JPH0692022B2 - Light reduction method for continuous cast slab - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a continuous solid casting of steel, in which the casting speed varies, the initial stage, the final stage, and the continuous solidification of a non-steady-state slab during continuous casting of different steel types. Regarding the method.

(Prior Art) Conventionally, as a measure against center segregation of cast slabs, miniaturization of solidification structure (dispersion of concentrated molten steel) by low temperature casting or application of electromagnetic stirring has been the mainstream. Recent studies have considered that the cause of central segregation is molten steel flow due to slab bulging or solidification shrinkage in the final stage of solidification, and the light reduction at the final stage of solidification that directly prevents this molten steel flow is the most effective in preventing segregation. Is known to be.

For example, Japanese Patent Publication No. 59-16862 describes 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 drafting rolls. .

For the method of light reduction, for example, in JP-A-61-37356, a method of detecting the crater end shape in the width direction of the slab, and performing the reduction at the crater end position located most upstream of casting, In addition, Japanese Patent Publication No. 59-39225 discloses that
A method of rolling down 0.5 to 2.0 mm / m near the crater end is
Further, in JP-A-63-108955, the reduction amount / unsolidified thickness ratio at the time of reduction is 0.5 to 1.0, and the solid fraction at the center of the slab is 0.5 to 0.
8. The method of reducing the rolling strain to 0.2% or less is described.

As described above, it is known that application of light reduction under certain appropriate conditions is effective for improving the center segregation of the slab,
The method of applying the light reduction to the unsteady part slab in the early stage, the final stage of casting and continuous casting of different steel types has not been established yet, although it is extremely useful in actual operation.

(Problems to be solved by the invention) Generally, the crater end position at the time of applying a reduction is said to have an appropriate central solid fraction (fs) in the range of 0.2 to 0.7. It is affected by casting conditions, etc., and is most strongly affected by casting speed. Here, the central part solid phase ratio means the solid phase volume ratio in the central part solid-liquid coexisting phase of the slab cross section.

In continuous casting of steel, it is customary to apply a casting pattern as shown in FIG. In the figure, the horizontal axis represents time and the vertical axis represents casting speed (Vc). That is, the casting pattern is composed of a speed-up section in the early stage of casting of the front ladle, a steady section, a deceleration section at the end of the front ladle, a pinch roll stop section of the continuous casting seam portion, a speed-up section of the rear ladle, and the like.

Here, the purpose of gradually increasing the casting speed in the initial stage of casting is
For example, in order to prevent the shell from breaking (breakout) by growing the solidified shell of the slab that is connected to the dummy bar, the purpose of deceleration at the end of casting is to reduce the depth of the shrinkage pipe at the top of the slab of the first pan. This is to reduce the amount of slag in the tundish and to prevent the slag from flowing into the mold.
In addition, the purpose of stopping the pinch roll at the continuous seam part is to insert a partition iron plate to prevent mixing of molten steel at the seam part and to start casting after storing a predetermined amount of molten steel in the rear pot in the tundish. Is. This stop time is generally about several minutes, although it depends on the size of the slab and the method of inserting the iron plate. Next, the reason why the speed at the start of casting of the ladle after continuous casting is gradually increased is to make the solidified shell at the seam solid and prevent the molten steel from flowing out (breakout) from the seam.

In the steady section where the casting speed is constant, the center segregation is improved by fixing the light reduction roll at an appropriate position. However, in the unsteady section, there is a problem that the position of the crater end to be rolled down significantly changes, and in order to apply light reduction to the slab under appropriate conditions, the position of the rolling down roll is set to the upstream side or the steady state. It will be necessary to change to the downstream side.

(Means for Solving the Problems) The present invention is to solve the above problems.

That is, when a plurality of pairs of light reduction roll groups are arranged in the vicinity of the crater end of the slab and the reduction is applied to the slab, the slab and the casting speed in the initial stage of casting until the casting speed reaches the steady speed are the steady speed. For the slab in the final stage of casting, which slows down from
By reducing with one or more pairs of rolls closer to the meniscus side than the rolls that apply the reduction to the in-steady part cast pieces of the light reduction roll group, the center segregation of the unsteady part cast pieces in the initial and final stages of casting is reduced.

In addition, when applying a reduction to a slab that has temporarily stopped the pinch roll due to continuous casting of different steel types or top treatment, etc., rather than the steady reduction position in accordance with the movement of the crater end according to the stop time of the pinch roll. By pressing down with one or more pairs of rolls close to the meniscus side, the center segregation of the unsteady part slab that solidifies during the stop is reduced.

Also, when applying a reduction to the slab in the redrawing process after temporarily stopping the pinch roll for the top treatment etc. at the end of casting, a steady reduction of the movement distance of the crater end according to the stop time of the pinch roll is performed. By pressing down with one or more pairs of rolls closer to the meniscus side than the position, the center segregation of the unsteady part slab in the redrawing process after the pinch roll is stopped is reduced.

Further, when applying reduction to the unsteady part slab in continuous casting of different steel types, it is closer to the meniscus side than the roll that applies the reduction to the inner stationary part slab of the light reduction roll group in the deceleration section at the end of casting of the pan. Press down the slab with one or more pairs of rolls,
While the pinch roll is stopped, it is rolled down by one or more pairs of rolls closer to the meniscus side than the steady rolling position according to the movement of the crater end according to the time, and during the re-pulling process after the pinch roll is stopped, the pinch roll stop time Depending on the moving distance of the crater end, the slab is rolled down by one or more pairs of rolls closer to the meniscus side than the steady rolling position, and then in the speed-up zone in the early stage of casting of the subsequent ladle, redrawing after stopping the pinch roll. By sequentially setting one or more pairs of reduction rolls from the reduction position in the process to the steady reduction position on the downstream side, the center segregation of the unsteady part slab in continuous casting of different steel types is reduced.

(Operation) As described above, in the present invention, the position of the reduction roll is adjusted according to the position of the crater end moving upstream or downstream depending on the change in casting speed. The solid fraction at one center is fs = 0.
A range of 2 to 0.7 is appropriate. The reason is that if fs <0.2, the segregation reduction effect is small, and internal cracking is likely to occur due to the reduction. Also, if fs> 0.7, dendrite develops and molten steel flow is less likely to occur.
This is because a large rolling capacity is required when rolling down.

A phenomenon in which a crater end corresponding to this appropriate condition (hereinafter, abbreviated as a crater end) moves due to a change in casting speed will be described below.

The solidification shell thickness at the crater end is D (mm), the distance from the meniscus is L (m), and the solidification coefficient is k (mm / mi).
n ^1/2 ), and the casting speed is Vc (m / min), from the solidification equation, D = k (L / Vc) ^1/2 (1) or L = (D / k) ² Vc (2) Since the speed dx / dt (mm / min) when the slab crater end enters the light pressure zone at the beginning of casting is approximately equal to the casting speed, dx / dt = Vc ...... (3) Next, in the deceleration section in the final stage of casting, when the moving distance to the upstream side of the crater end when decelerating from the steady casting speed Vc by ΔVc (m / min) is x (m), D = k {(L- x) / (Vc-ΔVc)} ^1/2 (4) From formulas (2) and (4), x = (D / k) ² ΔVc (5) That is, deceleration starts at the end of casting. After (D / k) ² (min), the crater end moves upstream by x (m) given by the equation (5).

On the other hand, in the pinch roll stop section, if the moving distance to the upstream side of the crater end after T (min) from the pinch roll stop is x (m), D = k {(L−x) / Vc + T} ^1/2 …… (6) From formulas (2) and (6), x = VcT …… (7) That is, the crater end is T after the pinch roll is stopped.
After (min) elapses, VcT (m) moves upstream.

As described above, it is known that the crater end moves upstream or downstream during unsteady casting. still,
The above calculation is based on the solidification equation with the surface temperature of the slab being constant, and it is considered that it actually varies depending on the continuous casting machine and casting conditions.

Therefore, the inventors obtained the position of the crater end in the unsteady part when casting a bloom having a cross-sectional size of 350 mm × 560 mm in the pattern shown in FIG. 1 by the anchoring method or solidification calculation. As a result, it was confirmed that the change was as shown in FIG.

In FIG. 2, the bottom slab of the pan reaches the light pressure lower zone after t (min) has elapsed, and its crater end moves from point A to point B in the figure. Therefore, in the present invention, the bottom slab is rolled by the roll between the points A and B in the light reduction roll group.

For the slab in the steady section, the crater end is rolled by a roll located at a certain distance (about 27m in Fig. 2) from the meniscus, and then the crater end is point C in the figure in the deceleration section at the end of the last pan casting. To the point D, and further to the point E in the pinch roll stop section, the rolling of the slab in these sections is sequentially performed by the roll corresponding to the position.

According to the measurement results of the inventors, it was known that the point E moves to the upstream side in proportion to the stop time of the pinch roll as shown in FIG. According to the movement of the crater end, the rolling is applied by the roll on the more upstream side. Further, after the start of the drawing of the rear pan, the crater end moves from point E to point F and point G in FIG. 2, so the rolling roll is further changed to the upstream side.
The H point is the crater end position near the seam (partition iron plate insertion part) that existed in the mold at the start of casting of the rear ladle, and these cast pieces were rolled down by the rolls corresponding to the G point to the H point. To do.

Further, the slab in the steady section of the rear ladle is subjected to reduction with a roll arranged near the point I, and from the end of casting of the rear ladle to points C ', D', E'and F'in the figure. Since the crater end changes and the point E'changes according to the relationship shown in FIG. 3, the crater end is sequentially rolled down by the roll corresponding to the position.

In the present invention, the number of rolling rolls is one pair or more. The reason for this is to reduce the rolling strain and prevent internal cracking near the solidification interface due to rolling.

According to the above method, it is possible to apply a suitable light reduction even to a cast from the early stage to the end of casting and a non-steady part cast such as continuous casting of different steel types, and good over the entire length of the cast. The center segregation level can be secured.

(Example) An example will be described with reference to the drawings. When manufacturing a slab with a cross-sectional size of 350 mm × 560 mm by the Bloom continuous casting machine shown in Fig. 4, a light reduction device is installed behind the mold 1, the guide roll 2, the secondary cooling zone 3, and the straightening machine 4. , Machine structural steel (S45C) was cast.

The light reduction device has 10 pairs of light reduction rolls 5 arranged at a pitch of 400 to 880 mm within a range of 25 to 30 m from the meniscus, and each light reduction roll (upper surface roll) has a hydraulic cylinder 6 that operates at a maximum of 180 kg / cm ^2. Combined. Further, a hydraulic control device 7 is installed so that the slab 9 can be pressed down by four pairs of rolls which are adjacent to each other at the same time according to a command from the computer 8.

That is, S45C is cast according to the casting pattern shown in FIG. 1, and the position of the crater end from the start to the end of casting changes according to the pattern shown in FIG. A reduction was applied.

Fig. 5 (a) shows the state of reduction when the casting speed is the steady speed V ₁ .
Shows the total reduction amount by 4 pairs of light reduction rolls 5 in the shaded area, 6.5 mm, and the reduction zone length from the 1st pair to the 4th pair.
1.76 m, and the rolling gradient in this case was 3.7 mm / m. When the casting speed is reduced to V ₂ which is slower than the steady speed, the appropriate reduction position moves to the upstream side as described above, so that the reduction roll also moves to the upstream side as shown in FIG. 5 (b). Hydraulically controlled.

The center segregation degree of C in the bloom slab cast according to the present invention is shown in FIG. 6 in comparison with the conventional material. The present invention fixes four pairs of light reduction rolls within a range of 28 to 30 m from the meniscus, and
It is the result when S45C was pressed down by 6.5 mm in total in the casting pattern shown in the figure. As is clear from this result, the light reduction method of the present invention has a large effect of improving the segregation, which is due to the effect of improving the unsteady cast piece. In the case of the conventional material, the effect of light reduction in the unsteady cast slab cannot be enjoyed, so that a large variation in the degree of segregation is recognized.

(Effects of the Invention) Since it is possible to carry out light reduction in the unsteady part cast slab, a good center segregation level is secured. As a result, there is no need for soaking / diffusion treatment and specification change to lower grade steel, which were conventionally performed for unsteady part cast slabs, and the effects on quality improvement and manufacturing cost reduction are extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing a casting pattern, FIG. 2 is a diagram showing a crater end position, FIG. 3 is a diagram showing a relationship between a pinch roll stop time and a crater end position, FIG. 4 is a diagram showing an embodiment, FIG. 5 (a) and 5 (b) are diagrams showing a method of controlling the reduction roll according to a change in casting speed, and FIG. 6 is a diagram showing effects. 1 ... Mold, 2 ... Guide roll, 3 ... Secondary cooling zone,
4 ... Straightener, 5 ... Light reduction roll, 6 ... Hydraulic cylinder, 7 ... Hydraulic control device, 8 ... Computer, 9
...... Slabs.

Claims (4)

[Claims] 1. In continuous casting of steel, when a plurality of pairs of light reduction roll groups are arranged near the crater end of a slab and a reduction is applied to the slab, casting at the initial stage of casting until the casting speed reaches a steady speed. For the cast piece and the cast piece at the final stage of casting, in which the casting speed decelerates from the steady speed, it is closer to the meniscus side than the roll that applies the reduction to the cast piece in the steady part of the light reduction roll group.
A method for lightly rolling down continuously cast slabs, characterized by reducing the center segregation of unsteady part slabs in the initial and final stages of casting by rolling with more than one pair of rolls. 2. In continuous casting of steel, when a reduction is applied to a slab in which the pinch roll is temporarily stopped, the meniscus side from the steady reduction position in accordance with the movement of the crater end according to the stop time of the pinch roll. A light reduction method for continuously cast slabs, characterized in that center segregation of unsteady part slabs that solidify during stoppage is reduced by rolling with one or more pairs of rolls close to each other. 3. In the continuous casting of steel, when the pinch roll is temporarily stopped at the end of casting and then a reduction is applied to the slab in the redrawing process, it is steady for a moving distance of the crater end according to the stop time of the pinch roll. By rolling with one or more pairs of rolls closer to the meniscus side than the rolling position,
A method for lightly rolling down a continuous cast slab, which reduces center segregation of a non-steady part slab in a redrawing process after a pinch roll is stopped. 4. When applying reduction to the unsteady part slab in continuous casting of different steel types, the slab is reduced by the method according to claim 1 in the deceleration section at the end of casting of the front ladle, and while the pinch roll is stopped. In the redrawing process after the pinch roll is stopped, the slab is pressed down by the method described in claim 2, and then the slab is pressed down by the method described in claim 3, and then, in the speed-up section in the initial stage of casting of the rear ladle, after the pinch roll is stopped. Continuous casting characterized by reducing the center segregation of the unsteady part slab in continuous casting of different steel types by sequentially setting one or more pairs of rolling rolls from the rolling position to the steady rolling position in the redrawing process of Light reduction method for cast slab.