DESCRIPTION
Title of Invention: High Strength Steel Sheet and High
Strength Galvanized steel ~h6et ~xcellent ,in Shapeability
and Methods of Production of Same
Technical Field
[OOOl] The present invention relates to high strength
steel sheet and high strength galvanized steel sheet
which are excellent in shapeability and to methods of
production of the same.
Background Art
[0002] In recent years, there have been increasing
demands for higher strength in the steel sheet which is
used for automobiles etc. In particular, for the purpose
of improving collision safety etc., high strength steel
sheet with a tensile maximum stress of 900 MPa or more is
also being used. Such high strength steel sheet is
inexpensively formed in large volumes by press working it
in the same way as soft steel sheet and is being used as
structural members.
[0003] However, in recent years, along with the rapid
increase in strength of high strength steel sheet, in
particular in high strength steel sheet with a tensile
maximum stress of 900 MPa or more, the problem has arisen
of the shapeability becoming insufficient and of working
accompanied with local deformation such as stretchformability
becoming difficult. Further, when a high
speed tension force acts on a steel material, there was
the problem that the fracture mode would easily change
from ductile fracture to brittle fracture.
[OOOS] In the past, as one example of the art for
strengthening a steel material, a high strength steel
material which was hardened by causing the fine
precipitation of Cu was known. PLT 1 discloses a Cu
precipitation hardening type high strength steel material
which contains C, Si, P, S, Al, N, and Cu in
predetermined ranges, contains one or both of Mn: 0.1 to
3.0% and Cr: 0.1 to 3.0%, has an (Mn+Cr)/Cu of 0.2 or
more, and has a balance of iron and unavoidable
impurities, has an average ferrite crystal grain size of
5 3 pm or more, and has a ferrite area ratio of 60% or
more.
[0005] Further, as one example of high strength steel
sheet which achieves both shapeability and hole
expandability, PLT 2 discloses high strength steel sheet
10 which is excellent in shapeability and hole expandability
which contains C, Si, Cu, and Mn in predetermined mass%,
further suitably contains at least one of Al, Ni, Mo, Cr,
V, B, Ti, Nb, Ca, and Mg, and has a hardness of the
ferrite phase of Hv 150 to 240, has a volume ratio of
15 residual austenite in the steel structure of 2 to 20%,
and exhibits a tensile strength of 600 to 800 MPa.
[0006] PLT 3 discloses, as one example of high
strength cold rolled steel sheet for working use which is
excellent in fatigue characteristics, high strength cold
20 rolled steel sheet for working use which is excellent in
fatigue characteristics which is comprised of steel sheet
containing C: 0.05 to 0.30%, Cu: 0.2 to 2.0%, and B: 2 to
20 ppm and which has a microstructure comprised of a
volume ratio of 5% or more and 25% or less of residual
25 austenite and ferrite and bainite and which has Cu
present in the ferrite phase in the state of particles
which are comprised of Cu alone of a size of 2 nm or less
in a solid solution state and/or precipitated state.
[0007] PLT 4 discloses, as one example of composite
30 structure high strength cold rolled steel sheet which is
excellent in fatigue characteristics, composite structure
high strength cold rolled steel sheet which is comprised
of ferrite-martensite composite structure steel sheet
which contains C: 0.03 to 0.20%, Cu: 0.2 to 2.0%, and B:
35 2 to 20 ppm and which has Cu present in the ferrite phase
in the state of particles which are comprised of Cu alone
of a size of 2 nm or less in a solid solution state
and/or precipitated state.
[OOOS] PLT 5 discloses, as one example of super high
strength steel sheet which is excellent in delayed
fracture resistance, super high strength steel sheet
5 which contains, by wt%, C: 0.08 to 0.30, Si: less than
1.0, Mn: 1.5 to 3.0, S: 0.010 or less, P: 0.03 to 0.15,
Cu: 0.10 to 1.00, and Ni: 0.10 to 4.00, has a balance of
iron and unavoidable impurities, contains one or more
structures of martensite, tempered martensite, or bainite
10 structures in a volume ratio of 40% or more, and has a
strength of 1180 MPa or more.
[0009] PLT 6 discloses, as one example of high
strength steel sheet which is excellent in press
formability and corrosion resistance, high strength steel
15 sheet which is excellent in press formability and
corrosion resistance which satisfies the requirements of
C: 0.08 to 0.20%, Si: 0.8 to 2.0%, Mn: 0.7 to 2.5%, P:
0.02 to 0.15%, S: 0.010% or less, Al: 0.01 to 0.10%, Cu:
0.05 to 1.08, and Ni: 1.0% or less, has a balance of iron
20 and unavoidable impurities, and satisfies the
relationship of the following formula
"0.41(10P+Si) / (lOC+Mn+Cu+O. 5Ni)ll. 6" (wherein, the
notations of elements indicate the respective contents
( % ) ) , which steel sheet has residual austenite of 3 to
25 10% and a tensile strength of 610 to 760 MPa.
[OOlO] PLT 7 discloses, as one example of high
strength thin gauge steel sheet, high strength thin gauge
steel sheet which has a composition of ingredients which
contains C: 0.05 to 0.3%, Si: 2% or less, Mn: 0.05 to
30 4.0%, P: 0.1% or less, S: 0.1% or less, Cu: 0.1 to 2%,
and Si(%)/5 or more, Al: 0.1 to 2%, N: 0.01% or less, Ni:
Cu(%)/3 or more (when Cu is 0.5% or less, not necessarily
included) and satisfies "Si(%)+A1(%)20.5",
"Mn(%)+Ni(%)20.5", has a structure which contains a
35 volume ratio of 5% or more of residual austenite, and
exhibits a tensile strength of 650 to 800 MPa.
Citations List
Patent Literature
[OOll] PLT 1: Japanese Patent Publication No. 2004-
100018A
5 PLT 2: Japanese Patent Publication No. 2001-355044A
PLT 3: Japanese Patent Publication No. 11-279690A
PLT 4: Japanese Patent Publication No. 11-199973A
PLT 5: Japanese Patent Publication No. 08-311601A
PLT 6: Japanese Patent Publication No. 08-199288A
10 PLT 7: Japanese Patent Publication No. 05-271857A
Summary of Invention
Technical Problem
[0012] Conventional high strength steel sheet is hot
rolled, pickled, and cold rolled, then is continuously
15 annealed under predetermined conditions to make
predetermined crystal phases precipitate in predetermined
ratios in the steel sheet structure and thereby achieve
both high strength and high workability.
[0013] However, in low alloy steel with low contents
20 of added elements, the phase transformation proceeds
quickly at the time of annealing treatment, so the extent
of the operating range at which predetermined crystal
phases can be made to precipitate at predetermined ratios
becomes narrow and, as a result, the high strength steel
25 sheet does not become stable in properties and varies in
quality.
[0014] Further, conventional tensile strength 900 MPa
or more high strength steel sheet was insufficient in
workability. It was desired to improve the stretch
30 flangeability and otherwise enhance the workability.
[0015] The present invention was made in consideration
of this situation and has as its object the provision of
tensile strength 900 MPa or more high strength steel
sheet where the stretch flangeability is improved to
35 improve the local deformation ability and where the
tensile strength can be improved when high speed tension
acts, and a method of production of the same.
Solution to Problem
[0016] The inventors etc. engaged in intensive studies
on the steel sheet structure and method of production so
as to achieve both improvement of the stretch
flangeability and improvement of the tensile strength
when high speed tension acts in high strength steel
sheet. As a result, they learned that by making Cu
efficiently precipitate in steel sheet, it is possible to
achieve both improvement of the stretch flangeability and
improvement of the tension strength when high speed
tension acts. Further, they discovered that to form such
a structure, it is sufficient to impart strain to the
steel sheet during annealing of the steel sheet.
[0017] The invention was made as a result of further
studies based on the above discovery and has as its gist
the following:
[0018] (1) High strength steel sheet which is
excellent in shapeability which contains, by mass%, C:
0.075 to 0.300%, Si: 0.30 to 2.50%, Mn: 1.30 to 3.50%, P:
0.001 to 0.030%, S: 0.0001 to 0.0100%, Al: 0.005 to
1.500%, Cu: 0.15 to 2.00%, N: 0.0001 to 0.0100%, and 0:
0.0001 to 0.0100%, contains, as optional elements, Ti:
0.005 to 0.150%, Nb: 0.005 to 0.150%, B: 0.0001 to
0.0100%, Cr: 0.01 to 2.00%, Ni: 0.01 to 2.00%, Mo: 0.01
to 1.00%, W: 0.01 to 1.00%, V: 0.005 to 0.150%, and one
or more of Ca, Ce, Mg, and REM: total 0.0001 to 0.50%,
and has a balance of iron and unavoidable impurities,
wherein the steel sheet structure contains a ferrite
phase and martensite phase, a ratio of Cu particles
incoherent with bcc iron is 15% or more with respect to
the Cu particles as a whole, a density of Cu particles in
the ferrite phase is 1.0x10'*/m3 or more, and an average
particle size of Cu particles in the ferrite phase is 2.0
nm or more.
35 [0019] (2) The high strength steel sheet which is
excellent in shapeability of the (1) characterized in
that the structure in a range of 1/8 thickness to 3/8
thickness of the high strength steel sheet comprises, by
volume fraction, a ferrite phase: 10 to 75%, bainitic
ferrite phase and/or bainite phase: 50% or less, tempered
martensite phase: 50% or less, fresh martensite phase:
15% or less, and residual austenite phase: 20% or less.
[0020] (3) High strength galvanized steel sheet which
is excellent in shapeability characterized by comprising
the high strength steel sheet of the (1) or (2) on the
surface of which a galvanized layer is formed.
[0021] (4) A method of production of high strength
steel sheet which is excellent in shapeability
characterized by comprising a hot rolling process of
heating a slab which contains, by mass%, C: 0.075 to
0.300%, Si: 0.30 to 2.50%, Mn: 1.30 to 3.50%, P: 0.001 to
0.030%, S: 0.0001 to 0.0100%, Al: 0.005 to 1.500%, Cu:
0.15 to 2.00%, N: 0.0001 to 0.0100%, 0: 0.0001 to
0.0100%, contains, as optional elements, Ti: 0.005 to
0.150%, Nb: 0.005 to 0.150%, B: 0.0001 to 0.0100%, Cr:
0.01 to 2.00%, Ni: 0.01 to 2.00%, Mo: 0.01 to 1.00%, W:
0.01 to 1.00%, V: 0.005 to 0.150%, and one or more of Ca,
Ce, Mg, and REM: total 0.0001 to 0.50%, and has a balance
of iron and unavoidable impurities, directly, or after
cooling once, to 1050°C or more, rolling with a lower
limit of a temperature of 800°C or the Ar3 transformation
point, whichever is higher, and coiling it at 500 to 700°C
in temperature and an annealing process of heating the
coiled steel sheet by an average heating rate at 550 to
700°C of 1.0 to 10.O°C/sec up to a maximum heating
temperature of 740 to 1000°Cf then cooling by an average
cooling rate from the maximum heating temperature to 700°C
of 1.0 to 10.O0C/sec, imparting strain to the steel sheet
from the maximum heating temperature to 700, and cooling
by a cooling rate from 700°C to the Bs point or 500°C of
5.0 to 200.0°C/sec.
[0022] (5) The method of production of high strength
steel sheet which is excellent in shapeability of the
above (5) characterized by having a cold rolling process,
after the hot rolling process and before the annealing
process, of pickling the coiled steel sheet, then rolling
it by a screwdown rate of a screwdown rate 35 to 75%.
[0023] (6) The method of production of high strength
steel sheet which is excellent in shapeability of the
above (4) or (5) characterized by the strain being
imparted to the steel sheet in the annealing process by
applying 5 to 50 MPa of tension to the steel sheet while
bending one time or more in a range giving an amount of
tensile strain at the outermost circumference of 0.0007
to 0.0910.
[0024] (7) The method of production of high strength
steel sheet which is excellent in shapeability of the
above (6) characterized in that the bending is performed
by pressing the steel sheet against a roll with a roll
diameter of 800 mm or less.
100251 (8) A method of production of high strength
galvanized steel sheet which is excellent in shapeability
characterized by producing high strength steel sheet by
the method of production of high strength steel sheet of
any of the above (4) to (7), then electrogalvanizing it.
[0026] (9) A method of production of high strength
galvanized steel sheet which is excellent in shapeability
characterized by producing high strength steel sheet by
the method of production according to any one of (4) to
(8) after the cooling to the Bs point or 500°C of which
performing hot dip galvanization.
100271 (10) A method of production of high strength
30 galvanized steel sheet which is excellent in shapeability
according to (9) characterized by performing alloying
treatment at 470 to 650°C in temperature after the hot dip
galvanization.
Advantageous Effects of Invention
35 100281 According to the present invention, it is
possible to provide high strength steel sheet which
secures a tensile maximum strength 900 MPa or more high
strength while has excellent stretch flangeability and
other shapeability and also has excellent high strength
tensile properties. Further, it is possible to provide
high strength galvanized steel sheet which secures a
tensile maximum strength 900 MPa or more high strength
while has excellent stretch flangeability and other
shapeability and also has excellent high strength tensile
properties.
Description of Embodiments
[0029] First, the structure of the high strength steel
sheet of the present invention will be explained. The
structure of the high strength steel sheet of the present
invention is not particularly limited so long as a
tensile maximum strength 900 MPa or more strength can be
secured.
[0030] For example, the structure may be any of a
single phase structure of martensite, a dual phase
structure comprised of martensite and bainite, a dual
phase structure comprised of ferrite and martensite, a
composite phase structure comprised of ferrite, bainite,
and residual austenite and other such structures
including ferrite, bainite, martensite, and residual
austenite alone or compositely. Alternatively, it may be
a structure of these structures further including a
pearlite structure.
[0031] The ferrite phase which is included in the
structure of the high strength steel sheet may be any of
precipitation strengthened ferrite, as-worked
nonrecrystallized ferrite, or partial dislocationrestored
ferrite.
[0032] The steel sheet structure of the high strength
steel sheet of the present invention is preferably
comprised of, in the range of 1/8 to 3/8 thickness
centered on 1/4 of the sheet thickness, by volume
fraction, ferrite phase: 10 to 75%, bainitic ferrite
phase and/or bainite phase: 50% or less, tempered
martensite phase: 50% or less, fresh martensite phase:
15% or less, and residual austenite phase: 20% or less.
If the high strength steel sheet has such a steel sheet
structure, high strength steel sheet which has a more
excellent shapeability results.
[ 0 0 3 3 ] Here, the structure is made one in the range of
1/8 to 3/8 thickness because this range of structure may
be considered to represent the structure of the steel
sheet as a whole. If such a steel sheet structure in the
range of 1/8 to 3/8 thickness, it can be judged that the
steel sheet as a whole has such a structure.
[0034] The phases which can be included in the
structure of the steel sheet will be explained.
[0035] Ferrite Phase
The ferrite phase is a structure which is effective for
improving the ductility and is preferably contained in
the steel sheet structure in a volume fraction of 10 to
75%. The volume fraction of the ferrite phase in the
steel sheet structure, from the viewpoint of the
ductility, is more preferably 15% or more, still more
preferably 20% or more. The ferrite phase is a soft
structure, so to sufficiently raise the tensile strength
of steel sheet, the volume fraction of the ferrite phase
which is contained in the steel sheet structure is more
preferably made 65% or less, still more preferably made
50% or less.
[0036] Bainitic Ferrite Phase and/or Bainite Phase
The bainitic ferrite phase and/or bainite phase is a
structure with a good balance of strength and ductility
and is preferably contained in the steel sheet structure
30 in a volume fraction of 10 to 50%. Further, the bainitic
ferrite phase and/or bainite phase is a microstructure
which has a strength intermediate to that of a soft
ferrite phase and hard martensite phase and tempered
martensite phase and residual austenite phase. From the
35 viewpoint of the stretch flangeability, inclusion of 15%
or more is more preferable and inclusion of 20% or more
is further preferable. If the volume fraction of the
bainitic ferrite phase and/or bainite phase rises, the
yield stress becomes higher, so from the viewpoint of the
shape freezability, the volume fraction of the bainitic
ferrite phase and/or bainite phase is preferably 50% or
5 less.
COO371 Tempered Martensite Phase
The tempered martensite phase is a structure which
greatly improves the tensile strength. From the viewpoint
of the tensile strength, the volume fraction of the
10 tempered martensite is preferably 10% or more. If the
volume fraction of the tempered martensite which is
contained in the steel sheet structure increases, the
yield stress becomes higher, so from the viewpoint of the
shape freezability, the volume fraction of tempered
martensite phase is preferably 50% or less.
[ 0 0 3 8 ] Fresh Martensite Phase
The fresh martensite phase greatly improves the tensile
strength. On the other hand, it forms starting points of
fracture and greatly degrades the stretch flangeability,
so it preferably limited to a volume fraction of 15% or
less. To raise the stretch flangeability, it is more
preferable to make the volume fraction of the fresh
martensite phase 10% or less, still more preferably 5% or
less.
[0039] Residual Austenite Phase
The residual austenite phase greatly improves the
strength and ductility. On the other hand, it becomes
starting points of fracture and sometimes causes the
stretch flangeability to deteriorate, so is preferably
made a volume fraction of 20% or less. To raise the
stretch flangeability, the volume fraction of the
residual austenite phase is more preferably made 15% or
less. To obtain the effect of improvement of the strength
and ductility, the volume fraction of the residual
35 austenite phase is preferably 3% or more, more preferably
5% or more.
[OOSO] Others
The steel sheet structure of the high strength steel
sheet of the present invention may further contain a
pearlite phase and/or coarse cementite phase or other
structure. However, if the steel sheet structure of high
strength steel sheet contains a large amount of pearlite
phase and/or coarse cementite phase, the bendability
deteriorates. Therefore, the volume fraction of the
pearlite phase and/or coarse cementite phase which is
contained in the steel sheet structure is preferably a
total of 10% or less, more preferably 5% or less.
[0041] The volume fractions of the different
structures which are contained in the steel sheet
structure of the high strength steel sheet of the present
invention can, for example, be measured by the following
method:
[0042] The volume fraction of the residual austenite
phase is obtained by examining the plane parallel to the
sheet surface of the steel sheet and at 1/4 thickness by
X-ray analysis, calculating the area fraction, and
deeming that value as the volume fraction.
[0043] The volume fractions of the ferrite phase,
bainitic ferrite phase, bainite phase, tempered
martensite phase, and fresh martensite phase which are
contained in the steel sheet structure of the high
strength steel sheet of the present invention are
obtained by obtaining samples with sheet thickness crosssections
parallel to the rolling direction as observed
surfaces, polishing the observed surfaces, etching them
by Nital, then examining the range of 1/8 thickness to
3/8 thickness centered at 1/4 of sheet thickness by using
a field emission scanning electron microscope (FE-SEM) to
measure the area fraction, and deeming that value as the
volume fraction.
[0044] Next, the microstructure of the high strength
steel sheet of the present invention will be explained.
[0045] The microstructure of the high strength steel
sheet of the present invention has to be one where the
density of Cu particles is 21.0x10~*/m~t,h e average
particle size of the Cu particles is 2.0 nm or more, and
the ratio of Cu particles where the Cu particles and
surrounding bcc iron are incoherent in the total Cu
particles is 15% or more.
[0046] The "bcc iron" is the general term for ferrite,
bainite, and bainitic ferrite with crystal structures of
body centered cubic lattices. If the Cu particles are
coherent with the bcc iron, the strength is greatly
improved. Cu particles which are not coherent with the
bcc iron obstruct the development of the dislocation
substructure at the bcc iron. Along with this,
aggregation of dislocations at the time of large strain
deformation becomes difficult, the formation of voids is
suppressed, and as a result the stretch flangability is
improved.
[0047] The density of Cu particles is preferably
5.0x10~*/m'o r more, more preferably 1 . 0 ~ 1 0 ~ ~o/r mm~or e.
[0048] Fine Cu particles easily maintain coherence
20 with the bcc iron and are small in contribution to the
stretch flangeability, so the lower limit of the average
particle size of the Cu particles is made 2.0 nm or more.
The average particle size of the Cu particles is more
preferably 4.0 nm or more, still more preferably 6.0 nm
25 or more.
[0049] If the number of Cu particles which are
incoherent with the bcc iron is less than 15%, the
improvement of the stretch flangeability becomes
insufficient. Therefore, the number of Cu particles has
30 to be 15% or more, preferably is 25% or more, more
preferably is 35% or more.
[OOSO] The average particle size, coherence, and
density of the Cu particles can be evaluated as follows:
[OOSl] A sample is cut out from the steel sheet at 1/4
35 thickness and is examined using a high resolution
transmission electron microscope (HRTEM). Electron
energy-loss spectroscopy (EELS) is used to confirm the
- 13 -
composition of the Cu particles. These are investigated
for particle size and coherence with the bcc iron. The
size of the particles was made the average of the
particle sizes of 20 or more particles. Further, the
ratio of the precipitates which are incoherent with the
bcc iron in the number of particles observed was found.
[0052] The Cu particle density is measured by two
methods in accordance with the average particle size. If
the average particle size is less than 3 nm, a threedimensional
atom probe (3D-AP) is used to cut out and
test samples from 1/4 thickness of the steel sheet. The
test is performed until 20 or more Cu particles are
obtained or until the measured volume exceeds 50000 nm3.
The density is obtained by dividing the number of
particles by the measured volume. On the other hand, if
the average particle size is 3 nm or more, the number of
Cu particles in a 10000 nm2 to 1 pm2 field is measured,
convergent-beam electron diffraction (CBED) is used to
measure the thickness of the observed part of the test
piece, this is multiplied with the observed area to find
the observed volume, and the number of Cu particles is
divided by the observed volume to find the Cu particle
density.
[0053] The means for measuring the composition,
particle size, and coherence of the Cu particles are not
limited to the above techniques. For example, the
particles may be observed using a field-emission
transmission electron microscope (FE-TEM) etc.
[0054] Next, the composition of ingredients of the
high strength steel sheet of the present invention will
be explained. Note that in the following explanation, " % "
shall mean "mass%".
[0055] C: 0.075 to 0.300%
C is included for raising the strength of the high
strength steel sheet. If the content of C exceeds 0.300%,
the weldability becomes insufficient. From the viewpoint
of the weldability, the content of C is preferably 0.250%
or less, more preferably 0.220% or less. If the content
of C is less than 0.075%, the strength falls and a 900
MPa or more tensile maximum strength cannot be secured.
To raise the strength, the content of C is preferably
0.090% or more, more preferably 0.100% or more.
[0056] Si: 0.30 to 2.50%
Si is an element which suppresses the formation of ironbased
carbides in steel sheet and is required for raising
the strength and shapeability. If the content of Si
exceeds 2.50%, the steel sheet becomes brittle and the
ductility deteriorates. From the viewpoint of the
ductility, the content of Si is preferably 2.20% or less,
more preferably 2.00% or less. On the other hand, if the
content of Si is less than 0.30%, a large amount of
coarse iron-based carbides form in the annealing process,
and the strength and shapeability deteriorate. From this
viewpoint, the lower limit of Si is preferably 0.50% or
more, more preferably 0.70% or more.
100571 Mn: 1.30 to 3.50%
20 Mn is added to raise the strength of the steel sheet. If
the content of Mn exceeds 3.50%, coarse Mn concentrated
parts form at the center of thickness of the steel sheet,
embrittlement easily occurs, and trouble such as cracking
of the cast slab easily occurs. Further, if the content
25 of Mn exceeds 3.50%, the weldability also deteriorates.
Therefore, the content of Mn has to be made 3.50% or
less. From the viewpoint of the weldability, the content
of Mn is preferably 3.20% or less, more preferably 3.00%
or less. On the other hand, if the content of Mn is less
30 than 1.30%, soft structures are formed in large amounts
during the cooling after the annealing, so it becomes
difficult to secure a 900 MPa or more tensile maximum
strength. Therefore, the content of Mn has to be made
1.30% or more. To raise the strength, the content of Mn
35 is more preferably 1.50% or more, still more preferably
1.70% or more.
[0058] P: 0.001 to 0.030%
P tends to precipitate at the center of thickness of
steel sheet and causes embrittlement of the weld zone. If
the content of P exceeds 0.030%, the weld zone becomes
greatly brittle, so the content of P is limited to 0.030%
or less. The lower limit of the content of P is not
particularly limited so long as the effect of the present
invention is exhibited. However, if making the content of
P less than 0.001%, the manufacturing costs greatly
increase, so 0.001% is made the lower limit.
[0059] S: 0.0001 to 0.0100%
S has a detrimental effect on the weldability and the
manufacturability at the time of casting and at the time
of hot rolling. Accordingly, the upper limit of the
content of S is made 0.0100% or less. S bonds with Mn to
form coarse MnS which lowers the ductility and stretch
flangeability, so 0.0050% or less is preferable, while
0.0025% or less is more preferable. The lower limit of
the content of S is not particularly limited so long as
the effects of the present invention are exhibited.
20 However, if the content of S is less than 0.0001%, the
manufacturing costs greatly increase, so 0.0001% is made
the lower limit.
[0060] Al: 0.005 to 1.500%
A1 suppresses the formation of iron-based carbides and
25 raises the strength and shapeability of the steel sheet.
If the content of A1 exceeds 1.500%, the weldability
becomes poor, so the upper limit of the content of A1 is
made 1.500%. From the viewpoint of the weldability, the
content of A1 is preferably made 1.200% or less, more
30 preferably 0.900% or less. A1 is an element which is
effective as a deoxidizing material as well, but if the
content of A1 is less than 0.005%, the effect as a
deoxidizing material is not sufficiently obtained, so the
lower limit of the content of A1 is made 0.005% or more.
35 To sufficiently obtain the effect of deoxidation, the
amount of A1 is preferably made 0.010% or more.
[0061] N: 0.0001 to 0.0100%
N forms coarse nitrides which cause the ductility and
stretch flangeability to deteriorate, so has to be kept
down in content. If the content of N exceeds 0.0100%,
this tendency becomes more remarkable, so the content of
N is made 0.0100% or less. Further, N becomes a cause of
formation of blowholes at the time of welding, so the
smaller the content, the better. The lower limit of the
content of N is not particularly set so long as the
effect of the present invention is exhibited. However, if
the content of N is made less than 0.0001%, the
manufacturing costs greatly increase, so 0.0001% is made
the lower limit value.
[0062] 0: 0.0001 to 0.0100%
0 forms oxides which cause the ductility and stretch
flangeability to deteriorate, so has to be kept down in
content. If the content of 0 exceeds 0.0100%, the
deterioration of the stretch flangeability becomes
remarkable, so the content of 0 is made 0.0100% or less.
The content of 0 is preferably 0.0080% or less, more
preferably 0.0060% or less. The lower limit of the
content of 0 is not particularly limited so long as the
effect of the present invention is exhibited. However, if
the content of 0 is less than 0.0001%, the manufacturing
costs greatly increase, so 0.0001% is made the lower
limit.
[0063] Cu: 0.15 to 2.00%
Cu is an important element in the present invention. Cu
is present in the steel as fine particles. The Cu
particles which are coherent or semi-coherent with the
surrounding bcc phase in particular increase the strength
of steel sheet. Cu particles are incoherent with the
surrounding bcc iron in particular suppress the formation
of dislocation substructures inside the steel sheet to
thereby raise the shapeability. In the present invention,
to sufficiently obtain the effect of the Cu particles,
the content of Cu has to be made 0.15% or more. The
content of Cu is preferably 0.30% or more, more
preferably 0.40% or more. On the other hand, if the
content of Cu exceeds 2.00%, the weldability is impaired,
so the content of Cu is made 2.00% or less. From the
viewpoint of the weldability, the content of Cu is
preferably 1.80% or less, more preferably 1.50% or less.
[0064] The high strength steel sheet of the present
invention may further, in accordance with need, contain
the following elements:
[0065] Ni: 0.01 to 2.00%
Ni suppresses embrittlement which occurs due to addition
of Cu in the high temperature region, so may be
deliberately added for the purpose of improving the
productivity. To obtain this effect, the content of Ni
has to be made 0.01% or more. Making it 0.05% or more is
more preferable, while making it 0.10% or more is still
more preferable. If the content of Ni exceeds 2.00%, the
weldability is impaired, so the content of Ni is made
2.00% or less.
[00661 Ti: 0.005 to 0.150%
Ti is an element which contributes to the rise in
strength of the steel sheet through precipitation
strengthening, fine grain strengthening by suppression of
growth of ferrite crystal grains, and dislocation
strengthening through suppression of recrystallization.
If the content of Ti exceeds 0.150%, precipitation of
carbonitrides increases and the shapeability
deteriorates, so the content of Ti is made 0.150% or
less. From the viewpoint of the shapeability, the content
of Ti is more preferably 0.100% or less, still more
preferably 0.070% or less. To sufficiently obtain the
effect of the rise in strength by Ti, the content of Ti
has to be made 0.005% or more. To raise the strength of
the steel sheet, the content of Ti is preferably 0.010%
or more, more preferably 0.015% or more.
[0067] Nb: 0.005 to 0.150%
Nb is an element which contributes to the rise in
strength of the steel sheet through precipitation
strengthening, fine grain strengthening by suppression of
growth of ferrite crystal grains, and dislocation
strengthening through suppression of recrystallization.
If the content of Nb exceeds 0.150%, precipitation of
carbonitrides increases and the shapeability
deteriorates, so the content of Nb is made 0.150% or
less. From the viewpoint of the shapeability, the content
of Nb is more preferably 0.100% or less, still more
preferably 0.060% or less. To sufficiently obtain the
effect of the rise in strength by Nb, the content of Nb
has to be made 0.005% or more. To raise the strength of
the steel sheet, the content of Nb is preferably 0.010%
or more, more preferably 0.015% or more.
[0068] V: 0.005 to 0.150%
V is an element which contributes to the rise in strength
of the steel sheet by precipitation strengthening, fine
grain strengthening by suppression of growth of ferrite
crystal grains, and dislocation strengthening through
suppression of recrystallization. If the content of V
exceeds 0.150%, precipitation of carbonitrides increases
and the shapeability deteriorates, so the content is made
0.150% or less. To sufficiently obtain the effect of
raising the strength by V, the content has to be 0.005%
or more.
[0069] B: 0.0001 to 0.0100%
B is an element which suppresses phase transformation at
a high temperature and is effective for increasing the
strength and can be added in place of part of the C
and/or Mn. If the content of B exceeds 0.0100%, the
workability while hot is impaired and the productivity
falls, so the content of B is made 0.0100% or less. From
the viewpoint of the productivity, the content of B is
preferably 0.0050% or less, more preferably 0.0030% or
less. To sufficiently obtain higher strength by B, the
content of B has to be made 0.0001% or more. To
effectively increase the strength of the steel sheet, the
content of B is preferably 0.0003% or more, more
preferably 0.0005% or more.
[0070] Mo: 0.01 to 1.00%
Mo is an element which suppresses phase transformation at
a high temperature and is effective for increasing the
5 strength and can be added in place of part of the C
and/or Mn. If the content of Mo exceeds 1.00%, the
workability when hot is impaired and the productivity
falls, so the content of Mo is made 1.00% or less. To
sufficiently obtain higher strength by Mo, the content
10 has to be 0.01% or more.
[0071] W: 0.01 to 1.00%
W is an element which suppresses phase transformation at
a high temperature and is effective for increasing the
strength and can be added in place of part of the C
15 and/or Mn. If the content of W exceeds 1.00%, the
workability when hot is impaired and the productivity
falls, so the content of W is made 1.00% or less. To
sufficiently obtain higher strength by W, the content has
to be 0.01% or more.
20 100721 Cr: 0.01 to 2.00%
Cr is an element which suppresses phase transformation at
a high temperature and is effective for increasing the
strength and can be added in place of part of the C
and/or Mn. If the content of Cr exceeds 2.00%, the
25 workability when hot is impaired and the productivity
falls, so the content of Cr is made 2.00% or less. To
sufficiently obtain higher strength by Cr, the content
has to be 0.01% or more.
100731 One or More of Ca, Ce, Mg, Zr, Hf, and REM:
30 Total 0.0001 to 0.5000%
Ca, Ce, Mg, and REM are elements which are effective for
improving the shapeability. One or more can be added. If
the content of the one or more elements which are
selected from Ca, Ce, Mg, and REM exceeds a total of
35 0.5000%, the ductility is liable to be impaired, so the
total of the contents of the elements is made 0.5000% or
less. To sufficiently obtain the effect of improvement of
the shapeability of the steel sheet, the total of the
contents of the elements has to be 0.0001% or more. From
the viewpoint of the shapeability, the total of the
contents of the elements is preferably 0.0005% or more,
more preferably 0.0010% or more.
100741 "REM" is an abbreviation for "rare earth metal"
and indicates the elements which belong to the lanthanoid
series. In the present invention, the REM or Ce is often
added as a Misch metal. Sometimes, elements of the
lanthanoid series in addition to La or Ce are contained
compositely. Further, even when elements of the
lanthanoid series other than La and Ce are included as
unavoidable impurities, the effects of the present
invention are exhibited. Further, even if adding metal La
or Ce, the effects of the present invention are
exhibited.
[0075] Above, the composition of ingredients of the
present invention was explained, but so long as in a
range not impairing the properties of the steel sheet of
the present invention, for example, elements other than
the essential added elements may also be included as
impurities which are derived from the starting materials.
[0076] The high strength steel sheet of the present
invention can also be made high strength galvanized steel
sheet on the surface of which a galvanized layer or
galvannealed layer is formed. By forming a galvanized
layer on the surface of the high strength steel sheet,
steel sheet which has excellent corrosion resistance
results. Further, by forming a galvannealed layer on the
surface of the high strength steel sheet, steel sheet
which has excellent corrosion resistance and which has
excellent coating adhesion results.
100771 Next, the method of production of the high
strength steel sheet of the present invention will be
explained.
[0078] To produce the high strength steel sheet of the
present invention, first, a slab which has the abovementioned
composition of ingredients is cast. As the slab
which is used for hot rolling, for example, it is
possible to use a continuously cast slab or a slab which
is produced by a thin slab caster etc. For the method of
production of the high strength steel sheet of the
present invention, it is preferable to use a process such
as continuous casting-direct rolling (CC-DR) where the
steel is cast, then immediately hot rolled.
[0079] The slab heating temperature in the hot rolling
process has to be 1050°C or more. If the slab heating
temperature is low, the finish rolling temperature falls
below the Ar3 point. As a result, two-phase rolling of the
ferrite phase and austenite phase results, so the hot
rolled sheet structure becomes an uneven mixed grain
structure. The uneven structure is not eliminated even
after the cold rolling and annealing process and
therefore the ductility and bendability deteriorate.
Further, if the finish rolling temperature falls, the
rolling load increases and the rolling becomes difficult
or shape defects are liable to be invited in the steel
sheet after rolling. The upper limit of the slab heating
temperature is not particularly set so long as the effect
of the present invention is exhibited, but it is not
preferable economically to set the heating temperature to
an excessively high temperature, so the upper limit of
the slab heading temperature is preferably made 1350°C or
less.
[0080] The Ar3 point can be calculated by the following
formula :
[0081] Ar3 (OC) =901-325xC+33xSi-
92x(Mn+Ni/2+Cr/2+Cu/2+M0/2)+52xAl
[0082] In the above formula, C, Si, Mn, Ni, Cr, Cu,
Mo, and A1 are the contents of the different elements
(mass%).
[0083] The finish rolling temperature of the hot
rolling is made the higher of 800°C or the Ar3 point as
the lower limit and 1000°C as the upper limit. If the
finish rolling temperature is less than 800°C, the rolling
load at the time of finish rolling becomes high, the
rolling becomes difficult, and shape defects are liable
to be invited in the hot rolled steel sheet which is
obtained after rolling. If the finish rolling temperature
is less than the Ar3 point, the hot rolling becomes twophase
region rolling of the ferrite phase and austenite
phase and the hot rolled steel sheet structure will
sometimes become an uneven mixed grain structure.
[0084] The upper limit of the finish rolling
temperature is not particularly set so long as the effect
of the present invention is exhibited, but if the finish
rolling temperature is made excessively high, to secure
that temperature, the slab heating temperature has to be
made excessively high. Therefore, the upper limit
temperature of the finish rolling temperature is
preferably made 1000°C or less.
[0085] The steel sheet after rolling is coiled at 500
to 700°C. If coiling the steel sheet at a temperature
exceeding 700°C, the oxides which are formed on the steel
sheet surface excessively increase in thickness and the
pickling ability deteriorates. To raise the pickling
ability, the coiling temperature is preferably 680°C or
less, more preferably 660°C or less. If the coiling
temperature becomes less than 500°C, the hot rolled steel
sheet becomes excessively high in strength and cold
rolling becomes difficult. From the viewpoint of
lightening the load in cold rolling, the coiling
temperature is preferably made 550°C or more. 600°C or
more is more preferable.
[0086] The coiled steel sheet is preferably cooled by
a cooling rate of 25"C/hour or less. This is to promote
the precipitation of Cu.
100871 The thus produced hot rolled steel sheet is
pickled. Due to the pickling, the oxides on the steel
sheet surface can be removed. This is important from the
point of improving the chemical convertability of the
cold rolled high strength steel sheet of the final
product or the hot dip coatability of cold rolled steel
sheet for hot dip galvanized or galvannealed steel sheet
use. The pickling may be just a single treatment or may
be divided into a plurality of treatments.
[0088] The pickled steel sheet may be supplied as is
to the annealing process, but by cold rolling it by a
screwdown rate of 35 to 75%, steel sheet with a high
thickness precision and excellent shape is obtained. If
the screwdown rate is less than 35%, it is difficult to
hold the shape flat and the final product becomes poor in
ductility, so the screwdown rate is made 35% or more. If
the screwdown rate exceeds 75%, the cold rolling load
becomes too great and cold rolling becomes difficult.
From this, the upper limit of the screwdown rate is made
75%. The number of rolling passes and the screwdown rate
for each pass are not particularly prescribed so long as
the effect of the present invention is exhibited.
[0089] Next, the obtained hot rolled steel sheet or
cold rolled steel sheet is subjected to annealing
treatment.
[0090] First, the steel sheet was heated by an average
heating rate from 550 to 700°C of 1.0 to 10.O°C/sec,
preferably 2.0 to 5.0°C/sec, up to the maximum heating
temperature. The maximum heating temperature was made 740
to 1000°C. Due to this treatment, the crystal structure of
30 the Cu precipitates formed in the previous hot rolling
process is made an fcc (face-centered cubic lattice).
Part of the Cu precipitates made an fcc at this point of
time dissolve in the austenite and/or ferrite in the
heating process and hold the fcc structure even in the
35 later cooling process, so can be utilized as Cu
precipitates incoherent with the bcc iron.
[0091] If the maximum heating temperature is less than
740°C, coarse iron-based carbides remain undissolved in
the steel sheet and act as starting points of fracture,
so the shapeability is remarkably degraded. To decrease
the remaining undissolved iron-based carbides, the
maximum heating temperature is preferably made 760°C or
more. If the maximum heating temperature exceeds 1000°C,
the Cu particles melt during the heating and the number
of Cu particles which are incoherent with the bcc iron
becomes smaller, so the stretch flangeability
deteriorates. To leave a large number of Cu particles
incoherent with the bcc iron, the maximum heating
temperature is preferably 970°C or less, more preferably
950°C or less.
[0092] Next, the steel sheet is cooled by an average
cooling rate from the maximum heating temperature to 700°C
of 1.0 to 10.O°C/sec. Furthermore, in the temperature
region from maximum heating temperature to 700°C, the
steel sheet is given strain. As the method of giving
strain, for example, it is possible to use the method of
applying 5 to 50 MPa tension while bending one or more
times in a range giving a tensile strain at the outermost
circumference of 0.0007 to 0.0910. Due to this, it is
possible to newly promote the formation of nuclei for Cu
precipitates which are coherent or semi-coherent with the
surrounding bcc phase. The bent steel sheet may be bent
back.
[0093] If the tension which is applied to the steel
sheet is less than 5 MPa, the precipitation of Cu
particles is sometimes not sufficiently promoted. To
promote the precipitation of Cu particles and raise the
shapeability more, the tension is more preferably made 10
MPa or more, still more preferably 15 MPa or more. If the
tension exceeds 50 MPa, the steel sheet may plastically
deform and the shape may not be held.
[0094] If the amount of strain is less than 0.0007,
sufficient formation of nuclei does not occur and the
shapeability easily deteriorates. From the viewpoint of
the shapeability, the amount of stress is preferably
0.0010 or more. If the amount of strain exceeds 0.0910,
the shape is not held, so the amount of strain is
preferably made 0.0910 or less. To maintain the shape of
the steel sheet, the amount of strain is more preferably
0.0500 or less, still more preferably 0.0250 or less.
[0095] The thickness of the steel sheet is preferably
0.6 mm to 10.0 mm. If the thickness is less than 0.6 mm,
the shape of the steel sheet sometimes cannot be held. If
the thickness exceeds 10.0 mm, the temperature inside of
the steel sheet becomes hard to control.
[0096] The bending may be performed by, for example,
applying tension while pressing against a roll. The
diameter of the roll is preferably 800 mm or less to
obtain a sufficient amount of strain. Further, if using a
roll with a diameter less than 50 mm, the maintenance
costs of the facility increase, so making the roll
diameter 50 mm or more is preferable.
[0097] After this, the steel sheet is cooled from 700°C
to the Bs point (bainite transformation start
temperature) or 500°C by a cooling rate of 5.0 to
200.0°C/sec. Bainite or bainitic ferrite starts to form at
a temperature below the Bs point, so the cooling rate may
also be slowed. Even at a temperature higher than the Bs
point, if 500°C or less, the ferrite does not grow much at
all, so the cooling rate may be slowed. The Bs point can
be calculated by the following formula:
[0098] ~s(~C)=820-290C/(1-VF)-37Si-90Mn-65Cr-50Ni+7OAl
In the above formula, VF is the volume fraction of
ferrite, while C, Mn, Cr, Ni, Al, and Si are the amounts
of addition of these elements (mass % ) .
[0099] Note that, it is difficult to directly measure
the volume fraction of the ferrite phase during
production of high strength steel sheet, so in the
present invention, a small piece of the cold rolled steel
sheet is cut out before running the sheet through the
continuous annealing line, that small piece is annealed
by the same temperature history as the case of running it
through the continuous annealing line, the change in
volume of the ferrite phase of the small piece is
measured, the result is used to calculate a numerical
value, and that value is used as the volume fraction VF
of the ferrite. This measurement may be performed using
10 the result of the first measurement operation when
producing steel sheet under the same conditions. The
value does not have to be measured each time. Measurement
is performed again when greatly changing the production
conditions. Of course, it is also possible to observe the
15 microstructure of the actually produced steel sheet and
feed back the results to the production the next time and
on.
[OlOO] The annealed steel sheet is held at 250 to 500°C
for 60 to 1000 seconds to form hard structures, then is
20 cooled down to room temperature. After cooling it down to
room temperature, the steel sheet may be cold rolled by
0.05 to 3.00% for the purpose of correcting the shape.
[OlOl] The annealed steel sheet may be electroplated
to obtain a plated steel sheet. Further, during the
25 cooling from maximum heating temperature to room
temperature, for example, after the cooling down to 500°C
or after holding, it may be dipped in a galvanization
bath to obtain hot dip galvanized steel sheet. After
dipping the steel sheet in the galvanization bath, it may
30 be treated for alloying in a range of 470 to 650°C.
Furthermore, a film comprised of P oxides and/or
composite oxides containing P may be formed.
Examples
[0102] Slabs which have the chemical ingredients
35 (compositions) of A to AL which are shown in Tables 1 and
2 were cast, then immediately after casting were hot
rolled, cooled, coiled, and pickled under the conditions
which are shown in Tables 3 to 5. After that, Experiments
4, 9, 14, 19, 25, 29, 87, and 90 left the hot rolled
steel sheets as they were, while the other experiments
cold rolled them under the conditions which are described
in Tables 3 to 6 after pickling. After that, an annealing
process was applied under the conditions which are shown
in Tables 7 to 10 to obtain the steel sheets of
Experiments 1 to 114.
[0103] Note that, Experiment 102 is an example in
which the upper limit of the amount of Cu is exceeded.
The results of the weldability test conducted after the
hot rolling were poor, so the subsequent tests were
suspended.
- 28 -
[ 0 10 4 ] Table 1
[0105] Table 2
I
kl
w
I
- 30 -
[0106] Table 3
- 31 -
[0107] Table 4
- 32 -
[0108] Table 5
- 33 -
[0109] Table 6
[OllO] Table 7
[Olll] Table 8
[0112] Table 9
[0113] Table 10
[ 0 11 4 ] In the heating process, the steel sheets were
heated by the average heating rates described in Table 7
to Table 10 in the interval from 550 to 700°C until the
the maximum heating temperatures described in Table 7 to
Table 10.
[0115] After that, in the first cooling process from
the maximum heating temperature to 700°C, the steel sheets
were cooled by the average cooling rates described in
Table 7 to Table 10. In the temperature region from the
maximum heating temperature to 700°C, while applying the
tensions which are described in Table 7 to Table 10, in
Experiments 1 to 20, a radius 600 mm roll was used to
bend the steel sheets six times by a maximum tensile
strain of 0.0020. Similarly, in Experiments 21 to 39, a
radius 450 mrn roll was used to bend the steel sheets two
times by a maximum tensile strain of 0.0055, in
Experiments 41 to 75, a radius 730 mrn roll was used to
bend the steel sheets seven times by a maximum tensile
strain of 0.0010, and in Experiments 76 to 114, a radius
500 mm roll was used to bend the steel sheets five times
by a maximum tensile strain of a 0.0040. The thickness of
the steel sheet at the time of bending was 1.2 mm in
Experiments 1 to 20, 2.5 mrn in Experiments 21 to 39, 0.7
mm in Experiments 41 to 75, and 2.0 mm in Experiments 76
to 114.
[0116] In the second cooling process from 700°C to
500°C or the Bs point, the steel sheets were cooled by the
average cooling rates described in Table 7 to Table 10,
then were further cooled from 250 to 500°C in range, were
held for exactly the times described in Table 7 to Table
10, then were cooled to room temperature.
[0117] After cooling down to room temperature, in
Experiments 6 to 20 and 70 to 114, the steel sheets were
cold rolled by 0.15%, in Experiment 22, the steel sheet
was cold rolled by 1.50%, in Experiment 28, the steel
sheet was cold rolled by 1.00%, and in Experiments 31 to
54, the steel sheet was cold rolled at 0.25%.
[ 0 11 8 ] Experiments 29, 33, 43, 60, and 69 are examples
in which the steel sheets are electrolyticaly plated
after the annealing process to obtain galvanized steel
5 sheets (EG) .
[0119] Experiments 13, 54, 57, 63, 75, and 78 are
examples in which the steel sheets are cooled down to
500°C or the Bs point in the second cooling process, then
are held at 250 to 500°C in range during which they are
10 dipped in a galvanization bath to obtain hot dip
galvanized steel sheets (GI).
[0120] Experiments 18, 21, 81, and 84 are examples in
which the steel sheets are held at 250 to 500°C in range,
then dipped in a galvanization bath, then cooled down to
15 room temperature to obtain hot dip galvanized steel
sheets (GI).
[0121] Experiments 3, 8, 14, 25, 93, and 96 are
examples in which the steel sheets are cooled down to
500°C or the Bs point in the second cooling process, then
20 are held at 250 to 500°C in range during which they are
dipped in a galvanization bath and are further treated
for alloying at the described temperatures to obtain hot
dip galvannealed steel sheets (GA).
[0122] Experiments 38, 48, 51, 66, 72, 87, and 90 are
25 examples in which after the holding treatment at 250 to
500°C in range, the steel sheets are dipped in a
galvanization bath and treated for alloying at the
described temperatures to obtain hot dipped galvannealed
steel sheets (GA). Experiments 38 and 72 are examples in
30 which the surfaces of the plating layers are given films
comprised of P-based composite oxides.
[0123] Table 11 to Table 14 give the results of
measurement of the fractions of the microstructures of
the steel sheets of Experiments 1 to 114 in the range of
35 1/8 thickness to 3/8 thickness. In the microstructure
fractions, the amounts of residual austenite (residual y)
were measured by X-ray diffraction. The rest were found
by cutting out sheet thickness cross-sections parallel to
the rolling direction, polishing them to mirror surfaces,
etching the cross-sections by Nital, then examining them
5 using a field emission scanning electron microscope (FESEMI
.
4 [0124] Table 11
- 42 -
[0125] Table 12
[0126] Table 13
- 44 -
[0127] Table 14
[0128] Table 15 to Table 18 show the results of
observation of the Cu precipitates.
[0129] Samples cut out from the steel sheets at 1/4
thickness were observed for Cu precipitates using a high
resolution transmission electron microscope (HRTEM).
Electron energy-loss spectroscopy (EELS) was used to
confirm the composition of the Cu particles. These were
investigated for particle size and coherence with the bcc
iron. The size of the particles was made the average of
the particle sizes of 25 particles. Further, the ratio of
the precipitates which are incoherent with the bcc iron
in the number of particles which were observed was found.
[0130] In these experiments, there were no test pieces
with average sizes of precipitates of 3 nm or less, so it
was assumed the average particle size was 3 nm or more,
the number of Cu particles in a 10000 nm2 to 1 pm2 field
was measured, convergent-beam electron diffraction (CBED)
was used to measure the thickness of the observed part of
the test piece, this was multiplied with the observed
area to find the observed volume, and the number of Cu
particles was divided by the observed volume to find the
Cu particle density.
[0131] Table 15
Experiment
1
2
3
4
Chemical
ingredients
A
A
A
A
Steel type
CR
CR
GA
HR
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Cu particles
Density
No. /m3
9.9~10'~
1.5~10~'
7 . 0 ~ 1 0 ~ ~
1.6~10~'
Average
size
nm
7.6
6.2
7.2
7.0
Ratio of
incoherent
particles
%
3 6
2 8
2 4
4 8
- 46 -
101321 Table 16
Experiment
31
32
3 3
3 4
Chemical
ingredients
G
G
G
G
Steel type
CR
CR
EG
CR
Inv. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Cu particles
Density
No. /m3
1 . 1 ~ 1 0 ~ ~
1.6~10'~
2 . 0 ~ 1 0 ~ ~
3.7~10"
Average
size
nm
6.8
4.8
4.5
11.8
Ratio of
incoherent
particles
%
4 0
2 0
2 8
100
- 47 -
[0133] Table 17
- 48 -
[0134] Table 18
[0135] Table 19 to Table 22 show the results of
evaluation of properties of the steel sheets of
Experiments 1 to 114. Tensile test pieces based on JIS Z
2201 were taken from the steel sheets of Experiments 1 to
114 and were subjected to tensile tests based on JIS Z
2241 to measure the yield strength (YS), tensile strength
(TS) , total elongation (EL) , and hold expansion rate (h) .
- 49 -
[0136] Table 19

- 51 -
[0138] Table 21
- 52 -
[0139] Table 22
[0140] Experiment 5 is an example in which the end
temperature of the hot rolling is low. The microstructure
is stretched in one direction making it uneven, so the
ductility and stretch flangeability are poor.
[0141] Experiment 10 is an example in which the
cooling rate after coiling is high. The Cu particles
insufficiently precipitate in the hot rolling process,
the ratio of Cu particles incoherent with the bcc iron is
small, and the stretch flangeability is poor.
[0142] Experiment 15 is an example in which the
heating rate is large. The Cu particles insufficiently
grow, the ratio of Cu particles incoherent with the bcc
iron is small, and the stretch flangeability is poor.
[Ox431 Experiment 20 is an example in which the
maximum heating temperature in the annealing process is
low. A large number of coarse iron-based carbides which
form starting points of fracture are included, so the
ductility and the stretch flangeability are poor.
[0144] Experiment 23 is an example in which the
maximum heating temperature in the annealing process is
high. The Cu particles form solid solutions once during
the heating and there are few Cu particles incoherent
with the bcc iron, so the stretch flangeability is poor.
[0145] Experiment 30 is an example in which the
average cooling rate of the first cooling process is
high. The Cu particles insufficiently precipitate, so the
ductility and the stretch flangeability are poor.
[0146] Experiment 34 is an example in which the
average cooling rate of the first cooling process is low.
Coarse iron-based carbides are formed, and the stretch
flangeability is poor.
[0147] Experiment 35 is an example in which there is
no tension in the first cooling process. The
precipitation of Cu is insufficient, and the stretch
flangeability is poor.
[ 0 14 8 ] Experiment 39 is an example in which the
cooling rate in the second cooling process is low. Coarse
iron-based carbides are formed, and the stretch
flangeability is poor.
[0149] Experiment 40 is an example in which no bending
is applied in the first cooling process. The
precipitation of Cu is insufficient, and the stretch
flangeability is poor.
[0150] Experiment 44 is an example in which the
holding time at 250 to 500°C is long. Iron-based carbides
30 excessively form, and the stretch flangeability is poor.
[0151] Experiment 45 is an example in which the
holding time at 250 to 500°C is short. Martensite
excessively forms, and the stretch flangeability is poor.
[0152] Experiment 97 to 100 are examples in which the
35 compositions of ingredients deviate from the
predetermined range. In each case, sufficient properties
could not be obtained.
[01531 Experiment 101 is an example in which the lower
limit of the amount of Cu is exceeded. The density of Cu
particles is low, and the stretch flangeability is poor.

- 55 -
CLAIMS
Claim 1. High strength steel sheet which is
excellent in shapeability which contains,
by mass%,
C: 0.075 to 0.300%,
Si: 0.30 to 2.50%,
Mn: 1.30 to 3.50%,
P: 0.001 to 0.030%,
S: 0.0001 to 0.0100%,
Al: 0.005 to 1.500%,
Cu: 0.15 to 2.00%,
N: 0.0001 to 0.0100%, and
0: 0.0001 to 0.0100%,
contains, as optional elements,
Ti: 0.005 to 0.150%,
Nb: 0.005 to 0.150%,
B: 0.0001 to 0.0100%,
Cr: 0.01 to 2.00%,
Ni: 0.01 to 2.00%,
Mo: 0.01 to 1.008,
W: 0.01 to 1.00%,
V: 0.005 to 0.150%, and -
one or more of Ca, Ce, Mg, and REM: total 0.0001 to
0.50%, and
25 has a balance of iron and unavoidable impurities,
wherein
said steel sheet structure contains a ferrite phase
and martensite phase,
a ratio of Cu particles incoherent with bcc iron is
30 15% or more with respect to the Cu particles as a whole,
a density of Cu particles in the ferrite phase is
1.0x10~*/mo~r more, and
an average particle size of Cu particles in the
ferrite phase is 2.0 nm or more.
35 Claim 2. The high strength steel sheet which is
excellent in shapeability of claim 1 characterized in
that the structure in a range of 1/8 thickness to 3/8
ORIGIN
- 56 -
thickness of said high strength steel sheet comprises, by
volume fraction,
a ferrite phase: 10 to 75%,
bainitic ferrite phase and/or bainite phase: 50% or
less,
tempered martensite phase: 50% or less,
fresh martensite phase: 15% or less, and
residual austenite phase: 20% or less.
Claim 3. High strength galvanized steel sheet which
is excellent in shapeability characterized by comprising
the high strength steel sheet of claim 1 or 2 on the
surface of which a galvanized layer formed.
Claim 4. A method of production of high strength
steel sheet which is excellent in shapeability
characterized by comprising
a hot rolling process of heating a slab which
contains,
Si: 0.30 to 2.50%,
Mn: 1.30 to 3.50%,
P: 0.001 to 0.030%,
Al: 0.005 to 1.500%,
Cu: 0.15 to 2.00%,
contains, as optional elements
Ti: 0.005 to 0.150%,
Nb: 0.005 to 0.150%,
B: 0.0001 to 0.0100%,
Cr: 0.01 to 2.00%,
Ni: 0.01 to 2.00%,
Mo: 0.01 to 1.00%,
V: 0.005 to 0.150%, and
one or more of Ca, and REM: total
I
0
0.50%, and
has a balance of iron and unavoidable impurities,
directly, or after cooling once, to 1050°C or more,
rolling with a lower limit of a temperature of 800°C or
5 the Ar3 transformation point, whichever is higher, and
coiling it at 500 to 700°C in temperature and
an annealing process of heating the coiled steel
sheet by an average heating rate at 550 to 700°C of 1.0 to
10.O°C/sec up to a maximum heating temperature of 740 to
10 1000°C, then cooling by an average cooling rate from the
I
maximum heating temperature to 700°C of 1.0 to 10.O°C/sec,
imparting strain to the steel sheet from the maximum
I heating temperature to 700, and cooling by a cooling rate
from 700°C to the Bs point or 500°C of 5.0 to 200.0°C/sec.
15 Claim 5. The method of production of high strength
steel sheet which is excellent in shapeability of claim 4
characterized by having a cold rolling process, after
said hot rolling process and before said annealing
process, of pickling the coiled steel sheet, then rolling
20 it by a screwdown rate of a screwdown rate 35 to 75%.
Claim 6. The method of production of high strength
steel sheet which is excellent in shapeability of claim 4
l or 5 characterized by the strain being imparted to the
steel sheet in said annealing process by applying 5 to 50
25 MPa of tension to the steel sheet while bending one time
or more in a range giving an amount of tensile strain at
the outermost circumference of 0.0007 to 0.0910.
Claim 7. The method of production of high strength
steel sheet which is excellent in shapeability of claim 6
30 characterized in that said bending is performed by
pressing the steel sheet against a roll with a roll
diameter of 800 mrn or less.
Claim 8. A method of production of high strength
galvanized steel sheet which is excellent in shapeability
35 characterized by producing high strength steel sheet by
the method of production of high strength steel sheet of
any one of claims 4 to 7, then electrogalvanizing it.
Claim 9. A method of production of high strength
galvanized steel sheet which is excellent in shapeability
characterized by producing high strength steel sheet by
the method of production according to any one of claims 4
to 8 after the cooling to the Bs point or 500°C of which
performing hot dip galvanization.
Claim 10. A method of production of high strength
galvanized steel sheet which is excellent in shapeability
according to claim 9 characterized by performing alloying
treatment at 470 to 650°C in temperature after the hot dip
galvanization.
THE APPLICANT[S]