JP3395717B2 - 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 steel in which a slab including a non-solidified portion is rolled down after bulging the slab.

[0002]

2. Description of the Related Art An internal defect called center segregation may occur at the center of the thickness of a steel slab obtained by continuous casting. This center segregation causes C in the final solidified portion of the slab,
The segregation components such as S, P and Mn appear in a concentrated form. It is known that the center segregation of a slab causes a decrease in toughness of a thick plate which is a product and causes hydrogen-induced cracking of a large-diameter steel pipe manufactured by bending and welding the thick plate.

The generation mechanism of the center segregation of the cast slab is considered as follows. As the solidification progresses, the segregation component is concentrated between the dendrite trees which are the solidification structure. The molten steel enriched with the segregation component flows out from between the dendrite trees due to shrinkage of the slab during solidification or swelling of the slab called bulging. The concentrated molten steel that has flowed out flows toward the solidification completion point of the final solidification portion and solidifies as it is as a thickened zone. Therefore, center segregation occurs.

As a measure for preventing this center segregation, it is effective to prevent the concentrated molten steel from moving between the dendrite trees and to prevent the localized accumulation of the concentrated molten steel. Is taken. One of them is a light reduction method using a reduction roll group, but there is a limit to the improvement of the center segregation at a light reduction slightly exceeding the amount of solidification shrinkage.

In order to effectively improve the center segregation, a method of applying a large reduction with a pair of reduction rolls is adopted. For example, in Japanese Unexamined Patent Publication No. 9-57410, a slab containing an unsolidified portion of 20 to 20 is produced. A method has been proposed in which bulging is performed for about 100 mm and the amount corresponding to the bulging amount is reduced by at least one pair of reduction rolls before the solidification completion position. In this method, the effect of improving center segregation can be expected,
Further, since the both ends of the completely solidified slab in the width direction are not pressed, it is not necessary to use excessive equipment.

However, according to this method, when a slab having a wide width and a large amount of reduction is used, the rolls of the reduction roll pair are liable to bend, which causes internal cracks at the solidification interface and also causes the center segregation. In some cases, the improvement effect of may not be obtained.

[0007]

DISCLOSURE OF THE INVENTION The present invention provides a continuous steel product capable of obtaining a slab having a good internal quality with little center segregation even in a wide slab without causing internal cracking. An object is to provide a casting method.

[0008]

The gist of the present invention resides in the continuous casting method for steel described in (1) and (2) below.

(1) The slab thickness at the start of bulging is 5 to 5
After bulging by a thickness corresponding to 25%, until the completion of solidification, a reduction roll pair in which each roll is divided into at least two is used so that the reduction ratio α satisfies the following formula (A). A continuous casting method for steel in which a slab is rolled down.

0.5 ≤ α ≤ 1.3 (A) where α = (R-d) / L R: amount of reduction of cast slab (mm) d: between divided rolls (Mm) L between the bearing and the roll surface L: Thickness of unsolidified portion with solid fraction of 0.8 or less at the rolling position (mm) (2) Roll down the slab using two or more rolling roll pairs In the continuous casting method for steel according to (1) above, the bearings of the divided portions of the rolling roll pairs are arranged such that they do not overlap at the position in the width direction of the slab on the same surface of the slab. .

In the method of the present invention, the thickness L (mm) of the unsolidified portion of the slab at the start of the reduction is determined by the solid fraction of 0.8 in the slab.
Is the thickness between the solidification interfaces of. The reason why the position where the solid phase ratio is 0.8 is the solidification interface that is the boundary between the solidification shell and the unsolidified part is that the force acting on this solidification interface is effectively transmitted to the solidified part outside this interface. This is because that. The solidification interface having a solid fraction of 0.8 can be calculated by solidification heat transfer analysis, and can also be calculated by a solidification shell thickness calculation formula represented by a proportional formula of 1/2 of the solidification time.

The present inventor has solved the above-mentioned problems of the present invention as follows. (A) In the method of the present invention, the slab containing the unsolidified portion is rolled down using a rolling roll pair in which each roll is divided into at least two rolls. Since the rolls divided by the bearings are used, it is possible to prevent the rolls from bending even when pressing down a wide slab. Since the rolls of the pressing roll pair do not bend, the slab can be uniformly pressed. The wide slab targeted by the method of the present invention means a slab having a width of about 1800 mm or more.

(B) Due to the structure of the roll, the bearing portion in which the roll is divided has a shape recessed from the surface of the roll. When using a rolling roll pair having such a bearing portion, when rolling down the slab containing the unsolidified portion after bulging, the slab corresponding to the position of the bearing portion, without causing internal cracking, In addition, in order to effectively improve the center segregation, it is necessary to perform the reduction with an appropriate reduction ratio in accordance with the shape of the bearing portion recessed from the roll surface.

(C) In the method of the present invention, the above-mentioned (A) is used.
Since the reduction is performed within the range of the reduction ratio represented by the formula, the reduction force acting on the solidification interface inside the cast piece corresponding to the position of the bearing portion can propagate between the solidification interfaces. Therefore, the force acting on the solidification interface does not become a tensile force along the casting direction but becomes a compressive force in the thickness direction of the cast piece. Therefore, it is possible to prevent the occurrence of internal cracks during rolling.

Further, due to the compressive force acting on the solidification interface and the effect of reducing the volume of the unsolidified portion during rolling, the concentrated molten steel in the final solidified portion is discharged upstream in the casting direction. Therefore, it is possible to prevent the occurrence of center segregation at the center of the thickness of the slab.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram for explaining an example of the apparatus configuration of a continuous casting machine when carrying out the method of the present invention. Molten steel 6 injected into the mold 1 through the immersion nozzle 7.
Solidifies into a solidified shell 2a in the mold. The thickness of the solidified shell that is drawn from the mold and cooled by the water sprayed from the spray nozzle group (not shown) below the solidified shell gradually increases. The solidified shell having the increased thickness, that is, the cast slab 2, is pulled out by the pinch roll 5 after passing through the guide roll 3 and the pressing roll pair 4. In the bulging zone, bulging is caused in the slab by gradually increasing the roll interval corresponding to the thickness of the slab of the guide roll. Then, in the rolling zone, the slab containing the unsolidified portion 2b is rolled down by the rolling roll pair under the condition that the formula (A) defined in the present invention is satisfied.

In the method of the present invention, the slab is reduced by using a pair of reduction rolls in which each roll is divided into at least two.

FIG. 2 is a view showing a situation in which the slab is being pressed by a pair of pressing rolls whose divided portions are supported by bearings.
The pressing roll pair used in the present invention has a structure in which the rotating shafts of the rolls are supported by the dividing portions, and FIG. 2 shows an example in which each roll has bearing portions at two positions. FIG. 2A shows an example of a pair of pressing rolls in which the bearing portions of the upper and lower rolls are arranged so as not to overlap at a position in the width direction of the slab in the drawing. Figure 2
(B) shows an example of a pair of pressing rolls arranged so that the bearing portions of the upper and lower rolls overlap each other.

These bearings 8 may be fixed to the segment 10 as is usually done. Further, these bearing tops are located lower than the roll surface 9. The step d between the roll surface and the bearing top is 10
-30 mm is desirable. If it is less than 10 mm, the number of times the roll surface of each roll can be cut and reused is reduced,
The so-called life is shortened. When it exceeds 30 mm, the swelling of the surface of the cast piece after reduction becomes large, and surface defects are likely to occur on the surface of the product hot-rolled from this cast piece.

In the method of the present invention, the amount of bulging is 5 to 25% of the thickness of the slab at the start of bulging. The thickness of continuously cast slabs such as slabs and blooms is 200-35.
Generally, it is about 0 mm. Therefore, when rolling down a slab that has been bulged by more than 25%, an excessive rolling-down equipment is required, and the equipment cost becomes huge. Therefore, the upper limit is 25
%. Further, in order to secure a reduction thickness necessary to give a compressive force to the solidification interface inside the slab by reduction,
It is necessary to bulge 5% or more. Below 5%,
The thickness that can be rolled is small, and it is difficult to apply sufficient compressive force to the solidification interface. Therefore, the bulging amount is 5 to 25% of the thickness of the slab at the start of bulging. The thickness of the slab at the start of bulging is the thickness of the slab when the slab is pulled out of the mold and reaches the guide roll immediately before the bulging zone.

In the method of the present invention, the rolling-down ratio α is 0.5 to 1.3, as shown in the above-mentioned formula (A). The reason will be described below.

As described above, when the cast slab is pressed,
If the reduction force acting on the solidification interface inside the slab can propagate between the facing solidification interfaces, the force acting on the solidification interface will not be a tensile force along the casting direction, so internal cracking of the slab will occur. Can be prevented. In addition, since the concentrated molten steel in the final solidified portion is discharged to the upstream side in the casting direction with the effect of reducing the volume of the unsolidified portion during the reduction, it is possible to prevent the center segregation of the slab.

In order to roll down the slab in the above-mentioned state, when a rolling roll pair supported by bearings and having no divided portion is used, it is necessary to roll down so as to satisfy the following formula (B). .

0.5 ≤ α ≤ 1.55 (B) where α = R / L R: amount of reduction of cast slab (mm) L: solid fraction at the reduction position of 0.8 or less Thickness of solidified portion (mm) Ratio R / L of the amount of reduction R and the thickness L between the solidification interfaces with a solid fraction of 0.8 is an index showing the degree of approach of the solidification interfaces on both sides in the thickness direction of the slab due to the reduction. Is. This ratio R / L is 0.5
In the range of up to 1.55, due to the reduction, the state of the slab as described above, that is, the reduction force acting on the solidification interface can be propagated between the solidification interfaces, and the volume of the unsolidified portion decreases. .

When using a pair of reduction rolls having a divided portion supported by bearings, if the step between each roll surface of the reduction roll pair and each bearing is d, the slab of the slab corresponding to the position of the bearing is Since the reduction amount is (Rd), the index indicating the degree of approach of the solidification interfaces on both sides of the slab corresponding to the position of the bearing is the ratio (Rd) / L. Therefore, if the above formula (B) is pressed down so as to satisfy the above formula (A), internal cracking will not occur in the slab corresponding to the position of the bearing portion, and the occurrence of center segregation can be prevented.

Here, the upper limit of the ratio (R-d) / L is 1.3.
The reason for this is that when the rolling reduction exceeds 1.3, the slab excessively rolls down the bearing portion, so that excessive stress acts on the bearing portion and the bearing portion is easily damaged.

Below, the reduction ratio α = (R−d) / L is set to 0.
The specific reason for setting 5 to 1.3 will be described. FIG. 3 shows C
Steel with a content of 0.20% by weight, thickness 200 mm, width 20
It is a figure which shows the relationship between the maximum length of the internal crack of the cast piece at the time of the reduction which was continuously cast in a slab of 00 mm, and was investigated by the method mentioned later, and the reduction ratio α. It can be seen that when the reduction ratio α defined by the above-mentioned formula (A) becomes 0.5 or more, internal cracking does not occur during the reduction. This is because the rolling-down force acts on the solidification interface to propagate between the facing solidification interfaces, and the compression force acts on the solidification interface.

FIG. 4 shows the center segregation degree C / C ₀ of C at the thickness center of the slab, which was obtained by continuously casting the same steel as described in FIG. 3 into a slab having the same cross-sectional shape and investigated by the method described later. It is a figure which shows the relationship with the rolling reduction ratio (alpha). It can be seen that the center segregation is remarkably improved when the reduction ratio α exceeds 0.5. Due to the compressive force acting on the solidification interface and the effect of shrinking the volume of the unsolidified portion during rolling, the concentrated molten steel in the final solidified portion was discharged to the upstream side in the casting direction, and the center segregation of the slab was prevented. Is. From the above, the rolling reduction ratio α is set to 0.5 or more in order to prevent the occurrence of internal cracking and center segregation of the slab.

The upper limit of the reduction ratio α = (R-d) / L is 1.3.
The reason is as described above, and is for preventing excessive stress from acting on the bearing portion. From the above,
The reduction ratio α = (R−d) / L is 0.5 to 1.3. When the slab is rolled by two or more rolling roll pairs, as described later, each rolling roll pair is rolled so as to fall within the range of this rolling reduction ratio.

According to the method of the present invention, when the slab is reduced by using two or more pairs of the reduction rolls, the bearing portions of the divided portions of the reduction rolls are in the slab width direction on the same surface of the slab. It is desirable to arrange and roll down so that they do not overlap at the position. That the bearing portions do not overlap here means that the gaps between the rolls on both sides of the bearing portion do not overlap. When pressed, a step corresponding to the shape of the bearing portion is formed on the surface of the slab corresponding to the position of the bearing portion.
Therefore, when the bearing portions of each rolling roll overlap at the position in the slab width direction, the step difference of these slabs after reduction becomes large, and the surface of the product hot-rolled using these slabs as a raw material. This is because surface defects easily occur.

Further, when the rolling is performed by one rolling roll pair, it is desirable to use the rolling roll pair arranged so that the bearing portions of the rolls do not overlap each other at the position in the width direction of the cast piece. That the bearing portions do not overlap here means that the gaps between the rolls on both sides of the bearing portion do not overlap. When rolling down, the surface of the slab corresponding to the position of the bearing part, when each bearing part of each bearing part roll overlaps, the roll becomes slightly flexible and the effect of improving center segregation is slightly reduced. . Further, the positions of the steps on the surface of the cast piece after reduction overlap on the upper and lower surfaces of the cast piece, and the surface of the product hot-rolled from the cast piece is likely to have surface defects.

In the method of the present invention, it is desirable to electromagnetically stir the unsolidified portion upstream of the solidification completion point in the casting direction. The upstream of the solidification completion point in the casting direction may be electromagnetically stirred in the molten steel in the mold, or may be electromagnetically stirred in the uncooled portion inside the cast in the secondary cooling region of the cast. Means This is because in any case, the solidified structure at the thickness center of the cast slab has an equiaxed structure. When such equiaxed crystals are formed, even if the rolling reduction ratio α is smaller than 1, the equiaxed crystals are in contact with each other and the rolling force can be transmitted more effectively. Therefore, even if the reduction ratio α is smaller than 1, the force acting on the solidification interface becomes a compressive force, internal cracks are less likely to occur during the reduction, and the center segregation of the slab can be prevented. The strength of the electromagnetic force is not particularly limited. It may be determined depending on the position of stirring, the thickness of the slab, and the like. When electromagnetically stirring the molten steel in the mold, it is 30
0 to 500 gauss is preferable.

A specific method of carrying out the method of the present invention is carried out, for example, as follows. The installation position of the rolling roll pair is
Generally, the position is fixed. Further, the step d between the surface of each roll and each bearing also has a value determined by the pair of rolling rolls to be arranged. The thickness L of the unsolidified portion having a solid fraction of 0.8 or less at the rolling position changes depending on the casting speed if the secondary cooling conditions of the steel and the slab are appropriately selected. Therefore, if the thickness of the slab after bulging and the secondary cooling conditions of the slab are kept constant, the reduction ratio α = (R
-D) / L can be adjusted to any value.

Although the vertical continuous casting machine is shown in FIG. 1, the method of the present invention can be applied to a curved continuous casting machine.

[0035]

EXAMPLE A slab continuous casting machine having the apparatus configuration shown in FIG.
Further, a casting test was conducted using a continuous casting machine with an electromagnetic stirring device in the mold. The width of the slab was fixed to 2000 mm and the thickness was set to two types of 200 and 300 mm. Steel for planks having a C content of 0.15 to 0.20% by weight was cast. The casting speed was 0.5 to 1.1 m / min, and the secondary cooling specific water content of the slab was 1 to 2 liters / kg-steel.

In the test, a reduction test using one reduction roll pair and a reduction test using two reduction roll pairs were performed. When using one pair of reduction rolls, FIG.
A pair of reduction rolls in which the bearings do not overlap each other in the width direction of the slab, as shown in (a), or FIG. 2 (b).
A rolling-down roll pair was used in which each bearing portion overlaps with each other as shown in FIG.

When two pairs of reduction rolls are used, the test is arranged so that the bearing portions of each reduction roll pair do not overlap at the position in the width direction of the slab, and the bearing portions are overlapped at the position in the width direction of the slab. The arranged test was performed.

The diameter of the roll body was 480 mm, and the step between the roll surface and the bearing was 10 mm. further,
In some of the tests, one pair of reduction rolls having a flat roll shape with a bearing portion having a diameter of 480 mm was used.

An electromagnetic stirrer was placed in the mold, and the electromagnetic force was 300 gauss at the center of thickness of the mold, and was used except for some tests.

A longitudinal section sample having a length of 4 m in the casting direction was sampled from the slab in a steady state at a portion corresponding to the position of the width center portion of the slab and the bearing portion of the roll. The cross-section of the longitudinal cross-section sample was subjected to sulfaprinting, and the presence or absence of internal cracking was investigated. If there were internal cracks, the maximum length of cracks that occurred was determined. Next, with a drill blade having a diameter of 2 mm from the center of the thickness of the slab of the longitudinal cross-section sample, cuttings were taken at a pitch of 500 mm over a length of 4 m in the casting direction, C was chemically analyzed, and the average value was calculated. C content C in the thickness center portion of This average value C is used as the Ladle analysis value C _0.
The value C / C ₀ divided by was taken as the center segregation degree of C. Table 1 shows the test conditions and Table 2 shows the test results. The thickness of the unsolidified portion was calculated by solidification heat transfer analysis.

[0041]

[Table 1]

[0042]

[Table 2]

Test No. of the present invention example 1-No. In 6,
Each roll was rolled down by a pair of rolling rolls having two bearings, of which the test No. 1-No. In No. 5, a pair of reduction rolls arranged so that the bearings of the rolls do not overlap each other in the width direction of the cast slab, Test No. In No. 6, the rolling roll pair in which the bearing portions of the rolls overlap was tested. The thickness of the slab at the start of bulging was set to No. 1 to N
o. No. 4, 200 mm, test No. 5 and test No.
In No. 6, it was set to 300 mm. Both were tested at 0.5 to 1.2 within the range of the reduction ratio α specified by the method of the present invention.

Test No. in which the electromagnetic stirrer in the mold was not operated In No. 3, no internal cracking occurred, but the center segregation degree C / C ₀ of C was 1.01 to 1.2 for both the center of the width of the slab and the slab corresponding to the position of the bearing of the reduction roll pair.
Occurrence of a very slight center segregation of about 02 was observed. Test No. tested with a pair of reduction rolls in which the bearings of each roll overlap. In No. 6, no internal cracking occurred, but a very slight center segregation with a C center segregation degree C / C ₀ of about 1.05 occurred in the slab corresponding to the position of the bearing portion. This is because the pressing roll pair is slightly bent. In addition, a slight step was generated on the surface of the slab. Other No. 1, No. 2, N
o. 4 and No. In the test of No. 5, no internal cracking occurred, and the center segregation degree C / C ₀ of the cast piece corresponding to the position of the bearing was 0.97 to 1.00, indicating that center segregation occurred. However, a slab with good internal quality was obtained.

Test No. of the present invention example 7 and No. In No. 8, each roll was rolled down by two rolling roll pairs having one bearing portion, and among them, test No. In Test No. 7, the rolling bearing pairs of the rolling roll pairs were arranged such that the bearing portions of the rolling roll rolls did not overlap each other at the position in the width direction of the cast slab. In No. 8, a test was performed using a pair of reduction rolls in which each bearing portion of each pair of reduction rolls overlaps. The thickness of the slab and the amount of bulging at the start of bulging are 300 mm and 50 mm, respectively, and the reduction amount is 2
Two rolling roll pairs were evenly spaced by 25 mm.

Test No. In No. 7, internal cracking did not occur, and the center segregation degree C / C ₀ of the slab corresponding to the position of the bearing was 1.01, which showed no center segregation and good internal quality. The slab was obtained. Test No. In No. 8, a slight step was generated on the surface of the slab, but other than that, a slab of good quality was obtained.

Test No. of the comparative example. 9 and No. In No. 10, a test was performed by operating the electromagnetic stirrer in the mold using a pair of reduction rolls arranged so that the bearings of the rolls did not overlap each other in the width direction of the cast slab. The reduction ratio α is 0.2
It was set to 5 or 0.40 and tested at a value outside the lower limit specified by the method of the present invention.

Test No. 9 and No. In No. 10, a slab corresponding to the position of each bearing of each roll has a length of 15
Internal cracks of about 17 mm occur, and the center segregation degree C / C ₀ of C of the cast piece at that position is 1.19 to 1.4.
4, and significant center segregation occurred. Since the reduction ratio is small, the reduction effect of the slab corresponding to the position of each bearing of each roll cannot be obtained.

Test No. of the comparative example. In No. 11, each bearing part of each roll uses a pair of reduction rolls arranged so as not to overlap at a position in the slab width direction, the electromagnetic stirring device in the mold is operated, and the reduction ratio α is set to 4.00. The test was conducted at a large value outside the upper limit specified by the method of the invention. Casting was stopped because it became difficult to roll down the slab.

Test No. of the comparative example. In 12, a single roll having a flat bearing surface without a bearing is used,
When the reduction ratio α was 1.00 within the range of the reduction ratio defined by the method of the present invention, the electromagnetic stirrer in the mold was operated and tested. Test No. In No. 12, a slight internal crack having a length of about 5 mm occurred near the width center of the slab, and a center segregation having a C center segregation degree C / C ₀ of about 1.22 occurred at the width center portion. . This is because the roll is bent during rolling down, and particularly in the vicinity of the width center of the slab, a sufficient rolling down effect was not obtained.

[0051]

By applying the method of the present invention, it is possible to obtain a sound slab with a small center segregation without causing internal cracks in the slab, even if the slab has a wide width.

[Brief description of drawings]

FIG. 1 is a diagram for explaining an example of a device configuration of a continuous casting machine when carrying out the method of the present invention.

FIG. 2 is a view showing a situation in which a cast piece is pressed by a pressing roll pair having divided rolls.

FIG. 3 is a diagram showing a relationship between a maximum internal crack length of a cast slab and a reduction ratio.

FIG. 4 is a diagram showing a relationship between a center segregation degree C / C ₀ of C and a reduction ratio.

[Explanation of symbols]

1: Mold 2: Cast piece 2a: Solidified shell 2b: Unsolidified part 3: Guide roll 4: Rolling roll pair 5: Pinch roll 6: Molten steel 7: Immersion nozzle 8: Bearing part 9: Roll surface 10: Segment d: Roll Step between surface and bearing

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-9-99348 (JP, A) JP-A-5-50200 (JP, A) JP-A-5-8006 (JP, A) JP-A-8- 257714 (JP, A) JP-A-6-297125 (JP, A) JP-A-9-206903 (JP, A) JP-A-11-123513 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) B22D 11/128 350 B22D 11/128 340 B22D 11/16 B22D 11/20

Claims (2)

(57) [Claims] 1. The slab thickness at the start of bulging is 5 to 25%.
After bulging by a thickness corresponding to, the rolls are divided into at least two rolls by the time the solidification is completed, and a slab is produced so that the reduction ratio α satisfies the following formula (A). Is a continuous casting method for steel. 0.5 ≤ α ≤ 1.3 (A) where α = (R-d) / LR R: amount of reduction of cast slab (mm) d: bearing portion between divided rolls Between roll and roll surface (mm) L: Thickness (mm) of unsolidified portion with solid fraction of 0.8 or less at the rolling position 2. When rolling down a slab using two or more rolling roll pairs, the bearings of the divided portions of the rolling roll pairs are
The continuous casting method for steel according to claim 1, wherein the steel is placed on the same surface of the slab so as not to overlap at a position in the width direction of the slab and rolled down.