JPH0763817B2 - Continuous casting method for steel - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a continuous casting method for steel, and more specifically to a steel for producing a slab having excellent surface properties by using a powder having an appropriate viscosity and an appropriate solidification temperature. It relates to a continuous casting method.

(Prior Art) When manufacturing a slab by a continuous casting method, an electromagnetic stirrer is installed in or under the mold to prevent center segregation and skinning of the meniscus, and molten metal (hereinafter, molten steel) Note) is performed. However, when electromagnetic stirring is performed, a difference in height can be created on the molten steel surface in the mold, as shown in FIG. 1 (longitudinal sectional view of the mold for manufacturing a round cast product). That is, the molten steel 1 is swirled by the electromagnetic stirrer 2, and due to the centrifugal force generated at that time, the molten steel level near the immersion nozzle 3 (A part) becomes low and becomes high near the mold 4 (B part). For this reason, the layer of the molten powder 5 becomes thin in the vicinity of the mold, and if the level of the molten metal fluctuates even slightly, the unmelted powder 6 comes into contact with the molten steel 1 and is captured by it. Further, the friction between the swirling molten steel 1 and the molten powder 5 causes a part thereof to be torn, and the molten powder pieces 5a are mixed in the molten steel (C portion). The powder pieces 5a collide with the molten steel flow discharged from the dipping nozzle, flow around in the mold, and are captured by the solidification shell 7.

As described above, when electromagnetic stirring is performed during continuous casting, the surface properties of the slab deteriorate. A slab having a surface flaw must be cared for, which increases the maintenance cost, and a product such as a pipe using the slab as a raw material causes a significant surface defect.

Therefore, various methods have been proposed to solve such a problem. For example, a method of strongly stirring molten steel with an electric mixer [CAMP-ISIJ, Vol. 1 (1988) -312] or a method of increasing the mold taper [CAMP-ISIJ, Vol.
8) -313], etc. However, even by these methods, it is difficult to make the layer thickness of the molten powder uniform and to eliminate the breakage.

(Problems to be Solved by the Invention) An object of the present invention is to prevent thinning of a molten powder and its entrainment that occur when electromagnetically stirring molten steel,
It is an object of the present invention to provide a continuous casting method of a slab that produces a sound slab without surface defects.

(Means for solving the problem) As described above, the deterioration of the surface quality caused by the electromagnetic stirring during continuous casting is caused by the thinning of the molten powder that occurs on the molten metal surface near the mold and the tearing of the molten powder due to the swirling flow. To do.

Therefore, the present inventor conducted various tests described below, considering that it is necessary to know the physical properties (viscosity and solidification temperature) of the powder in more detail.

First, a water model test was conducted to investigate the relationship between the powder viscosity and the powder thinning on the molten metal surface near the mold and the powder entrainment due to the swirling flow.

The test results are shown in FIGS. 2 and 3. FIG. 2 is a diagram showing the relationship between the electromagnetic stirring current and the powder layer thickness near the mold when the powder viscosity is changed variously. In the figure, ○ means viscosity is 0.046 poise, ◎ means 0.48 poise, and ● means 4.8 poise.
This is the case of 5 poises. As is apparent from this figure, the lower the powder viscosity, the thicker the layer thickness, and the higher the powder viscosity, the thinner the layer thickness. The layer thickness decreases as the electric current is increased. FIG. 3 shows the relationship between the electric current and the powder entrainment frequency when the powder viscosity was changed. In the figure,
○, ◎, ● are the cases where the viscosities are 0.046 poise, 0.48 poise and 4.85 poise, respectively. As can be seen from FIG. 3, the lower the viscosity is, the less often the powder is entrapped,
The higher the viscosity, the more frequently it occurs. Further, the entrainment frequency also increases as the electric stirring current increases.

The results shown in FIGS. 2 and 3 due to the difference in powder viscosity are considered to be due to the following causes.
That is, the molten steel is swirled by electromagnetic stirring, but the swirling speed is higher on the mold side than on the dipping nozzle side. In addition, when the powder viscosity is high, the nozzle-side velocity becomes slow because the frictional force between the powder and the immersion nozzle is large. On the other hand, the mold side velocity becomes relatively faster and the molten metal surface becomes higher, so that the powder layer thickness becomes thinner. In addition, the swirling speed increases and the frictional force increases, so that the powder entrapment frequency increases.

Next, a round slab having a diameter of 210 mm was manufactured using powders having different viscosities and solidification temperatures, and the state of flaw generation on the surface of the slab was examined. The results are shown in FIG. In the figure, ◯ indicates that the surface flaw generation index is less than 2 and the surface quality is good, and Δ indicates that it is 2 or more and the surface quality is poor. The surface flaw generation index is the ratio of the number of cast slabs to the number of flaws.

As is clear from FIG. 4, the powder viscosity at 1,300 ° C is 1.0 to 6.0 poise and the solidification temperature is 1,000 to
At 1,200 ° C, the surface properties of the slab are good. However, if it deviates from the range, the surface quality becomes poor. It is considered that this is due to the following.

When the viscosity exceeds 6.0 poise, the layer thickness of the molten powder near the mold becomes thin and the unmelted powder is captured by the molten steel, and the velocity difference of the molten steel swirling flow is large, as revealed by the water model test. Therefore, the entrainment of the molten powder increases.

b. If the solidification temperature is higher than 1,200 ° C, the powder will not be sufficiently solidified, resulting in uneven powder flow into the mold surface and poor lubrication.

c. When the viscosity is less than 1.0 poise or the solidification temperature is lower than 1,000 ° C, the amount of powder flowing into the mold surface increases and the molten powder layer on the molten metal surface becomes thin, and molten steel and unmelted powder contact each other. Part of it mixes with the molten steel.

As can be seen from the above description, when applying electromagnetic stirring during continuous casting, by using a powder whose viscosity at 1,300 ° C. is 1.0 to 6.0 poise and solidification temperature is 1,000 to 1,200 ° C. An excellent cast piece can be manufactured.

(Example) Hereinafter, the continuous casting method of this invention is demonstrated with an Example.
Using a powder having the composition, viscosity and solidification temperature shown in the column of the present invention method in Table 1, a round slab having a diameter of 210 mm was produced under the casting conditions shown in Table 2, and the surface of the slab was visually observed. Inspected for defects. Further, in order to know the effect of the method of the present invention, the powder shown in the column of the conventional method in Table 1 was used to cast under the same casting conditions, and the generation state of surface flaws was examined.

The result is shown in FIG. In the figure, ◯ indicates the case of the method of the present invention, and Δ indicates the case of the conventional method. As is clear from this figure, the surface defect generation index is very low in the case of the method of the present invention. Moreover, the surface flaw generation index is 2 or less even during high speed casting. On the other hand, in the conventional method, the flaw generation index is remarkably high, and it rapidly increases as the casting speed increases.

As is clear from the above, by using a powder having a viscosity and a solidification temperature in the above range, it is possible to greatly reduce the slab surface flaw even in the case of continuous casting in which electromagnetic stirring is performed.

(Effects of the Invention) As described above, according to the continuous casting method of the present invention,
It is possible to significantly reduce the slab surface flaws that occur during continuous casting. In addition, high-speed casting is possible, which greatly contributes to improvement in productivity.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical cross-sectional view of a mold part showing that a height difference occurs in a molten metal surface by electromagnetic stirring in continuous casting, and FIG. 2 is an electromagnetic stirring current when a powder viscosity changes. And the powder thickness of the molten metal near the mold, FIG. 3 shows the relationship between the electromagnetic stirring current and the powder entrainment frequency when the powder viscosity changes, and FIG. 4 shows the powder viscosity and its FIG. 5 is a diagram showing the relationship between the solidification temperature and the quality of the surface properties of the slab, and FIG. 5 is a diagram showing the relationship between the casting speed and the surface flaw generation index in the method of the present invention and the conventional method. 1 is molten steel, 2 is an electromagnetic stirrer, 3 is an immersion nozzle, 4 is a mold, 5 is molten powder, 6 is unmelted powder, and 7 is a solidified shell.

Claims (1)

[Claims] 1. Continuous steel casting characterized by using a powder having a viscosity at 1,300 ° C. of 1.0 to 6.0 poise and a solidification temperature of 1,000 to 1,200 ° C. when continuously casting steel while performing electromagnetic stirring. Casting method.