Method for smelting ultra-low phosphorus steel through intermediate frequency furnace and converter duplex method

Abstract

The invention provides a method for smelting ultralow-phosphorus steel through an intermediate frequency furnace and converter duplex method, and relates to the technical field of steel smelting. The intermediate frequency furnace is adopted for smelting low-phosphorus waste steel, and pre-dephosphorized slag is made during steel pouring; before ultra-low phosphorus steel is smelted in a converter, slag changing operation is carried out in the molten iron and scrap steel mode mixed smelting process in advance, the P2O5 content and alkalinity in slag are controlled, and part of slag is reserved during slag pouring; the method comprises the following steps: smelting in an intermediate frequency furnace to obtain low-phosphorus and low-sulfur molten steel, controlling the components and temperature of the molten steel in a steel ladle before entering a converter, adopting short-time and high-flow oxygen blowing, controlling the alkalinity of final slag, turning down a furnace to discharge slag before tapping, and adding a low-phosphorus alloy in the tapping process; according to the method, smelting is continuously conducted in the single converter, and the low-phosphorus molten steel with the P content smaller than or equal to 0.003% and the extremely low harmful element content can be stably obtained in batches. The quality of raw materials is reliable and easy to control, the converter blowing time is short, the metal yield is high, and the carbon emission is only 1% of the carbon emission of molten iron-converter; and large-scale production of the ultra-pure and ultra-low phosphorus steel is facilitated.

Description

Technical Field

[0001] The present invention relates to the technical field of steel smelting, and particularly to a method for smelting ultra-low phosphorus steel by a combined process of medium-frequency furnace and converter. Background Art

[0002] Phosphorus is one of the main harmful elements to be removed in the steelmaking process. The main harm of phosphorus is that it affects the low-temperature impact performance of steel and is prone to form "cold brittleness". With the increasing demand for high-clean steel in the market, especially in the fields of pressure vessel steel and hydroelectric steel, the requirements for the phosphorus content in steel are more stringent. The required finished phosphorus content is controlled below 0.004%, and generally below 0.003%. Conventional converter smelting processes are greatly affected by the quality conditions of hot metal and scrap. Coupled with the limited dephosphorization efficiency of converters, they cannot meet the finished product quality requirements, or have low smelting efficiency and high production costs, making it difficult to achieve mass production. For example, the Chinese patent with the application number 201711158046.6 discloses a production method for ultra-low phosphorus hydrogen-resistant steel. Its production process flow is cumbersome and requires two LF refinings. This process is limited to small-batch production, has certain limitations, and the converter smelting cycle of this process is long, which is not conducive to the control of the production rhythm of subsequent processes. The medium-frequency furnace has a small equipment investment and low smelting cost, but it has no metallurgical function and cannot be directly used for steel smelting.

[0003] In view of this, the present invention is specifically proposed. Summary of the Invention

[0004] One of the purposes of the present invention is to provide a method for smelting ultra-low phosphorus steel by a combined process of medium-frequency furnace and converter, so as to solve at least one of the technical problems existing in the prior art.

[0005] In order to achieve the above object of the present invention, the following technical solutions are specifically adopted:

[0006] The present invention provides a method for smelting ultra-low phosphorus steel by a combined process of medium-frequency furnace and converter, including the following steps:

[0007] A. When smelting ultra-low phosphorus steel by a combined process of medium-frequency furnace and converter, slag replacement of the converter is carried out in advance: In the converter, hot metal and scrap are mixed and smelted according to a mass ratio of 4:1 to 6:1, and slag replacement is carried out. The content of P2O5 in the slag after slag replacement is <0.5%, and the basicity is controlled at 3 to 5;

[0008] B. Put clean scrap steel as raw material into the medium-frequency furnace for melting to obtain preliminarily melted clean molten steel with low phosphorus and sulfur. When the amount of molten steel melted from scrap steel in the medium-frequency furnace reaches 70% - 80% of the furnace capacity, detect the composition of the molten steel. According to the composition of the molten steel, adjust the silicon content and carbon content in the molten steel so that the carbon content at the end of the medium-frequency furnace or when tapping into the ladle is 0.4% - 0.6%, and the silicon content is 0.2% - 0.40%. Then continue to add scrap steel to make the amount of melted molten steel reach the furnace capacity until the tapping temperature is 1570 - 1620 °C;

[0009] The method for adjusting the silicon content and carbon content in the molten steel includes: if Si≥0.4% and C<0.4% in the molten steel, first add iron oxide scale for desiliconization operation. When the silicon content reaches 0.4% - 0.6%, then add carbon powder to the molten steel for carbon increase in the medium-frequency furnace, or add carbon powder during the process of tapping from the medium-frequency furnace into the ladle; if 0.2≤Si<0.4% and C<0.4%, directly add carbon powder with the steel flow in the medium-frequency furnace or during the process of pouring steel from the medium-frequency furnace to increase the carbon content of the molten steel; if Si<0.2% and C<0.4%, add ferrosilicon alloy and carbon powder with the steel flow in the medium-frequency furnace or during the process of pouring steel from the medium-frequency furnace to increase the silicon and carbon content of the molten steel;

[0010] The P of the clean scrap steel ≤0.03%, and the S of the clean scrap steel ≤0.02%;

[0011] C. Make pre-dephosphorization slag when pouring steel from the medium-frequency furnace: When tapping from the medium-frequency furnace into the ladle, add active lime and light-burned dolomite with the steel flow, continuously blow argon during the pouring process, and continue to blow argon into the ladle for 3 - 5 minutes after tapping. Control the slag composition: CaO: 40 - 50%, MgO: 5 - 10%, SiO2: 10 - 20%, basicity 2.0 - 4.0. Add carbonized rice husk for heat preservation according to the amount of 0.5 - 1 Kg / t, and then transfer the ladle to the converter;

[0012] D. Converter blowing: Control the temperature of the molten steel in the ladle before entering the converter between 1530 - 1580 °C. Start blowing after pouring into the converter, and then add active lime and light-burned dolomite in batches. Control the final slag basicity at 3.0 - 5.0, and the P2O5 content in the slag is less than 0.15%; During the smelting process, control the lance position of the oxygen lance between 1.0 - 1.5 meters below the liquid surface. For a 100-ton converter, control the oxygen flow rate between 18000 - 20000 m 3 / h, the total blowing time is 3 - 5 minutes, and the tapping temperature at the end of the converter is 1560 - 1610 °C, C≤0.06%, P≤0.003%;

[0013] E. Before tapping, first tilt the furnace to remove slag, and the slag removal amount ≥40% of the total slag amount. Then tilt the furnace to tap. When the tapping reaches 60 - 75%, add a slag stopper or use a slide plate to block the slag. Add low-phosphorus ferromanganese alloy during the tapping process, and P≤0.003% in the molten steel after tapping;

[0014] F. Repeat steps B to E for continuous smelting in the same converter, stably control P2O5 in the slag of the converter to be <0.15%, and P at the end of tapping from the converter to be ≤0.003% to obtain low-phosphorus molten steel.

[0015] Further, step A includes: adding 15 - 25 Kg / t of active lime and 5 - 10 Kg / t of calcined dolomite in the first stage. When blowing for 3 - 5 minutes, control the temperature of the molten steel to be 1340 - 1400 °C, carry out slag tapping operation, and the slag tapping amount is 40 - 60% to discharge the high-phosphorus slag; adding 10 - 20 Kg / t of active lime and 5 - 10 Kg / t of calcined dolomite in the second stage, and at the same time adding sinter, mill scale or iron oxide. The end temperature of the converter is 1560 - 1580 °C, and control the slag composition: CaO: 50 - 60%, MgO: 5 - 10%, SiO2: 10 - 15%; carry out secondary slag tapping before tapping, and the slag tapping amount is 50% - 70%. Carry out slag splashing operation after tapping is completed;

[0016] Smelt 1 - 2 furnaces according to the above method.

[0017] Further, the tapping temperature in step B is between 1580 - 1610 °C;

[0018] In step D, control the temperature of the molten steel in the ladle before entering the converter to be 1540 - 1570 °C, and the tapping temperature at the end of the converter is 1560 - 1580 °C.

[0019] Further, control the temperature of the molten steel in the first stage to be 1360 - 1380 °C.

[0020] Further, the addition amount of the active lime in step C is 5 - 10 Kg / t, and the addition amount of the calcined dolomite is 3 - 5 Kg / t.

[0021] Further, the adding of the active lime and the calcined dolomite in batches in step D includes: before the molten steel is poured into the converter, first pre-add 1 / 3 - 1 / 2 of the addition amount of the active lime and 1 / 3 - 1 / 2 of the addition amount of the calcined dolomite into the converter, then pour in the molten steel from the intermediate frequency furnace, and add the remaining active lime and calcined dolomite in batches during the blowing process of the converter;

[0022] The addition amount of the active lime in step D is 20 - 25 kg / t, and the addition amount of the calcined dolomite is 8 - 12 kg / t.

[0023] Further, in step D, if the end temperature of the converter > 1580 °C, then add temperature-reducing materials for adjustment;

[0024] The temperature-reducing materials include at least one of slag materials, sinter or iron pellets.

[0025] Further, in step D, if the converter end point P > 0.003% or C > 0.06%, it is necessary to blow again for 30 - 90 s.

[0026] Further, the P content of the low - phosphorus ferromanganese alloy described in step E is < 0.030%.

[0027] Further, it further includes step G. According to the alloy content and carbon content of the steel grade to be smelted, at least one of the operations of deoxidizing molten steel, alloying, removing inclusions or removing harmful gases is performed on the low - phosphorus molten steel in the LF furnace and the VD furnace.

[0028] The method for smelting ultra - low - phosphorus steel by the combined process of intermediate - frequency furnace and converter provided by the present invention has reliable raw material quality and is easy to control, which is beneficial to providing high - quality primary molten steel for the converter. The converter blowing time is short, the blowing loss of Fe element in the molten steel is extremely small, and the metal yield is high. Moreover, the additional carbon element content in the whole smelting process is only about 2 Kg / t, and the oxidation decarburization amount in the blowing process is only about 5 Kg / t, which is much lower than the 10 - 20 Kg / t oxidation decarburization amount of the electric arc furnace and much lower than the 700 - 900 Kg / t oxidation decarburization amount level of the long - process. It has important significance for environmental protection and carbon emission reduction. The overall process is short, the dephosphorization efficiency is high, the smelting cost is low, the production rhythm is fast, and it is beneficial to large - scale production of ultra - pure and ultra - low - phosphorus steel. Detailed implementation mode

[0029] Unless otherwise defined herein, scientific and technical terms used in conjunction with the present invention shall have the meanings commonly understood by those of ordinary skill in the art. The meanings and scopes of the terms should be clear. However, in any case of potential ambiguity, the definitions provided herein shall prevail over any dictionary or extrinsic definition. In this application, unless otherwise specified, the use of "or" means "and / or". In addition, the use of the term "comprising" and other forms is non - restrictive.

[0030] The technical solutions of the present invention will be described clearly and completely below in conjunction with the embodiments. Obviously, the described embodiments are some but not all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0031] The present invention provides a method for smelting ultra - low - phosphorus steel by the combined process of intermediate - frequency furnace and converter, including the following steps:

[0032] A. When smelting ultra-low phosphorus steel by duplex smelting in an intermediate frequency furnace and a converter, slag replacement in the converter is carried out in advance: In the converter, molten iron and scrap steel are mixed and smelted according to a mass ratio of 4:1 to 6:1, slag replacement is carried out, and the content of P2O5 in the slag after slag replacement is <0.5%, and the basicity is controlled at 3 to 5; Through the in-furnace slag replacement operation, the slag inside the converter and the splashing slag layer on the surface of the furnace lining are all replaced with slag with a low phosphorus content to ensure better dephosphorization ability when smelting ultra-low phosphorus steel. The preferred mass ratio of molten iron to scrap steel is 4:1.

[0033] B. Clean scrap steel is put into the intermediate frequency furnace as raw material for melting to obtain preliminarily smelted low-phosphorus and low-sulfur clean molten steel. When the amount of molten steel melted from scrap steel in the intermediate frequency furnace reaches 70% - 80% of the furnace capacity, the composition of the molten steel is detected. According to the composition of the molten steel, the silicon content and carbon content in the molten steel are adjusted so that the carbon content at the end of the intermediate frequency furnace or when tapping into the ladle is 0.4% - 0.6%, and the silicon content is 0.2% - 0.40%. Then, scrap steel is continuously added to make the amount of molten steel melted reach the furnace capacity until the tapping temperature is 1570 - 1620 °C;

[0034] The method for adjusting the silicon content and carbon content in the molten steel includes: If Si ≥ 0.4% and C < 0.4% in the molten steel, then ferric oxide scale is first added for desiliconization operation. At this time, the molten steel temperature is relatively low, and it is relatively easy to desiliconize with ferric oxide scale at a lower molten steel temperature. By timely discharging the slag, the slag with a higher silicon content is discharged. When the silicon content reaches 0.4% - 0.6% to meet the requirements, then carbon powder is added to the molten steel in the intermediate frequency furnace for carbon increase, or carbon powder is added during the process of tapping from the intermediate frequency furnace to the ladle; If 0.2 ≤ Si < 0.4% and C < 0.4%, then according to the C and Si contents in the molten steel, carbon powder is directly added into the intermediate frequency furnace or along with the steel flow during the process of pouring steel from the intermediate frequency furnace to increase the carbon content of the molten steel; If Si < 0.2% and C < 0.4%, then ferrosilicon alloy and carbon powder are added along with the steel flow in the intermediate frequency furnace or during the process of pouring steel from the intermediate frequency furnace to increase the silicon and carbon contents of the molten steel;

[0035] The P ≤ 0.03% and S ≤ 0.02% of the clean scrap steel.

[0036] C. When pouring steel from the intermediate frequency furnace, a pre-dephosphorization slag is made: When pouring steel from the intermediate frequency furnace into the ladle, quicklime and calcined dolomite are added along with the steel flow, and argon is continuously blown during the pouring process. After the pouring is completed, argon is continuously blown into the ladle for 3 - 5 minutes. The composition of the slag is controlled as CaO: 40 - 50%, MgO: 5 - 10%, SiO2: 10 - 20%, and the basicity is 2.0 - 4.0. Carbide rice husk is added at a rate of 0.5 - 1 Kg / t for heat preservation, and then the ladle is transported towards the converter.

[0037] D. Converter blowing: Control the molten steel temperature in the ladle before entering the converter between 1530 and 1580 °C. Start blowing after pouring into the converter, and then add active lime and lightly burned dolomite in batches. Control the final slag basicity to be 3.0 - 5.0, and the P2O5 content in the slag is less than 0.15%. During the smelting process, control the lance position of the oxygen lance between 1.0 and 1.5 meters below the liquid surface. For a 100-ton converter, control the oxygen flow rate between 18000 and 20000 m 3 / h. The total blowing time is 3 - 5 minutes, and the tapping temperature at the end of the converter is 1560 - 1610 °C, C ≤ 0.06%, P ≤ 0.003%. Among them, for converters with other tonnages different from 100 tons, the oxygen flow rate can be adjusted proportionally with reference to the 100-ton converter.

[0038] E. Before tapping, tilt the furnace to remove slag first, and the slag removal amount ≥ 40% of the total slag amount. Removing slag in advance can prevent rephosphorization. Then tilt the furnace to tap steel. When the tapping reaches 60 - 75%, add a slag stopper or use a slide plate to block the slag. Add low-phosphorus ferromanganese alloy during the tapping process. After the tapping is completed, P ≤ 0.003% in the molten steel. Weigh to measure the metal yield, which is 97% - 98.5%.

[0039] F. Repeat steps B - E to continuously smelt in the same converter. Because the phosphorus content in the slag in step A is relatively low, and at the same time the phosphorus content of the clean scrap used in step B is very low, far lower than the phosphorus content level of about 0.1% in hot metal, it can ensure that the phosphorus content in the slag of the continuously smelted converter remains continuously stable at a low level, stably control P2O5 < 0.15% in the slag of the converter, control the composition and basicity of the slag according to step D. At this time, the slag has extremely high phosphorus dissolution ability, and P ≤ 0.003% at the end of tapping from the converter, obtaining low-phosphorus molten steel.

[0040] The method provided by the present invention has relatively low C, Si, and Mn contents in the raw material scrap, and the composition can be adjusted and controlled. At the same time, the P content is low, and it is easier to control the P content to a lower level; by adopting the method of changing the slag in the converter in advance and continuously smelting low-phosphorus molten steel in a single converter, the phosphorus content in the slag can be controlled at an extremely low level, which provides a good foundation for smelting ultra-low phosphorus steel; through methods such as slag component, smelting temperature, and slag blocking control, the content of low-melting-point elements such as Zn and Pb in the scrap can be reduced to a trace level (<0.001%), and ultra-pure and ultra-low phosphorus steel can be stably produced in batches with obvious quality control advantages; for a single smelting, the total blowing time of the converter is only 3 - 5 minutes, only about 20% of the normal blowing time, the blowing loss of Fe element in the molten steel is extremely small, and the metal yield is as high as 97 - 98%, far higher than the metal yield level of 90 - 91% in the conventional smelting of the converter;

[0041] Using this smelting method, the raw material quality is reliable and easy to control, which is beneficial to providing high-quality primary molten steel for the converter. The converter blowing time is short, the blowing loss of Fe element in the molten steel is extremely small, and the metal yield is high. Moreover, the content of additional carbon element added during the whole smelting process is only about 2 Kg / t, and the decarburization amount during the blowing process is only about 5 Kg / t, far lower than the 10 - 20 Kg / t decarburization amount of the electric arc furnace and much lower than the 700 - 900 Kg / t decarburization amount level of the long process, which is of great significance to environmental protection and carbon emission reduction. The overall process is short, the dephosphorization efficiency is high, the smelting cost is low, the production rhythm is fast, which is beneficial to large-scale production of ultra-pure and ultra-low phosphorus steel.

[0042] Among them, the carbon content at the end of the intermediate frequency furnace or when tapping into the ladle can be, but is not limited to, 0.4%, 0.42%, 0.44%, 0.46%, 0.48%, 0.50%, 0.52%, 0.54%, 0.56%, 0.58% or 0.6%, and can also be any value between 0.4% and 0.6%.

[0043] The silicon content at the end of the intermediate frequency furnace or when tapping into the ladle can be, but is not limited to, 0.2%, 0.22%, 0.24%, 0.26%, 0.28%, 0.30%, 0.32%, 0.34%, 0.36%, 0.38% or 0.4%, and can also be any value between 0.2% and 0.40%.

[0044] The tapping temperature described in step B can be, but is not limited to, 1570°C, 1580°C, 1590°C, 1600°C, 1610°C or 1620°C, and can also be any value between 1570 and 1620°C, preferably 1580 - 1610°C.

[0045] The time for continuing to blow argon into the ladle after tapping can be, but is not limited to, 3 min, 3.2 min, 3.4 min, 3.6 min, 3.8 min, 4 min, 4.2 min, 4.4 min, 4.6 min, 4.8 min or 5 min, and can also be any value between 3 and 5 min.

[0046] The temperature of the molten steel in the ladle before entering the converter can be, but is not limited to, 1530°C, 1540°C, 1550°C, 1560°C, 1570°C or 1580°C, and can also be any value between 1530 and 1580°C, preferably 1540 - 1570°C.

[0047] The tapping temperature at the end of the converter can be, but is not limited to, 1560°C, 1570°C, 1580°C, 1590°C, 1600°C or 1610°C, and can also be any value between 1560 and 1610°C, preferably 1560 - 1580°C.

[0048] In some specific embodiments, step A includes: adding 15 - 25 Kg / t of active lime and 5 - 10 Kg / t of lightly burned dolomite in the first stage. When blowing for 3 - 5 minutes, control the molten steel temperature at 1340 - 1400 °C, perform slag tapping operation, with the slag tapping amount being 40 - 60%, and discharge the high-phosphorus slag; adding 10 - 20 Kg / t of active lime and 5 - 10 Kg / t of lightly burned dolomite in the second stage, and simultaneously adding sinter, scale or iron oxide. The converter end temperature is 1560 - 1580 °C, and control the slag composition: CaO: 50 - 60%, MgO: 5 - 10%, SiO2: 10 - 15%; perform secondary slag tapping before tapping, with the slag tapping amount being 50% - 70%, and perform slag splashing operation after tapping is completed;

[0049] Smelt 1 - 2 furnaces according to the above method.

[0050] Among them, in the first stage, the controlled molten steel temperature can be, but is not limited to, 1340 °C, 1350 °C, 1360 °C, 1370 °C, 1380 °C, 1390 °C or 1400 °C, and can also be any value between 1340 - 1400 °C, preferably 1360 - 1380 °C.

[0051] It should be noted that the total content of elements C, Si, Mn, P, and S in the primary molten steel after smelting in the medium-frequency furnace is low, and it is quickly oxidized and finished during the converter blowing process. As the main component, Fe undergoes a chemical reaction with O2, and it is easier to obtain a higher FeO content. And the density of P2O5 generated after P oxidation is small and is almost insoluble in the molten steel. Once generated, it floats up and transfers into the slag phase. The P2O5 entering the slag combines with FeO to form iron phosphate, but the iron phosphate is unstable and will decompose as the molten pool temperature rises, causing the phosphorus to return to the molten steel. Therefore, the slag is designed with a higher alkalinity, increasing the content of the strong basic oxide CaO in the slag, and then obtaining stable calcium phosphate.

[0052] The dephosphorization reaction is a strongly exothermic reaction. Lowering the temperature is beneficial to dephosphorization, while raising the temperature will reduce the dephosphorization efficiency. However, a higher temperature is beneficial to the decrease in slag viscosity, accelerating the slag formation speed of lime and the diffusion speed of each component in the slag, and strengthening the transfer of phosphorus from the metal liquid to the slag. Therefore, a lower initial blowing temperature and a tapping end temperature below 1580 °C can ensure the maximization of dephosphorization efficiency.

[0053] In some specific embodiments, the addition amount of the active lime in step C is 5 - 10 Kg / t, and the addition amount of the lightly burned dolomite is 3 - 5 Kg / t.

[0054] In some specific embodiments, the step of adding active lime and lightly burned dolomite in batches in step D includes: before pouring molten steel into the converter, first adding 1 / 3 to 1 / 2 of the addition amount of active lime and 1 / 3 to 1 / 2 of the addition amount of lightly burned dolomite into the converter, then pouring the molten steel from the intermediate frequency furnace. During the blowing process of the converter, the remaining active lime and lightly burned dolomite are added in batches; in some specific embodiments, the addition amount of the active lime in step D is 20 - 25 kg / t, and the addition amount of the lightly burned dolomite is 8 - 12 kg / t.

[0055] The Si content in the molten steel poured into the converter is about 0.30%, and coupled with the relatively small amount of slag reserved in the furnace, compared with the conventional smelting method, a small amount of active lime and lightly burned dolomite can produce a slag with high alkalinity and low P2O5 content.

[0056] In some specific embodiments, in step D, if the converter end temperature > 1580 °C, cooling materials are added for adjustment; in some specific embodiments, the cooling materials include at least one of slag materials, sintered ore, or iron pellets.

[0057] In some specific embodiments, in step D, if the converter end P > 0.003% or C > 0.06%, it is necessary to blow again for 30 - 90 s.

[0058] In some specific embodiments, the P content of the low - phosphorus ferromanganese alloy in step E < 0.030%.

[0059] In some specific embodiments, it further includes step G, and according to the alloy content and carbon content of the steel grade to be smelted, at least one operation of deoxidizing the low - phosphorus molten steel, alloying, removing inclusions, or removing harmful gases is carried out in the LF furnace and the VD furnace.

[0060] The present invention will be further described below through examples. Unless otherwise specified, the materials in the examples are prepared according to existing methods or directly purchased from the market.

[0061] Example 1: Smelting steel grade 07MnNiMoDR

[0062] The required composition of the steel grade is: C: 0.06 - 0.08%, Si: 0.20 - 0.35%, Mn: 1.40 - 1.45%, Ni: 0.4 - 0.5%, Mo: 0.2 - 0.3%, P: ≤0.007% (target P content, ≤0.003%), S: ≤0.003%. Other alloy components are omitted, and the rest are Fe and trace elements.

[0063] The specific operation is carried out according to the following method:

[0064] 1) Converter slag change before smelting ultra-low phosphorus steel: Before smelting 07MnNiMoDR in the converter, the "hot metal + scrap steel" mode is adopted for smelting one furnace in advance for slag change operation. 81 tons of hot metal (temperature: 1342°C, Si: 0.35%, P: 0.105%) is charged, and 19 tons of recycled scrap is added. During the converter slag change, a large slag volume operation is carried out: In the first stage, 2300 Kg of active lime and 800 Kg of lightly burned dolomite are added. When blowing for 3 minutes and 35 seconds, the molten steel temperature is measured at 1476°C, and slag pouring operation is carried out, and the slag pouring volume is about 50% to discharge the high-phosphorus slag. In the second stage, slag materials are re-added for secondary slag making, 2100 Kg of active lime and 700 Kg of lightly burned dolomite, and at the same time 800 Kg of sinter is added to promote slag melting. The converter end temperature is 1570°C, and secondary slag pouring is carried out before tapping, with a slag pouring of about 50%. After tapping, slag splashing operation is carried out.

[0065] The slag composition is detected as CaO: 58%, MgO: 7.2%, SiO2: 12.7%, P2O5: 0.42%; the basicity is 4.5.

[0066] 2) Scrap steel smelting in intermediate frequency furnace: Prepare 98 tons of clean and oil-free recycled cut heads and cut edges from rolled steel. Randomly select three pieces of surplus materials and detect the composition by direct reading spectrometer. The average content is C: 0.161%, Si: 0.27%, Mn: 0.86%, P: 0.019%, S: 0.005%, Al: 0.025%.

[0067] Intermediate frequency furnace smelting: Use a 100-ton intermediate frequency furnace for simultaneous smelting. The temperature of the molten steel ladle at the start of production is 970°C. When 85% of the melting is completed, the molten steel composition is detected, C: 0.16%, Si: 0.15%, the molten steel temperature is measured at 1560°C. 320 kg of carburizer and 150 Kg of ferrosilicon are added to the intermediate frequency furnace for carburizing and silicon increasing operations for the molten steel. The tapping temperature at the end of smelting is 1590°C.

[0068] 3) Tapping from intermediate frequency furnace: During tapping, 1275 kg of active lime and 780 kg of lightly burned dolomite are cumulatively added along with the molten steel flow. Argon blowing operation is carried out during the steel pouring process, and argon blowing continues into the ladle for 4 minutes after the steel pouring ends. The argon blowing flow rate is 350 Nl / min, and the argon pressure is 0.6 Mpa; at this time, the molten steel temperature is 1551°C. Samples of the molten steel in the ladle are taken for composition detection, C: 0.46%, Si: 0.27%, Mn: 0.75%, P: 0.018%, S: 0.004%. Then 80 kg of carbonized rice husk is added for heat preservation, and the molten steel ladle is lifted to the front of the converter by the overhead crane. Samples of the slag are taken for detection, where CaO: 46.3%, MgO: 7.92%, SiO2: 16.4%, and the basicity is 2.83.

[0069] 4) Converter smelting: The molten steel temperature measured when lifted to the converter is 1548 °C. After the molten steel is poured into the converter, oxygen blowing starts. During the converter smelting process, 1350 kg of active lime and 570 kg of light-burned dolomite are added in batches. The lance position during the process is controlled at 1.1 - 1.2 meters, and the oxygen flow rate is controlled at 18000 - 20000 m 3 / h. The oxygen supply time is 3 minutes and 11 seconds. When smelting reaches the end point, the sublance is used to measure the temperature and take samples to detect the P content of 0.002% and C content of 0.041% in the molten steel, and the measured temperature is 1574 degrees; C, P, and temperature meet the tapping requirements. Final slag detection of the converter slag: CaO: 55%, SiO2: 13%, P2O5: 0.14%, MgO: 9.2%, FeO: 19%, basicity 4.2.

[0070] 5) Before tapping, about 60% of the slag is poured out first. During the tapping process, 1340 kg of ferromanganese and 280 kg of ferrosilicon are added. When the tapping reaches 60 - 75%, a slag stopper is added, and the tapping ends. The final molten steel in the converter is weighed by the ladle car and shows 96.8 tons, and the overall metal yield is 97.25%.

[0071] After alloying in the converter, samples are taken to detect the composition: C: 0.05%, Si: 0.27%, Mn: 1.34%, P: 0.003%, S: 0.006%.

[0072] 6) Steps 2) - 5) are continuously smelted for 10 furnaces to stably obtain low-phosphorus molten steel with a lower P content (0.001 - 0.003%). The basic composition and key data of the molten steel are shown in Table 1.

[0073] 7) LF refining: After the low-phosphorus molten steel arrives at LF refining, deoxidation, alloying, and inclusion removal operations are carried out. 110 Kg of ferromanganese, 5 Kg of carbon powder, 460 Kg of nickel plate, and 410 Kg of ferromolybdenum are added; 860 Kg of lime and 280 Kg of fluorite are added in 2 batches; argon blowing in the ladle is 21 minutes.

[0074] Finished product composition detection: C: 0.07%, Si: 0.25%, Mn: 1.43%, P: 0.002%, Ni: 0.45%, Mo: 0.25%, S: 0.001%, trace elements: Sn: 0.002, As: 0.002, Sb: 0.0001, Pb: 0.0002, Zn: 0.001.

[0075] Table 1

[0076]

[0077]

[0078] Example 2 Steel grade 15CrMoR(H)

[0079] Steel grade required composition, C: 0.13 - 0.15%, Si: 0.20 - 0.35%, Mn: 0.45 - 0.55%, Cr: 0.95 - 1.05%, Mo: 0.4 - 0.5%, P: ≤0.003% (control target P content, ≤0.002%), S: ≤0.003%, the rest of the alloying elements are omitted.

[0080] The specific operation is carried out according to the following method:

[0081] 1) Converter slag-changing operation before smelting ultra-low phosphorus steel: Before smelting 15CrMoR(H) in the converter, the "hot metal + scrap" mode is adopted for smelting 2 furnaces in advance for slag-changing operation; when smelting the first furnace, 82 tons of hot metal (temperature: 1345°C, Si: 0.37%, P: 0.101%) are charged, 19 tons of recycled scrap are added, and a large slag volume operation is adopted during slag-making: in the first stage, 2200 Kg of quicklime and 850 Kg of light-burned dolomite are added. When the molten steel temperature is measured at 1470°C after blowing for 3 minutes and 50 seconds, the slag-dumping operation is carried out, and the slag-dumping volume is about 50% to discharge the high-phosphorus slag; in the second stage, the slag materials are re-added for secondary slag-making, 2200 Kg of quicklime, 800 Kg of light-burned dolomite, and 900 Kg of sinter are added simultaneously to promote slag melting. The converter end temperature is 1579°C, and the secondary slag-dumping is carried out before tapping, with a slag-dumping of about 50%. The slag splashing operation is carried out after tapping ends;

[0082] During the second slag-changing of the converter, 80 tons of hot metal (temperature: 1340°C, Si: 0.39%, P: 0.107%) are charged, 21 tons of recycled scrap are added, and a large slag volume operation is adopted: in the first stage, 2250 Kg of quicklime and 880 Kg of light-burned dolomite are added. When the molten steel temperature is measured at 1480°C after blowing for 4 minutes and 10 seconds, the slag-dumping operation is carried out, and the slag-dumping volume is about 60% to discharge the high-phosphorus slag; in the second stage, the slag materials are re-added for secondary slag-making, 2150 Kg of quicklime, 850 Kg of light-burned dolomite, and 950 Kg of sinter are added simultaneously to promote slag melting. The converter end temperature is 1575°C, and the secondary slag-dumping is carried out before tapping, with a slag-dumping of about 60%. The slag splashing operation is carried out after tapping ends;

[0083] The furnace slag composition is detected as CaO: 57%, MgO: 7.3%, SiO2: 12.6%, P2O5: 0.35%; the basicity is 4.5.

[0084] 2) Scrap steel smelting in the intermediate frequency furnace: Prepare 97 tons of clean and oil-free scrap steel leftovers. Randomly select three leftovers and detect the composition by a direct-reading spectrometer. The average content is C: 0.172%, Si: 0.47%, Mn: 0.91%, P: 0.017%, S: 0.006%.

[0085] Medium-frequency induction furnace smelting: Using a 100-ton medium-frequency induction furnace for simultaneous smelting. The temperature of the molten steel ladle at the start of charging is 980°C. When 80% of the melting is completed, the composition of the molten steel is detected, with C: 0.16%, Si: 0.45%. 100 kg of iron oxide scale is added to the molten steel for desiliconization operation; the tapping temperature at the end of smelting is controlled at 1605°C.

[0086] 3) Tapping from the medium-frequency induction furnace: During tapping, 1260 kg of active lime, 770 kg of lightly burned dolomite, and 300 kg of carburizer are added cumulatively along with the steel flow. Argon blowing is carried out during the steel pouring process. After the steel pouring is completed, argon is continuously blown into the ladle for 5 minutes at an argon flow rate of 320 Nl / min and an argon pressure of 0.6 Mpa; at this time, the temperature of the molten steel is detected to be 1555°C. Samples of the molten steel in the ladle are taken for composition detection, with C: 0.457%, Si: 0.353%, Mn: 0.75%, P: 0.018%, S: 0.004%. Then, 80 kg of carbonized rice husk is added for heat preservation, and the molten steel ladle is lifted to the converter by the overhead crane. Samples of the slag are taken for detection, with CaO: 44.8%, MgO: 7.15%, SiO2: 14.7%, and basicity 3.05.

[0087] 4) Converter smelting: The temperature of the molten steel lifted to the converter is approximately 1562°C. After the molten steel is charged into the converter, oxygen blowing for smelting starts. During the converter smelting process, 1410 kg of active lime and 610 kg of lightly burned dolomite are added cumulatively in batches. The lance position during the process is controlled at 1.1 - 1.2 meters, and the oxygen flow rate is controlled at 18000 - 20000 m 3 / h, and the oxygen supply time is 3 minutes and 15 seconds. The P content in the molten steel is detected to be 0.003% and the C content is 0.049% by using the sublance for temperature measurement and sampling. The measured temperature is 1577°C; the C, P, and temperature meet the tapping requirements. Final slag detection of the converter slag: CaO: 54%, SiO2: 13%, P2O5: 0.13%, MgO: 8.5%, FeO: 18.6%, and basicity 4.2.

[0088] 5) Before tapping, 2 / 3 of the slag is first poured out. During tapping, 390 kg of ferromanganese and 280 kg of ferrosilicon are added. When tapping reaches 60 - 75%, a slag stopper is added. After tapping is completed, samples are taken from the ladle to detect the Mn content of 0.41% and the P content of 0.002%. The smelting cycle of the converter is 15 minutes. Finally, the molten steel is weighed by the steel ladle car, showing 95.2 tons, and the overall metal yield is 97.5%.

[0089] After alloying in the converter, samples are taken for composition detection, with C: 0.08%, Si: 0.23%, Mn: 0.41%, P: 0.003%, S: 0.007%.

[0090] 6) Steps 2) - 5) are continuously carried out for 5 furnaces to stably obtain low-phosphorus molten steel with a lower P content (0.001 - 0.003%). The basic composition and key data of the molten steel are shown in Table 2.

[0091] 7) LF refining: After the molten steel arrives at LF for refining, deoxidation, alloying, and inclusion removal operations are carried out. 70 Kg of metallic manganese, 50 Kg of carbon powder, 1530 Kg of low-carbon ferrochrome, and 800 Kg of ferromolybdenum are added; 850 Kg of lime and 270 Kg of fluorite are added in two batches; argon blowing in the ladle is carried out for 20 min.

[0092] Finished product composition detection: C: 0.13%, Si: 0.30%, Mn: 0.46%, P: 0.002%, Cr: 0.96%, Mo: 0.47%, S: 0.001%, other components are omitted, trace elements: Sn: 0.001, As: 0.002, Sb: 0.0001, Pb: 0.0002, Zn: 0.001.

[0093] Table 2

[0094]

[0095] Comparative Example 1

[0096] The difference from Example 2 is that the steel grade 15CrMoR(H) is smelted according to the production method of ultra-low phosphorus hydrogen-resistant steel disclosed in the Chinese patent with the application number 201711158046.6.

[0097] The specific operation is carried out according to the following method:

[0098] Hot metal pretreatment: After the hot metal is desulfurized by the KR method, the temperature is 1350 °C, the S content is 0.010%, and the P content is 0.12%; during this process, the initial S content of the hot metal is 0.039%, the P content is 0.12%, and the C content is 4.7%; the rotation speed of the stirring head is 80 r / min, and the addition amount of desulfurizer calcium oxide is 450 kg;

[0099] Converter smelting: The converter charge is 105 tons, including 80 tons of hot metal and 25 tons of scrap steel. During smelting, the oxygen content is controlled at 1000 ppm. The lance position in the early stage of blowing is controlled at 1.3 m, and in the middle and late stages, it is controlled between 1.0 m. The temperature at the first pour is 1550 °C. The spot blowing time is controlled within 2 minutes, and the number of spot blowing is 2 times. At the end of converter smelting, the carbon content in the molten steel is 0.03%, and the phosphorus content is 0.012%. During this process, no deoxidizer, slag-making material, or alloy is added, and the argon blowing process is not carried out. After tapping, the molten steel is temperature-measured at the argon station after the converter, and the temperature is 1600 °C. Samples are taken, and the weight of the molten steel is 94.5 tons. The metal yield of this process is about 89 - 91%, which is much lower than the metal yield of 97 - 98% of this application; the entire smelting cycle of the converter is 42 minutes, which is nearly 3 times that of 14 - 16 minutes of this application; this comparative example adopts the hot metal mode, the carbon content in the hot metal is 4.7%, the carbon blowing loss during blowing is about 4.6%, the overall oxygen supply time is long, the oxygen blowing of iron materials is large, and the metal yield is low; and there are two spot blows, and the required carbon content at the end point is 0.03%. Because the carbon content does not drop, it is difficult to dephosphorize. Therefore, the carbon end point requirements are harsh, the overall smelting cycle is long, and at the same time, the phosphorus content of the obtained molten steel is 0.012%; for every 1 ton of steel produced, about 800 Kg of coke is consumed in the whole process, and the CO2 emission is about 1830 Kg; while using the method of this application, the CO2 emission for producing 1 ton of steel is about 13 Kg, and its carbon emission is about 60 times that of this application.

[0100] Primary LF refining: The molten steel is lifted to the refining furnace for primary refining. The measured temperature is 1579 °C. During this process, no deoxidizer is added to the molten steel. 8 kg / t of molten steel of lime, 5 kg / t of molten steel of iron oxide scale, and 3.0 kg / t of molten steel of fluorite balls are added through the high-level bunker. The carbon content is controlled at 0.7%, and the silicon content is 1.0%. The current is adjusted to the maximum value for rapid heating. When the temperature of the molten steel rises to 1585 °C, the electrode is lifted, and the argon flow rate is adjusted to 1000 NL / min for strong argon blowing for 5 minutes. Samples are taken for analysis, and the phosphorus content in the molten steel is 0.003%, reaching the dephosphorization control value of primary LF refining.

[0101] Slag skimming treatment: The molten steel is lifted to the slag skimming station for slag skimming, and the oxidizing slag amount on the molten steel surface is controlled ≤ 4.0 kg / t of molten steel to prevent the phosphorus in the slag from being re-reduced to the molten steel in the reducing atmosphere of secondary LF refining.

[0102] Secondary LF refining: The molten steel is lifted back to the LF furnace for secondary refining to adjust the deoxidation, alloying, and inclusion removal operations of the molten steel; a total of 570 Kg of ferromanganese, 130 Kg of carbon powder, 400 Kg of ferrosilicon, 1560 Kg of low-carbon ferrochrome, and 810 Kg of ferromolybdenum are added; 910 Kg of lime and 290 Kg of fluorite are added in 2 batches; the ladle is blown with argon for 22 minutes.

[0103] Final product component detection: C: 0.14%, Si: 0.29%, Mn: 0.47%, P: 0.003%, Cr: 0.98%, Mo: 0.46%, S: 0.001%, other components are omitted, trace elements: Sn: 0.001, As: 0.002, Sb: 0.0001, Pb: 0.0003, Zn: 0.001.

[0104] This comparative example requires "secondary LF refining operation and slag removal operation at the first slag removal position", which greatly occupies the lifting resources of the LF refining station and the liquid crane, and has a great interference with the production in the steelmaking and smelting bay. It is only suitable for smelting with a small number of continuous casting and pouring furnaces, or for ingot casting smelting, and is not suitable for the production of large batches of contracts.

[0105] 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 them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for smelting extra-low phosphorus steel by a combined process of an intermediate frequency furnace and a converter, characterized in that, It includes the following steps: A. When smelting ultra-low phosphorus steel by duplex smelting with an intermediate frequency furnace and a converter, slag replacement of the converter is carried out in advance: In the converter, hot metal and scrap steel are mixed and smelted according to a mass ratio of 4:1 to 6:1, and slag replacement is carried out. The content of P2O5 in the slag after slag replacement is <0.5%, and the basicity is controlled at 3 - 5; B. Clean scrap steel is put into the intermediate frequency furnace as raw material for melting to obtain preliminarily smelted low phosphorus and low sulfur clean molten steel. When the molten steel amount of melting scrap steel in the intermediate frequency furnace reaches 70% - 80% of the furnace capacity, the composition of the molten steel is detected. According to the composition of the molten steel, the silicon content and carbon content in the molten steel are adjusted so that the carbon content at the end point of the intermediate frequency furnace or when tapping into the ladle is 0.4% - 0.6%, and the silicon content is 0.2% - 0.40%. Then, scrap steel is continuously added to make the molten steel amount reach the furnace capacity until the tapping temperature is 1570 - 1620 °C; The method for adjusting the silicon content and carbon content in the molten steel includes: If Si≥0.4% and C<0.4% in the molten steel, first add scale for desiliconization operation. When the silicon content reaches 0.4% - 0.6%, then add carbon powder to the molten steel for carbon increase in the intermediate frequency furnace, or add carbon powder during the process of tapping from the intermediate frequency furnace to the ladle; If 0.2≤Si<0.4% and C<0.4%, then directly add carbon powder with the steel flow in the intermediate frequency furnace or during the process of pouring steel from the intermediate frequency furnace to carry out carbon increase for the molten steel; If Si<0.2% and C<0.4%, then add ferrosilicon alloy and carbon powder with the steel flow in the intermediate frequency furnace or during the process of pouring steel from the intermediate frequency furnace to carry out silicon increase and carbon increase for the molten steel; The P of the clean scrap steel ≤0.03%, and the S of the clean scrap steel ≤0.02%; C. When pouring steel from the intermediate frequency furnace, pre - dephosphorization slag is made: When tapping from the intermediate frequency furnace to the ladle, add active lime and light burned dolomite with the steel flow, continuously blow argon during the pouring process, and continue to blow argon into the ladle for 3 - 5 min after tapping. Control the slag composition: CaO: 40 - 50%, MgO: 5 - 10%, SiO2: 10 - 20%, the basicity is 2.0 - 4.0, add carbonized rice husk for heat preservation according to the amount of 0.5 - 1 Kg / t, and then transport the ladle to the converter; D. Converter blowing: Control the molten steel temperature in the ladle before entering the converter between 1530 and 1580 °C. After pouring into the converter, start blowing, and then add active lime and lightly burned dolomite in batches. Control the final slag basicity to be 3.0 - 5.0, and the P2O5 content in the slag to be less than 0.15%; During the smelting process, control the lance position of the oxygen lance between 1.0 and 1.5 meters below the liquid surface. For a 100-ton converter, control the oxygen flow rate between 18000 and 20000 m 3 / h, the total blowing time is 3 - 5 minutes, and the tapping temperature at the end of the converter is 1560 - 1610 °C, C ≤ 0.06%, P ≤ 0.003%; E. Before tapping, first tilt the furnace to discharge slag, and the slag discharge amount ≥40% of the total slag amount. Then tilt the furnace to tap. When tapping to 60 - 75%, add a slag - stopping cone or adopt a slide plate for slag stopping. Add low - phosphorus ferromanganese alloy during the tapping process. After tapping, the P in the molten steel ≤0.003%; F. Repeat steps B - E for continuous smelting in the same converter, stably control the P2O5 in the slag of the converter <0.15%, and the P at the end point of tapping from the converter ≤0.003% to obtain low - phosphorus molten steel.

2. The method according to claim 1, wherein Step A includes: adding 15 - 25 Kg / t of quicklime and 5 - 10 Kg / t of lightly burned dolomite in the first stage. When blowing for 3 - 5 minutes, control the molten steel temperature at 1340 - 1400 °C, conduct slag tapping operation, with the slag tapping amount being 40 - 60%, and discharge the high - phosphorus slag; in the second stage, add 10 - 20 Kg / t of quicklime and 5 - 10 Kg / t of lightly burned dolomite, and at the same time add sinter ore, scale or iron oxides. The converter end - point temperature is 1560 - 1580 °C, and control the slag composition: CaO: 50 - 60%, MgO: 5 - 10%, SiO2: 10 - 15%; conduct secondary slag tapping before tapping steel, with the slag tapping amount being 50% - 70%, and conduct slag splashing operation after tapping steel is completed; Smelt 1 - 2 heats according to the above method.

3. The method according to claim 1, wherein The tapping temperature in Step B is between 1580 - 1610 °C; In Step D, control the molten steel temperature in the ladle before entering the converter at 1540 - 1570 °C, and the converter end - point tapping temperature is 1560 - 1580 °C.

4. The method according to claim 2, wherein The controlled molten steel temperature in the first stage is 1360 - 1380 °C.

5. The method according to claim 4, wherein In Step C, the addition amount of quicklime is 5 - 10 Kg / t, and the addition amount of lightly burned dolomite is 3 - 5 Kg / t.

6. The method according to claim 1, wherein The step of adding quicklime and lightly burned dolomite in batches in Step D includes: before the molten steel is poured into the converter, first pre - add 1 / 3 - 1 / 2 of the addition amount of quicklime and 1 / 3 - 1 / 2 of the addition amount of lightly burned dolomite into the converter, then pour in the molten steel from the intermediate - frequency furnace, and add the remaining quicklime and lightly burned dolomite in batches during the converter blowing process; In Step D, the addition amount of quicklime is 20 - 25 kg / t, and the addition amount of lightly burned dolomite is 8 - 12 kg / t.

7. The method according to claim 1, characterized in that In Step D, if the converter end - point temperature > 1580 °C, then add cooling materials for adjustment; The cooling materials include at least one of slag materials, sinter ore or iron pellets.

8. The method according to claim 1, wherein In Step D, if the converter end - point P > 0.003% or C > 0.06%, then it is necessary to blow again for 30 - 90 s.

9. The method according to claim 1, wherein In Step E, the P content of the low - phosphorus ferromanganese alloy < 0.030%.

10. The method according to any one of claims 1 to 9, characterized in that, It also includes Step G, according to the alloy content and carbon content of the steel grade to be smelted, conduct at least one of the operations of molten steel deoxidation, alloying, inclusion removal or harmful gas removal on the low - phosphorus molten steel in the LF furnace and VD furnace.

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