JPH0947852A - Continuous casting method and immersion nozzle - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] In continuous casting, the flow of molten metal is controlled and the W-shaped profile, which is an obstacle to the improvement of center segregation, is eliminated in order to effectively perform the reduction casting to improve the center segregation. Moreover, it is possible to easily and surely prevent center segregation with a relatively simple structure. SOLUTION: A plurality of side discharge ports 10 whose discharge direction is directed laterally from the nozzle center axis are provided on the side part in the lower part of the body 1a of the immersion nozzle 1, and the bottom discharge of the nozzle bottom part whose discharge direction is directed downward. By providing a plurality of outlets 11 and discharging molten metal from the side discharge port 10 and the bottom discharge port 11, a downward flow is formed in the slab width direction central portion in the unsolidified region of the slab to form the unsolidified region in the unsolidified region. Control the solidified shell thickness in the width direction of the slab to eliminate the conventional W-shaped profile,
The final solidified portion in this unsolidified region is continuously pressed to prevent center segregation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method and a dipping nozzle for eliminating uneven solidification in a width direction of a continuously cast slab and preventing center segregation.

[0002]

In the continuous casting, the molten steel cast in the mold is primarily cooled by the cooling water in the mold to form a solidified shell on the outer skin, and then in the guide roll group. Secondary cooling to promote solidification, is a method of continuously producing a slab by pulling out a completely solidified slab with a pinch roll, in such continuous casting, often,
Internal defects called central segregation pose a problem. This center segregation is caused by C, S, and C at the center of the slab in the thickness direction (final solidified portion).
This is a phenomenon in which molten steel components such as P, Si and Mn are positively segregated. Center segregation is a particularly serious problem in thick plate materials, and is known to cause deterioration of toughness in the segregated portion and hydrogen-induced cracking.

Such center segregation causes the residual molten steel between dendrites (dendritic crystals) at the final stage of solidification to be macroscopic toward the crater end of the final solidification part due to solidification shrinkage of the slab or bulging of the solidification shell. It is known that this occurs due to the local migration of concentrated molten steel and the local accumulation of concentrated molten steel. Therefore, as a measure for preventing center segregation, there is a method of compensating for the solidification shrinkage of the final solidification portion by preventing the movement of residual molten steel and the accumulation of concentrated molten steel by reducing the vicinity of the solidification front end by some method, Methods based on various ideas have been proposed.

For example, in Japanese Unexamined Patent Publication No. 63-252655, the surface temperature of the slab at the final solidification portion of the slab is 700 to 800 ° by increasing the amount of cooling water sprayed on the surface of the slab.
The bulging between rolls is suppressed by increasing the solidified shell thickness within the temperature range of C, and a rolling force of a strain rate of 0.2 to 0.4% per minute is applied to the slab by the light rolling roll group. Therefore, a light reduction method has been proposed which prevents the flow of concentrated molten steel and prevents center segregation. Further, in Japanese Patent Laid-Open No. 61-42460, an electromagnetic stirrer or an ultrasonic wave applicator is installed upstream of the solidification completion point to cut dendrite by molten steel flow to form an equiaxed crystal region near the crater end. In addition, a pair of reduction rolls placed immediately before the completion point of solidification gives a large reduction of 3 mm or more to forcibly form the completion point of solidification, which eliminates center segregation without cracking. A method has been proposed.

However, with the above roll reduction, since it can be reduced only in a dot shape in the longitudinal direction of the slab, solidification shrinkage and bulging cannot be sufficiently prevented, and since each reduction acts as a concentrated load, solidification occurs. There is a problem that internal cracks are likely to occur at the interface and a large reduction amount cannot be obtained.
Further, JP-A-1-170566 discloses a large reduction method in which the solidification completion point of a cast slab is continuously forged with a flat forging die, but the equipment cost is very high, Further, since the amount of reduction is large, the concentrated molten steel is difficult to enter the central portion of the slab and conversely tends to cause negative segregation.

Further, in the conventional center segregation improvement method by the above-mentioned reduction, whichever method of roll reduction or die reduction is adopted, the cast slab 4 as shown in FIGS. When there is uneven solidification in the width direction, that is, the width center part (1/4
When the solidification progresses slowly at the width ends (1 / 4W to the edge side and 3 / 4W to the edge side), the solidification is delayed because uniform rolling cannot be performed in the width direction of the slab. The center segregation A is deteriorated at both end portions in the width direction of the cast slab.

The uneven solidification in the width direction of the cast piece is usually caused by forming two discharge holes 10 shown in FIG. 5 (a) slightly downward in the width direction of the cast piece in the conventional continuous casting machine. Since the molten metal 3 is discharged into the water-cooled mold 2 from the immersion nozzle 1 called a two-hole nozzle, a large circulating downward flow F is formed in the unsolidified region B, and the circulating downward flow F uniformly cools the slab. Because it becomes impossible.

As a result, in the cross section of the slab, the thickness of the solidified shell S at the final solidified portion of the unsolidified region B is thicker at the central portion in the width direction of the slab and thinner at both ends, and in plan view of the slab,
The so-called W-shaped profile in which the crater end shape is recessed at the central portion in the width direction of the slab and projected at both end portions to be non-uniform (Fig.
(See (b)) occurs. As shown in FIG. 6, the mechanism for generating the W-shaped profile is due to the remelting of the solidified shell near the short side of the cast piece by the discharge flow F _A and the delay of solidification due to the downward flow F _B near the short side of the cast piece. .

[0009] In this state, when the rolling roll group receives the rolling reduction, the rolling force corresponding to the solidification shrinkage cannot be obtained at both widthwise ends of the slab 4, and the concentrated molten steel flows into and accumulates at the both ends. Central segregation A will occur.

As a method for improving center segregation for eliminating such uneven solidification in the width direction of the cast slab, Japanese Patent Laid-Open No. 18518/1993
No. 6, as shown in FIG. 7, the length of the long side 2a of the long side 2a of the mold 2 is 5 at the central portion of the long side 2a where the solidification progresses rapidly.
Depth Δd = 1.0-5.0 over a length of 0-80%
There is proposed a step thickness mold in which a concave portion 2b of mm is formed, and a convex portion formed in the widthwise central portion of the slab 4 ensures a uniform unsolidified molten steel thickness in the slab width direction. However, with this method, it is necessary to set the slab convexity to 3 mm or more in order to obtain a sufficient effect, so it becomes difficult to set the slab pass line, and slab bulging at the stepped portion becomes difficult. Therefore, there is a problem that internal cracks are easily induced.

Further, as a similar method for solving the uneven solidification, Japanese Patent Laid-Open No. 62-270260 discloses a dipping nozzle rotatably supported with respect to a tundish nozzle so that the lower end discharge port of the dipping nozzle is By making the discharge direction an angle to the radial direction from the center and rotating the immersion nozzle by the reaction of the molten steel to be discharged, the molten steel discharge flow is swirled and the cooling and solidification of the molten steel central part is promoted. It has been proposed to prevent center segregation, but it is difficult to make the discharge port shape of the immersion nozzle, the connection part between the tundish nozzle and the immersion nozzle becomes complicated, and uneven flow occurs and uniform solidification occurs. There is a problem that it becomes impossible. Further, in Japanese Patent Application No. 6-237059, it is proposed that two or more immersion nozzles are arranged in the width direction of the slab to eliminate the uneven solidification in the slab width direction due to the downward flow. However, the nozzle cost is increased, and the nozzle cassette seal is required at two or more places, which causes a problem of deterioration of cleanability.

Materials and Process Vol. 2 (1989), P1
159 (Japanese Patent Laying-Open No. 1-178355), the slab thickness is set to a mold by forcibly causing bulging in the slab by gradually increasing the interval between the guide rolls of the guide roll group in the slab thickness direction. The diameter of the concentrated molten steel is reduced by a small diameter roll of the reduction roll group near the crater end, which is 2 to 3 times the short side, and the concentrated molten steel is suspended and diffused by bulging. However, since the drum-shaped slab is pressed down, the amount of reduction at the width center portion becomes large, and it is considered difficult to perform uniform rolling down in the width direction.

As another method for improving center segregation,
As described in JP-A-63-157749, a method of applying electromagnetic stirring within a specific range, and JP-A-1-
As described in Japanese Patent No. 113157, there is a method of applying ultrasonic vibration to a slab, but in all cases, when there is uneven solidification in the width direction, a fundamental solution has not been reached.

The present invention has been made to solve the above-mentioned problems, and an object thereof is to control the flow of molten metal and to perform center segregation in order to effectively perform the reduction casting for improving center segregation. The object of the present invention is to eliminate the W-shaped profile, which is an obstacle to improvement, and to easily and surely prevent center segregation with a relatively simple structure.

[0015]

The continuous casting method according to the present invention, as shown in FIG. 1, is a continuous casting method in which a molten metal 3 supplied through a submerged nozzle 1 into a mold 2 is withdrawn while being cooled. By discharging the molten metal from the side discharge port 10 and the bottom discharge port 11 of the immersion nozzle 1, a downflow is formed in the slab width direction central part in the unsolidified region of the slab to solidify the shell in the unsolidified region. While controlling the thickness in the width direction of the slab, the final solidified portion of this unsolidified region is continuously rolled down to prevent center segregation.

The immersion nozzle according to the present invention is the immersion nozzle in which the lower part is immersed in the molten metal 3 in the mold 2.
A plurality of side discharge ports 10 whose discharge directions are directed from the central axis of the nozzle to the sides are provided on the side of the lower part of the nozzle body 1a
The nozzle bottom portion 1b is characterized in that a plurality of bottom discharge ports 11 whose discharge direction is downward are provided.

In a conventional immersion nozzle for slabs,
Each of the two side discharge ports 10 is formed slightly downward toward the short side of the slab, and the bottom discharge port 11 is formed in the bottom part 1b of such an immersion nozzle in the width direction of the slab with the central axis of the nozzle interposed therebetween. One (all four-hole nozzle) or four (all six-hole nozzle) may be arranged.

The operation of the present invention having the above structure will be described. 2 with conventional two side outlets
In the case of the hole nozzle, as shown in FIG. 6B, the discharge flow forms an ascending flow to the meniscus portion and a descending flow to the short side of the slab, and the crater end shape has a W-shaped profile in plan view. . When a cast slab having such a non-solidified region is non-uniformly rolled, the solidification retarded portion, that is, the vicinity of the cast slab edge is not rolled down, and there is a problem that center segregation near the cast slab edge is deteriorated.

On the other hand, in the present invention, as shown in FIG. 1A, the molten metal 3 is discharged from the side discharge port 10 and the bottom discharge port 11 and is supplied to the central portion and the upper portion of the mold 2. It The discharge flow toward the center becomes a downward flow F _{2 at the} central portion in the width direction of the slab, and the central portion is much larger than the heat supply on the short side of the slab. Therefore, the solidified shell thickness in the slab cross-section has a solidification form in which it is thin at the center of the slab width direction and thick at the ends of the slab width direction, and the W-shaped profile that has conventionally occurred is eliminated. Such a slab having an unsolidified region having a spindle-shaped cross section can be rolled down according to the amount of solidification shrinkage, the inflow and accumulation of the concentrated molten steel can be prevented, and the central segregation A can be prevented.

The upward flow F from the side discharge port 10
₁ raises the temperature of the meniscus and solves problems such as skinning.

The velocity of the downward flow F _{2 at} the central portion in the width direction of the cast slab can be controlled by adjusting the diameter and the discharge angle of the bottom discharge port 11. By providing two bottom discharge ports 11 (all four-hole nozzle), flow control becomes possible,
In particular, the heat supply to the central portion of the slab width direction in the downward flow can be increased, and by providing four bottom discharge ports 11 (all 6-hole nozzle), the flow can be made larger than that of all 4-hole nozzle. In addition, the discharge speed can be increased.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to an embodiment shown in the drawings. This is an example applied to continuous casting of steel slabs, and as shown in FIG.
Molten steel 3 from a tundish is cast into a water-cooled mold 2 by a dipping nozzle 1 and a slab 4 is obtained by primary cooling in the mold 2.
The solidified shell S is formed on the surface, and solidification is promoted by the subsequent secondary cooling of the support roll group 5 by spray water or the like, complete solidification by the pressure roll group 6 and extraction by the pinch roll 7.

In such a continuous casting facility, in the present invention, as shown in FIGS. 1 and 2, an immersion nozzle 1 attached to a tundish via a sliding nozzle device is provided at the bottom of the body 1a on two sides. Part discharge port 10 is provided,
The four-hole nozzle 1-I or the all-six-hole nozzle 1-II in which the bottom portion 1b is provided with two or four bottom discharge ports 11 is used.

The two side discharge ports 10 are arranged so that the discharge direction is toward both sides of the slab width direction, that is, the short side of the slab, and the discharge angle is several downwards with respect to the horizontal, as in a normal dipping nozzle. It is formed to have an angle of °. The bottom discharge ports 11 are arranged on both sides in the slab width direction with the central axis of the nozzle interposed therebetween, and the discharge direction is the vertical direction. The diameter and the discharge angle of the bottom discharge port 11 are as shown in FIG.
Optimal unsolidified region B having a spindle-shaped cross section as shown in (a)
It is appropriately determined so as to obtain the cast slab 4 having.

Further, the crater end portion in the final stage of solidification is reduced by the reduction roll group 6 over a predetermined range in the front and rear.

With the above structure, continuous casting was performed under the following conditions.

Equipment Specifications / Casting Conditions (1) Continuous casting machine: Curved continuous casting machine (curving radius: 12.5
m) (2) Slab size: 250 mm (thickness) x 2000 mm
(Width) (3) Steel type: C 0.15 to 0.20% for thick plate 4
0K steel (4) Superheated molten steel: ΔT = 20 ° C (5) Casting speed: 0.8m / min (6) Final solidification rolling reduction: Rolling zone length 5m, rolling gradient
1 mm / m C, S, P, Si, M is formed in the center of the thickness of the cast piece after solidification.
Although molten steel components such as n segregate, in this example, attention was paid to C having a high concentration, and evaluation was made by the degree of C segregation.
FIG. 4 shows the results. In the figure, Example I of the present invention
Shows the case where all four-hole nozzles are used, and Example II of the present invention shows the case where all six-hole nozzles are used, which are carried out under the specifications and conditions of (1) to (6). In addition, as an implementation comparative example,
Continuous casting using a conventional two-hole immersion nozzle was also performed.

As is clear from FIG. 4, in the present invention,
The degree of segregation in the conventional two-hole immersion nozzle was improved, and the center segregation at the end portion in the slab width direction was improved. Further, the center segregation was improved in all 4-hole nozzles as compared with all 6-hole nozzles. As a result, the W-shaped profile was eliminated, and a slab having a uniform composition in the width direction could be manufactured.

[0029]

As described above, according to the present invention, when continuous casting is performed, a plurality of side discharge ports are provided on the lower side of the immersion nozzle, and a plurality of bottom discharge ports are provided on the bottom of the nozzle, whereby A downward flow can be formed in the central portion of the slab in the width direction in the unsolidified region to optimally control the shape of the unsolidified region of the slab, and the W-shaped profile generated in the conventional two-hole nozzle can be eliminated. As a result, it is possible to uniformly reduce the unsolidified region, and it is possible to manufacture a good cast product having a uniform composition in the entire width direction of the cast product and having no center segregation.
Moreover, since the immersion nozzle needs only a slight modification to the normal immersion nozzle, the cost can be reduced and the center segregation can be easily and surely prevented.

[Brief description of drawings]

FIG. 1 (a) is a longitudinal sectional view and a transverse sectional view showing a flow / uniform solidification state of molten metal in a continuous casting method according to the present invention, and FIG. 1 (b) is a longitudinal section showing an example of a dipping nozzle according to the present invention. A plan view and a cross-sectional view, and (c) is a longitudinal cross-sectional view and a cross-sectional view showing another similar example.

FIG. 2 is a submerged nozzle according to the present invention, in which (a) is a vertical cross-sectional view and a bottom view showing a case of an all-four-hole nozzle, (b).
[Fig. 3] is a vertical cross-sectional view and a bottom view showing the case of all 6-hole nozzles.

FIG. 3 is a schematic sectional view showing a continuous casting machine according to the present invention.

FIG. 4 is a graph showing a C segregation degree distribution in the slab width direction.

5 (a) is a longitudinal sectional view, FIG. 5 (b) is a perspective view, FIG. 5 (c) is a lateral sectional view, and FIG. 5 (d) is a slab. FIG. 6 is a cross-sectional view of the center segregation situation of FIG.

6A and 6B are a flow chart and a cross-sectional view showing a flow and a non-uniform solidification state of molten metal, respectively, showing a conventional cause mechanism of a W-shaped profile.

FIG. 7 is a cross-sectional view showing a conventional mold for eliminating uneven solidification in the width direction.

[Explanation of Codes] A ... Central Segregation B ... Unsolidified Region S ... Solidified Shell 1 ... Immersion Nozzle 1a ... Body 1b ... Bottom, 1-I ... All 4-hole Nozzle, 1-II ... All 6-hole Nozzle, 2 ... Mold 3 ... Molten metal (molten steel) 4 ... Cast slab 5 ... Support roll group 6 ... Reduction roll group 7 ... Pinch roll 10 ... Side discharge port, 11 ... Bottom discharge port.

Claims (2)

[Claims] 1. In a continuous casting method in which molten metal supplied into a mold through a dipping nozzle is drawn while cooling, the molten metal is discharged from a side discharge port and a bottom discharge port of the dipping nozzle to form a slab. While controlling the solidified shell thickness in the unsolidified region in the slab width direction by forming a downward flow in the central part of the unsolidified region in the unsolidified region, the final solidified part of this unsolidified region is continuously rolled down. , A continuous casting method characterized by preventing center segregation. 2. In an immersion nozzle in which a lower part is immersed in a molten metal in a mold, a side part at a lower part of a nozzle body is provided.
An immersion nozzle characterized in that a plurality of side discharge ports whose discharge direction is directed laterally from the central axis of the nozzle are provided, and a plurality of bottom discharge ports whose discharge direction is directed downward are provided at the bottom of the nozzle.