KR101064988B1 - Fluorine free boron oxide system 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. A fluorine-free B ₂ O ₃ -based mold flux which can omit the fluorine (F) ion removal step and can suppress breakout when casting molten steel due to the small change in physical properties due to hydrogen in the steel. , By weight%, B ₂ O ₃ 30-50%, CaO 20-45%, SiO ₂ 15-40%, Na ₂ O 15% or less, Al ₂ O ₃ 10% or less, MgO 10% or less, The rest provides a B ₂ O ₃ -based mold flux, which contains unavoidable impurities and does not contain fluorine.

Mold Flux, Ｂ₂Ｏ₃, Continuous Casting, Fluorine, Calcium Borosilicate, Slag

Description

Fluorine-free Ｂ₂O₃-based mold flux {FLUORINE FREE BORON OXIDE SYSTEM 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;

4 is a graph comparing the crystallization temperature of the conventional material and the B ₂ O ₃ -based mold flux according to the present invention in a dry atmosphere and a wet atmosphere;

5 is a graph comparing the viscosity of the prior art and B ₂ O ₃ -based mold flux according to 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 B ₂ O ₃ -based mold flux that does not contain fluorine. More specifically, less immersion nozzles are immersed by the liquid mold flux in the continuous casting mold. F) By eliminating ion dissolution fundamentally, corrosion of cooling water pipes can be prevented, and the fluorine (F) ion removal process by water treatment can be omitted, and there is little change in physical properties by hydrogen in steel, so breakout occurs when casting high-hydrogen molten steel. The present invention relates to a B ₂ O ₃ -based mold flux that does not contain fluorine capable of suppressing 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.

The function of the mold flux 6 in the continuous casting mold 3 will be described in more detail with reference to FIG. 2. 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 15ppm 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, in the case of applying the conventional flux containing fluorine to the continuous casting process, equipment such as immersion nozzle (immersion nozzle) loss and the limitation of lead-fuel ratio and cooling water piping due to dissolution of fluorine (F) ion into secondary cooling water Problems include the generation of fluorine (F) ions in the cooling water by corrosion and water treatment, and the generation of hydrogen breakout during casting of molten steel (H) due to the promotion of crystallization behavior by hydrogen in the 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 the problems caused by the conventional mold flux, the present invention is designed to form a crystal phase other than Cuspidine in an fluorine-free 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 B ₂ O ₃ -based 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 in the mold flux, by weight%, B ₂ O ₃ 30-50%, CaO 20-45%, SiO ₂ 15-40%, Na ₂ O 15% or less, Al ₂ O ₃ and the composition to less than 10%, MgO less than 10%, and the other provides a B ₂ O ₃ based mold flux, characterized in that comprises the inevitable impurities, and which does not contain fluorine.

In addition, the present invention provides a B ₂ O ₃ -based mold flux characterized in that to form Ca ₂ SiB ₂ O ₇ in the calcium borate Silicate crystal phase by the mold flux.

The present invention also provides a B ₂ O ₃ -based mold flux characterized in that the crystallization behavior is not affected by the (OH ⁻ ) group in the liquid slag.

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 to the mold flux was prepared in the composition, Calcium Borate as a crystal phase Silicate (Ca ₂ SiB ₂ O ₇ ) was allowed to crystallize.

Table 1 and Table 2 show the comparison between the chemical composition and the crystalline 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 conventional mold flux. In the conventional mold flux for forming Cuspidine, the activation energy is about 80 KJ / mol, whereas in the mold flux of the present invention for forming Calcium Borate Silicate, the activation energy is about 37 KJ / mol, which is only about 50% of conventional materials. The lower the activation energy, the easier the formation of crystal nuclei, and therefore the shorter the time required for the completion of crystallization from 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 mold flux according to the present invention according to the basicity (CaO / SiO ₂ ) compared with the conventional material, both the conventional material and the invention material tends to increase the crystallization temperature with increasing the basicity Indicates. This is because the glass structure is broken in the liquid mold flux due to the increase of the basicity, so that the crystal phase is easily formed. Both the invention and the prior art exhibit crystallization temperatures in the range of about 800 to 1200 ° C. Therefore, it can be seen that the invention without fluorine can be designed to have a crystallization temperature range similar to that of the existing mold flux.

In particular, in the case of the conventional material, the change in the crystallization temperature according to the change of basicity in the case of the conventional material was investigated as a more rapid than the invention material, which acts as a cause of the change in the physical properties of the mold flux during operation. That is, since the SiO ₂ in the mold flux reacts with Al in molten steel during continuous casting, the basicity tends to increase. However, in the case of conventional materials, it is difficult to precisely control the crystallization temperature because the crystallization temperature is sensitive to the basicity. There was this.

In addition, in the case of conventional materials, the crystallization temperature is greatly influenced by the content of F, and thus, the crystallization temperature increases as F increases. Loss due to volatilization of F occurs during casting, which also causes a drastic change in the crystallization temperature during casting.

On the contrary, in the mold flux according to the present invention, since the change in crystallization temperature with respect to the change in basicity is relatively small, the change in crystallization temperature during the operation is small, thereby making it possible to operate more stably.

In addition, as shown in Figure 5, it can be seen that the basicity 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 basicity and the viscosity can be controlled by the component fraction of Na ₂ O or Al ₂ O ₃ .

More 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 is easily increased by increasing Na ₂ O or reducing Al ₂ O ₃ . In this case, the crystallization temperature is not significantly affected.

On the other hand, in the case of conventional materials, as the basicity and F content increase, the crystallization temperature increases rapidly and the viscosity decreases rapidly. Therefore, it is difficult to control the crystallization temperature and the viscosity independently. have.

In addition, Fig. 4 shows a comparison of the crystallization behavior of the inventive and conventional mold flux in a dry atmosphere and a wet atmosphere. In a humid atmosphere, H ₂ O in the atmosphere is dissolved into the liquid mold flux to form a (OH ⁻ ) group as follows.

^{_{H 2 O + (O 2-)}} → 2 (OH -)

Thus, the wet atmosphere has the same effect as the 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 mold flux of the related art, the risk of breakout during the summer or high hydrogen molten steel casting is increased.

On the other hand, in the mold flux according to the present invention there is almost no difference in the crystallization behavior according to the atmosphere. This is because Calcium Borate Silicate formed from the inventive mold flux has no affinity for the (OH ⁻ ) group and is not affected by crystallization. Accordingly, when the present invention mold flux 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 hydrogenated breakout.

Recently, as the demand for productivity improvement increases, the casting speed during continuous casting has also increased. Increasing the casting speed encourages uneven growth of the initial solidification layer directly under the meniscus, resulting in an increase in defects on the surface of the slab such as duty free cracks. In the case of the mold flux according to the present invention, as the mold flux film introduced into the mold during the continuous casting operation is early crystallization is completed, it is advantageous for the initial cooling of the initial solidification layer, so that the uniform solidification layer growth is achieved even during the high speed casting operation. Make it possible.

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 : 30-50% by weight

; As the most important component for determining the crystallization temperature in the mold flux according to the present invention, when the B ₂ O ₃ component is 30% or more, the crystallization temperature increases. Therefore, in the present invention, since the crystallization temperature ranges from 800 to 1200 ° C., the composition is formed up to 30 to 50 wt% in consideration of the loss of the components.

2) CaO: 20 ~ 45 wt%

; The CaO component controls the basicity of the mold flux together with the SiO ₂ component and is limited to at least 20% by weight and at most 45% by weight to obtain the required slag viscosity.

3) SiO ₂ : 15 ~ 40 wt%

; The SiO ₂ component is a component that forms the basicity of the mold flux together with the CaO component described above, and is limited to 15 to 40% by weight depending on the amount of the CaO component that forms the mold flux of the present invention.

4) Na ₂ O: 15 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 is more than 15% 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 : 10 wt% or less

; Like the Na ₂ O component described above, the Al ₂ O ₃ component is a component for controlling the viscosity of the mold flux. When the composition is more than 10% by weight, the viscosity of the flux is excessively increased and the absorption capacity of the non-metallic 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.

As shown in Table 3 and Table 4, although the fluorine was not added as shown in the table, the mold flux according to the present invention showed a crystallization temperature, a viscosity, and a 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 37KJ / mole, which can be seen to be about 1/2 lower than that of the conventional 78 ~ 79 KJ / mole. Accordingly, since the liquid mold flux introduced between the mold and the solidification shell forms a stable crystal layer at an early stage, it is possible to prevent uneven growth of the initial solidification layer.

As described above, the mold flux according to the present invention can significantly reduce the immersion nozzle melt by applying the present invention as fluorine is not added as a constituent component, it is possible to increase the soft fuel ratio. In addition, it is possible to prevent corrosion of piping equipment of continuous casting cooling water, and it does not require any water treatment cost for cooling water discharge. In addition, there is an effect that can fundamentally solve the occurrence of hydrogen breakout when casting high-hydrogen molten steel.

Claims (3)

In the mold flux, By weight%, B ₂ O ₃ 30-50%, CaO 20-45%, SiO ₂ 15-40%, Na ₂ O over 0% up to 15%, Al ₂ O _{3 over} 0% up to 10%, MgO 0% B ₂ O ₃ -based mold flux, characterized in that the composition is in excess of 10%, the rest contains inevitable impurities and does not contain fluorine. The method according to claim 1, wherein the mold flux is calcium silicate, light beams (Borate Calcium Silicate) as a crystalline phase Ca ₂ SiB ₂ O B ₂ O as to form a _73-based mold flux. delete

Images