[Technical Field of the Invention]
[0001]
The present invention particularly relates to a slab of steel containing a large
amount of Al and a continuous casting method thereof.
Priority is claimed on Japanese Patent Application No. 2020-069306, filed
April 7, 2020, the content of which is incorporated herein by reference.
[Related Art]
[0002]
In recent years, as high-strength iron and steel materials for thin sheets, a
number of alloy steels containing a large amount of Al have been manufactured in order
to improve mechanical properties. However, as the amount of Al added increases, in
continuous casting, transverse crackings are more likely to be initiated in the surface
layers of casting slabs, which has been a problem in terms of operation and product
quality.
[0003]
At straightening points in curved or vertical bending-type continuous casting
machines, straightening stress is applied to casting slabs. It is known that transverse
crackings are initiated along prior austenite grain boundaries in the surface layers of
casting slabs, and straightening stress concentrates on film-like ferrite that is formed
along austenite grain boundaries embrittled due to precipitation of AlN, NbC, or the like
and prior austenite grain boundaries, whereby transverse crackings are initiated. In
addition, these transverse crackings are likely to be initiated particular! y in temperature
- 1 -
ranges slightly higher than the phase transformation region from austenite to ferrite, but
transverse crackings are also initiated even in non-transformation compositions in the
same manner. Therefore, usually, a method in which the surface temperature of a
casting slab is controlled at a straightening point so as to avoid a temperature region
(poor ductility temperature region) where ductility deteriorates and the initiation of
transverse crackings is suppressed is adopted.
[0004]
However, in many cases, it is difficult to control the surface temperature of a
casting slab to avoid the poor ductility temperature region because there are significant
operational restrictions on attempts therefor. Therefore, Patent Document 1 discloses a
technique in which more than 0.010 mass% and 0.025 mass% or less of Ti is added and
the surface temperature of a casting slab in the upper portion of a secondary cooling
zone where the thickness of a solidified shell of the casting slab is 10 mm to 30 mm is
set to equal to or higher than the precipitation start temperature of AlN.
[Prior Art Document]
[Patent Document]
[0005]
[Patent Document 1] Japanese Patent No. 6347164
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0006]
However, in recent years, in order to further improve mechanical properties,
high-Al steel containing 0.20 mass% or more of Al also has been manufactured. An
increase in the Al concentration precipitates AlN at higher temperatures and expands the
poor ductility temperature region. Therefore, when 0.20 mass% or more of Al is
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contained, since the poor ductility temperature region is significantly expanded, it is
almost impossible in usual operation to avoid poor ductility temperature regions and
perform bending and straightening, and it is impossible to avoid transverse crackings.
In addition, when 0.50 mass% or more of Al is contained, since the poor ductility
temperature region is more significantly expanded, even in operation where cooling
conditions have been improved, it is almost impossible to avoid poor ductility
temperature regions and perform bending and straightening, and it is impossible to
avoid transverse crackings. For slabs where transverse crackings are initiated, not only
is maintenance such as a grinder required, but defects attributed to the transverse
crackings after hot rolling are also confirmed, which makes it impossible to avoid the
deterioration of the yield. An object of the present invention is to provide a slab
having exceptional manufacturability that is obtained by continuous casting and does
not require maintenance for transverse crackings.
[0007]
In addition, the method described in Patent Document 1 is applicable to lowcarbon
aluminum killed steel having anAl concentration of 0.063 mass% to 0.093
mass%, and whether or not this method is effective for high-Al steel containing 0.20
mass% or more of Al is not clear. Addition of a large amount of Ti in accordance with
an increase in the Al concentration is conceivable, but a large amount of Ti coarsens TiN
and causes a decrease in fatigue strength, and thus there are limitations on the amount of
Ti added.
[0008]
The present invention has been made in consideration of the above-described
problems, and an object of the present invention is to provide a slab that is a casting slab
of high-Al steel containing 0.20 mass% or more of Aland has exceptional surface crack
- 3 -
resistance and a continuous casting method the slab.
[Means for Solving the Problem]
[0009]
The present inventors paid attention to the fact that high-temperature
embrittlement in casting slabs of high-Al steel is attributed to precipitation of a large
amount of AlN and studied the precipitation control of nitrides. Specifically, the hightemperature
ductility of steel to which Zr having a higher N-fixing capability than Al
was added was investigated. As a result, it was found that addition of a small amount
of Zr significantly improves high-temperature ductility. It was found that, since Zr
forms ZrN and fixes N immediately after solidification, precipitation of a large amount
of AlN in grain boundaries is suppressed, and high-temperature embrittlement of highAl
steel can be fundamentally improved.
[0010]
Based on what has been described above, the present invention is as described
below.
(1)
A slab of high-Al steel containing C: 0.02 mass% to 0.50 mass% and Al: 0.20
mass% to 2.00 mass%,
in which a Zr content satisfies the following formula (1).
[Zr] ~ 4/3 x [Al] x [N] .. · (1)
Here, [Zr], [Al], and [N] each represent a content (mass%) in the slab.
[0011]
(2)
The slab according to (1), in which a mass ratio of ZrN in all nitrides in a
surface layer area of the slab is 50.0 mass% or more.
- 4 -
[0012]
(3)
The slab according to (1) or (2), further containing
Si: 0.20 mass% to 3.00 mass%, and
Mn: 0.50 mass% to 4.00 mass%.
[0013]
(4)
A continuous casting method of the slab according to any one of (1) to (3),
in which, when the slab is straightened, the straightening is performed at a
surface temperature within a range of 800°C to 1000°C.
[0014]
(5)
The continuous casting method of the slab according to ( 4 ), in which an
average cooling rate in a surface layer area of the slab is set to 60 °C/min or slower.
[Effects of the Invention]
[0015]
According to the present invention, it is possible to provide a slab that does not
include cracks attributed to straightening stress.
[Brief Description of the Drawings]
[0016]
FIG. 1 is a diagram showing changes in reductions in area at tensile
temperatures within a range of 700°C to 11 00°C.
FIG. 2 is a diagram showing a relationship between [Al] x [N] and [Zr] at a
tensile temperature of 900°C.
[Embodiments of the Invention]
- 5 -
[0017]
Hereinafter, the present invention will be described with reference to the
drawings. In the present embodiment, numerical ranges expressed using "to" include
numerical values before and after "to" as the lower limit and the upper limit.
Numerical values expressed using "more than" or "less than" are not included as lower
limits or upper limits.
In order to manufacture high-Al steel containing 0.20 mass% or more of Al, it
is necessary to prevent the initiation of transverse crackings due to straightening stress
at a straightening point during continuous casting. Since it is difficult to deviate
temperatures from the poor ductility temperature region at the straightening point, the
present inventors studied addition of Zr in order to straighten casting slabs in ordinary
temperature ranges at the straightening point.
[0018]
(First experiment)
First, a high-temperature tensile test was performed to confirm the amount of
Zr that improves high-temperature ductility. In this test, experiments were performed
with two types of steel (slabs), that is, a steel grade A and a steel grade B shown in Table
1. The units of all numerical values indicated in Table 1 are "mass%", and, as shown
in Table 1, while the steel grade A does not contain Zr, the steel grade B contains Zr and
has almost the same composition as the steel grade A except for Zr. The remainder
includes Fe and impurities in all of the types of steel. The "impurity" refers to an
element that is contained by accident from ore or scrap that is a raw material or from
manufacturing environments or the like at the time of industrially manufacturing the
slab.
[0019]
- 6 -
[Table 1]
Steel grade c Si Mn p s Ti Zr AI N
A 0.23 1.01 2.47 0.011 0.002 0.002 0 0.70 0.0019
B 0.22 1.01 2.48 0.011 0.002 0.002 0.033 0.68 0.0020
[0020]
Next, the tensile temperature was changed within a range of 700°C to 11 00°C,
and reductions in area (R. A.) were obtained for these two types of steel. Specifically,
based on JIS G0567: 2020, cogging was performed on each grade of steel produced by
vacuum melting of 25 kg up to
C is an element that improves the strength of steel, and, when the C content is
less than 0.02 mass%, the slab does not satisfy conditions for use as a high strength steel
sheet. In addition, when the C content exceeds 0.50 mass%, the hardness becomes
excessive, and bendability cannot be guaranteed. Therefore, the C content is set to
0.02 mass% to 0.50 mass%.
[0029]

Si is an element that improves the strength of steel, and, when the Si content is
- 10 -
less than 0.20 mass%, the slab does not satisfy a use as a high strength steel sheet. In
addition, when the Si content exceeds 3.00 mass%, the weldability is adversely affected.
Therefore, the Si content is preferably set to 0.20 mass% to 3.00 mass%.
[0030]

Mn is an element that improves the strength of steel, and, when the Mn content
is less than 0.50 mass%, the slab does not satisfy a use as a high strength steel sheet.
In addition, when the Mn content exceeds 4.00 mass%, since Mn is a segregation
element, there is a possibility that the strength may become uneven in casting slabs or
steel sheets. Therefore, the Mn content is preferably set to 0.50 mass% to 4.00 mass%.
The remainder other than the above-described elements is iron and impurities, but the
slab may contain several components instead of some of the iron. Here, the "impurity"
refers to, as described above, an element that is contained by accident from ore or scrap
that is a raw material or from manufacturing environments or the like at the time of
industrially manufacturing the slab. Therefore, the slab according to the present
embodiment contains, by mass%, for example, Al: 0.20% to 2.00%, Zr: 0.1% or less, N:
0.0010% to 0.0080%, C: 0.02% to 0.50%, Si: 0.20% to 3.00%, Mn: 0.50% to 4.00%, P:
0.0005% to 0.1 %, S: 0.0001% to 0.05%, Mo: 0% to 0.1 %, Nb: 0% to 0.1 %, V: 0% to
0.1 %, B: 0% to 0.005%, Cr: 0% to 0.1 %, Ni: 0% to 0.5%, Cu: 0% to 0.5%, and a
remainder including iron and the impurities and, furthermore, satisfies the abovedescribed
formula (1).
[0031]
Furthermore, as described above, since Zr forms ZrN and fixes N immediately
after solidification, precipitation of a large amount of AlN in grain boundaries is
suppressed, high-temperature embrittlement of high-Al steel can be fundamentally
- 11 -
improved, and it becomes possible to avoid transverse cracking in the slab. From such
a viewpoint, the mass ratio of ZrN in all nitrides in the 5 mm surface layer area where
the surface structure of the slab is uniformly present is preferably 50.0 mass% or more,
more preferably 60.0 mass% or more, and still more preferably 75.0 mass% or more.
[0032]
Here, the mass ratio of ZrN in the surface layer area of the slab is measured by
the following method. A sample for observing the surface layer of the casting slab (for
example, a sample that is 25 mm in width, 25 mm in length and 25 mm in thickness
from the widthwise center of the casting slab) is cut out from the manufactured slab,
and the surface at a depth position of 5 mm from the surface of the casting slab is
mirror-polished, thereby preparing an observed section. Next, the exposed surface
(observed section) is observed with a scanning electron microscope with an energy
dispersive X-ray analyzer (SEM/EDS). Element mapping on the observed section is
performed by the observation, and all nitrides having a size of 200 to 5000 nm
(equivalent circle diameter) on the observed section are specified. Here, examples of
nitrides that can be observed include ZrN, AlN, TiN, NbN, BN, VN, and the like. In
addition, with an assumption that all of the nitrides are uniformly distributed in the
surface layer area of the slab, the area proportion of ZrN in all of the nitrides obtained
based on the specification results can be regarded as the volume fraction, and the mass
ratio of ZrN in all of the nitrides is obtained from the volume fraction. ZrN is defined
as a nitride containing 50 mass% or more of Zr with respect to the total mass of nitride
particles.
[0033]
Next, a continuous casting method of the above-described slab will be
described. In the present embodiment, since there is no need to avoid the poor
- 12 -
ductility temperature region, it is possible to use, particularly, an ordinary method in
continuous casting. The results of the above-described first experiment show that, at
the time of straightening the casting slab, in a case where the straightening is performed
when the surface temperature of the casting slab is 800°C to 1000°C, particularly, the
effect becomes significant, which is preferable.
[0034]
Here, the average cooling rate in the surface layer area of the slab is preferably
set to 120 °C/min or slower and more preferably set to 60 °C/min or slower. In this
case, the mass ratio of ZrN in the surface layer area can be set to 50.0 mass% or more.
In particular, when the average cooling rate in the surface layer area of the slab is set to
60 °C/min or slower, it is possible to set the mass ratio of ZrN in the surface layer area
to 60.0 mass% or more. The average cooling rate in the surface layer area of the slab
is measured by the following method. That is, the temperature of the surface of the
slab in the center portion in the width direction is measured by a thermocouple or the
like, and the average cooling rate from 1450°C to 1 ooooc at a position 5 mm deep from
the position (measurement position) is calculated by two-dimensional heat transfer
calculation. Specifically, the difference between these temperatures ( 450°C) is divided
by the time necessary to cool the temperature at the measurement position from 1450°C
to 1000°C. Therefore, the average cooling rate in the surface layer area of the slab is
measured. The average cooling rate in the surface layer area of the slab can be
adjusted with the amount of secondary cooling water. The lower limit of the average
cooling rate needs to be, for example, 20 °C/min.
[Examples]
[0035]
Next, examples of the present invention will be described, but these conditions
- 13 -
are examples of conditions adopted to confirm the feasibility and effect of the present
invention, and the present invention is not limited to the description of these examples.
The present invention can be performed by a variety of means for achieving the object
of the present invention without departing from the gist of the present invention.
[0036]
Sixteen types of molten steel having a C content of 0.3 mass%, a Si content of
1.5 mass%, a Mn content of 2.0 mass%, and anAl content, aN content, and a Zr
content that were mutually different were prepared, each poured into a mold, and
continuously cast with a continuous casting machine. As the continuous casting
machine, a vertical bending-type continuous casting machine having mold sizes that
were 250 mm in thickness and 1200 mm in width was used, and the casting speed was
set to 1.2 m/min. In addition, at a straightening point, the surface temperatures of all
casting slabs were set to 850°C. In addition, the average cooling rates in the surface
layer areas were set to values shown in Table 3 (60 °C/min or 120 °C/min).
[0037]
In each of the slabs produced under the above-described conditions, the mass
ratio of ZrN in the surface layer area of the slab was measured by the above-described
method. Furthermore, in some of the slabs, the reductions in area (R. A.)(%) at 900°C
were obtained in the same manner as in the first experiment. Furthermore, transverse
crackings in the slabs were evaluated according to the following evaluation criteria.
That is, after the front and rear surfaces of the slab were ground 0.7 mm, and then the
presence or absence of transverse crackings was visually confirmed. Furthermore,
slabs from which transverse crackings could not be confirmed were heated to 1200°C in
a heating furnace in a hot rolling step without performing any maintenance for a defect,
roughly rolled, hot-rolled under conditions of a finish temperature of 880°C and a sheet
- 14 -
thickness of 2.8 mm, and the presence or absence of defects attributed to transverse
crackings after the hot rolling was visually confirmed. Slabs where no defects
attributed to transverse crackings were confirmed even after the hot rolling were
evaluated as very good (VG), slabs where defects attributed to transverse crackings
could be confirmed after the hot rolling were evaluated as good (G), and slabs where
transverse crackings could be confirmed before the hot rolling were evaluated as bad
(B). The experiment results are shown in Table 3.
- 15 -
[0038]
[Table 3]
4--< ~~ .s z 0 4--< 0 ~ 4--< r/J &' &' ~~ 1:l 8o 1=1 ~ X &' o~ ,......, 0 ~ "(jj 0 ~
ell~ 1=10 ;::i
~ ;..., u u