Note: Descriptions are shown in the official language in which they were submitted.
CA 03058488 2019-09-30
DESCRIPTION
A X80 pipeline steel plate with high carbon equivalent and high toughness at
low
temperatures used for bent pipes as well as a manufacturing method thereof
Technical Field
The present application relates to a technical field of manufacturing X80
pipeline
steel plate for bent pipes, especially relates to a X80 pipeline steel plate
for bent pipes
with high carbon equivalent and high toughness at low temperatures and a
manufacturing method thereof.
Background Art
With the dominance of petrochemical energy in the world's energy demand and
the rapid growth of petrochemical energy demand under the roaring world
economy
nowadays, the long-distance pipelines are largely improved for higher
transmission
efficiency and less investment, and the development trend of long-distance oil
and gas
pipelines steel is to be in high strength or super high grade. At present, the
highest
grade of pipeline steel used in the world is X80. In addition to the straight
pipe, a
pipeline project also includes the bent pipes for the direction change of the
pipeline,
the bent pipes in stations, and so on. Bent pipes are usually produced by two
kinds of
processes, namely, cold bending and hot bending, and the cold bending process
has
been gradually replaced by hot bending process as cold bending process is
under the
influence of manufacturing process and service environment. The hot bending of
bent
pipes usually are to heat the main pipe to Ac3 or above by induction heating
equipment, and then the heating zone rotates around the fixed center to bend
the bent
pipe with the required radius of curvature by a fixed turning panel and under
the
action of back thrust; after bending, the bent pipe outer ring is cooled by an
annular
cooling ring, and the heated part of the bent pipe undergoes an accelerated
cooling
similar to ACC because of the limited cooling capacity of the annular cooling
ring.
The steel plate after cooling treatment is treated by tempering in a
continuous furnace
in consideration of the property uniformity. From the view of the whole
process, the
whole hot bending and cooling bending process are similar to the TMCP process,
which is the combination of high-temperature deformation (bending) and ACC
cooling process. Because the Whole hot bending process is deformed to a small
extent
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CA 03058488 2019-09-30
at a high temperature and the subsequent cooling capacity is weak, the hot
bending
steel plate for bent pipes can only be designed with high carbon-equivalent
composition. The high-carbon-equivalent design will bring two main problems to
the
whole production: 1) the impact toughness is reduced; 2) there would be a
defective
shape of the steel plate due to the large structural stress in the cooling
process, which
brings difficulties to the subsequent straightening and affects the production
efficiency
at the same time.
At home and abroad, there are patent reports on X80 steel grade bent pipes and
the steel plate for bent pipes, such as the patent application of
CN201410239039.9,
which refers to a hot-rolling plate steel for X80 steel grade bent pipe
wherein: 1) only
the impact toughness of steel plate at -20 C is referred in the patent
application, but in
fact, more steel plates for bent pipes need the impact toughness at -30 C or
even
lower temperature at present; 2) slowly stack cooling is required after
rolling of
steel plate in the application patent, which is unfavorable to mass production
efficiency of steel plate and the follow-up production process at the same
time.
Another patent application of No. CN201110245761.X has the following
characteristics: 1) the patent mainly emphasizes the hot bending process but
does not
mention what kind of steel plate production process is used for raw material
steel
plate; 2) the low-temperature toughness impact value at -45 C obtained by the
patent
is not higher than 200J, and the numerical fluctuation is large.
Considering the whole process of austenite deformation in the hot bending
process and that subsequent cooling is relatively simple and insufficient, the
design of
steel plate for bent pipes usually always adopts a high carbon equivalent
composition
compared with that of steel plate for straight pipe. The design of
high-carbon-equivalent components will lead to a lower impact toughness under
low
temperatures.
Disclosure of Invention
According to the characteristics of the hot bending process for bent pipes,
the
chemical composition in the application is still designed with high carbon
equivalent,
but through the innovation of the cooling process in the production process, a
X80
pipeline steel plate with high carbon equivalent and high toughness at low
temperatures used for bent pipes is obtained with a concise process and a high
yield.
The technical scheme adopted by the invention to solve the above technical
problems is to manufacture X80 pipeline steel plate for bent pipes with HIC
resistance,
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wherein the chemical constituents by mass percentage are as follows: C:
Mn: 1.55-1.90%, Si: <0.45%, S: <0.001%, P: <0.010%, Nb: 0.045-0.08%,
Ti: <0.015%, V: <0.008%, Alt: <0.06%, N: <0.0040%, 0: <0.004%, Mo:
<0.40%, Cu: <0.30%, Ni: 0.20--1.5%, Cr: <0.35%, Mo+Cu+Ni+Cr <1.5%,
Ceq: 0.35-0.53%, Pcm: 0.17-0.27%, , the balance is Fe and unavoidable impurity
elements.
Furthermore, the thickness of the steel plate is 18.4-42mm, the yield
strength>600Mpa, the tensile strength >710Mpa, the yield ratio <0.93, the
elongation
is >35%, the impact energy at -30 C >350J, the impact energy at -50 C>250J,
and the
impact energy at -60 C >230J.
The composition of the steel in the invention is based on the design principle
of
high carbon equivalence, and comprises appropriate amount of C, Mn, trace
elements
of Nb, V, Ti and other micro-alloying elements, as well as a small amount of
Mo, Cu,
Ni, and other elements, combined with a specific TMCP rolling process, thereby
the
ultimate mechanical properties are ensured, especially the low-temperature
impact
toughness under the condition of high carbon equivalence. The principle of
adding the
elements mentioned above is as follows:
C: C is the most economical and basic strengthening element in steel, and can
obviously improve the strength of steel by solid solution strengthening and
precipitation strengthening, but have adverse effects on the toughness,
ductility, and
weldability of steel. Therefore, the development trend of pipeline steel is to
reduce C
content, but in order to match the hot bending process, strength and
toughness, the C
content is controlled to be 0.05-0.10%.
Mn: Mn is the most important element to compensate for the loss of strength
caused by the decrease of C content in pipeline steel, enhances the strength
of steel by
solid solution strengthening; Mn is an element to enlarge the y phase and
reduce the
y a transformation temperature of steel, which helps to obtain fine
transformation
products, improve the toughness of steel, reduce the ductile-brittle
transition
temperature; Mn is also an element to improve the hardenability of steel.
Considering
the harm of Mn segregation to HIC resistance in the process of inspection and
with
due consideration for both the hot bending process and the strength
requirement, the
Mn content in the present invention is designed to be 1.55-1.9%, and soft
reduction is
adopted in continuous casting to alleviate the central segregation caused by
high Mn
content.
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Nb: Nb is one of the most important micro-alloying elements in modern
microalloyed steels, especially pipeline steels, and has an obvious effect on
grain
refinement. The recovery and recrystallization of deformed austenite can be
hindered
by Nb solid solution dragging and Nb (C, N) strain-induced precipitation
during hot
rolling; the deformed austenite which is not rolled in the recrystallization
zone can be
transformed into fine phase transformation products during phase
transformation by
TMCP to give the steel high strength and toughness, and the range of Nb
content is
determined mainly based on the relationship between C and Nb content in the
invention.
V: V has higher precipitation strengthening and weaker grain refinement, and
plays a major role in precipitation strengthening when combined with the
microalloying elements of Nb, V, and Ti.
Ti: Ti is a strong N-fixing element, and the stoichiometric ratio of Ti/N is
3.42,
that is, N below 60 ppm in steel can be fixed by using about 0.02% Ti, and TiN
precipitates can be formed during slab continuous casting, and the fine
precipitate is
an indispensable element in pipeline steel, which can effectively prevent the
growth
of austenite grains in slab during heating, help to improve the solid
solubility of Nb in
austenite, and can improve the impact toughness of the welding heat affected
zone,
but too much Ti will form large TiN particles, which will affect the falling
impact
performance, so Ti content will be controlled to be <0.015% in this patent.
Mo: Mo can delay the formation of the preliminary ferrite phase in the y
transformation, is the major element to promote the formation of acicular
ferrite, and
plays an important role in controlling phase transformation and improving the
hardenability of steel. Obvious acicular ferrite or bainite can be obtained by
adding a
certain amount of Mo at a certain cooling rate and final cooling temperature
and
considering TMCP process and hot bending process, Mo content can be controlled
at
no less than 0.15%.
S, P: S and P are unavoidable impurity elements in pipeline steel, and the
lower
the content of S and P, the higher impact toughness pipeline steels have by
changing
the sulfide morphology through ultra-low sulfur and Ca treatment.
Cu, Ni: As the strength of steel can be improved by solution strengthening,
the
adding of Ni not only improve the toughness of steel but also prevent the hot
brittleness easily caused by Cu in steel, and Ni content is controlled at no
less than
0.2%.
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Cr: The addition of Cr can improve the hardenability of steel and is
relatively
economical.
The invention also aims to provide the preparation method of a
high-carbon-equivalent, low-temperature, and high-toughness X80 pipeline steel
plate
for bent pipes, which is as follows in sequence: ratio preparation ¨>
converter or
electric furnace smelting ¨> out-of-furnace refining ¨> continuous casting ¨>
slab
reheating ¨> specific TMCP process + stack cooling after
cooling¨*straightening.
The specific process steps are as follows:
The raw materials are processed in sequence by KR molten steel pretreatment,
converter smelting, LF refining, RH vacuum refining, continuous casting,
wherein
Ca-treatment process is used for refining, with a Ca/S molar ratio of not less
than 1,
and the type B inclusions are controlled to be Grade 1.5 or less. When
continuous
casting, soft reduction is used to prevent core segregation caused by a high
Mn
content, to manufacture a continuous casting slab which satisfies composition
requirement and has a thickness of not less than 350 mm and a compression
ratio of
not less than 10.
The continuous casting slab is reheated with multiple heating stages, and the
temperature of the second heating stage is controlled at 1250-1300 C; after
the
furnace is discharged, a specific TMCP process including two-stage rolling and
intermediate slab cooling is performed: the first stage is the rolling in the
recrystallization zone, wherein the final rolling temperature is not higher
than 1200 C,
and when rolling in the recrystallization zone, the single-pass rolling
reduction ratio
of continuous two or three passes is controlled to be not less than 20%; the
intermediate slab cooling is to moderately cool the intermediate slab to the
temperature of a start rolling temperature in the non-recrystallization zone
at the
second stage, by a Mild cooling system, wherein the cooling method is to cool
the
tank body by swinging back and forth, and the cooling rate is 6-12 C/s, to
ensure that
the austenite grains no longer grow after rolling and shaping in the
recrystallization
zone, and the temperature difference between the surface of the intermediate
slab and
the core is small; the second stage is the rolling in the non-
recrystallization zone,
wherein the start rolling temperature is not higher than 880 C, and the final
rolling
temperature is controlled at 790-850 C until the reaching of the final
thickness.
After rolling, the steel plate is cooled by water, wherein the start cooling
temperature is controlled to be not higher than 810 C, the final cooling
temperature is
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controlled to be not higher than 500 C, and the cooling rate is 10 to 35
C/s; after
cooling, the steel plate is straightened, and then the final product is
obtained by
directly cooling to room temperature. In this cooling process, considering
that the
finished steel sheet has a high carbon equivalent, a stepwise gradient cooling
process
is adopted, that is, the cooling water amount of each cooling unit of the ACC
is set to
be different: the amount of cooling water in the first 1-6 segments is the
largest with a
corresponding cooling rate of 25-35 C/s and the amount of cooling water in
the ACC
cooling unit in the last 7-12 segments decreases in sequence with a
corresponding
cooling rate change of 10-20 C/s.
After the final rolling temperature ends, the first 1-6 ACC cooling segments
of
the steel plate are cooled to the temperature close to Ac3 line in CCT curve
by means
of the stepwise gradient cooling process. On one hand, it can get a larger
degree of
supercooling through a fast cooling speed, and obtain more phase-
transformation
nuclei and finally obtain finer phase-transformation crystal grains. On the
other side,
the cooling method mention above can shorten the time needed to arrive at the
same
average cooling speed and the final cooling temperature. Once the temperature
is near
the Ar3 line, lower cooling speed is adopted to reduce the phase-
transformation stress
and to reduce the sensitivity of temperature stress in the phase
transformation under
the condition of a high carbon equivalence. Thereby, the structure in the
steel plate is
relatively small and the ultimate residual stress is relatively small, and
ultimately the
steel plate still has high toughness at low temperatures.
The Mild Cooling cooling system adopted in the invention is arranged between
the roughing mill and the finishing mill of the rolling mill production line,
and the
system is a box structure with a total length of 18 m, and spray nozzles are
densely
distributed on the top of the box to moderately cool the roughed intermediate
slab;
Corresponding to different thickness of the intermediate slab, the cooling
speed of the
intermediate slab is obtained to be 4-18 C/s; the thickness of intermediate
slab is
usually about 40-180 mm according to product and production requirements, and
the
intermediate slab whose thickness of is less than 40 mm is not opened for
moderate
cooling unless necessary, because it is thinner; for thick intermediate slabs,
considering the design limit, the maximum cooling speed is 4 C/s, while for
thin
slabs, the maximum cooling speed can reach 18 C/s.
Furthermore, the operation process of the Mild cooling system is as follows:
the
intermediate slab is obtained by the rolling in the recrystallization zone,
and the
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intermediate slab swings back and forth after entering the Mild cooling system
in
which the corresponding roller path operates the swing mode, meanwhile the
nozzle
sprays the water to the intermediate slab, in order to cool the intermediate
slab to the
start rolling temperature of the second stage at a specific cooling rate, and
when the
intermediate slab has been cooled to the start rolling temperature of the
second stage,
the intermediate slab is delivered out of the Mild cooling system and is sent
to the
rolling process at the second stage.
The invention has the following characteristics:
1) The technical problems of defective or unstable low-temperature impact
performance under high carbon equivalent are solved by adopting appropriate
composition and specific production technology, which makes the steel plate
for bent
pipes have excellent low-temperature impact toughness.
2) In the invention, a stepwise gradient cooling process is adopted for steel
plate
cooling, which can be realized on the spot without additional equipment
investment
with characteristics of high production efficiency and simple process.
Brief Description of Figures in the Drawings
Figure 1 is a comparison of ACC stepwise gradient cooling and a conventional
cooling in an embodiment of the invention;
Figure 2 is a structural diagram of a steel plate according to an embodiment
of
the invention.
Mode(s) for Carrying Out the Invention
The present invention is further described in detail with reference to
embodiments.
The preparation process of a X80 pipeline steel plate with high carbon
equivalent
and high toughness at low temperatures, which is used for bent pipes, is as
follows in
sequence: ratio preparation ¨> converter or electric furnace smelting ¨> out-
of-furnace
refining ¨> continuous casting ¨> slab reheating ¨) rolling ¨> ACC gradient
cooling
¨> straightening.
The specific process steps are as follows: the raw materials are processed in
sequence by KR molten steel pretreatment, converter smelting, LF refining, RH
vacuum refining, continuous casting, wherein in refining a Ca/S molar ratio is
controlled to be not less than 1, and the type B inclusions are controlled to
be Grade
1.5 or less. When continuous casting, soft reduction is used to prevent core
¨ 7 ¨
segregation caused by high Mn content. The continuous casting slab
manufactured
satisfies composition requirements, has a thickness of 350 mm; when the slab
is
heated, the temperature of the second-step heating section is controlled at
not more
than 1300 C, and the residence time in this heating section is not less than
4 hours;
then the process of rolling, ACC stepwise gradient cooling and straightening
are
sequentially conducted.
The specific TMCP process includes two-stage rolling and intermediate slab
cooling: the first stage is the rolling in the recrystallization zone, wherein
the final
rolling temperature is not higher than 1200 C, and when rolling in the
recrystallization zone, the single-pass rolling reduction ratio of continuous
two or
three passes is controlled to be not less than 20%;
The intermediate slab cooling is quickly cooling the intermediate slab to the
temperature of a start rolling temperature in the non-recrystallization zone
at the
second stage, by a Mild cooling system, wherein the cooling method is to cool
the
tank body by swinging back and forth, and the cooling rate is 6-12 C/s, to
ensure that
the austenite grains no longer grow after rolling and shaping in the
recrystallization
zone, and the temperature difference between the surface of the intermediate
slab and
the core is small;
The second stage is the rolling in the non-recrystallization zone, wherein the
start
rolling temperature is not higher than 900 C, and the final rolling
temperature is
controlled to be no higher than 850 C; after rolling, the steel plate is
cooled by water,
wherein the start cooling temperature is controlled to be not higher than 800
C, the
final cooling temperature is controlled to be not higher than 500 C, and the
cooling
rate is 10 to 35 C/s; considering that the finished steel sheet has a high
carbon
equivalent, a stepwise gradient cooling process is adopted in this cooling
process, that
is, the cooling water amount of each cooling unit of the ACC is set to be
different: the
amount of cooling water in the first 1-6 segments is the largest with a
corresponding
cooling rate of 25-35 C/s and the amount of cooling water in the ACC cooling
unit in
the last 7-12 segments decreases in sequence with a corresponding cooling rate
change of 10-20 C/s; after cooling, the steel plate is straightened, and then
the final
product is obtained by directly cooling to room temperature.
The specific chemical composition of the steel plate involved in each
embodiment is shown in Table 1, the specific TMCP process parameters are shown
in
Table 2, and the main mechanical properties are shown in Table 3.
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Table 1
Embodiment C Mn Si S P Nb Ti V Alt Mo+Cu+Ni+Cr
1 <0.1 1.55-1.9 <0.35 0.0005 0.01 <0.08 <0.015 <0.007 <0.06 1.5
2 0.10 1.65 0.25 0.0005 0.008
0.070 0.018 0.008 0.028 <1.5
3 0.05 1.9 0.20 0.0005 0.009
0.065 0.018 0.007 0.030 <1.5
Table 2
Embodiment Temperature Rolling in the Cumulative
Start Final Cooling Final
of the recrystallizati deformation temperature temperatu
speed of cooling
second-step on zone, the rate of the of the re of the water
temperature
heating final rolling rolling in the rolling in rolling in cooling
C temperature recrystallizat the the Cis
C ion zone non-recrysta non-recrys
% llization tallization
zone zone
C
1 <1300 <1100 <55 <900 <850 10-35 <500
2 <1300 <1100 <55 <900 <850 10-35 <500
3 <1300 <1100 <55 <900 <850 10-35 <500
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Table 3
Energy absorption under transverse
Embodi Yield Strength Tensile Strength Elongation after
Yield ratio Re/Rm impact ( V ) J
ment Rt0.5 Rm fracture %
Temperature 1 2 3
1 623 720 41.5 0.87 -30 351 367 355
2 631 719 42.3 0.88 -30 357 350 360
3 598 733 39.2 0.82 -30 389 378 395
Energy absorption under transverse
Embodi Yield Strength Tensile Strength
Elongation after Yield ratio Re/Rm impact (V) J
ment Rt0.5 Rm
fracture % Temperature 1 2 3
1 623 720 41.5 0.87 -45 267 272 251
2 631 719 42.3 0.88 -45 255 255 264
3 598 733 39.2 0.82 -45 361 375 379
Energy absorption under transverse
Embodi Yield Strength Tensile Strength Elongation after
Yield ratio Re/Rm impact ( V ) J
ment Rt0.5 Rm fracture %
Temperature 1 2 3
1 623 720 41.5 0.87 -50 253 257 251
2 631 719 42.3 0.88 -50 262 255 258
3 598 733 39.2 0.82 -50 353 349 357
Energy absorption under transverse
Embodi Yield Strength Tensile Strength Elongation after
Yield ratio Re/Rm impact ( V ) J
ment Rt0.5 Rm fracture %
Temperature 1 2 3
1 623 720 41.5 0.87 -60 233 246 239
2 631 719 42.3 0.88 -60 245 253 244
3 598 733 39.2 0.82 -60 339 327 331
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