JPS6313653A - Continuous casting method for steel - Google Patents

Abstract

PURPOSE:To stably obtain a cast slab having good surface characteristic at little energy consumption by giving the specific strong ultrasonic wave vibration only at the time of condition when casting velocity is out of the stationary velocity. CONSTITUTION:A continuous casting mold 2 fitting the ultrasonic wave vibrator 1, 1 is used and the casting velocity is monitored and only at the time of occurring >=+20% and <=-20% velocity difference to the stationary casting velocity, a control circuit is formed, so as to become to >=15kHz ultrasonic wave vibrator. In this way, a lubricity between the mold and the cast slab is further easily and stably maintained.

Description

[Detailed Description of the Invention] <Industrial Application Field> This invention effectively uses ultrasonic vibration energy to effectively ensure uniform 2 (powder lubrication) and stably produce slabs with good surface properties. The purpose of this invention is to produce steel with low energy consumption.

<Background technology> In continuous casting of steel, in order to reduce the friction between the mold and the slab to prevent seizure and obtain a stable casting condition,
Along with the use of lubricant (casting powder), mesillation is being used to create vertical movement of the mold.

However, these measures alone are not sufficient for lubrication, and it is inevitable that lubrication will occur when the casting speed is stable, such as when the casting speed increases after the start of pouring or when the casting speed decreases before the end of pouring. There was a concern that the lubricity deteriorated and C seizure was more likely to occur, leading to breakouts. In particular, this tendency is even stronger at the start of pouring because the temperature of the lubricant is low, which poses a major problem in terms of stable operation of continuous casting.

Therefore, in actual operations, front powders with different properties are used as a lubricant for about 30 minutes at the start of casting, that is, until the casting speed stabilizes at the target level (steady casting speed).

This "front powder" has very good sludge-forming properties and rich lubricity, so it is effective in preventing breakouts at the start of casting, but on the other hand, it has poor heat retention properties. It is not expected to have a sufficient effect, so it should only be used for about 30 minutes after the start of casting, and then switched to a casting powder for steady casting (casting at a steady speed) that also takes into account heat retention. This method is generally adopted. However, as mentioned above, front powder was developed with a focus on improving slag formation, so when it is melted, it flows unevenly between the mold and the slab, which causes problems. Therefore, the problem of frequent occurrence of vertical cracks on the surface of the slab was unavoidable.

- On the other hand, when the casting speed decreases before the end of casting, the type of powder is not changed,
Ya 4.1. l: J Casting speed A Due to the stability of ζ, the inflow of molten powder is unrestricted, and even if it does not lead to 'seizing', it is extremely difficult to suppress the occurrence of 1iC on the surface. Man:.

Therefore, the continuous casting of steel j1 has a radius near its top and bottom! 1) It is necessary to carry out severe scarfing in the steady casting part, and there are also parts that have to be cropped and removed due to surface flaws. He was forced to look down.

Of course, constant! /7 Even when the casting speed reaches a steady state, if the casting speed becomes unstable due to large bulging or nozzle clogging, poor lubrication occurs due to uneven powder inflow, and in this case, the surface It was difficult to avoid the occurrence of defects, scratches, and scratches.

For these reasons, in order to improve the lubricity between the mold and the slab and the surface condition of the fJf slab, we have recently applied regular ultrasonic vibration to the mold. A continuous casting method was proposed (Japanese Unexamined Patent Publications No. 11156/1983 and
-11153).

However, in this proposal, [the frequency of the applied ultrasonic vibration is 5
If the frequency is less than KHz, it becomes an audible noise, whereas if it exceeds 50 KHz, it does not function as mechanical vibration, so the frequency should be between 5 and 50 KHz.'' However, when actually implementing this method, it is necessary to use a large ultrasonic vibrator and However, the applied energy was also enormous, and it was not a method that could be easily adopted.

This invention takes into consideration the above-mentioned problems in continuous casting of steel, and has developed a method for achieving good surface quality without requiring a large ultrasonic generator that is difficult to introduce or consuming a huge amount of energy. In order to find a method to stably produce continuously cast slabs, we have developed a system in which ``during steady casting, there is almost no lubrication failure between the mold and the slab, and therefore no surface flaws that pose a particular problem occur.'' This was completed through research such as Invention H, which was carried out with a focus on the findings of Until the filling speed stabilizes, after the casting speed drops below the steady speed as the end of casting approaches, and until the end of casting, or even when casting at a steady speed, due to factors such as nozzle clogging. Only when the casting speed deviates from the steady casting speed, such as when the casting speed becomes unstable, a strong ultrasonic vibration of 15 KIIz or more is applied to create a bond between the mold and the cast J1. It is characterized by maintaining lubrication of 1/1 and making it possible to produce slabs with a good surface ('I-shape) even in unsteady parts at a stable temperature with low energy consumption.

In addition, in this invention, the reason why the ultrasonic vibration intensity (frequency) is limited to 15 KHz or less only when the casting speed deviates from the steady casting speed is because the ultrasonic vibration is relatively weak when casting at a steady speed. This is because while a sufficient lubrication state can be maintained by sonic vibration, when the speed deviates from a steady state, ultrasonic vibration of less than 15 KHz will not maintain a lubrication state sufficient to prevent surface flaws.

Now, this invention is based on the findings of the present inventors obtained through intensive research from various angles on the lubrication characteristics brought about when ultrasonic vibration is applied to a continuous casting mold.
When ultrasonic vibration is superimposed on the oscillation motion of the mold, as long as the casting speed is kept constant, sufficient lubricity can be maintained even if the ultrasonic vibration to be superimposed is 10 KHz or less, but if the casting speed is unstable This method was created based on the fact that lubricity cannot be maintained unless ultrasonic vibrations of at least 15 kHz or more are applied to the lubrication process. A continuous casting mold 2 is used (in FIG. 1, numeral 3 indicates molten steel, 4 indicates casting powder, and 5 indicates solidified shell). In this case, the casting speed is first monitored. By configuring a circuit that controls the ultrasonic vibration to 15 KIIz or more only when a speed difference of ±20% or more from the steady pouring speed occurs, the lubricity between the mold and the slab can be further improved. It can be easily and stably maintained. This is because molten casting powder tends to flow unevenly when the casting speed deviates by more than 20% from the steady pouring speed, which tends to impair the lubricity between the mold and the slab and cause scratches on the slab surface. #1 - Because it is.

By the way, FIG. 2 is a graph showing an example of application of ultrasonic vibration to obtain a continuously cast slab having sufficiently satisfactory surface properties. This graph shows [At the start of casting and at the end of casting]
Although ultrasonic vibrations of 18KIIz were applied in the section where the casting speed decreased by more than 20% at time r and during casting (1e l), only ultrasonic vibrations of 1OKHz were applied at other times when the casting speed was constant. However, even under conditions where the energy applied to sonic vibrations is minimized without applying constant large ultrasonic vibrations throughout the entire casting process, casting Even when the speed is unstable, by applying ultrasonic vibration of a predetermined 141 degrees, sufficient lubricity can be maintained throughout the entire casting process, as found by the present inventors, and continuously cast slabs with good surface properties can be obtained. be.

<Effects of the Invention> The representative effects brought about by this invention configured as described above are fa) It is no longer necessary to use front powder at the start of casting, and cracks at the top of the slab are minimized. fb) Uneven inflow of casting powder at the end of casting is alleviated, surface flaws at the bottom of the slab are reduced, and therefore scarfing and crop loss can be reduced, improving yield. (C) Sufficient lubricity is maintained between the mold and the slab even when trouble occurs during casting.
It is possible to suppress and prevent the occurrence of surface flaws and breakouts, and the use efficiency of ultrasonic vibration energy is extremely high, so there is no significant increase in energy consumption. The specific effects will be explained using examples and comparisons with comparative examples.

<Example> First, the component composition is C + 0.15% (hereinafter, the component ratio is expressed as weight %), Sl + 0.45%, Mn: 1.
41%, P: 0.02%, s: 0.009%, old: 0.01%, Cr: 0.03%, Cu: 0
．． 01%, N: 0.0035%6, balance: After melting medium carbon steel, which is essentially Fe, in a 250-ton converter,
A 2-strand continuous casting machine with a curvature radius of 15rn casts a slab of thickness: 200u x width: 1600 mm at 1.8 m/
Five continuous castings were performed at a casting speed of min (at steady state).

At that time, for the No. and L strands, the normal casting method of only oscillating the S front mold (no superimposition of educational wave vibration) was adopted, and during the initial casting stage, 20 g or 10 kg of commonly used front powder was added. After that, I switched to steady powder and started casting at wl and vt.

On the other hand, in No. 2 S[Land], four ultrasonic vibrators are installed on the long side of the mold and one in the center of the short side, and 18KHz ultrasonic vibration is applied at the start and end of casting. and 1 OKHz low energy ultrasonic vibration is set to be superimposed during constant casting, and from the casting speed monitor, 18Ktlz ultrasonic vibration is applied if the actual casting speed fluctuates by ±20% from the target. I set up the control circuit and started casting. In addition, for No. 2 strand, casting was carried out using steady powder from the initial stage of casting.

At this time, FIG. 3 shows the occurrence of vertical cracks in each slab at the start of casting, and FIG. 4 shows the occurrence of horizontal corner cracks at the end of casting.

As is clear from the above figures 3 and 4, No.
When 20 kg of front powder is introduced in the form of a strand, even if the casting speed becomes stable, the influence of the front powder remains, and vertical cracks occur up to a casting length of approximately 60 m. When the amount of front powder was reduced to 10 kg, the vertical cracks were reduced and their effect became relatively short, but when the obtained slab was observed, traces of breakout in the mold were clearly observed on its surface. . On the other hand, the No. 2 strand had no vertical cracks because no front powder was used, and no breakout occurred even after 1 minute of lubrication using ultrasonic vibration.

Furthermore, there was no horizontal cracking at the end of casting, which was caused by a force (N) in the No. 1 strand (1 and 2 strands).

In addition, this time's 1! Including] Number, 1 The casting speed in steady state is stable and there are no large speed changes during pouring.
The surface quality of the No. 1 strand 1 and No. 2 strand I land was good.

As explained in Section 2, this invention applies ultrasonic vibrations to an oscillating mold during continuous casting of steel, at the start of casting, at the end of casting, and even at regular intervals. The ultrasonic vibration is strengthened to maintain the lubricity of each part, and in addition, the casting powder is uniformly inflowed to suppress surface defects of the cast slab, so it is possible to create a large-sized product that can constantly produce high output. It is possible to stably produce high-yield castings with few surface defects without the need for an ultrasonic vibration generator, and with as little ultrasonic vibration energy consumption as possible.''2. This brings about extremely useful effects industrially, such as increasing the speed of the sludge and tube and quickly achieving a steady casting speed state.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram showing how an ultrasonic vibrator is attached to a continuous casting mold, Fig. 2 is a graph showing how ultrasonic vibration is applied according to the present invention, and Fig. 3 is a start of casting in an example. FIG. 4 is a graph showing changes in casting speed at the end of casting and occurrence of horizontal cracks in the corners in Examples. In the drawings, l... Ultrasonic vibrator, 2... Continuous casting mold, 3...
... Molten steel, 4... Casting powder, 5...
solidified shell.

Claims (2)

[Claims] (1) A method for continuously casting steel while applying ultrasonic vibrations to an on-rate mold, characterized in that strong ultrasonic vibrations of 15 KHz or more are applied only when the casting speed deviates from a steady speed. Continuous casting method for steel. (2) Applying strong ultrasonic vibrations of 15 KHz or more between the start of casting and the time when the steady casting speed is reached, and from the time when the casting at the steady speed ends until the end of the casting.
A continuous casting method for steel according to claim 1. (2) The continuous steel casting method according to claim 1, wherein ultrasonic vibration of 15 KHz or more is applied only when there is a speed difference of ±20% or more with respect to the steady casting speed.