IF steel containing rare earth and preparation method thereof

Abstract

The preparation method comprises the following steps that S1, BOF smelting is conducted, specifically, converter smelting is adopted, and the oxygen content in molten steel is controlled to be smaller than or equal to 550 ppm during tapping; s2, LF furnace refining is carried out, reducing slag is made in an LF furnace, and the content of FeO and MnO in top slag after refining is completed is smaller than or equal to 1%; s3, RH vacuum refining is carried out, oxygen blowing forced decarburization is adopted, deoxidation and alloying are carried out after decarburization, and a rare earth alloy is added at the final stage of alloying; and S4, continuous casting CC, wherein molten steel is subjected to continuous casting after refining is finished. According to the method, the number of Al2O3 inclusions in the steel is reduced, the size of the inclusions is refined, the purity of molten steel is guaranteed, the problems of crystallizer liquid level fluctuation and water gap distillation in the IF steel casting process are solved, and stable casting of the IF steel containing rare earth is guaranteed.

Description

Technical Field

[0001] The invention relates to the technical field of metallurgy, and in particular to IF steel containing rare earth and a preparation method thereof. Background Art

[0002] IF steel is a kind of steel that is made by adding strong carbon and nitrogen compound-forming elements such as Ti or Nb to ultra-low carbon steel. The elements react with C and N atoms in the steel to form carbon and nitrogen compounds, making the matrix in an interstitial atom-free state. It is also called interstitial atom-free steel. In IF steel, Al2O3 inclusions are the main non-metallic inclusions in the steel. Clustered Al2O3 is easily exposed to produce linear defects during the rolling process. In order to reduce the occurrence of defects, rare earth elements are usually added to the molten steel to modify the Al2O3 inclusions to make them smaller in size. However, small inclusions are prone to cause problems such as nozzle nodules and crystallizer liquid level fluctuations. There are relevant literature and patent reports that improving the nozzle structure and material can reduce nozzle nodules, but there are no reports on the impact of fine inclusions on the fluctuation of the liquid level in the crystallizer, and the fluctuation of the liquid level in the crystallizer is more sensitive and difficult to control than the nozzle nodules. The main reason for the liquid level fluctuation and nozzle nodules is the use of the traditional BOF-RH-CC process, which has the high oxidation of the ladle top slag and the difficulty in stably controlling the number of inclusions, thus failing to fundamentally solve the problem of stable casting of rare earth-containing IF steel.

[0003] Chinese patent application No. 202010364353.5 discloses a method for rare earth treatment to refine Al2O3 inclusions in the entire casting and rolling process of ultra-low carbon IF steel. Rare earth Ce is added at the end of the RH refining process to make the Ce content in the molten steel reach 15ppm, and the average size of Al2O3 inclusions in all thickness directions of the ingot is reduced from 5-7μm to 2-5μm; this technology reduces the size of inclusions through rare earth treatment, but does not mention the problems of inclusion aggregation at the water inlet and crystallizer liquid level fluctuation that are easily induced by the reduction in inclusion size. Therefore, there are doubts as to whether on-site mass production can be achieved.

[0004] Chinese patent application No. 201811398543.8 discloses a smelting method for controlling the oxidizability of IF steel top slag, which includes four major processes: converter smelting LD, LF furnace refining, RH refining and continuous casting CC. At the end of RH refining, the oxidizability of the top slag and the oxygen content in the steel are more stable. In this method, the oxidizability of the top slag wT.Fe decreases from 6% to 12% to 6% to 8%, and the oxygen content [O] in the steel decreases from 18 to 30 ppm to 18 to 23 ppm. The cleanliness of the molten steel is better and more stable, and the quality of the finished product is also better. This technology reduces the number of inclusions to a certain extent and improves the purity of the molten steel, but the size distribution of the inclusions is not clearly explained.

[0005] Chinese patent application No. 202111451022.6 discloses a method and device for inhibiting nozzle blockage during the continuous casting of rare earth steel, which is used to inhibit nozzle nodules during the casting of rare earth steel; this technology reduces the electrochemical reaction between molten steel and nozzle, improves the wettability of rare earth steel and nozzle, improves the flow state of molten steel, and improves the anti-blocking effect of nozzle by adding pulse current generating device, nozzle heating, nozzle argon blowing device, improving nozzle material and other technical means. Although this technology can improve the inhibition of nozzle nodules, it requires the transformation of continuous casting equipment, which increases production costs.

[0006] In view of the above situation, it is urgent to develop a method for preparing IF steel by adding rare earth elements, which can reduce the number of Al2O3 inclusions, solve the problems of crystallizer liquid level fluctuation and water inlet distillation during the casting of IF steel, and ensure the stable casting of IF steel containing rare earths. Summary of the invention

[0007] In view of the problems in the prior art such as adding rare earth elements to refine IF steel inclusions leading to nozzle distillation and crystallizer liquid level fluctuation, the purpose of the present invention is to provide a rare earth-containing IF steel and a preparation method thereof, adopting a BOF-LF-RH-CC process, modifying the inclusions by adding rare earth alloys in the RH refining process, and modifying the ladle top slag after the LF furnace makes reducing slag and the RH refining is completed, thereby reducing the number of Al2O3 inclusions, refining the size of the inclusions, ensuring the purity of the molten steel, solving the problems of crystallizer liquid level fluctuation and nozzle distillation during the casting of IF steel, and ensuring the stable casting of IF steel containing rare earth.

[0008] To achieve the above object, the present invention adopts the following technical solution:

[0009] A first aspect of the present invention provides a method for preparing rare earth-containing IF steel, comprising the following steps:

[0010] S1, BOF smelting, using converter smelting, controlling the oxygen content in molten steel to ≤550ppm during steel tapping;

[0011] S2, LF furnace refining, making reducing slag in the LF furnace, so that the content of FeO and MnO in the top slag after refining is ≤1%;

[0012] S3, RH vacuum refining, forced decarburization by oxygen blowing, deoxidation and alloying after decarburization, rare earth alloy added at the end of alloying;

[0013] S4, continuous casting CC, after refining is completed, the molten steel is continuously cast.

[0014] Preferably, in step S1, during the steel tapping process of the converter, the steel tapping temperature is 1640±15°C.

[0015] Preferably, in step S1, during the steel tapping process of the converter, aluminum particles are added to the molten steel for pre-deoxidation, and the amount of aluminum particles added is controlled within 200 kg / furnace.

[0016] Preferably, in step S1, during the steel tapping process of the converter, 300-500 kg / furnace of lime is added to the slag to adjust the alkalinity.

[0017] Preferably, in step S2, after the LF furnace refining is completed, the composition of the top slag is: CaO: 50-55%, SiO2: 6-8%, Al2O3: 23-32%, MgO: 6-8%, FeO and MnO content ≤1%, calculated in mass percentage.

[0018] Preferably, in step S2, heat compensation is performed on the molten steel during the LF furnace refining process, and the temperature of the molten steel is controlled at 1640±10°C after the LF furnace refining is completed.

[0019] Preferably, in step S3, during the oxygen blowing forced decarburization process, the oxygen blowing amount is 150-200m 3 .

[0020] Preferably, in step S3, after RH vacuum refining, the Ce content in the molten steel is 0.0004-0.0012 wt%.

[0021] Preferably, in step S3, the rare earth alloy is a Ce-La alloy or a Fe-Ce alloy; in the rare earth alloy, the content of Ce is ≥ 67wt%.

[0022] Preferably, in the rare earth alloy, the added amount of the rare earth alloy is 5 to 20 kg / furnace.

[0023] Preferably, in step S3, after RH refining is completed, the top slag is modified, and aluminum particles are added to the slag surface so that the content of FeO and MnO in the top slag is less than 2%.

[0024] Preferably, the amount of aluminum particles added is 50-100 kg / furnace.

[0025] Preferably, in step S4, during the continuous casting process, more than 90% of the crystallizer liquid level fluctuations are controlled within ±5 mm.

[0026] The second aspect of the present invention provides a rare earth-containing IF steel prepared by the method for preparing a rare earth-containing IF steel as described in the first aspect of the present invention, wherein the number density of inclusions with a size of 2 to 5 microns is ≤4.5 pieces / mm 2 , the number density of inclusions larger than 5 microns is ≤ 0.5 / mm 2 .

[0027] Compared with the existing production technology, the present invention has the following beneficial effects:

[0028] 1. The present invention adopts the BOF-LF-RH-CC process to produce IF steel. Since the converter tapping temperature is lower than that of the traditional BOF-RH-CC process, the final oxygen content in the molten steel can be reduced by about 100 ppm;

[0029] 2. The present invention adopts the BOF-LF-RH-CC process to produce IF steel, produces reducing slag in the LF furnace and reforms the ladle top slag after the RH refining, which effectively ensures that the oxidation of the ladle top slag in the whole process from refining to casting is kept at a low level. The low oxidation top slag avoids the secondary oxidation of molten steel, reduces the number of Al2O3 inclusions, and reduces the problems of crystallizer liquid level fluctuation and nozzle nodule formation caused by abnormal molten steel cleanliness.

[0030] 3. The present invention adopts the BOF-LF-RH-CC process to produce IF steel, wherein reducing slag is produced and the temperature is increased in the LF furnace, and rare earth elements are added in the RH refining process to modify Al2O3 inclusions to make them smaller, thereby ensuring that the molten steel meets the requirements of composition, temperature and cleanliness, and can be smoothly cast on the casting machine;

[0031] 4. The crystallizer liquid level fluctuation is ±5, and the slab qualification rate can be stably controlled at more than 90%;

[0032] 5. The present invention adds rare earth elements at the end of RH refining, which has a modification effect on Al2O3 inclusions, reduces the aggregation and growth of inclusions, and thus reduces the number of large-sized Al2O3 inclusions in the ingot, making the number density of inclusions with a size of 2 to 5 microns in the steel ≤ 4.5 / mm 2 、Inclusions with a size greater than 5 microns have a number density of ≤0.5 / mm 2 . BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0034] Figure 1 It is a schematic flow chart of the rare earth-containing IF steel and its preparation method of the present invention;

[0035] Figure 2 1 are scanning images of typical inclusions in the IF steel prepared in Example 1 of the present invention, (a) is an electron microscope scanning image of the inclusions, (b) is a distribution diagram of the Al element in the inclusions, (c) is a distribution diagram of the O element in the inclusions, and (d) is a distribution diagram of the Ce element in the inclusions. DETAILED DESCRIPTION

[0036] In order to better understand the above technical solution of the present invention, the technical solution of the present invention is further described below in conjunction with embodiments.

[0037] In the production process of IF steel, rare earth is added to modify the inclusions to make them smaller in size, which leads to problems such as water nozzle distillation and crystallizer fluctuation. The reason is that the traditional BOF-RH-CC process has high oxidation of the ladle top slag, and the number of inclusions is difficult to stably control, so it is impossible to fundamentally solve the problem of stable casting of IF steel containing rare earth. The present invention adopts the BOF-LF-RH-CC process to produce IF steel containing rare earth, which can meet the requirements of obtaining IF steel with small inclusion size and stable continuous casting castability.

[0038] The present invention adopts the BOF-LF-RH-CC process. After the steel is tapped from the converter, reducing slag is made in the LF furnace, and rare earth elements are added in the RH vacuum refining process to modify Al2O3 inclusions to make them smaller and reduce the number of inclusions, thereby ensuring that the molten steel meets the requirements of composition, temperature and cleanliness and can be smoothly cast on a casting machine.

[0039] Combination Figure 1 As shown, the method for preparing the rare earth-containing IF steel of the present invention comprises the following steps:

[0040] S1, BOF smelting, using converter smelting, controlling the oxygen content in molten steel to ≤550ppm during steel tapping;

[0041] Specifically, the converter smelting is adopted, the tapping temperature is controlled at 1640±15℃ when the converter is tapping, and part of the heating pressure is transferred to the LF furnace refining process, which can effectively reduce the overoxidation of the converter molten steel, and the oxygen content of the molten steel is controlled at ≤550ppm when tapping. Specifically, aluminum particles can be added to the molten steel for pre-deoxidation, wherein the amount of aluminum particles added is determined according to the oxygen content of the molten steel when tapping, and the amount of aluminum particles added is controlled within 200kg / furnace. In the process of converter tapping, lime is added to the slag to adjust the alkalinity of the slag, so that the dispersibility of the slag is better, and the amount of lime added is 300-500kg / furnace.

[0042] S2, LF furnace refining, during the refining process, reducing slag is produced so that the sum of the contents of FeO and MnO in the top slag after refining is ≤1%;

[0043] Specifically, LF furnace refining is adopted, and reducing slag is produced during the LF furnace refining process, so that the content of FeO and MnO in the top slag after refining is less than 1%, which effectively reduces the secondary oxidation of the top slag to the molten steel. Among them, after the LF furnace refining is completed, the composition of the top slag is: CaO: 50-55%, SiO2: 6-8%, Al2O3: 23-32%, MgO: 6-8%, and the sum of the content of FeO and MnO is ≤1%, in terms of mass percentage. In addition, heat compensation is performed on the molten steel during the LF furnace refining process, and the temperature of the molten steel is controlled at 1640±10℃ after the LF furnace refining is completed.

[0044] S3, RH vacuum refining, forced decarburization by oxygen blowing, deoxidation and alloying after decarburization, and rare earth alloy added at the end of alloying.

[0045] Specifically, oxygen blowing is used for forced decarburization during RH vacuum refining, and deoxidation and alloying are carried out after decarburization. Rare earth alloys are added at the end of alloying. The rare earth alloys are composed of Ce-La alloys or Fe-Ce alloys, and the amount added is 5 to 20 kg / furnace; wherein, in the rare earth alloy, the Ce content is ≥67wt%, and a stable Ce addition is ensured, such as a Ce-La alloy with a Ce content of 67wt% and a La content of 33wt%.

[0046] In a specific embodiment, after adding the rare earth alloy, the rare earth content in the molten steel is 0.0004-0.0012 wt%.

[0047] In the RH refining process, oxygen blowing is used for forced decarburization, and the oxygen blowing amount is 150-200m 3 Considering that oxygen blowing decarburization will cause oxygen transfer from molten steel to top slag, the FeO content in the top slag increases after RH refining. Therefore, the top slag is further modified after RH refining, and aluminum particles are added to the slag surface. The amount of aluminum particles added is determined according to the slag amount to ensure that the content of FeO and MnO in the top slag is less than 2%. In a specific embodiment, the amount of aluminum particles added is 50-100 kg / furnace.

[0048] S4, continuous casting CC, after refining is completed, the molten steel is continuously cast.

[0049] After refining, the molten steel is moved to the continuous casting process for casting. During the continuous casting process, more than 90% of the crystallizer liquid level fluctuations are controlled within ±5mm.

[0050] The present invention also provides a rare earth-containing IF steel prepared by the above-mentioned rare earth-containing IF steel preparation method, wherein the Ce content is 0.0004-0.0012wt%. The inclusions in the rare earth-containing IF steel are distributed as follows: the number density of inclusions with a size of 2-5 microns is ≤4.5 / mm 2, the number density of inclusions larger than 5 microns is ≤ 0.5 / mm 2 .

[0051] The rare earth-containing IF steel and its preparation method of the present invention are further introduced below with reference to specific examples.

[0052] Example

[0053] Examples 1 to 6 adopt the method for preparing rare earth-containing IF steel of the present invention, using the BOF-LF-RH-CC process route, and the specific process is as follows;

[0054] The BOF tapping temperature is controlled at 1640℃±15℃, and the oxygen content of the tapping is controlled below 550ppm. Lime is added during the tapping process to control the slag basicity, and aluminum particles are added for pre-deoxidation. The amount added is determined according to the oxygen content of the tapping, and is within 200kg. The LF furnace produces reducing slag, and the FeO+MnO content in the top slag is controlled to be ≤1%. At the end of the LF furnace refining, the composition of the top slag is: CaO: 52.2%, SiO2: 7%, Al2O3: 32%, MgO: 8%, FeO: 0.5%, MnO: 0.3%, in weight percentage. The LF furnace performs heat compensation on the molten steel, and the molten steel temperature is controlled to be 1640±10℃ at the end of LF. RH vacuum refining adopts forced decarburization, and the oxygen blowing amount is 50~200m 3 After decarburization, deoxidation and alloying are carried out. At the end of alloying, Ce-La alloy is added, with a Ce content of 67wt% and a La content of 33wt%. The amount of Ce-La alloy added is 5-20kg / furnace, so that the Ce content in the molten steel is 0.0004-0.0012wt% after refining. After the RH treatment, aluminum particles are added to the slag surface to modify the top slag, and the amount added is 50-100kg / furnace to keep the FeO+MnO content in the top slag less than 2%. After the refining treatment, the molten steel is cast into continuous casting. The fluctuation of the liquid level in the crystallizer during casting is monitored, and the number density of inclusions in the ingot is analyzed.

[0055] Comparative Example

[0056] In Comparative Examples 1 to 6, IF steel was produced by adopting the BOF-RH-CC process route, and the specific process is as follows:

[0057] The BOF tapping temperature is controlled at 1680℃±10℃, the oxygen content of the tapping is controlled below 650ppm, lime is added during the tapping process to control the slag basicity, aluminum slag is added to modify the ladle top slag, and the FeO+MnO content is controlled to be ≤10%. RH vacuum refining and decarburization, oxygen blowing volume is 50~100m according to the temperature 3After decarburization, deoxidation and alloying are performed. Rare earth alloy is added at the end of alloying, and the amount of rare earth alloy added is 5-20 kg / furnace. It is particularly noted that no rare earth is added in Comparative Example 1. After refining treatment, the molten steel is cast into continuous casting. The fluctuation of the crystallizer liquid level during casting is monitored, and the number density of inclusions in the ingot is analyzed.

[0058] Table 1 shows the chemical composition of IF produced by Examples 1 to 6 and Comparative Examples 1 to 6. It can be seen from Table 1 that the IF steel produced by the BOF-LF-RH-CC process has similar C, Si, Mn, P, S, Al, and Ti chemical compositions as the BOF-RH-CC process. Table 2 shows the process parameters involved in the preparation process of Examples 1 to 6. The O content in the IF steel prepared by the BOF-LF-RH-CC process in Examples 1 to 6 is significantly lower than that in the IF steel prepared by the BOF-RH-CC process in Comparative Examples 1 to 6, which is due to the LF furnace reduction slag and the top slag modification effect at the end of RH.

[0059] Table 1 Chemical composition of IF steel prepared in Examples and Comparative Examples

[0060]

[0061] Table 2 Process parameters during the preparation of the embodiment

[0062]

[0063] Table 3 shows the crystallizer liquid level fluctuation during the continuous casting process of Examples 1 to 6 and Comparative Examples 1 to 6. The method for determining the liquid level fluctuation is: taking the crystallizer liquid level fluctuation of ±5 mm as the standard, the slabs cast under this condition are qualified slabs, and the qualified rate of the slabs after each furnace casting is calculated. It can be seen from the table that the qualified rate of the liquid level fluctuation during the entire tundish casting process in the embodiment is stable, reaching more than 90%. In the comparative example, the liquid level fluctuation of several furnaces is between 30% and 70%, and the qualified rate is obviously low. Therefore, the use of the BOF-LF-RH-CC process can reflect better casting stability for rare earth-containing IF steel.

[0064] Table 3 Qualified rate of slabs meeting the liquid level fluctuation ±5 during continuous casting of the embodiment and the comparative example

[0065]

[0066] Combination Figure 2 The typical inclusion surface scan in Example 1 shown in the figure shows that due to the rare earth treatment in the RH process, the Al2O3 inclusion contains Ce content, indicating that under the rare earth addition conditions in Example 1, the Al2O3 inclusion has been effectively transformed. It should be noted that Figure 1As just a typical example, Ce-containing Al2O3 inclusions were found in Examples 1 to 6 and Comparative Examples 2 to 6.

[0067] Table 4 shows the statistical results of the number density of inclusions in IF steel produced by Examples 1 to 6 and Comparative Examples 1 to 6. Comparative Example 1 shows the number density of inclusions in IF steel produced without rare earth in RH. In Examples 1 to 6 and Comparative Examples 2 to 6, by adding rare earth alloying elements during the RH refining process, the number density of inclusions is significantly reduced, reflecting the refinement effect of rare earth on inclusions, which can effectively reduce the defects caused by large-sized Al2O3 inclusions exposed during the rolling process.

[0068] Table 4 Inclusion number density in IF steel produced in Examples and Comparative Examples

[0069]

[0070] In summary, in the present invention, IF steel within the traditional BOF-RH-CC composition range can be produced by the BOF-LF-RH-CC process. At the same time, due to the addition of rare earth alloy elements in the RH process, the Al2O3 inclusions are denatured to miniaturize their size, reducing the number density of large-sized Al2O3 inclusions and the probability of defects caused by rolling exposure of large-sized Al2O3. Since reducing slag is produced in the refining LF furnace and the ladle top slag is modified at the end of RH, the cleanliness of the molten steel is guaranteed, the stability of the crystallizer liquid level fluctuation during the casting process is guaranteed, and the production stability of the rare earth-containing IF steel is improved, which has good promotion prospects and application value.

[0071] It should be noted that those skilled in the art should recognize that the above embodiments are only used to illustrate the present invention and are not intended to limit the present invention. As long as they are within the spirit of the present invention, any changes or modifications to the above embodiments will fall within the scope of the claims of the present invention.

Claims

1. A method for preparing rare earth-containing IF steel, characterized in that: The following steps are involved: S1, BOF smelting, using converter smelting, controlling the oxygen content in molten steel to ≤550ppm during steel tapping; S2, LF furnace refining, making reducing slag in the LF furnace, so that the content of FeO and MnO in the top slag after refining is ≤1%; S3, RH vacuum refining, forced decarburization by oxygen blowing, deoxidation and alloying after decarburization, rare earth alloy added at the end of alloying; S4, continuous casting CC, after refining is completed, the molten steel is continuously cast.

2. The method for preparing rare earth-containing IF steel according to claim 1, characterized in that: In the step S1, during the steel tapping process of the converter, the steel tapping temperature is 1640±15°C.

3. The method for preparing rare earth-containing IF steel according to claim 1, characterized in that: In the step S1, during the steel tapping process of the converter, aluminum particles are added to the molten steel for pre-deoxidation, and the amount of the aluminum particles added is controlled within 200 kg / furnace.

4. The method for preparing rare earth-containing IF steel according to claim 1, characterized in that: In the step S1, during the steel tapping process of the converter, 300-500 kg / furnace of lime is added to the slag to adjust the basicity.

5. The method for preparing rare earth-containing IF steel according to claim 1, characterized in that: In the step S2, after the LF furnace refining is completed, the composition of the top slag is: CaO: 50-55%, SiO2: 6-8%, Al2O3: 23-32%, MgO: 6-8%, FeO and MnO content ≤1%, calculated by mass percentage.

6. The method for preparing rare earth-containing IF steel according to claim 1, characterized in that: In the step S2, heat compensation is performed on the molten steel during the LF furnace refining process, and the temperature of the molten steel is controlled at 1640±10°C after the LF furnace refining is completed.

7. The method for preparing rare earth-containing IF steel according to claim 1, characterized in that: In step S3, during the oxygen blowing forced decarburization process, the oxygen blowing amount is 150-200m 3 .

8. The method for preparing rare earth-containing IF steel according to claim 1, characterized in that: In the step S3, after RH vacuum refining, the Ce content in the molten steel is 0.0004-0.0012wt%.

9. The method for preparing rare earth-containing IF steel according to claim 1, characterized in that: In step S3, the rare earth alloy is Ce-La alloy or Fe-Ce alloy; in the rare earth alloy, the content of Ce is ≥67wt%.

10. The method for preparing rare earth-containing IF steel according to claim 9, characterized in that: In the rare earth alloy, the addition amount of the rare earth alloy is 5 to 20 kg / furnace.

11. The method for preparing rare earth-containing IF steel according to claim 1, characterized in that: In the step S3, after the RH refining is completed, the top slag is modified, and aluminum particles are added to the slag surface so that the content of FeO and MnO in the top slag is less than 2%.

12. The method for preparing rare earth-containing IF steel according to claim 11, characterized in that: The amount of aluminum particles added is 50-100 kg / furnace.

13. The method for preparing rare earth-containing IF steel according to claim 1, characterized in that: In the step S4, during the continuous casting process, more than 90% of the crystallizer liquid level fluctuations are controlled within ±5 mm.

14. A rare earth-containing IF steel prepared by the method for preparing a rare earth-containing IF steel according to any one of claims 1 to 13, characterized in that: The number density of inclusions with a size of 2 to 5 microns is ≤ 4.5 / mm 2 , the number density of inclusions larger than 5 microns is ≤ 0.5 / mm 2 .

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