JP2020146719A - Mold powder and method of producing medium-carbon steel - Google Patents

Abstract

To provide a mold powder that is compatible with both of inhibiting cracks in a surface of a cast slab and lubrication between a solidification shell and a mold to a higher degree, in continuous casting of medium-carbon steel having a carbon concentration of 0.08-0.25 mass%.SOLUTION: A mold powder contains SiO2 and CaO as main ingredients in which a mass ratio of CaO to SiO2 (CaO/SiO2) is 1.1 or greater and 2.5 or smaller, and has a K2O content of 1.0-10.0 mass%, Na2O and Li2O contents of 1.0-18.0 mass% in total, F, MgO, Al2O3 and total carbon contents of respectively 3.0-15.0 mass%, 0.5-3.0 mass%, 0.5-10.0 mass% and 1.0-20.0 mass%, a viscosity at 1300°C of 0.03-0.7 Pa s, and a crystallization temperature of 1080-1280°C, in which a primary crystal seed is of Cuspidine (Cuspidine: 3CaO2SiO2 CaF2).SELECTED DRAWING: None

Description

The present invention relates to a method for producing a mold powder and a medium carbon rope having a carbon concentration of 0.08 to 0.25% by mass.

Continuous steel casting is the continuous casting of steel by pouring molten steel into a mold of a continuous casting machine, cooling and solidifying it, and continuously pulling out the solidified solidified shell from below the mold. Say that. Powdered or granular mold powder is added to the surface of the molten steel in the mold. The mold powder is melted by the heat of the molten steel (hereinafter, the state in which the mold powder is melted is called "powder slag" or "slag"), and the slag flows between the solidified shell and the mold, and the film (hereinafter, It changes to slag film). The driving force that the slag flows between the solidified shell and the mold is the oscillation (vibration) of the mold, the pull-in by pulling out the solidified shell, and the weight of the slag. The main roles of the mold powder are (1) heat retention and oxidation prevention of the molten steel surface, (2) absorption of non-metal inclusions floating from the molten steel and cleaning of the molten steel, and (3) maintenance of lubrication between the solidified shell and the mold. , (4) Suppression and homogenization of heat removal from the solidified shell to the mold.

By the way, carbon steel having a carbon concentration of 0.08 to 0.25% by mass (hereinafter, referred to as "medium carbon steel", sometimes referred to as "subcapsular steel") shrinks because it undergoes phase transformation during solidification. The amount is large (see Patent Documents 1 and 2 and Non-Patent Document 1). For this reason, in continuous casting of medium carbon steel, the solidification shell is bent or delayed in solidification, stress is concentrated, and the surface of the slab is likely to be cracked. Further, in medium carbon steel, the gap between the solidified shell and the mold becomes large at the lower part of the mold due to solidification shrinkage, and an air gap is likely to be formed. For this reason, uneven coating of slag is likely to occur in the solidified shell, such as a portion that does not locally get wet with the slag or the attached slag becomes thin. If uneven coating of slag occurs, heat removal becomes non-uniform and cracks are likely to occur on the surface of the slag. As described above, the productivity of medium carbon steel tends to decrease due to deterioration of yield and regulation of casting speed to prevent it.

As a countermeasure, a method of raising the crystallization temperature of the mold powder, thickly precipitating a crystal layer that acts as a thermal resistance in the slag film flowing between the solidified shell and the mold, and slowing the solidification rate of steel, so-called slow cooling. Has been oriented.

Japanese Unexamined Patent Publication No. 2018-153831 WO2017 / 078178

Hiroshi Murakami and 3 others, "Control of non-uniform solidification of subcapsular carbon steel in continuous casting mold", Iron and Steel, The Iron and Steel Institute of Japan, Vol.78, No.1, p.105-112, 1992

However, it has been pointed out that slow cooling hinders lubrication between the solidified shell and the mold and sufficient thickness of the solidified shell. Furthermore, although caspidine (Cuspidine: 3CaO, 2SiO ₂ , CaF ₂ ) is generally used as the crystal species that precipitates in the slag film and contributes to slow cooling, the promotion of caspidine precipitation by optimizing the mold powder composition is improved. There is almost no room. Therefore, there is a strong demand for a slab crack suppression technique that focuses on methods and characteristics different from slow cooling.

Some aspects of the present invention have been made in view of the above circumstances, and an object of the present invention is to continuously cast medium carbon steel having a carbon concentration of 0.08 to 0.25% by mass on the surface of a slab. To provide a mold powder capable of suppressing cracks and lubricating between a solidified shell and a mold to a higher degree, and to provide a method for producing a medium carbon steel having continuous casting using the same. is there.

When the mold powder is heated to the molten steel temperature, chemical reactions such as decomposition and oxidation occur, so that the chemical composition fluctuates before and after heating. Therefore, in the present specification, the chemical composition of the mold powder is expressed by mass% in terms of oxide for components other than F and C, F in mass% in terms of simple substance, and C as a carbon raw material. What is added is expressed in mass% in terms of simple substance, and C (C of calcium carbonate, etc.) other than those added as a carbon raw material is assumed to disappear (0% by mass).

(1) One aspect of the present invention comprises SiO ₂ and CaO as main components, a mass ratio of SiO ₂ CaO (CaO / SiO ₂₎ is 1.1 to 2.5, the _{K 2} O The content is 1.0 to 10.0% by mass, the total content of Na ₂ O and Li ₂ O is 1.0 to 18.0% by mass, and F, MgO, Al ₂ O ₃ and total. The carbon contents are 3.0 to 15.0% by mass, 0.5 to 3.0% by mass, 0.5 to 10.0% by mass and 1.0 to 20.0% by mass, respectively, and 1300 ° C. The mold is characterized by having a viscosity of 0.03 to 0.7 Pa · s, a crystallization temperature of 1080 to 1280 ° C., and a primary crystal species of Caspidine (Cuspidine: 3CaO · 2SiO ₂ · CaF ₂ ). Regarding powder.

K 2 _O is Li 2 _O is the same alkali metal _oxide, the viscosity as with Na 2 _O, crystallization temperature, has the effect of lowering the surface tension, viscosity, decrease the effect of crystallization temperature is the smallest, the surface The effect of reducing tension is greatest. That is, K ₂ O greatly reduces the surface tension without significantly changing the viscosity and the crystallization temperature. By mold powder satisfies all the above requirements, in particular, by the K 2 _O content is 1.0 to 10.0 wt%, the viscosity of the slag, crystallization temperature without widely changing the surface tension Is greatly reduced. Since the viscosity and crystallization temperature of the slag do not change significantly, slow cooling can be maintained at a high level and uneven coating of the slag can be reduced. Therefore, uniform heat removal from the solidified shell to the mold is maintained, and cracking on the surface of the slab can be suppressed. Further, since the surface tension is greatly reduced, it is considered that the slag is well wetted with the solidified shell and easily flows into the solidified shell and the mold. Therefore, the lubrication between the solidified shell and the mold can be maintained. In this way, it is possible to achieve both suppression of cracks on the surface of the slab and lubrication between the solidified shell and the mold.

(2) In one aspect of the present invention, the surface tension at 1300 ° C. is preferably 190 to 300 mN / m. Lubrication between the solidified shell and the mold can be further improved, coating unevenness at the lower part of the mold is further reduced, and heat removal from the solidified shell to the mold becomes more uniform, so that cracks on the slab surface are further reduced. This is because it can be reduced.

(3) Another aspect of the present invention includes a step of continuously casting medium carbon steel, and the medium carbon steel has a carbon concentration of 0.08 to 0.25% by mass, and in the above step, one of the present inventions. The present invention relates to a method for producing a medium carbon steel, which comprises using the mold powder according to the above embodiment.

By using the mold powder of one aspect of the present invention, even in the continuous casting of medium carbon steel having a large shrinkage amount, it is possible to suppress cracks on the surface of the slab and to lubricate between the solidified shell and the mold. Therefore, it is possible to stably produce slabs of medium carbon steel having good surface quality.

Hereinafter, preferred embodiments of the present invention will be described in detail. It should be noted that the present embodiment described below does not unreasonably limit the content of the present invention described in the claims, and all the configurations described in the present embodiment are essential as a means for solving the present invention. Is not always the case.

(1) mold powder mold powder of the present embodiment includes a SiO ₂ and CaO as main components, a mass ratio of SiO ₂ CaO (CaO / SiO ₂₎ is 1.1 to 2.5, _{K 2} The content of O is 1.0 to 10.0% by mass, the total content of Na ₂ O and Li ₂ O is 1.0 to 18.0% by mass, and F, MgO, Al ₂ O ₃ And the total carbon content is 3.0 to 15.0% by mass, 0.5 to 3.0% by mass, 0.5 to 10.0% by mass and 1.0 to 20.0% by mass, respectively. The viscosity at 1300 ° C. is 0.03 to 0.7 Pa · s, the crystallization temperature is 1080 to 1280 ° C., and the primary crystal species is caspidine (Cuspidine: 3CaO · 2SiO ₂ · CaF ₂ ).

[Mass ratio (CaO / SiO ₂ )]
The mold powder contains SiO ₂ and CaO as main components. Weight ratio of SiO ₂ CaO (CaO / SiO ₂₎ is preferably 1.1 or more and 2.5 or less, more preferably 1.2 or more 2.1 or less. If the mass ratio (CaO / SiO ₂ ) is less than 1.1, the amount of crystals in the slag film is small, and a sufficient slow cooling effect cannot be obtained. On the other hand, when the mass ratio (CaO / SiO ₂ ) exceeds 2.5, the viscosity of the slag is significantly reduced, and the slag droplets are separated and caught in the molten steel, which tends to cause defects in the slab. Further, since caspidine decreases and crystals other than caspidine increase, a sufficient slow cooling effect cannot be obtained.

[K ₂ O]
The K ₂ O content is 1.0 to 10.0 wt%, preferably from 1.5 to 6.0 wt%. K 2 _O is Li 2 _O is the same alkali metal _oxide, the viscosity as with Na 2 _O, crystallization temperature, has the effect of lowering the surface tension, viscosity, decrease the effect of crystallization temperature is the smallest, the surface The effect of reducing tension is greatest. That is, K ₂ O greatly reduces only the surface tension without significantly changing the viscosity and the crystallization temperature. Therefore, when the K ₂ O content is in the above range, the decrease in surface tension can promote the inflow of slag between the solidified shell and the mold, and maintain the lubrication between the solidified shell and the mold. .. Furthermore, since the viscosity and crystallization temperature of the slag do not change significantly, slow cooling can be maintained at a high level and cracking on the surface of the slab can be suppressed. If the content of K ₂ O is less than 1% by mass, the effect of lowering the surface tension cannot be obtained and the inflow of slag cannot be promoted, so that the lubrication between the solidified shell and the mold cannot be maintained, and the yield tends to deteriorate. On the other hand, if the K ₂ O content exceeds 10.0% by mass, the viscosity of the slag is significantly reduced, and slag entanglement defects are likely to occur. Furthermore, since caspidine is reduced, a sufficient slow cooling effect cannot be obtained.

[Na ₂ O + Li ₂ O]
The total content of Na ₂ O and Li ₂ O is 1.0 to 18.0% by mass, preferably 3.0 to 14.0% by mass. Na ₂ O and Li ₂ O have the effect of lowering the surface tension of the slag, but the effect is smaller than that of K ₂ O. If Na ₂ O or Li ₂ O is excessively added in order to sufficiently reduce the surface tension of the slag, the crystallization of the slag is inhibited, a sufficient slow cooling effect cannot be obtained, or the viscosity of the slag is significantly reduced. It is prone to slag entanglement defects. If the total content of Na ₂ O and Li ₂ O is less than 1.0% by mass, the solidification temperature of the slag becomes high and the lubrication between the solidified shell and the mold is impaired. On the other hand, when the total content of Na ₂ O and Li ₂ O exceeds 18.0% by mass, the viscosity of the slag is significantly reduced, and the slag entrainment defect and the melting damage of the immersion nozzle are increased.

[F]
The content of F is 3.0 to 15.0% by mass, preferably 5.0 to 14.0% by mass. If the content of F is less than 3.0% by mass, the precipitation of caspidine in the slag film that gives a slow cooling effect is insufficient, and crystals other than caspidine increase. On the other hand, when the F content exceeds 15.0% by mass, the viscosity of the slag is significantly reduced, and the slag entrainment defect and the melting damage of the immersion nozzle are increased.

[MgO]
The content of MgO is 0.5 to 3.0% by mass, preferably 0.8 to 2.5% by mass. If the MgO content exceeds 3.0% by mass, the precipitation of caspidine is remarkably reduced, and the slow cooling effect cannot be obtained.

[Al ₂ O ₃ ]
The content of Al ₂ O ₃ is 0.5 to 10.0% by mass, preferably 1.0 to 6.0% by mass. If the content of Al ₂ O ₃ exceeds 10.0% by mass, it becomes difficult for caspidine to precipitate, and the precipitation of gerenite, which is a refractory crystal, increases, resulting in poor melt properties and non-uniform heat removal.

[Total carbon]
The total carbon content is 1.0 to 20.0% by mass, more preferably 2.0 to 12.0% by mass. If the total carbon content is less than 1.0% by mass, the slag rate becomes excessively high and the molten layer of slag in the mold becomes excessively thick, resulting in the formation of slag bears and slag adhering to the molten metal level sensor. However, it may be damaged. On the other hand, when the total carbon content exceeds 20% by mass, the slagging rate becomes low, the thickness of the molten layer of slag is insufficient, the heat retention of the molten steel surface is insufficient, the lubrication between the solidified shell and the mold is insufficient, and the molten steel is affected. Operational abnormalities such as entrainment of unmelted mold powder occur.

[viscosity]
The viscosity of the slag at 1300 ° C. is 0.04 to 0.7 Pa · s, preferably 0.04 to 0.50 Pa · s. If the viscosity of the slag at 1300 ° C. is less than 0.04 Pa · s, the slag entrainment defect increases, and if it exceeds 1.0 Pa · s, the inflow of slag decreases, resulting in insufficient lubrication.

[Crystalization temperature]
The crystallization temperature of the mold powder is 1080 to 1280 ° C, preferably 1100 to 1220 ° C. If the slag crystallization temperature is less than 1080 ° C., the slow cooling effect of the solidified shell is insufficient and the slab surface is liable to crack. On the other hand, when the slag crystallization temperature exceeds 1280 ° C., the crystal layer becomes excessively thick, so that the inflow of slag is reduced, the solidified shell is broken, and breakout is likely to occur.

[Primary crystal species]
The primary crystal species needs to be caspidine (Cuspidine: 3CaO, 2SiO ₂ , CaF ₂ ). If crystals other than caspidyne are primary crystal species, the slow cooling effect cannot be obtained.

[surface tension]
The surface tension of the slag at 1300 ° C. is 190 to 300 mN / m, preferably 210 to 280 mN / m. When the surface tension of the slag exceeds 300 mN / m, the inflow of the slag between the solidified shell and the mold is small, resulting in insufficient lubrication, non-uniform heat removal, insufficient slow cooling, and the like. Further, crystallization is not promoted, slow cooling is insufficient, and slab cracking is likely to occur. The lower the surface tension, the more preferable, but if it is less than 190 mN / m, other properties such as viscosity and crystallization temperature are unsuitable for casting medium carbon steel.

[Raw material for mold powder]
Raw materials for mold powder of the present embodiment CaO-SiO ₂ substrate material, silica material, _{K 2} O ingredients, flux material, and a carbon source, and / or other raw materials. Examples of the CaO-SiO ₂ base material include cements such as synthetic calcium silicate, wollastonite, rinse slag, blast furnace slag, dicalcium silicate, calcium carbonate, limestone, quicklime, and Portland cement. Examples of the silica raw material include pearlite, fly ash, silica sand, feldspar, silica stone, diatomaceous earth, glass powder, silica fume, silica flower and the like. The K 2 _O using, for example, potassium carbonate, potassium nitrate, potassium feldspar, potassium hydrogen carbonate, potassium cryolite, potassium fluoride, or synthetic raw materials containing these is. The flux raw material has a role of adjusting the softening point, viscosity and / or crystallization temperature, and for example, fluoride such as sodium fluoride, lithium fluoride, cryolite, fluorite, magnesium fluoride, sodium carbonate, etc. Examples thereof include carbonates such as lithium carbonate and magnesium carbonate, boric acid, boar sand, and cryolite. The carbon raw material has a role of adjusting the melting rate, and examples thereof include coke, graphite, and carbon black. Examples of other raw materials include magnesia and alumina. The form of the mold powder is not particularly limited, and examples thereof include powder, extruded granules, hollow spray granules, and agitated granules.

(2) Method for producing medium carbon steel The method for producing medium carbon steel of the present embodiment includes a step of continuously casting medium carbon steel, and the medium carbon rope has a carbon concentration of 0.08 to 0.25% by mass. Yes, the mold powder of this embodiment is used in the above step.

By using the mold powder of the present embodiment, it is possible to highly achieve both suppression of cracks on the surface of the slab and lubrication between the solidified shell and the mold even in continuous casting of medium carbon steel having a large shrinkage amount. Therefore, it is possible to stably produce medium carbon steel slabs having good surface quality.

Hereinafter, examples of the present invention will be described in detail.

[experimental method]
Continuous casting of medium carbon steel was performed using mold powder. Table 1 shows the composition of the mold powder. Examples 1 to 10 are examples of the present invention, and Comparative Examples 1 to 9 are comparative examples of the present invention.

Examples 1 to 10, weight ratio of SiO ₂ CaO (CaO / SiO ₂₎ is 1.1 to 2.5, the content of _{K 2} O is 1.0 to 10.0 wt% , Na ₂ O and Li ₂ O total content is 1.0 to 18.0 mass%, and F, MgO, Al ₂ O ₃ and total carbon content are 3.0 to 15.0 mass, respectively. %, 0.5 to 3.0% by mass, 0.5 to 10.0% by mass, and 1.0 to 20.0% by mass. On the other hand, in Comparative Examples 1 to 4, the content of K ₂ O does not satisfy 1.0% by mass. Further, in Comparative Example 1, the mass ratio (CaO / SiO ₂ ) does not satisfy 1.1, and in Comparative Example 2, the mass ratio (CaO / SiO ₂ ) exceeds 2.5. In Comparative Examples 5 to 8, the contents of F, MgO, Al ₂ O ₃ and K ₂ O exceed 15.0% by mass, 3.0% by mass, 10.0% by mass and 10.0% by mass, respectively. In Comparative Example 9, the content of F does not satisfy 3.0% by mass.

Table 2 shows the casting conditions for continuous casting, that is, the mold size, casting speed, and carbon concentration of medium carbon steel.

The casting conditions of Examples and Comparative Examples were the same.

[Evaluation method]
The following items were evaluated for the results of mold powder (slag) and continuous casting.

<Viscosity>
The viscosity of the mold powder (slag) was measured by the ball pulling method. That is, the viscosity was obtained from the load when a platinum ball having a diameter of 10 mm was suspended in a slag at 1300 ° C. and the platinum ball was pulled up at a speed of 0.85 cm / s.

<Crystallization temperature>
The crystallization temperature of the mold powder was measured by the differential thermal method. That is, after raising the temperature of about 150 g of the mold powder and melting it, the temperature of the mold powder (slag) was measured while lowering the temperature at 4 ° C./min, and the temperature at the start of heat generation was defined as the crystallization temperature.

<Primary crystal species>
The primary crystal species of the mold powder (slag) were identified by X-ray diffraction.

<Surface tension>
The surface tension of the mold powder (slag) was measured by the ring method. That is, a platinum ring having a diameter of 10 mm is immersed in a slag at 1300 ° C., the platinum ring is pulled up at a speed of 0.85 cm / s, and the maximum load shown at the moment when the platinum ring separates from the slag liquid surface and the droplet is cut. The surface tension was calculated from.

<Type of surface crack>
The type of crack on the surface of the slab was investigated by visual observation of the surface of the slab.

<Crack evaluation>
The slab cracks generated after the slabs were hot-rolled were evaluated by the rate of downgrade due to cracks. That is, if the disqualification rate is less than 2%, it is "excellent: ◎", if it is 2 to 5%, it is "good: ○", if it is 5 to 10%, it is "possible: △", and if it is 10% or more, it is "impossible: ×". I evaluated it.

<Operational stability (presence or absence of restraint)>
The operational stability (presence or absence of restraint) was evaluated as "no problem: ○" when there was no problem in the continuous casting operation, and "problem: ×" when slab restraint occurred due to insufficient lubrication.

[Evaluation results]
The evaluation results are shown in Table 3.

In Examples 1 to 10, the crystallization temperature was 1080 ° C. or higher, and the primary crystal species was caspidine (Cuspidine: 3CaO, 2SiO ₂ , CaF ₂ ). The type of surface crack was "none" or "fine", and the crack evaluation was "◎" or "○". It is considered that this is because caspidyne was appropriately precipitated in the slag, a sufficient cooling effect of the solidified shell was obtained, and cracks were less likely to occur on the surface of the slab.

In each of Examples 1 to 10, the viscosity and surface tension at 1300 ° C. were 0.03 to 0.7 Pa · s and 190 to 300 mN / m, respectively. In addition, the operational stability (presence or absence of restraint) was "○", and slab restraint was not recognized. It is considered that this is because the slag flows in between the solidified shell and the mold appropriately, and lubrication and uniform heat removal are stably maintained.

From the above, in each of Examples 1 to 10, it was possible to achieve both suppression of cracks on the surface of the slab and lubrication between the solidified shell and the mold.

On the other hand, vertical cracks or horizontal cracks occurred in Comparative Examples 1, 4 and 7. This is because the viscosity is relatively high at 0.11 to 0.75 Pa · s and the surface tension is high at 300 mN / m or more, so that slag does not sufficiently flow between the solidified shell and the mold, and slow cooling is possible. Probably because of the shortage. A shallow dent was formed in Comparative Examples 2 and 5. It is considered that this is because the viscosity is relatively low at 0.03 Pa · s, the surface tension is relatively high at 240 mN / m or more, and the slag has flowed in excessively. In Comparative Example 3, vertical cracking of the slab and restraint of the slab occurred. This is thought to be due to insufficient lubrication and uneven heat removal. In Comparative Example 6, fine cracks and slab restraints occurred. This is because the viscosity is relatively high at 0.11 Pa · s, the surface tension is high at 355 mN / m or more, the crystallization temperature is high at 1285 ° C., and the primary crystal species is dicalcium silicate, so that lubrication is insufficient. It is probable that the slag did not flow sufficiently between the solidified shell and the mold, and the slow cooling was insufficient. In Comparative Example 7, vertical cracks and slab restraints occurred. It is considered that this is because the viscosity and surface tension are high, so that crystal precipitation is insufficient, slag flow is insufficient, and lubrication is insufficient. In Comparative Example 8, a deep oscillation mark was formed in addition to the vertical dent. In Comparative Example 9, vertical and horizontal cracks and slab restraint occurred. It is probable that this is because the content of F is low, so that caspidine does not precipitate, but crystalline minerals composed of Ni ₂ O, Ca O, and SiO ₂ precipitate, resulting in non-uniform heat removal and insufficient lubrication.

Although the present embodiment has been described in detail as described above, those skilled in the art will easily understand that many modifications that do not substantially deviate from the novel matters and effects of the present invention are possible. Therefore, all such modifications are within the scope of the present invention. For example, in the specification, a term described at least once with a different term having a broader meaning or a synonym may be replaced with the different term at any part of the specification. Further, the configuration and operation of the manufacturing apparatus and the like of the present embodiment are not limited to those described in the present embodiment, and various modifications are possible.

Claims (3)

Contains SiO ₂ and CaO as main components
Weight ratio of SiO ₂ CaO (CaO / SiO ₂₎ is 1.1 to 2.5,
The K ₂ O content is 1.0 to 10.0 wt%,
The total content of Na ₂ O and Li ₂ O is 1.0 to 18.0% by mass.
The contents of F, MgO, Al ₂ O ₃ and total carbon are 3.0 to 15.0% by mass, 0.5 to 3.0% by mass, 0.5 to 10.0% by mass and 1.0 to 1.0 to respectively. 20.0% by mass,
The viscosity at 1300 ° C. is 0.03 to 0.7 Pa · s.
The crystallization temperature is 1080-1280 ° C.
A mold powder characterized in that the primary crystal species is caspidine (Cuspidine: 3CaO, 2SiO ₂ , CaF ₂ ). In the mold powder according to claim 1,
A mold powder having a surface tension of 190 to 300 mN / m at 1300 ° C. Has a process of continuously casting medium carbon steel
The medium carbon class has a carbon concentration of 0.08 to 0.25% by mass.
A method for producing medium carbon steel, which comprises using the mold powder according to claim 1 or 2 in the step.