JPH02151354A - Method for improving segregation in continuously cast slab - Google Patents

Abstract

PURPOSE:To make equi-axes crystal and fine crystal of the solidified structure and to disperse segregation by applying rolling reduction to a heated cast slab after secondary cooling zone under specific conditions. CONSTITUTION:In order to attain equi-axes crystal and the fine crystal of the solidified structure, over-heat temperature of molten steel in a tundish 3 is controlled to 5-50 deg.C to realize the low temp. casting and further by stirring the molten steel with an electromagnetic stirring devices 4, 5 the development of the crystal and the stabilization of the developed crystal are promoted. Further, operational troubles, such as deterioration of the quality in the continuously cast material caused by inclusion at the time of low temp. casting, nozzle clogging, are prevented. In order to realize slow cooling, temp. holding zone 7 and heating zone 8 are set, and in order to compensate solidified shrinkage quantity and heat shrinkage quantity, which are the main causes of the molten steel flowing and restrain the development of the segregation accompanied with the molten steel flowing at the end period of the solidification, the rolling reduction is executed to the cast slab at >=4mm in the range of 0.3-0.8 solid phase ratio in the center part of the sectional face in the cast slab 13.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention reduces micro, semi-macro, and macro segregation formed inside slabs produced by continuous casting, and eliminates abnormalities that appear in finished products due to segregation. This is intended to prevent the formation of structures and the deterioration of the mechanical properties of finished products.

Conventional technology Continuous casting of steel has been carried out for a long time, and slabs produced by this continuous casting method have micro-segregation between dendrite trees and granular segregation in the center of the thickness of the slab. V-shaped, linear, or band-like V segregation or center segregation was formed, and quality deterioration due to these segregations was unavoidable.

Micro-segregation is generated due to solute distribution between solid and liquid when a metal with a solid-liquid coexistence phase solidifies, while V-segregation and center segregation are caused by mechanical factors such as bulging and misalignment of the solidified shell due to static pressure of molten steel. Due to suction caused by solidification shrinkage, solidification shrinkage, and thermal contraction, molten steel enriched between trees accumulates in the center of the slab and is generated.

Conventionally, various countermeasures against segregation generated by such a mechanism have been proposed and implemented.

As a method to improve segregation, harmful segregated elements (e.g. P, S
etc.), methods to reduce segregation in advance at the molten steel stage, methods to disperse segregation by making the solidified structure equiaxed and finer using electromagnetic stirring, and methods to suppress bulging by shortening the roll interval of a continuous casting machine. It is being done.

In addition, a method of compensating for solidification shrinkage and thermal shrinkage by rolling down slabs at the final stage of solidification using rolls, flat or bar-shaped rolling terminals, suppressing the flow of concentrated molten steel, and improving segregation (Japanese Patent Publication No. 59-1983) 1
8882, Special Publication No. 59-18541. Special Public Service 1986-39
225, Japanese Unexamined Patent Publication No. 58-45258) is being carried out.

Japanese Patent Publication No. 82-344 EiO discloses a method of preventing semi-macro segregation, which forms macro central segregation and V segregation, by electromagnetic stirring and reduction at the final stage of solidification.

On the other hand, a method for separating and dispersing segregation by reducing the cooling rate in continuously cast slabs and promoting solute redistribution and diffusion within the solid phase during δ→γ transformation (Japanese Patent Laid-Open No. 80-188150, Kaisho 8
2-8744) is further disclosed in Japanese Patent Application Laid-open No. 81-154748.
proposed a segregation improvement method by adding No as a method of increasing the segregation effect due to solute redistribution during δ→γ transformation.

However, due to the continuous casting of high-grade steel, which is a material with severe segregation, as well as improvements in the quality of conventional materials and process omissions, the allowable segregation level for slabs has become increasingly strict, and the segregation countermeasures described above are insufficient. In some cases,

Problems to be Solved by the Invention The present invention satisfies the segregation level that has been difficult to achieve with conventional segregation countermeasures, and prevents abnormal structures and deterioration of mechanical properties caused by segregation that occur with conventional countermeasures. This technology provides a segregation improvement technology that enables continuous casting of high-grade steel, higher quality of conventional materials, and process omissions, which were previously unachievable due to segregation. This is what we are trying to provide.

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

(1) When manufacturing slabs by the single casting method, the degree of superheating of molten steel in the tundish is controlled to 5 to 50°C,
Casting is carried out while stirring the molten steel using a mold or an electromagnetic stirrer installed between the mold and the subsequent secondary cooling zone and the solidification completion position, and at least two of the secondary cooling zone and subsequent soft cooling zone exit side to the rolling zone entry side are cast. A heating zone or a heat insulation zone and a heating zone are provided in between to heat or insulate and heat the slab, and to apply 41 layers to the slab where the solid phase ratio at the center of the slab cross section is in the range of 0.3 to 0.8. A method for improving segregation of continuously cast slabs, characterized by applying a reduction of the above amount.

(2) In the method described in item 1, the slab is rolled down by a plurality of pairs of rolls with a rolling zone length of 2 m or more and a roll pitch of 500 m or less.

(3) In the method described in item 1, a rolling down device having a slab conveying mechanism is provided, and the slab is rolled down using a planar or par-shaped rolling terminal.

(4) A continuous casting method in which the method described in item 1 is applied, in which the tundish is provided with an induction heating device, a cold material addition device, or a re-device in order to control the degree of superheating of molten steel in the tundish to 5 to 50°C.

(5) Primary and secondary cooling operation conditions such as the degree of superheating of molten steel (or molten steel temperature) in the tundish, the amount of mold cooling water and the temperature change of mold cooling water, the amount of secondary cooling water, the heat retention capacity of the heat insulation zone, and the operation of the heating zone Solidification calculations were performed based on process information including conditions, ambient temperature, slab size, and casting speed, and the solid fraction at the center of the slab cross section in the rolling zone was 0.3.
The amount of secondary cooling water, heating zone operating conditions, and casting speed are controlled so that the solid phase ratio at the center of the slab cross section is within the range of 0.8 to 0.8, or the rolling zone includes a solid phase ratio of 0.3 to 0.8 at the center of the slab cross section. 2. The method according to item 1, which combines the methods of:

(6) Primary and secondary cooling operation conditions such as the degree of superheating of molten steel (or molten steel temperature) in the tundish, the amount of mold cooling water and the temperature change of mold cooling water, the amount of secondary cooling water, the heat retention capacity of the heat insulation zone, and the operation of the heating zone Solidification calculations are performed based on process information including conditions, ambient temperature, slab size, and casting speed, and the reduction zone has a solid phase ratio of 0.3 to 0.0 at the center of the slab cross section.
The method according to item 1, which combines the method of controlling the rolling band device so that the solid phase ratio at the center of the slab cross section is in the range of 0.8 to 0.8, or so that the solid phase ratio at the center of the slab cross section is in the range of 0.3 to 0.8. .

(7) The method described in item 1, which combines both the control methods described in items 5 and 6 above.

The present invention will be described in further detail below.

The basic structure of the first aspect of the present invention is to prevent center segregation of continuously cast slabs and
There is a method to suppress the flow and accumulation of concentrated molten steel, which is the main cause of segregation, and a method to disperse segregation by making the solidified structure equiaxed and finer. This method consists of a method of diffusing or separating segregation that is forming on a piece, and provides a segregation improvement technique that can have a significant improvement effect by appropriately combining these methods.

In the invention of Section S1, in order to make the solidified structure equiaxed and refined, the degree of superheating of the molten steel in the tundish is controlled to 50°C or less to achieve low-temperature casting, and the molten steel is further stirred by an electromagnetic stirring device. The degree of superheating of molten steel in the tundish is set to 5 to promote crystal formation and stabilization of the formed crystals.
The reason for controlling the temperature above 0.degree. C. is to prevent operational troubles such as deterioration of the quality of continuously cast materials and nozzle clogging due to inclusions during low-temperature casting.

The purpose of installing heat insulation zones and heating zones is to achieve gradual cooling.
A slab reduction of 41 mm or more when the solid phase ratio at the center of the slab cross section is in the range of 0.3 to 0.8 compensates for the amount of solidification shrinkage and heat shrinkage, which are the main causes of molten steel flow, and improves solidification. This is to suppress the generation of segregation that accompanies the flow of molten steel at the final stage.

The invention described in Items 2 and 3 is a method for rolling down a slab at the final stage of solidification, which effectively prevents the flow of molten steel at the final stage of solidification, and the method described in Item 4 is a method for controlling the temperature of molten steel in the tundish and applying gloss. This invention relates to a method for easily controlling the degree of superheating of molten steel in a tundish manner and improving its accuracy.

The inventions set forth in Items 5, 6, and 7 provide the method described in Item 1, which applies rolling at the final stage of solidification to suppress the flow of molten steel, in which the rolled zone falls within a solid phase ratio range effective for improving segregation. The present invention provides a method for controlling this.

Specific examples will be described below with reference to the drawings.

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

1 is an induction heating device, 2 is a cold material addition device, 3 is a tundish, 4 is a mold and an electromagnetic stirring device in the mold, 5 is an electromagnetic stirring device installed in a secondary cooling zone, 6 is a secondary cooling zone, 7 8 is a heating zone, 9 is a rolling zone, 10 is a rolling roll, 11
and 12 indicate equal solid fraction lines with a solid fraction of 0.3 and 0.8, respectively, and 13 is a slab.

First of all, in the method described in item 1 of the present invention, a method for suppressing flow and accumulation of concentrated molten steel, and equiaxed weighting of a solidified structure,
The reason for combining the method of dispersing segregation by miniaturization and the method of dispersing and separating segregation by slow cooling will be explained.

The present inventors have conducted various studies in order to establish a segregation improvement method that reduces micro, semi-macro, and macro segregation formed inside slabs produced by continuous casting, and have obtained the following knowledge. Ta.

Figure 2 shows the segregation improvement effect due to equiaxed loading of the solidified structure,
a A diagram showing the results of a study conducted by the inventors on the segregation improvement effect of suppressing flow by reducing the slab in the final stage of solidification. The equiaxed weight ratio on the top side of the horizontal axis in Figure 2 is an index showing the degree of equiaxed weight on the top side of the slab cast by the curved continuous caster, and the center segregation score on the vertical axis is an index of the degree of equiaxed weight on the top side of the slab cast by the curved continuous caster. 0 This survey is an indicator of the degree of the cast slab (slab size :16
The test was carried out for 2 mm thick x lB 2 mm wide).

When casting slabs of this size, the slab's aspect ratio (ratio of slab width to slab thickness) is as small as 1, and flow of residual molten steel due to bulging rarely occurs, such as in slabs with a large aspect ratio. No, as is clear from Figure 2? l! Ti
Center segregation can be improved by making the solidified structure equiaxed and finer by IW and stirring, but there is a limit to the degree of improvement.

On the other hand, in order to prevent the flow of concentrated molten steel due to solidification shrinkage and thermal contraction, the center segregation of slabs rolled down at the final stage of solidification is significantly greater than when the flow is not suppressed when equiaxed loading is the same. has been improved. Furthermore, even when the slab is rolled to suppress flow, center segregation tends to be reduced as the upper surface side equiaxed crystallinity increases, although not as much as when it is not rolled.

Therefore, a better center segregation level can be achieved by making the solidified structure sufficiently equiaxed and suppressing the flow of the remaining molten steel by reducing the steel at the final stage of solidification and preventing bulging. In principle, if the flow at the final stage of solidification is completely prevented, the accumulation of concentrated molten steel in the center of the slab should be prevented and center segregation will not occur, but in reality, fluctuations in operating conditions and casting Due to variations in the state of piece solidification, etc., it is difficult to always precisely reproduce ideal conditions that can completely prevent the flow of molten steel.

In Fig. 2, even when the slab was rolled down at the end of solidification to suppress the flow of molten steel, the reason why segregation improved as the equiaxed load increased was because the flow of molten steel was not completely prevented.
This is thought to be due to the effective contribution of the segregation and dispersion effect of equiaxed loads. Therefore, it is necessary to make the solidified structure as equiaxed and fine as possible, also from the perspective of countermeasures against segregation that occurs when the flow cannot be completely stopped.

In the range of experiments conducted by the present inventors, even when combining the method of suppressing the flow of concentrated molten steel and the method of improving segregation by equiaxed weighting and refinement of the solidified structure, there are several Considering that there are segregated grains on the order of 2 and the micro-segregation between dendrite trees is on the order of several tens to hundreds of #Lm, the generation of central segregation has not been completely prevented.

One way to further improve the above-mentioned segregation is to precisely control the rolling conditions at the final stage of solidification to more ideal conditions to prevent molten steel flow, but there are limits to this, and there are no other methods other than those described above. It is considered necessary to apply a method that improves segregation through this mechanism. One of these is to reduce the cooling rate inside the slab and maintain the slab at a higher temperature, thereby promoting the diffusion of segregation and the separation of segregated elements using solute redistribution during solid phase transformation. were considered to be effective, and we investigated their effects on improving segregation.

FIG. 3 is a diagram showing the relationship between the central segregation score at the center of the slab and the area ratio of the P-high concentration area, and the improvement effect on the grain-like segregation by keeping the slab warm, heating it, and slowly cooling it.

According to the results of the study conducted by the present inventors on the segregation improvement effect of the above-mentioned slow cooling, even when slow cooling of the slab is applied alone, micro-segregation between dendrite trees and spots existing in the center of the slab Shape segregation (semi-macro segregation) can be improved by the effect of slow cooling, but especially for spot segregation in the center of the slab, even if we try to promote solute diffusion by slow cooling, the final level will not reach the final level. It has become clear that this depends strongly on the conditions immediately after the segregation is formed, that is, on the conditions in which the segregation is formed before it is dispersed by subsequent diffusion.

In other words, if the center segregation rating is poor, such as when remarkable macro segregation is generated in the stopped part of the slab and there are many coarse spot segregations in the center of the slab, slow cooling is performed as shown in Fig. 3. Even if such segregation is attempted to be diffused, the area ratio of the high P concentration portion (P/Pa≧8, Po: molten steel P concentration) cannot be sufficiently reduced.

On the other hand, if the slab at the final stage of solidification is rolled down to suppress its flow and improve the formation of macro segregation (center segregation) that forms at the center of the slab, slow cooling is possible, as shown in Figure 3. Coupled with the segregation and diffusion effect caused by P, the area ratio of the high concentration portion of P is significantly reduced, making it possible to achieve a good segregation level.

Next, we will discuss the results of examining the conditions that promote equiaxed weighting and refinement of the solidified structure.

FIG. 4 is a diagram showing the range of the degree of superheating of tundish molten steel in which equiaxed loads 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 being cast, the degree of superheating of the molten steel in the tundish, and the stirring of the molten steel by electromagnetic stirring, etc. The lower the molten steel is, the more stable equiaxed crystals can be produced by stirring the molten steel using electromagnetic stirring or the like.

Therefore, as a result of investigating the relationship between the solidification structure of the slab and the degree of superheating of molten steel in the tundish, we found that
The results shown in the figure were obtained.

The steel types investigated were 348G, which is the most difficult to equiaxed load among ordinary steels, 935G, which is easier to equiaxed load, and 5IOC, which is intermediate in the ease of equiaxed load between the above two steel types.

As can be seen from Figure 14, the dependence of equiaxed solidification structure on the degree of superheating of molten steel in the tundish differs depending on the steel type, but even when electromagnetic stirring is applied to equiaxed the solidification structure, the dependence of the molten steel in the tundish For all steel types, it is necessary to limit the degree of superheating to 50°C or less. If the degree of superheating is higher than that, there is a considerable possibility that columnar crystals will develop to the center of the slab.

The reason why the degree of superheating of molten steel in tanditshu is limited to 5℃ or more is that if the operation is carried out at a degree of superheating of molten steel of less than 5℃, inclusions will increase due to increased viscosity of the molten steel, which will affect the surface and internal properties of the continuously cast material. It may cause deterioration, and
Problems such as nozzle clogging make stable casting difficult. Therefore.

It is preferable to avoid such low-temperature casting from the viewpoint of quality and operation.

71! Regarding the position in the strand direction where magnetic stirring is applied, the electromagnetic force acting on the molten steel decreases 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 the thinnest. Furthermore, an electromagnetic stirring device installed between the mold and the subsequent secondary cooling zone to the solidification completion position promotes crystal formation and stabilizes the crystals through multi-stage stirring, thereby stabilizing more equiaxed loads. It becomes possible to generate the

Next, in order to suppress the flow of molten steel at the final stage of solidification, the reason why a reduction of 41 cm or more is applied to the slab when the solid phase ratio at the center of the slab cross section is in the range of 0.3 to 0.8 will be explained below. 0 The present inventors used a curved continuous casting machine shown in Fig. 1 to suppress the flow of molten steel when rolling down slabs at the final stage of solidification with rolls, and found an appropriate solid phase ratio range in which a segregation improvement effect can be obtained. In order to examine the amount of reduction, tests were conducted by varying the casting speed and amount of reduction.

In this test, the steel type was 548G without a heat insulation zone or heating zone.
The slab size was 182 mm thick and 2 mm wide. A roll reduction method was adopted, and the reduction was applied using five wooden rolls arranged at a pitch of 500 mm near the final solidification part. In this case, the length of the rolling zone is 2 m from the rolling zone inlet roll to the outlet roll. The results of examining the appropriate solid fraction range in this test are shown in FIG.

The solid line in the figure shows the relationship between the solid fraction and center segregation at the center of the slab at the rolling zone entry roll position and the relationship between the solid phase ratio and center segregation at the rolling zone exit roll position when the slab is not kept warm or heated in this test. The broken line in Figure 0, which shows the relationship between the solid fraction at the center of the piece and the center segregation, shows the relationship when the slab is kept warm and heated, which will be described later.

As is clear from this figure, the solid fraction at the center of the slab at the rolling zone entry roll position is 0.3 or more, and the solid fraction at the slab center at the rolling zone exit roll position is 0.8 or less. In this case, center segregation is almost at the best level, and when aiming to improve segregation by reduction at the end of solidification, the appropriate reduction range is a solid phase ratio of 0.3 to 0.8 at the center of the slab. I understand that there is something.

Figure 6 shows the above-mentioned solid phase ratio in the rolling zone in the appropriate range (0
, 3 to 0.8), and shows the results of examining the amount of reduction required to improve segregation.As is clear from Part 6@, the center segregation score improves as the amount of reduction increases, especially when the amount of reduction increases. The improvement margin for segregation is large at 4 mm or more.

As mentioned above, when aiming to improve center segregation by reduction at the final stage of solidification, it is necessary to ensure a reduction amount of 41 cm or more when the solid phase ratio at the center of the slab is in the range of 0.3 to 0.8. There was found.

Next, a heat insulating zone and a heating zone were installed between the outlet side of the secondary cooling zone and the inlet side of the rolling zone as shown in Figure 1 of the same continuous caster as in the above test, to keep the slab warm and to improve segregation by heating and slow cooling. Confirming the effect and
The results of a test conducted to investigate the appropriate rolling reduction conditions at the final stage of solidification when combined with slow cooling will be explained. In addition, prior to this test, we conducted a solidification calculation to examine the appropriate location of the heat insulating zone and heating zone to reduce the cooling rate within the slab. As a result, in order to reduce the cooling rate especially in the center of the slab and disperse segregation through diffusion, it is not enough to simply insulate and heat the area near the point where solidification is completed; the cooling rate must be sufficiently reduced. It was discovered that the slab must be kept warm and heated from the upstream side.

One of the reasons for this is that it takes a certain amount of time to raise the temperature of the slab surface through heat retention and heating and reduce the temperature gradient inside the solidified shell, and another reason is that the unsolidified phase This is because, if the unsolidified phase disappears from the surface, the latent heat that accompanies solidification of the unsolidified phase will not be released, and the temperature within the slab will drop rapidly.

Therefore, in order to improve segregation by slow cooling, the slab should be kept warm and heated from the upstream side to raise the slab surface temperature.
It is preferable that the unsolidified phase exists in the center of the slab for as long as possible.
This is the reason that a heating zone or a heating zone and a heat insulating zone are provided after the next cooling zone and at least between the exit side of the secondary cooling zone and the inlet side of the rolling zone to heat or heat the slab.

The results of a study on the appropriate solid phase ratio range when the slab is kept warm and heated are shown by the broken line in the above-mentioned FIG. 5. As can be seen from Figure 5, the appropriate solid phase ratio range when the slab is kept warm and heated seems to be slightly expanded compared to when it is not kept warm or heated, but the solidity at the center of the slab cross section still remains. The phase ratio is 0,
Segregation is best in the range of 3 to 0.8.

The results of examining the relationship between slab reduction and center segregation in this test are shown in Figure 6.The results of this test, which was conducted using five reduction rolls and a roll pitch of 500 layers, are as follows:
There was no significant difference between the cases of heat retention and heating and those without heating. Therefore.

Even when combining the method of improving segregation by slow cooling, if the solid phase ratio at the center of the slab cross section is in the range of 0.3 to 0.8,
Applying a reduction amount of m or more is an appropriate condition for suppressing the flow of molten steel at the final stage of solidification.

Furthermore, the results of investigating the micro-segregation of the dendrite part of the slab after cooling the slab slowly by keeping it warm and heating it and applying pressure under the above-mentioned appropriate conditions are shown in Figure 7. Also shown are the results of investigating micro-segregation at the same location in slabs that were normally cooled.

High concentration of P (P/
The area ratio of Pa≧8) is significantly lower than the area ratio of the micro-segregation portion of the coolant. Therefore, the method according to claim 1 is effective in improving the micro-segregation in the slab. is also found to be effective.

Following the above test, a test continuous casting machine was used to investigate the necessary roll length and appropriate roll pitch when rolling is applied by the method described in claim 1. The casting speed is adjusted so that the solid phase ratio at the center of the slab is 0.
．． It was carried out at one level of speed ranging from 3 to 0.8. In examining the length of the rolling zone, tests were conducted in which rolls were rolled down with 3 rolls at the center of the rolling zone, and tests were rolled with 4 rolls with 1 water added, and compared with the case where rolls were rolled down with 5 rolls. In the examination of the roll pitch, a test was conducted in which three of the five rolling rolls were used at every other stage, and two rolls were used at the entrance and exit sides of the rolling zone.
A rolling test was conducted using only wood, and the roll pitch was 5.
A comparison was made with the case of 00 shoes (5 rolls). In all tests, the amount of reduction of each roll was set so that the total amount of reduction in the reduction zone was the same.

Figure 8 shows the result of examining the roll length, and Figure 9 shows the result of examining the roll pitch.From these figures, it can be seen that in the range of roll length of 2 m, as the roll length increases, The center segregation is improved, and the reduction roll pitch also tends to improve as the roll pitch decreases up to 500 square meters, which was the value tested.

Figure 9 shows the results of the same study as above for the case where no insulation or heating is performed.When the rolling roll pitch is 500 mm, it is not so great, but 1000 layers,
When the thickness is as large as 2,000 mm, the degree of segregation improvement due to rolling reduction is lower when thermal insulation or heating is performed than when not.

This is due to the increase in temperature of the solidified shell due to heat retention and heating.
The rigidity of the solidified shell decreases, and the deformation applied on the slab surface
This is thought to be because the range extending in the thickness direction and longitudinal direction of the slab is reduced, and the effect of suppressing the flow of molten steel is reduced. Taking this into consideration, it is necessary to reduce the rolling roll pitch to some extent.

For the reasons explained above, as stated in claim 2, “
The rolling strip length should be at least 2 m or more, and the roll pitch should be 50.
It is necessary to set it to 0 mm or less.

Furthermore, when applying this technology to slabs with a large aspect ratio, such as slabs, the decrease in rigidity of the solidified shell promotes the bulging phenomenon, and the flow of molten steel due to this bulging causes aggravation of segregation, so this should be avoided. For this reason, it is desirable to make the roll pitch smaller.

As a method to avoid local deformation during rolling due to heat retention and heating as described above, the range of contact between the slab and the rolled terminal is expanded, and the shape ratio (contact length between the slab and rolled terminal) is increased. The method of rolling down using a planar or bar-shaped rolling terminal described in claim 3, which facilitates securing the ratio of /(plate thickness), is effective.

As is well known in the fields of plastic working, particularly rolling and forging, increasing the shape ratio can achieve more uniform deformation of the workpiece. Therefore, when carrying out the method described in item 1, by adopting a method of rolling down the slab at the final stage of solidification with a flat or bar-shaped rolling terminal, it is more uniform and efficient compared to rolling down with rolls etc. It is possible to reduce the pressure.

In that case as well, the position where the slab is rolled should be in a range where the flow of molten steel can be effectively suppressed, and the position should be set at a solid phase ratio of 0 at the center of the slab cross section, as in the case of roll rolling. 3-0
．． It is between 8. In addition, in the case of rolling down with a planar or bar-shaped rolling terminal, by adding a slab conveying mechanism to the rolling device, it becomes possible to roll down continuously drawn slabs.

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

For this reason, and in order to prevent deterioration of quality due to inclusions, the degree of superheating of molten steel in the tundish is limited to 5 to 50°C in the first item. In actual operations, depending on the variation in the molten steel temperature adjustment in the previous process and the transition (change over time) of the molten steel superheating degree in the tundish as shown in Figure 1O, the molten steel superheating degree may be set as the target value throughout casting.
It may not be possible to control the temperature to ~50°C.

To solve this problem, it is desirable to have a means to reduce the degree of superheating of molten steel, a means to increase and maintain the degree of superheating of molten steel, or both. By adding this device, it becomes possible to adjust the temperature of the molten steel in the tundish, and the degree of superheating of the molten steel can be precisely controlled within the target range throughout casting.

The inventions set forth in claims 5, 6, and 7 will be described below. In order to suppress the flow of molten steel at the final stage of solidification, it is necessary to apply an appropriate rolling reduction amount so that the solid fraction at the center of the cross section of the slab is in the range of 0.3 to 0.8. The position within the strand where this solid phase ratio is in the range of 0.3 to 0.8 is determined by cooling conditions such as casting speed and amount of secondary cooling water.
Furthermore, when insulating or heating the slab, it changes depending on the heat retaining ability of the heat insulating zone and the heating conditions of the heating zone.

Therefore, when applying the method described in Section 1, solidification calculations should be performed taking into account the casting speed, conditions related to cooling of the slab, and conditions related to heat retention and heating, as described in Section 5, to reduce the reduction. In addition, always know where the appropriate solid fraction range is located within the strand, and adjust the casting speed and cooling conditions so that the solid fraction in the rolling zone is within the appropriate range, or the rolling zone is within the appropriate range. By adjusting the heating conditions, a good segregation level can be stably achieved.

However, in actual casting operations, there are cases where the casting speed is unavoidably significantly reduced or increased at the end of casting or at the beginning of casting, and there also occur situations in which the casting speed must be changed significantly even in the middle of casting. In such cases, it may not be possible to control the solid fraction in the rolling zone within an appropriate range simply by adjusting the cooling conditions and heating conditions. The invention set forth in claim 6 was devised as a countermeasure against such a situation.

In order to cope with large changes in casting speed, rolls with a rolling down mechanism are installed in advance, or a mechanism that can move in the strand direction is added to the rolling down device, so that the solid fraction can be kept within the appropriate solid fraction range estimated by solidification calculations. The rolling band device can be controlled to effectively suppress the flow of molten steel by using a certain roll as a rolling roll, or by using a wider group of rolls as rolling rolls, or by moving the rolling device to that area. , it becomes possible to improve segregation. Furthermore, similar effects can be expected from the method described in Item 7, which combines the control methods described in Items 5 and 6.

As described in detail of the invention, when manufacturing slabs by continuous casting, the present invention can be applied to make the solidified structure equiaxed, have a dispersion effect on segregation due to refinement, and suppress the flow of molten steel at the final stage of solidification. This prevents the accumulation of concentrated molten steel, and also makes the segregation diffusion and separation effect caused by slow cooling during solidification of the slab more effective, reducing micro, semi-macro, and macro segregation formed inside the slab. This makes it possible to achieve a good segregation level, which has been difficult to achieve with conventional segregation countermeasures.

This makes it possible to prevent the generation of abnormal structures caused by segregation and deterioration of mechanical properties that occur with conventional countermeasures, as well as to enable continuous casting of high-grade steel, which was previously impossible to achieve mainly due to segregation, and to achieve high quality of conventional materials. This makes it possible to simplify processes and omit processes.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram showing an embodiment of the present invention, Figure 2 is a diagram showing the relationship between the upper surface side equiaxed crystallinity and center segregation score, and the segregation improvement effect due to rolling reduction at the end of solidification, and Figure 3 is a diagram showing the center of the slab. Figure 4 shows the relationship between the center segregation score and the area ratio of the high concentration area of P in the area and the improvement effect of spot segregation by keeping the slab warm, heating it and slowly cooling it. 7
Figure 5 is a diagram showing the range of superheating degree of molten steel, Figure 5 is a diagram showing the study results regarding the appropriate solid phase ratio range when rolling down the slab in the invention described in item 1, and Figure 6 is the study regarding the appropriate reduction amount. Figure 7 is a diagram showing the results, and Figure 7 is a diagram showing the effect of improving the micro-segregation of the dendrite part of the slab by slow cooling when the method described in Item 1 is applied. Figure 9 shows the results of the study on the required rolling strip length during rolling down, Figure 9 shows the results of the study on the appropriate roll pitch for rolling down, and Figure 1O shows the overheating of tanditsu molten steel when it is not controlled by cold material addition or induction heating. Example of degree transition (4 examples)
FIG. 1... Induction heating device, 2... Cold material addition device, 3...
・Tandish, 4.5...Mold and electromagnetic stirring device in the mold, 6...Secondary cooling zone, 7...Heat insulation zone, 8φ・
・Heating zone, 911 meeting・Reduction zone, 10...φ reduction roll,
11.12...Solid fraction line, 13...Slab.

Claims (7)

[Claims] (1) Molten steel is overheated in the tundish when producing slabs by the continuous casting method using a continuous casting machine equipped with a tundish, a mold, a secondary cooling zone, a heating zone or a heat insulation zone, a heating zone, and a reduction zone. Casting is carried out while stirring the molten steel using a mold or an electromagnetic stirrer installed between the secondary cooling zone and the solidification completion position. A heating zone or an insulating zone and a heating zone are provided between the exit side of the next cooling zone and the input side of the rolling zone to heat the slab, keep it warm, and heat it, and the solid fraction at the center of the slab cross section in the rolling zone is 0.3. A method for improving segregation of continuously cast slabs, characterized by applying a reduction of 4 mm or more to the slabs in the range of -0.8. (2) Roll length is 2m or more, roll pitch is 500
2. The method according to claim 1, wherein the slab is rolled down by a plurality of pairs of rolls set to a diameter of 1 mm or less. (3) The method according to claim 1, wherein a rolling down device having a slab conveying mechanism is provided, and the slab is rolled down by a flat or bar-shaped rolling terminal. (4) The method according to claim 1, wherein the tundish is provided with an induction heating device, a cold material addition device, or both devices in order to control the degree of superheating of the molten steel in the tundish to 5 to 50°C. (5) Primary and secondary cooling operation conditions such as the degree of superheating of molten steel (or molten steel temperature) in the tundish, the amount of mold cooling water and the temperature change of mold cooling water, the amount of secondary cooling water, the heat retention capacity of the heat insulation zone, and the temperature of the heating zone. Solidification calculations were performed based on process information consisting of operating conditions, ambient temperature, slab size, and casting speed, and the solid fraction at the center of the slab cross section in the rolling zone was 0.3.
2 so that the solid phase ratio at the center of the slab cross section is in the range of 0.8 to 0.8, or so that the solid phase ratio at the center of the slab cross section is in the range of 0.3 to 0.8.
2. The method of claim 1, further comprising controlling the amount of secondary cooling water, heating zone operating conditions, and casting speed. (6) Primary and secondary cooling operation conditions such as the degree of superheating of molten steel (or molten steel temperature) in the tundish, the amount of mold cooling water and the temperature change of mold cooling water, the amount of secondary cooling water, the heat retention capacity of the heat insulation zone, and the temperature of the heating zone. Solidification calculations are performed based on process information consisting of operating conditions, ambient temperature, slab size, and casting speed, and the reduction zone has a solid phase ratio of 0.3 to 0.0 at the center of the slab cross section.
8. The method according to claim 1, further comprising a method of controlling a rolling band device so that the solid phase ratio at the center of the slab cross section is in the range of 0.3 to 0.8. . (7) The method according to claim 1, which combines both the control methods according to claims 5 and 6.