Free-cutting die steel and smelting methods for precisely controlling the sulfur content of free-cutting die steel

Abstract

This invention discloses a free-cutting die steel and a smelting method for precisely controlling the sulfur content of the free-cutting die steel, belonging to the technical field of steelmaking. The free-cutting alloy die steel contains: S 0.08%-0.12%, Alt≤0.003%, Ti≤0.0015%. The smelting method includes: blast furnace smelting: high-sulfur iron ore is selected and smelted in a blast furnace; after tapping, the molten iron is not skimmed off; converter tapping: silicon alloy is first added to the ladle for deoxidation, followed by alloying; sulfur-containing slag is added first, then calcium silicate synthetic slag and lime are added for slag formation; LF refining: bottom blowing argon is maintained throughout the process; manganese alloy, ferrochrome alloy, and ferromolybdenum alloy are added to adjust the steel composition and temperature to the required standards; after all alloy components meet the standards, lime is added; after the alloy composition, temperature, and slag composition of the molten steel meet the standards, it is transported to RH treatment; RH vacuum refining: carbon powder is first added to adjust the carbon content to the required standards, then ferrosilicon alloy and ferrosulfide alloy are added, followed by RH net circulation and vacuum tapping, and then transported to continuous casting. This invention enables precise control of the sulfur content in free-cutting mold steel.

Description

Technical Field

[0001] This invention relates to free-cutting die steel and a smelting method for precisely controlling the sulfur content of free-cutting die steel, belonging to the technical field of steelmaking. Background Technology

[0002] Conventional processes for producing sulfur-containing free-cutting alloy mold steel, while not desulfurizing in the KR (Killing Refinery) or using some high-sulfur scrap, employ high-basicity slag in the converter for dephosphorization, resulting in the removal of some sulfur. Simultaneously, the addition of alloys and carbon powder for full deoxidation at the converter tapping, and even the addition of aluminum or other strong deoxidizing alloying elements to reduce the oxygen content of the molten steel, creates a medium-to-high basicity slag system. The LF (Lead-Stage Refining) process, involving slag diffusion deoxidation and calcium treatment, further desulfurizes the molten steel, necessitating the addition of large amounts of sulfur wire or ferrisulfide during LF refining, leading to poor sulfur control stability. The continuous casting process uses low superheat combined with medium-to-low intensity cooling, resulting in low molten steel temperature control, which is detrimental to the melting of the mold flux and its separation from the billet. Furthermore, the control and reduction of the secondary cooling water are not precisely designed according to the cooling characteristics of each section of the billet, resulting in low overall production efficiency, poor control of sulfur and inclusions, and high production costs for sulfur-containing free-cutting alloy mold steel.

[0003] The inventors, after searching the literature in this field, discovered that:

[0004] Patent CN101580912B provides a production process for low-carbon, high-sulfur free-cutting steel. The main components of the steel include C 0.14%-0.20%, Si 0.17%-0.37%, Mn 1.30%-1.60%, P < 0.040%, and S 0.080%-0.130%. The process employs an electric furnace for oxygen blowing without electricity. The ratio of hot-charged molten iron to scrap steel is 7:1-3. Oxygen is alternately blown into the furnace through the carbon-oxygen lance at the furnace door and the coal-oxygen lance on the furnace wall for carbon-rich smelting. Steel is tapped with slag left at the eccentric furnace bottom. After the furnace is opened, one or more pre-deoxidation and alloying agents, such as steel-core aluminum, Ba-Ca-Si, or composite refining deoxidizers, are added, followed by slag formation. A refining furnace produces low-basicity slag. The weight composition of the refining slag is: CaO 40%-50%, SiO2 15%-30%, Al2O3 20%-35%, MgO... 5%-15%; steel is fed from the refining furnace using Ca or Ca-Si wire, with a soft blowing time of 8-15 minutes before being transferred from the ladle to the continuous casting platform; continuous casting employs a four-strand arc-shaped continuous casting machine, using long nozzle argon seam sealing protection and argon blowing protection in the tundish; secondary cooling uses a weak cooling process, and the crystallizer uses electromagnetic stirring with a current of 600-700A and a frequency of 4-5Hz. This yields continuously cast billets with stable chemical composition and good quality.

[0005] The research revealed the following conclusions:

[0006] While patent CN101580912B provides a production process for low-carbon, high-sulfur free-cutting steel, it fails to utilize high-sulfur molten iron and scrap steel in the electric arc furnace, resulting in significant subsequent sulfur additions and high costs. Furthermore, the steel grade described in this patent does not contain Al, but it adds steel-core aluminum, Ba-Ca-Si, or composite refining deoxidizers for deoxidation and alloying during both the electric arc furnace tapping and LF refining processes. These composite deoxidizers can significantly reduce the oxygen content in molten steel and slag, but also introduce high-melting-point alumina inclusions into the molten steel, negatively impacting product processing performance. Additionally, the introduction of a Ca-Si wire during refining allows Ca to directly react with S for desulfurization, hindering stable S content control. The use of high-basicity protective slag, with its high melting point and poor fluidity, provides unsatisfactory lubrication and heat conduction for the billet, negatively impacting surface quality control. Moreover, the absence of a pressing process also hinders control of central porosity and segregation.

[0007] Free-cutting alloy mold steels are widely used in various molds and mold frames, facilitating machining and extending tool life. Due to their extremely high requirements for machinability and product quality, very strict requirements are placed on the control of sulfide morphology, billet purity, billet shape, and the surface and internal quality of the billet. Therefore, to obtain high-quality free-cutting alloy mold steel, the key is to control the stability of sulfur content in the steel, the morphology of sulfide inclusions, the oxygen content and purity of the billet, and the quality of the billet. Based on this, this invention provides a free-cutting mold steel and a smelting method for precisely controlling the sulfur content of free-cutting mold steel. Summary of the Invention

[0008] To address the aforementioned problems, this invention discloses a free-cutting die steel and a smelting method for precisely controlling the sulfur content of the free-cutting die steel. The specific technical solution is as follows:

[0009] A free-cutting mold steel, wherein the chemical composition of the free-cutting alloy mold steel, by mass percentage, comprises: C: 0.25%-0.55%, Si: 0.20%-0.60%, Mn: 1.25%-1.85%, Cr: 1.7%-2.2%, Mo: 0.10%-0.40%, Ni≤0.02%, Cu≤0.015%, P≤0.015%, N≤0.0025%, H≤0.0002%, S: 0.08%-0.12%, TO≤0.002%, Alt≤0.003%, Ti≤0.0015%, and Fe and other unavoidable components.

[0010] A smelting method for precisely controlling the sulfur content of the aforementioned free-cutting die steel, following a process of blast furnace smelting—converter smelting—converter tapping—LF refining—RH vacuum refining—continuous casting, is detailed below:

[0011] Step 1: Blast furnace smelting: High-sulfur iron ore is selected and smelted in a blast furnace to obtain molten iron. The molten iron is tapped into an iron ladle. After tapping, the molten iron is not scraped off and is then transported to a converter for smelting. The composition of the molten iron in the iron ladle, by mass percentage, includes: C: 4.0%-4.4%, Si: 0.25%-0.65%, P≤0.11%, S: 0.05%-0.10%, at a temperature of 1350-1400℃.

[0012] Step 2: Converter Smelting: The converter charge is 200±5t, with a scrap steel ratio of 10-15%. Ordinary scrap steel is used for smelting. In the early stage of converter smelting, medium-low basicity and high oxidizing slag are used. In the early stage of converter blowing, the slag basicity CaO / SiO2 is controlled at 1.2-1.6, the T.Fe content in the slag is 20-30%, and the molten steel temperature is controlled at ≤1450℃. Then, more than 70% of the slag is dumped. In the second stage of blowing, lime, lightly calcined dolomite, pellets, and sulfur-containing slag are added in 2-4 batches to form slag. The amount of slag (except for...) In addition to these slag-forming materials, the total slag volume (including iron oxide formed after iron oxidation) is controlled at 50-70 kg / t, slag basicity at 2.0-3.0, T.Fe content in slag at 15-25%, and when the C content reaches 0.03-0.06%, the molten steel temperature is 1600-1650℃, oxygen content is 0.040-0.075%, and S content is 0.03-0.09%, blowing is stopped, and then the steel is tapped. Slag is blocked by a sliding plate, and slag is left after tapping the steel for the next heat of smelting.

[0013] Step 3: Converter tapping: At the start of tapping, the bottom blowing flow rate is 800-1200 NL / min. First, silicon alloy is added to the ladle for deoxidation. After the silicon alloy is added, manganese alloy, ferrochrome alloy, ferromolybdenum alloy, and carbon powder are added for alloying. When 60% of the steel has been tapped, sulfur-containing slag is added first. After tapping, the mixture is stirred for 3-5 minutes, and then calcium silicate synthesis slag and lime are added to form slag. The slag basicity CaO / SiO2 is controlled to be 0.6-1.0. When adding calcium silicate synthesis slag and lime, the bottom blowing flow rate is 200-400 NL / min. After stirring for 1-3 minutes, the mixture is transported to the LF furnace for processing.

[0014] Step 4: LF Refining: Bottom-blown argon gas is used throughout the process, and the temperature is increased by power. Manganese alloy, ferrochrome alloy, and ferromolybdenum alloy are added to adjust the steel composition and temperature to meet the standards. During the addition and alloying, the argon flow rate through the permeable bricks is 250-350 NL / min. During the heating period, the argon flow rate through the permeable bricks is 150-250 NL / min, and at other times, the argon flow rate through the permeable bricks is 100-150 NL / min. After all the alloy components in the refined steel meet the standards (the steel composition meets the requirements for free-cutting die steel), lime is added to control the basicity of the ladle slag at 1.0-1.5, 4≤T.Fe+MnO≤7%. After the alloy composition, temperature, and slag composition of the steel meet the standards (the core of the slag composition is that the basicity and T.Fe+MnO meet the requirements), the steel is transported to RH treatment.

[0015] Step 5: RH Vacuum Refining: Rapidly evacuate the RH inlet. When the vacuum level is below 50 mbar, begin adding carbon powder and increase the gas flow rate to 150-200 Nm. 3 / h, adjust the carbon content to meet the standard, then add ferrosilicon alloy and ferrous sulfate alloy. After alloying, increase the gas flow rate to 100-150 Nm. 3 / h, the vacuum degree is reduced to below 2mbar, the processing time is ≥15min, then RH net circulation, vacuum breaking and steel tapping, soft stirring and calming treatment after tapping, and then transported to continuous casting for pouring;

[0016] Step 6: Continuous casting: Slab continuous casting machine is used for protective pouring.

[0017] Furthermore, in step 2 of the converter smelting, the ordinary scrap steel contains 0.01%-0.05% S, with the remainder being Fe and other conventional components;

[0018] The sulfur-containing slag contains 2.5%-4.5% S, ≥85% CaO, and other unavoidable components, with an addition amount of 10-20 kg / t.

[0019] The lime contains ≥95% CaO, along with other unavoidable components, and is added at a rate of 10-15 kg / t.

[0020] Furthermore, in step 2 of the converter smelting, the light-calcined dolomite contains 50%-55% CaO, 30%-35% MgO, and other unavoidable components, with an addition amount of 5-10 kg / t; the pellets contain 80%-90% T.Fe, and other unavoidable components, with an addition amount of 10-15 kg / t.

[0021] Furthermore, in step 3, when the steel is tapped from the converter, 30%-40% silicon alloy is added when the steel is 20%-30% tapped from the converter. After the silicon alloy is added, 50%-60% manganese alloy, all ferrochrome alloy, ferromolybdenum alloy, and 40%-50% carbon powder are added.

[0022] Furthermore, the silicon alloy used in step 3 (converter tapping) and step 4 (LF refining) contains 75%-80% Si, ≤0.015% P, ≤0.0050% Al, and ≤0.0035% Ti, with the remainder being Fe and other unavoidable components.

[0023] The main components of manganese alloys include Mn≥98%, P≤0.010%, Al≤0.0035%, Ti≤0.0025%, with the remainder being Fe and other unavoidable impurities.

[0024] The chromium-iron alloy contains 55%-65% Cr, P≤0.015%, Al≤0.003%, Ti≤0.003%, with the remainder being Fe and other unavoidable impurities.

[0025] The ferromolybdenum alloy contains 45%-55% Mo, ≤0.018% P, ≤0.008% Al, ≤0.006% Ti, with the remainder being Fe and other unavoidable impurities.

[0026] Furthermore, in step 3, the amount of sulfur-containing slag added during converter tapping is 1.5-2.5 kg / t, of which the sulfur content is 2.0%-4.0% by mass.

[0027] The amount of calcium silicate synthesis slag added is 10-15 kg / t, and the mass percentage content includes CaO 25%-35%, SiO2 45%-55%, Al2O3≤2%, MgO 3%-6%, and other unavoidable components.

[0028] A free-cutting die steel obtained by the above-described smelting method.

[0029] The core inventive point of this invention and its technical principles and effects are as follows:

[0030] The KR pretreatment of molten iron does not involve desulfurization, resulting in high-sulfur molten iron. Simultaneously, the slag is not skimmed off and is mixed with ordinary sulfur-containing scrap steel before being added to the converter for smelting, increasing the sulfur content of the initial raw materials. In the early stages of converter smelting, medium-low basicity, high-oxidizing slag is used, which facilitates rapid slag melting and dephosphorization of the molten steel. The dephosphorized slag is then removed, which is beneficial for controlling the phosphorus content of the molten steel. At the same time, the medium-low basicity, high-oxidizing slag promotes the incorporation of sulfur (S) from the slag into the molten steel and reduces the desulfurization effect of the slag, resulting in high-sulfur steel. The converter enters the second stage of smelting (i.e., the blowing in step 2). In the later stage of smelting, medium-alkalinity slag is also used (this patent selects lime, lightly calcined dolomite and pellets) to prevent phosphorus reversion on the one hand, and to reduce the desulfurization effect of slag in combination with the high oxidation control of molten steel and slag on the other hand. At the same time, sulfur-containing slag is added to form slag, increase the sulfur content in the slag, promote the addition of sulfur from slag to molten steel, or avoid slag desulfurization and increase the sulfur content of molten steel when it is tapped from the converter.

[0031] During the converter tapping process, a portion of silicon alloy is added for deoxidation to prevent the complete removal of oxygen from the molten steel. Simultaneously, no slag-forming materials such as lime are added during tapping. The deoxidation product is primarily SiO2, forming an ultra-low basicity acidic slag, which avoids slag desulfurization. Sulfur-containing slag is also added. When it encounters the ultra-low basicity slag formed by deoxidation, combined with strong bottom blowing agitation, it promotes the rapid return of sulfur (S) from the sulfur-containing slag to the molten steel. After the low-basicity synthetic slag (calcium silicate synthetic slag) is added to the surface of the molten steel, the bottom blowing agitation intensity is appropriately reduced to prevent slag desulfurization.

[0032] The LF refining process utilizes bottom blowing throughout, ensuring the molten steel temperature and composition, as well as the slag composition, reach the target levels before adjusting the slag. This avoids the excessive desulfurization caused by bottom blowing agitation when adjusting slag basicity first and then molten steel composition. Furthermore, maintaining appropriate oxidizing properties in the slag further reduces desulfurization. Simultaneously, strong deoxidizing elements such as Si and C are adjusted under RH vacuum to avoid the strong deoxidation during LF refining, which would enhance slag formation and desulfurization. When adding strong deoxidizing elements like Si and C under RH vacuum, C is added first. Under vacuum, it reacts with oxygen in the molten steel to form a gas that is then removed, increasing the yield of subsequent silicon alloy addition, reducing Si oxidation, improving steel purity, and lowering costs. If sulfur levels are already controlled to the target, ferrosulfuron is unnecessary. If sulfur levels are insufficient, ferrosulfuron is added in the RH vacuum furnace. Steel circulation homogenizes the composition without bottom blowing agitation, preventing reaction between the molten steel and slag and thus avoiding desulfurization. This improves sulfur yield and allows for precise control of the sulfur content in the molten steel. Detailed Implementation

[0033] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that the following specific embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0034] The chemical composition of this free-cutting die steel, by mass percentage, includes: C: 0.25%-0.55%, Si: 0.20%-0.60%, Mn: 1.25%-1.85%, Cr: 1.7%-2.2%, Mo: 0.10%-0.40%, S: 0.08%-0.12%, Ni≤0.02%, Cu≤0.015%, P≤0.015%, TO≤0.002%, N≤0.0025%, H≤0.0002%, Alt≤0.003%, Ti≤0.0015%, as well as Fe and other unavoidable components. It is produced by a process of blast furnace smelting—converter smelting—LF refining—RH vacuum treatment—continuous casting.

[0035] Blast furnace smelting: High-sulfur iron ore is selected and smelted in blast furnace slag to obtain molten iron. The molten iron is tapped into an iron ladle. After tapping, the molten iron is not scraped off and is then transported to a converter for smelting. The composition of the molten iron in the iron ladle, by mass percentage, includes: C: 4.1%-4.5%, Si: 0.2%-0.6%, P≤0.12%, S: 0.05%-0.10%, at a temperature of 1350-1400℃.

[0036] The following are specific examples of the application of this method. The composition and temperature of the molten iron during blast furnace smelting are shown in Table 1.

[0037] Table 1

[0038]

[0039] Converter smelting: The converter charge is 200±5t, with a scrap steel ratio of 10-15%. Ordinary scrap steel is used for smelting. In the early stage of converter blowing, the slag basicity CaO / SiO2 is controlled at 1.2-1.6, the T.Fe content in the slag is 20-30%, and the molten steel temperature is controlled at ≤1450℃. Then, more than 70% of the slag is dumped. In the second stage of blowing, lime, lightly calcined dolomite, pellets, and sulfur-containing slag are added in multiple batches to form slag. The slag amount is controlled at 50-70kg / t, the slag basicity is 2.0-3.0, and the T.Fe content in the slag is 15-25%. When the C content reaches 0.03-0.06%, the molten steel temperature is 1600-1650℃, the oxygen content is 0.040-0.075%, and the S content is 0.03-0.09%, blowing is stopped, and then the steel is tapped. Slag is blocked by a sliding plate, and slag is left after tapping the steel for the next heat of smelting.

[0040] Ordinary scrap steel contains 0.01%-0.05% S, with the remainder being Fe and other conventional components; sulfur-containing slag contains 2.5%-4.5% S, ≥85% CaO, and other unavoidable components, with an addition amount of 10-20 kg / t; lime contains ≥95% CaO, and other unavoidable components, with an addition amount of 10-15 kg / t; lightly calcined dolomite contains 50%-55% CaO, 30%-35% MgO, and other unavoidable components, with an addition amount of 5-10 kg / t; pellets contain 80%-90% T and Fe, and other unavoidable components, with an addition amount of 10-15 kg / t.

[0041] The following are specific examples of the application of this method. For parameter control during converter smelting, please refer to Table 2.

[0042] Table 2

[0043]

[0044] Converter tapping: At the start of tapping, the bottom blowing flow rate is 800-1200 NL / min. First, silicon alloy is added to the ladle for deoxidation. After the silicon alloy is added, manganese alloy, ferrochrome alloy, ferromolybdenum alloy, and carbon powder are added for alloying. When 60% of the steel has been tapped, sulfur-containing slag is added first. After tapping, the mixture is stirred for 3-5 minutes, and then calcium silicate synthetic slag and lime are added to form slag. The slag basicity CaO / SiO2 is controlled to be 0.6-1.0. When adding synthetic slag and lime, the bottom blowing flow rate is 200-400 NL / min. After stirring for 1-3 minutes, the mixture is transported to the LF furnace for processing.

[0045] When the steel is tapped from the converter, alloys are added. When the steel is 20%-30% tapped from the converter, 30%-40% silicon alloy is added. After the silicon alloy is added, 50%-60% manganese alloy, all ferrochrome alloy, ferromolybdenum alloy, and 40%-50% carbon powder are added.

[0046] The following are specific examples of the application of this method. The parameter control during the converter tapping process is shown in Table 3.

[0047] Table 3

[0048]

[0049] LF Refining: Bottom-blown argon gas is used throughout the process, and the temperature is raised by electricity. Manganese alloy, ferrochrome alloy, and ferromolybdenum alloy are added to adjust the composition and temperature of the molten steel to meet the standards. During the addition and alloying, the argon flow rate of the permeable brick is 250-350 NL / min. During the heating period, the argon flow rate of the permeable brick is 150-250 NL / min, and at other times, the argon flow rate of the permeable brick is 100-150 NL / min. After all the alloy composition of the refined steel meets the standards, lime is added to control the basicity of the ladle slag at 1.0-1.5, 4%≤T.Fe+MnO≤7%. After the alloy composition, temperature, and slag composition of the molten steel meet the standards, it is transported to RH treatment.

[0050] The silicon alloys used in converter steelmaking and LF refining consist of 75%-80% Si, ≤0.015% P, ≤0.0050% Al, and ≤0.0035% Ti, with the remainder being Fe and other unavoidable components. The manganese alloys mainly consist of Mn ≥98%, P ≤0.010%, Al ≤0.0035%, and Ti ≤0.0025%, with the remainder being Fe and other unavoidable impurities. The ferrochrome alloys consist of 55%-65% Cr, ≤0.015% P, ≤0.003% Al, and ≤0.003% Ti, with the remainder being Fe and other unavoidable impurities. The ferromolybdenum alloys consist of 45%-55% Mo, ≤0.018% P, ≤0.008% Al, and ≤0.006% Ti, with the remainder being Fe and other unavoidable impurities. Sulfur-containing slag is added at a rate of 1.5-2.5 kg / t, with an S content of 2.0%-4.0%. Calcium silicate synthesis slag is added at a rate of 10-15 kg / t, with CaO as the main component. 25%-35%, SiO2 45%-55%, Al2O3≤2%, MgO 3%-6%, and other unavoidable components.

[0051] The following are specific implementation examples of this method. The parameter control in the LF refining process is shown in Table 4.

[0052] Table 4

[0053]

[0054] RH vacuum refining: Rapid vacuuming is performed upon RH inlet treatment. Carbon powder is added when the vacuum level is below 50 mbar, and the gas flow rate is increased to 150-200 Nm. 3 / h, adjust the carbon content to meet the standard, then add ferrosilicon alloy and ferrous sulfate alloy. After alloying, increase the gas flow rate to 100-150 Nm.3 / h, the vacuum degree is reduced to below 2mbar, the processing time is ≥15min, then RH net circulation, vacuum breaking and steel tapping, soft stirring and calming treatment after tapping, and then transported to continuous casting for pouring.

[0055] The following are specific implementation examples of this method. The parameter control in the RH vacuum refining process is shown in Table 5.

[0056] Table 5

[0057]

[0058] Continuous casting: Slab continuous casting machine is used for protective pouring.

[0059] The properties of the free-cutting mold steel billets obtained from the above three embodiments are shown in Table 6 below.

[0060] Table 6

[0061]

[0062] The technical means disclosed in this invention are not limited to those disclosed above, but also include technical solutions composed of any combination of the above technical features.

[0063] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A smelting method for precisely controlling the sulfur content of free-cutting die steel, characterized in that, The chemical composition of the free-cutting alloy mold steel, by mass percentage, includes: C: 0.25%-0.55%, Si: 0.20%-0.60%, Mn: 1.25%-1.85%, Cr: 1.7%-2.2%, Mo: 0.10%-0.40%, Ni≤0.02%, Cu≤0.015%, P≤0.015%, N≤0.0025%, H≤0.0002%, and Fe and other unavoidable components. It is characterized by the following element contents: S: 0.08%-0.12%, TO≤0.002%, Alt≤0.003%, Ti≤0.0015%. It is produced according to the blast furnace smelting—converter smelting—converter tapping—LF refining—RH vacuum refining—continuous casting process, specifically as follows: Step 1: Blast furnace smelting: High-sulfur iron ore is selected and smelted in a blast furnace to obtain molten iron. The molten iron is tapped into an iron ladle. After tapping, the molten iron is not scraped off and is then transported to a converter for smelting. The composition of the molten iron in the iron ladle, by mass percentage, includes: C: 4.0%-4.4%, Si: 0.25%-0.65%, P≤0.11%, S: 0.05%-0.10%, at a temperature of 1350-1400℃. Step 2: Converter smelting: The converter charge is 200±5t, with a scrap steel ratio of 10-15%. Ordinary scrap steel is used for smelting. In the early stage of converter smelting, medium-low basicity and high oxidizing slag are used. In the early stage of converter blowing, the slag basicity CaO / SiO2 is controlled at 1.2-1.6, the T.Fe content in the slag is 20-30%, and the steel temperature is controlled at ≤1450℃. Then, more than 70% of the slag is dumped. In the second stage of blowing, lime is added in 2-4 batches. The process involves slag formation using lightly calcined dolomite, pellets, and sulfur-containing slag, with a slag quantity controlled at 50-70 kg / t. The slag basicity is 2.0-3.0, and the T.Fe content in the slag is 15-25%. When the carbon content reaches 0.03-0.06%, the molten steel temperature is 1600-1650℃, the oxygen content is 0.040-0.075%, and the sulfur content is 0.03-0.09%. At this point, the blowing is stopped, and the steel is tapped. A sliding plate is used to block the slag, leaving slag behind, for the next heat of smelting. Step 3: Converter tapping: At the start of tapping, the bottom blowing flow rate is 800-1200 NL / min. First, silicon alloy is added to the ladle for deoxidation. After the silicon alloy is added, manganese alloy, ferrochrome alloy, ferromolybdenum alloy, and carbon powder are added for alloying. When 60% of the steel has been tapped, sulfur-containing slag is added first. After tapping, the mixture is stirred for 3-5 minutes, and then calcium silicate synthesis slag and lime are added to form slag. The slag basicity CaO / SiO2 is controlled to be 0.6-1.

0. When adding calcium silicate synthesis slag and lime, the bottom blowing flow rate is 200-400 NL / min. After stirring for 1-3 minutes, the mixture is transported to the LF furnace for processing. Step 4: LF Refining: Bottom-blown argon gas is used throughout the process, and the temperature is increased by power. Manganese alloy, ferrochrome alloy, and ferromolybdenum alloy are added to adjust the composition and temperature of the molten steel to meet the standards. During the addition and alloying, the argon flow rate of the permeable brick is 250-350 NL / min. During the heating period, the argon flow rate of the permeable brick is 150-250 NL / min, and at other times, the argon flow rate of the permeable brick is 100-150 NL / min. After the alloy composition of the refined steel meets the standards, lime is added to control the basicity of the ladle slag at 1.0-1.5, and 4≤T.Fe+MnO≤7%. After the alloy composition, temperature, and slag composition of the molten steel meet the standards, it is transported to RH treatment. Step 5: RH Vacuum Refining: Rapidly evacuate the RH inlet. When the vacuum level is below 50 mbar, begin adding carbon powder and increase the gas flow rate to 150-200 Nm³. 3 / h, adjust the carbon content to meet the standard, then add ferrosilicon alloy and ferrous sulfate alloy. After alloying, increase the gas flow rate to 100-150 Nm. 3 / h, the vacuum degree is reduced to below 2mbar, the processing time is ≥15min, then RH net circulation, vacuum breaking and steel tapping, soft stirring and calming treatment after tapping, and then transported to continuous casting for pouring; Step 6: Continuous casting: Use a slab continuous casting machine for protective pouring.

2. The smelting method according to claim 1, characterized in that, In step 2 of the converter smelting, the ordinary scrap steel contains 0.01%-0.05% S, with the remainder being Fe and other conventional components; The sulfur-containing slag contains 2.5%-4.5% S, ≥85% CaO, and other unavoidable components, with an addition amount of 10-20 kg / t. The lime contains ≥95% CaO, along with other unavoidable components, and is added at a rate of 10-15 kg / t.

3. The smelting method according to claim 1, characterized in that, In step 2 of the converter smelting, the lightly calcined dolomite contains 50%-55% CaO, 30%-35% MgO, and other unavoidable components, with an addition amount of 5-10 kg / t; the pellets contain 80%-90% T and Fe, and other unavoidable components, with an addition amount of 10-15 kg / t.

4. The smelting method according to claim 1, characterized in that, In step 3, when the steel is tapped from the converter, 30%-40% silicon alloy is added when the steel has been tapped to 20%-30% of its original volume. After the silicon alloy is added, 50%-60% manganese alloy, all ferrochrome alloy, ferromolybdenum alloy, and 40%-50% carbon powder are added.

5. The smelting method according to claim 1, characterized in that, The silicon alloy used in step 3 (converter tapping) and step 4 (LF refining) contains 75%-80% Si, ≤0.015% P, ≤0.0050% Al, and ≤0.0035% Ti, with the remainder being Fe and other unavoidable components. The main components of manganese alloys include Mn≥98%, P≤0.010%, Al≤0.0035%, Ti≤0.0025%, with the remainder being Fe and other unavoidable impurities. The chromium-iron alloy contains 55%-65% Cr, P≤0.015%, Al≤0.003%, Ti≤0.003%, with the remainder being Fe and other unavoidable impurities. The ferromolybdenum alloy contains 45%-55% Mo, ≤0.018% P, ≤0.008% Al, ≤0.006% Ti, with the remainder being Fe and other unavoidable impurities.

6. The smelting method according to claim 1, characterized in that, In step 3, the amount of sulfur-containing slag added during converter tapping is 1.5-2.5 kg / t, of which the sulfur content is 2.0%-4.0% by mass. The amount of calcium silicate synthesis slag added is 10-15 kg / t, and the mass percentage content includes CaO 25%-35%, SiO2 45%-55%, Al2O3≤2%, MgO 3%-6%, and other unavoidable components.

7. A free-cutting die steel obtained by the smelting method according to any one of claims 1-6.

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