JPH04178240A - Continuous casting method for stainless steel - Google Patents

Abstract

PURPOSE:To reduce surface defects of stainless steel ingot by supplying heat to a meniscus part of molten metal in a casting mold from a heating part installed on the inner surface of the casting mold and making oscillation of the casting mold into non-sinusoidal oscillation. CONSTITUTION:Heat is supplied from the heating part 15 installed on the inner surface of the casting mold 10 to the meniscus part of molten metal II in the casting mold to give non-sinusoidal oscillation to the casting mold. The wave form of this non-sinusoidal oscillation is normally called a bias sinusoidal wave form. When the normal bias sinusoidal wave form is compared with a sinusoidal oscillation wave form, it is the same in an amplitude, a number of oscillation, and in a displacement wave form of one cycle, a sinusoidal wave form displacement having a short cycle continues from a top dead center to a bottom dead center, then, a sinusoidal wave form displacement having a long cycle connected with the bottom dead center continues from bottom dead center to the top dead center. In this way, inflow of powder into the casting mold, clearances of the ingot is secure sufficiently, a trap ratio of floated non-metal inclusion to molten powder is improved to reduce surface defects of the stainless billet and ingot.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a continuous casting method for stainless steel, and more particularly to a method for improving surface properties.

[Conventional technology and issues] Continuously cast stainless steel slabs have a smaller amount of scale that peels off from the surface than ordinary continuous cast carbon steel slabs, so oscillation marks remain on the product. There's a big fear.

In order to reduce the depth of the oscillation mark, it is necessary to set the mold vibration to a so-called short-stroke, high-cycle operating condition in which the amplitude is small and the frequency is large. Fluctuations in the melt level are an operational factor that affects the depth of the fusion mark, and the most effective way to reduce this is to reduce the flow of the molten metal in the mold. However, by doing this, there is a tendency that the heat supply from the inflowing molten metal to the powder and molten metal near the molten metal surface is insufficient. Therefore, the amount of melted powder decreases. As a result, the amount of molten powder that flows between the inner surface of the mold and the slab decreases, the thickness of the powder that flows in becomes thinner, and the amount of heat removed from the slab increases. In other words, it becomes easier to cool the slab.

It is well known that the solidified shell that forms near the molten metal surface bends inward due to mold vibration, forming so-called claws and causing oscillation marks. If it becomes easy, the mold develops and becomes larger. As described above, the conventional so-called short stroke/high cycle operating conditions are not aimed at reducing the oscillation mark, and may end up increasing the oscillation mark, contrary to the intended purpose.

The present invention was made in view of the above circumstances, and aims to reduce fluctuations in the hot water level and reduce oscillation marks.
It is an object of the present invention to provide a continuous casting method for stainless steel that can reduce surface flaws.

[Means and effects for solving the problems]] The continuous casting method for stainless steel of the present invention supplies heat from a heat generating part provided on the inner surface of the mold to the meniscus of the molten metal in the mold, and converts vibrations of the mold into non-sine vibrations. It is characterized by:

The waveform of the non-sine vibration is usually called a biased sine waveform. When comparing a normal deviation sine waveform with a sine vibration waveform, the amplitude and frequency are the same, and the displacement waveform with a - period is a sine waveform displacement with a short period from top dead center to bottom dead center, followed by a sine waveform displacement with a short period from top dead center to bottom dead center. Let the displacement from to the top dead center be a sine waveform displacement with a long period connected at the bottom dead center.

[Embodiments] Examples of the present invention will be described with reference to the attached drawings. FIG. 1 is a longitudinal sectional view of the vicinity of a mold for continuous casting in which the method of the present invention is carried out. In the figure, numeral 10 denotes a mold into which molten metal 11 is poured from a ladle (not shown). The injected molten metal is cooled on the inner surface of the mold to form a solidified shell 12. The solidified shell 12 descends as the slab is pulled out by pinch rolls (not shown), is further cooled in a cooling zone under the mold, gradually grows, and finally solidifies to the center, where the molten metal part disappears and reaches complete solidification. . Reference numeral 15 denotes a heat generating part provided at the upper part of the mold, which supplies heat to the molten metal and powder near the meniscus.

The powder for continuous casting that is supplied into the mold from above is heated and melted from the part that comes into contact with the molten metal. The molten powder prevents air oxidation on the surface of the molten metal, and also acts as a lubricant that flows between the mold and the slab and reduces the friction between them that occurs when the slab is pulled out and the mold vibrates. In FIG. 1, 13 is powder powder, and 14 is molten powder. During this time, there is a semi-molten powder,
It is omitted for brevity.

FIG. 2(a) is a graph showing the vibration of the mold, with time and amplitude taken on the horizontal and vertical axes, respectively, and showing a sine vibration 21 and a non-sine vibration 22 which is a biased waveform. FIG. 2(b) shows the vibration velocity V of the mold for a sine vibration 23 and a non-sine vibration 24 which is a biased waveform, on the same time axis as FIG. 2(a). Further, in the same figure, the drawing speed of the slab is shown in VC (constant value). T shown on the time axis indicates a period. For stable casting, it is necessary to pull out the slab without applying compressive force to the fragile solidified shell directly below the meniscus, and in order to apply compressive force to the slab, it is necessary to ensure a certain degree of negative strip as a vibration condition. is necessary. Said negative strip occurs in FIG. 2(b) during the time when the mold is lowered by vibration at a rate faster than the drawing speed Vc of the slab. In the figure, j+ and is indicate negative strip times in sine vibration and non-sine vibration, respectively. Further, in FIG. 2(b), the areas surrounded by Vc and graphs 23 and 24 represent the negative trip amount (length) of sine vibration and non-sine vibration, respectively.

By making the vibration of the mold a biased waveform, as shown in Fig. 2(b)
As shown in Figure 2, under vibration conditions with the same period and amplitude, the negative strip time is reduced compared to the sine vibration, and the growth of the nail at the tip of the solidified shell is suppressed. The wind is generated due to the vibration of the mold, and if the wind is small, the depth of the oscillation mark will be shallow. In addition, the positive strip time, which has a positive correlation with the powder inflow amount, increases, and the inflow amount also increases.

Moreover, the mold vibration of the biased waveform increases the positive stripping time, and the inflow amount of powder, which is positively correlated with this, increases. On the other hand, the amount of compression to the tip of the shell (corresponding to the above-mentioned negative trip amount) is also larger than that of the sine waveform, which strengthens the solidified shell and promotes the flow of molten powder 14 into the mold and the gap between the slab. Ru. This increase in the amount of powder consumed increases the amount of powder consumed, which reduces the frictional force for drawing out the slab and at the same time increases the amount of powder consumed by the mold.
The powder layer in the gap between the slabs becomes thicker, reducing the amount of heat removed from the slabs, and suppressing the growth of claws due to the solidified shell near the mold surface. In this way, the tendency for the inflow amount of the molten powder 14 to decrease due to short strokes and high cycles of mold vibration is alleviated.

Furthermore, in order to suppress fluctuations in the molten metal level, the flow of the molten metal within the mold is minimized, but this results in insufficient heat supply to the vicinity of the surface of the self-molten metal in the mold. Furthermore, since the area near the meniscus in contact with the mold is cooled by the mold, protrusions called slag rim may adhere to the inner surface of the mold, which may prevent powder from flowing into the mold and the gap between the slab. Furthermore, due to the insufficient supply of heat, there is a risk that the fluidity of the molten powder may be reduced and the flow of the molten powder into the gap may not be carried out smoothly.Furthermore, non-metallic inclusions in the molten metal in the mold may float to the surface and cause the molten powder to flow into the gap. 1
4 is expected to be trapped, but it is possible that this trap rate will decrease.

A heat generating part 15 is provided in the upper part of the mold to supply heat to the molten metal and powder near the meniscus, thereby increasing the fluidity of the molten powder and preventing the generation of slag rim.
We aim to promote the flow of powder into the mold and the gaps between slabs,
This prevents the trap rate from decreasing.

The initial crystallization temperature of the powder is 1000°C or higher, 120°C
It is desirable that the temperature is less than 0°C and the basicity is 1.1 or more and less than 12. Outside this range, the solid film of powder that is formed when the molten powder solidifies between the mold and the slab will decrease, the heat removal rate from the slab surface will increase, and there is a risk of cracking on the slab surface. be.

[Effects of the Invention] According to the method for continuous casting of stainless steel of the present invention, heat is supplied to the meniscus of the molten metal in the mold and the mold vibration is made into non-sine vibration, so that the powder does not flow into the gap between the mold and the slab. A sufficient amount of inflow is ensured, and the trapping rate of floating nonmetallic inclusions into the molten powder is improved, reducing surface defects in the stainless steel slab.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal cross-sectional view of the vicinity of a mold used in the practice of the present invention, and FIG. 2 is a graph showing vibrations of the mold. lO...-type, 11... Molten metal, 12... Solidified shell, 13... Molten powder, 14... Powder powder, 15... Exothermic part.

Claims (1)

[Claims] A continuous casting method for stainless steel characterized by supplying heat from a heat generating part provided on the inner surface of the mold to a meniscus of molten metal in the mold, and making the vibration of the mold non-sine vibration.