U.S. patent application number 14/402745 was filed with the patent office on 2015-06-25 for hollow stabilizer, and steel pipe for hollow stabilizers and method of producing the same.
The applicant listed for this patent is NHK SPRING CO., LTD., NIPPON STEEL & SUMITOMO METAL CORPORATION. Invention is credited to Tetsuo Ishitsuka, Masamichi Iwamura, Motofumi Koyuba, Kiyoshi Kurimoto, Ken Takahashi, Akira Tange, Yutaka Wakabayashi.
Application Number | 20150176101 14/402745 |
Document ID | / |
Family ID | 49623526 |
Filed Date | 2015-06-25 |
United States Patent
Application |
20150176101 |
Kind Code |
A1 |
Ishitsuka; Tetsuo ; et
al. |
June 25, 2015 |
HOLLOW STABILIZER, AND STEEL PIPE FOR HOLLOW STABILIZERS AND METHOD
OF PRODUCING THE SAME
Abstract
A hollow stabilizer having an excellent fatigue property and
higher strength compared to the conventional ones has a chemical
composition containing 0.26% to 0.30% of C, 0.05% to 0.35% of Si,
0.5% to 1.0% of Mn, 0.05% to 1.0% of Cr, 0.005% to 0.05% of Ti,
0.0005% to 0.005% of B, and 0.0005% to 0.005% of Ca, wherein; Al,
P, S, N, and O are limited to 0.08% or less, 0.05% or less, less
than 0.0030%, 0.006% or less, and 0.004% or less, respectively, a
remainder of the chemical composition consists of Fe and
unavoidable impurities, a value of a product of the Mn content and
the S content is 0.0025 or less, and a critical cooling rate Vc90
represented by a predetermined equation is 40.degree. C./s or less;
and wherein a metallic structure comprises a tempered martensite, a
length of elongated MnS present at a center part in a thickness
direction of the hollow stabilizer is 150 .mu.m or less, a HRC is
from 40 to 50, a thickness to outer diameter ratio is 0.14 or more,
and a depth of a decarburized layer at an inner surface part is 20
.mu.m or less from the inner surface; and a steel pipe for hollow
stabilizers used as a material for the hollow stabilizer.
Inventors: |
Ishitsuka; Tetsuo;
(Chiyoda-ku, JP) ; Koyuba; Motofumi; (Chiyoda-ku,
JP) ; Iwamura; Masamichi; (Chiyoda-ku, JP) ;
Tange; Akira; (Yokohama-city, JP) ; Takahashi;
Ken; (Yokohama-city, JP) ; Kurimoto; Kiyoshi;
(Yokohama-city, JP) ; Wakabayashi; Yutaka;
(Yokohama-city, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL & SUMITOMO METAL CORPORATION
NHK SPRING CO., LTD. |
Chiyoda-ku, Tokyo
Yokohama-city, Kanagawa |
|
JP
JP |
|
|
Family ID: |
49623526 |
Appl. No.: |
14/402745 |
Filed: |
February 25, 2013 |
PCT Filed: |
February 25, 2013 |
PCT NO: |
PCT/JP2013/054815 |
371 Date: |
November 21, 2014 |
Current U.S.
Class: |
148/504 ;
148/330 |
Current CPC
Class: |
C21D 9/50 20130101; B60G
21/055 20130101; B60G 2206/427 20130101; C21D 7/06 20130101; C22C
38/24 20130101; C21D 2211/008 20130101; C21D 9/08 20130101; C22C
38/26 20130101; B21C 37/08 20130101; C22C 38/50 20130101; C22C
38/02 20130101; C22C 38/06 20130101; C22C 38/002 20130101; C21D
8/10 20130101; B60G 2206/72 20130101; C21D 2211/009 20130101; C21D
2211/004 20130101; C22C 38/28 20130101; C22C 38/04 20130101; C21D
9/085 20130101; C21D 2211/005 20130101; C22C 38/32 20130101; C21D
8/105 20130101; C22C 38/001 20130101; C22C 38/54 20130101; C22C
38/22 20130101; B21C 51/00 20130101; C21D 7/13 20130101 |
International
Class: |
C21D 9/50 20060101
C21D009/50; C22C 38/50 20060101 C22C038/50; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; C22C 38/32 20060101
C22C038/32; C22C 38/28 20060101 C22C038/28; C22C 38/26 20060101
C22C038/26; C22C 38/24 20060101 C22C038/24; C22C 38/22 20060101
C22C038/22; C21D 9/08 20060101 C21D009/08; C21D 8/10 20060101
C21D008/10; C21D 7/13 20060101 C21D007/13; C21D 7/06 20060101
C21D007/06; C22C 38/54 20060101 C22C038/54 |
Foreign Application Data
Date |
Code |
Application Number |
May 25, 2012 |
JP |
2012-119869 |
Claims
1. A hollow stabilizer having a chemical composition comprising, as
chemical components, in terms of % by mass: 0.26% to 0.30% of C,
0.05% to 0.35% of Si, 0.5% to 1.0% of Mn, 0.05% to 1.0% of Cr,
0.005% to 0.05% of Ti, 0.0005% to 0.005% of B, and 0.0005% to
0.005% of Ca, wherein: Al, P, S, N, and O are limited to 0.08% or
less, 0.05% or less, less than 0.0030%, 0.006% or less, and 0.004%
or less, respectively, a remainder of the chemical composition
consists of Fe and unavoidable impurities, a value of a product of
the Mn content and the S content is 0.0025 or less, and a critical
cooling rate Vc90 represented by the following Equation (1) is
40.degree. C./s or less: log Vc90=2.94-0.75.beta. Equation (1):
wherein 13=2.7C+0.4Si+Mn+0.8Cr; and wherein: a metallic structure
comprises a tempered martensite, a length of elongated MnS present
at a center part in a thickness direction of the hollow stabilizer
is 150 .mu.m or less, a Rockwell C hardness (HRC) is from 40 to 50,
a wall thickness/outer diameter ratio is 0.14 or more, and a depth
of a decarburized layer at an inner surface part of the hollow
stabilizer is 20 .mu.m or less from the inner surface.
2. The hollow stabilizer according to claim 1, further comprising,
in terms of % by mass, one or more of: 0.05% to 0.5% of Mo, 0.01%
to 0.1% ofNb, 0.01% to 0.1% of V, or 0.1% to 1.0% of Ni, wherein,
in Equation (1), .beta.=2.7C+0.4Si+Mn+0.8Cr+2.0Mo+0.8Ni.
3. The hollow stabilizer according to claim 1, wherein a maximum
compressive residual stress on an outer surface is 400 MPa or
more.
4. The hollow stabilizer according to claim 3, wherein the outer
surface and the inner surface are subjected to shot peening.
5. A steel pipe for a hollow stabilizer used as a material for the
hollow stabilizer according to claim 1, the steel pipe having a
chemical composition comprising, as chemical components, in terms
of % by mass: 0.26% to 0.30% of C, 0.05% to 0.35% of Si, 0.5% to
1.0% of Mn, 0.05% to 1.0% of Cr, 0.005% to 0.05% of Ti, 0.0005% to
0.005% of B, and 0.0005% to 0.005% of Ca, wherein: Al, P, S, N, and
O are limited to 0.08% or less, 0.05% or less, less than 0.0030%,
0.006% or less, and 0.004% or less, respectively, the chemical
composition optionally comprises one or more of: 0.05% to 0.5% of
Mo, 0.01% to 0.1% of Nb, 0.01% to 0.1% of V, or 0.1% to 1.0% of Ni,
a remainder of the chemical composition consists of Fe and
unavoidable impurities, a value of a product of the Mn content and
the S content is 0.0025 or less, and a critical cooling rate Vc90
represented by the following Equation (1) is 40.degree. C./s or
less: log Vc90=2.94-0.75.beta. Equation (1): wherein
.beta.=2.7C+0.4Si+Mn+0.8Cr+2.0Mo+0.8Ni; and wherein: a metallic
structure comprises a mixed structure of ferrite and perlite, a
length of elongated MnS present at a center part in a thickness
direction of the steel pipe is 150 .mu.m or less, a Rockwell B
hardness (HRB) is 95 or less, a wall thickness/outer diameter ratio
is 0.14 or more, and a depth of a decarburized layer at an inner
surface part of the steel pipe is 20 .mu.m or less from the inner
surface.
6. A method of producing the steel pipe for a hollow stabilizer
according to claim 5, wherein: the steel pipe is an electric
resistance-welded steel pipe; and the method comprises a process of
subjecting the electric resistance-welded steel pipe to heating,
after electric resistance-welding, to a temperature of from
800.degree. C. to 1200.degree. C. and diameter-reduction hot
rolling to a reduction in cross sectional area of from 40% to
80%.
7. A method of producing the steel pipe for a hollow stabilizer
according to claim 5, wherein: the steel pipe is an electric
resistance-welded steel pipe; and the method comprises a process of
elongating the electric resistance-welded steel pipe by
cold-drawing after electric resistance-welding.
8. The hollow stabilizer according to claim 2, wherein a maximum
compressive residual stress on an outer surface is 400 MPa or
more.
9. The hollow stabilizer according to claim 8, wherein the outer
surface and the inner surface are subjected to shot peening.
10. A steel pipe for a hollow stabilizer used as a material for the
hollow stabilizer according to claim 2, the steel pipe having a
chemical composition comprising, as chemical components, in terms
of % by mass: 0.26% to 0.30% of C, 0.05% to 0.35% of Si, 0.5% to
1.0% of Mn, 0.05% to 1.0% of Cr, 0.005% to 0.05% of Ti, 0.0005% to
0.005% of B, and 0.0005% to 0.005% of Ca, wherein: Al, P, S, N, and
O are limited to 0.08% or less, 0.05% or less, less than 0.0030%,
0.006% or less, and 0.004% or less, respectively, the chemical
composition optionally comprises one or more of: 0.05% to 0.5% of
Mo, 0.01% to 0.1% of Nb, 0.01% to 0.1% of V, or 0.1% to 1.0% of Ni,
a remainder of the chemical composition consists of Fe and
unavoidable impurities, a value of a product of the Mn content and
the S content is 0.0025 or less, and a critical cooling rate Vc90
represented by the following Equation (1) is 40.degree. C./s or
less: log Vc90=2.94-0.75.beta. Equation (1): wherein
.beta.=2.7C+0.4Si+Mn+0.8Cr+2.0Mo+0.8Ni; and wherein: a metallic
structure comprises a mixed structure of ferrite and perlite, a
length of elongated MnS present at a center part in a thickness
direction of the steel pipe is 150 .mu.m or less, a Rockwell B
hardness (HRB) is 95 or less, a wall thickness/outer diameter ratio
is 0.14 or more, and a depth of a decarburized layer at an inner
surface part of the steel pipe is 20 .mu.m or less from the inner
surface.
11. A method of producing the steel pipe for a hollow stabilizer
according to claim 10, wherein: the steel pipe is an electric
resistance-welded steel pipe; and the method comprises a process of
subjecting the electric resistance-welded steel pipe to heating,
after electric resistance-welding, to a temperature of from
800.degree. C. to 1200.degree. C. and diameter-reduction hot
rolling to a reduction in cross sectional area of from 40% to
80%.
12. A method of producing the steel pipe for a hollow stabilizer
according to claim 10, wherein: the steel pipe is an electric
resistance-welded steel pipe; and the method comprises a process of
elongating the electric resistance-welded steel pipe by
cold-drawing after electric resistance-welding.
Description
TECHNICAL FIELD
[0001] The present invention relates to a hollow stabilizer used
for a vehicle such as an automobile, and a steel pipe for hollow
stabilizers used as a material for the hollow stabilizer and a
method of producing thereof.
BACKGROUND ART
[0002] Stabilizers are applied to vehicles such as automobiles for
the purpose of securing the running stability of vehicle bodies
during high speed running while suppressing the rolling of vehicle
bodies during cornering. The stabilizer is conventionally
manufactured by processing a solid material such as a steel bar
into an intended shape. In recent years, hollow stabilizers using a
hollow material such as a seamless steel pipe or an electric
resistance-welded steel pipe are increasingly used for the purpose
of promoting weight reduction.
[0003] In the case of changing the design from a solid stabilizer
to a hollow stabilizer, the outer diameter of the hollow stabilizer
necessarily becomes larger than that of the solid stabilizer in
order to maintain the same roll rigidity. As a result, the
generated stress with respect to the same load becomes larger in
the hollow stabilizer, and therefore, it is necessary to increase a
wall thickness/outer diameter (t/D) ratio for suppressing an
increase in the generated stress.
[0004] Thin-walled hollow stabilizers with a t/D of from 0.10 to
0.17 are conventionally applied to compact cars with low design
stresses. However, in order to apply hollow stabilizers to a large
car with a high design stress, t/D is required to be increased. For
this purpose, a method of producing a hollow stabilizer in which an
electric resistance-welded steel pipe is subjected to
diameter-reduction hot rolling and then drawing (for example, see
Patent Document 1), and a thick-walled steel pipe for hollow
stabilizers produced by subjecting an electric resistance-welded
steel pipe to diameter-reduction hot rolling (for example, see
Patent Document 2) have been suggested.
[0005] In the case of the hollow stabilizer, fatigue fracture was
sometimes generated from the inner surface which is absent in the
solid stabilizer. This is because fatigue fracture is generated
from a decarburized layer on the inner surface even when the
fatigue strength of the outer surface of the steel pipe is improved
by increasing the strength to the steel pipe. In order to solve
this problem, a steel pipe for hollow stabilizers in which the t/D
is 0.20 or more and the generation of the decarburized layer on the
inner surface is suppressed has been suggested (for example, see
Patent Document 3).
[0006] Patent Document 1: Japanese Patent Application Laid-Open
(JP-A) No. 2000-233625
[0007] Patent Document 2: International Publication No. WO
2007-023873
[0008] Patent Document 3: JP-A No. 2007-270349
SUMMARY OF INVENTION
Technical Problem
[0009] The inventors found that quenching cracks are generated when
the C content is increased in order to improve the strength of an
electric resistance-welded pipe used as a material for producing
hollow stabilizers, and that the deterioration in fatigue strength
in the vicinity of the electric resistance-welded portion becomes
evident when the material is highly strengthened. FIG. 1(a) is a
perspective view of an electric resistance-welded steel pipe; FIG.
1(b) is a magnified view of a metal flow 18 in a base metal 17,
observed in the cross section of an electric resistance-welded
steel pipe 16 surrounded by circle S1 in FIG. 1(a); FIG. 1(c) is a
magnified view of a metal flow 18 in a welded portion 19, observed
in the cross section of the electric resistance-welded steel pipe
16 surrounded by circle S2 in FIG. 1(a); and FIG. 1(d) is a
magnified view showing a state of presence of MnS in the
longitudinal section along the extending direction (L direction) of
an electric resistance-welded abutting portion of the electric
resistance-welded steel pipe 16. These figures are schematic
diagrams. As is clear from a comparison of FIGS. 1(b) and 1(c), in
the vicinity of the electric resistance-welded portion 19, the
metal flow 18 is formed such that the center segregation in the
steel plate stands perpendicularly, in the wall thickness
direction, due to the intensive upset of abutting surfaces during
welding. Therefore, in a case in which MnS 20 elongated in a
longitudinal direction by rolling is present at the center
segregation of the steel plate, elongated MnS 20 occurs on the
surface close to the electric resistance-welded portion 19 after
cutting off a welding bead as shown in FIG. 1(d) and is an origin
of fatigue fracture.
[0010] The present invention is made in consideration of the above
situation, and the object of the present invention is to provide a
hollow stabilizer having higher strength compared to the
conventional hollow stabilizers and having excellent fatigue
properties and a steel pipe for hollow stabilizers, which is used
as a material for the hollow stabilizer.
Solution to Problem
[0011] Hollow stabilizers are produced through quenching and
tempering for adjusting quality of material. As a result of the
study by the inventors, it was found that quenching cracks are
generated during quenching in a case in which the C content is too
high. However, since the strength is insufficient with a low C
content, Cr is added to ensure hardenability in the invention.
Furthermore, in order to suppress the deterioration in fatigue
strength caused by MnS in the vicinity of the electric
resistance-welded portion, it is necessary to restrict Mn, S, Ca,
and O. It is preferable to restrict t/D and the thickness of a
decarburizing layer in order to suppress the generation of fatigue
cracks from the inner surface, and it is more preferable to impart
compressive residual stress by shot peening.
[0012] The summary of the invention is as follows.
[0013] (1) A hollow stabilizer having a chemical composition
comprising, as chemical components, in terms of % by mass: 0.26% to
0.30% of C, 0.05% to 0.35% of Si, 0.5% to 1.0% of Mn, 0.05% to 1.0%
of Cr, 0.005% to 0.05% of Ti, 0.0005% to 0.005% of B, and 0.0005%
to 0.005% of Ca, wherein: Al, P, S, N, and O are limited to 0.08%
or less, 0.05% or less, less than 0.0030%, 0.006% or less, and
0.004% or less, respectively, a remainder of the chemical
composition consists of Fe and unavoidable impurities, a value of a
product of the Mn content and the S content is 0.0025 or less, and
a critical cooling rate Vc90 represented by the following Equation
(1) is 40.degree. C./s or less:
log Vc90=2.94-0.75.beta. Equation (1): [0014] wherein
.beta.=2.7C+0.4Si+Mn+0.8Cr; and wherein: a metallic structure
comprises a tempered martensite, a length of elongated MnS present
at a center part in a thickness direction of the hollow stabilizer
is 150 .mu.m or less, a Rockwell C hardness (HRC) is from 40 to 50,
a wall thickness/outer diameter ratio is 0.14 or more, and a depth
of a decarburized layer at an inner surface part of the hollow
stabilizer is 20 .mu.m or less from the inner surface.
[0015] (2) The hollow stabilizer according to (1), further
including, in terms of % by mass, one or more of: 0.05% to 0.5% of
Mo, 0.01% to 0.1% of Nb, 0.01% to 0.1% of V, or 0.1% to 1.0% of Ni,
wherein, in Equation (1), 0=2.7C+0.4Si+Mn+0.8Cr+2.0Mo+0.8Ni.
[0016] (3) The hollow stabilizer according to (1) or (2), in which
a maximum compressive residual stress on an outer surface is 400
MPa or more.
[0017] (4) The hollow stabilizer according to (3), in which the
outer surface and the inner surface are subjected to shot
peening.
[0018] (5) A steel pipe for a hollow stabilizer used as a material
for the hollow stabilizer according to any one of claims 1 to 5,
the steel pipe having a chemical composition comprising, as
chemical components, in terms of % by mass: 0.26% to 0.30% of C,
0.05% to 0.35% of Si, 0.5% to 1.0% of Mn, 0.05% to 1.0% of Cr,
0.005% to 0.05% of Ti, 0.0005% to 0.005% of B, and 0.0005% to
0.005% of Ca, wherein: Al, P, S, N, and O are limited to 0.08% or
less, 0.05% or less, less than 0.0030%, 0.006% or less, and 0.004%
or less, respectively, the chemical composition optionally
comprises one or more of: 0.05% to 0.5% of Mo, 0.01% to 0.1% of Nb,
0.01% to 0.1% of V, or 0.1% to 1.0% of Ni, a remainder of the
chemical composition consists of Fe and unavoidable impurities, a
value of a product of the Mn content and the S content is 0.0025 or
less, and a critical cooling rate Vc90 represented by the following
Equation (1) is 40.degree. C./s or less:
log Vc90=2.94-0.75.beta. Equation (1): [0019] wherein
.beta.=2.7C+0.4Si+Mn+0.8Cr+2.0Mo+0.8Ni; and wherein: a metallic
structure comprises a mixed structure of ferrite and perlite, a
length of elongated MnS present at a center part in a thickness
direction of the steel pipe is 150 .mu.m or less, a Rockwell B
hardness (HRB) is 95 or less, a wall thickness/outer diameter ratio
is 0.14 or more, and a depth of a decarburized layer at an inner
surface part of the steel pipe is 20 .mu.m or less from the inner
surface.
[0020] (6) A method of producing the steel pipe for a hollow
stabilizer according to (5), wherein: the steel pipe is an electric
resistance-welded steel pipe, and the method includes a process of
subjecting the electric resistance-welded steel pipe to heating,
after electric resistance-welding, to a temperature of from
800.degree. C. to 1200.degree. C. and diameter-reduction hot
rolling to a reduction in cross sectional area of from 40% to
80%.
[0021] (7) A method of producing the steel pipe for a hollow
stabilizer according to (5), wherein: the steel pipe is an electric
resistance-welded steel pipe, and the method includes a process of
elongating the electric resistance-welded steel pipe by
cold-drawing after electric resistance-welding.
Advantageous Effects of Invention
[0022] According to the invention, there can be provided a hollow
stabilizer for automobiles having an excellent fatigue endurance
and higher strength compared to the conventional ones, while
maintaining fatigue properties and delayed fracture properties
equivalent to those of the conventional hollow stabilizer for
automobiles.
BRIEF DESCRIPTION OF DRAWINGS
[0023] FIGS. 1(a) to 1(c) are diagrams showing the relationship
between the surface layer of the electric resistance-welded portion
and MnS in the center segregation. FIG. 1(a) shows the electric
resistance-welded steel pipe; FIG. 1(b) is magnified view of a
metal flow in a base metal, observed in the cross section of an
electric resistance-welded steel pipe surrounded by circle S1 in
FIG. 1(a); FIG. 1(c) is magnified view of a metal flow in the
welded portion, observed in the cross section of the electric
resistance-welded steel pipe surrounded by circle S2 in FIG. 1(a);
and FIG. 1(d) is magnified view of the longitudinal sectional along
the extending direction (L direction) of the electric
resistance-welded abutting portion of the electric
resistance-welded steel pipe.
[0024] FIG. 2 shows an example of the stabilizer.
[0025] FIGS. 3(a) and 3(b) are diagrams illustrating the method of
producing a plate for a plane bending fatigue test specimen from
the electric resistance-welded steel pipe. FIG. 3(a) is a
perspective view of the electric resistance-welded steel pipe after
making a slit in a longitudinal direction; and FIG. 3(b) is a
perspective view of the electric resistance-welded steel pipe of
FIG. 3(a) developed in a plane.
[0026] FIGS. 4(a) and 4(b) are diagram illustrating the plane
bending fatigue test specimen produced using the plate of FIG.
3(b). FIG. 4(a) is a plane view, and FIG. 4(b) is a side view.
[0027] FIGS. 5(a) and 5(b) show a fracture surface of the specimen
after the fatigue test. FIG. 5(a) is the SEM photograph showing the
fracture surface of the specimen, and FIG. 5(b) shows the result of
the EDX analysis at the position enclosed with the dashed oval in
FIG. 5(a).
[0028] FIG. 6 is the photograph showing the metal flow of the cross
section perpendicular to the fracture surface of the specimen after
the fatigue test shown by sticking the photographs at the fracture
position.
[0029] FIG. 7 show an example of the relationship between the
cooling rate and the hardness during quenching.
[0030] FIG. 8 shows an example of the process of manufacturing the
stabilizer by cold forming.
[0031] FIG. 9 shows an example of the process of manufacturing the
stabilizer by hot forming.
[0032] FIG. 10 shows an example of the temper-softening curve of
the steel pipe for hollow stabilizers.
DESCRIPTION OF EMBODIMENTS
[0033] Configuration of Stabilizer
[0034] As shown in FIG. 2, a stabilizer 10 includes a torsion
portion 11 which is extended in a width direction of a vehicle body
(not shown) and a left-and-right pair of arm portions 12 which are
connected to either end of the torsion portion 11. The torsion
portion 11 is fixed to the body side via a bush 14 or the like. The
terminals 12a of the arm portions 12 are connected to suspension
mechanisms 15 on the left and right via stabilizer links (not
shown) or the like. In the torsion portion 11 and the arm portions
12, several portions or ten or more portions are usually subjected
to bending to avoid interference with other components.
[0035] When a vehicle turns, an upside down phase force is input to
the suspension mechanisms 15. At this time, in the case of a
vehicle at which the stabilizer 10 is mounted, the left and right
arm portions 12 are bowed in opposite directions and the torsion
portion 11 is twisted, and the stabilizer 10 functions as a spring
for suppressing excessive inclination (rolling) of the vehicle
body. During vehicle travel, straight travel and turning are
repeated. Therefore, stabilizers are required to have sufficient
hardness and fatigue properties.
[0036] Considering that the upper limit of the C content is 0.30%,
the hollow stabilizer according to the invention has a maximum
hardness of HRC 50 as an achievable hardness and a minimum hardness
of HRC 40 which is a substantial upper limit of the conventional
material.
[0037] In the hollow stabilizer according to the invention, a wall
thickness/outer diameter ratio (t/D) is set to 0.14 or more, such
that fatigue fracture initiates from the outer surface. In a case
in which the t/D is less than 0.14, the difference in stress
between the outer surface and the inner surface is small, and thus
fatigue fracture tends to initiate form the inner surface at which
the pre-existent origin of fatigue fracture is hardly detected. The
upper limit of the t/D is not particularly limited. Since the
stabilizer with a t/D of 0.5 is theoretically solid, the upper
limit of the t/D in the invention is substantially less than 0.5.
Since the weight saving effect is reduced and the production
becomes hard when the t/D is 0.25 or more, the t/D is preferably
less than 0.25 from a practical point of view. Here, each of the
HRC and the t/D is a value at a portion not subjected to bending
during the production of the hollow stabilizer.
[0038] In the electric resistance-welded portion, elongated MnS
sometimes is an origin of fatigue fracture. The inventors produced
a plate 22 for a plane bending fatigue test specimen from an
electric resistance-welded pipe 21 as shown in FIGS. 3(a) and 3(b).
Further, a plane bending fatigue test was conducted using a
specimen 24 in which an electric resistance-welded portion 23 of
the electric resistance-welded pipe 21 is located at a central
position in a longitudinal direction of the specimen 24 for the
plane bending fatigue test, in which the electric resistance-welded
portion 23 extends in a direction perpendicular to the longitudinal
direction of the specimen 24, as shown in FIGS. 4(a) and 4(b).
After the test, the fracture surface of the specimen 24 was
observed under a scanning electron microscope (SEM), and the
composition of an inclusion present at a fracture origin was
analyzed using an energy dispersive X-ray spectrometry (EDS)
attached to the SEM.
[0039] As a result, as shown in FIGS. 5(a) and 5(b), it was
confirmed that MnS was present at the fracture origin in the
fractured specimen. The observation result of metal flow of the
cross section perpendicular to the fracture surface of the specimen
after the fatigue test is shown in FIG. 6. As shown in FIG. 6, the
fracture surface in the specimen was located at a position somewhat
removed from the position of the welded portion, rather than the
welded portion. It was also confirmed that the surfaces of portions
in the vicinity of both sides sandwiching the electric
resistance-welded portion therebetween corresponded to the
segregation zone located at the center part in a thickness
direction of the base metal. Furthermore, as a result of the
examination by the inventors, it was found that the length of
elongated MnS present at the center part in a thickness direction
of the base metal is required to be limited for preventing the
generation of fatigue fracture from the electric resistance-welded
portion. In order to suppress the stretching of MnS, it is
effective to form CaS by the addition of Ca.
[0040] In the invention, the length of elongated MnS present at the
center part in a thickness direction is set to 150 .mu.m or less.
In a case in which the length of elongated MnS exceeds 150 .mu.m,
elongated MnS serves as an origin of fatigue fracture of the
electric resistance-welded portion. Regarding the presence or
absence of elongated MnS having a length exceeding 150 .mu.m, a 10
mm-length segment, as a specimen for observing the cross sectional
structure, is cutout from the hollow stabilizer in a longitudinal
direction, and at a center part in a thickness direction of the
hollow stabilizer present in the cross section of the specimen, the
length of MnS is confirmed with an optical microscope. The presence
of MnS may be confirmed by electron scanning microscopy together
with energy dispersive X-ray spectrometry. Here, "the length of
MnS" is determined by observing the center part in a thickness
direction present in the cross section with an optical microscope
or a scanning electron microscope with respect to 10 samples for
each, and measuring the length of MnS having the largest size among
the MnS present in the observed region.
[0041] Furthermore, in order to suppress the generation of fatigue
fracture from the inner surface of the hollow stabilizer, the depth
of a decarburized layer at the inner surface part is set to 20
.mu.m or less from the inner surface. It is preferable to include
no decarburized layer since the decarburized layer has lower
strength than the base metal and tends to serve as the origin of
fatigue fracture. However, in a case in which the t/D is set to
0.14 or more, the generation of fatigue fracture from the inner
surface can be prevented by setting the depth of the decarburized
layer at the inner surface part to 20 .mu.m or less. In the hollow
stabilizer according to the invention having a tempered martensite
structure, the depth of the decarburized layer means a maximum
depth from the inner surface of ferrite present at the inner
surface.
[0042] The grain size of ferrite on the inner surface of the steel
pipe for stabilizers before quenching is approximately from 10
.mu.m to 20 .mu.m. When the width corresponding to the grain size
of the ferrite grains connectively present on the inner surface is
regarded as the width of a layer, the depth of the ferrite
decarburized layer of the steel pipe can be set to 20 .mu.m or less
by limiting the width of the layer up to the one-layer size. In
order to suppress the generation of the decarburized layer, it is
preferable to lower a temperature of the inner surface, decrease
the holding time, and increase the cooling rate during quenching.
By appropriately selecting the quenching condition during the
production of the hollow stabilizer, the depth of the decarburized
layer can be set to 20 .mu.m or less. The decarburized layer is
formed in the dual phase range during cooling from a high
temperature to the room temperature. The dual phase range is a
temperature range below the Ar.sub.3 transformation temperature at
which austenite-to-ferrite transformation starts, and is a
temperature range at which austenite and ferrite coexist.
[0043] In the hollow stabilizer according to the invention, fatigue
strength is improved when compressive residual stress is imparted
to the outer surface, and the effect can be obtained significantly
when the maximum compressive residual stress on the outer surface
is 400 MPa or more. While the stress generated at the inner surface
of the hollow stabilizer is lower compared to the outer surface, it
is sometimes preferable to impart compressive residual stress to
the inner surface in order to improve fatigue endurance. The method
of imparting compressive residual stress is not particularly
limited, and shot peening is the simplest method. The compressive
residual stress can be determined by an X-ray diffraction
method.
[0044] Hereinbelow, the reason to limit respective components
included in the hollow stabilizer according to the invention is
explained. Here, "%" indicating the content of respective
components means "% by mass".
[0045] C is an element that determines the strength of the hollow
stabilizer. In order to realize higher strength compared to the
conventional hollow stabilizer, it is necessary to set the C
content to 0.26% or more. However, when the C content exceeds
0.30%, quenching cracks are generated. Therefore, the upper limit
of the C content is set to 0.30%.
[0046] Si is a deoxidizing element and contributes to
solid-solution strengthening. Si also has the effect of increasing
temper-softening resistance. In order to obtain these effects, the
content of Si is required to be 0.05% or more. However, when the Si
content exceeds 0.35%, toughness is decreased. Therefore, the Si
content is set to a range of from 0.05% to 0.35%. It is preferable
that the lower limit of the Si content is set to 0.20% and the
upper limit of the Si content is set to 0.30%.
[0047] Mn is an element that improves hardenability. In a case in
which the Mn content is less than 0.5%, the sufficient effect of
improving hardenability cannot be ensured. On the other hand, in a
case in which the Mn content exceeds 1.0%, delayed fracture
properties tends to be deteriorated and MnS easily precipitates,
thereby decreasing fatigue strength in the vicinity of the electric
resistance-welded portion. Therefore, the Mn content is set to a
range of form 0.5% to 1.0%, and preferably to 0.5% or more and less
than 0.8%.
[0048] P is an element that has an adverse effect on weld crack
resistance and toughness. Therefore, the P content is limited to
0.05% or less. The P content is preferably 0.03% or less.
[0049] S deteriorates toughness and precipitates as MnS to decrease
fatigue strength in the vicinity of the electric resistance-welded
portion. Therefore, the S content is limited to less than 0.0030%.
The S content is preferably 0.0026% or less.
[0050] In the invention, in order to suppress the precipitation of
MnS, it is necessary to reduce the S content in relation to the Mn
content in addition to reduce the S content alone. In particular,
the value of the product of the Mn content and the S content is
limited to 0.0025 or less. This is because, in a case in which the
value of the product of the Mn content and the S content exceeds
0.0025, sufficient fatigue strength cannot be obtained in the
vicinity of the electric resistance-welded portion even when each
of the Mn content and the S content satisfies the above appropriate
range.
[0051] Cr is an element that improves hardenability. In a case in
which the Cr content is less than 0.05%, these function and effect
cannot be expected sufficiently. In a case in which the Cr content
exceeds 1.0%, defects are easily generated during electric
resistance-welding. Therefore, the Cr content is set to a range of
from 0.05% to 1.0%.
[0052] Al is an element that is useful as an agent for deoxidizing
a molten steel, and it is preferable to add 0.01% or more of Al. Al
is also an element for fixing N and, hence, the Al content has a
significant influence on crystal grain sizes and mechanical
properties of the steel. In a case in which the Al content exceeds
0.08%, large amounts of non-metallic inclusions are formed and
surface defects are easily generated in the final product.
Therefore, the Al content is set to 0.08% or less. The Al content
is preferably 0.05% or less, and more preferably 0.03% or less.
[0053] Ti acts to stably and effectively improve hardenability
obtained by the addition of B, by suppressing the precipitation of
BN by fixing nitrogen in the steel in the form of TiN. Therefore,
in order to satisfy the stoichiometry of TiN, Ti needs to be added
in an amount that is at least 3.42 or more times the N content, and
the range of the Ti content is also automatically determined based
on the range of the N content. However, since some Ti may
precipitate to form a carbide, the Ti content is set to be larger
than a theoretical value, i.e., a range of from 0.005% to 0.05%, in
order to more surely fix N. The Ti content is preferably from 0.01%
to 0.02%.
[0054] B is an element for significantly enhancing the
hardenability of the steel material with addition in a small
quantity. However, in a case in which the B content is less than
0.0005%, the effect of improving hardenability cannot be expected.
In a case in which the B content exceeds 0.005%, a coarse phase
containing B tends to be formed and embrittlement easily occurs.
Therefore, the B content is set to a range of from 0.0005% to
0.005%. The B content is preferably from 0.001% to 0.002%.
[0055] N is an element that has the function of enhancing steel
strength via the precipitation in the form of nitrides or
carbonitrides. However, in a B-added steel, a deterioration of
hardenability caused by the precipitation of BN, or deterioration
of hot workability, fatigue strength, or toughness caused by the
precipitation of TiN as a result of Ti added to prevent the
precipitation of BN as described above, becomes problematic. On the
other hand, TiN has the effect of suppressing the coarsening of
.gamma.-grains at a high temperature to improve toughness.
Therefore, in order to achieve an optimum balance between hot
workability, fatigue strength, and toughness, the N content is set
to 0.006% or less. The N content is preferably from 0.001% to
0.005%, and more preferably from 0.002% to 0.004%.
[0056] Ca is an element that improves toughness and that suppresses
the reduction of fatigue strength caused by MnS in the vicinity of
the electric resistance-welded portion, by the fixation of S in the
form of CaS. In a case in which the Ca content is less than
0.0005%, these effects cannot be expected sufficiently. On the
other hand, in a case in which the Ca content exceeds 0.005%,
toughness is deteriorated due to the increase in oxides in the
steel. Therefore, the Ca content is set to a range of from 0.0005%
to 0.005%.
[0057] O is an element that neutralizes the effect obtained by the
addition of Ca via the formation of CaO. Therefore, the 0 content
is limited to 0.004% or less.
[0058] The hollow stabilizer according to the invention has the
chemical composition including the above essential components, and
may further include Mo, Nb, V, and Ni if necessary.
[0059] Mo is an element that has the effect of improving
hardenability. In a case in which the Mo content is less than
0.05%, the effect cannot be expected sufficiently. On the other
hand, in a case in which the Mo content exceeds 0.5%, the alloy
cost is increased. Therefore, the Mo content is set to a range of
from 0.05% to 0.5%.
[0060] Nb has the effect of precipitation strengthening by forming
Nb carbonitrides, and also has the effect of improving toughness by
reducing the crystal grain size of the steel material. In a case in
which the Nb content is less than 0.01%, sufficient effect of
improving strength and toughness cannot be obtained. On the other
hand, in a case in which the Nb content exceeds 0.1%, more effects
cannot be expected and merely the alloy cost is increased.
Therefore, the Nb content is set to a range of from 0.01% to
0.1%.
[0061] V has the effect of precipitation strengthening by V
carbonitrides. In a case in which the V content is less than 0.01%,
the effect cannot be expected sufficiently. On the other hand, in a
case in which the V content exceeds 0.1%, more effects cannot be
expected and merely the alloy cost is increased. Therefore, the V
content is set to a range of from 0.01% to 0.1%.
[0062] Ni is an element that has the effect of improving
hardenability and toughness. In a case in which the Ni content is
less than 0.1%, the effect cannot be expected sufficiently. On the
other hand, in a case in which the Ni content exceeds 1%, the alloy
cost is increased. Therefore, the Ni content is set to a range of
from 0.1% to 1.0%.
[0063] In the invention, it is necessary to sufficiently ensure the
hardenability of the material in order to obtain the hollow
stabilizer having a structure composed of martensite. As an index
of hardenability, the critical cooling rate Vc90 (.degree. C./s)
conventionally known by "TETSU-TO-HAGANE" 74 (1998), P. 1073, may
be used for example. This is an index represented by the following
Equation (1), and means the cooling rate at which the volume ratio
of martensite is 90% or more. Therefore, hardenability is higher as
Vc90 is lower, and the martensite structure can be obtained even
when the cooling rate is reduced.
log Vc90=2.94-0.75.beta.. Equation (1):
[0064] Here, .beta.=2.7C+0.4Si+Mn+0.8Cr+2.0Mo+0.8Ni.
[0065] The inventors produced electric resistance-welded steel
pipes with various compositions and examined the relationship
between Vc90 and the hardness after quenching. As a result, it was
found that, in a case in which Vc90 is 40.degree. C./s or less, the
martensite structure can be surely formed up to the inside by water
quenching. For this reason, the upper limit of Vc90 is limited to
40.degree. C./s in the invention. The inventors further examined
the relationship between the cooling rate and the Rockwell C
hardness at a center part in a thickness direction using the
electric resistance-welded steel pipe of steel No. 1 containing
0.30% of C, 0.30% of Si, and 0.35% of Cr and having a Vc90 of
27.1.degree. C./s as shown in Table 1. The Rockwell C hardness
(HRC) was measured in accordance with JIS Z 2245. As shown in FIG.
7, in a case in which the cooling rate is 20.degree. C./s or more,
the hardness corresponding to that of the structure composed of 90%
of martensite can be obtained. Since the cooling rate during water
quenching is 20.degree. C./s or more, the structure composed of 90%
or more of martensite can be obtained by water quenching.
[0066] The metallic structure of the hollow stabilizer according to
the invention is limited to a tempered martensite. This is because
variation in the structure and hardness is suppressed, and the
hardness is easily adjusted. In order to ensure the formation of
the martensite structure up to the inside by quenching, Vc90 is set
to 40.degree. C./s or less to obtain the sufficient hardenability
of the material. Whether the metallic structure of the hollow
stabilizer is a tempered martensite or not can be determined by the
observation under an optical microscope.
[0067] It is preferable that the metallic structure of the steel
pipe for hollow stabilizers, which is used as a material for the
hollow stabilizer according to the invention, comprises a mixed
structure of ferrite and perlite. Whether the metallic structure of
the steel pipe for hollow stabilizers comprises a mixed structure
of ferrite and perlite or not can be determined by the observation
under an optical microscope. The hollow stabilizer is often
produced by cold bending of the steel pipe. Therefore, in order to
ensure sufficient workability, the Rockwell B hardness (HRB) is
preferably 95 or less. In a case in which the metallic structure
comprises a mixed structure of ferrite and perlite, workability can
be ensured. The Rockwell B hardness (HRB) of the steel pipe for
hollow stabilizers can be measured in accordance with JIS Z
2245.
[0068] The depth of the ferrite decarburized layer on the inner
surface of the steel pipe for hollow stabilizers according to the
invention is set to 20 .mu.m or less. As a result, the depth of the
decarburized layer on the inner surface of the hollow stabilizer
after quenching can be reduced to less than 20 .mu.m. Here, the
depth of the ferrite decarburized layer is a maximum depth measured
from the inner surface of a region at which only ferrite grains are
arranged and no cementite is present in an L direction, when the
metallic structure of the longitudinal section (L section) of the
steel pipe is observed with an optical microscope.
[0069] The decarburized layer on the inner surface of the steel
pipe for hollow stabilizers is, for example, formed in the dual
phase range during cooling to the room temperature after subjecting
the electric resistance-welded steel pipe to diameter-reduction hot
rolling. The decarburized layer is formed easily on the inner
surface of the steel pipe for hollow stabilizers when the steel
pipe passes through the dual phase range during cooling from a high
temperature at which the metallic structure is composed of an
austenite single phase. The decarburized layer has a metallic
structure composed of ferrite, since the content of C, which is an
element for stabilizing austenite, is reduced therein. In order to
suppress the formation of the decarburized layer on the inner
surface of the steel pipe for hollow stabilizers, it is preferable
to shorten the time required for passing through the dual phase
range.
[0070] The depth of the decarburized layer generated at the inner
surface of the steel pipe for a hollow stabilizer can be reduced to
20 .mu.m or less by, for example, supplying water to the outer
surface of the steel pipe obtained by subjecting the electric
resistance-welded steel pipe to diameter-reduction hot rolling and
adjusting the cooling rate during passage through the dual phase
range to 5.degree. C./s or more. While the water for cooling may be
supplied only to the outer surface of the steel pipe for hollow
stabilizers, it is possible to supply water to the inner surface in
addition to the outer surface. By increasing the cooling rate of
the inner surface of the steel pipe for hollow stabilizers, the
depth of the decarburized layer can be further reduced.
[0071] Hereinbelow, the method of producing the steel pipe for
hollow stabilizers according to the invention is described. First,
a molten steel, smelted to provide a required chemical composition,
is casted as a slab, or the molten steel is formed as an ingot and
subsequently processed into a billet by hot rolling, and the slab
or the billet are then subjected to hot rolling to obtain a hot
rolled steel sheet. The hot rolled steel sheet is processed into an
electric resistance-welded steel pipe by a method of producing the
conventional electric resistance-welded steel pipe, for example,
hot or cold electric resistance-welding or high-frequency induction
heating. The obtained electric resistance-welded steel pipe may be
further subjected to diameter-reduction hot rolling to produce a
thick walled steel pipe.
[0072] The diameter-reduction rolling may be conducted using a
stretch reducer. The stretch reducer is a rolling machine provided
with a plurality of rolling stands arranged in series with the
rolling axis, the plurality of rolling stands each having 3 or 4
rolls disposed around the rolling axis. The tension in the pipe
axis direction (rolling direction) of the steel pipe and the
compression force in the circumferential direction of the steel
pipe are controlled by adjusting the number of revolutions of the
rolls and the rolling force in each of the rolling stands of the
rolling machine, whereby the diameter-reduction rolling for
increasing the wall thickness/outer diameter ratio can be
achieved.
[0073] That is, in diameter-reduction rolling, the outer diameter
is reduced by the rolling force with respect to the outer diameter
of the steel pipe and the wall thickness is increased at the same
time. On the other hand, the wall thickness is reduced by tension
acting in the pipe axis direction of the steel pipe. Therefore, the
final wall thickness is determined by the balance therebetween.
Since the wall thickness of the steel pipe obtained by
diameter-reduction rolling is mainly determined in accordance with
tension between the rolling stands, it is necessary to calculate
tension between the rolling stands for obtaining the desired wall
thickness based on the rolling theory or the like, and to set the
number of revolutions of a roll in each of the rolling stands for
exerting the tension.
[0074] The diameter-reduction rolling is preferably conducted at a
reduction in cross sectional area of from 40% to 80% using the
electric resistance-welded steel pipe heated to a temperature of
from 800.degree. C. to 1200.degree. C. The steel pipe for hollow
stabilizers is preferably an electric resistance-welded steel pipe
obtained by diameter-reduction hot rolling, but not limited
thereto. The steel pipe for hollow stabilizers may be an electric
resistance-welded steel pipe as electric resistance-welded state or
a drawn pipe obtained by cold drawing after electric
resistance-welding.
[0075] Production Example 1 of Stabilizer: Cold Forming
[0076] Hereinbelow, production example 1 of the stabilizer is
explained with reference to FIG. 8. A steel pipe (e.g., an electric
resistance-welded steel pipe, a seamless pipe, a pipe obtained by
diameter-reduction hot rolling, or a drawn pipe thereof) cut to a
predetermined length is subjected to bend forming (bend forming
process) to provide the intended shape shown in FIG. 2. The bend
formed steel pipe is heated to an austenitizing temperature region
(heating process) by furnace heating or electric heating, or using
a high-frequency heater, and then subjected to quenching (quenching
process) in water (or another quenching medium). Subsequently, the
shape of the heat-deformed stabilizer bar is corrected to an
intended stabilizer shape (shape correction process), and is
subjected to tempering (tempering process). The tempered pipe is
subjected to shot peening (shot peening process) with respect to
only the outer surface, or the outer and inner surfaces, and then
coated using an intended coating material (coating process). Here,
the shape correction (shape correction process) may be omitted if
restrained quenching is performed.
[0077] Production Example 2 of Stabilizer: Hot Forming
[0078] Hereinbelow, production example 2 of the stabilizer is
explained with reference to FIG. 9. A steel pipe (e.g., an electric
resistance-welded steel pipe, a seamless pipe, a pipe obtained by
diameter-reduction hot rolling, or a drawn pipe thereof) cut to a
predetermined length is heated to an austenitizing temperature
region (heating process) by furnace heating or electric heating, or
using a high-frequency heater, and then subjected to bend forming
(bend forming process) to provide the intended shape shown in FIG.
2. The bend formed steel pipe is then subjected to quenching
(quenching process) in water (or another quenching medium).
Subsequently, the shape of the heat-deformed stabilizer bar is
corrected to an intended stabilizer shape (shape correction
process), and is subjected to tempering (tempering process). The
tempered pipe is subjected to shot peening (shot peening process)
with respect to only the outer surface, or the outer and inner
surfaces, and then coated using an intended coating material
(coating process). Here, the shape correction (shape correction
process) may be omitted if restrained quenching is performed.
[0079] In the hot forming, quenching is performed after bend
forming to transform the metallic structure of the hollow
stabilizer to martensite. Therefore, it is necessary to finish bend
forming at the temperature of transformation point Ac.sub.3 or
higher. In the quenching after the cold forming, the heating
temperature is preferably 900.degree. C. or higher, and more
preferably 950.degree. C. or higher. The tempering temperature is
determined based on the temper-softening curve of the steel pipe
for hollow stabilizers. FIG. 10 shows the temper-softening curve of
the electric resistance-welded steel pipe of steel No. 1 containing
0.30% of C, 0.30% of Si, and 0.35% of Cr and having a Vc90 of
27.1.degree. C. as shown in Table 1. The tempering temperature at
which the Rockwell C hardness (HRC) of from 40 to 50 is obtained
can be determined based on the temper-softening curve shown in FIG.
9.
EXAMPLES
Example 1
[0080] Hereinbelow, the present invention is described in more
detail with reference to the Examples.
[0081] Each of steels having the compositions shown in Table 1 was
melted and cast into a slab. The slab was then heated to
1200.degree. C. and hot-rolled into a steel sheet of 5 mm in
thickness at a hot finishing temperature of 890.degree. C. and a
coiling temperature of 630.degree. C. The obtained steel sheet was
slit to a predetermined width, roll-formed into a tubular shape,
and then subjected to high frequency electric resistance welding to
produce a steel pipe of 90 mm in outer diameter. The obtained
electric resistance welded steel pipe was subsequently heated to
980.degree. C. by high frequency induction heating and then
subjected to diameter reduction rolling, thereby producing a steel
pipe (steel pipe for hollow stabilizers) of 30 mm in outer diameter
and 4.5 mm in wall thickness (t/D: 0.15) or a steel pipe (steel
pipe for hollow stabilizers) of 22 mm in outer diameter and 4.5 mm
in wall thickness (t/D: 0.20). Immediately after the diameter
reduction rolling, the pipe was water-cooled from the outer surface
side at a cooling rate of from 1.degree. C./s to 5.degree.
C./s.
TABLE-US-00001 TABLE 1 Steel Components (% by mass) No. C Si Mn P S
Cr Ni Mo Nb V Ti 1 0.30 0.30 0.80 0.006 0.0020 0.35 -- -- -- --
0.017 2 0.28 0.22 0.81 0.010 0.0025 0.33 -- -- -- -- 0.015 3 0.26
0.29 0.77 0.007 0.0026 0.33 -- -- -- -- 0.016 4 0.28 0.34 0.64
0.009 0.0025 0.56 -- -- -- -- 0.015 5 0.28 0.30 0.95 0.005 0.0015
0.23 -- -- -- -- 0.012 6 0.28 0.30 0.79 0.013 0.0025 0.35 -- -- --
-- 0.020 7 0.28 0.30 0.79 0.013 0.0025 0.35 -- -- -- -- 0.020 8
0.30 0.30 0.80 0.006 0.0020 0.35 -- -- 0.017 -- 0.017 9 0.28 0.22
0.81 0.010 0.0025 0.33 -- 0.057 -- -- 0.015 10 0.26 0.29 0.77 0.007
0.0026 0.33 0.19 -- -- -- 0.016 11 0.28 0.34 0.64 0.009 0.0025 0.56
-- -- -- 0.035 0.015 12 0.22 0.20 0.55 0.015 0.0032 0.35 -- -- --
-- 0.015 13 0.28 0.22 0.93 0.009 0.0028 0.35 -- -- -- -- 0.015 14
0.28 0.25 1.10 0.015 0.0040 0.20 -- -- -- -- 0.015 15 0.32 0.25
1.30 0.015 0.0062 0.33 -- -- -- -- 0.015 Steel Components (% by
mass) [Mn] .times. Vc90 No. Al N B Ca O [S] (.degree. C./s) Note 1
0.020 0.0045 0.0015 0.0021 0.0032 0.0016 27.1 Adapted steel 2 0.020
0.0035 0.0015 0.0019 0.0028 0.0020 31.7 3 0.022 0.0048 0.0015
0.0019 0.0022 0.0020 35.6 4 0.022 0.0038 0.0017 0.0025 0.0034
0.0016 28.5 5 0.020 0.0029 0.0013 0.0020 0.0025 0.0014 27.1 6 0.021
0.0056 0.0015 0.0025 0.0022 0.0020 30.2 7 0.021 0.0056 0.0015
0.0025 0.0022 0.0020 30.2 8 0.020 0.0045 0.0015 0.0021 0.0032
0.0016 27.1 9 0.020 0.0035 0.0015 0.0019 0.0028 0.0020 26.1 10
0.022 0.0048 0.0015 0.0019 0.0022 0.0020 27.4 11 0.022 0.0038
0.0017 0.0025 0.0034 0.0016 28.5 12 0.029 0.0048 0.0015 -- 0.0030
0.0018 64.9 Comparative 13 0.020 0.0045 0.0015 0.0018 0.0045 0.0026
25.1 steel 14 0.030 0.0036 0.0011 -- 0.0028 0.0044 22.5 15 0.030
0.0044 0.0014 0.0032 0.0022 0.0081 11.1 "--" in the table means
that the component is not added intentionally. The underline in the
table means that the value is outside the range of the
invention.
[0082] The metallic structure of the obtained steel pipes for
hollow stabilizers were observed under an optical microscope, and
it was confirmed that all of the obtained steel pipes have a
metallic structure composed of a mixed structure of ferrite and
perlite with depth of a decarburized layer at the inner surface of
15 .mu.m or less. The Rockwell hardness was determined in
accordance with JIS Z 2245, and as a result of which the Rockwell
hardness of all of the steel pipes for hollow stabilizers found to
have a Rockwell B hardness (HRB) of 95 or less. Furthermore, the
presence of MnS having a length exceeding 150 .mu.m was confirmed
using an optical microscope, an SEM, and an EDS in combination, and
as a result of which MnS having a length exceeding 150 .mu.m was
present in Comparative Examples of Nos. H to K as shown in Table
2.
[0083] The obtained steel pipes for hollow stabilizers were cut at
the electric resistance-welded portion or at a position located at
180.degree. opposite side of the electric resistance-welded portion
as shown in FIG. 3(a) and cold developed, thereby obtaining a
plate-like piece as shown in FIG. 3(b). Furthermore, the steel
pipes for hollow stabilizers were heated at 950.degree. C. for 10
minutes and water-quenched, and then tempered at different
temperature. The Rockwell C hardness (HRC) of the resultant was
determined in accordance with JIS Z 2245 to obtain a
temper-softening curve.
[0084] The plate-like piece obtained by cutting the steel pipe for
a hollow stabilizer at a position located at a 180.degree. opposite
side of the electric resistance-welded portion and spread out as
shown in FIGS. 3(a) and 3(b) was processed into a plane bending
fatigue test specimen in which the electric resistance-welded
portion is located at a central position in a longitudinal
direction as shown in FIGS. 4(a) and 4(b). In a similar manner, the
plate-like piece obtained by cutting the steel pipe for hollow
stabilizers at the electric resistance-welded portion and
developing was processed into a plane bending fatigue test specimen
in which the base metal is located at a central position in a
longitudinal direction. In the central position in a longitudinal
direction, the thickness (ta) was set to 3 mm and the width (Wa)
was set to 15 mm. Each of the specimens was tempered to impart a
Rockwell C hardness (HRC) of 40 based on the temper-softening
curve, and subjected to a plane bending fatigue test with a fatigue
limit of 5 million cycles. The results are shown in Table 2. Here,
the holding time for tempering was set to 30 minutes.
[0085] From the results shown in Table 2, it was found that each of
the steel pipes of Nos. A to K of the examples of the invention has
good fatigue properties at the electric resistance-welded portion
since the difference in fatigue limit between the base metal and
the electric resistance-welded portion is as small as 15 MPa or
less. On the other hand, it was found that each of the steel pipes
of Nos. L to O of the comparative examples has significantly
deteriorated fatigue properties at the electric resistance-welded
portion compared to the base metal since the difference in fatigue
limit between the base metal and the electric resistance-welded
portion is as large as 140 MPa or more. The fracture surface of the
specimen after the plane bending fatigue test was observed under an
SEM, and the composition of the inclusion present at the fracture
origin of fatigue fracture was analyzed using an EDS attached to
the SEM. As a result, the presence of MnS at the fracture origin
was confirmed in comparative examples L to O in which the electric
resistance-welded portion is located at a central position in a
longitudinal direction.
TABLE-US-00002 TABLE 2 Presence of Wall Outer MnS having Depth of
Steel thickness diameter a length decarburized pipe Steel (t) (D)
t/D Metallic exceeding layer in inner No. No. mm mm ratio HRB
structure 150 .mu.m surface A 1 4.5 30 0.15 94 Ferrite and Perlite
Absent Absent B 2 4.5 30 0.15 92 Ferrite and Perlite Absent 5 .mu.m
C 3 4.5 30 0.15 90 Ferrite and Perlite Absent 10 .mu.m D 4 4.5 30
0.15 92 Ferrite and Perlite Absent Absent E 5 4.5 30 0.15 92
Ferrite and Perlite Absent Absent F 6 4.5 30 0.15 92 Ferrite and
Perlite Absent Absent G 7 4.5 30 0.15 92 Ferrite and Perlite Absent
Absent H 8 4.5 22 0.20 94 Ferrite and Perlite Absent Absent I 9 4.5
30 0.15 93 Ferrite and Perlite Absent Absent J 10 4.5 30 0.15 91
Ferrite and Perlite Absent Absent K 11 4.5 30 0.15 92 Ferrite and
Perlite Absent Absent L 12 4.5 30 0.15 85 Ferrite and Perlite
Present 15 .mu.m M 13 4.5 30 0.15 92 Ferrite and Perlite Present
Absent N 14 4.5 30 0.15 92 Ferrite and Perlite Present Absent O 15
4.5 30 0.15 95 Ferrite and Perlite Present Absent P 1 4.5 30 0.15
94 Ferrite and Perlite Absent 25 .mu.m Q 1 4.0 30 0.13 94 Ferrite
and Perlite Absent Absent R 1 4.5 30 0.15 98 Ferrite and Perlite
Absent Absent fatigue limit (MPa) in plane bending fatigue test
Electric Steel resistance- pipe Production Base welded Difference
No. method HRC metal B portion W (B - W) Note A Hot rolling 40 480
465 15 Example of B Hot rolling 40 480 465 15 Invention C Hot
rolling 40 480 465 15 D Hot rolling 40 480 465 15 E Hot rolling 40
480 465 15 F Hot rolling 40 480 465 15 G Hot rolling 40 480 465 15
H Cold drawing 40 480 465 15 I Hot rolling 40 480 470 10 J Hot
rolling 40 480 465 15 K Hot rolling 40 480 465 15 L Hot rolling 40
485 310 175 Comparative M Hot rolling 40 480 340 140 Example N Hot
rolling 40 485 300 185 O Hot rolling 40 480 300 180 P Hot rolling
40 480 465 15 Q Hot rolling 40 480 465 15 R Hot rolling 40 480 465
15 Hot rolling: diameter reduction hot rolling The underline in the
table means that the value is outside the range of the
invention.
Example 2
[0086] A 1000 mm-length segment was cut out from each of the steel
pipes A to R produced in Example 1, and cold bended to a 90.degree.
angle in a position at 200 mm from the each pipe end, thereby
forming to have a U-shape. At this time, the U-shape pipe was
formed such that the electric resistance-welded portion was located
at the part that can be seen when viewed from the direction giving
a U-shaped appearance, that is, along the side surface of the
U-shape pipe, thereby obtaining respective Test Materials a to v.
The Test Materials a to n (but not Test Material c) as examples of
the invention were heated to 950.degree. C. for 10 minutes and
subjected to water quenching, and then tempered at 200.degree. C.
for 30 minutes, thereby adjusting the HRC thereof to 49. The Test
Material c was heated to 950.degree. C. for 10 minutes and
subjected to water quenching, and then tempered at 350.degree. C.
for 30 minutes, thereby adjusting the HRC thereof to 43.
Furthermore, with respect to Test Materials a to v, after heating,
the outer surface of each Test Material was subjected to shot
peening to impart a compressive residual stress on 450 MPa. In
addition, with respect to Test Materials b and f, the inner surface
thereof was also subjected to shot peening to have compressive
residual stress on 450 MPa.
[0087] Test Materials r and s were produced using, as a material,
the comparative steel (Steel 12 in Table 1) having a typical
chemical composition for conventionally used hollow stabilizers,
and thus contained low levels of C in the steel and could not
achieve an HRC of 49. Therefore, Test Material r was adjusted to
have an HRC of 47, which is the upper limit of achievable hardness,
and Test Material s was adjusted to have an HRC of 40, which is the
upper limit of hardness in practical use.
[0088] For comparison, the following test materials using as a
material Steel 1, which is an adapted steel shown in Table 1, were
prepared: Test Material o in which the depth of the decarburized
layer at the inner surface part is 25 .mu.m; Test Material p in
which the wall thickness/outer diameter ratio is 0.13; and Test
Material q having a bainite structure with a HRC of 35 produced
with a cooling rate during quenching of 15.degree. C./s, which is
slower compared to the case of water quenching.
[0089] Each of Test Materials a to v was fixed at a central
position in a longitudinal direction, and fatigue endurance was
tested up to 1 million cycles by vibrating respective ends thereof
in the opposite direction under a condition of a maximum main
stress on the outer surface of the bending portion of 500 MPa. Each
test was conducted with respect to 20 samples for each Test
Material. The observation of the metallic structure of Test
Materials and the measurement of the depth of the decarburized
layer at the inner surface part were carried out using an optical
microscope. The Rockwell hardness was measured in accordance with
JIS Z 2245. The presence of MnS having a length exceeding 150 .mu.m
was confirmed using an SEM and an EDS in combination.
[0090] As shown in Table 3, in all of Test Materials a to n as
examples of the invention, the number of cycles to fatigue exceeded
half-million, which meets a standard for fatigue endurance. In Test
Materials c to e, g and h in which no shot pinning was performed
with respect to the inner surface, breakage was sometimes initiated
from the inner surface side, while the number of cycles to fatigue
in respective materials exceeded half-million.
[0091] On the other hand, in Test Material o in which the depth of
the decarburized layer at the inner surface part exceeds 20 .mu.m
and Test Material p in which t/D is as small as 0.13, the number of
samples exhibiting inner surface breakage is large, and the number
of cycles to fatigue was not always reached to half-million, which
did not meet a standard for fatigue endurance. In the Test Material
q having a bainite structure with an HRC as low as 35, the number
of cycles to fatigue was significantly low.
[0092] In each of Test Materials r to v as comparative examples
produced using the comparative steels, MnS having a length
exceeding 150 .mu.m was present in the steel, and therefore,
fracturing sometimes occurred at an early stage, from the vicinity
of the electric resistance-welded portion on the outer surface, in
more than 2 samples among 20 samples.
TABLE-US-00003 TABLE 3 Presence of MnS having a Test Steel Wall
Outer length material Steel pipe thickness diameter t/D exceeding
No. No. No. mm .mu.m ratio HRC 150 .mu.m Structure a 1 A 4.5 30
0.15 49 Absent Tempered martensite b 1 A 4.5 30 0.15 49 Absent
Tempered martensite c 1 A 4.5 30 0.15 43 Absent Tempered martensite
d 2 B 4.5 30 0.15 49 Absent Tempered martensite e 3 C 4.5 30 0.15
49 Absent Tempered martensite f 3 C 4.5 30 0.15 49 Absent Tempered
martensite g 4 D 4.5 30 0.15 49 Absent Tempered martensite h 5 E
4.5 30 0.15 49 Absent Tempered martensite i 6 F 4.5 30 0.15 49
Absent Tempered martensite j 7 G 4.5 30 0.15 49 Absent Tempered
martensite k 8 H 4.5 22 0.20 49 Absent Tempered martensite l 9 I
4.5 30 0.15 49 Absent Tempered martensite m 10 J 4.5 30 0.15 49
Absent Tempered martensite n 11 K 4.5 30 0.15 49 Absent Tempered
martensite o 1 P 4.5 30 0.15 49 Absent Tempered martensite p 1 Q
4.0 30 0.13 49 Absent Tempered martensite q 1 R 4.5 30 0.15 35
Absent Bainite r 12 L 4.5 30 0.15 47 Present Tempered martensite s
12 L 4.5 30 0.15 40 Present Tempered martensite t 13 M 4.5 30 0.15
49 Present Tempered martensite u 14 N 4.5 30 0.15 49 Present
Tempered martensite v 15 O 4.5 30 0.15 49 Present Tempered
martensite Number of Number of cycles to breakage Test Depth of
fatigue at 500 MPa (number) material decarburized Shot (cycles)
Outer Inner No. layer position minimum maximum face face Note a
Absent Outer >1000000 >1000000 0 0 Example of b Absent Outer
+ Inner >1000000 >1000000 0 0 Invention c Absent Outer 946400
>1000000 0 1 d 5 .mu.m Outer 788300 >1000000 0 2 e 10 .mu.m
Outer 675200 >1000000 0 4 f 10 .mu.m Outer + Inner >1000000
>1000000 0 0 g Absent Outer 866900 >1000000 0 1 h Absent
Outer 911200 >1000000 0 1 i Absent Outer >1000000 >1000000
0 0 j Absent Outer >1000000 >1000000 0 0 k Absent Outer
>1000000 >1000000 0 0 l Absent Outer >1000000 >1000000
0 0 m Absent Outer >1000000 >1000000 0 0 n Absent Outer
>1000000 >1000000 0 0 o 25 .mu.m Outer 483900 >1000000 0 6
Comparative p Absent Outer 455600 >1000000 0 17 Example q Absent
Outer 78600 91200 20 0 r 15 .mu.m Outer 262700 875200 16 4 s 15
.mu.m Outer 162700 512700 20 0 t Absent Outer 255000 >1000000 2
2 u Absent Outer 283000 >1000000 2 0 v Absent Outer 122700
>1000000 2 1 "t/D ratio": wall thickness/outer diameter ratio;
"Outer": outer surface; and "Outer + Inner": outer surface and
inner surface. The underline in the table means that the value is
outside the range of the invention.
INDUSTRIAL APPLICABILITY
[0093] According to the invention, there can be provided a hollow
stabilizer for automobiles having an excellent fatigue endurance
and higher strength compared to the conventional ones, while
maintaining fatigue properties and delayed fracture properties
equivalent to those of the conventional hollow stabilizers for
automobiles. Such a hollow stabilizer can contribute considerably
to weight reduction in automobiles.
DESCRIPTION OF REFERENCE NUMERALS
[0094] 10: stabilizer [0095] 11: torsion portion [0096] 12: arm
portion [0097] 12a: terminal (of arm portion) [0098] 14: bush
[0099] 15: suspension mechanism [0100] 16: electric
resistance-welded steel pipe [0101] 17: base metal [0102] 18: metal
flow [0103] 19: welded portion [0104] 20: MnS [0105] 21: electric
resistance-welded steel pipe [0106] 22: plate for plane bending
fatigue test specimen [0107] 23: electric resistance-welded portion
[0108] 24: fatigue test specimen
* * * * *