ORIGINAL
Process for Manufacturing a Steel Tube for Air Bags
Technical Field
This invention relates to a process for inexpensively manufacturing a
5 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 ofvTrs100
(the lowest Charpy fracture appearance transition temperature at which the percent
ductile fracture is 100%) of -600 C or below.
10
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.
15 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 ofthe 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-
20 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
25 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
30 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, work ability, and weld ability as well as a
high strength and excellent bursting resistance.
Recently, there are increasing demands for decreases in the weight of
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
5 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 of3.55 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
10 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.
15 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 -600 C is ductile
(namely, vTrslOO is -600 C or below) and preferably such that fracture in a Charpy
20 impact test at -800 C is ductile (vTrslOO is -800 C or below).
Concerning a seamless steel tube for air bag systems having a high strength
and high toughness, Patent Document I, 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
25 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 10500 C followed by quenching, and then tempering it at a
temperature in the range of at least of4500 C to at most the ACt point.
It is purported that this process provides a seamless steel tube which has
30 excellent work ability and weld ability at the time of manufacture of an air bag
in flator, which has a tensile strength of at least 900 MPa when used as an in flator,
and which has high toughness such that it exhibits ductility in a dropping test
performed at -600 C on a steel tube cut in half. However, the fact that it exhibits
ductility in a dropping test at -600 C does not necessarily mean that it is ductile in a
bursting test at -600 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
5 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 ACl
10 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 -800 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
15 strength of at least 1000 MPa and a high toughness, it was necessary to contain a
large amount ofexpensive 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
20 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.
25 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
30 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
Patent Document I: JP 2004-76034 Al
Patent Document 2: WO 20041104255 Al
Patent Document 3: US 2005/0076975 Al
Patent Document 4: WO 2002/079526 Al
5
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
10 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
15 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
20 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
25 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
30 investigated an alloy composition and a manufacturing process which suppress the
use ofthese alloy elements as much as possible and which can guarantee a high
strength as expressed by a tensile strength ofat least 900 MPa and excellent lowtemperature
toughness as expressed by vTrs I00 of-600 C or below.
As a result, they obtained the following knowledge and completed the
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
10 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
15 is produced by hot tube forming from a steel comprising, in mass %, C: 0.040.20%,
Si: 0.10 - 0.50%, Mn: 0.10 - 1.00%, P: at most 0.025%, S: at most 0.005%,
AI: 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
20 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
25 least in a temperature range of 850 - 500° C and then to tempering at a temperature
of at most the Ac} 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
30 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%.
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
5 of those elements in mass percent. When Mo is not contained, Mo = O.
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.
10 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
15 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 -600
C or below, while the content ofexpensive Mo is restricted to 0 or a low level. In
addition, the strength ofthe 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
20 compared to a conventional process, and the number oftimes 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.
25
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.
30 (A) Chemical composition ofthe steel
In this description, percent with respect to the chemical composition of a
steel means mass percent. The remainder ofthe chemical composition ofa steel
other than the elements described below is Fe and unavoidable impurities.
C: 0.04 - 0.20%
7
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
5 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% ofC.
Si: 0.10 - 0.50%
Si is an element which has a deoxidizing action and which also increases
10 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%
15 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. Ifits content exceeds 1.00%, coarsening ofMnS takes place, the
coarse MnS being elongated at the time ofhot rolling, leading to a decrease in
20 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
25 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
30 particularly in the T direction of a steel tube (the direction perpendicular to the
rolling direction (the lengthwise direction) ofa steel tube). Ifthe S content
exceeds 0.005%, there is a marked decrease in the toughness in the T direction ofa
steel tube, so the S content is made at most 0.005%. A preferred S content is at
most 0.003%.
AI: 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, ifAl is contained in
an amount exceeding 0.10%, there is marked occurrence of sand marks.
5 Accordingly, the Al content is made at most 0.10%. The Al content may be on
the level ofan 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. AI).
Cr: 0.01 - 0.50%
10 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 ofcold
15 drawing operation, so there is an increased necessity to perform cold drawing a
plurality oftimes 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
20 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
25 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%. Ifthe 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 ofthe seamless steel tube to become too high.
30 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 oftimes 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
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 of0.10% or higher is an
impediment to achieving the objects ofthe present invention. Accordingly, when
Mo is contained, its content is made less than 0.10%, and a preferred content ofMo
5 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%
10 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%
15 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
20 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 ofthe contents ofCu 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 ofthe present invention, the contents of Cu, Ni,
25 Cr, and Mo in steel are adjusted so as to satisfy the following Equation (1).
Cu +Ni 2: (Cr + Moi + 0.3 (1)
The symbols for elements in Equation (1) indicate the value ofthe content
ofeach element in mass percent. When the steel does not contain Mo, Mo is O.
Cr and Mo interfere with spheroidization of cementite which precipitates
30 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.
In a preferred embodiment ofthe 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 ofstrongly
pinning grain boundaries. As a result, it refmes crystal grains and increases the
toughness of steel. However, ifNb 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 ofNb
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%
15 Ti has the effect offixing 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, ifTi is contained in an
amount larger than 0.050%, nitrides coarsen and toughness ends up decreasing.
Accordingly, the content ofTi when it is added is made at most 0.050%. The
20 effect ofTi 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 ofensuring toughness and increasing strength by
25 precipitation strengthening. However, a V content exceeding 0.20% leads to a
decrease in toughness. Accordingly, the content ofV when it is added is made at
most 0.20%. The effect ofV 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%.
30 Ca: at most 0.005%
Ca has the effect offixing S, which is present in steel as an unavoidable
impurity, as sulfides and improving the anisotropy oftoughness, thereby increasing
the toughness in the T direction ofa steel tube and hence increasing the resistance
to bursting thereof. However, ifCa is contained in excess of0.005%, inclusions
II
e increase and toughness ends up decreasing. Accordingly, the content ofCa when
it is added is made at most 0.005%. The above-described effect ofCa 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%.
5 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
10 is made at most 0.0030%. The effect ofB 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
15 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
20 given by the following Equation (2) based on the stoichiometric ratios ofB, 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 ofthe contents ofthose
elements in mass percent.
25 (B) Tube forming step
A steel ingot ofa 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
30 preparation ofa 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
e 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 ofpreventing center cracks is sufficiently obtained, so
the applicability ofthe present invention to a round CC is sufficiently high. As a
5 result, it is possible to eliminate a step ofworking 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
10 cooling rate such as air cooling in order to facilitate cold drawing. There are no
particular limitations on the shape ofthe resulting seamless steel tube, but a
diameter of32 - 50 mm and a wall thickness ofaround 2.5 - 3.0 mID, for example,
are suitable.
(C) Cold drawing step
15 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
20 exploit the characteristics ofthe 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%. Ifthe 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
25 cold drawing is carried out two or more times in the cold drawing step, the working
ratio in at least one ofthe times should be at least 40%, and it is possible to
combine cold drawing having a working ratio ofat 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
30 area (decrease in cross section) d'efined by the following formula.
% reduction in area = (So - Sf) x 100lSo
where, So is the cross-sectional area ofthe steel tube before cold drawing,
and Sf is the cross-sectional area ofthe steel tube after the completion ofcold
drawing.
13
The cross-sectional area of a steel tube is the cross-sectional area ofjust the
tube wall and excludes the hollow portion ofthe tube cross section.
The working ratio (or reduction in area) in one time ofcold drawing
operation can be the total working ratio when cold drawing is performed a plurality
5 oftimes 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
10 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
15 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 ofthe
tube. A steel tube for air bags preferably has a diameter of at most 30 mm and a
20 wall thickness ofat most 2 rom, 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
25 perform working with a reduction in area of46%, for example, by single
occurrence ofcold drawing. Therefore, when the final dimensions ofa steel tube
for air bags are a wall thickness of 1.7 rom and an outer diameter of25 rom, ifthe
dimensions ofa mother tube to undergo cold drawing are, for example, an outer
diameter of31.8 rom and a wall thickness of 2.5 rom, it is possible to obtain a
30 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
reduction in area of at least 40%, there is a tendency for the strength ofthe steel
tube after cold drawing to be higher than for a conventional steel, and in some
cases, there is the possibility ofthe steel tube developing bending such as
springback after cold drawing.
5 As explained below, in order to achieve a high strength and high toughness,
a steel tube which is given predetennined dimensions by cold drawing is heated to
at least the AC3 transfonnation 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,
10 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
15 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 ofthe 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 fonn of bending forth and back. The higher the working ratio in
20 bending forth and back at this time, the higher is the effect of straightening. From
this standpoint, the amount ofoffset (the amount of deviation ofthe roll axis
between adjacent roll pairs) is made at least 1% ofthe outer diameter ofthe steel
tube, and the roll gap is preferably made at most 1% smaller than the outer diameter
ofthe steel tube. In order to avoid problems such as cracking ofthe steel tube, the
25 amount ofoffset is preferably made at most 50% ofthe outer diameter ofthe steel
tube, and the roll gap is preferably made at least 5% smaller than the outer diameter
ofthe steel tube.
(E) Heat Treatment
After carrying out the straightening described above in (D) as required, the
30 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 ofat least 900 MPa and excellent low temperature
toughness or bursting resistance, heat treatment is carried out by quench hardening
)S
(
after heating to a temperature of at least the AC3 (transformation) point and
subsequent tempering at a temperature of at most the ACt (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. Ifthe 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
20 ofa 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 ofat 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
25 seconds. Ifthe holding time is too short, the uniformity ofmechanical properties
is sometimes inferior. Ifthe holding time is too long, particularly ifthe holding
temperature is on the high side, it easily leads to coarsening ofthe austenite grain
diameter. Refinement of grain diameter is necessary to guarantee extremely high
toughness.
30 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 vTrsl00 a value of -80° C or below, the cooling rate is
preferably made at least 150° C per second. Ifthe cooling rate is too low, quench
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
5 temperature of at most the ACl point in order to impart a tensile strength ofat least
900 MPa and sufficient bursting resistance. Ifthe tempering temperature exceeds
the ACl 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
10 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 of350 - 5000 C and a holding time of 20 - 30 minutes. After
tempering, bends can be straightened using a suitable straightener or the like in the
15 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 ofthe cut tube is subjected to
diameter reduction by press working or spinning (this is referred to as bottling) for
20 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 ofthe steel tube by welding.
25
Examples
Steels having the chemical compositions shown in Table 1 with ACl points
in the range of720 - 7350 C and AC3 points in the range of835 - 8600 C were
prepared in a converter, and cylindrical billets having an outer diameter of 191 mm
30 were manufactured by continuous casting (round CC). Each round CC billet was
cut to a desired length and heated to 12500 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 of2.5
mm and a second mother tube having an outer diameter of 42.7 mm and a wall
"
• 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 fmished to form steel tubes with an outer
5 diameter of25.0 mm and a wall thickness of 1.7 mm. For comparative steels G
and H in Table 1, 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.
10 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 6300
C for 20 minutes, and then it was finished to an outer diameter of25.0 mm and a
wall thickness of 1.7 mm by performing drawing a second time.
15 Each steel tube which underwent cold drawing was straightened using a
straightener, and then it was subjected to water quenching by heating to 9200 C at
an average heating rate of300° C per second (the average value in the temperature
range of200 - 9000 C) using a high-frequency induction heating apparatus, holding
at 9200 C for 2 seconds, and water cooling (at an average cooling rate of 1500 C per
20 second in the temperature range of 850 - 5000 C). Subsequently, in order to
temper the steel tube, it was soaked for 30 minutes at 350 - 5000 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 ofa fixed length was cut from each resulting steel tube, and it was
25 cut in the lengthwise direction ofthe 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-400 C and below. By means ofthis test, the lowest
30 temperature at which the percent ductile fracture was 100% (vTrsl00) was
obtained.
Using a No. 11 test piece prescribed by ns 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 ns Z 2241 was carried out. The results ofthe
-
Table 1
*Outside the range defined herein.
...D
Steel Steel composition (mass %, remainder of Fe and impurities) Ou+Ni (Or+Mo)2 Remark
0 Si Mn P S Or Mo Ou Ni Nb Ti V sol.AI Oa 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
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 0.14 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 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
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 Oomppara-
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
N ~
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Table 2
First cold rol ling Second cold rol ling Total Heating Cool-
Run Dimensions work- conditions ing TS vTrs100
No. Steel of mother Dimensions %area Re- Dimensions %area Re- ing for quench rate Remark
tube re~uc- su It reduc- ratio (MPa) (OC)
(mm) tlon (mm) tion su It (%) hardening (OC/s)
1 A 0 - - - 1098 -120
2 B 0 - - - 920°Cx2s 1070 -120
3 C 0 - - - (high 1101 -120 This
frequency 150 inven-
4 D OD:31. 8mm OD:25.0mm 0 - - - induction 1022 -75 tion
5 E x2.5mm t xl.7mm t 46 0 - - - 46 heating) 1028 -100
6 F 0 " - - - 1053 -110
7 G x *** *** *** *** *** *** ***
8 H x *** *** *** *** *** *** *** Compara-
9 G OD:42.7mm OD:32.0mm 0 OD:25.0mm 39. 6
0
63. 3 9200Cx2s 1075 -110 tive
x2. 7mm t x2.2mm t 39.3 xl. 7mm t (HF-IH) 150
10 H 0 0 1040 -110
***Due to cracking which occurred during cold drawing, subsequent steps could not be preformed.
HF-IH =high frequency induciton heating
'~,.~ ,~.-,~q ",5" W7F54..,·'4.,,#Eb"" lim; ·.$liV,",,,.g ·LU;SA LL ;'" M," M"~dA,_.\,,,5d~ . .A, 2. .J.l.$. AS" .3L.,.-.h!ALll4.;;N.-MX.i&JUL M Mi.", LXi ,,.$M],, LtG, A -.JJ~j J.., 4LUhUiSSU, X,LEA .,4 OJ XL "_",,KiLL 0.." g, L t,t,IX ¥ ,it.t, UJ.l.uit".hK~ "ii ,.~k . ,,_.' ,MM,.; __ bJL1PL;U."~;,,,.w;'(k k ,,$l31. ,J,X ..,L 4Sb': a !
As is apparent from Table 2, when steels A - F having the chemical
composition ofa 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 ofcold 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 ofproduct 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), vTrsl00 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 F and G, which were comparative examples, contained a large
amount ofMo, 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 ofa steel tube for air bags also increase.

We claim:
1. A process for manufacturing a steel tube for air bags characterized by
including:
5 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%, AI: 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,
10 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 ofat least 40% in one
time ofcold drawing 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 ofat least the AC3 point at a rate of
15 temperature increase ofat 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 ofat most the ACt point.
2. A process for manufacturing a steel tube for air bags as set forth in
20 claim 1 wherein the steel further contains less than 0.10% ofMo.
3. A process for manufacturing a steel tube for air bags as set forth in
claim 1 or claim 2 wherein the steel contains at least one ofNb: at most 0.050%,
Ti: at most 0.050%, and V: at most 0.20%.
25
4. A process for manufacturing a steel tube for air bags as set forth in any
of claims 1 - 3 wherein the steel contains at least one ofCa: at most 0.005% and B:
at most 0.0030%.
30 5. A process for manufacturing a steel tube for air bags as set forth in any
of claims 1 - 4 wherein the contents ofCu, Ni, Cr and Mo in the steel satisfy the
following Equation (1):
Cu +Ni 2: (Cr +Moi + 0.3 (l)
wherein the symbols for elements in Equation (1) mean the values ofthe
1..2
content ofthe respective elements in mass percent, and Mo = 0 when the steel does
not contain Mo.
6. A process for manufacturing a steel tube for air bags as set forth in any
5 ofclaims 1 - 5 wherein the wall thickness ofthe steel tube after completion ofthe
cold drawing step is at most 2.0 mm.
7. A process for manufacturing a steel tube for air bags as set forth in
claim 6 wherein the cold drawing step is carried out by performing cold drawing
10 one time.
8. A process for manufacturing a steel tube for air bags as set forth in any
of claims 1 - 7 wherein heating for quench hardening in the heat treatment step is
carried out by high-frequency induction heating.
15
9. A process for manufacturing a steel tube for air bags as set forth in
claim 8 wherein the steel tube obtained in the cold drawing step is straightened
before heating for the quench hardening.
Dated this 29th day ofNovember, 2012.
N~ Steel & Sumitomo Metal Corporation
(Ni~ilamani)
of Amarchand & Mangaldas &
Suresh A. Shroff& Co.
Attorneys for the Applicant