JP2001018041A - Continuous casting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel continuous casting method with which a large-width cast slab having little center segregation and good internal quality without development of internal cracks can be obtd. SOLUTION: In this method for continuously casting steel, rolling reduction is applied to a steel slab 2 so that a draft αsatisfies the formula 0.5<=α<=1.3 by using rolling reduction roll pairs 4 each of which is divided at least into two until completing solidification after bulging by a thickness corresponding to 5-25% of the cast slab thickness at the starting time of the bulging. Here, α=(R-d)/L, R is the draft (mm) of the cast slab, (d) is the level difference (mm) between a bearing part and the roll surface at the interval between divided rolls and L is the thickness (mm) of the unsolidified portion at <=0.8 solid ratio in the position of the rolling reduction.

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 an unsolidified portion is reduced 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 is caused by C,
The segregated components such as S, P, and Mn appear in a concentrated manner. It is known that the center segregation of the slab causes a decrease in toughness of a thick plate as a product and a hydrogen-induced cracking of a large-diameter steel pipe manufactured by bending and welding a thick plate.

[0003] The mechanism of generation of center segregation in a slab is considered as follows. As the solidification progresses, segregated components are concentrated between dendrite trees, which are solidified structures. The molten steel in which the segregated components are concentrated flows out from between the dendrite trees due to shrinkage of the slab during solidification or swelling of the slab called bulging. The outflowing concentrated molten steel flows toward the solidification completion point of the final solidification part, and solidifies as a thick enriched zone as it is. Therefore, center segregation occurs.

As measures to prevent the center segregation, it is effective to prevent the movement of the concentrated molten steel remaining between the dendrite trees and to prevent the local accumulation of the concentrated molten steel. Is adopted. As one of them, there is a light reduction method using a reduction roll group, but there is a limit to the improvement of center segregation under a light reduction slightly exceeding the solidification shrinkage.

In order to effectively improve the center segregation, a method of applying a large reduction with a reduction roll pair has been adopted. For example, in Japanese Patent Application Laid-Open No. 9-57410, a slab containing an unsolidified portion is reduced to 20 to 100%. A method has been proposed in which bulging is performed by about 100 mm, and 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,
In addition, since both ends of the completely solidified slab in the width direction are not reduced, excessive equipment does not need to be used.

However, in this method, when the slab is wide and the amount of reduction is large, the rolls of the pair of reduction rolls are liable to bend, so that internal cracks occur at the solidification interface and the center segregation occurs. May not be able to achieve the improvement effect.

[0007]

DISCLOSURE OF THE INVENTION The present invention relates to a continuous steel which can obtain a cast slab of good internal quality with less occurrence of center segregation without causing internal cracks even in a wide cast slab. It is intended to provide a casting method.

[0008]

The gist of the present invention resides in a continuous casting method of steel shown in the following (1) and (2).

(1) The thickness of the slab 5 to 5 at the start of bulging
After the bulging is performed by a thickness corresponding to 25%, until the solidification is completed, the roll ratio α satisfies the following formula (A) using a roll pair in which each roll is divided into at least two rolls. A continuous casting method of steel that reduces slabs.

0.5 ≦ α ≦ 1.3 (A) where α = (R−d) / LR R: amount of reduction of cast slab (mm) d: between divided rolls Step (mm) between the bearing portion and the roll surface (mm) L: Thickness (mm) of the unsolidified portion having a solid fraction of 0.8 or less at the rolling position (mm) (2) Rolling down the slab using two or more rolling roll pairs The continuous casting method for steel according to the above (1), wherein the rolling portions are arranged so that the bearing portions of the divided portions of the rolling roll pair are arranged so as not to overlap on the same surface of the slab at the position in the slab width direction. .

[0011] 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 adjusted to 0.8% of the solid fraction inside the slab.
Thickness between the solidification interfaces. The reason for setting the solid phase ratio of 0.8 as the solidification interface, which is the boundary between the solidification shell and the unsolidified portion, is that the force acting on this solidification interface is effectively transmitted to the solidified portion outside this interface. This is because that. The solidification interface having a solid fraction of 0.8 can be calculated by the solidification heat transfer analysis, and can also be obtained by the thickness calculation formula of the solidified shell expressed by a proportional expression of 1/2 of the solidification time.

The inventor has solved the above-mentioned problem of the present invention as follows. (A) In the method of the present invention, a slab including an unsolidified portion is reduced by using a reduction roll pair in which each roll is divided into at least two. Since the roll divided by the bearing portion is used, even when a wide cast piece is rolled down, the roll can be prevented from bending. Since each roll of the pair of reduction rolls does not bend, the reduction of the cast slab can be performed uniformly. The wide cast slab to which the method of the present invention is applied means a cast slab having a width of about 1800 mm or more.

(B) Due to the structure of the roll, the shape of the bearing part into which the roll is divided is concave from the roll surface. Using a rolling roll pair having such a bearing portion, when rolling down a slab including an unsolidified portion after bulging, the slab corresponding to the position of the bearing portion, without generating internal cracks, In addition, in order to effectively improve the center segregation, it is necessary to perform reduction at 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)
Since the reduction is performed within the range of the reduction ratio represented by the equation, the reduction force acting on the solidification interface inside the slab 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 a compressive force in the slab thickness direction. Therefore, it is possible to prevent the occurrence of internal cracks during rolling.

Further, the concentrated molten steel in the final solidified portion is discharged to the upstream side in the casting direction by the compressive force acting on the solidified interface and the effect of reducing the volume of the unsolidified portion during rolling. Therefore, the occurrence of center segregation at the center of the slab thickness can be prevented.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram for explaining an example of an apparatus configuration of a continuous casting machine when implementing a method of the present invention. Molten steel 6 injected into mold 1 through immersion nozzle 7
Solidifies in the mold to form a solidified shell 2a. The thickness of the solidified shell that is pulled out of the mold and cooled by water sprayed from a group of spray nozzles (not shown) below the mold gradually increases. The thickened solidified shell, that is, the slab 2, is pulled out by the pinch roll 5 through the guide roll 3 and the pressing roll pair 4. In the bulging zone, the slab is bulged by gradually increasing the roll interval corresponding to the thickness of the slab of the guide roll. Thereafter, in the reduction zone, the slab including the unsolidified portion 2b is reduced by the pair of reduction rolls under conditions that satisfy the formula (A) defined in the present invention.

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

FIG. 2 is a view showing a situation where the slab is reduced by a pair of reduction rolls in which the divided portion is supported by a bearing.
The pressing roll pair used in the present invention has a structure in which a split portion supports a rotating shaft of the roll. FIG. 2 shows an example in which each roll has a bearing portion at each of two locations. 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 the position in the slab width direction in the drawing. FIG.
(B) shows an example of a pair of pressing rolls arranged so that the bearings of the upper and lower rolls overlap.

The bearings 8 may be fixed to the segments 10 as usual. The tops of these bearings are positioned 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 that the roll surface of each roll can be cut and reused decreases,
The so-called life is shortened. If it exceeds 30 mm, the swelling of the slab surface after rolling becomes large, and surface defects are likely to occur on the surface of a product which is hot-rolled using this slab as a raw material.

In the method of the present invention, the bulging amount is 5 to 25% of the thickness of the slab at the start of bulging. The thickness of continuous cast slabs such as slabs and blooms is 200-35
About 0 mm is common. Therefore, when rolling down a bulged slab exceeding 25%, excessive rolling down equipment is required and the equipment cost becomes enormous.
%. In addition, in order to secure the required reduction thickness to apply a compressive force to the solidification interface inside the slab by reduction,
Bulging must be 5% or more. If less than 5%
The thickness that can be reduced is small, and it is difficult to apply sufficient compressive force to the solidification interface. Therefore, the bulging amount is set to 5 to 25% of the thickness of the slab at the start of bulging. The thickness of the slab at the start of bulging refers to 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, as shown in the above-mentioned equation (A), the rolling reduction α is 0.5 to 1.3. The reason will be described below.

As described above, when the slab is reduced,
If the rolling force acting on the solidification interface inside the slab can be propagated between the confronting solidification interfaces, the force acting on the solidification interface will not be a tensile force along the casting direction, so the occurrence of internal cracks in the slab Can be prevented. In addition to the effect of reducing the volume of the unsolidified portion at the time of rolling, the concentrated molten steel in the final solidified portion is discharged to the upstream side in the casting direction, so that the occurrence of center segregation of the slab can be prevented.

In order to reduce the cast slab in the above-described state, it is necessary to reduce the slab so that the following formula (B) is satisfied when a pair of reduction rolls having no divided portion supported by a bearing is used. .

0.5 ≦ α ≦ 1.55 (B) where α = R / LR R: Reduction amount of cast slab (mm) L: Not more than 0.8 of solid phase ratio at reduction position Thickness of solidified part (mm) The ratio R / L of the reduction amount R and the thickness L between solidification interfaces with a solid fraction of 0.8 is an index indicating the degree of approach of the solidification interfaces on both sides in the thickness direction of the slab due to the reduction. It is. This ratio R / L is 0.5
In the case of １．５1.55, the rolling reduces the state of the slab as described above, that is, the rolling force acting on the solidification interface can propagate between the solidification interfaces, and the volume of the unsolidified portion decreases. .

In the case of using a pressing roll pair having a divided portion supported by a bearing, if a step between each roll surface of the pressing roll pair and each bearing portion is d, a cast slab corresponding to the position of the bearing portion is formed. Since the reduction amount is (Rd), the index indicating the degree of approach of the solidification interface on both sides of the slab corresponding to the position of the bearing portion is the ratio (Rd) / L. Accordingly, if the pressure is reduced so as to satisfy the above-mentioned equation (A) from the above-mentioned equation (B), internal cracks do 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 (Rd) / L is 1.3.
The reason for this is that when the rolling reduction exceeds 1.3, the slab excessively lowers the bearing, so that excessive stress acts on the bearing and the bearing is likely to be damaged.

Hereinafter, the reduction ratio α = (R−d) / L is set to 0.
The specific reason for setting the value to 5 to 1.3 will be described. FIG.
A steel sheet having a content of 0.20% by weight was prepared by adding
It is a figure which shows the relationship between the maximum length of the internal crack of the slab at the time of rolling and the rolling ratio (alpha) at the time of rolling which was cast continuously in the slab of 00mm and investigated by the method mentioned later. It can be seen that when the reduction ratio α defined by the above-mentioned formula (A) becomes 0.5 or more, no internal cracks occur during the reduction. This is because the rolling force acting on the solidification interface due to the reduction propagates between the facing solidification interfaces, and the force acting on the solidification interface becomes a compressive force.

FIG. 4 shows that the same steel as that described in FIG. 3 is continuously cast into a slab having the same sectional shape, and the center segregation degree C / C ₀ of C at the center of the thickness of the slab, which is investigated by a method described later, is shown. It is a figure which shows the relationship with reduction ratio (alpha). It can be seen that when the rolling reduction α exceeds 0.5, the center segregation is significantly improved. Because of the effect of the compressive force acting on the solidification interface and the contraction of the volume of the unsolidified part during rolling, the concentrated molten steel in the final solidified part was discharged upstream in the casting direction, preventing the occurrence of center segregation in the slab. It is. From the above, the rolling reduction α is set to 0.5 or more in order to prevent the occurrence of internal cracks 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, in order to prevent excessive stress from acting on the bearing portion. From the above,
The reduction ratio α = (R−d) / L is set to 0.5 to 1.3. When the cast slab is reduced by two or more pairs of reduction rolls as described later, each reduction roll pair is reduced so as to be within the range of the reduction ratio.

In the method of the present invention, when the slab is reduced by using two or more reduction roll pairs, the bearings of the divided portions of the reduction roll pairs are arranged in the same direction in the width direction of the slab on the same surface of the slab. It is desirable to reduce the pressure by arranging them so that they do not overlap each other. Here, that the bearing portions do not overlap means that the intervals between the rolls on both sides of the bearing portion do not overlap. When rolling down, a step corresponding to the shape of the bearing is formed on the surface of the slab corresponding to the position of the bearing.
Therefore, when the bearings of each reduction roll pair overlap each other at the position in the slab width direction, the steps of the slabs after the reduction become large, and the surface of the hot-rolled product using these slabs as a raw material is increased. This is because surface defects tend to occur.

Further, when rolling down by one rolling roll pair, it is desirable to use a rolling roll pair in which each bearing portion of each roll is arranged so as not to overlap at a position in the slab width direction. Here, that the bearing portions do not overlap means that the intervals between the rolls on both sides of the bearing portion do not overlap. When rolling down, on the surface of the slab corresponding to the position of the bearing portion, if each bearing portion of each roll of the bearing portion overlaps, the rolls are slightly easily bent, and the effect of improving center segregation is slightly reduced. . Further, the positions of the steps on the surface of the cast slab after the rolling are overlapped on the upper and lower surfaces of the cast slab, and surface defects are likely to occur slightly on the surface of the product hot-rolled using the slab.

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. With the upstream side in the casting direction from the solidification completion point, the molten steel in the mold may be electromagnetically stirred, or in the secondary cooling area of the slab, the unsolidified portion inside the slab may be electromagnetically stirred. Means In any case, this is because the solidified structure at the center of the thickness of the slab has an equiaxed crystal structure. If such an equiaxed crystal is formed, even if the reduction ratio α is a value smaller than 1, the equiaxed crystals will be in contact with each other, and the reduction 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, so that internal cracks are less likely to occur at the time of reduction, and the occurrence of center segregation of the slab can be prevented. The strength of the electromagnetic force is not particularly limited. It may be determined according to the stirring position, the thickness of the slab, etc., but when the molten steel in the mold is electromagnetically stirred, 30 mm is required at the center of the thickness of the mold.
About 0 to 500 Gauss is good.

A specific method for carrying out the method of the present invention is, for example, as follows. The installation position of the roll pair is
Generally, it is a fixed position. Further, the step d between each roll surface and each bearing portion also has a value determined by the roll-down roll pair 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 by appropriately selecting the secondary cooling condition of the steel and the slab. Therefore, if the thickness of the slab after bulging and the condition of the secondary cooling of the slab are kept constant, the reduction ratio α = (R
-D) / L can be adjusted to any value.

Although FIG. 1 shows a vertical continuous casting machine, the method of the present invention can be applied to a curved continuous casting machine and the like.

[0035]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A slab continuous casting machine having an apparatus configuration shown in FIG.
Further, a casting test was performed using a continuous casting machine to which an electromagnetic stirring device in a mold was added. The width of the slab was constant at 2000 mm, and the thickness was 200 and 300 mm. Steel for thick plates 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 volume of the slab was 1 to 2 liter / kg-steel.

In the test, a test was performed using one roll pair and a test was performed using two roll pairs. In the case of using one reduction roll pair, FIG.
FIG. 2B shows a pair of pressing rolls in which each bearing portion does not overlap at a position in the slab width direction in each roll as shown in FIG.
As shown in Fig. 5, a pair of rolling rolls in which each bearing portion overlaps with each roll was used.

When two pressing roll pairs are used, a test is carried out so that the bearing portions of each pressing roll pair do not overlap at the position in the slab width direction, and the test is performed so that the bearing portions overlap at the position in the slab width direction. Placed tests were 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 tests, one flat roll-shaped roll pair having a diameter of 480 mm without bearings was used.

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

From the slab in the steady state, a longitudinal section sample having a length of 4 m in the casting direction was collected at a portion corresponding to the center of the width of the slab and the position of the bearing of the roll. The cross section of the vertical cross section sample was subjected to sulfur printing, and the presence or absence of occurrence of internal cracks was examined. When there was an internal crack, the maximum length of the crack that occurred was determined. Next, cuttings were taken at a pitch of 500 mm over a length of 4 m in the casting direction by a drill blade having a diameter of 2 mm from the center of the thickness of the slab of the sample of the longitudinal section, and C was chemically analyzed, and the average value was used as the slab. Was determined as the C content C at the center of the thickness. This average value C is used as the ladle analysis value C _0.
The value C / C ₀ divided by the above was defined as the degree of central segregation 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 to No. In 6,
Each roll was reduced by one pair of reduction rolls having two bearing portions. 1 to No. In Test No. 5, a pair of rolling rolls in which each bearing portion of each roll is arranged so as not to overlap at a position in the slab width direction, test No. In No. 6, the test was performed using a pair of rolling rolls in which the respective bearing portions of the rolls overlap. The thickness of the slab at the start of bulging 1 to N
o. No. 4, 200 mm, test no. 5 and test no.
6 was 300 mm. All were tested at 0.5 to 1.2 within the reduction ratio α defined by the method of the present invention.

Test No. 3 in which the electromagnetic stirring device in the mold was not operated. In No. 3, no internal cracks occurred, but the center segregation degree C / C ₀ of C was 1.01 to 1.0 in both the slab corresponding to the center of the width of the slab and the position of the bearing portion of the roll pair.
Occurrence of a very slight center segregation of about 02 was observed. Test No. was conducted using a pair of rolling rolls in which each bearing portion of each roll overlaps. In No. 6, no internal cracking occurred, but a very slight center segregation with a center segregation degree C / C ₀ of C of about 1.05 occurred in the slab corresponding to the position of the bearing portion. This is because the pressing roll pair slightly bent. In addition, a slight step was generated on the slab surface. Other Nos. 1, No. 2, N
o. 4 and no. In the test of No. 5, there was no occurrence of internal cracks, and the center segregation degree C / C ₀ of C was 0.97 to 1.00 in a slab corresponding to the position of the bearing portion. No slab of good internal quality was obtained.

Test No. of the present invention example 7 and No. 7 In Test No. 8, each roll was rolled down by two pairs of rolling rolls each having one bearing portion. In Test No. 7, a pair of rolling rolls in which each bearing portion of each rolling roll pair is arranged so as not to overlap at a position in the slab width direction, test no. In No. 8, the test was performed using a roll pair in which each bearing portion of each roll pair overlaps. The slab thickness and bulging amount at the start of bulging are 300 mm and 50 mm, respectively, and the reduction amount is 2 mm.
Each of the two rolling roll pairs was uniformly set to 25 mm.

Test No. In No. 7, no internal cracking occurred, and the degree of center segregation C / C ₀ of C was 1.01 in the slab corresponding to the position of the bearing portion. Was obtained. Test No. In No. 8, although a slight step was generated on the surface of the slab, a slab of good quality was obtained otherwise.

Test No. of Comparative Example 9 and No. 9 In No. 10, a test was performed by operating a magnetic stirrer in a mold using a pair of pressing rolls arranged such that each bearing portion of each roll did not overlap at a position in the slab width direction. Reduction ratio α is 0.2
5 or 0.40, and the test was performed at a value outside the lower limit specified by the method of the present invention.

Test No. 9 and No. 9 In the case of 10, the cast slab corresponding to the position of each bearing portion of each roll has a length of 15
An internal crack of about 17 mm occurs, and the degree of center segregation C / C ₀ of C of the slab at that position is 1.19 to 1.4.
4 and significant center segregation occurred. Since the rolling reduction ratio was small, the rolling effect of the slab corresponding to the position of each bearing portion of each roll could not be obtained.

Test No. of Comparative Example In No. 11, the rolling stirrer in the mold was operated using a rolling roll pair in which each bearing portion of each roll was arranged so as not to overlap at the position in the slab width direction, and the rolling reduction α was 4.00. The test was performed at a large value outside the upper limit specified in the method of the invention. It became difficult to reduce the slab, and the casting was stopped.

Test No. of Comparative Example In 12, using a single roll with a flat roll surface without bearings,
When the reduction ratio α was 1.00 within the range of the reduction ratio defined in the method of the present invention, the test was conducted by operating the electromagnetic stirring device in the mold. Test No. In No. 12, a slight internal crack having a length of about 5 mm occurred near the center of the width of the slab, and a center segregation C / C ₀ of C of about 1.22 occurred in the center of the width. . This is because when rolling down, the roll is bent, and especially in the vicinity of the center of the width of the slab, a sufficient rolling-down effect cannot be obtained.

[0051]

According to the method of the present invention, a sound slab having a small center segregation can be obtained without causing internal cracks in the slab even with a wide slab.

[Brief description of the drawings]

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

FIG. 2 is a diagram showing a situation where a slab is being reduced by a reduction roll pair having divided rolls.

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

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

[Explanation of symbols]

1: mold 2: cast slab 2a: solidified shell 2b: unsolidified portion 3: guide roll 4: reduction roll pair 5: pinch roll 6: molten steel 7: immersion nozzle 8: bearing 9: roll surface 10: segment d: roll Step between surface and bearing

Claims (2)

[Claims] 1. A slab thickness at the start of bulging of 5 to 25%
After the bulging is performed by the thickness corresponding to the thickness of the slab, until the solidification is completed, the slab is cast using a reduction roll pair in which each roll is divided into at least two so that the reduction ratio α satisfies the following formula (A). A continuous casting method of steel, characterized in that: 0.5 ≦ α ≦ 1.3 (A) where α = (R−d) / LR R: reduction of casting slab (mm) d: bearing between divided rolls (Mm) between the roll and the roll surface L: Thickness (mm) of the unsolidified portion having a solid phase ratio of 0.8 or less at the pressing position 2. When a slab is reduced by using two or more pairs of reduction rolls, a bearing portion of a divided portion between the pair of reduction rolls includes:
2. The continuous casting method for steel according to claim 1, wherein the slabs are arranged so as not to overlap each other at the position in the slab width direction on the same surface of the slabs and the slabs are lowered.