Low-aluminum refining method for high-cleanliness low-alloy high-strength steel

Abstract

This invention belongs to the field of iron and steel metallurgy technology, and more specifically relates to a low-aluminum refining method for high-purity low-alloy high-strength steel. The invention involves adding molten steel smelted in an electric arc furnace to an LF refining furnace, adding lime and fluorite to form a basic slag system, then adding a carbonaceous reducing agent and ferrosilicon powder deoxidizer to form a refining slag. Subsequently, low-aluminum ferrovanadium is added for alloying treatment, followed by vacuum degassing, soft argon blowing, and casting. This yields low-alloy high-strength steel with an Al2O3 mass fraction of <10 wt% in the micro-inclusions, fundamentally solving the industry problem of high ultrasonic flaw detection failure rates caused by large-sized, non-deformable Al2O3 inclusions.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy technology, and more specifically relates to a low-aluminum refining method for high-purity low-alloy high-strength steel. Background Technology

[0002] The cleanliness of low-alloy high-strength steel directly determines the ultrasonic flaw detection pass rate and final service safety of forgings. It requires extremely low levels of residual elements such as silicon and aluminum in the steel, and imposes extremely stringent requirements on the quantity and type of non-metallic inclusions. Among these, high-melting-point, non-deformable Al2O3 (alumina) inclusions pose the greatest threat and are the main factor leading to flaw detection failures and performance degradation.

[0003] In production practice, the LF and VD refining processes for low-alloy high-strength steel require the addition of large amounts of alloys such as ferrovanadium, ferrochrome, and ferromanganese to achieve the target composition. However, conventional alloy materials, especially ferrovanadium, often contain high levels of aluminum (Al) (usually present as residue or deoxidizer, with a content exceeding 3.0%). During refining, some of this aluminum dissolves into the molten steel, becoming a direct source of uncontrollable aluminum content at the final stage and the subsequent formation of Al2O3 inclusions. Existing technologies primarily focus on optimizing the refining slag composition to adsorb already formed Al2O3 inclusions, but neglect the fact that the large amount of aluminum introduced by the alloy material itself (especially ferrovanadium) is also a fundamental source of aluminum and Al2O3 inclusions in the molten steel.

[0004] Reducing the aluminum content in alloy materials (such as using low-alumina ferrovanadium alloys) can decrease the introduction of aluminum into molten steel, thereby reducing Al2O3 inclusions. However, the sources of Al2O3 inclusions in molten steel are diverse, including not only alloy introduction but also refractory material erosion, slag-steel interface reactions, and secondary oxidation. In other words, the source of aluminum is not solely controlled by the alloy.

[0005] Furthermore, the composition and basicity of the refining slag have a decisive influence on the quantity and size of inclusions. Optimizing the slag system (such as increasing the basicity ratio) can significantly reduce the inclusion content, while simply reducing aluminum input cannot effectively improve inclusion removal behavior. Therefore, using only low-aluminum ferrovanadium steel cannot fundamentally solve the Al2O3 inclusion problem in low-alloy high-strength steel.

[0006] It is evident that the total aluminum content and Al2O3 inclusion levels in molten steel fluctuate significantly, becoming a bottleneck restricting the stable improvement of product cleanliness. Therefore, providing a low-aluminum refining method for high-cleanliness low-alloy high-strength steel, achieving strict control of aluminum content from the source of alloy raw materials, has significant industrial value for the stable production of ultra-high-cleanliness low-alloy high-strength steel. Summary of the Invention

[0007] The purpose of this invention is to provide a low-aluminum refining method for high-purity low-alloy high-strength steel. By controlling the aluminum content of key alloy materials and adjusting the composition of refining slag to optimize the basicity of the refining slag system, the alumina inclusions in the steel are significantly reduced, thereby solving the problems existing in the prior art and realizing the low-aluminum refining of high-purity low-alloy high-strength steel.

[0008] To achieve the above objectives, the present invention provides the following solution: One of the technical solutions of this invention is to provide a low-aluminum refining method for high-purity low-alloy high-strength steel, comprising the following steps: Molten steel smelted in an electric arc furnace is added to an LF refining furnace, along with lime and fluorite to form a basic slag system. Then, carbonaceous reducing agent and ferrosilicon powder deoxidizer are added to form a refining slag. Subsequently, low-aluminum ferrovanadium is added for alloying treatment, followed by vacuum degassing, soft argon blowing, and casting to obtain the low-alloy high-strength steel.

[0009] Furthermore, the amount of lime added is 10-20 kg / t.

[0010] Furthermore, the amount of fluorite added is 3-10 kg / t.

[0011] Furthermore, the amount of carbonaceous reducing agent added is 5-25 kg / t.

[0012] Furthermore, the amount of the ferrosilicon powder deoxidizer added is 5-30 kg / t.

[0013] Furthermore, the amount of low-aluminum ferrovanadium added is 1-6 kg / t.

[0014] In this invention, the amount of low-aluminum ferrovanadium added is determined by the V content in the low-alloy high-strength steel. That is, the amount of low-aluminum ferrovanadium added is determined by material balance calculation based on the target vanadium content requirement of the low-alloy high-strength steel and the vanadium content of the low-aluminum ferrovanadium alloy. For example, the amount of low-aluminum ferrovanadium added = molten steel mass × (target vanadium content - vanadium content in molten steel) / (vanadium recovery rate × mass fraction of vanadium in low-aluminum ferrovanadium).

[0015] Furthermore, the mass percentage content of aluminum (Al) in the low-aluminum ferrovanadium is <1.5 wt%, preferably <0.5 wt%.

[0016] Furthermore, the carbonaceous reducing agent includes at least one of activated carbon, carbon powder, and graphite electrode powder (carbon content >90%, particle size less than 1.0 mm, moisture content less than or equal to 0.3%).

[0017] Furthermore, the vacuum degree of the vacuum degassing treatment is 0.5-2 Torr (approximately 67~267 Pa), the time is ≥30 minutes, and the argon stirring flow rate is 80-300 NL / min.

[0018] Furthermore, the soft blowing argon operation takes 15-25 minutes and the argon flow rate is 50-150 NL / min.

[0019] The second technical solution of the present invention provides a low-alloy high-strength steel obtained by the above-mentioned low-aluminum refining method for high-purity low-alloy high-strength steel, wherein the mass fraction of Al2O3 in the micro-inclusions of the low-alloy high-strength steel is <10 wt%.

[0020] The third technical solution of the present invention provides an application of the above-mentioned low-alloy high-strength steel in the preparation of forgings.

[0021] The present invention discloses the following technical effects: This invention establishes the aluminum content of ferrovanadium in the alloying process as a core control parameter (Al < 1.5%, preferably < 0.5%), thereby precisely blocking the input of aluminum impurities into the steel from the source of raw materials. This provides a fundamental guarantee for reducing the total aluminum content at the end of the molten steel and the total amount of Al2O3 inclusions formed subsequently to extremely low levels, solving the long-neglected problem of aluminum introduced by alloy materials as a source of pollution.

[0022] This invention clarifies a precise and reproducible clean refining process by defining a "low-aluminum ferrovanadium" standard and systematically integrating it with the "diffusion deoxidation-slag system regulation-VD vacuum treatment-soft blowing argon" process. This process has a stable process window and clearly defined key parameters, ensuring the large-scale and stable production of high-purity low-alloy high-strength steel with Al2O3 content below 10wt% in micro-inclusions. This fundamentally solves the industry problem of high ultrasonic flaw detection failure rates caused by large-sized, non-deformable Al2O3 inclusions.

[0023] The addition of carbonaceous reducing agents creates a reducing atmosphere during refining. Through the reaction of carbon with oxides such as FeO and MnO in the slag, the oxygen potential in the slag is reduced, increasing metal yield and reducing alloy loss. Simultaneously, the diffusion deoxidation effect of carbon promotes the release of oxygen as CO gas, preventing the formation of solid oxide inclusions and reducing Al2O3 inclusion formation at the source. Lime and ferrosilicon powder deoxidizers act as slag-forming agents, adjusting the CaO and SiO2 content in the slag system to maintain a reasonable slag basicity, forming a low-melting-point, high-fluidity refining slag. This enhances the wettability and solubility of Al2O3 inclusions, promoting their migration and adsorption into the slag, thereby improving inclusion removal efficiency. The addition of fluorite reduces slag viscosity and improves fluidity, facilitating the mass transfer process at the steel-slag interface, making it easier for inclusions to enter the slag phase, further enhancing inclusion removal efficiency.

[0024] The low-alumina ferrovanadium and slag used in this invention are all commercially available products, eliminating the need to develop special raw materials. The entire method is fully compatible with the mainstream LF-VD refining equipment and production rhythm of existing steel mills, without increasing additional equipment investment or process time. This "standard materials combined with optimized processes" approach enables this invention to be rapidly promoted and applied in steel enterprises in a short period of time with extremely low implementation costs, demonstrating significant industrial economic benefits. Attached Figure Description

[0025] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 The diagram shows the Al2O3-SiO2-MnO ternary phase diagram of inclusions in the low-alloy high-strength steel of Example 1.

[0026] Figure 2 The diagram shows the Al2O3-SiO2-MnO ternary phase diagram of inclusions in the low-alloy high-strength steel of Comparative Example 1. Detailed Implementation

[0027] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0028] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0029] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0030] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be readily apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0031] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0032] Unless otherwise specified, all raw materials and reagents involved in the specific embodiments of this invention are commercially available products.

[0033] Unless otherwise specified, room temperature and ambient temperature in the specific embodiments of this invention refer to 20-30℃.

[0034] It should be noted that any aspects not described in detail in this invention are conventional practices in the field and are not the focus of this invention.

[0035] In some specific embodiments, the present invention provides a low-aluminum refining method for high-purity low-alloy high-strength steel, the steps of which include: S1. After the electric furnace smelting is completed, the molten steel is poured into the LF refining furnace; S2 and LF refining initial slag formation: Add lime (10-20 kg / t) and fluorite (3-10 kg / t) to form a basic Al2O3-SiO2-CaO-MgO slag system; Lime is added to adjust the CaO content in the slag, so that the alkalinity of the slag system is within a reasonable range. Fluorite is added to reduce the viscosity of the slag and improve its fluidity, forming a low-melting-point, high-fluidity refining slag, which enhances the adsorption capacity for Al2O3 inclusions, thereby improving the inclusion removal efficiency. S3. Subsequently, carbonaceous reducing agent (addition amount of 5-25 kg / t) and ferrosilicon powder deoxidizer (addition amount of 5-30 kg / t) are added to the molten steel that forms the basic slag system to carry out diffusion deoxidation and adjust the slag composition, thereby forming a refining slag with good adsorption capacity; the carbonaceous reducing agent includes at least one of activated carbon, carbon powder and electrode powder. The main purpose of adding carbonaceous reducing agents is to reduce the oxygen potential of the slag-steel system through diffusion deoxidation. Carbonaceous reducing agents can react with oxidizing components such as FeO and MnO in the refining slag, promoting the release of oxygen as CO gas, thereby reducing the dissolved oxygen content in the molten steel and inhibiting the reaction of residual aluminum with oxygen to form Al2O3 inclusions. Adding ferrosilicon powder as a deoxidizer adjusts the SiO2 content in the refining slag, optimizes the slag composition and basicity, and gives the refining slag suitable fluidity, stability, and inclusion adsorption capacity. S4. Add low-aluminum ferrovanadium to the molten steel after step S3 for alloying treatment; the mass percentage content of aluminum (Al) in the low-aluminum ferrovanadium is <1.5 wt%, and the addition amount is 1-6 kg / t; The main purpose of adding low-aluminum ferrovanadium is to minimize the amount of aluminum input brought in by ferrovanadium during the alloying process, while meeting the vanadium requirements of the low-alloy high-strength steel composition design, and to reduce the formation of Al2O3 inclusions by aluminum combining with oxygen in the molten steel. S5. After the alloying treatment is completed, vacuum degassing is performed under argon stirring (flow rate of 80-300 NL / min). The vacuum degree of vacuum degassing is 0.5-2 Torr (approximately 67~267 Pa), and the time is ≥30 minutes. Then, soft argon blowing is performed at an argon flow rate of 50-150 NL / min for 15-25 minutes. S6. After the soft argon blowing operation is completed, the steel is transported to the casting station for casting to obtain high-purity low-alloy high-strength steel.

[0036] The mass fraction of Al2O3 in the micro-inclusions of the high-purity low-alloy high-strength steel obtained by the above-mentioned low-aluminum refining method for high-purity low-alloy high-strength steel is <10 wt%.

[0037] This invention systematically selects low-aluminum ferrovanadium as alloy raw material, fundamentally reducing the input of external aluminum. Combined with diffusion deoxidation, vacuum degassing and soft argon blowing operations in the refining process, it achieves efficient control of Al2O3 inclusions in steel, thereby stably obtaining ultra-high purity low-alloy high-strength molten steel.

[0038] In this invention, the high purity of high-purity low-alloy high-strength steel refers to the mass fraction of Al2O3 in the microscopic inclusions of low-alloy high-strength steel being <10 wt%.

[0039] In this embodiment of the invention, lime, fluorite, and ferrosilicon powder deoxidizer are all conventional metallurgical raw materials that can be purchased from existing industrial raw material suppliers. The lime used has CaO ≥ 90.0%, MgO ≤ 2.5%, SiO2 ≤ 2.5%, S ≤ 0.030%, and P ≤ 0.02%; the fluorite has CaF2 ≥ 90.0%, SiO2 ≤ 9.0%-14.0%, S ≤ 0.10%-0.15%, and P ≤ 0.06%; and the ferrosilicon powder deoxidizer has SiO2 (≥ 95.0%, Al2O3 ≤ 1.5%, Fe2O3 ≤ 0.5%, CaO ≤ 0.8%, MgO ≤ 0.3%, S ≤ 0.02%, and P ≤ 0.03%).

[0040] The main components of the low-aluminum ferrovanadium used in the specific embodiments of the present invention are: V≥50.0%, C:0.40%max, Si:2.0%max, P:0.060%max, S:0.040%max, Al:1.50%max, and particle size:10-50mm.

[0041] Example 1 The low-aluminum refining steps for high-purity low-alloy high-strength steel include: The amount of molten steel is approximately 43 tons.

[0042] S1. After the electric furnace smelting is completed, the molten steel is poured into the LF refining station; S2 and LF refining initial slag formation: Add lime (18 kg / t) and fluorite (8.0 kg / t) to form the basic slag system; S3. Subsequently, carbonaceous reducing agent (activated carbon, 6.2 kg / t) and ferrosilicon powder deoxidizer (8.2 kg / t) are added to the molten steel that forms the basic slag system to carry out diffusion deoxidation and adjust the slag composition, thereby forming a refining slag with good adsorption capacity. S4. Low-aluminum ferrovanadium is added to the molten steel after step S3 for alloying treatment; the amount of low-aluminum ferrovanadium added is 3.5 kg / t. S5. After alloying, transfer to VD station and perform vacuum degassing under argon stirring (flow rate of 100 NL / min). The vacuum degree of vacuum degassing is between 0.5-2 Torr, and the time is 50 minutes. Then return to LF refining station and perform soft argon blowing operation at argon flow rate of 150 NL / min for 20 minutes. S6. After the soft argon blowing operation is completed, the steel is transported to the casting station for casting at a casting temperature of 1630 ℃, resulting in high-purity low-alloy high-strength steel.

[0043] Comparative Example 1 The amount of molten steel was approximately 96 tons. After LF refining and slag formation, the alloying process used conventional ferrovanadium (with an aluminum content of approximately 3.6%) as the source of vanadium addition. The rest of the steps were the same as in Example 1, as follows: S1 and LF refining initial slag formation: Lime (12 kg / t) and fluorite (4.2 kg / t) are added to form the basic slag system; S3. Subsequently, carbonaceous reducing agent (activated carbon, carbon powder or electrode powder, added at a rate of 14.3 kg / t) and ferrosilicon powder deoxidizer (added at a rate of 8.9 kg / t) are added to the molten steel that forms the basic slag system to carry out diffusion deoxidation and adjust the slag composition, thereby forming a refining slag with good adsorption capacity. S4. Low-aluminum ferrovanadium is added to the molten steel after step S3 for alloying treatment; the amount of low-aluminum ferrovanadium added is 2.6 kg / t. S5. After alloying, the sample is transferred to the VD station and vacuum degassing is performed under argon stirring (flow rate of 50 NL / min). The vacuum degree of the vacuum degassing is between 0.5-2 Torr, and the time is 40 minutes. Then it is returned to the LF refining station and soft blowing argon is performed at an argon flow rate of 100 NL / min for 25 minutes.

[0044] Test case Microscopic inclusions were extracted and analyzed from the low-alloy high-strength steels obtained after refining in the examples and comparative examples. The results are shown in Table 1.

[0045] Table 1 As can be seen from the data in Table 1, the Al2O3 content in the micro-inclusions of the low-alloy high-strength steel prepared in the embodiments of the present invention is about 7.30%, which is much lower than 10%. The main components of the inclusions are SiO2 and MnO, which are typical low-melting-point plastic inclusions. In Comparative Example 1, Al2O3 > 50%, and the contents of SiO2 and MnO are relatively low.

[0046] Plot the components in Table 1 on the Al2O3-SiO2-MnO ternary phase diagram, as follows: Figures 1-2 As shown.

[0047] Figure 1 The figure shows the Al2O3-SiO2-MnO ternary phase diagram of inclusions in the low-alloy high-strength steel of Example 1. As can be seen from the figure, the inclusions are significantly located away from the Al2O3 angle, indicating a substantial reduction in Al2O3 content within the steel.

[0048] Figure 2 The figure shows the Al2O3-SiO2-MnO ternary phase diagram of inclusions in the low-alloy high-strength steel of Comparative Example 1. As can be seen from the figure, the inclusions are located near the Al2O3 corner region, indicating that the use of conventional ferrovanadium leads to the formation of a large number of harmful alumina inclusions in the steel, severely affecting its cleanliness.

[0049] Figure 1 and Figure 2 The comparison shows that, under similar steel grades and basic process flows, simply by replacing the "conventional ferrovanadium" in the alloying process with the "low-aluminum ferrovanadium" (Al<0.5%) required by this invention and through process synergy, the composition of inclusions in the final steel can be fundamentally transformed: from harmful inclusions dominated by high Al2O3 (… Figure 2 ), transforming into inclusions mainly composed of SiO2-MnO ( Figure 1This transformation directly and powerfully demonstrates that the core method of this invention—controlling the aluminum content of the alloy at the source—has a remarkable technical effect on reducing alumina inclusions in steel, providing a stable and reliable solution for achieving high cleanliness in low-alloy high-strength steel.

[0050] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0051] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for refining high-purity low-alloy high-strength steel with low aluminum content, characterized in that the steps include: include: Molten steel smelted in an electric arc furnace is added to an LF refining furnace, and lime and fluorite are added to form a basic slag system. Then, carbonaceous reducing agent and ferrosilicon powder deoxidizer are added to form a refining slag. Subsequently, low-aluminum ferrovanadium is added for alloying treatment, followed by vacuum degassing, soft argon blowing, and casting to obtain the low-alloy high-strength steel. The aluminum content in the low-aluminum ferrovanadium is <1.5 wt%.

2. The low-aluminum refining method as described in claim 1, characterized in that, The amount of lime added is 10-20 kg / t.

3. The low-aluminum refining method as described in claim 1, characterized in that, The amount of fluorite added is 3-10 kg / t.

4. The low-aluminum refining method as described in claim 1, characterized in that, The amount of carbonaceous reducing agent added is 5-25 kg / t.

5. The low-aluminum refining method as described in claim 1, characterized in that, The amount of ferrosilicon powder deoxidizer added is 5-30 kg / t.

6. The low-aluminum refining method as described in claim 1, characterized in that, The amount of low-aluminum ferrovanadium added is 1-6 kg / t.

7. The low-aluminum refining method as described in claim 1, characterized in that, The carbonaceous reducing agent includes at least one of activated carbon, carbon powder, and graphite electrode powder.

8. The low-aluminum refining method as described in claim 1, characterized in that, The vacuum degree of the vacuum degassing process is 0.5-2 Torr, the time is ≥30 minutes, and the argon stirring flow rate is 80-300 NL / min; And / or, the soft blowing argon operation time is 15-25 min, and the argon flow rate is 50-150 NL / min.

9. A low-alloy high-strength steel obtained by the low-aluminum refining method of the high-purity low-alloy high-strength steel according to any one of claims 1-8, characterized in that, The mass fraction of Al2O3 in the micro-inclusions of the low-alloy high-strength steel is <10 wt%.

10. The application of the low-alloy high-strength steel according to claim 9 in the preparation of forgings.

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