U.S. patent application number 11/317174 was filed with the patent office on 2006-06-29 for high strength thin steel sheet having high hydrogen embrittlement resisting property.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). Invention is credited to Hiroshi Akamizu, Tomohiko Hojo, Shushi Ikeda, Junichiro Kinugasa, Yoichi Mukai, Kenji Saito, Koichi Sugimoto, Fumio Yuse.
Application Number | 20060137768 11/317174 |
Document ID | / |
Family ID | 35739879 |
Filed Date | 2006-06-29 |
United States Patent
Application |
20060137768 |
Kind Code |
A1 |
Yuse; Fumio ; et
al. |
June 29, 2006 |
High strength thin steel sheet having high hydrogen embrittlement
resisting property
Abstract
The purpose of the present invention is to provide a high
strength thin steel sheet that has high hydrogen embrittlement
resisting property. In order to achieve the above purpose, a high
strength thin steel sheet having high hydrogen embrittlement
resisting property comprises: C: 0.10 to 0.25%; Si: 1.0 to 3.0%;
Mn: 1.0 to 3.5%; P: 0.15% or less; S: 0.02% or less; and Al: 1.5%
or less (higher than 0%) in terms of percentage by weight, with
balance of iron and inevitable impurities; and the metal structure
comprises: residual austenite; 1% by area or more in proportion to
the entire structure; bainitic ferrite and martensite: 80% or more
in total; and ferrite and pearlite: 9% or less (may be 0%) in
total, while the mean axis ratio (major axis/minor axis) of said
residual austenite grains is 5 or higher, and the steel has tensile
strength of 1180 MPa or higher.
Inventors: |
Yuse; Fumio; (Kobe-shi,
JP) ; Ikeda; Shushi; (Kobe-shi, JP) ; Mukai;
Yoichi; (Kakogawa-shi, JP) ; Akamizu; Hiroshi;
(Kakogawa-shi, JP) ; Kinugasa; Junichiro;
(Kobe-shi, JP) ; Saito; Kenji; (Kobe-shi, JP)
; Sugimoto; Koichi; (Nagano-shi, JP) ; Hojo;
Tomohiko; (Nagano-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Kobe-shi
JP
SHINSHU TLO CO., LTD.
Ueda-shi
JP
|
Family ID: |
35739879 |
Appl. No.: |
11/317174 |
Filed: |
December 27, 2005 |
Current U.S.
Class: |
148/320 ;
420/103 |
Current CPC
Class: |
C22C 38/12 20130101;
C21D 2211/002 20130101; C22C 38/02 20130101; C22C 38/06 20130101;
C22C 38/08 20130101; C22C 38/16 20130101; C22C 38/04 20130101 |
Class at
Publication: |
148/320 ;
420/103 |
International
Class: |
C22C 38/06 20060101
C22C038/06 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 28, 2004 |
JP |
2004-381230 |
Dec 28, 2004 |
JP |
2004-381231 |
Dec 28, 2004 |
JP |
2004-381232 |
May 19, 2005 |
JP |
2005-147238 |
May 19, 2005 |
JP |
2005-147239 |
May 19, 2005 |
JP |
2005-147240 |
Claims
1. A high strength thin steel sheet having high hydrogen
embrittlement resisting property, which comprises: C: 0.10 to
0.25%; Si: 1.0 to 3.0%; Mn: 1.0 to 3.5%; P: 0.15% or less; S: 0.02%
or less; and Al: 1.5% or less (higher than 0%) in terms of
percentage by weight, with balance of iron and inevitable
impurities; wherein the metal structure comprises: residual
austenite; 1% by area or more in proportion to the entire
structure; bainitic ferrite and martensite: 80% or more in total;
and ferrite and pearlite: 9% or less (may be 0%) in total, while
the mean axis ratio (major axis/minor axis) of said residual
austenite grains is 5 or higher, and the steel has tensile strength
of 1180 MPa or higher.
2. The high strength thin steel sheet according to claim 1, wherein
the metal structure further satisfies the requirements that: mean
length of minor axes of said residual austenite grains is 1 .mu.m
or less; and minimum distance between said residual austenite
grains is 1 .mu.m or less.
3. The high strength thin steel sheet according to claim 1, wherein
0.5% or less (higher than 0%) by weight of Al is contained.
4. The high strength thin steel sheet according to claim 1, wherein
0.003 to 0.5% of Cu and/or 0.003 to 1.0% of Ni in terms of
percentage by weight are further contained.
5. The high strength thin steel sheet according to claim 1, wherein
0.003 to 1.0% of Ti and/or V in terms of percentage by weight are
further contained.
6. The high strength thin steel sheet according to claim 1, wherein
1.0% or less (higher than 0%) of Mo and 0.1% or less (higher than
0%) of Nb in terms of percentage by weight are further
contained.
7. The high strength thin steel sheet according to claim 1, wherein
0.0002 to 0.01% of B in terms of percentage by weight is further
contained.
8. The high strength thin steel sheet according to claim 1, wherein
at least one element selected from the group consisting of: 0.0005
to 0.005% of Ca; 0.0005 to 0.01% of Mg; and 0.0005 to 0.01% of REM
in terms of percentage by weight is further contained.
9. A high strength thin steel sheet having high hydrogen
embrittlement resisting property, which comprises: C: 0.10 to
0.25%; Si: 1.0 to 3.0%; Mn: 1.0 to 3.5%; P: 0.15% or less; S: 0.02%
or less; and Al: 1.5% or less (higher than 0%) in terms of
percentage by weight, with balance of iron and inevitable
impurities, wherein the metal structure comprises: residual
austenite of 1% by area or more in proportion to the entire
structure, while the mean axis ratio (major axis/minor axis) of
said residual austenite grains is 5 or higher; mean length of minor
axes of said residual austenite grains is 1 .mu.m or less; and
minimum distance between said residual austenite grains is 1 .mu.m
or less; and the steel has tensile strength of 1180 MPa or
higher.
10. The high strength thin steel sheet according to claim 9,
wherein 0.5% or less (higher than 0%) by weight of Al is
contained.
11. The high strength thin steel sheet according to claim 9,
wherein 0.003 to 0.5% of Cu and/or 0.003 to 1.0% of Ni in terms of
percentage by weight are further contained.
12. The high strength thin steel sheet according to claim 9,
wherein 0.003 to 1.0% of Ti and/or V in terms of percentage by
weight are further contained.
13. The high strength thin steel sheet according to claim 9,
wherein 1.0% or less (higher than 0%) of Mo and 0.1% or less
(higher than 0%) of Nb in terms of percentage by weight are further
contained.
14. The high strength thin steel sheet according to claim 9,
wherein 0.0002 to 0.01% of B in terms of percentage by weight is
further contained.
15. The high strength thin steel sheet according to claim 9,
wherein at least one element selected from the group consisting of:
0.0005 to 0.005% of Ca; 0.0005 to 0.01% of Mg; and 0.0005 to 0.01%
of REM in terms of percentage by weight is further contained.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a high strength thin steel
sheet that has high hydrogen embrittlement resisting property
(particularly the hydrogen embrittlement resisting property after
being subjected to forming process) and high workability,
especially to a high strength thin steel sheet that has high
resistance against fractures due to hydrogen embrittlement such as
season crack and delayed fracture that pose serious problems for
steel sheets having tensile strength of 1180 MPa or higher, and has
high workability.
[0003] 2. Description of the Related Art
[0004] There are increasing demands for the steel sheet, that is
pressed or bent into a form of a high-strength component of
automobile or industrial machine, to have both high strength and
high ductility at the same time. In recent years, there are
increasing needs for high strength steel sheets having strength of
1180 MPa or higher, as the automobiles are being designed with less
weight. A type of steel sheet that is regarded as promising to
satisfy these needs is TRIP (transformation induced plasticity)
steel sheet.
[0005] The TRIP steel sheet includes residual austenite structure
and, when processed to deform, undergoes considerable elongation
due to induced transformation of the residual austenite (residual
.gamma.) into martensite by the action of stress. Known examples of
the TRIP steel include TRIP type composite-structure steel (TPF
steel) that consists of polygonal ferrite as the matrix phase and
residual austenite; TRIP type tempered martensite steel (TAM steel)
that consists of tempered martensite as the matrix phase and
residual austenite; and TRIP type bainitic steel (TBF steel) that
consists of bainitic ferrite as the matrix phase and residual
austenite. Among these, the TBF steel has long been known
(described, for example, in NISSIN STEEL TECHNICAL REPORT, No. 43,
December 1980, pp 1-10), and has such advantages as the capability
to readily provide high strength due to the hard bainitic ferrite
structure, and the capability to show outstanding elongation
because fine residual austenite grains can be easily formed in the
boundary of lath-shaped bainitic ferrite in the bainitic ferrite
structure. The TBF steel also has such an advantage related to
manufacturing, that it can be easily manufactured by a single heat
treatment process (continuous annealing process or plating
process).
[0006] In the realm of high strength of 1180 MPa upward, however,
the TRIP steel sheet is known to suffer a newly emerging problem of
delayed fracture caused by hydrogen embrittlement, similarly to the
conventional high strength steel. Delayed fracture refers to the
failure of high-strength steel under stress, that occurs as,
hydrogen originating in corrosive environment or the atmosphere
infiltrates and diffuses in microstructural defects such as
dislocation, void and grain boundary, and makes the steel brittle.
This results in decreases in ductility and toughness of the
metallic material.
[0007] It has been well known that the high strength steel that is
widely used in the manufacture of PC steel wire and line pipe
experiences hydrogen embrittlement (pickling embrittlement, plating
embrittlement, delayed fracture, etc.) caused by the infiltration
of hydrogen into the steel when tensile strength of the steel
becomes 980 MPa or higher. Accordingly, most of technologies of
improving hydrogen embrittlement resisting property have been
developed aiming at steel members such as bolt. "New Development in
Elucidation of Delayed Fracture" (published by The Iron and Steel
Institute of Japan in January, 1997), for example, describes that
it is effective in improving the resistance against delayed
fracture to add element such as Cr, Mo or V that demonstrates
resistance against temper softening to the metal structure that is
based on tempered martensite as the major phase. This technology is
intended to cause the delayed fracture to take place within grains
instead of in the grain boundaries, thereby to constrain the
fracture from occurring, by precipitating alloy carbide and making
use thereof as the site for trapping hydrogen.
[0008] Thin steel sheets having strength higher than 780 MPa have
rarely been used for the reason of workability and weldability.
Also hydrogen embrittlement has rarely been regarded as a problem
for thin steel sheets where hydrogen that has infiltrated therein
is immediately released due to the small thickness. For these
reasons, much efforts have not been dedicated to counter the
hydrogen embrittlement. In recent years, however, higher strength
is required of the reinforcement members such as bumper, impact
beam and seat rail, etc., in order to meet the requirement of
weight reduction of the automobile and to improve the collision
safety. As a result, there have been increasing demands for high
strength steel sheet having strength of 980 MPa or higher for the
manufacture of these parts. This makes it necessary to improve
hydrogen embrittlement resisting property of the high strength
steel sheet.
[0009] Use of the technology addressed to the bolt steel described
above may be considered for improving the hydrogen embrittlement
resisting property of the high strength steel sheet. However, in
the case of "New Development in Elucidation of Delayed Fracture"
(published by The Iron and Steel Institute of Japan in January,
1997), for example, 0.4% or higher C content and much alloy
elements are contained, and therefore application of this
technology to a thin steel sheet compromises the workability
required of the thin steel sheet. The technology also has a
drawback related to the manufacturing process, since it takes
several hours or longer period of heat treatment to cause the alloy
carbide to precipitate. Therefore, improving the hydrogen
embrittlement resisting property of a thin steel sheet requires it
to develop a novel technology.
[0010] Japanese Unexamined Patent Publication (Kokai) No. 11-293383
describes a technology to improve the hydrogen embrittlement
resisting property of steel sheet, where hydrogen-induced defects
can be suppressed by having oxides that include Ti and Mg exist as
the main components in the structure. However, this technology is
intended for thick steel sheets and, although consideration is
given to delayed fracture after welding with a large input heat, no
consideration is given to the environment (for example, corrosive
environment, etc.) in which automobile parts manufactured by using
thin steel sheets are used. Japanese Unexamined Patent Publication
(Kokai) No. 2003-166035 describes that it is made possible to
improve the ductility and delayed fracture resistance after being
subjected to forming process, by controlling the mutual
relationships between 1)the form (standard deviation and mean grain
size) in which oxide, sulfide, composite crystallization product or
composite precipitate of Mg is dispersed, 2)volumetric proportion
of residual austenite and 3)strength of the steel sheet. However,
it is difficult to improve the hydrogen embrittlement resisting
property in such an environment as hydrogen is generated through
corrosion of the steel sheet simply through the trapping effect
achieved by controlling the form of precipitate.
[0011] Tomohiko HOJO et. al "Hydrogen Embrittlement of High
Strength Low Alloy TRIP Steel (Part 1: Hydrogen Absorbing
Characteristic and Ductility", The Society of Materials Science,
Japan, proceedings of 51.sup.st academic lecture meeting, 2002,
vol. 8, pp 17-18 and Tomohiko HOJO et. al "Influence of
Austempering Temperature on Hydrogen Embrittlement of High, for
example, describe investigations into the hydrogen embrittlement
resisting property of the TRIP steel. It is pointed out that, among
the TRIP steels, TBF steel has particularly high hydrogen absorbing
capacity, and observation of a fracture surface of the TBF steel
shows the restriction of quasi cleavage fracture due to storage of
hydrogen. However, the TBF steels reported in the documents
described above show delayed fracture characteristic of about 1000
seconds at the most in terms of the time before crack occurrence
measured in cathode charging test, indicating that these steels are
not meant to endure the harsh operating environment such as that of
automobile parts over a long period of time. Moreover, since the
heat treatment conditions reported in the documents described above
involve heating temperature being set higher, there are such
problems as low efficiency of practical manufacturing process. Thus
it is strongly required to develop a new species of TBF steel that
provides high production efficiency as well.
SUMMARY OF THE INVENTION
[0012] The present invention has been made with the background
described above, and the object of the present invention is to make
available a high strength thin steel sheet that shows high hydrogen
embrittlement resisting property with workability improved under
the tensile strength of 1180 MPa or higher.
[0013] Hydrogen-induced delayed fracture is believed to occur in
such a steel that contains tempered martensite or martensite
+ferrite, that has been commonly used as a high-strength steel ever
before, because hydrogen is concentrated in grain boundaries of
prior austenite thereby to form voids or other defects that become
the starting points of the fracture. Common practice that has been
employed to decrease the sensitivity to delayed fracture is to
diffuse fine grains of carbide or the like uniformly as the site
for trapping hydrogen, thereby to decrease the concentration of
diffusive hydrogen. However, even when a large number of carbide
grains or the like are diffused as the trap site for hydrogen,
there is a limitation to the hydrogen trapping capability and
delayed fracture attributable to hydrogen cannot be fully
suppressed.
[0014] Thus, as a result of a hard study of the present inventors,
they found that (a) decreasing the points of destroying grains and
(b) neutralizing hydrogen by improving the ability of trapping
hydrogen are satisfied in order to make it possible to show a
satisfactory hydrogen embrittlement resisting property (delayed
fracture resisting property) with its use environment sufficiently
considered.
[0015] In order to achieve condition (a), it is desirable to form
the matrix phase of the steel structure after processing from a
binary phase structure of bainitic ferrite and martensite with the
bainitic ferrite acting as the main phase, instead of the single
phase structure of martensite that is generally used for high
strength steels. Because in the case of the single phase structure
of martensite, a carbide (for example, film-like cementite) is
likely to precipitate in the grain boundaries, thus making
intergranular fracture likely to occur. On the other hand, in the
case of the binary structure of bainitic ferrite and martensite
acting as the main phase, the bainitic ferrite is easy to increase
the strength of the entire structure as in the case of the single
phase of martensite, because a dislocation density of the bainitic
ferrite is high as it is in the form of plates, differing from
polygonal ferrite used commonly. The hydrogen embrittlement
resisting property can also be improved as much hydrogen is trapped
in the dislocations. It also has such an advantage that coexistence
of the bainitic ferrite and the residual austenite which will be
described later prevents the generation of carbide that acts as the
intergranular fracture initiating points.
[0016] The bainitic ferrite is a hard phase and therefore it is
easy to increase the strength of the entire structure. It can also
absorb much hydrogen compared to the other TRIP steel as much
hydrogen is trapped in the dislocations. It also has such an
advantage that in the boundaries of the lath-shaped bainitic
ferrite, it becomes easier to produce the lath-shaped residual
austenite, thus providing an excellent elongation with it.
Accordingly, it is found that it is required in the present
invention that the binary structure of bainitic ferrite and
martensite occupy 80% or more in order to effectively achieve the
above action.
[0017] In order to achieve condition (b), it is desirable to form
the lath-shaped residual austenite. In the past it has been thought
that the residual austenite exerts a negative impact on its
hydrogen embrittlement resisting property and fatigue. The present
inventor has studied that although the residual austenite which is
in the form of cluster in the submicron order exerts a negative
impact on its hydrogen embrittlement resisting property and
fatigue, if the residual austenite above described is controlled to
be produced the lath-shaped residual austenite in the order of
submicron, it can absorb and trap much hydrogen, thus improving the
hydrogen embrittlement resisting property to a large degree due to
the ability of absorbing hydrogen which the residual austenite
naturally has. In particular, hydrogen embrittlement risk index
sharply decreases when the mean axis ratio (major axis/minor axis)
of the residual austenite grains increases beyond 5. This is
supposedly because, when the mean axis ratio of the residual
austenite grains becomes 5 or higher, intrinsic capability of the
residual austenite to absorb hydrogen is put into full play, so
that the residual austenite attains far higher capacity of trapping
hydrogen than carbide and substantially neutralizes the hydrogen
that infiltrates from the outside through atmospheric corrosion
thereby to achieve remarkable achievement in hydrogen embrittlement
resisting property.
[0018] The metal structure may include other structure such as
ferrite (the term ferrite used herein refers to polygonal ferrite,
that is a ferrite structure that includes no or very few
dislocations) or pearlite to such an extent-that the effect of the
present invention is not compromised. The less the concentration of
additional components is, more preferable it is. It is found that
in particular, its concentration is preferably within 9%.
[0019] The present inventors conducted a research on a steel sheet
to find that if it is controlled so that the below conditions are
satisfied at a time, high hydrogen embrittlement resisting property
is achieved even if too much alloys are not added and they brought
to a completion of the present invention.
[0020] bainitic ferrite and martensite: 80% or more in total;
[0021] the mean axis ratio (major axis/minor axis) of said residual
austenite grains is 5 or higher; and
[0022] ferrite and pearlite: 9% or less (may be 0%) in total.
[0023] Thus, a first high strength thin steel sheet having high
hydrogen embrittlement resisting property according to the present
invention is constituted from 0.10 to 0.25% of C (contents of
components given in terms of percentage in this patent application
all refer to percentage by weight), 1.0 to 3.0% of Si, 1.0 to 3.5%
of Mn, 0.15% or less P, 0.02% or less S and 1.5% or less (higher
than 0%) of Al, with balance of iron and inevitable impurities,
wherein the metallurgical structure comprises:
1% or more residual austenite;
80% or more in total of bainitic ferrite and martensite; and
[0024] 9% or less (may be 0%) in total of ferrite and pearlite in
the proportion of area to the entire structure, and wherein the
mean axis ratio (major axis/minor axis) of the residual austenite
grains is 5 or higher, and the steel has tensile strength of 1180
MPa or higher.
[0025] The present inventors also found that if its average length
of minor axis of the lath-shaped grains in the residual austenite
is 1 .mu.m or less (submicrometer order), the surface area
(interface) of the grains in the residual austenite becomes larger,
thus improving the ability of trapping hydrogen and effectively
improving high hydrogen embrittlement resisting property. And it
has been found that hydrogen embrittlement resisting property can
be improved further by controlling the minimum distance between
adjacent residual austenite grains so that it is 1 .mu.m or less.
This is supposedly because propagation of cracks is suppressed so
that the structure demonstrates higher resistance against fracture,
when a large number of fine lath-shape grains of residual austenite
are dispersed in proximity to each other.
[0026] Thus, a second high strength thin steel sheet having high
hydrogen embrittlement resisting property according to the present
invention is constituted from 0.10 to 0.25% of C, 1.0 to 3.0% of
Si, 1.0 to 3.5% of Mn, 0.15% or less P, 0.02% or less S, 0.5% or
less (higher than 0%) Al, with balance of iron and inevitable
impurities, and wherein the metal structure comprises:
1% or more residual austenite;
the mean axis ratio (major axis/minor axis) of the residual
austenite grains is 5 or higher;
mean length of minor axes of the residual austenite grains is 1
.mu.m or less;
minimum distance between the residual austenite grains is 1 .mu.m
or less; and
tensile strength is 1180 MPa or higher.
[0027] Preferably, the high strength thin steel sheet according to
the present invention may contain 0.5% or less (higher than 0%) by
weight of Al. If the Al content increases over 0.5%, inclusions
such as alumina increase and thus workability becomes poorer. With
the content of Al to be restricted within 0.5% or less, it can
prevent a steel sheet from having a poorer workability.
[0028] Preferably, the high strength thin steel sheets according to
the present invention may further contain 0.003 to 0.5% of Cu
and/or 0.003 to 1.0% of Ni in terms of percentage by weight. The
effect of improving hydrogen embrittlement resisting property
through control of the structure can be achieved further by
containing 0.003 to 0.5% of Cu and/or 0.003 to 1.0% of Ni.
[0029] Further, it is preferred that the high strength thin steels
sheet according to the present invention may further contain 0.003
to 1.0% of Ti and/or V. Ti and/or V has/have the effect of
assisting in the generation of protective rust, the effect of
rendering steel high corrosion resistance, and the effect of
cleaning the steel. And V has the effect of increasing the strength
of the steel sheet and decreasing the size of crystal grains, in
addition to having the effect of improving hydrogen embrittlement
resistance.
[0030] Preferably, the high strength thin steels sheet according to
the present invention may further contain;
[0031] 1.0% or less (higher than 0%) of Mo and 0.1% or less (higher
than 0%) of Nb,
[0032] 0.0002 to 0.01% of B, or
[0033] at least one kind selected from the group consisting of:
0.0005 to 0.005% of Ca;
0.0005 to 0.01% of Mg; and
0.0005 to 0.01% of REM
in terms of percentage by weight.
[0034] According to the present invention, it is made possible to
manufacture, with a high level of productivity, a high strength
thin steel sheet having tensile strength of 1180 MPa or higher that
neutralizes hydrogen that infiltrates from the outside after the
steel sheet has been formed into a part thereby to maintain
satisfactory hydrogen embrittlement resisting property, and
demonstrates high workability during the forming process. Use of
the high strength thin steel sheet makes it possible to manufacture
high strength parts that hardly experience delayed fracture, such
as bumper, impact beam and other reinforcement members and other
automobile parts such as seat rail, pillar, etc.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 is a graph showing the relationship between the mean
axis ratio of the residual austenite grains and hydrogen
embrittlement risk index.
[0036] FIG. 2 is a diagram schematically showing the minimum
distance between residual austenite grains.
[0037] FIG. 3 is a schematic perspective view of a part used in
pressure collapse test in Example 1.
[0038] FIG. 4 is a side view schematically showing the setup of
pressure collapse test in Example 1.
[0039] FIG. 5 is a schematic perspective view of a part used in
impact resistance test in Example 1.
[0040] FIG. 6 is a sectional view along A-A in FIG. 5.
[0041] FIG. 7 is a side view schematically showing the setup of
impact resistance test in Example 1.
[0042] FIG. 8 is a photograph of TEM observation (magnification
factor 15000) of No. 101 (inventive steel) of Example 1.
[0043] FIG. 9 is a photograph of TEM observation (magnification
factor 15000) of No. 120 (comparative steel) of Example 1.
[0044] FIG. 10 is a photograph of TEM observation (magnification
factor 15000) of No. 201 (inventive steel) of Example 2.
[0045] FIG. 11 is a photograph of TEM observation (magnification
factor 15000) of No. 220 (comparative steel) of Example 2.
[0046] FIG. 12 is a photograph of TEM observation (magnification
factor 15000) of No. 301 (inventive steel) of Example 3.
[0047] FIG. 13 is a photograph of TEM observation (magnification
factor 60000) of No. 301 (inventive steel) of Example 3.
[0048] FIG. 14 is a photograph of TEM observation (magnification
factor 15000) of No. 313 (comparative steel) of Example 3.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0049] The high strength thin steel sheet of non-limiting
embodiment according to the present invention is explained in more
detail below -with reference to the drawing. The inventions can be
made variable in the scope of the present invention.
First Embodiment
[0050] The first high strength thin steel sheet according to the
present invention comprises higher than 0.10 and up to 0.25% of C
(contents of components given in terms of percentage in this patent
application all refer to percentage by weight), 1.0 to 3.0% of Si,
1.0 to 3.5% of Mn, 0.15% or less P, 0.02% or less S, 1.5% or less
(higher than 0%) of Al, 1.0% or less (higher than 0%) of Mo and
0.1% or less (higher than 0%) of Nb, with balance of iron and
inevitable impurities, and
[0051] the metal structure comprises:
[0052] 1% or more residual austenite;
[0053] 80% or more in total of bainitic ferrite and martensite;
and
[0054] 9% or less (may be 0%) in total of ferrite and pearlite in
terms of the proportion of area to the entire structure, and
[0055] the mean axis ratio (major axis/minor axis) of the residual
austenite grains is 5 or higher.
(1% or more residual austenite in the area proportion to the entire
structure)
[0056] It is necessary that the metal contains 1% or more residual
austenite in the area proportion to the entire structure, because
residual austenite contributes not only to the improvement of
hydrogen embrittlement resisting property, but also to the
improvement of total elongation. Content of the residual austenite
is preferably 3% or higher, and more preferably 5% or higher.
[0057] Since the desired level of high strength cannot be obtained
when an excessive amount of residual austenite is contained, it is
recommended to set an upper limit of 15% (more preferably 10%) to
the residual austenite content.
[0058] It is recommended to increase the C content in the steel
sheet as described above, thereby to maintain the concentration of
C in the residual austenite (C.gamma.R) of 0.8% or higher.
Controlling the value of C.gamma.R to 0.8% or higher enables it to
effectively improve the elongation property, which is preferably
1.0% or higher and more preferably 1.2% or higher. While it is
preferable that C.gamma.R is as high as possible, it is considered
that in practice there is an upper limit of around 1.6%.
[0059] The residual austenite refers to a region that is observed
as FCC (face centered cubic lattice) by the FE-SEM/EBSP method
which will be described later. Measurement by the EBSP may be done,
for example, by measuring a measurement area (about 50 by 50 .mu.m)
at an arbitrarily chosen position in a surface parallel to the
rolled surface at a position of one quarter of the thickness at
measuring intervals of 0.1 .mu.m. The measuring surface is prepared
by electrolytic polishing in order to prevent the residual
austenite from transforming. Then the test piece is set in the lens
barrel of an FE-SEM equipped with the EBSP detector (of which
details will be described later) and is irradiated with electron
beam. An EBSP image projected onto a screen is captured by a high
sensitivity camera (VE-1000-SIT manufactured by Dage-MTI Inc.) and
is sent to a computer. The computer carries out image analysis and
generates color mapping of the FCC phase through comparison with a
structural pattern simulated with a known crystal system (FCC (face
centered cubic lattice) phase in the case of residual austenite).
Area proportion of the region that is mapped as described above is
taken as the area proportion of the residual austenite. This
analysis was carried out by means of hardware and software of OIM
(Orientation Imaging Microscopy.TM.) system of TexSEM Laboratories
Inc.
(Mean axis ratio (major axis/minor axis) of the residual austenite
grains: 5 or higher)
[0060] In the past it has been thought that the residual austenite
exerts a negative impact on its hydrogen embrittlement resisting
property and fatigue. The present inventor's studies show that
although the residual austenite which is in the form of cluster in
the order of submicron exerts a negative impact on its hydrogen
embrittlement resisting property and fatigue, Lath-shaped grains of
residual austenite have far higher capacity of trapping hydrogen
than carbide, if the residual austenite above described is
controlled to be produced the lath-shaped residual austenite.
[0061] FIG. 1 is a graph showing the relationship between the mean
axis ratio of the residual austenite grains measured by a method to
be described later and hydrogen embrittlement risk index (measured
by a method to be described later in an example, lower value of
this index means better hydrogen embrittlement resisting property).
From FIG. 1, it can be seen that hydrogen embrittlement risk index
sharply decreases when the mean axis ratio (major axis/minor axis)
of the residual austenite grains increases beyond 5. This is
supposedly because, when the mean axis ratio of the residual
austenite grains becomes 5 or higher, intrinsic capability of the
residual austenite to absorb hydrogen is put into full play, so
that the residual austenite attains far higher capacity of trapping
hydrogen than carbide and substantially neutralizes the hydrogen
that infiltrates from the outside through atmospheric corrosion
thereby to achieve remarkable achievement in hydrogen embrittlement
resisting property. The mean axis ratio of the residual austenite
grains is preferably 10 or higher, and more preferably 15 or
higher.
[0062] While no upper limit of the mean axis ratio is specified for
the consideration of improvement in hydrogen embrittlement
resisting property, the residual austenite grains are required to
have certain level of thickness in order to achieve the TRIP effect
during processing. Thus it is preferable to set an upper limit to
30, more preferably to 20 or less.
[0063] The mean axis ratio was determined by observing with TEM
(magnification factor of 15000) in each of three arbitrarily chosen
fields of view and averaging the distances measured in the three
fields of view.
(80% or more in total of bainitic ferrite and martensite)
[0064] In the case of the binary structure of bainitic ferrite and
martensite (with the bainitic ferrite acting as the main phase in
the binary structure), the bainitic ferrite is a hard phase and
therefore it is easy to increase the strength of the entire
structure. The hydrogen embrittlement resisting property can also
be improved as much hydrogen is trapped in the dislocations. It
also has such an advantage that coexistence of the bainitic ferrite
and the residual austenite which will be described later prevents
the generation of carbide that acts as the intergranular fracture
initiating points, and it becomes easier to create the lath-shaped
residual austenite in the boundaries of lath-shaped bainitic
ferrite.
[0065] Accordingly, it is required in the present invention that
the binary structure of bainitic ferrite and martensite occupy 80%
or more, preferably 85% or more and more preferably 90% or more.
Upper limit of the proportion may be determined by the balance with
other structure (residual austenite), and is set to 99% when the
other structures (ferrite, etc.) than the residual austenite is not
contained.
[0066] The bainitic ferrite referred to in the present invention is
plate-shaped ferrite having a lower structure of high density of
dislocations. It is clearly distinguished from polygonal ferrite
that has lower structure including no or very low density of
dislocations, by SEM observation as follows.
[0067] Area proportion of bainitic ferrite structure is determined
as follows. A test piece is etched with Nital etchant. A
measurement area (about 50 by 50 .mu.m) at an arbitrarily chosen
position in a surface parallel to the rolled surface at a position
of one quarter of the thickness is observed with SEM (scanning
electron microscope) (magnification factor of 1500) thereby to
determine the area proportion.
[0068] Bainitic ferrite is shown with dark gray color in SEM
photograph (bainitic ferrite, residual austenite and martensite may
not be distinguishable in the case of SEM observation), while
polygonal ferrite is shown black in SEM photograph and has
polygonal shape that does not include residual austenite and
martensite inside thereof.
[0069] The SEM used in the present invention is a high-resolution
FE-SEM (Field Emission type Scanning Electron Microscope XL30S-FEG
manufactured by Philips Inc.) equipped with an EBSP (Electron Back
Scattering Pattern) detector, that has a merit of being capable of
analyzing the area observed by the SEM at the same time by means of
the EBSP detector. EBSP detection is carried out as follows. When
the sample surface is irradiated with electron beam, the EBSP
detector analyzes the Kikuchi pattern obtained from the reflected
electrons, thereby to determine the crystal orientation at the
point where the electron beam has hit upon. Distribution of
orientations over the sample surface can be measured by scanning
the electron beam two-dimensionally over the sample surface while
measuring the crystal orientation at predetermined intervals. The
EBSP detection method has such an advantage that different
structures that are regarded as the same structure in the ordinary
microscopic observation but have different crystal orientations can
be distinguished by the difference in color tone.
(9% or less (may be 0%) in total of ferrite and pearlite)
[0070] The steel sheet after the processing may be constituted
either from only the structures described above (namely, a mixed
structure of bainitic ferrite+martensite and residual austenite),
or may include other structure such as ferrite (the term ferrite
used herein refers to polygonal ferrite, that is a ferrite
structure that includes no or very few dislocations) or pearlite to
such an extent that the effect of the present invention is not
compromised. Such additional components are structures that can
inevitably remain in the manufacturing process of the present
invention, of which concentration is preferably as low as possible,
within 9%, preferably less than 5% and more preferably less than 3%
according to the present invention.
[0071] As described above, the present high strength steel is
characterized in that the amount and form of residual austenite are
controlled and the easy control of the amount and form of residual
austenite can be achieved to provide the desired high strength
steel according to the following composition.
<C: 0.10 to 0.25%>
[0072] C is an element required to achieve a high strength of 1180
MPa or higher. And C is an important element that can remain the
desired austenite at room temperature by providing a sufficient
amount of C into the phase of austenite. In the present invention,
0.10% or higher of C is contained, and 0.12% or more, preferably
0.15% or more C is contained. However, in order to ensure corrosion
resistance and weldability, concentration of C is limited within
0.25%, preferably 0.23% or lower in the present invention.
<Si: 1.0 to 3.0%>
[0073] Si is an important element that effectively suppresses the
residual austenite from decomposing and carbide from being
generated, and is also effective in enhancing substitution solid
solution for hardening the material. In order to make full use of
these effects, it is necessary to include Si in a concentration of
1.0% or higher, preferably 1.2% or higher and more preferably 1.5%
or higher. However, excessively high content of Si leads to
conspicuous formation of scales due to hot rolling and makes it
necessary to remove flaws, thus adding up to the manufacturing cost
and resulting in economical disadvantage. Therefore Si content is
controlled within 3.0%, preferably within 2.5% and more preferably
within 2.0%.
<Mn: 1.0 to 3.5%>
[0074] Mn is an element required to stabilize austenite and obtain
desired residual austenite. In order to make full use of this
effect, it is necessary to add Mn in concentration of 1.0% or
higher, preferably 1.2% or higher, and more preferably 1.5% or
higher. However, adding an excessive amount Mn leads to conspicuous
segregation and poor workability. Therefore upper limit to the
concentration of Mn is set to 3.5% and more preferably to 3.0% or
less.
<P: 0.15% or Lower (Higher Than 0%)>
[0075] P intensifies intergranular fracture due to intergranular
segregation, and the content thereof is therefore preferably as low
as possible. Upper limit to the concentration of P is set to 0.15%,
preferably 0.1% or less and more preferably to 0.05% or less.
<S: 0.02% or Lower (Higher Than 0%)>
[0076] S intensifies the absorption of hydrogen into the steel
sheet in corrosive environment, and the content thereof is
therefore preferably as low as possible. Upper limit to the
concentration of S is set to 0.02%.
<Al: 1.5% or Less (Higher Than 0%)>(In the Case of Inventive
Steel 1)
<Al: 0.5% or Less (Higher Than 0%)>(In the Case of Inventive
Steel 2)
[0077] 0.01% or higher content of Al may be included for the
purpose of deoxidation. In addition to deoxidation, Al also has the
effects of improving the corrosion resistance and improving
hydrogen embrittlement resisting property.
[0078] The mechanism of improving the corrosion resistance is
supposedly based on the improvement of corrosion resistance of the
matrix phase per se and the effect of formation rust generated by
atmospheric corrosion, while the effect of the formation rust
presumably has greater contribution. This is supposedly because the
formation rust is denser and better in protective capability than
ordinary iron rust, and therefore checks the progress of
atmospheric corrosion so as to decrease the amount of hydrogen
generated by the atmospheric corrosion, thereby to effectively
suppress the occurrence of hydrogen embrittlement, and hence the
delayed fracture.
[0079] While details of the mechanism of improvement of the
hydrogen embrittlement resistance by Al is not known, it is
supposed that condensing of Al on the surface of the steel makes it
difficult for hydrogen to infiltrate into the steel, and the
decreasing diffusion rate of hydrogen in the steel makes it
difficult for hydrogen to migrate so that hydrogen embrittlement
becomes less likely to occur. In addition, stability of lath-shaped
residual austenite improved by the addition of Al is believed to
contribute to the improvement of hydrogen embrittlement resisting
property.
[0080] In order to effectively achieve the effects of Al in
improving the corrosion resistance and improving the hydrogen
embrittlement resisting property, Al content is controlled to 0.2%
or higher, preferably 0.5% or higher.
[0081] However, Al content must be controlled within 1.5% in order
to keep inclusions such as alumina from increasing in number and
size so as to ensure satisfactory workability, ensure the
generation of fine residual austenite grains, suppress corrosion
from proceeding from the inclusion containing Al as the starting
point, and prevent the manufacturing cost from increasing. In view
of the manufacturing process, it is preferable to control so that
A3 point is not higher than 1000.degree. C.
[0082] As the Al content increases, inclusions such as alumina
increase and workability becomes poorer. In order to suppress the
generation of the inclusions such as alumina and make a steel sheet
having higher workability, Al content is restricted within 0.5%,
preferably within 0.3% and more preferably within 0.1%.
[0083] While constituent elements (C, Si, Mn, P, S, Al, Mo, Nb) of
the steel of this embodiment is as described above with the rest
substantially being Fe, it may include inevitable impurities
introduced into the steel depending on the stock material,
production material, manufacturing facility and other
circumstances, containing 0.001% or less N (nitrogen). In addition,
other elements as described below may be intentionally added to
such an extent that does not adversely affect the effects of the
present invention.
[0084] The case in which 1.5% or less (higher than 0%) of Al is
contained is referred to as an inventive steel 1. And the case in
which 0.5% or less (higher than 0%) of Al is contained is referred
to as an inventive steel 2
(Cu: 0.003 to 0.5% and/or Ni: 0.003 to 1.0%)
[0085] It was found that by including 0.003 to 0.5% of Cu and/or
0.003 to 1.0% of Ni, the generation of hydrogen leading to the
hydrogen embrittlement and the infiltration of hydrogen that has
been generated can be sufficiently suppressed.
[0086] Specifically, presence of Cu and Ni improves the corrosion
resistance of the steel, and effectively suppresses the generation
of hydrogen due to corrosion of the steel sheet. These elements
also have the effect of promoting the generation of iron oxide,
a-FeOOH, that is believed to be particularly stable
thermodynamically and have protective property among various forms
of rust generated in the atmosphere. By assisting the generation of
this rust, it is made possible to suppress hydrogen that has been
generated from infiltrating into the spring steel thereby to
sufficiently improve the hydrogen embrittlement resisting property
to endure in harsh corrosive environment. This effect can be
achieved particularly satisfactorily when Cu and Ni are contained
at the same time.
[0087] In order to achieve the effects described above,
concentration of Cu, if added, should be 0.003% or higher,
preferably 0.05% or higher and more preferably 0.1% or higher.
Concentration of Ni, if added, should be 0.003% or higher,
preferably 0.05% or higher and more preferably 0.1% or higher.
[0088] Since excessively high concentration of either Cu or Ni is
detrimental to workability, it is preferable to limit the Cu
content to 0.5% or lower and limit the Ni content to 1.0% or
lower.
<Ti and/or V: 0.003 to 1.0% in Total>
[0089] Ti has the effect of assisting in the generation of
protective rust, similarly to Cu and Ni. The protective rust has a
very valuable effect of suppressing the generation of .beta.-FeOOH
that appears in chloride environment and has adverse effect on the
corrosion resistance (and hence on the hydrogen embrittlement
resisting property). Formation of such a protective rust is
promoted particularly by adding Ti and V (or Zr). Ti renders the
steel high corrosion resistance, and also has the effect of
cleaning the steel.
[0090] V is effective in increasing the strength of the steel sheet
and decreasing the size of crystal grains, in addition to having
the effect of improving hydrogen embrittlement resistance through
cooperation with Ti, as described previously.
[0091] In order to fully achieve the effect of Ti and/or V
described above, it is preferable to add Ti and/or V in total
concentration of 0.003% or higher (more preferably 0.01% or
higher). For the purpose of improving hydrogen embrittlement
resisting property, in particular, it is preferable to add more
than 0.03% of Ti, more preferably 0.05% or more Ti. However, the
effects described above reach saturation when an excessive amount
of Ti is added, resulting in economical disadvantage. Excessive V
content also increases the precipitation of much carbonitride and
leads to poor workability and lower hydrogen embrittlement
resisting property. Therefore, it is preferable to control the
total concentration of Ti and/or V to within 1.0%, more preferably
within 0.5%.
<Zr: 0.003to 1.0%>
[0092] Zr is effective in increasing the strength of the steel
sheet and decreasing the crystal grain size, and also has the
effect of improving hydrogen embrittlement resisting property
through cooperation with Ti. In order to sufficiently achieve these
effects, it is preferable that 0.003% or more Zr is contained.
However, excessive Zr content increases the precipitation of
carbonitride and leads to poor workability and lower hydrogen
embrittlement resisting property. Therefore, it is preferable to
control the concentration of Zr to within 1.0%.
<Mo: 1.0% or Less (Higher Than 0%)>
[0093] Mo has the effects of stabilizing austenite so as to retain
the residual austenite, and suppress the infiltration of hydrogen
thereby to improve hydrogen embrittlement resisting property. Mo
also has the effect of improving the hardenability of the steel
sheet. In addition, Mo strengthens the grain boundary so as to
suppress hydrogen embrittlement from occurring. It is recommended
to add 0.005% or more Mo in order to achieve these effects. More
preferably 0.1% or more Mo is added. However, since the effects
described above reach saturation when the Mo content exceeds 1.0%,
resulting in economical disadvantage, Mo content is limited to 0.8%
or less and more preferably to 0.5% or less.
<Nb: 0.1% or Less (Higher Than 0%)>
[0094] Nb is very effective in increasing the strength of the steel
sheet and decreasing the grain size of the structure. Nb achieves
these effects particularly effectively in cooperation with Mo. In
order to achieve these effects, it is recommended to include 0.005%
or more Nb. More preferably 0.01% or more Nb is added. However,
since the effects described above reach saturation when an
excessive Nb content is included, resulting in economical
disadvantage, Nb content is limited to 0.1% or less and more
preferably to 0.08% or less.
<B: 0.0002 to 0.01%>
[0095] B is effective in increasing the strength of the steel
sheet, and it is preferable that 0.0002% or more (more preferably
0.0005% or more) B is contained in order to achieve these effects.
However, an excessive content of B leads to poor hot processing
property. Therefore, it is preferable to control the concentration
of B to within 0.01% (more preferably within 0.005%).
<At least one kind selected from among a group consisting of Ca:
0.0005% to 0.005%, Mg: 0.0005% to 0.01% and REM: 0.0005% to
0.01%)
[0096] Ca, Mg and REM (rare earth element) are effective in
suppressing an increase in hydrogen ion concentration, that is, a
decrease in pH in the atmosphere of the interface due to corrosion
of the steel sheet surface, thereby to improve the corrosion
resistance of the steel sheet. It is also effective in controlling
the form of sulfide in the steel and improving the workability of
the steel. In order to achieve the effects described above, it is
recommended to add each of Ca, Mg and REM in concentration of
0.0005% or higher. However, since excessive contents of these
elements leads to poor workability, it is preferable to keep the
concentrations of Ca within 0.005%, Mg and REM each within
0.01%.
[0097] While the present invention does not specify the
manufacturing conditions, it is recommended to apply heat treatment
in the following procedure after hot rolling or cold rolling
conducted thereafter, in order to form the structure described
above that can be easily worked and has high strength and high
hydrogen embrittlement resistance after the processing, by using
the steel material of the composition described above.
[0098] The recommended procedure is to keep the steel the
composition described above at a temperature (T1) in a range from
A3 point to (A3 point+50.degree. C.) for a period of 10 to 1800
seconds (t1), cool down the steel at a mean cooling rate of
3.degree. C./s or higher to a temperature (T2) in a range from Ms
point to Bs point and keep the material at this temperature for a
period of 60 to 3600 seconds (t2).
[0099] It is not desirable that the temperature T1 becomes higher
than (A3 point+50.degree. C.) or the period t1 is longer than 1800
seconds, in which case austenite grains grow resulting in poor
workability (elongation flanging property). When the temperature T1
is lower than A3 point, on the other hand, desirable bainitic
ferrite structure cannot be obtained. When the period t1 is shorter
than 10 seconds, austenitization does not proceed sufficiently and
therefore cementite and other alloy carbides remain. The period t1
is preferably in a range from 30 to 600 seconds, more preferably
from 60 to 400 seconds.
[0100] Then the steel sheet is cooled down. The steel is cooled at
a mean cooling rate of 3.degree. C./s or higher, for the purpose of
preventing pearlite structure from being generated while avoiding
the pearlite transformation region. The mean cooling rate should be
as high as possible, and is preferably 5.degree. C./s or higher,
and more preferably 10.degree. C./s or higher.
[0101] After quenching to the temperature between Ms point and Bs
point at the rate described above, the steel is subjected to
isothermal transformation so as to transform the matrix phase into
binary phase structure of bainitic ferrite and martensite. When the
heat retaining temperature T2 is higher than Bs, much pearlite that
is not desirable for the present invention is formed, thus
hampering the formation of the predetermined bainitic ferrite
structure. When T2 is below Ms, on the other hand, the amount of
residual austenite decreases.
[0102] When the temperature holding period t2 is longer than 1800
seconds, density of dislocations in bainitic ferrite becomes low,
the amount of trapped hydrogen decreases and the desired residual
austenite cannot be obtained. When t2 is less than 60 second, on
the other hand, desired bainitic ferrite structure cannot be
obtained. The length of t2 is preferably from 90 to 1200 seconds,
and more preferably from 120 to 600 seconds. There is no
restriction on the method of cooling after maintaining the heating
temperature, and air cooling, quenching or air-assisted water
cooling may be employed.
[0103] In the practical manufacturing process, the annealing
process described above can be carried out easily by employing a
continuous annealing facility or a batch annealing facility. In
case a cold rolled sheet is plated with zinc by hot dipping, the
heat treatment process may be replaced by the plating process by
setting the plating conditions so as to satisfy the heat treatment
conditions. The plating may also be alloyed.
[0104] There is no restriction on the hot rolling process (or cold
rolling process as required) that precedes the continuous annealing
process described above, and commonly employed process conditions
may be used. Specifically, the hot rolling process may be carried
out in such a procedure as, after hot rolling at a temperature
above Ar3 point, the steel sheet is cooled at a mean cooling rate
of about 30.degree. C./s and is wound up at a temperature from
about 500 to 600.degree. C. In case the hot rolled steel sheet has
unsatisfactory appearance, cold rolling may be applied in order to
rectify the appearance. It is recommended to set the cold rolling
ratio in a range from 1 to 70%. Cold rolling beyond 70% leads to
excessive rolling load that makes it difficult to carry out the
cold rolling.
[0105] While the present invention is addressed to thin steel
sheet, there is no limitation to the form of product, and may be
applied, in addition to steel sheet made by hot rolling or steel
sheet made by cold rolling, to those subjected to annealing after
hot rolling or cold rolling, followed by chemical conversion
treatment, hot-dip coating, electroplating, vapor deposition,
painting, priming for painting, organic coating treatment or the
like.
[0106] The, plating process may be either galvanizing or aluminum
plating. The method of plating may be either hot-dip coating or
electroplating, and the plating process may also be followed by
alloying heat treatment or multi-layer plating. A steel sheet, that
is plated or not plated, may also be laminated with a film.
[0107] When the coating operation described above is carried out,
chemical conversion treatment such as phosphating or
electrodepositing coating may be applied in accordance to the
application. The coating material may be a known resin that can be
used in combination with a known hardening agent such as epoxy
resin, fluorocarbon resin, silicone acrylic resin, polyurethane
resin, acrylic resin, polyester resin, phenol resin, alkyd resin,
or melamine resin. Among these, epoxy resin, fluorocarbon resin or
silicone acrylic resin is preferably used in consideration of
corrosion resistance. Known additives that are added to coating
materials such as coloring agent, coupling agent, leveling agent,
sensitization agent, antioxidant agent, anti-UV protection agent,
flame retarding agent or the like may be used.
[0108] There is also no restriction on the coating and
solvent-based coating, powder coating, water-based coating,
water-dispersed coating, electrodeposition coating or like may be
employed. Desired coating layer of the coating material described
above can be formed on the steel by a known technique-such as
dipping, roll coater, spraying, or curtain flow coater. The coating
layer may have any proper thickness.
[0109] The high strength thin steel sheet of the present invention
may be applied to high-strength automotive components such as
bumper, door impact beam, pillar and other reinforcement members
and interior parts such as seat rail, etc. Automobile components
that are manufactured by forming process also have sufficient
properties (strength) and high hydrogen embrittlement resisting
property.
Second Embodiment
[0110] The second high strength thin steel sheet according to the
present invention comprises:
[0111] C: higher than 0.10 up to 0.25%; Si: 1.0 to 3.0%; Mn: 1.0 to
3.5%; P: 0.15% or less; S: 0.02% or less; and Al: 1.5% or less
(higher than 0%) in terms of percentage by weight, with balance of
iron and inevitable impurities, wherein the metal structure
comprises:
residual austenite; 1% by area or more in proportion to the entire
structure;
while the mean axis ratio (major axis/minor axis) of said residual
austenite grains is 5 or higher;
mean length of minor axes of said residual austenite grains is 1
.mu.m or less; and
[0112] a minimum distance between said residual austenite grains is
1 .mu.m or less; and the steel has tensile strength of 1180 MPa or
higher. The structure may further contain 80% or more bainitic
ferrite and martensite in total and/or 9% or less (may be 0%)
ferrite and pearlite in total, and the structure may not contain
them.
[0113] The reason why the above requirements are defined and a
measuring method are explained in detail below. Requirements
explained in the first embodiment are omitted below.
<Mean Length of Minor Axes of the Residual Austenite Grains is 1
.mu.m or Less>
[0114] According to the present invention, it has been found that
hydrogen embrittlement resisting property can be effectively
improved by dispersing fine grains of residual austenite of lath
shape. Specifically, hydrogen embrittlement resisting property can
be surely improved by dispersing the lath-shape grains of residual
austenite having sizes of 1 .mu.m or less (submicrometer order).
This is supposedly because surface area of the residual austenite
grains (interface) increases resulting in larger hydrogen trapping
capability, when larger number of fine lath-shape grains of
residual austenite having smaller mean length of minor axis are
dispersed. Mean length of minor axes of the residual austenite
grains is preferably 0.5 .mu.m or less, more preferably 0.25 .mu.m
or less.
[0115] According to the present invention, hydrogen trapping
capability of the fine lath-shape grains of residual austenite can
be made far greater than that in the case of dispersing carbide,
and thereby to substantially neutralize hydrogen that infiltrates
from the outside through atmospheric corrosion, even when the same
proportion by volume of residual austenite is contained, by
controlling the mean axis ratio and mean length of minor axes of
the residual austenite grains as described above.
<Minimum Distance Between Residual Austenite Grains is 1 .mu.m
or Less>
[0116] According to the present invention, it has been found that
hydrogen embrittlement resisting property can be improved further
by controlling the minimum distance between adjacent residual
austenite grains, in addition to the above. Specifically, hydrogen
embrittlement resistance can be surely improved when the minimum
distance between residual austenite grains is 1 .mu.m or less. This
is supposedly because propagation of cracks is suppressed so that
the structure demonstrates higher resistance against fracture, when
a large number of fine lath-shape grains of residual austenite are
dispersed in proximity to each other. Minimum distance between
adjacent residual austenite grains is preferably 0.8 .mu.m or less,
and more preferably 0.5 .mu.m or less.
[0117] The present invention will now be described below by way of
examples, but the present invention is not limited to the examples.
Various modifications may be conceived without departing from the
technical scope of the present invention.
EXAMPLE 1
[0118] Sample steels A-1 through Y-1 having the compositions
described in Table 1 were melt-refined in vacuum to make test
slabs. The slabs were processed in the following procedure (hot
rolling.fwdarw.cold rolling.fwdarw.continuous annealing) thereby to
obtain hot-rolled steel plates measuring 3.2 mm in thickness. The
steel plates were pickled to remove scales from the surface and
then cold rolled so as to reduce the thickness to 1.2 mm.
<Hot Rolling>
Starting temperature (SRT): Held at a temperature between 1150 and
1250.degree. C. for 30 minutes.
Finishing temperature (FDT): 850.degree. C.
Cooling rate: 40.degree. C./s
Winding-up temperature: 550.degree. C.
<Cold Rolling>
Rolling ratio: 50%
<Continuous Annealing>
[0119] Each steel specimen was kept at a temperature of A3
point+30.degree. C. for 120 seconds, then cooled in air at a mean
cooling rate of 20.degree. C./s to temperature T0 shown in Table 2,
and was kept at T0 for 240 seconds, followed by air-assisted water
cooling to the room temperature.
[0120] No. 116 shown in Table 2 was made by heating a cold-rolled
steel sheet to 830.degree. C., keeping at this temperature for 5
minutes followed by quenching in water and tempering at 300.degree.
C. for 10 minutes, thereby to form a martensite steel as a
comparative example of the high-strength steel of the prior art.
No. 120 was made by heating a cold-rolled steel sheet to
800.degree. C., keeping at this temperature for 120 seconds,
cooling down at a mean cooling rate of 20.degree. C./s to
350.degree. C. and keeping at this temperature for 240 seconds.
[0121] The metal structures of steel sheets obtained as described
above were observed, and their tensile strength (TS) and elongation
(total elongation E1) and hydrogen embrittlement resisting property
were measured by the following procedures.
Observation of Metal Structure
[0122] Metal structures of the test pieces were observed before and
after the processing as follows. A measurement area (about 50 by 50
.mu.m) at an arbitrarily chosen position in a surface parallel to
the rolled surface at a position of one quarter of the thickness
was photographed at measuring intervals of 0.1 .mu.m, and area
proportions of bainitic ferrite (BF), martensite (M) and residual
austenite (residual .gamma.) were measured by the method described
previously. Then similar measurements were made in two fields of
view that were arbitrarily selected, and the measured values were
averaged. Area proportions of other structures (ferrite, pearlite,
etc.) were subtracted from the entire structure. Mean axis ratio of
the residual austenite grains of the steel sheet before and after
the processing were measured by the method described previously.
Test pieces having mean axis ratio of 5 or higher were regarded to
satisfy the requirements of the present invention (o), and those
having mean axis ratio of lower than 5 were regarded to fail to
satisfy the requirements of the present invention (x).
Measurement of Tensile Strength (TS) and Elongation (E1)
[0123] Tensile test was conducted on the JIS No. 5 test piece
before processing, so as to measure the tensile strength (TS) and
elongation (E1). Stretching speed of the tensile test was set to 1
mm/sec. Among the steel sheets having tensile strength of 1180 MPa
as measured by the method described previously, those which showed
elongation of 10% or more were evaluated as high in elongation
property.
Evaluation of Hydrogen Embrittlement Resisting Property
[0124] In order to evaluate hydrogen embrittlement resisting
property, the JIS No. 5 test piece was stretched. Then after
bending with a radius of curvature of 15 mm, load of 1000 MPa was
applied and the test piece was immersed in 5% solution of
hydrochloric acid, and the time before crack occurred was
measured.
[0125] Hydrogen-charged 4-point bending test was also conducted for
some steel species. Specifically, a rectangular test piece
measuring 65 mm by 10 mm made of each steel sheet elongated by 3%
was immersed in a solution of 0.5 mol of H.sub.2SO.sub.4 and 0.01
mol of KSCN and was subjected to cathode hydrogen charging. Maximum
stress endured without breaking for 3 hours was determined as the
critical fracture stress (DFL).
[0126] Results of these tests are shown in Table 2.
Evaluation of Weldability
[0127] Test of weldability was conducted on No. 101 and No. 114
which are representative steel species.
[0128] The test on weldability was conducted on the test pieces
made according to the procedures of JIS Z 3136 and JIS Z 3137. And
then spot welding was conducted on these pieces under the following
conditions. Then tensile shear test (in which ultimate load was
measured in the tensile velocity of 20mm/min) and cross tension
test (in which ultimate load was measured in the tensile velocity
of 20 mm/min) was conducted on these pieces, so as to measure the
tensile shear strength (TSS) and cross tension strength (CTS). And
if the ductibility ratio (CTS/TSS) of cross tension strength (CTS)
to the tensile shear strength (TSS) is 0.2 or higher, it was
evaluated that the test piece has a better weldability. As a
result, it was found that No. 101 (present invention) is better
than No. 114 (prior art) in reference to weldability because the
ratio of ductibility in sample No. 114 is 0.19 while that in sample
No. 101 is 0.22.
[Conditions of Spot Welding]
[0129] Initial pressurization time: 60 cycles/60 Hz, Pressurized
force 450 kgf (4.4 kN) [0130] Power distribution time: 1 cycle/60
Hz
[0131] Power current for welding: 8.5 kA. TABLE-US-00001 TABLE 1
Steel species Chemical composition (mass %)* Ac3 Bs Ms Symbol C Si
Mn P S Al Nb Mo Others (.degree. C.) (.degree. C.) (.degree. C.)
A-1 0.20 2.01 2.01 0.012 0.002 0.033 0.05 0.2 -- 876.7 578.5 395.7
B-1 0.17 1.50 2.50 0.011 0.002 0.032 0.05 0.2 -- 845.2 542.5 393.7
C-1 0.13 2.01 2.54 0.011 0.002 0.031 0.06 0.2 -- 876.9 549.7 411.4
D-1 0.20 2.52 2.51 0.011 0.002 0.030 0.05 0.2 -- 882.6 533.5 379.2
E-1 0.23 2.02 1.43 0.011 0.002 0.031 0.04 0.2 -- 886.4 622.6 400.6
F-1 0.19 2.00 3.20 0.011 0.002 0.030 0.05 0.2 -- 840.9 474.1 361.1
G-1 0.20 2.02 2.50 0.011 0.002 0.031 0.08 0.2 -- 860.9 534.4 379.5
H-1 0.22 1.98 2.48 0.011 0.002 0.033 0.05 0.8 -- 875.0 481.0 358.1
I-1 0.22 2.02 2.50 0.011 0.002 0.031 0.05 0.2 B: 0.0005 856.5 529.0
370.0 J-1 0.20 1.98 2.53 0.011 0.002 0.033 0.05 0.2 Ca: 0.004 859.0
531.7 378.5 K-1 0.20 1.49 2.48 0.012 0.002 0.033 0.04 0.2 Mg: 0.005
839.3 536.2 380.2 L-1 0.23 1.50 2.50 0.011 0.002 0.032 0.06 0.2
REM: 0.005 831.5 526.3 365.3 M-1 0.20 1.50 2.50 0.011 0.002 0.033
0.05 0.2 Ca: 0.004, Mg: 0.005, B: 0.0005 838.5 534.4 379.5 N-1 0.30
2.00 2.00 0.014 0.005 0.031 0.06 0.2 -- 856.7 552.4 348.6 O-1 0.25
2.50 0.90 0.014 0.005 0.031 0.05 0.2 -- 921.8 664.9 408.6 P-1 0.20
0.16 1.98 0.014 0.002 0.043 0.05 0.2 -- 800.3 581.2 396.7 Q-1 0.05
2.01 2.01 0.012 0.002 0.033 0.06 0.2 -- 922.1 619.0 466.8 R-1 0.21
2.02 1.60 0.012 0.002 0.033 0.2 0.2 -- 887.2 612.7 404.5 S-1 0.20
2.00 1.20 0.012 0.002 0.031 0.05 1.5 -- 940.7 543.5 395.1 T-1 0.17
1.50 2.50 0.011 0.002 0.051 0.05 0.2 -- 852.8 542.5 393.7 U-1 0.18
1.49 2.50 0.012 0.002 0.320 0.05 0.2 -- 958.2 539.8 389.0 V-1 0.17
1.50 2.51 0.011 0.002 0.415 0.05 0.2 Ca: 0.004 998.1 541.6 393.4
W-1 0.17 1.50 2.51 0.011 0.002 0.550 0.05 0.2 Ca: 0.004 1052.1
541.6 393.4 X-1 0.17 1.49 2.55 0.012 0.002 0.730 0.05 0.2 Mg:
0.005, B: 0.0005 1123.1 538.0 392.1 Y-1 0.18 1.50 2.60 0.011 0.002
1.65 0.05 0.2 -- 1486.9 530.8 385.7 *The balance consists of iron
and inevitable impurities.
[0132] TABLE-US-00002 TABLE 2 Steel Mean axis Hydrogen Test species
To Residual .gamma. ratio of BF + M Others TS El embrittlement DFL
No. Symbol .degree. C. % residual .gamma. % % MPa % h MPa 101 A-1
320 8 .largecircle. 92 0 1476 11 Over 24 -- 102 B-1 300 3
.largecircle. 97 0 1495 11 Over 24 405 103 C-1 300 6 .largecircle.
92 2 1193 15 Over 24 -- 104 D-1 320 7 .largecircle. 93 0 1376 12
Over 24 -- 105 E-1 350 8 .largecircle. 92 0 1356 12 Over 24 -- 106
F-1 300 7 .largecircle. 92 1 1487 11 Over 24 -- 107 G-1 300 6
.largecircle. 91 3 1531 11 Over 24 -- 108 H-1 300 8 .largecircle.
91 1 1540 10 Over 24 -- 109 I-1 300 8 .largecircle. 91 1 1522 11
Over 24 -- 110 J-1 320 7 .largecircle. 93 0 1487 12 Over 24 -- 111
K-1 320 7 .largecircle. 93 0 1454 11 Over 24 -- 112 L-1 320 8
.largecircle. 92 0 1551 11 Over 24 -- 113 M-1 320 7 .largecircle.
93 0 1491 11 Over 24 -- 114 N-1 320 12 .largecircle. 88 0 1543 10
12 -- 115 O-1 320 <1 X 99 <1 1279 7 8 -- 116 P-1 -- <1 X
99 <1 1511 5 3 -- 117 Q-1 370 2 .largecircle. 98 0 891 19 Over
24 -- 118 R-1 350 3 .largecircle. 97 0 1370 1 -- -- 119 S-1 350 4
.largecircle. 96 0 1341 2 -- -- 120 A-1 350 12 X 20 68 960 13 Over
24 -- 121 T-1 300 3 .largecircle. 97 0 1507 11 Over 24 505 122 U-1
300 4 .largecircle. 96 0 1511 12 Over 24 630 123 V-1 320 6
.largecircle. 94 0 1493 12 Over 24 660 124 W-1 320 6 .largecircle.
94 0 1505 11 Over 24 680 125 X-1 320 8 .largecircle. 92 0 1513 11
Over 24 700 126 Y-1 320 9 X 60 31 1210 15 12 300
[0133] The results shown in Tables 1 and 2 can be interpreted as
follows (numbers in the following description are test Nos. in
Table 2).
[0134] Test pieces Nos. 101 through 113 (inventive steel sheets 2)
and test pieces Nos. 121 through 125 (inventive steel sheets 1)
that satisfy the requirements of the present invention have high
strength of 1180 MPa or higher, and high hydrogen embrittlement
resisting property in harsh environment after the forming process.
They also have high elongation property required of the TRIP steel
sheet, thus providing steel sheets best suited for reinforcement
parts of automobiles that are exposed to corrosive atmosphere. Test
pieces Nos. 121 through 125, in particular, show even better
hydrogen embrittlement resisting property.
[0135] Test pieces Nos. 114 through 120 and 126 that do not satisfy
the requirements of the present invention, in contrast, have the
following drawbacks.
[0136] No. 114 made of steel species N-1 that includes excessive
amounts of C content does not have good weldability.
[0137] No. 115 made of steel species O-1 that includes insufficient
Mn content does not retain sufficient residual austenite and is
inferior in hydrogen embrittlement resisting property after the
processing.
[0138] No. 116, martensite steel that is a conventional high
strength steel made of steel species P-1 that includes insufficient
Si content, hardly contains residual austenite and is inferior in
hydrogen embrittlement resisting property. It also does not show
the elongation property required of a thin steel sheet.
[0139] No. 117 made of steel species Q-1 that includes excessive C
content has precipitation of carbide and is inferior in both
forming workability and hydrogen embrittlement resisting property
after processing.
[0140] No. 118 made of steel species R-1 that includes excessive Mo
content and No. 119 made of steel species S-1 that includes
excessive Nb content are inferior in forming workability. Nos. 118
and 119 could not undergo the processing, making it impossible to
investigate the property after the processing.
[0141] No. 120, that was made of a steel that has the composition
specified in the present invention but was not manufactured under
the recommended conditions, resulted in the conventional TRIP
steel. As a result, the residual austenite does not have the mean
axis ratio specified in the present invention, while the matrix
phase is not formed in binary phase structure of bainitic ferrite
and martensite, and therefore sufficient level of hydrogen
embrittlement resisting property is not achieved.
[0142] No. 126 includes Al content higher than that specified for
the inventive steel sheet 1. As a result, although the
predetermined amount of residual austenite is retained, the
residual austenite does not have the mean axis ratio specified in
the present invention, the desired matrix phase is not obtained and
inclusions such as AlN are generated thus resulting in poor
hydrogen embrittlement resisting property.
[0143] Then parts were made by using steel species A-1, J-1 shown
in Table 1 and comparative steel sheet (590 MPa class high strength
steel sheet of the prior art). Performance (pressure collapse
resistance and impact resistance) of the formed test piece were
studied by conducting pressure collapse test and impact resistance
test as follows.
Pressure Collapse Test
[0144] The part 1 (hat channel as test piece) shown in FIG. 1 was
made by using steel species A-1, J-1 shown in Table 1 and the
comparative steel sheet, and was subjected to pressure collapse
test. The part was spot welded at the positions 2 of the part shown
in FIG. 1 at 35 mm intervals as shown in FIG. 1 by supplying
electric current of a magnitude less than the expulsion generating
current by 0.5 kA from an electrode measuring 6 mm in diameter at
the distal end. Then a die 3 was pressed against the part 1 from
above-the mid portion thereof in the longitudinal direction as
shown in FIG. 2, and the maximum tolerable load was determined.
Absorbed energy was determined from the area under the
load-deformation curve. The results are shown in Table 3.
TABLE-US-00003 TABLE 3 Evaluation of test piece Steel sheet used
Maximum Energy Steel species TS EL Residual .gamma. load absorbed
Symbol (MPa) (%) (Area %) (kN) (kJ) A-1 1470 11 8 13.8 0.66 H-1
1540 10 8 14.3 0.7 Comparative steel 613 22 0 5.7 0.33 sheet
[0145] From Table 3, it can be seen that the part (test piece) made
from the steel sheet of the present invention has higher load
bearing capability and absorbs greater energy than a part made of
the conventional steel sheet having lower strength, thus showing
high pressure collapse resistance.
Impact Resistance Test
[0146] The parts 4 (hat channel as test piece) shown in FIG. 3 were
made by using steel species A-1, J-1 shown in Table 1 and the
comparative steel sheet, and were subjected to impact resistance
test. FIG. 4 is a sectional view along A-A of the part 4 shown in
FIG. 3. In the impact resistance test, after the part was spot
welded at the positions 5 of the part 4 similarly to the pressure
collapse test, the part 4 was placed on a base 7 as schematically
shown in FIG. 5. A weight 6 (weighing 10 kg) was dropped onto the
part 4 from a height of 11 meters, and the energy absorbed before
the part 4 underwent deformation of 40 mm in the direction of
height. The results are shown in Table 4. TABLE-US-00004 TABLE 4
Steel sheet used Evaluation of test piece Steel species TS EL
Residual .gamma. Energy absorbed Symbol (MPa) (%) (Area %) (kJ) A-1
1476 11 8 6.58 J-1 1540 10 8 6.87 Comparative 613 22 0 3.56 steel
sheet
[0147] From Table 4, it can be seen that the part (test piece) made
from the steel sheet of the present invention absorbs greater
energy than a part made of the conventional steel sheet that has
lower strength, thus showing higher impact resistance.
[0148] TEM photograph of the test piece made in this example is
shown as reference. FIG. 6 is a photograph of TEM observation of
No. 101 of the present invention. From FIG. 6, it can be seen that
the high strength thin steel sheet of the present invention
contains lath-shaped residual austenite (black portion of bar shape
in FIG. 6) specified in the present invention dispersed therein.
FIG. 7 is a photograph of TEM observation of No. 120 of a
comparative example. From FIG. 7, it can be seen that the high
strength thin steel sheet of No. 120 contains residual austenite
(black portion of somewhat round shape in FIG. 7), although the
residual austenite has a block shape that does not satisfy the
requirements of the present invention.
EXAMPLE 2
[0149] By using sample steels A-2 through Y-2 having the
compositions described in Table 5, test slabs were produced under
the same requirements as that in Example 1(hot rolling, cold
rolling and continuous annealing). In addition, No. 217 in Table 6
was prepared by the procedure of No. 116 in Table 2 according to
Example 1 to produce a known high strength martensite steel as a
comparative example. No. 220 was prepared by the procedure of No.
120 in Table 2 according to Example 1.
[0150] The metal structures of steel sheets obtained as described
above, their tensile strength (TS) and elongation (total elongation
El) and hydrogen embrittlement resisting property were measured by
the procedures in Example 1 and the following procedures.
Evaluation of Hydrogen Embrittlement Resisting Property
[0151] In order to evaluate hydrogen embrittlement resisting
property, test pieces were produced by the same procedures as in
Example 1. Then the test pieces were immersed in the same solution
of hydrochloric acid as that in Example 1, and the time before
crack occurred was measured.
[0152] The bent test pieces prepared as described in Example 1 were
subjected to accelerated exposure test in which 3% solution of NaCl
was sprayed once every day for 30 days simulating the actual
operating environment, and the number of days before crack occurred
was determined. In addition, hydrogen-charged 4-point bending test
was also conducted for some steel species as is the case with
Example 1. Maximum stress endured without breaking for 3 hours was
determined as the critical fracture stress (DFL). Then the ratio
(DFL ratio) of this value to the value of DFL of test No. 203
(steel species C-2) shown in Table 6 was determined.
[0153] Results of these tests are shown in Table 6.
Evaluation of Weldability
[0154] Test of weldability was conducted on No. 201 and No. 215
which are representative steel species. As a result, it was found
that No. 201(present invention) is better than No. 215(prior art)
in reference to weldability because the ratio of ductibility in
sample No. 215 is 0.19 while that in sample No. 201 is 0.22.
TABLE-US-00005 TABLE 5 Steel species Chemical composition (mass %)*
Ac3 Bs Ms Symbol C Si Mn P S Al Cu u Ni Ti V Nb Mo Others (.degree.
C.) (.degree. C.) (.degree. C.) A-2 0.24 2.04 2.01 0.012 0.002
0.033 0.3 -- -- -- -- -- -- 857.0 584.3 380.9 B-2 0.13 2.01 2.48
0.011 0.002 0.031 -- 0.3 -- -- -- -- -- 867.8 560.6 412.4 C-2 0.21
2.54 2.51 0.011 0.002 0.030 -- 0.3 -- -- -- -- -- 870.4 536.3 373.5
D-2 0.22 2.02 1.20 0.011 0.002 0.031 -- 0.3 -- -- -- -- -- 884.6
651.5 412.0 E-2 0.20 2.00 3.20 0.011 0.002 0.030 -- 0.3 -- -- -- --
-- 827.8 476.9 355.5 F-2 0.20 1.98 2.53 0.011 0.002 0.033 0.3 0.05
-- -- -- -- -- 846.0 546.5 381.9 G-2 0.19 2.02 2.50 0.011 0.002
0.031 0.3 0.2 0.05 -- -- -- -- 867.9 546.3 385.0 H-2 0.22 1.98 2.48
0.011 0.002 0.033 0.3 0.2 -- 0.05 -- -- -- 846.0 540.0 371.5 I-2
0.22 2.02 2.50 0.011 0.002 0.031 0.3 0.2 0.05 0.05 -- -- -- 866.3
538.2 370.8 J-2 0.20 1.49 2.48 0.012 0.002 0.032 0.3 0.2 0.035 --
-- -- Zr: 0.02 837.6 545.4 381.0 K-2 0.19 2.00 2.50 0.011 0.002
0.032 0.3 0.2 0.05 -- 0.05 0.2 -- 873.7 529.7 380.8 L-2 0.21 2.01
2.50 0.011 0.002 0.033 0.3 0.2 0.05 -- 0.06 0.2 -- 870.0 524.3
371.4 M-2 0.17 1.98 2.50 0.011 0.002 0.032 0.3 0.2 0.05 -- 0.05 0.2
B: 0.0005 877.6 535.1 390.3 N-2 0.20 2.01 2.50 0.011 0.002 0.033
0.3 0.2 0.05 -- 0.05 0.2 Ca: 0.004, 872.2 527.0 376.1 Mg: 0.005 O-2
0.30 2.00 2.00 0.014 0.005 0.031 -- 0.3 -- -- -- -- -- 845.9 557.9
347.7 P-2 0.25 2.50 0.90 0.014 0.005 0.031 -- 0.3 -- -- -- -- --
910.9 670.4 407.7 Q-2 0.20 0.16 1.42 0.014 0.002 0.043 -- 0.3 -- --
-- -- -- 806.2 637.1 414.2 R-2 0.05 2.01 2.01 0.012 0.002 0.033 --
0.3 -- -- -- -- -- 911.2 624.5 465.9 S-2 0.21 2.02 1.20 0.012 0.002
0.033 -- -- -- -- -- -- -- 892.9 665.3 421.9 T-2 0.24 2.04 2.01
0.011 0.002 0.052 0.3 -- -- -- -- -- -- 863.9 584.3 380.9 U-2 0.23
2.01 2.00 0.012 0.002 0.276 0.3 0.2 -- -- -- -- -- 952.3 580.5
382.6 V-2 0.25 1.99 2.02 0.011 0.002 0.355 0.3 0.3 0.05 -- -- -- --
996.0 569.6 370.7 W-2 0.23 2.02 1.99 0.011 0.002 0.751 0.3 0.2 0.05
-- 0.05 0.2 -- 1168.6 564.8 378.7 X-2 0.23 2.00 2.02 0.012 0.002
1.02 0.3 0.2 0.05 -- 0.05 0.2 Ca: 0.004 1275.1 562.1 377.7 Mg:
0.005 Y-2 0.24 1.98 2.00 0.011 0.002 1.66 0.3 0.2 0.05 -- 0.05 0.2
-- 1528.0 561.2 373.6 *The balance consists of iron and inevitable
impurities.
[0155] TABLE-US-00006 TABLE 6 Hydrochloric Steel Mean axis acid
Exposure DFL Test species To Residual .gamma. ratio of BF + M other
TS El immersion test ratio No. Symbol .degree. C. % residual
.gamma. % % MPa % h day -- 201 A-2 320 8 .largecircle. 92 0 1492 12
Over 24 Over 30 -- 202 B-2 320 3 .largecircle. 97 0 1210 14 Over 24
Over 30 1.00 203 C-2 300 7 .largecircle. 92 1 1234 14 Over 24 Over
30 -- 204 D-2 320 7 .largecircle. 93 0 1533 11 Over 24 Over 30 --
205 E-2 350 8 .largecircle. 92 0 1467 12 Over 24 Over 30 -- 206 F-2
320 7 .largecircle. 93 0 1511 11 Over 24 Over 30 -- 207 G-2 320 8
.largecircle. 92 0 1492 11 Over 24 Over 30 -- 208 H-2 320 8
.largecircle. 92 0 1532 12 Over 24 Over 30 -- 209 I-2 320 8
.largecircle. 92 0 1567 11 Over 24 Over 30 -- 210 J-2 320 7
.largecircle. 93 0 1461 12 Over 24 Over 30 -- 211 K-2 320 7
.largecircle. 93 0 1519 11 Over 24 Over 30 -- 212 L-2 320 7
.largecircle. 93 0 1495 11 Over 24 Over 30 -- 213 M-2 320 6
.largecircle. 94 0 1490 12 Over 24 Over 30 -- 214 N-2 320 7
.largecircle. 93 0 1503 11 Over 24 Over 30 -- 215 O-2 300 8
.largecircle. 92 0 1562 11 12 15 -- 216 P-2 320 <1 X 99 <1
1313 8 8 9 -- 217 Q-2 -- <1 X 99 <1 1488 3 3 5 -- 218 R-2 370
<1 X 99 <1 992 17 Over 24 Over 30 -- 219 S-2 320 5
.largecircle. 94 1 1448 10 Over 24 17 -- 220 A-2 350 12 X 20 68 961
14 Over 24 Over 30 -- 221 T-2 300 7 .largecircle. 93 0 1495 11 Over
24 Over 30 1.24 222 U-2 300 8 .largecircle. 92 0 1509 12 Over 24
Over 30 1.56 223 V-2 320 8 .largecircle. 92 0 1512 11 Over 24 Over
30 1.66 224 W-2 320 8 .largecircle. 92 0 1513 12 Over 24 Over 30
1.71 225 X-2 320 9 .largecircle. 91 0 1504 11 Over 24 Over 30 1.78
226 Y-2 320 10 X 61 29 1230 14 13 16 0.73
[0156] The results shown in Tables 5 and 6 can be interpreted as
follows (numbers in the following description are test Nos. in
Table 6).
[0157] Test pieces Nos. 201 through 214 (inventive steel sheets 2)
and test pieces Nos. 221 through 225 (inventive steel sheets 1)
that satisfy the requirements of the present invention have high
strength of 1180 MPa or higher, and high hydrogen embrittlement
resisting property in harsh environment after the forming process.
They also have high elongation property required of the TRIP steel
sheet, thus providing steel sheets best suited for reinforcement
parts of automobiles that are exposed to corrosive atmosphere. Test
pieces Nos. 221 through 225, in particular, show even better
hydrogen embrittlement resisting property.
[0158] Test pieces Nos. 215 through 220 and 226 that do not satisfy
the requirements of the present invention, in contrast, have the
following drawbacks.
[0159] No. 215 made of steel species O-2 that includes insufficient
C content has the amount of residual austenite significantly
decreased after the processing, and fails to show the required
level of hydrogen embrittlement resisting property of the present
invention.
[0160] No. 216 made of steel species P-2 that includes insufficient
Mn content does not retain sufficient residual austenite and is
inferior in hydrogen embrittlement resisting property after the
processing.
[0161] No. 217, martensite steel that is a conventional high
strength steel made of steel species Q-2 that includes insufficient
Si content, hardly contains residual austenite and is inferior in
hydrogen embrittlement resisting property. It also does not show
the elongation property required of a thin steel sheet.
[0162] No. 218 made of steel species R-2 that includes excessive C
content has precipitation of carbide and is inferior in both the
forming workability and the hydrogen embrittlement resisting
property after processing. No. 219 made of steel species S-2 that
does not include Cu and/or Ni shows insufficient corrosion
resistance and fails to show the required level of hydrogen
embrittlement resisting property of the present invention.
[0163] No. 220, that was made of a steel that has the composition
specified in the present invention but was not manufactured under
the recommended conditions, resulted in the conventional TRIP
steel. As a result, the residual austenite does not have the mean
axis ratio specified in the present invention, while the matrix
phase is not formed in binary phase structure of bainitic ferrite
and martensite, and therefore sufficient level of hydrogen
embrittlement resisting property is not achieved.
[0164] No. 226 includes Al content higher than that specified for
the inventive steel sheet 1. As a result, although the
predetermined amount of residual austenite is retained, the
residual austenite does not have the mean axis ratio specified in
the present invention, the desired matrix phase is not obtained and
inclusions such as AlN are generated thus resulting in poor
hydrogen embrittlement resisting property.
[0165] Then parts were made by using steel species A-2, I-2 shown
in Table 5 and comparative steel sheet (590 MPa class high strength
steel sheet of the prior art). Performance (pressure collapse
resistance and impact resistance) of the formed test piece were
studied by conducting pressure collapse test and impact resistance
test as follows.
Pressure Collapse Test
[0166] Maximum tolerable load was determined similarly to Example 1
by using steel species A-2, K-2 shown in Table 5 and the
comparative steel sheet. Absorbed energy was determined from the
area lying under the load-deformation curve. The results are shown
in Table 7. TABLE-US-00007 TABLE 7 Evaluation of test piece Steel
sheet used Maximum Energy Steel species TS EL Residual .gamma. load
absorbed Symbol (MPa) (%) (Area %) (kN) (kJ) A-2 1492 12 8 13.9
0.68 I-2 1567 11 8 14.6 0.71 Comparative steel 613 22 0 5.7 0.33
sheet
[0167] From Table 7, it can be seen that the part (test piece) made
from the steel sheet of the present invention has higher load
bearing capability and absorbs greater energy than a part made of
the conventional steel sheet that has lower strength, thus showing
higher pressure collapse resistance.
Impact Resistance Test
[0168] The impact resistance test was conducted similarly to
Example 1 on the steel sheets made of steel species A-2, K-2 shown
in Table 5 and the comparative steel sheet. The results are shown
in Table 8. TABLE-US-00008 TABLE 8 Steel sheet used Evaluation of
test piece Steel species TS EL Residual .gamma. Energy absorbed
Synbol (MPa) (%) (Area %) (kJ) A-2 1492 12 8 6.65 I-2 1567 11 8
6.99 Comparative steel sheet 613 22 0 3.56
[0169] From Table 8, it can be seen that the part (test piece) made
from the steel sheet of the present invention absorbs greater
energy than a part made of the conventional steel sheet having
lower strength, thus showing higher impact resistance.
[0170] TEM photograph of the test piece made in this example is
shown as reference. FIG. 8 is a photograph of TEM observation of
No. 201 of the present invention. From FIG. 8, it can be seen that
the high strength thin steel sheet of the present invention
contains lath-shaped residual austenite (black portion of bar shape
in FIG. 8) specified in the present invention dispersed therein.
FIG. 9 is a photograph of TEM observation of No. 220 of a
comparative example. From FIG. 9, it can be seen that the high
strength thin steel sheet of No. 220 contains residual austenite
(black portion of somewhat round shape in FIG. 9), although the
residual austenite has a block shape that does not satisfy the
requirements of the present invention.
EXAMPLE 3
[0171] By using sample steels A-3 through R-3 having the
compositions described in Table 9, test slabs were produced under
the same conditions as that in Example 1(hot rolling, cold rolling
and continuous annealing). In addition, No. 312 in Table 10 was
prepared by the procedure of No. 116 in Table 2 according to
Example 1 to produce a known high strength martensite steel as a
comparative example. No. 313 was prepared by the procedure of No.
120 in Table 2 according to Example 1.
[0172] The metal structures of steel sheets obtained as described
above, their tensile strength (TS) and elongation (total elongation
E1) and hydrogen embrittlement resisting property were measured by
the procedures in Example 1 and the following procedures.
Evaluation of Hydrogen Embrittlement Resisting Property
[0173] In order to evaluate the hydrogen embrittlement resisting
property, flat test piece 1.2 mm in thickness was subjected to slow
stretching rate test (SSRT) with a stretching speed of
1.times.10.sup.-4/sec, to determine hydrogen embrittlement risk
index (%) defined by the equation shown below. Hydrogen
embrittlement risk index (%)=100.times.(1-E1/E0)
[0174] E0 represents the elongation before rupture of a steel test
piece that does not substantially contain hydrogen, E1 represents
the elongation before rupture of a steel test piece that has been
charged with hydrogen electrochemically in sulfuric acid. Hydrogen
charging was carried out by immersing the steel test piece in a
mixed solution of H.sub.2SO.sub.4 (0.5 mol/L) and KSCN (0.01 mol/L)
and supplying constant current (100 A/m.sup.2) at room
temperature.
[0175] A steel sheet having hydrogen embrittlement risk index
higher than 50% is likely to undergo hydrogen embrittlement during
use. In the present invention, steel sheets having hydrogen
embrittlement risk index not higher than 50% were evaluated to have
high hydrogen embrittlement resisting property. Results of the test
are shown in Table 10.
Evaluation of Weldability
[0176] Test of weldability was conducted on No. 201 and No. 215
which are representative steel species. As a result, it was found
that No. 301 (present invention) is better than No. 311 (prior art)
in reference to weldability because the ratio of ductibility in
sample No. 311 is 0.19 while that in sample No. 301 is 0.22.
TABLE-US-00009 TABLE 9 Steel species Chemical composition (mass %)*
Ac3 Bs Ms Symbol C Si Mn P S Al Cu Ni Ti V Nb Mo Other (.degree.
C.) (.degree. C.) (.degree. C.) A-3 0.21 2.02 2.50 0.011 0.002
0.031 -- -- -- -- -- -- -- 852.4 548.3 379.0 B-3 0.22 1.98 2.52
0.011 0.002 0.033 -- 0.2 -- -- -- -- -- 845.6 536.4 370.2 C-3 0.24
2.04 2.01 0.012 0.002 0.033 0.3 0.2 0.035 -- -- -- -- 868.0 576.9
377.5 D-3 0.19 1.98 2.53 0.011 0.002 0.033 0.3 0.2 0.05 0.05 -- --
-- 871.2 543.6 384.1 E-3 0.21 2.02 2.51 0.011 0.002 0.030 -- --
0.05 0.05 -- -- -- 876.9 547.4 378.6 F-3 0.22 1.98 2.48 0.011 0.002
0.031 -- -- -- -- 0.05 0.2 -- 855.3 530.8 370.7 G-3 0.20 2.00 2.53
0.011 0.002 0.030 -- -- -- -- -- -- B: 0.0005 852.4 548.3 382.7 H-3
0.18 1.98 2.50 0.011 0.002 0.032 -- -- -- 0.05 0.05 0.2 B: 0.0005
869.4 539.8 389.0 I-3 0.21 2.01 2.50 0.011 0.002 0.033 0.3 0.3 0.05
0.05 0.2 Ca: 0.004, 868.5 520.6 369.7 Mg: 0.005 J-3 0.22 1.98 2.48
0.011 0.002 0.033 -- -- -- -- -- -- REM: 0.005 849.8 547.4 374.9
K-3 0.40 2.00 2.00 0.014 0.005 0.031 -- -- -- -- -- -- -- 833.2
542.0 305.4 L-3 0.20 0.16 1.42 0.014 0.002 0.043 -- -- -- -- -- --
-- 810.8 648.2 419.3 M-3 0.21 2.00 2.48 0.011 0.002 0.051 -- -- --
-- -- -- -- 860.1 550.1 379.6 N-3 0.20 2.02 2.50 0.012 0.002 0.251
0.3 0.2 -- -- -- -- -- 934.3 543.6 380.3 O-3 0.22 1.99 2.51 0.012
0.002 0.374 0.3 0.2 0.05 0.05 -- -- -- 997.4 537.3 370.5 P-3 0.21
2.01 2.51 0.012 0.002 0.750 0.3 0.3 0.05 0.05 -- -- -- 1149.4 536.3
373.5 Q-3 0.21 2.02 2.55 0.012 0.002 1.03 0.3 0.3 0.05 0.05 0.05
0.2 Ca: 0.004, 1266.9 516.1 368.0 Mg: 0.005 R-3 0.21 2.00 2.60
0.011 0.002 1.68 -- -- -- -- -- -- -- 1508.1 539.3 375.7 *The
balance consists of iron and inevitable impurities.
[0177] TABLE-US-00010 TABLE 10 Mean Hydrogen Steel Mean axis of
Minimum distance axis ratio embrittlement Test species To Residual
.gamma. residual .gamma. between residual of BF + M Other TS El
risk index No. Symbol .degree. C. % (nm) .gamma. grains (nm)
residual .gamma. % % MPa % (%) 301 A-3 350 8 160 320 10 92 0 1280
14 28 302 B-3 350 9 170 340 7 91 0 1310 14 30 303 C-3 320 8 140 280
15 92 0 1480 12 25 304 D-3 320 8 130 260 20 92 0 1495 11 23 305 E-3
320 8 120 240 25 92 0 1460 11 18 306 F-3 320 8 140 280 15 92 0 1490
11 30 307 G-3 320 7 120 240 25 93 0 1470 10 20 308 H-3 320 7 110
220 30 93 0 1480 11 18 309 I-3 300 6 90 180 40 93 1 1470 10 15 310
J-3 300 6 80 160 50 93 1 1470 10 12 311 K-3 350 15 270 520 20 85 0
1520 10 40 312 L-3 350 <1 -- -- -- 99 <1 1400 8 90 313 A-3
350 11 1300 1200 1.5 20 69 960 15 85 314 M-3 350 8 190 380 10 92 0
1290 14 28 315 N-3 320 7 140 280 13 93 0 1410 13 22 316 O-3 320 7
130 260 12 93 0 1480 12 19 317 P-3 320 7 130 260 12 93 0 1509 11 19
318 Q-3 320 7 120 240 11 93 0 1513 11 14 319 R-3 350 12 1000 1200 3
59 29 1295 14 75
[0178] The results shown in Tables 9 and 10 can be interpreted as
follows (numbers in the following description are test Nos. in
Table 10).
[0179] Test pieces Nos. 301 through 310 (inventive steel sheets 2)
and test pieces Nos. 314 through 318 (inventive steel sheets 1)
that satisfy the requirements of the present invention have high
strength of 1180 MPa or higher, and show high hydrogen
embrittlement resisting property in harsh environment after the
forming process. They also have high elongation property required
of the TRIP steel sheet, thus providing steel sheets best suited
for reinforcement parts of automobiles that are exposed to
corrosive atmosphere.
[0180] Test pieces Nos. 311 through 313 and 319 that do not satisfy
the requirements of the present invention, in contrast, have the
following drawbacks.
[0181] No. 311 made of steel species K-3 that includes excessive C
content has carbide precipitated and residual austenite of longer
mean length of minor axis, thus resulting poor performance in both
workability and hydrogen embrittlement resisting property after
processing.
[0182] No. 312, martensite steel that is a conventional high
strength steel made of steel species L-3 that includes insufficient
Si content, hardly contains residual austenite and is inferior in
hydrogen embrittlement resisting property. It also does not show
the elongation property required of a thin steel sheet.
[0183] No. 313, that was made of a steel that has the composition
specified in the present invention but was not manufactured under
the recommended conditions, resulted in the conventional TRIP
steel. As a result, the residual austenite does not have the mean
axis ratio and the mean length of minor axis specified in the
present invention, while the matrix phase is not formed in binary
phase structure of bainitic ferrite and martensite, thus resulting
in low strength and poor hydrogen embrittlement resisting
property.
[0184] No. 319 includes Al content higher than that specified for
the inventive steel sheet 1. As a result, although the
predetermined amount of residual austenite is retained, the
residual austenite does not have the mean axis ratio specified in
the present invention, the desired matrix phase is not obtained and
inclusions such as AlN are generated thus resulting in poor
hydrogen embrittlement resisting property.
[0185] Then parts were made by using steel species A-3, D-3 shown
in Table 9 and comparative steel sheet (590 MPa class high strength
steel sheet of the prior art). Performance (pressure collapse
resistance and impact resistance) of the formed test piece were
studied by conducting pressure collapse test and impact resistance
test as follows.
Pressure Collapse Test
[0186] Maximum tolerable load was determined similarly to Example 1
by using steel species A-3, D-3 shown in Table 9 and the
comparative steel sheet. Absorbed energy was determined from the
area under the load-deformation curve. The results are shown in
Table 11. TABLE-US-00011 TABLE 11 Evaluation of test piece Steel
sheet used Maximum Energy Steel species TS EL Residual .gamma. load
absorbed Symbol (MPa) (%) (Area %) (kN) (kJ) A-3 1280 14 8 12 0.6
D-3 1495 11 8 13.9 0.67 Comparative steel 613 22 0 5.7 0.33
sheet
[0187] From Table 11, it can be seen that the part (test piece)
made from the steel sheet of the present invention has higher load
bearing capability and absorbs greater energy than a part made of
the conventional steel sheet having lower strength, thus showing
high pressure collapse resistance.
Impact Resistance Test
[0188] The impact resistance test was conducted similarly to
Example 1 on the steel sheets made of steel species A-3, D-3 shown
in Table 9 and the comparative steel sheet. The results are shown
in Table 12. TABLE-US-00012 TABLE 12 Evaluation of Steel sheet used
test piece Steel species TS EL Residual .gamma. Energy absorbed
Symbol (MPa) (%) (Area %) (kJ) A-3 1280 14 8 5.95 D-3 1495 11 8
6.77 Comparative steel 613 22 0 3.56 sheet
[0189] From Table 12, it can be seen that the part (test piece)
made from the steel sheet of the present invention absorbs greater
energy than a part made of the conventional steel sheet that has
lower strength, thus showing high impact resistance.
[0190] TEM photographs of the test pieces made in this example are
shown as reference. FIG. 12 is a photograph of TEM observation
(magnification factor 15000) of No. 301 of the present invention.
FIG. 13 is a photograph of TEM observation (magnification factor
60,000) of a portion shown in the photograph of FIG. 12. From FIGS.
12, 13, it can be seen that the high strength thin steel sheet of
the present invention contains fine residual austenite grains
(black portion of bar shape in FIGS. 12, 13) specified in the
present invention dispersed therein, and that the residual
austenite has the lath shape specified in the present invention.
FIG. 14 is a photograph of TEM observation of No. 313 of a
comparative example. From FIG. 14, it can be seen that the high
strength thin steel sheet of No. 313 contains residual austenite
(black portion of somewhat round shape in FIG. 14), although the
residual austenite has a block shape that does not satisfy the
requirements of the present invention.
* * * * *