U.S. patent application number 10/957605 was filed with the patent office on 2005-04-14 for low carbon alloy steel tube having ultra high strength and excellent toughness at low temperature and method of manufacturing the same.
This patent application is currently assigned to TENARIS CONNECTIONS A.G.. Invention is credited to Altschuler, Eduardo, Lopez, Edgardo Oscar.
Application Number | 20050076975 10/957605 |
Document ID | / |
Family ID | 34426131 |
Filed Date | 2005-04-14 |
United States Patent
Application |
20050076975 |
Kind Code |
A1 |
Lopez, Edgardo Oscar ; et
al. |
April 14, 2005 |
Low carbon alloy steel tube having ultra high strength and
excellent toughness at low temperature and method of manufacturing
the same
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.degree. C.
Inventors: |
Lopez, Edgardo Oscar;
(Veracruz, MX) ; Altschuler, Eduardo; (Bergamo,
IT) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Assignee: |
TENARIS CONNECTIONS A.G.
SCHAAN
LI
|
Family ID: |
34426131 |
Appl. No.: |
10/957605 |
Filed: |
October 5, 2004 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60509806 |
Oct 10, 2003 |
|
|
|
Current U.S.
Class: |
148/593 ;
420/90 |
Current CPC
Class: |
C21D 8/10 20130101; C22C
38/20 20130101 |
Class at
Publication: |
148/593 ;
420/090 |
International
Class: |
C21D 009/08; C22C
038/20 |
Claims
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.degree. 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.
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. 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:
4 Inclusion Type Thin Heavy A 0.5 0 B 1.5 1.0 C 0 0 D 1.5 0.5
14. The low carbon alloy steel tube of claim 13, wherein oversize
inclusion content with 30 .mu.m 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.
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.degree. 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 feast about 100.degree. C. per second; 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.degree. 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%.
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.degree. 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.degree. 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.degree. 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.degree. 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.degree. 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.
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 firnace to a temperature between about 920-1050.degree.
C., at a heating rate of at least about 200.degree. 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.degree. C. per
second; and after the quenching step, tempering the steel tubing
for about 4-20 minutes at a temperature between about
450-550.degree. 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.degree. C. and
a good surface appearance.
Description
RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 60/509,806, filed on Oct. 10, 2003.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a low carbon alloy steel
tube 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.
[0003] 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.
[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.
[0006] Cold forming properties are very important in tubular
members used to manufacture accumulators 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.
[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
[0008] 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%, and most preferably less than about 0.56%, in
order to better guarantee weldability.
[0009] 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.
[0010] 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.
[0011] 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 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.
[0012] 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
[0013] The present invention relates to a low carbon alloy steel
tube suitable for cold forming having ultra high strength (UTS 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.degree. C., i.e., a
ductile-to-brittle transition temperature (DBTT) below -60.degree.
C., and possibly even as low as -100.degree. C. The present
invention also relates to a process of manufacturing such a steel
tube.
[0014] 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
[0015] 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.
[0016] 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.degree. C., i.e., a ductile-to-brittle
transition temperature below -60.degree. C.
[0017] 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.
[0018] A schematic illustration of a method of producing the
seamless low carbon ultra high strength steel could be as
follows:
[0019] 1. Steel making
[0020] 2. Steel casting
[0021] 3. Tube hot rolling
[0022] 4. Hot-rolled hollow finishing operations
[0023] 5. Cold drawing
[0024] 6. Heat treating
[0025] 7. Cold-drawn tube finishing operations
[0026] 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.
[0027] 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:
1 Inclusion Type Thin Heavy A 0.5 0 B 1.5 1.0 C 0 0 D 1.5 0.5
[0028] Furthermore, the extreme clean practice allows obtaining
oversize inclusion content with 30 .mu.m or less in size. These
inclusion contents are obtained limiting the total oxygen content
to 20 ppm.
[0029] 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.
[0030] The chemical composition of the obtained steel shall be as
follows, in each case means "mass percent":
[0031] Carbon (C)
[0032] 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%.
[0033] Manganese (Mn)
[0034] 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%.
[0035] Silicon (Si)
[0036] 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%.
[0037] Sulfur (S)
[0038] 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%.
[0039] Phosphorous (P)
[0040] 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%.
[0041] Nickel (Ni)
[0042] 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%.
[0043] Chromium (Cr)
[0044] 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 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%.
[0045] Molybdenum (Mo)
[0046] 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%.
[0047] Vanadium (V)
[0048] 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%.
[0049] Titanium (Ti)
[0050] 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%.
[0051] Copper (Cu)
[0052] 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%.
[0053] Aluminum (Al)
[0054] 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%.
[0055] Preferred ranges for other elements not listed above are as
follows:
2 Element Weight % Niobium 0.05% max Sn 0.05% max Sb 0.05% max Pb
0.05% max As 0.05% max
[0056] Residual elements in a single ladle of steel used to produce
tubing or chambers shall be:
Sn+Sb+Pb+As.ltoreq.0.15% max, and
S+P.ltoreq.0.025
[0057] 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.
[0058] The solid cylindrical billet of ultra high clean steel is
heated to a temperature of about 1200.degree. C. to 1300.degree.
C., and at this point undergoes the rolling mill process.
Preferably, the billet is heated to a temperature of about
1250.degree. 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.
[0059] 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.
[0060] 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.
[0061] 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.
[0062] 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.
[0063] 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.
[0064] 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.degree. C.), but preferably above about
920.degree. C. and below about 1050.degree. 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.degree. C. per second, but more
preferably at least about 200.degree. 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.
[0065] 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.
[0066] 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.degree. C. per second, more
preferably at a rate of at least about 200.degree. C. per second.
This extremely high cooling rate is crucial for obtaining a
complete microstructure transformation.
[0067] The steel tube is then tempered with an appropriate
temperature and cycle time, at a temperature below Ac1. Preferably,
the tempering temperature is between about 400-600.degree. C., and
more preferably between about 450-550.degree. 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/or oxidation of the tube.
[0068] 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.
[0069] 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.
[0070] 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:
3 Yield Strength about 125 ksi (862 MPa) minimum, more preferably
about 135 ksi (930 MPa) minimum Tensile Strength about 145 ksi
(1000 MPa) minimum Elongation about 9% minimum Hardness about 40
HRC maximum, more preferably about 37 HRC maximum.
[0071] The yield strength, tensile strength, elongation, and
hardness test shall be performed according to the procedures
described in the Standards ASTM E8 (yield strength, tensile
strength, and elongation) and ASTM A370 (hardness). For the tensile
test, a full size specimen for evaluating the whole tubular section
is preferred.
[0072] 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.
[0073] 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.
[0074] The steel tube obtained by the described method shall have
the stated properties in order to comply with the requirements
stated for the invention.
[0075] 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.
[0076] 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.
[0077] 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.degree. C. Tests
performed on the samples produced show that this grade has a
guaranteed ductile behavior at -60.degree. C., with a
ductile-to-brittle transition temperature below -60.degree. C.
[0078] 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.
* * * * *