COLD-ROLLED STEEL SHEET AND PROCESS FOR
MANUFACTURING SAME
Technical Field
5 The present invention relates to a cold-rolled steel sheet and a process for
manufacturing the same. More particularly, the present invention relates to a
cold-rolled steel sheet having excellent workability in addition to a high strength,
and a process for manufacturing the same with excellent stability.
1 0 Background Art
Regarding techniques for improving the mechanical properties of cold-rolled
steel sheets, following Patent Document 1 indicated discloses a high-strength steel
sheet having bainitic ferrite as a main phase, and containing at least 3% lath type
austenite and 1% to (lath type retained austenite area occupancy ratiox 112) block-
15 type austenite, the steel sheet being described as having excellent ductility and
stretch flangeability. However, the block-type austenite in this steel sheet has
grain diameters of around 2.2 pm to 20 pm and thus is coarse, and therefore can be
considered as adversely affecting the formability of the steel sheet.
Patent Document 2 discloses a method for performing cold rolling using a
20 hot-rolled steel sheet manufactured by hot rolling and then starting cooling in a
short period of time after the hot rolling. For example, Patent Document 2
discloses that a hot-rolled steel sheet having a fine structure containing ferrite
having a small average grain diameter as a main phase is manufactured by
performing cooling to at most 720°C at a cooling rate of at least 400°C/sec within
25 0.4 seconds after hot rolling and the hot-rolled steel sheet is subjected to usual cold
rolling and annealing to manufacture a cold-rolled steel sheet. Hereinafter, the
above hot-rolled steel sheet production process may also be referred to as
immediate cooling method.
30 Prior Art Documents
Patent Document
Patent Document 1 : JP 2007-32 1236 A
Patent Document 2: W02007/015541 A
Summary of Invention
According to the method disclosed in Patent Document 2, structure may be
5 refined without inclusion of precipitating elements and thus a cold-rolled steel
sheet having excellent ductility can be manufactured. The manufactured coldrolled
steel sheet also has a fine structure even after cold rolling and
recrystallization because a hot-rolled steel sheet, which is a starting material of the
cold-rolled steel sheet, has a fine structure. Thus, the produced austenite also
10 becomes fine and as a result, a cold-rolled steel sheet having a fine structure can be
obtained. However, since the usual annealing is performed after cold rolling,
recrystallization occurs in a heating process during the annealing, and after
completion of the recrystallization, austenite transformation occurs as grain
boundaries in the structure after the recrystallization function as nucleus forming
15 sites. In other words, after most preferred nucleus forming sites for austenite
transformation such as high angle grain boundaries, fine carbide grains and a low
temperature transformation phase existing in the hot-rolled steel sheet have
disappeared during the heating in the annealing, austenite transformation occurs.
Accordingly, although a cold-rolled steel sheet obtained by the method disclosed in
20 Patent Document 2 has a fine structure, refining austenite grain in an annealing
process is restrictedly premised on the structure after recrystallization, and thus, the
fine structure cannot be easily obtained after cold rolling and annealing even if the
hot-rolled steel sheet has the fine structure. In particular, when annealing is
carried out for a single-phase austenite region, it is difficult to utilize the fine
25 structure of the hot-rolled steel sheet in order to refine the structure after cold
rolling and annealing.
An object of the present invention is to provide a cold-rolled steel sheet
having excellent ductility and stretch flangeability in addition to a high strength by
enabling to effectively refine a structure after cold rolling and annealing even if a
30 large amount of precipitating elements such as Ti and Nb, which are known as
being effective for structure refinement is not added, and a process for
manufacturing the same.
The present inventors employed a composite-structure having a main phase
of either or both of martensite and bainite, which are low temperature
transformation phases and focused on suppression of growth of a particular texture,
in order to obtain a structure for providing excellent ductility and stretch
5 flangeability in addition to high strength.
Furthermore, generally, a decrease in stretch flangeability (hole expanding
formability) for a structure containing a soft phase, such as ferrite, and retained
austenite intermixed therein is concerned, and thus, investigation is performed
based on the material quality design concept that such decrease in stretch
10 flangeability is minimized by refining ferrite and/or controlling retained austenite
form.
In order to obtain such structure, the present inventors conceived of the new
concept of promoting austenitic transformation before completion of
recrystallization in an annealing process after cold rolling, as opposed to the
15 conventional annealing method in which austenitic transformation is promoted after
completion of recrystallization, and performing annealing in an adequate high
temperature range for suppression of growth of a particular texture, and conducted
test.
As a result, the present inventors obtained the following new knowledge.
20 1) In the conventional annealing method for promoting austenitic transformation
after completion of recrystallization, since austenitic transformation occurs with
grain boundaries in the structure after the recrystallization as nucleus forming sites,
refining austenite grains (prior austenite grains after annealing; hereinafter also
referred to as "prior austenite grains") in the annealing process receives a restriction
25 that the refining is premised on performing austenitic transformation from the
structure after recrystallization.
On the other hand, in the annealing method for promoting austenitic
transformation by rapid heating to a temperature range in which austenite is
produced before completion of recrystallization, since austenitic transformation
30 occurs from high angle grain boundaries, fine carbide grains and low-temperature
transformation phases, which are preferred nucleus forming sites for austenitic
transformation, in the hot-rolled steel sheet, the austenite grains are dramatically
refined during the annealing process. As a result, the structure of the cold-rolled
steel sheet after the annealing is effectively refined.
2) In such annealing method for promoting austenitic transformation by rapid
heating to a temperature range in which austenite is produced before completion of
recrystallization, since a worked ferrite structure intends to retain, a particular
texture grows, as a result, g and workability of the steel sheet intend to decrease.
On the other hand, if annealing is carried out in an adequate high
temperature range, recrystallization and austenitizing of the worked ferrite structure
is promoted, whereby while the fine structure is maintained, growth of the
particular texture is suppressed in connection with the structure refinement to
enable ensuring of excellent ductility and stretch flangeability.
3) Although containing ferrite having an excellent ductility enables improvement in
ductility of the cold-rolled steel sheet, in general, concern about a structure
containing a soft phase such as ferrite is a decrease in stretch flangeability because
when the steel sheet is worked, clacking easily occurs in an interface between the
soft phase and a hard phase.
However, as stated above, by refining the structure of the cold-rolled steel
sheet after annealing, the ferrite is also refined. Consequently, the formation and
development of fine cracks at the time of working of a steel sheet are effectively
suppressed, whereby the decrease in stretch flangeability is prevented. Thus,
containing fine ferrite enables ductility improvement and ensuring of excellent
stretch flangeability.
4) Ductility of the cold-rolled steel sheet is further improved by containing retained
austenite which exhibits a ductility improvement effect due to strain induced
transformation. However, it is concerned that generally, a structure containing
retained austenite results in decreasing stretch flangeability because retained
austenite is transformed to hard martensite due to the strain induced transformation,
which may cause cracking when the steel sheet is worked, .
In this regard, in the case of a steel sheet obtained by the annealing method
for promoting austenitic transformation before completion of recrystallization in an
annealing process after cold rolling, a fraction of lump-like retained austenite
having an aspect ratio of less than 5 in all the retained austenite increases. This is
because by refining prior-austenite grain, a retained austenite existing on the prioraustenite
grain boundaries, the packet boundaries and the block boundaries
increases and a retained austenite produced among laths of bainite and/or
martensite decreases. Such lump-like retained austenite has higher stability
against work strain than the retained austenite produced among laths of bainite
andlor martensite and thus increases the work hardening coefficient .in high strain
regions. Thus, the ductility of the steel sheet can effectively be improved.
Then, as described above, refining retained austenite and increasing fraction
of lump-like retained austenite having an aspect ratio of less than 5 resulting fiom
effective refinement of the structure of a cold-rolled steel sheet after annealing
prevents a decrease in stretch flangeability of the cold-rolled steel sheet. Thus, by
containing fine and low-aspect ratio retained austenite, ductility can be improved
and excellent stretch flangeability of the cold-rolled steel sheet can be maintained.
5) As stated above, in the annealing method in which austenitic transformation is
promoted before completion of recrystallization in an annealing step after cold
rolling, prior-austenite grains are effectively refined because nuclei of austenitic
transformation forms from the high angle grain boundaries, fine carbide grains, and
the low-temperature transformation phases, which are preferred nucleus forming
sites of austenitic transformation, in the hot-rolled steel sheet. Thus, as a process
for manufacturing a hot-rolled steel sheet, the production method described in
Patent Document 2, which provides a hot-rolled steel sheet containing preferred
nucleus forming sites of austenitic transformation in high density, is preferable.
Employment of the above annealing method for a hot-rolled steel sheet obtained by
the production method described in Patent Document 2 provides firther refining
austenite grains in the annealing process and firher refining the structure of the
cold-rolled steel sheet after the annealing.
The present inventors found that as a result of the above structure refinement,
ductility of the cold-rolled steel sheet and the balance between the ductility and the
stretch flangeability is significantly improved.
An aspect of the present invention provides a cold-rolled steel sheet
characterized by having: a chemical composition comprising, in mass% of C: 0.06
to 0.3%, Si: 0.6 to 2.5%, Mn: 0.6 to 3.5%, P: at most 0. I%, S: at most 0.05%, Ti: 0
to 0.08%, Nb: 0 to 0.04%, a total of Ti and Nb: 0 to 0.10%, sol.Al: 0 to 2.0%, Cr: 0
to I%, Mo: 0 to 0.3%, V: 0 to 0.3%, B: 0 to 0.005%, Ca: 0 to 0.003%, REM : 0 to
0.003%, and a remainder of Fe and impurities; a microstructure having a main
phase of either or both of martensite and bainite which comprising at least 40
5 area% in total; and a texture in which ratio of the average X-ray intensity for the
{ 100)<011> to (2 1 1)<011> orientations relative to the average X-ray intensity of
a random structure which does not have a texture at a depth of 112 of the sheet
thickness is less than 6.
A main phase in a microstructure means a phase having a largest area
10 fiaction, and a second phase means to any of phases other than the main phase.
It is preferable that the cold-rolled steel sheet according to the present
invention W e r provides one or more of the following features (1) to (8).
(1) The microstructure has the second phase of ferrite which comprises at least
3% and satisfies Equation (1):
15 dF < 4.0 ... (1).
where dF is an average grain diameter (unit: pm) of ferrite defined by high
angle grain boundaries having a tilt angle of at least 15'.
(2) The microstructure has the second phase of retained austenite which
comprises at least 3 area%., and satisfies Equations (2) and (3):
20 dh < 1.5 ... (2); and
rA, > 50 (31,
where dAs is an average grain diameter (unit: pm) of retained austenite
having an aspect ratio of less than 5 and r ~is, an area fraction (%) of the retained
austenite having an aspect ratio of less than 5 relative to all retained austenite.
25 (3) The .chemical composition contains, in mass%, one or two selected from Ti:
0.005 to 0.08% and Nb: 0.003 to 0.04%.
(4) The chemical composition contains, in mass%, sol.Al: 0.1 to 2.0%.
(5) The chemical composition contains one or more selected from, in mass%, of
Cr: 0.03 to 1%, Mo: 0.01 to 0.3% and V: 0.01 to 0.3%.
30 (6) The chemical composition contains, in mass%, B: 0.0003 to 0.005%.
(7) The chemical composition contains one or two selected from, in mass%,
0.0005 to 0.003% and REM: 0.0005 to 0.003%.
(8) The cold-rolled steel sheet has a plating layer on the surface.
Another aspect of the present invention provides process for manufacturing a
cold-rolled steel sheet characterized by comprising the following steps (A) and (B):
(A) a cold rolling step in which a hot-rolled steel sheet having the above
chemical composition is subjected to cold rolling to obtain a cold-rolled steel sheet;
and
(B) an annealing step in which the cold-rolled steel sheet obtained in Step
(A) is subjected to heat treatment under conditions that the cold-rolled steel sheet is
heated at an average heating rate condition of at least lS°C/sec so that the
proportion of unrecrystallization relative to a region not transformed to austenite
when the temperature (Acl point +lO°C) is reached is at least 30 area%, and is then
held in a temperature range of at least (0.3 x Acl point + 0.7 x Ac3 point) and at
most (Ac3 point +lOO°C) for at least 30 seconds, and the steel sheet is then cooled
at an average cooling rate of at least 10°C/sec for a temperature range of at most
650°C and at least 500°C.
It is preferable that the process for manufacturing the cold-rolled steel sheet
according to the present invention provides one or more of following features (9) to
(9) The hot-rolled steel sheet is obtained by coiling at a temperature of at most
300°C after completion of hot rolling and subsequent heat treatment in a
temperature range of 500°C to 700°C.
(10) The hot-rolled steel sheet is a steel sheet with average grain diameter of a
BCC phase defined by high angle grain boundaries having a tilt angle of at least
15" is at most 6 pm, the steel sheet being obtained by a hot rolling step of cooling
at a cooling rate (Crate) satisfying Equation (4) below for a temperature range from
a temperature at the completion of rolling to (temperature at the completion of
rolling -100°C) after completion of hot rolling in which hot rolling is completed at
at least an Ar3 point.
In the above equation, Crate (T) is a cooling rate ("CIS) (positive value),
T is a relative temperature with the temperature at the completion of rolling
as zero (T = (temperature of steel sheet during cooling - temperature at completion
of rolling) OC, negative value), and
if a temperature at which Crate is zero exists, a value obtained by dividing a
holding time (At) at the temperature by IC (T) is added as an integral for the section.
(1 1) The cooling for the temperature range in above (10) includes starting cooling
at a cooling rate of at least 400°C/sec and cooling at the cooling rate for a
temperature range of at least 30°C.
(12) The cooling for the temperature range in above (10) includes starting water
cooling at a cooling rate of at least 400°C/sec and cooling at the cooling rate for a
temperature range of at least 30°C and at most 80°C, and then stopping a water
cooling for 0.2 to 1.5 seconds to measure a shape of the sheet during stopping water
cooling, and subsequently cooling at a rate of at least 50°C/sec.
(13) The process for manufacturing the cold-rolled steel sheet M e r has the step
of plating the cold-rolled steel sheet after the (B) step.
The present invention provides effectively refining a structure after cold
rolling and annealing without addition of a large amount of elements which
precipitate such as Ti and Nb, and thus provides a high-strength cold-rolled steel
sheet having excellent ductility and stretch flangeability and a process for
manufacturing the same. Since the structure refinement mechanism which is
different from that of the conventional method is adopted in the present invention, a
fine structure can be obtained even if a holding time for annealing is made long
enough to obtain a stable material.
Description of Embodiment
The cold-rolled steel sheet according to the present invention and the process
for manufacturing the same will be described below. In the below description,
each of "%"s in chemical compositions is "mass%." Also, each of average grain
diameters in the present invention means an average Heywood diameter value
obtained according to Equation (4), which will be described later, using SEMEBSD.
1. Cold-rolled steel sheet
1 - 1 : Chemical composition
[C: 0.06 to 0.3%]
C has the effect of increasing the strength of steel. Also when C is
concentrated in austenite, C has the effect of obtaining the stable austenite,
5 increasing the area fraction of retained austenite in the cold-rolled steel sheet and
thereby increasing the ductility. Furthermore, in the hot rolling process and the
annealing process, C has the effect of refining the microstructure.
In other words, C has the effect of lowering a transformation point. As a
result, in the hot rolling process, hot rolling can be completed in a lower-
10 temperature range to refrne the microstructure of the hot-rolled steel sheet. In an
annealing step, due to the effect of C by which recrystallization of ferrite is
suppressed in the course of temperature increase, it is facilitated to reach a
temperature range of at least (Acl point +lO°C) by rapid heating while maintaining
a state with a high percentage of unrecrystallized ferrite. As a result, it becomes
15 possible to refine the microstructure of a cold-rolled steel sheet.
If the C content is less than 0.06%, it is difficult to obtain the abovedescribed
effects. Accordingly, the C content is made at least 0.06%. It.is
preferably at least 0.08% and more preferably at least 0.10%. If the C content
exceeds 0.3%, there is a marked decrease in workability and weldability.
20 Accordingly, the C content is made at most 0.3%. Preferably it is at most 0.25%.
[Si: 0.6 to 2.5%]
Si has the effect of promoting the formation of hard phases such as
martensite and bainite, which is a main phase of a cold-rolled steel sheet according
to the present invention, and thereby increasing the strength of the steel.
25 Furthermore, Si has the effect of promoting production of retained austenite and
thereby increasing the ductility of the steel.
If the Si content is less than 0.6%, it is difficult to obtain the above-described
effects. Therefore, the Si content is at least 0.6%, preferably at least 0.8%, M e r
preferably at least 1.0%. On the other hand, if the Si content exceeds 2.5%, a
30 substantial ductility decrease may occur or platability may be deteriorated.
Accordingly, the Si content is at most 2.5%, preferably at most 2.0%.
[Mn: 0.6 to 3.5%]
Mn has the effect of increasing the strength of steel. Mn also has the effect
of decreasing a transformation temperature. As a result, during an annealing step,
it is facilitated to reach a temperature range of at least (Acl point +lO°C) by rapid
heating while maintaining a state with a high percentage of unrecrystallized ferrite,
5 and it becomes possible to refine the microstructure of a cold-rolled steel sheet.
If the Mn content is less than 0.6%, it becomes difficult to obtain the abovedescribed
effects. Accordingly, the Mn content is made at least 0.6%. On the
other hand, if the Mn content exceeds 3.5%, the strength of the steel is excessively
increased, which may result in substantial ductility loss. Therefore, the Mn
10 content is at most 3.5%.
[P: At most 0.1 %]
P, which is contained as an impurity, has the action of embrittling the
material by segregation at grain bouridaries. If the P content exceeds 0.1%,
embrittlement due to the above action becomes marked. Accordingly, the P
15 content is made at most 0.1%. Preferably it is at most 0.06%. The P content is
preferably as low as possible, so it is not necessary to set a lower limit therefor.
From the standpoint of costs, it is preferably at least 0.001%.
[S: At most 0.05%]
S, which is contained as an impurity, has the action of lowering the ductility
20 of steel by forming sulfide-type inclusions in steel. If the S content exceeds
0.05%, there may be a marked decrease in ductility due to the above-described
action. Accordingly, the S content is made at most 0.05%. It is preferably at
most 0.008% and more preferably at most 0.003%. The S content is preferably as
low as possible, so it is not necessary to set a low limit therefor. From the
25 standpoint of costs, it is preferably at least 0.001%.
[Ti: 0 to 0.08%, Nb: 0 to 0.04% and a total of Ti and Nb: 0 to 0.10%]
Ti and Nb each have the effect of precipitating in steel as carbides or nitrides
and suppressing austenite grain growth in the annealing step, thereby promoting
refining the structure of the steel. Therefore, the chemical composition of the
30 steel may contain either or both of Ti and Nb as desired.
However, if the content of each of the elements exceeds the above upper
limit value or the total content exceeds the above upper limit value, a ductility may
markedly decrease. Therefore, the content of each of the elements and the total
content are set as above. Here, the Ti content is preferably at most 0.05%, M e r
preferably at most 0.03%. Also, the Nb content is preferably at most 0.02%.
The total content of Ti and Nb is preferably at most 0.05%, M e r preferably at
5 most 0.03%. In order to obtain t the above effect with greater certainty, it is
preferably to satisfl either of the conditions of at least 0.005% Ti and at least
0.003% Nb.
[sol.Al: 0 to 2.0%]
A1 has the effect of increasing the ductility of steel. Accordingly, Al may
10 be contained in the steel composition. However, since A1 has the effect of
increasing an Ar3 transformation point, if the sol.Al content exceeds 2.0%, it
becomes necessary to complete hot rolling in a higher temperature range. As a
result, it becomes difficult to refine the structure of a hot-rolled steel sheet and it
therefore becomes difficult to refine the structure of a cold-rolled steel sheet. In
15 addition, continuous casting sometimes becomes difficult. Accordingly, the sol. A1
content is made at most 2.0%. In order to obtain the above-described effect of A1
with greater certainty, the sol. Al content is preferably at least 0.1 %.
[Cr: 0 to I%, Mo: 0 to 0.3% and V: 0 to 0.3%]
Cr, Mo and V each have the effect of increasing the strength of steel. Also,
20 Mo has the effect of suppressing the growth of grains and refining the structure,
and V has the effect of promoting transformation to ferrite and increasing the
ductility of the steel sheet. Therefore, one or more of Cr, Mo and V may be
contained.
However, if the Cr content exceeds I%, the ferrite transformation may
25 excessively be suppressed, and as a result, it is impossible to ensure a desired
structure. Also, if the Mo content exceeds 0.3% or if the V content exceeds 0.3%,
an amount of precipitates may increase in the heating step in the hot rolling process,
which can substantially decrease the ductility. Accordingly, the contents of the
respective elements are set as above. The Mo content is preferably at most 0.25%.
30 In order to obtain the above effects with greater certainty, it is preferable to satisfy
any of the conditions of at least 0.03% Cr, at least 0.01% Mo and at least 0.01% V.
[B: 0 to 0.005%]
B has the effect of increasing the hardenability of steel and promoting the
formation of low-temperature transformation phases, thereby increasing the
strength of the steel. Therefore, B may be contained. However, if the B content
exceeds 0.005%, the steel excessively may harden, which can result in a significant
5 ductility decrease. Therefore, the B content is at most 0.005%. In order to
obtain the above effects with greater certainty, the B content is preferably at least
0.0003%.
[Ca: 0 to 0.003% and REM: 0 to 0.003%]
Ca and REM each have the effect of refining oxides and nitrides precipitated
10 during solidification of molten steel and thereby increasing the soundness of a slab.
Accordingly, one or more of these elements may be contained. However, each of
these elements is expensive, so the content of each element is made at most 0.003%.
The total content of these elements is preferably at most 0.005%. In order to
obtain the above-described effects with greater certainty, the content of either
15 element is preferably at least 0.0005%.
REM indicates the total of 17 elements including Sc, Y, and lanthanoids.
Lanthanoids are industrially added in the form of a rnish metal. The content of
REM in the present invention means the total content of these elements. .
The remainder other than the above is Fe and impurities.
20 1-2: Microstructure a d texture
[Main phase]
A microstructure has a main phase of either or both of martensite and bainite
which comprising at least 40 area% in total, which are hard low temperature
transformation phases.
25 As a result of the main phase of either or both of martensite and bainite,
which are hard low temperature transformation phases, the strength of the steel
sheet can be increased, and in addition, the hardness distribution in the
microstructure is equalized, the stretch flangeability of the cold-rolled steel sheet
can be increased.
30 If the area fraction of the main phase is less than 40%, the hardness
distribution in the structure becomes large in change, making fine cracks easily
occur during working deformation, resulting in difficulty to achieve excellent
stretch flangeability. Therefore, the area fiaction of the main phase (martensite
and/or bainite) is at least 40%. The area fiaction is preferably at least 50%, more
preferably at least 60%. The main phase does not need to contain both of
martensite and bainite, and may contain only either of them or both of them.
Bainite includes bainitic ferrite.
[Second phase]
A second phase preferably contains at least 3 area% ferrite and satisfies
above Equation (1). The second phase preferably fbther contains at least 3 area%
retained austenite and satisfies above Equations (2) and (3).
If the second phase contains at least 3 area% ferrite, the ductility of the coldrolled
steel sheet can be increased. In this case, since an average grain diameter of
ferrite defmed by high angle grain boundaries with a tilt angle of at least 15' is at
most 4.0 p and thus, fine (that is, satisfies Equation (1) above), the formation and
development of fine cracks during working of a steel sheet can effectively be
suppressed, whereby the stretch flangeability of the cold-rolled steel sheet is firher
increased. Hereinafter, the average grain diameter of the ferrite defined by the
high angle grain boundaries is simply referred to as "average grain diameter" of
ferrite..
Since retained austenite (retained y) has the effect of increasing the ductility
of steel sheet, the ductility can be increased by increasing the area fraction of
retained austenite. Setting the area fraction of retained austenite to at least 3%
makes it easy to guarantee excellent ductility, and thus the second phase preferably,
contains at least 3% by area fraction of retained austenite. The area fraction of
retained austenite is further preferably at least 5%. In this case, lump-like retained
austenite having an aspect ratio of less than 5 (hereinafter simply referred to as
"lump-like austenite") satisfies above Equations (2) and (3) (that is, a high area
fraction, i.e., at least 50% by area fraction of lump-like austenite relative to the
entire retained austenite is contained and an average grain diameter thereof is fine,
i.e., at most 1.5 pm), and further excellent stretch flangeability can be guaranteed.
Because lump-like retained austenite occupying the majority of the retained
austenite is fine, after transformation of the retained austenite to martensite during
working of the cold-rolled steel sheet, the formed martensite is fine. Thus, the
decrease in stretch flangeability caused by martensite transformation is prevented.
Also, since lump-like retained austenite tends to be produced adjacent to ferrite,
work hardening caused by strain induced transformation fiuther appears noticeably.
Thus, lump-like retained austenite has the highly effective for increasing the
ductility, in particular, the uniform ductility and the n-value, compared to elongated
ones having an aspect ratio exceeding 5, which are formed among laths of, e.g.,
martensite. Since the lump-like retained austenite having such properties occupies
the majority of the retained austenite, the workability of the cold-rolled steel sheet
can be improved. For the above reasons, the retained austenite contained in the
second phase preferably satisfies above Equations (2) and (3), and more preferably
satisfies following Equations (2a) and (3a):
dAsI1.O ...( 2a);and
r~~ 2 60 ... (3a).
Here, although the second phase may be contaminated by pearlite andlor
cementite, such contamination is allowed if a total area fraction of them is at most
10%.
An average grain diameter of ferrite that can be contained as the second
phase is determined using an SEM-EBSD for those ferrite grains which are
surrounded by high angle grain boundaries having a tilt angle of at least 15'.
SEM-EBSD is a method of carrying out measurement of the orientation of a minute
region by electron backscatter diffraction (EBSD) in a scanning electron
microscope (SEM). It is possible to measure the grain diameter from the resulting
orientation map.
The average grain diameter of the lump-like retained austenite having an
aspect ratio of less than 5 can be calculated by a method similar to the above.
The area fractions of the main phase and the ferrite can be measured by
structure analysis using SEM-EBSD. Also, the volume fraction of the retained
austenite determined by X-ray diffraction is used as the area fraction of the retained
austenite as it is.
In the present invention, the above-described average grain diameter and
area fraction are the values measured at a depth of 114 the sheet thickness of the
steel sheet.
[Texture]
The cold-rolled steel sheet according to the present invention has a texture
where ratio of the average of the X-ray intensities for { 100)<0 1 1> to (2 1 1 1x0 1 1>
orientations relative to an average of the X-ray intensities of a random structure not
having a texture is less than 6 at a depth of 112 the sheet thickness.
If the texture for {100)<011> to (21 1)<011> orientation grows, the
workability of the steel decreases. Thus, the X-ray intensity ratio of the
orientation group is decreased to decrease the workability of the steel. If the
average of the X-ray intensities for the orientation group relative to the average of
the X-ray intensities of the random structure not having a texture is at least 6, it is
difficult to guarantee good ductility and stretch flangeability.
Therefore, the ratio of the average of the X-ray intensities of the orientations
relative to the average of the X-ray intensities of the random structure not having a
texture is less than 6. The ratio is preferably less than 5, more preferably less than
4. Here, {hkl) of a texture represent an crystal orientation in which a
vertical direction of the sheet and the normal to {hkl) are parallel to each other and
a rolling direction and are parallel to each other.
The X-ray intensity of the particular orientation can be obtained by
chemically polishing the steel sheet to the depth of 112 the sheet thickness using
hydrofluoric acid and subsequently measuring pole figures of the {200), { 1 10) and
(2 1 1 ) planes of the ferrite phase on the sheet and analyzing an orientation
distribution function (ODF) by series expansion method using the measurement
values.
The X-ray intensities of the random structure not having a texture are
determined by measurement like that described above using a powdered sample of
the steel.
1-3 : Plating layer
With the object of improving corrosion resistance and the like, a plating
layer may be provided on the surface of the above-described cold-rolled steel sheet
to obtain a surface treated steel sheet. The plating layer may be an electroplated
layer or a hot-dip plating layer. Examples of an electroplating are
electrogalvanizing and Zn--Ni alloy electroplating. Examples of a hot-dip plating
are hot-dip galvanizing, galvannealing, hot-dip aluminum plating, hot-dip Zn--A1
alloy plating; hot-dip Zn--A--Mg alloy plating, and hot-dip Zn--Al--Mg--Si alloy.
plating. The plating weight is not limited, and it may be a usual value. It is also
possible to form a suitable chemical conversion treatment coating on the plating
5 surface (such as one formed by applying a silicate-based chromium-fiee chemical
conversion solution followed by drying) to further improve corrosion resistance.
It is also possible to cover the plating with an organic resin coating.
2. Process for Manufacturing
10 2- 1: Hot rolling and cooling aRer rolling
In the present invention, the structure of the cold-rolled steel sheet is refined
by the below-described annealing, and thus, a hot-rolled steel sheet provided for
cold rolling may be carried out in a conventional m&er. However, in order to
further refine the structure of the cold-rolled steel sheet, it is preferable to refine the
15 structure of a hot-rolled steel sheet provided for cold rolling to increase nucleus
forming sites for austenitic transformation. More specifically, this means refining
grains surrounded by high angle grain boundaries having a tilt angle of at least 15O
and refined dispersion of the second phase such as cementite andlor martensite.
When a hot-rolled steel sheet having a fine structure is subjected to cold
20 rolling and then to annealing by rapid heating, nucleus forming site disappearance
due to recrystallization in a heating process can be suppressed by the rapid heating,
and thus, the number of nuclei formed in austenite and recrystallized ferrite
increases, and it is facilitated to refine the final structure.
In the present invention, a hot-rolled steel sheet that is preferable for a
25 starting material for a cold-rolled steel sheet specifically has an average grain
diameter of the BCC phase defined by high angle grain boundaries having a tilt
angle of at least 15O, namely at most 6 p. The average grain diameter of the
BCC phase is further preferably at most 5 ym. This average grain diameter can
also be obtained by SEM-EBSD.
30 If the average grain diameter of the BCC phase in the hot-rolled steel sheet is
at most 6 pm, the cold-rolled steel sheet can further be refined to further improve
mechanical property. Here, since the average grain diameter of the BCC phase in
the hot-rolled steel sheet is preferably as small as possible, a lower limit is not
recited, but the average grain diameter is normally at least 1.0 p. The BCC
phase mentioned here may include ferrite, bainite and martensite, and consists of
one or more of ferrite, bainite and martensite. Martensite is precisely not a BCC
phase, but is included in a BCC phase in the Description considering that the
aforementioned average grain diameter is obtained by a SEM-EBSD analysis.
Such a hot-rolled steel sheet having a fine structure can be manufactured by
performing hot rolling and cooling by the method described below.
A slab having the above-described chemical composition is manufactured by
continuous casting, and is provided for hot rolling. Here, the slab may be used in
a high temperature state aRer the continuous casting or may be first cooled to room
temperature and then reheated.
The temperature of the slab which is subjected to hot rolling is preferably at
least 100d"~. If the heating temperature of the slab is lower than 1000°C,
excessive load is imposed on a rolling mill, and further, the temperature of the steel
may decrease to a ferrite transformation temperature during rolling, whereby the
steel can be rolled in a state in which transformed ferrite contained in the structure.
Therefore, the heating temperature of the slab is preferably sufficiently high so that
hot rolling can be completed in the austenite temperature range.
The hot rolling is preferably carried out using a reverse mill or a tandem mill.
From the standpoint of industrial productivity, it is preferable to use a tandem mill
for at least the fmal number of stands. Since it is necessary to maintain the steel
sheet in the austenite temperature range during rolling, the temperature at the
completion of the rolling is preferably made at least the Ar3 point.
Rolling reduction in hot rolling is preferably such that the percent reduction
in the sheet thickness when the slab temperature is in the temperature range from
the AT3 point to (AT3 point +150°C) is at least 40%. The percent reduction in
thickness is more preferably at least 60%. It is not necessary to carry out rolling
in one pass, and rolling may be carried out by a plurality of sequential passes.
Increasing the rolling reduction is preferable because it can introduce a larger
amount of strain energy into austenite, thereby increasing the driving force for
transformation to BCC phase and refining BCC phase more greatly. However,
doing so increases the load on rolling equipment, so the upper limit on the rolling
reduction per pass is preferably 60%.
Cooling after the completion of the rolling is preferably carried out by the
method described in detail below.
Cooling fkom the temperature at the completion of rolling is preferably
carried out at a cooling rate (Crate) satiseing Equation (4) below in a temperature
range from the temperature at the completion of rolling to (temperature at the
completion of rolling - 100°C).
The meanings of the symbols in the equation have been stated above.
Equation (4) above indicates a condition to be cooled to an austenite
unrecrystallization temperature range (temperature at the completion of rolling -
100°C) before strain energy accumulated in the steel sheet during hot rolling is
consumed by recovery and recrystallization after completion of the hot rolling.
More specifically, IC (T) is a value that can be obtained by calculation of body
diffusion of Fe atoms, and represents a period of time fkom completion of hot
rolling to a start of recovery of austenite. Furthermore, (l/(Crate(T).IC(T))) is a
value of a period of time required for cooling by 1 "C at a cooling rate (Crate(T)),
the period of time being normalized by IC(T), that is, represents a fraction of
cooling time relative to a period of time until disappearance of strain energy by
recovery and recrystallization. Therefore, a value that can be obtained by
integrating (l/Crate(T).IC(T)) in a range of T = 0 to -lOO°C serves as an index
representing an amount of strain energy disappeared during cooling. By limiting
the value, cooling conditions (cooling rate and holding time) required for cooling
by 100°C before disappearance of a certain amount of strain energy. The value of
the right side of Equation (4) is preferably 3.0, more preferably 2.0, further
preferably 1 .O.
In a preferred cooling method satisfying above Equation (4), primary cooling
is preferably started from the temperature at the completion of rolling at a cooling
rate of at least 400°C/sec and is preferably carried out in a temperature range of at
least 30°C at this cooling rate. The temperature range is preferably at least 60°C.
If a water cooling stop time which will be described later is not set, the temperature
range is further preferably at least 100°C. The cooling rate for the primary
cooling is more preferably at least 600°C/sec, particularly preferably at least
5 800°C/sec.
The primary cooling can be started after holding at the temperature at the
completion of rolling for a short length of time of at most 5 seconds. The time
from completion of the rolling to start of the primary cooling is preferably less than
0.4 seconds so as to satisfl above Equation (4).
10 Also, water cooling is preferably started at a cooling rate of at least
400°C/sec (preferably at least 600°C/sec, more preferably at least 800°C/sec), and
is carried out at this cooling rate in a temperature range of at least 30°C and at most
80°C, and then a water cooling stop period of at least 0.2 seconds and at most 1.5
seconds (preferably at most 1 second) is set, and during that period, the sheet shape
15 such as the sheet thickness or sheet width are measured, and after that, cooling
(secondary cooling) is carried out at a rate of at least 50°C/sec. Since feedback of
the sheet shape can be controlled by such sheet shape measurement, the
productivity is improved. During the water cooling stop period, the sheet may be
subjected to natural cooling or air cooling.
20 Industrially, the primary cooling and secondary cooling above are carried out
by water cooling.
When the cooling conditions for cooling from the temperature at the
completion of rolling to the temperature of (temperature at the completion of
rolling - 100°C) satisfy above Equation (4), the consumption of the strain by
25 recovery and recrystallization introduced to austenite as a result of the hot rolling,
can be suppressed as much as possible, as a result, the strain energy accumulated in
the steel can be used as a driving force for transformation from austenite to the
BCC phase to a maximum extent. A reason to make the cooling rate of the
primary cooling from the temperature at the completion of rolling at least
30 400°C/sec is also the same as above, that is, an increase in the transformation
driving force. Consequently, an amount of formed nuclei for transformation from
austenite to the BCC phase increases, thereby refining the structure of the hotrolled
steel sheet. By using a hot-rolled steel sheet having a fine structure
manufactured as described above for a starting material, the structure of the coldrolled
steel sheet can further be refined.
After the primary cooling, or the primary cooling and the secondary cooling
have been carried out as described above, structure control such as ferrite
transformation or precipitation of fine grains consisting of Nb and/or Ti may be
carried out by holding the temperature of the steel sheet in an desired temperature
range for an desired length of time before cooled to a coiling temperature. The
"holding" mentioned here includes natural cooling and retaining heat.
Considering the temperature and the holding time suitable for the structure control,
for example, natural cooling is carried out in a temperature range of 600°C to
680°C for around 3 to 15 seconds, which can introduce frne ferrite to the hot-rolled
sheet structure.
Subsequently, the steel sheet is cooled to the coiling temperature. For a
cooling method in this step, cooling can be carried out at a desired cooling rate by a
method selected from water cooling, mist cooling and gas cooling (including air
cooling). The coiling temperature for the steel sheet is preferably at most 650°C
from the standpoint of refining the structure with greater certainty.
The hot-rolled steel sheet manufactured by the above heat-rolling process
has a structure in which a sufficiently large number of high angle grain boundaries
has been introduced, an average grain diameter of grains defined by high angle
grain boundaries having a tilt angle of at least 15" is at most 6 pm and second
phases such as martensite and/or cementite are finely dispersed. As described
above, it is favorable that the hot-rolled steel sheet in which a large number of high
angle grain boundaries exists and the second phases are finely dispersed, is
subjected to cold rolling and annealing. This is because since these high angle
grain boundaries and fme second phases are preferred nucleus forming sites for
austenitic transformation, the structure can be refined by producing a large number
of austenite and recrystallized ferrite from these positions by rapid heating
annealing.
The structure of the hot-rolled steel sheet can be a ferrite structure containing
pearlite as a second phase, a structure consisting of bainite and martensite, or a
structure of a mixture thereof.
2-2: Heat treatment of hot-rolled steel sheet
The above hot-rolled steel sheet may be subjected to annealing at a
temperature of 500°C to 700°C. The annealing is particularly suitable for a hotrolled
steel sheet coiled at a temperature of at most 300°C.
The annealing can be carried out by a method in which a heat-rolled coil is
made to pass through a continuous annealing line or a method in which the coil is
put as it is in a batch annealing furnace. In heating the hot-rolled steel sheet, a
heating rate up to an annealing temperature of 500°C can be a desirable rate in a
range fkom slow heating of around 10°C/hour to rapid heating of 30°C/sec.
A soaking temperature (annealing temperature) is in a temperature range of
500°C to 700°C. A holding time in this temperature range does not need to be
specifically limited; however, the holding time is preferably at least 3 hours.
From the standpoint of suppressing coarsening of carbide, an upper limit of the
holding time is preferably at most 15 hours, more preferably at most 10 hours.
As a result of such annealing of the hot-rolled steel sheet, fine carbides can
be dispersed in the grain boundaries, the packet boundaries and the block
boundaries in the hot-rolled steel sheet, and carbides can further finely be dispersed
by a combination of the annealing and the above-described rapid cooling for an
extremely short length of time immediately after completion of hot rolling. As a
result, austenite nucleus forming sites can be increased during annealing to refine a
final structure. The annealing of the hot-rolled steel sheet also has the effect of
softening the hot-rolled steel sheet to decrease the load on the cold rolling
equipment.
2-3: pickling and cold rolling
The hot-rolled steel sheet manufactured by the method described above is
subjected to pickling and then to cold rolling. Each of the pickling and the cold
rolling may be carried out in a conventional manner. The cold rolling can be
carried out using lubricating oil. The cold rolling ratio does not need to be
specifically determined, but is normally at least 20%. If the cold rolling reduction
exceeds 85%, load on the cold rolling equipment becomes large, and thus, the cold
rolling ratio is preferably at most 85%.
5
2-4: Annealing
A cold-rolled steel sheet which is obtained by the above-described cold
rolling is subjected to annealing by heating at an average heating rate of at least
1 S°C/sec so that the unrecrystallization ratio of a region not transformed to
10 austenite at apoint of time of reaching (Acl point +lO°C) is at least 30%.
As described above, by heating up to (Acl point +lO°C) in a state in which
unrecrystallization structure remains, a large number of fine austenite nuclei to be
formed as the high angle grain boundaries and/or the second phases of the hotrolled
steel sheet as nucleus forming sites. Here, the hot-rolled steel sheet
15 preferably has a fine structure because a large number of nuclei can be formed.
The increase in the number of austenite nuclei formed enables significantly refining
austenite grains during the annealing, enabling refining ferrite, low-temperature
transformation phases and retained austenite, which are produced subsequently.
On the other hand, if the unrecrystallization ratio of the region not
20 ' transformed to austenite at the time of reaching (Acl point +lO°C) is less than 30%,
in most regions, austenitic transformation have been promoted after completion of
recrystallization. As a result, in such regions, austenitic transformation is
promoted from the grain boundaries of the recrystallized grains, and thus, the
austenite grains during annealing are coarsened and the final structure is also
25 coarsened.
Therefore, the average heating rate is at least 1 S°C/sec so that the
unrecrystallization ratio of the regions not transformed to austenite at the time of
reaching (Acl point + 10°C) becomes at least 30 area%. The average heating rate
is preferably at least 30°C/sec, further preferably at least 80°C/sec, particularly
30 preferably at least 100°C/sec. An upper limit of the average heating rate is not
specifically defined, but is preferably at most 1 OOO°C/sec to avoid temperature
control difficulty.
23
The above temperature for starting the rapid heating at a rate of at least
lS°C/sec may be any desired temperature if the recrystallization has not started
yet, and may be, T,-30°C relative to a the temperature for the start of softening
(the temperature for the start of recrystallization) T, measured under a heating rate
5 of 10°C/sec. The heating rate in the temperature range before such temperature is
reached can arbitrarily be determined. For example, even if rapid heating is
started fiom around 600°C, effect of sufficiently refining grain can be obtained.
Also, even if rapid heating is started fiom room temperature, nit does not have an
adverse effect on the cold-rolled steel sheet after annealing.
10 It is preferable to use electrical heating, resistance heating or induction
heating in order to obtain a sufficiently rapid heating rate, but as long as the abovedescribed
temperature increase conditions are satisfied, it is also possible to adopt
heating by a radiant tube. By using such a heating device, the time for heating a
steel sheet is greatly decreased, and it is possible to make annealing equipment
15 more compact, whereby effects such as a decrease in investment in equipment can
be expected. It is also possible to add a heating device to an existing continuous
annealing line or a hot-dip plating line to carry out the heating.
After heating to (Acl point + 10°C), heating is further carried out to an
annealing temperature (soaking temperature) in a range of at least (0.3 x Acl point
20 + 0.7 x Ac3 point) and at most (Ac3 point +lOO°C). The heating rate in this
temperature range can be any desired rate. Decreasing a heating rate can obtain
sufficient time to promote recrystallization of ferrite. Also, the heating rate can be
varied in such a manner that rapid heating (for example, at a rate that is the same as
that of the above rapid heating) is first carried out at any in the temperature range
25 and subsequently the heating rate is lowered.
In the annealing process, transformation to austenite is sufficiently promoted
to eliminate the deformed ferrite structure and dissolve carbides in the steel sheet.
Thus, the annealing temperature is at least (0.3 x Acl + 0.7 x Ac3 point). If
annealing is carried out at a temperature that is lower than that annealing
30 temperature, a single-phase austenite state is not achieved during the annealing or
recrystallization of ferrite does not occur, and as a result, deformed ferrite structure
retaining. In this case, in the texture of the cold-rolled steel sheet, the orientation
group fiom {100)<011> to (21 1)<011> becomes stronger, resulting in a decrease
in workability of the steel sheet. On the other hand, if annealing is carried out at a
temperature exceeding (Ac3 point +lOO°C), abrupt grain growth takes place,
resulting in coarsening of the final structure. Thus, the annealing temperature is at
most (Ac3 point +lOO°C), preferably (at most Ac3 point +50°C).
The Acl and Ac3 points in the present invention are values that can be
determined fiom a thermal expansion chart measured when the temperature of the
steel sheet which was cold rolled is heated to 1 100°C at a heating rate of 2"C/sec.
If an annealing holding time (soaking holding time) for the temperature
range is at most 30 seconds, dissolution of the carbides and transformation to
austenite are not sufficiently promoted, resulting in a decrease in workability of the
cold-rolled steel sheet. Also, temperature unevenness during the annealing easily
occurs, causing a problem in production stability. Therefore, it is necessary to
determine an annealing holding time of at least 30 seconds to sufficiently promote
transformation to austenite. An upper limit of the holding time is not specifically
determined; however, excessively-long time holding makes it difficult to satisfl a
final grain diameter of at most 5 pm, which is required in the present invention,
because of growth of austenite grains, and thus, the annealing holding time is
preferably less than 10 minutes.
Cooling after the soaking is carried out at a cooling rate of at least 10°C/sec
for a temperature range of at most 650°C and at least 500°C. Setting the cooling
rate for the temperature range to at least 10°C/sec can increase area fraction of low
temperature transformation phases in the structure of the cold-rolled steel sheet.
On the other hand, if the cooling rate is less than 10°C/sec, a large amount of ferrite
is formed during the cooling, resulting in deterioration in stretch flangeability.
Thus, the cooling rate for the temperature range after the annealing is at least
1 O°C/sec, preferably at least 20°C/sec.
During the cooling, overaging heat treatment or hot-dip plating (e.g., hot-dip
galvanizing or alloying hot-dip galvanizing) can be carried out. By controlling,
e.g., the soaking temperature and holding time, low temperature transformation
phases having an appropriate area fraction are formed in the cold-rolled steel sheet
and diffusion of carbon atoms to untransformed austenite is promoted to produce
retained austenite. Heat treatment conditions preferable for overaging are a
temperature range of 300°C to 500°C and a holding time range of 100 to 600
seconds.
Because of an excessively-low cooling rate and/or high-temperature and
long-time soaking, it is impossible to obtain a desired structure fiaction and
workability of the steel sheet deteriorates because of transformation of retained
austenite to carbides.
Thus, the holding time (including plating and/or averaging) during cooling is
preferably less than 2000 seconds. The cooling method can be various methods
such as gas, mist or water cooling.
Examples
Each ingot of steel types A to M each having the chemical composition
indicated in Table 1 was melted in a vacuum induction furnace. Table 1 indicates
15 Acl and Ac3 points for each of steel types A to M. These transformation
temperatures are determined fiom a thermal expansion chart measured when a steel
sheet subjected to cold rolling under the below-described manufacturing conditions
was heated to 1 100°C at a heating rate of 2OCIsec. Table 1 also indicates each
value of (Acl point + 10°C), (0.3 x Acl point + 0.7 x Ac3 point) and (Ac3 point
+1OO0C).
The resulting ingots underwent hot forging, and then they were cut to the
shape of slabs in order to subject them to hot rolling. These slabs were heated for
approximately one hour to a temperature of at least 1000°C and then hot rolling
was carried out at the hot temperature at the completion of rolling indicated in
Table 2, using a small test mill for trials. After the rolling completion, a hotrolled
steel sheet having a sheet thickness of 2.0 to 2.6 rnm was manufactured
under the cooling time, water cooling rate and coiling temperature conditions
indicated in the table.
The cooling after completion of the rolling were all water cooling and were
each carried out by any of the following methods:
1) carrying out only primary cooling for a temperature decrease amount of at least
100°C immediately after completion of the rolling;
2) carrying out only primary cooling for a temperature decrease amount of at least
100°C after holding (natural cooling) at the temperature at the completion of rolling
(FT) for a predetermined period of time; and
3) carrying out primary cooling immediately after completion of the rolling,
stopping the primary cooling when the relevant steel sheet was cooled by 30°C to
80°C fiom the temperature at the completion of rolling (FT), and held at the
temperature (allowed to naturally cool) for a predetermined length of time, and then
carrying outing secondary cooling.
The steel sheet was naturally cooled for 3 to 15 seconds after stoppage of
primary cooling if primary cooling was carried out alone, and after stoppage of
secondary cooling if secondary cooling was carried out, and subsequently was
water cooled at a cooling rate of 30°C to 100°C/sec to the coiling temperature.
Subsequently, the steel sheet was put in a furnace and subjected to slow cooling
simulated for coiling. A value of the left side of Equation (4) and an average
grain diameter of a BCC phase of the hot-rolled steel sheet are also indicated in
Table 2.
Measurement of an average grain diameter of a BCC phase in the hot-rolled
steel sheet was carried out by analyzing grain diameters of the BCC phase defined
by high angle grain boundaries having a tilt angle of at least 15" in a cross-section
of the structure of the steel sheet, the cross-section being parallel to a rolling
direction and the sheet thickness direction of the steel sheet, using an SEM-EBSD
apparatus (JSM-700 1F manufactured by EOL Ltd.). The average grain diameter
d of the BCC phase was obtained using following Equation (5). Here, Ai
represents the area of an i-th grain, and di represents a Heywood diameter of the i-
5 th grain.
For some of the hot-rolled steel sheets, hot-rolled plate annealing was carried
out under the conditions indicated in Table 2 using a heating h a c e .
Each of the hot-rolled steel sheets obtained as described above was subjected
10 to pickling using a hydrochloric acid and cold rolling at the rolling reduction
indicated in Table 2 in a conventional manner to make the steel sheet have a
thickness of 1.0 to 1.2 mm. Subsequently, using a laboratory scale annealing
equipment, annealing was carried out at the heating rate, the soaking temperature
(annealing temperature) and the soaking time (holding time) indicated in Table 2,
15 and cooling was carried out under a condition that makes the cooling rate for a
temperature range of fiom 650°C to 500°C become the "Cooling rate" indicated in
Table 2, whereby the resulting cold-rolled steel sheet was obtained. . Cooling after
the soaking was carried out using a nitrogen gas. Furthermore, in a'cooling
process, as indicated in Table 2, each steel sheet was subjected to any of heat
20 treatments indicated in A to I below, which are simulated for overaging or alloying
hot-dip galvanizing, and then cooled to room temperature at 2°C/sec, whereby the
resulting cold-rolled steel sheet was obtained. Conditions for these heat
treatments were indicated below.
A: Holding at 375°C for 330 seconds
25 B: Holding at 400°C for 330 seconds
C: Holding at 425°C for 330 seconds
D: holding at 480°C for 15 seconds, then cooling to 460°C for simulation of hotdip
galvanizing bath immersion, and further heating to 500°C for simulation of
alloying
E: Holding at 480°C for 60 seconds, then cooling to 460°C for simulation of hotdip
galvanizing bath immersion, and further heating to 520°C for simulation of
alloying
F: Holding at 480°C for 60 seconds, then cooling to 460°C for simulation of hotdip
galvanizing bath immersion, and further heating to 540°C for simulation of
alloying.
Table 2 indicates a proportion of an unrecrystallization of regions not
transformed to austenite in ferrite at the time of reaching (Acl point +lO°C). This
value was obtained by the following method. In other words, each steel sheet that
has been subjected to cold rolling according to the manufacturing conditions in the
present invention was heated to the temperature (Acl point + 10°C) at the heating
rate indicated in the relevant steel sheet number and then immediately cooled by
water cooling. The structure of the steel sheet was photographed using an SEM,
and on the structure photograph, the fractions of a recrystallization structure and a
deformed structure of each of regions except martensite, that is, regions other than
regions transformed to austenite at the time of reaching (Acl point +lO°C) were
measured to obtain the proportion of the unrecrystallization .
The microstructure and mechanical properties of each of the cold-rolled
steel sheets manufactured as described above were investigated as follows. The
results of the investigation are collectively indicated in Table 3.
An average grain diameter of ferrite and a grain diameter of retained
5 austenite having an aspect ratio of less than 5 in each cold-rolled steel sheet were
obtained using an SEM-EBSD equipment, by referring to a structure of a crosssection
in a rolling direction at a depth of 114 the sheet thickness of the steel sheet,
as in the case of the hot-rolled steel sheets. For an EBSD analysis of a structure
containing the retained austenite phase, the retained austenite is concernedly not
10 correctly measured because of disturbance at the time of sample preparation (e.g.,
transformation of retained austenite to martensite). Thus, in the present example,
the evaluation premise that an area fiaction of retained austenite obtained by an
EBSD analysis (yEBSD) satisfies (yEBSDIyXRD) > 0.7 relative to a volume
fiaction of retained austenite obtained by X-ray diftiactometry (yXRD) was
15 provided for an analysis accuracy index.
Area fractions of ferrite and the low temperature transformation phase were
obtained by a structure analysis using SEM-EBSD. Also, a volume ratio of the
austenite phase was obtained by means of X-ray diffiactometry using the laterdescribed
equipment to use the volume ratio as an area fiaction of retained
20 austenite (retainedy).
Measurement of a texture of each cold-rolled steel sheet was carried out by
X-ray diffiaction on a plane at a depth of 112 of the sheet thickness of a steel sheet.
Intensities in the { 100) <0 1 1 > to (2 1 1 )<0 1 1> orientation group were obtained
using ODF (orientation distribution function) obtained by analyzing the measured
25 results of pole figures of 12001, { 1 10) and (2 1 1 ) of ferrite. From the analysis
results, an ratio in intensity of each of { 100)<0 1 1 >, (4 1 1 ) <0 1 1 > and (2 1 1 )<0 1 1>
orientations relative to a random structure not having a texture was obtained, and
an average value of the ratios of the intensity was used as an average ratio of the
intensity in the { 100)<0 1 1> to (2 1 1 )<0 1 1> orientation group. X-ray intensities
30 of the random structure not having a texture were obtained by X-ray dieaction of
powdered steel. The apparatus used for X-ray diffiaction was RINT-2500HLlPC
manufactured by Rigaku Corporation.
The mechanical properties of each cold-rolled steel sheet after annealing
were investigated by a tensile test and a hole expanding test. The tensile test was
carried out using a JIS No. 5 tensile test piece to determine a tensile strength (TS)
and elongation at rupture (total elongation, El). The hole expanding test was
carried out in conformity of JIS Z 2256:2010 to determine a percent hole expansion
h (%). A value of TSxEl was calculated as an index for balance between the
strength and the ductility, and a value of TSxh was calculated as an index for
balance between the strength and the stretch flangeability. The respective values
are indicated in Table 3.
[Table 3-21
2) Texture = avarage X-ray intensity of a (100) 4 1 1s to (21 1 )CO11> orientations
An underline means that the relevant steel type or value falls outside the scope of the present invention.
Of steel sheet Nos. l'to 10 manufactured from steel type A, in steel sheets
Nos. 1 to 3 and 6, 7, 9 and '1 0 in which heating rates during annealing are at least
1 5"C/sec, a cold-rolled steel sheet having a microstructure according to the present
invention was each obtained. In particular, in steel sheets Nos. 1 to 3,6 and 7
which use a frne grain hot-rolled steel sheet satisfying the conditions for cooling
after hot rolling in Equation (4) as a base material, finer retained austenite grains
were obtained compared to steel sheets Nos. 9 and 10 not satisfying Equation (4).
On the other hand, in steel sheet No. 4, the soaking temperature during
annealing was high, and in steel sheets Nos. 5 and 8, the heating rate during
annealing was low, resulting in the proportion of retained lump-like y having an
aspect ratio of less than 5 relative to retained austenite (retained y) becoming less
than SO%, and thus, above Equation (2) was not satisfied and the grain diameters of
ferrite, which is a second phase, were coarse.
Results similar to the above were obtained for the other steel types, and high
workability was obtained in each of the examples of the invention.
On the other hand, in steel sheets Nos. 5, 8, 1 1, 14, 16, 19,22,25,28, 33, 35,
37,41,43,48, 50 and 52, the heating rate during annealing was less than lS°C/sec,
5 and thus, the proportion of the unrecrystallization at Acl+lOOC was less than 30%.
Thus, the microstructure of the cold-rolled steel sheet coarsened and the average
grain diameter of ferrite exceeds the upper limit specified in the present invention.
As a result, the mechanical properties were insufficient.
In steel sheets Nos. 4 and 30, rapid heating was carried out during annealing,
10 but since the annealing temperature exceeded Ac3+1000C, the microstructure of the
cold-rolled steel sheet coarsened and the grain diameter of ferrite exceeded the
upper limit specified in the present invention. As a result, good mechanical
properties were not obtained.
Since for steel sheet No. 27, the annealing temperature was lower than (0.3 x
15 Acl + 0.7 x Ac3), and as a result, the texture did not satisfy the requirement of the
present invention. Accordingly, good mechanical properties could not be
obtained.
For steel sheets No. 46 and 47, which have an Nb content of 0.123%, the
steel was excessively hardened and thereby the workability deteriorated. As a
20 result, the mechanical properties of the cold-rolled steel sheet were low irrespective
of the heating rate.
Furthermore, in steel sheets Nos. 48 and 49, which have an Si content of
0.06%, no retained austenite was produced in the cold-rolled steel sheet. Thus,
the ductility remained low regardless of the low strength. As a result, the
25 mechanical properties of the cold-rolled steel sheet were low irrespective of the
heating rate.
-N
We claim-:
1. A cold-rolled steel sheet characterized by having:
a chemical composition comprising, in mass% of C: 0.06 to 0.3, Si: 0.6 to
5 2.5%, Mn: 0.6 to 3.5%, P: at most 0.1%, S: at most 0.05%, Ti: 0 to 0.08%, Nb: 0 to
0.04%, total of Ti and Nb: 0 to 0.10%, sol.Al: 0 to 2.0%, Cr: 0 to I%, Mo: 0 to
0.3%, V: 0 to 0.3%, B: 0 to 0.005%, Ca: 0 to 0.003%, REM: 0 to 0.003% and the
remainder of Fe and impurities;
a microstructure having a main phase of either or both of martensite and
10 bainite which comprising at least 40 area% in total; and
a texture in which proportion of an average X-ray intensity for
{ 100)<0 1 1> to {2 1 1 ) orientations relative to the average X-ray intensity of
a random structure which does not have a texture at a depth of 112 of a sheet
thickness is less than 6.
15
2. The cold-rolled steel sheet as set forth in claim 1, wherein the
microstructure has a second phase of ferrite which comprises at least 3 area% and
satisfies Equation (1):
dF 5 4.0 ... (1)
20 where dp is an average grain diameter (unit: pm) of ferrite defined by high
angle grain boundaries having a tilt angle of at least 15'.
3. The cold-rolled steel sheet as set forth in claim 1 or 2, wherein the
microstructure has a second phase of retained austenite which comprises at least 3
25 area% and satisfies Equations (2) and (3):
dAs 5 1.5 . . . (2); and
r~~ 2 50 ... (31,
where dA, is an average grain diameter (unit: pm) of retained austenite
having an aspect ratio of less than 5 and r,+ is an area fraction (%) of the retained
30 austenite having an aspect ratio of less than 5 relative to all retained austenite.
4. The cold-rolled steel sheet as set forth in any of claims 1 to 3, wherein
the chemical composition comprises one or two elements selected fiom, in mass%
Ti: 0.005 to 0.08% and Nb: 0.003 to 0.04%
5 5. The cold-rolled steel sheet as set forth in any of claims 1 to 4, wherein
the chemical composition comprises, in mass%, sol.Al: 0.1 to 2.0%.
6. The cold-rolled steel sheet as set forth in any of claims 1 to 5, wherein
the chemical composition comprises one or more elements selected from, in mass%
10 Cr; 0.03 to 1%, Mo: 0.01 to 0.3% and V: 0.01 to 0.3%.
7. The cold-rolled steel sheet as set forth in any of claims 1 to 6, wherein
the chemical composition comprises, in mass%, B: 0.0003 to 0.005%.
15 8. The cold-rolled steel sheet as set forth in any of claims 1 to 7, wherein
the chemical composition comprises one or two elements selected from , in mass%,
Ca: 0.0005 to 0.003% and REM: 0.0005 to 0.003%.
9. The cold-rolled steel sheet as set forth in any of claims 1 to 8,
20 comprising a plating layer on a sheet surface.
10. Process for manufacturing a cold-rolled steel sheet characterized by
comprising the following steps (A) and (B):
(A) a cold rolling step in which a hot-rolled steel sheet having a chemical
25 composition as set forth in any of claims 1 and 4 to 8 is cold rolled to obtain a
cold-rolled steel sheet; and
(B) an annealing step in which the cold-rolled steel sheet obtained in the
step (A) is heated under conditions that the cold-rolled steel sheet is heated at an
average heating rate condition of at least 1 5OCIsec so that an proportion of an
30 unrecrystallization of a region not transformed to austenite at a time of reaching
(Acl point +lO°C) becomes at least 30 area%, and is then held in a temperature
range of at least (0.3 x Acl point + 0.7 x Ac3 point) and at most (Ac3 point +lOO°C)
for at least 30 seconds, and the steel sheet is then cooled at an average cooling rate
of at least 10°C/sec for a temperature range of at most 650°C and at least 500°C.
11. The process for manufacturing a cold-rolled steel sheet as set forth in
5 claim 10, wherein after completion of hot rolling, the hot-rolled steel sheet is coiled
at a temperature of at most 300°C and then subjected to heat treatment & a
temperature range of 500°C to 700°C.
12. The process for manufacturing a cold-rolled steel sheet as set forth in
10 claim 10 or 1 1, wherein the hot-rolled steel sheet is a steel sheet in which average
grain diameter of a BCC phase defined by high angle grain boundaries having a tilt
angle of at least 15" is at most 6 p, the steel sheet being obtained by a hot rolling
step of cooling at a cooling rate (Crate) satisfying following Equation (4) for a
temperature range fiom a temperature at the completion of rolling to (temperature
15 at the completion of rolling -1 00°C) after completion of hot rolling in which hot
rolling is completed at at least Ar3 point:
where Crate (T) is a cooling rate ("CIS) (positive value),
T is a relative temperature ("C, negative value) with the temperature at the
20 completion of rolling as zero, and
if there is a temperature at which Crate is zero, a value obtained by dividing
a holding time (At) at the temperature by IC (T) is added as an integral for the
section.
25 13. The process for manufacturing a cold-rolled steel sheet as set forth in
claim 12, wherein the cooling in the temperature range includes starting cooling at
a cooling rate of at least 400°C/sec and cooling at the cooling rate in a temperature
range of at least 30°C.
14. The process for manufacturing a cold-rolled steel sheet production as
set forth in claim 12, wherein the cooling in the temperature range includes starting
water cooling at a cooling rate of at least 400°C/sec and cooling at the cooling rate
for a temperature section of at least 30°C and at most 80°C, and then stopping a
5 water cooling stop time of 0.2 to 1.5 seconds to measure a shape of the sheet during
the time, and subsequently cooling at a rate of at least 50°C/sec.
15. The process for manufacturing a cold-rolled steel sheet production as
set forth in any of claims 10 to 14, fi.u-ther comprising the step of plating the cold-
10 rolled steel sheet after the step (B).