[Title of the Invention] HOT-ROLLED STEEL SI-IEET AND METHOD FOR
PRODUCING SAME
[Technical Field of the Invention]
[0001]
The present invention relates to a hot-rolled steel sheet and a method for
producing the satne. More specifically, the present invention relates to a highstrength
hot-rolled steel sheet having excellent elongation and hole expandability and a
method for producing tlie same.
[Related Art]
[0002]
In recent years, due to growing worldwide awareness of the e~lvirolunent,
there has been strong denland in the automotive field to reduce carbon dioxide
emission and inlpmve file1 consumption. For solving these tasks, weight reduction of
a vehicle body may be very effective, and application of a high-strength steel sheet is
being promoted to achieve the weight reduction. At present, a hot-rolled steel sheet
having a tensile strcngth of a 440 MPa class is often used in automotive suspension
parts. However, in order to achieve \vcight reduction of a vehicle body, application of
a steel sheet having a higher strength is desired.
[0003]
Many suspension nlenibers of an automobile have a complicated shape in
order to secure 11igh rigidity. Accordingly, tnultiple kinds of works sucli as burring,
stretch flanging, and clongation are applied thereto during press fornling, and thus,
\vorkability responding to these kinds of works is required in the hot-rolled steel sheet
as a material. Generally, the burring workability and the stretch flanging workability
have a correlation wit11 a hole expanding ratio measured in a hole expanding test, and
Inany studies for increasing the hole expanding ratio have been heretofore advanced.
[OOO4]
While dual phasc steel (hereinafter, referred to as "DP steel") consisting of
ferrite and martensite bas hig11 strength and excellent elongation, the hole
expandability thereof is lolv. This is because high a~llountso f strain and stress
concentration occur in the ferrite near the martensite with foulling due to a large
difference in the strength between the ferrite and the nlartensite and thus cracks are
generated. Fro111 this finding, a hot-rolled stecl sheet with an inlproved hole
expanding ratio done by reducing the difference in strength between structures bas
been developed.
[OOOS]
In Patent Docunlent 1, a steel sheet that i~lcludesb ainite or bainitic ferrite as a
primary phase so as to secure the strength and significantly inlprovc holc cxpandability
thereof is proposed. When single struch~rest eel is fornled, the above-described strain
and stress concentration do not occur and a high expanding ratio can be obtained.
I-Io~vevcrC, ~\~C\II\ ten the singie structure steel conlposed of bainite or bainitic ferrite is
fonned, it is difficult to secure high elongation and thus high levels of both elongation
and hole expatidability are not easily attained.
~OOOS]
In recent years, steel sheets in \vl~ichf errite having excellent elongation is
used as a structure ofsingle structure steel and high strength is achieved by using
carbides such as Ti and Mo are proposed (for esample, refer to Patent Documents 2
and 3). Ho\\~cvcr, the steel sheet proposed in Patent Document 2 contains a large
atnount of Mo and the stecl sheet proposed in Patent Documcnt 3 contains a large
amount of V.
[0007]
In addition, iii Patent Document 4, a coniplcx structure stcel sheet in which
~nartensitein DP steel is changed into bainite and tlie difference in strength between
structures of ferrite and bainite is reduccd to itliprove liole expandability has been
proposed. However, ~vlienth e area fraction of the bainite structure is increased to
secure tlie strength, as a result, it is difficult to secure high elongation and tl111s high
levels of both elongation and hole expandability are not easily attained. Furthel; in
Patent Docu~nen5t , a high-strength steel sheet having excellent liole expandability and
formability by attaining both strength and liole expandability using fcrrite llaving
excellent ductility and tempered martensite by controlling tlie amount of C solidsoluted
in ferrite before quenching, in addition to quenching and tempering inartensite
after quenching in order to attain hole expandability and formability is disclosed.
Ho\vevel; in recent years, it has been desired to further improve tlie balance between
elongation and hole expandability.
[Prior Art Docutnetit]
[Patent Document]
[0008]
[Patent Doculllent 11 Japanese Unexamined Patent Application, First
Publication No. 2003-193190
[Patent Docunietit 21 Japanesc U~lexanlinedP atent Application, First
Publication No. 2003-089848
[Patent Document 31 Japanese Unexamined Patent Application, First
Publication No. 2007-063668
[Patent Docunient 41 Japanese Unexamined Patent Application, First
Publication No. 2004-204326
[Patent Docutnent 51 Japanese Unexamined Patent Application, First
Publication No. 2007-30291 8
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0009]
The present illvcntio~iis to provide a high-strength hot-rolled steel sheet
capable of attaining excellent elo~lgatiolal nd hole cxpatldability without containing an
expensive element, and a method for produciilg the same.
[Means for Solvirig the Proble~~~]
[OOlO]
The itiwrentors have conducted a detailed investigation of the relationship
between the structural conipositiotl of DP steel havitlg high stre~lgtlal nd high
elongation and hole expandabilitj: and examined a method for improving both
elongation and hole expandability with respect to the type of steel in the related art.
As a result, the i~lwre~ltohrasw re found a mcthod for improving hole expandability while
maintaining high elo~igatioo~fl t he DP steel by colltrolli~lgth e dispersion state of
martensite thereiti. That is, it has been found that even in a DP structure in which a
difference in strength is large like in structures of ferrite and martensite, and the hole
expandability is generally low, wvhen the relationship of K/I)~: 2 1.00, which will be
described later, is satisfied by cont~~ollittihge area fraction of martensite and the
average diameter, the hole expandability can bc improved while high elongation is
maintained.
[OOll]
The presetlt invention is made based on the above findings and the gist thereof
is as follo\vs.
[OO 121
(1) According to a first aspect of the present invention, there is provided a
hot-rolled steel sheet including, as a cl~e~niccaolm position, by mass%, C: 0.030% to
0.10%, Mn: 0.5% to 2.5%, Si+Al: 0.100% to 2.5%, P: 0.04% or less, S: 0.01% or less,
N: 0.01% or less, Nb: 0% to 0.06%, Ti: 0% to 0.20%, V: 0% to 0.20%, W: 0% to 0.5%,
Mo: 0% to 0.40%, Cr: 0% to 1.0%, Cu: 0% to 1.2%, Ni: 0% to 0.6%, B: 0% to 0.005%,
REM: 0% to 0.01%, Ca: 0% to 0.01%, and a remainder consisting of Fe and impurities,
in \vhich the steel sheet has a microstructure including, by area fiaction, ferrite: 80% or
more, martensite: 3% to 15.0%, and pearlite: less than 3.0%, in \vhic11 the llunlber
density of 111al-tensite having an equivalent circle diatneter of 3 pm or more at a
position which is at a depth of 114 of the steel sheet thickness fs0111 the surface of the
steel sheet, is 5.0 piecesllOOO0 pm2 or less, and the following Expression (A) is
satisfied.
R/D~: 2 1 .OO ... Expression (A)
Here, R is an average martensite intes\~a(lC IIII)d efined by the following
Expression (B), and Dhf is a martensite average diameter (ptn).
R = (12.5 x (?r/6~h1)'- (213)~~x )D h, ...E xpressio~l( B)
Here, VM is a mai-tensitc area fiaction (%) and DM is the martensite average
diameter (~III).
(2) The hot-rolled steel sheet according to (1) nlay ful-thcr include, as a
chemical co~llpositionb, y mass%, at least one of Nb: 0.005% to 0.06% and Ti: 0.02%
to 0.20%.
(3) The hot-rolled steel sheet according to (1) or (2) map further inclttde, as
a chen~icacl omposition, by mass%, at least one of V: 0.02% to 0.20%, W: 0.1% to
0.5%, and Mo: 0.05% to 0.40%.
(4) The hot-rolled steel sheet according to any one of ( 1 ) to (3) may further
include, as a chemical composition, by mass%, at least one of Cr: 0.01% to 1.0%, Cu:
0.1% to 1.2%, Ni: 0.05% to 0.6%, and B: 0.0001% to 0.005%.
(5) The hot-rolled steel sheet according to any one of (1) to (4) may fi~rtller
include, as a chetnical co~npositionb, y mass%, at least one ofR EM: 0.0005% to
0.01% and Ca: 0.0005% to 0.01%.
(6) According to a second aspect of the present invention, there is provided
a tnethod for producing a hot-rolled steel sheet including heating a slab having the
cllenlical composition according to any one of (1) to (5) to 1150°C to 1300°, then
subjecting the slab to multipass rongh rolling and rolling the slab with four or nlore
final passes of rolling in a temperature range of 1000°C to 1050°C at a total reduction
of 30% or more to f o m ~a rough bar, starting rolling on the rougll bar within 60
seconds after completing the rough rolling and subjecting the rough bar to finish
rolling to complete rolling in a tcnlperature range of 850°C to 950°C to obtain a finishrolled
steel sheet, and after cooling the finish-rolled steel sheet to a te~nperaturer ange
of 600°C to 750°C at an average cooling rate of 50 'CIS or tnore and air-cooling the
steel sheet for 5 seconds to 10 seconds, cooling the steel sheet to a temperature range
of 400°C or lower at an average cooling rate of 30 'CIS or more and coiling the steel
sheet to obtain a hot-rollcd steel sheet.
[Effects of the Invention]
[0013]
According to the present invention, it is possible to obtain a high-strength hotrolled
steel sheet having excellent elongation and hole expandability without
containing an expensive element, and the present invention significantly contributes to
tlie industry.
[Brief Description of the Drawings]
[0014]
FIG. 1 is a diagram showing the relationship between a niartensite average
dianieter (pm) DM and a martensite area fraction VM (%) and nutnerical values in
parentheses represent hole expanding ratios (%).
FIG. 2 is a diagram showing the relationsliip between R/D~: obtaineci by
dividing an average inartensite interval R by the square of a Inartensite average
diameter DM and a hole expanding ratio (%).
FIG. 3 is a diagram showing tlie relationship between a number density Nh,1
(pieces/10000 l ~ n ~o2fn )i al-tetisite having an eq~~ivalecnitr cle diameter of 3 pni or
more at a position which is at a depth of 114 of the thickness froni the surface of a steel
sheet, and a hole expanding ratio (%).
[Embodintents of the Invention]
[0015]
DP steel is a steel sheet in which hard mastensite is dispersed in soft ferritc
and high strength and high elongation are realized. Howevel; strain and stress
concentration resulting from a difference in strength between ferrite and martensite
occurs during deformation and voids which cause ductile fractures are easily formed.
'Therefore, tlie hole expandability is very lo\v. I-Iowever, a detailed investigation ol
void for~iiationb ehavior has not been conducted and a relationship between tlie
microstructure of the DP steel and ductile fractures has not been always clear.
[00 161
I-Iere, the present inventors have conducted a detailed investigation of a
relationship between structures and void fortnation behavior and a relationship
between void formation behavior and hole expandability in DP steel having various
structural conlpositions. As a result, it has been found that the hole expandability of
the DP steel is significantly affected by the dispersion state of martcnsite, which is a
hard second phase structure. Further, it has been found that when a value obtained by
dividing the average martensite interval obtained using Expression (1) by the square of
a martensite average diameter is set to 1.00 or more, even in str~~cturheasv ing a large
difference in strength between the structures like the DP steel, high hole expandability
can be obtained.
[O017]
Cracks during hole expanding are generated and propagated by ductile
fractures having an elenlentary process of fornling, expanding, and cotlnecting voids.
In the structure having a large difference in strength between structures like DP steel,
high levels of strain and stress concentration caused by hard tmartensite are generated
and thus voids are easily fornled and the hole expandability is lo\v.
[OO 1 81
Howevel; whetl the relationship between the structure and the void formation
behavior and the relationship between the void formation behavior and the hole
expandability are investigated, it has beeti found that there may be a case in which the
forn~ationg, rowth, and connection of voids is delayed depending on the dispersion
state of martensite, \vhich is a hard second phase, and high hole expandability can be
obtained.
[00 1 91
Specificall>: it has been found that void formation is delayed by refining the
grain size of martensite. It is thought that this is because the grain size of the
tnartensite is reduced and a strain and stress concentration region forn~edn ear the
niartensite is narrowed. I11 addition, it has been also found that when an interval
betwveen martensite grains, wl~ichis cllanged according to the number density and
average dianlcter of the martensite, is increased, the distance between voids formed
using the inartensite as a starting point is increased and the voids are not easily coupled.
[0020]
The investigation of the DP structure having high hole expandability has been
conducted based on the above findings. As a result, as showvn in FIG. 1 showing the
relationship between the ina~-teiisitea verage diameter (~111D) Ma nd the martensite area
fraction Vh, (%), it has been found that high hole expandability can be obtained by
controlling the area fraction and grain size of the ~na~tensittoe f all within a
predetertnined range. I11 addition, in FIG. 1, numerical values in parentheses
represent hole expanding ratios (%).
[0021]
Further, a relationship between WD~: obtained by dividing the average
tnartensitc interval R by the square of a maitensite averagc diameter DL, and a hole
expanding ratio (%) are sliowv~l. As shown it1 FIG. 2, i t has been found that WD~: 011
tlie left side in the followi~lgE xpression (1) has a clear correlation wvith the hole
expanding ratio (%) and \?'hen WD~? is 1.00 or tilore, high hole expandability can be
obtained even in the DP structure so as to obtain a hot-rolled steel sheet having
excelle~iet longation and hole expandability.
RID~?? 1.00 ... Expression (1)
I-Iere, R is an average illartensite interval (pn) defined by the following
Expression (2), and Dhl is a tllartensite average diameter (pm).
R = { 12.5 x (1rl6~h,)~-.' ( 213)O.~)x Dh, ...E xpressiotl(2)
Here, Vh, is a nlartensite area fraction (%) and Dh, is tlie martensite average
diameter (111n).
[0022]
In Expression (I), difficulty in formation and connection of voids is expressed
and the average martensite intel-val R obtained from the area fraction and the average
diameter of martensite by Expression (2) is divided by the square of the average
diameter of martensite. In the specification, the average diameter of martensite refers
to an arithmetic average of martensite having an equivalent circle diameter of 1.0 pm
or more. This is because formation and co~luectio~f v~o ids are not affected by
martensite having an equivalent circle diameter of less tllan 1.0 pm. As the distance
between martensite grains increases, voids formed using martensite as a starting point
are not easily coupled and forlllation and connectionl of voids are suppressed by
refining the maltensite.
[0023]
The reason for suppressing the cotlllectio~ol fv oids by refining the martensite
is not clear but it is thought that the reason is that the growth of voids is delayed.
When the grain size of nllartensite is small, the size ofv oids fornled using ma~tensitea s
a starting point is also rcfincd. The for~ned voids grow to be connccted to each other.
I-Iowevel; a ratio behvcen a void surface area and a void volume is increased with
refinement of the size of the voids, that is, the surface tension is increased, and thus the
gro\vth of voids is delayed.
[0024]
However, as s11owv11 in FIG. 3 showing the relationship betwccn a number
density Nh, (pieces11 0000 p1n2) of martensite having an equivalent circle diameter of 3
11111 or more at a position wvhich is at a depth of 114 of the steel sheet thickness from the
surface of thc steel sheet and a hole expanding ratio (%), it has been found that even in
the case in ~vbic11E xpression (1) is satisfied, when coarse martcnsite is present, local
fractures are propagated and the hole expandability is lowered. In order to prevent
thc hole expandability from being lo~~cscidt ,is necessary that the number density of
martensite having an equivalent circle diarneter of 3 p111 or Inore at a dcpth position
which is at a depth of 114 of the steel sheet thickness be 5.0 pieces110000 Ll~no2r less.
In addition, FIG, 3 shows that when the nunlber density (pieces/10000 pn2) of
nlartensite having an equivalent circle dianletcr of 3 p n or Inore is 5.0 or more, the
hole expandability is lowered. In this graph, 0111~1d ata in which RID^,; is 1.00 or
more is shown.
[0025]
Hereinafter, the chemical composition of the hot-rolled steel sheet of the
present invention will be described in detail. " % representing the amount of each
element included means mass%.
[0026]
(C: 0.030% to 0.10%)
C is an inlportant eletnent which contributes to strengthening by forming
~nartensite. When the amount of C is less than 0.030%, it is difficult to fornl
martensite. Accordingly, the amount of C is set to 0.030% or tnore. The anlount of
C is preferably 0.04% or more. On the other hand, \vhen the amount of C is more
than 0.10%, the area fraction of nlartensite is increased and the hole expandability is
lo\vered. Accordingly, the anlount of C is set to 0.10% or less. The amount of C is
preferably 0.07% or less.
[0027]
(Mn: 0.5% to 2.5%)
Mn is an inll~ortanet lement related to the strengthening of ferrite and
liardetlability. When tlie amount of Mn is less thaa 0.5%, the hardenability is
increased and it is difficult to fonn martensite. Accordingly, the amotint of Mn is set
to 0.5% or more. The alnount of Mn is preferably 0.8% or more and more preferably
1.0% or tnore. 011 the other hand, wlien the amounit of Mn is more than 2.5%, it is
difficult to fi0111 a sufficient amount of ferrite. Therefore, the amount of Mn is set to
2.5% or less. The amount of Mtl is preferably 2.0% or less and more preferably 1.5%
or less.
[0028]
(Si+AI: 0.100% to 2.5%)
Si and A1 are iniporta~iet lements related to tlie strerigthe~li~oigf ferrite and
for~nationo f ferrite. When the total alnoutlt of Si and A1 is less than 0.100%, the
amount of ferrite to be formed is not sufficient and thus it is difficult to obtain a desired
microstructure. Accordingly, the total a~lioutlot f Si and A1 is set to 0.100% or more.
The total amount of Si and Al is preferably 0.5% or tnore and tnore preferably 0.8% or
more. On the other hand, wlien the total amount of Si and Al is more than 2.5%, the
effects are saturated and costs incrcasc. Thcrcforc, the total anlount of Si and A1 is set
to 2.5% or less. The total a~nounot f Si and A1 is preferably 1.5% or less atid more
preferably 1.3% or less.
Here, Si has high performance in strengthening ferrite and is capable of more
effectively strengthening ferrite than Al. Therefore, fiotn tlie viewpoirit ofetl'ectively
strengthening ferrite, the amount of Si is preferably 0.30% or more. More preferably,
the atnou~iot f Si is 0.60% or more. On the other hand, \vI~ent he alllourit of Si is large,
red scale is generated on the surface of tlie steel sheet and the appearance is
deteriorated in some cases. Therefore, from the vie\vpoint of suppressing generation
of red scale, the alnoutlt of Si is preferably 2.0% or less. More preferably, tlie amount
of Si is 1.5% or less.
Since A1 has an action of strengthening ferrite and promoting the forn~ationo f
ferrite like Si, the atnoluttt of Si cat1 be suppressed by incrcasitig the amount ofAl, and
as a result, generation of the above-mentioned red scale is easily suppressed.
Therefore, front the vicwpoillt of easily suppressing tlie red scale, the amount of A1 is
preferably 0.010% or more. More preferably, the amoutit of A1 is 0.040% or ntore.
On the other liand, front the viewpoint of strengthelling ferrite as described above, it is
preferable that the amount of Si is increased. Accordingly, fiom the viewpoint of
strengtliening ferrite, the aruount of A1 is preferably less tltat~0 .300%. More
preferably, the attiount of A1 is less than 0.200%.
[0029]
(P: 0.04% or less)
P is an element that is generally contained as an impurity and ~ ~ ~ lttheet t
amount of P is tilore than 0.04%, the welding zotie is remarkably enibrittled.
Therefore, tlie atnoutit of P is set to 0.04% or less. The lower litnit of tlie amoutlt of P
is not particularly limited. Ho\vever, when the amo~tttot f P is less than 0.0001%, it is
econoniically disadvantageolis. Therefore, the atnoutit of P is preferably 0.0001% or
Inore.
[0030]
(S: 0.01% or less)
S is an elentetit that is generally contained as an impurity and adversely
affects the weldability atid productivity during casting and hot rolling. Accordingly,
the attiourit of S is set to 0.01% or less. In addition, ~vlietta n excessive amount of S is
contained, coarse MIIS is for~necla nd tlie hole expandability is lowered. Tltns, in
order to i~iiproveth e hole espattdability, the amomnt of S is preferably reduced. The
lower limit of the anlount of S is not pal-ticularly limitcd. Ho\vevel; wvlien the amount
of S is less than 0.0001%, it is econo~llicallyd isadvantageous. Tlierefore, the amount
of S is preferably 0.0001 % or more.
[003 I]
(N: 0.01% or less)
N is an element that is generally contained as an impurity and \vIien the
amount of N is more than 0.01%, coarse nitrides are formed and tlie bendability and
the hole expandability are deteriorated. Accordinglj: tlie amount of N is set to 0.01%
or less. In addition, when the amount of N is increased, N generates blow holes
during welding and thus the amount of N is preferably reduced. The lower limit of
the anlount of N is not particularly limited and the less, the more preferable. When
setting the amount of N to less than 0.0005%, production costs increase. Therefore,
the amount of N is preferably 0.0005% or more.
[0032]
The chemical composition of the steel sheet of the present itivetltion may
further contain Nb, Ti, V, W, Mo, Cr, Cu, Ni, B, REM, and Ca as optional elements.
Since these elements are contained in tlie steel as optional eleinents, the lower Ii~ilits
thereof are not particularly defined.
[0033]
(Nb: 0% to 0.06%)
(Ti: 0% to 0.20%)
Nb and Ti arc clc~~~crenltaste d to tlie precipitation strengthening of ferrite.
Accordingl~:e ither or both of tliese eler~ientsm ay be contained. Ho\vcver, when tlie
amount of Nb to be contained is nlore than 0.06%, ferrite transfortllation is
significantly delayed and thus elongation is deteriorated. Accordingly, the amount of
Nb is set to 0.06% or less. The atnount of Nb is prcfcrably 0.03% or less and more
preferably 0.025% or less. In addition, when the amount of Ti contai~ledis lilore than
0.20%, the ferrite is excessively strengthened and thus high elongation cannot be
obtained. Therefore, the amount of Ti is set to 0.20% or less. Tlie amount of Ti is
preferably 0.16% or less and more preferably 0.14% or less. In order to tllore reliably
stret~gtlienth e ferrite, the amount of Nb is preferably 0.005% or more, Illore preferably
0.01% or tnore, and particularly preferably 0.015% or more. F~uthert,h e amount of
Ti is preferably 0.02% or more, niore preferably 0.06% or more, and particularly
preferably 0.08% or more.
[0034]
(V: 0% to 0.20%)
(W: 0% to 0.5%)
(Mo: 0% to 0.40%)
V, W, atid Mo are elements contributing to the strengtl~eeningo f steel.
Accordinglj: the steel may contain at least one element alllong these elements.
Ho\vever, \when these ele~lle~iatrsc exccssi\rely contained, tlie formability is
deteriorated in some cases. Therefore, the aalilount of V is set to 0.20% or less, the
atuoutit of 1' is set to 0.5% or less, and the amount of Ado is set to 0.40% or less. 111
order to obtain a more reliable effect of increasing t11c strcngth of steel, the atiloullt of
V is prcfcrably 0.02% or more, the amount of W is preferably 0.02% or more, and tlie
amoout of Mo is preferably 0.01% or more.
[0035]
(Cr: 0% to 1.0%)
(Cu: 0% to 1.2%)
mi: 0% to 0.6%)
(B: 0% to 0.005%)
Cr, Cu, Ni and B are elements liaving an action of increasing the strength of
steel. Accordingly, tlie steel may contain at least one element alnong these elements.
Howevel; when these elements are excessively contained, thc fortilability is
deteriorated in sonie cases. Therefore, the amount of Cr is set to 1.0% or less, tlie
amount of Cu is set to 1.2% or less, tlie amoutit of Ni is set to 0.6% or less and the
a~nounot f B is set to 0.005% or less. In order to obtain a more reliable effect of
iticreasirig the strength of steel, the aliiou~lot f Cr is prcfcrably 0.01% or inore, tlie
atiiou~lot f Cu is preferably 0.01% or illore, tlie amount of Ni is preferably 0.01% or
more and the amount of B is prcferably 0.0001% or more.
[0036]
(REM: 0% to 0.01%)
(Ca: 0% to 0.01%)
I a M and Ca are elements effective in controlling the shape of oxides and
sulfides. Accordingly, the steel niay contain at least one element atilong these
ele~nents. However, wl~ettih ese ele~iie~iatrse cxccssively contained, tlie foniiability
is deteriorated in sonie cases. Therefore, the atliount of IEM is set to 0.01% or less,
and the amount of Ca is set to 0.01% or less. In order to Illore reliably co~ltrotlh c
shapc of oxides and sulfides, the amount of REM is preferably 0.0005% or more, and
the atilount of Ca is preferably 0.0005% or more. 111 the present invention, REM
refers to La and elements in tlie lantlianoid series. REh? is added in the form of tnisch
nietal in many cases and there is a case in \vl~icha combination of La and elelllcnts in
tlie lanthanoid series such as Ce are colitailled tliercin. Mctallic La and Ce may be
co~ltaitiedth erein. A remainder includes Fe and impurities.
[0037]
Hereinafter, the nlicrostruct~~oref the present invention will be described in
detail.
(Ferrite: 80% or more)
Ferrite is the nlost inlportarit structure for securing the elongation. When the
area fraction of ferrite is less than 80%, high elongation of the Dl' steel of the related
art ca~u~boet r ealized. Accordingly, the area fraction of ferrite is set to 80% or more.
On the other hand, the upper limit of the area fraction of ferrite is determined by the
area fiaction of martensite, which will be described latel; and when the area fraction of
ferrite is lilore than 97%, the amount of martensite is too small and thus it is difficult to
utilize strengthening througl~m artetlsite. Eve11 \vhen another method, for example, a
method of increasing the amount of precipitation strengthening, is used to secure the
strength thereof, uniform elongation is deteriorated and thus it is difficult to obtain
high elongation.
lo0381
(Mastensite: 3% to 15.0%)
(Number density of martensite havit~ga verage diameter of 3 pm or more: 5.0
pieces11 0000 Cun2 or less)
Martensite is an important structure for securing the strength and the
elongation of steel. When the area fraction of ~nartensitcis less tlian 3%, it is difficult
to secure excellent ~~niforernlo ngation. Accordingly, the area fiaction of martensite is
set to 3% or more. On the other hand, \vl~e~letnh e area fraction of martensite is more
than 15%, the hole expandability is deteriorated. Therefore, the area fraction of
martensite is set to 15.0% or less.
In addition \vhen coarse rnartensite is present, local fracture is propagated and
the hole expandability is lowered. In order to prevent such fractures, the nmnber
density of martensite having an average diameter of 3 pm or niore is set to 5.0
pieces/10000 pm2 or less.
[0039]
(Pearlite: less than 3.0%)
Pearlite deteriorates tlie hole expandability and tlius it is preferable that
pearlite is not present. Howvevel; wwrl~cn tlie area fraction of pearlite is less than 3.0%,
there is no actual damage to thc steel and thus this value is allowable as an upper limit.
[0040]
(Another Stiucture)
As for another structure, bainite may be present. Bainite is not essential and
the area fraction of bainite may be 0%. Bainite is a structure contributing to
iricreasing tlie strengtli. However, when a large ainount of bainite is used to increase
the strength, it is difficult to secure the above-nientioned area fraction of ferrite ar~d
high elongation cannot be achieved.
[0041]
The tensile strcngth of the hot-rolled steel sheet of the present invention is
prefe1,ablp 590 MPa or more. The tensile strength is more preferably 630 MPa or
inore and pal-titularly preferably 740 MPa or more.
[0042]
Hcrciilaficr, a mcthod for producing thc hot-rolled steel sheet according to the
present invention will be described.
[0043]
First, a slab is prepared by melting steel by a routine procedure arid casting
tlie steel, atid blooniing the steel according to tlie circ~~mstances. As for the casting,
continuous casting is preferable fioni the viewvl~oinot f productivity.
[0044]
The slab having the above-described chemical composition is heated to
1150°C to 13OO0C and then subjected to nlultipass rough mlling. When the
temperature of the slab to be subjected to rough rolling is lower than 115O0C, the
rolling load is sigtlificantly increased during rough rolling and thus the productivity is
deteriorated. Therefore, the tenlperature of the slab to be subjected to rough rolling is
set to 1150°C or highel: On the other hand, it is not preferable that the temperature of
the slab to be subjected to rough rolling is higher than 1300°C from the viewpoint of
production costs. Accordi~lgll: the temperature of the slab to be subjected to rough
rolling is set to 1300°C or lo\ver. As for the slab to be subjected to rough rolling, a
cast slab nlay be subjected to direct rolling as being hot-rolled. I11 order to obtain an
effect of i~lcreasingth e stretlgth by precipitation strengthening, it is necessaly to nlelt
elements such as Nb and Ti. Thus, the ternperature of the slab to be subjected to
rough rolling is preferably 1200°C or higher.
[0045]
The above-described slab is subjected to nlnltipass rough rolli~lga nd is rolled
with four or nlore final passes of rolling at a temperature range of 1000°C to 1050°C
for a total reduction of 30% or more to form a rougll bar.
It is impol-tant to refine austenite it1 a hot rolling process to suppress formation
of rough martensite. In order to refine austenite, it is eifective to repeatedly
recrystallize austenite in a rongh rolling process before fifinish rolling. Here, the
grains after recrystallization gm\v fast during rolling in a temperature range of higher
than 1050°C and thus it is difficult to refine anstenite. On the other hand, since the
grains are not co~llpletelyr ccrpstallized during rolling in a temperature range of lower
than 1000°C and then subjected to the following rolling, the grain diameter is not
uniform in au uncrystallized portion and a recrystallized portion. As a result, the
number density of martensite having an average dianleter of 3 pin or more is increased.
In addition, when the total reduction is less than 30%, austenite cannot be sufficiently
refined. Further, even when rolling is perfomled for a total reduction of 30% or more,
with less than fonr rolling passes, the grain diameter ofaustenite is not uniform and as
a result, coarse martensite is fornied.
Accordingly, the above-described slab is rolled by multipass rough rolling
with four or more final passes of rolling in a temperature range of 1000°C to 1050°C
for a total reduction of 30% or no re to fonn a rough bar.
[0046]
The above-mentioned rough bar is subjected to finish rolling in wvl~ich rolliug
is conlpleted in a tetnperature range of 850°C to 950°C while rolling is started within
60 seconds after the rough rolling is completed, and thus a finish-rolled steel sheet is
obtained.
As described above, it is inlportant to refine austenite in a hot rolling process
to suppress Sonnation of rough inartensite. Even when the above described rough
rolling is perfor~neda nd the time from the start of finish rolling after completion of
rough rolling is nlore than 60 seconds, the austenite is coarsened. Accordingl]: the
titne from the start of finis11 rolling after the completion of rough rolling is within 60
seconds.
When the finishing temperature is higher that1 950°C, the austenite after the
finish rolling is completed is coarsened and thus the nucleation site of ferrite
transfortnation is reduced to renlarkably delay ferrite transformation. Accordingly,
the finishing temperature is set to 950°C or lower. On the other hand, \\'hen the
finishing telnperature is lower than 85OoC, the rolling load increases. Therefore, the
finisliing tcniperature is set to 850°C or Iiighen
[0047]
Then, t11c finish-rolled steel sheet is subjected to primary cooling and ail<
cooled, and further subjected to secondary cooling and coilcd. The primary cooling
rate is set to an average cooling rate of 50 'CIS or more. When the prinlary coolillg
rate is low, the grain diameter of ferrite is coarsened. Martensite is obtained by
transformation of residual austcnite in which ferrite trai~sfom~atiopnro ceeds. When
the grain diameter of ferrite is coarsened, the residual martensite is also coarsened.
The upper limit of tlie primary cooling rate is not pal-ticularly limited. When the
primary cooling rate is more than 100 "CIS, excessive facility costs are required and
thus a primary cooling rate of more than 100 "CIS is not preferable.
[0048]
Tlie primary cooling stop temperature is set to 600°C to 750°C. When tlie
prinlary cooling stop temperature is lower than 600°C, ferrite transformation cannot
sufficiently procccd during air-cooling. In addition, when the pritilary cooling stop
temperature is higher than 750°C, ferrite transformation excessively proceeds and
pcarlite transfor~nationo ccurs during the following cooling. 'fliercforc, the hole
expandability is deteriorated.
[0049]
'l'lic air cooling titne is set to 5 seconds to 10 seconds. When the air cooling
titile is shol-ter than 5 seconds, ferrite transforn~ationc annot sufficiently proceed. In
addition, when tlie air cooling time is longer than 10 seconds, pearlite transformation
occurs anct thus tlie hole expandability is deteriorated.
[OOSO]
Tlie secondary cooling rate is set to an averagc cooling rate of 30 'CIS or morc.
When the secondary cooling rate is less than 30 'CIS, bainite transformation
exeessi\rely proceeds during cooling and a sufficient area fiaction of ferrite cannot be
obtained. Thus, uniform elongation is deteriorated. The upper litnit thereof is not
particularly limited. When the secondary coolingrate is Inore than 100 "Cls,
excessive facility costs are required and thus a secondary cooling rate of more than
100 "CIS is not preferable.
[005 11
The coiling tetnperatt~reis set to 400°C or lo\ver. When the coiling
tetnperature is higher than 400°C, bainite transfornlation excessively proceeds and a
suflicient amount of ma~tcnsitec atmot be obtained. Thus, highly unifonn elongation
catmot be secured. The tetnperature range is preferably 250°C or lower and tnore
preferably 100°C or lower, and the tetnperature may be room temperature.
[Examples]
[0052]
Steels A to AJ having chemical con~positionss hown in Tables 1 and 2 as
Exanlples 1 to 48 were tnelted and cast to obtain slabs. The slabs were rolled under
thc conditions shown in Tables 3 and 4.

[0054]
[Table 21
[Table 31
to 1050°C
between
rough
rolling and
finish
Primary
cooling stop cooling
(tempeRrature time cooling temperature
[Table 41
finish
Finishing
temperature , Primary
cooling
rate
Primary
cooling stop
temperahire
Air
cooling
time
Secondary
cooling
rate
Coiling
temperature
[0057]
A satnple was collected from each of the obtained steel sl~eetsa nd the
metallographic structure was observed at a position which was at 114 of the steel sheet
thickness using an optical microscope. For preparation of the sa~llplet,h e cross
section of the stcel sheet thickness in a rolling direction was polished as a surface to bc
observed and was etched with a nital reagent and a Le Pera reagent. The itllage of the
sample etched with a nital reagent w\~l~icwha s obtained by observation through an
optical microscope at 500 times was analyzed to obtain area fractions of ferrite and
pearlitc. In addition, the itnage of the sample etched with a Le Pera reagent \vllich
\vas obtained by observatioll tlxougli an optical microscope at 500 times was at~alyzed
to obtain an area fractiotl and the average diarileter of the martensite. The average
diameter is obtained by number-averaging tllc equivalent circle diameter of each of the
grains of martensite. A martensite grain of less than 1.0 pm was exclndcd fs0111
number coutlting. The area fraction of bainite was obtained as the remainder of
ferrite, pearlite and martensite.
[0058]
The tensile strength (TS) \\'as evaluated according to JIS Z 2241:2011 using a
No. 5 test piece tiescribed in JIS Z 220 1 : 1998 collected fsotll each steel shect at a
position, wltich was at 114 in the steel sheet \\kith direction, in a direction
perpendicular to the rollitlg direction. The unifor~el longation (u-El) and total
elongation (t-El) werc measured together \vitl~th e tensile strength (TS). A Itole
expanding test was perfortned according to a test tlletllod described in Japan Iron and
Steel Federation Standard JFS TI00 I- 1996 to cvaluate l~olcx pandability. The
structures and mechanical properties of the steel shccts were sho\vn in Tables 5 and 6.
In Tables 5 and 6, VF rel)resetlts the area fraction (96) of ferrite, Vg represents the area
fraction (%) of bainite, Vp represents the area fraction (%) of pearlite, and VM
represents the area fraction (%) of martensite, respectively. DD,r\e, presents a
martensite average diameter (pm) and NM represents the nunlber density of mal-tensite
having an equivalent circle dianletcr of 3 pm or Illore per 10000 LunZa t a position
which is at a depth of 114 of the steel sheet thickness from the surface of the steel sheet.

[0061]
The results will be described. Examples 3 to 8, 16, 18, 19,21,22,24,26 to
28,30 to 32, 37,39,40, and 42 to 48 are examples of the present invention. In these
examples, the chc~nicacl ompositions of steel components, production conditions and
microstructnres satisfied the requirenlents of the present invention and both the
elongation and hole expandability were excellent. On the other hand, Examples 1,2, 9
to 15, 17,20, 23, 25, 29, 33 to 36, 38, and 41 are comparative exalnples. In these
conlparative cxanlples, effects were not able to be obtained due to the reasons shown
below.
[0062]
In Example 1, since Steel No. A in ~vhichth e anlount of M11 was large was used,
ferrite transfornlation did not suficielltly proceed. Therefore, the area fraction of
ferrite wvas less than 80% and thus the unifoiln elongation was lo~w~.
[0063]
In Example 2, since Steel No. B in which the amount of Nb was large was used,
ferrite transforlnation did not sufficiently proceed. Therefore, thc area fraction of
ferrite was less than 80% and thus the uniform elongation was low.
[0064]
I11 Exalnple 9, since the air cooling time ww7as too long, the forn~edp earlite
exceeded an appropriate range. Therefore, the hole expandability was low.
[0065]
In Example 10, since the finishing telnperature was too high, ferrite
tra~isfor~natidoild~ n ot sufficie~ltlyp roceed. Tl~crcforet,h e area fraction of ferrite was
less than 80% and thus the uniform elongation was lo\v.
[0066]
I11 Example 11, since the air cooling time was too short, ferrite transfor~l~ation
did not sufficie~ltlyp roceed. Therefore, the area fiaction of ferrite was less than 80%
and thus the unifortii elongation was low.
[0067]
In Example 12, since the primary cooli~lgra te was low, the average diameter of
martensite was large and as a result, Expression 1 was not satisfied. Therefore, the
hole expandability was low.
[0068]
In Exanlples 13 and 20, since the nnmber of rolling passes in a tenlpcrature
range of 1000°C to 1050°C was small, the number density of coarse maltensite was
high. Therefore, the hole expandability was low.
[0069]
I11 Exatilple 14, since the reductiotl in a tc~ilperaturera nge of 1000°C to
1050°C was low, the average diameter of marteasite was large and as a result,
Expression 1 was not satisfied. Therefore, the hole expandability was low.
[0070]
In Example 15, since the time fiom the end of rough rolling to the start of
finish rolli~~wga s long, austenite was coarse~tcda tid the average diameter of nlarte~lsite
was large. Therefore, R/D~? was decreased and the hole cxpandability was lo\v.
[0071]
In Exatllple 17, since Steel No. I in \\lliich the amount of C was large was used,
thc area fiaction of inartensite was high. Therefore, the hole expandability was lo\v.
[0072]
In Exan~ple2 3, since Steel No. 0 in which the amount of Si+Al was snlall was
used, ferrite transfor~liationd id riot sufficic~itlyp roceed. Therefore, the utlifortll
elongation was low.
[0073]
In Exaniple 25, since the primary cooling rate was lo~vt,h e average diameter of
maltensite was large and as a result, Expression 1 was not satisfied. Therefore, tlie
hole expandability was low.
[0074]
In Example 29, since Steel No. U in \vliich tlie a~nounot f Ti was large was
used, ferrite was excessively strengthened. Therefore, the uniform elotigation was loxv.
[0075]
In Exatnple 33, since tlie priniary cooling rate was high, pearlite was formed.
Therefore, the hole expandability was lo\v.
[0076]
In Example 34, since the coiling temperature was too high, martensite was
rarely formed. Tlierefore, tlie iuiiforni elongation was low.
[0077]
In Exatnple 35, since the primary cooliilg stop temperature was too low, ferrite
transforniation did not sufficiently proceed. Therefore, the area fraction of ferrite was
less than 80% and the uiliforrn elongation was low.
[0078]
I11 Exatnple 36, since tlie secondary cooling rate was lo\v, bainite was fonned.
Therefore, the area fraction of fcrritc was less than 80% and the uniforn elongation was
Io\v.
[0079]
In Exatnple 38, since Steel No. Y in which the amount of C was small was used,
tlie area fraction of nlarletrsite was less than 3%. Tlierefore, tlie uniforin elongation
was low.
[0080]
In Example 41, since Steel No. AC in which the atlloutlt of Mn was small was
used, martensite was not for~i~edT. herefore, the u~liforne~lo ngation was low.
[I~ldustriaAl pplicability]
[008l]
Accordi~igto the present i~ir~e~ltiiot nis, possible to provide a high-strength hotrolled
steel sheet capable of attainit~ge xccllcnt eloligation atid hole expandability
without containing an expensive element and a mnetliod for producilig the same.

[Document Type] CLAIMS
1. A hot-rolled steel sheet comprising, as a chemical conlposition, by mass%:
C: 0.030% to 0.10%;
Mn: 0.5% to 2.5%;
Si+AI: 0.100% to 2.5%;
P: 0.04% 01. less;
S: 0.01% or less;
N: 0.01% or less;
Nb: 0% to 0.06%;
Ti: 0% to 0.20%;
V: 0% to 0.20%;
W: 0% to 0.5%;
Mo: 0% to 0.40%;
Cr: 0% to 1.0%;
Cu: 0% to 1.2%;
Ni: 0% to 0.6%;
B: 0% to 0.005%;
REM: 0% to 0.01%;
Ca: 0% to 0.01%; and
a remainder consisting of Fe and i~llplrrities,
\vl~ereint he steel sheet has a microstructure iincluding, by area fraction, ferrite:
80% or nlorc, martensite: 3% to 15.0%, and pearlite: less than 3.0%, in m~llical~ n utllber
density of ~nartcllsitch aving an equivalent circle diameter of 3 pm or more at a position
\vl~ichi s at a dcpth of 114 of the steel shcct tllic!u~cssf ro~ltlh e surface of the steel sheet,
is 5.0 piecesllOOO0 pm2 or less, and the following Expression (1) is satisfied,
R/D~,? 2 1 .OO ... Expressio~(l1 ),
here, R is an average ~llarte~isiitnet erval (pm) defined by the followitig
Expression (2), and DM is a martensite average diatileter (pnl),
R= (12.5 x ( ~ / 6 ~ i , ~ ) ~ - ~x( D 2j-,, / .3..E )x~pr~es~si)ot i (2),
here, VM is a martensite area fraction (%) and Dh, is the martensite average
diatneter (p~ii).
2. The hot-mlled steel slieet accordiilg to Claitn 1, fi~rthecr omprising, as a
cl~e~niccaoln iposition, by mass%,
at least one ofNb: 0.005% to 0.06% and Ti: 0.02% to 0.20%.
3. The hot-rollcd steel sheet according to Claim 1 or 2, fi~rthecr otnprising, as
a che~liicalc o~ilpositionb, y mass%,
at least one of V: 0.02% to 0.20%, W: 0.1% to 0.5%, and Mo: 0.05% to 0.40%.
4. The hot-rolled steel shcct according to any one of Claitns 1 to 3, fi~rtlier
comprising, as a chcmical composition, by tllass%,
at least one of Cr: 0.01% to 1.0%, Cu: 0.1% to 1.2%, Ni: 0.05% to O.6%, and
B: 0.0001% to 0.005%.
5. The hot-rolled steel sheet accordi~igto ally one of Claims 1 to 4, furtliet
comnprisiag, as a clicmical co~iipositiotib, y mass%,
at least one of E M : 0.0005% to 0.01% and Ca: 0.0005% to 0.01%.
6. A method for producing a hot-rolled steel sheet co~iiprisitig:
heating a slab having the chemical conlposition according to any one o-f .Cl aims
1 to 5, to 1150°C to 1300°, then subjecting the slab to multipass rough rolling and
rolling the slab with four br more final passes of rolling in a temperature range of
1000°C to 1050°C at a total reduction of 30% or more to form a rough bar;
starting rolling on the rough bar within 60 seconds after completing the rough
rolling and subjecting the rough bar to finish rolling to complete rolling in a temperature
range of 850°C to 950°C to obtain a finish-rolled steel sheet; and
after cooling the finish-rolled steel sheet to a temperature range of 600°C to
750°C at an average cooling rate of 50 'CIS or more and air-cooling the steel sheet for 5
seconds to 10 seconds, cooling the steel sheet to a temperature range of 400°C or lower
at an average cooling rate of 30 "CIS or more and coiling the steel sheet to obtain a hotrolled
steel sheet.