JP2001259808A - Method for continuously casting steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To drastically reduce the center segregation over all positions in a cast slab in a continuous casting steel. SOLUTION: Rolling reduction is applied in a continuous caster to a part in the casting direction of the cast slab 2 having unsolidified phase 4 in the inner part to press-stick both of the faced solidified shells 3. Successively, the cast slab after press-sticking is solidified while applying the light rolling- reduction with plural pairs of light rolling-reduction rolls 13, afterward, the press-sticking part is cut to form the cast slab. At this time, both faced solidified shells are desirable to press-stick by applying the light rolling-reducing at the point of time when the solid phase ratio at the center part in the thickness direction of the cast slab is <=0.4 and by applying the rolling-reduction when the thickness of unsolidified phase at the center part in the thickness direction of the cast slab is 2-10 mm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for continuously casting steel capable of preventing component segregation occurring at the center of a continuously cast slab.

[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 macro component segregation of the steel.

[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 not enough molten steel in the unsolidified phase at the end of solidification in continuous cast slabs, the concentrated molten steel between dendrite trees, which is the final solidified part, flows along with solidification shrinkage, and it flows in the slab thickness direction. It accumulates in the center and solidifies, producing 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, a method has been proposed in which unsolidified slabs are reduced by a plurality of pairs of light reduction 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 speed corresponding to the solidification shrinkage speed of the slab so that the flow of concentrated molten steel between dendrite trees does not occur. The purpose is to prevent center segregation.

[0006]

However, in the final solidification part where light reduction is performed, a static iron pressure having a height of usually 10 m or more acts on the solidified shell. Further, in the light reduction method, the slab is reduced by a plurality of pairs of light reduction rolls. However, between adjacent rolls (between the rolls), the slab is not supported. In this case, swelling of the solidified shell (hereinafter referred to as "bulging") occurs due to static iron pressure acting on the solidified shell. The bulging causes a change in the volume of the unsolidified phase and causes the molten steel to flow, so that the bulging is one of the causes of center segregation. In the light pressure reduction method, if the installation interval between adjacent rolls (called the roll pitch) is reduced, the amount of bulging decreases, but there is a limit to reducing the roll pitch in terms of roll strength. There is a problem that bulging occurs more or less inevitably between the bulging portions and the conventional light reduction method cannot prevent the center segregation due to the bulging.

[0007] Further, if the amount of light reduction is too large in the light reduction method, the concentrated molten steel between the dendrite trees is squeezed out in the direction opposite to the casting direction, and carbon, phosphorus, and sulfur are located at the center in the thickness direction of the slab. Segregation with a low solute element concentration (called negative segregation in this case) is generated, while if the amount of light reduction is too small, the molten steel is sucked by volume shrinkage accompanying solidification, so the concentrated molten steel between dendrite trees Center segregation is generated without suppressing the flow of

[0008] As described above, in the light rolling method, the optimum rolling conditions for preventing molten steel from flowing are very limited conditions. However, due to the above-mentioned bulging, measures to prevent slab segregation by light rolling are still available. It is hardly enough. On the other hand, demands on steel quality from customers are becoming more stringent, and further reduction of center segregation is desired.

The present invention has been made in view of the above circumstances,
The aim is to continuously reduce the segregation of the center over all the parts of the slab, and to produce a slab that can cope with recent severe quality requirements. It is to provide.

[0010]

According to a first aspect of the present invention, there is provided a continuous casting method for steel, in which a part of a slab having an unsolidified phase therein in a casting direction is reduced in a continuous casting machine so that opposed solidified shells are separated from each other. The method is characterized in that the slab is pressed and then solidified while being lightly reduced by a plurality of pairs of light reduction rolls, and then the crushed portion is cut into a slab.

[0011] The continuous casting method for steel according to a second aspect of the present invention is the continuous casting method according to the first aspect, wherein the solid fraction at the center in the thickness direction of the slab is 0.1.
It is characterized in that the pressure is reduced slightly from the point of time 4 or less.

[0012] The continuous casting method of steel according to the third invention is characterized in that, in the first invention or the second invention, when the thickness of the unsolidified phase at the center in the thickness direction of the slab is 2 mm to 10 mm, the solidified shell is reduced. It is characterized in that they are pressed together.

A continuous casting method for steel according to a fourth aspect of the present invention is the method according to any one of the first to third aspects, wherein the drawing gradient of the roll under light pressure is constant, and 0.2 mm / min to 0.2 mm / min.
It is characterized in that light reduction is performed at a light reduction speed of 1.0 mm / min.

According to a fifth aspect of the present invention, there is provided the continuous casting method of steel according to any one of the first to third aspects, wherein the solid phase ratio at the center in the thickness direction of the slab is 0 to 0.6. .2mm
/ Min to 1.0 mm / min with a light reduction speed, and a range where the solid fraction in the center of the slab thickness direction exceeds 0.6 is a 0.6 mm / min to 1.5 mm / min light reduction speed. And a slight reduction in pressure.

In the present invention, a part of a slab having an unsolidified phase therein in the casting direction is pressed down in a continuous casting machine to press the opposing solidified shells together. By pressing the solidified shell,
The static iron pressure on the upstream side of the slab withdrawal direction from the crimped portion does not act on the solidified shell on the downstream side of the slab withdrawal direction from the crimped portion, and therefore, on the solidified shell on the downstream side of the slab withdrawal direction from the crimped portion. The working iron pressure is greatly reduced, for example in the horizontal part of a continuous casting machine, the iron pressure is substantially zero.

After pressing, the slab in which the unsolidified phase is sealed solidifies, but when solidified as it is, the volume of the unsolidified phase decreases due to solidification shrinkage, and center segregation and porosity occur. I do. Therefore, in the present invention, the slab in which the unsolidified phase is sealed is lightly reduced until the internal unsolidified phase is completely solidified. That is, a rolling force is applied from the slab surface so as to match the volume that decreases with solidification shrinkage.

As described above, in the present invention, the bulging of the solidified shell becomes substantially zero, and the movement of the unsolidified phase is prevented by light pressure, so that it is possible to produce a cast piece with extremely small center segregation. . The light reduction in the present invention means to reduce the drawing gradient of each light reduction roll to 0.2 to 2.0% of the thickness of the slab per 1 m in the drawing direction of the slab.

It is preferable that light reduction be started when the solid fraction at the center in the thickness direction of the slab is 0.4 or less.
This is because even when light reduction is started after the solid phase ratio in 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. is there.

Further, when the thickness of the unsolidified phase at the center in the thickness direction of the slab is 2 mm to 10 mm, it is preferable to compress the solidified shells by pressing down. If the unsolidified phase is less than 2 mm, the time required for solidifying the center in the thickness direction of the slab after crimping is short, making it difficult to perform sufficient light reduction. When the solidified shell of the piece is pressed, the surface of the molten steel (the molten metal surface) in the mold rises during the pressure bonding, and the amount of the rise increases. The reason for this is that the quality of the cast slab is likely to be deteriorated due to the entanglement of the mold powder or the like due to the fluctuation of the molten metal level.

Furthermore, the drawing gradient of the light reduction roll is kept constant, and the reduction is performed at a light reduction speed of 0.2 mm / min to 1.0 mm / min. The range from to 0.6 is 0.2 mm / min
Light reduction at a light reduction speed of ~ 1.0 mm / min. The range where the solid fraction at the center of the slab thickness direction exceeds 0.6 is 0.6 m.
It is preferable to perform light reduction at a light reduction speed of m / min to 1.5 mm / min or to perform light reduction by either method.
This is because when the drawing gradient of the light reduction roll is constant, the light reduction speed is slow at a light reduction speed of less than 0.2 mm / min, and the movement of the unsolidified phase cannot be prevented.
At a light reduction speed exceeding 0 mm / min, the unsolidified phase is squeezed out and the center segregation deteriorates. Also, in the range where the solid fraction in the center of the slab thickness direction exceeds 0.6, the movement of the unsolidified phase does not occur even if the light reduction speed is increased, so the drawing gradient of the light reduction roll is changed from the middle of the reduction. In this case, the light reduction speed in the range where the solid fraction in the center of the slab thickness direction exceeds 0.6 is 0.6 mm / min to 1.5 m.
By increasing the value to m / min, the porosity at the center of the slab in the thickness direction can be reduced.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. FIG. 1 is a view showing an example of an embodiment of the present invention, and is a schematic side sectional view of a vertical bending slab continuous casting facility having a rectangular slab section.

In FIG. 1, a plurality of sets of support rolls 10, guide rolls 11, pinch rolls 12, and
A guide roll group that supports the slab 2 and that includes a light pressure roll 13 is installed, and on the downstream side of the guide roll group,
A plurality of transport rolls 15 and a gas cutting machine 14 that is located above the transport rolls 15 and moves in synchronization with the drawing speed of the slab 2 are installed. A secondary cooling zone (not shown) made of air mist spray or water spray is installed in the guide roll group.

A pair of pressure bonding rolls 6 are provided on the downstream side of the lower straightening belt 9 in the guide roll group in the slab drawing direction. The pressure roll 6 is connected to a hydraulic cylinder (not shown) that moves in synchronization with the speed at which the slab 2 is pulled out, and the distance between the pressure roller 6 and the opposing pressure roll 6 can be increased or decreased by the hydraulic cylinder. That is, the slab 2 can be lowered by the press roll 6 in synchronization with the drawing speed of the slab 2. On the downstream side of the slab pulling direction of the pressure roll 6, a light pressure lowering band 7 including a plurality of pairs of light pressure lowering rolls 13 is provided. In addition, the pressure bonding roll 6
The structure is not limited to the above. For example, the pressing roll 6 is connected to a hydraulic cylinder fixed to a continuous casting machine, and the pressing roll 6 is driven by an electric motor or a hydraulic motor to synchronize with the drawing of the slab 2. It is also possible to adopt a method of lowering the pressure at a fixed place without moving it.

The continuous casting method of the present invention in the continuous casting facility having such a configuration will be described below. Molten steel is continuously injected into the mold 1 from a tundish (not shown) installed at a predetermined position above the mold 1. Usually, this injection
This is performed through an immersion nozzle (not shown) whose tip is immersed in the molten metal surface 5 in the mold 1. The molten steel poured into the mold 1 is cooled in the mold 1 to form a solidified shell 3, and the support roll 10, the guide roll 11, and the cast piece 2 having the solidified shell 3 on the outer periphery and the unsolidified phase 4 on the inside are formed. The guide roll group including the pinch rolls 12 is continuously pulled downward by the driving force of the pinch rolls 12. During that time, the slab 2 is straightened from a flat plate shape to an arc shape in the upper straightening band 8, and is bent back from the arc shape to the flat plate shape in the lower straightening band 9.

During the drawing, the slab 2 is removed by the gas cutting machine 1.
The pressing roll 6 is pressed against the surface at the position where the cutting is performed by the pressing member 4, and the slab 2 is pressed down, so that the opposing solidified shells 3 are pressed together. The position at which the slab 2 is cut by the gas cutting machine 14 can be determined by the total casting length and the required length of the individual slab 2A after cutting.

The thickness of the unsolidified phase 4 at the time of pressing is 2 mm to 10 mm.
mm. Therefore, the secondary cooling strength from immediately below the mold to the pressure roll 6 is adjusted so that the thickness of the unsolidified phase 4 during the pressure bonding falls within this range, and further, the drawing speed of the slab 2 is adjusted as necessary. .

Next, the cast slab 2 in which a part of the solidified shell 3 is press-bonded and the unsolidified phase 4 is sealed therein is lightly reduced by the light reduction band 7. The drawing speed of the slab 2 is set so that the slab 2 is completely solidified within the range of the low-pressure band 7. This can be determined in advance by heat transfer calculation from the slab thickness, the slab withdrawal speed, the secondary cooling strength, and the like.

As described above, the timing of starting the light pressure reduction is preferably the time when the solid fraction at the center of the slab thickness is 0.4 or less. Light reduction is continued until the center of the slab thickness is solidified. This is because if the reduction is stopped during the solidification, the effect of reducing the center segregation is small.

The drawing gradient of the roll 13 under light pressure (mm /
m) is fixed, and 0.2 mm / min to 1.0 mm / m
The light reduction rate is a light reduction speed of in, or the drawing gradient of the light reduction roll 13 is changed from the middle of the reduction.
Light reduction is performed at a light reduction speed of 2 mm / min to 1.0 mm / min, and when the solid phase ratio at the center in the slab thickness direction exceeds 0.6, the reduction is 0.6 mm / min to 1.5 mm / min. It is preferable to reduce the pressure at a reduced speed or by either method.

The light reduction speed is a value obtained by multiplying the slab withdrawing speed by the drawing gradient (mm / m) of the roll interval of the light reduction rolls 13. Therefore, the light reduction speed is determined based on the slab withdrawing speed determined as the casting condition. The narrowing gradient of the roll 13 (m
m / m) may be set. By changing the drawing gradient of the light reduction roll 13 from the middle of the reduction, the range where the solid phase ratio in the center of the slab thickness direction exceeds 0.6 is 0.6 mm / min.
When the reduction is performed at a light reduction speed of 1.5 mm / min, a slab 2 having a small center segregation and a small porosity can be cast. Then, the slab 2 is completely solidified to the center of the slab 2 while slightly reducing the pressure. After the slab 2 is completely solidified, the crimping portion is pulled out to the position of the transport roll 15 by the gas cutting machine 14. Cut to produce a cast piece 2A.

The thickness of the unsolidified phase 4 when reduced by the pressure roll 6 is set to 2 mm to 10 mm, and
In order to completely solidify the slab 2 within the range, the length of the light pressure reduction zone 7 is necessary correspondingly, and this is also obtained in advance by heat transfer calculation, and the setting range of the light pressure reduction zone 7 is set. good.
Further, in order to make the bulging force acting on the solidified shell 3 substantially zero, it is preferable that the light pressure lowering zone 7 is installed in a horizontal portion of the continuous casting machine.

By performing the casting in this manner, the bulging force does not substantially act on the solidified shell 3 of the slab 2, and therefore, it is possible to perform light reduction corresponding to the shrinkage amount of the unsolidified phase 4. Thus, center segregation at all portions of the slab 2 can be significantly reduced.

Although the above description is for the case of using a vertical bending type continuous casting machine, the present invention is not limited to the vertical bending type continuous casting machine, and the present invention can be applied to a curved type continuous casting machine in accordance with the above description. Can be implemented. Although the above description relates to a continuous slab caster, the present invention is not limited to a slab cast, and can be applied to a bloom continuous caster and a billet continuous caster.

[0034]

EXAMPLE Using a continuous slab casting machine shown in FIG. 1, a slab drawing speed and a drawing gradient (mm /
m) was changed, and a test was conducted to investigate the relationship between the slab withdrawal speed and the center segregation, the relationship between the light reduction speed and the center segregation, and the relationship between the unsolidified phase thickness during crimping and the variation in the mold surface in the mold. The center segregation was evaluated by taking a 1 mm slice sample from the center of the cast slab in the thickness direction over a range of 30 mm in the thickness direction, analyzing the carbon, analyzing the maximum carbon concentration C _max and the carbon content of the molten steel. the ratio of the concentration C ₀ (C _{ma x} / C
₀ ) was evaluated as a center segregation degree. In this case, the center segregation is reduced as the degree of center segregation approaches 1.0.

The continuous casting machine used was a mold (950 m long)
m) There is a vertical portion of 2.8 m directly below, the radius of the curved portion following it is 10 m, and the upper straightening band is 3.
8m to 5.0m, lower straightening belt is 18m from top of mold
This is a vertical bending type slab continuous casting machine having a length of 49 m and a length of m to 20 m. 24m to 24.
A pressure roll was installed at a position of 5 m (including the range of the synchronous movement with the slab), and a low pressure lower band extending about 8 m in the casting direction was installed on the downstream side of the slab withdrawal direction of the slab. Then, an Al-killed steel slab having a slab thickness of 250 mm, a slab width of 2100 mm, a carbon concentration of 0.08 mass%, and a Mn concentration of 1.4 mass% was cast.

FIG. 2 shows that the speed of drawing the slab is from 1.30 to 1.
It is a figure which shows the result of having investigated the center segregation degree of the slab when changing in the range of 1.60 m / min. In this case, the squeezing gradient (mm) of the light reduction roll is adjusted in accordance with the slab drawing speed so that the light reduction speed is 0.5 mm / min.
/ M).

As shown in FIG. 2, when the slab drawing speed is 1.30 m / min, the degree of center segregation is high and the center segregation is not reduced, but the slab drawing speed is 1.35 m / min.
/ Min, the center segregation was slightly reduced, and when the slab drawing speed was 1.40 m / min or more, the center segregation was found to be reduced. This reason was considered from the relationship with the thickness of the unsolidified phase at the center in the thickness direction of the slab at the time of press bonding.

FIG. 3 shows that the speed of drawing the slab is from 1.30 to 1.
It is a figure which shows the result of having calculated the unsolidified phase thickness of the center part of the slab thickness direction at the time of press bonding at 1.60 m / min from heat transfer calculation. As is clear from FIG. 3, when the slab drawing speed was 1.30 m / min, solidification had already been completed at the time of press bonding, and it was found that the present invention could not be implemented. When the slab drawing speed was 1.35 m / min, a solid phase and a liquid phase were mixed at the center in the slab thickness direction, and the solid phase ratio was about 0.3. On the other hand, when the slab drawing speed was 1.40 m / min or more, the solid phase ratio was zero at the center in the slab thickness direction, and it was found that a liquid phase was present.

From these results, when a sufficient liquid phase, that is, an unsolidified phase exists in the center of the slab in the thickness direction, the slab is pressure-bonded.
Thereafter, it was confirmed that the segregation at the central portion in the thickness direction of the slab was remarkably improved by light reduction.

On the other hand, if the thickness of the unsolidified phase at the time of pressing is large, the unsolidified phase is extruded at the time of pressing at a direction opposite to the casting direction.
It reaches the molten metal level in the mold and appears as a change in the level of the molten metal level in the mold. Therefore, the slab drawing speed is set to 1.4 to 2.0.
The range was changed within the range of m / min, and the amount of change in the level of the molten metal in the mold during press bonding was investigated. Fluctuations in the level of the molten metal were measured with an eddy current type distance meter, and the thickness of the unsolidified phase at the time of compression was calculated by heat transfer calculation, and the relationship between the thickness of the unsolidified phase at the time of compression and the amount of variation in the molten metal level was investigated. . FIG. 4 shows the results of the investigation.

As shown in FIG. 4, the thickness of the unsolidified phase was 10 m.
When the slabs of m or more are pressure-bonded, the level change
over m. The present inventors have empirically known that when the level of the molten metal level exceeds 5 mm, the mold powder covering the molten metal surface in the mold is involved. Therefore, it is preferable to adjust the casting conditions so that the thickness of the unsolidified phase at the time of press bonding is 10 mm or less, in order to suppress the fluctuation amount of the level of the molten metal in the mold.

FIG. 5 shows the drawing gradient (m) of the roll under light pressure.
m / m) is selected from the range of 0 to 1.0 mm / m and the slab drawing speed is set to 1.40 to 1.0 mm / m.
Change the speed in the range of 1.60 m / min and set the light reduction speed to 0.
It is a figure which shows the result of having investigated the center segregation degree of the slab when changing in the range of -1.5 mm / min. As shown in FIG. 5, it was found that the center segregation was reduced when the light reduction speed was 0.2 to 1.0 mm / min. 1.0mm / m
At a reduction speed exceeding in, the unsolidified phase was squeezed out.

However, the drawing gradient (mm
/ M) is constant and the light reduction speed is 0.
When it was less than 6 mm / min, the center segregation was reduced, but the frequency of occurrence of porosity was high at the center in the thickness direction of the slab. Therefore, the narrowing gradient (m
m / m).

In the state where the solid phase and the liquid phase coexist, the solid phase ratio at which the liquid phase is said to be flowable is about 0.6. A light reduction at a light reduction speed of ~ 1.0 mm / min. A range in which negative segregation does not occur even if the light reduction speed is increased. A range where the solid phase ratio exceeds 0.6 is a drawing gradient of the light reduction roll ( mm / m), and was lightly reduced at a light reduction speed of 0.6 to 2.0 mm / min. As a result, the region where the solid fraction exceeds 0.6
Although light reduction was possible at a light reduction speed of up to m / min, it was difficult to reduce the light reduction speed at a light reduction speed exceeding 1.5 mm / min due to equipment restrictions. However, 0.6-
The porosity at the center of the slab thickness direction disappeared at a light reduction speed of 1.5 mm / min, and it was confirmed that no further light reduction speed was necessary.

[0045]

According to the present invention, since the slab can be lightly reduced in a state where bulging force does not substantially act on the solidified shell, the center segregation is greatly reduced over all the parts of the slab. As a result, it is possible to stably produce a slab that can cope with recent strict quality requirements, and an industrially beneficial effect is brought.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of an embodiment of the present invention.

FIG. 2 is a diagram showing the results of an investigation on the relationship between the slab withdrawal speed and the degree of center segregation.

FIG. 3 is a diagram showing the thickness of an unsolidified phase at the center in the slab thickness direction at the time of press bonding when the slab drawing speed is changed.

FIG. 4 is a diagram showing the results of an investigation of the relationship between the thickness of the unsolidified phase at the center of the slab thickness direction at the time of press bonding and the level change in the mold level in the mold.

FIG. 5 is a diagram showing the results of an investigation on the relationship between the speed of light reduction and the degree of center segregation.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 mold 2 cast slab 3 solidified shell 4 unsolidified phase 5 molten surface 6 pressure roll 7 light pressure lower band 8 upper correction band 9 lower correction band 10 support roll 11 guide roll 12 pinch roll 13 light pressure lower roll 14 gas cutting machine 15 transport roll

Claims (5)

[Claims] 1. A part of a slab having an unsolidified phase therein in a casting direction is reduced in a continuous casting machine so that opposing solidified shells are pressure-bonded to each other. A continuous casting method for steel, characterized in that the steel is solidified while being lightly reduced by a roll, and thereafter, the crimped portion is cut into a cast piece. 2. The continuous casting method for steel according to claim 1, wherein the steel is lightly reduced from a point in time when the solid phase ratio at the center of the slab thickness direction is 0.4 or less. 3. The steel according to claim 1, wherein when the thickness of the unsolidified phase at the center in the thickness direction of the slab is 2 mm to 10 mm, the solidified shells are pressed to each other. Continuous casting method. 4. The method according to claim 1, wherein the reduction gradient of the light reduction roll is constant, and the reduction is performed at a light reduction speed of 0.2 mm / min to 1.0 mm / min. The continuous casting method for steel according to any one of the first to third aspects. 5. A solid phase ratio of 0 to 0.
The range up to 6 is 0.2 mm / min to 1.0 mm / mi
n in the range of 0.6 mm / min to 1.5 mm where the solid fraction at the center in the thickness direction of the slab exceeds 0.6.
The continuous casting method for steel according to any one of claims 1 to 3, wherein the steel is lightly reduced at a light reduction speed of mm / min.