JP3099158B2 - Continuous casting method of defect-free slab - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method for defect-free slabs.

[0002]

2. Description of the Related Art In general, when a slab is cast in continuous casting, a vertical crack is generated on the long side surface of the slab. May cause production difficulties.

[0003] In particular, the carbon content is 0.08 to 0.1
The so-called medium carbon steel of 6% has a high frequency of occurrence of vertical cracks.

Generally, a slab shrinks with solidification, but when the slab is constrained by a mold, the slab cannot shrink and a tensile stress is applied to the long side surface of the slab.

[0005] At this time, in the early stage of solidification where the thickness of the solidified shell is small, the stress cannot withstand the stress and vertical cracks occur in the slab.

In particular, when the solidified shell is non-uniform, stress is concentrated on a thin portion of the shell, and a vertical crack is generated.

[0007] In the case of medium carbon steel having a carbon content of 0.08 to 0.16%, a peritectic reaction occurs upon cooling, and the solidified shell is not uniform because the solidified shell and the mold are separated from each other. It is a type of steel that easily occurs.

Therefore, vertical cracks can be prevented by preventing the slabs from being restrained by the mold or by making the solidified shell uniform so that stress is not concentrated.

As a countermeasure, the use of a mold powder has been conventionally used from the viewpoint of preventing the slab from being restrained by the mold and ensuring uniform cooling of the slab (Japanese Patent Laid-Open No. 3-2785).
0) has been performed.

[0010] This is to make the powder flow uniformly between the slab and the mold to prevent the slab from being restrained by the mold and at the same time to ensure slow cooling of the slab.
Due to the low viscosity of the powder, powder entrainment occurs and the powder is trapped as inclusions in the slab, causing problems such as internal defects.Therefore, it can be applied only to steel types where internal defects are not a problem. There was a problem.

Further, from the viewpoint of dispersing solidification nuclei and ensuring uniform solidification, a coating agent on the inner surface of the mold (Japanese Patent Laid-Open No.
3-180347) or the provision of irregularities on the mold surface (Japanese Patent Application Laid-Open No. 63-160752) has been proposed.
There were problems such as peeling off of the surface coating agent, frequent removal of the surface due to loss of unevenness due to abrasion, and shortening the life of the mold and reducing productivity.

Further, a method of reducing the carbon content to 0.10% or less has also been proposed. However, in this method, another alloy element such as manganese is added to increase the strength of the reduced carbon content. However, there are problems such as an increase in alloy cost.

[0013]

The conventional technology relating to the prevention of vertical cracks causes internal defects, which are other slab defects, in order to prevent vertical cracks, thereby reducing the life of the mold or reducing the alloy cost. There was a problem such as raising, and it was not industrially satisfactory.

The present invention does not cause internal defects,
An object of the present invention is to provide a method for preventing vertical cracking without reducing the mold life and without increasing the alloy cost.

[0015]

DISCLOSURE OF THE INVENTION The present invention has been made to advantageously solve the above-mentioned problems, and the gist of the present invention is to provide a method for continuously casting a slab in which molten steel flows in a mold. The present invention relates to a method for casting a defect-free slab, wherein the direction of rotation of molten steel flow is given alternately in left and right directions.

[0016]

The present inventors have carried out various experiments and studies for the uniformization of the solidified shell, and have found that the uniformity of the solidified shell can be achieved by applying molten steel flow to the solidification interface.

That is, as described above, when unevenness occurs in the solidified shell due to uneven inflow of the mold powder,
When molten steel flow is applied, the solidification tip is supplied with higher temperature molten steel than the solidification delay part, the solidification speed of the solidification tip is reduced, and at the same time, the solidification tip is scraped off and the solidification interface is leveled. It was found.

The method of applying high-temperature molten steel to the solidification interface can be easily achieved by using an in-mold electromagnetic stirring device.

However, as a result of further study, even if the conventional electromagnetic stirring in the mold was applied as it was, no effect could be exerted on the prevention of vertical cracks on the entire long side of the slab.

That is, as shown in FIG. 1, the molten steel is normally discharged to the long side of the slab through the immersion nozzle 4.

Therefore, the reverse flow 3 of the discharge flow 7 flows in the direction of the nozzle 4 near the meniscus.

Therefore, as shown in FIG. 2, on the slab surfaces a and c, the direction of the reversal flow of the discharge flow 3 and the direction of the electromagnetic stirring flow 5 in the mold coincide, and the solidification interface flow velocity can be secured.
In the case of d, the reverse flow 3 of the discharge flow and the electromagnetic stirring flow 5 in the mold cancel each other out, and the flow velocity at the solidification interface could not be secured.

Next, the in-mold electromagnetic stirring flow 5 which overcomes the reverse flow 3 of the discharge flow was applied, but mold powder was entrained on the slab surfaces a and c, causing internal defects.

Then, the rotation direction of the molten steel flow at the solidification interface by the electromagnetic stirring device 1 in the mold was given to the left and right alternations, and as a result, vertical cracks could be prevented on the slab surfaces a, b, c and d.

The molten steel flow velocity at the solidification interface is preferably 10 to 25 cm / s at a portion where the reverse flow 3 of the discharge flow and the electromagnetic stirring flow 5 in the mold are in the same direction.

If it is less than 10 cm / s, the application of high-temperature molten steel to the solidification interface is small, and leveling of the solidification interface may be insufficient. If it is more than 25 cm / s, the meniscus flow rate becomes too fast, and powder entrainment may occur. It is not preferable because there is a possibility that the internal defect may increase.

The cycle of the left and right alternation is 15 to 25.
s is preferred. In the portion where the reverse flow 3 of the discharge flow and the electromagnetic stirring flow 5 in the mold cancel each other, the molten steel flow velocity at the solidification interface is 0 cm / s.
May occur, but the cycle of the left and right
If it is set to s, vertical cracks can be prevented.

That is, if the time is less than 15 seconds, it is difficult to sufficiently level uneven solidification, which is not preferable.

On the other hand, if the flow rate of the molten steel at the relevant site is 10 cm / s due to the reverse electromagnetic force, if the flow rate exceeds 25 s, uneven solidification proceeds too much, and sufficient leveling cannot be achieved.

[0030]

Example 1 Component C held in a ladle: 0.13%
Si: 0.02%, Mn: 0.45%, P: 0.016
%, S: 0.015%, Al: 0.035% molten steel through a tundish and a reverse Y-shaped 35 ° immersion nozzle at a casting speed of 1.5 m / min, a width of 1700 mm, and a thickness of 24.
It was cast into a 5 mm mold.

At this time, using the in-mold electromagnetic stirring device 1 shown in FIG. 2, the molten steel flow velocity at the solidification interface is such that the reverse flow 3 of the discharge flow and the in-mold electromagnetic stirring flow 5 are in the same direction (a, c). ) And (b, d), the flow rate was set to 20 cm / s, and the direction of the electromagnetic stirring flow 5 in the mold was changed to left and right at 20 s.

After casting, the surface of the slab was visually observed to measure the length of vertical cracks and the amount of inclusions in the slab by the slime method. No crack was observed on the slab surface. There was no large inclusion of 250 μm due to the mold powder.

[0033]

Comparative Example 1 Component C held in a ladle: 0.13%,
Si: 0.02%, Mn: 0.45%, P: 0.016
%, S: 0.015%, Al: 0.035% molten steel through a tundish and a reverse Y-shaped 35 ° immersion nozzle at a casting speed of 1.5 m / min, a width of 1700 mm, and a thickness of 24.
It was cast into a 5 mm mold. At this time, the in-mold electromagnetic stirring device was not used.

After casting, the surface of the slab was visually observed to measure the length of vertical cracks and the amount of inclusions in the slab by the slime method. A vertical crack of 0.3 m per 1 m of the slab length occurred on each of the slab surfaces a, b, c, and d. There was no large inclusion of 250 μm due to the mold powder.

[0035]

Comparative Example 2 Component C retained in a ladle: 0.13%,
Si: 0.02%, Mn: 0.45%, P: 0.016
%, S: 0.015%, Al: 0.035% molten steel through a tundish and a reverse Y-shaped 35 ° immersion nozzle at a casting speed of 1.5 m / min, a width of 1700 mm, and a thickness of 24.
It was cast into a 5 mm mold.

At this time, using the in-mold electromagnetic stirring device 1 shown in FIG. 2, the molten steel flow velocity at the solidification interface is set at a position where the reverse flow of the discharge flow and the in-mold electromagnetic stirring flow are in the same direction.
cm / s, and the electromagnetic stirring in the mold provided a flow in a certain direction.

After casting, the surface of the slab was visually observed to measure the length of vertical cracks and the amount of inclusions in the slab by the slime method.

No cracks were observed on the slab surfaces a and c, but a vertical crack of 0.5 m per slab length of 1 m occurred on the slab surfaces b and d. It was found that there was a portion where the vertical cracks were worse than in the case where it was not provided.

There were no large inclusions of 250 μm due to the mold powder.

[0040]

Comparative Example 3 Component C held in a ladle: 0.13%,
Si: 0.02%, Mn: 0.45%, P: 0.016
%, S: 0.015%, Al: 0.035% molten steel through a tundish and a reverse Y-shaped 35 ° immersion nozzle at a casting speed of 1.5 m / min, a width of 1700 mm, and a thickness of 24.
It was cast into a 5 mm mold.

At this time, using the in-mold electromagnetic stirring apparatus 1 shown in FIG. 2, the molten steel flow velocity at the solidification interface is set to 20 cm / cm at a portion where the reverse flow of the discharge flow and the in-mold electromagnetic stirring flow cancel each other.
s, and the electromagnetic stirring in the mold provided a flow in a certain direction.

After casting, the surface of the slab was visually observed to measure the length of vertical cracks and the amount of inclusions in the slab by the slime method. No cracks were observed on any of the slab surfaces a, b, c, and d, but four large inclusions of 250 μm due to mold powder were detected per 10 kg of molten steel, and it was found that internal defects were worse.

[0043]

As described above, according to the method of casting a defect-free slab according to the present invention, the electromagnetic stirring device is installed in the mold, and the rotation direction of the molten steel flow at the solidification interface is simply given to the left and right alternations. It is possible to prevent vertical cracks without causing internal defects of the piece, reducing the life of the mold, and increasing the cost of the alloy.
A continuous casting method that is industrially effective, that is, improves productivity, has become possible.

[Brief description of the drawings]

FIG. 1 is an elevational view showing a method of giving a flow to molten steel in a mold.

FIG. 2 is an explanatory view showing a method of giving a flow to molten steel in a mold, and is a plan view.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 In-mold electromagnetic stirring apparatus 2 Mold 3 Hot water flow by reverse flow 4 Immersion nozzle 5 Hot water flow by electromagnetic stirring 6 Mold powder 7 Discharge flow 8 Downflow 9 Molten steel 10 Solidified shell

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-63-16841 (JP, A) JP-A-63-104763 (JP, A) JP-A-57-75270 (JP, A) 64953 (JP, A) JP-A-55-158859 (JP, A) JP-A-6-71403 (JP, A) JP-A-6-182518 (JP, A) JP-A-6-226409 (JP, A) JP-A-60-37251 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/115 B22D 11/04 311

Claims (1)

(57) [Claims] 1. A method for casting a defect-free slab, wherein the direction of rotation of molten steel flow at a solidification interface is alternately provided in a continuous casting of a slab that imparts molten steel flow in a mold.