JPH0344858B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a continuous casting method for steel. When casting slabs such as billets and blooms using the continuous casting method, it is necessary to perform low-temperature casting, electromagnetically stir the molten steel in the mold, or electromagnetically stir the unsolidified molten steel in the secondary cooling zone of the slab. By doing so, a large amount of equiaxed crystals are generated in the final solidified part of the slab, thereby preventing center segregation or a decrease in the density of the central structure, thereby improving the structure at the center of the slab. is commonly practiced. However, even if a large amount of equiaxed crystals are produced in the final solidified part of the slab by such a method, the structure of the central part of the slab cannot be completely improved. This is because the fluidity of the equiaxed crystals in the final solidification zone is not sufficient for the volumetric shrinkage of the final solidification zone upon completion of solidification, resulting in V segregation and local porosity. As a method for solving the above-mentioned problem, a method has been proposed in which the unsolidified molten steel in the final solidification zone is electromagnetically stirred to improve the fluidity of equiaxed crystals in the final solidification zone. As a result of various studies on electromagnetic stirring methods for unsolidified molten steel in the final solidification section, the inventors of the present application have found that unsolidified molten steel is continuously electromagnetically stirred in one direction around the central axis of the slab using a rotating electromagnetic stirring device. Conventional stirring methods cannot improve the internal quality of slabs, but if unsolidified molten steel is stirred by so-called forward-reverse alternating stirring, in which the stirring direction is reversed at regular intervals, as shown in Figure 1, casting can be improved. It was found that the internal quality of the pieces can be improved, and that the effect of stirring is lost when the half-cycle time of forward and reverse alternating stirring, that is, Δt in FIG. 1, is out of a predetermined range. The present invention was made based on the above knowledge, and includes the following steps: When continuously casting steel to produce a cylindrical or prismatic billet, a large amount of equiaxed crystals are generated in the unsolidified molten steel in the final solidified part of the slab; Then, the unsolidified molten steel is swirled about the center axis of the slab by an electromagnetic stirrer to generate a circulation flow in the unsolidified molten steel in a plane perpendicular to the slab drawing direction. In the continuous steel casting method, the unsolidified molten steel is stirred for a stirring time (ΔT) determined by the following formula while reversing the stirring direction at predetermined time intervals, and ΔT=L/V≧ 0.5 (min) However, L: Length of the electromagnetic coil of the electromagnetic stirring device (m), V: Casting speed (m/min). Half cycle stirring time ΔT in the above stirring
(sec) is in the range of 5≦Δt≦30 (sec), and the number of stirring cycles n is in the range of n≧1. Next, the reasons for limiting the numerical values in this invention will be explained. The inventors cast steel having the composition shown in Table 1 at a molten steel superheating degree of 30°C using a vertical continuous casting machine equipped with an electromagnetic stirring device and a mold with a short side of 400 mm and a long side of 520 mm. did. At this time, 16m below the mold
Various types of rotary electromagnetic stirring devices of 60 to 1200 KVA were installed in the final solidification section to stir the unsolidified molten steel in the final solidification section.

[Table] The stirring time ΔT at this time is calculated by setting the length of the electromagnetic coil of the electromagnetic stirring device to L (m) and the casting speed to V (m/m).
min), ΔT=L/V(min)... (1) Defined as follows. Under these conditions, casting was carried out with various changes in ΔT and Δt (half-cycle stirring time (sec) at ΔT), and center segregation and porosity present in the center of the slab after casting were investigated. As a result, when ΔT and Δt are within the shaded range shown in Figure 2, that is, when ΔT and Δt satisfy the following formula, a healthy slab without center segregation or porosity can be obtained. I understood. Δ≧=0.5 (min) …(2) 5≦Δt≦30 (sec) …(3) In addition, the number of stirring cycles n (A to B in Figure 1 is 1)
cycle) requires one or more cycles,
This condition is shown by the straight line l in FIG. From the above, in this invention, ΔT, Δt, and n are limited as described above. Next, embodiments of the invention will be described. Short side 400mm, long side 520mm with electromagnetic stirring device installed
A vertical continuous casting machine with a rectangular cylindrical mold allows
Steel having the composition shown in Table 1 above was cast at a molten steel superheating degree of 30 DEG C. under conditions 1 to 4 shown in Table 2 at a casting speed of 0.5 m/min. At this time, 1000mm is added to the final solidification part 16m below the mold.
An electromagnetic stirring device was installed with an electromagnetic coil having a length of . Then, the porosity occurrence index in the center of the cast slab was investigated. The results are shown in FIG. In Table 2, Condition 1 is the case where the unsolidified molten steel in the final solidification zone is not stirred, Conditions 2 and 3 are the cases where Δt is outside the range of the present invention, and Condition 4 is the case where Δt is outside the range of the present invention.
This is the case when cast by the method of the present invention.

[Table] As is clear from Fig. 3, the porosity index of the slab cast by the method of the present invention is significantly smaller than that of the slab cast by a method other than the method of the present invention. It can be seen that a slab with excellent internal quality was cast. Next, another example will be described. Using an arc type continuous casting machine with a 210φ cylindrical mold equipped with an electromagnetic stirring device and a curved orbit of 11.5mR, steel having the composition shown in Table 1 was cast at a molten steel superheating degree of 30°C. Conditions 5~ shown in the table
8 at a casting speed of 1.8 m/min. At this time, the solidification end point (crater end) at the center of the slab was located 27 m along the slab from the molten steel surface in the mold, and reached within the horizontal orbit of the slab. The length of the electromagnetic coil of the electromagnetic stirrer is
It was 1800 mm and was installed in the final solidification section. Then, the porosity index of the cast slab and its center was investigated. The results are shown in FIG. In Table 3, Condition 5 is the case where the unsolidified molten steel in the final solidification zone is not stirred, Conditions 6 and 7 are the case where Δt is within the range of the present invention, and Condition 8 is the case where Δt is within the range of the present invention.
This is the case when cast by the method of the present invention.

[Table] As is clear from Fig. 4, the porosity index of the slab cast by the method of the present invention is significantly smaller than that of the slab cast by a method other than the method of the present invention. It can be seen that a slab with excellent internal quality could be cast. In addition, if the final solidified part of the slab reaches the horizontal orbit as in the case of the above example, the equiaxed crystals will settle on the way and a sufficient amount of equiaxed crystals will not be present in the final solidified part. In this case, good results can be obtained by installing another electromagnetic stirring device in the middle of the circular arc orbit to increase the formation of equiaxed crystals. As explained above, according to the present invention, useful effects such as being able to obtain a slab of excellent internal quality free of defects such as V segregation and porosity are brought about.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the stirring pattern of the final solidification section of the slab according to the present invention, Fig. 2 is a diagram showing the relationship between ΔT and Δt, and Figs. 3 and 4 are diagrams showing the porosity occurrence index. It is.

Claims (1)

[Scope of Claims] 1. When continuously casting steel to produce a cylindrical or prismatic billet, a large amount of equiaxed crystals are generated in the unsolidified molten steel in the final solidified part of the slab, and the unsolidified molten steel is electromagnetically cast. In a continuous casting method for steel, the method comprises rotating the slab about its central axis by a stirring device to generate a circulation flow in the unsolidified molten steel in a plane perpendicular to the direction of drawing the slab. , While reversing the stirring direction at predetermined time intervals, the unsolidified molten steel is stirred for a stirring time (Δt) determined by the following formula, Δt=L/V≧0.5 (min) provided that L : Length of electromagnetic coil of electromagnetic stirring device (m), V: Casting speed (m/min). A method for continuous casting of steel, characterized in that the half-cycle stirring time Δt in the stirring is within the range of 5≦Δt≦30 (sec), and the number of cycles n of the stirring is n≧1.