DESCRIPTION

TITLE OF THE INVENTION

CONTINUOUS CASTING METHOD AND CONTINUOUS CASTING MOLD

TECHNICAL FIELD

The present invention relates to a method of continuous casting for continuously casting molten metal and a continuous casting mold.

BACKGROUND ART

When molten metal is poured into a mold in the continuous casting of steel or other molten metal, the portion of the molten metal in contact with the mold solidifies to form a solidified shell that is extracted downward of the mold into a secondary cooling zone below the mold where solidification proceeds to finally form a continuously cast slab. The sides of the mold that contact the molten metal are formed of water-cooled copper plates. A continuous casting machine for casting slab has two wide-side mold plates and two narrow-side mold plates, the width of the narrow-side mold plates is substantially the same as the thickness of the cast slab, and the continuous casting mold is formed by assembling the two wide-side mold plates to sandwich the two narrow-side mold plates.

In the process of moving the solidified shell downward as the solidification of the solidified shell proceeds inside the mold, the solidified shell progressively solidifies while also undergoing solidification contraction. Therefore, the solidified shell that begins to solidify at the meniscus position of the molten metal in the mold becomes contracted by the time it arrives at the lower end of the mold, so that the slab under solidification is smaller in width and thickness than at the meniscus position. Since the slab is greater in width than thickness in slab continuous casting, the amount of solidification contraction is large in the slab width direction. In the course of the contraction of the solidified shell by solidification, if a gap should form between the mold and the solidified shell downstream in the mold, heat removal from the solidified shell to the mold is inhibited, making it impossible to adequately cool the mold and sometimes causing outward bulging of the solidified shell that has lost support from the mold.

So the practice is to taper at least the narrow side of the mold. By tapering is meant to narrow the distance between the two opposed short sides to make the distance at the bottom end of the mold narrower than the distance at the meniscus position at the top of the mold.

In the present invention, as shown in FIG. 1(c), where an upper position and a lower position are defined at arbitrary points in the casting direction, the distance between the two narrow sides is defined as W1 at the upper position and W2 at the lower position, and the distance from the upper position to the lower position is defined as AL, the amount of taper (%) and taper rate (%/m) are defined and termed as

Amount of taper (%) = {(W1 - W2) / AL} x 100 (3)
Taper rate (%/m) = {(W1 - W2) / W0 / AL} x 100 (4), where W0 can be anywhere insofar as it is a length prescribed in accordance with a certain width. It can, for example, be the width of the upper end of the mold or the width of the lower end of the mold. Here W0(m) is defined as the meniscus width (Ww).

When the amount of narrow-side taper is too small, contact between the solidified shell and the mold narrow-side plates becomes uneven, so that unbalanced cooling occurs to give rise to uneven solidified shell length and cracking of the slab surface by molten metal static pressure. Especially when the amount of narrow-side taper is smaller than the suitable amount, particularly thin regions tend to occur in the thickness distribution of the solidified shell in the vicinity of the solidified shell corners near the lower end of the mold on the wide sides, as shown in FIG. 8, and longitudinal cracks tend to occur in the slab surfaces corresponding to these regions. And when the amount of narrow-side taper is too large, contact between the solidified shell and the mold narrow-side plates becomes strong, so that excessive stress acts on the solidified shell to cause solidified shell breakage and breakout accompanying shell breakage. Or shortening of mold life may occur with increasing frictional force between the solidified shell and the mold.

As regards appropriate narrow-side taper, Japanese Unexamined Patent Publication (Kokai) No. 2005-211936, for example, points out that operation is conducted with the narrow-side taper rate Bn set at 0.7 to 1.3%/m. As shown in FIG. 1(c), the surface of the conventional mold narrow-side plate 2 facing the solidified shell (hereinafter called "taper surface 6") is formed as a flat surface from the upper portion toward the lower portion. However, the contraction rate of the solidified shell is not constant among all locations in the casting-direction of the mold, and the contraction rate is fast in the vicinity of the meniscus and the contraction rate slows with increasing proximity to the mold lower end. It is therefore thought that the surfaces of the solidified shell in contact with the mold narrow-side plates are not flat but that the amount of shell taper grows smaller in the downward direction of the mold.

Japanese Unexamined Patent Publication (Kokai) No. 2-247059 teaches a taper control method for controlling the taper of the mold short sides as curved surfaces. The narrow-side mold is supported at at least three points on a rearward surface and imparted with deformation. A pressure device is installed at at least one place among the three points, for example, at the central portion, and still more uniform heat removal is enabled by making the narrow-side copper plate surfaces and the free contraction profiles coincide in advance and also during operation. Under application of a force of 2 to 5 tons at the central load point, the maximum amount of bending becomes as great as 0.33 to 0.83 mm, which is deemed a sufficient amount in view of the amount of solidification contraction of the molten steel.

By determining the optimum narrow-side taper through theoretical analysis, Japanese Unexamined Patent Publication (Kokai) No. 56-53849 finds that the optimum narrow-side taper depends on distance Z along the casting direction from the meniscus and the casting speed V, and that the optimum taper rate (%/m) at each distance Z is proportional to Z-1/2 and also proportional to (4 - V) (m/min). According to Example 1 and FIG. 2 of this Publication, the short sides of a mold of 20.8 cm x 105 cm cross-section are shaped with three stages of taper whose taper rates are from above 2%/m, 0.7%/m and 0.4%/m. Further, according to Example 2 and FIG. 3, the short sides of a mold of 22 cm x 124 cm cross-section are shaped with three stages of taper whose taper rates are from above 4%/m, 1.3%/m and 0.8%/m. A mold having two stages or three or more stages of taper in the casting direction in this manner will be called a multistage tapered mold, and a mold narrow-side plate having such tapers will be called a multistage tapered mold narrow-side plate.

In continuous casting, productivity can be improved in proportion as the casting speed is faster. Even in continuous casting of slab, the casting speed has recently increased from around 2.0 m/min to a casting speed of about 3.0 m/min. In continuous casting using multistage tapered mold narrow-side plates, the optimum shape of the multistage tapered mold narrow-side plates changes as the casting speed increases, and the casting method using the multistage tapered mold narrow-side plates also changes. According to Japanese Unexamined Patent Publication (Kokai) No. 3-210953, for example, when the casting speed is high, the degree of curvature of the multistage tapered mold narrow-side plates is reduced and the overall inclination is decreased.

On the other hand, however, since in the continuous casting of slabs the cast slabs have various widths for the respective purposes, the width of the slab being cast is varied while continuing the continuous casting. As shown in FIG. 7, slab width can be varied during casting by having narrow-side drive devices 4 for moving the mold narrow-side plates 2 in the wide-side direction and varying the positions of the mold narrow-side plates 2 with the mold narrow-side plates 2 still sandwiched by the mold wide-side plates 3. In other words, it is possible to cast slabs having various widths with the same continuous casting mold 1, without changing either the mold wide-side plates 3 or the mold narrow-side plates 2.

Further, Japanese Unexamined Patent Publication (Kokai) No. 2006-346735 and Japanese Unexamined Patent Publication (Kokai) No. 2006-346736 set out methods for estimating slab solidification behavior in a mold by calculation, in which the thickness of the solidified shell at each of the mold's four surrounding regions is calculated for when the inclination of the casting direction of the mold and the casting speed are set at

arbitrary values and, based on the result, it is said to be possible to determine the ratio between the maximum value and minimum value of the solidified shell thickness at the lower end of the mold, the constraining force between the solidified shell and the mold, and the gap size.

DISCLOSURE OF THE INVENTION

At the time of conducting continuous casting using multistage tapered mold narrow-side plates, the optimum curvature of the mold narrow-side plates decreases as the casting speed increases. Therefore, as the mold narrow-side plates used in continuous casting when the maximum casting speed VM (m/min) of the casting is fast, there are used ones whose curvature of the narrow-side surfaces formed by the narrow-side taper is small.
In slab continuous casting, casting can be conducted responsive to various casting widths using one and the same mold narrow-side plates by varying the positions of the mold narrow-side plates in the mold. When slabs of various casting widths were cast using mold narrow-side plates with small narrow-side surface curvature matched to the aforesaid fast casting speed, it was found that good continuous casting was possible at narrow widths and medium widths but that the effect of using the multistage tapered mold narrow-side plates sometimes could not be adequately achieved during wide-width casting.

The object of the present invention is to provide a continuous casting method, a multistage tapered mold narrow-side plate, and a continuous casting machine that in continuous casting using multistage tapered mold narrow-side plates enable the effect of the multistage tapered mold narrow-side plates to be realized at every casting width from narrow width to wide width even if the maximum casting speed is fast. In other words, the gist of the present invention is as set out below.

(1) A continuous casting method using multistage tapered mold narrow-side plates having two or more different tapers in the casting direction, which continuous casting method is characterized in that where the maximum casting speed of the casting is defined as VM (m/min) and the distance in the casting direction from the meniscus position to the first taper inflection point is defined as inflection position x (mm), x as a function of VM falls within the ranges of Inequation (1) and Inequation (2) below

50 < x < 300 : VM < 2.5 ... Ineq. (1)
50 < x < 300 - 200 (VM - 2.5) : 2.5 < VM < 3.75 ... Ineq. (2).

(2) A continuous casting method set out in (1), further characterized in that slabs of multiple slab widths are cast.

(3) A continuous casting method set out in the aforesaid (1) or (2), characterized in that the narrow-side mold plates are two-stage taper narrow-side mold plates.

(4) A multistage tapered mold narrow-side plate that is used for continuous casting whose maximum casting speed of the casting is VM (m/min) and has two or more different tapers in the casting direction, which narrow-side mold plate for continuous casting is characterized in that where the distance in the casting direction from the meniscus position to the first taper inflection point is defined as inflection position x (mm), x as a function of VM falls within the ranges of Inequation (1) and Inequation (2) below

50 < x < 300 : VM < 2.5 ... Ineq. (1)
50 < x < 300 - 200 (VM - 2.5) : 2.5 < VM < 3.75 ... Ineq. (2).

(5) A narrow-side mold plate for continuous casting set out in (4), characterized in that it is a two-stage taper narrow-side mold plate.

(6) A continuous casting mold comprising mold wide-side plates 3 and multistage tapered mold narrow-side plates 2 having two or more different tapers in the casting direction, characterized in that where the maximum casting speed of the casting is defined as VM (m/min) and the distance in the casting direction from the meniscus position of said multistage tapered mold narrow-side plates 2 to the first taper inflection point is defined as inflection position x (mm), x as a function of VM falls within the ranges of Inequation (1) and Inequation (2) below

50 < x < 300 : VM < 2.5 ... Ineq. (1)
50 < x < 300 - 200 (VM - 2.5) : 2.5 < VM < 3.75 ... Ineq. (2).

(7) A continuous casting mold set out in (6),

characterized in that the multistage tapered mold narrow-side plates 2 are two-stage taper narrow-side mold plates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a set of diagrams for explaining the tapered surface of a narrow-side mold plate, where (a) is view showing a two-stage taper narrow-side plate, (b) a three-stage taper narrow-side mold plate, and (c) a single-stage taper narrow-side mold plate.
FIG. 2 is a set of diagrams showing change in solidification uniformity and constraining force at 1,100 mm slab width when the upper/lower taper ratio and casting speed were varied.

FIG. 3 is a set of diagrams showing change in solidification uniformity and constraining force at 2,200 mm slab width when the upper/lower taper ratio and casting speed were varied.

FIG. 4 is a set of diagrams showing change in solidification uniformity and constraining force at 1,100 mm slab width when the inflection point position x and casting speed were varied.

FIG. 5 is a set of drawings showing change in solidification uniformity and constraining force at 2,200 mm slab width when the inflection point position x and casting speed were varied.

FIG. 6 is a set of diagrams showing change in solidification uniformity and constraining force when the total taper rate was varied.

FIG. 7 is a set of diagrams showing a continuous casting mold of the present invention, where (a) is a plan view and (b) is a cross-sectional view.

FIG. 8 is a diagram showing the solidified shell shape at the lower end of the mold as determined by calculation.

EFFECT OF THE INVENTION

In continuous casting using multistage tapered mold narrow-side plates, the present invention shortens the taper inflection point position from the meniscus with increasing maximum casting speed, thereby making it possible to maintain both solidification uniformity and constraining force in optimum ranges over a broad casting width range from narrow width to wide width.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, the maximum casting speed of the casting is defined as VM (m/min) and the distance in the casting direction from the meniscus position to the first taper inflection point of a multistage tapered mold narrow-side plate is defined as inflection position x (mm).

The present invention further defines total taper rate TT, upper taper rate Tu, lower taper rate TL, and upper/lower taper ratio as follows.

Defining the distance between the two narrow sides at the meniscus position as WM (mm), at the lower end of the mold as WB (mm) , and the distance from the meniscus position to the lower end of the mold as L (m) (FIG. 1(a), (b)), the total taper rate TT (%/m) is defined as TT (%/m) = { (WM - WB) / WM / L} x 100 ... Eq. (5).

Where the upper position and the lower position of the upper taper surface 6u of the uppermost part of the multistage tapered mold narrow-side plate in the casting direction are arbitrarily determined, and defining the distance between the two short sides at the upper position as W1 (m) , at the lower position as W2 (m) , and the distance from the upper position to the lower position as AL (m) (FIG. 1(a), (b)), the upper taper rate Tu (%/m) is defined as

Tu (%/m) = { (W1 - W2) / WM / AL} x 100 ... Eq. (6).

Where the upper position and the lower position of the lower taper surface 6L of the lowermost part of the multistage tapered mold narrow-side plate in the casting direction are arbitrarily determined, and defining the distance between the two short sides at the upper position as W3 (m) , at the lower position as W4 (m), and the distance from the upper position to the lower position as AL (m) (FIG. 1(a), (b)), the lower taper rate TL is defined as

TL (%/m) = { (W3 - W4) / WM / AL} x 100 ... Eq. (7).

The upper/lower taper ratio is defined as

Upper/lower taper ratio = upper taper rate / lower taper rate = Tu / TL ... Eq. (8) .

As mentioned above, Japanese Unexamined Patent Publication (Kokai) No. 2006-346735 and Japanese Unexamined Patent Publication (Kokai) No. 2006-346736 set out methods for estimating slab solidification behavior in a mold by calculation, in which the thickness of the the lower end of the mold determined by the sulfur printing were in good agreement. It is therefore possible to find an optimum continuous casting method with the solidification uniformity determined by calculation as an indicator.

If the value of the solidification uniformity (B/A) determined by calculation is 0.7 or greater, good solidification uniformity can be realized even in actual casting. If the constraining force determined by calculation (reference value at each width; value normalized to constraining force when taper rate at first stage taper is 1.0%/m) is 2.0 or less, good casting low in constraint can be conducted even in actual casting. Further, it was ascertained from the results of actual continuous casting that occurrence of breakout is avoided by bringing the solidification uniformity (B/A) and constraining force into the aforesaid preferred ranges.

Next, solidification uniformity and constraining solidified shell at each of the mold's four surrounding regions is calculated as in FIG. 8 for when the inclination of the casting direction of the mold and the casting speed are set at arbitrary values. Based on the result, it is possible to determine the ratio B/A between the maximum value A and minimum value B of the solidified shell thickness at the lower end of the mold, the constraining force between the solidified shell and the mold, and the gap size. And the calculation methods set out in these patent publications were used with respect to continuous casting utilizing multistage tapered mold narrow-side plates to determine the solidified shell shape and constraining force between the solidified shell and mold at the lower end of the mold. The shape of the solidified shell at the lower end of the mold derived by calculation was as in FIG. 8. Regions thin in solidified shell thickness may be formed on the wide sides of the solidified shell near the slab corners, and the solidified shell thickness of these regions can be used as the minimum value B of the shell thickness. Then the ratio B/A of the maximum value A and minimum value B of the solidified shell thickness is here termed the "solidification uniformity". When casting good in solidification uniformity is conducted, the shell thickness of the regions thin in solidified shell thickness on the long sides near the slab corners can be brought closer to the shell thickness of the other thick regions.

Molten steel was actually continuously cast with S being added to the molten steel in the mold steel during casting, and when the thickness distribution of the solidified shell at the lower end of the mold was evaluated by sulfur printing of the slab after solidification, it was found that the solidification uniformity determined by the aforesaid calculation and the ratio of the maximum and minimum shell thicknesses at the lower end of the mold determined by the sulfur printing were in good agreement. It is therefore possible to find an optimum continuous casting method with the solidification uniformity determined by calculation as an indicator.

If the value of the solidification uniformity (B/A) determined by calculation is 0.7 or greater, good solidification uniformity can be realized even in actual casting. If the constraining force determined by calculation (reference value at each width; value normalized to constraining force when taper rate at first stage taper is 1.0%/m) is 2.0 or less, good casting low in constraint can be conducted even in actual casting. Further, it was ascertained from the results of actual continuous casting that occurrence of breakout is avoided by bringing the solidification uniformity (B/A) and constraining force into the aforesaid preferred ranges.

Next, solidification uniformity and constraining force will be calculated and the optimum shape of a multistage tapered mold narrow-side plate examined using a calculation method (hereinafter called the "calculation method of the present invention") based on the aforesaid two patents (Japanese Unexamined Patent Publication (Kokai) No. 2006-346735 and Japanese Unexamined Patent Publication (Kokai) No. 2006-346736).

In the conventional multistage tapered mold narrow-side plate, particularly the two-stage tapered mold narrow-side plate, the distance L from the meniscus position to the lower end of the mold was approximately 900 mm, and the inflection point position x was about 300 mm. And when a casting speed of a maximum casting speed VM of up to around 2.5 m/min was adopted, a taper of an upper/lower taper ratio of about 4.0 was adopted and casting good in both solidification uniformity and constraining force could be achieved. This point can be confirmed by the aforesaid calculation method of the present invention.

With casting width fixed at 1,100 mm (narrow width), total taper rate at 1.6%/m and two-stage tapered mold narrow-side plate inflection point position x at 300 mm, solidification uniformity and constraining force were calculated by the calculation method of the present invention with casting speed varied from 1.0 to 3.0 m/min and narrow-side mold plate curvature condition varied by varying the upper/lower taper ratio of the two-stage tapered mold narrow-side plate.

As shown in FIG. 2, for the same upper/lower taper ratio, solidification uniformity improves with faster casting speed but constraining force also increases. It can be seen that for maintaining both solidification uniformity and constraining force in favorable ranges, the upper/lower taper ratio is preferably lowered with increasing casting speed. When the taper ratio range enabling both solidification uniformity and constraining force to be favorably maintained was investigated for each casting speed, the result was that the optimum upper/lower ratio range at a casting speed of 2.0 m/min is 5.0 or less, the optimum upper/lower ratio range at a casting speed of 2.5 m/min is 4.0 or less, and the optimum upper/lower ratio range at a casting speed of 3.0 m/min is 3.0 or less.

Next, the narrow-side mold plate shape that was good in solidification uniformity and constraining force at 1,100 mm casting width (mold shape with upper/lower taper ratio of 3.0 whose performance optimized in the casting speed range up to 3.0 m/min) was used with the casting width expanded to 2,200 mm. When the width was changed with the total taper rate still maintained at 1.6 %/m, the upper/lower taper ratio at 2,200-mm width became 1.7.

When solidification uniformity and constraining force were calculated for 2,200-mm casting width (wide width) by the calculation method of the present invention, it was found that when the slab width was expanded with the total taper rate kept fixed, the optimum upper/lower taper ratio range at casting speed of 3.0 m/min fell to less than 1.7 and the solidification uniformity also declined (FIG. 3). In other words, it was found that a mold optimized for 1,100-mm width high-speed casting at up to 3.0 m/min casting speed fell outside the optimum range when the casting width was expanded to 2,200 mm.

Therefore, the attempt to optimize multistage taper at 1,100-mm width for the individual casting speeds was pursued not by fixing the inflection point position x and varying the upper/lower taper ratio but by varying the inflection point position x with the upper/lower taper ratio fixed at 4.0. With total taper rate set at 1.6%/m, solidification uniformity and constraining force at varying inflection point position x were calculated by the calculation method of the present invention. The results are shown in FIG. 4. At casting speed of 2.5 m/min or less, the optimum inflection point position range was 300 mm or less, and at casting speed of 3.0 m/min, the optimum inflection point position range was 200 mm or less.

Next, the narrow-side mold plate having an inflection point position x that was optimum at 1,100-mm width (mold shape with inflection point position of 200 mm whose performance optimized in the casting speed range up to 3.0 m/min) was used to calculate casting at 2,200-mm width. When, with total taper rate maintained at 1.6%/m, slab width was expanded at a fixed total taper rate, upper/lower taper ratio at 2,200-mm casting width was 2.5. So the results of calculating solidification uniformity and constraining force at 2,200-mm width by the calculation method of the present invention are shown (FIG. 5) for varying inflection point position x, with total taper rate at 1.6%/m and upper/lower taper ratio maintained fixed at 2.5 similarly to in the foregoing. As is clear from FIG. 5, it was found that a favorable range can be secured at casting speed of 3.0 m/min or less even at 2,200-mm casting width insofar as inflection point position x is 200 mm or less. Therefore, when casting of a maximum casting speed of 3.0 m/min is conducted, favorable continuous casting can be achieved insofar as the displacement point position is 200 mm or less.

Similarly, insofar as the inflection point position x is 50 mm or less, good casting can be conducted at a casting speed of 3.75 m/min at either 1,200-mm width (FIG. 4) or 2,200-mm width (FIG. 5). Therefore, when casting of a maximum casting speed of 3.75 m/min is conducted, favorable continuous casting can be achieved insofar as the displacement point position is 200 mm or less.

Upon comparing the optimum upper limit taper ratios in the case where, as set out in the foregoing, the mold optimized by varying the upper/lower taper ratio at the time of narrow width for the individual casting speeds was used and applied at the time of wide width, it was found that in the case where a mold optimized by changing the inflection point position x was used and applied at the time of wide width, the optimum upper/lower taper ratio could be raised, and that when the inflection point position x was changed, the optimum upper/lower taper ratio decreased more at the time of wide width than at the time of narrow width but that, to the contrary, the solidification uniformity rose. In other words, it was found that, as shown in FIG. 4 and FIG. 5, in deciding the optimum taper shape of the multistage tapered mold narrow-side plate when the casting speed becomes fast, good solidification uniformity and constraining force can be better maintained when the casting width is wide by raising the inflection point position x higher in proportion as the casting speed is faster than by changing the upper/lower taper ratio.

In this connection, as regards the relationships in FIG. 4 and FIG. 5, similar relationships are also exhibited in the range of 600-mm to 2,500-mm slab widths anticipated from the industrial viewpoint, and have been confirmed by calculation and real machine tests.

The following Inequation (1) and Inequation (2) are derived from the relationships in FIG. 4 and FIG. 5 to express the conditions for obtaining the aforesaid optimum ranges defined as solidification uniformity of 0.7 or greater and constraining force of 2.0 or less by Inequation (1) and Inequation (2) having maximum casting speed VM as a variable.

50 < x < 300 : VM < 2.5 ... Ineq. (1)
50 < x < 300 - 200 (VM - 2.5) : 2.5 < VM < 3.75 ... Ineq. (2).

The lower limit of x is defined as 50 mm because when the inflection point position is higher on the mold than this, the effect of the multistage taper cannot be adequately obtained and is almost no different from an ordinary single-stage taper. In light of Inequation (2), there is no solution when VM exceeds 3.75 m/min. In other words, the upper limit of VM in the present invention is 3.75 m/min. Further, the reason for defining the upper limit of x as 300 mm is that when it is attempted to realize an upper/lower taper ratio of greater than a certain value, the taper rate of the lower taper portion becomes small when the upper strong taper region becomes long, and when narrow casting is done by changing the width at a fixed total taper rate, the lower taper rate becomes extremely small to make reverse taper (taper increasing progressively downward) more likely, so that an issue of the slab bulging at the lower part of the mold tends to arise.

In the present invention, the effect is more apparent as the maximum casting speed VM increases. A particularly remarkable effect can be exhibited in high-speed casting of a maximum casting speed VM greater than 2.5 m/min.

Next, using the same two-stage taper narrow-side plate as above (mold with inflection point position of 200 mm), casting speed was fixed at 1.5 m/min and casting width at 1,100 mm, and solidification uniformity and constraining force were determined by calculation at varying total taper rates. Slab thickness was set at 240 mm. The results are shown in FIG. 6. As is clear from the drawing, solidification uniformity can be favorably maintained insofar as the total taper rate is made 0.5%/m or greater. Further, constraining force can be favorably maintained small insofar as the total taper rate is made 2.0%/m or less.

Although a mold plate having three or more tapers can be utilized as the multistage tapered mold narrow-side plate used in the present invention, the two-stage taper narrow-side mold plate can adequately exhibit its effect as a result of the high positioning of the inflection point position.

In the present invention the thickness of the cast slab is preferably 220 mm to 300 mm, more preferably 240 mm to 300 mm. When the slab thickness exceeds 300 mm, excessively large equipment in necessary for the continuous casting mold that changes width during casting, making realization practically difficult. Further, when the slab thickness is less than 240 mm, uniform pouring of molten metal becomes difficult because the diameter of the submerged nozzle for pouring molten metal from the tundish must be made small. Uniform pouring becomes still more difficult when the slab thickness is less than 220 mm.

Next, a continuous casting mold for realizing the casting method of the present invention will be explained based on FIG. 7.

The continuous casting mold 1 of the present invention has mold wide-side plates 3 and multistage tapered mold narrow-side plates 2 having two or more different narrow-side taper rates (unit: %/m) in the casting direction. The mold wide-side plates 3 and multistage tapered mold narrow-side plates 2 consist of sets of two each and their sides facing the solidified shell are advisably water-cooled copper plates and their opposite sides steel back frames. The width of the multistage tapered mold narrow-side plates 2 is nearly the same as thickness of the cast slab. A mold having a rectangular casting space is formed by sandwiching the multistage tapered mold narrow-side plates 2 between the two mold wide-side plates 3.

When maximum casting speed of the casting with this continuous casting mold is VM (m/min), the distance in the casting direction from the meniscus position of the multistage tapered mold narrow-side plates 2 to the first taper inflection point is defined as inflection position x (mm), and x as a function of VM is made to fall within the ranges of Inequation (1) and Inequation (2) below. As a result, both solidification uniformity and constraining force can be maintained in optimum ranges over a broad casting width range from narrow width to wide width.

50 < x < 300 : VM < 2.5 ... Ineq. (1)
50 < x < 300 - 200 (VM - 2.5) : 2.5 < VM < 3.75 ... Ineq. (2).

The continuous casting mold 1 of this invention further comprises narrow-side drive devices 4 capable of changing the cast slab width and the inclination of the narrow sides, and a controller 5 of the narrow-side drive devices. The uppermost narrow-side taper rate in the casting direction is termed as the upper taper rate, the lowermost narrow-side taper rate as the lower taper rate and the narrow-side taper rate connecting the meniscus portion of the narrow-side surface and the lower end of the mold by a straight line as the total taper rate, and the value obtained by dividing the upper taper rate by the lower taper rate is defined as the upper/lower taper ratio, which point is the same as in the aforesaid continuous casting method of the present method.

In the actual operating mode, the controller 5 of the narrow-side drive devices preferably controls the driving of the narrow-side mold plates so that during casting the total taper rate is the same at every slab width and the upper/lower taper ratio is 4 or less at every slab width.

The narrow-side drive devices 4 each comprises, for example, two vertically tiered drive actuators 9, and the multistage tapered mold narrow-side plates 2 are held from the back frame sides by the drive actuators 9. The total taper rate of the multistage tapered mold narrow-side plates 2 can be set at a prescribed value for each casting width by using the movement of the individual upper/lower drive actuators 9 to set the position of the narrow-side mold plates. Electric cylinders, hydraulic cylinders or the like can be used as the drive actuators.

9. Otherwise, as the narrow-side drive device it is possible to use a device having drive means for conducting reciprocating movement and oscillating movement of the narrow-side mold plate.

Note that when the casting width is changed during continuous casting, the casting is required to be continuously changed while conducing normal casting. In the course of carrying out such width change, it is necessary to change the total taper rate in order to carry out a smooth width change, so that the total taper rate cannot be kept constant.

Casting of slabs having a broad range of widths can be preferably enabled if the minimum slab width that can be cast with the continuous casting mold of the present invention is 1,100 or less and the maximum slab width that can be cast is 2,200 mm or greater. Preferably, the minimum castable slab width is 800 mm or smaller. A minimum castable slab width of 600 mm is practical. A maximum castable slab width of 2,500 mm is practical.

INDUSTRIAL APPLICABILITY

In continuous casting using multistage tapered mold narrow-side plates, the present invention shortens the taper inflection point position from the meniscus with increasing maximum casting speed, thereby making it possible to maintain both solidification uniformity and constraining force in optimum ranges over a broad casting width range from narrow width to wide width.

CLAIMS

1. A continuous casting method using multistage tapered mold narrow-side plates having two or more different tapers in the casting direction, which continuous casting method is characterized in that where the maximum casting speed of the casting is defined as VM (m/min) and the distance in the casting direction from the meniscus position to the first taper inflection point is defined as inflection position x (mm), x as a function of VM falls within the ranges of Inequation (1) and Inequation (2) below

50 < x < 300 : VM < 2.5 ... Ineq. (1)
50 < x < 300 - 200 (VM - 2.5) : 2.5 < VM <3.75 ... Ineq. (2).

2. A continuous casting method set out in claim 1, further characterized in that slabs of multiple slab widths are cast.

3. A continuous casting method set out in claim 1 or 2, characterized in that the narrow-side mold plates are two-stage taper narrow-side mold plates.

4. A multistage tapered mold narrow-side plate that is used for continuous casting whose maximum casting speed of the casting is VM (m/min) and has two or more different tapers in the casting direction, which narrow-side mold plate for continuous casting is characterized in that where the distance in the casting direction from the meniscus position to the first taper inflection point is defined as inflection position x (mm), x as a function of VM falls within the ranges of Inequation (1) and Inequation (2) below

50 < x < 300 : VM < 2.5 ... Ineq. (1)
50 < x < 300 - 200 (VM - 2.5) : 2.5 < VM <3.75 ... Ineq. (2) .

5. A narrow-side mold plate for continuous casting set out in claim 4, characterized in that it is a two-stage taper narrow-side mold plate.

6. A continuous casting mold comprising mold wide-side plates and multistage tapered mold narrow-side plates having two or more different tapers in the casting direction, characterized in that where the maximum casting speed of the casting is defined as VM (m/min) and the distance in the casting direction from the meniscus position of said multistage tapered mold narrow-side plates to the first taper inflection point is defined as inflection position x (mm), x as a function of VM falls within the ranges of Inequations (1) and (2) below 50 < x < 300 : VM < 2.5 .... Ineq. (1) 50 < x < 300 - 200 (VM - 2.5) : 2.5 < VM < 3.75 ... Ineq. (2).

7. A continuous casting mold set out in claim 6, characterized in that the multistage tapered mold narrow side plates are two-stage taper narrow-side mold plates.