JPH0628790B2 - Continuous casting method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention prevents segregation of impurity elements found in the center of thickness of continuously cast slabs, that is, in the case of steel slabs, sulfur, phosphorus, manganese and the like. The present invention relates to a continuous casting method capable of obtaining a homogeneous metal.

[Prior art and problems to be solved by the invention]

In recent years, offshore structures, storage tanks, steel pipes for oil and gas transportation,
The requirements for material properties such as high-strength wire rods are becoming more and more severe, and it is an important issue to provide homogeneous steel products. Originally, steel should be homogeneous in cross section, but steel generally contains impurity elements such as sulfur, phosphorus, and manganese, and these segregate and partially concentrate during the casting process. Becomes vulnerable. Particularly in recent years, continuous casting has been generally used for the purpose of improving productivity, yield, and energy saving, but a remarkable segregation of components is usually observed in the thickness center of the slab obtained by continuous casting. .

The above-mentioned component segregation significantly impairs the homogeneity of the final product,
Reduction in stress is desired because it causes serious defects such as cracks caused by stress acting on steel in the use process of products and the drawing process of wire rods. Such component segregation occurs when the residual molten steel flows at the final stage of solidification due to solidification shrinkage force and the like, the concentrated molten steel near the solid-liquid interface is washed out, and the residual molten steel progressively concentrates. Therefore, in order to prevent the segregation of the components, it is important to eliminate the cause of the flow of the residual molten steel.

Such molten steel flow causes include, in addition to the flow caused by solidification shrinkage, the flow caused by slab bulging between rolls and roll alignment irregularity, but the most serious of these is solidification shrinkage, In order to prevent segregation, it is necessary to roll down the slab by an amount that compensates for this.

Attempts have been made to reduce segregation by rolling down the slab, and in the continuous casting process, solidification shrinkage of the slab occurs during the period from the liquidus temperature to the solidus temperature of the slab center temperature. A method is known in which the rolling is carried out at a constant rate above the amount to be compensated.

[Problems to be solved by the invention]

However, the conventional continuous casting method has a problem that the segregation improving effect is hardly recognized depending on the conditions, and in some cases, segregation rather deteriorates, and it is difficult to sufficiently improve the component segregation. .

As a result of various investigations on the cause of the problem of the conventional method, the present inventors find that the segregation improving effect is not recognized in the case of the conventional method, or the segregation rather deteriorates. We found that it was due to the improper coagulation time and its range.

The present inventor previously mentioned in JP-A-62-275556,
The area from the time when the central part of the slab reaches the temperature corresponding to the solid fraction of 0.1 to 0.3 to the temperature corresponding to the flow limit solid fraction of 0.5 mm / min or more and less than 2.5 mm / min per unit time The continuous casting method in which the rolling is continuously reduced in a ratio, and the region from the time when the temperature of the slab center reaches the solid phase limit to the solidus temperature is not substantially reduced Proposed.

Furthermore, the present inventor has proposed a further advanced continuous casting method by assuming some formulas from a large number of experimental results and comparing them with the experimental results.

An object of the present invention is to prevent the segregation of the impurity element found in the thickness center portion of the continuously cast slab and obtain a homogeneous metal.

[Means for Solving the Problems]

According to the present invention, the unsolidified portion crater vicinity of the continuous cast slab is a continuous casting method of pulling out while continuously rolling down with a plurality of roll pairs, where the central solid fraction of the slab is 0.1 to 0.3 A pair of rolls is provided with a spacer that can be inserted into and removed from a pressing device that applies a required pressing force. The thickness of the slab was adjusted in the pressing device in a state in which the spacer was fitted when the slab passed through while adjusting the thickness so as to continuously decrease at 1.5 to 3.0 mm / m. A continuous casting method is provided which is characterized by applying a pressing force sufficient to be equal to the roll gap.

[Action]

According to the continuous casting method of the present invention, the central solid fraction of the slab is 0.
The roll band in the area of 1 to 0.3 is provided with a removable spacer, and it is 1.5 to 3.0 mm / m per unit length in the casting direction.
A light reduction is performed.

As a result, it is possible to prevent the segregation of the impurity elements found in the central portion of the thickness of the continuously cast slab and obtain a homogeneous metal.

〔Example〕

First, an example of a continuous casting machine in which the continuous casting method according to the present invention is used will be schematically described with reference to FIG.

FIG. 1 is a diagram showing a continuous casting machine to which the continuous casting method according to the present invention is applied, specifically, an example of a twin cast arc type continuous casting machine. As shown in the figure, in this continuous casting machine, the ladle 1 filled with molten steel is placed above the tundish 2, and the molten steel in the ladle 1 is poured into the tundish 2 through the sliding nozzle 11 at the bottom. It is designed to be done. Here, the opening of the sliding nozzle 11 is controlled according to the total weight of the tundish 2 including the ladle poured from the ladle 1 so that the meniscus (the position of the molten metal in the tundish) M becomes constant. Has been done.

The molten steel in the tundish 2 is poured into the mold 3 at a constant rate by vertically moving the stopper 21 that closes the bottom of the tundish. The bottom of the mold 3 is also opened, and the molten steel injected into the mold 3 is cooled by the side wall of the mold 3 to which cooling water is supplied and solidified (primary cooling) from the outside. The molten steel (cast slab) primarily cooled by the mold 3 is continuously drawn out by the roller.

The cast piece pulled out from the mold 3 is spray-cooled, for example, in a spray band, further bent by a plurality of group rolls and pinch rolls, and supplied to the reduction band.

The reduction band includes a plurality of reduction rolls R ₄₂ , R ₄₃ , R ₄₄ ,
And is configured to perform a predetermined reduction on the cast slab. Here, in the continuous casting method of the present invention,
The insertable / removable spacer is a roll band in which the central solid fraction of the slab (the ratio of the liquid phase inside the slab during casting) is in the range of 0.1 to 0.3, specifically, for example, the rolling roll R in FIG. ₄₂ ,
Provided for R ₄₃ and R ₄₄ , and per unit length with respect to the casting direction (downstream in the slab drawing direction) 1.
It is designed to apply a light reduction of 5 to 3.0 mm / m.

FIG. 2 is a side view showing the positional relationship between the dummy bar, the slab, and the roll. As shown in the same figure, the slab 4 is formed at the beginning of casting.
The tail end of is connected to the dummy bar 5. In the continuous casting equipment according to the present invention, the horizontal roll band applies hydraulic pressure to the roll 7 by the hydraulic cylinder 6 to exert a rolling down force on the slab 4 and the dummy bar 5. Normally, the rolling force acting on the dummy bar 5 is about 100 kgf / cm ² so that the dummy bar 5 does not slip, and the rolling force acting on the slab 4 is 180- Give about 200kgf / cm ² . Therefore, the thickness of the slab 4 is thicker than the thickness of the dummy bar 5, and the thickness of the dummy bar 5 gradually increases as it approaches the slab 4.

FIG. 3 is a front view showing the positional relationship between the dummy bar, the slab, and the roll. There is a fixed spacer 9 between the upper and lower rolls 7. When the upper and lower rolls 7 are kissed, the fixed spacers 9
By pressing, the space between the upper and lower rolls 7 becomes substantially the same as the thickness of the dummy bar 5. The spacer fitting / removing device 81 is laid on each roll band and operates when the cast piece 4 passes to insert and attach the insertion spacer 8 to the upper portion of the fixed spacer 9. Here, the thickness of the insertion spacer 8 is several mm to several tens.
It is about cm.

FIG. 4 is a diagram showing the positional relationship between rolls and spacers.
When the insertion spacer 8 is not provided, the distance between the upper and lower rolls 7 is almost the same as the thickness of the dummy bar 5. When the insertion space 8 is provided, the distance between the upper and lower rolls 7 becomes substantially the same as the thickness of the cast piece 4.

FIG. 5 is a diagram showing an optimum vertical roll interval gradient row in the lower portion of the light pressure. In the figure, the horizontal axis represents the distance from the meniscus, and the vertical axis represents the vertical roll interval. The casting thickness is initially equal to the mold thickness and gradually decreases slightly according to solidification shrinkage, and reaches the light pressure lower region in the region where the central solid fraction of the slab is 0.1 to 0.3. The center solid fraction of the slab is 0 in the light pressure lower region.
It is described in JP-A-62-275556 that the optimum value in the range of 1 to 0.3 is excellent in terms of the internal quality of the slab.
In this light pressure lower region, a light pressure reduction of 1.5 to 3.0 mm / m per unit length (from the meniscus) is applied in the casting direction. 1.5mm / m per unit length for casting method
Considering the minute fluctuation of the hydraulic pressure applied to the roll and the fluctuation of the thickness of the slab during casting, it is impossible to uniformly apply the light reduction to the slab under a light pressure of less than less than. When a light reduction of 3.0 mm / m or more per unit length is applied to the casting direction, the liquid phase fluctuation inside the cast piece is remarkably increased and the occurrence of internal cracks in the cast piece increases. Therefore, it is preferable to apply a light pressure reduction of 1.5 to 3.0 mm / m per unit length in the casting direction in the light pressure lower region.

Table 1 shows the effect of the removable spacer of the present invention.
If there is no insertable / removable spacer, center segregation and internal cracking are slightly generated as the quality of the slab. The reason for this is considered to be that the fluctuation of the roll interval is ± 1.3 mm regardless of applying the light reduction, and the liquid phase flow inside the slab is remarkable. On the other hand, when a light reduction is applied by the insertable / removable spacer of the present invention, the fluctuation of the roll interval is suppressed to ± 0.1 mm, and the center segregation and internal cracking as the slab quality are completely eliminated.

[Effects of the Invention] As described above in detail, the continuous casting method according to the present invention,
By providing a removable spacer on the roll band in the region where the central solid fraction of the slab is 0.1 to 0.3, and performing a light reduction of 1.5 to 3.0 mm / m per unit length in the casting direction, It is possible to prevent the segregation of the impurity element in the central portion of the thickness of the continuously cast slab and obtain a homogeneous metal.

[Brief description of drawings]

FIG. 1 is a view showing an example of a continuous casting machine to which the continuous casting method according to the present invention is applied, FIG. 2 is a side view showing a positional relationship between a dummy bar, a cast piece and a roll, and FIG. 3 is a dummy bar, a cast piece. FIG. 4 is a front view showing the positional relationship between the rolls and the rolls, FIG. 4 is a view showing the positional relationship between the rolls and the spacers, and FIG. (Explanation of symbols) 1 ... Ladle, 2 ... Danish, 3 ... Mold, 4 ... Cast piece, 5 ... Dummy bar, 6 ... Hydraulic cylinder, 7 ... Roll, 8 ... Inserting spacer , 9: Fixed spacer, 11: Sliding nozzle, 21: Stopper, 81: Spacer fitting / removing device, M: Meniscus.

Front Page Continuation (72) Inventor Yasuo Maruki 1 Kimitsu, Kimitsu-shi, Chiba Inside Nippon Steel Works Ltd. (56) References JP 63-63561 (JP, A)

Claims (1)

[Claims] 1. A continuous cast slab, in the vicinity of the crater of an unsolidified portion,
It is a continuous casting method in which a plurality of roll pairs are continuously pressed down and pulled out, and the slab has a central solid fraction of 0.1 to 0.3. A spacer is provided freely, and the thickness is adjusted so that the roll gap when the spacer is fitted into the pressing device continuously decreases toward the downstream in the slab drawing direction at 1.5 to 3.5 mm / m per unit length. In addition, the pressing device in the state where the spacer is fitted during passage of the slab is such that a sufficient pressing force is applied so that the thickness of the slab becomes equal to the adjusted roll gap. Characteristic continuous casting method.