Note: Descriptions are shown in the official language in which they were submitted.
CA 02800991 2012-11-27
1
Process for Manufacturing a Steel Tube for Air Bags
Technical Field
This invention relates to a process for inexpensively manufacturing a
seamless steel tube which is suitable as a steel tube for air bags (air bag
systems)
and of which are required a high strength as expressed by a tensile strength
of at
least 900 MPa and a high level of toughness as expressed by a value of vTrs100
(the lowest Charpy fracture appearance transition temperature at which the
percent
ductile fracture is 100%) of -60 C or below.
Background Art
In recent years, the automotive industry has actively promoted the
introduction of safety equipment. One example of such equipment which has
been developed is an air bag system, which has been installed in many
automobiles.
At the time of a collision, an air bag system inflates an air bag with a gas
or the like
between a passenger and the steering wheel, the instrument panel, or the like
before
the passenger impacts these objects and reduces injuries of the passenger by
absorbing the kinetic energy thereof. Air bag systems were initially of a type
which used explosive chemicals, but in recent years, a type which uses a high-
pressure filling gas has been developed and is being increasingly widely used.
In air bag systems which use a high-pressure filling gas, an inflating gas
such as an inert gas (such as argon) which is blown into an air bag at the
time of a
collision is always maintained at a high pressure inside an accumulator
connected
to the air bag, and at the time of a collision, the gas is blown all at once
from the
accumulator into the air bag in order to inflate the air bag. An accumulator
is
typically manufactured by welding a lid to both ends of a steel tube which has
been
cut to a suitable length and if necessary subjected to diameter reduction.
Accordingly, a stress at a high strain rate is applied to a steel tube used
for
an accumulator of an air bag system (referred to below as an air bag
accumulator or
simply as an accumulator) in an extremely short length of time. Therefore,
unlike
structures such as conventional pressure cylinders or line pipes, this type of
steel
tube requires high dimensional accuracy, workability, and weldability as well
as a
high strength and excellent bursting resistance.
Recently, there are increasing demands for decreases in the weight of
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,
2
automobiles. From this standpoint, there is also a desire to decrease the wall
thickness and the weight of steel tubes for air bags for mounting on
automobiles.
In order to guarantee a high bursting pressure even with a decreased wall
thickness,
accumulators are now manufactured from high-strength seamless steel tubes
having
a tensile strength of at least 900 MPa or even at least 1000 MPa. Taking an
accumulator manufactured from a seamless steel tube having an outer diameter
of
60 mm and a wall thickness of 3.55 mm as an example, if its tensile strength
is 800
MPa, its bursting pressure is at most around 100 MPa, but if its tensile
strength is
1000 MPa, its bursting pressure increases to 130 MPa. At the same time, when
the outer diameter of an air bag accumulator and the required bursting
pressure are
constant, it is possible to decrease the wall thickness by around 20%.
An accumulator also needs to have excellent low-temperature toughness so
that even in cold regions, the accumulator does not undergo brittle fracture
at the
time of a collision which can lead to secondary accidents.
For this reason, a seamless steel tube for an accumulator has been imparted
a high strength and a high toughness by carrying out quench hardening and
tempering thereon. Specifically, it is desired that an accumulator have low-
temperature toughness such that fracture in a Charpy impact test at -60 C is
ductile
(namely, vTrs100 is -60 C or below) and preferably such that fracture in a
Charpy
impact test at -80 C is ductile (vTrs100 is -80 C or below).
Concerning a seamless steel tube for air bag systems having a high strength
and high toughness, Patent Document 1, for example, proposes a process for
manufacturing a seamless steel tube for air bags comprising forming a seamless
steel tube by hot working using a steel material having a chemical composition
in a
prescribed range, cold drawing the seamless steel tube so as to give
predetermined
dimensions, heating the steel tube to a temperature in the range of at least
the Ac3
point to at most 1050 C followed by quenching, and then tempering it at a
temperature in the range of at least of 450 C to at most the Aci point.
It is purported that this process provides a seamless steel tube which has
excellent workability and weldability at the time of manufacture of an air bag
inflator, which has a tensile strength of at least 900 MPa when used as an
inflator,
and which has high toughness such that it exhibits ductility in a dropping
test
performed at -60 C on a steel tube cut in half However, the fact that it
exhibits
ductility in a dropping test at -60 C does not necessarily mean that it is
ductile in a
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bursting test at -60 C.
Patent Document 2 proposes a process for manufacturing a steel tube for
air bag systems having a tensile strength exceeding 1000 MPa by carrying out
quench hardening by high-frequency induction heating to achieve grain
refinement
by rapid heating. When using a seamless steel tube as a mother tube, the
seamless
steel tube is prepared by hot tube forming using a steel material having a
chemical
composition in a prescribed range, and the seamless steel tube is subjected to
cold
drawing to obtain a steel tube having predetermined dimensions. After the
steel
tube is heated, it is quenched and then tempered at a temperature of at most
the Ac
to transformation point. By carrying out tempering after quench hardening,
the steel
tube is given a desirable high toughness so as to exhibit ductility in a
bursting test
even at -80 C or below.
However, in the processes disclosed in Patent Documents 1 and 2, as
specifically disclosed therein, in order to obtain a steel tube having a
tensile
strength of at least 1000 MPa and a high toughness, it was necessary to
contain a
large amount of expensive alloying metals such as Cr and Mo. In Patent
Document 1, the (Cr + Mo) content is from 1.0 to 2.5 mass %, and in Patent
Document 2, a steel material is employed for which in many cases the (Cr + Mo)
cotent is 0.92 mass %. If large amounts of Cr and Mo are contained, in
addition
to a high material cost particularly due to expensive Mo, after forming a
seamless
steel tube in a hot state, the resulting steel tube tends to have a high
strength which
makes the subsequent cold drawing difficult. Therefore, softening treatment
becomes necessary before cold drawing, thereby making the manufacturing
process
complicated and manufacturing costs high.
Patent Document 3, which utilizes a steel in which the (Cr + Mo) content is
1.0 - 1.18 mass %, has the same problems as Patent Documents 1 and 2.
Patent Document 4 discloses a steel composition for a seamless steel tube
having excellent bursting resistance and which contains Cr, Mo, Cu, and Ni.
However, its properties are evaluated with respect to a seamless steel tube in
which
the (Cr + Mo) content is at least 0.76 mass %, and the tensile strength of
that tube is
at most 947 MPa.
Prior Art Documents
Patent Documents
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Patent Document 1: Japanese patent application publication No. JP 2004-76034
Al
Patent Document 2: PCT patent application publication No. WO 2004/104255 Al
Patent Document 3: United States patent application publication No. US
-- 2005/0076975 Al
Patent Document 4: PCT patent application publication No. WO 2002/079526 Al
Summary of the Invention
In a conventional steel tube for air bags, in order to provide it with a high
strength
-- and a high toughness, strengthening was achieved by adding Cr and Mo.
However, that
technique not only increases the alloy cost but also makes it difficult to
carry out cold
drawing after tube forming. Therefore, when there is a large difference
between the size
of a seamless steel tube used as a mother tube and the size of a steel tube
for air bags as a
final product, it becomes necessary to repeat cold drawing multiple times in a
cold
-- drawing step. In this case, the steel tube is finished to a product with
desired dimensions
while carrying out softening between successive times of cold drawing, so the
overall
manufacturing costs increase.
An object of the present invention is to provide a process for manufacturing a
steel tube for air bags having a high strength and high toughness by less
expensive means
-- than the prior art techniques and which is less expensive than conventional
products by
simplifying a drawing step or decreasing the alloy cost.
From another standpoint, an object of the present invention is to provide a
process for manufacturing a steel tube for air bags having a wall thickness
and diameter
which are the same as or smaller than those of conventional products using a
starting
-- material and a manufacturing process with lower costs than in the past.
The present inventors noted that as a result of relying on strengthening by Cr
and
Mo in a conventional high-strength steel tube for air bags, the strength after
the
completion of hot tube forming becomes high, thereby leading to a decrease in
productivity during cold drawing, and the alloy cost increases. Therefore,
they
-- investigated an alloy composition and a manufacturing process which
suppress the use of
these alloy elements as much as possible and which can guarantee a high
strength as
expressed by a tensile strength of at least 900 MPa and excellent low-
temperature
toughness as expressed by vTrs100 of -60 C or below.
As a result, they obtained the following knowledge and completed the
CA 02800991 2012-11-27
present invention.
(a) In the manufacture of a steel tube for air bags by carrying out cold
drawing followed by quench hardening and tempering, if the heating conditions
and cooling conditions at the time of quench hardening are appropriately set,
it is
5 possible to guarantee a high strength and low-temperature toughness even
if the
steel tube does not contain a large amount of Cr and Mo. It is particularly
effective for the steel to contain Cu and Ni in place of Cr and Mo.
(b) A steel having a reduced content of Cr and Mo and in place
containing Cu and Ni easily undergoes cold drawing after hot tube forming. As
a
o result, it is possible to increase the working ratio (reduction in area)
in one time of
cold drawing operation in a cold drawing step, thereby simplifying the cold
drawing step.
The present invention is a process for manufacturing a steel tube for air
bags characterized by including a tube forming step in which a seamless steel
tube
is produced by hot tube forming from a steel comprising, in mass %, C: 0.04 -
0.20%, Si: 0.10 - 0.50%, Mn: 0.10 - 1.00%, P: at most 0.025%, S: at most
0.005%,
Al: at most 0.10%, Cr: 0.01 - 0.50%, Cu: 0.01 - 0.50%, Ni: 0.01 - 0.50%, and a
remainder of Fe and unavoidable impurities, a cold drawing step in which the
resulting seamless steel tube is subjected to cold drawing at least one time
with a
reduction in area of at least 40% in one time of cold drawing operation to
obtain a
steel tube having predetermined dimensions, and a heat treatment step in which
the
cold drawn steel tube is subjected to quench hardening by heating it to a
temperature of at least the Ac3 point at a rate of temperature increase of at
least 50
C per second followed by cooling at a cooling rate of at least 50 C per
second at
least in a temperature range of 850 - 500 C and then to tempering at a
temperature
of at most the Aci point.
Preferred embodiments of a process for manufacturing a steel tube for air
bags according to the present invention are as follows.
The steel may optionally further contain one or more of the following
elements:
Mo: less than 0.10%,
at least one of Nb: at most 0.050%, Ti: at most 0.050%, and V: at most
0.20%; and
at least one of Ca: at most 0.005% and B: at most 0.0030%.
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The contents of Cu, Ni, Cr, and Mo in the steel preferably satisfy the
following Equation (1).
Cu + Ni (Cr + Mo)2 + 0.3 (1)
The symbols for elements in Equation (1) indicate the values of the content
of those elements in mass percent. When Mo is not contained, Mo = 0.
The wall thickness of the steel tube after completion of the cold drawing
step is preferably at most 2.0 mm.
The cold drawing step is preferably carried out by performing cold drawing
a single time.
The heating for quench hardening in the heat treatment step is preferably
carried out by high-frequency induction heating. In this case, before being
heated
for quench hardening, the steel tube obtained in the cold drawing step
preferably
undergoes straightening.
According to the present invention, it is possible to manufacture a steel
tube for air bags having a high strength as expressed by a tensile strength of
at least
900 MPa and excellent low-temperature toughness as expressed by vTrs100 of -60
C or below, while the content of expensive Mo is restricted to 0 or a low
level. In
addition, the strength of the seamless steel tube obtained by hot tube forming
is not
too high, so the working ratio in the subsequent cold drawing step can be
increased
compared to a conventional process, and the number of times that cold drawing
operation must be carried out with intervening softening between cold rolling
operations can be decreased. Therefore, according to the present invention, it
is
possible to decrease both the alloy cost and the manufacturing cost of a steel
tube
for air bags compared to the prior art.
Modes for Carrying Out the Invention
The chemical composition and the manufacturing process for a steel tube
for air bags according to the present invention will be explained more
specifically
below.
(A) Chemical composition of the steel
In this description, percent with respect to the chemical composition of a
steel means mass percent. The remainder of the chemical composition of a steel
other than the elements described below is Fe and unavoidable impurities.
C: 0.04 - 0.20%
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C is an element which is effective at inexpensively increasing the strength
of steel. If its content is less than 0.04%, it is difficult to obtain a high
strength
(tensile strength), and if it exceeds 0.20%, workability and weldability
decrease.
Accordingly, the C content is made at least 0.04% and at most 0.20%. A
preferred range for the C content is at least 0.07% to at most 0.20%, and a
more
preferred range is at least 0.12% to at most 0.17%. When it is desired to
obtain a
tensile strength of at least 1000 MPa, it is preferable to contain at least
0.06% of C.
Si: 0.10 - 0.50%
Si is an element which has a deoxidizing action and which also increases
the strength of steel by increasing its hardenability. With this object, the
Si
content is made at least 0.10%. However, if its content exceeds 0.50%,
toughness
decreases, so the Si content is made at most 0.50%. A preferred range for the
Si
content is at least 0.20% to at most 0.45%.
Mn: 0.10- 1.00%
Mn is an element which has a deoxidizing action and which is also
effective at increasing the strength and toughness of steel by increasing its
hardenability. If its content is less than 0.10%, a sufficient strength and
toughness
are not obtained. If its content exceeds 1.00%, coarsening of MnS takes place,
the
coarse MnS being elongated at the time of hot rolling, leading to a decrease
in
toughness. Therefore, the Mn content is made at least 0.10% and at most 1.00%.
A preferred Mn content is at least 0.30% and at most 0.80%.
P: at most 0.025%
P, which is contained in steel as an impurity, produces a decrease in
toughness due to grain boundary segregation. In particular, if the P content
exceeds 0.025%, toughness is markedly decreased. Accordingly, the P content is
made at most 0.025%. The P content is preferably at most 0.020% and more
preferably at most 0.015%.
S: at most 0.005%
S, which is contained in steel as an impurity, also decreases toughness
particularly in the T direction of a steel tube (the direction perpendicular
to the
rolling direction (the lengthwise direction) of a steel tube). If the S
content
exceeds 0.005%, there is a marked decrease in the toughness in the T direction
of a
steel tube, so the S content is made at most 0.005%. A preferred S content is
at
most 0.003%.
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Al: at most 0.10%
Al is an element which has a deoxidizing action and which is effective at
increasing the toughness and workability of steel. However, if Al is contained
in
an amount exceeding 0.10%, there is marked occurrence of sand marks.
Accordingly, the Al content is made at most 0.10%. The Al content may be on
the level of an impurity, so there is no particular lower limit, but it is
preferably at
least 0.005%. The Al content in the present invention is expressed as the
content
of acid-soluble Al (so-called sol. Al).
Cr: 0.01 - 0.50%
Cr has the effect of increasing the strength and toughness of steel by
increasing the hardenability and resistance to temper softening. This effect
appears when the Cr content is at least 0.01%. However, because Cr is an
element
which improves hardenability, it causes hardening of steel in the cooling
stage after
hot tube forming, thereby limiting the working ratio in a single time of cold
drawing operation, so there is an increased necessity to perform cold drawing
a
plurality of times in a cold drawing step with intervening softening
treatment.
Furthermore, an increase in the Cr content leads to an increase in the alloy
cost.
For the above reasons, the Cr content is made at least 0.01% and at most
0.50%.
A preferred Cr content is at least 0.15% to at most 0.45%, and a more
preferred
content is at least 0.18% to at most 0.35%.
Mo: 0% to less than 0.10 mass %
Mo has the effect of increasing the strength and toughness of steel by
increasing the hardenability and resistance to temper softening. This effect
appears when its content is at least 0.01%. However, in the present invention,
the
necessary strength and toughness are achieved by Ni and Cu, and it is not
essential
to add Mo. Namely, Mo may be 0%.
When Mo is added, its content is made less than 0.10%. If the Mo content
is higher, even if a seamless steel tube obtained by hot tube forming is air
cooled,
there is a tendency for the strength of the seamless steel tube to become too
high.
As a result, in the subsequent cold drawing step, it becomes necessary to
carry out
softening before working, and the working ratio (reduction in area) in cold
drawing
is limited. Therefore, the number of times of cold drawing and softening prior
to
cold drawing necessary to obtain a steel tube having predetermined dimensions
increases. This tendency becomes marked when Mo is 0.10% or greater. Mo is
= CA 02800991 2012-11-27
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an extremely expensive metal, so an increase in the Mo content is tied to a
marked
increase in the alloy cost. Namely, an Mo content of 0.10% or higher is an
impediment to achieving the objects of the present invention. Accordingly,
when
Mo is contained, its content is made less than 0.10%, and a preferred content
of Mo
is at least 0.01% and at most 0.05%.
Cu: 0.01 - 0.50%
Cu has the effect of increasing the strength and toughness of steel by
increasing its hardenability. This effect is exhibited if the Cu content is at
least
0.01% and preferably at least 0.03%. However, a Cu content in excess of 0.50%
to leads to an increase in the alloy cost. Accordingly, the Cu content is
made at least
0.01% and at most 0.50%. A preferred Cu content is at least 0.03% and
particularly at least 0.05%, and more preferably at least 0.15%. The upper
limit
on the Cu content is preferably 0.40% and more preferably 0.35%.
Ni: 0.01 - 0.50%
Ni has the effect of increasing the strength and toughness of steel by
increasing its hardenability. This effect appears if the Ni content is at
least 0.01%
and preferably at least 0.03%. However, an Ni content exceeding 0.50% leads to
an increase in the alloy cost. Accordingly, the Ni content is made at least
0.01%
and at most 0.50%. The Ni content is preferably at least 0.03%, more
preferably
at least 0.05%, and most preferably at least 0.15%. The upper limit on the Ni
content is preferably 0.40% and more preferably 0.35%.
The sum of the contents of Cu and Ni (Cu + Ni) is preferably at least 0.20%
and at most 0.65%, and more preferably at least 0.28% and at most 0.60%.
In a preferred embodiment of the present invention, the contents of Cu, Ni,
Cr, and Mo in steel are adjusted so as to satisfy the following Equation (1).
Cu + Ni > (Cr + Mo)2 + 0.3 (1)
The symbols for elements in Equation (1) indicate the value of the content
of each element in mass percent. When the steel does not contain Mo, Mo is 0.
Cr and Mo interfere with spheroidization of cementite which precipitates
during tempering. Particularly in a steel containing B, they easily form
compounds with B (borides) at grain boundaries, so they easily cause a
decrease in
toughness particularly in a high-strength steel. By suppressing Cr and Mo and
containing Cu and Ni so as to satisfy Equation (1), it becomes easy to
manufacture
a steel tube for air bags having a high strength and a high toughness.
CA 02800991 2012-11-27
In a preferred embodiment of the present invention, at least one element
selected from one or both of the following groups (i) and (ii) can be further
contained.
(i ) Nb, Ti, V
5 (ii) Ca, B
Nb: at most 0.050%
Nb, which is finely dispersed in steel as carbides, has an effect of strongly
pinning grain boundaries. As a result, it refines crystal grains and increases
the
toughness of steel. However, if Nb is contained in an amount exceeding 0.050%,
10 carbides coarsen and toughness ends up decreasing. Accordingly, when Nb
is
added, its content is made at most 0.050%. The above-described effect of Nb
appears even with an extremely small content, but in order to adequately
obtain this
effect, the Nb content is preferably at least 0.005%.
Ti: at most 0.050%
Ti has the effect of fixing N in steel and thereby increasing toughness.
Finely-dispersed Ti nitrides strongly pin grain boundaries and refine crystal
grains,
thereby increasing the toughness of steel. However, if Ti is contained in an
amount larger than 0.050%, nitrides coarsen and toughness ends up decreasing.
Accordingly, the content of Ti when it is added is made at most 0.050%. The
effect of Ti appears even when it is added in a minute amount, but in order to
adequately obtain its effect, its content is preferably at least 0.005%. A
preferred
Ti content is 0.008 - 0.035%.
V: at most 0.20%
V has the effect of ensuring toughness and increasing strength by
precipitation strengthening. However, a V content exceeding 0.20% leads to a
decrease in toughness. Accordingly, the content of V when it is added is made
at
most 0.20%. The effect of V appears even when it is added in a minute amount,
but in order to obtain an adequate effect, its content is preferably at least
0.02%.
A preferred range for the V content is 0.03 - 0.10%.
Ca: at most 0.005%
Ca has the effect of fixing S, which is present in steel as an unavoidable
impurity, as sulfides and improving the anisotropy of toughness, thereby
increasing
the toughness in the T direction of a steel tube and hence increasing the
resistance
to bursting thereof. However, if Ca is contained in excess of 0.005%,
inclusions
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11
increase and toughness ends up decreasing. Accordingly, the content of Ca when
it is added is made at most 0.005%. The above-described effect of Ca is
observed
even when it is added in an extremely small amount, but in order to obtain an
adequate effect, its content is preferably at least 0.0005%.
B: at most 0.0030%
When B is added in a minute amount, it segregates at grain boundaries in
steel and markedly increases the hardenability of steel. However, if the B
content
is 0.0030% or higher, coarse borides precipitate at grain boundaries and a
tendency
for toughness to decrease is observed. Accordingly, when B is added, its
content
to is made at most 0.0030%. The effect of B is observed even when it is
added in a
minute amount, but in order to guarantee an adequate effect, its content is
preferably made at least 0.0005%.
In the present invention, when it is desired to obtain a tensile strength of
at
least 1000 MPa, it is preferable to add B in order to increase strength by
improving
hardenability.
B does not segregate at grain boundaries unless it is present in solid
solution in steel. Accordingly, N, which easily forms a compound with B, is
preferably fixed by Ti, and B is preferably contained in at least an amount
which is
fixed by N. For this reason, the B content preferably satisfies the
relationship
given by the following Equation (2) based on the stoichiometric ratios of B,
Ti, and
N.
B - (N - Ti/3.4) x (10.8/14) ? 0.0001 ¨ (2)
In Equation (2), B, N, and Ti represent the values of the contents of those
elements in mass percent.
(B) Tube forming step
A steel ingot of a steel having its chemical composition adjusted as set
forth above in (A) is used as a starting material to obtain a seamless steel
tube by
hot tube forming.
There are no particular limitations on the form or the method for the
preparation of a steel ingot which is used as a starting material for hot tube
forming. For example, it may be a cast member (a round CC billet) obtained by
casting using a continuous casting machine having a cylindrical mold, or it
may be
an ingot which is cast into a rectangular mold and then hot forged to obtain a
cylindrical shape. As a result of suppressing the addition of ferrite-
stabilizing
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elements such as Cr and Mo and adding austenite-stabilizing elements such as
Cu
and Ni, even when continuous casting is employed into a round shape to form a
round CC billet, the effect of preventing center cracks is sufficiently
obtained, so
the applicability of the present invention to a round CC is sufficiently high.
As a
result, it is possible to eliminate a step of working to form a round billet
by
blooming or the like which is necessary when casting into a rectangular mold.
There are no particular limitations on a hot tube forming method for
obtaining a seamless steel tube. For example, the mandrel-Mannesmann method
can be used. Cooling after hot tube forming is preferably cooling with a low
cooling rate such as air cooling in order to facilitate cold drawing. There
are no
particular limitations on the shape of the resulting seamless steel tube, but
a
diameter of 32 - 50 mm and a wall thickness of around 2.5 - 3.0 mm, for
example,
are suitable.
(C) Cold drawing step
A seamless steel tube which is obtained by hot tube forming generally has a
large wall thickness and a large diameter with an inadequate dimensional
accuracy.
In order to obtain predetermined dimensions (the outer diameter and wall
thickness
of a steel tube) and good surface condition, the seamless steel tube which is
used as
a mother tube is subjected to cold drawing. In the present invention, in order
to
exploit the characteristics of the steel being used, the working ratio
(reduction in
area) in at least one time of cold drawing operation which is performed in the
cold
drawing step is made greater than 40%. If the working ratio in one time of
cold
drawing operation exceeds 50%, inner surface wrinkles and cracks easily
develop,
so the working ratio is preferably 42 - 48% and more preferably 43 - 46%. When
cold drawing is carried out two or more times in the cold drawing step, the
working
ratio in at least one of the times should be at least 40%, and it is possible
to
combine cold drawing having a working ratio of at least 40% with cold drawing
having a working ratio of less than 40%.
The working ratio in cold drawing is synonymous with the reduction in
area (decrease in cross section) defined by the following formula.
% reduction in area = (So - Sf) x 100/S0
where, So is the cross-sectional area of the steel tube before cold drawing,
and Sf is the cross-sectional area of the steel tube after the completion of
cold
drawing.
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The cross-sectional area of a steel tube is the cross-sectional area of just
the
tube wall and excludes the hollow portion of the tube cross section.
The working ratio (or reduction in area) in one time of cold drawing
operation can be the total working ratio when cold drawing is performed a
plurality
of times with no softening intervening between occurrences of cold drawing.
Using a steel according to the present invention, the working ratio in one
time of
cold drawing can exceed 40%, so if the finished dimensions of a seamless steel
tube obtained by hot tube forming are suitably selected, it is possible to
manufacture a thin-walled steel tube of predetermined dimensions in a single
occurrence (one time) of cold drawing. Manufacture can thus be greatly
simplified compared to the conventional process for manufacturing a thin-
walled
steel tube, which requires two occurrences of cold drawing and requires
intervening
softening between them.
Methods of cold drawing are well known, and cold drawing can be carried
out in a conventional manner. For example, when a seamless steel tube prepared
by the mandrel-Mannesmann method as described above is used as a mother tube,
the resulting tube may be allowed to cool to room temperature and then
subjected
to drawing with a die and a plug to reduce the diameter and wall thickness of
the
tube. A steel tube for air bags preferably has a diameter of at most 30 mm and
a
wall thickness of at most 2 mm, for example. As long as a steel tube having
the
necessary dimensions can be obtained from the seamless steel tube used as a
mother tube by cold drawing, there are no particular limitations on the
working
method, but the above-described drawing method is preferable.
With a steel composition used in the present invention, it is possible to
perform working with a reduction in area of 46%, for example, by single
occurrence of cold drawing. Therefore, when the final dimensions of a steel
tube
for air bags are a wall thickness of 1.7 mm and an outer diameter of 25 mm, if
the
dimensions of a mother tube to undergo cold drawing are, for example, an outer
diameter of 31.8 mm and a wall thickness of 2.5 mm, it is possible to obtain a
product having predetermined dimensions by performing cold drawing a single
time.
(D) Straightening
Since a steel tube for air bags manufactured in the present invention has a
tensile strength of at least 900 MPa and has undergone cold drawing with a
= CA 02800991 2012-11-27
14
reduction in area of at least 40%, there is a tendency for the strength of the
steel
tube after cold drawing to be higher than for a conventional steel, and in
some
cases, there is the possibility of the steel tube developing bending such as
springback after cold drawing.
As explained below, in order to achieve a high strength and high toughness,
a steel tube which is given predetermined dimensions by cold drawing is heated
to
at least the Ac3 transformation point by rapid heating for the purpose of
quench
hardening. This rapid heating is typically carried out by high-frequency
induction
heating. If there are bends in a steel tube which is to undergo quench
hardening,
to the problem may occur that the steel tube is unable to pass straight
through the
high-frequency coils used for high-frequency induction heating. Accordingly,
in a
preferred embodiment, straightening is carried out after cold drawing to
remove
bends in the steel tube.
There are no particular limitations on the straightening method, and
straightening can be carried out in a conventional manner. For example, a
preferred method is one in which four 2-roll stands having an adjusted roll
gap are
provided with the center of the roll gap in each stand being slightly deviated
or
offset with respect to each other and a steel tube is passed through the rolls
to apply
working in the form of bending forth and back. The higher the working ratio in
bending forth and back at this time, the higher is the effect of
straightening. From
this standpoint, the amount of offset (the amount of deviation of the roll
axis
between adjacent roll pairs) is made at least 1% of the outer diameter of the
steel
tube, and the roll gap is preferably made at most 1% smaller than the outer
diameter
of the steel tube. In order to avoid problems such as cracking of the steel
tube, the
amount of offset is preferably made at most 50% of the outer diameter of the
steel
tube, and the roll gap is preferably made at least 5% smaller than the outer
diameter
of the steel tube.
(E) Heat Treatment
After carrying out the straightening described above in (D) as required, the
steel tube is subjected to heat treatment in order to impart the required
tensile
strength to the steel tube and increase the toughness in the T direction,
thereby
guaranteeing bursting resistance. In order to give a steel tube a high
strength as
expressed by a tensile strength of at least 900 MPa and excellent low
temperature
toughness or bursting resistance, heat treatment is carried out by quench
hardening
CA 02800991 2012-11-27
after heating to a temperature of at least the Ac3 (transformation) point and
subsequent tempering at a temperature of at most the Ac (transformation)
point.
If the heating temperature before quenching is lower than the Ac3 point at
which an austenite single phase forms, it is not possible to guarantee good
5 toughness in the T direction (and accordingly good bursting resistance).
On the
other hand, if the heating temperature is too high, austenite grains abruptly
start to
grow and become coarse grains, and toughness decreases. Therefore, the heating
temperature is preferably made at most 1050 C. More preferably it is at most
1000 C.
10 Heating to at least the Ac3 point for quench hardening is carried out
by
rapid heating at a heating rate of at least 50 C per second. This heating
rate can
be the average heating rate in a temperature range from at least 200 C to the
heating temperature. If the heating rate is lower than 50 C per second, it is
not
possible to achieve refinement of austenite grain diameters, and the tensile
strength
15 and low-temperature toughness or bursting resistance decrease. In order
to obtain
a steel tube with a tensile strength of at least 1000 MPa and vTrs100 of -80
C or
below, the heating rate is preferably at least 80 C per second and more
preferably
at least 100 C per second. This rapid heating can be achieved by high-
frequency
induction heating. In this case, the heating rate can be adjusted by the feed
speed
of a steel tube passing through high-frequency coils.
A steel tube which has been heated to a temperature of at least the Ac3
point by rapid heating is held for a short period at a temperature of at least
the Ac3
point, and then it is rapidly cooled to carry out quench hardening. The
holding
time is preferably in the range of 0.5 - 8 seconds. More preferably it is 1 -
4
seconds. If the holding time is too short, the uniformity of mechanical
properties
is sometimes inferior. If the holding time is too long, particularly if the
holding
temperature is on the high side, it easily leads to coarsening of the
austenite grain
diameter. Refinement of grain diameter is necessary to guarantee extremely
high
toughness.
The cooling rate for quench hardening is controlled so as to be at least 50
C per second at least in a temperature range of 850 - 500 C. This cooling
rate is
preferably at least 100 C per second. In order to make the tensile strength
at least
1000 MPa and make vTrs100 a value of -80 C or below, the cooling rate is
preferably made at least 150 C per second. If the cooling rate is too low,
quench
CA 02800991 2012-11-27
16
hardening becomes incomplete, and the proportion of martensite decreases, so a
sufficient tensile strength is not obtained.
A steel tube which has undergone the above-described rapid cooling and
cooled to the vicinity of room temperature is then subjected to tempering at a
temperature of at most the Aci point in order to impart a tensile strength of
at least
900 MPa and sufficient bursting resistance. If the tempering temperature
exceeds
the Aci point, it becomes difficult to stably obtain the desired tensile
strength and
low-temperature toughness with certainty.
There are no particular limitations on a method for tempering, and it can be
carried out by, for example, soaking in a heat treatment furnace such as a
hearth
roller type continuous furnace or by using high-frequency induction heating or
the
like followed by cooling. Preferred soaking conditions in a heat treatment
furnace
are a temperature of 350 - 500 C and a holding time of 20 - 30 minutes. After
tempering, bends can be straightened using a suitable straightener or the like
in the
manner described in (D).
In order to form an air bag accumulator from a steel tube for air bags
manufactured in this manner, the steel tube is cut to a predetermined length
to
obtain a short tube, and if necessary at least one end of the cut tube is
subjected to
diameter reduction by press working or spinning (this is referred to as
bottling) for
final working to a shape necessary for mounting of an initiator or the like.
Accordingly, the predetermined dimensions and dimensional accuracy for a steel
tube for air bags referred to in this description mean the dimensions and
dimensional accuracy with respect to the tube thickness and diameter. Finally,
a
lid is mounted on each end of the steel tube by welding.
Examples
Steels having the chemical compositions shown in Table 1 with MI points
in the range of 720 - 735 C and Ac3 points in the range of 835 - 860 C were
prepared in a converter, and cylindrical billets having an outer diameter of
191 mm
were manufactured by continuous casting (round CC). Each round CC billet was
cut to a desired length and heated to 1250 C, and then it underwent piercing
and
rolling by the usual Mannesmann piercer-mandrel mill type technique to obtain
a
first mother tube having an outer diameter of 31.8 mm and a wall thickness of
2.5
mm and a second mother tube having an outer diameter of 42.7 mm and a wall
CA 02800991 2012-11-27
17
thickness of 2.7 mm.
The two types of mother tubes which were obtained in this manner
underwent cold drawing one or two times by a usual method which carries out
drawing using a die and a plug and were finished to form steel tubes with an
outer
diameter of 25.0 mm and a wall thickness of 1.7 mm. For comparative steels G
and H in Table I, when it was attempted to manufacture a steel tube having the
above-described shape by performing cold drawing one time on the first mother
tube having an outer diameter of 31.8 mm and a wall thickness of 2.5 mm,
fracture
developed and manufacture could not be carried out.
In Comparative Examples 9 and 10, the second mother tubes were used.
A steel tube having an outer diameter of 32.0 mm and a wall thickness of 2.2
mm
was formed by performing drawing a first time, then it underwent softening at
630
C for 20 minutes, and then it was finished to an outer diameter of 25.0 mm and
a
wall thickness of 1.7 mm by performing drawing a second time.
Each steel tube which underwent cold drawing was straightened using a
straightener, and then it was subjected to water quenching by heating to 920
C at
an average heating rate of 300 C per second (the average value in the
temperature
range of 200 - 900 C) using a high-frequency induction heating apparatus,
holding
at 920 C for 2 seconds, and water cooling (at an average cooling rate of 150
C per
second in the temperature range of 850 - 500 C). Subsequently, in order to
temper the steel tube, it was soaked for 30 minutes at 350 - 500 C in a
bright
annealing furnace and then cooled to room temperature by natural cooling
initially
in the furnace and then outside the furnace to obtain a steel tube for air
bags.
A tube of a fixed length was cut from each resulting steel tube, and it was
cut in the lengthwise direction of the tube at room temperature and unrolled.
A
rectangular member having a length of 55 mm, a height of 10 mm, and a width of
1.7 mm which was taken in the T direction from the unrolled tube and which had
a
2-mm V-notch was used as a test piece for a Charpy impact test which was
carried
out at various temperatures of -40 C and below. By means of this test, the
lowest
temperature at which the percent ductile fracture was 100% (vTrs100) was
obtained.
Using a No. 11 test piece prescribed by JIS Z 2201 which was taken from
the L direction of each steel tube, a tensile test in accordance with the
tensile test
method for metals prescribed by JIS Z 2241 was carried out. The results of the
Ci)
C/)
Table 1
co
Steel composition (mass %, remainder of Fe and impurities)
Steel
Cu+Ni (Cr+Mo)2
Remark
C Si Mn P S Cr Mo Cu Ni Nb Ti V sol.A1 Ca B +0.3
A 0.14 0.29 0.50 0.012 0.003 0.30 0.01 0.25 0.26 0.025 0.024
- 0.031 0.0016 0.0014 0.51 0.40
0
B 0.15 0.28 0.48 0.012 0.002 0.29
- 0.26 0.28 0.024 0.024 - 0.035 0.0011 0.0013 0.54 0.38
0
0
C 0.14 0.26 0.52 0.013 0.002 0.30 0.01 0.27 0.25 0.024 0.026
- 0.042 0.0015 0.0014 0.52 0.40 This
inven-
0
D 0.13 0.25 0.47 0.011 0.002
0.36 0.04 0.26 0.06 - 0.023 0.018 0.042 0.0013 0.0015 0.32 0.46
tion -=
0 oo
Cf)
E 0.13 0.26 0.48 0.012 0.002 0.22 - 0.26 0.25 - - -
0.034 - - 0.51 0.35
F 0.15 0.26 0.40 0.013 0.003 0.35 0.02 0.29 0.30 - 0.022 -
0.040 - 0.0010 0.59 0.44
G 0.12 0.25 1.29*
0.014 0.003 0.61* 0.28* 0.27 0.25 0.023 0.024 - 0.036 0.0015 0.0003 0.52
1.09 Comp-
para-
H 0.15 0.23 0.54
0.013 0.002 0.74* 0.35* 0.29 0.31 0.024 0.008 - 0.033 0.0022 0.0002 0.60
1.49 tive cr.
co
*Outside the range defined herein.CD
(1)
fa.
cr
'F17
Table 2
First cold rolling Second cold
rolling Total
Heating Cool-
Dimensions work-
IS vTrs100
Run
No. Steel of mother Dimensions % area Re_ Dimensions % area De_ ing
conditions ing
for quench rate Remark
tube reduc-
reduc- " ratio(MPa) ( C)
OW tion OW tion
sult suit
hardening ( C/s)
(%)
,
1 A C) - - -
1098 -120
2 B C) - - - 920
Cx2s 1070 -120 n
3 C C) - - -
(high 1101 -120 This 0
frequency 150 inven- "
co
4 D _____________________ OD:31.8mm OD:25.0mm C) _ __ _ _
46 induction 1022 -75 tion 0
46
0
E x2.5mm t x1.7mm t C) _ _ - heating)
1028 -100
H
KJ
C) - - -
1053 -110 'Z' 0
6 F
H
KJ
I
7 G x *** *** ***
*** *** *** *** H
H
I
KJ
8 H x *** *** ***
*** *** *** *** Com- -,
para-
9 G OD:42.7mm OD:32.0mm 0 OD:
25. Omm C) 920 Cx2s 1075 -110 tive
39.3 _______________________________________________ 39.6 _____ 63.3
150
H x2.7mm t x2.2mm t 0 x1.7mm t C) (HF-IH)
1040 -110
***Due to cracking which occurred during cold drawing, subsequent steps could
not be preformed.
HF-IH = high frequency induciton heating
CA 02800991 2012-11-27
As is apparent from Table 2, when steels A - F having the chemical
composition of a steel according to the present invention were used, in spite
of a
low alloy cost due to the amount of expensive Mo which was zero or a small
amount of less than 0.10%, it was possible to perform working to predetermined
5 product dimensions by one time of cold drawing even with a working ratio
as
expressed by a reduction in area of 46%. Furthermore, by carrying out rapid
heating and rapid cooling in the subsequent quench hardening step, it was
possible
to achieve a high level of product performance as a steel tube for air bags.
In
particular, when using steels A - C, E, and F having a composition which
satisfies
10 above-described Equation (1), vTrs100 was -100 C or below, so it is
apparent that
the low-temperature toughness is extremely high and excellent bursting
resistance
in a low-temperature environment can be expected.
Steels G and H, which were comparative examples, contained a large
amount of Mo, so the alloy cost was high. Furthermore, cracks developed when
15 cold drawing was carried out with a reduction in area of at least 40%.
Therefore, it
is necessary to carry out cold drawing at least 2 times with a reduction in
area of
less than 40%, and softening between cold drawing is necessary, so the
manufacturing costs of a steel tube for air bags also increase.
