JPH10211549A - Mold for continuous casting and continuous casting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the W-shaping of a crater end part and to reduce the segregation in the center part of thickness at both widths of a slab by pouring molten metal into a mold for continuous casting through a two-hole nozzle and secondary-cooling so that solidified shell growing speed in the cast slab just after coming out from the mold becomes quicker than that in the mold. SOLUTION: The outlet side at the center parts in the width direction of the long side surfaces in the mold is formed as projecting shape to restrain the growth of solidified shell at the center part in the slab width. The molten metal is poured by using the two hole nozzle into this continuous casting mold so as not to entrap powder covering a meniscus. After the solidified shell comes out from the mold, the secondary cooling is arranged to strengthen the cooling more than that in the mold. The growing speed of the solidified shell in the secondary cooling zone is quicker than that in the mold. By this method, after forming the sound cast slab surface in the mold, the cooling is strengthened just after coming out from the mold and the solidified shell thickness is secured, and the solidified shell is made to stand up to deformation, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting mold and a continuous casting method capable of reducing the segregation of the center of a metal slab, especially a steel slab, in a quarter of its width.

[0002]

2. Description of the Related Art A conventional plate-type copper plate mold includes a copper plate and a backup plate for reinforcing the copper plate from the outside thereof. Cooling water is usually passed through a backup water passage through a cooling water passage provided in a rectangular copper plate. Structure.

FIG. 7 is a view schematically showing a longitudinal section of a conventional continuous casting apparatus.

FIG. 6 (a) shows the A of the mold in FIG.
6A is a cross-sectional view, and FIG.
Ba shows a sectional view. In this type of mold, a backup plate 2 is superimposed on a back surface of a copper mold plate 1 and fastened with bolts, and cooling water supplied from a cooling water pipe 4 is introduced into the mold copper plate 1 through the backup rate 2 and cooled. Structure. As shown in FIG. 6 (b), for example, the cooling water passage 3 of the continuous casting mold having such a structure is formed by forming a rectangular slit groove 3 on the back surface of the mold copper plate 1 and stacking the backup plate 2 on the back surface. The method of forming is common.

When a slab having a rectangular cross section (hereinafter referred to as a slab) is continuously cast by using the above-mentioned conventional mold, the molten metal 8 poured into the mold is cooled through the mold, the molten powder 6 and the like. Forming a solidified shell 8a. Thereafter, the unsolidified molten metal exits the mold, and then ends solidification in the secondary cooling zone where the mist is cooled while being supported by the roll 9. The contour of the solidification end position is called a crater end.

In a wide slab, the crater end has the same distance from the meniscus in the width direction,
That is, it is difficult to realize a flat crater end for the following reasons.

The molten metal 8 is discharged from the immersion nozzle 5 usually called a two-hole nozzle into the water-cooled mold 1, and a large circulating downward flow 8b is formed in the molten metal.

FIG. 5 is a view showing a downward flow of the molten metal discharged from the two-hole nozzle. Since the molten metal that forms the downflow is hotter than the surrounding metal, uniform cooling in the slab becomes impossible if the downflow occurs.

FIG. 4A is a diagram schematically showing a cross section immediately before the final solidification position. FIG. 4B is a diagram showing the flow of the unsolidified molten metal. According to FIG.
The solidified shell is thicker at the center of the mold and thinner around 1/4 in width. Therefore, the crater end has a W-shaped shape.

When the crater end has a W-shape, when the center of the thickness at the center of the width solidifies, molten steel enriched with an element having a high liquid phase distribution ratio is transferred to the unsolidified portion at the 1/4 width position. It is discharged and causes center segregation of 1/4 width.

Japanese Patent Application Laid-Open No. Hei 5-237621 proposes a method in which a magnetic field is formed in a mold and the discharge flow rate of an immersion nozzle is braked by an electromagnetic force in order to prevent such segregation at a quarter of the width. ing. However, such an electromagnetic brake for a mold has the disadvantage that the equipment cost is high and the weight of the mold is very heavy.

Regardless of the prevention of segregation of 1/4 of the width, there has been proposed that the amount of cooling water is changed for each part in order to locally change the cooling capacity of the mold wall for each part (Japanese Patent Application Laid-Open No. HEI 3 (1991) -1991). However, it is difficult to control the growth of the solidified shell for each part.

[0013]

SUMMARY OF THE INVENTION An object of the present invention is to provide a continuous casting mold and a continuous casting mold capable of reliably reducing center segregation in both quarters of the slab width without requiring high equipment costs. It is to provide a method.

[0014]

Means for Solving the Problems The present inventors have studied the method of preventing the crater end shape from being W-shaped while utilizing the current cooling method using a mold copper plate, and have confirmed the following items. Was.

(A) Even if a downward flow occurs, if the growth of the solidified shell is suppressed at the center of the slab width, the crater end will not be W-shaped but will be flat in the width direction, and the slab width will be reduced. Center segregation concentrated in a quarter part is dispersed.

(B) As described later, since the growth rate of the solidified shell is higher in the secondary cooling zone than in the mold,
If the central width can be allowed to pass more time in the mold than the other parts, the growth of the solidified shell in the central width of the slab is suppressed.

FIG. 1 is a drawing showing a continuous casting mold according to the present invention in which the length of the mold at the center of the width is increased. FIG.
(A) shows a mold whose center in the width is elongated in the advancing direction of a slab according to a trigonometric function curve, FIG. 1 (b) shows a mold whose length is increased in an inverted trapezoidal shape, and FIG. 1 (c) shows an inverted triangle. Represents a mold elongated in shape. As shown in these figures, if the width of the center of the mold is increased toward the exit side of the slab, the time required for the center of the slab to elapse in the mold is longer than that of the other parts, and the transition to the secondary cooling zone is made accordingly. Since the time of the formation is delayed, the growth of the solidified shell is delayed, and a flat crater end shape is realized in the width direction.

The present invention has been completed by repeating calculations and experiments on the basis of the above-mentioned matters and confirming its effects at the manufacturing site, and is characterized by the following continuous casting mold and continuous casting method ( (See FIG. 1).

(1) A casting mold for continuous casting of a slab having a rectangular cross section, wherein the center of the width of the long side of the casting mold is longer than the end of the width in the advancing direction of the slab. ([Invention 1]
And).

(2) The molten metal is poured into the continuous casting mold of the above [Invention 1] by means of a two-hole nozzle, and immediately after the slab has left the mold, the solidification shell growth rate is changed to the solidification in the mold. A continuous casting method characterized by performing secondary cooling so as to be higher than the shell growth rate (referred to as [Invention 2]).

In [Invention 1] and [Invention 2], the metal to be subjected to continuous casting is mainly steel such as carbon steel, low alloy steel and stainless steel, but may be a non-ferrous alloy such as copper.

The casting mold for continuous casting which is the subject of [Invention 1] and [Invention 2] can be applied to all continuous casting apparatuses in which the solidification shell growth rate is higher in the secondary cooling zone than in the casting mold. The magnitude of the solidified shell growth rate is specifically described by a solidification coefficient k described later. The optimal “length of the center of the width of the long side of the mold” is, as described below,
Characteristics of the secondary cooling zone of the continuous casting apparatus to which the mold of the present invention is applied and the ability to supply cooling water to the mold, the shape of the immersion nozzle, the specific gravity of the molten metal, the slab withdrawing speed, and the like are the results of the crater end W It is determined according to the actually measured value of the character-shaped unevenness.

The "slab" in [Invention 2] also includes a slab immediately after the mold is released, that is, a slab containing an unsolidified molten metal therein.

"Long side surface" refers to the surface of the mold on the side where the long side of the rectangle in the cross section of the continuous casting slab is formed by contact. The “slab traveling direction” refers to the direction in which the slab is continuously pulled out.

The immersion nozzle in [Invention 2] corresponds to any nozzle that immerses a discharge port in a molten metal pool formed in a mold and pours the molten metal.

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION

1. [Invention 1] In [Invention 1], the reason why the exit side at the center in the width direction of the long side surface of the mold is made to have a convex shape is to suppress the growth of the solidified shell at the width center of the slab as described above. . Cooling is also performed in the mold, and the solidified shell grows. As shown in the optimum value of the convex length, the growth rate of the solidified shell is higher in the secondary cooling zone than in the mold. Therefore, the length of the central portion of the long side surface of the mold is increased in order to obtain the same solidified shell thickness as the width 1/4 portion where the solidified shell growth rate is slow due to the influence of the downward flow.

It is necessary to quantitatively determine the extent of the convex shape in the longitudinal direction based on the actually measured value of the crater end. The optimum length at the center of the width of the long side surface of the mold will be described in detail.

2. [Invention 2] In [Invention 2], using a two-hole nozzle [Invention 1]
Pour into a continuous casting mold. The reason why the two-hole nozzle is used is to supply hot water to the mold without involving the powder covering the meniscus (the surface of the molten metal).

Secondary cooling is provided after leaving the mold, and the cooling is strengthened more than in the mold. After a sound slab surface is formed in the mold, the cooling is strengthened immediately after leaving the mold. This is for ensuring the solidified shell thickness to withstand deformation and the like and for improving efficiency. To achieve that goal,
The solidification shell growth rate in the secondary cooling zone should be greater than that in the mold. Usually, the solidification shell growth rate is indicated by the solidification coefficient k (mm / (min) ^0.5 ). Solidification coefficient km in the mold is 10 to 30 mm / (min)
^0.5 compared to 20 in the secondary cooling zone.
４０40 mm / (min) ^0.5 is desirable.

3. Optimum length of the center of the width of the long side of the mold Next, in [Invention 1], how much the exit side of the center of the width of the long side of the mold depends on the shape of the crater end, which is the result of the characteristics of the continuous casting apparatus. The calculation method of the optimum length at the center of the width or the convex shape is shown.

FIG. 3 is a drawing showing the thickness of the solidified shell and the condition of the slab according to the distance 1 from the meniscus along the longitudinal direction of the slab. "Distance from the meniscus along the longitudinal direction of the slab" refers to the length along the slab.

In general, in continuous casting of metal, the thickness of the solidified shell is proportional to the square root of time and is expressed by the following equation (1).

D = kt0.5 ^0.5 (1) D : Solidification shell thickness (mm) k: solidification coefficient (mm / (min ^0.5 )) t: time (min) In the following, the above formula (1) is divided into a mold and a secondary cooling zone. Coagulation factor is also discussed separately thereto k _S in which k _M and a secondary cooling zone in the mold.

The value of the coagulation factor k _M in the mold by methods observing analyzed during breakout occurs which solidified shell broken molten steel flows out during casting or the cross section of sulfur was added S as a tracer in Sulfur Print Sought, about 2
0 mm / (min ^0.5 ).

On the other hand, the solidification coefficient k _{S in the} secondary cooling zone after leaving the mold is about 28.6 mm / (min ^0.5 ) as a measured value.

Based on the above relationship, the time t from the start of solidification
The thickness of the solidified shell after one minute can be expressed by equations (2) and (3), separately for the part in the mold and the part in the secondary cooling zone.

[0037] ^{_{D M = k M · t 0.5}} (0 ≦ t ≦ l M / V C) ·················· (2) D S = k S · { t- (l ₀ / V _C )} ^0.5・ ・ ・ ・ ・ ・ ・ ・ ・ (3) (l _M / V _C ≦ t ≦ l _E / V _C ) Here, d ₁ : thickness of solidified shell (mm) at the time of leaving the mold in the conventional mold (CASE 1) d _E : thickness of solidified end (mm) = slab thickness / 2 l _M : mold Length of long side surface (mm) (in CASE1 of FIG. 3, it is indicated as l _M1 ) l _E : distance from meniscus at the end of solidification (mm) V _c : slab withdrawal speed (mm / min) l ₀ is next State. Since the solidified shell thickness in equations (2) and (3) must be the same when transitioning from the mold to the secondary cooling zone, the time in the mold, t = l _M1 / V _C
The thickness of the solidified shell after the passage is set as D _M = D _S = d ₁ . Further, at the final solidification time t = (l _E / V _C ), the solidified shell thickness is D _S = d _E. From these conditions, k _s and l ₀
Is obtained as follows.

K _S = d _E / (l ₀ / V _C ) ^0.5 (4) l ₀ = _{(l E -l M) · d} E 2 / (d E 2 -d 1 2) ···················· (5) l 0 is 3 It means a point P in the relationship diagram between the distance l from the middle meniscus and the solidified shell thickness D.

Unless affected by the downward flow from the immersion nozzle, the thickness of the solidified shell determined by the equations (2) to (5) is constant in the width direction, and the crater end shape is flat in the width direction. However, the crater end shape is actually W-shaped due to the influence of the downward flow as in CASE1 of FIG.

Assuming that the difference in the distance from the meniscus to the solidification end position in the slab width direction is Δl _E (mm),
As shown in the figure of CASE1, solidification is delayed by Δl _E a (= Δl _E ) (mm) to cancel the center of the slab width where solidification is completed early due to the downward flow. The length of the mold may be increased as described above.

CASE 2 shows the solidification state in the case where there is no influence of the descending flow of the immersion nozzle.
It is set as 1 _M2 longer than 1 _{M1 of} SE1. At this time, if there is no descending flow, the crater end shape becomes convex if the width center part of the long side surface of the mold is made convex on the output side.

However, because of the effect of the downward flow, CASE1 and CASE2 are superimposed by the method of the present invention, and the former W-shaped concave portion and the latter convex portion just at the coagulation end position are formed. As a result, the solidification end position is finally aligned with a fixed position flat in the width direction as in CASE3.

In order to realize such a crater end shape, the width center length l _M2 (mm) of the mold is determined as follows.

[0044] coagulation end position, .DELTA.l _E a as in FIG. 3
When there is a difference in (mm), the thickness of the solidified shell after a time t from the start of solidification (formation of meniscus) in the mold at the center of the width can be expressed by equation (6).

D _M = k _M · t ^0.5 (0 ≦ t ≦ l _M2 / V _C ) (6) In the secondary cooling zone at this time the solidified shell thickness D _S a (7)
It is expressed by an equation.

[0046] _{D S a = k S · {} t- (l 0 / V C) - (Δl E a / V C)} 0.5 (l M2 / V C ≦ t ≦ (l E + Δl E a) / V _C ) (7)
Therefore, the time t = l _M2 / V _C at the time of exiting the center of the width of the mold.
In, placing the solidified shell thickness _{D M = D S a = d} 2, the following
Equations (8) and (9) are obtained.

D ₂ = k _M · (l _M2 / V _C ) ^0.5 (8) d ₂ = k _S・ {(L _M2 / V _C )-(l ₀ / V _C )-(Δl _E a / V _C )} ^0.5・ ・ ・ ・ ・ ・ (9) By transforming the equations (8) and (9), Mold center length l _M2 (mm)
Is determined by the following equation (10).

L _M2 = k _S ² (l ₀ + Δl _E a) / (k _S ² -k _M ² ) (10) d ₂ = k _M · [k _S ² (l ₀ + Δl _E a) / {(k _S ² -k _M ² ) · Vc}] ^0.5 · (11) From the above equation (10), the crater using the conventional mold Based on the actual measured value を l _E a of the end, it is possible to derive the width central length l _M2 of the long side surface of the mold according to the present invention, which is optimal for the continuous casting apparatus, immersion nozzle, metal to be cast, drawing speed, etc. it can.

[0049] Using a continuous casting apparatus conventional steel, it can occur under normal operating conditions △ l _E a is within 300 mm, in this case, approximately 500mm width central portion of the mold
It only has to be longer than other parts.

[0050]

EXAMPLES Next, the effects of the present invention will be described with reference to examples.

Table 1 shows a continuous casting machine equipped with a mold according to the present invention used in the practice and a conventional rectangular mold for comparison, and continuous casting conditions.

[0052]

[Table 1]

Using a continuous casting machine having the specifications shown in the table, a low-carbon aluminum killed steel (0.05 wt% C-0.05 wt% Si-0.20 wt% Mn)
Was continuously cast at a casting speed of 5.0 m / min. As a continuous casting mold, a comparison was made between a conventional mold having a long side surface of a rectangular shape and a mold according to the present invention in which the outlet side central portion had a convex shape. The mold according to the present invention was an inverted trapezoidal mold as shown in FIG. In order to make the height of this trapezoid 290 mm, that is, to make the length l _M2 of the long side of the mold 1190 mm, the solidification coefficient in the mold k _M = 20.0 mm / (mi
n) ^0.5 , solidification coefficient k _S of the secondary cooling zone = 28.58 m
m / (min) ^0.5 , Δl _E a = 0.200 m, l _O = 0.4
085 m was used.

FIG. 2 is a graph showing the results of measuring the segregation degree of the carbon concentration in the thickness direction at a quarter of the width of the cross section of the slab. The degree of segregation on the vertical axis indicates local concentration / average concentration.

According to the present invention, it has been found that the carbon concentration can be reduced from 1.3 times the average value at the center of the thickness to slightly less than 1.1 times.

[0056]

According to the present invention, the crater end shape W
Characterization can be prevented, segregation at the center of the thickness at a quarter of the width is reduced, and a high-quality continuous cast slab can be obtained.

[Brief description of the drawings]

FIG. 1 is a view showing a long side surface shape of a continuous casting mold of the present invention. (A) shows the case of following a triangular function, (b) shows the case of an inverted trapezoidal shape, and (c) shows the case of an inverted triangular shape.

FIG. 2 is a view showing a measurement result of a distribution of carbon in a thickness direction in a central portion of a width of a steel slab.

FIG. 3 is a view schematically showing a growth behavior of a solidified shell and crater end shapes when using various shapes of molds.

FIG. 4A is a diagram showing an unsolidified portion of a thick section near a crater end. (B) is a figure which shows the flow of a molten metal.

FIG. 5 is a diagram showing a downward flow generated from a two-hole nozzle.

6A is a cross-sectional view of the mold taken along line A-Aa, and FIG. 6B is a cross-sectional view taken along line B-Ba of FIG.

FIG. 7 is a view schematically showing a longitudinal section of a continuous casting apparatus.

[Brief description of reference numerals]

 1: Mold copper plate 2: Backup plate 3: Cooling channel 4: Water supply pipe 5: Two-hole nozzle 6: Melting powder 7: Tundish 8: Molten metal 8a: Solidified shell 8b: Downflow 9: Support roll 10: Crater end

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masafumi Hanao 4-33, Kitahama, Chuo-ku, Osaka-shi, Osaka Sumitomo Metal Industries, Ltd.

Claims (2)

[Claims] 1. A casting mold for continuous casting of a slab having a rectangular cross section, characterized in that the center of the width of the long side of the casting mold is longer than the end of the width in the advancing direction of the slab. Continuous casting mold. 2. The molten metal is supplied by a two-hole nozzle.
And the secondary cooling is performed immediately after the slab leaves the mold, so that the solidification shell growth rate becomes higher than the solidification shell growth rate in the mold. Continuous casting method.