EP0134030A1

EP0134030A1 - Method for reducing or widening mold width during continuous casting

Info

Publication number: EP0134030A1
Application number: EP84109550A
Authority: EP
Inventors: Wataru c/o Sakai Seitetsusho of Ohashi; Takeyoshi c/o Sakai Seitetsusho of Ninomiya; Masami c/o Sakai Seitetsusho of Temma
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1983-08-11
Filing date: 1984-08-10
Publication date: 1985-03-13
Also published as: ES8505567A1; AU3166284A; BR8404013A; ES535020A0; AU551521B2

Abstract

A variable width continuous casting method in which the wide side walls are stationary and the narrow side walls (1) are movable. The center (P) of the rotation radius of the narrow side walls (1) is set below and above the mold on the basis of the casting speed and the moving speed of the narrow side walls (1). The method provides lessening of air gaps and shell deformation resistance and of the size of the width-varied part of the slab, and an increase in yield and a stable operation.

Description

BACKGROUND OF THE INVENTION:

This invention relates to a method of reducing or widening mold width during continuous casting.
In the prior art of varying the width of a mold during a continuous casting, the method in which the narrow side walls of the mold are moved while the wide walls are fixed has been known.
When the inclination angle of the narrow side walls is changed, generally, the narrow side walls are inclined and supported on the ends of the vertical shaft of a cylinder or the upper or lower end point of the narrow side walls. The inclination angle changing rate is set and controlled at a constant rate established experimentally on the basis of the moving speed of the upper and lower ends of the narrow side walls. Therefore, when the casting speed or the inclination angle changing rate leaves a predetermined range, and thus, the casting speed becomes high, or the inclination angle changing rate becomes low, an air gap tends to break out. This outbreak of an air gap entails a risk of insufficient cooling of a casting strand and production of break out after drawing. The inventors experimentally have obtained the result that when the air gap at the lower part of the mold is more than 2 mm, there is much risk of break out and the like. On the other hand, it has been intended recently that a continuous casting strand be conducted at a high temperature, and a direct rolling which connects the casting with a succeeding-process, namely a rolling line, has found a wide use. This has caused the development of a tendency for the casting speed to be more and more increased, and there have been various attempts to increase the translational moving speed so as to vary the width at the possible highest speed and to increase the yield by reducing in size the tapered strand produced by so varying the width. In order to increase the translational moving speed, the amount of change in inclination should be increased. That is, if the inclination angle changing rate is set at a constant value as in the prior art, the outbreak rate of an air gap and break out increases very unfavorably in terms of practical use.
As is widely known, in the continuous casting method, the method of enlarging and reducing the width of the casting strand by moving the narrow side walls constantly in a continuous casting and direct rolling for the purpose of increasing productivity has been proposed and has found a very spreading practical use. In particular, from the point of saving energy, direct rolling has recently been conducted by some parties, such that continuously cast strand is directly fed to a soaking furnace or a heating furnace without being cooled, or is directly fed to a hot rolling process without being fed to the heat retaining furnace or the heating furnace. For the purpose of an effective direct rolling in a continuous casting and direct rolling, a casting strand needs to be produced in accordance with the rolling plan, and the method of reducing the size of a casting strand according to the rolling plan has usually been adopted.
However, in the prior art, an air gap was apt to break out at the time of reducing the size of a casting strand which was the cause of cracks and break out, failed to increase productivity, and in addition involved heavy energy loss.

SUMMARY OF THE INVENTION:

Accordingly it is an object of the invention to eliminate the drawbacks of the prior art and to provide a method for reducing or widening mold width during continuous casting with movable narrow side walls and fixed wide side walls, comprising, in changing the inclination angle, setting the center of rotation radius of the narrow side walls below and above a mold on the basis of the casting speed and the translational moving speed of the narrow side walls, and changing the inclination angle so as to lessen air gaps and shell deformation resistance and lessen the size of the width-varied part of the slab by varying the width in a short time thus leading to an increase in yield and a stable operation without cracking or break out.
It is another object to provide a method for reducing or widening mold width during continuous casting wherein the inclination of the narrow side walls is moved such as to be along the rotating surface concentric with the center of the rotation radius, that is, the center of the rotation radius is fixed to be outside the mold.
It is still another object to provide a method for reducing or widening mold width during continuous casting in which the narrow side walls are inclined while the center of the rotation radius is being moved.
It is a further object to provide a method for reducing or widening mold width during continuous casting in which the narrow side walls are inclined such that either of the upper or lower ends of the narrow side walls is set at a constant speed and the speed of the other end is set such as to gradually reduce the difference in velocity between the two.
It is a still further object to provide a method for reducing or widening mold width during continuous casting method wherein the narrow side wall is inclined such that each speed of the upper and lower ends of said narrow side wall changes and the difference in velocity between the two is gradually reduced.
It is a still further object to provide a stable method for reducing or widening mold width during continuous casting comprising: dividing the movement step of said narrow side wall into a forward inclination step (1), a translational movement step (2) and a backward inclination step (3); in said forward inclining step, increasing the moving speed of the upper part la of said narrow side wall more than that of the lower end lb of said narrow side wall, starting the moving of said upper and lower ends simultaneously or substantially simultaneously, and gradually reducing the difference in the velocity between said upper and lower ends, finally making said movement speeds of said upper and lower ends equal, until said translational movement step has been reached while constantly changing the inclination position; in the translational movement step, moving said narrow side wall with the equal and constant moving speeds of said upper and lower ends; and in said backward inclination step, lowering said moving speed of said upper part more than that of said lower end of said narrow side wall, and gradually increasing the distance of said moving speeds of said upper end and said lower end of said narrow side wall while constantly changing said inclination position and moving said narrow side wall until said narrow side wall reaches the normal inclination position; whereby to make the moving speed of the narrow side wall more than two times that in the prior art, and eliminate the risk of outbreak of air gaps between the narrow wall of the mold and the casting strand in the forward and backward inclination section of the narrow wall of the mold, which is the cause of production of cracking and break out.

BRIEF DESCRIPTION OF THE DRAWINGS:

These and other objects as well as advantages of the present invention will become clear by the following description of preferred embodiments of the present invention with reference to the accompanying drawings, wherein:

Fig. 1 is a sectional view showing the system of the structure of the method for reducing or widening mold width during continuous casting method in the prior art;
Fig. 2 is a view showing the structure of a means of reducing a width;
Fig. 3 is a schematic view showing the moving speed of a width-reduced narrow side wall;
Figs. 4, 5(a) and 5(b) are views showing the conception of the structure of the outbreak of an air gap in the prior art;
Figs. 6, 7 (a), 7(b), 7(c), 8, 9 (a), 9(b), 9(c) and 10 show embodiments according to this invention, in which:
Figs. 6 and 7 are views explaining the basic structures of change of the inclination angle at the stage of beginning to vary a width;
Figs. 8 and 9 are views explaining the basic structures of change of the inclination angle at the finishing stage of varying a width;
Fig. 10 is a view of an embodiment according to the invention;
Figs. 11 and 12 are graphs showing moving patterns of narrow side walls;
Fig. 13 is a graph showing the change of the force required for deformation of a casting strand;
Figs. 14 and 15 are graphs showing the change of the force required for transformation of a casting strand; and
Figs. 16(a), 16(b), 17(a) and 17(b) schematically illustrate deformations in the translational movement and in the forward inclination.

PREFERRED EMBODIMENTS OF THE INVENTION:

Fig. 1 shows an embodiment of a width variable mold in the prior art in which the narrow side walls are made to be movable. The narrow side wall 1 can be moved forward or backward in the widthwise direction of the casting strand 2 by the cylinder means 31, 32 connected to the upper and lower ends of the casting strand 1. It is known that, to reduce the width in this width variable mold, at the beginning stage of the translational movement of the narrow side wall 1, the inclination angle (a') is made steeper than a predetermined angle (a) in the normal operation and in the final stage of the translational movement it is restored to the predetermined angle (a). Generally, the inclination angle is represented as the difference between the distance from the center of the mold 0 to the upper end point la_l and the distance from the center of the mold 0 to the lower end point lbl, (t = x_l- x₂, hereinunder "the inclination amount t"), and the detection and the control of the inclination angle is thereby conducted. The inclination angle changing rate is the rate of the amount of change in inclination AT (t (before changing a width) - t (after changing the width)) in proportion to changing time S (
) and hereinunder, the inclination angle changing rate at the beginning stage of the movement is represented as V_ml, and the inclination angle changing rate at the finishing stage of the movement as V_m3. Fig. 3 schematically shows the movement of the narrow side wall at the time of the reduction of the width in the diagram of velocity. In Fig. 3 at the beginning stage of the movement, the narrow side wall 1 is moved in the width reducing direction of the casting strand 2 at the inclination angle changing rate of V_m1 around the lower point lbl, and the inclination angle a' is made steeper than the predetermined angle. The narrow side wall 2 is next translationally moved at the rate of V_m2, and when the upper end la of the narrow side wall has reached a predetermined width, the angle change enters that of the final stage of the movement. At the final stage of the movement, the narrow side wall 1 is moved at the inclinatio angle changing rate of V_m3 around the upper end point la_l, and the inclination angle a' is restored to the predetermin ed angle a, thereby to reduce the width. It is for the purpose of reducing the indentation resistance in the translational movement that the inclination angle of the narrow wall 1 is made steeper than the angle a in the norma operation, and the amount of change in inclination AT for obtaining the inclination angle a' has a close relation to the translational moving speed V_m2 and the casting speed V , as may be generally formulized as follows:

ΔT: the amount ot change in inclination
V_m2: the translational moving speed of a narrow side wall
k: coefficient of change in inclination
V_c: casting speed
ℓ: effective length of a narrow side wall

Next, the conception of outbreak of an air gap will be explained as an example with reference to Figs. 4 and 5 in which narrow side walls are width-reduced. Fig. 4 schematically shows the movements of the narrow side wall 1 and the solidified shell of a casting strand 2a at the time of change of the inclination angle at the beginning stage of the movement of the narrow side wall 1, the width of which has started changing. While the upper end la of the narrow side wall moves forward from the point A to the point C at the inclination angle changing rate of V_m1, the solidified shell of the casting strand 2a travels from the point A to A' at the inclination angle changing rate of V. If the center of the center of rotation radius of the narrow side wall 1 is fixed to the lower end point lb₁, the correlation between the casting speed V and the inclination angle changing rate V_m1 changes and if the travelling distance (AA') of the solidified shell 2a becomes greater than the length of the narrow side wall 1, an air gap 5 shown by the hatched lines is produced between the narrow side wall BC and the solidified shell surface A'C of the casting strand. Fig. 5 illustrates the change of the inclination angle at the final stage of changing the width. While the lower end lb of the narrow side wall moves forward from the point B to the point D at the inclination angle changing rate of V_n3, the solidified shell 2a of the casting strand travels from the point A to the point A', and from the point B to the point B'. At this final stage of width varying, also, if the rotation supporting point is fixed to the upper end point la_l, an air gap shown by the hatched lines is produced between the narrow side wall of the mold AD and the solidified shell surface AA'B' of the casting strand. The size of the air gap increases in proportion to the travelling distance (AA') of the solidified shell 2a of the casting strand during the change of the inclination angle of the narrow wall 1 from AB to AD, as is shown in Fig. 5(a). (Fig. 5(b) shows the case in which the travelling distance of the solidified shell 2a of the casting strand is shorter than in Fig. 5(a).)
As stated above, in the prior art, wherein the rotation supporting point is fixed to the upper end point la₁ or the lower end point lbl, there was a problem in that the air gap became enlarged and consequently such trouble as break out caused by this often occurred and, in addition, width variation at high speed was impossible in view of the need to the prevent generation of break out.
Figs. 6 to 9 schematically show the movement of the narrow side wall and the movement of the solidified shell of the casting strand as an explanation of the basic concept of changing an inclination angle when the width is reduced. Fig. 6 shows the change of the inclination angle at the beginning stage of the movement, the upper end la of the narrow side wall moves forward by the length ΔT and the solidified shell 2a of the casting strand travels from the point A to the point A' while the inclination angle is being changed. However it was found that the inclination angle can be changed without outbreak of an air gap if the point A' which the solidified shell 2a reaches at the end of changing the inclination angle is set as the center of the rotation radius and the inclination of the narrow side wall 1 is moved so as to be along the rotation surface which shares the center of the rotation radius (hereinunder "the rotation center P"). That is, the point A' is the distance the solidified shell 2a of the casting strand 1 travels at the casting speed of V while the narrow side wall 1 changes the inclination angle (ΔT/V_m1). It has been confirmed by our experiments that by setting the point A', namely the rotation point P, below the mold based on the casting speed V and the travelling speed of the narrow side wall 1 V_m1, the narrow side wall surface CD always coincides with the solidified shell surface of the casting strand CD resulting in no air gap breaking out as is shown in Fig. 8. The length from the upper part of the narrow side wall lb to the rotation center P is obtained using the following formula:

the length from the lower end of the narrow side wall to the rotation center
L: rotation radius of the narrow side wall
ℓ: effective length of the narrow diew wall
ΔT: the amount of change in inclination
V_ml: rate of change in inclination angle
^V _c: casting speed

By substituting T from formula (1) in formula (2), the following formula (3) results
Consequently by selecting an appropriate coefficient of change in inclination based on a casting speed V_c, the rotation center P corresponding to the variation of the translational moving speed V 2 can be set. Similarly, in the change of the inclination angle at the final stage of the movement, outbreak of an air gap can also be minimized by setting the rotation center of the narrow side wall 1 at the point P' above the narrow side wall 1 and inclining the narrow side wall 1 such as to be along the rotation surface concentric with the rotation center P', as is shown in Fig. 8. It has been found that β, which is the length from the upper end la to the rotation center P, may be represented by the following formula:
Fig. 9 shows the narrow side wall 1 in the state of moving at the time of change in the inclination angle at the finishing stage of the movement. The narrow side wall 1 at the time of change in the inclination angle subsequently moves from Fig. 9(a) to Fig. 9(b) subsequently. At this time, the starting point of the movement A' of the solidified shell 2a of the casting strand narrow side wall 1 moves vertically, as is shown by the hatched lines in Fig. 8, which is the cause of the outbreak of an air gap 5 (in the hatched area) until the starting point of the movement A' passes the lower end of the narrow side wall, but the amount can be reduced remarkably compared with the case in which the upper end point la_l is set as the rotation supporting point, which does not substantially lead to break out.
In this invention the narrow side wall 1 moves such as to be along the rotation surface concentric with the rotation center P or P', and the inclination angle changing rate V_m1 V_m3 should be maintained at a predetermined speed established by the translational moving speed V_m2 and so on. For this reason, the moving speed at the beginning stage of change in the inclination angle becomes the following formula (4) and (5) as is shown in Figs. 6 and 8:

moving speed of the upper end
moving speed of the lower end

The moving speed at the final stage of the change in inclination angle becomes the following formula (6) and (7) : moving speed of the upper end
moving speed of the lower end
Therefore, the narrow side wall 1 can move along the rotation surface concentric with the rotation center P, P', if each of the upper and the lower ends of the narrow side wall is made to move at the above moving speed for example by controlling the indentation speed with the cylinder means 31, 32.

Example:

This invention was used for reducing the width of a casting strand of a size of 1,000 mm x 250 mm to a width of 900 mm during continuous casting. In this example, as is shown in Fig. 10 (ℓ = 772 mm, ℓ₁ = 85 mm. ℓ₂ = 585 mm, ℓ₃ = 103 mm, S = ℓ' = 51.5 mm), it was reduced at a casting speed V_c of 1.6 mm/min, at an inclination angle changing rate V_m1 = V_m3 of 21 mm/min and at a translational short wall moving speed V_M2 of 32 mm/min.
The coefficient of change of inclination was 0.7 and the effective length of the short wall was 772 mm. Accordingly, the deviation of the lower or upper part of the rotation center (P, P') for a change in the inclination angle β = β', and the moving speed of the upper and lower ends of the short wall V'_m1 ⁼ V'_m3' V'_m11 = V'_m33 are as follows:
In fact, since the position of the upper and lower ends of the effective narrow wall and the cylinder shaft 310, 320 deviates, the speed of the cylinder was corrected by this deviation to obtain the following values.
The above example varied the width with a minimal air gap, and thus made it possible to eliminate such trouble as break out.
Incidentally, in the example above described, the rotation center P was fixed at a predetermined position below or above the mold and the difference in the speed of the upper and lower molds is constant. However, the rotation center P need not be limited to a determined position, and it is possible to incline the short wall while moving the rotation center P. Figs. 11 and 12 show embodiments for reducing the width while moving the rotation center.
Fig. 11 illustrates the width reducing method in which the moving speed of a lower compressing cylinder is increased as greatly as possible so as to minimize the increase in time at the beginning stage of the movement, namely in the forward inclination section (1), and at the final stage of the movement, namely in the backward inclination section (3). The movement of an upper compressing cylinder 5_4a which is connected to the upper part of the back of the narrow side wall of a casting strand starts at a high moving speed V₁₁ approximately equal to a translational moving speed V , and at the same time the lower compressing cylinder 5_4b which is connected to the lower end of the back of the narrow side wall of the casting strand starts to move at an initial moving speed of V₂₀ and the speed is gradually increased at an acceleration of a₂₁ whereby the inclination is changed until a preset time t_a is reached, or until the inclination angle becomes a set angle θ relative to the short wall of the casting strand. The moving speed V₂₁ of the lower compressing cylinder 5_4b when the movement has finished, namely when it has reached the translational movement section, is made equal to the moving speed V₁₁ of the upper compressing cylinder 5_4a.
When the inclination angle of the narrow side wall reaches a preset angle θ, or when a preset time t_a is reached, the narrow side wall is translationally moved to a preset position while increasing the speed of the upper compressor cylinder 5₄a and the lower compresser cylinder 5_4b.
After the narrow side wall is translationally moved to the predetermined position, the speed of the lower compressing cylinder 5_4b is slightly decreased and is moved at a moving speed of V₂₂, and the speed of the upper compressing cylinder 5_4a is gradually reduced from the moving speed V₁₂ to the reduced speed of ₁ and the inclination is restored, namely in the backward inclination until the inclination angle of the narrow side wall reaches a preset angle θ, or when a preset time t_b is reached. After the restoration of the inclination, namely when the backward inclination starts, the moving speed V₁₂ of the upper compressing cylinder is made the same as or slightly lower than the moving speed V₃₂ of the lower compressing cylinder 5_4b.
As is obvious from Fig. 11, it is very characteristic of this invention that the inclination position of the narrow side wall in the forward and backward inclination sections subsequently changes. In order to change the inclination position, the rotation center P is moved below and above the mold. In the method shown in Fig. 11, it is preferable to set the value of the initial moving speed V₂₀ of the lower compressing cylinder 5_4b and the moving speed v₁₃ of the upper compressing cylinder 5_4a at the end of the backward inclination within the range shown by formula 10:

V_m: translational moving speed
It is preferable not to set the value at a figure greater than 0.2 V , because when greater than 0.2 V_m in the forward inclining section (1) and the backward inclining section (3) the gap between the casting strand and the narrow side wall of the mold becomes unfavorably large in terms of operation and quality. On the other hand, the value of the V₂₀ and V₁₂ is preferably not greater than 0.3 V , because if it is more than 0.3 V , the increase in m m deformation resistance of the casting strand becomes large in the backward inclining section (3) and the time span in the forward and backward inclination section increases.
In an embodiment of a width reducing method according to the invention, as shown in Fig. 12, in the forward inclination section the lower compressing cylinder 5_4b starts to move a preset lag time t₁ after the upper compressing cylinder 5₄a starts to move or when the inclination angle of the narrow side wall of the mold becomes a preset angle θ₁, in order to minimize the increase of time span in the forward inclining section (1) and the backward inclination section (3). In the backward inclination section (3), the lower compressing cylinder 5_4b is moved and the inclination is restored, namely in the backward inclination, until a preset lag time after the upper compressing cylinder 54a starts to move, or until the inclination angle of the narrow side wall of the mold becomes a preset angle θ₀. In order to minimize the force required for the transformation of the casting strand in the forward inclination section (1) and the backward inclination section (3), the speed of the upper compressing cylinder 5₄a is gradually increased in the forward inclination section (1) at an initial speed of V₁₀ and an acceleration of all, and the speed of the lower compressing cylinder 5_4b is gradually increased at an initial speed of V₂₀ and an acceleration of α₂₁.
However, in this case, the moving speed of the lower compressing cylinder 5_4b does not exceed the moving speed of the upper compressing cylinder 5₄a and the moving speed V₁₁ of the upper compressing cylinder 5₄a and the moving speed V₂₁ of the lower compressing cylinder 5_4b is made equal to, or slightly lower than, the translational moving speed V_m. In the backward inclination section (3), the moving speed of the upper compressing cylinder 5_4a is gradually decreased from V₁₁ to a reduced moving speed V₁₂ and the moving speed of the lower compressing cylinder 5_4b is gradually decreased from the initial moving speed V₁₁ to a reduced moving speed V₂₂.
Those conditions under which an air gap does not break out between the casting strand and the narrow side wall of the mold are shown in the following formulae, (11), (12), (13) and (14).
When V₂₀, V₁₃ are less than 0.2 m, an air gap breaks out and when they are more than 0.3 V_m, the deformation resistance is unfavorably large. On the other hand, if V₁₀, V₂₃ are less than 0.6 V_m, the inclination angle changing rate becomes long, and further if they are more than 0.8 V , the deformation resitance is unfavorably great.

t₂: (time in the forward inclination section) minus ^t ₁
t₃: (time in the backward inclination section) minus ^t ₄
V₁₃: the moving speed of the upper compresser cylinder at the end of the backward inclination minus moving speed of 5_4a
V₂₃: the moving speed of the lower compresser cylinder at the end of the backward inclination minus moving speed of 5_4b

Figs. 13, 14 and 15 show the force required for the transformation of the mold in embodiments of a method for reducing the width of a casting strand in the prior art and that according to the invention. Fig. 13 shows the force required for the transformation of the mold in the prior art. When the forward moving speed V , is made equal to the translational moving speed V , a maximum value Fmax of the transformation force required for the casting strand is produced in the backward inclination section of the mold.
In the example of the inventors, Fmax was confirmed under the following conditions:
The maximum value Fmax of the transformation force required for the casting strand and the maximum value Gmax of an air gap between the narrow side wall of the mold and the casting strand in the backward inclination section were as follows:
Figs. 14 and 15 show the force required for the transformation of the casting strand by the speed patterns shown in Figs. 11 and 12. Referring to Fig. 11, the maximum value approximately equal to the maximum value Fmax required for the transformation of the casting strand in the prior art is produced under the following conditions (19) to (21). In this case an air gap between the casting strand and the narrow side wall of the mold did not break out.
In the following example using the method of Fig. 12, no air gap broke out between the casting strand and the narrow side wall of the mold.
Example:
In this invention, the movement section is divided into three sections, namely the forward inclination section (1), the translational movement section (2) and the backward inclination section (3). The forward and the backward sections are provided for the purpose of decreasing the deformation resistance. Referring to Figs. 16 (a) and (16(b), when the reduction of the width of the mold is conducted in the normal casting state at a translational movement rate of V , a solidified shell 55 formed at the m point A ought to move downwrad by V_c.t mm and to reach the point B after t seconds. However, since a narrow side wall 53 has been moved, it reaches the point C, not B, moving to the left side by V_mt. In this case, if the original casting width is represented as the deformation rate & of is as follows:
However, if the inclination of the narrow side wall 53 is changed as in Fig. 17(b) and moved transla- tionally at a rate of V_m , and 8 is
, the shell produced at the point A moves downward by V_c.t mm to reach the point B after t second. The narrow side wall 53 moves from the point C to the point B, which means = 0 and the deformation resistance of the shell in the translational movement reaches zero.
Nevertheless, since when is too large, an air gap tends to break out as described above, the appropriate value of should be selected.
As described in detail, according to this invention, the width-change of a mold is enabled in the shortest possible time. Therefore, the width-changed portion of the casting strand produced so varying the width can be reduced so as to remarkably increase the yield.
In addition, the amount of an air gap and the deformation resistance of a shell is possible to be constantly less than the allowable values, and a stable operation without cracking or break out is enabled.
While there has been described what is at present considered to be preferred embodiments of the invention, it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

Claims

1. A method for reducing or widening mold width during continuous casting in which a wide side wall(2)is stationary while a narrow side wall(1)is movable in order to vary the width of casting, characterized in

that at the time of change in the inclination angle, the center P of the rotation radius is set below and above the mold on the basis of a casting speed and a moving speed of said narrow side wall.

2. A method according to claim 1, wherein the inclination of said narrow side wall is made to move such as to be along the rotating surface concentric with said center of the rotation radius.

3. A method according to claim ₁or ₂, _wherei_nsaid narrow side wall is inclined while said center is being moved.

4. A method according to claim 3, wherein said narrow side wall is inclined such that either of the upper or lower ends of said narrow side wall is set at a constant speed and the speed of the other end is set such as to gradually reduce the difference in velocity between the two.

5. A method according to claim 3, wherein said narrow side wall is inclined such that each speed of said upper and lower ends of said narrow side wall changes and the difference in velocity between the two is gradually reduced.

6. A method for reducing or widening mold width during continuous casting method according to claim 3, further comprising: dividing the movement step of said narrow side wall into a forward inclination step (1), a translational movement step (2) and a backward inclination step (3); in said forward inclining step, increasing the moving speed of the upper part(la)of said narrow side wall more than that of the lower end(lb)of said narrow side wall, starting the moving of said upper and lower ends simultaneously or substantially simultaneously, and gradually reducing the difference in the velocity between said upper and lower ends, finally making said movement speeds of said upper and lower ends equal, until said translational movement step has been reached while constantly changing the inclination position; in the translational movement step, moving said narrow side wall with the equal and constant moving speeds of said upper and lower ends; and in said backward inclination step, lowering said moving speed of said upper part more than that of said lower end of said narrow side wall, and gradually increasing the distance of said moving speeds of said upper end and said lower end of said narrow side wall while constantly changing said inclination position and moving said narrow side wall until said narrow side wall reaches the normal inclination position.