Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION:
The present invention relates to a high
strength bolt and a method of manufacturing the high
strength bolt, and more particularly, to a nonheat-
refined high strength bolt excellent in toughness aswell as tool life and having a tensile strength not
smaller than 80 kg f/mm2 and a method of manufacturing
the same which method is characterized by austenitizing
a low or medium carbon-manganese steel wire material,
and then subjecting the austenitic steel wire material
to isothermal transformation, wire drawing and cold
forging successively, without quenching and tempering
steps.
In general, a high strength bolt having a
tensile strength not smaller than 80 kgf/mm2 is manu-
factured by a method wherein a medium carbon steel wire
rod is subjected to spheroidizing annealing and wire
drawing, followed by bolt forming by means of cold
forging and is then subjected to hardening and temper-
ing, thereby to provide the bolt with a necessarystrength and toughness, as shown in United States
Patent No. 3532560 dated Oct. 6, 1970.
Attention has recently been paid to a method
by which a steel product having a high strength and
toughness can be manufactured without requiring hardening
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1 and tempering after the cold forging, as describe~ in
United States Patent No. 3340102 dated Sept. 5, 1967.
However, this method can not practically apply to the
mass production of bolts because of the necessity to
effect mechanical deformation at a relatively high
temperature range defined by the metastable austenite
region of a material to be worked.
SUMMARY OF THE INVENTION:
Accordingly, it is a primary object of the
invention to obtain a high strength bolt having a
"as-forged" structure and a tensile strength not smaller
than 80 kgf/mm2 and to provide a method of manufacturing
a nonheat-refined high strength bolt equal or higher
in quality to or than conventional bolt subjected to
hardening and tempèring after cold forging, without
requiring such hardening and tempering, as well as
excellent in life of cold forging tool and having a
tensile strength not smaller than 80 kgf/mm2.
Another object of the present invention is to
obtain a high strength bolt having superior fatigue
strength and being small with respect to variation
in mechanical strength, in comparison with these of
conventional bolts.
According to the present invention, there is
obtained a high strength bolt having a tensile strength
not less than 80 kgf/mm2 made of a steel alloy consisting
essentially, by weight, of 0.15 to 0.3~ C, 1 to 2% Mn
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1 and the balance Fe and inevitable impurities, the bolt
being provided with a as-forged structure having being
superior in fatigue strength and toughness and being
small in variation in mechanical strength.
A method of manufacturing a high strength
bolt embodying the present invention, comprises the
steps of:
heating a steel alloy consisting essentially,
by weight, of 0.15 to 0.3~ C, 1 to 2% Mn and the
balance ~e and inevitable impurities, to a temperature
not less than Ac3 transformation point;
subjecting the steel alloy to isothermal
transformation at 450 to 580C;
subjecting, after cooling, the steel alloy to
cold wire drawing at a reduction ratio not more than
40%; and
bolt-forming the steel alloy.
In the case of the method of the present
invention, the life of tools used therein is substantial-
ly equal to that of convention bo:Lt producing methodsin which hardening and tempering are effected.
Moreover, according to another aspect of the
invention, there is provided a method of manufacturing
. a high strength bolt, comprising the steps of: subject-
ing a steel containing 0.15 to 0.3 wt % C and 1 to 2
wt % Mn to cold wire drawing at a reduction rate not
larger than 40%; heating the steel to a temperature
not lower than Ac3 transformation point; subjecting
_ 3 _
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1 the steel to isothermal transformation at 450 to 580C;
subjecting, after cooling, the steel to cold wire
drawing at a reduction rate not larger than 40%; and
bolt-forming the steel.
The high strength bolt and the method for
producing the same in accordance with the invention
will be described hereinbelow in detail.
First of all, the reason that a steel containing
0.15 to 0.3 wt % C and 1 to 2 wt % Mn is most suitable
for manufacturing a high strength bolt will be explained
hexeinbelow through the components and the component
ratio thereof.
Carbon is essential for increasing the strength
of the steel. A carbon content less than 0.15 wt ~
~5 is not effective in increasing the strength, while a
carbon content in excess of 0.3 wt % lowers the tough-
ness of the steel, causing cold forging formability
and tool life to be remarkably lowered. Thereore,
the C content is limited to fall between 0.15 and 0.3
wt ~.
Mn is an element which strengthens the
steel by existing in a ferrite in a solid solution
state, but an Mn content less than 1 wt % is not suf-
ficient for ensuring the strength. The reason that Mn
is contained in order to ensure the strength is as
follows. Namely, the lowering of toughness caused by
containing Mn for strengthening is smaller than that
caused by containing other elements, such as C or Si.
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1 In addition, Mn is smaller than C or Si in the rate of
damaging the cold forging formability. Moreover, an
Mn content in excess of 2 wt % reduces the toughness
improving effect and damages the cold forging formability
as well as gives a,rise in the production cost.
Accordingly, the Mn content is limited to fall between
1 and 2 wt %.
Besides C and Mn, Si is essential for refining
the steel. An Si content up to 0.5 wt % is allowable.
Moreover, as alloy components the following elements
may be contained according to circumstances: Ni of not
larger than 1 wt %; Cr of not larger than 1 wt %;
Mo of not larger than 0.5 wt %; AQ o not larger tharl
0.1 wt ~; Ti of not larger than 0.1 wt %; B of not larger
than 0.005 wt %; and so forth.
In ~he method of manufacturing a high
strength bolt in accordance with the invention, the
following steps are indispensable for obtaining a nonheat
re,ined high strength bolt excellent in tough~ess as
well as cold forging formability and having a tensile
strength not smaller than 80 kgf/mm2: namely, heating
a steel having the components and the component ratio
such as described above to a temperature not lower than
Ac3 transformation point; subjecting the heated steel
to isothermal transformation in lead bath or salt bath
of 450 to 580C; and subjecting the treated steel to
cold wire drawing and cold forging.
More specifically, if the above-described
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1 steel is heated to a temperature not lower than Ac3
transformation point and then subjected to isothermal
transformation at a temperature ranging ~etween 450 and
580C, the structure of the steel is transformed into
a fine structure having bainite, thereby allowing the
steel to improve in elongation and drawing. In the
above-mentioned temperature range, a temperature ranging
between 550 and 570C is preferable to obtain a fine
structure having bainite. The steel having been sub-
jected to the isothermal transformation is improvedin toughness but is unsatisfactory in strength, i.e.,
the steel has a tensile strength not satisfying the
condition of not smaller than 80 ~gf/mm2. Therefore/
the transformed steel is subjected to cold wire drawing
for obtaining desired mechanical properties and for
sizing. In this case, a reduction rate less than 15%
is insufficient for ensuring a tensile strength not
smaller than 80 kgf/mm2, so that a desired strength
cannot be obtained. On the other hand, a reduction rate
exceeding 40% deteriorates the toughness unfavorably.
Therefore, i.t is preferable to select the reduction rate
to be 15% - 40%.
By bolt-forming the drawn wire material by
means of cold forging, a bolt high in strength and
toughness can be obtained.
The steel having the above-described components
and component ratio may be used as it is cold-forging.
However, after the steel has been heated to a temperature
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l not lower than Ac3 transformation point and before the
heated steel is subjected to isothermal transformation,
if the heated steel is subjected to cold wire drawing
at a reduction rate not larger than 40% so as to cause
dislocations into the structure, the dislocation
becomes an austenitic core in the subse~uent step of
austenitizing the drawn steel to allow the austenitic
grains to be finer than those of the steel not sub-
jected to cold wire drawing. Therefore, a much finer
transformed structure can be cbtained in the subsequent
isothermal transformation treatment. As a result, the
steel is improved in toughness more than the steel not
subjected to cold wire drawing. It is desirable that
the reduction rate in such a case should be not larger
than 40% so that the drawing can be completed by one
pass, and the steel should be wire-drawen at a reduc-
tion rate not smaller than 10% for making the grains
finer.
In general, a high strength bolt is hardly
used as it has been subjected to hardening and tempering
after cold forging, and in most cases the bolt is
subjected to plating before being used. In such a
case, the bolt is subjected to baking for about four
hours at 190C for dehydrogenation. However, in the
case of the bolt obtained by the method of manufacturing
a high strength bolt in accordance with the invention,
if the bolt is subjected to plating after cold forging
and is then subjected to stress relief annealing for
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1 not less than 30 minutes at 200 to 400C instead of
baking, then, it is possible to effect dehydrogenation
as well as to obtain a yield ratio of 88 to 90%, which
is substantially equal to that of the conventional bolt
subjected to hardening and tempering after cold forging,
while the permanent elongation is improved in the case
of the present invention.
The invention will be more easily understood
from the following description taken in connection with
the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
Fig. 1 is a graph showing the tool life (A)
in the case of a bolt formed by the method of manufactur-
ing a high strength bolt in accordance with the
invention and the tool life (B) in the case of a bolt
for comparison formed by the conventional method.
Fig. 2 is a graph showing the values of
fatigue strength of bolts embod~ing the present inven-
tion in comparison with the fatigue strength of
conventional technique.
Fig. 3 is a graph showing ~he manner of
fatîgue test eff~cted to obtain the values shown in
Fig. 2.
DESCRIPTION OF PREFERRED EMBDOIMENTS:
Examples of high strength bolts and method
fo~ manufacturing the same in accordance with the
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1 present invention will be described hereinbelow in
comparison with an example of the conventional ones.
Examples:
For each of test steels having components and
component ratios shown in Table 1, an upset head bolt
having nominal thread diameter of 10 mm was produced
according to the following manufacturing steps.
Table
(wt %)
Sample C Si ~n S Cr Ni I
A 0.23 0.241.38 O.Q18 0.014 0.02 0.02
B 0.2~ 0.201.44 0.015 0.017 0.02 0.03
_ _
C 0.42 0.200.75 0.013 0.012 0.03 0.03
(Manufacturing Steps)
(1) A method of manufacturing a high strength bolt
in accordance with the invention:
hot rolling into a material having a diameter of 11.0 mm
~ reheating (950C for 6 minutes) ~ inserting in lead
bath (or salt bath) (560C for 5 minutes) and air
cooling ~ cold wire drawing (32% in reduction of area)
~ cold forming (nominal thread diameter of 10 mm and
pitches of 1.25 mm, upset head bolt) ~ zinc-chromate
plating ~ stress relief annealing (200C for 4 hours).
(2) A method of manufacturing a high strength bolt
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1 is accordance with the invention:
hor rolling into a material having a diameter of 13.0 mm
) cold wire drawing (28% in reduction of area) ~ reheating
(95QC for 6 minutes3 ~ inserting into lead bath
(560C for 5 minutes) ~ cold wire drawing (32~ in
reduction of area) ~ cold forging (nominal thread
diameter of lO mm and pitches of 1.25 mm, upset head
bolt) ~ zinc chromate plating ~ stress relief annealing
(200C for 4 hours).
(3) A conventional method:
hot rolling into a material ha~ing a diameter of
10.3 mm ~ spheroidizing annealing ~ cold wire drawing
(22% in reduction of area) ~ cold forging (nominal
thread diameter of 10 mm and pitches of 1.25 mm, head
bolt) ) hardening and tempering (850C for 30 minutes
~ o.Q., 570C for 60 minutes ~ A.C.) ~ zinc chromate
plating ~ baking (190C for 4 hours).
Various properties of the high strength bolts
produced according to the above described manufacturing
steps are shown in Table 2. In addition, the tool
life in the case of each of th~ bolts is shown in
Fig. 1.
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-- 11 --
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1 As will be clear from Table 2, according to
the method of manufacturing a high strength bolt in
accordance with the invention, it is possible to obtain
a bolt equal or higher in quality to or than the bolt
formed by the conventional method, without requiring
hardening and tempering, and particularly excellent in
impact resistance value.
Next, as will be apparent from Fig. 1, it can
be said that the tool life in the case of the bolt
formed by the method B of manufacturing a high strength
bolt in accordance with the invention is substantially
equal to that in the case of the bolt formed by the
conventional method C. More specifically, if isothermal
transformation treatment is carried out instead of
spheroidizing annealing, a bolt having an excellent
toughness can be obtained without effecting hardening
and tempering. Moreover, as mentioned above, the
method of manufacturing a high strength bolt in
accordance with the invention is substantially equal
in tool life to the conventional method, and the step
of spheroidizing annealing and the step of isothermal
transformation are regarded as corresponding to each other.
Also, the step of baking and the step of stress relief
annealing are regarded as corresponding to each other. In
addition, the method of manufacturing a high strength
bolt in accordance with the invention can omit hardening
and temper.ing. Therefore, the method of the invention
advantageously makes it possible to save energy and
~2~3~6
1 reduce the production cost through omission of manufactur-
ing steps.
Fig. 2 shows the superior fatigue limit o~
the bolts embodying the present invention in comparison
with conventional bol~s, that is, the bolts of the
present invention have fatigue limit larger about 100
~han that of conventional bolts when compar~d by use
of Bordwin type Fatigue test Machine in which each of
test pieces is subjected to the repetition of a cycle
of the maximum stress ~max and the minimum stress ~min
of one-tenth of ~ max as shown in Fig. 3.
In addition, since in the present invention
there is no step of hardening treatment, the variation
in resultant mechanical strength of bolt products of
the present invention becomes within a range of 5 kgf/mm2
in the case of tensile strength which is very small
in comparison with the variation range of 10 - 15 kgf/mm2
in tensile strength in the case of the conventional
bolts.
Further, in the present invention, there is
no fear of any bend of the bolt due to hardening which
is often found in a conventional long bolt produced
by the conventional process.
