JPS6163348A - Continuous casting method of steel - Google Patents

Abstract

PURPOSE:To decrease the central segregation in a billet by limiting the product of a mold thickness and billet drawing speed to a specific value and heating the meniscus part of a molten steel to the solidification start point or above. CONSTITUTION:The value of the product D.V of the thickness Dmm of the mold and the drawing speed Vmm/min of the billet 3 is limited to <= 1.0X10<5> mm<2>/min. A heating flask 7 provided with a heater 6 is provided in the upper part of the mold 1 to heat the meniscus part B of the molten steel 2 to the solidification start point or above. The equiaxed crystal to be generated mainly in the meniscus part B is suppressed by the adequate D.V value and the heating of the meniscus part to the solidification start point temp. or above. The generation of the central segregation in the unsolidified billet 3 is decreased by such method.

Description

[Detailed description of the invention] [Technical backbone of the invention] The present invention relates to a continuous casting method for steel.

[Prior art and its problems]

Conventionally, the following methods have been used to reduce center segregation that occurs in flakes produced by continuous casting.

■ Perform low-temperature preparation or electromagnetically stir the melt in the mold, thereby equiaxing the solidified structure near the center of the slab and dispersing center segregation.

■ Unexamined Japanese Patent Publication No. 49-121738 and % Publication No. 1973
As disclosed in Japanese Patent Publication No. 22777, the interval between the rolls for pulling the slab is gradually narrowed in the direction of pulling the slab, and the melting column due to solidification shrinkage near the completion of solidification at the final stage of solidification of the slab. prevent the movement of

However, according to the above-mentioned method, although it is possible to reduce the center segregation occurring in the slab to some extent, it is not possible to significantly reduce the center segregation.

In addition, JP-A-56-331-ri No. 8 Gong Cave and JP-A-5
Publication No. 8-103942 discloses a method in which a copper piece continuously cast by a conventional method is mechanically divided into two parts at the center cutting/segregation boundary, and the center segregation appearing on the surface layer is removed. According to the above-mentioned method, center segregation can be completely removed, but it is difficult to implement industrially because it requires a special cutting device to divide the slab into two parts. When cutting high-temperature slabs, problems remain in the lifespan of the cutting device.

Furthermore, Japanese Patent Publication No. 55-46265 discloses a method for preventing center segregation by maintaining the temperature of the inner surface of the mold at the mold exit portion of a continuous casting machine at a temperature higher than the solidification temperature of molten steel.

However, according to the above-described method, stable casting cannot be performed because the mold exit position becomes the solidification start point of the molten steel. That is, unless the drawing speed of the slab is carefully controlled, a so-called breakout occurs, in which the shell breaks and the molten steel inside flows out.

[Purpose of the invention]

Therefore, an object of the present invention is to provide a method for casting steel that can easily and significantly reduce the center segregation that occurs in continuously cast slabs.

[Summary of the invention]

In a continuous steel casting method, in which a slab is produced by continuously pulling molten steel poured into a mold downward from a lower part of the mold, the thickness D of the mold and the drawing speed V of the slab The product D−V is 1. OXI
O'mA-m1n-1, and the molten steel cast into the mold is heated so that the temperature of the meniscus portion thereof becomes equal to or higher than the solidification start point of the molten steel, and thus,
It has a special feature in reducing the middle 7uN segregation that occurs in the slab.

[Structure of the invention]

As shown in FIG. 1, in a continuous steel casting method, molten steel 2 is continuously poured into a mold 1, and unsolidified slabs 3 are continuously pulled out from the lower part of the mold 1 to produce slabs. The crater end (solidification completion point) A varies depending on the slab withdrawal speed, but is usually 12 mm from the meniscus B.
It is located ~20m away. Therefore, since a large static pressure of molten steel is applied to the unsolidified slab 3, a phenomenon in which the unsolidified slab 3 swells, so-called bulging, tends to occur, and the thickness of the solid-liquid coexistence zone 4 in the unsolidified slab 3 is thin. , since it is widely distributed,
In the solid-liquid coexistence region 4, a phenomenon in which the shell on the upper surface side and the 7L on the lower surface side come into close contact, so-called bridging, occurs (7 easily).

The above bulging and bridging are the main causes of center segregation.

Therefore, the present invention aims to prevent the occurrence of the above-mentioned bulging 2 and bridging, which are the main causes of center segregation, and obtain a slab free of center segregation.

The present invention will be explained in detail below.

In Fig. 1, the distance between meniscus B and crater end A is L (ms), and the thickness of mold 1 is o.
(ms), the degree of ease with which bridging occurs in the solid-liquid coexistence region 4 can be evaluated by D/L. In other words, the larger the value of D/I, the less likely it is that bridging, which is the cause of segregation, will occur.However, considering actual operation, D/L as a parameter
Since it is more convenient to evaluate the degree of bulging susceptibility using an actual operational parameter than using @, in this invention, instead of D/L, mold thickness CD) u and Slab drawing speed v (shoulder j/min
) The degree of susceptibility to the occurrence of bridging was evaluated by the product DV. That is, the smaller the value of DV, the less likely bridging and bulging, which cause center segregation, will occur.

FIG. 2 shows the relationship between the thickness D of the mold 1 and the above-mentioned DV. As is clear from Fig. 2, the conventional DV range is 1.6 x 105rua・m1n-1≦D-V≦
3.6 x 105j x m1n-1. This is because the main purpose was to increase the rate of drawing slabs and increase productivity.

According to the research of the present inventors, the above DV is 1.0×10
It was found that center segregation can be significantly reduced by reducing the concentration to 5-.m1n-1 or less. However, depending on the casting conditions, a large amount of material may be present in the center of the unsolidified steel billet 3 during casting.
When the equiaxed crystals of 2 are multiplied, a cylindrical segregation 5 may occur in a V-shape as shown in FIG. 31.

In order to prevent the occurrence of the above-mentioned 7-shaped middle □ segregation,
The four steels 2 of the mace cuff 8 portion are heated to a temperature higher than the solidification start temperature of stone steel, that is, higher than the liquidus temperature.

This is because the equiaxed crystals that cause center segregation in the semi-hardened slab 3 are mainly generated in the molten steel 2 in the meniscus 8 part, so the molten steel 2 in the mace cuff 8 part starts solidifying. This is because if heated above the temperature, the generation of equiaxed crystals can be suppressed to Daifuku.

In the present invention, since the above-mentioned D.■ is made smaller than the conventional method, the productivity decreases to some extent.

However, even if productivity is high, if the required quality level is not met, there is no industrial value at all. Therefore, even if -C1 productivity is relatively low, the industrial value of this invention is extremely large.

In order to heat the molten steel in the meniscus to a temperature higher than the solidification start temperature, for example, as shown in FIG. The melt @ 2 is fixed from the top of the heating frame 7 through the casting nozzle 8, and the upper surface level is different, 1 each frame 7
The molten steel 2 in the mace cuff 8 is heated to the solidification start temperature using the heater 6. JB is hot. As a result, it is possible to suppress the equiaxed crystals that mainly occur in the meniscus area, so as shown in Fig. [Effects of the Invention] As explained above, according to the present invention, the product of the mold thickness D and the slab drawing speed V, DV.

1. The occurrence of center segregation in the slab can be reduced by keeping OX 10'IIj-min-1 or less and heating the molten steel in the meniscus to a temperature higher than its solidification start temperature.

[Brief explanation of drawings]

Fig. 1 is a longitudinal sectional view of a continuously cast slab, and Fig. 2 is a diagram showing the relationship between mold thickness D, the product of mold thickness D and slab withdrawal speed V, and D-V. , FIG. 5 is a longitudinal cross-sectional view of a slab produced by a conventional method, FIG. 4 is a cross-sectional view showing a method of heating molten steel in the meniscus portion, and FIG. 5 is a method of the present invention. FIG. In the drawing,

Claims (1)

[Claims] In a continuous steel casting method, in which slabs are continuously produced by continuously pulling molten steel cast into a mold downward from a lower part of the mold, the thickness D of the mold and the slab The product D・V with the drawing speed V of is 1.0
×10^5mm^2・min^-^1, and heating the molten steel cast in the mold so that the temperature of the meniscus portion thereof becomes equal to or higher than the solidification start point of the molten steel,
A continuous steel casting method characterized by reducing center segregation occurring within the slab.