COLD-ROLLED STEEL SHEET AND PROCESS FOR
MANUFACTURING SAME
Technical Field
5 The present invention relates to a cold-rolled steel sheet and process for
manufacturing the same. ~articularl~,.tphree sent invention relates to a cold-rolled
steel sheet having excellent workability in addition to a high strength, and a process
for manufacturing the same.
10 Background Art
Conventionally, refining structure has been studied as a method for
improving the mechanical properties of cold-rolled steel sheets.
Patent Document 1 indicated below discloses a cold-rolled steel sheet having
a structure including a low-temperature transformation phase consisting of one or
15 more of ferrite, martensite, bainite and retained y (retained austenite), in which the
volume fiaction of the low-temperature transformation phase being 10 to 50% and
an average grain diameter of the low-temperature transformation phase is at most 2
pm..
Patent Document 2 indicates a method in which a cold-rolled steel sheet is
20 manufactured using a hot-rolled steel sheet manufactured by hot rolling followed by
cooling in a short length of time after the hot rolling. For example, Patent
Document 2 discloses that a hot-rolled steel sheet having a.microstructure
containing ferrite having a small average grain diameter as a main phase is
manufactured by carrying out cooling to at most 720°C at a cooling rate of at least
25 400°C/sec within 0.4 seconds after hot rolling and the hot-rolled steel sheet is
subjected to usual cold rolling and annealing.
Prior Art Documents
Patent Document 1 : Japanese Patent Laid-Open No. 2008-23 1480
30 Patent Document 2: International Publication No. W02007/0 1554 1, pamphlet
Summary of Invention
Patent Document 1 discloses that a cold-rolled steel sheet having a fine
structure is obtained. ow ever, in order to refme structure, it is necessary to
contain one or more of Ti, Nb and V, which are precipitating elements. If the steel
sheet contains a large amount of such precipitating elements, the ductility of the
5 steel sheet is deteriorated, and thus, it becomes difficult to guarantee excellent
ductility and thus excellent workability far the cold-rolled steel sheet disclosed in
Patent Document 1.
In this regard, according to the method disclosed in Patent Document 2,
structure can be refined without containing precipitating elements and thus a cold- .
10 rolled steel sheet having excellent ductility can be manufactured. The
manufactured cold-rolled steel sheet 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, austenite manufactured fiom the
hot-rolled steel sheet also becomes fme and therefore a cold-rolled steel sheet
15 having a fine structure can be obtained. However, since the method of the
annealing after cold rolling is a usual one, recrystallization occurs in a heating step
during the annealing, and after completion of the recrystallization, austenitic
transformation occurs with grain boundaries in the. structure after the
recrystallization as nucleus forming sites. In other words, after most preferred
20 nucleus forming sites for austenitic 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,
austenitic transformation occurs. Accordingly, although a cold-rolled steel sheet
obtained by the method disclosed in Patent Document 2 has a fine structure,
25 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, where annealing'is carried out for a single-phase austenite
region, it is difficult to utilize the fine structure of the hot-rolled steel sheet fin
30 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
large amount of precipitating elements such as Ti and Nb is not added, and a
process for manufacturing the same.
The present inventors employed a composite-structure having a main phase
of ferrite in order to obtain a structure for providing excellent ductility and stretch
flangeability in addition to high strength and a second phase containing a lowtemperature
transformation phase to guarantee the strength of the steel sheet and
retained austenite to obtain effect of increasing a ductility due to transformation
induced plasticity.
Furthermore, generally, a decrease in stretch flangeability (hole expanding
formability) is concerned for a structure containing a soft phase such as ferrite and a
hard phase such as low temperature transformation phase or retained austenite
intermixed therein, and thus, investigation is carried out based on the material
quality design concept that such decrease in stretch flangeability is minimized by
refining ferrite and hard phase 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
conventional annealing method in which austenitic transformation is promoted after
completion of recrystallization, and conducted test.
As a result, the present inventors obtained the following new knowledge.
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
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 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 that the refining is premised on performing austenitic
transformation from the structure after recrystallization.
2) A steel sheet obtained by the annealing method in which austenitic
5 transformation is promoted before completion of recrystallization in an annealing
step 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 prior-austenite
grain boundaries, the packet boundaries and the block boundaries increases and a
10 retained austenite produced among laths of bainite and/or martensite decreases.
Such lump-like retained austenite exists in grain boundaries on which stress is
easily concentrated when working the steel sheet compared to retained austenite
formed among the laths of bainite and/or martensite. Thus, since ductility can
effectively be increased due to transformation induced plasticity, ductility of the
15 steel sheet is increased.
In general, stretch flangeability in structures in which a soft phase, such as
ferrite, and retained austenite are intermixed, may concernedly decrease. However,
as stated above, in the structure of a cold-rolled steel sheet after annealing, since the
ferrite, the low-temperature transformation phase and the retained austenite &e
20 effectively refined, the decrease in stretch flangeability is prevented. Thus,
excellent stretch flangeability can also be guaranteed.
3) 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
25 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
30 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 hrther refining
austenite grains in the annealing step and fbrther refining the ferrite, the lowt
temperature transformation phase and the retained austenite of the structure of the
cold-rolled steel sheet after the annealing.
The present inventors found that as a result of the above structure refinement,
5 ductility of the cold-rolled steel sheet and the balance between the ductility and the
stretch flangeability is significantly improved.
The present invention based on the above new findings provides a cold-rolled
steel sheet including a chemical composition consisting in mass% C: 0.06 to 0.3%,
Sk0.4 to 2.5%, Mn: 0.6 to 3.5%, P: at most O.l%, S: at most 0.05%, Ti: 0 to 0.08%,
10 Nb: 0 to 0.04%, a total content of Ti and Nb: 0 to 0.10%, sol.Al: 0 to 2.0%, Cr: 0 to
l%, 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 a remainder of Fe and impurities; a microstructure having a main
phase of at least 40 area% ferrite, and a second phase of a low-temperature
transformation phase consisting of either or both of martensite and bainite at least
15 10 area% in total and retained austenite at least 3 area%, the microstructure
satisfying Equations (1) to (4):
dF I 5.0 ... (1);
dhl+~52.0 ...( 2);
dAs I 1.5 ... (3); and
r~~ 2 50 --.(4 1,
where dF is an average grain diameter (pm) of ferrite defined by high angle
grain boundaries having a tilt angle of at least 15';
dM+B is an average grain diameter (pm) of the low-temperature
transformation phase;
25 dAs is an average grain diameter (pm) of retained austenite having an aspect
ratio of less than 5; and
rAs is an area fraction (%) of retained austenite having an aspect ratio of less
than 5 relative to all the retained austenite.
A main phase in the microstructure means a phase having a largest area
30 fraction, and the second phase means any of phases and structures other than the
main phase. Each of the average grain diameters means an average Heywood
diameter value obtained according to Equation (6), which will be described later,
using SEM-EBSD.
It is preferable that the cold-rolled steel sheet according to the present
invention further include one or more features of (1) to (7) below.
(1) The cold-rolled steel sheet has a texture in which ratio of an average X-ray
intensity for the { 100)<0 1 1> to (2 1 1 }<0 1 1> orientations relative to the average Xray
intensity of a random structure which does not have a texture at a depth of 112 of
the sheet thickness is less than 6.
(2) The chemical composition contains one or two selected fiom, in mass%, Ti:
0.005 to 0.08% and Nb: 0.003 to 0.04%.
(3) The chemical composition contains, in mass%, sol.Al: 0.1 to 2.0%.
(4) The chemical composition contains one or more selected fiom, in mass%, Cr:
0.03 to 1%, Mo: 0.01 to 0.3% and V: 0.01 to 0.3%.
(5) The chemical composition contains, in mass%, B: 0.0003 to 0.005%.
(6) The chemical composition contains one or two selected from, in mass%, Ca:
0.0005 to 0.003% and REM: 0.0005 to 0.003%.
(7) The cold-rolled steel sheet has a plating layer on the surface.
Another aspect of the present invention provides a process for manufacturing
the above-described 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 of at least 15"CIsec 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 (0.9 x Acl point + 0.1 x Ac3 point) and at most (Ac3
point + 100°C) for 30 seconds.
Here, the Acl point and the Ac3 point are transformation points determined
from a thermal expansion chart measured when the temperature of the steel sheet is
heated at a heating rate of 2"CIsec.
It is preferable that the process for manufacturing the cold-rolled steel sheet
5 according to the present invention provides one or more of following features (8) to
(12).
(8) The hot-rolled steel sheet is obtained from 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 (9) The hot-rolled steel sheet is a steel sheet whose 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 Equation (5) below for a temperature range from the
completion of rolling to (temperature at the completion of rolling - 100°C) after
15 completion of hot rolling in which hot rolling is completed at at least an AT3 point.
1 ~ ( ~ ) = 0 . 1 - 3 ~ 1. T0 +- ~~ X ~ O - ~ .-T5~~1 0-'- T3+ 5 ~ 1 0-- ~T -~7~ 10-l1-T 5
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 ("C, negative value), and
20 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.
(10) The cooling for the temperature range in above (9) 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.
25 (1 1) The cooling for the temperature range in above (9) includes starting water
cooling at a cooling rate of at least 400°C/sec and cooiing 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 the stopping
water cooling, and subsequently cooling at a rate of at least 50°C/sec.
(12) The process for manufacturing the cold-rolled steel sheet hrther has the step
of plating the cold-rolled steel sheet after the Step (B).
The present invention provides effectively refining a structure after cold
rolling and annealing without addition of a large amount of elements which
5 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, a fine structure can effectively be
obtained even for performing annealing of a single-phase austenite region and a fine
10 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
15 for manufacturing the same will be described below. In the below description,
each of "%"s in chemical compositions is "mass%" unless particularly notified.
Also, each of average grain diameters in the present invention means an average
Heywood diameter value obtained according to Equation (5), which will be
described later, using'SE~-EBSD.
20 1. Cold-rolled steel sheet
1-1 : Chemical composition
[C: 0.06 to 0.3%]
C has the effect of enhancing the strength of steel. Also when C is
concentrated in austenite, C has the effect of obtaining the stable austenite,
25 increasing the fraction of retained austenite in the cold-rolled steel sheet and thereby
increasing the ductility of steel. Furthermore, in the annealing step, a temperature
range of at least (Acl point + 10°C) can easily be reached while maintaining a state
with the high percentage of an unrecrystallization by rapid heating, due to the effect
of C by which recrystallization of ferrite is suppressed in the course of temperature
30 increase, and the microstructure of the resulting cold-rolled steel sheet is refined.
Furthermore, since C has the effect of lowering an A3 point, in the hot rolling
process, hot rolling can be completed in a lower-temperature range to easily refme
the microstructure of the hot-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%. Ifthe C content
exceeds 0.3%, there is a marked decrease in workability and weldability.
Accordingly, the C content is made at most 0.3%. Preferably it is at most 0.25%.
[Si: 0.4 to 2.5%]
Si has the effect of promoting formation of low-temperature transformation
phases such as martensite and bainite, and thereby increasing the strength of the
steel. Si also has the effect of promoting formation of retained austenite and
thereby increasing the ductility of the steel. If the Si content is less than 0.4%, it is
difficult to obtain the above-described effects. Therefore, the Si content is at least
0.4%, preferably at least 0.6%, Mher preferably at least 0.8%, particularly
preferably at least 1.0%. On the other hand, if the Si content exceeds 2.5%, a
substantial ductility decrease may occur or platability may be deteriorated.
Accordingly, the Si content is at most 2.5%, preferably at most 2.0%.
[Mix 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,
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 above-described
effects. Therefore, the Mn content is 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 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 boundaries. If the P content exceeds 0.194, the
embrittlement due to the above action may become marked. Therefore, the P
content is at most 0.1%, preferably, at most 0.06%. Since the P content is
preferably as low as possible, there is no need to provide a lower limit; however,
fiom the standpoint of cost, the P content 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
5 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.
Therefore, the S content is made at most 0.05%, preferably at most 0.008%, W e r
preferably at most 0.003%. Since the S content is preferably as low as possible,
there is no need to provide a lower limit; however, fiom the standpoint of cost, the S
10 .content is preferably at least 0.00 1 %.
[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 steel
15 may contain either or both of these elements. However, if the content of each
element exceeds the above upper limit or the total content exceeds the above upper
limit, the above-described effect is saturated, which results in a disadvantage in
costs. Therefore, the content of each element and the total content are set as above.
The Ti content is preferably at most 0.05%, hrther preferably at most 0.03%. The
20 Nb content is preferably at most 0.02%. Also, the total content of Nb and Ti is
preferably at most 0.05%, further preferably at most 0.03%. In order to obtain the
above-described effect of these elements with greater certainty, it is preferably to
satisfy either of the conditions of Ti: at least 0.005% and Nb: at least 0.003%.
[sol.Al: 0 to 2.0%]
25 A1 has the effect of increasing the ductility of steel. Accordingly, A1 may be
contained. However, since A1 has the effect of increasing an AT3 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
30 structure of a cold-rolled steel sheet. In 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 Al with greater certainty, 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,
5 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
excessively be suppressed, and as a result, it is impossible to ensure a desired
10 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%.
Also, in order to obtain the above effects with greater certainty, it is preferable to
15 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
20 of the steel. Therefore, B may be contained. However, if the B content exceeds
0.005%, the steel may excessively hardens, which can result in a significant
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%]
2 5 Ca and REM each have the effect of refining oxides and nitrides precipitated
during solidification of molten steel and thereby increasing the soundness of a slab.
Therefore, one or more of these elements may be contained. However, each of
these elements is expensive, and thus, the content of each element is made at most
0.003%. The total content of these elements is preferably at most 0.005%. In
30 order to obtain the above-described effects with greater certainty, at least 0.0005%
Ca or REM is preferably contained.
Here, REM includes a total of 17 elements including, Sc, Y and lanthanoids,
and industrially, lanthanoids are generally added in the form of misch metal. The
content of REM in the present invention refers to a total content of these elements.
The remainder other than the above is Fe and impurities.
5 1-2: Microstructure and texture
[Main phase]
A main phase includes at least 40 area% ferrite and satisfies Equation (1)
above.
Employment of soft ferrite for the main phase can increase the ductility of the
10 cold-rolled steel sheet. Furthermore, an average grain diameter dF of ferrite
defined by high angle grain boundaries having a tilt angle of at least 15" satisfies
Equation (I), whereby a hard second phase finely disperses on the grain boundaries
of the ferrite, and the formation of fine cracks at the time of working of the steel
sheet is suppressed . Also, concentration of stress on the edges of the fine cracks
15 is reduced by refining ferrite, which can suppress the development of the cracks.
As a result, the stretch flangeability of the cold-rolled steel sheet is increased.
If the area fraction of the ferrite is less than 40%, it is difficult to guarantee
excellent ductility. Therefore, the area fraction of the ferrite is at least 40%. The
area fraction of the ferrite is preferably at least 50%.
20 If the average grain diameter dF of ferrite defmed by the high angle grain
boundaries having a tilt angle of at least 15" does not satisfy above Equation (I),
the second phase does not uniformly disperse, and thus it is difficult to guarantee
excellent stretch flangeability. Therefore, the average grain diameter dF of the
ferrite is set to satisfy above Equation (1). The value of dF preferably satisfies
25 following Equation (1 a).
dFS4.0 ...( la)
The average grain diameter dF of ferrite surrounded by the high angle grain
boundaries having a tilt angle of at least 15" is used as an index because small angle
grain boundaries having a tilt angle of less than 15" are low-energy interfaces
30 having a small difference in orientation between adjoining grains, as a result, it
becomes difficult that the second phase precipitates, the effect of finely dispersing
the second phase decreases, and contribution to stretch flangeability increasing
becomes little.
Hereinafter, the average grain diameter of ferrite defmed by the high angle
grain boundaries having a tilt angle of at least 15' is simply referred to as the
average grain diameter of ferrite. In the present invention, the average grain
diameter of ferrite is at most 5.0 pm, preferably at most 4.0 p.m.
[Second phase]
The second phase contains a low-temperature transformation phase
consisting of either or both of rnartensite and bainite at least 10 area% in total and
retained austenite at least 3 area%, and satisfies above Equations (2) to (4).
Containing a hard phase or structure formed by low-temperature
transformation, such as martensite andlor bainite in the second, which can increase
strength of the steel. Also, since retained austenite has the effect of increasing the
ductility of the steel sheet, increasing area fiaction of retained austenite can
guarantee excellent ductility. Furthermore, as a result of the low-temperature
transformation phase and the retained austenite being fme enough to satis@ above
Equations (2) and (3), the formation and development of fme cracks is suppressed
when working the steel sheet and the stretch flangeability of the steel sheet is
increased. Furthermore, since lump-like retained austenite having an aspect ratio
of less than 5 often exists on the grain boundaries, stress concentration during
working can effectively be reduced. Accordingly, by satis@ing Equation (4),
ductility (in particular, uniform elongation) of the steel sheet can significantly be
increased.
If the total area fraction of the low-temperature transformation phase
consisting of either or both of rnartensite and bainite is less than lo%, it is difficult
to guarantee high strength. Therefore, the total area fraction of the lowtemperature
transformation phase is set at least 10%. The low-temperature
transformation phase does not need to contain both of martensite and bainite, and
may contain only either of them. Also bainite includes bainitic ferrite.
Also, if an average grain diameter dM+Bof the low-temperature
transformation phase (rnartensite andlor bainite) does not satisfy above Equation (2),
it is difficult to suppress the formation and development of fine cracks during
stretch flanging, and thus it is difficult to guarantee excellent stretch flangeability.
Therefore, the average grain diameter dM+Bo f the low-temperature transformation
phase needs to satisfy above Equation (2). The value of dM+Bp referably satisfies
following Equation (2a):
5 d + 1 6 ...( 2a)
If the area fraction of the retained austenite is less than 3%, it is difficult to
guarantee excellent ductility. Therefore, the area fraction of the retained austenite
is at least 3%, preferably at least 5%.
If an average grain diameter dAs of lump-like retained austenite having an
10 aspect ratio of less than 5 does not satisfy above Equation (3), coarse lump-like
martensite is formed by transformation of the retained austenite at the time of
working of the steel sheet, and as a result, stretch flangeability of the steel decreases.
Therefore, the average grain diameter dAs of retained austenite having an aspect
ratio of less than 5 needs to satisfy above Equation (3). The value of dA,
15 preferably satisfies above Equation (3a).
dAs51.0 ...( 3a)
If an area fraction rAs of the retained austenite having an aspect ratio of less
than 5 relative to all the retained austenite does not satisfy Equation (4), ductility
hardly increases. Therefore, the area fraction rAS of the retained austenite having
20 an aspect ratio of less than 5 relative to all the retained austenite needs to satisfy
Equation (4). The value of rAs preferably satisfies following Equation (4a).
rAsr60 ...( 4a)
By satisfying Equations (3) and (4), it is possible to exhibit effect of
increasing ductility to the maximum extent and suppress decreasing the stretch
25 flangeability (hole expansibility) to the minimum extent.
Here, if the second phase may be contaminated by pearlite andlor cementite,
a total area fraction of them should be at most 10%.
The average grain diameter DF of ferrite is determined by obtaining an
average grain diameter of ferrite surrounded by high angle grain boundaries having
30 a tilt angle of at least 15' using SEM-EBSD. SEM-EBSD refers to a method for
measuring an orientation of a minute region by electron backscatter diffraction
(EBSD) in a scanning electronic microscope (SEM). An obtained orientation map
is analyzed to calculate an average.grain diameter. The average grain diameters of
the low-temperature transformation phase and the retained austenite having an
aspect ratio of less than 5 can be calculated by a method similar to the above.
Further, the area fiactions of the ferrite and the low-temperature
5 transformation phase are also measured using SEM-EBSD. For the area fraction
of the retained austenite, the volume fraction of the austenite obtained by X-ray
diffractometry is used as the area fraction as it is.
In the present invention, for each of the average grain diameters and the area
fractions above, a value of measurement at a depth of 114 of a sheet thickness of the
10 steel sheet is employed.
[Texture]
The cold-rolled steel sheet according to the present invention preferably has a
texture where ratio of an average of X-ray intensities for {100)<011> to
(21 1 ) orientations relative to an average of X-ray intensities of a random
15 structure not having texture is less than 6 at a depth of 112 of the sheet thickness.
If growth of a texture of {100)<011> to (21 1)<011> orientations is
suppressed, the workability of the steel is increased. Thus, by reducing an X-ray
intensity ratio of the orientations, the workability of the steel is increased. The
ratio of an average X-ray intensity of the orientations relative to an average X-ray
20 intensity of random structure not having texture is set to less than 6, and ductility
and stretch flangeability can hrther increase. Therefore, the ratio of the average
X-ray intensity of the orientations relative to the average X-ray intensity of the
random structure not having texture is preferably less than 6. The ratio is more
preferably less than 5, most preferably less than 4. Here, {hkl) of a texture
25 represents an crystal orientation in which 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 of the sheet thickness using
30 hydrofluoric acid and subsequently measuring pole figures of the (2001, { 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 texture are
determined by carrying out measurement similar to the described above using a
5 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
10 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-Al-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
15 form a suitable chemical conversion treatment coating on the plating 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
20 2-1 : Hot rolling and cooling after rolling
In the present invention, the structure of the cold-rolled steel sheet is refined
by the later-described annealing, and thus, for a hot-rolled steel sheet provided for
cold rolling may be carried out in a conventional manner. However, in order to
hrther refine the structure of the cold-rolled steel sheet, it is preferable to refine the
25 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 15'
and refined dispersion of the second phase such as cementite and/or martensite.
When a hot-rolled steel sheet having a fine structure is subjected to cold
30 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, it is facilitated to refine the final structure.
In the present invention, a hot-rolled steel sheet that is preferable for a
starting material for a cold-rolled steel sheet is specifically has an average grain
5 diameter of the BCC phase defined by high angle grain boundaries having a tilt
angle of at least 1 5 O , namely at most 6 pm. The average grain diameter of the
BCC phase is further preferably at most 5 pm. This average grain diameter can
also be obtained by SEM-EBSD.
If the average grain diameter of the BCC phase in the hot-rolled steel sheet is
10 at most 6 pm, the cold-rolled steel sheet can further be refmed to M e r 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 pm. The BCC
phase mentioned here may include ferrite, bainite and martensite, and consists of
15 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 fme structure can be manufactured by
carrying out hot rolling and cooling by t&e method described below.
20 A slab having the above-described chemical composition is manufactured by
continuous casting, and is provided for hot rolling. Here, the slab can be used with
a high temperature during 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
25 least 1000°C. 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 temperature of the slab is preferably sufficiently high so that hot
30 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 t the standpoint of industrial productivity, it is preferable to use a tandem mill
for at least the final 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
5 in the sheet thickness when the slab temperature is in the temperature range from
the AT3 point to (Ar3 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
10 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
15 method described in detail below.
Cooling fiom the temperature at the completion of rolling is preferably
carried out at a cooling rate (Crate) satisfying Equation (4) below in a temperature
range fiom the temperature at the completion of rolling to (temperature at the
completion of rolling - 100"~).
Equation (5) 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.
25 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 from 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
30 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 -100°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
5 100°C before disappearance of a certain amount of strain energy. The value of the
right side of Equation (5) is preferably 3.0, more preferably 2.0, further preferably
1 .o.
In a preferred cooling method satisfying above Equation (5), primary cooling
is preferably started fiom the temperature at the completion of rolling at a cooling
10 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 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 800°C/sec.
15 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
fiom completion of the rolling to start of the primary cooling is preferably less than
0.4 seconds so as to satisfy above Equation (5).
~ l s op;r eferably, water cooling is started at a cooling rate of at least
20 400°C/sec immediately after the rolling completion, and cooling is carried at this
cooling rate for a temperature range of at least 30°C and at most 80°C, and then a
water cooling stop time of 0.2 to 1.5 seconds (preferably at most 1 second) is set,
and during that time, the sheet shape such as the sheet thickness or sheet width are
measured, and then cooling (secondary cooling) is carried out at a rate of at least
25 50°C/sec. Since feedback of the sheet shape can be controlled by such sheet shape
measurement, the productivity is improved. The water cooling stop time is
preferably at most 1 second. During the water cooling stop time, the sheet may be
subjected to natural cooling or air cooling.
Industrially, the primary cooling and secondary cooling above are carried out
30 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 (9, the consumption of the strain by 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 fiom austenite to the BCC
phase to a maximum extent. A reason to make the cooling rate of the primary
cooling fiom the temperature at the completion of rolling at least 400°C/sec is also
the same as above, which is an increase in the transformation driving force.
Consequently, an amount of formed nuclei for transformation fiorn austenite to the
BCC phase increases, thereby refming the structure of the hot-rolled 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 cold-rolled steel sheet can firther
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 fme grains consisting of Nb andlor 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 fine 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 fiom 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 suficiently 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 15O is at most 6 p and second phases such
as martensite andlor 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
-xboundaries
exists and the'second phases are fmely 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
5 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
10 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
15 put as it is in a batch annealing hrnace. 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 fiom slow heating of around 10°C/hour to rapid heating of 30°C/sec.
Annealing temperature (soaking temperature) is in a temperature range of
500°C to 700°C. A holding time in this temperature range does not need to be
20 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
25 boundaries in the hot-rolled steel sheet, and carbides can hrther 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
30 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
5 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%.
2-4: Annealing
A cold-rolled steel sheet which is obtained by the above-described cold
10 rolling is subjected to annealing by heating at an average heating rate of at least
1 5"CIsec so that the unrecrystallization ratio of a region not transformed to austenite
at a point 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 fme austenite nuclei to be
15 formed as the high angle grain boundaries andlor the second phases of the hot-rolled
steel sheet as nucleus forming sites. Here, the hot-rolled steel sheet preferably has
a fme 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
20 phases and retained austenite, which are produced subsequently.
On the other hand, if the unrecrystallization ratio of the region not
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
25 promoted fiom the grain boundaries of the recrystallized grains, and thus, the
austenite grains during annealing are coarsened and the final structure is also
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
30 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
preferably at least 100°C/sec. An upper limit of the average heating rate is not
specifically defined, but is preferably at most 1000°C/sec to avoid temperature
control difficulty.
The above temperature for starting the rapid heating at a rate of at least
1 S°C/sec may be any desired temperature if the recrystallization has not started yet,
5 and may be, T,-30°C relative to a the temperature for the start of softening (the
temperature for the start of recrystallization) Ts measured under a heating rate 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,
10 even if rapid heating is started fkom room temperature, nit does not have an adverse
effect on the cold-rolled steel sheet after annealing.
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
15 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
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.
20 After heating to (Acl point + 10°C), heating is carried out to an annealing
temperature in a range of at least (0.9 x Acl point + 0.1 x Ac3 point) and at most
(Ac3 point + 100°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
25 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 and subsequently the
heating rate is lowered.
In the annealing process, transformation to austenite is sufficiently promoted
and carbides are dissolved in the steel. Thus, the annealing temperature is at least
30 (0.9 x Acl + 0.1 x Ac3 point). The annealing temperature is preferably at least (0.3
x Acl point + 0.7 x Ac3 point), and in this case, in particular, the texture of the coldrolled
steel sheet, the strength of { 100) <0 1 1> to (2 1 1 ) <0 1 1 > orientations is
lowered and the workability of the steel sheet is increased. On the other hand, if
soaking holding is carried out at a temperature exceeding (Ac3 point + 100°C) as the
annealing temperature, sharp growth of austenite grains occur, and as a result, the
final structure becomes coarse. Thus, the annealing temperature is at most (Ac3
5 point + 100°C), preferably at most (Ac3 point + 50°C).
The Acl and Ac3 points in the present invention are values that can be
determined from 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 2OCIsec.
If an annealing holding time for the temperature range is at most 30 seconds,
10 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
15 austenite and dissolution of the carbides. An upper limit of the holding time is not
specifically determined; however, from the standpoint of suppressing the austenite
grain growth, the annealing holding time is preferably less than 10 minutes.
In cooling after annealing, temperature history such as the cooling rate and '
the temperature and time of low-temperature holding are controlled to form proper
20 area fractions of ferrite, the low-temperature transformation phase and the retained
austenite, whereby the structure of the cold-rolled steel sheet is controlled. If the
cooling rate in the cooling after annealing is too low, the low-temperature
transformation phase is reduced to less than 10 area%, resulting in decrease in
strength of the steel sheet. Thus, an average cooling rate for a temperature range
25 of from 650°C to 500°C is preferably at least 1°C/sec. On the other hand, if the
cooling rate is too high, the area fraction of the low-temperature transformation
phase is excessively increased, deteriorating the ductility of the steel sheet. Thus,
the average cooling rate for the above temperature range is preferably at most
60°C/sec. The cooling may be performed by an arbitrary method. For example,
30 cooling using gas, mist or water or a combination thereof can be employed.
After the cooling in the temperature range, the cooling is stopped or the coldrolled
steel sheet is held in a low-temperature range for slow cooling, whereby a
proper area fiaction of the low-temperature transformation phase is formed in the
cold-rolled steel sheet, and diffusion of carbon atoms to untransformed austenite is
promoted to form retained austenite.
After the annealing, in a cooling process to room temperature, hot-dip plating
may be performed to provide a hot-dip plated steel sheet, or hot-dip plating or
electroplating may be performed in a separate process after the cooling to room
temperature to provide a hot-dip plated steel sheet or an electroplated steel sheet.
If hot-dip plating is performed in the cooling process to room temperature to
provide a hot-dip plated steel sheet, the steel sheet may be retained at a temperature
10 that is higher or lower than that of a hot-dip plating bath before the hot-dip plating.
The hot-dip plating layer, the electroplating layer or the plating adhesion amount
have been described above. In order to W e r improve the corrosion resistance,
proper chemical conversion coating may be performed after the plating.
Examples
Each ingot of steel types A to N each having the chemical composition
indicated in Table 1 was melted in a vacuum induction furnace. Table 1 indicates
Acl and also Ac3 points for each of steel types A to N. These transformation
temperatures are obtained fiom a thermal expansion curve measured when the
temperature of the relevant steel sheet subjected to cold rolling under the laterdescribed
production conditions is raised to 1 100°C at a heating rate of 2"CIsec.
Table 1 also indicates each value of (Acl point + 10°C), (0.9 x Acl point + 0.1 x
Ac3 point) and (Ac3 point + 100°C).
These ingots were hot forged and cut to the shape of slabs to be subjected to
hot rolling. These slabs were heated for one hour at a temperature of at least
1 OOO°C, and then subjected to hot rolling in which rolling was completed at the
5 temperature at the completion of rolling (indicated also as FT in Table 2) indicated
in Table 2 using a small test mill for trials, whereby a hot-rolled steel sheet having a
sheet thickness of 2.0 to 2.6 mm under the cooling conditions and at the coiling
temperature that are indicated in the table.
The cooling after the rolling completion was each carrying out by any of the
10 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 temperature at the completion of rolling
15 (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 from the temperature at the completidn of rolling (FT), and held at the
temperature (allowed to naturally cool) for a predetermined length of time, and then
20 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.
25 Subsequently, the steel sheet was put in a h a c e and subjected to slow cooling
simulated for coiling. A value of the left side of Equation (5) 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
30 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-7001F manufactured by JEOL Ltd.). The average grain diameter
d of the BCC phase was obtained using following Equation (6). Here, Ai
represents the area of an i-th grain, and di represents a Heywood diameter of the i-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 furnace.
Each of the hot-rolled steel sheets obtained as described above was subjected
to pickling using a hydrochloric acid and cold rolling at the rolling reduction
10 indicated in Table 2 in a conventional manner to make the steel sheet have a
thickness of 1.0 to 1.2 rnm. Subsequently, using a laboratory scale mealing
equipment, annealing was carried out at the heating rate, the annealing temperature
and the holding time indicated in Tabie 2,'and cooling was carried out under a
condition that makes the cooling rate for a temperature range of fiom 650°C to
15 500°C become the "Cooling rate" indicated in Table 2, whereby the resulting coldrolled
steel sheet was obtained. Furthermore, in a cooling process, as indicated in
Table 2, each steel sheet was subjected to any of heat treatments indicated in A to I
below, and then cooled to room temperature at 2"C/sec, whereby the resulting coldrolled
steel sheet was obtained. Cooling after the soaking was carried out using a
20 nitrogen gas. Underlined values in Tables 2 and 3 indicate values outside the
range of the present invention.
A: Holding at 375°C for 330 seconds
B: Holding at 400°C for 330 seconds
C: Holding at 425OC for 330 seconds
25 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
.al loying
E: Holding at 480°C for 60 seconds, then cooling to 460°C for simulation of hot-dip
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 hot-dip
galvanizing bath immersion, and further heating to 540°C for simulation of alloying.
G: Holding at 375°C for 60 seconds, then cooling to 460°C for simulation of hotdip
galvanizing bath immersion, and further heating to 500°C for simulation of
alloying.
H: Holding at 400°C for 60 seconds, then cooling to 460°C for simulation of hotdip
galvanizing bath immersion, and further heating to 500°C for simulation of
alloying.
I: Holding at 425°C for 60 seconds, then cooling to 460°C for simulation of hot-dip
galvanizing bath immersion, and further heating to 500°C for simulation of alloying.
Table 2 indicates a proportion of an unrecrystallization of regions not
transformed to austenite at the time of reaching (Acl point + 10°C). This value was
obtained by the following method. Each steel sheet that has been subjected to cold
rolling according to the manufacturing conditions in the present invention was
15 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, an average grain diameter of the low-
5 temperature transformation phase and an average grain diameter of retained
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
parallel to a rolling direction at a depth of114 of a sheet thickness and the
sheet thickness direction of the steel sheet. Area fractions of ferrite and the low-
10 temperature transformation phase were also obtained using the analysis results of
SEM-EBSD. Also, a volume fraction of the austenite phase was obtained by Xray
diffractometry using the later-described equipment to use the volume fraction as
an area fraction of retained austenite (retainedy). For an EBSD analysis of a
structure containing the retained austenite phase, the retained austenite is
15 concernedly not 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 fraction of retained austenite
obtained by an EBSD analysis (yEBSD) satisfies (yEBSD1yXR.D) > 0.7 relative to a
volume fraction of retained austenite obtained by X-ray di ffractometry (yXRD) was
20 provided for an analysis accuracy index.
Measurement of a texture of each cold-rolled steel sheet was carried out by
using X-ray diffraction on a plane at a depth of 112 of the sheet thickness of steel
sheets and then using ODF (orientation distribution function) obtained by analyzing
the measured results of pole figures of (2001, (1 10) and (21 1) of ferrite. From
25 the analysis results, a ratio in intensity of each of { 100) , (4 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) to (2 1 1 ) <0 1 1> orientation group. X-ray
intensities of the random structure not having a texture were obtained by X-ray
30 diffraqtion of powdered steel. The apparatus used for X-ray diffraction was RINT-
2500HL/PC 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
5 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.
10
[Table 3-21
') Lump-like retained y: Area fraction of retained y having an aspect ratio of less than 5 relative to all retained y
" Texture: Average X-ray intensity ratio of the { 100)<011> to {2 1 I ) orientation group
In steel sheets Nos. 5, 8, 11, 14, 16, 19, 22,25,27, 32, 34, 36, 40,42, 47 and
5 49, the heating rate during annealing was less than 1 S°C/sec, and thus, the
proportion of the unrecrystallization at Acl + 10°C 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 defined by the present invention. As a
result, the mechanical properties were inferior.
10 In steel sheets Nos. 4 and 29, the heating rate during annealing was at least
15"C/s, but since the annealing temperature exceeded Ac3 + 1 OO°C, the
microstructure of the cold-rolled steel sheet coarsened and the grain diameter of
ferrite exceeded the upper limit defined by the present invention. As a result, the
mechanical properties were inferior.
In steel sheets Nos. 45 and 46, the Nb content exceeded the upper limit, and
thus the steel was excessively hardened, resulting in deteriorating workability. As
5 a result, the mechanical properties of the cold-rolled steel sheet were low
irrespective of the heating rate.
In steel sheets Nos. 47 and 48, the Si content was lower than the lower limit,
and thus retained austenite was formed in the cold-rolled steel sheet. Thus, the
ductility was low. As a result, the mechanical properties of the cold-rolled steel
10 sheet were low irrespective of the heating rate.
On the other hand, the steel sheets having the chemical composition and
structure defmed by the present invention, while having high strength, had, in
particular, significantly excellent ductility compared to the comparative examples
and favorable stretch flangeability as can be seen in comparison with those of the
same steel types.
We claim:
1. A cold-rolled steel sheet characterized by having:
a chemical composition comprising, in mass%, C: 0.06 to 0.3%, Si: 0.4 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%, a total content 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 ferrite which comprising at least 40
10 area% , and a second phase of a low-temperature transformation phase consisting
either or both of martensite and bainite which comprising at least 10 area% in total
and retained austenite at least which comprising 3 area% , and satisfying
Equations (1) to (4):
dF I 5.0 ... (1);
15 dM+~<2.0 ...( 2);
dAs < 1.5 ... (3); and
rAS 1 50 ... (41,
where dF is an average grain diameter ( pm) of ferrite defined by high angle
grain boundaries having a tilt angle of at least 15';
20 dM+is~ a n average grain diameter (pm) of the low-temperature
transformation phase;
dAs is an average grain diameter (pm) of retained austenite having an aspect
ratio of less than 5; and
rAs is an area fraction (%) of retained austenite having an aspect ratio of less
25 than 5 relative to all the retained austenite.
2. The cold-rolled steel sheet as set forth in claim 1, wherein the cold-rolled
steel sheet has a texture whose ratio of an average X-ray intensity of { 1 OO} to
(2 1 1 }<0 1 12 orientations relative to an average X-ray intensity of a random
30 structure having no texture is less than 6 at a depth of 112 of a sheet thickness.
3. The cold-rolled steel sheet as set forth in claim 1 or 2, wherein the
chemical composition contains one or two selected fiom, in mass%, Ti: 0.005 to
0.08% and Nb: 0.003 to 0.04%.
5 4. The cold-rolled steel sheet as set forth in any of claims 1 to 3, wherein
the chemical composition contains, in mass%, sol.Al: 0.1 to 2.0%.
5. The cold-rolled steel sheet as set forth in any of claims 1 to 4, wherein
the chemical composition contains one or more selected from, in mass%, Cr: 0.03 to
10 1%, Mo: 0.01 to 0.3% and V: 0.01 to 0.3%.
6. The cold-rolled steel sheet as set forth in any of claims 1 to 5, wherein
the chemical composition contains, in mass%, B: 0.0003 to 0.005%.
15 7. The cold-rolled steel sheet as set forth in any of claims 1 to 6, wherein
the chemical composition contains one or two selected fiom, in mass%, Ca: 0.0005
to 0.003% and REM: 0.0005 to 0.003%.
8. The cold-rolled steel she& as set forth in any of claims 1 to 7, comprising
20 a plating layer on a sheet surface.
9. A process for manufacturing a cold-rolled steel sheet as set forth in any
of claims 1 to 8, the method comprising:
(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 3 to 7 is cold-rolled to obtain a coldrolled
steel sheet; and
(B) an annealing step in which the cold-rolled steel sheet obtained in the step
(A) is annealed under conditions that the cold-rolled steel sheet is heated at an
average heating rate of at least 1 5OCIsec so that a proportion of an
30 unrecrystallization of a region not transformed to austenite at a time of reaching
(Act point + 10°C) is at least 30 area%, and is then held in a temperature range of at
least (0.9 x Acl point + 0.1 x Ac3 point) and at most (Ac3 point + 100°C) for 30
seconds.
10. The process for manufacturing cold-rolled steel sheet as set forth in
5 claim 9, wherein the hot-rolled steel sheet, after completion of hot rolling, is coiled
at a temperature of at most 300°C and subsequent heat treatment in a temperature
range of 500°C to 700°C.
1 1. The process for manufacturing cold-rolled steel sheet as set forth in
10 claim 9 or 10, 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 pm, the steel sheet being obtained by a hot rolling
step of cooling at a cooling rate (Crate) satisfying following Equation (5) for a
temperature range from a temperature at the completion of rolling to (temperature at
15 the completion of rolling - 100°C) after completion of hot rolling in which hot
rolling is completed at at least an AT3 point:
1 ~ ( ~ ) = 0 . 1 - 3 ~ 1- 0T-+~~ x ~ o "-. 5T~~1 0 - ' -+~5~~ 1 0 --~7 ~- 1~0~-- T~5~
where Crate (T) is a cooling rate ("C/s) (positive value),
T is a relative temperature ("C, negative value) with the a 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
detention period (At) at the temperature by IC (T) is added as an integral for the
section.
25 12. The process for manufacturing cold-rolled steel sheet as set forth in
claim 11, wherein the cooling for the temperature range 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 3 0°C.
13. The process for manufacturing cold-rolled steel sheet as set forth in
claim 11, wherein the cooling for 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 range of at least 30°C and at most 80°C, and then stopping a water
5 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.
14. The stopping a water cooling stop time cold-rolled steel sheet as set
forth in any of claims 9 to 13, fixther comprising the step of plating the cold-rolled
10 steel sheet after the (B) step.