JP3111954B2 - Continuous casting method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a slab that is actively bulged between guide rolls of a continuous casting apparatus in order to reduce the center segregation of a steel slab cast by a continuous casting method. Thereafter, the present invention relates to a method of rolling down, and more particularly to a continuous casting method capable of preventing steel leakage at the end of casting and improving casting yield.

[0002]

2. Description of the Related Art When a steel slab is manufactured by a continuous casting method, there is a problem of occurrence of an internal defect often called center segregation. The occurrence of the center segregation is a phenomenon in which component elements such as C, Mn, S, and P in the molten steel are concentrated and positively segregated in the final solidified portion at the center in the thickness direction of the slab. This phenomenon is a particularly serious problem in thick steel materials, and is known to cause a decrease in toughness and hydrogen-induced cracking in segregated portions.

[0003] The cause of the center segregation is that molten steel in which component elements such as C, Mn, S, and P are concentrated between dendrites at the end of solidification of molten steel, and remains in the center of the slab in the thickness direction. This is because the molten steel macroscopically moves toward the solidification completion point of the final solidified portion due to solidification and the flow of the molten steel caused by shrinkage during solidification or swelling of the slab called bulging. Therefore, as a measure to prevent center segregation, it is necessary to prevent the movement of the concentrated molten steel between dendrites,
It is effective to prevent local accumulation of concentrated molten steel.

As described above, it has been said that when bulging occurs in a slab during casting, center segregation occurs.
A continuous casting method in which bulging is actively caused in a slab and then reduced (hereinafter, this method is referred to as “bulging-reduction method”).
Some inventions have been proposed to prevent the occurrence of center segregation as described below. FIG. 4 is a view schematically showing an example of a continuous casting method for explaining the principle of the above-mentioned "bulging-reduction method".

[0005] Liquid layer crater end 9a of mold 1 and slab 2
A bulging force is applied to the solidified shell 2a between the crater end 9a and the solid line crater end 9a.
Continuous casting method in which the slab is subjected to rolling reduction between
254).

In the continuous casting of a slab 2 having an aspect ratio of 1.6 or less, a plurality of sets of guide rolls 3a and 3b arranged immediately below the mold 1 have a roll interval larger than the inner thickness of the lower end of the mold, and the direction of the slab thickness is increased. A method of manufacturing a slab in which bulging is performed and the slab 2 is reduced by 0.04 to 10% by another roll 5 behind the bulging (see JP-A-60-21150).

[0007] The guide roll group 3 arranged in the drawing direction from immediately below the mold 1 is stepwise increased in the thickness direction of the slab 2 to cause bulging of the slab, and the thickness of the slab is reduced by the mold. A continuous casting method for slabs in which the length is reduced to two to three times the short side, and the slab is lightly reduced by a small-diameter roll 5 in the vicinity of the crater end 9 (see JP-A-1-178355).

A reduction zone consisting of a group of light reduction rolls 5 is provided at the position of the pass line of the crater end 9 of the slab 2, and a guide in which the roll interval is widened by 5.0 to 0.5% with respect to the reference roll interval before this reduction zone. A bloom continuous casting method in which a bulging forming block composed of a group of rolls 3 is continuously provided, bulging is formed on a cast piece, and then a 4.0-0.5% light reduction is performed (see JP-A-2-235558).

Bulging occurs at a position where the solid phase ratio at the center of the slab 2 is 0.1 or less, and the maximum thickness of the slab is made 20 to 100 mm thicker than the short side length of the mold 1. Just before 1
A continuous casting method in which a reduction of 20 mm or more per pair of reduction rolls 5 is applied to reduce the amount of bulging corresponding to the amount of bulging (JP-A-9-57410
Reference).

[0010] Bulging occurs in the slab before the unsolidified thickness of the slab 2 reaches a position of 30 mm or more, and the maximum thickness of the slab is increased by 10 to 50% of the length of the short side of the mold 1. A continuous casting method in which a reduction is applied by a reduction gradient of 80 mm / m or more per slab length using at least one pair of reduction rolls just before completion to reduce an amount corresponding to a bulging amount (see JP-A-9-206903).

In the above-mentioned "bulging-reduction method", the bulging zone is formed immediately below the mold 1, as shown in FIG. The rolling zone is defined between the liquid layer crater end 9a and the solid line crater end 9 in the above description, but is generally in the vicinity of the solid line crater end 9 in.

The molten steel 8 is injected into the mold 1 through the immersion nozzle 10, and is spray-water nozzle group provided between the water-cooled mold 1 and guide rolls (3a to 3n) arranged below the mold 1. The solidified shell 2a is formed by cooling with cooling water injected from a (not shown), and becomes a slab.

The slab is pulled out of the mold while holding the unsolidified portion 2b inside. In the bulging zone, the roll spacing of the guide roll group (3a to 3n) is gradually increased, so the central part in the longitudinal direction of the slab is gradually expanded in the short side direction by the molten steel static pressure, so-called bulging occurs. I do. The slab that has undergone bulging is stepped down by the pressing roll group 5 in the pressing zone, and the solidification interface of the slab is pressed and solidified. The solidified slab is a pinch roll group
Pulled out by 7.

The bulging zone may be provided with a device 4 for stirring the molten steel in the slab. Each of the pressing rolls is provided with a pressing device 6. The rolling zone may be a pair of rolling rolls 5a.

[0015] The above-mentioned "bulging-reduction method" only specifies various conditions during the operation of steady casting, and does not elucidate any special phenomenon at the end of casting.

In the continuous casting method, since the supply of molten steel is stopped at the end of casting, it is necessary to perform a special process different from that in the steady casting. FIG. 3 is a diagram showing an example of a change pattern of a casting speed (a speed of drawing a slab) in the casting end process. As shown in the figure, using a previously prepared deceleration pattern, the casting speed is reduced according to the residual molten steel weight and residual molten steel level in the tundish at the end of casting, leaving a predetermined amount of molten steel in the tundish. To stop the injection of molten steel into the mold. Then, the last end (bottom) of the molten steel in the mold
After a cooling process (metal particles, metal flakes, water, etc.) is performed and solidification is performed, the drawing speed is increased to pull out the cast slab. An invention relating to such a casting termination method is disclosed in, for example, Japanese Patent Publication No. 3-54025.

[0017]

In order to reduce the center segregation of the slab, in the continuous casting method to which the above-mentioned "bulging-reduction method" is applied, the casting speed is reduced at the end of the continuous casting method, and the bottom treatment is performed. If the method of pulling out the slab after applying is applied, the following problem occurs.

(A) By lowering the casting speed at the end of casting, the liquidus crater end and the solidus crater end move to the mold side. Therefore, under the condition of steady casting, the bulging position and the rolling-down position may be out of the proper range, so that internal cracks and center segregation occur in the slab, and the quality of the slab deteriorates.

(B) When the solidus crater end moves to the mold side, the slab may be in a completely solidified state at the position of the normal rolling roll. In that case, in the equipment which performs unsolidification reduction, reduction becomes impossible from the viewpoint of equipment capacity. Even if the rolling can be reduced, it does not help to improve the center segregation at all.

(C) Even after the bottom processing operation, if the bulging-reduction is continued under the same conditions as in the steady casting, the unsolidified molten steel squeezed out (pushed up) by the reduction is reduced to the bottom portion (the end portion of the slab). ) To leak steel. Steel leaks are not only dangerous, but also cause damage to the casting equipment and are a major obstacle to operation.

(D) If the reduction is stopped at the end of casting,
Although the problems (b) and (c) are solved, the cross section of the slab remains in a bulged shape, and the slab has center segregation.

An object of the present invention is to provide a casting end method in which a bottom processing is performed by reducing the casting speed.
It is an object of the present invention to provide a continuous casting method that can improve the quality of a slab by applying the "bulging-reduction method" to the end of the slab and has no problem of steel leakage.

[0023]

FIG. 1 is a schematic sectional view of a vertical continuous casting apparatus for explaining a continuous casting method of the present invention. The method of the present invention can be applied to a curved continuous casting apparatus and a vertical bending continuous casting apparatus regardless of the type. The slab may be a slab or a bloom (a billet), but is particularly suitable for application to continuous casting of a slab to be used as a material such as a thick steel plate.

The gist of the present invention resides in the following continuous casting method (see FIGS. 1 and 2 (b) and (c)).

In a predetermined range between the position corresponding to the liquidus crater end 9a of the slab 2 and the position corresponding to the solidus crater end 9, the interval between the guide rolls in the short side direction of the slab is increased. A continuous casting method in which a slab is subjected to bulging and then reduced by a reduction roll by a reduction amount equal to or less than the bulging amount, and the guide roll group and the reduction roll group are rolled together with a reduction in casting speed at the end of casting. A continuous casting method characterized by narrowing an interval, making a bulging amount and a reduction amount smaller than those at the time of steady casting, and moving a reduction position to a mold side. The final stage of casting is
After stopping the injection of molten steel into the mold.

In the above method of the present invention, a temporal change pattern of the casting speed from the start of the deceleration of the casting speed to the end of the casting is determined in advance, and the predicted pouring amount from the start of the deceleration to the end of the casting in this pattern is calculated. At the time when the measured value of the molten metal weight in the tundish is determined from the calculated predicted pouring amount, it is desirable to start the pouring speed deceleration process according to the pattern and perform a pattern following process for the pouring speed pattern. (See FIG. 3).

The guide rolls for performing the bulging and the reduction are provided with a liquid phase crater end of the slab at the time of steady casting.
It is provided between 9a and the solidus crater end 9, and it is desirable that the roll interval can be adjusted up to 1.5 times the short side length of the mold.

[0028]

FIG. 1 is a schematic sectional view of a vertical continuous casting apparatus for explaining the principle of a continuous casting method according to the present invention. As shown in the figure, in the method of the present invention, after the slab 2 is positively bulged between the liquidus crater end 9a and the solidus crater end 9, the squeezing roll group 5 causes the bulging amount to be less than the bulging amount. The pressure is reduced. Therefore, as shown in FIG. 1, the continuous casting apparatus includes a bulging roll group capable of adjusting the roll interval between the liquid layer crater end 9a and the solid phase crater end 9 in the short side direction of the slab. 3b and a roll group 5 are arranged. The roll 5 of the roll group is provided with a rolling device 6.

In the method of the present invention, the bulging zone is set to be below (below) the liquid layer crater end 9a. If bulging is performed before (upward), bending stress is applied to the thin shell and cracks are formed on the inner wall of the shell. Occurs, and there is a risk of breakout.

FIG. 2 is a schematic sectional view of a slab for explaining bulging and rolling conditions in the method of the present invention. Hereinafter, the method of the present invention will be described in more detail with reference to FIG.

Operation During Steady Casting FIG. 2 (a) is a diagram showing a situation during steady casting. As shown in the figure, at the time of steady casting, the slab is bulged in the bulging zone, for example, by 10 to 50% of the length of the short side of the mold (so that the maximum thickness of the slab is 1.1 to 1.5 times the short side of the mold). , A reduction of less than the bulging equivalent amount is applied in the reduction zone. In addition, the amount of reduction is usually equal to the bulging amount, and the cast piece has the same thickness as the short side of the mold.However, even if a slab slightly smaller than the short side of the mold is produced, the amount of reduction is smaller than the bulging amount. Good.

The first half of the final stage of casting (approximately period A in FIG. 3)
In the last stage of the casting, from the time when the casting speed is reduced to the middle of the speed reduction, as shown in FIG. 2 (b), the roll interval in the bulging zone and the reduction zone is gradually narrowed according to the decrease in the casting speed. Perform moderate bulging and rolling less than during steady casting. During this period, since the solidus crater end 9 is still in the reduction zone at the time of steady casting, the reduction zone may be moved to the mold side.

The latter half of the final stage of casting (approximately period B in FIG. 3)
The casting speed is further reduced, the drawing is stopped for bottom processing, and then the speed is increased to draw the slab. During this period, as shown in FIG. 2 (c), the solidus crater end 9 moves to the mold side and enters the bulging zone at the time of steady casting. The rolling zone is moved to the mold side, and the rolling is performed at the final part in the bulging zone during steady casting. The amount of bulging and the amount of reduction are set to be smaller than those described above.

In the above operation, the bulging amount is determined as follows.
It is desirable that the maximum thickness of the slab when bulging is set to be 1.01 to 1.07 times the short side length so as to be 1.0 to 7.0%. The rolling reduction is equal to or less than the bulging amount (however, 0.5 times or more is required).

The position L _SCE of the solidus crater end 9 in the continuous casting can be calculated using the following equation as the length from the meniscus.

L _SCE = (t / K) ² × Vc / 4 where t is the thickness (m) of the slab, K is the solidification coefficient, and Vc is the casting speed (m / min).

By performing the above-described processing, it is possible to prevent the occurrence of central segregation up to the end of the slab while preventing the steel from leaking from the end of the slab, and to perform casting with a high yield.

[0038]

Example: Using a curved continuous casting apparatus, aluminum-killed carbon steel (C: 0.16 to 0.18%, Si: 0.3 to 0.4%, Mn: 1.3 to 1.4)
5%, P ≦ 0.025%, S ≦ 0.0025%, Fe: balance). The inner cross-sectional dimension of the used mold is 23
0mm, width 2300mm, casting conditions, casting speed 1.0m
/ min, the bulging amount during steady casting is 20% (46mm), and the reduction amount is 20% (46mm), equivalent to the bulging amount. I let it. FIG. 3 is a diagram showing a deceleration pattern at the end of casting used in this test.

[0039]

[Table 1]

The present invention was evaluated based on the occurrence of steel leakage from the bottom of the slab and the maximum value of the degree of center segregation at the bottom of the slab.
The occurrence of steel leakage was evaluated based on the average amount of steel leakage.

The maximum degree of P segregation was determined by cutting the cross section perpendicular to the casting direction at a position 3 m from the end of the obtained slab, collecting a test piece from the center in the thickness direction, and setting the surface of the test piece to 200 μm. The mesh is divided into meshes, and the P concentration is measured using EPMA in each section, and the ratio ([Pmax] / [Pav] between the maximum P concentration [Pmax] and the P concentration [Pave] of the base molten steel therein.
e]).

The results of these investigations are also shown in Table 1.

In Test Nos. 1 to 6 of the invention examples, the casting speed was lowered at the end of casting, and at the same time, the bulging zone amount and the reduction amount were reduced from 0.5% as shown in Table 1 to 20% as shown in Table 1. The casting was completed by lowering to 7.0%. In these invention examples, in test No. 6 , only slight leakage of steel which did not affect the operation was observed, but in the other tests, occurrence of leakage was not observed, and the P maximum segregation degree was small. However, in Test No. 5, since the amount of bulging and reduction at the end of casting was reduced to 0.5%, the maximum P segregation degree was slightly larger at 5.25.

Test No. 7 is a comparative example in which bulging and reduction during steady casting were continued even at the end of casting.
In this example, since the bulging amount at the end of casting was as large as 20%, there was a leakage of 0.05 tons of steel. In addition, since the bulging and rolling position became inappropriate due to the movement of the solidus crater end, complete reduction was not possible (20% bulging amount and 10% reduction amount), and the degree of center segregation increased to 6.8. Was.

Test No. 8 is an example of a normal continuous casting method in which neither bulging nor reduction is performed, and the P maximum segregation degree is 13.3.
And very big.

[0046]

According to the method of the present invention, by performing rolling after bulging, not only can a high quality cast slab with less center segregation be produced at the time of steady casting, but also appropriate bulging and rolling can be achieved at the end of casting. You can continue. As a result, the segregation of the center of the slab at the end of casting is also reduced, the amount cut off as poor quality is reduced, and the casting yield is greatly improved.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view of a continuous casting apparatus for explaining a continuous casting method of reducing the pressure after bulging according to the present invention.

FIG. 2 is a schematic view of a cross section of a slab for explaining a casting completion processing method in the method of the present invention, where (a) is a diagram showing a time of steady casting, and (b) and (c) are diagrams showing a casting completion processing method. It is.

FIG. 3 is a diagram showing a casting speed pattern in a casting end process.

FIG. 4 is a cross-sectional view similar to FIG. 1 for explaining a conventional method of rolling down after bulging.

[Explanation of symbols]

1. 1. Mold Slab 2a. Solidified shell 2b. Unsolidified part 3a. Guide roll 3b. Guide roll in bulging zone 4. Electromagnetic stirrer Roll roll group 6. 6. Roll-down device 7. Pinch roll Molten steel 9. Solid phase crater end 9a. Liquid phase crater end 10. Immersion nozzle 11.
Casting direction

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-9-206903 (JP, A) JP-A-9-57410 (JP, A) JP-A-2-235558 (JP, A) JP-A-4- 33757 (JP, A) JP-A-60-6254 (JP, A) JP-A-60-21150 (JP, A) JP-A-1-178355 (JP, A) JP-A-9-314298 (JP, A) JP-A-11-156511 (JP, A) JP-A-8-47758 (JP, A) JP-A-8-47759 (JP, A) JP-A-7-223053 (JP, A) JP-A-6-335759 (JP, A) JP-A-6-226414 (JP, A) JP-A-5-50201 (JP, A) JP-A-11-156508 (JP, A) JP-A-9-314289 (JP, A) Kaihei 9-122845 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) B22D 11/128 350 B22D 11/128 B22D 11/10 B22D 11/16

Claims (1)

(57) [Claims] 1. A method of casting a slab in which a gap between guide roll groups in a short side direction of a slab is widened within a predetermined range from a position corresponding to a liquidus crater end to a position corresponding to a solidus crater end. A continuous casting method in which a bulging is caused on a piece and then reduced by a reduction roll with a reduction amount equal to or less than the bulging amount, wherein a casting speed is reduced at the end of casting and a roll interval between the guide roll group and the reduction roll group. Wherein the bulging amount and the reduction amount are made smaller than those at the time of steady casting, and the reduction position is moved to the mold side.