KR20030011419A - Tundish flux for stainless steel sheets having good inclusion absorptivity and low erosion of refractory - Google Patents

Abstract

PURPOSE: A tundish flux for stainless steel having superior absorption power of inclusions and lower erosion power of refractories is provided to produce stainless steel products having superior cleanliness by injecting the flux onto molten steel in a tundish containing a large quantity of Al2O3 and TiO2 inclusions to absorb the inclusions. CONSTITUTION: In a tundish flux which has superior absorption power of Al2O3 and TiO2 inclusions in molten stainless steel and lower erosion power of refractories and can be used in the continuous casting conditions, the tundish flux for stainless steel is manufacture by mixing powder type oxides comprising 6 to 8 wt.% of MgO, 4 to 6 wt.% of Al2O3 and a balance of CaO and SiO2, completely melting the mixed oxides, and crushing the melted material, wherein the flux is a CaO-SiO2-Al2O3-MgO based composition, basicity (%CaO/%SiO2) of the flux is in the range of 1.1 to 1.2, the crushed powder type flux has a particle size of 1 mm or less, and melting temperature of the powder type oxides is in the temperature range of 1300 to 1350 deg.C.

Description

Tundish flux for stainless steel sheets having good inclusion absorptivity and low erosion of refractory}

The present invention relates to a tundish flux for stainless steel, and in particular, a non-metal capable of producing a stainless steel product having excellent cleanliness by absorbing and removing inclusions in molten steel by inputting it on a tundish molten steel containing a large amount of Al ₂ O ₃ and TiO ₂ inclusions in the molten stainless steel. It relates to a tundish flux having good absorption of inclusions and a low tundish refractory erosion ability.

FIG. 1 schematically shows an example of a continuous casting process, in which scrap metal and ferroalloy are introduced into a molten metal in an electric furnace, which is a previous process, and then a molten steel having a target composition and temperature is secured in a refinery. The molten steel is then transferred to a continuous casting process in ladle. The molten steel transferred to the continuous casting process is supplied to the tundish 3 through the long nozzle 2 from the ladle 1 located above the tundish. At this time, a tundish flux (not shown) is added over the molten steel to dissolve and absorb and remove the non-metallic inclusions suspended in the molten steel. In this way, the molten steel from which the non-metallic inclusions are removed is injected into the mold 5 through the immersion nozzle 4 to be produced as the cast 6.

The tundish flux used in this way is a CaO-SiO ₂ system of A and CaO-Al ₂ O _{3 of} B, which are representative low melting point regions as shown in the ternary state diagram of CaO-SiO ₂ -Al ₂ O ₃ of FIG. _2. Generally used. In practice, CaO-SiO ₂ is called "A-based flux" and is mainly used for Si deoxidized steel, and CaO-Al ₂ O ₃ -based is called "B-based flux" and used for Al deoxidized steel. Table 1 shows the chemical compositions of A and B based tundish fluxes in actual use.

Unit (wt.%) division CaO SiO ₂ Al ₂ O ₃ Etc A flux 43 54 3 0 B flux 51 3 42 4

Theoretically, however, in the case of Al deoxidized steel, in order to absorb and remove Al ₂ O ₃ , which is the composition of the main inclusions, the absorption capacity increases as the content of Al ₂ O _{3 in} the initial tundish flux is as low as possible. However, since the B-based flux already contains about 42% of Al ₂ O ₃ in the tundish flux, the Al ₂ O ₃ absorption capacity cannot be greatly expected. Despite these characteristics, the reason for using CaO-Al ₂ O ₃ B flux instead of CaO-SiO ₂ A flux is due to the following equation of Al or Ti and SiO ₂ in molten steel. This is because there is a problem of forming a new inclusion.

SiO ₂ (in tundish slag) + Ti, Al (in molten steel) → TiO ₂ , Al ₂ O ₃ (inclusion) + Si (1)

Therefore, if the proper composition of CaO-SiO ₂ system that can suppress the reaction of formula (1) can be derived, the optimal tundish flux can be secured to maximize the absorption capacity of Al ₂ O ₃ inclusions in Al deoxidized steel. . Meanwhile, in the case of stainless steel, there are many steel grades that can be industrially produced by adding a large amount of Ti. In particular, these steel grades are steel grades which ensure the corrosion resistance of steel by adding a large amount of Ti in the range of 0.1-0.5 weight% normally, and typical is 321 stainless steel. As such, in Ti-containing stainless steel, TiO ₂ inclusions are present in the steel, which in severe cases adversely affects nozzle clogging and final product surface quality during casting. Therefore, it is necessary to develop a tundish flux having excellent absorption ability of TiO ₂ inclusions in the molten stainless steel, and to use it in actual operation. However, to date, the tundish fluxes developed for the purpose of removing the inclusions by using the absorption capability of the Al ₂ O ₃ and TiO ₂ inclusions are not known.

Therefore, the inventors of the present invention focus on research and experiment to improve the problems required in the above-described conventional techniques, and based on the results, the present invention proposes the present invention, the present invention reacts with molten steel. It is an object of the present invention to provide an optimum tundish flux for stainless steels having excellent absorption of Al ₂ O ₃ and TiO ₂ nonmetallic inclusions in molten steel and low tundish refractory erosion without forming new inclusions.

1 is a schematic diagram showing a ladle-tundish-mold of a continuous casting process.

2 is a ternary state diagram of CaO-SiO ₂ -Al ₂ O ₃ showing a low melting point composition region.

Figure 3 is a schematic diagram showing the measurement performance of Al ₂ O ₃ and TiO ₂ inclusion absorption in the present invention.

Figure 4 is a graph showing the measurement results of Al ₂ O ₃ and TiO ₂ absorption capacity by type of tundish flux.

5 is a schematic diagram showing the reactivity test according to the basicity of molten steel and tundish flux.

6 is a graph showing the results of reactivity test according to the basicity of molten steel and tundish flux.

7 is a state diagram showing a critical basicity preventing CaO-TiO ₂ crystallization in a CaO-SiO ₂ -TiO ₂ state diagram.

8 is a graph showing a measurement result of MgO saturation concentration of tundish slag derived from the present invention.

9 is a graph showing the effect of the amount of Al ₂ O ₃ addition to the melting temperature of the tundish flux of the present invention.

10 is a graph showing a comparison between inclusion absorbency and refractory erosion ability of the present invention tundish plus and comparative materials.

11 is a graph showing a comparison of the surface quality of hot rolled coils of stainless steel 321 steel when the present invention tundish flux and flux of the conventional material is used.

<Description of Signs of Major Parts of Drawings>

(1) Ladle (2) Long Nozzle (3) Tundish (4) Immersion Nozzle

(5) Mold (6) Casting Cast (7) Al ₂ O ₃ or MgO Crucible

(8) Melt Tundish Flux (9) Graphite Crucible

(10) TiO ₂ Pellets (11) Al ₂ O ₃ Crucibles (12) Tundish Slag

(13) Stainless steel containing Ti or Al

The present invention for achieving the above object is excellent in the absorption capacity of Al ₂ O ₃ and TiO ₂ inclusions in the molten stainless steel, low refractory erosion ability and in the tundish flux having an appropriate melting temperature that can be used in continuous casting conditions

By weight%, MgO is 6-8%, Al ₂ O ₃ is 4-6%, and the remainder is mixed with a powdery oxide having a composition range of CaO + SiO ₂ and then melted completely at a melting temperature of 1300-1350 ° C. The particle size is 1mm or less, and the basicity (% CaO /% SiO ₂ ) at this time satisfies the range of 1.1 to 1.2. Provide dish flux.

In addition, the flux of the present invention is characterized in that the CaO-SiO ₂ -Al ₂ O ₃ -MgO system.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

Figure 3 is a test method for measuring the absorption capacity of the Tundish flux and TiO ₂ inclusions in Figure 3 (a) is a method for measuring the absorption capacity of Al ₂ O _3, MgO, Figure 3 (b) shows a method for measuring the absorption capacity of TiO _2. will be. Al ₂ O ₃ is a molten tundish flux (8) in the Al ₂ O ₃ crucible (7) as shown in Figure 3 (a) and reacted for 2 hours at 1500 ℃ after the melting of Al ₂ O ₃ from the crucible The amount was defined as solubility. On the other hand, a known method for measuring the absorption capacity of TiO ₂ inclusions is not yet described. Therefore, in the present invention, the experimental method as shown in FIG. 3 (b) was developed in-house, and a cylindrical pellet 10 was formed by using a TiO ₂ reagent and sintered at a high temperature. The reaction was carried out for 2 hours at 1500 ℃ and the amount of TiO ₂ dissolved in the flux from the pellet was defined as the TiO ₂ absorption capacity of the flux.

Figure 4 shows the measurement results of the inclusion absorption of Al ₂ O ₃ and TiO ₂ for the A and B-based flux of Table 1 through the experimental method of FIG. As described above, it can be seen that the inclusion absorption of the A-based flux is much better than that of the B-based flux. This is because the A-based flux does not contain Al ₂ O ₃ in the flux, so the absorption capacity of Al ₂ O ₃ is high, whereas the B-based flux contains a large amount of Al ₂ O ₃ , which causes almost Al ₂ O _{3 to} become saturated. It is judged that the content of Al ₂ O ₃ that can additionally be absorbed is small. In addition, the absorption capacity for TiO2 also shows that the A-based flux is much better than the B-based flux. The reason for this is that the reaction surface of the test result is confirmed by electron microscopic analysis. As a result, the A-based flux easily dissolves the TiO ₂ pellets to form a liquid phase slag of glass, whereas the B-based flux has CaO and TiO ₂ of the inclusions in the flux. It is judged that there is little solubility in TiO ₂ since it forms CaO-TiO ₂ having a very high melting point and thus cannot form liquid slag. Based on the above investigations and experimental results, it was found that the composition of the tundish flux for effectively absorbing and removing Al ₂ O ₃ and TiO _{2, which} are representative inclusions of stainless steel, was basically CaO-SiO ₂ system.

Therefore, the present inventors urgently required to derive a CaO-SiO ₂ based tundish flux composition having low reactivity with Al and Ti in stainless molten steel. Accordingly, in order to investigate such reactivity, the reactivity between slag of Al and Ti in molten steel was investigated by the experimental method of FIG. 5 while varying the CaO / SiO ₂ content ratio of the tundish flux, that is, the basicity.

That is, after melting the molten stainless steel (13) containing Ti and Al in the Al ₂ O ₃ crucible (11), the CaO-SiO ₂ -based slag (12) having a changed basicity was added to the molten steel (13) and reacted for 30 minutes. After analyzing the components of the slag and molten steel to evaluate the reaction degree is shown in Figure 6 the results. Here, the degree of reaction is indexed using the concentration decrease of Ti and Al of the molten steel reduced by the reaction of the slag with SiO ₂ . As shown in FIG. 6, the basicity shows a sharp decrease in reactivity based on 1.1. This result is interpreted as due to activity (activity) of the SiO ₂ based on the CaO-SiO ₂ is rapidly decreased to 1.1, based on the basicity. Therefore, it was found that the basicity of CaO-SiO ₂ should be adjusted to at least 1.1 to minimize reactivity. However, such a high basicity is very advantageous in preventing reactivity of tundish slag and molten steel and absorbing ability of Al ₂ O ₃ , but increases the possibility of formation of harmful high melting point CaO-TiO ₂ inclusions in the absorption of TiO ₂ inclusions of the present invention. Therefore, in the present invention, the equilibrium diagram of CaO—SiO ₂ —TiO ₂ shown in FIG. 7 was considered to derive the conditions for preventing the formation of CaO—TiO ₂ . As can be seen from this state diagram, the maximum basicity for suppressing the formation of CaO-TiO ₂ is 1.2. Therefore, the appropriate basicity in the present invention was found to be 1.1 to 1.2.

The following is one of the conditions for proper tundish flux composition, which should be less corrosive to tundish refractories of MgO components. In order to reduce the erosion of such tundish refractory, MgO is added to the one tundish flux in advance as much as the MgO saturation concentration of the tundish flux. Therefore, the MgO saturation concentration of CaO-SiO ₂ slag having a basicity of 1.1 to 1.2 derived from the present invention was measured. As shown in FIG. 3, the amount of MgO dissolved from the crucible was investigated according to the reaction time after adding the slag 10 derived from the MgO crucible 9. Figure 8 shows the MgO saturation concentration test results of the tundish slag according to the reaction time shows that the optimum MgO saturation concentration is about 7%. Therefore, to prevent refractory erosion in the present invention 6-8% MgO It is preferable to add.

Through the above experiments and studies, the tundish flux composition of the present invention is CaO-SiO ₂ -MgO system, the basicity is 1.1-1.2, MgO is in the range of 6-8%, and the rest is composed of CaO-SiO ₂ composition.

However, the melting point was too high, such as 1399 ℃ to apply this composition to stainless steel as it is. Unlike ordinary carbon steel, stainless steel contains a large amount of Cr and Ni, so the solidification temperature of the steel is low, and as a result, the molten steel temperature in the tundish is 1480 ~ 1500 ℃ for 304 stainless steel, which is about 50 ℃ lower than that of general carbon steel. It can be seen that the melting temperature of 1300 ~ 1350 ℃ is required.

Therefore, the tundish flux composition derived from the present invention was finally required to reduce the melting point. As a method of reducing the melting point, a strong melting point reducing material such as fluorite (CaF2) or Na ₂ O may be added, but such a material has a fatal disadvantage due to severe tundish refractory erosion ability and problems such as environmental pollution (F gas). have.

Therefore, even if the absorption capacity of the present invention, although Al ₂ O ₃ is reduced by the addition of Al ₂ O ₃ was induced decrease the melting point. This is because Al ₂ O ₃ has a melting point reduction effect to some extent and is a very stable oxide for use as a tundish flux. FIG. 9 shows melting point behavior when Al ₂ O ₃ is added to a composition having a basicity of 1.1 to 1.2 and MgO 6 to 8%, as described above. As shown in FIG. 9, in order to obtain 1300 to 1350 ° C., which is the melting point of the tundish flux targeted in the present invention, 4 to 6% of Al ₂ O ₃ may be added to satisfy the flux composition required by the present invention.

As described above, the tundish flux of the present invention has excellent absorption ability of Al ₂ O ₃ and TiO ₂ inclusions in stainless molten steel, has low refractory erosion, and has an optimal melting temperature having a suitable melting temperature that can be used in continuous casting conditions. CaO-SiO ₂ -Al ₂ O ₃ -MgO system derived from a composition with basicity (CaO / SiO ₂ ) of 1.1-1.2, wt% with 6-8% MgO and 4-6% of Al ₂ O _3. .

In the present invention, the following effects on the tundish flux derived as described above were verified.

FIG. 10 is a comparison of Al ₂ O ₃ , TiO ₂ absorption ability, and MgO erosion ability of the conventional A and B based tundish fluxes shown in Table 1 and finally the composition derived from the present invention. As can be seen in FIG. 10, the conventional A flux shows better absorption of Al ₂ O ₃ and TiO ₂ than the B flux, but shows low MgO erosion ability, which has a low basicity. To show that it is On the other hand, the present invention is not only the most excellent absorption of the inclusions, but also has a very small MgO erosion ability, showing the optimal results.

FIG. 11 is a comparison of the tungsten flux B system using the tungsten flux of the present invention to 321 stainless steel containing a large amount of 0.3% Ti as compared with the conventional tungsten flux B system. I can see that the defect rate of hot rolled coils is significantly reduced compared to the existing B flux.

As described above, the tundish flux of the present invention derives the optimum composition of the tundish flux having an appropriate melting temperature that can be used in continuous casting conditions, so that the Al existing in the molten steel without forming a new inclusion without reacting with the molten stainless steel. Excellent absorption of ₂ O ₃ and TiO ₂ nonmetallic inclusions, less refractory erosion, and significantly reduce the defect rate of stainless hot rolled products.

Claims (5)

The present invention relates to a tundish flux having an excellent melting capacity of Al ₂ O ₃ and TiO ₂ inclusions in stainless molten steel, low refractory erosion, and an appropriate melting temperature that can be used in continuous casting conditions. MgO is 6 to 8% by weight, Al to 4 to 6% Al ₂ O ₃ , the remainder is mixed with a powdered oxide having a composition range of CaO + SiO ₂ and then completely melted and pulverized, characterized in that Tungsten flux for stainless steel with excellent refractory erosion. The method of claim 1, The flux composition is a tungsten flux for stainless steel with excellent inclusions absorbing ability and low refractory erosion ability, characterized in that the CaO-SiO ₂ -Al ₂ O ₃ -MgO system. The method according to claim 1 or 2, The basicity of the flux (% CaO /% SiO ₂ ) is a tundish flux for stainless steel with excellent inclusion absorption capacity and low refractory erosion ability, characterized in that it satisfies the range of 1.1 ~ 1.2. The method of claim 1, The pulverized powdered flux is a tundish flux for stainless steel having excellent inclusion absorption capacity and low refractory erosion ability, characterized in that the particle size is 1 mm or less. The method of claim 1, Melting temperature of the powdered oxide is a stainless steel tundish flux excellent in inclusion absorption, low refractory erosion ability, characterized in that it satisfies the range of 1300 ~ 1350 ℃.