U.S. patent application number 10/591475 was filed with the patent office on 2007-08-16 for high-strength bolt superior in delayed fracture and resistance and relaxation resistance.
This patent application is currently assigned to Honda Motor Co., Ltd.. Invention is credited to Takehiko Egawa, Nobuhiko Ibaraki, Zenji Iida, Yuichi Namimura, Kentaro Takada, Mitsuo Takashima, Katsuhiro Tsukiyama.
Application Number | 20070187003 10/591475 |
Document ID | / |
Family ID | 34909027 |
Filed Date | 2007-08-16 |
United States Patent
Application |
20070187003 |
Kind Code |
A1 |
Takashima; Mitsuo ; et
al. |
August 16, 2007 |
High-strength bolt superior in delayed fracture and resistance and
relaxation resistance
Abstract
Disclosed is a high-strength bolt having a tensile strength of
1,200 N/mm.sup.2 or more and superior in delayed fracture
resistance and relaxation resistance, prepared by wire-drawing a
bolt steel containing the following elements: C: 0.5 to 1.0% (mass
%, the same shall apply hereinafter), Si: 0.55 to 3%, Mn: 0.2 to
2%, P: 0.03% or less (but not 0%), S: 0.03% or less (but not 0%),
and Al: 0.3% or less (but not 0%), and containing proeutectoid
ferrite, proeutectoid cementite, bainite and martensite at a total
areal rate of less than 20% and pearlite in balance; cold-heading
the wire into a bolt shape; and then bluing the bolt in a
temperature range of 100 to 500.degree. C.
Inventors: |
Takashima; Mitsuo;
(Wako-shi, JP) ; Takada; Kentaro; (Wako-shi,
JP) ; Iida; Zenji; (Wako-shi, JP) ; Tsukiyama;
Katsuhiro; (Saga-shi, JP) ; Egawa; Takehiko;
(Saga-shi, JP) ; Namimura; Yuichi; (Kobe-shi,
JP) ; Ibaraki; Nobuhiko; (Kobe-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Honda Motor Co., Ltd.
Minato-ku
JP
107-8556
Saga Tekkohsho Co., Ltd.
Saga-shi
JP
840-0806
Kabushiki Kaisha Kobe Seko Sho
Kobe-shi
JP
651-8585
|
Family ID: |
34909027 |
Appl. No.: |
10/591475 |
Filed: |
March 1, 2005 |
PCT Filed: |
March 1, 2005 |
PCT NO: |
PCT/JP05/03393 |
371 Date: |
September 1, 2006 |
Current U.S.
Class: |
148/320 |
Current CPC
Class: |
F16B 35/00 20130101;
C22C 38/18 20130101; C22C 38/04 20130101; C22C 38/02 20130101 |
Class at
Publication: |
148/320 |
International
Class: |
C22C 38/00 20060101
C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 2, 2005 |
JP |
2004-057379 |
Claims
1. A high-strength bolt having a tensile strength of 1,200
N/mm.sup.2 or more that is superior in delayed fracture resistance
and relaxation resistance, wherein the bolt is prepared by:
wire-drawing a bolt steel containing the following elements: C: 0.5
to 1.0% (mass %, the same shall apply hereinafter), Si: 0.55 to 3%,
Mn: 0.2 to 2%, P: 0.03% or less (but not 0%), S: 0.03% or less (but
not 0%), and Al: 0.3% or less (but not 0%), and containing
proeutectoid ferrite, proeutectoid cementite, bainite and
martensite at a total areal rate of less than 20% and pearlite in
balance; cold-heading the wire into a bolt shape; and then bluing
the bolt in a temperature range of 100 to 500.degree. C.
2. The high-strength bolt according to claim 1, wherein the bolt
steel further comprises at least one of the following elements: Cr:
2.5% or less (but not 0%) and Co: 0.5% or less (but not 0%).
3. The high-strength bolt according to claim 1, wherein the bolt
steel further comprises Ni at 1.0% or less (but not 0%).
4. The high-strength bolt according to claim 1, wherein the bolt
steel further comprises Cu at 1.0% or less (but not 0%).
5. The high-strength bolt according to claim 1, wherein the bolt
steel further comprises at least one element selected from Mo, V,
Nb, Ti, and W in a total amount of 0.5% or less (but not 0%).
6. The high-strength bolt according to claim 1, wherein the bolt
steel further comprises B at 0.003% or less (but not 0%).
7. The high-strength bolt according to claim 2, wherein the bolt
steel further comprises Ni at 1.0% or less (but not 0%).
8. The high-strength bolt according to claim 2, wherein the bolt
steel further comprises at least one element selected from Mo, V,
Nb, Ti, and W in a total amount of 0.5% or less (but not 0%).
9. The high-strength bolt according to claim 2, wherein the bolt
steel further comprises B at 0.003% or less (but not 0%).
10. The high-strength bolt according to claim 7, wherein the bolt
steel further comprises at least one element selected film Mo, V,
Nb, Ti, and W in a total amount of 0.5% or less (but not 0%).
11. The high-strength bolt according to claim 1, wherein the
elements in balance are Fe and inevitable impurities.
Description
TECHNICAL FIELD
[0001] The present invention relates to a high-strength bolt mainly
for use in automobiles, and in particular to a high-strength bolt
having a tensile strength (strength) of 1,200 N/mm.sup.2 or more
that is superior in delayed fracture resistance and relaxation
resistance.
BACKGROUND ART
[0002] Carbon alloy steels (such as SCM435, SCM440, and SCr440) are
used for common high-strength bolts, and these bolts are given a
desirable strength by quenching and tempering. However, common
high-strength bolts used in automobiles and various industrial
machines have a risk of generating delayed fracture when they have
a tensile strength in the range above approximately 1,200
N/mm.sup.2, and thus, have restriction in use.
[0003] The delayed fracture includes both phenomena occurring in
corrosive and noncorrosive environments, and the reasons for its
occurrence are said to be complicated by various factors, and thus,
it is difficult to specify the reasons indiscriminately. The
regulatory factors responsible for the delayed fracture include
tempering temperature, structure, material hardness, grain size,
various alloy elements, and others, but there is currently no
effective means to prevent the delayed fracture, and various
methods are studied by trial and error even now.
[0004] There are proposed many methods of preventing the delayed
fracture (e.g., Patent Documents 1 to 3). In these methods, the
delayed fracture of high-strength bolts having a tensile strength
of 1,400 N/mm.sup.2 or more is reduced by adjusting the contents of
various major alloy elements, but the risk of the delayed fracture
is not completely eliminated, and the application thereof still
remains in extremely limited areas.
[0005] Patent Document 4 discloses a method of improving the
delayed fracture resistance further. In Patent Document 4, a
high-strength bolt having a tensile strength of 1,200 N/mm.sup.2 or
more is produced by preparing a high-strength bolt steel not in a
quenched and tempered structure but in a pearlite structure and
then by strongly wire-drawing the bolt steel. The pearlite
structure introduced in the high-strength bolt has an action to
trap hydrogen at the interface between cementite and ferrite and to
reduce hydrogen accumulated at the interface, and thus, improves
the delayed fracture resistance.
[0006] However, the pearlite steel has a problem of its own. That
is, tightening bolts used at high temperature occasionally show a
phenomenon of deterioration in proof stress ratio and thus in
tightening force during use, and such a phenomenon is called
relaxation (relaxation of stress). There is a concern about decline
in properties for the phenomenon (e.g., in relaxation property),
especially when a pearlite steel instead of a quenched and tempered
steel is used in production of bolt. Such a phenomenon may lead to
elongation of the bolt and possible deterioration in its initial
tightening force, and thus, bolts used, for example, around
automobile engine should also be superior in relaxation resistance.
However, the relaxation property is not considered well in
conventional high-strength bolts, except the bolt described in
Patent Document 5.
[0007] In Patent Document 5, a pearlite steel having a particular
composition is wire-drawn strongly and cold-headed into the bolt
shape, and the bolt is subject to a bluing treatment in a
temperature range of 100 to 400.degree. C. The bluing prevents
plastic deformation by the age hardening with C and N, improves the
strength and the proof stress ratio of the bolt, and also prevents
thermal settling at a temperature of 100 to 200.degree. C., and
thus, improves the relaxation resistance. Although Patent Document
5 is an invention aimed at improving relaxation resistance, it does
not disclose the relationship between Si content and the relaxation
resistance and claims that the Si content should be 0.5% or less
because an excessive Si content leads to deterioration of the
ductility of the steel material after wire-drawing as well as
significant deterioration of the cold-heading efficiency. [0008]
Patent Document 1: Japanese Unexamined Patent Publication No.
60-114551 [0009] Patent Document 2: Japanese Unexamined Patent
Publication No. 2-267243 [0010] Patent Document 3: Japanese
Unexamined Patent Publication No. 3-243745 [0011] Patent Document
4: Japanese Unexamined Patent Publication No. 2000-337332 [0012]
Patent Document 5: Japanese Unexamined Patent Publication No.
2001-348618
SUMMARY OF THE INVENTION
[0013] An object of the present invention, which was made under the
circumstance above, is to establish a method to further increase
the relaxation resistance of a high-strength bolt in a pearlite
structure having a tensile strength of 1,200 N/mm.sup.2 or more
that is superior in delayed fracture resistance.
[0014] The high-strength bolt according to the invention has a
tensile strength of 1,200 N/mm.sup.2 or more that is superior in
delayed fracture resistance and relaxation resistance,
characterized by being prepared by: wire-drawing a bolt steel
containing the following elements: C: 0.5 to 1.0% (mass %, the same
shall apply hereinafter), Si: 0.55 to 3%, Mn: 0.2 to 2%, P: 0.03%
or less (but not 0%), S: 0.03% or less (but not 0%), and Al: 0.3%
or less (but not 0%), having at a total areal rate of proeutectoid
ferrite, proeutectoid cementite, bainite and martensite of less
than 20%, and containing pearlite in balance; cold-heading the wire
into a bolt shape; and then bluing the bolt in a temperature range
of 100 to 500.degree. C.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 includes schematic explanatory views illustrating a
stud bolt used in a delayed fracture resistance test.
[0016] FIG. 2 is a photograph replacing drawing, showing the
bainite structure.
[0017] FIG. 3 is a photograph replacing drawing, showing the
proeutectoid cementite structure.
[0018] FIG. 4 is a graph showing the relationship among the Si
content, presence or absence of bluing, and the relaxation
value.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0019] After intensive studies to solve the problems above, the
inventors have completed the present invention by finding that,
although it was not possible to improve the relaxation resistance
only by bluing a bolt steel or only by increasing the Si content to
a certain value or more without bluing, it was possible to improve
the relaxation resistance significantly by combining an addition of
Si in an amount more than a particular value with bluing
treatment.
[0020] The bolt steel for use in the present invention
(high-strength bolt steel) has generally a wire or rod shape; more
specifically, it includes both a steel material (wire rod)
hot-worked into the wire or rod shape and then heat-treated and a
steel material (steel wire) obtained by cold-working the wire rod,
for example, mainly by wire drawing; and the steel wire is
preferable. The high-strength bolt steel is a kind of pearlite
steel, more specifically, a steel having at a total areal rate of
proeutectoid ferrite, proeutectoid cementite, bainite and
martensite of less than 20% and containing pearlite in balance
(i.e., the areal rate of the pearlite structure is more than 80%).
Increase in the amounts of proeutectoid ferrite and proeutectoid
cementite makes strong wire-drawing difficult because of breakage
of wire during wire-drawing, and makes it difficult to adjust the
strength of the bolt to a predetermined value or more. Increase in
the amounts of proeutectoid cementite and martensite leads to more
frequent disconnection during wire-drawing. In addition, bainite,
which is smaller in work-hardening amount than pearlite and
ineffective in increasing the strength by strong wire-drawing,
should be reduced in the amount. In contrast to these structures,
the amount of the pearlite structure, which is effective in
trapping hydrogen at the interface between cementite and ferrite
and in reducing the hydrogen accumulated in grain boundary, should
be increased for improvement in delayed fracture resistance. The
areal rate of the pearlite structure is recommended to be
preferably 90% or more, more preferably 95% or more.
[0021] The high-strength bolt steel according to the present
invention contains C at 0.5 to 1.0% (mass %, the same shall apply
hereinafter), Si at 0.55 to 3%, Mn at 0.2 to 2%, P at 0.03% or less
(but not 0%), S at 0.03% or less (but not 0%), and Al at 0.3% or
less (but not 0%). Hereinafter, the reason for restricting the
content of each component will be described.
[0022] C is an economical element effective in increasing the
strength of bolt, and increase in C content leads to increase in
strength. To achieve a desirable strength of bolt, the C content is
raised to 0.5% or more, preferably 0.55% or more, and more
preferably 0.60% or more. However, an excessive C content leads to
increase in the amount of the proeutectoid cementite precipitated
and thus to significant deterioration in toughness and ductility
and also in wire-drawing processability. Thus, the C content is
1.0% or less, preferably 0.9% or less, and more preferably 0.85% or
less. When the eutectoid carbon content is represented as Ce, the
most desirable C content is Ce.+-.0.2% (preferably Ce.+-.0.1% and
particularly preferably Ce.+-.0.05%).
[0023] Si increases the relaxation resistance of bluing-treated
bolt further. It is presumably because Si is solid-solubilized in
soft ferrite, the maximum cause of relaxation, and exerts an action
to strengthen the solid solution. Thus, the Si content is 0.55% or
more, preferably 0.7% or more, more preferably 1.0% or more, and
particularly preferably 1.5% or more. Si has an action to
accelerate decarburization of steel material during heating, for
example during hot rolling or patenting (e.g., lead patenting).
Although the operational condition is normally modified to prevent
decarburization, it is possible to soften the surface by
accelerating decarburization positively and to prevent cracking
during bolt forging, even when the Si content is increased.
However, an excessive Si content leads to deterioration in the
ductility of the core region. Thus, the Si content is 3% or less,
preferably 2.5% or less, and more preferably 2.0% or less.
[0024] Mn has an action as a deoxidizing agent and also an action
to improve the quenching efficiency of wire rod, and thus to
increase the uniformity of the cross sectional structure of the
wire rod. The Mn content is 0.2% or more, preferably 0.4% or more,
and more preferably 0.5% or more. However, an excessive Mn content
leads to generation of supercooled structures such as martensite
and bainite in the Mn segregation region and hence to deterioration
in wire-drawing processability. Thus, the Mn content is 2% or less,
preferably 1.5% or less, and more preferably 1.0% or less.
[0025] P is an element causing grain boundary segregation and
deterioration in delayed fracture resistance. Thus, the P content
is reduced to 0.03% or less, preferably 0.02% or less, more
preferably 0.015% or less, and particularly preferably 0.010% or
less.
[0026] S forms MnS in steel, which becomes the stress-concentration
point when a stress is applied. Thus, the S content is preferably
as small as possible, for improvement in delayed fracture
resistance. From the viewpoint above, the S amount is limited to
0.03% or less, preferably 0.02% or less, more preferably 0.015% or
less, and particularly preferably 0.010% or less.
[0027] Al generates nitride and oxide inclusions, lowering
wire-drawing efficiency. Thus, the Al content is 0.3% or less,
preferably 0.1% or less, and more preferably 0.05% or less; and, in
particular when the wire-drawing efficiency is considered more, it
is 0.03% or less (preferably 0.02% or less, in particular 0.010% or
less). On the other hand, Al is also effective in improving the
delayed fracture resistance by capturing N in steel to form AlN and
reducing the size of crystal grain, and thus, Al may be added
positively. Thus, the Al content is, for example, 0.01% or more,
preferably 0.02% or more, and more preferably 0.03% or more.
[0028] The high-strength bolt steel may contain other elements
additionally in the range that does not impair the advantageous
effects of the present invention, and examples thereof include
first supplementary elements (such as Cr and Co), second
supplementary elements (such as Ni), third supplementary elements
(such as Cu), fourth supplementary elements (such as Mo, V, Nb, Ti,
and W), and fifth supplementary elements (such as B); and these
supplementary element may be used alone or in combination of two or
more as needed. Hereinafter, the supplementary element will be
described.
[0029] The first supplementary elements, Cr and Co, may be added in
the ranges of Cr: 2.5% or less (but not 0%) and Co: 0.5% or less
(but not 0%). Cr and Co are effective in preventing precipitation
of proeutectoid cementite, and thus, particularly useful as the
additives for the high-strength bolt according to the present
invention aimed at reducing the content of proeutectoid cementite.
Such an action is amplified by increase in the amounts thereof
added, and thus, the Cr content is recommended to be 0.05% or more
(for example, 0.1% or more and particularly preferably 0.2% or
more), or the Co content, 0.01% or more (for example, 0.03% or more
and particularly preferably 0.05% or more), to make the action more
distinctive. An excessive addition leads to saturation of the
action, and is thus uneconomical. Thus, the Cr content is 2.5% or
less (preferably 2.0% or less and more preferably 1.2% or less),
and the Co content is 0.5% or less (preferably 0.3% or less and
more preferably 0.2% or less). Only one or both of Cr and Co may be
added.
[0030] The second supplementary element Ni may be added in an
amount in the range of 1.0% or less (but not 0%). Ni is not
effective in improving the strength of bolt, but effective in
increasing the toughness of drawn wire rod. Although the effect is
amplified when the Ni content is increased, the Ni content is
recommended to be preferably 0.05% or more, more preferably 0.1% or
more, and particularly preferably 0.15% or more, to make the action
more distinctive. However, an excessive Ni content leads to
elongation of the period until completion of transformation and
thus to expansion of facility and decrease in productivity. Thus,
the Ni content is 1.0% or less, preferably 0.5% or less, and more
preferably 0.3% or less.
[0031] The third supplementary element Cu may be added in an amount
in the range of 1.0% or less (but not 0%). Cu is an element
effective in improving the strength of bolt by its
precipitation-hardening action. The action is amplified by increase
of the Cu content, and, to make the action more distinctive, the Cu
amount is recommended to be preferably 0.05% or more, more
preferably 0.1% or more, and particularly preferably 0.2% or more.
However, an excessive Cu content causes embrittlement of grain
boundary and thus deterioration in delayed fracture resistance.
Thus, the Cu amount is 1.0% or less, preferably 0.5% or less, and
more preferably 0.3% or less.
[0032] The fourth supplementary elements Mo, V, Nb, Ti, W, and
others may be added in a total amount of 0.5% or less (but not 0%).
These elements Mo, V, Nb, Ti, and W form fine carbides/nitrides,
which are effective in improving delayed fracture resistance. The
action is amplified by increase of the total amount of these
elements, and the total amount is preferably 0.02% or more and more
preferably 0.05% or more. However, an excessive total amount of
these elements leads to inhibition of delayed fracture resistance
and also to deterioration in toughness. Thus, the total amount of
these elements is 0.5% or less, preferably 0.2% or less, and more
preferably 0.15% or less. These elements, Mo, V, Nb, Ti, W, and the
like, may be added alone or in combination of two or more.
[0033] The fifth supplementary element B may be added in an amount
in the range of 0.003% or less (but not 0%). B is added for
improvement in quenching efficiency. The action is amplified by
increase of the B content, and the B content is preferably 0.0005%
or more, and more preferably 0.0010% or more, to make the action
more distinctive. However, an excessive B content leads to
inhibition of toughness. Thus, the B content is 0.003% or less,
preferably 0.0025% or less, and more preferably 0.0020% or
less.
[0034] The elements in balance include Fe and unavoidable
impurities.
[0035] The bolt steel for use in the present invention preferably
has a tensile strength to a degree allowing the bolt after
processing such as wire-drawing or bluing to have a certain
strength, and specifically, has a tensile strength of approximately
1,000 N/mm.sup.2 or more, preferably 1,100 N/mm.sup.2 or more, more
preferably 1,200 N/mm.sup.2 or more, and particularly preferably
1,300 N/mm.sup.2 or more.
[0036] The bolt according to the present invention is produced by
wire-drawing the bolt steel (wire rod or steel wire), cold-heading
it into bolt, and then, bluing the bolt in a temperature range of
100 to 500.degree. C. Such a bolt has a favorable high tensile
strength of 1,200 N/mm.sup.2 or more (preferably 1,400 N/mm.sup.2
or more, more preferably 1,500 N/mm.sup.2 or more, and particularly
preferably 1,600 N/mm.sup.2 or more) and is also superior in
delayed fracture resistance and relaxation resistance.
[0037] The wire-drawing is performed, because a steel only rolled
or forged does not have the dimensional accuracy needed for
high-strength bolt and it is difficult to achieve a predetermined
final strength. The strong wire drawing results in improvement in
fine dispersion of part of the cementite grains in pearlite
structure and thus in improvement in hydrogen-trapping capacity,
and increases in cracking resistance due to the alignment of the
grains in the wire-drawing direction. The wire-drawing condition is
not particularly limited if it is the strong wire-drawing to a
degree giving a particular tensile strength, but, for example, it
is recommended to process the wire rod to a degree that the
reduction of area is approximately 30 to 85% (preferably
approximately 50 to 70%).
[0038] The bluing treatment is an essential step in the present
invention for utilizing the relaxation resistance endowed by Si.
That is, the bluing treatment is useful because it prevents plastic
deformation by the age hardening with C and N and increases the
strength and the proof stress ratio of bolt; and it is also useful
in amplifying the action by the Si addition above (reinforcement of
solid solution in relaxation-causing ferrite) and in improving the
relaxation resistance markedly, because it prevents thermal
settling at a temperature of 100 to 200.degree. C. Because the Si
content is increased in the present invention, it is also possible
to prevent deterioration in tensile strength and proof stress
during bluing, even when the bluing is performed at high
temperature. It is thus possible to improve tensile strength and
proof stress and additionally to raise relaxation resistance. An
excessively lower bluing temperature results in insufficient age
hardening and insufficient improvement in the strength and proof
stress ratio of bolt, and consequently, in insufficient improvement
in relaxation resistance. Thus, the bluing temperature is
100.degree. C. or higher, preferably 200.degree. C. or higher, and
more preferably 300.degree. C. or higher. On the other hand, an
excessively higher bluing temperature results in deterioration in
the strength of bolt by softening. Thus, the bluing temperature is
500.degree. C. or lower, preferably 450.degree. C. or lower, and
more preferably 400.degree. C. or lower.
EXAMPLES
[0039] Hereinafter, the present invention will be described more
specifically with reference to Examples, but it should be
understood that the present invention is not restricted by the
following Examples, and modifications can be made within the scope
of the description above and below, and such modification are also
included in the technical scope of the present invention.
Example 1
[0040] Test steels (A to M) respectively having the chemical
compositions shown in the following Table 1 were hot-rolled into
wires having the wire diameters (8.0 to 11.5 mm.phi.) shown in the
following Table 2, and the wires were patented in the conditions
shown in the following Table 2 (heating temperature: 940.degree.
C., constant temperature transformation: at 510 to 620.degree. C.
for 4 minutes). The test steel M was converted completely into the
martensite structure by quenching and tempering for comparison. The
structure, degree of decarburization, and tensile strength of the
steel wires obtained were determined. In studying the structure,
the proeutectoid ferrite, proeutectoid cementite, bainite and
martensite or pearlite structure was separated by the following
method; and the areal rate of each structure was determined.
[0041] [Separation of Each Structure]
[0042] The cross sectional face of a steel wire was embedded,
polished and corroded as it is immersed in alcoholic 5% picrate
solution for 15 to 30 seconds; and the structure of the D/4 region
(D: diameter) was observed under a scanning electron microscope
(SEM) JKA-89CORL manufactured by Japan electron Co., Ltd.
Photographs of 5 to 10 visual fields were taken at a magnification
of 1,000 to 3,000 times, for determining the pearlite structure
region, and the areal rate of each structure was determined by
using an image-analyzer FRMTOOL-KIT manufactured by Photron Ltd. As
for the bainite and proeutectoid cementite structures, which are
less easily differentiated from the pearlite structure, the
structure shown in FIG. 2 (photo replacing drawing) was regarded as
the bainite structure, and the structure shown in FIG. 3 (photo
replacing drawing) as the proeutectoid cementite structure. In
general tendency of these structures, proeutectoid ferrite and
proeutectoid cementite deposited along the original austenite grain
boundary, while martensite deposited in the bulky shape. Results
are summarized in Table 2. TABLE-US-00001 TABLE 1 STEEL CHEMICAL
COMPOSITION (mass %) .sup..asterisk-pseud. SYMBOL C Si Mn P S Al N
O OTHERS A 0.64 1.46 0.66 0.013 0.010 0.002 0.005 0.0014 Cr: 0.68 B
0.64 1.46 0.60 0.011 0.010 0.002 0.005 0.0019 V: 0.104 C 0.83 0.92
0.74 0.008 0.006 0.037 0:006 0.0008 D 0.64 2.05 0.90 0.010 0.008
0.003 0.005 0.0013 Cr: 0.98, Ni: 0.26, V: 0.092 E 0.60 1.94 0.93
0.012 0.005 0.033 0.005 0.0014 F 0.62 2.54 0.51 0.015 0.012 0.034
0.005 0.0010 Cr: 0.50, Mo: 0.10, Co: 0.05 G 0.60 2.89 0.51 0.010
0.008 0.035 0.006 0.0011 Cr: 0.51, B: 0.0015 H 0.82 0.55 0.78 0.009
0.007 0.034 0.005 0.0008 Cr: 0.70 I 0.65 1.50 0.65 0.015 0.014
0.035 0.005 0.0009 Cu: 0.25 J 0.85 0.25 0.77 0.010 0.006 0.048
0.004 0.0007 K 0.82 0.26 0.71 0.015 0.009 0.041 0.005 0.0007 Cr:
0.18 L 0.77 0.45 0.72 0.012 0.012 0.039 0.005 0.0008 Cr: 0.17 M
0.34 0.19 0.70 0.016 0.009 0.033 0.003 0.0009 Cr: 0.95, Mo: 0.18
.sup..asterisk-pseud.THE ELEMENTS IN BALANCE ARE FE AND UNAVOIDABLE
IMPURITIES.
[0043] TABLE-US-00002 TABLE 2 HOT- PATENTING ROLLED CONSTANT
STRUCTURE (AREA %) STEEL WIRE TEMPERA- PROEUTEC- PROEUTEC- TENSILE
TEST STEEL WIRE DIAMETER TURE TOID TOID BAI- MARTEN- PEAR- STRENGTH
NO. SYMBOL SYMBOL (mm) (.degree. C.) FERRITE CEMENTITE NITE SITE
LITE (N/mm.sup.2) 1 A WA1 10.5 600 10 0 0 0 90 1360 2 WA2 8.0 600 3
0 0 0 97 1358 3 WA3 8.0 620 5 0 0 0 95 1287 4 B WB1 10.5 600 10 0 0
0 90 1370 5 WB2 8.0 600 5 0 0 0 95 1346 6 WB3 8.0 620 7 0 0 0 93
1305 7 C WC 10.5 560 5 5 0 0 90 1314 8 D WD 10.5 600 10 0 0 0 90
1450 9 E WE 11.5 560 15 0 0 0 85 1220 10 F WF 10.5 600 10 0 0 0 90
1381 11 G WG 10.5 600 15 0 0 0 85 1398 12 H WH 10.5 560 5 5 0 0 90
1305 13 I WI 8.0 600 5 0 0 0 95 1399 14 J WJ 8.0 510 5 0 0 0 95
1268 15 K WK 8.0 525 5 0 0 0 95 1305 16 L WL 8.0 535 10 0 0 0 90
1310 17 M WM 11.0 QUENCHING(880.degree. C. .times. 30
min..fwdarw.OQ). TEMPERING(460.degree. C. .times. 90
min..fwdarw.WC), 100% MARTENSITE STRUCTURE OQ: OIL QUENCHING, WC:
WATER COOLING
[0044] The respective steel wires were wire-drawn tightly to the
wire diameters (7.06 mm.phi. or 5.25 mm.phi.) shown in the
following Table 3 (reduction of area: 55 to 62%); the wire-drawing
efficiency was evaluated; and the tensile strength of the strongly
drawn wirerods obtained and the delayed fracture resistance were
determined. The criteria for evaluation of wire-drawing efficiency
are as follows: The wire-drawing efficiency and the delayed
fracture resistance were evaluated by the following methods:
[Wire-Drawing Efficiency]
[0045] Favorable: Drawn to a predetermined wire diameter without
problem, and no abnormal breakage observed in the tensile test
after wire-drawing.
[0046] Unfavorable: Abnormal breakage such as cuppy breakage and
longitudinal crack observed in the tensile test during or after
wire-drawing
[Delayed Fracture Resistance]
[0047] From the strongly drawn wire rods, stud bolts of
M8.times.P1.25 shown in FIG. 1 [FIG. 1A, from steel wire having a
wire diameter of 7.06 mm.phi.)] and stud bolts of M6.times.P1.0
[FIG. 1B, from the steel wire having a wire diameter of 5.25
mm.phi.] were prepared, and the delayed fracture test was
performed. In the delayed fracture test, a bolt was immersed in
acid (15% HCl.times.30minute), washed with water, dried, and then,
left in air under stress (loaded stress: 90% of tensile strength);
and presence or absence of fracture evaluated after 100 hours
(.smallcircle.: no fracture, x: fracture). TABLE-US-00003 TABLE 3
DRAWN WIRE ROD STEEL STRONG WIRE-DRAWING STEP TENSILE DELAYED TEST
STEEL WAVE REDUCTION POST-WIRE-DRAWING WIRE-DRAWING STRENGTH
FRACTURE NO. SYMBOL SYMBOL OF AREA (%) WIRE DIAMETER (mm)
EFFICIENCY (N/mm.sup.2) RESISTANCE 1 A WA1 55 7.06 FAVORABLE 1609
.smallcircle. 2 WA2 57 5.25 FAVORABLE 1615 .smallcircle. 3 WA3 57
5.25 FAVORABLE 1561 .smallcircle. 4 B WB1 55 7.06 FAVORABLE 1619
.smallcircle. 5 WB2 57 5.25 FAVORABLE 1623 .smallcircle. 6 WB3 57
5.25 FAVORABLE 1584 .smallcircle. 7 C WC 55 7.06 FAVORABLE 1637
.smallcircle. 8 D WD 55 7.06 FAVORABLE 1699 .smallcircle. 9 E WE 62
7.06 FAVORABLE 1507 .smallcircle. 10 F WF 55 7.06 FAVORABLE 1622
.smallcircle. 11 G WG 55 7.06 FAVORABLE 1632 .smallcircle. 12 H WH
55 7.06 FAVORABLE 1624 .smallcircle. 13 I WI 57 5.25 FAVORABLE 1658
.smallcircle. 14 J WJ 57 5.25 FAVORABLE 1640 .smallcircle. 15 K WK
57 5.25 FAVORABLE 1631 .smallcircle. 16 L WL 57 5.25 FAVORABLE 1628
.smallcircle. 17 M WM 59 7.06 -- 1318 x
[0048] Each of the strongly drawn wire rods thus obtained was
subjected to a bluing treatment at a temperature of 200 to
400.degree. C. The tensile strength and the 0.2% proof stress of
the bluing and non-bluing treated wire rods were determined.
Separately, a relaxation test was performed according to JIS G3538.
The temperature of the relaxation test was 150.degree. C. In the
present test, a test piece was held at a suitable distance, and a
load (applied load) W1 equivalent to 80% of the load of the 0.2%
proof stress was applied; the applied load was gradually lowered,
while a movable weight (weight for adjusting elongation of the test
piece) was adjusted to make the grasp length of the test piece (GL:
300 mm) constant; the load W2 after 10 hours was determined; and
the relaxation value was calculated according to the following
Formula: Relaxation value (%)=[(W1W2)/W1].times.100
[0049] In addition, each stud bolt prepared above after bluing was
also evaluated in a delayed fracture test similar to that above.
Results are summarized in Table 4 and FIG. 4. TABLE-US-00004 TABLE
4 0.2% STEEL BLUING TENSILE PROOF APPLIED RELAXATION DELAYED TEST
STEEL WIRE TEMPERATURE STRENGTH STRESS LOAD VALUE FRACTURE NO.
SYMBOL SYMBOL (.degree. C.) (N/mm.sup.2) (N/mm.sup.2) (N/mm.sup.2)
(%) RESISTANCE 2 A WA2 -- 1615 1436 1149 10.55 -- .sup. 2A 200 1713
1642 1314 9.14 .smallcircle. .sup. 2B 300 1700 1638 1310 8.87
.smallcircle. .sup. 2C 400 1681 1628 1302 8.39 .smallcircle. 3 WA3
-- 1561 1363 1090 10.48 -- .sup. 3A 200 1671 1595 1276 7.72
.smallcircle. 5 B WB2 -- 1623 1453 1162 10.50 -- .sup. 5A 200 1746
1709 1367 8.76 .smallcircle. 6 WB3 -- 1584 1389 1111 10.30 -- .sup.
6A 200 1690 1617 1294 8.87 .smallcircle. .sup. 7A C WC 200 1754
1705 1364 9.11 .smallcircle. .sup. 7B 300 1734 1643 1314 8.89
.smallcircle. .sup. 7C 400 1728 1624 1299 8.47 .smallcircle. .sup.
8A D WD 200 1821 1765 1412 7.81 .smallcircle. .sup. 8B 300 1807
1715 1372 7.55 .smallcircle. .sup. 8C 400 1787 1699 1359 7.41
.smallcircle. .sup. 9A E WE 200 1628 1594 1275 8.21 .smallcircle.
.sup. 9B 300 1611 1542 1234 8.10 .smallcircle. .sup. 9C 400 1608
1538 1230 7.99 .smallcircle. 10A F WF 200 1741 1701 1361 7.51
.smallcircle. 10B 300 1738 1698 1358 7.40 .smallcircle. 11A G WG
200 1758 1724 1379 7.31 .smallcircle. 11B 300 1751 1715 1372 7.26
.smallcircle. 12A H WH 200 1711 1668 1334 9.55 .smallcircle. 12B
300 1698 1625 1300 9.26 .smallcircle. 13A I WI 200 1751 1711 1369
8.67 .smallcircle. 13B 300 1742 1708 1366 8.11 .smallcircle. 14A J
WJ 200 1758 1680 1344 12.31 .smallcircle. 14B 300 1750 1658 1326
10.49 .smallcircle. 15A K WK 200 1740 1660 1328 12.52 .smallcircle.
15B 300 1727 1551 1241 10.70 .smallcircle. 16A L WL 200 1745 1671
1337 11.01 .smallcircle. 16B 300 1734 1572 1258 10.51
.smallcircle.
[0050] The quenched and tempered steel (martensite steel) M was
insufficient in delayed fracture resistance (see Table 3).
[0051] The pearlite steels J to L were improved in delayed fracture
resistance (see Tables 3 and 4). However, these steels having a Si
content of less than 0.55% were limited in the relaxation
resistance (see Table 4).
[0052] In contrast, both bluing and unbluing-treated steels A to I,
which are all pearlite steels, were superior in delayed fracture
resistance (see Tables 3 and 4). In addition, the bluing-treated
steels having a Si content of 0.55% or more were more improved in
relaxation resistance (white and black circles in FIG. 4). As
apparent from FIG. 4, it is not possible to improve the relaxation
resistance without bluing treatment even if the Si content is 0.55%
or more (cross in FIG. 4), but possible to improve the relaxation
resistance by combination of increase in the Si content with bluing
treatment.
[0053] In summarizing the invention above, the high-strength bolt
according to the invention is characterized by having a relaxation
resistance significantly increased by addition of Si in an amount
of more than a particular value.
[0054] The high-strength bolt according to the invention has a
tensile strength of 1,200 N/mm.sup.2 or more that is superior in
delayed fracture resistance and relaxation resistance,
characterized by being prepared by: wire-drawing a bolt steel
containing the following elements: C: 0.5 to 1.0% (mass %, the same
shall apply hereinafter), Si: 0.55 to 3%, Mn: 0.2 to 2%, P: 0.03%
or less (but not 0%), S: 0.03% or less (but not 0%), and Al: 0.3%
or less (but not 0%), having at a total areal rate of proeutectoid
ferrite, proeutectoid cementite, bainite and martensite of less
than 20%, and containing pearlite in balance; cold-heading the wire
into a bolt shape; and then bluing the bolt in a temperature range
of 100 to 500.degree. C.
[0055] The high-strength bolt according to the present invention
may contain additionally Cr at 2.5% or less (but not 0%), Co at
0.5% or less (but not 0%), Ni at 1.0% or less (but not 0%), Cu at
1.0% or less (but not 0%), B at 0.003% or less (but not 0%), and
others, and may contain Mo, V, Nb, Ti, W, and others in a total
amount in the range of 0.5% or less (but not 0%).
[0056] The high-strength bolt according to the present invention
preferably contains additionally at least one element selected from
Cr at 2.5% or less (but not 0%) and Co at 0.5% or less (but not
0%). It more preferably contains additionally Ni at 1.0% or less
(but not 0%). It more preferably contains additionally at least one
element selected from Mo, V, Nb, Ti, and W in a total amount in the
range of 0.5% or less (but not 0%). It more preferably contains B
at 0.003% or less (but not 0%).
[0057] The high-strength bolt according to the present invention
more preferably contains additionally Ni at 1.0% or less (but not
0%).
[0058] The high-strength bolt according to the present invention
more preferably contains Cu at 1.0% or less (but not 0%).
[0059] The high-strength bolt according to the present invention
more preferably contains additionally at least one element selected
from Mo, V, Nb, Ti, and W in a total amount in the range of 0.5% or
less (but not 0%).
[0060] The high-strength bolt according to the present invention
preferably contains B at 0.003% or less (but not 0%).
[0061] The balance of the high-strength bolt according to the
present invention may be Fe and inevitable impurities.
[0062] The present invention overcomes various problems associated
with bolts prepared from pearlite steel materials superior in
delayed fracture resistance. That is, it is possible to improve the
relaxation resistance of bolt drastically by adding Si in a certain
amount or more.
[0063] This application is based on Japanese Patent Application No.
2004-057379 filed on Mar. 2, 2004, the contents of which are hereby
incorporated by reference.
INDUSTRIAL APPLICABILITY
[0064] By the present invention in the configuration above, it was
possible to produce a high-strength bolt having a tensile strength
of 1,200 N/mm.sup.2 or more that is superior in delayed fracture
resistance and relaxation resistance, by adding Si in a certain
amount or more.
* * * * *