[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. 2020-069313, filed
April 7, 2020, the content of which is incorporated herein by reference.
[Background 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]
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 embrittlement
temperature range. Therefore, when 0.20 mass% or more of Al is contained, since the
- 2 -
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 application 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.
[0008]
The present disclosure has been made in consideration of the above-described
problems, and an object of the present disclosure 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 resistance and a continuous casting method the slab.
[Means for Solving the Problem]
[0009]
- 3 -
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]
On the other hand, since Zr is an expensive metal, there is a desire to keep an
amount of Zr added as low as possible. Therefore, the inventors have found that it is
possible to suppress precipitation of large amounts of AlN at the grain boundary without
significantly increasing cost by adding Ti and Zr in appropriate amounts.
[0011]
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 and a Ti content satisfy the following formula (1), and
the Zr content satisfies the following formula (2).
[Zr] + 0.2 x [Ti] ~ 4/3 x [Al] x [N] .. · (1)
0.0010 mass%:::; [Zr] · · · (2)
Here, [Zr], [Ti], [Al], and [N] each represent a content (mass%) in the slab.
- 4 -
[0012]
(2)
The slab according to (1), further satisfies the following formula (3).
[Ti] I [Zr] ~ 1 .. · (3)
[0013]
(3)
The slab according to (1) or (2), in which a mass ratio of (Zr,Ti)N in all nitrides
in a surface layer area of the slab is 50.0 mass% or more.
[0014]
(4)
The slab according to any one of (1) to (3 ), further containing
Si: 0.20 mass% to 3.00 mass%, and
Mn: 0.50 mass% to 4.00 mass%.
[0015]
(5)
A continuous casting method of the slab according to any one of ( 1) to ( 4 ),
in which, when the slab is bent and straightened, the bending and the
straightening are performed at a surface temperature within a range of 800°C to 1000°C.
[0016]
(6)
The continuous casting method of the slab according to (5), 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]
[0017]
According to the present invention, it is possible to provide a slab that does not
- 5 -
include cracks attributed to straightening stress.
[Brief Description of the Drawings]
[0018]
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] + 0.2 x
[Ti] at a tensile temperature of 900°C.
[Embodiments of the Invention]
[0019]
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.
[0020]
On the other hand, since Zr is an expensive metal, there is a desire to keep the
amount of Zr added as low as possible. Therefore, the inventors studied adding Zr
and/or Ti, and performed the following experiments in order to find conditions under
which transverse crackings do not occur.
- 6 -
[0021]
(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 four types of steel (slabs), that is, types of steel A to D 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 contains only small amounts of both Zr and Ti, the steel grade
B contains relatively large amount of Zr and has almost the same composition as the
steel grade A except for Zr. The steel grade C contains relatively large amount of Ti
and has almost the same composition as the steel grade A except for Ti. On the other
hand, the steel gradeD is an example in which both Zr and Ti are relatively abundant.
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.
[0022]
[Table 1]
Steel grade c Si Mn p s Ti Zr AI N
A 0.23 1.0 2.50 0.011 0.002 0.002 0.0001 0.69 0.0035
B 0.22 1.0 2.48 0.010 0.002 0.002 0.0025 0.68 0.0033
c 0.23 1.0 2.51 0.010 0.002 0.015 0.0001 0.71 0.0032
D 0.23 1.0 2.50 0.012 0.002 0.015 0.0025 0.7 0.0036
[0023]
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 four 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 Ti > Al. Since with the simple addition of Ti alone, it is impossible to
precipitate TiN at high temperatures, a large amount of AlN precipitates, the
improvement in high temperature ductility is small and the effect cannot be obtained.
However, adding Ti together with Zr, as in the steel gradeD, fixes N as thermally stable
(Zr,Ti)N at high temperatures and greatly improves high temperature ductility. That is,
ZrN is precipitated immediately after solidification by adding both Zr and Ti, and N is
fixed at higher temperatures and high temperature ductility is improved compared to
adding Ti alone by promoting the precipitation of TiN in a form accompanying ZrN.
[0030]
For the above reasons, the slab according to the present embodiment contains
- 10 -
Zr to satisfy the following formula (2).
0.0010 mass%:::; [Zr] · · · (2)
[0031]
In addition, the upper limit of the Zr content is not particularly limited;
however, since Zr is an expensive metal, the Zr content is preferably 0.0050 mass% or
less from the view point of keeping the amount of Zr added as low as possible. In
addition, the upper limit and the lower limit of theN content are not particularly limited,
but theN content is preferably set to 0.0080 mass% or less as a range in which N is
contained after a usual refining step and a continuous casting step without intentionally
increasing the N content. In addition, when the cost in the refining step is taken into
account, theN content is preferably set to 0.0010 mass% or more. In addition,
although the present disclosure relates to high-Al steel, however, when the Al content
exceeds 2.0 mass%, the Zr content and the Ti content also increase according to the
formula (1), and the cost is vainly increased. Therefore, the Al content is 0.20 to 2.00
mass%, preferably 0.50 to 2.00 mass%, more preferably 0.55 to 2.00 mass%, and still
more preferably 0.60 to 2.00 mass%.
[0032]
Furthermore, from the viewpoint that it is desirable to reduce cost by using Ti
instead of Zr as much as possible, it is preferable that a ratio of [Ti] and [Zr] ([Ti] I [Zr])
satisfies the following formula (3). More preferably, the above ratio is 3 or higher.
The upper limit is not particularly limited, but the upper limit is preferably 10 or less.
When [Ti] I [Zr] is more than 10, the content of Zr decreases, so that (Zr,Ti)N which
fixing N may not be sufficient! y generated.
[Ti] I [Zr] ~ 1 .. · (3)
[0033]
- 11 -
As described above, in the slab according to the present embodiment, the
relationship among the Zr, Ti, Al, and N contents is made to satisfy the condition of the
above-described formulas (1) and (2). In addition, the upper limit of the Ti content is
not particularly limited, but even when an excessive amount ofTi is contained, the
effect is saturated, and the cost is vainly increased, and thus the Ti content is preferably
0.5 mass% or less. The lower limit of the Ti content is not particularly limited, but is
determined from the formulas (1) and (2), and the Ti content is preferably 0.0020
mass% or more. Incidentally, the contents of other elements are not particularly
limited, but C, Si, and Mn are preferably contained within the following ranges, and it
was confirmed that, in the present application, as long as C, Si, Mn, and the like are
contained within ranges shown in the present specification, the object of the invention
can be achieved.
[0034]

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%.
[0035]

Si is an element that improves the strength of steel, and, when the Si content is
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%.
- 12 -
[0036]

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: 0.0050% 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%, Ti:
0.0020% to 0.5%, and a remainder including iron and the impurities and, furthermore,
satisfies the above-described formulas (1) and (2), preferably formula (3).
[0037]
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
improved. Furthermore, N is fixed at higher temperatures and high temperature
ductility is improved compared to adding Ti alone by promoting the precipitation of TiN
in a form accompanying ZrN. In addition, Zr and Ti fix N in the composition of
- 13 -
(Zr,Ti)N. From such a viewpoint, the mass ratio of (Zr,Ti)N 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. As a result, transverse cracking in the slab can be
suppressed more reliably.
[0038]
Here, the mass ratio of (Zr, Ti)N 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 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 (Zr,Ti)N, AlN, NbN, BN, VN, and the like. In
addition, from the area proportion of (Zr,Ti)N in all of the nitrides obtained based on the
specification results, with an assumption that all of the nitrides are uniformly distributed
in the surface layer area of the slab, the area proportion can be regarded as the volume
fraction, and the mass ratio of (Zr, Ti)N in all of the nitrides is obtained from the volume
fraction. (Zr,Ti)N is defined as a nitride containing 50 mass% or more of Zr and Ti in
total with respect to the total mass of nitride particles and the mass% of Zr is 10 mass%
or more.
[0039]
- 14 -
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
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 bending and straightening the casting slab, in a case where the bending and
the straightening is performed when the surface temperature of the casting slab is 800°C
to 1 000°C, particularly, the effect becomes significant, which is preferable.
[0040]
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]
- 15 -
[0041]
Next, examples of the present invention will be described, but these conditions
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.
[0042]
Eighteen 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 Tables 3A and 3B (60 °C/min or 120 °C/min).
[0043]
In each of the slabs produced under the above-described conditions, the mass
ratio of (Zr,Ti)N in the surface layer area of the slab was measured by the abovedescribed
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 cracks was visually
confirmed. In addition, in a case where no transverse cracks were present, it was
- 16 -
evaluated as "0", in a case where one or more transverse cracks were present but could
be removed by light care (additional grinding of 0.7mm), it was evaluated as "1 ",and in
a case where one or more transverse cracks couldnot be removed by light care, it was
evaluated as "2", 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 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 Tables
3Aand 3B.
[0044]
[Table 3A]
z +~z !-<
~ X ~ ~ ~~X~ o~
~ E ~ C'l
~~ ~r:/J~~ ,......., '-"
r:/J r:/J ~ r:/J r:/J
r:/J r:/J ~~ '-" ro
0 r:/J r:/J r:/J r:/J ro ro r:/J r:/J ro ro r:/J ro r:/J r:/J - ro,.........
z § § ~ ~ § § ro S r:/J r:/J ""5 s s'---Jro ro ~ §S '----' ..... s s ~ s !-<
~ z !-< ~ !-< E-< '----' '----' !-< 0 N ~~~ 0~
~ ~
0 '-"
1 0.30 0.0040 0.0012 0.0015 0.0100 2.92 6.7 0
2 0.50 0.0052 0.0026 0.0020 0.0080 1.38 4.0 0
3 0.70 0.0040 0.0028 0.0030 0.0100 1.79 3.3 0
Present 4 0.90 0.0032 0.0029 0.0025 0.0120 1.70 4.8 0
Invention 5 1.10 0.0036 0.0040 0.0035 0.0150 1.64 4.3 0
Example 6 1.50 0.0032 0.0048 0.0040 0.0200 1.67 5.0 0
7 2.00 0.0028 0.0056 0.0050 0.0150 1.43 3.0 0
8 0.50 0.0035 0.0018 0.0020 0.0180 3.20 9.0 0
9 0.50 0.0041 0.0021 0.0020 0.0200 2.93 10.0 0
- 17 -
10 0.80 0.0029 0.0023 0.0015 0.0090 1.42 6.0 0
11 1.00 0.0034 0.0034 0.0030 0.0110 1.53 3.7 0
1 0.40 0.0035 0.0014 0.0002 0.0300 4.43 150.0 X
2 0.80 0.0042 0.0034 0.0020 0.0080 1.07 4.0 X
Comparat 3 0.20 0.0050 0.0010 0.0010 0.0010 1.20 1.0 X
ive 4 0.60 0.0042 0.0025 0.0020 0.0040 1.11 2.0 X
Example 5 1.00 0.0038 0.0038 0.0030 0.0050 1.05 1.7 X
6 1.50 0.0028 0.0042 0.0020 0.0100 0.95 5.0 X
7 2.00 0.0035 0.0070 0.0015 0.0150 0.64 10.0 X
Underlined value in Zr [mass%] does not satisfy the formula (2) and underlined
values in (Zr [mass%]+ 0.2 Ti [mass%]) I (AI [mass%] x N [mass%]) do not satisfy
the formula (1).
[0045]
[Table 3B]
Present
Invention
Example
0 z
1 0
2 0
3 0
1--4- -1----0- -1 Slow
1--5- -1----0- -1 cooling
6 0 60 oc/min
1---1-------1
7 0
8 0
9 0
10 0
11 0
1 1
2 1
Rapid
cooling
120 °C/min
3 1 Slow
Comparative
4 1 cooling
Example 1---5-1---1---1 60 oc/min
6 1
7 2
[0046]
70.0 90.0 VG
75.0 VG
80.0 85.0 VG
80.0 No VG
80.0 VG
65.0 85.0 No VG
55.0 75.0 VG
75.0 No VG
55.0 Yes G
50.0 Yes G
55.0 Yes G
30.0 B
35.0 B
30.0 45.0 B
30.0 Yes B
35.0 30.0 B
25.0 B
20.0 20.0 B
- 18 -
Underlines in Tables 3A and 3B indicate examples where the conditions of the
present invention are not satisfied. As shown in Tables 3A and 3B, in a case where the
conditions of the formulas (1) and (2) were satisfied, transverse cracks were not present
regardless of the Al or N content. On the other hand, in No. 1 of Comparative
Example that the formula (1) was satisfied and the formula (2) was not satisfied, it was
considered that the amount of Zr was insufficient and a large amount of AlN remained,
and transverse crackings were initiated. On the contrary, in Comparative Examples
No.2 to No.7 that the formula (2) was satisfied and the formula (1) was not satisfied, a
large amount of AlN was considered to remain, and transverse crackings were observed.
In a case where the formula (1) or the formula (2) was not satisfied, the mass ratio of
(Zr,Ti)N in the surface layer area of the slab was also below 50.0 mass%.
[0047]
Here, when the examples of the present invention were examined in more
detail, when the average cooling rate in the surface layer area of the slab was set to
60 °C/min or slower, it was possible to set the mass ratio of (Zr,Ti)N in the surface layer
area of the slab to 60.0 mass% or more. In this case, no defects attributed to transverse
crackings were confirmed even after hot rolling. On the other hand, in a case where
the average cooling rate in the surface layer area of the slab became 120 °C/min or in a
case where [Ti] I [Zr] is 10 or more even if the average cooling rate is 60 °C/min or
slower, the mass ratio of ZrN in the surface layer area of the slab became 50.0 mass% or
more and less than 60.0 mass%. In this case, no transverse crackings were confirmed
before hot rolling, but defects attributed to transverse crackings were confirmed after
hot rolling.
[0048]
Hitherto, the preferred embodiment of the present invention has been described
- 19 -
in detail with reference to the accompanying drawings, but the present invention is not
limited to such examples. It is evident that a person skilled in the art of the present
invention is able to conceive a variety of modification examples or correction examples
within the scope of the technical concept described in the claims, and such examples
should also be understood to be within the technical scope of the present invention.

CLAIMS
1. A slab of high-Al steel comprising:
C: 0.02 mass% to 0.50 mass%; and
Al: 0.20 mass% to 2.00 mass%,
wherein a Zr content and a Ti content satisfy the following formula (1 ),
and the Zr content satisfies the following formula (2).
[Zr] + 0.2 x [Ti] ~ 4/3 x [Al] x [N] .. · (1)
0.0010 mass%:::; [Zr] (2)
here, [Zr], [Ti], [Al], and [N] each represent a content (mass%) in the slab.
2. The slab according to claim 1,
wherein the following formula (3) is further satisfied.
[Ti] I [Zr] ~ 1 .. · (3)
3. The slab according to claim 1 or 2,
wherein a mass ratio of (Zr,Ti)N in all nitrides in a surface layer area of the
slab is 50.0 mass% or more.
to 4,
4. The slab according to any one of claims 1 to 3, further comprising:
Si: 0.20 mass% to 3.00 mass%; and
Mn: 0.5 mass% to 4.00 mass%.
5. A continuous casting method of the slab according to any one of claims 1
wherein, when the slab is bent and straightened, the bending and the
straightening is performed at a surface temperature within a range of 800°C to 1000°C.
6. The continuous casting method of the slab according to claim 5,
wherein an average cooling rate in a surface layer area of the slab is set to
60 °C/min or slower.