A method for controlling large-size inclusions in a rare earth alloy steel

Abstract

The application discloses a method for controlling large-size inclusions in rare earth alloy steel, and the steel grade is C 0.30-0.60%, Si≤1.0%, Mn≤1.0%, Cr 1.0-2.0%, Ni 1.0-3.0%, Mo 0.1-1.0%, and the total content of rare earth elements is RE 50-200ppm, and the rare earth elements are mainly Ce and La. The application optimizes the process of LF refining and RH vacuum treatment to improve the removal of large-particle inclusions in the steelmaking process, and ensures the continuous casting castability to avoid the falling of flocculation flow material. The rare earth elements are fed by using the continuous casting crystallizer wire feeding process, the S and O contents of the molten steel, the wire feeding position and the wire feeding speed are controlled to prevent the formation of solid slag blocks in the flow dead zone and the slag winding in the wire feeding process. Through the process, the qualified rate of the product blooming material (160mm*160mm) B grade ultrasonic flaw detection is 100%, and the qualified rate of the inner injury of the AA grade flaw detection of the rolled material is greater than or equal to 98%.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel smelting, and specifically relates to a method for controlling large-sized inclusions in rare earth alloy steel. Background Technology

[0002] The rare earth industry is one of my country's most distinctive and advantageous industries, and the application of rare earths in steel is one of the important areas for the promotion and application of rare earths. Adding an appropriate amount of rare earths to steel can purify molten steel, modify inclusions, and microalloy, which helps to improve the impact toughness and other properties of steel. It is used in many alloy steels.

[0003] However, because rare earth elements (mainly Ce and La) have a very strong binding ability with O and S, the yield of rare earths in the steelmaking process fluctuates greatly. At the same time, the rare earth oxides, sulfides or oxysulfides generated are very likely to cause nozzle clogging, affecting the continuous casting castability, and resulting in large rare earth inclusions of tens or even hundreds of micrometers in the product, which seriously affects the impact, fatigue and other performance properties of the product.

[0004] Currently, patents on rare earth steel mainly focus on the methods of rare earth addition, rare earth yield, and continuous casting castability. For example, patent CN 106609313 A, "A method for treating high-purity rare earth steel," uses a batch-addition method to improve the rare earth yield while utilizing rare earth modified inclusions to improve the cleanliness of the molten steel. Patent CN 110484811A, "An ultra-clean rare earth steel and a method for controlling inclusion modification," obtains small-sized, spherical, near-spherical, or granular rare earth inclusions by specifying the relationship between the rare earth content (REM) in ultra-clean rare earth steel and the total oxygen content, total sulfur content, and total oxygen content in the added rare earth metals or alloys. At the same time, the casting superheat is increased by 5-15°C compared to steel with the same composition but without rare earth to prevent casting nodules. Patent CN 110438389 A, "A Method for Producing High-Purity Rare Earth Steel," introduces rare earth elements into the RH process. By stabilizing the temperature and composition of the molten steel before the addition of rare earth elements, and optimizing the RH refining circulation flow rate, degassing time, and settling time before and during the addition of rare earth elements, the aim is to improve rare earth yield and stabilize continuous casting castability. Patent CN 111545717 A, "A Method for Casting Rare Earth Steel," controls the content of free O, S, Als, Ca, and rare earth elements in the molten steel before casting. It also precisely controls the process parameters of the continuous casting process and the composition and application of auxiliary materials, and refines the operation of the casting furnace and the protection of the molten steel during casting to improve rare earth yield and continuous casting castability. Patent CN 116219261A, "A Method for Preparing Rare Earth Steel," uses rare earth manganese alloy instead of rare earth ferroalloy to reduce losses from oxygen and sulfur combination and improve rare earth yield. Patent CN 115491463 A, "A Method for Improving Rare Earth Yield in the Smelting of Rare Earth Steel," strengthens refining, deoxidation, and desulfurization operations, while using a three-layer covering agent in the tundish to improve rare earth yield. Patent CN 111593252 A, "A Rare Earth Steel Smelting Method," adds rare earth using a dedicated container after RH cavitation. Patent CN 114716256 A, "A Refractory Material for Smelting Rare Earth Steel and a Method for Improving Rare Earth Yield," controls the dissolved oxygen [O] content in the molten steel to below 1.5 ppm before adding rare earth, and further optimizes the refractory material, top slag, and tundish covering agent to improve rare earth yield and continuous casting castability.Patents CN 111906266 A "A method for suppressing blockage of rare earth steel pouring nozzles using pulsed current", CN114309571 A "A method and apparatus for suppressing nozzle blockage during rare earth steel continuous casting", CN 114716850A "A coating for rare earth steel continuous casting nozzles and its preparation method", CN 115716126 A "A method for solving blockage of immersion nozzles in rare earth steel continuous casting", and CN 115945678A "An immersion nozzle for improving the castability of rare earth steel and its application method" work on immersion nozzles in continuous casting. They either increase the nozzle temperature and reduce condensation by installing an induction coil outside the nozzle, or reduce internal turbulence by optimizing the internal structure of the immersion nozzle, or suppress the adhesion of rare earth inclusions to the inner wall of the nozzle by using pulsed current, or improve the nozzle material or use argon blowing method for the nozzle. When rare earth elements are added during the steelmaking process, they react with elements such as O, S, Al, and Ca in the steel to form fine rare earth inclusions. They also react with existing large inclusions to generate larger rare earth inclusions. Due to the small density difference between the rare earth inclusions and the molten steel, the flotation removal efficiency is low, which is detrimental to the control of large inclusions in rare earth steel. Furthermore, the methods described in the above patents cannot completely solve the problem of rare earth inclusions adhering to the inner wall of the nozzle in the molten steel. When these inclusions adhere to a certain extent on the nozzle inner wall, they peel off, also leading to the problem of large inclusions in the steel.

[0005] Currently, there are some patented studies on feeding rare earth wire into a crystallizer. This method can effectively solve many problems caused by the turbulent flow of rare earth inclusions and does not cause rare earth loss. For example, patent CN 114700473A, "A method and system for uniformly adding rare earth to molten steel and its application," uses a feeding pipe to add rare earth into the crystallizer, but the tooling for this method is too complex. Patent CN 116274905A, "A method for producing rare earth steel," uses a crystallizer to feed rare earth. To improve the uniformity of rare earth, the submerged entry nozzle adopts a side-opening method, and the wire feeding point is selected in the central area of ​​the crystallizer, with a distance of <10mm from the inner wall of the nozzle, to promote the uniform distribution of rare earth elements in the billet with the circulation of molten steel. However, if the wire feeding method is improper, it is easy for rare earth elements to react with the protective slag, forming agglomerates, which can easily be drawn into the steel, resulting in the formation of ultra-large inclusions.

[0006] Controlling large particle inclusions in rare earth alloy steel requires addressing both the cleanliness of the molten steel before adding rare earth and the generation of large rare earth particle inclusions during and after the rare earth addition process. Currently, there are no patents that systematically, scientifically, and rationally study and control large particle inclusions in rare earth alloy steel. Summary of the Invention

[0007] The purpose of this invention is to provide a method for controlling large-sized inclusions in rare earth alloy steel. Combining key production processes that generate large inclusions, this invention optimizes the LF refining and RH vacuum treatment processes to improve the removal of large-particle inclusions during steelmaking and ensure continuous casting castability by preventing the shedding of fibrous material. Rare earth is fed into the continuous casting crystallizer using a wire feeding process. By controlling the S and O content of the molten steel, as well as the wire feeding position and speed, the formation of solid slag blocks and slag entrapment in the flow dead zone during the wire feeding process is prevented. This reduces the number of large-particle inclusions in the product and minimizes their negative impact on the fatigue, toughness, and other properties of the steel product.

[0008] This invention is achieved through the following technical solution:

[0009] The present invention discloses a method for controlling large-sized inclusions in rare earth alloy steel, comprising the following steps: converter primary refining - LF refining - RH vacuum treatment - continuous casting.

[0010] This invention discloses a method and production process for controlling large-size inclusions in rare earth alloy steel. Ni, Mo, and other alloys are added to the converter along with scrap steel; the final C content at the converter endpoint is ≥0.05%, and the endpoint temperature is ≥1600℃; aluminum blocks are used for pre-deoxidation during tapping, with an addition rate of 1.0 kg / t, followed by the addition of other alloys such as ferrosilicon, ferrosilicon, and ferrochrome to ensure that the composition of the first sample at the LF (sulfurized leaching) is within the steel grade requirements. After the alloys are added, 500 kg of lime and 600 kg of furnace protectant are added.

[0011] The present invention discloses a method and production process for controlling large-size inclusions in rare earth alloy steel, characterized in that: the LF refining process uses aluminum granules + SiC or calcium carbide slag surface deoxidation, the process adds ≤300kg of alloy and ≤200kg of slag, and all additions are completed within 30 minutes after the start of refining; the Al content of the final molten steel is controlled at 0.030-0.040%, the S content is ≤0.005%, and the final slag binary basicity (CaO / SiO2) is controlled at 4.0-6.0.

[0012] This invention discloses a method and production process for controlling large-size inclusions in rare earth alloy steel, with an RH boost gas flow rate of 100 Nm. 3 / h, holding time ≥20min under vacuum degree ≤133Pa. After vacuum treatment, calcium treatment is performed, with a target calcium content of 15-20ppm for the first furnace and 10-15ppm for the continuous casting furnace, ensuring continuous casting castability. Soft blowing time ≥15min, during which argon gas must not break the slag surface, expose molten steel, or cause excessive stirring.

[0013] This invention discloses a method and production process for controlling large-sized inclusions in rare earth alloy steel. Rare earth is fed into the crystallizer using a wire feeder during the continuous casting stage. The continuously cast billet is a square or rectangular billet with a cross-sectional length between 300-400 mm. The casting speed is 0.5-1.0 m / min, maintained at a constant speed. A crystallizer wire feeder is used to feed rare earth cored wire into the crystallizer. The feeding position is 20-40 mm from the outer wall of the nozzle. The feeding speed is controlled according to the rare earth (RE) content. The formula for calculating the feeding speed and rare earth content is as follows:

[0014] V 线 =0.078×V 拉 ×S 断面 ×[RE 产 %] / m 线

[0015] Among them, V 线 V is the wire feeding speed, in m / min. 拉 The continuous casting speed is expressed in m / min; S 断面 The dimensions are the cross-sectional dimensions of the continuously cast billet, in mm. 2 ;[RE 产 [%] represents the required percentage of rare earth content in the product; m 线 The weight per meter of wire is expressed in g / m. The length of the billet cut off in the first continuous casting furnace should be ≥2m to remove the unstable section during the initial feeding of rare earth elements.

[0016] This invention discloses a method and production process for controlling large-size inclusions in rare earth alloy steel. The steel composition includes C 0.30-0.60%, Si ≤ 1.0%, Mn ≤ 1.0%, Cr 1.0-2.0%, Ni 1.0-3.0%, Mo 0.1-1.0%, and a total rare earth content of 50-200 ppm. The main rare earth elements are Ce and La, with S ≤ 0.0050% and O ≤ 0.0010%. Other components such as V, Nb, Ti, Cu, and B can be added according to product requirements without affecting the effectiveness of this patent.

[0017] The technical principle of this invention:

[0018] (1) Controlling large inclusions larger than tens of micrometers in rare earth steel relies on two main aspects. First, it involves controlling the cleanliness of the molten steel before adding rare earth elements. This is primarily achieved by reducing the addition of foreign alloys and slag in the LF refining process, thus reducing the likelihood of slag entrapment and the formation of large foreign inclusions. Second, it requires appropriate duration of high vacuum treatment with RH and soft blowing to promote the flotation and removal of large inclusions, which can significantly reduce the probability of large inclusions. Therefore, this invention requires that the alloy addition during the LF refining process be ≤300 kg and the slag addition be ≤200 kg, and that all additions be completed within 30 minutes after the start of refining. The RH gas flow rate should be increased to 100 Nm. 3 / h, holding time at vacuum degree ≤133Pa ≥20min; soft blowing time ≥15min, the soft blowing process must not involve argon gas blowing through the slag surface, molten steel exposure, or excessive stirring. In addition, in order to achieve the following deoxidation and desulfurization effects, the Al content of the LF refined steel liquid must be controlled at 0.030-0.040%, and the binary basicity (CaO / SiO2) of the final slag must be controlled at 4.0-6.0.

[0019] (2) Controlling large particle inclusions of tens of micrometers or larger in rare earth steel is another aspect of how to avoid the generation of large-sized inclusions during and after the rare earth addition process. The rare earth addition method in the refining process cannot completely avoid the turbulence problem at the nozzle caused by the formation of rare earth oxides and sulfides, which will lead to the problem of large particle inclusions caused by the falling of turbulent material at the nozzle in stages; while rare earth alloying through the crystallizer wire feeding process has the problem of the wire feeding speed being too fast, which leads to the entrapment of protective slag, or the wire feeding position being improper and biased towards the nozzle side, which leads to the combined reaction of rare earth elements, O, S and other elements in the low-speed steel flow near the nozzle with the protective slag, which deteriorates the performance of the protective slag, causing the protective slag to agglomerate and be entrapped into the molten steel during the casting process. This is a point that has not been considered in other patents.

[0020] (3) Taking into account the advantages and disadvantages of different rare earth addition methods, this invention adopts the crystallizer wire feeding method to add rare earth, and further limits the S content in the molten steel to ≤0.0050% and dissolved O to ≤0.0010%, and standardizes the wire feeding position (20-40mm from the nozzle) to reduce the multiphase reaction of molten steel, protective slag and rare earth wire in the low flow rate area of ​​molten steel, and prevent the formation of slag lumps on the inner wall of the nozzle. In addition, this invention believes that the core role of rare earth is to purify the material grain boundaries by solidifying trace rare earth elements in steel, and has limited effect on refining inclusions or improving the crystallizer of molten steel. Therefore, the upper limit of the total rare earth content is limited to ≤200ppm, which is conducive to preventing the risk of slag entrapment caused by excessively fast wire feeding speed. However, if the wire feeding speed is too slow, it will aggravate the reaction between rare earth wire and protective slag. Therefore, the lower limit of rare earth content is required to be ≥50ppm. Attached Figure Description

[0021] Figure 1 Feeding the crystallizer with wire.

[0022] Figure 2 The rare earth inclusions are shown in the examples.

[0023] Figure 3 The image shows a relatively large rare earth inclusion found in the bar material of Comparative Example 1.

[0024] Figure 4 As a comparative example, yellow clumps were found on the outer wall of the submersible nozzle.

[0025] Figure 5 For comparison example 2, ultra-large rare earth inclusions were found in the billet. Detailed Implementation

[0026] The present invention will be further described in detail below with reference to specific implementation methods, but the scope of protection of the present invention is not limited to the content described herein.

[0027] Example

[0028] This example illustrates the actual control effect of the present invention on large particle inclusions in steel using the production process of a medium-carbon rare earth alloy steel. The actual controlled composition is C 0.35%, Si 0.20%, Mn 0.70%, Cr 1.5%, Ni 2.8%, Mo 0.5%, total rare earth content RE 0.012%, with Ce and La being the main rare earth elements, S 0.004%, and O 0.0007%.

[0029] (1) A 120t converter was used, with 4000kg of nickel plate and 1220kg of ferromolybdenum alloy added along with scrap steel; the final C content of the converter was 0.06%, and the final temperature was 1620℃; during the tapping process, 130kg of aluminum blocks were used for pre-deoxidation, followed by the addition of 1500kg of ferrosilicon manganese, 150kg of ferrosilicon, and 4000kg of ferrochrome alloy. After the alloys were added, 500kg of lime and 600kg of furnace protectant were added.

[0030] (2) In the LF refining process, aluminum granules + SiC slag surface deoxidation is used, 236 kg of alloy is added, and 50 kg of slag is added; the final steel liquid Al is 0.032%, the final slag binary basicity (CaO / SiO2) is 5.23, and the final S is 0.004%.

[0031] (3) RH increase gas flow rate by 100 Nm 3 / h, holding pressure for 20min under vacuum degree ≤133Pa. After vacuum treatment, calcium treatment is carried out, with soft blowing time of 20min. During the soft blowing process, there were no instances of argon gas breaking the slag surface, molten steel being exposed, or excessive stirring.

[0032] (4) The continuous casting billet has a rectangular cross-section with a length between 300mm and 325mm. The casting speed is 0.80m / min, and a constant casting speed is maintained after casting begins. The continuous casting uses a mold feeder to feed rare earth cored wire into the mold. The wire diameter is 2.5mm, the weight per meter is 33g / m, and the feeding position is 30mm from the outer wall of the nozzle. The feeding speed is controlled according to the rare earth RE content and is 2.21m / min. The first batch of billets in the continuous casting process has a cut-off length of 4.5m to remove the unstable section during the initial rare earth feeding.

[0033] Ultrasonic testing was performed on process products and final bar products (φ30mm in diameter). The pass rate for Grade B ultrasonic testing of billets (160mm×160mm) was 100%, and the pass rate for Grade AA internal defects in rolled products was 99.5%. Rare earth elements exist in the following forms: Figure 2As shown, the rare earth inclusions are all small-sized inclusions.

[0034] Comparative Example 1 (high S and O content)

[0035] The actual controlled composition of Comparative Example 1 was C 0.34%, Si 0.25%, Mn 0.69%, Cr 1.45%, Ni 2.8%, Mo 0.5%, and the total rare earth content RE 0.012ppm. The main rare earth elements were Ce and La, S 0.012%, and O 0.0023%.

[0036] (1) A 120t converter was used, with 4000kg of nickel plate and 1220kg of ferromolybdenum alloy added along with scrap steel; the final C content of the converter was 0.06%, and the final temperature was 1620℃; during the tapping process, 120kg of SiC was used for pre-deoxidation, followed by the addition of 1550kg of ferrosilicon manganese, 130kg of ferrosilicon, and 4000kg of ferrochrome alloy. After the alloys were added, 500kg of lime and 300kg of furnace protectant were added.

[0037] (2) The LF refining process uses SiC slag surface deoxidation, adds 150kg of alloy and 70kg of slag material; the final steel liquid Al is 0.0028%, the final slag binary basicity (CaO / SiO2) is 2.80, and the final S is 0.012%.

[0038] (3) RH increase gas flow rate by 100 Nm 3 / h, holding pressure for 20min under vacuum degree ≤133Pa. After vacuum treatment, calcium treatment is carried out, with soft blowing time of 20min. During the soft blowing process, there were no instances of argon gas breaking the slag surface, molten steel being exposed, or excessive stirring.

[0039] (4) The continuous casting billet has a rectangular cross-section with a length between 300mm and 325mm. The casting speed is 0.80m / min, and a constant casting speed is maintained after casting begins. The continuous casting uses a mold feeder to feed rare earth cored wire into the mold. The feeding position is 30mm away from the outer wall of the nozzle. The feeding speed is controlled according to the rare earth RE content and is 2.21m / min. The first batch of billets in the continuous casting process has a cut-off length of 4.5m to remove the unstable section during the initial rare earth feeding.

[0040] Ultrasonic testing was performed on both the process products and the final bar products (φ30mm in diameter). The pass rate for Grade B ultrasonic testing of the billet (160mm×160mm) was 100%, but the pass rate for Grade AA internal defects in the rolled products was 85%. Dissection revealed a certain number of rare earth inclusions with dimensions of tens of micrometers, with typical morphologies as shown below. Figure 3 As shown.

[0041] Comparative Example 2 (Feeding silk near the water inlet)

[0042] The actual controlled composition of Comparative Example 2 was C 0.35%, Si 0.23%, Mn 0.70%, Cr 1.49%, Ni 2.75%, Mo 0.5%, and the total rare earth content RE 0.012ppm. The main rare earth elements were Ce and La, S 0.004%, and O 0.0007%.

[0043] (1) A 120t converter was used, with 3950kg of nickel plate and 1230kg of ferromolybdenum alloy added along with scrap steel; the final C content of the converter was 0.07%, and the final temperature was 1610℃; during the tapping process, 130kg of aluminum blocks were used for pre-deoxidation, followed by the addition of 1500kg of ferrosilicon manganese, 160kg of ferrosilicon, and 4000kg of ferrochrome alloy. After the alloys were added, 500kg of lime and 600kg of furnace protectant were added.

[0044] (2) In the LF refining process, aluminum granules + SiC slag surface deoxidation is used, 212 kg of alloy is added, and 75 kg of slag is added; the final steel liquid Al is 0.035%, the final slag binary basicity (CaO / SiO2) is 5.61, and the final S is 0.003%.

[0045] (3) RH increase gas flow rate by 100 Nm 3 / h, holding pressure for 20min under vacuum degree ≤133Pa. After vacuum treatment, calcium treatment is carried out, with soft blowing time of 20min. During the soft blowing process, there were no instances of argon gas breaking the slag surface, molten steel being exposed, or excessive stirring.

[0046] (4) The continuous casting billet has a rectangular cross-section with a length between 300mm and 325mm. The casting speed is 0.80m / min, and a constant casting speed is maintained after initial casting. A mold feeder is used to feed rare earth cored wire into the mold during continuous casting. The feeding position is ≤10mm from the outer wall of the nozzle. The feeding speed is controlled according to the rare earth RE content, and is 2.21m / min. The first batch of billets is cut off to a length of 4.5m to remove the unstable section during the initial rare earth feeding. Agglomerates exist on the outer wall of the immersion nozzle in the mold, such as… Figure 4 As shown.

[0047] Ultrasonic testing was performed on process products and final bar products (φ30mm in diameter). The pass rate for Grade B ultrasonic testing of billets (160mm×160mm) was 85%, and the pass rate for Grade AA internal defects in rolled products was 83%. Dissection revealed the presence of oversized rare earth inclusions in the billets, such as... Figure 5 As shown.

Claims

1. A method for controlling large-size inclusions in rare earth alloy steel, characterized in that, The process includes converter primary refining, LF refining, RH vacuum treatment, and continuous casting, specifically comprising the following steps: (1) Nickel and molybdenum alloys are added to the converter along with scrap steel; the final C of the converter is ≥0.05% and the final temperature is ≥1600℃; aluminum blocks are used for pre-deoxidation during the tapping process, followed by the addition of silicon manganese, silicon ferrosilicon and high carbon ferrochrome alloys to ensure that the composition of the first sample at LF is within the range required by the steel grade. After the alloys are added, slag-forming materials and furnace protection agents are added. (2) In the LF refining process, aluminum granules + SiC are used for slag surface deoxidation. The alloy replenishment is ≤300kg and the slag replenishment is ≤200kg. All replenishment is completed within 30 minutes after the start of refining. The Al content of the molten steel is controlled at 0.030-0.040%, the S content is ≤0.005%, and the binary basicity of the final slag, CaO / SiO2, is controlled at 4.0-6.

0. (3) RH vacuum lift gas flow rate 100 Nm 3 / h, holding time ≥20min under vacuum degree ≤133Pa; calcium treatment after vacuum treatment to ensure continuous casting castability; soft blowing time ≥15min, and argon gas must not blow the slag surface, expose the molten steel, or cause excessive stirring during the soft blowing process; (4) During the continuous casting stage, a wire feeder is used to feed rare earth cored wire into the crystallizer. The feeding position is 20-40 mm away from the outer wall of the nozzle. The feeding speed is controlled according to the rare earth RE content. The formula for calculating the feeding speed and rare earth content is as follows: V 线 =0.078×V 拉 ×S 断面 ×[RE 产 %] / m 线 Among them, V 线 V is the wire feeding speed, in m / min. 拉 The continuous casting speed is expressed in m / min; S 断面 The dimensions are the cross-sectional dimensions of the continuously cast billet, in mm. 2 ;[RE 产 [%] represents the required percentage of rare earth content in the product; m 线 The weight of the thread per meter is expressed in g / m. The rare earth alloy steel composition, by percentage, includes: C 0.30-0.60%, Si≤1.0%, Mn≤1.0%, Cr 1.0-2.0%, Ni 1.0-3.0%, Mo 0.1-1.0%, total rare earth content RE 50-200ppm, S≤0.0050%, O≤0.0010%.

2. The method for controlling large-size inclusions in rare earth alloy steel according to claim 1, characterized in that, Step (1) The amount of aluminum blocks added is 1.0 kg / t; the slag material is lime, and the amount added is 500 kg; the amount of furnace protectant added is 600 kg.

3. The method for controlling large inclusions in rare earth alloy steel according to claim 1, characterized in that, Step (3) Target calcium content after calcium treatment: 15-20 ppm for the first furnace and 10-15 ppm for the continuous casting furnace.

4. The method for controlling large-size inclusions in rare earth alloy steel according to claim 1, characterized in that, The continuously cast billet is a rectangular billet with a cross-sectional length between 300-400mm. The continuous casting speed is kept constant, ranging from 0.5 to 1.0m / min.

5. The method for controlling large-size inclusions in rare earth alloy steel according to claim 1, characterized in that, The length of the billet cut in the first continuous casting furnace is ≥2m to remove the unstable section during the initial feeding of rare earth elements.

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