U.S. patent application number 12/303566 was filed with the patent office on 2010-09-02 for high-strength composite steel sheet having excellent moldability and delayed fracture resistance.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho. Invention is credited to Yoichi Mukai, Michiharu Nakaya, Koichi Sugimoto.
Application Number | 20100221138 12/303566 |
Document ID | / |
Family ID | 38801448 |
Filed Date | 2010-09-02 |
United States Patent
Application |
20100221138 |
Kind Code |
A1 |
Nakaya; Michiharu ; et
al. |
September 2, 2010 |
HIGH-STRENGTH COMPOSITE STEEL SHEET HAVING EXCELLENT MOLDABILITY
AND DELAYED FRACTURE RESISTANCE
Abstract
The present invention provides a high-strength composite steel
sheet which has a tensile strength of 980 MPa class as well as
excellent and excellent anti-delayed fraction property, and also
has excellent spot-weldability. The high-strength composite steel
sheet comprises a steel satisfying: C: 0.10 to 0.25% (% by mass in
case of a chemical component, the same shall apply hereinafter),
Si: 1.0 to 3.0%, Mn: 1.5 to 3.0%, P: 0.15% or less, S: 0.02% or
less, Al: 0.4% or less, and comprising the remnant made from iron
and unavoidable impurities; the contents of Si, Al, Mn and Cr
satisfy the relationship of "(Si+Al)/Mn or (Si+Al)/(Mn+Cr)=0.74 to
1.26"; and microstructure is specified.
Inventors: |
Nakaya; Michiharu; ( Hyogo,
JP) ; Mukai; Yoichi; ( Hyogo, JP) ; Sugimoto;
Koichi; (Nagano, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, L.L.P.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Kabushiki Kaisha Kobe Seiko
Sho
Kobe-shi
JP
Shinshu TLO Co., Ltd.
Ueda-shi
JP
|
Family ID: |
38801448 |
Appl. No.: |
12/303566 |
Filed: |
June 4, 2007 |
PCT Filed: |
June 4, 2007 |
PCT NO: |
PCT/JP2007/061301 |
371 Date: |
December 5, 2008 |
Current U.S.
Class: |
420/84 ; 420/103;
420/104; 420/117; 420/120; 420/8 |
Current CPC
Class: |
C22C 38/04 20130101;
C21D 9/46 20130101; C22C 38/02 20130101 |
Class at
Publication: |
420/84 ; 420/103;
420/104; 420/117; 420/120; 420/8 |
International
Class: |
C22C 38/28 20060101
C22C038/28; C22C 38/06 20060101 C22C038/06; C22C 38/18 20060101
C22C038/18; C22C 38/02 20060101 C22C038/02; C22C 38/04 20060101
C22C038/04; C22C 38/00 20060101 C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 5, 2006 |
JP |
2006-156442 |
Claims
1. A high-strength composite steel sheet having excellent
formability and anti-delayed fraction property, comprising a steel
satisfying: C: 0.10 to 0.25% (% by mass in case of a chemical
component, the same shall apply hereinafter), Si: 1.0 to 3.0%, Mn:
1.5 to 3.0%, P: 0.15% or less, S: 0.02% or less, Al: 0.4% or less,
and comprising the remnant made from iron and unavoidable
impurities, wherein the contents of Si, Al and Mn satisfy the
relationship of the following formula (I): (Si+Al)/Mn: 0.74 to 1.26
(I) and a microstructure in a longitudinal section comprises, by an
occupancy ratio based on the entire structure, 1) bainitic ferrite:
50% or more, 2) polygonal ferrite: 5 to 35%, 3) average grain size
of polygonal ferrite: 10 .mu.m or less], and 4) residual austenite:
5% or more.
2. A high-strength composite steel sheet having excellent
formability and anti-delayed fraction property, comprising a steel
satisfying: C: 0.12 to 0.25%, Si: 1.0 to 3.0%, Mn: 1.5 to 3.0%, Cr:
1.0% or less, P: 0.15% or less, S: 0.02% or less, Al: 0.4% or less,
and comprising the remnant made from iron and unavoidable
impurities, wherein the contents of Si, Al, Mn and Cr satisfy the
relationship of the following formula (II): (Si+Al)/(Mn+Cr): 0.74
to 1.26 (II) and a microstructure in a longitudinal section
comprises, by an occupancy ratio based on the entire structure, 1)
bainitic ferrite: 50% or more, 2) polygonal ferrite: 5 to 35%, 3)
average grain size of polygonal ferrite: 10 .mu.m or less], and 4)
residual austenite: 5% or more.
3. The high-strength composite steel sheet according to claim 1 or
2, wherein the steel further contains, as other elements, Ti: 0.15%
or less and/or Nb: 0.1% or less.
4. The high-strength composite steel sheet according to claim 1 or
2, wherein the steel further contains, Ca: 30 ppm or less and/or
REM: 30 ppm or less as other elements.
5. The high-strength composite steel sheet according to claim 1 or
2, which has a tensile strength of 980 MPa or higher.
Description
TECHNICAL FIELD
[0001] The present invention relates to a high-strength composite
steel sheet which has a tensile strength of 980 MPa or higher class
as well as excellent formability and excellent anti-delayed
fraction property, and also has excellent spot-weldability and is
useful as automotive structural parts (body flame members such as
pillar, member and reinforcement; bumper, door guard bar, sheet
parts, suspension parts, and other reinforcing members).
BACKGROUND ART
[0002] In recent years, for the purpose of reducing fuel
consumption due to saving body weight of automobiles and ensuring
safety upon collision, demands for high-strength steels have
increased more and more. Accordingly, steel sheets having a tensile
strength of 980 MPa or higher class have been required in place of
those having a tensile strength of 590 MPa class. However, in the
case of high-strength steel sheets having a tensile strength of 980
MPa or higher class, deterioration of formability cannot be avoided
and there is restriction on applications since it is possible to
apply to parts having complicated shapes.
[0003] In the case of high-strength steel sheets having a tensile
strength of 980 MPa or higher class, residual stress generated upon
press forming increases and thus a risk of delayed fracture
enhances. That is, delayed fracture is a phenomenon in which
hydrogen in the corrosion environment or atmosphere is diffused and
accumulated at dislocations, vacancies and grain boundaries in the
structure of steel materials, especially high-strength steel sheets
thereby causing embrittlement of the materials, leading to fracture
when stress is applied. Therefore, delayed fracture exerts a severe
influence on ductility and toughness of steel materials.
[0004] Thus, it is very important to improve, in addition to the
strength, formability (i.e. elongation and stretch flangeability)
and anti-delayed fraction property so as to response to the
above-described demand of increasing the strength.
[0005] Now various steel sheets including residual austenite in the
metal structure are put into practical use as high-strength steel
sheets which exhibit excellent formability.
[0006] For example, Non-Patent Document 1 discloses a steel sheet
in which a bore expansion property (i.e. stretch flangeability) is
enhanced while ensuring a high strength by constituting the metal
structure with a composite structure which mainly contains bainitic
ferrite and also contains lath-type residual austenite. However,
when a tensile strength (TS) becomes a tensile strength of 980 MPa
or higher class, this steel sheet shows TS.times.El as an indicator
of the strength (TS) and ductility (El) of 9,000 to 10,300 at most
and therefore it is hardly to say that the steel sheet is
satisfactory.
[0007] It is considered that, in a mass production line of a
practical operation using a continuous annealing furnace, a maximum
heating temperature is about 900.degree. C. and a heating time is 5
minutes or less. However, under the production conditions disclosed
in this document, it is required to cool to a temperature within
the range from 350 to 400.degree. C. in a salt bath after annealing
at 950.degree. C. for 1,200 seconds, and thus this method is not
suited for the practical operation.
[0008] In Patent Document 1, elongation of about 20% and stretch
flangeability (.lamda.) of 55% are attained while ensuring a
tensile strength of 980 MPa or higher by constituting a matrix
phase with a structure composed mainly of bainitic ferrite and 3%
or more of residual austenite. However, in this technique, the
addition of expensive alloy elements such as Mo, Ni and Cu is
indispensable and it leaves a room for improvement in cost.
[0009] Furthermore, in Patent Document 2, high-level elongation and
stretch flangeability are attained by constituting a matrix
structure with tempered martensite and ferrite and adjusting the
occupancy ratio of residual austenite within the range from 5 to
30%. However, since a microstructure before annealing is important
so as to obtain the required metal structure using this technique,
it is necessary to perform continuous annealing or annealing twice
or more after incorporating a proper metal structure by reeling up
at low temperature during a hot rolling step. However, in the case
of reeling up at low temperature during the hot rolling step, since
the structure before annealing is broken and the intended metal
structure is not obtained unless the subsequent cold rolling
reduction is controlled to the low value, severe restriction is
added to the thickness and thickness tolerance. When continuous
annealing is performed twice, although there is no restriction on
the thickness, the number of steps increases when compared with the
case of a conventional method and thus cost-up cannot be
avoided.
[0010] Furthermore, Patent Document 3 discloses steel sheets having
enhanced total elongation and stretch flangeability by mainly
constituting a matrix structure with tempered bainite. However,
since a study is mainly made on steels having a tensile strength of
900 MPa class in this steel type, delayed fracture, which is caused
in steels having a tensile strength of 980 MPa or higher class, is
not sufficiently studied.
[0011] Non-Patent Document 1: ISIJ International, Vol. 40 (2000),
No. 9, pp. 920 to 926
[0012] Patent Document 1: Japanese Unexamined Patent Publication
(Kokai) No. 2004-332099
[0013] Patent Document 2: Japanese Unexamined Patent Publication
(Kokai) No. 2003-171735
[0014] Patent Document 3: Japanese Unexamined Patent Publication
(Kokai) No. 2002-309334
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0015] The present invention has been made in view of the
above-mentioned prior arts, and an object thereof is to provide a
high-strength composite steel sheet which has a tensile strength of
980 MPa class suited for use as automotive structural parts and has
excellent formability (stretch flangeability), and also has
excellent spot-weldability and excellent anti-delayed fraction
property, without adding expensive alloying elements such as Mo, Ni
and Cu.
Means for Solving the Problems
[0016] The high-strength composite steel sheet of the present
invention, which could achieve the above object, is a high-strength
composite steel sheet having excellent formability and anti-delayed
fraction property, comprising a steel satisfying C: 0.10 to 0.25%,
Si: 1.0 to 3.0%, Mn: 1.5 to 3.0%, P: 0.15% or less (excluding 0%),
S: 0.02% or less (excluding 0%), Al: 0.4% or less (excluding 0%),
and comprising a remnant made from iron and unavoidable impurities,
wherein the contents of Si, Al and Mn satisfy the relationship of
the following formula (I):
(Si+Al)/Mn: 0.74 to 1.26 (I)
or comprising a steel satisfying C: 0.12 to 0.25%, Si: 1.0 to 3.0%,
Mn: 1.5 to 3.0%, Cr: 1.0% or less (excluding 0%), P: 0.15% or less
(excluding 0%), S: 0.02% or less (excluding 0%), Al: 0.4% or less,
and comprising a remnant made from iron and unavoidable impurities,
wherein the contents of Si, Al, Mn and Cr satisfy the relationship
of the following formula (II):
(Si+Al)/(Mn+Cr): 0.74 to 1.26 (II)
and a microstructure in a longitudinal section comprises, by an
occupancy ratio based on the entire structure,
[0017] 1) bainitic ferrite: 50% or more,
[0018] 2) polygonal ferrite: 5 to 35%,
[0019] 3) average grain size of polygonal ferrite: 10 .mu.m or
less], and
[0020] 4) residual austenite: 5% or more.
[0021] The high-strength composite steel sheet of the present
invention optionally contains, in addition to the above elements,
Ti: 0.15% or less (excluding 0%) and/or Nb: 0.1% or less (excluding
0%), or optionally contains Ca: 30 ppm or less (excluding 0%)
and/or REM: 30 ppm or less (excluding 0%) as other elements.
[0022] It is particularly preferred that the high-strength
composite steel sheet of the present invention has a tensile
strength of 980 MPa or higher so as to more effectively make use of
its high strength.
EFFECT OF THE INVENTION
[0023] According to the present invention, by specifying chemical
components of the steel material as described above, particularly
controlling a ratio (Si+Al)/Mn or a ratio (Si+Al)/(Mn+Cr), and
constituting the metal structure with a composite structure which
mainly contains bainitic ferrite (BF) and also contains polygonal
ferrite (PF) and residual austenite (residual .gamma.), it is
possible to provide a composite steel sheet which has good
formability (elongation-stretch flangeability) and also excellent
spot-wedability and anti-delayed fraction property while ensuring a
tensile strength of 980 MPa or higher class at cheap price.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a diagram for explaining a heat pattern of a heat
treatment employed in Test Example.
BEST MODE FOR CARRYING OUT THE INVENTION
[0025] In light of the problems described above, the present
inventors have focused on a TRIP steel sheet (Transformation
Induced Plasticity) having a tensile strength of 980 MPa or higher
class, containing bainitic ferrite as a matrix phase, and
intensively studied by paying attention to the form of the second
phase in the metal structure and chemical components, especially
Si, Al and Mn (and/or Cr) so as further improve formability,
spot-weldability and anti-delayed fraction property. Thus, the
following finings were obtained.
[0026] 1) When a predetermined amount of fine polygonal ferrite is
incorporated into a structure composed mainly of bainitic ferrite,
elongation is remarkably improved. Moreover, when the ferrite to be
incorporated is fine ferrite, deterioration of the strength and
stretch flangeability is suppressed and also the structure exhibits
excellent performances in anti-delayed fracture
characteristics.
[0027] 2) When (Si+Al) and Mn or (Mn+Cr) among chemical components
of the steel are adjusted so as to obtain a predetermined ratio, it
is possible to obtain a desired structure having a tensile strength
of 980 MPa or higher class while suppressing deterioration of
spot-weldability.
[0028] Based on these findings, the present inventors have
intensively studied about an influence of the contents of Si, Al,
Mn and Cr in steel components and the metal structure on the
strength, formability, spot-weldability and anti-delayed fracture
characteristics. As a result, they have confirmed that a
high-strength composite steel sheet having high performances, which
achieve the above object, can be obtained by controlling the
occupancy ratio of bainitic ferrite in the metal structure,
controlling the occupancy ratios of polygonal ferrite and residual
.gamma. and controlling an average grain size of the polygonal
ferrite to a specific value or less using a steel material having
specific component composition described above. Thus, the present
invention has been completed.
[0029] Specific constitutions of the present invention will be made
clear by way of reasons for decision of chemical components of the
steel material.
[0030] First, reasons for decision of chemical components of the
steel material are explained.
C: 0.10% or more and 0.25% or less
[0031] C is an element which is indispensable so as to ensure a
high strength and residual .gamma., and is an important element so
as to incorporate a sufficient amount of C in a .gamma. phase
thereby retaining a desired amount of the .gamma. phase at room
temperature. In order to effectively exert such an effect, the
content of C must be 0.10% or more, preferably 0.12% or more, and
0.15% or more. When the content of C is too large, a severe adverse
influence is exerted on spot-weldability, and thus the upper limit
was 0.25% in view of security of spot-weldability. The C content is
preferably 0.23% or less, and more preferably 0.20% or less.
Si: 1.0 to 3.0%
[0032] Si is an essential element which effectively serves as a
solution-hardening element and also suppresses formation of a
carbide as a result of decomposition of residual .gamma.. In order
to effectively exert such an effect, the content of Si must be 1.0%
or more, and preferably 1.2% or more. Since the effect is saturated
at 3.0% and problems such as deterioration of spot-weldability and
hot shortness arise when the content is more than the above value,
the content may be suppressed to 3.0% or less, and preferably 2.5%
or less.
Mn: 1.5 to 3.0%
[0033] Mn is an element required to suppress formation of excess
polygonal ferrite thereby forming a structure composed mainly of
bainitic ferrite. Also it is an important element required to
stabilize .gamma. thereby ensuring desired residual .gamma.. The
occupancy ratio of Mn is at least 1.5% or more, and preferably 2.0%
or more.
[0034] However, since excess addition causes deterioration of
spot-weldability and anti-delayed fraction property, the content is
suppressed to 3.0% at most, and preferably 2.5% or less.
P: 0.15% or less, S: 0.02% or less
[0035] These elements are inevitably incorporated into the steel
and cause deterioration of formability and spot-weldability when
the content increases, and thus the content must be suppressed to
the upper limit or less.
Al: 0.4% or less
[0036] Al is a useful element so as to suppress formation of a
carbide thereby ensuring residual .gamma. similar to Si. However,
if Al is too much, polygonal ferrite is likely to be produced;
therefore, the content should be suppressed to 0.4% at most, and
preferably 0.2% or less.
Cr: 1.0% or less
[0037] Since Cr has the effect of suppressing formation of
polygonal ferrite thereby enhancing the strength, it can be
optionally added. However, when it is excessively added, an adverse
influence may be exerted on formation of the target metal structure
in the present invention. Therefore, the content should be
suppressed to 1.0% at most.
(Si+Al)/Mn (or Mn+Cr): 0.74 to 1.26 (mass ratio)
[0038] It is necessary to suppress excess formation of polygonal
ferrire (PF) thereby promoting a bainitic ferrite (BF)
transformation so as to obtain the intended metal structure in the
present invention. Moreover, since C (carbon) discharged from
bainitic ferrite is concentrated in a lath-type residual .gamma.,
it is important to promote the bainitic ferrite transformation so
as to obtain the lath-type residual .gamma..
[0039] In the present invention, it is necessary to disperse an
appropriate amount of fine ferrite in the metal structure. It was
found that controlling a ratio of the content of Si and Al as
ferrite formation promoting elements to Mn (or Mn+Cr) as ferrite
formation suppressing elements so as to satisfy a given relation is
extremely effective. Moreover, it was confirmed that controlling
the content ratio of the ferrite formation promoting
element/ferrite formation suppressing element is effective so as to
enhance anti-delayed fraction property.
[0040] When the ratio (Si+Al)/Mn (or Mn+Cr) is less than 0.74, not
only it becomes difficult to ensure a proper degree of polygonal
ferrite, but also it becomes impossible to sufficiently ensuring
bainitic ferrite. Moreover, C cannot be sufficiently concentrated
in the residual .gamma. and stability of the residual .gamma.
deteriorates and elongation deteriorates, and also the amount of
martensite increases and stretch flangeability deteriorates.
[0041] Furthermore, anti-delayed fraction property is improved by
controlling the ratio of the elements within a proper range.
Details of this reason are not sure, but are considered as follows.
That is, Mn promotes delayed fracture by decreasing a grain
boundary strength by grain boundary segregation and also promotes
formation of voids serving as the starting point of delayed
fracture upon working, whereas, Si and Al have the effect of
increasing a tolerance amount of hydrogen which induces delayed
fracture. Therefore, it is considered that anti-delayed fraction
property varies by a ratio of both elements.
[0042] In contrast, when the ratio (Si+Al)/Mn (or Mn+Cr) exceeds
1.26, formation of polygonal ferrite is excessively promoted and
the occupancy ratio becomes excessive, and also the grain size of
ferrite is likely to exceed 10 .mu.m and thus both the strength and
stretch flangeability deteriorate. Further, since a ferrite grain
boundary decreases when the ferrite grain size increases,
anti-delayed fraction property deteriorates.
[0043] Therefore, it is important to adjust the component so as to
control the ratio (Si+Al)/Mn (or Mn+Cr) within the range from 0.74
to 1.26, and more preferably 0.84 or more and 1.16 or less.
Nb: 0.1% or less, Ti: 0.15% or less
[0044] Since these elements have the effect of enhancing toughness
by refinement of the metal structure, these elements can be
optionally added in a small amount. However, further effect is not
obtained to cause cost-up even if they are added in the amount of
more than the upper limit, therefore it is wasteful.
Ca: 0.01% or less, REM: 0.01% or less
[0045] Ca and REM have the effect of enhancing stretch
flangeability by adding in a small amount and therefore they may be
optionally added in a small amount. Since the effect is saturated
at about 0.01%, it is wasteful even if they are added in a larger
amount.
Mo, Cu, Ni: each about 0.1% or less
[0046] These elements are effective for improving the strength and
anti-delayed fraction property as described in the prior art. In
the present invention, excellent performances are sufficiently
obtained without adding these elements and it is not necessary to
add because these elements are expensive and cause cost-up. There
is no restriction on the content in a level of impurities and these
elements may be respectively added in the amount of about 0.1% or
less.
[0047] Next, reasons of limitation of the metal texture will be
explained.
Bainitic Ferrite.gtoreq.50%
[0048] Bainitic ferrite has not only the effect of easily achieving
a high strength because of somewhat high dislocation density, but
also the effect of decreasing a difference in hardness between
bainitic ferrite and the second phase thereby enhancing stretch
flangeability. Bainitic ferrite is a structure which is useful for
enhancing anti-delayed fraction property. This reason is considered
that bainitic ferrite does not contain or contains little cementite
serving as the starting point of delayed structure, and has a high
hydrogen absorbing effect because of a lot of dislocations. In
order to effectively exert these effects, it is necessary that the
content of bainitic ferrite exist at 50% or more. The content is
more preferably 60% or more.
[0049] The bainitic ferrite is different from a bainite structure
in that the structure does not include carbides, and is also
different from a polygonal ferrite structure having a lower bainite
structure which does not contain or contains little dislocation, or
a quasi-polygonal ferrite structure having a lower bainite
structure such as fine subgrain. These differences can be easily
identified by TEM (Transmission Electron Microscope)
observation.
Polygonal Ferrite (PF): 5 to 35%
[0050] When a steel sheet having a tensile strength of 980 MPa or
higher class, comprising bainite ferrite (BF) as a matrix phase
contains a predetermined amount of polygonal ferrite having an
average grain size described below, elongation is further improved.
The content of polygonal ferrite must be contained at 5% or more so
as to exert such an effect. However, since it becomes difficult to
ensure the tensile strength and stretch flangeability when the
content of polygonal ferrite is too large, the content should be
suppressed to 35% at most. Preferred occupancy ratio of polygonal
ferrite is 10% or more and 30% or less.
Average Grain Size of Polygonal Ferrite: 10 .mu.m or less
[0051] The average grain size of polygonal ferrite must be 10 .mu.m
or less by the following reason. That is, refinement of ferrite
enables uniform dispersion of the second phase, and thus both
stretch flangeability and strength are enhanced and also
anti-delayed fraction property is improved. This reason is
considered that refinement of ferrite enables trap of hydrogen at
the ferrite grain boundary and suppression of concentration of
hydrogen at a dangerous site. The average grain size of polygonal
ferrite as used herein means an average of an equivalent circle
diameter (diameter of a circle having the same area) of polygonal
ferrite.
Residual .gamma..gtoreq.5%
[0052] Residual .gamma. has the effect of promoting hardening of
the deformed part by transforming into martensite when the material
is deformed by application of strain, and thus preventing strain
concentration (TRIP effect). It is necessary that the content of
the residual .gamma. is 5% or more so as to effectively exert such
the effect. There is no restriction on the upper limit of the
amount of the residual .gamma.. Since a large amount of C is
required so as to form excessive residual .gamma., it becomes
difficult to reconcile with spot-weldability and workability,
especially stretch flangeability. Therefore, the content is
preferably suppressed to about 30% or less.
[0053] In the composite steel sheet of the present invention,
martensite, bainite and pearite can exist as the other balance
structure. The contents of these other structures are preferably
suppressed to 5% or less so as not to exert an adverse influence on
the above operations and effects.
[0054] There is no noticeable restriction on the production
conditions required to obtain the above metal structure defined in
the present invention. In usual production procedures of a steel
sheet, for example, continuous casting, hot rolling, pickling, cold
rolling and continuous annealing, a heating temperature, a heating
rate, a holding temperature, a cooling initiation temperature and a
cooling rate maybe properly controlled. In the case of a galvanized
steel sheet and a galvannealed steel sheet, proper temperature
control including a continuous galvanizing line may be performed.
Since heat treatment conditions in a continuous annealing line are
most important so as to obtain the metal structure, preferred heat
treatment conditions in the continuous annealing line will be
mainly explained.
Heating Temperature upon Annealing: Ac.sub.3+10.degree. C. or
higher
[0055] In order to obtain the bainitic ferrite-ricked metal
structure, the heating temperature upon annealing may be adjusted
to "Ac.sub.3+10.degree. C. or higher" so as to suppress formation
of polygonal ferrite. By the way, when continuous annealing
performed at an Ac.sub.3 point or lower, polygonal ferrite is
likely to be formed in the subsequent cooling step since the
residual ferrite serves as a nucleus, and thus it becomes difficult
to obtain the intended metal structure in the present invention.
Therefore, more preferred heating temperature is
"Ac.sub.3+30.degree. C. or higher".
Cooling Rate after Annealing
[0056] The larger the cooling rate after annealing, the better
since formation of polygonal ferrite is suppressed. Considering
equipment restriction and difficulty in temperature control, the
cooling rate is preferably 25.degree. C./sec or higher, and more
preferably 30.degree. C./sec or higher, so as to suppress the
amount of polygonal ferrite to a certain amount of less according
to each component system.
Quenching Termination Temperature after Annealing
[0057] The temperature at which quenching after annealing is
terminated should be controlled to the temperature at which fine
polygonal ferrite is formed or lower, and is preferably 650.degree.
C. or lower, and more preferably 600.degree. C. or lower. When the
quenching termination temperature becomes higher, coarse polygonal
ferrite is formed and it becomes impossible to achieve the object
of the present invention. However, since it becomes impossible to
obtain a sufficient amount of polygonal ferrite when the quenching
termination temperature becomes too low, the quenching termination
temperature should be about 360.degree. C. or higher, and more
preferably up to 400.degree. C.
Holding Temperature after Cooling
[0058] After cooling, since bainitic transformation proceeds by
holding at a given temperature and also concentration of C to
austenite proceeds to form residual .gamma., it is important to
properly control the holding temperature after cooling. The holding
temperature is preferably within the range from 360 to 440.degree.
C. so as to obtain the metal structure of the present invention.
The retention time is preferably one minute or more. It is
necessary that the holding temperature is lower than the quenching
termination temperature. After passing through the temperature
range where fine ferrite is likely to be formed, the material is
transferred to a bainitic ferrite transformation temperature
range.
[0059] In the high-strength composite steel sheet of the present
invention, a composite steel sheet having a high strength of 980
MPa or higher class, good spot-weldabaility and anti-delayed
fraction property can be provided at cheap price by using a steel
material having specified chemical components as described above
and employing proper heat treatment conditions including cooling
conditions and holding conditions thereby ensuring a predetermined
metal structure.
EXAMPLES
[0060] The present invention is further illustrated by the
following examples. It is to be understood that the present
invention is not limited to the examples, and various design
variations made in accordance with the purports described
hereinbefore and hereinafter are also included in the technical
scope of the present invention.
Test Example
[0061] Steel materials with compositions shown in Table 1 were
prepared, subjected to continuous casting, subjected to hot rolling
and cold rolling under the conditions described below and then
subjected to a heat treatment (annealing) under the conditions
shown in Table 2 (also refer to FIG. 1) to obtain cold rolled steel
sheets.
[Hot Rolling]
[0062] Heating temperature: 1,200.degree. C. for 60 minutes Finish
temperature: 880.degree. C. Cooling: Cooling to 720.degree. C. at
40.degree. C./sec, cooling for 10 seconds, cooling to 500.degree.
C. at 40.degree. C./sec and holding at 500.degree. C. for 60
minutes, followed by furnace cooling. Finish thickness: 3.2 mm
[Pickling, Cold Rolling]
[0063] After pickling, cold rolling was performed to obtain a cold
sheet having a thickness of 1.2 mm.
[Heat Treatment (Annealing)]
[0064] As shown in Table 2, each cold rolled sheet was heated to a
predetermined annealing temperature, held at the same temperature
for 180 seconds, cooled to a predetermined cooling termination
temperature at a predetermined cooling rate, held at a
predetermined temperature for 6 minutes and then
furnace-cooled.
[0065] The metal structure of the resultant cold rolled steel sheet
was confirmed by the following method and each test steel sheet was
subjected to a tension test, a bore expansion test, a spot-welding
test and an anti-delayed fracture test. The results collectively
shown in Table 3 were obtained.
[Metal Structure]
Structure Identification Method
[0066] A: Optical microscope observation (magnification:
.times.1,000) by repeller corrosion, 1 visual field B: SEM
observation (magnification: .times.4,000), 4 visual fields
Polygonal Ferrite (PF)
[0067] Polygonal ferrite is identified by a micrograph taken by A
described above. Since etched residual .gamma. and etched
martensite show a white color, whereas, etched PF shows a gray
color, PF can be identified. After tracing the periphery of
polygonal ferrite in the SEM micrograph taken by B described above,
an equivalent circle diameter was calculated from the resultant
trace image by image analysis. An average of the resultant
equivalent circle diameter was taken as an average grain size of
the polygonal ferrite.
Retained .gamma., Martensite (M) and Bainite (B)
[0068] After residual .gamma., M and B were confirmed by a
transition electron microscope (TEM: magnification of
.times.15,000), the occupancy ratio was calculated from the
micrograph taken by B described above.
Bainitic Ferrite (BF)
[0069] After confirming that the structure is not a structure of
bainite or pseudo-ferrite by a transition electron microscope (TEM:
magnification of .times.15,000), the occupancy ratio was calculated
by subtracting an amount of polygonal ferrite, an amount of
residual .gamma., and balance of martensite (M) and bainite (B)
from 100%.
[Performance Evaluation Test]
[0070] Tension test: The measurement was performed using JIS No. 5
tension test specimens. Bore expansion test: The test was performed
in accordance with the Japan Iron and Steel Federation Standard
(JFST) 1001.
Spot-Weldability:
[0071] Spot-welding was performed under the following conditions.
The case where a ductility ratio at a nugget diameter of 5 t is
0.25 or more was rated Good (.largecircle.).
<Welding Conditions>
[0072] Thickness of test material: 1.2 mm Electrode: Dome radius
type (tip diameter: 6 mm)
[0073] Pressure: 375 kg
Upslope: 1 cycle, electrification time: 12 cycles, hold: 1 cycle
(60 Hz) Adjustment of nugget: adjusted by welding current Ductility
ratio: Cross tensile strength/Shear tensile strength
[Anti-Delayed Fraction Property]
[0074] After performing V-shaped bending using a 60.degree. V-block
of R=3 mm, stress of 1,500 MPa was applied to the bent portion,
followed by immersion in an aqueous 5% hydrochloric acid solution.
Then, the time until cracking occurs was measured. The case where
cracking did not occur after 48 hours was rated good anti-delayed
fraction property (.largecircle.).
TABLE-US-00001 TABLE 1 Chemical components (% by mass) (Si + Al)/
Steel type C Si Mn P S Al Cr Nb Ti Others (Mn + Cr) A3 point A 0.17
1.7 2.3 0.010 0.002 0.045 0.76 858 B 0.23 1.8 2.3 0.005 0.002 0.045
Ca: 0.002 0.80 846 C 0.17 2.3 2 0.005 0.002 0.045 1.17 891 D 0.17
2.3 2.6 0.005 0.002 0.045 0.90 873 E 0.14 2.0 2.5 0.005 0.002 0.045
0.82 870 F 0.20 1.6 2.04 0.005 0.002 0.045 0.81 851 G 0.17 1.8 2.1
0.005 0.002 0.045 0.2 0.80 863 H 0.17 1.8 2.2 0.001 0.002 0.045
0.04 0.84 859 I 0.17 1.8 2.3 0.001 0.002 0.045 0.05 0.80 856 J 0.16
1.8 2.4 0.010 0.001 0.15 0.81 904 K 0.08 1.6 1.6 0.010 0.003 0.040
1.03 899 L 0.22 0.5 2.8 0.010 0.003 0.040 0.5 0.19 771 N 0.17 1.8
1.2 0.010 0.003 0.040 1.53 894 N 0.23 2.3 1.5 0.010 0.003 0.040
1.56 893 O 0.18 1.5 2.1 0.010 0.002 0.040 0.2 0.05 Ca: 0.002 0.73
849
TABLE-US-00002 TABLE 2 Heat treatment conditions T1 (Temperature T2
(Holding T3 (Holding Annealing at which initiation termination
temperature Cooling rate 1 Cooling rate 2 cooling rate temperature:
temperature: Test No. Steel type (.degree. C.) (.degree. C.)
(.degree. C.) varies: .degree. C.) .degree. C.) .degree. C.) 1 A
900 50 20 580 420 400 2 B 880 50 20 580 420 380 3 C 930 100 20 580
420 380 4 D 910 50 20 580 420 380 5 E 900 50 20 580 420 380 6 F 890
100 20 580 420 380 7 G 900 50 20 580 420 380 8 H 900 50 20 580 420
380 9 I 900 50 20 580 420 380 10 J 940 50 20 580 420 380 11 K 920
50 50 -- 440 340 12 L 820 50 50 -- 380 380 13 M 890 50 50 -- 380
380 14 N 920 50 50 -- 400 380 15 A 820 30 20 600 360 360 16 O 880
30 20 550 420 400
TABLE-US-00003 TABLE 3 Metal structure Physical properties Ferrite
Anti- Residual grain TS .times. EI TS .times. .lamda. Spot- delayed
Test Steel PF .gamma. BF Remnant size YP TS EI .lamda. (MPa .times.
(MPa .times. weld- fraction No. type (%) (%) (%) (%) (.mu.m) (MPa)
(MPa) (%) (MPa .times. %) YR %) %) ability property 1 A 18.2 12
69.3 <1 5.5 650 985 21.2 41 0.66 20890 40309 .largecircle.
.largecircle. 2 B 10.3 13 76.2 <1 3.7 735 1097 21.3 44 0.67
23400 48619 .largecircle. .largecircle. 3 C 29 15 56.5 <1 8.7
647 1096 20.4 38 0.59 22350 41880 .largecircle. .largecircle. 4 D
9.1 14 76.4 3.3 (M) 6.1 778 1235 19.2 43 0.63 23720 53025
.largecircle. .largecircle. 5 E 22.7 9 67.8 2.4 (M) 7.4 744 1163
16.7 50 0.64 19430 58388 .largecircle. .largecircle. 6 F 25.5 10 64
<1 8.6 643 1020 18.9 46 0.63 19304 46737 .largecircle.
.largecircle. 7 G 17.3 12 70.2 <1 6.1 651 1085 18.5 42 0.60
20120 46093 .largecircle. .largecircle. 8 H 16.8 13 69.7 <1 5.9
728 1056 19.4 48 0.69 20487 50299 .largecircle. .largecircle. 9 I
18.4 13 71.2 <1 5.6 738 1085 18.7 48 0.68 20330 51778
.largecircle. .largecircle. 10 J 20.1 12 67.4 2.5 8.4 746 1113 17.6
50 0.67 19548 56055 .largecircle. .largecircle. 11 K 50.3 6 43.2 18
(B + M) 9.7 518 836 21.4 34 0.62 17860 28595 .largecircle.
.largecircle. 12 L 11 0 69 20 (M + B) 4.1 783 1118 15.0 27 0.70
16740 29792 .largecircle. X 13 M 72 14 13.5 9 (M) 9.0 544 907 23.2
20 0.60 21013 18589 .largecircle. .largecircle. 14 N 35 8 55 2 15.7
533 920 21.2 25 0.58 19532 23063 X .largecircle. 15 A 60 14 25.5
3.5 (M) 9.9 676 1090 17.4 22 0.62 18912 23448 .largecircle. X 16 O
25 13 61.5 <1 3.5 688 1110 17.9 45 0.62 19912 50220
.largecircle. .largecircle. Note: PF: Polygonal ferrite, BF:
Bainitic ferrite Ramnant (M: Martensite, B: Bainite)
[0075] The following facts become apparent from the results shown
in Tables 1 to 3.
[0076] Tests Nos. 1 to 10 and 16 are Examples which satisfy all
defined features of the present invention. All steel materials have
a tensile strength of 980 MPa or higher class and have good
formability evaluated by strength.times.elongation characteristics
and strength.times.stretch flangeability characteristics, and also
have good spot-weldability and anti-delayed fraction property.
[0077] In contrast, in Test No. 11, since the steel material has a
low C content and also has a low bainitic ferrite content in the
metal structure, it has poor tensile strength and also has poor
formability evaluated by strength.times.elongation characteristics
and strength.times.stretch flangeability characteristics. In Test
No. 12, since the steel material has low Si content and also has a
ratio (Si+Al)/(Mn+Cr) which is not within a defined range, no
residual .gamma. exist in the metal structure and also has very
poor formability evaluated by strength.times.elongation
characteristics and strength.times.stretch flangeability
characteristics, and poor anti-delayed fraction property.
[0078] In Test No. 13, since the Mn content is not within a defined
range and the ratio (Si+Al)/Mn exceeds a defined range, polygonal
ferrite excessively increases and the amount of bainitic ferrite
drastically decreases, and thus the steel material has low strength
and very poor workability. In Test No. 14, although each content
satisfies defined constituents, the ratio (Si+Al)/Mn exceeds a
defined range and polygonal ferrite is coarse and an average grain
size exceeds a defined value, and thus the steel material has low
strength, poor workability and poor spot-weldability.
[0079] In Test No. 15, although the steel components satisfy
defined constituents, since the heat treatment conditions are not
proper, the amount of polygonal ferrite in the structure
excessively increases and the bainitic amount is low, and thus the
steel material has very poor formability evaluated by
strength.times.elongation characteristics and
strength.times.stretch flangeability characteristics, and also has
poor anti-delayed fraction property.
* * * * *