U.S. patent application number 10/468945 was filed with the patent office on 2004-04-15 for thin steel sheet for automobile excellent in notch fatigue strength and method for production thereof.
Invention is credited to Nakamoto, Takehiro, Sugiura, Natsuko, Tsuchihashi, Koichi, Yokoi, Tatsuo, Yoshinaga, Naoki.
Application Number | 20040069382 10/468945 |
Document ID | / |
Family ID | 26610025 |
Filed Date | 2004-04-15 |
United States Patent
Application |
20040069382 |
Kind Code |
A1 |
Yokoi, Tatsuo ; et
al. |
April 15, 2004 |
Thin steel sheet for automobile excellent in notch fatigue strength
and method for production thereof
Abstract
The present invention provides a thin steel sheet, for
automobile use, excellent in notch-fatigue strength, and a method
for producing said steel sheet. Specifically, the present invention
is a thin steel sheet for automobile use excellent in notch-fatigue
strength, said 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% Al, 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 said 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 and that the thickness of
said steel sheet is in the range from 0.5 to 12 mm, and a method
for producing said steel sheet by subjecting a steel slab
containing aforementioned chemical components to rolling at a total
reduction ratio of 25% or more in a temperature range of the
Ar.sub.3 transformation temperature+100.degree. C. or lower.
Inventors: |
Yokoi, Tatsuo; (Osaka,
JP) ; Sugiura, Natsuko; (Chiba, JP) ;
Yoshinaga, Naoki; (Chiba, JP) ; Tsuchihashi,
Koichi; (Oita, JP) ; Nakamoto, Takehiro;
(Oita, JP) |
Correspondence
Address: |
Robert T Tobin
Kenyon & Kenyon
One Broadway
New York
NY
10004
US
|
Family ID: |
26610025 |
Appl. No.: |
10/468945 |
Filed: |
August 22, 2003 |
PCT Filed: |
February 20, 2002 |
PCT NO: |
PCT/JP02/01498 |
Current U.S.
Class: |
148/650 ;
148/320 |
Current CPC
Class: |
C21D 8/0226 20130101;
C22C 38/06 20130101; C21D 8/0273 20130101; C22C 38/04 20130101;
C22C 38/002 20130101; C23C 2/40 20130101; C23C 2/02 20130101; C22C
38/02 20130101 |
Class at
Publication: |
148/650 ;
148/320 |
International
Class: |
C21D 008/00; C22C
038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 23, 2001 |
JP |
2001-049012 |
Aug 16, 2001 |
JP |
2001-247306 |
Claims
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}<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; 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 claim 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 claim 1, 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.
4. A thin steel sheet for automobile use excellent in notch-fatigue
strength according to claim 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.
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% Al, 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
{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 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 claim 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 claim 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 at 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
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 claims 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% Al, with the
balance consisting of Fe and unavoidable impurities, is subjected,
in a hot rolling 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 Ar.sub.3
transformation temperature +100.degree. 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}<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
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 claim 9,
characterized by cooling the steel sheet at a cooling rate of
20.degree. C./sec. or higher after the finish rolling and then
coiling it at a coiling temperature of 450.degree. C. or
higher.
11. A method for producing a thin steel sheet, for automobile use,
excellent in notch-fatigue strength according to claim 9,
characterized by retaining the steel sheet for 1 to 20 sec. in the
temperature range from the Ar.sub.1 transformation temperature to
the Ar.sub.3 transformation temperature after the finish rolling
then cooling it at a cooling rate of 20.degree. C./sec. or higher
and then coiling it at a coiling temperature in the range from
higher than 350.degree. C. to lower than 450.degree. C.
12. A method for producing a thin steel sheet, for automobile use,
excellent in notch-fatigue strength according to claim 11,
characterized by coiling the steel sheet at a coiling temperature
of 350.degree. 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 claims
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 claims
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% Al, with the
balance consisting of Fe and unavoidable impurities, is 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 Ar.sub.3 transformation temperature +100.degree. 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 Ac.sub.3 transformation
temperature +100.degree. 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 {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 and that the
thickness of the steel sheet is in the range from 0.5 to 12 mm.
16. A method for producing a thin steel sheet, for automobile use,
excellent in notch-fatigue strength according to claim 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 Ac.sub.1 transformation temperature to the Ac.sub.3
transformation temperature +100.degree. C. and then cooling it.
17. A method for producing a thin steel sheet for automobile use
excellent in notch-fatigue strength according to claim 15,
characterized by subjecting the steel sheet to a heat treatment
comprising the processes of, in sequence, retaining the cold-rolled
steel sheet for 5 to 150 sec. in said temperature range, cooling it
at a cooling rate of 20.degree. C./sec. or higher to the
temperature range from higher than 350.degree. C. to lower than
450.degree. C., retaining it for 5 to 600 sec. in said temperature
range, and then cooling it at a cooling rate of 5.degree. C./sec.
or higher to the temperature range of 200.degree. C. or lower.
18. A method for producing a thin steel sheet, for automobile use,
excellent in notch-fatigue strength according to claim 15,
characterized in 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.degree. C./sec. or higher to the temperature
range of 350.degree. 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 claims
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 claim 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 automobile use,
excellent in notch-fatigue strength according to claim 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 claim 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 claims 9 to
22, further applying galvanizing to the surfaces of the steel sheet
by dipping the steel sheet in a zinc plating bath.
24. A method for producing a thin steel sheet, for automobile use,
excellent in notch-fatigue strength according to claim 23,
characterized by further subjecting the steel sheet to an alloying
treatment after the galvanizing.
Description
TECHNICAL FIELD
[0001] 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
[0002] 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.
[0003] 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 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.
[0004] 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.
[0005] 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.
[0006] 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 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.
[0007] 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.
[0008] 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.
[0009] 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 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.
[0010] 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.
[0011] 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 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] Besides the above, no invention has been proposed up to now
wherein notch-fatigue properties are evaluated 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
[0016] 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.
[0017] 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 {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;
and that the thickness of the steel sheet is in the range from 0.5
to 12 mm.
[0018] The gist of the present invention, therefore, is as
follows:
[0019] (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}<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; and that the thickness of
the steel sheet is in the range from 0.5 to 12 mm.
[0020] (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.
[0021] (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.
[0022] (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.
[0023] (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% Al, 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 {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; and that the thickness of
the steel sheet is in the range from 0.5 to 12 mm.
[0024] (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.
[0025] (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 martensite as the second phase.
[0026] (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).
[0027] (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% Al, with
the balance consisting of Fe and unavoidable impurities, is
subjected, in a hot rolling 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 Ar.sub.3
transformation temperature +100.degree. 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}<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;
and that the thickness of the steel sheet is in the range from 0.5
to 12 mm.
[0028] (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.degree. C./sec. or higher after the finish rolling; and
then coiling it at a coiling temperature of 450.degree. C. or
higher.
[0029] (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 Ar.sub.1 transformation
temperature to the Ar.sub.3 transformation temperature after the
finish rolling; then cooling it at a cooling rate of 20.degree.
C./sec. or higher; and coiling it at a coiling temperature in the
range from higher than 350.degree. C. to lower than 450.degree.
C.
[0030] (12) A method for producing a thin steel sheet for
automobile use excellent in notch-fatigue strength according to the
item (11), characterized by coiling the steel sheet at a coiling
temperature of 350.degree. C. or lower after the cooling.
[0031] (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.
[0032] (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.
[0033] (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% Al, with the balance consisting of Fe and unavoidable
impurities, is 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 Ar.sub.3 transformation
temperature +100.degree. 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 Ac.sub.3 transformation temperature +100.degree. 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 {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; and that the thickness of the steel sheet is
in the range from 0.5 to 12 mm.
[0034] (16) 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 Ac.sub.1 transformation temperature to
the Ac.sub.3 transformation temperature +100.degree. C. and then
cooling it.
[0035] (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 steel sheet for 5 to 150 sec. in said temperature
range, cooling it at a cooling rate of 20.degree. C./sec. or higher
to the temperature range from higher than 350.degree. C. to lower
than 450.degree. C., retaining it for 5 to 600 sec. in said
temperature range, and then cooling it at a cooling rate of
5.degree. C./sec. or higher to the temperature range of 200.degree.
C. or lower.
[0036] (18) 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 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.degree. C./sec. or higher to the
temperature range of 350.degree. C. or lower.
[0037] (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.
[0038] (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.
[0039] (21) A method for producing a thin steel sheet for
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.
[0040] (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.
[0041] (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.
[0042] (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
[0043] 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.
[0044] FIG. 2 is a graph showing the result of a preliminary test
that leads to the present invention in terms of the relationship
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 (the fatigue strength for finite life after
10.sup.7 cycles of repetition, namely the fatigue limit).
BEST MODE FOR CARRYING OUT THE INVENTION
[0045] In the first place, the results of preliminary studies that
lead to the present invention are explained hereafter.
[0046] 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.
[0047] The present inventors investigated the influences of 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 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% Al, 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
Ar.sub.3 transformation temperature, and then coiling the
hot-rolled steel sheet.
[0048] For the purpose of measuring 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 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 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.
[0049] 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 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).
[0050] 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 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}.
[0051] 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
.phi.2=45.degree. 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}<011> to {223}<110> to random X-ray
diffraction strength is the arithmetic average of the ratios in all
the above orientation components.
[0052] 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}<011>, {116}<110>,
{114}<110>, {112}<110> and {223}<110> may be used
as a substitute.
[0053] Likewise, 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 can be obtained from the three-dimensional texture
calculated in the same manner as explained above.
[0054] 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.
[0055] 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}<011> 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 over
notch-fatigue strength. The numeral in a circle in the figure
indicates the fatigue limit (the fatigue strength for finite life
after 10.sup.7 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-fatigue
strength.
[0056] 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.
[0057] 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.
[0058] 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.
[0059] The reason for the above is not altogether clear, but it is
presumed to be as follows.
[0060] 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.
[0061] Next, the reasons for limiting the thickness of a steel
sheet in the present invention are explained.
[0062] 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.
[0063] 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 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.
[0064] The microstructure of a steel sheet according to the present
invention is explained hereafter.
[0065] 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, for 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.
[0066] 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-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.
[0067] 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.
[0068] 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:
V.gamma.=(2/3){100/(0.7.times..alpha.(211)/.gamma.(220)+1)}+(1/3){100/(0.7-
8.times..alpha.(211)/.gamma.(311)+1)},
[0069] where, .alpha.(211), .gamma.(220) and .gamma.(311) are the
X-ray reflection intensities of the indicated lattice planes of
ferrite (.alpha.) and austenite (.gamma.), respectively.
[0070] 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%.
[0071] 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
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.
[0072] Next, the reasons for limiting the chemical components in
the present invention are explained.
[0073] 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.
[0074] 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%.
[0075] 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.ltoreq.20 is satisfied in
terms of mass percentage. Further, Mn is an element that 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 0.1% 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.
[0076] 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.
[0077] 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.
[0078] Al 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.
Al increases the formation of non-metallic inclusions and
deteriorates elongation when added excessively and, for this
reason, a desirable content of Al is 0.5% or less.
[0079] 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.degree. 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.
[0080] B is added as occasion demands, as B has an effect of
raising fatigue limit when added in combination with 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%.
[0081] 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%.
[0082] 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 0.0005 to 0.002% and REM by 0.0005 to
0.02%.
[0083] 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.
[0084] 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.
[0085] Now, the reasons for limiting the conditions of the
production method according to the present invention are explained
in detail hereafter.
[0086] A steel sheet according to the present invention can be
produced through any of the following process routes: 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.
[0087] 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 reheating furnace.
[0088] No limit is particularly set to the temperature of
reheating, but it is desirable that a reheating temperature is
lower than 1,400.degree. C., since, when it is 1,400.degree. C. or
higher, the descale amount becomes large and the product yield
decreases. It is also desirable that a reheating temperature is
1,000.degree. C. or higher, since a reheating temperature lower
than 1,000.degree. C. remarkably deteriorates the operation
efficiency of a rolling mill in terms of rolling schedule.
[0089] 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).times.L(1/cm.sup.2).gtoreq.0.0025,
[0090] where, P (MPa) is an impact pressure of high-pressure water
on a steel sheet surface, and L (1/cm.sup.2) a flow rate of
descaling water.
[0091] An impact pressure P of high-pressure water on a steel sheet
surface is expressed as follows (see Tetsu-to-Hagan, 1991, Vol. 77,
No. 9, p.1450):
P(MPa)=5.64.times.P.sub.0.times.V/H.sup.2,
[0092] where, P.sub.0 (MPa) is a pressure of liquid, V (l/min.) a
liquid flow rate of a nozzle, and H (cm) a distance between a
nozzle and the surface of a steel sheet.
[0093] The flow rate L (l/cm.sup.2) is expressed as follows:
L(1/cm.sup.2)=V/(W.times.V),
[0094] where, V (l/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.
[0095] 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.
[0096] It is preferable, further, that the maximum roughness height
Ry of a steel sheet after finish rolling is 15 .mu.m (15 .mu.m Ry,
l 2.5 mm, ln 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.
[0097] Besides the above, finish rolling may be carried out
continuously by welding sheet bars together after rough 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.
[0098] 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 Ar.sub.3
transformation temperature +100.degree. C. or lower during the
latter half of the finish rolling. Here, the Ar.sub.3
transformation temperature can be expressed, in a simplified
manner, in relation to steel chemical components, for instance, by
the following equation:
Ar.sub.3=910-310.times.% C+25.times.% Si-80.times.% Mn.
[0099] When the total reduction ratio in the temperature range of
the Ar.sub.3 transformation temperature +100.degree. 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
Ar.sub.3 transformation temperature +100.degree. C. or lower is 35%
or more.
[0100] 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 Ar.sub.3 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
Ar.sub.3 transformation 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 Ar.sub.3 transformation
temperature is acceptable.
[0101] The present invention does not specify an upper limit of the
total reduction ratio in the temperature range of the Ar.sub.3
transformation temperature +100.degree. 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.
[0102] Here, when the friction between a hot-rolling roll and a
steel sheet is large during hot rolling in the temperature range of
the Ar.sub.3 transformation temperature +100.degree. 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.
[0103] 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 Ar.sub.3
transformation temperature +100.degree. 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
rolling in the temperature range of the Ar.sub.3 transformation
temperature +100.degree. C. or lower.
[0104] 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.
[0105] 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
Ar.sub.3 transformation temperature. This is because, if a rolling
temperature is lower than the Ar.sub.3 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
Ar.sub.3 transformation temperature.
[0106] The present invention does not specify an upper limit of a
finishing temperature, but, if a finishing temperature exceeds the
Ar.sub.3 transformation temperature +100.degree. C., it becomes
practically impossible to carry out rolling at a total reduction
ratio of 25% or more in the temperature range of the Ar.sub.3
transformation temperature +100.degree. C. or lower. For this
reason, it is desirable that an upper limit of a finishing
temperature is the Ar.sub.3 transformation temperature +100.degree.
C. or lower.
[0107] 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
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.degree. 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.degree. 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.degree. C./sec. or
higher.
[0108] 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 Ar.sub.3 transformation temperature
to the Ar.sub.1 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 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.
[0109] 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
Ar.sub.1 transformation temperature to 800.degree. 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.degree. C./sec. or higher after
completing finish rolling.
[0110] 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.degree. 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 Ar.sub.1
transformation temperature or lower, losing the ductility
improvement effect. For this reason, a cooling rate here is,
desirably, 150.degree. C./sec. or lower.
[0111] Subsequently, a steel sheet is cooled at a cooling rate of
20.degree. C./sec. or higher from the above temperature range to a
coiling temperature (CT). When a cooling rate is lower than
20.degree. 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 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.degree. C./sec. or lower.
[0112] 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 Ar.sub.3 transformation temperature to
the Ar.sub.1 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.
[0113] 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
Ar.sub.1 transformation temperature to 800.degree. 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.degree. 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.degree. 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 Ar.sub.1 transformation temperature or lower.
For this reason, a cooling rate here is, desirably, 150.degree.
C./sec. or lower.
[0114] Subsequently, a steel sheet is cooled at a cooling rate of
20.degree. C./sec. or higher from the above temperature range to a
coiling temperature (CT). When a cooling rate is lower than
20.degree. 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.degree. C./sec. or
lower.
[0115] 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 sec. in the temperature range from the
Ar.sub.3 transformation temperature to the Ar.sub.1 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.
[0116] 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
Ar.sub.1 transformation temperature to 800.degree. 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.degree. 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.degree. 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 Ar.sub.1 transformation temperature or lower.
For this reason, a cooling rate here is, desirably, 150.degree.
C./sec. or lower.
[0117] Subsequently, a steel sheet is cooled at a cooling rate of
20.degree. C./sec. or higher from the above temperature range to a
coiling temperature (CT). When a cooling rate is lower than
20.degree. 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.
[0118] 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.degree. C./sec. or
lower.
[0119] 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 Ar.sub.3 transformation temperature
+100.degree. C. or lower, it is desirable to coil a steel sheet at
the coiling temperature To shown below or lower. Note that it is
unnecessary to set the temperature T.sub.0 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:
T.sub.0=-650.4.times.% C+B,
[0120] where, B is determined as follows:
B=-50.6.times.Mneq+894.3,
[0121] where, Mneq is determined from the mass percentages of the
component elements as shown below:
Mneq=% Mn+0.24.times.% Ni+0.13.times.% Si+0.38.times.%
Mo+0.55.times.% Cr+0.16.times.% Cu-0.50.times.% Al-0.45.times.%
Co+0.90.times.% V.
[0122] Note that the influences on T.sub.0 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.
[0123] 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.degree. C.
[0124] 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.degree. C. or higher. This is because, when a
coiling temperature is lower than 450.degree. 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.
[0125] 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.degree. C./sec. or higher to a
temperature of 200.degree. 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.
[0126] 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.degree. C. This is because, when a coiling temperature is
450.degree. 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.degree. 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.degree. C.
[0127] 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.degree. C./sec. or higher up to a
temperature of 200.degree. 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.
[0128] 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 coiling temperature has to be
restricted to 350.degree. C. or lower. This is because, when a
coiling temperature exceeds 350.degree. 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.degree. C.
[0129] 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.
[0130] 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 Ar.sub.3
transformation temperature +100.degree. 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 Ar.sub.3
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 treatment.
[0131] 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 {111} 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.
[0132] 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.
[0133] In the first place, a steel sheet is subjected to a heat
treatment for 5 to 150 sec. in the temperature range of the
Ac.sub.3 transformation temperature +100.degree. C. or lower. When
an upper limit of a heat treatment temperature exceeds the Ac.sub.3
transformation temperature +100.degree. 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 Ac.sub.3 transformation temperature +100.degree. C. or
lower.
[0134] The Ac.sub.1 and Ac.sub.3 transformation temperatures
mentioned herein can be expressed in relation to steel chemical
components using, for example, the expressions according to p. 273
of the Japanese translation of The Physical Metallurgy of Steels by
W. C. Leslie (published by Maruzen in 1985, translated by Hiroshi
Kumai and Tatsuhiko Noda).
[0135] 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 is worse, productivity is
lowered. For this reason, a retention time is determined to be in
the range from 5 to 150 sec.
[0136] 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.
[0137] 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.sub.1 transformation
temperature or higher. When a lower limit of a heat treatment
temperature is lower than the Ac.sub.1 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 Ac.sub.1 transformation temperature to the Ac.sub.3
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 Ac.sub.3 transformation temperature to the Ac.sub.3
transformation temperature +100.degree. C. in order to increase the
volume percentage of bainite.
[0138] 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 Ac.sub.1 transformation
temperature to the Ac.sub.3 transformation temperature, it is
desirable to cool a steel sheet at a cooling rate of 20.degree.
C./sec. or higher to the temperature range from higher than
350.degree. C. to the temperature To specified herein earlier. This
is because, when a cooling rate is lower than 20.degree. 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.degree. 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 is desirable that a
cooling end temperature is higher than 350.degree. 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.
[0139] Finally, when a cooling rate to the cooling end temperature
is 20.degree. 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.degree. C./sec. Further, when a
cooling end temperature is higher than 200.degree. C., aging
properties may deteriorate, and, for this reason, it is desirable
that a cooling end temperature is 200.degree. 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.degree. C.
[0140] On the other hand, in the case where above mentioned heat
treatment temperature is in the range from higher than the Ac.sub.3
transformation temperature to the Ac.sub.3 transformation
temperature +100.degree. C., it is desirable to cool a steel sheet
at a cooling rate of 20.degree. C./sec. or higher to a temperature
of 200.degree. C. or lower. This is because, when a cooling rate is
lower than 20.degree. 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.degree. C., aging properties may
deteriorate. For this reason, it is desirable that a cooling end
temperature is 200.degree. 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.degree. C.
[0141] 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 Ac.sub.1
transformation temperature to the Ac.sub.3 transformation
temperature +100.degree. 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.degree. C. to 850.degree. C. When a cooling rate after
retention is lower than 20.degree. 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.degree. C./sec. or higher.
[0142] 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.degree. C., retained austenite is decomposed into bainite or
pearlite containing much carbide, and 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 cooling end temperature is not
higher than 350.degree. 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.degree. C. to lower
than 450.degree. C.
[0143] 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.
[0144] Finally, when a cooling rate up to the end of cooling is
lower than 5.degree. 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 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.degree. C./sec. or higher.
[0145] In addition, when a cooling end temperature is higher than
200.degree. C., aging properties may deteriorate and, for this
reason, a cooling end temperature must be 200.degree. 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.degree. C.
[0146] 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
Ac.sub.1 transformation temperature to the Ac.sub.3 transformation
temperature +100.degree. 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 steel sheet to a temperature in the range from
780.degree. C. to 850.degree. C.
[0147] When a cooling rate after retention is lower than 20.degree.
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.degree.
C./sec. or higher. When a cooling end temperature is higher than
350.degree. 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.degree.
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.degree. C.
[0148] Thereafter, skin pass rolling may be applied, if
required.
[0149] 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
[0150] The present invention is further explained hereafter based
on Example 1.
[0151] 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 5.5 mm in thickness, and then
coiled. Note that the chemical components in the table are
expressed in terms of mass percentage.
[0152] 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 Ar.sub.3
transformation temperature +100.degree. 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 Ar.sub.3
transformation temperature +100.degree. C. or lower.
[0153] In the column of "coiling", .largecircle. means that the
coiling temperature (CT) is equal to or lower than T.sub.0, and X
that the coiling temperature is higher than T.sub.0. 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.
[0154] 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.
[0155] 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", .largecircle. means that
the annealing temperature is within the range from the recovery
temperature to the Ar.sub.3 transformation temperature +100.degree.
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 l/cm.sup.2 after the rough rolling. Further,
among the steels mentioned above, steels G and F-5 were subjected
to zinc plating.
[0156] 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 (.sigma.Y), tensile
strength (.sigma.B) and breaking elongation (El) of the steel
sheets are shown also in Table 2.
[0157] 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).
[0158] 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}<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}.
[0159] 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
.phi.2=45.degree. 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}<011> to {223}<110> to random X-ray
diffraction strength is the arithmetic average of the ratios in all
the above orientation components.
[0160] 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}<011>, {116}<110>,
{114}<110>, {112}<110> and {223}<110> may be used
as a substitute.
[0161] Likewise, 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 can be obtained from the three-dimensional texture
calculated in the same manner as explained above.
[0162] 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 {100}<011> 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 {554}<225>,
{111}<112> and {111}<110> to random X-ray diffraction
strength.
[0163] 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 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
electro-hydraulic servo type fatigue tester and methods conforming
to JIS Z 2273-1978 and z 2275-1978. The notch-fatigue limit
(.sigma.WK) and notch-fatigue limit ratio (.sigma.WK/.sigma.B) of
each of the steel sheets are shown also in Table 2.
[0164] 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
{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; 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%.
[0165] All the steels other, than those mentioned above, in the
tables were outside the ranges of the present invention for the
following reasons.
[0166] In steel B, the content of C was outside the range specified
in the present invention and, as a consequence, a sufficient
strength (.sigma.B) 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 notch-fatigue strength ratio
(.sigma.WK/.sigma.B) 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 (El) was not obtained. In
steel F-3, as the total reduction ratio in the temperature range of
the Ar.sub.3 transformation temperature +100.degree. 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
(.sigma.WK/.sigma.B) was not obtained.
[0167] 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 (.sigma.WK/.sigma.B) 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 (.sigma.WK/.sigma.B) 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
(.sigma.WK/.sigma.B) 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 (.sigma.WK/.sigma.B) was not obtained.
Example 2
[0168] The present invention is hereafter explained in more detail
based on Example 2.
[0169] Slabs of two steels G and H having the chemical components
shown in Table 1 were reheated to the reheating 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/cm.sup.2 after the rough rolling.
[0170] Table 3 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 Ar.sub.3
transformation temperature +100.degree. 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 Ar.sub.3
transformation temperature +100.degree. 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.
[0171] The hot-rolled and cold-rolled steel sheets thus prepared
were subjected to a tensile test in the same manner as described
earlier.
[0172] The yield strength (.sigma.Y), tensile strength (.sigma.B),
breaking elongation (El), yield ratio (YR) and strength-ductility
index (.sigma.B.times.El) 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
of the Japan Iron and Steel Federation JFS T 1001-1996. Table 4
also shows the hole expansion ratio (.lambda.).
[0173] 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.
[0174] 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 K.alpha. ray of Mo on their lattice
planes:
V.gamma.=(2/3){100/(0.7.times..alpha.(211)/.gamma.(220)+1)}+(1/3){100/(0.7-
8.times..alpha.(211)/.gamma.(311)+1)},
[0175] where, .alpha.(211), .gamma.(220) and .gamma.(311) are the
X-ray reflection intensities of the indicated lattice planes of
ferrite (.alpha.) and austenite (.gamma.), 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.
[0176] The X-ray diffraction strength was measured by the same
method as described earlier.
[0177] The fatigue test was carried out also in the same manner as
described earlier. The notch-fatigue limit (.sigma.WK) and
notch-fatigue limit ratio (.sigma.WK/.sigma.B) of the steel sheets
are shown also in Table 4.
[0178] 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
{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; 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 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%.
[0179] All the steels, other than those mentioned above, in the
table were outside the ranges of the present invention for the
following reasons.
[0180] In steel g-4, as the finish rolling end temperature (FT) and
the total reduction ratio in the temperature range of the Ar.sub.3
transformation temperature +100.degree. 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
(.sigma.WK/.sigma.B) 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 (.sigma.WK/.sigma.B) 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 Ar.sub.3 transformation
temperature +100.degree. 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 (.sigma.WK/.sigma.B) was
not obtained.
1 TABLE 1 Chemical composition (in mass %) Steel C Si Mn P S Al
Others Remarks A 0.041 0.02 0.26 0.012 0.0011 0.033 REM: 0.008
Invented steel B 0.002 0.01 0.11 0.011 0.0070 0.044 Ti: 0.057
Comparative steel C 0.022 0.02 0.22 0.300 0.0015 0.012 Comparative
steel D 0.018 0.04 0.55 0.090 0.0400 0.033 Comparative steel E
0.058 0.92 1.16 0.008 0.0009 0.041 Cu: 0.48, B: 0.0002 Invented
steel F 0.081 0.88 1.24 0.007 0.0008 0.031 Invented steel G 0.049
0.91 1.27 0.006 0.0011 0.025 Cu: 0.78, Ni: 0.33 Invented steel H
0.094 1.89 1.87 0.008 0.0007 0.024 Ti: 0.071, Nb: 0.022 Invented
steel I 0.060 1.05 1.16 0.007 0.0008 0.033 Mo: 0.11 Invented steel
J 0.061 0.91 1.21 0.006 0.0011 0.030 V: 0.02, Cr: 0.08 Invented
steel K 0.055 1.21 1.10 0.008 0.0007 0.024 Zr: 0.03 Invented steel
L 0.050 1.14 1.00 0.007 0.0009 0.031 Ca: 0.0005 Invented steel
[0181]
2 TABLE 2 Ratios of X-ray Production conditions diffraction Cold
rolling and strength to annealing processes random X-ray Hot
rolling process Cold diffraction Reduc- reduc- strength tion tion
Strength Strength Classi- SRT FT ratio Lubrica- ratio Anneal- Time
ratio ratio Steel fication (.degree. C.) (.degree. C.) (%) tion
Coiling (%) ing (S) 1 2 A Hot- 1250 880 42 Not .largecircle. -- --
-- 5.8 0.7 rolled applied B Hot- 1250 890 30 Applied .largecircle.
-- -- -- 1.3 6.1 rolled C Hot- 1200 880 30 Not .largecircle. -- --
-- 0.8 1.3 rolled applied D Hot- 1200 880 30 Not .largecircle. --
-- -- 1.2 0.9 rolled applied E Hot- 1150 870 42 Not .largecircle.
-- -- -- 8.1 1.8 rolled applied F-1 Hot- 1200 870 42 Not
.largecircle. -- -- -- 7.2 2.1 rolled applied F-2 Hot- 1200 870 42
Applied .largecircle. -- -- -- 8.3 1.4 rolled F-3 Hot- 1300 950 0
Not .largecircle. -- -- -- 1.8 1.5 rolled applied F-4 Hot- 1300 970
0 Not X -- -- -- 1.8 1.7 rolled applied F-5 Cold- 1200 860 --
Applied -- 65 .largecircle. 90 4.2 2.3 rolled F-6 Cold- 1200 860 --
Applied -- 80 .largecircle. 90 2.8 4.2 rolled F-7 Cold- 1200 860 --
Applied -- 65 X 90 1.7 2.6 rolled F-8 Cold- 1200 860 -- Applied --
65 .largecircle. 2 1.8 2.2 rolled G Hot- 1150 870 71 Not
.largecircle. -- -- -- 8.5 0.8 rolled applied H Hot- 1250 870 30
Applied .largecircle. -- -- -- 8.7 0.9 rolled I Hot- 1200 870 42
Not .largecircle. -- -- -- 6.7 2.0 rolled applied J Hot- 1200 870
71 Not .largecircle. -- -- -- 5.9 2.1 rolled applied K Hot- 1200
870 71 Not .largecircle. -- -- -- 7.8 1.0 rolled applied L Hot-
1150 790 71 Not .largecircle. -- -- -- 11.0 1.4 rolled applied
Mechanical Fatigue properties properties .delta.Y .delta.B E1
.delta.Wk .delta.Wk/.alpha.B Steel (MPa) (MPa) (%) (MPa) (%)
Remarks A 221 311 47 100 32 Invented steel B 161 281 56 75 27
Comparative steel C 220 369 42 90 24 Comparative steel D 195 306 44
75 25 Comparative steel E 422 637 29 230 36 Invented steel F-1 438
668 28 230 34 Invented steel F-2 423 655 29 240 37 Invented steel
F-3 431 660 28 150 23 Comparative steel F-4 400 622 32 150 24
Comparative steel F-5 418 671 28 240 36 Invented steel F-6 433 667
28 150 22 Comparative steel F-7 552 721 20 150 21 Comparative steel
F-8 570 710 21 150 21 Comparative steel G 441 661 30 235 36
Invented steel H 776 986 16 340 34 Invented steel I 404 638 27 220
34 Invented steel J 431 623 26 220 35 Invented steel K 425 627 30
220 35 Invented steel L 401 588 25 210 36 Invented steel
[0182]
3 TABLE 3 Production conditions Cold rolling and annealing
processes Hot rolling process Cold Reduc- reduc- tion tion Classi-
SRT FT Ar.sub.3 + 100 ratio Lubri- CT TO ratio Ac.sub.1 ST Ac.sub.3
+ 100 Time 1 OA Time 2 CR Steel fication (.degree. C.) (.degree.
C.) (.degree. C.) (%) cation (.degree. C.) (.degree. C.) (%)
(.degree. C.) (.degree. C.) (.degree. C.) (S) (.degree. C.) (S)
(.degree. C/S) g-1 Hot 1150 870 916 71 Applied 50 782 -- -- -- --
-- -- -- -- rolled g-2 Hot 1150 870 916 71 Applied 400 782 -- -- --
-- -- -- -- -- rolled g-3 Hot 1150 890 916 42 Applied 600 782 -- --
-- -- -- -- -- -- rolled g-4 Hot 1250 930 916 0 Not 600 782 -- --
-- -- -- -- -- -- rolled applied g-5 Cold- 1150 870 -- -- Applied
-- -- 65 730 800 982 90 -- -- 5 rolled g-6 Cold- 1150 870 -- --
Applied -- -- 65 730 800 982 400 180 30 rolled g-7 Cold- 1150 870
-- -- Applied -- -- 65 730 800 982 90 -- -- 30 rolled g-8 Cold-
1150 870 -- -- Not -- -- 80 730 800 982 90 -- -- 5 rolled applied
h-1 Hot 1230 860 879 30 Applied 50 727 -- -- -- -- -- -- -- --
rolled h-2 Hot 1230 860 879 30 Applied 400 727 -- -- -- -- -- -- --
-- rolled h-3 Hot 1230 860 879 30 Applied 600 727 -- -- -- -- -- --
-- -- rolled h-4 Hot 1230 930 879 0 Not 730 727 -- -- -- -- -- --
-- -- rolled applied Ratios of X-ray diffraction strength to random
X-ray diffraction strength Strength Strength Classi- ratio ratio
Steel fication 1 2 g-1 Hot 8.2 1.1 rolled g-2 Hot 8.0 1.0 rolled
g-3 Hot 8.4 0.9 rolled g-4 Hot 1.8 1.5 rolled g-5 Cold- 4.4 2.2
rolled g-6 Cold- 4.6 2.4 rolled g-7 Cold- 4.8 2.6 rolled g-8 Cold-
2.6 4.3 rolled h-1 Hot 8.6 1.2 rolled h-2 Hot 8.5 0.9 rolled h-3
Hot 8.4 1.3 rolled h-4 Hot 1.8 2.1 rolled
[0183]
4TABLE 4 Microstructure Fatigue Retained Fatigue properties
properties Ferrite Bainite Martensite austenite Others .delta.Y
.delta.B E1 .lambda. .delta.B .times. E1 YR .delta.Wk
.delta.Wk/.alpha.B (%) (%) (%) (%) (%) (MPa) (MPa) (%) (%) (MPa*%)
(%) (MPa) (%) Remarks 85 0 13 2 0 470 772 26 52 20072 61 280 36
Invented steel 80 8 0 12 0 512 646 37 67 23902 79 220 34 Invented
steel 67 30 0 0 3 478 576 27 130 15552 83 190 33 Invented steel 65
35 0 0 0 502 588 28 141 16464 85 120 20 Comparative steel 70 28 0 0
2 482 584 25 87 14600 83 190 33 Invented steel 79 10 0 11 0 480 660
36 50 23760 73 230 35 Invented steel 87 0 10 3 0 444 731 26 42
19006 61 270 37 Invented steel 50 45 0 2 3 495 591 25 76 14775 84
135 23 Comparative steel 67 5 21 4 3 613 991 21 18 20811 62 330 33
Invented steel 63 15 3 17 2 694 902 27 26 24354 77 285 32 Invented
steel 35 55 3 4 3 670 823 18 76 14814 81 255 31 Invented steel 30
63 0 3 4 673 796 20 70 15920 85 180 23 Comparative steel
[0184] Effect of the Invention
[0185] 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.
* * * * *