JP5131992B2 - Continuous casting method for medium carbon steel - Google Patents

Description

  In the present invention, when steel having a carbon content of 0.08 to 0.16% by mass (hereinafter also simply referred to as “%”), which is generally referred to as medium carbon steel, is continuously cast, The present invention relates to a continuous casting method capable of preventing occurrence and performing stable operation.

  In recent years, there has been a remarkable improvement in productivity in the steel industry. In the continuous casting field, ensuring stable operation during high speed casting has become an important issue. In particular, in the medium carbon steel (subperitectic steel), the solidification of the slab becomes non-uniform due to the large volume shrinkage caused by the transformation from the δ iron phase to the γ iron phase.

  In particular, the above phenomenon during initial solidification in the mold forms irregularities in the solidified shell. Since the thin part of the solidified shell is easily re-dissolved by the discharge flow from the immersion nozzle, there is a high possibility that a breakout from the short side surface (side surface in the width direction of the slab) occurs.

  Breakout of a continuous cast slab is a serious accident that leads to a shutdown of operations, and is a major impediment to stable operation in the continuous casting process, and its occurrence must be avoided by all means.

  In order to prevent the occurrence of breakout due to such remelting of the solidified shell, it is effective to suppress nonuniform solidification of the solidified shell in the mold or to suppress remelting of the solidified shell. It is known.

  As countermeasures, methods have been proposed in which the thickness of the solidified shell is made uniform by improving the properties of the mold flux and the degree of superheated molten steel is adjusted.

  As one of the methods for suppressing the above-mentioned non-uniform solidification, there is a method aimed at suppressing the non-uniform formation of initial solidified shells in the mold by adjusting the physical properties or component composition of the mold flux.

For example, Patent Document 1 contains CaO, SiO ₂ , Na ₂ O, Al ₂ O ₃ , F, C, and 0.7 <CaO / SiO ₂ <1.8 at 1300 ° C. with a viscosity of 1 to 4 poise. The powder for continuous casting whose solidification temperature is 900-1300 degreeC is disclosed. This continuous casting powder has a characteristic that when the powder flows between the mold and the solidified shell, the side in contact with the mold crystallizes the crystal phase.

  Patent Document 2 discloses a continuous casting method in which powders having one or more properties for preventing vertical cracking of a slab, preventing internal defects of a slab, and preventing breakout are separately produced and selectively used during casting. Is disclosed.

Further, in Patent Document 3, the oscillation conditions are adjusted so that the consumption of mold powder having a viscosity at 1300 ° C. of 0.1 to 5.0 poise and CaO / SiO ₂ of 1.2 or more is a predetermined value or less. A method of casting medium carbon steel at a speed of 1.6 m / min or more is disclosed. According to this method, vertical cracking can be prevented and stable high speed casting can be performed.

  Patent Document 4 discloses a high-speed casting method in which the amount of heat removal is controlled to 4.0 to 9.0 MJ / t when medium carbon steel is continuously cast at a casting speed of 2.0 m / min or higher. ing.

  However, in order to prevent the occurrence of breakout due to cracks and fractures starting from the non-uniform portion of the solidified shell, the conditions between the inner surface of the mold and the solidified shell are defined, and the nonuniformity is suppressed from the outer surface of the solidified shell. It is not enough to do. Measures that take into account the remelting phenomenon from the inner surface of the solidified shell due to the discharge flow of molten steel from the immersion nozzle are also required.

Adjustment of the degree of superheat of the molten steel is one of the methods for suppressing the uneven formation of the solidified shell.
For example, in Patent Document 5, the casting speed is changed based on the measurement temperature result of the molten metal in the mold to prevent breakout of the continuous cast slab, and surface defects and internal defects of the slab after casting are disclosed. A continuous casting method for maintaining the above is disclosed.

  However, in the method disclosed in Patent Document 5, since it is necessary to install a measuring device, the equipment cost is increased, and the measuring device is continuously immersed in the molten steel in the mold. May cause operational troubles and defective slab quality.

  Further, Patent Document 5 does not clearly show the relationship between the casting speed and the degree of superheat of the molten steel. For example, in order to prevent breakout, the casting speed may be reduced more than necessary. It can lead to a decline in ability.

As described above, the following problems remain in the conventional continuous casting technique for medium carbon steel.
That is,
(1) In order to prevent the occurrence of breakout due to cracks and breakage starting from the non-uniform portion of the solidified shell, it is not sufficient to prevent unevenness from the outer surface of the solidified shell. It is also necessary to suppress the remelting of the solidified shell due to.

(2) As a response to the above (1), a casting method for adjusting the superheat degree and casting speed of molten steel has been proposed, but the proper operating conditions including the molten steel superheat degree and casting speed are not clear. Moreover, there are concerns about equipment problems, and there is still room for improvement as a casting method for realizing stable operation.

JP 2003-94150 A JP-A-9-192805 JP 2003-170259 A JP 2003-334635 A JP-A-6-170511

  The problem to be solved by the present invention is that in the conventional continuous casting technique for medium carbon steel, the outer surface of the solidified shell is used to prevent the occurrence of breakout due to cracks and breakage starting from the non-uniform portion of the solidified shell. It is a point that the non-uniformity suppression measure from is not sufficient.

  Moreover, in the case of the casting method which adjusts the superheat degree and casting speed of molten steel, it is a point that proper operation conditions including a molten steel superheat degree, a casting speed, etc. are not clear, and the trouble on an installation is also a concern.

  In order to solve the above-mentioned problems, the inventors have secured a solid shell thickness on the mold exit side of a medium carbon steel slab whose thickness exceeds 240 mm, which is likely to break out due to bulging stress at the short side of the slab. The continuous casting method using a mold having a length of 1.1 m or less, which is difficult to perform, has been studied. As a result, the following knowledge was acquired about the continuous casting method which can be stably cast with a curved type or a vertical bending type continuous casting machine.

  When the thermal energy of the discharge flow flowing from the immersion nozzle toward the short side of the mold is Ef, and the energy necessary for remelting the initial solidified shell formed in the mold is Es, Casting so that the solidified shell soundness index A is 190 or more.

  If it does in this way, the breakout of a slab short side part can be prevented. The thermal energy Ef can be obtained by speed × area × density × temperature × specific heat, and the energy Es can be obtained by solidified shell volume × density × heat of fusion.

A method for calculating the solidified shell soundness index A will be described.
Since it is difficult to measure the molten steel temperature and flow velocity in the mold close to 1600 ° C., the flow velocity and temperature distribution of the molten steel in the mold are obtained from a numerical analysis simulation of the thermal flow in the mold.

  FIG. 1 shows a schematic diagram of the molten steel flow in the mold. The thermal energy Ef per unit time for colliding with the solidified shell 2 formed in the mold 1 and re-dissolving the solidified shell 2 is expressed by the following formula 2. In FIG. 1, 3 is an immersion nozzle, 4 is molten steel in the mold 1, and a molten steel flow discharged from the immersion nozzle 3 (hereinafter referred to as “molten steel discharge flow”) is indicated by arrows.

  In Equation 2, u is the velocity (m / sec) of the molten steel flow that collides with the solidified shell, and T is the temperature (° C.) of the molten steel flow. u and T have different values depending on the position, and integration is performed with the area dS of the region finely divided by the numerical analysis simulation. Further, the flow velocity in the direction of colliding with the solidified shell is positive, and u = 0 in the case of the reverse flow velocity.

  In the continuous casting machine, the solidified molten steel is drawn at the speed Vc, so the volume of the solidified shell that flows into the position where the molten steel discharge flow collides per unit time is (solidified shell cross-sectional area) x (casting speed). .

However, since the molten steel discharge flow flows out from the discharge hole 3a of the immersion nozzle 3 as shown in FIG. 1, the cross-sectional area of the solidified shell at the collision position is typically defined as follows. The thickness of the solidified shell calculates the solidified shell thickness at the lower end position of the molten steel discharge flow δ from the following equation 4, the solidified shell width was 2 times the immersion nozzle discharge port width t _0. When 2t ₀ exceeds the mold thickness t, the solidified shell width is defined as the mold thickness t. The thermal energy Es required per unit time required to dissolve the solidified shell is expressed by the following Equation 3.

  When the dimension of A, which is the ratio of Es and Ef defined as described above (= Es / Ef), is confirmed, the dimension of the numerator and the denominator is a dimensionless number having the same dimension as shown in the above equation 1.

In addition, typical values of heat of fusion ΔH, molten steel density ρ _L , ρ _S , and specific heat Cp in Formulas 2 and 3 above are, for example, ΔH = 65 (kcal / kg) when the C content is about 0.1%, Values of ρ _L , ρ _S = 7.8 (kg / m ³ ) and Cp = 0.15 (kcal / kg / K) can be used.

  When the density of the molten steel is unknown, the density difference between the solidified shell and the molten steel is small near the solidification temperature, and therefore the density may be omitted when calculating the solidification shell soundness index A. The solidification rate coefficient k was determined by measuring the thickness distribution of the solidified shell remaining in the mold during past breakout troubles.

The continuous casting method of medium carbon steel of the present invention is based on the above knowledge,
In order to completely prevent remelting breakout caused by remelting of the solidified shell at the short side of the slab, which is a concern during the casting of medium carbon steel,
A medium carbon steel having a carbon content of 0.08 to 0.16% by mass is cast using a mold having a thickness corresponding to the slab thickness of more than 240 mm and a length in the casting direction of 1.1 m or less. When
Ratio of CaO mass% to SiO ₂ mass%, CaO / SiO ₂ is 1.2 to 2.5, mold flux having a solidification temperature of 1200 to 1280 ° C., and an immersion nozzle whose discharge holes are directed downward from the horizontal direction Use
A static magnetic field applying device is disposed with the center position in the casting direction of the magnetic pole as a position below the discharge hole, and a static magnetic field having a magnetic field strength of 0.15 T or more applied to the molten steel at the center in the mold thickness direction, The most important feature is that casting is performed under the condition that the solidification shell soundness index A of the slab defined by Equation 1, Equation 2, and Equation 3 is 190 or more.

  In the present invention, since the medium carbon steel is cast under the optimum conditions including the properties of the mold flux, the flow control of the molten steel by electromagnetic force, and the casting speed, the remeltability due to the remelting of the solidified shell at the short side of the slab Breakout can be completely prevented, and continuous casting of medium carbon steel can be performed stably.

  Therefore, the continuous casting method of the present invention can greatly contribute to the continuous casting technology of medium carbon steel in terms of both improvement in productivity and stable slab quality due to stable operation of the process.

It is a schematic diagram explaining the method of calculating the solidification shell soundness index A, (a) is a vertical sectional view, (b) is a horizontal sectional view. FIG. 5 is a diagram showing a relationship between a casting width and a solidified shell soundness index A under each casting condition. It is a figure explaining the investigation method of the solidification property in a slab short side part. It is the microscope picture which showed an example of the observed white line. It is the figure which showed the relationship between the casting width and white line average thickness in each casting condition.

  In the present invention, the value of the solidification shell soundness index A (= Es / Ef) of the slab is set to 190 or more for the purpose of completely preventing the remeltability breakout which is likely to occur during the casting of the medium carbon steel. That was realized.

Hereinafter, the continuous casting method for medium carbon steel of the present invention will be described.
As described above, the present invention is a continuous casting machine equipped with a mold having a carbon content of 0.08 to 0.16% and a mold having a thickness of over 240 mm and a length of 1.1 m or less. In this method, casting is performed using an immersion nozzle in which the discharge hole faces downward from the horizontal direction.

In the present invention, a mold flux having a basicity (CaO / SiO ₂ ) of 1.2 to 2.5 and a solidification temperature of 1200 to 1280 ° C. is used. A static magnetic field having a magnetic field strength of 0.15 T or more at the central portion in the mold thickness direction is applied to the molten steel in the mold using a magnetic field application device installed below the discharge hole of the immersion nozzle.

Under the above conditions, casting is performed so that the solidified shell soundness index A of the slab represented by Equation 1 is 190 or more.
Hereinafter, the continuous casting method for medium carbon steel of the present invention will be described in more detail.

(1) Mold thickness and length The present invention uses a continuous casting machine provided with a mold having a mold thickness (slab thickness) exceeding 240 mm and a mold length of 1.1 m or less. It is applied when casting medium carbon steel where the slab is likely to break out due to the bulging stress of the short side.

  The main types of continuous casting machines are a vertical type, a vertical bending type, a curved type, and the like, but the present invention can be applied regardless of the type.

  When the mold length exceeds 1.1 m, the passage time of the slab in the mold can be lengthened, and the thickness of the solidified shell in the slab can be sufficiently secured on the mold exit side, so that a breakout occurs. The risk of doing so is greatly reduced.

  Therefore, the inventors have developed an optimum continuous casting method of medium carbon steel that can prevent breakout when a continuous casting machine having a mold length of 1.1 m or less that is likely to cause breakout is used. Repeated research.

(2) Properties of mold flux The proper range of the basicity of the mold flux used in the present invention is 1.2 to 2.5, and the proper range of the solidification temperature is 1200 to 1280 ° C.

  When the basicity is less than 1.2 or the solidification temperature is less than 1200 ° C., the amount of crystal phase deposited in the solidification process of the mold flux is small, and the proportion of the glass phase is high. Therefore, the heat transfer coefficient between the mold and the solidified shell is increased, the amount of heat removed from the slab is increased, and slow cooling cannot be achieved.

  On the other hand, when the basicity exceeds 2.5 or the solidification temperature exceeds 1280 ° C., the lubrication between the mold and the solidified shell is hindered, and a constraining breakout due to seizure of the solidified shell to the mold occurs. This is because the possibility of occurrence increases.

(3) Static magnetic field flow control conditions The means to control molten steel flow using a static magnetic field application device such as a static magnetic field flow control device in a mold lowers the discharge flow velocity from the immersion nozzle, and is effective in preventing breakout. Means.

  The inventors have confirmed that the effect is more exhibited when a static magnetic field having a magnetic field strength of 0.15 T or more is applied at the central portion in the thickness direction of the mold.

  In addition, when casting is performed using an immersion nozzle having a downward discharge hole, a static magnetic field application device is provided with a position where the maximum value of the magnetic field is at the immersion nozzle in order to effectively apply a braking force to the molten steel discharge flow. It is necessary to install so that it may be located below.

  The inventors have confirmed that it is more effective to dispose the static magnetic field applying device at the position of the passage path of the molten steel discharge flow.

  In the present invention, an immersion nozzle having a downward discharge hole is used when a horizontal or upward discharge hole is used, and mold powder is caught in the solidified shell, resulting in a breakout. This is because of the circumstances. Moreover, it is because a quality defect occurs even if it does not lead to a breakout.

(4) Appropriate range of solidified shell soundness index A The inventors investigated the casting performance of slabs where breakout actually occurred (no static magnetic field applied), and based on the results of this investigation, the solidified shell soundness index A Was calculated. In addition, simulations were performed for a casting speed of 1.6 m / min and 1.7 m / min (both with a static magnetic field applied) to obtain a solidified shell soundness index A. These results are shown in FIG.

  As a result of calculating the solidified shell soundness index A based on the actual casting of the slab where breakout actually occurred, a maximum value of 186 was obtained as the index (see FIG. 2). Therefore, in the present invention, the appropriate range of the solidified shell soundness index A is set to 190 or more as a condition that breakout does not occur.

  Further, according to the present invention, the molten steel discharge flow rate from the submerged nozzle increases, so in the wide and high casting speed regions where it has been considered difficult to improve the casting speed, the solidified shell soundness index A is beyond a certain casting width. It was confirmed from the result of simulation that the value of This suggested the possibility of improving the casting speed in the wide region (see FIG. 2). In this simulation result, there is a minimum point of the solidified shell soundness index A in the vicinity of the width of 1450 mm.

  In order to confirm the validity of the present invention, the inventors conducted the following continuous casting test, investigated the longitudinal section of the obtained slab, and evaluated the result.

  Using a vertical bending type continuous casting machine having a machine length of 43 m equipped with a mold having a thickness of 270 mm and a length of 0.9 m, the main component composition of the steel was mass%, and C: 0.10 to 0.11 %, And Mn: 1.0% of medium carbon steel was cast. The casting speed was varied between 1.3 and 1.8 m / min.

  For casting, a mold flux having a basicity of 1.8 and a solidification temperature of 1235 ° C. was used, and an immersion nozzle having discharge holes directed 30 ° below the horizontal direction was used.

  In addition, a static magnetic field flow control device is installed outside the long side of the mold and below the discharge hole of the immersion nozzle, and the flow velocity of the molten steel in the mold is adjusted by adjusting the strength of the static magnetic field applied to the molten steel in the mold. Controlled.

  The test conditions and test results are shown in Table 1 below.

  Test Nos. 1 to 4 are tests for invention examples that satisfy the conditions defined in the present invention. Test numbers 5 to 8 are tests for comparative examples that do not satisfy the conditions defined in the present invention.

In the inventive examples of Test Nos. 1 to 4, breakage of the slab did not occur, a stable casting operation of medium carbon steel could be achieved, and extremely good results were shown in terms of the stable casting operation.
On the other hand, the comparative examples of Test Nos. 5 to 8 that do not satisfy the conditions defined in the present invention all had breakouts with a certain probability, and were inferior to the inventive examples.

Next, in order to confirm the soundness of the initial solidified shell, the slab was investigated.
FIG. 3 is a diagram showing a method for investigating the solidification property in the short side portion of the slab.

  As shown in FIG. 3, a longitudinal section sample 6 at a half thickness position of the short side portion of the slab 5 is cut out from the short side portion of the slab 5, and dendrite is formed on the surface indicated by the hatched portion in FIG. 3. Etched. At this position, a white line was confirmed along the casting direction within a range of several mm to 20 mm from the short side surface of the slab 5. An example of the observation result is shown in FIG.

  This white line is observed when the molten steel flow in the mold causes the segregation of concentrated segregated components between the dendrite branches on the front of the solidified shell to be washed away to form negative segregation. It becomes an important clue to grasp.

  Using the observed average thickness of the white line as the initial solidified shell thickness, the relationship with the casting width was investigated. The average thickness of the white line is an average value of values obtained by measuring several distances from the slab surface to the white line as a representative. The result is shown in FIG.

  Similar to the simulation result of the solidified shell soundness index A shown in FIG. 1, it was confirmed that there was a minimum value in the vicinity of the width of 1450 mm. This is considered to be because the solidification shell remelting due to the discharge flow from the immersion nozzle is maximized in the vicinity of 1450 mm. In other words, it was confirmed that the casting speed can be improved in a wide region, which has been considered difficult in the past.

  The solidification shell soundness index A is minimized in the vicinity of the 1450 mm width because, in the region of 1450 mm or less, the flow rate of molten steel from the immersion nozzle to the short side increases as the casting width increases, and the remelting amount of the solidified shell increases. It is.

  Further, in the region of 1450 mm or more, the molten steel flow velocity from the immersion nozzle to the short side increases as the casting width increases, but the molten steel deceleration effect by the electromagnetic brake is exerted and sufficiently slowed down until it collides with the short side. This is because the amount of shell remelting decreases as the casting width increases.

  According to the present invention, since the medium carbon steel is cast under the optimum conditions including the properties of the mold flux, the flow control of the molten steel by electromagnetic force, the casting speed, etc., the re-melting caused by the remelting of the solidified shell at the short side of the slab. Soluble breakout is completely prevented, and medium carbon steel can be stably cast continuously.

  Therefore, the continuous casting method of the present invention can be widely applied to a continuous casting process of medium carbon steel, which requires stable operation of the process and improvement of slab quality. In addition, it is possible to improve the casting speed in a wide region that has been considered difficult in the past, which contributes to the improvement of efficiency.

  Needless to say, the present invention is not limited to the above-described examples, and the embodiments may be appropriately changed within the scope of the technical idea described in the claims.

1 Mold 2 Solidified Shell 3 Immersion Nozzle 3a Discharge Hole 4 Molten Steel

Claims (1)

A medium carbon steel having a carbon content of 0.08 to 0.16% by mass is cast using a mold having a thickness corresponding to the slab thickness of more than 240 mm and a length in the casting direction of 1.1 m or less. When
A mold flux having a CaO / SiO ₂ ratio of CaO to SiO ₂ mass% of 1.2 to 2.5, a solidification temperature of 1200 to 1280 ° C., and a submerged nozzle whose discharge holes are directed downward from the horizontal direction. use,
A static magnetic field application device is arranged with the center position in the casting direction of the magnetic pole located below the discharge hole, and a static magnetic field having a magnetic field strength of 0.15 T or more at the center in the mold thickness direction is applied to the molten steel. A continuous casting method of medium carbon steel, characterized in that casting is performed under a condition that a solidification shell soundness index A of a slab defined by Equations 1, 2 and 3 is 190 or more.