Preparation method of ultralow-carbon low-phosphorus low-sulfur bloom industrial pure iron

Abstract

The invention discloses a preparation method of ultralow-carbon low-phosphorus low-sulfur bloom industrial pure iron. The preparation method comprises the steps of molten iron pretreatment, converter smelting, LF refining, RH vacuum and continuous casting. The converter smelting adopts a high-drawing and reblowing smelting mode, the end point [C] of the converter is less than or equal to 0.045%, and the end point [Mn] of the converter is less than or equal to 0.027%; lime and fluorite are added in the tapping process, and calcium carbide and / or C powder are / is added to the slag surface for pre-deoxidation; in the LF refining, an Al line segment is added to the slag surface during outbound, and the oxidability of the slag is inhibited; in the continuous casting, a straight-through type water gap is adopted, the inner diameter of the water gap is large in top and small in bottom, a tundish protection pouring technology is adopted, and it is guaranteed that tundish residual oxygen is smaller than or equal to 1%; according to RH vacuum and RH station entering oxygen determination, Al particles are added according to the oxygen position of molten steel, decarburization and deoxidation are carried out at the same time, and the target oxygen position is 30-50 ppm. By means of the method, continuous pouring of industrial pure iron on the bloom is achieved.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, specifically relating to a method for preparing industrial pure iron in large square billets with ultra-low carbon, low phosphorus, and low sulfur content. Background Technology

[0002] Currently, industrial pure iron is steel with fewer impurity elements and is an important industrial raw material, widely used in the production of high-end special steel, high-temperature alloys, electrolytic aluminum, high-temperature alloys and stainless steel.

[0003] The purity of industrial pure iron is closely related to its quality. Generally speaking, the higher the purity of industrial pure iron, the better. Currently, the smelting of industrial pure iron generally adopts a production method of molten iron pretreatment—converter—(LF)—RH—continuous casting. Oxygen is blown into the molten steel in the RH vacuum stage to decarburize it. After decarburization, a modifier is added to the slag surface to modify the slag. A large amount of Al is added to the molten steel to remove oxygen. In this process, on the one hand, external oxygen is required, failing to fully utilize the oxidizing properties of the molten steel for decarburization; on the other hand, excessive oxygen blowing necessitates the addition of a large amount of Al for deoxidation. Furthermore, the deoxidation product Al2O3 cannot be fully removed from the molten steel, accumulating at the nozzle during the subsequent continuous casting process. This prevents continuous casting in multiple heats, making it impossible to continuously cast large billets, and only slabs or small billets can be produced.

[0004] Patent applications with publication numbers CN 113774277 A, CN 113512619 A, and CN118127268 A all involve additional oxygen blowing into RH for decarburization, and then adding Al to remove oxygen from the molten steel after decarburization. However, the deoxidation products cannot be completely removed, which limits the continuous casting section and the number of continuous casting furnaces.

[0005] The patent application with publication number CN 113215476A uses Si deoxidation after steelmaking in the converter, performs slag removal in the LF, and uses Al+Ti deoxidation after decarburization in the RH. After deoxidation, 200-300m of calcium wire is added for modification treatment, realizing continuous casting of industrial pure iron. However, due to the addition of elements such as Si, Ti, and Ca, the cost of molten steel increases, and the content of elements such as C, Si, Mn, P, Ti, and Ca in the steel is relatively high. Furthermore, the paper does not specify the use of a cast billet cross-section.

[0006] The patent application with publication number CN 117265210 A adopts the LF+RH dual smelting process, in which oxygen is not blown in the RH and oxygen barrier, calcium carbide and Al particles are added in the LF to make white slag. Although it can meet the requirements of continuous casting of small square billets, the oxygen barrier needs to be prepared in advance, the process is complicated and the cost is increased. In addition, Al is added for deoxidation after the RH decarburization task is completed. The amount of Al particles added is large, the yield is low, and the deoxidation product Al2O3 is not fully removed. It can only meet the requirements of continuous casting of small square billets.

[0007] Patent CN 114908281 A uses a hot metal pretreatment-converter-RH-LF process for production. After the decarburization task is completed in RH, white slag is made in LF and Al is added to deoxidize the slag and molten steel respectively. However, the deoxidation product Al2O3 cannot be fully removed. In addition, the casting section is not clearly indicated in the text.

[0008] Patent CN 118186182 A adopts a production method of LF refining + RH refining + VD refining + VD refining, which has a long production process and is extremely complex, resulting in increased production costs and making it impractical for widespread application. Summary of the Invention

[0009] The purpose of this invention is to provide a method for preparing industrial pure iron in large square billets with ultra-low carbon, low phosphorus, and low sulfur.

[0010] To achieve the above objectives, the technical solution of the present invention is as follows: A method for preparing ultra-low carbon, low phosphorus, and low sulfur large billet industrial pure iron includes hot metal pretreatment, converter smelting, LF refining, RH vacuum casting, and continuous casting.

[0011] Furthermore, the molten iron pretreatment adopts the KR desulfurization + slag removal process, and the desulfurizing agent is lime and magnesium powder. After the molten iron pretreatment, the [S] in the molten iron is ≤0.001%.

[0012] Furthermore, in the converter smelting, the Mn content in the scrap steel is ≤0.3%, the smelting is carried out using the double slag method, and the smelting method of high-pulling and supplementary blowing is adopted. The final temperature of the converter is controlled at 1600-1650℃, [C]≤0.045%, [Si]≤0.005%, [Mn]≤0.027%, [P]≤0.003%, and [S]≤0.005%. During the tapping process, lime and fluorite are added, and calcium carbide and / or C powder are added to the slag surface for pre-deoxidation. While inhibiting the oxidizing properties of the slag, the deoxidation product CO has a good stirring effect on the molten steel, which is conducive to the rapid melting of the slag.

[0013] Furthermore, in the LF refining process, lime, fluorite, and refining slag are added during the heating process. The outlet temperature is 1650-1680℃. Al segments are added to the slag surface at the outlet to further inhibit the oxidative properties of the slag. The FeO content in the outlet slag is ≤4%.

[0014] Furthermore, the RH vacuum process does not involve oxygen blowing for decarburization; instead, it utilizes the reaction between carbon and oxygen in the molten steel itself. The RH inlet oxygen level is controlled, and Al particles are added based on the oxygen level of the molten steel to allow decarburization and oxygen removal to occur simultaneously, ensuring the complete removal of the deoxidation product Al₂O₃. The target oxygen level is 30-50 ppm. If the oxygen level is higher, a small amount of Al is added to continue deoxidation until the target oxygen level is reached. After the RH exits the station, it is allowed to stand for at least 10 minutes to allow all inclusions to float to the surface.

[0015] Furthermore, the continuous casting process involves a superheat of 30-50℃; a straight-through nozzle with a larger inner diameter at the top and smaller at the bottom; tundish protection casting technology; monitoring of residual oxygen in the tundish using a flue gas analyzer to control residual oxygen ≤1%; the use of a carbon-free tundish covering agent to prevent carbon increase in the molten steel; and the use of a low-carbon, high-basicity protective slag with a viscosity of 0.35±0.15 Pa•s; casting of 5-8t of residual steel; and the use of a stepped ladle bottom to prevent slag from flowing downwards; the continuous casting process uses large square billets with a cross-sectional dimension of 320mm*430mm.

[0016] Furthermore, the main components of the industrial pure iron described in this invention are: [C]≤0.003%, [Si]≤0.005%, [Mn]≤0.03%, [P]≤0.006%, [S]≤0.007%, [Al]≤0.008%, total oxygen≤0.008%, and the remainder is Fe.

[0017] The inventive principle of this invention lies in: This invention reduces the sulfur content in molten iron to 0.001% through pretreatment. Pre-deoxidation with calcium carbide or carbon powder is performed on the steel tapped from the converter. This suppresses slag oxidation while the deoxidation product CO has a good stirring effect on the molten steel, facilitating rapid slag melting. Furthermore, the addition of Ca to the calcium carbide reduces the need for lime in the refining process. The Al addition line segment of the slag in the LF refining furnace is modified to have low oxidizing properties while ensuring that the molten steel's own oxygen content is sufficient to meet the RH decarburization requirements. During RH inlet oxygen determination, Al is added simultaneously with decarburization based on the oxygen level and composition of the molten steel. This simultaneous decarburization and deoxidation ensures thorough removal of deoxidation products. After RH treatment, the steel is allowed to stand for at least 10 minutes. A straight-through nozzle with a larger inner diameter at the top and smaller at the bottom is used in continuous casting. Tundish protection casting technology is employed to control the residual oxygen in the tundish to no more than 1%, reducing secondary oxidation of the molten steel. In addition, the use of a sprue with a larger inner diameter at the top and a smaller inner diameter at the bottom increases the flow velocity of molten steel at the sprue outlet, quickly flushing away any build-up at the sprue and enabling the casting of large square billets of industrial pure iron.

[0018] The beneficial effects of this invention are as follows: (1) High-pulling and supplementary blowing are adopted in the tapping process, and the task of removing Mn is completed in the converter. By adding calcium carbide and Al segments in the converter and refining respectively, the oxidizing property of the slag is suppressed throughout the process. In the RH, the decarburization reaction is carried out by relying solely on the oxidizing property of the molten steel itself, without the need for external oxygen blowing, thus reducing oxygen consumption.

[0019] (2) RH enters the station for oxygen fixation. While RH is decarburizing, Al particles are added for deoxidation. This can reduce the amount of oxygen blown in and ensure that the deoxidation products are fully removed, thus preventing the sprue blockage during the casting process.

[0020] (3) A straight-through nozzle is used during continuous casting, with the inner diameter of the nozzle being larger at the top and smaller at the bottom, which increases the flow velocity of the molten steel at the nozzle outlet and quickly flushes away the lumps on the nozzle. The tundish sealing technology is adopted, and with the help of a flue gas analyzer, the residual oxygen in the tundish is controlled to be ≤1%, which prevents secondary oxidation during the continuous casting process and realizes the continuous casting of industrial pure iron in large billets. Attached Figure Description

[0021] Figure 1 The stopper rod curve is shown in Example 1, which describes the process of casting industrial pure iron. Detailed Implementation

[0022] The technical solution of the present invention will be further described in detail below through embodiments. Example

[0023] The preparation method of ultra-low carbon, low phosphorus, and low sulfur large billet industrial pure iron in this embodiment is as follows: (1) Desulfurization of molten iron The molten iron was 120t, and the KR desulfurization + slag removal process was adopted. The desulfurizing agents were lime and magnesium powder. After desulfurization, the [S] in the molten iron was 6ppm and the temperature was 1366℃.

[0024] (2) 130t converter smelting 4 tons of scrap steel with a [Mn] content of 0.26% were added. The smelting process was carried out using the double slag method and the high-pulling and supplementary blowing method. The final converter temperature was 1605℃. The composition was [C]: 0.042%, [Si]: 0.0038%, [Mn]: 0.026%, [P]: 0.0027%, [S]: 0.0034%. During the tapping process, 398 kg of lime and 89 kg of fluorite were added, and 35 kg of calcium carbide was added to the slag surface for pre-deoxidation.

[0025] (3) LF Refining The slag entering the station has a temperature of 1527℃. During the heating process, 287kg of lime, 200kg of fluorite, and 605kg of refining slag are added. Upon exiting the station, 30kg of Al segments are added to the slag surface to inhibit slag oxidation. The slag composition is: CaO: 45.3%, Al2O3: 17.5%, SiO2: 10.62%, MgO: 6.86%, FeO: 3.8%, MnO: 0.95%, with the remainder being unavoidable impurities. The molten steel composition is: [C]: 0.034%, [Si]: 0.0018%, [Mn]: 0.024%, [P]: 0.0023%, [S]: 0.0035%, and the exit temperature is 1675℃. After exiting the station, the RH slag is allowed to stand for 12 minutes to allow all inclusions to float to the surface.

[0026] (4) RH vacuum The inlet temperature was 1669℃, and the oxygen content of the molten steel was 414ppm. 20kg of Al particles were added upon entry, and decarburization and deoxidation were carried out simultaneously. After 11 minutes under high vacuum (133Pa), the oxygen content was stabilized at 33ppm, and the molten steel temperature was 1630℃. The composition of the molten steel exiting the station was: [C]: 0.0022%, [Si]: 0.0028%, [Mn]: 0.027%, [P]: 0.005%, [S]: 0.006%.

[0027] (5) Continuous casting The continuous casting process uses a 320mm*430mm cross-section. This is the first heat to be cast. The superheat is controlled slightly higher at 45℃. A straight-through nozzle is used, with an inner diameter that is larger at the top and smaller at the bottom. The upper nozzle is 55mm and the lower nozzle is 52mm. Tundish protection casting technology is used. Using a flue gas analyzer, the residual oxygen in the tundish is 0.38%. The casting speed is 0.65m / min. The remaining 7 tons of steel in the ladle are transferred to the next ladle for casting. A total of 8 heats were cast in this batch. The stopper rod curve is smooth with no obvious rod expansion. Figure 1 As shown.

[0028] According to the test, the composition of the finished steel in this embodiment is: [C]: 0.0026%, [Si]: 0.0025%, [Mn]: 0.026%, [P]: 0.005%, [S]: 0.006%, [Als]: 0.005%, total oxygen 0.0045%, and the remainder is Fe. Example

[0029] The preparation method of ultra-low carbon, low phosphorus, and low sulfur large billet industrial pure iron in this embodiment is as follows: (1) Desulfurization of molten iron The molten iron was 125t, and the KR desulfurization + slag removal process was adopted. The desulfurizing agents were lime and magnesium powder. After desulfurization, the [S] in the molten iron was 5ppm and the temperature was 1372℃.

[0030] (2) 130t converter smelting 6 tons of scrap steel with a [Mn] content of 0.26% was added. The smelting process was carried out using the double slag method and the high-pulling and supplementary blowing method. The final converter temperature was 1647℃. The composition was [C]: 0.028%, [Si]: 0.0020%, [Mn]: 0.025%, [P]: 0.0027%, [S]: 0.0030%. During the tapping process, 400 kg of lime and 98 kg of fluorite were added, and 20 kg of C powder was added to the slag surface for pre-deoxidation.

[0031] (3) LF Refining The slag was fed into the furnace at a temperature of 1520℃. During the heating process, 316 kg of lime, 164 kg of fluorite, and 608 kg of refining slag were added. Upon exiting the furnace, 30 kg of Al fraction was added to the slag surface to inhibit oxidation. The slag composition was: CaO: 48.9%, Al₂O₃: 14.4%, SiO₂: 10.9%, MgO: 4.09%, FeO: 3.9%, MnO: 0.86%, with the remainder being unavoidable impurities. The molten steel composition was: [C]: 0.029%, [Si]: 0.0026%, [Mn]: 0.024%, [P]: 0.0027%, [S]: 0.0035%, with an exit temperature of 1657℃. After exiting the furnace, the RH slag was allowed to stand for 18 minutes to allow all inclusions to float to the surface.

[0032] (4) RH vacuum The inlet temperature was 1650℃, and the oxygen content of the molten steel was 440ppm. 33kg of Al particles were added upon entry, and decarburization and deoxidation were carried out simultaneously. After 12 minutes under high vacuum (133Pa), the oxygen content was stabilized at 39ppm, and the molten steel temperature was 1621℃. The composition of the molten steel exiting the station was: [C]: 0.0028%, [Si]: 0.0027%, [Mn]: 0.023%, [P]: 0.0034%, [S]: 0.0038%.

[0033] (5) Continuous casting The continuous casting process uses a 320mm*430mm cross-section. This is the fourth heat cast in this cycle, with a superheat of 36℃. A straight-through nozzle is used, with an inner diameter wider at the top and narrower at the bottom (55mm for the top nozzle and 52mm for the bottom nozzle). Tundish protection casting technology is employed. Flue gas analyzer testing showed a residual oxygen level of 0.53% in the tundish, and a casting speed of 0.7m / min. The remaining 5.8 tons of steel from the ladle are transferred to the next ladle for casting. A total of 8 heats were cast in this cycle.

[0034] According to the test, the composition of the finished steel in this embodiment is: [C]: 0.0027%, [Si]: 0.0018%, [Mn]: 0.022%, [P]: 0.004%, [S]: 0.004%, [Als]: 0.004%, total oxygen 0.0058%, and the remainder is Fe. Example

[0035] The preparation method of ultra-low carbon, low phosphorus, and low sulfur large billet industrial pure iron in this embodiment is as follows: (1) Desulfurization of molten iron The molten iron was 120t, and the KR desulfurization + slag removal process was adopted. The desulfurizing agents were lime and magnesium powder. After desulfurization, the [S] in the molten iron was 7ppm and the temperature was 1386℃.

[0036] (2) 130t converter smelting 8 tons of scrap steel with a [Mn] content of 0.26% were added. The smelting process was carried out using the double slag method and the high-pulling and supplementary blowing method. The final converter temperature was 1603℃. The composition was [C]: 0.026%, [Si]: 0.0038%, [Mn]: 0.021%, [P]: 0.0024%, [S]: 0.0029%. During the tapping process, 410 kg of lime and 100 kg of fluorite were added. 15 kg of calcium carbide and 10 kg of carbon powder were added to the slag surface for pre-deoxidation.

[0037] (3) LF Refining The slag entering the station has a temperature of 1532℃. During the heating process, 331 kg of lime, 151 kg of fluorite, and 616 kg of refining slag are added. Upon exiting the station, 30 kg of Al segments are added to the slag surface to inhibit slag oxidation. The slag composition is: CaO: 49.1%, Al₂O₃: 13.9%, SiO₂: 10.7%, MgO: 4.76%, FeO: 3.2%, MnO: 0.79%, with the remainder being unavoidable impurities. The molten steel composition is: [C]: 0.028%, [Si]: 0.0025%, [Mn]: 0.018%, [P]: 0.0024%, [S]: 0.0033%, and the exit temperature is 1650℃. After exiting the station, the RH slag is allowed to stand for 14 minutes to allow all inclusions to float to the surface.

[0038] (4) RH vacuum The inlet temperature was 1642℃, and the oxygen content of the molten steel was 423 ppm. 27 kg of Al particles were added upon entry, and decarburization and deoxidation were carried out simultaneously. After 12 minutes under high vacuum (133 Pa), the oxygen content was stabilized at 41 ppm, and the molten steel temperature was 1626℃. The composition of the molten steel exiting the station was: [C]: 0.0023%, [Si]: 0.0028%, [Mn]: 0.019%, [P]: 0.0032%, [S]: 0.0037%.

[0039] (4) Continuous casting The continuous casting process uses a 320mm*430mm cross-section. This is the sixth heat cast in this cycle, with a superheat of 36℃. A straight-through nozzle is used, with an inner diameter wider at the top and narrower at the bottom (55mm for the top nozzle and 52mm for the bottom nozzle). Tundish protection casting technology is employed. Flue gas analyzer testing showed a residual oxygen level of 0.23% in the tundish, and a casting speed of 0.7 m / min. The remaining 6.3 tons of steel from the ladle are transferred to the next ladle for casting. Due to good control of secondary oxidation in the tundish, a total of 10 heats were cast in this cycle.

[0040] According to the test, the composition of the finished steel in this embodiment is: [C]: 0.0029%, [Si]: 0.0014%, [Mn]: 0.018%, [P]: 0.004%, [S]: 0.004%, [Als]: 0.003%, total oxygen 0.006%, and the remainder is Fe.

[0041] The above embodiments are only used to illustrate and not limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention without departing from the spirit and scope of the present invention. Any modifications or partial substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing ultra-low carbon, low phosphorus, and low sulfur large square billet industrial pure iron, characterized in that, It includes hot metal pretreatment, converter smelting, LF refining, RH vacuum and continuous casting; the converter smelting adopts a high-pulling and blowing smelting method, and the converter endpoint [Mn] ≤ 0.027%; lime and fluorite are added during the tapping process, and calcium carbide and / or C powder are added to the slag surface for pre-deoxidation.

2. The method for preparing ultra-low carbon, low phosphorus, and low sulfur large square billet industrial pure iron according to claim 1, characterized in that, The molten iron pretreatment results in a [S] content of ≤0.001% in the molten iron.

3. The method for preparing ultra-low carbon, low phosphorus, and low sulfur large billet industrial pure iron according to claim 1, characterized in that, In the LF refining process, Al segments are added to the slag surface upon exiting the station to inhibit slag oxidation, resulting in FeO ≤ 4% in the slag exiting the station.

4. The method for preparing ultra-low carbon, low phosphorus, and low sulfur large billet industrial pure iron according to claim 1, characterized in that, The RH vacuum system, RH inlet oxygen level, and Al particles are added according to the oxygen level of the molten steel to allow decarburization and deoxidation to proceed simultaneously, with a target oxygen level of 30-50 ppm.

5. The method for preparing ultra-low carbon, low phosphorus, and low sulfur large square billet industrial pure iron according to claim 1, characterized in that, The continuous casting process employs a straight-through nozzle with a larger inner diameter at the top and a smaller inner diameter at the bottom. It also utilizes tundish protection casting technology to ensure that the residual oxygen in the tundish is ≤1%.

6. The method for preparing ultra-low carbon, low phosphorus, and low sulfur large billet industrial pure iron according to claim 1, characterized in that, The main components of the industrial pure iron are: [C]≤0.003%, [Si]≤0.005%, [Mn]≤0.03%, [P]≤0.006%, [S]≤0.007%, [Al]≤0.008%, total oxygen≤0.008%, and the remainder is Fe.

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