JP2012183569A - Continuous casting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent longitudinal cracks associated with uneven solidification of a continuously cast slab and lateral cracks generated on an oscillation mark bottom part.SOLUTION: In molding of the round cross sectional continuously cast slab as the stock for a seamless steel tube having the carbon concentration in steel of 0.08 mass% or more and 0.18 mass% or less, mold powder having viscosity at 1,573 K of 0.2 Pa s to 0.8 Pa s is used, and continuous casting is performed at a speed of 2.0 m/min or more and 4.0 m/min or less, and friction force between a mold and the cast slab is 60 kN/mor less. It is possible to improve both of the cast slab quality and operation efficiency.

Description

  The present invention relates to a method for continuously casting a round cross-section slab used in the Mannesmann method.

  As a method for manufacturing a seamless steel pipe, there is a Mannesmann method in which a central portion is perforated while rotating a round cross-section slab. When a slab having surface defects is used in this Mannesmann method, fractures or cover-like wrinkles occur starting from the surface defects at the time of pipe production, and maintenance and scrapping after pipe production are necessary. It causes cost deterioration.

  Therefore, the surface quality of the slab to be manufactured is important when continuously casting a round cross-section slab that is a raw material for a seamless steel pipe.

  By the way, the surface defect of the continuous cast slab occurs due to the initial solidification abnormality, and there are vertical cracks due to non-uniform cooling of the solidified shell and lateral cracks generated mainly from the valleys of the oscillation mark of the mold. Further, it is known that in the subperitectic steel region having a carbon concentration of 0.08% by mass or more and 0.18% by mass or less, volume shrinkage during initial solidification is large and slab surface defects are likely to occur.

  As countermeasures for the surface defects, in general, a uniform flow of powder due to a high cycle of mold oscillation, a reduction in oscillation mark depth due to a short stroke, a decrease in heat extraction from the mold due to the use of a slow cooling mold powder, etc. A method has been adopted.

  However, when the casting speed is increased for the purpose of improving efficiency, the amount of mold powder flowing between the mold and the slab decreases as the above-mentioned high cycle and short stroke increase, so the amount of heat extracted from the slab increases. Thus, the surface quality of the slab cannot be ensured.

  Therefore, in Patent Document 1, oscillation vibration is used for the purpose of reducing the depth of the oscillation mark (depth from the slab surface to the bottom of the oscillation mark) and securing the inflow amount of mold powder, which is the starting point of the transverse crack. A method of applying an alternating magnetic field that sometimes changes the magnetic field strength in synchronization with the vibration is disclosed.

  However, in the method disclosed in Patent Document 1, it is necessary to install an electromagnetic coil in the vicinity of the meniscus and apply a magnetic field whose strength changes, which is suitable for high-speed casting in which the molten metal surface fluctuation (meniscus vertical movement) increases. In addition, a new electromagnetic coil for generating an alternating magnetic field that changes the magnetic field strength is required, and the installation cost of the equipment becomes a problem.

  Moreover, in patent document 2, the vibration of a casting_mold | template is made to vibrate also in a horizontal direction in addition to the normal direction (casting direction) which is a normal oscillation direction, the deformation | transformation of a solidification shell is prevented, and an oscillation mark of A method of performing suppression is disclosed.

  However, unlike the assembly mold, the method disclosed in Patent Literature 2 cannot be applied to a tubular mold used for a round cross-section mold or the like.

On the other hand, in Patent Document 3, when high-speed continuous casting is performed at 2 to 10 m / min, occurrence of breakout and occurrence of vertical cracks on the surface of the slab can be prevented, and a slab having good surface quality can be stably obtained. There has been proposed a continuous casting method for thin cast pieces. In this method, the frictional force between the inner wall of the mold and the solidified shell is 6.37 × 10 ⁻² N / mm ² or less, and the heat removal amount Q (MW / m ² ) from the solidified shell by the mold is determined as the casting speed. The value is adjusted to a value between 0.7 times and 1.1 times the value of B defined by Vc (m / min) (= 0.30 × (1 + Vc)).

  However, the method disclosed in Patent Document 3 is intended for thin slabs, and there is no description on the physical properties of the mold powder for reducing the frictional force between the inner wall of the mold and the solidified shell. Further, there is no description of the frictional force between the inner wall of the mold and the solidified shell in the round section continuous cast slab.

Japanese Patent Laid-Open No. 5-115595 Japanese Patent Laid-Open No. 6-198409 JP 2001-129648 A

  In order to carry out the method disclosed in Patent Documents 1 and 2 for the purpose of producing continuous cast slabs with good surface quality, a new electromagnetic coil is required or the oscillation device is complicated. There is a problem of doing.

The continuous casting method of the present invention comprises:
In the casting of a round cross-section continuous casting slab that has a large volumetric shrinkage during initial solidification, in which the carbon concentration in the steel is 0.08 mass% or more and 0.18 mass% or less, which is the material of the seamless steel pipe,
In order to prevent vertical cracks due to non-uniform cooling of the solidified shell and lateral cracks starting from the oscillation mark while solving the problems of the conventional method,
Using a mold powder with a viscosity at 1573K of 0.2 Pa · s or more and 0.8 Pa · s or less, continuously casting at a speed of 2.0 m / min or more and 4.0 m / min or less, and between mold and slab The most important feature is that the frictional force of the steel sheet is 60 kN / m ² or less.

  In the present invention, a continuous cast slab having good surface quality can be produced without requiring a new electromagnetic coil or complicating the oscillation device.

In the present invention, the freezing point is 1273 K or more, the mass ratio (basicity) in the mold powder represented by CaO / SiO ₂ is 0.8 to 1.0, the amount of Na ₂ O is 3.0 mass% or less, F It is desirable to use a mold powder having a concentration of 5.0% by mass or less, an MgO content of 8.0% by mass or less, and an AL ₂ O ₃ content of 5.0% by mass or less.

In the present invention, a low-viscosity mold powder having a viscosity of 0.2 Pa · s or more and 0.8 Pa · s or less is cast at a high speed at a speed of 2.0 m / min or more and 4.0 m / min or less. By setting the frictional force between the pieces to be 60 kN / m ² or less, it is possible to achieve both improvement in slab quality and operational efficiency. And when implementing this invention, a new electromagnetic coil is not required or an oscillation apparatus is not complicated.

It is the figure which showed the relationship between casting speed and molten steel superheat degree. It is the figure which showed the relationship between a casting speed and an initial stage solidification shell. It is the figure which showed the relationship between casting speed and a thermocouple variation standard deviation. It is the figure which showed the relationship between a negative strip time rate and an oscillation mark depth. It is the figure which showed the relationship between the viscosity of mold powder, and the frictional force between a casting_mold | template and a slab.

In the present invention, a mold and a slab are formed by casting a low-viscosity mold powder at a high speed for the purpose of preventing vertical cracks and transverse cracks without requiring a new electromagnetic coil or complicating the oscillation device. This was realized by setting the frictional force between them to 60 kN / m ² or less.

  Hereinafter, an experiment conducted by the inventors to solve the conventional problems and an embodiment of the present invention established based on the experiment result will be described with reference to FIGS.

  Defects in the continuous cast slab that occur due to abnormalities in initial solidification include vertical cracks due to non-uniform cooling of the solidified shell and transverse cracks that occur due to deepening of the oscillation mark.

  The non-uniform solidification that occurs due to non-uniform cooling during the initial solidification is because the cooling conditions differ depending on the position of the solidified shell due to the difference in the amount of mold powder locally flowing between the mold and the slab and the difference in the mold shape. appear.

  Conventionally, in response to these problems, oscillation crack valleys in the solidified shell have been shortened by using a short stroke to prevent vertical cracking by using non-cooled mold powder to suppress non-uniform solidification and reduce oscillation amplitude. The method of suppressing the depth of the crack and preventing the transverse crack has been taken.

  However, as described above, in this method, when the casting speed is increased, the amount of mold powder flowing between the mold and the slab decreases when the high cycle or short stroke is performed. The amount of heat removed from the piece increases and the surface quality of the slab cannot be ensured.

  Therefore, to solve the above problems, mold powder flows uniformly between mold and slab during high-speed casting, uniform growth due to thinning of the initial solidified shell, and friction between mold and slab by applying low-viscosity mold powder. We tried to reduce resistance and to prevent both vertical cracks and transverse cracks in high-speed casting.

  Causes of non-uniform cooling in the initial solidification of continuous casting include non-uniform flow of mold powder between the mold and the slab and volume shrinkage of the initial solidified shell.

  Mold powder used for continuous casting is supplied in the form of powder into the mold, melted by heat supply from molten steel, and flows between the mold and the slab. For this reason, when the casting speed is low or when the molten steel superheat degree in the mold (difference from the liquidus temperature of the molten steel temperature) is insufficient, the mold powder will not hatch due to insufficient heat supply to the mold powder. It becomes complete, and the inflow of mold powder between the mold and the slab becomes uneven.

  On the other hand, if the casting speed is increased, as shown in FIG. 1, the heat supply to the molten steel surface in the mold increases, so that the hatching of the mold powder is promoted and the inflow of the mold powder is stabilized. . FIG. 1 shows the result when a mold powder having a viscosity of 0.8 Pa · s is supplied into a mold having an inner diameter of 191 mm.

  The initial solidified shell formed during continuous casting transforms from the δ phase to the γ phase by cooling from the mold, and causes volume shrinkage due to the difference in volume of the crystal structure. At this time, if the taper amount provided in the mold is not appropriate, the solidified shell is separated from the mold due to volume shrinkage, which causes uneven solidification.

However, the taper amount of the mold cannot be changed every time the steel type is changed.
Therefore, the inventors have found that the initial solidified shell thickness that can be calculated from the following formula 1 using the solidification coefficient and casting speed unique to the continuous casting machine is as the casting speed increases as shown in FIG. Focused on thinning. In addition, the initial solidified shell thickness in the present specification indicates a calculated value at a position of 50 mm from the meniscus in the casting direction.

  The inventors tried to suppress non-uniform solidification due to a decrease in volume shrinkage by increasing the casting speed and reducing the initial solidified shell thickness, and in a high-speed casting region of 2.0 m / min or more. A casting test was conducted. At that time, since there is a risk of breakout due to the thinning of the initial solidified shell accompanying high-speed casting, the upper limit of the casting speed was set to 4.0 m / min.

  As a result, the inventors found that the inside of the mold exhibiting uneven solidification of the slab surface as shown in FIG. 3 by a continuous casting test described later at a high speed of 2.0 m / min to 4.0 m / min. It was found that the temperature fluctuation standard deviation of the thermocouple can be suppressed. The temperature variation standard deviation in FIG. 3 was obtained by measuring the temperature with a thermocouple embedded at a position 200 mm away from the upper end of the mold and downstream of the casting direction, and calculating from the measured value of the steady casting portion.

  In continuous casting, the mold is vibrated (oscillated) along the slab according to the casting speed for the purpose of reducing the friction between the mold and the slab and optimizing the inflow of mold powder.

  By extending the oscillation stroke, the negative strip time rate increases and the amount of mold powder supplied also increases, but at the same time, the deformation time of the collapse of the solidified shell toward the molten steel increases. Therefore, the depth of the oscillation mark is increased (see FIG. 4). The negative strip time rate refers to the ratio of the difference between the casting speed and the average mold lowering speed to the casting speed, as shown in Equation 2 below.

  Since the solidified shell receives resistance in the direction opposite to the drawing direction (casting direction) of the solidified shell due to friction with the mold, if there is a deep oscillation mark on the surface of the solidified shell, the solidified shell is notched due to the notch effect in the valley. Breaks and lateral cracks occur.

  As a countermeasure against lateral cracking, it is effective to reduce the depth of the oscillation mark by shortening the stroke. However, shortening the stroke results in a decrease in the amount of mold powder supplied, and the amount of mold powder supplied is insufficient when casting at high speed. Cause deterioration of surface quality.

  This insufficient supply of mold powder can be dealt with by reducing the frictional force between the mold and the slab by using low-viscosity mold powder. However, when the viscosity of the mold powder is significantly reduced, surface quality such as powder biting is deteriorated due to excessive inflow of the mold powder.

  In order to obtain the optimum viscosity range of this mold powder, the inventors have used a round steel of a molten steel having a carbon concentration of 0.08% by mass or more and 0.18% by mass or less as a material for a seamless steel pipe. It was cast into a mold and continuously cast at a high speed of 2.0 m / min to 4.0 m / min.

As a result, by using a low-viscosity mold powder having a viscosity at 1573 K of 0.2 Pa · s or more and 0.8 Pa · s or less, the friction force between the mold and the slab can be controlled to 60 kN / m ² or less. It was possible to prevent the occurrence of longitudinal cracks and transverse cracks while ensuring proper inflow of mold powder. This is the invention of claim 1.

As described above, the mold powder used in the continuous casting method of the present invention is required to satisfy both the viscosity in the above range and the optimization of heat removal from the mold by stable crystallization in the mold. For this reason, in addition to the above viscosity, the freezing point is 1273 K or more, the mass ratio (basicity) in the mold powder represented by CaO / SiO ₂ is 0.8 to 1.0, and the amount of Na ₂ O is 3.0 mass. %, F concentration is 5.0 mass% or less, MgO content is 8.0 mass% or less, and AL ₂ O ₃ content is 5.0 mass% or less. This is the invention of claim 2. The viscosity was measured from the viscous resistance during vibration by immersing the vibrating piece in molten mold powder held at 1573K.

The inventors conducted an intensive investigation to grasp the frictional force between the mold and the slab and the viscosity of the mold powder in the round section mold.
FIG. 5 shows the frictional force between the mold (inner diameter: 191 mm) and the slab for each viscosity of the mold powder when the casting speed is 2.8 m / min. The frictional force between the mold and the slab shown in FIG. 5 is obtained by measuring the hydraulic pressure of a hydraulic cylinder that generates oscillation vibration, and obtaining the average load per area acting on the mold from the surface area of the mold. It is obtained from the difference in load from the state.

  Molten steel adjusted to the target component and temperature was cast in a mold having an inner diameter of 191 mm to 225 mm through a ladle, tundish, and immersion nozzle. 3. Mold powder which is the component range of the invention of claim 2, wherein the casting speed is in the range of 2.0 m / min to 4.0 m / min and the viscosity is 0.2 Pa · s to 0.8 Pa · s. Was cast, and the slab out of the secondary cooling zone was cut to the pipe making length, and the slab was piped by the Mannesmann method without maintenance.

  The results are shown in Table 1 below and FIG. In addition, the evaluation of the vertical crack and the horizontal crack in the following Table 1 was evaluated as ◯ when there was no crack, Δ when there was a crack that could be removed by maintenance, and × when there was a crack that could not be removed by maintenance.

  As shown in FIG. 3, with the increase in casting speed, the standard deviation of temperature variation of the thermocouple installed on the mold wall surface, which indicates the occurrence of non-uniform solidification of the cast slab, decreased.

  Moreover, as shown in Table 1, invention examples 1-5 did not generate | occur | produce the vertical crack and the horizontal crack in the slab in any case.

On the other hand, in Comparative Examples 1, 4, 7, and 8 where the viscosity is higher than the range specified in the present invention, the friction force between the mold and the slab is 60 kN / m ² or more even in the high-speed casting in which the casting speed is within the range of the present invention. As a result, transverse cracks originated from the oscillation mark. In addition, in Comparative Examples 4 and 8, vertical cracks due to non-uniform solidification also occurred.

On the contrary, in Comparative Example 3 in which the viscosity is lower than the range specified in the present invention, the casting speed is high speed casting within the range of the present invention, and the frictional force between the mold and the slab is 60 kN / m ² or less. Vertical cracks occurred due to excessive inflow of mold powder, and surface quality deteriorated.

  Moreover, for Comparative Examples 1, 2, 5, and 6 that are not high-speed casting within the range of the present invention, the Comparative Examples 2, 5, and 6 in the case of low viscosity in the range specified by the present invention, Even in Comparative Example 1 having a height higher than the range specified in the present invention, vertical cracks due to non-uniform solidification during initial solidification occurred.

  The present invention is not limited to the above example, and it goes without saying that the embodiments may be changed as appropriate within the scope of the technical idea described in each claim.

Claims (2)

In the casting of a round cross-section continuous cast slab having a carbon concentration in the steel of 0.08% by mass or more and 0.18% by mass or less, which is a material of the seamless steel pipe,
Using a mold powder with a viscosity at 1573K of 0.2 Pa · s or more and 0.8 Pa · s or less, continuously casting at a speed of 2.0 m / min or more and 4.0 m / min or less, and between mold and slab The continuous casting method is characterized in that the frictional force is 60 kN / m ² or less. The mold powder has a freezing point of 1273 K or more, a mass ratio (basicity) in the mold powder represented by CaO / SiO ₂ of 0.8 to 1.0, an amount of Na ₂ O of 3.0% by mass or less, F _2. The continuous casting method according to claim 1, wherein the concentration is 5.0 mass% or less, the MgO content is 8.0 mass% or less, and the AL ₂ O ₃ content is 5.0 mass% or less.

Images