JPH10286652A - Method for continuously casting square billet and mold for continuous casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the development of deformation, crack, etc., of solidified shell and to improve the billet quality by setting the ratio of take-out heat flow speed of corner parts and side surface parts of a mold at a specific range and changing drawing-out speed and/or electromagnetic stirring flow speed according to the heat conductive ratio. SOLUTION: Electromotive forces generated in thermocouples 2 embedded at the side surface parts and the corner parts of the mold 1 existing in the same distance from the solidified starting point 5 of poured molten steel, are converted into mold temp. data with a converter and through an arithmetic device, the heat flow speed qf at the side surface part and the heat flow speed qc at the corner part are obtd. with qf =Kf ×(Tf -Tw ) and qc ×(Tc -Tw ), respectively. Wherein, Kf , Kc and Tf , Tc are overall coefficient of heat transfers and temps. of thermocouples at the side surface part and the corner parts, respectively. This permissible range of the take-out heat flow speed ratio is made 30-70% and when the ratio exceeds the range, the drawing-out speed is lowered and the flowing-in of powder is promoted or the electromagnetic stirring flow speed is reduced and the molten metal surface is raised, and thereby the uniformity of the solidification is recovered.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method for a square billet and a casting mold for the continuous casting, and more particularly, to prevent uneven solidification in a cross section of the billet which occurs during continuous casting of medium carbon steel. The present invention relates to a continuous casting method of a square billet and a continuous casting mold thereof.

[0002]

2. Description of the Related Art In conventional continuous casting of rectangular billets, uneven solidification occurs in the cross section of a strand, and defects such as rhombic deformation, dents (depression), and cracks are likely to occur. In particular, in the casting of medium carbon steel, when the casting speed is increased, these defects occur frequently, so that there was a problem that the casting speed could not be increased and the productivity was low.

[0003] For this reason, as a method of making the solidification uniform, for example, a method of lowering the viscosity of the powder for continuous casting to promote powder inflow between the mold and the strand, or a method of electromagnetically applying molten metal in the mold. Stirring methods and the like have been generally used. However, in the method of adjusting the viscosity of the powder, it is difficult to control the distribution of the powder inflow in the cross section. In addition, agitation by electromagnetic force requires a large-scale device, and when strong agitation is performed, uneven swelling occurs in the meniscus shape,
There are problems such as uneven coagulation.

[0004] Japanese Patent Publication No. 59-39220 discloses a method in which a multiple taper portion is provided on the upper portion of a mold to change the level of molten metal in the mold according to the shrinkage ratio of the solidified shell. However, in this method, since the shrinkage form of the molten metal is not uniform in the casting direction and in the cross section, it is difficult to set an appropriate tapered shape.

Japanese Patent Application Laid-Open No. 54-116330 discloses a method in which predetermined cooling is applied to the obtuse and acute angles of the strand in the secondary cooling behind the mold, depending on the degree of cross section deviation. .

However, since the solidified shell develops behind the mold, the surface temperature decreases, and the rigidity is high, it is difficult to change the cross-sectional shape by adjusting the cooling.
Also, if the cross-sectional shape is forcibly deformed when the solidified shell becomes thicker to some extent, internal cracks will occur inside the solidified shell, causing quality problems. In the secondary cooling, only the corners are cooled. Requires a large-scale device, and there is a problem that equipment costs and running costs increase.

[0007] Japanese Patent Application Laid-Open No. 6-31401 discloses that
A method is disclosed in which the cross-sectional shape of the mold opening on the inlet side of the billet is different from the cross-sectional shape on the outlet side, and the space between them is continuously changed. With this method, it is difficult to determine an appropriate shape in advance when casting under conditions with different casting speeds and types, and because of the complicated mold shape, processing is difficult, and equipment costs are reduced. There is a problem of increasing.

In any of these methods, a large-scale apparatus is required, and it is difficult to find appropriate conditions. Therefore, it is desired to develop a mold capable of adjusting solidification in a cross section of the mold by a simple method. I have.

[0009]

SUMMARY OF THE INVENTION Accordingly, the present invention provides a method for continuously casting a square billet, which can prevent deformation and cracking of a solidified shell by appropriately cooling a mold.
An object of the present invention is to provide a continuous casting method of a square billet and a continuous casting mold thereof.

[0010]

The gist of the present invention to achieve the above object is as follows.

(1) In the continuous casting method of a square billet, the ratio of the heat flux at the mold corner to the heat flux at the mold face at a position where the distance from the solidification start point to the casting direction is equal is defined as a specific range. (2) The method of (2), wherein the ratio of the heat removal flux is such that the heat removal flux at the corner portion is 30 to 70% of the heat removal flux at the surface portion. (1) The continuous casting method of a square billet according to (1), (3) the drawing speed and / or the electromagnetic stirring flow rate is changed according to the ratio of the heat flux at the corner portion and the heat flux at the face portion. The continuous casting method of a square billet according to the above (1) or (2), (4) R
In a continuous casting mold for a square-shaped billet having a cylindrical cross section having a processed corner portion, the thickness of the corner portion is made thicker than the thickness of the surface portion. (5) A continuous casting mold for a square billet, comprising: a plurality of grooves in the casting direction on a cooling water channel side of a surface portion of the square billet; The mold for continuous casting of a square billet according to the above (4), wherein a plurality of grooves are provided in a casting direction on a cooling water channel side of the surface portion.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the continuous casting of a square billet, the present invention is capable of reducing the deformation of the solidified shell by appropriately setting the heat flux at the corner and the face.
Continuous casting of high-quality billets becomes possible.

FIG. 1 shows a mold for continuous casting of a square billet used in the method of the present invention. First, FIG.
FIG. 3 is a view showing a vertical cross section of the mold, in which a thermocouple 2 is embedded in a mold cylinder, and a molten metal 3 is continuously drawn out while being cooled by the mold 1 to form a solidified shell 4. FIG. 1B is a cross-sectional view taken along the line AA of FIG.
In this example, thermocouples 2 are embedded at the center of each face and at each corner, and the mold temperatures of the face and the corner at the same distance from the solidification start point 5 can be measured simultaneously.

FIG. 2 shows a method of continuously casting a square billet of the present invention using the mold of FIG. The electromotive force generated in the thermocouple 2 embedded in the face portion and the corner portion of the mold 1 equidistant from the solidification start point 5 is converted into mold temperature data by the converter 7, and the face heat flux is calculated by the arithmetic unit 8. qf and the heat flux qc at the corner are calculated as follows.

Qf = kf × (Tf−Tw) (1) qc = kc × (Tc−Tw) (2) Here, kf and kc are obtained in advance by, for example, heat conduction calculation. , Tf and Tc are the temperatures (K) of thermocouples embedded in the face and corners, respectively, and the total heat transfer coefficient (W / m ² K) of the face and corners,
Tw is the cooling water temperature (K).

Here, thermocouples 2 may be embedded in a plurality of faces and corners equidistant from the solidification starting point 5, for example, as shown in FIG. 1B, each face of a square billet mold. And when the temperature is measured at each corner,
The maximum value max (qf) and the minimum value mi of the heat flux of each surface
n (qf), the maximum value of the heat flux at each corner max (q
c) and a minimum value min (qc) are calculated, and then the maximum value max and the minimum value min of the heat flux ratio (qc / qf) are calculated as follows:
Calculate as follows:

Max (qc / qf) = max (qc) / min (qf) (3) min (qc / qf) = min (qc) / max (qf) (4) The calculated heat flux ratio is equal to the allowable minimum value Cmin (=
0.3) and the allowable maximum value Cmax (= 0.7), and if it is within the allowable range, the casting is continued under the current operating conditions. The uniformity of coagulation can be reduced by reducing the drawing speed to promote powder inflow, or, when using the electromagnetic stirrer 6, by reducing the electromagnetic stirring flow rate to reduce the rise of the molten metal surface. To recover.

These operating condition changes may be carried out alone or in combination. Here, when the heat flux ratio is smaller than the allowable minimum value (0.3), the solidification of the corner portion is extremely delayed, and the risk of internal cracking, breakage of the solidified shell and breakout sharply increases. Conversely, the heat flux ratio is the maximum allowable value (0.7).
If it becomes larger, the rigidity of the corner part will increase,
The risk of dents in the surface and rhombic deformation increases.

FIG. 3 shows a mold 1 for continuous casting of a square billet, which is formed into a cylindrical shape having a rounded corner.
FIG. 3 shows a cross-sectional view of the mold of the present invention in which the thickness of the corner portion is larger than the thickness of the surface portion. Here, assuming that the thickness of the surface portion is t1, the thickness of the corner portion is t2, and the inner radius is R, t2 is:
It is desirable to set the following range.

√2 × t1 ≦ t2 ≦ (√2-1) R + √2 × t1 (5) In FIG. 4, a plurality of grooves are formed on the cooling water channel side of the surface in the casting direction. 1 shows a sectional view of a continuous casting mold 1 for a square billet according to the present invention. Here, the depth d of the groove 9, the width w of the groove,
Assuming that the groove period is L, d <0.5 × t
It is desirable to set L <w + t1-d to make the temperature of the inner surface of the mold uniform.

[0021]

Next, a description will be given of an example of continuous casting of medium carbon steel using the continuous casting method for a square billet of the present invention and a continuous casting mold made of Cu. The casting conditions are as follows.

・ Slab size: 180 × 180 mm cross section, corner R = 1
0 mm ・ Mold thickness: 10 mm (one face and one corner) ・ Steel type: medium carbon steel ([C] = 0.1 wt%) FIGS. 5 and 6 show various casting conditions, that is, casting speed and electromagnetic stirring device. The figure shows the heat flux ratio and the number of occurrences of cracks, breakouts, dents, and rhombic deformation (the slab cross section deformed into a rhombic shape) when the coil current was changed.

As shown in FIG. 5, when the heat flux ratio falls below 0.3, the occurrence of cracks and breakouts increases.
As shown in the figure, when the heat flux ratio is increased from 0.7, the occurrence of dents and rhombic deformation increases. By maintaining the heat flux ratio at 0.3 or more and 0.7 or less, it is possible to manufacture a high-quality slab without defects.

Next, an embodiment in which casting is performed using the continuous casting mold of the present invention will be described. The following items were changed in the slab conditions described above. Inventive Example 1: Corner mold thickness = 14.1 mm (mold outer surface corner R = 10 mm). Invention Example 2: Processing of cooling grooves only on the surface (full length of mold, width direction 1)
A groove having a width of 2.0 mm, a depth of 3.0 mm, a groove bottom R of 1.0 mm, and a pitch of 4.0 mm was formed on each surface over 42 mm.
Book / surface processing).

FIG. 7 shows the average casting speed (one month average of the same steel type) when casting was performed using the mold of the present invention. In a conventional mold, when the above-mentioned heat flux ratio exceeds a specific range, the casting speed is reduced to operate so as to obtain an appropriate heat flux ratio. In contrast, when casting using the mold of the present invention, the heat flux ratio may exceed a specific range,
The drastic reduction has made it possible to manufacture without reducing the casting speed.

FIG. 8 shows the results of examining the number of surface inclusions in the slab when casting was performed using the mold of the present invention. In the conventional mold, when the above-mentioned heat flux ratio exceeds a specific range, the electromagnetic stirring is reduced so that the operation is performed to obtain an appropriate heat flux ratio, so there are many surface inclusions, and there is a problem in quality. Had become. In contrast, when casting was performed using the mold of the present invention, since the heat flux ratio greatly decreased beyond the specified range, casting was possible without reducing electromagnetic stirring, and inclusions were significantly reduced. did.

[0027]

As described in detail above, according to the present invention,
In the continuous casting of square billets, it is possible to prevent cracks, breakouts, dents and deformation of the slab, thereby preventing a reduction in casting speed, improving productivity, and reducing electromagnetic stirring current. Prevention and quality improvement can be expected.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a continuous casting mold for a rectangular billet according to an embodiment of the method of the present invention, wherein a) is a longitudinal cross-sectional view,
(B) is a transverse sectional view (AA section).

FIG. 2 is a conceptual diagram showing a method for continuously casting a square billet according to an embodiment of the present invention.

FIG. 3 is a cross-sectional view of a mold for continuous casting of a square billet according to an embodiment of the present invention.

FIG. 4 is a cross-sectional view of a square billet continuous casting mold according to another embodiment of the present invention.

FIG. 5 is a diagram showing a relationship between a heat flux ratio and the number of occurrences of cracks and breakouts in the present embodiment.

FIG. 6 is a diagram showing a relationship between a heat flux ratio, a dent, and the number of occurrences of rhombic deformation in the present embodiment.

FIG. 7 is a diagram showing an example of comparison of the average casting speed between the mold of the present invention and the mold of the prior art.

FIG. 8 is a diagram showing an example of comparison of the number of inclusions between the mold of the present invention and the mold of the prior art.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Mold 2 Thermocouple 3 Molten metal 4 Solidified shell 5 Meniscus (solidification starting point) 6 Electromagnetic stirring coil 7 Converter 8 Arithmetic unit 9 Groove

Claims (6)

[Claims] In a continuous casting method of a square billet,
A continuous casting method of a square billet, wherein a ratio of a heat flux at a mold corner portion and a heat flux at a mold surface portion at a position where a distance from a solidification start point to a casting direction is equal is within a specific range. 2. The continuous casting method of a square billet according to claim 1, wherein the ratio of the heat removal flux is such that the heat removal flux at the corner portion is 30 to 70% of the heat removal flux at the face portion. . 3. The corner according to claim 1, wherein the drawing speed and / or the electromagnetic stirring flow rate is changed in accordance with the ratio of the heat flux at the corner portion and the heat flux at the face portion. Continuous casting method for die billets. 4. A continuous casting mold for a square-shaped billet having a cylindrical section having a rounded corner portion, wherein the corner portion has a greater thickness than the surface portion. A mold for continuous casting of billets. 5. A continuous casting mold for a square billet, comprising: a plurality of grooves in a casting direction on a cooling water channel side of a surface of the casting mold. 6. The mold for continuous casting of a rectangular billet according to claim 4, wherein a plurality of grooves are provided in a casting direction on a cooling water channel side of a surface portion of the mold.