JPH0663715A - Method for executing rolling reduction to continuously cast bloom at end stage of solidification - Google Patents

Abstract

PURPOSE:To prevent the generation of internal cracks in a cast bloom by light rolling reduction in the end period of the solidification and to improve the center segregation in a continuous casting for the bloom and billet. CONSTITUTION:Plural pairs of the light rolling reduction rolls 7 are arranged in the vicinity of the crater end of a cast bloom. One or more pairs of guide rolls 8 setting the inclination corresponding to solidified shrinkage of the cast bloom to the gap of each guide roll pairs are disposed just before the light rolling reduction roll 7. The gap between the final guide rolls are set as the equal as the gap between the light rolling reduction roll 7 at the tip part, and after guiding the cast bloom, the light rolling reduction is applied.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a light-end final solidification of a slab for preventing internal cracking of the slab due to light-end final softening and improving center segregation in continuous casting of bloom billets. Concerning the law.

[0002]

2. Description of the Related Art Conventionally, as a measure against center segregation of continuously cast slabs, refining of solidification structure by applying low temperature casting or electromagnetic stirring, that is, dispersion of concentrated molten steel has been the mainstream. Recent studies suggest that the cause of central segregation is molten steel flow due to solidification contraction in the final stage of solidification, and that light reduction at the final stage of solidification, which directly prevents this molten steel flow, is most effective in preventing segregation. Are known. For example, Patent No. 128
The invention relating to No. 7382 is a method of rolling down between the tip of the liquidus crater and the tip of the solidus crater of the slab in a steady drawing process by one or a plurality of pairs of rolling rolls.

Regarding the method of light pressure reduction, for example, Japanese Patent Publication No.
No. 9-39225 discloses a method of rolling down at a gradient of 0.5 to 2.0 mm / m near the crater end, and in Japanese Laid-Open Patent Publication No. 63-108955, the rolling reduction amount / unsolidified thickness ratio at rolling down. Is 0.5 to 1.0, the solid fraction of the slab center is 0.5 to 0.8, and the rolling strain is 0.2% or less.
Further, Japanese Patent Application Laid-Open No. 60-184455 describes a method of rolling down a final-stage solidified slab having a thickness solidification rate of 55% or more by a crown roll.

As described above, it is known that application of a light reduction under a certain appropriate condition is effective for improving the center segregation. Mutual relations, etc. are not mentioned although they are useful in operation.

[0005]

Generally, the light reduction at the end of solidification for improving the center segregation is influenced by steel type, slab size, central solid fraction (fs), etc. Receive strongly.

In a bloom / billet continuous casting machine, since the slab size is relatively small, bulging and bulging straight cracking due to the static pressure of molten steel are less likely to occur. Therefore, the guide roll zone length is generally short, and it is not necessary to install the guide roll until the end of solidification or the completion of solidification as in a slab continuous casting machine. As described above, in the conventional bloom / billet continuous casting machine, the slab itself can be expected to have a self-supporting effect. Therefore, as shown in FIG. The guide roll 3 is generally not arranged.

However, when a light reduction roll is installed at the final stage of solidification as a measure against center segregation, its proper position is, for example, the solid fraction of the central part of the slab, fs = 0.5 to 0.8, or the thickness. It is said that the solidification rate is preferably 55% or more, and the position is on the downstream side, which is considerably distant from the support rolls and guide rolls following the lower end of the mold. Therefore, it is usual that no roll is installed in the middle stage of solidification between the guide roll 3 and the light reduction roll.

The inventor investigated the relationship between the cast piece thickness and the casting speed in the conventional continuous casting machine shown in FIG.
Since the length of the guide roll 3 is short and there is no support roll for the slab 6 except the pinch roll 4 and the straightening roll 5, the slab thickness is likely to vary due to variations in the casting speed and the slab cooling conditions.

When a light reduction is applied to a slab with such a bloom billet continuous casting machine, the thickness of the slab that has passed through the guide roll is larger than the target size due to some casting conditions. However, there is a problem in that the slab is subjected to an excessive reduction due to the slab passing through a light reduction roll set at a predetermined interval and a reduction gradient, which causes internal cracking. On the contrary, when the thickness of the slab that has passed through the guide roll is smaller than the target dimension due to the influence of some casting conditions, the slab is set to a light reduction roll set at a predetermined interval and a reduction gradient. However, there is a problem in that a predetermined amount of reduction cannot be secured even when the slab passes, the reduction gradient is small, and the center segregation is not improved.

As described above, the thickness of the cast slab is closely related to the occurrence of internal cracks during light reduction and the effect of improving segregation. Therefore, it is extremely important to adjust the target thickness.

[0011]

Means for Solving the Problems The gist of the present invention is, in continuous casting of a bloom or a billet, when applying a reduction to a slab with a light reduction roll installed near the crater end of the slab, immediately before the light reduction roll. One or more pairs of guide rolls are arranged in the slab, and the gap between each pair of guide rolls is gradually reduced along the drawing direction of the slab so that all of the guide rolls come into contact with the light reduction application surface of the slab. In addition, the final guide roll gap is set to be equal to the gap of the leading light reduction roll to guide the slab, and then the light reduction is applied, which is a final solidification light reduction method for a continuous cast slab.

[0012]

In the present invention, the guide roll is arranged immediately before the light reduction roll, and the gap of the final guide roll is set to be equal to the gap of the leading light reduction roll or set slightly wider toward the upstream side of the continuous casting machine. The thickness of the slab just before the light reduction is adjusted to a predetermined value by guiding the slab with the slab. In other words, the thickness of the cast piece that is cast into the light draft zone is adjusted by the guide roll immediately before, and is equal to or slightly larger than the interval between the leading light draft rolls, so ensure a predetermined draft amount. Is possible.

The present invention, as shown in FIG.
In the billet continuous casting machine, the solid fraction of the central part is fs = 0.2 to
At the final stage of coagulation, which corresponds to 0.7, install the light reduction roll 7,
Immediately before the light pressure roll 7, one or more pairs of guide rolls 8 are arranged. The length of the guide roll zone, that is, the number of installed rolls is determined according to the bulging amount determined by the slab size, steel type, roll pitch, casting speed, and the like.
That is, when the expected amount of bulging is large, the number of guide rolls is increased until the guide rolls 3 are closest to each other.

The interval between the exit rolls of the guide rolls 8 is made equal to the leading light pressure roll interval, and the roll intervals within the guide roll group are set by giving a predetermined inclination in anticipation of solidification shrinkage of the slab. . That is, the guide roll has a role of adjusting the thickness of the cast slab within a predetermined range and guiding it to the light reduction roll group.

[0015]

EXAMPLES When manufacturing a slab having a cross-sectional size of 250 mm × 300 mm with the Bloom / Billet continuous casting machine shown in FIG. 1, a mold 1, a support roll 2, a guide roll 3 (installed up to a distance of 6 m from the meniscus), Pinch roll 4,
A light reduction roll 7 is installed behind the straightening roll 5, eight pairs of guide rolls 8 are arranged immediately in front of the light reduction roll 7, and a slab 6 of bloom for high carbon steel wire rod (0.80% C steel) is provided. Was cast at a casting speed of 1.2 m / min.

Here, the guide rolls 8 are arranged at a pitch of 350 to 400 mm within a range of 15.0 to 17.6 m from the meniscus, and the roll spacing is 258 mm on the entrance side.
It was set to 257 mm on the delivery side. On the other hand, the light reduction roll 7 is 400 within the range of 18.0 to 22.0 m from the meniscus.
Eight pairs were arranged at a pitch of ˜700 mm, and the roll interval was set to 257 mm on the entrance side and 250 mm on the exit side. That is,
The amount of light reduction was 7 mm. Further, a hydraulic cylinder 9 which operates at a maximum of 180 kg / cm ² was connected to each of the rolling rolls, and a hydraulic control device 10 was installed so that the cast slab 6 was rolled down by a command from a computer 11.

The internal cracking and the center segregation level of the cast piece cast by this method are shown in FIG. 3 together with a comparative example in which the guide roll 8 is opened in the continuous casting machine of FIG. As is clear from FIG. 3, in the method of the present invention, generation of internal cracks was not recognized, and a good level of center segregation was obtained.

[0018]

According to the present invention, it is possible to reliably secure a predetermined amount of reduction, and solve the problems such as the occurrence of internal cracks due to excessive reduction and the decrease in segregation improvement effect due to insufficient reduction. Further, by obtaining the slab having a good center segregation level, the soaking diffusion treatment which has been conventionally performed is not required, and the quality can be improved and the manufacturing cost can be reduced.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention.

FIG. 2 is a view showing a conventional bloom billet continuous casting machine.

FIG. 3 is a diagram showing internal cracking and center segregation degree.

[Explanation of symbols]

 1 mold 2 support roll 3 guide roll 4 pinch roll 5 straightening roll 6 slab 7 light reduction roll 8 guide roll 9 hydraulic cylinder 10 hydraulic control device 11 computer

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hirofumi Said 12 Nakamachi, Muroran-shi, Hokkaido Nippon Steel Corporation Muroran Works

Claims (1)

[Claims] 1. In continuous casting of a bloom or billet, when applying a reduction to a slab with a light reduction roll installed near the crater end of the slab, one or more pairs of guide rolls are provided immediately before the light reduction roll. Then, the gap between each pair of guide rolls is gradually reduced and set along the drawing direction of the slab so that all of the guide rolls come into contact with the light reduction application surface of the slab, and the final guide roll gap is set at the top. A light reduction method at the end of solidification of continuous cast slabs, which is characterized by guiding the slab at the same setting as the gap of the reduction roll and then applying a light reduction.