JP4527693B2 - Continuous casting method of high Al steel slab - Google Patents

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 to produce a slab of good surface quality. 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 effects of this mold powder include (a) ensuring lubricity between the mold and the solidified shell, and (b) slow cooling by suppressing the rate of heat removal from the solidified shell to the mold. .

  First, in order to ensure lubricity between the mold and the solidified shell mentioned in (a) above, the viscosity and solidification temperature so that an appropriate amount of molten slag obtained from mold powder flows into the gap between the mold and the solidified shell. It is important to set properly. 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. In the case of a steel type such as hypoperitectic steel, where slab surface cracks are likely to occur, particularly slow cooling is required. 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 a crystal for this purpose, caspidine (3CaO.2SiO ₂ .CaF ₂ ) is generally used.

However, in continuous casting of high aluminum steel having a dissolved Al content of 0.1% or more, it becomes difficult to ensure the lubricity of (a) and to slowly cool (a). Because in the continuous casting of high aluminum steel, the following formula (6):
4Al + 3SiO ₂ → 2Al ₂ O ₃ + 3Si (6)
This is because SiO ₂ is consumed by the chemical reaction represented by Therefore, the basicity [CaO] / [SiO ₂ ] is increased in the molten slag, and as a result, the solidification temperature is significantly increased. And a slag bear is formed in a mold wall surface, and inflow of molten slag is inhibited. Therefore, lubricity is impaired, the solidified shell and the mold are seized, and breakout occurs. The slag bear is generally composed of a part formed by cooling and solidifying molten slag and a part formed by melting a layer obtained by sintering molten slag or mold powder.

Variation in the composition of the molten slag caused by the chemical reaction of the above formula (6) can deteriorate the surface quality of the slab. Therefore, in Patent Document 1, in the flux for continuous casting, the amount of SiO ₂ is reduced as much as possible, and Al ₂ O ₃ is adjusted to an appropriate amount, thereby suppressing the reaction of the above formula (6) and preventing composition variation. (Claims, paragraph [0009]). However, when the amount of SiO ₂ is small, it is difficult to produce cuspidyne and it is difficult to achieve slow cooling.

Further, in the continuous casting of high aluminum steel, the composition of the molten slag varies, so that it is difficult to stably generate cuspidyne. In order to crystallize caspidyne crystals in continuous casting of high aluminum steel, Patent Document 2 discloses that CaO, SiO ₂ , Li ₂ O, F, Na ₂ O, K ₂ O, and Al ₂ O ₃ contents are predetermined. A mold powder having a specific composition that satisfies the formula is proposed (claims, paragraphs [0011] and [0017]).

On the other hand, Patent Document 3 discloses a mold powder containing two or more kinds of oxides of elements belonging to Group IA of the Periodic Table in order to produce a complex crystal different from caspidyne to achieve slow cooling ( Claims and paragraph [0013]). In the invention of Patent Document 3, LiCa ₂ FSiO ₄ and NaCa ₂ FSiO ₄ are disclosed as assumed composite crystals, but 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 3 is characterized by containing two or more 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] ).

However, even in a high Al steel, particularly in a steel 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 dents 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 that have a large amount of peritectic reaction or δ / γ transformation, the effects of alloy elements such as Si, Mn, Al, Ni, Cr, and Mo are taken into account, and equilibrium thermodynamic It is known that the following formulas (1) to (3) are defined based on the calculation (Non-patent Document 1). In addition, the subperitectic 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] −0.01951 [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] are the contents (mass%) of Si, Mn, Ni, Cr and Mo in the steel, respectively. Represents. ]
JP-A-9-85404 (Claims, paragraph [0009]) JP 2002-346708 A (Claims, paragraphs [0011] and [0017]) JP-A-10-216907 (claims, paragraphs [0013], [0020], [0024] and [0030]) "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 caspidyne (3CaO · 2SiO ₂ · CaF ₂ ) in the slag film obtained from the mold powder and 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 the 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 is a continuous casting method of high Al steel using mold powder,
Al content is 0.1 to 3.0% (meaning mass%, the same shall apply hereinafter), and Si, Mn, Ni, Cr and Mo are each included in an amount of 4.0% or less (excluding 0%). And the C content [C] is represented by the following formulas (1) to (3):
f1-0.10 ≦ [C] ≦ f2 + 0.05 (1)
f1 = 0.0828 [Si] −0.01951 [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] are the contents (mass%) of Si, Mn, Ni, Cr and Mo in the steel, respectively. Represents. ]
When continuously casting molten steel that satisfies the relationship
T-CaO: 35~60%, SiO 2: 5~20%, Al 2 O 3: 15~30%, MgO: 0.2~1.0%, Li 2 O: 7~13%, F: 7 0.0 to 13%, C: 10.5 to 14% and inevitable impurities, and the following formula (4):
2.5 ≦ [T-CaO] / [SiO ₂ ] ≦ 12.0 (4)
[Wherein [T-CaO] and [SiO ₂ ] represent the contents (mass%) of T-CaO and SiO _{2 in} the mold powder, respectively. ]
Using mold powder that satisfies the relationship
The molten metal surface level fluctuation speed in the mold is set to 14 mm / second or less, and 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,
Furthermore, the stroke of the amplitude is set to more than 2 mm and not more than 8 mm, and the following formula (5):
t _N = (1 / π × f) cos ⁻¹ (V _c / π × f × s) (5)
[Where f is the mold frequency (Hz), s is the distance (mm) between the top and bottom stop points of the mold during mold vibration, and V _c is the slab drawing speed (mm / second) ). ]
The mold strip vibration is applied such that the negative strip time t _N determined in (1) is 0.25 seconds or less, and the mold is operated while performing magnetic stirring in the mold at a magnetic flux density of 1200 gauss or less.

  Here, the requirement “perform magnetic stirring in the mold with a magnetic flux density of 1200 gauss or less” is intended to limit the upper limit of magnetic flux density when performing magnetic stirring to 1200 gauss, and the scope of the present invention includes: The case where the magnetic flux density is 0, that is, the case where electromagnetic stirring in the mold is not performed is also included. Further, 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 gauss or more.

  According to the present invention, a mold powder having a properly adjusted composition is used, and by appropriately controlling the operating conditions of continuous casting, dents and cracks on the slab surface are prevented and high surface quality is excellent. Al steel can be manufactured.

  The inventors of the present invention have made extensive studies to achieve the above object. As a result, the inventors have found that the above object can be achieved brilliantly by using mold powder having a properly adjusted composition and appropriately controlling the continuous casting conditions, and completed 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. Therefore, the present inventors examined the generation of crystals instead of caspidyne in the slag film.

However, when crystals such as dicalcium silicate (2CaO · SiO ₂ ), mayenite (12CaO · 7Al ₂ O ₃ ) and gelenite (2CaO · SiO ₂ · Al ₂ O ₃ ) are crystallized for slow cooling, the temperature of the mold copper plate There is a problem that the fluctuation of the slab becomes large, and 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 unevenness (air layer) is formed on the surface on the mold side, and as a result, the heat extraction rate varies. 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 dents and cracks in 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 slab dents and cracks is unknown, but can be estimated as follows.

Since LiAlO ₂ crystallizes uniformly as fine crystals on the mold side surface of the slag film, a uniform air layer is formed. As a result, uniform heat removal is achieved, fluctuations in the mold copper plate temperature are small, uniform solidified shells are formed by uniform slow cooling, and unevenness and cracks in the slab due to transformation shrinkage are suppressed. Conceivable. However, the present invention is not limited to such an estimation mechanism.

The mold powder of the present invention is characterized in that the amount of each component and the basicity [T-CaO] / [SiO ₂ ] are adjusted to an appropriate range in order to crystallize LiAlO ₂ instead of caspidine. . Further, from the viewpoint of ensuring lubricity by adjusting the solidification temperature of molten slag (mold powder) to an appropriate range, it is also a feature of the mold powder of the present invention that each component composition is adjusted to an appropriate range. . Hereinafter, the amount of each component in the mold powder of the present invention and the basicity [T-CaO] / [SiO ₂ ] will be described.

<T-CaO: 35-60%>
In the present invention, “T-CaO” represents the CaO amount (% by mass) when all 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, 60% or less, preferably 55% or less, more preferably 50% or less. When the amount of T-CaO is less than 35%, the amount of SiO ₂ is relatively increased, and gehlenite (2CaO · SiO ₂ · Al ₂ O ₃ ) is more easily generated than LiAlO ₂ . On the other hand, when the amount of T-CaO exceeds 60%, the amount of Li ₂ O is relatively reduced and LiAlO ₂ is hardly formed, and coarse crystals of mayenite (12CaO · 7Al ₂ O ₃ ) are preferentially used. Crystallize. Furthermore, the solidification temperature of the molten slag may become too high.

<SiO _2: 5~20%>
The amount of SiO ₂ is 5% or more, preferably 6% or more, more preferably 7% or more, 20% or less, preferably 17% or less, more preferably 14% or less. When the amount of SiO ₂ that is a glass-forming compound is small, coarse crystals are easily crystallized from the liquid phase molten slag. Since LiAlO ₂ is generally composed of LiO ₂ that lowers the melting point of inclusions, it is estimated that its melting point is considerably lower than that of mayenite (12CaO · 7Al ₂ O ₃ ). Therefore, if the amount of SiO ₂ is less than 5% by mass, coarse crystals of mayenite having a melting point higher than that of LiAlO ₂ and not containing SiO ₂ are preferentially formed. It is thought that uniform unevenness is formed. In addition, the solidification temperature rises, the lubricity is impaired, and slag bear is easily generated. Also the amount of SiO ₂ exceeds 20% Conversely, gehlenite composed of _{SiO 2 (2CaO · SiO 2 ·} Al 2 O 3) and Dicalcium silicate (2CaO · SiO ₂₎ is more produced.

_{<Al 2 O 3: 15~30%} >
The amount of Al ₂ O ₃ is 15% or more, preferably 16% or more, more preferably 17% or more, and 30% or less, preferably 28% or less, more preferably 26% or less. By containing Al ₂ O ₃ at a high concentration of 15% or more, LiAlO ₂ is easily formed. Further, when the Al ₂ O ₃ content in the mold powder is 15% or more, the Al ₂ O ₃ concentration in the molten slag becomes close to saturation, and the reaction of the above formula (6) is suppressed, and the composition of the molten slag. In particular, the amount of SiO ₂ can be maintained within an appropriate range. If the amount of Al ₂ O ₃ is less than 15%, coarse crystals of dicalcium silicate (2CaO · SiO ₂ ) that does not contain an Al ₂ O ₃ component are likely to be formed, resulting in variations in the heat removal rate. It may adversely affect the quality of the slab surface. On the other hand, if the amount of Al ₂ O ₃ exceeds 30%, mayenite (12CaO · 7Al ₂ O ₃ ) is likely to be formed, and the solidification temperature of the molten slag will rise too much to ensure proper lubricity. Becomes difficult.

<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 content exceeds 1.0%, the number of nuclei becomes too large to control the crystallization of crystals properly, and in particular, depending on the mold powder composition, dicalcium silicate (2CaO · SiO ₂ ) is preferential. May crystallize. 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. Slow cooling is difficult to achieve. 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: 7~13%>
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. If 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, so that sufficient 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 slag is greatly reduced, and the molten slag may locally flow excessively or pulsate, which may adversely affect the stable operation of continuous casting.

<F: 7.0 to 13%>
The F amount is 7.0% 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 content is less than 7%, the solidification temperature and viscosity of the molten slag increase, and lubricity may not be ensured. On the other hand, since F has an action of suppressing crystal crystallization, if the amount of F is excessive, sufficient crystals cannot be crystallized on the mold side surface of the molten slag, and it is difficult to achieve slow cooling. become. In particular, when the amount of F exceeds 13%, the crystallization amount of LiAlO ₂ is drastically reduced.

<C: 10.5-14%>
The amount of C specified in the present invention represents all the amount of C contained in the mold powder. That C content of the present invention, such as is added as a raw material for mold powder, carbon content alone (free C amount) and, for example, the amount of carbon compounds in such Li ₂ CO ₃ added as Li ₂ O feed And the total. 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 of the present invention comprises the above components and inevitable impurities. In order to reduce the viscosity and the solidification temperature, Na ₂ O and K ₂ O are added to a general mold powder, but the mold powder of the present invention is also characterized by not containing these. Because in the continuous casting of high aluminum steel, the following reaction formulas (7) and (8):
2Al + 3Na ₂ O → Al ₂ O ₃ + 6Na (7)
2Al + 3K ₂ O → 2Al ₂ O ₃ + 6K (8)
Therefore, Na ₂ O and K ₂ O are consumed, and these effects are not fully exhibited. Conversely, Al ₂ O ₃ more than expected by the present invention is generated and melted. This is because it adversely affects the solidification temperature of the slag. In addition, when Na ₂ O or K ₂ O is added, the difference from the softening start temperature of the mold powder to the melting temperature widens, so that it becomes easy to form a molten slag or a mold powder sintered layer. Be encouraged. In addition, when Na ₂ O is present, Na—Al—O crystals may be crystallized non-uniformly, and unevenness (air layer) of the slag film may occur.

<2.5 ≦ [T-CaO] / [SiO ₂ ] ≦ 12.0>
The basicity [T-CaO] / [SiO ₂ ] is 2.5 or more, preferably 3.0 or more, more preferably 3.5 or more, 12.0 or less, preferably 11.0 or less, more preferably Is 10.0 or less. When the basicity is less than 2.5, the amount of SiO ₂ is relatively increased, and dicalcium silicate (2CaO · SiO ₂ ) and gehlenite (2CaO · SiO ₂ · Al ₂ O ₃ ) are generated rather than LiAlO _2. It becomes easy. Conversely, even when the basicity exceeds 12.0, the amount of SiO ₂ that is a glass-forming compound is relatively decreased, and dicalcium silicate (2CaO · SiO ₂ ), mayenite, which is considered to have a melting point higher than LiAlO _2. Coarse crystals of (12CaO · 7Al ₂ O ₃ ) and gehlenite (2CaO · SiO ₂ · Al ₂ O ₃ ) are preferentially generated. On the other hand, if the basicity is too high, the coagulation temperature becomes high and the lubricity can be adversely affected.

  The solidification temperature of the mold powder (molten slag) of the present invention is preferably 1050 to 1220 ° C, more preferably 1050 to 1190 ° C. If the solidification temperature is lower than 1050 ° 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 1220 ° C., slag bear is generated, and breakout and cracking of the slab surface may occur due to non-uniform inflow by the slag bear.

The amount of Al dissolved in the continuously cast steel (Al content in the molten steel) is 0.1% or more, preferably 0.3% or more, more preferably in order to sufficiently exhibit the effect of the mold powder of the present invention. It is 0.5% or more, 2.5% or less, preferably 2.0% or less, 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 does not include the amount of Al present as a compound such as Al ₂ O ₃ .

  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 (not including 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 inevitable impurities such as S, P, and Cu.

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

<Fuel level fluctuation speed in mold: 14 mm / second 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 becomes large, and dents and cracks 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.

<Use a submerged nozzle with a discharge angle of 0 ° or more and 55 ° or less with respect to the horizontal while discharging molten steel in the mold width direction>
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 situation of the corresponding part is different from other parts. It is easy to become. The discharge angle (discharge direction angle) of the immersion nozzle at this time 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), since the discharged molten steel goes directly to the interface between the molten mold powder and the molten steel bath surface, the interface becomes hot and stirred, and the dissolved Al in the molten steel The reaction of the above formula (6) that occurs with SiO ₂ in the mold powder proceeds violently and cannot be maintained at an appropriate mold powder composition. If the discharge angle of the immersion nozzle exceeds 55 ° in the horizontal direction downward, 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, a slag bear may be generated, causing inflow non-uniformity of mold powder and causing vertical cracks.

<Amplitude stroke: More than 2 mm, 8 mm or less, negative strip time t _N defined by the following formula (5): 0.25 seconds or less>
When performing continuous casting, it is common to pull the slab downward while vibrating the mold. The mold vibration condition is the amplitude determined by the distance between the top and bottom stop points of the mold. Is controlled to be within a range of more than 2 mm and not more than 8 mm, and it is necessary to apply mold vibration such that the negative strip time t _N defined by the following formula (5) is 0.25 seconds or less.
t _N = (1 / π × f) cos ⁻¹ (V _c / π × f × s) (5)
[Where f is the mold frequency (Hz), s is the distance (mm) between the top and bottom stop points of the mold during mold vibration, and V _c is the slab drawing speed (mm / second) ). ]

  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 is increased, and the risk of breakout is increased, making it difficult to achieve stable casting. On the other hand, if 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, and concentrates on the oscillation mark portion, thereby causing dents.

The negative strip time t _N defined by the above equation (5) 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), p. 638), the smaller the value, the smaller the oscillation mark depth (for example, “Iron and Steel”, 67 (1981), p. 1190). Moreover, when continuously casting a normal steel material, the negative strip time t _N 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 which is the object of the present invention, the negative strip time t _N defined by the above formula (5) is controlled to 0.25 seconds or less. There is a need. That is, when the negative strip time t _N is greater than 0.25 seconds, the downward kinetic energy of the mold is transmitted by the powder, and the pressure of the meniscus powder is generated, resulting in the depth of the oscillation mark. Becomes larger, deformation stress accompanying solidification and transformation concentrates in the valley of the oscillation mark, and transverse cracks occur. The preferable upper limit of the negative strip time t _N is 0.23 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, dents and vertical cracks can be prevented. In order to exert such an effect, the magnetic flux density at the time of electromagnetic stirring is preferably 300 gauss or more, 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 violently and may not be maintained at an appropriate mold powder composition. Must be less than Gauss.

  EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and appropriate modifications are made within a range that can meet the above and the following purposes. Of course, it is possible to implement them, and they are all 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 steel (steel type) having the chemical composition shown in Table 1 below was used, and mold powder having the chemical composition shown in Table 2 below was used. The mold size in continuous casting is 240 × 1230 mm, and the casting speed is 1.4 m / min.

  The solidification temperature was calculated as a guide for lubricity of the mold powder (molten slag). 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 results are also shown in Table 2 below.

The following table shows the steel types used in each test, mold powder, and operating conditions (rate level fluctuation speed in mold, discharge angle of immersion nozzle, magnetic flux density of electromagnetic stirring, mold amplitude stroke, negative strip time t _N ). 3 shows. Table 3 below also shows the results of the X-ray diffraction intensity of the crystals present in the slag film, the mold heat flux, the temperature fluctuation of the mold copper plate, the dents and cracks on the slab surface, measured as follows.

LiAlO ₂ present in slag film obtained from mold powder, caspidyne (3CaO · 2SiO ₂ · CaF ₂ ), dicalcium silicate (2CaO · SiO ₂ ), mayenite (12CaO · 7Al ₂ O ₃ ) and gelenite (2CaO · SiO ₂ ) _In order to investigate the amount of ( ₂ · Al ₂ O ₃ ), a slag film was taken from the mold after the casting was finished, and the X-ray diffraction intensity of each crystal was measured by X-ray diffraction (Cu tube 40 kV, 200 mA). .

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 exceeding 2 mm was evaluated as "there is 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”.

As is clear from these results, those satisfying the requirements defined in the present invention (Test Nos. 1 to 10) can realize slow cooling even if caspidyne is not formed in the slag film, and dents and cracks. It is possible to produce a slab having no surface quality and excellent surface quality. Test No. Slow cooling at 1-10 is believed to be achieved by LiAlO ₂ in the slag film. In addition, Test No. In 1-10, there are few temperature fluctuations and it can operate stably. Furthermore, test no. Mold powder No. 1 used in 1-10. As for 1-10, it turns out that the coagulation | solidification temperature exists in an appropriate range, and has appropriate lubricity.

  On the other hand, mold powder No. which lacks the requirements specified in the present invention. Test Nos. 11 to 25 were used. In Nos. 11 to 25, only slabs with dents and cracks were obtained for the reasons described below.

Test No. In No. 11, since the amount of SiO ₂ was small, the mayenite crystallized preferentially and cracking of the slab occurred. In addition, due to the increase in the solidification temperature, the lubricity of the molten slag was impaired and the temperature fluctuation increased.
Test No. In No. 12, since the amount of SiO ₂ was large, a large amount of gehlenite was crystallized, uniform slow cooling could not be achieved, and dents and cracks occurred on the surface of the slab.
Test No. In No. 13, since the amount of CaO was small, much gehlenite was crystallized, uniform slow cooling could not be achieved, and dents and cracks occurred on the slab surface.
Test No. In the case of No. 14, since the amount of CaO was large, a large amount of mayenite was crystallized and cracking of the slab occurred. In addition, due to the increase in the solidification temperature, the lubricity of the molten slag was impaired and the temperature fluctuation increased.

Test No. In No. 15, since the amount of Al ₂ O ₃ was large and coarse crystals such as mayenite were formed, the heat removal rate varied, and dents and cracks occurred on the surface of the slab.
Test No. In No. 16, since the amount of Al ₂ O ₃ was small and a large number of coarse crystals of dicalcium silicate were formed, dents and cracks occurred on the surface of the slab.
Test No. In the case of No. 17, since there was much Li ₂ O, a sufficient amount of LiAlO ₂ was not crystallized out of the optimum range of LiAlO ₂ crystallization, and the slab surface was not dented due to a decrease in the viscosity of the molten slag. Cracking occurred.
Test No. In No. 18, since the amount of Li ₂ O was small, LiAlO ₂ did not crystallize, the heat extraction speed varied, and dents and cracks occurred on the slab surface.

Test No. In No. 19, since the amount of F was large, sufficient crystals were not crystallized and could not be slowly cooled, and excessive inflow due to a decrease in viscosity caused dents and cracks on the surface of the slab.
Test No. In No. 20, since the amount of F was low, the solidification temperature of the molten slag increased and slag bear was generated. As a result, vertical slabs of the slab occurred due to uneven flow.
Test No. In No. 21, coarse crystals were crystallized with a small amount of MgO, the heat extraction speed varied, and dents and cracks occurred on the surface of the slab.
Test No. In the case of No. 22, the amount of MgO was large and a large number of coarse crystals of dicalcium silicate were formed, resulting in variations in the heat removal rate, and dents and cracks occurred on the surface of the slab.

Test No. In No. 23, since Na ₂ O and K ₂ O were present, many layers of molten slag or mold powder were formed, and as a result, slag bear was generated. Moreover, since the Na—Al—O crystal was crystallized non-uniformly, the heat extraction rate varied, and the slab surface was dented or cracked.
Test No. In the case of No. 24, since the amount of C was large and the melting rate was insufficient, a portion where the slag film was not sufficiently formed was generated, the portion was rapidly cooled, and cracks occurred on the surface of the slab.
Test No. In No. 25, since the amount of C was small and the melting rate increased, excessive inflow and non-uniform inflow occurred, and dents and cracks occurred on the surface of the slab.

<Example 2>
Using a vertical bending type continuous casting machine, molten steel of 240 tons per heat was cast. In this example, molten steel (steel types No. C and D) having the chemical composition shown in Table 1 was used, and mold powder (mold powder No. 5 to 8) having the chemical composition shown in Table 2 was used. . The mold size in continuous casting is 240 × 1230 mm, and the casting speed is 1.4 m / min.

Steel types used in each test, mold powder, and continuous casting conditions (rate level fluctuation speed in mold, discharge angle of immersion nozzle, magnetic flux density of electromagnetic stirring, mold amplitude stroke, negative strip time t _N ) Table 4 shows. Table 4 below also shows the results of mold heat flux, mold copper plate temperature fluctuation, slab surface dents and cracks measured in the same manner as in Example 1.

  As is clear from these results, those satisfying the requirements stipulated in the present invention (test Nos. 26 to 37) can realize slow cooling and stabilization of the temperature fluctuation of the mold copper plate, and have a surface free from dents and cracks. A slab excellent in quality can be manufactured. On the other hand, in the thing (test No. 38-44) which deviates from the operation condition prescribed | regulated by this invention, only the slab with a dent and a crack was obtained for the reason described below.

Test No. In No. 38 and No. 39, since the mold surface level fluctuation speed was large, the heat removal speed became non-uniform, and as a result, the temperature fluctuation of the mold copper plate became large and dents and cracks occurred.
Test No. In No. 40, since the immersion nozzle discharge angle was −5 °, the heat removal rate became non-uniform. As a result, the temperature fluctuation of the mold copper plate was increased, and dents and cracks were generated.
Test No. In the case of No. 41, the magnetic flux density in electromagnetic stirring was large, and the heat removal rate became non-uniform. As a result, the temperature fluctuation of the mold thermocouple became large and dents and cracks were generated.

Test No. In 42, since the amplitude stroke was small, cracking occurred due to insufficient inflow.
Test No. No. 43 had a large amplitude stroke and a large interval between the oscillation marks, and therefore, dents and cracks along the oscillation marks occurred.
In Test 44, since the negative strip time t _N was large and the oscillation mark depth was large, dents and cracks along the oscillation mark occurred.

Claims (1)

A continuous casting method of high Al steel slab using mold powder,
Al content is 0.1 to 3.0% (meaning mass%, the same shall apply hereinafter), and Si, Mn, Ni, Cr and Mo are each included in an amount of 4.0% or less (excluding 0%). And the C content [C] is represented by the following formulas (1) to (3):
f1-0.10 ≦ [C] ≦ f2 + 0.05 (1)
f1 = 0.0828 [Si] −0.01951 [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)
[In the formula, [Si], [Mn], [Al], [Ni], [Cr] and [Mo] are the contents (mass of Si, Mn, Al, Ni, Cr and Mo, respectively) in steel. %). ]
When continuously casting molten steel that satisfies the relationship
T-CaO: 35~60%, SiO 2: 5~20%, Al 2 O 3: 15~30%, MgO: 0.2~1.0%, Li 2 O: 7~13%, F: 7 0.0 to 13%, C: 10.5 to 14% and inevitable impurities, and the following formula (4):
2.5 ≦ [T-CaO] / [SiO ₂ ] ≦ 12.0 (4)
[Wherein [T-CaO] and [SiO ₂ ] represent the contents (mass%) of T-CaO and SiO _{2 in} the mold powder, respectively. ]
Using mold powder that satisfies the relationship
While maintaining the stability of the mold powder melting pool melt surface level fluctuation rate of the mold in less 14 mm / sec, the ejected molten steel in the mold width direction,
Using an immersion nozzle whose discharge angle is 0 ° or more and 55 ° or less downward with respect to the horizontal,
Furthermore, the stroke of the amplitude is set to more than 2 mm and not more than 8 mm, and the following formula (5):
t _N = (1 / π × f) cos ⁻¹ (V _c / π × f × s) (5)
[Where f is the mold frequency (Hz), s is the distance (mm) between the top and bottom stop points of the mold during mold vibration, and V _c is the slab drawing speed (mm / second) ). ]
Is applied with mold vibration such that the negative strip time t _N determined by 0.25 is 0.25 seconds or less, and the magnetic flow rate is less than 1.5 MW / m ² while performing electromagnetic stirring in the mold at a magnetic flux density of 1200 gauss or less. A method for continuous casting of a high Al steel slab characterized by performing slow cooling of the steel.