JPH0433757A - Method for executing light rolling reduction to continuously cast slab - Google Patents

Abstract

PURPOSE:To reduce center segregation in unsteady part of a cast slab at the initial stage and end stage of casting by executing the rolling reduction of the cast slab at the end stage of casting with one or more pairs of rolls near meniscus side rather than with the rolls applying the rolling reduction to the steady part of cast slab in the light rolling reduction roll group. CONSTITUTION:The light rolling reduction apparatus sets the light rolling reduction rolls 5 in the prescribed range from the meniscus and to each light rolling reduction roll (upper roll), a hydraulic cylinder 6 is connected. The rolling reduction is applied to the cast slab through a computer and a hydraulic control device. In the case casting velocity is the steady velocity, the rolling reduction is executed with four pairs of the light rolling reduction rolls 5 with slanting line part in the figure. In the case the casting velocity is reduced to V2 slower than the steady velocity, as the suitable rolling reduction position is shifted to the upstream side, the hydraulic control is executed so that the rolling reduction rolls shift to the upstream side, too. By enabling the light rolling reduction at the unsteady part of cast slab, good center segregation level is secured.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to continuous casting of steel, in which the casting speed changes during the initial and final stages of casting, and during continuous casting of different steel types, light reduction at the final stage of solidification of a slab in an unsteady region. Regarding the method.

(Prior Art) Conventionally, the main methods of countering the center segregation of continuously cast slabs have been to refine the solidified structure (dispersion of concentrated molten steel) by applying low-temperature casting or electromagnetic stirring. Recent research has shown that the cause of center segregation is thought to be molten steel flow due to slab bulging and solidification shrinkage at the final stage of solidification, and light reduction at the end of solidification, which directly prevents this molten steel flow, is the most effective method for preventing segregation. It is known that

For example, Japanese Patent Publication No. 59-16862 describes a method of rolling down between the liquidus crater tip and the solidus crater tip of a slab in a steady drawing process using one or more pairs of rolling down rolls. ing.

Regarding the method of light pressure, for example, Japanese Patent Application Laid-Open No. 61-3735
Japanese Patent Publication No. 59-39225 describes a method of detecting the shape of the crater end in the width direction of the slab and rolling down at the crater end position located on the most upstream side of casting. 0.5-2.0mm near the crater end
The method of reducing the pressure by / m is further disclosed in JP-A-63-10895.
Publication No. 5 states that the reduction amount/unsolidified thickness ratio during rolling is 0.5.
~1.0, a method in which the solid fraction in the center of the slab is 0.5 to 0.8, and the rolling strain is 0.2% or less is described.

As mentioned above, is it known that applying light reduction under certain appropriate conditions is effective for improving center segregation of slabs?
Although it is extremely useful in actual operations, the method of applying light reduction to unsteady parts of slabs in the initial and final stages of casting and in continuous casting of different steel types has not yet been established.

(Problems to be Solved by the Invention) Generally, it is said that the appropriate position of the crater end when applying pressure is such that the solid fraction (fs) in the center is in the range of 0.2 to 0.7. It is affected by the slab size, steel type, casting conditions, etc., and is most strongly affected by the casting speed. Here, the central homomorphic ratio means the solid phase volume ratio within the solid-liquid coexistence phase at the center of the cross section of the slab.

In continuous steel casting, a casting pattern as shown in FIG. 1 is usually applied. In the figure, the horizontal axis is time and the vertical axis is casting speed (Vc). That is, the casting pattern is composed of an increasing speed section at the beginning of pre-casting, a steady section, a decelerating section at the end of pre-casting, a pinch roll stopping section at the continuous casting joint, an increasing speed section at the rear pot, etc.

Here, the purpose of gradually increasing the casting speed in the early stages of casting is, for example, to grow the solidified shell of the slab connected to the dummy bar to prevent breakout of the shell, and also to slow down the speed in the final stage of casting. The purpose of this is to reduce the depth of the shrinkage pipe at the top of the preassembled slab and to suppress the slag in the tundish from flowing into the mold. In addition, the purpose of stopping the pinch rolls at the casting joint is to insert a partition iron plate to prevent molten steel from mixing at the joint, and to start pouring after collecting a predetermined amount of molten steel in the rear pot in the tundish. be. This stopping time depends on the slab size and the iron plate insertion method, but it is generally about several minutes. Next, the reason why the speed at the start of casting of the ladle is gradually increased after successive castings is to invert the solidified shell at the joint and prevent breakout of molten steel from the joint.

In a steady zone where the casting speed is constant, center segregation can be improved by fixing the light reduction roll at an appropriate position. However, in the above-mentioned unsteady section, there is a problem that the position of the crater end to be rolled changes significantly, and in order to apply a light reduction to the slab under appropriate conditions, the position of the rolling roll must be moved upstream from the steady state. Or it will be necessary to make changes downstream.

(Means for solving problems) The present invention solves the above problems.

In other words, when a plurality of pairs of light reduction rolls are arranged near the crater end of the slab to apply reduction to the slab, the slab and casting speed at the initial stage of casting until the casting speed reaches a steady speed are constant. For slabs at the final stage of casting, where the speed decreases,
By rolling down with one or more pairs of rolls closer to the meniscus than the rolls that apply reduction to the steady section slab in the group of light reduction rolls, center segregation of the unsteady section slab at the initial and final stages of casting is reduced.

In addition, when applying reduction to slabs whose pinch rolls have been temporarily stopped for continuous casting of different steel types or top treatment, etc., it is necessary to move from the steady reduction position in accordance with the movement of the crater end according to the stop time of the pinch rolls. By rolling down with one or more pairs of rolls close to the meniscus side, center segregation of the unsteady part of the slab that solidifies during suspension is reduced.

In addition, when applying pressure to the slab during the re-pulling process after the pinch rolls have been temporarily stopped for top treatment etc. at the end of casting, the pressure is reduced by the distance traveled by the crater end according to the stop time of the pinch rolls. By rolling down with one or more pairs of rolls closer to the meniscus side than the rolling position, center segregation of the unsteady part of the slab during the re-drawing process after the pinch rolls are stopped is reduced.

Furthermore, when applying reduction to the unsteady part of the slab in continuous casting of different steel types, the deceleration section at the end of the casting of the previous set is closer to the meniscus side than the roll that applies reduction to the steady part of the group of light reduction rolls. The slab is rolled down with one or more pairs of rolls,
While the pinch rolls are stopped, the pressure is reduced by one or more pairs of rolls closer to the meniscus side than the steady reduction position in accordance with the movement of the crater end according to the time, and in the re-pulling process after the pinch rolls are stopped, the stop time of the pinch rolls is The slab is rolled down by one or more pairs of rolls closer to the meniscus side than the steady rolling position by the travel distance of the crater end corresponding to By sequentially setting one or more pairs of roll-down rolls from the roll-down position during the re-pulling process to the steady roll-down position on the downstream side,
Reduces center segregation of slabs in unsteady areas in continuous casting of different steel types.

(Function) As described above, in the present invention, the position of the reduction roll is adjusted in accordance with the position of the crater end that moves upstream or downstream due to changes in the casting speed. As the solid phase ratio at the center of the slab, fs=
A range of 0.2 to 0.7 is appropriate. The reason is f
This is because rolling down at s < 0.2 has a small segregation reduction effect and tends to cause internal cracks due to rolling down, and fs >
This is because in the case of 0.7, dendrites are developed, making it difficult for molten steel to flow, and requiring a large rolling capacity when rolling down.

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

The thickness of the solidified shell at the crater end is D (mm), the distance from the meniscus is L (m), and the solidification coefficient is k (mm/mm).
min"), and the casting speed is Vc (m/m1n), then from the solidification equation, D = k (L / Vc)1"...
・・・・・・・・・ (1) Or, L − (D/k
)2Vc ・・・・・・・・・・・・・・・・・・
(2) The velocity dx/dt (mm/m1n) when the slab crater end enters the light reduction zone at the initial stage of casting is:
Since it is approximately equal to the casting speed, dx/dt=Vc ・・・・・・・・・・・・・・・
(3) Next, in the deceleration section at the end of casting, the steady casting speed Vc to △Vc (m, /
If the moving distance of the crater end to the upstream side when decelerating by m1n) is x (m), then D=k [(L-x) / (Vc-ΔVc)]
”・・・・・・・・・・・・・・・ (.1:1(2
) and (4), X2(D/k)2△Vc ・・・・・・・・・・・・
... (5i, that is, at the end of casting, from the start of deceleration (D/
”k)2 (min), x(m
) move to Crater End or upstream.

On the other hand, in the pinch roll stop section, if the moving distance of the crater end upstream after T (min) from the pinch roll stop is x (m), then D=k ((L-
x) /Vc+T] l/2- (6) (2) formula,
From formula (6), x=VcT ・・・・・・・・・・・・・・・・・・
71 That is, the crater end moves upstream by VcT (m) after T (min) has elapsed since the pinch roll stopped.

As described above, it is known that the crater end moves upstream or downstream during unsteady casting. still,
The above calculation is based on a solidification formula assuming a constant slab surface temperature, and it is thought that the actual value differs depending on the continuous casting machine and casting conditions.

Therefore, the inventors determined the position of the crater end in the unsteady part when casting a bloom with a cross-sectional size of 350 mm x 560 mm in the pattern shown in Fig. 1.
It was determined using the anchoring method and coagulation calculations. As a result, it was confirmed that the temperature changed as shown in Figure 2.

In FIG. 2, the bottom slab of the previous set reaches the light reduction 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 down by the rolls located between points A and B in the group of light rolling rolls.

For the slab in the steady section, the crater end is rolled down by a roll located at a certain distance from the meniscus (approximately 27 m in Figure 2), and then in the deceleration section at the end of pre-casting, the crater end is rolled down at a certain distance from the meniscus (about 27 m in Figure 2). Since it moves from point C to point D, and further to point E in the pinch roll stop section, the rolling of the slab in these sections is carried out sequentially by the rolls corresponding to the positions.

According to the measurement results of the inventors, it was found that point E moves upstream in proportion to the stopping time of the pinch roll as shown in Figure 3. As the crater end moves, a roll on the more upstream side applies pressure. Furthermore, since the crater end moves from point E to point F and then point G in FIG. 2 after the start of drawing out the rear pot, the reduction roll is further moved to the upstream side.

Point H is the crater end position near the joint (inserted part of the partition iron plate) that existed in the mold at the start of casting of the after pot, and for these slabs, the rolls corresponding to point G and H point are Press down.

In addition, the slab in the steady section of the after-ladder is rolled down by rolls placed near one point, and from the end of casting of the after-ladder, it is placed at points C', D', E', and F' in the figure. Since the crater end changes and the E' point changes according to the relationship shown in FIG. 3, rolling is performed successively with the roll corresponding to that position.

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

According to the above method, it is possible to apply an appropriate light reduction to slabs from the early stage to the final stage of casting, as well as during unsteady casting such as continuous casting of different steel types, and it is possible to achieve a good centering over the entire length of the slab. Segregation level can be ensured.

(Example) An example will be described with reference to the drawings. The bloom continuous casting machine shown in Fig. 4 has a cross-sectional size of 350 mm x 56
When producing a 0 mm slab, a light reduction device was installed behind the mold 1, guide roll 2, secondary cooling zone 3, and straightening machine 4, and machine structural steel (345C) 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 (top roll) has a maximum of 18
A hydraulic cylinder 6 operating at 0 kg/cmf was coupled. In addition, a hydraulic control device 7 is installed, and according to instructions from a computer 8, four pairs of adjacent rolls are simultaneously moved to form a slab 9.
It is now possible to press down.

That is, 545C 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. pressure was applied to.

The reduction state when the casting speed is a steady speed Vl is shown in Figure 5 (a
), the total reduction by the four pairs of light reduction rolls 5 in the shaded area is 6.5 mm, the reduction zone length from the first pair to the fourth pair is 1.76 m, and the reduction gradient in this case is 3.5 mm. 7
mm/m. V2 where the casting speed is slower than the steady speed
When decelerating to , the appropriate rolling position moves upstream as described above, so the rolling roll was also hydraulically controlled to move upstream as shown in FIG. 5(b).

FIG. 6 shows a comparison of the center segregation degree of C in bloom slabs cast according to the present invention with conventional materials. The present invention fixes four pairs of light reduction rolls within a range of 28 to 30 m from the meniscus,
This is the result when 345C was rolled down by a total of 6.5 mm using the casting pattern shown in FIG. As is clear from this result,
The light reduction method of the present invention has a large effect of improving segregation, and this is due to the effect of improving the unsteady part of the slab. In the case of conventional materials, it is not possible to enjoy the effect of light reduction in slabs in unsteady areas, so a large variation in the degree of segregation is observed.

(Effects of the Invention) A good center segregation level is ensured by making it possible to perform light reduction on the slab in the unsteady part. As a result, it is no longer necessary to perform soaking diffusion treatment or to change specifications to lower grade steel, which were conventionally performed on unsteady part slabs, and the effects on quality improvement and manufacturing cost reduction are extremely large.

[Brief explanation of drawings]

Figure 1 is a diagram showing the casting pattern, Figure 2 is a diagram showing the crater-end position, Figure 3 is a diagram showing the relationship between the pinch roll stop time and the crater-end position, and Figure 4 is a diagram showing an example. , FIGS. 5(a) and 5(b) are diagrams showing a method of controlling the reduction roll as the casting speed changes, and FIG. 6 is a diagram showing the effect. ■...mold, 2... guide roll, 3... secondary cooling zone, 4... straightening machine, 5... light reduction roll, 6...
- Hydraulic cylinder, 7... Hydraulic control device, 8... Computer, 9... Slab. Agent Patent attorney Masamitsu Aki Sawa and one other person
Figure 4 Meniz n Sky et al. 30 sparrows -1 Figure 71-5 (b) O60

Claims (4)

[Claims] (1) In continuous casting of steel, when applying reduction to a slab by arranging multiple pairs of light reduction rolls near the crater end of the slab, the slab during the initial casting until the casting speed reaches a steady speed. For slabs at the final stage of casting, where the casting speed slows down from the steady speed, by rolling down the slab with one or more pairs of rolls closer to the meniscus side than the rolls that apply the rolling to the steady section of the light rolling roll group, A method for lightly rolling continuously cast slabs, characterized by reducing center segregation of slabs in unsteady areas at the initial and final stages of casting. (2) In continuous casting of steel, when applying a reduction to a cast slab whose pinch rolls have been temporarily stopped, a position closer to the meniscus side than the steady reduction position is applied in accordance with the movement of the crater end according to the stop time of the pinch rolls. 1. A light reduction method for continuously cast slabs, characterized in that center segregation of unsteady portions of slabs that solidify during stoppage is reduced by rolling down with a pair or more of rolls. (3) In continuous casting of steel, when applying a reduction to the slab during the re-pulling process after the pinch rolls are temporarily stopped at the end of casting, the steady reduction position is adjusted by the distance the crater end moves according to the stopping time of the pinch rolls. A light reduction of a continuously cast slab characterized by reducing center segregation of the slab in an unsteady area during the re-drawing process after stopping the pinch rolls by rolling down with one or more pairs of rolls closer to the meniscus side than the Method. (4) Unsteady part in continuous casting of different steel types When applying a reduction to a slab, the method described in claim 1 is used to reduce the slab in the deceleration section at the end of casting of the tip pan, and the method described in claim 2 is applied when the pinch roll is stopped. The slab is rolled down by the method described in claim 3, and in the re-pulling process after the pinch rolls are stopped, the slab is rolled down by the method set forth in claim 3,
Next, in the speed-up section at the initial stage of casting of the rear pot, one or more pairs of reduction rolls are sequentially set from the reduction position in the re-drawing process after the pinch rolls stop to the steady reduction position, thereby reducing the non-conformity in continuous casting of different steel types. A rolling reduction method for continuously cast slabs, characterized by reducing center segregation of slabs in a steady section.