DESCRIPTION
TITLE OF THE INVENTION: METHOD FOR MANUFACTURING
FORGED STEEL ROLL
TECHNICAL FIELD
[00011
The present invention relates to a method for manufacturing a forged
steel roll for cold or warm use, and particularly relates to a method for
manufacturing a forged steel roll which can maintain satisfactory surface
properties even when cutting of the roll surface is repeated in association
with its long-term use.
BACKGROUND ART
[00021
In general, forged steel rolls are manufactured, due to their large
diameter, by casting large -scaled ingots (steel ingots) by the ingot-making
method and forging the ingots. In the large-scaled ingots, a macro
segregation called as ghost segregation tends to occur from the center to the
, vicinity of the surface during casting, and this ghost segregation remains
inside the manufactured forged steel rolls as a segregation even after
passing through a forging step and a heat-treatment step.
[OOO~]
Fig. 1 is a longitudinal sectional view of a general ingot obtained by
the ingot-making method. As shown in this figure, V segregation and ghost
segregation appear inside the ingot as general macro segregations. The V
segregation is formed of V shape in the central part of the ingot, and includes
dense V segregation in the upper portion and pale V segregation in the lower
portion. Settled crystals exist below the pale V segregation. The ghost
segregation, in which C, P, Mn or other alloy components are thickened, is
located in an area extending from the outside of the V segregation to a
position of about 112 of the radius of the ingot, and has a linear segregation
line shape extending in the vertical direction of the ingot.
[0004]
Since the generation position of the ghost segregation is closer to the
ingot surface than that of the V segregation, cracks starting from the ghost
segregation can be caused, in the forging and heat-treatment steps following
the casting of the ingot, by stresses in processing deformation and thermal
stresses in heat treatment to cooling.
[00051
Further, forged steel rolls, when the surface of the forged steel rolls is
worn or abraded during use, are repaired by cutting the roll surface to
restore the smoothness into a regulated range. If the ghost segregation is
left in the surface vicinity of the forged steel rolls on that occasion,
segregation lines can be exposed to the surface of the rolls by this cutting
repair, even if no defects such as cracks are caused in the original
manufacturing process. When a roll with exposed segregation lines is used
for processing such as rolling, the roll itself becomes unsuited for reuse since
the segregation lines are transferred onto a workpiece.
[0006]
Therefore, it is strongly requested to establish a technique for
manufacturing a forged steel roll, which can be stably used over a long period
of time without cracking in the forging and heat treatment steps and without
exposure of segregation lines by repeated cutting repairs of the surface of the
forged steel roll.
[00071
When ingots obtained by the ingot-making method are used as a
material for forged steel rolls as they are, the quality of the resulting forged
steel rolls is noticeably deteriorated, particularly, resulting from the ghost
segregation. In this regard, steel ingots obtained by the electroslag
remelting (hereinafter referred to as "ESR") method are generally known to
have a solidified structure with less segregation. Therefore, as the material
for forged steel rolls, the steel ingots obtained by the ESR method are
generally applied.
[00081
Fig. 2 is a longitudinal sectional view of a general steel ingot obtained
by the ESR method. Inside the steel ingot, freckle defects appear in the
vicinity of an area of about 112 of the radius of the steel ingot where the
curvature radius of molten steel pool is increased, depending on the depth of
the molten steel pool. The freckle defects appearing inside the steel ingots
by the ESR method is minor, compared with the V segregation and ghost
segregation appearing inside the ingots by the ingot-making method.
Therefore, the application of the steel ingots obtained by the ESR method as
the material for forged steel rolls holds promise for improving the quality of
forged steel rolls in a fashion.
[00091
However, the freckle defect is a channel type segregation having the
same generation mechanism as the ghost segregation. Thus, even when the
steel ingots obtained by the ESR method are used as the material for forged
steel rolls, deterioration in the quality of forged steel rolls resulting from the
freckle defects becomes obvious, similarly to that resulting from the ghost
segregation.
[00101
The generation mechanism of freckle defects can be explained as
follows.
[OOllI
In a forging process, light elements such as C, P, and Si in steel are
micro-segregated between dendrite trees in the course of solidification.
Such micro-segregation molten steel is lower in density than bulk (base
metal) molten steel since these light elements are thickened, and receives a
vertically upward force opposite to the gravity by buoyancy.
Loo 121
Although the micro-segregation molten steel stops between
branch-like dendrite trees in the early stage of generation, it is then slightly
moved upward by buoyancy, integrated with another micro-segregation
molten steel located further upward, and developed into an aggregate of
micro-segregation molten steels, whereby its volume is increased. Such
micro-segregation molten steel is further increased in volume through
further upward movement and promotion of the integration, and ascended
by large buoyancy produced thereby while crossing branches of dendrites
existing upward and breaking the branches to further collect other
micro-segregation molten steels.
Lo01 31
This micro-segregation molten steel freezes in accordance with the
progress of solidification during ascending between dendrite trees, and
remains a segregation line inside the steel ingot, and this emerges as a
freckle defect.
[0014]
.It goes without saying that the freckle defect is more likely to occur
as the content of light elements in molten steel is larger, from the point of its
generation mechanism.
[OO 151
When the dendrite structure that is a solidified structure is coarse,
the volume of the micro-segregation molten steel tends to increase, and the
freckle defects tend to be coarsened. This is attributed 'to that, when the
dendrite structure is coarse, an upward flow of molten steel is easily
generated due to an increased volume of the micro-segregation molten steel
which is generated first between dendrite trees and a small resistance when
the micro-segregation molten steel starts ascending by buoyancy.
[0016]
In general, when the radius of a steel ingot is represented by R,
freckle defects tend to occur in the vicinity of Rl2 of the steel ingot where the
curvature radius of molten steel pool is increased to facilitate apical
extension of dendrite arm spacing. However, when the steel ingot is
large-sized and high in the content of light elements, the freckle defects tend
to be generated also near the surface of the steel ingot, causing a problem
such as generation of cracks in the heat treatment step, similarly to the case
of the above-mentioned ghost segregation.
[00171
As described above, it is strongly requested to establish the technique
capable of preventing generation of cracks in the forging and heat treatment
steps, in manufacturing of forged steel rolls, and preventing segregation
lines from being exposed even when the surface of the forged steel rolls is
repeatedly repaired by cutting, so that the forged steel rolls can be stably
used over a long period of time. To meet this request, it is necessary to
perfectly suppress the freckle defects in the casting stage of steel ingots or
sealing the freckle defects at least nearer the center in relation to the surface
of the steel ingots.
[00181
It is supposed that the generation of freckle defects can be
suppressed by miniaturizing the dendrite structure, from a standpoint of its
generation mechanism. Although the miniaturization of the dendrite
structure can be attained by increasing the cooling rate in casting, even the
manufacturing of small-diameter steel ingots at high cooling rate, for
example, involves problems such as restrictions on roll diameter of product
and an insufficient forge ratio in forging of the steel ingots.
[00191
Patent Document 1 describes a method for miniaturizing the
dendrite structure by setting the content of P to 0.025 to 0.060 wt%, as a
method for improving the surface roughing of a work roll for cold rolling mill
since the surface roughing of the roll is caused by the dendrite structure
generated during casting. However, since P is generally an impurity
element, and causes embrittlement of iron and steel material, it is not
preferred to increase the content of P. Further, P is a light element which
causes freckle defects as described above, and an increased content of P is
considered to encourage the generation of freckle defects..
[00201
Patent Document 2 proposes a determination method in a simulator
for casting process, which is characterized by simultaneously evaluating a
freckle defect evaluation index (Ra number (Rayleigh number)) with
consideration for a segregation molten steel flow, or a hetero-crystal defect
evaluation index with consideration for a hetero-crystallization mechanism
from the concentration or temperature calculated in a casting process
simulation based on an optional casting plan to determine the quality of the
casting plan. As described in Lo0571 of this document, although it can be
suggested from the calculation example of Fig. 12 in this document that
freckle defects are likely to occur at a site where Ra number is 0.07 or larger,
defect evaluation reference values must be newly set when the casting
material is changed.
CITATION LIST
PATENT DOCUMENT
Lo02 11
Patent Document 1: Japanese Patent Application Publication No.
61-009554
Patent Document 2: Japanese Patent Application Publication No.
2003-033864
SUMMARY OF THE INVENTION
TECHNICAL PROBLEM
Lo0221
As described above, the miniaturization of dendrite structure in steel
ingots as the material for forged steel rolls has problems such as the
restrictions on roll diameter and the occurrence of embrittlement or
segregation due to increased light element contents. The present invention
is achieved in view of such problems, and has an object to provide a method
for manufacturing a forged steel roll, capable of perfectly suppressing freckle
defects, in casting of a steel ingot as the material for forged steel rolls by the
ESR method, or sealing the freckle defects at least nearer the center in
relation to a position where freckle defects emerge in conventional steel
ingots.
SOLUTION TO PROBLEM
Lo0231
As a result of the earnest examinations to attain the
above-mentioned object, the present inventors found that the dendrite
structure can be miniaturized while suppressing the generation of freckle
defects by adding Bi to molten steel, in the process of casting by the ESR
method, to cast a steel ingot containing a predetermined amount of Bi. The
content of the examinations will be described later.
[00241
The present invention is achieved based on this knowledge, and the
gist thereof is the following method for manufacturing a forged steel roll.
Namely, the method for manufacturing a forged steel roll of the present
invention is characterized by casting, by the ESR method, a steel ingot which
contains, by mass%, C: 0.3% or more, Si: 0.2% or more, Cr: 2.0-13.0% and Mo:
0.2% or more, and further contains Bi at 10-100 ppm by mass; and forging
the steel ingot to manufacture the roll.
LO0251
In the following description, with respect to the component
composition of steels, "%" means "% by mass (mass%)", and "ppm" means
"ppm by mass", unless otherwise noted.
ADVANTAGEOUS EFFECTS OF THE INVENTION
Lo0261
According to the method for manufacturing a forged steel roll of the
present invention, freckle defects that are a macro-segregation generated in
casting of a steel ingot by the ESR method can be sealed nearer the center in
relation to the surface of the steel ingot. Since cracks starting from the
segregation can be thus suppressed during forging and heat treatment of the
steel ingot, and segregation lines of the freckle defect are hardly exposed
even when the roll is repaired by cutting to reuse the roll, the roll can be
stably used over a long period of time.
BRIEF DESCRIPTION OF DRAWINGS
Lo0271
[Fig. 11 Fig. 1 is a longitudinal sectional view of a general ingot
obtained by the ingot-making method.
[Fig. 21 Fig. 2 is a longitudinal sectional view of a general steel ingot
obtained by the ESR method.
[Fig. 31 Fig. 3 is a schematic view showing, in the method for
manufacturing a forged steel roll of the present invention, one example of
casting of a steel ingot used as the material by the ESR method.
[Fig. 41 Fig. 4 is a view showing the relationship between Bi content
and dendrite primary arm spacing.
[Fig. 51 Fig. 5 is a view showing the relationship between the radial
distance from steel ingot surface and the dendrite primary arm spacing.
[Fig. 61 Fig. 6 is a view showing the relationship between the radial
distance &om steel ingot surface and the value of RaIRao.
DESCRIPTION OF EMBODIMENTS
The method for manufacturing a forged steel roll of the present
invention is characterized by: casting, by the ESR method, a steel ingot
which contains C: 0.3% or more, Si: 0.2% or more, Cr: 2.0-13.0% and Mo:
0.2% or more, and further contains Bi at 10-100 ppm; and forging the steel
ingot to manufacture the roll.
[00291
The reasong to specify the method for manufacturing a forged steel
roll of the present invention as described above and preferred embodiments
thereof will be then described.
[00301
1. Casting of Steel Ingot by ESR Method
Fig. 3 is a schematic view showing, in the method for manufacturing
a forged steel roll of the present invention, one example of a state for casting
a steel ingot used as the material by the ESR method.
[00311
As shown in this figure, in the ESR method, a stub 4 is connected by
welding to the upper end of a cylindrical consumable electrode 2 that is a
base metal of a steel ingot 1, and the electrode is moved down in accordance
with the lowering of the stub 4 by a raising and lowering mechanism not
shown. A molten slag 7 is held in a casting mold (water-cooled copper mold)
6 within a chamber 5, and energization is performed with the consumable
electrode 2 being immersed in the molten slug 7, whereby electricity is
carried to the molten slug 7, and the molten slug 7 generates heat. The
consumable electrode 2 is successively molten from the lower end by the
Joule heat of the molten slug 7. The molten consumable electrode 2 settles
out through the molten slug 7 as droplets, and solidifies in layers while being
retained as a pool of molten steel 3 within the casting mold 6. The
consumable electrode 2 is successively molten up to the upper end, and the
molten steel 3 is successively solidified in the casting mold 6, whereby the
steel ingot 1 for the forged steel roll is obtained.
Lo0321
In the present invention, since the steel ingot 1 obtained by the ESR
method contains a predetermined amount of Bi, the molten steel 3 must be
caused to contain Bi in the process of casting by'the ESR method. As a
method therefor, Bi may be added to the molten steel 3 in a casting stage by
the ESR method, or Bi may be added, at a stage prior to the casting by the
ESR method or in the stage of producing the consumable electrode 2 that is
the base metal by the ingot-making method, to the molten steel of the
electrode.
LO0331
When Bi is added to the molten steel 3 in the casting stage by the
ESR method as the former, the addition of Bi can be attained by supplying a
Bi wire 8 containing Bi to the molten steel 3 as shown in Fig. 3. Besides, it
can be attained also by preliminary welding the Bi wire to the side surface of
the consumable electrode 2 along the axial direction.
Lo0341
In the casting by the ESR method, the temperature of molten steel
exceeds 1,600°C. On the other hand, the pure boiling point of Bi is only
1,564OC which falls below the molten steel temperature. Therefore, when
the Bi wire is composed of Bi single body, Bi cannot be effectively retained in
the molten steel since Bi is evaporated during casting. Thus, the Bi wire is
appropriately composed of an alloy of Bi with Ni or the like. The inclusion
of Ni or the like leads to an apparent rise of the boiling point of Bi. When
Ni-Bi series is selected as the alloy, the content of Bi in the Bi wire is
preferably set to 20 to 70 mass% so that Bi is present in a liquid phase state
in the molten steel.
Lo0351
When Bi is added to the molten steel in the stage of producing the
consumable electrode 2 as the latter, Bi can be added in prospect of the
evaporation amount of Bi during the casting by the ESR method.
LO0361
2. Component Composition of Forged Steel Roll and Determination Reason
Thereof
C: 0.3% or more
C enhances the hardenability of steel. C also enhances the wear
resistance of steel by bonding to Cr or V to form a carbide. Therefore, the
content of C is set to 0.3% or more, more preferably to 0.5% or more, further
preferably to 0.85% or more. The upper limit of the C content is not
particularly limited, but when C is excessively contained, sufficient hardness
particularly as forged steel rolls for cold rolling cannot be secured, and the
toughness and machinability of steel are deteriorated due to uneven
distribution of the carbide. Thus, the content of C is preferably set to 1.3%
or less, more preferably to 1.05% or less.
Lo0371
Si: 0.2% or more
Si is an element effective for deoxidizing steel. Si also enhances the
resistance to temper softening of steel and enhances the hardness of steel by
being solid-dissolved in the steel. Therefore, the content of Si is set to 0.2%
or more, more preferably to 0.3% or more. Although the upper limit of Si
content is not particularly limited, the cleanliness of steel is deteriorated
when Si is excessively contained. Thus, the Si content is preferably set to
1.1% or less, more preferably to 0.85% or less, further preferably to 0.6% or
less.
[00381
Cr: 2.0-13.0%
Cr enhances the hardenability of steel. Cr also enhances the wear
resistance of steel by forming a carbide. On the other hand, when Cr is
excessively contained, the ductility or toughness of steel is deteriorated due
to uneven distribution of the carbide. Thus, the content of Cr is set to 2.0 to
13.0%, more preferably to 2.5 to 10.0%.
[00391
Mo: 0.2% or more
Mo enhances the hardenability of steel. Mo also enhances the
resistance to temper softening. Therefore, the content of Mo is set to 0.2%
or more, more preferably to 0.3% or more. The upper limit of the Mo
content is not particularly limited. However, when Mo is excessively
contained, the ductility or toughness of steel is deteriorated due to formation
of a carbide. Thus, the Mo content is set preferably to 1.0% or less, more
preferably 0.7% or less.
[OO~O]
Bi: 10-100 ppm
Since C and Si are light elements, freckle defects tend to occur when
0.2% or more Si is contained in high-carbon carbon steel having a C content
of 0.3% or more. However, Bi is contained in molten steel in the process of
casting by the ESR method to set the content of Bi to 10 ppm or more, as will
be described below, whereby the generation of freckle defects can be
suppressed. When the content of Bi exceeds 100 ppm, the embrittlement
becomes problematic, even if it is a trace amount, in forming a roll by forging.
Therefore, the Bi content is set to 100 ppm or less.
[00411
The forged steel roll can further contain the following elements, in
addition to the above-mentioned essential elements.
LO0421
Mn: 0.4-1.5%
Mn enhances the hardenability of steel. Further, Mn is an element
effective for deoxidizing steel. When Mn is excessively contained, the crack
resistance of steel is deteriorated. Therefore, when Mn is aggressively
contained, the content thereof is set to 0.4 to 1.5%.
100431
Ni: 2.5% or less
Ni enhances the toughness of steel. Ni also enhances the
hardenability of steel. On the other hand, when Ni is excessively contained,
hydrogen cracking tends to occur after heat treatment. Since Ni is an
austenite forming element, the hardness of steel is deteriorated when Ni is
excessively contained. Therefore, when Ni is aggressively contained, the
content of Ni is set to 2.5% or less, more preferably to 0.8% or less.
Loo441
V 1.0% or less
V enhances the wear resistance of steel by forming a carbide.
However, when V is excessively contained, the ductility or toughness of steel
is deteriorated due to formation of the carbide. Therefore, when V is
aggressively contained, the content .thereof is set to 1.0% or less, preferably
to 0.2% or less.
Loo451
In steel ingots having the above-mentioned composition, the dendrite
structure becomes fine by casting by the ESR method. Therefore, in forged
steel rolls manufactured by forging these steel ingots as the material, freckle
detects are perfectly suppressed, or the freckle defects are sealed near the
center of the steel ingots, compared with a case in which no Bi is contained,
so that no segregation lines are exposed even when the surface of the forged
steel rolls is repeatedly repaired by cutting, and the forged steel rolls can be
thus stably used also as recycled rolls.
LO0461
3. Effects of Inclusion of Bi
The present inventors found, by the following unidirectional
solidification test, that the dendrite structure can be miniaturized to
suppress the generation of freckle defects by causing molten steel to contain
Bi in the process of casting by the ESR method so that a resulting steel ingot
contains a trace amount (10 ppm or more) of Bi.
[00471
3-1. Test Condition
A test was performed for casting of a columnar steel ingot having a
diameter of 15 mm and a height of 50 mm by the ESR method. In that
regard, steel ingots having Bi contents of 10 ppm, 21 ppm and 38 ppm were
produced respectively by adding Bi to molten steels, and a steel ingot free
from Bi was also produced without addition of Bi. The cooling rate was set
to 5 to 15°C Imin in accordance with the condition of real operation.
[00481
With respect to each of the obtained steel ingots, spacings each
between about 10 primary arms extending substantially in parallel to the
axial direction in a longitudinal section passing through the center were
measured, and an arithmetic average value thereof was taken as the
dendrite primary arm spacing of each steel ingot.
[00491
3-2. Test Result
Fig. 4 is a view showing the relationship between Bi content and
dendrite primary arm spacing; In this figure, dendrite primary arm
spacing (d) was shown in the vertical axis as the ratio (d/d~)to dendrite
primary arm spacing ( d ~o)f Bi-free steel ingot. It is found from this figure
that as the Bi content is higher, the dendrite primary arm spacing of carbon
steel is narrower, and the dendrite structure is finer. This is attributed to
that Bi is an element having an effect to reduce the interface energy of
solid-liquid interface of the carbon steel, and shows an effect on the
miniaturization of dendrite primary arm spacing even if its content is trace.
If the Bi content is 10 ppm or more, the generation of freckle defects can be
effectively suppressed, as shown in examples to be described later.
[0050]
4. Index of Freckle Defect Generation
The present inventors focused attention on the use of Ra number as a
index of freckle defect generation. The Ra number is a dimensionless
number indicating a convective flow in temperature field, or a product of Pr
number (Prandtl number) and Gr number (Grashof number), and is
represented by the following equation (1).
Ra=Pr.Gr=gp (Ts-Tw)L3/va ... (1)
In the equation, g [m/s2]: gravity acceleration, P [l/KI: volume
expansion coefficient, Ts [a: object surface temperature, Tw [K]:
temperature of fluid, v [m2/s] : kinetic viscosity coefficient, a [m2/s] thermal
diffusivity, and L [m] : typical length.
[00511
The Ra number is considered physically to be a ratio of buoyancy that
is flow-driving force to flow-resisting force, and is proportional to the cube of
typical length as shown in the above-mentioned equation (1). If the
criticality of freckle defect generation is contemplated, the typical length in
the Ra number should be set to the magnitude of micro-segregation between
dendrite trees. Since micro-segregation molten steel is filled between
. dendrite trees in the early state of generation, the magnitude of
micro-segregation can be regarded as the dendrite primary arm spacing.
Accordingly, the typical length in the Ra number can be set to the dendrite
primary arm spacing. Thus, the Ra number can be said to be proportional
to the cube of the dendrite primary arm spacing.
[00521
As described above, since freckle defects are more likely to be
coarsened as the dendrite structure is coarser, the freckle defects are
considered to more easily occur as the Ra number is larger. If generation
results of freckle defects in actual steel ingots are compared with the Ra
number, the Ra number can be taken as an index for the criticality of freckle
defect generation. Since the Ra number is proportional to the cube of the
dendrite primary arm spacing even if the reduction of the dendrite primary
arm spacing by containing a trace amount of Bi in steel ingots is relatively
small, the inclusion of Bi in the steel ingots is effective for the reduction in
Ra number, and thus extremely effective for suppressing the generation of
freckle defects.
EXAMPLES
LO0531
The effects of the present invention were evaluated by a preliminary
test performed actually using steel ingots and a simulation by numerical
calculation.
Lo0541
1. Preliminary Test
A casting test of a steel ingot 800 mm in diameter by the ESR method
was performed as the preliminary test. As the object steel, a high-carbon
steel of 0.87%C-0.30%Si-0.41%Mn-O.10%Ni-4.95%Cr-0.41%Mo-O.O1%V
(Bi-free) was adopted. The liquidus-line temperature of this steel is 1460°C,
and the solidus-line temperature thereof is 1280°C. As the casting
conditions, a molten steel scale of 9 t(ton) and a steel ingot length of 2.3 m
were adopted.
100551
As a result, no freckle defects were generated up to a position 133
mm radially inward from the steel ingot surface, and freckle defects were
generated on the inner side thereof. Namely, the critical point of freckle
defect generation was the position 133 mm radially inward from the steel
ingot surface. The dendrite primary arm spacing and Ra number at this
freckle defect generation critical point were represented by do and Rao,
respectively, and used as reference values of the following simulation by
numerical calculation.
100561
2. Simulation by Numerical Calculation
Evaluation conditions of the numerical calculation simulation were
set as follows. The object steel has the same composition as the
above-mentioned preliminary test of
0.87%C-0.30%Si-0.41%Mn-0.10%Ni-4.95%Cr-0.41%Mo-O.O1%V, with the
content of Bi being 0 pprn (Bi-free), 10 ppm, 21 ppm, and 38 ppm. The
diameter of the object steel ingot was set to 800 mm similarly to the
preliminary test.
[00571
In the above-mentioned evaluation conditions, the solidification rate
and cooling rate of each part of the steel ingot were calculated by radial
unidimensional non-steady heat transfer analysis of the steel ingot, and
distribution of dendrite primary arm spacings in the radial direction from
the surface of the steel ingot was calculated by the following equation (2)
("Solidification of Iron and Steel", The Iron and Steel Institute of Japan-Iron
and Steel Basic Joint Research, Division of Solidification, 1997, Appendix-4).
The equation (2) is an experimental expression of dendrite primary arm
spacing d (pm) using solidification rate V (cmlmin) and temperature gradient
G ("Clem) as parameters in a case that a Cr-Mo steel is adopted.
d=1620V-O.2G-O.4 . . . (2)
[00581
Fig. 5 is a view showing the relationship between the radial distance
from the steel ingot surface and the dendrite primary arm spacing.
Dendrite primary arm spacing ( d ~i)n the Bi-free case, shown in this figure,
was calculated from the above-mentioned equation (2). Dendrite primary
arm spacing (dl in the Bi-containing case was calculated by multiplying the
ratio (d/d~o)f dendrite primary arm spacing with respect to each Bi content
(10 ppm, 21 pprn and 38 ppm) shown in the above-mentioned Fig. 4 by the
value of d~ which was calculated from the equation (2).
[00591
Fig. 6 is a view showing the relationship between the radial distance
from the steel ingot surface and the value of RaIRao. With respect to the Ra
number (Ra) in each Bi content, RaIRao can be said to be the cube of &do, as
shown in the following equation (3) derived from the above-mentioned
equation (1). The Ra/Rao shown in this figure was calculated based on the
equation (3).
Ra/Rao=(d/do)3 . . . (3)
In the equation, Ra/Rao is the ratio of Ra number ( ~ ai)n each Bi
content to basic Ra number (Rao determined in the above-mentioned
preliminary test), and ddo is the ratio of dendrite primary arm spacing d of
each Bi-containing steel ingot to dendrite primary arm spacing do at freckle
defect generation critical point of the Bi-free steel ingot.
[OO~O]
It is found from the above-mentioned Fig. 5 that the dendrite
primary arm spacing do at freckle defect generation critical point of the
Bi-free steel ingot is about 400 pm. In the inside of the steel ingot in which
the dendrite primary arm spacing d is larger than do, freckle defects are
generated. On the other hand, when Bi is contained in trace amounts (10
ppm, 21 ppm, and 38 ppm), the dendrite primary arm spacing d becomes
smaller than the above-mentioned arm spacing at critical point do almost
over the whole area extending radially from the steel ingot surface. In this
case, or when d/do