Method for horizontal type continuous casting

Abstract

A method for horizontal type continuous casting, which comprises: withdrawing a cast strand from a mold provided horizontally at the lower part of a tundish at a prescribed withdrawing speed for a prescribed period of withdrawing time; then, pushing back said cast strand for a prescribed period of pushback time in the direction opposite to the direction of withdrawal; and, withdrawing said cast strand again from said mold at said prescribed withdrawing speed for said prescribed period of withdrawing time; repeating said withdrawal and said pushing back, the distance of said withdrawal being longer than that of said pushing back; thereby intermittently withdrawing said cast strand from said mold; said method being characterized by comprising: limiting said prescribed withdrawing speed within the range of from 1.0 to 10.0 m/minute; limiting said prescribed period of withdrawing time within the range of from 0.05 to 1.5 second; and, limiting said prescribed period of pushback time within the range of from 0.1 to 0.6 second; thereby preventing a breakout and a crack from occurring in the trailing end portion of the solidified shell of the cast strand in said mold.

Description

FIELD OF THE INVENTION

The present invention relates to a method for a horizontal type continuous casting, which eliminates the risk of a breakout and a crack occurring in the trailing end portion of the solidified shell of a cast strand in a mold when intermittently withdrawing the cast strand from the mold, and permits casting of a cast strand of satisfactory quality.

BACKGROUND OF THE INVENTION

In place of the vertical type continuous casting process which comprises casting steel by vertically withdrawing a cast strand from a vertical mold installed below a tundish, the horizontal type continuous casting process which comprises casting steel by horizontally withdrawing a cast strand from a horizontal mold installed at the lower part of the side wall of a tundish is recently being industrialized because of the low installation costs and other advantages.

One of the problems involved in the continuous casting process is that molten steel may adhere or stick to the inner wall of the mold when withdrawing a cast strand from the mold, and thereby the cast strand may not sometimes be withdrawn properly from the mold.

In the vertical type continuous casting process, in which a mold is installed below a tundish not directly connected to the tundish, it is possible to prevent molten steel from adhering or sticking to the mold by vibrating the mold itself while withdrawing the cast strand from the mold.

In the horizontal type continuous casting process, in contrast, a mold is horizontally installed onto the side wall of a tundish, in direct connection with the tundish. Unlike the vertical type continuous casting process, it is difficult, from the point of view of equipment, to vibrate only the mold while withdrawing the cast strand from the mold, and this practice is not practicable. Alternatively, it is conceivable to vibrate the tundish and the mold as an integral body. This practice is also problematic from the consideration of equipment and is not therefore practicable.

When withdrawing a cast strand from a mold in the horizontal type continuous casting process, therefore, a practicable method comprises either continuously withdrawing the cast strand from the mold at a prescribed withdrawing speed, or withdrawing the cast strand from the mold at a prescribed withdrawing speed for a prescribed period of withdrawing time, then, discontinuing withdrawal of the cast strand for a prescribed period of time, then, withdrawing the cast strand from the mold at said prescribed withdrawing speed for said prescribed period of withdrawing time, and repeating this cycle of withdrawal and discontinuance, i.e., intermittently withdrawing the cast strand from the mold.

However, the method comprising continuously withdrawing the cast strand from the mold at a prescribed withdrawing speed has the following problem. When withdrawing the cast strand from the mold, the cast strand is always pulled by pinch rolls, and movement of the cast strand in the mold is restricted by frictional resistance between the cast strand and the mold. As a result, a breakout or a crack may be caused in brittle parts of the solidified shell which is made of molten steel by a single withdrawing action of the cast strand at a molten steel supply end portion in the mold (hereinafter referred to as the "trailing end portion of solidified shell"), thus resulting in a considerably deteriorated shape of the cast strand.

On the other hand, the method comprising intermittently withdrawing the cast strand from the mold is problematic in the following point. After the cast strand is withdrawn from the mold at a prescribed withdrawing speed for a prescribed period of withdrawing time, the cast strand shrinks by cooling. However, because the cast strand is constrained by pinch rolls, and movement of the cast strand is restricted by frictional resistance between the cast strand and the mold as mentioned above, tension acts on the cast strand. As a result, breakouts and cracks are caused in the brittle parts in the trailing end portion of the solidified shell of the cast strand in the mold, thus resulting in a considerably deteriorated shape of the cast strand.

A method capable of solving the above-mentioned problem comprises, after withdrawing the cast strand from the mold at a prescribed withdrawing speed for a prescribed period of withdrawing time, pushing back the cast strand for prescribed period of pushback time in the direction opposite to the direction of the withdrawal, and then withdrawing again the cast strand from the mold at said prescribed withdrawing speed for said prescribed period of withdrawing time, and thus intermittently withdrawing the cast strand from the mold by repeating the cycle of said withdrawal and pushing back. According to this method, since a pushback force is imparted to the cast strand in the same direction as the shrinking direction of the cast strand even when the cast strand shrinks by cooling after withdrawing the cast strand from the mold at a prescribed withdrawing speed for a prescribed period of withdrawing time, no tension acts on the cast strand. As a result, breakouts and cracks do not occur to brittle parts in the trailing end portions of the solidified shell of the cast strand in the mold.

The period of pushback time of the above-mentioned pushing back of the cast strand is limited within a certain range, and a cast strand with a satisfactory quality cannot be obtained with a period of pushback time of under or over this range.

However, since a particular optimum period of pushback time of a cast strand has not as yet been known, a cast strand with a satisfactory quality has not always been obtained even by the application of the method comprising, after withdrawing the cast strand from the mold at a prescribed withdrawing speed for a prescribed period of withdrawing time, then, pushing back the cast strand for a prescribed period of pushback time in the direction opposite to the direction of the withdrawal, and then withdrawing again the cast strand from the mold at said prescribed withdrawing speed for said prescribed period of withdrawing time, and thus intermittently withdrawing the cast strand from the mold by repeating the cycle of withdrawal and pushback.

SUMMARY OF THE INVENTION

A principal object of the present invention is therefore to provide a method for intermittently withdrawing a cast strand from a mold without causing a breakout and a crack in brittle parts in the trailing end portion of the solidified shell of the cast strand.

In accordance with one of the features of the present invention, there is provided a method for horizontal type continuous casting, which comprises: withdrawing a cast strand from a mold provided horizontally at the lower part of a tundish at a prescribed withdrawing speed for a prescribed period of withdrawing time; then, pushing back said cast strand for a prescribed period of pushback time in the direction opposite to the direction of withdrawal; and, withdrawing said cast strand again from said mold at said prescribed withdrawing speed for said prescribed period of withdrawing time; repeating said withdrawal and said pushing back, the distance of said withdrawal being longer than that of said pushing back; thereby intermittently withdrawing said cast strand from said mold; said method being characterized by comprising: limiting said prescribed withdrawing speed within the range of from 1.0 to 10.0 m/minute; limiting said prescribed period of withdrawing time within the range of from 0.05 to 1.5 second; and, limiting said prescribed period of pushback time within the range of from 0.1 to 0.6 second, thereby preventing a breakout and a crack from occurring in the trailing end portion of the solidified shell of the cast strand in said mold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinally partially lower section of the trailing end portion of the solidified shell of a cast strand in a mold;

FIG. 2 is a perspective view illustrating the thinnest part in the trailing end portion of the solidified shell of a cast strand in a mold; and,

FIG. 3 is a drawing illustrating the relationships of the cast strand pushback time "Tp" with the stress ".sigma." occurring in the section of the thinnest part in the trailing end portion of the solidified shell and with the cast strand breakout rate "y".

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the method comprising, after withdrawing a cast strand from a mold at a prescribed withdrawing speed for a prescribed period of withdrawing time, pushing back the cast strand for a prescribed period of pushback time in the direction opposite to the direction of said withdrawal, and then, withdrawing the cast strand again from the mold at said prescribed withdrawing speed for said prescribed period of withdrawing time, thus intermittently withdrawing the cast strand from the mold by repeating said withdrawal and said pushing back, the pushback time of said cast strand in the direction opposite to the direction of withdrawal exerts an important effect on the quality of the cast strand, as mentioned above.

We carried out extensive studies to determine the optimum period of pushback time of the cast strand in the direction opposite to the direction of withdrawal in the method for intermittently withdrawing the cast strand from the mole mentioned above, and obtained the following finding.

The probability of a breakout or a crack occurred to the trailing end portion of the solidified shell of the cast strand in the mold when intermittently withdrawing the cast strand from the mold depends upon the stress occurring in the section of the thinnest part in the trailing end portion of the solidified shell.

The present invention was made on the basis of the above-mentioned finding, and, in the method comprising, after withdrawing a cast strand from a mold at a prescribed withdrawing speed for a prescribed period of withdrawing time, pushing back the cast strand for a prescribed period of pushback time in the direction opposite to the direction of said withdrawal, and then, withdrawing the cast strand again from the mold at said prescribed withdrawing speed for said prescribed period of withdrawing time, thus intermittently withdrawing the cast strand from the mold by repeating said withdrawal and said pushing back, the present invention is characterized by comprising: limiting said prescribed withdrawing speed within the range of from 1.0 to 10.0 m/minute, limiting said prescribed period of withdrawing time within the range of from 0.05 to 1.5 second, and, limiting said prescribed period of pushback time within the range of from 0.1 to 0.6 second, thereby preventing a breakout and a crack from occurring in the trailing end portion of the solidified shell of the cast strand in said mold.

The following paragraphs describe the reasons why the withdrawing speed of a cast strand from a mold, the withdrawing time of the cast strand from the mold and the pushback time of the cast strand toward the mold are limited as mentioned above.

A withdrawing speed of a cast strand from a mold of under 1.0 m/minute results in a lower average withdrawing speed, thus impairing the productivity. At a withdrawing speed of the cast strand from the mold of over 10 m/minute, on the other hand, a sharp tension is applied to the solidified shell of the mold, thus bringing about the risk of causing a breakout. The withdrawing speed of the cast strand from the mold should therefore be limited within the range of from 1.0 to 10.0 m/minute.

A withdrawing time of the cast strand from the mold of under 0.05 second does not allow normal operations since the withdrawing time comes to an end before the prescribed withdrawing speed is reached. With a withdrawing time of the cast strand from the mold of over 1.5 second, on the other hand, the surface condition of the cast strand is deteriorated. The withdrawing time of the cast strand from the mold should therefore be limited within the range of from 0.05 to 1.5 second.

In the method of the present invention, the reasons why the period of pushback time of the cast strand toward the mold is limited within the range of from 0.1 to 0.06 second are described with reference to FIGS. 1 to 3.

When intermittently withdrawing a cast strand from a mold, the average withdrawing speed "Vc" (m/minute) of the cast strand is calculated by the following formula: ##EQU1## where, Tw: withdrawing time time (sec.) of the cast strand in the case where the cast strand is withdrawn from the mold at a prescribed withdrawing speed for a prescribed period of withdrawing time;

Vw: withdrawing speed (m/minute) of the cast strand in the case where the cast strand is withdrawn from the mold at prescribed withdrawing speed for a prescribed period of withdrawing time; and, Tp: pushback time (sec.) for pushing back the cast strand in the direction opposite to the direction of said withdrawal.

The amount of pushing back distance of the cast strand under the influence of pushing back pressure of cast strand, being very slight, is not taken into account in the above-mentioned average withdrawing speed "Vc" of cast strand.

In the case where, at a constant average withdrawing speed "Vc" of the cast strand, the cast strand is withdrawn from the mold at a prescribed withdrawing speed for a prescribed period of withdrawing time with various withdrawing times "Tw" of cast strand and various pushback times "Tp" of cast strand, when considering the relationships of the frictional force "F" produced between the trailing end portion of the solidified shell and the inner surface of the mold with the stress ".sigma." occurring in the section of the thinnest part "D" of the solidified shell, a smaller value of stress ".sigma." leads to a lower possibility of breakout of crack occurring in the trailing end portion of the solidified shell.

More specifically, in the case where the cast strand is withdrawn from the mold at a prescribed withdrawing speed for a prescribed period of withdrawing time, the withdrawing length "L" of the cast strand can be calculated by the following formula:

L=Tw.multidot.Vw.multidot.1/60 (2)

The smallest thickness "D" in the trailing end portion of the solidified shell is calculated by the following formula:

D=K.multidot..sqroot.Tp (3) where, K: solidification coefficient of molten steel (K.apprxeq.3.5).

The frictional force "F" is calculated by the following formula:

F=.mu..multidot.M (4) where, .mu.: frictional coefficient; and M: static pressure of molten steel acting on the entire trailing end portion of the solidified shell, which is expressed by M=a.times.P a: inner peripheral area of the trailing end portion of the solidified shell; P: static pressure of molten steel (P.apprxeq.0.3 kg/cm.sup.2).

The sectional area "S" of the thinnest part of the trailing end portion of the solidified shell is calculated by the following formula:

S=l.multidot.D (5) where, l: circumferential length of the thinnest part of the trailing end portion of the solidified shell.

The stress ".sigma." occurring in the thinnest part in the trailing end portion of the solidified shell is therefore calculated by the following formula:

.sigma.=F/S (6)

By substituting all the preceding formulate into the above-mentioned formula (6) and arranging properly, ".sigma." can be expressed by the following formula: ##EQU2## where, k: constant ##EQU3##

The relationships of the pushback time "Tp" of cast strand with the stress ".sigma." appearing in the section of the thinnest part in the trailing end portion of the solidified shell and with the breakout rate "y" of cast strand are calculated by means of the formula (7) under the following conditions:

Size of cast strand: 115 (mm square) Vc: 2.0 (m/minute) Tw: 0.2 (sec.) Tp: 0.1 to 1.0 (sec.) Vw: 3 to 12 (m/minute)

The results of this calculation are shown in FIG. 3.

The cast strand breakout rate "y" is expressed by X.sub.1 /X.sub.2, where,

X.sub.1 : the number of cast strand withdrawals with breakouts in the case where a plurality of cast strand withdrawals are carried out; X.sub.2 : the total number of cast strand withdrawals in the case where a plurality of cast strand withdrawals are carried out.

As is clear from FIG. 3, the cast strand breakout rate "y" is larger when the cast strand pushback time "Tp" is under 0.1 second and over 0.6 second. The reason is as follows. With a constant average withdrawing speed "Vc" of cast strand, if the cast strand pushback time "Tp" is under 0.1 second, the cast strand is withdrawn before the thickness of the solidified shell of cast strand grows sufficiently thick because of the short cast strand pushback time "Tp". If the cast strand pushback time "Tp" is over 0.6 second, on the other hand, it is necessary to employ a cast strand withdrawing speed "Vw" higher by a degree corresponding to the increment of the cast strand pushback time "Tp", and as a result, larger stress ".sigma." occurs in the trailing end portion of the solidified shell.

For these reasons, the pushback time "Tp" of the cast strand toward the mold is limited within the range of from 0.1 to 0.6 second in the method of the present invention.

The ratio of the withdrawing time "Tw" of the cast strand from the mold to the pushback time "Tp" of the cast strand toward the mold, i.e., the ratio "Tw/Tp" should preferably be up to 2.0. With a "Tw/Tp" of over 2.0, the period of withdrawing time becomes excessively longer than the period of pushback time, so that a breakout is easily caused in the trailing end of the solidified shell during withdrawal, thus deteriorating the cast strand surface condition.

The sum of the period of withdrawing time "Tw" of the cast strand from the mold and the period of pushback time "Tp" of the cast strand toward the mold, i.e., one withdrawing cycle time "Tw+Tp" should preferably be within the range of from 0.3 to 1.6 second. With a withdrawing cycle time of under 0.3 second, the shell thickness at the trailing end of the solidified shell of the cast strand in the mold does not reach the thickness suitable for the withdrawal operation. This tends to easily cause a breakout at the trailing end of the solidified shell during withdrawal of the cast strand. With a withdrawing cycle time of over 1.6 second, on the other hand, the junction between the solidified shell formed during one withdrawing cycle and the solidified shell formed during the next withdrawing cycle becomes difficult to weld with each other, thus easily causing cracks on the cast strand surface.

Now, the present invention is described below by means of examples as compared with comparison cases.

EXAMPLES

Cast strands with cross-sections of 75.times.75 mm, 115.times.115 mm and 150.times.150 mm were subjected to the horizontal type continuous casting under conditions including an average withdrawing speed "Vc", a withdrawing speed "Vw", a period of withdrawing time "Vw" and a period of pushback time "Tp" as shown in Table 1.

TABLE 1______________________________________ CastCast strandstrand Vc Vw break-size (m/ (m/ out(mm .times. min- min- Tw Tp rate Tw/mm) ute) ute) (sec.) (sec.) (%) Tp______________________________________Ex- 1 75 3.0 10.0 0.26 0.6 0 0.43ample 2 75 3.0 5.0 0.15 0.1 0 1.5 3 115 2.5 10.0 0.2 0.6 0 0.33 4 115 2.5 3.0 0.05 0.1 0 0.5 5 150 2.0 10.0 0.15 0.6 0 0.25 6 150 2.0 3.0 0.2 0.1 0 2Com- 7 115 2.5 5.0 2.0 2.0 0.5 1.0parison 8 115 2.5 12.5 0.2 0.8 1.0 0.25cases______________________________________

Nos. 1 to 6 shown in Table 1 are examples carried out in accordance with the method of the present invention, whereas Nos. 7 and 8 are comparison cases outside the scope of the method of the present invention.

In all the examples Nos. 1 to 6, stable operations could be conducted without causing cracks or breakout in the trailing end of the solidified shell of a cast strand in the mold, with a cast strand breakout rate of null, and a value of "Tw/Tp" of up to 2 in all cases.

In the comparison case No. 7, in contrast, operations were carried out with a withdrawing speed "Vw" of within the range of the limitation of the present invention, but with a period of withdrawing time "Vw" and a period of pushback time "Tp" outside the scope of the present invention. In the comparison case No. 7, while the ratio "Tw/Tp" was lower than 2, the value of "Tw+Tp" exceeded 1.6 second and the cast strand breakout rate showed a value of 0.5%.

In the comparison case No. 8, operations were carried out at a withdrawing speed "Vw" and a period of pushback time "Tp" which are outside the scope of the present invention. In the comparison case No. 8, the ratio "Tw/Tp" was lower than 2 and "Tw+Tp" took a value within the range of from 0.3 to 1.6 second, whereas because of the withdrawing speed exceeding the range of the limitation of the present invention, the breakout rate was 1.0%, suggesting frequent occurrence of breakout when withdrawing the cast strand.

According to the method of the present invention, as described above in detail, it is possible to withdraw intermittently a cast strand from a mold without causing a breakout or a crack in brittle part of the trailing end portion of the solidified shell in the mold, thus giving a cast strand of a satisfactory quality and providing industrially useful effect.

Claims

1. A method for horizontal type continuous steel casting, which comprises:

withdrawing a cast steel strand from a copper mold provided horizontally at the lower part of a tundish at a prescribed withdrawing speed for a prescribed period of withdrawing time; then, pushing back said cast steel strand for a prescribed period of pushback time in the direction opposite to the direction of withdrawal; and, withdrawing said cast steel strand again from said mold at said prescribed withdrawing speed for said prescribed period of withdrawing time; repeating said withdrawal and said pushing back, the distance of said withdrawal being longer than that of said pushing back; thereby intermittently withdrawing said cast steel strand from said mold;

said method being characterized by comprising:

said prescribed withdrawing speed being within the range of from 1.0 to 10.0 m/minute;

said prescribed period of withdrawing time being within the range of from 0.5 to 1.5 second; and,

said prescribed period of pushback time being within the range of from 0.1 to 0.6 second;

thereby preventing a breakout and a crack from occurring in the trailing end portion of the solidified shell of the cast steel strand in said mold.