A high-purity high-aluminum steel and its preparation method

Abstract

The present invention relates to a high-purity high-aluminum steel and a preparation method thereof, belonging to the technical field of steelmaking. In the chemical composition of the high-purity high-aluminum steel, the mass ratio of Al is 0.60% - 0.90%, and the mass ratio of S is ≤0.005%. The preparation method is carried out according to the process flow of KR hot metal pretreatment → converter smelting → converter tapping → LF furnace refining → RH furnace refining → continuous casting. By optimizing the design of the deoxidation alloying and slag-making process of the high-aluminum steel, the LF refining slag system, the RH vacuum treatment system and other measures, the oxidation of metallic aluminum in the smelting process is reduced, and the removal of alumina inclusions in the molten steel is strengthened, so as to obtain a high-purity high-aluminum steel product. Calculating the total dissolved aluminum amount based on the metallic aluminum content in the continuous casting billet and comparing it with the total added amount of metallic aluminum, the recovery rate of the metallic aluminum element is above 85%.

Description

Technical Field

[0001] The present invention relates to a high-purity high-aluminum steel and a preparation method thereof, belonging to the technical field of steelmaking. Background Art

[0002] 10CrMoAl is a seawater corrosion-resistant steel plate, mainly used for manufacturing pipelines and structural parts for conveying water, oil, gas, etc., and is widely used in coastal oil fields, power plants, petrochemical natural gas and other fields. Since it serves in a marine environment, seawater contains a large amount of salts, chlorine and other substances, which are highly corrosive to it. Aluminum in the material can be oxidized to form a dense aluminum oxide film to prevent material corrosion; chromium ions and molybdenum ions in the material can automatically supplement the voids formed by pitting corrosion of the steel, form a dense protective layer, prevent pitting corrosion from developing in depth, and further play a role in corrosion resistance and extend the service life.

[0003] In recent years, with the rapid development of science and technology, the development and utilization of the ocean by people has gradually become in-depth, and the consumption of seawater corrosion-resistant steel plates has also increased significantly. Since the Al content in corrosion-resistant steel is much higher than that of conventional steel grades, the control stability of the Al content is poor during the smelting process, resulting in low production efficiency, high cost, and the cleanliness of molten steel cannot be guaranteed.

[0004] The patent application with the publication number CN116590619A provides a production method of a seawater corrosion-resistant steel 10CrMoAl. The key production steps include: converter smelting, LF refining, VD vacuum treatment, continuous casting, rolling, and heat treatment. For VD vacuum degassing treatment, the vacuum is pumped to below 0.5 torr, and the vacuum holding time is 15 min, which can control a lower gas content. However, during the VD vacuum treatment process, the slag-metal reaction is intense, slag entrainment is serious, the yield stability of metallic aluminum is poor, and it is difficult to stably control the Al2O3 inclusions in the steel, the cleanliness of molten steel is poor, and there is a risk of nodulation in the submerged entry nozzle during the continuous casting and continuous casting process, affecting the number of continuous casting furnaces and production efficiency.

[0005] The patent application with the publication number CN115961118A provides a high-aluminum steel, a preparation method thereof and products, adopting an LF refining + continuous casting production process, mainly designing a two-step aluminum addition operation: the first step, adding aluminum during converter tapping for deoxidation and desulfurization; the second step, adding aluminum in 3 - 5 times during the LF refining process, and opening the bottom blowing for stirring at 500 - 800 NL / min. It solves the technical problems such as large addition amount and low efficiency of metallic aluminum during the production process of high-aluminum steel, and obtains high-aluminum molten steel with stable composition. However, adding aluminum blocks multiple times during the LF refining process and opening the bottom blowing for stirring result in a long refining time, serious oxidation loss of metallic aluminum, affecting the cleanliness of molten steel, increasing production costs and reducing production efficiency.

[0006] The patent application with the publication number CN116024485A provides a preparation method for high-aluminum steel and high-aluminum steel. The production steps include: converter smelting, LF refining, RH vacuum treatment, and continuous casting. In the converter blowing process, decarburization and dephosphorization are carried out. After the blowing is completed, the molten steel is transported to the LF refining furnace; during the LF furnace refining process, lime and pre-melted slag are used to make slag for desulfurization; metallic aluminum is added, and lime is added during the melting process of metallic aluminum to reduce the oxidation of the molten steel; then the alloyed molten steel is transported to the RH vacuum treatment, and degassing treatment is carried out under the lowest vacuum pressure of 69 Pa, and the lifting gas flow rate is increased to accelerate the molten steel circulation. High-aluminum steel molten steel with P≤0.025%, S≤0.015%, and N≤0.01% is obtained. However, the treatment time in the refining process is long, and the oxidation amount of metallic aluminum is large, which is not conducive to the cleanliness of the molten steel; in the RH process, a process with a large lifting gas flow rate and a low vacuum chamber pressure is used for treatment, the stirring intensity of the molten steel is large, metallic aluminum is easily vaporized, and the decarburization effect in the net circulation treatment process of the vacuum furnace is not ideal, resulting in poor control of impurity elements such as P, S, and N in the product.

[0007] The patent application with the publication number CN116042958A provides an efficient refining preparation method for high-aluminum steel and high-aluminum steel. During the converter tapping process, 0.4 - 0.6 t of aluminum is first added, and then low-silicon steel hot refining slag, lime, and low-silicon pre-melted refining slag are added to make slag when entering the LF refining furnace, and then the remaining 9.4 - 9.6 t of aluminum is added for deoxidation alloying to improve the slag system composition of the ladle slag, which can reduce the secondary oxidation problem of aluminum in the refining process. However, metallic aluminum is also mainly added during the refining process, the power-on time is long, and it depends on bottom blowing stirring and power-on to stir metallic aluminum into and melt it into the molten steel. Metallic aluminum reacts violently with the slag and air, resulting in large consumption of metallic aluminum and deteriorating the cleanliness of the molten steel. The high-aluminum content molten steel is then transported to the RH deep vacuum treatment. Under high vacuum conditions, Al is easily vaporized, and it is more difficult to stably control the Al content during the steelmaking process.

[0008] It can be seen that in the field of high-purity high-aluminum steel, there is still an urgent need to propose a preparation method for high-purity high-aluminum steel with high process operability, stable product performance, and stable process. Summary of the Invention

[0009] In order to solve the above existing problems, the present invention discloses a high-purity high-aluminum steel and its preparation method. By optimizing and designing measures such as the deoxidation alloying and slag-making process of high-aluminum steel, the LF refining slag system, and the RH vacuum treatment system, the oxidation of metallic aluminum during the smelting process is reduced, and the removal of alumina inclusions in the molten steel is strengthened, so as to obtain a high-purity high-aluminum steel product.

[0010] The specific technical solutions are as follows:

[0011] A preparation method of high-purity high-aluminum steel is carried out according to the process flow of KR hot metal pretreatment → converter smelting → converter tapping → LF furnace refining → RH furnace refining → continuous casting. Calculating the total dissolved aluminum amount based on the metal aluminum content in the continuous casting billet and comparing it with the total added amount of metal aluminum, the recovery rate of metal aluminum element is above 85%. Specifically, it includes:

[0012] Step 1: KR hot metal pretreatment. Before desulfurization, the chemical composition of hot metal by mass percentage includes: C: 3.8% - 4.5%, Si: 0.2% - 0.7%, P ≤ 0.12%, S ≤ 0.03%, and the rest are Fe and other inevitable impurity components; the temperature is 1350 - 1430 °C; after desulfurization, the S content of hot metal is ≤ 0.0030%, and the slag skimming rate is ≥ 95%;

[0013] Step 2: Converter smelting. Using the hot metal and clean scrap steel from Step 1, the temperature of the molten steel at the end of the converter is 1630 - 1670 °C, the oxygen mass content is 0.045% - 0.065%, the S mass content is ≤ 0.005%, and a slide gate slag stopper is used during the tapping process;

[0014] Step 3: Converter tapping. When 30% of the steel is tapped, aluminum ingots, silicon alloy, manganese alloy, and ferrochrome alloy are added to the ladle in sequence for deoxidation alloying, and the bottom blowing flow rate is 200 - 300 NL / min; when 65% - 75% of the steel is tapped, all the alloys are added, and then lime and calcium aluminate synthetic slag are added to make slag, controlling the CaO / Al2O3 ratio of the slag: 1.7 - 2.2. At the end of tapping, the bottom blowing flow rate is 300 - 500 NL / min. After stirring for 3 - 5 min, it is transported to the LF furnace for treatment;

[0015] Step 4: LF furnace refining. The ladle is blown with small and medium bottom argon gas throughout the process. During the refining process, lime, calcium aluminate synthetic slag, and alkali metal carbonate are added to make slag. The alkalinity of the final refining slag is controlled above 20, and the slag contains CaO: 50% - 60%, Al2O3: 30% - 40%, SiO2 ≤ 2%, Na2O: 5% - 10%, T.Fe + MnO ≤ 1%. During the refining process, electric heating is used to raise the temperature, and metal aluminum, silicon alloy, manganese alloy, and ferrochrome alloy are added as supplements. After adjusting the composition and temperature of the molten steel to meet the standards, it is transported to the RH furnace for treatment;

[0016] Step 5: RH furnace refining. The pressure in the RH furnace vacuum chamber is controlled at 5 - 50 mbar, and the gas flow rate in the riser pipe of the RH vacuum chamber is 80 - 120 Nm 3 / h, and the holding time is above 12 min. Then, the pressure is released for calcium treatment. The feeding amount of pure calcium wire is 120 - 180 m / furnace, followed by soft stirring and calming treatment, and then it is transported to the continuous casting for pouring;

[0017] Step 6: Continuous casting. A slab continuous caster is used for pouring, with full protection during pouring. Control the superheat of molten steel in the tundish at 25 - 35°C, control the liquid level fluctuation in the mold within ±2 mm, and the frequency of electromagnetic stirring in the mold is 5 - 10 Hz, and the current is 400 - 600 A.

[0018] Further, in the converter smelting of Step 2, the proportion of clean scrap steel added is 10% - 15%. The composition of the clean scrap steel used in the converter includes: the mass content of P ≤ 0.015%, the mass content of S ≤ 0.010%, and the rest are elements such as C, Si, Al, Mn, Fe, and other inevitable impurity elements. At the same time, 2.5 - 3.5 kg / t of ferromolybdenum is equipped.

[0019] Further, the following relationship is satisfied between the mass M of aluminum ingots added during the tapping of the converter in Step 3 and the converter end point:

[0020] ,

[0021] When the mass content of S at the converter end point is greater than or equal to 0.01%:

[0022] ,

[0023] When the mass content of S at the converter end point is less than 0.01%:

[0024] ,

[0025] In the formula: w [O] represents the oxygen content in the converter tapping, unit ppm; M1 represents the weight of molten steel, unit t; w [Al] represents the target Al content, unit %; represents the Al recovery rate, unit %; Δ w [Al] represents the aluminum burning loss amount such as furnace slag air, unit %; λ represents the Al loss coefficient, which is related to the converter end point temperature and the end point S content; T represents the converter end point temperature, unit K; w [S] represents the converter end point S content, unit %.

[0026] Further, start adding aluminum ingots, 70% - 85% of ferromanganese alloy, 70% - 85% of ferrosilicon alloy, and all ferrochromium alloy when 30% of the converter tapping in Step 3.

[0027] Further, the chemical composition of the aluminum ingots in Step 3 includes by mass percentage: Al ≥ 98%, P ≤ 0.010%, and the rest are Fe and other inevitable impurity components;

[0028] The chemical composition of the ferromanganese alloy includes by mass percentage: Mn ≥ 98%, P ≤ 0.010%, and the rest are Fe and other inevitable impurity components;

[0029] The chemical composition of the silicon alloy by mass percentage includes: Si: 75% - 80%, P ≤ 0.015%, and the rest are Fe and other inevitable impurity components;

[0030] The chemical composition of the ferrochrome alloy by mass percentage includes: Cr: 55% - 65%, P ≤ 0.015%, C: 0.1% - 0.5%, and the rest are Fe and other inevitable impurity components;

[0031] The addition amount of calcium aluminate synthetic slag is 7 - 9 kg / t, and its chemical composition by mass percentage includes: CaO: 45% - 55%, Al2O3: 30% - 40%, SiO2 ≤ 5%, and other inevitable components.

[0032] Furthermore, in the refining process of the LF furnace in step 4, the mass content of CaO in the lime added is ≥ 95%, and the rest are inevitable impurity components; the addition amount of calcium aluminate synthetic slag is 0.5 - 1 kg / t, and its components by mass percentage include: CaO: 45% - 55%, Al2O3: 30% - 40%, SiO2 ≤ 5%, and other inevitable components; the chemical composition of the alkali metal carbonate component by mass percentage includes: Na2CO3: 60% - 70%, CaO: 20% - 30%, and other inevitable components.

[0033] Furthermore, in step 5, after the RH calcium treatment, the soft stirring time is ≥ 10 min and the calming time is ≥ 5 min.

[0034] Furthermore, in step 6, the whole continuous casting process is protected by pouring. The argon flow rate of the long nozzle of the tundish is 150 - 200 NL / min, the argon flow rate of the upper nozzle of the intermediate tundish is 3 - 8 NL / min, and the argon flow rate of the stopper rod is 5 - 10 NL / min; the starting casting tonnage of the continuous casting intermediate tundish is ≥ 45 t; during normal casting, the tonnage of the intermediate tundish is ≥ 60 t; when changing the tundish during continuous casting, the tonnage of the intermediate tundish is ≥ 45 t; the immersion depth of the submerged nozzle is 120 - 180 mm, and the continuous casting casting speed is controlled at 1.2 - 1.5 m / min.

[0035] A high-purity high-aluminum steel is obtained by the preparation method of the high-purity high-aluminum steel described above. The chemical composition of the high-purity high-aluminum steel by mass percentage includes: C: 0.06% - 0.15%, Si: 0.20% - 0.50%, Mn: 0.45% - 0.85%, Cr: 0.8% - 1.3%, Mo: 0.15% - 0.25%, Al: 0.60% - 0.90%, S ≤ 0.005%, P ≤ 0.015%, Ni ≤ 0.02%, Cu ≤ 0.015%, T.O ≤ 0.0010%, N ≤ 0.0025%, H ≤ 0.0002%, and the rest are Fe and other inevitable impurity components.

[0036] The working principle of the present invention is as follows:

[0037] In order to reduce the fluctuation of the recovery rate of metallic aluminum during the smelting process of high-aluminum steel and thus obtain high-aluminum steel products with stable chemical composition and high purity, strict control is carried out on the smelting process.

[0038] Firstly, deep desulfurization treatment of hot metal is carried out by using KR, and combined with clean scrap steel for converter smelting to obtain low-sulfur molten steel, reducing desulfurization during the LF refining process, that is, avoiding large bottom-blow stirring, which can reduce the oxidation of metallic aluminum and improve the cleanliness of molten steel.

[0039] Secondly, during the tapping process of the converter, a slide gate slag blocking operation is adopted to strictly control the slag flowing into the ladle from the converter, reducing the oxidability of the ladle top slag and reducing the oxidation of the aluminum ingots added to the ladle by the slag, thereby affecting the recovery rate of Al element and deteriorating the cleanliness of molten steel. During the tapping process of the converter, the addition amount of aluminum ingots is accurately controlled according to the end-point conditions to improve the recovery rate of Al element, so that the generated Al2O3 inclusions float up and are removed in time. Immediately after alloying is completed, lime and calcium aluminate synthetic slag are added for slag making, and the molten steel mixing and stirring during the tapping process can be used to quickly make white slag for deoxidation, desulfurization and inclusion adsorption, improving the cleanliness of molten steel.

[0040] Thirdly, during the LF refining process, lime and calcium aluminate synthetic slag and a low-melting-point, high-alkalinity calcium aluminate slag system containing alkali metal carbonate are used to improve the fluidity of the slag, which is beneficial to the adsorption of inclusions and can improve the cleanliness of molten steel. In addition, the CO2 gas generated by the decomposition of Na2CO3 is beneficial to the foaming of the refining slag, thus better isolating the air to prevent secondary oxidation of the molten steel and improving the recovery rate of aluminum element.

[0041] Finally, during the RH vacuum treatment process, the lowest vacuum chamber pressure is controlled at 5 - 50 mbar, and at the same time, the gas flow rate in the riser is reduced, so that the molten steel is circulated and treated for more than 12 minutes, which can ensure good removal of inclusions in the steel and reduce the erosion of the RH refractories by the molten steel; it can also avoid the gasification loss of aluminum element in the steel under high vacuum and improve the recovery rate of aluminum element. Combining measures such as soft stirring, calming treatment, and full-process protected casting during continuous casting, high-purity high-aluminum steel molten steel is obtained comprehensively.

[0042] The beneficial effects of the present invention are as follows:

[0043] (1) Deep desulfurization treatment of hot metal is carried out by using KR, and combined with low-sulfur scrap steel for converter smelting to obtain low-sulfur molten steel, reducing desulfurization during the LF refining process, that is, avoiding large bottom-blow stirring, which can reduce the oxidation of metallic aluminum and improve the cleanliness of molten steel.

[0044] (2) The high-aluminum steel is an aluminum-deoxidized steel. During the LF refining process, a high-alkalinity calcium aluminate-based refining slag is used for deoxidation, desulfurization, and inclusion adsorption. However, due to the high CaO content in the high-alkalinity slag, the viscosity of the refining slag is high and the fluidity is poor, which affects the effects of deoxidation, desulfurization, and inclusion adsorption, and further affects the cleanliness of the molten steel. The present invention uses lime, calcium aluminate synthetic slag, and alkali metal carbonate to make foam slag and reduce the melting point of the refining slag, which is not only beneficial to improving the cleanliness of the molten steel but also beneficial to increasing the recovery rate of aluminum element.

[0045] (3) During the RH high-vacuum treatment process, the molten steel is not treated under an extremely low vacuum pressure. The lowest vacuum pressure is controlled at 5 - 50 mbar to reduce the vaporization loss of aluminum in the molten steel, increase the recovery rate of aluminum element, and combine measures such as soft stirring and calming treatment to ensure the cleanliness of the molten steel. Specific Embodiments

[0046] The following further clarifies the present invention in conjunction with specific embodiments. It should be understood that the following specific embodiments are only used to illustrate the present invention and not to limit the scope of the present invention.

[0047] The present invention provides a high-purity high-aluminum steel and its preparation method. By optimizing the design of the deoxidation alloying and slag-making processes, LF refining slag system, RH vacuum treatment system, etc. of the high-aluminum steel, the oxidation of metallic aluminum during the smelting process is reduced, and the removal of alumina inclusions in the molten steel is strengthened to obtain a high-purity high-aluminum steel product.

[0048] First, deep desulfurization treatment of hot metal is carried out using KR, and combined with low-sulfur scrap steel for converter smelting to obtain low-sulfur molten steel, reducing desulfurization during the LF refining process, that is, avoiding large bottom blowing stirring, which can reduce the oxidation of metallic aluminum and improve the cleanliness of the molten steel.

[0049] Secondly, during the tapping process of the converter, alloys such as metallic aluminum, ferrosilicon, and metallic manganese are added to the molten steel. The alloys are quickly dissolved into the molten steel by the mixing of the molten steel, and a large amount of lime, synthetic slag, etc. are added during the later stage of tapping to protect the molten steel from being oxidized by air, that is, reducing the oxidation of alloying elements and increasing the recovery rate of alloying elements such as metallic aluminum.

[0050] Thirdly, during the LF refining process, an ultra-high-alkalinity, low-oxidation calcium aluminate slag is made, and an appropriate amount of calcium carbonate is added for slag melting treatment to improve the fluidity of the slag, reduce the oxidation of aluminum in the molten steel by the slag, and increase the recovery rate of metallic aluminum and the cleanliness of the molten steel.

[0051] Finally, under the conditions of high pressure and low lifting gas flow rate in RH, the molten steel is circulated for treatment to strengthen the removal of inclusions, reduce the vaporization of metallic aluminum, and then combined with measures such as calcium treatment, soft stirring, calming, and continuous casting protective casting to comprehensively improve the cleanliness of the molten steel, reduce the secondary oxidation of metallic aluminum, and obtain high-quality high-aluminum steel molten steel.

[0052] The specific process is as follows:

[0053] Step 1: KR hot metal pretreatment. Before desulfurization, the hot metal has the following composition: C: 3.8% - 4.5%, Si: 0.2% - 0.7%, P ≤ 0.12%, S ≤ 0.03%, and the temperature is 1350 - 1430°C; after desulfurization, the S content of the hot metal is ≤ 0.0030%, and the slag skimming rate is ≥ 95%.

[0054] Step 2: Converter smelting. The scrap ratio in the converter is 10% - 15%. The composition of the scrap used in the converter includes: P ≤ 0.015%, S ≤ 0.010%, and the rest are conventional elements such as C, Si, Al, Mn, Fe, and other inevitable impurity elements. At the same time, 2.5 - 3.5 kg / t of ferromolybdenum is equipped. The temperature of the molten steel at the end of the converter is 1630 - 1670°C, the oxygen content is 0.045% - 0.065%, the S content is ≤ 0.005%, and a slide gate slag stopper is used during tapping.

[0055] Step 3: Tapping from the converter. When 30% of the tapping is completed, aluminum ingots, 70% - 85% of ferromanganese, 70% - 85% of ferrosilicon, and all of the ferrochromium alloy are sequentially added to the ladle for deoxidation alloying. The composition of the aluminum ingots includes Al ≥ 98%, P ≤ 0.010%, and the rest are Fe and other inevitable impurity components; the composition of the ferromanganese includes Mn ≥ 98%, P ≤ 0.010%, and the rest are Fe and other inevitable impurity components; the ferrosilicon has Si: 75% - 80%, P ≤ 0.015%, and the rest are Fe and other inevitable impurity components; the ferrochromium alloy has Cr: 55% - 65%, P ≤ 0.015%, C: 0.1% - 0.5%, and the rest are Fe and other inevitable impurity components; the addition amount of calcium aluminate synthetic slag is 7 - 9 kg / t, and the composition includes CaO: 45% - 55%, Al2O3: 30% - 40%, SiO2 ≤ 5%, and other inevitable components. The bottom blowing flow rate is 200 - 300 NL / min; when 65% - 75% of the tapping is completed, all the alloys are added, and then lime and calcium aluminate synthetic slag are added to make slag, controlling the CaO / Al2O3 ratio of the slag: 1.7 - 2.2. At the end of tapping, the bottom blowing flow rate is 300 - 500 NL / min. After stirring for 3 - 5 minutes, it is transported to LF for treatment;

[0056] The mass M of the aluminum ingots added during tapping from the converter and the end point of the converter satisfy the following relationship:

[0057] ,

[0058] When the S content at the end point of the converter is higher than 0.01%:

[0059] ,

[0060] When the S content at the end point of the converter is lower than 0.01%:

[0061] ,

[0062] In the formula: w [O] represents the oxygen content in the molten steel tapped from the converter, in ppm; M1 represents the weight of the molten steel, in t; w [Al] represents the target Al content, in %; represents the Al recovery rate, in %; Δ w [Al] represents the aluminum burning loss amount such as that of slag and air, in %; λ represents the Al loss coefficient, which is related to the converter end temperature and the end S content; T represents the converter end temperature, in K; w [S] represents the converter end S content, in %.

[0063] Step 4: LF furnace refining. Argon gas is blown at a medium and small rate through the bottom of the ladle throughout the process. During the refining process, lime (CaO ≥ 95%, and the rest are inevitable impurity components), calcium aluminate synthetic slag (added amount 0.5 - 1 kg / t, the composition includes CaO: 45% - 55%, Al2O3: 30% - 40%, SiO2 ≤ 5%, and other inevitable components), and alkali metal carbonate are added to make slag. In the alkali metal carbonate, Na2CO3: 60% - 70%, CaO: 20% - 30%, and other inevitable components. The basicity of the final refining slag is controlled above 20, and in the slag, CaO: 50% - 60%, Al2O3: 30% - 40%, SiO2 ≤ 2%, Na2O: 5% - 10%, T.Fe + MnO ≤ 1%. During the refining process, electric heating is used to raise the temperature, and metallic aluminum, silicon alloy, manganese alloy, and ferrochrome alloy are additionally added. After adjusting all the molten steel composition and temperature to meet the standards, it is transported to RH for treatment.

[0064] Step 5: RH furnace refining. The pressure in the vacuum chamber is controlled at 5 - 50 mbar, the gas flow rate in the riser pipe is 80 - 120 Nm 3 / h, the holding time is 12 min or more, RH calcium treatment is carried out, the soft stirring time after treatment is ≥ 10 min, and the calming time is ≥ 5 min. Then it is transported to continuous casting for pouring.

[0065] Step 6: Continuous casting. A slab continuous caster is used for pouring, with full protection during the whole process. The argon gas flow rate of the long nozzle of the tundish is 150 - 200 NL / min, the argon gas flow rate of the upper nozzle of the tundish is 3 - 8 NL / min, and the argon gas flow rate of the stopper rod is 5 - 10 NL / min; the starting pouring tonnage of the tundish in continuous casting is ≥ 45 t; during normal pouring, the tundish tonnage is ≥ 60 t; when changing the tundish during continuous casting, the tundish tonnage is ≥ 45 t; the immersion depth of the submerged nozzle is 120 - 180 mm, and the continuous casting drawing speed is controlled at 1.2 - 1.5 m / min. The superheat of the molten steel in the tundish is controlled at 25 - 35 °C, the liquid level fluctuation in the mold is controlled within ± 2 mm, and the frequency of the electromagnetic stirring in the mold is 5 - 10 Hz and the current is 400 - 600 A.

[0066] The following are several specific embodiments of the present invention:

[0067] The high-purity high-aluminum steel is produced according to the technological process of KR hot metal pretreatment → converter smelting → converter tapping → LF furnace refining → RH furnace refining → continuous casting.

[0068] (1)KR hot metal pretreatment: Before KR desulfurization, the chemical components of the hot metal are shown in Table 1 in terms of mass percentage for C, Si, P, and S contents, and the rest are Fe and other inevitable impurity components; the hot metal temperature, the S content of the hot metal after desulfurization, and the slag skimming rate are also shown in Table 1.

[0069] Table 1 Conditions of hot metal in each heat and S content of hot metal after desulfurization and slag skimming rate

[0070]

[0071] (2)Converter smelting: The scrap ratio added to the converter and the mass contents of P and S elements in the clean scrap are shown in Table 2. The rest in the scrap are elements such as C, Si, Al, Mn, Fe, and other inevitable impurity elements. At the same time, ferromolybdenum is equipped and added together with the scrap, as shown in Table 2. The molten steel temperature, the oxygen mass content, and the S mass content at the end point of the converter are also shown in Table 2. The slide gate slag blocking is adopted during the tapping process.

[0072] Table 2 Converter scrap, scrap ratio, and converter end point parameters

[0073]

[0074] (3)Converter tapping: When 30% of the tapping is completed, aluminum ingots, part of the manganese alloy, part of the silicon alloy, and all of the ferrochrome alloy are sequentially added to the ladle for deoxidation alloying. The appropriate bottom blowing flow rate is controlled for deoxidation alloying, as shown in Table 3; when 65% - 75% of the tapping is completed, all the alloys are added, and then lime and calcium aluminate synthetic slag are added to make slag, controlling the CaO / Al2O3 of the slag. At the end of the tapping, the appropriate bottom blowing flow rate is controlled, and after stirring for a certain time, it is transported to the LF for furnace treatment. The detailed parameters are shown in Table 4. The chemical components of aluminum ingots, manganese alloy, silicon alloy, and ferrochrome alloy are shown in Table 5. The main chemical components of the calcium aluminate synthetic slag are shown in Table 6.

[0075] Table 3 Alloys and bottom blowing flow rate for deoxidation alloying in each heat

[0076]

[0077] Table 4 Alloys and bottom blowing flow rate for deoxidation alloying in each heat

[0078]

[0079] Table 5 Main chemical components of alloys in each heat

[0080]

[0081] Table 6 Main Chemical Compositions of Calcium Aluminate Synthetic Slag

[0082]

[0083] (4)LF furnace refining: During the whole process of the ladle, medium and small bottom argon blowing is carried out. During the refining process, lime, calcium aluminate synthetic slag and alkali metal carbonate are added to make slag. The basicity of the final refining slag is controlled above 20, and the composition of the final slag is shown in the table; during the refining process, the temperature is increased by electrifying, and metallic aluminum, silicon alloy, manganese alloy and ferrochrome alloy are additionally added. After adjusting the composition and temperature of the molten steel to meet the standards, it is transported to RH for treatment. The chemical compositions of alkali metal carbonate, lime and calcium aluminate synthetic slag are shown in the table.

[0084] Table 7 Calcium Aluminate Synthetic Slag Addition Amount during Refining Process and Main Chemical Compositions of End Point Slag

[0085]

[0086] Table 8 Main Chemical Compositions of Alkali Metal Carbonate, Lime and Calcium Aluminate Synthetic Slag

[0087]

[0088] (5)RH furnace refining: The pressure in the vacuum chamber of the RH furnace is controlled at 5 - 50 mbar, the gas flow rate in the riser pipe is 80 - 120 Nm 3 / h, and the holding time is more than 12 min. Then, the pressure is released for calcium treatment. The feeding amount of pure calcium wire is 120 - 180 m / furnace. After the treatment, the soft stirring time is ≥10 min, and the calming time is ≥5 min. Then it is transported to continuous casting for pouring. The specific parameters of each example are shown in Table 9.

[0089] Table 9 RH Vacuum Treatment, Calcium Treatment and Soft Stirring Calming Time Conditions

[0090]

[0091] (6)Continuous casting: A slab continuous casting machine is used for pouring. During the whole process of continuous casting, protective pouring is carried out. The argon gas flow rates of the long nozzle of the tundish, the upper nozzle of the intermediate tundish and the stopper rod are shown in Table 10; the tapping tonnage of the intermediate tundish at the start of casting, the tonnage of the intermediate tundish during normal pouring and the tonnage of the intermediate tundish when changing the tundish during continuous casting are shown in Table 10; the immersion depth of the submerged nozzle is 120 - 180 mm, and the continuous casting drawing speed is controlled at 1.2 - 1.5 m / min. The superheat of the molten steel in the intermediate tundish is controlled at 25 - 35 °C, the liquid level fluctuation in the mold is controlled within ±2 mm, and the frequency of the electromagnetic stirring in the mold is 5 - 10 Hz and the current is 400 - 600 A.

[0092] Table 10 Argon Gas Flow Rates of Each Component and Intermediate Tundish Tonnage under Each Condition

[0093]

[0094] Table 11 Parameters such as continuous casting speed and superheat

[0095]

[0096] (7) Through the above steps, the chemical composition of the high-aluminum steel and the recovery rate of aluminum element are shown in Table 12. The test results of the mechanical properties of the steel plate are shown in Table 13.

[0097] Table 12 Chemical composition of high-aluminum steel

[0098]

[0099] Continued: Table 12 Chemical composition of high-aluminum steel and recovery rate of aluminum element

[0100]

[0101] Table 13 Test results of mechanical properties of steel plate

[0102]

[0103] Taking the ideal embodiments of the present invention described above as an inspiration, through the above description, relevant staff can completely make various changes and modifications without departing from the technical idea of the present invention. The technical scope of the present invention is not limited to the content in the specification, and its technical scope must be determined according to the scope of the claims.

Claims

1. A method for preparing high-purity high-aluminum steel, characterized in that: The chemical composition of the high-purity high-aluminum steel includes, by mass percentage, C: 0.06%-0.15%, Si: 0.20%-0.50%, Mn: 0.45%-0.85%, Cr: 0.8%-1.3%, Mo: 0.15%-0.25%, Al: 0.60%-0.90%, S≤0.005%, P≤0.015%, Ni≤0.02%, Cu≤0.015%, TO≤0.0010%, N≤0.0025%, H≤0.0002%, and the rest are Fe and other inevitable impurity components; the high-purity high-aluminum steel is produced according to the KR molten iron pretreatment → converter smelting → converter steel tapping → LF furnace refining → RH furnace refining → continuous casting process flow, and the total amount of dissolved aluminum is calculated based on the metal aluminum content in the continuous casting billet to the total amount of metal aluminum added, and the recovery rate of the metal aluminum element is above 85%, specifically including: Step 1: KR hot metal pretreatment, before desulfurization, the chemical composition of hot metal includes by mass percentage: C: 3.8%-4.5%, Si: 0.2%-0.7%, P≤0.12%, S≤0.03%, and the rest is Fe and other inevitable impurities; temperature 1350-1430℃; after desulfurization, the S content of hot metal is ≤0.0030%, and the slag removal rate is ≥95%; Step 2: Converter smelting, using the molten iron and clean scrap steel from step 1, the converter terminal molten steel temperature is 1630-1670°C, the oxygen mass content is 0.045%-0.065%, the S mass content is ≤0.005%, and the slide plate is used to block the slag during the steel tapping process; Step 3: When the converter taps steel, when 30% of the steel is tapped, aluminum ingots, silicon alloys, manganese alloys, and ferrochrome alloys are added to the ladle in sequence for deoxidation and alloying, and the bottom blowing flow rate is 200-300NL / min; when 65%-75% of the steel is tapped, all the alloys are added, and then lime and calcium aluminate are added to synthesize slag to control the slag CaO / Al2O3 ratio of 1.7-2.

2. When the tapping is completed, the bottom blowing flow rate is 300-500NL / min, and after stirring for 3-5 minutes, it is transported to the LF for furnace treatment; Step 4: LF furnace refining, the ladle is opened with small and medium bottom blowing argon throughout the whole process, lime, calcium aluminate synthetic slag and alkali metal carbonate are added in the refining process to make slag, the basicity of the final slag is controlled above 20, CaO: 50%-60%, Al2O3: 30%-40%, SiO2≤2%, Na2O: 5%-10%, T.Fe+MnO≤1% in the slag, the refining process is powered on to increase the temperature, and metal aluminum, silicon alloy, manganese alloy, chromium iron alloy are added, and after the composition and temperature of the molten steel are all adjusted to meet the standards, it is transported to the RH for treatment; The components of calcium aluminate synthetic slag include, by mass percentage, CaO: 45%-55%, Al2O3: 30%-40%, SiO2≤5%, and other inevitable components; the components of alkali metal carbonate include, by mass percentage, Na2CO3: 60%-70%, CaO: 20%-30%, and other inevitable components; Step 5: RH furnace refining, the RH furnace vacuum chamber pressure is controlled at 5-50mbar, and the riser gas flow rate of the RH vacuum chamber is 80-120Nm 3 / h, keep for more than 12min, then break the air for calcium treatment, the feeding amount of pure calcium wire is 120-180m / furnace, soft stirring, calming treatment, and then transported to continuous casting; Step 6: Continuous casting, using slab continuous casting machine for casting, protecting the casting throughout the whole process, controlling the superheat of the molten steel in the tundish to 25-35°C, the crystallizer liquid level fluctuation to ±2mm, the crystallizer electromagnetic stirring frequency to 5-10Hz, and the current to 400-600A.

2. The method for preparing high-purity high-aluminum steel according to claim 1, characterized in that: The clean scrap steel added in the converter smelting in step 2 accounts for 10%-15%. The components of the clean scrap steel used in the converter include: P mass content ≤0.015%, S mass content ≤0.010%, and the rest are C, Si, Al, Mn, Fe elements, and other unavoidable impurity elements, and 2.5-3.5 kg / t of ferromolybdenum is also provided.

3. The method for preparing high-purity high-aluminum steel according to claim 1, characterized in that: In step 3, the mass M of aluminum ingot added to the converter for steel production and the converter end point satisfy the following relationship: ， When the S mass content at the converter end point is greater than or equal to 0.01%: ， When the S mass content at the converter end point is less than 0.01%: ， Where: w [O] represents the oxygen content of steel tapped from the converter, in ppm; M1 represents the weight of molten steel, in t; w [Al] represents the target Al content, unit: %; ƞ represents the Al yield, unit: %;Δ w [Al] represents the aluminum loss from slag, air, etc., in units of %; λ represents the Al loss coefficient, which is related to the converter end temperature and the end S content; T represents the converter end temperature, in units of K; w [S] represents the S content at the converter end point, unit: %.

4. The method for preparing high-purity high-aluminum steel according to claim 1, characterized in that: In step 3, when 30% of the steel is tapped from the converter, aluminum ingots, 70%-85% of the manganese alloy, 70%-85% of the silicon alloy and all of the ferrochrome alloy are added.

5. The method for preparing high-purity high-aluminum steel according to claim 1, characterized in that: The chemical composition of the aluminum ingot in step 3 includes, by mass percentage: Al≥98%, P≤0.010%, and the rest is Fe and other inevitable impurity components; The chemical composition of manganese alloy includes, by mass percentage: Mn ≥ 98%, P ≤ 0.010%, and the rest is Fe and other inevitable impurity components; The chemical composition of silicon alloy includes by mass percentage: Si: 75%-80%, P≤0.015%, and the rest is Fe and other inevitable impurity components; The chemical composition of ferrochrome alloy includes, by mass percentage: Cr: 55%-65%, P≤0.015%, C: 0.1%-0.5%, and the rest is Fe and other inevitable impurities; The addition amount of calcium aluminate synthetic slag is 7-9kg / t, and the chemical composition by mass percentage includes: CaO: 45%-55%, Al2O3: 30%-40%, SiO2≤5%, and other inevitable components.

6. The method for preparing high-purity high-aluminum steel according to claim 1, characterized in that: In the step 4, the CaO mass content in the lime added during the LF furnace refining process is ≥95%, and the rest are inevitable impurity components; the amount of calcium aluminate synthetic slag added is 0.5-1 kg / t.

7. The method for preparing high-purity high-aluminum steel according to claim 1, characterized in that: After the RH calcium treatment in step 5 is completed, the soft stirring time is ≥10 min, and the sedation time is ≥5 min.

8. The method for preparing high-purity high-aluminum steel according to claim 1, characterized in that: In step 6, the continuous casting is protected throughout the casting process, the argon flow rate of the long water inlet of the large ladle is 150-200NL / min, the argon flow rate of the upper water inlet of the medium ladle is 3-8NL / min, and the argon flow rate of the stopper rod is 5-10NL / min; the tonnage of the continuous casting tundish is ≥45t; the tonnage of the tundish during normal casting is ≥60t; the tonnage of the tundish when continuously casting and changing the large ladle is ≥45t; the insertion depth of the immersion water inlet is 120-180mm, and the continuous casting pulling speed is controlled at 1.2-1.5m / min.

9. A high-purity high-aluminum steel, characterized in that: The steel is prepared by the method for preparing high-purity high-aluminum steel according to any one of claims 1 to 8.

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