The present invention relates to a high strength hot-rolled steel sheet \vl~icli
has excelletit external appearance and excellent balance between elongation and hole
expansibility and has a tensile strength of 590 MPa or higher, and a production method
of therefor.
[Related At]
[0002]
In recent years, for the purpose of an improwrement in the fuel efficiency of a
vehicle and an enhancement in collision safety, a reduction in the weight of the vehicle
body has been actively achieved by the application of a high strength steel sheet. In a
case where a high strength steel sheet is applied to the vehicle body or the like of a
vehicle, it is important to secure press formability. In addition, for example, for an
enhancement in surface designability of a vehicle wheel disk, it is required to eliminate
Si scale patterns as much as possible. hi addition, since elongating and burring are
performed, a steel sheet as a material requires excellent external appearance and high
elongation and hole expansibility.
[0003]
Patent Docu~iletit 1 suggests a hot-rolled steel sheet in \vl~ichth e st~ucture
fiaction of mal-tensite is 3% or higher and lower than 10%: In Patent Document 1, it
is disclosed that a hot-rolled steel sheet having excellent balance between elongation
and hole expansibility is obtained by enhancing strengtl~th rough precipitation
strengthening of ferrite using Ti and Nb.
[0004]
Patent Docunlent 2 discloses a steel \vhich has a combined structure of ferrite
and martensite in which the proportion of the ferritc in a microsttucture is caused to be
40% or higher by adding A1 thcrcto in order to prevent the generation of Si scale,
which is a cause of deterioration of chemical conversion properties.
[Prior Art Docun~ent]
[Patent Document]
[0005]
[Patent Document 11 Japanese Unexamined Patent Application, First
Publication No. 201 1- 184788
[Patent Docurnetit 21 Japanese Unexamined Patent Application, First
Publicatiot~N o. 2005-120438
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0006]
In the technique described in Patent Document 1, Ti or Nb is added for
precipitation strengthening of ferrite. Therefore, a texture is developed during botrolling,
and plastic anisotropy of the ferrite becomes strong. As a result, sufficient
hole expansibility cannot be obtained.
In addition, in the technique described in Patent Docutllent 1, 0.5% or more of
Si is added. Therefore, due to scale generated during hot-rolling, a stripe pattern
(hereinaftel; referred to as scale pattern) is generated in the steel sheet, and excellent
external appearance catnlot be obtained.
[0007]
In the tecl~niqoe described in I'atent Docut~le~2l,t external appearance 01
chemical conversion properties are enhanced by adding A1 as an alternative to Si to a
steel sheet. Howcvcr, \vhenAl is added, a ferrite transformation start temperature
becomes a high temperature, and coarse ferrite and nlartensite are formed. As a result,
in the steel sheet described in Patent Document 2, cracking easily occurs at the
interface between the ferrite and the martensite, and elorlgation and hole expansibility
are insufficient.
[OOOS]
It1 view of the above-described circumstances, an object of the present
invention is to provide a high strength hot-rolled steel sheet which has excellent
external appearance and excellent balance between elongation and hole expansibility
and has a tensile strength of 590 MPa or highel; and a production nlethod of therefor.
In the present invention, excellent external appearance indicates less
generation of scale patterns on a surface, and excellent balance between elongation and
hole expansibility indicates an elongation of 20% or higher and a hole expansion ratio
of 100% or highel; which are sin~ultaneous.
[Means for Solving the Problem]
[0009]
The inventors conducted various examinations on means for solving the
problems.
When a n~icrostructurec ontains martensite, strength is enhanced, but a
reduction in hole expansibility is a concern. Therefore, in order to enhance strength,
using precipitation strengthening of Ti or Nb instead of the enhancement in the
strength by martensite (transfornlation strengthening) is considered. However, \vhen
Ti or Nb is contained, a texture is formed drrring hot-rolling.
I11 addition, in order to improve external appearance, whcn A1 is contained as
an alternative to Si, which is a cause of generation of scalc patterns, coarse martensite
is fonned, resulting it1 a deterioratioti in hole expansibility. The inventors newly
found that it is important to control an austenitic structure it~~tnediateblye fore
transformation in order to solve these two problems.
Specificallj~,it was found that by causing a rolling reduction to be 20% or
higher in the final pass of finish rolling and by causing a finish rolling temperature to
be 880°C to 1000°C, recrystallization of austetiite can be promoted, and accordinglj:
au improvetiie~iti n a texture can be achieved. Furthermore, it was found that by
starting water cooling of a steel sheet at a time between 0.01 seconds to 1.0 seconds
after the end of the finish rolling, the recrystallization can be completed within a short
period of time, and accordingly, finely recrystallized austenite can be made. During
trausformation from tile finely recrystallized austenite, tliere are many ferrite
riucleation sites, and transformation rapidly proceeds. Therefore, by performing air
cooling after the completion of the cooling, fine ferrite is formed, and residual
austenite during air cooling finely remains. As a result, it becomes possible to refine
martensite after the transformation.
[OOIO]
The present invention was obtained on the basis of the above-described
knowledge. The gist of the present invention is as follows.
[OOl I]
(I) That is, accorditig to an aspect of the present invention, a hot-rolled steel
sheet includes, as chemical con~positionb, y mass%: C: 0.02% to 0.10%, Si: 0.005% to
0.1%, Mn: 0.5% to 2.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.2% to 0.8%, N:
0.01% or less, Ti: 0.01% to 0.1 I%, Nb: 0% to 0.10%, Ca: 0% to 0.0030%, Mo: 0.02%
to 0.5%, Cr: 0.02% to 1.0%, and Fe and impurities as a remainder, in \vhich the sunl of
a Si content and an Al content is higher than 0.20% and lo\ver than O.Sl%, a
tnicrostructure includes, by area fraction, 90% to 99% of a ferrite, 1% to 10% of a
martensite, and a bainite limited to 5% or less, a grain size of the martensite is 1 to 10
11m, an X-ray randoin intensity ratio of a {211}<011> orientation which is parallel to a
rolled surface of the steel sheet and is parallel to a rolling direction is 3.0 or lower, and
the tensile strength is 590 MPa or higher.
[0012]
(2) The hot-rolled steel sheet described in (1) may include one or tnore of, as
chemical composition, by mass%: Nb: 0.01% to 0.10%, Ca: 0.0005% to 0.0030%, Mo:
0.02% to 0.5%, and Cr: 0.02% to 1.0%.
[0013]
(3) According to another aspect of the present i~lventiona, production method
of a hot-rolled steel sheet includes: a casting process of obtaining a slab by
contitluously casting a steel having the chemical composition described in (1) or (2); a
heating process of heating the slab to a temperature range of 1200°C or higher; a rough
rolling process of performing a rough rolling on the heated slab; a finish rolling
process of, after the rough rolling process, performing a continuous finish rolling on
the slab using a finishing mill row having a plurality of rolling mills connected in
series to cause a rolling reduction in a final pass to be 20% or higher and cause a finish
rollillg temperature to be 880°C to 1000°C, thereby obtaining a steel sheet; a primary
cooling process of performing a water cooling, which is started after 0.01 to 1.0
secot~dsfr om con~pletiono f the finish rolling process, on the steel sheet to a
temperature range of 600°C to 750°C at a cooling rate of 30 "CIS or higher; an air
cooling process of performing an air cooli~igo n the steel sheet for 3 to 10 seconds after
the primary cooling process; a secondary cooling process of, after the air cooling
process, perfonning a water cooling on the steel sheet to 200°C or lowcr at a cooling
rate of 30 "CIS or higher; and a coiling process of coiling the steel shcct after the
seconda~yc ooling process.
[Effects of the Invention]
[0014]
According to the aspects of the present invention, the hot-rolled steel sheet
having the predetermined chemical composition, in which, in the tnicrostructure, the
structure fraction of a ferrite is 90% to 99%, the grain size of a martensite is 1 pm or
greater and 10 pm or smaller, and the structure fraction of the martensite is 1% to lo%,
the X-ray random intensity ratio of the (21 1)<011> orientation which is parallel to the
rolled surface and is parallel to the rolling direction is 3.0 or lower, and the tensile
strength is 590 MPa or higher can be obtained. The hot-rolled steel sheet has
excellent external appearance and excellent balance between elongation and hole
expansibility.
[0015]
In addition, when the slab having the predetermined chemical composition is
hot-rolled, by causing the finish rolling temperature to be 880°C to 1000°C,
recr~~stallizationf austenite is promoted, and thus an improvement in the texh~reca n
be achieved. Furthermore, by causing the finish rolling reduction (the rolling
reduction in the final pass) to be 20% or higher and ,starting water cooling for a time of
0.01 to 1.0 seconds after the end of the rolling, the recrystallization is completed within
a short period of time, and finely recrystallized austenite can be made. During
transforlnation from the fiilely recrystallized austenite, there are many ferrite
nucleation sites, and transfortnation rapidly proceeds. Therefore, by performing air
cooling tl~crcafterf, ine ferrite is forn~ed. In addition, since residual austenite during
air cooling finely rcmains, it becomes possible to refine martensite after the
transformation. That is, according to the aspects of tlie present invention, a high
strength hot-rolled steel sheet which has a predeternlitled microstruch~re and an X-ray
randoni intensity ratio, excellent external appearance and excellent balance between
elongation and hole expansibility, and a tensile strength of 590 MPa or higher can be
produced.
[Brief Description of the Drawings]
[0016]
FIG. 1 is a view showing tlie relationship between an X-ray random intensity
ratio and a hole expansion ratio.
FIG. 2 is a flowchart showing an exaniple of a production method of a liotmlled
steel sheet according to an embodiment.
[Embodiment of the Invention]
[0017]
Hereinaftel; a hot-rolled steel sheet according to an embodiment of the present
invention (hereinafter, sometimes referred to as a hot-rolled steel sheet according to
this embodiment) will be described.
The hot-rolled steel sheet according to this enlbodiment is aimed at high
strength hot-rolled steel sheets having a tensile strength of 590 MPa or higher.
Regarding such a high strength hot-rolled steel sheet, in order to realize an
enhancement in hole expansibility, it is effective that in tlie microstructure
(metallographic sttucture) thereof the structure fiaction (area fraction) of ferrite is 90%
or higher and the stn~cturefr action (area fiaction) of ~nartensiteis 10% or lowet For
example, tlie structure fraction and grain size of each structure may be obtained by
perfomling image analysis on a structure photograph obtai~lcdf rom an optical
micrograph (visual ficld: a visual field of 500 x 500 ~rmo) f the steel sheet wvllich is
appropriately subjected to etching. For obtaining this structure, for example, as
described it1 Patent Document 1, a ~liethodo f performing air cooling (intermediate air
cooling) on a steel sheet containing 0.5% or more of Si on a run-out table (hereinafter,
referred to as ROT) in a hot-rolling process to promote ferritic transformation is
considered. However, Si is a cause of generation of scale patterns due to Si scale.
Therefore, when Si is contained, there is a problem of poor external appearance during
use of the steel sheet.
On the other hatld, in a case where Si is not added, in order to promote ferritic
transformation, there is a need to reduce a fiish rollit~gte mperature. However, a
reduction in the finish rolling temperature causes the developtnent of the texture of the
steel sheet. Specifically, {211)<110> which is parallel to a rolled surface and is
parallel to a rolling direction is developed. When the texture is developed, anisotropy
of plastic deformation becomes strong, and hole expansibility is deteriorated.
That is, an enhancetnent in balance between elongation and hole expansibility
in a steel sheet which does not contain Si added thereto has not been achieved in the
related art.
[OO 1 81
In the hot-rolled steel sheet of this embodiment, as an alternative to Si, ferritic
transformation is promoted using Al. By causing a predetermined amount of A1 to be
contained, ferrite is transformed fiom fine austenite, and it becomes possible to avoid
coarsening of the ferrite.
In addition, during finish rolling, a finish temperature is set to 880°C to
1000°C and a rolling reduction in the final pass is set to 20% or higher. At a time
between 0.01 to 1.0 seconds after the end of the finish rolling, primary cooling is
started. During the primary cooling, cooling is perfonned to 600°C to 750°C at a
cooling rate of 30 "CIS or higher. After the primary cooling, air cooling is perfornled
for 3 to 10 seconds. After the air cooling, secondary cooling is perfortned to 200°C
or lower at a cooling rate of 3O0Cls or highel; and the resultant is coiled. In tlie
production nlethod described above, a hot-rolled steel sheet in which the structure
fraction of ferrite is 90% to 99%, the grain size of nlartensite is 1 to 10 Inn, the
structure fraction of martensite is 1% to lo%, an X-ray rand0111 intensity ratio of a
(21 1)<01 I> orientation which is parallel to the rolled surface and is parallel to tlie
rolling direction in the texture of the steel sheet is 3.0 or lower, and the tensile strength
is 590 MPa or higher cat1 be obtained. The hot-rolled steel sheet has excellent
external appearance and excellent balance between elongation and hole expansibility.
[OOI 91
Hereinafter, the hot-rolled steel sheet according to this etnboditnent will be
described in detail.
First, the reason that the clieniical composition limited will be described.
[0020]
C: 0.02% to 0.10%
C is an important element to enhance the strength of the steel sheet. In order
to obtain this effect, the lower limit of the C content is set to 0.02%. A preferable
lower limit of the C content is 0.04%. On the other hand, when the C content is more
than 0.10%, toughness is deteriorated, and the fundatnental properties of the steel sheet
cannot be secured. Therefore, the upper limit of the C content is set to 0.10%.
[0021]
Si: 0.005% to 0.1%
Si is an ele~nennt ecessary for pre-deoxidation. Therefore, the lower litnit of
the Si content is set to 0.005%. On the other hand, since Si is an element that causes
poor external appearance, atid tt~usth e upper limit of the Si content is set to 0.1%.
The Si content is preferably less than 0.1%, more preferably 0.07% or less, and even
more preferably 0.05% or less.
[0022]
Mti: 0.5% to 2.0%
Mn is an element which contributes to an increase in the strength of tlie steel
sheet by enhancing hardenability and causing solid solution strengthenitig. In order
to obtait~a desired strength, the lower limit of the Mn content is set to 0.5%.
However, when the Mn content is excessive, MnS which is hannfi~tlo isotropy of
toughness forms. Therefore, the upper limit of tlie Mn content is set to 2.0%.
LO0231
P: 0.1% or less
P is an impurity and is an element which has an adverse effect on workability
and weldability and reduces fatigue properties. Therefore, tlie P content is preferably
as low as possible. However, in view of dephospliorizing costs, the lower limit
thereof may be set to 0.0005%. When tlie P content is more than 0.1%, the adverse
effect becomes significatit, and thus the P content is limited to 0.1% or less.
[0024]
S: 0.01% or less
S forms inclusiot~ss uch as MtiS which is harmful to isotropy of tougliness.
: Therefore, the S content is preferably as low as possible. However, in view of
desulfurizing costs, the lower litnit thereof may be set to 0.0005%. M'hen the S
content is more than 0.01%, the adverse effect becomes significant, and thus the S
content is limited to 0.01% or less. In a case \vliere particularly strict lo\v tenll~erature
tougliness is required, the S content is preferably limited to 0.006% or less.
[0025]
Al: 0.2% to 0.8%
A1 is an important element for the hot-rolled steel sheet according to this
embodiment. In order to promote ferritic transfor~llationd uring cooling on tlie ROT
after tlie finish rolling, tlie lowver limit of the A1 content is set to 0.2%. However,
when the A1 content is excessive, alumina precipitated in a cluster form fonns,
resulting it1 a deterioration in toughness. Therefore, the upper limit of the A1 content
is set to 0.8%.
[0026]
N: 0.01% or less
N is at1 element that forms precipitates of Ti in a higher temperature range
than that of S. When the N content is excessive, not only is the amount of Ti effective
in fixing S reduced, bat also coarse Ti nitrides forms, resulting in a deterioration in the
toughness of the steel sheet. Therefore, the N content is litnited to 0.01% or less.
[0027]
Ti: 0.01% to 0.11%
Ti is an elenlent that enhances the strength of the steel sheet through
precipitation strengthening. In order to achieve precipitation strengthening of ferrite
and excelletit balance between elongation and hole expansibility, tlie lower limit of the
Ti content is set to 0.01%. However, wvl~ent he Ti content is more than 0.1 1%,
inclusions caused by TiN form, and hole expansibility is deteriorated. Therefore, the
upper limit of the Ti content is set to 0.1 1%.
[0028]
0.20% orientation wvliich is parallel to the rolled
surface and is parallel to the rollilig direction is caused to be 3.0 or lower. By causing
the structure fraction atid the texture to be in optimal ranges, high elorlgatio~al ud hole
espatlsibility can be compatible with each other.
In addition, bainite is poorer in elongation and hole expansibility than ferrite
and thus causes a smaller increase in stretlgth than martensite. Therefore, for the
reason that it is difficult to cause elongation and hole expansibility to be compatible
nrit11 each other, it is preferable that the area fraction of the baiuite is limited to 5% or
lower. 111 the hot-rolled steel sheet according to this embodiment, the area fractions of
structures other than the ferrite, martensite, and bainite do not need to be specified.
[0037]
Next, a production metllod of the hot-rolled steel sheet according to this
embodiment will be described.
[0038]
First, by co~itinuouslyc asting a steel having the above-described chemical
composition, a continuously cast slab (hereinafter, refelred to as a slab) is obtained
(casting process). Before hot-rollitig, the slab is heated to 1200°C or higher (heating
process). In a case where the slab is heated at a temperature of lower than 1200°C,
Tic is not sufficiently melted in the slab, and thus the amount of Ti necessary for
precipitation strengthening of ferrite is insufficient. On the other hand, when the
heating temperature is 1300°C or higher, the amount of scale generated or maintenance
costs for a heating furllace increase, which is not preferable.
[0039]
The heated slab is subjected to rough rolling (rough rolling process), and is
further subjected to contitluous finish rolling in a finishing mill row having a plurality
of rolling mills connected in series (fiuish rolling process). At this time, a final
rolling reduction of the finish rolling (a rolling reduction in the final pass of the finish
rolling) is caused to be 20% or higher, and a finish temperature FT (a temperature at
the co~npletiono f the filial pass) of the filial finish rolling is caused to be 880°C to
1000°C. In order to cause recrystallizatioti of austenite to occur at a high temperature,
as the rolling reduction of the filial pass, a rollirig reduction of 20% or higher is
necessary. When the rolling reduction of the final pass is lower than 20%, driving
power necessary for rec~ystallizationis insufficient, and grain growth occurs at a time
between the completioti of the final pass of the finish rolling and the start of cooling.
As a result, martensite becotlies coarsened and hole expansibility is deteriorated.
When the finish rolling temperature is lower than 880°C, recrystallization of austenite
does not proceed, the texture of the steel sheet is developed, and the X-ray random
intensity ratio of the (21 1)<01 I> orientation which is parallel to the rolled surface and
is parallel to the rolling direction becomes higher than 3.0, resulting in the
deterioratio~in hole expansibility. When the finish rolling temperature is higher than
100O0C, the grain size of austenite is coarsened, a dislocatio~dl etisity rapidly decreases,
and thus ferritic tratisforniation is significantly delayed. As a result, a ferrite structure
fraction of 90% or highcr cannot be obtained.
In order to more reliably recrystallize austenite, the finish rolling temperature
is preferably set to 900°C or higher.
[0040]
Subsequent to the finish rolling, primary cooling is performed (primary
cooling process). The primary coolillg is started at a time between 0.01 to 1.0
seconds after the completio~ol f the finish rolling. Although water coolilig is
performed during the pritnary coolitig, in order to complete the recrystallization of
austenite after the rolling, air cooling needs to be performed for 0.01 seconds or lotiger
from the cotiipletion of the finish rolling to the start of the primary cooling. In order
to reliably complete the rccrystallization, the titiie fro111 the co~ilplctiono f the finish
rolling to the start of the primary cooling is preferably set to 0.02 seconds or longel
and more preferably 0.05 seconds or longer. Howevcl; \vl~enth e air cooling time
increases, grains of the recrystallized austenite become coarsened, ferritic
transformation is significantly delayed, and coarse martensite for~ils. I11 order to
suppress voids generated at the interface between ferrite and martensite and obtain
excellent hole expansibilit): it is importa~ito cause the grain size of the martensite to
be 10 pm or sniallec For this, there is a need to suppress grain coarsening of the
austenite. Therefore, the primary cooling is started witliiti 1.0 seconds after the
completion of the finish rolling.
[0041]
The pritl~atyc ooling after the f ~ srohlli ng is perfomled to cause a cooling
stop temperature to be it] a temperature range of 60OoC to 750°C at a cooling rate of
30 "CIS or higher. In addition, after the completion of the primary cooling,
intermediate air cooling is performed for 3 to 10 seconds in this temperature range (air
cooling process). Fine aostenite has a fast rate of grain elongation, and grain growth
occurs during cooling at a cooling rate of lower than 30 "CIS, resulting in a coarse
structure. On the other hand, when the cooling rate of the primary cooling is too fast,
a temperature distribution easily occurs in the thickness direction of the steel sheet.
When a temperature distribution is present in the thickness direction, the grain sizes of
ferrite and martensite vary between the steel sheet central part and the surface part, and
there is concern that n~atcriavl ariations increase. Therefore, the cooling rate of the
;primary cooling is prcfcrably set to 100 OC/s or lo\\ler. I When the cooling stop
temperature and a temperature range in which the air cooling is perfomled are lo\ver
than 600°C, feslitic transformation is delayed, a high ferrite fiaction is not obtained,
and elongation is dctcrioratcd. On the other liand, \\.hen the cooling stop temperature
and the temperature range in which the air cooling is performed are higher than 750°C,
coarse Tic is precipitated in the ferrite. Therefore, precipitation strengthening of the
ferrite is not suficiently achieved, and a tensile strength of 590 MPa is not obtained.
The intermediate air cooling needs to be performed 3 seconds or longer in order to
cause ferritic transformation. Nowewrer, during air cooling for longer than 10 seconds,
precipitation of bainite proceeds, and elongation and hole expansibility are deteriorated.
[0042]
After the intermediate air cooling, secondary cooling for cooling the steel
sheet to 200°C or lower is performed at a coolit~gra te of 30 "CIS or higher (secondary
cooling process) and the resultant is coiled (coiling process). When the cooling rate
of the secondary cooling is lower than 30 "CIS, bainitic transformation proceeds, and
martensite cannot be obtained. In this case, the tensile strength is decreased, and
elongation is deteriorated. On the other hand, when the cooling rate of the secondary
cooling is too fast, a temperature distribution easily occurs it1 the thickness direction of
the steel sheet. When a temperahlre distribution is present in the thickness direction,
the grain sizes of ferrite and martensite vary between the steel sheet central part and
the surface part, and there is concern that material variations increase. Therefore, the
cooling rate of the secondary cooling is preferably set to 100 'CIS or lower. When the
cooling stop temperature is higher than 20OoC, a self-ten~peringe ffect of martensite
occurs. When the self-tempering occurs, the tensile strength is decreased, and
elongation is deteriorated.
[Example]
[0043]
Steel containing components shown in Table 1 was melted in a converter and
was continnously cast into a slab having a thickness of 230 111111. Thereafter, the slab
was heated to a temperature of 1200°C to 1250°C and was subjected to rough rolling
and finish rolling by a cotltinuous hot-rolling apparatus, and the resultant was coiled
after ROT cooling, thereby producitlg a hot-rolled steel sheet. Table 2 shows steel
type syrnbols used, hot-rolling conditions, and steel sheet thicknesses. In Table 2,
"FTG" is the tetnperature at the time of the completion of the final finish pass, "cooling
start tinle" is the time from the finish rolling to the start of primary cooling, "primaty
cooling" is the average coolitlg rate until an intermediate air cooling temnperature is
reached after the end of the finish rolling, "intertnediate tetnperature" is the
intermediate air cooling temperature after the pri~narpc ooling, "intet.~nediatet ime" is
the intermediate air cooling time after the primary cooling, "seconda~y cooling" is the
average cooling rate until coiling is performed after the intermediate air cooling, and
"coiling tetnperah~re" is the temperature after the end of the secondary cooling.
1 1 Components (mass%)
[Table 21

- 23 -
24
25
22 1 93 1 4
26
27
- . . 7
94
92
91
29
30
3 l
32
??
2.6
28 92 8 0 6 2.4 681 858 23 119 G
Prcscnt lnvcntion
94
91
659 581
6
8
9
90
93
92
X5
6
9
- 18
0
0
0
47 1
10
7
8
1
0
0
-42
---6 --
- 14
6
0
0
0
0
14
G I Comparative Esamplc
--ppppp
4
2
2.7
a
2.1
6
2
2
21
4
2.3
2.2
633 133
-. . . . -
2.2
2.7
S
2.2
2 5
G
ppp
G
G
598
680
622
5 64
836
859
676
663
601
686
638 1
481
488
458
ricscnt Inventlo"
Example
Cornparativc Enarnplc
Present lnvcntion
Fvamnlr
770
757
864
792
-8
22
28
24
17
22
24
21
23
-64
107
a
6L
58
61
114
144
124
G
-B 1
G
G
G
Cornpantive Esamplc
Cornpantivc Example
Cornpmtivc Esarnplc
Comnmtivc Esarnnle
G
G
Prcscnt lnvcntion
Example
Prcscnt lnvcntion
--Examplc
[0047]
The structure fractions of ferrite, bainite, and martensite and the texture of the
obtained steel sheet were analyzed sing an optical microscope. In addition, the grain
size of the martensite was inspected.
[0048]
Regarding the structure fractions of the ferrite and bainite of the steel sheet,
the area fractions thereof were obtained by perfonnit~gim age analysis on a structure
photograph obtained from a visual field of 500 x 500 pm after nital etching using the
optical n~icroscope. Regarding the grain size and structure fraction of the marten site,
the area fiaction and grain size thereof were obtained using image at~alysisp erformed
on a structure photograph obtained from a visual field of 500 x 500 p n a~ft er lepera
etching using the optical n~icroscope.
[0049]
For analysis of the texture, the X-ray random intensity ratio of a {211}<01 I>
orientation which was parallel to the rolled surface and \vas parallel to the rolling
direction at a sheet thickness 114 portion which is a 114 position fiotn the surface in the
thickness direction \vas evaluated. Using the electron back scattering diffraction
patter11 (EBSD) method, at a pixel measuretl~etlti titerval of 115 of the average grain
size or smallel; measurement was performed on a region where 5000 or more grains
could be measured, and the X-ray random intensity ratio was measured from the
distribution of the orientation distribution function (ODF). I11 addition, an X-ray
random intensity ratio of 3.0 or lower was evaluated as pass.
[0050]
In a tensile test of the steel sheet, a JIS 5 test piece was extracted in a rolling
width direction (C direction) of the steel sheet, atid yield strength: YP (MPa), tensile
strength: TS (MPa), and elongation: EI, (%) were evaluated on the basis of JIS Z 2241.
[0051]
Hole expansion ratio: regarding h (%), evaluation was perfomled according to
a method specified in IS0 16630.
[eon]
For evaluation of the external appearance of the steel sheet, a steel sheet was
cut into 500 Inn1 in the longitudinal direction at a 10 m position of the outer
circun~ferenceo f a hot-rolled coil, and the area fraction of a scale pattern was
measured. Those having a scale pattern area fraction of 10% or loxver were evaluated
as "G: GOOD. On the other hand, those having a scale pattern area fraction of
higher than 10% were evaluated as "B: BAD".
[0053]
Table 3 sho\vs evaluation results of the structure fraction (area kaction) of
each structure, the martensite grain size, the texture, the material quality, and the
external appearance.
[0054]
As shown in Table 3, in present invention examples, the tensile strength was
590 MPa or higher, the stn~cturef raction of ferrite was 90% or higher, the grain size of
martensite was 10 Fun or smaller, the structure fraction thereof was 1% to lo%, and the
X-ray rand0111i ntensity ratio of the (211 }<011> orientation wl~c11w as parallel to the
rolled surface and was parallel to the rolling direction \vas 3.0 or lower. That is, all of
the present invention exanlplc had excellent external appearance and excellent balance
between elongation and hole expansibility,
[OOSS]
Contrary to this, in No. 2, since the intermediate air cooling temperature was
11ig11, coarse Ti was precipitated in ferrite, and sufficient precipitation stre~lgtllcning
could not be obtained. Therefore, the tensile strength was lower that1 590 MPa.
[0056]
In No. 5, since the finish temperature was lower tlian 880°C, the steel sheet
texture had strolig anisotropy, and hole expansibility was deteriorated.
[0057]
111 No. 8, since the time after the finish rolling to the start of the primary
cooling was longer tlian 1.0 seconds, coarsening of the austenite stmcture had
proceeded, and ferritic transformation was significantly delayed. Therefore,
elongation and hole expansibility were deteriorated.
[O05S]
In No. 12, since the intermediate air cooling time \vas shorter than 3 seconds,
ferritic transfornlation could not be sufficiently proceeded. Therefore, elongation and
hole expansibility were deteriorated.
[O059]
In No. 16, since the internlediate air cooling time was longer than 10 seconds,
bainitic transformation had proceeded, and thus the structure fraction of martensite
could not be obtained. Therefore, elongation and hole expansibility were deteriorated.
[0060]
In No. 17, since the intermediate air cooling temperature was lower than
600°C, the structure fraction of ferrite could not be obtained. Therefore, elongation
and hole expa~isibilityw ere deteriorated.
.[O061] I
In No. 20, since the finis11 temperature was higher than 1000°C, feuitic
transformation was delayed due to coarsening of the austenite structure. Thcrcfore,
elongation and hole expansibility were deteriorated.
[0062]
I11 No. 22, since the coiling temperature was higher than 20OoC, martensite
could not be obtained but bainite had fortiled. Therefore, the tensile strength was
lo\ver than 590 MPa, and elongation and hole expansibility \yere deteriorated.
[0063]
In No. 24, since the rolling reduction in the final pass was lower than 20%,
martensite becoti~ec oarsened and exceeded 10 pn. Therefore,h ole expansibility was
deteriorated. In addition, since recrystallization of austenite was insoficient, the
anisotmpy of the steel sheet texture :eras strong, and thus hole expansibility was
deteriorated.
[0064]
In No. 29, since the Al content was less than 0.2 mass%, fenitic
tratisformation did not proceed, and elongation and hole expansibility were
deteriorated.
[0065]
In No. 30, since the Si content was more than 0.1 inass%, a large number of
scale patterns could be seen fiotn the external appearance, and the area fraction of the
scale patterns was higher than 10% wih respect to the total area fraction.
[0066]
In No. 31, since the time after the finish rolling to the start of the pritnary
cooling was shorter than 0.01 seconds, recrystallization could not be sufficiently
proceeded, and the texture was developed. Therefore, hole expansibility was
deteriorated.
[0067]
I11 No. 32, since the cooling rate of the primary cooling n7as lower than
30 "CIS, the grain size of marteasite was greater than 10 Ltm, and hole expansibility
was deteriorated.
[0068]
In No. 33, since the cooling rate of the secondary cooling was lower than
30 'CIS, bainite during cooling exceeded 5%. Therefore, elongation and hole
expansibility were deteriorated.
[Industrial Applicability]
[0069]
According to the embodiment of the present invention, a hot-rolled steel sheet
having predetermined chemical composition, in which, regarding the proportions of
structures, the structure fraction of ferrite is 90% to 99%, the grain size of martensite is
1 pm to 10 ptn and the stlucture fiaction thereof is 1% to 1096, the X-ray randotn
intensity ratio of a {211)<011> orientation which is parallel to a rolled surface and is
parallel to a rolling direction is 3.0 or lo\vel; and the tensile strength is 590 MPa or
higher can be obtained. The hot-rolled steel sheet has excellent external appearance
and excellent balance between elongation and hole expansibility.

What is clainied is:
1. A hot-rolled steel sheet comprising, as che~nicacl ompositiot~b, y mass%:
C: 0.02% to 0.10%,
Si: 0.005% to 0.1%,
Ah: 0.5% to 2.0%,
P: 0.1% or less,
S: 0.01% or less,
Al: 0.2% to O.X%,
N: 0.01% or less,
Ti: 0.01% to O.11%,
Nb: 0% to 0.10%,
Ca: 0% to 0.0030%,
Mo: 0.02% to 0.5%,
Cr: 0.02% to 1.0%, and
Fe and impurities as a remainder,
wherein a sum of a Si content and an A1 conterlt is higher than 0.20% and
lower than 0.81%,
a microst~~~ctiuncrleu des, by area fraction, 90% to 99% of a ferrite, 1% to
10% of a martensite, and a bainite limited to 5% or less,
a grain size of the martensite is 1 to 10 pm,
an X-ray random intensity ratio of a {211)<011> orientation which is parallel
to a rolled surface of the steel sheet aud is parallel to a mlling direction is 3.0 or lower,
and
a tensile strength is 590 A4Pa or higher.
2. The hot-rolled steel sheet according to clainl 1, conlprising one or Inore
of, as chemical composition, by mass%:
Nb: 0.01% to 0.10%,
Ca: 0.0005% to 0.0030%,
Mo: 0.02% to 0.5%, and
Cr: 0.02% to 1.0%.
3. A production metl~odo f a hot-rolled steel sheet comnprising:
a casting process of obtaimling a slab by co~ititiuouslyc asting a steel having the
chemical composition according to claitn 1 or 2;
a heating process of heating the slab to a temperature range of 1200°C or
higher;
a rough rollitig process of perfornling a rough rolling on the heated slab;
a finish rollirig process of, after the rough rolling process, performitlg a
continuous finish rolling on the slab using a finishing mill row having a plurality of
rolling mills connected in series to cause a rolling reduction in a final pass to be 20%
or higher and cause a fiuish rolling temperature to be 880°C to 1000°C, thereby
obtaining a steel sheet;
a pritnary cooling process of performing a water cooling, which is started
after 0.01 to 1.0 seconds fiorn a conlpletion of the finish rolling process, on the steel
sheet to a tetnperature range of 600°C to 750°C at a cooling rate of 30 "CIS 01. higher;
an air cooling process of performing an air cooling on the steel sheet for 3 to
10 seconds after the pri~naryc ooling process;
a secondary cooling process of, after the air cooling process, perfor~ilinga
\\rater cooling oil tile steel sheet to 200°C or lower at a coolii~gra te of 30 "C/s or
higher; atid
a coiling process of coiling the steel sheet after the secondary cooling process.