JPS58112642A - Continuous casting method for steel - Google Patents

Abstract

PURPOSE:To obtain an ingot of excellent internal quality free from defects in the stage of casting steel continuously by specifying stirring time, the stirring time in a half cycle and the number of stirring cycles while inverting the stirring directions at every prescribed time interval. CONSTITUTION:A large amt. of cubic crystals are formed in the unsolidified molten steel in the final solidifying part of an ingot, and the molten steel is cast continuously while the unsolidified molten steel is turned and stirred around the central axial line of the ingot with an electromagnetic stirrer. In such a case, the stirring directions are inverted at every prescribed time interval, and the stirring time DELTAT is set at DELTAT=L/V>=0.5min. Here, L: the length of the electromagnetic coil of the electromagnetic stirrer (m), V: casting speed (m/min). The stirring time DELTAt second in the half cycle of the stirring is set in a 5<=DELTAt<= 30sec. range. The number n of cycles of the stirring is set at n>=1. Thus the excellent ingot free from defects such as V segregation and porosity is obtained.

Description

[Detailed description of the invention] This invention relates to a continuous casting method for steel.

When casting slabs such as billets and plumes 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. It is generally done to generate a large amount of equiaxed crystals 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. ing.

However, even if a large amount of equiaxed crystals are generated 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.

The inventors of the present application have conducted various studies on electromagnetic stirring methods for unsolidified molten steel in the final solidification zone, and have found that unsolidified molten steel is continuously stirred in one direction around the central axis of the slab using a rotating electromagnetic stirring device. The conventional method of electromagnetic stirring cannot improve the internal quality of the slab, 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, It was found that the internal quality of the slab can be improved, and that the effect of stirring is lost when the half-cycle time of forward and reverse alternating stirring, that is, jt in FIG. 1, is out of a predetermined range.

This invention was made based on the above knowledge, and when continuously casting steel, a large amount of equiaxed crystals are generated in the unsolidified molten steel in the final solidification part of the slab, and the unsolidified molten steel is heated by an electromagnetic stirring device. In a continuous steel casting method in which the slab is stirred by rotating around its central axis, the unsolidified molten steel is stirred for a stirring time (Δ'T) determined by the following formula while reversing the stirring direction at predetermined time intervals.
) Stir for a while.

ΔT! =L/V≧0.5 (mtn) However, L: Electromagnetic coil of electromagnetic stirring device le length -), V: Casting speed (m/min).

Half cycle stirring time Δt (sec) in the above stirring
is in the range of 5≦Δt≦30 (sec), and the number of stirring cycles is set in the range of ≧1.

Next, the reasons for limiting the numerical values in this invention will be explained.

The inventors of the present application have installed a magnetic stirring device on the short side 400.
Steel having the composition shown in Table 1 was cast at a molten steel superheat degree of 30° C. using a vertical continuous casting machine equipped with a mold having a long side of 520*es+. At this time, various types of rotary electromagnetic stirring devices of 60 to 1200 KVA were installed in the final solidification zone 16 m below the mold to stir the unsolidified molten steel in the final solidification zone.

The stirring time ΔT at this time is the length of the electromagnetic coil of the electromagnetic stirring device L (m), the casting speed @ V (yyz/m1n)
If so.

It was defined as ΔT = L/V (min) - (1).

Under these conditions, casting was carried out by variously changing ΔT and Δt (half-cycle stirring time (sec) at ΔT),
The center segregation and porosity present in the center of the slab after casting was 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 understand.

ΔT ≧ 0.5 (min) River (
2) 5≦Δt≦30 (sec) −・(3) Furthermore,
The number of stirring cycles 71 (A-B in Figure 1 is one cycle) requires at least 1 cycle A, and this condition is a straight line in Figure 1! It is shown in

From the above, in this invention, ΔT, Δt and n
f is limited as described above.

Next, embodiments of the invention will be described.

Short side 400iem, long side 520iem with electromagnetic stirring device installed
Using a vertical continuous casting machine equipped with a M rectangular cylindrical mold, steel having the composition shown in Table 1 was cast at a molten steel superheating degree of 30°C and a casting rate of 1 in accordance with conditions 1 to 4 shown in Table 2. Each was cast at a rate of 0.5 m/min. At this time, an electromagnetic stirring device having an electromagnetic coil having a length of 1000 m+ was installed in the final solidification part 16 m below the mold. Then, the porosity generation 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 solidified part 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 when the unsolidified molten steel in the final solidified part is cast by the method of the present invention. This is the case.

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 the method other than the method of the present invention. It can be seen that a slab of excellent quality was cast.

Next, another example will be described.

Equipped with a 210φ cylindrical mold equipped with an electromagnetic stirring device,
Steel having the composition shown in Table 3 was cast at a molten steel superheating degree of 30°C according to conditions 5 to 8 shown in Table 4 using an arc type continuous casting machine having a curved trajectory of 11.5 mH. Each was cast at a speed of 1-8 rn/min. In this case, the solidification end point (crater end) at the center of the slab is located 27 m along the slab from the molten steel surface in the mold.
It had reached the horizontal orbit of the slab. The electromagnetic coil of the electromagnetic stirrer had a length of 1800 m and was installed in the final solidification section. Then, the porosity index at the center of the cast slab was investigated. The results are shown in FIG.

In Table 2, Condition 5 is the case where the unsolidified molten steel in the final solidification part is not stirred, Conditions 6 and 7 are the cases where Δt is outside the range of the present invention, and Condition 8 is the case when the unsolidified molten steel in the final solidified part is cast by the method of the present invention. This is the case.

As is clear from Table 3 and Figure 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 the method other than the method of the present invention. It can be seen that a slab with excellent internal quality was cast.

In addition, as in the case of the above example, if the final solidified part of the slab reaches the horizontal orbit, the equiaxed crystals will settle midway, and a sufficient amount of equiaxed crystals will reach the final solidified part. Since no crystals are produced, in this case, good results can be obtained by installing another electromagnetic stirrer in the middle of the circular arc orbit to increase the production of equiaxed crystals.

As explained above, according to the present invention, useful effects such as being able to obtain a cast slab with excellent internal quality free of defects such as V segregation and porosity are brought about.

[Brief explanation of the drawing]

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 FIG.
The figure and FIG. 4 are diagrams showing the porosity generation index. Applicant Nippon Kokan Co., Ltd. Agent Keitabe Tsutsumi (1 other person)

Claims (1)

[Scope of Claims] When delivering steel continuously, a large amount of equiaxed crystals are generated in the unsolidified molten steel in the final solidification section of the slab, and the unsolidified lh+ molten steel is stirred using an electromagnetic stirring device to align the central axis of the slab. In a method of continuously casting steel while stirring the dust at the center. Mti while reversing the stirring direction at predetermined time intervals.
The unsolidified molten steel is stirred for a stirring time (Δ
Stir for ΔT = L/V≧0.5 (min), where L: length of the electromagnetic coil of the electromagnetic stirring device (rrL). V: Casting speed (m/m1n). Half cycle stirring time Δt (sec) in the above stirring
A continuous casting method for steel, characterized in that Δt is in the range of 5≦Δt≦30 (sec), and the number of stirring cycles is ≧1.