[Document Type] Specification
[Title of the Invention] COLD-ROLLED STEEL SHEET, PLATED STEEL SHEET,
AND METHOD FOR MANUFACTURING THE SAME
[Technical Field of the Invention]
[OOOl]
lie present inventiorl relates to a 11ig11-strength cold-rolled steel sheet and a
plated steel sheet, which have excellent ductility and stretch flangeability and are
suitable for an automobile steel sheet, particularly suitable for a structural member (for
example, a bracket), and a method for manufactt~ringth e same.
Priority is claimed on Japanese Patent Application No. 2012-028271, filed on
February 13,2012, the content ofwhich is incorporated Iierein by reference.
[Related Art]
[0002]
In recent years, in order for autonlakers to cope wit11 the tightening of COz
en~issionre gulations in Europe ia 2012, fuel economy regulations in Japan in 2015,
and stricter collision regulations in Europe, 11ig11-strengtl~eni~of~ gst eel to be used has
rapidly progressed to improve file1 economy tl~rouglla decrease in the weight of an
automobile body and improve collision safety. Such a high-strength steel sheet is
called a "high tensile strength steel sheet", and the alliount of orders of thin steel sheets
mainly having a tensile strength of 440 MPa to 590 MPa, and recently more than 590
MPa, tends to increase every year. Among them, excellent ductility and stretch
flangeability are demanded for a structural nlenlber such as a bracket iu view of the
working method. Generally, it is considered that when the product of tensile strength
and total elongation is 17000 MPa.% or more, ductility is excellent, and, regarding a
grade of 590 MPa of tensile strength, when hole expansion ratio is 80% or more,
stretch flangeability is excellent.
[0003]
Generally, when tensile strength increases, yield strength also increases.
Tllns, ductility is decreased, and fi~rthers, tretch flange for~nabilityis deteriorated. In
the related art, in a case of dual phase (DP) steel including two phases of ferrite and
martensite, the ductility is excellent, but micro-cracks caused by local strain
concentration in the vicinity of a boundary behveen ferrite which is a soft phase and
ma~te~~swihtiec h is a hard phase easily occur and propagate, and thus, it is consideled
that the dual phase is a disadva~ltageousn licrostructure in liole expansibility.
Accordingly, it is considered that the sn~alletrh e hardness difference between the
microstructt~resis , the more advantageous it is in hole expansibility improveme~~atn, d
thus, a steel sheet having a uniform structure such as ferrite single phase steel or
bainite single phase steel is considered to be superior, From the above viewpoint, it is
inlportant to control a constituent phase fraction matched with a desired tensile
strength to attain both ductility and hole expansibility.
[0004]
As a high-strength steel sheet in which both of dnctility and stretch
flangeability are attained, a steel sheet it1 which precipitation strengthening is actively
utilized has been proposed so far (for example, refer to Patent Documents 1 and 2).
However, since the cold-rolled steel sheet proposed in Patent Document 1 is
n~ostlya nnealed alnlost within a ferrite single phase region, strncture strengthening by
bainite is hardly utilized. Tlius, in order to facilitate high-strengthening, a large
amount of Ti and precipitation elements other than Ti have to be added to actively
utilize precipitation strengthening. Subsequently, a higher alloy cost is required. In
addition, precipitation elements such as Ti and Nb also fnnction as recrystallization
inhibiting elen~entsa, nd thus, wlien these elements are added in a large amount,
recrystallization is reniarkably delayed in aanealitig. Accordingly, in order to have
the area ratio of non-recrystallization ferrite of 25% or less, it is assumed that a
temperature rising rate needs to become extremely slow in tlie annealing step or that a
holding time at the maximum heating temperature needs to become extre~ilelylo ng,
atid thus, productivity is deteriorated. In addition, since precipitation strengthening is
actively utilized in a cold-rolled steel sheet proposed in Patent Document 2 as in Patent
Document 1, a large aaniount of Ti and precipitation elements other than Ti have to be
added to tlie cold-rolled steel sheet proposed in Patent Document 2. Subsequently, a
higher alloy cost is required and also when these elements are added in a large amoutit,
recrystallization is remarkably delayed in annealing. Thus, in order to have the area
ratio of non-rec~ystallizationf errite of 25% or less, the maxinlu~nh eating temperature
in the annealing step becomes extremely high. Alternatively, when the maximum
heating temperature is just higher than an Acl transformation temperature, a
temperature rising rate becomes extremely slow. Alternatively, it is assumed that a
holding time at tlie maximum heating temperature needs to becon~ee xtremely long,
and thus, productivity is deteriorated.
[0005]
In addition, a steel sheet having improved stretch flangeability by actively
utilizing non-reclystallization ferrite to reduce the hardness difference between ferrite
and a hard phase has been proposed (for example, refer to Patent Docutnents 3 to 5).
Ho\vever, since it is necessary to add a large aalnount of recrystallization
inhibiting elements such as Nb and Ti to actively utilize tion-recrystallization ferrite, a
higher alloy cost is required and also a temperature rising rate needs to be increased in
an annealing step. Thus, facility investtnent is needed.
[Prior Art Document]
[Patent Document]
[0006]
[Patent Docunient 11 Japanese Unexanlined Patent Application, First
Publication No. 2010-285656
[Patent Document 21 Japanese Unexamined Patent Application, First
Publication No. 2010-285657
[Patent Document 31 Japanese Unexamined Patent Application, First
PublicationNo. 2008-106352
[Patent Document 41 Japanese Unexamined Patent Application, First
PoblicationNo. 2008-190032
[Patent Document 51 Japanese Unexamined Patent Application, First
Publication No. 2009-1 14523
Disclosure of the Invention]
[Technical Probleni]
[0007]
The present invention is to stably provide a high-strength cold-rolled steel
sheet and a plated steel sheet which have excellent ductility and stretch flangeability,
without deterioration in productivity.
[Solution to Problem]
[0008]
The present invention is a finding obtained fronl an investigation that has been
conducted to solve the above ~nentionedp roblems of improving ductility and stretch
flangeability of a liigh-strength cold-rolled steel sheet, a hot-dip galvanized steel sheet,
and a galvannealed steel sheet which have a tensile strength of 590 MPa or more.
That is, an appropriate microstructure is attained by optinlizing the a~nounot f alloying
elements, particularly, optimizing the amount of Nb, and Ti and positively adding Si.
In addition, in an annealing process, the maximum heating temperature is controlled
within a ternperah~rer ange ofAcl PC] or more and (Acl + 40) ["C] or less and an end
temperature and a cooling rate of a primary cooling after annealing are determined.
Accordingly, a sufficient recrystallization suppressing eEect can be obtained, and thus,
while utilizing bainite, the amount of non-recrystallization ferrite is appropriately
controlled by cor~trolli~a~n ge quivalent circle diameter of the carbonitrides including
one or both of Nb and Ti to fine. The present invention is made based on the findings
that it is possible to produce a steel sheet having excellerlt ductility and stretch
flangeability compared to steel sheets of the related art, and the sunlmary thereof is
described as follows.
[0009]
(1) According to a first aspect of the present invention, there is provided a
cold-rolled steel sheet including, by mass%: C: 0.020% or more and 0.080% or less;
Si: 0.20% or more and 1.00% or less; Ma: 0.80% or more and 2.30% or less; P:
0.0050% or more and 0.1500% or less; S: 0.0020% or more and 0.0150% or less; AI:
0.010% or more and 0.100% or less; N: 0.0010% or more and 0.0100% or less; and
finther including: one or more of Nb and Ti which satis@ a require~nenot f 0.005% 5
Nb +Ti < 0.030%; and a reminder including Fe and unavoidable impurities, in which a
structure consists of, a ferrite, a bainite, and an other phase, the other phase includes a
pearlite, a residual austenite, and a martensite, an area ratio of the ferrite is 80% or
more and less thaa 95%, an area ratio of a non-recrystallizatio~f~e rrite in the ferrite is
1% or more and less than lo%, an area ratio of the bainite is 5% to 20%, a total
amount of the other phase is less than 8%, an equivalent circle diameter of a
carbonit~idein cluding one or both of Nb and Ti is 1 nm or more and 10 Inn or less, and
a tensile strength is 590 MPa or more.
[OOlO]
(2) The cold-rolled steel sheet according to (1) nay further include one or
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; B: 0.0005% or Inore and
0.0100% or less; Ni: 0.05% or more and 1.50% or less; Cu: 0.05% or more and 1.50%
or less; and Cr: 0.05% or Inore and 1.50% or less.
[0011]
(3) According to a second aspect of the present invention, there is provided
a plated steel sheet in which plating is provided on a surface of the cold-rolled steel
sheet according to (1) or (2).
[0012]
(4) According to a thi~das pect of the present invention, there is provided a
method for manufactnring a cold-rolled steel sheet including: heating a slab having a
chemical co~npositiona ccording to (1) or (2) to 1150°C or more and 1280°C or less;
finishing a finish rolling under a temperature ofAr3OC or more and 1050°C or less;
pickling and then cold-rolling a hot-rolled steel sheet, which is coiled under a
temperature range of 45OoC or more and 650°C or less, tinder a reduction of 40% or
more aand 70% or less; thereafter heating into a temperature range of Acl°C or more
and (Acl+ 40) "C or less under a rate of 2"C/sec or more and S°C/sec or less;
annealing the cold-rolled steel sheet under the temperature range ofAcl°C or more and
(Acl + 40) "C or less and for a holding time of 10 sec or more aand 200 sec or less; and
prima~yc ooling immediately after the annealing into a steel sheet temperature range of
600°C or more and 720°C or less under a cooling rate of 10°C/sec or less in a course
from the annealing to arriving at a nornlal temperature, in wllich the&"C and the
AclOC is a Ar3 transformation temperah~rea nd a Ac~tr ansfortnation temperature,
respectively, calculated from Expressions 1 and 2,
Ar3 = 910 - 325 x [C] + 33 x [Si] + 287 x [PI + 40 x [All - 92 x ([MI] + [Mo]
+ [Cu]) - 46 x ([Cr] + [Nil) (Expression I),
Acl = 723 + 212 x [C] - 10.7 x [Mn] + 29.1 x [Si] (Expression 2), and
ele~nentsn oted in brackets represents an amount of the elements by mass%,
respectively.
[00 131
(5) According to a fourth aspect of the present invention, there is provided a
method for manufacturing a plated steel sheet including plating the cold-rolled steel
sheet manufactured by the method according to (4) after the annealing and the cooling.
[0014]
(6) The method for ~nanufacturi~alg p lated steel sheet according to (5) may
further include heat treating the plated steel slieet under a temperature range of 450°C
or more and 600°C or less with 10 seconds or longer.
[Effects of the Invention]
[0015]
According to the present invention, it is possible to provide a high-strengtl~
cold-rolled steel sheet, a hot-dip galvanized steel sheet, and a galvannealed steel sheet
which have a tensile strength of 590 MPa or more, and excellent ductility and stretch
flangeability. Therefore, the present invention makes an extremely significant
contribution to the indust~y.
[Brief Description of the Dra\ving]
[00 161
FIG. 1 is a graph sl~owinga relationship behveen the maximum heating
temperature, particularly, an Act transformation point or higher, in annealing and an
area ratio of non-recrystallization ferrite.
FIG. 2 is a graph showing a relationship betweell an area ratio of nonrecrystallization
ferrite and a hole expansion ratio h.
FIG 3 is a graph showing a relationship between the maximum heating
tetnperature, particularly, an Acl transforn~ationp oint or higher, in annealing and an
area ratio of bainite.
FIG. 4 is a glaph showing a relationship between an area ratio of bainite and a
hole expansion ratio h.
FIG. 5 is a graph showing a relationship between the maximum heating
temperature, patticularly, an Act transformation point or higher, in annealing and an
equivalent circle diameter of carbonitrides.
FIG. 6 is a graph sho\vii~ga relationship between an equivalent circle diameter
of carbonitrides and an area ratio of non-recrystallization ferrite.
FIG. 7 is a graph showing a relationship between a total amount of other phase
and a hole expansion ratio h.
[Embodiments of the Invention]
[0017]
Hereinafter, the present invention \vill be described in detail.
First, the reasons why steel cotnponents are limited in the present invention
will be described.
C is an elenlent \vhich contributes to an increase in tensile strength and yield
strength, and added in appropriated amounts according to a targeted strength level. In
addition, C is also effective in obtaining baitlite. When the amount of C is less than
0.020%, it is difficult to obtain a target tensile strength atid yield strengtli, and thus, the
lower limit is set to 0.020%. On the other hand, when the amount of C is more than
0.080%, deterioration in the ductility, hole expansibility, and wveldability is caused.
Thus, the upper li~iiiits set to 0.080%. In addition, in order to stably secure the
tensile strength and yield strength, the lower limit of C may be preferably 0.030% or
0.040%, and the upper limit of C may be preferably 0.070% or 0.060%.
[OOI 81
Si is an extretnely itnpol-tant element in the present invention. Si is effective
in improving the stretch flangeability by hardening ferrite throng11 solid solution
strengthening to reduce the hardness difference with a bard phase. In order to obtain
the effect, it is tiecessary to set the anlount of Si to 0.20% or more, and thus, tlie lower
limit is set to 0.20%. On the other hand, Si may cause a problem of a decrease in
plating wettability when hot dip galvanizing is carried out and a proble~iol f a decrease
in productivity due to the delay of alloying reaction. Therefore, tlie upper limit of tlie
amount of Si is set to 1.00%. In addition, Si is a ferrite stabilizing element. Tlie
lower limit of Si may be set to 0.30% or 0.40% and tlie upper limit of Si Inay be set to
0.90% or 0.80% to obtain an appropriate amount of bainite.
[0019]
Mn acts as an element that contributes to solid solution strengthening, and
thus has an effect of increasing the strength. In addition, Mn is effective in obtainitig
beinite. In addition, it is necessary to contain 0.80% o; more of Mn to improve hole
expansibility. On tlie other hand, when the amount of Mn is liiore than 2.30%,
deterioration in hole expansibility and weldability is caused, and thus, the upper limit
thereof is set to 2.30%. In addition, in order to stably obtain bainite, the lower limit
of Mn may be set to 1.00%, 1.20%, or 1 .SO%, and tlie upper limit of Mn may be set to
2.10%, or 2.00%.
[0020]
P is an impurity, and is segregated at grain boundaries to cause a decrease in
the touglmess of the steel sheet and deterioration in the ~veldability. Further, the
alloying reaction becomes extremely slow during hot dip galvanizing, and the
set to 0.1500%. Since P is an element which increases strength at a lo111 price, the
lower limit of the alnount of P is preferably set to 0.0050% or more. In order to
further inlprove the toughness and the weldability, the lower linlit of P nlay be set to
0.0060% or 0.0070%, and the upper limit of P may be set to 0.1000% or 0.0850%.
[0021]
S is an impurity and when the content thereof is more than 0.0150%, hot
cracking is induced or workability is deteriorated. Thus, the npper lirnit of the
amount of S is set to 0.0150%. Due to restriction on production costs, the lower limit
of the amount of S is set to 0.0020%. In order to inlprove the workability, the lower
limit of S may be set to 0.0025%, and the upper limit of S may be set to 0.0100% or
0.0080%.
[0022]
A1 is a ferrite stabilizing element similar to Si. Al is a deoxidizing element
and the lower limit is set to 0.010% or more in view of deoxidation. In addition,
excessive addition of Al causes deterioration in the weldability, and thus, the upper
litnit thereof is set to 0.100%. The lower linlit ofAl may be set to 0.015% or 0.025%,
and the npper litnit ofAl may be set to 0.080%, 0.060% or 0.040%.
[0023]
N is an impurity. When the amount of N is more than 0.0100%,
deterioratiori in touglnless and ductility and occurrence of cracking in a slab are
significant. Since N is effective in increasing ter~siles trength and yield strength,
similar to C, N may be positively added as the upper limit of the amount of N is set to
0.0100%. In addition, N is effective in obtaining bainite. Due to restriction on
production costs, the lower limit of the amount of N is set to 0.0010%. The lower
0.0080%, 0.0060%, or 0.0050%.
[0024]
Further, Nb and Ti are extremely iniporta~iet leriients in the present invention.
These elements have an effect of delaying the progress of recrystallization in an
anliealing process to allow non-recrystallization ferrite to remaul, Since tlie nolirecrystallization
ferrite contributes to hardening of ferrite, the amount of the nonrecrystallization
ferrite is appropriately controlled to reducing the hardness difference
between ferrite and the hard phase, thereby obtaining a11 effect of improving the stretch
flangeability. When one or niore of Nb and Ti are contained so as to satisfy the
condition of 0.005% 5 Nb +Ti < 0.030%, the reason why the upper limit of at least
one of Nb and Ti is less than 0.030% is that when one or more ofNb and Ti are added
at a content more than the upper limit, the non-recrystallization ferrite remains
excessively, and tlie ductility is decreased. In addition, the reasoil ~vliyth e lower limit
of one or more of Nb and Ti are set to 0.005% is that when one or more of Nb and Ti
are added at a content less than the lo\ver limit, a recrystallization suppressing effect is
small, and the non-recrystallization ferrite hardly remains. 111 addition, in order to
itriprove the stretch flangeability, the lower limit of one or more of Nb acid Ti may be
set to 0.010%, and the upper limit of one or more of Nb and Ti may be set to 0.025%.
[0025]
All of Mo, W, and V are recrystallization inhibiting elements, and one or more
of these elements may be added as required. In order to obtain the effect of strength
improvement, 0.005% of Mo, 0.005% of W, and 0.005% of V are preferably added
respectively as the lower limits. 011 the other hand, since excessive addition carrses
an increase in an alloy cost, the upper limits are preferably set to 1.000% of Mo,
and 1.000% respectively.
All of B, Ni, Cu, and Cr are ele~nentsw hich increase hardenability, and one or
more of these eleme~~mtsa y be added as required. In order to obtain the effect of
strength i~nprove~ne~0.i0t,0 05% of B, 0.05% ofNi, 0.05% of Cu, and 0.05% of Cr are
preferably added respectively as the lower limits. On the other hand, since excessive
addition causes an increase in an alloy cost, the upper limits are preferably set to
0.0100% of B, 1.50% ofNi, 1.50% of Cu, and 1.50% of Cr, respectively.
The high-strengt11 cold-rolled steel sheet containing the above-described
chemical composition may contain impurities unavoidably incorporated in a
production process within the range in wliich a reminder including iron as a main
compone~lt does not impair the properties of the present invention.
[0026]
Next, the reasons why a production method is limited will be described.
A slab having the above-described co~npositionis heated at a temperature of
1150°C or higher. The slab may be a slab im~nediatelya fter being produced by a
co~ltinuousc asting facility or a slab produced by an electric fi~nlace. The reason why
the temperature is limited to 115OoCo r higher is to sufficiently decon~posea nd
dissolve carbonitride forming elements and carbon in the steel. In order to dissolve
the precipitated carbonitrides, the temperature is preferably 1200°C or higher.
However, when the heating te~nperatureis higher than 1280°C, the temperature is not
preferable in view of production costs, and thus, 1280°C is preferably set as the upper
limit.
[0027]
When a finishing temperature in hot rolling is lower than an Ar3
transfo~~natiotenm perature, carbonitrides are precipitated and the particle size is
decreased after the annealing, so that the Ar3 transfor~nationt emperature is set as the
lower limit. A te~nperah~oref 900°C or higher is preferable to stably precipitate the
precipitates of carbonitrides with an equivalent circle diameter of 10 nm or less. The
upper limit of the finishing temperature is substantially 1050°C in view of the slab
heating temperature.
Here, h ° C i s an Ar3 tra~sformationte mperature obtained by the followving
Expression 1.
Ar3 = 910 - 325 x [C] + 33 x [Si] + 287 x [PI + 40 x [All - 92 x ([Mn] +
[Mo] + [Cu]) - 46 x ([Cr] + [Nil) (Expression 1)
Wherein, ele~nentsn oted in brackets represent an amount of the elements by
mass%, respectively.
[0028]
A coiling te~nperah~arfete r finishing rolling is an extrennely important
production condition in the present invention. In the present invention, the
soppression of the precipitation of carbonitrides at the stage of the hot-rolled steel
sheet with setting the coiling temperature to 650°C or lowver is important, and the
properties of the present invention is not deteriorated by the history up to that time.
When the coiling temperature is higher than 650°C, carbonitrides are precipitated and
coarsened in the hot-rolled steel sheet, sufficient rec~~~stallizatsiuop~plr essing effects
cannot be attained during annealing, and thus, 650°C is set as the upper limit. Further,
when the coiling temperature is lower tlian 450°C, tlie strength of the hot-rolled steel
sheet is increased and rolling load is increased during cold rolling. Therefore, 450°C
is set as the lower limit.
[0029]
Cold rolling after typical pickling is carried out under a reduction of 40% to
70%. When the reduction is less than 40%, the driving force of recrystallization
becomes s~ilaldl uring tlie annealing, and thus, non-recrystallizatio~if errite remains
excessively after the annealing, which causes a decrease in the ductility. Tlius, the
lower limit is set to 40%. In addition, when the reduction is more than 70%, the
driving force of recrystallizatio~b~ec ornes large during the annealing, and tllus, a s~iiall
amount of non-recrystallization ferrite remains, which causes a decrease in the tensile
strength and the stretch flangeability. Therefore, the upper limit is set to 70%.
[0030]
The an~iealingis preferably carried out by the continuous annealing facility to
co~itrotlh e heating temperature and the heating time. The maximum heating
temperature in the annealing is an extre~iielyim portant production condition in the
present invention. The lower limit of the maximum heating temperature is set to an
Acl transfor~nationte mperature, and the upper limit is set to (Acl transformation
temperature + 40)"C. When tlie maxiliiurn heating temperature is lower than the Acl
transfo~mationte mperature, a sufficient aniount of a hard phase and nonrecrystaliization
ferrite are not obtained, and a decrease in tlie tensile strength is caused.
On tile other hand, when the maxitnum heating temperature is higher tlian (Acl
transformation temperature + 40)"C, tlie amount of tlie non-recrystallization ferrite is
reduced as shown in FIG. 1, and thus, the stretch flangeability is decreased as shown in
- 14 -
FIG. 2. The amount of bainite is increased as sho~vnin FIG. 3, and thus, the stretch
flangeability is decreased as shown in FIG. 4. Since the carbonitrides are coarsened
as shown in FIG. 5, the amount of the lion-recrystallization ferrite is reduced as shown
in FIG. 6, and the stretch flangeability is decreased as shown in FIG 2. Therefore,
(Acl transformation temperature + 40)"C is set as the upper limit.
Here, Acl°C is an ACI transfonnation temperature obtained by the following
Expression 2.
Acl = 723 + 212 x [C] - 10.7 x Wn] + 29.1 x [Si] (Expression 2)
Wherein, elements noted in brackets represent an aniouut of the elements by
mass%, respectively.
[003 11
A teniperature ~isingra te is set to 2 OCIsec to 5 "C/sec in the annealing.
When the temperature rising rate is less than 2 "C/sec, not only is the productivity
deteriorated, but also reclystallization substantially proceeds to reduce the amount of
non-recrystallization fenite, and thus, the tensile strength and tlie stretch flangeability
are decreased. Therefore, the lower limit is set to 2 "C/sec. In addition, when the
temperature rising rate is niore than 5 OC/sec, non-recrystallization fe~ritere mains
excessively, and the ductility is decreased. Thus, the upper limit is set to 5 OC/sec.
Aholding time at the lnaxiinuln heating temperature in the annealing is an
extremely important production condition in the present invention. The holding time
of the steel sheet within the temperature range of theAc1 transformation temperature to
(Acl transfor~nationte mperature + 40) "C is set to 10 seconds to 200 seconds. This is
because when the holding time of the steel sheet at the maximuin heating temperature
is shorter tltan 10 seconds, non-recrystallization ferrite remains excessively, and thus,
tlie ductility is decreased. On the other hand, when the holding time of the steel sheet
at the maximum heating temperature is increased, a decreased in the productivity is
caused and also the aniount of non-recrystallizatio~if errite is reduced. Then, tlie
tensile strength and the stretch flangeability are decreased, and thus, the upper limit is
set to 200 seconds.
[0032]
In addition, aaer tlie annealing, primary cooling for cooling the steel within a
steel sheet temperature range of 600°C to 72OoC may be canied out nuder a cooling
rate of 10 "Clsec or less. Then, the steel sheet may be cooled and controlled to an
appropriate temperature through forced cooling with spraying of a coolant, such as
water, air blowing, or mist or the like, and over-aging or tempering is additionally
carried out during the cooling as required. At a temperature of lower than 600°C, the
fiaction of bainite is insufficient and the ductility is decreased. At a temperature of
higher than 72OoC, tlie fraction of bainite is excessive, and the ductility is decreased.
In addition, when the cooling rate is more than 10 OC/sec, the precipitation of ferrite is
small and the fraction of bainite beconles excessive, and thus, the ductility is decreased.
The lower limit of the cooling rate is not particularly limited, but is preferably set to
1 "Clsec or more in view of the productivity and the cooling controllability.
[0033]
When hot dip galvanizing or galvannealing after the cooli~iga fter the
annealing is carried out, tlie composition of zinc plating is not particularly limited, and
in addition to Zn, Fe, Al, Mn, Cr, Mg, Pb, Sn, Ni, and the like may be added as
required. The plating may be carried out as a separate process froill annealing, but is
preferably carried out through a continuous annealing-hot dip galvanizing line in
\vhic11 annealing, cooli~lga nd plating are continuously carried out in view of the
productivity. When the following alloying treatliient is not carried out, the steel sheet
is cooled to a normal temperature after the plating.
[0034]
When an alloying treatment is carried out, it is preferable that the alloying
treatment be carried outwithin a temperature range of 450°C to 600°C after tlie plating,
and then, the steel sheet be cooled to a normal temperahrre. This is because alloying
does not sufficiently proceed at a temperature of lower than 450°C, and alloying
excessively proceeds at a temperature of higher than 600°C such that the plated layer is
embrittled to cause a probleln of exfoliation of the plating by working such as pressing
or the like. When an alloying treatment time is shorter than 10 seconds, alloying does
not sufficiently proceed, and tllus, 10 seconds or longer is preferable. In addition, the
upper limit of the alloying treatment time is not particularly limited, but preferably
within 100 seconds in view of productivity.
[003 51
In view of productivity, it is preferable that an alloying treatment firrnace be
provided continoously to the continuous antlealing-hot dip galvanizing line to carly out
antlealing, cooling, plating and an alloying treatment, and cooling in a contiuuous
manner.
Exatnples of the plated layer shown in examples include hot dip galvanizing
and galvannealing, and electrogalvanizing is also included.
[003 61
Skin pass rolling is carried out to correct the shape and secure the surface
properties, and is preferably carried out in a range of an elongation ratio of 0.2% to
2.0%. The reason \vhy the lo~verli mit of the elongation ratio of the skin pass rolling
is set to 0.2% is that sufficient improvement in the surface roughness is not attained at
an elongation ratio of less than 0.2%, and thus, the lower limit is set to 0.2%. On the
other hand, \vIien the skin pass rolling is carried out at tlie elongation ratio of more
than 2.0 %, the steel sheet is excessively work-hardened to deteriorate the press
foniiability. Tlins, the upper limit is set to 2.0%.
[0037]
Next, a metallographic structare will be described.
composed of mainly felrite and bainite. When tlie area ratio of ferrite is less than
80%, bainite is increased and st~fficiendt uctility cannot be obtained. Thus, tlie lower
limit of tlie area ratio of fe~riteis set to 80%. When the area ratio of ferrite is 95% or
more, a tensile strength of 590 MPa or more cannot be secured in some cases, and thus,
the upper limit of the area ratio of ferrite is set to less than 95%. Further, the area
ratio of ferrite is preferably 90% or less.
Since the non-recrystallization ferrite contributes to hardening of tlie ferrite,
tlie effect of improving the stretch flangeability is obtained by reducing the hardness
difference with the bainite with appropriately co~~trollinthge area ratio of the nonrecrystallization
ferrite within a range of 1% or more and less than 10%. When the
ratio of the non-rec~ystallization ferrite in tlie ferrite is less than 1%, the nonrecrystallization
ferrite does not contribute to hardening of tlie ferrite, and thus, the
lower linlit of the area ratio of tlie non-recrystallization ferrite is set to 1% or more.
When the ratio of tlie lion-recrystallization ferrite in the ferrite is 10% or more, a
decrease in the hole expansion ratio or the like is caused, and thus, tlie upper limit is
set to less tllan 10%.
Bainite contributes to 11ig11-strengtheniag. However, when the amount of
bainite is excessive, a decrease in the ductility is caused, and thus, the lower limit is set
to 5% and the upper limit is set to 20%.
In addition, as sl~ownin FIG. 7, as other phase, there are pearlite, residual
austenite, and martensite. Whetl a total amount (area ratio or volu~nera tio) of these
conlponents is 8% or more, the hardness difference with the ferrite is large, and thus,
the hole expansio~rla tio or the like is decreased. Therefore, the upper limit of the
total amount of the pearlite, residual austenite, and martensite is set to less than 8%.
the preseut invetltion, a tensile strength of 590 MPa or more can be obtained. The
upper limit of the tensile strength is not particularly limited. However, considering
the lower limit of the area ratio of the ferrite of the present invention, the upper limit
may be set to about 780 MPa.
[0038]
The equivalent circle diameter of the carbonitrides including one or both of
Nb and Ti is set to 10 nm or less. As shown in FIG. 6, the average particle diameter
of the carbollitrides is extremely important to appropriately control the amount of the
non-recrystallization ferrite, and when the equivalent circle diameter is more than 10
nm, a sufficient rec~ystallizations uppressing effect cannot be obtained and at1
appropriate amount of the non-recrystallization ferrite cannot be obtained. Thus, the
upper limit is set to 10 nm. 111 addition, the lower limit is set to 1 111n or more in terms
of accuracy in nleasnrement.
[0039]
The microstructure may be observed with an optical n~icroscopeb y collecting
a sample having an observatio~ls urface wl~ichis a cross section parallel to the rolling
direction and the thickness direction, polisl~itlgth e observation surface, aud carrying
out nital etching, and as required, Le Pera etching. In the observation of the
microstructure, regarding the sample collected from an arbitrary position of the steel
sheet, a poltion which is at a 114 portion along the thickness direction was imaged at a
magnification of I000 times in a range of 300 x 300 pm. The image of the
microstroctnre obtained by the optical nlicroscope is analyzed by binarizing the image
to white and black so that a total area ratio of any one or two or more of pearlite,
bainite, and martensite can be obtained as an area ratio of phases other than the ferrite.
steel sheet with by the above method imaging a 114 portion along the thickness
direction at a ~nagnificatiorio f 1000 tinles in a range of 300 x 300 pnl and having 3 or
more imaged view fields. It is difficult to distinguish residual austenite from
nlartensite with the optical microscope, but the volume ratio of the residual aostenite
can be ~neasuredb y an X-ray diffraction method. The san~pleu sed in the
aforenlentioned n~icrostn~ctuoreb servation is used for obtaining the fraction of the
residual austenite. The non-recrystallization ferrite and ferrite other than the nonrecrystallization
ferrite can be deter~ninedb y analyzing the measurement data of the
orientation of an electron back scattering pattern (EBSP) by the Kernel Average
Misorie~ltationm nethod (KAM method). In the grains of the non-recrystallization
ferrite, dislocations are recovered, but a continuous change of the orientation, which is
caused by plastic deformation during the cold rolling, is present. 011 the other hand,
the change in the orientation in the grains of the ferrite other than the nonrecrystallization
ferrite becomes extretnely small. In the KAM method, it is possible
to quantitatively indicate the orientation difference with an adjacent measurement point.
In the present invention, when an area between measurement points, the measurement
points having 5" or more of average orientation difference, is defined as grain
boundary, a grain in which the average orientation difference with an adjacent
measurement point is lo or less and of \vhich the grain size is more than 0.51un is
defined as the ferrite other than the non-rec~ystallizationf errite. That is, the area ratio
of the non-recrystallization ferrite is an area ratio obtained by subtracting the area ratio
of ferrite other than the non-recrystallization ferrite from the area ratio of total ferrite.
The area ratio obtained from the microstnicture is the same as the volume ratio.
[0040]
Ti is measured by preparing an extraction replica sample extracted from a portion
whiclich is at a depth of 114 of the sheet thickness from a surface of arbitrary position of
the steel sheet, and observing carbonitrides as a target with a transtnission type electron
microscope (TEM) to obtain the average particle size of the carbonitrides. The
average particle size was obtained by imaging an image at a magnification of 10000
times in a range of 10 x 10 pm, and counting 100 rando~np articles of alloy carbides.
It is difficult to count a particle having a size of 1 nm, and 100 large particles are
counted, not exactly in descending order, but in random order.
A test method of each mechanical property will be described below. A
tensile test sample according to JIS Z 2201 No. 5 was taken from a steel sheet after
being manufactured in which the width direction (referred to as the TD direction) is
considered as the longitudinal direction, and the tensile properties in the TD direction
were evaluated according to JIS Z 2241. The stretch flangeability was evaluated
according to Japan Iron and Steel Federation Standard JFS T 1001. Each of the
obtained steel sheets was cut to 100 tntn x 100 nun size pieces and then punched to
have a hole with a diameter of 10 nun with a clearance being 12% of the thickness.
Then, in a state in ~vlvhich blank holding pad were suppressed with a force of 88.2 kN
and in which a die with an inner diameter of 75 mm is used, a 60" conical punch was
forced tlvhrough the hole to measure a hole diameter in a crack initiation limit. A
litniting hole expansion ratio [%] was obtained from the following (Expression 3), and
the stretch flangeability was evaluated based on the limiting hole expansion ratio.
Limiting hole expansion ratio h [YO] = {(Dr- Do)/Do) x 100 (Expression 3)
Here, Df represents a hole diameter [mm] at the time of crack initiation, and
Do represents an initial hole diameter [mm]. In addition, plating adhesion is evalnated
portion bent by a bending test.
[0041]
Steel sheets were obtained by melting the steels having the compositions
shown in Table 1, casting to obtain the slabs, and manufacturing the steel sheetsunder
the conditions shown in Tables 2-1 and 2-2. "[-I" in Table 1 indicates that the
analyzed value of a component is less than a detection limit. In addition, calcnlation
values of As3 ["C] and As1 ["C] are also showvn in Table 1.
[0042]
A tensile test sample according to JIS Z 2201 No. 5 was taken from a steel
sheet after being manufactored in which the width direction (referred to as the TD
direction) is considered as the longitudinal direction, and the tensile properties in the
TD direction were evalnated according to JIS Z 2241. In addition, tlie stretch
flangeability w\fas evalnated according to Japan Iron and Steel Federation Standard JFS
T 1001. Each of the obtained steel sheets was cut to 100 mm x 100 mm size pieces
and then punched to have a hole with a diameter of 10 mn with a clearance being 12%
of the thickness. Then, in a state in which blank holding pad were suppressed with a
force of 88.2 kN and in which a die with an inner diameter of 75 mm is used, a 60"
conical punch was forced tl~roughth e hole to measure a hole diameter in a crack
initiation limit. A limiting hole expansion ratio [Oh] was obtained fro111 the following
(Expression 3), and the stretch flangeability was evaluated based on the limiting hole
expansion ratio
Limiting hole expansion ratio h [Oh] = {(Df- Do)/Do) x 100 ... (Expression 3)
Here, Df represents a hole diameter [mm] at the time of crack initiation, and
according to JIS H 0401 by visually observing a surface state of a plating fill11 at a
portion bent by a bending test
[0043]
The tnicrostruct~~roef the sheet thickness cross section of the steel sheet was
observed by the above-described manner, and the area ratio of bainite was obtained as
a total area ratio of phase which is not ferrite and other phases.
The result is shown in Tables 3-1 and 3-2. In the present invention, a sample
having 17000 mPa.%] or more of a product of tensile strength TS [MPa] and total
elongation El [%I, i.e. TS x El [ma.%], \vlliclich is a ductility index, is considered as
acceptance. A sample having 75% or nlore of, and preferably 80% or more of the
hole expansion ratio h [Oh], which is a hole expansibility index, is considered as
acceptance. In a case of a hot-dip galvanized steel sheet or a galvannealed steel sheet,
plating adhesion is also set as a target. The plating adhesion was evaluated according
to JIS H 0401 by visually observing a surface state of a plating film at a portion bent
by a bending test.
[0044]
As shown in Tables 3-1 and 3-2, it is possible to obtain a high-strength steel
sheet, a hot-dip galvanized steel sheet, and a galvannealed steel sheet which have
excellent ductility and stretch flangeability by subjecting steel having the chemical
co~npositiono f the present invention to hot rolling, cold rolling, and annealing under
appropriate conditions.
011 tlie other hand, for Steel No. M, since the amount of C is large, tlie total
elongation is decreased, the product of tensile strength and total elongation is
decreased, and tlie hole expansiotl ratio is also decreased.
reduced, the tensile strengtli is decreased, and the product of tensile strengtll and total
eloilgation is decreased.
For Steel No. 0, since tlie amount of Si is small, tlie hole expansion ratio is
decreased.
For Steel No. P, since tlie anlount of Si is large, the area ratio of bainite is
reduced, the tensile strengtli and tlie total elongation are decreased, tlie product of
tensile strength and total elongation is decreased, and the plating adhesion is also
decreased.
For Steel No. Q, since tlie amount of Mn is small, the area ratio of bainite is
reduced, the tensile strength and tlie total elongation are decreased, the product of
tensile strengtli and total elongation is decreased, and tlie hole expansion ratio is also
decreased.
For Steel No. R, since the amount of Mn is large, the area ratio of bainite is
increased, the tensile strengtli is increased, the total elongation is decreased, the
product of tensile strength and total elongation is decreased, and tlie hole expansion
ratio is also decreased.
For Steel No. S, since the amoutit of A1 is large, the area ratio of bainite is
reduced, tlie tensile strength is decreased, tlie product of tensile strength and total
elongation is decreased, and the hole expansion ratio is also decreased.
For Steel No. T, since the amount of N is large, the area ratio of bainite is
increased, tlie total elongation is decreased, the product of tensile strength and total
elongation is decreased, and the hole expansibility is also decreased.
For Steel No. U, since the amount of Ti and Nb is small, the area ratio of nonrecrystallization
ferrite is reduced, tlie tensile strength and the hole expansion ratio are
decreased.
For Steel No. V, since the amount of Ti arid Nb is large, the area ratio of nonrecrystallization
ferrite is increased, the total elongation is decreased, the product of
tensile strength and total elongation is decreased, and the hole expansion ratio is also
decreased.
For Steel No. W, since the an~ounot f Nb is small, the area ratio of nonrecrystallization
ferrite is reduced, the tensile strength and the hole expansion ratio are
decreased.
For Steel No. X, since the amount of Ti is large, the area ratio of aonrecrystallization
ferrite is increased, tlie total elongation is decreased and the product of
the tensile strengt11 and the total elongation is decreased. Also, the Ilole expansion
ratio is decreased.
For Steel No. Y, since the amount of Nb is large, the area ratio of nonrec~
ystallizationf errite is increased, the total elongation is decreased, the product of
tensile strength and total elongation is decreased, and the hole expansion ratio is also
decreased.
[0045]
For Production No. 3, since the heating teniperature during the hot rolling is
low, the carbonitrides are coarsened and the recrystallization suppressing effect during
t11e annealing is small, and thus, the area ratio of the non-recrrystallization ferrite is
reduced and the tensile strength and the l~olex pansion ratio are decreased.
For Production No. 6, since the finishing terilperatnre during the hot rolling is
slightly low, the carbonitrides are coarsened and the recrystallization suppressing effect
during the annealing is small, and thus, the area ratio of non-recrystallization ferrite is
reduced and the tensile strength and the hole expansion ratio are decreased.
slightly low, the carbonitrides are coarsened and the recrystallization suppressing effect
during the annealing is sniall, and thus, the area ratio of non-recrystallization ferrite is
reduced and the tensile strength and the hole expansion ratio are decreased.
For Production No. 12, since the finishing temperature during the hot rolling
is low, the carbonitrides are coarsened and the recrystallization suppressing effect
during the annealing is small, and thus, the area ratio of non-recrystallization ferrite is
reduced and the tensile strength and the hole expansion ratio are decreased.
For Production No. 15, since the coiling teniperature is high, the carbonitrides
are coarsened and the rec~ystallizations uppressing effect during the annealing is small,
and thus, the area ratio of non-rec~~~stallizatifoernr ite is reduced and the tensile
strength and the hole expansion ratio are decreased.
For Production No. 18, since the cold rolling reduction is low, the area ratio of
non-recrystallization ferrite is increased and the total elongation is decreased, and tllus,
the product of tensile strength and total elongation is decreased, and the hole expansion
ratio is also decreased.
For Production No. 21, since the ~llaxinlunh~ea ting temperature is high during
the annealing, the carbonitrides are coarsened, and the recrystallizatio~s~u ppressing
effect during the annealing is small, the area ratio of non-rec~ystallization ferrite is
reduced. The area ratio of bainite is increased, and thus, the 11ole expansion ratio is
decreased.
For Production No. 24, since the maximum heating temperature during tlie
annealing is low, the area ratio of bainite is reduced, and thus, the tensile strength and
the total elongation are decreased, aid the product of tensile strength and total
elongation is decreased. Also, the hole expansion ratio is decreased.
annealing is excessively high, the area ratio of ferrite does not reach a predetermined
value, and relatively, the area ratio of bai~~iitse i ncreased. The hole expansion ratio is
decreased.
For Production No. 28, the holding time at the ~naxi~nuhmea ting teniperature
during the annealing is short, the amount of bainite is reduced and the area ratio of
non-recrystallization ferrite is increased. Thus, the total elongation is decreased, the
product of tensile strength and total elongation is decreased and the hole expansion
ratio is also decreased.
For Production No. 29, since the end tenlperature of primary cooling after
annealing is excessively low, the area ratio of ferrite is increased excessively, and
relatively, the area ratio of bainite is reduced excessively. While the hole expansion
ratio is satisfied, the tensile strength does not reach a predetermined value, and also the
balance behveen tensile strength and total elongation is poor. The product of tensile
strength and total elongation is also decreased.
For Production No. 32, since the holding time at the maximum heating
teniperature during the annealing is long, tlie carbonitrides are coarsened and the
recrystallization suppressing effect during the annealing is small, and thus, the area
ratio of non-recrystallizatio~fe~r rite is reduced and the area ratio of bainite is increased.
Thus, the hole expansion ratio is decreased.
- 27 -
For Production No. 33, since the annealing primary cooling rate is excessively
high, the area ratio of ferrite does not reach a predetermined value and relatively, the
area ratio of hair~iteis increased. Thus, the hole expansion ratio is decreased and also
the balance between tensile stre~lgtha nd total elongation is poor. The product of
tensile strength and total elongation is also decreased.
recrystallization suppressing effect by carbonitrides during the annealing is large, the
area ratio of non-recrystallizatior~f errite is increased and the total elongation is
decreased. The product of tensile strength and total elongation is decreased and the
hole expansion ratio is also decreased.
For Production No. 39, sincethe ten~peraturer isiug rate during the armealing
is high, the area ratio of non-recrystallization ferrite is increased and the total
elongation is decreased. The product of tensile strength and total elongation is
decreased and the hole expansion ratio is also decreased.
[0046]
[Table I]
[0047]
[Table 2-11
[0048]
[Table 2-21
[0049]
[Table 3-11
[OO50]
[Table 3-21
[Industrial Applicability]
[005 11
According to the present invention, it is possible to provide a high-strength
cold-rolled steel sheet and a plated steel sheet which have a tensile strength of 590
MPa or more, and excellent ductility and stretch flangeability, and the present
invention makes an extremely significant contribution to the industry.
TABLE 1
W
X
Y
(ANNOTATION 1) THE UNDERLINED VALUES ARE OUT OF THE RANGE OF THE PRESENT INVENTION
0.045
0.055
0.050
0.40
0.45
0.50
1.90
2.00
1.85
0.0078
0,0080
0.0068
0.0032
0.0029
0.0031
0.033
0.031
0.038
0.0041
0.0035
0.0036
-
0.035 -
eeQ2
-
0.035
eeQ2
QJE
0.035
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
737
727
744
724
726
728
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
TABLE 2-1
HOT ROLLING 1 COLD I ROLLING ANNEALING
- . . . . . . . . - -
1 ROLLING AFTER 1
ANNOTATION 1 THE UNDERLINED VALUES ARE OUT OF THE RANGE OF THE PRESENT INVENTION
STEEL
No.
MANUFACTURING
No. HEATING
TEMPERATURE FINISHING REDUCTION
TEMPERATURE
COILING
TEMPERATURE
ANNEALING
ELONGATION
RAT 10
TEMPERATURE
RATE TEMPERATURE
HOLDING
TIME
END
TEMPERATURE
OF PRIMARY
COOLING
RATE OF
PRIMARY
COOLING
ALLOYING
TREATMENT
TEMPERATURE
ALLOYING
TREATMENT
TIME
TABLE 2-2
HOT ROLLING 1 COLD 1 ROI I ING ANNEALING
SKIN ?:;k:R1 PASS I
52 1 1200 1 900 I 550 I 60 I 3 1 760 680 530 1 35 I 0.6 I
(ANNOTATION 1) THE UNDERLINED VALUES ARE OUT OF THE RANGE OF THE PRESENT INVENTION
STEEL
No.
MANUFACTURING
No. HEATING
TEMPERATURE
-
FINISHING REDUCTION
TEMPERATURE
COILING
TEMPERATURE
ANNEALING
RAT I0
RATE OF
PRIMARY
COOLING
TEMPERATURE
RATE
HOLDING
TEMPERATURE TIME
ALLOYING
TRMTMENT
TEMPERATURE
END
TEMPERATURE
OF PRIMARY
COOLING
ALLOYING
TREATMENT
TIME
TABLE 3-1
L3
'4
-P -. VI
(ANNOTATION 1) THE UNDERLINED VALUES ARE OUT OF THE RANGE OF THE PRESENT INVENTION
TABLE 3-2
'4
-C r vl
(ANNOTATION 1) THE UNDERLINED VALUES ARE OUT OF THE RANGE OF THE PRESENT INVENTION
STEEL
No.
MANUFACTURING
No.
MICROSTRUCTURE CARBONITRIDE
EQUIVALENT
CIRCLE
DIAMETER
AREA
RATIO OF
FERRITE
MECHANICAL PROPERTIES
AREA OF
RECRYSTALLIZATION
FERRITE
PLATING
ADHESION
AREA
RATIO OF
BAINITE
AMOUNT OF
THE OTHER
PHASE
YIELD
STRENGTH
YP
TOTAL
ELONGATION
El
TENSILE
STRENGTH
TS
TS x El
HOLE
EXPANSION
RATIO A
[Document Type] CLAIMS
[Claim 11
A cold-rolled steel sheet comprising, by mass%:
C: 0.020% or more and 0.080% or less;
Si: 0.20% or more and 1.00% or less;
Mn: 0.80% or more and 2.30% or less;
P: 0.0050% or more and 0.1500% or less;
S: 0.0020% or more and 0.0150% or less;
Al: 0.010% or more and 0.100% or less;
N: 0.0010% or more and 0.0100% or less; and hrther comprising:
one or more of Nb and Ti which satisfy a requireme~ito f 0.005% 5Nb + Ti <
0.030%; and
a reminder includi~lgF e and u~lavoidableim purities,
wherein a structure consists of, a ferrite, a baiaite, and an other phase,
the other phase includes a pearlite, a residual austenite, and a martensite,
an area ratio of the ferrite is 80% or more and less than 95%,
an area ratio of a non-recrystallization ferrite in the ferrite is 1% or more and
less than lo%,
an area ratio of the bainite is 5% to 20%,
a total amount of the other phase is less than 8%,
an equivalent circle diameter of a carbonitride including one or both of Nb
and Ti is 1 nm or more and 10 11111 or less, and
a tensile strength is 590 MPa or more.
[Claim 21
The cold-rolled steel sheet according to claim I , further comprising one or
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;
B: 0.0005% or more and 0.0100% or less;
Co: 0.05% or more and 1.50% or less; and
Cr: 0.05% or more and 1.50% or less.
[Claim 31
Aplated steel sheet, wherein plating is provided on a surface of the coldrolled
steel sheet according to claim 1 or 2.
[Claim 41
Amethod for tnanufach~ringa cold-rolled steel sheet comprising:
heating a slab having a co~npositiona ccording to claim I or 2 to 1150°C or
more and 1280°C or less;
finishing a finish rolling under a temperature of Ar3"C or more and 1050°C or
less;
pickling and then cold-rolling a hot-rolled steel sheet, which is coiled under a
temperature range of 450°C or more and 650°C or less, under a reduction of 40% or
more and 70% or less; thereafter
heating into a temperature range ofAc~"Co r more and (Act + 40) OC or less
under a rate of 2"CIsec or more and 5"CIsec or less;
anllealirlg the cold-rolled steel sheet under the temperature range of Acl0C or
more and (Acl + 40) "C or less and under a holding time of 10 sec or more and 200 sec
or less: and
primary cooling immediately after the annealing into a steel sheet temperature
range of 600°C or Inore and 720°C or less under a cooling rate of 10°C/sec or less in a
course from the annealing to arriving at a normal temperature,
wherein the Ar3"C and the AcloC is aAr3 transformation temperature and a
Acl transfonnation temperature, respectively, calculated from Expressions 1 arid 2,
Ar3 = 910 - 325 x [C] + 33 x [Si] + 287 x [PI t 40 x [All - 92 x ([Mil] + [Mo]
+ [Cu]) - 46 x ([Cr] + [Nil) (Expression I),
Ac, = 723 + 212 x [C] - 10.7 x [Mn] + 29.1 x [Si] (Expression 2), and
elements noted in brackets represents an amount of the elements by mass%,
respectively.
[Claim 51
A method for manufacturing a plated steel sheet, comprising:
plating the cold-rolled steel sheet manufactured by the method according to
claim 4 after the annealing and the cooling.
. . [Claim 61
The method for manufacturing a plated steel sheet according to claim 5,
comprising:
heat treating the plated steel sheet under a temperature range of 450°C or
more and 600°C or less with 10 seconds or longer.
Dated this 22.07.2014
IMNJNA MEHTA-Dun]
OF N%FRY & SAGAR
ATTORNEY FOR THE APPLICANTLSJ
[Docunlent Type] Abstract
A cold-rolled steel sheet includes, by mass%: C: 0.020% or more and 0.080%
or less; Si: 0.20% or more and 1.00% or less; Mn: 0.80% or more and 2.30% or less;
and Al: 0.010% or more and 0.100% or less; and further includes: one or 1110re of Nb
and Ti which satisfy a requirement of 0.005%