JPH079099A - Continuous casting method for defect-free slabs - Google Patents

Abstract

(57) [Summary] [Purpose] The purpose of the present invention is to prevent longitudinal cracking of so-called medium carbon steel having a carbon content of 0.08 to 0.16% and cast defect-free cast pieces by continuous casting. In a continuous casting of a slab in which a molten steel flow is given in a mold, a method for casting a defect-free slab is characterized in that the rotation direction of the molten steel flow at the solidification interface is applied to the left and right alternations. [Effect] An electromagnetic stirrer is installed in the mold, and the rotation direction of the molten steel flow at the solidification interface is imparted to the left and right alternations, without causing internal defects in the slab and shortening the mold life, and alloying. Vertical cracking can be prevented without increasing costs, and a continuous casting method that is industrially effective in that maintenance can be omitted and productivity can be improved has become possible.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art Generally, when a slab is cast in continuous casting, vertical cracks occur on the long side surface of the slab, and the maintenance of the cracks not only leads to a decrease in the yield of the slab but also leads to breakout, which is a major problem. This may cause production failure.

In particular, the carbon content is 0.08 to 0.1.
The occurrence of vertical cracking is high in 6% so-called medium carbon steel.

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

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

In particular, when the solidified shell is non-uniform, stress concentrates on the thin shell portion and vertical cracking occurs.

In the case of medium carbon steel having a carbon content of 0.08 to 0.16%, a peritectic reaction occurs with cooling, and the solidified shell and the mold are separated from each other, so that the solidified shell becomes nonuniform and vertical cracking occurs. It is a type of steel that is easily generated.

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

As a preventive measure, use of mold powder has been conventionally used from the viewpoint of preventing restraint of a cast piece by a mold and ensuring uniform cooling of the cast piece (Japanese Patent Laid-Open No. 3785/1993).
0) is performed.

This is to inject powder evenly between the slab and the mold to prevent the slab from being restrained by the mold and at the same time to ensure gentle cooling of the slab.
Since the viscosity of the powder is low, there is a problem that powder entrapment occurs and the powder is trapped as inclusions in the slab and causes internal defects, so it can only be applied to steel types where internal defects do not pose a problem. There was a problem.

Further, a coating agent for the inner surface of the mold is disclosed from the viewpoint of dispersing the solidification nuclei to ensure uniform solidification (Japanese Patent Laid-Open Publication No. Sho 6-62).
3-180347) or imparting irregularities to the mold surface (Japanese Patent Laid-Open No. 63-160752), etc.
There are problems that the surface coating agent is peeled off, the unevenness disappears due to abrasion, the frequency of reworking of the surface is increased, the life of the mold is shortened, and productivity is lowered.

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

[0013]

In order to prevent vertical cracks, the conventional technology for preventing vertical cracks causes internal defects, which are other slab defects, to shorten mold life or reduce alloy cost. There were problems such as raising it, and it was not industrially satisfactory.

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

[0015]

The present invention has been made to advantageously solve the above-mentioned problems, and its purpose is to provide a solidification interface in continuous casting of a slab for imparting molten steel flow in a mold. The present invention relates to a method for casting defect-free cast slabs, characterized in that the rotating direction of the molten steel flow is applied to the right and left alternations.

[0016]

The present inventors have conducted various experiments and studies for making the solidified shell uniform, and found that the solidified shell can be made uniform by giving a molten steel flow to the solidification interface.

That is, as described above, when the solidification shell is non-uniform due to the non-uniformity of the inflow of the mold powder,
When molten steel flow is applied, higher temperature molten steel is supplied to the solidification tip compared to the solidification delay part, the solidification rate of the solidification tip slows down, the solidification tip is scraped off, and the solidification interface is leveled. I found it.

As a method for applying the high temperature molten steel to the solidification interface, it is possible to easily use the electromagnetic stirring device in the mold.

However, as a result of further study, the effect of preventing vertical cracks on the entire long side of the cast piece could not be exhibited even if the conventional electromagnetic stirring in the mold was applied as it was.

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

Therefore, the reverse flow 3 of the discharge flow 7 becomes a flow in the direction of the nozzle 4 in the vicinity of the meniscus.

Therefore, as shown in FIG. 2, on the slab surfaces a and c, the reversal flow of the discharge flow 3 and the electromagnetic stirring flow 5 in the mold coincide with each other to secure the solidification interface flow velocity.
In d, the reversal flow 3 of the discharge flow and the electromagnetic stirring flow 5 in the mold were canceled out, and the flow velocity at the solidification interface could not be secured.

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

Therefore, as a result of imparting the rotating direction of the molten steel flow at the solidification interface by the in-mold electromagnetic stirring device 1 to the left and right alternating, vertical cracks could be prevented on all of the slab surfaces a, b, c, d.

The molten steel flow velocity at the solidification interface is preferably 10 to 25 cm / s at the 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 high-temperature molten steel is not applied to the solidification interface so much that the leveling of the solidification interface may be insufficient. If it exceeds 25 cm / s, the meniscus flow velocity becomes too fast, and powder entrapment may occur. It is not preferable because it may increase internal defects.

The right and left alternation cycle 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.
There is a case where it becomes
If it is set to s, vertical cracking can be prevented.

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

On the other hand, if it exceeds 25 s, when the molten steel flow velocity at the site due to the reverse electromagnetic force is 10 cm / s, the uneven solidification proceeds too much, and sufficient leveling cannot be achieved, which is not preferable.

[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% through a tundish and an inverted 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 in a 5 mm mold.

At this time, by 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). ), (B, d), the pressure was set to 20 cm / s, and the direction of the electromagnetic stirring flow 5 in the mold was set to be left-right alternating for 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 cracks were observed on the surface of the slab. There was no large inclusion of 250 μm due to the mold powder.

[0033]

[Comparative Example 1] Component C held in the ladle: 0.13%,
Si: 0.02%, Mn: 0.45%, P: 0.016
%, S: 0.015%, Al: 0.035% through a tundish and an inverted 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 in a 5 mm mold. At this time, the electromagnetic stirring device in the mold 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. Vertical cracks of 0.3 m per 1 m of slab length were generated on the slab surfaces a, b, c, d. There was no large inclusion of 250 μm due to the mold powder.

[0035]

[Comparative Example 2] Component C held in the ladle: 0.13%,
Si: 0.02%, Mn: 0.45%, P: 0.016
%, S: 0.015%, Al: 0.035% through a tundish and an inverted 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 in a 5 mm mold.

At this time, by using the in-mold electromagnetic stirring device 1 shown in FIG. 2, the molten steel flow velocity at the solidification interface is 20 at a portion where the reversal 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 imparted flow in a fixed 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 vertical slabs of 0.5 m per 1 m of slab length were generated on the slab surfaces b and d, and flow was imparted by electromagnetic stirring in the mold. It was found that there was a portion where vertical cracking was worse than when not.

There was no large inclusion of 250 μm due to the mold powder.

[0040]

[Comparative Example 3] Component C held in the ladle: 0.13%,
Si: 0.02%, Mn: 0.45%, P: 0.016
%, S: 0.015%, Al: 0.035% through a tundish and an inverted 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 in 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 20 cm / cm at a portion where the reverse flow of the discharge flow and the electromagnetic stirring flow in the mold cancel each other.
s, and electromagnetic stirring in the mold imparted flow in a fixed 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. Although no cracks were observed on the slab surfaces a, b, c, d, four large inclusions of 250 μm due to the mold powder were detected per 10 kg of molten steel, and it was found that the internal defects were aggravated.

[0043]

As described above, according to the method for casting a defect-free cast piece according to the present invention, the casting can be performed simply by installing an electromagnetic stirrer in the mold and imparting the rotating direction of the molten steel flow at the solidification interface to the left and right alternations. It is possible to prevent vertical cracks without generating internal defects in the piece, shortening the mold life, and increasing the alloy cost.
A continuous casting method that has an industrially great effect of improving productivity has become possible.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a method of imparting a flow to molten steel in a mold, and is an elevation view.

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

[Explanation of symbols] 1 electromagnetic stirring device in mold 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 solidification shell

Claims (1)

[Claims] 1. A method for casting a defect-free slab, characterized in that, in continuous casting of a slab in which a molten steel flow is imparted in a mold, the direction of rotation of the molten steel flow at the solidification interface is applied to the left and right alternations.