High-phosphorus-sulfur free-cutting steel and production method thereof

Abstract

This invention discloses a high-phosphorus and sulfur free-cutting steel and its production method. The chemical composition of the steel is: C 0.51-0.56%, Si 0.06-0.13%, Mn 0.80-1.0%, P 0.041-0.065%, S 0.08-0.10%, Als≤0.010%, Ca 0.0015-0.0030%, Mo 0.16-0.22%, Cr 1.90-1.98%, and Mn / S: 8-12. Before converter blowing, high-phosphorus slag and scrap iron are added to the converter, followed by molten iron, ensuring that the phosphorus and sulfur content in the final molten steel meets the finished product requirements. The final molten steel temperature is 1630-1660℃. The steel then enters the LF refining process to produce low-basicity slag, with basicity controlled at 1.0-1.2, and the composition and manganese-sulfur ratio stabilized. The molten steel exits the LF at a temperature of 1530-1540℃. This invention eliminates the need to add ferrous sulfide and ferrophosphorus alloys after the converter, avoiding sulfur content fluctuations caused by unstable sulfur recovery during refining, reducing manganese-sulfur ratio fluctuations, eliminating cracks, improving the quality of continuously cast billets, and lowering production costs. It also enables the effective utilization of waste resources.

Description

Technical Field

[0001] This invention belongs to the field of steelmaking continuous casting technology, and specifically relates to a high-phosphorus and sulfur free-cutting steel and its production method. Background Technology

[0002] As steel products face increasingly stringent requirements for phosphorus and sulfur content, the utilization of desulfurized slag iron is restricted, with most of it being stored in slag yards, making it difficult to use iron-containing resources. The dephosphorization slag from the early stages of converter production is slag that was discharged when the phosphorus content was high. This type of steel slag is also difficult to utilize effectively in subsequent production and is usually stored in slag yards, resulting in a waste of resources.

[0003] my country began producing free-cutting steel in the 1950s, with relatively low annual output at that time. Although output has increased since then, the demand for free-cutting steel is also growing rapidly as the machining industry moves towards automation. Currently, my country's production level, output, quality, and variety of free-cutting steel lag significantly behind those of the world's leading steel-producing countries. Most steel mills use ingot casting (electric furnace + ingot casting) to produce free-cutting steel ingots (billets) or spray forming to produce free-cutting die steel. Ingot casting processes are energy-intensive, have low metal yield, and cause significant environmental pollution, which is not in line with the current social development direction of energy conservation, emission reduction, and environmental protection. Moreover, sulfur segregation is a major cause of uneven machinability, while continuous casting does not produce this segregation. Continuous casting has a faster solidification rate, so the sulfide particles are smaller than those in ingot casting.

[0004] Producing free-cutting steel using continuous casting methods brings new problems such as high viscosity and poor fluidity of molten steel. To ensure its castability, the casting temperature must be increased. At the same time, free-cutting steel is a crack-sensitive steel, which is very prone to surface longitudinal cracks, leading to the scrapping of the billet. The patent application number CN202010958724.2, entitled "A High-Sulfur, Low-Aluminum Free-Machining Steel Continuous Casting Slab and Its Production Method," describes a steel grade with the following chemical composition: C, Si, Mn, P, S, Cr, Mo, Ti, Ca, and Alt. The production method involves: no lime slag washing during converter tapping, stable control of S content, addition of aluminum blocks for pre-deoxidation, and control of slag discharge; low-basicity beige slag produced in an LF furnace, controlling the top slag basicity to <2, ensuring stable and precise control of S content and guaranteeing an active oxygen content ≤5ppm; after vacuum treatment in a VD / RH furnace to break up voids, adding a top slag modifier to deoxidize the slag surface, controlling Fe0+Mn0 in the slag to ≤2%, and strictly controlling the argon flow rate; employing weak cooling and low casting speed technology during casting, along with protective slag; through the above smelting and continuous casting process, the incidence of longitudinal cracks on the surface of the high-sulfur, low-aluminum free-machining steel slab is less than 0.5%, and the scrap rate due to surface cracks in the rolled steel plate is reduced to below 0.3%. This patent uses ferrous sulfide added during LF refining. There is no lower limit requirement for phosphorus content, and there is no need to add ferrophosphorus. The amount of ferrous sulfide added is large, the yield is unstable, and the manganese-sulfur ratio fluctuates greatly. Because the manganese-sulfur ratio is difficult to control, the produced continuous casting billets often crack. Since the location of the cracks is random in both length and width, the steel plates rolled later cannot be effectively utilized, resulting in a large number of scraps. Summary of the Invention

[0005] The purpose of this invention is to provide a high-phosphorus and sulfur free-cutting steel and its production method. When producing high-phosphorus and sulfur free-cutting steel in a converter, the consumption of ferrophosphorus and ferrous sulfide is reduced by adding high-phosphorus slag and desulfurized slag iron into the converter, thereby reducing the alloying cost. At the same time, by using desulfurized slag iron, the sulfur content between slag and steel is brought into balance during the refining process, the sulfur content is stably controlled, the composition fluctuation is reduced, and the manganese-sulfur ratio is kept stable, thus avoiding hot cracking defects in the continuously cast billet when the manganese-sulfur ratio fluctuates greatly.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] One of the technical solutions of this invention is to provide a high-phosphorus-sulfur free-cutting steel, wherein the chemical composition by weight percentage of the free-cutting steel is as follows: C: 0.51%~0.56%, Si: 0.06%~0.13%, Mn: 0.80%~1.0%, P: 0.041%~0.065%, S: 0.08%~0.10%, Als≤0.010%, Ca: 0.0015%~0.0030%, Mo: 0.16%~0.22%, Cr: 1.90%~1.98%, with the remainder being Fe and unavoidable impurities, and satisfying the Mn / S ratio of 8~12.

[0008] The second technical solution of this invention provides a method for producing high-phosphorus and sulfur free-cutting steel, including converter smelting, LF refining, and continuous casting. Before converter blowing, high-phosphorus slag and scrap iron are added to the converter, and then molten iron is added to ensure that the phosphorus and sulfur in the molten steel at the end of the converter meet the requirements of the finished product. The temperature of the molten steel at the end of the converter is controlled at 1630-1660℃, and the amount of slag discharged during the converter tapping process is controlled to be less than 5 kg / t of steel. The steel enters the LF refining process to produce low-basicity slag, with the slag basicity controlled at 1.0-1.2, and the composition and manganese-sulfur ratio stabilized. The temperature of the molten steel discharged from the LF is controlled at 1530-1540℃.

[0009] Further, the high-phosphorus slag composition is as follows: CaO: 29%–34%, SiO2: 16%–21%, MgO: 6%–8%, Al2O3: 1%–2%, MnO: 4%–6%, P2O5: 4%–6%, S: 0.03%–0.06%, FeO: 16%–18%, MFe: 2%–4%, slag particle size 5–50 mm, and the addition amount accounts for 0.8%–1.2% of the total steel material weight; the high-phosphorus slag is the recovery of the high-phosphorus slag discharged in the early stage of the converter.

[0010] Further, the composition of the slag scraping iron is: C: 2.9%~3.4%, Si: 0.02%~0.05%, Mn: 0.06%~0.14%, P: 0.09%~0.13%, S: 0.70%~0.80%, slag particle size 5~500mm, and the amount added accounts for 10%~14% of the total steel material. The slag scraping iron is the recovery of slag scraping iron generated during the desulfurization pretreatment of molten iron.

[0011] Further, the composition of the molten iron is: C: 4.1% to 4.5%, Si: 0.20% to 0.30%, Mn: 0.10% to 0.20%, P: 0.10% to 0.13%, S: 0.03% to 0.05%, with the balance being Fe and unavoidable impurities.

[0012] Furthermore, during the initial stage of converter blowing, the high-oxygen lance position is controlled at 2.4–2.6 m, and the oxygen flow rate is controlled at 3.2–3.5 Nm³. 3 / (min·t), the amount of lime and dolomite added is 3.8% to 4.0% and 0.5% to 0.7% of the total weight of the steel material, respectively, and is added in 2 to 3 times in the early and middle stages of blowing.

[0013] Furthermore, the argon flow rate at the bottom of the converter is controlled at 0.05–0.2 Nm³. 3 / (min·t), the composition of the molten steel at the end of the blowing process is: C: 0.2%~0.3%, Si: 0.01%~0.02%, Mn: 0.02%~0.06%, P: 0.04%~0.06%, S: 0.080%~0.10%, and the composition of the slag at the end of the process is: CaO: 40%~45%, SiO2: 16%~18%, MgO: 8%~10%, Al2O3: 1%~2%, MnO: 2%~3%, P2O5: 1.5%~2.5%, S: 0.3%~0.5%, FeO: 17%~19%.

[0014] Furthermore, in LF refining, a mixed slag of lime and silica is added at 0.5% to 1.5% of the weight of molten steel, wherein the mass ratio of lime to silica in the mixed slag is 1:1. 0.04% to 0.08% silicon carbide and 0.013% to 0.015% aluminum segments, respectively, are added to the slag surface for diffusion deoxidation.

[0015] Furthermore, during LF refining, the argon flow rate at the bottom of the ladle is controlled at 210–260 L / min, and the refining time is 40–50 min. After refining, the silicon-calcium wire fed in accounts for 0.03%–0.045% of the weight of the molten steel. The composition of the molten steel removed from the LF refining process is as follows: C: 0.52%–0.55%, Si: 0.07%–0.12%, Mn: 0.82%–0.98%, P: 0.043%–0.063%, S: 0.08%–0.10%, Als: 0.005%–0.010%, Ca: 0.0015%–0.0030%, Mo: 0.17%–0.21%, Cr: 1.92%–1.98%, with the remainder being Fe and unavoidable impurities, and satisfying the Mn / S ratio of 8–12.

[0016] Furthermore, during the continuous casting process, the superheat of the molten steel is controlled at 30–40°C, the billet pulling speed is controlled at 0.90–1.1 m / min, and electromagnetic stirring is used during the continuous casting process with a current of 400–450 A and a frequency of 2.0–2.5 Hz.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0018] 1. This invention uses desulfurized slag iron and dephosphorized slag from the early stage of the converter to increase the sulfur and phosphorus content of molten steel. It eliminates the need to add ferrous sulfide and ferrophosphorus alloy during the LF refining process to adjust the phosphorus and sulfur content, while also making effective use of waste resources.

[0019] 2. This invention enables sulfur enrichment of molten steel in the converter, directly achieving the target sulfur content without readjustment during LF refining. This avoids sulfur content fluctuations caused by unstable sulfur recovery during refining, ensures uniform sulfur distribution in the steel, reduces manganese-sulfur ratio fluctuations, eliminates cracks, improves the quality of continuously cast billets, and reduces production costs.

[0020] 3. This invention utilizes high-phosphorus slag from the early stage of the converter to produce high-phosphorus and sulfur free-cutting steel, thereby achieving effective utilization of high-phosphorus slag within the converter, reducing waste accumulation, and increasing the phosphorus content at the converter endpoint without requiring adjustment of the phosphorus content during the refining process.

[0021] 4. This invention produces high-phosphorus and sulfur free-cutting steel, making full use of existing equipment and raw materials, reducing steel consumption, being easy to operate, and requiring no additional investment. Detailed Implementation

[0022] It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined with each other. The described embodiments are merely some, not all, of the embodiments of this invention. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the invention or its application or use. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0023] To further describe the present invention, the following detailed description is provided in conjunction with embodiments:

[0024] A high-phosphorus and sulfur free-cutting steel and its production method utilize high-phosphorus slag discharged during the early stage of converter blowing and desulfurized iron removed during the desulfurization process of molten iron pretreatment. The high-phosphorus slag discharged during the early stage of blowing is slag discharged at a lower temperature and higher phosphorus content in the early stage of producing steel grades with high phosphorus content requirements, in order to improve the dephosphorization effect of the converter. This steel slag is difficult to effectively utilize in subsequent processes and is usually stockpiled. Desulfurized slag iron is a mixture of slag and iron removed during the pretreatment process. After magnetic separation, desulfurized iron is obtained. Desulfurized iron is enriched with a large amount of sulfur. With users having increasingly higher requirements for steel quality, most of the desulfurized slag iron is stockpiled in slag yards, making it difficult to utilize iron-containing resources. When producing high-phosphorus and sulfur free-cutting steel, this invention fully utilizes high-phosphorus slag and desulfurized iron during converter smelting, ensuring stable control of steel composition, improving the utilization rate of iron resources, and reducing the occupation of large amounts of land and environmental pollution. During LF refining, there is no need to add ferrous sulfide and ferrophosphorus, the sulfur content fluctuates little, the manganese-sulfur ratio is controlled stably, and the generation of low-melting-point ferrous sulfide is prevented during the solidification of the continuous casting billet, effectively avoiding the occurrence of cracks in the continuous casting billet.

[0025] The example uses high-phosphorus slag and scraped iron to increase the phosphorus and sulfur content in the steel, while the comparative example uses conventional smelting and LF refining to add ferrophosphorus and ferrous sulfide. All continuously cast billets are 230mm thick slabs.

[0026] The chemical composition and weight percentage of the high phosphorus and sulfur free-cutting steel are as follows: C: 0.51%–0.56%, Si: 0.06%–0.13%, Mn: 0.80%–1.0%, P: 0.041%–0.065%, S: 0.08%–0.10%, Als≤0.010%, Ca: 0.0015%–0.0030%, Mo: 0.16%–0.22%, Cr: 1.90%–1.98%, with the remainder being Fe and unavoidable impurities, and satisfying the Mn / S ratio of 8–12.

[0027] 1) Utilization of high-phosphorus slag

[0028] The high-phosphorus slag discharged in the early stage of the converter is recovered. The composition of the high-phosphorus slag is as follows: CaO: 29%~34%, SiO2: 16%~21%, MgO: 6%~8%, Al2O3: 1%~2%, MnO: 4%~6%, P2O5: 4%~6%, S: 0.03%~0.06%, FeO: 16%~18%, MFe: 2%~4%. The particle size of the slag is controlled to be 5~50mm, and the amount added accounts for 0.8%~1.2% of the total weight of the steel material.

[0029] 2) Desulfurization slag and iron utilization

[0030] The scraped iron generated during the desulfurization pretreatment of molten iron is recovered. The scraped iron composition is: C: 2.9%–3.4%, Si: 0.02%–0.05%, Mn: 0.06%–0.14%, P: 0.09%–0.13%, S: 0.70%–0.80%. The slag particle size is controlled at 5–500 mm, and the scraped iron accounts for 10%–14% of the total steel feedstock. It is added to the converter along with high-phosphorus slag using a scrap steel trough, reducing the consumption of converter steelmaking flux and steel feedstock.

[0031] 3) Converter smelting

[0032] Before converter blowing, high-phosphorus slag and scrap iron are added to the converter together through a scrap steel trough, followed by molten iron. The composition of the molten iron is: C: 4.1%–4.5%, Si: 0.20%–0.30%, Mn: 0.10%–0.20%, P: 0.10%–0.13%, S: 0.03%–0.05%. During the initial stage of converter blowing, a high lance position is adopted, controlled at 2.4–2.6 m, and the oxygen flow rate is controlled at 3.2–3.5 Nm. 3 / (min·t), during the early stage of blowing, silicon and manganese in the steel are oxidized. To ensure the basicity of the slag and protect the furnace lining, lime and dolomite are added. The amount of lime and dolomite added accounts for 3.8% to 4.0% and 0.5% to 0.7% of the total weight of the steel material, respectively. It is added in 2 to 3 times during the early and middle stages of blowing. Bottom blowing argon is used during the smelting process, and the argon flow rate is controlled at 0.05 to 0.2 Nm³. 3 / (min·t), the final steel composition at the blowing point is C: 0.2%~0.3%, Si: 0.01%~0.02%, Mn: 0.02%~0.06%, P: 0.04%~0.06%, S: 0.08%~0.10%, and the final slag composition is CaO: 40%~45%, SiO2: 16%~18%, MgO: 8%~10%, Al2O3: 1%~2%, MnO: 2%~3%, P2O5: 1.5%~2.5%, S: 0.3%~0.5%, FeO: 17%~19%. The final steel temperature is controlled at 1630~1660℃, and the slag discharge during the converter tapping process is controlled to be less than 5kg / t steel.

[0033] 4) LF Refining

[0034] The molten steel enters the LF refining process. A mixed slag of lime and silica (lime:silica mass ratio = 1:1) is added at 0.5% to 1.5% of the molten steel weight to create a low-basicity slag. Silicon carbide (0.04% to 0.08% of the molten steel weight) and aluminum wire segments (0.013% to 0.015% of the molten steel weight) are added to the slag surface for diffusion deoxidation. The basicity of the refining slag is controlled at 1.0 to 1.2. The argon flow rate at the bottom of the ladle is controlled at 210 to 260 L / min to promote the flotation of oxide inclusions in the steel. The refining time is 40 to 50 minutes. After refining, silicon-calcium wire (0.03% to 0.045% of the molten steel weight) is fed in. The molten steel temperature at the LF outlet is controlled at 1530 to 1540℃. Since phosphorus and sulfur are already adjusted to the required range during converter smelting, the phosphorus and sulfur content in the steel slag approaches equilibrium during fine-tuning in LF refining. This eliminates the problem of sulfur content fluctuations caused by the addition of ferrous sulfide during LF refining, which would affect the manganese-sulfur ratio in the steel and lead to cracking in continuously cast billets. The composition of the molten steel from LF refining is: C: 0.52%–0.55%, Si: 0.07%–0.12%, Mn: 0.82%–0.98%, P: 0.043%–0.063%, S: 0.08%–0.10%, Als: 0.005%–0.010%, Ca: 0.0015%–0.0030%, Mo: 0.17%–0.21%, Cr: 1.92%–1.98%, with the remainder being Fe and unavoidable impurities, and meeting the Mn / S ratio of 8–12.

[0035] 5) Continuous casting

[0036] During continuous casting, free-cutting steel protective slag is used to ensure proper casting protection, inhibit oxygen absorption by the molten steel, prevent secondary oxidation of the molten steel, ensure the tundish is thoroughly cleaned, and control the online baking time to ensure 2 hours. The covering agent is added evenly to ensure that the molten steel in the tundish is not exposed. The casting speed is kept constant during casting to reduce liquid level fluctuations. A weak cooling method is adopted for continuous casting, and the superheat of the molten steel is controlled at 30-40℃. The casting speed is controlled at 0.90-1.1m / min. Electromagnetic stirring is used during continuous casting with a current of 400-450A and a frequency of 2.0-2.5Hz to improve the compositional segregation in the continuously cast billet.

[0037] The chemical composition of the finished products of the examples and comparative examples is shown in Table 1.

[0038] Table 1 Chemical composition of the examples and comparative examples

[0039] Example Example 1 Example 2 Example 3 Example 4 Example 5 Comparative Example 1 Comparative Example 2 C / ％ 0.51 0.52 0.54 0.55 0.56 0.53 0.54 Si / % 0.06 0.08 0.10 0.12 0.13 0.09 0.10 Mn / % 0.98 0.94 0.90 0.86 0.82 0.92 0.86 P / ％ 0.065 0.060 0.054 0.047 0.041 0.056 0.054 S / ％ 0.082 0.094 0.10 0.093 0.088 0.12 0.108 Als / % 0.003 0.005 0.007 0.008 0.010 0.005 0.006 Ca / % 0.0015 0.0018 0.0020 0.0024 0.0029 0.0020 0.0023 Mo / % 0.22 0.20 0.18 0.17 0.16 0.19 0.18 Cr / ％ 1.90 1.92 1.94 1.96 1.98 1.94 1.95 Mn / S 11.95 10 9.0 9.2 9.3 7.67 7.96

[0040] The process parameters for the high-phosphorus slag in the example are shown in Table 2.

[0041] Table 2 Process parameters for high-phosphorus slag

[0042]

[0043]

[0044] The process parameters for slag removal and iron scraping in the example are shown in Table 3.

[0045] Table 3 Slag Removal and Iron Process Parameters

[0046] Example Example 1 Example 2 Example 3 Example 4 Example 5 C，％ 2.9 3.0 3.1 3.3 3.4 Si,% 0.05 0.04 0.04 0.03 0.02 Mn, % 0.06 0.08 0.10 0.12 0.14 P，％ 0.09 0.10 0.11 0.12 0.13 S，％ 0.80 0.78 0.76 0.73 0.70 Percentage of additions, % 14 13 12 11 10

[0047] The process parameters for converter smelting in the examples and comparative examples are shown in Table 4.

[0048] Table 4 Converter Smelting Process Parameters

[0049]

[0050] The process parameters for the converter endpoints of the examples and comparative examples are shown in Table 5.

[0051] Table 5 Converter final process parameters

[0052]

[0053]

[0054] During the converter smelting process, the slag is a high-basicity oxidizing slag, which removes some of the phosphorus and sulfur from the molten steel. For comparison, if ferrous sulfide and ferrophosphorus alloy are added in the converter, most of them will be removed, resulting in a very low yield. Only by adding alloys during LF refining can a certain yield be guaranteed, and even then, the yield is unstable.

[0055] The process parameters for LF refining in the examples and comparative examples are shown in Table 6. In the comparative example, ferrophosphorus and ferrous sulfide were added during LF refining, while ferrophosphorus alloy and ferrous sulfide were added during LF refining for adjustment.

[0056] Table 6 LF Refining Process Parameters

[0057]

[0058] Phosphorus and sulfur content reaches the required levels within the converter, requiring no adjustment during refining. Sulfur content fluctuates minimally, and the manganese-sulfur ratio is stably controlled, preventing the formation of low-melting-point ferrous sulfide during continuous casting solidification and effectively avoiding cracks in the continuous casting billet. In contrast, the proportion that adds ferrous sulfide to increase sulfur content during LF refining suffers from unstable sulfur recovery due to excessive addition, resulting in higher sulfur content and a lower manganese-sulfur ratio.

[0059] The continuous casting process parameters for the examples and comparative examples are shown in Table 7.

[0060] Table 7 Continuous Casting Process Parameters

[0061]

[0062]

[0063] The high-phosphorus and sulfur free-cutting steel produced by the above process has phosphorus and sulfur reaching the required composition in the converter and does not need to be adjusted during the refining process. The sulfur content fluctuates little, and the manganese-sulfur ratio is stably controlled, which prevents the formation of low-melting-point ferrous sulfide during the solidification of the continuous casting billet and can effectively avoid the occurrence of cracks in the continuous casting billet.

[0064] It is beneficial to the stable control of composition, and the produced continuous casting billets are free of cracks and defects.

[0065] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A high-phosphorus, high-sulfur free-cutting steel, characterized in that, The chemical composition of the steel by weight percentage is as follows: C: 0.51%~0.56%, Si: 0.06%~0.13%, Mn: 0.80%~1.0%, P: 0.041%~0.065%, S: 0.08%~0.10%, Als≤0.010%, Ca: 0.0015%~0.0030%, Mo: 0.16%~0.22%, Cr: 1.90%~1.98%, with the remainder being Fe and unavoidable impurities. And it meets the Mn / S ratio of 8~12; the production method includes converter smelting, LF refining, and continuous casting. Before converter blowing, high-phosphorus slag and scraped iron are added to the converter, and then molten iron is added to ensure that the phosphorus and sulfur in the molten steel at the end of the converter meet the finished product requirements; the temperature of the molten steel at the end of the converter is controlled at 1630~1660℃, and the slag amount is controlled to be less than 5kg / t of steel during the converter tapping process; it enters LF refining to produce low-basicity slag, with the slag basicity controlled at 1.0~1.2, and the composition and manganese-sulfur ratio are controlled and stabilized; L The temperature of the molten steel being removed is controlled at 1530~1540℃; the high-phosphorus slag composition is: CaO: 29%~34%, SiO2: 16%~21%, MgO: 6%~8%, Al2O3: 1%~2%, MnO: 4%~6%, P2O5: 4%~6%, S: 0.03%~0.06%, FeO: 16%~18%, MFe: 2%~4%, slag particle size 5~50mm, and the addition amount accounts for 0.8% of the total steel material weight. 1.2%; the high-phosphorus slag is the recovery of high-phosphorus slag discharged in the early stage of converter; the composition of the slag scraping iron is: C: 2.9%~3.4%, Si: 0.02%~0.05%, Mn: 0.06%~0.14%, P: 0.09%~0.13%, S: 0.70%~0.80%, slag particle size 5~500mm, the amount added accounts for 10%~14% of the total steel material, and the slag scraping iron is the recovery of slag scraping iron generated during the desulfurization pretreatment of molten iron.

2. The high-phosphorus, high-sulfur free-cutting steel according to claim 1, characterized in that, The composition of the molten iron is: C: 4.1%~4.5%, Si: 0.20%~0.30%, Mn: 0.10%~0.20%, P: 0.10%~0.13%, S: 0.03%~0.05%, with the balance being Fe and unavoidable impurities.

3. The high-phosphorus and sulfur free-cutting steel according to claim 1, characterized in that, In the early stage of converter blowing, a high-oxygen lance position is used, controlled at 2.4~2.6m, and the oxygen flow rate is controlled at 3.2~3.5Nm. 3 The amount of lime and dolomite added is 3.8%~4.0% and 0.5%~0.7% of the total weight of the steel material, respectively, and is added in 2~3 times in the early and middle stages of blowing.

4. The high-phosphorus and sulfur free-cutting steel according to claim 1, characterized in that, The flow rate of bottom-blown argon gas in the converter is controlled at 0.05~0.2 Nm. 3 / (min·t), the composition of the molten steel at the end of the blowing process is: C: 0.2%~0.3%, Si: 0.01%~0.02%, Mn: 0.02%~0.06%, P: 0.04%~0.06%, S: 0.080%~0.10%, and the composition of the slag at the end of the process is: CaO: 40%~45%, SiO2: 16%~18%, MgO: 8%~10%, Al2O3: 1%~2%, MnO: 2%~3%, P2O5: 1.5%~2.5%, S: 0.3%~0.5%, FeO: 17%~19%.

5. The high-phosphorus and sulfur free-cutting steel according to claim 1, characterized in that, In LF refining, lime and silica mixed slag is added at 0.5% to 1.5% of the weight of molten steel, wherein the mass ratio of lime to silica in the mixed slag is 1:

1. 0.04% to 0.08% silicon carbide and 0.013% to 0.015% aluminum segments, respectively, are added to the slag surface for diffusion deoxidation.

6. The high-phosphorus and sulfur free-cutting steel according to claim 1, characterized in that, In LF refining, the argon flow rate at the bottom of the ladle is controlled at 210~260 L / min, and the refining time is 40~50 min. After refining, the silicon-calcium wire fed in accounts for 0.03%~0.045% of the weight of the molten steel. The composition of the molten steel removed from LF refining is C: 0.52%~0.55%, Si: 0.07%~0.12%, Mn: 0.82%~0.98%, P: 0.043%~0.063%, S: 0.08%~0.10%, Als: 0.005%~0.010%, Ca: 0.0015%~0.0030%, Mo: 0.17%~0.21%, Cr: 1.92%~1.98%, with the remainder being Fe and unavoidable impurities, and satisfying the Mn / S ratio of 8~12.

7. The high-phosphorus and sulfur free-cutting steel according to claim 1, characterized in that, During continuous casting, the superheat of molten steel is controlled at 30~40℃, the billet pulling speed is controlled at 0.90~1.1m / min, and electromagnetic stirring is used during continuous casting with a current of 400~450A and a frequency of 2.0~2.5Hz.

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