CA 02546009 2006-05-12
1
HIGH-RIGIDITY HIGH-STRENGTH THIN STEEL SHEET AND
METHOD FOR PRODUCING SAME
TECHNICAL FIELD
[0001] This invention relates to a high-stiffness high-strength thin
steel sheet suitable mainly as a vehicle body for automobiles and a
method for producing the same. Moreover, the high-stiffness high-
strength thin steel sheet according to the invention is a column-shaped
structural member having a thickness susceptibility index of the
stiffness near to 1 such as center pillar, locker, side flame, cross
member or the like of the automobile and is widely suitable for
applications requiring a stiffness.
RELATED ART
[0002] As a result of recent heightened interest in global environ-
ment problems, the exhaust emission control is conducted even in the
automobiles, and hence the weight reduction of the vehicle body in the
automobile is a very important matter. For this end, it is effective to
attain the weight reduction of the vehicle body by increasing the
strength of the steel sheet to reduce the thickness thereof.
[0003] Recently, the increase of the strength in the steel sheet is
considerably advanced, and hence the use of thin steel sheets having a
thickness of less than 2.0 mm is increasing. In order to further reduce
the weight by the increase of the strength, it is indispensable to
simultaneously control the deterioration of the stiffness in parts
through the thinning of the thickness. Such a problem of deteriorating
the stiffness of the parts through the thinning of the thickness in the
steel sheet is actualized in steel sheets having a tensile strength of not
less than 590 MPa, and particularly this problem is serious in steel
sheets having a tensile strength of not less than 700 MPa.
[0004] In general, in order to increase the stiffness of the parts, it is
effective to change the shape of the parts, or to increase the number of
welding points or change the welding condition such as changeover to
CA 02546009 2006-05-12
-2-
laser welding or the like in the spot-welded parts. However, when
these parts are used in the automobile, there are problems that it is not
easy to change the shape of the parts in a limited space inside the
automobile, and the change of the welding conditions causes the
increase of the cost and the like.
[0005] Consequently, in order to increase the stiffness of the parts
without changing the shape of the parts or the welding conditions, it
becomes effective to increase the Young's modulus of the material
used in the parts.
[0006] In general, the stiffness of the parts under the same shape of
parts and welding conditions is represented by a product of Young's
modulus of the material and geometrical moment of inertia of the part.
Further, the geometrical moment of inertia can be expressed so as to
be approximately proportionate to t'' when the thickness of the material
is t. In this case, ~, is a thickness susceptibility index and is a value
of 1-3 in accordance with the shape of the parts. For example, in
case of one plate shape such as panel parts for the automobile, ~, is a
value near to 3, while in case of column-shape such as structural parts,
~, is a value near to 1.
[0007] When ~, of the parts is 3, if the thickness is made small by
10% while equivalently maintaining the stiffness of the parts, it is
required to increase the Young's modulus of the material by 37%,
while when ~, of the parts is 1, if the thickness is made small by 10%,
it may be enough to increase the Young's modulus by 11%.
[0008] That is, in case of the parts having ~, near to 1 such as column-
shaped parts, it is very effective to increase the Young's modulus of
the steel sheet itself for the weight reduction. Particularly, in case of
steel sheets having a high strength and a small thickness, it is strongly
demanded to highly increase the Young's modulus of the steel sheet.
[0009] In general, the Young's modulus is largely dependent upon
the texture and is known to become high in a closest direction of atom.
Therefore, it is effective to develop X112}<110> in order to develop
an orientation advantageous for the Young's modulus of steel being a
05845HH (2/34)
CA 02546009 2006-05-12
-3-
body-centered cubic lattice in a steel making process comprising the
rolling through rolls and the heat treatment, whereby the Young's
modulus can be increased in a direction perpendicular to the rolling
direction.
[0010] There have hitherto been variously examined steel sheets by
controlling the texture to increase the Young's modulus.
[0011] For example, the patent article 1 discloses a technique wherein
a steel obtained by adding Nb or Ti to an extremely low carbon steel is
hot-rolled at a rolling reduction at Ar3-(Ar3+150°C) of not less than
85%
to promote transformation from non-crystallized austenite to ferrite to
thereby render the texture of ferrite at the stage of the hot-rolled sheet
into {311}<Oll> and X332}<113>, which is an initial orientation and
is subjected to a cold rolling and a recrystallization annealing to
render X211}<O11> into a main orientation to thereby increase the
Young's modulus in a direction perpendicular to the rolling direction.
[0012] Also, the patent article 2 discloses a method for producing a
hot rolled steel sheet having an increased Young's modulus in which
Nb, Mo and B are added to a low carbon steel having a C content of
0.02-0.15% and the rolling reduction at Ar3-950°C is made to not less
than 50% to develop [211]<O11>.
[0013] Further, the patent article 3 discloses a method for producing
a hot rolled steel sheet in which Si and Al are added to a low carbon
steel having a C content of not more than 0.05% to enhance Ar3 trans-
formation point and the rolling reduction below Ar3 transformation
point in the hot rolling is made to not less than 60% to increase
Young's modulus in a direction perpendicular to the rolling direction.
[0014] Patent article 1: JP-A-H05-255804
Patent article 2: JP-A-H08-311541
Patent Article 3: JP-A-H09-53118
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED IN THE INVENTION
[0015] However, the aforementioned techniques have the following
problems.
05845HH (3/34)
CA 02546009 2006-05-12
-4-
In the technique disclosed in the patent article 1, the Young's
modulus of the steel sheet is increased by using the extremely low
carbon steel having a C content of not more than 0.01 % to control the
texture, but the tensile strength is low as about 450 MPa at most, so
that there is a problem in the increase of the strength by applying this
technique.
[0016] In the technique disclosed in the patent article 2, since the C
content is as high as 0.02-0.15%, it is possible to increase the strength,
but as the target steel sheet is the hot rolled steel sheet, the control of
the texture through cold working can not be utilized, and hence there
are problems that it is difficult to further increase the Young's modulus
but also it is difficult to stably produce high-strength steel sheets
having a thickness of less than 2.0 mm through low-temperature finish
rolling.
[0017] Further, in the technique disclosed in the patent article 3, the
crystal grains are coarsened by conducting the rolling at the ferrite
zone, so that there is a problem that the workability is considerably
deteriorated.
[0018] Thus, the increase of the Young's modulus in the steel sheet
by the conventional techniques is targeted to hot rolled steel sheets
having a thick thickness or soft steel sheets, so that it is difficult to
increase the Young's modulus of high-strength thin steel sheet having a
thickness of not more than 2.0 mm by using the above conventional
techniques.
[0019] As a strengthening mechanism for increasing the tensile
strength of the steel sheet to not less than 590 MPa, there are mainly a
precipitation strengthening mechanism and a transformation texture
strengthening mechanism.
[0020] When the precipitation strengthening mechanism is used as
the strengthening mechanism, it is possible to increase the strength
while suppressing the lowering of the Young's modulus of the steel
sheet as far as possible, but the following difficulty is accompanied.
That is, when utilizing the precipitation strengthening mechanism for
05845HH (4/34)
CA 02546009 2006-05-12
-5-
finely precipitating, for example, a carbonitride of Ti, Nb or the like,
in the hot rolled steel sheet, the increase of the strength is attained by
conducting the fine precipitation in the coiling after the hot rolling, but
in the cold rolled steel sheet, the coarsening of the precipitate can not be
avoided at the step of recrystallization annealing after the cold rolling
and it is difficult to increase the strength through the precipitation
strengthening.
[0021] When utilizing the transformation texture strengthening
mechanism as the strengthening mechanism, there is a problem that the
Young's modulus of the steel sheet lowers due to strain included in a
low-temperature transformation phase such as bainite phase,
martensite phase or the like.
[0022] It is, therefore, an object of the invention to solve the above
problems and to provide a high-stiffness high-strength thin steel sheet
having a tensile strength of not less than 590 MPa, preferably not less
than 700 MPa, a Young's modulus of not less than 230 GPa, preferably
not less than 240 GPa and a thickness of not more than 2.0 mm as well
as an advantageous method for producing the same.
MEANS FOR SOLVING PROBLEMS
[0023] In order to achieve the above object, the gist and
construction of the invention are as follows.
(I) A high-stiffness high-strength thin steel sheet comprising
C: 0.02-0.15%, Si: not more than 1.5%, Mn: 1.0-3.5%, P: not more than
0.05%, S: not more than 0.01%, Al: not more than 1.5%, N: not more
than 0.01% and Ti: 0.02-0.50% as mass%, provided that C, N, S and Ti
contents satisfy the relationships of the following equations (1) and (2):
Ti* = Ti-(47.9/14)xN-(47.9/32.1)xS s 0.01 ~ ~ ~ ~ ~ (1)
0.01 s C-(12/47.9)xTi* s 0.05 ~ ~ ~ ~ ~ (2)
and the remainder being substantially iron and inevitable impurities,
and having a texture comprising a ferrite phase as a main phase and
having a martensite phase at an area ratio of not less than 1 %, and
having a tensile strength of not less than 590 MPa and a Young's
modulus of not less than 230 GPa.
05845HH (5/34)
CA 02546009 2006-05-12
-6-
[0024] (II) A high-stiffness high-strength thin steel sheet
according to the item (I), which further contains one or two of Nb:
0.005-0.04% and V: 0.01-0.20% as mass% in addition to the above
composition and satisfies the relationships of the above equation (1)
and the following equation (3) instead of the equation (2):
0.01 s C-(12/47.9)xTi*-(12/92.9)xNb-(12/50.9)xV s 0.05 ~ ~ ~ (3)
[0025] (III) A high-stiffness high-strength thin steel sheet
according to the item (I) or (II), which further contains one or more of
Cr: 0.1-1.0%, Ni: 0.1-1.0%, Mo: 0.1-1.0%, Cu: 0.1-2.0% and B:
0.0005-0.0030% as mass% in addition to the above composition.
[0026] (IV) A method for producing a high-stiffness high-strength
thin steel sheet comprising subjecting a starting material of steel
comprising C: 0.02-0.15%, Si: not more than 1.5%, Mn: 1.0-3.5%, P:
not more than 0.05%, S: not more than 0.01%, Al: not more than 1.5%,
N: not more than 0.01% and Ti: 0.02-0.50% as mass%, provided that C,
N, S and Ti contents satisfy the relationships of the following
equations (1) and (2):
Ti* = Ti-(47.9/14)xN-(47.9/32.1)xS z 0.01 ~ ~ ~ ~ ~ (1)
0.01 s C-(12/47.9)xTi* s 0.05 ~ ~ ~ ~ ~ (2)
to a hot rolling step under conditions that a total rolling reduction
below 950°C is not less than 30% and a finish rolling is terminated at
800-900°C, coiling the hot rolled sheet below 650°C, pickling,
subjecting to a cold rolling at a rolling reduction of not less than 50%,
raising a temperature to 780-900°C at a temperature rising rate from
500°C of 1-30°C/s to conduct soaking, and then cooling at a
cooling
rate up to 500°C of not less than 5°C/s to conduct annealing.
[0027] (V) A method for producing a high-stiffness high-strength
thin steel sheet according to the item (IV), wherein the starting
material of steel further contains one or two of Nb: 0.005-0.04% and
V: 0.01-0.20% as mass% in addition to the above composition and
satisfies the relationships of the above equation (1) and the following
equation (3) instead of the equation (2):
0.01 s C-(12/47.9)xTi*-(12/92.9)xNb-(12/50.9)xV s 0.05 ~ ~ ~ (3)
05845HH (6/34)
CA 02546009 2006-05-12
-7-
[0028] (VI) A method for producing a high-stiffness high-strength
thin steel sheet according to the item (IV) or (V), wherein the staring
material of steel further contains one or more of Cr: 0.1-1.0%, Ni:
0.1-1.0%, Mo: 0.1-1.0%, Cu: 0.1-2.0% and B: 0.0005-0.0030% as
mass% in addition to the above composition.
EFFECT OF THE INVENTION
[0029] According to the invention, it is possible to provide a high-
stiffness high-strength thin steel sheet having a tensile strength of not
less than 590 MPa and a Young's modulus of not less than 230 GPa.
[0030] That is, the starting material of low carbon steel added with
Mn and Ti is roll-reduced below 950°C in the hot rolling to
promote
the transformation from non-recrystallized austenite to ferrite and then
cold rolled to develop a crystal orientation useful for the improvement
of Young's modulus and thereafter a low-temperature transformation
phase suppressing the lowering of the Young's modulus is produced
and a greater amount of ferrite phase useful for the improvement of the
Young's modulus is retained in the cooling stage by the control of the
heating rate in the annealing step and the soaking at two-phase region,
whereby the thin steel sheet satisfying higher strength and higher
Young's modulus can be produced, which develops an effective effect
in industry.
[0031] Further explaining in detail, the starting material of low
carbon steel added with Mn and Ti is roll-reduced at an austenite low-
temperature region in the hot rolling to increase the non-recrystallized
austenite texture having a crystal orientation of {112}<111>, and
subsequently the transformation from the non-recrystallized austenite
of {112}<111> to ferrite is promoted in the cooling stage to develop
ferrite orientation of { 113 } <110>.
[0032] In the cold rolling after the coiling and pickling, the rolling
is carried out at a rolling reduction of not less than 50% to turn the
crystal orientation of {113}<110> to {112}<110> useful for the
improvement of the Young's modulus, and in the temperature rising
stage at the subsequent annealing step, the temperature is raised from
05845HH (7/34)
CA 02546009 2006-05-12
_g_
500°C to the soaking temperature at a heating rate of 1-30°C/s
to
promote the recrystallization of ferrite having an orientation of
{112}<110> and provide a two-phase region at a state of partly
retaining the non-recrystallized grains of {112}<110>, whereby the
transformation from the non-recrystallized ferrite of {112}<110> to
austenite can be promoted.
[0033] Further, in the transformation from austenite phase to ferrite
phase at the cooling after the soaking, ferrite grains having an
orientation of {112}<110> is grown to enhance the Young's modulus,
while the steel enhancing the hardenability by the addition of Mn is
cooled at a rate of not less than 5°C/s to produce the low-temperature
transformation phase, whereby it is attempted to increase the strength.
[0034] Moreover, the low-temperature transformation phase is
produced by retransforming the austenite phase transformed from
ferrite having an orientation of { 112} <110> during the cooling, so that
{112}<110> can be also developed even in the crystal orientation of
the low-temperature transformation phase.
[0035] Thus, the Young's modulus is enhanced by developing
{112}<110> of ferrite phase, and particularly {112}<110> is increased
in the orientation of the low-temperature transformation phase largely
exerting on the lowering of the Young's modulus, whereby the strength
can be increased by the formation of the low-temperature transforma-
tion phase and the lowering of the Young's modulus accompanied with
the formation of the low-temperature transformation phase can be
largely suppressed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 is a graph showing an influence of a total rolling
reduction below 950°C on Young's modulus;
FIG. 2 is a graph showing an influence of a final temperature
in hot finish rolling on Young's modulus;
FIG. 3 is a graph showing an influence of a coiling
temperature on Young's modulus;
FIG. 4 is a graph showing an influence of a rolling reduction
05845HH (8/34)
CA 02546009 2006-05-12
-9-
in cold rolling on Young's modulus; and
FIG. 5 is a graph showing an influence of an average
temperature rising rate from 500°C to soaking temperature in
annealing on Young's modulus.
BEST MODE FOR CARRYING OUT THE INVENTION
[0037] The high-stiffness high-strength thin steel sheet according to
the invention is a steel sheet having a tensile strength of not less than
590 MPa, preferably not less than 700 MPa, a Young's modulus of not
less than 230 GPa, preferably not less than 240 GPa, and a thickness of
not more than 2.0 mm. Moreover, the steel sheet to be targeted in the
invention includes steel sheets subjected to a surface treatment such as
galvanization inclusive of alloying, zinc electroplating or the like in
addition to the cold rolled steel sheet.
[0038] The reason of limiting the chemical composition in the steel
sheet of the invention will be described below. Moreover, the unit for
the content of each element in the chemical composition of the steel
sheet is "% by mass", but it is simply shown by "%" unless otherwise
specified.
[0039] C:0.02-0.15%
C is an element stabilizing austenite and can largely
contribute to increase the strength by enhancing the hardenability at
the cooling stage in the annealing after the cold rolling to largely
promote the formation of the low-temperature transformation phase.
Further, C can contribute to increase the Young's modulus by
promoting the transformation of ferrite grains having {112}<110>
after the cold rolling from the non-recrystallized ferrite to austenite in
the temperature rising stage at the annealing step.
[0040] In order to obtain such effects, the C content is required to be
not less than 0.02%, preferably not less than 0.05%, more preferably not
less than 0.06%. On the other hand, when the C content exceeds 0.15%,
the fraction of hard low-temperature transformation phase becomes
large, and the strength of the steel is extremely increased but also the
workability is deteriorated. Also, the greater amount of C suppresses
05845HH (9/34)
CA 02546009 2006-05-12
-10-
the recrystallization of the orientation useful for the increase of the
Young's modulus at the annealing step after the cold rolling. Further,
the greater amount of C brings about the deterioration of the
weldability.
Therefore, the C content is required to be not more than
0.15%, preferably not more than 0.10%.
[0041] Si: not more than 1.5%
Si raises the Ar3 transformation point in the hot rolling, so
that when the rolling is terminated at 800-900°C, if Si is contained in
an amount exceeding 1.5%, the rolling at austenite region becomes
difficult and the crystal orientation required for the increase of the
Young's modulus can not be obtained. Also, the greater amount of Si
deteriorates the weldability of the steel sheet but also promotes the
formation of fayalite on a surface of a slab in the heating at the hot
rolling step to accelerate the occurrence of surface pattern so-called as
a red scale. Furthermore, in case of using as a cold rolled steel sheet,
Si oxide produced on the surface deteriorates the chemical conversion
processability, while in case of using as a galvanized steel sheet, Si
oxide produced on the surface induces non-plating. Therefore, the Si
content is required to be not more than 1.5%. Moreover, in case of
steel sheets requiring the surface properties or the galvanized steel
sheet, the Si content is preferable to be not more than 0.5%.
[0042] Also, Si is an element stabilizing ferrite and promotes the
ferrite transformation at the cooling stage after the soaking of two-
phase region in the annealing step after the cold rolling to enrich C in
austenite, whereby austenite can be stabilized to promote the formation
of the low-temperature transformation phase. For this end, the
strength of steel can be increased, if necessary. In order to obtain
such an effect, the Si content is desirable to be not less than 0.2%.
[0043] Mn: 1.0-3.5%
Mn is one of important elements in the invention. Mn has
an action of suppressing the recrystallization of worked austenite in
the hot rolling. Also, Mn can promote the transformation from the
05845HH (10/34)
CA 02546009 2006-05-12
-11-
non-recrystallized austenite to ferrite to develop {113}<110> and
improve the Young's modulus in the subsequent cold rolling and
annealing steps.
[0044] Furthermore, Mn as an austenite stabilizing element lowers
Acl transformation point in the temperature rising stage at the
annealing step after the cold rolling to promote the transformation
from the non-recrystallized ferrite to austenite, and can develop the
orientation useful for the improvement of the Young's modulus to
control the lowering of the Young's modulus accompanied with the
formation of the low-temperature transformation phase with respect to
the orientation of the low-temperature transformation phase produced
in the cooling stage after the soaking.
[0045] Also, Mn enhances the hardenability in the cooling stage after
the soaking and annealing at the annealing step to largely promote the
formation of the low-temperature transformation phase, which can
largely contribute to the increase of the strength. Further, Mn acts as a
solid-solution strengthening element, which can contribute to the increase
of the strength in steel. In order to obtain such an effect, the Mn
content is required to be not less than 1.0%, preferably not less than 1.5%.
[0046] On the other hand, when the Mn content exceeds 3.5%, Ac3
transformation point is excessively lowered in the temperature rising
stage at the annealing step after the cold rolling, so that the recrystal-
lization of ferrite phase at the two-phase region is difficult and it is
required to raise the temperature up to an austenite single-phase region
above Ac3 transformation point. As a result, ferrite of {112}<110>
orientation useful for the increase of the Young's modulus obtained by
the recrystallization of worked ferrite can not be developed to bring
about the lowering of the Young's modulus. Further, the greater
amount of Mn deteriorates the weldability of the steel sheet.
Moreover, the greater amount of Mn enhances the deformation
resistance of steel in the hot rolling to increase the rolling load, which
causes the difficulty in the operation. Therefore, the Mn content is
not more than 3.5%.
05845I-IH (11/34)
CA 02546009 2006-05-12
-12-
[0047] P: not more than 0.05 %
Since P segregates in the grain boundary, if the P content
exceeds 0.05%, the ductility and toughness of the steel sheet lower but
also the weldability is deteriorated. In case of using the alloyed
galvanized steel sheet, the alloying rate is delayed by P. Therefore,
the P content is required to be not more than 0.05%. On the other
hand, P is an element effective for the increase of the strength as a
solid-solution strengthening element and has an action of promoting
the enrichment of C in austenite as a ferrite stabilizing element.
In the steel added with Si, it has also an action of suppressing the
occurrence of red scale. In order to obtain these actions, the P
content is preferable to be not less than 0.01%.
[0048] S: not more than 0.01%
S considerably lowers the hot ductility to induce hot tearing
and considerably deteriorate the surface properties. Further, S hardly
contributes to the strength but also forms coarse MnS as an impurity
element to lower the ductility and drill-spreading property. These
problems become remarkable when the S content exceeds 0.01%, so
that it is desirable to reduce the S content as far as possible.
Therefore, the S content is not more than 0.01%. From a viewpoint
of improving the drill-spreading property, it is preferable to be not
more than 0.005%.
[0049] Al: not more than 1.5%
Al is an element useful for deoxidizing steel to improve the
cleanness of the steel. However, A1 is a ferrite stabilizing element,
and largely raises the Ar3 transformation of the steel, so that when the
finish rolling is terminated at 800-900°C, if the A1 content exceeds
1.5%, the rolling at austenite region becomes difficult to suppress the
development of the crystal orientation required for the increase of the
Young's modulus. Therefore, the Al content is required to be not more
than 1.5%. From this viewpoint, A1 is preferable to be made lower,
and further preferable to be limited to not more than 0.1%. On the
other hand, A1 as a ferrite forming element promotes the formation of
05845HH (12/34)
CA 02546009 2006-05-12
-13-
ferrite in the cooling stage after the soaking at the two-phase region in
the annealing step after the cold rolling to enrich C in austenite,
whereby austenite can be stabilized to promote the formation of the
low-temperature transformation phase. As a result, the strength of
the steel can be enhanced, if necessary. In order to obtain such an
effect, the A1 content is desirable to be not less than 0.2%.
[0050] N: not more than 0.01 %
N is a harmful element because slab breakage is
accompanied in the hot rolling to cause surface defect. When the N
content exceeds 0.01%, the occurrence of slab breakage and surface
defect becomes remarkable. Further, when a carbonitride forming
element such as Ti, Nb or the like is added, N forms a coarse nitride at
a high temperature to suppress the effect by the addition of the
carbonitride forming element. Therefore, the N content is required to
be not more than 0.01 %.
[0051] Ti:0.02-0.50%
Ti is a most important element in the invention. That is, Ti
controls the recrystallization of worked austenite at the finish rolling
step in the hot rolling to promote the transformation from the non-
recrystallized austenite to ferrite and develop X113}<110>, and can
increase the Young's modulus at the subsequent cold rolling and
annealing steps. Also, the recrystallization of worked ferrite is
suppressed in the temperature rising stage at the annealing step after
the cold rolling to promote the transformation from the non-
recrystallized ferrite to austenite, and the orientation useful for the
increase of the Young's modulus can be developed with respect to the
orientation of the low-temperature transformation phase produced in
the cooling stage after the soaking to suppress the lowering of the
Young's modulus accompanied with the formation of the low-
temperature transformation phase. Further, a fine carbonitride of Ti
can contribute to the increase of the strength. In order to obtain such
actions, the Ti content is required to be not less than 0.02%, preferably
not less than 0.03%.
05845HH (13/34)
CA 02546009 2006-05-12
-14-
[0052] On the other hand, when the Ti content exceeds 0.50%, all
the carbonitride can not be solid-soluted in the re-heating at the usual
hot rolling step and a coarse carbonitride remains, and hence the effect
of suppressing the recrystallization of worked austenite at the hot
rolling step or the effect of suppressing the recrystallization of worked
ferrite at the annealing step after the cold rolling can not be obtained.
Also, even if the hot rolling of the slab after the continuous casting is
started as it is without conducting the re-heating after the continuously
cast slab is cooled, when the Ti content exceeds 0.50%, the improve-
ment of the effect of suppressing the recrystallization is not recognized
and also the increase of the alloy cost is brought about. Therefore,
the Ti content is required to be not more than 0.50%, preferably not
more than 0.20%.
[0053] In the invention, the contents of C, N, S and Ti are required
to satisfy the relationship of the following equations (1) and (2):
Ti* = Ti-(47.9/14)xN-(47.9/32.1)x5 >_ 0.01 ~ ~ ~ ~ ~ (1)
0.01 s C-(12/47.9)xTi* s 0.05 ~ ~ ~ ~ ~ (2)
[0054] Ti is liable to easily form coarse nitride and sulfide at a high
temperature region. The formation of such nitride and sulfide brings
about the reduction of the effect of suppressing the recrystallization
through the addition of Ti. Therefore, the amount of Ti* = Ti-
(47.9/14)xN-(47.9/32.1)xS as an amount of Ti not fixed as the nitride
and sulfide is required to be not less than 0.01%, preferably not less
than 0.02%.
[0055] If C not fixed as a carbonitride is existent in an amount
exceeding 0.05%, the introduction of strain in the cold rolling becomes
non-uniform and further the recrystallization of the orientation useful
for the increase of the Young's modulus is suppressed, so that the C
amount not fixed as the carbide calculated by (C-(12/47.9)xTi*) is
required to be not more than 0.05%. On the other hand, when the C
amount not fixed as the carbide is less than 0.01%, the C content in
austenite decreases in the annealing at two-phase region after the cold
rolling to suppress the formation of martensite phase after the cooling
05845HH (14/34)
CA 02546009 2006-05-12
-15-
and hence it is difficult to increase the strength. Therefore, the
amount of C-(12/47.9)xTi* as the C amount not fixed as the carbide is
0.01-0.05 %.
[0056] Moreover, the term "the remainder being substantially iron
S and inevitable impurities" used herein means that steels containing
slight amounts of other elements without damaging the action and
effect of the invention are included within the scope of the invention.
In case of further increasing the strength, one or two of Nb: 0.005-
0.04% and V: 0.01-0.20% and one or more of Cr, Ni, Mo, Cu and B
may be added, if necessary, in addition to the above definition of the
chemical composition.
[0057]~ Nb:0.005-0.04%
Nb is an element contributing to the increase of the strength
by forming a fine carbonitride. Also, it is an element contributing to
the increase of the Young's modulus by suppressing the recrystalliza-
tion of worked austenite at the finish rolling step in the hot rolling to
promote the transformation from the non-recrystallized austenite to
ferrite. In order to obtain such actions, the Nb content is preferable
to be not less than 0.005%. On the other hand, when the Nb content
exceeds 0.04%, the rolling load considerably increases in the hot
rolling and cold rolling and the difficulty is accompanied in the
production, so that the Nb content is preferably not more than 0.04%,
more preferably not more than 0.01%.
[0058] V:0.01-0.20%
V is an element contributing to the increase of the strength
by forming a fine carbonitride. Since it has such an action, the V
content is preferable to be not less than 0.01%. On the other hand,
when the V content exceeds 0.20%, the effect of increasing the
strength by the amount exceeding 0.20% is small and the increase of
the alloy cost is caused.
Therefore, the V content is preferable to be 0.01-0.20%.
[0059] In the invention, when Nb and/or V are included in addition
to Ti, the contents of C, N, S, Ti, Nb and V are required to satisfy the
05845HH (15/34)
CA 02546009 2006-05-12
-16-
relationship of the following equation (3) instead of the equation (2):
0.01 s C-(12147.9)xTi*-(12/92.9)xNb-(12/50.9)xV s 0.05 ~ ~ ~ (3)
[0060] Nb and V form the carbide to decrease the C content not
fixed as the carbide. Therefore, in order to render the C content not
fixed as the carbide into 0.01-0.05%, when Nb and/or V are added, the
value of C-(12/47.9)xTi*-(12/92.9)xNb-(12/50.9)xV is required to be
0.01-0.05 %.
[0061] Cr:0.1-1.0%
Cr is an element enhancing the hardenability by suppressing
the formation of cementite and can largely contribute to the increase of
the strength by largely promoting the formation of the low-temperature
transformation phase in the cooling stage after the soaking at the
annealing step. Further, the recrystallization of worked austenite is
suppressed in the hot rolling step to promote the transformation from
non-recrystallized austenite to ferrite and develop {113}<110>, and
the Young's modulus can be increased at the subsequent cold rolling
and annealing steps. In order to obtain such an effect, Cr is
preferable to be included in an amount of not less than 0.1%. On the
other hand, when the Cr content exceeds 1.0%, the above effect is
saturated and the alloy cost increases, so that Cr is preferable to be
included in an amount of not more than 1.0%. Moreover, when the
thin steel sheet of the invention is used as a galvanized steel sheet, the
oxide of Cr produced on the surface induces the non-plating, so that Cr
is preferable to be included in an amount of not more than 0.5%.
[0062] Ni:0.1-1.0%
Ni is an element stabilizing austenite to enhance the harden-
ability, and can largely contribute to the increase of the strength by
largely promoting the formation of the low-temperature transformation
phase in the cooling stage after the soaking at the annealing step.
Further, Ni as an austenite stabilizing element lowers Acl transforma-
tion point in the temperature rising stage at the annealing step after the
cold rolling to promote the transformation from the non-recrystallized
ferrite to austenite, and develops the orientation useful for the increase
05845HH (16/34)
CA 02546009 2006-05-12
-17-
of the Young's modulus with respect to the orientation of the low-
temperature transformation phase produced in the cooling stage after
the soaking, whereby the lowering of the Young's modulus accompanied
with the formation of the low-temperature transformation phase can be
suppressed. Moreover, Ni suppresses the recrystallization of worked
austenite in the hot rolling to promote the transformation from the
non-recrystallized austenite to ferrite to thereby develop {113}<110>,
whereby the Young's modulus can be increased at the subsequent cold
rolling and annealing steps. In case of the steel added with Cu, the
surface defect is induced by cracking accompanied with the lowering
of the hot ductility in the hot rolling, but the occurrence of the surface
defect can be controlled by composite addition of Ni. In order to
obtain such an action, Ni is preferable to be included in an amount of
not less than 0.1%.
[0063] On the other hand, when the Ni content exceeds 1.0%, Ac3
transformation point is extremely lowered in the temperature rising
stage at the annealing step after the cold rolling and the recrystalliza-
tion of ferrite phase at the two-phase region is difficult, and hence it is
required to raise the temperature up to austenite single phase region
above Ac3 transformation point. As a result, ferrite of orientation
obtained by the recrystallization of worked ferrite and useful for the
increase of the Young's modulus can not be developed to bring about
the decrease of the Young's modulus. And also, the alloy cost
increases. Therefore, Ni is preferable to be included in an amount of
not more than 1.0%.
[0064] Mo:0.1-1.0%
Mo is an element enhancing the hardenability by making
small the mobility of the interface, and can largely contribute to the
increase of the strength by largely promoting the formation of the low-
temperature transformation phase in the cooling stage at the annealing
step after the cold rolling. Further, the recrystallization of worked
austenite can be suppressed, and the transformation from the non-
recrystallized austenite to ferrite is promoted to develop f 113}<110>
05845HH (17134)
CA 02546009 2006-05-12
-18-
and the Young's modulus can be increased at the subsequent cold
rolling and annealing steps. In order to obtain such an action, Mo is
preferable to be included in an amount of not less than 0.1%. On the
other hand, when the Mo content exceeds 1.0%, the above effect is
saturated and the alloy cost increases, so that Mo is preferable to be
included in an amount of not more than 1.0%.
[0065] B:0.0005-0.0030%
B is an element suppressing the transformation from austenite
phase to ferrite phase to enhance the hardenability, and can largely
contribute to the increase of the strength by largely promoting the
formation of the low-temperature transformation phase in the cooling
stage at the annealing step after the cold rolling. Further, the
recrystallization of worked austenite can be suppressed, and the
transformation from the non-recrystallized austenite to ferrite is
promoted to develop ~113~<110> and the Young's modulus can be
increased at the subsequent cold rolling and annealing steps. In order
to obtain such an effect, B is preferable to be included in an amount of
not less than 0.0005%. On the other hand, when the B content
exceeds 0.0030%, the deformation resistance in the hot rolling is
enhanced to increase the rolling load and the difficulty is accompanied
in the production, so that B is preferable to be included in an amount
of not more than 0.0030%.
[0066] Cu:0.1-2.0%
Cu is an element enhancing the hardenability, and can
largely contribute to the increase of the strength by largely promoting
the formation of the low-temperature transformation phase in the
cooling stage at the annealing step after the cold rolling. In order to
obtain such an effect, Cu is preferable to be included in an amount of
not less than 0.1%. On the other hand, when the Cu content exceeds
2.0%, the hot ductility is lowered and the surface defect accompanied
with the cracking in the hot rolling is induced and the hardening effect
by Cu is saturated, so that Cu is preferable to be included in an amount
of not more than 2.0%.
05845HH (18/34)
CA 02546009 2006-05-12
-19-
[0067) The reason on the limitation of the texture according to the
invention will be described below.
In the thin steel sheet of the invention, it is required to have
a texture comprising a ferrite phase as a main phase and having a
martensite phase at an area ratio of not less than 1%.
The term "ferrite phase as a main phase" used herein means
that the area ratio of the ferrite phase is not less than SO%.
[0068] Since the ferrite phase is less in the strain, useful for the
increase of the Young's modulus, excellent in the ductility and good in
the workability, the texture is required to be the ferrite phase as a main
phase.
Also, in order to render the tensile strength of the steel sheet
into not less than 590 MPa, it is required that the low-temperature
transformation phase as a hard phase is formed in a portion other than
the ferrite phase as a main phase or a so-called second phase to
provide a composite phase. At this moment, the feature that a hard
martensite phase among the low-temperature transformation phases is
particularly existent in the texture is advantageous because the
fraction of the second phase for obtaining the target tensile strength
level is made small and the fraction of ferrite phase is made large,
whereby the increase of the Young's modulus is attained and further
the workability can be improved. For this end, the martensite phase
is required to be not less than 1 % as an area ratio to the whole of the
texture. In order to obtain the strength of lot less than 700 MPa, the
area ratio of the martensite phase is preferable to be not less than 16%.
[0069] The texture of the steel sheet according to the invention is
preferable to be a texture comprising ferrite phase and martensite
phase, but there is no problem that phases other than the ferrite phase
and martensite phase such as bainite phase, residual austenite phase,
pearlite phase, cementite phase and the like are existent at the area
ratio of not more than 10%, preferably not more than 5%. That is, the
sum of area ratios of ferrite phase and martensite phase is preferably
not less than 90%, more preferably not less than 95%.
05845HH (19/34)
CA 02546009 2006-05-12
- 20 -
[0070] Next, the reason on the production conditions limited for
obtaining the high-stiffness high-strength thin steel sheet according to
the invention and preferable production conditions will be explained.
The composition of the starting material of steel used in the
production method of the invention is the same as the composition of
the aforementioned steel sheet, so that the description of the reason on
the limitation of the starting material of steel is omitted.
[0071] The thin steel sheet according to the invention can be
produced by successively conducting a hot rolling step of subjecting
the starting material of steel having the same composition as the
composition of the steel sheet to a hot rolling to obtain a hot rolled
sheet, a cold rolling step of subjecting the hot rolled sheet after
pickling to a cold rolling to obtain a cold rolled sheet, and an
annealing step of attaining the recrystallization and composite texture
in the cold rolled sheet.
[0072] (Hot rolling step)
Finish rolling: total rolling reduction below 950°C is not less
than 30%, and the rolling is terminated at 800-900°C.
In the final rolling at the hot rolling step, the rolling is
conducted at a lower temperature to develop a non-recrystallized
austenite texture having a crystal orientation of {112}<111>, and the
{112}<111> non-re crystallized austenite can be transformed to ferrite
in the subsequent cooling stage to develop ferrite orientation of
{113}<110>. This orientation advantageously acts to the improve-
ment of the Young's modulus in the formation of the texture at the
subsequent cold rolling and annealing steps. In order to obtain such
an action, it is required that the total rolling reduction below 950°C
(total rolling reduction) is not less than 30% and further the finish
rolling is terminated below 900°C. On the other hand, when the final
temperature of the finish rolling is lower than 800°C, the rolling load
considerably increases due to the increase of the deformation
resistance and the difficulty is accompanied in the production.
Therefore, the final temperature of the finish rolling is required to be
05845HH (20/34)
CA 02546009 2006-05-12
-21-
not lower than 800°C.
[0073] Coiling temperature: not higher than 650°C
When the coiling temperature after the finish rolling exceeds
650°C, the carbonitride of Ti is coarsened and the effect of
suppressing
the recrystallization of ferrite becomes small in the temperature rising
stage at the annealing step after the cold rolling and it is difficult to
transform the non-recrystallized ferrite into austenite. As a result,
the orientation of the low-temperature transformation phase
transformed in the cooling stage after the soaking can not be
controlled, and the Young's modulus is largely lowered by the low-
temperature transformation phase having such a strain. Therefore,
the coiling temperature after the finish rolling is required to be not
higher than 650°C. Moreover, when the coiling temperature is too
low, a great amount of the hard low-temperature transformation phase
is produced and the load in the subsequent cold rolling is increased to
cause the difficulty in the production, so that it is preferable to be not
lower than 400°C.
[0074] (Cold rolling step)
Cold rolling is carried out at a rolling reduction of not less
than 50% after the pickling.
After the hot rolling step, the pickling is carried out for
removing scale formed on the surface of the steel sheet. The pickling
may be conducted according to the usual manner. Thereafter, the cold
rolling is conducted. By the cold rolling at a rolling reduction of not
less than 50% can be turned the orientation of {113}<110> developed
on the hot rolled steel sheet to an orientation of {112}<110> effective
for the increase of the Young's modulus. Thus, as the orientation of
{112}<110> is developed by the cold rolling, the orientation of
{112}<110> in ferrite is enhanced in the texture after the subsequent
annealing step and further the orientation of {112}<110> is developed
in the low-temperature transformation phase, whereby the Young's
modulus can be increased. In order to obtain such an effect, the
rolling reduction in the cold rolling is required to be not less than 50%.
05845HH (21/34)
CA 02546009 2006-05-12
-22-
[0075) (Annealing step)
Temperature rising rate from 500°C to soaking temperature:
1-30°C/s, Soaking temperature: 780-900°C
The temperature rising rate at the annealing step is an
important process condition in the invention. In the course of raising
the temperature to a soaking temperature of two-phase region or a
soaking temperature of 780-900°C at the annealing step, the recrystal-
lization of ferrite having an orientation of f 112}<110> is promoted,
while a part of ferrite grains having an orientation of f 112}<110> is
arrived to a two-phase region at a non-recrystallized state, whereby the
transformation from the non-recrystallized ferrite having an
orientation of {112}<110> can be promoted. Therefore, the Young's
modulus can be increased by promoting the growth of ferrite grains
having an orientation of {112}<110> when austenite is transformed
into ferrite in the cooling after the soaking. Further, when the
strength is increased by producing the low-temperature transformation
phase, austenite phase transformed from ferrite having an orientation
of f 112}<110> is re-transformed in the cooling, so that f 112}<110>
can be also developed with respect to the crystal orientation of the
low-temperature transformation phase. By developing {112}<110> of
ferrite phase is increased the Young's modulus, while {112}<110> is
particularly developed in the orientation of the low-temperature
transformation phase largely influencing the lowering of the Young's
modulus, whereby the lowering of the Young's modulus accompanied
with the formation of the low-temperature transformation phase can be
suppressed while forming the low-temperature transformation phase.
When austenite is transformed from the non-recrystallized ferrite
while promoting the recrystallization of ferrite in the temperature
rising stage, an average temperature rising rate largely exerting on the
recrystallization behavior from 500°C to 780-900°C as a soaking
temperature is required to be 1-30°C/s. Also, the reason why the
soaking temperature is 780-900°C is due to the fact that when it is
lower than 780°C, the non-recrystallized texture remains, while when
05845HH (22/34)
CA 02546009 2006-05-12
-23-
it exceeds 900°C, the amount of austenite produced becomes large and
it is difficult to develop ferrite having an orientation of {112}<110>
useful for the increase of the Young's modulus.
Moreover, the soaking time is not particularly limited, but it
is preferable to be not less than 30 seconds for forming austenite,
while it is preferable to be not more than about 300 seconds because
the production efficiency is deteriorated as the time is too long.
[0076] Cooling rate to 500°C after soaking: not less than 5°C/s
In the cooling stage after the soaking, it is required to form
the low-temperature transformation phase containing martensite for
increasing the strength. Therefore, an average cooling rate to 500°C
after the soaking is required to be not less than 5°C/s.
[0077] In the invention, steel having a chemical composition in
accordance with the target strength level is first melted. As the
melting method can be properly applied a usual converter process, an
electric furnace process and the like. The molten steel is cast into a
slab, which is subjected to a hot rolling as it is or after the cooling and
heating. After the finish rolling under the aforementioned finish
conditions in the hot rolling, the steel sheet is coiled at the afore-
mentioned coiling temperature and then subjected to usual pickling
and cold rolling. As to the annealing, the temperature is raised under
the aforementioned condition, and in the cooling after the soaking, the
cooling rate can be increased within a range of obtaining a target low-
temperature transformation phase. Thereafter, the cold rolled steel
sheet may be subjected to an overaging treatment, or may be passed
through a hot dip zinc in case of producing as a galvanized steel sheet,
or further in case of producing as an alloyed galvanized steel sheet, a
re-heating may be conducted up to a temperature above 500°C for the
alloying treatment.
EXAMPLES
[0078] The following examples are given in illustration of the
invention and are not intended as limitations thereof.
At first, a steel A having a chemical composition shown in
05845HH (23/34)
CA 02546009 2006-05-12
-24-
Table 1 is melted in a vacuum melting furnace of a laboratory and
cooled to room temperature to prepare a steel ingot (steel raw
material).
[0079] Table 1
KindChemical
composition
of Remarks
steelC Si Mn P S A1 N Ti Ti* SC
A 0.06 0.2 2.50.02 0.0010.030.0020.120.11 0 Acceptable
03
. example
(Note) Ti* = Ti-(47.9/14)xN-(47.9/32.1)xS
SC = C-(12/47.9)xTi*
[0080] Thereafter, the hot rolling, pickling, cold rolling and
annealing are successively conducted in the laboratory. The basic
production conditions are as follows. After the steel ingot is heated
at 1250°C for 1 hour, the hot rolling is conducted under conditions
that
the total rolling reduction below 950°C is 40% and the final rolling
temperature (corresponding to a final temperature of finish rolling) is
860°C to obtain a hot rolled sheet having a thickness of 4.0 mm.
Thereafter, the coiling condition (corresponding to a coiling temper-
ature of 600°C) is simulated by leaving the hot rolled sheet up to
600°C and keeping in a furnace of 600°C for 1 hour and then
cooling
in the furnace. The thus obtained hot rolled sheet is pickled and
cold-rolled at a rolling reduction of 60% to a thickness of 1.6 mm.
Then, the temperature of the cold rolled sheet is raised at 10°C/s
on
average up to 500°C and further from 500°C to a soaking
temperature
of 820°C at 5°C/s on average. Next, the soaking is carried out
at
820°C for 180 seconds, and thereafter the cooling is carried out at an
average cooling rate of 10°C/s up to 500°C, and further the
temperature
of 500°C is kept for 80 seconds, and then the sheet is cooled in air.
[0081] In this experiment, the following conditions are further
individually changed under the above production conditions as a basic
condition. That is, the experiment is carried out under the basic
condition except for the individual changed conditions that the total
rolling reduction below 950°C is 20-60% and the final temperature of
05845HH (24/34)
CA 02546009 2006-05-12
-25-
the hot finish rolling is 800-920°C and the coiling temperature is S00-
670°C and the rolling reduction of the cold rolling is 40-75% and the
average temperature rising rate from 500°C to the soaking temperature
(820°C) in the annealing is 0.5-35°C/s.
[0082] From the sample after the annealing is cut out a test specimen
of 10 mm x 120 mm in a direction perpendicular to the rolling
direction as a longitudinal direction, which is finished to a thickness
of 0.8 mm by a mechanical polishing and a chemical polishing for
removing strain, and thereafter a resonance frequency of the sample is
measured by using a lateral vibration type internal friction measuring
device to calculate a Young's modulus therefrom. With respect to the
sheet subjected to a temper rolling of 0.5%, a tensile test specimen of
JIS No. S is cut out in the direction perpendicular to the rolling
direction and subj ected to a tensile test. Further, the sectional texture
is observed by a scanning type electron microscope (SEM) after the
corrosion with Nital to judge the kind of the texture, while three
photographs are shot at a visual region of 30 pm x 30 um and then
area ratios of ferrite phase and martensite phase are measured by an
image processing to determine an average value of each phase as an
area ratio (fraction) of each phase.
[0083] As a result, the values of the mechanical characteristics
under the basic condition in the experiment according to the
production method of the invention are Young's modulus E: 242 GPa,
TS: 780 MPa, El: 23%, fraction of ferrite phase: 67% and fraction of
martensite phase: 28%, from which it is clear that the thin steel sheet
has an excellent balance of strength-ductility and a high Young's
modulus.
Moreover, the remainder of the texture other than ferrite
phase and martensite phase is either of bainite phase, residual
austenite phase, pearlite phase and cementite phase.
[0084] Then, the relationship between the production conditions and
Young's modulus is explained based on the above test results with
reference to the drawings. Even in any experimental conditions, the
05845HH (25/34)
CA 02546009 2006-05-12
-26-
tensile strength is 730-820 MPa, and the fraction of ferrite phase is
55-80%, the fraction of martensite phase is 17-38%, and the remainder
of the texture is either of bainite phase, residual austenite phase,
pearlite phase and cementite phase.
[0085] In FIG. 1 is shown influences of the total rolling reduction
below 950°C upon Young's modulus, respectively. When the total
rolling reduction is not less than 30% being the acceptable range of the
invention, the Young's modulus indicates an excellent value of not less
than 230 GPa.
[0086] In FIG. 2 is shown an influence of the final temperature of
the hot finish rolling upon the Young's modulus. When the final
temperature is not higher than 900°C being the acceptable range of the
invention, the Young's modulus indicates an excellent value of not less
than 230 GPa.
[0087] In FIG. 3 is shown an influence of the coiling temperature
upon the Young's modulus. When the coiling temperature is not
higher than 650°C being the acceptable range of the invention, the
Young's modulus indicates an excellent value of not less than 230 GPa.
[0088] In FIG. 4 is shown an influence of the rolling reduction of the
cold rolling upon the Young's modulus. When the rolling reduction is
not less than 50% being the acceptable range of the invention, the
Young's modulus indicates an excellent value of not less than 230 GPa.
[0089] In FIG. 5 is shown an influence of the average temperature
rising rate from 500°C to the soaking temperature of 820°C in
the
annealing upon the Young's modulus. When the temperature rising
rate is 1-30°C/s being the acceptable range of the invention, the
Young's modulus indicates an excellent value of not less than 230 GPa.
[0090] Furthermore, steels B-Z and AA-AI having a chemical
composition as shown in Table 2 are melted in a vacuum melting
furnace of a laboratory and cooled to room temperature to prepare a
steel ingot (steel raw material). Thereafter, it is successively
subjected to the hot rolling, pickling, cold rolling and annealing under
conditions shown in Table 3, respectively. After the steel ingot is
05845HH (26/34)
CA 02546009 2006-05-12
~ - '7'7
heated at 1250°C for 1 hour, the hot rolling is carried out at various
rolling temperatures to obtain a hot rolled sheet having a thickness of
4.0 mm. Then, the coiling condition after a target coiling temper-
ature is simulated by keeping in a furnace of the coiling temperature
for 1 hour and then cooling in the furnace. The hot rolled sheet is
pickled, cold-rolled at various rolling reductions to a thickness of
0.8-1.6 mm, and the temperature is raised up to 500°C at 10°C on
average and further up to a target soaking temperature at an average
temperature rising rate shown in Table 3. After the soaking is carried
out at the soaking temperature for 180 seconds, the cooling is carried
out at various average cooling rates shown in Table 3, and the sheet is
kept at 500°C for 80 seconds and then cooled to room temperature in
air.
In Table 4 are shown characteristics obtained by the
aforementioned tests. At this moment, the residual texture other than
ferrite phase and martensite phase in the tables is either of bainite
phase, residual austenite phase, pearlite phase and cementite phase.
05845HH (27/34)
CA 02546009 2006-05-12
-28-
[0091] Table 2
KofdChemical
composition
(mass%)
Ti SC Remarks
Steel Si Mn P S AI N Ti other components
C
B 0.020.202.50.020.001 0.0030.06- 0 0 c
0.02 05 01 e
. . St
el
C 0.040.013.00.010.0020.030.0020.03- 0 0 Accepta
02 03 a
. . Steel
D 0.010.202.50.020.0010.030.0030.05- 0 0 om arative
04 00 S
. . teel
E 0.110.202.50.020.0020.040.0040.15- 0 0 Coup aratme
13 08
. . Steel
F 0.070.202.50.020.0020.040.0040.05- 0 0 Com aranve
03 06 S
. . teel
G 0.060.502.00.030.0010.050.0050.10- 0 0 Accepta
08 04 a
. . Steel
H 0.061.503.50.030.0010.050.0050.15- 0 0 ccepta
13 03 a
. . Steel
I 0.060.201.50.030.0010.050.0050.15- 0 0 c
13 03 e
a
. . St
el
J 0.060.201.40.030.0010.050.0020.15- 0 0 Accepta
14 02 a
. . Steel
K 0.060.303.60.030.0010.050.0010.12- 0 0 Coms
12 03 eltlve
. . te
L 0.050.202.50.030.0020.100.0020.10- 0 0 c
09 03 e
a
. . St
el
M 0.040.302.00.010.0030.500.0030.08- 0 0 Accepta
07 02 a
. . Steel
N 0.040.503.00.010.0011.500.0010.09- 0 0 c
09 02 e
a
. . St
el
O 0.050.102.50.010.0010.030.0060.03- 0 0 c
01 05 e
a
. . St
el
P 0.050.102.50.010.0010.030.0010.02- 0 0 c
02 05 e
a
. . St
el
Q 0.060.202.50.020.0010.030.0020.05N6:0.03 0 0 cce to
04 05 a
~t
H
e
. . e1
St
R 0.060.202.50.020.0010.030.0020.05Nb:0.03 0 0 Accepts
V:0 04 02 a
10
, . . Steel
.
S 0.040.013.00.020.0020.020.0020.08Cr:0.3 0 0 c
07 02 e
a
. . St
el
T 0.080.032.00.010.0020.030.0030.15Ni:0.2 0 0
14 05 p ~t
Acce to
a
. . Steel
U 0.050.201.50.020.0010.030.0030.12Mo:0.2 0 0 ccepta
11 02 a
. . Steel
V 0.040.302.80.030.0010.020.0030.08Cu:0.3 0 0 ccepta
07 02 a
. . Steel
W 0.060.202.50.020.0010.030.0020.13B:0.0010 0 0
12 03 w
Acce to
a
e
. . St
el
X 0.050.202.50.010.0010.020.0030.10Nb:0.03 0 0 ccepta
Mo:0 09 02 a
15
, . . Steel
.
Y 0.060.302.40.010.0020.020.0010.05Cr:0.2 0 0 cce to
Ni:0 04 05 a
2 ~l
e
H
, . . e1
. St
Z 0.050.202 0 0 0 0 0 Nb:0.04, Mo:0.15, Acceptable
4 02 001 02 002 04
. . . . . . B:0.0010 0,030.04Steel
Nb:0.02, V:0.05,
AA 0.070.202.90.020.0010.020.0030.05Cr:0.l, Ni:0.02,0 0 Acceptable
04 05
Mo:0.2, Cu:0.3,, . Steel
B:0.0015
AB 0.110.011.70.010.0010.020.0010.23- 0 0 c
23 05 e
a
. . St
el
AC 0.140.201.80.010.0010.030.0010.40- 0 0 c
40 04 e
a
. . St
el
AD 0.160.012.00.020.0010.030.0010.07- 0 0 om arative
07 14 S
. . teel
AE 0.050.021.l0.010.0010.030.0010.08- 0 0 c
08 03 e
a
. . St
el
AF 0.050.030.90.01.0010.02.0010.05- 0 0 om aratme
0 0 05 04 S
. . teel
AG 0.060.032Ø01.0010.04.0010.03Nb:0.01 0 05 cce to
0 0 0 03 a
0 e
. . St
el
AH .07.201.8.01.001.03.001.07Nb:0.01 0 05 cce to
0 0 0 0 0 0 0 07 a
0 e
. . St
el
AI .05.012Ø02.001.03.002.01- 0 00 05 om araUve
0 0 0 0 0 0 0 0 S
. . teel
(Note) Ti* = Ti - (47.9/14) x N - (47.9/32.1) x S
SC = C - (12/47.9) x Ti*- (12/92.9) x Nb - (12/50.9) x V
05845HH (28/34)
CA 02546009 2006-05-12
-29-
rnna~7 TahlP 3
Cold
Hot rollingAnnealing
roiling onditionconditions
conditions
c
Kind Total Final
of rollinem oiling Temper- oakingoolingRemarks
steelt er- olling S C ate
re g C emper-a ttire emper-u~
ductionp re ductionrising r
a ttire attire t 500
below of (C) (%) rake attireC
C r t from to (C/s)
95 finish 500C (C)
ol (C/s)
Cng
B ~a 550 65 10 840 15 rivention
50 830 Exam le
C 45 840 500 70 15 gp0 2p Invention
Exam le
D 50 850 530 70 8 800 25 Examajeve
E 45 870 600 60 10 810 20 omParative
Exam le
F 50 850 550 65 10 820 10 omparative
Exam 1e
G 35 880 650 70 10 gpp 2p Invention
Exam le
nvention
H 45 860 540 75 10 860 15 Exam le
Invention
I 50 830 550 70 10 g7p 30 Exam le
50 830 550 70 10 870 30 Invention
g Exam le
55 800 500 60 12 810 15 omparative
Exam le
Invention
L 40 870 55p 70 10 870 20 Exam le
Invention
M 45 880 540 75 30 870 25 Exam 1e
Invention
N 45 890 550 70 15 880 20 Exam le
5p 830 550 65 10 g2p 15 Invention
Exam le
Invention
P 50 820 500 75 10 gap 1p Exam le
4p 850 550 60 10 820 15 Invention
Exam le
I
i
R 40 850 550 60 10 820 15 le
S Sp 840 570 80 25 840 30 Exam
Invention
Exam le
Invention
T 3p 870 600 60 10 g35 12 Exam le
LT 40 850 580 65 15 84p 10 Invention
Exam le
V 45 845 550 65 15 820 17 Invention
Exam le
Invention
W 35 860 600 60 15 gap 10 Exam le
X 30 840 550 65 10 g6p 15 Invention
Exam le
y 40 850 570 65 15 840 10 Invention
Exam le
4p 860 600 60 10 840 15 Inventoon
Exam le
A 30 870 630 60 10 845 13 Invention
Exam le
Invention
Ag 35 850 650 60 10 830 1p Exam le
Invention
AC 45 870 630 55 10 g4p 10 Exam le
AD 40 860 600 60 10 860 15 omparative
Exam le
AE 40 840 600 65 15 g60 1p Invention
Exam le
AF 30 820 600 60 10 850 10 omparative
Exam le
AG 35 840 550 50 10 810 10 Invention
Exam le
AH 35 860 580 50 10 830 15 invention
Exam le
AI 40 870 600 60 10 840 10 omparative
Exam le
OS$45HH (29/34)
CA 02546009 2006-05-12
-30-
[0093] Table 4
Steel texture Mechanical
Properties
Kind Fraction Fraction Remarks
of of of ferriteTS El E
steel martensitephase (MPa) (%) (GPa)
phase (%)
(%)
B 5 93 610 30 251 Invention Example
C 11 87 680 25 245 Invention Example
D 0 100 540 33 252 Comparative Example
E 70 30 1200 11 218 Comparative Example
F 45 55 1030 15 222 Comparative Example
G 35 61 830 20 243 Invention Example
H 25 73 8S0 18 245 Invention Example
I 20 80 760 24 243 Invention Example
J 15 85 740 25 235 Invention Example
K 40 60 850 18 225 Comparative Example
L 25 70 760 22 243 Invention Example
M 22 72 700 20 245 Invention Example
N 20 75 750 21 245 Invention Example
O 30 65 800 21 234 Invention Example
P 35 60 810 20 233 Invention Example
Q 35 65 820 20 245 Invention Example
R 25 72 780 22 247 Invention Example
S 25 75 760 23 245 Invention Example
T 35 65 890 18 242 Invention Example
U 26 71 790 23 243 Invention Example
V 19 81 750 25 245 Invention Example
W 30 70 900 17 243 Invention Example
X 30 68 890 17 248 Invention Example
Y 40 60 920 16 243 Invention Example
Z 35 65 980 15 245 Invention Example
AA 45 55 1030 14 243 Invention Example
AB 30 65 920 16 235 Invention Example
AC 25 70 940 15 231 Invention Example
AD 70 25 1100 11 210 Comparative Example
AE 5 90 630 30 230 Invention Example
AF 2 96 570 33 223 Comparative Example
AG 20 80 780 19 233 Invention Example
AH 25 70 780 20 238 Invention Example
AI 20 75 750 20 208 Comparative Example
05845HH (30/34)
CA 02546009 2006-05-12
-31-
[0094] In the steel D, the C content is as small as 0.01%, and the
fraction of martensite is 0%, and TS is smaller than the acceptable
range of the invention. In the steel E, the C content not fixed as the
carbide (SC) is as high as 0.08% and the fraction of ferrite phase is as
small as 30%, and the Young's modulus is smaller than the acceptable
range of the invention. In the steel F, SC is as high as 0.06%, and the
Young's modulus is smaller than the acceptable range of the invention.
In the steel K, the Mn content is as high as 3.6%, and the Young's
modulus is smaller than the acceptable range of the invention. In the
steel AD, the C content is as high as 0.16% and SC is as high as 0.14%
and the fraction of ferrite phase is as small as 25%, and the Young's
modulus is smaller than the acceptable range of the invention. In the
steel AF, the Mn content is as low as 0.9%, and TS and the Young's
modulus are smaller than the acceptable range of the invention.
In the steel AI, the Ti content is as low as 0.01% and Ti* is as small as
0.00%, and the Young's modulus is smaller than the acceptable range
of the invention.
With respect to the other steels, all items are within the
acceptable range of the invention, and TS and Young's modulus satisfy
the acceptable range of the invention.
INDUSTRIAL APPLICABILITY
[0095] According to the invention, it is possible to provide high-
stiffness high-strength thin steel sheets having a tensile strength of not
less than 590 MPa and a Young's modulus of not less than 230 GPa.
05845HH (31/34)
