DESCRIPTION
Title of Invention
HIGH-STRENGTH STEEL SHEET AND HIGH-STRENGTH ZINC-COATED STEEL
5 SHEET WHICH HAVE EXCELLENT DUCTILITY AND
STRETCH-FLANGEABILITY AND MANUFACTURING METHOD THEREOF
Technical Field
[0001]
10 The present invention relates to a high-strength steel sheet and a high-strength
zinc-coated steel-sheet which have excellent ductility and stretch-flangeability and a
manufacturing method thereof
Priority is claimed on Japanese Patent Application Nos. 2010-208329 and
2010-208330, filed September 16, 2010, the content of which is incorporated herein by
15 reference.
Background Art
[0002]
In recent years, there has been an increasing demand for a high-strength steel
sheet used in a vehicle or the like, and a high-strength cold-rolled steel sheet with a
20 maximum tensile stress of 900 MPa or more is also being used.
Generally, as the strength of a steel sheet is enhanced, ductility and
stretch-flangeability are lowered, and workability is degraded. However, a
high-strength steel sheet with sufficient workability has been demanded in recent years.
[0003]
25 As a conventional technique for enhancing ductility and stretch-flangeability of
2
a high-strength steel sheet, a high-tensile galvainzed steel sheet, which has a composition
containing by mass percentage, C: 0.05 to 0.20%, Si: 0.3 to 1.8%, Mn: 1.0 to 3.0%, S:
0.005% or less, the remainder composed of Fe and inevitable impurities, has a composite
structure including ferrite, tempered martensite, retained austenite, and low temperature
5 transformation phase, and contains by volume percentage 30% or more of ferrite, 20% or
more of tempered martensite, 2% or more of retained austenite, in which average crystal
grain sizes of ferrite and tempered martensite are 10 (j,m or less, is an exemplary example
(see Patent Document 1, for example).
[0004]
10 In addition, as a conventional technique for enhancing workability of a
high-strength steel sheet, a high-tensile cold-rolled steel sheet, in which amounts of C, Si,
Mn, P, S, Al, and N are adjusted, which further contains 3% or more of ferrite and a total
of 40% or more of bainite containing carbide and martensite containing carbide as metal
strutures of the steel sheet containing one or more of Ti, Nb, V, B, Cr, Mo, Cu, Ni, and Ca
15 as necessary, in which the total amount of ferrite, bainite, and martensite is 60% or more,
and which further has a structure in which the number of ferrite grains containing
cementite, martensite, or retained austenite therein corresponds to 30% or more of the
total number of ferrite grains and has tensile strength of 780 MPa or more, is an
exemplary example (see Patent Document 2, for example).
20 [0005]
Moreover, as a conventional technique for enhancing stretch-flangeability of a
high-strength steel sheet, a steel sheet in which a difference in hardness between a hard
part and a soft part of the steel sheet is reduced is an exemplary example. For example.
Patent Document 3 discloses a technique in which the standard deviation of hardness in
25 the steel sheet is reduced and uniform hardness is given to the entire steel sheet. Patent
3
Document 4 discloses a technique in which hardness in the hard part is lowered by heat
treatment and the difference in hardness from that in the soft part is reduced. Patent
Document 5 discloses a technique in which the difference in hardness from the soft part
is reduced by configuring the hard part of relatively soft bainite.
5 [0006]
Furthermore, as a conventional technique for enhancing stretch-flangeability of
a high-strength steel sheet, a steel sheet, which has a structure containing by an area ratio
40 to 70% of tempered martensite and a remainder composed of ferrite, in which a ratio
between an upper limit value and a lower limit value of Mn concentration in a
10 cross-section in a thickness direction of the steel sheet is reduced (see Patent Document 6,
for example) may be exemplified.
Citation List
Patent Documents
[0007]
15 [Patent Document 1 ] Japanese Unexamined Patent Application, First
Publication No. 2001-192768
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. 2004-68050
[Patent Document 3] Japanese Unexamined Patent Application, First
20 Publication No. 2008-266778
[Patent Document 4] Japanese Unexamined Patent Application, First
Publication No. 2007-302918
[Patent Docviment 5] Japanese Unexamined Patent Application, First
Publication No. 2004-263270
25 [Patent Document 6] Japanese Unexamined Patent Application, First
4
Publication No. 2010-65307
Summary of Invention
Technical Problem
[0008]
5 According to the conventional techniques, however, workability of the
high-strength steel sheet with a maximum tensile strength of 900 MPa or more is
insufficient, and it has been desired to further enhance ductility and stretch-flangeability
and to thereby further enhance workability.
The present invention is made in view of such circumstances, and an object
10 thereof is to provide a high-strength steel sheet, which has excellent ductility and
stretch-flangeability and has excellent workability while high strength is secured such
that the maximum tensile strength becomes 900 MPa or more, and a manufacturing
method thereof
Solution to Problem
15 [0009]
The present inventor conducted intensive study in order to solve the above
problems. As a result, the present inventor found that it is possible to secure a
maximum tensile strength as high as 900 MPa or more and significantly enhance
ductility and stretch-flangeability (hole expanding property) by allowing the steel sheet to
20 have a large hardness difference by increasing a micro Mn distribution inside the steel
sheet and have a sufficiently small average crystal grain size by controlling dispertion in
the hardness distribution.
[0010]
[1] A high-strength steel sheet which has excellent ductility and
25 stretch-flangeability, including by mass percentage: 0.05 to 0.4% of C; 0.1 to 2.5% of Si;
5
^ 1.0 to 3.5% of Mn; 0.001 to 0.03% of P; 0.0001 to 0.01% of S; 0.001 to 2.5% of Al;
0.0001 to 0.01% of N; 0.0001 to 0.008% of O; and a remainder composed of iron and
inevitable impurities, wherein a steel sheet structure contains by volume fraction 10 to
50% of a ferrite phase, 10 to 50% of a tempered martensite phase, and a remaining hard
5 phase, wherein when a plurality of measurement regions with diameters of 1 |xm or less
are set in a range from 1/8 to 3/8 of thickness of the steel sheet, hardness measurement
values in the plurality of measurement regions are arranged in an ascending order to
obtain a hardness distribution, an integer NO.02, which is a number obtained by
multiplying a total number of the hardness measurement values by 0.02 and, if present,
10 by rounding up a decimal number, is obtained, a hardness of a measurement value which
is an N0.02-th largest value from a smallest hardness measurement value is regarded as a
2% hardness, an integer NO.98 which is a number obtained by multiplying the total
number of the hardness measurement values by 0.98 and, if present,by rounding down
the decimal number is obtained, and a hardness of a measurement value which is an
15 N0.98-th largest value from the smallest hardness measurement value is regarded as a
98% hardness, the 98% hardness is 1.5 or more times as high as the 2% hardness,
wherein a kurtosis K* of the hardness distribution between the 2% hardness and the 98%
hardness is equal to or more than -1.2 and equal to or less than -0.4, and wherein an
average crystal grain size in the steel sheet structure is 10|j.m or less.
20 [2] The high-strength steel sheet which has excellent ductility and
stretch-flangeability according to [1], wherein a difference between a maximum value
and a minimum value of Mn concentration in a base iron in a thickness range from 1/8 to
3/8 of the steel sheet is equal to or more than 0,4% and equal to or less than 3,5% when
converted into the mass percentage.
25 [3] The high-strength steel sheet which has excellent ductility and
6
stretch-flangeability according to [1] or [2], wherein when a section from the 2%
hardness to the 98% hardness is equally divided into 10 parts, and 10 1/10-sections are
set, a number of the hardness measurement values in each 1/10-section is 2 to 30% of a
number of all measurement values.
5 [4] The high-strength steel sheet which has excellent ductility and
stretch-flangeability according to any one of [1] to [3], wherein the hard phase includes
any one of or both a bainitic ferrite phase and a bainite phase of 10 to 45% by a volume
fraction, and a fresh martensite phase of at 10% or less.
[5] The high-strength steel sheet which has excellent ductility and
10 stretch-flangeability according to any one of [1] to [4], wherein the steel sheet structure
further includes 2 to 25% of a retained austenite phase.
[6] The high-strength steel sheet which has excellent ductility and
stretch-flangeability according to any one of [1] to [5], further including by mass
percentage one or more of 0.005 to 0.09% of Ti; and 0.005 to 0.09% of Nb.
15 [7] The high-strength steel sheet which has excellent ductility and
stretch-flangeability according to any one of [1] to [6], further including by mass
percentage one or more of 0.0001 to 0.01% of B; 0.01 to 2.0% of Cr; 0.01 to 2.0% of Ni;
0.01 to 2.0% of Cu; and 0.01 to 0.8% of Mo.
[8] The high-strength steel sheet which has excellent ductility and
20 stretch-flangeability according to any one of [1] to [7], further including by mass
percentage: 0.005 to 0.09% of V.
[9] The high-strength steel sheet which has excellent ductility and
stretch-flangeability according to any one of [1] to [8], further including one or more of
Ca, Ce, Mg, and REM at 0.0001 to 0.5% by mass percentage in total.
25 [10] A high-strength zinc-coated steel sheet which has excellent ductility and
7
stretch-flangeability, wherein the high-strength zinc-coated steel sheet is produced by
forming a zinc-coated layer on a surface of the high-strength steel sheet according to any
oneof[l]to[9].
[11] A manufacturing method of a high-strength steel sheet which has an
5 excellent ductility and a stretch-flangeability, the method including: a hot rolling process
in which a slab containing the chemical constituents according to any one of [1] or [6] to
[9] is heated up to 1050°C or higher directly or after cooling once, a hot rolling is
performed thereon at a higher temperature of one of 800°C and an Ars transformation
point, and a winding is performed in a temperature range of 750°C or lower such that an
10 austenite phase in a structure of a rolled material after rolling occupies 50% by volume or
more; a cooling process in which the steel sheet after the hot rolling is cooled from a
winding temperature to (the winding temperature - 100) °C at a rate of 20°C/hour or
lower while a following Equation (1) is satisfied; and a process in which continuous
annealing is performed on the steel sheet after the cooling, wherein in the process in
15 which continuous annealing is performed, the steel sheet is annealed at a maximum
heating temperature of 750 to 1000°C, a first cooling in which the steel sheet is cooled
from the maximum heating temperature to a ferrite transformation temperature range or
lower and maintained in the ferrite transformation temperature range for 20 to 1000
seconds is subsequently performed, a second cooling in which the steel sheet is cooled at
20 a cooling rate of 10°C/second or higher on average in a bainite transformation
temperature range and cooling is stopped within a range from a martensite transformation
start temperature - 120°C to the martensite transformation start temperature is
subsequently performed, the steel sheet after the second cooling is maintained in a range
from a second cooling stop temperature to the martensite transformation start temperature
8
for 2 to 1000 seconds, the steel sheet is subsequently reheated up to a reheating stop
temperature, which is equal to or more than a bainite transformation start temperature -
100°C, at a rate of temperature increase of 10°C/second or higher on average in the
bainite transformation temperature range, and a third cooling in which the steel sheet
5 after the reheating is cooled from the reheating stop temperature to a temperature which
is lower than the bainite transformation temperature range and maintained in the bainite
transformation temperature range for 30 seconds or more is performed:
[Equation 1]
il? 9A7xn^-&xp,~^^^-tiT)'dr] ^ >1J - CI)
*^e-!«) *^i^ T + 273} I
10 [where t{T) in Equation (1) represents maintaining time (seconds) of the steel
sheet at a temperature T°C in the cooling process after the winding.]
[12] The manufacturing method of the high-strength steel sheet which has
excellent ductility and stretch-flangeability according to [11], wherein the winding
temperature after the hot rolling is equal to or more than a Bs point and equal to or less
15 than750°C.
[13] The manufacturing method of the high-strength steel sheet which has
excellent ductility and stretch-flangeability according to [11] or [12], further including
between the cooling process and the continuous annealing process: a cold rolling process
in which the steel sheet is subjected to acid pickling and a cold rolling at rolling
20 reduction from 35 to 80%.
[14] The manufacturing method of the high-strength steel sheet which has
excellent ductility and stretch-flangeability according to any one of [11] to [13], wherein
a sum of a time during which the steel sheet is maintained in the bainite transformation
temperature range in the second cooling and a time during which the steel sheet is
9
maintained in the bainite transformation temperature range in the reheating is 25 seconds
or less.
[15] A manufacturing method of a high-strength zinc-coated steel sheet which
has excellent ductility and stretch-flangeability, wherein the steel sheet is dipped into a
5 zinc plating bath in the reheating in manufacturing the high-strength steel sheet based on
the manufacturing method according to any one of [11] to [14].
[16] A manufacturing method of a high-strength zinc-coated steel sheet which
has excellent ductility and stretch-flangeability, wherein the steel sheet is dipped into a
zinc plating bath in the bainite transformation temperature range in the third cooling in
10 manufacturing the high-strength steel sheet based on the manufacturing method
according to any one of [11] to [14].
[17] A manufacturing method of a high-strength zinc-coated steel sheet which
has excellent ductility and stretch-flangeability, wherein a zinc electroplating is
performed after manufacturing the high-strength steel sheet based on the manufacturing
15 method according to any one of [ 11 ] to [ 14].
[18] A manufacturing method of a high-strength zinc-coated steel sheet which
has excellent ductility and stretch-flangeability, wherein a hot-dip zinc-plating is
performed after manufacturing the high-strength steel sheet based on the manufacturing
method according to any one of [11] to [14].
20 Advantageous Effects of Invention
[0011]
The high-strength steel sheet of the present invention contains predetermined
chemical constituents, when a plurality of measurement regions with diameters of 1 |im
or less are set in a range from 1/8 to 3/8 of a thickness of the steel sheet, hardness
25 measurement values in the plurality of measurement regions are arranged in ascending
10
order to obtain a hardness distribution, an integer N0.02 which is a number obtained by
multiplying a total number of the hardness measurement values by 0.02 and, if present,
by rounding up a decimal number, is obtained, a hardness of a measurement value which
is an N0.02-th largest value from the smallest hardness measurement value is regarded as
5 a 2% hardness, an integer N0.98 which is a number obtained by multiplying the total
number of the hardness measurement values by 0.98 and, if present, rounding down a
decimal number, is obtained, and a hardness of a measurement value which is an
N0.98-th largest value from the smallest hardness measurement value is regarded as a
98% hardness, the 98% hardness is 1.5 or more times as high as the 2% hardness, a
10 kurtosis K* of the hardness distribution between the 2% hardness and the 98% hardness
is equal to or less than -0.40, an average crystal grain size in the steel sheet structure is
lO^m or less, and therefore, the steel sheet which has excellent ductility and
stretch-flangeability is obtained while tensile strength which is as high as 900 MPa or
more is secured.
15 [0012]
In addition, a micro Mn distribution inside the steel sheet increases by winding
the steel sheet after the hot rolling around a coil at 750°C and cooling the steel sheet from
the winding temperature to (the winding temperature - 100) °C at a cooling rate of
20°C/hour or lower while the above Equation (1) is satisfied, in the process for producing
20 a hot-rolled coil from the slab containing the predetermined chemical constituents in the
manufacturing method of the high-strength steel sheet according to the present invention.
In addition, since the process in which continuous annealing is performed on the
steel sheet with increased Mn distribution includes a heating process in which the steel
sheet is annealed at a maximum heating temperature of 750 to 1000°C, a first cooling
11
process in which the steel sheet is cooled from the maximum heating temperature to a
ferrite transformation temperature range or lower and maintained in a ferrite
transformation temperature range for 20 to 1000 seconds, a second cooling process in
which the steel sheet after the first cooling process is cooled at a cooling rate of
5 10°C/second or higher on average in a bainite transformation temperature range and
cooling is stopped within a range from a martensite transformation start temperature -
120°C to the martensite transformation start temperature, a maintaining process in which
the steel sheet after the second cooling process is maintained in a range from a second
cooling stop temperature to the Ms point or lower for 2 to 1000 seconds, a reheating
10 process in which the steel sheet after the maintaining process is reheated up to a reheating
stop temperature, which is equal to or more than a bainite transformation start
temperature - 80°C, at a rate of temperature increase of 10°C/second or higher on
average in the bainite transformation temperature range, and a third cooling process in
which the steel sheet after the reheating process is cooled from the reheating stop
15 temperature to a temperature which is lower than the bainite transformation temperature
range and maintained in the bainite transformation temperature range for 30 seconds or
more, the steel sheet structure is controlled such that the hardness difference inside the
steel sheet is large and the average crystal grain size is sufficiently small, and it is
possible to obtain the high-strength cold-rolled steel sheet which has excellent ductility
20 and stretch-flangeability (hole expanding property) and has excellent workability while
securing a maximum tensile strength of 900 MPa or more.
Furthermore, it is possible to obtain the high-strength zinc-coated steel sheet
which has excellent ductility and stretch-flangeability (hole expanding property) and has
excellent workability while securing the maximum tensile strength as high as 900 MPa or
25 more by adding the process for forming the zinc-pated layer.
12
Brief Description of Drawings
[0013]
FIG. 1 is a graph showing a relationship between hardness classified into a
plurality of levels and a number of measurement values in each level, which is obtained
5 by converting each measurement value while a difference between a maximum hardness
measurement value and a minimum hardness measurement value is regarded as 100%, in
relation to an example of a high-strength steel sheet according to the present invention.
FIG. 2 is a diagram for comparing the hardness distribution in the high-strength
steel sheet according to the present invention with a normal distribution.
10 FIG. 3 is a graph schematically showing a relationship between a transformation
rate and elapsed time of transformation treatment when the difference between a
maximum value and a minimum value of Mn concentration in base iron is relatively
large.
FIG. 4 is a graph schematically showing a relationship between a transformation
15 rate and elapsed time of transformation treatment when a difference between a maximum
value and a minimum value of Mn concentration in base iron is relatively small.
FIG. 5 is a graph illustrating temperature history of a cold-rolled steel sheet
when the sheet is made to pass through a continuous annealing line, which shows a
relationship between the temperature of the cold-rolled steel sheet and time.
20 Description of Embodiments
[0014]
The high-strength steel sheet according to the present invention is a steel sheet,
which includes predetermined chemical components, in which an average crystal grain
size in the structure thereof is 10 (j.m or less, 98% hardness is 1.5 or more times as high
25 as 2% hardness in a hardness distribution when a plurality of measurement regions with
13
diameters of 1 |j,m or less is set in a thickness range from 1/8 to 3/8 thereof, and
measurement values of hardness in the plurality of measurement regions are aligned in an
order from a smallest measurement value, and kurtosis K* of the hardness distribution
between the 2% hardness region and the 98% hardness region is -0.40 or less. An
5 example of hardness distribution in the high-strength steel sheet according to the present
invention is shown in FIG. 1.
[0015]
(Definition of Hardness)
Hereinafter, definition of hardness will be described, and 2% hardness and 98%
10 hardness will be described first. Measurement values of hardness are obtained in the
plurality of measurement regions set in a thickness range from 1/8 to 3/8 of the steel
sheet, and an integer N0.02, which is a number obtained by multiplying the total number
of the measurement values of hardness by 0.02 and, if present, by rounding up a decimal
number, is obtained. In addition, when a number obtained by multiplying the total
15 number of the measurement values of hardness by 0.98 includes a decimal number, an
integer N0.98 is obtained by rounding down the decimal number. Then, hardness of an
NO.02-th largest measurement value from the minimum hardness measurement value in
the plurality of measurement regions is regarded as the 2% hardness. In addition, a
hardness of an N0.98-th largest measurement value from the minimum hardness
20 measurement value in the plurality of measurement regions is regarded as the 98%
hardness. In the high-strength steel sheet of the present invention, the 98% hardness is
preferably 1.5 or more times as high as the 2% hardness, and the kurtosis K* of the
hardness distribution between the 2% hardness and the 98% hardness is preferably -0.40
or less.
25 [0016]
14
Each diameter of the measurement regions is limited to 1 ^m or less in setting
the plurality of measurement regions in order to exactly evaluate dispertion in hardness
resulting from a steel sheet structure including a ferrite phase, a bainite phase, a
martensite phase, and the like. Since the average crystal grain size in the steel sheet
5 structure is 10 )j.m or less in the high-strength steel sheet of the present invention, it is
necessary to obtain hardness measurement values in narrower measurement regions than
the average crystal grain size in order to exactly evaluate the dispertion in hardness
resulting from the steel sheet structure, and specifically, it is necessary to set regions with
diameters of 1 ^m or less as the measurement regions. When the hardness is measured
10 using an ordinary Vickers tester, an indentation size is too large to exactly evaluate the
dispertion in hardness resulting from the structure.
[0017]
Accordingly, the "hardness measurement value" in the present invention
represents a value evaluated based on the following method. That is, a measurement
15 value obtained by measuring hardness under an indentation load of 1 g using a dynamic
micro-hardness tester provided with a Berkovich type three-sided pyramid indenter based
on an indentation depth measurement method is used for the high-strength steel sheet of
the present invention. The hardness measurement position is set to a range from 1/8 to
3/8 around 1/4 of a sheet thickness in the sheet thickness cross-section which is parallel
20 to a rolling direction of the steel sheet. In addition, the total number of the hardness
measurement values ranges from 100 to 10000, and is preferably equal to or more than
1000. The thus measured indentation size has a diameter of 1 \xm or less on the
assumption that the indentation shape is a circular shape. When the indentation shape is
rectangular shape or a triangular shape other than the circular shape, the dimension of the
15
indentation shape in the longitudinal direction may be 1 jim or less.
[0018]
In addition, the "average crystal grain size" in the present invention represents
the size measured by the following method. That is, a grain size measured based on an
5 EBSD (Electron BackScattering Diffraction) method is preferably used for the
high-strength steel sheet of the present invention. A grain size observation surface
ranges from 1/8 to 3/8 around 1/4 of the sheet thickness in the sheet thickness
cross-section which is parallel to the rolling direction of the steel sheet. In addition, it is
preferable to calculate the average crystal grain size by applying a intercept method to a
10 grain boundary map for the observation surface obtained by regarding a boundary, at
which a crystal orientation difference between adjacent measurement points in a bcc
crystal orientation becomes 15° or more, as a grain boundary.
[0019]
In order to obtain a steel sheet which has excellent ductility, it is important to
15 utilize a structure such as ferrite, which has excellent ductility, as the steel sheet structure.
However, the structure which has excellent ductility is soft. Accordingly, it is necessary
to employ a steel sheet structure containing a soft structure and a hard structure such as
martensite in order to obtain a steel sheet with high ductility while having sufficient
strength.
20 [0020]
In the steel sheet with the steel sheet structure including both the soft structure
and the hard structure, strain caused by deformation is more easily accumulated in the
soft part and is not easily distributed to the hard part when a hardness difference between
the soft part and the hard part is larger, and therefore ductility is enhanced.
25 [0021]
16
Since the 98% hardness is 1.5 or more times as high as the 2% hardness in the
high-strength steel sheet of the present invention, the hardness difference between the
soft part and the hard part is sufficiently large, and therefore, it is possible to obtain
sufficiently high ductility. In order to obtain further higher ductility, the 98% hardness
5 is preferably 3.0 or more times as high as the 2% hardness, more preferably more than
3.0 times, further more preferably 3.1 or more times, further more preferably 4.0 or more
times, and still further more preferably 4.2 or more times. When the measurement value
of the 98% hardness is less than 1.5 times of the measurement value of the 2% hardness,
the hardness difference between the soft part and the hard part is not sufficiently large,
10 and therefore, ductility is insufficient. Meanwhile, the measurement value of the 98%
hardness is 4.2 or more times of the measurement value of the 2% hardness, the hardness
difference between the soft part and the hard part is sufficiently large, and both ductility
and a hole expanding property are further enhanced, which is preferable.
[0022]
15 As described above, the hardness difference between the soft part and the hard
part is preferably larger from the standpoint of ductility. However, if regions with the
large hardness difference are in contact with each other, a strain gap caused by
deformation of the steel sheet occurs at the border part, and a micro-crack is easily
generated. Since the micro-crack may become a start point of cracking,
20 stretch-flangeability is degraded. In order to suppress the degradation of
stretch-flangeability resulted from the large hardness difference between the soft part and
the hard part, it is effective to reduce number of borders at which the regions with the
large hardness difference are in contact with each other and shorten the length of each
border at which the regions with the large hardness difference are in contact with each
25 other.
17
[0023]
Since the average crysal grain size of the high-strength steel sheet of the present
invention, which is measured by the EBSD method, is 10 )^m or less, the border, at which
the regions with the large hardness differences are in contact with each other, in the steel
5 sheet is shortened, degradation of stretch-flangeability resulting from the large hardness
difference between the soft part and the hard part is suppressed, and excellent
stretch-flangeability can be obtained. In order to obtain further excellent
stretch-flangeability, the average crystal grain size is preferably 8 i^m or less, and more
preferably 5 |j,m. If the average crystal grain size exceeds 10 ^im, the effect of
10 shortening the border, at which the regions with the large hardness difference are in
contact with each other, in the steel sheet is not sufficient, and it is not possible to
sufficiently suppress the degradation of stretch-flangeability.
[0024]
In addition, in order to reduce the number of the borders at which the regions
15 with the large hardness difference are in contact with each other, the steel sheet structure
having a variety of narrow distribution of hardness, in which dispertion of the hardness
distribution in the steel sheet is small, may be employed.
[0025]
According to the high-strength steel sheet of the present invention, the dispertion
20 in the hardness distribution in the steel sheet is reduced by setting the kurtosis K* of the
hardness distribution to be -0.40 or less, it is possible to reduce the borders at which the
regions with the large hardness difference are in contact with each other and thereby to
obtain excellent stretch-flangeability. In order to obtain further excellent
stretch-flangeability, the kurtosis K* is preferably -0.50 or less, and more preferably
25 -0.55 or less. Although the effects of the present invention can be achieved without
18
particularly determining the lower limit of the kurtosis K*, it is difficult to set K* to be
less than -1.20, and therefore, this value is regarded as the lower limit.
[0026]
In addition, the kurtosis K* is a value which can be obtained by the following
5 Equation (2) based on the hardness distribution and is a numerical value obtained as a
result of evaluation of the hardness distribution by comparing the hardness distribution
with the normal distribution. A case in which the kurtosis is a negative value denotes
that a hardness distribution curve is relatively flat, and a large absolute value denotes that
the hardness distribution deviates further from the normal distribution.
10 [0027]
[Equation 2]
j^, », I ,i%2? - ^ «.ll^i^«Z.^iiiilL_, I. yfam I t^tZ f
^'%.m'' ^'sipy ,
i^'Q m ~ %c:" ^X-^'o n ~ ^'a»2 ~ -) "" * 2)
Hi: hardness of an i-th largest measurement point from a measurement value of
the minimum hardness
15 H*: average hardness from the N0.02-th largest measurement point from the
minimum hardness to the NO.98-th largest measurement point
s*: standard deviation from the N0.02-th largest measurement point from the
minimum hardness to the N0.98-th largest measurement point
[0028]
20 In addition, when the kurtosis K* exceeds -0.40, the steel sheet structure is not a
structure which has a sufficient variety of sufficiently narrow distribution of hardness,
dispertion in the hardness distribution in the steel sheet becomes larger, the number of the
borders at which the regions with the large hardness difference are in contact with each
19
other increases, and it is not possible to sufificiently suppress degradation of
stretch-flangeability.
[0029]
Next, detailed description will be given of the dispertion in the hardness
5 distribution in the steel sheet with reference to FIG. 1. FIG. 1 is a graph showing a
relationship between hardness classified into a plurality of levels and a number of
measurement values in each level, which is obtained by converting each measurement
value while a difference between a maximum hardness measurement value and a
minimum hardness measurement value of the hardness is regarded as 100%, in relation to
10 an example of a high-strength steel sheet according to the present invention. In the
graph shown in FIG. 1, the horizontal axis represents hardness, and the vertical axis
represents a number of measurement values in each level. In addition, a solid line of the
graph shown in FIG. 1 is obtained by connecting the point representing the numbers of
the measurement values in each level.
15 [0030]
In the high-strength steel sheet of the present invention, it is preferable that all
numbers of the measurement values in divided ranges D, which are obtained by equally
dividing a range from the 2% hardness to the 98% hardness into 10 parts, in the graph
shown in FIG. 1 be within a range from 2% to 30% of the number of all measurement
20 values.
[0031]
In such a high-strength steel sheet, the line joining up the numbers of the
measurement values in the levels becomes a smooth curve with no steep peaks and
valleys in the graph shown in FIG. 1, and the dispertion in the hardness distribution in the
25 steel sheet is significantly reduced. Accordingly, such a high-strength steel sheet has
20
less borders at which the regions with large hardness difference are in contact with each
other, and excellent stretch-flangeability can be obtained.
[0032]
In addition, if any of the numbers of the measurement values in a divided range
5 D, which has been equally divided into 10 parts, is outside the range from 2% to 30% of
the number of total measurement values in the graph shown in FIG. 1, the line joining up
the numbers of the measurement values in the levels may easily include a steep peak or a
valley, and an effect that stretch-flangeability is enhanced due to low dispertion in the
hardness distribution in the steel sheet is reduced.
10 [0033]
Specifically, for example, when only a number of the measurement values in a
divided range D near the center exceeds 30% of the number of all measurement values
among the equally divided 10 regions D, the line joining up the numbers of the
measurement numbers in the levels has a peak in the divided range D near the center.
15 [0034]
In addition, if only a number of the measurement values in the divided range D
near the center are less than 2% of the number of all measurement values, the line joining
up the numbers of the measurement values in the levels has a valley in the divided range
D near the center, and many structures have large hardness differences, in which the
20 hardness in different divided ranges D arranged on both sides of the valley is included.
[0035]
In the high-strength steel sheet of the present invention, all numbers of the
measurement values in the divided ranges D are preferably 25% or less of the number of
all measurement values, and more preferably 20% or less, in order to further enhance
25 stretch-flangeability. In order to still further enhance stretch-flangeability, all numbers
21
of the measurement values in the divided ranges D are preferably 4% or more of the
number of all measurement values, and more preferably 5% or more.
[0036]
The hardness distribution in the high-strength steel sheet of the present invention
5 will be compared with a general normal distribution and described in detail. The
kurtosis K* of the normal distribution is generally considered to be 0. On the other
hand, the kurtosis of the hardness distribution in the steel sheet according to the present
invention is -0.4 or less, and therefore, it is obvious that the distribution is different from
the normal distribution. The hardness distribution in the steel sheet according to the
10 present invention is flatter and has a wider bottom as compared with the normal
distribution as shown in FIG. 2. Since the high-strength steel sheet of the present
invention has such a hardness distribution, and the ratio of the 98% hardness to the 2%
hardness, which correspond to both sides of the bottom of the distribution, is 1.5 or more
times which is extremely large, the hardness difference between the soft part and the hard
15 part in the steel sheet structure is sufficiently large, and high ductility can be obtained.
That is, the present inventor found that the hole expanding property is further enhanced
when the ratio between the 98% hamess and the 2% hardness is larger in the hardness
distribution in which the kurtosis is -0.4 or less unlike the conventional hardness
distribution. On the other hand, the hole expanding property is considered to be further
20 enhanced as the hardness ratio in the structure is smaller, according to the conventional
technique. The conventional technique was based on the assumption of the hardness
distribution which is close to the normal distribution, which is basically different from
the technique proposed in the present invention.
[0037]
25 (Mn Distribution)
22
In the high-strength steel sheet of the present invention, it is preferable that a
difference between a maximum value and a minimum value of Mn concentration in the
base iron at a thickness from 1/8 to 3/8 of the steel sheet be equal to or more than 0.40%
and equal to or less than 3.50% when converted into a mass percentage in order to obtain
5 the aforementioned hardness distribution.
[0038]
The difference between the maximum value and the minimum value of the Mn
concentration in the base iron at the thickness from 1/8 to 3/8 of the steel sheet is defined
as 0.40% or more when converted into a mass percentage because phase transformation
10 proceeds more slowly during continuous annealing after cold rolling as the difference
between the maximum value and the minimum value of the Mn concentration is larger
and it is possible to reliably generate each transformation product at a desired volume
fraction and to thereby obtain the high-strength steel sheet with the aforementioned
hardness distribution. More specifically, it is possible to generate a transformation
15 product with relatively high hardness such as martensite in place of a transformation
product with relatively low hardness such as ferrite in a balanced manner, and therefore,
a sharp peak is not present in the hardness distribution in the high-strength steel sheet,
that is, the kurtosis decrease, and a flat hardness distribution curve as shown in FIG. 1 can
be obtained. In addition, the width of the hardness distribution is widened by
20 generating various transformation products in a balanced manner, and it is thus possible
to set the 98% hardness to be 1.5 or more times as high as the 2% hardness, preferably
3.0 or more times, more preferably more than 3.0 times, further more preferably 3.1 or
more times, still further preferably 4.0 or more times, and still further preferably 4.2 or
more times.
25 [0039]
23
For example, transformation of a ferrite phase will be described as an example.
In a heat treatment process for causing transformation of the ferrite phase, the phase
transformation from austenite to ferrite starts relatively early in a region where the Mn
concentration is low. On the other hand, the phase transformation from austenite to
5 ferrite starts relatively slowly in the region where the Mn concentration is high as
compared with the region where the Mn concentration is low. Therefore, the phase
transformation from the austenite to ferrite proceeds more slowly in the steel sheet as the
Mn concentration in the steel sheet is more non-uniform and the concentration difference
is larger. In other words, a transformation rate, during a period when the volume
10 percentage of the ferrite phase reaches, for example,50% from 0%, becomes lower.
The above phenomenon similarly occurs in the tempered martensite phase and
the remaining hard phase as well as the ferrite phase.
[0040]
FIG. 3 schematically shows a relationship between a transformation rate and
15 elapsed time of transformation treatment. In the case of the phase transformation from
austenite to ferrite, for example, the transformation rate represents a volume percentage
of ferrite in the steel sheet structure, and the elapsed time of the transformation treatment
represents elapsed time of heat treatment for causing ferrite transformation. In the
example of the present invention shown in FIG. 3, the difference between the maximum
20 value and the minimum value of the Mn concentration is relatively large, and a gradient
of the curve showing the transformation rate in the entire steel sheet is small (the
transformation rate is low). On the other hand, in the comparative example shown in
FIG. 4, the difference between the maximum value and the minimum value of the Mn
concentration is relatively small, and the gradient of the curve showing the
25 transformation rate in the entire steel sheet is large (the transformation rate is high). For
24
this reason, although the transformation treatment may be terminated during a period
from xi to X2 in order to control the transformation rate (volume percentage) in a range
from yi to y2 (%) in the example shown in FIG. 3, it is necessary to terminate the
transformation treatment during a period from X3 to X4 and it is difficult to control
5 treatment time in the example shown in FIG. 4.
[0041]
When the difference in the Mn concentration is less than 0.40%, it is not
possible to sufficiently suppress the transformation rate and achieve a sufficient effect,
and therefore, this is set as the lower limit. The difference in the Mn concentration is
10 preferably 0.60% or more, and more preferably 0.80% or more. Although the phase
transformation can be more easily controlled as the difference in the Mn concentration is
larger, it is necessary to excessively increase the amount of Mn added to the steel sheet in
order that the difference in the Mn concentration exceeds 3.50%, and it is preferable that
the difference in the Mn concentration be 3.50% or less since there is a concern of
15 cracking of a cast slab and degradation of a welding property. In view of the welding
property, the difference in the Mn concentration is more preferably 3.40% or less, and
more preferably 3.30% or less.
[0042]
A method of determining a difference between the maximum value and the
20 minimum value of Mn at the thickness from 1/8 to 3/8 is as follows. First, a sample is
obtained while a sheet thickness cross-section which is parallel to the rolling direction of
the steel sheet is regarded as an observation surface. Then, EPMA analysis is
performed in a thickness range from 1/8 to 3/8 around a thickness of 1/4 to measure an
Mn amount. The measurement is performed while a probe diameter is set to 0.2 to 1.0
25 |am and measurement time per one point is set to 10 ms or longer, and the Mn amounts
25
are measured at 1000 or more points based on line analysis or surface analysis.
In the measurement results, points at which the Mn concentration exceeds three
times the added Mn concentration are considered to be points at which inclusions such as
manganese sulfide are observed. In addition, points at which the Mn concentration is
5 less than 1/3 times the added Mn concentration are considered to be points at which
inclusions such as aluminum oxide are observed. Since such Mn concentrations hardly
affect the phase transformation behavior in the base iron, the maximum value and the
minimum value of the Mn concentration are respectively obtained after the measurement
results of the inclusions are excluded from the measurement results. Then, the
10 difference between the thus obtained maximum value and minimum value of the Mn
concentration is calculated.
The method of measuring the Mn amount is not limited to the above method.
For example, an EMA method or direct observation using a three-dimensional atom
probe (3D-AP) may be performed to measure the Mn concentration.
15 [0043]
(Steel Sheet Structure)
In addition, the steel sheet structure of the high-strength steel sheet of the
present invention includes 10 to 50% of a ferrite phase and 10 to 50% of a tempered
martensite phase and a remaining hard phase by volume fractions. In addition, the
20 remaining hard phase includes 10 to 60% of one of or both a bainitic ferrite phase and a
bainite phase and 10% or less of a fresh martensite phase by volume fractions.
Furthermore, the steel sheet structure may contain 2 to 25% of a retained austenite phase.
When the high-strength steel sheet of the present invention has such a steel sheet
structure, the hardness difference inside the steel sheet becomes much larger, the average
25 crystal grain size becomes sufficiently small, and therefore, the high-strength steel sheet
26
has further higher strength and excellent ductility and strength-flangeability (hole
expanding property).
[0044]
"Ferrite"
5 Ferrite is a structure which is effective in enhancing ductility and is preferably
contained in the steel sheet structure at 10 to 50% by a volume fraction. The volume
fraction of ferrite contained in the steel sheet structure is preferably 15% or more, and
more preferably 20% or more in view of ductility. In addition, the volume fraction of
ferrite contained in the steel sheet structure is preferably 45% or less, and more
10 preferably 40% or less in order to sufficiently enhance the tensile strength of the steel
sheet. When the volume fraction of ferrite is less than 10%, there is a concern that
sufficient ductility may not be achieved. On the other hand, ferrite has a soft structure,
and therefore, yield stress is lower in some cases when the volume fraction exceeds 50%.
[0045]
15 "Bainitic Ferrite and Bainite"
Bainitic ferrite and bainite are structures with a hardness between the hardness
of soft ferrite and the hardness of hard tempered martensite and fresh martensite. The
high-strength steel sheet of the present invention may contain any one of bainitic ferrite
and bainite or may contain both. In order to flatten the hardness distribution inside the
20 steel sheet, a total amount of bainitic ferrite and bainite contained in the steel sheet
structure is preferably 10 to 45% by volume fraction. The sum of volume fractions of
bainitic ferrite and bainite contained in the steel sheet structure is preferably 15% or more,
and more preferably 20% or more in view of stretch-flangeability. In addition, the sum
of the volume fractions of bainitic ferrite and bainite is preferably 40% or less, or more
25 preferably 35% or less in order to obtain a satisfactory balance between ductility and
27
yield stress.
[0046]
When the sum of the volume fractions of bainitic ferrite and bainite is less than
10%, bias occurs in the hardness distribution, and there is a concern that
5 stretch-flangeability may be degraded. On the other hand, when the sum of the volume
fractions of bainitic ferrite and bainite exceeds 45%, it becomes difficult to generate
appropriate amounts of ferrite and tempered martensite, and the balance between
ductility and yield stress is degraded, which is not preferable.
[0047]
10 "Tempered Martensite"
Tempered martensite is a structure which greatly enhances the tensile strength
and is preferably contained in the steel sheet structure at 10 to 50% by a volume fraction.
When the volume fraction of tempered martensite contained in the steel sheet structure is
less than 10%, there is a concern that sufficient tensile strength may not be obtained.
15 On the other hand, when the volume fraction of the tempered martensite contained in the
steel sheet structure exceeds 50%, it becomes difficult to secure ferrite and retained
austenite necessary for enhancing ductility. In order to sufficiently enhance the ductility
of the high-strength steel sheet, the volume fraction of tempered martensite is preferably
45% or less, and more preferably 40% or less. In addition, in order to secure tensile
20 strength, the volume fraction of tempered martensite is preferably 15% or more, and
more preferably 20% or more.
[0048]
"Retained Austenite"
Retained austenite is a structure which is effective in enhancing ductility and is
25 preferably contained in the steel sheet structure at 2 to 25% by a volume fraction. When
28
the volume fraction of retained austenite contained in the steel sheet structure is 2% or
more, more sufficient ductility can be obtained. In addition, when the volume fraction
of retained austenite is 25% or less, the welding property is enhanced without a need for
adding a large amount of austenite stabilizer such as C or Mn. In addition, although it is
5 preferable that retained austenite be contained in the steel sheet structure of the
high-strength steel sheet according to the present invention since retained austenite is
effective in enhancing ductility, retained austenite may not be contained when sufficient
ductility can be obtained.
[0049]
10 "Fresh Martensite"
Since fresh martensite flinctions as a start point of fracture and degrades
stretch-flangeability while fi"esh martensite greatly enhances tensile strength, fresh
martensite is preferably contained in the steel sheet structure at 10% or less by a volume
fraction. In order to enhance stretch-flangeability, the volume fraction of fresh
15 martensite is preferably 5% or less, and more preferably 2% or less.
[0050]
"Others"
The steel sheet structure of the high-strength steel sheet according to the present
invention may contain structures such as pearlite and coarse cementite other than the
20 above structures. However, when large amounts of pearlite and coarse cementite are
contained in the steel sheet structure of the high-strength steel sheet, ductility is degraded.
For this reason, the volume fraction of pearlite and coarse cementite contained in the
steel sheet structure is preferably 10% or less in total, and more preferably 5% or less.
[0051]
25 The volume fraction of each structure contained in the steel sheet structure of the
29
high-strength steel sheet according to the present invention can be measured based on the
following method, for example.
[0052]
In relation to the volume fraction of retained austenite. X-ray analysis is
5 performed while a surface at a thickness of 1/4, which is parallel to the sheet surface of
the steel sheet, is regarded as an observation surface, an area fraction is calculated, and
the result thereof can be regarded as the volume fraction.
[0053]
In relation to the volume fractions of ferrite, bainitic ferrite, bainite, tempered
10 martensite, and fresh martensite, a sample is obtained while a sheet thickness
cross-section which is parallel to the rolling direction of the steel sheet is regarded as an
observation surface, the observation surface is ground, subjected to nital etching, and
observed with a Field Emission Scanning Electron Microscope (FE-SEM) in a thickness
range from 1/8 to 3/8 around 1/4 of the sheet thickness to measure area fractions, and the
15 results thereof can be regarded as the volume fractions.
[0054]
In addition, an area of the observation surface observed with the FE-SEM can be
a 30 [xm sided square, for example, and each structure in the observation surface can be
distinguished from each other as follows.
20 [0055]
Ferrite is a lump of crystal grains and is a region inside which iron carbide with
a long diameter of 100 nm or more is not present. In addition, the volume fraction of
ferrite is a sum of the volume fraction of ferrite remaining at the highest heating
temperature and the volume fraction of ferrite which is newly produced in a ferrite
25 transformation temperature range. However, it is difficult to directly measure the
30
volume fraction of ferrite during the production. For this reason, a small piece of the
cold-rolled steel sheet before passing though the continuous annealing line is cut, the
small piece is annealed based on the same temperature history as that when the small
piece is made to pass through the continuous annealing line, dispertion in the volume of
5 ferrite in the small piece is measured, and a numerical value calculated with the use of
the resuh is regarded as the volume fraction, in the present invention.
[0056]
In addition, bainitic ferrite is a group of lath-shaped crystal grains, and iron
carbide with a long diameter of 20 nm or more is not contained inside the lath.
10 In addition, bainite is a group of lath-shaped crystal grains, and a plurality of
compounds of iron carbide with a long diameter of 20 nm or more is contained inside the
lath, and carbide belongs to a single variant, namely an iron carbide group extending in a
same direction. Here, the iron carbide group extending in the same direction denotes
that the differences in the extending direction of the iron carbide group are within 5°.
15 [0057]
In addition, tempered martensite is a group of lath-shaped crystal grains, a
plurality of compounds of iron carbide with a long diameter of 20 nm or more is
contained inside the lath, and carbide belongs to a plurality of variants, namely a plurality
of iron carbide groups extending in different directions.
20 Moreover, bainite and tempered martensite can be easily distinguished from
each other by observing iron carbide inside the lath-shaped crystal grain using the
FE-SEM and examining the extending directions thereof
[0058]
In addition, fresh martensite and retained austenite are not sufficiently eroded by
25 the nital etching. Therefore, fresh martensite and retained austenite are apparently
31
distinguished from the aforementioned structures (ferrite, bainitic ferrite, bainite,
tempered martensite) in the observation with the FE-SEM.
Accordingly, the volume fraction of fresh martensite is obtained as a difference
between an area fraction of a region observed with the FE-SEM, which has not yet been
5 eroded, and an area fraction of retained austenite measured with X rays.
[0059]
(Concerning Definition of Chemical Compositions)
Next, description will be given of chemical constituents (compositions) of the
high-strength steel sheet of the present invention. In addition, [%] in the following
10 description represents [mass %].
[0060]
"C: 0.050 to 0.400%"
C is contained in order to enhance the strength of the high-strength steel sheet.
However, if the C content exceeds 0.400%, a sufficient welding property is not obtained.
15 In view of the welding property, the C content is preferably 0.350% or less, and more
preferably 0.300% or less. On the other hand, if the C content is less than 0.050%, the
strength is lowered, and it is not possible to secure the maximum tensile strength of 900
MPa or more. In order to enhance the strength, the C content is preferably 0.060% or
more, and more preferably 0.080% or more.
20 [0061]
"Si: 0.10 to 2.50%"
Si is added in order to suppress temper softening of martensite and enhance the
strength of the steel sheet. However, if the Si content exceeds 2.50%, embrittlement of
the steel sheet is caused, and ductility is degraded. In view of ductility, the Si content is
25 preferably 2.20% or less, and more preferably 2.00% or less. On the other hand, if the
32
Si content is less than 0.10%, hardness of tempered martensite is lowered to a large
degree, and it is not possible to secure a maximum tensile strength of 900 MPa or more.
In order to enhance the strength, the lower limit value of Si is preferably 0.30% or more,
and more preferably 0.50%o or more.
5 [0062]
"Mn: 1.00 to 3.50%"
Since Mn is an element which enhances the strength of the steel sheet, and it is
possible to control the hardness distribution in the steel sheet by controlling the Mn
distribution in the steel sheet, Mn is added to the steel sheet of the present invention.
10 However, if the Mn content exceeds 3.50%, a coarse Mn concentrated part is generated at
the center in the sheet thickness of the steel sheet, embrittlement easily occurs, and
problems such as cracking of a cast slab easily occur. In addition, if the Mn content
exceeds 3.50%, the welding property is also degraded. For this reason, it is necessary
that the Mu content be 3.50% or less. In view of the welding property, the Mn content
15 is preferably 3.20% or less, and more preferably 3.00% or less. On the other hand, if
the Mn content is less than 1.00%, a large amount of soft structures are formed during
cooling after annealing, which makes it difficult to secure the maximum tensile strength
of 900 MPa or more, and therefore, it is necessary that the Mn content be 1.00% or more.
In order to enhance the strength, the Mn content is preferably 1.30% or more, and more
20 preferably 1.50% or more.
[0063]
"P: 0.001 to 0.030%"
P tends to be segregated at the center in the sheet thickness of the steel sheet and
brings about embrittlement of a welded part. If the P content exceeds 0.300%,
25 significant embrittlement of the welded part occurs, and therefore the P content is limited
33
to 0.030% or less. Although the effects of the present invention can be achieved
without particularly determining the lower limit of the P content, 0.001% is set as the
lower limit value since manufacturing costs greatly increase when the P content is less
than 0.001%.
5 [0064]
"S: 0.0001 to 0.0100%"
S adversely affects the welding property and manufacturability during casting
and hot rolling. For this reason, the upper limit of S content is set to 0.0100% or less.
In addition, since S is bonded to Mn to form coarse MnS and lowers the
10 stretch-flangeability, S is preferably contained at 0.0050% or less, and more preferably
contained at 0.0025% or less. Although the effects of the present invention can be
achieved without particularly determining the lower limit of S content, 0.0001% is set as
the lower limit value since manufacturing costs greatly increase when the S content is
less than 0.0001%.
15 [0065]
"Al: 0.001% to 2.500%"
Al is an element which suppresses production of iron carbide and enhances the
strength. However, if an Al content exceeds 2.50%, a ferrite fraction in the steel sheet
excessively increases, and the strength is rather lowered, therefore the upper limit of the
20 Al content is set to 2.500%. The Al content is preferably 2.000% or less, and more
preferably 1.600% or less. Although the effects of the present invention can be
achieved without particularly determining the lower limit of the Al content, 0.001% is set
as the lower limit since an effect as a deoxidizing agent can be obtained when the Al
content is 0.001% or more. In order to obtain sufficient effect as the deoxidizing agent,
25 the Al content is preferably 0.005% or more, and more preferably 0.010% or more.
34
[0066]
"N: 0.0001 to 0.0100%"
Since N forms coarse nitride and degrades the stretch-flangeability, it is
necessary to suppress the added amount thereof. If the N content exceeds 0.0100%, this
5 tendency is more evident, and therefore, the range of the N content is set to 0.0100% or
less. In addition, since N causes a blow hole during welding in many cases, it is
preferable that the amount of N is as small as possible. Although the effects of the
present invention can be achieved without particularly determining the lower limit of the
N content, 0.0001% is set as the lower limit value since manufacturing costs greatly
10 increase when the N content is less than 0.0001%.
[0067]
"O: 0.0001 to 0.0080%"
Since O forms oxide and degrades the stretch-flangeability, it is necessary to
suppress the added amount thereof If the O content exceeds 0.0080%, the degradation
15 of the stretch-flangeability is more evident, and therefore, the upper limit of the O
content is set to 0.0080% or less. The O content is preferably 0.0070% or less, and
more preferably 0.0060% or less. Although the effects of the present invention can be
achieved without particularly determining the lower limit of the O content, 0.0001% is
set as the lower limit value since manufacturing costs greatly increase when the O
20 content is less than 0.0001%.
[0068]
The high-strength steel sheet of the present invention may further contain the
following elements as necessary.
[0069]
25 "Ti: 0.005 to 0.090%"
35
Ti is an element which contributes to enhancement of the strength of the steel
sheet by precipitation strengthening, fine grain strengthening by suppressing growth of
the ferrite crystal grains, and dislocation strengthening by suppressing recrystallization.
However, if a Ti content exceeds 0.090%, the number of precipitate of carbonitride
5 increases, formability is degraded, and therefore, the Ti content is preferably 0.090% or
less. In view of the formability, the Ti content is preferably 0.080% or less, and more
preferably 0.70% or less. Although the effects of the present invention can be achieved
without particularly determining the lower limit of the Ti content, the Ti content is
preferably 0.005% or more in order to sufficiently obtain the effect of Ti enhancing the
10 strength. In order to further enhance the strength of the steel sheet, the Ti content is
preferably 0.010% or more, and more preferably 0.015% or more.
[0070]
"Nb: 0.005 to 0.090%"
Nb is an element which contributes to enhancement of the strength of the steel
15 sheet by precipitation strengthening, fine grain strengthening by suppressing growth of
ferrite crystal grains, and dislocation strengthening by suppressing recrystallization.
However, if theNb content exceeds 0.090%, the number of precipitate of carbonitride
increases, formability is degraded, and therefore, the Nb content is preferably 0.090% or
less. In view of formability, the Nb content is preferably 0.070% or less, and more
20 preferably 0.050% or less. Although the effects of the present invention can be
achieved without particularly determining the lower limit of the Nb content, the Nb
content is preferably 0.005% or more in order to sufficiently obtain the effect of Nb
enhancing the strength. In order to further enhance the strength of the steel sheet, the
Nb content is preferably 0.010% or more, and more preferably 0.015% or more.
25 [0071]
36
"V: 0.005 to 0.090%"
V is an element which contributes to enhancement of the strength of the steel
sheet by precipitation strengthening, fine grain strengthening by suppressing growth of
ferrite crystal grains, and dislocation strengthening by suppressing recrystallization.
5 However, if the V content exceeds 0.090%, the number of precipitate of carbonitride
increases, formability is degraded, and therefore, the Nb content is preferably 0.090% or
less. Although the effects of the present invention can be achieved without particularly
determining the lower limit of the V content, the V content is preferably 0.005% or more
in order to sufficiently obtain the effect of V enhancing the strength.
10 [0072]
"B: 0.0001 to 0.0100%"
Since B delays phase transformation from austenite in a cooling process after
hot rolling, it is possible to effectively cause distribution of Mn to proceed by adding B.
If the B content exceeds 0.0100%, workability at a high temperature deteriorates,
15 productivity is lowered, and therefore, the B content is preferably 0.0100% or less. In
view of the productivity, the B content is preferably 0.0050% or less, and more
preferably 0.0030% or less. Although the effects of the present invention can be
achieved without particularly determining the lower limit of the B content, the B content
is preferably 0.0001% or more in order to sufficiently obtain the effect of B delaying the
20 phase transformation. In order to delay the phase transformation, the B content is
preferably 0.0003% or more, and more preferably 0.0005% or more.
[0073]
"Mo: 0.01 to 0.80%"
Since Mo delays phase transformation from austenite in a cooling process after
25 hot rolling, it is possible to effectively cause distribution of Mn to proceed by adding Mo.
37
If the Mo content exceeds 0.80%, workability at a high temperature deteriorates,
productivity is lowered, and therefore, the Mo content is preferably 0.80% or less.
Although the effects of the present invention can be achieved without particularly
determining the lower limit of the Mo content, the Mo content is preferably 0.01%» or
5 more in order to sufficiently obtain the effect of Mo delaying the phase transformation.
[0074]
"Cr: 0.01 to 2.00%" "Ni: 0.01 to 2.00%" "Cu: 0.01 to 2.00%"
Cr, Ni, and Cu are elements which enhance contribution to the strength, and one
kind or two or more kinds therefrom can be added instead of a part of C and/or Si. If
10 the content of each element exceeds 2.00%), the acid pickling property, the welding
property, the workability at a high temperature, and the like are degraded, and therefore,
the content of Cr, Ni, and Cu is preferably 2.00% or less, respectively. Although the
effects of the present invention can be achieved without particularly determining the
lower limit of the content of Cr, Ni, and Cu, the content of Cr, Ni, and Cu is preferably
15 0.10% or more, respectively, in order to sufficiently obtain the effect of enhancing the
strength of the steel sheet.
[0075]
"Total Content of one kind or two or more kinds from Ca, Ce, Mg, and REM
from 0.0001 to 0.5000%"
20 Ca, Ce, Mg, and REM are elements which are effective in enhancing formability,
and it is possible to add one kind or two or more kinds therefrom. However, if the total
amount of one or more of Ca, Ce, Mg, and REM exceeds 0.5000%, there is a concern
that ductility may deteriorate, on the contrary, and therefore, the total content of the
elements is preferably 0.5000% or less. Although the effects of the present invention
25 can be achieved without particularly determining the lower limit of the content of one or
38
more of Ca, Ce, Mg, and REM, the total content of the elements is preferably 0.0001% or
more in order to sufficiently obtain the effect of enhancing formability of the steel sheet.
In view of the formability, the total content of one or more of Ca, Ce, Mg, and REM is
preferably 0.0005% or more, and more preferably 0.0010% or more. In addition, REM
5 is an abbreviation for Rare Earth Metals and represents an element belonging to
lanthanoid series. In the present invention, REM and Ce are added in the form of misch
metal in many cases, and there is a case in which elements in the lanthanoid series are
contained in combination in addition to La and Ce. Even if such elements in the
lanthanoid series other than La and Ce are included as inevitable impurities, the effects of
10 the present invention can be achieved. In addition, the effects of the present invention
can be achieved even if metal La and Ce are added.
[0076]
In addition, the high-strength steel sheet of the present invention may be
configured as a high-strength zinc-coated steel sheet by forming a zinc-plated layer or an
15 alloyed zinc-plated layer on the surface thereof. By forming the zinc-plated layer on the
surface of the high-strength steel sheet, the high-strength steel sheet obtains excellent
corrosion resistance. The high-strength steel sheet has excellent corrosion resistance,
and excellent adhesion of a coating can be obtained, since the alloyed zinc-plated layer is
formed on the surface thereof
20 [0077]
(Manufacturing Method of High-Strength Steel Sheet)
Next, description will be given of a manufacturing method of the high-strength
steel sheet of the present invention.
Firstly, in order to manufacture the high-strength steel sheet of the present
25 invention, slab containing the aforementioned chemical constituents (compositions) is
39
firstly casted.
As the slab subjected to hot rolling, continuous cast slab or slab manufactured
by a thin slab caster can be used. The manufacturing method of the high-strength steel
sheet of the present invention can be adapted to a process such as continuous
5 casting-direct rolling (CC-DR) in which hot rolling is performed immediately after the
casting.
[0078]
In the hot rolling process, it is necessary that a slab heating temperature be
1050° C or higher. If the slab heating temperature is excessively low, a finish rolling
10 temperature is below an A13 transformation temperature, two phase region rolling of
ferrite and austenite is performed, a hot-rolled sheet structure becomes a duplex grain
structure in which non-uniform grains are mixed, the non-uniform structure remains even
after cold rolling and annealing processes, and therefore, ductility and bendability are
degraded. In addition, since lowering of the finish rolling temperature causes excessive
15 increase in rolling load, and there is a concern that it may become difficult to perform
rolling or a shape of the steel sheet after the rolling may be defective, it is necessary that
the slab heating temperature be 1050°C or higher. Although the effects of the present
invention can be achieved without particularly determining the upper limit of the slab
heating temperature, it is preferable that the upper limit of the slab heating temperature
20 be I350°C or lower since setting of an excessively high heating temperature is not
economically preferable.
[0079]
In addition, the A13 temperature is calculated based on the following equation.
Ar3 = 901 - 325 X C + 33 X Si - 92 X (Mn + Ni/2 + Cr/2 + Cu/2 + Mo/2) + 52 x Al
40
[0080]
In the above equation, C, Si, Mn, Ni, Cr, Cu, Mo, and Al represent content
[mass %] of the elements.
[0081]
5 In relation to the finish rolling temperature of the hot rolling, a higher
temperature among 800°C and the Ars point is set as a lower limit thereof, and 1000°C is
set as an upper limit thereof If the finish rolling temperature is lower than 800°C, the
rolling load during the finish rolling increases, and there is a concern that it may become
difficult to perform the hot rolling or the shape of the hot-rolled steel sheet obtained after
10 the hot rolling may be defective. In addition, if the finish rolling temperature is lower
than the Ars point, the hot rolling becomes two phase region rolling of ferrite and
austenite, and the structure of the hot- rolled steel sheet becomes a structure in which
non-uniform grains are mixed.
On the other hand, although the effects of the present invention can be achieved
15 without particularly determining the upper limit of the finish rolling temperature, it is
necessary to set the slab heating temperature to an excessively high temperature when the
finish rolling temperature is set to an excessively high temperature in order to secure the
finish rolling temperature. For this reason, it is preferable that the upper limit
temperature of the finish rolling temperature be 1000°C or lower.
20 [0082]
A winding process after the hot rolling and a cooling process before and after the
winding process are significantly important to distribute Mn. The above Mn
distribution in the steel sheet can be obtained by causing the micro structure during slow
cooling after the winding to be a two phase structure of ferrite and austenite and
25 performing processing thereon at a high temperature for long time to cause Mn to be
41
diffused from ferrite to austenite.
[0083]
In order to control the distribution of the Mn concentration in the base iron at the
thickness from 1/8 to 3/8 of the steel sheet, it is necessary that the volume fraction of
5 austenite is 50% or more at the thickness from 1/8 to 3/8 when the steel sheet is wound
up. If the volume fraction of austenite at the thickness from 1/8 to 3/8 is less than 50%,
austenite disappears immediately after the winding due to progression of the phase
transformation, and therefore, the Mn distribution does not sufficiently proceed, and the
above Mn concentration distribution in the steel sheet cannot be obtained. In order that
10 the Mn distribution effectively proceeds, the volume fraction of austenite is preferably
70% or more, and more preferably 80% or more. On the other hand, if the volume
fraction of austenite is 100%, the phase transformation proceeds after the winding, ferrite
is produced, the Mn distribution is started, and therefore the upper limit is not
particularly provided for the volume fraction of austenite.
15 [0084]
In order to enhance the austenite fraction when the steel sheet is wound up, it is
necessary that the cooling rate during a period from completion of the hot rolling to the
winding be 10°C/second or higher on average. If the cooling rate is lower than
10°C/second, ferrite transformation proceeds during the cooling, and there is a possibility
20 that the volume fraction of austenite during the winding may become less than 50%. In
order to enhance the volume fraction of austenite, the cooling rate is preferably
13°C/second or higher, and more preferably 15°C/second or higher. Although the
effects of the present invention can be achieved without particularly determining the
upper limit of the cooling rate, it is preferable that the cooling rate be 200°C/second or
42
lower since a special facility is required to obtain a cooling rate of higher than
200°C/second and manufacturing costs significantly increase.
[0085]
Since a thickness of oxide formed on the surface of the steel sheet excessively
5 increases and the acid pickling property is degraded if the steel sheet is wound up at a
temperature which exceeds 800°C, the winding temperature is set to 750°C or lower In
order to enhance the acid pickling property, the winding temperature is preferably 720°C
or lower, and more preferably 700°C or lower. On the other hand, if the winding
temperature is lower than Bs point, the strength of the hot-rolled steel sheet is excessively
10 enhanced, it becomes difficult to perform cold rolling, and therefore, the winding
temperature is set to the Bs point or higher. In addition, the winding temperature is
preferably 500°C or higher, more preferably 550°C or higher, and further more
preferably 600°C or higher in order to enhance the austenite fraction after the winding.
[0086]
15 Moreover, since it is difficult to directly measure the volume fraction of
austenite during the production, a small piece is cut from the slab before the hot rolling,
the small piece is rolled or compressed at the same temperature and rolling reduction as
those in the final pass of the hot rolling and cooled with water immediately after cooling
at the same cooling rate as that during a period from the hot rolling and the winding,
20 phase fractions of the small piece are measured, and a sum of the volume fractions of
as-quenched martensite, tempered martensite, and retained austenite is regarded as a
volume fraction of austenite during the winding, in determining the volume fi-action of
austenite during the winding according to the present invention.
[0087]
43
The cooling process of the steel sheet after the winding is important to control
the Mn distribution. The Mn distribution according to the present invention can be
obtained by cooling the steel sheet from the winding temperature to (winding
temperature - 100)° at a rate of 20°C/hour or lower while the austenite fraction is set to
5 50% or more during the winding and the following equation (3) is satisfied. Equation
(3) is an index representing the degree of progression of the Mn distribution between
ferrite and austenite and represents that the Mn distribution further proceeds as the value
of the left side becomes greater. In order to fiirther cause the Mn distribution to proceed,
the value of the left side is preferably 2.5- or more, and more preferably 4.0 or more.
10 Although the effects of the present invention can be achieved without particularly
determining the upper limit of the value of the left side, it is preferable that the upper
limit is 50.0 or less since it is necessary to retain heat for long time to keep the value over
50.0 and the manufacturing costs significantly increase.
[0088]
15 [Equations]
& %47y^m'-m4-^^^]-tiT)-dT >}.0 •••*$}
Tc: winding temperature (°C)
T: steel sheet temperature (°C)
t(T): maintaining time at temperature T (second)
20 [0089]
In order to cause the Mn distribution to proceed between ferrite and austenite, it
is necessary to maintain a state where both the two phases coexist. If the cooling rate
from the winding temperature to (winding temperature - 100)°C exceeds 20°C/hour, the
44
phase transformation excessively proceeds, austenite in the steel sheet may disappear,
and therefore, the cooling rate from the winding temperature to (winding temperature -
100)°C is set to 20°C/hour or lower. In order to cause the Mn distribution to proceed,
the cooling rate from the winding temperature to (winding temperature - 100)°C is
5 preferably 17 °C/hour or lower, and more preferably 15°C/hour or lower. Although the
effects of the present invention can be achieved without particularly determining the
lower limit of the cooling rate, it is preferable that the lower limit be l°C/hour or higher
since it is necessary to perform heat retaining for a long period of time in order to keep
the cooling rate at lower than l°C/hour and the manufacturing costs significantly
10 increase.
In addition, the steel sheet may be reheated after the winding within a range of
satisiying Equation (3) and the cooling rate.
[0090]
Acid pickling is performed on the thus manufactured hot-rolled steel sheet.
15 Acid pickling is important to enhance a phosphatability of the cold-rolled high-strength
steel sheet as a final product and a hot dipping zinc-plating property of the cold-rolled
steel sheet for a galvanized steel sheet or a galvaimealed a steel sheet since oxide on the
surface of the steel sheet can be removed by pickling. In addition, the acid pickling
may be performed once or a plurality of times.
20 [0091]
Next, the hot-rolled steel sheet after the acid pickling is subjected to cold rolling
at rolling reduction from 35 to 80% and is made to pass through a continuous annealing
line or a continuous galvanizing line. By setting the rolling reduction to 35% or higher,
it is possible to maintain the flattened shape and enhance the ductility of the final
45
product.
In order to enhance the stretch-flangeability, it is preferable that regions where
the Mn concentration is high and regions where the Mn concentration is low have a
narrow distribution in distributing Mn in the subsequent process. In order to do so, it is
5 effective to increase the rolling reduction during the cold rolling, recrystallize ferrite
during temperature increase, and make grain diameters be fine. In such a viewpoint, the
rolling reduction is preferably 40% or higher, and more preferably 45% or higher.
On the other hand, in the case of cold rolling at the rolling reduction of 80% or
lower, the cold rolling load is not excessively large, and it is not difficult to perform the
10 cold rolling. For this reason, the upper limit of the rolling reduction is set to 80% or
lower. In view of the cold rolling load, the rolling reduction is preferably 75% or lower.
In addition, the effects of the present invention can be achieved without
particularly determining the number of rolling passes and rolling reduction of each pass.
In addition, the cold rolling may be omitted.
15 [0092]
Next, the obtained cold-rolled steel sheet is caused to pass through the
continuous annealing line to manufacture the high-strength cold-rolled steel sheet. In
relation to a process in which the cold-rolled steel sheet is caused to pass through the
continuous annealing line, a detailed description will be given of a temperature history of
20 the steel sheet when the steel sheet is caused to pass through the continuous annealing
line, with reference to FIG. 5.
FIG. 5 is a graph illustrating the temperature history of the cold-rolled steel sheet
when the cold-rolled steel sheet is caused to pass through the continuous annealing line,
which is a graph showing the relationship between the temperature of the cold-rolled
25 steel sheet and time. In FIG. 5, a range from (the Ae3 point - 50°C) to the Bs point is
46
shown as a "ferrite transformation temperature region", a range from the Bs point to the
Ms point is shown as the "bainite transformation temperature range", and a range from
the Ms point to a room temperature is shown as the "martensite transformation
temperature range".
5 [0093]
In addition, the Bs point is calculated based on the following equation:
Bs point [°C] - 820 - 290C/(1 - VF) - 37Si - 90Mn - 65Cr - 50Ni + 70A1
In the above equation, VF represents the volume fraction of ferrite, and C, Mn,
Cr, Ni, Al, and Si represent added amounts [mass %] of the elements.
10 [0094]
In addition, the Ms point is calculated based on the following equation:
Ms point [ °C] = 541 - 474C/(1 - VF) - 15Si - 35Mn - 17Cr - 17Ni+ 19A1
[0095]
In the above equation, VF represents a volume fraction of ferrite, C, Si, Mn, Cr,
15 Ni, and Al represent added amounts [mass %] of the elements. In addition, since it is
difficult to directly measure the volume fraction of ferrite during the production, a small
piece of the cold-rolled steel sheet before the cold-rolling sheet is made to pass through
the continuous annealing line is cut and annealed based on the same temperature history
as that when the small piece is caused to pass through the continuous annealing line,
20 dispertion in the volume of ferrite in the small piece is measured, and a numerical value
calculated using the result of the measurement is regarded as the volume fraction VF of
ferrite, in determining the Ms point in the present invention.
[0096]
As shown in FIG. 5, a heating process for annealing the cold-rolled steel sheet at
25 a maximum heating temperature (Ti) ranging from 750°C to 1000°C is firstly performed
47
in causing the cold-rolled steel sheet to pass through the continuous annealing line. If
the maximum heating temperature Ti in the heating process is lower than 750°C, the
amount of austenite is insufficient, and it is not possible to secure a sufficient amount of
hard structures in the phase transformation during the subsequent cooling. From this
5 viewpoint, the maximum heating temperature Ti is preferably 770°C or higher. On the
other hand, if the maximum heating temperature Tj exceeds 1000°C, the grain diameter
of austenite becomes coarse, the transformation hardly proceeds during the cooling, and
it becomes difficult to sufficiently obtain a soft ferrite structure, in particular. From this
viewpoint, the maximum heating temperature Ti is preferably 900°C or lower.
10 [0097]
Next, a first cooling process for cooling the cold-rolled steel sheet from the
maximum heating temperature Ti to the ferrite transformation temperature range or lower
is performed as shown in FIG. 5. In the first cooling process, the cold-rolled steel sheet
is maintained in the ferrite transformation temperature range for 20 seconds to 1000
15 seconds. In order to sufficiently produce a soft ferrite structure, it is necessary that the
cold-rolled steel sheet be maintained for 20 seconds or longer in the ferrite
transformation temperature range in the first cooling process, and the cold-rolled steel
sheet is preferably maintained for 30 seconds or longer, and more preferably maintained
for 50 seconds or longer On the other hand, if the time during which the cold-rolled
20 steel sheet is maintained in the ferrite transformation temperature range exceeds 1000
seconds, the ferrite transformation excessively proceeds, an amount of untransformed
austenite decreases, and it is not possible to sufficiently obtain a hard structure.
[0098]
In addition, a second cooling process in which the cold-rolled steel sheet after
48
being maintained in the ferrite transformation temperature range for 20 seconds to 1000
seconds to cause ferrite transformation in the first cooling process is cooled at a second
cooling rate and the cooling is stopped within a range from the Ms point -120°C to the
Ms point (the martensite transformation start temperature) is performed as shown in FIG.
5 5. By performing the second cooling process, it is possible to cause the martensite
transformation of the untransformed austenite to proceed.
[0099]
If the second cooling stop temperature Tj at which the second cooling process is
stopped exceeds the Ms point, martensite is not produced. On the other hand, if the
10 second cooling stop temperature T2 is lower than the Ms point - 120°C, most parts of the
untransformed austenite become martensite, and it is not possible to obtain a sufficient
amount of bainite in the subsequent processes. In order to cause a sufficient amount of
untransformed austenite to remain, the second cooling process stop temperature T2 is
preferably the Ms point -80°C or higher, and more preferably the Ms point - 60°C or
15 higher.
[0100]
In addition, it is preferable to prevent the bainite transformation from
excessively proceeding in the bainite transformation temperature range, which is a
temperature range between the ferrite transformation temperature range and the
20 martensite transformation temperature range, in cooling the steel sheet from the ferrite
transformation temperature range to the martensite transformation temperature range at
the second cooling rate in the second cooling process. For this reason, it is necessary to
set the second cooling rate in the bainite transformation temperature range to
10°C/second or higher on average, and the second cooling rate is preferably 20°C/second
49
or higher, and more preferably 50°C/second or higher.
[0101]
After performing the second cooling process which stops the cooling in a range
from the Ms point - 120 °C to the Ms point, as shown in FIG. 5, a maintaining process in
5 which the steel sheet is maintained within a range from the second cooling stop
temperature to the Ms point for 2 seconds to 1000 seconds in order to cause the
martensite transformation to further proceed is performed. In the maintaining process,
it is necessary to maintain the steel sheet for 2 seconds or longer in order to cause the
martensite transformation to sufficiently proceed. If the time during which the steel
10 sheet is maintained exceeds 1000 seconds in the maintaining process, hard lower bainite
is produced, an amount of untransformed austenite is reduced, and bainite with a
hardness which is close to that of ferrite cannot be obtained.
[0102]
Moreover, after maintaining the steel sheet in within the range from the second
15 cooling stop temperature to the Ms point and causing the martensite transformation to
proceed as shown in FIG. 5, a reheating process for reheating the steel sheet is performed
in order to produce bainite with a hardness between the hardness of ferrite and the
hardness of martensite. A temperature T3 (reheating stop temperature) at which the
reheating is stopped in the reheating process is set to the Bs point (Bainite transformation
20 start temperature (the upper limit of the bainite transformation temperature range)) -
100°C or higher in order to reduce the dispertion in the hardness distribution in the steel
sheet.
[0103]
In order to fiirther reduce the dispertion in the hardness distribution in the steel
25 sheet, it is preferable to produce soft bainite with a small hardness different from that of
50
ferrite. In order to produce soft bainite, the bainite transformation is preferably caused
to proceed at a temperature which is as high as possible. Accordingly, the reheating
stop temperature T3 is preferably the Bs point - 60°C or higher, and is more preferably
the Bs point or higher as shown in FIG. 5.
5 [0104]
In the reheating process, it is necessary that the rate of temperature increase in
the bainite transformation temperature range be 10°C/second or higher on average, and
the rate of temperature increase is preferably 20°C/second or higher, and more preferably
40°C/second or higher. Since the bainite transformation excessively proceeds in a state
10 of the low temperature range if the rate of temperature increase in the bainite
transformation temperature range is low in the reheating process, hard bainite with a
large hardness difference from that of ferrite is easily produced, and soft bainite with a
small hardness difference from that of ferrite, which can reduce the dispertion in the
hardness distribution in the steel sheet, is not easily produced. Accordingly, it is
15 preferable that the rate of temperature increase in the bainite transformation temperature
range be high in the reheating process.
[0105]
According to this embodiment, a sum (total maintaining time) of the time during
which the steel sheet is maintained in the bainite transformation temperature range in the
20 second cooling process and the time during which the steel sheet is maintained in the
bainite transformation range in the reheating process is preferably 25 seconds or shorter,
and more preferably 20 seconds or shorter, in order to suppress the excessive progression
of the bainite transformation in the second cooling process and the reheating process.

CLAIMS
1. A high-strength steel sheet which has an excellent ductility and a
stretch-flangeability, the steel sheet comprising by mass percentage:
5 0.05 to 0.4% of C;
0.1 to 2.5% of Si;
1.0 to 3.5% of Mn;
0.001 to 0.03% of P;
0.0001 to 0.01% of S;
10 0.001 to 2.5% of Al;
0.0001 to 0.01% of N;
0.0001 to 0.008% of O; and
a remainder composed of iron and inevitable impurities,
wherein a steel sheet structure contains by volume fraction 10 to 50% of a ferrite
15 phase, 10 to 50% of a tempered martensite phase, and a remaining hard phase,
wherein when a plurality of measurement regions with diameters of 1 [xm or less
are set in a range from 1/8 to 3/8 of a thickness of the steel sheet, hardness measurement
values in the plurality of measurement regions are arranged in ascending order to obtain a
hardness distribution, an integer N0.02 which is a number obtained by multiplying a total
20 number of the hardness measurement values by 0.02 and, if present, by rounding up a
decimal number, is obtained, a hardness of a measurement value which is an N0.02-th
largest value from a smallest hardness measurement value is regarded as a 2%) hardness,
an integer NO.98 which is a number obtained by multiplying the total number of the
hardness measurement values by 0.98 and, if present, by rounding down the decimal
25 number is obtained, and a hardness of a measurement value which is an N0.98-th largest
103
value from the smallest hardness measurement value is regarded as a 98% hardness, the
98% hardness is 1.5 or more times as high as the 2% hardness,
wherein a kurtosis K* of the hardness distribution between the 2% hardness and
the 98% hardness is equal to or more than -1.2 and equal to or less than -0.4, and
5 wherein an average crystal grain size in the steel sheet structure is 10p,m or less.
2. The high-strength steel sheet which has excellent ductility and
stretch-flangeability according to Claim 1,
wherein a difference between a maximum value and a minimum value of Mn
10 concentration in a base iron in a thickness range liom 1/8 to 3/8 of the steel sheet is equal
to or more than 0.4% and equal to or less than 3.5% when converted into the mass
percentage,
3. The high-strength steel sheet which has excellent ductility and
15 stretch-flangeability according to Claim 1 or 2,
wherein when a section fi-om the 2% hardness to the 98% hardness is equally
divided into 10 parts, and 10 1/10-sections are set, a number of the hardness
measurement values in each 1/10-section is 2 to 30% of a number of all measurement
values.
20
4. The high-strength steel sheet which has excellent ductility and
stretch-flangeability according to any one of Claims 1 to 3,
wherein the hard phase includes any one of or both a bainitic ferrite phase and a
bainite phase of 10 to 45% by a volume fraction, and a fresh martensite phase of 10% or
25 less.
io4-
5. The high-strength steel sheet which has excellent ductility and
stretch-flangeability according to any one of Claims 1 to 4,
wherein the steel sheet structure further includes 2 to 25% of a retained
5 austenite.
6. The high-strength steel sheet which has excellent ductility and
stretch-flangeability according to any one of Claims 1 to 5, further comprising by mass,
percentage one or more of;
10 0.005 to 0.09% of Ti; and
0.005 to 0.09% of Nb.
7. The high-strength steel sheet which has excellent ductility and
stretch-flangeability according to any one of Claims 1 to 6, further comprising by mass
15 percentage one or more of
0.0001 to 0.01% of B;
0.01 to 2.0% of Cr;
0.01 to 2.0% of Ni;
0.01 to 2.0% of Cu; and
20 0.01 to 0.8% of Mo.
8. The high-strength steel sheet which has excellent ductility and
stretch-flangeability according to any one of Claims 1 to 7, further comprising by mass
percentage:
25 0.005 to 0.09% of V.
105"
9. The high-strength steel sheet which has excellent ductility and
stretch-flangeability according to any one of Claims 1 to 8, fUrther comprising one or
more of Ca, Ce, Mg, and REM at 0.0001 to 0.5% by mass percentage in total.
5
10. A high-strength zinc-coated steel sheet which has excellent ductility and
stretch-flangeability,
wherein the high-strength zinc-coated steel sheet is produced by forming a
zinc-plated layer on a surface of the high-strength steel sheet according to any one of
10 Claims 1 to 9.
11. A manufacturing method of a high-strength steel sheet which has an
excellent ductility and a stretch-flangeability, the method comprising:
a hot rolling process in which a slab containing the chemical constituents
15 according to any one of Claims 1 and 6 to 9 is heated up to 1050°C or higher directly or
after cooling once, a hot rolling is performed thereon at a higher temperature of one of
800°C and an A13 transformation point, and a winding is performed in a temperature
range of 750°C or lower such that an austenite phase in a structure of a rolled material
after rolling occupies 50% by volume or more;
20 a cooling process in which the steel sheet after the hot rolling is cooled from a
winding temperature to (the winding temperature - 100) °C at a rate of 20°C/hour or
lower while a following Equation (1) is satisfied; and
a process in which continuous annealing is performed on the steel sheet after the
cooling,
wherein in the process in which continuous annealing is performed,
the steel sheet is annealed at a maximum heating temperature of 750 to 1000°C,
a first cooling in which the steel sheet is cooled from the maximum heating
temperature to a ferrite transformation temperature range or lower and maintained in the
5 ferrite transformation temperature range for 20 to 1000 seconds is subsequently
performed,
a second cooling in which the steel sheet is cooled at a cooling rate of
10°C/second or higher on average in a bainite transformation temperature range and
cooling is stopped within a range from a martensite transformation start temperature -
10 120°C to the martensite transformation start temperature is subsequently performed,
the steel sheet after the second cooling is maintained in a range from a second
cooling stop temperature to the martensite transformation start temperature for 2 to 1000
seconds,
the steel sheet is subsequently reheated up to a reheating stop temperature,
15 which is equal to or more than a bainite transformation start temperature - 100°C, at a
rate of temperature increase of 10°C/second or higher on average in the bainite
transformation temperature range, and
a third cooling in which the steel sheet after the reheating is cooled from the
reheating stop temperature to a temperature which is lower than the bainite
20 transformation temperature range and maintained in the bainite transformation
temperature range for 30 seconds or more is performed:
[Equation 1]
t!^ ?.47Kl#-exi|--i^^|-?(r)-rfrl >1.0 J^c~m \ T^m) J "• ii)
[where, t(T) in Equation (1) represents maintaining time (seconds) of the steel
sheet at a temperature T°C in the cooUng process after the winding.]
12. The manufacturing method of the high-strength steel sheet which has
5 excellent ductility and stretch-flangeability according to Claim 11,
wherein the winding temperature after the hot rolling is equal to or more than a
Bs point and equal to or less than 750°C.
13. The manufacturing method of the high-strength steel sheet which has
10 excellent ductility and stretch-flangeability according to Claim 11 or 12, further
comprising between the cooling process and the continuous annealing process:
a cold rolling process in which the steel sheet is subjected to acid pickling and a
cold rolling at rolling reduction from 35 to 80%.
15 14. The manufacturing method of the high-strength steel sheet which has
excellent ductility and stretch-flangeability according to any one of Claims 11 to 13,
wherein a sum of a time during which the steel sheet is maintained in the bainite
transformation temperature range in the second cooling and a time during which the steel
sheet is maintained in the bainite transformation temperature range in the reheating is 25
20 seconds or less.
15. A manufacturing method of a high-strength zinc-coated steel sheet which
has excellent ductility and stretch-flangeability,
wherein the steel sheet is dipped into a zinc plating bath in the reheating in
25 manufacturing the high-strength steel sheet based on the manufacturing method
10g>
according to any one of Claims 11 to 14.
16. A manufacturing method of a high-strength zinc-coated steel sheet which
has excellent ductility and stretch-flangeability,
5 wherein the steel sheet is dipped into a zinc plating bath in the bainite
transformation temperature range in the third cooling in manufacturing the high-strength
steel sheet based on the manufacturing method according to any one of Claims 11 to 14.
•17. A manufacturing method of a high-strength zinc-coated steel sheet,
10 wherein a zinc electroplating is performed after manufacturing the high-strength
' steel sheet based on the manufacturing method according to any one of Claims 11 to 14.
18. A manufacturing method of a high-strength zinc-coated steel,
wherein a hot-dip zinc-plating is performed after manufacturing the high-strength steel
15 sheet based on the manufacturing method according to any one of Claims 11 to 14.