JP2727205B2 - Method for improving segregation of continuous cast slab - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention reduces micro, semi-macro and macro segregation formed inside a slab produced by a continuous casting method, and anomalies appearing in products due to segregation. The purpose is to prevent generation of tissue and deterioration of mechanical properties of the product.

2. Description of the Related Art Continuous casting of steel has been conventionally performed, and micro-segregation occurs between dendrite trees of slabs produced by this continuous casting method, and granular segregation occurs in the center of slab thickness. V-shaped or linear or band-shaped V segregation and center segregation were formed, and quality deterioration due to these segregation was inevitable. Micro-segregation is generated due to solute distribution between solid and liquid when a metal having a solid-liquid coexisting phase is solidified.
Segregation and center segregation are produced by mechanical factors such as bulging and misalignment of the solidified shell due to the molten steel static pressure, and by suction (suction) caused by solidification shrinkage and heat shrinkage, and the dense wood melt is accumulated in the center of the slab. You.

Conventionally, for segregation generated by such a mechanism,
Various measures have been proposed and implemented.

As a method of improving segregation, harmful segregation elements (for example, P,
S) in advance in the molten steel stage, a method of dispersing segregation by equiaxed crystallization and refinement of the solidified structure by electromagnetic stirring, and a method of reducing bulging by shortening the roll interval of a continuous casting machine. Generally done.

In addition, a method of reducing the slab in the final stage of solidification with rolls, sheet-like or bar-shaped reduction terminals to compensate for solidification shrinkage and heat shrinkage, to suppress the flow of concentrated molten steel, and to improve segregation. 16
No. 862, JP-B-59-16541, JP-B-59-39225, JP-A-56
−45256) is taking place.

Japanese Patent Publication No. 62-34460 discloses a method for preventing semi-macro segregation which forms macro center segregation and V segregation by electromagnetic stirring and reduction at the end of solidification.

On the other hand, a method of lowering the cooling rate in the continuous cast slab, promoting redistribution of solute during δ → γ transformation and diffusion in the solid phase, and separating or dispersing segregation (Japanese Patent Application Laid-Open No. 60-166150, Kaisho 62-674
4) In addition, Japanese Patent Application Laid-Open No. 61-154748 proposes a segregation improvement method in which Mo is added as a method for increasing the segregation separation effect by solute redistribution at the time of δ → γ transformation.

However, continuous casting of high-grade steel, which is a strict segregation material,
Due to the high quality of the conventional material and the omission of the process, etc., the segregation level allowed for the cast slab becomes increasingly severe, and the segregation measures as described above may not be sufficient.

Problems to be Solved by the Invention The present invention satisfies the segregation level that was difficult to achieve with conventional segregation countermeasures, and prevents the segregation-induced abnormal structure and deterioration of mechanical properties caused by conventional countermeasures. It provides segregation improvement technology, and this segregation improvement technology provides a method that enables continuous casting of high-grade steel that could not be achieved mainly due to segregation, higher quality of conventional materials, and omission of processes. It is something to offer.

Means for Solving the Problems The present invention for solving the above problems is the following seven items.

(1) In producing a slab by the continuous casting method, the degree of superheat of molten steel in a tundish is controlled at 5 to 50 ° C., and a magnetic stirring is provided between a mold or a mold and a subsequent secondary cooling zone to a solidification completion position. Casting is performed while stirring the molten steel by the device, and at least the secondary cooling zone exit side after the secondary cooling zone
A heating zone or a heating zone and a heating zone are provided between the reduction zone entrance side to heat or heat and heat the slab, and the slab is 4 mm or more when the solid phase ratio at the center of the slab cross section is 0.3 to 0.8. A method for improving segregation of continuously cast slabs, characterized by applying a reduction of (a).

(2) The method according to (1), wherein the slab is reduced by a plurality of pairs of rolls having a reduction band length of 2 m or more and a roll pitch of 500 mm or less.

(3) The method according to (1), further comprising the step of providing a pressing device having a slab conveying mechanism, and reducing the slab by a planar or bar-shaped pressing terminal.

(4) A continuous casting method in which an induction heating device or a cooling material adding device or both devices are provided in a tundish in order to control the degree of superheat of molten steel in the tundish to 5 to 50 ° C., and the method described in item 1 is applied.

(5) Primary cooling operation conditions such as the temperature of molten steel in the tundish, mold cooling water amount and temperature change of mold cooling water, secondary cooling operation conditions such as secondary cooling water amount, heat insulation capacity of the heat insulation zone, and operation conditions of the heating zone. The solidification calculation is performed based on the process information consisting of the ambient temperature, the slab size, and the casting speed, so that the solid phase ratio at the center of the slab cross section in the reduction zone is in the range of 0.3 to 0.8, or the reduction zone is the slab cross section. 2. The method according to claim 1, wherein a method of controlling the secondary cooling water amount, the operating conditions of the heating zone, and the casting speed is combined so that the solid fraction in the center portion is in the range of 0.3 to 0.8.

(6) The temperature of the molten steel in the tundish is determined by the primary cooling operation conditions such as the amount of mold cooling water and the temperature change of the mold cooling water, the secondary cooling operation conditions such as the amount of secondary cooling water, the heat insulation capacity of the heat insulation zone, and the operation of the heating zone. Perform solidification calculation based on process information consisting of conditions, ambient temperature, slab size and casting speed,
The reduction zone is such that the solid phase ratio at the center of the slab cross section is in the range of 0.3 to 0.8, or the reduction zone is 0.
2. The method according to claim 1, wherein a method of controlling the position of the rolling band is included so as to include a range of 3 to 0.8.

(7) The method according to (1), wherein the control methods described in (5) and (6) are combined.

 Hereinafter, the present invention will be described in more detail.

The basic constitution of the first aspect of the present invention is a method for suppressing the flow and accumulation of concentrated molten steel, which is the main cause of central segregation and V segregation of continuous cast slabs, and segregation due to equiaxed crystallization and refinement of solidification structure. And a method of diffusing or separating the segregation that is being formed in the slab while maintaining the slab at a high temperature by slow cooling. It is intended to provide a segregation improvement technique having a significant improvement effect.

In the invention of the first aspect, in order to achieve equiaxed crystallization and refinement of the solidification structure, the superheat degree of the molten steel in the tundish is controlled to 50 ° C. or less, low-temperature casting is realized, and the molten steel is further stirred by an electromagnetic stirrer. This promotes crystal generation and stabilization of the generated crystal. Superheat degree of molten steel in tundish is 5
The reason for controlling the temperature to at least ° C. is to prevent operational troubles such as deterioration of the quality of the continuous cast material and nozzle clogging due to inclusions during low-temperature casting.

The heat insulation zone and heating zone are installed to achieve slow cooling, and the solid phase ratio at the center of the slab section is 0.3 to 0.8.
The slab reduction of 4 mm or more is intended to compensate for the solidification shrinkage and the heat shrinkage, which are the main causes of the molten steel flow, and to suppress the generation of segregation accompanying the molten steel flow at the end of solidification.

The invention described in the second and third aspects is a method of rolling down a slab at the end of solidification to effectively prevent the flow of molten steel at the end of solidification,
The method according to the fourth aspect is an invention relating to a method of providing a tundish with a function of adjusting the temperature of molten steel, facilitating control of the degree of superheat of the molten steel in the tundish, and improving the accuracy thereof.

Item 5, Item 6 and Item 7 provide the method according to Item 1, wherein the reduction at the end of solidification is applied to suppress the flow of molten steel, and the reduction zone comes to a solid phase ratio range effective for improving segregation. The control method is provided.

 Hereinafter, specific examples will be described with reference to the drawings.

FIG. 1 is an explanatory view showing an embodiment of the present invention, and also shows an outline of a curved type continuous test caster used in the following examples.

1 is an induction heating device, 2 is a cooling material addition device, 3 is a tundish, 4 is a mold and an electromagnetic stirring device in a mold, 5 is an electromagnetic stirring device installed in a secondary cooling zone, 6 is a secondary cooling zone, 7 Is a heat retention zone, 8 is a heating zone, 9 is a reduction zone, 10 is a reduction roll, 11 and 12 show isosolid fraction lines having solid fractions of 0.3 and 0.8, respectively.
13 is a slab.

First, in the method according to the first aspect of the present invention, a method for suppressing the flow and accumulation of the concentrated molten steel, a method for dispersing segregation by equiaxed crystallization and refining of the solidified structure, and a method for segregating by slow cooling. The reason for combining the methods for dispersing and separating is described.

The present inventors have conducted various studies to establish a segregation improvement method for reducing micro, semi-macro and macro segregation formed inside a slab manufactured by a continuous casting method, and obtained the following knowledge. Was.

FIG. 2 is a diagram showing the results of investigations by the present inventors on the effect of improving segregation by equiaxed crystallization of the solidification structure and the effect of suppressing segregation by rolling down the slab at the end of solidification. . The upper-axis equiaxed crystal ratio on the horizontal axis in FIG. 2 is an index indicating the degree of equiaxed crystallization on the upper surface side of a slab cast by a curved continuous caster, and the center segregation score on the vertical axis is the center segregation score. It is an index indicating the degree. In this study, S45C-S48C were cast with the test continuous caster (radius of the curved part 12m) shown in Fig. 1 without heat insulation zone and heating zone (cast size: 162mm thickness x 16).
2 mm width).

When casting a slab of this size, the flattening ratio of the slab (the ratio of the slab width to the slab thickness) is as small as 1 and almost no flow of the residual molten steel due to bulging occurs as in a slab or the like having a large flattening ratio. . As is clear from FIG. 2, the center segregation is improved by equiaxed crystallization and refinement of the solidified structure by electromagnetic stirring or the like, but the degree of improvement is limited.

On the other hand, in order to prevent the flow of concentrated molten steel due to solidification shrinkage and heat shrinkage, the center segregation of the slab that was rolled down at the end of solidification is significantly greater when the equiaxed crystallization is about the same as when the flow was not suppressed. Has been improved. Further, even when the slab is rolled down to suppress the flow, the center segregation tends to be reduced as the upper surface side equiaxed crystal ratio increases, although this is not the case.

Therefore, the solidified structure is sufficiently equiaxed and the flow of the residual molten steel is suppressed by reducing the pressure at the end of solidification or preventing bulging, thereby achieving a better center segregation level.
In principle, if the flow at the end of solidification is completely prevented, the accumulation of concentrated molten steel in the center of the slab should be prevented, and no center segregation should occur. It is difficult to always accurately reproduce ideal conditions for completely preventing the flow of molten steel due to variations in the state of flake solidification.

In FIG. 2, even when the slab was lowered at the end of solidification to suppress the flow of molten steel, the reason why segregation was improved with the increase of equiaxed crystals was that the flow of molten steel that was not completely prevented, It is presumed that the effect of segregation and dispersion of equiaxed crystals effectively contributed. Accordingly, it is necessary to make the solidified structure as equiaxed as possible and finer as much as possible from the viewpoint of measures against segregation that occurs when the flow cannot be completely stopped.

In the range tested by the present inventors, even when combining the method of suppressing the flow of concentrated molten steel and the method of equiaxed crystallization of the solidified structure, the method of improving segregation by refining, the center of the slab thickness is several mm. Considering that segregated grains of the order exist and the micro-segregation between the dendrite trees is of the order of several tens to several hundreds μm, the generation of the central segregation has not yet been completely prevented.

As a method for further improving the segregation as described above, there is a case where the rolling conditions at the end of solidification for preventing the flow of molten steel are accurately controlled to more ideal conditions. It is considered necessary to apply a method of improving segregation by the above mechanism. One of them is to reduce the cooling rate in the slab and maintain the slab at a higher temperature to promote the diffusion of segregation and the separation of segregated elements using solute redistribution during solid phase transformation. Was considered to be effective, and the segregation improvement effect by them was examined.

FIG. 3 is a diagram showing the relationship between the center segregation score at the center of the slab and the area ratio of the P-rich portion, and the effect of improving spot-like segregation by keeping the slab warm, heated and slowly cooled.

According to the effects studied by the present inventors on the segregation improvement effect due to the slow cooling, even when the slow cooling of the slab is applied alone, micro segregation between dendrite trees and the slab exists in the center of the slab Spot-like segregation (semi-macro segregation)
It is improved by the effect of slow cooling, but especially for spot-like segregation in the center of the slab, even if the solute diffusion is promoted by slow cooling, the final attained level is immediately after the segregation is formed. , That is, it strongly depends on the state of segregation generation before being dispersed by the subsequent diffusion.

That is, when significant macrosegregation is generated in the slab discontinuation portion and the center segregation score is poor such that a large number of coarse spot-like segregations are present in the center of the slab, the cooling is performed by slow cooling as shown in FIG. Even if such segregation is diffused, the area ratio of the high concentration portion of P (P / Po ≧ 8, Po: molten steel P concentration) cannot be sufficiently reduced.

On the other hand, the flow is suppressed by applying rolling to the slab at the end of solidification,
When the generation of macro-segregation (center segregation) formed in the center of the slab is improved, as shown in FIG. It is greatly reduced, and a good segregation level can be achieved.

Next, the results of examining conditions for promoting equiaxed crystallization and refinement of the solidified structure will be described below.

FIG. 4 is a view showing the range of the degree of superheating of a tundish molten steel in which equiaxed crystals are generated. It is well known that the solidification structure of slabs produced in continuous casting of ordinary steel changes depending on the type of steel to be cast and the degree of superheat of the molten steel in the tundish or the stirring of the molten steel by electromagnetic stirring. As the degree of superheat is reduced, the equiaxed crystal can be more stably generated by stirring the molten steel by electromagnetic stirring or the like.

Therefore, as a result of investigating the relationship between the solidification structure of the slab and the superheat degree of molten steel in the tundish for three steel types, the results shown in FIG. 4 were obtained. The steel types to be investigated were S48C, which is the least likely to be equiaxed among ordinary steels, S35C, which is easy to be equiaxed, and S10C, which has an intermediate degree of ease of equiaxed crystallization.

As can be seen from FIG. 4, the dependence of the equiaxed crystallization of the solidified structure on the degree of superheat of the molten steel in the tundish varies depending on the steel type. It is necessary to limit the degree of superheat of molten steel in a dish to 50 ° C. or less for all steel types. At a higher degree of superheat, the possibility that columnar crystals develop up to the center of the slab becomes considerably high.

The reason for limiting the superheat of molten steel in a tundish to 5 ° C or higher is that when the molten steel superheat is operated at less than 5 ° C, inclusions of inclusions due to an increase in the viscosity of the molten steel increase, and the surface and internal properties of the continuous cast material , And troubles such as noise clogging make stable casting difficult. Therefore, it is preferable to avoid such low-temperature casting in terms of quality and operation.

Regarding the position in the strand direction where the electromagnetic stirring is applied, the electromagnetic force acting on the molten steel is attenuated as the thickness of the solidified shell increases, so the most efficient method is to stir at the position of the mold where the solidified shell is thinnest. In addition, the electromagnetic stirring device provided between the mold and the subsequent secondary cooling zone to the solidification completion position promotes crystal formation and stabilizes crystals by multi-stage stirring, stabilizing more equiaxed crystals And generate it.

Next, in order to suppress the flow of molten steel at the end of solidification, the solid phase ratio at the center of the slab cross section is in the range of 0.3 to 0.8, 4mm to the slab
The reason for applying the above reduction will be described below. The inventor uses a curved type continuous casting machine shown in FIG. 1 to suppress the flow of molten steel in the case where the slab at the end of solidification is rolled down by a roll, and a proper solid phase ratio range in which an effect of improving segregation is obtained. In order to examine the rolling reduction, a test was performed with changing the casting speed and the rolling reduction.

In this test, the steel type was S48C and the slab size was 162 mm thick x 162 mm width without the heat insulation zone and the heating zone. A rolling method using rolls was adopted, and rolling was performed with five rolls arranged at a pitch of 500 mm near the final solidification part. In this case, the length of the rolling band is 2 m, which is the distance between the roll on the entering side and the roll on the exit side. FIG. 5 shows the results of examination of the appropriate solid phase ratio range in this test.

The solid line in the figure shows the relationship between the solid phase ratio and center segregation in the center of the slab at the roll entry side roll position and the casting at the reduction band exit roll position when the slab in this test is not kept warm or heated. The relationship between the solid fraction at the center of one side and the center segregation is shown. The broken line in the figure shows the relationship when a slab, which will be described later, is kept warm and heated.

As is clear from this figure, when the solid phase ratio at the center of the slab at the roll entry side roll position is 0.3 or more and the solid phase ratio at the slab center at the roll exit side roll position is 0.8 or less, the center segregation occurs. Is almost at the best level, and it can be seen that the appropriate rolling range for improving segregation by rolling at the end of solidification is in the range of 0.3 to 0.8 in terms of the solid fraction at the center of the slab.

FIG. 6 shows the results of examining the amount of reduction necessary for improving segregation by controlling the solid phase ratio in the reduction zone to an appropriate range (0.3 to 0.8). As can be seen in FIG. 6, the center segregation score was improved with the increase of the rolling reduction.
Above m, the margin of segregation is large.

As described above, when the center segregation was improved by the reduction at the end of solidification, it was found that it was necessary to secure a reduction amount of 4 mm or more when the solid fraction at the center of the slab was in the range of 0.3 to 0.8.

Next, a heat insulation zone and a heating zone were installed between the secondary cooling zone exit side and the reduction zone entrance side shown in FIG. 1 of the same continuous caster as in the above test, and the segregation was improved by maintaining the cast slab and heating slowly cooling. The results of a test performed to confirm the effect and to investigate the appropriate rolling conditions at the end of solidification when combined with slow cooling will be described. Prior to the present test, a solidification calculation was carried out to determine a proper heat retaining zone and heating zone installation position for reducing the cooling rate in the slab. As a result, in order to reduce the cooling rate, especially in the center of the slab, and to disperse the segregation by diffusion, it is not enough to keep the temperature near the position where solidification is completed and to heat, and to sufficiently reduce the cooling rate. , It was found that the slab had to be kept warm and heated from the more upstream side.

One reason for this is that it takes a certain amount of time to raise the temperature of the slab surface by heat retention and heating to reduce the temperature gradient in the solidified shell. When it disappears from the portion, the latent heat accompanying the solidification of the unsolidified phase is not released, and the temperature in the slab rapidly decreases.

Therefore, in order to improve segregation by slow cooling, the slab is kept warm and heated from the more upstream side to increase the slab surface temperature,
It is preferred that the unsolidified phase be present at the center of the slab as long as possible. The above is described in claim 1 as “2.
A heating zone or a heating zone and a heat insulation zone are provided at least between the secondary cooling zone exit side and the reduction zone entrance side after the next cooling zone to heat or keep and heat the slab.

The results of the examination of the appropriate solid phase ratio range when the slab was kept warm and heated are shown by broken lines in FIG. 5 described above. As can be seen from FIG. 5, the range of the appropriate solid phase ratio when the slab is heated and heated is slightly larger than that when the slab is not heated and heated. Phase ratio is 0.
The segregation is best in the range of 3 to 0.8.

FIG. 6 shows the results of an examination of the relationship between the slab reduction and the center segregation in this test. In the results of this test performed using all five rolling rolls at a roll pitch of 500 mm, no significant difference was observed between the case where the heat was retained and the case where the heating was not performed. Therefore, even when a method for improving segregation by slow cooling is combined, the solid fraction at the center of the slab section is 0.3 to 0.8.
Applying a rolling amount of 4 mm or more within the range is an appropriate condition for suppressing the flow of molten steel at the end of solidification.

Further, FIG. 7 shows the results of investigating the micro-segregation of the slab dendrite portion in which the slab was slowly cooled by keeping the slab warm and heated, and under the above-mentioned appropriate conditions, where the reduction was applied. This figure also shows the results of an investigation on microsegregation at the same position in a normally cooled slab without taking measures against slow cooling.

High-concentration P (P /
The area ratio of Po ≧ 8) is usually much lower than the area ratio of the micro-segregated portion of the coolant. Therefore, the method according to claim 1 is effective in improving the micro-segregation in the slab. Is also effective.

Subsequent to the above-mentioned test, a test continuous caster was used to examine the required reduction band length when applying a reduction by a roll and an appropriate roll pitch by the method described in claim 1. In this study, the casting speed and the solid fraction at the center of the slab were
The test was performed at a speed of 1 level, which ranged from 0.3 to 0.8. In examining the length of the rolling band, a test was performed with three rolls at the center of the rolling band, and a test was performed with one additional roll and four rolls.
A comparison was made with the case where the entire book was reduced. In examining the roll pitch, a test was conducted in which three of the five rolls were rolled down every other stage, and a test was performed in which only two rolls were used, one on the entrance side and the other on the exit side.
(5 pressure reductions). In each of the tests, the reduction amount of each roll was set so that the total reduction amount in the reduction zone was the same.

FIG. 8 shows the results of the study on the reduction band length, and FIG. 9 shows the results of the study on the roll pitch. From these figures, when the rolling band length is in the range of 2 m, the center segregation is improved with the increase of the rolling band length, and the rolling roll pitch also decreases as the roll pitch decreases up to the tested 500 mm. It tends to be improved.

FIG. 9 shows the results of the same study as above when heat insulation and heating were not performed. When the rolling roll pitch is 500 mm, it is not so large. However, when the rolling roll pitch is as large as 1000 mm or 2000 mm, the degree of improvement of segregation by rolling is reduced as compared with the case where heat insulation and heating are not required. This is because the rigidity of the solidified shell decreases as the temperature of the solidified shell increases due to heat retention and heating, and the deformation applied on the slab surface reduces the range of the slab in the thickness and longitudinal directions, reducing the flow of molten steel. It is considered that the suppression effect is reduced. In consideration of this, it is necessary to reduce the rolling roll pitch to some extent.

For the reasons described above, as described in claim 2, "the length of the rolling band is at least 2 m or more, and the roll pitch is 500 m.
m or less. " Furthermore, when the present technology is applied to a slab or other cast slab having a large aspect ratio, a decrease in the rigidity of the solidified shell promotes the bulging phenomenon, and the molten steel flow due to the bulging causes deterioration of segregation.
To avoid this, it is desirable to make the roll pitch smaller.

As a method of avoiding the local deformation at the time of rolling down due to the heat retention and heating as described above, the range of contact between the slab and the rolling down terminal is enlarged, and the shape ratio, that is, (the contact length of the slab and the rolling down terminal) The method of rolling down with a planar or bar-shaped rolling-down terminal described in claim 3 which is easy to secure the ratio of / (plate thickness) is effective.

As is well known in the field of plastic working, in particular, rolling and casting, more uniform deformation of a workpiece can be realized by increasing the shape ratio. Therefore, in carrying out the method described in item 1, the method of rolling down the slab at the end of solidification with a planar or bar-shaped rolling down terminal is adopted, whereby the method is more uniform and efficient than when rolling down with a roll or the like. It becomes possible to perform a specific reduction.

In this case as well, the position where the slab is reduced should be in a range where the flow of molten steel can be effectively suppressed. 0.8
Between. In the case of rolling down with a planar or bar-shaped rolling down terminal, it is possible to continuously roll down a slab that is continuously drawn by adding a slab transport mechanism to the rolling down device.

Next, the invention described in claim 4 will be described. As described above, even when combining the method of suppressing the flow of molten steel to improve segregation and the method of improving segregation by slow cooling, in order to achieve a better segregation level, the solidification structure is made as equiaxed as possible, It needs to be miniaturized.

For that purpose, and in order to prevent the deterioration of quality due to inclusions, etc., the first paragraph sets a limit of 5 to 50 ° C. on the degree of superheat of molten steel in a tundish. In actual operation, depending on the variation of the molten steel temperature adjustment in the previous process and the transition (temporal change) of the molten steel superheat degree of the tundish as shown in FIG.
Control may not be possible at 50 ° C.

In order to solve this problem, it is desirable to have means for reducing the degree of superheat of the molten steel, means for increasing and maintaining the degree of superheat of the molten steel, or both means. By adding an addition device or induction heating device or both devices that add cold material to the tundish, the temperature of the molten steel can be adjusted in the tundish, and the degree of superheat of the molten steel can be accurately controlled to the target range throughout casting. Become like

The invention described in claims 5, 6, and 7 will be described below. In order to suppress the flow of molten steel at the end of solidification, it is necessary to add an appropriate amount of reduction when the solid fraction at the center of the slab cross section is in the range of 0.3 to 0.8.
The position in the strand where the solid phase ratio is in the range of 0.3 to 0.8 depends on the cooling conditions such as the casting speed and the amount of secondary cooling water, and when the slab is heated and heated, the heat retaining capacity of the heat insulating zone and the heating of the heating zone It changes depending on conditions.

Therefore, when applying the method described in item 1, the solidification calculation is performed in consideration of the casting speed, the condition related to cooling of the slab, the heat retention, and the condition related to heating as described in item 5, and the reduction is reduced. In addition, always know where the appropriate solid phase ratio range is located within the strand, and set the casting speed and cooling conditions so that the solid phase ratio of the reduction zone falls within the appropriate range, or that the reduction zone includes the appropriate range. By adjusting the heating conditions, a good segregation level can be stably achieved.

However, in the actual casting operation, the casting speed may inevitably decrease or increase inevitably in the last stage of casting or in the early stage of casting, and a situation in which the casting speed must be significantly changed even in the middle stage of casting occurs. In such a case, it may not be possible to control the solid phase ratio in the reduction zone to an appropriate range only by adjusting the cooling conditions and the heating conditions. Claim 6 was created as a countermeasure against such a situation.
The invention described in the paragraph.

A roll with a reduction mechanism is arranged in advance so that it can respond to a large change in casting speed, or a mechanism that can move in the strand direction is added to the reduction device, and the appropriate solid fraction ratio estimated by solidification calculation is added. Use a certain roll as a reduction roll, or use a wider group of rolls as a reduction roll, or move a reduction device to that range to control the position of the reduction band and effectively suppress molten steel flow. It is possible to improve segregation. A similar effect can also be expected in the method of the seventh item in which the control methods of the fifth and sixth items are combined.

Effect of the Invention As described above, in producing a slab by the continuous casting method, by applying the present invention, an equiaxed crystal of a solidified structure, a dispersing effect of segregation due to refining, and suppression of molten steel flow at the end of solidification. The effect of preventing the accumulation of concentrated molten steel due to slag and the effect of segregation diffusion and separation by slow cooling during solidification of the slab more effectively act to reduce the micro, semi-macro and macro segregation formed inside the slab. It is possible to achieve a good segregation level which has been difficult to obtain with the above segregation measures.

As a result, it is possible to prevent the occurrence of abnormal structure due to segregation and the deterioration of mechanical properties, which have been caused by conventional countermeasures, as well as to continuously cast high-grade steel, which could not be achieved mainly due to segregation, and to improve the quality of conventional materials. And the omission of steps can be achieved.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing an embodiment of the present invention, FIG. 2 is a view showing the relationship between the upper surface side equiaxed crystallinity and the center segregation score, and the effect of improving segregation by rolling down at the end of solidification, and FIG. Fig. 4 shows the relationship between the center segregation score in the area and the area ratio of the high concentration part of P, and the effect of improving spot-like segregation by keeping, heating and slowly cooling the slab. Fig. 5 shows the range of the superheat degree of the dish molten steel, Fig. 5 shows the study results on the appropriate solid phase ratio range when rolling down the slab in the invention described in item 1, and Fig. 6 shows the study on the appropriate rolling reduction. FIG. 7 is a diagram showing the results, FIG. 7 is a diagram showing the effect of improving the micro-segregation of the slab dendrite portion by slow cooling when the method described in item 1 is applied, and FIG. Fig. 9 shows the results of the study on the length of the required rolling band when rolling down. Shows the study results on pressure roll pitch, FIG. 10 is a diagram showing an example of a tundish molten steel superheat transition when no control with cold material addition and induction heating (4 cases). DESCRIPTION OF SYMBOLS 1 ... Induction heating apparatus, 2 ... Cold material addition apparatus, 3 ... Tundish, 4,5 ... Mould and electromagnetic stirring apparatus in a mold,
6: secondary cooling zone, 7: warming zone, 8: heating zone, 9 ...
… Reduction band, 10… Reduction roll, 11, 12 …… Solidity ratio line, 13
…… a slab.

Claims (7)

(57) [Claims] (1) When producing a slab by a continuous casting method using a continuous casting machine provided with a tundish, a mold, a secondary cooling zone, a heating zone or a heating zone and a heating zone, and a reduction zone, a tundish is prepared. The superheat degree of the molten steel is controlled to 5 to 50 ° C., and the casting is performed while stirring the molten steel with the electromagnetic stirring device provided between the mold or the mold and the subsequent secondary cooling zone and the solidification completion position. A heating zone or a heat insulation zone and a heating zone are provided at least between the secondary cooling zone exit side and the reduction zone entrance side to heat or insulate and heat the slab, and the solid phase ratio at the center of the slab cross section in the reduction zone is 0.3. A method for improving segregation of a continuously cast slab, wherein a reduction of 4 mm or more is applied to the slab in the range of 0.8 to 0.8. 2. The roll belt length is 2 m or more and the roll pitch is 50
The method according to claim 1, wherein the slab is reduced by a plurality of pairs of rolls set to 0 mm or less. 3. The method according to claim 1, further comprising the step of providing a pressing device having a slab conveying mechanism, and reducing the slab by a planar or bar-shaped pressing terminal. 4. The method according to claim 1, wherein an induction heating device and / or a cooling material adding device are provided in the tundish for controlling the superheat degree of the molten steel of the tundish to 5 to 50 ° C. 5. Primary cooling operation conditions such as molten steel temperature of a tundish, mold cooling water amount and temperature change of mold cooling water;
The solidification calculation is performed based on the secondary cooling operation conditions such as the amount of secondary cooling water, the heat retention capacity of the heat insulation zone and the operation conditions of the heating zone, the ambient temperature, the slab size, and the process information including the casting speed. Part of solid phase ratio is 0.3 ~ 0.8
Combining a method for controlling the amount of secondary cooling water, operating conditions of the heating zone, and the casting speed such that the reduction zone is within the range or the solid phase ratio at the center of the slab cross section is in the range of 0.3 to 0.8. The method of claim 1. 6. The molten steel temperature of the tundish is determined by primary cooling operation conditions such as mold cooling water amount and temperature change of mold cooling water, secondary cooling operation conditions such as secondary cooling water amount, heat insulation capacity of heating zone and heating zone. Solidification calculation is performed based on process information consisting of operating conditions, ambient temperature, slab size and casting speed, and the reduction zone is adjusted so that the solid phase ratio at the center of the slab cross section is in the range of 0.3 to 0.8, or the reduction zone is 2. The method according to claim 1, wherein a method of controlling the position of the rolling band is combined so that the solid fraction at the center of the slab cross section includes a range of 0.3 to 0.8. 7. The method according to claim 1, wherein both control methods according to claim 5 and 6 are combined.