KR870002050B1 - Electromagnetic stirring method of molten steel in continuous casting - Google Patents

Abstract

The method presents the production of a satisfactory solidification structure of steel in continous casting. The electromagnetic stirrer is placed at a position at 15 to 45%, preferably 20 to 35%, of the thickness of the unsolidified portion in drawing such that the magnetic flux density (in gauss) multiplied by stirring time (min. defined by the ratio of the effective distance for stirring to the casting speed-m/min) is arranged to be more than 1600gauss/min per cubic meter (m3) of the unsolidified molten steel in the portion from the stirrer to the drawing section.

Description

Electromagnetic Stirring Method of Molten Steel in Continuous Casting

1 is a graph showing the relationship between the carbon concentration and the percentage of isotropic crystal parts of the upwardly convex curve during continuous casting.

2 is a schematic diagram showing the effect of the present invention.

3 to 7 are schematic diagrams showing how the present invention is realized.

8 is a graph showing the relationship between the non-solidified steel volume and the stirring force with or without the effect of the present invention.

9 is a graph showing the effect of the present invention on carbon segregation.

The present invention relates to an electronic stirring method for providing a satisfactory solidification structure in continuous casting.

In addition to iron, molten steel contains various alloying components, and in solidification of molten steel, segregation components such as C, P, and S sometimes segregate to a degree corresponding to the final solidified portion of the steel goth or cast slab and bloom. It is important to reduce segregation of products made of materials having such segregation parts because of inferior product characteristics due to the nonuniformity of their mechanical properties and also difficult cases during welding.

In continuous casting, significant segregation proceeds in the direction perpendicular to the drawing direction of the cast slab and the bloom.

However, although several operating conditions have been studied in the past, it has not been successful in improving the mechanical properties of casting slabs and blooms.

The most anticipated method taken so far was to electrostirr the molten steel during solidification.

However, this method has been recognized to have some effect of destroying columnar tablets growing during solidification, but the destruction of columnar tablets is not enough to remove the waste-rock enough.

Therefore, in order to enhance the stirring effect, attempts have been made to increase the electronic stirring force to provide an increased stirring force, but there is a drawback of exhibiting white bands in the form of segregation.

(mm), 주조속도를 V(m/min), 및 교반시간을 T(min)=

을 나타낼 경우 주조방향의 교반을 BXT가 전자교반기가 놓여 있는 위치로부터 인발부까지 연장되어 있는 부분에 있는 비응고용강의 전체적의 ㎡당 1600gauss-min 또는 그 이상의 양이 되게끔하는 것이 특징인 것이다.Considering this serious situation, the present invention seeks to form an electron agitation condition that can enhance the effect of collapsing columnar tablets to reduce segregation and avoid white band formation. Therefore, the electrostirring method of molten steel during continuous casting according to the present invention is an electronic stirrer between drawing parts having a non-solidified thickness of 45% to 15%, preferably 35% to 20%, respectively, in the thickness direction of the casting slab and the bloom. The magnetic flux density at the interface between the non-solidification and solidification portions (this interface is referred to as the solidification interface below) is represented by B (gauss), and the stirring effective distance of the electromagnetic stirrer is shown.

(mm), casting speed is V (m / min), and stirring time is T (min) =

It is characterized by causing the stirring in the casting direction to be 1600gauss-min or more per m2 of the total non-solidified steel in the part where the BXT extends from the position where the electromagnetic stirrer is placed to the drawing part.

The conditions are determined by the flow of molten steel during solidification. The arrangement, function and effects of the present invention will be described according to the improvement process of the present invention.

In continuous casting, the causes of the waste-rock occurring in the center of the casting slab and the bloom are generally considered as follows.

Although in the casting direction (drawing direction) the center of the casting slab and the bloom has little temperature gradient, the flow of solid-liquid coexisting layer in this area is called suction (the solid-liquid layer of the last solidification stage of the molten steel). Contraction phenomenon).

However, not all of the solid-liquid coexisting layers are flowing at the same time, and because of the solidification contraction advancing from the bottom (draw side), the upper part (mold axis) flows downward as well, and as this flow part solidifies, the upper part is also lower. Flows into and solidifies. By repeating this stepwise flow, periodicity of V segregation is formed. We will now describe this situation more schematically.

The solid-liquid coexistence state is formed in several parts according to the drawing direction of the casting slab and the bloom, but these parts flow into the block, but first, the lower part flows, and after some time, these parts flow continuously. Therefore, in accordance with the delay of the flow time between the adjacent portions, the dendrites are separated from each other to form casting holes having some periodicity.

The casting hole has a temperature gradient in a direction perpendicular to the drawing direction of the casting slab and the bloom, and a flow of molten steel is formed between the dendrites, so that the above suction effect increases toward the center of the casting slab and the bloom.

Under this influence, when the casting hole is V-shaped inclined toward the central axis, the liquid phase with segregation around the dendrite is likely to flow into the V-type casting hole to form V segregation.

Based on this analysis, it was thought that by adjusting the electronic stirring force to change the solidification mechanism described above, segregation in the center of the casting slab and the bloom could be reduced.

The part where V segregation occurs becomes the part with little temperature gradient eventually. Factors that determine the size of this part include the composition of molten steel (particularly carbon concentration) and superheat degree of molten steel.However, statistical analysis of the portion where V segregation occurs shows that the maximum value is obtained in the thickness direction of the casting bloom. It does not exceed 45% of the thickness.

The present invention will be described in more detail with reference to the accompanying drawings.

As can be seen in FIG. 1, the percentage of isotropic crystals in the upwardly convex curve is low in the low and high carbon ranges but very high in the intermediate carbon range.

γ상으로 되는 응고가 발생하여 이 변태과정에서 시간이 길게 연장되어 등방성결정을 위한 핵이 더많이 잔존해 있게 되기 때문이라 생각된다. 또한, 공석반응에 의해 국부적으로 생겨나는 열에 의해 분지상결정의 그 분지를 재용융하여 등방성결정을 위한 핵을 제공해 준다고 생각할 수도 있다.This results in less isotropic crystals due to coagulation of single phases δ and γ in the low and high carbon ranges, while the two phases in the intermediate carbon range, i.e., the liquid + δ phase

It is thought that the coagulation of γ phase occurs and the time is prolonged in this transformation process, leaving more nuclei for isotropic crystallization. It is also conceivable to provide a nucleus for isotropic crystals by remelting the branches of the branched phase crystals by the heat generated locally by the vacancy reaction.

The theory is about this.

The percentage of isotropic crystal parts corresponds to the distance from the central axis of the casting bloom to the portion where V segregation takes place, expressed as the thickness ratio of the thickness of the casting bloom.

는 용강의 과열도이다)에서의 연속주조의 결과로서 V편석이 발생하는 부위가 주조불룸의 두께방향에서 볼 때 중앙측으로부터 그 두께의 45%양호하게는 35% 까지 연장된다는 결론을 얻을 수 있다.Under the state seen in the drawing (V is the casting bloom draw rate

As a result of the continuous casting in the molten steel, it can be concluded that the area where V segregation occurs extends from the center side to 45%, preferably 35%, of the thickness in the casting direction. .

Therefore, in order to achieve the purpose of removing the V segregation by electrostirring, it is considered necessary to stir the part, where the thickness of the non-solidified part is closer to the drawn part in the thickness direction of the casting bloom, That is, it was concluded that it is appropriate to place the stirrer at a position of 45% or less, preferably 35% or less.

For this reason, the upper limit of the thickness ratio of the non-solidified portion to the thickness of the cast slab or the bloom should be 45%, preferably 35%, and the lower limit should be 15%, preferably 20%. The reason for this is as follows. Since the amount of remaining non-solidified molten steel of the cast slab or bloom is relatively small and the temperature is apart in the portion where the ratio is below the lower limit, the fact that the molten steel itself is high, that is, the stirring is difficult, and the quality of the cast slab or the bloom This is due to the fact that the improvement effect is reduced.

2 is a schematic diagram illustrating a V segregation reducing mechanism according to the present invention, in which A is a case where electron agitation is not performed, B is a case using a conventional electron agitation technique, and C is a case according to the present invention. In each case, the casting bloom is moved vertically downward.

In the case of A, the macrostructure test shows that the columnar tablet extends to the middle of the thickness of the casting bloom and forms a central pore at the contact part.In the case of B, the columnar tablet is destroyed to increase the isotropic crystal and the solidification structure of the central section is greatly reduced. It can be seen that V segregation and fines are not enough to eliminate voids.

However, in the case of C according to the method of the present invention, the V-shaped segregation angle becomes extremely sharp, that is, the end edge is almost parallel to the casting bloom surface and is oriented in the drawing direction of the casting bloom.

Therefore, the flow of the V segregation portion in the casting direction diffuses without gathering at the center portion by the electron stirring according to the present invention. In particular, due to the contracting force applied at the last stage of solidification, the flow forms a temperature gradient in a direction perpendicular to the drawing direction of the casting bloom and is artificially diffused in this vertical direction.

Therefore, the segregated liquid formed in the final stage of solidification diffuses into the circumference and solidifies without V type segregation. In addition, such an artificial flow may be generated in a direction opposite to that of the casting bloom, but it is uneconomical in terms of power supply capacity, and therefore, should be created in the casting direction of the casting bloom.

3 to 7 are schematic diagrams showing how the present invention is realized.

One or more electronic stirrers 2 are provided at positions closer to the one foot than positions satisfying the above conditions.

However, in order to achieve the intended purpose of the present invention, it is necessary to establish more specific conditions for electron stirring. Here, it was concluded that the product (B · T) of the magnetic flux density (B gauss) at the solidification interface and the stirring time (T · min) should be 1600gauss · min per volume m3 of non-solidified molten steel.

The conditions that led to this conclusion were based on the experimental results described below.

Table 1 shows that in continuous casting of a casting bloom of 380mm × 550mm cross section, the stirring stirrer distance ℓ is 1,300mm, 13m (Test Nos. 1 to 8) or 17m (Test Nos. 9 to 12) from the curved recess. And the block status with the changed out.

The mm mark indicates the thickness of the solidified part.

For example, when the casting speed is 0.45 m / min, the solidification percentage is calculated as follows.

=0.22(㎥). 계산에 이용된 가우스값은 표 2에 나타내었다.Also, the non-coagulated volume from the stirrer can be calculated as follows on the assumption that this part is pyramidal. (0.38-2 × 0.125) × (0.55-2 × 0.125) × 17 ×

= 0.22 (m 3). The Gaussian values used in the calculations are shown in Table 2.

TABLE 1

TABLE 2

On the other hand, the magnetic flux density B at the solidification interface is given by the following equation. In other words,

Where Bo is the magnetic flux density (Gauss on the surface of the electronic stirrer)

τ: pole pitch of electronic stirrer (mm)

δ: penetration depth (mm)

ρ: non-directed (μΩ)

f: frequency (Hz)

8 is a graph showing the values of Table 1, where the vertical axis represents stirring force (B · T), and the horizontal velocity represents non-solidified steel volume. The O mark is the case where the center V segregation is reduced and the O mark is the case where there is no such effect. The vertical / horizontal ratio (unit: gauss · min / ㎥) can also be seen on the graph.

From FIG. 8, it was concluded that the V segregation reduction effect was remarkable when the value of B · T / m 3 became 1600 or more.

9 shows an embodiment in the case of continuously casting a casting bloom having a superheat degree ΔT of molten steel of 15 to 40 ° C. and a cross section of 380 × 550 (mm) at a casting speed of 0.6 m / min.

The O mark is a comparative example in which no electron agitation is performed, and the O mark is an embodiment of the present invention in which the electronic stirrer is placed at a position where the thickness of the non-solidified part is 40%.

As can be seen in the drawings, the comparative example shows a marked C segregation, but the embodiment of the present invention produced a casting bloom with little C segregation. Moreover, no segregation was developed and no discoloration bands were formed.

Since the present invention is aligned as described above, it is possible to prevent formation of not only V-type segregation but also segregation in the center of the casting bloom, which can successfully improve the mechanical properties of the continuous casting product.

Claims (2)

In the manufacture of casting slabs and blooms, in the method of applying electromagnetic stirring force to the cast slabs and the non-solidified molten steel of the blooms, the thickness of the non-solidified portion is 45% to 15% of the thickness observed in the thickness direction of the casting slabs and the bloom Place the magnetic stirrer in the range of%, and the magnetic flux density (unit: Gauss) at the interface between the non-solidified and solidified parts and the stirring time [the ratio of stirring effective distance (mm) and casting speed (m / min) of the electronic stirrer Non-solidified steel so that the value of the product of the unit is min. 1600gauss · min or more per m3 of the total volume (unit: ㎥) of non-solidified steel in the portion extending from the place where the electronic stirrer is located to the draw section. A stirring method for molten steel in continuous casting, characterized in that the steel is stirred in the casting direction. 2. The molten steel in continuous casting according to claim 1, wherein an electron stirrer is placed between the drawing portions in which the thickness of the non-solidified portion is in the range of 35% to 20% of the thickness observed in the casting slab and the thickness direction of the bloom. Electronic Stirring Method.

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