JP3114679B2 - 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 continuous casting method for reducing the segregation at the center of a steel slab. More particularly, the present invention relates 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 steel plates, 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 countermeasure for preventing center segregation, it is effective to prevent the movement of the concentrated molten steel between dendrites and to prevent the local accumulation of the 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. 3 is a view schematically showing an example of a continuous casting apparatus 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).

As shown in FIG. 3, in a normal continuous casting method, molten steel 8 is injected into a mold 1 through an immersion nozzle 10 and is cooled by a water-cooled mold 1 and a guide roll group 3 arranged below the mold. Cooled by cooling water injected from a spray nozzle group (not shown) provided between the rolls,
A solidified shell 2a is formed and becomes a slab 2 and a pinch roll group 7
It is pulled out by.

In the continuous casting apparatus that implements the above-mentioned "bulging-reduction method", the guide rolls 3a to 3n provided between immediately below the mold 1 and the solidus crater end 9 are provided. The roll interval of the guide roll group 3 is gradually increased, and a roll-down roll group 5 is provided between the guide roll 3n and the solid-phase crater end 9. The range of the guide roll group 3 is called a bulging zone, and the range of the roll group 5 is called a rolling zone. 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.

The slab is pulled out while holding the unsolidified portion 2b therein, and the central portion in the longitudinal direction of the slab is gradually expanded in the short side direction in the bulging zone, so-called bulging occurs. The bulging slab is reduced step by step by the reduction roll group 5 in the reduction zone,
The solidification interface of the slab is pressed and solidified. The solidified slab is pulled out by the pinch roll group 7.

The above-mentioned "bulging-reduction method" has only specified various conditions during the operation of steady pouring, and has not clarified any special operation at the end of pouring.

In general, at the end of continuous casting, the casting speed (drawing speed) is gradually reduced, casting is stopped once, and an operation called bottom material injection is performed. Such processing at the end of time is troublesome, not only lowering the work efficiency but also deteriorating the quality of the slab end (bottom). In order to solve this problem, there is a method of maintaining a steady drawing speed without decreasing the casting speed even at the end of casting (see, for example, JP-A-6-262323). This method is referred to herein as the “steady speed casting end method”.

It is considered that an ideal continuous casting method can be obtained by combining the above-mentioned "bulging-reduction method" and the above-mentioned "steady speed casting end method". However, the combination has a major problem to be solved. is there.

[0017]

The above-mentioned "bulging-
In the "rolling-down method", the molten steel in the bulging portion is squeezed out by a subsequent rolling-down and is pushed up above the slab. Therefore, in the state where hot water supply to the mold is stopped at the end of casting, if the slab is continuously reduced while the steady drawing speed is maintained, the pushed molten steel overflows from the final end (bottom) of the slab. So-called "leaking steel" occurs. This steel leakage is a serious problem that is not only dangerous to workers, but also damages casting equipment including guide rolls. In order to avoid steel leakage, it is only necessary to stop rolling down the slab at the end of casting, but this makes it impossible to eliminate center segregation, and eventually a considerable length of the end of the slab remains center segregation Poor quality results in a significant deterioration in casting yield.

It is an object of the present invention to improve the quality of a slab by applying the "bulging-down method" to the end of the slab even in the "steady speed casting end method". An object of the present invention is to provide a continuous casting method which does not have the above problems.

[0019]

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 lies in the following continuous casting method (see FIGS. 1 and 2).

The distance between the position corresponding to the liquidus crater end 9a of the slab 2 and the position corresponding to the solidus crater end is extended within a predetermined range of the guide roll group 3b in the direction of the short side of the slab. This is a continuous casting method in which a bulging is caused on a piece, and then a reduction amount equal to or less than the bulging amount is provided by a reduction roll group 5.In the final stage of casting, the guide roll is drawn while maintaining a constant drawing speed. Wherein the bulging volume is larger than the bulging volume during steady casting. Here, the end of casting refers to a state after the injection of molten steel into the mold is stopped.

In the method of the present invention, the bulging volume to be increased in the last stage of the casting is a volume V ₂ (hereinafter referred to as “additional bulging volume”), which is a bulging volume V ₁ during steady casting (hereinafter referred to as “additional bulging volume”). Steady bulging volume "
It is desirable to determine the value so that it is 1.0 times to 2.0 times.

[0023]

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. Then, at the end of casting, the bulging volume is increased without changing the casting speed, that is, while maintaining the casting speed (drawing speed) at the time of steady casting.

In order to terminate the casting while keeping the casting speed at the final stage of the casting at the steady casting speed, the above-mentioned JP-A-6-262323 is used.
The method proposed in the above publication, that is, a method of controlling a drawing speed pattern or a water amount pattern of secondary cooling water immediately below a mold based on a predetermined drawing length can be adopted.

There are the following methods for increasing the bulging volume. Of course, these may be used in combination.

The bulging range (the range in which the interval between the guide rolls 3b is increased) is the same as that in the case of steady casting, and the roll interval in that range is made larger than that in the case of steady casting.

The bulging range is extended to the mold side from the range at the time of steady casting. However, this extended bulging range must also be between the liquidus crater end 9a and the solidus crater end 9.

FIGS. 2A and 2B are diagrams schematically showing an embodiment of the "bulging-reduction method" in a curved continuous casting apparatus, wherein FIG. 2A shows a state at the time of steady casting, and FIG. In this example, the bulging range is expanded by the above method at the end of casting.

In the continuous casting method in which the slab is positively bulged and then reduced, if the slab is drawn at a constant speed in a state where molten steel is not injected into the mold, shrinkage of the slab due to solidification and reduction of the slab are reduced. As a result, the cross-sectional area of the slab becomes smaller,
The phenomenon of squeezing out molten steel (leakage) occurs.

In the method of the present invention, for example, as shown in FIG. 2 (b), the length L ₁ of the bulging range in the steady state is set to prevent the occurrence of the squeezing phenomenon (steel leakage) of molten steel at the end of casting. extending by a length L ₂ on the upstream side of the casting mold side.

It should be noted, steady bulging volume is determined by integrating the distance enlargement of the guide rolls in the range of a length L ₁ (the difference between the roll clearance when not roll distance and bulging at which to bulging). In fact, the average value of the distance enlargement of pairs of guide rolls as T _1,
The steady bulging volume may be approximately determined by T ₁ × L ₁ .
The additional bulging volume can be calculated in a similar manner.

FIG. 2 shows an example in which the bulging range is extended in the direction of the mold. However, the present invention is not limited to this method, and the bulging range is the same as in the case of steady casting (that is, as shown in FIG. 2A). Then, the interval between the guide rolls in the range may be enlarged to secure an additional bulging volume. In either case, the additional bulging volume V ₂ is equal to the steady bulging volume V ₁
It is desirable to choose from 1.0 times to 2.0 times of.

If the additional bulging volume V ₂ is less than 1.0 times the steady bulging volume, steel leakage may occur,
If it exceeds twice, the shrinkage cavity length at the end of the slab becomes large, and the yield of the slab deteriorates.

The timing at which additional bulging is performed is also important. If the time is too early and before hot water supply is stopped, the level of the molten metal in the mold temporarily drops, but the effect of preventing steel leakage cannot be obtained because hot water is still being supplied. On the other hand, if the timing of performing the additional bulging is too late, steel leakage occurs because the tip of the solidified shell comes off the mold at a time when the level of the molten metal is insufficient. Therefore, it is desirable to perform the additional bulging within 20 seconds after the completion of the molten steel injection.

In the method of the present invention, as shown in FIG. 1, the bulging zone is set between the liquidus crater end 9a and the solidus crater end 9 of the slab. That is, one feature is that the roll interval of the guide roll group 3a is widened in a predetermined range between the liquidus crater end 9a and the solidus crater end 9. It is desirable to increase the roll interval stepwise (that is, to gradually increase the roll interval from the top to the bottom).

The slab 2 drawn from the mold is drawn from the lower part of the mold to the liquidus crater end 9a while narrowing the roll interval so as not to cause bulging.
Guide roll 3b at position past liquidus crater end 9a
Bulging occurs between -1 and guide roll 3b-n. As described above, when the bulging range is extended at the end of casting, the enlarged range is set to be within the bulging zone.

FIG. 4 is a diagram schematically showing a cross section on the short side of the slab when bulging is caused. FIG. 4A shows bulging between immediately below the mold and the liquidus crater end. (The proposed method), (b)
FIG. 4 is a diagram showing a case where bulging is performed after the liquidus crater end (the method of the present invention).

When bulging is performed before the liquidus crater end 9a, as shown in FIG.
Since the material strength is low, the short side S is bent by bulging, a tensile stress F acts on the solidification interface side of the solidified shell 2a-1 on the short side S side, and a crack C is generated inside the solidified shell.

On the other hand, when bulging is performed after the liquidus crater end 9a, as shown in FIG. 4B, the short side S does not bend due to bulging, and the solidified shell 2a on the long side L side does not deform. Due to the gentle deformation, stress does not concentrate at a specific position on the solidification interface, and no crack occurs inside the solidification shell.

In the case of bulging as described above, the length of the bulging zone should be short, and the location of the bulging zone should be far from the mold in view of the alignment control of the continuous casting machine and the ease of operation.

The bulged slab is reduced in a reduction zone. This reduction is also performed at the end of casting. The reduction amount is equal to or less than the bulging amount (the maximum thickness of the slab increased by the bulging). An amount of reduction exceeding the bulging amount is not necessary for reducing center segregation, but also reduces the length of the short side of the already solidified slab, so an extremely large reduction force is required. And the burden on the equipment increases.

[0042]

EXAMPLE Using a curved continuous casting equipment shown in FIG. 2, an aluminum-killed carbon steel (C: 0.16 to 0.18%, Si: 0.3 to 0.4%, M
n: 1.3-1.45%, P ≦ 0.020%, S ≦ 0.004%, Fe: balance) slab casting test was performed. The inner cross-sectional dimension of the used mold is 235 mm in thickness and 2260 mm in width. Steady casting speed
The casting speed was maintained at 0.85 m / min at the end of casting.
The amount of cooling water in the guide rolls is 2 liters in specific water / molten steel
kg.

The length L ₁ of the bulging zone during steady casting is
1.5 m, the average bulging amount T ₁ in this range was 30 mm, the reduction amount was 30 mm, and the length L ₂ of the bulging zone added at the end of casting and the average bulging amount T ₂ in the range were changed as shown in Table 1. Was. The time for expanding the roll interval for additional bulging was set at the same time as the end of pouring.

[0044]

[Table 1]

As a comparative example, a test was conducted in which casting was completed at a constant drawing speed without additional bulging.

The effect was evaluated based on the shrinkage cavity length at the terminal end (bottom) of the slab and the maximum segregation degree of phosphorus (P).

FIGS. 5A and 5B are diagrams showing a method for examining the shrinkage cavity length, wherein FIG. 5A is a perspective view of the end portion of the cast slab, and FIG. 5B is a sectional view taken along line AA. The center of the end of the slab was cut in the casting direction (AA) as shown in FIG. (A), and the shrinkage cavity length shown in (b) was measured. That there is a large shrinkage cavities length, compared bulging amount V ₁ of the steady state casting means that bulging amount V ₂ was added at the end casting is large, therefore means a decrease in the yield of the slab. On the other hand, a shrinkage cavity length of 0 (zero) mm indicates that steel leakage has occurred. Therefore,
It is desirable that the length of the shrinkage nest exceeds 0 and is as short as possible.

The maximum P segregation degree was obtained by cutting a section perpendicular to the casting direction at a position 5 m from the end of the obtained slab.
Take a test piece from the center in the thickness direction and apply 20
0μm mesh, and P
The concentration was measured, and the ratio ([Pmax] / [Pav] between the maximum P concentration [Pmax] and the P concentration [Pave] of the base molten steel therein was measured.
e]).

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

In Test No. 1 of the invention, the bulging zone length L ₂ to be added at the end of casting is 3 m, the average bulging amount T ₂ is 0.02 m, and the bulging capacity V ₂ (3.
0 × 0.02 = 0.060m ² ) is the bulging capacity V during steady casting
₁ (1.5 × 0.03 = 0.045m ² ) 1.33 times,
The shrinkage cavity length of the slab is as good as 0.2 m.

In Test No. 2, the additional bulging zone length L ₂ is 3 m, the average bulging amount T ₂ is 0.03 m, and the additional bulging capacity V ₂ (3.0 × 0.03 = 0.090 m ² ) at the time of steady casting. This is the case where the bulging capacity V ₁ is set to 2.0 times of 0.045 m ² , and the shrinkage cavity length of the slab is 0.5 m.

In test No. 3, the additional bulging zone length L ₂ was 1.5 m, the average bulging amount T ₂ was 0.05 m, and the additional bulging capacity V ₂ (1.5 × 0.05 = 0.075 m ² ) was obtained during steady casting. The bulging capacity V ₁ (0.045 m ² ) is 1.67 times, and the shrinkage cavity length of the slab is as good as 0.35 m.

As is clear from Table 1, the maximum segregation degree of P is 4 or less for the test pieces of Test Nos. 1 to 3 of the invention examples, which is less than the allowable maximum segregation degree of the steady casting part.

On the other hand, in the comparative example, the maximum P segregation degree is
Although it was good at 3.7, the shrinkage cavity length was 0 m, and steel leakage occurred at the end of casting. This indicates that the molten steel squeezed out by the reduction flowed out from the slab end (bottom) because the normal “bulging-reduction method” was continued without performing additional bulging.

[0055]

According to the method of the present invention, in a continuous casting method in which a slab during casting is bulged and then reduced, the amount of bulging is increased at the end of casting, or the bulging zone is located upstream of the mold side. By extending the length, it is possible to prevent steel leakage from the final pouring portion without lowering the casting speed, and furthermore, it is possible to maintain the center segregation level of the final casting portion of the slab at the same level as during steady casting. .

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of a vertical continuous casting apparatus for explaining the principle of the method of the present invention.

FIGS. 2A and 2B are diagrams schematically illustrating a curved continuous casting apparatus for carrying out the method of the present invention, in which FIG. 2A shows the position of a bulging zone during steady casting, and FIG. FIG. 7 is a diagram showing an example of extension of a bulging zone of FIG.

FIG. 3 is a view similar to FIG. 1 for illustrating a conventional continuous casting method using a “bulging-reduction method”.

FIG. 4 is a view showing a cross section of a short side of a slab when bulging is caused. FIG. 4A shows a case where bulging is caused from immediately below a mold to an end of a liquidus crater. FIG. 4 is a diagram showing a case where bulging is performed after the liquidus crater end.

5A and 5B are diagrams showing a method for examining shrinkage cavity length, wherein FIG. 5A is a diagram showing a final portion of a slab, and FIG. 5B is a cross-sectional view taken along line AA of FIG.

[Explanation of Codes] 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-9-10899 (JP, A) JP-A-7-206 112255 (JP, A) JP-A-2-235558 (JP, A) JP-A-10-244347 (JP, A) JP-A-6-335759 (JP, A) JP-A-60-6254 (JP, A) JP-A-60-21150 (JP, A) JP-A-9-314289 (JP, A) JP-A-8-314298 (JP, A) JP-A-11-156511 (JP, A) JP-A-11-156508 JP-A-1-178355 (JP, A) JP-A-4-33757 (JP, A) JP-A-8-47758 (JP, A) JP-A-8-47759 (JP, A) JP-A-7-223053 (JP, A) JP-A-6-226414 (JP, A) JP-A-5-50201 (JP, A) JP-A-9-122845 (JP P, A) (58) investigated the field (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 piece is subjected to bulging and then reduced by a reduction roll with a reduction amount equal to or less than the bulging amount, and in the final stage of casting, the bulging is performed while drawing a slab while maintaining a steady drawing speed. The continuous casting method characterized in that the volume of the bulging is made larger than the bulging volume at the time of steady casting.