JPH09314298A - Continuous casting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the center segregation without developing negative segregation zone by increasing the intervals of guide rolls in the thickness direction of a cast slab step by step, developing the bulging and successively, regulating rolling reduction till the completing point of solidification. SOLUTION: Molten steel 8 is poured into a mold 1 to form solidified shell 2a and drawn out as the cast slab 2 through pinch rolls 7. At this time, in a bulging zone, the guide roll group 3b between the liquidus crater end 9a of the cast slab 2 and the end part of the bulging zone increases intervals βin the thickness direction of the cast slab 2 step by step in the casting direction to develop the bulging and the max. thickness of the cast slab 2 is made 10-50% of the short side length γ of the mold 1. Further, the rolling reduction of >=10% of the short side length γ or the mold 1 per one pair of rolling reduction rolls is given in the thickness direction of the fast slab 2 by using at least a pair of rolling reduction rolls 5 in the interval from the end part of the bulging zone to the solidification completing point 9b. By this method, the center segregation can efficiently be reduced with the small rolling reduction force.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a continuous casting method capable of reducing center segregation of a steel slab.

[0002]

2. Description of the Related Art When a steel slab is produced by a continuous casting method, an internal defect called center segregation often becomes a problem. This center segregation is a phenomenon in which the molten steel components such as C, S, P and Mn are positively segregated in the thickness direction central portion (final solidified portion) of the cast slab. This phenomenon is a particularly serious problem in thick plate materials, and is known to cause deterioration of toughness in the segregated portion and hydrogen-induced cracking.

The cause of such center segregation is that the residual molten steel between dendrites (dendritic crystals) at the final stage of solidification is caused by the solidification shrinkage of the molten steel or the bulging of the solidification shell, etc., at the solidification completion point of the final solidification part. There is a macro-movement toward the steel, and the concentrated molten steel locally accumulates.

Therefore, as a measure to prevent center segregation,
There is a method of preventing the movement of the residual molten steel and the accumulation of the concentrated molten steel by reducing the vicinity of the solidification point by some method using a roll or a mold, and methods based on various ideas have been proposed.

For example, Japanese Patent Application Laid-Open No. 63-252655 proposes the following method for preventing center segregation.
This is to increase the amount of secondary cooling water sprayed on the surface of the slab to bring the surface temperature of the final solidified portion of the slab to a range of 700 to 800 ° C., and to increase the thickness of the solidified shell to prevent bulging between rolls. The flow of the concentrated molten steel is prevented by suppressing and further applying a reduction force having a strain rate of 0.2 to 0.4% per minute to the slab with the light reduction roll group.

In the light reduction by the above-mentioned reduction roll group, since it can be reduced only in a dot shape in the longitudinal direction of the slab, solidification shrinkage and bulging cannot be sufficiently prevented.
Further, since each reduction acts as a concentrated load, internal cracks are likely to occur at the solidification interface, and there is a drawback that the reduction amount cannot be large.

The method of continuously forging in the vicinity of the solidification completion point of the slab with a flat die has a drawback that the equipment cost becomes very high. A continuous casting method for solving this has been proposed in Japanese Patent Application Laid-Open No. 61-42460.

The method disclosed in the above-mentioned JP-A-61-42460 uses an electromagnetic stirrer or an ultrasonic wave applying device installed on the upstream side of the solidification completion point to cause molten steel to flow and cut the dendrites, and near the solidification completion point. In order to form an equiaxed crystal region, the rolling roll pair placed immediately before the solidification completion point applies a large reduction of 3 mm or more to forcibly form the solidification completion point and generate internal cracks. Instead, the center segregation is eliminated.

[0009]

However, even in the method according to the above-mentioned JP-A-61-42460,
Large reduction requires a considerable reduction force, which may not be applicable or sufficient reduction may not be secured depending on the conditions. In other words, in this method, the solidification part at both ends of the slab with large deformation resistance is plastically deformed by rolling, so when the steel type with large deformation resistance or when the solidification part temperature becomes low and the deformation resistance becomes large, the reduction roll The intended effect cannot be obtained due to bending and breakage of the frame or bending of the frame.

To solve this problem, Japanese Patent Laid-Open No. 61-2324
No. 7 publication proposes a method of locally reducing the unsolidified portion at the center of the slab width direction by a stepped roll, which is called a camel crown roll and has a large-diameter roll portion provided in the center in a protruding shape. There is. However, also in this method, since the rolling is locally performed by the stepped roll, a recess is formed on the surface of the slab, and this recess causes a surface flaw of the product in the subsequent rolling step. Further, due to the flow of the unsolidified portion in the slab and the variation of the secondary cooling, the unsolidified portion is not always located in the center in the width direction near the solidification completion point of the slab, and the position of the unsolidified portion and the large diameter roll portion There is a drawback in that the position of the pressure does not match, and the rolling position cannot be properly maintained.

An object of the present invention is to solve the conventional problem that occurs when a slab obtained by continuous casting is largely rolled by a roll just before the solidification completion point of the slab, and to roll by a smaller rolling force. It is an object of the present invention to provide a method for effectively reducing center segregation by a rolling method.

[0012]

The summary of the present invention is as follows.
It is in the continuous casting method of (1) and (2).

(1) A continuous casting method for a steel slab characterized by using the following means. Hereinafter, this is referred to as a first method of the present invention.

The interval between the guide rolls arranged in the bulging zone in the slab withdrawal direction in the thickness direction of the slab is
Increase in stages.

By the above, bulging is caused in the slab between the liquidus crater end of the slab and the end of the bulging zone.

According to the above, the maximum thickness of the slab is increased by 10 to 50% of the length of the short side of the mold.

Next, between the end of the bulging zone and the solidification completion point, at least one pair of reduction rolls is used to reduce in the thickness direction of the slab 10% or more of the short side length of the mold per pair. give.

A desirable upper limit of the amount of reduction by the pair of reduction rolls at this time is about 50%. The most desirable reduction amount is the amount corresponding to the above bulging amount.

Further, it is desirable that this reduction is performed after the unsolidified portion near the solidification completion point is equiaxed.

(2) A method for continuously casting a steel slab, characterized by using the following means 1) to 4). Hereinafter, this is referred to as a second method of the present invention.

1) The interval in the thickness direction of the slab of the guide rolls arranged in the bulging zone in the slab withdrawal direction is
Increase in stages.

2) According to the above 1), bulging is caused in the slab at the position where the solidified shell thickness of the slab is 70 mm or more.

At this time, the desirable upper limit of the thickness of the solidified shell is about 45% of the length of the short side of the mold.

3) According to 2) above, the maximum thickness of the slab is increased by 10 to 50% of the length of the short side of the mold.

At this time, the desirable upper limit of the thickness of the solidified shell at the end of the bulging zone is about 45% of the length of the short side of the mold.

4) Next, between the end of the bulging zone and the completion point of solidification, at least one pair of reduction rolls is used to apply a reduction at a reduction rate of 60 mm / min or more in the thickness direction of the slab to obtain the above-mentioned 3 ) The amount corresponding to the bulging amount is reduced.

The desirable upper limit of the rolling speed at this time is 15
It is about 0 mm / min. Further, this reduction is preferably performed after the unsolidified portion near the solidification completion point is equiaxed.

The term "stepwise" in the method of the present invention usually means
In the guide roll group consisting of a plurality of segment pairs consisting of a plurality of guide rolls, (a) continuous,
(b) continuous within a pair of segments, stepwise in segment pair units, (c) stepwise in segment pair units, and (d) combinations thereof.

[0029]

BEST MODE FOR CARRYING OUT THE INVENTION A first method of the present invention will be described with reference to FIGS.

FIG. 1 is a schematic side sectional view showing a structural example of a continuous casting machine for realizing the first method of the present invention. In FIG. 1, reference numeral 1 is a mold, 2 is a slab, 2a is a solidified shell, 2b is a non-solidified portion, 3a is a guide roll group before the liquidus crater end 9a, and 3b is a liquidus crater end 9a to bulging zone. Guide roll groups between the ends, 4 is an electromagnetic stirring means, 5 is a reduction roll group, 6 is a reduction means group, 7 is a pinch roll group, 8 is molten steel, 10 is a dipping nozzle, and 11 is a casting direction. The reduction roll 5 is a so-called flat roll.

In the case of FIG. 1, the vertical type continuous casting machine is used.
A curved continuous casting machine or the like may be used. The electromagnetic stirring means 4 is a means for stirring the unsolidified portion 2b to make it equiaxed as described later, and is not an essential means.

The molten steel 8 injected into the mold 1 through the immersion nozzle 10 is cooled by the water-cooled mold 1 and spray water sprayed from a spray nozzle group (not shown) arranged below the mold 1 to form a solidified shell 2a. The slab 2 is formed to be a slab 2, which is pulled out by the pinch roll 7 group through the guide rolls 3a and 3b group and the pressing roll 5 group while holding the unsolidified portion 2b therein.

The liquid phase crater end 9a is the slab 2
The solid phase ratio in the thickness center of the cast iron 2 is greater than 0, and the solidification completion point 9b is the position where the solid phase ratio in the thickness center of the slab 2 is 0.8 or more. These can be calculated by a one-dimensional unsteady heat transfer analysis in the thickness direction of the slab 2.

In the first method of the present invention, in the bulging zone of the continuous casting machine configured as described above, the group of guide rolls 3b between the liquid line crater end 9a of the slab 2 and the bulging zone end is The slabs 2 are arranged so that the distance between them in the thickness direction is increased stepwise in the casting direction. That is, when the short side length of the lower end of the mold 1 is γ, and the gap in the thickness direction of the slab 2 of the guide roll 3b immediately below the position of the liquidus line crater end 9a is α, the bulging zone shown in FIG. Then, the interval between the guide rolls 3b is gradually expanded in the drawing direction of the slab 2, and the range of the interval β in the thickness direction of the slab 2 of the guide roll 3b at the end of the bulging zone is set to 1.10γ to 1.50γ. This β = 1.10γ
˜1.50γ is the target maximum thickness of the slab before reduction, as described later.

Regarding the group of guide rolls 3a before the liquidus line crater end 9a, the intervals in the thickness direction of the slabs 2 may be determined according to the usual method according to the shrinkage and bulging of the slabs 2 and are limited. do not do.

The group of guide rolls 3b usually has a plurality of pairs of segment rolls, each of which has a plurality of pairs of guide rolls. An example of a stepwise increase method of the interval α in this case will be described with reference to FIG.

FIG. 2 is a schematic view for explaining an example of a method of stepwise increasing the interval between the guide rolls 3b in the thickness direction of the slab in the case of the segment structure. In FIG. 2, reference numeral 12 is a segment.

FIG. 2 (a) is continuous, FIG. 2 (b) is continuous within a pair of segments, and is stepwise in segment pair units, and FIG. 2 (c) is stepwise in segment pair units. , In each case. It is also possible to use these methods in combination. It can be selected from the above methods depending on the conditions such as the type of cast steel and the continuous casting machine, but normally, as shown in Fig. 1 and Fig. 2 (a),
It is preferable that the guide rolls 3b in each stage are formed in a completely continuous and stepwise manner so that the intervals between them are substantially evenly spread.

Further, the interval α may be increased either by a method of making it uniform in both sides as shown in FIGS. 1 and 2 or a method of making it only on one side in the thickness direction of the slab, depending on conditions such as a continuous casting machine. It is desirable to select by.

In the first method of the present invention, the slab 2 is bulged between the liquid phase line crater end 9a of the slab and the bulging zone end by the arrangement of the guide rolls 3b group as described above. Then, the maximum thickness of the slab 2 is made 10 to 50% thicker than the short side length γ of the lower end of the mold 1.

FIG. 3 is a cross-sectional view showing the state of the cast slab 2 in the bulging zone along the line AA 'shown in FIG. The bulging of the slab 2 shown in FIG. 3 is a phenomenon that occurs only when the unsolidified portion 2b exists in the slab 2 where the static pressure of the molten steel 8 acts. Even if the unsolidified portion 2b is not in the center of the slab 2 in the width direction, bulging occurs where the unsolidified portion 2b exists, so that a roll called a camel crown roll is not used in the subsequent reduction zone and the flatness is maintained. Even if a roll is used, it is possible to effectively reduce only the region where the unsolidified portion 2b exists.

When β is less than 1.10γ (10%),
It is not possible to secure a necessary minimum reduction amount of 10%, which will be described later, with the reduction roll 5 group. On the other hand, when it exceeds 1.50γ (50%), it becomes difficult to set the interval between the guide rolls 3b for preventing the occurrence of internal cracks in the bulging zone.

The reason for setting the bulging position between the liquidus crater end of the cast slab and the bulging zone end will be described with reference to FIGS. 1 and 4.

If bulging is performed before the liquidus crater end 9a, internal cracks will occur in the slab 2.

FIG. 4 (a) is a partial cross-sectional view showing the state of the slab when internal cracking occurs when bulging is performed before the liquidus crater end 9a. As shown in FIG. 4 (a), since the solidified shell 2a has a small strength, the short side S is dented by bulging, and the solidified shell 2a 'on the short side S side is recessed.
A large tensile stress F acts on the solidification interface side of and internal cracking occurs.

On the other hand, when bulging is performed between the liquid phase line crater end 9a and the bulging zone end, no internal cracking occurs in the cast piece 2.

FIG. 4 (b) is a partial cross-sectional view showing the state of the slab when internal cracking does not occur when bulging is performed between the liquid phase line crater end 9a and the bulging zone end. As shown in Fig. 4 (b), the short side S is not dented due to bulging, and the solidified shell 2 on the long side L side is formed.
Due to the gentle deformation of a, stress does not concentrate at a specific position on the solidification interface, and internal cracking does not occur.

In the case of bulging as described above, it is preferable that the length of the bulging zone is short and the position of the bulging zone is far from the mold also in terms of alignment management and operation of the continuous casting machine. The arrangement of the bulging zone end should be such that the thickness of the solidified shell is 45 times the short side length of the mold.
It is desirable to set the position to be less than or equal to%. This is because when the solidification shell thickness at the bulging position becomes thicker than 45% of the short side length of the mold, the thickness of the unsolidified portion of the slab is too small, so the concentrated molten steel at the final stage of solidification is sucked by the predetermined bulging. This is because even if a desired large reduction described later is applied in the subsequent reduction zone, the concentrated molten steel is not discharged, and there is a location where segregation or porosity deteriorates locally.

As described above, the maximum thickness of the slab is increased by 10 to 50% of the length of the short side of the mold, and then, between the end of the bulging zone and the completion point 9b of solidification by at least one pair of reduction rolls 5. In the thickness direction of the slab 2, a reduction of 10% or more of the short side length of the lower end of the mold is applied per pair in the thickness direction, and most desirably, the bulging amount equivalent to the above-mentioned thickness is reduced, and as shown in FIG. Roll down as shown.

FIG. 5 is a cross-sectional view showing an example of the state of the cast piece 2 in the reduction zone along the line BB 'shown in FIG.
By the reduction shown in the figure, the solidification interface of the solidification shell 2a 'on the short side S side of the slab 2 becomes a compressive stress field, so that no crack occurs at the solidification interface and a malignant negative segregation zone is formed in the center. Central segregation including semi-macro segregation is effectively improved by a relatively simple rolling-down means without any generation. A desirable upper limit of the amount of reduction by the pair of reduction rolls is about 50%. Such a reduction is what is called a large reduction. The desirable logarithm (stage number) of the reduction roll 5 is 1
About 10 to 10.

In the conventional roll reduction method, it is considered that the amount of reduction cannot exceed a certain value because of the fear of cracking at the solidification interface, and the amount of solidification shrinkage at the completion point of solidification is compensated by the reduction. It was limited to light pressure.

However, when the reduction amount is further increased, the solidification interface becomes in an elongated state, but since it is constrained in the casting direction in terms of stress, it rather compresses and cracks hardly occur. At this time, it was experimentally revealed that the required amount of reduction per pair of reduction rolls was 10% or more in the thickness direction of the slab. In addition, it is said that internal cracking can be prevented by forcibly forming the solidification completion point in the conventional large reduction method using rolls, but it was also tested that the formation of the solidification completion point does not affect the prevention of internal cracking. Became clear.

A considerable amount of rolling force is required to roll down the solidification completion point. Particularly in the case of a large slab, the pressing means for that purpose is required to be so large that it cannot be industrially put to practical use. For this reason, in the reduction in the first method of the present invention, it is desirable to reduce the diameter of the reduction roll and the size of the reduction means so that the solidification completion point is not forcibly formed. Further, it is desirable that the rolling reduction is equivalent to the amount of bulging in the bulging zone to such an extent that the solidified portions at both ends of the slab with large deformation resistance are not plastically deformed by rolling.

According to the first method of the present invention, from the viewpoint of preventing center segregation, the position of the unsolidified portion to be reduced can be exposed by bulging, and that portion can be effectively reduced.

The solidification structure at the center of the slab usually becomes a columnar crystal structure, but in the first method of the present invention, as shown in FIG. 1, the electromagnetic stirring means provided before the solidification completion point 9b of the slab. Four
Thus, it is desirable to stir the unsolidified portion 2b to generate an equiaxed crystal in the unsolidified portion 2b at the center of the slab near the solidification completion point 9b, and then perform the above reduction.

In the case of the columnar crystal structure, the effect of locally improving segregation may be locally reduced due to bridging in the width direction of the cast slab. However, in the case of the equiaxed crystal structure, molten steel flow is likely to occur due to reduction, and local accumulation of concentrated molten steel is prevented.

The desirable position of the electromagnetic stirring means 4 is in the bulging zone, and it is preferable that it does not overlap with the pressure reduction zone. The desirable range of frequency during electromagnetic stirring is 1.0 to 3.0 Hz, and the desirable range of current value is 600 to
It is 900A.

The method of generating equiaxed crystals does not necessarily have to be electromagnetic stirring. For example, a method in which ultrasonic waves are applied to the slab 2 via the guide roll 3 group or the reduction roll 5 group may be used, or in addition to this, low temperature casting or steel wire in a mold in consideration of simplicity and effect from the operation side. A method such as addition may be used.

Next, the second method of the present invention will be described with reference to FIG.

FIG. 6 is a schematic side sectional view showing a structural example of a continuous casting machine for realizing the second method of the present invention. The configuration shown in FIG. 6 is basically the same as that of the first method of the present invention shown in FIG. However, reference numeral 3'a is a guide roll group at a position where the thickness δ of the solidified shell 2a of the slab 2 is less than 70 mm, and 3'b is a guide roll group at a position where the thickness δ of the solidified shell 2a is 70 mm or more.

Reference numeral α'in FIG. 1 indicates a guide roll 3'at a position where the thickness δ of the solidified shell 2a of the cast piece 2 is 70 mm or more.
It is a space | interval in the thickness direction of the cast piece 2 of b. The symbol β is the distance in the thickness direction of the slab 2 of the guide roll 3′b at the end of the bulging zone.

The case of FIG. 6 is also a vertical continuous casting machine,
A curved continuous casting machine or the like may be used. The electromagnetic stirring means 4 is a device for stirring the unsolidified portion 2b to make it equiaxed as described above, and is not essential.

Regarding the group of guide rolls 3'a, the interval between the slabs 2 in the thickness direction may be in accordance with a usual method and is not limited.

In the second method of the present invention, the thickness δ of the solidified shell 2a of the cast slab 2 is set in the continuous casting machine having the above construction.
The guide roll 3′b group provided at a position of 70 mm or more has a distance α ′ in the thickness direction of the slab extracted from the slab 2 in the bulging zone as in the case of the first method of the present invention shown in FIG. It arranges so that it may increase gradually in the direction. The relationship between β and the short side length γ of the mold 1 shown in FIG. 6 is the same as in the case of the first method of the present invention described above.

In the second method of the present invention, the continuous casting machine configured as described above is used, and the interval between the guide rolls 3'b in the slab thickness direction is gradually increased in the bulging zone.

As a result, bulging is caused in the slab at the position where the thickness δ of the solidified shell 2a of the slab 2 is 70 mm or more, and the maximum thickness of the slab 2 is 1 of the short side length γ of the lower end of the mold 1.
Increase the thickness by 0-50%. At this time, the desirable upper limit of the solidified shell thickness δ is about 45% of the short side length γ.
Furthermore, the desirable upper limit of the solidified shell thickness δ at the end of the bulging zone is about 45% of the short side length γ.

The reason why the bulging is generated at the position where the solidified shell thickness δ of the cast slab is 70 mm or more is as follows.

That is, the solidified shell thickness δ of the cast piece is 70.
When bulging is performed at less than mm, in addition to the large thermal stress applied to the solidification interface side of the solidified shell on the short side, internal cracking occurs due to the reason explained in FIG. 4 (a) above. On the other hand, when the solidified shell thickness δ of the slab is bulged at a position of 70 mm or more, internal cracking does not occur for the reason explained in FIG. 4 (b) above.

When the thickness of the solidified shell at the end of the bulging zone exceeds about 45% of the short side length γ, the unsolidified thickness of the slab is too small, so the concentrated molten steel at the final stage of solidification is sucked by bulging. Even if a desired large reduction is applied in the subsequent reduction zone, the concentrated molten steel is not discharged, and there arises a local location where the positive segregation or the porosity deteriorates.

Desirable conditions for the length of the bulging zone and its position are the same as those for the first method of the present invention.

The reason for limiting the maximum thickness of the cast slab 2 after bulging to 10 to 50% of the short side length γ of the lower end of the mold 1 is as follows.

That is, if it is less than 10%, it is impossible to secure a necessary minimum rolling speed of 60 mm / min, which will be described later, in the rolling roll group. On the other hand, when it exceeds 50%, it becomes difficult to set the interval between the guide rolls 3'b for preventing internal cracking in the bulging zone.

The state of the slab at the position of line C-C 'shown in FIG. 6 and the significance of bulging are the same as those at the position of line AA' of the first method of the present invention shown in FIG. Become.

Then, between the end of the bulging zone and the completion point of solidification, at least one pair of reduction rolls is used to apply a reduction at a reduction rate of 60 mm / min or more in the thickness direction of the slab, whereby the above bulging is performed. Reduce the amount equivalent. The desirable upper limit of the rolling speed at this time is 15
It is about 0 mm / min. The amount of reduction corresponding to the amount of bulging is set so that the solidified portions at both ends of the slab with large deformation resistance are not plastically deformed by the reduction. Such a reduction is also called a large reduction. A desirable logarithm (stage number) of the reduction roll 5 is about 1 to 10.

In the second method of the present invention as well, the diameter of the pressing roll is reduced and the size of the pressing device is also reduced.
It is desirable to reduce the solidification completion point 9b shown in (4) so that the solidification completion point 9b is not forcibly formed. Furthermore, for the same reason as in the case of the first method of the present invention, it is desirable to perform this reduction after equiaxing the unsolidified portion near the solidification completion point 9b.

The state of the slab at the position of line D-D 'in the reduction zone shown in FIG. 6 is the same as that of the first embodiment of the present invention shown in FIG.
This is the same as in the case of the position of the line BB 'in the method.

According to the second method of the present invention as described above,
From the viewpoint of reducing center segregation, the position of the unsolidified portion to be reduced can be exposed by bulging, and that portion can be effectively reduced.

As described above, the conventional roll reduction method is limited to the light reduction in which the amount of solidification shrinkage at the completion point of solidification is compensated by the reduction. However, if the amount of reduction is further increased, the solidification interface becomes in an elongated state, but since it is constrained in the casting direction in terms of stress, it rather compresses and cracks hardly occur.

One of the methods for realizing this is to make the rolling speed appropriate as in the second method of the present invention. At this time, the required rolling speed is 60 mm / min or more,
It was empirically revealed that it is not always necessary to forcibly form the solidification completion point.

In the second method of the present invention, the first method of the present invention
The reason for not considering the liquid layer line crater end used in the method is as follows.

The liquid layer line crater end changes depending on the degree of superheat of molten steel. If this degree of superheat changes during operation, it is difficult to predict. On the other hand, the solidified shell thickness used in the case of the second method of the present invention is relatively easy to predict if the cooling conditions and the like are determined.

Therefore, in the first method of the present invention, when the casting conditions such as the molten steel superheat degree can be kept constant during the operation, the second method of the present invention makes the casting conditions constant. It is desirable to apply each when it is difficult to keep.

In the method of the present invention, the desirable range of the casting speed is about 0.3 to 3 m / min, and the desirable range of the cast piece size is about 800 to 2500 mm in width and 80 to 40 in thickness.
It is about 0 mm.

[0084]

【Example】

(Test 1) Using a continuous casting machine having the configuration shown in FIG.
Steel slabs were cast under the three conditions A, B, and C shown in FIG. The position of the final reduction roll is 100 mm from the solidification completion point
The upper part.

[0085]

[Table 1]

Condition A is a comparative example in which the central part of the slab is made to be an equiaxed crystal by electromagnetic stirring without bulging, and then large reduction is performed.
Condition B is an example of the present invention in which 10% bulging is performed and the central portion of the slab is made to be an equiaxed crystal by electromagnetic stirring, followed by a large reduction of 10%.
Condition C is an example of the present invention in which 50% bulging is performed without using magnetic stirring, and then a large reduction of 20 to 30% is performed.

The evaluation was performed by the maximum segregation degree of [P] and the number of semi-macro segregated grains. The maximum degree of segregation of [P] was obtained by cutting the obtained slab in a cross section perpendicular to the casting direction and taking a test piece from the center in the thickness direction.
The average concentration of [P] in each mesh was investigated by dividing the roughness into μm mesh, and this [P] and P of the molten steel
The ratio P / P ₀ to the concentration [P ₀ ] was set. The number of segregated grains is 50
The number of granular segregation in the range of mm × 1000 mm was magnified 50 times, and the case where P / P ₀ was 3 or more was investigated. The results are shown in FIGS. 7 and 8.

FIG. 7 shows [P] under conditions A, B and C.
8 is a graph showing the maximum degree of segregation, and FIG. 8 is a graph showing the relationship between the number of segregated grains and the semi-macro segregated grain size.

As is apparent from FIGS. 7 and 8, in Examples B and C of the present invention, proper bulging is performed at a proper position to expose a region in which an unsolidified portion is present, and a reduction is applied. The large reduction in the region where the unsolidified portion of the cast piece exists can be effectively performed, and the center segregation is improved as compared with Comparative Example A.

(Test 2) Using a continuous casting machine having the configuration shown in FIG. 6, steel cast pieces were cast under the three conditions of D, E and F shown in Table 2. However, the logarithm of the reduction rolls was changed as shown in Table 2, and the position of the final reduction roll was 100 mm above the solidification completion point as in Test 1.

[0091]

[Table 2]

Condition D is a comparative example in which the central portion of the slab is made equiaxed by electromagnetic stirring without bulging, and then large reduction is performed.
Condition E is an example of the present invention in which 10% bulging is performed and the central portion of the slab is made equiaxed by electromagnetic stirring, and then large reduction is performed. Condition F is 50% bulging without electromagnetic stirring, and then large reduction is performed. Here is an example.

The evaluation was based on the method of Test 1. However, the microscope range of the number of segregated particles is 50 mm × 500 mm, and P / P
_A survey was conducted for those in which ₀ was 2 or more. The results are shown in FIGS. 9 and 10.

FIG. 9 shows [P] under the conditions D, E and F.
And FIG. 10 is a diagram showing the relationship between the number of segregated grains and the semi-macro segregated grain size.

As is apparent from FIGS. 9 and 10, in Examples E and F of the present invention, appropriate bulging is performed at appropriate positions to expose the region where the unsolidified portion is present, and the roll is pressed. As a result, large reduction in the region where the unsolidified portion of the slab was present could be effectively performed, and center segregation was improved as compared with Comparative Example D.

[0096]

According to the method of the present invention, a large reduction is effectively applied to an unsolidified portion with a relatively small reduction load, so that a center segregation including a semi-macro segregation can be achieved without causing a negative segregation zone. Can be reduced.

[Brief description of drawings]

FIG. 1 is a schematic side cross-sectional view showing a configuration example of a continuous casting machine for realizing the first method of the present invention.

FIG. 2 is a schematic view illustrating an example of a method of gradually increasing the interval between the guide roll groups in the slab thickness direction in the casting direction. (a) is continuous, (b) is continuous within a pair of segments, and is segment-wise in steps, (c)
Is a step-like case for each segment pair.

FIG. 3 is a transverse cross-sectional view of the cast piece taken along the line AA ′ shown in FIG.

FIG. 4 is a partial transverse cross-sectional view of a slab for explaining a difference in the state of the slab according to a bulging position. (a) is when internal cracking occurs before the liquid phase crater end, and (b) is when internal cracking does not occur between the liquid phase crater end and the end of the bulging zone.

5 is a transverse cross-sectional view showing an example of a state of a cast piece along a line BB 'shown in FIG.

FIG. 6 is a schematic side sectional view showing a structural example of a continuous casting machine for implementing the second method of the present invention.

FIG. 7 is a graph showing the maximum degree of segregation of [P] in Test 1 of Example.

FIG. 8 is a diagram showing the relationship between the number of segregated grains and the semi-macro segregated grain size in Test 1 of Example.

FIG. 9 is a diagram showing the maximum degree of segregation of [P] in Test 2 of Example.

FIG. 10 is a diagram showing the relationship between the number of segregated grains and the semi-macro segregated grain size in Test 2 of Example.

[Explanation of symbols]

1: mold, 2: slab, 2a, 2a ': solidified shell, 2b: unsolidified part, 3a: guide roll before liquidus crater end, 3'a: solidified shell thickness 70m
Guide roll at a position less than m, 3b: Guide roll between liquidus crater end and solidification completion point, 3'b: Guide roll at a position where solidification shell thickness is 70 mm or more, 4: Electromagnetic stirrer, 5: Reduction Roll, 6: rolling down means,
7: Pinch roll, 8: Molten steel, 9a: Liquid line crater end, 9b: Solidification completion point, 10: Immersion nozzle,
11: Casting direction, 12: Segment, α: Spacing in the thickness direction of the guide roll immediately below the liquidus crater end position, α ′: Thickness direction of the guide roll at the solidified shell thickness of 70 mm or more Spacing, β: guide roll spacing in the thickness direction of the slab at the end of the bulging zone,
γ: short side length of mold, δ: solidified shell thickness, S:
Short side of slab, L: Long side of slab, F: Tensile stress

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kozo Ota Sumitomo Metal Industry Co., Ltd.

Claims (2)

[Claims] 1. An interval between guide rolls arranged in the bulging zone in the slab drawing direction in the thickness direction of the slab is gradually increased so as to be between the liquidus crater end of the slab and the bulging zone end. By causing bulging in the slab, the maximum thickness of the slab can be increased by 10 to the short side length of the mold.
50% thicker, and then between the end of the bulging zone and the completion point of solidification, at least 10% or more of the short side length of the mold is used per pair in the thickness direction of the slab using at least one pair of reduction rolls. A continuous casting method for a steel slab, which comprises applying a reduction. 2. A slab is formed at a position where the solidified shell thickness of the slab is 70 mm or more by gradually increasing the interval in the slab thickness direction of the guide rolls arranged in the bulging zone in the slab withdrawal direction. By causing bulging, the maximum thickness of the slab is increased by 10 to 50% of the short side length of the mold, and then at least one pair of reduction rolls are used between the end of the bulging zone and the completion point of solidification. A continuous casting method for a steel slab, comprising reducing the amount corresponding to the bulging amount by applying a reduction at a reduction rate of 60 mm / min or more in the thickness direction of the slab.