U.S. patent application number 10/848119 was filed with the patent office on 2005-01-06 for steel pipe for an airbag system and a method for its manufacture.
Invention is credited to Arai, Yuji, Kondo, Kunio.
Application Number | 20050000601 10/848119 |
Document ID | / |
Family ID | 33475138 |
Filed Date | 2005-01-06 |
United States Patent
Application |
20050000601 |
Kind Code |
A1 |
Arai, Yuji ; et al. |
January 6, 2005 |
Steel pipe for an airbag system and a method for its
manufacture
Abstract
A high strength steel pipe for an airbag system has a steel
composition comprising C: 0.05-0.20%, Si: 0.1-1.0%, P; at most
0.025%, S: at most 0.010%, Cr: 0.05-1.0%, Al: at most 0.10% at
least one of Ti and Mn satisfying (1) Ti.ltoreq.0.02% and (2)
0.4%.ltoreq.Mn+40Ti.ltoreq.1.2%, and a remainder of Fe. The
composition may further include one or more of (i) at least one of
Mo: 0.05-0.50%, Ni: 0.05-1.5%, V: 0.01-0.2%, and B: 0.0003-0.005%,
(ii) at least one of Cu: 0.05-0.5% and Nb: 0.003-0.1%, and (iii) at
least one of Ca: 0.0003-0.01%, Mg: 0.0003-0.01%, and REM:
0.0003-0.01%. The steel pipe can be manufactured by forming a pipe
from the above-described steel composition to obtain prescribed
dimensions, heating to at least the Ac.sub.1 transformation point
and quenching, and then tempering at the Ac.sub.1 transformation
point or below.
Inventors: |
Arai, Yuji; (Amagasaki-shi,
JP) ; Kondo, Kunio; (Kobe-shi, JP) |
Correspondence
Address: |
CLARK & BRODY
1750 K STREET NW
SUITE 600
WASHINGTON
DC
20006
US
|
Family ID: |
33475138 |
Appl. No.: |
10/848119 |
Filed: |
May 19, 2004 |
Current U.S.
Class: |
148/320 ;
148/590; 148/593 |
Current CPC
Class: |
C22C 38/24 20130101;
C22C 38/26 20130101; C22C 38/54 20130101; B60R 21/26 20130101; C21D
8/10 20130101; C22C 38/06 20130101; C21D 9/08 20130101; C22C 38/00
20130101; C21D 8/105 20130101; C22C 38/20 20130101; C22C 38/02
20130101; C22C 38/04 20130101; C22C 38/22 20130101; C22C 38/32
20130101 |
Class at
Publication: |
148/320 ;
148/590; 148/593 |
International
Class: |
C22C 038/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 21, 2003 |
JP |
2003-143764 |
Claims
What is claimed is;
1. A steel pipe for an airbag system having a steel composition
which consists essentially, in mass %, of: C: 0.05-0.20%, Si:
0.1-1.0%, P: at most 0.025%, S; at most 0.010%, Cr: 0.05-1.0%, Al;
at most 0.10%, at least one of Ti and Mn in amounts satisfying the
following equations (1) and (2), Mo: 0-0.50%, Ni: 0-1.5%, V:
0-0.2%, B: 0-0.005%, Cu: 0-0.5%, Nb: 0-0.1%, Ca: 0-0.01%, Mg:
0-0.01%, REM: 0-0.01%, and a remainder of Fe and impurities,
Ti.ltoreq.0.02% (1)0.4%.ltoreq.Mn+40Ti.ltoreq.1.2% (2). wherein the
symbols for elements in the above equation indicate the mass % of
the elements, the steel pipe having a tensile strength of at least
1000 MPa.
2. A steel pipe for an airbag system as claimed in claim 1 wherein
the steel composition contains at least 0.20 mass % of Mn.
3. A steel pipe for an airbag system as claimed in claim 1 wherein
the steel composition contains, in mass %, at least one of Mo:
0.05-0.50%, Ni: 0.05-1.5%, V: 0.01-0.2%,and B: 0.0003-0.005%.
4. A steel pipe for an airbag system as claimed in claim 1 wherein
the steel composition contains, in mass %, at least one of Cu:
0.05-0.5% and Nb: 0.003-0.1%
5. A steel pipe for an airbag system as claimed in claim 1 wherein
the steel composition contains, in mass %, at least one of Ca:
0.0003-0.01%, Mg: 0.0003-0.01%, and REM: 0.0003-0.01%.
6. A steel pipe for an airbag system as claimed in claim 1 wherein
the steel has a microstructure having a .gamma. grain size number
which is 11 or larger.
7. A method of manufacturing a steel pipe for an airbag system
comprising forming a pipe from a steel having a composition as
recited in claim 1 by a process including pipe making and
subsequent cold working, then heating the cold-worked pipe to a
temperature of at least the Ac.sub.1 transformation point of the
steel followed by quenching, and then tempering the quenched pipe
at a temperature no higher than the Ac.sub.1 transformation
point.
8. A method as claimed in claim 7 wherein the heating temperature
in the heating step of the cold-worked pipe is at least the
Ac.sub.3 transformation point of the steel.
9. A method as claimed in claim 8 wherein the heating temperature
is in the range of 900-1000.degree. C.
10. A method as claimed in claim 7 wherein the heating is performed
by rapid heating at a rate of temperature increase of at least
10.degree. C./sec.
11. A method as claimed in claim 10 wherein the heating is
performed by high frequency induction heating.
12. A method as claimed in claim 7 wherein the quenching is
performed such that the cooling rate at least in the temperature
range of from 850.degree. C. to 500.degree. C. is at least
20.degree. C./sec.
Description
TECHNICAL FIELD
[0001] This invention relates to a steel pipe which is suitable for
use with an airbag system and which has a high tensile strength of
at least 1000 MPa along with a high toughness. It also relates to a
method for the manufacture of such a steel pipe. In particular, the
present invention relates to a steel pipe for au airbag system and
to a method for its manufacture, the pipe having a high strength
and high toughness to such a degree that when a pipe sample having
a small wall thickness is subjected to an internal pressure
bursting test at -40.degree. C. and even at -80.degree. C., it does
not undergo brittle fracture
BACKGROUND ART
[0002] In recent years, there has been much emphasis in the
automotive industry on the introduction of equipment for increasing
the safety of vehicles. An example of such equipment is an airbag
system, which causes an airbag to be inflated with gas between
passengers and the steering wheel, the dashboard, or other portions
of the vehicle interior before the passengers can impact therewith
at the time of a collision and thereby absorbs the kinetic energy
of the passengers so as to decrease the likelihood and severity of
injuries.
[0003] A conventional airbag system employs an explosive chemical
to generate a gas to inflate an airbag. However, due to a desire
for a quick response to airbag inflation and for adjustment of the
pressure of an inflating gas, another type of airbag system which
employs a gas accumulated in a steel pipe under high pressure to
inflate an airbag has been developed and is being increasingly
employed.
[0004] An airbag system of the latter type normally maintains an
inflating gas at a high pressure in an accumulator. When a
collision occurs, the inflating gas in the accumulator is
discharged all at once into an airbag. As a result, a steel pipe
which is used as an accumulator for the high pressure inflating gas
undergoes stress in an extremely short period of time at a high
strain rate. Therefore, in contrast to a simple structural member
such as a conventional pressure cylinder or line pipe, the steel
pipe used as the inflating gas accumulator in such an airbag system
is required to have high dimensional accuracy, workability, and
weldability, and it is also required to have high strength and
excellent resistance to bursting.
[0005] A steel pipe which is suitable for use with an airbag system
to form an inflating gas accumulator and a method for its
manufacture are described in the following published Japanese
patent applications: JP P10-140238A, JP P10-140249A, JP
P10-140250A, JP P10-140283A, JP P10-212549A, JP P2002-294339A, JP
P11-199929A, JP P2001-49343A, and JP P2002-194501A.
[0006] Although the above-listed patent applications aim at
providing a high strength, high toughness steel pipe suitable for
use with an airbag system, the target tensile strength described in
those applications is on the level of 590 MPa or above, and the
values of tensile strength obtained in the examples of those
applications are no higher than 947 MPa.
[0007] The steel pipes disclosed in the above patent applications
can provide is adequate performance for existing airbag systems.
However, as a result of the trend in recent years towards decreases
in the weight of automobiles, there has also come to be a demand
for decreases in the size and weight of airbag systems.
Accordingly, there is a demand for steel pipes for airbag systems
to be able to accumulate an inflating gas at a higher pressure
while having a smaller wall thickness.
SUMMARY OF THE INVENTION
[0008] The present invention provides a high strength, high
toughness steel pipe suitable for use with airbag systems (i.e.,
for use in forming inflating gas accumulators for airbag systems)
which can satisfy properties which are demanded today as described
above. The present invention also provides a method for the
manufacture of such a steel pipe.
[0009] The present inventors found that in order to provide a steel
pipe for airbag systems with a high tensile strength and excellent
resistance to bursting so as to meet the demand for higher
accumulator pressures and smaller wall thicknesses, it is necessary
for the steel pipe to have the following properties:
[0010] (a) a tensile strength of at least 1000 MPa, and
[0011] (b) a sufficient toughness as demonstrated in a Charpy
impact test by exhibiting 100% ductile fracture at -40.degree. C.,
preferably 100% ductile fracture at -60.degree. C., and more
preferably 100% ductile fracture at -80.degree. C.
[0012] From the standpoint of safety, it is also important for the
steel pipe to have good resistance to bursting, which is
demonstrated in an internal pressure bursting test under
hydrostatic pressure at -40.degree. C. by there being no occurrence
of brittle fracture and there being no cracks which extend over the
entire length of the steel pipe. It is still more desirable that
such resistance to bursting be demonstrated in an internal pressure
bursting test at -80.degree. C.
[0013] According to the present invention, a steel pipe having a
high strength of at least 1000 MPa and a high toughness as
evidenced by 100% ductile fracture at -40.degree. C. in a Charpy
impact test as well as the above-described good resistance to
bursting can be obtained by selecting a specific steel
composition.
[0014] According to one form of the present invention, a steel pipe
suitable for use with an airbag system (to form an inflating gas
accumulator for the airbag system) is made of a steel having a
composition which consists essentially, in mass %, of: C:
0.05-0.20%, Si: 0.1-1.0%, P: at most 0.025%, S: at most 0.010%, Cr:
0.05-1.0 %, Al: at most 0.10%, one or both of Ti and Mn in amounts
sufficient to satisfy the following equations (1) and (2), and a
remainder of Fe and impurities:
Ti.ltoreq.0.02% (1)
0.4%.ltoreq.Mn+40Ti.ltoreq.1.2% (2).
[0015] The steel pipe has a tensile strength of at least 1000
MPa.
[0016] Optionally, the steel composition may further comprise one
or more of Mo: 0.05-0.50%, Ni: 0.05-1.5%, V: 0.01-0.2%, and B:
0.0003-0.005%.
[0017] Optionally the steel composition may further comprise one or
two of Cu: 0.05-0.5% and Nb: 0.003-0.1%.
[0018] Optionally the steel composition may further comprise one or
more of Ca: 0.0003-0.01%, Mg: 0.0003-0.01%, and REM:
0.0003-0.01%.
[0019] According to another form of the present invention, a method
of manufacturing a steel pipe for an airbag system comprises
forming a pipe having prescribed dimensions from a steel having the
above-described composition by a process including pipe making and
subsequent cold working, then heating the cold-worked pipe to at
least the Ac.sub.1 transformation point of the steel followed by
quenching, and tempering the quenched pipe at a temperature no
higher than the Ac.sub.1 transformation point of the steel.
[0020] In a preferred embodiment of the method, the cold-worked
steel pipe is heated to at least the Ac.sub.3 transformation point
of the steel and more preferably from 900.degree. C. to
1000.degree. C. This heating is preferably performed by rapid
heating at a rate of temperature increase of at least 10.degree.
C./sec by means of high frequency induction heating, for example.
The subsequent quenching is preferably performed such that the
cooling rate at least in the temperature range of from 850.degree.
to 500.degree. C. is at least 20.degree. C./sec. In this manner, a
steel pipe having an as-quenched fine microstructure with a .gamma.
grain size number of 11 or larger (the larger the number, the finer
the .gamma. grain size). Such a steel pipe exhibits extremely good
resistance to bursting as demonstrated by no cracks progressing
significantly even in an internal pressure bursting test at
-80.degree. C.
[0021] None of the above-listed patent documents discloses a steel
which has the above-described specific steel composition while
satisfying the above equation, and which has both a high strength
of at least 1000 MPa and a high toughness in the form of 100%
ductile fracture at -40.degree. C. The tensile strengths of steels
disclosed in these patent documents are shown in FIG. 2 as a
function of the value of (Mn+40Ti).
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a graph showing the tensile strength in the
circumferential direction of a steel pipe as a function of the
value of vTrs100, which is defined below, and
[0023] FIG. 2 is a graph showing the tensile strength in the
circumferential direction of a steel pipe according to the present
invention and according to the prior art as a function of the value
of (Mn+40Ti).
DESCRIPTION OF PREFERRED EMBODIMENTS
[0024] The present invention will be described below more fully. In
the following description, unless otherwise indicated, percent with
respect to a steel composition refers to mass percent.
[0025] (A) Chemical Composition of the Steel
[0026] According to one form of the present invention, a steel pipe
for an airbag system is made of a steel which has the
above-described specific steel composition in which the Ti and Mn
contents satisfy the following Equations (1) and (2):
Ti.ltoreq.0.02% (1)
0.4%.ltoreq.Mn+40Ti.ltoreq.1.2% (2).
[0027] The content of either one of Ti and Mn may be zero
percent.
[0028] The reasons for the ranges for the various elements in the
specific steel composition according to the present invention are
as follows.
[0029] C: 0.05-0.20%
[0030] Carbon (C) is an element which is effective for
inexpensively increasing the strength of steel. If the content
thereof is less than 0.05%, it is difficult to obtain a desired
tensile strength of at least 1000 MPa, while if its content exceeds
0.20%, workability and weldability decrease. A preferred range for
the C content is 0.08-0.20%, and a more preferred range is
0.12-0.17%.
[0031] Si: 0.1-1.0%
[0032] Silicon (Si) is an element which has a deoxidizing action
and which also increases the hardenability and strength of steel.
In view of these activities of Si, its content is made at least
0.1%. However, if the Si content exceeds 1.0%, toughness decreases.
A preferred range for the Si content is 0.2-0.5%.
[0033] P: at most 0.025%
[0034] Phosphorus (P) brings about a decrease in toughness caused
by grain boundary segregation. The decrease in toughness becomes
particularly marked when the P content exceeds 0.025%. The P
content is preferably at most 0.020% and more preferably at most
0.015%.
[0035] S: at most 0.010%
[0036] Sulfur (S) decreases toughness particularly in the T
direction, i.e., the circumferential direction of a steel pipe. In
particular, if the S content exceeds 0.010%, there is a marked
decrease in toughness in the T direction of a steel pipe. The S
content is preferably at most 0.005%, and still more preferably at
most 0.003%.
[0037] Cr: 0.05-1.0%
[0038] Chromium (Cr) is an element which is effective for
increasing the strength and toughness of steel. If the content
thereof is less than 0.05%, it becomes difficult to obtain a
tensile strength of at least 1000 MPa. However, if the Cr content
exceeds 1.0%, it leads to a decrease in the toughness of welded
portions. A preferred range for the Cr content is 0.2-0.8%, and a
more preferred range is 0.4-0.7%.
[0039] Al: at most 0.10%
[0040] Aluminum (Al) is an element which has a deoxidizing action
and which is effective for increasing toughness and workability.
However, if the content thereof exceeds 0.10%, the occurrence of
scratch marks in the steel during rolling becomes marked. 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%. A
preferred range for the Al content is 0.005-0.05%. In the present
invention, the Al content refers to the content of acid soluble Al
(so called sol. Al).
[0041] In order to guarantee toughness for a steel pipe for an
airbag system having the above-described specific steel composition
and to obtain a strength of at least 1000 MPa, it is necessary to
control the Mn and Ti content so as to satisfy the above-described
Equations (1) and (2).
[0042] Ti: 0-0.02%
[0043] The addition of titanium (Ti) to the steel composition of a
steel pipe according to the present invention is optional. If Ti is
added, the content is at most 0.02% so as to satisfy Equation (1).
There is no particular lower limit, and it may be present on the
level of an impurity.
[0044] Ti has a deoxidizing action. In addition, it has a strong
affinity for N, and at high temperatures, it stably exists as a Ti
nitride. Accordingly, it suppresses crystal grain growth at the
time of hot rolling and contributes to an increase in toughness. In
order to obtain such an effect from Ti, the Ti content is
preferably at least 0.003%. However, if the Ti content exceeds
0.02%, toughness ends up decreasing. Accordingly, when Ti is added,
the content thereof is preferably 0.003-0.02%.
[0045] Mn: up to 1.2%
[0046] Manganese (Mn) is an element which has a deoxidizing action
and which is effective for increasing the hardenability of steel
and for increasing strength and toughness, so Mn may be present in
an amount of up to 1.0%. A sufficient hardness and toughness are
not obtained when the content thereof is less than 0.20%, so the Mn
content is preferably at least 0.20%. On the other hand, if the Mn
content exceeds 1.0%, coarse MnS grains may form in the steel and
they may be extended during hot rolling, thereby causing toughness
to decrease. Therefore, the Mn content is preferably 0.20-1.0% and
more preferably 0.4-0.8%.
[0047] The Ti and Mn content is controlled so as to satisfy
Equation (2). If the value of (Mn+40Ti) is less than 0.4% or
greater than 1.2%, the desired high tensile strength and/or high
toughness cannot be obtained. The value of (Mn+40Ti) is preferably
in the range of 0.4-0.8%.
[0048] When it is desired to further improve the strength,
resistance to bursting, and/or weldability of a steel, in addition
to the above-described elements, one or more of Mo, Ni, Cu, V, Nb,
B, Ca, Mg, and REM may optionally be added within their respective
ranges described below to the steel composition for a steel pipe
according to the present invention.
[0049] Mo, Ni, B, V:
[0050] Each of molybdenum (Mo), nickel (Ni), boron (B), and
vanadium (V) has the effect of increasing hardenability, and one or
more of these elements may be added as optional elements.
[0051] Mo also has the effect of increasing strength through
solid-solution hardening and precipitation hardening. These effects
of Mo are obtained even when its content is on the level of an
impurity, but in order to obtain a more marked effect, the content
of Mo is preferably at least 0.05%. However, if the content of Mo
exceeds 0.50%, welded portions are hardened and toughness
decreases. Therefore, when it is added, the content of Mo is
preferably 0.05-0.50% and more preferably 0.1-0.35%.
[0052] Ni also has the effect of increasing toughness. This effect
of Ni is obtained even when its content is on the level of an
impurity, but in order to obtain a more marked effect, the content
of Ni is preferably at least 0.05%. However, Ni is an expensive
element, and costs increase markedly when the content thereof
exceeds 1.5%. Therefore, when Ni is added, its content is
preferably 0.05-1.5% and more preferably 0.1-1.0%.
[0053] The effect of B on increasing hardenability is obtained even
when its content is on the level of an impurity, but in order to
obtain a more marked effect, the content of B is preferably at
least 0.0003%. However, if the content of B exceeds 0.005%,
toughness decreases. Therefore, when B is added, its content is
preferably 0.0003-0.005%. A more preferred range for the B content
is 0.0003-0.002%.
[0054] V also has the effect of increasing strength through
precipitation hardening. This effect of V is obtained when its
content is at least 0.01%, but if the V content exceeds 0.2%,
toughness decreases. Accordingly, when V is added, its content is
preferably 0.01-0.2%. A more preferred range for the content of V
is 0.03-0.10%.
[0055] Cu, Nb:
[0056] Each of copper (Cu) and niobium (Nb) has the effect of
increasing toughness, is and one or both of these elements may be
added as optional elements.
[0057] The effect of Cu on increasing toughness can be obtained
even when its content is on the level of an impurity, but in order
to obtain a more marked effect, the Cu content is preferably at
least 0.05% and more preferably at least 0.1%. However, Cu
decreases the hot workability of steel, so when Cu is added, Ni is
preferably also added in order to guarantee hot workability. If the
content of Cu exceeds 0.5%, it may not be possible to guarantee
good hot workability even if Ni is also added. Accordingly, the
content of Cu when it is added is preferably 0.05-0.5%.
[0058] Nb has the effect of increasing toughness even when present
on the level of an impurity, but in order to obtain a more marked
effect, its content is preferably at least 0.003% and more
preferably at least 0.005%. However, if the content of Nb exceeds
0.1%, toughness ends up decreasing. Accordingly, when Nb is added,
its content is preferably 0.003-0.1%. A more preferred range for
the Nb content is 0.003-0.03% and a still more preferred range is
0.005-0.02%.
[0059] Ca, Mg, REM:
[0060] When it is desired to guarantee even better resistance to
bursting for a high strength steel pipe for an airbag system, one
or more of calcium (Ca), magnesium (Mg), and rare earth metals
(REM) may be added as optional elements.
[0061] Each of these elements improves the anisotropy of toughness,
and increases the toughness in the T direction of a steel pipe, and
as a result they have the effect of further increasing the
resistance to bursting. This effect can be obtained even when the
content is on the level of an impurity, but in order to obtain a
more marked effect, the content of any of these elements which is
added is preferably at least 0.0003%. However, if the content of
any of these elements exceeds 0.01%, the formation of inclusions in
the form of clusters occurs, leading to the occurrence of scratch
marks. Accordingly, when any of these elements is added, the
content of each added element is preferably 0.0003-0.01% and more
preferably 0.0005-0.003%.
[0062] (B) Pipe Manufacture
[0063] In order to obtain a steel pipe for an airbag system
according to the present invention, a seamless steel pipe or a
welded steel pipe is made from a steel having the above-described
chemical composition. A seamless steel pipe is preferred from the
standpoint of reliability. There are no particular limitations on a
method of manufacturing a seamless pipe or a welded pipe.
[0064] (C) Cold Working
[0065] A seamless or welded steel pipe made from the above steel is
subjected to cold working under conditions selected so as to
provide the pipe with a prescribed dimensional accuracy and surface
quality. The cold working may be any type as long as it can provide
a prescribed dimensional accuracy and surface quality. Examples of
useful types of cold working are cold drawing and cold rolling.
There are no particular restrictions on the degree of cold working,
but it is preferably at least 3% in terms of reduction in area. In
order to decrease the working force during cold working, softening
heat treatment may be performed prior to cold working.
[0066] (D) Heat Treatment
[0067] After the cold working described above in (C), the steel
pipe is subjected to heat treatment in order to guarantee that the
steel pipe has the necessary tensile strength and in order to
increase toughness in the T direction so as to also guarantee
resistance to bursting. In order to provide the steel pipe with a
high strength in the form of a tensile strength of 1000 MPa and
resistance to bursting, the steel pipe is quenched after heating at
a temperature of at least the Ac.sub.1 transformation point of the
steel, and then tempering is carried out at a temperature no higher
than the Ac.sub.1 transformation point of the steel.
[0068] If the heating temperature prior to quenching is less than
the Ac.sub.1 transformation point, good toughness in the T
direction and therefore a good resistance to bursting cannot be
guaranteed. The heating temperature is preferably a temperature of
at least the Ac.sub.3 transformation point of the steel, which is
in the austenite region.
[0069] Heating at a high temperature for a long period causes a
large amount of scale to be formed on the surface of a steel pipe
and decreases the dimensional accuracy and surface quality and also
leads to a decrease in resistance to bursting. Therefore, the
heating is preferably rapid heating to a predetermined temperature
followed by a short retention period at that temperature. The rapid
heating is preferably carried out such that the rate of temperature
increase is at least 10.degree. C./sec. Such rapid heating can be
achieved by high frequency induction heating or direct resistance
heating, although there are no particular restrictions on the
heating method. A particularly preferred heating method is high
frequency induction heating.
[0070] Particularly in the case of such short-period heating, a
preferred heating temperature is in the range of from 900.degree.
C. to 1000.degree. C. and more preferably from 900.degree. C. to
960.degree. C. If the heating temperature is lower than 900.degree.
C., complete autstenitization may not be achieved during the
short-term heating, thereby making it impossible to form a desired
microstructure. Heating to a temperature exceeding 1000.degree. C.
may cause the .gamma. grains to be coarsened to such a degree that
the toughness is decreased.
[0071] The heating to a temperature of at least the Ac.sub.1
transformation point of the steel is preferably performed in an
atmosphere having as low an oxygen potential as possible from the
standpoint of suppressing the formation of surface scale during
heating. A reducing atmosphere is even more preferred.
[0072] After the steel pipe is heated at a temperature of at least
the Ac.sub.1 transformation point and preferably at least the
Ac.sub.3 transformation point of the steel, it is then quenched in
order to obtain a desired tensile strength of at least 1000 MPa in
a stable and reliable manner. Specifically, the cooling rate for
quenching is at least approximately 5.degree. C./sec in the
temperature range of 850-500.degree. C. Preferably, the cooling
rate is at least 20.degree. C./sec. Such quenching may be performed
by water quenching.
[0073] Rapid heating to a temperature in the range of
900-1000.degree. C. by high frequency induction heating followed by
quenching at a cooling rate of at least 20.degree. C./sec in the
temperature range of 850-500.degree. C. makes it possible to form a
dense as-quenched microstructure characterized by the the size of
.gamma. grains (size of pre-existing austenite grains as measured
by the Bechet-Beaujard method according to JIS G0551) having a size
number of 11 or larger in a stable manner. A steel pipe having such
a microstructure has a particularly improved toughness and exhibits
an excellent resistance to bursting as demonstrated by no cracks
progressing significantly even in an internal pressure bursting
test performed at -80.degree. C.
[0074] A steel pipe which is cooled to approximately room
temperature by quenching is then tempered at a temperature of no
higher than the Ac.sub.1 transformation point of the steel in order
to provide the pipe with the desired tensile strength of at least
1000 MPa and resistance to bursting. The .gamma. grain size does
not vary by such tempering. It is difficult to obtain the
above-described characteristics in a stable and a reliable manner
if the temperature of tempering exceeds the Ac.sub.1 transformation
point. The tempering is preferably carried out by maintaining the
steel pipe for at least 10 minutes at a temperature in the range of
450-700.degree. C. After tempering, bends can be removed by a
suitable straightener or similar device.
[0075] In this manner, according to the present invention, it is
possible to provide a steel pipe for an airbag system which has a
tensile strength of at least 1000 MPa and which has a high
toughness as indicated by 100% ductile fracture at -40.degree. C.,
preferably at -60.degree. C., and more preferably at -80.degree. C.
in a Charpy impact test and by no cracks progressing significantly
in an internal pressure bursting test at -40.degree. C. and
preferably at -80.degree. C. The steel pipe can adequately cope
with the recent trend in steel pipes for airbag systems towards
higher accumulator pressures and lower wall thicknesses.
EXAMPLES
[0076] The following examples illustrate the present invention and
demonstrate the effects thereof. These examples are to be
considered in all respects as illustrative and not restrictive. The
steels used in the examples had an Ac.sub.1 transformation point in
the range of from 700.degree. C. to 760.degree. C. and an Ac.sub.3
transformation point in the range of from 820.degree. C. to
880.degree. C.
Example 1
[0077] Using steel billets having the chemical compositions shown
in Table 1, a seamless steel pipe having nominal dimensions of an
outer diameter of 70 mm and a wall thickness of 4.1 mm was
manufactured from each billet by a hot working process which
included heating the billet to 1250.degree. C. and performing
piercing and rolling by a conventional Mannesmann piercer-mandrel
mill system. Each seamless steel pipe was then finished by cold
drawing so as to have an outer diameter of 60.33 mm and a wall
thickness of 3.35 mm. The pipe was then heated to 920.degree. C. in
a conventional walking beam furnace (heating rate: about
0.3.degree. C. per second) with a temperature retention period of
10 minutes and then water quenched, after which it was tempered at
a temperature which was no higher than the Ac.sub.1 transformation
point of the steel by heating in a conventional walking beam
furnace (atmosphere: air). In this manner, three seamless steel
pipes for airbag systems having different tensile strengths were
manufactured for each steel composition by varying the conditions
of tempering. The water quenching was performed such that the
cooling rate in the temperature range of 850-500.degree. C. was at
least 20.degree. C./sec.
[0078] A certain length of each steel pipe was cut open and
unrolled at room temperature by cutting the pipe in a longitudinal
direction thereof. A V-notch Charpy test piece having a width of
2.5 mm in accordance with JIS Z 2002 was taken from the opened pipe
in the T direction, and a Charpy impact test was then performed on
the test piece. A tensile test in accordance with the tensile test
method for metallic materials set forth in JIS Z 2241 was carried
out using a Number 11 test piece in accordance with JIS Z 2201,
which was also taken from the opened pipe in the T direction. The
lower temperature limit for guaranteeing a ductile fracture rate of
100% in the Charpy impact test (referred to below as vTrs 100) and
the tensile strength of each pipe are shown in Table 2 and FIG.
1.
[0079] In addition, a bursting test was performed using a pipe
piece having a length of 250 mm, which was cut from each steel
pipe. Both ends of the 250 mm long pipe piece were closed by
welding, and an increasing internal pressure was applied with a
liquid at -40.degree. C. until bursting of the pipe occurred. The
resistance to bursting was evaluated based on the presence or
absence of a crack progressing to either end of the burst pipe. The
results are also shown in Table 2.
1TABLE 1 Steel Steel Composition (mass %) Type C Si Mn P S Cr Mo Cu
Ni Nb Ti sol. Al Mn + 40Ti A 0.11 0.30 1.62 0.015 0.002 0.82 -- --
-- -- 0.042 0.031 3.3* B 0.13 0.31 1.42 0.013 0.002 0.61 0.31 0.31
0.25 0.021 0.031 0.031 2.66* C 0.16 0.31 0.74 0.015 0.002 0.61 0.31
0.30 0.24 0.021 0.011 0.031 1.18 D 0.15 0.30 0.50 0.014 0.002 0.61
0.31 0.31 0.25 0.021 0.007 0.032 0.78 E 0.11 0.30 0.18 0.012 0.002
0.62 -- -- 0.07 0.008 0.005 0.031 0.38* *outside the range
according to this invention
[0080]
2TABLE 2 Steel vTrs 100 Progress of brittle cracks in Type Pipe No.
TS (MPa) (.degree. C.) bursting test at -40.degree. C..sup.1) A a
1085 -15 Yes b 1054 -20 Yes c 1005 -35 Yes B d 930 -40 No e 862 -55
No f 775 -60 No C g 1075 -40 No h 1035 -55 No i 1007 -65 No D j
1102 -80 No k 1054 -100 No l 1008 -110 No E m 800 -65 No n 762 -75
No o 684 -80 No .sup.1)The presence of brittle cracks extending to
either end of the pipe in a bursting test at -40.degree. C.
[0081] As can be seen from Table 2 and FIG. 1, for a steel having a
high Mn content such that (Mn+40Ti) exceeded the range defined by
Equation (2), if it was attempted to make the strength at least
1000 MPa, vTrs100 was higher than -40.degree. C. Therefore, brittle
cracks developed during the bursting test under hydrostatic
pressure at -40.degree. C. with Steel Pipes a, b, and c of Steel A,
indicating that this steel was unsuitable for a steel pipe for an
airbag system. With Steel Pipes d, e, and f of Steel B, if it was
attempted to satisfy vTrs100.ltoreq.-40.degree. C., it was not
possible to obtain a strength of at least 1000 MPa. On the other
hand, for Steel E in which Mn was reduced such that (Mn+40Ti) was
below the range defined by Equation (2), a 20 uniform hardened
structure was not obtained, and even if the tempering temperature
was adjusted, not only was a strength of at least 1000 MPa not
obtained, but the strength level did not even reach that of Steel
Pipes d, e, and f of Steel B.
[0082] A tendency for vTrs 100 to increase as the tensile strength
increases can be seen. For Steel A, the tensile strength exceeded
1000 MPa, but vTrs 100 greatly exceeded -40.degree. C. However, for
Steel Pipes g to l of Steel C and Steel D which had the specific
steel composition according to the present invention and also
satisfied Equations (1) and (2), the strength was at least 1000
MPa, and vTrs was -40.degree. C. or lower.
Example 2
[0083] Steel billets having the chemical compositions shown in
Table 3 were each heated to 1250.degree. C., after which each
billet was pierced and hot rolled with a conventional Mannesmann
piercer-mandrel mill system to obtain a seamless steel pipe with
nominal dimensions of an outer diameter of 70 mm and a wall
thickness of 4.1 mm. Each pipe was then subjected to cold drawing
by a conventional method to provide the pipe with finished
dimensions of an outer diameter of 60.33 mm and a wall thickness of
3.35 mm.
[0084] Steel Numbers 1-22 in Table 3 are steels for which the steel
compositions is satisfy the conditions prescribed by the present
invention, and Steel Numbers 23-27 are comparative steels for which
the contents of some elements do not satisfy the conditions
prescribed for the present invention.
[0085] Each steel pipe which had been subjected to cold drawing was
heated to 920.degree. C. in a conventional walking beam furnace,
and it was held in the furnace for 10 minutes and then water
quenched, after which it was heated at a temperature of lower than
the Ac.sub.1 transformation point of the steel for 30 minutes in a
conventional walking beam furnace to perform tempering.
[0086] For each of the heat treated steel pipes, a tensile test, a
Charpy impact test, and a bursting test were performed.
[0087] The Charpy impact test was carried out using a V-notch
Charpy test piece with a width of 2.5 mm as prescribed by JIS Z
2202 taken in the circumferential direction (the T direction) from
a sample of each steel pipe, which was cut open at room temperature
in the same manner as described in Example 1, and toughness was
evaluated based on vTrs100.
[0088] The tensile test was performed in accordance with the
tensile test method prescribed by JIS Z 2241 using a Number 11 test
piece prescribed by JIS Z 2201 taken in the same manner as for the
Charpy impact test.
[0089] For the internal pressure bursting test, five steel pipe
pieces each having a length of 250 mm were cut from each steel
pipe, both ends of each 250 mm steel pipe piece were closed by
welding, internal pressure was applied with a liquid, and the
extent to which cracks had progressed at the time of bursting at
-40.degree. C. was observed. The resistance to bursting was
evaluated based on the number of pipes for which a crack progressed
to either end among the five pipes which were tested.
[0090] The results of each of the above-described tests are shown
in Table 4.
Example 3
[0091] Seamless steel pipes having steel compositions shown in
Table 3 were manufactured in the same manner as described in
Example 2 except that the conditions for heat treatment were
changed.
[0092] In this example, a steel pipe, which had been finished by
piercing, hot rolling, and cold working as described in Example 2,
was heated to 920.degree. C. using a high frequency induction
heating apparatus at a heat-up rate of about 20.degree. C./sec, and
the induction heating was continued for 5 seconds after the
temperature of the steel pipe reached 920.degree. C. Thereafter,
using the same procedure as in Example 2, the heated steel pipe was
water quenched and tempered at a temperature of lower than the
Ac.sub.1 transformation point of the steel for 30 minutes in a
conventional walking beam furnace.
[0093] For each of the steel pipes, the .gamma. grain size number
of the steel was determined by the Bechet-Beaujard method according
to JIS G0551. The tensile strength and vTrs100 of the steel pipe
were determined in the same manner as described in Example 2. An
internal pressure bursting test was performed in the same manner as
described in Example 1 but at a temperature of -80.degree. C.
rather than -40.degree. C., and the resistance to bursting was
evaluated based on the presence or absence of a crack progressing
to either end of the burst pipe. The results are also shown in
Table 4.
[0094] From Table 4, it can be seen that for Steel Numbers 1-22
which had a composition according to the present invention, even
when the heating for quenching was performed by furnace heating as
in Example 2, the tensile strength was at least 1000 MPa and the
value of vTrs100 in the T direction in the Charpy impact test was
-40.degree. C. or lower. In addition, in the bursting test at
-40.degree. C., cracks did not progress to the ends of the pipes.
When the heating for quenching was rapid heating by high frequency
induction heating with a short period of temperature retention as
in Example 3, the steel had a fine as-quenched microstructure
having a .gamma. grain size number of 11 or larger. The steel pipe
generally had an increased tensile strength, and its toughness was
also further improved as demonstrated by the value of vTrs100,
which was -90.degree. C. or lower. As a result, no crack
progression was found in the bursting test at -80.degree. C.
[0095] When the composition included at least one of Mo, Ni, V, and
B, the hardenability was superior compared to when these elements
were not present, so a uniform hardened-tempered structure was
easily obtained, and there was an excellent balance between
strength and toughness. As a result, the strength could be further
increased without a decrease in toughness.
[0096] When at least one of Cu, Nb, Ca, Mg, and REM was present,
vTrs 100 was lower compared to when these elements were not
present, and the toughness was even better.
[0097] For Steel Number 23, the content of Mn was higher than the
range for the present invention and Equation (2) was not satisfied,
so its toughness decreased, vTrs 100 was -35.degree. C. in the case
of quenching after furnace heating, and the resistance to bursting
decreased.
[0098] For Steel Number 24, the value of (Mn+40Ti) exceeded the
upper limit defined by Equation (2) and the toughness decreased, so
vTrs was -20.degree. C. in the case of quenching after furnace
heating, and the resistance to bursting decreased.
[0099] For Steel Number 25, the value of (Mn+40Ti) was below the
lower limit defined by Equation (2), so even when the tempering
temperature was adjusted, a tensile strength of 1000 MPa could not
be obtained.
[0100] Steel Number 26 had a Cr content which was higher than the
range for the present invention and the toughness of welded
portions was decreased, so vTrs was -20.degree. C. in the case of
quenching after furnace heating, and the resistance to bursting was
decreased.
[0101] For Steel Number 27, the Cr content was lower than the range
for the present invention, and the hardenability was decreased.
Therefore, a non-uniform structure resulted, and even when the
tempering temperature was adjusted, a strength of 1000 MPa could
not be obtained. In addition, the resistance to bursting was not
satisfactory.
[0102] In each of these comparative steels, when quenching was
performed by high frequency induction heating as in Example 3, vTrs
was not -80.degree. C. or lower, so cracks progressed in the
bursting test at -80.degree. C.
[0103] In order to compare the present invention with the prior
art, FIG. 2 shows the relationship between the value of (Mn+40Ti)
in Equation (2) and the tensile strength for examples of the
present invention and examples of the prior art in the
above-mentioned patent documents having the specific steel
composition according to the present invention except for Equation
(2). As can be seen from FIG. 2, when Equation (2) is satisfied,
the strength can be increased to an extremely high level of 1000
MPa.
[0104] Although the present invention has been described with
respect to preferred embodiments, they are merely illustrative and
not intended to limit the present invention. It should be
understood by those skilled in the art that various modifications
of the embodiments described above can be made without departing
from the scope of the present invention as set forth in the
claims.
3 TABLE 3 Chemical Composition (mass %) Mn + Steel No. C Si Mn P S
Cr Ti Al Mo Ni V B Cu Nb Ca Mg REM 40Ti 1 0.15 0.31 0.81 0.008
0.001 0.60 0.008 0.035 -- -- -- -- -- -- -- -- -- 1.13 2 0.15 0.30
0.55 0.015 0.002 0.56 0.001 0.035 0.29 -- -- -- -- -- -- -- -- 0.59
3 0.14 0.31 0.45 0.015 0.002 0.52 0.002 0.029 -- 0.22 -- -- -- --
-- -- -- 0.53 4 0.16 0.29 0.53 0.011 0.003 0.64 0.009 0.032 -- --
0.05 -- -- -- -- -- -- 0.89 5 0.16 0.34 0.41 0.012 0.003 0.61 0.011
0.033 -- -- -- 0.0011 -- -- -- -- -- 0.85 6 0.09 0.31 0.47 0.009
0.002 0.60 0.013 0.033 0.32 0.25 -- -- -- -- -- -- -- 0.99 7 0.12
0.35 0.44 0.011 0.004 0.12 0.012 0.028 0.31 0.24 0.03 -- -- -- --
-- -- 0.92 8 0.15 0.32 0.45 0.008 0.003 0.56 0.011 0.033 0.17 -- --
0.0021 -- -- -- -- -- 0.89 9 0.16 0.35 0.45 0.009 0.003 0.58 0.013
0.025 -- -- -- -- 0.33 -- -- -- -- 0.97 10 0.13 0.27 0.43 0.012
0.003 0.55 0.011 0.022 -- -- -- -- -- 0.018 -- -- -- 0.87 11 0.14
0.33 0.51 0.012 0.003 0.21 0.009 0.035 0.33 -- -- -- 0.31 -- -- --
-- 0.87 12 0.15 0.30 0.50 0.008 0.001 0.60 0.011 0.035 0.32 0.24 --
-- 0.32 0.025 -- -- -- 0.94 13 0.15 0.30 0.51 0.015 0.002 0.62
0.008 0.029 -- -- -- -- -- -- 0.0023 -- -- 0.83 14 0.14 0.32 0.44
0.013 0.002 0.66 0.009 0.032 -- -- -- -- -- -- -- 0.0008 -- 0.80 15
0.16 0.29 0.55 0.012 0.003 0.64 0.011 0.033 -- -- -- -- -- -- -- --
0.0015 0.99 16 0.14 0.33 0.49 0.011 0.003 0.33 0.011 0.022 -- -- --
-- -- -- 0.0015 0.0012 -- 0.93 17 0.16 0.34 0.52 0.012 0.003 0.59
0.013 0.033 0.31 -- -- -- -- -- 0.0021 -- -- 1.04 18 0.09 0.36 0.47
0.009 0.002 0.60 0.012 0.028 0.15 -- 0.05 0.0022 -- 0.018 -- -- --
0.95 19 0.16 0.36 0.51 0.011 0.004 0.60 0.011 0.033 0.29 -- -- --
0.23 0.010 0.0022 -- -- 0.95 20 0.15 0.32 0.45 0.008 0.003 0.56
0.013 0.025 -- -- -- -- 0.25 0.021 0.0021 0.0011 -- 0.97 21 0.16
0.33 0.43 0.009 0.003 0.58 0.011 0.022 0.30 0.25 -- -- 0.30 0.022
0.0023 -- -- 0.87 22 0.14 0.34 0.45 0.015 0.003 0.63 0.013 0.025
0.30 0.31 0.04 -- 0.18 0.008 0.0012 0.0008 0.0007 0.97 23 0.15 0.31
1.51 0.008 0.001 0.60 0.009 0.035 -- -- -- -- 0.31 0.019 -- -- --
1.87.dagger-dbl. 24 0.16 0.30 0.62 0.015 0.002 0.56 0.019 0.035
0.29 -- -- -- -- -- -- -- -- 1.38.dagger-dbl. 25 0.12 0.31 0.23
0.008 0.001 0.56 0.004 0.035 -- -- -- -- -- -- -- -- --
0.39.dagger-dbl. 26 0.15 0.30 0.70 0.015 0.002 1.54.dagger-dbl.
0.011 0.035 -- -- 0.02 -- -- -- -- -- -- 1.14 27 0.15 0.30 0.65
0.015 0.002 --.dagger-dbl. 0.011 0.035 0.29 -- -- -- -- 0.021 -- --
-- 1.09 .dagger-dbl.Outside the range according to this
invention.
[0105]
4 TABLE 4 Example 2: quenching by furnace heating (920.degree. C.
.times. 10 Example 3: quenching by high frequency minutes)
induction heating (920.degree. C. .times. 5 seconds) Results of
.gamma. grain Results of Steel TS vTrs100 bursting test size TS
vTrs100 bursting test No. (MPa) (.degree. C.) at -40.degree.
C..sup.1) number (MPa) (.degree. C.) at -80.degree. C..sup.2)
Remarks 1 1011 -40 0 11.0 1023 -90 No This 2 1011 -40 0 12.0 1050
-100 No Invention 3 1005 -40 0 11.5 1021 -100 No 4 1012 -40 0 12.0
1025 -100 No 5 1008 -45 0 12.0 1026 -100 No 6 1025 -65 0 11.5 1035
-110 No 7 1033 -65 0 12.0 1045 -110 No 8 1015 -45 0 12.0 1021 -100
No 9 1022 -50 0 11.5 1037 -90 No 10 1015 -70 0 12.0 1023 -100 No 11
1053 -70 0 11.5 1017 -110 No 12 1073 -80 0 12.5 1112 -120 No 13
1015 -50 0 11.0 1010 -100 No 14 1013 -45 0 11.0 1012 -90 No 15 1011
-45 0 11.0 1019 -90 No 16 1021 -50 0 11.5 1030 -90 No 17 1053 -50 0
11.5 1070 -90 No 18 1056 -70 0 11.5 1086 -90 No 19 1071 -100 0 12.0
1120 -120 No 20 1087 -80 0 12.5 1134 -110 No 21 1131 -80 0 12.5
1162 -90 No 22 1150 -80 0 12.5 1170 -90 No 23 1023 -15 4 11.5 1058
-60 Yes Comparative 24 1008 -20 3 11.0 1027 -60 Yes Examples 25 840
-40 0 10.5 984 -60 Yes 26 1005 -20 3 12.5 1180 -60 Yes 27 765 -35 1
10.0 954 -70 Yes .sup.1)Number of steel pipes having cracks
extending to either end thereof among 5 pipes tested.
.sup.2)Presence of cracks extending to either end of the steel pipe
tested.
* * * * *