JPH0649217B2 - Forming method of cast slab in continuous casting machine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a light-weight reduction device at the final stage of solidification and an in-machine slab molding device provided behind the device for adjusting the thickness of the slab after the complete solidification, thereby forming a square shape. The present invention relates to a slab shape forming method in a continuous casting machine for obtaining a slab shape close to the above.

[Conventional technology]

A square cross section is usually required as the shape of a steel slab used for rolling a steel bar, a wire rod, or the like. Generally, when a billet is manufactured from an ingot through a breakdown process, a billet shape with a small dimensional difference between thickness and width (hereinafter referred to as deviation deviation) and a shape close to a square can be obtained. There was no problem in rolling work.

[Problems to be Solved by the Invention] By the way, when a slab obtained from a bloom / billet continuous casting machine is directly supplied to rolling, a deviation of the slab may be a problem.

For example, in a bloom / billet continuous casting machine, there is a problem in the case where a light reduction at the end of solidification is applied as a measure against center segregation.
For improving segregation by light reduction, see “Iron and Steel” 73
(1987), S207, it is known that the segregation becomes smaller as the light reduction amount increases. On the other hand, as reported in “Iron and Steel” 73 (1987), S910, internal cracking may occur due to light reduction when the light reduction strain at the solidification interface is larger than the critical strain of the slab. It is clear. Therefore, depending on the type of steel, there are cases where light reduction is applied and cases where light reduction is not applicable, and as a result, the thickness of the slab will differ depending on whether light reduction is applied. For example, in a cast that does not apply light reduction using a square cross-section mold, a slab with the same thickness and width can be obtained, but if light reduction is applied, the slab thickness becomes a slab shape that is smaller than the width. I will end up.

If a slab with a large deviation is obtained in this way, the problem of lowering the coverage rate due to ultrasonic deep scratches for the purpose of detecting internal defects, and the twisting and caliber insufficient filling of bar steel and wire rods, etc. It causes work problems and is not preferable. Therefore, it is extremely important to obtain a slab shape with a small deviation between edges and a shape as close as possible to a square, but an apparatus and a method for forming a slab shape in a continuous casting machine have not been found to date.

The present invention has been made in view of the above problems, and provides a slab shape forming method in a continuous casting machine that obtains a square cross-section slab capable of eliminating twist rolling and performing a stable rolling operation while satisfying the caliber filling degree.

[Means for Solving the Problems]

The present invention relates to a steel bloom having a light-rolling device at the end of solidification.
A slab shape forming method in a billet continuous casting machine,
A slab is cast by a mold having a cross-sectional size thickness / width ratio of 1.03 to 1.06, and the slab is pressed in the thickness direction by a final solidification light reduction device, and inside the continuous casting machine behind the final solidification light reduction device. A method for molding a cast shape in a continuous casting machine, characterized in that a cast molding apparatus is provided, and the cast piece is pressed down in the thickness direction so that the cross-sectional shape of the cast piece finally becomes square by the cast molding apparatus. Yes, [% Mn] / [% S] <20
For steel types below, the reduction amount of the slab by the final stage solidification light reduction device is 3% or less, the reduction amount of the slab after complete solidification by the in-machine slab molding device is 5% or less, and the sum is 3 to the slab thickness.
A slab shape with a square cross section is obtained by adjusting the range to 6%.

[Action]

 The present invention will be described in detail below along with the operation.

The condition for light reduction to improve the center segregation of the slab is that the concentrated molten steel does not flow, depending on the size of the slab and the shape of the reduction roll when the solid fraction (fs) in the central part is in the range of 0.3 to 0.7. A reduction amount corresponding to the amount of solidification contraction is given. According to the investigation by the present inventors, the appropriate amount of reduction is 3 to 6% of the thickness of the cast piece for the reason described below. That is, when the reduction amount is less than 3%, the solidification shrinkage flow remains after the reduction to cause V segregation, and when the reduction amount exceeds 6%, the molten steel flow based on the overpressure is induced to induce reverse V segregation. This is because the formation of

If the final solidification light reduction is applied to all steel grades, cast pieces with a mold cross-sectional size of thickness / width ratio of 1.03 to 1.06, and a light reduction of 3 to 6% at the end of solidification, in this case There is no problem in obtaining a square slab.

However, in the case of a steel type having a [% Mn] / [% S] ratio of less than 20 and having a high cracking susceptibility, such as sulfur free-cutting steel, internal cracking occurs when a light reduction amount exceeding a certain limit value is applied, which is a problem. That is, FIG. 1 shows the measurement results of the present inventors regarding the relationship between the total length of internal cracks generated by the light reduction at the end of solidification and the reduction amount (reduction amount / slab thickness × 100,%). There is no internal cracking under the light reduction, while in the case of more than 3% light reduction, the internal cracking increases as the light reduction amount increases. Therefore, for low [Mn] / [S] steel, a light reduction of 3% or less at the end of solidification is possible, but a light reduction of more than 3% is not applicable.

On the other hand, all steel grades were cast in the same mold with a square cross section, and light reduction was performed with a light reduction of 3 to 6% and low [Mn].
If the / S steel grade is not subjected to light reduction to prevent internal cracking, the thickness of the cast piece becomes 3 to 6% thinner in the light reduction applied steel grade. In this way, in a steel bloom / billet continuous casting machine with a final solidification stage light reduction device, if there is a final solidification stage light reduction applicable steel grade and an inapplicable steel grade, a slab shape with a square cross section can be obtained with either steel grade. There is a problem that can not be.

For steel types applicable to light reduction, as described above, the mold cross-sectional size was made in advance so that the thickness / width ratio = 1.03 to 1.06, and if a 3 to 6% final solidification light reduction was performed, a square slab would be obtained. can get. On the other hand, for steel grades with a [% Mn] / [% S] ratio of <20, a light reduction of 3% or less is performed at the end of solidification, and a reduction of 5% or less is added after complete solidification, and the sum is If the amount of reduction is adjusted to 3 to 6%, it is possible to finally obtain a square slab shape.

When rolling the cast piece after complete solidification, it is necessary to prevent the occurrence of flaws on the surface of the cast piece due to rolling strain. Surface strain and strain rate are mentioned as an index showing the occurrence of surface cracks.
In the reduction model by the reduction roll 2 of the cast slab 1 shown in FIG. 2, the surface strain (ε) and the strain rate () are respectively given by the following equations.

_{ε = 1n (1 2/1} 1) = 1n (h 1 / h 2) ...... (1) = ε / t = ε / {R (h 1 / h 2)} 1/2 / VC ...... (2 ) where 1 _1: pressure before long, 1 _2: after reduction length, h _1: pressure before the thickness, h _2: pressure thickness after, R: reduction roll radius, Vc: the casting speed.

Slab reduction (h ₁ -h ₂ ) / h ₁ × 100 (%) and surface strain (ε),
FIG. 3 shows the measurement results regarding the strain rate () and the presence or absence of cracks on the surface of the slab. It was found that when the reduction amount is 5% or less, the surface strain and the strain rate are small, and the slab surface cracks do not occur.

From the above, in the present invention, first, the mold cross-sectional size is set in advance so that the thickness / width ratio falls within the range of 1.03 to 1.06. Then, for the steel types applied to the final solidification stage reduction, after applying a light reduction of 3 to 6%, which is necessary for improving the center segregation, the ingot molding device after complete solidification is not positively reduced and the slab shape with a square cross section is not positively applied. To get

For steel grades with a [% Mn] / [% S] ratio of less than 20, a reduction of 3% or less is applied with a light reduction device in the final stage of solidification, and a reduction of 5% or less is applied with an in-machine forming device after solidification. The sum is 3-6%
The slab shape with a square cross section is obtained by adjusting so that

The in-machine forming apparatus that enables the above-described method is one in which several pairs of reduction rolls are incorporated in the segment, and both ends of the upper roll of the upper and lower rolls can be moved up and down by hydraulic cylinders, and the driving force for withdrawing the slab is also provided. When it is necessary to supplement the above, the upper and lower rolls are driven by an electric motor via a gear box. Further, a mechanical stopper is installed between the cylinder rod and the segment frame so that the vertical pressing roll is kept at a predetermined interval.

According to the method described above, it is possible to obtain a slab with a square cross section for both the end-solidification light reduction applicable steel grade and the inapplicable steel grade from the same mold cross-sectional size with the same continuous casting machine.
Therefore, problems related to the adjusting work such as ultrasonic deep scratches on the slab due to the deviation of the deviation, and problems related to rolling such as twist rolling and insufficient caliber filling can be solved.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings. The billet continuous casting machine shown in Fig. 4 has a cross sectional size of 162 mm.
When manufacturing a slab of × 162 mm, thickness 169 mm × width 162 mm
Steel for machine structural use (S45C) using mold 3, guide roll and secondary cooling zone 4 with a cross-sectional size of (thickness / width ratio = 1.043)
Was cast at a casting speed of 2.5 m / min.

In the final solidification light reduction device 5 installed at the position of L ₁ = 21.3 m from the meniscus, a 7 mm light reduction was applied to the cast slab 6 with a hydraulic cylinder for the purpose of improving center segregation [reduction amount = (7/169) x 100 = 4.
1%]. In this casting, the mechanical stopper was set so that the vertical interval of the pressing roll 8 in the in-machine forming device 7 installed at the position of L ₂ = 29.5 m from the meniscus was 162 mm, and the casting after the final stage of light solidification Since the piece is allowed to pass, the cross-sectional dimension of the obtained cast piece 9 is 162 mm.
It had a square cross section of × 162 mm.

Next, sulfur free-cutting steel (1.00% Mn, 0.30% S, [% Mn] /
[% S] = 3.3) with the same continuous casting machine, thickness 169 mm x width 162 mm
The same mold 3 of No. 1 was used for casting at a casting speed of 2.5 m / min. Since this steel type has a high cracking susceptibility, based on the results shown in Fig. 1, the steel plate 6 has a diameter of 3 mm (rolling down amount = (3/169) x 100) by controlling the hydraulic pressure of the rolling down roll by the light rolling down device 5 at the final stage of solidification. = 1.8%]
After the light reduction, the complete roll was passed through the in-machine molding device 7 in which the interval between the reduction rolls 8 was set to 162 mm. As a result, a reduction of 4 mm [Reduction amount = (4/166) x 100 =
2.4%]. In this case, the total reduction amount is 7 mm
[(7/169) × 100 = 4.1%], and the obtained slab 9 is 1
It had a square cross section of 62 mm × 162 mm, and at the same time, had good quality with no internal solidification or surface cracking caused by unsolidified light reduction and post solidification reduction.

Cross-sectional dimensions of the billet slab cast according to the present invention (thickness,
The distribution of (width) is shown in FIG. 5 together with a comparative example (conventional method). The slab according to the present invention has a square cross section whose thickness and width are almost equal, but in the comparative example, a square cross section is used as a mold, and no in-machine molding device is applied. Since thin slabs were generated in the rolled steel, the variation in thickness was large.

〔The invention's effect〕

As described above, according to the present invention, as a result of obtaining a slab shape having an accurate square cross section, the coverage factor in ultrasonic deep scratches is improved and the dead zone is reduced, so that the internal defect detection capability is improved. In addition, since problems such as twist rolling and insufficient caliber filling are eliminated in rolling of steel bars and wire rods, stabilization of rolling work and the surface flaw level of the product are remarkably improved, and the effect is extremely large. Further, since it is not necessary to replace the mold and the support roll depending on whether or not the light reduction is applied, the production capacity of the continuous casting machine is improved.

[Brief description of drawings]

Fig. 1 is a drawing showing the relationship between the total length of internal cracks and the amount of reduction due to light reduction at the end of solidification, Fig. 2 is a perspective view showing the reduction model of a slab, and Fig. 3 is the reduction amount and surface strain of a slab after solidification. FIG. 4 is a schematic side view showing the continuous casting machine in the embodiment, and FIG. 5 is a drawing showing the distribution of the cross-sectional dimension of the cast slab. 1, 6 ... Slab, 2, 8 ... Reduction roll, 3 ... Mold, 4 ... Guide roll and secondary cooling zone, 5 ... End-solidification light reduction device, 7 ... In-machine forming device

Claims (2)

[Claims] 1. A method for forming a shape of a slab in a bloom / billet continuous casting machine of a steel having a light-rolling final stage solidification device, wherein the slab is cast by a mold having a cross-sectional size thickness / width ratio of 1.03 to 1.06. , The slab is reduced in the thickness direction by a final solidification light reduction device, and a slab molding device in a continuous casting machine is provided behind the final solidification light reduction device, and the slab cross-sectional shape is finalized by the slab molding device. 1. A method of forming a shape of a cast piece in a continuous casting machine, which comprises pressing the cast piece in a thickness direction so that the cast piece has a square shape. 2. In a steel grade of [% Mn] / [% S] <20, the reduction amount of the cast piece by the final stage solidification light reduction apparatus is 3% or less, and the reduction rate of the cast piece after complete solidification by the in-machine cast piece forming apparatus. The method for forming a cast piece in a continuous casting machine according to claim 1, wherein the cast piece shape having a square cross section is obtained by adjusting the amount to 5% or less and adjusting the sum within a range of 3 to 6% of the cast piece thickness. .