JPS58128254A - Method for stirring molten steel electromagnetically in continuous casting - Google Patents

Abstract

PURPOSE:To prevent the V-shaped dense segregation and negative segregation of an ingot and to make mechanical properties homogeneous by limiting the mounting positions of electromagnetic stirrers, and the product of the magnetic flux density at the interface between an unsolidified part and a solidified part and stirring time. CONSTITUTION:One a number of pieces of electromagnetic stirrers 2 are provided below a casting mold 1, in the positions nearer the drawing direction side than the drawing position where the unsolidified thickness with respect to the thickness of the ingot in the thickness direction thereof is 45%. Molten steel is stirred in the casting direction in such a way that the product of the magnetic flux density at the interface between an unsolidified part and a solidified part and stirring time attains >=1,600 gauss min/m<3> with respect to the volume of the entire unsolidified molten steel existing in the drawing direction side from the positions where the electromagnetic stirrers exist.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electromagnetic stirring method for providing a good solidified structure in continuous casting.

In addition to Fe, various alloying elements and impurity elements are mixed into molten steel, and during the solidification process of molten steel, c, p.

Segregating elements such as S may be concentrated and segregated in the final solidification part of steel ingots and slabs. Products manufactured from materials with such segregation areas exhibit deterioration due to non-uniform mechanical properties, and also suffer from frequent defects during welding, making segregation countermeasures an important issue. Particularly in the continuous casting method, significant segregation occurs in the direction perpendicular to the direction of slab drawing, and although various operating conditions have been investigated, it has not been possible to homogenize the mechanical properties of the slab. I haven't seen it.

Among the conventional countermeasures, one that is considered to be the most promising is the method of stirring the molten steel during the cooling and solidification process using electromagnetic stirring, which has been confirmed to be effective in breaking the columnar crystals that grow during the solidification process. However, since the columnar crystals are broken to some extent, it is insufficient to eliminate the Fi-rich segregation. Therefore, in order to make the stirring effect more noticeable, it was considered to increase the stirring force by increasing the electromagnetic force, but this also has the disadvantage of forming a white band of negative segregation. In the white band area, the alloy composition is lower than the average value, which not only becomes a material defect but also has an unfavorable appearance.

The present invention was developed in consideration of the current situation, and aims to reduce negative segregation by improving the effect of columnar crystal rupture, and to perfect electromagnetic stirring conditions that do not involve the formation of white bands. This is the purpose. Therefore, in the electromagnetic stirring method for molten steel during continuous casting according to the present invention, (an electromagnetic stirring device is provided at a position on the drawing direction side from the drawing position where the unsolidified thickness with respect to the thickness in the thickness direction of the slab is 4511, The magnetic flux density at the interface between the unsolidified part and the solid part of the hood (solidified interface under H) is B (Gauss) The effective stirring length of the electromagnetic stirring device is l' (A.) $6 The withdrawal speed is V (m / sin ) Stirring time T (欝in')
= l/v, the point at which stirring is applied in the casting direction so that BXT is 1600 Gauss/cm3 or more for the total unsolidified molten steel volume existing in the drawing direction from the electromagnetic stirring fracture setting position. The main points are as follows.

The above conditions were set in consideration of the flow situation of molten steel during the solidification process, and the configuration and effects of the present invention will be reviewed below based on the progress of the research.

The causes of segregation in the center of slabs during continuous casting are generally considered as follows.

When looking at the center of the slab in the casting direction (drawing direction), the temperature gradient is extremely small, but the flow of the solid-liquid coexistence layer in such a region is due to the so-called suction (solid-liquid coexistence layer that occurs at the final stage of solidification of molten steel). However, in this case, not all of the solid-liquid coexistence layer flows at once, but due to solidification and contraction that proceeds in advance in the lower part (the drawing direction (Ill)). 1) The upper part of the shingle (mold side) flows downward, and as the flowing part of the face flows, the part directly above it flows further downward and solidifies. By repeating this stepwise flow, the periodicity of segregation is reduced. To explain this situation more schematically, the solid-liquid coexistence state is formed in several regions along the slab drawing direction, and each region flows together. Flow occurs with some time lag, and flows sequentially from the bottom side. Therefore, as the flow timing lags between each region, the gap between dendrite poles opens, and there is also a certain periodicity. Also, within this gap there is a temperature gradient in the direction perpendicular to the slab drawing direction, and a flow of molten steel is formed between the dendrite poles. As a result of these effects overlapping, the above-mentioned void becomes V-shaped with an inclination toward the axis. It is thought that it flows into the figure 7-shaped void, solidifies, and becomes V-segregation as it is.

Based on such analysis, the present inventors thought of changing the above-mentioned solidification mechanism by adjusting the electromagnetic stirring force to reduce segregation at the slab axis/0 section.

That is, the region where V segregation occurs is, after all, a region with a small temperature gradient. Although factors such as molten steel composition (particularly coal purple concentration) and molten steel superheating temperature are thought to influence the size of this region, we have statistically investigated the region where V segregation actually forms. It has been found that the maximum thickness in the thickness direction of the slab does not exceed 45 mm. In other words, Figure 1 is a graph showing the relationship between I# elemental concentration and upper surface equiaxed porosity.As seen in the figure, the upper surface equiaxed porosity is low in the low # elemental region and the high carbon region, whereas Extremely high in the carbon region.

The reason for this is that in the low-order and high-order regions, there is heavy phase solidification of the δ phase and γ phase, so the formation of equiaxed holes is -7x less, whereas in the middle leg region, the liquid + δ phase → r phase It is thought that this is because it takes a long time during this transformation to cause two-phase solidification, and as a result, more equiaxed hole nuclei remain. It is also possible that the heat locally generated by the peritectic reaction remelts the dendrite branches from their roots, which becomes the nucleus of the equiaxed pore. Regardless of these reasons, the top equiaxed porosity shown in Figure 1 is expressed as the ratio of the distance from the slab axis where V segregation occurs to the thickness in the slab thickness direction. Correspondingly, according to the results of continuous casting under the conditions shown in the same figure (V is the slab drawing speed, Δt is the degree of superheating of molten steel), the area where V segregation is formed is the thickness of the flange piece in the thickness direction. It was concluded that this was the part up to 45 meters from the axis. Therefore, in order to achieve the above-mentioned objective of eliminating segregation by electromagnetic stirring, we considered that it was necessary to stir this area, and we determined that the unsolidified thickness relative to the thickness of the slab in the thickness direction was 45 mm. We have reached the conclusion that it is better to provide the electromagnetic stirring device at a position closer to the drawing direction than the drawing position.

FIG. 2 is a schematic diagram for explaining the mechanism for reducing V segregation in the present invention, and (A) shows that when no electromagnetic stirring is applied, CB
) shows the case of the conventional electromagnetic stirring technique, and (C) shows the case of the present invention, and in both cases, casting proceeds from top to bottom. Looking at the macrostructure of (A) without electromagnetic stirring, the columnar crystals reached the center of the thickness of the slab, creating center porosity at the meeting area, but in ω), the columnar crystals were divided, resulting in equiaxed Although the pores have been increased and the coagulation structure in the center has been significantly reduced, it has not yet been possible to completely eliminate segregation and microporosity. However, in (C) according to the method of the present invention, it was possible to polarize the V-shaped segregation angle to an extremely acute angle, in other words, to make it parallel to the surface of the slab, that is, to polarize the light in the direction of drawing the slab. In other words, the electromagnetic stirring of the present invention attempts to disperse the flow phenomenon of the V-segregation forming part in the casting direction in the direction of the center 1b without any noticeable effect. The flow phenomenon is artificially dispersed in the direction perpendicular to the slab drawing direction by forming a temperature gradient in the direction perpendicular to the drawing direction. Therefore, the concentrated liquid formed at the final stage of solidification is dispersed and solidified in the circumferential direction without v5 segregation in the axial center. When applying this kind of artificial flow, it is necessary to apply a force that can be applied in the opposite direction to the direction in which the slab is pulled out, that is, in the opposite direction.
It is disadvantageous in terms of cost such as power supply capacity, and it is more advantageous to provide it in the direction of drawing the slab. 8 to 7 are conceptual diagrams showing the casting process of the present invention, in which one to several electromagnetic stirring devices 2 are installed below the mold 1 at a position closer to the drawing direction than the position that satisfies the above-mentioned conditions. will be established.

However, in order to achieve the intended purpose of the present invention, it is necessary to determine more specific electromagnetic stirring conditions. Since it was concluded that T) should be 1600 Gauss·m/m3 or more relative to the volume of unsolidified molten steel, the circumstances during this time will be explained based on experimental results.

Table 1 shows the effective stirring length / = 11100 in continuous casting of slabs with a cross section of R80 + mXfi 50 m.
This shows all the conditions when the previous electromagnetic stirring device was installed at a position 18 m from the meniscus [a position that satisfies the above-mentioned installation conditions (45 inches or less)] and the output was changed. The resistance indication of the solidified part indicates the thickness, for example, the drawing speed is 0.45 m.
The coagulation ratio (qb) at /m was determined by the following calculation.

80 Also, the unsolidified volume of the Stark (electromagnetic stirring device) is the same considering that the part is pyramid-shaped (calculated as follows: round).

(0,88-2X0.125)X(0,55-2X0
．． 125) Mowing 7 x 0.22 m3 Also, the Gauss values shown in Table 2 were used in the calculations.

Magnetic flux q at the same solidification interface! ! ! The degree B was calculated using the following formula.

B=Boe 7 However, BO: Magnetic flux density P on the surface of the electromagnetic stirring device! (Gauss) τ: Pole pitch in the electromagnetic stirrer (drunk) δ: Deep penetration depth tm ) δ = 5.04F ρ: Specific resistance (μΩ) f double layer wave number (Hz) Plot the values in Table 1 on a graph This is shown in the ts8 diagram, in which the vertical axis represents the stirring force (B-T), and the horizontal axis represents the volume of unsolidified molten steel (m). The ○ mark in the figure is an example in which center V segregation was reduced, and the . mark indicates the effect level (Nigauss m/m). From FIG. 8, it was concluded that when the B-17m value is 1600 or more, the effect of reducing V segregation is significant.

Figure 9 shows a cross section of 380° when the molten steel superheat degree ΔT is 15 to 40°C.
A slab of X 550 (mm ) was pulled out at a speed of 0. Pi
This is an example of continuous casting at mAix, where the * mark is a comparative example without electromagnetic stirring, and the ○ mark is an example where an electromagnetic stirring device was installed at the unsolidified thickness of 40q6 position, and electromagnetic stirring was performed under the condition of B - T / m3 = = 1840. An example of the present invention is shown below. As is clear from the figure, whereas the comparative example had extremely high C segregation, the inventive example produced a slab with low C segregation. Further, no negative segregation occurred, and no formation of white pantoids was observed.

Since the present invention is configured as described above, it is possible to prevent not only the V-shaped concentrated segregation at the axial center of the slab, but also the formation of negative segregation, thereby homogenizing the mechanical properties of continuous cast products. succeeded in doing so.

[Brief explanation of the drawing]

Fig. 1 is a graph showing the relationship between carbon concentration and upper surface equiaxed porosity in continuous casting, Fig. 2 is a schematic diagram showing the effects of the present invention,
8 to 7 are conceptual diagrams showing the implementation status of the present invention,
Figure 9 is a graph showing the effect of the present invention on the relationship between unsolidified molten steel volume and stirring force, and Figure 9 is a graph showing the effect of the present invention on the C segregation score. 2nd rI! J Figure 7

Claims (1)

[Claims] (1) When producing slabs by continuous casting method,
i@ An electromagnetic stirring method that applies an electromagnetic stirring force to unsolidified molten steel in a slab, which is on the side in the pulling direction from the pulling position where the unsolidified thickness is 45 times the thickness in the thickness direction of the slab. An electromagnetic stirring device is installed at the location. The magnetic flux density (in Gauss) at the interface between the unsolidified part and the solidified part and the stirring time [however, the stirring time is given by the ratio of the effective stirring length (m) of the electromagnetic stirring device/the drawing speed in meters (m/m). Stir in the casting direction so that the product of the total unsolidified molten steel volume C (unit: 2 m) existing in the drawing direction from the electromagnetic stirring device setting position is 1600 Gauss xi/m3 or more. A method for electromagnetic stirring of molten steel in continuous casting, characterized by adding: