JPS6142460A - Continuous casting method - Google Patents

Abstract

PURPOSE:To eliminate central segregation by making a billet to regular crystal near the solidification completion point and subjecting the billet just before the completion of solidification to a large rate of reduction by means of screw-down rolls. CONSTITUTION:At least a pair of large screw-down rolls 5 are disposed near the solidification end 2c of the billet 2 and an electromagnetic stirrer 4 is provided near the upper side thereof. Dendrite settles and the regular crystal zone is formed near the crater end when the stirrer 4 is operated near the solidification point of the billet 2 in the stage when said billet is continuously cast. The billet 2 just before the solidifaction completion point is subjected to the large rate of reduction of >=3mm. by means of the screw-down rollers 5 in the state of sandwiching the regular crystal zone in the central part. Since the billet is subjected to the large reduction near te solidification point in the regular crystal state of the unsolidified part 2b, the central segregation of the billet 2 is effectively eliminated without generating the crack within the boundary face.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The technical content described in this specification in relation to the reduction of center segregation that occurs at the center of the plate thickness of continuously cast slabs is based on electromagnetic stirring, which has been attempted in the past. Light reduction by rolls, application of ultrasonic waves, addition of steel wire, and low-temperature casting have all been aimed at, but by combining at least one of these methods of axial crystallization with large reduction by rolls, This paper proposes the results of research aimed at reducing central segregation, which is particularly advantageous.

(Prior art) Center segregation of continuously cast slabs means that molten steel components such as c, s, and p are present at the center of the thickness of the final solidification zone of the slab. This is a phenomenon that appears as positive segregation. This center segregation causes deterioration of mechanical properties in the thickness direction of thick plates and cracks caused by concentrated hydrogen, but it is one of the quality defects that cannot be avoided in conventional casting methods. .

The mechanism of center segregation is that the pores created by solidification shrinkage and bulging of the solidified shell at the solidification tip near the solidification completion point of the slab act as a suction force, sucking the concentrated molten steel to the solidification completion point. This is caused by positive segregation remaining in the center of the slab, and the problem becomes even worse if the point at which solidification is completed is uneven in the width direction of the slab.

Measures to prevent center segregation have been attempted, such as electromagnetic stirring or application of ultrasonic waves within the secondary cooling zone, but they have not been able to reduce even microscopic segregation and cannot be said to be sufficiently effective.

On the other hand, there is also a method in which the vicinity of the solidification tip is lightly rolled down using a plurality of pairs of rolls, and the amount of solidification shrinkage at the solidification tip is compensated for by the rolling down. However, in the case of roll reduction, since the slab can only be reduced in points in the thickness section in the casting direction, it is not possible to sufficiently prevent solidification shrinkage and bulging that occur between the lower pair of rolls. In particular, since each reduction acts as a concentrated load, internal cracks occur at the solidification interface and the reduction cannot be large enough. The reality is that there have been cases where this has encouraged severe segregation.

In addition to these, a method has also been proposed in which the area near the solidification completion point of the slab is continuously forged using a flat forging die, and furthermore, with forging alone, it is difficult for concentrated molten steel to enter the center of the slab. On the contrary, a negative segregation zone is formed in the center.
Before the solidification completion point, an electromagnetic stirring device or an ultrasonic application device is installed to make the central unsolidified part solidify as an equiaxed product instead of the normal dendrite solidification.''2 Apply forging to form a negative segregation zone. (It is possible to reduce center segregation.

However, near the solidification completion point of the continuous casting machine, the
In practice, it is very difficult to move a large-scale anvil with a thrust of up to 1,000,000'' thrust at a pitch of several seconds, and it has not yet been industrialized.

(Problems to be Solved by the Invention) Continuous casting makes it easier to obtain sound continuously cast slabs while avoiding the formation of negative segregation bands by taking advantage of the advantages of the forging method and the roll reduction method. It is an object of this invention to provide a method.

(Means for Solving the Problems) This invention provides, in continuous casting of steel, in addition to equiaxed crystallization of the unsolidified portion near the solidification completion point of the slab, immediately before the solidification completion point of the slab. This is a continuous casting method characterized by applying a large reduction of 311 or more per pair using at least one pair of reduction rolls to forcibly form a solidification completion point.

Here, center segregation, including semi-micro segregation, can be effectively improved with relatively simple equipment by causing cracks at the solidification interface of the slab, without forming a negative segregation zone at the center of the slab.

In the conventional roll reduction method, it was thought that increasing the reduction amount could not exceed a certain value for fear of cracking at the solidification interface, and the reduction amount was limited to so-called light reduction. When the reduction is further increased and the solidification is forcibly pushed to the point where the solidification completion point is formed, the solidification interface is in an elongated state, but since the stress is restrained in the casting direction, it becomes compressed. The basis for the idea of this invention is the new understanding of the phenomenon in which cracks almost no longer occur.

In other words, although the roll reduction method is much more advantageous in terms of equipment than the forging method, it could not become a mainstream technology because it was thought that the amount of reduction could not be obtained sufficiently. Since it was found that a large reduction could be achieved with a roll, the drawbacks of the roll reduction method were eliminated, leading to the completion of this invention. If the reduction amount is less than 3N, it will not be possible to eliminate uneven solidification in the width direction of the slab, and there is a high possibility that solidification interface cracks will occur, so the required reduction amount per pair of rolls is 3N or less. It was specified as two.

In Fig. 1, 1 is the mold, 2 is the slab, 28 is its solidified part, 2b is the unsolidified part, 3 is the support roll, 4 is the electromagnetic stirring device, 5 is the large reduction roll, and 6 is the reduction device. ,7
8 indicates the slab drawing roll, and 8 indicates the casting direction.

If the slab is rolled down only by the large reduction roll 5, in this case, the solidification interface is columnar crystals, so the concentrated molten steel between the trees is easily squeezed out, and a negative segregation zone is formed in the center. tend to form.

However, if an electromagnetic stirrer 4 is installed above and operated as shown in the figure, the cut dendrites will settle due to the flow of molten steel, creating a crater to be crimped.
An equiaxed crystal region is formed near the end. In this respect, the ultrasonic wave applying device has the same effect.

In this way, if the vicinity of the solidified end 2c of the unsolidified portion 2b is subjected to a large roll roll with the equiaxed crystal zone sandwiched in the center, center segregation in the center will be prevented without forming a negative segregation zone in the center. It has been experimentally shown that this improves the

As explained above, the basic knowledge of the present invention is that it is sufficient to apply a large roll reduction with an equiaxed crystal region formed in the center, and therefore electromagnetic stirring is not necessarily the method for coagulating the equiaxed product. For example, a method of applying ultrasonic waves to the slab through rolls may be used, or other methods such as low-temperature casting or adding steel wire into the mold may be used as considerations for operation.

(Example) (A) and (B) shown in Table 1 including the case where the electromagnetic stirring device 4 shown in FIG. 1 and the large reduction roll 5 are combined
.

Casting was carried out under three conditions (C).

Table 1 A comparison was made of case (A) without electromagnetic stirring or large roll reduction, case (B) with only large roll reduction, and case (C) with both electromagnetic stirring and large roll reduction. The installation position of device 4 is 1 from the meniscus in the mold.
The mounting position was selected so that the crystal nuclei generated by stirring the molten steel would not be remelted and a considerable amount of unsolidified area would exist.

The obtained slab was cut in a cross section perpendicular to the casting direction 5, and
The degree of segregation of phosphorus (P) was investigated as a representative sample using a μφ macro analyzer, and the macro segregation of P over a 50'6ffl in the casting direction in the plate thickness direction was also investigated using a 3'''' drill sample. At this time, in the conventional casting method (case A), there is positive segregation S as illustrated in Figure 2, and as shown in Figure 3, semi-micro segregation investigated with a macro analyzer, the maximum segregation degree of P (P/P, , ) was approximately 12. Here, P is the P concentration in the semi-micro segregation region, and Po is P1 degree in the steel. On the other hand, this invention (case C
), it was confirmed that the maximum segregation degree was improved to 7. In case B, where only large reduction was carried out, clear negative segregation was observed in the center as shown in Figure 4, and this caused the molten steel to gradually thicken as shown in Figure 5. A phenomenon occurs and the quality is 'second unfavorable.

In other words, if only large roll reduction is applied, there is a drawback that macroscopic negative segregation occurs in the center, but when applied in combination with a technique for producing equiaxed products in the center, such as electromagnetic stirring, for example. It has become clear that this drawback can be eliminated and that semi-micro segregation can also be significantly improved.

(Effects of the Invention) According to the present invention, it is assumed that the unsolidified portion near the solidification completion point of a continuously cast slab is equiaxed crystallized, and internal cracks are likely to occur at the conventional solidified shell interface near the solidification point. Breaking away from the prevailing conventional ideas, it is possible to effectively eliminate center segregation in continuously cast steel slabs by applying a large reduction with rolls without causing the above-mentioned interfacial inner surface cracks.

[Brief explanation of drawings]

Fig. 1 is a sectional view of a slab along the casting direction showing an embodiment of the present invention, Fig. 2 is a sectional view of the slab perpendicular to the casting direction, and Fig. 3 shows the conventional method of semi-micro segregation and the method of this invention. A graph showing the difference. Figure 4 is a graph showing macro segregation in the plate thickness direction when only large roll reduction is applied. Figure 5 is a comparison graph showing changes in macro segregation in the casting direction. . I... Mold 2... Slab 3... Support roll 4... Electromagnetic stirring device 5... Large reduction roll. CLQ ¥ 1 ＠ ＠ ＠ CLQ; 9JJ Joy G Kuku ■ Procedure Correction Book August 241, 2, 25. Invention continuous casting method 3, Patent applicant to the case (1,25 ) Kawasaki Steel Corporation Telephone number (581) 2241 (Representative) 1 other person Correct 5M of M-side medium pit as shown in the paper correction diagram. Fig. 5 Ofo 20 30 40 50 7F? )

Claims (1)

[Claims] 1. During continuous casting of steel, in addition to equiaxed crystallizing the unsolidified portion near the solidification completion point of the slab, one
A continuous casting method for steel, characterized by applying a large reduction of 3 mm or more per pair to forcibly form a solidification completion point.