Note: Descriptions are shown in the official language in which they were submitted.
+ CA 02755271 2011-09-12
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HIC-Resistant Thick Steel Plate and UOE Steel Pipe
Technical Field
This invention relates to a thick steel plate and a UOE steel pipe. More
specifically, it relates to a high strength thick steel plate of at least X60
grade which
is improved in resistance to hydrogen induced cracking (HIC resistance) and
which
is suitable for use in applications such as line pipe, offshore structures,
and pressure
vessels. It also relates to a UOE steel pipe made from this thick steel plate.
In
this description, a thick steel plate means a steel plate with a thickness of
at least 6.0
mm.
Background Art
Line pipe used for transporting crude oil and natural gas requires not only
ordinary properties such as strength, toughness, and weldability, but it is
also
important for it to have resistance to hydrogen induced cracking (abbreviated
below
as HIC) because it is sometimes used in corrosive environments containing
hydrogen sulfide.
In the past, oil leaks, breakage, and explosions have actually taken place in
connection with line pipes and oil country tubular goods due to HIC.
Therefore,
intensive research has been performed concerning HIC. As a result, it has been
found that HIC occurs by the following mechanism. Hydrogen ions which are
formed by a corrosion reaction are adsorbed by the surface of steel and
diffuse into
the steel in the form of atomic hydrogen, which accumulates around MnS or
oxide
inclusions in the steel and becomes molecular hydrogen thereby forming a gas.
Cracks develop due to the internal pressure of the gas. Accordingly,
countermeasures for preventing HIC which have been disclosed are generally
classified into the following categories.
(1) If MnS is present in steel, cracks develop with the MnS as the starting
point and that cracking sensitivity increases as MnS is elongated at the time
of
3o rolling. Based on these finding, Patent Document 1 discloses that S is
contained in
steel in the form of fine spheroidized CaS or REM sulfides by decreasing the S
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content of steel and adding Ca or REM to steel.
(2) Hard phases such as martensite or bainite are formed at a location
corresponding to the region of center segregation in a cast slab due to
segregation of
C, Mn, P, and the like and become pathways for propagation of cracks. Taking
this into consideration, Patent Document 2 discloses that the formation of
such hard
phases is prevented by decreasing the concentration of C, Mn, P, and the like
in
steel, by carrying out soaking treatment in order to decrease segregation by
diffusion, and by increasing the cooling rate after rolling.
Patent Documents 3 and 4 disclose that center segregation itself can be
lo eliminated by causing bulging of a cast slab in a stage of continuous
casting in
which unsolidified molten steel remains and then subjecting the slab to
reduction in
thickness.
(3) As the strength specifications which are recently being demanded of
steel have become more stringent, individual countermeasures against the
above-described center segregation or formation of MnS are often inadequate.
Patent Documents 5 - 7 disclose that the formation of a protective coating on
the
steel surface by adding Cu or Ni to the steel in order to suppress the
infiltration of
hydrogen into the steel is combined with the addition of Cr, Mo or similar
elements
or application of a thermo-mechanical control process (TMCP) to rolling.
Prior Art Documents
Patent Documents
Patent Document 1: JP 54-110119 Al
Patent Document 2: JP 61-60866 Al
Patent Document 3: JP 09-57410 A 1
Patent Document 4: JP 2002-105604 Al
Patent Document 5: JP 06-220577 Al
Patent Document 6: JP 09-209037 Al
Patent Document 7: JP 2003-226922 Al
Summary of the Invention
The present inventors discovered that a mechanism of the occurrence of HIC
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which cannot be prevented by the conventional countermeasures explained above
in
(1) - (3) exists in a high strength thick steel plate of at least X60 grade
(having a YS
on the order of 60 ksi and YP on the order of 70 ksi) which is primarily used
for line
pipe and the like.
The object of the present invention is to provide a thick steel plate having
excellent HIC resistance (referred to below as a HIC-resistant thick steel
plate) and
a UOE steel pipe which can prevent HIC, which could not be prevented by
previously known countermeasures against HIC, in a thick steel plate having a
strength level of at least X60 grade and particularly in a thick steel plate
with a
lo thickness of at least 10 mm in which marked segregation easily occurs.
The present invention is based on the important finding that in addition to
MnS, C, Mn, and P which were conventionally thought to be the main impediments
to HIC resistance, carbonitrides of Nb become starting points of HIC.
The present invention is a thick steel plate having improved HIC resistance
characterized by having a chemical composition consisting essentially of, in
mass
percent, C: 0.02 - 0.07%, Si: 0.05 - 0.50%, Mn: 1.1 - 1.6%, P: at most 0.015%,
S: at
most 0.002%, Nb: 0.005 - 0.060%, Ti: 0.005 - 0.030%, Al: 0.005 - 0.06%, Ca:
0.0005 - 0.0060%, N: 0.00 15 - 0.007%, at least one of Cu, Ni, Cr, and Mo in
an
amount satisfying Equation (1), V: 0 - 0.10%, and a remainder of Fe and
impurities,
wherein the degree of Nb segregation is at most 2.0, the ratio a of the degree
of Nb
segregation to the degree of Mn segregation (a = the degree of Nb
segregation/the
degree of Mn segregation) is at least 1.0 and at most 1.5, the degree of Ti
segregation is at most 2.0, and the ratio 0 of the degree of Ti segregation to
the
degree of Mn segregation (f = the degree of Ti segregation/the degree of Mn
segregation) is at least 1.0 and at most 1.5.
Equation (l ): 0.1% < (Cu + Ni + Cr + Mo) < 1.5%
In the above equation, the symbols for elements indicate the content (mass
percent) of each element.
In the present invention, the degree of Nb segregation, the degree of Mn
segregation, and the degree of Ti segregation mean the values determined by
measuring the concentration of Nb, Mn, and Ti at 50 or more points and
preferably
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at 100 or more points above and below the center (midpoint) of the plate
thickness
in a cross section cut parallel to the rolling direction of a steel plate (a
cross section
perpendicular to the plate surface), and for each of Nb, Mn, and Ti, dividing
the
maximum concentration which is the maximum measured value among all the
measurement points by the average concentration which is the arithmetic mean
of
all the measured values. Namely, the degree of Nb segregation is (the maximum
Nb concentration) / (the average Nb concentration). Similarly, the degree of
Mn
segregation is (the maximum Mn concentration) / (the average Mn
concentration),
and the degree of Ti segregation is (the maximum Ti concentration) / (the
average
1 o Ti concentration).
The 50 or more and preferably 100 or more measurement points are
preferably positioned so that in a region having a length of at least 20% of
the
overall thickness in the plate thickness direction (namely, in a region having
a
length of at least 10% of the overall plate thickness above and below the
center of
the plate thickness), the points have a distance which increases by
approximately
the same rate above and below the center of the plate thickness. For example,
at
or more points and preferably at 50 or more points at intervals of 100 m (0.1
mm) each of above and below the center of the plate thickness, the
concentrations
of Nb, Mn, and Ti are measured. In all the cases, the degree of segregation of
each
2o element can be calculated by finding the average concentration and the
maximum
concentration of Nb, Mn, and Ti from the measured values obtained at 50 or
more
points. As described below, in the examples of the present invention, the
degree of
segregation is determined by measurement at 60 points at intervals of 0.1 mm
above
and below the center of the plate thickness (along the total measured length
of 12
25 mm).
As stated above, a thick steel plate means a steel plate having a plate
thickness of at least 6.0 mm. A preferred plate thickness is at least 10 mm in
which segregation occurs particularly easily. There is no particular upper
limit on
the plate thickness, but the present invention can achieve a HIC-resistant
thick steel
plate having a thickness up to 40 mm.
From another standpoint, the present invention is a UOE steel pipe
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characterized by being formed from the above-described HIC-resistant thick
steel
plate.
A UOE steel pipe is manufactured by press forming a thick steel plate into
the shape of a U and then into the shape of an 0, joining the abutting ends of
the
5 plate by submerged arc welding, and then expanding the pipe from the inner
side
with an expander to finish the pipe to predetermined dimensions.
A HIC-resistant thick steel plate and UOE steel pipe according to the present
invention can achieve a high strength of at least X60 grade (YP on the order
of 70
ksi). In addition, elements such as Mn, C, P, and S which have been proposed
in
lo the past as causes of HIC are controlled, and HIC caused by carbonitrides
of Nb or
Ti can be prevented. Therefore, the occurrence of HIC in a high strength thick
steel plate can be prevented with certainty.
A thick steel plate according to the present invention having improved HIC
resistance can be used in structures such as offshore structures or pressure
vessels,
or it can be used for the manufacture of UOE steel pipes. A UOE steel pipe
according to the present invention having improved HIC resistance is
particularly
suitable for line pipe, but it can also be used for offshore structures.
Because the
occurrence of HIC can be prevented with certainty, the reliability of products
is
markedly increased.
Brief Explanation of the Drawings
Figure 1 is a graph showing the results of investigation of the degree of
segregation of Mn, P, S, Nb, and C by the laser ICP method.
Figure 2(a) is a graph showing the relationship between the Mn
concentration and the Nb concentration, and Figure 2(b) is a graph showing the
relationship between the Mn concentration and the Ti concentration.
Modes for Carrying Out the Invention
The chemical composition of a thick steel plate according to the present
invention is as explained below. In the following explanation, percent with
respect
to the chemical composition of steel always means mass percent.
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[C: at least 0.02% and at most 0.07%]
C is generally known as an element which has a great effect on the strength
of steel. If the C content is less than 0.02%, it is difficult to obtain a
sufficient
strength required in applications such as line pipe. On the other hand, if the
C
content exceeds 0.07%, as stated above, macroscopic segregation takes place at
the
center of the thickness of a cast slab during continuous casting and causes
HIC.
Therefore, the C content is at least 0.02% and at most 0.07%. From the above
standpoints, the lower limit on the C content is preferably 0.03%, and the
upper
limit is preferably 0.06%.
[Si: at least 0.05% and at most 0.50%]
When present with a content of at least 0.05%, Si generally acts as a
deoxidizing element in the manufacture of steel and is effective at decreasing
the
oxygen concentration in steel. Si also has an effect of increasing the
strength of
steel. However, an Si content exceeding 0.50% leads to the formation of
martensite-austenite constituent, which deteriorates the toughness of HAZs
(heat
affected zones) in welding. In addition, due to the strong interaction of Si
with Ti,
Si affects the formation of TiN notwithstanding it is not a constituent
element of
TiN. Nb carbonitrides which have been identified as starting points for HIC in
the
present invention have a high probability of precipitating with TiN as nuclei.
Therefore, if the Si concentration becomes too high, a deterioration in HIC
resistance may occur. Accordingly, the Si content is at least 0.05% and at
most
0.50%. A preferred Si content is at least 0.05% and less than 0.3%.
[Mn: at least 1.1 % and at most 1.6%]
Mn is an element which generally has a large effect on the strength of steel.
It is difficult to obtain sufficient strength if the Mn content is less than
1.1 %. If the
Mn content exceeds 1.6%, as discussed previously, Mn concentrates in the
region of
center segregation, and HIC resistance deteriorates. Therefore, the Mn content
is
at least 1.1% and at most 1.6%. In order to guarantee HIC resistance in the
region
of center segregation with certainty, the Mn content is preferably at least
1.1 % and
less than 1.5%.
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[P: at most 0.015%]
The content of P, which is an impurity unavoidably contained in steel, is
preferably as low as possible. Due to its low distribution coefficient at a
solid-liquid interface at the time of solidification, P has a marked tendency
to
segregate, so as discussed previously, it concentrates in the region of center
segregation and causes HIC resistance to deteriorate. Therefore, the upper
limit on
the P content is 0.015%. In order to prevent a deterioration in HIC resistance
due
to segregation of P in the region of center segregation with certainty, the P
content
is preferably less than 0.008%. There is no lower limit on the P content.
lo However, in view of the fact that reducing the P content to an extremely
low level is
accompanied by a corresponding increase in costs, the P content is preferably
at
least 0.004%.
[S: at most 0.002%]
The content of S, which is an impurity which is unavoidably contained in
steel, is preferably as low as possible. In the same manner as P, S has a low
distribution coefficient at a solid-liquid interface at the time of
solidification and
hence a marked tendency to segregate. Furthermore, as discussed previously, in
the region of segregation, it forms MnS which becomes a starting point for
HIC.
Therefore, the upper limit on the S content is 0.002%. In order to obtain
stable
2o HIC resistance under more severe conditions such as are required with high
strength
steel, the upper limit on the S content is preferably 0.001%. There is no
lower
limit on the S content, but lowering the S content to an extremely low level
is
accompanied by a corresponding increase in costs. Therefore, the S content is
preferably at least 0.0003%.
[Nb: at least 0.005% and at most 0.060%]
Nb is an element which forms carbonitrides in steel, thereby increasing the
strength of steel and which is also effective at improving the toughness of
steel.
For this purpose, the Nb content is at least 0.005%. Particularly in TMCP, Nb
is
used in order to control the microstructure of a thick steel plate by
controlling the
formation of solid solution and precipitation. Also in order to obtain this
effect,
the Nb content is made at least 0.005%. However, if the Nb content exceeds
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0.060%, Nb does not completely dissolve to form a solid solution at the
heating
stage and the structure can no longer be controlled. At the same time, an
increase
in the Nb content means an increase in the amount of Nb carbonitrides, and
this
may cause a decrease in resistance to HIC. Therefore, the Nb content is at
least
0.005% and at most 0.060%. In order to stably guarantee resistance to HIC
under
more severe conditions which are demanded of high strength steel, for example,
the
Nb content is preferably at least 0.010% and at most 0.040%.
[Ti: at least 0.005% and at most 0.030%]
Ti is effective at increasing the strength of steel, and it fixes N in steel
as TiN
leading to a decrease in the amount of precipitation of NbN or A1N. Therefore,
it
has the effect of preventing surface cracking in a cast slab caused by dynamic
precipitation of NbN or A1N at austenite grain boundaries at the time of
bending or
straightening of a continuously cast slab. In order to achieve these effects,
the Ti
content is at least 0.005%. However, with a Ti content exceeding 0.030%, a
large
number of Ti carbide is formed, resulting in a decrease in the toughness of
HAZs
and causing the formation of coarse TiN. In addition, as described above,
there is
a high possibility of the precipitation of Nb carbonitrides occurring with TiN
as
nuclei, and the presence of coarse TiN causes a decrease in resistance to HIC.
Therefore, the Ti content is at least 0.005% and at most 0.030%. A preferred
Ti
content is at least 0.010% and at most 0.025%.
[Al: at least 0.005% and at most 0.06%]
Al is an element which is effective as a deoxidizing element which decreases
the oxygen concentration in steel. The Al content necessary for deoxidation is
at
least 0.005%. If the Al content is below this level, desulfurization becomes
inadequate, and the yield of added Ca worsens so that the effect thereof can
no
longer be adequately obtained. As a result, HIC caused by segregation of
sulfides
or S in steel takes place. However, alumina which is formed by deoxidation
with
Al sometimes becomes the cause of HIC. Therefore, the Al content is at most
0.06%. For the same reason, the Al content is preferably at most 0.04%.
[Ca: at least 0.0005% and at most 0.0060%]
In HIC-resistant steel, Ca is a kind of essential element for preventing the
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formation of MnS by decreasing the concentration of S as well as for
controlling the
shape of sulfides. Therefore, at least 0.0005% of Ca is added. However, with a
Ca content exceeding 0.0060%, its effects saturate and an increase in
manufacturing
costs results. Therefore, the Ca content is at least 0.0005% and at most
0.0060%.
[N: at least 0.0015% and at most 0.007%]
N is an element which is unavoidably incorporated in steel when steel
undergoes melting in air such as in a convertor. N is a constituent element of
coarse Nb carbonitrides on which the present invention is focused. Nb
carbonitrides is not directly preferentially linked to N, but it is known that
Nb
carbonitrides precipitate with crystallized TiN as nuclei. N is an element
which
forms nitrides with Al, Ti, or the like in steel, and these nitrides have the
effect of
refining crystal grains as pinning particles during hot working. Therefore, N
has
an effect on the mechanical properties of steel and also on the formation of
the
microstructure. For this reason, it is necessary for the N concentration to be
at
least 0.0015%. On the other hand, as stated above, in view of the fact that
surface
cracking of a cast slab is caused due to dynamic precipitation of these
nitrides at the
austenite grain boundaries during continuous casting, the upper limit on N is
0.007%.
[0.1% < (Cu + Ni + Cr + Mo) < 1.5%]
As stated with respect to Equation (1), the symbols for elements in the above
equation means the contents of those elements (in mass %).
In a HIC-resistant steel, the upper limits on the contents of C and Mn are
determined with the object of decreasing the formation of MnS and the
segregation
of C. Therefore, at least one alloying element selected from Cu, Ni, Cr, and
Mo is
added for ensuring strength. In order to achieve an effect on improvement in
strength with certainty, it is effective for the overall content of these
alloying
elements to be larger than 0.1%. However, a too high content of these alloying
elements is accompanied by an increased hardenability, which increases the
strength
and causes the structure to locally harden, resulting in a deterioration in
HIC
3o resistance. Therefore, the overall content of these alloying elements in
the present
invention is less than 1.5%.
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f
The individual contents of these alloying elements will be explained. As
can be seen from these explanations, it is preferable to add all four of these
elements,
but depending upon the desired strength level, it is also possible to add one
to three
of these. In any case, the total content of these elements is greater than
0.1% and
5 less than 1.5%.
Cu with a content of at least 0.1 % increases the hardenability of the steel.
On the other hand, if the Cu content exceeds 0.5%, the hot workability and
machinability of the steel decrease, and at the time of continuous casting,
surface
cracking referred to as copper checking is induced. Accordingly, the Cu
content is
lo preferably at least 0.1% and at most 0.5%. When at least 0.2% of Cu is
added, Ni
in an amount of at least one-third of the Cu content is preferably also added
in order
to prevent copper checking.
Ni has the effects of increasing the strength of steel by solid solution
strengthening and improving its toughness. These effects are obtained with a
Ni
content of at least 0.1%. However, these effects reach a limit and weldability
worsens when the Ni content exceeds 1.0%. Therefore, the Ni content is
preferably at least 0.1 % and at most 1.0%.
Cr increases the strength and toughness of steel. Therefore, the addition of
Cr is effective particularly for steel which requires high strength. The fact
that
2o adding even a small amount of Cr greatly contributes to increasing strength
can be
seen from the equation for carbon equivalent Ceq = C + Mn/6 + (Cr + Mo)/5 +
(Cu
+ Ni)/15. This effect is obtained with a Cr content of at least 0.05%. On the
other hand, addition of Cr in an amount exceeding 0.5% causes weld cracking to
occur. Therefore, the Cr content is preferably at least 0.05% and at most
0.5%.
Mo increases the hardenability of steel, thereby contributing to an increase
in
strength. In addition, since Mo is an element which does not readily undergo
microscopic segregation, it has the effect of suppressing the occurrence of
HIC
caused by center segregation. These effects of Mo are obtained with a Mo
content
of at least 0.02%. However, Mo is not only an expensive element which leads to
3o an increase in costs, but if greater than 0.5% of Mo is added, a hard phase
such as a
bainite phase or a martensite phase is formed, and HIC resistance ends up
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worsening. Therefore, the Mo content is preferably at least 0.02% and at most
0.5%. In view of its effect on HIC resistance which is large compared to that
of
the other three above-described elements, the Mo content is more preferably at
most
0.3%.
Next, optional elements will be explained.
[V: at most 0.10%]
V is effective at increasing the strength of steel by forming a solid solution
in
ferrite and carbonitrides in steel. Therefore, V may be added when a
particularly
high strength is demanded. However, if the V content exceeds 0.1 %, it has an
i o adverse effect on toughening of HAZs. Accordingly, when V is added, its
content
is at most 0.10%. In order to obtain the above-described effect of V with
certainty,
the V content is preferably at least 0.01 M.
A remainder of the composition other than the above-described elements is
Fe and impurities.
[Degree of Nb segregation: at most 2.0, (degree of Nb segregation) / (degree
of Mn segregation) (a): at least 1.0 and at most 1.5]
[Degree of Ti segregation: at most 2.0; (degree of Ti segregation) / (degree
of
Mn segregation) ((3): at least 1.0 and at most 1.5]
The degree of Mn segregation, the degree of Nb segregation, and the degree
of Ti segregation are the degrees of segregation in the central portion of the
plate
thickness and are determined as described above.
The present inventors found that in a thick steel plate used in line pipe or
the
like of at least X60 grade (YP on the order of 70 ksi), HIC sometimes occurs
even
when the contents of MnS and C, Mn, and P, which were conventionally thought
to
be the main causes of impairment of HIC resistance, are decreased. This was
thought to be because even if conventional countermeasures against HIC due to
MnS or macroscopic segregation were carried out, carbonitrides of Nb or Ti
remaining in the steel became starting points which cause cracks to develop.
By carrying out HIC tests on thick steel plates with varying degree of
segregation to investigate the effect of segregation, it was found that the
causes of
HIC in a thick steel plate include not only segregation of C, Mn, P, and S but
also
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segregation of Nb and Ti. Namely, in addition to relying on conventional
knowledge, by additionally controlling segregation of Nb and Ti, the HIC
resistance
of a thick steel plate of at least X60 grade which is used in line pipe or the
like can
be improved.
The present inventors carried out HIC tests on test pieces taken from thick
steel plate products. Cracks all developed in central portions in the
thickness
direction of the plates. This fact shows that HIC occurs in the region of
center
segregation. The regions where cracks developed in the test pieces which
showed
cracking were investigated in detail. Upon analysis by SEM/EDS of inclusions
lo which became the starting points of cracks, it was found that although the
concentration may varied, the inclusions were primarily carbonitrides
containing Nb
and indicated as Nb(C,N) (containing at most 10 volume % of Ti and referred to
in
this description as Nb carbonitrides).
The extent of concentration of each element (degree of segregation) can be
known by investigating the region of center segregation over a wider range of
steel
compositions. Investigation of the degree of segregation can be carried out by
the
EPMA (electron probe microanalyzer) method, by the laser ICP (laser ablation
inductively coupled plasma) method, or by chemical analysis.
The present inventors investigated the degree of segregation of each element
in the central portion of the plate thickness using a thick steel plate
produced by
rolling and investigated the relationship of the degree of segregation to the
occurrence of HIC.
The degree of segregation of each element was determined by the laser ICP
method. The apparatus which was used was a laser ICP analyzer manufactured by
Shimadzu Corp. In the laser ICP method which is a kind of emission
spectroscopy,
a cut cross section of a sample is irradiated with a laser beam to generate
vapor, the
vapor is transported by a carrier gas into an induction plasma to cause
emission, and
the wavelength and intensity of the emission spectrum are analyzed for
quantitative
analysis of each element. By repeating the analysis while moving the sample in
one direction and irradiating it with a laser beam, the variation in the
concentration
of each element over a certain length can be investigated.
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In a laser ICP apparatus, it is possible to perform measurement every 100 gm
(0.1 mm; measurement is carried out by moving the sample by 100 gm at a time),
so
measurement can be obtained at 100 points over a length of 10 mm. In the
present
invention, measured values of the concentration of each element are obtained
at 120
points over a total length of 12 mm having a distance of 6 mm above and below
the
center of the plate thickness in a cut cross section using a sample of a cross
section
cut perpendicular to the plate surface and parallel to the rolling direction
of a steel
plate. In this manner, macroscopic segregation can be adequately evaluated.
The
degree of segregation of each element is determined by taking the arithmetic
mean
lo of the measured values obtained at the 120 points as the average
concentration of
the element in the thick plate and dividing the maximum of the measured values
(the maximum concentration) by the average concentration.
The degree of segregation of each element can be obtained by actually
determining the concentration (mass %) of each element from the measured
values
(emission peak strength) at the 120 points obtained by the laser ICP method
and
calculating the ratio of the maximum concentration to the average
concentration.
However, due to the fact that the concentration and the emission peak strength
assigned to each element obtained by measurement are proportional to each
other,
the degree of segregation can be easily found as the ratio of the maximum peak
strength to the average peak strength for each element.
The diameter of a laser beam is approximately 1 mm, so all inclusions of 1
mm or less are reflected in the measured data. The size of inclusions is
usually on
the order of several micrometers and even large inclusions measure around
several
tens of micrometer. Therefore this measurement technique makes it possible to
adequately investigate the degree of segregation which is accompanied by the
formation of inclusions.
Figure 1 is a graph showing the degree of segregation for Mn, P, S, Nb, and
C obtained by the laser ICP method for a thick steel plate having 8.8%
occurrence
of HIC. In the graph of Figure 1, the abscissa shows the locations of the
measurement points in the plate thickness direction (the center of the plate
thickness
was at 60 in units of 0.1 mm, the measurement was performed over a length of 6
CA 02755271 2011-09-12
14
mm above and below the center of the plate thickness, and the total number of
measurement points was 120), and the ordinate shows the measured results for
each
element (emission peak strength in arbitrary unit). The level on the ordinate
is
different from element to element, but the average composition and the degree
of
segregation can be calculated by this measurement method.
As stated above, it is possible to use methods other than the laser ICP
method.
For example, it is possible to determine the degree of segregation by
measuring the
concentration of elements in a location of interest using chemical analysis or
the
like and comparing the results of analysis with the results for portions where
there is
lo no segregation. It is also possible to determine the degree of segregation
by the
EPMA method or by the laser ablation inductive coupled plasma mass
spectroscopy
(LA-ICP-MS) method in which quantitative determination in the laser ICP method
is carried out by mass spectrometry instead of by emission spectroscopy.
In the example shown in the graph of Figure 1, it can be seen from the graph
that the degree of Nb segregation is 2.1, the degree of Ti segregation is 1.8,
and the
degree of Mn segregation is 1.3. Thus, the ratio of the degree of Nb
segregation to
the degree of Mn segregation (a) is calculated at 1.7 and the ratio of the
degree of Ti
segregation to the degree of Mn segregation ((3) is calculated at 1.4.
Figure 2(a) is a graph using the data of Figure 1 in which the abscissa is the
Mn concentration and the ordinate is the Nb concentration, and Figure 2(b) is
a
graph in which the abscissa is the Mn concentration and the ordinate is the Ti
concentration. The concentration of each element is actually indicated by the
emission peak intensity. These graphs make it possible to compare the degree
of
Nb segregation or the degree of Ti segregation with the degree of Mn
segregation.
From the graphs shown in Figures 2(a) and 2(b), it can be seen that the
degree of Mn segregation is representative of the segregation of each element
in a
cast slab, and if the degree of segregation of a cast slab (degree of Mn
segregation)
worsens, the degree of Nb segregation and the degree of Ti segregation
similarly
worsen. Furthermore, it can be seen that if the degree of Mn segregation
exceeds a
certain critical point, as shown by the regions surrounded by circles in these
graphs,
Nb and Ti show a tendency to abruptly precipitate as inclusions.
CA 02755271 2011-09-12
The present inventors thought that inclusions which develop in large
quantities could become the cause of HIC, and they investigated the
relationship
between the occurrence of HIC and the ratio of the degree of Nb segregation to
the
degree of Mn segregation (a) and the ratio of the degree of Ti segregation to
the
5 degree of Mn segregation ((3). As a result, they found that the occurrence
of HIC
can be markedly suppressed if the degree of Nb segregation is at most 2.0 and
a,
which is the ratio of (the degree of Nb segregation) / (the degree of Mn
segregation),
is at least 1.0 and at most 1.5, and if the degree of Ti segregation is at
most 2.0 and
0, which is the ratio of (the degree of Ti segregation) / (the degree of Mn
lo segregation9, is at least 1.0 and at most 1.5.
The criterion for the occurrence of HIC according to the present invention is
particularly effective with respect to the resistance to HIC of high strength
steels of
at least X60 grade for which the conventional criterion was ineffective. If
the
criterion based on the degree of segregation of Nb and Ti according to the
present
15 invention is employed together with a decrease in C, Mn, P, and S which has
been
employed in the past, the occurrence of HIC can of course be effectively
prevented.
Even when thick steel plates have the same chemical composition, if the
manufacturing conditions are different, their state of segregation differs
from each
other. As a result, there are cases in which the above-described requirements
concerning the degree of segregation (the degree of segregation of Nb and Ti
and
the values of a and (3) may not be satisfied. Accordingly, a thick steel plate
according to the present invention can be manufactured by selecting not only
the
chemical composition but also the steel making conditions and the rolling
conditions so that segregation does not readily take place. Operating
conditions
which are effective for decreasing segregation will be briefly explained
below.
In the steel making stage, application of IR (injection refining) and shape
control of oxides by addition of Ca are effective at reducing segregation.
In the next continuous casting (CC) stage, segregation in the thickness
direction of a plate can be decreased by providing the cast slab with a slope
3o approximately corresponding to the amount of contraction due to
solidification of
the slab when the central portion of the slab solidifies or with a slightly
greater
CA 02755271 2011-09-12
16
slope. Employing suitable water cooling conditions or selecting the casting
speed
so that marked non-uniformity of solidification does not develop in the
widthwise
direction of casting and the lengthwise direction of casting is also effective
at
decreasing segregation.
In hot rolling of the cast slab into a thick plate (this may be direct rolling
carried out in succession with CC), it is effective to first perform heating
to at least
1100 C and at most 1200 C. This heating causes Nb which crystallized out in
the cast slab to go into solid solution, thereby preventing Nb from forming Nb
carbonitrides. In order to cause Nb to form a solid solution, it is effective
to adjust
lo the heating temperature or the duration of heating in accordance with the
Nb content.
When the Nb content is high, the heating temperature and/or the duration of
heating is increased.
After rolling, water cooling is started at a temperature equal to or higher
than
the Ara point. The reason why the temperature at the start of water cooling is
controlled in this manner is because if water cooling is started at a
temperature
below the Ara point, the formation of ferrite which is accompanied by
discharge of
carbon begins before water cooling. As a result, water cooling causes a hard
structure containing an increased amount of carbon to form, thereby
deteriorating
resistance to HIC.
Water cooling is effective because diffusion of elements such as C and P is
decreased as much as possible and C can be prevented from combining with Nb.
As the temperature decreases, the speed of diffusion of each element
decreases. If
the steel is let cool after rolling without water cooling, the length of time
for which
the steel is exposed to a higher temperature increases and diffusion of
elements may
be promoted. The diffused elements segregate at grain boundaries and in
inclusions.
The rate of water cooling is preferably at least 5 C per second and at most
C per second. At a rate of water cooling which is less than 5 C per second,
diffusion of elements is promoted, while if it is greater than 30 C per
second,
3o hardening is excessively introduced to the steel and a hard structure is
formed.
A UOE steel pipe manufactured from a thick steel plate according to the
CA 02755271 2011-09-12
17
present invention has increased reliability when used in a corrosive
environment
because it does not develop HIC due to coarse carbonitrides of Ti and Nb.
Methods of manufacturing a UOE steel pipe are known to those skilled in the
art.
In the present invention as well, a UOE steel pipe can be manufactured by the
same
methods as in the prior art.
Example 1
The present invention will be explained more specifically while referring to
examples.
Steels Nos. 1 - 14 having the chemical compositions and Ara points shown in
Table 1 was subjected to continuous casting using a continuous casting machine
of
a vertical bending type with a thickness of 300 mm and a width of 2300 mm at a
casting rate of at least 0.7 meters per minute and at most 0.8 meters per
minute to
obtain a cast slab.
The resulting cast slab was heated to at least approximately 1100 C and at
most approximately 1200 C, and then it was subjected to hot rolling to give a
plate
thickness of around 25.4 mm under conditions such that the finish rolling
temperature was at least approximately 750 C and at most approximately 850
C.
Immediately after hot rolling, the plate was water cooled. Water cooling was
stopped at a temperature of at least approximately 450 C and at most
approximately 550 C and followed by air cooling. The cooling rate during
water
cooling was 10 - 30 C per second.
The resulting thick steel plate was subjected to investigation of the degree
of
segregation using the above-described laser ICP method (120 measurement
points)
and to a tensile test and a HIC test. The test results and the chemical
compositions
of the steels are also shown in Table 1.
The HIC test was carried out by a NACE test prescribed by NACE
TM-02-84, and the crack area ratio (CAR) was measured as the rate of
occurrence
of HIC. A value of CAR of at most 3% was considered satisfactory, while a
value
3o exceeding 3% indicated that substantial HIC occurred and was determined to
be
unsatisfactory.
CA 02755271 2011-09-12
18
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CA 02755271 2011-09-12
19
As can be seen from Table 1, with thick steel plates made from the inventive
steels in which the ratios a and (3 of the degree of segregation of Nb and Ti,
respectively were small values of at most 1.5, the strength was of at least
X60 grade
with a tensile strength of at least 520 MPa and the rate of occurrence of HIC
(CAR)
was at most 3%. Accordingly, it is obvious to those skilled in the art that
UOE
steel pipes of high reliability can be manufactured from these thick steel
plates.
In contrast, thick steel plates made from the comparative steels were
unsatisfactory with respect to either strength or the rate of occurrence of
HIC. In
particular, Steels Nos. 4, 11, and 12 had a chemical composition which
satisfied the
lo requirements of the present invention, but the value of the degree of Nb
segregation,
the ratio (a) of (the degree of Nb segregation) / (the degree of Mn
segregation), or
the ratio ((3) of (the degree of Ti segregation) / (the degree of Mn
segregation) did
not satisfy the ranges for the present invention, and for each of these
examples, the
strength was inadequate or the rate of occurrence of HIC was high.
