[Document Type] Specification
[Title of the Inve~~tion] STEEL SHEET AND METHOD OF PRODUCING THE
SAME
[Tech~icaFl ield of the Inve~ltio~l]
[OOOl]
The present invention relates to a high strength steel sheet which has a low
jlield ratio and excellent elongation, and a method of producing the same.
Priority is clainled 011 Japanese Patent Application No. 2011-221904, filed on
Octobcr 6,2011, and the content of \\rl~icha rc incorporated herein by reference
[Related Art]
[0002]
In recent years, there has been an increasing demand for weight saving of
vehicle bodies to itnprove fuel consotuption and for improving collision safety to
protect the passengers at the time ofcollision in automobiles or the like. Therefore,
the use of high strength steel sheets has increased but excellent workability (ductility
and the like) that is necessary for the fornlation of vehicle bodies and parts is required
for high stret~gtlsl teel sheets used in auton~obileso r the like, as well as required
strength.
[0003]
As one of the indices for evaluating the workability of high strength steel
sheets, there is a yield ratio (a ratio ofyield strength (YP) to tensile strength (TS):
YPITS x loo(%)). Usually, when a yield ratio decreases, deterioration in shape
fixability which tends to deteriorate along with strengthening and the occurrence of
wrinkles can be suppressed. In addition, it is possible to reduce a press load
[0004]
As a high strength steel sheet supplied for an application in which sstisfactoly
elongation (ductility) is needed, there has been known dual phase steel (hereinafter,
referred to as "DP steel" in some cases) having a dual phase structure of ferrite and
martensite, and the steel has been widely used for structural n~e~nbefrosr automobiles.
The DP steel has an excellent balance of strength and ductility conlpared to a soli~te
strengthening type steel sheet and a precipitation strengthening type steel sheet, and
also has a feature of a low yield ratio (for example, refer to Patent Documents 1 to 6).
[0005]
la Patent Documcnt 1, a teclu~iqnei s disclosed in which a dual phase structure
of ferrite and marteasite is formed by holding steel in a telnperaturc range ofAc1 or
higher and Ac1 + 75OC or loiver for 15 seconds or longer, and then, cooling the steel to
a te~nperat~o~fr 2e0 0°C or lo\ver at a cooling rate of 10 "CIS or more.
[0006]
In Patent Document 2, a teclinique is disclosed in which a doal phase strt~ch~re
of ferrite and martensite is fornied by cooling steel to 700°C to 600°C frotii an
annealing soaking teniperatt~rca t 15 'CIS or less, subsequently, cooling to room
temperatiwe at 100 "CIS or more, and reheating the steel to hold the stccl at 150°C to
250°C.
[0007]
In Patent Document 3, a tecliniqt~eis disclosed in which an amount of solidsoluted
C and martensite hardness in steel is adjnsted while the steel has a dual phase
stroch~reo f ferrite and martensite fornled by cooling the steel to a Ms point or lower
from a dual phase region tenlperature (preferably 201s or more) and transfor~ning
austenite to martensite, and then, liolding the steel ill a temperature range of 100°C to
250°C for 10 secot~dso r longer.
[OOOS]
In Patent Document 4, a tecluiiqoe is disclosed in wl~icha dual phase structure
of ferrite and martensite is formed by holding to anneal steel at a dual phase region
temperature ofAc1 point or higher and lower thanAc3 for 30 seconds to 90 seconds,
and then, cooling the steel to 550°C at 5 OCIs or more.
[0009]
In Patent Document 5, a technique isdisclosed in \\~l~ical id ual phase structure
of ferrite and martensite is fornied by annealilig a cold-rolled steel sheet at a required
temperatore, and then cooling the steel sheet at a cooling rate of 10 "CIS or more,
preferably 20 "CIS or more.
[do lo]
111 Patent Document 6, a techniqoe is disclosed in \\,hicli a dual phase structure
of ferrite atid martensite is formed by annealing a cold-rolled stcel sheet at a required
teniperature for 3 seconds or longer, and then cooling the steel sheet to lower than
40OoC at a cooling rate of 2 "CIS to 200 "CIS.
[OOl 11
As disclosed in Patent Docume~its 1 to 6 above, it is kiown that it is i~iiportant
to control a coolirig rate atid a cooling elid temperature after annealing in a dual phase
region to obtain a dual phase structure (DP steel) \vhich satisfies required i~~echanical
properties.
[PriorArt Doct~ment]
[Patent Docnment]
[0012]
[Patent Document 11 Japanese Unesainined Patent Application, First
Publication No. H09-287050
[Patent Docoment 21 Japanese Unexanii~ied Patent Application, First
PnblicationNo. H10-147838
[Patent Docu~nen3t 1 Japanese Unexamined Patent Application, First
PoblicationNo. H11-350063
[Patent Docu~iien4t 1 Japanese Unexamined Patent Application, First
Publication No. 2001-335890
[Patent Docutuent 51 Japanese Unexamined Patent Application, First
Publicatio~iNo.2 002-226937
[Patent Docu~~ict6i1t Jal)anese Unexamined Patent Application, First
Publication No. 2003-213370
[Disclosure of tlie Invention]
[Problems to be Solved by the Invention] .
[0013]
I-to\vever, in the manufacturing niethods in Patent Doctunents 1 to 6, in order
-to produce a steel sheet having a dual phase structure of ferrite and martensite;.a rapid
cooling apparatus and a large anioi~not f Mn that itliproves hardenability are osed.
Therefore, there is a proble~iin that workability is deteriorated caused by local
material deterioration dne to coriiponent segregation.
Usually, when steel is not cooled at a rapid cooling rate after being soaked in a
dual phase region, pearlite is precipitated froni a hardened structure such as martensite,
bainite, or tlie like, and thus, it is dific~~tolt e nsure the reqoired strength. In addition,
when a steel sl~eeits annealed and cooled in a continuous annealing fi~~iiahcaev ing a
usual overaging section, the cooling end temperature is held around 400°C, and tli~~s,
forn~edn iartensite is tempered and decomposed into pearlite.
.- [0014] . .
I11 a case where a large amount of an austenite fornler (Mn is generally used)
is used so that steel is easily transfor~~~wehde,n a coolitig rate after annealing is not
optimized, workability is deteriorated due to compoaent segregation and also,
deterioratio~in~ d uctility (elotlgatiotl) is caused by ~l~artensiitne a Mn segregated
portion.
[OOl 51
As described above, in order to obtain a dual phase structure wl~ichs hows a
low yield ratio and excellent elongatiotl, it is importat~to co~ltrotlh e cooling rate atld
the cooli~lgc t~dte mperature after amlealing in the dual phase region, but it is difficult
to stably obtain a high stre~lgths teel which shows a low yield ratio and excelletit
elo~lgatioo~ill ly by cooling after a~ulealing.
[00 161
The present invetltiot~is made in consideration of the above circumstances
and an object thereof is to provide a high streilgth steel sheet which has a structure
shoning a lo~vyi eld ratio and excellent elongation atltl a method of protiocing the same.
In tllc prcseilt invention, the lo\\, yield ratio refers to a yield ratio of 65% or less, and
the high strength refers to a tc~lsiles tretigth of 590 MPa or more.
I11 addition, \\hen it is considered that the steel sheet is used as a member for
automobiles or t11c like, TS x El, which is a product of tensile strength TS and
elongation El, is preferably 17500 (MPa.%) or more in terms of \\rorkability.
[Means for Solving the Problen~]
[00 171
The itlve~~tohrasv e conducted itite~~sivsteu dies of a method for solvitlg the
above problem. As a result, it has been fouttd that it is effective to strictly tnanage a
coolitlg rate atid a cooli~ige nd tcmperaturc after annealing in a dual phase regioti and
fi~rtliert, o perforni retention in tlie optinlum teniperatnre range after performing the
cooling. That is, tlie inventols have found tlie following. Here, tlie letention may
denote not only isotliernial holding but also a temperature change in tlie temperature
range.
[0018]
(i) It is possible to forni a steel sheet which niainly incl~rdesf errite and
~iiartensitea s a structure (a so-called dual phase struchlre) by decreasing tlie cooling
rate of a steel slieet after antlealing (a pritilary coolitig rate) and niaking a cooling end
temperature fall within a required teniperature range. Therefore, it is effective to
plod~lcca steel sheet of 590 MPa or nlore having a low yield ratio arid excellent
elongation.
[0019]
(ii) Ho\vever, \vhen tlie prinla~yc ooling rate is slow, since martensite is
tliflicult to be fornled it is difficult to obtain the dual phase structore. On the other
hand, wlien the amount of Mn is increased so as to for111 martensite, Mn is segregated
and tlitts, detcrioration in ductility is caused by the niartcnsite in a Mn segregated
portion and a yield point i~icreascs. On tlie contra~y,e ven wlien a large aniount of Mn
is contau~edi,f a soaking ti~iiein annealing is increased, Mn is unifornily diffused and
tlie segregation is eliminated. Therefore, the nlartensitc is uniformly fornied in a
thickness direction and a width dilection and tlie quality of the tilaterial becomes
uniform.
[0020]
(iii) Further, by perfornii~igs oaking and retention in which a retention time
and a retention tenlperature after priniary cooling are co~itrolledi, t is possible to obtain
a sttuctorc suitable for a steel sheet of 590 MPa or niore \\~liicllh as a low yield ratio
and excelie~iet longation.
[0021]
The presetit i~wentioi~s l~ iiadeb ased on the above fi~iditigsa id tlie gist thereof
is as follows.
[0022]
(I) According to an aspect of tlie present invention, there is provided a steel
sheet including, by mass%, C: 0.04% or more and 0.15% or less, Si: 0.3% or more and
0.7% or less, Mn: 1.0% or tiiore and 3.0% or less, Al: 0.005% or more and 0.10% or
less, P: li~nitedto 0.03% or less, S: limited to 0.01% or less; N: litliited to 0.01% or
less, and a remainder consisting of Fe and unavoidable impurities, wherein tlie steel
sheet has a steel structure obtained by perfortiii~iga soaking for a soaking time of 15
secotids or lo~igera nd 35 seconds or shorter at a dual phase region temperature of Acl
temperatare or higher and lowcr than Ac3 temperature, nest, performing a prinlary
cooling to a temperature range of 250°C or higher and 380°C or lower at a cooling rate
of 0.5 "CIS or more and 30 "CIS or less within 3 seconds, and perfo~minga retention in
a temperature range of 260°C or higher and 370°C or lower for 180 secottds or lo~iger
and 540 sccoiids or shorter after the pritiia~yc ooling, a yield ratio is 65% or less and
tetisile strength is 590 MPa or more, and the Ac1 tetnperat~~irse a telnperature
expressed by a following Expression (a) in 11nit.so f "C, and the Ac3 tetliperature is a
temperature expressed by a follovving Expression (b) ill units of OC.
Acl =732-26.6 x [C]+ 17.6 x [Sil- 11.6 x [Mn] ...( a)
Ac3 = 924 + 56.1 x [Si] - 19.7 x [Mn] - 436.5 x [C] ...( b)
here, [C], [Si], and [Mn] represent a C content, an Si cotitetit, and an Mn
content respectively, and a unit thereof is mass%.
[0023]
(2) 111 the steel sheet according to (I), the cooling rate may be 0.5 "CIS or
niore and 15 "CIS or less.
[0024]
(3) hl the steel sheet according to (1) and (2), y wvhicli is a product of a
retention temperature and a retention time in the retention and x ~w~hicisli t he cooling
rate in the prinlaty cooling may satisfy a follow\~ingE xpression (c).
Y <- 796700 x X(.O-~") ... (c).
[OOZS]
(4) The steel shcet accortlitlg to any one of (1) to (3), nlay further include, by
mass%, any one or two or more of Cr: 0.01% or tllore and 0.5% or less, Mo: 0.01% or
niore and 0.5% or less, and B: 0.0005% or more and 0.005% or less, and the Acl
tenlperature may a tetnperatttre expressed by a following Expression (d) in units of "C,
and the Ac3 tenlperatltre niap be a temperature expressed by a following Expressioti
(e) in units of "C,
Ac1 = 732 - 26.6 x [C] + 17. 6 x [Si] - 11.6 x [Mn] + 24.1 x [Cr] ... (d)
Ac3 =924 + 56.1 x [Si] - 19.7 x [Mn] -4.9 x [Cr] -436.5 [C] ... (e)
here, [C], [Si], [fii], anti [Cr] reprcscnt a C content, an Si contcnt, at1 MI
content, and a Cr content respectively, and a unit thereof is mass%.
(5) The steel sheet according to (4), ma)r fi~rtherin clude, by mass%, one or
two or more of Nb, Ti, and V of 0.005% or more and 0.05% or less in total.
[0027]
(6) The steel sheet according to any one of (1) to (3), may further include, by
mass%, one or hvo or more of Nb, Ti, and V of 0.005% or more and 0.05% or less in
total.
[0028]
(7) In the steel sheet according any one of (1) to (3), the steel structllre may be
a structure that contains, by an area fraction, a bainite and a martensite in a total of 3%
or more and 10% or less, a residual austenite of 1% or more and 3% or less, and a
remainder consisting of a fe~rite.
[0029]
(8) In the steel sheet according to (7), in the steel structure, by an area fraction,
the bainite may be limited to 1% or less.
[003 01
(9) According to another aspect of the present invention, a nietl~od is provided
in ,11icli a steel sheet is produced osing a continooos annealing line, the method
including a fitst retention process of retaining a base steel sheet having the component
compositioti according to Claim I at a dual phase region ten~peratureo fAcl
temperatu~eo r higher and lower than Ac3 tenipeiature for 15 seconds or longer and 35
seconds or shorter; a priniary cooling process of primarily cooling the steel sheet to a
temperature range of 25OoC or higher and 380°C or lower within 3 seconds at a
cooling rate of 0.5 "CIS or ~11orea nd 30 "CIS or less after the first tete~ltiotpl rocess; and
a second rete~itionp rocess of retaining the steel sheet while passing through an
overagit~gse ction atranged in the continuous annealing line \\,hose te~nperatureis set to
260°C or higher and 370°C or lo\wrer for a retention time of 1 80°C or longer and 540
seconds or shorter after the primary cooling.
[003 I]
(10) In the method of producing a steel sheet according to (9), in the second
retention process, y that is a product of an overaging section passing temperahlre
\vhich is the retention tempetatore itthen the steel sheet passes through the overaging
section, and an overaging section passing time which is the retenti011 time, and x that is
the cooling rate in the primary cooling process map satisfy a following Expression (0.
< 796700 Y - x xC097L...) (f)
[0032]
(11) The method of producing a steel sheet according to (9) or (lo), may
fi~rtherin clude a prelin~inarys heet passing process of, before starting the printa~y
cooling process, passing a required amount or more of a temperature adjusted steel
sheet hose primary cooling stop te~nperatoreis set to 330°C or lower through the
continuous annealing line.
[0033]
(12) In the method of producing a steel sheet according to (ll), the required
anlount may 30 tons.
100341
(1 3) In the method of producing a steel sheet according to (9) or (lo), the base
steel sheet may fi~rtherc ontain, by inass%, any one or hvo or more of Cr: 0.01% or
tilore and 0.5% or less, Mo: 0.01% or more and 0.5% or less, and B: 0.0005% or nlore
and 0.005% or less.
(14) In the method of producing a steel sheet according to (13), the base steel
sheet may ftirther contain, by mass%, one or two or inore of Nb, Ti, and V of 0.005%
or more and 0.05% or less in total.
100361
(1 5) In the tnethod of producing a steel sheet according to (9) or (lo), the base
steel sheet may further contain, by mass%, one or h ~oor n lore of Nb, Ti, and V of
0.005% or Inore and 0.05% or less in total.
[Effect of the Invention]
[0037]
According to the present invention, it is possible to provide a high strength
steel sheet \vhich is suitable for vehicle bodics and parts for automobiles and has a low
yield ratio and excellent elongation.
[Brief Description of the Drawings]
[003 81
FIG. 1 is a view showing a relationship between y which is a product of a
retenti011 tenlperature and a retention time at the time of retention in a temperature
range of 260°C or higher and 370°C or lo\ver ( at the time of passage through an
overaging section) and x which is a prilnary cooling rate.
FIG 2 is a flow chart showing a neth hod of prodocing a steel sheet according
to an embodiment of the present invention.
[Embodiments of the Invention]
[0039] . .. ,
Hereinafter, an etnbodiment of the present invention will be described based
on the above findings.
~.
Ahigh strength steel sheet according to the cniboditnent which has a low
yield ratio and excellent elongation (hereinafter, referred to as a "steel sheet according
to the embodiment" in some cases) includes, by mass%, C: 0.04% or more and 0.15%
or less, Si: 0.3% or more and 0.7% or less, Mn: l .O% or more and 3.0% or less, Al:
0.005% or more and 0.10% or less, P: limited to 0.03% or less, S: limited to 0.01% or
less, N: limited to 0.01% or less, and a remainder consisting of Fe and unavoidable
impurities, and has a steel strtlcture obtained by performing soaking for a soaking titile
. . of 15 seconds or longer and 35 seconds or shorter at a dual phase region temperature of , . .
Acl temperature or higl~era nd lower than Ac3 temperature, next, performing pritnaly
cooling to a temperature range of 250°C or higher and 380°C or lo\ver at a cooling rate
of 0.5 "CIS or more and 30 "CIS or less within 3 seconds, and perfortnitlg retention in a
tenlperature range of 260°C or higher and 370°C or lower for 180 seconds or longer
and 540 seconds or shorter after the prinla~yc ooling.
[0040]
First, the reason for limiting the cornpotlent composition in the steel sheet
according to the cmbodiment ivill be described. Here, %related to the cotnponcnt
cot~~positiorenp rcsents mass%.
[0041]
C: 0.04% or more and 0.15% or less
C is an element effective to contribute to the forn~ationo f bainite and
martensite to achieve a low yield ratio and high strength. When the C content is less
than 0.04%, the effect cannot be obtained, and thrls, the lo\ver limit is set to 0.04%.
On the other hand, \vl~ent he C content exceeds 0.15%, bainite and martensite are
excessively forn~eda, nd thus, the upper linlit is set to 0.15%. In addition, \vl~ent he C
content is high, \veldability is deteriorated and a problenl arises in practical use. The
C content is preferably 0.07% or more and 0.12% or less.
[0042]
Si: 0.3% or more and 0.7% or less
Si is ail element effective to increase ~neclianicals trength (TS) witl~out
deterioration in ductility. However, when the Si content is less than 0.3%, the
addition effect is not exhibited sufficiently and tllus, the lower lin~iot f the content is
set to 0.3%. On the other hand, when the content exceeds 0.7%, ductility is
deteriorated and thus, the upper liniit is set to 0.7%. I11 addition, when the Si content
exceeds 0.7%, there is a concern of excessive for~nationo f residual austenite. The Si
content is preferably 0.4% or more and 0.6% or less.
[0043]
Mn: 1.0% or lnore 3.0% or less
MII is an elenle~ltw hich stabilizes austenite and contributes to uniforin
forination of martensite and i~nproven~eonft ductility even when the cooling rate is
slolv. Ilowever, when the Mn content is less than 1.0%, the addition effect is not
exhibited sufficiently, and thus, the lowcr lin~iits set to 1.0%.
[0044]
On the other hand, when the Ma content excecds 3.0%, Mn is segregated.
The martensite formed in the segregated portiotl causes deterioration in ductility and
deterioration in workability due to at1 increase of a yield point. In addition, when the
Mn content exceeds 3.0%, martensite is excessively forn~eda nd ductility is
deteriorated. Therefore, the upper limit of the Mn content is set to 3.0%. The upper
limit is p~eferably2 .6% or less.
[0045]
P: 0.03% or less
P is an impurity eletncnt and thus, the lowcr the colltent is, the more
preferable it is. Ho\vever, up to 0.03%, mechanical properties are not impaited, and
thas, the upper limit ofthe P content is set to 0.03%. The upper litnit is preferably
0.01% or less. Here, it is difficult to set the P content to 0% in operation, and thus,
0% is not included.
[0046]
S: 0.01% or less
S is an itnpority elen~cnat nd thus, the lower the content is, the Inore
preferable it is. Ho\\~ever, up to 0.01%, mechanical properties are not impaired, atid
thus, the upper limit of the S content is set to 0.01%. The upper litnit is preferably
0.005% or less. I-lere, it is difficult to set the S content to 0% in operation, atid tlius,
0% is not included.
[0047]
Al: 0.005% or more and 0.10% or less
Al is an element wliich is usually used for deoxidation, but, si~iiilatro MI), is
an element which contributes to in~proveme~ilnt hardenability. Ho\vcver, when tlie
A1 content is less tliati 0.005%, deosidatiotl is tiot sufficie~iat tid ductility is
deteriorated. Thus, the lower liniit is set to 0.005%. In addition, when tlic A1
cotitetit is less than 0.005%, I~ardenability is deteriorated atid tensile strength is
deteriorated. Therefore, there is a concern of i~lcreasit~ag y ield ratio. 011 the other
hand, \\'hen the A1 content esceeds 0.10%, the addition effect is saturated and tlius, the
upper limit is set to 0.10%. The A1 content is preferably 0.01% or more and 0.06% or
less. .
[0048]
N: 0.01% or less
N is an eletiletlt which contributes to tlie for~~latiootfl niartensite, sitllilar to C,
Ho\vever, when Al as the deoxidizing element is present, A1 nitrides are fortlied atid
ductility is deteriorated. Thus, tlie N content is set to 0.01% or less. The lower the
N content is, the more preferable it is. Howve\rer, in order to set tlie N content to less
than 0.001%, a denitrification process is required and production cost increases, and
thus, the lower liniit is preferably set to 0.001%. The cotitelit is more preferably
0.001% or more and 0.005% or less.
[0049] . . . . , . . .. . . . .
The steel sheet according to the en~bodinlenmt ay further contain, by mass%,
any one or two or nlore of Cr: 0.01% or more and 0.5% or less, Mo: 0.01% or more
and 0.5% or less, and B: 0.0005% or more and 0.005% or less.
[0050]
Cr: 0.01% or more and 0.5% or less
Cr is an elenlent which increases tlie I~ardenabilityo f steel and contributes to
the fortilation of martetlsite. However, when the Cr content is less than 0.01%, the
addition effect is not suEcicnt and tlius, the lower linlit when Cr is added is set to
0.01%. On the other hand, when the content exceeds 0.5%, formability and
wveldability are deteriorated and thus, the upper limit is set to 0.5%. The content is
preferably 0.05% or more and 0.3% or less.
[0051]
Mo: 0.01% or tnore and 0.5% or less
Mo is an element which increases the hardenability of steel and contributes to
the formation of martensite, similar to Cr. However, \when the Mo content is less that1
0.01%, the addition effect is not sufkicient and thus, the lower limit when Mo is added
is set to 0.01%. On the other hand, wlicn the content exceeds 0.5%, formability and
weldabilit~a~re deteriorated atid thos, the upper lin~iits set to 0.5%. The content is
preferably 0.05% or Illore and 0.3% or less.
[0052]
B: 0.0005% or more and 0.005% or less
B is an element which increases the hardenability of steel and contributes to
the formation of martensite, sirl~ilatro Cr and Mo. However, \\.hen the B content is
less thau 0.0005%, the addition effect is not sufficient and thus, the lower limit when B
- .. .~~. ., . , . . . is added~iss et to 0.0005%. On the other hand, when the content exceeds 0.005%, the . . . .
a~nounot f ferrite is too snlall and workability is deteriorated. Thus, the upper limit is
set to 0.005%. The colltellt is preferably 0.0008% or Illore and 0.003% or less.
[0053]
The steel sheet accordi~~tog t he en~bodime~mlta y further co~ltainb, y mass%,
one or hvo or more of Nb, Ti, and V of 0.005% or more and 0.05% or less, in total.
[0054]
Nb, Ti and V are ele~ue~iwtsh ich form carbonitrides to be precipitated in the
steel and contribute to i~iiproveme~iu~ ~t i~echauicparlo perties in the stccl shcct.
Wl1e11 the total content of one or two or more of Nb, Ti and V is less than 0.005%, the
addition effect is hardly obtained and tfius, tl~elo \ver limit when one or hvo or more of
Nb, Ti and V are added is set to 0.005%. On the other hand, \vIien the total content
exceeds 0.05%, workability is deteriorated and thus, the upper limit is set to 0.05%.
The co~~tcis~ pirte ferably 0.008% or more and 0.03% or less.
lO055l
The steel sheet according to the embodiment may further contain elements
other than the above elements (for example, Cu, Ni, Zr, Sn, Co, As and the like) as
lavoi voidable impurities as long as the properties are not deteriorated.
[0056]
Next, the metallographic structure (microstructore) of the steel sheet
according to the eriibodimetit will be described.
The steel sheet accordi~igto tile embodiment has a steel stri~ctoreo btained by
soaking a base steel slieet having the above component composition at a dual phase
region temperature of Acl teluperature or higher and lo\\~etrh an Ac3 temperature for a
soaking time of 15 seconds or.lo~~gaenrd 35 seco~~odrs s horter, next, primarily cooling
. ... . the steel sheet to a temperature range of 250°C or higher alld380°C or lower within 3
seconds at a coolitig rate of 0.5 "CIS or more and 30 "Cis or less, and after the primary
cooling, retaining the steel sheet in a telllperature range of 260°C or higher and 370°C
or lower for 180 seco~~odrs l onger and 540 seconds or shorter. The steel sheet which
has a yield ratio of 65% or less, a tensile strength of 590 MPa, and excellent elongation
is obtained by fornling the above-described structure.
In the steel sheet according to the etnbodiment, for example, the steel
structrlre may be a structure that contains, by an area fraction, bainite ant1 n~artensitein
a total of 3% or more and 10% or less, residual austenite of 1% or nlore and 3% or lcss,
and a retnainder consisting of ferrite. III the case of the str~tctrlreh aving such area
fractions, low yield ratio, liigli elongation and high strength are easily acliicved.
By containing the bainite and n~artensitein a total of 3% or more, it is
possible to obtain a desired high strengtl~. Ho\vever, when the bainite and martetisite
are contained more tlian 10% there is uticvetiness in tlie strengtli of the structure ant1
thereby ductility is locally deteriorated, and thus, more tlian 10% of the bainite and
martensite is not preferable. U'hen tlie residual austenite is uiiiformly present,
ductility is improved. Since the effect is weak at less than 1% of the residual
austenite, tl~elo wer liti~iits preferably 1%. However, the bainite and mattensite, and
the residual austenite have a competitive relationship, that is, when the area fraction of
the residual austenite ii~creases,t he area fraction of tlie bainite and n~artensite
decreases. When the area fraction of the residual austenite exceeds 3%, the area
fraction of the bainite and ~uattensited ecreases, and the yield ratio is increased by
deterioration in the tensile strength. 'I'hus, the area fraction exceeding 3% is not
preferable. In addition, the bainite deteriorates a balance between strengtli and
ductility compared to the martensite, the bainite of 1% or less is preferable. In tlie
~~,.~ .. structure containing pearlite, a sufficient tensile strength to yield strength cannot be . .
obtained, that is, a yield ratio is increased in some cases. In addition, C is suppressed
to being collce~~trateodn to non-transformed austenite due to the fonuation of the
pearlite, the for~llationo f the residual austenite is l~iudered. Therefore, it is preferable
not to i~lclr~dpea rlite.
The observation and deternlination of the structure may be performed in such
a manner that 1000 grains or more in a sample which is etched using a nital reagent are
observed at a magnification of 400 tinles at three visual fields or more with an optical
~nicroscope.
[0057]
Next, a method of producing a steel sheet according to an embodi~~lewntil l be
described.
First, a base steel sheet having the above cornpollent composition is heated to
a dual phase region temperature, that is, a tenlperatare of Acl te~npcratt~orer higher
and lower than Ac3 temperature, and is soaked at the dual phase region temperature for
a soaking time of 15 seconds or longer and 35 seconds or sl~orte(rf irst retention).
When the soaking time is shorter than 15 seco~~dths,e segregation of Mn and the like
cannot be uniformized and unevenness is caused in material of the base steel sheet.
As a result, since pearlite is forn~eda t a portion in which sufficient segregation is not
obtained, a soaking time of shorter than 15 seconds is not preferable.
Here, as the above base steel sheet, a steel sheet that is produced by a kno\vn
casting method, and hot rolling inethod can be used.
[OOSS]
Asubstitutional elen~enst uch as Mn or the like has a low diffusion rate.
Therefore, \)%en the cooling rate after soaking is slolv, martensite and residual
, . . . austenite arc fornled around the Mn segregated portion, Thos, ~llartensitea nd residual
austenite are hardly fol-med in portions otlier than the Mi segregated portion atid there
is a concern that a aon-unifrom structure niay be for~iied. I-Iowever, when a sufficient
soaking time is given and the substitutional eletnent such as Mn or the like is
uniformly diffi~seda s described above, ~iiartensitei s uniformly fo~niedin the thickness
direction and the width direction of the steel sheet and tlios, it is possible to suppress
local conce~itrationin processitig.
[0059]
When the soaking teliiperature is lower thanAc1 temperature, tlie diffi~sion
rate of MI is slo\\f and MI is 1101 co~lcentrated, and thus, pearlitc is fomed at the
cooli~igr ate of the embodiment. III addition, wheli the soaking temperaturc is Ac3 or
higher, C co~ice~itration~tlo austenite (y) does not proceed ui soaking, and thus,
pearlite is fornied. Therefore, the soaking temperatnre is set to Acl te~nperatureo r
higher and lo\ver than Ac3 temperature.
[0060]
BJJt aking a sutxcient soaking time, residual austenite is uuifor~nlyf ormed in
the structure. The residual austenite contributes to the iniprovement of ductility.
[0061]
On the other hand, when the soaking time is too long, an atiiount of scale is
increased and the yield is dccreased. Therefore, tlie soaking time is set to 35 seco~ids
or shorter.
[0062]
After tlie soaking, primary cooling to a temperature range of 250°C or higher
and 380°C or lo\ver at a cooli~lgra te of 0.5 "CIS or more and 30 "CIS or less, is
perfomled. When it takes a long time before tlie coolitig is started, non-transformed
a~tsteriitei s transfornied.to ferrite, and thereby bait~itea nd rna~tensitec annot be . .
obtained afrer cooling in sonle cases. Therefore, after the soaking is completed, it is
preferable that the prhilary cooli~~isg p erfortiled within 3 seconds. The prinlaty
coolit~gis preferably started in a titne as sl~oorta s possible after the soaking, but it is
difficult to set the time to be shorter than 1.5 seconds in actual production, and thus, a
time shorter than 1.5 secol~dsis the actual lower limit.
[0063]
When the cooling rate after the soaking (primary coolit~gra te) is less than
0.5 "CIS, even when the amount of Mn is within the range of the present invention, Mn
segregates and the structure is not oniforn~. In addition, required strength cannot be
obtained due to pearlite precipitated from the hardened structure and the like.
[0064]
011 the other hand, when the coolic~gr ate exceeds 30 "Cls, since the cooii~lg
rate is too fast, tuartcnsite is excessively formed and thus, the balance betwcen strength
and ductility is deteriorated. Therefore, the cooling rate after the soaking is set to
0.5 "CIS or more and 30 'CIS or less. The coolingrate is preferably 0.5 'CIS or more
and 15 "CIS or less.
[0065]
hi the cooling after the soaking, in additiot~to the coolit~gla te of 0.5 "Cls or
more and 30 "CIS or less, it is important to make a cooling end tenlperature fall within
a temperature range of 250°C or higher and 380°C or lower. When the cooling end
temperatore is lower than 250°C, a structure consisting of ferrite and martensite is
fortlied and a uniforni stt uch~rec annot be obtained. l'hus, cracking occuls at the time
of processing and workability is deteriorated.
[0066]
On the other hand, when the cooling end temperature exceeds 380°C, fonned
n~artensiteis tempered and is decon~posedi nto pearlite and thus, required strength
cantlot be obtained. Therefore, the cooli~~engd te~nperatureis set to a temperature in
a temperature range of 250°C or higher and 380°C or lower. The cooling end
temperature is preferably 280°C or higher and 350°C or lo\ver.
[0067]
Further, after the primary cooling, in a temperature range of 260°C or higher
and 370°C or lo\\rer, retention (second retention) is perfornied for 180 seconds or
longer and 540 seconds or shorter. After the primary cooling, by perforn~ing
re ten ti or^ under the above conditions, it is possible to for111 a steel structure in which
strength and elongation are more balanced (TS x El is high).
When the retention temperature is lower than 260°C, the area fraction of
bainite a ~mda rtensite is excessive and ductility is deteriorated. On the other hand,
\\rhea the r e t e ~ ~ ttieoni~pc~ra ture exceeds 370°C, bainitc and martensite are tetnpered
and are deco~nposed into pearlite and tllos, the retention temperature exceeding 370°C
is not preferable. ...
I11 addition, when the rete~~tiotinm e is shorter than 180 secotlds, C
concentration on non-transformed austet~iteis not sufficiently pron~otcda nd pearlite is
fortlied and, thus, a retention ti~nesl ~ortetrh an 180 seco~~idss n ot preferable. On the
other hand, when the retention time exceeds 540 seconds, productivity is deteriorated,
and thus, a retention time exceeding 540 seconds is not preferable.
In the above retention, when the steel sheet acco~dingto the embodiment is
s~ibjectedto n~icrostrt~ctucreo ntrol using a continuous annealing line, the steel sheet
nlay be retained suc11 that the te~nperatureo f an overaging section of the continuous
annealing line is set to 260°C or higher and 370°C or lower and the steel sheet passes
througb the overaging section for 180 seconds or longer and 540 scco~~odrs s horter.
After tlie second retention, the steel slieet may be cooled to room temperature
using an arbitrary iiiethod to form a product.
[0068]
Further, the inventors have found that when the steel slieet is retained in the
overaging section, if y which is a product of retention teniperatl~re( overaging section
passing temperah~re)a nd retenti011 time (overaging sectio~pi assing time) and x which
is a pri~na~coyo ling rate satisfy the following Expression, it is possible to fi~rther
impmve the balance bet\veen strength and elongation
>r -< 796700 x X(-0.977')
FIG. 1 is a relatiotiship betsveen (overagitig section passing telnperature x
overaging section passing ti111e):y and primary cooling rate: x, which are examined by
tlie inventors with actual inacliine.
[0069]
It is possible to obtain a high strength steel sheet \vhicIi has a low yield ratio
and excellent elongation due to organic coordination of the soaking temperature,
soaking time, prin~atyc ooli~~tegm perature, priniaty cooling stop temperature, retentiotl
temperature, and retention time in the steel slieet according to the embodi~nent.
[0070]
The method of producing a steel sheet accordi~~tog t he embodinient can
obtain tlie eKect without limiting tlie apparatus, but fson~th e viewpoint of promoting
structure refineme~itb y rapid heating and cooling and material ho~nogenizationin a
coil, it is preferable to use tlie continuous annealing line.
In addition, in a case in which the continuous annealing line is used, \vIien a
steel slieet in which tlie prinla~yc ooling stop temperature of tlie steel sheet according
to tlie embodinient (pri~naryc ooling outlet side slieet temperature) is set to 250°C or
11igher and 380°C or lower passes through the overaging section, it is preferable that a
required anlount of steel sheets in which the priniary cooling stop tetnperature is set to
330°C or lower (temperature adjusted steel sheet), for example, 30 tons or Inore of the
steel sheets pass tlxough the overaging section before tlie prin~a~coyo ling starts in
order to adjust the temperature of the overaging section to 260°C or higher and 370°C
or lo\ver. Accordingly, since equipment such as a blo\ver for adjusting the
tetnperature of the overaging section is not necessacy, it is possible to reduce the size of
the equipment and also, possible to reduce construction cost. Therefore, a steel sheet
\v11icl1 has a low yield ratio, a tensile strength of 590 MPa or more, and excellent
elongation can be easily obtained by the continoous annealing line.
When the ten~peratureo f the tenlperature adjusted steel sheet exceeds 330°C,
the atmospheric tenlperature of the overaging section cannot be not sufficiently
decreased, and thos, a teniperature exceeding 330°C is not preferable. Contrarily,
when the temperature of the temperature adjusted steel sheet is lo\ver than 30OoC, the
atmospl~ericte mperature is excessively decreased, and thus, a temperahlre of I'o\verthan
300°C is not preferable.
When 100 tons or Inore of steel sheets pass thro~~gthhe overaging section, the
tetnperature of tlie overaging section is excessively decreased in sonie cases, and thus,
it is preferable that the upper lin~iot f the temperature adjusted steel sheet to pass be set
to 100 tons. In addition, \vhen a time fron~th e pass of the temperature adjusted steel
sheet is completed and to when the pri~natyc ooling starts exceeds 30 minutes, there is
a concern that the above effect is hardly obtained and thus, it is preferable that the
temperature adjusted steel sheet pass within 30 tnin~~tbeesf ore the primary corning
starts.
. . . . . . , ..~. . . . . [Exatnples] ~ - . ,. .. . ~. ~. , .
[007 11
Next, exan~pleso f the present inve11tio11 will be described but the conditions
in the examples are simply an example of conditions employed to confirm the
feasibility and effect of the present invention, and the present invention is not limited
to the example of conditions. TIie present invention can employ a variety of
conditions within the scope of the present invention as long as the objective of the
present inventiot~c an be achieved.
[0072]
(Example 1)
Steel sheets having componel~ct ompositions shown in Table 1 were subjected
to heat treatruent itnder the soaking conditions and retention conditions (overaging
. section passing conditions) sho\vn in Table 2. The results are shown together in Table
2.
In the example, when the yield ratio \\!as 65% or less, TS was 590 MPa or
mole, and TS x El was 17500 MPa.% or more, it is judged that the steel sheet was a
high strength steel sheet \vbich has a low yield ratio, and excellent elongation.
In the tensile tcst, a JIS 5 test piece was made by cutting each of the steel
sheets in the perper~dicular direction of the steel sheets to evaluate tensile strength
according to J1S Z 2241:2011.
The observatioll and determination ofthe structure \\,as performed in such a
manner that samples etclicd using a nital reagent were observed at a magnification of
400 times at twenty visual fields with an optical microscope, and an area fractiot~o f
each strtlchtre was obtained by image analysis.
The remainder of the components in Table 1 refer to Fe and tulavoidable
ilnpilrities and "-" represents that there is nothing detected.
[0073]
In the example of the preselit invention, it is possible to stably obtain the high
strength steel sheets n~llichl~av e a low yield ratio, excellellt elongation, and a tensile
strength of 590 MPa or more.

[0076]
(Example 2)
Before the steel sheet of steel type A in Table 1 passed through the overaging
section of tlie continuous annealing line after primary cooling, a telnperature adjusted
steel sheet passed though the continuous arineali~igli ne under the conditions sliowv~iln
Table 3. Then, the steel slieet of steel type Aiti Table 4 passed the overagilig section.
The results are shown in Table 5. The temperature control of the overaging section
was not performed except during the passage of tlie steel sheet. It was found that the
temperature of the overaging section could be decreased in all appropriate range by
allowing the teniperature adjusted steel shcct to pass through the overaging section of
the continuous ali~lealingli ne in advance, and the steel sheet of the present invention
could be obtained without cooling by a blower and the like.
[0077]
[Table 31
[0078]
[Table 41

[Industrial Applicability]
[OOSO]
As described above, according to the present invention, it is possible to
provide the high strength steel sheet which is suitable for vehicle bodies aid pa~tsfo r
autoniobiles and has a low yield ratio and excelle~let longation. Therefore, the
present invention has high industrial applicability in the steel industry and the
automobile nianufactt~ringi ~idt~stry,
Table 1
Table 2-1(1/2)
RETENTION CONDITIONS
(OVERAGING SECTION)
W
-LLL Z Z
2 Z o RETENTION LO
TEMPERATURE szzs
:c x RETENTION ;;~cc
W I W
LLW LL TIME ", % C LU
austenite 6: Bainite P: Pearlite
Table 2-2(2/2)
Table 3
Table 4
+ Z
0 a E
A
Oc
714
OC
872
SOAKING
TEMPERATURE
"C
740
SOAKING
TIhlE
sec
17
COOLING
RATE
"C/sec
12
PRlhlARY
COOLING
STOP
TEMPERATURE
"C
260
Table 5
NO
43
44
45
46
TENSILE
STRENGTH
MPa
61 1
606
61 5
622
YIELD
STRENGTH
MPa
342
330
434
444
YR
%
56
54
71
71
Z
E!
Z w
Z
0
_J
W
%
32
33
28
27
TSXEl
MPa.%
19552
19998
17220
16794
-
EXAMPLE
EXAMPLE
COMPARATIVE EXAMPLE
COMPARATIVE EXAMPLE
STRUCTURE
M + B
(MARTENSITE
+ BAINITE)
%
7
7
5
5
RESIDUAL y:
RESIDUAL
AUSTENITE
%
ff
f e r r i t e
%
P
p e a r l i t e
%
0
0
7
7
2 91
2
0
0
91
88
88

[Document Type] CLAMS
[Claim 11
A steel sheet comprising, by mass%,
C: 0.04% or Inore and 0.15% or less,
Si: 0.3% or Illore and 0.7% or less,
Mn: 1.0% or lllore and 3.0% or less,
Al: 0.005% or more and 0.10% or less,
P: limited to 0.03% or less,
S: linlited to 0.01% or less,
N: linlited to 0.01% or less, and
a reniainder consisting of Fe and unavoidable inipurities,
wherein the steel sheet has a steel structure obtai11ed.b~pe rforming a soaking
for a soaking time of 15 seconds or longer and 35 seconds or shorter at a dual phase
region temperatore ofAcl temperature or higher and lower than Ac3 temperature, next,
performing a primary cooling to a temperature range of 250°C or liiglier and 3SO°C or
less at a cooling rate of 0.5 "CIS or more and 30 'CIS or less within 3 seconds, and
perfornling a retention in a tenlperature rangc of 260°C or higller and 37OoC or lo\ver
for 180 seconds or longer and 540 secor~dso r shorter after the primaty cooling,
a yicld ratio is 65% or less and tensile strength is 590 MPa or more, and
the Acl temperature is a temperature expressed by a following Expression (1)
in units of "C, and the Ac3 tenlperatt~reis a temperah~ree xpressed by a follo\ving
Expression (2) in units of 'C,
Acl=732-26.6x[C]+17.6x[Si]-11.6x[Mn] ...( l),
Ac3 =924 + 56.1 x [Si] - 19.7 x [Mn] -436.5 x [C] ...( 2),
here, [C], [Si], and [Mn] represent a C content, an Si content, and at1 Mn
content respectively, and a lniit thereof is mass%.
[Claim 21
The steel sheet according to Claini 1,
wherein the cooling rate is 0.5 "CIS or niore and 15 "CIS or less.
[Claim 31
The steel sheet according to Claini 1,
wherein y which is a product of a retention temperah~rea nd a retention time in
the retention arid x which is the cooling rate in the priniary cooling satisfy a following
Expression (3),
y i 796700 x d'"97.').. (3).
[Claim 41
The steel sheet according to any one of Claims 1 to 3, fi~rtlierc omprising, by
mass%, any one or t\vo or more of
Cr: 0.01% 01. more and 0.5% or less,
Mo: 0.01% or more and 0.5% or less, and
B: 0.0005% or more and 0.005% or less,
wherein the Acl temperature is a tcnlperature expressed by a following
Expression (4) in u~~iotfs " C, atid tlieAc3 temperature is a tetiiperatk~re xpressed by a
following Expression (5) in units of OC,
Acl =732-26.6 x [C] + 17. 6 x [Si] - 11.6 x [Mti] +24.1 x [Cr] ... (4),
Ac3 = 924 + 56.1 x [Si] - 19.7 x [Mn] - 4.9 x [Cr] - 436.5 x [C] ... (5),
here, [C], [Si], [Mn], and [Cr] represent a C content, an Si content, an Mn
content, and a Cr content respectively, and a unit tliereof is mass%.
[Claim 51
The steel sheet according to Clainl4, fnrtlier comprising, by mass%,
one or two or more of Nb, Ti, and V of 0.005% or more and 0 05% or less in
total.
[Claim 61
The steel sheet according to any one of Clail~ls1 to 3, further comprising, by
~nass%,
one or t\vo or tilore of Nb, Ti, and V of 0.005% or more and 0.05% or less in
total.
[Claim 71
The steel sheet according to any one of Claims 1 to 3,
wherein the steel structure is a structure that contains, by an area fraction, a
bainite and a martensite in a total of 3% or rnore and 10% or less, a residual austenite
of 1% or more and 3% or less, and a remainder consisting of a ferrite.
[Claim 81
The steel sheet according to Claim 7,
whetein in tlie steel structore, by an area fraction, the bainite that is limited to
1% or less.
[Claim 91
An~etl~ofd p roducing a steel sheet using a contini~ousa nnealing line, the
method comprising:
a first retention process of tetaining a base steel sheet having the component
composition according to Claim I at a dual phase region temperahlre of Acl
temperatore or lligher and lower than Ac3 temperature for 15 seconds or longer and 35
seconds or shorter;
a primary cooling process of plimarily cooling the steel sheet to a tetnperahlre
range of 250°C or higher and 380°C or lowcr within 3 seconds at a cooling rate of
0.5 "Cls or nlore and 30 "CIS or less after the first retention process; and
a second retention process of retaining tlle steel sheet while passing through
an overaging section arranged in the continuous annealing line wl~osete mperature is
set to 260°C or higher and 370°C or lower for a rctctltion time of 180°C or longcr and
540 seconds or sl~ortear fter tlie primary cooling.
[Claim 101
The method of prodncing a steel sheet according to Claim 9,
wherein, in the second retention process, y that is a product of an overaging
section passing teltlpcrature which is the retctition temperature \vI~enth e steel sheet
passes througi~th e overaging section, and an overaging section passing time \\,l~ichi s
the retention time, and x that is tlie cooling rate in tlie priniary cooling process satisfy a
following Expression (6),
Jr <- 796700 x s(-0.971) ... (6).
[Claim 111
The inetIi,od of producing a steel sheet according to Claim 9 or 10, further
comprising:
a preliminary slicet passing process of, before starting the primary cooling
process, passing a required amount or more of a tenlpcrature adjusted steel shect whose
pritiiary cooling stop tcmperature is set to 330°C or lower throng11 the continuous
annealing line.
[Claim 121
The method of producing a steel sheet according to Claim 11,
~vhereintl ie required amount is 30 tons.
[Claim 131
Tile method of producing a steel shect according to Claini 9 or 10, . . . ~ ., ~- .
wherein the base steel sheet further co~itltaulsb, y mass%, any one or two or
Illore of
Cr: 0.01% or more and 0.5% or less,
Mo: 0.01% or liiore and 0.5% or less, and
B: 0.0005% or Illore and 0.005% or less.
[Clailn 141
The rnetliotl of producing a steel sheet according to Clailil 13,
herein the base steel slieet fi~rtherc ontains, by inass%, ol~cor t\vo or more
of Nb, Ti, and V of 0.005% or more and 0.05% or less in total.
[Clailii 151
The method of producing a steel sheet accordi~lgto Claim 9 or 10,
~\4iereinth e base steel sheet further contains, by mass%, one or t\vo or mole
ofNb,'Ti, and V of 0.005% or more and 0.05% or less ill total.