Cu-l5Ni-
5Cr+38Mo+125V+136Ti-l9Nb+198A1+3315B (formula 2)
wherein C, Si, Mn, Cu, Ni, Cr, Mo, V, Ti, Nb, A1 and B
10 represent the contents of respective elements, and the
unit is mass%.
(5)
The method for producing a hot-rolled steel sheet
15 with excellent press formability as set force in (4),
wherein the total pass-to-pass time of final 4 stands in
said finish rolling is 3 seconds or less.
(6)
20 The method for producing a hot-rolled steel sheet
with excellent press formability as set force in (4) or
(5), wherein said slab further comprises one or more of,
in mass%,
Nb: 0.08% or less,
Ti: 0.2% or less,
V: 0.2% or less,
W: 0.5% or less,
Mo: 0.4% or less,
Cu: 1.2% or less,
Ni: 0.6% or less,
Cr: 1.0% or less,
B: 0.005% or less,
Ca: 0.01% or less, and
REM: 0.01% or less.
The method for producing a hot-rolled steel sheet
with excellent press formability as set force in (4) or
(5), wherein with regard to respective average intervals,
average diameters and number densities of a cementite, an
5 inclusion and either one or both of martensite and
retained austenite in the metallic structure of said
steel sheet, the void formation/connection index L
defined by formula 1 is 11.5 or more:
(formula 1) ~ F I ,
ni and n,: number densities of a cementite, an inclusion
and either one or both of martensite and retained
austenite, respectively, and the unit is pieces/p.m2;
Do, Di and Dm: average diameters of a cementite, an
inclusion and either one or both of martensite and
retained austenite, respectively, and the unit is pm; and
Lo, Li and L,: average intervals of a cementite, an
inclusion and either one or both of martensite and
retained austenite, respectively, and the unit is p.m.
(8)
The method for producing a hot-rolled steel sheet
with excellent press formability as set force in (6),
wherein with regard to respective average intervals,
25 average diameters and number densities of a cementite, an
inclusion and either one or both of martensite and
retained austenite in the metallic structure of said
steel sheet, the void formation/connection index L
defined by formula 1 is 11.5 or more:
(formula 1)
no, ni and n,: number densities of a cementite, an
inclusion and either one or both of martensite and
retained austenite, respectively, and the unit is
Do, Di and Dm: average diameters of a cementite, an
inclusion and either one or both of martensite and
retained austenite, respectively, and the unit is pm; and
5 Lo, Li and Lm: average intervals of a cementite, an
inclusion and either one or both of martensite and
retained austenite, respectively, and the unit is pm.
ADVANTAGEOUS EFFECTS OF INVENTION
10 [0022]
According to the present invention, a high-strength
hot-rolled steel sheet excellent in the ductility, hole
expandability and stretch flangeability can be obtained.
15 BRIEF DESCRIPTION OF DRAWINGS
[0023]
[Fig. 11 Fig. 1 is a view showing the relationship
between the void formation/connection index and the side
bend elongation, where data having TS (tensile strength)
20 of 540 MPa or more, h of 110% or more and elongation at
break of 30% or more are plotted.
DESCRIPTION OF EMBODIMENTS
[0024]
25 The present invention pays attention to the actual
stretch flanging as well, and an object of the present
invention is to provide a hot-rolled steel sheet with
excellent press formability, which can be kept from
cracking at the stretch flanging and has good hole
30 expandability comparable to conventional techniques, and
a production method thereof. Accordingly, as for the
characteristics other than stretch flange workability,
the aim may be to have characteristics equivalent to
those of conventional steel sheets. Specifically, the
35 following numerical values equivalent to those of a
conventional steel having a tensile strength of a 540 MPa
d level may be set as the goals for targeted mechanical
characteristics.
Tensile strength: 540 MPa
Elongation at break: 30%
Hole expansion ratio: 110%
The stretch flanging workability may be evaluated by
sand bend elongation.
The present invention may be described in detail
below.
10 100251
[Void formation/Connection Index L]
As described above, even a hot-rolled steel sheet
improved in hole expandability by forming a structure
composed of phases small in the strength difference
15 between respective phases in the crystalline structure
may have low side bend elongation in some cases. In the
course of determining the reason thereof, it has been
found that the side bend elongation is governed by the
existing state (dispersed state) of either one or both of
20 martensite and retained austenite (hereinafter, sometimes
referred to as MA), a hard second phase such as
cementite, and a hard second phase particle such as
inclusion. The present inventors have discovered a void
formation/connection index L defined by formula 1 as an
25 indicator of existing state (dispersed state) of such a
second phase or inclusion or the like. The void
formation/connection index L that may become a key part
of the present invention is described below.
100261
The hole expanding may be a working to expand a
bored hole and in the hole expanding, the punching edge
part may be severely worked. The stretch flanging may be
a working to stretch a steel sheet marginal part when
forming a flange by bending a steel sheet edge part. The
35 stretch flanging may be a working establishing a small
strain gradient as compared with the hole expanding and
therefore, a fine crack generated in the punching edge
part may be likely to develop to the inside, leading to
fracture with a smaller strain amount than in the hole
expanding.
100271
Crack propagation may be caused due to connection of
voids formed starting from MA, a hard second phase such a
cementite, and a hard second particle such as inclusion
(hereinafter, unless otherwise indicated, the hard second
phase and the hard second particle are collectively
referred to as "hard second phase and the like").
Therefore, in the stretch flanging, control of this hard
second phase and the like is important more than in the
hole expanding. In other words, even when high hole
expandability may be realized by constituting a metallic
structure having phases small in the strength difference
between respective phases, only with this configuration,
high stretch flanging workability may not be obtained
depending on the distribution of MA, cementite and an
inclusion.
100281
From the results of investigation, the present
inventors have deduced that ease of connection of voids,
i.e, ease of crack propagation, is greatly affected by
the void formation/connection index L determined from the
dispersed state of the hard second phase and the like.
(formula 1)
no, ni and nm: number densities (pieces/p2) of a
cementite, an inclusion and either one or both of
martensite and retained austenite, respectively,
Do, Di and Dm: average diameters (pm) of a cementite,
an inclusion and either one or both of martensite and
retained austenite, respectively, and
Lo, Li and Lm: average intervals (pm) of a cementite,
an inclusion and either one or both of martensite and
retained austenite, respectively.
[0029]
In formula 1, with respect to each of MA, a
cementite and an inclusion, a value obtained by dividing
the average interval by the square of the average
5 diameter may be taken as the effective interval, and the
weighted average of effective intervals of MA, a
cementite and an inclusion may be taken as the void
formation/connection index L. The void
formation/connection index L may be qualitatively
10 described as follows. The probability of void generation
may be proportional to the surface area (D~)of the hard
second phase, and ease of connection of voids may be
inversely proportional to the distance between respective
phases (interval Lo between respective phases) .
15 Accordingly, (D~/L~ma)y be considered as an indicator of
ease of void formation/connection. The reciprocal
thereof may become an indicator of difficulty of void
formation/connection of, that is, an indicator of good
stretch flanging workability.
20 Here, using subscripts 8, i and MA for a cementite,
an inclusion and MA, respective average intervals Lo, Li
and Lm may be determined according to formula 3. In
formula 3, fe, fi and fm may represent area fractions of
cementite, an inclusion and MA, respectively, and Do, Di
25 and Dm may represent average diameters (pm) of a
cementite, an inclusion and MA, respectively. The area
fraction may be a ratio of each of a cementite, an
inclusion and MA, in the whole investigation range. The
average diameter may be an average value of a major axis
30 and a minor axis of each of a cementite, an inclusion and
MA investigated. The methods for measuring the area
fraction, number density and average interval may be
described in Examples later.
In formula 3, an average interval (pm) assuming an
35 isotropic distribution may be obtained.
[0030]
In the case where the hard second phase and the like
have the same size, ease of connection of voids formed
starting from such a phase may depend on the effective
interval, because as the effective interval is large,
voids may become more difficult to connect. Also, in the
present invention, a quotient obtained by dividing the
average interval by the square of the average diameter
may be taken as the effective interval (unit may be pm-I).
This is to reflect the finding that ease of connection of
voids may not be determined merely by an average interval
and as the size of the hard second phase and the like is
smaller, voids formed starting from such a phase may
become finer and difficult to connect. The reason why as
the size of the hard second phase and the like is
smaller, voids become difficult to connect may not be
clearly known but may be considered because as the void
size is smaller, the surface area of a void per unit
volume is larger, i.e, the surface tension is increased,
as a result, a void does not easily occur.
Also, when the hard second phase and the like are
small, not only a void may become difficult to grow but
also connection of voids may be less likely to occur.
Accordingly, as the hard second phase and the like are
smaller and as the void formation/connection index L is
larger, the strain amount leading to fracture may be
increased. The reason for the square of the average
diameter may be considered because stress generated
around the hard second phase and the like by working is
proportional to the size but, on the other hand, the
stress per unit surface area of the hard second phase and
the like is reduced and a void becomes difficult to grow.
[0032]
In addition, ease of void formation may differ
depending on the kind of the hard second phase and the
like, and it is confirmed that an inclusion may readily
form a void as compared with MA and cementite. Because
of this, the term of an inclusion on weighted averaging
may be multiplied by a coefficient. The coefficient may
be a ratio between the number of voids formed per one
inclusion and the number of voids formed per one
MA/cementite and was set to 2.1 from the observation
results.
As shown in Fig. 1, it has been confirmed that a
strong correlation exists between the void
formation/connection index L taking into account ease of
void formation and the side bend elongation.
Furthermore, it has been confirmed that the percentage
increase in the side bend elongation rises when the void
formation/connection index becomes 11.5 (pm-l) or more. In
other words, the stretch flanging workability can be
greatly improved by setting the void formation/connection
index L to 11.5 (pm-l) or more.
[0033]
The reason why the side bend elongation is greatly
enhanced when the void formation/connection index becomes
11.5 (pm-l) or more may be considered because connection
of voids is inhibited, but detailed reasons thereof may
not be clear. However, it is believed that the size of
the hard second phase and the like may affect the void
formation, more specifically, fine formation of the hard
second phase and the like may produce an effect that not
only connection of voids is less likely to occur but also
a void itself is hardly formed. Furthermore, the strain
amount leading to fracture may be attributed to
production/connection of voids originated in a hard
second phase and the like present in the steel material
structure and may be determined by the kind, amount and
size of the hard second phase and the like. Accordingly,
even when the ingredients of the steel material are
changed, the critical void formation/connection index at
which the effects of the present invention are obtained
may not be changed.
[0034]
Incidentally, MA and cementite of which area
fraction, average interval and average diameter must be
taken into account may be those having an area of 0.1 pn2
or more in the cross-section of the hot-rolled steel
sheet, because MA and cementite smaller than that may be
unlikely to significantly affect the side bend
elongation. The inclusion of which area fraction,
average interval and average diameter must be taken into
account may be an inclusion having an area of 0.05 pm2 or
more in the cross-section of the hot-rolled steel sheet,
15 because an inclusion smaller than that may be unlikely to
significantly affect the side bend elongation.
The area fraction, average interval and average
diameter may be determined by image analysis. A
measurement sample may be prepared by LePera etching in
20 the case of MA and picral etching in the case of
cementite, an optical micrograph of the sample may be
binarized, and the area fraction and the average diameter
can be determined using an image analysis software (for
example, Image Pro). As for the inclusion, the area
25 fraction and the average diameter can be determined using
a particle analysis software (for example, particle
finder) by FE-SEM. From the values obtained, the
interval assuming an isotropic distribution can be
obtained as the average interval.
30 [0035]
As described above with respect to the void
formation/connection index L, the stretch flanging
workability of a steel sheet may be evaluated also by the
void formation/connection index. The stretch
35 flangeability can be evaluated by the void
formation/connection index without confirming it by
actually testing the steel sheet, so that the quality
control efficiency for a steel sheet can be remarkably
enhanced.
[0036]
[Ingredients of Steel Sheet]
5 The hot-rolled steel sheet of the present invention
and the ingredients of a steel used for the production
thereof are described in detail below. Incidentally, " % "
that is the unit for the content of each ingredient means
"mass%".
10 [0037]
C: 0.03 to 0.10%
C may be an important ingredient for securing the
strength. If the C content is less than 0.03%, it may be
difficult to obtain sufficient strength, for example, a
15 strength of 540 MPa or more. On the other hand, if the C
content exceeds 0.10%, the hard second phase and the
like, such as cementite, may be excessively increased to
deteriorate the hole expandability. For this reason, the
C content is specified to be from 0.03 to 0.10%.
20 Incidentally, from the standpoint of securing the
strength, the C content may be preferably 0.05% or more,
more preferably 0.06% or more. Also, in order to
suppress an excessive increase of the hard second phase
and the like, such as cementite, as much as possible, the
25 C content may be preferably 0.08% or less, more
preferably 0.07% or less.
[0038]
Si: 0.5 to 1.5%
Si may be an important element for more successfully
30 securing the strength by solid solution strengthening.
If the Si content is less than 0.5%, it may be difficult
to obtain sufficient strength, for example, a strength of
540 MPa or more. On the other hand, if the Si content
exceeds 1.5%, the hole expandability may deteriorate,
35 because when Si is added in a large amount, the toughness
may be reduced to cause brittle fracture before
undergoing a large deformation. For this reason, the Si
content is specified to be from 0.5 to 1.5%.
Incidentally, from the standpoint of securing the
strength, the Si content may be preferably 0.7% or more,
more preferably 0.8% or more. Also, from the standpoint
5 of suppressing an excessive increase of the hard second
phase and the like as much as possible, the Si content
may be preferably 1.4% or less, more preferably 1.3% or
less.
[ 0 0 3 9 ]
10 Mn: 0.5 to 2.0%
Mn may be an important element for ensuring the
quenchability. If the Mn content is less than 0.5%,
bainite cannot be adequately produced and it may be
difficult to obtain sufficient strength, for example, a
strength of 540 MPa or more. Because, Mn is an austenite
former and may have an effect of suppressing ferrite
transformation, that is, if the Mn content is small,
ferrite transformation may excessively proceed, failing
in obtaining bainite.
On the other hand, if the Mn content exceeds 2.0%,
transformation may be extremely delayed, making it
difficult to produce ferrite, and ductility may
deteriorate. Because, Mn that is an austenite former may
have an effect of lowering the Ae3 point. For this
reason, the Mn content is specified to be from 0.5 to
2.0%. Furthermore, the Mn content may be preferably 1.0%
or more and preferably 1.6% or less.
[0040]
Al: 0.30% or less
A1 may function as a deoxidizing element, but if the
A1 content exceeds 0.3%, many inclusions such as alumina
may be formed and the hole expandability and stretch
flanging workability may deteriorate. A1 may be an
element that is desired to be eliminated, and even when
this element is unavoidably contained, the A1 content is
limited to 0.3% or less. The content may be preferably
limited to 0.15% or less, more preferably to 0.10% or
less. The lower limit of the A1 content may not be
particularly specified, but it may be technologically
difficult to reduce the content to less than 0.0005%.
[ 0 0 4 1 ]
5 P: 0.05% or less
P may be an impurity element, and if the P content
exceeds 0.05%, in the case of applying welding to the
hot-rolled steel sheet, embrittlement of the welded part
may become conspicuous. Accordingly, the P content may
10 be preferably as low as possible and is limited to 0.05%
or less. The content may be preferably limited to 0.01%
or less. Incidentally, the lower limit of the P content
may not be particularly specified, but reducing the
content to less than 0.0001% by a dephosphorization (P)
15 step or the like may be economically disadvantageous.
[0042]
S: 0.01% or less
S may be an impurity element, and if the S content
exceeds 0.01%, an adverse effect on the weldability may
20 become conspicuous. Accordingly, the S content may be
preferably as low as possible and is limited to 0.01% or
less. The content may be preferably limited to 0.005% or
less. If S is excessively contained, coarse MnS may be
formed and the hole expandability and stretch flanging
25 workability may be liable to deteriorate. Incidentally,
the lower limit of the S content may not be particularly
specified, but reducing the content to less than 0.0001%
by a desulfurization (S) step or the like may be
economically disadvantageous.
30 [0043]
N: 0.01% or less
N may be an impurity element and if the N content
exceeds 0.01%, coarse nitride may be formed and the hole
expandability and stretch flanging workability may
35 deteriorate. Accordingly, the N content may be
preferably as low as possible and is limited to 0.01% or
less. The content may be preferably limited to 0.005% or
4? less. As the N content is increased, a blow hole may be
more likely to be formed at the welding. The lower limit
of the N content may not be particularly specified, but
when the content is reduced to less than 0.0005%, the
5 production cost may significantly rise.
[ 0 0 4 4 1
In the hot-rolled steel sheet of the present
invention and the steel used for the production thereof,
the balance is Fe. However, at least one element
10 selected from Nb, Ti, V, W, Mo, Cu, Ni, Cr, B, Ca and REM
(rare earth metal) may be contained.
[0045]
Nb, Ti, V, W and Mo may be elements contributing to
more increasing the strength. The lower limits of the
15 contents of these elements are not particularly
specified, but for effectively increasing the strength,
the Nb content may be preferably 0.005% or more, the Ti
content may be preferably 0.02% or more, the V content
may be preferably 0.02% or more, the W content may be
20 preferably 0.1% or more, and the Mo content may be
preferably 0.05% or more. On the other hand, for
securing the moldability, the Nb content may be
preferably 0.08% or less, the Ti content may be 0.2% or
less, the V content may be preferably 0.2% or less, the W
25 content may be preferably 0.5% or less, and the Mo
content may be preferably 0.4% or less.
[0046]
Cu, Ni, Cr and B may be also elements contributing
to increasing the strength. The lower limits may not be
30 particularly specified, but in order to obtain an effect
of increasing the strength, it may be preferred to add
Cu: 0.1% or more, Ni: 0.01%, Cr: 0.01%, and B: 0.0002% or
more. However, the upper limits are Cu: 1.2%, Ni: 0.6%,
Cr: 1.0%, and B: 0.005%, because excessive addition may
35 deteriorate the moldability.
[0047]
Ca and REM may be elements effective in controlling
the morphologies of oxide and sulfide. The lower limits
of contents of these elements may not be particularly
specified, but in order to effectively perform the
morphology control, both the Ca content and the REM
content may be preferably 0.0005% or more. On the other
hand, for securing moldability, both the Ca content and
the REM content may be preferably 0.01% or less. Here,
REM as used in the present invention indicates La and a
lanthanoid series element. As REM, for example, a misch
metal may be added at the steelmaking stage. The misch
metal may contain La and an element of this series, such
as Ce, in a composite form. It may be also possible to
add metal La and/or metal Ce.
[ 0 0 4 8 ]
[Metal Texture]
The structure of the hot-rolled steel sheet
according to the present invention may be described in
detail below.
[0049]
Area Fraction of Ferrite: 70% or more
Ferrite may be a very important structure for
securing ductility. If the area fraction of ferrite is
less than 70%, sufficiently high ductility may not be
obtained. For this reason, the area fraction of ferrite
is specified to be 70% or more and may be preferably 75%
or more, still more preferably 80% or more. On the other
hand, if the area fraction of ferrite exceeds 90%,
bainite may lack, failing in securing the strength.
Also, C enrichment into austenite may proceed, as a
result, the strength of bainite may be excessively
increased and the hole expandability may deteriorate.
For this reason, the area fraction of ferrite may be
preferably 90% or less, more preferably 88% or less, and
the area fraction may be still more preferably 85% or
less, because deterioration of the hole expandability may
not occur.
[0050]
Area Fraction of Bainite: 30% or less
Bainite may be an important structure contributing
to strengthening. If the area fraction of bainite is
less than 5%, it may be difficult to obtain a
sufficiently high tensile strength, for example, a
tensile strength of 540 MPa or more. For this reason,
the area fraction of bainite may be preferably 5% or
more, more preferably 7% or more. On the other hand, if
the area fraction of bainite exceeds 30%, the area
fraction of ferrite may lack, failing in obtaining
adequate ductility. Accordingly, the area fraction of
bainite may be preferably 30% or less and from the
standpoint of securing ductility by ferrite, the area
fraction may be more preferably 27% or less, still more
preferably 25% or less.
[0051]
Area Fraction of MA (martensite-retained austenite): 2%
or less
MA may be either one or both of martensite and
retained austenite and can be observed, for example, as a
white part in an optical microscopic image of a sample
subjected to etching with a LePera reagent. Also, the
inclusion may include an oxide, a sulfide and the like,
such as MnS and A1203. These may contain, for example, an
impurity ingredient or an ingredient added for
deoxidization.
MA may be a structure that forms a void along with
deformation to deteriorate the hole expandability.
Accordingly, if the area fraction of MA exceeds 2%, such
deterioration of hole expandability may become
conspicuous. For this reason, the area fraction of MA is
specified to be 2% or less. The area fraction of MA may
be preferably smaller and may be preferably 1% or less,
more preferably 0.5% or less.
[0052]
Due to the structure control described above, a hotrolled
steel sheet with excellent press formability,
which is high in all of ductility, hole expandability and
side bend elongation, may be obtained. Accordingly,
application of a high-strength steel sheet to automotive
underbody components may be encouraged, and contribution
5 to improvement of fuel consumption and reduction of
carbon dioxide emission may be quite noticeable.
Furthermore, by controlling the following texture, a hotrolled
steel sheet with excellent press formability,
where the material anisotropy is small, may be obtained.
10 That is, in a steel having a predetermined
ingredient composition, when the steel is produced to
have a predetermined textural structure and have a void
formation/connection index L in a predetermined range (in
the present invention, 11.5 or more), a hot-rolled steel
15 sheet excellent not only in the hole expandability but
also in the stretch flanging workability can be produced.
[0053]
The texture may be an important factor relevant to
the material anisotropy. When there is a difference of
20 10% or more between the side bend elongation in the sheet
width direction and that in the rolling direction, for
example, a crack may be generated depending on the
forming direction of an actual component. In the steel
sheet, the X-ray random intensity ratios of (211) planes
25 parallel to steel sheet surfaces (rolling surfaces) at
the 1/2 thickness position, the 1/4 thickness position
and the 1/8 thickness position are specified to be 1.5 or
less, 1.3 or less, and 1.1 or less, respectively, whereby
the anisotropy of the side bend elongation can be reduced
30 and the difference thereof can be made to be 10% or less.
Here, the 1/2 thickness position, the 1/4 thickness
position and the 1/8 thickness position mean that the
distance in the thickness direction from the surface of
the hot-rolled steel sheet is located at the position of
35 1/2, the position of 1/4, and the position of 1/8,
respectively, of the thickness of the hot-rolled steel
sheet. In the side bend test, the strain amount allowing
@ a generated crack to penetrate in the sheet thickness
direction may be measured. Accordingly, in order to
decrease the anisotropy, it may be effective to reduce
the X-ray random intensity ratios at all sheet thickness
5 positions.
100541
[Production Method]
The production method for a hot-rolled steel sheet
of the present invention may be described below.
10 [0055]
A slab (steel billet) may be obtained by performing
ingot making and casting of a steel composed of the
above-described ingredients. As the casting, continuous
casting may be preferably performed in view of
15 productivity. Subsequently, the slab may be reheated at
a temperature of 1,150°C or more, held for 120 minutes or
more, and then hot-rolled. Reheating may be done because
heating at a temperature of 1,150°C or more for 120
minutes or more melts an inclusion such as MnS in the
20 slab and an inclusion even when produced in the
subsequent cooling process becomes fine. If the
reheating temperature is less than 1,150°C or the
reheating time is less than 120 minutes, a coarse
inclusion present in the slab may be not fully melted and
25 many inclusions may remain, failing in obtaining high
stretch flangeability. The upper limit of the reheating
temperature may be not particularly specified, but in
view of production cost, the temperature may be
preferably 1,300°C or less. The upper limit of the
30 holding time of reheating may be also not particularly
specified, but in view of the production cost, the
holding time may be preferably 180 minutes or less.
However, these may not apply when a slab cast by
continuous casting is hot transferred and directly
35 rolled. In this case, it may be sufficient when a
temperature state of 1,150°C or more including the
0 temperature after continuous casting is continuously held
for 120 minutes or more before rolling.
[0056]
In the hot rolling, rough rolling and then finish
5 rolling may be performed. At this time, the finish
rolling may be preferably performed such that the end
temperature (finish rolling temperature) becomes from Ae3-
30°C to Ae3+300C. If the finish rolling temperature
exceeds Ae3+300C, an austenite grain after
10 recrystallization may be coarsened, making it difficult
to cause ferrite transformation. On the other hand, if
the finish rolling temperature is less than Ae3-30°C,
recrystallization may be significantly delayed and the
anisotropy of side bend elongation may become large. In
15 order to eliminate these concerns, the finish rolling may
be preferably performed such that the end temperature
becomes from Ae3-25°C to Ae3+25"C, more preferably from
Ae3-20°C to Ae3+200C. Incidentally, Ae3 can be determined
according to the following formula 2:
20 Ae3 = 937-477C+56Si-20Mn-16Cu-l5Ni-
5Cr+38Mo+125V+136Ti-l9Nb+198A1+3315B (formula 2)
wherein C, Si, Mn, Cu, Ni, Cr, Mo, V, Ti, Nb, A1 and B
represent the contents (mass%) of respective elements.
[0057]
25 Also, in the finish rolling, the total of pass-topass
times in final 4 stands (in the case of a four-stand
tandem rolling mill, the total of transit times between
respective stands (three sections)) may be preferably 3
seconds or less. If the total pass-to-pass time exceeds
30 3 seconds, recrystallization may occur between passes and
since the strain cannot be accumulated, the
recrystallization rate after finish rolling may be
reduced. As a result, the X-ray random intensity ratio
of {211) plane may become high and the side bend
35 anisotropy may be increased.
[0058]
After the hot rolling, cooling of the rolled steel
sheet may be performed in two stages. These cooling
operations in two stages may be referred to as primary
cooling and secondary cooling, respectively.
[0059]
In the primary cooling, the cooling rate for the
steel sheet is specified to be 50°C/s or more. If the
cooling rate in the primary cooling is less than 50°C/s, a
ferrite grain may grow large and the nucleation site of
cementite may decrease, as a result, cementite may be
coarsened, failing in obtaining a void
formation/connection index L of 11.5 (pn-') or more. In
order to more reliably prevent the coarsening of
cementite, the lower limit of the cooling rate may be
preferably 60°C/s or more, more preferably 70°C/s or more.
The upper limit of the cooling rate in the primary
cooling may be not particularly specified, but the upper
limit may be preferably set to 300°C/s or less in the
practical range.
[0060]
The primary cooling may be preferably started
between 1.0 seconds and 2.0 seconds after the completion
of hot rolling. If the cooling is started before the
elapse of 1.0 seconds, recrystallization may not proceed
sufficiently, as a result, the random intensity ratio may
become large and the anisotropy of side bend elongation
may be increased. On the other hand, if the cooling is
started after the elapse of 2.0 seconds, the y grain after
recrystallization may be coarsened and therefore, the
strength can be hardly secured. In order to more
unfailingly achieve these effects, the lower limit of the
elapse time after hot rolling to start of primary cooling
may be preferably 1.2 seconds, more preferably 1.3
seconds, and the upper limit of the elapse time may be
preferably 1.9 seconds, more preferably 1.8 seconds.
[0061]
The primary cooling stop temperature is specified to
be from 510 to 700°C. When the cooling is stopped at a
temperature of more than 700°C, ferrite grain growth may
proceed and the nucleation site of cementite may
5 decrease, as a result, cementite may be coarsened,
failing in obtaining a void formation/connection index L
of 11.5 (pm-l) or more. Also, sufficient side bend
elongation may not be obtained.
For the fine formation of cementite or MA, the
10 primary cooling stop temperature may be preferably as low
as possible. For this reason, the primary cooling stop
temperature may be preferably 650°C or less, more
preferably 620°C or less. The stop temperature may be
still more preferably 600°C or less, because finer
15 cementite or MA may be obtained.
On the other hand, if the cooling is stopped at a
temperature of less than 510°C, ferrite transformation may
not proceed and since the volume percentage of bainite
may be increased, ductility may deteriorate. For the
20 fine formation of cementite or MA, the primary cooling
stop temperature may be preferably as low as possible
but, in view of ferrite transformation ratio, the
temperature cannot be too much low. For this reason, the
lower limit of the primary cooling stop temperature may
25 be preferably 52O0Cf more preferably 530°C. The primary
cooling stop temperature may be still more preferably
550°C or more, and in this case, ferrite transformation
may proceed and the effect of subsequent air cooling may
be obtained easily.
30 [0062]
Between the primary cooling and the secondary
cooling, air cooling for 2 to 5 seconds is performed. If
the air cooling time is less than 2 seconds, ferrite
transformation may not proceed sufficiently and adequate
35 elongation may not be obtained. On the other hand, if
the air cooling time exceeds 5 seconds, pearlite may be
0 produced and bainite may not be obtained, leading to
decrease in the strength. Here, air cooling means
leaving to stand in the air, so-called radiational
cooling, and the cooling rate may be approximately from 4
Thereafter, secondary cooling is performed. The
cooling rate in the secondary cooling is specified to be
30°C/s or more. If the cooling rate is less than 30°C/s,
10 the growth of cementite may be promoted, and a void
formation/connection index L of 11.5 (pm-l) or more may
not be obtained. In order to unfailingly prevent the
growth of cementite, the cooling rate may be preferably
40°C/s or more, more preferably 50°C/s or more. The upper
15 limit of the cooling rate in the secondary cooling may be
not particularly specified, but the upper limit may be
preferably set to 300°C/s or less in the practical range.
[0064]
After the secondary cooling, the steel sheet may be
20 wound into a coil form. Accordingly, the end temperature
of secondary cooling may be almost the same as the
coiling start temperature. The coiling start temperature
can be set to be from 500 to 600°C. If the coiling start
temperature exceeds 600°C, bainite may lack and sufficient
25 strength cannot be secured. From the standpoint of
eliminating these concerns, the upper limit of the
coiling start temperature may be preferably 590°C, more
preferably 580°C.
[0065]
30 On the other hand, if the coiling start temperature
is less than 500°C, bainite may become excessive and not
only the hole expandability may deteriorate but also the
stretch flanging workability ma be worsened.
Furthermore, if the coiling start temperature is a low
35 temperature of less than 500°C, production of acicular
ferrite may be readily promoted. As described above,
acicular ferrite may be likely to allow for production of
a void working out to a starting point of a crack, which
may lead to worsening of the stretch flangeability and
reduction in the ductility. In order to eliminate these
concerns, the coiling start temperature may be preferably
510°C, more preferably 520°C or more, and when the
temperature is 530°C or more, production of acicular
ferrite can be greatly suppressed.
[0066]
The average cooling rate from the coiling start
temperature until reaching 200°C may be 30°C/h or more.
If this average cooling late is less than 30°C/h,
cementite may excessively grow, and a void
formation/connection index L of 11.5 (pn-l) or more may
not be obtained. In turn, adequate side bend elongation
may not be obtained. Incidentally, the method for
controlling the cooling rate may not be particularly
limited. For example, a coil obtained by coiling may be
cooled directly with water. In addition, as the mass of
the coil is larger, the cooling rate may be lower, and
therefore, it may be also possible to reduce the mass of
the coil and thereby increase the cooling rate.
While the invention has bee described in detail in
the foregoing pages, the present invention may not be
limited to these embodiments. Any embodiment may be
employed without limitation as long as it has the
technical characteristics of the present invention.
Also, the production line may have its inherent
characteristics and therefore, in the production method,
minor adjustments may be made in the characteristics
inherent in the production line based on the abovedescribed
production method so that the void
formation/connection index L proposed in the present
invention can fall in the predetermined range (in the
present invention, 11.5 or more).
EXAMPLES
[0067]
Examples performed by the present inventors may be
5 described below. In these Examples, the conditions and
the like may be an example employed for verifying the
practicability and effects of the present invention, and
the present invention may not be limited thereto.
[0068]
10 First, a slab (Steels A to R) was produced by
casting a steel having chemical ingredients shown in
Table 1. Subsequently, the slab was hot-rolled under the
conditions shown in Table 2 (Table 2 includes Table 2-1
and Table 2-2) to obtain a hot-rolled steel sheet (Test
15 Nos. 1 to 40).

[0071]
[Table 2-21
Table 2-2
Test
No.
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
Steel
I
I
I
I
I
I
J
K
L
M
N
0
P
Q
R
S
T
U
V
W
W
W
W
W
SRT
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
Heating
Time
135
126
125
129
128
127
121
145
130
137
131
135
148
130
137
135
123
130
121
123
125
130
123
122
Total
Passto-
Pass
Time
2 . 1 5
2 . 6 1
2.42
2 . 6 5
2 . 7 5
2 . 1 6
2 . 5 9
2.48
2.17
2 . 2 3
2 . 1
2 . 2 6
2 . 4 3
2 . 5 5
2.42
2.88
2 . 1
2 . 1 6
2 . 1
2 . 4 5
2.48
2.17
2 . 6 9
2 . 6 9
End
Temperature
of Finish
Rolling
952
928
94 6
965
92 3
9 3 8
948
953
928
962
97 9
94 3
975
972
972
92 6
983
932
952
942
9 3 5
94 4
968
92 9
Time Until
Start of
Primary
Cooling
1 . 5
1 . 7
1 . 7
1 . 8
1 . 9
1 . 5
1 . 5
1 . 7
1 . 9
1 . 7
1 . 6
1 . 9
1 . 5
1 . 9
1 . 7
1 . 9
1 . 6
1 . 8
1 . 6
1 . 9
1 . 7
1 . 7
1 . 8
2
Primary
Cooling
Rate
6 5
6 5
5 7
6 1
5 1
5 4
5 8
5 4
5 2
5 2
5 9
5 0
5 4
62
5 7
5 5
5 1
5 5
5 6
5 4
4 7
5 2
4 6
5 8
Primary
Cooling
Stop
Temperature
6 6 1
54 4
6 12
561
5 98
640
5 6 6
532
581
514
543
596
5 3 3
618
658
64 0
683
644
632
64 9
618
616
64 0
614
Air
Cooling
Time
5
4
5
3
4
2
4
2
3
2
3
3
4
3
4
3
4
4
4
4
4
4
4
4
Secondary
Cooling
Rate
4 0
4 3
3 9
4 1
3 2
4 5
4 4
3 5
4 0
4 3
3 9
4 1
3 3
3 1
3 5
3 4
3 5
3 8
3 9
4 0
3 4
4 4
4 4
3 1
'Oiling
Temperature
630
521
5 0 0
529
574
599
545
512
559
500
526
574
512
579
5 7 8
566
574
577
597
589
5 98
581
598
589
Cooling
Rate from
CT to
2OO0C
31
25
4 1
4 1
4 6
4 7
4 8
5 0
3 7
3 8
3 8
4 7
4 4
4 7
3 5
4 1
3 7
3 8
3 3
3 8
33
27
48
25
Remarks
Comp. Ex.
Comp.Ex.
Invent ion
Invent ion
Invention
Invention
Invent ion
Invent ion
Invent ion
Invention
Invention
Invent ion
Invent ion
Invent ion
Invention
Invention
Invent ion
Invent ion
Invention
Invention
Comp. Ex.
Comp. Ex.
Comp. Ex.
Comp. Ex.
A sample was collected from each hot-rolled steel
sheet, and the cross-section of the sheet thickness in
the rolling direction, which was taken as the observation
5 surface, was polished and then subjected to etching by
various reagents to observe the metallic structure,
whereby evaluations of MA, cementite (carbide) and an
inclusion were preformed. The results obtained are shown
in Table 3 (Table 3 includes Table 3-1 and Table 3-2).
10 [0073]
The area fraction of ferrite and the area fraction
of pearlite were measured by an optical micrograph at the
1/4 thickness position of the sample etched by Nital
reagent. The area fraction (fm), average diameter (Dm)
15 and number density (nm) of MA were measured by image
analysis of an optical micrograph at the magnification of
500 time at the 1/4 thickness position of the sample
etched by LePera reagent. At this time, the measurement
visual field was set to 40,000 pm2 or more, and MA having
20 an area of 0.1 pm2 or more was taken as the measuring
object. The area fraction of the remaining structure
except for ferrite, pearlite and MA was used as the area
fraction of bainite.
[0074]
25 The area fraction (fe), average diameter (Do) and
number density (no) of cementite were measured by image
analysis of an optical micrograph at the magnification of
1,000 time at the 1/4 thickness position of the sample
etched by picral reagent. The measurement visual field
30 was set to 10,000 p.m2 or more, and measurement of two or
more visual fields was performed per one sample.
Cementite having an area of 0.1 pm2 or more was taken as
the measuring object.
[0075]
35 The area fraction (fi), average diameter (Di) and
number density (ni) of an inclusion were measured by
42 particle analysis (particle finder method) in the region
of 1.0 mm x 2.0 rnm at the 1/4 thickness position of the
cross-section of sheet thickness in the rolling
direction. At this time, an inclusion having an area of
5 0.05 pm2 or more was taken as the measuring object.
[0076]
Incidentally, MA and cementite having area of 0.1 pm2
or more were taken as the measuring object, because, as
described above, MA and cementite smaller than that may
10 not greatly affect the side bend elongation. On the
other hand, an inclusion having an area of 0.05 pm2 or
more was taken as the measuring object, because an
inclusion may more readily form a void than MA and
cementite and affect the side bend elongation.
15 [0077]
The void formation/connection index was calculated
according to formula 1 and formula 2.

(Continued)
a
rn
X
L4
rd
$
p:
Id
a u - ; a c *
4 2 . 2 -
C,
W
$
.d
m
W
z n
.d
3
0
H - >
4 g
P.W.- .
.d
. . . . . . . . . . . . . . . . . . . . . . . .
w w m m m m o m m w m m m w r - w m w w w w w m w
;:cG
.d -4
.
4 2 . 2 -
U
.d
m m K r w w N m 4 N
~ ~ - ~ ~ ~ ~ ~ m w ~ w m w m m ~ ~ ~ m w r d d 4 4 4 4 4 4 d 4 4 d d 4 4 d d d d d d d d d
. . . . . . . . . . . . . . . . . . . . . . . .
.d
. . . .
3 -4
C, Q
U G
- O. m. m. m. 4. 4.m. m. w. N. w. m. w. m. ~. m. m. m. .m .N .m.m.w.w 4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
. . . . . . . . . . . . . . . . . . . . . . . .
4
a,
aJ H H H H H H b ~ d ~ z o b o ~ r n b 3 > ~ 5 3 3 5
C,
V)
C, ; 6 w r - w m o d ~ m - m w ~ m m o d ~ m - m w r - w m
B 2
~ ~ ~ ~ m m m m m m m m m m - - - - - - - - - -
a,:
%+I -
> a - 4 g
a ,
rn
c \ I m o ] o w ~ o ~ m w ~ m ~ m m w c J w
. P W P W : W W ~ W P
0
4 W d - O m d W
.d
. . . . . . . . . . . . . . . . . .
o o o O O ~ o o ~ O ~ o o o o ~ d d O O o o o o
e. w. .r .- .w .w.m. .o .~.~.~. w. .w .w. o. w. .m.~. ~. o. ~ m m m m
. . . . . . . .
d a-J U o 3! ,
4h4J
.d
. . . . .
~ ~ ~ ~ ~ ~ r - r - ~ ~ ~ w ~ ~ ~ r - r w ~ r - r .,
.4
C , + J C , C , U U + J + J C , ~ C , 4 J C , C , ~ U U C ,
U U H H H H H H H H H H H H H H H H H H V U V V
d O 4 d O d 4 d O O 4 O r l O O O d O ~ 0 O d O d
o o o o o o o o o o o o o o o o o o q o o o o o
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ° ~ 0 0 0 0
m d m m q w ~ ~
0 0 0 0 0 0 0 0 ° 0 0 0 0 0 0 0 0 0 0 0 0 ~ 0 0
w.w.m.m.m.m.d .m .m .r -
0 0 0 0 0 0 d 0 d 0
. C C C C G C d C G C C G G G C C C d - - . .
X X 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 X X X X
4
. C C C C C C C G C G C C C C C C C C - . . a
a a a ~ a ~ a ~ a ~ a , a , a ~ a a , a a ~ a ~ a , a ~ a a , a , a , a a a a
E E > > > > > > > > > > > > > > > > > > E E E E
O O C G C C C C G C C C G C C C C C C 6 O O O O
. W W ~ T : ~ P W P
m ~ d m d 4 r - W W N
.,
.4
d w m N . . . .
. . . . . . . . . .
,m.,m., . .
,0000
.4
~ C D W . . .
.,
.d
0.,-r.- .
0 4 0 0
4
.,
.d 4 4 W W
m m d m d
. . . . .
W W
(Continued)
a
Also, various mechanical characteristics were
evaluated. The results obtained are shown in Table 4.
[ 0 0 8 0 ]
The tensile strength and elongation at break were
measured in accordance with JIS Z 2241 by using No. 5
test specimen of JIS Z 2201 collected perpendicularly to
the rolling direction from the center in the sheet width
direction.
The hole expansion percentage was evaluated in
accordance with the test method described in JFST 1001-
1996 of JFS Standard by using a hole expansion test
specimen collected from the center in the sheet width
direction.
15 The side bend elongation was evaluated by the method
described in Kokai No. 2009-145138. In this method, a
strip-like steel billet was collected from the hot-rolled
steel sheet in two directions, that is, the rolling
direction and a direction (sheet width direction)
20 perpendicular to the rolling direction, and scribe lines
were drawn on a surface of the steel billet.
Subsequently, the widthwise edge part in the longitudinal
center part of the steel billet was punched out in a
semicircular shape, and the punched end face was
25 subjected to tensile bending to generate a crack
penetrating the sheet thickness. The strain amount until
generation of the crack was measured based on the
previously drawn scribe lines.
[ 0081 1
[Table 41
Table 4
As seen in Tables 3 and 4, in the tests where the
conditions of the present invention were satisfied, all
5 of tensile strength, elongation, hole expandability and
side bend elongation were excellent. However, in Test
Nos. 8, 12 and 18, anisotropy of the side bend elongation
was confirmed due to slight difference in the production
conditions.
10 [ 0 0 8 3 ]
On the other hand, in Test No. 1 where the C content
was lower than the range of the present invention, a
strength of 540 MPa or more was not obtained.
In Test No. 2 where the C content exceeded the range
15 of the present invention, the area fraction of bainite
became higher than the range of the present invention,
and the ductility and hole expansion percentage were low.
[ 0 0 8 4 1
In Test No. 3 where the Si content was lower than
20 the range of the present invention, cementite was
excessively produced, and the void formation/connection
index L became lower than the range of the present
invention. Therefore, despite a high hole expansion
percentage, a side bend elongation of 70% or more was not
25 obtained.
In Test No. 4 where the Si content was higher than
Test
No. tion at
Strength
tion in Remarks
sion Sheet
Break
(MPa)
( % )
Percent- Width
age ( % ) Direc-
Steel
Mechanical Characteristics
Tensile
Elonga-
Hole
Expan-
Side
Bend
Elongation
in
Side
Bend
Elonga-
Side Bend
Anisotropy,
@ the range of the present invention, hole expandability of
110% or more was not obtained.
[0085]
In Test No. 5 where the Mn content was lower than
5 the range of the present invention, bainite was little
produced, and a strength of 540 MPa or more was not
obtained.
In Test No. 6 where the Mn content was higher than
the range of the present invention, a hard second phase
10 was excessively produced, and an elongation of 30% or
more was not obtained. That is, the ductility was low.
[0086]
In Test No. 7 where the reheating temperature of the
slab was lower than the range of the present invention,
15 the void formation/connection index L became smaller than
the range of the present invention, and a side bend
elongation of 70% or more was not obtained.
[ 0 0 8 7 ]
In Test No. 16 where the cooling rate of secondary
20 cooling was lower than the range of the present
invention, coarse cementite was produced, the void
formation/connection index L became smaller than the
range of the present invention, and a side bend
elongation of 70% or more was not obtained.
25 [0088]
In Test No. 17 where the reheating time of the slap
was shorter than the range of the present invention, the
void formation/connection index L became smaller than the
range of the present invention, and a side bend
30 elongation of 70% or more was not obtained.
[0089]
In Test No. 19 where the end temperature of finish
rolling was higher than the range of the present
invention, ferrite transformation was greatly delayed,
35 and the elongation was low. That is, the ductility was
low.
[0090]
43 In Test Nos. 20, 46 and 48 where the cooling rate of
primary cooling was lower than the range of the present
invention, a coarse carbide was produced, the void
formation/connection index L became smaller than the
5 range of the present invention, and a side bend
elongation of 70% or more was not obtained.
[0091]
In Test No. 21 where the primary cooling stop
temperature was lower than the range of the present
10 invention, ferrite transformation did not proceed, and
the elongation was low. That is, the ductility was
worsened.
In Test No. 22 where the primary cooling stop
temperature was higher than the range of the present
15 invention, a second phase was coarsened, and the side
bend elongation was reduced.
[0092]
In Test No. 23 where the air cooling time was
shorter than the range of the present invention, ferrite
20 transformation did not proceed, and the elongation was
low. That is, the ductility was worsened.
In Test No. 24 where the air cooling time was longer
than the range of the present invention, pearlite was
produced, and bainite was not obtained, as a result, the
25 strength was reduced.
In Test No. 25 where the coiling temperature was
lower than the range of the present invention, bainite
became excessive, and the ductility was low. In Test No.
26 where the coiling temperature was higher than the
30 range of the present invention, a strength of 540 MPa or
more was not obtained. Also, a carbide was coarsened,
and the side bend elongation was low.
[0093]
In Test Nos. 27, 47 and 49 where the cooling rate
35 after coiling was lower than the range of the present
invention, cementite was coarsened, the void
formation/connection index L became smaller than the
0 range of the present invention, and a side bend
elongation of 70% or more was not obtained.
[0094]
Fig. 1 shows the results where out of the
5 measurement results obtained in these tests, the tensile
strength was 540 MPa or more and at the same time, the
hole expansion percentage was 110% or more.
[0095]
The present invention has bee described in detail in
10 the foregoing pages. Needless to say, implementation of
the present invention may not be limited to the
embodiments illustrated in the description of the present
invention.
15 INDUSTRIAL APPLICABILITY
[0096]
According to the present invention, in regard to a
high-tensile steel not lower than 540 MPa class, a steel
sheet with excellent press formability, which is easily
20 workable and has not only hole expandability but also
stretch flanging workability, can be produced.
Accordingly, the present invention can be utilized not
only in the iron and steel industry but also in wide
range of industries such as the automobile industry using
25 a steel sheet.

CLAIMS
[Claim 11
A hot-rolled steel sheet with excellent press
formability, comprising, in mass%,
5 C: 0.03 to 0.10%,
Si: 0.5 to 1.5%,
Mn: 0.5 to 2.0%, and
with the balance of Fe and unavoidable impurities,
as impurities,
10 P: limited to 0.05% or less,
S: limited to 0.01% or less,
Al: limited to 0.30% or less,
N: limited to 0.01% or less,
wherein in the metallic structure of said steel sheet,
15 the area fraction of ferrite is 70% or more, the area
fraction of bainite is 30% or less, the area fraction of
either one or both of martensite and retained austenite
is 2% or less, and
with regard to respective average intervals, average
20 diameters and number densities of a cementite, an
inclusion and either one or both of martensite and
retained austenite, a void formation/connection index L
defined by formula 1 is 11.5 or more:
(formula 1)
ne, ni and nm: number densities of a cementite, an
inclusion and either one or both of martensite and
retained austenite, respectively, and the unit is
30 pieces/pm2;
Do, Di and Dm: average diameters of a cementite, an
inclusion and either one or both of martensite and
retained austenite, respectively, and the unit is pm; and
Lo, Li and Lm: average intervals of a cementite, an
inclusion and either one or both of martensite and
retained austenite, respectively, and the unit is pm.
[Claim 21
The hot-rolled steel sheet with excellent press
5 formability as set force in claim 1, wherein said steel
sheet further comprises one or more of, in mass%,
Nb: 0.08% or less,
Ti: 0.2% or less,
V: 0.2% or less,
W: 0.5% or less,
Mo: 0.4% or less,
Cu: 1.2% or less,
Ni: 0.6% or less,
Cr: 1.0% or less,
B: 0.005% or less,
Ca: 0.01% or less, and
REM: 0.01% or less.
[Claim 31
The hot-rolled steel sheet with excellent press
formability as set force in claim 1 or 2, wherein in said
steel sheet, the X-ray random intensity ratios of (211)
plane parallel to a surface of the steel sheet at the 1/2
thickness position, the 1/4 thickness position and the
1/8 thickness position in the thickness direction from
the surface are 1.5 or less, 1.3 or less, and 1.1 or
less, respectively.
[Claim 41
A method for producing a hot-rolled steel sheet with
excellent press formability, comprising:
a step of subjecting a slab made of a steel
comprising, in mass%,
C: 0.03 to 0.10%,
Si: 0.5 to 1.5%,
Mn: 0.5 to 2.0%, and
the balance of Fe and unavoidable impurities,
O as impurities,
P: limited to 0.05% or less,
S: limited to 0.01% or less,
Al: limited to 0.30% or less,
N: limited to 0.01% or less,
reheating the slab to a temperature of 1,150°C or more and
holding the slab for 120 minutes or more, thereafter
performing rough rolling the slab,
a step of performing finish rolling such that the
10 end temperature becomes between Ae3-30°C and Ae3+300C,
a step for performing primary cooling to a
temperature between 510 and 700°C at a cooling rate of
50°C/s or more,
a step of performing air cooling for 2 to 5 seconds,
15 a step of performing secondary cooling at a cooling
rate of 30°C/s or more,
a step of performing coiling at a temperature of 500
to 600°C, and
a step of performing cooling to 200°C or less at an
20 average cooling rate of 30°C/h or more to obtain a steel
sheet, wherein:
Ae3 = 937-477C+56Si-20Mn-16Cu-l5Ni-
5Cr+38Mo+125V+136Ti-l9Nb+198A1+3315B (formula 2)
wherein C, Si, Mn, Cu, Ni, Cr, Mo, V, Ti, Nb, A1 and B
25 represent the contents of respective elements, and the
unit is mass%.
[Claim 51
The method for producing a hot-rolled steel sheet
30 with excellent press formability as set force in claim 4,
wherein the total pass-to-pass time of final 4 stands in
said finish rolling is 3 seconds or less.
[Claim 61
35 The method for producing a hot-rolled steel sheet
with excellent press formability as set force in claim 4
a or 5, wherein said slab further comprises one or more of,
in mass%,
Nb: 0.08% or less,
Ti: 0.2% or less,
V: 0.2% or less,
W: 0.5% or less,
Mo: 0.4% or less,
Cu: 1.2% or less,
Ni: 0.6% or less,
Cr: 1.0% or less,
B: 0.005% or less,
Ca: 0.01% or less, and
REM: 0.01% or less.
15 [Claim 71
The method for producing a hot-rolled steel sheet
with excellent press formability as set force in claim 4
or 5, wherein with regard to respective average
intervals, average diameters and number densities of a
20 cementite, an inclusion and either one or both of
martensite and retained austenite in the metallic
structure of said steel sheet, the void
formation/connection index L defined by formula 1 is 11.5
or more:
25
(formula 1)
no, ni and nm: number densities of a cementite, an
inclusion and either one or both of martensite and
30 retained austenite, respectively, and the unit is
Do, Di and Dm: average diameters of a cementite, an
inclusion and either one or both of martensite and
retained austenite, respectively, and the unit is pm; and
Lo, Li and Lm: average intervals of a cementite, an
- 53 -
a inclusion and either one or both of martensite and
retained austenite, respectively, and the unit is pm.
[Claim 81
5 The method for producing a hot-rolled steel sheet
with excellent press formability as set force in olhim 6,
wherein with regard to respective average intervals,
average diameters and number densities of a cementite, an
inclusion and either one or both of martensite and
10 retained austenite in the metallic structure of said
steel sheet, the void formation/connection index L
defined by formula 1 is 11.5 or more:
(formula 1)
no, n, and nm: number densities of a cementite, an
inclusion and either one or both of martensite and
retained austenite, respectively, and the unit is
Do, Di and Dm: average diameters of a cementite, an
inclusion and either one or both of martensite and
retained austenite, respectively, and the unit is pm; and
LO, Li and Lm: average intervals of a cementite, an
25 inclusion and either one or both of martensite and
retained austenite, respectively, and the unit is pm.
- -
- - - - - _ __ - -
-- ,
Dated this 30/07/2013
RAN'MEHTA-DUTT
OF REMFRY & SAGAR .
ATTORNEY FOR THE APPLICANTS *