JPH0899155A - Continuous casting method - Google Patents

Abstract

(57) [Summary] [Purpose] The optimum shape of the unsolidified region is eliminated by eliminating uneven solidification in the width direction of the slab due to the downward flow along both short sides of the slab in the unsolidified region of the continuously cast slab. To obtain a slab without center segregation. [Structure] When continuously drawing a continuously cast slab by pulling out the molten metal supplied into the mold while cooling, the two single-hole dipping nozzles 1-1 are arranged into the molten metal mold 2. By providing two or more supply locations in the width direction of the slab, a downward flow F is formed in the center portion of the slab in the slab width direction in the unsolidified region B, and the solidified shell thickness in the unsolidified region B is changed to the slab width. The final solidified portion of the unsolidified region B is continuously pressed down while optimizing in the direction 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 capable of preventing center segregation which occurs in the central portion of the plate thickness of a continuously cast slab produced by continuously casting molten metal in a mold. It is about.

[0002]

2. Description of the Related Art In continuous casting, molten steel cast in a mold is primarily cooled by cooling water in the mold to form a solidified shell on the outer skin, and secondary cooling is then performed in a group of guide rolls to accelerate solidification. However, it is a method of continuously producing a cast piece by pulling out a completely solidified cast piece with a pinch roll. In such continuous casting, an internal defect called center segregation often becomes a problem.

This center segregation is a phenomenon in which molten steel components such as C, S, P, Si and Mn are positively segregated in the center portion (final solidified portion) in the thickness direction of the cast slab. 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 to prevent center segregation, there is a method of preventing the movement of the residual molten steel or the accumulation of the 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 Patent Laid-Open No. 63-252655, the surface temperature of the slab at the final solidified 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 method of preventing the flow of the concentrated molten steel and preventing center segregation has been proposed.

[0006] Further, in the light reduction by the reduction roll group, 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 each reduction works as a concentrated load. Therefore, there is a drawback that internal cracks are likely to occur at the solidification interface and the reduction amount cannot be made large.Furthermore, the method of continuously forging the vicinity of the solidification completion point of the slab with a flat forging die is very expensive. However, there is a drawback that the concentrated molten steel is less likely to enter the center of the slab due to a large reduction amount, and conversely negative segregation is liable to occur.
The continuous casting method of Japanese Patent Publication has been proposed.

This is done by installing an electromagnetic stirrer or an ultrasonic wave applying device upstream of the solidification completion point to cut the dendrite by molten steel flow to form an equiaxed crystal region near the crater end. A pair of reduction rolls arranged immediately before the solidification completion point applies a large reduction of 3 mm or more to forcibly form the solidification completion point and eliminate the central segregation without causing cracking.

[0008]

However, the above-described conventional center segregation improving method, in which the roll pressing or the die pressing is adopted, does not affect the width direction of the slab 4 as shown in FIG. If there is uneven solidification, that is, the width center (1 /
4 to 3/4 W), when the solidification progresses slowly at the width end portions (1/4 W to edge side, 3/4 W to edge side), uniform rolling in the width direction of the slab cannot be performed, so It has a drawback that the center segregation A is deteriorated at both end portions of the delayed slab width direction.

This uneven solidification in the slab width direction is usually caused by a conventional continuous casting machine, in which two discharge holes 1a are formed slightly downward toward the outside in the slab width direction. 1 is used, and when molten steel 3 is discharged from the immersion nozzle 1 into the mold 2, a downward flow F _0d that descends along the short side of the slab and an upflow F that rises at the center of the slab. _{This is} because a large circulating _downflow F _{0 of 0 u} is formed in the unsolidified region B, and this circulating _downflow F ₀ makes it impossible to uniformly cool the slab.

As a result, the thickness of the solidified shell S in the final solidified portion of the unsolidified region B is large in the central portion in the width direction of the slab,
It becomes thin at both ends, and the crater end CE shape is recessed at the center of the slab width direction and protrudes at both ends to become uneven. In this state, when the rolling roll group receives the rolling reduction, a rolling reduction force corresponding to the solidification shrinkage cannot be obtained at both widthwise end portions of the slab 4, and the concentrated molten steel flows into and accumulates at the both end portions to cause central segregation A. Will occur.

As a method of improving center segregation for eliminating such uneven solidification in the width direction of the slab, as shown in FIG. 5, the long side 2a of the mold 2 has a long side length at the center of the slab width direction. 50-
A concave portion 2b having a depth of 1.0 to 5.0 mm is formed over a length of 80%, and a convex portion formed in the widthwise central portion of the slab 4 provides a uniform unsolidified molten steel thickness in the slab width direction. It is proposed to secure it (Japanese Patent Laid-Open No. 5-185186).
Issue).

However, in this method, it is necessary to set the projection of the cast slab to 3 mm or more in order to obtain a sufficient effect, so it becomes difficult to set the pass line of the cast slab, and the step line of the cast slab is difficult to set. There is a problem that internal cracking is easily induced by slab bulging.

Materials and Process Vol. 2 (1989), P1
Alternatively, in Japanese Patent Laid-Open No. 1-178355, there is a problem that FIG.
As shown in FIG. 5, the interval between the guide rolls in the guide roll group 5 ′ in the thickness direction of the slab is increased step by step to forcibly cause bulging in the slab 4, thereby reducing the slab thickness to 2 to the short sides of the mold. Around the tripled crater end, light reduction is performed by the small-diameter rolls of the reduction roll group 6 ', and the concentrated molten steel is floated and diffused by bulging, and the reduced concentration prevents the concentrated molten steel from falling and prevents center segregation. A method has been proposed.

However, in this method, since the drum type slab is rolled down, the amount of rolling down 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,
A method of applying electromagnetic stirring within a specific range (Japanese Patent Laid-Open No. 63-15
7749) and a method of applying ultrasonic vibration to the cast slab (Japanese Patent Application Laid-Open No. 1-113157). However, in any case, when there is uneven solidification in the width direction, a fundamental solution is not reached. There wasn't.

The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to obtain a flow of molten metal, particularly an inconsistency in the width direction of a cast product due to a downward flow along both short sides of the cast product. It is an object of the present invention to provide a continuous casting method capable of eliminating uniform solidification and obtaining a slab without center segregation due to the optimum shape of the unsolidified region.

[0017]

The present invention will be described with reference to FIGS.
As shown in FIG. 2, when the molten metal supplied into the mold is pulled out while being cooled to continuously produce a continuously cast slab, the molten metal is supplied into the mold 2 at two or more supply positions in the width direction of the slab. As a result, a downward flow F is formed in the central portion of the slab in the unsolidified region B in the width direction of the slab to optimize the solidified shell thickness in the unsolidified region B in the slab width direction, and The solidified portion is continuously pressed to prevent center segregation.

An immersion nozzle is usually used to supply the molten metal into the mold. In this immersion nozzle, in order to accelerate the floating of non-metallic inclusions in the molten steel, 2
One discharge hole is formed slightly downward toward the short side of the slab. For example, when two immersion nozzles are used, the immersion nozzle has one discharge hole 1a as shown in FIG. The hole immersion nozzle 1-1 is provided, and the discharge holes 1a are arranged so as to face inward and face each other. In the case of three or more, as shown in FIG. 2, only the both end portions in the width direction of the cast slab are set as the single-hole dipping nozzles 1-1, the inside of each discharge hole 1a is directed, and the dipping nozzle in the middle portion is a normal two-hole dipping nozzle. Let it be nozzle 1-2.

[0019]

When molten metal is applied from two or more dipping nozzles 1 in the above structure, the unsolidified region B of the cast slab 4 is obtained.
A downward flow F is formed at the center of the slab in the width direction. For example, when hot water is supplied from two locations using the two single-hole dipping nozzles 1-1 of FIG. 1, the discharge flows from the opposite discharge holes 1a collide at the center of the width direction of the slab, resulting in a molten steel flow pattern. Is a relatively strong downward flow F ₁ at the center of the slab width direction and an ascending flow f ₁ near the short side, and a velocity distribution V ₁ is obtained in which the center is downwardly convex, and accordingly downwardly convex. The crater end CE shape of is obtained.

When hot water is supplied from three or more locations using the three or more immersion nozzles 1-1 and 1-2 shown in FIG. 2, the discharge flows from the plurality of opposed discharge holes 1a collide with each other, and in the case of FIG. is decelerated flow rate than, and homogenized downward flow F _2, and the velocity distribution V ₂ of smooth convex downward is obtained which smooth convex crater end CE shape to bottom according to the obtained To be

As described above, the thickness of the solidified shell S in the final solidified portion of the unsolidified region B is thin in the central portion in the width direction of the slab,
It becomes thicker at both ends, and the non-solidified region B has an optimum cross-sectional spindle shape. The cast slab 4 having such an unsolidified region B can be rolled according to the solidification shrinkage amount thereof, and the rolled roll group 6
When the slab 4 is subjected to a reduction by, the slab 4 is reduced with a reduction force commensurate with its solidification shrinkage, and this reduction force prevents the inflow and accumulation of the concentrated molten steel and prevents the center segregation A.

By changing the number of dipping nozzles, the discharge hole diameter, the discharge angle, and the distance between the dipping nozzles, the descending flow F in the unsolidified region, that is, the crossing of the solidified shell S in the final solidified portion of the unsolidified region B is performed. The surface shape can be controlled in various ways.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to an illustrated embodiment. This is an example applied to continuous casting of a steel slab. As shown in FIG. 3, molten steel 3 from a ladle and a tundish is cast into a mold 2 by a dipping nozzle 1 and cast by primary cooling in the mold 2. The solidified shell S is formed on the surface of the piece 4, and the solidification is promoted by the subsequent secondary cooling of the support roll group 5 by the spray water or the like, the solidification shell S is completely solidified, and the pinch roll 7 pulls it out.

In such a continuous casting facility, in the present invention, as shown in FIG. 1, the immersion nozzle 1 attached to a tundish through a sliding nozzle device (not shown) is a one-hole immersion nozzle 1-1, Two one-hole dipping nozzles 1-1 are arranged at intervals in the width direction of the cast piece so that the molten metal can be poured into the mold 2 from two locations. Furthermore, the discharge holes 1a of the one-hole dipping nozzles 1-1 are made to face inward,
A downward flow F ₁ is formed so that the central portion in the width direction of the slab is convex downward.

Further, as shown in FIG. 2, a large number of immersion nozzles 1 are arranged in the width direction of the slab so that the molten metal can be poured into the mold 2 from a large number of places. The dipping nozzle 1 is a single-hole dipping nozzle 1-1 similar to that shown in FIG. 1 and a normal 2-hole dipping nozzle 1-2. The single-hole dipping nozzle 1-1 is arranged at both ends so that the discharge hole 1a faces inward. and setting, as the intermediate portion of the two-hole immersion nozzle 1-2 has two discharge holes 1a arranged so as to face in the slab width direction, downward flow F ₂ in smooth convex downward is formed To do.

Further, the crater end CE portion at the final stage of coagulation is subjected to a light reduction 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.

(1) Equipment specifications and casting conditions Continuous casting machine: Curved continuous casting machine (curving radius: 12.5
m) Slab size: 250 mm (thickness) x 2000 mm
(Width) Steel type: C 0.15 to 0.20% for thick plate 4
0K Steel Molten Steel Superheat ΔT: 20 ° C Casting Speed: 0.8m / min Final Solidification Reduction: Reduction Zone Length 5m, Reduction Gradient
1mm / m (2) Specifications of immersion nozzle Discharge hole diameter: 90mm × 90mm square Discharge angle: 5 ° downward Distance between nozzles: 1500mm (Fig. 1), 400mm (Fig. 2) Casting in the center of the thickness direction of the slab after complete solidification Table 1 shows the results of the C segregation degree distribution in the one-width direction. In Table 1, Example I of the present invention is the case where the two immersion nozzles of FIG. 1 are used, and Example II of the present invention is the case of the use of the five immersion nozzles of FIG. For comparison, continuous casting of hot water supply at one location using a conventional two-hole immersion nozzle was also performed. In addition,
In each case, the reduction roll group 6 was used to perform light reduction at the end of coagulation.

[0029]

[Table 1]

As is apparent from this table, in the present invention, by changing the hot water supply method and improving the shape of the unsolidified region of the cast piece, the reduction can be effectively performed, and compared with the conventional comparative example. The center segregation near the slab edge was significantly improved, and a slab with a uniform composition in the width direction could be manufactured.

[0031]

As described above, according to the present invention, when continuously casting molten metal, there are two hot water supply points for the molten metal in the mold.
By providing more than one place, it is possible to form a downward flow in the central portion of the slab in the unsolidified region in the width direction of the slab to optimally control the shape of the unsolidified region of the slab, and to uniformly reduce the unsolidified region. Thus, it is possible to manufacture a cast product having a uniform composition in the entire width direction of the cast product and having no center segregation.

[Brief description of drawings]

FIG. 1 shows an example of a continuous casting method according to the present invention,
(A) is a longitudinal sectional view, (b) is a perspective view, and (c) is a lateral sectional view.

FIG. 2 shows another example of the continuous casting method according to the present invention,
(A) is a longitudinal sectional view and (b) is a perspective view.

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

FIG. 4 shows a conventional continuous casting method, in which (a) is a longitudinal sectional view, (b) is a perspective view, (c) is a lateral sectional view, and (d) is a central segregated lateral sectional view.

FIG. 5 is a cross-sectional view showing a conventional mold for eliminating solidification unevenness in the width direction.

FIG. 6A is a schematic side view showing a continuous casting machine for eliminating the conventional uneven solidification in the width direction, and FIG. 6B is a sectional view showing the cast piece.

[Explanation of symbols]

 A ... Center segregation B ... Unsolidified region S ... Solidified shell 1 ... Immersion nozzle 1a ... Discharge hole 1-1 ... Single-hole immersion nozzle 1-2 ... 2-hole immersion nozzle 2 ... Mold 3 ... Molten steel 4 ... Cast piece 5 ... Support Roll group 6 ... Rolling roll group 7 ... Pinch roll

Claims (1)

[Claims] 1. When continuously producing a continuously cast slab by pulling out the molten metal supplied into the mold while cooling, the molten metal should be supplied to the mold at two or more positions in the width direction of the slab. Thus, a downward flow is formed in the slab width direction central portion in the unsolidified region of the slab to optimize the solidified shell thickness in the unsolidified region in the slab width direction, and the final solidified portion of this unsolidified region is continuously formed. The continuous casting method is characterized in that the center segregation is prevented by rolling down.