Continuous steel casting process

A continuous steel casting process adapted to produce steel castings of satisfactory quality with less center segregations is described. A molten steel is electromagnetically stirred in at least two of three locations, viz., a casting mold and intermediate and final solidifying zones of a continuously cast strand. In the casting mold, is applied a magnetic field induced by alternate current of a frequency f=1.5.about.10 Hz and having G in the range of 195.times.e.sup.-0.18f .about.1790.times.e.sup.-0.2f at the inner surface of the casting mold. The intermediate solidifying zone employs a magnetic field induced by alternate current of a frequency f=1.5.about.10 Hz and having a magnetic flux density G in the range of 195.times.e.sup.-0.18f .about.1790.times.e.sup.-0.2f at the surface of the strand or a magnetic field induced by alternate current of a frequency f=50.about.60 Hz and having a magnetic flux density G in the range of 0.6.times.10.sup.6 /(D-107).sup.2 .about.1.8.times.10.sup.6 /(D-100).sup.2 (in which D=the thickness of a solidified shell layer of the strand) at the surface of the strand. For electromagnetic stirring in the final solidifying zone, a magnetic field induced by alternate current of a frequency f=1.5.about.10 Hz and having a magnetic flux density in the range of 895.times.e.sup.-0.2f .about.2137.times.e.sup.-0.2f is applied.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method for producing steel castings by 
continuous casting process. 
2. Description of the Prior Art 
In continuous steel casting, there arise problems of defects as detected by 
ultrasonic test, e.g., inclusions occurring in a sub-surface or internal 
portion of a continuously cast strand (hereinafter referred to as "c.c. 
strand" for brevity) in its solidifying stage or shrinkage cavities 
produced in axial center portions of the c.c. strand. In addition, strong 
segregation occurs in c.c. strands cast at high temperature in continuous 
casting operations, impairing cold forgeability due to lowered reduction 
ratio. 
Various attempts have thus far been made to eliminate the internal defects 
of c.c. strands, including the center segregations and shrinkage cavities, 
through single electromagnetic stirring either within a mold or in a 
secondary cooling zone, severing tip ends of growing crystals with fluidic 
movements of molten steel to produce a large quantity of equiaxed crystal 
nuclei, thereby expanding the equiaxed crystal zone in the center portion 
of c.c. strands. However, none of them has succeeded in sufficiently 
reducing the rate of center segregation and irregularities of center 
segregation in the axial direction of c.c. strands, failing to produce 
steel castings of satisfactory quality. 
SUMMARY OF THE INVENTION 
It is a primary object of the present invention to provide a method which 
overcomes the above-mentioned problems and which is capable of producing 
steel castings of satisfactory quality with less center segregations in 
continuous steel casting processes. 
In order to attain this object, the method of the present invention 
includes, in its preferred form, the step of electromagnetically stirring 
molten metal in at least two of three locations, viz., a casting mold and 
intermediate and final solidifying zones of a continuously cast strand, by 
application of: 
for electromagnetic stirring in the casting mold, a magnetic field induced 
by alternate current of a frequency f=1.5.about.10 Hz and having G (Gauss) 
in the range of 195.times.e.sup.-0.18f .about.1790.times.e.sup.-0.2f at 
the inner surface of the casting mold; 
for electromagnetic stirring in the intermediate solidifying zone, a 
magnetic field induced by alternate current of a frequency f=1.5.about.10 
Hz and having a magnetic flux density G in the range of 
195.times.e.sup.-0.18f .about.1790.times.e.sup.0.2f at the surface of the 
strand or a magnetic field induced by alternate current of a frequency 
f=50.about.60 Hz and having a magnetic flux density G in the range of 
0.6.times.10.sup.6 /(D-107).sup.2 .about.1.8.times.10.sup.6 /(D-100).sup.2 
(in which D= the thickness of a solidified shell layer of the strand (mm)) 
at the surface of the strand; and 
for electromagnetic stirring in the final solidifying zone, a magnetic 
field induced by alternate current of a frequency f=1.5.about.10 Hz and 
having a magnetic flux density in the range of 895.times.e.sup.-0.2f 
.about.2137.times.e.sup.-0.2f at the surface of the strand.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The electromagnetic stirring which provokes motive forces in molten steel 
in a continuous steel casting process, if too weak, fails to reduce in a 
sufficient degree the aforementioned inclusions in molten steel and the 
negative and center segregations. On the other hand, excessively intense 
stirring will contrarily act to increase abruptly the amounts of 
inclusions and the negative segregations in c.c. strands. Therefore, in 
consideration of the inclusion levels as well as the ratios of negative 
and center segregations, extensive experiments and studies of various 
factors in electromagnetic stirring were carried out for producing steel 
materials of satisfactory quality by the continuous casting process, thus 
attaining the present invention. 
The method of the present invention is now illustrated by way of an example 
which applies the invention to a low carbon killed steel. Molten steel was 
prepared by the use of an LD converter, which substantially had, after 
adjustments of Al and FeMn components at the time of tapping, a chemical 
composition of C=0.13%, Mn=0.45%, Si=0.06%, P=0.014%, S=0.017%, Cu=0.01%, 
Ni=0.01%, Cr=0.02%, Mo=0.01% and Al=0.035%. After refining treatment, the 
molten steel was continuously fed into a casting mold through a submerged 
nozzle, establishing a non-oxidizing state by Ar-seal from the ladle to 
the tundish and mold to prevent production of inclusions at the time of 
casting while continuously supplying the molten steel to the mold through 
the submerged nozzle. 
The molten steel in the casting mold is added to lubricant type powder, for 
example, powder of SiO.sub.2 =33.9%, CaO=34.0%, Al.sub.2 O.sub.3 =4.3%, 
Fe.sub.2 O.sub.3 =2.0%, Na.sub.2 O=8.4%, K.sub.2 O=0.6%, MgO=0.9%, F=5.1%, 
and C=5.5%. The molten steel in the casing mold, by the cooling effect of 
mold wall surfaces, begins to solidify from its outer peripheral surface 
and is continuously drawn out downward of the mold for transfer to a 
secondary cooling zone. An electromagnetic coil is provided around the 
outer periphery of the casting mold, which is imparted with alternate 
current to induce a magnetic field for electromagnetic stirring. 
According to the method of the present invention, for the electromagnetic 
stirring within the casting mold, a frequency of 1.5-10 Hz which is 
smaller in attenuation is used so that the magnetic force will reach the 
molten steel through the copper walls of the mold of low magnetic 
permeability. In order to have suitable electromagnetic stirring within 
the mold, the magnetic flux density at the inner wall surface of the mold, 
which is induced by the electromagnetic coil, is an important factor in 
addition to the frequency. 
FIG. 1 is a diagram of the index number of inclusions in c.c. strands 
occurring when the magnetic flux density which represents the intensity of 
stirring is varied in a number of ways at each frequency of applied 
current. It is seen therefrom that the magnetic flux density should be 
restricted to a certain range in view of the allowable limit of the index 
number of inclusions in practically acceptable c.c. strands. Namely, in 
order to provoke predetermined movements in the molten steel by stirring, 
the values dictated by the frequency and magnetic flux density is required 
to fall within predetermined ranges. In the diagram of FIG. 1, the value 
of frequency f should be in the range of 1.5.about.10.0 Hz while the value 
of magnetic flux density G in the range of 
EQU 195.times.e.sup.-0.18f &lt;&lt;G&lt;&lt;1790.times.e.sup.-0.2f 
In other words, outside those ranges the c.c. strands contain inclusions in 
increased amounts which reflect low cold forgeability, so that cracks are 
easily produced, thus increasing the proportion of defective products. 
The electromagnetic stirring in the above-mentioned ranges urges production 
of equiaxed crystal nuclei in the molten steel. More particularly, the 
production of equiaxed crystal nuclei by the stirred molten steel takes 
place more easily in the initial stage of solidification where the 
columnar dendrites growing from the outer surface of c.c. strand are still 
very fine and readily severed, permitting fine equiaxed crystal nuclei to 
be produced in a large quantity. Further, the production of equiaxed 
crystal nuclei is accelerated by the chilling effect resulting from molten 
steel flows in the meniscus portions of the mold. 
With regard to the frequency of current to be applied to a production of a 
c.c. strand of a sectional area larger than 400 cm.sup.2, it is 
recommended to set the frequency preferably in the range of 1.5.about.4 Hz 
in view of the strong magnetic permeability which is required to achieve a 
suitable intensity of electromagnetic stirring. In this connection, FIG. 2 
illustrates the intensities of the electromagnetic stirring actions at 
different frequencies occurring in c.c. strands of large sectional areas. 
It is seen therefrom that a suitable intensity of electromagentic stirring 
can be obtained by setting the frequency in the range of 1.5.about.4 Hz. 
Of course, the magentic flux density in such cases is restricted to the 
range governed by the abovementioned formula. 
The c.c. strand which is drawn out through the lower end of the mold after 
the electromagnetic stirring in the mold is again subjected to 
electromagnetic stirring in the intermediate solidifying zone of the c.c. 
strand upon passage through a magnetic field induced by an electromagnetic 
coil which is located around the c.c. strand for further stirring 
unsolidified molten steel in the strand. In this instance, the 
electromagnetic stirring is required to employ a low frequency 
(1.5.about.10 Hz) in view of the magnetic permeability and a magnetic flux 
density G (gauss) in the range of 195.times.e.sup.-0.18f 
&lt;&lt;G&lt;&lt;1790.times.e.sup.-0.2f at the surface of the c.c. strand. In a case 
where the electromagnetic coil can approach the c.c. strand, a commercial 
frequency of 50.about.60 Hz may be used instead of low frequency. In such 
a case, the range of appropriate magnetic flux density G (gauss) for a 
c.c. strand with a solidified shell thickness of Dmm is 
##EQU1## 
By effecting the electromagnetic stirring in the intermediate solidifying 
zone of a c.c. strand in addition to that within the casting mold, the 
inclusions are reduced in a broader area across the width of the c.c. 
strand, improving its cold forgeability all the more. Further, the 
electromagnetic stirring in the intermediate solidifying zone contributes 
to the production of equiaxed crystal nuclei in that area. FIG. 3 
illustrates the numbers of macrostreak flaws (in index numbers) in c.c. 
strands with no electromagnetic stirring (symbol "o"), single stirring in 
the mold (symbol "*") and dual stirring in the mold and intermediate 
solidifying zone according to the present invention (symbol ".DELTA.") in 
relation with the distance from the surface layer to the center axis of 
each strand. It is observed therefrom that the number of macrostreak flaws 
is suppressed inwardly from the surface layer in the strand obtained by 
the method of the present invention. 
In the production of a low carbon steel by the continuous casting process, 
there arises a problem of shrinkage cavities occurring in the center 
portions of c.c. strands, which is a problem inherent in low carbon 
steels, in addition to the above-mentioned problem of inclusions. This 
problem can be eliminated by an electromagnetic stirring treatment in a 
final solidifying zone of the c.c. strand further to the stirring 
treatment in the mold and/or in the intermediate solidifying zone. 
The term "final solidifying zone" of molten steel as used herein refers to 
that stage where, as a result of progress of solidification into equiaxed 
crystals, the shorter diameter of the molten steel pool has become smaller 
than 100 mm in the case of c.c. strands greater than 200 
mm.sup..quadrature. or become smaller than 1/2 the length of the shorter 
side of the strand in the case of c.c. strands smaller than 200 
mm.sup..quadrature.. 
The so-called "bridging" phenomenon occurs in the low carbon steel due to 
rapid growth of columnar crystals. However, the above-described 
electromagnetic stirring in the mold and/or in the intermediate 
solidifying zone has the effect of severing the columnar crystals, 
increasing the amount of equiaxed crystals. The electromagnetic stirring 
of the pool of molten steel in the final solidifying stage serves to 
disperse the molten steel between the individual equiaxed crystal grains 
and thus to reduce the temperature gradient. Then, the entire unsolidified 
portions are solidified almost simultaneously, so that the shrinkage 
cavities are dispersed to suppress production of consecutive cavities in 
the center portion. Appropriate conditions for the electromagnetic 
stirring in the final solidifying zone essentially include a frequency in 
the range of 1.5.about.10 Hz and a magnetic flux density G(gauss) at the 
surface of the c.c. strand in the range of 895.times.e.sup.-0.2f 
&lt;&lt;G&lt;&lt;2137.times.e.sup.-0.2f. FIG. 4 shows photos of macrostructures in 
section of c.c. strands (A) and (B) by single electromagnetic stirring in 
the mold and by dual or combined electromagnetic stirring in the mold and 
final solidifying zone, respectively. As clear therefrom, shrinkage 
cavities in the center portion is conspicuously suppressed in the c.c. 
strand (B) according to the method of the present invention. 
As clear from the foregoing description, synergistic effects are produced 
in the method of the present invention which subjects the c.c. strand to 
electromagnetic stirring at least at two positions along its passage 
through the casing mold, intermediate solidifying zone and final 
solidifying zone under particular frequency and magnetic flux density 
conditions. Although the foregoing description deals with a low carbon 
steel, the present invention is also applicable to medium and high carbon 
steels. 
In an application to a medium or high carbon steel, where reductions of 
negative and center segregations are desired, it is recommended to set the 
frequency, for the electromagnetic stirring in the mold, in the range of 
1.5.about.10 Hz and the magnetic flux density G(gauss) at the surface of 
the c.c. strand in the range of 
EQU 268.times.e.sup.-0.18f .ltoreq.G.ltoreq.745.times.e.sup.-0.2f (1) 
and, for the electromagnetic stirring in the intermediate solidifying zone 
of the c.c. strand, to set the frequency in the range of 1.5.about.10 Hz 
and the magnetic flux density at the surface of the c.c. strand in the 
range of 
EQU 268.times.e.sup.-0.18f .ltoreq.G.ltoreq.745.times.e.sup.0.2f (2) 
or to use commercial frequency of 50.about.60 Hz to produce a magnetic flux 
density at the surface of the c.c. strand in the range of 
EQU 750,000/(D-107).sup.2 .ltoreq.G.ltoreq.750,000/(D-100).sup.2 (3) 
The following embodiment explains the above-defined ranges from the 
standpoint of center segregation. FIG. 5 is a diagram of the ratio of 
center segregation vs. ratio of segregation in surface layer produced 
under different intensities of electromagnetic stirring, namely, by 
varying the magnetic flux density at each frequency of applied alternate 
current in electromagnetic stirring in the mold, using molten steel which 
was obtained by 3-charge blowing in an LD converter and which, after 
adjustments of Al and Fe components at the time of tapping, had a chemical 
composition of C=0.61%, Mn=0.90%, Si=1.65%, P=0.020%, S=0.015%, Cu=0.13%, 
Ni=0.01%, Cr=0.02%, Mo=0.01% and Al=0.030%. It is seen therefrom that the 
magnetic flux density should be restricted to a certain range in view of 
the allowable ranges of the ratio of center segregation and the ratio of 
negative segregation in the surface layer for this sort of c.c. strands. 
Namely, in order to impart predetermined stir in the molten steel, it is 
necessary for the magnetic flux density to fall in a certain range 
dictated by the frequency. As seen in the diagram of FIG. 5, the 
appropriate frequency f of the alternate current is in the range of 
1.5.about.10 Hz and the appropriate magnetic flux density G (gauss) at the 
surface of the c.c. strand is in the range of 
EQU 268.times.e.sup.-0.18f .ltoreq.G.ltoreq.745.times.e.sup.-0.20f (1) 
Values in excess of the above-mentioned range result in c.c. strands which 
are inferior in cold forgeability due to increases of center segregations 
and which have low quench hardness due to increases of negative 
segregations in the surface layer, which will be reflected by a 
practically unacceptable high proportion of defective products. 
More particularly, FIG. 5 shows the effects of in-mold low-frequency 
stirring (1.5.about.10 Hz) on center segregation of carbon and negative 
segregation in white band in continuous casting of 0.60%C blooms, in which 
the ratio of center segregations on the left ordinate drops sharply with 
increases in a particular range of the magnetic flux density on the 
abscissa. On the other hand, the negative segregation in white band, 
plotted on the right ordinate, linearly increases with the magnetic flux 
density. FIG. 5 indicates by hatching an optimum zone of electromagnetic 
stirring where the center segregation ratio of C is less than 1.2 and the 
negative segregation ratio of C is less than -0.10. The optimum range of 
magnetic flux density becomes narrower and lower at a higher frequency, it 
being 187-500 at 2 Hz and 130-335 at 4 Hz. The hatched area in FIG. 6 
indicates the optimum range in the relations between the frequency and 
magnetic flux density, which is expressed by Formula (1) given 
hereinbefore. 
For further reduction of irregularities in the center segregation in the 
axial direction of c.c. strands after the in-mold electromagnetic 
stirring, it is effective to subject the strands once again to 
electromagnetic stirring of predetermined conditions in the intermediate 
solidifying zone, which improves the center segregation by producing a 
greater amount of equiaxed crystals. The electromagnetic stirring in the 
intermediate solidifying zone should be carried out at the above-defined 
frequency and in the magnetic flux density range ((2) or (3)) mentioned 
hereinbefore. The optimum range (2) is determined by the same reasons as 
considered for the in-mold stirring. However, the shell thickness in the 
intermediate solidifying zone has to be considered in a case where 
commercial frequency is used. Similarly to FIG. 5, FIG. 7 illustrates the 
magnetic flux density of the electromagnetic stirring in the intermediate 
solidifying zone in relation with center segregations and negative 
segregations in the white band with regard to c.c. strands with shell 
thicknesses of 20 mm and 60 mm, indicating the respective optimum ranges 
by hatching. The optimum range of the magnetic flux density is shown in 
relation with the solidified shell thickness (Dmm) in FIG. 8. 
As mentioned hereinbefore, the application of the electromagnetic stirring 
subsequent to the in-mold stirring has the effect of reducing segregations 
in c.c. strands. This effect is illustrated in terms of reduction ratio of 
drawing in FIG. 9, from which it will be seen that the drawing reduction 
rate of a sample (C) according to the invention is improved distinctively 
as compared with a sample (A) with no stirring and a sample (B) with 
in-mold stirring alone. 
Although the irregularities of center segregations in the axial direction 
of c.c. strands can be improved by the combined electromagnetic stirring 
in the mold and the intermediate solidifying zone, the rate of center 
segregation (mean concentration in axial center portion) can be improved 
further by producing an electromagnetic stir in the final solidifying zone 
in addition to the stirring in the mold and/or in the intermediate 
solidifying zone. Upon provoking a flow in the pool of molten steel by 
electromagnetic stirring in the final solidifying zone, the molten steel 
is stirred within the equiaxed crystal zone of molten steel. The stirring 
in the final solidifying zone where the residual molten steel has almost 
no temperature gradient as compared with the stirring of the columnar 
crystal zone causes the molten steel undergoing densification at the 
interface of solidification to be distributed between the individual 
crystal grains while preventing further forward or backward movement of 
the molten steel. Therefore, the solidification proceeds almost 
simultaneously in the molten steel pool, occluding densified molten steel 
between the individual crystal grains, thereby broadening the white band 
to reduce the possibility of segregation. In this connection, the magnetic 
flux density should also be limited to a certain range in consideration of 
the allowable ranges of the rate of center segregation and the rate of 
negative segregation in the white band of practically acceptable c.c. 
strands of this sort. Namely, in order to provoke a predetermined stir in 
the molten steel, the magnetic flux density of the electromagnetic 
stirring should be in a certain range relative to the frequency. As seen 
in the diagram of FIG. 10, the optimum range of the magnetic flux density 
G (gauss) at the surface of a c.c. strand for alternate current of a 
frequency of 1.5.about.10 Hz is 
EQU 895.times.e.sup.-0.20f .ltoreq.G.ltoreq.2137.times.e.sup.-0.20f (4) 
In other words, a magnetic flux density in excess of that range will result 
in c.c. strands which are inferior in cold forgeability due to a large 
amount of center segregation or which have low quench hardness owing to 
increased negative segregation in the white band, increasing the 
proportion of practically unacceptable, defective products. 
More particularly, similarly to FIGS. 5 and 7, FIG. 10 illustrates the 
effects of circumferentially applied low-frequency power (1.5.about.10 Hz) 
stirring on the center segregation and negative segregation in the white 
band in continuous casting of 0.60%C steel blooms. From these relations, 
the optimum range of the magnetic flux density was obtained as shown in 
FIG. 11, which is defined by Formula (4). 
FIG. 12 plots mean values of carbon contents in the draw direction across 
the width of a c.c. strand of 0.60%C steel obtained after electromagnetic 
stirring in the mold and in the final solidifying zone under the 
above-described conditions. It is clear therefrom that the electromagnetic 
stirring of molten steel in the mold (M) final solidifying zone (F) (o) 
reduces the formation of the negative segregation generally referred to as 
white band and considerably minimize the center segregation in contrast to 
no stirring () and stirring in the mold alone (.DELTA.). The combination 
of the in-mold electromagnetic stirring and the electromagnetic stirring 
in the final solidifying zone of the c.c. strand produces synergistic 
effects, thereby not only suppressing irregularities of center 
segregations in the axial direction of c.c. strand but also lowering the 
rate of center segregation, to improve various properties of the resulting 
c.c. strands, including the cold forgeability. Needless to say, further 
improved results can be obtained by subject c.c. strands in each of the 
casting mold, intermediate solidifying zone and final solidifying zone. 
FIG. 13 shows the ratio of center segregation and maximum values in 
irregularities of center segregation in the axial direction of c.c. 
strands against a white band negative segregation ratio of -0.10 in 
continuous casting of (200-300).times.400 blooms of 0.60%C steel with 
regard to a situation employing no electromagnetic stirring, a situation 
effecting single electromagnetic stirring in the mold (M), intermediate 
solidifying zone (S) or final solidifying zone (F) alone, and a case 
effecting combined electromagnetic stirring at least at two positions in 
the mold and intermedial and final solidifying zones of c.c. strands 
according to the method of the present invention. It is observed therefrom 
that the combined electromagnetic stirring at least at two of the three 
positions, i.e. a position in the casting mold, a position in the 
intermediate solidifying zone and a position in the final solidifying 
zone, manifests synergistic effect in improving the ratio of center 
segregation and irregularities in center segregation as compared with 
non-stirring and single stirring at one position. 
The continuously cast strands produced with the combined electromagnetic 
stirring at all of the positions in the casing mold, intermediate 
solidifying zone and final solidifying zone, c.c. strands produced with 
the combined electromagnetic stirring in the casting mold and intermediate 
solidifying zone, and c.c. strands produced with the combined 
electromagnetic stirring in the casing mold and final solidifying zone are 
excellent in that order with regard to the ratio of center segregation as 
well as irregularity of center segregation. 
As clear from the foregoing description, the method of the present 
invention effectively reduces inclusions of both high and medium carbon 
steels, effectively suppressing the ratio of and irregularities center 
segregation by the combinined electromagnetic stirring especially in a 
case where the center segregation is problematic, thereby ensuring 
production of c.c. strands of satisfactory quality. 
Thus, the method of the present invention permits the production of c.c. 
strands which are improved as to segregation, inclusions, surface quality, 
cold forgeability, machinability and quench hardness, by the continuous 
casting process relatively at a low cost. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.