JPH0994645A - Method for cleaning surface of continuously cast slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely achieve the surface cleaning of a continuously cast slab particularly without developing secondary surface defect by heating and remelting the surface layer of the continuously cast slab continuously drawn out from the lower end of a continuous cast mold on the way of cooling with heating device. SOLUTION: At the step on the way of cooling the high temp. in the inner part of the continuously cast slab 7, the heating and the remelting are executed to clean the cast slab. For example, the heating devices 8, 8' are arranged along a carrying course of continuously cast slab 7 at the position just before the upstream side or just after the downstream side of a cast slab cutting device 9 in a continuous caster to execute the heating and the remelting. In the treatment at this step, since the heating and the remelting are executed by effectively utilizing the high temp. of the continuously cast slab 7, this treatment is extremely profitable to the reducing point of the thermal energy and to the eliminating point of the surface defect. The heating devices 8, 8' have the constitution shifting a plasma torch, etc., along the width direction on the surface of the cast slab 7. Therefore, this treatment can be treated by small scale device and a holding time in the molten state can be shortened.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for cleaning a surface of a continuously cast slab, and more particularly to a continuously cast slab (hereinafter simply referred to as "continuous cast slab") having no surface defects such as surface cracks and frostiness.
It is a technology that is effectively used for manufacturing.

[0002]

2. Description of the Related Art In general, continuous cast slabs such as slabs, blooms, and billets may have surface defects such as surface cracks (longitudinal cracks, lateral cracks, corner cracks), mortar, or oscillation marks. . For these large surface defects, it is necessary to carry out maintenance by performing fusing with a gas burner or cutting with a grinder. The maintenance of such continuous cast slabs not only increases the work load and lowers the product yield, but also makes it impossible to perform "maintenance-free charging" of the high-temperature slab into the heating furnace and "direct rolling" by the rolling mill. There was a problem.

Particularly, the surface layer portion of the continuous cast slab tends to collect non-metallic inclusions and gas bubbles (Ar gas, N ₂ gas, etc.) whose main component is alumina or mold powder, and these are heated in the heating process. It is not usually removed by the treatment of rolling or rolling process. For this reason, for example, a thick plate product has surface unevenness scratches (balding), and a thin plate product has streaky scratches (balding, sliver), which leads to a reduction in product yield.

In order to solve such a problem, Japanese Patent Publication No.
No. 23974 proposes a surface modification maintenance method of arc melting a cast slab under vacuum. However, since this method is arc melting, it has a wide melting region,
Moreover, it is not economical in that it is held in the molten state for a long time, requires a large current, and needs to be performed under reduced pressure, and performs continuous casting slabs online during cooling of the continuous casting machine. There were many difficulties to do.

In addition, Japanese Laid-Open Patent Publication No. 3-42145 proposes a method of remelting the surface of a thin steel sheet by using twin rolls. In this method, the surface layer portion (several hundreds of microns) is fixed type. This is a technique for melting fine particles and internal defects in the subsequent roller rolling step while melting with the heating means. However, this method is effective for heating and remelting only the surface layer part or a local part, but when remelting the entire surface of the slab over a depth of 2 mm or more, an extremely large amount of energy is required. , Impractical.

[0006]

The problem with the prior art is that, in addition to the above-mentioned contents, these technologies mainly deal with cold cast pieces or cold slabs after cooling. Therefore, the thermo-economic effect and the surface cleaning effect are poor. That is, in these conventional techniques, continuous casting slabs whose internal temperature is still high are targeted, and not a method of performing a cleaning treatment during cooling, and only the surface layer portion where surface defects appear at this stage is concentrated. Since it is not a method of heating and remelting it, the original effect aimed at by the present invention cannot be obtained. Further, in many of these conventional techniques, there is also a risk of recurrence of secondary surface defects during heating and remelting.

An object of the present invention is to propose an effective surface cleaning treatment technique for continuous cast slabs. Another object of the present invention is to reduce the energy cost in addition to producing a continuous casting slab excellent in surface cleaning by performing surface cleaning in the stage of hot cast slab. Still another object of the present invention is to uniformly and intensively treat only the slab surface layer where surface defects are likely to appear, thereby preventing recurrence of secondary surface defects and improving the cleaning effect.

[0008]

SUMMARY OF THE INVENTION The present invention is a surface cleaning technique capable of reliably solving the above-mentioned problems of the prior art, and its characteristic constitution is that of a continuous casting mold. It is a method for cleaning the surface of a continuously cast slab, characterized in that the surface layer portion of the continuously cast slab that is continuously pulled out from the lower end is heated and remelted by a heating device during cooling. Then, in the present invention, it is effective to perform the above heating in the vicinity of the slab cutting device; that is, immediately before the upstream side or immediately after the downstream side of the cutting device. The reason is that the surface of the slab can be observed in detail and the slab temperature is as high as 800 ° C or higher, so that the energy required for heating until the surface melting is small.
Further, in the present invention, the heating device is provided so as to be movable along the surface of the continuously cast slab, particularly in the width direction, and uses plasma or laser as a heat source.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The first feature of the present invention resides in that the inside of a continuously cast slab is heated and remelted in the course of cooling at a high temperature for cleaning. For example, a heating device is installed along the transport path of the continuous cast slab at a position immediately upstream or immediately downstream of the slab cutting device of the continuous casting machine to heat and remelt the slab. In the treatment at this stage, the high temperature of the continuous cast slab can be effectively utilized to perform heating and remelting, which is extremely advantageous in terms of reducing thermal energy and eliminating surface defects. is there.

In the present invention, what is important in the above heat remelting treatment is that most of the surface defects are generated only in the surface layer portion of the continuous cast slab, so that this portion is effectively heated. The method of remelting is advantageous. The inventors of the present invention have found that the depth of heating and remelting the surface layer portion of the continuous cast slab using a heating device is such that the deeper the cleaning is, the closer the cleaning becomes to perfection, and the maximum depth is about 10 mm. It was In particular, in order to repair fine cracks formed on the surface of the cast slab and to make it possible to scale off the inclusions of blowholes (defects due to alumina clusters and mold powder) immediately below the surface layer before rolling, the surface layer below the surface layer 2
It was found that it was necessary to uniformly heat and remelt in the range of mm.

In this respect, in order to melt the surface of the continuously cast slab or deepen the melting depth with a fixed heating device, more heat energy needs to be supplied, and the device becomes large in size, which is economical. Not.

As described above, as such a heating device, a plasma torch or a laser torch arranged so as to be movable in the width direction of the slab is used, but these have a high incident energy density and the energy is applied to the surface layer of the slab. Because it is convenient to focus. In addition, control of incident energy by plasma torch and laser torch is
From the following perspectives. Now, the required temperature rise: Δθ
(° C), energy input speed: P (J / S), energy input area: S (m ² ), input depth: Δh (m), energy input time: Δt (s) Is given by equation (1) below.

[Equation 1] Where δ is the density of steel (kg / cm ³ ), C is the specific heat of steel (J / kg
・ K) K is the thermal conductivity (W / m · k) of steel. Therefore, above
As is clear from the equation (1), if the charging time Δt is made small and the charging depth Δh is made small, if the charging time Δt is the same,
It can be seen that a higher temperature rise amount Δθ can be obtained with a small energy input speed P.

In the present invention, such a heating device is not a fixed type, but a structure in which a plasma torch or a laser torch is moved along the width direction along the surface of the slab is adopted. With such a heating means, a small heating device can be used, and the holding time in the molten state can be shortened, so the risk of oxidation is reduced and secondary surface defects can be achieved despite the treatment during standby. The occurrence of can be suppressed.

[0014]

【Example】

<Example 1> In this example, a plasma heating device with an output of 50 KW and a plasma frame diameter of 20 mm (the surface of the slab) was attached to the surface of the slab immediately before reaching the slab cutting device during the slab cooling of the continuous casting device. In this example, one device is installed on each side of the back surface, and the apparatus is continuously scanned in the direction perpendicular to the casting direction at a speed of reciprocating once per second in the width direction of the slab. In the continuous casting, an aluminum-killed ultra-low carbon steel slab having a thickness of 220 mm and a width of 1200 mm was cast at a casting speed of 2 m / min. As a result, the surface of the continuous cast slab is smooth with no oscillation marks, dents, or cracks, and the continuous cast slab after cutting is extremely beautiful compared to conventional continuous cast slabs that do not heat or remelt during cooling. A surface texture was obtained. Further, the rates of surface defects caused by inclusion bubbles were compared for cold-rolled coils obtained by heating and rolling these slabs without care. When the present invention was not applied, the occurrence rate was 0.8% per coil, but the occurrence rate was 0% for the coil obtained from the slab that was remelted by plasma heating.
Met. 1 shows a continuous casting apparatus used in this example, in which 1 is a ladle, 2 is a long nozzle, 3 is a tundish, 4 is a dipping nozzle, 5 is a continuous casting mold, and 6 is A slab support roll, 7 is a continuous cast slab, 8, 8'is a slab surface heating device (laser torch), and 9 is a slab cutting device.

<Example 2> An aluminum-killed continuous cast slab for a thick plate having a thickness of 215 mm and a width of 1600 mm obtained by casting at a casting speed of 1.0 m / min was cut with a slab cutting device to a size of about 3 m. Disconnected. The continuous cast slab in the hot piece state after cutting was immediately supplied into a laser heating device having an output of 10 KW, and a laser beam of 5 mm beam system was continuously applied to the surface at a rate of 6 times (3 reciprocations) per second. And scanned and heated in the same manner as shown in Example 1. As a result, the oscillation mark found in the conventional slab that was not irradiated with laser light disappeared, and the lateral cracks that occurred in the oscillation valley were completely repaired. Furthermore, when observing a plank product to which this method was applied, there were no heggling scratches and brick entrapment scratches.

[0016]

As described above, according to the present invention, it is possible to eliminate surface defects of the continuous casting slab as it is, and it becomes unnecessary to take care of the surface again. In particular, the surface cleaning of the continuous cast slab can be reliably achieved without generating secondary surface defects. Moreover, since the method of the present invention is processed during cooling of the continuous cast slab, it can be processed by a method with less heat energy, which is effective in reducing product cost and improving productivity by stable supply of raw materials.

[Brief description of drawings]

FIG. 1 is a continuous casting apparatus showing an example of an embodiment of the present invention.

[Explanation of symbols]

 1 Ladle 2 Long nozzle 3 Tundish 4 Immersion nozzle 5 Continuous casting mold 6 Slab support roll 7 Continuous casting slab 8, 8'Slab surface heating device 9 Slab cutting device

Claims (3)

[Claims] 1. The surface of a continuous cast slab, characterized in that the surface layer portion of the continuous cast slab that is continuously drawn from the lower end of the continuous casting mold is heated and remelted by a heating device during the cooling process. Cleaning method. 2. The surface cleaning method according to claim 1, wherein the heating is performed at a position near a slab cutting device. 3. The heating device is provided so as to be movable along the surface of a continuously cast slab, and uses plasma or laser as a heat source. Cleaning method.