JP2007290007A - Method for continuous casting of high aluminum steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for continuous casting, which enables the manufacture of a cast ingot having excellent surface quality by preventing the generation of shrinkages and cracks even when continuously casting high aluminum steel containing 0.1% or more of aluminum. <P>SOLUTION: When high aluminum molten steel having required chemical components is manufactured by continuous casting using mold powder, the continuous casting is carried out by using the mold powder, which contains 35 to 55% of T-CaO, 10 to 30% of SiO<SB>2</SB>, 4.0% or less (except 0%) of Al<SB>2</SB>O<SB>3</SB>, 0.2 to 1.0% of MgO, 7 to 13% of Li<SB>2</SB>O, 7 to 13% of F, 10.5 to 14% of C, and unavoidable impurities, and satisfies the following conditions: (4) 1.6≤[T-CaO]/[SiO<SB>2</SB>]≤5; and (5) 0.2≤[Li<SB>2</SB>O]/[SiO<SB>2</SB>]≤1.1, and further by controlling the conditions, such as a fluctuating speed of the level of molten metal in a casting mold, a discharge angle of the molten metal in the direction of the width of the casting mold, a magnitude of an amplitude, a negative strip time tN determined by a specified relational expression. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing high Al steel from molten steel having a dissolved Al content of 0.1% by mass or more by a continuous casting method, and in particular, a continuous casting method for producing a slab having good surface quality. It is about.

In continuous casting of steel, mold powder is added onto the molten steel surface in the mold. This melts and melts with the heat from the molten steel, forms a molten slag layer, and then flows into the gap between the mold and the solidified shell and is consumed. The mold powder is mainly composed of CaO and SiO _2, and Al ₂ O ₃ , MgO, Na ₂ O, F, Li ₂ O, etc. to adjust the viscosity and solidification temperature of the molten slag, and the slag melting rate C or the like is added to adjust the value. The main roles of this mold powder include (a) ensuring lubricity between the mold and the solidified shell, and (b) slow cooling by suppressing the heat removal rate from the solidified shell to the mold. .

  First, in order to ensure the lubricity between the mold and the solidified shell mentioned in (a) above, the viscosity is appropriately adjusted so that the molten slag obtained from the mold powder flows into the gap between the mold and the solidified shell. It is important to set. In general, the higher the casting speed, the lower the viscosity of the molten slag in order to ensure the inflow.

In addition, the slow cooling (b) is important because it directly affects the surface quality of the resulting slab. For slow cooling, it is effective to crystallize the slag film obtained from the mold powder, particularly on the mold side surface. This is because when crystals crystallize on the mold side surface of the slag film, irregularities are formed between the film and the mold, and the air layer contained in the irregularities acts as a heat insulating layer. As this crystal, caspidine (3CaO · 2SiO ₂ · CaF ₂ ) is generally used.

However, when producing a slab from a molten steel having a dissolved Al content of 0.1% or more by a continuous casting method, it becomes difficult to ensure the lubricity of (a) and to slowly cool (a). Because, in such continuous casting of high Al steel, the following reaction formula (7):
4Al + 3SiO ₂ → 2Al ₂ O ₃ + 3Si (7)
This is because SiO ₂ is consumed by the chemical reaction represented by Therefore, the basicity [CaO] / [SiO ₂ ] in the molten slag is increased, and the solidification temperature is remarkably increased. And a hard sintered material called a slag bear is made on the mold wall surface, and the inflow of molten slag is hindered. As a result, the lubricity is impaired, the solidified shell and the mold are seized, and breakout occurs.

  Further, the reaction of the above formula (7) causes the molten slag to undergo composition fluctuations, so that it is difficult to stably generate cuspidyne. Thus, in the continuous casting of high Al steel, composition fluctuations due to the reaction of the above formula (7) occur, and it is difficult to stably produce a slab having excellent surface quality.

  Therefore, Patent Document 1 describes a composition with low basicity and high viscosity that makes it difficult for crystals to crystallize in order to produce a slab excellent in surface quality even in continuous casting of high Al steel, in particular to suppress the formation of slag bear. And mold powders having physical properties are proposed (claims, paragraphs [0004] and [0007]).

Patent Document 2 discloses a mold powder containing two or more kinds of oxides of elements belonging to Group IA of the Periodic Table in order to achieve a slow cooling by generating a composite crystal different from caspidine. And paragraph [0013]). In the invention of Patent Document 2, LiCa ₂ FSiO ₄ and NaCa ₂ FSiO ₄ are disclosed as assumed composite crystals, but the oxides of elements belonging to group IA of the periodic table used in the examples are disclosed. Among them, since the amount of Na ₂ O is the largest, it is considered that NaCa ₂ FSiO ₄ is assumed as the main composite crystal (paragraphs [0020] and [0030]). The invention of Patent Document 2 is characterized by containing two or more kinds of oxides of elements belonging to Group IA of the periodic table because the purpose is to reduce the softening temperature of the mold powder (paragraph [0024] ).

In Patent Document 3, in the continuous casting of high aluminum steel, when the Al ₂ O ₃ content increases due to the reaction between Al and SiO ₂ [the above formula (7)], the solidification temperature and viscosity increase, In order to prevent the occurrence of out and deterioration of the surface quality of the slab, the contents of CaO, SiO ₂ , Li ₂ O, F, Na ₂ O, K ₂ O and Al ₂ O ₃ satisfy the prescribed formula and melt A mold powder has been proposed having a composition such that crystals of caspidyne crystallize in a film in which the layer has solidified (claims, paragraphs [0011] and [0017]).

However, even in a high Al steel, particularly in steels having a composition range in which the peritectic reaction or the amount of δ / γ transformation is large, even if the mold powder as described above is used, the depletion accompanying transformation shrinkage on the surface of the resulting slab ( There is a problem that unevenness and cracks are likely to occur. Such a steel type is called hypoperitectic steel, and its chemical composition range is generally determined by the C content [C] based on the Fe-C or Fe-Fe ₂ C ₃ binary equilibrium diagram. Is done. The range is approximately C: 0.09 to 0.18%.

However, in the case of alloy steel, the phase diagram itself changes due to the influence of the additive element, and both the maximum solid solution C concentration and the peritectic point of the δ phase move, so the composition range of the subperitectic steel with only the C content. There is a circumstance that cannot be uniformly defined. For this reason, even for high Al steels, especially for compositions with a large peritectic reaction or δ / γ transformation, the effects of alloying elements such as Si, Mn, Al, Ni, Cr and Mo are taken into account, and the equilibrium thermodynamics are considered. It is known that the following formulas (1) to (3) are defined based on the calculation (Non-patent Document 1). In addition, the hypoperitectic steels subject to these formulas each have a basic component content of Si, Mn, Al, Ni, Cr and Mo of 4.0% or less (excluding 0%). The Al content is 0.1 to 3.0%.
f1-0.10 ≦ [C] ≦ f2 + 0.05 (1)
f1 = 0.0828 [Si] −0.0195 [Mn] +0.07398 [Al] −0.04614 [Ni] +0.02447 [Cr] +0.01851 [Mo] +0.090
... (2)
f2 = 0.2187 [Si] −0.03291 [Mn] +0.2017 [Al] −0.06715 [Ni] +0.04776 [Cr] +0.04601 [Mo] +0.173
... (3)
[Wherein, [Si], [Mn], [Al], [Ni], [Cr] and [Mo] indicate the contents (mass%) of Si, Mn, Ni, Cr and Mo, respectively. ]
JP 2003-53496 A (claims, paragraphs [0004] and [0007]) JP-A-10-216907 (claims, paragraphs [0013], [0020], [0024] and [0030]) JP 2002-346708 A (Claims, paragraphs [0011] and [0017]) "Solidification" -373 (1985) Japan Society for the Promotion of Science Steelmaking 19th Committee 3rd Subcommittee Solidification Phenomenon Council 10670

In the steel type in which slab surface cracks are likely to occur, such as hypoperitectic steels defined by the above formulas (1) to (3), in order to suppress cracking, the heat removal rate is lowered and the steel is slowly cooled. This is very important. Therefore, conventionally, by generally crystallizing caspodyne (3CaO · 2SiO ₂ · CaF ₂ ) in a slag film obtained from mold powder, by forming irregularities (heat insulation layer by air) on the mold surface, Slow cooling was achieved. However, in the case of high Al steel, it is difficult to stably produce cuspidyne due to composition variation.

  Moreover, in order to manufacture the above steel types while maintaining good surface quality, it is important to use an appropriate mold powder, but it is also necessary to appropriately control the conditions in continuous casting. However, in reality, it cannot be said that optimum casting conditions in the case of continuously casting high Al steel having an Al content of 0.1% or more have been established.

  The present invention has been made by paying attention to the above-described circumstances, and the purpose thereof is to produce a dent and a high Al steel having an Al content of 0.1% or more by continuous casting. An object of the present invention is to provide a continuous casting method capable of producing a slab having excellent surface quality by preventing occurrence of cracking of the slab.

The continuous casting method of the present invention that has achieved the above object has an Al content of 0.1 to 3.0% (meaning mass%, hereinafter the same), Si, Mn, Ni, Cr, and Mo. When the molten steel containing 4.0% or less (excluding 0%) and the C content [C] satisfying the relations of the following formulas (1) to (3) is continuously cast using mold powder: ,
f1-0.10 ≦ [C] ≦ f2 + 0.05 (1)
f1 = 0.0828 [Si] −0.0195 [Mn] +0.07398 [Al] −0.04614 [Ni] +0.02447 [Cr] +0.01851 [Mo] +0.090
... (2)
f2 = 0.2187 [Si] −0.03291 [Mn] +0.2017 [Al] −0.06715 [Ni] +0.04776 [Cr] +0.04601 [Mo] +0.173
... (3)
[Wherein, [Si], [Mn], [Al], [Ni], [Cr] and [Mo] indicate the contents (mass%) of Si, Mn, Ni, Cr and Mo, respectively. ]
Examples mold powder, T-CaO: 35~55%, SiO 2: 10~30%, Al 2 O 3: 4.0% or less (not including 0%), MgO: 0.2~1.0% Li ₂ O: 7-13%, F: 7-13%, C: 10.5-14%, and inevitable impurities, satisfying the following formulas (4) and (5),
1.6 ≦ [T-CaO] / [SiO ₂ ] ≦ 5 (4)
0.2 ≦ [Li ₂ O] / [SiO ₂ ] ≦ 1.1 (5)
[In the formula, [T-CaO], [SiO ₂ ] and [Li ₂ O] represent the contents (mass%) of T-CaO, SiO ₂ and Li ₂ O in the mold powder, respectively. ]
The molten steel surface level fluctuation speed in the mold is set to 14 mm / second or less, the molten steel is discharged in the mold width direction, and the discharge angle is 0 ° or more and 55 ° or less downward with respect to the horizontal. It has a gist in that the stroke is set to more than 2 mm and not more than 8 mm, and operation is performed while applying mold vibration such that the negative strip time tN defined by the following formula (6) is not more than 0.28 seconds.
tN = (1 / π · f) cos ⁻¹ (Vc / π · f · s) (6)
[Where f: mold frequency (Hz), s: distance between top and bottom mold stop points (mm) during mold vibration, Vc: slab drawing speed (mm / sec), respectively]

  In the method of the present invention, it is preferable to operate while performing in-mold electromagnetic stirring at a magnetic flux density of 300 to 1200 gauss.

  According to the present invention, by making the composition of the mold powder appropriate and controlling the continuous casting conditions appropriately, the slab surface can be prevented from being dented or cracked, and a high Al steel having excellent surface quality can be produced. I was able to.

  The inventors of the present invention have made extensive studies to solve the above problems. As a result, the inventors have found that the above object can be achieved brilliantly by making the composition of the mold powder appropriate and controlling the continuous casting conditions appropriately, thereby completing the present invention. First, the mold powder used in the present invention will be described.

  In the conventionally proposed mold powder, when applied to high Al steel, it is difficult to stably generate cuspidyne due to composition variation. Accordingly, the present inventors have studied to crystallize crystals instead of cuspidine in a slag film.

However, when crystals such as dicalcium silicate (2CaO · SiO ₂ ), mayenite (12CaO · 7Al ₂ O ₃ ) and gelenite (3CaO · 2SiO ₂ · Al ₂ O ₃ ) are crystallized for slow cooling, the template In addition to the problem that the temperature fluctuation of the copper plate becomes large, it is not effective in preventing dents and cracks in the slab. Since these crystallize non-uniformly in the slag film as coarse crystals, non-uniform irregularities (air layers) are formed on the surface on the mold side, resulting in variations in the heat removal rate. Then, since the thickness of the solidified shell becomes non-uniform, it is considered that dents and cracks are generated on the slab surface due to transformation shrinkage.

As a result of intensive studies, it was found that unevenness and cracking of the slab can be effectively prevented by crystallizing LiAlO ₂ in the slag film instead of caspidine. The exact mechanism by which LiAlO ₂ is crystallized to prevent unevenness and cracking of the slab is unknown, but can be estimated as follows.

Since LiAlO ₂ is uniformly crystallized as fine crystals on the surface of the slag film mold, a uniform air layer is formed. As a result, uniform heat removal is achieved, fluctuations in the mold copper plate temperature are small, and cracking is prevented by slow cooling, and a solidified shell of uniform thickness is formed, resulting in transformation shrinkage. It is thought that dents and cracks in the slab are also suppressed. However, the present invention is not limited to such an estimation mechanism.

In the mold powder used in the present invention, Li ₂ O from the mold powder is reacted with Al ₂ O ₃ formed by the reaction of SiO ₂ from the mold powder and Al from the molten steel to crystallize LiAlO ₂ . Is intended to be That is, this mold powder crystallizes LiAlO ₂ by utilizing the reaction between SiO ₂ and Al in the above formula (7), which causes composition fluctuations, in continuous casting of high Al steel. In order to crystallize LiAlO ₂ , this mold powder is used for the amount of each component, particularly T-CaO, SiO ₂ and Li ₂ O, and their mass ratio [Li ₂ O] / [SiO ₂ ] and The basicity [T-CaO] / [SiO ₂ ] needs to be adjusted to an appropriate range.

In addition, from the viewpoint of ensuring lubricity by adjusting the solidification temperature of molten slag (mold powder) to an appropriate range, the characteristics of the mold powder used in the present invention are also adjusted to an appropriate range. It is. Hereinafter, each component amount, basicity [T-CaO] / [SiO ₂ ] and mass ratio [Li ₂ O] / [SiO ₂ ] in the mold powder of the present invention will be described.

[T-CaO: 35 to 55%]
In the mold powder used in the present invention, “T-CaO” represents the CaO amount (% by mass) when all the Ca contained in the mold powder is converted to CaO. The amount of T-CaO in the mold powder is 35% or more, preferably 38% or more, more preferably 40% or more, 55% or less, preferably 50% or less, more preferably 48% or less. When the amount of T-CaO is less than 35%, SiO ₂ and the amount relatively increase. As a result, the amount of Al ₂ O ₃ increases due to the reaction of formula (7), and LiAlO ₂ is easily crystallized. This makes it difficult to crystallize LiAlO ₂ . Also, gehlenite (3CaO.2SiO ₂ .Al ₂ O ₃ ) is easily generated. On the contrary, even if the amount of T-CaO exceeds 55%, the amount of Li ₂ O and SiO ₂ is relatively decreased, and as a result, the amount of Al ₂ O ₃ is decreased due to the reaction of formula (7). An amount of LiAlO ₂ cannot be secured. Also, the solidification temperature of the molten slag becomes too high.

[SiO ₂ : 10 to 30%]
The amount of SiO ₂ is 10% or more, preferably 15% or more, 30% or less, preferably 28% or less, more preferably 25% or less. If the amount of SiO ₂ that is a glass-forming element is less than 10%, crystals are likely to develop, so that coarse crystals are formed, and uneven irregularities are formed on the mold surface side of the slag film. In addition, the solidification temperature rises, the lubricity is impaired, and slag bear is easily generated. On the contrary, when the amount of SiO ₂ exceeds 30%, more gehlenite (3CaO · 2SiO ₂ · Al ₂ O ₃ ) and dicalcium silicate (2CaO · SiO ₂ ) are crystallized than LiAlO ₂ .

[Al ₂ O ₃ : 4.0% or less (excluding 0%)]
In order to prevent the solidification temperature and viscosity of the molten slag from increasing, the amount of Al ₂ O ₃ is 4.0% or less, preferably 3% or less, more preferably 2% or less. However, since Al ₂ O ₃ is mixed as an inevitable impurity in mold powder production, it is industrially difficult to reduce this amount to 0%.

[MgO: 0.2 to 1.0%]
The amount of MgO is 0.2% or more, preferably 0.3% or more, more preferably 0.4% or more, 1.0% or less, preferably 0.9% or less, more preferably 0.8%. It is as follows. MgO acts as a nucleus for crystallizing in the slag film. Therefore, if the MgO amount exceeds 1.0%, the number of nuclei increases so that the crystallization of crystals cannot be controlled appropriately. In particular, depending on the mold powder composition, dicalcium silicate (2CaO.SiO ₂ ) or mayenite ( 12CaO · 7Al ₂ O ₃ ) may crystallize preferentially. On the other hand, if the amount of MgO is less than 0.2%, the number of crystal nuclei is too small, so that the crystal does not crystallize sufficiently until the low temperature equilibrium temperature is reached. It is difficult to perform slow cooling. Also, when the equilibrium temperature is reached, coarse crystals are crystallized at a time, resulting in variations in the heat removal rate.

[Li ₂ O: 7 to 13%]
The amount of Li ₂ O is 7% or more, preferably 7.5% or more, more preferably 8.0% or more, and 13% or less, preferably 12% or less, more preferably 11% or less. When the amount of Li ₂ O is less than 7%, it is difficult to crystallize a sufficient amount of LiAlO ₂ , and the solidification temperature and viscosity of the molten slag increase, and lubricity may not be ensured. On the other hand, even if the Li ₂ O content exceeds 13%, the LiAlO ₂ may deviate from the optimum range for crystallization, the crystallization amount may decrease, and slow cooling may not be achieved. Further, the viscosity of the molten powder is greatly reduced, and the molten slag may locally flow excessively or pulsate, which may adversely affect the stable operation of continuous casting.

[F: 7-13%]
The amount of F is 7% or more, preferably 7.5% or more, more preferably 8.0% or more, and 13% or less, preferably 12% or less, more preferably 11% or less. If the F amount is less than 7%, the viscosity of the molten slag increases, and lubricity may not be ensured. On the other hand, F has an effect of suppressing crystallization of LiAlO ₂ , and particularly when the F amount exceeds 13%, the crystallization amount of LiAlO ₂ is rapidly reduced.

[C: 10.5-14%]
This C amount represents the total amount of C contained in the mold powder. That is, the C amount, such as added as a raw material of the mold powder, carbon content alone (the free C content), for example, the amount of carbon compound such as Li ₂ CO ₃ added as Li ₂ O feed Represents the sum of The amount of C in the mold powder is 10.5% or more, preferably 11.0% or more, more preferably 11.5% or more, 14% or less, preferably 13.5% or less, more preferably 13%. It is as follows. If the amount of C is less than 10.5%, the melting rate of the mold powder becomes too high, resulting in excessive inflow and non-uniform inflow. As a result, the vertical crack of the slab is likely to occur. On the other hand, if the amount of C exceeds 14%, the melting rate becomes too low to ensure a sufficient thickness of the slag film. As a result, slag film breakage occurs when the molten metal level in the mold inevitably occurs in industrial production, causing seizure and rapid cooling due to direct contact of the molten steel with the mold. Surface quality is degraded.

The mold powder used in the present invention comprises the above components and inevitable impurities. In addition, Na ₂ O and K ₂ O are added to general mold powder in order to reduce the viscosity and the solidification temperature, but the mold powder of the present invention is also characterized by not containing these. . Because in the continuous casting of high aluminum steel planned by the present invention, the following reaction formulas (8) and (9):
2Al + 3Na ₂ O → Al ₂ O ₃ + 6Na (8)
2Al + 3K ₂ O → 2Al ₂ O ₃ + 6K (9)
Therefore, Na ₂ O and K ₂ O are consumed, and these effects are not fully exhibited, and more Al ₂ O ₃ is generated than expected, and the solidification temperature of the molten slag, etc. It is because it adversely affects In addition, when Na ₂ O is present, Na—Al—O crystals may be crystallized non-uniformly, and unevenness in the slag film (air layer) may occur.

[1.6 ≦ [T-CaO] / [SiO ₂ ] ≦ 5]
The basicity [T-CaO] / [SiO ₂ ] is 1.6 or more, preferably 1.8 or more, more preferably 2.0 or more, 5 or less, preferably 4 or less, more preferably 3 or less. is there. When the basicity is less than 1.6, the amount of SiO ₂ is relatively increased, and LiAlO ₂ deviates from the composition range in which LiAlO ₂ is easily crystallized, and LiAlO ₂ is hardly crystallized. Also, gehlenite (3CaO.2SiO ₂ .Al ₂ O ₃ ) is easily generated. Conversely, even if the basicity exceeds 5, the amount of SiO ₂ is relatively decreased, and the amounts of Al ₂ O ₃ and LiAlO ₂ are decreased accordingly. Moreover, mayenite (12CaO · 7Al ₂ O ₃ ) develops excessively as the amount of SiO ₂ that is a glass forming component decreases. Furthermore, the solidification temperature is increased, which can adversely affect the lubricity.

[0.2 ≦ [Li ₂ O] / [SiO ₂ ] ≦ 1.1]
The mass ratio [Li ₂ O] / [SiO ₂ ] is 0.2 or more, preferably 0.3 or more, more preferably 0.4 or more, 1.1 or less, preferably 1.0 or less, more preferably Is 0.9 or less. When [Li ₂ O] / [SiO ₂ ] is less than 0.2, the amount of Li ₂ O becomes insufficient, so that LiAlO ₂ is not sufficiently generated. On the other hand, even if [Li ₂ O] / [SiO ₂ ] exceeds 1.1, it is out of the optimum range for LiAlO ₂ crystallization, so that LiAlO ₂ is difficult to crystallize.

  The solidification temperature of the mold powder (molten slag) of the present invention is preferably 950 to 1200 ° C, more preferably 1000 to 1150 ° C. If the solidification temperature is less than 950 ° C., the crystals are difficult to crystallize, and the effect of slow cooling may not be fully exhibited. On the other hand, when the solidification temperature exceeds 1200 ° C., slag bear is generated, and breakout or cracking of the slab surface may occur due to non-uniform inflow by the slag bear.

The Al content of the continuously cast steel (Al content in the molten steel) is 0.1% or more, preferably 0.3% or more, more preferably 0.00%, in order to fully exhibit the effect of the mold powder. It is 5% or more, 2.5% or less, preferably 2.0% or less, and more preferably 1.7% or less. Here, the amount of dissolved Al in the steel represents the amount of Al dissolved in the molten steel used for continuous casting, and this amount includes the amount of Al deposited as Al ₂ O ₃ (ie, not dissolved). Is not included.

  Moreover, in the hypoperitectic steel targeted by the method of the present invention, the basic component contents of Si, Mn, Al, Ni, Cr and Mo are 4.0% or less (excluding 0%), respectively. This satisfies the above formulas (1) to (3). In addition to the above components, it is substantially made of iron, but may contain unavoidable impurities such as S, P, Cu, etc., and a small amount of acceptable components (for example, Ti, Nb of 0.2% or less). May also be included.

  In the present invention, in order to achieve the above object, it is necessary to appropriately control the continuous casting conditions. Next, these conditions will be described.

[Velocity fluctuation level in mold: 14 mm / sec or less]
It is necessary to control the fluctuation level of the molten metal level in the mold within an appropriate range in order to maintain the stability of the mold powder melting pool. When this fluctuation speed exceeds 14 mm / sec, the mold powder molten pool is cut and the molten steel comes into direct contact with the mold copper plate, and the mold heat removal rate becomes non-uniform. As a result, the temperature fluctuation of the mold thermocouple is increased, and depletion and cracking are likely to occur. The fluctuation speed is preferably 10 mm / second or less. In order to control the fluctuation level of the molten metal surface level in the mold within the above range, the nozzle clogging prevention Ar gas flow rate may be optimized according to the casting conditions, and the discharge hole shape of the immersion nozzle may be optimized.

[Using molten steel in the mold width direction and using an immersion nozzle whose discharge angle is 0 ° or more and 55 ° or less downward with respect to the horizontal]
The immersion nozzle used in the mold requires that the molten steel discharge direction be the width direction of the mold. When the molten steel discharge direction is the thickness direction, the molten steel discharge flow hits a specific part of the mold wide side solidified shell, and the heat removal status of the corresponding part is different from other parts. It is easy to become. At this time, the discharge angle (discharge direction angle) of the immersion nozzle is preferably 0 ° or more and 55 ° or less downward with respect to the horizontal direction. When the discharge angle of the immersion nozzle is less than 0 ° (that is, upward), the discharged molten steel goes directly to the interface between the molten mold powder and the molten steel bath surface, so that the interface becomes hot and stirred, and the dissolved Al in the molten steel The reaction of the above formula (7) that occurs with SiO ₂ in the mold powder proceeds violently and cannot be controlled to an appropriate mold powder composition. If the discharge angle of the immersion nozzle is 55 ° or more downward in the horizontal direction, the hot molten steel discharge flow is centered on the flow toward the lower side of the mold, and the molten steel bath surface temperature in the mold is excessively lowered. In such a case, in the present mold powder having a relatively high solidification temperature, slag bear is generated, which may cause uneven inflow of mold powder and cause vertical cracks.

[Amplitude stroke: More than 2 mm, not more than 8 mm, negative strip time tN defined by the following formula (6): not more than 0.28 seconds]
When performing continuous casting, it is common to pull the slab downward while vibrating the mold. This mold vibration condition is the amplitude determined by the distance between the top dead center and the bottom dead center of the mold. It is necessary to apply a mold vibration such that the negative strip time tN defined by the following formula (6) is 0.28 seconds or less while controlling the stroke of 2 mm to 8 mm or less.
tN = (1 / π · f) cos ⁻¹ (Vc / π · f · s) (6)
[Where f: mold frequency (Hz), s: distance between top and bottom mold stop points (mm) during mold vibration, Vc: slab drawing speed (mm / sec), respectively]

  When the stroke is 2 mm or less, the amount of mold powder inflow is extremely reduced, the seizure frequency between the mold and the slab increases, and the risk of breakout increases, making it difficult to achieve stable casting. On the other hand, when the stroke exceeds 8 mm, the interval between the oscillation marks becomes wide, the shrinkage stress at the initial stage of casting is not dispersed, concentrates on the oscillation mark portion, and causes depletion.

  The negative strip time tN defined by the above formula (6) is known as an index indicating the depth of the oscillation mark taking into account the amplitude (for example, “Third Edition Steel Handbook II Steel Making / Steel Making”). (Japan Steel Association), p638), the smaller the value, the smaller the oscillation mark depth (for example, “Iron and Steel”, 67 (1981), p1190). In addition, when a normal steel material is continuously cast, the negative strip time tN is set to about 0.35 seconds or less. According to the study by the present inventors, in order to continuously cast the high Al steel targeted in the present invention, the negative strip time tN defined by the above equation (6) is controlled to 0.28 seconds or less. There is a need. That is, when the negative strip time tN is greater than 0.28 seconds, the downward kinetic energy of the mold is transmitted by the powder, and the pressure of the meniscus powder is generated. As a result, the deformation stress due to solidification and transformation concentrates in the valleys of the oscillation marks, and transverse cracks occur. The preferable upper limit of the negative strip time tN is 0.25 seconds.

  Although the basic casting conditions in the method of the present invention are as described above, it is also effective to perform electromagnetic stirring in the mold if necessary. By performing magnetic stirring, the molten steel flow in the mold is made uniform and the molten steel temperature that collides with the solidified shell is made uniform, so that the heat input in the width direction of the slab is made uniform, and a uniform solidified shell is formed. As a result, depletion and vertical cracking can be prevented. In order to exhibit such an effect, the magnetic flux density when performing electromagnetic stirring is preferably 300 gauss or more, and more preferably 500 gauss or more. However, if the magnetic flux density becomes too high, the molten steel flow velocity on the surface of the molten steel becomes too high, and the reaction shown by the above formula (6) proceeds vigorously and may not be controlled to an appropriate mold powder composition. It is preferable to be less than Gauss.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and both are included in the technical scope of the present invention.

[Example 1]
Using a vertical bending type continuous casting machine, molten steel of 240 tons per heat was cast. In this example, molten steels (steel types) having various chemical composition compositions shown in Table 1 below were used, and mold powders having compositions shown in Table 2 below were used. At this time, the mold size in continuous casting is 240 × 1230 mm, and the casting speed is 1.4 m / min.

  As a guideline for lubricity, the solidification temperature of the mold powder (molten slag) was calculated. The solidification temperature (° C.) was calculated from the viscosity η and temperature T of the molten slag. Specifically, the viscosity η of the molten slag is continuously measured while raising the temperature by the vibrating piece method, the logarithm log η of the viscosity η is taken as the Y axis, and the inverse 1 / T of the viscosity measurement temperature T is taken as the horizontal axis. The temperature T corresponding to the inflection point of this graph was determined as the solidification temperature.

The mold heat flux (MW / m ² ) was calculated as a guide for slow cooling. The mold heat flux was calculated by obtaining the total heat removal amount in the mold from the flow rate of the mold cooling water and the temperature difference between the inlet and outlet, and dividing this by the contact area between the mold copper plate and the slab. A sample having a heat flux value of 1.5 MW / m ² or more was determined as “strong cooling”, and a sample having a heat flux value of less than 1.5 MW / m ² was determined as “slow cooling”.

  As a guideline for stable operation of continuous casting, the temperature fluctuation (° C.) in a fixed section cast at a constant speed was measured using a thermocouple embedded in a mold copper plate. In continuous casting, if the temperature fluctuation exceeds 15 ° C., the casting speed may be reduced, and if the fluctuation still does not stop, the casting stoppage may be taken.

  As an indicator of the surface quality of the slab, dents and cracks were evaluated. As for the dent on the surface of the slab, two slabs of the part that could be cast in a steady state are arbitrarily extracted from one heat, and the front and back surfaces of the wide surface of the slab are visually inspected, and the dent depth is measured at the part where the dent is recognized And the thing with a dent with a depth of 2 mm or more was evaluated as "with a dent". The crack on the surface of the slab was evaluated by visually observing the front and back surfaces of the wide surface of the slab and having one crack having a length of 100 mm or more as “having a crack”.

  These results are shown in the following Table 3 together with the continuous casting conditions (the mold surface level fluctuation speed in the mold, the immersion nozzle discharge angle, the electromagnetic stirring magnetic flux density, the mold vibration stroke, and the negative strip time tN).

As is clear from these results, those satisfying the requirements defined in the present invention (Test Nos. 1 to 10) can achieve slow cooling or stabilization of mold thermocouple temperature fluctuations, and can be free from dents and cracks. It is possible to produce a slab having excellent surface quality. On the other hand, in the case of using the mold powder lacking the requirements defined in the present invention (test Nos. 11 to 23), slow cooling cannot be performed, many crystals other than LiAlO ₂ are crystallized, and lubricity is improved. As a result of the difficulty, it can be seen that the slab has dents and cracks.

  Specifically, Test No. In No. 11, since the C content in the mold powder was excessive and the melting was insufficient, the portion where the slag film was not sufficiently formed was rapidly cooled, causing vertical cracks. Test No. In No. 12, the MgO content in the mold powder was small, and coarse crystals were crystallized, resulting in variations in the heat removal rate, and indentations and cracks in the slab.

Test No. In No. 13, the MgO content in the mold powder was increased, and mayenite and the like were preferentially crystallized, resulting in variations in the heat removal rate, and indentations and cracks in the slab. Test No. In the samples of Nos. 14 and 15, the SiO ₂ content in the mold powder was small, a large amount of slab bear was generated, and dents and cracks in the slab were generated.

Test No. In the case of No. 16, the Li ₂ O content in the mold powder is increased, and as a result, [Li ₂ O / SiO ₂ ] is increased, resulting in excessive inflow due to a decrease in viscosity, resulting in pulsation of the inflow. The mold thermocouple temperature variation is large. Moreover, the dent and the crack of slab generate | occur | produced without ensuring appropriate lubricity.

Test No. In the case of No. 17, the Li ₂ O content in the mold powder is reduced, and as a result, [Li ₂ O / SiO ₂ ] is reduced, the viscosity and the solidification temperature are high, and sufficient consumption basic unit is secured. Inability to ensure lubricity, many mayenite and dicalcium silicate were crystallized, and the heat removal rate varied, resulting in dents and cracks.

Test No. In No. 18, the F content in the mold powder was low, the viscosity increased, and sufficient lubricity could not be ensured, so dents and cracks occurred. Test No. In No. 19, the F content in the mold powder was increased, and since the amount of LiAlO ₂ was extremely decreased, slow cooling was not achieved, and dents and cracks were generated.

Test No. In No. 20, the basicity [T-CaO] / [SiO ₂ ] was low, and a large amount of coarse gehlenite crystallized, resulting in variations in the heat removal rate and cracking of the slab. Test No. In the case of No. 21, the Li ₂ O content in the mold powder was low, the solidification temperature was too high, and proper lubricity could not be ensured, and cracks occurred in the slab.

Test No. In No. 22, the Li ₂ O content in the mold powder was increased, the heat removal rate varied, and the slab was cracked. Test No. In No. 23, the basicity [T-CaO] / [SiO ₂ ] was low, and a large amount of coarse gehlenite crystallized, resulting in variations in the heat removal rate and cracking of the slab. Further, since Na ₂ O is also present, Na—Al—O crystals are crystallized non-uniformly, which is considered to have also affected the variation in the heat removal rate. Furthermore, a large amount of gehlenite is generated, the crystal becomes unstable, and slow cooling is not achieved.

[Example 2]
The steels shown in Table 1 were cast in the same manner as in Example 1 except that molten steels (steel types) having various chemical composition compositions shown in Table 1 were used and mold powders having compositions shown in Table 4 below were used. At this time, the continuous casting conditions (mold surface level fluctuation speed, immersion nozzle discharge angle, magnetic stirring magnetic flux density, mold vibration stroke, negative strip time tN) were controlled as shown in Table 5 below.

  These were evaluated in the same manner as in Example 1 for lubricity (solidification temperature), slow cooling (mold heat flux), stable operation (temperature fluctuation), surface quality of the slab (dents and cracks), and the like. The results are also shown in Table 5 above.

  As is clear from these results, those satisfying the requirements specified in the present invention (Test Nos. 24, 25, 28, 30 to 34, and 36 to 39) exhibit slow cooling or mold thermocouple temperature fluctuations. Stabilization can be realized, and a slab excellent in surface quality free from dents and cracks can be produced. On the other hand, it turns out that the dent and crack generate | occur | produce in the thing (test No. 26, 27, 29, 35, 40-42) which remove | deviates from the casting conditions prescribed | regulated by this invention.

  Specifically, Test No. In the case of Nos. 26 and 27, the mold surface level fluctuation speed in the mold is large, the heat removal speed becomes non-uniform, and as a result, the mold thermocouple temperature fluctuation becomes large and dents and cracks are generated. Test No. In No. 29, the submerged nozzle discharge angle is −5 °, and the heat removal rate becomes non-uniform. As a result, the temperature fluctuation of the mold thermocouple becomes large, and dents and cracks are generated.

  Test No. In the case of 35, the magnetic stir magnetic flux density, which is a preferable requirement of the present invention, is large, the heat removal speed is non-uniform, and as a result, the mold thermocouple temperature variation is large and dents and cracks are generated. .

Test No. In the case of 40, the mold amplitude stroke was 2 mm, and cracking occurred due to insufficient inflow. Test No. In the case of 41 and 42, since the interval between the oscillation marks is large, the dent and the crack are generated along the oscillation mark.

Claims (2)

The Al content is 0.1 to 3.0% (meaning mass%, the same shall apply hereinafter), and each of Si, Mn, Ni, Cr and Mo is 4.0% or less (not including 0%), And, when continuously casting molten steel satisfying the relationship of the following formulas (1) to (3) with the C content [C] using mold powder,
f1-0.10 ≦ [C] ≦ f2 + 0.05 (1)
f1 = 0.0828 [Si] −0.0195 [Mn] +0.07398 [Al] −0.04614 [Ni] +0.02447 [Cr] +0.01851 [Mo] +0.090
... (2)
f2 = 0.2187 [Si] −0.03291 [Mn] +0.2017 [Al] −0.06715 [Ni] +0.04776 [Cr] +0.04601 [Mo] +0.173
... (3)
[Wherein [Si], [Mn], [Al], [Ni], [Cr] and [Mo] indicate the contents (mass%) of Si, Mn, Ni, Cr and Mo, respectively. ]
Examples mold powder, T-CaO: 35~55%, SiO 2: 10~30%, Al 2 O 3: 4.0% or less (not including 0%), MgO: 0.2~1.0% Li ₂ O: 7-13%, F: 7-13%, C: 10.5-14%, and inevitable impurities, satisfying the following formulas (4) and (5),
1.6 ≦ [T-CaO] / [SiO ₂ ] ≦ 5 (4)
0.2 ≦ [Li ₂ O] / [SiO ₂ ] ≦ 1.1 (5)
[Wherein [T-CaO], [SiO ₂ ] and [Li ₂ O] represent the contents (mass%) of T-CaO, SiO ₂ and Li ₂ O in the mold powder, respectively. ]
The molten steel surface level fluctuation speed in the mold is set to 14 mm / second or less, molten steel is discharged in the mold width direction, and the discharge angle is 0 ° or more and 55 ° or less downward with respect to the horizontal. Continuous operation of high Al steel, characterized in that the stroke is more than 2 mm and not more than 8 mm, and the mold is vibrated so that the negative strip time tN defined by the following formula (6) is 0.28 seconds or less. Casting method.
tN = (1 / π · f) cos ⁻¹ (Vc / π · f · s) (6)
[Where f: mold frequency (Hz), s: distance between top and bottom mold stop points (mm) during mold vibration, Vc: slab drawing speed (mm / sec), respectively]   The continuous casting method according to claim 1, wherein the operation is performed while performing electromagnetic stirring in the mold at a magnetic flux density of 300 to 1200 gauss.