DESCRIPTION
HIGH TENSILE STEEL SHEET SUPERIOR IN FORMABILITY AND
METHOD OF MANUFACTURING THE SAME
TECHNICAL FIELD
[0001] The present invention is directed to a high
tensile steel sheet superior in a formability
suitable for a vehicle body or the like and a method
of manufacturing the same.
BACKGROUND ART
[0002] In recent years, weight reduction of a
vehicle body is increasingly required for the sake of
improvement of automobile fuel efficiency. Though a
steel sheet with a high strength is used for weight
reduction of the vehicle body, press forming becomes
difficult as the strength becomes high. This is
because, in general., a yield stress of a steel sheet.
increases and an elongation is reduced as a strength
of the steel sheet becomes high. Further, as a high
tensile'steel sheet for a vehicle body, one to which
a galvanizing treatment or a chemical treatment such
as a phosphating treatment is performed, such as a
galvanized steel sheet, is sometimes used.
Therefore, such a high tensile steel sheet is
required also of a good galvanizing property and a
chemical treatment property.
[0003] With regard to improvement of an elongation,
- 1 -
a TRIP (transformation induced plasticity) steel
sheet, in which strain induced transformation of a
retained austenite is used, is described in Patent
Literature 1 and Patent Literature 2. However, since
a large amount of C is contained in a TRIP steel
sheet, there is, a problem in welding such as nugget
cracking. Further, in a TRIP steel sheet with a
tensile strength equal to or more than 980 MPa in
particular, a„yield stress is so high that there is a
problem that a shape fixability at a time of press
forming or the like is low.
[0004] Further, there is a concern that a delayed
fracture occurs in the high tensile TRIP steel sheet
with the tensile strength equal to or more than 980
MPa. Since the TRIP steel sheet contains a large
amount of a retained austenite, a void and a
dislocation are apt to occur frequently in an
interface between a martensite generated by induced
transformation at a time of processing and a
surrounding phase thereof. Then, hydrogen is
accumulated in such places, thereby generating the
delayed fracture.
[0005] Further, with regard to reduction of a yield
stress, DP (dual phase) steel, which includes a
ferrite, is described in Patent Literature 3.
However, in order to manufacture the DP steel, it is
necessary that a cooling speed after
recrystallization annealing is as quite high as equal
to or more than 30°C/s. Accordingly, application to
manufacturing of a galvanized steel sheet using a
common manufacturing line is difficult.
[0006] Though Patent Literatures 3 to 6 describe
various indexes about a formability, it is difficult
to make a formability of elongation flanging of an
automobile component sufficient by only adjusting
those indexes within predetermined ranges.
CITATION LIST
PATENT LITERATURE
[0007]. Patent Literature 1: Japanese Laid-open
Patent Publication No. 61-157625
Patent Literature 2: Japanese Laid-open Patent
Publication No. 10-130776
Patent Literature 3: Japanese Laid-open Patent
Publication No. 57-155329
Patent Literature 4: Japanese Laid-open Patent
Publication No. 2001-355043
Patent Literature 5: Japanese Laid-open Patent
Publication No. 2007-302918
Patent Literature 6: Japanese Laid-open Patent
Publication No. 2008-63604
SUMMARY OF THE INVENTION
TECHNICAL PROBLEM
[0008] An object of the present invention is to
provide a high tensile steel sheet superior in a
formability in which the formability and a
galvanizing treatment property can be made compatible
- 3 -
with each other, and a method of manufacturing the
same.
SOLUTION TO PROBLEM
[0009] The present inventors find out that, with
regard to a DP steel sheet having a low yield
strength, a formability and a galvanizing treatment
property maybe made compatible with each other by
making a relation between a Si content and an A]
content appropriate and making a hardness
distribution appropriate. Then, -the present
inventors have reached ideas of embodiments of the
invention described below.
[0010] (1) A high tensile steel sheet superior in
a formability, containing, in mass %:
C: 0.03% to 0.20%;
Si: 0.005% to 1.0%;
Mn: 1.0% to 3.1%; and
Al: 0.005% to 1.2%,
a P content being over 0% and equal to or less
than 0.06%,
an_S content being over 0% and equal to or less
than 0.01%,
an N content being over 0% and equal to or less
than 0.01%, and
a balance being composed of Fe and inevitable
impurities,
wherein
a metal structure includes a ferrite and a
.- 4 -
martensite,
a relation of a formula (A) is established about
an Al content (%) and a Si content (%), and
an average value Yave defined by a formula (B)
regarding hardnesses measured at 100 points or more
with a nanoindenter is equal to or more than 40.
0.3 0.7x[Si] + [Al] S 1.5 ... (A)
Yave = E('180x(Xi - 3)-2/n) ...(B)
([Al] indicates the Al content (%), [Si]
indicates the Si content (%), n indicates a total
number of the measuring points of the hardnesses, and
X indicates the hardness. (GPa) at the i-th measuring
point (i is a natural number equal to or less than
n).
[0011] (2) The high tensile steel sheet superior
in a formability according to (1), which further
contains:
at least one selected from a group consisting of,
in mass %,
B: 0.00005° to 0.005%,
Me: 0.01% to 0.5%,
Cr: 0.01% to 1.0%,
V: 0.01% to 0.1%,
Tie 0.01% to 0.1%,
'Nb: 0.005% to 0.05%,
Ca: 0.0005% to 0.005%, and
REM: 0.0005% to 0.005%.
[0012] (3) The high tensile steel sheet superior
in a formability according to (1) or (2), wherein the
- 5 -
high tensile steel sheet is a cold-rolled steel
sheet.
[0013] (4) The high tensile steel sheet superior
in a formability according to any one of (l)to (3),
wherein the high tensile steel sheet is a galvanized
steel sheet.
[0014] (5) The high tensile steel sheet superior
in a formability according to any one of (1) to (4),
wherein a martensitefractio'n in the steel structure
is over 5%.
[0015]. (6)A method of manufacturing a high
tensile steel sheet superior in a formability,
including:
obtaining a hot-rolled steel strip by performing
hot rolling;
next, performing acid pickling of the hot-rolled
steel strip;
next, obtaining a cold-rolled steel strip by
performing cold rolling of a steel strip with a
tandem rolling mill having a plurality of stands;
next, performing continuous annealing of the
cold-rolled steel strip in a continuous annealing
line; and
next, performing temper rolling of the coldrolled
;steel strip,
wherein
the steel strip contains, in mass %.
C: 0.03% to 0.200;
Si: 0.005% to 1.0%;
Mn: 1.0% to 3.1%; and
Al: 0.005% to 1.2%,
a P content being over 0% and equal to or
less than 0.06%,
an S content being over 0% and equal to or
less than 0.01%,
an N content being over 0% and equal to or
less than 0-01%, and
a balance being composed Fe and an
inevitable impurity, and
a,relation of a formula (C) being established
about a cold-rolling reduction in the first stand
among the plurality of stands and a temperature
increasing rate in a first heating zone in the
continuous annealing line.
50 <_ rlo.esxV 5 300 (C)
(rl indicates the cold-rolling reduction
(%), and V indicates the temperature increasing rate
(°C/s)
[0016 ) ( 7) The method of manufacturing a high
tensile steel sheet superior in a formability
according to (6), further including, after said
performing the continuous annealing;
performing a galvanizing treatment to the coldrolled
steel strip; and
next, performing a temper rolling of the coldrolled
steel strip.
[00171 (8) The method of manufacturing a high
tensile steel sheet superior in a formability
- 7 -
according to (7), further including, after said
performing the galvanizing treatment, holding the
cold-rolled steel strip at a temperature of 400°C to
650°C for t seconds, wherein a relation of a formula
(D) is established.
t < 60x[C] -+ 20x[Mn] + 24x[Cr] + 40x[Mo) ... (D)
([C] indicates a C content (%), [Mn] indicates an
Mn content.(l), [Cr] indicates a Cr content (o), and
[Mo] indicates'. an Mo content (%).)
ADVANTAGEOUS EFFECTS OF INVENTION
[0018] According to the present invention, since a
relation between an At content and a Si content are
made appropriate and a hardness distribution is made
appropriate, a formability and a galvanizing
treatment property can be made compatible with each
other,
BRIEF DESCRIPTION OF DRAWINGS
[0019] [Fig. 1] Fig. 1 is a graph representing a
relation among an Al content and a Si content, and a
formability, and a galvanizing treatment property and
a chemical treatment property;
[Fig. 2] Fig. 2 is a graph representing a
relation between an average value rave of a formula
(B) and a formability;
[Fig. 3] Fig. 3 is a diagram illustrating a test
piece used for a side bend test;
[Fig. 4] Fig. 4 is a graph representing a
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relation between a cold-rolling reduction r and a
temperature increasing rate V, and a formability; and
[Fig. 5] Fig. 5 is a graph representing a
relation between a C content, a Mn content, a Cr
content and an Mo content, and a holding time.
DESCRIPTION OF EMBODIMENTS
[0020] Hereinafter, an embodiment of the present
invention will; he described in detail with reference
to the attached drawings.
[0021] A steel sheet according to the embodiment of
the present invention contains, in mass %, C: 0.03%
to 0.20%, Si: 0.005% to 1.0%, Mn: 1.0% to 3.1%, and
Al: 0.005% to 1.2%, a P content being over 0% and
equal to or less than 0.06%, an S content being over
0% and equal to or less than 0.01%, an N content
being over 0% and equal to or less than 0.01%, and
the balance being composed of Fe and an inevitable
impurity.
[0022] Here, a reason for a limit of the content of
such a component will be explained.
[00231 --, C secures a strength and stabilizes a
martensite. If a C content is less than 0.03%, it is
difficult to obtain a sufficient strength and the
martensite is hard to be formed. On the other hand,
if the ;C content is over 0.2%, the strength becomes
too high and a sufficient ductility is hard to be
obtained and sufficient weldability is hard to be
obtained. Therefore, a range of the C content is
0.03% to 0.2%. Here, it is preferable that the C
content is equal to or more than 0.06%, and it is
more preferable that the C content is equal to or
more than 0.070. Further, it is preferable that the
C content is equal to or less than 0.15% and it is
more preferable that the C content is equal to or
less than 0.12%.
[0024] Si secures a strength and a ductility,
exhibits a deoxidation effect, and improves a
quenching property. If a Si content is less than
0.005%, it is difficult to obtain a sufficient
deoxidationeffect, and it is difficult to obtain a
sufficient quenching property. On the other hand, if
the Si content is over 1.0%, it is difficult to
obtain a sufficient chemical treatment property and a
galvanizing treatment property. Therefore, a range
of the Si content is 0.005° to 1.0%. Here, it is
preferable that the Si content is equal to or more
than 0.01%, and it is more preferable that the Si
content is equal to or more than 0.05%. Further, in
a case that a good galvanizing treatment property is
regarded, as important in particular, it is preferable
that the Si content is equal to or less than 0.7%.
Further, it is more preferable that the Si content is
equal to or less than 0.6%, and it is further
preferable that the Si content is equal to or less
than 01%.
[0025] Mn secures a strength, delays generation of
a carbide, and is effective in generation of a
- 10 -
ferrite, If aMn content is less than 1.0%, it is
difficult to obtain a sufficient strength, and
generation of the ferrite becomes insufficient,
making it hard to obtain a sufficient ductility. On
the other hand, if the Mn content is over 3.1%, a
quenching property is too high, generating a
martensite excessively and the strength is too high.
Consequently, a sufficient ductility is hard to be
obtained, and a large variation in the property is
apt to occur. Therefore, a range of the Mn content
is 1.0,% to 341< Here, it is preferable that the Mn
content is equal to or more than 1.2% and it is more
preferable that the Mn content is equal to or more
than 1.5%. Further, it is preferable that the Mn
content is equal to or less than 2.8% and it is more
preferable that the Mn.content is equal to or less
than 2.6%.
[0026] Al accelerates generation of a ferrite,
improves a ductility, and exhibits a deoxidation
effect. If an Al content is less than 0.005%, it is.!
difficult to obtain a sufficient deoxidation effect.
On the-other hand, if the Al content is over 1.2%, an
inclusion such as alumina increases, and it is hard
to obtain a sufficient processability. Therefore, a
range of the Al content is 0.005% to 1.20. Here, it
is preferable that the Al content is equal to or more
than 0`.02% and it is more preferable that the Al
content is equal to or more than 0.1%. Further, it
is preferable that the Al content is equal to or less
- 11 -
than 1.0% and it is more preferable that the Al
content is equal to or less than 0.8%. It should be
noted that, even if a large amount of Al is
contained, a chemical treatment property and a
galvanizing treatment property are hard to be
reduced.
[0027] Since P contributes to improvement of a
strength, P may be contained in correspondence with a
required strength level. However, if the P content
is over 0.06%, segregation in a grain boundary occurs
and a local ductility is apt to be reduced, and a
weldabilityi.s apt to be reduced. Therefore, the P
content is equal to or less than 0.06%. Here, it is
preferable that the P content is equal to or less
than 0.03%, and it is more preferable that the P
content is equal to or less than 0.02%. On the other
hand, in order to make the P content less than
0.001%, an intensive cost increase in a steel forming
stage is necessary, and in order to make the P
content 0%, a further intensive cost increase is
necessary. Therefore, it is preferable that the P
content is over 0% and equal to or more than 0.001%.
[0028] S generates MnS and reduces a local
ductility and weldability. In particular, if the S
content is over 0.01%, these are prominent.
Accordingly, the S content is 0.010. Here, it is
preferable that the S content is equal to or less
than 0.007%, and it is more preferable that the S
content is equal to or less than 0.005%. On the
- 12 -
other hand, in order to make the S content less than
0.001%, an intensive cost increase in a steel forming
stage is necessary, and in order to make the S
content 0%, a further intensive cost increase is
necessary. Therefore, it is preferable that the S
content is over 0% and equal to or more than 0-001%.
[0029] N is inevitably contained, and an N content
over 0.01% reduces an aging property. Further, AlN
is generated i,n a large quantity and an effect of Al
is reduced. Accordingly, the N content is equal to
or less than 0.01%. Here, it is preferable that the
N content is equal to or less than 0.007%, and it is
more preferable that the N content is equal to or
less than 0.0050. On the other hand, in order to
make the N content less than 0.0005%, an intensive
cost increase in a steel forming stage is necessary,
and in order to make the N content 0 %, a further
intensive cost increase is necessary. Therefore, it
is preferable that the N content is over 0% and equal
to or more than 0.0005%.
[0030] It should be noted that the steel sheet
according to the present embodiment may contain one
or more selected from a group consisting of B, Me,
Cr, V, Ti, Nb, Ca, and rare earth metals (REM) within
a range indicated below.
[0031] B contributes to securing of a quenching
property, generates BN, and increases effective Al.
In general, when a ferrite fraction increases, a
superior elongation may be secured, but a layered
- 13 -
structure is made and sometimes a local ductility is
reduced. B suppresses such reduction of the local
ductility. If a B content is less than 0.00005%, the
effect is hard to be obtained. On the other hand, if
the B content is over 0.005%, an elongation in a
tensile test and an elongation distortion amount
(value of a fracture elongation distortion) in a side
bend test are reduced significantly. Accordingly, it
is preferable that a range of the B content is
0.00005% to 0.005%. Here, it is more preferable that
the B content is equal to or more than 0.0001%, and
it is further preferable that the B content is equal
to or more than 0.0005%. Further, it is more
preferable that the B content is equal to or less
than 0.003%, and it is further preferable that the B
content is equal to or less than 0.002%,
[0032] Me contributes to securing of a strength and
improvement of a quenching property. If a Me content
is less than 0.010, these effects are hard to be
obtained. On the other hand, if the Me content is
over 0.5%, generation of a ferrite is suppressed, so
that a_ductility is reduced. Further, if the Mo
content is over 0.5%, obtaining a sufficient chemical
treatment property and a galvanizing treatment
property sometimes becomes difficult. Accordingly,
it is preferable that a range of the Me content is
0.01% to 0.5%. Here, it is more preferable that the
Mo content is equal to or more than 0.03%, and it is
further preferable that the Me content is equal to or
- 14 -
more than 0.05%. Cr contributes to securing of a
strength and improvement of a quenching property. If
a Cr content is less than 0.01%, these effects are
hard to be obtained. On the other hand, if the Cr
content is over 1.0%, generation of a ferrite is
suppressed and a ductility is reduced. Further, if
the Cr content is over 1.0%, obtaining a sufficient
chemical treatment property and a galvanizing
treatment property sometimes becomes difficult.
Accordingly, it is preferable that a range of the Cr
content is 0.01% to 1.0 %. Here, it is more
preferable that the Cr content is equal to or more
than 0.1% and it is further preferable that the Cr
content is equal to or more than 0.20. Further, it
is more preferable that the Cr content is equal to or
less than 0.7% and it is further preferable that the
Cr content is equal to or less than 0.5%.
[0033] V, Ti, and Nb contribute to securing of a
strength. If a V content is less than 0.01%, a Ti
content is less than 0.01%, and an Nb content is less
than 0.005%, the effect is hard to be obtained. On
the other hand, if the V content is over 0.1%, the Ti
content is over 0.1%, and the Nb content is over
0.05%, an elongation in a tensile test and an amount
of an elongation distortion in a side bend test are
reduced significantly. Accordingly, it is preferable
that a? range of the V content is 0.01% to 0.1%, and
it is preferable that a range of the Ti content is
0.01% to 0.10, and it is preferable that a range of
- 15 -
the Nb content is 0.005% to 0.05%.
[0034] Ca and REM contribute to control of an
inclusion and improvement of a hole-expanding
property. If a Ca content is less than 0.0005% and
an REM content is less than 0.0005%, these effects
are hard to be obtained. On the other hand, if the
Ca content is over 0.005% and the REM content is over
0.005%, an elongation in a tensile test and an amount
of an elongation distortion in a side bend test are
reduced significantly. Accordingly, it is preferable
that a range of the Ca content is 0.0005% to 0.005%,
and it is preferable that a range of the REM content
is 0.0005% to 0.005%.
[0035] Incidentally, as the inevitable impurity, Sn
and the like can be cited. If a content of such an
inevitable impurity is equal to or less than 0.01%,
an effect of the embodiment is not impaired.
[0036] In the steel sheet according to the present
embodiment, a relation of a formula (A) is
established between the Al content and the Si
content.
0.3._< 0.7x[Si] + [Al] <_ 1.5 _. (A)
Here, [Al] indicates the Al content (%) and [Si]
indicates the Si content (%).
[0037] A large amount of elements are added to
conventional high tensile steel, and formation of a
ferrite is suppressed. Therefore, a ferrite fraction
of a structure is low and a fraction of another phase
(second phase) is high. Accordingly, an elongation
- 16 -
is considerably reduced particularly in DP steel with
a tensile strength equal to or more than 980 MPa. In
contrast, it is possible to make an elongation larger
by increasing the Si content, by lowering the Mn
content, or the like. However, if the Si content is
made high, a chemical treatment property and a
galvanizing treatment property are apt to be reduced.
Further, if the Mn content is made low, securing of a
strength becomes difficult,
[0038] Under the circumstances, the present
inventors found out the above-described effect of Al,
as a result of earnest study. Further, as a result
of investigation of a relation among a Si content and
an Al content, a formability, and a galvanizing
treatment property (a plating treatment property) and
a chemical treatment property, a result represented
in Fig. I was obtained. In other words, if a value
of "0.7x[Si] + [Al]" was less than 0.3, a formability
was insufficient.. Further, if a value of "0.7x[Si] '±
[Al]" was over 1,5, a good chemical treatment
property and a galvanizing treatment property failed
to be obtained. From those results, it may be said
that when the relation of the formula (A) is
satisfied it is possible to secure a sufficient
ferritefraction thereby to obtain a superior
elongation while securing a plating treatment
property and a chemical treatment property.
Incidentally, a result of an investigation of a
relation between a formability and a result of a
- 17 -
tensile test indicated that when the formability was
sufficient., with regards to an elongation EL (%) and
a tensile strength TS (MPa) obtained by the tensile
test, a value of "EL x TS" was equal to or more than
16000% MPa and that when the formability was
insufficient the value of "EL x TS" was less than
16000% MPa.
[0039] It should be noted that an evaluation of the
formability and an evaluation of the chemical
treatment property and the galvanizing property may
be performed similarly to an evaluation, for example,
in .later-described examples No. 1 to No. 27 and
comparative examples No. 28 to No. 43.
[0040] Further, a metal structure of the steel
sheet according to the present embodiment includes a
ferrite and a martensite. The ferrite includes a
polygonal ferrite and a bainitic ferrite- The
martensite includes a normal martensite obtained by
quenching and a martensite obtained by tempering
performed to a temperature equal to or lower than
600°C. In the present embodiment, because of such a
metal structure, a tensile strength and a ductility
may be made compatible with each other.
[0041] The ferrite fraction and the martensite
fraction are not limited in particular, but it is
preferable that the martensite fraction is over 5%.
This is because a martensite fraction of less than 5%
makes it hard to obtain a tensile strength of equal
to or more than 500 MPa. It should be noted that
- 18 -
more preferable ranges of the ferrite fraction and
the martensite fraction are different in
correspondence with required tensile strengths and
elongations. In other words, since heightening of
the ferrite fraction enables securing of the
elongation and heightening of the martensite fraction
enables securing of the tensile strength, it is
preferable to adjust each range based on a balance of
the elongation, and the tensile strength. For
example, if the tensile strength is 500 MPa to 800
MPa, it is preferable that the range of the ferrite
fraction is 50% to 90%, and it is preferable that the
range of the martensite fraction is 10% to 40%. If
the tensile strength is 800 MPa to 1100 MPa, it is
preferable that the range of the ferrite fraction is
20% to 60%, and it is preferable that the range of
the martensite fraction is 30% to 60 If the
tensile strength is over 1100 MPa, it is preferable
that the ferrite fraction is equal to or less than
30% and it is preferable that the martensite fraction
is equal to or more than 40%.
[0092] Further, it is preferable that the metal
structure of the steel sheet according to the present
embodiment also includes a bainite, and it is
preferable that a range of a bainite fraction is 10%
to 40%.; Incidentally, in order to secure a tensile
strength, it is more effective to increase the
martensite fraction than to increase the bainite
fraction, the martensite being able to secure a
19
required tensile strength by smaller fraction. Thus,
it becomes possible to increase the ferrite fraction
by that portion thereby to increase an elongation.
Therefore, it is preferable that the martensite
fraction is higher than the bainite fraction. It
should be noted that if an austenite remains in a
metal structure, a secondary processing brittleness
and a delayed fracture property are apt to be
reduced. Therefore, it is preferable that a retained
austenite is not substantially contained, but the
retained austenite of less than 3% may be inevitably
contained,
[0043] Further, in the steel sheet according to the
present embodiment, an average value Yave defined by a
formula (B) regarding hardnesses measured at 100
points or more with a nanoindenter is equal to or
more than 40.
rave = E(180x(Xi - 3) -'/n) ...( B)
Here, n indicates a total number of measuring
points of hardnesses, and Xi indicates a hardness
(GPa) at the i-th (i is a natural number equal to or
less than n) measuring point.
[0044] The present inventors found out that as an
index indicating a formability of a steel sheet used
for a vehicle body or the like an elongation
distortion amount s measured in a side bend test is
superior to an elongation and a hole-expanding value.
Further, the present inventors found out that the
larger an elongation distortion amount a is made the
- 20 -
better a formability becomes.
[0045] Further, the present inventors found out
that as represented in Fig. 2 the larger the average
value Yave of the formula (B) is made the larger a
value of "s x TS " being a product of an elongation
distortion amount s (%) and a tensile strength TS
(MPa) becomes. Besides, when the value of "e x TS"
was equal to or more than 40000% MPa, a good
formability could be obtained. Hence, it may be said
that if an average value Yave is equal to or more than
40, a good formability may be obtained. It should be
noted that an upper limit of the average value rave is
not limited in particular, but a maximum value of the
average value Yave obtained in the test conducted by
the present inventors is 250.
[0046] Further, it was also found out that in a
case that the value of the product "s x TS" is equal
to or more than 40000% MPa, it is more preferable and
superior in terms of,a formability if further a value
"EL x TS" being a product of the elongation EL (%)
and the tensile strength TS (MPa) is equal to or more
than 16000% MPa.
[0047] It should be noted that in the side bend
test, an in-plane bending is applied to an end face
on which a cutout is formed, and an elongation
distortion amount at a time that a through crack
occurs`:.is measured. Fig. 3 illustrates a shape of a
test piece. In order to evaluate an elongation
flange property, a cutout 2 with a large curvature
21 -
radius is provided in the test piece 1. Further, in
order to measure an elongation distortion amount
after the test, a marking line is provided. Once the
test is started, the test piece 1, while receiving a
tensile stress in a circumferential direction, is
bent and fractured. In the side bend test, it is
judged that a "fracture" occurs when a through crack
occurs in athickness direction. In other words,
unlikely to in,' the hole-expanding test, the
elongation distortion after the through crack is not
influenced by a size of a crack. Hence, variation of
crack judgment does not occur.
[0048] According to the present embodiment, since
the relation between the Si content and the Al
content represented by the formula (A) is made
appropriate and a hardness distribution represented
by the formula (B) is made appropriate, the
formability, and the galvanizing treatment property
and the chemical treatment property may be made
compatible with each other..
[0049] Further, the hardness distribution
represented by the formula (B) reflects a result of
the side bend test, and the result of the side bend
test may represent a formability of an automobile
part or, the like with a higher degree of accuracy
than an elongation and a hole-expanding property
being conventional indexes representing a
formability.
[0050] It should be noted that though a strength of
- 22 -
the steel sheet according to the present embodiment
is not limited in particular, but a tensile strength
of, for example, about 590 MPa to 1500 MPa may be
obtained in correspondence with a composition. An
effect of compatibility of the formability, and the
galvanizing treatment property and the chemical
treatment property is prominent particularly in a
high tensile steel sheet of equal to or more than 980
MPa.
[0051] In order to manufacture the steel sheet
according to the present embodiment described above,
a steel with the above-described composition may be
used, and a processing similar to that of, for
example, a method of manufacturing a hot-rolled steel
sheet, a method of manufacturing a cold-rolled steel
sheet, or a method of manufacturing a plated steel
sheet which are generally performed may be performed.
For example, obtaining of a cold-rolled steel strip
by cold rolling of a steel strip, . and. continuous
annealing of the cold-rolled steel strip may be
performed. Further, there may be performed obtaining
of a hot-rolled steel strip by hot rolling of steel,
acid pickling of the hot-rolled steel strip,
obtaining of a cold-rolled steel strip by cold
rolling.of the hot-rolled steel strip, continuous
annealing of the cold-rolled steel strip, and temper
rolling of the cold-rolled steel strip, in that
sequence. Further, it is possible to perform a
galvanizing treatment after continuous annealing. In
- 23
such a case, for example, the temper rolling may be
performed after the galvanizing treatment.
[0052] For example, hot rolling may be performed
under a general condition. Incidentally, in order t
prevent reduction of processability as a result that
a strain is excessively applied to a ferrite grain,
it is preferable to perform hot rolling at a
temperature equal to or more than a point Ara.
Further, if hot rolling is performed at a temperature
over 940°C, a recrystallized grain diameter after
annealing sometimes become coarse' excessively.
Accordingly, it is preferable that hot rolling is
performed at equal to or less than 940°C. The higher
a coiling temperature of hot rolling is, the more
recrystallization and grain growth are accelerated,
so that processability is improved. However, if the
coiling temperature is over 550°C, generation of a
scale occurring at a time of hot rolling is
accelerated. Thus, a time necessary for acid
pickling is sometimes prolonged. Further, a ferrite
and a pearlite are generated in layers, so that C is
apt to diffuse. Accordingly, it is preferable that
the coiling temperature is equal to or less than
550°C. On the other hand, if the coiling temperature
is less than 400°C, a steel sheet is hardened and a
load at a time of cold rolling becomes high.
Accordingly, it is preferable that the coiling
temperature is equal to or more than 400°C.
[0053] Acid pickling may be performed under a
- 24 -
general condition,
[0054] Cold rolling after acid pickling may also be
performed under a general condition. It should be
noted that it is preferable that a range of a rolling
reduction of cold rolling is 30% to 70%. It is
because if the rolling reduction is less than 30%,
correction of a shape of a steel sheet sometimes
becomes difficult, and if the rolling reduction is
over 70%, a crack occurs in an edge portion of the
steel sheet or a deviation of the shape occurs.
[0055] Further, it is preferable that cold rolling
is performed continuously with a tandem rolling mill
having a plurality of stands and that a cold rolling
reduction rl (%) in the first stand and a temperature
increasing rate V (°C/Sec) in a first heating zone in
a continuous annealing line satisfy a relation of a
formula (C). Here, the continuous annealing line
includes a continuous annealing line provided in a
manufacturing line of a cold-rolled steel sheet and a
continuous annealing line provided in a manufacturing
line of a continuous galvanized steel sheet.
50 < rlo.e5xV 5 300 ... (C)
[0056) As a result that the present inventors
investigated the relation between the cold rolling
reduction rl and the temperature increasing rate V, a
result=represented in Fig. 4 was obtained. As
described above, if the value of "s x TS" is equal to
or more than 40000% MPa, a good formability may be
obtained. Thus, in Fig. 4, a condition under which
- 25 -
the value of "e x TS" is equal to or more than 40000%
MPa is indicated by "O" while a condition under which
the value of °ex TS" is less than 40000% MPa is
indicated by "X". If the value of "rlo.esxV" is less
than 50, a ferrite becomes too soft and a hardness
difference from a hard phase is large. On the other
hand, if the value of "rlo_85xV" is over 300, a rate of
unrecrystallization is too high and a formability is
reduced. It should be noted' that it is more
preferable that the value of "rl0-85xV" is equal to or
more than 100 and that it is more preferable that the
value of "rl°'85xV" is equal to or less than 250.
[0057] It is preferable that continuous annealing
is performed in a range equal to or more than a point
A,1 and equal to or less than a point A,3 + 100°C. If
continuous annealing is performed at a temperature
less than the point A,1, a structure is apt to become
uneven. On the other hand, if continuous annealing
is performed at a temperature over the point A13 -k
100°C, generation of a ferrite is suppressed by
coarsening of an austenite, leading to reduction of
an elongation. Further, it is desirable that the
annealing temperature is equal to or lower than 900°C
from an economical viewpoint. With regard to an
annealing time, it is preferable that the temperature
is held for equal to or more than 30 seconds in order
to eliminate a layered structure. On the other hand,
if the temperature is held for over 30 minutes, an
effect is saturated and a productivity is reduced.
- 26 -
Accordingly, it is preferable that a range of the
annealing time is 30 seconds to 30 minutes.
[0058] In cooling of continuous annealing, it is
preferable that a finish temperature is equal to or
less than 600°C. If the finish temperature is over
600°C, an austeni.te is apt to remain and a secondary
processing brittleness and a delayed fracture
property are apt to be reduced.
[0059] It should be noted that a tempering
treatment at equal to or less than 600°C may be
performed after continuous annealing. By performing
such a tempering treatment, for example, a holeexpanding
property and a brittleness can be made
better.
[0060] The present inventors consider, when
performing a galvanizing treatment after continuous
annealing, that it is preferable that after the
galvanizing treatment the cold-rolled steel strip is
held at a temperature of 400°C to 650°C for a time (t
second) satisfying a relation of a formula (D).
t <_ 60x[C] + 20x[Mn] + 24x[Cr] + 40x[Mo] ... (D)
Here, [C] indicates a C content (o), [Mn]
indicates a Mn content (o), [Cr] indicates a Cr
content (o), and [Me] indicates a Mo content (o)
[0061] The present inventors, as a result of
investigating a holding time in holding the coldrolledsteel
strip at a temperature of 400°C to 650°C
after the galvanizing treatment, obtained a result
represented in Fig. 5. A mark 0 in Fig. 5 indicates
- 27 -
that a sufficient tensile strength was obtained and a
mark X indicates that the tensile strength was
comparatively low. As represented in Fig. 5, if a
value of the holding time t (s) was over a value of a
right side (mass %) of the formula (D), the tensile
strength was comparatively low. This is because a
bainite is generated excessively thereby to make it
difficult to secure a sufficient martensite fraction.
EXAMPLE
[0062] Next, an experiment conducted by the present
inventors will be explained.
[0063) First, steel of examples No. 1 to No. 34 and
of comparative examples No. 35 to No. 52 having
compositions represented in a table 1 was fabricated
with a vacuum melting furnace. Next, after the steel
was cooled and solidified, the steel was reheated to
1200°C and finish rolling of hot rolling was performed
at 880°C. Thereafter, the steel was cooled to 500°C .
and a temperature was held at 500°C for one hour,
thereby a hot-rolled steel plate was obtained.
Holding-of the temperature at 500°C for one hour
simulates a heat treatment at a time of coiling in
hot rolling. Subsequently, a scale was removed from
the hot-rolled steel plate by acid pickling, and
thereafter, cold rolling was performed at a coldrolling:
reduction r represented in a table 4, thereby
a cold-rolled steel plate was obtained. Next, with a
continuous annealing simulator, the temperature of
- 28 -
the cold-rolled steel plate was increased at a
temperature increasing rate V represented in the
table 4 and annealing was performed at 770°C for 60
seconds. Thereafter, galvanizing was performed and
an alloying treatment was performed in an alloying
furnace, thereby an alloyed galvanized steel sheet
was manufactured.
[0064] Then, an elongation EL (%) and a tensile
strength TS (Mpa) were measured in a tensile test,
and an elongation distortion amount c (%) was measured
in a side bend test. In the tensile test, a JIS 5
test piece was used. The side bend test was
performed according to a procedure described above.
Then, a value of "EL x TS" and a value of "s x TS"
were found. Results thereof are represented in a
table 2. If at least the value of "s x TS" is equal
to or more than 40000% MPa, it may be. said that the
tensile strength and a ductility are compatible with
each other, and if the value of "EL x TS" is equal to
or more than 16000% MPa, it may be said that the
tensile strength and the ductility are better.
[0065]` Further, a metal structure was observed with
an optical microscope. On this occasion a ferrite
was observed after nital etching, and a martensite
was observed after repeller etching. Then, a ferrite
fraction and a martensite fraction were calculated.
Further, a surface having been chemically polished to
be 1/4 thickness from a surface layer of the steel
sheet was subjected to X-ray diffraction and a
29 -
retained austenite fraction was calculated. Results
thereof are represented in the.table 2.
[0066] Further, hardnesses X1 to X300 were measured
at 300 points per a test piece with a nanoindenter.
On this occasion, as the nanoindenter,
"TRIBOINDENTER" of HYSITRON was used and a measuring
interval was 3 μm. Then, an average value Ya„e was
calculated from the hardnesses X1 to X300. A result
thereof is represented in a table 3.
[0067] Further, evaluations of the chemical
treatment property and the galvanizing treatment
property were also performed. In the evaluation of
the chemical treatment property, after a treatment
with phosphate treatment chemicals according to a
standard specification, an aspect of a chemical
coating was observed by-visual observation and by a
scanning electron microscope. Then,.one which
covered a steel sheet base densely was judged to be''.
good and one which did not was judged to be poor. A z
the phosphate treatment chemicals, "Bt3080" of Nihon
Parkerizing Co., Ltd. being common automotive
chemicals was used. In the evaluation of the
galvanizing treatment property, after annealing was
performed under a condition satisfying the formula
(C), a galvanizing treatment was performed with a
galvanizing simulator and visual observation was
done. Then, one in which a plating film was evenly
formed in an area equal to or more than 90% of a
plated surface was judged to be good and one in which
- 30 -
the plating film was not evenly formed was judged to
be poor. Then, one which was good in both the
evaluation of the chemical treatment property and the
evaluation of the galvanizing treatment property is
indicated as and one which was poor in at least
one of the above is indicated as "X" in the table 3.
Further, after the galvanizing treatment, a
temperature was held at 500°C for a time indicated in
a table 4.
[0068] [Table 1]
- - - - - - - - - - - - -
- - - - 0
1 I 1 1 I 1 1 1 1 1. 1 1 1 1
N
1 1 1 1 { I I I I I I I B 1 1 1 1 1 1 1 1 1
O
O m N b O O
I f
(q 1 1 1 I I 1 1 1 I 1 I ! I I I I o 1 I 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 p 1
O O
-
O
- -
O O O
- - - - - - P- - M- - - - N N
ro
U I 1 1 1
O
O 1
OO
1 1 1 I I 1 1 I 1 1 1 1 1
O
O 1 1 I 1 1 1 1 1 I 1 1 1 1 I 1 1 1 1 1
O
O 1 1 1 1 1
O O O O
N 1n H N H N H
1 1 I 1 I I 1 1 1 1 1 1 1 I O,
O
O,
O
1 1 1 I 1 I i 9
O
I I 1
O
I I 1 1 O, 1 9
O
1 1 1 1 I I c
O
1 1 1
N
iy I I 1 I 1 1 I 1 1 1 1 1 1 1 1 1 O, 1 1 1 -v 1 1 I I 1 1 I 1 1 1 1 1 1 i I 1 1 '4 1
p O
.1
.^ 1 I 1 I 1 1 I 1 1 'I
)
1 1 I 1 1 1 I 1 I I I I 1 9 I 1 1 O, 1 I I 1 1 1 1 I 1 I 1 I 1 1 I 1 1 I I I O
O O O
1t1 V1 U) H 1-1 'i N H H
N
U) N H N b N m V1 N N H N N M H
O I I . - I l I I •-1 0 I N ri f V I N I O I 1 ri N I I N i 1 1 O I I N N M I N 1 I . i 1 I N 1
"I C ! "TV
0 0 0 I
O O o O O o o O o O O 0 o O O O
- -
O
-
O
- -
O
- - -
O
-
O
- -
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4I V I 1 I N 1 M 1 1. 1 1 M 1 I 1 1 1 I 1 I 1 N 1 1 M 1 1 1 1 1 1 I I 1 1 1 N I M N 1 1 1 1 1 1 1 1 N I
0
O
N O O O O O O ' O o O
I N P N N U) N E' m N N W C N N N O (1 N N O O O O N N t N N b N N m N M M M 0 0 0 0 N 0 0
'
N
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HN
O
@ N 10 M N 1•I N
'
N m N N W N N N
IO
VJ
NN
N (•1 M rf N N N N 10
rI
N
NN
P N N N P N N V1 0 N N 10 111 N 4' O p
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U ^ 1O h N a U) OI IO P O m P b P M 91 O M p1 N lO N M O M Ni N tO N
9
In tO M m lO rY b h m l9 nI P N O O. .. I-. . . . .
• 0 0 0 0 0 0 0 0 050 0 0 0 0 0 0 0 0 0 0 0 0 0 0 09090 0 0 0 0 0 0 0 H 0 0 0 0 0 0 0 0 H O H0 o
V1 C' N I[J M h N 0 U, N V1 N N N 0 0 N N P N 0 N N 0 0 U) 0 01 N OI W m d' N V) h N N 0 H OI O IO N H H H OJ ''0' O m 0
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0
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0
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OO
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O
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0
N
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0
O O O O O O O O O O O O O O O O-
- O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O 0 O o - -
O
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0
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0 10 Ol M P M 0 0 0' M 03 01 M N b M 10 Al
O O O O O O O O O O O O O O O 0 O O O O O O O O O O O 0 O O O O O O O O O O O O N H O O O O O O O O O O
w o 0 0 0 o B o 0 0 0 o O o 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 o B o 0 0 0 0
O O O O O O O O O O O O O O O O O O O 0 O O O O O O O O 0 O O O O O O 0 O O O O O O O O O O O O O O O O
V1 M m h W b N W W h m N H OI M Y) N b M m IO C M N C U, ' h N O H 0 U) H N m h 6) 01 N 04 H m N m 0 0 10 H U) H II)
O N O O O O M 0 0 0 0 - H O N O H H H H H H H 04 0 0 0 0 0 N P M O N 0 0 0 0 0 N O H H O 0 O U, 0 14) 0 11) 0
w 0 0 0 0 O o O 0 0 o O 0 B 0 o 0 0 0 O 0 B O O O 0 O O o O O O 0 0 O O O O 0 O 0 O o O O O O O o
0 0
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0 0 0 0 0 0 0 0
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0 0
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0 0
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0-
0-
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m N O) '4) M M In H m P II) O N Y) N Y) m N 1T U) H H N N O) V) N O N b N O Y) ID Y) O1 1p N ®
10 0 i H P IO 0 N N M H O N 10 9 T H 0 0) 0) C - U) M N P N H N N O M 10 0 1-i M M O N N In N 4T 10 N M O In M N H In
H N N H N H H H H H H N H N H N H N H N N N H N N N N N N H N N N N H N N O M N N H H N N N N H N H N H
- -
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N M W N m W In In N O' N N H 0 M ID N O) U) m H H b W M H N N b lO O) In N O M N N O' M N N ^T ^ N N N U) N H N P 4'
y H H H O O H O N H N M IO N O N U) N H N I H N N H N M H N d' W O - N N N H N P N M H H N H O) M UI O1 # N O4
O O O O O O O O O O O 0 O O O O O G O O O O O O O O O O O O O O O O O O H H O O O O O O O O O O O O D O
U) H O1 H N H In OI U P H N In H n I In H N O N C U) 10 N W U) N U) M N N N O O N m b N G' N U) b (7 1 N (.'.4 CO O H N b
a' R d O IO IO N H M a' M P N N H O m m Ol h m O) P H H H M - M N IO M IT OI h 4rv i'I h M N OJ O
U O O O
1
O H H 0 0 0 0 0 H H O H H H H H H H H H H 0 0 0 H H H O H O M H H H H O O H H O H H O H H O H
O O O 8181,+161818 O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O 0 O O O O
N M a' U) - - W - O H N M d' In lO h m m O H N M 4 U, IO N O I O H N M C U) IO h OJ M O H N M M UI b h
'
0 6) O H N z H H H H H H H H H H N N N N N N N N N N M M M M M M M M n n 0 0 0 0 0 0 W V 0 0 n N U)
NN
X
W N
Hro
[0069] [Table 2]
No. Ts (MPd EL & e(4 EL x TS e z TS Ferrite fraction eainite fraction Martensite traction Retained austenite fraction
1 577 33.2 86 19156,4 49622 60 B 22 2.0
2 576 32,5 82 18720 47232 68 6 23 2,7
3 585 31.2 76 18252 95630 69 1 22 2.1
^ -
4 222 29,5 69 18349 42918 65 8 25 1.8
- - - 5 612 29.8 71 18237.6 43452 64 8 26 2,4
6 635 29.4 86 18669 54610 59 6 33 1.9
7 612 30.1 68 18722.2 42296 56 9 31 2,2
8 638 28,5 71 18183 95296 59 10 30 1.0
9 552 28.1 69 18321.1 99988 55 12 31 2,2
50 685 27.2 62 18632 42470 52 ..; 16 31 1.0
11 1•34 26.4 58 19377.6 42572 52 10 - - - 36 2.3
12 795 24.5 88 19477.5 69960 52 16 32 0.0
13 789 24.2 55 19093.8 43395 51 12 35 2,2
14 825 22.2 49 16315 40425 50 13 34 2,7
15 788 23.5 56 18518 44128 2 -10 36 2,1
16 553 21.5 52 18339.5 44356 55 5 38 2.0
Exam le
17 832 22.4 66 10636.8 54912 52 p 6 41 1.5
18 874 21.2 51 18526.8 44574 51 11 36 2.3
19 873 20.1 11 17547.3 53253 48 12 38 2.2
20 953 19,2 66 18297.6 43838 44 44 41 1,5
21 987 18,5 43 18259.5 42441 42 44 42 2,3
22 981 17,2 48 16873.2 47088 37 17 44 2,0
23 988 16,5 62 16302 61256 36 18 46 0.0
24 993 18.3 56 18171.9 55608 41 18 41 0.0
25 1005 16,5 52 16582.5 52260 42 24 32 2.5
26 1 15 16,8 49 17052 49735 40 28 30 1.8
27 1018 17.2 51 17509.6 51918 43 25 30 2.3
28 1023 16,5 55 16979.5 56265
-
40 27 31 2.2
29 1035 17,4 48 180 9 49680 39 24 35 2,1
30 1252 13,5 92 16902 52584 38 14 48 0.0
31 1356 12,3 39 16678.8 52884 15 23 62 0.0
32 1512 11.3 33 17085.6 49896 12 13 75 0.0
33 998 1 ,9 42 168 66.2 41916 42 18 38 2.0
34 1 12 1 ,5 41 16698 41492 90 18 al 1.5
35 335 33.2 65 11122 21775 - 92 6 0 1.9
36 1623 9,2 Ti 14931.6 34083 5 3 90 2.5
37 985 1 ,5 59 19207.5 58115 44 13 42 1,0
38 885 22.3 62 19735.5 54870 55 12 32 1,0
39 1 35 10,2 25 12597 30875 30 18 - 52 0,0
40 795 20.1 31 15979.5 24645 51 12 37 0,0
41 587 26,5 42 15555.5 24654 66 9 21 1,8
42 557 28A 52 15818,8 28964 69 0 ZS 2.1
Comma=ative example
43
44
1470
1480
7.1
11.2
27
45
10437
16576
39690
66600
21
22
10
9
68
69
1.0
0.0
45 880 16.5
-- -
45 14520 39600 25 9 65 1.5
46 990 7 2 52 17028. 51980 72 15 11 2.1
47 1010 17.5
- -
32 17675 32320 42 28 30 0.0
48 750 772 35 17400 26250 52 10 36 2,5
49 899 10,2 42 9169.8 37758 48 14 38 0,0
50 964 13.2
-- '-
40 12988 .8. ..-.39360 45 11 42 2.3
51 602 2 9. 4 42-- 5892.8 25284 62 25 12 1.2
52 778 19.5 40 15111 31120 41 32 25 2.3
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formability were obtained. In other words, the
strength and the' ductility were compatible with each
other. In particular, in the examples No. 1 to No,
32 satisfying the formula (D), the value of "El x TS"
and the value of "s x TS" were higher than in the
examples No. 33 and No. 34.
(0073) On the other hand, in the comparative
examples No. 35, 36 and No. 39 to No. 43, in which a
component of the steel was out of a range of the
present invention, the value of "El x TS" was less
than 16000% MPa, the value of "a x TS" was less than
40000% MPa, and the formability and the tensile
strength were not made compatible with each other.
Further, in the comparative examples No. 37, No. 38
and No. 44, in which a component of the steel was out
of the range of the present invention, the
galvanizing property and the chemical treatment
property were low.
[0074] In the comparative example 45, which did not
satisfy the formula (A), the value of "El x TS" was
less than 16000% MPa, the value of "s x TS" was less
than 40000% MPa, and the formability and the tensile
strength were not made compatible with each other,
and the galvanizing property and the chemical
treatment property were also low. Further, in the
comparative example No.. 46, which did not satisfy the
formulaJ(A), the galvanizing property and the
chemical treatment property were low.
[0075] In the comparative examples No. 47 and No.
- 36 -
48, which did not satisfy the formula (B) nor the
formula (C), the value of "s x TS" was less than
40000% MPa, and the formability and the tensile
strength were not made compatible with each other.
[0076] In the comparative examples No. 49 and No.
50, which did not satisfy the formula (C), the value
of "El x TS" was less than 16000% MPa and the value
of "e x TS" as less than 40000% MPa, and the
formability and the tensile strength were not made
compatible with each other.
[0077] In the comparative examples No. 51 and Noe
52, which did not satisfy the formula (D), the value
of "El x TS" was less than 16000% MPa and the value
of "e x TS" was less than 40000% MPa, and the
formability and the tensile strength were not be made
compatible with each-other.
INDUSTRIAL APPLICABILITY
[0078] The present invention may be used in, for
example, an industry related to a high tensile stee
sheet superior in a formability which is used for a
vehicle body.
CLAIMS
1, A high tensile steel sheet superior in a
formability, containing, in mass %:
C: 0.03% to 0.200;
Si: 0.005% to 1.0%;
Mn: 1.0% to 3.1%; and
Al: 0.005% to 1.2%,
a P content being over 0% and equal to or less
than 0.06%,
ent being over 0% and equal to or less
than 0.01
an N content being over 0% and equal to or less
than 0.01%, and
a balance being composed of Fe and inevitable
impurities,
wherein
a metal structure comprises a ferrite and a
martensite,
a relation of a formula (A) is established about
an Al content (%) and a Si content (%), and
anaverage value Ya„e defined by a formula (B)
regarding hardnesses measured at 100 points or more
with a nanoindenter is equal to or more than 40.
0.3 0.7x [Si] + [Al] _< 1.5 ... (A)
Yave = E(180x(Xi - 3)_z/n) ...(B)
([Al] indicates the Al content (%), [Si]
indicates the Si content (%), n indicates a total
number of the measuring points of the hardnesses, and
- 38 -
Xj indicates the hardness (GPa) at the i-th measuring
point (i
n) .
is a natural number equal to or less than
2. The high tensile steel sheet superior in a
formability according to claim 1, further contains:
at least one selected from a group consisting of,
in mass %,
B: 0'.00005% to 0.005%,
Mo: 0;.,01% to 0.5%,
Cr: 0.01% to 1.0%,
V: 0.01% to 0.1%,
Ti: O.01% to 0.1%,
Nb: 0.005% to 0,05%,
Ca: 0.0005% to 0.005%, and
REM: 0.0005% to 0.005%.
3. the high tensile steel sheet superior in a
formability according to claim 1 or 2., wherein the
high tensile steel sheet is a cold-rolled steel
sheet.
4. The high, tensile steel sheet superior in a
formability according to any one of claim 1 to claim
3, wherein the high tensile steel sheet is a
galvanized steel sheet.
5. The high tensile steel sheet superior in a
formability according to any one of claim 1 to claim
4, wherein a martensite fraction in the steel
structure is over 5%.
6. A method of manufacturing a high tensile
steel sheet superior in a formability, comprising:
- 39 -
obtaining a hot-rolled steel strip by performing
hot rolling;
next, performing acid pickling of the hot-rolled
steel strip;
next, obtaining a cold-rolled steel strip by
performing cold rolling of a„steel strip with a
tandem rolling mill having a plurality of stands;
next, performing continuous annealing of the
cold-rolled steel strip in a continuous annealing
line; and
next, performing temper rolling of the coldrolled
steel strip,
wherein
the steel strip contains, in mass %.
C: 0.03% to 0.20%;
Si: 0.005% to 1.0 %;
Mn: 1.0% to 3.1%; and
Al: 0.005% to 1.2%,
a P content being over 0 % and equal to or
less than 0.06%,
an S content being over 0% and equal to or
less then 0.01%,
an N content being over 0% and equal to or
less than 0.01%, and
a balance being composed Fe and an
inevitable impurity; and
a 'relation of a formula (C) being established
about a cold-rolling reduction in the first stand
among the plurality of stands and a temperature
- 40 -
increasing rate in a-first heating zone in the
continuous annealing lines
50 < rl°.8SxV < 300 (C)
(rl indicates the cold-rolling reduction
(5), and V indicates the temperature increasing rate
(°C/s)
7.- The method of manufacturing a high tensile
steel sheet superior in a formability according to
claim 6, further comprising, after said performing
the'lcontinuous annealing:
performing a galvanizing treatment to the coldrolled
steel strip; and
next, performing a temper rolling of the coldrolled
steel strip.
8. The method of manufacturing a high tensile
steel sheet superior in a formability according to
claim 7, further comprising; aftersaid performing
the galvanizing treatment, holding the cold-rolled
steel strip at a temperature of 400°C to 650°C for
seconds, wherein-a relation of a formula (D) is
established.
t 60x [C] + 20x [Mn] + 24x [Cr] + 40x [Mo] ... (D)
([C] indicates a C content (%), [Mn] indicates an
Mn-content (%), [Cr] indicates a Cr content (%), and
[Me] indicates an No content (%).)