LOW CARBON ALLOY STEEL TUBE HAVING ULTRA HIGH STRENGTH AND EXCELLENT TOUGHNESS AT LOW TEMPERATURE AND METHOD OF MANUFACTURING THE SAME

TUBE EN ALLIAGE D'ACIER A FAIBLE TENEUR EN CARBONE PRESENTANT UNE RESISTANCE TRES ELEVEE ET UNE TENACITE EXCELLENTE A BASSE TEMPERATURE, ET SON PROCEDE DE FABRICATION

English Abstract

A low carbon alloy steel tube and a method of manufacturing the same, in which
the steel tube consists essentially of, by weight: about 0.06% to about 0.18%
carbon; about 0.5% to about 1.5% manganese; about 0.1% to about 0.5% silicon;
up to about 0.015% sulfur; up to about 0.025% phosphorous; up to about 0.50%
nickel; about 0.1% to about 1.0% chromium; about 0.1% to about 1.0%
molybdenum; about 0.01% to about 0.10% vanadium; about 0.01% to about 0.10%
titanium; about 0.05% to about 0.35% copper; about 0.010% to about 0.050%
aluminum; up to about 0.05% niobium; up to about 0.15% residual elements; and
the balance iron and incidental impurities. The steel has a tensile strength
of at least about 145 ksi and exhibits ductile behavior at temperatures as low
as -60 ~C.

French Abstract

Tube en alliage d'acier à faible teneur en carbone et son procédé de fabrication. Ce tube en acier est essentiellement constitué de : environ 0,06 à 0,18 % en poids de carbone; environ 0,5 à 1,5 % en poids de manganèse; environ 0,1 à 0,5 % en poids de silicium; jusqu'à environ 0,015 % en poids de soufre ; jusqu'à environ 0,025 % en poids de phosphore; jusqu'à environ 0,50 % en poids de nickel; environ 0,1 à 1,0 % en poids de chrome; environ 0,1 à 1,0 % en poids de molybdène; environ 0,01 à 0,10 % en poids de vanadium; environ 0,01 à 0,10 % en poids de titane; environ 0,05 à 0,35 % en poids de cuivre; environ 0,010 à environ 0,050 % en poids d'aluminium; jusqu'à environ 0,05 % en poids de niobium ; jusqu'à environ 0,15 % en poids d'éléments résiduels; le solde étant composé de fer et des impuretés inévitables. Cet acier présente une résistance à la traction d'au moins 145 ksi environ, et présente un degré de ductilité à des températures aussi basses que -60 ·C.

Claims

11
We claim:
1. A low carbon alloy steel tube consisting essentially of, by weight: about
0.06% to
about 0.18% carbon; about 0.5% to about 1.5% manganese; about 0.1% to about
0.5%
silicon; up to about 0.015% sulfur; up to about 0.025% phosphorous; up to
about 0.50%
nickel; about 0.1% to about 1.0% chromium; about 0.1% to about 1.0%
molybdenum: about
0.01% to about 0.10% vanadium; about 0.01% to about 0.10% titanium: about
0.05% to
about 0.35% copper; about 0.010% to about 0.050% aluminum; up to about 0.05%
niobium;
up to about 0.15% residual elements; and the balance iron and incidental
impurities, wherein
the steel tube has a tensile strength of at least about 145 ksi and has a
ductile-to-brittle
transition temperature below-60°C.
2. The low carbon alloy steel tube of claim 1, wherein the steel tube consists
essentially
of, by weight: about 0.07% to about 0.12% carbon; about 1.00% to about 1.40%
manganese:
about 0.15% to about 0.35% silicon; up to about 0.010% sulfur; up to about
0.015%
phosphorous; up to about 0.20% nickel: about 0.55% to about 0.80% chromium;
about 0.30%
to about 0.50% molybdenum; about 0.01% to about 0.07% vanadium: about 0.01% to
about
0.05% titanium; about 0.15% to about 0.30% copper; about 0.010% to about
0.050%
aluminum; up to about 0.05% niobium; up to about 0.15% residual elements; and
the balance
iron and incidental impurities.
3. The low carbon alloy steel tube of claim 1, wherein the steel tube consists
essentially
of, by weight: about 0.08% to about 0.11% carbon; about 1.03% to about 1.18%
manganese;
about 0.15% to about 0.35% silicon: up to about 0.003% sulfur : up to about
0.012%
phosphorous; up to about 0.10% nickel; about 0.63% to about 0.73% chromium:
about 0.40%
to about 0.45% molybdenum; about 0.03% to about 0.05% vanadium; about 0.025%
to about
0.035% titanium; about 0.15% to about 0.30% copper; about 0.010% to about
0.050%
aluminum; up to about 0.05% niobium; up to about 0.15% residual elements; and
the balance
iron and incidental impurities.
4. The low carbon alloy steel tube of claim 1, wherein the steel tube has a
yield strength
of at least about 125 ksi.

12
5. The low carbon alloy steel tube of claim 1, wherein the steel tube has a
yield strength
of at least about 135 ksi
6. The low carbon alloy steel tube of claim 1, wherein the steel tube has an
elongation at
break of at least about 9%.
7. The low carbon alloy steel tube of claim 1, wherein the steel tube has a
hardness of no
more than about 40 HRC.
8. The low carbon alloy steel tube of claim 1, wherein the steel tube has a
hardness of no
more than about 37 HRC.
9. The low carbon alloy steel tube of claim 1, wherein the steel tube has a
carbon
equivalent of less than about 0.63%, the carbon equivalent being determined
according to the
formula:
Ceq = %C + %Mn/6 + (%Cr + %Mo + %V)/5 + (%Ni + %Cu)/15.
10. The low carbon alloy steel tube of claim 9, wherein the steel tube has a
carbon
equivalent of less than about 0.60%.
11. The low carbon alloy steel tube of claim 9, wherein the steel tube has a
carbon
equivalent of less than about 0.56%.
12. The low carbon alloy steel tube of claim 1, wherein the steel tube has a
maximum
microinclusion content of 2 or less --thin series--, and level 1 or less --
heavy series--,
measured in accordance with ASTM E45 Standard - Worst Field Method (Method A).

13
13. The low carbon alloy steel tube of claim 1, wherein the steel tube has a
maximum
microinclusion content measured in accordance with ASTM E45 Standard - Worst
Field
Method (Method A), as follows:
<IMG>
14. The low carbon alloy steel tube of claim 13, wherein oversize inclusion
content with
30 µm or less in size is obtained.
15. The low carbon alloy steel tube of claim 14, wherein the total oxygen
content is
limited to 20 ppm.
16. The low carbon alloy steel tube of claim 1, wherein the tube has a
seamless
configuration.
17. A stored gas inflator pressure vessel comprising the low carbon alloy
steel tube of
claim 1.
18. An automotive airbag inflator comprising the low carbon alloy steel tube
of claim 1.

14
19. A low carbon alloy steel tube consisting essentially of, by weight: about
0.08% to
about 0.11% carbon; about 1.03% to about 1.18% manganese; about 0.15% to about
0.35%
silicon; up to about 0.003% sulfur; up to about 0.012% phosphorous; up to
about 0.10%
nickel; about 0.63% to about 0.73% chromium; about 0.40% to about 0.45%
molybdenum;
about 0.03% to about 0.05% vanadium; about 0.025% to about 0.035% titanium;
about
0.15% to about 0.30% copper; about 0.010% to about 0.050% aluminum; up to
about 0.05%
niobium; up to about 0.15% residual elements; and the balance iron and
incidental impurities,
wherein the steel tube has a yield strength of at least about 135 ksi, a
tensile strength of at
least about 145 ksi, an elongation at break of of at least about 9%, a
hardness of no more than
about 37 HRC, and has a ductile-to-brittle transition temperature below -60
°C.
20. The low carbon alloy steel tube of claim 19, wherein the tube has a
seamless
configuration.
21. A stored gas inflator pressure vessel comprising the low carbon alloy
steel tube of
claim 19.
22. An automotive airbag inflator comprising the low carbon alloy steel tube
of claim 19.
23. A method of manufacturing a length of steel tubing for a stored gas
inflator pressure
vessel, comprising the following steps:
producing a length of tubing from a steel material consisting essentially of,
by weight:
about 0.06% to about 0.18% carbon, about 0.5% to about 1.5% manganese, about
0.1% to
about 0.5% silicon, up to about 0.015% sulfur, up to about 0.025% phosphorous,
up to about
0.50% nickel, about 0.1 % to about 1.0% chromium, about 0.1 % to about 1.0%
molybdenum,
about 0.01% to about 0.10% vanadium, about 0.01% to about 0.10% titanium,
about 0.05%
to about 0.35% copper, about 0.010% to about 0.050% aluminum, up to about
0.05%
niobium, up to about 0.15% residual elements, and the balance iron and
incidental impurities;
subjecting the steel tubing to a cold-drawing process to obtain desired
dimensions;
austenizing by heating the cold-drawn steel tubing in an induction-type
austenizing
furnace to a temperature of at least Ac3, at a heating rate of at least about
100 °C per second;

15
after the heating step, quenching the steel tubing in a quenching fluid until
the tubing
reaches approximately ambient temperature, at a cooling rate of at least about
100 °C per
second; and
after the quenching step, tempering the steel tubing for about 2-30 minutes at
a
temperature below Ac1.
24. The method of claim 23, wherein the steel tubing produced consists
essentially of, by
weight: about 0.07% to about 0.12% carbon, about 1.00% to about 1.40%
manganese, about
0.15% to about 0.35% silicon, up to about 0.010% sulfur, up to about 0.015%
phosphorous,
up to about 0.20% nickel, about 0.55% to about 0.80% chromium, about 0.30% to
about
0.50% molybdenum, about 0.01% to about 0.07% vanadium, about 0.01% to about
0.05%
titanium, about 0.15% to about 0.30% copper, about 0.010% to about 0.050%
aluminum, up
to about 0.05% niobium, up to about 0.15% residual elements, and the balance
iron and
incidental impurities.
25. The method of claim 23, wherein the steel tubing produced consists
essentially of, by
weight: about 0.08% to about 0.11% carbon, about 1.03% to about 1.18%
manganese, about
0.15% to about 0.35% silicon, up to about 0.003% sulfur, up to about 0.012%
phosphorous,
up to about 0.10% nickel, about 0.63% to about 0.73% chromium, about 0.40%
to.about
0.45% molybdenum, about 0.03% to about 0.05% vanadium, about 0.025% to about
0.035%
titanium, about 0.15% to about 0.30% copper, about 0.010% to about 0.050%
aluminum, up
to about 0.05% niobium, up to about 0.15% residual elements, and the balance
iron and
incidental impurities.
26. The method of claim 23, wherein the finished steel tubing has a yield
strength of at
least about 125 ksi.
27. The method of claim 23, wherein the finished steel tubing has a yield
strength of at
least about 135 ksi.
28. The method of claim 23, wherein the finished steel tubing has a tensile
strength of at
least about 145 ksi.
29. The method of claim 23, wherein the finished steel tubing has an
elongation at break
of at least about 9%.

16
30. The method of claim 23, wherein the finished steel tubing has a hardness
of no more
than about 40 HRC.
31. The method of claim 23, wherein the finished steel tubing has a hardness
of no more
than about 37 HRC.
32. The method of claim 23, wherein the finished steel tubing has a ductile-to-
brittle
transition temperature below -60 °C.
33. The method of claim 23, wherein in the austenizing heating step, the steel
tubing is
heated to a temperature between about 920-1050 °C.
34. The method of claim 33, wherein in the austenizing heating step, the steel
tubing is
heated at a rate of at least about 200 °C per second.
35. The method of claim 23, wherein in the quenching step, the steel tubing is
cooled at a
rate of at least about 200 °C per second.
36. The method of claim 23, wherein in the tempering step, the steel tubing is
tempered at
a temperature between about 400-600 °C.
37. The method of claim 36, wherein in the tempering step, the steel tubing is
tempered
for about 4-20 minutes.
38. The method of claim 23, further comprising a finishing step wherein the
tempered
steel tubing is pickled, phosphated, and oiled.

17
39. A method of manufacturing a length of steel tubing for a stored gas
inflator pressure
vessel, comprising the following steps:
producing a length of tubing from a steel material consisting essentially of,
by weight:
about 0.08% to about 0.11% carbon, about 1.03% to about 1.18% manganese, about
0.15%
to about 0.35% silicon, up to about 0.003% sulfur, up to about 0.012%
phosphorous, up to
about 0.10% nickel, about 0.63% to about 0.73% chromium, about 0.40% to about
0.45%
molybdenum, about 0.03% to about 0.05% vanadium, about 0.025% to about 0.035%
titanium, about 0.15% to about 0.30% copper, about 0.010% to about 0.050%
aluminum, up
to about 0.05% niobium, up to about 0.15% residual elements, and the balance
iron and
incidental impurities;
subjecting the steel tubing to a cold-drawing process to obtain desired
dimensions;
austenizing by heating the cold-drawn steel tubing in an induction-type
austenizing
furnace to a temperature between about 920-1050 °C, at a heating rate
of at least about 200
°C per second;
after the heating step, quenching the steel tubing in a water-based quenching
solution
until the tubing reaches approximately ambient temperature, at a cooling rate
of at least about
200 °C per second; and
after the quenching step, tempering the steel tubing for about 4-20 minutes at
a
temperature between about 450-550 °C,
a finishing step wherein the tempered steel tubing is pickled, phosphated, and
oiled,
wherein the finished steel tubing has a yield strength of at least about 135
ksi, a
tensile strength of at least about 145 ksi, an elongation at break of at least
about 9%, a
hardness of no more than about 37 HRC, a ductile-to-brittle transition
temperature below -60
°C and a good surface appearance.

Description

CA 02540000 2006-03-22
WO 2005/035800 PCT/IB2004/003311
LOW CARBON ALLOY STEEL TUBE HAVING ULTRA HIGH STRENGTH
AND EXCELLENT TOUGHNESS AT LOW TEMPERATURE AND METHOD OF
MANUFACTURING THE SAME
Related Application
[0001] This application claims the benefit of U.S. Provisional Patent
Application No.
60/509,806, filed on October 10, 2003, and U.S. Non-Provisional Patent
Application No.
filed on October 5, 2004.
Background of the Invention
[0002] The present invention relates to a low carbon alloy steel tube having
ultra high
strength and excellent toughness at a 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 system components, an example of which is
an
automotive airbag inflator.
[0003] Airbag inflators for vehicle occupant restraint systems are required to
meet strict
structural and functional stand~.rds. 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.
[0004] 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.
[0005] 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,
workability,
and weldability, and it must also have high strength, toughness, and excellent
resistance to
bursting. The dimensional accuracy is important to ensure a very precise
volume of gas that
blows the airbag.
CONFIRMATION COPY

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2
[0006] Cold forming properties are very important in tubular members used to
manufacture
accumulators since they are formed to final shape after the seamless 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.
[0007] The steel that has been developed has very good weldability, not
requiring for this
application either preheating prior to welding, or post weld heat treatment.
The carbon
equivalent, as defined by the formula,
Ceq = %C + %Mn/6 + (%Cr+%Mo+%V)/5 + (%Ni+%Cu)/15
should be less than about 0.63% in order to obtain the required weldability.
In the preferred
embodiment of this invention, the carbon equivalent as defined above should be
less than
about 0.60%, in order to better guarantee welbability.
[0008] To produce a gas container, a cold-drawn tube made according 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.
[0009] The inflators are tested to assure that they retain their structural
integrity during
airbag deployment. One of such tests is the so call 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.
[0010] 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 outturned 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

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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.
[0011] 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
[0012] The present invention relates to a low carbon alloy steel tube suitable
for cold
forming having ultra high strength (IJTS 145 ksi minimum), 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 even as low as -100 °C. The present
invention also relates to a
process of manufacturing such a steel tube.
[0013] The material of the present invention is designed to make components
for containers
for automotive restraint system components, an example of which is an
automotive airbag
inflator.
Description of the Preferred Embodiments
[0014] 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.
[0015] The present invention relates to a 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.
[0016] 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.
[0017] 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

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4
3. Tue hot rolling
4. Hot-rolled hollow finishing operations
5. Cold drawing
6. Heat treating
7. Cold-drawn tube finishing operations
[0018] 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.
[0019] 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:
Inclusion Thin Heavy
Type
A 0.5 0
B 1.5 1.0
C 0 0
D 1.5 ~ 0.5
[0020] Furthermore, the extreme clean practice allows obtaining oversize
inclusion content
with 30 pm or less in size. These inclusion contents are obtained limiting the
total oxygen
content to 20 ppm.
[0021] 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.
[0022] The chemical composition of the obtained steel shall be as follows, in
each case "%"
means "mass percent":
Carbon (C)

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[0023] 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 0.08 to 0.11%.
Manganese (Mn)
[0024] Mn is an element which is effective in increasing the hardenability of
the steel, and
therefore it increases strength and toughness. If its content is less than
0.5% it is difficult to
obtain the desired strength, whereas if it exceeds 1.5%, then banding
structures become
marked, and toughness decreases. Accordingly, the Mn content is 0.5% to 1.5%.
However, a
preferred Mn range is 1.00% to 1.40%, and a more preferred range is 1.03% to
1.18%.
Silicon (Si)
[0025] 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 0.10%, the steel
is susceptible to
oxidation, on the other hand if it exceeds 0.50%, then both toughness and
workability
decrease. Therefore, the Si content is 0.1% to 0.5%. A preferred Si range is
0.15% to 0.35%.
Sulfur (S)
[0026] 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%, and
a more
preferred maximum value is 0.003%.
Phosphorous (P)
[0027] 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 0.015%, and
a more
preferred maximum value is 0.012%.
Nickel (Ni)
[0028] Ni is an element that increases the strength and toughness of the
steel, but it is very
costly, therefore the Ni is limited to 0.50% maximum. A preferred maximum
value is 0.20%
and a more preferred maximum value is 0.10%.
Chromium (Cr)
[0029] Cr is an element which is effective in increasing the strength,
toughness, and
corrosion resistance of the steel. If its content is less than 0.10% it is
difficult to obtain the

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6
desired strength, whereas if it exceeds 1.0%, then toughness at the welding
zones decreases
markedly. Accordingly, the Cr content is 0.1 % to 1.0%. However, a preferred
Cr range is
0.55 to 0.80%, and a more preferred range is 0.63% to 0.73%.
Molybdenum (Mo)
[0030] Mo is an element which is effective in increasing the strength of the
steel and
contributes to retard the softening during tempering. If its content is less
than 0.10% it is
difficult to obtain the desired strength, whereas if it exceeds 1.0%, then
toughness at the
welding zones decreases markedly. Accordingly, Mo content is 0.1 % to 1.0%.
However, this
ferroalloy is expensive, forcing the necessity to lower the maximum content.
Therefore, a
preferred Mo range is 0.30% to 0.50%, and a more preferred range is 0.40% to
0.45%.
Vanadium (V)
[0031] 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. V
content is found to
be optimum from 0.01% to 0.10%. However, this ferroalloy is expensive, forcing
the
necessity to lower the maximum content. Therefore, a preferred V range is
0.01% to 0.07%,
and a more preferred range is 0.03% to 0.05%.
Titanium (Ti)
(0032] Ti is an element which is effective in increasing the strength of the
steel, even if
added in small amounts. Ti content is found to be optimum from 0.01% to 0.10%.
However,
this ferroalloy is expensive, forcing the necessity to lower the maximum
content. Therefore,
a preferred Ti range is 0.01 % to 0.05%, and a more preferred range is 0.025%
to 0.035%.
Copper (Cu)
[0033] This element improves the corrosion resistance of the pipe, therefore
the Cu content
is in the range of 0.05% to 0.35%, and a preferred range is 0.15% to 0.30%.
Aluminum (Al)
[0034] This element is added to the steel during the steel making process to
reduce the
inclusion content and to refine the steel grain. A preferred Al content is
0.010% to 0.050%.
[0035] Preferred ranges for other elements not listed above are as follows:
Element Weight
Niobium 0.05% max
Sn 0.05% max

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Sb 0.05% max
Pb 0.05% max
As 0.05% max
[0036] 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
[0037] 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.
[0038] 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 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.
[0039] 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 15:1, with preferred
and most
preferred minimum cross-sectional area reductions of 20:1 and 25:1,
respectively.
[0040] 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.
[0041] 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.
[0042] 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

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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.
[0043] The seamless tube is so cold drawn at least once, each pass reducing
both the outside
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.
[0044] 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.
[0045] 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.
[0046] 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

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per second. This extremely high cooling rate is crucial for obtaining a
complete
microstructure transformation.
[0047] 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. 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. Therefore, soaking times of between about 2-30
minutes,
preferably between about 4-20 minutes, have been utilized. The tempering
process is
performed preferably in a protective reducing or neutral atmosphere to avoid
decarburizing
and oxidation of the tube.
[0048] 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.
[0049] 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 phosphate,
and oiled using a petroleum-based oil, a water-based oil, or a mineral oil.
[0050] The steel tube obtained by the described method shall have the
following mechanical
properties in order to comply with the requirements stated for the invention:
Yield Strength about 125 ksi (862 MPa) minimum,
more preferably 135 ksi (930 Mpa) minimum
Tensile Strength about 145 ksi (1000 MPa) minimum
Elongation about 9% minimum
Hardness about 40 HRC maximum,
more preferably 37 HRC maximum.
[0051] The yield strength, tensile strength, elongation, and hardness test
shall be performed
according to the procedures described in the Standards ASTM E8 and ASTM A370.
For the
tensile test, a full size specimen for evaluating the whole tubular section is
preferred.
[0052] 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.

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[0053] In order to obtain a good balance between strength and toughness, 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-112 Standard. This
is
accomplished thanks to the extremely short heating cycle during austenitizing.
[0054] The steel tube obtained by the described method shall have the stated
properties in
order to comply with the requirements stated for the invention.
[0055] 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.
[0056] 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.
[0057] 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
°C.
[0058] 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 Charily
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 Charily impact test can be machined from the tube in the
transverse direction.
Moreover, in order to get this subsize Charily 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.