JP2001259809A - Continuous casting method - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To reduce the segregation of a continuous cast slab by a light reduction method, without causing cracks at the solid-liquid interface of the slab, to the limit where concentrated molten steel does not squeeze out. Increase light reduction to prevent center segregation. SOLUTION: In a continuous casting method in which a slab 1 having an unsolidified phase 2 therein is lightly reduced by a plurality of pairs of rolls 11 to reduce center segregation of the slab, from the start of light reduction to the end of light reduction. While applying a compressive force in the slab withdrawal direction to the slab, it is lightly reduced. This compressive force can be exerted by giving a difference in the peripheral speed of the pinch rolls 10 provided on the upstream side and the downstream side of the light pressure lowering zone 4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method for steel, and more particularly, to a method for preventing segregation of components in a central portion of a continuously cast slab in a casting process.

[0002]

2. Description of the Related Art In the final solidification part in the solidification process of steel,
Solute elements such as carbon, phosphorus and sulfur are concentrated in the uncoagulated phase.
Molten steel in which the solute elements are concentrated (referred to as concentrated molten steel) flows and accumulates. When solidified in this state, the concentration becomes much higher than the initial concentration, and a component segregation portion is generated. Such flow and accumulation of the concentrated molten steel is a main cause of steel component segregation.

[0003] When the steel solidifies, volume shrinkage occurs. With this solidification shrinkage, in the case of continuous casting, molten steel is sucked and flows in the direction of drawing the slab. Since there is no sufficient amount of molten steel in the unsolidified phase at the end of solidification of continuous cast slabs, the concentrated molten steel between the dendrite trees, which is the final solidified part, flows along with the solidification shrinkage, and it flows in the center of the slab. And solidifies to form so-called center segregation.

[0004] This center segregation degrades the quality of steel products. For example, in line pipe materials for oil transportation and natural gas transportation, hydrogen-induced cracking occurs from the center segregation as a starting point due to the action of sour gas, and in deep drawing materials used for can products for drinking water, Anisotropy appears in workability due to segregation of components. Therefore, many measures have been proposed to reduce center segregation from the casting process to the rolling process.

[0005] Among them, as means for reducing the center segregation of the cast slab inexpensively and effectively, for example, Japanese Patent Laid-Open No. 8-132 is disclosed.
As disclosed in JP-A-203-203 and JP-A-8-192256, there has been proposed a method of reducing the unsolidified slab with a plurality of pairs of rolls (hereinafter referred to as "light reduction"). This light reduction method reduces the volume of the unsolidified phase by gradually reducing the volume of the slab at a reduction speed corresponding to the amount of solidification shrinkage of the slab, so as not to cause the flow of concentrated molten steel between dendrite trees. The purpose is to prevent center segregation.

[0006]

In the light reduction method, since the solidified shell of the slab undergoes bending deformation by the roll, a tensile stress acts on the solid-liquid interface of the solidified shell. The limit of tensile strain at which cracking occurs at the solid-liquid interface is about 1%. Therefore, if the amount of light reduction is too large, cracking occurs at the solid-liquid interface. It is sucked and forms segregation with a high degree of segregation. When the amount of light reduction is further increased, the concentrated molten steel between the dendrite trees is squeezed out in the direction opposite to the casting direction, and segregation with a low concentration of solute elements such as carbon, phosphorus, and sulfur in the center of the slab (in this case, Is called negative segregation). On the other hand, if the amount of light reduction is too small, the molten steel is sucked by the volume shrinkage accompanying solidification, so that the flow of the concentrated molten steel between the dendrite trees cannot be suppressed and center segregation is generated.

In order to prevent center segregation, it is preferable to increase the amount of light reduction to such an extent that the concentrated molten steel is not squeezed out. However, in the past, light reduction was carried out in order to prevent cracking of the solid-liquid interface due to tensile strain. The upper limit of the amount is limited, and from this viewpoint, it is difficult to say that measures for preventing slab segregation by light reduction are sufficient.

Further, in continuous casting, the slab is supported by a plurality of pairs of rolls, and the slab is not supported between the rolls. Therefore, the slab is not supported between the rolls by the static pressure of the molten steel acting on the solidified shell. Then, the swelling of the solidified shell (hereinafter "bulging")
Occurs). The molten steel flows along with the volume change of the unsolidified phase due to the bulging, and therefore bulging generated between the rolls is also one of the causes of the center segregation. In the light reduction method, bulging occurs between the rolls because the rolls are used, and there is a problem that the center segregation due to the bulging cannot be prevented.

[0009] On the other hand, demands for steel quality from customers are becoming increasingly severe, and further reduction of center segregation is desired.

The present invention has been made in view of the above circumstances,
The aim is to reduce the segregation of the center of continuous cast slabs by the light reduction method, which makes it possible to expand the optimum conditions of the light reduction method, and to meet the severe quality requirements in recent years. It is to provide a continuous casting method by which pieces can be manufactured.

[0011]

According to a first aspect of the present invention, there is provided a continuous casting method in which a slab having an unsolidified phase therein is continuously cast while being lightly reduced by a plurality of pairs of rolls. Until the end of the reduction, the slab is lightly reduced while applying a compressive force in the slab withdrawal direction to the slab.

A continuous casting method according to a second aspect of the present invention is a method for continuously casting a slab having an unsolidified phase therein while lightly reducing the slab with a plurality of pairs of rolls. While pulling the slab while lowering the peripheral speed of the pinch roll installed on the upstream and downstream in the slab drawing direction, the peripheral speed of the pinch roll installed on the downstream side with respect to the peripheral speed of the pinch roll installed on the upstream side It is characterized by light reduction.

[0013] The method continuous casting according to a third invention, in the second invention, the pinch rolls installed in the upstream side to the circumferential velocity (V _PR1), the pinch rolls disposed downstream peripheral speed of (V _PR2) The ratio (V _PR2 / V _PR1 ) is controlled to a range of less than 1.0 and 0.8 or more.

According to a fourth aspect of the present invention, there is provided the continuous casting method according to any one of the first to third aspects, wherein at least a period from the start of light reduction to the end of light reduction, the surface temperature of the slab and the solid-liquid interface The temperature difference from the temperature is maintained at 600 ° C. or more.

According to a fifth aspect of the present invention, there is provided the continuous casting method according to any one of the first to fourth aspects, wherein
It is characterized in that the slab is lightly reduced in the range of the reduction speed of mm / min.

According to a sixth aspect of the present invention, in the continuous casting method according to any one of the first to fifth aspects, light reduction is started from a point in time when a solid fraction in a central portion in a slab thickness direction is 0.4 or less. ,
Light reduction is continued until the center of the slab in the thickness direction is solidified.

According to the present invention, the roll is lightly reduced while applying a compressive force to the slab in the slab withdrawal direction. That is,
Since the compressive force in the slab pull-out direction acts on the slab during the light reduction, the tensile force acting on the solidified shell by the light reduction is canceled. As described above, the limit of tensile strain at which cracking occurs at the solid-liquid interface is about 1%.
In the present invention, since a compressive force is acting, it is possible to apply a light reduction amount that far exceeds the limit value of the conventional light reduction amount before the solidified shell is cracked by the tensile force due to the light reduction. Become. Incidentally, the light reduction amount of the present invention is equal to the reduction gradient of the roll, and the light reduction casting is the reduction gradient of each roll, that is, the light reduction amount is 1 m in the drawing direction of the slab.
That is, casting is performed while reducing the thickness of the slab to 0.2 to 2.0% of the thickness of the slab.

The compressive force acting on the slab in the drawing direction can be applied, for example, as follows. That is, the peripheral speed of the pinch rolls installed upstream and downstream in the slab drawing direction with the light reduction zone for performing the light reduction being set on the downstream side with respect to the peripheral speed of the pinch rolls installed on the upstream side. By reducing the peripheral speed of the pinch roll, a compressive force in the drawing direction can be applied to the slab. This principle will be described with reference to FIG. FIG. 1 is a schematic diagram showing a positional relationship between a slab, a low-pressure band, and a pinch roll.

As shown in FIG. 1, there is an unsolidified phase 2 inside.
Of the solidified shell 3 of the slab 1 to be cast from a plurality of pairs of light pressure rolls 11
While being reduced by the light pressure reduction zone 4
Placed on the upstream pinch roll 10A and placed on the downstream side
The slab 1 is continuously drawn by the downstream pinch roll 10B
When punching, the peripheral speed (V
_PR1 ), The peripheral speed of the downstream pinch roll 10B
(V_PR2 ), The downstream pinch roll 10B
It acts to brake the removal of the slab 1. That
Therefore, a compression force in the drawing direction is applied to the slab 1 of the light pressure lowering band 4.
To use. According to this method, the compression force in the drawing direction can be easily obtained.
To use this method
Is preferred.

The peripheral speed of the downstream pinch roll 10B (V
_PR2 ) to the peripheral speed (V _PR1 ) of the upstream pinch roll 10A.
If it is slower than that, the difference in peripheral speed will be the driving force that generates the compression force, but how the compression force generated by this difference in peripheral speed is distributed in the solidified shell is because the solidified shell has a temperature distribution. I do not know exactly. Further, it is also difficult to calculate how much stress or strain is generated at the solid-liquid interface. Therefore, an experiment was performed in which the peripheral speed of the pinch roll was changed before and after the low pressure zone, and the conditions under which cracking occurred at the solid-liquid interface were obtained.

In the experiment, a continuous casting machine having a light pressure lowering zone having a total length of 12 m was used, and the drawing speed (Vc) of the slab was 1.
And a constant 3m / min, the peripheral speed ratio of the pinch rolls (V _{PR 2} / V _PR1) changed in the range of 0.78 to 1.0, also soft reduction amount, i.e., the roll clearance of soft reduction rolls The narrowing gradient (mm / m) was changed in the range of 0.3 to 1.6 mm / m. The peripheral speed of the upstream pinch roll 10A (V
_PR1 ) is equal to the drawing speed (Vc), so the peripheral speed ( _VPR2 ) of the downstream pinch roll 10B was changed. Table 1 shows the experimental conditions and the results of the investigation. The diameter of the roll under light pressure used was 280 mm, and a split roll was used.

[0022]

[Table 1]

As is clear from Table 1, as the peripheral speed ratio (V _PR2 / V _PR1 ) of the pinch roll becomes smaller, that is, as the compressive force increases, the limit drawing gradient of the crack at the solid-liquid interface becomes larger. Without generating cracks at the liquid interface,
The light reduction amount can be increased. However, it was found that if the peripheral speed (V _PR2 ) of the downstream pinch roll was too low, the slip occurred between the slab and the roll, so that the generated compressive force had a limit. The limit value of the compressive force depends on the motor torque for driving the pinch roll, the pressing force of the pinch roll, and the drawing speed, but is remarkable when the peripheral speed ratio (V _PR2 / V _PR1 ) of the pinch roll is less than 0.8. there were. Therefore, in the present invention, the preferable pinch roll peripheral speed ratio (V _PR2 / V _PR1 ) is less than 1.0 and 0.8 or more.

Further, in the continuous casting method of the present invention, the temperature difference between the surface temperature of the slab and the solid-liquid interface temperature under light pressure is 600
It is preferred that the temperature be maintained at a temperature of at least ° C. By setting the temperature difference between the slab surface temperature and the solid-liquid interface temperature to be 600 ° C. or more, the limit narrowing gradient of cracks at the solid-liquid interface can be further increased. The reason why the temperature difference between the slab surface temperature and the solid-liquid interface temperature is ΔT will be described with reference to FIG.
FIG. 2 is a diagram schematically showing the temperature gradient of the solidified shell and the stress generated by the temperature gradient, wherein (a) shows the temperature gradient,
(B) shows the stress distribution.

Usually, the temperature difference (ΔT) between the temperature (T _L ) of the solid-liquid interface 3A and the temperature (T _S ) of the slab surface 1A is 400 to 400 ° C.
It is about 500 ° C. This temperature distribution is shown by a broken line in FIG. On the other hand, the case where a temperature difference (ΔT) of 600 ° C. or more is applied (the surface temperature is set to 600 ° C. in FIG. 2) is shown by a solid line. Here, the solid-liquid interface 3A
Temperature (T _L) is generally equal to the solidus temperature.

FIG. 2B shows that the solidified shell 3
Is a result of calculating a stress distribution acting on the temperature difference (ΔT) of 400 to 500 ° C., and a solid line shows a case where the temperature difference (ΔT) is 600 ° C. or more. FIG.
As shown in (b), the temperature difference (ΔT) is 400 to 500.
Although a compressive force acts on the solid-liquid interface 3A even at a temperature of 0 ° C, the solid-liquid interface 3A
Has a large compressive force. This is because the slab surface 1A tends to shrink due to the temperature drop, but the inside of the solidified shell 3 does not drop so much, so that the inside of the solidified shell 3 becomes a resistance to shrinkage, a tensile force acts on the slab surface 1A side, and the solid-liquid interface This is because a compressive force acts on the 3A side. Therefore, in the present invention, as a preferable slab temperature distribution, the difference between the slab surface temperature and the solid-liquid interface temperature is maintained at least 600 ° C. at least during the period from the start of light reduction to the end of light reduction.

The rolling speed is 0.8 to 1.5 mm / mi.
It is preferable to control in the range of n. 0.8 reduction speed
If it is less than mm / min, the flow of the concentrated molten steel due to solidification shrinkage cannot be sufficiently prevented.
If it exceeds 5 mm / min, the concentrated molten steel is squeezed out in the direction opposite to the casting direction, and negative segregation may be generated at the center of the slab.

Further, the solid fraction at the center of the slab in the thickness direction is 0.1%.
It is preferable to start the light reduction from a time point of 4 or less and continue the light reduction until the center of the slab thickness direction is completely solidified.
Even if light reduction is started after the solid phase ratio at the center of the slab thickness direction exceeds 0.4, the movement of the concentrated molten steel has already occurred, and the effect of reducing center segregation is small. When the light reduction is stopped, the effect of reducing the center segregation is similarly small.

[0029]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. FIG. 3: is a figure which shows the example of embodiment of this invention, and is a side schematic diagram of a slab continuous casting machine.

As shown in FIG. 3, the molten steel cast in the mold 6 through the immersion nozzle 5 is cooled in the mold 6 to form a solidified shell 3 and a cast slab having an unsolidified phase 2 therein. As 1, while being supported by the support roll 8, the guide roll 9, and the pinch roll 10 provided below the mold 6, it is continuously pulled out below the mold 6 by the driving force of the pinch roll 10. While passing through these rolls, the slab 1 is cooled in a secondary cooling zone (not shown) composed of water spray or air mist spray to increase the thickness of the solidified shell 3 and eventually reach the inside. Complete coagulation.

On the downstream side in the drawing direction of the continuous casting machine, there is provided a light pressure lowering band 4 composed of a plurality of pairs of light pressure lowering rolls 11. The pinch roll 10 installed on the mold 6 side) is called an upstream pinch roll 10A, and the pinch roll 10 installed on the opposite downstream side is called a downstream pinch roll 10B. In the present embodiment, the upstream pinch roll 10A has four
Although two pairs of the downstream pinch rolls 10B are provided as a pair, the present invention can be implemented as long as one or more pairs are provided. However, in order to accurately obtain a desired compression force in the light pressure lowering zone 4, it is preferable that both the upstream pinch rolls 10A and the downstream pinch rolls 10B have large numbers.

A water spray 7 capable of strongly cooling the slab 1 is disposed between the light pressure lower zone 4 and the lower straightening roll 12. As shown in FIG. 3, the installation position of the water spray 7 capable of strongly cooling the slab 1 is preferably on the downstream side of the lower straightening roll 12. The slab 1 on the upstream side of the lower straightening roll 12
When the slab is strongly cooled, the surface temperature of the slab when passing through the lower straightening roll 12 is excessively lowered, and lateral cracks may occur in the slab 1 due to straightening distortion. This can be prevented beforehand.

Under various casting conditions, the thickness of the solidified shell 3 and the solid fraction at the center in the thickness direction of the slab are determined in advance by heat transfer calculation or the like. Adjust casting conditions such as drawing speed and secondary cooling strength. Then, the slab 1 is slightly reduced while casting under the adjusted casting conditions.

At this time, preferably, 0.8 to 1.5 mm /
The slab 1 is lightly reduced in the range of the reduction speed of min. The reduction speed is a product of the slab drawing speed and the drawing gradient (mm / m) of the roll interval of the light reduction roll 11, that is, the light reduction amount. Therefore, the drawing reduction (mm) is determined based on the drawing speed determined as the casting condition. / M) may be set. Further, it is preferable to start the light reduction from the time when the solid phase ratio at the center of the slab thickness direction is 0.4 or less. In this case, the solid fraction at the center of the slab thickness direction at the entrance of the low pressure lower zone 4 is 0.1%.
4 and a sufficient length of the light reduction zone 4 is required to complete the solidification in the light reduction zone 4.

The circumferential speed (V _PR2 ) of the downstream pinch roll 10B is adjusted to be lower than the circumferential speed (V _PR1 ) of the upstream pinch roll 10A, and the slab 1 is continuously lowered. Pull out. At that time, as described above, the ratio of the peripheral speed (V _PR2 ) of the downstream pinch roll 10B to the peripheral speed (V _PR1 ) of the upstream pinch roll 10A (V _PR2 /
V _PR1 ) is preferably controlled within a range of less than 1.0 and 0.8 or more. In addition, a plurality of pairs of upstream pinch rolls 10A
The peripheral speed (V _PR1 ) between the pairs and the peripheral speed (V _PR2 ) between the plural pairs of downstream pinch rolls 10B are adjusted to be substantially the same.

Further, in order to further apply a compressive force to the solid-liquid interface to prevent the solid-liquid interface from cracking under light pressure, the slab 1 that has passed through the lower straightening roll 12 is rapidly cooled by a water spray 7, The temperature difference (ΔT) between the slab surface temperature (T _s ) and the solid-liquid interface temperature (T _L ) at the time of entering the light pressure lowering zone 4 is set to 600 ° C. or more. Preferably, 1 is cooled.

Specifically, for example, it is preferable that the slab surface temperature when passing through the lower straightening roll 12 be 900 ° C. or higher, and the slab surface temperature when the pressure is lightly reduced be maintained at about 600 to 800 ° C. In order to rapidly cool the slab 1 in this way, the water spray 7 has a cooling water amount of 100 minutes per 1 m ^{2 of} the slab surface, depending on the installation length.
It is necessary to be within a range of ２０００2000 l (hereinafter referred to as “l / m ² · min”). The means for rapidly cooling the slab 1 is not limited to the water spray 7, but may be, for example, a cooling method in which laminar cooling water flows on the slab surface. Further, if the lower straightening roll 12 is a steel type having low lateral cracking susceptibility, the temperature difference (ΔT) between the slab surface temperature (T _S ) and the solid-liquid interface temperature (T _L ) at the time of entering the light reduction zone 4 is reduced. The secondary cooling strength may be adjusted from the secondary cooling zone immediately below the mold so as to be 600 ° C. or higher.

By casting in this manner, it is possible to increase the light reduction amount to the limit where the squeezing of the concentrated molten steel does not occur without causing cracks at the solid-liquid interface of the slab 1, and as a result In addition, the flow of the concentrated molten steel accompanying the solidification shrinkage of the slab 1 is prevented, and the center segregation of the slab 1 can be greatly reduced. When the temperature difference (ΔT) between the slab surface temperature (T _S ) and the solid-liquid interface temperature (T _L ) is set to 600 ° C. or more, the strength of the solidified shell 3 increases and the light pressure zone 4 In this case, the bulging between the rolls can be reduced, and the center segregation caused by the bulging between the rolls can be reduced.

Although the above description relates to a continuous slab caster, the present invention is not limited to a slab cast, but can be applied to a bloom continuous caster and a billet continuous caster. Is not limited to a rectangular shape but may be a circle.

[0040]

EXAMPLE Using the slab continuous casting machine shown in FIG. 3, the timing of starting the light reduction, the peripheral speed ratio of the pinch roll (V _PR2 / V
_PR1 ), and light reduction (mm / m), that is, a sample was taken from a slab slab cast by changing the drawing gradient of the light reduction roll, and the center segregation of each sample was investigated. Pinch roll peripheral speed ratio (V _PR2 / V
_PR1 ) and the effect of light reduction on center segregation were investigated.

The continuous casting machine used was 2.8 m directly below the mold.
This is a vertical bending type slab continuous casting machine having a vertical portion of 10 m and a radius of a curved portion following the vertical portion is 10 m. A light pressure lowering zone is set within a range of 20 to 32 m from the molten steel surface in the mold, and a medium carbon steel having a carbon concentration of 0.08 to 0.1 mass% is applied to a thickness of 250 to
mm, width 2100 mm slab, drawing speed 1.3
It was cast at m / min. Then, the crystallization start position of the solid phase in the center of the slab thickness direction is about 22 m from the molten steel surface in the mold, and the fully solidified position in the center of the slab thickness direction is about 30 m from the molten steel surface in the mold. The secondary cooling strength before entering the low pressure zone was adjusted. In some tests, 200 to 6 water sprays were placed on the upper surface of the slab from the water spray installed just before the low pressure zone.
00l / m ² · min, 300-1200l /
The cooling water of m ² · min was sprayed to cool strongly, the slab surface temperature was changed, and the effect of the slab surface temperature on the center segregation was investigated.

The degree of segregation of the center segregation was determined by taking a macrostructure photograph of the cross section in the slab cross section and using the photograph to determine the length of the macrosegregated portion (SL = ΣSA _i / L, SA _i : area of macro segregation, L: Slab width) by measuring and evaluating the slab thickness center by 1 mm over a range of 30 mm in the thickness direction.
, And analyzed for carbon, and the ratio (C _max / C _max) between the maximum value C _max of the carbon concentration and the carbon concentration C _{0 of the} molten steel was obtained.
C ₀ ) was evaluated as a center segregation degree. In this case, the center segregation decreases as the degree of center segregation approaches 1.

FIG. 4 shows a drawing speed of 1.3 m / m.
and the ratio of the peripheral speed of the pinch roll (V _PR2 / V _PR1 )
FIG. 6 is a graph showing the results of investigation of the relationship between the pinch roll peripheral speed ratio (V _PR2 / V _PR1 ) and the limit light reduction amount at the solid-liquid interface when the light reduction amount (mm / m) is changed. It is. If the peripheral speed (V _PR2 ) of the downstream pinch roll is made slower than the peripheral speed (V _PR1 ) of the upstream pinch roll,
The critical light reduction of crack generation increases. This means that the smaller the pinch roll peripheral speed ratio (V _PR2 / V _PR1 ), the more the compressive force acts on the solid-liquid interface, and the less likely it is for the solid-liquid interface to crack.

FIG. 5 shows a drawing speed of 1.3 m / mi.
n, the pinch roll peripheral speed ratio (V _PR2 / V _PR1 ) is set to 0.
9. The calculated solid phase ratio at the center of the slab thickness is less than 0, 0.02, 0.1, 0.2,
It is a figure which shows the result of having investigated the relationship between the timing of light reduction start and center segregation when reducing lightly from 0.3, 0.4, 0.5, 0.6 to complete solidification. The horizontal axis in FIG. 5 shows the solid fraction (calculated value) and the liquid phase thickness (calculated value) at the center of the slab thickness at the start of light reduction. In this case, in each test, the slab was only supported without light reduction until the calculated solid fraction at the center of the slab thickness direction in the light reduction zone reached the above-mentioned predetermined value. As shown in FIG. 5, when the solid phase ratio at the center of the slab thickness is 0.4 or less and light reduction is started, there is an effect of reducing center segregation. In the case of starting, the effect of improving the center segregation was small.

FIG. 6 shows a drawing speed of 1.3 m / mi.
n, the roll narrowing gradient of all rolls in the low pressure zone is 0.9
mm / m (converted to rolling speed 1.17 mm / mi
n), the light reduction start time is defined as the time when the solid phase ratio at the center of the slab thickness direction is 0.1, and the peripheral speed ratio of the pinch roll (V _PR2
/ V _PR1 ) and the pinch roll peripheral speed ratio (V _PR2 / V _PR1 ) when light reduction is performed until complete solidification
FIG. 6 is a diagram showing a result of investigating a relationship between and center segregation. As shown in FIG. 6, as the peripheral speed ratio (V _PR2 / V _PR1 ) is reduced, the degree of center segregation is reduced, but when the ratio is less than 0.8, the effect almost disappears. This is because slip occurs at the interface between the roll and the slab, and the compression force does not increase. In this experiment, the compression force was limited due to the roll slip, but the combination of the improvement of the roll drive motor, the roll pressing force, and the pulling speed,
The limits of compression force vary.

FIG. 7 shows a drawing speed of 1.3 m / mi.
n, the pinch roll peripheral speed ratio (V _PR2 / V _PR1 ) is set to 0.
FIG. 9 is a diagram showing the result of examining the effect of the reduction speed on center segregation by changing the reduction speed (= light reduction amount × drawing speed) as 95. As shown in FIG. 7, it was found that the center segregation was improved when the rolling speed was in the range of 0.8 to 1.5 mm / min.

FIG. 8 shows a drawing speed of 1.3 m / mi.
n, the pinch roll peripheral speed ratio (V _PR2 / V _PR1 ) is set to 0.
Investigate the relationship between the temperature difference (ΔT) between the slab surface temperature and the solid-liquid interface temperature when the slab is strongly cooled just before the light reduction zone as 95, and the limit light reduction amount at which cracking occurs at the solid-liquid interface. It is a figure which shows the result. As shown in FIG. 8, as the temperature difference (ΔT) becomes larger, the crack reduction limit light reduction becomes larger. By setting the temperature difference (ΔT) to 600 ° C. or more, the temperature difference (ΔT) becomes larger.
T) is 1.3 compared to the normal case where 400-500 ° C.
About twice as large.

FIG. 9 shows a drawing speed of 1.3 m / mi.
n, the light reduction amount was 1.0 mm / m, the pinch roll peripheral speed ratio (V _PR2 / V _PR1 ) was 0.95, and when the slab was strongly cooled just before the light reduction zone, the slab surface temperature and solidification It is a figure which shows the result of having investigated the relationship between temperature difference ((DELTA) T) with a liquid interface temperature, and center segregation. By strongly cooling the slab surface as shown in FIG. 9, the center segregation could be further reduced.

[0049]

According to the present invention, since the slab is given a compressive force in the drawing direction to reduce the slab lightly, no cracking occurs at the solid-liquid interface of the slab and the concentrated molten steel does not squeeze out. It is possible to increase the light reduction amount to the limit,
Center segregation can be significantly reduced. As a result, it is possible to stably produce a slab that can cope with recent strict quality requirements.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a positional relationship between a slab, a low pressure band, and a pinch roll.

FIG. 2 is a diagram schematically showing a temperature gradient of a solidified shell and a stress generated by the temperature gradient, wherein (a) shows a temperature gradient,
(B) is a figure which shows a stress distribution.

FIG. 3 is a view showing an example of an embodiment of the present invention and is a schematic side view of a slab continuous casting machine.

FIG. 4 is a diagram showing the results of an investigation on the relationship between the peripheral speed ratio of a pinch roll and the critical reduction in crack generation at the solid-liquid interface.

FIG. 5 is a diagram showing the results of an investigation of the relationship between the start time of light reduction and center segregation.

FIG. 6 is a diagram showing the results of an investigation of the relationship between the peripheral speed ratio of pinch rolls and center segregation.

FIG. 7 is a diagram showing the results of investigating the effect of reduction speed on center segregation.

FIG. 8 shows a temperature difference (Δ) between a slab surface temperature and a solid-liquid interface temperature.
FIG. 4 is a diagram showing the results of an investigation of the relationship between T) and the amount of light reduction under the crack initiation limit at the solid-liquid interface.

FIG. 9 shows a temperature difference (Δ) between a slab surface temperature and a solid-liquid interface temperature.
It is a figure which shows the result of having investigated the relationship between T) and center segregation.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Cast piece 2 Unsolidified phase 3 Solidified shell 4 Light pressure lower zone 5 Immersion nozzle 6 Mold 7 Water spray 8 Support roll 9 Guide roll 10 Pinch roll 11 Light pressure lower roll 12 Lower straightening roll

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification FI FI Theme Court ゛ (Reference) B22D 11/20 B22D 11/20 C (72) Inventor Masayuki Nakata 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. F-term (reference) 4E004 KA13 MC06 MC07 MC19

Claims (6)

[Claims] 1. A method of continuously casting a slab having an unsolidified phase therein while lightly reducing the slab by a plurality of pairs of rolls,
A continuous casting method characterized in that from the start of light reduction to the end of light reduction, light reduction is performed while applying compressive force in the direction of drawing the slab to the slab. 2. A method of continuously casting a slab having an unsolidified phase therein while lightly reducing the slab with a plurality of pairs of rolls under a light reduction zone. The peripheral speed of the pinch roll installed on the upstream side is reduced while the peripheral speed of the pinch roll installed on the downstream side is made slower than the peripheral speed of the pinch roll installed on the upstream side while lightly reducing while pulling out the slab. Continuous casting method. 3. The ratio (V _PR2 / V _PR1 ) of the peripheral speed (V _PR2 ) of the pinch roll installed on the downstream side to the peripheral speed (V _PR1 ) of the pinch roll installed on the upstream side is less than 1.0; The continuous casting method according to claim 2, wherein the control is performed in a range of 0.8 or more. 4. The temperature difference between the surface temperature of the slab and the solid-liquid interface temperature is maintained at 600 ° C. or more at least during a period from the start of light reduction to the end of light reduction.
The continuous casting method according to claim 1. 5. The continuous casting method according to claim 1, wherein the slab is lightly reduced in a range of a reduction speed of 0.8 to 1.5 mm / min. 6. A light reduction is started at a time point when a solid phase ratio in a center portion of the slab thickness direction is 0.4 or less, and the light reduction is continued until solidification of the center portion of the slab thickness direction is completed. The continuous casting method according to any one of claims 1 to 5.