Description
Title of Invention: CONTINUOUS-CAST SLAB, METHOD AND APPARATUS OF
MANUFACTURING THE SAME, AND METHOD AND APPARATUS OF
5 MANUFACTURING THICK STEEL PLATE
Technical Field
[0001] The present invention relates to a continuous-cast slab, a method and
apparatus of manufacturing the same, and a method and apparatus of manufacturing a
10 thick steel plate. Particularly, the present invention relates to: a continuous-cast slab
where central porosity and segregation, which inevitably form in the center of a cast
slab, are reduced, a method and apparatus of manufacturing the same; and a method and
apparatus of manufacturing a thick steel plate of a few flaws under ultrasonic testing,
manufactured using the continuous-cast slab and used for a nuclear reactor, a boiler, a
15 pressure vessel and so on.
Background Art
[0002] An outer surface, which is supported by support rolls, of a cast slab
solidifies first upon cast in continuous casting equipment. Thus, the center of the
20 castin~ thickness (in the cast slab thickness direction) solidifies last. In addition, when
molten steel solidifies, volume shrinkage of 3 to 4% occurs. Therefor, a micro-cavity
that is called central porosity inevitably forms in the cast slab center which solidifies
last. This central porosity remains even after rolling, and can be detected under
ultrasonic testing in the stage of a thick steel plate. Internal flaws caused by this
25 central porosity are harmful especially according to a use of a nuclear reactor, a boiler, a
2
pressure vessel or the like. Thus, to reduce the central porosity volume in a cast slab is
conventionally carried out.
[0003] Segregation is easy to form in addition to porosity in the cast slab center
which solidifies last. Especially, it is difficult to reduce both the porosity volume and
5 segregation at the same time in a slab having granular equiaxed crystals in its center.
The following are some reasons considered: (\) segregation is easy to form around
granular equiaxed crystals; (2) if granular equiaxed crystals move at the end of
solidification, a segregation portion also moves along with the granular equiaxed
crystals, and segregating elements are easy to gather at a portion surrounded by a
10 plurality of gathering granular equiaxed crystals, which makes segregation easy to be
large; and (3) porosity is easy to form at a portion surrounded by segregation that forms
around granular equiaxed crystals. Therefore, so far, porosity and segregation have
been tried to be remedied by making columnar crystals with which both porosity and
segregation are easier to be reduced at the same time than with granular equiaxed
15 crystals, easy to grow.
[0004] In a case where the central porosity volume is reduced by heavily rolling a
cast slab in the later process, conventionally, it is necessary to carry out heavy rolling of
no less than 0.7 in shape factory in the later process in order to reduce central porosity
of a conventional cast slab of 230 to 380 mm in thickness (casting thickness) D to a
20 level With passing ultrasonic testing at the stage of a thick steel plate. To carry out
such rolling, the cast slab has to be heated at a high temperature to 1250°C or more,
which requires a high cost. The shape factor y is an indicator used for showing a
degree of rolling, and a value thereof is defined by the formula: contact arc length of a
reduction roll with a steel plate/mean plate thickness= (R(ho-hl))0
·
5/(0.5(ho+hi)), where
25 R is a roll diameter (mm), ho is entry side plate thickness (mm) and h1 is delivery side
3
plate thickness (mm).
[0005] In order to reduce the amount of forming central porosity at the casting stage,
for example, Patent Literature 1 discloses the art that after perfect solidification of a slab
when the surface temperature of the slab is 700 to 1000°C, the slab is sandwiched
5 between upper and lower rolls each having a projecting portion at the center and is
subjected to rolling reduction to be crushed, to reduce central porosity.
[0006] Patent Literature 2 discloses the art that before completing the solidification
after bulging the cast slab into 10·mm or more, the center part ofthe width is subjected
to reduction, and successively, the vicinity of either edge part is subjected to reduction,
10 so that the solidified interfaces are press-stuck.
[0007] Patent Literature 3 discloses the art that the center part in the thickness of
the cast slab is subjected to reduction in the continuous casting equipment in the
condition of 1400°C to the solidified temperature thereat.
15 Citation List
Patent Literature
[0008] Patent Literature 1: JP2009-279652A
Patent Literature 2: JP2001-334353A
Patent Literature 3: JP H07-227658A
20
Summary of Invention
Technical Problem
[0009] An object of the art disclosed in Patent Literature 1 is so-called a bloom that
is cast to be narrow, and whose ratio (D/W) of the casting thickness (thickness) D to the
25 casting width W is 0.7. Ifthis art is applied to a wide slab whose ratio (D/W) ofthe
4
casting thickness D to the casting width W is 0.1 to 0.3, applied loads to the upper and
lower rolls are very large, and therefore, the durability of the rolls is insufficient and the
productivity deteriorates, which is problematic.
[0010] While the art disclosed in Patent Literature 2 is effective if the last
5 unsolidified part forms near edge parts of the cast slab width, it is not effective if the last
unsolidified part forms at the center part of the cast s1ab width, which is problematic.
[0011] The art disclosed in Patent Literature 3 is not effective if the last unsolidified
part forms near edge parts of the cast slab width, which is problematic.
[0012] As described above, the art of reducing the central porosity volume of a slab
10 having a large casting thickness D at the continuous casting stage is not established.
The art of reducing segregation generated around central porosity at the continuous
casting stage is not established as well. Thus, it is the actual circumstance that heavy
rolling is carried out in the later process, to reduce central porosity to a level with
passing ultrasonic testing specified in Japanese Industrial Standards (JIS) G 0801:2008
15 shown in Table 3, which is done at the stage of a thick steel plate. However, in heavy
rolling in the later process, although the central porosity volume can be reduced, it is
difficult to reduce segregation.
[0013] An object of the present invention is to solve the problems the above
:.~
described prior arts have, and to provide a continuous-cast slab where central porosity is
20 surely'reduced, and also, segregation is reduced during casting by crushing the slab, and
a method and apparatus of manufacturing the same. It is also an object of the present
invention to solve the above described conventional problems, and to provide a method
of manufacturing a thick steel plate that passes ultrasonic testing, where central porosity
and segregation are reduced at the continuous casting stage without heavy rolling of no
25 less than 0.7 in shape factory at low cost.
5
Solution to Problem
[OOI4] The inventors of the present invention found that: it is possible to check
movement of granular equiaxed crystals at the end of solidification by forming the
5 granular equiaxed crystals so that crystals in the upper surface side of a cast slab and
those in the lower surface side are symmetrical (hereinafter referred to as "horizontally
symmetrical" or "uniform") with respect to the thickness center of the cast slab; and as a
result, it is possible to reduce central porosity and segregation. Here, "horizontally
symmetrical" means that difference between the equiaxed crystal ratios of the upper half
10 and the lower half of the cast slab bordered by the thickness center of the cast slab is no
more than 5%. "Equiaxed crystal ratio" means the ratio of thickness of a zone where
equiaxed crystals form in the upper half of the cast slab in the thickness direction, to the
I/2 cast slab thickness. The inventors further found that it is possible to reduce central
porosity more than conventional arts by carrying out proper reduction at the continuous
15 casting stage. The present invention was completed based on these findings.
The present invention made for solving the above problems will be described.
In the following description, the fraction solid XI to X2 represents the fraction solid
within the range of no less than XI and no more than X2 unless otherwise mentioned.
In addition, YI to Y2 that refers to another except the fraction solid (for example, the
20 ratio FJ/W, casting thickness, the dent amount, the dent rate, distance, the maximum
shape factor, steel plate thickness, the ratio dt!D, the ratio dVD, casting width and
heating temperature) represents a value within the range of no less than YI and no more
than Y2 unless otherwise mentioned.
[OOI5] A first aspect of the present invention is a continuous-cast slab of O.I to 0.3
25 in ratio D/W, the ratio D/W being a ratio of casting thickness D to casting width W, and
6
230 to 380 mm in casting thickness D, the continuous-cast slab having a horizontally
symmetrical granular equiaxed crystals at least in a center in a thickness direction, the
continuous-cast slab comprising: a first reduction dent and a second reduction dent that
is narrower than the first reduction dent at least on one long side surface, the second
5 reduction dent denting further from a bottom surface of the first reduction dent, wherein
a dent amount d1 of the first reduction dent from ali end surface of the continuous-cast
slab is 0.08 to 1.1 mm, and a dent amount d2 of the second reduction dent from the
bottom surface of the first reduction dent is 1.2 to 12 mm.
[0016] A second aspect of the present invention is a continuous-cast slab of 0.1 to
10 0.3 in ratio D/W, the ratio DIW being a ratio of casting thickness D to casting width W,
and 230 to 380 mm in casting thickness D, the continuous-cast slab having a
horizontally symmetrical granular equiaxed crystals at least in a center in a thickness
direction, the continuous-cast slab comprising: a first reduction dent and a second
reduction dent that is narrower than the first reduction dent at least on one long side
15 surface, the second reduction dent denting further from a bottom surface of the first
reduction dent, wherein a dent rate of the first reduction dent from an end surface of the
continuous-cast slab, to the casting thickness D is 0.03 to 0.36%, and a dent rate of the
second reduction dent from the bottom surface of the first reduction dent, to the casting
thickness D is 0.6 to 4%.
20 [0017j' In the present invention, "dent rate" means the reduction rate at a dent on the
basis of the thickness before the dent is formed. That is, "a dent rate of the first
reduction dent from an end surface of the continuous-cast slab, to the casting thickness
D" represents "the amount of denting the first reduction dent diicasting thickness D x
1 00(% )". "A dent rate of the second reduction dent from the bottom surface of the first
25 reduction dent, to the casting thickness D" represents "the amount of denting the second
7
reduction dent d2/casting thickness D x 1 00(%)".
[0018] In the first aspect of the present invention, preferably, a dent rate of the first
reduction dent from the end surface of the continuous-cast slab, to the casting thickness
Dis 0.03 to 0.36%, and a dent rate of the second reduction dent from the bottom surface
5 of the first reduction dent, to the casting thickness Dis 0.6 to 4%.
[0019] In the first and second aspects of the present invention, preferably, distance
between either end of the first reduction dent and the end surface of the continuous-cast
slab is 0.37 x the casting thickness D to 1.0 x the casting thickness D, and distance
between either end of the second reduction dent and the end surface of the
10 continuous-cast slab is 0.5 x the casting thickness D to 1.2 x the casting thickness D.
[0020] In the first and second aspects of the present invention, preferably, a
maximum porosity volume is no more than 1.5 x 104 cm3 /g.
[0021] A third aspect of the present invention is a method of manufacturing a
continuous-cast· slab, the method comprising: a first step of forming a first reduction
15 dent at least in one long side surface of the continuous-cast slab by carrying out
reduction with first reduction rolls on the continuous-cast slab, the continuous-cast slab
being 0.1 to 0.3 in ratio DIW, the ratio DIW being a ratio of casting thickness D to
casting width W, and 230 to 380 mm in casting thickness D, the continuous-cast slab
having a horizontally symmetrical granular equiaxed crystals at least in a center in a
20 thickn"ess direction; and a second step of forming a second reduction dent that is
narrower than the first reduction dent by carrying out further reduction on a bottom
surface of the first reduction dent with second reduction rolls that are narrower than the
first reduction rolls, wherein in the first step, the reduction is carried out on the
continuous-cast slab so that a dent amount d1 of the first reduction dent from an end
25 surface of the continuous-cast slab is 0.08 to 1.1 mm, and in the second step, the
8
reduction is carried out on the continuous-cast slab so that a dent amount d2 of the
second reduction dent from the bottom surface of the first reduction dent is 1.2 to 12
mm.
[0022] A fourth aspect of the present invention is a method of manufacturing a
5 continuous-cast slab, the method comprising: a first step of forming a first reduction
dent at least in one long side surface of the continuous-cast slab by carrying out
reduction with first reduction rolls on the continuous-cast slab, the continuous-cast slab
being 0.1 to 0.3 in ratio D/W, the ratio D/W being a ratio of casting thickness D to
casting width W, and 230 to 380 mm in casting thickness D, the continuous-cast slab
10 having a horizontally symmetrical granular equiaxed crystals at least in a center in a
thickness direction; and a second step of forming a second reduction dent that is
narrower than the first reduction dent by carrying out further reduction on a bottom
surface of the first reduction dent with second reduction rolls that are narrower than the
first reduction rolls, wherein in the first step, the reduction is carried out on the
15 continuous-cast slab so that a dent rate of the first reduction dent from an end surface of
the continuous-cast slab, to the casting thickness D is 0.03 to 0.36%, and in the second
step, the reduction is carried out on the continuous-cast slab so that a dent rate of the
second reduction dent from the bottom surface of the first reduction dent, to the casting
thickness D is 0.6 to 4%.
20 [0023j' In the third aspect of the present invention, preferably, in the first step, the
reduction is carried out on the continuous-cast slab so that a dent rate of the first
reduction dent from the end surface of the continuous-cast slab, to the casting thickness
D is 0.03 to 0.36%, and in the second step, the reduction is carried out on the
continuous-cast slab so that a dent rate of the second reduction dent from the bottom
25 surface of the first reduction dent, to the casting thickness Dis 0.6 to 4%.
9
[0024] In the third and fourth aspects of the present invention, preferably, the first
reduction rolls are provided for a zone where a fraction solid is 0.3 to 0.7, and the
second reduction rolls are provided for a zone where a fraction solid is 0.7 to 1.0,
downstream from the first reduction rolls.
5 [0025] Here, the fraction solid can be obtained by, for example, heat transfer
calculation, a change in transmissivity of a horizontal electromagnetic acoustic wave.
[0026] In the third and fourth aspects of the present invention, preferably, distance
between either end of the first reduction dent and an end surface of the continuous-cast
slab is 0.37 x the casting thickness D to 1.0 x the casting thickness D, and distance
10 between either end of the second reduction dent and the end surface of the
continuous-cast slab is 0.5 x the casting thickness D to 1.2 x the casting thickness D.
[0027] In the third and fourth aspects of the present invention, preferably, a
maximum porosity volume of the continuous-cast slab manufactured through the first
and second steps is no more than 1.5 x 104 cm3/g.
15 [0028] A fifth aspect of the present invention is an apparatus of manufacturing a
continuous-cast slab, the apparatus comprising: first reduction rolls that shape an
intermediate shaped product having a first reduction dent at least on one long side
surface of the continuous-cast slab, the continuous-cast slab being 0.1 to 0.3 in ratio
D/W, the ratio D/W being a ratio of casting thickness D to casting width W, the
20 continuous-cast slab being 230 to 380 rnrn in casting thickness D, and having
horizontally symmetrical granular equiaxed crystals at least in a center in a thickness
direction; and second reduction rolls that shape a second reduction dent denting further
from a bottom surface of the first reduction dent of the intermediate shaped product and
being narrower than the first reduction dent, the second reduction rolls having shapes
25 narrower than the first reduction rolls and being arranged downstream from the first
10
reduction rolls, wherein the first reduction rolls are provided so that a dent amount d1 of
the first reduction dent from an end surface of the continuous-cast slab is 0.08 to 1.1
mm, and the second reduction rolls are provided so that a dent amount dz of the second
reduction dent from the bottom surface of the first reduction dent is 1.2 to 12 mm.
5 [0029] A sixth aspect of the present invention is an apparatus of manufacturing a
continuous-cast slab, the apparatus comprising: first reduction rolls that shape an
intermediate shaped product having a first reduction dent at least on one long side
surface of the continuous-cast slab, the continuous-cast slab being 0.1 to 0.3 in ratio
D/W, the ratio D/W being a ratio of casting thickness D to casting width W, the
10 continuous-cast slab being 230 to 380 mm in casting thickness D, and having
horizontally symmetrical granular equiaxed crystals at least in a center in a thickness
direction; and second reduction rolls that shape a second reduction dent denting further
from a bottom surface of the first reduction dent of the intermediate shaped product and
being narrower than the first reduction dent, the second reduction rolls having shapes
15 narrower than the first reduction rolls and being arranged downstream from the first
reduction rolls, wherein the first reduction rolls are provided so that a dent rate of the
first reduction dent from an end surface of the continuous-cast slab, to the casting
thickness Dis 0.03 to 0.36%, and the second reduction rolls are provided so that a dent
rate of the second reduction dent from the bottom surface of the first reduction dent, to
20 the ca~ting thickness D is 0.6 to 4%.
[0030] In the fifth aspect of the present invention, preferably, the first reduction
rolls are provided so that a dent rate of the first reduction dent from an end surface of ·
the continuous-cast slab, to the casting thickness D is 0.03 to 0.36%, and the second
reduction rolls are provided so that a dent rate of the second reduction dent from the
25 bottom surface of the first reduction dent, to the casting thickness Dis 0.6 to 4%.
11
[0031] In the fifth and sixth aspects of the present invention, preferably, the first
reduction rolls are provided for a zone where a fraction solid is 0.3 to 0.7, and the
second reduction rolls are provided for a zone where a fraction solid is 0.7 to 1.0,
downstream from the first reduction rolls.
5 [0032] In the fifth and sixth aspects of the present invention, preferably, the first
reduction rolls are provided so that distance between either end of the first reduction
dent and the end surface ofthe continuous-cast slab is 0.37 x the casting thickness D to
1.0 x the casting thickness D, and the second reduction rolls are provided so that
distance between either end of the second reduction dent and the end surface of the
10 continuous-cast slab is 0.5 x the casting thickness D to 1.2 x the casting thickness D.
[0033] In the fifth and sixth aspects of the present invention, preferably, a
maximum porosity volume of the continuous-cast slab is no more than 1.5 x 10-4 cm3/g.
[0034] A seventh aspect of the present invention is a method of manufacturing a
thick steel plate comprising: a continuous-cast slab manufacturing step of
15 manufacturing a continuous-cast slab according to the method of manufacturing a
continuous-cast slab of the above described third or fourth aspect of the present
invention; and a rolling step of rolling, within the range of 0.2 to 0.65 in maximum
shape factor, the continuous-cast slab manufactured in the continuous-cast slab
manufacturing step, the continuous-cast slab being no more than 2.5 x 10-4 cm3/g in
20 -maximum porosity volume.
[0035] Here, "maximum shape factor" means the maximum shape factor in one
pass in a case where hot-rolling is carried out on a thick steel plate with multi passes.
[0036] In the seventh aspect of the present invention, preferably, steel plate
thickness to casting thickness D after the rolling step is ended is 50% to 80% by the
25 rolling step.
12
[0037] In the seventh aspect· of the present invention, preferably, the steel plate
thickness after the rolling step is ended is 150 to 300 mm by the rolling step.
The steel plate manufactured according to the method of manufacturing a thick
steel plate of the seventh aspect of the present invention can be manufactured by an
5 apparatus of manufacturing a thick steel plate of the present invention, described later.
[0038] An eighth aspect of the present invention is an apparatus of maimfacturing a
thick steel plate, the apparatus comprising: the apparatus of manufacturing a
continuous-cast slab according to the above described fifth or sixth aspect of the present
invention; and a rolling mill that rolls the continuous-cast slab manufactured by the
10 apparatus of manufacturing a continuous-cast slab, wherein the rolling mill rolls, within
the range of 0.2 to 0.65 in maximum shape factor, the continuous-cast slab of no more
than 2.5 x 104 cm3 /g in maximum porosity volume.
[0039] In the eighth aspect of the present invention, preferably, the rolling mill
makes steel plate thickness after said rolling 50% to 80% to casting thickness D.
15 [0040] In the eighth aspect of the present invention, preferably, the rolling mill
makes steel plate thickness after said rolling 150 to 300 mm. .
Advantageous Effects of Invention
[0041] According to the continuous-cast slab of the present invention and the
20 metho1:l and apparatus of manufacturing the same, it is possible to provide the
continuous-cast slab where maximum porosity volume and segregation are reduced to a
low level even if the slab is so wide as to be 0.1 to 0.3 in ratio D/W of the casting
thickness D to the casting width Wand 230 to 380 mm in casting thickness D.
[0042] According to the apparatus and method of manufacturing the
25 continuous-cast slab of the present invention, two stages of reduction are carried out,
13
which brings about an effect of not applying an excessive load to a reduction roll.
[0043] According to the method and apparatus of manufacturing a thick steel plate
of the present invention, a continuous-cast slab where maximum porosity volume (the
maximum central porosity volume) is reduced in a cast slab manufacturing step can be
5 obtained. Thus, in a rolling step following the cast slab manufacturing step, the steel
plate where internal flaws caused by central porosity are reduced to a level with passing
ultrasonic testing can be manufactured even under a condition of 0.2 to 0.65 in
maximum shape factor. In this case, it is not necessary to heat the cast slab at a high
temperature as conventional prior arts. Thus, the manufacturing costs of the thick steel
10 plate can be greatly reduced.
Brief Description of Drawings
[0044] Fig. 1 is a schematic v1ew showing a cross-sectional shape of the
continuous-cast slab of the present invention.
15 Fig. 2 is an explanatory view showing an example of steps included in the
method of manufacturing the continuous-cast slab of the present invention.
Fig. 3 is a graph showing the influence of the dent amount of the first reduction
dent and the dent amount of the second reduction dent on the central porosity volume.
Fig. 4 is a graph showing the influence of the dent rate of the first reduction
20 dent and the dent rate of the second reduction dent on the central porosity volume.
Fig. 5 is an explanatory view schematically showing an example of part of the
apparatus of manufacturing the continuous-cast slab of the present invention.
Fig. 6 is an explanatory view schematically showing the structure of an
apparatus 0 of manufacturing a thick steel plate of the present invention.
25 Fig. 7 is a schematic explanatory view of a transversal cross section of the cast
14
>_>
slab.
Fig. 8 shows an example of granular crystals and the maximum segregation
thickness.
Fig. 9 shows an example of branching dendrites and the maximum segregation
5 thickness.
Fig. 10 is a graph showing the relationship between the maximum porosity
volume, the maximum shape factor upon rolling with rolls, and a pass or fail of
ultrasonic testing.
10 Description of Embodiments
[0045] The present invention will be described with reference to the accompanying
drawings properly. The embodiments below are examples of the present invention,
and the present invention is not limited thereto.
[0046] 1. Continuous-cast slab 1 of Present Invention
15 Fig. 1 is a schematic view· showing a cross-sectional shape of the
continuous-cast slab of the present invention. In Fig. 1, dents (first reduction dent 2
and second reduction dent 3) are exaggeratedly illustrated.
[0047] The continuous-cast slab 1 of the present invention is a cast slab that: the
ratio D/W of the casting thickness D to the casting width W is 0.1 to 0.3; the casting
20 thickn't:ss Dis 230 to 380 mm; and horizontally symmetrical granular equiaxed crystals
are included at least at the center in the thickness direction which solidification from
ends does not affect when the thickness direction is regarded as a vertical axis and the
width direction of long sides is regarded as a horizontal axis. As shown in Fig. 1, the
continuous-cast slab 1 has the first reduction dent 2 and the second reduction dent 3 that
25 dents further from the bottom surface of the first reduction dent 2 and that is narrower
15
than the first reduction dent 2, at least in one surface. Where solidification from ends
does not affect as described above is a zone except a portion of columnar crystals,
which solidifies from ends. This zone almost corresponds to the rest of a long side
which is obtained by deducting the casting thickness D from each end of the long side.
5 [0048] A cast slab having a cross-sectional shape like the cast slab of 0.1 to 0.3 in
ratio D/W of the casting thickness D to the casting width W and 230 to 380 mm in
casting thickness D is called a slab. The lower limit of the ratio D/W is 0.1 because
the casting width W is approximately 2500 mm or more under a condition that the
casting thickness Dis 230 to 380 mm, which makes it difficult to carry out reduction on
10 the wide cast slab uniformly in the width direction. The upper limit thereof is 0.3
because solidification from the end has great influence and thus sufficient reduction
cannot be carried out due to restrictions on equipment. In the present invention, the
maximum value of the casting width W is not especially limited. The casting width W
is preferably 1320 to 2360 mm.
15 [0049] If the casting thickness Dis over 380 mm, reaction force to reduction rolls is
strengthened, which makes the rolls easy to deform. Therefore, it is necessary to make
the reduction rolls and a segment that supports the reduction rolls highly rigid, and the
cost of equipment goes up, which is not preferable. If the casting thickness D is less
than 230 mm, the casting speed has to be slowed down, and the productivity also
20 deteriGrates, which is not preferable. In such a view, the casting thickness D is 230 to
380mm.
[0050] The continuous-cast slab 1 has horizontally symmetrical granular equiaxed
crystals at least in the center in the thickness direction which solidification from the
ends does not affect when the thickness direction is regarded as a vertical axis and the
25 width direction of long sides is regarded as a horizontal axis. Such a configuration
16
makes it possible to uniformly transmit force from the upper and lower sides of the cast
slab to the center thereof in the thickness direction upon reduction for forming the first
reduction dent 2 and the second reduction dent 3. As a result, sheering force that has
potential for driving force of moving granular equiaxed crystals can be checked, and
5 thus, the movement of granular equiaxed crystals can be checked. Checking the
movement of granular equiaxed crystals makes it 'possible to check the movement of
segregation elements, and thus, segregation can be checked. In addition, checking the
movement of granular equiaxed crystals makes a size of a zone sandwiched between
(surrounded by) a plurality of granular equiaxed crystals small, and thus, the porosity
10 (central porosity) volume forming in the zone can be reduced.
Further, a diameter of each granular equiaxed crystal is made to be small,
which makes the resistance against movement of granular equiaxed crystals increase
when sheering stress operates, and which makes the zone surrounded by granular
equiaxed crystals smaller. The size of a granular equiaxed crystal is no more than 1.5
15 mm, and preferably no more than 1.3 mm in equivalent circle diameter.
[0051] According to the present invention, as described above, the continuous-cast
slab where the central porosity volume is reduced and segregation is held down during
casting can be made even if the cast slab is wide.
[0052] Reduction corresponding to solidification shrinkage is carried out on the
20 contirruous-cast slab of the present invention by forming the wide first reduction dent 2
in continuous-casting equipment, to prevent molten steel flow from occurring.
Whereby, it is possible to make the initial diameter of porosity small. Next, reduction
is further carried out on the bottom surface of the first reduction dent 2, to form the
second reduction dent 3 narrower than the first reduction dent 2. Whereby,
25 contact-bonding can be subjected to forming porosity by reduction. Such two-stage
17
reduction makes it possible to reduce the maximum porosity volume of the slab to a low
level without applying an excessive load to reduction rolls.
[0053] It is general in continuous casting equipment that one surface of a cast slab
that hangs down from a mold is defined as a reference plane, and support rolls are
5 arranged so that the other surface corresponds to solidification shrinkage to incline.
Fig. 2 illustrates an embodiment of steps included In the method of manufacturing the
continuous-cast slab of the present invention. Because a first reduction roll 4 and a
second reduction roll 5 are arranged in the opposite side to the reference plane in this
embodiment, Fig. 1 shows the continuous-cast slab 1, only one surface of which the
10 wide first reduction dent 2 and the narrow second reduction dent 3 are formed on. The
present invention is not limited to this embodiment, and the first reduction dent 2 and
the second reduction dent 3, which is narrower than the first reduction dent 2, may be
formed on both surfaces of the continuous-cast slab.
[0054] In the present invention, the dent amount d1 of the first reduction dent 2
15 from each end surface ofthe continuous-cast slab 1 is 0.08 to 1.1 rnm. The lower limit
of the dent amount d1 is 0.08 rnm in order to reduce formation of porosity due to the
volume shrinkage. The upper limit thereof is 1.1 rnm in order to reduce center
segregation and formation of porosity due to the movement of equiaxed crystals. In
the present invention, the dent amount d2 of the second reduction dent 3 from the
20 botton1. surface of the first reduction dent 2 is 1.2 to 12 rnm. The lower limit of the
dent amount d2 is 1.2 rnm in order to obtain an effect of reducing central porosity. The
upper limit thereof is 12 rnm in order to check occurrence of surface cracking.
[0055] In the present invention, the dent rate can be specified instead of, or in
addition to the dent amount. The dent rate of the first reduction dent 2 from each end
25 surface of the cast slab, to the casting thickness D is 0.03 to 0.36%. That is, the ratio
18
dJID of the dent amount d1 of the first reduction dent 2 from each end surface of the cast
slab, to the casting thickness D is specified as dd:O = 0.03 to 0.36%. The lower limit
of the dent rate is 0.03% in order to reduce formation of porosity due to the volume
shrinkage. The upper limit thereof is 0.36% in order to reduce center segregation and
5 formation of porosity due to the movement of equiaxed crystals. The dent rate of the
second reduction dent 3 from the bottom surface of the first reduction dent 2, to the
casting thickness D is 0.6 to 4%. That is, the ratio d2/D of the dent amount d2 of the
second reduction dent from the bottom surface of the first reduction dent, to the casting
thickness Dis specified as d2/D = 0.6 to 4%. If the dent rate of the second reduction
10 dent from the bottom surface of the first reduction dent, to the casting thickness D is
less than 0.6%, the effect of reducing central porosity is insufficient, which is not
preferable. If the dent rate thereof is over 4%, the possibility of surface cracking
becomes high, which is not preferable. Therefore, the dent rate of the second
reduction dent 3 from the bottom surface of the first reduction dent 2, to the casting
15 thickness D is specified as 0.6 to 4%.
[0056] Specifying the dent rate and the dent amount of the first reduction dent, and
those of the second reduction dent within the above described ranges makes it possible
to reduce the maximum porosity volume of the slab to a low level with 1.5x104 cm3/g
or less.
20 [0057j' Preferably, the first reduction dent 2 exists at a position so that distance a1
between an end of the first reduction dent 2 and a corresponding end surface of the cast
slab is 0.37 x the casting thickness D to 1.0 x the casting thickness D. The lower limit ·
of the distance a1 is preferably 0.37 x the casting thickness D in order to make the
influence of each end of the cast slab, which have a high strength, small, to obtain a
25 high reduction efficiency. The upper limit of the distance a1 is preferably 1.0 x the
19
casting thickness D in order to make the length in the vicinity of the ends of the cast
slab, on which rolling with rolls is not carried out, short. Preferably, the second
reduction dent 3 exists at a position so that distance a2 between an end of the second
reduction dent 3 and a corresponding end surface of the cast slab is 0.5 x the casting
5 thickness D to 1.2 x the casting thickness D. The lower limit of the distance a2 is
preferably 0.5 x the casting thickness D in order to- make the influence of each end of
the cast slab, which have a high strength, small, to obtain a high reduction efficiency.
The upper limit of the distance a2 is preferably 1.2 x the casting thickness D in order to
make the length in the vicinity of the ends of the cast slab, on which rolling with rolls is
10 not carried out, short.
[0058] It is assumed that reduction at the first stage is carried out just before a
position of the fraction solid of the liquid limit of molten steel in the cast slab flowing
out of the mold can make the initial diameter of central porosity small. Here,
solidification shrinkage occurs almost all over the cast slab except both ends of the
15 casting width. Thus, it is necessary in the present invention that the first reduction
dent 2 is wide.
[0059] On the other hand, at the stage after the first reduction dent 2 is formed, the
zone where central porosity forms shrinks around the center of the casting width.
Therefore, preferably, the second reduction dent 3 is narrower than the first reduction
20 dent 2~ to have an aspect of applying more concentrative reduction.
[0060] Both ends of the casting width W are excluded as described above because
solidification progresses from the ends of the cast slab as well. A roll having the
length same as or more than the casting width W is easy to deform due to reaction force
of reduction. Therefore, in the present invention, preferably, the reduction rolls have
25 narrower reduction width than the reduction casting width.
20
[0061] Preferably, the maximum porosity volume of the continuous-cast slab 1 of
the present invention is no more than 1.5 x 104 cm3/g.
[0062] The central porosity volume of a conventional material is about 6 to 10 x
104 cm3/g. If such a volume of central porosity remains inside the cast slab, internal
5 flaws occur in an end product, which results in a critical problem with which destruction
is started off, unless rolling with a high shape factor such as no less than 0.7 in
maximum shape factor in heavy plate rolling is carried out. Preferably, the remaining
amount of central porosity in the continuous-cast slab of the present invention is low,
that is, the maximum porosity volume of the slab is no more than 1.5 x 104 cm3/g.
10 The central porosity volume of no more than 1.5 x 104 cm3/g makes it possible to
obtain effects of reducing the shape factor in heavy plate rolling and of reducing internal
flaws in products with the low shape factor.
[0063] The central porosity volume Pv can be obtained by: Pv = (1/p) - (1/po)
( cm3 /g) where the density of a representative sample of a 114 thickness portion of the
15 same kind of a cast slab is po and the density of a sample of the center part is p.
[0064] Preferably, a size of the representative sample is 50 mm in length, 100 mm
in width and 7 mm in thickness. As the precision of surface finishing of the sample, a
smooth finishing surface is preferable. Conforming to JIS B 0601 :2013, preferably,
the surface roughness is no more than 1.6 (J..tm) in arithmetic average roughness Ra, and
20 more preferably no more than 0.8 (J..tm) therein. If the surface is rough, there is a case
that when the sample is immersed in water, bubbles are trapped on the surface, and the
accuracy of Pv is not high, which is not preferable. In the present invention, all the
center of the cast slab thickness in the cast slab width direction, excluding portions
within D/2 from short sides of the cast slab is cut out as surfaces in length and width of
25 the sample, and the maximum value of the porosity volume in the width direction is
21
defined as the maximum central porosity volume. The density po of the 114 thickness
portion can be the mean value of the porosity volumes of the samples cut out from six
points in the width direction.
[0065] The center of the continuous-cast slab 1 of the present invention, which is at
5 a high temperature, deforms first. Thus, dendrite structure forming in the top layer of
the cast slab upon solidification is linear. However, if reduction is can:'ied out after
completion of solidification, the top layer also deforms, and dendrite structure curves.
Thus, the continuous-cast slab 1 is distinguishable from a conventional product on
which reduction is carried out after the completion of solidification.
10 [0066] 2. Apparatus of Manufacturing Continuous-cast slab 1 according to Present
Invention
Continuous casting equipment that is the apparatus of manufacturing the
continuous-cast slab 1 according to the present invention includes the first reduction
rolls 4 and the second reduction rolls 5 that are narrower than the first reduction rolls 4.
15 [0067] Fig. 5 is an explanatory view schematically showing an example of part of
the apparatus of manufacturing the continuous-cast slab according to the present
invention. In Fig. 5, the first reduction rolls 4 and the second reduction rolls 5 are
arranged beneath the mold of the continuous casting equipment. Fig. 5 shows an
aspect that reduction is carried out on the cast slab in the thickness direction in the
20 vicinity of the solidification end point. While Fig. 5 illustrates the first reduction rolls
4 consisting of rolls of six stages which include backup rolls 6, and the second reduction
rolls 5 consisting of rolls of three stages, the apparatus of manufacturing the
continuous-cast slab 1 according to the present invention is not limited to this aspect.
[0068] Fig. 2 is an explanatory view showing an example of steps included in the
25 method of manufacturing the continuous-cast slab according to the present invention.
22
As shown in Fig. 2, the first reduction dent 2 is formed by pressing the surface
of the cast slab with the first reduction rolls 4 equipped with the continuous casting
equipment. The second reduction dent 3 is formed by pressing the bottom surface of
the first reduction dent 2 with the second reduction rolls 5 disposed beneath (at the later
5 stage of) the first reduction rolls 4.
[0069] (1) Continuous Casting Equipment
A type of the continuous casting equipment of manufacturing the
continuous-cast slab 1 according to the present invention is not especially limited. The
-~
present invention can be applied to any of a vertical bending-type, a bending-type and a
10 vertical type. A vertical type is preferable in view of achieving an aspect of easily
manufacturing the continuous-cast slab 1 having horizontally symmetrical granular
equiaxed crystals in the center in the thickness direction. In the case of either vertical
bending-type or bending-type, for example, the continuous-cast slab 1 having
horizontally symmetrical granular equiaxed crystals can be manufactured through
15 electromagnetic stirring or the like. Electromagnetic stirring is applicable to a vertical
type as well. Application of electromagnetic stirring to a vertical type makes it easier
to manufacture the continuous-cast slab 1 having horizontally symmetrical granular
equiaxed crystals in the center in the thickness direction.
Further, it is also effective for adjustment of equiaxed crystals thickness to
20 adjust"the strength of electromagnetic stirring on the upper and lower surfaces while
adjusting the degree of superheat of molten steel (difference between temperature of
molten steel in the cast slab and solidification start temperature during casting), and to
adjust the strength of multi-stage electromagnetic stirring on the upper and lower
surfaces.
25 [0070] (2) First Reduction Rolls 4
23
The first reduction rolls 4 shape the first reduction dent at least on one long
side surface of the cast slab by carrying out reduction on the cast slab.
[0071] Preferably, the first reduction rolls 4 are disposed just before a position of
the fraction solid of the flow limit of molten steel in the cast slab flowing out of the
5 mold. Reduction corresponding to solidification shrinkage, that is, reduction (light
reduction) so as to lessen the thickness of the cast slab as much as solidification
shrinkage that causes formation of porosity is carried out, to prevent molten steel flow
from occurring. Specifically, the fraction solid of the cast slab just before the above
described position is about 0.3 to 0.7. If reduction with the first reduction rolls 4 is
10 carried out at a position where the fraction solid of the cast slab is less than 0.3, because
the fraction solid of less than 0.3 brings about the same behavior as absolute liquid,
liquid is just pushed out upstream in the casting direction, and there is no influence on
center segregation and porosity. If reduction with the first reduction rolls 4 is carried
out at a position where the fraction solid of the cast slab is over 0.7, deformation
15 resistance suddenly increases, which makes it difficult to carry out reduction due to
restriction on the equipment. 'l'hus, preferably, reduction with the first reduction rolls
4 is carried out at a position where the fraction solid of the cast slab is 0.3 to 0.7 in order
to avoid such a situation. It is presumed that reduction at the first stage at this position
can make the initial diameter of central porosity small.
20 [00721 Solidification shrinkage occurs almost all over the cast slab except both ends
of the casting width. Thus, it is necessary that the first reduction dent 2, which is
formed by reduction with the first reduction rolls 4, is wide. Preferably, the distance ar
between an end of the first reduction dent 2 and a corresponding end surface of the cast
slab is 0.37 x the casting thickness D to 1.0 x the casting thickness D. Here, both ends
25 of the casting width Ware excluded because solidification also progresses from the ends
24
of the cast slab. A normal roll that has the length same as or more than the casting
width W is easy to deform due to reaction force of reduction. Thus, it is necessary that
the first reduction rolls 4 are rolls having reduction width shorter than the reduction
casting width.
5 [0073] Reduction with the first reduction rolls 4 is carried out on the cast slab of 0.1
to 0.3 in ratio D/W of the casting thickness D to th~-casting width W, 230 to 380 mm in
casting thickness D, which has horizontally symmetrical granular equiaxed crystals at
least in the center in the thickness direction, so that the dent amount d1 of the first
reduction dent 2 from each end surface of the cast slab is 0.08 to 1.1 mm. This
10 reduction is also carried out so that the dent rate of the first reduction dent from an end
surface of the cast slab, to the casting thickness Dis 0.03 to 0.36%.
[0074] (3) Second Reduction Rolls 5
The second reduction rolls 5 have shapes narrower than the first reduction rolls
4. The second reduction rolls 5 shape the second reduction dent 3 narrower than the
15 first reduction dent 2 by carrying out further reduction on the bottom surface of the first
reduction dent 2 of an intermediate shaped product.
[0075] Preferably, the second reduction rolls 5 are arranged in the downstream side
of the first reduction rolls 4 between the position of the fraction solid of the flow limit
of molten steel in the cast slab flowing out of the mold and the complete solidification
20 point"' Contact-bonding is carried out on porosity forming in the cast slab by reduction
with the second reduction rolls 5, which makes central porosity reduced. Specifically,
the fraction solid of the cast slab between the position of the fraction solid of the flow ·
limit of molten steel in the cast slab flowing out of the mold and the complete
solidification place is approximately 0.7 to 1.0. If reduction with the second reduction
25 rolls 5 is carried out at the position where the fraction solid of the cast slab is less than
25
0.7, equiaxed crystals greatly move. Thus, center segregation and porosity deteriorate.
Therefore, preferably, reduction with the second reduction rolls 5 is carried out at the
position where the fraction solid of the cast slab is 0.7 to 1.0 in order to avoid such a
situation. Central porosity can be subjected to contact-bonding, to be reduced by
5 reduction at the second stage with the second reduction rolls 5 at this position.
[0076] When the fraction solid of the cast slab- is 0.7 to 1.0, a zone where central
porosity forms shrinks around the center of the casting width. Thus, the second
reduction dent 3 is shaped so as to be narrower than the first reduction dent 2 by
applying more concentrative reduction. Whereby, contact-bonding can be subjected to
10 central porosity firmly. Preferably, the distance a2 between an end of the second
reduction dent 3 (that is, an end of each second reduction roll 5) and a corresponding
end surface of the cast slab is 0.5 x the casting thickness D to 1.2 x the casting thickness
D.
[0077] Reduction with the second reduction rolls 5 is carried out on the cast slab of
15 0.1 to 0.3 in ratio D/W of the casting thickness D to the casting width W, 230 to 380 mm
in casting thickness D, which has horizontally symmetrical granular equiaxed crystals at
least in the center in the thickness direction, so that the dent amount d2 of the second
reduction dent 3 from the bottom surface of the first reduction dent 2 is 1.2 to 12 mm.
This reduction is also carried out so that the dent rate of the second reduction dent 3
20 from the bottom surface of the first reduction dent 2, to the casting thickness Dis 0.6 to
4%.
[0078] Specifying the dent rate and dent amount of the first reduction dent 2, and ·
those of the second reduction dent 3 within the above described ranges makes it
possible to reduce the maximum porosity volume of the slab to a low level with no
25 more than 1.5 x 104 cm3/g.
26
[0079] It is general in continuous casting equipment that one surface of a cast slab
that hangs down from a mold is defined as a reference plane, and support rolls are
arranged so that the other surface corresponds to solidification shrinkage to incline.
Here, in the embodiment shown in Fig. 5, the first reduction rolls 4 and the second
5 reduction rolls 5 are arranged in the opposite side to the reference plane. Therefore, in
Fig. 1, the first reduction dent 2 and the second reduction dent 3 that is narrower than
the first reduction dent 2 are formed only on one surface of the continuous-cast slab 1.
In short, the embodiment shown has an aspect of arranging the first reduction rolls 4
and the second reduction rolls 5 only on one surface. The present invention is not
10 limited to this embodiment, and the first reduction rolls 4 and the second reduction rolls
5 can be provided with both surfaces of the continuous-cast slab.
[0080] As shown in Fig. 5, a plurality of the first reduction rolls 4 and the plurality
of the second reduction rolls 5 can be used. In this case, preferably, each pitch
between adjacent reduction rolls is same as that between support rolls of the continuous
15 casting equipment.
[0081] 3. Method of Manufacturing Continuous-cast slab 1 according to Present
Invention
The method of manufacturing the continuous-cast slab 1 according to the
present invention includes the first step of forming the first reduction dent 2 on the cast
20 slab and the second step of forming the second reduction dent 3 thereon.
[0082] Such two-stage reduction makes it possible to reduce the maximum porosity
volume of the slab to a low level without applying an excessive load to reduction rolls.
[0083] (1) First Step
In the first step, the wide first reduction dent 2 is formed at least on one long
25 side surface of the cast slab by reduction on the cast slab with the above described first
27
reduction rolls 4.
[0084] Preferably, the first reduction rolls 4 are provided with a zone where the
fraction solid is 0.3 to 0.7. That is, preferably, the first step is carried out in a zone
where the fraction solid of the cast slab is 0.3 to 0.7.
5 [0085] In the first step, reduction with the first reduction rolls 4 is carried out on the
cast slab of 0.1 to 0.3 in ratio D/W of the casting thickness D to the casting width W,
230 to 380 mm in casting thickness D, which has horizontally symmetrical granular
equiaxed crystals at least in the center in the thickness direction, so that the dent amount
d1 ofthe first reduction dent 2 from an end surface ofthe cast slab is 0.08 to 1.1 mm.
10 This reduction is also carried out so that the dent rate of the first reduction dent 2 from
an end surface of the cast slab, to the casting thickness Dis 0.03 to 0.36%.
[0086] (2) Second Step
In the second step, the narrow second reduction dent 3 is formed by further
reduction with the above described second reduction rolls 5 on the bottom surface of the
15 first reduction dent 2, which is formed in the first step.
[0087] Preferably, the second reduction rolls 5 are provided with a zone where the
fraction solid is 0.7 to 1.0 in the downstream side of the first reduction rolls 4. That is,
preferably, the second step is carried out downstream from the first step in a zone where
the fraction solid of the cast slab is 0.7 to 1.0.
20 [0088T In the second step, reduction with the second reduction rolls 5 is carried out
on the cast slab of0.1 to 0.3 in ratio D/W ofthe casting thickness D to the casting width
W, 230 to 380 mm in casting thickness D, which has horizontally symmetrical granular
equiaxed crystals at least in the center in the thickness direction, so that the dent amount
dz of the second reduction dent 3 from the bottom surface of the first reduction dent 2 is
25 1.2 to 12 mm. This reduction is also carried out so that the dent rate of the second
28
reduction dent 3 from the bottom surface of the first reduction dent 2, to the casting
thickness D is 0.6 to 4%.
[0089] Specifying the dent rate and dent amount of the first reduction dent 2, and
those of the second reduction dent 3 makes it possible to reduce the maximum porosity
5 volume of the slab to a low level with no more than 1.5 x 104 cm3 I g.
[0090] 4. Apparatus 0 of Manufacturing Thick Steel Plate
Fig. 6 is an explanatory view schematically showing the structure of the
apparatus p of manufacturing a thick steel plate according to the present invention. Fig.
5 is an explanatory view of the apparatus of manufacturing the continuous-cast slab
10 which is provided with the apparatus 0 of manufacturing a thick steel plate. In Fig. 6,
the first reduction rolls 4, the second reduction rolls 5 and support rolls are denoted as
rolls 65 without distinction. Details on the rolls are illustrated in Fig. 5. In Fig. 5, the
first reduction rolls 4 and the second reduction rolls 5 are arranged beneath a mold 69 of
the continuous casting equipment. Fig. 5 shows an aspect that reduction is carried out
15 on the cast slab in the thickness direction in the vicinity of the solidification end point.
[0091] As shown in Figs. 5 and 6, the apparatus 0 of manufacturing a thick steel
plate according to the present invention includes the apparatus of manufacturing the
continuous-cast slab of the present invention including the first reduction rolls 4 and the
second reduction rolls 5, and a rolling mill63.
20 A continuous-cast slab 61 where the maximum porosity volume is no more
than 2.5 x 104 cm3/g and segregation is reduced is manufactured using the first
reduction rolls 4 and the second reduction rolls 5 provided with the continuous casting
equipment. Rolling is carried out on this continuous-cast slab 61 by the rolling mill 63
provided in the downstream side of the continuous casting equipment under the
25 condition that the maximum shape factor is 0.2 to 0.65. Whereby, a thick steel plate 62
29
that has a level with passing ultrasonic testing is manufactured.
[0092] As shown in Fig. 6, in the apparatus 0 of manufacturing a thick steel plate
according to the present invention, molten steel 69 that is poured from a ladle not shown
to a tundish 66 is poured into a (water-cooled) mold 67, and a solidified shell forms in
5 the mold 67, to be a cast slab 60 having an unsolidified portion inside. The cast slab
60 is withdrawn by a plurality of the rolls 65 (in d~tail, support rolls, the first reduction
rolls 4 and the second reduction rolls 5) toward the downstream side while being cooled,
and at the same time, reduction is carried out thereon, to manufacture the
continuous-cast slab 61. After that, the continuous-cast slab 61 is cut into a
10 predetermined length by a cutting machine 68, inserted into a furnace and heated to a
predetermined temperature, and then, rolling is carried out thereon with the rolling mill
63 to be a slab, to manufacture the steel plate 62.
[0093] The first reduction rolls 4 and the second reduction rolls 5 are as described
above. Thus, the rolling mill 63 will be described in detail hereinafter.
15 [0094] (1) Rolling Mill 63
The rolling mill 63 rolls the cast slab within the range of 0.2 to 0.65 in
maximum shape factor. Preferably, the rolling mill 63 is configured so that the rolled
steel plate thickness to the casting thickness Dis 50% to 80%.
[0095] Specifically, the rolling mill 63 is preferably provided so that the steel plate
20 thickrfess after rolling the cast slab of230 to 380 mm in casting thickness D and 0.1 to
0.3 in ratio D/W of the casting thickness D to the casting width W, which has
horizontally symmetrical granular equiaxed crystals at least in the center in the
thickness direction is 150 to 300 mm.
[0096] Heating to preferably 1050 to 1240°C, and more preferably 1050 to 1230°C
25 is applicable to rolling conditions. Conventionally, heavy rolling of no less than 0. 7 in
30
shape factor y is necessary, and thus the cast slab has to be heated at a high temperature
to 1250°C or more. In contrast, according to the present invention, the thick steel plate
where internal flaws due to central porosity are reduced to a level with passing
ultrasonic testing can be manufactured even at 1240°C or less. In addition, it is not
5 necessary to heat the cast slab at a high temperature to 1250°C or more as conventional
one, and thus, the manufacturing costs can be greatly reduced.
[0097] The rolling mill 63 is not especially limited, and a well-known rolling mill is
applicable. Because such a rolling mill is well-known and commonly used for the
person skilled in the art, the specification of the rolling mill 63 is omitted.
10 [0098] According to the present invention, the continuous-cast slab 61 where
central porosity and segregation are reduced can be obtained by reduction with the first
reduction rolls 4 and the second reduction rolls 5, and thus, it is not necessary to carry
out heavy rolling by the rolling mill 63.
[0099] 5. Method of Manufacturing Thick Steel Plate
15 The method of manufacturing a thick steel plate of the present invention
includes a cast slab manufacturing step of manufacturing the continuous-cast slab 61
according to the method of manufacturing the continuous-cast slab of the present
invention, and a rolling step of manufacturing the steel plate 62 by rolling the obtained
continuous-cast slab 61. The method of manufacturing the continuous-cast slab of the
20 present invention is as described above, and the description thereof is omitted here.
The rolling step will be described hereinafter.
[0100]
In the rolling step, the continuous-cast slab 61 where central porosity and
segregation are reduced, which is obtained in the cast slab manufacturing step of
25 manufacturing the continuous-cast slab 61 according to the method of manufacturing
31
the continuous-cast slab of the present invention, is rolled by the above described
rolling mill63 within the range of0.2 to 0.65 in maximum shape factor.
[0101] Preferably, by the rolling step, the steel plate thickness to the casting
thickness D after the rolling step is ended is 50% to 80%.
5 [0102] Preferably, rolling is carried out in the rolling step so that the steel plate
thickness after the rolling step is 150 to 300 mm.
[0103] The maximum porosity volume of a cast slab manufactured with a
conventional method is approximately 6 x 10-4 cm3/g or more. Thus, conventionally,
the cast slab cannot pass ultrasonic testing unless heavy rolling of 0.7 or more in
10 maximum shape factor is carried out thereon besides heating the cast slab at a high
temperature. In contrast, the central porosity volume of the cast slab manufactured
according to the method of manufacturing the continuous-cast slab of the present
invention is held down to be no more than 2.5 x 10-4 cm3/g. Therefore, the thick steel
plate where central porosity is reduced to a level with passing ultrasonic testing can be
15 manufactured by rolling within the range of 0.2 to 0.65 in maximum shape factor in the
rolling step. In this case, conventional heating to no more than 124o·c has only to be
carried out on the cast slab. Thus, the manufacturing costs can be reduced. Here, the
maximum shape factor means the shape factor of the maximum value in one pass in a
case where hot-rolling is carried out on a thick steel plate with multi passes.
20 [0104j' The thick steel plate manufactured according to the present invention is a
thick steel plate where internal flaws caused by central porosity are reduced to a level
with passing ultrasonic testing. Moreover, this thick steel plate has the advantage of its
possible manufacturing at lower cost than conventional one.
[0 1 05] 6. Thick Steel Plate Manufactured according to Present Invention
25 The thick steel plate manufactured according to the present invention is a
32
hot-rolled steel plate of no less than 150 mm in thickness. The thick steel plate
manufactured according to the present invention is a thick steel plate where internal
flaws detected through ultrasonic testing are a few, and thus, preferably used for a
nuclear reactor, a boiler, a pressure vessel or the like.
5
Examples
[0106] Examples of the present invention will be described hereinafter. The
present invention is not limited to such Examples.
[0 1 07] 1) Casting Test for Continuous-cast slab
10 A cast slab of 300 mm in casting thickness D, 2000 mm in casting width Wand
0.15 in DIW value was cast in vertical type continuous casting equipment while
subjected to strand-electromagnetic stirring of0.05'to 0.2 in center fraction solid fs.
[0108] Reduction was carried out on the cast slab with six wide reduction rolls
arranged at certain pitches in a zone where the fraction solid of the cast slab was 0.3 to
15 0.7. Reduction was further carried out thereon with three narrow reduction rolls
arranged at certain pitches downstream from the above mentioned zone in a zone where
the fraction solid of the cast slab was 0.7 to 1.0.
The fraction solid was obtained by heat transfer calculation according to the
common finite difference method.
20 [01091 On a surface of the cast slab (slab) cast by the vertical type continuous
casting equipment, a wide first reduction dent of 200 mm in distance from an end
surface ofthe cast slab, and a narrow second reduction dent of 300 mm in distance from
an end surface of the cast slab were formed. The dent amount of the first reduction
dent from an end surface of the cast slab was 0.4 mm. The dent amount of the second
25 reduction dent from the first reduction dent was 3.8 mm.
33
[0110] The dent rate of the first reduction dent from an end surface of the cast slab
was 0.13%. The dent rate of the second reduction dent from the first reduction dent
was 1.27%.
[0111] A sample of 50 rnrn in length, 100 rnrn in width and 7 rnrn in thickness was
5 cut out from each 1/4 thickness portion and center part of this slab, and the central
porosity volume Pv was obtained with the above 'described method. The maximum
value thereof was 1.0 x 10-4 cm3/g. This value was one sixth of that of a conventional
slab.
[0112] Other than the above, test casting was carried out on cast slabs of230 to 380
10 rnrn in casting thickness D, 1500 to 2400 rnrn in casting width Wand 0.1 to 0.3 in DIW,
which have horizontally symmetrical granular equiaxed crystals at least in each center
in the thickness direction. In this test casting, the dent amounts were variously
changed, and each central porosity volume was obtained in the same way. The results
are shown in the graph of Fig. 3. In Fig. 3, the vertical axis shows the dent amount d1
15 (rnrn) of the first reduction dent and the horizontal axis shows the dent amount d2 (rnrn)
of the second reduction dent. In this test casting, a range where the maximum central
porosity volume ofthe cast slab was no more than 1.5 x 10-4 cm3/g is surrounded by a
solid line.
[0113] The graph in Fig. 4 shows the results represented by the vertical axis
20 showing the dent rate of the first reduction dent and the horizontal axis showing the dent
rate of the second reduction dent. In this test casting, a range where the maximum
central porosity volume of the cast slab was no more than 1.5 x 10-4 cm3/g is surrounded
by a solid line. The dent rate of the first reduction dent is dJID and the dent rate of the
second reduction dent is d2/D where the the cast slab thickness is D rnrn, the dent
25 amount of the first reduction dent is d1, and the dent amount of the second reduction
34
dent from the bottom surface of the first reduction dent is d2. However, because values
of both dent rates are small, the vertical axis and the horizontal axis in Fig. 4 show the
values 100 times as large as original ones, which is converted into a percentage.
[0114]
5 It was confirmed that the maximum porosity volume of the slab was reduced to
a low level according to the present invention.
[0115] Specifically, it was confirmed that to specify the dent rate and the dent
amount of the first reduction dent and those of the second reduction dent made it
possible to reduce the maximum porosity volume of the slab to a low level with no
10 more than 1.5 x 104 cm3/g. The central porosity volume Pv of a conventional slab
was 6 to 10 x 104 cm3/g. Thus, according to this result, it was confirmed that the cast
slab whose maximum central porosity volume was reduced to no more than a fraction of
a conventional one was able to be provided.
[0 116] 2) Manufacturing Test for Thick Steel Plate
15 A cast slab whose casting thickness D, casting width Wand D/W satisfied the
conditions shown in Table 1, which had horizontally symmetrical granular equiaxed
crystals at least in the center in the thickness direction was cast in vertical type
continuous casting equipment. Reduction was carried out on the cast slab with six first
reduction rolls (250 mm in diameter) arranged in a zone where the fraction solid of the
20 cast sl~b was as shown in Table 1. Reduction was further carried out with three second
reduction rolls (500 mm in diameter) arranged in a zone where the fraction solid of the
cast slab was as shown in Table 1, downstream from the first reduction rolls.
Conditions of the first reduction rolls and the second reduction rolls such as the dent
amount and the dent rate were as shown in Table 1. Rolls whose reduction width was
25 narrower than the casting width W and whose distance from an end surface of the cast
35
slab was within the range of 105 to 320 mm were used as the first reduction rolls.
Rolls whose reduction width was narrower than that of the first reduction rolls and
whose distance from an end surface of the cast slab was within the range of 155 to 370
mm were used as the second reduction rolls. The second reduction rolls having a
5 larger diameter than the first reduction rolls had were used in order to make it easy to
carry out reduction to the center in the thickness 'airection of the cast slab when the
reduction was carried out on the cast slab at a lower temperature than with the first
reduction rolls.
[0117] A sample of 50 mm in length, 100 mm in width and 7 mm in thickness was
10 cut out from each 114 thickness portion and center part of the cast slab (slab), and the
central porosity volume was obtained with the above described method. The obtained
central porosity volume was as shown in Table 2.
[0118] Next, a thick steel plate was manufactured by heating each cast slab (slab)
and carrying out rolling of various shape factors as shown in Table 1 using rolls of 600
15 mm in diameter. Heating conditions were as shown in Table 2.
[0119] Ultrasonic testing was carried out on obtained each thick steel plate of 150
to 300 mm in thickness. An ultrasonic testing method is defined by "Ultrasonic testing
of steel plates for pressure vessels" in JIS G 0801:2008. In this testing, "Standard A"
and "Standard B", which were stricter standards as shown in Table 3, were used to
20 decide' whether to pass the testing.
[0120] Comparing "Standard A" and "Standard B", "Standard B" was a stricter
standard. Examples mentioned as "Pass Standard B" also passed "Standard A".
[0121]
[Table 1]
36
I Casting Casting Casting
Degree of First Reduction Dent Second Reduction Dent
········· Direction WidthW DIW Speed Vc
Superheat Slab End Dent Dent Fraction Slab End Dent Dent Fraction
D(mm) (mm) (m/min)
ofMohen Surface Amountdt Rate
SoM
Surface Amount Rate
Solid
Steei(Cl Distance (mm) (mm) lr%l Distance lmm) ch(mm) II%)
Example I 230 2120 0.11 0.36 28 200 0.46 0.20 0.3-0.7 250 1.4 0.61 0.7-1.0
Example 2 250 2120 0.12 0.31 40 200 0.31 0.12 0.3-0.7 250 3.44 1.38 0.7-1.0
Example 3 25( 2050 0.12 0.31 36 165 0.63 0.25 0.3-0.7 215 3.13 1.25 0.7-1.0
Example 4 280 2120 0.13 0.25 35 200 0.84 0.30 0.3-0.7 250 3.08 1.1 0.7-1.0
Example 5 280 1930 0.15 0.25 37 105 0.28 0.10 0.3-0.7 !55 5.46 1.95 0.7-1.0
Example 6 280 1930 0.15 0.25 35 105 0.08 0.03 0.3-0.7 !55 10.08 3.6 0.7-1.0
Exa!11ple 7 280 2120 0.13 0.25 38 200 1.01 0.36 0.3-0.7 250 11.06 3.95 0.7-1.0
Example 8 300 1890 0.16 0.22 31 295 0.75 0.25 0.3-0.7 345 7.5 2.5 0.7-1.0
Example 9 300 1890 0.16 0.22 31 295 0.75 0.25 0.3-0.7 345 7.5 2.5 0.7-1.0
Example 10 380 1320 0.29 0.16 40 295 0.68 0.18 0.3-0. 7 345 3.8 I 0.7-1.0
Example II 380 2120 0.18 0.16 32 200 0.68 0.18 0.3-0.7 250 2.28 0.6 0.7-1.0
Example 12 380 2360 0.16 0.16 36 320 ·- 0.87 0.23 0.3-0.7 370 6.3 1.66 0.7-1.0
Example 13 250 2050 0.12 0.31 38 165 0.63 0.25 0.3-0.7 215 3.44 1.38 0.7-1.0
Example 14 380 2360 0.16 0.16 31 320 0.87 0.23 0.3-0.7 370 11.78 3.1 0.7-1.0
Example 15 380 2360 0.16 0.16 31 320 0.87 0.23 0.3-0.7 370 6.3 1.66 0.7-1.0
Comparative Example a 230 2120 0.11 0.28 30 20< 0.46 0.20 0.3-0.7 250 0 0 0.7-1.0
Comparative Example b 250 2050 0.12 0.35 39 165 I 0.40 0.2-0.5 215 10.5 4.2 0.5-0.7
Comparative Example c 280 1930 0.15 0.18 44 105 0 0.00 0.7-1.0 155 0 0 1.0
Comparative Example d 300 1890 0.16 0.22 53 295 0 0.00 0.3-0.7 345 7.5 2.5 0.7-1.0
Comparative Example e 380 2360 0.16 0.16 40 320 0.87 0.23 0.3-0.7 370 6 1.58 0.7-1.0
Comparative Example f 250 2050 0.12 0.31 35 165 0 0.00 0.3-0.7 215 9.2 3.68 0.7-1.0
Comparative Example g 380 1320 0.29 0.16 40 320 0 0.00 0.7-1.0 370 0 0 1.0
Comparative Example h 380 2360 0.16 0.16 40 32( 0.87 0.23 0.3-0.7 370 0 0 0.7-1.0
Comparative Example i 380 2120 0.18 0.16 30 200 0.46 0.12 0.3-0.7 250 0 0 0.7-1.0
Comparative Example j 230 2120 0.11 0.28 61 - 2.2 0.% 0.3-0.7 - - - -
Comparative Example k 300 1890 0.16 0.22 65 - 2.7 0.90 0.3-0.7 345 15 5 0.7-1.0
Comparative Example I 300 1890 0.16 0.22 31 - 4.5 1.50 0.3-0.7 345 14 4.67 0.7-1.0
Comparative Example m 280 1930 0.15 0.25 35 105 0.05 0.02 0.3-0.7 155 9.8 3.5 0.7-1.0
Comparative Example n 380 2120 0.18 0.16 32 200 0.48 0.13 0.3-0.7 250 2.2 0.58 0.7-1.0
Comparative Example o 280 2120 0.13 0.25 38 200 1.5 0.54 0.3-0.7 250 13.2 4.71 0.7-1.0
[0122] In Table 1, "Degree of Superheat of Molten Steel CC)" is a temperature
added to the liquidus temperature that is determined by components of steel in a tundish.
In Examples 1 to 15 and Comparative Examples a to i, m and n, rolls of convex surfaces
5 were used as the first reduction rolls and the second reduction rolls. On the other hand,
in Comparative Example j, rolls of smooth surfaces were used as the first reduction rolls,
and no second reduction rolls were used. In Comparative Examples k to 1, rolls of
smooth surfaces were used as the first reduction rolls and the second reduction rolls.
In every Example and Comparative Example, the value shown in Table 1 was
10 multi plied by "1 04 ", to be the dent amount.
[0123]
[Table 2]
37
Heating Maximum Steel Plate
Configuration of Thickness Center Solidification Maximwn
I Porosity Ultrasonic Reduction Segregation Over'aU
Temp. Shape Thickness
Equiaxed Equiaxed Volume Testing
(C) Factor (mm)
Rate(%) Solidifying Structure Crystal Crystal Unifonnity Thickness
(cm3/g) Result
Evaluation
Ratio(%) Diameter (mm) (mm)
Example I 1230 0.63 ISO 34.8 Granular IS.2 0.6 Uniform 0.2S 1.88 PassB Pass
Example 2 1220 0.63 ISO 40.0 Granu1ar 22.0 0.8 Unifonn O.S 1.88 Pass B Pass
Example 3 122S 0.63 ISO 40.0 Granular 23.S 0.7 Unifonn O.S 1.16 PassB Pass
Example4 1140 0.41 210 2S.O Granular 2S.6 0.8 Unifonn 0.2S I~()( PassB Pass
ExampleS 1110 0.37 210 2S.O Granular 2S.O 0.8 Uniform 0.2S 0.73 PassB Pass
Example 6 1180 0.46 210 2S.C Granular 24.8 0.8 Uniform 0.2S 1.00 PassB Pass
Example 7 1170 0.44 210 2S.O GranuJar 2S.3 0.8 Uniform O.S 0.7S PassB Pass
Example 8 1130 0.41 210 30.0 Granular 31.2 0.9 Uniform O.S ].()( PassB Pass
Example 9 1210 O.SS 170 43.3 Granular 28.8 0.9 Uniform 0.2S 0.7S PassB Pass
Example 10 JJSO 0.42 260 31.6 Granular SI.O 1.3 Uniform 0.2S 0.7S PassB Pass
Example II 1120 0.37 300 21.1 Granular S2.S 1.2 Uniform 0.2S 0.7S PassB Pass
Example 12 1230 0.64 200 47.4 Granu1ar 19.6 1.2 Uniform O.S 2.72 PassB Pass
Example 13 123 0.53 170 32.0 Granular 22.2 0.7 Uniform 0.2S 1.88 Pass A Pass
Example 14 !OS 0.20 300 21.1 Granular 49.1 - 1.2 Uniform 0.2S 1.00 Pass A Pass
Example IS 1230 O.S3 200 47. Granular SO.O 1.2 Uniform 0.2S 2.72 Pass A Pass
Comparative Example a 1230 O.SS ISO 34.8 Granular IS.7 2.1 Ununiform J.S S.89 X Fail
Comparative Example b JJSO 0.43 170 32.0 Granular 23.S 0.8 Uniform 0.7S 2.90 X Fail
Comparative Example c 1120 0.33 210 2S.O Granular 31.0 0.9 Unifonn J.S 2.% X Fail
Comparative Example d 123 O.S3 210 30.1 Granular 30.4 1.1 Uniform I 2.89 X Fail
Comparative Example e 123 O.Sl 320 IS.8 Granular 28.5 1.3 Uniform O.S 2.90 X Fail
Comparative Example f 1260 0.70 ISO 40.0 Granular 20.8 0.8 Uniform I S.89 Pass A Fail
Comparative Example g 12SS 0.70 ISO 60.S Granular 47.0 1.3 Uniform I 2.96 PassB Fail
Comparative Example h 12SS 0.71 ISO 60.S Granular 48.6 1.4 Uniform 0.2S 2.96 PassB Fail
Comparative Example i 1270 0.80 ISO 60.S Granular 49.S 1.3 Unifonn 0.2S S.89 PassB Fail
Comparative Example j 1230 O.SS ISO 34.8 Branching Dendrites 7.3 S.7 Uniform J.S S.89 X Fail
Comparative Example k 1130 0.41 210 30.0 Branchin~ Dendrites 31.2 6.2 Uniform J.S 3.20 X Fail
Comparative Example I 121( O.SS 170 43.3 Granular 21.2 0.9 Uniform I 2.7S X Fail
Comparative Example m 118( 0.46 210 25.0 Granular 19.3 2.3 Ununiform 1.25 2.88 X Fail
Comparative Examjlle n 1120 0.37 300 21.1 Granular 18.S 3.7 Ununiform 1.25 3.05 X Fail
Comparative Example o 1170 0.44 210 25. Granular 25.3 0.8 Uniform 1.5 6.10 X Fail
[0124] In Table 2, "Configuration of Thickness Center Solidification" shows results
that: a sample was cut out from the cast slab after the cast slab manufacturing step
before the rolling step, and solidifying structure that emerged using an etchant prepared
5 with cupric chloride, aqueous solution saturated with picric acid, and hot water at 80°C
was observed in an equiaxed crystal zone of 50 mm in cast slab width center and 100
mm in 1/2 thickness.
"Equiaxed Crystal Ratio (% )" is a proportion of thickness of a zone in the top
half of the cast slab in the thickness direction where equiaxed crystals formed, to the 112
10 thickness of the cast slab. "Equiaxed Crystal Diameter (mm)" is a mean value of
equivalent circle diameters of approximately 100 equiaxed crystals that were measured
by binarization image processing on the solidifying structure. In "Uniformity of
Solidifying Structure", "Uniform" means that difference between the equiaxed crystal
ratios of the upper half and the lower half of the cast slab bordered by the thickness
15 center of the cast slab was no more than 5%, and "Ununiform" means that the difference
38
was over 5%. Fig. 7 is a schematic view of a transversal cross section of the cast slab.
"Maximum Segregation Thickness" is the maximum value of segregation
thickness of a sample cut out from the cast slab after the cast slab manufacturing step
before the rolling step, specified by observation on whole of the cast slab in the width
5 direction. Fig. 8 shows an example of granular crystals and the maximum segregation
thickness. Fig. 9 shows an example of branching dendrites and the maximum
segregation thickness.
The value shown in Table 2 was multiplied by "104 ", to be "Porosity Volume".
"Reduction Rate" is a proportion of reduction thickness in the rolling step to
10 the cast slab thickness before rolling (= cast slab thickness before rolling - slab
thickness after reduction).
In addition, "x" in each "Ultrasonic Testing Result" field means "Fail A" and
"Fail B".
In "Over-all Evaluation", Examples that satisfied "the maximum segregation
15 thickness:::; 0.5 mm", "the maximum shape factor< 0.7", "the porosity volume< 2.5 x
104 cm3/g" and "Ultrasonic Testing Result was other than 'x'" were determined to
"pass".
[0125] As shown in Tables 1 and 2, the cast slab manufactured with the method of
manufacturing the continuous-cast slab of the present invention (hereinafter may be
20 referred to as "cast slab of Example") had uniform and small granular equiaxed crystals
of 1.3 mm in diameter. The cast slab of Example was 0.50 mm in maximum
segregation thickness, and thus, segregation therein was reduced. Moreover, the cast
slab of Example was no more than 2.5 x 104 cm3/g in porosity volume. A
conventional cast slab was approximately 6 to 10 x 104 cm3/g in.porosity volume.
25 Thus, according to the present invention, the porosity volume was able to be reduced.
39
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mm and no more tban 15 nun in len~~ and no
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volume, the maximum shape factor upon rolling with rolls, and a pass or fail of
ultrasonic testing.
[0128] As shown in the distribution map of Fig. 1 0, when a conventional cast slab
of approximately 6 x 10-4 cm3/g in maximum porosity volume was used, it was
5 impossible to pass ultrasonic testing by Standard A unless heavy rolling of no less than
0.7 in maximum shape factor was carried out. It ~as also impossible to pass ultrasonic
testing by Standard B unless heavy rolling of no less than 0.7 in maximum shape factor
was carried out even when the cast slab of 3 x 10-4 cm3 /g in maximum porosity volume
was used.
10 [0129] In contrast, the cast slabs manufactured by adjusting reduction with the first
reduction rolls and the second reduction rolls had no more than 2.5 x 10-4 cm3/g in
maximum porosity volume although some variation arose. When these cast slabs were
used, they passed ultrasonic testing even if the maximum shape factor in rolling in the
later process was reduced to no more than 0.65 in Standard B.
15 [0130] The heating temperature in the rolling then was within the range of 1050 to
1230°C.
Specifically, it is found that it was possible to satisfy Standard A by reducing
the maximum porosity volume to a level with 1.0 x 10-4 cm3/g even if the maximum
shape factor was 0.2 as shown in Fig. 10.
20 [0131J In view of these results, it was found that according to the present invention,
a thick steel plate of a level with passing ultrasonic testing was able to be manufactured
even ifthe maximum shape factor in rolling was within the range of0.2 to 0.65. The
heating temperature in rolling might be within the range of 1050 to 1230°C, and it was
not necessary for the cast slab to be heated at a high temperature to no less than 1250°C
25 · as a conventional one. Thus, the manufacturing costs of a thick steel plate was able to
41
be greatly reduced.
[0132] As described above, according to the present invention, a thick steel plate of
a level with passing ultrasonic testing can be manufactured at low cost without heavy
rolling ofno less than 0.7 in shape factory.
5
Reference Signs List
[0133] 0: apparatus of manufacturing a thick steel plate

We claim:
1. A continuous-cast slab of 0.1 to 0.3 in ratio D/W, the ratio D/W being a ratio of
casting thickness D to casting width W, and 230 to 380 mm in casting thickness D, the
5 continuous-cast slab having a horizontally symmetrical granular equiaxed crystals at
least in a center in a thickness direction, the continuous-cast slab comprising:
a first reduction dent and a second reduction dent that is narrower than the first
reduction dent at least on one long side surface, the second reduction dent denting
further from a bottom surface of the first reduction dent,
10 wherein a dent amount d1 of the first reduction dent from an end surface of the
continuous-cast slab is 0.08 to 1.1 mm, and a dent amount d2 of the second reduction
dent from the bottom surface of the first reduction dent is 1.2 to 12 mm.
2. A continuous-cast slab of 0.1 to 0.3 in ratio D/W, the ratio D/W being a ratio of
15 casting thickness D to casting width W, and 230 to 380 mm in casting thickness D, the
continuous-cast slab having a horizontally symmetrical granular equiaxed crystals at
least in a center in a thickness direction, the continuous-cast slab comprising:
a first reduction dent and a second reduction dent that is narrower than the first
reduction dent at least on one long side surface, the second reduction dent denting
20 further from a bottom surface of the first reduction dent,
wherein a dent rate of the first reduction dent from an end surface of the
continuous-cast slab, to the casting thickness D is 0.03 to 0.36%, and a dent rate of the
second reduction dent from the bottom surface of the first reduction dent, to the casting
thickness D is 0.6 to 4%.
25
43
3. The continuous-cast slab according to claim 1, wherein
a dent rate of the first reduction dent from the end surface of the
continuous-cast slab, to the casting thickness D is 0.03 to 0.36%, and a dent rate of the
second reduction dent from the bottom surface of the first reduction dent, to the casting
5 thickness Dis 0.6 to 4%.
4. The continuous-cast slab according to any one of claims 1 to 3, wherein
distance between either end of the first reduction dent and the end surface of
the continuous-cast slab is 0.37 x the casting thickness D to 1.0 x the casting thickness
10 D, and distance between either end of the second reduction dent and the end surface of
the continuous-cast slab is 0.5 x the casting thickness D to 1.2 x the casting thickness D.
5. The continuous-cast slab according to any one of claims 1 to 4, wherein a
maximum porosity volume is no more than 1.5 x 104 cm3 /g.
15
6. A method of manufacturing a continuous-cast slab, the method comprising:
a first step of forming a first reduction dent at least in one long side surface of
the continuous-cast slab by carrying out reduction with first reduction rolls on the
continuous-cast slab, the continuous-cast slab being 0.1 to 0.3 in ratio D/W, the ratio
20 D/W 5eing a ratio of casting thickness D to casting width W, and 230 to 380 mm in
casting thickness D, the continuous-cast slab having a horizontally symmetrical granular
equiaxed crystals at least jn a center in a thickness direction; and
a second step of forming a second reduction dent that is narrower than the first
reduction dent by carrying out further reduction on a bottom surface of the first
25 reduction dent with second reduction rolls that are narrower than the first reduction
44
rolls,
wherein in the first step, the reduction is carried out on the continuous-cast slab
so that a dent amount d1 of the first reduction dent from an end surface of the
continuous-cast slab is 0.08 to 1.1 mm, and
5 in the second step, the reduction is carried out on the continuous-cast slab so
10
that a dent amount d2 of the second reduction dent 'from the bottom surface of the first
reduction dent is 1.2 to 12 mm.
7. A method of manufacturing a continuous-cast slab, the method comprising:
a first step of forming a first reduction dent at least in one long side surface of
the continuous-cast slab by carrying out reduction with first reduction rolls on the
continuous-cast slab, the continuous-cast slab being 0.1 to 0.3 in ratio D/W, the ratio
D/W being a ratio of casting thickness D to casting width W, and 230 to 380 mm in
casting thickness D, the continuous-cast slab having a horizontally symmetrical granular
15 equiaxed crystals at least in a center in a thickness direction; and
a second step of forming a second reduction dent that is narrower than the first
reduction dent by carrying out further reduction on a bottom surface of the first
reduction dent with second reduction rolls that are narrower than the first reduction
rolls,
20 wherein in the first step, the reduction is carried out on the continuous-cast slab
so that a dent rate of the first reduction dent from an end surface of the continuous-cast
slab, to the casting thickness Dis 0.03 to 0.36%, and
in the second step, the reduction is carried out on the continuous-cast slab so
that a dent rate of the second reduction dent from the bottom surface of the first
25 reduction dent, to the casting thickness Dis 0.6 to 4%.
45
8. The method according to claim 6, wherein
in the first step, the reduction is carried out on the continuous-cast slab so that a
dent rate of the first reduction dent from the end surface of the continuous-cast slab, to
5 the casting thickness D is 0.03 to 0.36%, and
in the second step, the reduction is carried- out on the continuous-cast slab so
that a dent rate of the second reduction dent from the bottom surface of the first
reduction dent, to the casting thickness D is 0.6 to 4%.
10 9. The method according to any one of claims 6 to 8, wherein
the first reduction rolls are provided for a zone where a fraction solid is 0.3 to
0.7, and the second reduction rolls areprovided for a zone where a fraction solid is 0.7
to 1.0, downstream from the first reduction rolls.
15 10. The method according to any one of claims 6 to 9, wherein
distance between either end of the first reduction dent and an end surface of the
continuous-cast slab is 0.37 x the casting thickness D to 1.0 x the casting thickness D,
and distance between either end of the second reduction dent and the end surface of the
continuous-cast slab is 0.5 x the casting thickness D to 1.2 x the casting thickness D.
20
11. The method according to any one of claims 6 to 10, wherein
a maximum porosity volume of the continuous-cast slab manufactured through
the first and second steps is no more than 1.5 x w-4 cm3/g.
25 12. An apparatus of manufacturing a continuous-cast slab, the apparatus
46
comprising:
first reduction rolls that shape an intermediate shaped product having a first
reduction dent at least on one long side surface of the continuous-cast slab, the
continuous-cast slab being 0.1 to 0.3 in: ratio D/W, the ratio D/W being a ratio of casting
5 thickness D to casting width W, the continuous-cast slab being 230 to 380 mm in
casting thickness D, and having horizontally symmetrical granular equiaxed crystals at
least in a center in a thickness direction; and
second reduction rolls that shape a second reduction dent denting further from
a bottom surface of the first reduction dent of the intermediate shaped product and being
10 narrower than the first reduction dent, the second reduction rolls having shapes
narrower than the first reduction rolls and being arranged downstream from the first
reduction rolls,
wherein the first reduction rolls are provided so that a dent amount d1 of the
first reduction dent from an end surface of the continuous-cast slab is 0.08 to 1.1 mm,
15 and the second reduction rolls are provided so that a dent amount d2 of the second
reduction dent from the bottom surface of the first reduction dent is 1.2 to 12 mm.
13 An apparatus of manufacturing a continuous-cast slab, the apparatus
comprising:
20 first reduction rolls that shape an intermediate shaped product having a first
reduction dent at least on one long side surface of the continuous-cast slab, the
continuous-cast slab being 0.1 to 0.3 in ratio D/W, the ratio D/W being a ratio of casting
thickness D to casting width W, the continuous-cast slab being 230 to 380 mm in
casting thickness D, and having horizontally symmetrical granular equiaxed crystals at
25 least in a center in a thickness direction; and
47
second reduction rolls that shape a second reduction dent denting further from
a bottom surface of the first reduction dent of the intermediate shaped product and being
narrower than the first reduction dent, the second reduction rolls having shapes
narrower than the first reduction rolls and being arranged downstream from the first
5 reduction rolls,
wherein the first reduction rolls are provided so that a dent rate of the first
reduction dent from an end surface of the continuous-cast slab, to the casting thickness
Dis 0.03 to 0.36%, and the second reduction rolls are provided so that a dent rate of the
second reduction dent from the bottom surface of the first reduction dent, to the casting
10 thickness D is 0.6 to 4%.
14. The apparatus according to claim 12, wherein
the first reduction rolls are provided so that a dent rate of the first reduction
dent from an end surface of the continuous-cast slab, to the casting thickness D is 0.03
15 to 0.36%, and the second reduction rolls are provided so that a dent rate of the second
reduction dent from the bottom surface of the first reduction dent, to the casting
thickness D is 0.6 to 4%.
15. The apparatus according to any one of claims 12 to 14, wherein
20 . .,. the first reduction rolls are provided for a zone where a fraction solid is 0.3 to
0.7, and the second reduction rolls are provided for a zone where a fraction solid is 0.7
to 1.0, downstream from the first reduction rolls.
16. The apparatus according to any one of claims 12 to 15, wherein
25 the first reduction rolls are provided so that distance between either end of the
48
first reduction dent and the end surface of the continuous-cast slab is 0.37 x the casting
thickness D to 1.0 x the casting thickness D, and the second reduction rolls are provided
so that distance between either end of the second reduction dent and the end surface of
the continuous-cast slab is 0.5 x the casting thickness D to 1.2 x the casting thickness D.
5
17. The apparatus according to any one of cl~1ms 12 to 16, wherein a maximum
porosity volume of the continuous-cast slab is no more than 1.5 x 10-4 cm3/g.
18. A method of manufacturing a thick steel plate comprising:
10 a continuous-cast slab manufacturing step of manufacturing a continuous-cast
slab according to the method of manufacturing a continuous-cast slab of any one of
claims 6 to 11; and
a rolling step of rolling, within the range of 0.2 to 0.65 in maximum shape
factor, the continuous-cast slab manufactured in the continuous-cast slab manufacturing
15 step, the continuous-cast slab being no more than 2.5 x 10-4 cm3/g in maximum porosity
volume.
19. The method according to claim 18, wherein steel plate thickness to casting
thickness D after the rolling step is ended is 50% to 80% by the rolling step.
20
20. The method according to claim 18 or 19, wherein the steel plate thickness after
the rolling step is ended is 150 to 300 mm by the rolling step.
21. An apparatus of manufacturing a thick steel plate, the apparatus comprising:
25 the apparatus of manufacturing a continuous-cast slab according to any one of
49
claims 12 to 17; and
a rolling mill that rolls the continuous-cast slab manufactured by the apparatus
of manufacturing a continuous-cast slab,
wherein the rolling mill rolls, within the range of 0.2 to 0.65 in maximum
5 shape factor, the continuous-cast slab of no more than 2.5 x 10-4 cm3/g in maximum
10
porosity volume.
22. The apparatus according to claim 21, wherein the rolling mill makes steel plate
thickness after said rolling 50% to 80% to casting thickness D.
23. The apparatus according to claim 21 or 22, wherein the rolling mill makes steel
plate thickness after said rolling 150 to 300 mm.