HIGH-STRENGTH STEEL SHEET AND
HIGH-STRENGTH GALVANIZED STEEL SHEET EXCELLENT IN
SHAPE FIXABILITY, AND MANUFACTURING METHOD THEREOF
[Technical Field]
[0001] The present invention relates to a high-strength steel sheet and a
high-strength galvanized steel sheet excellent in shape fixability, and a
manufacturing method thereof. This application is based upon and claims
the benefit of priority of the prior Japanese Patent Application No.
2011-167689, filed on July 29, 2011, the entire contents of which are
incorporated herein by reference.
[Background Art]
[0002] In recent years, a demand for high-strengthening of steel sheets
used for automobiles and the like has been increasing, and high-strength steel
sheets having a maximum tensile stress of 900 MPa or more are also being
used.
These high-strength steel sheets are formed in large quantities and in
an inexpensive manner through presswork similar to mild steel sheets, and are
provided as members. However, in accordance with a rapid acceleration of
high-strengthening in recent years, there has been a problem that in a
high-strength steel sheet having a maximum tensile stress of 900 MPa or
more, a springback is caused right after press forming, and it is difficult to
form a target shape.
[0003] As a technique of improving a shape fixability of a conventional
high-strength steel sheet, there can be cited a hot-dip galvanized steel sheet
with high strength and high ductility excellent in shape fixability being a steel
sheet containing, in mass%, C: 0.0001 to 0.3%, Al: 0.001 to 4%, Mn: 0.001 to
3%, Mo: 0.001 to 4%, P: 0.0001 to 0.3%, and S: 0.01% or less, having a
plating layer containing Al: 0.001 to 0.5%, Mn: 0.001 to 2%, Fe: less than
20%, and a balance composed of Zn and inevitable impurities, containing
ferrite or ferrite and bainite of 50 to 97% in total in volume fraction as a main
phase, containing austenite of 3 to 50% in total in volume fraction as a second
phase, and having a yield ratio of 0.7 or less (refer to Patent Document 1, for
example).
[0004] Further, as a technique of improving a shape fixability of a
conventional high-strength steel sheet, there can be cited a high-strength steel
sheet excellent in workability and shape fixability having a structure which
contains, in mass%, each of C: 0.06 to 0.6%, Si + Al: 0.5 to 3%, Mn: 0.5 to
3%, P: 0.15% or less (0% is not included), and S: 0.02% or less (including
0%), contains tempered martensite of 15% or more in an area ratio with
respect to the entire structure, contains ferrite of 5 to 60% in an area ratio with
respect to the entire structure, contains a retained austenite phase of 5% or
more in a volume ratio with respect to the entire structure, and may further
contain bainite and/or martensite, in which a proportion of retained austenite
phase, out of the retained austenite phase, that transforms into martensite by
applying a strain of 2% is 20 to 50% (refer to Patent Document 2, for
example).
[0005] Further, as a technique of improving a shape fixability of a
conventional high-strength steel sheet, there can be cited a manufacturing
method of a high-strength cold-rolled steel sheet excellent in impact property
and shape fixability in which a slab having a composition of C: 0.08 to 0.18
mass%, Si: 1.00 to 2.0 mass%, Mn: 1,5 to 3.0 mass%, P: 0.03 mass% or less,
S: 0.005 mass% or less, and T.A1: 0.01 to 0.1 mass%, and having a
segregation degree of Mn with respect to a cast slab of 1.05 to 1.10 is
hot-rolled, the resultant is further-cold-rolled, the resultant is then heated for a
retention time of 60 seconds or more in a two-phase region or a single-phase
region at 750 to 870°C in a continuous annealing line, cooling is then
performed in a temperature region of 720 to 600°C at an average cooling rate
of 10°C/s or less, cooling is then performed until the temperature reaches 350
to 460°C at an average cooling rate of 10°C/s or more, retention is performed
for 30 seconds to 20 minutes, and cooling is then performed until the
temperature reaches a room temperature to obtain a five-phase structure of
polygonal ferrite, acicular ferrite, bainite, retained austenite phase, and
martensite (refer to Patent Document 3, for example).
[0006] Further, as a technique of improving a shape fixability of a
conventional high-strength steel sheet, there can be cited a high-strength steel
sheet excellent in formability and shape fixability characterized in that it is
mainly formed of a ferrite phase of 20 to 97% in volume fraction and a
retained austenite phase of 3% or more in volume fraction, in which a
proportion of a part other than the ferrite phase having an aspect ratio of
crystal grains of 2.5 or less is 50 to 95%, and the steel sheet preferably
contains C: 0.05 to 0.30 mass%, Si: 2.0 mass% or less, Mn: 0.8 to 3.0 mass%,
P: 0.003 to 0.1 mass%, S: 0.01 mass% or less, Al: 0.01 to 2.50 mass%, and N:
0.007 mass% or less, in which Si and Al satisfy a relation of Si + Al ^ 0.50
mass% (refer to Patent Document 4, for example).
[0007] Further, the present applicant discloses a high-strength steel sheet
excellent in ductility and stretch flangeability, containing predetermined
components, and having a steel sheet structure composed of,. in volume
fraction, a ferrite phase of 10 to 50%, a tempered martensite phase of 10 to
50%, and a remaining hard phase (refer to Patent document 5, for example).
[Prior Art Document]
[Patent Document]
[0008] Patent Document 1: Japanese Patent Publication No. 2003-253386
Patent Document 2: Japanese Patent Publication No. 2004-218025
Patent Document 3: Japanese Patent Publication No. 2004-300452
Patent Document 4: Japanese Patent Publication No. 2007-154283
Patent Document 5: International Publication WO2012/036269A1
[Disclosure of the Invention]
[Problems to Be Solved by the Invention]
[0009] However, in Patent Document 1, there was a problem that a
manufacturing cost is increased, since it is essential to add a large amount of
expensive Mo.
Further, in Patent Document 2, the manufacturing steps become
complicated since the annealing step after the hot rolling is performed in
divided two steps, and meanwhile, it was difficult to stably secure the shape
fixability in the high-strength steel sheet having a maximum tensile strength
of 900 MPa or more.
Further, in Patent Document 3, it is required to perform processing of
controlling casting conditions to reduce a center segregation of Mn in the slab
when manufacturing the slab, and there was a possibility of reducing the
production efficiency.
Further, in Patent Document 4, the steel sheet structure and the aspect
ratio of the crystal grains are specified to improve the shape fixability, but, no
specification is made for securing the ductility.and the tensile strength, so that
the securement of high-strength steel sheet with the maximum tensile strength
of 900 MPa or more was unstable. Further, the shape fixability in the
high-strength region of 900 MPa or more as above is insufficient, and thus it
has been desired to further improve the shape fixability.
Further, in Patent Document 5, it is basically required to have the
tempered martensite phase of 10 to 50%, so that there was a concern that the
workability becomes inferior.
[0010] Accordingly, the present invention was made in view of such
circumstances, and an object thereof is to provide a high-strength steel sheet
and a high-strength galvanized steel sheet having excellent shape fixability
and workability while securing a high strength of maximum tensile strength
of 900 MPa or more, and a manufacturing method thereof.
[Means for Solving the Problems]
[0011] The present inventors conducted earnest studies for solving the
above-described problems. As a result of this, they found out that it is
possible to obtain a steel sheet having excellent shape fixability and
workability with large work hardening amount in an initial stage of forming
while securing a high strength of maximum tensile strength of 900 MPa or
more, by making a microstructure of steel sheet to be a microstructure having
a retained austenite phase, and by concentrating Si and Mn in the retained
austenite phase.
[0012] The gist of the present invention to solve the above-described
problems is as follows.
[0013] (1)
A high-strength steel sheet excellent in shape fixability, contains, in
mass%, C: 0.075 to 0.300%, Si: 0.30 to 2.5%, Mn: 1.3 to 3.50%, P: 0.001 to
0.030%, S: 0.0001 to 0.0100%, Al: 0.080 to 1.500%, N: 0.0001 to 0.0100, O:
0.0001 to 0.0100, and a balance composed of Fe and inevitable impurities, in
which a steel sheet structure contains a retained austenite phase of 5 to 20% in
volume fraction in a range of 1/8 thickness to 3/8 thickness of the steel sheet,
an amount of solid-solution C contained in the retained austenite phase is 0.80
to 1.00% in mass%, WSiy defined as an amount of solid-solution Si contained
in the retained austenite phase is 1.10 times or more WSi* defined as an
average amount of Si in the range of 1/8 thickness to 3/8 thickness of the steel
sheet, WMny defined as an amount of solid-solution Mn contained in the
retained austenite phase is 1.10 times or more WMn* defined as an average
amount of Mn in the range of 1/8 thickness to 3/8 thickness of the steel sheet,
and when a frequency distribution is measured, by setting a plurality of
measurement regions each having a diameter of 1 um or less in the range of
1/8 thickness to 3/8 thickness of the steel sheet, with respect to a sum of a
ratio between Wsi defined as a measured value of an amount of Si in each of
the plurality of measurement regions and Wsi* being the average amount of Si
and a ratio between WAi defined as a measured value of an amount of Al in
each of the plurality of measurement regions and WAi* being the average
amount of Al, a mode value of the frequency distribution is 1.95 to 2.05, and a
kurtosis is 2.00 or more.
(2)
In the high-strength steel sheet excellent in shape fixability according
to (1), the steel sheet structure further contains a ferrite phase of 10 to 75% in
volume fraction, and either or both of a bainitic ferrite phase and a bainite
phase of 10 to 50% in total, a tempered martensite phase is limited to less
than 10%.in volume fraction, and a fresh martensite phase is limited to 15%
or less in volume fraction.
(3)
- -The high-strength steel sheet excellent in shape fixability further
contains, in mass%, one or two or more of Ti: 0.005 to 0.150%, Nb: 0.005 to
0.150%, V: 0.005 to 0.150%, and B: 0.0.001 to 0.0100%.
(4)
The high-strength steel sheet excellent in shape fixability according to
(1) further contains, in mass%, one or two or more of Mo: 0.01 to 1.00%, W:
0.01 to 1.00%, Cr: 0.01 to 2.00%, Ni: 0.01 to 2.00%, and Cu: 0.01 to 2.00%.
' (5)
The high-strength steel sheet excellent in shape fixability according to
(1) further contains, in mass%, one or two or more of Ca, Ce, Mg, Zr, Hf, and
REM of 0.0001 to 0.5000% in total.
(6)
i A high-strength galvanized steel sheet excellent in shape fixability has
the high-strength steel sheet according to (1) having a galvanized layer
formed on a surface thereof.
(7)
In the high-strength galvanized steel sheet excellent in shape fixability
) according to (6), a coating film made of a composite oxide containing a
phosphorus oxide and/or phosphorus is formed on a surface of the galvanized
layer.
(8)
A manufacturing method of a high-strength steel sheet excellent in
> shape fixability includes: a hot-rolling step being a step of heating a slab
containing,- in mass%, C: 0.075 to 0.300%, Si: 0.30 to 2.5%, Mn: 1.3 to
3.50%, P: 0.001 to 0.030%, S: 0.0001 to 0.0100%, Al: 0.080 to 1.500%, N:
0.0001 to 0.0100, O: 0.0001 to 0.0100, and a balance composed of Fe and
inevitable impurities to 1100°C or more, performing hot rolling on the slab in
a temperature region in which a higher temperature between 850°C and an Ar3
temperature is set to a lower limit temperature, performing first cooling of
performing cooling in a range from a completion of rolling to a start of
coiling at a rate of 10°C/second or more on average, performing coiling in a
range of coiling temperature of 600 to 750°C, and performing second cooling
of cooling the coiled steel sheet in a range of the coiling temperature to (the
coiling temperature - 100)°C at a rate of 15°C/hour or less on average; and a
continuous annealing step of performing annealing on the steel sheet at a
maximum heating temperature (Aci + 40)°C to 1000°C after the second
cooling, next performing third cooling at an average cooling rate of 1.0 to
10.0°C/second in a range of the maximum heating temperature to 700°C, next
performing fourth cooling at an average cooling rate of 5.0 to 200.0°C/second
in a range of 700°C to 500°C, and next perfomiing retention process of
retaining the steel sheet after being subjected to the fourth cooling for 30 to
1000 seconds in a range of 350 to 450°C.
(9)
The manufacturing method of the high-strength steel sheet excellent in
shape fixability according to (8) includes a cold-rolling step of performing
pickling and then performing cold rolling at a reduction ratio of 30 to 75%,
between the hot-rolling step and the continuous annealing step.
(10)
The manufacturing method of the high-strength steel sheet excellent in
shape fixability according to (8) includes a temper rolling step of performing
rolling on the steel sheet at a reduction ratio of less than 10%, after the
continuous annealing step.
(11)
A manufacturing method of a high-strength galvanized steel sheet
excellent in shape fixability includes forming, after performing the retention
process when manufacturing the high-strength steel sheet in the
manufacturing method according to (8), a galvanized layer on a surface of the
steel sheet by conducting electrogalvanization.
(12)
A manufacturing method of a high-strength galvanized steel sheet
excellent in shape fixability includes forming, between the fourth cooling and
the retention process, or after the retention process when manufacturing the
high-strength steel sheet in the manufacturing method according to (8), a
galvanized layer on a surface of the steel sheet by dipping the steel sheet in a
galvanizing bath.
(13)
In the manufacturing method of the high-strength galvanized steel
sheet excellent in shape fixability according to (12), the steel sheet after being
dipped in the galvanizing bath is reheated to 460 to 600°C, and retained for
two seconds or more to make the galvanized layer to be alloyed.
(14)
In the manufacturing method of the high-strength galvanized steel
sheet excellent in shape fixability according to (11), after the galvanized layer
is formed, a coating film made of a composite oxide containing either or both
of a phosphorus oxide and phosphorus is given to a surface of the galvanized
l a y e r . - • --,. -.- ...,...,. „ , ; . , . • - -
In the manufacturing method of the high-strength galvanized steel
sheet excellent in shape fixability according to (13), after the galvanized layer
is alloyed, a coating film made of a composite oxide containing either or both
of a phosphorus oxide and phosphorus is given to a surface of the alloyed
galvanized layer.
[Effect of the Invention]
[0014] Each of a high-strength steel sheet and a high-strength galvanized
steel sheet of the present invention contains predetermined chemical
components, and when a frequency distribution is measured, in a range of 1/8
thickness to 3/8 thickness of the steel sheet, with respect to a sum of a ratio
between a measured value of Si amount and an average Si amount and a ratio
between a measured value of Al amount and an average Al amount, a mode
value of the frequency distribution is 1.95 to 2.05, and a kurtosis is 2.00 or
more, so that it is possible to create a distribution state where either Si or Al
exists in an amount being an equal amount or more of an average amount in
the entire area of the steel sheet. Accordingly, a generation of iron-based
carbide is suppressed, and C can be prevented from being consumed as
carbide. For this reason, it is possible to stably secure a retained austenite
phase, resulting in that a shape fixability, a ductility and a tensile strength can
be largely improved.
Further, in each of the high-strength steel sheet and the high-strength
galvanized steel sheet of the present invention, the retained austenite phase
occupies 5 to 20% in volume fraction, a Si amount contained in the retained
austenite phase is 1.10 times or more an average Si amount, a Mn amount
contained in the retained austenite phase is 1.10 times or more an average Mn
amount, and a C amount contained in the retained austenite phase is 0.80 to
1.00% in mass%, so that it is possible to obtain a steel sheet having excellent
shape fixability and workability while securing a high strength of 900 MPa or
more of a maximum tensile strength.
[0015] Further, in a manufacturing method of a steel sheet of the present
invention, a step of making a slab containing predetermined chemical
components to be a hot-rolled coil includes a first cooling step in which a
cooling rate from when hot rolling is completed to when coiling is conducted
is set to 10°C/second or more, a coiling step of making the steel sheet to be a
coil at 600 to 700°C, and a second cooling step in which an average cooling
rate from a coiling temperature to (the coiling temperature - 100)°C is set to
15°C/hour or less, so that solid-solution Si and solid-solution Al in the inside
of the steel sheet can be distributed in a symmetric manner, namely, an Al
amount is reduced at a portion where a Si amount is large, and a portion
where solid-solution Si is concentrated and a portion where solid-solution Mn
is concentrated can be set to the same.
Further, in the manufacturing method of the steel sheet of the present
invention, a step of making the steel sheet pass through a continuous
annealing line includes a step of performing annealing at a maximum heating
temperature (Aci + 40)°C to 1000°C, a third cooling step of cooling the steel
sheet from the maximum heating temperature to 700°C at 1.0 to 10.0°C/sec
on average, a fourth cooling step of cooling the steel sheet after being
subjected to the third cooling step from 700°C to 500CC at 5.0 to 200.0°C/sec
on average, and a step of retaining the steel sheet after being subjected to the
fourth cooling step for 30 to 1000 seconds in a range of 350 to 450°C, so that
a microstructure of the steel sheet contains the,retained austenite phase of 5 to
20%, and Si, Mn, and C having predetermined concentrations can be
solid-solved in the retained austenite phase, resulting in that a high-strength
steel sheet or a high-strength galvanized steel sheet capable of securing a high
strength of 900 MPa or more of the maximum tensile strength and having
excellent shape fixability and workability, can be obtained.
Mode for Carrying out the Invention]
0016] Hereinafter, a high-strength steel sheet and a high-strength
galvanized steel sheet excellent in shape fixability, and a manufacturing
method thereof of the present invention will be described in detail.
[0017]
A high-strength steel sheet of the present invention is a steel sheet that
contains, in mass%, each of C: 0.075 to 0.300%, Si: 0.30 to 2.5%, Mn: 1.3 to
3.50%, P: 0.001 to 0.030%, S: 0.0001 to 0.0100%, Al: 0.080 to 1.500%, N:
0.0001 to 0.0100, O: 0.0001 to 0.0100, and a balance composed of Fe and
inevitable impurities, in which a steel sheet structure contains a retained
austenite phase of 5 to 20% in volume fraction in a range of 1/8 thickness to
3/8 thickness of the steel sheet, an amount of solid-solution C contained in the
retained austenite phase is 0.80 to 1.00% in mass%, WSir defined as an
amount of solid-solution Si contained in the retained austenite phase is 1.10
times or more WSi* defined as an average amount of Si in the range of 1/8
thickness to 3/8 thickness of the steel sheet, WMny defined as an amount of
solid-solution Mn contained in the retained austenite phase is 1.10 times or
more WMn* defined as an average amount of Mn in the range of ,1/8 thickness
to 3/8 thickness of the steel sheet, and when a frequency distribution is
measured, by setting a plurality of measurement regions each having a
.„ diameter of 1 um or less in the range of 1/8 thickness to 3/8 thickness of the
13
steel sheet, with respect to a sum of a ratio between WS; defined as a
measured value of an amount of Si in each of the plurality of measurement
regions and WSi* being the average amount of Si and a ratio between WAi
defined as a measured value of an amount of Al in each of the plurality of
measurement regions and WAi* being the average amount of Al, a mode value
of the frequency distribution is 1.95 to 2.05, and a kurtosis is 5.00 or more.
Hereinafter, reasons of limiting the steel sheet structure and the
chemical components (composition) in the present invention will be described.
Note that the notation of % means volume fraction regarding the structure,
and means mass% regarding the composition, unless otherwise noted.
[0018] The steel sheet structure of the high-strength steel sheet of the
present invention contains predetermined chemical components, in which, in
the range of 1/8 thickness to 3/8 thickness of the steel sheet, the retained
austenite phase of 5 to 20% in volume fraction is contained, the amount of
solid-solution C in the retained austenite phase is 0.80 to 1.00% in mass%,
WMIIY / WMn* being the ratio between Wiviny being the amount of solid-solution
Mn in the retained austenite phase and WMn* being the average amount of Mn
in the range of 1/8 thickness to 3/8 thickness of the steel sheet is 1.10 or more,
and WsiY / Wsi* being the ratio between the amount of solid-solution Si WSiY in
the retained austenite phase and Ws;* being the average amount of Si in the
range of 1/8 thickness to 3/8 thickness of the steel sheet is 1.10 Or more, so
that the steel sheet having excellent shape fixability and workability while
securing a,high strength of 900 MPa or more of tensile strength is obtained.
Note that it is desirable that the retained austenite phase of 5 to 20% in
volume fraction is contained in the entire steel sheet structure. However, a
. metaLstructure in the range of 1/8 thickness to 3/8 thickness with.1/4 of the
sheet thickness of the steel sheet being the center represents the structure of
the entire steel sheet. Therefore, if retained austenite of 5 to 20% in volume
fraction is contained in the range of 1/8 thickness to 3/8 thickness-of the steel
sheet, it can be regarded that retained austenite of 5 to 20% in volume fraction
is substantially contained in the entire structure of the steel sheet. For this
reason, in the present invention, a range of volume fraction of retained
austenite in the range of 1/8 thickness to 3/8 thickness of the base steel sheet
is defined.
Regarding the volume fraction of the retained austenite phase, an
X-ray analysis is conducted by setting a surface parallel to and at 1/4
thickness from the sheet surface of the steel sheet as an observation surface to
calculate an area fraction, and a result of the calculation can be regarded as
the volume fraction.
Note that a microstructure in the range of 1/8 thickness to 3/8
thickness has high homogeneity, and if measurement is performed on a
sufficient large region, even when the measurement is performed at any
position in the range of 1/8 thickness to 3/8 thickness, a fracture of
microstructure representing the range of 1/8 thickness to 3/8 thickness can be
obtained.
An X-ray diffraction test is performed on an arbitrary surface parallel
to and at 1/8 thickness to 3/8 thickness from the sheet surface of the steel
sheet to calculate an area fraction of the retained austenite phase, and a result
of the calculation can be regarded as the volume fraction in the range of 1/8.
thickness to 3/8 thickness. Concretely, it is preferable to perform the X-ray
diffraction test on a surface parallel to and at 1/4 thickness from the sheet
. .surface of the steel sheet in a range of 250000 square urn or more. .,•„, .,. ,.
Hereinafter, solid-solution elements and amounts of solid-solution
elements solid-solved in the retained austenite phase will be described in
detail.
[0019] (Retained austenite phase)
Amounts of elements solid-solved in the retained austenite phase
determine a stability of the retained austenite phase, and change a strain
amount required when the retained austenite phase is transformed into hard
martensite. For this reason, it is possible to control a work hardening
behavior by controlling the amounts of solid-solution elements in the retained
austenite phase, resulting in that the shape fixability, the ductility and the
tensile strength can be largely improved.
[0020] Solid-solution C in the retained austenite phase is an element that
increases the stability of the retained austenite phase, and increases a strength
of transformed martensite. If the amount of solid-solution C is less than
0.80%, it is not possible to sufficiently achieve the effect of improving the
ductility obtained by retained austenite, so that in the present embodiment, a
lower limit of the amount of solid-solution C is set to 0.80%. Note that in
order to sufficiently increase the ductility, the amount of solid-solution C is
preferably 0.85% or more, and is more preferably 0.90% or more. On the
other hand, if the amount of solid-solution C exceeds 1.00%, the strength of
transformed martensite is increased too much, and the martensite acts as a
starting point of destruction with respect to processing in which a large strain
is locally applied such..as stretch flange deformation, which only deteriorates
the formability, so that an upper limit of the amount of solid-solution C is set
to 1.00% or less. From this point of view, the amount of solid-solution C is
preferably 0.98% or less, and is more preferably 0.96% or less.
[0021] Note that the amount of solid-solution C (Cy) in the retained
austenite phase can be determined through the following equation (1) by
conducting an X-ray diffraction test under the same conditions as those of the
measurement of the area fraction of the retained austenite phase to determine
a lattice constant a of the retained austenite phase.
[0022] [Mathematical equation 1]
c _ (a-0.3556),, 12.01
7 0.00095 55.84
[0023] Solid-solution Mn in the retained austenite phase is an element
that increases the stability of the retained austenite phase. If an amount of
solid-solution Mn in the retained austenite phase is set to WM^, and an
average Mn amount in the range of 1/8 thickness to 3/8 thickness of the steel
sheet is set to WMMS a lower limit of Wp^y / WMH* being a ratio of the both
amounts is set to 1.1 or more in the present embodiment. Note that in order
to increase the stability of the retained austenite phase, the WMny / WMn* is
preferably 1.15 or more, and is more preferably 1.20 or more.
[0024] Further, solid-solution Si in the retained austenite phase is an
element that moderately destabilizes the retained austenite phase, increases a
work hardening performance, and increases the shape fixability in a low strain
region. Specifically, by concentrating Si in the retained austenite phase, it is
possible to give a moderate instability to the retained austenite phase, so that
it is possible to cause easy transformation when applying a strain, and to
cause sufficient work hardening in an initial stage at a time of processing.
On the other hand, solid-solution Si in the retained austenite phase is an
element that increases the stability of the retained austenite phase and
contributes to a local ductility in a high strain region.
In the present embodiment, by setting WSiY / WSj* being a ratio
between WSiY defined as an amount of solid-solution Si in the retained
austenite phase and Wsi* defined as an average amount of Si in the range of
1/8 thickness to 3/8 thickness of the steel sheet to 1.10 or more, the influence
of solid-solution Si described above is obtained. Note that the WSiY / Wsi* is
preferably 1.15 or more, and is more preferably 1.20 or more.
[0025] Further, the amount of solid-solution Mn and the amount of
solid-solution Si in the retained austenite phase are obtained by first collecting
a sample by setting a thicknesswise cross section parallel to a rolling direction
of the steel sheet as an observation surface, in the range of 1/8 thickness to
3/8 thickness of the steel sheet. Next, an EPMA analysis is performed in the
range of 1/8 thickness to 3/8 thickness with 1/4 thickness being the center to
measure Mn and Si amounts. The measurement is performed while a probe
diameter is set to 0.2 to 1.0 um, and a measurement time per one point is set
to 10 ms or more, and the Mn and Si amounts are measured at 2500 points or
more based on area analysis, to thereby create Si and Mn concentration maps.
Here, in results of the measurement described above, a point at which
the Mn concentration exceeds three times an added Mn concentration can be
considered to be a point at which an inclusion such as Mn sulfide is measured.
Further, a point at which the Mn concentration is less than 1/3 times the added
Mn concentration can be considered to be a point at which an inclusion such
as Al oxide is measured. Since the Mn concentrations in these inclusions do
not affect a phase transformation behavior in the base iron almost at all, the
measurement results of the inclusions are set to be excluded from the
above-described measurement results.. Note that measurement results of Si
are also processed in a similar manner, and measurement results of inclusions
are set to be excluded from the above-described measurement results.
Further, the region analyzed either before or after the above-described
EPMA analysis is observed through an EJ3SD analysis method, distributions
of FCC iron (retained austenite phase) and BCC iron (ferrite) are mapped, the
obtained map is overlapped with the Si and Mn concentration maps, and Si
and Mn amounts in a region overlapped with a region of FCC iron, namely,
retained austenite are read. Accordingly, the amount of solid-solution Si and
the amount of solid-solution Mn in the retained austenite phase can be
determined.
[0026] Solid-solution Si in the retained austenite phase is the element that
moderately destabilizes the retained austenite phase, increases the work
hardening performance, and increases the shape fixability in the low strain
region, and is the element that increases the stability of the retained austenite
phase and contributes to the local ductility in the high strain region as
described above, and in addition to that, it is also an element of suppressing a
generation of iron-based carbide.
Normally, when Si is just concentrated in the retained austenite phase,
the iron-based carbide is generated in a portion where Si is not concentrated,
and C being an austenite stabilizing element is consumed as carbide, resulting
in that the retained austenite phase cannot be sufficiently secured and the
shape fixability is deteriorated, which is a problem.
Accordingly, in the present embodiment, Al being an element of
suppressing the generation of iron-based carbide, similar to Si, is added in an
appropriate amount, and processing is performed based on a predetermined
thermal history in the hot-rolling step, resulting in that Si can be efficiently
concentrated in retained austenite. Further, at this time, Al exhibits the
concentration distribution opposite to the concentration distribution of Si, so
that a region with low Si concentration has higher Al amount. For this
reason, in retained austenite, it is possible to suppress the generation of
iron-based carbide by Si in a region with high Si concentration, and in a
region with low Si concentration, the generation of iron-based carbide can be
suppressed by Al, instead of Si. Accordingly, it is possible to prevent C
from being consumed as carbide in the retained austenite phase, resulting in
that the retained austenite phase can be efficiently obtained. Further, the
generation of coarse iron-based carbide which becomes a starting point of
destruction at the time of processing can be suppressed, which contributes to
the improvement of the shape fixability, the ductility and the tensile strength.
Si is the element that destabilizes austenite, and generally, Mn is
concentrated in the retained austenite phase, and Si is concentrated in ferrite.
However, in the present invention, Al is added, and through the predetermined
manufacturing conditions, Al is concentrated in ferrite, and Si is concentrated
in the retained austenite phase.
[0027] Further, when, in the thicknesswise cross section parallel to the
rolling direction of the steel sheet according to the present embodiment, a
frequency distribution (histogram) of F (WSi, WAi) = Wsi / Wsi* + WAi / WAi*
being a sum of a ratio between Wsi defined as a measured value of an amount
of Si in each of measurement regions at 1/8 thickness to 3/8 thickness with
1/^k thickness being the center and WSi* defined as an average, amount of Si at
1/8 thickness to 3/8 thickness, and a ratio between WAi defined as a measured
value of an amount of Al in each of the measurement regions at 1/8 thickness
to, 3/8..thickness with 1/4 thickness being the center and WA|* defined as an
average amount of Al at 1/8 thickness to 3/8 thickness is created, a mode
value is set to fall within a range of 1.95 to 2.05, and a kurtosis K of the
histogram defined by the following equation (2) is set to 2.00 or more. Note
that the measurement region is set to have a diameter of 1 urn or less, and a
plurality of such measurement regions are set to measure the Si amount and
the Al amount.
By creating a distribution state as described above in which either Si
or Al exists in an amount being an equal amount or more of an average
amount in the entire area of the steel sheet, the generation of iron-based
carbide is suppressed, so that it is possible to stably secure the retained
austenite phase, resulting in that the shape fixability, the ductility and the
tensile strength can be largely improved.
In any of a case where the mode value becomes less than 1.95, a case
where the mode value exceeds 2,05, and a case where the kurtosis K becomes
less than 2.00, there exists a region where a generation suppression
performance of iron-based carbide is small in the measurement range, and
there is a possibility that sufficient shape fixability, formability and/or
strength cannot be achieved. From this point of view, the kurtosis K is
preferably 2.50 or more, and is more preferably 3.00 or more.
[0028] Here, the kurtosis K is a number determined by the following
equation (2) from data, and is a numerical value evaluated by comparing a
frequency distribution of data with a normal distribution. When the kurtosis
is a negative number, this represents that a frequency distribution curve of
data is relatively flat, and it is meant that the larger an absolute value is, the
more the frequency distribution is deviated from the normal distribution.
- Note that Fi in the following equation (2) indicates a value of F (WSjr
WAI) at i-th measurement point, F* indicates an average value of F (WSi, WAI),
s* indicates a standard deviation of F (WSi, WAI), and N indicates a number of
measurement points in the obtained histogram.
[0029] [Mathematical equation 2]
[0030] Note that the method of measuring the amounts of solid-solution
C, Mn, Si and Al is not limited to the above-described method. For example,
an EMA method or a direct observation using a three-dimensional atom probe
(3D-AP) may be performed to measure the concentrations of the various
elements.
[0031] (Microstructure)
It is preferable that the steel sheet structure of the high-strength steel
sheet of the present invention contains, in addition to the above-described
retained austenite phase, a ferrite phase of 10 to 75% in volume fraction, and
either or both of a bairiitic ferrite phase and a bainite phase of 10 to 50% in
total in volume fraction, a tempered martensite phase is limited to less than
10% in volume fraction, and a fresh martensite phase is limited to 15%o or less
in volume fraction. When the high-strength steel sheet of the present
invention has the steel sheet structure as described above, it becomes a steel
sheet having further excellent shape fixability and formability.
[0032] "Ferrite phase"
The ferrite phase is a structure effective for improving the ductility,
and is preferably contained in the steel sheet structure in an amount of 10 to
75% in volume fraction. The volume fraction of the ferrite phase contained
in the steel sheet structure is more preferably 15% or more, and is still more
preferably 20% or more from a point of view of the ductility. Further, in
order to sufficiently increase the tensile strength of the steel sheet, the volume
fraction of the ferrite phase contained in the steel sheet structure is more
preferably set to 65% or less, and is still more preferably set to 50% or less.
When the volume fraction of the ferrite phase is less than 10%, there is a
chance that the sufficient ductility cannot be achieved. On the other hand,
the ferrite phase is a soft structure, so that when the volume fraction thereof
exceeds 75%, the sufficient strength may not be obtained.
[0033] "Bainitic ferrite phase and/or bainite phase"
Bainitic ferrite and/or bainite are/is structure(s) necessary for
efficiently obtaining the retained austenite phase, and preferably contained in
the steel sheet structure in an amount of 10 to 50% in total in volume fraction.
Further, the bainitic ferrite phase and/or bainite phase are/is microstructure(s)
having a strength which is in the middle of a strength of a soft ferrite phase
and hard martensite phase, tempered martensite phase and retained austenite
phase, and the bainitic ferrite phase and/or bainite phase are/is more
preferably contained in an amount of 15% or more, and still more preferably
contained in an amount of 20% or more, from a point of view of the stretch
flangeability. On the other hand, it is not preferable that the volume fraction
of the bainitic •ferrite phase and/or the bainite phase exceeds 50%, since there
is a concern that a yield stress is excessively increased and the shape fixability
is deteriorated.
[0034] "Tempered martensite phase"
.. = The tempered martensite phase is a structure of improving the tensile
strength. However, martensite is generated by preferentially consuming
non-transformed austenite with a large Si content, so that there is a tendency
that a steel sheet containing a large amount of tempered martensite has a
small amount of retained austenite with a large Si content. Further, it is not
preferable that an amount of tempered martensite is 10% or more, since there
is a concern that the yield stress is excessively increased, and the shape
fixability is deteriorated. For this reason, in the present invention, tempered
martensite is limited to less than 10% in volume fraction. The tempered
martensite phase is preferably 8% or less, and is more preferably 6% or less.
[0035] "Fresh martensite phase"
The fresh martensite phase largely improves the tensile strength, but,
on the other hand, it becomes a starting point of destruction to deteriorate the
stretch flangeability. Further, martensite is generated by preferentially
consuming non-transformed austenite with a large Si content, so that there is a
tendency that a steel sheet containing a large amount of fresh martensite has a
small amount of retained austenite with a large Si content. From a point of
view of the stretch flangeability and the shape fixability, the fresh martensite
phase in the steel sheet structure is preferably limited to 15% or less in
volume fraction. In order to further increase the stretch flangeability, the
volume fraction of fresh martensite is more preferably set to 10%) or less, and
is still more preferably set to 5% or less.
[0036] "Other microstructures"
It is also possible, that the steel sheet structure of the high-strength
steel sheet of the present invention contains a structure other than the above,
such as pearlite and/or coarse cementite. However, when an amount of
pearlite and/or.coarse, cementite is increased in the steel sheet structure of the
high-strength steel sheet, the ductility is deteriorated. For this reason, a
volume fraction of pearlite and/or coarse cementite contained in the steel
sheet structure is preferably 10% or less in total, and is more preferably 5% or
less in total.
[0037] The volume fraction of each structure contained in the steel sheet
structure of the high-strength steel sheet of the present invention can be
measured by a method described below, for example.
[0038] Regarding the volume fractions of ferrite, bainitic ferrite, bainite,
tempered martensite and fresh martensite contained in the steel sheet structure
of the high-strength steel sheet of the present invention, a sample is collected
while a thicknesswise cross section perpendicular to the rolling direction of
the steel sheet is set as an observation surface, the observation surface is
polished and subjected to nital etching, and a range of 1/8 thickness to 3/8
thickness with 1/4 of the sheet thickness being the center is observed with an
FE-SEM (Field Emission Scanning Electron Microscope) to measure area
fractions, and results of the measurement can be regarded as the volume
fractions.
As described above, the fractions of microstructures except the
retained austenite phase can be measured by performing observation with the
electron microscope at an arbitrary position at 1/8 thickness to 3/8 thickness.
Concretely, an observation with the electron microscope is performed in three
or more of fields of view set, on a surface which is perpendicular to the sheet
surface of the base steel sheet and parallel to the rolling direction, while
providing an interval of 1 mm or more therebetween in the range of 1/8
thickness to 3/8 thickness to calculate an area fraction of each structure in a
range where the observation,area,is 5000 square urn or more in total, and a
result of the calculation can be regarded as the volume fraction in the range of
1/8 thickness to 3/8 thickness.
[0039] Ferrite is a mass of-erystal grains, and is a region in which no
iron-based carbide with a major axis of 100 nm or more exists in its inside.
Note that the volume fraction of ferrite is a sum of a volume fraction of ferrite
remaining at the maximum heating temperature and a volume fraction of
ferrite newly generated in a ferrite transformation temperature region.
Bainitic ferrite is an aggregation of lath-shaped crystal grains, and
does not contain, in the inside of the lath, iron-based carbide with a major axis
of 20 nm or more.
Bainite is an aggregation of lath-shaped crystal grains, contains, in the
inside of the lath, a plurality of iron-based carbides with a major axis of 20
nm or more, and those carbides belong to a single variant, namely, an
iron-based carbide group stretching in the same direction. Here, the
iron-based carbide group stretching in the same direction means one having a
difference of 5° or less in stretch direction of the iron-based carbide group.
Tempered martensite is an aggregation of lath-shaped crystal grains,
contains, in the inside of the lath, a plurality of iron-based carbides with a
major axis of 20 nm or more, and those carbides belong to a plurality of
variants, namely, a plurality of iron-based carbide groups stretching in
different directions.
Note that bainite and tempered martensite can be easily distinguished
by observing iron-based carbide inside of the lath-shaped crystal grain using
the FE-SEM and examining the stretch direction thereof.
[0040] Further, fresh martensite and retained austenite are not sufficiently
corroded by the nital etching. Accordingly, fresh martensite and retained
austenite are clearly distinguished from the aforementioned structures (ferrite,
bainitic ferrite, bainite, tempered martensite) in the observation with the
FE-SEM.
Therefore, the volume fraction of fresh martensite is obtained as a
5 difference between an area fraction of a non-corroded region observed with
the FE-SEM, and an area fraction of retained austenite measured with X-ray.
[0041] (Chemical components)
Next, chemical components (composition) of the high-strength steel
sheet of the present invention will be described. Note that in the description
10 hereinbelow, [%] indicates [mass%].
[0042] "C: 0.075 to 0.300%"
C is contained to increase the strength of the high-strength steel sheet.
However, if a C content exceeds 0.300%, a weldability becomes insufficient.
From a point of view of the weldability, the C content is preferably 0.250% or
15 less, and is more preferably 0.220% or less. On the other hand, if the C
content is less than 0.075%, the strength is lowered, and it is not possible to
secure the maximum tensile strength of 900 MPa or more. In order to
increase the strength, the C content is preferably 0.090% or more, and is more
preferably 0.100% or more.
20 [0043] "Si: 0.30 to 2.50%"
Si is an element required for suppressing the generation of iron-based
carbide in an annealing step to obtain a predetermined amount of retained
austenite. However, if a Si content exceeds 2.50%,..the steel sheet becomes
brittle, and the ductility is deteriorated. From a point of view of the ductility,
25 the Si content is preferably 2.20% or less, and is more preferably 2.00% or
less. On the other hand, if the Si content is less than 0.30%, a large amount
of iron-based carbides is generated in the annealing step, resulting in that a
sufficient amount of retained austenite phase cannot be obtained, and it is not
possible to realize both of the maximum tensile strength of 900 MPa or more
and the shape fixability. In order to increase the shape fixability, a lower
limit value of Si is preferably 0.50% or more, and is more preferably 0.70%
or more.
[0044] "Mn: 1.30 to 3.50%"
Mn is added to the steel sheet of the present invention to increase the
strength of the steel sheet. However, if a Mn content exceeds 3.50%, a
coarse Mn concentrated portion is generated at a center portion in the sheet
thickness of the steel sheet, embrittlement occurs easily, and a trouble such as
breaking of a cast slab occurs easily. Further, if the Mn content exceeds
3.50%, the weldability is also deteriorated. Therefore, it is required to set
the Mn content to 3.50% or less. From a point of view of the weldability,
the Mn content is preferably 3.20% or less, and is more preferably 3.00% or
less. On the other hand, if the Mn content is less than 1.30%, a large amount
of soft structures is formed during cooling after annealing, which makes it
difficult to secure the maximum tensile strength of 900 MPa or more, so that
it is required to set the Mn content to 1.30% or more. In order to increase
the strength, the Mn content is preferably 1.50% or more, and is more
preferably 1.70% or more.
[0045] "P: 0.001 to 0.030%"
P tends to be segregated at the center portion in the sheet thickness of
the steel sheet, and embrittles a weld zone. If a P content exceeds 0.030%,
significant embrittlement of the weld zone occurs, so that the P content is
Jimited to 0.030% or less. Although the effect of the present invention is
exhibited without particularly determining a lower limit of the P content,
0.001% is set as a lower limit value since manufacturing costs greatly
increase when the P content is set to less than 0.001 %.
[0046] "S: 0.0001 to 0.0100%"
S exerts an adverse effect on the weldability and manufacturability
during casting and hot rolling. For this reason, an upper limit value of S
content is set to 0.0100% or less. Further, S couples with Mn to form coarse
MnS and lowers the ductility and the stretch flangeability, so that the S
content is preferably set to 0.0050% or less, and is more preferably set to
0.0025% or less. Although the effect of the present invention is exhibited
without particularly determining a lower limit of the S content, 0.0001% is set
as a lower limit value since manufacturing costs greatly increase when the S
content is set to less than 0.0001%.
[0047] "Al: 0.080% to 1.500%"
Al is an element which suppresses the generation of iron-based
carbide to make it easy to obtain the retained austenite phase. Further, by
adding an appropriate amount of Al, it is possible to increase an amount of
solid-solution Si in the retained austenite phase to increase the shape fixability.
However, if an Al content exceeds 1.500%, the weldability worsens, so that
an upper limit of the Al content is set to 1.500%. From this point of view,
the Al content is preferably set to 1.200% or less, and is more preferably set to
0.900% or less. On the other hand, if the Al content is less than 0.080%, the
effect of increasing the amount of solid-solution Si in the retained austenite
phase is insufficient, and it is not possible to secure sufficient shape fixability.
When Al is increased, Si is easily concentrated in the retained austenite phase,
,rSQ. that .the Al content-is preferably 0.100% or more, and is, more .preferably
0.150% or more.
[0048] "N: 0.0001 to 0.0100%"
N forms a coarse nitride and-deteriorates the ductility and the stretch
flangeability, so that an added amount thereof is required to be suppressed.
If an N content exceeds 0.0100%, this tendency becomes evident, so that a
range of the N content is set to 0.0100% or less. Further, since N causes a
generation of blowhole during welding, the content of N is preferably small.
Although the effect of the present invention is exhibited without particularly
determining a lower limit of the N content, 0.0001% is set as a lower limit
value since manufacturing costs greatly increase when the N content is set to
less than 0.0001%.
[0049] "O: 0.0001 to 0.0100%"
O forms an oxide and deteriorates the ductility and the stretch
flangeability, so that an added amount thereof is required to be suppressed.
If an O content exceeds 0.0100%, the deterioration of stretch flangeability
becomes noticeable, so that an upper limit of the O content is set to 0.0100%
or less. The O content is preferably 0.0080% or less, and is more preferably
0.0060% or less. Although the effect of the present invention is exhibited
without particularly determining a lower limit of the O content, 0.0001% is
set as the lower limit since manufacturing costs greatly increase when the O
content is set to less than 0.0001 %.
[0050] The high-strength steel sheet of the present invention may further
contain the following elements according to need.
"Ti: 0.005 to 0.150%"
Ti is an element which contributes to strength increase of the steel
sheet by. precipitate strengthening, fine grain strengthening by growth.
suppression of ferrite crystal grains, and dislocation strengthening through
suppression of recrystallization. However, if a Ti content exceeds 0.150%,
precipitation of carbonitride increases, and the formability is deteriorated, so
that the Ti content is preferably 0.150% or less. From a point of view of the
formability, the Ti content is more preferably 0.100% or less, and is still more
preferably 0.070%) or less. Although the effect of the present invention is
exhibited without particularly determining a lower limit of the Ti content, in
order to sufficiently obtain the effect of increasing the strength provided by Ti,
the Ti content is preferably 0.005% or more. In order to increase the
strength of the steel sheet, the Ti content is more preferably 0.010%) or more,
and is still more preferably 0.015% or more.
[0051] "Nb: 0.005 to 0.150%"
Nb is an element which contributes to strength increase of the steel
sheet by precipitate strengthening, fine grain strengthening by growth
suppression of ferrite crystal grains, and dislocation strengthening through
suppression of recrystallization. However, if a Nb content exceeds 0.150%,
precipitation of carbonitride increases, and the formability is deteriorated, so
that the Nb content is preferably 0.150%) or less. From a point of view of the
formability, the Nb content is more preferably 0.100% or less, and is still
more preferably 0.060% or less. Although the effect of the present invention
is exhibited without particularly determining a lower limit of the Nb content,
in order to sufficiently obtain the effect of increasing the strength provided by
Nb, the Nb content is preferably 0.005%) or more. In order to increase the
strength of the steel sheet, the Nb content is more preferably 0.010% or more,
and is still more preferably 0.015%o or more.
[0052] ...... "V:Q.005,to,Q,150%"^ ..
V is an element which contributes to strength increase of the steel
sheet by precipitate strengthening, fine grain strengthening by growth
suppression of ferrite crystal grains, and dislocation strengthening through
suppression of recrystallization. However, if a V content exceeds 0.150%,
precipitation of carbonitride increases, and the formability is deteriorated, so
that the V content is preferably 0.150%) or less. Although the effect of the
present invention is exhibited without particularly determining a lower limit
of the V content, in order to sufficiently obtain the effect of increasing the
strength provided by V, the V content is preferably 0.005% or more.
[0053] "B: 0.0001 to 0.0100%"
B is an element effective for increasing strength, and may be added
instead of a part of C and/or Mn. If a B content exceeds 0.0100%, the
workability during hot working is impaired and the productivity is lowered,
so that the B content is preferably 0.0100% or less. From a point of view of
the productivity, the B content is more preferably 0.0050% or less, and is still
more preferably 0.0030% or less. Although the effect of the present
invention is exhibited without particularly determining a lower limit of the B
content, in order to sufficiently increase the strength with the use of B, the B
content is preferably set to 0.0001%) or more. To increase strength, the B
content is more preferably 0.0003% or more, and is still more preferably
0.0005% or more.
[0054] "Mo: 0.01 to 1.00%"
Mo is an element effective for increasing strength, and may be added
instead of a part of C and/or Mn. If a Mo content exceeds 1.00%, the
workability during hot working is impaired and the productivity is lowered,
so that the Mo content is preferably 1.00% or less. Although the effect of
the present invention is exhibited without particularly determining a lower
limit of the Mo content, in order to sufficiently increase the strength with the
use of Mo, the Mo content is preferably 0.01% or more.
[0055] "W: 0.01 to 1.00%"
W is an element effective for increasing strength, and may be added
instead of a part of C and/or Mn. If a W content exceeds 1.00%, the
workability during hot working is impaired and the productivity is lowered,
so that the W content is preferably 1.00% or less. Although the effect of the
present invention is exhibited without particularly determining a lower limit
of the W content, in order to sufficiently increase the strength with the use of
W, the W content is preferably 0.01% or more.
[0056] "Cr: 0.01 to 2.00%"
Cr is an element effective for increasing strength, and may be added
instead of a part of C and/or Mn. If a Cr content exceeds 2.00%, the
workability during hot working is impaired and the productivity is lowered,
so that the Cr content is preferably 2.00% or less. Although the effect of the
present invention is exhibited without particularly determining a lower limit
of the Cr content, in order to sufficiently increase the strength with the use of
Cr, the Cr content is preferably 0.01% or more.
[0057] "Ni: 0.01 to 2.00%"
Ni is an element effective for increasing strength, and may be added
instead of a part of C and/or Mn. If a Ni content exceeds 2.00%, the
weldability is impaired, so that the Ni content is preferably 2.00% or less.
Although the effect of the present invention is exhibited without particularly
determining a lower limit of the Ni content, in order to sufficiently increase
the strength with the use of Ni, the Ni content is preferably 0.01% or more.
[0058] "Cu: 0.01 to 2.00%"
Cu is an element that exists in the steel as a fine particle to increase
the strength, and may be added instead of a part of C and/or Mn. If a Cu
:ontent exceeds 2.00%, the weldability is impaired, so that the Cu content is
>referably 2.00% or less. Although the effect of the present invention is
exhibited without particularly determining a lower limit of the Cu content, in
)rder to sufficiently increase the strength with the use of Cu, the Cu content is
3referably 0.01% or more.
;0059] "One or two or more of Ca, Ce, Mg, Zr, Hf, and REM of 0.0001 to
3.5000% in total"
Ca, Ce, Mg, and REM are elements effective for improving the
formability, and one or two or more of them can be added. However, if a
total content of one or two or more of Ca, Ce, Mg and REM exceeds 0.5000%,
the ductility may be impaired, on the contrary, so that a total content of the
respective elements is preferably 0.5000% or less. Although the effect of the
present invention is exhibited without particularly determining a lower limit
of the content of one or two or more of Ca, Ce, Mg and REM, in order to
sufficiently achieve the effect of improving the formability of the steel sheet,
the total content of the respective elements is preferably 0.0001%) or more.
From a point of view of the formability, me total content of one or two or
more of Ca, Ce, Mg and REM is more preferably 0.0005%) or more, and is
still more preferably 0.0010% or more.
Note that REM stands for Rare Earth Metal, and represents an element
belonging to lanthanoid series. In the present invention, REM and Ce are
often added in misch metal, and there is a case in which elements in the
lanthanoid series are contained in a complex form, in addition to La and Ce.
Even if these elements in the lanthanoid series other than La and Ce are
contained as inevitable impurities, the effect of the present invention is
exhibited. Further, the effect of the present invention is exhibited even if
metal La and Ce are added.
0060] Further, the high-strength steel sheet of the present invention may
De configured as a high-strength galvanized steel sheet by forming a
galvanized layer or an alloyed galvanized layer on a surface thereof. By
[brming the galvanized layer on the surface of the high-strength steel sheet,
;he high-strength steel sheet becomes one with excellent corrosion resistance.
Further, by forming the alloyed galvanized layer on the surface of the
ligh-strength steel sheet, the high-strength steel sheet becomes one with
excellent corrosion resistance and with excellent adhesiveness of coating.
Further, the galvanized layer or the alloyed galvanized layer may contain Al
as an impurity.
[0061] The alloyed galvanized layer may contain one or two or more of
Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, Sr, I, Cs,iand REM,
or one or two or more of the elements may be mixed in the alloyed galvanized
layer. Even if the alloyed galvanized layer contains one or two or more of
the above-described elements, or one or two or more of the elements is (are)
mixed in the alloyed galvanized layer, the effect of the present invention is not
impaired, and there is sometimes a preferable case where the corrosion
resistance and the workability are improved depending on the content of the
element. -
[0062] A coating weight of the galvanized layer or the alloyed galvanized
layer is not particularly limited, but, it is desirably 20 g/m or more from a
point of.-view of the corrosion resistance, .and is desirably.!50 g/m" m- less
from a point of view of economical efficiency. Further, an average thickness
of the galvanized layer or the alloyed galvanized layer is set to not less than
1.0 jim nor more than 50 um. If the average thickness is less than 1.0 urn, it
is not possible to achieve sufficient corrosion resistance. The average
thickness is preferably set to 2.0 \xm or more. On the other hand, to set the
average thickness to more than 50.0 |um is not economical, and impairs the
strength of the steel sheet, and thus it is not preferable. From a point of view
of raw material cost, the thinner the thickness of the galvanized layer or the
alloyed galvanized layer is, the. more favorable it is, and thus the thickness is
preferably 30.0 urn or less.
Regarding the average thickness of the plating layer, a thicknesswise
cross section parallel to the rolling direction of the steel sheet is finished to be
a mirror surface and observed with the FE-SEM, thicknesses of plating layer
are measured at five points on a front surface and at five points on a rear
surface, namely, 10 points in total of the steel sheet, and an average value of
the thicknesses is set as the thickness of plating layer.
[0063] Note that when performing alloying treatment, a content of iron in
the alloyed galvanized layer is set to 8.0% or more, and is preferably 9.0% or
more for securing good flaking resistance. Further, the content of iron in the
alloyed galvanized layer is set to 12.0% or less, and is preferably 11.0% or
less for securing good powdering resistance.
[0064] Further, in the high-strength steel sheet of the present invention, a
coating film made of a composite oxide containing a phosphorus oxide and/or
phosphorus may be formed on a surface of the galvanized layer.
Accordingly, the coating film can be functioned as a lubricant when
.performing processing on the steel sheet, resulting in that the galvanized layer
formed on the surface of the steel sheet can be protected.
[0065]
Next, a manufacturing method of the high-strength steel sheet of the
present embodiment will be described.
The manufacturing method of the high-strength steel sheet of the
present embodiment includes: a hot-rolling step being a step of heating a slab
containing the aforementioned chemical components to 1100°C or more,
performing hot rolling on the slab in a temperature region in which a higher
temperature between 850°C and an Ar3 temperature is set to a lower limit
temperature, performing first cooling of performing cooling in a range from a
completion of rolling to a start of coiling at a rate of 10°C/second or more on
average, performing coiling in a range of coiling temperature of 600 to 750°C,
and performing second cooling of cooling the coiled steel sheet in a range of
the coiling temperature to (the coiling temperature - 100)°C at a rate of
15°C/hour or less on average; and a continuous annealing step of performing
annealing on the steel sheet at a maximum heating temperature (Aci + 40)°C
to 1000°C after the second cooling, next performing third cooling at an
average cooling rate of 1.0 to 10.0°C/second in a range of the maximum
heating temperature to 700°C, next performing fourth cooling at an average
cooling rate of 5.0 to 200.0°C/second in a range of 700°C to 500°C, and next
performing retention process of retaining the steel sheet after being subjected
to the fourth cooling for 30 to 1000 seconds in a range of 350 to 450°C.
Hereinafter, reasons for limiting the above-described manufacturing
conditions will be described.
[0066] In order to manufacture the high-strength steel sheet of the present
embodiment*-,a slab containing the above-described chemical components
(composition) is firstly casted.
As the slab subjected to hot rolling, it is possible to employ a
continuously cast slab or a slab manufactured by a thin slab caster or the like.
The manufacturing method of the high-strength steel sheet of the present
invention is compatible with a process like continuous casting-direct rolling
(CC-DR) in which hot rolling is performed right after the casting.
[0067] (Hot-rolling step)
In the hot-rolling step, a slab heating temperature is required to be set
to 1100°C or more. If the slab heating temperature is excessively low, a
finish rolling temperature is below an Ar3 temperature, two-phase rolling of
ferrite and austenite is performed, a hot-rolled sheet structure becomes a
heterogeneous duplex grain structure, and the heterogeneous structure
remains even after being subjected to cold rolling and annealing steps,
resulting in that the ductility and the bendability are deteriorated. Further,
the lowering of the finish rolling temperature causes an excessive increase in
rolling load, and there is a concern that the rolling may become difficult to be
performed or a shape of the rolled steel sheet may be defective, so that the
slab heating temperature is required to be set to 1100°C or more. Although
the effect of the present invention is exhibited without particularly
determining an upper limit of the slab heating temperature, it is desirable to
set the upper limit of the slab heating temperature to 1350°C or less since it is
not economically preferable to set the heating temperature to an excessively
high temperature.
[0068] Note that the Ar3 temperature is calculated based on the following
equation.
Ar3 = 901—325 x.C + 3 3 * -Si- 92 x (Mn + Ni / 2 + Cr / 2 + Cu / 2-fc
In the above equation, C, Si, Mn, Ni, Cr, Cu, Mo, and Al represent
contents [mass%]- of respective elements. An element which is not
contained is calculated as 0.
[0069] A lower limit of the finish rolling temperature being a completion
temperature of hot rolling is set to a higher temperature between 850°C and
the Ar3 temperature. If the finish rolling temperature is less than 850°C, the
rolling load during the finish rolling increases, and there is a concern that the
hot rolling may becorhe difficult to be performed or the shape of the
hot-rolled steel sheet obtained after the hot rolling may be defective. Further,
if the finish rolling temperature is less than the An temperature, the hot
rolling becomes two-phase rolling of ferrite and austenite, and the structure of
the hot-rolled steel sheet sometimes becomes a heterogeneous duplex grain
structure.
On the other hand, although the effect of the present invention is
exhibited without particularly determining an upper limit of the finish rolling
temperature, when the finish rolling temperature is set to an excessively high
temperature, it is necessary to set the slab heating temperature to an
excessively high temperature for securing the finish rolling temperature. Fo*
this reason, it is desirable to set the upper limit temperature of the finish
rolling temperature to 1000°C or less.
[0070] Next, first cooling of performing cooling in a range from the
completion of rolling to the start of coiling at a rate of 10°C/second or more
on average is conducted, and coiling is performed in a range of coiling
temperatui'e of 600 to 750°C. Further, second cooling of cooling the coiled
steel sheet in a range of the coiling temperature to (the coiling temperature -
100)°C at a rate of 15°C/hour or less on average is conducted.
The reason why the coiling condition after the hot rolling and the
cooling conditions before and after the coiling are defined as above will be
described in detail.
[0071] In the present embodiment, the coiling step after the hot rolling
and the first and second cooling steps before and after the coiling step are
very important steps for distributing Si, Mn and Al.
In the present embodiment, in order to control the distributions of Si,
Mn, and Al concentrations in the base iron at 1/8 thickness to 3/8 thickness of
the steel sheet, it is required that the volume fraction of austenite is 50% or
more at 1/8 thickness to 3/8 thickness after the steel sheet is coiled. If the
volume fraction of austenite at 1/8 thickness to 3/8 thickness is less than 50%,
austenite disappears right after the coiling due to a progress of phase
transformation, so that the distributions of Si and Mn do not sufficiently
proceed, resulting in that the concentration distributions of solid-solution
elements of the steel sheet according to the present embodiment as described
above cannot be obtained. In order to effectively facilitate the distribution of
Mn, the volume fraction of austenite is preferably 70% or more, and is more
preferably 80% or more. On the other hand, even if the volume fraction of
austenite is 100%, the phase transformation proceeds after the coiling, ferrite
is generated, and the distribution of Mn is started, so that an upper limit of the
volume fraction of austenite is not particularly provided.
[0072] As described above, in order.to increase the austenite fraction
when coiling the steel sheet, it is required to set the cooling rate in the first
cooling in the temperature range from the completion of hot rolling to the
coiling to 10°C/second or more on average. If the average cooling rate in
the first cooling is less than 10°C/second, ferrite transformation proceeds
during the cooling, and there is a possibility that the volume fraction of
austenite during the coiling becomes less than 50%. In order to increase the
volume fraction of austenite, the cooling rate is preferably 13°C/second or
more, and is more preferably 15°C/second or more. Although the effect of
the present invention is exhibited without particularly determining an upper
limit of the cooling rate, the cooling rate is preferably set to 200°C/second or
less, since a special facility is required to obtain the cooling rate of more than
200°C/second, and manufacturing costs significantly increase.
[0073] If the steel sheet is coiled at a temperature exceeding 800°C after
the first cooling, a thickness of oxide formed on the surface of the steel sheet
excessively increases, and picklability is deteriorated, so that the coiling
temperature is set to 750°C or less. In order to increase the picklability, the
coiling temperature is preferably 720°C or less, and is more preferably 700°C
or less. On the other hand, if the coiling temperature is less than 600°C, a
distribution of alloying element is insufficient, so that the coiling temperature
is set to 600°C or more. Further, in order to increase the austenite fraction
after the coiling, the coiling temperature is preferably set to 615°C or more,
and is more preferably set to 630°C or more.
[0074] Note that since it is difficult to directly measure the volume
fraction of austenite during the manufacture, for determining the volume
fraction of austenite at the time of the coiling in the present invention, a small
piece is cut out from the slab before the hot rolling, the small piece is rolled or
compressed at a temperature and a reduction ratio same as those in the finish
rolling (final pass) of the hot rolling, the resultant is cooled with water right
after being cooled at a cooling, rate same as that during a period of time from
the completion of hot rolling to the completion of coiling, phase fractions of
the small piece are then measured, and a sum of the volume fractions of
as-quenched martensite, tempered martensite and retained austenite phase is
set as a volume fraction of austenite during the coiling.
[0075] The second cooling being the cooling step for the coiled steel
sheet is an important step for controlling the distributions of Si, Mn and Al
elements.
In the present embodiment, the conditions of the first cooling
described above are controlled to set the austenite fraction during the coiling
to 50% or more, and then slow cooling is conducted in a range of the coiling
temperature to (the coiling temperature - 100)°C at a rate of 15°C/hour or less.
By conducting the slow cooling after the coiling as described above, the steel
sheet structure can be set to have a two-phase structure of ferrite and austenite,
and further, it is possible to obtain the distributions of Si, Mn and Al of the
present invention.
Since the distribution of Mn after the coiling is more likely to proceed
as the temperature becomes higher, it is required to set the cooling rate of the
steel sheet to 15°C/hour or less particularly in a range of the coiling
temperature to (the coiling temperature - 100°C).
Further, in order to make the distribution of Mn from ferrite to
austenite proceed to obtain the Mn distribution as described above, it is
required to create a state where two phases of ferrite and austenite coexist,
and to retain this state for a long period of time. If the cooling rate from the
coiling temperature to (the coiling temperature - 100)°C exceeds 15°C/hour,
the phase transformation excessively proceeds, and austenite in the steel sheet
may, disappear, so that the cooling rate from the coiling temperature to (the
coiling temperature - 100)°C is set to 15°C/hour or less. In order to make
the distribution of Mn from ferrite to austenite proceed, the cooling rate from
the coiling temperature to (the coiling temperature — 100)°C is preferably set
to 14°C/hour or less, and is more preferably set to 13°C/hour or less.
Although the effect of the present invention is exhibited without particularly
determining a lower limit of the cooling rate, it is preferable to set the lower
limit to l°C/hour or more, since it becomes required to perform heat retaining
for a long period of time to set the cooling rate to less than l°C/hour, and
manufacturing costs significantly increase.
Further, there is no problem if the steel sheet is reheated after the
coiling within a range of satisfying the average cooling rate of the second
cooling.
[0076] Pickling is performed on the hot-rolled steel sheet manufactured
as above. An oxide on the surface of the steel sheet can be removed by the
pickling, so that the pickling is important to improve a conversion property of
the cold-rolled high-strength steel sheet as a final product and a hot-dip
platability of the cold-rolled steel sheet for a hot-dip galvanized steel sheet or
an alloyed hot-dip galvanized steel sheet. Further, the pickling may be
performed once or a plurality of times separately.
[0077] It is also possible to perform cold rolling on the hot-rolled steel
sheet after being subjected to the pickling, for the purpose of sheet thickness
adjustment and shape correction. When performing the cold rolling, a
reduction ratio is set to fall within a range of 30 to 75%. If. the-reduction
ratio is less than 30%, it is difficult to keep the shape flat, and the ductility of
the final product becomes very poor, so that the reduction ratio is set to 30%
.-...,w, ,- ,.or,more. In order to simultaneously increase the strength andithe ductility, it
is effective to recrystallize ferrite during temperature increase, and to reduce
grain diameters. From this point of view, the reduction ratio is preferably
40% or more, and is more preferably 45% or more.
On the other hand, in the cold rolling in which the reduction ratio
exceeds 75%, a cold-rolling load is increased too much, ^resulting in that it
becomes difficult to perform the cold rolling. For this reason, an upper limit
of the reduction ratio is set to 75%. From a point of view of the cold-rolling
load, the reduction ratio is preferably 70% or less.
[0078] (Continuous annealing step)
Next, the steel sheet is made to pass through a continuous annealing
line to perform a continuous annealing step, thereby manufacturing the
high-strength cold-rolled steel sheet.
First, annealing is performed by setting that a maximum heating
temperature is from (Act + 40)°C to 1000°C. Such a temperature range is a
range in which two phases of ferrite and austenite coexist, and it is possible to
further facilitate the distributions of Si, Mn, and Al as described above.
If the maximum heating temperature is less than (Aci + 40)°C, a large
number of coarse iron-based carbides is remained in the steel sheet in an
insoluble state, and the formability is significantly deteriorated, so that the
maximum heating temperature is set to (Aci + 40)°C or more. From a point
of view of the formability, the maximum heating temperature is preferably set
to (Ac) + 50)°C or more, and is more preferably set to (Acj + 60)°C or more.
On the other, hand, if the maximum heating temperature exceeds 1000°C, a
diffusion of atom is facilitated, and the distributions of Si, Mn, and Al are
reduced, so that the maximum heating temperature is set to 1000°C or less.
Inorder to control the amounts of Si, Mn, and Al in the retained, austenite
phase, the maximum heating temperature is preferably the Ac3 temperature or
less.
[0079] Next, there is performed third cooling of cooling the steel sheet
from the above-described maximum heating temperature to 700°C. In the
third cooling, if an average cooling rate exceeds 10.0°C/second, a ferrite
fraction in the steel sheet becomes easily non-uniform, and the formability is
deteriorated, so that an upper limit of the average cooling rate is set to
10.0°C/second. On the other hand, if the average cooling rate is less than
1.0°C/second, a large amount of ferrite and pearlite is generated, and it is not
possible to obtain the retained austenite phase, so that a lower limit of the
average cooling rate is set to 1.0°C/second. In order to obtain the retained
austenite phase, the average cooling rate is preferably set to 2.0°C/second or
more, and is more preferably set to 3.0°C/second or more.
[0080] After the third cooling, there is further performed fourth cooling
of cooling the steel sheet from 700°C to 500°C. In the fourth cooling, if an
average cooling rate becomes less than 5.0°C/second, a large amount of
pearlite and/or iron-based carbide is generated, and the retained austenite
phase is not remained, so that a lower limit of the average cooling rate is set
to 5.0°C/second or more. From this point of view, the average cooling rate
is preferably 7.0°C/second or more, and is more preferably 8.0°C/second or
more. On the other hand, although the effect of the present invention is
exhibited without particularly determining an upper limit of the average
cooling rate, the upper limit of the average cooling rate is set to
200.0°C/second from a point of view of the cost, since a special facility is
required to obtain the average cooling rate of more than 200°C/second.
..„ ... Note that a cooling stop temperature in the fourth coolingis preferably.
set to (Ms - 20)°C or more. This is because, if the cooling stop temperature
is largely below an Ms point, non-transformed austenite is transformed into
martensite, -and it is not possible to sufficiently obtain retained austenite in
which Si is concentrated. From this point of view, the cooling stop
temperature is more preferably set to the Ms point or more.
The Ms point is calculated based on the following equation.
Ms point [°C] = 541 - 474C / (1 - VF) - 15Si - 35Mn - 17Cr - 17Ni
+ 19A1
In the above equation, VF represents a volume fraction of ferrite, and
C, Si, Mn, Cr, Ni, and Al represent added amounts [mass%] of the respective
elements. Note that since it is difficult to directly measure the volume
fraction of ferrite during the manufacture, for determining the Ms point in the
present invention, a small piece of the cold-rolled steel sheet before the steel
sheet is made to pass through the continuous annealing line is cut out and
annealed based on a temperature histoiy same as that when the small piece is
made to pass through the continuous annealing line, a change in the volume
of ferrite in the small piece is measured, and a numerical value calculated
using the result of the measurement is set as the volume fraction VF of ferrite.
[0081] Further, in order to make the bainite transformation proceed to
obtain the retained austenite phase, there is performed retention process in
which the steel sheet is retained in a range of 350 to 450°C for 30 to 1000
seconds after the fourth cooling. If a retention time is short, the bainite
transformation does not proceed, resulting in that the concentration of C in the
retained austenite phase becomes insufficient, and a sufficient amount of
retained austenite cannot be remained. From this point of view, a lower limit
of the retention timeis, setlo 30 seconds. The retention time is preferably40
seconds or more, and is more preferably 60 seconds or more. On the other
hand, if the retention time is excessively long, iron-based carbide is generated,
C is consumed as the iron-based carbide, and a sufficient amount of retained
austenite phase cannot be obtained, so that the retention time is set to 1000
seconds or less. From this point of view, the retention time is preferably 800
seconds or less, and is more preferably 600 seconds or less.
Further, in order to set tempered martensite to less than 10%, the
average cooling rate in the fourth cooling is preferably set to 10 to 190°C/s as
the manufacturing method. Further, in the retention process after the fourth
cooling, the retention time is preferably set to 50 to 600 seconds.
Note that by performing cooling without conducting reheating of over
600°C as in the present application, the concentration of Si concentrated in
the retained austenite phase can be maintained as it is. If the temperature
exceeds 600°C, a speed of the diffusion of alloying element becomes very fast,
and a redistribution of Si is caused between retained austenite and a
microstructure in the periphery of retained austenite, resulting in that the Si
concentration in austenite is lowered.
[0082] Further, in the present invention, it is also possible to form a
high-strength galvanized steel sheet by performing electrogalvanization, after
the above-described retention process, on the high-strength steel sheet
obtained by making the steel sheet pass through the continuous annealing line
through the aforementioned method.
[0083] Further, in the presentinvention, it is also possible to manufacture
a high-strength galvanized steel sheet through the following method by using
the high-strength steel sheet obtained by the above-described method.
Specifically, the .high-strength galvanized steel- sheet can be
manufactured in a similar manner to the case where the above-described
hot-rolled steel sheet or cold-rolled steel sheet is made to pass through the
continuous annealing line except that the obtained high-strength steel sheet is
dipped in a galvanizing bath between the fourth cooling and the retention
process or after the retention process.
Accordingly, it is possible to obtain a high-strength galvanized steel
sheet having a galvanized layer formed on a surface thereof, and having high
ductility and high stretch flangeability.
[0084] Further, it is also possible to perform alloying treatment in which
the steel sheet after being dipped in the galvanizing bath is reheated to 460°C
to 600°C and is retained for two seconds or more, to thereby make the plating
layer on the surface to be alloyed.
By performing such alloying treatment, Zn-Fe alloy formed by
alloying the galvanized layer is formed on the surface, resulting in that a
high-strength galvanized steel sheet having the alloyed galvanized layer on a
surface thereof is obtained.
[0085] The galvanizing bath is not particularly limited, and even if one or
two or more of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, Sr, I,
Cs, and REM, is (are) mixed in the galvanizing bath, the effect of the present
invention is not impaired, and there is sometimes a preferable case where the
corrosion resistance and the workability are improved depending on the
content of the element. Further, Al may also be contained in the galvanizing
bath. In this case, it is preferable to set anAl concentration in the bath to not
less than 0.05% nor more than 0.15%.
Further, a temperature in the alloying treatment is preferably 480 to
560°C, and a retention time, in the.alloying treatment is preferably 15 to 60
seconds.
[0086] Further, there is no problem if a coating film made of a composite
oxide containing a phosphorus oxide and/or phosphorus is given 4o-a surface
layer of each of these galvanized steel sheets.
[0087] Note that the present invention is not limited to the
above-described examples.
For example, in the manufacturing method of the high-strength
galvanized steel sheet of the present invention, it is also possible to perform
plating with one kind or a plurality of kinds selected from Ni, Cu, Co, and Fe,
on the steel sheet before being subjected to the annealing, in order to improve
the plating adhesiveness.
[0088] Further, in the present embodiment, there is no problem if temper
rolling is performed on the steel sheet after being subjected to the annealing,
for the purpose of shape correction. However, when a reduction ratio after
the annealing exceeds 10%, work hardening of soft ferrite part is caused and
the ductility is largely deteriorated, so that the reduction ratio is preferably set
to less than 10%.
[0089] With the use of the high-strength steel sheet according to the
present invention as described above, since Mn is concentrated in the retained
austenite phase, it is possible to stabilize the retained austenite phase, and to
increase the tensile strength.
Further, in the high-strength steel sheet according to the present
invention, since„Si is also concentrated in the retained austenite phase, similar ......
to Mn, it is possible to moderately destabilize the retained austenite phase,
easily cause the transformation when applying a strain, and to cause sufficient
- work harderiingin the initial-stage at the time of processing in4he low strain ^
region. As a result of this, it is possible to achieve excellent shape fixability.
On the other hand, in the high strain region, it is possible to increase the
stability of the retained austenite phase, and to make Si contribute to the local
ductility.
[0090] Further, in the high-strength steel sheet according to the present
invention, Al being an element of suppressing the generation of iron-based
carbide is added in an appropriate amount, and processing is performed based
on a predetermined thermal history in the hot-rolling step, resulting in that Si
can be efficiently concentrated in retained austenite. Further, at this time, Al
exhibits the concentration distribution opposite to the concentration
distribution of Si, so that it is possible to create a distribution state where
either Si or Al exists in an amount being an equal amount or more of an
average amount in the entire area of the steel sheet. Accordingly, the
generation of iron-based carbide is suppressed and C can be prevented from
being consumed as carbide, so that it is possible to stably secure the retained
austenite phase, resulting in that'the shape fixability, the ductility and the
tensile strength can be largely improved.
[0091] Further, in the manufacturing method of the high-strength steel
sheet according to the present invention, by controlling the coiling step after
the hot rolling and the first and second cooling steps before and after the
coiling step, it is possible to secure the sufficient retained austenite phase, and
to distribute Si, Mn and Al in the steel sheet.
EXAMPLES .......
[0092] Hereinafter, the effect of the present invention will be. described
based on examples, but, the present invention is not limited to conditions
employed in the following examples. .
[0093] Slabs containing chemical components (composition) of A to AD
presented in Tables 1 and 2 were cast, hot rolling was performed under
conditions (slab heating temperature, hot-rolling completion temperature)
presented in Tables 3 to 5 right after the casting, cooling was performed under
conditions of average cooling rate in the first cooling from the completion of
hot rolling to the start of coiling presented in Tables 3 to 5, coiling was
performed at coiling temperatures presented in Tables 3 to 5, cooling was
performed under conditions of average cooling rate in the second cooling
after the coiling presented in Table 2, and then pickling was performed.
Note that experimental examples 6, 49, and 87 were left as they were after the
pickling, and the other experimental examples were subjected to cold rolling
at reduction ratios described in Tables 3 to 5, and subjected to annealing under
conditions presented in Tables 6 to 8, to thereby obtain steel sheets of
experimental examples 1 to 93.
Further, after cooling the steel sheets after being subjected to the
annealing to a room temperature, cold rolling at a reduction ratio of 0.15%
was performed in the experimental examples 9 to 28, and cold rolling at a
reduction ratio of 0.55% was performed in the experimental examples 47 to
67.
Thereafter, in each of the experimental examples 15 and 85, a coating
film made of a composite oxide containing P was given to a surface layer of a
galvanized steel sheet.
[0094] Note that Acl5 and A.c3 in Tables 6 to 8 were calculated based on
the following empirical formulas.
Ad [°C] = 723 - 10.7Mn + 19.1Si + 29.1 Al - 16.9Ni + 16.9Cr
[0095] Ac3 [°G] = 9.10.=r 203iC + 44.7SU 30Mn + 200A1 - 20Ni - 1 OCr
[0096] The annealing conditions presented in Tables 6 to 8 include the
maximum heating temperature in the heating step, the average cooling rate in
the third cooling step in which cooling is performed from-the maximum
heating temperature to 700°C, the average cooling rate in the fourth cooling
step in which cooling is performed from 700°C to 500°C, and the retention
time in the retention process in a range of 350°C to 450°C for making the
bainite transformation proceed.
Further, a steel type CR shown in Tables 6 to 8 indicates a cold-rolled
steel sheet obtained by performing the cold rolling after the pickling, a steel
type HR indicates a hot-rolled steel sheet being the steel sheet which is left as
it is after the pickling, a steel type GI indicates a hot-dip galvanized steel
sheet obtained by performing hot-dip galvanizing on a surface of the steel
sheet, a steel type GA indicates an alloyed hot-dip galvanized steel sheet
obtained by performing alloying treatment after performing the hot-dip
galvanizing, and a steel type EG indicates an electrogalvanized steel sheet
obtained by performing electrogalvanization on a surface of the steel sheet.
Note that an alloying temperature when performing the alloying treatment
was set to a temperature presented in Table 3, and a retention time in the
alloying was set to 25 seconds.
Further, when manufacturing the electrogalvanized steel sheet (EG),
alkaline degreasing, water washing, pickling, and water washing were
conducted in order as preprocessing of electroplating, on the steel sheet after
being subjected to the annealing. Thereafter, electrolytic treatment was
performed on the steel sheet after being subjected to the preprocessing using a
liquid circulation type electroplating device with a plating bath containing
zinc sulfate, sodium sulfate* and. sulfuric.acid at a current density of 100
experimental examples 1 to 93, fractions of microstructures when a surface
parallel to and at 1/4 thickness from a sheet surface of the steel sheet was set
as an observation surface. Out of the fractions of microstructures, an amount
of retained austenite phase (retained y) was measured based on X-ray analysis,
ind the fractions of ferrite (F), bainite (B), bainitic ferrite (BF), tempered
nartensite (TM) and fresh martensite (M) being the other microstructures
were obtained by cutting out a thicknesswise cross section parallel to the
•oiling direction, performing nital etching on the cross section polished to be a
nirror surface, and observing the cross section using the FE-SEM (field
amission scanning electron microscope).
64
[0109] Tables 12 to 14 represent analysis results of components in the
obtained steel sheets. Out of the analysis results of components, an amount
of solid-solution carbon (Cy) in the retained austenite phase was determined
based on X-ray analysis.
5 [0110] An amount of solid-solution Mn in the retained austenite phase
was determined in the following manner.
First, a thicknesswise cross section parallel to the rolling direction was
cut out from each of the obtained steel sheets in a range of 1/8 thickness to
3/8 thickness of the steel sheet, an EPMA analysis was performed on the cross
10 section polished to be a mirror surface to create a Mn concentration map, and
an average Mn amount (WM„*) was determined. Further, in the same range,
a distribution of retained austenite phase was mapped using an EBSD
analyzing device provided together with the FE-SEM, the resultant was
overlapped with the Mn concentration map, and only the analysis result of
15 component in the retained austenite phase was extracted, to thereby determine
the amount of solid-solution Mn (WM^) in the retained austenite phase.
[0111] An amount of solid-solution Si in the retained austenite phase was
also determined in a similar manner to that of the amount of solid-solution
Mn.
20 First, the EPMA analysis and analytical research were conducted to
determine a Si concentration map, an average Si amount (WSJ*)> and an
amount of solid-solution Si (Wsir) in retained austenite.
[0112] An amount of solidrsolution Al in the retained austenite phase was
also determined in a similar manner to that of the amount of solid-solution
25 Mn. '
First, the EPMA analysis was conducted to determine an Al
concentration map, and an average Al amount (WAi*)-
Note that "-" representing the amount of solid-solution C, the amount
of solid-solution Mn, and the amount of solid-solution Si in the experimental
examples 89 and 90 indicates that the measurement was impossible to be
performed. This is because the volume fraction of the retained austenite
phase was 0% in both of the experimental examples 89 and 90 as presented in
Tables 9 to 11, and accordingly, it was impossible to measure an amount of
any solid-solution element.
[0113] Next, from the results of EPMA analysis, a sum (F) of normalized
Si amount (WSJ / WSi*) and normalized Al amount (WAi / WAi*) at each
measurement point was determined, a histogram thereof was created, and a
mode value and a kurtosis K were determined.
Results thereof are presented in Tables 12 to 14.
[0117] Next, property evaluation results of the steel sheets of the
experimental examples 1 to 93 are shown in Tables 15 to 17.
Tensile test pieces based on JIS Z 2201 were collected from the steel
sheets of the experimental examples 1 to 93, and a tensile test was conducted
based on JIS Z 2241 to measure a yield strength (YS), a tensile strength (TS),
and a total elongation (EL).
Further, a hole expansion test for evaluating the stretch flangeability
was conducted based on JFST 1001 to determine a hole expansion limit value
(X) being an index of the stretch flangeability.
Further, for evaluating the shape fixability, a 90-degree V bending test
was conducted. A test piece with 35 mm x 100 mm was cut out from each
of the steel sheets of the experimental examples 1 to 92, a shear cut surface
was mechanically polished, and the bending test was conducted while a bend
radius was set to double the sheet thickness of each of the steel sheets, in
which an angle made by the test piece after the forming was measured, and a
return angle from 90° was measured.
Note that the test example having "X" in the test results in Tables 15
to 17 had conditions in which a crack and/or necking were (was) observed on
an edge line of the test piece, and the forming could not be realized.
Note that as a method of evaluating the properties, the example having
the tensile strength of less than 900 MPa, the example having the total
elongation of less than 10%, the example having the hole expansion limit
valucof less than 20%, and the example having the shape fixability of more
than 3.0 degrees, were evaluated as failed.
Note that underlined numerical value and symbol in Tables 1 to 17
indicate a range out of the present invention.
[0121] The experimental examples 6 and 87 are examples of the present
invention in which the hot rolling and the coiling were conducted based on
the conditions according to the present invention and the annealing processing
was performed. Further, the experimental example 49 is an example of the
present invention in which the hot rolling and the coiling were conducted
based on the conditions according to the present invention, the steel sheet was
dipped in a zinc bath during cooling in the annealing step, and the alloying
treatment of plating layer was further conducted. The experimental example
satisfies the manufacturing conditions of the present invention, and shows
excellent shape fixability, ductility, and formability.
Further, the experimental examples 11, 23, 35, 46, 55, 64, 73, and 82
are examples of the present invention in which the respective processings of
the hot rolling, the coiling, the cold rolling and the annealing were conducted
based on the conditions according to the present invention, and the
electroplating processing was then conducted to obtain the high-strength
galvanized steel sheets. These experimental examples satisfy the
manufacturing conditions of the present invention, and show excellent shape
fixability, ductility, and formability.
Further, the experimental examples 7, 19, 31, 43, 52, 61, 70, 79, and
88 are examples of the present invention in which the hot rolling, the coiling
and the cold rolling were conducted based on the conditions according to the
present invention, and the steel sheets were then dipped in a zinc bath in the
middle of the cooling in the annealing step, to thereby obtain the.
high-strength hot-dip galvanized steel sheets. These experimental examples
satisfy the manufacturing conditions of the present invention, and show
excellent shape fixability, ductility, and formability.
Further, the experimental examples 3, 15, 27, 39, 58, 67, 76, and 85
are examples of the present invention in which the hot rolling, the coiling and
the cold rolling were conducted based on the conditions according to the
present invention, the steel sheets were then dipped in a zinc bath in the
middle of the cooling in the annealing step, and the alloying treatment of
plating layer was further conducted, to thereby obtain the high-strength
alloyed hot-dip galvanized steel sheets. These experimental examples
satisfy the manufacturing conditions of the" present invention, and show
excellent shape fixability, ductility, and formability.
Further, the experimental examples 15 and 85 are examples in which a
coating film made of a composite oxide containing P was given to a surface of
the alloyed galvanized layer, and obtain good properties.
[0122] Examples of the present invention other than the above are
examples in which the hot rolling and the coiling were conducted based on
the conditions according to the present invention, the steel sheets were cooled
to 100°C or less, surfaces were subjected to the pickling, the cold rolling was
conducted at described reduction ratios, and then the annealing processing
was conducted. Each of the examples of the present invention shows
excellent shape fixability, ductility, and formability.
[0123] In the experimental example 89, the added amount of C is small,
and it is not possible to obtain bainite, bainitic ferrite, tempered martensite
and fresh martensite being hard microstructures, so that the strength is
inferior.
[0124] In the experimental example 90, the added amount of Si is small,
and the retained austenite phase cannot be obtained, so that the shape
fixability is inferior.
[0125] In the experimental example 91, bainite, bainitic ferrite, tempered
martensite and fresh martensite being hard microstructures cannot be
sufficiently obtained since the added amount of Mn is small, and since the
amount of solid-solution Mn in the retained austenite phase is small, the
strength and the shape fixability are inferior.
[0126] In the experimental example 92, the added amount of Al is small,
so that Si cannot be sufficiently concentrated in the retained austenite phase,
and distributions of Si and Al concentrations are not predetermined
distributions, resulting in that the shape fixability is inferioi.
[0127] The experimental example 4 is an example in which the
completion temperature of the hot rolling is low, and since the microstructure
becomes a heterogeneous one in which the structure stretches in one direction,
the ductility and the shape fixability are inferior.
[0128] The experimental example 8 is an example in which the
temperature at which the steel sheet is coiled into a coil after the hot rolling is
low, and since Mn and Si are not sufficiently concentrated in the retained
austenite phase, the shape fixability is inferior.
[0129] The experimental example 12 is an example in which the cooling
rate after the hot rolling and after the coiling is low, and since Mn and Si are
not sufficiently concentrated in the retained austenite phase, the shape
fixability is inferior.
[0130] The experimental example 16 is an example in which the
maximum heating temperature in the annealing step is high, and since the
volume fraction of soft ferrite is small, the ductility, the stretch flangeability
and the shape fixability are inferior.
On the other hand, the experimental example 20 is an example in
which the maximum heating temperature in the annealing step is low, and
since a large number of coarse iron-based carbide to be a starting point of
destruction is remained in an insoluble state, bainite, bainitic ferrite, tempered
martensite and fresh martensite being hard microstructures and retained
austenite cannot be sufficiently obtained, resulting in that the ductility, the
stretch flangeability and the shape fixability are inferior.
[0131] In the experimental example 24, the average cooling rate in the
third cooling step up to 700°C is low, a large number of coarse iron-based
carbide and ferrite is generated, and bainite, bainitic ferrite, tempered
martensite and fresh martensite being hard microstructures cannot be
sufficiently obtained, resulting in that the strength is inferior.
On the other hand, in the experimental example 28, the average
cooling rate in the third cooling step up to 700°C is high, and the volume
fraction of soft ferrite is small, so that the ductility and the shape fixability are
inferior.
[0132] In the experimental example 32, the cooling rate in the fourth
cooling step from 700°C to 500°C is low, a large number of coarse iron-based
carbide is generated, and bainite, bainitic ferrite, tempered martensite and
fresh martensite being hard microstructures cannot be sufficiently obtained,
resulting in that the strength is inferior.
[0133] In the experimental example 36, since the retention time from
450°C to 350°C is short, C is not sufficiently concentrated in the retained
austenite phase, and the retained austenite phase cannot be, sufficiently
remained, and since a large amount of martensite to be a starting point of
destruction is contained, the ductility, the stretch flangeability and the shape
fixability are inferior.
On the other hand, in the experimental example 40, since the retention
time from 450°C to 350°C is long, the iron-based carbide is generated during
the retention process, and the volume fraction of retained austenite phase is
small, so that the ductility and the shape fixability are inferior.
[0134] The experimental example 93 is an example in which the
maximum heating temperature in the annealing step is high, and the average
cooling rate in the third cooling step after the annealing step is high, and since
the volume fraction of soft ferrite is small, the stretch flangeability is inferior.
The experimental example 94 is an example in which the average
cooling rate in the first cooling from the completion of hot rolling to the start
of coiling is low, and since the ferrite transformation excessively proceeds,
the distributions of Mn, Si, and Al cannot be made to proceed after the coiling,
and the Mn, Si, and Al amounts in the retained austenite phase obtained in the
annealing step are out of the range of the present invention, so that the shape
fixability is inferior.

We claim:
[Claim 1) high-strength steel sheet excellent in shape fixability, comprisi1 g3. SFP 20\1
in mass%,
C: 0.075 to 0.300%;
Si: 0.30 to 2.5%;
Mn: 1.3 to 3.50%;
P: 0.001 to 0.030%;
S: 0.0001 to 0.0100%;
Al: 0.080 to 1.500%;
N: 0.0001 to 0.0100%;
0: 0.0001 to 0.0100%; and
a balance composed of Fe and inevitable impurities, wherein:
a steel sheet structure contains a retained austenite phase of 5 to 20% in volume
fraction in a range of 118 thickness to 318 thickness of the steel sheet;
an amount of solid-solution C contained in the retained austenite phase is 0.80
to 1.00% in mass%;
Wsiy defined as an amount of solid-solution Si contained in the retained
austenite phase is 1.10 times or more Wsi* defined as an average amount of Si in the
range of 118 thickness to 318 thickness of the steel sheet;
WMny defined as an amount of solid-solution Mn contained in the retained
austenite phase is 1.1 0 times or more WM,* defined as an average amount of Mn in the
range of 118 thickness to 318 thickness of the steel sheet; and
when a frequency distribution is measured, by setting a plurality of
measurement regions each having a diameter of 1 pm or less in the range of 118
thickness to 318 thickness of the steel sheet, with respect to a sum of a ratio between Ws,
defined as a measured value of an amount of Si in each of the plurality of measurement
regions and Wsi* being the average amount of Si and a ratio between WA1 defined as a
measured value of an amount of A1 in each of the plurality of measurement regions and
WAI*d efined as an average amount of Al, a mode value of the frequency distribution is
1.95 to 2.05, and a kurtosis is 2.00 or more. 2 3 ?EP 2014
[Claim 21 The high-strength steel sheet excellent in shape fixability according to
claim 1, wherein:
the steel sheet structure further contains a ferrite phase of 10 to 75% in volume
fraction, and either or both of a bainitic ferrite phase and a bainite phase of 10 to 50% in
total; and
a tempered martensite phase is limited to less than 10% in volume fraction, and
a fresh martensite phase is limited to 15% or less in volume fraction.
[Claim 31 The high-strength steel sheet excellent in shape fixability according to
claim 1, further comprising
in mass%,
one or two or more of
Ti: 0.005 to 0.150%,
Nb: 0.005 to 0.150%,
V: 0.005 to 0.150%,
B: 0.0001 to 0.0100%,
Mo: 0.01 to 1.00%,
W: 0.01 to 1.00%,
Cr: 0.01 to 2.00%,
Ni: 0.01 to 2.00%, and
Cu: 0.01 to 2.00%, and 1 or
one or two or more of Ca, Ce, Mg, Zr, Hf, and REM of 0.0001 to 0.5000% in
total.
[Claim 41 A high-strength galvanized steel sheet excellent in shape fixability,
comprising
the high-strength steel sheet according to claim 1 having a galvanized layer
formed on a surface thereof.
[Claim 51 The high-strength galvanized steel sheet excellent in shape fixability
according to claim 4, wherein
a coating film made of a composite oxide containing a phosphorus oxide and/or
phosphorus is formed on a surface of the galvanized layer. 2 9 YEP 2Nr
[Claim 61 A manufacturing method of a high-strength steel sheet excellent in shape
fixability, comprising:
a hot-rolling step being a step of heating a slab containing:
in mass%,
C: 0.075 to 0.300%;
Si: 0.30 to 2.5%;
Mn: 1.3 to 3.50%;
P: 0.001 to 0.030%;
S: 0.0001 to 0.0100%;
Al: 0.080 to 1.500%;
N: 0.0001 to 0.01 00;
0: 0.0001 to 0.0100; and
a balance composed of Fe and inevitable impurities to llOO°C or more,
performing hot rolling on the slab in a temperature region in which a higher temperature
between 850°C and an Ar3 temperature is set to a lower limit temperature, performing
first cooling of performing cooling in a range from a completion of rolling to a start of
coiling at a rate of 10°C/second or more on average, performing coiling in a range of
coiling temperature of 600 to 750°C, and performing second cooling of cooling the
coiled steel sheet in a range of the coiling temperature to (the coiling temperature -
100)"C at a rate of 15"Ckour or less on average; and
a continuous annealing step of performing annealing on the steel sheet at a
maximum heating temperature (Acl + 40)"C to 1000°C after the second cooling, next
performing third cooling at an average cooling rate of 1.0 to 10.O0C/second in a range of
the maximum heating temperature to 700°C, next performing fourth cooling at an
average cooling rate of 5.0 to 200.0°C/second in a range of 700°C to 500°C, and next
performing retention process of retaining the steel sheet after being subjected to the
fourth cooling for 30 to 1000 seconds in a range of 350 to 450°C.
oR4% l~-*:
2 3 SEP 20" 0 1 j 4
[Claim 71 The manufacturing method of the high-strength steel sheet excellent in
shape fixability according to claim 6, wherein the slab comprises, in mass%,
one or two or more of
Ti: 0.005 to 0.150%,
Nb: 0.005 to 0.1 50%,
V: 0.005 to 0.150%, and
B: 0.0001 to 0.0100%,
Mo: 0.01 to 1.00%,
W: 0.01 to 1 .OO%,
Cr: 0.01 to 2.00%,
Ni: 0.01 to 2.00%, and
Cu: 0.01 to 2.00%, and / or
one or two or more of Ca, Ce, Mg, Zr, Hf, and REM of 0.0001 to 0.5000% in
total.
[Claim 81 The manufacturing method of the high-strength steel sheet excellent in
shape fixability according to claim 6, further comprising
a cold-rolling step of performing pickling and then performing cold rolling at a
reduction ratio of 30 to 75%, between the hot-rolling step and the continuous annealing
step.
[Claim 91 The manufacturing method of the high-strength steel sheet excellent in
shape fixability according to claim 6, further comprising
a temper rolling step of performing rolling on the steel sheet at a reduction ratio
of less than lo%, after the continuous annealing step.
[Claim 101 A manufacturing method of a high-strength galvanized steel sheet
excellent in shape fixability, comprising
forming, after performing the retention process when manufacturing the
high-strength steel sheet in the manufacturing method according to claim 6, a
galvanized layer on a surface of the steel sheet by conducting electrogalvanization.
0-1 52';':'"" 4 O R ~ ~ ~ ~ A L 2 q2wii\ K-
[Claim 111 A manufacturing method of a high-strength galvanized steel sheet
excel lent in shape fixability, comprising
forming, between the fourth cooling and the retention process, or after the
retention process when manufacturing the high-strength steel sheet in the manufacturing
method according to claim 6, a galvanized layer on a surface of the steel sheet by
dipping the steel sheet in a galvanizing bath.
[Claim 121 The manufacturing method of the high-strength galvanized steel sheet
excellent in shape fixability according to claim 1 1, wherein
the steel sheet after being dipped in the galvanizing bath is reheated to 460 to
600°C, and retained for two seconds or more to make the galvanized layer to be alloyed.
[Claim 131 The manufacturing method of the high-strength galvanized steel sheet
excellent in shape fixability according to claim 10, wherein
after the galvanized layer is formed, a coating film made of a composite oxide
containing either or both of a phosphorus oxide and phosphorus is given to a surface of
the galvanized layer.
[Claim 141 The manufacturing method of the high-strength galvanized steel sheet
excellent in shape fixability according to claim 12, wherein
after the galvanized layer is alloyed, a coating film made of a composite oxide
containing either or both of a phosphorus oxide and phosphorus is given to a surface of
the alloyed galvanized layer.