JP2010042425A - Mold powder for continuously casting steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent occurrence of the detection of brake out caused by the defect of the stability in the temperature of a mold frequently shown in the case the content of SiO<SB>2</SB>in mold powder is reduced for dealing with the problem of contamination of the powder to a slab causing the surface flaw of a cold-rolled steel sheet. <P>SOLUTION: In the mold powder for continuous casting containing SiO<SB>2</SB>, CaO, Al<SB>2</SB>O<SB>3</SB>, Na<SB>2</SB>O, B<SB>2</SB>O<SB>3</SB>, F and C , as chemical components, the SiO<SB>2</SB>content is defined as 20 to 32%, the Al<SB>2</SB>O<SB>3</SB>content is defined as 5 to 15%, the Na<SB>2</SB>O content is defined as 1 to 8%, the B<SB>2</SB>O<SB>3</SB>content is defined as 1 to 10% and the F content is defined as 3 to 11% by mass. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a powder for continuous casting which is used by being added to a mold in continuous casting of steel, and is used for casting steel for a thin steel plate, particularly a cold-rolled steel plate. That can be easily cast.

  In the continuous casting of molten steel, powder or granular powder is sprayed on the molten metal surface in the mold. This powder melts on the surface of the molten steel to form a molten layer of slag that keeps the molten steel warm, prevents oxidation and absorbs non-metallic inclusions, and flows between the solidified shell of steel and the mold to form a slag film. To do. The slag film flowing between the mold and the solidified shell of the slab performs lubrication and controls the heat removal from the slab, thereby obtaining a slab having excellent surface properties and producing a clean and good slab. do.

  The continuous casting of steel has been widely applied to ultra-low carbon steel and low-carbon aluminum killed steel, which are cold-rolled steel materials with strict quality requirements. When the non-metallic inclusions are exposed on the surface of the cold-rolled steel sheet, it becomes a linear ridge called a sliver ridge, and it is difficult to cover it even by painting products such as electric products and automobiles. However, since the cold-rolled steel sheet is rolled to a thickness of several millimeters or less, the surface area is greatly expanded as compared with the state of the slab. Therefore, slabs for cold-rolled steel sheets are required not only to have surface quality, but also to be excellent in the quality of the slabs that should be the surface of cold-rolled steel sheets, especially without slag entrainment and few inclusions It is necessary to produce a slab.

Japanese Patent Application Laid-Open No. 2006-192440 (Patent Document 1) describes a mold powder suitable for manufacturing a steel plate that requires high quality such as a steel plate for automobiles. For this purpose, it is necessary to prevent the entrainment of mold powder inside the molten steel, and the inventors have found that the reactivity of the mold powder melt with aluminum in the molten steel is related to entrainment. And by restricting the content of SiO ₂ , Na ₂ O, Li ₂ O, which is the oxidation source of aluminum in the molten steel, in relation to the viscosity of the slag, the entrainment of the powder in the molten steel is significantly reduced. The coil surface defects after rolling can be reduced to 1 or less per coil.

Japanese Patent Application Laid-Open No. 2006-175472 (Patent Document 2) describes a mold powder for continuously casting steel containing at least one of Al and Ti. And if Al and Ti are contained in the steel, the SiO ₂ in the powder reacts with the Al and Ti in the molten steel, the physical properties of the powder change, and the lubricity of the powder and the entrainment of the powder in the steel deteriorate. Yes. Therefore, in Patent Document 2, the activity of SiO ₂ at 1550 ° C. is limited to 0.4 or less. The above activity is obtained by multiplying the concentration of the component by the activity coefficient determined by the interaction with other coexisting components, and shows chemical reactivity, but primarily depends on the concentration of the component To do. In fact, looking at the examples of Patent Document 2, when SiO ₂ of the mold powder of the present invention embodiment is 18.8 to 33.0%, SiO ₂ of the comparative example becomes 38.3 to 40.1% ing.
JP 2006-192440 A JP 2006-175472 A

In order to cope with the problem of entrainment of mold powder in a slab, the present inventors have changed the content of SiO _{2 from} about 40% by mass (hereinafter referred to as “%”) to 32% or less in steels with strict quality requirements. A reduced powder was produced. As a result, the problem of powder entrainment has been improved, but breakout detection has occurred frequently. To detect breakout, a plurality of temperature sensors are mounted at regular intervals on the mold at intervals, and an alarm is issued when the temperature of these sensors changes abnormally. For this reason, even if the breakout is not actually reached, the breakout is detected if there is a disturbance in the transition of the mold temperature. An object of this invention is to provide the mold powder which does not generate | occur | produce such a breakout detection.

The present invention has been made to solve the problems the above problems, as chemical _{components, SiO 2, CaO, Al 2} O 3, Na 2 O, B 2 O 3, F and continuous casting mold of steel containing C in powders, SiO ₂ is 20 to 32% _Al 2 _{O 3} is 5 to 15% Na ₂ O is 1 to _8% B 2 _{O 3} is 1-10%, the F is 3-11% It is a featured powder for continuous casting of steel.
Here, the above-mentioned steel continuous casting powder has a viscosity at 1300 ° C. of 0.5 to 5.0 poise, and among chemical components other than C, at least two types of components are pre-melted blended raw materials It is also characterized by being for casting steel for a thin steel plate. Furthermore, the chemical component is characterized by CaO: 30 to 50% and C: 0.3 to 5%.

While the powder for continuous casting of the present invention has improved the problem of entrainment of powder in the slab by reducing the content of SiO ₂ , it detects breakouts found in conventional powders of such components. There is no problem that often occurs. For this reason, it is possible to obtain a slab of good quality in continuous casting of ultra-low carbon steel and low-carbon aluminum killed steel, which are raw materials for cold-rolled steel sheets with strict quality requirements, and smooth casting work can be performed.

The powder for continuous casting of the present invention is for casting a thin steel plate having a thickness of 3.2 mm or less, particularly a steel for cold rolling steel plate, and is used to prevent the entrainment of mold powder into a slab. One of the features is to reduce the content of ₂ to 32% or less. The inventors of the present invention have studied various means for preventing breakout detection that frequently occurs when such a powder having a reduced content of SiO ₂ is used. As a result, it has been found that the occurrence of breakout detection can be significantly reduced by adding 1% or more of B ₂ O ₃ to the powder having a reduced content of SiO ₂ .

  The powder for continuous casting is melted by adding it into the mold to form a slag and flows between the mold and the slab, and is always cooled from the mold. For this reason, the slag that has flowed in is solidified on the mold side to form a solid film and is in contact with the mold, and on the slab side is a liquid film melted by the solidified shell. Therefore, for example, the inflow of slag between the mold and the slab becomes a problem, or the glassy film rapidly cooled by the mold is partially crystallized to lower the thermal conductivity, resulting in variations in heat transfer of the slag film. If this occurs and the solidification becomes uneven, the temperature variation of the mold becomes excessive and a breakout detection alarm is generated.

From the above, it can be seen that the occurrence of breakout detection is related to the viscosity of the molten slag and the solidification temperature. However, according to the findings of the present inventors, the effect of reducing the occurrence of breakout detection by the addition of B ₂ O ₃ is considered based on the effect unique to the B ₂ O ₃ rather than optimizing just the viscosity and solidification temperature It is done. That is, if the viscosity of the slag is related, the viscosity can be adjusted by changing the amount of components other than B ₂ O ₃ , such as fluoride, so that the same effect can be obtained. It is. The same applies to the solidification temperature, and the amount of other components can be changed without addition of B ₂ O _3, and it is not always used as a component of powder for continuous casting. _It can also be adjusted by adding ₂ or the like.

However, since components other than B ₂ O ₃ cannot obtain the effect of reducing the occurrence of breakout detection, it is determined that the component is based on the operational effects unique to B ₂ O ₃ . From the viewpoint of the phenomenon, the present inventors consider that the occurrence of breakout detection due to the reduction in the content of SiO ₂ is due to the slag film falling into solid state and in contact with the mold. ₂ O ₃ is considered to have an effect of preventing such slag film from falling off. In other words, SiO ₂ is a component that works to make slag vitreous, but if this is reduced, the glassy properties of the slag film will be degraded and easily fall off, whereas B ₂ O ₃ should be added. It is presumed that vitrification is promoted by this to strengthen the slag film.

The mold powder for continuous casting according to the present invention contains SiO ₂ , CaO, Al ₂ O ₃ , Na ₂ O, B ₂ O ₃ , F and C as chemical components including the above-mentioned SiO ₂ and B ₂ O _3. However, the reasons for limiting these components will be described.

SiO ₂ is the main component of the powder, but as described above, the content of SiO ₂ is set to 32% or less in order to prevent the powder from being caught in the slab. However, when it is less than 20%, the viscosity of the slag is too low. Therefore, SiO ₂ is a 20 to 32 percent.

Al ₂ O ₃ is used for the purpose of lowering the solidification temperature. However, if Al ₂ O ₃ is less than 5%, the solidification temperature cannot be lowered sufficiently. On the other hand, if it exceeds 15%, the solidification temperature becomes extremely low, which does not meet the casting conditions, and the alumina inclusions may increase. Thus _Al 2 _{O 3} is 5 to 15%.

Na ₂ O is a component that lowers the viscosity and is used for the purpose of maintaining the viscosity within a specified viscosity range. If the amount of Na ₂ O is less than 1%, the effect of decreasing the viscosity is insufficient, while if it exceeds 8%, the viscosity is excessively decreased. Therefore, Na ₂ O is 1 to 8%.

B ₂ O ₃ needs to be 1% or more in order to obtain the effect of reducing the occurrence of breakout detection as described above, but it is preferable to add 2% or more in order to ensure the effect. On the other hand, if added over 10%, boron is reduced from the molten slag and boron is contained in the slab surface layer, which may increase the cracking susceptibility of the slab. Moreover, it is expensive in terms of price, and even if added over 5%, the effect is saturated, so it is preferable to make it 5% or less. Therefore, B ₂ O ₃ is 1 to 10%, and particularly preferably, the lower limit is 2% and the upper limit is 5%.

F is a component that lowers the viscosity and is used for the purpose of maintaining the viscosity within a specified range. If the amount of F is less than 3%, the effect of decreasing the viscosity is insufficient, while if it exceeds 11%, the viscosity is excessively decreased. Therefore, F is 3 to 11%. Note F amount is F equivalent of F compounds such as CaF ₂ or NaF.

Also CaO is a main component of the powder with SiO _2, the content is preferably 30-50%, containing in particular as CaO / SiO ₂ in relation to the amount of SiO ₂ is in the range of 0.9 to 1.8 It is preferable to do so. If CaO / SiO ₂ is less than 0.9, the viscosity becomes high and it is difficult to adjust the physical properties. If it exceeds 1.8, it is difficult to obtain a predetermined viscosity and the glassy properties of the slag film are likely to deteriorate.

  C is added in the form of carbon powder or the like, and has the effect of slowing the melting rate of the powder according to the amount added. The content is preferably 1 to 5%. If the content is less than 0.3%, the melting rate is too fast, and if it exceeds 5%, the melting rate becomes slow, so that the melt pool thickness becomes thin and the lubricating function is hindered. This C does not contain C contained as a compound, such as when a carbonate of Ca is used as a blending raw material.

  The viscosity of the molten slag needs to be an appropriate value depending on the casting speed. In the continuous casting powder of the present invention, the viscosity at 1300 ° C. is preferably 0.5 to 5.0 poise. If it is less than 0.5 poise, the surface of the slab may be rough due to excessive inflow of molten slag. On the other hand, if the viscosity is higher than 5.0 poise, the slab may be cracked due to non-uniform inflow of molten slag. . The solidification temperature of the solidified slag is suitably 1000 to 1250 ° C. If it is lower than this, there is a risk of cracking of the slab due to supercooling, and if it is high, the mold may be poorly lubricated.

In addition, in order to achieve rapid and uniform melting of the powder in response to the high speed of continuous casting, at least two kinds of chemical components other than C, which is a melting rate adjusting agent, such as SiO ₂ and CaO, are pre-melted. It is preferable to use the formulated raw material. Moreover, it is preferable to use 40% or more of the premelted raw material in order to exhibit the effect. The pre-melt raw material is manufactured by melting components in an arc furnace or the like and then hydrolyzing them with a jet water stream.

Hereinafter, the present invention will be described in detail by way of examples.
Various types of mold powders for continuous casting having the chemical components shown in Table 1 were prepared and used for continuous casting of ultra-low carbon steel (carbon content 0.03%) for producing cold rolled sheets. The casting results, the viscosity of the molten slag, and the solidification temperature are also shown in Table 1. In addition, about the powder of the symbols A and C, the base material which pre-melted about Si oxide and Ca oxide was used among the mixing | blending raw materials.

  In Table 1, “steel surface flaw generation” means that the surface flaw per cold-rolled coil is extremely good when the average is 1 or less on average, the mark is ◎, the average is over 1 and 2 or less is good, and the mark is over 2 X was marked as defective. Continuous casting was performed at a casting speed of 1.7 m / min. However, when no breakout warning was issued after 200 tons, the "mold temperature stability" was very good, and a single alarm was given. When it came out, it was marked as “Good”, and when it exceeded 2 times, it was marked as “Poor” as bad.

In addition to the occurrence of surface flaws on the steel sheet and the mold temperature stability, the points described in the “Others” column of Table 1 are combined, and “Comprehensive evaluation” indicates “very good”, “good”, and “bad”. It was. Symbols A to D are powders according to the present invention, both the state of occurrence of surface flaws on the steel sheet and the mold temperature stability are good. In particular, the powders of symbols B to D to which 2% or more of B ₂ O ₃ is added are mold temperature stability. Was very good. In addition, it was confirmed that the powders of symbols A and C using the pre-melted blended raw material were melted quickly and smoothly in the mold of the powder and were suitable for high-speed casting.

On the other hand, the powder of symbol E, which is a comparative example, had a poor mold temperature stability because of its low B ₂ O ₃ content. Further, since the powder of symbol F has a B ₂ O ₃ content that is too high, the amount of boron on the surface of the slab increased and cracking of the slab occurred.

The powder symbol G is a comparative example the viscosity of the slag is low because SiO ₂ content is too low, surface roughening slab surface due to excessive flow of molten slag occurs. On the other hand, since the powder of symbol H has a high SiO ₂ content, there were many occurrences of surface flaws on the steel sheet due to the entrainment of the powder.

Further, the powder of symbol I, which is a comparative example, had a low content of Al ₂ O _3, so the solidification temperature was high, and cracking of the slab due to poor lubrication occurred. On the other hand, the symbol J had a too high Al ₂ O ₃ content, so the solidification temperature was low and cracking of the slab due to supercooling occurred.

Moreover, the powder of the symbol K which is a comparative example had a high viscosity because of its low Na ₂ O content, and there were many slab surface defects due to the uneven flow of molten slag. On the other hand, the powder of symbol L had a low viscosity because the Na ₂ O content was too high, and the slab surface was rough due to excessive inflow of molten slag.

Moreover, the powder of the symbol M which is a comparative example has a high viscosity because the F content is low, and there are many slab surface defects due to uneven flow of molten slag. On the other hand, the powder of symbol N had a low viscosity because the F content was too high, and the slab surface was rough due to excessive inflow of molten slag.

Claims (5)

In the mold powder for continuous casting of steel containing SiO ₂ , CaO, Al ₂ O ₃ , Na ₂ O, B ₂ O ₃ , F and C as chemical components, by mass%, SiO ₂ is ₂₀ to 32%, A powder for continuous casting of steel, wherein Al ₂ O ₃ is 5 to 15%, Na ₂ O is 1 to 8%, B ₂ O ₃ is 1 to 10%, and F is 3 to 11%. The powder for continuous casting of steel according to claim 1, wherein the viscosity at 1300 ° C is 0.5 to 5.0 poise. The powder for continuous casting of steel according to claim 1 or 2, wherein a pre-melted blended raw material is used for at least two kinds of chemical components other than C. 4. The powder for continuous casting of steel according to claim 1, wherein the powder is used for casting steel for a thin steel plate. The powder for continuous casting of steel according to any one of claims 1 to 4, characterized in that, by mass%, CaO: 30 to 50% and C: 0.3 to 5%.