JP3389449B2 - Continuous casting method of square billet - Google Patents

Description

Detailed Description of the Invention

[0001]

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

[0002]

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

Therefore, as a method for making the solidification uniform, for example, a method of lowering the viscosity of the powder for continuous casting to promote the powder inflow between the mold and the strand, or a method of electromagnetically applying the molten metal in the mold Methods such as stirring have been commonly used. However, it is difficult to control the distribution of the powder inflow amount in the cross section by the method of adjusting the powder viscosity. Further, stirring by electromagnetic force requires a large-scale device, and when strong stirring is performed, uneven swelling occurs in the meniscus shape,
There are problems such as non-uniform coagulation.

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

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

However, behind the mold, the solidified shell develops, the surface temperature decreases, and the rigidity is high, making it difficult to change the cross-sectional shape by adjusting cooling.
Also, if the shape of the cross section is forcibly deformed when the solidified shell becomes thick to some extent, internal cracks will occur inside the solidified shell, which will cause quality problems. Also, in the secondary cooling, only the corners will be cooled. A large-scale device is required to control the temperature, and there is a problem that equipment cost and running cost increase.

Further, 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 billet-entry side and the cross-sectional shape on the exit side are different, and the interval between them is continuously changed. In this method, when casting under different conditions of casting speed and product type, it is difficult to determine an appropriate shape in advance, and because of the complicated mold shape, it is difficult to process and the equipment cost is high. There is a problem of increasing.

Since any of these methods requires a large-scale apparatus and it is difficult to find out appropriate conditions, it is desired to develop a mold capable of adjusting solidification in the mold cross section by a simple method. There is.

[0009]

Therefore, in the present invention, in the continuous casting of a square billet, by appropriately cooling the inside of the mold, it is possible to prevent the solidified shell from being deformed or cracked.
An object of the present invention is to provide a method for continuously casting a square billet and a mold for the continuous casting.

[0010]

The gist of the present invention for achieving the above object is as follows.

(1) In the continuous casting method for a square billet, the ratio of the heat removal flux of the mold corner portion to the heat removal flux of the mold surface portion at a position where the distance from the solidification start point to the casting direction is the same is within a specific range. A method for continuously casting a rectangular billet characterized by changing the drawing speed and / or the electromagnetic stirring flow rate according to the ratio, (2) the ratio of the heat removal flux to the heat removal flux at the corners of the mold It is 30-70% of the heat removal flux of a mold surface part, It is the continuous casting method of the square billet of Claim 1 characterized by the above-mentioned.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention, in continuous casting of a rectangular billet, by appropriately setting the heat removal flux of the corner portion and the surface portion, the deformation of the solidified shell is extremely small.
It enables continuous casting of high quality billets.

FIG. 1 shows a mold for continuous casting of a square billet used in the method of the present invention. First, FIG. 1A shows a mold 1
FIG. 3 is a view showing a vertical section of FIG. 1, in which a thermocouple 2 is embedded in a mold cylinder, and the molten metal 3 is continuously drawn out while being cooled by the mold 1 to form a solidified shell 4. Further, 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 surface and at each corner, and the mold temperatures of the surface and the corner equidistant from the solidification start point 5 can be measured simultaneously.

FIG. 2 shows a method for 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 surface 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 surface heat flux is calculated by the arithmetic unit 8. The qf and the corner heat flux qc are calculated by the following equations.

Qf = kf × (Tf−Tw) (1) qc = kc × (Tc−Tw) (2) Here, kf and kc are obtained in advance by, for example, heat conduction calculation. a overall heat transfer coefficient of the surface portion and the corner portion ^{(W / m 2 K),} Tf and Tc are the surface portion and the corner portion embedded thermocouple temperature (K),
Tw is the cooling water temperature (K).

Here, the thermocouples 2 may be embedded in a plurality of surface portions and corner portions that are equidistant from the solidification starting point 5. For example, as shown in FIG. 1 (b), each surface of the square billet mold may be embedded. And when measuring temperature at each corner,
Maximum value max (qf) and minimum value mi of heat flux on each surface
n (qf), maximum value of heat flux max (q
c) and the minimum value min (qc), and then the maximum value max and the minimum value min of the heat flux ratio (qc / qf) are
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 the minimum allowable value Cmin (=
0.3) and the maximum allowable value Cmax (= 0.7), and if it is within the allowable range, casting is continued under the current operating conditions, and if the allowable range is exceeded, Reduces the extraction speed to promote powder inflow, or, if an electromagnetic stirrer 6 is used, reduces the electromagnetic stirrer flow rate to reduce swelling of the molten metal surface, thereby increasing the uniformity of solidification. To recover.

These changes in operating conditions may be carried out alone or in combination. Here, if the heat flux ratio becomes smaller than the allowable minimum value (0.3), the solidification of the corner portion is extremely delayed, and the risk of internal cracking or breakage of the solidified shell rapidly increases. On the contrary, the heat flux ratio is the maximum allowable value (0.7)
If it is larger than this, the rigidity of the corner part will increase,
The risk of dents on the surface and diamond deformation increases.

FIG. 3 shows a mold 1 for continuous casting of a square billet, which is formed into a cylindrical shape with a corner portion having a rounded shape.
FIG. 3 is 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, if the plate thickness of the surface portion is t1, the plate thickness of the corner portion is t2, and the inner radius is R, then t2 is
The following ranges are desirable.

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

[0021]

EXAMPLES Next, examples of continuous casting of medium carbon steel using the method for continuously casting square billets and the continuous casting mold made of Cu according to the present invention will be described. The casting conditions are as follows.

-Slab size: 180 x 180 mm cross section, corner R = 1
0 mm ・ Mold thickness: 10 mm (one surface and corner joint) ・ Steel type: Medium carbon steel ([C] = 0.1 wt%) In FIG. 5 and FIG. 6, various casting conditions, that is, casting speed, electromagnetic stirrer The heat flux ratio and the number of times of occurrence of cracks, breakouts, dents, and rhombus deformation (the slab cross section is transformed into a rhombus) when the coil current is changed are shown.

As shown in FIG. 5, when the heat flux ratio is lower than 0.3, cracks and breakouts increase, and FIG.
As shown in (1), when the heat flux ratio increases from 0.7, the occurrence of dents and rhombus 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, examples of casting using the continuous casting mold of the present invention will be shown. The following items were changed in the above-mentioned casting condition. Invention Example 1: Corner mold thickness = 14.1 mm (mold outer surface corner R = 10 mm). Inventive Example 2: Cooling groove processing only on the surface portion (mold length, 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 is formed on each surface of 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 the conventional mold, when the above-mentioned heat flux ratio exceeds a specific range, the casting speed is reduced to operate so that the heat flux ratio becomes appropriate. On the other hand, when cast using the mold of the present invention, the heat flux ratio may exceed a specific range,
The significant reduction has made it possible to manufacture without slowing down the casting speed.

FIG. 8 shows the results of examining the number of surface inclusions in a cast piece 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, it was operated so as to have an appropriate heat flux ratio by reducing the electromagnetic stirring, so there were many surface inclusions, which caused quality problems. Was becoming. On the other hand, in the case of casting using the mold of the present invention, the heat flux ratio exceeding the specific range was significantly reduced, so that casting was possible without reducing electromagnetic stirring, and inclusions were significantly reduced. did.

[0027]

As described in detail above, according to the present invention,
Since it is possible to prevent cracks, breakouts, dents and deformations in the continuous casting of rectangular billets, it is possible to prevent lowering of casting speed, improve productivity, and reduce electromagnetic stirring current. Preventing and improving quality can be expected.

[Brief description of drawings]

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

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

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

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

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

FIG. 6 is a diagram showing the relationship between the heat flux ratio and the number of occurrences of dents and rhomboid deformations in the present embodiment.

FIG. 7 is a diagram showing an example of comparison of average casting speeds of 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]

1 mold 2 thermocouple 3 Molten metal 4 solidification shell 5 Meniscus (starting point of solidification) 6 electromagnetic stirring coil 7 converter 8 arithmetic unit 9 grooves

Continuation of front page (56) Reference JP-A-6-31418 (JP, A) JP-A-57-206555 (JP, A) JP-A-55-84253 (JP, A) JP-A-7-88598 (JP , A) JP 58-97471 (JP, A) JP 7-116783 (JP, A) JP 6-320245 (JP, A) JP 58-151943 (JP, A) JP 58-41661 (JP, A) JP-A-6-31401 (JP, A) JP-A-54-116330 (JP, A) JP-A-9-262641 (JP, A) Practical application Sho-64-38131 (JP, A) U) Actual development 1-118845 (JP, U) Actual development 5-93644 (JP, U) Actual development 61-36340 (JP, U) Japanese public examination 59-39220 (JP, B2) (58) Survey Areas (Int.Cl. ⁷ , DB name) B22D 11/055 B22D 11/04 311 B22D 11/20 B22D 11/22

Claims (2)

(57) [Claims] 1. A method of continuously casting a square billet, comprising:
The ratio between the heat removal flux of the mold corner portion and the heat removal flux of the mold surface portion at a position where the distance from the solidification start point to the casting direction is the same is set to a specific range, and the drawing speed and / or
Alternatively, a method for continuously casting a rectangular billet is characterized by changing the electromagnetic stirring flow rate. The proportion of wherein said dissipation heat flux, continuous prismatic billet of claim 1, wherein the heat extraction flux of the mold corners, characterized in that 30 to 70% of the heat removal flux mold surface Casting method.