KR101064987B1 - Fluorine free mold flux for continuous casting of steel slab - Google Patents

Abstract

In the present invention, there is little loss of immersion nozzle by liquid mold flux in continuous casting mold, and it is possible to prevent corrosion of cooling water pipe by fundamentally eliminating fluorine (F) ion dissolution to secondary cooling water during continuous casting. The fluorine (F) ion elimination process can be omitted, and the fluorine-free mold flux that can suppress the breakout during casting of high-hydrogen molten steel due to the small change in physical properties of hydrogen in steel, CaO, SiO ₂ and that the B ₂ O ₃ main components, including calcium beam light silicate (calcium Borate silicate) crystalline phase and containing no fluorine in the weight% as _{characterized, B 2 O 3 10 ~ 30} %, CaO 25 ~ 50%, Providing a mold flux characterized in that the composition of SiO ₂ 20-40%, Na ₂ O 20% or less, Al ₂ O ₃ 15% or less, MgO 10% or less, the rest contains inevitable impurities and does not contain fluorine do.

Mold flux, continuous casting, fluorine, calcium borosilicate, basicity, slag

Description

Fluorine-free mold flux {FLUORINE FREE MOLD FLUX FOR CONTINUOUS CASTING OF STEEL SLAB}

1 is a schematic diagram showing a general continuous casting process;

Figure 2 is a schematic diagram for explaining the presence of the mold flux in the continuous casting mold;

3 is a process chart for explaining a general continuous casting cooling water treatment process;

Figure 4 is a graph comparing the crystallization temperature of the mold flux according to the prior art and the present invention in a dry atmosphere and a wet atmosphere;

5 is a graph comparing the viscosity of the mold flux according to the prior art and the present invention;

6 is a graph comparing the thermal conductivity of the mold flux according to the prior art and the present invention.

♣ Explanation of symbols for main part of drawing ♣

1: tundish 2: immersion nozzle

3: performance mold 4: tundish molten steel

5: solidification cell 6: molten steel

7: mold flux

The present invention relates to a mold flux that does not contain fluorine, and more particularly, less dissolution loss of the immersion nozzle by the liquid mold flux in the continuous casting mold, and fluorine (F) ion dissolution into secondary cooling water during continuous casting. By eliminating it fundamentally, it is possible to prevent corrosion of the cooling water pipe and to omit the fluorine (F) ion removal process by water treatment, and to suppress breakout when casting high-hydrogen molten steel due to the small change in physical properties due to hydrogen in steel. It relates to a mold flux that does not contain fluorine.

In the continuous casting process of the steel mill, molten steel in a tundish (1) is injected into the mold (3) through the immersion nozzle (2) to produce a cast steel of a predetermined shape, wherein the water is cooled Lubricant to prevent breakout phenomenon in which the solidified cell 5 breaks due to friction between the solidified cell 5 and the mold 3. As a powder or granular synthetic slag (Mlag Flux) 7 is added.

Referring to Figure 2 will be described in more detail the function of the mold flux (7) in the continuous casting mold (3). The mold flux has a lubrication function and a heat transfer control function for controlling the heat flow rate from the cast piece to the mold 3. That is, the mold flux is injected onto the molten steel 6 in the continuous casting mold 3 to melt while forming the unmelted layer, the semi-melted layer, and the molten slag layer, and the molten slag vibrates up and down between the mold and the solidification cell. It is introduced into the gap to form a thin film slag film (Slag Film).

The slag film is divided into two phases again. The side close to the mold is solidified to exist as a solid glass or crystalline, and the side close to the solidification cell is in a liquid state. Mold flux film is the most important factor in determining the heat transfer from the cast to the mold, especially when the solid slag film is present as a crystalline has a significant effect of lowering the heat flux (Heat Flux Density).

Increasing the amount of heat transfer in the mold generally causes the following problems.

First, as the coagulation cell grows unevenly, the surface crack of the cast steel increases, and secondly, the depth of the oscillation hook increases and the sliver defect increases, and the third, solidification of the mold corner part. Lifting occurs between the cell and the mold, causing the coagulation cell to re-dissolve to push the liquid phase out of the solid phase, thereby increasing the blowing breakout, and fourthly, the mold level fluctuation due to bulging is increased. .

In order to solve the above problems, the conventional mold flux has been designed so that Cuspidine (3CaO-2SiO ₂ -CaF ₂ ) is crystallized in the main crystalline phase so as to secure a crystalline mold slag film and to suppress the heat transfer during the playing operation. (F) component is necessarily contained, and the content is generally 3 to 15% by weight.

As mentioned above, the conventional mold flux inevitably containing the fluorine (F) component exhibits the following problems.

First, the conventional liquid mold flux covering the mold bath surface severely damages the immersion nozzle. Therefore, the immersion nozzles in contact with the mold flux are generally made of a material capable of suppressing melting loss as much as ZrO ₂ -C, and the remaining parts are generally made of Al ₂ O ₃ -C. ZrO ₂ -C is more expensive than Al ₂ O ₃ -C, and the cost of nozzle manufacturing is increased by using heterogeneous materials. In addition, even if the surface is protected by ZrO ₂ -C, the slag line is not prevented from being melted during the operation time, so that the so-called short change is performed to change the immersion nozzle depth.

In general, the immersion depth of the immersion nozzle has an optimum value depending on the operating conditions such as casting width, molten steel discharge angle, molten steel discharge amount. Therefore, the immersion depth is out of the optimum value in accordance with the change of the stage, thereby impairing the stability of the operation. In addition, if the change is no longer possible, the casting operation must be stopped, thus limiting the performance of the cast, thus lowering the productivity.

Second, the conventional fluorine-containing mold flux is a point that the fluorine component is dissolved in cooling water. Since the cast slab out of the mold is directly cooled by the cooling water sprayed through the nozzle or a mist, a mixture of cooling water and air, fluorine dissolves in the cooling water while the mold flux film on the surface of the cast steel is washed with the cooling water. Figure 3 schematically shows a general continuous casting cooling water treatment process, the fluorine concentration in the cooling water is more than 100ppm has a problem that the cooling water pipe is easily corroded. In addition, the discharged cooling water cannot be discharged as it is, so proper fluorine concentrations should be lowered below the environmental regulations (eg 15 ppm in Korea).                         

Currently, the fluorine treatment method generally carried out is a method in which rare earth metals are chemically combined with fluorine in cooling water by adding a fluorine treatment agent, which is a main component, and then coagulated using a polymer. In this case, fluorine treatment agent, polymer and other water costs are high, and a large apparatus such as a reaction tank and a concentration tank must be constructed and operated, and a fluorine elution problem occurs when the treated fluoride is buried in a cake state.

Third, it causes hydrogen breakout of the coagulation cell. In the fluorine-containing mold flux, the crystallization behavior is greatly affected by hydrogen in the steel. Hydrogen and oxygen in the steel together with the oxygen ions in the mold flux cause the following reactions.

[H] + [O] + (O 2-) → 2 (OH -)

(OH ^-) in the mold flux, if any groups known to be rapidly it promotes crystallization of Cuspidine. Therefore, in the conventional mold flux, the crystallization behavior is drastically changed according to the hydrogen concentration in the steel, causing instability of the operation. Specifically, as the crystallization of the mold flux is excessively accelerated during the summer when the hydrogen concentration in the steel is high, the heat transfer amount is excessively reduced, and the growth of the coagulation cell is slowed, causing breakout, which is commonly called hydrogen breakout. .

As described above, when the conventional flux containing fluorine is applied to the continuous casting process, the immersion nozzle (immersion nozzle) is impaired, the limitation of the lead-fuel ratio, and the corrosion of equipment such as cooling water piping due to the dissolution of fluorine (F) ions into the secondary cooling water. And generation of fluorine (F) ions in the cooling water by water treatment and generation of hydrogen breakout during casting of molten steel (H) molten steel due to promotion of crystallization behavior by hydrogen in steel.

As a prior art for solving the above problems caused by fluorine in the mold flux, Korean Patent Publication No. 6664 1993 provides a pyrogenic solvent that does not contain fluorine to prevent equipment corrosion. The so-called Front Powder, which is used to prevent the temperature drop of the molten metal, is limited to its use. In 1995, Korean Patent Publication No. 7131 restricted the fluorine content to 6% by weight or less to improve the quality of cast steel.In 1993, Korean Patent Publication No. 8444 contained 5-10% by weight of fluorine, but BaO 5 to 15% by weight and 5% by weight of Li ₂ O were added to reduce the amount of fluorine eluted in the cooling water. In addition, Japanese Patent No. 2001-353561 and European Patent No. 1063035 attempt to prevent the nozzle damage and equipment corrosion as much as possible by suppressing the content of fluorine in the mold flux to 3 wt% or less and 2 wt% or less, respectively.

However, none of these prior arts provides a mold flux from which fluorine is completely removed, and does not provide a crystalline phase containing no fluorine. Therefore, the above-described prior art cannot fundamentally solve the above-mentioned problems caused by fluorine.

In order to fundamentally solve various problems arising from the conventional mold flux, the present invention is designed to form a crystal phase other than Cuspidine in an fluorine-containing chemical system so that the amount of heat can be effectively controlled. In addition, the physical properties such as viscosity, thermal conductivity, and solidification temperature are also designed to be within the range similar to the existing mold flux, thereby limiting the immersion nozzle loss and its associated lead ratio, incurring facility corrosion and water treatment costs, and hydrogenous breakout during high hydrogen molten steel casting. It is an object of the present invention to provide a mold flux that does not contain fluorine that can fundamentally solve problems such as generation.

In order to achieve the above object, the present invention provides a mold flux containing a calcium borate silicate (Calcium Borate Silicate) crystal phase containing CaO, SiO ₂ and B ₂ O ₃ as a main component and does not contain fluorine.

In addition, the present invention in weight%, B ₂ O ₃ 10-30%, CaO 25-50%, SiO ₂ 20-40%, Na ₂ O 20% or less, Al ₂ O ₃ 15% or less, MgO 10% or less And a remainder which provide inevitable impurities and a fluorine-free mold flux.

In the present invention, the mold flux has a basicity (CaO / SiO ₂ ) in the range of 0.7 to 2.2 and forms Ca ₂ SiB ₂ O ₇ or Ca ₁₁ Si ₄ B ₂ O ₂₂ as a calcium borate silicate crystal phase. , (OH ^-) of the liquid slag, characterized in that by groups not affected by the crystallization behavior.

In addition, the mold flux according to the present invention is controlled by controlling the content of B ₂ O ₃ crystallization temperature, the viscosity is controlled by adjusting the component ratio of Na ₂ O or Al ₂ O ₃ .

EMBODIMENT OF THE INVENTION Hereinafter, the structure of this invention is demonstrated in detail.

The greatest influence on the heat transfer through the mold flux during continuous casting is the solidification and crystallization behavior of the mold flux. That is, as shown in Figure 2, the mold flux completely dissolved at high temperature is sandwiched between the coagulation cell and the mold in the form of a film and then solidified from the low temperature mold side is changed from glass to crystalline (Crystalline). When the mold flux is crystallized, an air gap occurs due to volume shrinkage between the copper mold and the crystalline mold flux film, thereby preventing heat transfer due to conduction, and the crystalline mold flux is radiated. It blocks the heat transfer by (Radiation).

Therefore, the heat transfer from the molten steel to the mold is greatly hindered by the crystallization of the mold flux. Therefore, the formation of an appropriate crystal layer mold flux film is indispensable for the stability of the continuous casting operation and the quality of the cast steel.

The crystallization behavior was investigated for various composition systems to find a composition system having a suitable crystalline phase that could substitute Cuspidine without containing fluorine. Based on the results, in the present invention, CaO, SiO ₂ , B ₂ O ₃ as a main component and Na ₂ O, Al ₂ O ₃ , MgO and the like was added to the mold flux was prepared in the composition, Calcium Borate as a crystal phase Silicate (Ca ₂ SiB ₂ O ₇ or Ca ₁₁ Si ₄ B ₂ O ₂₂ ) was crystallized.

Table 1 and Table 2 show the comparison between the chemical composition and the crystal phase of the mold flux according to the prior art and the present invention.

Table 2 shows the activation energy of the crystallization reaction with respect to the invention and the prior art mold flux. In the conventional mold flux for forming Cuspidine, the activation energy is about 80 KJ / mol, whereas in the inventive mold flux for forming Calcium Borate Silicate, the activation energy is about 15 KJ / mol, which is only about 20% of conventional materials. When the activation energy is lowered, it is easy to generate crystal nuclei, thereby shortening the time required from the start of crystallization to completion.                     

In addition, as the type of crystal phase is changed, the difference in physical properties including the crystallization behavior of the mold flux according to the present invention was investigated and compared with the conventional materials.

First, Figure 4 shows the crystallization temperature of the inventive mold flux according to the B ₂ O ₃ fraction compared with the conventional materials, within the range of 10 to 25% by weight B ₂ O ₃ fraction increases the crystallization temperature decreases 25% the above shows a tendency to increase the crystallization temperature in accordance with the B ₂ O ₃ fraction. This is because the B ₂ O ₃ fraction of Ca ₁₁ Si ₄ B ₂ O ₂₂ formed as the main crystal phase in the range of 10 to 25% is about 7.5% by weight, and thus the composition of the crystal phase increases with increasing concentration.

Of note in the contents shown in Figure 4 is that the crystallization temperature range of the invention material is 500 ℃ ~ 1200 ℃, the width of the change is larger than the conventional material. That is, by controlling only the content of B ₂ O ₃ it is possible to manufacture a mold flux having a wide range of crystallization temperature, it is possible to easily design the optimum mold flux for various casting operation conditions such as steel grade, casting speed, width, casting temperature Will be.

In the case of conventional materials, the method of controlling the crystallization temperature was mainly to change the basicity (CaO / SiO ₂ ) or the content of F. In this case, the following problems exist. First, since the basicity and the F content have a great influence on the viscosity value, which is the main physical property of the mold flux, it is difficult to freely control only the crystallization temperature. In addition, since the SiO ₂ reacts with Al in molten steel during the continuous casting operation and F is lost by volatilization, it is difficult to precisely control the crystallization temperature.

On the contrary, in the mold flux according to the present invention, the crystallization temperature is controlled by the fraction of B ₂ O ₃ , which is an element having extremely low component change during operation, while maintaining the basicity (CaO / SiO ₂ ) in the range of 0.7 to 2.2, as described above. Not only can control the crystallization temperature of a wide range, there is an advantage that the change in the crystallization temperature during the operation is smaller and more stable operation is possible.

In addition, Fig. 4 shows the comparison between the crystallization behavior of the inventive mold and the flux of the conventional mold in a dry atmosphere and a wet atmosphere. In the wet atmosphere, H ₂ O is dissolved in a liquid mold flux and thus (OH ⁻ To form a group.

^{_{H 2 O + (O 2-)}} → 2 (OH -)

Therefore, the wet atmosphere has the same effect as contacting molten steel with high hydrogen concentration. In the figure, the conventional mold flux shows a rapid acceleration of crystallization in a wet atmosphere. This is because the (OH ⁻ ) group present in the liquid mold flux promotes crystallization of Cuspidine. The fact that natural Cuspidine contains (OH ^– ) groups supports this trend.

Accordingly, in the conventional mold flux, the risk of breakout during the summer or high hydrogen molten steel casting is increased, whereas the mold flux according to the present invention has little difference in crystallization behavior according to the atmosphere. This is because Calcium Borate Silicate formed from the mold flux of the present invention is not affected by crystallization because it has no affinity for (OH ⁻ ) groups. Accordingly, when the mold flux according to the present invention is applied, it is possible to maintain a constant crystallization behavior irrespective of the hydrogen concentration in molten steel, so that stable casting operation can be performed without the risk of generating a hydrogen breakout.

In addition, as shown in Figure 5, it can be seen that the B ₂ O ₃ content in the mold flux according to the present invention does not significantly affect the viscosity value. Accordingly, two main properties of the mold flux, crystallization temperature and viscosity, can be controlled independently. That is, the crystallization temperature is controlled by the fraction of B ₂ O ₃ and the viscosity can be controlled by the component fraction of Na ₂ O or Al ₂ O ₃ . Specifically, if the viscosity is to be increased, Na ₂ O or Al ₂ O ₃ is increased. On the contrary, if the viscosity is to be decreased, the viscosity can be easily increased by increasing Na ₂ O or reducing Al ₂ O ₃ . In this case, the crystallization temperature is not significantly affected.

Therefore, in the case of the inventive mold flux, the mold flux film introduced into the mold during the continuous casting operation is advantageously completed for the initial cooling of the initial solidification layer as crystallization is completed early, so that the uniform solidification layer growth is achieved even during the high speed casting operation. Make it possible.

FIG. 6 shows the thermal conductivity of the inventive mold flux in comparison with the conventional materials. As in the conventional materials, the thermal conductivity of the crystalline material is higher than that of the glass material because the heat transfer due to the lattice vibration occurs actively in the crystalline material. to be. In general, it can be seen that the thermal conductivity of the invention is similar to that of the conventional art.

Hereinafter, the reason for limiting the composition of the mold flux according to the present invention will be described in more detail.

1) B ₂ O ₃ : 10 ~ 30% by weight

; As a component for determining the crystallization temperature in the present invention, the crystallization temperature decreases as the B ₂ O ₃ fraction increases to 10 to 25% by weight, and at 25% or more, the crystallization temperature increases according to the B ₂ O ₃ fraction. have. Therefore, in the present invention, since the crystallization temperature ranges from 500 to 1200 ° C., the amount of the crystallization temperature may be set to 10 to 30% by weight in consideration of the loss of components.

2) CaO: 25 ~ 50 wt%

; The CaO component is a component that forms the basicity of the mold flux together with the SiO ₂ component. Since the basicity (CaO / SiO ₂ ) of the mold flux of the present invention is in the range of 0.7 to 2.2, in order to obtain the required slag viscosity while satisfying this range, It is limited to 25 wt% or more and up to 50 wt% or less.

3) SiO ₂ : 20 ~ 40 wt%

; The SiO ₂ component is a component that forms the basicity of the mold flux together with the CaO component described above. The basic flux (CaO / SiO ₂ ) of the mold flux of the present invention is in the range of 0.7 to 2.2, so that 20 to 40 wt% is satisfied. Limit to scope

4) Na ₂ O: 20 wt% or less

; The Na ₂ O component in the mold flux is a component that is formed to control the melting point of the mold flux. In the present invention, when the composition exceeds 20% by weight, the melting point of the mold flux is lowered, so that the viscosity and surface tension are excessively lowered. Decreases significantly.

5) Al ₂ O ₃ : 15 wt% or less

; The Al ₂ O ₃ component, like the Na ₂ O component described above, is a component that controls the viscosity of the mold flux. When the composition is more than 15% by weight, the viscosity of the flux is excessively increased and the absorption capacity of nonmetallic inclusions in the molten steel is reduced. Therefore, to increase the viscosity, the Na ₂ O component is decreased and the Al ₂ O ₃ component is increased. To decrease the viscosity, the Na ₂ O component is increased and the Al ₂ O ₃ component is decreased.

6) MgO: 10 wt% or less

; It is inevitably contained in the mold flux of the present invention and is a basic component together with the CaO component. Therefore, in the present invention, the basicity of the flux is limited to 10 wt% or less in order to control the CaO component.

Hereinafter, the operation of the present invention through the preferred embodiment.                     

[Example]

Inventive material and comparative material mold flux of the composition shown in Table 2 was prepared and the physical properties such as crystallization behavior, viscosity, thermal conductivity and the like are shown in Table 3.

Although no fluorine was added as shown in Table 3 and Table 4, the mold flux according to the present invention showed a crystallization temperature, viscosity, and thermal conductivity in a range similar to that of the prior art. However, the activation energy required for crystallization in the inventive material is about 16 KJ / mole, which is about 1/5 lower than that of the conventional 78 ~ 79 KJ / mole. Accordingly, since the liquid mold flux introduced between the mold and the coagulation cell forms a stable crystal layer at an early stage, it is possible to prevent uneven growth of the initial coagulation layer.

As described above, since the mold flux according to the present invention does not form a fluorine component as a constituent, it is possible to drastically reduce the melting loss of the immersion nozzle, and to increase the soft casting ratio. In addition, it is possible to prevent corrosion of the piping equipment of the continuous casting cooling water, there is an effect that the water treatment cost for cooling water discharge is significantly reduced. In addition, there is an effect that can fundamentally solve the occurrence of hydrogen breakout during high-hydrogen molten steel casting.

Claims (7)

In the mold flux, A mold flux comprising CaO, SiO ₂ and B ₂ O ₃ components, comprising a calcium borate silicate crystal phase and free of fluorine. In the mold flux, By weight%, B ₂ O ₃ 10-30%, CaO 25-50%, SiO ₂ 20-40%, Na ₂ O more than 0% 20% or less, Al ₂ O ₃ 0% or more 15% or less, MgO 0% Mold flux, characterized in that it is made up to more than 10%, and the rest contains unavoidable impurities and does not contain fluorine. The mold flux according to claim 1 or 2, wherein the mold flux has a basicity (CaO / SiO ₂ ) of 0.7 to 2.2. The mold flux of claim 1 or 2, wherein the mold flux forms Ca ₂ SiB ₂ O ₇ or Ca ₁₁ Si ₄ B ₂ O ₂₂ as a Calcium Borate Silicate crystal phase. delete The mold flux according to claim 1 or 2, wherein the mold flux controls the crystallization temperature by adjusting the content of B ₂ O ₃ . The mold flux of claim 1 or 2, wherein the mold flux controls viscosity by adjusting a component ratio of Na ₂ O or Al ₂ O ₃ .

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