High-oxygen high-sulfur free-cutting steel 1215MS and oxygen control LF smelting method thereof

Abstract

This invention provides a high-oxygen, high-sulfur free-machining steel 1215MS and its oxygen-controlled LF smelting method, relating to the field of metal smelting; the method includes: 1) obtaining high-sulfur molten steel by converter smelting; 2) tapping the steel from the converter using a weak deoxidation process, including: pouring the molten steel into a ladle, sequentially adding ferroaluminum alloy, low-carbon ferromanganese, and ferrophosphorus during the tapping process, and adding synthetic slag to the slag surface of the ladle after tapping to obtain molten steel to be LF refined; 3) subjecting the molten steel to LF refined under controlled oxygen to obtain refined molten steel; wherein, the chemical composition of the refined molten steel... The composition is as follows: C: 0.05-0.08%, Mn: 1.20-1.45%, S: 0.30-0.50%, Si≤0.03%, P: 0.04-0.06%, O: 0.004-0.006%, Alt≤0.005%, Ti≤0.002%, with the remainder being Fe and unavoidable impurities; 4) Continuous casting to obtain high-oxygen, high-sulfur free-machining steel 1215MS; This invention achieves the technical effect of precisely controlling the oxide inclusion morphology in molten steel and improving the machinability of the product by precisely controlling the oxygen content of molten steel.

Description

Technical Field

[0001] This invention relates to the field of metal smelting technology, specifically to a high-oxygen, high-sulfur free-machining steel 1215MS and its oxygen-controlled LF smelting method. Background Technology

[0002] Sulfur-containing free-cutting steel requires precise control of oxygen content during LF smelting, with a target oxygen value of 45-60 Ppm for LF tapped steel. For this type of high-oxygen steel, unstable oxygen control during smelting, resulting in fluctuating oxygen content, leads to two main problems: firstly, it prolongs the LF smelting cycle, causing severe stopper rod erosion and hindering steel casting; secondly, when the oxygen content in the molten steel is too low, a large number of high-melting-point inclusions appear, resulting in inclusion morphology during continuous casting that does not meet the requirements for sulfur-containing free-cutting steel; and thirdly, when the oxygen content in the molten steel is too high, severe subsurface blistering occurs in the cast billet, affecting the surface quality of the product and reducing its machinability. Summary of the Invention

[0003] The purpose of this invention is to provide a high-oxygen, high-sulfur free-cutting steel 1215MS and its oxygen-controlled LF smelting method, which solves the problem of large fluctuations in oxygen content control during the smelting of sulfur-containing free-cutting steel, achieves precise control of oxygen content in molten steel, obtains good manganese sulfide inclusion morphology in the billet, and improves the machinability of the product.

[0004] To achieve the above objectives, the present invention proposes the following technical solution:

[0005] In the first aspect, a high-oxygen, high-sulfur free-machining steel 1215MS is proposed, comprising the following chemical composition by mass percentage: C: 0.05-0.08%, Mn: 1.20-1.45%, S: 0.30-0.50%, Si≤0.03%, P: 0.04-0.06%, O: 0.004-0.006%, Alt≤0.005%, Ti≤0.002%, with the remainder being Fe and unavoidable impurities.

[0006] Secondly, a controlled-oxygen LF smelting method for high-oxygen, high-sulfur free-machining steel 1215MS is proposed, including the following steps:

[0007] 1) High-sulfur steel is obtained through converter smelting;

[0008] 2) The weak deoxidation process is used for converter tapping, including: pouring molten steel into a ladle, adding aluminum-iron alloy, low-carbon ferromanganese and ferrophosphorus in sequence during the tapping process, and adding synthetic slag to the slag surface of the ladle after tapping to obtain molten steel to be refined by LF.

[0009] 3) The LF-refined molten steel is subjected to controlled oxygen refining to obtain refined molten steel; wherein the chemical composition of the refined molten steel is: C: 0.05-0.08%, Mn: 1.20-1.45%, S: 0.30-0.50%, Si≤0.03%, P: 0.04-0.06%, O: 0.004-0.006%, Alt≤0.005%, Ti≤0.002%, with the remainder being Fe and unavoidable impurities;

[0010] 4) Continuous casting to obtain high-oxygen, high-sulfur free-machining steel 1215MS.

[0011] Furthermore, the specific process of using a weak deoxidation process for converter tapping in step 2) is as follows:

[0012] When 10% of the steel is tapped from the converter, an aluminum-iron alloy is added; the amount of the aluminum-iron alloy added is M. Al The calculation is as follows:

[0013] M Al = 9 / 8 × [ M 钢 ×( w [O] -0.015%]×η / w [Al] (1),

[0014] when w [O] When ≤0.055%, η=1.1-1.3; when w [O] When >0.055%, η = 1.35 - 1.55;

[0015] Among them, M Al : Indicates the amount of aluminum-iron alloy added, in kg; w [Al] : Indicates the mass percentage of aluminum in an aluminum-iron alloy, %; M 钢 : Indicates the weight of molten steel, in kg; w [O] : Represents the mass fraction of oxygen in molten steel, %; η: Correlation coefficient between converter slag charge and slag oxidizability;

[0016] After adding aluminum-iron alloy to the steel produced from the converter, low-carbon ferromanganese and ferrophosphorus are added until 70% of the alloy is completely added to the steel produced from the converter; wherein the amount of low-carbon ferromanganese added is 13-17 kg / t, and the amount of ferrophosphorus added is 0.15-0.30 kg / t.

[0017] After tapping, 10-15 kg / t of synthetic slag is added to the slag surface of the ladle. The composition of the synthetic slag, by mass percentage, includes CaO: 65-70%, SiO2: 5-10%, Al2O3: 10-15%, CaF2: 3-5%, MgO: 4-8%, with the remainder being unavoidable impurities.

[0018] Furthermore, during the process of adding aluminum-iron alloy to ferrophosphorus during the tapping of the converter, the bottom blowing flow rate of the ladle is 500-800 NL / min; during the process of adding the synthetic slag after tapping, the bottom blowing flow rate of the ladle is 200-300 NL / min.

[0019] Furthermore, the specific process of step 3) of oxygen-controlled refining of the LF-refined steel is as follows:

[0020] The oxygen content of the molten steel entering the LF refining station should be controlled at 0.008-0.015%;

[0021] First, carbon powder, low-carbon ferromanganese, and ferrophosphorus are added during the LF refining process to adjust the composition of the molten steel, and then electricity is applied to raise the temperature and control the temperature of the molten steel.

[0022] Subsequently, calcium carbide and silicon carbide are added during the LF refining process to deoxidize and slag the ladle slag surface; wherein the amount of calcium carbide added is 0.2-0.4 kg / t, and the amount of silicon carbide added is 0.5-1.0 kg / t; wherein the silicon carbide, by mass percentage, includes SiC: 85-95%, Si: 5-10%, C≤5%, and the remainder being unavoidable impurities; the composition of the refined slag obtained by deoxidation and slag formation, by mass percentage, includes: CaO: 45-55%, SiO2: 20-30%, Al2O3: 20-30%, CaF2: 1-3%, MgO: 3-6%, T.Fe+MnO: 2-4%;

[0023] The molten steel is fed into a sulfur wire at a feeding speed of 5-8 m / s, and the sulfur content and oxygen content in the molten steel are controlled at 0.30-0.50% and 0.004-0.006%, respectively.

[0024] Finally, after soft stirring for 10-15 minutes, continuous casting is carried out.

[0025] Furthermore, the outer layer of the sulfur wire is pure iron sheet with an outer diameter of 12-15 mm and a thickness of 3-5 mm; wherein, the composition of the iron sheet, by mass percentage, includes Alt ≤ 0.003%, Si: 0.2-0.4%, Mn: 0.3-0.5%, with the remainder being Fe and unavoidable impurities;

[0026] The sulfur line has an inner diameter of 8-10 mm and is filled with iron sulfide powder. The iron sulfide powder contains 60-80% sulfur by mass, with the remainder being iron sulfide and unavoidable impurities.

[0027] Furthermore, during the LF refining process, when carbon powder, low-carbon ferromanganese, and ferrophosphorus are added, the bottom blowing flow rate of the ladle is 300-500 NL / min; when the refined molten steel is fed into the sulfur line, the bottom blowing flow rate of the ladle is 200-300 NL / min, and the bottom blowing flow rate is reduced to 100-150 NL / min 3-5 minutes after the refined molten steel is fed into the sulfur line; during the soft stirring, the bottom blowing flow rate of the ladle is 30-80 NL / min.

[0028] Furthermore, the composition of the aluminum-iron alloy, by mass percentage, includes Al: 55-65%, with the remainder being Fe and unavoidable impurities;

[0029] The composition of the low-carbon ferromanganese, by mass percentage, includes Mn: 80-85%, C ≤ 0.8%, with the remainder being Fe and unavoidable impurities;

[0030] The composition of the phosphorus iron, by mass percentage, includes P: 30-40%, with the remainder being Fe and unavoidable impurities.

[0031] Furthermore, the process of obtaining high-sulfur steel through converter smelting in step 1) is as follows:

[0032] Undesulfurized molten iron and high-sulfur scrap steel are mixed into a converter for smelting. The proportion of high-sulfur scrap steel is 10-20%. After the converter smelting is completed, high-sulfur steel is obtained. The temperature of the undesulfurized molten iron is not lower than 1320℃. Its composition, by mass percentage, includes C: 4.2-4.5%, Si: 0.15-0.35%, P≤0.12%, S≥0.03%, with the remainder being Fe and unavoidable impurities. The composition of the high-sulfur scrap steel, by mass percentage, includes S: 0.02-0.15%, with the remainder being Fe and conventional C, Al, Si, Mn and unavoidable impurities.

[0033] Furthermore, the converter tapping process employs a sliding plate slag barrier, with a slag discharge rate ≤2.5 kg / t;

[0034] The final slag basicity of the converter is CaO / SiO2 = 2.0-3.0, and the mass percentage of T.Fe is 13-16%.

[0035] The tapping temperature of the converter is 1620-1660℃, and the content of O, C, P and S elements in the molten steel is as follows by mass percentage: O: 0.035-0.075%, C: 0.03-0.06%, Mn: 0.04-0.12%, P: 0.02-0.05%, S: 0.02-0.055%.

[0036] As can be seen from the above technical solutions, the technical solutions of the present invention have achieved the following beneficial effects:

[0037] This invention discloses a high-oxygen, high-sulfur free-machining steel 1215MS and its oxygen-controlled LF smelting method. First, high-sulfur molten steel is obtained through converter smelting. Second, a weak deoxidation process is used for converter tapping to reduce the Al content in the molten steel, ensuring the oxygen content and reducing the need for steel and slag desulfurization. Subsequently, LF refining is performed, while precisely controlling the O and S content in the molten steel. Finally, the molten steel with precisely controlled O and S content is continuously cast. This invention solves the problems of excessively low oxygen content leading to a large number of high-melting-point inclusions in the molten steel and inclusion morphology during continuous casting that does not meet the requirements for sulfur-containing free-machining steel. It also avoids the problem of severe subsurface bubbles in the cast billet caused by excessively high oxygen content. This results in a good manganese sulfide inclusion morphology in the cast billet, improving the product's machinability.

[0038] Specifically, the advantages of the method described above in this invention are as follows:

[0039] (1) By designing a whole-process sulfur control technology, the sulfur and phosphorus content of the molten steel from the converter was increased, the use of sulfur wire or iron-sulfur and iron-phosphorus alloys was reduced, and the production cost was lowered.

[0040] (2) A precise weak deoxidation process for converter tapping and a step-by-step deoxidation and alloying process for LF refining were creatively developed. Combined with the adoption of a weak oxidizing calcium aluminosilicate ternary slag-making process and sulfur line property control, the O content, S content and P content of molten steel were simultaneously and precisely controlled, effectively shortening the LF smelting cycle, improving the stopper rod erosion problem and the problem of poor casting, and obtaining high-quality free-cutting steel billets.

[0041] It should be understood that all combinations of the foregoing concepts and the additional concepts described in more detail below can be considered part of the inventive subject matter of this disclosure, provided that such concepts do not contradict each other.

[0042] The foregoing and other aspects, embodiments, and features of the teachings of the present invention will be more fully understood from the following description in conjunction with the embodiments. Other additional aspects of the invention, such as features and / or beneficial effects of exemplary embodiments, will become apparent from the following description or may be learned through practice of specific embodiments according to the teachings of the present invention. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the described embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Unless otherwise defined, the technical or scientific terms used herein should have the ordinary meaning understood by those skilled in the art to which this invention pertains.

[0044] The terms "first," "second," and similar words used in the specification and claims of this patent application do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Similarly, unless the context clearly indicates otherwise, the singular forms of "an," "a," or "the," etc., do not indicate a quantity limitation, but rather indicate the presence of at least one. Terms such as "comprising" or "including" mean that the element or object preceding "comprising" encompasses the features, integrals, steps, operations, elements, and / or components listed following "comprising" or "including," and do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components, and / or collections thereof. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; these relative positional relationships may change accordingly when the absolute position of the described object changes.

[0045] The current practice of LF smelting for sulfur-containing cutting steel suffers from the inability to precisely control oxygen content, resulting in significant fluctuations in oxygen content during smelting. This leads to inconsistent control over the morphology of oxide inclusions in the molten steel, resulting in suboptimal performance of the produced sulfur-containing cutting steel. This invention aims to address the problem of unstable oxygen content control during LF smelting of sulfur-containing cutting steel by proposing a high-oxygen, high-sulfur free-machining steel, 1215MS, and its oxygen-controlled LF smelting method. The method employs a weak deoxidation process in converter tapping combined with stepwise deoxidation and alloying in LF refining to achieve simultaneous and precise control of O, S, and P content in the molten steel, yielding high-quality free-machining steel billets.

[0046] The oxygen-controlled LF smelting method for high-oxygen, high-sulfur free-machining steel 1215MS includes the following steps:

[0047] 1) High-sulfur steel is obtained through converter smelting;

[0048] Specifically, undesulfurized molten iron and high-sulfur scrap steel are mixed into a converter for smelting, with the proportion of high-sulfur scrap steel being 10-20%. After converter smelting, high-sulfur steel is obtained. The temperature of the undesulfurized molten iron is not lower than 1320℃, and its composition, by mass percentage, includes C: 4.2-4.5%, Si: 0.15-0.35%, P≤0.12%, S≥0.03%, with the remainder being Fe and unavoidable impurities. The composition of the high-sulfur scrap steel, by mass percentage, includes S: 0.02-0.15%, with the remainder being Fe and conventional C, Al, Si, Mn, and unavoidable impurities.

[0049] The method disclosed in this scheme for stabilizing and controlling the O content during the smelting of high-sulfur free-cutting steel firstly selects high-sulfur molten iron and high-sulfur scrap steel for smelting, thereby increasing the sulfur content in the molten steel, reducing the total amount of converter slag, and reducing the desulfurization and dephosphorization of the converter final slag. This achieves the effect of comprehensively reducing the sulfur and phosphorus capacity of the converter slag and increasing the S and P content in the molten steel.

[0050] 2) The weak deoxidation process is used for converter tapping, including: pouring molten steel into a ladle, adding aluminum-iron alloy, low-carbon ferromanganese and ferrophosphorus in sequence during the tapping process, and adding synthetic slag to the slag surface of the ladle after tapping to obtain molten steel to be refined by LF.

[0051] Specifically, the tapping temperature of the converter is 1620-1660℃, and the content of O, C, P and S elements in the molten steel, by mass percentage, is: O: 0.035-0.075%, C: 0.03-0.06%, Mn: 0.04-0.12%, P: 0.02-0.05%, S: 0.02-0.055%; the tapping process of the converter first adopts a sliding plate to block slag, and the slag discharge is ≤2.5kg / t;

[0052] Then, when 10% of the steel has been tapped from the converter, an aluminum-iron alloy is added; the amount of the aluminum-iron alloy added is M. Al The calculation is as follows:

[0053] M Al = 9 / 8 × [ M 钢 ×( w [O] -0.015%]×η / w [Al] (1),

[0054] when w [O] When ≤0.055%, η=1.1-1.3; when w [O] When >0.055%, η = 1.35 - 1.55;

[0055] Among them, M Al: Indicates the amount of aluminum-iron alloy added, in kg; w [Al] : Indicates the mass percentage of aluminum in an aluminum-iron alloy, %; M 钢 : Indicates the weight of molten steel, in kg; w [O] : represents the mass fraction of oxygen in molten steel, %; η: the correlation coefficient between the amount of slag added to the converter and the oxidizability of the slag; and the composition of the aluminum-iron alloy, by mass percentage, includes Al: 55-65%, with the remainder being Fe and unavoidable impurities;

[0056] After adding aluminum-iron alloy to the steel produced from the converter, low-carbon ferromanganese and ferrophosphorus are added until 70% of the alloy is completely added to the steel produced from the converter. The low-carbon ferromanganese is added at a rate of 13-17 kg / t, and its composition, by mass percentage, includes 80-85% Mn, ≤0.8% C, with the remainder being Fe and unavoidable impurities. The ferrophosphorus is added at a rate of 0.15-0.30 kg / t, and its composition, by mass percentage, includes 30-40% P, with the remainder being Fe and unavoidable impurities.

[0057] After tapping, 10-15 kg / t of synthetic slag is added to the slag surface of the ladle; the composition of the synthetic slag, by mass percentage, includes CaO: 65-70%, SiO2: 5-10%, Al2O3: 10-15%, CaF2: 3-5%, MgO: 4-8%, and the remainder being unavoidable impurities.

[0058] During the process of adding aluminum-iron alloy to ferrophosphorus during the tapping of the converter, the bottom blowing flow rate of the ladle is 500-800 NL / min; after the tapping is completed and the synthetic slag is added, the bottom blowing flow rate of the ladle is 200-300 NL / min.

[0059] The converter has a final slag basicity of CaO / SiO2 of 2.0-3.0 and a T.Fe mass percentage of 13-16%.

[0060] The above process, through precise control of aluminum-iron alloy deoxidation, can ensure the O content in molten steel with high sulfur content and reduce the desulfurization effect of the alloying process. Experiments show that using aluminum-iron alloy can control the O content in molten steel to about 80-150 ppm, and reduce oxygen by about 100 ppm per 27 kg of aluminum-iron alloy (due to the 60% Al content in aluminum-iron alloy). At the same time, by increasing the bottom blowing flow rate during deoxidation and alloying, the uniformity of alloy composition is accelerated; while by reducing the bottom blowing flow rate during slag addition, the slag-metal reaction is weakened, reducing slag desulfurization.

[0061] 3) The LF-refined steel is subjected to controlled oxygen refining to obtain refined steel; wherein the chemical composition of the refined steel is: C: 0.05-0.08%, Mn: 1.20-1.45%, S: 0.30-0.50%, Si≤0.03%, P: 0.04-0.06%, O: 0.004-0.006%, Alt≤0.005%, Ti≤0.002%, with the remainder being Fe and unavoidable impurities;

[0062] The specific process is as follows: control the oxygen content of the molten steel entering the LF refining station to be 0.008-0.015%;

[0063] First, carbon powder, low-carbon ferromanganese, and ferrophosphorus are added during the LF refining process to adjust the composition of the molten steel, and the temperature of the molten steel is controlled by heating with electricity; and when adding carbon powder, low-carbon ferromanganese, and ferrophosphorus, the bottom blowing flow rate of the ladle is controlled to be 300-500 NL / min.

[0064] Subsequently, calcium carbide and silicon carbide are added during the LF refining process to deoxidize and slag the ladle slag surface; wherein the amount of calcium carbide added is 0.2-0.4 kg / t, and the amount of silicon carbide added is 0.5-1.0 kg / t; wherein the silicon carbide, by mass percentage, includes SiC: 85-95%, Si: 5-10%, C≤5%, and the remainder being unavoidable impurities; the composition of the refined slag obtained by deoxidation and slag formation, by mass percentage, includes: CaO: 45-55%, SiO2: 20-30%, Al2O3: 20-30%, CaF2: 1-3%, MgO: 3-6%, T.Fe+MnO: 2-4%;

[0065] The molten steel is fed into a sulfur wire at a feeding speed of 5-8 m / s. The sulfur content in the molten steel is controlled at 0.30-0.50%, and the oxygen content is controlled at 0.004-0.006%. The bottom blowing flow rate of the ladle is 200-300 NL / min, and the bottom blowing flow rate is reduced to 100-150 NL / min 3-5 minutes after the sulfur wire feeding is completed. The outer layer of the sulfur wire is pure iron sheet with an outer diameter of 12-15 mm and a thickness of 3-5 mm. The composition of the iron sheet, by mass percentage, includes Alt ≤ 0.003%, Si: 0.2-0.4%, Mn: 0.3-0.5%, with the remainder being Fe and unavoidable impurities. The inner diameter of the sulfur wire is 8-10 mm, and the interior is filled with ferrous sulfate powder. The ferrous sulfate powder contains 60-80% sulfur by mass, with the remainder being Fe and unavoidable impurities.

[0066] Finally, after soft stirring for 10-15 minutes, continuous casting is carried out. During soft stirring, the bottom blowing flow rate of the ladle is controlled at 30-80 NL / min.

[0067] The above process involves first adding carbon powder, low-carbon ferromanganese, and ferrophosphorus alloys to the molten steel to be refined (LF) to adjust the Mn and P composition. Due to the low Si content in the alloy composition and the weak deoxidizing ability of the Mn alloy, the oxygen content of the molten steel can be guaranteed to meet the requirements. Then, calcium carbide and silicon carbide are used for deoxidation to appropriately reduce the oxygen content of the molten steel and slag, and the slag is controlled to be a calcium aluminosilicate ternary slag system, containing a small amount of T.Fe+MnO, thus weakening the slag's desulfurization ability. Simultaneously, the weakly oxidizing calcium aluminosilicate ternary slag system can reduce the need for sulfur feeding. The process utilizes slag desulfurization during wire feeding; thick iron sheet sulfur wire is used to enhance protection; and a high-speed feeding process is employed to feed the sulfur wire to the bottom of the molten steel, increasing the sulfur element recovery rate and preventing it from being fed to the top of the molten steel, where it would churn and vaporize, thus reducing steel oxidation. This allows for precise control of the O, S, and P content in the molten steel simultaneously. It effectively addresses the problem of low O content in the molten steel leading to precipitates and inclusions that do not meet the requirements for sulfur-containing free-cutting steel, and the problem of high O content in the molten steel causing severe subsurface bubbles in the billet, while also shortening the LF refining cycle.

[0068] 4) Continuous casting to obtain high-oxygen, high-sulfur free-machining steel 1215MS.

[0069] Through the above process, precise control of the oxygen content in molten steel is achieved. When the oxygen content in the steel is sufficiently high, sulfides and oxides combine to form large manganese sulfide inclusions in a complex form during the early stages of solidification, thus improving the machinability of the steel. Conversely, when the al content in the steel increases and the oxygen content decreases, the solubility of sulfur in the molten steel increases, causing sulfides to precipitate in a eutectic form only towards the end of the solidification process, resulting in smaller sulfides that are detrimental to improving the machinability of the steel. Therefore, this invention can produce high-oxygen, high-sulfur free-machining steel. Steel 1215MS, a high-oxygen, high-sulfur free-machining steel, comprises the following chemical composition by mass percentage: C: 0.05-0.08%, Mn: 1.20-1.45%, S: 0.30-0.50%, Si≤0.03%, P: 0.04-0.06%, Alt≤0.005%, Ti≤0.002%, with the remainder being Fe and unavoidable impurities. This high-oxygen, high-sulfur free-machining steel 1215MS exhibits well-formed manganese sulfide inclusions, resulting in excellent machinability.

[0070] The oxygen-controlled LF smelting method for high-oxygen, high-sulfur free-machining steel 1215MS disclosed in this invention will be further described below with reference to specific embodiments.

[0071] Example

[0072] 1) Converter smelting

[0073] The smelting process involves mixing undesulfurized molten iron and high-sulfur scrap steel in a 120t converter. The undesulfurized molten iron contains only C, Si, P, and S, with the remainder being Fe and unavoidable impurities. The high-sulfur scrap steel contains only S, with the remainder being Fe, conventional C, Al, Si, Mn, and unavoidable impurities.

[0074] Table 1 Information on Undesulfurized Molten Iron and Scrap Steel

[0075]

[0076] High-sulfur steel is obtained through converter smelting. The tapping temperature of the converter and the chemical composition of the molten steel and slag are controlled, as shown in Table 2 below:

[0077] Table 2. Finished Steel and Slag Conditions at Converter End

[0078]

[0079] 2) Deoxidation, alloying, and slag formation at the converter tapping point

[0080] The furnace output is 118-122 tons. A weak deoxidation process is used during tapping. When 10% of the converter steel is tapped, aluminum-iron alloy, low-carbon ferromanganese, ferrophosphorus, and synthetic slag are added sequentially. The alloy is added when 70% of the steel is tapped. The aluminum-iron alloy contains 55-65% Al, with the remainder being Fe and unavoidable impurities. The bottom blowing intensity of the ladle is controlled during tapping. From the addition of the aluminum-iron alloy to the addition of ferrophosphorus, the ladle bottom blowing flow rate is 700 NL / min. At the end of tapping, when the synthetic slag is added, the ladle bottom blowing flow rate is 240 NL / min. (See Table 3 below.)

[0081] Table 3. Addition of various alloys and slag materials during converter tapping process

[0082]

[0083] 3) LF refining process

[0084] Oxygen-controlled refining is performed on LF refined steel to obtain refined steel.

[0085] The oxygen content of the molten steel is measured upon entering the LF refining station. Subsequently, a step-by-step deoxidation and alloying process is carried out during the LF refining process. First, carbon powder, low-carbon ferromanganese, and ferrophosphorus are added to adjust the composition of the molten steel, and the temperature is controlled by electric current. Then, calcium carbide and silicon carbide are added to the ladle slag surface for deoxidation and slag formation. The main operations are shown in Table 4. Combined with the use of a weakly oxidizing calcium aluminosilicate ternary slag formation process and sulfur line property control, the O, S, and P contents in the molten steel are accurately determined. After the chemical composition of the molten steel meets the standards, soft stirring treatment is performed. During the LF refining process, the slag composition, molten steel temperature, and soft stirring are adjusted, and the ladle bottom blowing flow rate is adjusted according to different operations. The bottom blowing adjustment is shown in Table 5. The chemical composition of the slag after LF refining is shown in Table 6.

[0086] Table 4 Main Operating Parameters of LF Refining

[0087]

[0088] Table 5 Bottom blowing conditions for each operation in LF refining

[0089]

[0090] Table 6. Composition of LF refining end slag (%)

[0091]

[0092] The chemical composition of the molten steel after LF refining is shown in Table 7.

[0093] Table 7 Chemical composition of molten steel at the end of LF refining

[0094]

[0095] 4) Continuous casting to obtain 1215MS billet of high oxygen and high sulfur free machinable steel.

[0096] The high-oxygen, high-sulfur free-machining steel 1215MS billets obtained in Examples 1-3 above were rolled into wire rods according to existing technology, and their cutting performance was tested. As shown in Table 8 below, it is clear that the cutting performance of the high-oxygen, high-sulfur free-machining steel 1215MS obtained by the method of the present invention is better than that of sulfur-containing cutting steel obtained by existing technology.

[0097] Table 8. Cutting performance parameters of products in Examples 1-3

[0098]

[0099] In addition, longitudinal electron microscopy analysis of the wire rod obtained by rolling the high-oxygen and high-sulfur free-machining steel 1215MS billet prepared in Example 1 of the present invention revealed that the proportion of sulfide inclusions with an aspect ratio ≥4 reached more than 80%, which is very beneficial to the machinability of the product and meets the product quality requirements.

[0100] While the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the invention. Those skilled in the art can make various modifications and refinements without departing from the spirit and scope of the invention. Therefore, the scope of protection of the present invention shall be determined by the claims.

Claims

1. A method for smelting high-oxygen, high-sulfur free-machining steel 1215MS with controlled oxygen LF, characterized in that, Includes the following steps: Step 1) The process of obtaining high-sulfur steel through converter smelting is as follows: Undesulfurized molten iron and high-sulfur scrap steel are mixed into a converter for smelting. The proportion of high-sulfur scrap steel is 10-20%. After converter smelting, high-sulfur steel is obtained. The temperature of the undesulfurized molten iron is not lower than 1320℃, and its composition, by mass percentage, includes C: 4.2-4.5%, Si: 0.15-0.35%, P≤0.12%, S≥0.03%, with the remainder being Fe and unavoidable impurities. The composition of the high-sulfur scrap steel, by mass percentage, includes S: 0.02-0.15%, with the remainder being Fe and conventional C, Al, Si, Mn, and unavoidable impurities. Step 2) The converter tapping process is carried out using a weak deoxidation process, including: pouring molten steel into a ladle, adding aluminum-iron alloy, low-carbon ferromanganese, and ferrophosphorus in sequence during the tapping process, and adding synthetic slag to the slag surface of the ladle after tapping to obtain molten steel to be refined by LF. The specific process of tapping steel from the converter is as follows: When 10% of the steel is tapped from the converter, an aluminum-iron alloy is added; the amount of aluminum-iron alloy added, MA1, is calculated as follows: M Al =9 / 8 × [M 钢 ×( w [O] -0.015%)] ×η / w [Al] (1), when w [O] When ≤0.055%, η=1.1-1.3; when w [O] When >0.055%, η = 1.35 - 1.55; Among them, M Al : Indicates the amount of aluminum-iron alloy added, in kg; w [Al] : indicates the mass percentage of aluminum in the aluminum-iron alloy, %; M steel: indicates the weight of molten steel, kg; w [O] : Represents the mass fraction of oxygen in molten steel, %; η: Correlation coefficient between converter slag charge and slag oxidizability; After adding aluminum-iron alloy to the steel produced from the converter, low-carbon ferromanganese and ferrophosphorus are added until 70% of the alloy is completely added to the steel produced from the converter; wherein, the amount of low-carbon ferromanganese added is 13-17 kg / t, and the amount of ferrophosphorus added is 0.15-0.30 kg / t. After tapping, 10-15 kg / t of synthetic slag is added to the slag surface of the ladle; the composition of the synthetic slag, by mass percentage, includes CaO: 65-70%, SiO2: 5-10%, Al2O3: 10-15%, CaF2: 3-5%, MgO: 4-8%, and the remainder being unavoidable impurities. Step 3) The molten steel to be refined by LF is subjected to oxygen-controlled refining to obtain refined molten steel; wherein the chemical composition of the refined molten steel is: C: 0.05-0.08%, Mn: 1.20-1.45%, S: 0.30-0.50%, Si≤0.03%, P: 0.04-0.06%, O: 0.004-0.006%, Alt≤0.005%, Ti≤0.002%, with the remainder being Fe and unavoidable impurities; the specific process of oxygen-controlled refining of the molten steel to be refined by LF is as follows: The oxygen content of the molten steel entering the LF refining station should be controlled at 0.008-0.015%; First, carbon powder, low-carbon ferromanganese, and ferrophosphorus are added during the LF refining process to adjust the composition of the molten steel, and then electricity is applied to raise the temperature and control the temperature of the molten steel. Subsequently, calcium carbide and silicon carbide are added during the LF refining process to deoxidize and slag the ladle slag surface; wherein the amount of calcium carbide added is 0.2-0.4 kg / t, and the amount of silicon carbide added is 0.5-1.0 kg / t; wherein the silicon carbide, by mass percentage, includes SiC: 85-95%, Si: 5-10%, C≤5%, and the remainder being unavoidable impurities; the composition of the refined slag obtained by deoxidation and slag formation, by mass percentage, includes: CaO: 45-55%, SiO2: 20-30%, Al2O3: 20-30%, CaF2: 1-3%, MgO: 3-6%, T.Fe+MnO: 2-4%; The molten steel is fed into a sulfur wire at a feeding speed of 5-8 m / s, and the sulfur content and oxygen content in the molten steel are controlled at 0.30-0.50% and 0.004-0.006%, respectively. Finally, after soft stirring for 10-15 minutes, continuous casting was performed to obtain high-oxygen, high-sulfur free-machining steel 1215MS.

2. The method for smelting high-oxygen, high-sulfur free-machining steel 1215MS oxygen-controlled LF according to claim 1, characterized in that, During the process of adding aluminum-iron alloy to ferrophosphorus during the tapping of the converter, the bottom blowing flow rate of the ladle is 500-800 NL / min; after the tapping is completed and the synthetic slag is added, the bottom blowing flow rate of the ladle is 200-300 NL / min.

3. The method for smelting high-oxygen, high-sulfur free-machining steel 1215MS oxygen-controlled LF according to claim 1, characterized in that, The outer layer of the sulfur wire is pure iron sheet with an outer diameter of 12-15 mm and a thickness of 3-5 mm. The composition of the iron sheet, by mass percentage, includes Alt ≤ 0.003%, Si: 0.2-0.4%, Mn: 0.3-0.5%, with the remainder being Fe and unavoidable impurities. The inner diameter of the sulfur wire is 8-10 mm, and the interior is filled with ferrous sulfate powder. The ferrous sulfate powder contains 60-80% sulfur by mass, with the remainder being Fe and unavoidable impurities.

4. The method for smelting high-oxygen, high-sulfur free-machining steel 1215MS oxygen-controlled LF according to claim 1, characterized in that, When adding carbon powder, low-carbon ferromanganese, and ferrophosphorus during the LF refining process, the bottom blowing flow rate of the ladle is 300-500 NL / min; when feeding the refined molten steel into the sulfur line, the bottom blowing flow rate of the ladle is 200-300 NL / min, and the bottom blowing flow rate is reduced to 100-150 NL / min 3-5 minutes after the feeding of the refined molten steel into the sulfur line ends; during the soft stirring, the bottom blowing flow rate of the ladle is 30-80 NL / min.

5. The method for smelting high-oxygen, high-sulfur free-machining steel 1215MS oxygen-controlled LF according to claim 1, characterized in that, The composition of the aluminum-iron alloy, by mass percentage, includes Al: 55-65%, with the remainder being Fe and unavoidable impurities; The composition of the low-carbon ferromanganese, by mass percentage, includes Mn: 80-85%, C ≤ 0.8%, with the remainder being Fe and unavoidable impurities; The composition of the phosphorus iron, by mass percentage, includes P: 30-40%, with the remainder being Fe and unavoidable impurities.

6. The method for smelting high-oxygen, high-sulfur free-machining steel 1215MS oxygen-controlled LF according to claim 5, characterized in that, The converter tapping process uses a sliding plate to block slag, with a slag discharge rate of ≤2.5kg / t; The final slag basicity of the converter is CaO / SiO2 = 2.0-3.0, and the mass percentage of T.Fe is 13-16%. The tapping temperature of the converter is 1620-1660℃, and the content of O, C, P and S elements in the molten steel is as follows by mass percentage: O: 0.035-0.075%, C: 0.03-0.06%, Mn: 0.04-0.12%, P: 0.02-0.05%, S: 0.02-0.055%.

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