Low-sulfur content production method for low-temperature steels

Abstract

The application discloses a low-sulfur-content production method of low-temperature steel. The method comprises the following steps: in a converter smelting process, tapping molten steel at 1590-1630 DEG C, carrying out deoxidization and alloying during the tapping process, adding 3-5 kg / t of lime, 1.5-2.5 kg / t of fluorite and 10-15 kg / t of calcium aluminate synthetic slag for slagging after the alloying is completed, and adding all the calcium aluminate synthetic slag when the tapping is 80-90%; the mass percentage of 12CaO·7Al2O3 in the component of the calcium aluminate synthetic slag is more than 60%; in an LF refining process, the tapping molten steel is at 1610-1630 DEG C, calcium wire is fed at 1-1.5 m / t after the tapping is completed, the feeding speed of the calcium wire is 4-6 m / s, and the molten steel is transported to a RH vacuum furnace after the feeding of the calcium wire is completed; the inside of the calcium wire is passivated calcium powder with Ca content being more than 95%, and the outer layer is iron sheet with an outer diameter of 12-15 mm and a thickness of 3-5 mm.

Description

Technical Field

[0001] This invention belongs to the field of steel material preparation technology, and relates to a method for producing low-temperature steel, particularly a method for producing low-sulfur steel. Background Technology

[0002] Sulfur (S) has a significant impact on the properties of steel, especially for cryogenic steel, where it is a key harmful element affecting low-temperature toughness. Specifically, the inner shell of liquefied natural gas (LNG) storage tanks is welded from cryogenic steel. Because it operates at temperatures below -162°C, it has special performance requirements, primarily including excellent low-temperature toughness, high strength, high structural stability, and weldability. However, sulfur reacts with metallic manganese to form MnS precipitates, which significantly reduces the low-temperature toughness of cryogenic steel. Therefore, effectively controlling the sulfur content is a hot topic in cryogenic steel research. Summary of the Invention

[0003] The purpose of this invention is to provide a method for producing low-temperature steel, and more particularly a method for producing low-sulfur steel with low sulfur content.

[0004] To achieve the above-mentioned objective, one embodiment of the present invention provides a method for producing low-sulfur steel at low temperatures. The method includes:

[0005] KR desulfurization process: The temperature of the molten iron leaving the station after desulfurization is 1300~1350℃, and the sulfur content is ≤0.0010% by mass percentage;

[0006] Converter smelting process: Smelting is carried out using molten iron accounting for more than 89% by weight and nickel plates accounting for less than 11% by weight. The tapping temperature of the molten steel is 1590–1630℃, and the tapping steel has a carbon content of 0.02–0.05%, an oxygen content of 0.045–0.085%, a phosphorus content of ≤0.005%, and a sulfur content of ≤0.0015% by weight. Deoxidation and alloying are carried out during tapping. After alloying, 3–5 kg / t of lime, 1.5–2.5 kg / t of fluorite, and 10–10 kg / t of other additives are added. A 5 kg / t calcium aluminate synthesis slag is used for slag formation, and all of it is added when the steel is tapped to 80-90% concentration. The composition of the calcium aluminate synthesis slag includes, by mass percentage, 50-55% CaO, 30-35% Al2O3, 5-10% CaF2, less than 3% SiO2, and 2-5% MgO. Among them, the mass percentage of the 12CaO·7Al2O3 phase exceeds 60%, the mass percentage of the Al2O3 phase is ≤5%, and the remainder is a single phase or composite phase of CaO, CaF2, SiO2, and MgO.

[0007] LF refining process: The molten steel temperature at tapping is 1610~1630℃. After tapping, calcium wire is fed in at a rate of 1~1.5m / t and a feeding speed of 4~6m / s. After feeding, the molten steel is transported to the RH vacuum furnace. The interior of the calcium wire is passivated metallic calcium powder with a Ca content of more than 95%, and the outer layer is iron sheet with an outer diameter of 12~15mm and a thickness of 3~5mm.

[0008] RH vacuum refining process: The molten steel transported to the RH vacuum furnace is smelted in a vacuum atmosphere, and desulfurization is carried out using a desulfurizing agent during the smelting process;

[0009] Continuous casting process: The molten steel from the RH vacuum refining process is hoisted to the continuous casting platform and poured under full protection to obtain a continuously cast billet.

[0010] Preferably, the RH vacuum refining process includes:

[0011] An RH vacuum furnace employs a first-stage steam pump, a second-stage steam pump, a third-stage steam pump, a fourth-stage steam pump, and two-stage water circulation pumps arranged sequentially in the vacuum exhaust pipeline of the vacuum chamber.

[0012] After the molten steel is transported to the RH vacuum furnace, the two-stage water circulation pumps are turned on within 1 minute to maintain the vacuum degree above 200 mbar. Then, desulfurizing agent is added for desulfurization treatment. After the desulfurizing agent is added, treatment continues for 5 to 10 minutes. Then, the fourth-stage steam pump, the third-stage steam pump, the second-stage steam pump, and the first-stage steam pump are turned on in sequence. After the vacuum degree drops below 1.5 mbar, treatment continues for 10 to 15 minutes before breaking the vacuum and tapping the steel.

[0013] Before adding the desulfurizing agent, maintain the gas flow rate at 80–100 Nm. 3 / h, but after adding desulfurizing agent, the gas flow rate is increased to 150-200 Nm. 3 / h.

[0014] Preferably, in the RH vacuum refining process: before adding desulfurizing agent for desulfurization treatment, the molten steel is sampled and the S content is tested. If the S content is ≤0.0010%, 1.5-2.5 kg / t of desulfurizing agent is added to the molten steel; if the S content of the molten steel is >0.0010%, 3.0-4.0 kg / t of desulfurizing agent is added to the molten steel.

[0015] The desulfurizing agent comprises, by mass percentage: 50-60% CaO, 25-35% CaF2, and 5-10% calcium-iron alloy; the calcium-iron alloy comprises, by mass percentage: 30-40% Ca, with the remainder being iron and impurities.

[0016] Preferably, the converter smelting process involves sequentially adding metallic aluminum, ferrosilicon, metallic manganese, and nickel to the molten steel when the steel reaches 10-20% for deoxidation and alloying, until the addition is completed when the steel reaches 60-70%, thus ending the alloying process.

[0017] Preferably, in the converter smelting process: during the tapping process, the flow rate of bottom-blown argon gas in the ladle is 400-600 NL / min, and after tapping is completed, the flow rate of bottom-blown argon gas in the ladle is increased to 800-1000 NL / min. At the same time, after tapping is completed, the molten steel is continuously stirred for 2-5 minutes before being transported to the LF refining furnace.

[0018] Preferably, the LF refining process includes a sequential energizing and heating stage, an alloying stage, and a slag-forming stage, adjusting the slag composition to contain 50-55% CaO, 30-35% Al2O3, 3-8% CaF2, less than 3% SiO2, 4-7% MgO, less than 1% (T.Fe+MnO), and other unavoidable impurity components by mass percentage.

[0019] Preferably, the LF refining process involves bottom blowing argon throughout. During the energizing and heating stage, the flow rate of bottom blowing argon is 400–500 NL / min; during the alloying stage, the flow rate is 300–400 NL / min; during the slag-forming stage, the flow rate is 500–600 NL / min; during the calcium wire feeding stage, the flow rate is 100–200 NL / min; and at other times, the flow rate is 150–250 NL / min.

[0020] Preferably, in the continuous casting process: the molten steel is allowed to stand still for more than 5 minutes after arriving at the continuous casting station before being poured; during the pouring process, the casting speed is 0.8 to 1.1 m / min, the argon blowing flow rate of the long nozzle is 150 to 250 L / min, and the argon blowing flow rate of the stopper rod and the submerged nozzle is 3 to 5 L / min. Argon blowing begins in the tundish 5 minutes before pouring and stops after the first round of tundish covering agent addition.

[0021] Preferably, in the LF refining process, the chemical composition of the iron sheet of the calcium wire, by mass percentage, includes: Al: 0.005-0.035%, Si: 0.2-0.4%, Mn: 0.3-0.5%, P≤0.001%, S≤0.003%, with the remainder being Fe and unavoidable impurities.

[0022] Preferably, the chemical composition of the continuously cast billet, in mass percentage, includes: TO≤10ppm, P≤0.005%, S≤0.0005%, N≤0.002%, and H≤1.5ppm.

[0023] Compared with the prior art, the beneficial effects of the present invention are: after deoxidation and alloying during converter tapping, slag formation is carried out by using synthetic slags of lime, fluorite and calcium aluminate with special composition. The calcium aluminate synthetic slag with special composition takes the low-melting-point phase 12CaO·7Al2O3 as the main component, so as to achieve rapid and super desulfurization effect and greatly reduce the S content in the molten steel; in addition, calcium wire is fed after the completion of the LF furnace refining process, and combined with the specific composition, feeding speed and feeding amount of the calcium wire, Ca elements are accumulated at the bottom of the molten steel in the ladle, ensuring that the Ca elements fully stay in the molten steel, thereby greatly improving the desulfurization effect of the molten steel, and further realizing the control of low S content in the molten steel on the whole, so that the final obtained continuous casting billet has a low S content. Brief Description of the Drawings

[0024] Figure 1 is a flow chart of the production method according to one embodiment of the present invention;

[0025] Figure 2 is a partial structural schematic diagram of the RH vacuum furnace in one embodiment of the present invention. Detailed Description of the Embodiments

[0026] To make the purposes, technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the specific embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work shall fall within the protection scope of the present invention.

[0027] One embodiment of the present invention provides a production method of low-temperature steel, which can obtain low-temperature steel with low S content. The process route adopted by the production method is KR desulfurization process - converter smelting process - LF refining process - RH vacuum refining process - continuous casting process, so as to produce the low-temperature steel continuous casting billet.

[0028] Ref Figure 1 , specifically, the production method includes the following contents ——

[0029] KR desulfurization process: the temperature of the molten iron exiting the station after desulfurization is 1300~1350℃, and S is ≤0.0010% by mass percentage;

[0030] Converter smelting process: Smelting is carried out using molten iron accounting for more than 89% by weight and nickel plates accounting for less than 11% by weight. The tapping temperature of the molten steel is 1590–1630℃, and the tapping steel has a carbon content of 0.02–0.05%, an oxygen content of 0.045–0.085%, a phosphorus content of ≤0.005%, and a sulfur content of ≤0.0015% by weight. Deoxidation and alloying are carried out during tapping. After alloying, 3–5 kg / t of lime, 1.5–2.5 kg / t of fluorite, and 10–10 kg / t of other additives are added. A 5 kg / t calcium aluminate synthesis slag is used for slag formation, and all of it is added when the steel is tapped to 80-90% concentration. The composition of the calcium aluminate synthesis slag includes, by mass percentage, 50-55% CaO, 30-35% Al2O3, 5-10% CaF2, less than 3% SiO2, and 2-5% MgO. Among them, the mass percentage of the 12CaO·7Al2O3 phase exceeds 60%, the mass percentage of the Al2O3 phase is ≤5%, and the remainder is a single phase or composite phase of CaO, CaF2, SiO2, and MgO.

[0031] LF refining process: The molten steel temperature at tapping is 1610~1630℃. After tapping, calcium wire is fed in at a rate of 1~1.5m / t and a feeding speed of 4~6m / s. After feeding, the molten steel is transported to the RH vacuum furnace. The interior of the calcium wire is passivated metallic calcium powder with a Ca content of more than 95%, and the outer layer is iron sheet with an outer diameter of 12~15mm and a thickness of 3~5mm.

[0032] RH vacuum refining process: The molten steel transported to the RH vacuum furnace is smelted in a vacuum atmosphere, and desulfurization is carried out using a desulfurizing agent during the smelting process;

[0033] Continuous casting process: The molten steel from the RH vacuum refining process is hoisted to the continuous casting platform and poured under full protection to obtain a continuously cast billet.

[0034] Thus, compared with the prior art, the beneficial effects of the present invention are as follows: After deoxidation and alloying of the steel produced in the converter, slag is formed by using lime, fluorite, and a special calcium aluminate synthetic slag. This special calcium aluminate synthetic slag has a low-melting-point phase 12CaO·7Al2O3 as its main component, thereby achieving a rapid and powerful desulfurization effect and greatly reducing the S content in the molten steel. In addition, after the refining process in the LF furnace, calcium wire is fed in, and combined with the specific composition of the calcium wire and the feeding speed and amount, Ca elements accumulate at the bottom of the molten steel in the ladle, ensuring that the Ca elements remain fully in the molten steel, thereby greatly improving the desulfurization effect of the molten steel. As a result, the overall low S content control of the molten steel is achieved, resulting in a low S content in the final continuously cast billet. For example, the chemical composition of the continuously cast billet, including the S content by mass percentage, does not exceed 0.0005%.

[0035] More specifically, the various steps of a preferred embodiment are described in detail below.

[0036] <KR Desulfurization Process>

[0037] Desulfurization treatment is performed on blast furnace molten iron using the KR desulfurization technology.

[0038] In the present invention, the temperature of the molten iron exiting the station after desulfurization is 1300 to 1350°C, and S is ≤0.0010% in mass percentage. Specifically, the KR desulfurization technology can be implemented by any feasible technology disclosed in the art, and no redundant details are described herein.

[0039] <Converter Smelting Process>

[0040] The molten iron exiting the station from the KR desulfurization process, together with nickel plates, are charged into a converter for smelting. The weight proportion of the exiting molten iron is 89% or more, that is, the weight of the molten iron accounts for 89% or more of the total weight of the molten iron and the nickel plates; correspondingly, the weight proportion of the nickel plates is 11% or less, that is, the weight of the nickel plates accounts for 11% or less of the total weight of the molten iron and the nickel plates. The specific actual charging amount of the nickel plates can be determined within the above weight proportion range according to the target chemical composition of the low-temperature steel planned to be produced.

[0041] Therein, the chemical composition of the used nickel plates may include, in mass percentage, Ni ≥ 99%, P ≤ 0.015%, S ≤ 0.003%, with the balance being Fe and other unavoidable impurities.

[0042] In this converter smelting process, the temperature of the tapped molten steel is 1590 to 1630°C, and the tapped molten steel has a C content of 0.02 to 0.05%, an O content of 0.045 to 0.085%, a P content of ≤ 0.005%, and an S content of ≤ 0.0015% in mass percentage.

[0043] Deoxidation and alloying are carried out during the tapping process. After the alloying is completed, 3 to 5 kg / t of lime, 1.5 to 2.5 kg / t of fluorite, and 10 to 15 kg / t of calcium aluminate synthetic slag are added for slag forming, and all of them are added when 80 to 90% of the tapping is completed.

[0044] Therein, the components of the calcium aluminate synthetic slag include, in mass percentage, 50 to 55% of CaO, 30 to 35% of Al₂O₃, 5 to 10% of CaF₂, 3% or less of SiO₂, and 2 to 5% of MgO, wherein the mass percentage of the phase 12CaO·7Al₂O₃ exceeds 60%, the phase Al₂O₃ is ≤ 5%, and the rest are single phases or composite phases of CaO, CaF₂, SiO₂ and MgO. Apart from these components, all the remaining components of the calcium aluminate synthetic slag are unavoidable impurities.

[0045] As described above, after deoxidation and alloying in converter tapping, slagging is carried out by using a synthetic slag composed of lime, fluorite and calcium aluminate with a special composition, wherein the calcium aluminate synthetic slag with the special composition takes the low-melting-point phase 12CaO·7Al₂O₃ as the main component, so as to achieve a rapid and super-strong desulfurization effect and greatly reduce the S content in the molten steel.

[0046] Further, the deoxidation and alloying treatment in this process is roughly as follows: when the tapping reaches 10 to 20%, metallic aluminum, ferrosilicon, metallic manganese and nickel are sequentially added into the molten steel for deoxidation and alloying, and the feeding is completed when the tapping reaches 60 to 70%, thus ending the alloying.

[0047] Still more preferably, the flow rate of argon bottom-blown into the ladle during the tapping process is 400 to 600 NL / min, after the tapping is completed, the flow rate of argon bottom-blown into the ladle is increased to 800 to 1000 NL / min, and meanwhile, stirring is continued for 2 to 5 minutes after the tapping is completed, and then the molten steel is transported to the LF refining furnace. In this way, gas absorption by the molten steel can be further reduced, so as to avoid high O and N contents.

[0048] <LF Refining Process>

[0049] The tapped molten steel from the converter smelting process is refined by using an LF refining furnace, which, for example, generally includes a sequential electrification heating stage, an alloying stage and a slagging stage.

[0050] In the present invention, the temperature of the tapped molten steel is 1610 to 1630°C. Moreover, 1 to 1.5 m / t of calcium wire is fed after tapping is completed, the wire feeding speed is 4 to 6 m / s, and after wire feeding is finished, the molten steel is transported to an RH vacuum furnace.

[0051] Wherein, the interior of the calcium wire is passivated metallic calcium powder with a Ca content of 95% or more, and the outer layer is an iron sheet with an outer diameter of 12 to 15 mm and a thickness of 3 to 5 mm.

[0052] In this way, the calcium wire is fed after the LF furnace refining process is completed, and combined with the specific composition of the calcium wire as well as the wire feeding speed and wire feeding amount, Ca elements are accumulated at the bottom of the molten steel in the ladle, and the Ca elements are ensured to fully stay in the molten steel, thereby greatly improving the desulfurization effect of the molten steel. Combined with the specific operations of the previous converter smelting, the low S content control of the molten steel is realized on the whole, so that the S content of the final obtained continuous casting slab is low.

[0053] Preferably, the chemical composition of the iron sheet of the calcium wire includes, in mass percentage: Al: 0.005 to 0.035%, Si: 0.2 to 0.4%, Mn: 0.3 to 0.5%, P ≤ 0.001%, S ≤ 0.003%, with the balance being Fe and unavoidable impurities. The structure of the iron sheet of the calcium wire of the present invention is not limited thereto.

[0054] Further, in the slag-forming stage of the LF refining process, the slag composition is adjusted to contain, by mass percentage, 50-55% CaO, 30-35% Al₂O₃, 3-8% CaF₂, no more than 3% SiO₂, 4-7% MgO, no more than 1% (T.Fe+MnO), and other unavoidable impurity components. In this way, this slag system has ultra-strong desulfurization capacity, which can greatly reduce the S content; at the same time, this large slag amount operation can also protect the molten steel and reduce gas absorption by the molten steel during the refining process.

[0055] More preferably, argon is bottom-blown throughout the entire LF refining process, the flow rate of bottom-blown argon in the electrification heating stage is 400-500 NL / min, the flow rate of bottom-blown argon in the alloying stage is 300-400 NL / min, the flow rate of bottom-blown argon in the slag-forming stage is 500-600 NL / min, the flow rate of bottom-blown argon during calcium wire feeding is 100-200 NL / min, and the flow rate of bottom-blown argon in the remaining time is 150-250 NL / min. In this way, N absorption by the molten steel can be reduced, thereby improving the purity of the low-temperature steel as a whole.

[0056] <RH vacuum refining process>

[0057] The molten steel transported to the RH vacuum furnace is smelted in a vacuum atmosphere, and desulfurization treatment is carried out using a desulfurizing agent during the smelting. Specifically, any known feasible technology can be adopted for implementation.

[0058] In a preferred embodiment, an RH vacuum furnace sequentially equipped with a first-stage steam pump, a second-stage steam pump, a third-stage steam pump, a fourth-stage steam pump, and two stages of water circulation pumps in the vacuum exhaust pipeline of the vacuum chamber is adopted. This type of RH vacuum furnace is a known technology in the art, and for the convenience of description herein, by the attached Figure 2 schematic diagram shown, the positional relationship of the first-stage steam pump E1, the second-stage steam pump E2, the third-stage steam pump E3, the fourth-stage steam pump E4, the two stages of water circulation pumps W1&W2 and the vacuum chamber 10 is briefly illustrated, so as to facilitate the understanding of the technical content of the RH vacuum refining process.

[0059] In the RH vacuum refining process, after the molten steel is transported to the RH vacuum furnace, the two stages of water circulation pumps are turned on within 1 minute to maintain the vacuum degree at 200 mbar or above, and then a desulfurizing agent is added for desulfurization treatment; after the desulfurizing agent is added, the treatment is continued for 5-10 minutes, and then the fourth-stage steam pump, the third-stage steam pump, the second-stage steam pump, and the first-stage steam pump are turned on in sequence; after the vacuum degree drops below 1.5 mbar, the treatment is continued for 10-15 minutes, followed by breaking the vacuum and tapping. Moreover, before adding the desulfurizing agent, the lifting gas flow rate is maintained at 80-100 Nm 3 / h, and after adding the desulfurizing agent, the lifting gas flow rate is increased to 150-200 Nm3 / h. Thus, after the molten steel arrives at the station, the vacuum level in the vacuum chamber is controlled to be relatively high (e.g., maintained above 200 mbar), and a low lifting gas flow rate is used, so that the Ca element fed in the LF refining process can be further desulfurized in the vacuum cycle, thereby further improving the desulfurization effect.

[0060] Furthermore, before adding desulfurizing agent for desulfurization treatment, the molten steel is sampled and the S content is tested. If the S content is ≤0.0010%, 1.5 to 2.5 kg / t of desulfurizing agent is added to the molten steel. If the S content of the molten steel is >0.0010%, 3.0 to 4.0 kg / t of desulfurizing agent is added to the molten steel.

[0061] Optionally, the desulfurizing agent comprises, by mass percentage: 50-60% CaO, 25-35% CaF2, and 5-10% calcium-iron alloy; wherein the calcium-iron alloy comprises, by mass percentage: 30-40% Ca, with the remainder being iron and impurities.

[0062] <Continuous casting process>

[0063] The molten steel from the RH vacuum refining process is hoisted to the continuous casting platform and poured under full protection to obtain a continuous casting billet, which is a low-temperature steel continuous casting billet, and its chemical composition includes N ≤ 0.0005% by mass percentage.

[0064] Preferably, the molten steel is allowed to stand still for more than 5 minutes after arriving at the continuous casting station before being poured. During the pouring process, the casting speed is 0.8 to 1.1 m / min, the argon blowing flow rate of the long nozzle is 150 to 250 L / min, and the argon blowing flow rate of the stopper rod and the submerged nozzle is 3 to 5 L / min. Argon blowing begins in the tundish 5 minutes before pouring and stops after the first round of tundish covering agent addition.

[0065] In one specific embodiment, the chemical composition of the continuously cast billet, by mass percentage, includes: C: 0.03–0.10%, Si: 0.15–0.35%, Mn: 0.5–1.6%, Ni: 0.4–10.0%, Al: 0.015–0.055%, Cu ≤ 0.015%, Mo ≤ 0.50%, Cr ≤ 0.70%, Nb ≤ 0.035%, TO ≤ 10 ppm, P ≤ 0.005%, S ≤ 0.0005%, N ≤ 0.002%, and H ≤ 1.5 ppm. Wherein, Cu is an impurity element rather than an alloying element, and Mo, Cr, and Nb may be impurities rather than alloying elements in some specific embodiments, while in other embodiments they may be alloying elements. For example, in one embodiment where Mo, Cr, and Nb are impurity elements, the chemical composition of the continuously cast billet, by mass percentage, is: C: 0.03–0.10%, Si: 0.15–0.35%, Mn: 0.5–0.9%, Ni: 0.4–10.0%, Al: 0.015–0.055%, Cu≤0.015%, Mo≤0.010%, Cr≤0.015%, Nb≤0.006%, TO≤10ppm, P≤0.005%, S≤0.002%, N≤0.002%, H≤1.5ppm, with the remainder being iron and unavoidable impurities.

[0066] Furthermore, the thickness of the continuously cast billet is 220mm or 320mm, and the width is 1500-2300mm.

[0067] Thus, compared with the prior art, the beneficial effects of one embodiment of the present invention are as follows: after deoxidation and alloying of the steel produced in the converter, slag is formed by using lime, fluorite, and a special calcium aluminate synthetic slag. This special calcium aluminate synthetic slag has a low-melting-point phase 12CaO·7Al2O3 as its main component, thereby achieving a rapid and powerful desulfurization effect and greatly reducing the S content in the molten steel. In addition, after the refining process in the LF furnace, calcium wire is fed in, and combined with the specific composition of the calcium wire and the feeding speed and amount, Ca elements accumulate at the bottom of the molten steel in the ladle, ensuring that the Ca elements remain fully in the molten steel, thereby greatly improving the desulfurization effect of the molten steel. As a result, the overall low S content control of the molten steel is achieved, resulting in a low S content in the final continuously cast billet. For example, the chemical composition of the continuously cast billet, including the S content by mass percentage, does not exceed 0.0005%.

[0068] Several embodiments are provided below, in which continuously cast billets are prepared using the method described in this invention. The chemical composition and thickness of the continuously cast billets in these embodiments are shown in Table 1 below.

[0069] [Table 1]

[0070]

[0071] [Table 1 continued]

[0072]

[0073]

[0074] The continuously cast billets of each embodiment are rolled into steel plates. For example, the continuously cast billets are fed into a heating furnace for heating, with the temperature in the preheating section rising to 750-850℃, the heating section to 1100-1200℃, and the soaking section to 1150-1200℃; then rolled, with an initial rolling temperature of 1030-1130℃ and a final rolling temperature of 800-850℃, a rolling amount of 10-15% per pass, and a plate thickness of 5-60mm; the rolled plate is then subjected to a second quenching, with the first quenching at 800-900℃ and the second quenching at 700-800℃; tempering is then performed at a tempering temperature of 560-620℃ to obtain the finished low-temperature steel plate.

[0075] The mechanical properties and low-temperature performance of the finished low-temperature steel plates from each embodiment were tested, and the results are shown in Table 2.

[0076] [Table 2]

[0077]

[0078] As can be seen, the method of the present invention can obtain continuously cast billets with low S content and high purity, which can be used to prepare low-temperature steel plate products with excellent low-temperature toughness, thus meeting the market's performance requirements for low-temperature steel.

Claims

1. A method for producing low-sulfur steel at low temperatures, characterized in that, The method includes: KR desulfurization process: The temperature of the molten iron leaving the station after desulfurization is 1300~1350℃, and the sulfur content (S) is ≤0.0010% by mass. Converter smelting process: Smelting is carried out using molten iron accounting for more than 89% by weight and nickel plates accounting for less than 11% by weight. The tapping temperature of the molten steel is 1590~1630℃, and the tapping steel has a carbon content of 0.02~0.05%, an oxygen content of 0.045~0.085%, a phosphorus content of ≤0.005%, and a sulfur content of ≤0.0015% by weight. Deoxidation and alloying are carried out during tapping. After alloying, 3~5 kg / t of lime, 1.5~2.5 kg / t of fluorite, and 10~1... A 5 kg / t calcium aluminate synthesis slag is used for slag formation, and all of it is added when the steel is tapped to 80-90% concentration. The composition of the calcium aluminate synthesis slag includes, by mass percentage, 50-55% CaO, 30-35% Al2O3, 5-10% CaF2, less than 3% SiO2, and 2-5% MgO. Among them, the mass percentage of the 12CaO•7Al2O3 phase exceeds 60%, the mass percentage of the Al2O3 phase is ≤5%, and the remainder is a single phase or composite phase of CaO, CaF2, SiO2, and MgO. LF refining process: The molten steel temperature at tapping is 1610~1630℃. After tapping, calcium wire is fed in at a rate of 1~1.5m / t at a speed of 4~6m / s. After feeding, the molten steel is transported to the RH vacuum furnace. The interior of the calcium wire is passivated metallic calcium powder with a Ca content of more than 95%, and the outer layer is an iron sheet with an outer diameter of 12~15mm and a thickness of 3~5mm. The chemical composition of the iron sheet of the calcium wire, by mass percentage, includes: Al: 0.005~0.035%, Si: 0.2~0.4%, Mn: 0.3~0.5%, P≤0.001%, S≤0.003%, with the remainder being Fe and unavoidable impurities. RH vacuum refining process: The molten steel transported to the RH vacuum furnace is smelted in a vacuum atmosphere, and desulfurization is carried out using a desulfurizing agent during the smelting process; Continuous casting process: The molten steel from the RH vacuum refining process is hoisted to the continuous casting platform and poured under full protection to obtain a continuously cast billet with a mass percentage S≤0.0005%.

2. The method for producing low-sulfur steel according to claim 1, characterized in that, The RH vacuum refining process: An RH vacuum furnace employs a first-stage steam pump, a second-stage steam pump, a third-stage steam pump, a fourth-stage steam pump, and two-stage water circulation pumps arranged sequentially in the vacuum exhaust pipeline of the vacuum chamber. After the molten steel is transported to the RH vacuum furnace, the two-stage water circulation pumps are turned on within 1 minute to maintain the vacuum degree above 200 mbar. Then, desulfurizing agent is added for desulfurization treatment. After the desulfurizing agent is added, treatment continues for 5 to 10 minutes. Then, the fourth-stage steam pump, the third-stage steam pump, the second-stage steam pump, and the first-stage steam pump are turned on in sequence. After the vacuum degree drops below 1.5 mbar, treatment continues for 10 to 15 minutes before breaking the vacuum and tapping the steel. Before adding the desulfurizing agent, maintain the gas flow rate at 80~100 Nm. 3 / h, but after adding desulfurizing agent, the gas flow rate is increased to 150~200Nm 3 / h.

3. The method for producing low-sulfur steel according to claim 2, characterized in that, The RH vacuum refining process involves the following steps: Before desulfurization, the molten steel is sampled and tested for sulfur content. If the sulfur content is ≤0.0010%, 1.5~2.5 kg / t of desulfurizing agent is added to the molten steel. If the sulfur content is >0.0010%, 3.0~4.0 kg / t of desulfurizing agent is added to the molten steel. The desulfurizing agent comprises, by mass percentage: 50-60% CaO, 25-35% CaF2, and 5-10% calcium-iron alloy; the calcium-iron alloy comprises, by mass percentage: 30-40% Ca, with the remainder being iron and impurities.

4. The method for producing low-sulfur steel according to claim 1, characterized in that, The converter smelting process involves sequentially adding aluminum, ferrosilicon, manganese, and nickel to the molten steel when the steel reaches 10-20% of its capacity for deoxidation and alloying. This process continues until the steel reaches 60-70% of its capacity, at which point the alloying process is complete.

5. The method for producing low-sulfur steel according to claim 1, characterized in that, The converter smelting process is as follows: during the tapping process, the flow rate of bottom-blown argon gas in the ladle is 400~600 NL / min. After tapping is completed, the flow rate of bottom-blown argon gas in the ladle is increased to 800~1000 NL / min. At the same time, after tapping is completed, the molten steel is continuously stirred for 2~5 minutes before being transported to the LF refining furnace.

6. The method for producing low-sulfur steel according to claim 1, characterized in that, The LF refining process includes a sequential heating stage, an alloying stage, and a slag-forming stage, adjusting the slag composition to contain 50-55% CaO, 30-35% Al2O3, 3-8% CaF2, less than 3% SiO2, 4-7% MgO, less than 1% T.Fe+MnO, and other unavoidable impurities by mass percentage.

7. The method for producing low-sulfur steel according to claim 1, characterized in that, The LF refining process is as follows: argon is blown into the bottom throughout the process. During the power-on heating stage, the flow rate of argon is 400~500 NL / min; during the alloying stage, the flow rate of argon is 300~400 NL / min; during the slag-forming stage, the flow rate of argon is 500~600 NL / min; during the calcium wire feeding period, the flow rate of argon is 100~200 NL / min; and at other times, the flow rate of argon is 150~250 NL / min.

8. The method for producing low-sulfur steel according to claim 1, characterized in that, The continuous casting process is as follows: when the molten steel arrives at the continuous casting station, it should be allowed to stand still for more than 5 minutes before being poured; during the pouring process, the casting speed is 0.8~1.1m / min, and the argon blowing flow rate of the long nozzle is 150~250L / min, while the argon blowing flow rate of the stopper rod and the submerged nozzle is 3~5L / min. Argon blowing begins in the tundish 5 minutes before pouring begins and stops after the first round of tundish covering agent addition is completed.

9. The method for producing low-sulfur steel according to claim 1, characterized in that, The chemical composition of the continuously cast billet, expressed as a percentage by mass, includes: TO≤10ppm, P≤0.005%, N≤0.002%, and H≤1.5ppm.

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