JPH05228597A - Method for controlling segregation in continuously - Google Patents

Abstract

PURPOSE:To restrain center segregation developed in a cast slab (continuously cast slab) obtd. by continuous casting. CONSTITUTION:In a method for continuously squeezing the cast slab 1 drawn out from a mold 7 for continuous casting at the step before the inner part of the cast slab completes the solidification by using a squeezing die 4, a horizontal direction rotary magnetic field system electromagnetic stirring device 3 is arranged within 3000mm from the squeezing part by the die 4 to stir the molten steel in the incomplete solidified range of the cast slab 1. By this method, the high concn. liquid phase discharged by squeezing is uniformly dispersed in the width and thickness directions of the incomplete solidified range and the canon. in the cast slab after squeezing can be controlled to the constant and the desirable center segregation improving effect is obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a segregation control method for continuously cast slabs, which controls center segregation generated in slabs (continuously cast slabs) obtained by continuous casting to an appropriate range.

[0002]

2. Description of the Related Art Center segregation of a continuous cast slab of steel appears as a positive segregation due to the concentration of molten steel components such as C, P and S at the center of thickness, which is the final solidified portion of the slab. This is one of the quality defects that is difficult to avoid by the conventional continuous casting method. The generation mechanism of center segregation is that, in addition to solidification contraction of the solidification tip of the slab obtained by continuous casting, vacuum suction force of vacancy generated by bulging of the solidification shell is also added to suck the concentrated molten steel into the solidification tip. It is considered that positive segregation remains at the center of the thickness of the slab.

For example, in a bloom slab for hard steel wire rods having a slab size of 400 mm × 560 mm, significant V segregation near the axis,
Creates intermittent cavities. V segregation is generated in a region with a width of about 100 mm centering on the shaft center, and the center segregation and the negative segregation body adjacent to it are clearly recognized, and the solute-concentrated molten steel moves about 100 mm width centering on the shaft center. You can see that there is. Such a technique for reducing center segregation of slabs and blooms obtained by continuous casting is to reduce the degree of superheat of molten steel or add wire rods to molten steel in the mold to solidify the structure of equiaxed crystals. The method of forming in the cast, by electromagnetic stirring molten steel in the mold and molten steel in the strand,
The method of obtaining equiaxed crystals is widespread. All of these methods attempt to finely disperse the solute by obtaining a solidified structure composed of equiaxed crystals and reduce center segregation, but each has advantages and disadvantages and is widely used. However, the segregation improving effect was not always sufficient.

Further, Japanese Patent Laid-Open Nos. 52-104220 and 5
Japanese Unexamined Patent Publication No. 4-107831 discloses a method for reducing center segregation by a light roll reduction method, but this method is difficult to adjust the reduction amount. That is, when the reduction amount is small, it is insufficient to prevent downward movement of the concentrated molten steel caused by bulging or solidification shrinkage, while when the reduction amount is excessive, cracks occur at the solidification interface. There is a fatal drawback.

Further, it prevents the movement of a wide range of concentrated molten steel,
JP-A-63-183765 discloses a method for advantageously preventing center segregation.
It is proposed in the publication that forging is performed at a stage before the solidification completion point of a slab obtained by continuous casting. This proposal is a stage before the solidification of the inside of the slab in the continuous casting of the slab pulled out from the continuous casting mold, and δ / d ≧ at the position where the solid fraction is 0.5 to 0.9. 0.5
[Here, δ: total reduction amount (mm) by forging process, d: unsolidified thickness (mm) at forging position] is applied. According to this method, central segregation can be easily reduced without causing internal cracking or significant negative segregation.

[0006] Further, JP-A-63-183765, JP-A-SHO
In the 60-148651 publication, electromagnetic stirring is performed on the upstream side far away from the rolling position to obtain fine equiaxed crystals, which makes the solidification interface in the incompletely solidified region more columnar than the solidified interface. This suggests that unsolidified molten steel with a high solute concentration is difficult to squeeze out, the formation of the negative segregation zone is eliminated, and the central segregation is improved more effectively.

[0007]

In the above-described method of continuous forging pressure, the forging pressure is applied before the incompletely solidified region where the unsolidified molten steel remains in the center of the slab is completely solidified, and the state is changed to a completely solid phase or a state close to it. To do. For this reason, the unsolidified molten steel having a high solute concentration due to incomplete solidification in the forging portion is forcibly squeezed from the forging portion toward the incompletely solidified region and mixed with the molten steel in this portion. At this time, the unsolidified molten steel with a high solute concentration squeezed out does not mix sufficiently with the unsolidified molten steel in the incompletely solidified region, and the center of the incompletely solidified region of the slab with a low solid phase ratio (solid fraction) While re-melting the solid phase in the vicinity, a region of unsolidified molten steel with a high solute concentration is formed. In this unsolidified molten steel region, the solute concentration is further concentrated by the unsolidified molten steel having a high solute concentration which is newly squeezed out as the casting progresses (the forging pressure progresses), and changes in the casting direction. As a result, the solute concentration in the central portion of the slab after forging was changed significantly with the progress of casting (progress of forging), and the desired effect of improving center segregation could not be obtained in some cases.

[0008] In recent years, attempts have been made to form an appropriate negative segregation in the central portion of the slab in order to improve the workability of the material such as the improvement of the wire drawing workability of the wire, and it is desired to form a negative segregation zone. Some steel types are starting to appear. In this case, JP-A-63-183765
The fine equiaxed crystal as described in Japanese Patent Laid-Open No. 60-148651 and Japanese Patent Laid-Open No. 60-148651 is not always necessary, and the electromagnetic stirrer may be unnecessary. In the continuous forging pressure of the steel type for which the formation of the negative segregation zone is desired, since the amount of unsolidified molten steel having a high solute concentration is squeezed out, the concentration of the solute concentration in the unsolidified molten steel portion further proceeds, and There was a problem that the solute concentration in the central part of the slab was more changed.

Further, depending on the type of steel, fine equiaxed crystals cannot be obtained even if electromagnetic stirring is performed, and during long-time casting, the solute concentration in the center of the slab after forging changes with the progress of casting, which is desirable. In some cases, the effect of improving the center segregation could not be obtained.

[0010]

The present invention has been made to solve the above problems, and its gist is as follows. That is, the slab pulled out from the mold for continuous casting, in the method of continuously forging the casting inside the slab before the completion of solidification, within 3000 mm upstream from the forging processing portion, the electromagnetic stirrer By disposing and stirring the molten steel in the incompletely solidified region of the cast slab, the liquid phase having a high concentration of molten steel components discharged by forging pressure is uniformly dispersed in the width of the incompletely solidified region and in the thickness direction. Then, the segregation control method of the continuous cast slab is characterized in that the molten steel component concentration of the slab after forging is controlled to be constant.

[0011]

FIG. 1 schematically shows an example of a continuous casting machine suitable for applying forging processing according to the present invention. Also, FIG.
In addition, immediately before the forging pressure of a conventional continuous casting machine equipped with an electromagnetic stirrer at a position exceeding 3000 mm from the continuous forging device, the cross-sectional state of the forging portion is shown in FIG. 3 just before the continuous forging device according to the present invention. Immediately before forging pressure of a continuous casting machine equipped with an electromagnetic stirring device,
The state of the cross section of a forging part is shown. In FIGS. 2 and 3, 1 is a slab, 1a is a completely solidified shell, 1b is an incompletely solidified region in which a solid phase and a liquid phase coexist, and 1c is forced from a forging portion to an incompletely solidified region by forging pressure. The unsolidified molten steel with a high solute concentration that has been squeezed out as a target, 1d is a region of unsolidified molten steel with a high solute concentration that is formed near the center of the incompletely solidified region due to the squeezing of the unsolidified molten steel, and 1e is an incompletely solidified region. Area after forging pressure, 2 is a guide roll, 3 is an electromagnetic stirrer, 4 is a forging die for pressing down a slab, 5 is a pressure cylinder for driving the forging die 4, 6 is a pinch roll, and 7 is It is a mold for continuous casting.

According to the present invention, as shown in FIG. 3, a horizontal rotating magnetic field type electromagnetic stirrer 3 is arranged in the immediate vicinity of the forging device between the mold 7 and the forging part processed by the forging die 4. By horizontally stirring the unsolidified molten steel in the incompletely solidified region 1b near the forging pressure of the slab 1, unsolidified with a high solute concentration, which is forcibly squeezed out from the forging pressure portion toward the incompletely solidified region 1b. The molten steel 1c can be sufficiently mixed in the width of the incompletely solidified region 1b and in the thickness direction. As a result, a region 1 of unsolidified molten steel having a high solute concentration near the center of the incompletely solidified region formed by the conventional method shown in FIG.
The formation of c can be suppressed, and the concentration of solute in the unsolidified molten steel in the incompletely solidified region can be suppressed along with the progress of casting (progress of forging pressure). As a result, the slab center portion 1e after forging
The solute concentration does not change with the progress of casting, and the desired effect of improving center segregation can be obtained.

In the present invention, the position of the electromagnetic stirrer 3 is preferably close to the forging device, but can be separated up to 3000 mm. If it exceeds 3000 mm, the formation of the unsolidified molten steel region 1c having a high solute concentration in the vicinity of the center of the incompletely solidified region 1b cannot be suppressed, so that the effect of the present invention becomes small. Further, vertical rotation and electromagnetic stirring of a moving magnetic field method can also obtain similar effects although the effect is smaller than that in the horizontal direction.

It is desirable that the magnetic flux density on the surface of the slab during electromagnetic stirring is large. Generally, at least 200 Gauss (Gauss) or more is required, depending on the size of the slab to be manufactured. If it is less than 200 gauss, sufficient stirring force cannot be obtained due to stirring at the final stage of coagulation. Further, the frequency is preferably 50 Hz or less, and above 50 Hz, the magnetic flux does not penetrate deeply into the slab, and a sufficient stirring force cannot be obtained.

Further, it is needless to say that the number of electromagnetic stirring devices is not limited to one set, and a plurality of sets may be arranged.

[0016]

Example: C: 0.72 wt%, Si: 0.21 wt%, Mn: 0.50 wt
%, P: 0.010 wt%, S: 0.010 wt%, and Al: 0.0
Molten steel was continuously cast at a casting speed of 1.0 m / min to obtain a wire rod bloom with a slab size of 270 x 340 mm containing 25 wt% and the balance being substantially Fe composition, as shown in Fig. 1. By using a continuous forging device composed of the forging die 4 and the pressure cylinder 5 as described above, the solid fraction of the central portion of the cast piece at the forging pressure position (the solid fraction referred to here is the temperature of the central portion of the cast piece determined by the steel type). Casting was performed while adjusting the casting conditions (secondary cooling rate, casting rate) so that the position between the liquidus temperature and the solidus temperature was about 0.85. Further, continuous forging was performed so that the central portion became a completely solid phase.

At this time, as shown in FIG. 3, a horizontal rotating magnetic field type electromagnetic stirrer 3 placed 700 mm in front of the continuous forging device is arranged at a frequency of 2 Hz at the center of the opposing device 3. A magnetic field with a magnetic flux density of 2000 Gauss was generated, and the effect of mixing unsolidified molten steel with high solute concentration squeezed out by electromagnetic stirring in the immediate vicinity of the forging device and the incompletely solidified region before forging was investigated. In addition, as a comparative example, as shown in FIG. 2, a case where an electromagnetic stirrer of the same system was arranged at a position 4000 mm in front of the forging device was also performed.

Under these two conditions, forging pressure casting lengths of 5 m and 50 m
The center segregation index in the thickness direction of the slab at the end of the forging pressure at two levels was investigated. In addition, as shown in FIGS. 2 and 3, the survey position is a position 700 mm forward from the final forging pressure position (BB ′, D).
-D ') and the position (AA', CC ') after forging. Here, the center segregation index is the analysis value of molten steel in the tundish and 2 mm thickness 20 mm from the center of the slab to the thickness direction of the slab.
It is the ratio of the length sample to the analyzed value.

FIG. 4 shows an example of the present invention and a conventional comparative example P.
It shows using the distribution of the center segregation index (P / P ₀ ) in the thickness direction of the cast piece. If an electromagnetic stirrer is installed 4000 mm from the forging pressure position, an unsolidified molten steel region with a high solute concentration is formed near the center of the incompletely solidified region (BB '), so at a position 700 mm forward from the forging pressure position. The higher the concentration and the longer the casting length (forging length), the higher the concentration changes. As a result, in the vicinity of the slab center (A-A ') after forging, the solute concentration becomes higher in the slab center, and the longer the casting length (forging length), the higher the solute concentration and the desired center segregation improvement. No effect.

On the other hand, when an electromagnetic stirrer is installed at 700 mm from the forging pressure position, the incompletely solidified region immediately before forging pressure is agitated and mixed in the thickness and width directions. Since the area of unsolidified molten steel with a high solute concentration formed in D ') is suppressed, the concentration at the position 700 m ahead from the forging pressure position does not increase even if the casting length increases. As a result, the vicinity of the slab center after forging (CC ')
Then, the solute concentration does not become high, and the concentration becomes almost the same even if the casting length becomes long.

FIG. 5 shows changes in the center segregation index of P in the casting direction at the center position of the slab. The center segregation index of the center of the slab due to the continuous forging pressure is remarkably improved as compared with the case where there is no forging pressure, but if the electromagnetic stirrer is not arranged in the immediate vicinity of the forging pressure position, P / P ₀ over 10 m from the forging pressure position
Tend to change. However, in the case where it is arranged near the forging pressure position, P / P ₀ becomes almost constant at 5 m from the forging pressure start position. According to the present invention, the concentration of the cast slab after forging can be controlled to be constant, and a good cast slab with significantly improved center segregation was obtained.

[0022]

The segregation control method for continuous slabs according to the present invention is
In the method of continuously forging the cast slab drawn from the casting mold for continuous casting in the stage before the solidification of the cast slab is completed, the forging press working is performed between the casting forging work part and the forging work part. By placing a horizontal rotating magnetic field type electromagnetic stirrer within 3000 mm from the part to stir the molten steel in the incompletely solidified region of the slab, the liquid phase with high concentration discharged by forging pressure is incompletely solidified. The region is uniformly dispersed in the width and thickness directions, the concentration of the cast slab after forging can be controlled to be constant, and a desired center segregation improving effect can be obtained.

[Brief description of drawings]

FIG. 1 is a schematic view showing an example of a continuous casting machine suitable for applying forging processing according to the present invention.

FIG. 2 is a schematic diagram showing a state of a cross section of a forging portion immediately before forging in a conventional continuous casting machine equipped with a continuous forging device.

FIG. 3 is a schematic diagram showing a state of a cross section of a forging portion immediately before forging pressure of a continuous casting machine equipped with the continuous forging device according to the present invention.

FIG. 4 is a graph showing a comparison of the center segregation index (P / P ₀ ) of P in the thickness direction of the slab according to the example of the present invention and the comparative example of the conventional method.

FIG. 5 is a comparison of an example according to the present invention and a comparative example according to a conventional method by a change in the center segregation index of P in the casting direction at the center position of the slab.

[Explanation of symbols]

1 cast slab 1a fully solidified shell 1b incompletely solidified region where solid phase and liquid phase coexist 1c unsolidified molten steel with a high solute concentration that is forcibly squeezed from the forging portion toward the incompletely solidified region by forging pressure 1d Region of unsolidified molten steel with high solute concentration formed near the center of incompletely solidified region by squeezing unsolidified molten steel 1e Region after forging of incompletely solidified region 2 Guide roll 3 Electromagnetic stirrer 4 For pressing down the slab Forging die 5 Pressure cylinder for driving forging die 4 6 Pinch roll 7 Mold for continuous casting

Claims (1)

[Claims] 1. A method for continuously forging a cast piece drawn from a casting mold for continuous casting in a stage before the solidification of the inside of the cast piece is completed.
By disposing an electromagnetic stirrer within and stirring the molten steel in the incompletely solidified region of the slab, the liquid phase having a high molten steel component concentration discharged by the forging pressure has a width of the incompletely solidified portion, and
A method for controlling segregation of continuously cast slabs, which comprises uniformly dispersing in the thickness direction and controlling the molten steel component concentration of the slabs after forging to be constant.