[0001] This invention relates to a continuous casting method applicable for steels such
as bearing steels and spring steels according to the features as laid out in claim
1, which is based upon EP-A- 0 211 422.
[0002] The continuous casting process has remarkable advantages in the high yield rate and
in the high productivity because a blooming process is not required, and is widely
utilized as a casting method which is possible to continuously manufacture the final
cast piece such as a slab, a bloom, a billet and so on directly from molten steel.
[0003] However, in a case of casting the molten steel through the continuous casting process,
carbon, phosphorus, sulfur and the like have a tendency to segregate (concentrate)
in the center part of the cast piece, accordingly there has been a problem in that
it is difficult to introduce the continuous casting process for steels containing
chemical elements easy to segregate, especially for the steels containing a large
amount of carbon, such as bearing steels, spring steels and so on.
[0004] As a method for inhibiting the segregation, the method to apply soft reduction against
the cast piece drawn out from the mould is well known.
[0005] The segregation and the concentration phenomena of the chemical elements such as
carbon in the center part of the cast piece is considered to be caused by the solidification
of the molten steel proceeding toward the center part from the outer peripheral part
of the cast piece, in particular, it is considered that flowing of the concentrated
molten steel is produced by solidification shrinkage at the time a liquid phase remaining
in the center part of the cast piece is finally solidified, and linked up with the
concentrated segregation of carbon and the like.
[0006] The aforementioned soft reduction treatment is to prevent the flowing of the concentrated
molten steel caused by the solidification shrinkage and to inhibit the segregation
at the time of solidification by compressively deforming the unsolidified portion
in the center of the cast piece at least an amount corresponding to the volume reduction
at the time of solidification.
[0007] However, in the soft reduction treatment, the cast piece is applied with the compressive
deformation in the state where the unsolidified portion remains in the center part
of the cast piece, therefore the soft reduction treatment is sometimes accompanied
with a risk of generation of cracks at a front surface of the solidification according
to magnitude of tensile stress produced by the deformation of unsolidified interface
of the molten steel.
[0008] The cracks generated at the front surface of the solidification brings penetration
into the cracks of the molten steel concentrated with carbon, sulfur, phosphorus and
so on at the front surface of the solidification, and may be a defect harmful as much
as center segregation in the finished products.
[0009] EP-A-0211422 discloses a continuous casting method wherein the thickness of a strand
is continuously reduced in its unsolidified region (generally called a soft reduction
treatment). The document further discloses casting the metal into a bloom having a
rectangular cross section.
[0010] It is necessary to apply a soft reduction to a cast piece effectively so that high
tensile stress may not be generated at the unsolidified interface of the molten steel
and the unsolidified center portion of the cast piece may change in the volume (compressive
deformation) sufficiently in order to prevent the aforementioned cracks at the soft
reduction treatment. Therefore, it has been hoped to establish the technical method
possible to realize the effective soft reduction treatment without generating the
cracks.
[0011] This invention is made in order to solve his kind of problem of the prior art.
[0012] The present invention provides a continuous casting method for steels comprising:
pouring molten steel into a water cooled mould through an upper opening of the mould;
solidifying the molten steel to form a cast piece in the mould and drawing continuously
the cast piece through a lower opening of said mould at the same time, wherein said
cast piece is subjected to soft reduction treatment with a flat part of a roll at
a position of a solid phase ratio from 0.2 to 0.8 before the molten steel in the center
part of the cast piece is completely solidified, and wherein said cast piece has a
circular cross section, and the flat part of the roll forms a plane on a depressed
surface of the cast piece.
[0013] Preferred embodiments of the present invention will now be described hereinbelow
by way of example only with reference to the accompanying drawings, in which:
FIG.1 is a graph illustrating distribution of the carbon concentration on a cross
section of a cast piece obtained in an embodiment according to this invention in comparison
with that of a cast piece obtained without soft reduction;
FIG. 2 is a graph illustrating distribution of the carbon concentration on a cross
section of a cast piece obtained in another embodiment according to this invention
in comparison with that of a cast piece obtained without soft reduction;
FIGS.3A and 3B are perspective views illustrating testing methods for confirming an
effect of soft reduction in the continuous casting method according to this invention;
FIG.4 is a graph illustrating results obtained through the testing method shown in
FIGS.3A and 3B; and
FIG.5 is a schematic view illustrating a position to be applied with the soft reduction
in the continuous casting method according to this invention.
[0014] In the continuous casting, the molten steel undergoes a change in state from perfect
liquid phase A to perfect solid phase C through solid-liquid mixed phase B as shown
in FIG.5.
[0015] In the continuous casting process according to this invention, the solid phrase ratio
is defined as a weight ratio occupied by the solid phase on a cross section of the
cast piece (the solid phase ratio 1 means the state of perfect solid phase).
[0016] It is possible to estimate the solid phase ratio from the thermal distribution on
the cross section of the cast piece obtained according to the heat transfer calculation
based on data such as specific heat and thermal conductivity of the cast piece, the
temperature of the molten steel, for example.
[0017] Then the continuous casting method according to this invention is characterized by
subjecting to the soft reduction treatment against the cast piece cast into a shape
having a circular cross section with the flat part of the roll R at the position of
the solid phase ratio from 0.2 to 0.8 in the solid-liquid mixed phase as shown in
FIG.5.
[0018] The roll having the flat part means a roll possible to form a plane on the depressed
surface of the cast piece with the circular cross section, and the roll is applicable
without distinction of the shape so long as it is possible to form a plane on the
contact surface of the cast piece.
[0019] As mentioned above, in the soft reduction treatment, it is necessary to apply the
soft reduction to the cast piece effectively so as to sufficiently vary the volume
of the unsolidified center portion (compressive deformation) without producing the
high tensiled stress on the unsolidified interface of the molten steel for preventing
the cast piece from the internal cracks.
[0020] In order to find appropriate conditions for the soft reduction treatment, the inventors
carried out stress-strain analysis at the time of applying the soft reduction with
the flat part of the roll R against a cast piece D having a circular cross section
(350mm diameter) and a cast piece E having a square cross section (350mm square) shown
in FIGS.3A and 3B through the finite element method using computer, and results were
obtained as shown in FIG.4. Additionally, symbol F denotes the unsolidified portion,
and symbol G denotes the completely solidified portion of the cast pieces D and E
in FIGS. 3A and 3B.
[0021] In graph A of FIG.4, the axis of abscissas shows a reduction ratio applied to the
cast piece, and the axis of ordinates shows volume reduction of the unsolidified center
portion with an index. The reduction ratio means the decrease percentage of area on
the cross section of the cast piece before and after the soft reduction.
[0022] The other side, in graph B and graph C of FIG.4, the tensile stress generated on
the unsolidified interface of the molten steel in the directions of X-axis (perpendicular
to the axis of the cast piece) and Y-axis (axial direction of the cast piece) are
plotted as ordinates against the reduction ratio as abscissas.
[0023] The results show that the reduction ratio required for obtaining the some volume
reduction at the unsolidified center portion of the cast piece is remarkably different
between the cases of the cast piece D having the circular section and the cast piece
E having the square cross section, and the reduction ratio to be applied on the cast
piece D having the circular cross section is not required so much in comparison with
that to be applied on the cast piece E having the square cross section.
[0024] Furthermore, when the volume reduction is equivalent at the unsolidified center portion
between the cast pieces D and E, it is found that the tensile stress generated at
the unsolidified interface of the molten steel in the cast piece D having the circular
cross section is remarkably low as compared with that in the cast piece E having the
square cross section from the results shown in graphs B and C of FIG.4.
[0025] In view of the aforementioned results, it is seen that it is possible to reduce the
volume of the unsolidified center portion effectively at the same time of controlling
the tensile stress generated at the unsolidified interface of the cast piece very
low by the compression with low reduction ratio when the cast piece is cast into the
shape having the circular cross section and subjected to the low reduction with the
flat part of the roll.
[0026] This invention is made on basis of the above-mentioned findings, it is possible to
prevent the cast piece from the segregation of carbon and the like at the same time
of controlling the generation of the tensile stress in the cast piece in a low level
and preventing the cast piece from the generation of cracks, therefore it is possible
to improve the quality of the cast piece according to this invention.
[0027] This invention is especially effective when it is applied to steels containing carbon
not lower than 0.5 %. The continuous casting method according to this invention is
especially characterized by performing the soft reduction at the position of the solid
phase ratio from 0.2 to 0.8.
[0028] In this invention, the soft reduction is applied on the cast piece at the position
of the solid phase ratio from 0.2 to 0.8 for the reason that a liquid phase in the
center portion of the cast piece loses its fluidity substantially and it is not possible
to obtain the sufficient effect by the low reduction treatment even if the low reduction
is applied on the cast piece at the position of solid phase ratio more than 0.8, and
in contrast with this, the liquid phase occupied most of the cross section of the
cast piece at the position of solid phase ratio less than 0.2 and it is not possible
similarly to obtain the sufficient effect by the low reduction treatment owing to
the excessive fluidity of the liquid phase.
[0029] In this invention, the low reduction is performed for the purpose of preventing the
cast piece from the cracks and the concentrated segregation of the chemical components
such as carbon, phosphorus, sulfur and so on at the center portion of the cast piece.
In this sense, it is preferable to apply the low reduction to the cast piece in a
reduction ratio range of 1.0 to 3.0 %, and further preferable in the reduction ratio
range of 1.5 to 2.5 %.
[0030] This invention will be described below in detail on basis of the following non-limiting
examples.
EXAMPLE 1
[0031] Steel including 1.0 % of carbon and specified as SUJ 2 in JIS G 4805 (High Carbon
Chromium Bearing Steels) was cast into a cast piece having a circular cross section
with a diameter of 350 mm at an extraction speed(Vc) of 0.4 m/min through the continuous
casting process.
[0032] In this time, the cast piece was subjected to the low reduction treatment with a
reduction ratio of 2 % at a position of a solid phase ratio(fs) from 0.4 to 0.5 using
a double roll disposed with two flat rolls on the upper and lower sides. Additionally,
the roll is available so long as it has a flat part even a roll having a V-shaped
recess partially.
[0033] As a result of analyzing the carbon content on the cross section of the obtained
cast piece, a graph on the upper side in FIG.1 was obtained. Furthermore, it is confirmed
that there were not internal cracks in the cast piece.
[0034] The other side, a result shown in a graph on the lower side in FIG.1 was obtained
by analyzing the carbon content of a cast piece cast in the same manner without applying
the low reduction treatment in order to make comparison.
[0035] According to the comparison between both graphs of FIG.1, it is apparent that it
is possible to inhibit the center segregation without generating the internal cracks
by casting the molten steel into the cast piece having the circular cross section
and applying the low reduction to the cast piece with the flat roll.
EXAMPLE 2
[0036] Steel including 0.6 % of carbon and specified as SUP 7 in JIS G 4801 (Spring Steels)
was cast into a cast piece through the continuous casting process under the same condition
as that of Example 1, and a graph on the upper side in FIG.2 was obtained as a result
of analyzing the carbon content on the cross section of the obtained cast piece .
Furthermore, there were not internal cracks at all. A graph on the lower side in FIG.2
shows a result of a cast piece obtained without applying the low reduction treatment.
[0037] By comparing results shown in both graphs of FIG.2, it is seen that it is possible
to inhibit the segregation of carbon in the center portion effectively also in the
case of the steel specified as SUP 7 by casting the cast piece having the circular
cross section and applying the low reduction to the cast piece using the flat roll.
1. Stranggußverfahren für Stahl, welches umfaßt:
Gießen von Stahlschmelze in ein wassergekühltes Formwerkzeug durch eine obere Öffnung
des Formwerkzeugs;
Verfestigen der Stahlschmelze zur Bildung eines Gußstücks im Formwerkzeug und gleichzeitig
kontinuierliches Ziehen des Gußstücks durch eine untere Öffnung des Formwerkzeugs,
wobei das Gußstück einer Weichreduktionsbehandlung mit einem flachen Teil einer Walze
an einer Stelle mit einem Festphasenverhältnis von 0,2 bis 0,8 unterworfen wird, bevor
die Stahlschmelze im Mittelteil des Gußstücks vollständig verfestigt ist, dadurch
gekennzeichnet, daß das Gußstück einen kreisförmigen Querschnitt besitzt und der flache
Teil der Walze eine Ebene auf einer eingedrückten Fläche des Gußstücks bildet.
2. Stranggußverfahren für Stähle nach Anspruch 1, bei welchem das Gußstück einer Weichreduktionsbehandlung
mit einem Reduktionsverhältnis von 1,0 bis 3,0% unterworfen wird.
3. Stranggußverfahren für Stähle nach Anspruch 2, bei welchem das Gußstück einer Weichreduktionsbehandlung
mit einem Reduktionsverhältnis von 1,5 bis 2,5% unterworfen wird.
1. Procédé de coulée continue de l'acier, comprenant:
le versement de l'acier fondu dans un moule refroidi par eau à travers une ouverture
supérieure du moule ;
la solidification de l'acier fondu pour former une pièce coulée dans le moule et le
démoulage en continu, dans le même temps, de la pièce coulée à travers une ouverture
inférieure dudit moule, dans lequel ladite pièce coulée est soumise à un traitement
de réduction légère avec une partie plate d'un cylindre dans une position d'un rapport
de phase solide de 0,2 à 0,8 avant la solidification complète de l'acier fondu dans
la partie centrale de la pièce coulée, caractérisé en ce que ladite pièce coulée présente
une section transversale circulaire, et la partie plate du cylindre forme un plan
sur une surface en creux de la pièce coulée.
2. Procédé de coulée continue pour des aciers selon la revendication 1, dans lequel ladite
pièce coulée est soumise à un traitement de réduction légère d'un taux de réduction
allant de 1,0 à 3,0%.
3. Procédé de coulée continue pour des aciers selon la revendication 2, dans lequel ladite
pièce coulée est soumise à un traitement de réduction légère d'un taux de réduction
allant de 1,5 à 2,5%.