Note: Descriptions are shown in the official language in which they were submitted.
CA 02885696 2015-03-20
High-Frequency Straight Welded Pipe and
Manufacturing Method Thereof
Technical Field
The invention relates to a steel pipe and a method for manufacturing the
same, particularly to a high frequency welded pipe and a method for
manufacturing the same.
Background Art
In the field of production and transportation of petroleum and natural gas,
high frequency straight welded pipes (HFW) are used widely owing to their
advantages of low manufacture cost, high dimension precision, easy control
over specified lengths, etc. They are employed mainly for transportation of
petroleum, natural gas, ore slurry on the land and under the sea, and have a
broad prospect in application.
Along with the ever growing global demand on petroleum and natural gas,
the exploitation conditions in petroleum and natural gas wells tend to be
worse
and more complicated. As oil and gas fields characterized by badly corrosive
environment with high contents of H and S are developed successively, it is
urgent for steel pipe manufacturers to develop and produce pipes suitable for
transporting petroleum and natural gas under this kind of acidic operation
condition. At present, there are more than one hundred sets of high frequency
straight welded pipe (HFW) machines in China. However, most manufacturers
have to purchase plate volumes from large steel makers that mainly produce
steel pipes of conventional steel grades. No domestic plant produces HIC
(hydrogen induced crack) resistant pipeline pipes of higher steel grades
having
superior properties that are needed raringly in the market. Up to now, there
still
CA 02885696 2015-03-20
remains a domestic blank area for HIC-resistant high-frequency straight welded
pipes of grade L360MCS and a process of manufacturing the same.
Summary
The object of the invention is to provide a high frequency straight welded
pipe and a method for manufacturing the same, wherein the high frequency
straight welded pipe possesses good HIC resistance, properties of steel grade
L360MCS, high yield strength, high tensile strength, high impact toughness and
good weldability.
In order to achieve the above object of the invention, there is provided a
high frequency straight welded pipe, comprising the following chemical
elements in mass percentages
C: 0.042-0.056%;
Si: 0.18-0.22%;
Mn: 0.75-0.95%;
P: 0.0064-0.015%;
S: 0.0006-0.002%;
Ti: 0.012-0.018%;
V: 0.001-0.002%;
Al: 0.026-0.038%;
Ni: 0.080-0.13%;
Nb: 0.020-0.029%;
Cu: 0.125-0.135%;
Cr: 0.018-0.03%;
Mo: 0.004-0.008%;
B: 0-0.0005%;
Ca: 0.001-0.003%;
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CA 02885696 2015-03-20
the balance of Fe and other unavoidable impurities.
The main chemical elements in the high frequency straight welded pipe of
the invention are designed according to the following principles:
C: Carbon is the main solid solution strengthening element in the pipeline
pipe. A good number of experiments conducted by the researchers demonstrate
that the sensitivity of strip steel to hydrogen induced crack (HIC) increases
as
the carbon content in the strip steel increases. Thus, it is necessary to
control the
carbon content appropriately to an acceptable low range that will not affect
the
strength of the strip steel. The carbon content in the composition of the
invention is reduced suitably, from about 0.07wt% which is generally used in
the prior art to not more than 0.056wt%. As such, the carbon in the technical
solution of the invention is controlled in the range of 0.042-0.056wt%.
S: In low sulfur steel, the crack length ratio is reduced and MnS appears at
the fracture face, indicating that the generation of cracks may be controlled
effectively by reduction of N, S in strip steel. Nevertheless, it is
unnecessary to
seek reduction of the sulfur content blindly because cracks in the strip steel
cannot be avoided completely even if the sulfur content is reduced to an
extremely low level. The inventors have carried out a lot of experiments and
found that, when the mass percentage of sulfur is controlled in the range of
0.0006-0.002%, not only the requirement of corrosion resistance can be
fulfilled,
but also cracking in the strip steel can be prevented.
Ca: Calcium treatment has an important influence on the hydrogen induced
crack (HIC) resistance of strip steel. In the technical solution of the
invention, a
suitable amount of calcium is added into the composition. CaS precipitated at
the final solidification position is converted into spherical inclusions after
rolling, so that the hydrogen induced crack (HIC) resistance of the strip
steel is
improved. Yet, control over the calcium content is related with the sulfur
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content. Hence, the calcium content in the technical solution of the invention
is
controlled in the range of 0.001-0.003wt%.
Cu: Among the variety of alloy elements, copper is the only element that is
beneficial to the hydrogen induced crack (HIC) resistance. When an amount of
copper is added into pipeline steel, the sensitivity to hydrogen induced crack
is
decreased remarkably. The main reason is that copper facilitates the formation
of a passivation film which blocks invasion of hydrogen element and thus
inhibits formation of hydrogen induced crack. According to the technical
solution of the invention, an amount of copper is added to improve the
hydrogen
induced crack (HIC) resistance. Therefore, the copper content is controlled in
the range of 0.125-0.135wt%.
Mn: The effect of manganese on the sensitivity of pipeline steel to
hydrogen induced crack principally resides in the influence of manganese on
the
phase transition of the strip steel. If the manganese content exceeds 1.0wt%,
the
sensitivity to hydrogen induced crack (HIC) increases. Therefore, the
manganese content in the technical solution of the invention is controlled in
the
range of 0.75-0.95wt%.
Accordingly, the invention also provides a method for manufacturing the
above high frequency straight welded pipe, comprising the following steps:
in a shell forming step, the squeeze is controlled in the range of 2-3% of
the outer diameter of the welded pipe;
in a welding step, the welding speed is controlled in the range of 18-
20m/min;
in a post-welding heat treatment step, the welding line is normalized at
930-970 C, followed by air cooling to below 380 C and water cooling to below
80 C.
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Further, in the shell forming step of the method for manufacturing a high
frequency straight welded pipe according to the invention, the opening angle
is
controlled in the range of 3-4.2 .
In the technical solution of the invention, the squeeze measured before and
after welding is controlled in the range of 2-3% of the outer diameter of the
welded pipe, wherein the squeeze refers to the difference between the
perimeter
of the shell before squeezing and the perimeter after the squeezing. In a
molten
state, the molten pool at the welding line is exposed to air and susceptible
to
oxidation, wherein the product of the oxidation reaction is closely related
with
the chemical composition of the strip steel. Hence, a relatively large squeeze
needs to be applied to squeeze the resulting product with high melting point
to
the surface of the welding line on the strip steel and remove the same by
deburring. If the squeeze is lower than 2%, flaws such as cold welding will
occur, such that the inclusions in the strip steel cannot be expelled to the
surface
of the welding line for removal, and the quality of the strip steel will be
affected.
The welding speed is set in the range of 18-20m/min for the reason that the
welding speed is generally inversely proportional to the welding power, such
that a rapid welding speed tends to counteract the expellability of the
inclusions
resulted from a high welding power and a large squeeze, leading to decreased
effect in expelling the inclusions. Therefore, the inventors control the
welding
speed in the range of 18-20m/min according to the technical solution of the
invention.
According to the method for manufacturing a high frequency straight
welded pipe in the invention, on the basis of the utilization of a
manufacturing
method of high frequency straight induction welding (HFW), a high frequency
straight welded pipe that meets the requirements of HIC resistance, tensile
property, impact toughness and microstructure is manufactured by reasonably
CA 02885696 2015-03-20
regulating the squeeze and the forming process, setting the high frequency
weld
forming and welding parameters, and controlling the technical parameters of
the
subsequent heat treatment.
Compared with the prior art, the high frequency straight welded pipe of the
invention has good HIC resistance, properties of steel grade L360MCS, high
yield strength, high tensile strength, high impact toughness and good
weldability, and is suitable for use as a transporting pipe in a harsh
operation
environment where the contents of H and S are high or acidic corrosion exists.
Detailed Description of the Invention
Examples 1-6
The high frequency straight welded pipe of the invention was
manufactured in accordance with the following steps:
The head and tail portions of unrolled steel rolls were cut off, and cuts
were formed flush at an angle of 30 relative to the traverse direction of the
steel
rolls. The tail of a preceding steel roll was welded to the head of a
succeeding
steel roll by carbon dioxide shielded welding. The steel strip was used to
manufacture a high frequency straight welded pipe, wherein the edge of the
plate was treated by edge milling to control the width of the steel strip and
the
plumbness of the plate edge precisely. The steel strip was formed into a shell
by
a cage roll forming process. The squeeze was controlled in the range of 2-3%
of
the outer diameter of the welded pipe, and the opening angle was controlled in
the range of 3-4.2 . The welding speed was controlled in the range of 18-
20m/min during welding. After welding, the welding line was normalized at
930-970 C, followed by air cooling to below 380 C and water cooling to below
80 C. The wall thickness of the high frequency straight welded pipe obtained
by
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the above process was 6.4mm-9.5mm, and the pipe diameter was 219.7mm-
406.4mm.
Table 1 lists the chemical compositions of the high frequency straight
welded pipes of Examples 1-6.
Table 1 (The balance is Fe and other unavoidable impurities, wt%)
Ex. 1 2 3 4 5 6
C 0.0551 0.0553 0.0499 0.0494 0.0517 0.0421
Si 0.18 , 0.182 0.0182 0.182 0.183 0.22
Mn 0.886 0.886 0.0878 0.877 0.750 0.950
P 0.0136 0.0136 ., 0.0118 0.0121 0.0064
0.0149
S 0.002 0.0012 0.0014 0.0015 0.0006 0.0010
Ti 0.014 0.014 0.014 0.014 0.012 0.018
/ 0.001 0.001 0.001 0.001 0.002
0.001
Al 0.031 0.026 0.030 0.030 0.028 0.038
Ni 0.096 0.096 0.096 0.096 0.08 0.128
Nb 0.027 0.027 0.026 0.027 0.020 0.029
Cu 0.132 0.129 0.13 0.129 0.125 0.135
Cr 0.026 0.026 0.026 0.027 0.030 0.018
Mo 0.005 0.005 0.005 0.005 0.004 0.008
B 0.0001 0.0002 0.0002 0.0002 0 0.0005
Ca 0.0018 0.0020 0.0020 0.0023 0.0029 0.0010
Table 2 lists the detailed process parameters for manufacturing the high
frequency straight welded pipes of Examples 1-6.
Table 2
1
Water
Air Cooling C ooling Strip Wall
Welding Opening Normalizing Temperature Width Output Thickness
Diameter
Squeeze
Ex. Speed Angle Temperature for the Temperaturefor
t,ne After Power of Welded of Welded
(%)
(rnimin) ( ) ( C) Welding Milling (KW) Pipe Pipe
(mm)
Welding Line
Line("C) ("C) (mm) (mm)
1 2.9 20 3.8 950 360 42 . 700 450
6.4 219.7
2 2 18 4 955 365 46 1285 840 8.7 406.4
3 2.7 18 4 960 370 50 1281 906 9.5 . 406.4
4 3 18 4 960 375 38 1024 772 9.1 323.9
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2.62 20 3.8 950 350 35 1031 572 6.4 323.9
6 2.46 18 4 955 360 40 1289 698 7.1 406.4
Table 3 lists the performance parameters of the high frequency straight
welded pipes of Examples 1-6.
Table 3
Welding Line of Welded
Welded Pipe BodyWhole
Pipe
Impact Impact
Toughness/Char
Toughness/Char I-Iydrogen Induced Crack
Ex. Yield Tensile Elong- Yield py Impact
Ener gy (J) Tensile r Impact (IIIC)
Resistance
Strength Strength ation Ratio Strength ergy (J)
(MPa) (MPa) (%) (MPa) Crack
Crack Crack
AveAve Sensitiv Length Thickne
Minimum Mini mum
rage rage e
Rate Rate ss Rate
(CSR) (CLR) (CLR)
I 478 545 28 0.877 119 127 475 139 145 0 0 0
2 399 505 37 0.790 198 259 480 284 318 0 0 0
3 422 520 37 0.812 293 301 487 266 268 0 0 0
4 455 540 24 0.843 231 241 510 225 239 0 0 0
5 400 510 34 0.784 170 180 490 160 166 0 0 0
6 420 520 24 0.808 150 162 505 202 214 0 0 0
As shown by Table 3, the high frequency straight welded pipe of the
invention possesses superior mechanical properties and HIC resistance.
Specifically, the welded pipe body exhibits a yield strength >399MPa, a
tensile
strength >505MPa, an elongation >24%, and the welding line of the welded
pipe has a tensile strength >475MPa, indicating that the high frequency
straight
welded pipe as a whole meets the requirement of high strength and possesses
high tensile capability. With respect to impact toughness, the Charpy impact
energy of the welded pipe body has a minimum >119J and an average >127J,
and that of the welding line of the welded pipe has a minimum >139J and an
average >145J, indicating that the high frequency straight welded pipe has
good
toughness and weldability. The crack sensitive rate (CSR), the crack length
rate
(CLR) and the crack thickness rate (CTR), which are used for evaluating the
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hydrogen induced crack (HIC) resistance of a strip steel material, are all 0%
as
shown in Table 3, demonstrating the good hydrogen induced crack (HIC)
resistance of the high frequency straight welded pipe.
It is to be noted that the above specific examples of the invention are only
exemplary. Obviously, the invention is not limited to the above examples.
Rather, a number of variations can be made. All variations derived directly or
contemplated from the disclosure of the invention by one skilled in the art
fall
within the protection scope of the invention.
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