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NSC-K714
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DESCRIPTION
THIN STEEL SHEET FOR AUTOMOBILE USE
EXCELLENT IN NOTCH-FATIGUE STRENGTH
AND METHOD FOR PRODUCTNG THE SAME
Technical Field
The present invention relates to a thin steel sheet
for automobile use excellent in notch-fatigue strength,
and a method for producing the steel sheet, and, more
specifically, to a thin steel sheet for automobile use
excellent in notch-fatigue strength and suitable as the
material for undercarriage components of an automobile
and the like to overcome the problem of the propagation
of a fatigue crack from a site of stress concentration
such as a blanked or welded portion, and a method for
producing the steel sheet.
Background Art
The application of light metals such as aluminum
alloys and high-strength steel sheets to automobile
members has expanded recently for the purposes of
reducing automobile weight and thereby reducing the fuel
consumption and the like. However, while light metals
such as aluminum alloys have an advantage of high
specific strength, their application is limited to
special uses because they are far more costly than steel.
For further reducing the automobile weight, therefore, a
wider application of low-cost high-strength steel sheets
is required.
In response to the requirement for such high-
strength steel materials, in the field of cold-rolled
steel sheets used for a white body and panels which
account for about one fourth of the weight of an
automobile, a steel sheet having both high strength and
deep drawability, a steel sheet having bake-hardenability
and the like, have so far been developed and have
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contributed to the weight reduction of an automobile
body. However, the focus of the efforts for reducing the
weight of an automobile has shifted lately to structural
and undercarriage members which account for roughly 20%
of the automobile body weight. As a consequence, the
development of a high-strength steel sheet applicable to
those members has come to be required as a matter of
urgency.
However, as the strengthening of a steel material
usually leads to the deterioration of formability
{workability) and so on, a key issue in the development
of a high-strength steel sheet for those applications is
how to realize a high strength without sacrificing those
material properties. The important properties required
especially of a steel sheet for the structural and
undercarriage members of an automobile include shearing
and blanking workability, burring workability, fatigue
resistance, corrosion resistance and so forth, not to
mention elongation; it is essential to balance a high
strength with these properties at high levels.
For instance, an undercarriage component such as a
suspension arm is produced through the processes of
blanking and boring by shearing and punching, thereafter
press forming and, in some cases, welding. It is often
the case with such a component that a crack propagates
from a point near a sheared end face or a weld and causes
fatigue fracture. In other words, a sheared end face or
a weld acts as a stress concentration site like a notch
and a fatigue crack propagates therefrom.
Meanwhile, in general, the fatigue limit of a
material is lowered as a notch becomes acute. when the
acuteness of a notch surpasses a certain extent, however,
a fatigue limit does not lower any further. This is
because a fatigue limit shifts from being dominated by a
crack initiation limit toward being dominated by a crack
propagation limit as the acuteness of a notch increases.
when the strength of a material increases, while a crack
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initiation limit increases, a crack propagation limit
does not, and therefore the acuteness of a notch, at
which a fatigue limit shifts from being dominated by a
crack initiation limit toward being dominated by a crack
propagation limit, moves toward an acuter side. As a
result, when a material has an acute notch, even if the
strength of the material is increased, the decrease in
the fatigue limit resulting from the acuteness of the
notch becomes significant and thus the advantages of the
high strength are not secured. In other words, when the
strength of a material is increased, the sensitivity
thereof to a notch increases.
Thin steel sheets having strength of the 340 to 440
MPa class are presently used for the undercarriage
members of an automobile. However, the level of strength
required of the steel sheets for those members is rising
toward the 590 to 780 MPa class. Therefore, to
satisfactorily respond to such a requirement, it is
essential to develop a steel sheet with which the
advantages of high strength can be secured even when an
acute notch exists.
There are basically two methods for enhancing the
fatigue strength of a steel sheet having an end face
formed by blanking or shearing: one is to remove an acute
notch such as a burr formed at a blanking or shearing end
face, and the other is to enhance the resistance to the
propagation of a crack even when such an acute notch
exists.
There are the following methods as examples of
inventions based on the former method. Japanese
Unexamined Patent Publication No. H5-51695 discloses a
technology wherein the occurrence of a burr is suppressed
by reducing the addition amount of Si and forming
precipitates of Ti, Nb and v for lowering breaking
elongation and thereby the fatigue strength of an as-
blanked or as-sheared steel sheet is enhanced. Japanese
Unexamined Patent Publication No. H5-179346 discloses a
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technology wherein the upper limit of the volume
percentage of bainite is regulated by defining an upper
limit of a finish rolling temperature and, thereby, the
fatigue strength of an as-blanked or as-sheared steel
sheet is enhanced. Japanese Unexamined Patent
Publication No. H8-13033 discloses a technology wherein
the formation of martensite is suppressed by defining a
cooling rate after rolling and, thereby, the fatigue
strength of an as-blanked or as-sheared steel sheet is
enhanced.
Further, Japanese Unexamined Patent Publication No.
H8-302446 discloses a technology wherein strain energy
during blanking or shearing is reduced by regulating the
hardness of the second phase of a dual phase steel to at
least 1.3 times that of ferrite and, thereby, the fatigue
strength of an as-blanked or as-sheared steel sheet is
enhanced. Japanese Unexamined Patent Publication No. H9-
170048 discloses a technology wherein the occurrence of a
burr during blanking or shearing is suppressed by
regulating the length of intergranular cementite and
thereby the fatigue strength of an as-blanked or as-
sheared steel sheet is enhanced. Furthermore, Japanese
Unexamined Patent Publication No. H9-202940 discloses a
technology wherein blanking performance is improved by
regulating a parameter based on the addition amounts of
Ti, Nb and Cr and thereby the fatigue strength of an as-
blanked steel sheet is enhanced.
Meanwhile, there are the following methods as the
examples of the inventions based on the latter method.
Japanese Unexamined Patent Publication No. H6-88161
discloses a technology wherein the X-ray diffraction
strength ratio of a (100) plane parallel to the rolling
surfaces in the texture at a steel sheet surface layer is
regulated to 1.5 or more and, thereby, a fatigue crack
propagation speed is lowered. Further, Japanese
Unexamined Patent Publications No. H8-199286 and No. H10-
147846 disclose technologies wherein the area percentage
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of recovered or recrystallized ferrite is controlled in
the range from 15 to 40$ by regulating the X-ray
diffraction strength ratio of a (200) plane in the
thickness direction in the range from 2.0 to 15.0 and,
thereby, a fatigue crack propagation speed is lowered.
However, in the cases of the technologies of
suppressing an acute notch such as a burr generated at a
blanked or sheared end face as disclosed in the above
Japanese Unexamined Patent Publications No. H5-51695, No.
H5-179346, No. H8-13033, No. H8-302446, No. H9-170048,
No. H9-202940 and so forth, as the degree of a generated
burr largely varies with the clearance of tools at
blanking or shearing, the technologies are not ones that
can be employed under any conditions. Therefore, it must
be said that the technologies are insufficient when be
applied to a steel sheet excellent in notch-fatigue
strength.
On the other hand, technologies of enhancing the
resistance to crack propagation by controlling the
texture of a steel sheet as disclosed in the above
Japanese Unexamined Patent Publications No. H6-88161, No.
H8-199286 and No. H10-147846 are the inventions mainly
intended for steels used for large structures such as
construction machines, ships and bridges and are not
intended for a thin steel sheet, used for automobiles,
for which the present invention is intended.
In addition, the aforementioned technologies are
ones wherein a fatigue crack propagation speed is
controlled in a PARIS zone that is referred to in the
fracture mechanics of a fatigue crack mainly propagating
from a weld toe portion and therefore are insufficient as
technologies to be employed in such a case as a thin
steel sheet, for automobile use, where a crack
propagation zone is not included in the PARIS zone
because of the thickness of the steel sheet.
Besides the above, no invention has been proposed up
to now wherein notch-fatigue properties are evaluated
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using a test piece, as shown in Fig. 1(b), in a plane
bending fatigue test method applied to a thin steel
sheet.
Disclosure of the Invention
In view of the above situation, the present
invention relates to a technology wherein the notch-
fatigue strength of a thin steel sheet for automobile use
is improved by controlling the texture of the steel sheet
and thus enhancing the resistance to a fatigue crack
propagating from a notch such as an end face formed after
blanking or shearing, regardless of the conditions such
as the clearance of tools during blanking or shearing.
In other words, the object of the present invention is to
provide a thin steel sheet for automobile use excellent
in notch-fatigue strength and a method for producing the
steel sheet economically and stably.
The present inventors, in consideration of the
production processes of thin steel sheets presently
produced on an industrial scale using generally employed
production facilities, earnestly studied methods for
enhancing the notch-fatigue strength of a thin steel
sheet for automobile use. As a result, the present
invention has been established on the basis of a new
discovery that the following conditions are very
effective for enhancing notch-fatigue strength: that, on
a plane at an arbitrary depth within 0.5 mm from the
surface of a steel sheet in the thickness direction
thereof, the average of the ratios of the X-ray
diffraction strength in the orientation component group
of {100}<011> to X223}<110> to random X-ray diffraction
strength is 2 or more and the average of the ratios of
the X-ray diffraction strength in the three orientation
components of X554}<225>, X111}<112> and (111}<110> to
random X-ray diffraction strength is 4 or less; and that
the thickness of the steel sheet is in the range from 0.5
to 12 mm.
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The gist of the present invention, therefore, is as
follows:
(1) A thin steel sheet for automobile use excellent
in notch-fatigue strength, characterized in: that, on a
plane at an arbitrary depth within 0.5 mm from the
surface of the steel sheet in the thickness direction
thereof, the average of the ratios of the X-ray
diffraction strength in the orientation component group
of {100}<O11> to {223}<110> to random X-ray diffraction
strength is 2 or more and the average of the ratios of
the X-ray diffraction strength in the three orientation
components of {554}<225>, X111}<112> and X111}<110> to
random X-ray diffraction strength is 4 or less; and that
the thickness of the steel sheet is in the range from 0.5
to 12 mm.
(2) A thin steel sheet for automobile use excellent
in notch-fatigue strength according to the item (1),
characterized in that the microstructure of the steel
sheet is a compound structure containing bainite or
ferrite and bainite as the phase accounting for the
largest volume percentage.
(3) A thin steel sheet for automobile use excellent
in notch-fatigue strength according to the item (1),
characterized in that the microstructure of the steel
sheet is a compound structure containing retained
austenite by 5 to 25~ in terms of volume percentage and
having the balance mainly consisting of ferrite and
bainite.
(4) A thin steel sheet for automobile use excellent
in notch-fatigue strength according to the item (1),
characterized in that the microstructure of the steel
sheet is a compound structure containing ferrite as the
phase accounting for the largest volume percentage and
martensite as the second phase.
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(5) A thin steel sheet for automobile use excellent
in notch-fatigue strength, the steel sheet containing, in
mass, 0.01 to 0.3% C, 0.01 to 2% Si, 0.05 to 3% Mn, 0.1%
or less P, 0.01% or less S and 0.005 to 1% A1, with the
balance consisting of Fe and unavoidable impurities,
characterized in: that, on a plane at an arbitrary depth
within 0.5 mm from the surface of the steel sheet in the
thickness direction thereof, the average of the ratios of
the X-ray diffraction strength in the orientation
component group of X100}<O11> to {223}<110> to random X-
ray diffraction strength is 2 or more and the average of
the ratios of the X-ray diffraction strength in the three
orientation components of {554}<225>, X111}<112> and
{111}<110> to random X-ray diffraction strength is 4 or
less; and that the thickness of the steel sheet is in the
range from 0.5 to 12 mm.
(6) A thin steel sheet for automobile use excellent
in notch-fatigue strength according to the item (5),
characterized by further containing, in mass, one or more
of 0.2 to 2% Cu, 0.0002 to 0.002% B, 0.1 to 1% Ni, 0.0005
to 0.002% Ca, 0.0005 to 0.02% REM, 0.05 to 0.5% Ti, 0.01
to 0.5% Nb, 0.05 to 1% Mo, 0.02 to 0.2% V, 0.01 to 1% Cr
and 0.02 to 0.2% Zr.
(7) A thin steel sheet for automobile use excellent
in notch-fatigue strength according to the item (5) or
(6), characterized in that the microstructure of the
steel sheet is any one of 1) a compound structure
containing bainite or ferrite and bainite as the phase
accounting for the largest volume percentage, 2) a
compound structure containing retained austenite by 5 to
25% in terms of volume percentage and having the balance
mainly consisting of ferrite and bainite, and 3) a
compound structure containing ferrite as the phase
accounting for the largest volume percentage and
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martensite as the second phase.
(8) A thin steel sheet for automobile use excellent
in notch-fatigue strength, characterized in that the
steel sheet is produced by applying galvanizing to a thin
steel sheet for automobile use according to any one of
the items (1) to (7).
(9) A method for producing a thin steel sheet for
automobile use excellent in notch-fatigue strength
characterized in: that a steel slab containing, in mass,
0.01 to 0.3$ C, 0.01 to 2$ Si, 0.05 to 3$ Mn, 0.1$ or
less P, 0.01$ or less S and 0.005 to 1$ A1, with the
balance consisting of Fe and unavoidable impurities, is
subjected, in a hot roiling process, to rough rolling and
then to finish rolling at a total reduction ratio of 25$
or more in terms of steel sheet thickness in the
temperature range of the Ar3 transformation temperature +
100°C or lower; that, on a plane at an arbitrary depth
within 0.5 mm from the surface of the steel sheet in the
thickness direction thereof, the average of the ratios of
the X-ray diffraction strength in the orientation
component group of {100}<Oll> to X223}<110> to random X-
ray diffraction strength is 2 or more and the average of
the ratios of the x-ray diffraction strength in the three
orientation components of X554}<225>, X111}<112> and
~lil}<110> to random X-ray diffraction strength is 4 or
less; and that the thickness of the steel sheet is in the
range from 0.5 to 12 mm.
(10) A method for producing a thin steel sheet for
automobile use excellent in notch-fatigue strength
according to the item (9), characterized by: cooling the
steel sheet at a cooling rate of 20°C/sec. or higher
after the finish rolling; and then coiling it at a
coiling temperature of 450°C or higher.
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(11) A method for producing a thin steel sheet for
automobile use excellent in notch-fatigue strength
according to the item (9), characterized by: retaining
the steel sheet for 1 to 20 sec. in the temperature range
from the Arl transformation temperature to the Ar3
transformation temperature after the finish rolling; then
cooling it at a cooling rate of 20°C/sec. or higher; and
coiling it at a coiling temperature in the range from
higher than 350°C to lower than 450°C.
(12) A method for producing a thin steel sheet for
automobile use excellent in notch-fatigue strength
according to the item (9), characterized by coiling the
steel sheet at a coiling temperature of 350°C or lower
after the cooling.
(13) A method for producing a thin steel sheet for
automobile use excellent in notch-fatigue strength
according to any one of the items (9) to (12),
characterized by applying lubrication rolling to the
steel sheet in the hot rolling.
(14) A method for producing a thin steel sheet for
automobile use excellent in notch-fatigue strength
according to any one of the items (9) to (13),
characterized by applying descaling to the steel sheet
after the completion of the rough rolling in the hot
rolling.
(15) A method for producing a thin steel sheet for
automobile use excellent in notch-fatigue strength,
characterized in: that a steel slab containing, in mass,
0.01 to 0.3~ C, 0.01 to 2~ Si, 0.05 to 3~ Mn, 0.1~ or
less P, 0.01 or less S and 0.005 to 1~ A1, with the
balance consisting of Fe and unavoidable impurities, is
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subjected to rough rolling, then finish rolling at a
total reduction ratio of 25~ or more in terms of steel
sheet thickness in the temperature range of the Ar3
transformation temperature + 100°C or lower, pickling,
cold rolling at a reduction ratio of less than 80$ in
terms of steel sheet thickness, and then annealing for
recovery or recrystallization comprising the processes of
retaining the cold-rolled steel sheet for 5 to 150 sec.
in the temperature range from the recovering temperature
to the Ac3 transformation temperature + 100°C and then
cooling it; that, on a plane at an arbitrary depth within
0.5 mm from the surface of the steel sheet in the
thickness direction thereof, the average of the ratios of
the X-ray diffraction strength in the orientation
component group of X100}<O11> to X223}<110> to random X-
ray diffraction strength is 2 or more and the average of
the ratios of the X-ray diffraction strength in the three
orientation components of {554}<225>, {111}<112> and
{111}<110> to random X-ray diffraction strength is 4 or
less; and that the thickness of the steel sheet is in the
range from 0.5 to 12 mm.
(1G) A method for producing a thin steel sheet for
automobile use excellent in notch-fatigue strength
according to the item (15), characterized by subjecting
the steel sheet after the cold rolling to a heat
treatment comprising the processes of retaining the cold-
rolled steel sheet for 5 to 150 sec. in the temperature
range from the Acl transformation temperature to the Ac3
transformation temperature + 100°C and then cooling it.
(17) A method for producing a thin steel sheet for
automobile use excellent in notch-fatigue strength
according to the item (15), characterized by subjecting
the steel sheet to a heat treatment comprising the
processes of, in sequence, retaining the cold-rolled
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steel sheet for 5 to 150 sec. in said temperature range,
cooling it at a cooling rate of 20°C/sec. or higher to
the temperature range from higher than 350°C to lower
than 450°C, retaining it for 5 to 600 sec. in said
temperature range, and then cooling it at a cooling rate
of 5°C/sec. or higher to the temperature range of 200°C
or lower.
(18) A method for producing a thin steel sheet for
automobile use excellent in notch-fatigue strength
according to the item (i5), characterized by subjecting
the steel sheet to a heat treatment comprising the
processes of retaining the cold-rolled steel sheet for 5
to 150 sec. in said temperature range and then cooling it
at a cooling rate of 20°C/sec, or higher to the
temperature range of 350°C or lower.
(19) A method for producing a thin steel sheet for
automobile use excellent in notch-fatigue strength,
characterized in that the steel sheet produced by the
method according to any one of the items (11) to (18)
further contains, in mass, one or more of 0.2 to 2% Cu,
0.0002 to 0.002% B, 0.1 to 1% Ni, 0.0005 to 0.002% Ca,
0.0005 to 0.02% REM, 0.05 to 0.5% Ti, 0.01 to 0.5% Nb,
0.05 to 1% Mo, 0.02 to 0.2% V, 0.01 to 1% Cr and 0.02 to
0.2% zr.
(20) A method for producing a thin steel sheet for
automobile use excellent in notch-fatigue strength
according to the item (10) or (16), characterized in that
the microstructure of the steel sheet is a compound
structure containing bainite or ferrite and bainite as
the phase accounting for the largest volume percentage.
(21) A method for producing a thin steel sheet for
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automobile use excellent in notch-fatigue strength
according to the item (11) or (17), characterized in that
the microstructure of the steel sheet is a compound
structure containing retained austenite at 5 to 25~ in
terms of volume percentage and having the balance mainly
consisting of ferrite and bainite.
(22) A method for producing a thin steel sheet for
automobile use excellent in notch-fatigue strength
according to the item (12) or (18), characterized in that
the microstructure of the steel sheet is a compound
structure containing ferrite as the phase accounting for
the largest volume percentage and martensite as the
second phase.
(23) A method for producing a thin steel sheet for
automobile use excellent in notch-fatigue strength
characterized by, after producing a hot-rolled steel
sheet or a steel sheet annealed for recovery or
recrystallization according to any one of the items (9)
to (22), further applying galvanizing to the surfaces of
the steel sheet by dipping the steel sheet in a zinc
plating both.
(24) A method for producing a thin steel sheet for
automobile use excellent in notch-fatigue strength
according to the item (23), characterized by subjecting
the steel sheet further to an alloying treatment after
the galvanizing.
Brief Description of the Drawings
Fig. 1 consists of illustrations showing the shapes
of test pieces for fatigue test: Fig. 1(a) shows an
unnotched test piece for fatigue test, and Fig. 1(b) a
notched test piece for fatigue test.
Fig. 2 is a graph showing the result of a
preliminary test that leads to the present invention in
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terms of the relationship among: the average of the
ratios of the X-ray diffraction strength in the
orientation component group of X100}<O11> to 1223}<110>
to random X-ray diffraction strength; the average of the
ratios of the X-ray diffraction strength in the three
orientation components of {554}<225>, {111}<112> and
X111}<110> to random X-ray diffraction strength; and
notch-fatigue strength (the fatigue strength for finite
life after 10' cycles of repetition, namely the fatigue
limit).
Best Mode for Carrying out the Invention
In the first place, the results of preliminary
studies that lead to the present invention are explained
hereafter.
Generally speaking, a fatigue crack of a steel sheet
starts from the surface thereof; this is true also with
the case where a stress concentration site such as a
notch exists. In the case where an end face formed by
blanking or shearing exists, it is often observed that,
under a repeated load including a loading mode in the
out-of-plane bending direction, a fatigue crack starts
and propagates from an end of a steel sheet surface. It
is clear from this that, even in such a case, it is
effective for enhancing notch-fatigue strength to
increase resistance to crack propagation at the surface
of a steel sheet or in the layer from the surface to a
depth of several crystal grains or so. On the other
hand, even though resistance to crack propagation is
increased at the thickness center of a steel sheet, it is
difficult to arrest an already formed crack. For this
reason, in the present invention, the range of a steel
sheet texture effective in enhancing fatigue strength is
limited to the range from the surface to a depth of 0.5
mm in the thickness direction. The range is, more
adequately, to a depth of 0.1 mm.
The present inventors investigated the influences of
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the average of the ratios of the X-ray diffraction
strength in the orientation component group of {100}<O11>
to {223}<110> to random X-ray diffraction strength and
the average of the ratios of the X-ray diffraction
strength in the three orientation components of
.(554}<225>, {111}<112> and {111}<110> to random X-ray
diffraction strength on a plane at an arbitrary depth in
the range from the surface of a steel sheet to a depth of
0.5 mm in the thickness direction thereof over notch-
fatigue strength. The specimens for the investigation
were prepared by melting a steel and adjusting the
chemical components thereof so that the steel contained
0.08 C, 0.9~ Si, 1.2$ Mn, 0.01 P, 0.001 S, and 0.03
A1, casting it into a slab, hot rolling the slab to a
thickness of 3.5 mm so that the finish rolling was
completed at a temperature of not lower than the Ar3
transformation temperature, and then coiling the hot-
rolled steel sheet.
For the purpose of measuring the average of the
ratios of the X-ray diffraction strength in the
orientation component group of X100}<pll> to X223}<110>
to random X-ray diffraction strength and the average of
the ratios of the X-ray diffraction strength in the three
orientation components of {554}<225>, X111}<112> and
X111}<110> to random X-ray diffraction strength on a
plane at an arbitrary depth within 0.5 mm from the
surface of a steel sheet obtained as above in the
thickness direction thereof, a test piece was prepared by
cutting out a specimen sheet 30 mm in diameter from a
position of 1/4 or 3/4 of the width of a steel sheet,
grinding the surface of the specimen sheet to a depth of
about 0.05 mm from the surface so that the surface might
have the second finest finish, and then removing strain
by chemical polishing or electrolytic polishing.
Note that a crystal orientation component expressed
as ~(hkl}<uvw> means that the direction of a normal to the
plane of a steel sheet is parallel to <hkl> and the
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rolling direction of the steel sheet is parallel to
<uvw>. The measurement of a crystal orientation with X-
rays is conducted, for example, in accordance with the
method described in pages 274 to 296 of the Japanese
translation of Elements of X-ray Diffraction by B. D.
Cullity (published in 1986 by AGNE Gijutsu Center,
translated by Gentaro Matsumura).
Here, the average of the ratios of the X-ray
diffraction strength in the orientation component group
of {100}<O11> to {223}<110> to random X-ray diffraction
strength is obtained from the X-ray diffraction strengths
in the principal orientation components included in said
orientation component group, namely {100}<011>,
{116}<110>, {114}<110>, {113}<110>, {112}<110>,
{335}<110> and {223}<110>, in the three-dimensional
texture calculated either by the vector method based on
the pole figure of {110} or by the series expansion
. method using two or more (desirably, three or more) pole
figures out of the pole figures of {110}, {100}, {211}
and {310}.
For example, in the case of obtaining the ratios of
the X-ray diffraction strength in the above crystal
orientation components to random X-ray diffraction
strength by the latter method, the strengths of (001)[1-
10], (116)[1-10], (114)[1-10], (113)[1-10], (112)[1-10],
(335)[1-10] and (223)[1-10] at a ~2 = 45° cross section
in a three-dimensional texture may be used without
modification. Note that the average of the ratios of the
X-ray diffraction strength in the orientation component
group of {100}<O11> to {223}<110> to random X-ray
diffraction strength is the arithmetic average of the
ratios in all the above orientation components.
When it is impossible to obtain the strengths in all
these orientation components, the arithmetic average of
the strengths in the orientation components of
{100}<O11>, {116}<110>, {114}<110>, {112}<110> and
{223}<110> may be used as a substitute.
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Likewise, the average of the ratios of the X-ray
diffraction strength in the three orientation components
of X554}<225>, {111}<112> and {111}<110> to random X-ray
diffraction strength can be obtained from the three-
s dimensional texture calculated in the same manner as
explained above.
Next,,for the purpose of investigating the notch-
fatigue strength of the above steel sheet, a test piece
for fatigue test having the shape shown in Fig. 1(b) was
cut out from a position of 1/4 or 3/4 of the width of the
steel sheet so that the longitudinal direction of the
test piece coincided with the rolling direction of the
steel sheet, and was subjected to a fatigue test. It has
to be noted here that, whereas a test piece for fatigue
test shown in Fig. 1(a) is a common unnotched test piece
for evaluating the fatigue strength of a steel material,
a test piece for fatigue test shown in Fig. 1(b) is a
notched test piece prepared for evaluating notch-fatigue
strength. A test piece for fatigue test was ground to a
depth of about 0.05 mm from the surface so that the
surface might have the second finest finish, and a
fatigue test was carried out using an electro-hydraulic
servo type fatigue tester and the methods conforming to
JIS Z 2273-1978 and JIS Z 2275-1978.
Fig. 2 shows the results of an investigation of the
influences of the average of the ratios of the X-ray
diffraction strength in the orientation component group
of {100}<O11> to {223}<li0> to random X-ray diffraction
strength and the average of the ratios of the X-ray
diffraction strength in the three orientation components
of X554}<225>, X111}<112> and X111}<110> to random X-ray
diffraction strength over notch-fatigue strength. The
numeral in a circle in the figure indicates the fatigue
limit (the fatigue strength for finite life after 10'
cycles of repetition) obtained through a fatigue test
using a notched test piece having the shape shown in Fig.
1(b); the numeral is hereinafter referred to as a notch-
CA 02438393 2003-08-14
- 1$ -
fatigue strength.
It has been clarified that there is a strong
correlation among: the average of the ratios of the x-ray
diffraction strength in the orientation component group
of {100}<011> to {223}<110> to random X-ray diffraction
strength; the average of the ratios of the X-ray
diffraction strength in the three orientation components
of {554}<225>, {111}<112> and {111}<110> to random X-ray
diffraction strength; and notch-fatigue strength, and
that notch-fatigue strength is remarkably enhanced when
the above average figures are 2 or more and 4 or less,
respectively.
As a result of closely examining the results of
those tests, the present inventors have newly found that
it is very important, for enhancing notch-fatigue
strength, that, on a plane at an arbitrary depth within
0.5 mm from the surface of a steel sheet in the thickness
direction thereof, the average of the ratios of the X-ray
diffraction strength in the orientation component group
of {100}<011> to {223}<110> to random X-ray diffraction
strength is 2 or more and the average of the ratios of
the X-ray diffraction strength in the three orientation
components of {554}<225>, {111}<112> and {111}<110> to
random X-ray diffraction strength is 4 or less.
Further, for enhancing the resistance to the
occurrence of a fatigue crack not only in a notched test
piece but also in an unnotched test piece, it is
desirable that, on a plane at an arbitrary depth within
0.5 mm from the surface of a steel sheet in the thickness
direction thereof, the average of the ratios of the X-ray
diffraction strength in the orientation component group
of {100}<011> to {223}<110> to random X-ray diffraction
strength is 4 or more and the average of the ratios of
the X-ray diffraction strength in the three orientation
components of {554}<225>, {111}<112> and {111}<110> to
random X-ray diffraction strength is 2.5 or less.
The reason for the above is not altogether clear,
CA 02438393 2003-08-14
1~
but it is presumed to be as follows.
Generally speaking, in the case where an acute notch
exists, the fatigue limit of a material is determined by
the crack propagation limit of the material, namely the
degree of the resistance to the propagation of a crack
for arresting the crack. The propagation of a fatigue
crack is caused by the repetition of small plastic
deformation at the bottom of a notch or a stress
concentration site, and it is presumed that, when a crack
length is comparatively small and plastic deformation
occurs within a range comparable to the size of a crystal
grain, the crack propagation is significantly influenced
by crystallographic slip planes and slip directions.
Therefore, if the proportion of the crystal grains having
slip planes and slip directions that show a high
resistance to crack propagation is large in the crack
propagation direction and on the plane of a crack, then
the propagation of the fatigue crack is suppressed.
Next, the reasons for limiting the thickness of a
steel sheet in the present invention are explained.
When the thickness of a steel sheet is less than 0.5
mm, the conditions of allowing the occurrence of a small-
scale yield are not satisfied regardless of the extent of
stress concentration and therefore there is a danger that
monotonic ductile fracture is caused. In addition, as
the sufficient constraint of plastic deformation is
required from the viewpoint of arresting a crack, it is
desirable that the thickness of a steel sheet is 1.2 mm
or more for maintaining the state of plane strain.
When the thickness of a steel sheet exceeds 12 mm,
on the other hand, the deterioration of fatigue strength
resulting from thickness effect (size effect) becomes
significant. Further, when the thickness of a steel
sheet exceeds 8 mm, an excessive load may be required to
be imposed on production facilities for achieving the
conditions of hot or cold rolling that allow a texture
effective for enhancing notch-fatigue strength to be
CA 02438393 2003-08-14
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obtained. For that reason, a desirable thickness is 8 mm
or less. As a conclusion, the thickness of a steel sheet
is limited to 0.5 to 12 mm, or desirably 1.2 to 8 mm, in
the present invention.
The microstructure of a steel sheet according to the
present invention is explained hereafter.
In the present invention, it is not necessary to
specify the microstructure of a steel sheet for the
purpose of enhancing the notch-fatigue strength of the
steel sheet. The effect of enhancing notch-fatigue
strength in the present invention is obtained as far as a
texture falls in the range specified in the present
invention (a texture showing the ratios of the X-ray
diffraction strength in specific orientation components
to random X-ray diffraction strength falling in the
ranges specified in the present invention) in the
structures of ferrite, bainite, pearlite and martensite
forming in a commonly used steel material. Therefore, it
is desirable to regulate the microstructure of a steel
sheet in consideration of other required material
properties. It has to be noted, however, that the above
effect is further enhanced when a microstructure is a
specific microstructure, fox example, a compound
structure containing retained austenite by 5 to 25~ in
terms of volume percentage and having the balance mainly
consisting of ferrite and bainite, a compound structure
containing ferrite as the phase accounting for the
largest volume percentage and mainly martensite as the
second phase, or the like.
Note that the ferrite mentioned here includes
bainitic ferrite and acicular ferrite. Note also that,
when a structure which is not a bcc crystal structure,
such as retained austenite, is included in a compound
structure composed of two or more phases, such a compound
structure does not pose any problem insofar as the ratios
of the X-ray diffraction strength in the orientation
components and orientation component groups to random X-
CA 02438393 2003-08-14
- 21 -
ray diffraction strength converted by the volume
percentage of the other structures are within the
relevant ranges according to the present invention.
Besides, as pearlite containing coarse carbides may act
as a starting point of a fatigue crack and remarkably
deteriorate fatigue strength, it is desirable that the
volume percentage of the pearlite containing coarse
carbides is 15~ or less. When still better fatigue
properties are required, it is desirable that the volume
percentage of the pearlite containing coarse carbides is
5~ or less.
Here, the volume percentage of ferrite, bainite,
pearlite, martensite or retained austenite is defined as
the area percentage thereof in a microstructure observed
with an optical microscope under a magnification of 200
to 500 at a position in the depth of 1/4 of the steel
sheet thickness on a section surface along the rolling
direction of a specimen which is cut out from a position
of 1/4 or 3/4 of the width of the steel sheet, the
section surface being polished and etched with a nitral
reagent and/or the reagent disclosed in Japanese
Unexamined Patent Publication No. H5-163590. As it is
sometimes difficult to identify retained austenite by the
etching with the above reagents, the volume percentage
may also be calculated in the following manner.
Because the crystal structure of austenite is
different from that of ferrite, they can be easily
distinguished from each other crystallographically.
Therefore, the volume percentage of retained austenite
can be obtained experimentally by the X-ray diffraction
method too, namely by the simplified method wherein the
volume percentage thereof is calculated with the
following equation on the basis of the difference between
austenite and ferrite in the reflection intensity of the
Ka ray of Mo on their lattice planes:
Vy = (2/3)100/(0.7 x a(211)/y(220) + 1)} +
CA 02438393 2003-08-14
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(1/3).(100/(0.78 x a(211)/y(311) + 1)},
where, a(211), y(220) and y(311) are the X-ray reflection
intensities of the indicated lattice planes of ferrite
(a) and austenite (y), respectively.
For the purpose of obtaining a good burring
workability in addition to enhancing notch-fatigue
strength in the present invention, it is necessary that
the microstructure of a steel sheet is a compound
structure containing bainite or ferrite and bainite as
the phase accounting for the largest volume percentage.
Here, in this case, the present invention allows the
compound structure to contain unavoidably included
martensite, retained austenite and pearlite. For the
purpose of obtaining a good burring workability (a hole
expansion ratio), it is desirable that the total volume
percentage of hard retained austenite and martensite is
less than 5~. It is also desirable that the volume
percentage of bainite is 30$ or more. Further, for
realizing a good ductility, it is desirable that the
volume percentage of bainite is 70~ or less.
Further, for the purpose of obtaining a good
ductility in addition to enhancing notch-fatigue strength
in the present invention, it is necessary that the
microstructure of a steel sheet is a compound structure
containing retained austenite by 5 to 25$ in terms of
volume percentage and having the balance mainly
consisting of ferrite and bainite. Here, in this case,
the present invention allows the compound structure to
contain unavoidably included martensite and pearlite as
far as their total volume percentage is less than 5%.
Furthermore, for the purpose of obtaining a low
yield ratio for realizing a good shape-fixation property
in addition to enhancing notch-fatigue strength in the
present invention, it is necessary that the
microstructure of a steel sheet is a compound structure
containing ferrite as the phase accounting for the
CA 02438393 2003-08-14
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largest volume percentage and mainly martensite as the
second phase. Here, in this case, the present invention
allows the compound structure to contain unavoidably
included bainite, retained austenite and pearlite as far
as their total volume percentage is less than 5%. Note
that, for securing a low yield ratio of 70% or less, it
is desirable that the volume percentage of ferrite is 50%
or more.
Next, the reasons for limiting the chemical
components in the present invention are explained.
C is an indispensable element for obtaining a
desired microstructure. When a C content exceeds 0.3%,
however, workability deteriorates and, for this reason, a
C content is limited to 0.3% or less. Additionally, when
a C content exceeds 0.2%, weldability tends to
deteriorate and, for this reason, it is desirable that a
C content is 0.2% or less. On the other hand, when a C
content is less than 0.01%, steel strength decreases and,
therefore, a C content is limited to 0.01% or more.
Further, for the purpose of obtaining retained austenite
stably in an amount sufficient for realizing a good
ductility, it is desirable that a C content is 0.05% or
more.
Si is a solute-strengthening element and, as such,
it is effective for enhancing strength. An Si content
has to be 0.01% or more for obtaining a desired strength,
but, when an Si content exceeds 2%, workability
deteriorates. Therefore, an Si content is limited in the
range from 0.01 to 2%.
Mn is also a solute-strengthening element and, as
such, it is effective for enhancing strength. An Mn
content has to be 0.05% or more for obtaining a desired
strength. In the case where elements such as Ti, which
suppress hot cracking induced by S, are not added in a
sufficient amount in addition to Mn, it is desirable to
add Mn so that the expression Mn/S Z 20 is satisfied in
terms of mass percentage. Further, Mn is an element that
CA 02438393 2003-08-14
- 2~ -
stabilizes austenite and, therefore, in order to stably
obtain a sufficient amount of retained austenite in an
attempt to secure a good ductility, it is desirable that
an Mn addition amount is O.i% or more. When Mn is added
in excess of 3%, on the other hand, cracks occur to a
slab. For this reason, an Mn content is limited to 3% or
less.
P is an undesirable impurity, and the lower the P
content, the better. When a P content exceeds 0.1%,
workability and weldability are adversely affected, and
so are fatigue properties. Therefore, a P content is
limited to 0.1% or less.
S is also an undesirable impurity, and the lower the
S content, the better. When an S content is too high,
the A type inclusions detrimental to local ductility and
burring workability are formed and, for this reason, an S
content has to be minimized. A permissible content of S
is 0.01% or less.
A1 must be added by 0.005% or more for deoxidizing
molten steel, but its upper limit is set at 1.0% to avoid
a cost increase. A1 increases the formation of non-
metallic inclusions and deteriorates elongation when
added excessively and, for this reason, a desirable
content of A1 is 0.5% or less.
Cu is added as occasion demands, since Cu has an
effect of improving fatigue properties when it is in the
state of solid solution. No tangible effect is obtained
when a Cu addition amount is less than 0.2%, but the
effect is saturated when a Cu content exceeds 2%. Thus,
the range of a Cu content is determined to be from 0.2 to
2%. It has to be noted that, when a coiling temperature
is 450°C or higher and Cu is added in excess of 1.2%, Cu
may precipitate after coiling, drastically deteriorating
workability. For this reason, it is desirable to limit a
Cu content to 1.2% or less.
B is added as occasion demands, as B has an effect
of raising fatigue limit when added in combination with
CA 02438393 2003-08-14
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Cu. An addition of B by less than 0.0002% is not enough
for obtaining the effect, but, when B is added in excess
of 0.002%, cracks occur in a slab. For this reason, the
addition amount of B is limited to 0.0002 to 0.002%.
Ni is added as occasion demands for preventing hot
shortness caused by the presence of Cu. An Ni addition
amount of less than 0.1% is not enough for obtaining the
effect, but, even when it is added in excess of 1%, the
effect is saturated. For this reason, an Ni content is
limited in the range from 0.1 to 1%.
Ca and REM are the elements that modify the shape of
non-metallic inclusions, which serve as the starting
points of fractures and/or deteriorate workability, and,
by so doing, render them harmless. But no tangible
effect is obtained when either of them is added at less
than 0.0005%. When Ca is added in excess of 0.002% or
REM in excess of 0.02%, the effect is saturated. Thus,
it is desirable to add Ca by 0.0045 to 0.002% and REM by
0.0005 to 0.02%.
Additionally, one or more of precipitation-
strengthening and solute-strengthening elements, namely
Ti, Nb, Mo, v, Cr and Zr, may be added for enhancing
strength. However, when they are added at less than
0.05%, 0.01%, 0.05%, 0.02%, 0.01% and 0.02%,
respectively, no tangible effects are obtained and, when
they are added in excess of 0.5, 0.5%, 1%, 0.2%, 1% and
0.2%, respectively, their effects are saturated.
Note that Sn, Co, Zn, W and/or Mg may be added at 1%
or less in total to a steel containing aforementioned
elements as the main components. However, as Sn may
cause surface defects during hot rolling, it is desirable
to limit an Sn content to 0.05% or less.
Now, the reasons for limiting the conditions of the
production method according to the present invention are
explained in detail hereafter.
A steel sheet according to the present invention can
be produced through any of the following process routes:
CA 02438393 2003-08-14
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casting, hot rolling and cooling; casting, hot rolling,
cooling, pickling, cold rolling and annealing; heat
treatment of a hot-rolled or cold-rolled steel sheet in a
hot dip plating line; or, further, surface treatment
applied separately to a steel sheet produced through any
of the above process routes.
The present invention does not specify production
methods prior to hot rolling. That is, a steel may be
melted and refined in a blast furnace, an electric arc
furnace or the like, then the chemical components may be
adjusted in one or more of various secondary refining
processes so that the steel may contain desired amounts
of the components, and then the steel may be cast into a
slab through a casting process such as an ordinary
continuous casting process, an ingot casting process and
a thin slab casting process. Steel scraps may be used as
a raw material. Further, in the case of a slab cast
through a continuous casting process, the slab may be fed
to a hot-rolling mill directly while it is hot, or it may
be hot rolled after being cooled to room temperature and
then heated in a repeating furnace.
No limit is particularly set to the temperature of
repeating, but it is desirable that a repeating
temperature is lower than 1,400°C, since, when it is
1,400°C or higher, the descale amount becomes large and
the product yield decreases. It is also desirable that a
repeating temperature is 1,000°C or higher, since a
repeating temperature lower than 1,000°C remarkably
deteriorates the operation efficiency of a rolling mill
in terms of rolling schedule.
In a hot rolling process, a slab undergoes finish
rolling after completing rough rolling. When descaling
is applied after completing the rough rolling, it is
desirable to satisfy the following condition:
P (MPa) x L (1/cmZ) Z 0.0025,
where, P (MPa) is an impact pressure of high-pressure
CA 02438393 2003-08-14
- 27 -
water on a steel sheet surface, and L (1/cm2) a flow rate
of descaling water.
An impact pressure P of high-pressure water on a
steel sheet surface is expressed as follows (see Tetsu-
to-Hagane, 1991, Vol. 77, No. 9, p.1450):
P (MPa) - 5.64 x Po x V/H2,
where, Po (MPa) is a pressure of liquid, V (1/min.) a
liquid flow rate of a nozzle, and H (cm) a distance
between a nozzle and the surface of a steel sheet.
The flow rate L (1/cm2) is expressed as follows:
L (1/cm2) - V/(W x v),
where, V (1/min.) is a liquid flow rate of a nozzle, W
(cm) the width of liquid when the liquid blown from a
nozzle hits a steel sheet surface, and v (cm/min.) a
traveling speed of a steel sheet.
It is not necessary to specify an upper limit of the
product of the impact pressure P and the flow rate L for
the purpose of obtaining the effects of the present
invention. However, it is preferable that the product is
0.02 or less because, when the liquid flow rate of a
nozzle is raised, problems such as violent nozzle wear
occur.
It is. preferable, further, that the maximum
roughness height Ry of a steel sheet after finish rolling
is 15 N,m ( 15 ~.m Ry, ~ 2.5 mm, .fin 12 .5 mm) or less. The
reason for this is clear from the fact that the fatigue
strength of an as-hot-rolled or as-pickled steel sheet
correlates with the maximum roughness height Ry of the
steel sheet surface, as stated, for example, in page 84
of Metal Material Fatigue Design Handbook edited by the
Society of Materials Science, Japan. Further, it is
preferable that the subsequent finish hot rolling is done
within 5 sec. after high-pressure descaling so that
scales may be prevented from forming again.
Besides the above, finish rolling may be carried out
continuously by welding sheet bars together after rough
CA 02438393 2003-08-14
- 2$ -
rolling or the subsequent descaling. In this case, the
rough-rolled sheet bars may be welded together after
being coiled temporarily, held inside a cover having a
heat retention function as occasion demands, and then
uncoiled.
When a hot-rolled steel sheet is used as a final
product, it is necessary that the finish rolling is done
at a total reduction ratio of 25$ or more in the
temperature range of the Ar3 transformation temperature +
100°C or lower during the latter half of the finish
rolling. Here, the Ar3 transformation temperature can be
expressed, in a simplified manner, in relation to steel
chemical components, for instance, by the following
equation:
Ar3 = 910 - 310 x $C + 25 x ~Si - 80 x ~Mn.
when the total reduction ratio in the temperature
range of the Ar3 transformation temperature + 100°C or
lower is less than 25$, the rolled texture of austenite
does not develop sufficiently and, as a result, the
effects of the present invention are not obtained, no
matter how the steel sheet is cooled thereafter. For
obtaining the specified texture, it is desirable that the
total reduction ratio in the temperature range of the Ar3
transformation temperature + 100°C or lower is 35$ or
more.
The present invention does not specify a lower limit
of the temperature range in which rolling at a total
reduction ratio of 25~ or more is carried out. However,
when the rolling is done at a temperature lower than the
Ar3 transformation temperature, a work-induced structure
remains in ferrite having precipitated during the
rolling, and, as a result, ductility falls and
workability deteriorates. For this reason, it is
desirable that a lower limit of the temperature range in
which rolling at a total reduction ratio of 25~ or more
is carried out is not lower than the Ar3 transformation
CA 02438393 2003-08-14
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temperature. However, when recovery or recrystallization
advances to some extent during the subsequent coiling
process or a heat treatment after the coiling process, a
rolling temperature lower than the Ar3 transformation
temperature is acceptable.
The present invention does not specify an upper
limit of the total reduction ratio in the temperature
range of the Ar3 transformation temperature + 100°C or
lower. However, when a total reduction ratio exceeds
97.5$, the rolling load becomes too high and it becomes
necessary to increase the rigidity of a rolling mill
excessively, resulting in economical disadvantage. For
this reason, the total reduction ratio is, desirably,
97.5 or less.
Here, when the friction between a hot-rolling roll
and a steel sheet is large during hot rolling in the
temperature range of the Ar3 transformation temperature +
100°C or lower, crystal orientations mainly composed of
{110} planes develop at planes near the surfaces of the
steel sheet, causing the deterioration of notch-fatigue
strength. As a countermeasure, lubrication may be
applied for reducing the friction between a hot-rolling
roll and a steel sheet as occasion demands.
The present invention does not specify an upper
limit of the friction coefficient between a hot-rolling
roll and a steel sheet. However, when a friction
coefficient exceeds 0.2, crystal orientations mainly
composed of (110} planes develop conspicuously,
deteriorating notch-fatigue strength. For this reason,
it is desirable to control a friction coefficient between
a hot-rolling roll and a steel sheet to 0.2 or less at
least at one of the passes of the hot rolling in the
temperature range of the Ar3 transformation temperature +
100°C or lower. It is more desirable to control a
friction coefficient between a hot-rolling roll and a
steel sheet to 0.15 or less at all the passes of the hot
CA 02438393 2003-08-14
3p _
rolling in the temperature range of the Ar3
transformation temperature + 100°C or lower.
Here, a friction coefficient between a hot-rolling
roll and a steel sheet is the value calculated from a
forward slip ratio, a rolling load, a rolling torque and
so on on the basis of the rolling theory.
The present invention does not specify a temperature
at the final pass (FT) of finish rolling, but it is
desirable that the final pass is completed at a
temperature not lower than the Ar3 transformation
temperature. This is because, if a rolling temperature
is lower than the Ar3 transformation temperature during
hot rolling, a work-induced structure remains in ferrite
having precipitated before or during the rolling, and, as
a result, ductility lowers and workability deteriorates.
However, when a heat treatment for recovery or
recrystallization is applied during or after the
subsequent coiling process, a temperature at the final
pass (FT) of finish rolling is allowed to be lower than
the Ar3 transformation temperature.
The present invention does not specify an upper
limit of a finishing temperature, but, if a finishing
temperature exceeds the Ar3 transformation temperature +
100°C, it becomes practically impossible to carry aut
rolling at a total reduction ratio of 25$ or more in the
temperature range of the Ar3 transformation temperature +
100°C or lower. For this reason, it is desirable that an
upper limit of a finishing temperature is the Ar3
transformation temperature + 100°C or lower.
In the present invention, it is not necessary to
specify the microstructure of a steel sheet for only the
purpose of enhancing the notch-fatigue strength thereof
and, therefore, no specific limitation is set forth
regarding the cooling process after the completion of
finish rolling until the coiling at a prescribed coiling
temperature. Nevertheless, a steel sheet is cooled, as
CA 02438393 2003-08-14
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occasion demands, for the purpose of securing a
prescribed coiling temperature or controlling the
microstructure. The present invention does not specify
an upper limit of a cooling rate, but, as thermal strain
may cause a steel sheet to warp, it is desirable to
control a cooling rate to 300°C/sec. or lower. In
addition, when a cooling rate is too high, it becomes
impossible to accurately control the cooling end
temperature and an over-cooling may happen as a result of
overshooting to a temperature lower than a prescribed
coiling temperature. For this reason, a cooling rate
here is, desirably, 150°C/sec. or lower. No lower limit
of a cooling rate is specifically set forth, either. For
reference, the cooling rate in the case where a steel
sheet is left to cool by air without any intentional
cooling is 5°C/sec. or higher.
For the purpose of obtaining a good burring
workability in addition to enhancing notch-fatigue
strength in the present invention, it is necessary that
the microstructure of a steel sheet is a compound
structure containing bainite or ferrite and bainite as
the phase accounting for the largest volume percentage.
In that case, the present invention does not specify the
conditions of the process after the completion of finish
rolling until the coiling at a prescribed coiling
temperature, except for the cooling rate applied during
the process. However, in the case where a steel sheet is
required to have both a good burring workability and a
high ductility without sacrificing the burring
workability too much, a hot-rolled steel sheet may be
retained for 1 to 20 sec. in the temperature range from
the Ar3 transformation temperature to the Arl
transformation temperature (the ferrite-austenite two-
phase zone). Here, the retention of a hot-rolled steel
sheet is carried out for accelerating ferrite
transformation in the two-phase zone. When a retention
CA 02438393 2003-08-14
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time is less than 1 sec., ferrite transformation in the
two-phase zone is insufficient and a sufficient ductility
is not obtained.- However, when a retention time exceeds
20 sec., pearlite forms and an intended microstructure
having a compound structure containing bainite or ferrite
and bainite as the phase accounting for the largest
volume percentage is not obtained.
In addition, in order to facilitate the acceleration
of ferrite transformation, it is desirable that the
temperature range in which a steel sheet is retained for
1 to 20 sec. is from the Arl transformation temperature
to 800°C. Further, in order not to lower productivity
drastically, it is desirable that the retention time,
which has been defined earlier as in the range from 1 to
20 sec., is 1 to 10 sec. For satisfying all those
requirements, it is necessary to reach said temperature
range rapidly at a cooling rate of 20°C/sec. or higher
after completing finish rolling.
The present invention does not specify an upper
limit of a cooling rate, but, in consideration of the
capacity of cooling equipment, a reasonable cooling rate
is 300°C/sec. or lower. In addition, when a cooling rate
is too high, it becomes impossible to accurately control
the cooling end temperature and over-cooling may occur as
a result of overshooting to the Arl transformation
temperature or lower, losing the ductility improvement
effect. For this reason, a cooling rate here is,
desirably, 150°C/sec. or lower.
Subsequently, a steel sheet is cooled at a cooling
rate of 20°C/sec. or higher from the above temperature
range to a coiling temperature (CT). When a cooling rate
is lower than 20°C/sec., pearlite or bainite containing
carbides forms and an intended microstructure having a
compound structure containing bainite or ferrite and
bainite as the phase accounting for the largest volume
CA 02438393 2003-08-14
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percentage is not obtained. The effects of the present
invention can be enjoyed without specifying an upper
limit of the cooling rate down to the coiling temperature
but, to avoid warping caused by thermal strain, it is
desirable to control a cooling rate to 300°C/sec. or
lower.
For the purpose of obtaining a good ductility in
addition to enhancing notch-fatigue strength in the
present invention, it is necessary that the
microstructure of a steel sheet is a compound structure
containing retained austenite at 5 to 25$ in terms of
volume percentage and having the balance mainly
consisting of ferrite and bainite. For obtaining such a
compound structure, a hot-rolled steel sheet has to be
retained for 1 to 20 sec. in the temperature range from
the Ar3 transformation temperature to the Arl
transformation temperature {the ferrite-austenite two-
phase zone) in the first process after completing finish
rolling. Here, the retention of a hot-rolled steel sheet
is carried out for accelerating ferrite transformation in
the two-phase zone. When a retention time is less than 1
sec., ferrite transformation in the two-phase zone is
insufficient and a sufficient ductility is not obtained.
However, when a retention time exceeds 20 sec., pearlite
forms and an intended microstructure containing retained
austenite by 5 to 25$ in terms of volume percentage and
having the balance mainly consisting of ferrite and
bainite is not obtained.
In addition, in order to facilitate the acceleration
of ferrite transformation, it is desirable that the
temperature range in which a steel sheet is retained for
1 to 20 sec. is from the Arl transformation temperature
to 800°C. Further, in order not to lower productivity
drastically, it is desirable that the retention time,
which has been defined earlier as in the range from 1 to
20 sec., is 1 to 10 sec. To satisfy all those
CA 02438393 2003-08-14
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requirements, it is necessary to reach said temperature
range rapidly at a cooling rate of 20°C/sec. or higher
after completing finish rolling. The present invention
does not specify an upper limit of a cooling rate, but,
in consideration of the capacity of cooling equipment, a
reasonable cooling rate is 300°C/sec. or lower. In
addition, when a cooling rate is too high, it becomes
impossible to accurately control the cooling end
temperature and over-cooling may happen as a result of
overshooting to the Arl transformation temperature or
lower. For this reason, a cooling rate here is,
desirably, 150°C/sec. or lower.
Subsequently, a steel sheet is cooled at a cooling
rate of 20°C/sec. or higher from the above temperature
range to a coiling temperature (CT). When a cooling rate
is lower than 20°C/sec., pearlite or bainite containing
carbides forms and a sufficient amount of retained
austenite is not secured and, as a result, an intended
microstructure containing retained austenite at 5 to 25~
in terms of volume percentage and having the balance
mainly consisting of ferrite and bainite is not obtained.
The effects of the present invention can be enjoyed
without bothering to specify an upper limit of the
cooling rate down to the coiling temperature but, to
avoid warping caused by thermal strain, it is desirable
to control a cooling rate to 300°C/sec. or lower.
Further, for the purpose of obtaining a low yield
ratio for realizing a good shape-fixation property in
addition to enhancing notch-fatigue strength in the
present invention, it is necessary that the
microstructure of a steel sheet is a compound structure
containing ferrite as the phase accounting for the
largest volume percentage and mainly martensite as the
second phase. For obtaining such a compound structure, a
hot-rolled steel sheet has to be retained for 1 to 20
CA 02438393 2003-08-14
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sec. in the temperature range from the Ar3 transformation
temperature to the Arl transformation temperature (the
ferrite-austenite two-phase zone) in the first process
after completing finish rolling. Here, the retention of
a hot-rolled steel sheet is carried out for accelerating
ferrite transformation in the two-phase zone. When a
retention time is less than 1 sec., ferrite
transformation in the two-phase zone is insufficient and
a sufficient ductility is not obtained. However, when a
retention time exceeds 20 sec., pearlite forms and an
intended compound structure containing ferrite as the
phase accounting for the largest volume percentage and
mainly martensite as the second phase is not obtained.
In addition, in order to facilitate the acceleration
of ferrite transformation, it is desirable that the
temperature range in which a steel sheet is retained for
1 to 20 sec. is from the Arl transformation temperature
to 800°C. Further, in order not to lower productivity
drastically, it is desirable that the retention time,
which has been defined earlier as in the range from 1 to
20 sec., is 1 to 10 sec. To satisfy all those
requirements, it is necessary to reach said temperature
range rapidly at a cooling rate of 20°C/sec. or higher
after completing finish rolling. The present invention
does not specify an upper limit of a cooling rate, but,
in consideration of the capacity of cooling equipment, a
reasonable cooling rate is 300°C/sec. or lower. In
addition, when a cooling rate is too high, it becomes
impossible to accurately control the cooling end
temperature and over-cooling may happen as a result of
overshooting to the Arl transformation temperature or
lower. For this reason, a cooling rate here is,
desirably, 150°C/sec. or lower.
Subsequently, a steel sheet is cooled at a cooling
rate of 20°C/sec. or higher from the above temperature
CA 02438393 2003-08-14
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range to a coiling temperature (CT). When a cooling rate
is lower than 20°C/sec., pearlite or bainite forms and a
sufficient amount of martensite is not secured and, as a
result, an intended microstructure containing ferrite as
the phase accounting for the largest volume percentage
and martensite as the second phase is not obtained.
The effects of the present invention can be enjoyed
without specifying an upper limit of the cooling rate
down to the coiling temperature but, to avoid distortion
caused by thermal strain, it is desirable to control the
cooling rate to 300°C/sec. or lower.
In the present invention, it is not necessary to
specify the microstructure of a steel sheet only for the
purpose of enhancing the notch-fatigue strength thereof
and, therefore, the present invention does not specify an
upper limit of a coiling temperature. However, in order
to carry over the texture of austenite obtained by finish
rolling at a total reduction ratio of 25% or more in the
temperature range of the Ar3 transformation temperature +
100°C or lower, it is desirable to coil a steel sheet at
the coiling temperature Ta shown below or lower. Note
that it is unnecessary to set the temperature To to room
temperature or lower. To is the temperature defined
thermodynamically as that at which austenite and ferrite
having the same chemical components as the austenite have
the same free energy. It can be calculated in a
simplified manner by the following equation, taking the
influences of components other than C into consideration:
To = -650.4 x %C + B,
where, B is determined as follows:
B = -50.6 x Mneq + 894.3,
where, Mneq is determined from the mass percentages of
the component elements as shown below:
Mneq = %Mn + 0.24 x %Ni + 0.13 x %Si + 0.38 x %Mo +
0.55 x %Cr + 0.16 x %Cu - 0.50 x %A1 - 0.45 x %Co + 0.90
x %V.
CA 02438393 2003-08-14
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Note that the influences on To of the mass
percentages of the other components specified in the
present invention than those included in the above
equation are insignificant, and are negligible here.
Since it is not necessary to specify the
microstructure of a steel sheet only for the purpose of
enhancing the notch-fatigue strength thereof, it is not
necessary to specify the lower limit of a coiling
temperature. However, to avoid a poor appearance caused
by rust when a coil is kept wet with water for a long
period of time, it is desirable that a coiling
temperature is not lower than 50°C.
For the purpose of obtaining a good burring
workability in addition to enhancing notch-fatigue
strength in the present invention, it is necessary that
the microstructure of a steel sheet is a compound
structure containing bainite or ferrite and bainite as
the phase accounting for the largest volume percentage.
To obtain such a compound structure, the coiling
temperature has to be restricted to 450°C or higher.
This is because, when a coiling temperature is lower than
450°C, retained austenite or martensite considered
detrimental to burring workability may form in a great
amount and, as a consequence, an intended microstructure
having a compound structure containing bainite or ferrite
and bainite as the phase accounting for the largest
volume percentage is not obtained.
Further, although the present invention does not
specify a cooling rate to be applied after coiling, it is
desirable that a cooling rate after coiling is 30°C/sec.
or higher to a temperature of 200°C. Otherwise, when Cu
is added by 1.2% or more, it precipitates after coiling
and, as a result, not only workability is deteriorated
but also solute Cu effective for improving fatigue
properties may be lost.
CA 02438393 2003-08-14
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Further, for the purpose of obtaining a good
ductility in addition to enhancing notch-fatigue strength
in the present invention, it is necessary that the
microstructure of a steel sheet is a compound structure
containing retained austenite at 5 to 25% in terms of
volume percentage and having the balance mainly
consisting of ferrite and bainite. To obtain such a
compound structure, the coiling temperature is restricted
to lower than 450°C. This is because, when a coiling
temperature is 450°C or higher, bainite containing
carbides forms and a sufficient amount of retained
austenite is not secured and, as a result, an intended
microstructure containing retained austenite at 5 to 25%
in terms of volume percentage, and having the balance
mainly consisting of ferrite and bainite, is not
obtained. When a coiling temperature is not higher than
350°C, on the other hand, a great amount of martensite
forms and a sufficient amount of retained austenite is
not secured and, as a result, an intended microstructure
containing retained austenite by 5 to 25% in terms of
volume percentage and having the balance mainly
consisting of ferrite and bainite is not obtained. For
this reason, a coiling temperature is limited to higher
than 350°C.
Further, although the present invention does not
specify a cooling rate to be applied after coiling, it is
desirable that a cooling rate after coiling is 30°C/sec.
or higher up to a temperature of 200°C. Otherwise, when
Cu is added at 1% or more, it precipitates after coiling
and, as a result, not only is the workability
deteriorated but also solute Cu effective for improving
fatigue properties may be lost.
Further, for the purpose of obtaining a low yield
ratio for realizing a good shape-fixation property in
addition to enhancing notch-fatigue strength in the
CA 02438393 2003-08-14
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present invention, it is necessary that the
microstructure of a steel sheet is a compound structure
containing ferrite as the phase accounting for the
largest volume percentage and mainly martensite as the
second phase. For obtaining such a compound structure, a
coiling temperature has to be restricted to 350°C or
lower. This is because, when a coiling temperature
exceeds 350°C, bainite forms and a sufficient amount of
martensite is not secured and, as a result, an intended
microstructure containing ferrite as the phase accounting
for the largest volume percentage and martensite as the
second phase is not obtained. It is not necessary to
specify a lower limit of a coiling temperature but, to
avoid a poor appearance caused by rust when a coil is
kept wet with water for a long period of time, it is
desirable that a coiling temperature is not lower than
50°C.
After completing a hot rolling process, as occasion
demands, a steel sheet may be subjected to pickling and
then skin pass rolling at a reduction ratio of 10% or
less or cold rolling at a reduction ratio up to 40% or
so, either on-line or off-line.
Next, in the case where a cold-rolled steel sheet is
used as a final product, the present invention does not
specify the conditions of finish hot rolling. However,
in order to obtain a better notch-fatigue strength, it is
desirable that a total reduction ratio, in the
temperature range of the Ar3 transformation temperature +
100°C or lower, is 25% or more. Further, while the
temperature at the final pass (FT) of finish rolling is
allowed to be lower than the Ar3 transformation
temperature, in such a case, since an intensively work-
induced structure remains in ferrite having precipitated
before or during the rolling, it is desirable that the
work-induced structure is recovered and recrystallized
through the subsequent coiling process or a heat
CA 02438393 2003-08-14
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treatment.
A total reduction ratio at subsequent cold rolling
after pickling must be less than 80%. This is because,
when a total reduction ratio at cold rolling is 80% or
more, the ratios of the integrated X-ray diffraction
strengths in X111} and {554} crystallographic planes
parallel to the plane of a steel sheet, the
crystallographic planes having a texture usually obtained
through cold rolling and recrystallization, tend to rise.
A preferable total reduction ratio at cold rolling is 70%
or less. The effects of the present invention can be
enjoyed without specifying a lower limit of a cold
reduction ratio but, for controlling the X-ray
diffraction strengths in specific crystal orientation
components within appropriate ranges, it is desirable to
set a lower limit of a cold reduction ratio at 3% or
more.
The discussion here is based on the premise that the
heat treatment of a steel sheet cold rolled as specified
above is carried out in a continuous annealing process.
In the first place, a steel sheet is subjected to a
heat treatment for 5 to 150 sec. in the temperature range
of the Ac3 transformation temperature + 100°C or lower.
When an upper limit of a heat treatment temperature
exceeds the Ac3 transformation temperature + 100°C,
ferrite having formed through recrystallization
transforms into austenite, the texture formed by the
growth of austenite grains is randomized, and the texture
of ferrite finally obtained is also randomized. For this
reason, an upper limit of a heat treatment temperature is
set at the Ac3 transformation temperature + 100°C or
lower.
The Acl and Ac3 transformation temperatures
mentioned herein can be expressed in relation to steel
chemical components using, for example, the expressions
according to p. 273 of the ,7apanese translation of The
CA 02438393 2003-08-14
- 41 -
Physical Metallurgy of Steels by W. C. Leslie {published
by Maruzen in 1985, translated by Hiroshi Kumai and
Tatsuhiko Noda).
With regard to a lower limit of a heat treatment
temperature, it is acceptable if the temperature is equal
to or higher than the recovery temperature, because it is
not necessary to specify the microstructure of a steel
sheet for the purpose of enhancing the notch-fatigue
strength thereof. When a heat treatment temperature is
lower than the recovery temperature, however, a work-
induced structure is retained and formability is
significantly deteriorated. For this reason, a lower
limit of a heat treatment temperature is set to be equal
to or higher than the recovery temperature. Further,
with regard to a retention time in the above temperature
range, when a retention time is shorter than 5 sec., it
is insufficient for having cementite completely dissolve
again. However, when a retention time exceeds 150 sec.,
the effect of the heat treatment is saturated and, what
24 is worse, productivity is lowered. For this reason, a
retention time is determined to be in the range from 5 to
150 sec.
The present invention does not specify the
conditions of cooling after a heat treatment. However,
for the purpose of controlling the microstructure of a
steel sheet, cooling or the combination of retention at
an arbitrary temperature and cooling as explained later
may be employed as deemed necessary.
For the purpose of obtaining a good burring
workability in addition to enhancing notch-fatigue
strength in the present invention, it is necessary that
the microstructure of a steel sheet is a compound
structure containing bainite or ferrite and bainite as
the phase accounting for the largest volume percentage.
To obtain such a compound structure, a lower limit of a
heat treatment temperature is set at a temperature of the
Ac, transformation temperature or higher. when a lower
CA 02438393 2003-08-14
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limit of a heat treatment temperature is lower than the
Acl transformation temperature, an intended compound
structure containing bainite or ferrite and bainite as
the phase accounting for the largest volume percentage,
is not obtained. when it is intended to obtain both a
good burring workability and a high ductility without
sacrificing the burring workability too much, a heat
treatment temperature must be in the range from the Acl
transformation temperature to the Ac3 transformation
temperature (the ferrite-austenite two-phase zone) in
order to increase the volume percentage of ferrite.
Further, for the purpose of obtaining a still better
burring workability, it is desirable that the heat
treatment temperature is in the range from the Ac3
transformation temperature to the Ac3 transformation
temperature + 100°C in order to increase the volume
percentage of bainite.
The present invention does not specify the
conditions of a cooling process in heat treatment.
However, when a heat treatment temperature is in the
range from the Acl transformation temperature to the Ac3
transformation temperature, it is desirable to cool a
steel sheet at a cooling rate of 20°C/sec. or higher to
the temperature range from higher than 350°C to the
temperature To specified herein earlier. This is
because, when a cooling rate is lower than 20°C/sec., the
temperature history of steel is likely to pass through
the transformation nose of bainite or pearlite containing
much carbide. Further, when a cooling end temperature is
350°C or lower, martensite, which is considered
detrimental to burring properties, may form in a great
amount and, as a result, an intended microstructure
having a compound structure containing bainite or ferrite
and bainite as the phase accounting for the largest
volume percentage is not obtained. For this reason, it
CA 02438393 2003-08-14
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is desirable that a cooling end temperature is higher
than 350°C. In addition, in order to carry over the
texture obtained to the previous process, it is desirable
that a cooling end temperature is not higher than To.
Finally, when a cooling rate to the cooling end
temperature is 20°C/sec. or higher, martensite, which is
considered detrimental to burring properties, may form in
a great amount during the cooling and, as a result, an
intended microstructure having a compound structure
containing bainite or ferrite and bainite as the phase
accounting for the largest volume percentage may not be
obtained. For this reason, it is desirable that a
cooling rate is lower than 20°C/sec. Further, when a
cooling end temperature is higher than 200°C, aging
properties may deteriorate, and, for this reason, it is
desirable that a cooling end temperature is 200°C or
lower. If water cooling or mist cooling is applied and a
coil is kept wet with water for a long period of time, it
is desirable, to avoid a poor appearance caused by rust,
that a cooling end temperature is not lower than 50°C.
On the other hand, in the case where above mentioned
heat treatment temperature is in the range from higher
than the Ac3 transformation temperature to the Ac3
transformation temperature + 100°C, it is desirable to
cool a steel sheet at a cooling rate of 20°C/sec. or
higher to a temperature of 200°C or lower. This is
because, when a cooling rate is lower than 20°C/sec., the
temperature history of steel is likely to pass through
the transformation nose of bainite or pearlite containing
much carbide. In addition, when a cooling end
temperature exceeds 200°C, aging properties may
deteriorate. For this reason, it is desirable that a
cooling end temperature is 200°C or lower. If water
cooling or mist cooling is applied and a coil is kept wet
CA 02438393 2003-08-14
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with water for a long period of time, it is desirable, to
avoid a poor appearance caused by rust, that a cooling
end temperature is not lower than 50°C.
Further, for the purpose of obtaining a good
ductility in addition to enhancing notch-fatigue strength
in the present invention, it is necessary that the
microstructure of a steel sheet is a compound structure
containing retained austenite at 5 to 25$ in terms of
volume percentage and having the balance mainly
consisting of ferrite and bainite. To obtain such a
compound structure, a steel sheet must be subjected to a
heat treatment for 5 to 150 sec. in the temperature range
from the Acl transformation temperature to the Ac3
transformation temperature + 100°C, as described earlier.
In this case, when a temperature is too low within the
above temperature range and when cementite has
precipitated in an as-hot-rolled state, it takes too long
for the cementite to dissolve again. When a temperature
is too high, on the other hand, the volume percentage of
austenite increases excessively and the concentration of
C in austenite decreases, and, as a consequence, the
temperature history of steel is likely to pass through
the transformation nose of bainite or pearlite containing
much carbide. For this reason, it is desirable to heat a
steel sheet to a temperature in the range from 780°C to
850°C. When a cooling rate after retention is lower than
20°C/sec., the temperature history of steel is likely to
pass through the transformation nose of bainite or
pearlite containing much carbide, and, for this reason, a
cooling rate must be 20°C/sec. or higher.
Next, with respect to the process to accelerate
bainite transformation and stabilize a required amount of
retained austenite, when a cooling end temperature is not
lower than 450°C, retained austenite is decomposed into
bainite or pearlite containing much carbide, and an
CA 02438393 2003-08-14
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intended microstructure containing retained austenite at
to 25~ in terms of volume percentage and having the
balance mainly consisting of ferrite and bainite is not
obtained. When a cooling end temperature is not higher
5 than 350°C, on the other hand, martensite may form in a
great amount and a sufficient amount of retained
austenite cannot be secured and, as a result, an intended
microstructure containing retained austenite at 5 to 25~
in terms of volume percentage and the balance mainly
consisting of ferrite and bainite is not obtained. For
this reason, the cooling must be continued to a
temperature in the range from higher than 350°C to lower
than 450°C.
Further, with respect to a retention time in the
above temperature range, when a retention time is shorter
than 5 sec., bainite transformation for stabilizing
retained austenite is insufficient and, as a consequence,
unstable retained austenite may transform into martensite
at the end of the subsequent cooling, and, as a result,
an intended microstructure containing retained austenite
at 5 to 25$ in terms of volume percentage and having the
balance mainly consisting of ferrite and bainite is not
obtained. When a retention time exceeds 600 sec., on the
other hand, bainite transformation overshoots and a
required amount of stable retained austenite is not
formed, and, as a result, an intended microstructure
containing retained austenite at 5 to 25~ in terms of
volume percentage and having the balance mainly
consisting of ferrite and bainite is not obtained. For
this reason, a retention time in the above temperature
range must be from 5 to 600 sec.
Finally, when a cooling rate up to the end of
cooling is lower than 5°C/sec., bainite transformation
may overshoot during the cooling and a required amount of
stable retained austenite is not formed, and, as a
consequence, an intended microstructure containing
CA 02438393 2003-08-14
- 46 -
retained austenite by 5 to 25% in terms of volume
percentage and having the balance mainly consisting of
ferrite and bainite may not be obtained. For this
reason, a cooling rate is set at 5°C/sec. or higher.
In addition, when a cooling end temperature is
higher than 200°C, aging properties may deteriorate and,
for this reason, a cooling end temperature must be 200°C
or lower. The present invention does not specify a lower
limit for a cooling end temperature. However, if water
cooling or mist cooling is applied and a coil is kept wet
with water for a long period of time, it is desirable, to
avoid a poor appearance caused by rust, that a cooling
end temperature is not lower than 50°C.
Further, for the purpose of obtaining a low yield
ratio for realizing a good shape-fixation property in
addition to enhancing notch-fatigue strength in the
present invention, it is necessary that the
microstructure of a steel sheet is a compound structure
containing ferrite as the phase accounting for the
largest volume percentage and mainly martensite as the
second phase. To obtain such a compound structure, a
steel sheet must be subjected to a heat treatment for 5
to 150 sec. in the temperature range from the Acl
transformation temperature to the Ac3 transformation
temperature + 100°C as described before. In this case,
when the temperature is too low within the above
temperature range and when cementite has precipitated in
an as-hot-rolled state, it takes too long for the
cementite to dissolve again. When the temperature is too
high, on the other hand, the volume percentage of
austenite increases excessively and the concentration of
C in austenite decreases, and, as a consequence, the
temperature history of steel is likely to pass through
the transformation nose of bainite or pearlite containing
much carbide. For this reason, it is desirable to heat a
CA 02438393 2003-08-14
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steel sheet to a temperature in the range from 780°C to
850°C.
When a cooling rate after retention is lower than
20°C/sec., the temperature history of steel is likely to
pass through the transformation nose of bainite or
pearlite containing much carbide, and, for this reason, a
cooling rate must be 20°C/sec. or higher. When a cooling
end temperature is higher than 350°C, an intended
microstructure containing ferrite as the phase accounting
for the largest volume percentage and martensite as the
second phase is not obtained. For this reason, the
cooling must be continued down to a temperature of 350°C
or lower. The present invention does not specify a lower
limit of a cooling end temperature. However, if water
cooling or mist cooling is applied and a coil is kept wet
with water for a long period of time, it is desirable, to
avoid a poor appearance caused by rust, that a cooling
end temperature is not lower than 50°C.
Thereafter, skin pass rolling may be applied, if
required.
when galvanizing is applied to a hot-rolled steel
sheet after pickling or a cold-rolled steel sheet after
completing the above annealing for recrystallization, the
steel sheet is dipped in a zinc-plating bath. After
that, it may be subjected to an alloying treatment, if
required.
Example
(Example 1)
The present invention is further explained hereafter
based on Example 1.
Steels A to L having the chemical components shown
in Table 1 were melted and refined in a converter, cast
continuously into slabs, reheated and then rolled through
rough rolling and finish rolling into steel sheets 1.2 to
CA 02438393 2003-08-14
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5.5 mm in thickness, and then coiled. Note that the
chemical components in the table are expressed in terms
of mass percentage.
Table 2 shows the details of the production
conditions. In the table, "SRT" means the slab reheating
temperature, "FT" the finish rolling temperature at the
final pass, and "reduction ratio" the total reduction
ratio in the temperature range of the Ar3 transformation
temperature + 100°C or lower. Note that, in the case
where a hot-rolled steel sheet is cold rolled, it is not
necessary to restrict the reduction ratio of hot rolling
and, for this reason, the space of "reduction ratio" is
filled with a dash meaning "not applicable." Further,
"lubrication" indicates if or not lubrication is applied
in the temperature range of the Ar3 transformation
temperature + 100°C or lower.
In the column of "coiling", O means that the
coiling temperature (CT) is equal to or lower than To,
and X that the coiling temperature is higher than To.
Note that, in the case of a cold-rolled steel sheet, the
space is filled with a dash meaning "not applicable,"
because it is not necessary to restrict the coiling
temperature as one of the production conditions.
Some of the steel sheets were subjected to pickling,
cold rolling and annealing after hot rolling. The
thickness of the cold-rolled steel sheets ranged from 0.7
to 2.3 mm.
Also in the table, "cold reduction ratio" means the
total reduction ratio of the cold rolling, and "time" the
time of annealing. In the column of "annealing", O
means that the annealing temperature is within the range
from the recovery temperature to the Ar3 transformation
temperature + 100°C, and X that it is outside the range.
Steel L was subjected to descaling under the conditions
of an impact pressure of 2.7 MPa and a flow rate of 0.001
1/cm2 after the rough rolling. Further, among the steels
CA 02438393 2003-08-14
49 -
mentioned above, steels G and F-5 were subjected to zinc
plating.
The hot-rolled steel sheets thus prepared were
subjected to a tensile test in accordance with the test
method specified in JIS Z 2241, after forming the
specimens into No. 5 test pieces according to JIS Z 2201.
The yield strength (aY), tensile strength (aB) and
breaking elongation (E1) of the steel sheets are shown
also in Table 2.
Then, a test piece 30 mm in diameter was cut out
from a position of 1/4 or 3/4 of the width of each of the
steel sheets, the surfaces were ground to a depth of
about 0.05 mm so that the surfaces might have the three-
triangle grade finish (the second finest finish) and,
subsequently, strain was removed by chemical polishing or
electrolytic polishing. The test pieces thus prepared
were subjected to X-ray diffraction strength measurement
in accordance with the method described in pages 274 to
296 of the Japanese translation of Elements of X-ray
Diffraction by B. D. Cullity (published in 1986 by AGNE
Gijutsu Center, translated by Gentaro Matsumura).
Here, the average of the ratios of the X-ray
diffraction strength in the orientation component group
of {100}<011> to {223}<110> to random X-ray diffraction
strength is obtained from the X-ray diffraction strengths
in the principal orientation components included in the
orientation component group, namely {100}<O11>,
{116}<110>, {114}<110>, {i13}<110>, {112}<110>,
{335}<110> and {223}<110>, in the three-dimensional
texture calculated either by the vector method based on
the pole figure of {110} or by the series expansion
method using two or more (desirably, three or more) pole
figures out of the pole figures of {110}, {100}, {2i1}
and {310}.
For example, in the case of obtaining the ratios of
the X-ray diffraction strength in the above crystal
orientation components to random X-ray diffraction
CA 02438393 2003-08-14
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strength by the latter method, the strengths of (001)[1-
10], (116)[1-10], {114)[1-10], (113)[1-10], (112)[1-10],
(335)[1-10] and {223)[1-10] at a ~2 = 45° cross section
in a three-dimensional texture may be used without
modification. Note that the average of the ratios of the
X-ray diffraction strength in the orientation component
group of X100}<O11> to 1223}<110> to random X-ray
diffraction strength is the arithmetic average of the
ratios in all the above orientation components.
When it is impossible to obtain the strengths in all
these orientation components, the arithmetic average of
the strengths in the orientation components of
X100}<O11>, {116}<110>, {114}<110>, {112}<110> and
{223}<110> may be used as a substitute.
Likewise, the average of the ratios of the X-ray
diffraction strength in the three orientation components
of {554}<225>, X111}<112> and X111}<110> to random X-ray
diffraction strength can be obtained from the three-
dimensional texture calculated in the same manner as
explained above.
In Table 2, "strength ratio 1" under "ratios of X-
ray diffraction strength to random X-ray diffraction
strength" means the average of the ratios of the X-ray
diffraction strength in the orientation component group
of X100}<O11> to {223}<110> to random X-ray diffraction
strength, and "strength ratio 2" the average of the
ratios of the X-ray diffraction strength in the above
three orientation components of X554}<225>, X111}<112>
and dill}<110> to random X-ray diffraction strength.
Next, for the purpose of investigating the notch-
fatigue strength of the above steel sheets, a test piece
for fatigue test having the shape shown in Fig. 1(b) was
cut out from a position of 1/4 or 3/4 of the width of
each of the steel sheets so that the longitudinal
direction of the test piece coincided with the rolling
direction of the steel sheet, and subjected to a fatigue
test. The surfaces of the test pieces for fatigue test
CA 02438393 2003-08-14
- 51 -
were ground to a depth of about 0.05 mm so that the
surfaces might have the second finest finish, and the
fatigue test was carried out using an eiectro-hydraulic
servo type fatigue tester and methods conforming to JIS Z
2273-1978 and Z 2275-1978. The notch-fatigue limit
(o'WK) and notch-fatigue limit ratio (~WK/aB) of each of
the steel sheets are shown also in Table 2.
The samples according to the present invention are
11 steels, namely steels A, E, F-1, F-2, F-5, G, H, I, J,
K and L. In these samples, obtained are the thin steel
sheets for automobile use excellent in notch-fatigue
strength, each of the steel sheets being characterized in
that: the steel sheet contains prescribed amounts of
chemical components; on a plane at an arbitrary depth
within 0.5 mm from the surface of the steel sheet in the
thickness direction thereof, the average of the ratios of
the X-ray diffraction strength in the orientation
component group of X100}<O11> to .(223}<110> to random X-
ray diffraction strength is 2 or more and the average of
the ratios of the X-ray diffraction strength in the three
orientation components of X554}<225>, X111}<112> and
X111}<110> to random X-ray diffraction strength is 4 or
less; and the thickness of the steel sheet is in the
range from 0.5 to 12 mm. As a consequence, in the
evaluations by the methods according to the present
invention, the fatigue limit ratios of these steels were
superior to those of conventional steels which ranged
from 20 to 30~.
All the steels other, than those mentioned above, in
the tables were outside the ranges of the present
invention for the following reasons.
In steel B, the content of C was outside the range
specified in the present invention and, as a consequence,
a sufficient strength (oB) was not obtained. In steel
C, the content of P was outside the range specified in
the present invention and, as a consequence, a sufficient
CA 02438393 2003-08-14
- 52 _
notch-fatigue strength ratio (QWK/aB) was not obtained.
In steel D, the content of S was outside the range
specified in the present invention and, as a consequence,
a sufficient elongation (E1) was not obtained. In steel
F-3, as the total reduction ratio in the temperature
range of the Ar3 transformation temperature + 100°C or
lower was outside the range specified in the present
invention, the texture intended in the present invention
was not obtained and, as a consequence, a sufficient
notch-fatigue strength ratio (aWK/aB) was not obtained.
In steel F-4, as the finish rolling end temperature
(FT) and the coiling temperature were outside the
respective ranges specified in the present invention, the
texture intended in the present invention was not
obtained and, as a consequence, a sufficient notch-
fatigue strength ratio (oWK/aB) was not obtained. In
steel F-6, as the cold reduction ratio was outside the
range specified in the present invention, the texture
intended in the present invention was not obtained and,
as a consequence, a sufficient notch-fatigue strength
ratio (aWK/oB) was not obtained. In steel F-7, as the
annealing temperature was outside the range specified in
the present invention, the texture intended in the
present invention was not obtained and, as a consequence,
a sufficient notch-fatigue strength ratio (aWK/aB) was
not obtained. In steel F-8, as the annealing time was
outside the range specified in the present invention, the
texture intended in the present invention was not
obtained and, as a consequence, a sufficient notch-
fatigue strength ratio (cfWK/aB) was not obtained.
(Example 2)
The present invention is hereafter explained in more
detail based on Example~2.
Slabs of two steels G and H having the chemical
CA 02438393 2003-08-14
- 53 -
components shown in Table 1 were repeated to the
repeating temperatures shown in Table 3, rolled through
rough rolling and then finish rolling into steel sheets
1.5 to 5.5 mm in thickness, and then coiled. As shown in
Table 3, some of the steel sheets were subjected to
descaling under the conditions of an impact pressure of
2.7 MPa and a flow rate of 0.001 1/cm2 after the rough
rolling.
Table 3 shows the details of the production
conditions. In the table, "SRT" means the slab repeating
temperature, "FT" the finish rolling temperature at the
final pass, and "reduction ratio" the total reduction
ratio in the temperature range of the Ar3 transformation
temperature + 100°C or lower. Note that, in the case
where a hot-rolled steel sheet is cold rolled, it is not
necessary to restrict the reduction ratio of hot rolling
and, for this reason, the space "reduction ratio" is
filled with a dash meaning "not applicable." Further,
"lubrication" indicates if or not lubrication is applied
in the temperature range of the Ar3 transformation
temperature + 100°C or lower. Furthermore, "CT"
indicates the coiling temperature. Note that, in the
case of a cold-rolled steel sheet, the space is filled
with a dash meaning "not applicable," because it is not
necessary to restrict the coiling temperature as one of
the production conditions. Some of the steel sheets were
subjected to pickling, cold rolling and heat treatment
after the hot rolling. The thickness of the cold-rolled
steel sheets ranged from 0.7 to 2.3 mm. Also in the
table, "cold reduction ratio" means the total reduction
ratio of the cold rolling, "ST" the temperature of the
heat treatment and "time" the time thereof. Some of the
steels were subjected to galvanizing .
The hot-rolled and cold-rolled steel sheets thus
prepared were subjected to a tensile test in the same
manner as described earlier.
CA 02438393 2003-08-14
- 59 -
The yield strength (vY), tensile strength (aB),
breaking elongation (E1), yield ratio (YR) and strength-
ductility index (QB x E1) of each of the steel sheets
are shown in Table 4. Burring workability (hole
expansibility) was evaluated following the hole expansion
test method according to the Standard ofwthe Japan Iron
and Steel Federation JFS T 1001-1996. Table 4 also shows
the hole expansion ratio (~,).
Table 4 shows the microstructures of the steel
sheets, too. Here, "others" accounts for pearlite and
any other phase than ferrite, bainite, retained austenite
and martensite, which are listed individually in Table 4.
The volume percentage of ferrite, bainite, retained
austenite, pearlite or martensite is defined as the area
percentage thereof in the microstructure of each of the
steel sheets observed with an optical microscope under a
magnification of 200 to 500 at a position in the depth of
1/4 of the steel sheet thickness on a section surface
along the rolling direction of a specimen which is cut
out from a position of 1/4 or 3/4 of the width of the
steel sheet, the section surface being polished and
etched with a nitral reagent and the reagent disclosed in
Japanese Unexamined Patent Publication No. H5-163590.
Because the crystal structure of austenite is
different from that of ferrite, they can be easily
distinguished from each other crystallographically.
Therefore, the volume percentage of retained austenite
can be obtained experimentally by the X-ray diffraction
method too, namely by the simplified method wherein the
volume percentage thereof is calculated with the
following equation on the basis of the difference between
austenite and ferrite in the reflection intensity of the
xa ray of Mo on their lattice planes:
Vy = (2/3)100/(0.7 x a(211)/y(220) + 1)} +
(1/3)100/(0.78 x a(211)/y(311) + 1)},
CA 02438393 2003-08-14
- 55 -
where, a(211), y(220) and y(311) are the X-ray reflection
intensities of the indicated lattice planes of ferrite
{a) and austenite {y), respectively. The measurement
result of the volume percentage of retained austenite was
substantially the same either by the optical microscope
observation or the X-ray diffraction method, and, thus,
the measured values by any of the two methods may be
used.
The X-ray diffraction strength was measured by the
same method as described earlier.
The fatigue test was carried out also in the same
manner as described earlier. The notch-fatigue limit
{aWK) and notch-fatigue limit ratio {aWK/QB) of the
steel sheets are shown also in Table 4.
The samples according to the present invention are 9
steels, namely steels g-1, g-2, g-3, g-5, g-6, g-7, h-1,
h-2 and h-3. In these samples, obtained are thin steel
sheets, for automobile use, excellent in notch-fatigue
strength, each of the steel sheets being characterized in
that: the steel sheet contains prescribed amounts of
chemical components; on a plane at an arbitrary depth
within 0.5 mm from the surface of the steel sheet in the
thickness direction thereof, the average of the ratios of
the X-ray diffraction strength in the orientation
component group of {100}<011> to X223}<110> to random X-
ray diffraction strength is 2 or more and the average of
the ratios of the X-ray diffraction strength in the three
orientation components of X554}<225>, {111}<112> and
.(111}<110> to random X-ray diffraction strength is 4 or
less; the thickness of the steel sheet is in the range
from 0.5 to 12 mm; and the microstructure is a compound
structure containing bainite or ferrite and bainite as
the phase accounting for the largest volume percentage, a
compound structure containing retained austenite by 5 to
25$ in terms of volume percentage and having the balance
mainly consisting of ferrite and bainite, or a compound
CA 02438393 2003-08-14
- 56 -
structure containing ferrite as the phase accounting for
the largest volume percentage and mainly martensite as
the second phase. As a consequence, in the evaluations
by the methods according to the present invention, the
fatigue limit ratios of these steels were significantly
superior to those of conventional steels which ranged
from 20 to 30~.
All the steels, other than those mentioned above, in
the table were outside the ranges of the present
invention for the following reasons.
In steel g-4, as the finish rolling end temperature
(FT) and the total reduction ratio in the temperature
range of the Ar3 transformation temperature + 100°C or
lower were outside the respective ranges specified in the
present invention, the texture intended in the present
invention was not obtained and, as a consequence, a
sufficient notch-fatigue strength ratio (dWK/QB) was not
obtained. In steel g-8, as the cold reduction ratio was
outside the range specified in the present invention, the
texture intended in the present invention was not
obtained and, as a consequence, a sufficient notch-
fatigue strength ratio (aWK/aB) was not obtained. In
steel h-4, too, as the finish rolling end temperature
(FT) and the total reduction ratio in the temperature
range of the Ar3 transformation temperature + 100°C or
lower were outside the respective ranges specified in the
present invention, the texture intended in the present
invention was not obtained and, as a consequence, a
sufficient notch-fatigue strength ratio (crWK/aB) was not
obtained.
CA 02438393 2003-08-14
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CA 02438393 2003-08-14
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CA 02438393 2003-08-14
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CA 02438393 2003-08-14
- 61 -
Effect of the Invention
As has been explained in detail, the present
invention relates to a thin steel sheet, for automobile
use, excellent in notch-fatigue strength, and a method
for producing the steel sheet. The use of a thin steel
sheet according to the present invention makes it
possible to expect a significant improvement in notch-
fatigue strength that is one of the essential properties
of such a structural member including an undercarriage
component of an automobile to overcome the problem of
generating the propagation of a fatigue crack from a site
of stress concentration including a blanked or welded
portion and thus to require durability. For this reason,
the present invention is of a high industrial value.
