TECHNICAL FIELD
[0001] The present disclosure relates to a method of producing steel with a low amount of
oxide-based nonmetallic inclusions, namely, high-cleanliness steel, and particularly
relates to a method of producing calcium-added steel.
BACKGROUND
[0002] Because of the need for stricter product characteristics and higher-performance materials,
the demand for high-cleanliness steel with a reduced amount of oxide-based nonmetallic
inclusions in steel material is increasing. Moreover, high-strength steel pipes or
tubes for use in oil well pipes or tubes, etc. are used in acidified severe environments
(sour environments) containing hydrogen sulfide of corrosive gas, and therefore are
required to have excellent hydrogen-induced cracking resistance (HIC resistance) and
sulfide stress corrosion cracking resistance (SSC resistance).
[0003] For improvement in HIC resistance and SSC resistance, it is necessary to not only
reduce the amount of oxide-based nonmetallic inclusions in the molten steel stage,
but also reduce sulfide, such as MnS, which precipitates and crystallizes during solidification
of molten steel, and render it harmless. In particular, MnS is known to be harmful
to HIC resistance and SSC resistance, because MnS has high stretching property and,
in the subsequent rolling of the steel, stretches to become a hydrogen storage site.
[0004] A commonly known effective measure for this is to add Ca-containing metal in the
molten steel stage to change MnS to CaS. Regarding methods of adding Ca-containing
metal and its addition amounts, the following techniques are known.
[0005] JP 2002-60893 A (PTL 1) describes a production method for steel for an oil well having excellent
sulfide stress corrosion cracking resistance, wherein Ca or a Ca-containing substance
is added to molten steel after tapping of the molten steel from a converter and before
casting so that the Ca content in the molten steel will be 0.0005 mass% to 0.005 mass%,
and S, Al, Ca, and T.[O] (total oxygen) in the steel are controlled to satisfy the
following formula:

[0006] JP 2011-89180 A (PTL 2) describes a smelting method for steel material for a high-strength and high-corrosion-resistance
oil well pipe or tube, wherein T.[O] in molten steel is measured after completion
of secondary refining and, before starting pouring the molten steel into a tundish
of a continuous caster, Ca is added in an addition amount calculated based on the
measured value to control inclusions.
[0007] JP 2010-209372 A (PTL 3) describes the following production method for clean steel having excellent
sulfide corrosion cracking resistance: In a process of adding Al to molten steel to
deoxidize the molten steel during or after tapping of the molten steel from a converter
to a ladle, first adding CaO-containing flux to the molten steel in the ladle to perform
desulfurization treatment and adding Ca-containing metal during the desulfurization
treatment, thereafter subjecting the molten steel in the ladle to vacuum degassing
treatment and further adding Ca-containing metal to the molten steel in the ladle
after the vacuum degassing treatment, and thereafter casting the molten steel, pure
content of Ca in the Ca-containing metal in the desulfurization treatment is adjusted
depending on the Al concentration and the total oxygen concentration in the molten
steel.
CITATION LIST
Patent Literatures
SUMMARY
(Technical Problem)
[0009] By adding Ca-containing metal to molten steel, not only the formation of MnS can
be suppressed as described above, but also Al
2O
3-based inclusions can be changed to CaO-Al
2O
3-based inclusions. The techniques described in PTL 1 to PTL 3 each define the addition
amount of Ca-containing metal for the purpose of improving HIC resistance and SSC
resistance from this perspective. In detail, the techniques described in PTL 1 to
PTL 3 define the respective addition methods or addition amounts based on the concept
that, assuming only Al
2O
3-based inclusions are present before Ca addition, the Al
2O
3-based inclusions react with Ca and as a result change to appropriate CaO-Al
2O
3-based inclusions.
[0010] However, our study revealed that, with Ca addition based on such a concept, there
is a problem of nozzle clogging especially in an immersion nozzle of a small inner
diameter in a continuous casting line, and, for steel types required to have a high
strength of 110 psi (760 MPa) or more and strict SSC resistance, the formation of
large inclusions exceeding 200 µm cannot be completely prevented and such strict SSC
resistance requirement cannot be met.
[0011] It could therefore be helpful to provide a method of producing high-cleanliness steel
that can both prevent clogging of an immersion nozzle in a continuous casting line
and achieve superior sulfide stress corrosion cracking resistance (SSC resistance).
(Solution to Problem)
[0012] We closely studied an inclusion composition in steel for a high-strength seamless
pipe or the like used in a sour environment. Since such steel is typically required
to have extremely low S component and P component and low O component, the steel is
usually produced by the following process: First, Si and Al are added to molten steel
in a converter or a ladle subsequent to the converter, to perform deoxidation treatment.
Following this, CaO-containing flux is added to the molten steel in the ladle, to
perform ladle refining (desulfurization treatment) using a ladle furnace (LF). Next,
vacuum degassing treatment by an RH vacuum degasser is performed. Next, Ca treatment
of adding Ca-containing metal to the molten steel (hereafter also simply referred
to as "Ca addition") is performed. After this, the molten steel is transferred from
the ladle to a tundish, and continuously cast to form cast steel.
[0013] Regarding inclusions in the molten steel, Al
2O
3-based inclusions are predominant immediately after the deoxidation treatment. Steel
for a high-strength seamless pipe, a line pipe, etc. used in a sour environment is
required to have high strength, and thus typically has a composition having a Si content
of 0.1 % or more. In the case of producing such steel, for Si component, it is common
to add a large amount of a FeSi alloy simultaneously with Al as a deoxidizer and then
add, in the ladle that receives the molten steel tapped from the converter or in the
subsequent LF treatment and vacuum degassing, the FeSi alloy to the molten steel several
times so as to achieve the target Si content. About 1 % of Ca component is inevitably
contained in the FeSi alloy. Moreover, in the ladle refining, a reaction between CaO-Al
2O
3-SiO
2-based flux added for desulfurization and a refractory of MgO-C composition causes
Mg to enter into the molten steel. Thus, the inclusion composition at the end of the
ladle refining tends to be not Al
2O
3-based inclusions alone but have changed to CaO-MgO-Al
2O
3-based inclusions containing CaO and MgO.
[0014] Our study further revealed that, in the case where the CaO concentration varies among
a plurality of inclusions in the molten steel before Ca addition within one charge,
even when a predetermined amount of Ca-containing metal is added in the subsequent
Ca treatment, the composition of oxide inclusions in the molten steel in the tundish
stage varies. If there are variations in inclusion composition, nozzle clogging occurs
or a stricter SSC resistance requirement cannot be met.
[0015] As a result of conducting close research, we learned that, when the Ca concentration
in the molten steel after the vacuum degassing treatment and before the Ca treatment
is higher, the final inclusion composition is more likely to vary even though the
Ca addition amount in the subsequent Ca treatment is adjusted, and the probability
that giant inclusions of 200 µm or more are observed in cast steel is higher.
[0016] Regarding the production process, we learned the following: The foregoing problem
is more likely to occur in a rimmed steel tapping process of adding deoxidizers such
as Si and Al in the ladle subsequent to the converter (i.e. tapping without killing
a converter) than in a killed steel tapping process of adding deoxidizers such as
Si and Al in the converter (i.e. tapping with killing). Moreover, the foregoing problem
is particularly noticeable in the case where the FeSi alloy for Si component adjustment
is added in the latter half of the ladle refining (LF) or in the vacuum degassing
treatment. Analysis results in these cases indicate that the Ca concentration in the
molten steel after the vacuum degassing treatment and before the Ca treatment increased
to about 5 ppm to 10 ppm.
[0017] In view of this, we discovered the following: By satisfying all of: (1) performing
the deoxidation treatment not by rimmed steel tapping but by killed steel tapping;
(2) in the deoxidation treatment, adding the deoxidizers in the order of Si and Al;
and (3) in the case of additionally adding Si for chemical composition adjustment,
adding Si before or in the first half of the ladle refining and not in the latter
half of the ladle refining or in the vacuum degassing, the following effects (A) to
(C) can be achieved: (A) the Ca concentration in the molten steel from the converter
treatment to the vacuum degassing can be continuously maintained at a low concentration
of 4 ppm or less; (B) variations in the inclusion composition after the Ca addition
can be suppressed and the inclusion composition can be controlled within a 1600 °C
liquid phase composition range; and (C) the molten steel after the Ca addition contains
few large inclusions of 5 µm or more in diameter. Consequently, high-cleanliness steel
having superior SSC resistance can be produced while preventing clogging of an immersion
nozzle in a continuous casting line.
[0018] The present disclosure is based on these discoveries. We thus provide:
- [1] A method of producing high-cleanliness steel, the method comprising: adding Si
and then adding Al to molten steel in a converter to subject the molten steel to deoxidation
treatment; performing ladle refining of adding CaO-containing flux to the molten steel
and of subjecting the molten steel to desulfurization treatment using a ladle furnace;
thereafter subjecting the molten steel to vacuum degassing treatment using a vacuum
degasser; thereafter adding Ca-containing metal to the molten steel; and thereafter
continuously casting the molten steel, wherein Si is not added to the molten steel
in the ladle refining, or wherein in a case of adding additional Si for adjusting
a chemical composition of the molten steel, the additional Si is added in a first
half of a treatment period of the ladle refining and is not added in a latter half
of the treatment period of the ladle refining and in a period of the vacuum degassing
treatment.
- [2] The method of producing high-cleanliness steel according to [1], wherein the additional
Si is added within 10 minutes from a treatment start of the ladle refining.
- [3] The method of producing high-cleanliness steel according to [1] or [2], wherein
an interval between the addition of Si and the addition of Al in the deoxidation treatment
is 1 minute or more and 10 minutes or less.
- [4] The method of producing high-cleanliness steel according to any one of [1] to
[3], wherein a Ca concentration in the molten steel after the vacuum degassing treatment
and before the addition of the Ca-containing metal is 0.0004 mass% or less, and an
addition amount of the Ca-containing metal is set to satisfy the following Formula
(1):

where [%Ca], [%O], and [%S] are respective concentrations of elements Ca, O, and
S in the molten steel in a tundish, in mass%.
(Advantageous Effect)
[0019] It is thus possible to produce high-cleanliness steel having superior sulfide stress
corrosion cracking resistance (SSC resistance) while preventing clogging of an immersion
nozzle in a continuous casting line.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] In the accompanying drawings:
FIG. 1A is a production process flowchart of a method of producing high-cleanliness
steel according to one of the disclosed embodiments;
FIG. 1B is a production process flowchart of a method of producing high-cleanliness
steel according to a comparative example;
FIG. 2 is a diagram illustrating an example of changes in Ca concentration in molten
steel in each of the production processes of comparative methods 1 and 2 and a disclosed
method;
FIG. 3A is a diagram illustrating results of studying, in a plurality of charges,
the average composition of CaO-MgO-Al2O3-based inclusions in molten steel samples collected after RH treatment and before
Ca addition in each of the comparative methods 1 and 2 and the disclosed method;
FIG. 3B is a diagram illustrating results of studying, in each charge in FIG. 3A,
the average composition of CaO-MgO-Al2O3-based inclusions in molten steel samples collected after Ca addition;
FIG. 4 is a diagram illustrating results of studying the number of CaO-MgO-Al2O3-based inclusions of 5 µm or more in diameter in molten steel samples collected after
Ca addition in each of the comparative methods 1 and 2 and the disclosed method; and
FIG. 5 is a graph illustrating the relationship between the atomic concentration ratio
(ACR) index and the stress corrosion cracking (SSC) test fail rate.
DETAILED DESCRIPTION
[0021] A method of producing high-cleanliness steel according to one of the disclosed embodiments
comprises: adding deoxidizers to molten steel in a converter to subject the molten
steel to deoxidation treatment; performing ladle refining of adding CaO-containing
flux to the molten steel and of subjecting the molten steel to desulfurization treatment
using a ladle furnace; thereafter subjecting the molten steel to vacuum degassing
treatment using a vacuum degasser; thereafter adding Ca-containing metal to the molten
steel; and thereafter continuously casting the molten steel.
[0022] Types of deoxidation treatment include killed steel tapping treatment of adding deoxidizers
such as Si and Al in a converter (i.e. tapping with killing) as illustrated in FIG.
1A as an example and rimmed steel tapping treatment of adding deoxidizers such as
Si and Al in ladle refining or vacuum degassing treatment after converter refining
(i.e. tapping without killing a converter) as illustrated in FIG. 1B as an example.
In this embodiment, the killed steel tapping treatment is used as the deoxidation
treatment. With the rimmed steel tapping treatment, the Ca concentration in the molten
steel cannot be limited to 0.0004 % or less between the converter treatment and the
vacuum degassing treatment, the inclusion composition after Ca addition cannot be
controlled within a 1600 °C liquid phase composition range, and many large inclusions
of 5 µm or more in diameter form, as described later. This causes nozzle clogging
and insufficient SSC resistance. The deoxidation treatment can be performed by a typical
method of adding deoxidizers such as Si and Al to molten steel. A deoxidation product
formed as a result of the deoxidation treatment is Al
2O
3 (alumina).
[0023] In this embodiment, it is important to add Si and then add Al as the order of addition
of the deoxidizers in the deoxidation treatment. If Si is added after Al or Al and
Si are simultaneously added, the Ca concentration in the molten steel cannot be limited
to 0.0004 % or less between the converter treatment and the vacuum degassing treatment,
the inclusion composition after Ca addition cannot be stably controlled within the
1600 °C liquid phase composition range, and many large inclusions of 5 µm or more
in diameter form. This causes nozzle clogging and insufficient SSC resistance.
[0024] The interval between the Si addition and the Al addition in the deoxidation treatment
is not limited, but is preferably 1 minute or more and 10 minutes or less. If the
interval is less than 1 minute, the effects according to the present disclosure may
not be achieved sufficiently. If the interval is more than 10 minutes, giant SiO
2-MnO(-CaO) oxide may grow.
[0025] The ladle refining includes heating stirring treatment of introducing gas into the
molten steel while heating the molten steel by arc discharge using a ladle furnace
(LF). CaO-containing flux is added to the molten steel, and desulfurization treatment
is performed. Examples of the flux include quicklime (CaO) alone and a mixture of
quicklime and Al
2O
3 or SiO
2 as a slag formation accelerator for CaO. The vacuum degassing treatment can be performed,
for example, using a typical device such as a RH vacuum degasser. The treatment time
of the ladle refining and the treatment time of the vacuum degassing treatment are
not limited, and may be set as appropriate depending on the contents of O and S before
the treatment with respect to the target contents of O and S. Typically, the treatment
time of the ladle refining is about 30 minutes to 60 minutes, and the treatment time
of the vacuum degassing treatment is about 10 minutes to 40 minutes.
[0026] The chemical composition of the molten steel is eventually adjusted to the target
chemical composition as a result of adding an alloy in the vacuum degassing treatment.
Regarding Mn and Si components, typically the elements are added in large amounts
simultaneously with Al as a deoxidizer and then added several times until the ladle
refining or the vacuum degassing treatment ends to achieve the target content. In
this embodiment, on the other hand, it is important to, in the case of additionally
adding Si for chemical composition adjustment, add Si until the end of the first half
of the ladle refining and not in the latter half of the ladle refining and in the
vacuum degassing. It is also preferable to achieve the target Si content only by the
Si addition performed before the Al addition in the converter, without adding additional
Si from the ladle refining onward. Thus, the Ca concentration in the molten steel
from the converter treatment to the vacuum degassing can be continuously maintained
at a low concentration of 4 ppm or less, and high-cleanliness steel having superior
SSC resistance can be produced.
[0027] To further ensure the effects according to the present disclosure, in the case of
adding additional Si, the additional Si is preferably added within 10 minutes from
the treatment start of the ladle refining.
[0028] After the vacuum degassing treatment, Ca-containing metal is added to the molten
steel. The method of adding Ca is not limited. A commonly used method involves adding,
to the molten steel, a massive alloy containing Ca: 70 mass% and Si: 30 mass% or a
wire obtained by wrapping it in a Fe hoop. A Ca alloy reacts vigorously with the molten
steel and thus tends to form a molten steel reoxidation product when added. It is
therefore preferable to ensure perfect argon sealing upon the addition.
[0029] The Ca addition after the vacuum degassing treatment may be performed in a ladle
of the RH vacuum degasser, following the vacuum degassing treatment. However, it is
preferable to transfer the molten steel to a ladle dedicated to Ca treatment and then
add Ca to the molten steel in the ladle.
[0030] Experimental examples that led to the completion of the presently disclosed technique
will be described below.
(Disclosed method (method according to one of the disclosed embodiments))
[0031] In the process illustrated in FIG. 1A, steel having a chemical composition containing
C: 0.2 % to 0.3 %, Si: 0.22 % to 0.27 %, Mn: 0.4 % to 0.6 %, P: 0.005 % to 0.009 %,
S: 0.0005 % to 0.002 %, sol.Al: 0.03 % to 0.1 %, Ca: 0 % to 0.003 %, and O: 0.0010
% to 0.0020 % with the balance being Fe and inevitable impurities as molten steel
in the tundish was obtained by steelmaking. The converter treatment time was 60 minutes,
and a FeSi alloy of 2.2 kg/ton-steel was added 50 minutes after the treatment start
and then Al of 3.5 kg/ton-steel was added 5 minutes later. The treatment time of the
LF process was 30 minutes, and a FeSi alloy of 1.8 kg/ton-steel was added 10 minutes
after the treatment start as additional Si for chemical composition adjustment. Si
was not added in the latter half of the LF process and in the RH process. After the
RH process, Ca was added to the molten steel.
(Comparative method 1)
[0032] Steel was obtained by steelmaking in the same way as in the disclosed method, except
that the order of Al addition and Si addition in the converter was reversed. In detail,
in the comparative method 1, the converter treatment time was 60 minutes, and Al of
3.7 kg/ton-steel was added 50 minutes after the treatment start and then a FeSi alloy
of 2.2 kg/ton-steel was added 3 minutes later.
(Comparative method 2)
[0033] In the process of rimmed steel tapping illustrated in FIG. 1B, steel having a chemical
composition containing C: 0.2 % to 0.3 %, Si: 0.22 % to 0.27 %, Mn: 0.4 % to 0.6 %,
P: 0.005 % to 0.009 %, S: 0.0005 % to 0.002 %, sol.Al: 0.03 % to 0.1 %, Ca: 0 % to
0.003 %, and O: 0.0010 % to 0.0020 % with the balance being Fe and inevitable impurities
as molten steel in the tundish was obtained by steelmaking. In detail, Si and Al as
deoxidizers were not added in the converter. The treatment time of the subsequent
LF process was 45 minutes, and a FeSi alloy of 2.2 kg/ton-steel as deoxidizer Si was
added simultaneously with Al of 3.5 kg/ton-steel 5 minutes after the treatment start.
Further, an additional FeSi alloy was added for chemical composition adjustment 2
minutes after the start of the RH treatment. After the RH process, Ca was added to
the molten steel.
[0034] We collected molten steel samples for each of these production processes, and studied
the molten steel components, the amount of inclusions, and the inclusion composition.
The molten steel components were analyzed by Quantovac rapid analysis. The inclusions
were studied using a PSEM device produced by Aspex Corporation. Specifically, first,
a molten steel sample was collected from a depth position of 2 m or more from the
bath surface, and subjected to resin embedding and polishing to produce a sample for
SEM observation. SEM observation was performed on the sample, and the compositions
of all inclusions of 5 µm or more in inclusion diameter in an observation field of
15 mm × 15 mm were determined using an EDX and their average was calculated. In the
case where the inclusion cross-sectional shape was anisotropic, the square root of
the product of the major axis and the minor axis of an ellipse enclosing the cross-section
was taken to be the inclusion diameter.
[0035] Composition of inclusions are influenced by reactions among oxides produced by deoxidizer
(Al, Si, Mn, etc.) and elements entering from slag, and strongly deoxidizier (Ca,
Mg, Ti, etc.) contained in alloyes. Lastly, Ca treatment is performed in order to
suppress MnS inclusions which form during solidification, as a result of which an
oxide with high CaO content or a CaS-based sulfide is formed.
[0036] Our study has demonstrated that the inclusion composition mainly changes as follows:
- (1) before Al addition: Fe2O3 (+ MnO + SiO2 + CaO ...)
- (2) after Al addition: Al2O3 inclusions
- (3) during desulfurization treatment by CaO flux addition: MgO-Al2O3-based inclusions
- (4) Si addition: CaO-Al2O3-based inclusions increase
- (5) after ladle refining (LF), after vacuum degassing (RH): CaO-MgO-Al2O3-based inclusions
- (6) after Ca treatment: CaO-Al2O3-based inclusions + CaS.
[0037] Regarding the foregoing inclusion compositions, it is known that during desulfurization
treatment by CaO-based flux addition (3), Mg molten from a refractory or the like
into slag reacts with inclusions to form MgO·Al
2O
3 inclusions.
[0038] Si addition (4) is typically performed by adding a FeSi alloy for Si component adjustment.
Atypical FeSi alloy inevitably contains about 0.3 % to 1.5 % of Ca component, and
a slight amount of Ca component is added to the molten steel as a result of the Si
addition, so that CaO-Al
2O
3-based inclusions form. Another method for Si addition that can be used is to add
an alloy such as a SiMn alloy or Si scrap within a range in which the permissible
amounts of other components such as Mn are not exceeded.
[0039] In production of high-tensile-strength steel that involves an LF process for lowering
S while no Ca addition process is needed, for example, a refining process of "converter
→ LF process → RH process → tundish → mold" is performed. By controlling the timing
of adding a FeSi alloy in the same way as in the present disclosure, formation of
giant CaO·Al
2O
3-based inclusions in steel after continuous casting can be prevented.
[0040] Inclusions after each of ladle refining (LF) and vacuum degassing (RH) (5) are a
mixture of the foregoing MgO·Al
2O
3 inclusions and CaO·Al
2O
3 inclusions and exist as CaO-MgO-Al
2O
3-based inclusions. Considerable variations in composition were recognized, as described
later.
[0041] Ca treatment (6) is performed by charging the ladle after the vacuum degassing mainly
with a CaSi alloy. Typically, Ca is added to the molten steel so as to be 10 ppm or
more. The foregoing CaO-MgO-Al
2O
3-based inclusions become CaS sulfide or CaO-Al
2O
3-based inclusions with little MgO content.
[0042] The use of an alloy having a low Ca content of 0.1 % to 0.2 %, called high-purity
FeSi, is also effective in avoiding the above-mentioned entrance of Ca during FeSi
alloy addition. However, such an alloy is very expensive, and alloy types that can
be used in production are restricted. According to the present disclosure, an effective
method can be provided without using high-purity FeSi.
[0043] FIG. 2 illustrates changes in Ca concentration in molten steel in each of the comparative
methods 1 and 2 and the disclosed method. For the comparative method 1, the average
value of 22 charges is plotted. For the disclosed method and the comparative method
2, the average value of five charges is plotted. As is clear from FIG. 2, the Ca analysis
value before Ca addition is a low value of 4 ppm or less in the disclosed method,
whereas the Ca analysis value varies greatly from 5 ppm to 15 ppm in the comparative
methods 1 and 2.
[0044] The reason that the Ca concentration is low in the case of adding the deoxidizers
in the order of Si and Al in killed steel tapping is not clear, but is considered
to be as follows: In the case where FeSi is added in a high oxygen concentration state
with little deoxidation, Ca which has high oxidizability and easily evaporates oxidizes
instantaneously on the molten steel surface upon the addition, and either stays on
the molten steel surface or evaporates and discharges out of the system. In the case
where FeSi is added after or simultaneously with Al, on the other hand, in a state
in which oxygen in the steel decreases rapidly due to Al deoxidation and Al
2O
3 inclusions are formed, Ca reacts with Al
2O
3 inclusions and exists stably as CaO·Al
2O
3-based inclusions. In rimmed steel tapping, FeSi tends to be added several times in
ladle refining (LF) and vacuum degassing (RH) for chemical composition adjustment,
and, at each addition, a slight amount of Ca component enters the molten steel.
[0045] We further compared the average composition of all inclusions of 5 µm or more in
diameter between molten steel after RH treatment and before Ca addition and molten
steel after Ca addition. FIG. 3A illustrates results of studying, in a plurality of
charges, the average composition of CaO-MgO-Al
2O
3-based inclusions in each molten steel sample collected after RH treatment and before
Ca addition. FIG. 3B illustrates results of studying, in each charge in FIG. 3A, the
average composition of CaO-MgO-Al
2O
3-based inclusions in each molten steel sample collected in the tundish after Ca addition.
[0046] In all charges, Ca addition was performed with the Ca addition amount being determined
so that the inclusion composition after the Ca addition would be within the 1600 °C
liquid phase range in the tundish stage. As illustrated in FIG. 3A and FIG. 3B, however,
in the case of rimmed steel tapping (comparative method 1), the average composition
of inclusions before Ca treatment was already an inclusion composition containing
many CaO-Al
2O
3 inclusions, due to Ca which appeared to have derived from the FeSi alloy. In the
case of adding Si after Al in killed steel tapping (comparative method 2), too, the
change to CaO-Al
2O
3 inclusions progressed before Ca treatment, and the inclusion composition varied greatly.
[0047] In the case of killed steel tapping whereby Al addition was performed after Si addition
in the disclosed method, the inclusion composition before Ca addition was a uniform
composition with very little variations, containing MgO-·Al
2O
3 component as a main component and containing 10 wt% to 20 wt% of CaO. Consequently,
the inclusion composition of the tundish-collected sample after Ca treatment was able
to be controlled within the 1600 °C liquid phase range. With the comparative methods
1 and 2, on the other hand, the composition varied greatly and CaO-Al
2O
3 inclusions of a high CaO composition with a high melting point formed.
[0048] The following are the reasons for limiting the average composition of inclusions
in the tundish stage to the 1600 °C liquid phase range:
- (1) In the case where the average composition of inclusions in the molten steel stage
in the process is CaO-Al2O3 inclusion composition (3CaO·Al2O3 to CaO + CaS) of high CaO concentration associated with CaS precipitation, nozzle
clogging caused by CaS tends to occur in an immersion nozzle in the subsequent tundish
to mold stages upon a temperature decrease. Besides, large inclusions resulting from
coagulation drop from the nozzle adhesion site and are incorporated into the cast
steel. This causes significant degradation in HIC resistance and SSC resistance.
- (2) In the case where the average composition of inclusions in the molten steel stage
is a CaO-Al2O3 inclusion composition (especially, CaO·6Al2O3 to CaO·2Al2O3) of lower CaO concentration than the liquid phase inclusion composition (1600 °C
liquid phase range), too, nozzle clogging tends to occur. Moreover, harmful MnS is
likely to precipitate during solidification. This causes significant degradation in
HIC resistance and SSC resistance.
[0049] It is therefore important to control the inclusion composition to be an inclusion
composition of CaO·Al
2O
3 to 3CaO·Al
2O
3 and preferably an inclusion composition of 12CaO·7Al
2O
3.
[0050] FIG. 4 illustrates results of studying the inclusion cleanliness of each tundish-collected
sample used in FIG. 3B. The number of inclusions of 5 µm or more in diameter was considerably
improved by the disclosed method as compared with the comparative methods 1 and 2.
It seems that, with the disclosed method, the average composition of inclusions after
Ca addition was able to be controlled within the 1600 °C liquid phase range, so that
flotation and removal of inclusions were facilitated.
[0051] Next, the appropriate range of the Ca addition amount in the Ca treatment was determined
by studying Ca addition conditions and sulfide stress corrosion cracking (SSC) test
results beforehand.
[0052] FIG. 5 illustrates the relationship between the atomic concentration ratio (ACR value)
in the tundish when performing Ca addition after vacuum degassing treatment (RH) and
the fail rate in an SSC test in the disclosed method. In the SSC test, a uniaxial
tensile test was conducted for 720 hours by applying, to each test piece with the
same hardness of HRC = 27, stress of 85 % of the minimum yield point in a NACE test
liquid in which hydrogen sulfide of 1 atm had been saturated. In the SSC test, each
test piece fractured in an intermediate stage before the end of 720 hours was determined
as "fail". In the case of "fail", a fracture occurred mainly in a relatively short
time, i.e. within several tens of hours, from the test start (short-time fracture
type), and observation of the fractured surface revealed giant CaO-Al
2O
3 inclusions or CaS inclusions stretched to several hundred µm. The atomic concentration
ratio (ACR) is defined by the following formula:

where [%Ca], [%O], and [%S] are the respective concentrations (mass%) of elements
Ca, O, and S in the molten steel in the tundish.
[0053] The ACR value is an index used to control MnS sulfide which crystallizes during solidification
and CaS sulfide, CaO oxide, and calcium aluminate inclusion (CaO-Al
2O
3) composition which form during excessive Ca addition. It is commonly known that the
formation of MnS sulfide can be effectively suppressed when ACR ≥ 1.0, and the formation
of CaO-CaS inclusions caused by excessive Ca addition can be suppressed when ACR ≤
3.0.
[0054] However, our detailed evaluation using a pipe having a strength of 110 psi (760 MPa)
or more revealed that the SSC test fail rate sharply increases when ACR > 2.00, as
illustrated in FIG. 5. This result was obtained along with recognition that stress
corrosion cracking (SSC) is caused by CaS or CaO-Al
2O
3 inclusions having a higher melting point than the liquid phase state in the 1600
°C molten steel stage described above. Thus, the effectiveness of limiting the Ca
treatment condition to ACR = 1.00 to 2.00 was able to be confirmed.
[0055] According to the present disclosure, the composition of MgO-CaO-Al
2O
3-based inclusions before Ca addition is controlled to be in a less varying state,
to enable more accurate control of the subsequent oxide composition and sulfide composition.
Moreover, clogging of a tundish immersion nozzle caused by inclusions can be prevented,
and the formation of inclusions, such as oxide and sulfide, harmful to SSC resistance
can be sufficiently suppressed. The use of the presently disclosed technique makes
it possible to produce a steel pipe or tube having excellent SSC resistance without
clogging of an immersion nozzle caused by inclusions, and achieves production cost
reduction and yield rate stabilization.
EXAMPLES
[0056] Steel having a chemical composition containing C: 0.2 % to 0.3 %, Si: 0.22 % to 0.27
%, Mn: 0.4 % to 0.6 %, P: 0.005 % to 0.009 %, S: 0.0005 % to 0.002 %, sol.Al: 0.03
% to 0.1 %, Ca: 0 % to 0.003 %, and O: 0.0010 % to 0.0020 % with the balance being
Fe and inevitable impurities as molten steel in the tundish was obtained by steelmaking,
and cast using a round billet continuous casting machine of cast steel size 210Φ.
[0057] Table 1 lists the steel tapping form (killed steel tapping or rimmed steel tapping)
in the production, the FeSi alloy addition timing, the Ca concentration in the molten
steel before Ca treatment, the molten steel components in the tundish after Ca treatment,
and the ACR value. The converter treatment time was 60 minutes. In the case of killed
steel tapping, Si and Al were added to the molten steel in the converter, to perform
deoxidation treatment. The addition order is shown in Table 1. In the case of adding
Al after adding FeSi, a FeSi alloy of 2.2 kg/ton-steel was added 50 minutes after
the start of converter treatment, and then Al of 3.5 kg/ton-steel was added 5 minutes
later. In the case of adding FeSi after adding Al, Al of 3.7 kg/ton-steel was added
50 minutes after the start of converter treatment, and then a FeSi alloy of 2.2 kg/ton-steel
was added 3 minutes later. In the case of rimmed steel tapping, deoxidizers were not
added in the converter, and Si and Al were added 5 minutes after the start of LF treatment
to perform deoxidation treatment.
[0058] Next, CaO-Al
2O
3-SiO
2-based flux was added to the molten steel to perform ladle refining (desulfurization
treatment) using an LF. The treatment time of the LF process was 45 minutes. In FIG.
1, in each example in which Si was added in the LF first half, Si was added 5 minutes
after the start of LF treatment. In each example in which Si was added in the LF latter
half, Si was added 30 minutes after the start of LF treatment.
[0059] Next, vacuum degassing treatment using an RH vacuum degasser was performed. The molten
steel was then transferred to another ladle, and Ca was added to the molten steel.
The molten steel was then transferred from the ladle to the tundish, and continuously
cast to form cast steel.
<SSC resistance evaluation>
[0060] In a test of SSC resistance, a uniaxial tensile test was conducted for 720 hours
by applying, to each sample, stress of 85 % of the minimum yield point in a NACE test
liquid in which hydrogen sulfide of 1 atm had been saturated. Each sample used in
the SSC test had the same hardness of HRC = 27 as a result of heat treatment. The
SSC test was conducted on six samples for each condition, and the rate of the number
of samples that successfully passed the test without fracturing until the end of 720
hours is shown in Table 1 as the pass rate. A pass rate of 100 % is evaluated as good
SSC resistance.
<Nozzle clogging determination>
[0061] As a nozzle clogging determination method, the state of clogging was determined from
the opening degree (hereafter referred to as "SN opening") of a sliding nozzle in
an upper part of an immersion nozzle for pouring molten steel from the tundish into
the mold. In detail, in the case where the cross-sectional area of the flow passage
of the immersion nozzle decreases due to clogging, the SN opening approaches 100 %
by an inner-mold bath surface level automatic control function. With the present casting
conditions, operation with an SN opening of 60 % to 70 % corresponds to a stable casting
state, but the SN opening sharply increases to 80 % to 100 % if nozzle clogging occurs.
Hence, in the case where the SN opening was 80 % or more, it was determined that nozzle
clogging occurred.
Table 1
ID |
Category |
Steel tapping category |
FeSi alloy addition |
Before Ca treatment |
After Ca treatment (tundish) |
SSC test result |
SN opening (nozzle clogging degree evaluation) |
Converter (order of deoxidizers) |
LF first half |
LF latter half |
RH |
Ca amount (%) |
Ca (%) |
0 (%) |
S (%) |
ACR |
A |
Example |
Killed |
Added (FeSi, Al) |
Not added |
Not added |
Not added |
0.0001 |
0.0020 |
0.0010 |
0.0008 |
1.82 |
100% |
62% |
B |
Example |
Killed |
Added (FeSi, Al) |
Added |
Not added |
Not added |
0.0004 |
0.0021 |
0.0009 |
0.0008 |
1.94 |
100% |
65% |
C |
Example |
Killed |
Added (FeSi, Al) |
Added |
Not added |
Not added |
0.0004 |
0.0015 |
0.0010 |
0.0009 |
1.17 |
100% |
64% |
D |
Example |
Killed |
Added (FeSi, Al) |
Not added |
Not added |
Not added |
0.0002 |
0.0012 |
0.0012 |
0.0009 |
0.87 |
84% |
80% |
E |
Example |
Killed |
Added (FeSi, Al) |
Added |
Not added |
Not added |
0.0003 |
0.0025 |
0.0010 |
0.0008 |
2.32 |
50% |
98% |
F |
Comparative Example |
Killed |
Added (FeSi, Al) |
Added |
Added |
Added |
0.0006 |
0.0015 |
0.0010 |
0.0008 |
1.32 |
33% |
66% |
G |
Comparative Example |
Killed |
Added (Al, FeSi) |
Added |
Not added |
Added |
0.0005 |
0.0022 |
0.0010 |
0.0009 |
1.80 |
33% |
62% |
H |
Comparative Example |
Rimmed |
Not added |
Added |
Added |
Added |
0.0006 |
0.0021 |
0.0010 |
0.0008 |
1.92 |
17% |
62% |
I |
Comparative Example |
Rimmed |
Not added |
Added |
Added |
Added |
0.0008 |
0.0020 |
0.0010 |
0.0008 |
1.82 |
17% |
62% |
J |
Comparative Example |
Rimmed |
Not added |
Added |
Added |
Not added |
0.0005 |
0.0015 |
0.0010 |
0.0007 |
1.51 |
33% |
63% |
K |
Comparative Example |
Rimmed |
Not added |
Added |
Added |
Not added |
0.0004 |
0.0022 |
0.0012 |
0.0008 |
1.98 |
0% |
60% |
L |
Comparative Example |
Rimmed |
Not added |
Added |
Added |
Added |
0.0007 |
0.0011 |
0.0010 |
0.0007 |
1.05 |
33% |
96% |
M |
Comparative Example |
Killed |
Added (Al, FeSi) |
Added |
Not added |
Not added |
0.0006 |
0.0023 |
0.0010 |
0.0009 |
1.88 |
67% |
65% |
[0062] Steel sample IDs A, B, and C satisfied all conditions according to the present disclosure,
and were good in both SSC resistance and immersion nozzle clogging degree. Steel sample
ID D is an example in which the ACR value was less than the lower limit of the preferred
range, and clogging of the immersion nozzle caused by inclusions of CaO·6Al
2O
3 to CaO·2Al
2O
3 composition of a high melting point with a low CaO weight ratio increased and the
SSC test result worsened a little. Steel sample ID E is an example in which the ACR
value was more than the upper limit of the preferred range, and the SSC test result
decreased to 50 % (three out of six samples fractured) due to an increase of CaO-CaS-based
inclusions.
[0063] Steel sample ID F is a comparative example in which the FeSi addition timing did
not satisfy the conditions according to the present disclosure, and also the Ca concentration
before Ca treatment was more than the upper limit of the preferred range, so that
the SSC test result decreased to 33 % (four out of six samples fractured). Steel sample
ID G had the same results as steel sample ID F. Steel sample IDs H to L are comparative
examples in which rimmed steel tapping (non-deoxidized steel tapping) was used and
the FeSi addition timing did not satisfy the conditions according to the present disclosure,
and also the Ca concentration before Ca treatment was high, so that the SSC test result
was low. Steel sample ID M is a comparative example in which the addition order of
FeSi and Al did not satisfy the conditions according to the present disclosure, and
also the Ca concentration before Ca treatment was high, so that the SSC test result
was below the levels of steel sample IDs A, B, and C.
INDUSTRIAL APPLICABILITY
[0064] It is therefore possible to produce high-cleanliness steel having superior SSC resistance
while preventing clogging of an immersion nozzle in a continuous casting line.