Note: Descriptions are shown in the official language in which they were submitted.
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LOW CARBON ALLOY STEEL TUBE HAVING ULTRA HIGH STRENGTH
AND EXCELLENT TOUGHNESS AT LOW TEMPERATURE AND METHOD OF
MANUFACTURING THE SAME
This PCT application claims the benefit of U.S. Non-Provisional Application
No.
11/395,322, filed April 3, 2006.
BACKGROUND OF THE INVENTION
1. Field of the In>>ention
The present invention relates to low carbon alloy steel tubes having ultra
high strength
and excellent toughness at low temperature and also to a method of
manufacturing
such a steel tube. The steel tube is particularly suitable for making
components for
containers for automotive restraint systems, an example of which is an
automotive
airbag inflator.
In addition, alternative steel compositions in the low carbon, low alloy
category and
different heat treatment processes were developed and tested in order to
decrease the
manufacturing cost.
2. Brief Description of the Prior Art
Japanese Publication No. 10-140249 [Application date Nov. 5, 1996] and
Japanese
Publication No. 10-140283 [Application date Nov. 12, 1996] illustrate in
general
terms steel chemistry considered useful for an automotive airbag inflator.
These
documents
mention as a final condition the absence of heat treatment, a stress
relieving, and a
normalizing or a quenching and tempering. These publications do not mention
the
possibility of just a quenching as a heat treatment step. No mechanical
properties are
mentioned in the claims. In the various examples, only in example #21 is the
steel
quenched and tempered, but the reported UTS is only 686 MPa (99 ksi). Even the
highest stated mechanical properties, in example #26, are relatively low, with
a
maximum UTS of 863 MPa (125 ksi) . Hence, these publications relate to grades
which are relatively low (the intended target is 590 MPa (86 ksi). In addition
these
publications show ductility at low temperature with a flattening drop-weight
(DW)
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type test at -40 C. The currently accepted test for demonstrating ductility at
low
temperature is the burst test, which is more efficient in showing brittleness.
It is
believed that most of the examples shown in these documents that are alleged
to be
ductile after a DW test, would in fact not show ductile behavior at low
temperature in
a burst test and, therefore, would not qualify for certain airbag inflator
applications
due to a lack of compliance with governmental regulations (e.g. US DOT).
Japanese Publication No. 2001-49343 [Application date Oct. 8, 1999] is said to
address only steels for use in making electric-resistance-welded tubes (the
ERW
process). The claims specify various aspects of the ERW process and an
optional heat
treatinent for a normalizing or quench and temper, an optional ulterior cold
drawing,
an optional ulterior heat treatment (normalizing or quench and temper). This
document addresses only two different, very general steel chemistry, one being
a low
carbon steel, the other noting common limits in various alloying elements.
This
document does not suggest the possibility of just a quenching heat treatment.
Various
examples are given for a quench and temper material, but mechanical properties
obtained are relatively low. The maximum result achieved is 852 MPa (123 ksi)
in
the quench and temper test #18.
It is believed that the steel "chemistry" put forth by Sumitomo in each of JP
10-
140249 JP 10-140283; JP 2001-49343; as well as the chemistry later identified
in
Kondo et al., US 6878219 B2, or the continuation published as US 2005/ 0039826
Al, actually define steels with such broad ranges so as to include SAE 1010
general
purpose steel as made and sold in the US since long prior to 1990. Applicants
are
aware that for several years a SAE 1010 steel grade manufactured with modem
technologies normally guarantees that a P amount will be below 0.025 and an S
amount will be below 0.01 as described in the mentioned application.
Additional documents illustrating the state of the prior art in steels for air
bag
applications include Erike, US 6386583 B2 and various published continuations
thereof, including US 2004/0074570 Al and US 2005/0061404 Al. These documents
do not suggest any advantage as taught herein from an extremely rapid
induction
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austenitizing and an ulterior ultra fast water quenching, let alone using just
such a
rapid quench and not thereafter using a tempering step. In addition JP 10-
140283
discloses overlapping chemistry with US 6878219 B2, with only a slightly lower
maximum for P (0.02) and a slightly higher maximum for S (0.02). While Patent
Publication US20020033591A1 broadly suggests the possibility of quenching
without
tempering, claims 6 and 7 do not mention the necessity of quenching in order
to
achieve the mechanical properties claimed and instead these claims require at
least
two heat treatments.
Airbag inflators for vehicle occupant restraint systems are required to meet
strict
structural and functional standards. Therefore, strict procedures and
tolerances are
imposed on the manufacturing process. While field experience indicates that
the
industry has been successful in meeting past structural and functional
standards,
improved and/or new properties are necessary to satisfy the evolving
requirements,
while at the same time a continuous reduction in the manufacturing costs is
also
important.
Airbags or supplemental restraint systems are an important safety feature in
many of
today's vehicles. In the past, air bag systems were of the type employing
explosive
chemicals, but they are expensive, and due to environmental and recycling
problems,
in recent years, a new type of inflator has been developed using an
accumulator made
of a steel tube filled with argon gas or the like, and this type is
increasingly being
used.
The above-mentioned accumulator is a container which at normal times maintains
the
gas or the like at a high pressure which is blown into an airbag at the time
of the
collision of an automobile, in a single or multiple stage burst. Accordingly,
a steel
tube used as such an accumulator is to receive a stress at a high strain rate
in an
extremely short period of time. Therefore, compared with a simple structure
such as
an ordinary pressure cylinder, the above-described steel tube is required to
have
superior dimensional accuracy, excellent workability, and weldability, and
above all
must have high strength, toughness, and excellent resistance to bursting.
Dimensional
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accuracy also is important to ensure a very precise volume of gas will blow
into the
airbag.
Cold forming properties are very important in tubular members used to
manufacture
acciunulators since they are formed to final shape after the tube is
manufactured.
Different shapes depending on the vessel configuration shall be obtained by
cold
forming. It is crucial to obtain pressure vessels without cracks and
superficial defects
after cold forming. Moreover, it is also vital to have very good toughness
even at low
temperatures after cold forming.
The steels disclosed herein have very good weldability, and do not require,
for air bag
accumulator applications, either a preheating prior to welding, or a post weld
heat
treatment. The carbon equivalent, as defined by the formula,
Ceq=% C+% M11/6+(% Cr+% Mo+% V)/5+(% Ni+% Cu)/15
should be less than about 0.63% in order to obtain the required weldability.
As Ceq
diminishes, weldability improves. In the preferred embodiment of this
invention, the
carbon equivalent as defined above should be less than about 0.60%, preferably
less
than about 0.56%, and most preferably less than about 0.52%, or even less than
about
0.48%, in order to better guarantee weldability.
To produce a gas container, a cold-drawn tube made according to the present
invention is cut to length and then cold formed using different known
technologies
(such as crimping, swaging, or the like) in order to obtain the desired shape.
Alternatively, a welded tube could be used. Subsequently, to produce the
accumulator, an end cap and a diffuser are welded to each end of the container
by any
suitable technology such as friction welding, gas tungsten arc welding or
laser
welding. These welds are highly critical and as such require considerable
labor, and in
certain instances testing to assure weld integrity throughout the pressure
vessel and
airbag deployment. It has been observed that these welds can crack or fail,
thus,
risking the integrity of the accumulator, and possibly the operation of the
airbag.
The inflators are tested to assure that they retain their structural integrity
during airbag
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deployment. One of such tests is the so-called burst test. This is a
destructive-type test
in which a canister is subjected to internal pressures significantly higher
than those
expected during normal operational use, i.e., airbag deployment. In this test,
the
inflator is subjected to increasing internal pressures until rupture occurs.
In reviewing the burst test results and studying the test canister specimens
from these
tests, it has been found that fracture occurs through different alternative
ways: ductile
fracture, brittle fracture, and sometimes a combination of these two modes. It
has been
observed that in ductile fracture an outtumed rupture exemplified by an opened
bulge
(such as would be exhibited by a bursting bubble) occurs. The ruptured surface
is
inclined approximately 45 degrees with respect to the tube outer surface, and
is
localized within a subject area. In a brittle fracture, on the other hand, a
non-arresting
longitudinal crack along the length of the inflator is exhibited, which is
indicative of a
brittle zone in the material. In this case, the fracture surface is normal to
the tube outer
surface. These two modes of fracture have distinctive surfaces when observed
under a
scanning electron microscope--dimples are characteristic of ductile fracture,
while
cleavage is an indication of brittleness.
At times, a combination of these two fracture modes can be observed, and
brittle
cracks can propagate from the ductile, ruptured area. Because the whole
system,
including the airbag inflator, may be utilized in vehicles operating in very
different
climates, it is crucial that the material exhibits ductile behavior over a
wide
temperature range, from very cold up to warm temperatures.
SUMMARY OF THE INVENTION
First, the present invention first relates to certain novel low carbon alloy
steels
suitable for cold forming having more than high tensile strength (UTS 145 ksi
minimum) and preferably ultra high tensile strength (UTS 160 ksi minimum and
possibly 175 ksi or 220 ksi), and, consequently, a very high burst pressure.
Moreover,
the steel has excellent toughness at low temperature, with guaranteed ductile
behavior
at -60 C., i.e., a ductile-to-brittle transition temperature (DBTT) below -60
C., and
possibly as low as -100 C.
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Second, the present invention also relates to a process of manufacturing such
a steel
tube which essentially comprises a novel rapid induction austenizing/high
speed
quench/no temper technique. In a preferred method, there is an extremely rapid
induction austenizing with an ultra fast water quenching step that eliminates
any
tempering step, so as to create a low carbon alloy steel tube that also is
suitable for
cold forming having ultra high tensile strength (LTTS 145 ksi minimum and up
to220
ksi), and, consequently, a very high burst pressure. Moreover, the steel has
excellent
toughness at low temperature, with guaranteed ductile behavior at -60 C,
i.e., a
ductile-to-brittle transition temperature (DBTT) that is below -60 C, and
possibly
even as low as -100 C.
The material of the present invention has particular utility in components for
containers for automotive restraint system components, an example of which is
an
automotive airbag inflator. The chemistry used to create each of the steels
disclosed
herein is novel, hereafter will be identified as Steel A, Steel B, Steel C,
Steel D and
Steel E, with the compositions for each being summarized in the following
Table I:
Steel C Mn S P Cr Mo Ni V
A 0.10 1.23 0.002 0.008 0.11 0.05 0.34 0.002
B 0.10 1.09 0.001 0.011 0.68 0.41 0.03 0.038
C 0.11 1.16 0.001 0.010 0.64 0.47 0.03 0.053
D 0.11 1.07 0.002 0.008 0.06 0.04 0.03 0.083
E 0.10 0.47 0.001 0.011 0.04 0.02 0.05 0.001
Steel Ti Si Cu Al Carbon.eg
A 0.023 0.27 0.24 0.035 0.38
B 0.025 0.28 0.22 0.035 0.52
C 0.026 0.25 0.22 0.028 0.55
D 0.001 0.08 0.06 0.033 0.33
E 0.002 0.19 0.07 0.027 0.20
Test results using each of these steels in a novel rapid induction
austenizing/high
speed quench/no temper technique revealed surprising and differing results,
among
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the five steel compositions, as summarized in the following Table II:
Steel Yield UTS Elong. Hardness Flatten Burst
(MPa) (ksi) (MPa) ksi (%) (HRC (DOT) -60 C -100 C
A 920 133 1230 178 22 42 OK ductile ductile
B 940 136 1217 176 22 41 OK ductile N/A
C 997 144 1260 183 20 42 OK ductile N/A
D 781 113 1184 172 19 32 OK ductile N/A
E 552 80 827 120 26 17 OK ductile N/A
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention are described in detail below, by
example
only, with reference to the accompanying drawings, wherein:
FIG. I is a core microstructure for a high speed quench on Steel E;
FIG. II shows burst tests at -60 C for a high speed quench on Steel E.
FIG. III shows microstructure for a normal quench on Steel E;
Figure IV shows a high speed quench core microstructure on Steel D;
Figure V shows burst test at -60 C for a high speed quench on Steel D.
Figure VI sllows micro-structure for a normal quench on Steel D
DESCRIPTION OF THE PREFERRED EMBODIMENTS
While the present invention is susceptible of embodiment in various forms, it
will
hereinafter be described a presently preferred embodiment with the
understanding that
the present disclosure is to be considered an exemplification of the invention
and is
not intended to limit the invention to the specific embodiment illustrated.
The present invention relates to steel tubing to be used for stored gas
inflator pressure
vessels. More particularly, the present invention relates to a low carbon
ultra high
strength steel grade for seamless pressure vessel applications with guaranteed
ductile
behavior at -60 C., i.e., a ductile-to-brittle transition temperature below -
60 C, and
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possibly even as low as -100 .
More particularly, the present invention relates to a chemical composition and
a
manufacturing process to obtain a seamless steel tubing to be used to
manufacture an
inflator.
A schematic illustration of a method of producing the seamless low carbon
ultra high
strength steel could be as follows:
1. Steel making
2. Steel casting
3. Tube hot rolling
4. Hot-rolled hollow finishing operations
5. Cold drawing
6. Austenizing with Quenching (without tempering)
7. Cold-drawn tube finishing operations
One of the main objectives of the steel-making process is to refine the iron
by
removal of carbon, silicon, sulfur, phosphorous, and manganese. In particular,
sulfur
and phosphorous are prejudicial for the steel because they worsen the
mechanical
properties of the material. Ladle metallurgy is used before or after basic
processing to
perform specific purification steps that allow faster processing in the basic
steel
making operation.
The steel-making process is performed under an extreme clean practice in order
to
obtain a very low sulfur and phosphorous content, which in turn is crucial for
obtaining the high toughness required by the product. Accordingly, the
objective of an
inclusion level of 2 or less --thin series--, and level 1 or less --heavy
series--, under
the guidelines of ASTM E45 Standard-Worst Field Method (Method A) has been
imposed. In the preferred embodiment of this invention, the maximum
microinclusion
content as measured according to the above-mentioned Standard should be:
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Inclusion Thin Heavy
Type
A 0.5 0
B 1.5 1.0
C 0 0
D 1.5 0.5
Furthermore, the extreme clean practice allows obtaining oversize inclusion
content
with 30 m or less in size. These inclusion contents are obtained limiting the
total
oxygen content to 20 ppm.
Extreme clean practice in secondary metallurgy is performed by bubbling inert
gases
in the ladle furnace to force the inclusion and impurities to float. The
production of a
fluid slag capable of absorbing impurities and inclusions, and the inclusions'
size and
shape modification by the addition of SiCa to the liquid steel, produce high
quality
steel with low inclusion content.
EXAMPLES USING LOW CARBON, ALLOY STEELS
The chemical composition of the obtained steel shall be as follows, in each
case"%"
means "mass percent":
Carbon (C)
C is an element that inexpensively raises the strength of the steel, but if
its content is
less than 0.06% it is difficult to obtain the desired strength. On the other
hand, if the
steel has a C content greater than 0.18%, then cold workability, weldability,
and
toughness decrease. Therefore, the C content range is 0.06% to 0.18%. A
preferred
range for the C content is 0.07% to 0.12%, and an even more preferred range is
0.10
to 0.12%.
Manganese (Mn)
Mn is an element which is effective in increasing the hardenability of the
steel, and
therefore it increases strength and toughness. If it content is less than 0.3%
it is
difficult to obtain the desired strength, whereas if it exceeds 1.5%, then
banding
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structures become marked, and toughness decreases. Accordingly, the Mn content
is
0.3% to 1.5%, with a preferred Mn range of 0.60 to 1.40%.
Silicon (Si)
Si is an element which has a deoxidizing effect during steel making process
and also
raises the strength of the steel. If Si content is less than 0.05%, the steel
is susceptible
to oxidation, on the other hand if it exceeds 0.50%, then both toughness and
worlcability decrease. Therefore, the Si content is 0.05% to 0.5%., and a
preferred Si
range of 0.05% to 0.40%.
Sulfur (S)
S is an element that causes the toughness of the steel to decrease.
Accordingly, the S
content is limited to 0.015 % maximum. A preferred maximum value is 0.010%
Phosphorous (P)
P is an element that causes the toughness of the steel to decrease.
Accordingly, the P
content is limited to 0.025% maximum. A preferred maximum value is 0.02%,
Nickel (Ni)
Ni is an element that increases the strength and toughness of the steel, but
it is very
costly, therefore for cost reasons Ni is limited to 0.70% maximum. A preferred
maximum value is 0.50%.
Chromium (Cr)
Cr is an element which is effective in increasing the strength, toughness, and
corrosion resistance of the steel. If it exceeds 1% the toughness at the
welding zones
decreases markedly. Accordingly, the Cr content is limited to 1.0% maximum,
and a
preferred Cr maximum content is 0.80%,
Molybdenum (Mo)
Mo is an element which is effective in increasing the strength of the steel
and
contributes to retard the softening during tempering, but it is very costly.
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Accordingly, the Mo content is limited to 0.7% maximum, and a preferred Mo
maximum content is 0.50%
Vanadium (V)
V is an element which is effective in increasing the strength of the steel,
even if added
in small amounts, and allows to retard the softening during tempering.
However, this
ferroalloy is expensive, forcing the necessity to lower the maximum content.
Therefore, V is limited to 0.3% maximum, with a preferred maximum of 0.20%
Preferred ranges for other elements not listed above are as follows:
Element Weight %
Aluminum 0.10% max
Niobium 0.06% max
Sn 0.05% max
Sb 0.05% max
Pb 0.05% max
As 0.05% max
Residual elements in a single ladle of steel used to produce tubing or
chambers shall
be:
Sn+Sb+Pb+As <_ 0.15% max, and
S+P < 0.025
The next step is the steel casting to produce a solid steel bar capable of
being pierced
and rolled to form a seamless steel tube. The steel is cast in the steel shop
into a round
solid billet, having a uniform diameter along the steel axis.
The solid cylindrical billet of ultra high clean steel is heated to a
temperature of about
1200 C. to 1300 C., and at this point undergoes the rolling mill process.
Preferably,
the billet is heated to a temperature of about 1250 C., and then passed
through the
rolling mill. The billet is pierced, preferably utilizing the known Manessmann
process, and subsequently the outside diameter and wall thickness are
substantially
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reduced while the length is substantially increased during hot rolling. For
example, a
148 mm outside diameter solid bar is hot rolled into a 48.3 mm outside
diameter hot-
rolled tube, with a wall thickness of 3.25 mm.
The cross-sectional area reduction, measured as the ratio of the cross-
sectional area of
the solid billet to the cross-sectional area of the hot-rolled tube, is
important in order
to obtain a refined microstructure, necessary to get the desired mechanical
properties.
Therefore, the minimum cross-sectional area reduction is about 15:1, with
preferred
and most preferred minimum cross-sectional area reductions of about 20:1 and
about
25:1, respectively.
The seamless hot-rolled tube of ultra high clean steel so manufactured is
cooled to
room temperature. The seamless hot-rolled tube of ultra high clean steel so
manufactured has an approximately uniform wall thickness, both
circumferentially
around the tube and longitudinally along the tube axis.
The hot-rolled tube is then passed through different finishing steps, for
example cut in
length into 2 to 4 pieces, and its ends cropped, straightened at known rotary
straightening equipment if necessary, and non-destructively tested by one or
more of
the different known techniques, like electromagnetic testing or ultrasound
testing.
The surface of each piece of hot-rolled tube is then properly conditioned for
cold
drawing. This conditioning includes pickling by immersion in acid solution,
and
applying an appropriate layer of lubricants, like the known zinc phosphate and
sodium
estearathe combination, or reactive oil. After surface conditioning, the
seamless tube
is cold drawn, pulling it through an external die that has a diameter smaller
than the
outside diameter of the tube being drawn. In most cases, the internal surface
of the
tube is also supported by an internal mandrel anchored to one end of a rod, so
that the
mandrel remains close to the die during drawing. This drawing operation is
performed
without the necessity of previously heating the tube above room temperature.
The seamless tube is so cold drawn at least once, each pass reducing both the
outside
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diameter and the wall thickness of the tube. The cold-drawn steel tube so
manufactured has a uniform outside diameter along the tube axis, and a uniform
wall
thickness both circumferentially around the tube and longitudinally along the
tube
axis. The so cold-drawn tube has an outside diameter preferably between 10 and
70
mm, and a wall thickness preferably from 1 to 4 mm.
The cold-drawn tube is then heat treated in an austenizing furnace at a
temperature of
at least the upper austenizing temperature, or Ac3 (which, for the specific
chemistry
disclosed herein, is about 880 C.), but preferably above about 920 C. and
below
about 1050 C. This maximum austenizing temperature is imposed in order to
avoid
grain coarsening. This process can be performed either in a fuel furnace or in
an
induction-type furnace, but preferably in the latter. The transit time in the
furnace is
strongly dependent on the type of furnace utilized. It has been found that the
high
surface quality required by this application is better obtained if an
induction type
furnace is utilized. This is due to the nature of the induction process, in
which very
short transit times are involved, precluding oxidation to occur. Preferably,
the
austenizing heating rate is at least about 100 C. per second, but more
preferably at
least about 200 C. per second. The extremely high heating rate and, as a
consequence, very low heating times, are important for obtaining a very fine
grain
microstructure, which in turn guarantees the required mechanical properties.
Furthermore, an appropriate filling factor, defined as the ratio of the round
area
defined by the outer diameter of the tube to the round area defined by the
coil inside
diameter of the induction furnace, is important for obtaining the required
high heating
rates. The minimum filling factor is about 0.16, and a preferred minimum
filling
factor is about 0.36.
At or close to the exit zone of the furnace the tube is quenched by means of
an
appropriate quenching fluid. The quenching fluid is preferably water or water-
based
quenching solution. The tube temperature drops rapidly to ambient temperature,
preferably at a rate of at least about 100 C. per second, more preferably at
a rate of at
least about 200 C. per second. This extremely high cooling rate is crucial
for
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obtaining a complete microstructure transformation.
In a technique where a tempering step is employed, the steel tube is then
tempered
with an appropriate temperature and cycle time, at a temperature below Acl.
Preferably, the tempering temperature is between about 400-600 C., and more
preferably between about 450-550 C. Alternatively, the tempering temperature
may
be between 200 C to 600 C and more preferably between 250 C to 550 C. The
soaking time shall be long enough to guarantee a very good temperature
homogeneity,
but if it is too long, the desired mechanical properties are not obtained.
This tempering
step is performed preferably in a protective reducing or neutral atmosphere to
avoid
decarburizing and/or oxidation of the tube.
In a preferred method, the tempering step is eliminated and only a high speed
quench
using water or water based solutions , as described above, is employed.
In order to achieve a high speed quench, the following equipment is preferred,
but not
required.
A Quenching line with a full capacity of 2200 kg per hour, follows an
induction
furnace with a maximum power of inductor settled at 500 Kw. A head quencher
employs 421ines with 12 nozzles on each line. Water quenching flow is adjusted
into
a range of 10 to 60 m3 per hour, and the advance speed of the tube is
controlled from
to 25 meters per minute. Additionally, following pinch rollers are set up to
produce
a rotation over the tube.
The ultra high strength steel tube so manufactured is passed through different
finishing steps, straightened at known rotary straightening equipment, and non-
destructively tested by one or more of the different known techniques.
Preferably, for
this kind of applications tubes should be tested by means of both known
ultrasound
and electromagnetic techniques.
The tubing after heat treatment can be chemically processed to obtain a tube
with a
desirable appearance and very low surface roughness. For example, the tube
could be
pickled in a sulfuric acid and hydrochloric acid solution, phosphated using
zinc
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phosphate, and oiled using a petroleum-based oil, a water-based oil, or a
mineral oil.
A steel tube obtained by the first or second described methods have the
following
minimum mechanical properties:
Yield Strength about 110 ksi (758 MPa) minimum
Tensile Strength about 145 ksi (1000 MPa) minimum
Elongation about 9% minimum
The yield strength, tensile strength, and elongation are to be performed
according to
the procedures described in the Standards ASTM E8. For the tensile test, a
full size
specimen for evaluating the whole tubular section is preferred.
Flattening testing shall conform to the requirements of Specification DOT 39
of 49
CFR, Paragraph 178.65. Therefore, a tube section shall not crack when
flattened with
a 60 degree angled V-shaped tooling, until the opposite sides are 6 times the
tube wall
thickness apart. This test is fully met by the steel developed.
In order to obtain a good balance between strength and touglmess, the prior
(sometimes referred to as former) austenitic grain size shall be preferably 7
or finer,
and more preferably 9 or finer, as measured according to ASTM E-1 12 Standard.
This
is accomplished thanks to the extremely short heating cycle during
austenitizing.
The steel tube obtained by the described method shall have the stated
properties in
order to comply with the requirements stated for the invention.
The demand of the industry is continuously pushing roughness requirements to
lower
values. The present invention has a good visual appearance, with, for example,
a
surface finish of the finished tubing of 3.2 microns maximum, both at the
external and
internal surfaces. This requirement is obtained through cold drawing, short
austenizing times, reducing or neutral atmosphere tempering, and an adequate
surface
chemical conditioning at different steps of the process.
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A hydroburst pressure test shall be performed by sealing the ends of the tube
section,
for example, by welding flat steel plates to the ends of the tube. It is
important that a
300 mm tube section remains constraint free so that full hoop stress can
develop. The
pressurization of the tube section shall be performed by pumping oil, water,
alcohol or
a mixture of them.
The burst test pressure requirement depends on the tube size. When burst
tested, the
ultra high strength steel seamless tube has a guaranteed ductile behavior at -
60 C.
Tests performed on the samples produced show that this grade has a guaranteed
ductile behavior at -60 C., with a ductile-to-brittle transition temperature
below -60
oc.
The inventors have found that a far more representative validation test is the
burst
test, performed both at ambient and at low temperature, instead of Charpy
impact test
(according to ASTM E23). This is due to the fact that relatively thin wall
thicknesses
and small outside diameter in these products are employed, therefore no
standard
ASTM specimen for Charpy impact test can be machined from the tube in the
transverse direction. Moreover, in order to get this subsize Charpy impact
probe, a
flattening deformation has to be applied to a curved tube probe. This has a
sensible
effect on the steel mechanical properties, in particular the impact strength.
Therefore,
no representative impact test is obtained with this procedure.
EXAMPLES USING ALTERNATNE, LOW CARBON, LOW ALLOY STEELS
Applicants have discovered that a high speed quench without a temper is a
critical
aspect of the present invention. Steels which are lower alloy and less
expensive than
prior art chemistries when treated by a particular heating and high speed
quench can
meet or exceed the standards discussed hereinbefore.
CA 02650452 2008-10-24
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17
The above defined, novel Steels A, B, C, D and E are alternative steels that
were
analyzed using the preferred method, wherein a very fast induction furnace
austenizing with a high speed quench was used instead of adding a tempering
step.
Surprisingly, when control testing was done with certain of these novel steels
wherein less than a high speed quench, i.e, a normal quenching process was
employed or a tempering step, as described hereinbefore, was employed, the
tests
showed significantly poorer characteristics.
High Speed Quench And No Temper Process With Alternative Including Lower
Cost Steels According To The Preferred Method
The parameters used for high speed quench tests on Steel E samples were as
follows:
Water flow of 40 m3/hr ; Speed advance tube of 20 m/min. ; Inductor power of
80 %
Austenitizing temperature: 880 - 940 , aim 920 ; Martensite transformation
on OD
surface and core material was observed.
Figure 1 shows core material with 100 % Martensite transformation for Steel E.
Steel E, which has chemistry similar to a low alloy SAE 1010 grade steel, did
not
achieved minimum expected values. when subjected to high speed quenching.
Test results were as follows:
Sample YS YS % UTS UTS (Psi)
(Mpa) (Psi) Elo (Mpa)
20476 561 81414 26 835 121140
20477 570 82680 32 827 119988
20478 538 78086 32 802 116446
20479 552 80177 32 831 120613
Likewise, burst tests at low temperature (- 60 C) were performed in order to
observe
the behavior and type of crack. Figure II shows tested burst samples for Steel
E. Both
presented a ductile behavior.
A control test on Steel E involved a normal quenching process was performed,
results
CA 02650452 2008-10-24
WO 2007/113642 PCT/IB2007/000850
18
as follows:
Sample ySa Ps % Elo 'UTS UTS (Psi)
20480 478 69367 28 721 104683
20481 469 68059 32 713 103531
20482 497 72226 32 714 103574
20483 478 69367 32 703 102009
Figure III presents the core structures for Steel E using normal quenching
process.
Some ferrite structure is observed along the wall thickness.
Steel D was discovered to be very promising because of the high performance to
cost
value it presented. Steel D was selected to manufacture tubing according to
the
preferred method. Measured chemical composition of samples of Steel D that
were
used for high speed quench tests were as follows:
Element % Value
C 0.11
Mn 1.07
S 0.002
P 0.008
Si 0.08
V 0.08
Al 0.03
Nb 0.008
The parameters used for the high speed quench tests on samples of Steel D were
as
follows:
Quenching process was conducted controlling austenite temperature into 920 -
940
C. Water flow of 40 m3/hr
Speed advance tube of 10m/min.
Inductor power of 62 % total capacity (500 Kw)
A rotation over the tube was given with an angle of pinch rolls of 17
Test results for high speed quenched on samples of Steel D, were as follows:
CA 02650452 2008-10-24
WO 2007/113642 PCT/IB2007/000850
19
Sample YS YS % UTS UTS (Psi)
(Mpa) si Elo (Mpa)
19605 860 124810 20 1209 175388
19606 781 113360 19 1184 171860
Figure IV shows that a high speed quench Steel D microstructure that presents
Martensite at 100% and a completely quenched transformation.
Likewise, burst tests at low temperature (- 60 C) were performed in order to
observe
the behavior and type of crack. Figure V shows tested burst samples for Steel
D. Both
presented a ductile behavior.
A control test on Steel D involving a normal quenching process was performed,
results as follows:
UTS (Psi)
Sample Y a ps1 % Elo UTS (Mpa)
19609 618 89635 24 861 124952
19610 586 85060 24 882 127967
Figure VI presents the core structures for Steel D using normal quenching
process.
Steel B was selected to manufacture tubing according to the preferred method.
Measured chemical composition of samples of Steel B that were used for higli
speed
quench tests were as follows:
Element % Value
C 0.10
Mn 1.09
S 0.001
P 0.011
Si 0.28
V 0.038
Al 0.035
Cr 0.68
Mo 0.41
Nb 0.005
The parameters used for the high speed quench tests on samples of Steel B were
as
CA 02650452 2008-10-24
WO 2007/113642 PCT/IB2007/000850
follows:
Quenching process was conducted controlling austenite temperature into 920 -
940
C. Water flow of 40 m3/hr
Speed advance tube of 10m/min.
Inductor power of 70 % total capacity (500 Kw)
A rotation over the tube was given with an angle of pinch rolls of 17
Test results for high speed quenched on samples of Steel B, were as follows:
Sample YS YS % UTS UTS (Psi)
a (Psi) Elo (Mpa)
25222 940 136 22 1217 176
25002 914 132 24 1206 175
Likewise, burst tests at low temperature (- 60 C) were performed on Steel B
in order
to observe the behavior and type of crack.,. both presented a ductile
behavior.
Steel A was selected to manufacture tubing according to the preferred method.
Measured chemical composition of samples of Steel A that were used for high
speed
quench tests were as follows:
Element % Value
C 0.10
Mn 1.23
S 0.002
P 0.008
Si 0.27
V 0.002
Al 0.035
Cr 0.11
Mo 0.05
Ni 0.34
The parameters used for the high speed quench tests on samples of Steel A were
as
follows:
Quenching process was conducted controlling austenite temperature into 920 -
940
CA 02650452 2008-10-24
WO 2007/113642 PCT/IB2007/000850
21
C. Water flow of 50 m3/hr
Speed advance tube of 20m/min.
Inductor power of 90 % total capacity (500 Kw)
A rotation over the tube was given with an angle of pinch rolls of 17
Test results for high speed quenched on samples of Steel A, were as follows:
Sample YS YS % UTS UTS (Psi)
a (Psi) Elo (Mpa)
20313 920 133 22 1230 178
21442 883 128 20 1195 173
Likewise, burst tests at low temperature (- 60 C and -100 C) were performed
on
Steel A in order to observe the behavior and type of crack.,. both presented a
ductile
behavior.
Control Tests With A High Quench Followed By A Temper Process With
Alternative Lower Cost Steels
Once samples of the preferred Steel D were found to yield surprising
mechanical
values upon using a high speed quenching according to the preferred method, a
tempering then was performed in order to determine the effect of adding a
temper
upon the mechanical properties.
A tempering heat treatment was conducted at 580 C for total time of 15
minutes. The
UTS average was 116 Ksi (805 MPa), which do not meet the expected values
While preferred embodiments of our invention have been shown and described in
order to comply with the description and enablement requirements of 35 USC
112,
it is to be understood that the scope of the invention is not limited to any
embodiment
that has been described, but solely is to be defined by the scope of the
appended
claims.
