JPS6348619B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an electromagnetic stirring method for providing a good solidification structure in continuous casting. Molten steel contains various alloying elements and impurity elements other than Fe, 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 measures, one that is considered to be the most promising is the method of stirring the molten steel during the cooling and solidification process using an electromagnetic stirring method, which has been confirmed to be effective in breaking the columnar crystals that grow during the solidification process. However, even if the columnar crystals are slightly broken, it is not sufficient to eliminate the dense segregation. Therefore, in order to make the stirring effect more pronounced, it has been considered to increase the stirring force by increasing the electromagnetic force, but this has the disadvantage that a negative segregation white band is formed. The white band portion has an alloy composition lower than the average value, which not only becomes a material defect but also is not desirable in terms of appearance. The present invention was made in consideration of the current situation, and aims to improve the fracture effect of columnar crystals, reduce negative segregation, and complete electromagnetic stirring conditions that do not involve the formation of white bands. That is. However, in the electromagnetic stirring method for molten steel during continuous casting according to the present invention, an electromagnetic stirring device is installed at a position on the drawing direction side from the drawing position where the unsolidified thickness of the slab is 45% of the thickness in the thickness direction. , The magnetic flux density at the interface between the unsolidified part and the solidified part (hereinafter referred to as the solidified interface) is B (Gauss), the effective stirring length of the electromagnetic stirring device (mm), the casting withdrawal speed is v (m/min), the stirring time T (min) = /v, the point where stirring is applied in the casting direction so that B x T becomes 1600 Gauss min per 1 ^m3 or more for the total unsolidified molten steel volume existing in the drawing direction from the electromagnetic stirring device 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 explained below based on the research progress. 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 caused by the so-called suction (shrinkage phenomenon of the solid-liquid coexistence layer that occurs at the final stage of solidification of molten steel). ), but in this case, not all of the solid-liquid coexistence layer flows at once;
Due to the solidification shrinkage that progresses in advance in the lower part (the side in the drawing direction), the area just above it (mold side) flows downward, and as the flowing area solidifies, the area just above it further flows and solidifies downward. , the periodicity of V segregation is established by repeating the stepwise flow. 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, but this flow is caused by some time lag. It occurs with , and flows sequentially from the bottom side. Therefore, as the flow timing shifts between each region, the dendrite trees open up, and voids with a certain degree of periodicity are formed in the sheath. In addition, there is a temperature gradient in this gap in the direction perpendicular to the slab drawing direction, and a flow of molten steel is formed between the dendrite trees, so the suction effect described above becomes stronger toward the center of the slab. . As a result of the combination of these effects, the above-mentioned void becomes a V-shape inclined toward the axis, and the surrounding concentrated liquid existing between the dendrites flows into this V-shaped void and solidifies, resulting in V-segregation. It seems that it will become. Based on such analysis, the present inventors attempted to change the solidification mechanism described above by adjusting the electromagnetic stirring force to reduce segregation in the axial center of the slab. 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 carbon concentration) and molten steel superheating temperature are thought to influence the size of this region, a statistical investigation of the region where V segregation is actually formed reveals that It was found that the thickness does not exceed at most 45% of the thickness in the thickness direction of the slab. In other words, Figure 1 is a graph showing the relationship between carbon concentration and upper surface equiaxed crystallinity. As seen in the figure, the upper surface equiaxed crystallization ratio is low in the low carbon region and high carbon region, while it is low in the medium carbon region. Extremely high. The reason for this is that in the low coal and high coal regions, the formation of equiaxed crystals is small due to single phase solidification of the δ phase and γ phase, respectively, whereas in the medium coal region, liquid + δ phase→
It is thought that this is because it takes a long time during this transformation to cause two-phase solidification of the γ phase, and as a result, a large number of equiaxed nuclei survive. It is also conceivable that the heat locally generated by the peritectic reaction remelts the dendrite branches from their roots, each becoming the nucleus of an equiaxed crystal. Regardless of these reasons, the top equiaxed crystallinity shown in Figure 1 is expressed as the ratio of the distance from the slab axis to the part where V segregation occurs to the thickness in the slab thickness direction. Corresponding to the conditions shown in the same figure (v is the slab drawing speed,
According to the results of continuous casting (Δt is the degree of superheating of molten steel), it was concluded that the region where V segregation is formed is up to 45% from the axis of the slab in the thickness direction. Obtained. Therefore, in order to achieve the purpose of eliminating the V segregation mentioned above by electromagnetic stirring, we considered that it was necessary to stir this region, and we determined that the unsolidified thickness relative to the thickness of the slab in the thickness direction was 45. 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 where %. Figure 2 is a schematic diagram for explaining the mechanism for reducing V segregation in the present invention. (A) is when no electromagnetic stirring is applied, (B) is when conventional electromagnetic stirring technology is used, and (C) is according to the present invention. The following cases are shown, and in both cases, casting proceeds from top to bottom. Do not apply electromagnetic stirring (A)
Looking at the macrostructure of (B), the columnar crystals reached the center of the thickness of the slab, creating center porosity at the meeting area, but in (B), equiaxed crystals multiplied due to the division of the columnar crystals. Although the solidified structure in the center has been significantly reduced, V segregation and microporosity have not been completely eliminated. However, in (C) according to the method of the present invention, we succeeded in polarizing 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 it in the direction of drawing the slab. In other words, the electromagnetic stirring of the present invention is intended to disperse the flow phenomenon in the casting direction of the V-segregation forming part so as not to concentrate it toward the center. , by forming a temperature gradient in the direction perpendicular to the direction of drawing the slab. Therefore, the concentrated liquid formed at the final stage of solidification is dispersed and solidified in the circumferential direction without V-shaped segregation in the axial center. In addition, when applying such an artificial flow, it is possible to apply it in the opposite direction to the direction in which the slab is pulled out, that is, in the opposite direction, but this is disadvantageous in terms of costs such as power supply capacity, and it is not possible to It is more advantageous to give 3 to 7 are conceptual diagrams showing the state of implementation of the present invention, and one to several electromagnetic stirring devices 2 are provided below the mold 1 at a position closer to the drawing direction than the position that satisfies the above-mentioned conditions. However, in order to achieve the intended purpose of the present invention, it is necessary to define more specific electromagnetic stirring conditions. ) should be 1600 Gauss min/m ³ or more relative to the volume of unsolidified molten steel, so we will explain the circumstances during this time based on experimental results. Table 1 shows that in continuous casting of slabs with a cross section of 380 mm x 550 mm, an electromagnetic stirring device with an effective stirring length of 1300 mm is installed at a position 13 m from the meniscus [a position that satisfies the above installation conditions (45% or less)]. It collectively shows the conditions under which the output is changed. The mm representation of the solidified portion indicates the thickness, and for example, the solidification ratio (%) at a drawing speed of 0.45 m/min was determined by the following calculation. 125+125/380×100=65.8% Also, the unsolidified volume from the stirrer (electromagnetic stirring device) was determined by the following calculation, assuming that the part concerned was pyramid-shaped. (0.3-2 x 0.125) x 0.55-2 x 0.125) x 17 x 1/3 = 0.22 ^m The Gauss values shown in Table 2 were used for calculation.

【table】

【table】
(Unit Gauss)
The magnetic flux density B at the solidification interface was determined from the following equation. B=Boe−τ/δ where Bo: Magnetic flux density on the surface of the electromagnetic stirrer (Gauss) τ: Pole pitch inside the electromagnetic stirrer (mm) δ: Deep penetration depth (mm) ρ: Specific resistance (μΩ): Frequency (Hz) Figure 8 is a graph plotting the values in Table 1, where the vertical axis represents the stirring force (B・T) and the horizontal axis represents the unsolidified molten steel. The volume (mm ³ ) was calculated for each. The ○ mark in the figure is an example in which the center V segregation was reduced, and the ● mark is an example in which ^the effect was not achieved. Also listed. From FIG. 8, it was concluded that when the B·T/m ³ 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 △T is 15 to 40℃.
This is an example when a slab of ×550 (mm) was continuously cast at a drawing speed of 0.6 m/min. The ● mark is a comparative example without electromagnetic stirring, and the ○ mark is a comparative example with an electromagnetic stirring device installed at the unsolidified thickness position of 40%. An example of the present invention is shown in which electromagnetic stirring is performed under the condition of B·T/m ³ =1840. As is clear from the figure, C segregation in the comparative example was extremely high, whereas
In the examples of the present invention, slabs with less C segregation were obtained. Further, no negative segregation occurred, and no white band formation 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 drawings]

Figure 1 is a graph showing the relationship between carbon concentration and top equiaxed crystallinity in continuous casting, Figure 2 is a schematic diagram showing the effects of the present invention, and Figures 3 to 7 are conceptual diagrams showing the implementation status of the present invention. 8 is a graph showing the success or failure of the effect of the present invention in the relationship between unsolidified molten steel volume and stirring force, and FIG. 9 is a graph showing the effect of the present invention on C segregation score.

Claims (1)

[Claims] 1. In manufacturing slabs by continuous casting method,
This is an electromagnetic stirring method that applies an electromagnetic stirring force to the unsolidified molten steel in a drawn slab. An electromagnetic stirring device is installed at the
The magnetic flux density (unit: Gauss) at the interface between the unsolidified part and the solidified part and the stirring time [however, the stirring time is determined by the effective stirring length of the electromagnetic stirring device (mm)/casting withdrawal speed (m/
min) given as a ratio. Unit: min] is the product of
The electromagnetic stirring device is characterized in that it applies stirring in the casting direction to the total unsolidified molten steel volume (unit: m ³ ) existing in the drawing direction from the set position so that the stirring rate is 1600 Gauss min/m ³ or more. Electromagnetic stirring method for molten steel in continuous casting.