Cold-Rolled Steel Sheet and Process for Production Thereof
Technical Field
This invention relates to a cold-rolled steel sheet and a process for producing
5 the same. More particularly, the present invention relates to a cold-rolled steel
sheet having excellent v^orkability in addition to a high strength and a process for
manufacturing a cold-rolled steel sheet with excellent stability of material properties.
Background Art
10 From m the past, there have been many studies of methods of refinmg the
structure of a cold-rolled steel sheet in order to improve the mechanical properties of
the steel sheet.
These methods can be generally divided into the following categories (1) -
(3).
15 (1) A first method is a method in which a large amount of elements which
suppress grain growth such as Ti, Nb, and Mo are added to refine austenite grains
which are formed at the time of annealing after cold rolling, thereby refining ferrite
grains which are formed by transformation from austenite in subsequent cooling.
(2) A second method is a method in which heating to a single-phase
20 austenitic region in the above-described annealing is carried out by rapid heating
followed by holding for an extremely short length of time to prevent coarsening of
the structure.
(3) A third method is a method in which cold rolling and annealing are
carried out on a hot-rolled steel sheet manufactured by rapid cooling immediately
25 after hot rolling. Below, this method of manufacturing a hot-rolled steel sheet will
be referred to as the immediate cooling method.
With respect to the above-described first method. Patent Document 1, for
example, discloses a cold-rolled steel sheet having a steel structure primarily
comprising ferrite with an average grain diameter of at most 3.5 jjin. Patent
30 Document 2 discloses a cold-rolled steel sheet having a structure comprising ferrite
and a low temperature transformation phase constituted by one or more selected
from martensite, bainite, and retained y (retained austenite). The average grain
2
diameter of the low temperature transformation phase is at most 2 \mi and its
volume fraction is 10 - 50%.
Concerning the second method. Patent Document 3, for example, discloses a
method in which a hot-rolled steel sheet which was coiled at 500° C or above is
5 cold-rolled and then annealed by rapid heating at a rate of at least 30° C per second
in the temperature range from room temperature to 750° C and limitmg the holding
time at an annealing temperature in ttie range of 750 - 900° C, thereby causing
transformation from unrecrystallized ferrite to fine austenite, from which fine ferrite
is formed at the time of cooling. Patent Document 4 describes a method of
10 manufacturing a bake hardenable high strength cold-rolled steel sheet comprising
cold rolling a hot-rolled steel sheet obtained by usual hot rolling, and then subjecting
the steel sheet to continuous annealing by heating to a temperature range of 730 -
830° C at a rate of 300 - 2000° C per second in a temperature region of at least 500°
C followed by holding in the temperature range for at most 2 seconds.
15 Concerning the third method, Patent Document 5 discloses a method in
which cold rolling is carried out on a hot-rolled steel sheet produced by the
immediate rapid cooling method in which cooling is started a short time after hot
rolling. For example, a hot-rolled steel sheet having a fine structure and
predominantly comprising ferrite with a small average grainrdiameter is produced by
20 cooling to a temperature of 720° C or below at a cooling rate of at least 400° C per
second within 0.4 seconds after the completion of hot roUmg and is used as a
startmg material for cold rolling, and cold rolling and annealing are carried out in a
usual manner.
In Patent Document 5, a region which is surrounded by a high angle grain
25 boundary for which the misorientation (also referred to as the tilt angle) is at least
15° is defined as a single (crystal) grain. Accordingly, a hot-rolled steel sheet
having a fine structure disclosed by Patent Document 5 is characterized by having a
large number of high angle grain boundaries.
30 Prior Art Documents
Patent Documents
Patent Document 1: JP 2004-250774 A
3
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i
Patent Document 2: JP 2008-231480 A
Patent Document 3: JP 2007-92131 A
Patent Document 4: JP 7-34136 A
Patent Document 5: WO 2007/015541
5
Summary of the Invention
As stated above, in the prior art, there have been many studies of methods
for refining the structure of a cold-roiled steel sheet with the object of improving the
mechanical properties of the steel sheet. However, as stated below, none of the
10 conventional methods were completely satisfactory.
In the method disclosed in Patent Document 1 and Patent Document 2, due
to the addition of Ti, Nb, or similar element which is essential, problems remain
from the standpoint of resource conservation.
In the method disclosed in Patent Document 3, as shown by the examples, in
15 order to obtain a structure having fine grains such as a structure comprismg ferrite
grains with an average grain diameter of less than 3.5 |jm, it is necessary to make the
holding time at the time of annealing a short time of at most around 10 seconds.
Examples in which the holding time at the tune of annealing is 30 seconds or 200
seconds are shown therein, but the average grain diameter after annealing becomes
20 3.8 |4m or 4.4 |im, indicating that abrupt grain growth takes place. In order to
increase the stability of manufacture of a steel sheet, it has normally been considered
necessary for the holding time in an annealing step to be at least several tens of
seconds. Therefore, with the method disclosed in Patent Document 3, it is difficult
to obtain both manufacturing stability and an extremely fine structure of less than
25 3.5 |xm.
Similarly, the method disclosed in Patent Document 4 limits the holding
time during annealing to at most 2 seconds. Thus, it makes it necessary to carry
out annealing in an extremely short time and has the same problems as the method
disclosed in Patent Document 3.
30 A method employing immediate rapid cooling disclosed in Patent Document
5 is excellent as a means for refining the microstructure of a cold-rolled steel sheet.
However, the ferrite grain diameter of a cold-rolled steel sheet is approximately the
>^ X
same as or larger by 1 - 3 urn than the ferrite grain diameter of a hot-rolled steel
sheet which is the starting material. Therefore, there is a limit to refining the
microstructure of a cold-rolled steel sheet.
The object of the present invention is to solve the above-described problems
5 of the prior art with respect to a cold-rolled steel sheet having a refined structure.
More specifically, the object of the present invention is to provide a cold-rolled steel
sheet which has a fine structure even if Ti, Nb, or the like is not added and even if
the holding time for annealing is made long enough to obtain a stable material and
which has a ferrite grain diameter which is the same as or smaller tiian the ferrite
10 grain diameter of a hot-rolled steel sheet and a process for manufacturing such coldrolled
steel sheet.
The present inventors performed detailed investigations with the object of
solving the above-described problems.
First, they investigated the cause of why the ferrite grain diameter of the
15 cold-rolled steel sheet disclosed in Patent Document 5, which is excellent as a means
for refining the microstructure of a cold-rolled steel sheet, is approximately the same
as or 1 - 3 nm larger than the ferrite grain diameter of a hot-rolled steel sheet, and
they obtained the following knowledge (a) - (c).
(a) The method disclosed m Patent Document 5 is based on the technical
20 concept that when cold rolling and annealing are carried out on a hot-rolled steel
sheet which is obtained by the immediate rapid cooling method and which has a
i
thermally stable fine grain structure having a large number of high angle grain
boundaries, a large number of recrystallization nuclei form on the grain boundaries
of the hot-rolled steel sheet, thereby refining the structure after cold rolling and
25 annealing.
(b) However, the speed of grain growth of recrystallized grains which
grow from recrystallization nuclei which are formed on the grain boundaries of a
hot-rolled steel sheet at the time of annealing markedly increases as the structure of
a hot-rolled steel sheet is refined.
30 (c) The effect of refining the structure of a cold-rolled steel sheet by the
method disclosed in Patent Document 5 is decreased by the above-described active
grain growth of the recrystallized grains, and the ferrite grain diameter of a cold-
5
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rolled steel sheet ends up being nearly the same as or 1 - 3 ^m larger than the ferrite
grain diameter of a hot-rolled steel sheet.
Accordingly, the present inventors investigated how to suppress the abovedescribed
active gram growth of recrystallized grams and acquired the following
5 new knowledge (d) - (i).
(d) When performing cold rolling followed by annealing on a hot-rolled
steel sheet having a fine structure, by carrying out annealing by rapid heating so as
to reach a temperature at which ferrite and austenite coexist before reciystallization
of ferrite (which has a deformed texture due to cold rolUng) is completed, a fine
10 structure having a ferrite grain diameter which is the same as or smaller than the
ferrite grain diameter of the hot-rolled steel sheet is obtained.
(e) This is because the annealing by rapid heating causes a large number of
refined austenite grains to form fi-om locations which were high angle grain
boundaries of the hot-rolled steel sheet (prior grain boundaries) in a state in which
15 unrecrystallized ferrite remains. Due to the presence of the large number of refined
austenite grains, the growth of recrystallized ferrite grains beyond the prior grain
boundaries of the hot-rolled steel sheet is suppressed.
(f) By refining the structure of a hot-rolled steel sheet, it is possible to
refine the structure at the time of annealing after cold rolling. However, the more
20 the structure of a hot-rolled steel sheet is refined, the more the rate of grain growth
of recrystallized grains increases. Therefore, in order to obtain a refined structure
after annealing, it is necessary to perform annealing by rapid heating at a fiirther
increased rate of temperature increase.
(g) By using this grain growth suppressing mechanism, even if the holding
25 time during annealing is extended to, for example, from 30 seconds to several
hundred seconds, grain growth is suppressed, and a fine structure is maintained.
As a result, variations in material properties caused by variations in manufacturing
conditions such as the strip running speed can be suppressed, and a cold-rolled steel
sheet having stable material properties can be obtained.
30 (h) A cold-rolled steel sheet which is manufactured by such a
manufacturing process has a texture which is characterized in that the average X-ray
intensity for the {111 }<145>, {111 }<123>, and {554}<225> orientations at a depth
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A *
of 1/2 of the sheet thickness is at least 4.0 times the average X-ray intensity of a
random structure which does not have a texture. A cold-rolled steel sheet having
such a texture has good stretch flangeability (hole expanding formability).
(i) It is sufBcient for a hot-rolled sheet which is subjected to cold rolling to
5 have a fine structure, but it is preferable that it have excellent thermal stability.
The present invention which is based on these new findings is as follows.
(1) A cold-rolled steel sheet characterized by having:
a chemical composition comprising, in mass%, C: 0.01 - 0.3%, Si: 0.01 -
2.0%, Mn: 0.5 - 3.5%, P: at most 0.1%, S: at most 0.05%, Nb: 0 - 0.03%, Ti: 0 -
10 0.06%, V: 0 - 0.3%, sol. Al: 0 - 2.0%, Cr: 0 - 1.0%, Mo: 0 - 0.3%, B: 0 - 0.003%, I
Ca: 0 - 0.003%, REM: 0 - 0.003%, and a remainder of Fe and unpurities;
a microstructure having a main phase of ferrite which comprises at least
50% by area and a second phase which contains a total of at least 10% by area of a
low temperature transformation phase including one or more of martensite, bainite,
IS pearlite, and cementite and 0 - 3% by area of retained austenite, and satisfying the
following Equations (1) - (3); and
a texture in which the average X-ray intensity for the {111 }<145>,
{111 }<123>, and {554}<225> orientations at a depth of 1/2 of the sheet thickness is
at least 4.0 times the average X-ray intensity of a random structure which does not
20 have a texture:
d„ < 2.7 + 10000/(5+300xC+50xMn+4000xNb+2000xTi+400xV)^ - (1)
dm < 4.0 - (2)
ds<1.5 ••• (3)
wherein
25 C, Mn, Nb, Ti, and V indicate the contents (mass%) of the respective
elements,
dm is the average grain diameter (jxm) of ferrite defined by a high angle grain
boundary having a tih angle (the difference in the crystal orientation) of at least 15°,
and
30 ds is the average grain diameter (jxm) of the second phase.
(2) Acold-roUedsteelsheet as set forth above in (1) wherein the chemical
composition contains, in mass percent, one or more elements selected fi-om Nb: at
least 0.003%, Ti: at least 0.005%, and V: at least 0.01%, and the microstructure
satisfies the following Equation (4):
dm < 3.5 - (4)
wherein dm is as defined above.
5 (3) A cold-rolled steel sheet as set forth above in (1) or (2) wherein the
chemical composition contains, in mass percent, sol. Al: at least 0.1%.
(4) A cold-rolled steel sheet as set forth above in any of (1) - (3) wherein
the chemical composition contains, in mass percent, one or more elements selected
from Cn at least 0.03%, Mo: at least 0.01%, and B: at least 0.0005%.
10 (5) A cold-rolled steel sheet as set forth above in any of(l)-(4) wherein
the chemical composition contains, in mass percent, one or two elements selected
from Ca: at least 0.0005%, and REM: at least 0.0005%.
(6) A cold-rolled steel sheet as set forth above in any of (1) - (5) having a
plating layer on the surface of the steel sheet.
15 (7) A 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
composition as set forth above in any of (1) - (5) and having a microstructure which
satisfies the following Equations (5) and (6) is subjected to cold rolling to obtain a
20 cold-rolled steel sheet, and
(B) an annealing step in which the cold-rolled steel sheet obtained in Step
(A) is subjected to annealing by increasing the temperature of the steel sheet to a
temperature range of at least (Aei point + 10° C) to at most (0.95 xAcs point +
0.05 xAci point) under conditions such that the proportion of unrecrystallized ferrite
25 is at least 30% by area when the temperature (Aci point + 10° C) is reached and then
holding the steel sheet in this temperature range for at least 30 seconds:
d < 2.5 + 6000/(5 + 350xC + 40xMn)^ - (5)
d<3.5 • (6)
wherein,
30 C and Mn are the contents of the respective elements (mass percent); and
d is the average grain diameter (pm) of ferrite defined by a high angle grain
boundary having a tih angle of at least 15°.
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X
(8) A process for manufacturing a cold-rolled steel sheet as set forth above
in (7) wherein the hot-rolled steel sheet is obtained by a hot rolling step comprising
performing hot rolling with a temperature at the completion of rolling of at least the
Ara point on a slab having the above-described chemical composition and then
5 performing cooling to a temperature range of 750° C or below at an average cooling
rate of at least 400° C per second within 0.4 seconds after completion of rolling.
(9) A process for manufacturing a cold-rolled steel sheet as set forth above
in (7) or (8) fiirther including a step of applying plating to the cold-rolled steel sheet
after step (B).
10 In this description, the main phase means the phase or structure having the
largest percentage by volume (in the present invention, the volume percentage is
actually evaluated by the area percentage in a cross section), and a second phase
means a phase or structure other than the main phase.
Ferrite includes polygonal ferrite and bainitic ferrite. A low temperature ^
IS transformation phase (a phase formed by low temperature transformation) includes
martensite, bainite, pearlite, and cementite. Martensite includes tempered
martensite, and bainite includes tempered bainite.
A cold-rolled steel sheet according to the present invention has a structure
which is refined on the same level or more compared to the hot-rolled steel sheet
20 used as a starting material. Therefore, it has excellent workability while having a
high strength, and it is suitable as a steel sheet for automobiles. In addition, it does
not require the addition of a large amount of rare metals such as Nb or Ti, which is
advantageous from the standpoint of conservation of resources. Since this coldrolled
steel sheet is manufactured by a process according to the present invention
25 which does not make the annealing time a short length of time, it has stable material
properties.
Brief Explanation of the Drawings
Figure 1 is a graph showing the relationship between the average grain
30 diameter of a cold-rolled steel sheet and the rate of temperature increase for coldrolled
steel sheets made of steel types A, B, and C which were used in the examples
and which were annealed by heating to 750° C at various rates of temperature
y
increase and then holding for 60 seconds at that temperature.
Figure 2 is a graph showing the relationship between the tensile strength of a
cold-rolled steel sheet and the rate of temperature increase for cold-rolled steel
sheets made of steel types B and C which were used in the examples and which were
5 annealed by heating to 750° C at various rates of temperature increase and holding
for 60 seconds at that temperature, with the ordinate showing the percent increase in
the tensile strength compared to when the rate of temperature increase was 10° C per
second.
Figure 3 is a graph showing the relationship between the value of TS^EL
10 (tensile strength multiplied by total elongation) and the holding time during
annealing for steel B which was used in the examples and which was annealed by
heating to 750° C at 500° C per second and then soaking (temperature holding) for
from 15 seconds to 300 seconds followed by cooling to room temperature at 50° C
per second.
15
Modes for Carrymg Out the Invention
Below, a cold-rolled steel sheet according to the present invention and a
process for manufacturing the same will be described. In the following
explanation, percent with respect to chemical composition means mass percent.
20
1. Cold-rolled steel sheet
1.1 - Chemical Composition
C: 0.01-0.3%
C has the effect of increasing the strength of steel. In addition, it has the
25 effect of reJHning the microstructure during a hot rolling step and an annealing step.
Namely, C has the effect of lowering the transformation point. Therefore, during a
hot rolling step, it makes it possible to complete hot rolling in a lower temperature
range, thereby making it possible to refine the microstructure of a hot-rolled steel
sheet. In an annealing step, due to the effect of C by which recrystallization of
30 ferrite is suppressed in the course of temperature increase, it is facilitated to reach a
temperature range of at least (Acj point + 10° C) by rapid heating while maintaining
a state with a high percentage of unrecrystallized ferrite. As a result, it becomes
lo
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possible to refine the microstructure of a cold-rolled steel sheet. If the C content is
less than 0.01 %, it is difficult to obtain the above-described effects. Accordingly,
the C content is made at least 0.01%. It is preferably at least 0.03% and more
preferably at least 0.05%. If the C content exceeds 0.3%, there is a marked
5 decrease in workability and weldability. Accordingly, the C content is made at
most 0.3%. Preferably it is at most 0.2% and more preferably at most 0.15%.
Si: 0.01 - 2.0%
Si has the effect of increasing the ductility and strength of steel. In
addition, when it is added along with Mn, it promotes the formation of a hard second
10 phase such as martensite (a phase which is harder than ferrite forming the main
phase), and it has the effect of increasing the strength of steel. If the Si content is
less than 0.01%, it is difficuh to obtain the above-described effects. Accordingly,
the Si content is made at least 0.01%. Preferably it is at least 0.03% and more
preferably at least 0.05%. On the other hand, if the Si content exceeds 2.0%,
15 oxides are formed on the surface of the steel during hot rolling or annealing and the
surface condition is sometimes worsened. Accordingly, the Si content is made at
most 2.0%. Preferably it is at most 1.5% and more preferably at most 0.5%.
Mn: 0.5 - 3.5%
Mn has the effect of increasing the strength of steel. In addition, it has the
20 effect of decreasing the transformation temperature. As a result, during an
annealing step, it is facilitated to reach a temperature range of at least (Aei point +
10° 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 0.5%, it becomes difficult to
25 obtain the above-described effects. Accordingly, the Mn content is made at least
0.5%. Preferably it is at least 0.7% and more preferably at least 1%. However, if
the Mn content exceeds 3.5%, ferrite transformation is excessively delayed, and it
may not be possible to guarantee the desired area percentage of ferrite.
Accordingly, the Mn content is made at most 3.5%. Preferably it is at most 3.0%
30 and more preferably at most 2.8%.
P: at most 0.1%
P, which is contained as an impurity, has the action of embrittling the
11
- material by segregation at grain boundaries. If the P content exceeds 0.1 %,
embrittlement due to the above action becomes maiiced. Accordingly, the P
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.
5 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
of steel by forming sulfide-type inclusions in steel. If the S content exceeds 0.05%,
there may be a mariced decrease in ductility due to the above-described action.
10 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 standpoint of
costs, it is preferably at least 0.001%.
Mb: 0 - 0.03%, Ti: 0 - 0.06%, V: 0 - 0.3%
15 Mb, Ti, and V precipitate in steel as carbides or nitrides, and during cooling
in an annealing step, they suppress transformation from austenite to ferrite and
thereby have the effect of increasing the percent by area of a hard second phase and
increase the strength of steel. Accordingly, one or more of these elements may be
contained in the chemical composition of the steel. However, if the contents of
20 these elements exceed the above-described upper limits, there is sometimes a
marked decrease in ductility. Accordingly, the content of each element is as given
above. The Ti content is preferably at most 0.03%. The total content of Nb and
Ti is preferably at most 0.06% and more preferably at most 0.03%. The contents of
Nb, Ti, and V preferably satisfy the following Equation (7). In order to obtain the
25 above-described effects with greater certainty, the contents preferably satisfy any
one of Nb: at least 0.003%, Ti: at least 0.005%, and V: at least 0.01%.
(Nb + O.SxTi + 0.01 xV) < 0.02 - (7)
wherein Nb, Ti, and V are the contents (mjiss percent) of the respective elements,
sol. Al: 0 - 2.0%
30 Al has the effect of increasing ductility. Accordingly, Al may be contained
I in the steel composition. However, Al has the action of increasing the
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
refme the structure of a cold-rolled steel sheet. In addition, continuous casting
sometimes becomes difficult. Accordingly, the sol. Al content is made at most
5 2.0%. In order to obtain the above-described effect ofAl with greater certainty, the
sol. Al content is preferably at least 0.1%.
Cr: 0 - 1.0%, Mo: 0 - 0.3%, B: 0 - 0.003%
Cr, Mo, and B have the effect of increasing the strength of steel by
increasing the hardenability of steel and promoting the formation of a low
10 temperature transformation phase. Accordingly, one or more of these elements
may be contained in the steel composition. However, if the contents of these
elements exceed the above-described upper limits, there are cases in which ferrite
transformation is excessively suppressed and it is not possible to guarantee the
desired percent area of ferrite. Accordingly, the contents of these elements are as
15 set forth above. The Mo content is preferably at most 0.2%. In order to obtain
the above-described effects with greater certainty, the contents preferably satisfy any
one of Cr: at least 0.03%, Mo: at least 0.01%, and B: at least 0.0005%.
Ca: 0 - 0.003%, REM: 0 - 0.003%
Ca and REM have the effect of refining oxides and nitrides which
20 precipitate during solidification of molten steel and thereby increasing the soundness
j
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
25 content of either 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 mish metal. The content of REM in the present invention means
the total content of these elements.
30 1.2 - Microstmcture and texture
Main Phase: ferrite which is present in a proportion of at least 50% by area
and which satisfies above Equations (1) and (2).
By making the main phase ferrite which is soft, it is possible to increase the
ductility of a cold-rolled steel sheet. In addition, by making the main phase of
ferrite fine so that the average gram diameter d^ of ferrite which is defined by a high
angle grain boundary with a tilt angle of at least 15° satisfies above Equations (1)
5 and (2), the formation and development of fine cracks at the time of working of a
steel sheet are suppressed, and the stretch flangeability of the cold-rolled steel sheet
is increased. In addition, the strength of steel is increased by grain refmement
strengthening. Above-described Equation (1) is an index which represents the
extent of refinement of ferrite taking into consideration the effects of C, Mn, Nb, Ti,
10 and V on refining the structure.
If the percent by area of ferrite is less than 50%, it becomes difQcult to
guarantee excellent ductility. Accordingly, the percent by area of ferrite is made at
least 50%. The percent by area of ferrite is preferably at least 60% and more
preferably at least 70%.
15 If the average grain diameter dm of ferrite does not satisfy at least one of
above Equations (1) and (2), the main phase is not sufficiently fine. As a result, it
becomes difficuh to guarantee excellent stretch flangeability, and the effect of
increasing strength by grain refinement strengthening is not sufficiently obtained.
Accordingly, the average grain diameter dm of ferrite is made to satisfy above
20 Equations (1) and (2).
The average grain diameter of ferrite which is surrounded by a high (large)
angle (tilt) grain boundary having a tilt angle of at least 15° is used as an index
because a small angle grain boundary with a tilt angle of less than 15° has a small
difference in orientation between adjoining grains, and the effect of accumulating
25 dislocations is small, leading to little contribution to increasing strength. Below,
the average grain diameter of ferrite defined by a high angle grain boundary with a
tilt angle of at least 15° is referred to simply as the average grain diameter of ferrite.
When the steel has a chemical composition containing one or more elements
selected fi-om Nb: at least 0.003%, Ti: at least 0.005%, and V: at least 0.01%, the
30 average grain diameter dm of ferrite preferably satisfies the above-described
Equation (4).
Second Phase: Containing at least 10% by area of a low temperature
transformation phase including martensite, bainite, pearlite, and cementite and 0 -
3% by area of retained austenite, and satisfying above Equation (3).
When the second phase contains a hard phase or structure which is formed
by a low temperature transformation such as martensite, bamite, pearlite, and
5 cementite, it becomes possible to increase the strength of steel. In addition,
retained austenite has the action of lowering the stretch flangeability of a steel sheet.
Therefore, it is possible to guarantee excellent stretch flangeability by limiting the
percent by area of retained austenite. Furethermore, by refining the second phase
so as to satisfy above Equation (3), the formation and development of fine cracks
10 during working of a steel sheet are suppressed and the stretch flangeability of the
steel sheet is increased. The strength of steel is also increased by grain refinement
strengthening.
If the total percent by area of a low temperature transformation phase
including martensite, bainite, pearlite, and cementite is less than 10%, it is difficult
15 to guarantee a high strength . Accordingly, the total percent by area of a low
temperature transformation phase is made at least 10%. It is not necessary for the
low temperature transformation phase to contain all of martensite, bainite, pearlite,
and cementite, and it is sufficient for it to contain at least one of these phases.
If the percent by area of retained austenite exceeds 3%, it is difficult to
20 guarantee excellent stretch flangeability. Accordingly, the percent by area of
retained austenite is made 0 - 3%. Preferably it is at most 2%.
If the average grain diameter dg of the second phase does not satisfy above
Equation (3), the second phase is not sufficiently fine, and it becomes difficult to
guarantee excellent stretch flangeability. In addition, an effect of increasing the
25 strength of steel by grain refinement strengthening is not sufficiently obtained.
Accordingly, the average grain diameter dj of the second phase is made to satisfy
above Equation (3).
As explained in greater detail in the examples, the average grain diameter of
ferrite which is the main phase is determined using an SEM-EBSD for those ferrite
30 grains which are surrounded by a high angle grain boundary 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
\5
microscope (SEM). It is possible to measure the grain diameter from the resulting
orientation map.
The average grain diameter of the second phase can be determined by
counting the number of particles N of the second phase by observation of a cross-
5 section of a steel sheet with an SEM and calculating by equation: r = (A/NTC)*'^ using
the percent by area A of the second phase.
The percent by area of the main phase and that of the second phase can be
measured by observation of a cross section with an SEM. The percent by area of
the retained austenite is the same as the percent by volume determined by X-ray
10 diffraction. By subtracting the percent by area of retained austenite which is
determined in this manner from the percent by area of the second phase, it is
possible to find the total percent by area of the low temperature transformation phase
in the second phase.
In the present invention, the above-described average grain diameter and
15 percent by area are the values measured at a depth of 1/4 the sheet thickness of the
steel sheet.
Texture: At a depth of 1/2 the sheet thickness, the average of the X-ray
intensities in the {111 }<145>, {111 }<123>, and {554}<225> orientations is at least
4.0 times the average X-ray intensity of a random structure which does not have a
20 texture
By increasing the degree of integration of the {111 }<145>, {111 }<123>,
and {554}<225> orientations at a depth of 1/2 the sheet thickness in the above
manner, the stretch flangeability of the steel sheet is increased. If the average of
the X-ray intensities in the {111 }<145>, {111 }<123>, and {554}<225> orientations
25 is less than 4.0 times the average X-ray intensity of a random structure not having a
texture, it is difficult to guarantee excellent stretch flangeability. Accordingly, the
cold-rolled sheet is made to have the above-described texture.
The X-ray intensity for a particular orientation is determined by an
orientation distribution function (ODF), which is obtained by chemical polishing a
30 steel sheet with hydrofluoric acid to a depth of 1/2 the sheet thickness, measuring a
pole figure for each of the {200}, {111}, and {211} planes of the ferrite phase on
the sheet surface, and analyzing the measured values of the pole figure using the
I
series expansion method.
The X-ray intensity of a random structure not having a texture is determined
by measurement like that described above using a powdered sample of the steel.
By satisfying the above-described microstructure and texture, a high degree
5 of wOTkability which satisfies the following Equation (8) is obtained for a steel sheet
having a tensile strength (TS) of less than 800 MPa. With a steel sheet having a
tensile strength (TS) of at least 800 MPa, a high degree of workability which
satisfies the following Equation (9) is obtained.
SxTSxEl + TSxX > 105000 ••• (8)
10 3xTSxEl + TSxX> 85000 •• (9)
In the above equations, TS is the tensile strength (MPa), El is the total
elongation (elongation at rupture m%), and X is the percent hole expansion (%)
defined m JFS T 1001-1996 of Japan Iron and Steel Federation Standards.
15 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
20 electrogalvanizing and Zn-Ni alloy electroplating. Examples of a hot-dip plating
are hot-dip galvanizing, galvannealing, hot-dip aluminum plating, hot-dip Zn-Al
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 form a suitable chemical conversion treatment coating on the plating
25 surface (such as one formed by applying a silicate-based chromium-free 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 a cold-rolled steel sheet
30 2.1 - Chemical composition
The chemical composition is as set forth above in 1.1.
2.2 - Cold rolling step
By subjecting a hot-rolled steel sheet having a fine structure in which there
are a large number of high angle grain boundaries so as to satisfy above Equations
(5) and (6) to rapid heating annealing following cold rolling, a large amount of fine
5 austenite is formed fi-om the locations which were high angle grain boundaries of the
hot-rolled steel sheet in a state in which unrecrystallized ferrite remains. Because
the large number of fine austenite grains which are formed restrain recrystallized
ferrite grains firom growing with crossing the prior grain boundaries of the hot-rolled
steel sheet, it is possible to obtain a cold-rolled steel sheet having a fine structure.
10 When the average grain diameter d of ferrite defined by the high angle grain
boundaries in a hot-rolled steel sheet which is subject to cold rolling does not satisfy
above Equations (5) or (6), even if annealing after cold rolling is performed by rapid
heating annealing, the number of nucleus forming sites is small, and a small number
ofcoarse austenite grains are formed fi'om the deformation texture. The small
15 number of coarse austenite grains contribute ahnost nothing to suppressing the grain
growth of recrystallized ferrite, and the structure of the cold-rolled steel sheet
becomes coarse.
Accordingly, the structure of a hot-rolled steel sheet which is to be subjected
to cold rolling is made to satisfy above Equations (5) and (6).
20 In Equation (5), the average grain diameter d of ferrite is limited by the I
contents of C and Mn because as the contents of C and Mn increase, the ductility of
a cold-rolled steel sheet decreases. Therefore, by making the structure of a hotrolled
steel sheet which is subjected to cold rolling a finer structure, the structure of
the cold-rolled steel sheet becomes finer, and excellent ductility is guaranteed.
25 The average grain diameter d of ferrite in the hot-rolled steel sheet is
preferably as small as possible, and therefore there is no particular need to specify a
lower limit, but normally it is at least 1.0 |xm. Similarly, with respect to a coldrolled
steel sheet, the average grain diameter d^ of ferrite is normally at least 1.0
|im.
30 Cold rolling can be carried out in a conventional manner. There is no
particular limit on the reduction in cold rolling (cold rolling reduction), but from the
standpoints of promoting recrystallization during annealing and improving the
workability of a cold-rolled steel sheet, it is preferably at least 30%. From the
standpoint of decreasing the load on cold rolling equipment, it is preferably at most
85%.
From the standpoint of suppressing accumulation of excessive strains in the
5 surface due to friction and preventing abnormal grain growth in the surface at the
time of annealing, cold rolling may be carried out using lubricating oil.
2.3 - Annealing step
A cold-rolled steel sheet which is obtained by the above-described cold
10 rolling step is subjected to annealmg by heating to a temperature range of at least
(Aei point + 10° C) to at most (0.95 xAes point + 0.05 xAci point) under the
conditions that the percent by area of ferrite which remains unrecrystallized when
the temperature reached (Aci point + 10° C) is at least 30% by area, and then
holding in the temperature range for at least 30 seconds.
15 If the annealing temperature is lower than (Aei point +10° C), a large
amount of austenite grains for suppressing growth of the crystallized grains are not
formed, and it is difficult to obtain a cold-rolled steel sheet having a fine structure,
which is the object of the present invention. Accordingly, the annealing
temperature is made at least (Aci point +10° C). Preferably it is at least (Aei point
20 + 30° C).
On the other hand, if the annealing temperature is higher than (0.95 xAcs
point + 0.05 X Aei point), sudden growth of austenite grains may occur, thereby
coarsening the final structure. In particular, since annealing is carried out for at
least 30 seconds in order to guarantee manufacturing stability, coarsening of the
25 structure easily progresses. Accordingly, the annealing temperature is made at
most (0.95xAe3 point + O.OSxAci point). Preferably it is at most (O.SxAcs point +
0.2xAei point).
Heating to the annealing temperature is carried out by rapid heating. The
heating conditions at this time are based on the above-described new findings.
30 Since these findings are obtained fi-om the result of below-described Example 2, this
point will next be described in detail.
Figure 1 shows the average grain diameter d^ of ferrite of a cold-rolled steel
sheet as a function of the rate of temperature increase at the time of annealing for
some of the cold-rolled steel sheets of steel types A - C shown in Table 5. As
shown in Figure 1, as the rate of temperature increase becomes higher, the average
grain diameter of ferrite of a cold-rolled steel sheet decreases. As stated above, as
5 the average grain diameter of ferrite of a cold-rolled steel sheet decreases, the tensile
strength of the steel sheet increases.
In this connection. Figure 2 shows the relationship between the percent
increase in the tensile strength relative to the tensile strength when the rate of
temperature increase was 10** C per second and the rate of temperature increase at
10 the time of annealing. As shown in Figure 2, if the rate of temperature increase is
at least 50° C per second, an increase in tensile strength of at least 2% is stably
achieved. Namely, ifthe rate oftemperature increase is 50° C per second, the
effect attributed to an increase in the rate of temperature increase can be stably
achieved.
15 The higher the rate of temperature increase at the time of annealing of a
cold-rolled steel sheet, the higher the proportion of ferrite which remains
unrecrystallized (the percentage of unrecrystallized ferrite) when the annealing
temperature is reached. As a result of an investigation with respect to the
relationship between the rate of temperature increase and the percentage of
20 unrecrystallized ferrite at a temperature of (Aei point +10° C), it was found that the
percentage of unrecrystallized forite was at least 30% by area when the rate of
temperature increase was at least 50° C per second. In other words, by elevating
the temperature to the above-described annealing temperature range under such
conditions that the percent of unrecrystallized ferrite at a temperature of (Aei point +
25 10° C) is at least 30% by area, the effect of refining the structure formed by
performing cold rolling and subsequent rapid heating annealing on a hot-rolled steel
sheet having a fine structure can be stably obtained.
Accordingly, a cold-rolled steel sheet obtained by the above-described cold
rolling step is heated to a temperature range for annealing which is at least (Aci
30 point + 10° C) by rapid heating which satisfy the conditions that the percentage of
unrecrystallized ferrite at a temperature of (Aci point + 10° C) is at least 30% by
area. There is no particular upper limit on the percentage of unrecrystallized ferrite
T pxi
at this time. If the percent of unrecrystallized ferrite when reaching a temperature
of (Aei point + 10° C) is less than 30%, it is difficult to stably obtain the effect of
refining the structure when cold rolling and rapid heating annealing are carried out
on a hot-rolled steel sheet having a fine structure. It is sufficient to carry out rapid
5 heating annealing until the temperature reaches (Aei point + 10° C) at which ferrite
and austenite begin to coexist, and following this temperature, heatmg may be slow
heating or isothermal temperature holding.
Since the rate of temperature increase is a means of adjusting the percentage
of unrecrystallized ferrite at the temperature of (Aei point + 10° C), it is not
10 necessary to restrict the rate of temperature increase, but it is preferably at least 50°
C per second, more preferably at least 80° C per second, particularly preferably at
least 150° C per second, and most preferably at least 300° C per second. There is
no particular upper limit on the rate of temperature increase, but fi'om the standpoint
of controlling the annealing temperature, it is preferably at most 1500° C per second.
15 The above-described rapid heating can start fi'om a temperature before
reaching the recrystallization starting temperature. Specifically, if the temperature
for the start of softening which is measured at a rate of temperature increase of 10°
C per second is Ts, it is sufficient to start rapid heating fi-om (Ts - 30° C). In
actuality, it is sufficient to start rapid heating fi-oiji 600° C, and the rate of
20 temperature increase before reaching this temperature can be any desired value.
Even if rapid heating is started fi-om room temperature, it does not have an adverse
effect on the cold-rolled steel sheet after annealing.
There is no particular limit on a heating method as long as the necessary rate
of temperature increase can be achieved. It is preferable to use resistance heating
25 or induction heating, but as long as the above-described temperature increase
conditions are satisfied, it is also possible to use 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
30 a heating device to an existing continuous annealing line or a hot-dip plating line.
When the annealing temperature is a temperature range of at least (Aci point
+ 10° C) to at most (0.95 xAes point + 0.05 x Aei point), if the annealing time is less
2^X
than 30 seconds, recrystallization is not completed, and most of the grain boundaries
m the structure are constituted by small angle grain boundaries with a tilt angle of at
most IS*' or a state occurs in which dislocations which are introduced by cold rolling
remain. In this case, the workability of a cold-rolled steel sheet markedly
5 decreases. Accordingly, in order to sufficiently promote recrystallization, the
annealing time is made at least 30 seconds. Preferably it is at least 45 seconds and
more preferably at least 60 seconds.
It is not necessary to restrict an upper limit on the annealing time, but fix)m
the standpoint of suppressing coarsening of recrystallized ferrite grains with greater
10 certainty, it is preferably made less than 10 minutes.
Figure 3 shows the change in the value of TSxEl as a function of the holding
time for annealing when a cold-rolled steel sheet made of steel type B of Example 2
shown in Table 5 was annealed by heating to 750° C at a rate of temperature
increase of 500° C per second and then holding for 15 - 300 seconds. From this
15 result, it can be seen that even if a cold-rolled steel sheet according to the present
invention has a long annealing time of around 300 seconds, grain growth is
suppressed and stable material properties are obtained. On the other hand, if the
annealing time is less than 30 seconds, the structure of the steel'sheet does not
complete recrystallization, and an increase in the grain diameter still progresses, or
20 the phase transformation does not reach an equilibrium state with the transformation
in structure remaining in an intermediate state. As a result, workability
(elongation) is poor, and in actual operation, it becomes difficult to stably obtain a
uniform structure.
Cooling after annealing can be carried out at a desired cooling rate, and by
25 controlling the cooling rate, it is possible to precipitate a second phase such as
pearlite, bainite, or martensite in the steel. The cooling method can be any desired
method. For example, cooling with a gas, a mist, or water is possible. After
cooling from the annealing temperature to an appropriate temperature, overaging
heat treatment may be performed by supplemental reheating, if necessary, and
30 holding at a temperature of at least 200° C and at most 600° C. Alternatively, after
cooling the annealed steel sheet to an appropriate temperature, it can be subjected to
surface treatment such as plating. Specifically, a steel sheet which has undergone
i
annealing can be subjected to hot-dip galvanizing, galvannealing (hot-dip
galvanizing followed by annealing for alloying), or electrogalvanizing to obtain a
galvanized (zinc-plated) steel sheet.
5 2.3 - Hot Rolling Step
A hot-rolled steel sheet which is subjected to cold rolling has a
microstructure which satisfies the conditions stated in the section on cold rolling,
namely, it has the above-described chemical composition and a microstructure
satisfying above Equations (5) and (6). There are no particular limitations on a
10 manufacturing method of the hot-rolled steel sheet which is used, but preferably it
has excellent thermal stability. A preferred hot-rolled steel sheet can be
manufactured by a hot rolling step in which a slab having the above-described
chemical composition undergoes hot rolling with rolling being completed at the Ars
point or above, and then within 0.4 seconds of the completion of rolling it is cooled
15 to a temperature range of at most 750° C at an average cooling rate of at least 400° C
per second.
By employing such a hot rolling step, the strains which have been
introduced into austenite during rolling can be prevented from being consumed by
recovery and recrystallization as much as possible. As a result, strain energy
20 accumulated in the steel can be used to the maximum extent as a driving force for
transformation from austenite to ferrite, resulting in the formation of an increased
amount of nuclei for transformation from austenite to ferrite, thereby refining the
structure of the hot-rolled steel sheet and imparting excellent thermal stability to the
structure.
25 By subjecting a hot-rolled steel sheet which is manufactured in this manner
to cold rolling and then the above-described annealing, refinement of a cold-rolled
steel sheet can be effectively achieved.
From the standpoint of productivity, a slab which is subjected to hot rolling
is preferably manufactured by continuous casting. The slab may be used in a high
30 temperature state after continuous casting, or it may be first cooled to room
temperature and then reheated. From the standpoints of reducing the load on
rolling equipment and easily guaranteeing the temperature at the completion of
as
T ;a'
rolling, the temperature of the slab which is subjected to hot rolling is preferably at
least 1000° C. From the standpoint of suppressing a decrease in yield due to scale
loss, the temperature of a slab which is subjected to hot rolling is preferably at most
1400° C.
5 Hot rolling is preferably carried out using a reversing mill or a tandem mill.
From the standpomt of industrial productivity, it is preferable to use a tandem mill
for at least the final number of stands.
During rolling, because it is necessary to maintain the steel sheet in an
austenite temperature range, the temperature at the completion of rolling is made at
10 least the Aij point. In order to suppress as much as possible thermal recovery of
woiidng strains which are introduced into austenite, the temperature at the
completion of rolling is preferably just above the Ara point and specifically at most
(Ars point + 50" C).
The rolling reduction in hot rolling is preferably such that the percent
15 reduction in the sheet thickness when the slab temperature is in the temperature
range from the Ara point to (Ara point + 100° C) is at least 40%. The percent
reduction in thickness in this temperature range 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
20 preferable because it can introduce a larger amount of strain energy into austenite,
thereby increasing the driving force for ferrite transformation and refining ferrite
more greatly. However, doing so increases the load on rolling equipment, so the
upper limit on the rolling reduction per pass is preferably 60%.
As stated above, cooling after the completion of rolling is preferably carried
25 out by cooling to a temperature range of 750° C or below at an average cooling rate
of at least 400° C per second within 0.4 seconds of the completion of rolling.
It is more preferable to fiirther shorten the time required for cooling from the
completion of rolling to 750° C or below, to fiirther increase the cooling rate, and to
cool to a lower temperature since it can more greatly refine the structure of the hot-
30 rolled steel sheet. Specifically, the time for cooling from the completion of rolling
to a temperature range of 750° C or below is preferably made at most 0.2 seconds.
The average cooling rate at the time of cooling within 0.4 seconds after the
'2^H
completion of rolling to a temperature range of 750° C or below is preferably made
at least 600° C per second and is particularly preferably at least 800° C per second.
Cooling within 0.4 seconds after the completion of rolling to a temperature range of
720° C or below at an average cooling rate of at least 400° C is still more preferable.
5 The temperature range for cooling is preferably at least the Ms point. The cooling
method is preferably water cooling.
After carrying out the above-described cooling, the steel sheet may be held
at a temperature of 600 - 720° C for a desired length of time to allow ferrite
transformation to proceed and control the percent by area of ferrite in the structure.
10 In order to sufficiently form equiaxial grains of ferrite in the hot-rolled steel sheet, it
is preferable to hold the steel sheet for at least 3 seconds at a temperature of 600 -
720° C.
Then, until the steel sheet is coiled, cooling can be carried out at a desired
cooling rate by water cooling, mist cooling, or gas cooling. The steel sheet can be
IS coiled at a desired temperature.
The structure of the hot-rolled steel sheet which is subjected to cold rolling
preferably has ferrite as a main phase, and it may contain at least one hard phase
selected fi'om pearlite, bainite, and martensite as a second phase.
20 2.5 - Plating
With the object of improving corrosion resistance and the like, a plating
layer like that described above may be formed on the surface of the cold-rolled steel
I : sheet which is obtained by the above-described manufacturing process to form a
I surface treated steel sheet. Plating can be carried out in a conventional manner.
25 Following plating, a suitable chemical conversion treatment may be carried out.
I EXAMPLE 1
i
This example illustrates cold-rolled steel sheets according to the present
invention.
30 Ingots of steel types AA - AN having the chemical compositions shown in
Table 1 were prepared by melting in a vacuum induction ftimace. Table 1 shows
the AC] point and the Aea point ofeach steel type. These transformation
N
?^
temperatures were determined from a thermal expansion chart measured when a
steel sheet which was cold rolled under the below-described manufacturing
conditions was heated to 1000" C at a rate of temperature increase of 5" C per
second. Table 1 also shows the values of (Aci point + 10° C) and (0.05Aej +
s 0.95 Aca) and the calculated values of the right sides of above-described Equations
(l)and(5).
The right side of Equation (1) = 2.7 + 10000/(5 + 300xC + 50xMn +
4000xNb + 2000xTi + AOO^Wf .
The right side of Equation (5) = 2.5 + 6000/(5 + 350xC + 40xMn)^
:
^ S c o S * "S coco 0 0 0 0 00
d "
o
V O C O O O r ^ T - I O C ^ I O C M i - I O C S J C M ^ CM
+ . 0 T - O ^ ^ O T - C M O J T - O I - C M I - T -
, 0 . r^ r. r^^^
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o g i 3 o * * ' * * ' * o » o < o p » o q r * r ^ r » r * ; o o
•«»TB
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.0 L_ I I I I I I I P I I I I I 2 fO
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o o J o o o o o o o o o o o o o o r S
u d d d d d d d d d d d d d d iS
-5 3
g Q - o o o P P o o o o o o o o o f i
-c d d d ° ° d d d d d d d d d -5
r : c o r ^ e n T - a > o O | « q m < o o o ^ i o c o c M S
^ T - ^ » - ^ o c s i * ^ » - d l < ^ * * J * ^ < ^ c « i e N i i - : a
:-
C Q P p S i o i q " ^ S ' ^ 5 S S ' « ' 5 t : i o -
o o o o o " ^ o ^ ~ d d d C 3 o o c
|c
c o c < i o o o > o o o > o o c M c o r ~ . o > o o T - T- X
O o o ^ m c o i o < o < o t * - r « » i o r ~ o o c g «
' - ^ T - O O O O O O O O O O T - - O
J) d d d d d d d d d d d d d d^
ji _ I — — i — — — — — — - ^ . ^ — — — . 1 ^ -^
^ ^ < < < < < < < < < < < < < <
2.^
- t ^
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
and cooling were carried out using a small test mill with the temperature at the
5 completion of roUmg, the cooling time fix)m the completion of rolling to 750** C, the
cooling rate (water cooling), and the coiling temperature shown m Table 2 to
manufacture hot-rolled steel sheets having a thickness of 1.5 - 3.0 mm.
The average grain diameter d of ferrite in each hot-rolled steel sheet is
shown in Table 2. The grain diameter of ferrite in a hot-rolled steel sheet was
10 measured on a cross section in a widthwise direction at a depth of 1/4 of the
thickness of the steel sheet using an SEM-EBSD apparatus (model JSM-700 IF
manufactured by JEOL Ltd.) and determined by analyzing the grains defined by
high angle grain boundaries having a tilt angle of at least 15°.
The resulting hot-rolled steel sheets were pickled with a hydrochloric acid
15 solution and subjected to cold rolling with the cold rolling reduction shown in Table
2 (each at least 30%) to reduce the sheet thickness of the steel sheets to 0.6 mm - 1.0
mm, and then amiealing was carried out thereon with the heating rate (rate of
temperature increase), annealing temperature (soaking temperature), and holding
time for amiealing (soaking time) shown in Table 2 using a laboratory scale
20 annealing apparatus to obtain cold-rolled steel sheets. Cooling after soaking was
carried out with helium gas.
>2
T X
Table 2
Hot roiling conditions Cold rolling/annealing conditions
^'^ Steel Temp «tL^,.^TRateof I d Cold I lAnn^hl Holding
'^•«* type couple- . i) water ^emp (j^) rdling ^^e ,'"« ^ - ^ J^
•^o- ton """'. cooling ,^^ reduc- fon/^^ ^ temp, annealing
j CO ^^^^ CC/sec) ^^^ tion(%)^^^^°°^ (°C) (sec)
A1 AA 840 0.070 1300 RT 2.0 60 10 800 30
A2 AA 840 0.070 1300 RT 2.0 60 300 800 30
A3 AA 840 0.070 1300 RT 2.0 60 300 740 30
A4 AB 840 0.070 1300 RT 1.9 60 10 800 30
A5 AB 840 0.070 1300 RT 1.9 60 100 800 30
A6 AC 850 0.123 885 RT 2.1 50 10 800 60
A7 AC 850 0.123 885 RT 2.1 50 100 800 60
A8 AC 860 8.000 130 RT M 50 10 800 60
I A9 AC 860 8.000 130 RT M 50 500 800 60
A10 AD 810 0.065 930 RT 2.3 55 500 800 30
All AE 810 0.065 930 550 1.9 55 10 800 30
A12 AE 810 0.065 930 550 1.9 55 500 800 30
A13 AF 810 0.065 930 RT 2.5 55 10 800 60
A14 AF 810 0.065 930 RT 2.5 55 500 800 60
I A15 AQ 850 0.123 885 RT 3.2 50 10 800 60
A16 AQ 850 0123 885 RT 3.2 50 100 800 60
A17 AH 820 0.076 920 RT 1.8 55 10 820 30
A18 AH 820 0.076 920 RT 1.8 55 100 820 30
A19 AI 810 0.072 840 RT 2.1 55 10 800 30
A20 AI 810 0.072 840 RT 2.1 55 500 800 30
A21 AJ 810 0.072 840 RT 2.3 55 10 800 30
A22 AJ 810 0.072 840 RT 2.3 55 500 800 30
A23 AK 810 0.072 840 RT 2.0 55 10 800 30
A24 AK 810 0.072 840 RT 2.0 55 500 800 30
A25 AK 810 0.072 840 550 2.0 55 10 800 30
A26 AK 810 0.072 840 550 2.0 55 500 800 30
I A27 AL 810 0.072 840 550 2.2 55 10 760 30
i A28 AL 810 0.072 840 550 2.2 55 500 760 30
• A29 AM 810 0.072 840 RT 2.2 55 10 800 30
A30 AM 810 0.072 840 RT 2.2 55 500 800 30
'• A31 AN 820 0.084 840 RT 2.1 55 10 780 30
A32 AN 820 0.084 840 RT 2.1 55 500 780 30
Underlining indicates values outside the range of the present invention.
RT = room temperature
Cooling time from completion of rolling to 750° C.
i
I *
The microstructure and mechanical properties of the cold-rolled steel sheets
which were manufactured in this manner were investigated as follows.
The average grain diameter d^ of ferrite of the cold-rolled steel sheets was
determined in the same manner as described with respect to the hot-rolled steel
s sheets by analyzing the structure of a cross section in the widthwise direction at a
depth of 1/4 of the thickness of a steel sheet using an SEM-EB SD apparatus. The
average grain diameter ds of the second phase was determined by calculating the
equation: r = (A/NJC)^^ from the number of grains N of the second phase and the area
A of the second phase measured on the structure of a cross section in the widthwise
10 direction at a depth of 1/4 of the thickness of a steel sheet.
The percent by area of ferrite and the percent by area of the second phase
which was a phase other than ferrite were determined by the point count method in
an SEM photograph take in the widthwise direction of a cross section at a depth of
1/4 of the thickness of the steel sheet. The percent by volume of the austenite
15 phase was determined by X-ray diffractometry, and this value was taken as the
percent by area of retained austenite (retained y). By subtracting this percent by
I area from the above-described percent by area of the second phase, the percent by
{ area of the low temperature transformation phase which was the hard second phase
' was determined. This low temperature transformation phase contained at least one
20 of martensite, bainite, pearlite, and cementite.
Measurement of the texture of the cold-rolled steel sheets was carried out by
X-ray diffractometry on a plane at a depth of 1/2 of the sheet thickness of a steel
sheet. The average of the X-ray intensities in three orientations, i.e., {Ill }<145>,
{111}<123>, and {554}<225> orientations was determined using ODF (orientation
25 distribution function) which was obtained by analyzing the measured results of pole
I figures of {200}, {110}, and {211} of ferrite. Separately, the average X-ray
j intensity of a random structure not having a texture was determined by X-ray
diffraction of a powdered steel. The ratio of the average X-ray intensities in the
! above-described three orientations to the average X-ray intensity of the random
30 structure was calculated, and this ratio was made the average X-ray intensity. The
apparatus which was used was a RINT-2500HL/PC manufactured by Rigaku
Corporation.
3o
i
The mechanical properties of the 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 half-size ASTM tensile test piece, and the yield strength, the tensile
strength (TS) and the elongation at rupture (total elongation El) were determined.
3 The hole expanding test was carried out by expanding a hole with a punched
diameter do of 10 mm using a conical punch having a peak angle of 60'', and the
percent hole expansion X (%) was determined from the hole diameter di at the time
when a crack formed at the edge surface of the punched hole reached both surfaces
of the sheet as X, = (di - doVdo^ 100.
10 Table 3 shows the results of investigation of the structure and the
mechanical properties of the cold-rolled steel sheets. Compliance with Equations
(1) - (4) is shown by the mark o (compliance with all equations) or x (lack of
compliance with at least one equation).
T
^ I 5 S I S B S -I -I S I 5 -I S -I -I -I § -I § -I § --p 5 I 5 -p --^ I 5 "B S
a e 11 s I e I g g 111 s 1 21 a I s I 21 s I s I s e 81 c I
5 a « o a S a S a a $ a c a 9 o a a 9 a 9 o 9 a 9 a S a o Q « Q«
O o J 3 > s o > S S . S o o > 5 o « s o > s o o o > s o < s o > s o > s o > s o o o > a o >s
O O O O O 0 _ 0 _ 0 0 0 _ 0 _ _ 0 _ 0 _ 0 _ 0 0 0 _ 0 _
5 is
= . | ' ^ T X OO x O ' < O x x Q x O x O ^ >< x O x O x O ^ O x O x O ^ O x O
j ^ o o o o » « ' < f e M ' ^ e M e o c > j T - o o c M c > i ^ c s < c o o > i o e o r « - r ^ o r » u » i o ^ «*
* x^^ o o > r « ' i ( > < 3 r S i » o B « a o o ^ < D ^ o i ^ < o < o c M e M O i - a > m T - c o i o c ^ a > i o a > a>
« J^^S (p/IQ^w . <ceiiiTon-» (-ao«' 4<' <-oi4-«-><'o«o^'<j cci ;o4-i«2—r^'rS4«?-«oSo4ScmSo
m ^^ __
"5 _ 5 f
2 UJv T - a » < o ^ c M t n < o c M ' ^ ^ i o i o o o r > o O ( o i n i n i A < o ( 0 ( D < o o < o a > a > o o c o e o r»
" V C M O c o r » i o « o i o ^ c o e o « o o ^ c > | i — o » ^ r » r > < o ^ c M ^ c « i o < D r - > e 2 < O Q o o <»
—, '^ • c » * i o r » r - o » o o i o < M r » ^ r » < o r ~ ' * r » Q r » < o i o e M ^ ' ' T ' - r - c 5 « M t o r i Q O «o
2 COQ. co<4-racM«o(D' |J:i.:::::i:ii:i:ii:::i::::::::i:::::
•g ,-N * ^ ' ^ * H * ? « ? P • * : ' * ? ' T r « . e > i e o ^ c M P * H o ' - o ^ i o o e o e M c » j o o o o o e o » - o o < » c q ' ^ ^ i n a q c q e q c o < ^ c > i e v i o , '<4:^cdio
t . UJ£ ^ i o c > j c o ^ r ^ r ^ < d ^ K < D c d ( 0 < d o > a > < 4 ' r > ^ i r i r * ^ ^ i o i o r < ^ i o < b e o c 4 i o ( D O > ' r^
• cMeiicMeoeo»-'--^»-«-'r-'^T-T-^'.-T-»--F-T-eM
a.
e •
a ,. '<• i n i o o o o i o o i o i n o o o o o m p p p p i n p i o p p p p ^ l J ^ i q p i q ^
•5 f^Q. • f - ' e d i o r > > c 9 e d o ' a > < d i n o e d c o a > c N i c 4 a > T ^ a d T - i r i i o ^ ^ o r ^ S N < ^ i ' > < ^ < ^ _
o " " S ^ i o o > i o r ^ i o o o i p < O i - T - e M e o i O ' - c M ^ o o o o o > o » o i o r ^ o ) O o S i H a > T - f - c M 5
•g w i o i o i o i n i n ^ ^ 9 ' < - r ^ < o « D < o < o ^ ' ^ a > a > c o a o c o < » a o a o a o c o ^ ^ a o a > o o o o ^ .S
« ——^— — — — — — — — — ^ — — — — — — — — — — C u
o , . «o o o i o o o o i n i o o o o o o o o m p p o i n i o i o i o i o p i n i n i A i n i i > p c 4 ' ' g Q ,
2 > * 3 ^ r > m m a o r ~ o i ^ a > r » c M c o ^ ( o c o r a o o a > ^ r o i o i o r o c M o r » < o o o o o ^ < o c •"
•S ^ ^ i o ^ ^ c o ^ o o ^ ^ ' * > * ^ c o e o i o u 5 r » - r - « r > - i » > r > r » - r - « o < o < D < o r « > i o » o o _g
——— §0
2 2 « o > o < D i o i o i o c o u i < o < o u 5 r « - o c o o > ^ c M O > ' * o t o a i e M a > ^ e o o o i ~ - c ^ o o ' » - « 9 - g }"
% S, % ^ •^ "^ vi -^ vi •^ c< c< "•* m -^ vi •«r'»-ooeO'r-o>r»ooi-i— ' * c o o > a > < D r > r ~ o > r - - o o o i - o o T - o > o o o O ' ^ " og
• 3- ' - ^ o o c N i o ' ^ o e « i e v J d ^ o c v i ^ c s i c a i - ; o ' r - : o ^ c > ' - ' ^ " t - ' - ' ^ r - ' 7 - : c ) ^ T ^ g ^
J, — ^.^
S - r f § p p » - « o > o q ' < - c o o > i o c o « o i o i o ' * ^ « D i o c o ' < * i o ^ ' - c ^ p c v j r > - " * « o ^ < o « o c v i5
+j 3- «eviesicoc>i^eor~-'i«CNic«scjM i ' ^ ' i - c > i e M - ^ i - e M e M T - i - / \ t ( i
^_-±-± X^
^ i L « :: •
o A ^ ° r « - r > r > - c o c o ^ c o m ' « r a > c o c o c s i O r ^ < n < o t n o r - > r ^ i o r > c o m m ( o c > i c M r « ^ ' < t i-fi*
-o S E 55
ag *' _ _ _ _ .£ g
> o * 2-5
»« .a (B''cwcocor~r~-ooococ>ieO'*oo»-'>-coP^cocortcor»«Or-ooio F <»
^ S Coooooocooooooooooooooooor»ooo>a>ioio<-a
«> •= *J 5 fi
y~ S S n _ t M c « 5 T f i n < o r - . o o a > 0 ' - ^ « ^ ^ " ' * 0 ' ^ " ' 0 < ' > 0 ' - < ^ W ' * - i o « o i « » o o o > o ^ w «C0
Of steel sheets Nos. Al - A3 manufactured from steel type AA, with A2 and
A3 in which a hot-rolled steel sheet with a grain diameter of less than 3.5 \aa was
I used as a starting material and the heating rate at the time of annealing was at least
50** C per second, cold-rolled steel sheets having a microstructure according to the
I 5 present invention were obtained. On the other hand, with Al, due to the heating
rate at the time of annealing which was low, the grain diameter of ferrite and that of
the second phase m the cold-rolled steel sheet were coarse, and the average X-ray
intensity in the above orientations which is an indicator of a texture was less than 4.
As a result, with A2 and A3 which were examples of the present invention, a high
10 degree of workability which satisfied above Equation (8) was obtained.
Similar results were obtained for the other steel types. Based on whether
the tensile strength (TS) was less than 800 MPa or at least 800 MPa, a high degree
of woricability which satisfied Equation (8) or Equation (9) was obtained. With
AlO, A13, A14, A17 - A20, A23 - A26, and A29 - A32 to which one or more of Nb,
i
15 Ti, and V were added, when the heating rate was at least 50° C per second, the
feirite grain diameter satisfied Equation (4) (less than 3.5 ^m), and a cold-rolled
\ I steel sheet having a preferred microstructure was obtained.
! In contrast, with A8 and A9, due to the hot-rolled steel sheets used as a
starting material which had coarse grains with a grain diameter of 6.4 [an, in spite of
20 carrying out annealing by rapid heating, the microstructure of the cold-rolled steel
sheets coarsened, and the average grain diameter of ferrite and the average grain
diameter of the second phase both exceeded the upper limits defined by the present
invention. In addition, the X-ray intensity of the texture fell below 4.0. As a
result, the mechanical properties were insufficient.
25 A15 and A16 had an Mn content of 0.37%, and the cold-rolled steel sheet
had coarse grains because suppression of grain growth during annealing did not
work sufficiently. As a result, good mechanical properties were not obtained.
I A27 and A28 had an Nb content of 0.052%, and due to suppression of the
i
formation of recrystallization nuclei during annealing, a deformation texture
30 remained in the cold-rolled steel sheet. The extent to which such a deformation
texture remained became more marked as the heating rate at the time of annealing
increased. As a result, the mechanical properties of the cold-rolled steel sheet were
i •
poor regardless of the heating rate.
EXAMPLE 2
This example illustrates a process for manufacturing a cold-rolled steel sheet
s according to the present invention.
Ingots of steel types A - K having the chemical compositions shown in
Table 4 were prepared by melting in a vacuum induction fiimace. The resulting
ingots were hot forged and then cut into slabs to be subjected to hot rolling. The
slabs were heated for approximately 1 hour at a temperature of at least 1000° C, then
10 they underwent hot rolling using a small test mill with the temperature at the
completion of rolling, the cooling time from the completion of rolling to 750° C, the
cooling rate (water cooling), the holding time, and the temperature at the termination
of rapid cooling shown in Table 5, and then they were cooled to room temperature to
manufacture hot-rolled steel sheets having a sheet thickness of 1.5 - 3.0 mm.
15 Table 4 shows the Aci point and the Aes point for each steel type which was
determined by the method described in Example 1, the value of (Aei point +10° C),
the value of (O.OSAei + 0.95Ae3), and the calculated values of the right sides of
i Equation (1) and Equation (5).
• *
(^ a> ui a> •r- o a> o o o o o o o o>
Is
•tfg. W T - c o r ^ i o 00 ao a> o r--r-
S --
" "
^ < 3 ^ < ^ < 0 - ^ 0 00 C J O O ^ N i -
S W o O ' ^ C M C M i - T- « ^ « M « « T -
^ O s-' O O O O O O O O O O 00 O O Q O O O O O O)
go
o
Y ^ O ) o o i o o o a > r ~ M i O ' * ' - r»
+ y ( O i - O O O ) 0» O O J T - C M OI
e ° < o r « c o r « < o to r- to lo O J I O T - O J CM
^ ^.x oooOoOoooo 00 o o o o o o o o a>
S TP i o o o o a > o o 00 a > o o o i - oo
< ° ^ < D r > « o < o « D r > oo
I — — ^ ^ — ^ -^
^ " S o o P _o
O ^ .c
^ i - < o lor-x ^ e o i o < o o oc
. S J ^ I J ^ J ^ «^ c o c o c o o o c o ig
• Q P P ^ P O O O O ' ^ O W
( o O O O O o O O O i - ^ O 'S
^ °-
w s i i i i i p g p i i p * ;
C "^ O o O O O
•—* 4>
c *^
•^ j ^ r I I 1 I I I I p I p 2
o _ I
o c o o o o o o o o o o o o r s
^ O O O O O O O O O O O J 3
a 5
u o
•= ° J ^ 1 2 1 2 J 2 ° C O O O O O O OT
(D Q - p p p p p o O O O O O ®
J= O O O O O o o o d d o - < 5
C o » c s u 3 ^ o a j S O j - o o o c n | S E
^ • * O C » 0 0 ' * ' ^ ' « 1 0 0 > 0 + J '
:2 I
— • • - • * i o i o < o i — O i — o i — i n .E
( O i r j o o o o m i o i o ^ i o O h B ?
d o o d d d d d d d d c i
;c f
o o c o r o O ) ! — a > a > a o T - i f > c \ i X f
• * t ; p p p T - : c > 4 p p T - O T - g T 3 i
o d d d d d d d o ' d d d - ^ I
la I — — — — —I — — — — . — -^ P
i 2 •« « I
^^ I M I n I I M I I i
F I
t
35
« 4
^ X
The average grain diameter d of ferrite defined by high angle grain
boundaries having a tilt angle of at least 15° of each hot-rolled steel sheet, which
was determined in the same manner as described in Example 1, is shown in Table 5.
After the hot-rolled steel sheets were pickled with a hydrochloric acid
5 solution, they underwent cold rolling with a rolling reduction of at least 30% (shown
in Table 5) to reduce the thickness of the steel sheets to 0.6 -1.4 mm and then
annealing using a laboratory-scale annealing apparatus with the heating rate (rate of
temperature increase), annealing temperature, and annealing time shown in Table 5
to obtain cold-rolled steel sheets. Cooling after soaking (annealing) was carried out
10 in the same manner as in Example 1.
Table 5 shows the percentage of unrecrystallized ferrite at a temperature of
i the Aci point + 10** C (referred to below simply as the percent unrecrystallized
ferrite). This value was determined by the following method. A steel sheet which
underwent processing up to cold rolling in accordance with the manufacturing
15 conditions for each steel number was heated to a temperature of around Aei point +
10° C (error of ± 15° C) at the heating rate shown for each steel number, and it was
I immediately cooled by water cooling. The structure was photomicrographed with
j an SEM, and by measuring the fractions of recrystallized ferrite and deformed ferrite
on the resulting photomicrograph of the structure, the percentage of unrecrystallized
20 ferrite was determined as being equal to the fraction of deformed ferrite. As can be
seen from Table 5, the percentage of unrecrystallized ferrite correlates to the heating
rate during annealing, and when the heating rate is at least 50° C per second, the
percentage of unrecrystallized ferrite becomes at least 40%. In Example 1, the
i percentage of unrecrystallized ferrite was not measured, but it is certain that it
i 25 exhibits the same tendency as in Example 2.
The yield strength, tensile strength, and elongation at rupture (total
elongation) of the cold-rolled steel sheets which were manufactured in this manner
were determined by subjecting a half-size ASTM tensile test piece prepared from
I each steel sheet to a tensile test. The total elongation was evaluated as acceptable
30 if it is at least 20%. Since the strength of a steel sheet is highly dependent upon its
I chemical composition, the strength of steel sheets which were manufactured from
the same steel types but by different manufacturing processes were compared, and
the manufacturing processes were evaluated based on these results. The average
grain diameter dm of ferrite defined by high angle grain boundaries with a tilt angle
of at least 15° in the cold-rolled steel sheets after annealing was determined in the
same manner as described in Example 1. The results of measurement are shown in
5 Table 5.
I I al I I I 91 SI $1 Si Si I I I I Si I I Si Si SI $1 $1 $1 %.\ Si
2" I § § § B fl I .S .§ S § i I I § I I I I I I I I -I ,
a fi I I fl 5 c e 8 c -I I I I c -I ••& g c s c E c s c 5
^ a s S S a - S . a a a « S S S 9 ' S S 9 ' R - 9 ' 9 ' & 9 ' § - 9 ' . n'
J i i l i i I I I i i l i l i l l I I 11 i§ I I fS
o 0 0 0 0 0 o 0 0 0 0 0 0 0 0 * - . - — J — — — — — — — — — — — — — — — — — — — — — — — — s ^
"8 6 e « i o r » « o i o < « » « D r » « > ^ r * * e ^ o 9 « > « > » H M i « c « * i o i a i o q i e o a5
= "O W c J r ^ - i - ' « W W « < < » < « > C \ l < N C J ^ « > I C i l ^ ' « I ^ I O l ' * l ^ " * J « 2C 1 ^ . •§ «
jb'Vt —r-N o ^ < o < e ^ o < o a o c < 4 r » < O T - c o c o e o c o o i o u ^ r > o c 4 0 a > S ^
^x ;^. -g I
r «• ^ J^
? I I s s s s s s s s s s s s s ^ i l s s s s s s s s ^ l
|_Lr |J
• S ^ t?-2-' r - p ~ r - - r > o o | o o l o o | r « - r « . r » r - r ~ r » r « i ^ r « » « o | o o | o M r ~ r « ' r - r » « § "
•o I *" up
J i l l + S 3 l £ 5 S S ; « > t S 5 g « i « f 9 < : ' S S S S S i « > i ^ ^ S ; s S ? J - ^B
1 III 8iiii^ii2igigsi|iii|^isii8io,||
^ ^ ^ l§
O ! •• •
- a ' ^ ^ : § O O O Q O O O O O O O O O O O O O O O O O O Q O «_
i ^ 0 O § ^ « e ( O I O $ < O C O < O C D < O C D C O ( O C O < D C D < D ( 0 ( 0 < 0 < O C O ( 0 0 « O O .E
^•^ ^ 1 ?
C _ - g r i « E i o u > i o i o i o i o « o o o o o p p o o r - T - i - i p ( u > i T H ' i - 4 0 ' E - s
•g-^'S^ io
M f. M 5 g
^ O . ' S ^ ^ O Q O O Q O O O O O O O O O O O O O O O O O O O C "
*''''§ . lS 'S~'''2 • * ' « f t ' * ' * - ^ ^ c M C M « s o 4 c > 4 e > i c > ! c s ! e M C M C M c j i o i o e M e ^ " * o2
T> X "•*^^"^ 2 g
o «* -3 ~ • B
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1 ^ • Q C * o o o o o o o - > - ' « - T - i - i - i - T - » - - ^ , - - . - ^ p p ' ^ ' - : S 5g
o •§ J5 o o o o o o o o o o o o d o o d d o o «>i 01 " " o .3 ^
! _ii_! 5|
! I i s ° ° 2 2 2 2 2 9 § § § § § § § § § ? § § § g g § | |
I 1— 1 ^ 2 ' ^ o o o o o o o o o o a o o o e o o o o o o o o o c o a o o o o o c o o o c o o o o o o o e o o o S :=
I ' . =5 8
-£ J l < < < < < < < ( a o Q f f l f f i m f f i a ) a ) i n a ) a i m i D ( a ( Q a ) o ? Sc
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o 0 0 0 0 0 0 0 0 0 0 0 0 * _-
1 t"S _f o o i > » o » « o « c < i | r « - o » i o r - c o o o ' * r > ' M > « o « o « » c 4 « ' l 3 * 8 3 - i2
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• < M C > « C M C M C M e M e < C « i - « M < M C M C 4 e « » C < C > I C 0 C « I C i l C M ^ ' * ' f C c I
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— _ _ . ^ J_
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. f i ^ + 3 « 5 '- o>a>ji^ '-'- CO "g _± 5 g
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_J± ? E
u 9 - ^ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 & >»
CM '^ &^ o o o o o o o o o o o o o o o o o o o o a o c o o o o o c o o o c o a o o > a > a > a > a > ( J —
I I • • ^ o
10 _ — — — — — i—i — — ^ - » x - ^ ^ a — — . _ _ ^ . i _ _ _ _ _ i _ _ g ^
;S ^^ :i«|
S S d i o < o r ~ o o o > O T - c « i < < > ' « - ! 0 < o r « o o o > Q ' - c M c < > ' < t u i < o r ^ - S - Sc
j3 _C Z W C M e M C M C M C O C O C O O O C J C O C O P J COlcO * ' « * - T f ^ ' « * ^ ^ » « - c ! 2o
I <^ «» I I I I I I I I I I I I I I I I I I I I I I I I3 § "5
3^
*' I
Of cold-rolled steel sheets Nos. 1-7 which were manufactured using steel
type A, the tensile strength had a high value of 697 - 710 MPa for Nos. 2 - 4 which
were manufactured m accordance with the present invention. In addition, the total
elongation exceeded 20% for each steel sheet. On the other hand, for the steel of
3 steel sheet No. 1, the cooling rate at the time of annealing after cold rolling was
slow, and the percentage of unrecrystallized ferrite was less than 30%. For this
reason, the grain diameter of ferrite was large, and the tensile strength decreased.
For steel sheets Nos. 5 - 7 , due to the annealing temperature which was too high, the
grain diameter of ferrite did not fall into the range defined by the present invention,
10 and the tensile strength was around 100 MPa lower than for steel sheets 2 - 4 . I
The same tendency was observed with cold-rolled steel sheets manufactured
using steel type B. In addition, for steel sheet No. 14 of steel type B, because of
the annealing time which was too short, the total elongation was lower than for other
cold-rolled steel sheets using the same steel type B, and even when steel sheets were
15 manufactured a plurality of times under the same conditions as for No. 14, stable
manufacture was not possible with properties varying from one location to another
j within the same steel sheet. For steel sheet No. 17 of steel type B, due to the
annealing temperature after cold rolling which was a low value of 650" C, a
i sufficient amount of austenite was not formed, the grain diameter of ferrite became
20 large, and tensile strength decreased. For steel sheets Nos. 20 - 23 of steel type B,
since the rapid cooling after hot rolling was insufficient, the hot-rolled steel sheet
which was subjected to cold rolling had a large grain diameter of ferrite. As a
result, the grain diameter of ferrite after cold rolling became large, and tensile
strength decreased.
25 The above-described tendency which was observed with cold-rolled steel
sheets of steel types A and B was similarly observed for cold-rolled steel sheets
which were manufactured using the remaining steel types C - J having a chemical
i
composition in the range of the present invention.
For steel sheets Nos. 45 - 47 which were manufactured using steel type K,
30 since they did not have a chemical composition defined by the present invention,
even if hot rolling was carried out by immediate rapid cooling, the grain diameter of
ferrite in the hot-rolled steel sheets became large. As a result, the grain diameter of
H CJ I
ferrite in the cold-rolled steel sheet could not be refined by varying the annealing
temperature, and the tensile strength became extremely low.

We claim:
1. A cold-rolled steel sheet characterized by having:
a chemical composition comprising, in mass%, C: 0.01 - 0.3%, Si: 0.01 -
5 2.0%, Mn: 0.5 - 3.5%, P: at most 0.1%, S: at most 0.05%, Nb: 0 - 0.03%, Ti: 0 -
0.06%, V: 0 - 0.3%, sol. Al: 0 - 2.0%, Cr: 0 -1.0%, Mo: 0 - 0.3%, B: 0 - 0.003%,
Ca: 0 - 0.003%, REM: 0 - 0.003%, and a remainder of Fe and impurities;
a microstructure having a main phase of ferrite which comprises at least
50% by area and a second phase containing a total of at least 10% by area of a low
10 temperature transformation phase including one or more of martensite, bainite,
pearlite, and cementite and 0 - 3% by area of retained austenite, and satisfying the
following Equations (1) - (3); and
a texture in which the average X-ray intensity for the {111 }<145>,
{111 }<123>, and {554}<225> orientations at a depth of 1/2 of the sheet thickness is
15 at least 4.0 times the average X-ray intensity of a random structure which does not
have a texture:
d„, < 2.7 + 10000/(5+300xC+50xMn+4000xNb+2000xTi+400xV)^ - (1)
d„.<4.0 - (2)
d5<1.5 •• (3)
20 wherein
C, Mn, Nb, Ti, and V indicate the contents (mass%) of the respective
elements,
dm is the average grain diameter (|xm) of ferrite defined by a high angle grain
boundary having a tilt angle of at least 15", and
25 ds is the average grain diameter ([un) of the second phase.
2. A cold-rolled steel sheet as set forth in claim 1 wherein the chemical
composition contains, in mass percent, one or more elements selected from Nb: at
least 0.003%, Ti: at least 0.005%, and V: at least 0.01%, and the microstructure
30 satisfies the following Equation (4):
dm < 3.5 ••• (4)
wherein d^ is as defined in claim 1.
3. A cold-rolled steel sheet as set forth in claim 1 or 2 wherein the
chemical composition contains, in mass percent, sol. Al: at least 0.1%.
i
4. A cold-rolled steel sheet as set forth in any of claims 1 to 3 wherein the
s chemical composition contains, in mass percent, one or more elements selected from
Cr: at least 0.03%, Mo: at least 0.01%, and B: at least 0.0005%.
5. A cold-rolled steel sheet as set forth in any of claims 1 to 4 wherein the
chemical composition contains, in mass percent, one or two elements selected from
10 Ca: at least 0.0005%, and REM: at least 0.0005%.
6. A cold-rolled steel sheet as set forth in any of claims 1 to 5 which has a
plating layer on the surface of the steel sheet.
15 7. A process for manufacturing a cold-rolled steel sheet characterized by
j comprising the following steps (A) and (B):
(A) a cold rolling step in which a hot-rolled steel sheet having a chemical
composition as set forth in any of claims 1 to 5 and having a microstructure which
satisfies the following Equations (5) and (6) is subjected to cold rolling to obtain a
20 cold-rolled steel sheet, and
(B) an annealing step in which the cold-rolled steel sheet obtained in Step
I (A) is subjected to annealing by increasing the temperature of the steel sheet to a
temperature range of at least (Aei point + 10° C) to at most (0.95 xAe3 point +
0.05 xAei point) under conditions such that the proportion of unrecrystallized ferrite
25 is at least 30% by area when the temperature (Aei point + 10° C) is reached and then
holding the steel sheet in this temperature range for at least 30 seconds:
d < 2.5 + 6000/(5 + 350xC + 40xMn)^ - (5)
d<3.5 • (6)
wherein,
30 C and Mn are the contents of the respective elements (mass percent); and
d is the average grain diameter (^m) of ferrite defined by a high angle grain
boundary having a tilt angle of at least 15°.
M3
*
8. A process for manufacturing a cold-rolled steel sheet as set forth in
claim 7 wherein the hot-rolled steel sheet is obtained by a hot rolling step
comprising performing hot rolling with a temperature at completion of rolling of at
least the Aij point on a slab having the above-described chemical composition and
5 then performing cooling to a temperature range of 750° C or below at an average
cooling rate of at least 400" C per second within 0.4 seconds after completion of
rolling.
9. A process for manufacturing a cold-rolled steel sheet as set forth in
10 claim 7 or 8 further having a step of carrying out plating on the cold-rolled steel
sheet after step (B).