U.S. patent application number 11/340472 was filed with the patent office on 2006-08-03 for high strength bolt having excellent hydrogen embrittlement resistance.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). Invention is credited to Shushi Ikeda, Kenji Saito, Koichi Sugimoto, Fumio Yuse.
Application Number | 20060169366 11/340472 |
Document ID | / |
Family ID | 36337395 |
Filed Date | 2006-08-03 |
United States Patent
Application |
20060169366 |
Kind Code |
A1 |
Yuse; Fumio ; et
al. |
August 3, 2006 |
High strength bolt having excellent hydrogen embrittlement
resistance
Abstract
The present invention provides a high strength bolt that has
excellent hydrogen embrittlement resistance. The high strength bolt
having excellent hydrogen embrittlement resistance which bolt
comprises 0.20 to 0.60% of C, 1.0 to 3.0% of Si, 1.0 to 3.5% of Mn,
higher than 0% and not higher than 1.5% of Al, 0.15% or less P,
0.02% or less S, and balance of iron and inevitable impurities, and
the structure includes: 1% or more residual austenite; 80% or more
in total content of bainitic ferrite and martensite; and 10% or
less (may be 0%) in total content of ferrite and pearlite in the
proportion of area to the entire structure, and also the mean axis
ratio (major axis/minor axis) of the residual austenite grains is 5
or higher and the bolt has tensile strength of 1180 MPa or
higher.
Inventors: |
Yuse; Fumio; (Kobe-shi,
JP) ; Saito; Kenji; (Kobe-shi, JP) ; Ikeda;
Shushi; (Kobe-shi, JP) ; Sugimoto; Koichi;
(Nagano-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Kobe-shi
JP
SHINSHU TLO CO., LTD.
Ueda-shi
JP
|
Family ID: |
36337395 |
Appl. No.: |
11/340472 |
Filed: |
January 27, 2006 |
Current U.S.
Class: |
148/320 |
Current CPC
Class: |
C21D 6/005 20130101;
C21D 2211/002 20130101; C21D 6/008 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 |
Jan 28, 2005 |
JP |
2005-021501 |
Sep 6, 2005 |
JP |
2005-258346 |
Claims
1. A high strength bolt having excellent hydrogen embrittlement
resistance, which comprises: 0.20 to 0.60% of C, 1.0 to 3.0% of Si,
1.0 to 3.5% of Mn, higher than 0% and not higher than 1.5% of Al,
0.15% or less P, 0.02% or less S, and balance of iron and
inevitable impurities, wherein the structure includes: 1% or more
of residual austenite; 80% or more in total content of bainitic
ferrite and martensite; and 10% or less (may be 0%) in total
content of ferrite and pearlite in the proportion of area to the
entire structure, and wherein the mean axis ratio (major axis/minor
axis) of the residual austenite grains is 5 or higher and the steel
has tensile strength is 1180 MPa or higher.
2. A high strength bolt having excellent hydrogen embrittlement
resistance, which comprises: 0.20 to 0.60% of C, 1.0 to 3.0% of Si,
1.0 to 3.5% of Mn, higher than 0% and not higher than 0.5% of Al,
0.15% or less P, 0.02% or less S, and balance of iron and
inevitable impurities, wherein the structure includes: 1% or more
residual austenite; 80% or more in total content of bainitic
ferrite and martensite; and 10% or less (may be 0%) in total
content of ferrite and pearlite in the proportion of area to the
entire structure, and wherein the mean axis ratio (major axis/minor
axis) of the residual austenite grains is 5 or higher and the steel
has tensile strength is 1180 MPa or higher.
3. A high strength bolt having excellent hydrogen embrittlement
resistance according to the claim 1, which further comprises:
higher than 0% and not higher than 0.1% of Nb and/or higher than 0%
and not higher than 1.0% of Mo.
4. A high strength bolt having excellent hydrogen embrittlement
resistance according to the claim 2, which further comprises:
higher than 0% and not higher than 0.1% of Nb and/or higher than 0%
and not higher than 1.0% of Mo.
5. A high strength bolt having excellent hydrogen embrittlement
resistance according to the claim 1, which further comprises:
higher than 0% and not higher than 2% of Cu and/or higher than 0%
and not higher than 5% of Ni.
6. A high strength bolt having excellent hydrogen embrittlement
resistance according to the claim 2, which further comprises:
higher than 0% and not higher than 2% of Cu and/or higher than 0%
and not higher than 5% of Ni.
7. A high strength bolt having excellent hydrogen embrittlement
resistance according to the claim 3, which further comprises:
higher than 0% and not higher than 2% of Cu and/or higher than 0%
and not higher than 5% of Ni.
8. A high strength bolt having excellent hydrogen embrittlement
resistance according to the claim 4, which further comprises:
higher than 0% and not higher than 2% of Cu and/or higher than 0%
and not higher than 5% of Ni.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates a high strength bolt that has
excellent hydrogen embrittlement resistance, particularly to a high
strength bolt wherein hydrogen embrittlement, season crack and
delayed fracture, that cause troubles for bolts having tensile
strength of 1180 MPa or higher, are suppressed.
[0003] 2. Description of the Related Art
[0004] Low-alloy high-toughness steels, in particular SCM435 and
SCM440, are widely used for manufacturing ordinary bolts. While
these steels have a high tensile strength of 120 to 130
kgf/mm.sup.2 (1180 to 1270 MPa), these steels are subjected to a
refining treatment when higher strength is required.
[0005] However, refined bolts obtained through the refining
treatment have the problem of delayed fracture, in which the bolt
suddenly cracks upon long period of use after it was tightened in
place. Japanese Patent No. 2614659 describes a method of solving
this problem by adding an alloy element such as Cr, Mo, Ti and V to
the basic components, and controlling the proportions of these
components. Japanese Unexamined Patent Publication (Kokai) No.
2003-321743 describes that resistance against delayed fracture can
be ensured by making a metal structure constituted from a single
phase of tempered martensite and causing fine precipitate having a
grain size smaller than 10 nm to be dispersed in the structure.
With this technology, alloy element such as Ni, Ti, Mo or the like
is added so as to obtain the precipitate.
[0006] However, there is a limitation to the improvement in the
hydrogen storing capacity by simply controlling the form of
precipitation, and it is difficult to achieve higher hydrogen
embrittlement resistance by this method. Moreover, the micro alloy
elements such as Ti and V are expensive and it is difficult to
provide a high strength bolt that has a higher level of delayed
destruction resistance at a low price. There is also such a problem
that it is difficult to recycle a steel that includes much contents
of the alloy elements described above.
[0007] Japanese Patent No. 3494798 describes that a bolt having
tensile strength of 1300 MPa or higher and elongation of 8.8% to
12.2% can be made by controlling the structure without need for the
addition of the alloy elements described above. Specifically, it is
described that it suffices to form a bainite structure and control
the ratio of length to width of prior austenite grains to 1.2 or
higher in a region from 300 .mu.m to 630 .mu.m from the surface.
However, further improvements are required to achieve higher
hydrogen embrittlement resistance with this technology.
SUMMARY OF THE INVENTION
[0008] The present invention has been made in consideration of the
problems described above, and has an object of providing a high
strength bolt that has high tensile strength of 1180 MPa or higher
and has significantly improved hydrogen embrittlement
resistance.
[0009] The high strength bolt of the present invention comprises
0.20 to 0.60% of C (contents of components given in terms of
percentage in this patent application all refer to percentage by
weight), 1.0 to 3.0% of Si, 1.0 to 3.5% of Mn, higher than 0% and
not higher than 1.5% of Al, 0.15% or less P, 0.02% or less S, and
balance of iron and inevitable impurities, and wherein the
structure includes:
1% or more residual austenite;
80% or more in total content of bainitic ferrite and martensite;
and
[0010] 10% or less (may be 0%) in total content of ferrite and
pearlite in the proportion of area to the entire structure, wherein
the mean axis ratio (major axis/minor axis) of the residual
austenite grains is 5 or higher and the bolt has tensile strength
of 1180 MPa or higher. This bolt shall be hereinafter referred to
as the inventive bolt 1.
[0011] The high strength bolt of the present invention comprises
0.20 to 0.60% of C, 1.0 to 3.0% of Si, 1.0 to 3.5% of Mn, higher
than 0% and not higher than 0.5% of Al, 0.15% or less P, 0.02% or
less S, and balance of iron and inevitable impurities, and wherein
the structure includes:
[0012] 1% or more residual austenite;
[0013] 80% or more in total content of bainitic ferrite and
martensite; and
[0014] 10% or less (may be 0%) in total content of ferrite and
pearlite in the proportion of area to the entire structure, and
wherein the mean axis ratio (major axis/minor axis) of the residual
austenite grains is 5 or higher and the bolt has tensile strength
of 1180 MPa or higher. This bolt shall be hereinafter referred to
as the inventive bolt 2.
[0015] The high strength bolt of the present invention may also
comprise higher than 0% and not higher than 0.1% of Nb and/or
higher than 0% and not higher than 1.0% of Mo, or higher than 0%
and not higher than 2% of Cu and/or higher than 0% and not higher
than 5% of Ni.
[0016] According to the present invention, a high strength bolt
having tensile strength of 1180 MPa or higher in which hydrogen
infiltrating from the outside is neutralized and hydrogen
embrittlement resistance is improved can be manufactured with high
productivity without using expensive elements. It is also made
possible to provide a high strength bolt that hardly experiences
delayed fracture or such failures and is used as an automobile
component at a low price. The high strength bolt of the present
invention contains less alloy elements than in the prior art, and
can be therefore readily recycled.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0017] Hydrogen-induced delayed fracture is believed to occur, in
tempered martensite bolt and bolt based on martensite and ferrite
that have been commonly used as high strength bolts, as hydrogen is
concentrated in grain boundaries of prior austenite thereby to form
voids or other defects that become the start points of the
fracture. Common practice that has been employed to decrease the
sensitivity of delayed fracture is to diffuse fine carbide grains
or the like uniformly as the trap site for hydrogen, thereby to
decrease the concentration of diffusive hydrogen, as described
previously as the prior art technology. However, even when a large
number of carbide grains or the like are diffused uniformly as the
trap site for hydrogen, there is a limitation to the hydrogen
trapping capability and delayed fracture attributable to hydrogen
cannot be fully suppressed.
[0018] Accordingly, the inventors of the present application
studied the means of achieving higher hydrogen embrittlement
resistance (delayed fracture resistance) while fully taking into
account the environment where the bolt is used.
[0019] The inventors reached such a conclusion that the best way of
improving the hydrogen embrittlement resistance by decreasing the
number of intergranular fracture initiating points is to form the
matrix phase of the bolt from a binary structure of bainitic
ferrite and martensite with the bainitic ferrite acting as the main
phase, not the single phase structure of martensite that is
generally used for high strength bolts. In the single phase
structure of martensite, a carbide (for example, film-like
cementite) is likely to precipitate in the grain boundaries, thus
making intergranular fracture likely to occur. In the case of the
binary structure of bainitic ferrite and martensite with the
bainitic ferrite acting as the main phase, in contrast, the
bainitic ferrite takes the form of plate-like ferrite that has
higher dislocation density, unlike the ordinary (polygonal)
ferrite, and allows it to easily increase the strength of the
entire structure as in the case of the single phase of martensite
while improving the hydrogen embrittlement resistance as much
hydrogen is trapped in the dislocations. It also has such an
advantage that coexistence of the bainitic ferrite and residual
austenite which will be described later prevents the generation of
carbide that acts as the intergranular fracture initiating
points.
[0020] It was also found that it is very effective to form
lath-shaped residual austenite for increasing the hydrogen trapping
capacity and neutralizing hydrogen, thereby to improve hydrogen
embrittlement resistance. It has been believed in the past that
residual austenite has an adverse effect on hydrogen embrittlement
resistance and fatigue characteristic. However, according to the
research conducted by the present inventors, it was found that the
residual austenite has a form of blocks on the order of micrometers
which adversely affects the hydrogen embrittlement resistance and
fatigue characteristic, though controlling the form of residual
austenite in lath shape of size on the order of sub-micrometers
makes it possible to put the hydrogen storing capability of the
residual austenite into full play so as to store and trap much
hydrogen thereby to achieve significant improvement of hydrogen
embrittlement resistance.
[0021] The reason for specifying the structures of the bolt of the
present invention will now be described in detail below.
<Bainitic Ferrite (BF) and Martensite (M): 80% or More>
[0022] The bolt of the present invention is made in binary phase
structure of bainitic ferrite and martensite (bainitic ferrite is
the main phase). As described previously, bainitic ferrite is a
hard structure and enables it to achieve high strength of the bolt.
Also because the matrix phase has high density of dislocations so
that much hydrogen can be trapped in the dislocations, higher
hydrogen storing capability than other types of TRIP bolt is
obtained. Moreover there is such an advantage that the lath-shaped
residual austenite specified in the present invention is readily
generated in the boundary between the lath-shaped bainitic ferrite
grains, thereby giving the bolt excellent drawability. In order to
achieve these effects efficiently, total area of bainitic ferrite
and martensite is set to 80% or more, preferably 85% or more and
more preferably 90% or more in proportion to the entire structure.
Upper limit of the proportion may be determined by the balance with
other structure (residual austenite), and is set to 99% when other
structure (ferrite, etc.) than the residual austenite is not
contained.
[0023] The bainitic ferrite is a lower structure having high
density of dislocations consisting of plate-shaped ferrite. It is
clearly distinguished from polygonal ferrite that has a lower
structure containing no or very low density of dislocations, by SEM
observation as follows.
[0024] Area proportion of bainitic ferrite structure is determined
as follows. A test piece (columned shape) is cut off so that a
position one half of the diameter in the cross section can be
observed, and the surface is etched with Nital etchant. A
measurement area (about 50 by 50 .mu.m) at an arbitrarily chosen
position in the surface is observed with a magnification factor of
1500 by means of a scanning electron microscope (SEM).
[0025] Bainitic ferrite is shown with dark gray color in SEM
photograph (bainitic ferrite, residual austenite and martensite may
not be distinguishable in the case of SEM observation), while
polygonal ferrite is shown black in SEM photograph and has
polygonal shape that does not contain residual austenite and
martensite inside thereof.
[0026] The SEM used in the present invention is a high-resolution
FE-SEM (Field Emission type Scanning Electron Microscope XL30S-FEG
manufactured by Philips Inc.) equipped with an EBSP (Electron Back
Scattering Pattern) detector, that has a merit of being capable of
analyzing the area observed by the SEM at the same time with the
EBSP detector. EBSP detection is carried out as follows. When the
sample surface is irradiated with electron beam, the EBSP detector
analyzes the Kikuchi pattern obtained from the reflected electrons,
thereby to determine the crystal orientation at the point where the
electron beam has hit upon. Distribution of orientations over the
sample surface can be measured by scanning the electron beam
two-dimensionally over the sample surface while measuring the
crystal orientation at predetermined intervals. The EBSP detection
method has such an advantage that different structures that are
regarded as the same structure in the ordinary microscopic
observation but have different crystal orientations can be
distinguished by the color tone.
<Residual Austenite (Residual .gamma., .gamma..sub.R): 1% or
More>
[0027] Residual austenite, that contributes not only to the
improvement of total elongation as has been known in the prior art
but also to the improvement of hydrogen embrittlement resistance,
is contained in a proportion of 1% or more in the bolt of the
present invention. The proportion is preferably 2% or more, and
more preferably 3% or more. Excessive content of the residual
austenite makes it unable to achieve a desired strength, and
therefore it is recommended to set an upper limit of 20% to the
proportion. The upper limit is more preferably 15%.
[0028] The inventors found that, when the residual austenite has
lath shape, hydrogen trapping capability far exceeding that of
carbide is obtained and, in case the shape has mean axis ratio
(major axis/minor axis) of 5 or higher, hydrogen that infiltrates
through the so-called atmospheric corrosion can be substantially
neutralized thereby greatly improving the hydrogen embrittlement
resistance, as described previously. Mean axis ratio of the
residual austenite is preferably 10 or higher, more preferably 15
or higher.
[0029] In view of stability of the residual austenite, it is
recommended to control the C concentration (C.sub..gamma.R) in the
residual austenite to 0.8% or higher. When the value of
C.sub..gamma.R is controlled to 0.8% or higher, it is also made
possible to improve the elongation characteristic and other
properties effectively. C.sub..gamma.R is preferably 1.0 or higher
and more preferably 1.2% or higher. While it is preferable that
C.sub..gamma.R is as high as possible, it is considered that in
practice there is an upper limit of around 1.6%.
[0030] The residual austenite refers to a region that is observed
as FCC (face centered cubic lattice) by the FE-SEM/EBSP method.
Measurement by the EBSP may be done, for example, by measuring a
measurement area (about 50 by 50 .mu.m) at an arbitrarily chosen
position in the cross section of a test piece (columned shape) at a
position of one half of the diameter at measuring intervals of 0.1
.mu.m, as in the case of the observation of the bainitic ferrite
and martensite. The measuring surface is prepared by electrolytic
polishing in order to prevent the residual austenite from
transforming. Then the test piece is set in the lens barrel of the
FE-SEM equipped with the EBSP detector and is irradiated with
electron beam. An EBSP image projected onto a screen is captured by
a high sensitivity camera (VE-1000-SIT manufactured by Dage-MTI
Inc.) and is sent to a computer. The computer carries out image
analysis and generates color mapping of the FCC phase through
comparison with a structural pattern simulated with a known crystal
system (FCC (face centered cubic lattice) phase in the case of
residual austenite). Area proportion of the region that is mapped
as described above is taken as the area proportion of the residual
austenite. This analysis was carried out by means of hardware and
software of OIM (Orientation Imaging Microscopy.TM.) system of
TexSEM Laboratories Inc.
[0031] The mean axis ratio was determined by measuring the major
axis and minor axis of residual austenite crystal grain existing in
each of three arbitrarily chosen fields of view in the observation
by means of TEM (transmission electron microscope) with
magnification factor of 15000, and averaging the ratios of major
axis to minor axis.
<Ferrite (F)+Pearlite (P): 10% or Less (May be 0%)>
[0032] The bolt of the present invention may be constituted from
only the structure described above (mixed structure of bainitic
ferrite+martensite and residual austenite). However, other
structures of ferrite (which refers to polygonal ferrite, namely
ferrite that contains no or very low density of dislocations)
and/or pearlite to such an extent that does not compromise the
effects of the present invention. While these structures may
inevitably remain in the bolt through the manufacturing process of
the present invention, their content is preferably as small as
possible and is controlled to not higher than 10%, preferably below
5% and more preferably below 3%, according to the present
invention.
[0033] The present invention is characterized in that the
metallurgical structure is controlled as described above. In order
to form such a structure and improve the hydrogen embrittlement
resistance and increase the strength, it is necessary to control
the composition of the bolt as follows.
<C: from 0.20 to 0.60%>
[0034] C is an essential element required to ensure high strength
of 1180 MPa or higher and retain the residual austenite.
Particularly it is important to contain a sufficient content of C
in the austenite phase, so as to maintain the desired austenite
phase to remain at the room temperature. In order to make use of
this action, it is necessary to contain 0.20% or more C content,
preferably 0.25% or more. Since excessive C content decreases the
toughness and therefore leads to lower hydrogen embrittlement
resistance, C content is controlled within 0.60%, preferably 0.5%
or lower.
<Si: 1.0 to 3.0%>
[0035] Si is an important element that effectively suppresses the
residual austenite from decomposing and carbide from being
generated, and is also a substitution type solid solution
strengthening element that is effective for hardening the material.
In order to make full use of these effects, it is necessary to
contain Si in a concentration of 1.0% or higher, preferably 1.2% or
higher and more preferably 1.5% or higher. However, excessively
large content of Si decreases the toughness and leads to lower
hydrogen embrittlement resistance, Si content is controlled within
3.0%, preferably within 2.7% and more preferably within 2.5%.
<Mn: 1.0 to 3.5%>
[0036] Mn is an element required to stabilize austenite phase and
obtain the desired level of residual austenite. In order to make
full use of this effect, it is necessary to contain Mn in a
concentration of 1.0% or higher, and preferably 1.2% or higher and
more preferably 1.5% or higher. However, since excessive content of
Mn leads to conspicuous segregation and results in poor
machinability, Mn content is controlled within 3.5%, preferably
within 3.2% and more preferably within 3.0%.
<Al: 1.5% or Less (Higher than 0%)> (In the Case of Inventive
Bolt 1)
<Al: 0.5% or Less (Higher than 0%)> (in the Case of Inventive
Bolt 2)
[0037] 0.01% or higher content of Al may be contained for the
purpose of deoxidation. In addition to deoxidation, Al also has the
effects of improving the corrosion resistance and improving
hydrogen embrittlement resistance.
[0038] The mechanism of improving the corrosion resistance is
supposedly based on the improvement of corrosion resistance of the
matrix per se and the effect of formation rust generated by
atmospheric corrosion, while the effect of formation rust
presumably has greater contribution. This is supposedly because the
formation rust is denser and better in protective capability than
ordinary iron rust, and therefore checks the progress of
atmospheric corrosion so as to decrease the amount of hydrogen
generated by the atmospheric corrosion, thereby to effectively
suppress the occurrence of hydrogen embrittlement, and hence the
delayed fracture.
[0039] While details of the mechanism of improvement of the
hydrogen embrittlement resistance by Al is not known, it is
supposed that condensing of Al on the surface of the bolt makes it
difficult for hydrogen to infiltrate into the bolt, and the
decreasing diffusion rate of hydrogen in the bolt makes it
difficult for hydrogen to migrate so that hydrogen embrittlement
becomes less likely to occur. In addition, stability of lath-shaped
residual austenite improved by the addition of Al is believed to
contribute to the improvement of hydrogen embrittlement
resistance.
[0040] In order to effectively achieve the effects of Al in
improving the corrosion resistance and improving the hydrogen
embrittlement resistance, Al content is controlled to 0.02% or
higher, preferably 0.2% or higher and more preferably 0.5% or
higher.
[0041] However, Al content must be controlled within 1.5% in order
to keep inclusions such as alumina from increasing in number and
size so as to ensure satisfactory machinability, ensure the
generation of fine residual austenite, suppress corrosion from
proceeding with the inclusion containing Al as the starting point,
and prevent the manufacturing cost from increasing. In view of the
manufacturing process, it is preferable to control so that A.sub.3
point is not higher than 1000.degree. C.
[0042] As the Al content increases, inclusions such as alumina
increase and delayed fracture resistance becomes poorer. In order
to suppress the generation of the inclusions such as alumina and
make a bolt having higher delayed fracture resistance, Al content
is restricted within 0.5%, preferably within 0.3% and more
preferably within 0.1%.
<P: 0.15% or Lower)
[0043] P is an element that promotes intergranular fracture due to
intergranular segregation. Therefore, P content is preferably as
low as possible with an upper limit set to 0.15%. P content is
controlled to preferably within 0.1%, and more preferably within
0.05%.
<S: 0.02% or Lower>
[0044] S is an element that promotes absorption of hydrogen in the
bolt in corrosive environment. S content is controlled to within
0.02%, and preferably within 0.01%.
[0045] While composition of the bolt of the present invention is as
described above with the rest substantially consisting of Fe, it
may contain inevitable impurities introduced into the bolt
depending on the stock material, production material, manufacturing
facility and other circumstances, containing 0.01% or less
nitrogen. In addition, other elements as described below may be
intentionally added to such an extent that does not adversely
affect the effects of the present invention.
<Nb: 0.1% or Lower (Higher than 0%) and/or Mo: 1.0% or Lower
(Higher than 0%)
[0046] Nb has great effect in increasing the strength of the bolt
and decreasing the grain size, and the effects can be enhanced by
adding Nb and Mo together. It is recommended to add 0.005% or more
(preferably 0.01% or more) Nb in order to achieve the effects
described above. However, the effects described above reach
saturation when excessive Nb content is contained, resulting in
economical disadvantage. Therefore, Nb content is limited to 0.1%
or less.
[0047] Mo has the effects of stabilizing austenite so as to retain
residual austenite, impeding the infiltration of hydrogen so as to
improve hydrogen embrittlement resistance and improving the
hardenability of the bolt. It also has the effect of strengthening
the grain boundary so as to suppress hydrogen embrittlement from
occurring. It is recommended to add 0.005% or more (preferably
0.01% or more) Mo in order to achieve these effects. However, since
the effects described above reach saturation when excessive Mo
content is contained, resulting in economical disadvantage, Mo
content is limited to 1.0% or less.
<Cu: 2% or Lower (Higher than 0%) and/or Ni: 5% or Lower (Higher
than 0%)
[0048] Addition of Cu and/or Ni enables it to effectively suppress
the generation of hydrogen that causes hydrogen embrittlement, and
at the same time suppress hydrogen that has been generated from
infiltrating into the bolt. As a result, diffusive hydrogen
concentration in the bolt can be decreased to a harmless level by
the synergy effect of the effects of these elements and the effects
of the composition described above to improve the hydrogen trapping
capability of the bolt.
[0049] Specifically, Cu and Ni have the effect of improving the
corrosion resistance of the bolt itself thereby to suppress the
generation of hydrogen through corrosion of the bolt. These
elements also have the effect of promoting the generation of iron
oxide, .alpha.-FeOOH, that is believed to be particularly stable
thermodynamically and have protective property among various forms
of rust generated in the atmosphere. By assisting the generation of
this rust, it is made possible to suppress hydrogen that has been
generated from infiltrating into the bolt thereby to sufficiently
improve the hydrogen embrittlement resistance to endure in harsh
corrosive environment. This effect can be achieved particularly
satisfactorily when Cu and Ni are contained at the same time.
[0050] In order to achieve the effects described above,
concentration of Cu, if added, is preferably 0.03% or higher and
more preferably 0.1% or higher, and concentration of Ni, if added,
is preferably 0.03% or higher and more preferably 0.1% or
higher.
[0051] Since excessively high concentration of either Cu or Ni is
detrimental to machinability, it is preferable to limit the Cu
content to 2% or lower (more preferably 1.5% or lower) and limit
the Ni content to 5% or lower (more preferably 3% or lower).
<Cr: 2% or Lower (Higher than 0%)>
[0052] Cr is a useful element that improves hardenability without
hardly affecting the deformability, thereby to easily achieve high
strength. In order to fully achieve this effect, it is preferable
that 0.1% or more Cr is contained. However, excessively high
concentration of Cr leads to the generation of cementite that makes
it difficult for residual austenite to remain, and therefore
concentration of Cr is preferably controlled within 2%.
<Ti and/or V: 0.003 to 1.0% in Total>
[0053] Ti has the effect of assisting in the generation of
protective rust, similarly to Cu and Ni. The protective rust has a
very valuable effect of suppressing the generation of .beta.-FeOOH
that appears in chloride environment and has adverse effect on the
corrosion resistance (and hence on the hydrogen embrittlement
resistance). Formation of such a protective rust is promoted
particularly by adding Ti and V (or Zr). Ti renders the bolt
excellent corrosion resistance, and also has the effect of cleaning
the bolt.
[0054] V is effective in increasing the strength of the bolt and
decreasing the crystal grains, in addition to having the effect of
improving hydrogen embrittlement resistance through cooperation
with Ti, as described previously.
[0055] In order to fully achieve the effect of Ti and/or V
described above, it is preferable to add Ti and/or V to total
concentration of 0.003% or higher (more preferably 0.01% or
higher). For the purpose of improving hydrogen embrittlement
resistance, in particular, it is preferable to add more than 0.03%
of Ti, more preferably 0.05% or more Ti. However, the effects
described above reach saturation when an excessive amount of Ti is
added, resulting in economical disadvantage. Excessive V content
also increases the precipitation of much carbonitride and leads to
poor machinability and lower hydrogen embrittlement resistance.
Therefore, it is preferable to control the total concentration of
Ti and/or V to within 1.0%, more preferably within 0.8%.
<Zr: 0.003 to 1.0%>
[0056] Zr is effective in increasing the strength of the bolt and
decreasing the crystal grain size, and also has the effect of
improving hydrogen embrittlement resistance through cooperation
with Ti. In order to sufficiently achieve these effects, it is
preferable that 0.003% or more Zr is contained. However, excessive
Zr content increases the precipitation of carbonitride and leads to
poor machinability and lower hydrogen embrittlement resistance.
Therefore, it is preferable to control the concentration of Zr to
within 1.0%.
<B: 0.0002 to 0.01%>
[0057] B is effective in increasing the strength of the bolt, and
it is preferable that 0.0002% or more (more preferably 0.0005% or
more) B is contained. However, excessive content of B leads to poor
hot machinability. Therefore, it is preferable to control the
concentration of B to within 0.01% (more preferably within
0.005%).
[0058] The present invention does not specify the manufacturing
conditions, but asserts it that stud bolts or the like can be
manufactured by forging a steel that has the composition specified
in the present invention and forming threads by thread rolling on
both end portions, or forming a bolt head at one end by warm
forging and forming threads by thread rolling on both end portions,
or forming a bolt head at one end by warm forging or turning
operation. In this case, in order to form the structure described
above that can improve the hydrogen embrittlement resistance and
strength at the same time, it is recommended to carry out the
rolling operation at a finishing temperature of A.sub.3 point or
higher. When the finishing temperature is lower than A.sub.3 point,
C does not diffuse sufficiently and the desired bainitic ferrite
structure and residual austenite structure cannot be obtained.
[0059] When the finishing temperature is too high, austenite grains
grow and it becomes impossible to form fine residual austenite
structure. Therefore, the finishing temperature is preferably set
to (A.sub.3 point +100.degree. C.) or lower, and more preferably
(A.sub.3 point +50.degree. C.) or lower.
[0060] Then the material is cooled down. According to the present
invention, it is recommended to cool down the wire at a mean
cooling rate of 3.degree. C./s or higher to a temperature in a
range from (Ms point -50.degree. C.) to Bs point and keep the
material at this temperature for a period of 60 to 3600
seconds.
[0061] The reason for cooling down the material at the mean cooling
rate of 3.degree. C./s or higher is to form the desired bainitic
ferrite structure and avoid the formation of pealite structure that
is undesirable for the present invention. The mean cooling rate is
preferably as high as possible, and it is recommended to set it to
10.degree. C./s or higher (more preferably 20.degree. C./s or
higher).
[0062] Then the material is quenched to a temperature between (Ms
point -50.degree. C.) and Bs point, followed by isothermal
transformation, thereby to form the desired structure. When the
heat retaining temperature becomes higher than Bs point, pealite
structure that is undesirable for the present invention is formed,
thus making it impossible to obtain the desired bainitic ferrite
structure. When the heat retaining temperature is lower than (Ms
point -50.degree. C.), area proportion of residual austenite
becomes smaller.
[0063] When the heat retaining period is longer than 3600 seconds,
residual austenite decomposes and cementite is formed, leading to
failure to achieve the desired performance. When the heat retaining
period is shorter than 60 seconds, sufficient diffusion of C does
not occur and residual austenite cannot be formed, in which case
again leading to failure to achieve the desired performance. The
heat retaining period is preferably in a range from 100 to 3000
seconds, more preferably from 180 to 2400 seconds.
[0064] The high strength bolt of the present invention may be
high-tension bolt, torque shear-type bolt, galvanized high strength
bolt, rust-proof high strength bolt and flame resistant high
strength bolt, and can be used as bolts having high strength and
high hydrogen embrittlement resistance suitable for automobile,
architecture, industrial machinery and other fields.
[0065] The present invention will now be described below by way of
examples, but the present invention is not limited to the example.
Various modifications may be conceived without departing from the
spirit of the present invention.
EXAMPLE
[0066] Sample steels A through Q having the compositions shown in
Table 1 were heated, to (A.sub.3 point +30.degree. C.) for Nos. 1
through 14 and 16 through 21 and 780.degree. C. for No. 15, for a
period of 60 to 1800 seconds, quenched to the temperature of To
shown in Table 2 and were held at the temperature (To) for t
seconds as shown in Table 2, and was left to cool down. While the
samples were made in this procedure in the Example, such a
procedure may also be employed as the material is rolled at
(A.sub.3 point +30.degree. C.), cooled down to the room
temperature, then heated again to (A.sub.3 point +30.degree. C.),
held at this temperature for 60 to 1800 seconds, quenched to the
temperature of To shown in Table 2 and held at the temperature (To)
for t seconds as shown in Table 2, and is left to cool down.
[0067] The samples obtained as described above were investigated
for the metal structure, tensile strength (TS), elongation (total
elongation E1), hydrogen embrittlement resistance and fatigue
characteristic in the following procedure.
Observation of Metal Structure
[0068] The test pieces (10 mm in diameter) prepared as described
above were observed and photographed in a measurement area (about
50 by 50 .mu.m at measuring intervals of 0.1 .mu.m) at an
arbitrarily chosen position in the cross section of the test piece
at a position of one half of the diameter, and area proportion of
bainitic ferrite (BF) and martensite (M) and area proportion of
residual austenite (residual .gamma.) were measured by the method
described previously. Similar measurements were made in two fields
of view that were arbitrarily selected, and the measured values
were averaged. Proportions of other structures were determined by
subtracting the area proportions of these structures. Mean axis
ratio of the residual austenite crystal grains was determined by
the method described previously.
Measurement of Tensile Strength
[0069] The various types of bolts were machined to make tensile
strength test pieces measuring 8 mm in diameter. These test pieces
were subjected to tensile strength test to measure the tensile
strength (TS).
Evaluation of Hydrogen Embrittlement Resistance
[0070] Samples having tensile strength of 1180 MPa or higher were
subjected to the evaluation of hydrogen embrittlement resistance.
The various specimens were machined to make delayed fracture test
pieces with annular notch (measuring 8 mm in diameter in parallel
portion and 6 mm in diameter in notched portion). The test pieces
were subjected to hydrogen charge with current density of 1.0
mA/cm.sup.2 in dilute sulfuric acid of pH 3.0 (liquid temperature
30.degree. C.), and were subjected to loading with the load varied
at 10% steps in a range from 30% to 80% of the ST, and the time
before rupture was measured. Then the load stress with rupture time
of 200 hours was determined from the relationship between the load
and the rupture time. Crack fracture strength ratio was defined as
the load stress with rupture time of 200 hours of hydrogen-charged
test piece divided by the load stress with rupture time of 200
hours of test piece not subjected to hydrogen charge, and the crack
fracture strength ratio was used as an index of hydrogen
embrittlement resistance. In consideration of the fact that SCM435
that is widely used to manufacture bolts having tensile strength of
1000 MPa class has delayed fracture strength ratio of about 0.5 at
the most, samples having delayed fracture strength ratio of 0.5 or
higher were evaluated to have high hydrogen embrittlement
resistance.
[0071] Some kinds of bolt were subjected to hydrogen charge
four-point bending test. In this test, rectangular test pieces
measuring 65 mm by 8 mm made of the various types of bolts
described above were immersed in (0.5 mol/H.sub.2SO.sub.4+0.01
mol/KSCN) solution and were cathodically charged with hydrogen, so
as to measure the maximum stress that was endured for 1 hour
without rupture as the critical fracture load (DFL). Ratio of this
value to the value of DFL of experiment No. 1 (bolt A) shown in
Table 2 was determined.
[0072] The test results are shown in Table 2. TABLE-US-00001 TABLE
1 Type of Formulation of chemical components* (mass %) Ac3 Bs Ms
steel C Si Mn Al P S Nb Mo Cu Ni Ti Others (.degree. C.) (.degree.
C.) (.degree. C.) A 0.41 2.03 2.97 0.033 0.03 0.003 -- -- -- -- --
-- 815.9 452.0 268.7 B 0.38 2.01 2.52 0.031 0.03 0.003 0.05 -- --
-- -- -- 832.5 500.6 297.7 C 0.40 1.98 2.30 0.033 0.03 0.002 -- 0.3
-- -- -- -- 844.8 490.1 289.2 D 0.40 2.00 2.03 0.032 0.02 0.003 --
-- 0.3 -- -- -- 830.9 539.3 304.4 E 0.41 1.97 2.00 0.033 0.02 0.003
-- -- -- 0.3 -- -- 830.7 528.2 295.6 F 0.35 1.48 2.49 0.032 0.02
0.002 -- -- -- -- 0.05 -- 828.2 511.4 312.9 G 0.29 1.98 1.99 0.033
0.03 0.003 -- -- -- -- 0.08 -- 895.7 572.6 357.9 H 0.51 2.01 2.47
0.032 0.02 0.003 -- -- -- -- -- Cr: 0.05 807.6 470.0 237.8 I 0.15
2.52 3.02 0.031 0.02 0.003 0.05 -- -- -- -- -- 879.8 517.7 390.2 J
0.41 0.49 2.48 0.033 0.02 0.002 -- 0.5 -- -- -- -- 770.5 454.6
274.3 K 0.70 2.01 2.01 0.033 0.013 0.003 -- -- -- -- -- -- 792.0
460.1 162.9 L 0.41 2.02 2.48 0.051 0.03 0.003 -- -- -- -- -- --
837.3 496.1 284.8 M 0.40 1.98 2.50 0.321 0.03 0.002 0.05 -- -- --
-- -- 944.5 497.0 288.9 N 0.42 2.00 2.51 0.422 0.03 0.002 -- 0.3 --
-- -- -- 991.8 465.8 272.8 O 0.41 1.97 2.51 0.541 0.03 0.002 0.05
0.2 -- -- -- -- 1036.5 476.8 279.6 P 0.42 1.98 2.55 0.72 0.03 0.002
0.05 0.2 -- -- -- -- 1105.7 470.5 273.6 Q 0.40 2.01 2.60 1.60 0.03
0.002 0.05 0.2 -- -- -- -- 1460.8 471.4 281.4 *balance of iron and
inevitable impurities
[0073] TABLE-US-00002 TABLE 2 Mean axis ratio Strength of residual
ratio of Type Residual austenite delayed Test of To t .UPSILON.
crystal grains BF + M F TS fracture DFL No. steel (.degree. C.) Sec
Area % -- Area % Area % MPa -- ratio 1 A 300 1800 13 .smallcircle.
87 0 1490 0.67 1.00 2 B 300 1800 12 .smallcircle. 88 0 1510 0.65 --
3 C 300 1800 11 .smallcircle. 89 0 1560 0.63 -- 4 D 320 1800 11
.smallcircle. 89 0 1530 0.65 -- 5 E 320 1800 12 .smallcircle. 88 0
1520 0.67 -- 6 F 320 1200 9 .smallcircle. 91 0 1620 0.65 -- 7 G 320
600 8 .smallcircle. 92 0 1650 0.61 -- 8 H 300 2400 14 .smallcircle.
86 0 1710 0.59 -- 9 I 350 1200 5 .smallcircle. 92 3 1090 -- -- 10 J
320 1200 1 x 99 0 1330 0.44 -- 11 K 350 1200 15 .smallcircle. 85 0
1490 0.41 -- 12 A 550 1800 0* x 0* 40* 760 -- -- 13 A 350 7200 1 x
99 0 1380 0.42 -- 14 A 320 10 1 x 99 0 1820 0.27 -- 15 A 400 600 2
x 68 30 1540 0.38 -- 16 L 320 1800 13 .smallcircle. 87 0 1494 0.68
1.19 17 M 320 1800 13 .smallcircle. 87 0 1497 0.69 1.49 18 N 320
1800 14 .smallcircle. 86 0 1499 0.70 1.58 19 0 320 1800 14
.smallcircle. 86 0 1501 0.72 1.58 20 P 320 1800 14 .smallcircle. 86
0 1513 0.73 1.63 21 Q 320 1800 15 x 55 30 1211 0.35 0.79 *balance
of Pearlite
[0074] The test results shown in Tables 1 and 2 can be interpreted
as follows (numbers in the following description refer to the
experiment Nos. given in Table 2).
[0075] Nos. 1 through 8 and 16 through 20 that satisfy the
requirements of the present invention show high strength of 1180
MPa or higher and high hydrogen embrittlement resistance to endure
harsh corrosive environment. Nos. 16 through 20 show particularly
excellent hydrogen embrittlement resistance.
[0076] Nos. 9 through 15 and 21 that do not satisfy the
requirements of the present invention have the following
drawbacks.
[0077] No. 9 does not have the strength specified in the present
invention due to insufficient C content.
[0078] No. 10 has poor hydrogen embrittlement resistance because it
dose not have a desired residual .gamma. due to insufficient Si
content.
[0079] No. 11, that was made from bolt K having excessive C
content, showed poor hydrogen embrittlement resistance and poor
fatigue characteristic due to precipitation of carbide.
[0080] Nos. 12 through 15 were made of steels having the
composition specified in the present invention, but had some
defects because they were not manufactured under the recommended
conditions.
[0081] No. 12 failed to show a high strength, because it was
subjected to austempering treatment at an excessively high
temperature, and therefore bainitic ferrite, martensite and
residual .gamma. could not be retained.
[0082] No. 13 was subjected to austempering treatment for an
excessively long period of time, No. 14 was subjected to
austempering treatment for too short a period of time and No. 15
was subjected to heating in a two-phase region (780.degree. C.),
and therefore all of these samples developed polygonal form of
residual .gamma., resulting in poor hydrogen embrittlement
resistance.
[0083] No. 21 contained Al content higher than that specified for
the inventive bolt 1, and therefore retained the predetermined
amount of residual austenite, but the residual austenite did not
satisfy the requirement for the mean axis ratio specified in the
present invention, did not form the desired matrix phase while
inclusions such as AlN were formed, thus resulting in poor hydrogen
embrittlement resistance.
* * * * *