JP2001259810A - Continuous casting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce center segregation by applying a large quantity of light rolling reduction to the limit which does not cause the squeezing of concentrated molten steel, without caursing the crack in the interface between the solid and the liquid in a cast slab. SOLUTION: In the continuous casting method in which the light rolling reduction is applied to the cast slab 1 having unsolidified phase 2 in the inner part with plural pairs of rolls 11 to reduce the center segregation in the cast slab, while holding the temperature difference between the surface temperature and the temperature of the interface between the solid and the liquid of the cast slab for at least the period from the staring time of the light rolling reduction to the completing time of the light rolling reduction to >=800 deg.C, the light rolling reduction is applied. At this time, it is desirable that the rolling reduction speed is set to the range of 0.8-1.6 mm/min and further, the light rolling reduction is started from 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 continued until the center part in the thickness direction of the cast slab completes the solidification.

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 purpose is to reduce the center segregation of the continuous cast slab by the light reduction method, without causing cracks at the solid-liquid interface of the slab, and reducing the pressure to the limit where concentrated molten steel does not squeeze out. An object of the present invention is to provide a continuous casting method capable of increasing the amount, achieving a large reduction in center segregation, and producing a slab that can cope with recent severe quality requirements.

[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. During the period until the end of the light reduction, the temperature difference between the surface temperature of the slab and the solid-liquid interface temperature is maintained at 800 ° C. or more, and the light reduction is performed.

[0012] The continuous casting method according to the second invention is characterized in that, in the first invention, the slab is lightly reduced at a reduction speed of 0.8 to 1.6 mm / min.

[0013] The continuous casting method according to a third aspect of the present invention is the method according to the first or second aspect, wherein light reduction is started at a point where the solid phase ratio at the center in the thickness direction of the slab is 0.4 or less. Light reduction is continued until the center in the thickness direction is solidified.

When the temperature of the slab surface and that of the solid phase interface are increased by rapidly cooling the slab surface, tensile stress acts on the slab surface and compression acts on the solid-liquid interface. Here, the temperature difference between the slab surface temperature and the solid phase interface temperature is ΔT, and the stress generated by the temperature difference (ΔT) will be described with reference to FIG. FIG. 1 is a diagram schematically showing the temperature gradient of the solidified shell and the stress caused by the temperature gradient, wherein (a) shows the temperature gradient and (b) shows the stress distribution.

Conventionally, it has been common practice to control the surface temperature of the slab to 900 ° C. or more and reduce the pressure. In this case,
The temperature (T _L ) of the solid-liquid interface 3A and the temperature (T _L ) of the slab surface 1A
The temperature difference (ΔT) from _S ) is about 400 to 500 ° C. This temperature distribution is shown by a broken line in FIG.
On the other hand, for example, when the surface temperature is lowered to about 600 ° C. and the temperature difference (ΔT) is set to 800 ° C. or more (FIG. 1).
In this example, the surface temperature is set to 600 ° C.). Here, the temperature ( _TL ) of the solid-liquid interface 3A is usually equal to the solidus temperature.

FIG. 1B 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 indicates a case where the temperature difference (ΔT) is 800 ° 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 when the temperature is 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.

When a light pressure is applied while applying a compressive force to the solid-liquid interface, the tensile force acting on the solidified shell by the light pressure is canceled. As described above, the limit of tensile strain at which cracking occurs at the solid-liquid interface is about 1%. However, in the present invention, since a compressive force is acting, cracks are generated in the solidified shell by a tensile force under light pressure. By this time, it is possible to add a light reduction amount far exceeding the conventional limit value of the light reduction amount.
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, casting is performed while reducing the amount of light reduction to 0.2 to 2.0% of the thickness of the slab per 1 m in the drawing direction of the slab.

Qualitatively, a compressive force acts on the solid-liquid interface as the slab surface temperature is lowered, but it is difficult to calculate the magnitude with high accuracy. The reason is that there is no accurate data on the mechanical properties near the solidification point of steel. Therefore,
An experiment was conducted in which the surface cooling and the amount of light reduction, that is, the narrowing gradient (mm / m) of the roll interval of the light reduction roll were changed, and the limit light reduction amount at which solid-liquid interface cracking occurred was investigated.

In the experiment, the slab continuous casting machine used in Examples described later was used, and the drawing speed (Vc) of the slab was set at 1.
The temperature difference (ΔT) between the slab surface temperature and the solid-liquid interface temperature by adjusting the secondary cooling just before the low pressure zone at a constant 3 m / min.
Is changed within the range of 400 to 1050 ° C., and the light reduction amount, that is, the narrowing gradient of the roll interval of the light reduction roll (mm)
/ M) was changed in the range of 0.3 to 1.6 mm / m. Then, the presence or absence of a solid-liquid interface crack was determined from the macrostructure of the obtained slab slab. FIG. 2 shows the result of the investigation. The diameter of the roll under light pressure is 280 mm, and a split roll is used.

As shown in FIG. 2, as the temperature difference (ΔT) between the surface temperature of the slab and the solid-liquid interface increases, the critical light reduction amount at which solid-liquid interface cracking occurs increases, and solid-liquid interface cracking occurs. It turns out that it becomes difficult. And this temperature difference (ΔT)
Is set to 800 ° C. or more, the critical light reduction amount becomes the temperature difference (Δ
T) is much larger than in the case of 400 to 500 ° C., and the amount of light reduction can be lightly reduced to the range of 0.9 to 1.2 mm / m.

Therefore, in the present invention, the temperature difference (ΔT) between the slab surface temperature and the solid-liquid interface temperature is maintained at 800 ° C. or more at least during the period from the start of light reduction to the end of light reduction.
We decided to slightly reduce the pressure.

Further, the rolling speed is set to 0.8 to 1.6 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 6 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. The rolling speed is the slab drawing speed and the narrowing gradient (mm /
m), that is, a value multiplied by the light reduction amount.

Further, the solid fraction at the center in the thickness direction of the slab 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.

[0024]

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 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 comprising a plurality of pairs of light pressure lowering rolls 11. A water spray 7 capable of strongly cooling the piece 1 is arranged. 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 1 is strongly cooled, the surface temperature of the slab when passing through the lower straightening roll 12 is too low, and horizontal cracks may occur in the slab 1 due to the straightening distortion. By setting it downstream of 12, this can be prevented beforehand.

Under various casting conditions, the thickness of the solidified shell 3 and the solid phase ratio 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, the slab 1 that has passed through the lower straightening roll 12 is rapidly cooled by a water spray 7, and the slab surface temperature (T _S ) and the solid-liquid interface temperature (T _L ) when entering the low-pressure lower zone 4. )
The temperature difference (ΔT) is set to 800 ° C. or more, and the slab 1 is cooled in the light pressure lower zone 4 while maintaining the state.

Specifically, for example, the slab surface temperature when passing through the lower straightening roll 12 is set to 900 ° C. or higher, and then the slab surface temperature when rapidly cooled by the water spray 7 and lightly reduced to 500 to 700. C., and is maintained at this temperature during the passage through the low pressure zone 4. In order to rapidly cool the slab 1 in this way, the water spray 7 has a cooling water amount of 100 to 2000 l per 1 m ^{2 of} the slab surface (hereinafter referred to as “l”), depending on the installation length. / M ² · mi
n "). 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. Water spray 7 so that the temperature is 800 ° C or more.
The secondary cooling intensity may be adjusted from the secondary cooling zone immediately below the mold without installing the mold.

The rolling speed is preferably 0.8 to 1.6 m.
The slab 1 is lightly reduced under the control of m / 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.

By casting in this manner, a compressive force is applied to the solid-liquid interface of the slab 1, and no cracking occurs at the solid-liquid interface of the slab 1, and the concentrated molten steel does not squeeze out. The light reduction amount can be increased to the limit, and as a result, 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. The temperature difference between the slab surface temperature and the solid-liquid interface temperature (Δ
Since T) is set to 800 ° C. or higher, the strength of the solidified shell 3 is increased, and bulging between rolls in the light pressure lowering zone 4 is reduced, and center segregation due to bulging between rolls can also be reduced.

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

[0033]

EXAMPLE Using a continuous slab casting machine shown in FIG. 3, the slab surface temperature at the start of light reduction, the solid fraction at the center in the thickness direction of the slab, and the light reduction amount, that is, the narrowing gradient of the light reduction roll (mm / M). Samples were taken from the cast slab slabs, the center segregation of each sample was investigated, and the temperature difference between the slab surface temperature and the solid-liquid interface temperature (Δ
T), the influence of the solid phase ratio at the center of the slab thickness direction and the amount of light reduction on the 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 at the center of the slab thickness direction is about 22 m from the molten steel surface in the mold, and the fully solidified position at the center of the slab thickness direction is about 28 m from the molten steel surface in the mold. The secondary cooling strength before entering the low pressure zone was adjusted. Moreover, 200-600 l / m < ² >
min, spraying 300 to 1200 l / m ² · min of cooling water on the lower surface side to strongly cool and change the slab surface temperature,
The influence of the temperature difference (ΔT) on the center segregation was investigated.

The center segregation is performed by taking a slice sample of 1 mm over a range of 30 mm in the thickness direction of the center of the slab and analyzing the carbon, and analyzing the maximum carbon concentration C _max and the carbon concentration C of the molten steel. _The evaluation was performed by evaluating the ratio to ₀ (C _max / C ₀ ) as the degree of central segregation. In this case, the center segregation degree is 1
, The center segregation is reduced.

FIG. 4 shows that the calculated solid fraction at the center of the slab thickness is less than 0 to 0. 0, with the light reduction amount being 1.0 mm / m and the temperature difference (ΔT) between the surface and the solid-liquid interface being 950 ° C. 6 is a diagram showing the results of an investigation of the relationship between the timing of light reduction start and center segregation when light reduction is performed from time point 6 until complete solidification. FIG.
The horizontal axis indicates 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. 4, when the solid phase ratio at the center of the slab thickness direction 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. 5 shows the temperature difference between the surface and the solid-liquid interface (Δ
T) was set to 950 ° C., the light reduction start time was set to the time when the solid phase ratio in the center of the slab thickness direction was 0.3, and the reduction speed (= light reduction amount × drawing speed) was changed to complete the slab. FIG. 6 is a view showing the results of an investigation on the effect of the rolling speed on the center segregation when light reduction is performed until the solidification occurs. As shown in FIG. 5, it was found that the center segregation was improved when the rolling speed was in the range of 0.8 to 1.6 mm / min.

FIG. 6 shows that the light reduction amount is 0.8 mm / m, the light reduction start time is the time when the solid fraction in the center of the slab thickness direction is 0.3, and the temperature difference between the surface and the solid-liquid interface ( ΔT) from 400 to
It is a figure which shows the result of investigating the relationship between temperature difference ((DELTA) T) and center segregation at the time of changing into the range of 1050 degreeC, and lightly reducing until it completely solidifies. As shown in FIG. 6, the temperature difference (Δ
As T) increases, the center segregation decreases and the temperature difference (Δ
By setting T) to 800 ° C. or higher, center segregation could be stably reduced.

[0039]

According to the present invention, since a compressive force is applied to the solid-liquid interface of the slab to lower the slab lightly, cracking does not occur at the solid-liquid interface of the slab. It is possible to increase the amount of light reduction to the limit that does not occur. or,
Since the slab surface temperature is lowered, slab strength is increased, and bulging between rolls is suppressed. For this reason, center segregation can be greatly reduced, and a cast piece that can cope with recent severe quality requirements can be stably manufactured.

[Brief description of the drawings]

FIG. 1 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. 2 is a diagram showing a result of investigation on a relationship between a temperature difference between a slab surface temperature and a solid-liquid interface temperature and a limit reduction in crack generation at a solid-liquid interface.

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 of the relationship between the timing of light rolling start and center segregation.

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

FIG. 6 is a diagram showing the results of an investigation on the relationship between the temperature difference between the surface and the solid-liquid interface temperature 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

Continuation of the front page (72) Inventor Koichi Tsutsumi 1-2-1 Marunouchi, Chiyoda-ku, Tokyo F-term in Nihon Kokan Co., Ltd. 4E004 KA13 MC07 MC19

Claims (3)

[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,
At least the period from the start of light reduction to the end of light reduction,
A continuous casting method characterized in that a temperature difference between a surface temperature of a slab and a solid-liquid interface temperature is maintained at 800 ° C. or more and light reduction is performed. 2. The continuous casting method according to claim 1, wherein the slab is lightly reduced in a reduction speed range of 0.8 to 1.6 mm / min. 3. The method according to claim 1, wherein light reduction is started at a point where the solid phase ratio at the center of the slab thickness direction is 0.4 or less, and the reduction is continued until solidification of the center of the slab thickness direction is completed. The continuous casting method according to claim 1 or 2.