A method for smelting a high-cleanliness steel rare-earth steel

Abstract

This invention discloses a method for smelting high-purity rare earth steel, belonging to the field of iron and steel smelting technology. This invention reduces inclusions and improves purity by sequentially subjecting vacuum-treated molten steel to calcium and rare earth treatments. Calcium treatment transforms inclusions into liquid inclusions, promoting polymerization and growth, which are then removed by flotation during soft argon blowing, thus reducing the purity of the molten steel. Based on this, rare earth treatment further deoxidizes the steel, improving its purity and modifying inclusions. This invention fully considers the aluminum reduction in actual production processes. It utilizes the total aluminum, sulfur, and total oxygen content in the molten steel after vacuum degassing to limit the amount of solid pure calcium wire and rare earth added, thereby improving the purity of the molten steel while avoiding nozzle clogging or nodule formation during continuous casting.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel smelting technology, and specifically relates to a smelting method for high-purity rare earth steel. Background Technology

[0002] Inclusions are primarily classified according to their physical state at molten steel temperatures (e.g., 1550℃), which directly determines their morphology, distribution, and hazards. Liquid inclusions remain liquid at steelmaking temperatures and will extend into slender, plastic inclusions (such as sulfides) or thin films during rolling. Solid inclusions remain solid at steelmaking temperatures and do not deform with the matrix during rolling, retaining their angular shape.

[0003] Studies have found that pretreatment with calcium followed by rare earth treatment can accelerate deoxidation and desulfurization, modify high-melting-point inclusions, significantly improve rare earth yield, and suppress the formation of coarse rare earth inclusions. A method for refining rare earth inclusions in super stainless steel was disclosed in Chinese patent document CN114058767A (202111348815.5), achieving the refinement of rare earth inclusions. Although the pretreatment with calcium followed by rare earth treatment can synergistically modify inclusions, it also presents problems such as the easy formation of high-melting-point composite inclusions and the risk of nozzle blockage. Furthermore, improper matching of process parameters can lead to a decrease in steel toughness and fatigue performance, and a reduction in production stability. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention proposes a method for producing high-purity rare earth treated steel. The process route is as follows: KR desulfurization → electric furnace / converter smelting → LF furnace refining → vacuum degassing → calcium treatment → soft blowing → rare earth treatment → soft blowing → continuous casting. This invention fully considers the aluminum drop in the actual production process. It utilizes the total aluminum, sulfur, and total oxygen content in the molten steel after vacuum degassing to limit the amount of solid pure calcium wire and rare earth added, thereby improving the cleanliness of the molten steel while avoiding clogging or nodule formation during continuous casting.

[0005] The technical problem to be solved by the present invention is achieved by the following technical solution: a smelting method for high-purity rare earth steel, the process route of which is: KR desulfurization → electric furnace / converter smelting → LF furnace refining → vacuum degassing treatment → calcium treatment → soft blowing → rare earth treatment → soft blowing → continuous casting. The raw material used for calcium treatment is solid pure calcium wire, and the amount of calcium added is (0.1~0.2)*[Al]t+[S]+[O]t. Calcium treatment turns the inclusions into liquid inclusions. The raw material used for rare earth treatment is pure rare earth La, Ce or (La, Ce) alloy, and the amount of rare earth added is (1~2)*([S]+[O]t), where [Al]t, [S], and [O]t are the total aluminum, sulfur, and total oxygen contents in the molten steel after vacuum degassing, respectively. Argon blowing and stirring are performed during calcium and rare earth treatment processes; After refining in the LF furnace, the Al content is 0.030%~0.065%, S≤0.0025%, and O≤0.0020%. After vacuum degassing, the Al content is 0.015%~0.050%, S≤0.0015%, and O≤0.0009%. During continuous casting, the Al content in the molten steel ranges from 0.010% to 0.045%. Rare earth treatment causing nozzle blockage or nodule formation is a production bottleneck, significantly influenced by aluminum and rare earth elements. This invention utilizes the total aluminum, sulfur, and total oxygen content in the molten steel after vacuum degassing to limit the addition of solid pure calcium wire and rare earth elements. Besides [S] + [O]t, [Al]t multiplied by 0.1 to 0.2 primarily considers the aluminum drop during actual production, especially in the subsequent continuous casting process. Severe aluminum drop worsens the castability of the molten steel, making nozzle blockage or nodule formation more likely. By limiting the addition of solid pure calcium wire and rare earth elements, nozzle blockage or nodule formation during continuous casting is avoided.

[0006] Since calcium exists primarily in molten steel as oxides, sulfides, and complex oxygen sulfides, the amount of solid pure calcium wire added is limited by utilizing the total aluminum, sulfur, and total oxygen content in the molten steel after vacuum degassing. This invention fully treats the CaO-Al2O3 inclusions into liquid inclusions. These liquid inclusions are more prone to collision and growth, more easily combine with argon-blown bubbles to float, and are given sufficient time to float.

[0007] This invention uses solid-core pure calcium wire instead of silicon-calcium wire. This makes it easier to adjust the composition of inclusions, and because Al has a greater deoxidizing capacity than Si, it is easier to control the aluminum content, deoxidation effect, and castability of molten steel without silicon-calcium wire. The argon blowing flow rate varies in this invention. A high flow rate is used for both calcium and rare earth treatments to facilitate stirring and homogenization, removing large inclusions. A soft blowing process with a low flow rate is used after calcium and rare earth treatments to remove smaller inclusions.

[0008] In a preferred embodiment of this invention, KR desulfurization specifically involves: immersing the stirring head in a ladle of molten iron and rotating it at a speed of 80-120 rpm; adding a lime (CaO)-based desulfurizing agent and fluorite; and stirring and reacting for 8-15 minutes. The sulfur content of the treated molten iron is ≤0.002%. Clean steel generally has sulfur content requirements, which are related to calcium treatment and rare earth treatment.

[0009] In a preferred embodiment of the present invention, the electric furnace / converter smelting process specifically involves: employing combined blowing, controlling the final carbon content of the converter to 0.10%~0.35%; the final phosphorus content to 0.009%~0.015%; and requiring a tapping temperature of 1600~1635℃. Slag-blocking tapping is employed to minimize the entry of oxidizing slag into the ladle and prevent phosphorus reversion. Endpoint carbon and endpoint phosphorus are crucial indicators for clean steel production, relating to the oxidizability of molten steel, or more specifically, the oxygen content of converter steel, directly impacting calcium and rare earth treatment processes.

[0010] In a preferred embodiment of the present invention, the LF furnace refining process specifically involves: using strong deoxidizers such as aluminum for deep deoxidation, achieving a slag basicity of 4-10, and controlling the (FeO+MnO) content in the slag to be <0.5%. Adjust the argon flow rate according to the liquid level fluctuations to promote the flotation of inclusions. A slag concentration of (FeO+MnO) < 0.5% ensures that the oxygen content of the precipitated deoxidized steel is less than 10 ppm.

[0011] In a preferred embodiment of this invention, the vacuum degassing process specifically involves transferring the ladle into a VD furnace or RH furnace for vacuum circulation treatment, maintaining this state at a vacuum level ≤67 Pa for 15-25 minutes. These are all cleanliness indicators related to rare earth steel clogging or nozzle nodule formation.

[0012] Preferably, the calcium treatment in this invention specifically involves: during the feeding of solid pure calcium wire into the ladle, the argon blowing flow rate is 70~100 NL / min; Soft blowing: Bottom blowing with a low flow rate of argon gas (visibly causing slight fluctuations on the surface of the molten steel), at a flow rate of 40~60 NL / min, for a duration of ≥10 minutes, to promote the floating of any remaining small inclusions.

[0013] Preferably, the rare earth treatment in this invention specifically involves: the argon blowing flow rate during the process of adding the rare earth alloy into the ladle is 70~100 NL / min; Soft blowing: Bottom blowing with a low flow rate of argon gas (visibly causing slight fluctuations on the surface of the molten steel), at a flow rate of 40~60 NL / min, for a duration of ≥10 minutes, to promote the floating of any remaining small inclusions.

[0014] In a preferred embodiment of the present invention, continuous casting specifically involves: full protective casting.

[0015] The inventive concept of this invention is as follows: In processes such as vacuum degassing in a steel ladle, inert gases such as argon are blown into the bottom of the molten steel, forming a large number of rising bubbles. Inclusion particles in the molten steel collide with these rising bubbles, becoming wetted and adhering to the bubble surface, or possibly becoming encapsulated within the bubbles, rising together to the slag layer on the surface of the molten steel. Furthermore, the agitation of the bubbles promotes collisions, aggregation, and growth of the inclusion particles. According to Stokes' law, the buoyancy is proportional to the square of the radius; as the size of the inclusion increases, its buoyancy increases dramatically, making it easier to remove from the molten steel. To remove inclusions, argon needs to be blown into the molten steel at different stages. During calcium and rare earth treatments, a larger argon flow rate is used to promote the rapid homogenization of calcium and rare earth elements; after their addition, a smaller argon flow rate can be used to promote inclusion flotation.

[0016] Conventional steelmaking temperatures are 1520–1600℃. High-melting-point solid Al2O3 inclusions or CaO-Al2O3 calcium aluminates have melting points above 1800℃. Due to their high melting point and poor fluidity, they are difficult to polymerize and float, and the particles do not easily fuse and grow. After vacuum degassing, calcium treatment is performed using solid pure calcium wire as raw material. The amount of calcium added is 0.1*[Al]t+[S]+[O]t. The high-melting-point inclusions are transformed into low-melting-point 12CaO·7Al2O3, which has a melting point of approximately 1455℃. It is liquid in molten steel and has good fluidity, which is conducive to collision polymerization and floating separation.

[0017] Cerium and lanthanum have significantly stronger deoxidizing capabilities than calcium. Calcium can reduce the oxygen activity in molten steel, creating thermodynamic conditions for efficient deoxidation by rare earth elements. Rare earth elements, on the other hand, can stabilize calcium oxides, preventing premature volatilization or the formation of low-melting-point sulfides with sulfur. When rare earth metals are added to steel, they preferentially react with elements such as O and S compared to Al and Mn to form rare earth inclusions, thus altering the inclusions in the steel. Rare earth oxygen sulfides have low interfacial energy and good wettability, making them less prone to growth in molten steel and resulting in fine, dispersed particles within the steel matrix. Under the combined action of these three elements, a homogeneous and dense composite oxide (such as CaO-Ce₂O₃-Al₂O₃) can be formed.

[0018] Compared with the prior art, the beneficial effects of the present invention are: This invention reduces inclusions and improves cleanliness by sequentially treating molten steel with calcium and rare earth elements after vacuum treatment. The calcium treatment transforms inclusions into liquid inclusions, promoting polymerization and growth, which are then removed by flotation during soft argon blowing, thus reducing the cleanliness of the molten steel. Building upon this, the rare earth treatment further deoxidizes the steel, improving its cleanliness and altering inclusions. This invention fully considers the aluminum reduction in actual production processes. It utilizes the total aluminum, sulfur, and total oxygen content in the molten steel after vacuum degassing to limit the amount of solid pure calcium wire and rare earth elements added, thereby improving steel cleanliness while preventing nozzle blockage or nodule formation during continuous casting. This invention can be applied to the production of high-purity steels such as high-quality gear steel, bearing steel, automotive sheet steel, and die steel. Attached Figure Description

[0019] Figure 1 This is a morphological image of the inclusions in the steel in Example 1; Figure 2 for Figure 1 Energy dispersive spectroscopy (EDS) analysis of inclusions; Figure 3 This is a morphological image of the inclusions in the steel in Example 2; Figure 4 for Figure 3 Energy dispersive spectroscopy (EDS) analysis of inclusions. Detailed Implementation

[0020] The technical solutions in the embodiments of the present invention will now be clearly and completely described in conjunction with the accompanying drawings.

[0021] Example 1 This embodiment uses a high-purity rare earth steel smelting method to smelt gear steel. The process route is as follows: KR desulfurization → electric furnace / converter smelting → LF furnace refining → vacuum degassing treatment → calcium treatment → soft blowing → rare earth treatment → soft blowing → continuous casting. The details are as follows: KR desulfurization: The stirring head is immersed in the molten iron ladle and rotated at a speed of 80-120 rpm. Lime (CaO)-based desulfurizing agent and fluorite are added, and the mixture is stirred and reacted for 8-15 minutes. The sulfur content of the treated molten iron is ≤0.002%.

[0022] Electric furnace / converter smelting: adopts combined blowing, converter endpoint carbon control 0.10%~0.35%; endpoint phosphorus 0.009%~0.015%; tapping temperature required 1600~1635℃; slag-blocking tapping is adopted to minimize the entry of oxidizing slag into the ladle and prevent phosphorus return.

[0023] LF furnace refining: Deep deoxidation is performed using strong deoxidizers such as aluminum, with slag basicity of 4-10, and the (FeO+MnO) content in the slag is controlled to be <0.5%. Argon flow rate is adjusted according to liquid level fluctuations to promote inclusion flotation. Al content is 0.030%~0.065%, S≤0.0025%, O≤0.0020%. Vacuum degassing treatment: The ladle is transferred to a VD furnace or RH furnace for vacuum circulation treatment. It is maintained at a vacuum degree ≤67 Pa for 15-25 minutes; Al content 0.015%~0.050%, S≤0.0015%, O≤0.0009%; Calcium treatment: Feed solid pure calcium wire into the ladle at a rate of 0.0070%; during the addition process, the argon flow rate is 70~100 NL / min; Soft blowing: Bottom blowing with a low flow rate of argon gas (visibly causing slight fluctuations on the surface of the molten steel), at a flow rate of 40~60 NL / min, for a duration of ≥10 minutes, to promote the floating of any remaining small inclusions.

[0024] Rare earth treatment: Add (La, Ce) rare earth alloy to the ladle at a rate of 0.0040%, and blow argon at a rate of 70~100 NL / min during the addition process; Soft blowing: Bottom blowing with a low flow rate of argon gas (visibly causing slight fluctuations on the surface of the molten steel), at a flow rate of 40~60 NL / min, for a duration of ≥10 minutes, to promote the floating of any remaining small inclusions.

[0025] Continuous casting: Fully protected casting is carried out, with an Al content of 0.010%~0.045% in the molten steel, enabling continuous production of 6 heats and effectively avoiding clogging or nodule formation during continuous casting.

[0026] Through the above production process, especially the vacuum degassing treatment and continuous casting, the Al content in the molten steel fluctuates by less than 0.010%, and the obtained gear steel has [O]t ≤ 7 × 10⁻⁶. -6 Ca≤4×10 -6 The rare earth element (La, Ce) content is 0.0008~0.0011%. For example... Figure 1 As shown, the size of inclusions in the steel after rare earth treatment is ≤10μm, such as... Figure 2 As shown, the inclusions contain elements such as O, S, Al, Ca, La and Ce. Because rare earth elements have high atomic numbers, they appear bright white in the backscattered electron image.

[0027] Example 2 This embodiment uses a high-purity rare earth steel smelting method to smelt H13 mold steel. The process route is as follows: KR desulfurization → electric furnace / converter smelting → LF furnace refining → vacuum degassing treatment → calcium treatment → soft blowing → rare earth treatment → soft blowing → continuous casting. The details are as follows: KR desulfurization: The stirring head is immersed in the molten iron ladle and rotated at a speed of 80-120 rpm. Lime (CaO)-based desulfurizing agent and fluorite are added, and the mixture is stirred and reacted for 8-15 minutes. The sulfur content of the treated molten iron is ≤0.002%.

[0028] Electric furnace / converter smelting: adopts combined blowing, converter endpoint carbon control 0.10%~0.35%; endpoint phosphorus 0.009%~0.015%; tapping temperature required 1600~1635℃; slag-blocking tapping is adopted to minimize the entry of oxidizing slag into the ladle and prevent phosphorus return.

[0029] LF furnace refining: Deep deoxidation is performed using strong deoxidizers such as aluminum, with slag basicity of 4-10, and the (FeO+MnO) content in the slag is controlled to be <0.5%. Argon flow rate is adjusted according to liquid level fluctuations to promote inclusion flotation. Al content is 0.030%~0.065%, S≤0.0025%, O≤0.0020%. Vacuum degassing treatment: The ladle is transferred to a VD furnace or RH furnace for vacuum circulation treatment. It is maintained at a vacuum degree ≤67 Pa for 15-25 minutes; Al content 0.015%~0.030%, S≤0.0030%, O≤0.0020%; Calcium treatment: Feed solid pure calcium wire into the ladle at a rate of 0.0085%; during the addition process, the argon blowing flow rate is 80~100 NL / min; Soft blowing: Bottom blowing with a low flow rate of argon gas (visibly causing slight fluctuations on the surface of the molten steel), at a flow rate of 40~50 NL / min, for a duration of ≥10 minutes, to promote the floating of any remaining small inclusions.

[0030] Rare earth treatment: Add (La, Ce) rare earth alloy to the ladle at a rate of 0.0060%, and purge argon at a flow rate of 80~100 NL / min during the addition process; Soft blowing: Bottom blowing with a low flow rate of argon gas (visibly causing slight fluctuations on the surface of the molten steel), at a flow rate of 40~50 NL / min, for a duration of ≥10 minutes, to promote the floating of any remaining small inclusions.

[0031] Continuous casting: Fully protected casting is carried out, with an Al content of 0.010%~0.025% in the molten steel, enabling continuous production of 4 heats and effectively avoiding clogging or nodule formation during continuous casting.

[0032] Through the above production process, especially the vacuum degassing treatment and continuous casting, the Al content in the molten steel fluctuates by less than 0.010%, and the obtained H13 mold steel has [O]t ≤ 15 × 10⁻⁶. -6 Ca≤8×10 -6 The rare earth element (La, Ce) content is 0.0011~0.0015%. For example... Figure 3 As shown, rare earth-treated steel exhibits smaller inclusion sizes, with inclusion sizes ≤10mm; for example... Figure 4 As shown, the rare earth-treated inclusions contain elements such as S, La, and Ce. Rare earth elements have high atomic numbers, so they appear bright white in backscattered electron images.

Claims

1. A method for smelting high-purity rare earth steel, characterized in that, The process route adopted is: KR desulfurization → electric furnace / converter smelting → LF furnace refining → vacuum degassing treatment → calcium treatment → soft blowing → rare earth treatment → soft blowing → continuous casting. The raw material used for calcium treatment is solid pure calcium wire, and the amount of calcium added is (0.1~0.2)*[Al]t+[S]+[O]t. Calcium treatment turns the inclusions into liquid inclusions. The raw material used for rare earth treatment is pure rare earth La, Ce or (La, Ce) alloy, and the amount of rare earth added is (1~2)*([S]+[O]t), where [Al]t, [S], and [O]t are the total aluminum, sulfur, and total oxygen contents in the molten steel after vacuum degassing, respectively. Argon blowing and stirring are performed during calcium and rare earth treatment processes; After refining in the LF furnace, the Al content is 0.030%~0.065%, S≤0.0025%, and O≤0.0020%. After vacuum degassing, the Al content is 0.015%~0.050%, S≤0.0015%, and O≤0.0009%. During continuous casting, the Al content in the molten steel is 0.010%~0.045%.

2. The smelting method for high-purity rare earth steel as described in claim 1, characterized in that, KR desulfurization specifically involves immersing the stirring head in the molten iron ladle and rotating it at a speed of 80-120 rpm. Lime-based desulfurizing agent and fluorite are added, and the mixture is stirred and reacted for 8-15 minutes. The sulfur content of the treated molten iron is ≤0.002%.

3. The smelting method for high-purity rare earth steel as described in claim 1, characterized in that, The specific requirements for electric arc furnace / converter smelting are as follows: combined blowing is adopted, the final carbon content of the converter is controlled at 0.10%~0.35%; the final phosphorus content is controlled at 0.009%~0.015%; the tapping temperature is required to be 1600~1635℃; Slag-blocking is used for steel tapping.

4. The smelting method for high-purity rare earth steel as described in claim 1, characterized in that, The LF furnace refining process specifically involves: using a strong deoxidizer for deep deoxidation, maintaining a slag basicity of 4-10, and controlling the (FeO+MnO) content in the slag to be <0.5%. Adjust the argon flow rate according to the fluctuation of the liquid level to promote the floating of inclusions.

5. The smelting method for high-purity rare earth steel as described in claim 1, characterized in that, The vacuum degassing process involves transferring the ladle into a VD furnace or RH furnace for vacuum circulation treatment, maintaining the vacuum level at ≤67Pa for 15-25 minutes.

6. The smelting method for high-purity rare earth steel as described in claim 1, characterized in that, The calcium treatment process specifically involves feeding solid pure calcium wire into the ladle at an argon flow rate of 70-100 NL / min. Soft blowing: Bottom blowing is performed with a low flow rate of argon gas, at a flow rate of 40~60 NL / min, and the holding time is ≥10 minutes.

7. The smelting method for high-purity rare earth steel as described in claim 1, characterized in that, The rare earth treatment specifically involves adding rare earth alloys to the ladle at an argon flow rate of 70-100 NL / min. Soft blowing: Bottom blowing is performed with a low flow rate of argon gas, at a flow rate of 40~60 NL / min, and the holding time is ≥10 minutes.

8. The smelting method for high-purity rare earth steel as described in claim 1, characterized in that, Continuous casting specifically involves full protective casting.

Images