JPS6235855B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a continuous casting method for steel.

Conventionally, when continuously casting steel using a mold with both ends open, a method was adopted in which vibration was applied to the mold to prevent seizure between the mold and the shell, and to promote lubrication between them. That is, in this method, as shown in FIG.
When the slab with the shell 5 formed thereon is gradually pulled out from the lower part of the mold 1, vibration is applied to the mold 1 by the mechanical vibration means 6, thereby causing the mold 1 and the shell 5 to separate. This prevents seizure and promotes lubrication during casting.

The mechanical vibration means 6 cause the mold 1 to vibrate at a rate of several 10 per minute.
Vertical vibrations with an amplitude of several millimeters to several tens of millimeters are applied over 10 to 100 times. In this case, in order to apply compressive force to the solidified molten steel, promote healthy growth of shell 5, and enable continuous drawing, the negative strip ratio (NS) expressed by the following formula is set to 60 to 80%. The amplitude and frequency are selected as follows.

In addition, the negative stripping rate refers to the percentage of time when the descending speed of the mold is lowered by vibration in the same direction as the slab pulling direction, and the rate of descent is greater than the slab pulling speed, and the negative stripping time is: Tell me the time at that time.

However, V: casting speed, A: swing width, f: frequency of vibration.

However, in recent years, in continuous steel casting, the following problems have arisen due to the increasing need to speed up casting in order to improve production efficiency.

That is, in high-speed casting, the thickness of the shell solidified within the mold becomes thinner, so that even if slight restraint occurs between the shell and the mold wall, the shell will break. This phenomenon is particularly likely to occur in the solidified shell near the meniscus, and as casting continues, the fracture moves below the mold, causing breakout.

As a preventive measure for the above-mentioned accidents, it is necessary to select a molding powder that exhibits suitable melting characteristics and physical properties to improve the lubrication performance between the mold and the solidified shell. However, it is very troublesome to select the appropriate mold powder according to the casting steel type, and the lubrication performance between the mold and the solidified shell is greatly influenced by the amount and distribution of mold powder that flows in. Therefore, it is necessary to control not only the physical properties of the powder described above but also the powder inflow mechanism.

Therefore, in the past, breakouts frequently occurred under high-speed casting.

From the above-mentioned viewpoint, the inventors of the present application have determined that the consumption amount of mold powder, which greatly contributes to lubrication between the mold and the solidified shell, is determined by the positive strip time (t _p ) expressed by the following equation. We investigated the relationship between powder consumption and _tp .

t _p =1/πfcos ^-1 (-V/2πAf)...(2) However, V: casting speed, f: frequency, A: amplitude.

It should be noted that the positive strip time is the sum of the time during which the descending speed of the mold is lower than the withdrawal speed of the slab when the mold is lowered by vibration in the same direction as the slab drawing direction, and the mold rising time.

As a result, as shown in Fig. 2, increasing t _p can increase powder consumption, and therefore, by setting t _p to a predetermined value or more, lubrication between the mold and solidified shell can be increased. We have obtained knowledge that performance is improved and breakouts that occur during high-speed casting can be prevented.

The present invention has been made based on the above-mentioned knowledge, and includes a method for continuously casting steel by adding mold powder to a mold with open ends, which comprises: applying ultrasonic vibration to the mold; Casting is performed while applying mechanical vibration whose positive strip time (t _p ) satisfies the following formula: t _p >0.7/f (min), where f: frequency of vibration.

This feature improves the lubrication performance between the mold and the solidified shell and prevents the occurrence of breakouts.

An embodiment of the method of the present invention will be described with reference to the drawings.

FIG. 3 is a schematic explanatory diagram showing the method of the present invention.

In FIG. 3, the same numbers as in FIG. 1 indicate the same parts, 8 is an ultrasonic vibrator attached to the mold 1, and 9 is an ultrasonic generator.

This invention applies ultrasonic vibration to the mold in order to ensure a certain amount or more of mold powder inflow and improve the lubrication performance between the mold and the solidified shell, and a machine that satisfies the following conditions. This applies vibration to the mold.

t _p >0.7/f (minutes) ...(3) where, t _p : positive strip time, f : frequency of vibration.

In this invention, the reason why the mold 1 is subjected to ultrasonic vibration is to prevent excessive inflow of mold powder locally and to obtain a uniform powder film. The contact has improved, and casting is now possible even with a negative strip ratio of less than 60%, which was previously considered difficult. The ultrasonic vibration conditions are a frequency of around 20KHz and an amplitude of 2, taking into consideration the material of the mold wall.
~10μm is good.

The above equation (3) is a combination of the above-mentioned equations (1) and (2) and the condition of negative strip ratio NS < 60%, which is made possible by the combination of mechanical vibration and ultrasonic vibration. This was derived by

Figure 4 shows casting speed V = 2m/min, swing width A = ±
Frequency f, negative strip rate NS and positive strip time t _p when set to 4 mm
Indicates the relationship between

As is clear from Fig. 4, conventionally it was impossible to cast unless the negative strip ratio NS was 60% or more, but according to the method of this invention, it is possible to cast even if the NS is 60% or less. becomes possible,
Therefore, the positive strip time t _p can be increased, and thereby the amount of mold powder can be increased, which improves the lubrication between the mold and the solidified shell and allows high-speed processing without breakouts. It can be seen that casting can be performed.

Next, embodiments of the invention will be described.

26 ultrasonic transducers were installed in a water-cooled copper mold with a short side of 200 mm, a long side of 800 mm, and a length of 780 mm.
A vibration of 22KHz was applied to the mold at 10KW, and at the same time, the positive strip time t _p was 0.5.
Vibration conditions such that the frequency f=seconds, for example,
Mechanical vibration was applied to the mold under the conditions of 100 cpm, amplitude A = ±4 mm, and NS = 41%.

As a result, uneven inflow of mold powder,
Vertical cracks caused by uneven solidification of the shell are reduced, and the incidence of breakouts is reduced by the combined use of mechanical vibration and ultrasonic vibration, especially as shown in Figure 5. , the effect was noticeable during high-speed casting.

As explained above, according to the present invention, extremely useful effects such as the ability to prevent breakout accidents even during high-speed casting are brought about.

[Brief explanation of the drawing]

Figure 1 is a schematic explanatory diagram of the conventional casting method;
The figure shows the relationship between the positive strip time and the amount of mold powder, Figure 3 is a schematic explanatory diagram of the method of the present invention, and Figure 4 shows the relationship between the vibration frequency, the negative strip rate NS, and the positive strip rate. _FIG . 5 is a diagram showing the relationship between casting speed and breakout frequency. In the drawings, 1... Mold, 2... Tundish, 3... Immersion nozzle, 4... Molten steel, 5... Shell, 6
... Mechanical vibration means, 7... Mold powder, 8...
Ultrasonic transducer, 9... Ultrasonic generator.

Claims (1)

[Claims] 1. A method of continuously casting steel by adding mold powder to a mold with both ends open, in which ultrasonic vibration is applied to the mold and a positive strip time (t _p ) is applied to the mold. Casting is performed while applying mechanical vibration that satisfies the following formula, t _p >0.7/f (min), where f: frequency of vibration. A continuous steel casting method characterized by improving the lubrication performance between the mold and the solidified shell and preventing the occurrence of breakouts.