JP2005305533A - Dressing method for steel cast slab - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a dressing method for a steel cast slab, by which the crystal-grain size of the slab is made small by making the solidified structure of the slab as fine as possible after subjecting the slab to a melt-dressing treatment, so that surface cracking and the like in rolling are prevented and thus a cast slab excellent in surface characteristics is obtained. <P>SOLUTION: The dressing method for a steel cast slab restores defects in a surface layer of the steel cast slab by melting the surface layer of the slab with either of induction heating and plasma heating or both of them. In the dressing method, an inert gas is blown to a molten pool in the surface layer of the slab just after the melt treatment, and thus the molten pool is cooled. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for maintaining a continuous cast slab of steel, and particularly to a method for maintaining using at least one of induction heating and plasma heating.

Normally, as a method of cleaning a steel slab cast by continuous casting, the entire surface or partial scarfing of the slab surface by a cutting crater is known, but as shown in (Patent Document 1), a plasma arc is known. Proposal has also been made for a method of surface melting care for slabs. This (Patent Document 1) discloses a method in which a plasma arc is reciprocated in a flat arc shape using an external electromagnetic force in the slab width direction, and the surface of a moving stainless steel slab is melted and maintained over a certain width. .
JP 54-142154 A

In the above (Patent Document 1), it is said that good uniformity of the penetration depth is recognized in the portion melted and re-solidified by the plasma arc. However, with this method, the targeted uniformity of penetration depth has been achieved to some extent, but it has been difficult to sufficiently suppress the occurrence of microcracks on the surface with only melt resolidification treatment. In particular, for continuous cast slabs that require strict surface quality characteristics as at present, satisfactory surface properties cannot be obtained, and there has been a strong demand for improvement.
The object of the present invention was made in order to meet such a demand, and as much as possible, the solidification structure after the melt care treatment of the steel slab is made as fine as possible, the size of the crystal grains is reduced, the surface cracks during rolling, etc. An object of the present invention is to provide a method for caring for a steel slab that can prevent slabs and obtain a slab having excellent surface properties.

The steel slab care method according to the present invention for achieving the above-mentioned object is a method of repairing defects in a steel slab surface layer by melting the surface layer of the steel slab by one or both of induction heating and plasma heating. The slab care method is characterized in that the molten pool immediately after the surface layer is melted is cooled by one or both of spraying an inert gas and roll contact. As a result, the melted portion can be rapidly cooled and solidified, the solidified structure can be reduced, and the crystal grains can be miniaturized.
In the above method, the cooling after the melting treatment is performed at a cooling rate of 2 ° C./sec or more, and the degree of superheating of the melted portion sprayed with the inert gas is 10 ° C. or less, more preferably 5 ° C. or less. It is preferable. Furthermore, it is desirable that He gas is used as the inert gas, and the spraying amount is 10 liters / minute / m ² or more. It is also effective to contain 1 to 10% by volume of hydrogen in the inert gas atmosphere.

  According to the care method according to the present invention, the solidification structure after the melt care treatment of the steel slab is made as fine as possible, the size of the crystal grains is reduced, the surface cracking during rolling is prevented, and the surface properties are excellent. A slab can be obtained.

Hereinafter, the present invention will be described based on the best embodiment.
As a method for melting the steel slab surface layer, any one of induction heating alone, plasma heating alone, or a combination of induction heating and plasma heating may be selected. In order to select one of them, an optimum means may be selected in consideration of other conditions such as the type and size of the steel slab and the processing time.
In the case of performing induction heating alone, the slab obtained by the continuous casting machine is transported to the melting processing place after cutting, and as shown in FIG. 1, the induction coil 2 arranged so as to go around the slab 1. The surface layer part is melted by heating. As shown in the detailed view of FIG. 2, the induction coil 2 forms a stable molten portion surface by pressing the molten portion toward the inner surface by electromagnetic force in the slab surface layer molten portion 3, and then cooled again in the atmosphere and solidified. To do. In the figure, 4 is a support roll of the slab 1, 6 is an unmelted heating part, 7 is a magnetic field, and 8 is a solidified part that has been re-solidified after melting.

  In this invention, in this cooling, in order to accelerate | stimulate cooling, suppressing the oxidation of the fusion | melting part 3, an inert gas is sprayed on this fusion | melting part. In FIG. 1 and FIG. 2, reference numeral 5 denotes an inert gas spray nozzle directed toward the melting portion 3. By blowing this inert gas, the melting part temperature of the slab surface is changed to the liquidus temperature (Since iron contains other components, the melting state is not determined at one temperature, such as the melting point, and the temperature at which solidification begins. The solidus temperature is maintained at a temperature slightly higher than the liquidus temperature), and the solidification structure can be reduced by rapidly cooling and solidifying after processing, thereby reducing the size of the crystal grains. By doing so, it becomes possible to improve the surface properties of the rolled material, such as prevention of surface cracks during rolling. The inert gas blowing nozzle 5 may be arranged in the vertical direction as shown by the solid line in FIG. 1 or at an angle shown by the broken line.

  Here, the principle of generation of electromagnetic force by the induction coil is shown in FIG. Due to the interaction between the magnetic field 7 generated by the induction coil 2 and the current induced in the slab 1 which is an electric conductor, an electromagnetic force acts on the melted portion, and this electromagnetic force causes the melted portion called a pinch force to move to the inner surface side. There is an action to compress, contributing to stabilization of the surface of the melted part. In FIG. 3, 9 represents a coil current, 10 represents an alternating current, and 11 represents time.

In addition, when plasma is used alone, since the plasma is generally axisymmetric, in order to continuously treat the surface of the slab, a method of scanning the plasma torch in the slab width direction, The method described in (Patent Document 1) can be employed, for example, a flat reciprocating motion of plasma using electromagnetic force in the slab width direction. As shown in FIG. 4, the slab 1 obtained by the continuous casting machine is transferred to the melting processing place after cutting, and the slab surface layer is melted and repaired by the plasma torch 12. Then it is cooled again and solidifies. In the case of plasma alone, only the upper surface of the slab is melted.
Also in this plasma method, by blowing an inert gas to the melting part 3 by the nozzle 5, the melting part temperature of the slab surface layer is reduced to the liquidus temperature (Since iron contains other components, the melting point is The temperature does not determine the melted state, the temperature at which solidification begins is called the solidus temperature, and the temperature at which it becomes all liquid is called slightly higher than the liquidus temperature). By reducing the solidification structure and reducing the size of the crystal grains, it is possible to improve the surface properties of the rolled material, such as suppressing surface cracking during rolling.

  Furthermore, in the present invention, a mode in which the above-described induction heating and plasma heating are used together can be employed. In this combined method, for example, a method in which the slab surface layer is first heated to a certain temperature by induction heating and then heated to a target temperature by plasma heating to melt the slab surface layer is desirable. Of course, even in this combined system, the molten part temperature of the slab surface layer is kept at a temperature slightly higher than the liquidus temperature by spraying an inert gas on the molten part, and rapidly cooled and solidified after processing. Thus, the solidified structure can be reduced, and by reducing the size of the crystal grains, it is possible to improve the surface properties of the rolled material, such as suppressing surface cracking during rolling. The melted portion may be the entire surface of the cast slab, or may be a portion that clearly needs to be maintained from the surface state. The depth is about 3 mm to 5 mm.

  As described above, the present invention aims to make the solidified structure as small as possible and improve the surface properties by spraying an inert gas on the molten pool immediately after the slab melting process and rapidly cooling the molten pool. Therefore, the cooling rate employed in the present invention is preferably 2 ° C./sec or more. If the cooling rate is less than this, it is difficult to make the solidified structure small without rapid cooling. The upper limit of the cooling rate is not particularly defined and may be set as appropriate. In the present invention, the interval between the dendritic crystals of the solidified structure after rapid cooling is set to a structure having a secondary branch interval of 100 μm or less. If the particle size exceeds this, the desired surface properties cannot be obtained.

In addition, by blowing an inert gas to the melted part, the melted part temperature of the slab surface layer is maintained at a temperature slightly higher than the liquidus temperature. If the temperature is maintained at not higher than 5 ° C., more preferably not higher than 5 ° C., the solidified structure can be reduced.
Furthermore, in order to perform cooling while preventing oxidation more reliably, it is preferable that 1 to 10% by volume, preferably about 2% by volume, of hydrogen is contained in the inert gas atmosphere and melt reformed. In addition, although helium, argon, etc. can be used as an inert gas, helium with favorable thermal conductivity is suitable. The amount of inert gas sprayed is preferably 10 liters / minute / m ² or more. The upper limit of the inert gas spray amount is not particularly defined and may be set as appropriate.

  In the above-described present invention, the inert gas spraying means is exemplified and explained as the means for rapidly cooling the melted portion of the slab surface layer. However, the present invention is not limited to this, and the cooling is performed immediately after the melting process. It is also possible to cool the melted part by means of bringing it into contact with the roll. In some cases, roll cooling alone or in combination with inert gas spraying can be used. As this roll cooling means, for example, the inside of the roll body has a structure capable of supplying and discharging an arbitrary cooling medium (cooling water), and the roll is disposed so as to freely advance and retreat at a position immediately after the melting processing position, Quickly cool by contacting the melted part. In this case, it is important to be able to achieve the cooling rate and superheat conditions described above.

A melt repair treatment was performed on the upper surface 3 mm of a continuous cast slab of medium-carbon aluminum killed steel having a width of 1500 mm, a thickness of 250 mm, and a length of 10 m. The liquidus temperature of this steel slab was about 1500 ° C. Further, the melting treatment was performed by plasma heating under the following conditions.
[Plasma heating conditions]
Due to the transformation during casting, the surface of the slab was found to have minute cracks on the surface before the treatment, but it was repaired by the melting treatment. During this melting treatment, hot rolling after being sent to a heating furnace and held at 1200 ° C. for 2 hours, when helium gas is sprayed at 10 liter / min / m ² on the melting part The number of surface defects of the material was compared. Note that the temperature of the melted part when helium gas was sprayed was maintained at 1505 ° C. The melt temperature when not sprayed was 1515 ° C. As a result, it was recognized that the number of defects due to the melting treatment was reduced to 1/10 or less of the case where no spraying was performed by gas cooling control.

The schematic diagram at the time of performing the care method of the steel slab which concerns on this invention by induction heating. The enlarged view which shows the detail of the fusion | melting part of the steel slab in FIG. Explanatory drawing which shows the generation principle of the electromagnetic force by the induction coil in induction heating. The schematic schematic diagram at the time of performing the care method of the steel slab which concerns on this invention by plasma heating.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Cast piece 2 Induction coil 3 Melting part 4 Support roll 5 Inert gas spray nozzle 6 Unmelted heating part 7 Magnetic field 8 Solidification part 9 Coil current 10 AC current 11 hours 12 Plasma torch

Claims (5)

  In the method of cleaning a steel slab in which the surface layer of the steel slab is melted by one or both of induction heating and plasma heating to repair defects in the steel slab surface layer, the surface of the steel slab is inadequate in the molten pool immediately after the surface layer melting treatment. A method of cleaning a steel slab characterized by cooling by either or both of spraying of active gas and roll contact.   The cooling method according to claim 1, wherein the cooling after the melting treatment is performed at a cooling rate of 2 ° C./sec or more.   The care method according to claim 1 or 2, wherein the degree of superheat of the melted part sprayed with inert gas is set to 10 ° C or less. The maintenance method according to any one of claims 1 to 3, wherein the inert gas is He gas, and a spray amount thereof is 10 liters / minute / m ² or more.   The care method according to any one of claims 1 to 4, wherein 1 to 10% by volume of hydrogen is contained in an inert gas atmosphere.

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