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

Abstract

The invention discloses high-phosphorus and high-sulfur free-cutting steel and a production method thereof. The high-phosphorus and high-sulfur free-cutting steel comprises the following chemical components: 0.51 to 0.56 percent of C, 0.06 to 0.13 percent of Si, 0.80 to 1.0 percent of Mn, 0.041 to 0.065 percent of P, 0.08 to 0.10 percent of S, less than or equal to 0.010 percent of Als, 0.0015 to 0.0030 percent of Ca, 0.16 to 0.22 percent of Mo, 1.90 to 1.98 percent of Cr and 8 to 12 percent of Mn / S. Before converter blowing, high-phosphorus slag and drossing iron are added into a converter, then molten iron is added, phosphorus and sulfur in molten steel at the end point of the converter meet the requirements of a finished product, and the temperature of the molten steel at the end point is 1630-1660 DEG C; the low-alkalinity slag is manufactured through LF refining, the alkalinity is controlled to be 1.0-1.2, and component control and the manganese-sulfur ratio are stable; and the temperature of LF carried-out molten steel is 1530-1540 DEG C. According to the method, ferrous sulfide and ferrophosphorus alloy are not added behind the converter, the sulfur content fluctuation caused by unstable sulfur yield in the refining process is avoided, the manganese-sulfur ratio fluctuation is reduced, cracks are eliminated, the quality of the continuous casting billet is improved, and the production cost is reduced. Meanwhile, the effective utilization of waste resources is realized.

Description

Technical Field

[0001] The invention belongs to the technical field of steelmaking and continuous casting, and particularly relates to a high-phosphorus-sulfur free-cutting steel and a production method thereof. Background Art

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

[0003] my country has been producing free-cutting steel since the 1950s. At that time, the annual output was relatively low. Although the output has increased now, with the development of the mechanical processing industry towards automation, the demand for free-cutting steel is also growing rapidly. At present, the production level, output, quality and variety of free-cutting steel in my country are very different from those of the world's advanced steel producers. Most steel mills use die casting (electric furnace + ingot casting) to produce free-cutting steel ingots (billets) or spray forming to produce free-cutting die steel. The die casting process has high energy consumption, low metal recovery rate, and high environmental pollution. It is not suitable for the current social development direction of energy conservation, emission reduction and environmental protection. In addition, sulfur segregation is a major cause of its uneven cutting performance. This segregation will not occur when the continuous casting process is used. Continuous casting solidifies quickly, so the sulfide particles are smaller than the die casting sulfide particles.

[0004] The continuous casting method for producing free-cutting steel will bring new problems such as high viscosity and poor fluidity of the molten steel. In order to ensure its castability, the pouring temperature must be increased. At the same time, free-cutting steel is a crack-sensitive steel type, which is very easy to produce surface longitudinal cracks, resulting in the scrapping of the ingot. The patent application number is CN202010958724.2, and the name is "A high-sulfur, low-aluminum free-cutting steel continuous casting slab and its production method". The chemical composition of this steel includes C, Si, Mn, P, S, Cr, Mo, Ti, Ca, and Alt. The production method is as follows: no lime slag is added for washing when the converter is tapped, the S content is stably controlled, only aluminum blocks are added for pre-deoxidation, and the amount of slag is controlled; the LF furnace produces low-basicity beige slag, and the top slag basicity is controlled to be less than 2, to ensure stable and precise control of the S content and ensure that the active oxygen content is ≤5ppm; after the VD / RH vacuum treatment is broken, a top slag modifier is added to deoxidize the slag surface, control Fe0+Mn0 in the slag to be ≤2%, and strictly control the argon flow rate; weak cooling and low-speed drawing technology is adopted during casting, and protective slag is used at the same time; through the above smelting and continuous casting process methods, the occurrence rate of longitudinal cracks on the surface of high-sulfur, low-aluminum free-cutting steel slabs is less than 0.5%, and the scrap rate caused by surface cracks on the rolled steel plate is reduced to below 0.3%. This patent uses ferrous sulfide to add during LF refining. There is no lower limit requirement for the phosphorus content and no need to add ferrophosphorus. The amount of ferrous sulfide added is large, the yield is unstable, and the manganese-sulfur ratio fluctuates greatly. Since the manganese-sulfur ratio is difficult to control, the continuous casting billets produced often crack. Since the locations of the cracks are random in the length and width directions, the subsequently rolled steel plates cannot be effectively utilized, resulting in a large amount of waste. Summary of the invention

[0005] The object of the present invention is to provide a high-phosphorus-sulfur free-cutting steel and a production method thereof. When a converter is used to produce the high-phosphorus-sulfur free-cutting steel, high-phosphorus slag and desulfurized slag iron are added into the converter to reduce the consumption of ferrophosphorus and ferrous sulfide and reduce the alloying cost. At the same time, desulfurized slag iron is used to balance the sulfur content between slag and steel during the refining process, stably control the sulfur content, reduce component fluctuations, and thus maintain a stable manganese-sulfur ratio, thereby avoiding the generation of hot crack defects in the continuous casting billet when the manganese-sulfur ratio fluctuates greatly.

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

[0007] One of the technical solutions of the present invention is to provide a high-phosphorus-sulfur free-cutting steel, the chemical composition weight percentage of the free-cutting 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%, the rest is Fe and unavoidable impurities, and satisfies Mn / S: 8 ~ 12.

[0008] The second technical solution of the present invention is to provide a production method of high-phosphorus and sulfur free-cutting steel, including converter smelting, LF refining, and continuous casting. Before converter blowing, high-phosphorus slag and slag iron are first added to the converter, and then molten iron is added to make the phosphorus and sulfur in the molten steel at the converter end meet the requirements of the finished product; the temperature of the molten steel at the converter end is controlled at 1630-1660°C, and the slag amount is controlled to be less than 5kg / t steel during the converter tapping process; entering LF refining, low-basicity slag is produced, the slag basicity is controlled at 1.0-1.2, the composition is controlled and the manganese-sulfur ratio is stable; the temperature of the molten steel discharged from the LF is controlled at 1530-1540°C.

[0009] Furthermore, the high-phosphorus slag has the following components: CaO: 29% to 34%, SiO2: 16% to 21%, MgO: 6% to 8%, Al2O3: 1% to 2%, MnO: 4% to 6%, P2O5: 4% to 6%, S: 0.03% to 0.06%, FeO: 16% to 18%, MFe: 2% to 4%, the slag particle size is 5 to 50 mm, and the added amount accounts for 0.8% to 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] Furthermore, the slag 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%, slag particle size is 5 ~ 500mm, the added amount accounts for 10% ~ 14% of the total steel material, and the slag iron is the recovery of the slag iron produced in the pretreatment process of molten iron desulfurization.

[0011] Furthermore, 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%, and the remainder is Fe and unavoidable impurities.

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

[0013] Furthermore, the argon flow rate of the converter bottom blowing is controlled at 0.05-0.2 Nm 3 / (min·t), the composition of the molten steel at the end of blowing 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 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, lime and silica mixed slag is added at 0.5% to 1.5% of the weight of the molten steel, wherein the mass ratio of lime to silica in the mixed slag is 1:1, and 0.04% to 0.08% silicon carbide and 0.013% to 0.015% aluminum segments accounting for 0.04% to 0.08% of the molten steel 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, the molten steel refining time is 40-50 min, and after the refining is completed, the fed silicon-calcium wire accounts for 0.03%-0.045% of the weight of the molten steel; the composition of the molten steel discharged 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%, and the rest is Fe and unavoidable impurities, and satisfies Mn / S: 8-12.

[0016] Furthermore, during the continuous casting process, the superheat of the molten steel is controlled at 30-40°C, the billet drawing 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 present invention has the following beneficial effects:

[0018] 1. The present invention adopts the method of adding desulfurized slag iron and dephosphorized slag in the converter to increase the sulfur and phosphorus content of molten steel. It is not necessary to add ferrous sulfide and ferrophosphorus alloy in the LF refining process to adjust the phosphorus and sulfur content, and at the same time, it realizes the effective utilization of waste resources.

[0019] 2. The present invention realizes sulfur addition to molten steel in the converter, and the sulfur content directly reaches the target value without the need for readjustment in LF refining, thereby avoiding the sulfur content fluctuation caused by the unstable sulfur recovery rate during the refining process, uniformly distributing the sulfur element in the steel, reducing the fluctuation of the manganese-sulfur ratio, eliminating cracks, improving the quality of the continuous casting billet, and reducing the production cost.

[0020] 3. The present invention utilizes high-phosphorus slag in the early stage of the converter to produce high-phosphorus and sulfur free-cutting steel, thereby achieving effective utilization of the high-phosphorus slag in the converter, reducing waste storage, and at the same time increasing the phosphorus content at the end of the converter, without the need to adjust the phosphorus content during the refining process.

[0021] 4. The present invention produces high-phosphorus-sulfur free-cutting steel, fully utilizes existing equipment and raw materials, reduces steel material consumption, is easy to operate, and does not increase additional investment. DETAILED DESCRIPTION

[0022] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. The described embodiments are only a part of the embodiments of the present invention, not all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present invention and its application or use. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0023] To further describe the present invention, the present invention is described in more detail below in conjunction with embodiments:

[0024] A high-phosphorus-sulfur free-cutting steel and a production method thereof, which utilizes the high-phosphorus slag discharged in the early stage of converter blowing and the desulfurized iron removed in the process of pretreatment desulfurization of molten iron. The high-phosphorus slag discharged in the early stage of blowing is the slag discharged when the temperature is low and the phosphorus content is high in the early stage in order to improve the dephosphorization effect of the converter when producing steel with high phosphorus content requirements. This kind of steel slag is difficult to be effectively used in the subsequent period and is usually stored. Desulfurized slag iron is a mixture of slag and iron removed in the pretreatment process. After magnetic separation, desulfurized iron is obtained. A large amount of sulfur is enriched in the desulfurized iron. When users have higher and higher requirements for the quality of steel, most of the desulfurized slag iron is stored in slag yards, making it difficult to use iron-containing resources. When the present invention produces high-phosphorus and sulfur free-cutting steel, high-phosphorus slag and desulfurized iron are fully utilized during converter smelting, the composition of molten steel is stably controlled, the utilization rate of iron resources is improved, and occupation of a large amount of land and environmental pollution are reduced. During LF refining, it is not necessary to add ferrous sulfide and ferrophosphorus, the sulfur content fluctuates little, the manganese-sulfur ratio is stably controlled, and the generation of low-melting-point ferrous sulfide during the solidification process of the continuous casting billet is prevented, thereby effectively avoiding the occurrence of cracks in the continuous casting billet.

[0025] The embodiment adopts the method of high phosphorus slag and slag iron to increase phosphorus and sulfur in steel, and the comparative example adopts the method of conventional smelting and LF refining to add ferrophosphorus and ferrous sulfide. The continuous casting billet production is 230mm thick slab.

[0026] The chemical composition and weight percentage of the high-phosphorus-sulfur free-cutting steel are: 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 the rest are Fe and unavoidable impurities, and satisfy Mn / S: 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 high-phosphorus slag comprises 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 5-50 mm, and the added amount accounts for 0.8%-1.2% of the total weight of the steel material.

[0029] 2) Utilization of desulfurized slag iron

[0030] The scrap iron produced in the pretreatment process of hot metal desulfurization is recovered. The scrap iron contains 2.9% to 3.4% C, 0.02% to 0.05% Si, 0.06% to 0.14% Mn, 0.09% to 0.13% P, and 0.70% to 0.80% S. The particle size of the slag is controlled to be 5 to 500 mm. The amount of the scrap iron added accounts for 10% to 14% of the total steel material. The addition method is to add the scrap iron into the converter together with the high-phosphorus slag through a scrap steel tank, thereby reducing the consumption of converter steelmaking flux and steel material.

[0031] 3) Converter smelting

[0032] Before the converter blowing, high phosphorus slag and slag iron are added to the converter through the scrap steel tank, and then molten iron is added. The molten iron composition 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%. In the early stage of converter blowing, the high gun position is adopted, the gun position is controlled at 2.4 ~ 2.6m, and the oxygen flow rate is controlled at 3.2 ~ 3.5Nm 3 / (min·t), silicon and manganese in the steel are oxidized in the early stage of blowing. In order to ensure the basicity of the slag and protect the furnace lining, lime and dolomite are added. 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. They are added in 2 to 3 times in the early and middle stages of blowing. Bottom blowing of argon is used in the smelting process, and the argon flow rate is controlled at 0.05 to 0.2 Nm 3 / (min·t), the composition of the molten steel at the end of blowing 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%, the composition of the final slag 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 temperature of the final molten steel is controlled at 1630~1660℃, and the slag amount is controlled to be less than 5kg / t steel during the converter tapping process.

[0033] 4) LF refining

[0034] The molten steel enters LF refining, and a mixed slag of lime and silica (mass ratio of lime: silica = 1:1) is added at 0.5% to 1.5% of the weight of the molten steel to make low-basicity slag. 0.04% to 0.08% silicon carbide and 0.013% to 0.015% aluminum wire segments accounting for 0.04% to 0.08% of the molten steel are added to the slag surface for diffusion deoxidation. The basicity of the refining slag is controlled at 1.0 to 1.2, and the argon flow rate at the bottom of the ladle is controlled at 210 to 260 L / min to promote the floating of oxide inclusions in the steel. The molten steel refining time is 40 to 50 minutes. After the refining is completed, 0.03% to 0.045% of silicon-calcium wire is fed into the molten steel, and the temperature of the molten steel removed from the LF is controlled at 1530 to 1540°C. Since phosphorus and sulfur have been adjusted to the required range of composition during converter smelting, phosphorus and sulfur between steel slags are close to balance when LF refining fine-tunes the composition. There is no sulfur content imbalance between steel slags caused by adding ferrous sulfide during LF refining, which causes large fluctuations in sulfur content, thereby affecting the manganese-sulfur ratio in steel and causing continuous casting billets to crack easily. The composition of 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%, and the rest is Fe and unavoidable impurities, and meets Mn / S: 8 ~ 12.

[0035] 5) Continuous casting

[0036] During the continuous casting process, free-cutting steel protective slag is used to protect the casting, inhibit the oxygen absorption of the molten steel, prevent the secondary oxidation of the molten steel, ensure that the tundish is clean, and the online baking time is controlled to ensure 2 hours. The covering agent is evenly added to ensure that the molten steel in the tundish is not exposed. The pulling speed is constant during the casting process to reduce liquid level fluctuations. The continuous casting adopts a weak cooling method, the superheat of the molten steel is controlled at 30-40°C, the billet drawing speed is controlled at 0.90-1.1m / min, and electromagnetic stirring is used during the continuous casting process, with a current of 400-450A and a frequency of 2.0-2.5Hz to improve the component segregation in the continuous casting billet.

[0037] The chemical compositions of the finished products of the embodiments and comparative examples are shown in Table 1.

[0038] Table 1 Chemical compositions of the embodiments 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 of high phosphorus slag in the embodiment are shown in Table 2.

[0041] Table 2 High phosphorus slag process parameters

[0042]

[0043]

[0044] The process parameters of slag iron are shown in Table 3.

[0045] Table 3 Slag 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 Addition amount ratio, % 14 13 12 11 10

[0047] The process parameters of converter smelting in the embodiment and comparative example are shown in Table 4.

[0048] Table 4 Converter smelting process parameters

[0049]

[0050] The process parameters of the converter end point of the embodiment and the comparative example are shown in Table 5.

[0051] Table 5 Converter endpoint process parameters

[0052]

[0053]

[0054] During the converter smelting process, the slag is a high-basicity oxide slag, which will remove some of the phosphorus and sulfur in the molten steel. For the comparative example, if ferrous sulfide and ferrophosphorus alloy are added to the converter, most of them will be removed and the yield is extremely low. Only by adding alloys during LF refining can a certain yield be guaranteed, and the yield is still unstable.

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

[0056] Table 6 LF refining process parameters

[0057]

[0058] The phosphorus and sulfur contents reach the required composition in the converter, and no adjustment is required 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. In the comparative example, ferrous sulfide is added to increase sulfur during LF refining. Due to the large amount of addition, the sulfur recovery rate is unstable, resulting in a high sulfur content and a low manganese-sulfur ratio.

[0059] The continuous casting process parameters of the embodiments 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 the required composition of phosphorus and sulfur 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 process 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 the composition, and the continuous casting billets produced have no crack 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 aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein by 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 and sulfur free-cutting steel, characterized in that: The chemical composition of the steel is as follows by weight: C: 0.51% to 0.56%, Si: 0.06% to 0.13%, Mn: 0.80% to 1.0%, P: 0.041% to 0.065%, S: 0.08% to 0.10%, Als ≤ 0.010%, Ca: 0.0015% to 0.0030%, Mo: 0.16% to 0.22%, Cr: 1.90% to 1.98%, and the rest is Fe and unavoidable impurities, and satisfies Mn / S: 8 to 12.

2. A method for producing high-phosphorus-sulfur free-cutting steel according to claim 1, comprising converter smelting, LF refining, and continuous casting, characterized in that: Before blowing in the converter, high-phosphorus slag and slag iron are added into the converter, and then molten iron is added to make 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 slag amount is controlled to be less than 5kg / t steel during the converter tapping process; entering LF refining, low-basicity slag is produced, the slag basicity is controlled at 1.0-1.2, the composition is controlled and the manganese-sulfur ratio is stable; the temperature of the molten steel discharged from the LF is controlled at 1530-1540℃.

3. The method for producing high phosphorus and sulfur free-cutting steel according to claim 2, characterized in that: The high-phosphorus slag comprises: 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 slag particle size is 5-50mm, and the added 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.

4. The method for producing high phosphorus and sulfur free-cutting steel according to claim 2, characterized in that: The scrap iron comprises: 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 is 5-500mm, and the added amount accounts for 10%-14% of the total steel material. The scrap iron is the recovery of the scrap iron produced in the pretreatment process of molten iron desulfurization.

5. The method for producing high-phosphorus-sulfur free-cutting steel according to claim 2, 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%, and the remainder is Fe and inevitable impurities.

6. The method for producing high phosphorus and sulfur free-cutting steel according to claim 2, characterized in that: In the early stage of converter blowing, the high oxygen lance position is controlled at 2.4-2.6m, and the oxygen flow rate is controlled at 3.2-3.5Nm 3 / (min·t), the added amounts of lime and dolomite are 3.8% to 4.0% and 0.5% to 0.7% of the total weight of the steel material respectively, and are added in 2 to 3 times in the early and middle stages of blowing.

7. The method for producing high phosphorus and sulfur free-cutting steel according to claim 2, characterized in that: The argon flow rate of the converter bottom blowing is controlled at 0.05~0.2Nm 3 / (min·t), the composition of the molten steel at the end of blowing 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 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%.

8. The method for producing high phosphorus and sulfur free-cutting steel according to claim 2, 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, and 0.04% to 0.08% silicon carbide and 0.013% to 0.015% aluminum wire segments accounting for 0.04% to 0.08% of the molten steel are added to the slag surface for diffusion deoxidation.

9. The method for producing high phosphorus and sulfur free-cutting steel according to claim 2, characterized in that: During LF refining, the argon flow rate at the bottom of the ladle is controlled at 210-260 L / min, the molten steel refining time is 40-50 min, and after the refining, the fed silicon-calcium wire 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%, and the rest is Fe and unavoidable impurities, and meets Mn / S: 8-12.

10. The method for producing high phosphorus and sulfur free-cutting steel according to claim 2, characterized in that: During the continuous casting process, the superheat of the molten steel is controlled at 30-40°C, the billet drawing speed is controlled at 0.90-1.1 m / min, and electromagnetic stirring is used during the continuous casting process, with a current of 400-450A and a frequency of 2.0-2.5 Hz.

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