[Document Tj~pe] Specification
[Title of the Invention] STEEL SHEET, PLATED STEEL SI-IEET, AND METHOD
FOR PRODUCING THE SAME
[Technical Field of the Invention]
[OOOl]
The present invention relates to a high-strength steel sheet and a plated steel
sheet which have excellent fatigue properties, ductility, and hole expansibility, and
further, excellent collision properties, ww~l~icihs suitable for a steel sheet for a vehicle,
particularly suitable for a suspe~isiop~alr t, and a itlethod for producing the same.
Priority is claimed on Japanese Patent Application No. 2012-032591, filed on
February 17,2012, the content of n41ich is incorporated herein by refere~lce.
[Related At]
[0002]
I11 recent years, in order for auto~nakersto cope with tlie tiglitetening of C02
e~nissiroe~gu~l ations in Europe in 2012, fuel econollly regulatio~lsin Japan in 2015,
and stricter collision regulations in Europe, 11igh-strengtbeni~~ogf steel to be used has
rapidly progressed to improve fuel economy throi~gha decrease in tlie wwcight of a
vehicle body and improve collisio~si afety. Such a high-strength steel sheet is called a
"hig11 strength steel sheet", and orders of steel sheets mainly having a tensile strength
of 440 MPa to 590 MPa, and recently more than 590 MPa, tends to increase every year.
[0003]
Anloag the high stre~~gstele~l sheet, excellelit fatigue properties are required
for a suspension part such as a chassis frame from the viewvpoitit of the application
portion thereof, and fnrther, ductility, and hole expansibility are required from the
viewpoint of the shape of the parts. On the other hand, a hot-rolled steel sheet which
is thick and has a thickness of 2.0 111111 or lnore is ~llai~tulyse d for the suspe~lsiop~alr t,
and the quality is guaranteed by selecting a thick ~naterialf or securing rigidity. Thus,
thi~minga suspension part is being delayed compared to vehicle body parts or tlte like.
Accordingly, \vlten reduction in the thickness of the suspension part is promoted, a
corrosion thinning area thereof is reduced, and tht~si,t is expected that an application to
a hot-dip galva~tizeds teel sheet having high corrosior~r esistance fro111 the current hotrolled
steel sheet will be made.
[0004]
Generally, it is considered tltat \vhen a fatigue stre~igthr atio obtained by
dividing fatigue strength by tensile strengtlt is 0.45 or tnore, fatigue properties are
excellent. In addition, it is considered that when the product of tensile strength and
total elongatio~ils 17000 MPa% or more, ductility is excellent, and when hole
expansion ratio is 80% or more at a tensile stretlgth of 590 MPa class, hole
expansibility is excelleat. It is considered that \vhen a yield ratio obtained by
dividing yield strength by tensile strength is 0.80 or more, collisio~re~s istance is
escellellt.
[OOOS]
Generally, wlten tensile strength increases, yield strength also increases.
Thus, ductility is decreased, and further, stretch flangeability is deteriorated. 111 the
related art, in a case of dual phase (DP) steel including a dual phase of ferrite and
martensite, the ductility is excellent, but micro-cracks caused by local strain
concentration in the vicinity of a bounda~yb ehveen ferrite whicll is a soft phase and
martensite which is a hard phase easily occur or propagate, and thus, it is considered
tltat the dual phase is a disadvantageous microstructure in Itole expansibility.
Accordinglj: it is considered that tlte smaller the hardness difference between tlte
~~~icrostn~ctisu, rtehse lllore advantageous it is in hole expansibility improvement, and
thus, a steel sheet having a u11ifor111s tructure such as a ferrite or bainite single phase is
considered to be superior. On the other hand, sirice the ductility is decreased, it has
been difficult to attain both ductility and hole expansibility in the related art.
[0006]
In addition, generally, \\rhea telisile strength increases, fatigue strength also
tends to increase. However, \\then a material having a higher strength is used, a
fatigue strength ratio decreases. In addition, the fatigue strength ratio is obtained by
dividing the fatigue strength of a steel sheet by tensile streagth. Generally, the harder
the outermost surface of a steel sheet is, the more the fatigue strengtli of steel is
improved. Thus, the hardening of the outn~osstu rface of the steel sheet is important
to obtain excellent fatigue properties.
[0007]
As a steel sheet in which both hole expansibility and ductility are attained, for
example, in Patent Doculllent 1, a steel sheet to which A1 is positively added and,
carbonitride fornling elements such as Nb, Ti, and V are positively added has been
proposed so far. However, it is necessary to add 0.4% or nlore ofAl in a large
alilount to the steel sheet, and thus, the steel sheet proposed in Patent Document 1 has a
problen~o f a higher alloy cost and deterioration in \veldability. 111 addition, there is
110 description regarding fatigue properties or a yield ratio as a collision resistance
index is also not disclosed.
[OOOS]
In Patent Doct~n~en2t sa nd 3, high-strength steel sheets having excellent hole
expa~isibilityto which Nb and Ti are positively added have been proposed. However,
since Si is positively added to the high-strength steel sheets proposed in Patent
Docunie~~2t sa nd 3, the steel sheets have a problem of deterioration in plating
wettability. In addition, there is no description regarding fatigue properties or a yield
ratio as a collision resistance index is also not disclosed.
[0009]
In Patent Document 4, a steel sheet having both fatigue properties atid hole
expansibility to whic11 Nb and Ti are positively added has been proposed. However,
since 1F steel is used as a base, the steel sheet proposed in Patent Document 4 has a
problem that it is hard to achieve Iiigll-strengthening in \vliich the tensile strengtl~is
590 MPa or more. In addition, a yield ratio as a collisio~rie sistance index is not
disclosed.
[0010]
111 Patent Docu~lie~5l,t a hi&-strength steel sheet in which both fatigue
properties and hole expansibility are attained by controlling an inclusion in the steel
has bee11 proposed. Ho\vevel; since it is necessary to add a rare metal sucli as La or
Ce to the steel sheet proposed in Patent Document 5, a higher alloy cost is required and
a yield ratio as a collision resistance index is not disclosed.
[001 I]
In Patent Document 6, a steel slieet havitig excelletlt hole expansibility to
\vllicl~c arbonitride forming elenle~ltss ucli as Nb, Ti, Mo, and V are positively added
has been proposed. Ho\vever, the Vickers hardness of ferrite in the steel sheet
proposed in Patent Docoment 6 has to be 0.3 x TS + 10 or more. Since it is assumed
that the target tensile strength in the present invention is 590 MPa or higher, the
Vickers hardness of ferrite has to be at least 187 Hv or more and a large amount of
alloying elements (particularly, carbonitride fomiing elements sucli as C, Nb, atid Ti,
and ferrite stabilizing elements sucli as Si) has to be added to liarden ferrite, and thus, a
higher alloy cost is required and a yield ratio as a collision resistance index is not
disclosed.
[Prior Art Document]
[Patent Docuinent]
[0012]
[Patent Docutncnt I] Japanese Unexamined Patent Application, First
Publication No. 2004-204326
[Patent Docunnent 21 Japanese Unexamined Patent Applicatio~~Fi,r st
Publication No. 2004-225 109
[Patent Document 31 Japanese Unexamined Patent Application, First
Publication No. 2006-152341
[Patent Document 41 Japanese Uncsatnined Patent Application, First
Publication No. H7-090483
[Patent Document 51 Japanese Unexamined Patent Application, First
Publication No. 2009-299136
[Patent Docuinent 61 Japanese Unexamined Patent Application, First
Publication No. 2006-161 1 11
[Disclosure of the hlvention]
[Problems to be Solved by tlie I~nvention]
[0013]
TIle present invention is to stably provide a high-strength steel sheet a plated
steel sheet \vhicIn have excellent fatigue properties, ductility, and hole expansibility,
and further, eexellent collision properties, without deterioration in prodrtctivity.
[Means for Solving the Problem]
[0014]
The present invention is a finding obtained from an investigation that has been
conducted to solve the above inentioned problems of improving fatigue properties and
improvement in ductility-llolc expansibility balance of a high-strength steel slieet and a
plated steel slieet whose tensile strength is 590 MPa or more. That is, an appropriate
microstructore is attained by optinlizing tlie amount of alloying elements, particularly,
optimizing the amount of Nb and Ti added and by positively adding Al. In addition,
in an annealing process, tlle shape of cementite in ferrite is precisely controlled by
cooli~lgth e steel to an appropriate temperature, and holding the cooled steel after
heating to the maximum heating temperature. Tlien, the surface is hardened by
carrying out appropriate skin pass rolling on the steel after the annealing. The present
invention is made based on tlie findings in \vliicli a steel sheet having excellent fatigue
properties, ductility, and hole expansibility, and fiirther, excellent collisio~pl prpe~ties,
compared to steel sheets of the related art, can be produced in the above manner, and
the sulnmary thereof is described as follows. There is no upper limit in the te~~sile
strength of a steel sheet as a target of tlie present technology; l~o\vevel; it is difficult for
the tensile strengtli to be Illore than 980 MPa in reality.
[0015]
(1) According to a first aspect of tlie present invention, there is provided a
steel sheet including, by mass%: C: 0.020% or more and 0.080% or less; Si: 0.01% or
Inore and 0.10% or less; Mn: 0.80% or niore and 1.80% or less; Al: niore than 0.10%
and less than 0.40%; P: limited to 0.0100% or less; S: limited to 0.0150% or less; N:
limited to 0.0100% or less; Nb: 0.005% or more and 0.095% or less; Ti: 0.005% or
Inore and 0.095% or less; and a balance including Fe and unavoidable impurities, in
which a total amount of Nb and Ti is 0.030% or Inore and 0.100% or less, a
nietallograpliic structure of the steel slieet includes ferrite, bainite, and other phases,
the other phases inclnde a pearlite, a residual austenite, and a ma~te~lsitae1, area
fiaction of the ferrite is 80% or more and 95% or less, an area fraction of the bainite is
5% or nlore and 20% or less, a total fiaction of the other phases is less than 3%, an
equivalent circle diameter of a cementite in the ferrite is 0.003 prn or nlore and 0.300
pm or less, a number density of the cementite in the ferrite is 0.02 particles/pm2 or
more and 0.10 p a r t i c ~ e s / ~or~ l~esns,~ a tensile strength is 590 MPa or more, and a
fatigue strength ratio as a fatigue strength to the tensile strength is 0.45 or more.
[0016]
(2) The steel sheet according to (1) may filrther include one or two more of,
by nlass%: Mo: 0.005% or more and 1.000% or less; W: 0.005% or niore and 1.000%
or less; V: 0.005% or nlore and 1.000% or less; B: 0.0005% or more and 0.0100% or
less; Ni: 0.05% or inore and 1 .SO% or less; Cu: 0.05% or more and 1 .SO% or less; and
Cr: 0.05% or Inore and 1.50% or less.
[0017]
(3) According to a second aspect of the present invention, a plated steel
sheet is provided in n~l~ica hp lating is provided on a surface of the steel sheet
according to (1) or (2).
[0018]
(4) According to a third aspect of the present invention, a method is
provided for producing a steel sheet including: heating a slab having a chcmical
composition according to (I) or (2) to 1150°C or higher before the slab is hot-rolled;
finishing finish rolling at a temperature of Ar3"C or higher; pickling a hot-rolled steel
sheet which is coiled ~vithina tenlperatnre range of 400°C or higher and 600°C or
lower; heating the hot-rolled steel sheet within a temperature range of 600°C or higher
and Acl°C or lower; annealing the hot-rolled steel sheet for a holding time, in ~vhich
tlie temperature of the hot-rolled steel sheet is within the temperature range for 10
seconds or longer and 200 seco~idso r shorter; cooling tlie steel sheet to 35OoC or
higher and 550°C or lo\ver; and cooling the steel slieet after holding tlie steel slieet for
the holding time, in \\~hichth e te~nperatureo f the hot-rolled steel slieet is within a
temperature range of 350°C or higher and 550°C or lower for 10 seconds or longer and
500 seconds or shorter, in which the Ar3OC and tlie Acl°C are a Ar3 transformation
temperature and a Acl transfonnatio~te~m perature, respectively, obtained from
expressio~is 1 and 2,
Ar3 = 910 - 325 x [C] + 33 x [Si] + 287 x [PI + 40 x [All - 92([Mll] + No] +
[Cu]) - 46 x ([Cr] + [Nil) ... (Expression l),
Acl = 761.3 + 212[C] - 45.8[M1i] + 16.7[Si] ... (Expression 2), and
elements noted in brackets represent an amount of the elements by mass%.
[OO 1 91
(5) The method for producing a steel sheet according to (4) may fiirther
include carlying out skin pass rolling on the steel sheet at an elongation ratio of 0.4%
or more and 2.0% or less.
[0020]
(6) According to a fourth aspect of the present invention, tliere is provided a
method for producing a plated steel sheet i~icludingp lating and then cooli~lgth e steel
slieet after tlie annealing, the cooling, and holding according to (4) or (5).
[0021]
(7) The method for producing a plated steel sheet according to (6) may
further include carrying out a heat treatnlent within a temperature range of 450°C or
higher and 600°C or lower for 10 seconds or longer and tl~enc ooling the steel sheet
after the plating.
[Effects of the I~mention]
[0022]
According to the present invention, it is possible to provide a high-strength
steel sheet and a plated steel sheet, which have a tensile strength of 590 MPa or more,
a high yield ratio, and excellent fatigue properties and ductility-hole expansibility
balance, and further, excellent collisio~pl roperties, and which make an extremely
significant contribotion to the industry. Further, the present invention makes it
possible to reduce the sheet thickness of a suspensio~pl art of a vehicle and tllus
exhibits an extrelilely relilarkable effect that significalltly contributes to a decrease in
the weight of a vel~icleb ody.
[Brief Description of the Drawing]
LO0231
FIG. 1 is a graph showing a relationship betwveen an average equivalent circle
diameter of carbollitrides and a product of tensile strength and total elongatioa.
FIG. 2 is a graph slio\ving a relationship betwveen an average equivalent circle
diameter of carbonitrides and a hole espa~isior~atli o 1,.
FIG. 3 is a graph showi~iga relatiotisliip behveen at1 average equivalent circle
diameter of carbonitrides and a yield ratio.
FIG. 4 is a graph sliowiug a relationsliip behveen all average equivalent circle
diameter of carbonitrides and a fatigue strength ratio.
FIG. 5 is a graph showing a relationship between a holding te~iil~eratuareft er
a~lllealinga nd an equivalent circle diameter of cementite in ferrite.
FIG. 6 is a graph showing a relationship between a holding te~nperaturea fter
annealing and a nutnber density of ce~iientitein ferrite.
FIG. 7 is a graph showing a relationship between an equivalent circle diameter
of ce~i~etltiitne ferrite and a hole expansion ratio h.
FIG. 8 is a graph sl~o\vinga relationship between a number density of
cementite in ferrite and a hole expansion ratio h.
[Embodiments of the Invention]
[0024]
Hereinafter, the present invention will be described in detail.
First, the reasons \vhy steel cotnpositions are limited in the present invention
will be described.
C is an element \vhich contribntes to an increase in tensile strength and yield
strength, and the amount added is appropriately co~~trolleadcc ording to a targeted
strength level. In addition, C is also effective in obtaini~igb ainite. When the
amoont of C is less than 0.020%, it is difficult to obtain a target tensile strength and
yield strength, and tli~rst,l ie lotver limit is set to 0.020%. On the other hand, when the
anlount of C is more than 0.080%, deterioration in the ductility, hole expansibility, and
nleldability is caused. Thus, the upper limit is set to 0.080%. In addition, in order to
stably secure the te~lsiles trength and yield strength, the lou~erli mit of C may
preferably be 0.030% or 0.040%, and the upper li~iliot f C n1ay preferably be 0.070%
or 0.060%.
[0025]
Si is a deoxidizing element and the lower limit of tlie amount of Si is not
deter~nined. However, when the amount of Si is less than 0.01%, the production cost
increases, and thus, the lower limit is preferably set to 0.01%. Si is a ferrite
stabilizing element. In addition, Si may causes a problenl of a decrease in plating
\vettability when hot dip galvanizillg is carried out and a decrease i11 productivity due
to the delay of alloying reaction. Therefore, the upper lin~iot f the atnot~not f Si is set
to 0.10%. Further, in order to reduce the problem of a decrease in plating \vettability
and a decrease in productivity, the lower linlit of Si may be set to 0.020%, 0.030%, or
0.040%, and tlie upper liniit of Si tnay be set to 0.090%, 0.080%, or 0.070%.
[0026]
Mn has an action of illcreasing the strengtll as an element that contributes to
solid solution strengthening, and is thus effective in obtaining bainite. 'Illerefore, it is
necessasy to contain 0.80% or more of Mn. On the other hand, when tlie amount of
Mn is more than 1.80%, deterioration in hole expansibility and velda ability is caused,
and tlius, the upper linlit thereof is set to 1.80%. In addition, in order to stably obtain
bainite, tlie lo\ver limit of Mn may be set to 0.90%, 1.00%, or 1.10%, and tlie upper
limit of mi nlay be set to' 1.70%, 1.60%, or 1.50%.
[0027]
P is an impurity, and is segregated at grain boundaries and causes a decrease
in tlie totrgliness of the steel sheet and deterioration in the ~ieldability. Further, the
alloying reaction beconles extren~elys lo~vd uring hot dip galvanizing, and the
productivity is degraded. Froin the vie\vpoints, the upper linlit of the a~nounot f P is
set to 0.0100%. The lower linlit thereof is not patticlrlarly limited. However, since
P is an elenle~iwt hich increases strength at a low price, tlie a~nou~oift P is preferably
set to 0.0050% or more. In order to fi~rtlierim prove the tot~ghnessa nd the weldability,
tlie upper limit of P map be limited to 0.0090% or 0.0080%.
[0028]
S is an impurity and xvlien the anlount thereof is more than 0.0150%, hot
cracking is induced or workability is deteriorated. Thus, the upper limit of the
amot~not f S is set to 0.0150%. The lower limit thereof is not particularly limited, but
the amount of S is preferably set to 0.0010% or more from the viewpoint of a
desulfurization cost. In order to further reduce hot cracking, tlte upper limit of S may
be limited to 0.0100% or 0.0050%.
[0029]
Al is an extremely in~portanet lenlent in the present inventio~i. Although Al
is a ferrite stabilizing element similar to Si, Al is at1 important ele~nenwt hich promotes
ferrite for~ilationw ithout a decrease in plating wettabilitJ: thereby securing ductility.
In order to obtain tlte effect thereof, it is necessary to contain tnore than 0.10% ofAl.
I11 addition, wlten Al is excessively added, not only is tlte above-described effect
saturated, but also an excessive increase in an alloy cost and deterioration in
weldability are caused. Thus, the upper litnit is set to less than 0.40%. In order to
stably secure ductility, the lower liniit ofAl may be set to 0.15%, 0.20%, or 0.2594, and
the upper limit ofAl may be set to 0.35% or 0.30%.
[0030]
N is an impurity. Wlten the a~iiounot f N is kilore than 0.0100%,
deterioratio~ui l tonghlless and ductility and occurrence of cracking in a steel piece are
significant. Since N is effective in increasing tensile strength and yield strength,
similar to C, N may be positively added as the upper li~iliits set to 0.0100%.
[003 1 ]
Further, Nb and Ti are extrenlely iniportant elenie~ltsin the present invention.
These ele~ncntsa re necessary \when a steel sheet having excellent collisio~pi roperties is
prepared by fornting carbonitrides so as to increase the yield strength. The
precipitation strengtltening of tlte respective elenteats is different. However, ~vllen
both Nb and Ti are contained in total of 0.030% or more, the product of the tensile
strength TS and the total elongation El as sho\\a in FIG. 1 is excellent, and a tensile
strength of 590 MPa or more can be obtained. F-'urther,e xcelle~tht ole expansibility
(hole expansion ratio 1,) as shown in FIG. 2 can be obtained. Moreover, it is possible
to obtain a yield ratio as a collision property index of 0.80 or more and a fatigue
strength ratio, as a fatigue property index of 0.45 or more as shown in FIGS. 3 and 4.
The higher the fatigue strength ratio is, the more preferable it is. However, it is
difficult for tile fatigue strengtll ratio to be more than 0.60, and thus, 0.60 is the actilal
upper limit. Also, when Nb and Ti are added compositely, finer carbonitrides can be
obtained cotnpared to a case in \vhich Nb and Ti are added singly, and precipitation
strength is increased. TIILIS, it is important to add these elelnents compositely. I11
addition, the reason ~vlvhp the upper lilnit of the total amount of both Nb and Ti is set to
0.100% is not only that there is a limitation in precipitation strengthening and the
strength is actually not increased anj7 more even \irhen Nb and Ti are added more, but
also that the ductility and hole expansibility are decreased as shown in FIGS. 1 and 2.
In order to stably secure the product of tensile strength and total elongation, the hole
expansibility, the yield ratio, and the fatigue strength ratio, the lower lin~iot f the total
content of bot11Nb and Ti may be 0.032%, 0.035%, or 0.040%, and the upper limit of
the total colltent of bot11Nb and Ti may be 0.080%, 0.060%, or 0.050%.
'I'he reason why the lo\ver limit of each ofNb and Ti is set to 0.005% is that
few carbonitrides are forlned when the content is less than 0.005%, the effect of an
increase in yield strength is hardly obtained, and finer carbonitrides cannot be obtained.
In addition, hole expansibility is decreased. The upper limit of each of Nb and Ti
depends on the upper litnit of the total amount of both Nb and Ti.
[0032]
All of Mo, W, and V arc elements which form carbonitrides, and one or two or
no re of these elelnents may be used as required. In order to obtain the effect of
strength irnproveme~~0t.,0 05% or nlore of Mo, 0.005% or Inore of W, and 0.005% or
more of V are preferably added as the lower limits. 011 the other hand, since
excessive addition causes an increase in an alloying cost, the upper limits are
preferably set to 1.000% or less of Mo, 1.000% or less of W, and 1.000% or less of V,
respectively.
[0033]
All of B, Ni, Cu, and Cr are elements wliicll increase hardenability, and one or
two of more of these elements may be added as required. In order to obtain the effect
of strength inlprovement, 0.0005% or Inore of B, 0.05% or more ofNi, 0.05% or tiiore
of Cu, and 0.05% or more of Cr are preferably added as the lower linlits. On the
other hand, since excessive addition causes an increase in an alloyitig cost, tile npper
limits are preferably set to 0.0100% or less of B, 1.50% or less of Ni, 1.50% or less of
Cu, and 1.50% or less of C1; respectively.
In the high-strength steel sheet containing the above-described chemical
con~positions,a balance i~lcludingir on as a main conlposition lnap contain unavoidable
impurities nlixed in a production process n~itliinth e range (hat does not inipair the
properties of the present invention.
[0034]
Next, tlie reasons why a production method is limited will be described.
A slab liavi~igtl ie above-described composition is heated at a temperature of
11 50°C or higlies. As the slab, a slab imniediately after being produced by a
continuoos casting facility or a slab produced by an electric furnace may be used.
The reason n~hyth e temperah~reis limited to 1150°C or higher is to sufticie~ltlltl~~
deco~nposea nd dissolve carbonitride fonning elements and carbon. In such case, tile
tensile strength, the product of tensile stretlgth and total elongation, the yield ratio, and
the fatigue strength ratio become excellent. In order to dissolve the precipitated
carbonitrides, the temperature is preferably 1200°C or higher. However, \\rlien the
heating temperature is lligher than 128OoC, the temperatore is not preferable from tlie
viewpoint of productio~lc osts, and thus, 1200°C is preferably set as the upper limit.
[0035]
In order to prevent deterioration in fatigue properties doe to the fact that whea
a finishing temperature in hot rolling is lo\\rer than an Ar3 transformation temperature,
carbonitrides are precipitated and the particle size is coarsened on the surface, and the
strength of the surface is significantly decreased, A r 3 transformation temperature is set
as the lower limit of tlie finishing temperature iin hot rolling. The upper lin~iot f the
finishing temperatore is not particularly limited, but 1050°C is substantially set as the
upper limit.
I-lere, Ar3OC is an A r 3 transforniation temperatore obtained by the follo\\ing
Expression 1.
A r j = 910 - 325 x [C] + 33 x [Si] + 287 x [PI + 40 x [All - 92 x ([Mn] + [Mo]
+ [Cn]) - 46 x ([Cr] + [Nil) ... (Expression 1)
Wherein, elenlents noted in brackets represent an amount of the elenlents by
mass%.
[0036]
A coiling temperature after finishing rolling is an extrenlely iinportant
production cotldition in the present invention. In the present invention, the control of
the precipitation of carbonitrides by setting the coiling temperature to 600°C or lower,
is inlportant at the stage of the hot-rolled steel sheet, and the properties of tlie present
invention is not deteriorated by the past history up to that time. When the coiling
temperatore is higher than 60OoC, carbonitrides on the hot-rolled steel sheet are
precipitated, sufficient precipitation strengthening after atlnealing cannot be attained,
and thns, the tensile strength, the yield ratio, and the fatigue properties are deteriorated.
Therefore, 600°C is set as the upper litnit. Further, lit hen the coiling temperature is
600°C or lower, bainite is obtained, and it is effective in improving the strength. In
addition, when the coiling temperature is lower than 400°C, a sufficient amount of
ferrite cannot be obtained, and the ductility, the prodnct of tensile strength and total
elongation, and the hole expansibility are decreased. Thus, 400°C is set as the lower
limit.
[003 71
Since a hot-rolled steel sheet is nsed as a base ~nateriaflo r the steel sheet of
the present invention, the steel sheet is then subjected to typical pickling and annealing
\vithout cold rolling by a tanden1 rolling mill after hot rolling. Ho\vever, rolling such
as temper rolling (reduction of about 0.4% to 10%) Inay be carried out before
annealing for the purpose of improving the shape to avoid meandering or the like when
the steel sheet passes through a continuous alltlealing facility.
[003 81
The anllealing is preferably carried out by the continuous annealing facility to
control the heating temperature and the heating titne. The maximum heating
temperature in the annealing is an extremely important production condition in the
present invention. The lower linlit of the maxinlum heating te~nperature is set to
600°C, and the upper limit is set to an ACIt ransfor~nationte mperature. When the
maxia~umh eating temperature is lower than 60OoC, the precipitation of carbonitrides
is insufficient in the a~mealinga, nd the tensile strength and the yield strengtll are
decreased. Further, the fatigne properties are decreased. On the other hand, when
the maxin~um heating temperature is higher than the Acl transformation temperature,
the coarsening of the carbonitrides and the transforn~ationf rom ferrite to austenite
occur, and insufficient precipitation strengthening is attained. Thus, the Acl
transfor~iiationt emperature is set as the upper limit.
Here, Acl°C is an Acl transforniation temperature obtained by the follo~ving
Expression 2.
Ac~= 761.3 + 212[C] -45.8[Mn] + 16.7[Si] ... (Expression 2)
Wherein, eletiients noted in brackets represent an amount of the elements by
mass%.
[0039]
A holding time at the niaximum heating temperature in tlie annealing is an
extremely important production condition in the present invention. The holding time
of the steel sheet within the temperahire range of 600°C to the Ac~tra nsforniation
tenlperature is set to 10 seconds to 200 seconds. This is because when the holding
time of the steel sheet at the maximum heating temperature is shorter than 10 seconds,
the precipitation of carbonitrides is insufficient, and sufficient precipitation
strengthening cannot be attained. Thus, a decrease in the tensile strength, the yield
strength, and the fatigue strength is caused. On the other hand, when tile holding time
of tlie steel slieet at tlie tnaxitiiu~ihi eating temperahire is long, a decrease in the
productivity is caused, and also, coarsening of the carbonitrides is caused. Thus,
sufficient precipitation stre~~gthenincgan ~lobt e attained, and the tensile strength and
the yield strength are decreased. Further, the fatigue strength is decreased. Thus,
200 seconds are set as the upper limit.
[0040]
After the annealing, the steel slieet is cooled to 350°C to 550°C and held the
steel sheet within tlie above temperature range for 10 seconds to 500 seconds. The
holding in the above temperature range is extremely i~nportanitn the present invention,
and the hole expansibility can be improved through the precipitation of fine ce~nentite
in ferrite as far as possible by holding the steel sheet at 350°C to 550°C after the
annealing. When the holding temperature is higher than 550°C, the cementite in the
ferrite is coarsened as shown in FIG. 5, the number density of the cementite in the
ferrite is also increased as sho\vn in FIG. 6, and thus, the hole expansibility is
deteriorated as showtl in FIGS. 7 and 8. Therefore, the upper limit is set to 550°C.
In addition, when the holding tetnperature is set to lower that1 350°C, the effect of
precipitating fine cementite in the ferrite is reduced, and thus, the lower liltlit is set to
350°C. When the holding time within the above temperature range is longer than 500
seconds, the cementite in the ferrite is coarsened, the number density thereof is
increased, and the hole expansibility is deteriorated. Thus, the upper linlit is set to
500 seconds. When the holding time within the above temperattire rage is shorter
than 10 seconds, the effect of precipitating fine cementite in ferrite cannot be obtained
sufficiently, and thus, the lower limit is set to 10 seconds. After the holding of the
steel sheet, the steel sheet is cooled to root11 temperature.
In addition, the cooli~tgra te after the annealing nlay be appropriately
controlled through spraying of a coolant, such as water, air blowing, or forcible cooling
using mist or the like.
[0041]
When the steel sheet is subjected to hot dip galvanizing or galvattnealing after
the cooling after the annealing is carried out, the composition of zinc plating is not
particularly limited, and in addition to Zn, Fe, Al, Mn, Cr, Mg, Pb, Sn, Ni, and the like
!nay be added as required. The plating may be carried out as a separate process from
annealing, but is preferably carried out through a co~~tini~aonunse aling-hot dip
galva~~izinligli c in which annealing, cooling and plating are continuously carried out
from the viewpoint of the productivity. When the follo\ving alloying treatment is not
carried out, the steel sheet is cooled to room temperature after the plating.
[0042]
When an alloying treatment is carried out, it is preferable tliat the alloying
treatment is carried out within a temperature range of 450°C to 600°C after tlie plating,
and then, the steel sheet be cooled to rooai temperature. This is because alloying
does not sufficiently proceed at a temperature of lower than 450°C, and alloying
excessively proceeds at a telilperature of higher than 600°C such tliat the plated layer is
e~ilbrittledto cause a probleni of exfoliation of the plated layer during \orking such as
pressing or tlie like. When an alloying treatment time is shorter than 10 seconds,
alloying does not suficie~ltlyp roceed, and thus, 10 seconds or longer is preferable.
In addition, the upper liti~iot f tlie alloying treatment time is not particularly limited,
but preferably within 100 seconds from the viewpoint of prodoctivity.
Fro111 tlie viewpoint of productivity, it is preferable that an alloyi~igtr eat~iient
filrnace be provided contiuooosly to tlie co~iti~luoaunsn ealing-hot dip galvanizing line
to carqr out a~lliealingc, ooling, plating and ai alloying treatment, and cooli~igin a
cotiti~luousm anner.
Exa~ilpleso f the plated layer sho\vn in examples include hot dip galva~lizing
and galvanaealing, but electrogalvanizing is also included.
[0043]
Skin pass rolling is extrenielp important in the present invention. The skin
pass rolling has tlie effects of not only correcting the shape and securing surface
properties, but also improving the fatigue properties by hardening the surface. Thus,
the skin pass rolling is preferably carried oot in a range of an elongation ratio of 0.4%
to 2.0%. The reason why tlie lower linlit of tlie elongation ratio of tlie skin pass
rolling is set to 0.4% is that when the elongation ratio is less than 0.4%, sufficient
i~nprovernentin the surface roughness and working hardening of the only surface are
not attained, and the fatigue properties are not improved. Thus, 0.4% is set as the
lower limit. 011 the other hand, when the skin pass rolling is carried out at an
elongation ratio of more than 2.0%, the steel sheet is excessively worked and hardened
to deteriorate the press fornlability, and thus, 2.0% is set as the upper limit.
100441
Next, a ~netallographics tructure mill be described.
The microstructure of the steel sheet obtained by the present invention is
composed of mainly ferrite and bainite. When the area fi'action of ferrite is less than
80%, the fraction of bainite is increased and sufficient ductility carmot be obtained.
Thus, the lower litnit of the area fraction of ferrite is set to 80% or more. When the
area fraction of ferrite is Inore thao 95%, the tensile strength is decreased, and thus the
upper linlit of the area fraction of ferrite is set to 95% or less. However, the cementite
in the ferrite is not converted into an area.
Bainite contributes to high-stre~igtl~ening. Ho\vever, when the atnount of
bainite is excessive, a decrease in the ductility is caused, and thus, the lower limit is set
to 5% arid the llpper limit is set to 20%.
111 addition, as other phases, there are pearlite, residual austenite, and
maltensite, and when a total fraction (area fraction or volotne ratio) of these
conlpositions is 3% or more, the yield strength is decreased and it is difficult to
increase the yield ratio to 0.80 or more. Therefore, the total fraction of the pearlite,
residual aostenite, and martensite is set to less than 3%.
[0045]
The microstructure may be observed with an optical microscope by collecting
a sample having a sheet thich~essc ross section, which is parallel in a rolling direction,
as an observation surface, polishing the observation surface, and carrying out nital, and
as required, La Pera etching. In the observation of the microstructure, a portion
\vl~ichi s at a depth of 114 of the sa~npleco llected from an arbitrary position of the steel
sheet in the thickness direction was imaged at a tnag~~ificatioof~ 1l 000 times it1 a range
of 300 x 300 {im. By binarizi~lgth e image of the microstructure obtained by the
optical tnicroscope to white and black and analyzing the image, a total area fraction of
any one or hvo or nlore of pearlite, bainite, and ~nartensitec an be obtained as an area
fraction of phases other tllan the ferrite. It is difficult to distinguish residual austenite
from martensite wit11 the optical microscope, but the volume ratio of the residual
austenite can be measured by an X-ray difiactio~m~e thod. The area fraction obtained
from the microstructure is the same as the volume ratio.
[0046]
The shape of ce~ne~ltiitne ferrite is extremely important in the present
invention. When the equivalent circle diameter of cementite in ferrite is more than
0.300 pn~t,l ~ereis a high possibility of cementite being a starting point of cracking in a
hole expa~lsiot~els t, and the hole expansibility is deteriorated. Thus, the upper limit is
set to 0.300 pm. The lower limit is set to 0.003 ym in ter~nso f accuracy it1
measurenlent. In addition, when the nu~nberd ensity of the cementite having the
equivalent circle diacneter in ferrite is tilore than 0.10 particleslIu~n2t,h e cementite in
the ferrite may be a starting point of cracking in a hole expatlsion test, and thus, the
hole expansibility is deteriorated. Thus, the upper limit is set to 0.10 l)articleslp~n2.
It is difficult to co~ltrotlh e number density of cementite ill ferrite to be 0.02
particleslpm2, and thus, the lo\ver lintit is set to 0.02 particleslCun2. The equivalent
circle diameter and the number density of the cementite in the ferrite were deter~nined
from the observation result of 100 view fields obtained by preparing an extraction
replica sample which was extracted from a portion wl~iclis at a depth of 114 of a
sacnl~lec ollected from an arbitrary position of the steel sheet in the tliickliess direction,
and observing cementite in ferrite with a tra~is~~iisstiyopne electron microscope (TEM)
at a magnification of 10000 tinles in a range of 10 x 10 Lml. As for a count method,
100 view fields were arbitrarily selected.
A test method of each mechanical property will be described belo\\'. A
tensile test sanlple according to JIS Z 2201 No. 5 was taken from a steel sheet after
being produced co~~sidertih~e ~ivgid th directio~(ir eferred to as the TD direction) as the
longittrdinal direction, and the tensile properties in the TD direction were evaluated
according to JIS Z 2241. The fatigue strength was evaluated with the Schenk type
plane bending fatigue testing machine according to JIS Z 2275. Tlie stress load at tliis
time was set at a vibration frequency of reversed testing of 30 I-Iz. In addition,
according to the above description, a value obtained by dividing tlie fatigue streagth at
the cycle of 10' measured by the plane betiding fatigue test by the tensile strength
~neasured by the above-described tensile test was set to a fatigue strength ratio. The
hole expansibility was evaluated according to Japan iron and Steel Federation Standard
JFST 1001. Each of the obtained steel sheets was cut to 100 mm x 100 nitn size
pieces and then punched to have a hole with a diameter of 10 111111~ 14tha cleara~ice
being 12% of the thickness. Then, in a state in which wrinkles were suppressed with
a wrinkle suppressing force of 88.2 !d using a die with an inner diameter of 75 mm, a
GO0 conical punch mras forced througli the hole to measure a hole diameter in a fracture
initiation limit. A limit hole expa~lsionra tio [%] was obtained from tlie following
Expression 3, and the hole expansibility was evaluated based on the limit hole
expansio~ir atio.
Limit hole expa~lsiora~ti~o h [%I = {(Df- Do)/Do} x 100 ... (Expression 3)
Here, Drrepresents a hole diameter [mm] at the tinie of frach~rein itiation, and
Do represents all initial hole diameter [rn~n]. In addition, plating adhesion is evaluated
according to JIS H 0401 by visbally obse~vinga surface state of a plating film at a
portion bent by a bending test.
[Examples]
[0047]
Steel having the co~llpositio~sh~osm a ia Table 1 were melted and cast to for111
slabs. Steel sheets were prodoced using the obtained slabs under the conditions
shown in Tables 2-1 and 2-2. "[-Iinn T able 1 indicates that tlie analyzed value of a
co~llpositioi~s ll ess than a detection limit. In addition, calculation values in Table 1,
AT3 ["C] and Arl ["C] are also shown.
100481
A tensile test sanlple according to JIS Z 2201 No. 5 was taken fro111 a steel
sheet after being produced considering the width direction (referred to as the TD
direction) as the longihtdinal direction, and the tensile properties in the TD direction
were evaluated according to JIS Z 2241. The fatigue strength was evaluated with the
Schenk type plane bending fatigue testing machine according to JIS Z 2275. The
stress load at this time was set at a vibration frequency of reversed testing of 30 Hz
In addition, according to tlie above description, a value obtained by dividing the
fatigue strength at the cycle of lo7 measured by the plane bending fatigue test by the
tensile strength nleasured by the above-described tensile test was set to a fatigue
strength ratio. The hole expatisibility was evaluated accordicig to Japan Iron and Steel
Federation Standard JFST 1001. Each of the obtained stecl sheets was cut to 100 nlnl
x 100 mm size pieces and then punched to have a hole with diameter of 10 111111 with a
clearance being 12% of the thickness. Then, in a state in which wrinkles were
suppressed with a ivrinkle suppressing force of 88.2 kN using a die with an inner
dianteter of 75 mm, a 60" conical punch was forced tltrougll the ltole to measore a hole
dianleter in a fracture initiation limit. A li~niht ole expansion ratio [YO] was obtained
from the following Expression 3, and the hole expa~tsibilityw as evaluated based on the
limit ltole expansio~tr atio.
Limit ltole expansion ratio h [%I = {(Dc- Do)/Do) x 100 ... (Expression 3)
Here, Df represents a hole dianteter [~n~ant ]th e time of fracture initiation, and
Do represents an initial hole diameter [mm]. 111 addition, plating adl~esionis evaluated
according to .lIS H 0401 by vis~~alolyb serving a surface state of a plating fill11 at a
portion bent by a bending test.
[0049]
The ~nicrostructureo f the sheet thickness cross section of the steel sheet was
observed by the above-described manner, and the area fiaction of bainite was obtained
as a total area fraction of ferrite and phases other than ferrite.
Tlte result is shown in Tables 3-1 and 3-2. In the present invention, the
fatigue properties \Irere evaluated to be excellent in a case in which a fatigue strength
ratio as a fatigue property index was 0.45 or more. The ductility was evaluated to be
excellent in a case in n~ltichth e product of tensile strength TS [MPa] and total
elongation El [%I, that is, TS x El [MPa.%], as a ductility index was 17000 [MPa%]
or more. The ltole expansibility was evaluated to be excellent in a case in which a
hole espansion ratio h [YO] as a hole expansibility index was 80% or inore. Tlte
collision properties \Irere evaluated to be excellent in a case in which a yield ratio as a
collision property index n7as 0.80 or more.
As s11own in Tables 3-1 and 3-2, the result is that it is possible to obtain a
11ig11-strengtl~s teel sheet having excellent fatigue strength and collisio~p~ro perties, and
excellent ductility-hole expansibility balance, a hot-dip galvanized steel sheet, and a
galvat~nealeds teel sheet by snbjecting steel having tlte cl~ernicacl otnpositions of the
present invention to hot rolling and annealing under appropriate cotlditions.
[OOSO]
011 the other hand, for Steel No. M, since the amount of C is large, tlte
ductility and the hole expansibility are decreased.
For Steel No. N, since the anlonnt of C is small, the area fraction of bainitc is
reduced, the tensile strength is decreased, and the yield ratio and the product of tensile
strength and total elongation are decreased.
For Steel No. 0, since the an~ounot f Si is large, the area fraction of bainite is
reduced, the tensile strength is decreased, and the product oftensile strengtl~a nd total
elongation is decreased.
For Steel No. P, since the amount of Mn is small, the area fraction of bainite is
reduced, the tensile strength is decreased, and the product of tensile strength and total
elongation is decreased.
For Steel No. Q, since the amount of Mn is large, the area fraction of bainite is
increased, and the tensile strength is increased. I-Iowever, the ductility is decreased,
the product of tensile strength and total elongation is decreased, and the hole
expansibility is also decreased.
For Steel No. R, since the amount of A1 is small, the area fraction of bainite is
increased, the ductility is decreased, the product of tensile strength and total elongation
is decreased, and the hole expansibility is also decreased.
For Steel No. S, since the amount ofAI is large, the area fraction of bainite is
reduced, the tensile strength is decreased, and the product of tensile strengtlt and total
elongation is decreased.
For Steel No. T, since the total amount of Ti and Nb is small, the tensile
strength is decreased, the yield ratio, the product of tensile strength and total
elongation are decreased. Also, the fatigue strength and the liole expansibility are
decreased.
For Steel No. U, since the amount of Ti is small, the yield ratio and the hole
expansibility are decreased.
For Steel No. V, since the anloullt of Ti is large, the ductility is decreased, the
product of tensile strength and total elongatio~lis decreased, and the hole expansibility
is also decreased.
For Steel No. W, since the anlou~lot f Nb is small, the yield ratio and the hole
expansibility are decreased.
For Steel No. X, since the amount of Nb is large, the ductility is decreased, the
product of tensile strength and total elongation is decreased, and the liole expansibility
is also decreased.
For Steel No. Y, since Nb is not added, the tensile strength, the yield ratio and
the fatigue strength ratio are decreased.
For Steel No. 2, since the total amount of Ti and Nb is large, the ductility is
decreased, the product of tensile strength and total elongation is decreased, and the
hole expansibility is also decreased.
For Steel No. AA, since the amount of Ti and Nb is large, the ductility is
decreased, the product of tensile strength and total elongation is decreased, and the
hole expansibility is also decreased.
[005 11
For Production No. 3, since the heating temperatore is low during the hot
rolling, and tlie ariiount of precipitation strengthening by carbonitrides is small, the
tensile strength is decreased, tlie product of tensile strength and total elongation is
decreased, and tlie yield ratio and tlie fatigoe strength ratio are also decreased.
For Production No. 6, since the holding temperature after heating the steel
sheet to tlie maximum heating temperature in tlie annealing process and cooling is low,
the cementite in the ferrite is coarsened and tlie hole expansibility is decreased.
For Production No. 9, since tlie holding time after heating tlie steel sheet to
the maximum lieating temperature in the annealing process and cooling is sho~-tt,h e
cenie~ititein the ferrite is coarsened and tlie hole expansibility is decreased.
For Production No. 12, the finishing tcinperature during the hot rolling is low
and tlie fatigue strength is decreased due to softening of the surface of the steel sheet.
For Production No. 15, since the coiling teniperattlre is high, and the a~nount
of precipitation strengthening by carbonitrides is small, the tensile strength, the yield
ratio, and the fatigue strength ratio are decreased.
For Production No. 18, the coiling temperature is lowy the area fraction of
bainite is increased, the ductility is decreased, tlie product of tensile strength and total
elongation is decreased, and tlie hole expansibility is also decreased.
For Production No. 21, since the ~naxim~ihinea ting temperature during the
a~uiealingis liigl~a nd the amount of precipitation strengthening by carbo~iitridesis
sniall, tlie tensile strength, tlie yield ratio, and the fatigue strength ratio are decreased.
For Production No. 24, since the maximum heating temperature during the
an~iealingis low and the amount of precipitation strengthening by carbonitrides is
small, the tensile strengtli, the yield ratio, and the fatigue strength ratio are decreased.
For Production No. 27, since the holding tirtie at the maxi~~iuhmea ting
temperature during the annealing is short, and tlie amount of precipitation
strengthening by carbonitrides is small, tlie tensile strength, the yield ratio, and tlie
fatigue strength ratio are decreased.
For Production No. 30, since the holding time at the niaxinium heating
temperature during the anaealing is long and the atnoutlt of precipitation strengtl~e~iuig
by carbonitrides is small, the tensile strength, the yield ratio, and tlie fatigue stretigtli
ratio are decreased.
For Production No. 31, since the holding temperature after tile steel sheet is
held at the maximum heating temperature and tlleen cooled is high, tlie ce~nei~tiitne t he
ferrite is coarsened, and the number density is also increased, the hole expansibility is
decreased.
For Production No. 34, since the coiling temperature is high, the amount of
the ferrite is excessive and the tensile strength is decreased.
For Production No. 35, since tlie isothemial holding time after the steel sheet
is held at tlle maximum heating temperature and tliell cooled is long, the cenientite is
coarsened, and the nuniber density is iilcreased, the hole expansibility is decreased.
For Production No. 38, since the coilitlg temperature is low, a large aino~uiot f
precipitates are generated and tlie hole expalision ratio is low.
[0052]
[Table 11
TABLE 1
Cw
0 F
Gi
(NOTE 1) THE UNDERLINED VALUES INDICATE VALUES OUTSIDE THE RANGE OF THE PRESENT INVENTION.
TABLE 2-1
STEEL PRODUCTION
No. No. HEATING FINISHING COILING HOLDING HOLDING HOLDING
TREATMENT TREATMENT TIME TEMPERATURE TIME
TEMPERATURE TEMPERATURE TEMPERATURE TEMPERATURE
TEMPERATURE
ih
h,
3
(NOTE 1) THE UNDERLINED VALUES INDICATE VALUES OUTSIDE THE RANGE OF THE PRESENT INVENTION.
I SKIN PASS w
U
HOT ROLUNG ANNEALING ROLUNG AFTER
TABLE 2-2
, .
0
s
(NOTE 1) THE UNDERUNED VALUES INDICATE VALUES OUTSIDE THE RANGE OF THE PRESENT INVENTION.
HOT ROLUNG
STEEL
No.
PRODUCTION
No.
0
'4
ANNEALING C
SKIN PASS
ROLLING AFTER
HEATING
TEMPERATURE
FINISHING
TEMPERATURE
ANNEAUNG
ELONGATION
RATIO
COIUNG
TEMPERATURE
MAXIMUM
TEMPERATURE
HOLDING
TIME
HOLDING
TEMPERATURE
PLATING
5
HOLDING
TIME
TREATMENT
TEMPERATUR
TREATMENT
TIME
TABLE 3-1
~
MICROSTRUCTURE I CEMENTITE I MECHANICAL PROPERTIES
EQUIVALENT HOLE
Gs
La P
VI
(NOTE 1) THE UNDERLINED VALUES INDICATE VALUES OUTSIDE THE RANGE OF THE PRESENT INVENTION.
TABLE 3-2
MICROSTRUCTURE I CEMENTITE I MECHANICAL PROPERTIES
W
E
(NOTE 1) THE UNDERUNED VALUES INDICATE VALUES OUTSIDE THE RANGE OF THE PRESENT INVENTION.
STEEL
No.
PRODUCTION
No. Fg",'"
FRACTION
'f,"F
FRACTION
FRACTION
OF OTHER
PHASES
EQUIVALENT
DIAMETER
NUMBER
DENSITY
YIELD
STRENGTH
YP
TENSILE
STRENGTH
TS
TOTAL
ELONGATION
El
YIELD
RATIO
TS x El
FATIGUE
STRENGTH
FATIGUE
STRENGTH
RATIO
HOLE
RATIO A
[0056]
[Table 3-21
[Iadustrial Applicability]
[0057]
According to the present i~~ventioit~ ils, possible to provide a high-strength
steel sheet and a plated steel sheet, which have a tensile strength of 590 MPa or more,
a high yield ratio, and excellent fatigue properties and ductility-hole expausibility
balance, and fu~there, xcellent collision properties, and 1\~hic11m ake an extre~nely
significant coatribution to the industry. Further, the present invention makes it
possible to reduce the sheet thickness of a suspe~lsiolpl art of a vehicle and thus
eshibits at1 extremely re~narkablee ffect that sig~lifica~~ctolnyt ributes to a decrease in
the weight of a vehicle body.

[Document Type] CLAIMS
[Claim 11
A steel sheet comprising, by mass%:
C: 0.020% or more and 0.080% or less;
Si: 0.01% or more and 0.10% or less;
Mn: 0.80% or nlore and 1.80% or less;
Al: more than 0.10% and less than 0.40%;
P: limited to 0.0100% or less;
S: limited to 0.0150% or less;
N: limited to 0.0100% or less;
Nb: 0.005% or Inore and 0.095% or less;
Ti: 0.005% or nlore and 0.095% or less; and
a balance inclodiog Fe and unavoidable impurities, wherein
a total anlount of Nb and Ti is 0.030% or nlore and 0.100% or less,
a n~etallograpl~sictr ucttlre of the steel sheet includes ferrite, bainite, and other
phases,
the other pllases include a pearlite, a residual austenite, and a inartensite,
an area fiaction of the ferrite is 80% or more and 95% or less,
an area fraction of the bainite is 5% or more and 20% or less,
a total fraction of the other phases is less than 3%,
an equivalent circle diameter of a cementite in the ferrite is 0.003 [tnl or more
and 0.300 p111 or less,
a ntutlber density of the cementite in the ferrite is 0.02 particle~/~orrn m~ ore
and 0.10 particles/pn~2o r less,
a tensile strength is 590 MPa or inore, and
a fatigue strength ratio as a fatigue strength to the tensile strength is 0.45 or
more.
[Claim 21
The steel sheet according to clainl I, fi~rtl~ceorn lprisi~lgo ne or hvo more of,
by mass%:
Mo: 0.005% or more and 1.000% or less;
W: 0.005% or more and 1.000% or less;
V: 0.005% or more and 1.000% or less;
R: 0.0005% or more and 0.0100% or less;
Ni: 0.05% or inore and 1.50% or less;
Cu: 0.05% or more and 1.50% or less; and
Cr: 0.05% or nlore and 1.50% or less.
[Claim 31
Aplated steel sheet, in w\41ich a plating is provided on a surface of tl~est eel
sheet according to claim 1 or 2.
[Claim 41
A method for producing a steel sheet, the nlethod comprising:
heating a slab having a che~nicacl omposition according to claim 1 or 2 to
1150°C or higher before the slab is hot-rolled;
finishing finish rolling at a temperature of Ar3OC or higher;
pickling a hot-rolled steel sheet which is coiled within a temperature range of
400°C or higher and 600°C or lower;
heating the hot-rolled steel sheet within a tenlperature range of 600°C or
higher and Acl0C or lower;
annealing the hot-rolled steel sheet for a holding time, in \\.hich the
tenlperature of the hot-rolled steel sheet is \vithi~tih e ten~peraturer ange for 10 seconds
or longer and 200 seconds or shorter;
cooling the steel sheet to 350°C or higher and 550°C or lower; and
cooling the steel sheet after holding the steel sheet for the holding time, in
\vhich the temperature of the hot-rolled steel sheet is within a te~iiperat~r~arneg e of
350°C or higher and 550°C or lo\~efro r I0 seconds or longer and 500 seconds or
shorter,
wherein the Ar3"C and the Acl°C are a k 3t ransforn~ationt emperature and a
Ac~tra nsformation temperatt~re,r espectively, obtained fro111e xpressions 1 aid 2,
Ar3 = 910 - 325 x [C] + 33 x [Si] + 287 x [PI + 40 x [All - 92([Mn] + [Mo] +
[Cu]) - 46 x ([Cr] + wi]) ... (Expression l),
A c ~= 761.3 + 212[C] - 45.8[Ma] + 16.7[Si] ... (Expression 2), and
elements noted in brackets represent an amount of the elements by mass%.
[Claim 51
'rlie niethod for producing a steel sheet according to claim 4, further
comprising:
canying out skin pass rolling on the steel sheet at an elo~igationr atio of 0.4%
or more and 2.0% or less.
[Claim 61
A method for producing a plated steel sheet comprising:
plating and then cooling the steel slieet after the annealing, tlie cooling, and
the holding according to claim 4.
[Claim 71
A method for producing a plated steel sheet comprising:
plating and then cooling the steel sheet after tlie annealing, tlie cooling, and
tlie holding according lo clainl 5.
[Claim 81 s ;
.., ...
The method for producing a plated steel sheet according to claim 6 or 7,
fi~rtherc olnprising:
. .
carrying out a heat treatment within a temperature range of 450°C or higher . .:
. .
and 600°C or lower for 10 secorlds or longer and then coolil~gth e steel sheet after the
plating.