JPH0741388B2 - Method for producing continuously cast slabs with excellent internal quality - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a continuously cast slab having excellent internal quality, and more particularly to a method for producing a continuously cast slab having improved center segregation and porosity. .

[Conventional technology]

Conventionally, positive segregation of carbon or the like, so-called center segregation occurs in the center of a continuously cast slab (hereinafter referred to as slab), which is a problem in terms of slab quality. Then, this center segregation is generally considered to be generated as follows. That is, the component elements of the molten steel in the solid-liquid coexisting phase are concentrated by the distribution of solutes accompanying solidification, and this concentrated molten steel flows due to solidification shrinkage and bulging between rolls and accumulates at the center of the slab, resulting in casting at the end of solidification. At the central part of one piece, the concentrated concentrated molten steel immediately before solidification is sucked into the negative pressure part generated by solidification contraction or bulging between rolls, and central segregation is generated.

On the other hand, based on such a mechanism of center segregation generation, as a measure to improve center segregation, a slight reduction by rolls is applied to the final stage of solidification of the slab to suppress the generation of negative pressure parts caused by solidification contraction and bulging between rolls. Then, a technique for suppressing the flow of the liquid phase of the solid-liquid coexisting phase as much as possible to reduce the center segregation and the porosity has been proposed. For example, in Japanese Examined Patent Publication No. 62-34460, in addition to stirring the molten steel by electromagnetic stirring to refine the cast structure, at least from the crater end of the solidus line of the final stage of solidification of the slab toward the upstream side. Rolling rate of 2m range by rolling roll set to roll pitch less than 450mm
By lightly reducing at 0.5 mm / m or more, this prevents bulging that occurs in the slab and prevents the flow of molten steel in the solid-liquid coexisting phase at the center of the slab, producing a continuous cast slab without segregation. A method is provided.

[Problems to be Solved by the Invention]

By the way, as in the above-described method for producing a cast piece, when setting the reduction roll so that the roll pitch is 450 mm or less, the roll diameter is necessarily 450 mm or less and must be a small diameter roll. . On the other hand, as one of the indexes necessary for improving the center segregation, the difference in the thickness of the slab before and after the reduction [total reduction amount (mm)] divided by the length (m) of the reduction zone [that is, the reduction gradient (that is, the reduction gradient ( mm / m)] is commonly used,
As a result, the same amount of reduction (mm) is applied to each roll.

Therefore, by the method for producing a slab described above, when the solidification end portion of the slab slab having a larger width than the thickness is rolled down by a small-diameter roll, center segregation and porosity are reduced, and a good slab is obtained. When the rolling amount of the roll is large, the internal strain becomes large, so that a slab having internal cracks is sometimes produced.

On the other hand, when the slab is a bloom or billet, as compared to a slab, the slab has a square cross-sectional shape, so internal deformation does not occur easily even when the final solidification stage is rolled down by a small-diameter roll. In order to obtain the segregation improving effect, it is necessary to obtain a large amount of reduction with respect to the cast slab. However, when the amount of reduction is large, a large tensile strain (hereinafter referred to as internal strain) is easily generated in the solidified interface and the non-strengthened solidified shell in the vicinity of the solidified interface, and internal cracking easily occurs. This will hinder the rolling process in the next process. For this reason, when the cast piece is a bloom or billet, it has not yet been put to practical use.

The present invention has been made in view of the above problems,
It is an object of the present invention to provide a method for producing slabs such as slabs and blooms having excellent internal quality without causing internal cracking as well as reducing center segregation and porosity.

[Means for Solving the Problems]

In order to achieve the above-mentioned object, the method for producing a continuously cast slab excellent in internal quality according to the present invention has a thickness of 2 to 5 times the thickness of the slab at the end of solidification of the slab continuously cast by the mold. The reduction roll having the diameter of 2 is arranged in two stages, and the reduction ratio of the first reduction roll is set to 1.5 to 4.0%, and the reduction ratio of the second reduction roll is set to 2.0 to 4.5%. It is a thing.

Moreover, when arranging three reduction rolls having a diameter of 2 to 5 times the thickness of the slab, the reduction ratios of the first reduction roll and the second reduction roll are set to the above reduction ratios. Although it is not necessary to apply the reduction to the third stage, if the third stage reduction roll is used, it is desirable to set the reduction rate to 2.0 to 4.5% and perform the reduction.

Hereinafter, the present invention will be described in detail.

Conventionally, a slab pulled out from a mold in continuous casting has unsolidified molten steel inside, and so-called bulging occurs in which the slab bulges due to the static pressure of the molten steel. Therefore, in order to suppress this bulging, use a guide roll in the cooling zone under the mold that has a roll diameter that is as small as possible (usually less than 1.5 times the thickness of the slab) to minimize the roll gap. Is provided. From such a point, it is considered that it is preferable to provide the pressing roll for pressing down the final stage of solidification of the cast slab as narrowly as possible, but in order to narrow the roll interval, reduce the roll diameter. In this way, if the roll diameter of the reduction roll is made small in this way, in order to reduce the center segregation of the slab and the porosity, it is necessary to take a large reduction ratio for the slab, but on the other hand, the reduction ratio When the value is large, the internal strain becomes large, so that internal cracking of the slab becomes a concern.

Therefore, as a result of diligent research conducted by the present inventors, the thickness of the slab is 2 to 2 at the end of solidification of the slab continuously cast by the mold.
Two reduction rolls having a diameter of 5 times are arranged, and the reduction ratio of the first reduction roll is set to 1.5 to 4.0% and the reduction ratio of the second reduction roll is set to 2.0 to 4.5%. It was discovered that, if it is pressed, not only the center segregation and the reduction of porosity are reduced, but also a slab without internal cracks can be manufactured. Furthermore, in the case where three stages of reduction rolls having a diameter of 2 to 5 times the thickness of the slab are provided and the reduction by the third stage reduction roll is applied, the reduction by the third stage reduction roll is performed. It was found that a slab with further improved center segregation can be produced by adding 2.0 to 4.5% in the ratio.

In this way, if the reduction roll is a large-diameter roll, as shown in FIG. 1, in the case of the same reduction rate (reduction amount Δh / thickness of cast h before reduction h × 100%), it is changed to a small-diameter roll (1). Compared to,
The contact length (l) of the large diameter roll (2) is longer (l ₁ →
l ₂ ), and hence the contact area is increased, friction between the surface of the large diameter roll (2) and the surface of the cast slab (3) at the contact surface can be effectively obtained, and the surface of the large diameter roll (2) can be obtained. In the surface layer portion of the cast slab (3) which is in contact with, the deformation of the cast slab (3) in the longitudinal direction and the width direction is suppressed more than in the case of the small diameter roll (1). When the deformation of the cast slab (3) in the longitudinal direction and the width direction is largely suppressed in this way, a large amount of deformation in the rolling direction can be obtained, and thus, the large diameter roll (2) In this case, the rolling reduction is made smaller than in the case of the small diameter roll (1), and the same amount of deformation in the rolling direction as in the case of the small diameter roll (1) is obtained. In addition, since the reduction ratio can be made small, the flattening of the cast slab (3) due to the reduction of the slab can be small. Can also be rolled down.

Next, in the present invention, the diameter of the reduction roll is set to 2 to the thickness of the slab.
The reason for specifying the range of 5 will be described.

The graph shown in Fig. 2 is obtained by numerical analysis by the finite element method. The final solidification part of a 300 mm thick slab has the same rolling reduction (about 2.7%) and roll diameters of 400 mm, 600 mm and 1000 mm.
It is a graph showing changes in the internal strain of the slab, the amount of compression of the cross-sectional area of the unsolidified portion, and the amount of deformation of the unsolidified portion when the steel sheet is rolled down instead of. As is clear from FIG. 2, if the roll diameter is more than twice the ratio with the thickness of the slab for the same reduction ratio,
The amount of compression of the cross-sectional area of the unsolidified portion and the amount of deformation of the unsolidified portion dramatically increase, which greatly contributes to the effect of improving center segregation. The amount of deformation of the part is small and improvement effects such as center segregation cannot be expected. Therefore, the diameter of the reduction roll is specified to be twice the thickness of the cast piece or more. On the other hand, the upper limit of the diameter of the reduction roll is set to 5 of the cast piece thickness.
It is specified to be not more than twice, because if it is not less than 5 times, the reduction roll interval becomes too large, bulging of the slab occurs between the reduction rolls, and the reduction action at the final solidification stage of the slab cannot be sufficiently obtained. is there.

Next, the reason why the reduction rate of the reduction rolls arranged in two or three stages in the present invention is specified will be described.

The reduction ratio of the first-stage reduction roll is specified to be 1.5 to 4.0% because the reduction ratio is less than 1.5% and the reduction is small, especially V
Segregation occurs. On the other hand, if the rolling reduction exceeds 4.0%, the rolling will be too large and internal cracking will occur. Therefore, the reduction rate of the first-stage reduction roll is specified to be 1.5 to 4.0%.

The reduction ratio of the second-stage reduction roll is specified to be 2.0 to 4.5% because when the reduction ratio is less than 2.0%, the reduction is small and segregation, particularly V segregation occurs, as in the case of the first-stage reduction roll. On the other hand, if the reduction ratio exceeds 4.5%, the reduction becomes too large and internal cracking occurs, and unsolidified molten steel flows upstream and reverse V segregation occurs. Therefore, the reduction rate of the second-stage reduction roll is 2.0 to
Specified at 4.5%.

Further, in the present invention, if a third-stage reduction roll is provided and reduction is performed, it is preferable to add reduction of 2.0 to 4.5% as a reduction ratio in addition to the above-described first and second reductions. The reason is that the reduction by the second-stage reduction roll is sufficient, but the center segregation and porosity are further improved. If the reduction rate is less than 2.0%, the improvement effect cannot be obtained, and if it exceeds 4.5%. This is because the reduction becomes too large and internal cracking occurs, and unsolidified molten steel flows upstream and reverse V segregation occurs.

Further, the position where the reduction is applied is the end of solidification of the continuously cast slab, but preferably the solidification end point (crater) is determined before and after the equiaxed crystal start point obtained by the following commonly used solidification equation. The length (a) up to the end) is good, and the first-stage reduction roll is preferably provided in the range of a to a / 2.

(Coagulation formula) χ = K (A / V) ^-1/2 χ: Solidified shell thickness (mm) K: Solidification coefficient (mm ・ min ^-1/2 ) A: Distance from meniscus (m) V: Casting speed (m / min) [Examples] Examples of the present invention will be described below.

FIG. 3 is a schematic view of a three-stage reduction device applied to the method of the present invention, in which (3) is a cast piece at the final solidification stage, (4) is an unsolidified molten steel portion in the cast piece, (5) is a reduction roll having a roll diameter of 1000 mm, (6) is an upper reduction roll (5)
Shows a cylinder for rolling down.

A slab having a cross section of 300 × 430 mm ² was continuously produced using the steel types shown in the table below by using the above-described three-stage reduction device. The manufacturing conditions at this time are: casting speed: 0.69 m / min, rolling roll diameter: 1000 m
m, roll pitch: 1.2 m, reduction roll position (distance from meniscus) 1st stage: 22.6 m, 2nd stage: 23.8 m, 3rd stage: 25.0 m.

Under the above conditions, the rolling reduction of the first-stage rolling roll was changed, and the internal cracks at this time were investigated. Results are shown in FIG. According to the figure, when the rolling reduction of the rolling roll exceeds 4.0%, internal cracking occurs, and when it is 4.0% or less, there is no internal cracking.

Next, the first stage reduction is applied at a reduction rate of 1.5 to 4.0%, the reduction rate by the second stage reduction roll is changed, and the C segregation maximum ratio (maximum C segregation / average C segregation) was investigated. For comparison, the same investigation was conducted by adding the reduction of the first stage in a reduction rate of less than 1.5%. These results are shown in FIG. According to the figure, the former has
In the range of 2.0 to 4.5%, troubles due to segregation in the working process after casting (for example, breakage during wire drawing) were improved to within the limit that can prevent segregation.
Even in the range of 2.0 to 4.5%, the improvement of segregation was insufficient.

Next, the reduction rate of the first stage is 1.5 to 4.0%, the reduction rate of the second stage is 2.0 to 4.5%, and the reduction rate by the reduction roll of the third stage is changed. The maximum C segregation ratio of the slab was investigated. Results are shown in FIG. As shown in the figure, in the rolling reduction range of 2.0 to 4.5%, the segregation was improved more than the segregation improving effect up to the second stage.

Further, the slab obtained by the rolling down to the second step was rolled to a diameter of 8 mm, and the cross section of the obtained rolled product was evaluated by macro etching.

The evaluation results are shown in FIG. As is clear from the figure, in the example of the present invention, the number of (good) was large, whereas in the comparative example, it was (slightly) good.

〔The invention's effect〕

As described above, according to the method for producing a continuously cast slab excellent in internal quality according to the present invention, casting of slabs, blooms and the like having relatively low center segregation and porosity and excellent internal quality without internal cracking Pieces can be manufactured.

[Brief description of drawings]

FIG. 1 is an explanatory view for explaining the method of the present invention in comparison with a conventional method, and FIG. 2 is a roll diameter obtained by numerical analysis by a finite element method, internal strain of a cast piece, and an unsolidified portion. Fig. 3 is a graph showing the relationship between the amount of cross-sectional area compression and the amount of deformation of the unsolidified portion. Fig. 3 is a schematic diagram of a three-stage reduction device applied to the method of the present invention. Fig. 4 is the first-stage reduction. FIG. 5 is a diagram showing the relationship between the rolling reduction by the roll and the internal cracking of the slab, and FIG. 5 shows the rolling reduction by the second-stage rolling roll and the casting when the second-stage rolling is applied in addition to the first-stage rolling. The figure which shows the relationship with the C segregation maximum ratio of a piece,
FIG. 6 shows the relationship between the reduction ratio by the third-stage reduction roll and the maximum C segregation ratio of the slab when the third-stage reduction is applied in addition to the first-stage reduction and the second-stage reduction. And FIG. 7 are evaluation diagrams obtained by macro-etching the cross section of the rolled product obtained by the method of the present invention and the comparative method. 1 ... Small-diameter roll, 2 ... Large-diameter roll 3 ... Slab, 4 ... Unsolidified molten steel part 5 ... Rolling roll, 6 ... Rolling cylinder l ₁ , l ₂ ...... Contact length Δh …… Rolling amount

Claims (2)

[Claims] 1. A reduction roll having a diameter of 2 to 5 times the thickness of the slab is provided in two stages at the end of solidification of the slab continuously cast by a mold, and at the same time, The rolling reduction is 1.5 to 4.0%, and the rolling reduction of the second-stage rolling roll is 2.0.
A method for producing a continuously cast slab having excellent internal quality, which is characterized by setting to ~ 4.5% for reduction. 2. A reduction roll having a diameter of 2 to 5 times the thickness of the slab is arranged in three stages at the final stage of solidification of the slab continuously cast by a mold, and the first reduction roll is The rolling reduction is 1.5 to 4.0%, and the rolling reduction of the second-stage rolling roll is 2.0.
To 4.5%, the reduction rate of the third stage rolling roll is 2.0 to 4.5%
A method for producing a continuously cast slab having excellent internal quality, which is characterized in that it is set to a lower temperature.