KR20030023907A - Method for controlling refine slag of Grain-Oriented electrical Steel - Google Patents

Abstract

PURPOSE: A control method of slag for refining grain-oriented magnetic steel sheet is provided to minimize adhesion of slag to submerged pipe during degassing by injecting calcium oxide into slag when tapping molten steel of converter and injecting a small quantity of silica when bubbling molten steel, thereby making slag having low Al2O3. CONSTITUTION: In a method for tapping molten steel(5) for grain-oriented electrical steel sheet of high silicon steel from converter to ladle(3), bubbling molten steel received in the ladle and degassing molten steel using a degassing unit(1), the refining method of high silicon steel for preventing slag from adhering onto the inside of submerged pipe(2) of vacuum degassing apparatus comprises a step of injecting 2.5 to 5.5 kg/ton-steel of CaO into the ladle according to terminal point oxygen amount of the converter while molten steel of the converter is being tapped into the ladle; and a step of injecting 1.0 to 1.5 kg/ton-steel of silica into the ladle as strongly bubbling molten steel inside the ladle for 8 to 12 minutes using nitrogen, wherein 2.5 to 3.5 kg/ton-steel of the CaO is injected in case that the terminal point oxygen amount of the converter is 750 ppm or less, 3.6 to 4.5 kg/ton-steel of the CaO is injected in case that the terminal point oxygen amount of the converter is 751 to 900 ppm, and 4.6 to 5.5 kg/ton-steel of the CaO is injected in case that the terminal point oxygen amount of the converter is 901 ppm or more, and wherein injection of the silica is completed within 2 minutes after starting the strong bubbling.

Description

Method for controlling refine slag of Grain-Oriented electrical Steel}

The present invention relates to a method for refining oriented electrical steel of high silicon steel, and more specifically, first, quicklime is added to slag during the tapping of the converter molten steel, and a small amount of silica is injected into the slag during bubbling operation of the molten steel to lower Al ₂ O ₃ slag. It relates to a slag control method for refining oriented electrical steel sheet to minimize the adhesion of slag to the deposition pipe during the degassing process.

In order to maintain the cleanliness and degassing of oriented electrical steel, oriented electrical steel sheets have a bubbling process to carry out molten steel from the converter and carry out the first temperature adjustment, and then transfer to the degassing process to perform degassing and composition adjustment of molten steel. Finally, it is manufactured through continuous casting by adjusting to a temperature suitable for casting.

The degassing treatment is carried out in the vacuum degassing apparatus of FIG. 1, which comprises a vacuum chamber 1 for producing a vacuum state, a molten steel reflux apparatus (not shown) for refluxing the molten steel 5 in a vacuum chamber, and a water ladle ( 3) it is configured to include a immersion pipe (2) to be deposited in the suction of the molten steel (5). The immersion pipe 2 is constructed of refractory and has an internal diameter of about 450 mm, which is deposited in the molten steel before degassing, sucks the molten steel into the vacuum chamber through the rising pipe 2a, and returns the molten steel into the ladle through the down pipe 2b. Degassing takes about 25 to 35 minutes. During the degassing process, the immersion pipe is deposited in the molten steel of the slabs, so the slag on the upper part of the molten steel is attached to the outside of the immersion pipe as shown in FIG. 1, and when the amount of slag attachment is too high, the immersion pipe cannot be deposited into the ladle. There is a problem that the gas treatment becomes impossible.

Korean Laid-Open Patent Publication No. 1994-11655 is a technique for reducing the adhesion of slag to the deposition pipe of the degassing apparatus. In this technique, the CaO of 1.85 ~ 4.44kg / ton-steel is injected into the outflow slag at the time of 1/2 ~ 2/3 of the tapping of the converter, and then stirred for 4 ~ 8 minutes at the flow rate of 0.2 ~ 0.6Nm ³ / min. By blowing the gas, CaO is injected into the slag and stirred to make the melting point of the slag at a low melting point of 1350 to 1550 ° C. lower than molten steel to prevent the slag from adhering to the deposition tube.

As a result of applying this conventional technique to the refining of the oriented electrical steel sheet, there was still a problem that the slag was attached to the deposition pipe of the vacuum degassing apparatus, and the effect was confirmed in other general steels. The cause of the analysis was the difference between the slag of oriented electrical steel and that of ordinary steel.

The slag of the upper part of molten steel is refined slag of specific composition for the improvement of quality and stabilization of casting work by securing the cleanliness of molten steel through absorption of inclusions in molten steel. Inclusion capacity of refined slag differs depending on deoxidation method during converter down. In other words, the composition of the refined slag varies depending on Si-only deoxidation steel, Al deoxidation steel, Si + Al complex deoxidation steel, and the like. That is, according to the deoxidation method, the composition is made as a refined slag with excellent inclusion absorption ability. For example, the slag having the best inclusion absorption in Al deoxidized steel is preferably in the CaO / Al ₂ O ₃ ratio around 2.0, and the slag having FeO + MnO of at least 5% or less is ideally known. In the case of the grain-oriented electrical steel sheet, since the Si + Al complex deoxidation is performed as high silicon steel, the slag composition is CaO-SiO ₂ -Al ₂ O ₃ , and the optimum slag composition is CaO: 55 to 60% by weight, SiO ₂ ≤ 10, Al ₂ O ₃ : It is known as 25-35.

General steel has a deoxidation form of Al deoxidation or Si + Al + Mn, and the content of Si is relatively low so that the content of SiO ₂ in slag is 10% or less. On the other hand, in the case of the grain-oriented electrical steel sheet, the component of Si is very high while the component of Mn is very low, and a large amount of Fe-Si is added as a deoxidizer in the converter steel, and Fe-Mn is hardly added. In addition, in order to carry out the bubbling operation of molten steel (to add nitrogen to the steel while using nitrogen as a bubbling gas), the steel stirring operation is largely performed for about 4 minutes or more than usual. This causes the reoxidation of molten steel to cause an increase in Al ₂ O ₃ .

The fundamental difference in the slag composition of ordinary steel and oriented electrical steel sheet is the content of SiO ₂ . In the case of general steel, SiO ₂ is less than 10%, but in the case of high-silicon steel oriented electrical steel sheet, it is formed at least 18% by the large amount of Fe-Si. In other words, the electrical steel sheet is low carbon, low manganese, high silicon steel, which is excessively overcharged when the converter is blown, and a large amount of Fe-Si is added and the deoxidation is carried out by Si and Al. Due to the high production of ₂ and Al ₂ O ₃ , high melting point slag peculiar to the steel sheet, which is different from the optimum slag composition, is formed.

On the other hand, the slag composition of molten steel for high silicon oriented electrical steel sheet excessively attached to the deposition pipe during degassing was CaO: 24 to 33%, SiO ₂ : 15 to 22%, Al ₂ O ₃ : 36 to 48%, and MgO. 4 to 6%. Analysis of the slag state diagram, the melting point of the grain-oriented electrical steel slag is not a problem in the molten state near 1600 ℃, the molten steel temperature to start the degassing treatment to approximately 1560 ~ 1580 ℃. However, as the molten steel temperature and surface temperature decrease during the degassing process, the slag is excessively attached to the deposition tube and when the slag is removed, the refractory pipe refractory is dropped together to stop the continuous degassing treatment and replace or repair the deposition pipe. There is a problem that the slag with high viscosity to be left on the ladle after continuous casting is attached to the upper or bottom of the ladle, poor exhaustion and thus difficult to clean the ladle, thereby affecting the entire refining operation.

Therefore, the prior art exhibits excellent effects in Al + Si + Mn or Al + Mn deoxidized steels, but does not have much effect on Si + Al deoxidized steels, such as oriented electrical steel sheets of high silicon steel.

Accordingly, the present inventors have determined that the problem of easily adhering to a deposition pipe due to its high melting point in the degassing treatment of high-silicon grain oriented electrical steel sheet lies in the properties of the slag due to its unique characteristics. The present invention has been proposed based on the results of the application, and the present invention first applies quicklime to slag, and injects a small amount of silica sand during bubbling operation of molten steel to make slag with low Al ₂ O ₃ , and to deposit the slag during degassing treatment. The purpose is to minimize the attachment of slag to the.

1 is a schematic diagram showing a state that the slag is attached to the deposition pipe in the vacuum degassing apparatus.

Figure 2 is a ternary state diagram showing a conventional molten steel slag and molten steel slag composition range of the present invention

3 is a ternary state diagram showing the viscosity of conventional molten steel slag and the molten steel slag of the present invention.

In the refining method of the present invention for achieving the above object, the molten steel for directional electrical steel sheet of high silicon steel is pulled into the ladle in the converter, and then the molten steel is bubbling ladle and degassing the molten steel by the degassing apparatus In the method, the step of injecting CaO 2.5 ~ 5.5kg / ton-steel according to the amount of converter end point oxygen during the tapping of the molten steel of the converter,

It comprises a step of putting 1.0 ~ 1.5kg / ton-steel silica sand while bubbling the molten steel in the ladle with nitrogen for 8 to 12 minutes.

Hereinafter, the present invention will be described in detail.

The present inventors put CaO and silica sand into the slag of high melting point in the course of studying the method for converting the slag composition of the high melting point into the slag composition of the low melting point in the refining of high silicon oriented electrical steel as shown in Figure 2 The present invention has been completed by confirming that the amount of slag attached to the deposition pipe during degassing can be reduced to slag.

The present invention is applied to the refining of high silicon grain-oriented electrical steel sheet, the target composition of the steel by weight, C: 0.07% or less, Si: 3.0 to 3.25%, Mn: 0.06 to 0.43%, P: 0.024% or less, Typical examples include S: 0.024% or less, S. Al: 0.006 to 0.028%, and N: 0.0075 to 0.011%.

In the production of grain-oriented electrical steel sheets, quicklime is first introduced during converter exit. Quicklime is to make low-melting refined slag by increasing CaO component of slag and lowering Al ₂ O ₃ component relatively. In the present invention, quicklime is added CaO 2.5 ~ 5.5kg / ton-steel. If no quicklime is added or the amount is low, Al ₂ O ₃ in the refining slag is excessively high and high melting slag is formed. There is a problem.

In the present invention, it is preferable to add quicklime according to the amount of end point oxygen. Depending on the amount of end point oxygen, the degree of peroxidation of the converter end slag is determined, and the greater the peroxidation of the converter end slag, the greater the slag fluidity and the inflow into the ladle through molten steel through the tapping hole by the vortex formed on the tap flow during tapping. Since the amount of slag is increased and accordingly the composition of the deoxidation product is different, it is preferable to increase the amount of the end oxygen to increase the amount of quicklime injected into the ladle during tapping to correct. That is, when the end point oxygen is less than 750 ppm, the quicklime input amount is 2.5 to 3.5 kg / ts, and when the end point oxygen is more than 750 ppm to 900 ppm, the amount of quicklime is 3.6 to 4.5 kg / ts. Let it be 5.5kg / ts.

Molten steel tapping into the ladle is strongly bubbled with nitrogen gas, usually, the bubble is bubbled at a stirring pressure of 5kg / cm ² or more. In the present invention, silica sand is injected during strong bubbling. Silica sand is used to deoxidize slag spilled during tapping to prevent an increase in Al ₂ O ₃ concentration and reoxidation of molten steel. In other words, oxygen in the slag is deoxidized by silica sand injection to make SiO _{2 and} to produce Al ₂ O ₃ . The silica sand injection time is preferably added within 2 minutes after the start of bubbling, which is the initial bubbling, in order to minimize the reaction of oxygen in the slag with Al. The silica sand injection amount is suitably 1.0 to 1.5 kg / ts, more preferably 1.0 kg / ts. If there is too little or no silica injection, the slag spilled at the end of the tapping is generated as Al ₂ O ₃ , which is disadvantageous for low melting point slag formation.

Hereinafter, the present invention will be described in more detail with reference to Examples.

[Prior example]

The high-silicon directional electricity of the prior art (Korean Laid-Open Patent Publication No. 1994-11655) in which 1.85 to 4.44 kg / ton-steel of CaO is injected into the outflow slag at 1/2 to 2/3 of the converter exit. Table 1 shows the results of vacuum degassing the steel sheet.

Oxygen end point (ppm) Quicklime input (kg / ton-slag) Silica injection Slag adhesion amount (mm / ch) Refined slag component (% by weight) Remarks CaO SiO ₂ Al ₂ O ₃ MgO 680-980 0-3.0 Not input 45 24-32 18-22 37-48 4 ~ 5 Spinel

Conventional methods do not inject silica sand in the ladle during tapping, but also inject quicklime regardless of the amount of end point oxygen. The slag composition had high Al2O3 content and high viscosity as slag of high melting point, so that the flowability was very poor after bubbling, and the amount of slag adhered to the deposition tube during the degassing treatment.

[Inventive Example and Comparative Example]

Refining operation as shown in Table 2 and analyzed by the slag components are shown in Table 2 together.

number Oxygen end point (ppm) Quicklime input (kg / ton-slag) Bubbling work Degassing Condition Slag component (wt%) Stirring Pressure (kg / cm ² ) Silica injection amount (kg / ton-slag) Slag fluidity Processing status Slag adhesion amount (mm / charge) CaO Al ₂ O ₃ Comparative Example 1 730 1.5 8.0 1.0 Bad Bad 40 26.4 45.2 Comparative Example 2 920 3.0 9.0 1.0 Bad Bad 35 31.6 38.0 Comparative Example 3 730 4.5 9.0 1.0 usually usually 20 39.7 33.7 Comparative Example 4 680 5.5 9.0 1.5 Bad Bad 40 36.4 35.8 Comparative Example 5 850 4.0 10.0 0.9 usually usually 20 34.9 40.5 Comparative Example 6 960 5.0 9.0 1.6 good Good 5 39.8 32.1 Comparative Example 7 740 3.0 9.0 2.0 good Good 4 41.3 31.2 Comparative Example 8 820 4.0 4.0 1.0 Bad Bad 30 34.7 38.7 Comparative Example 9 850 4.0 5.0 1.5 Bad Bad 30 33.1 40.6 Inventive Example 1 680 2.7 9.0 1.0 good Good 4 43.9 30.5 Inventive Example 2 730 3.3 9.0 1.5 good Good 3 43.8 25.8 Inventive Example 3 820 4.0 10.0 1.0 good Good 5 40.3 32.1 Inventive Example 4 850 4.2 9.0 1.5 good Good 4 41.6 28.3 Inventive Example 5 920 5.0 10.0 1.0 good Good 3 39.5 32.2 Inventive Example 6 960 5.0 10.0 1.5 good Good 5 38.1 33.0

In Table 2, Comparative Example (1-2) used quicklime during converter winding work, but the amount of molten steel slag was low due to low CaO and high Al ₂ O _3. This is a case where slag with high adhesion amount is made during degassing due to its poor viscosity and high viscosity.

In Comparative Example (3-4), the quicklime was added during the conversion of the converter, but the slag was not completely released even though the bubbling work and the silica sand input amount were exceeded due to the excessive amount compared to the end point oxygen content. This is a case where slag with high adhesion amount is made during degassing treatment because of low CaO and high Al ₂ O ₃ , poor fluidity and high viscosity. In this case, the CaO component was increased in comparison with Comparative Example (1-2), but the CaO in the slag component was lower than the amount of quicklime added. This is because the slag hardened due to the incomplete loosening during the bubbling operation. In Comparative Example (5), the amount of silica sand injection during the bubbling operation did not effectively reduce the Al 2 O 3 component in the slag, but the slag fluidity was normal. In Comparative Example (6-7), the silica sand injection amount was increased during the bubbling operation, and the slag state was good, so that the degassing treatment could be easily performed. However, since the slag is not much better compared to the 1.0 ~ 1.5kg / ts input standard, it seems that the input is more than necessary when considering the cost aspect. In Comparative Example (8, 9), the quicklime input amount during the conversion of the converter was complied with the standard, and the bubbling operation was performed in the same manner as the general steel treatment, and the stirring pressure was reduced to about 4 to 5 kg / cm ^{2, and the} intermediate stirring was performed. This is the case that the slag loosening is poor and the work to be done is not done properly, so the slag fluidity and the nitrogen [N] component among the degassing components are too low, which causes difficulties in processing.

Except for Comparative Examples (6,7) in the Comparative Examples (1 to 9), the slag fluidity is not good, and thus, when degassing, the molten steel surface slag is solid and the slag is attached to the deposition pipe. The slag thickness attached to the immersion tube was about 20 ~ 40mm / charge.

Inventive example (1, 2) is a case where the input amount of quicklime is 2.5 ~ 3.5 kg / ts when the end point oxygen content is 750ppm or less when the silica sand 1.0 ~ 1.5 kg / ts during the bubbling operation, the existing bubbling through In contrast, the CaO concentration was greatly increased and the Al ₂ O ₃ concentration was significantly decreased, so the slag fluidity was very good, and the slag adhesion was hardly generated during the degassing treatment. Inventive example (3 ~ 4) is the case where the quicklime input amount is 3.6 ~ 4.5 kg / ts at the end point oxygen content of 751 ~ 900ppm, and 1.0 ~ 1.5 kg / ts of silica sand is injected during the bubbling operation. And the Al ₂ O ₃ concentration was lowered to obtain slag suitable for degassing. Inventive example (5, 6) is the case that the injection of the silica sand 1.0 ~ 1.5 kg / ts during bubbling operation with the input of quicklime as 4.6 ~ 5.5 kg / ts with a charge of the end point oxygen is greater than 901ppm Al2O3 is slightly higher than Honors (1-4), but overall good slag is obtained. In terms of silica sand input, it is not clear that 1.6 kg / ts more than 1.0 ~ 1.5 kg / ts is added, and 1.0 ~ 1.5 kg / ts is input from the cost side because slag composition is good by 1.0 ~ 1.5 kg / ts. It is desirable to.

In addition, the slag adhesion amount measurement and the number of times of use of the immersion pipe were investigated for the case prepared according to the present invention as described above and the case prepared according to the conventional method, and the results are shown in Table 3, and the viscosity according to the slag composition is shown in FIG. 3. Indicated.

division Slag melting point (℃) Slag adhesion amount (mm / charge) Number of times of immersion pipe use (times / EA) Remarks Conventional example 1580-1600 45 38 Starting temperature after degassing: 1560 ℃ Inventive Example 1500-1540 4 118

As shown in Table 3 and FIG. 3, in the conventional method, the slag melting point is higher than the degassing temperature of molten steel and the slag viscosity is high, so that the fluidity is poor. In the present invention, the slag melting point is lower than the degassing temperature of the molten steel. The slag viscosity was also significantly lowered, which improved the fluidity and greatly reduced slag adhesion to the deposition pipe during degassing treatment. Therefore, it can be seen that the present invention improves the life of the deposition tube compared to the conventional method.

As described above, the present invention injects the appropriate amount of quicklime during tapping according to the converter end point oxygen amount during the production of grain-oriented electrical steel sheet and the initial silica sand during bubbling operation to make the high melting point slag into a low melting point slag, the deposition pipe during the degassing treatment By reducing the amount of slag attached to the slab and improving the ladle slag excavation after casting, the operation was stabilized by reducing the workload and improving the life of the immersion pipe.

Claims (3)

In the method of tapping the molten steel for directional electrical steel sheet of high silicon steel into the ladle in the converter, and then bubbling the molten steel received ladle and degassing the molten steel by a degassing device, Injecting the CaO 2.5 ~ 5.5kg / ton-steel in accordance with the amount of converter end point oxygen during the tapping of the molten steel of the converter, The steel slag in the ladle for 8 to 12 minutes while bubbling with nitrogen, while the silica sand 1.0 to 1.5kg / ton-steel, comprising the step of preventing high slag adhesion in the deposition pipe, characterized in that it comprises a step Method of refining silicon steel. The method of claim 1, wherein the CaO input is 2.5 ~ 3.5kg / ton-steel injection when the converter end point oxygen content is less than 750ppm, 3.6 ~ 4.5kg / ton-steel when 751-900ppm, 901ppm In the above case 4.6 ~ 5.5kg / ton-steel refining method of high silicon steel for preventing slag adhesion in the deposition pipe of the vacuum degassing apparatus characterized in that the injection. The method of refining high silicon steel for preventing slag adhesion in a deposition tube of a vacuum degassing apparatus according to claim 1, wherein the injection of the silica sand is completed within 2 minutes of the start of the strong bubbling.