Ultra-low-nitrogen and ultra-low-sulfur steel and smelting method thereof

Abstract

The invention discloses ultralow-nitrogen ultralow-sulfur steel and a smelting method thereof. The smelting method comprises the following steps that S1, molten iron pretreatment is conducted, specifically, molten iron is deeply desulfurized till the S content is smaller than or equal to 20 ppm; s2, primary smelting is conducted, specifically, oxygen blowing decarburization is conducted through a primary smelting furnace, argon bottom blowing is conducted in the whole process to stir molten steel, a slagging material, an alloy and a deoxidizing agent are put into a steel ladle in the tapping process of the primary smelting furnace, and limestone is added into the steel ladle before tapping is finished; s3, LF refining is conducted, argon bottom blowing is conducted in the whole process, the molten steel is stirred, and bauxite clinker and active lime are added in the LF refining process for rapid slagging; and S4, pouring is conducted, specifically, in the molten steel pouring process, the water gap positions of the steel ladle and the tundish are sealed through argon. According to the method, the nitrogen increasing amount of the molten steel is effectively reduced while the sulfur content in the molten steel is controlled to reach the standard, and therefore the requirements for the nitrogen content and the sulfur content in the ultralow-nitrogen and ultralow-sulfur steel are met.

Description

Technical Field

[0001] The invention relates to the technical field of steelmaking in metallurgical science, and in particular to ultra-low nitrogen and ultra-low sulfur steel and a smelting method thereof. Background Art

[0002] For high-end thick plates, pipeline products and cold-rolled ultra-high strength steel, the KR-LD-LF-RH-CC process is generally used for smelting; the nitrogen content in the steel directly affects the quality of the ingot, which in turn affects the hot delivery rate of the ingot, the quality of subsequent hot rolling and cold rolling products. In addition, in order to control the amount of sulfides in the steel, the sulfur in the steel needs to be reduced to 20ppm or even below 10ppm. How to stably control the nitrogen content in the steel of such products below the specification at a low cost while ensuring that the finished sulfur in the molten steel meets the requirements is a common challenge faced by modern steel mills.

[0003] In the process of producing ultra-low sulfur and low nitrogen steel with the KR-LD-LF-RH-CC process, the sulfur content in the molten steel changes as follows: KR removes sulfur from the molten iron to an extremely low level, the sulfur content in the molten steel in the LF converter increases due to the influence of raw materials or desulfurization slag, and the LF furnace heats up and slags to remove sulfur from the steel to below the specification content. The nitrogen content in the molten steel changes as follows: the nitrogen content in the molten iron increases during KR desulfurization, the LD converter removes most of the nitrogen in the molten steel during the decarburization process, the nitrogen content in the molten steel rises sharply during steel tapping and LF furnace treatment, the nitrogen content in the molten steel decreases during RH vacuum treatment, and then rises slightly during continuous casting. It can be seen from this that desulfurization and nitrogen control of molten iron or steel are mutually exclusive.

[0004] Relevant technical searches were conducted using low-nitrogen steel / denitrification as keywords. Chinese patent CN101553583B uses aluminum combustion chemical heat to replace LF furnace electrical heating, thereby smelting ultra-low sulfur and ultra-low nitrogen steel with finished sulfur less than 10ppm and finished nitrogen less than 50ppm at low cost. Equivalent lime 6-16kg / t steel and Al 1.5-7kg / t steel are added during the steel tapping process after the furnace; the molten steel is heated in the ladle by top-blown oxygen aluminothermic method, and quantitative Ar gas is used to stir and mix the molten steel to remove sulfur from the molten steel while controlling the amount of nitrogen added to the molten steel; the actual use of aluminothermic method to adjust the temperature of the molten steel leads to an increase in the amount of slag, and RH degassing and refining treatment may not be possible for steel varieties with strict dehydrogenation requirements. Chinese patent CN105452504B uses special equipment to produce high manganese steel. Conventional molten steel is smelted in a converter and discharged into a ladle. At the same time, ferromanganese is melted in a conventional induction furnace and the temperature is maintained, and then poured into the ladle containing molten steel for pouring. When the ferromanganese alloy is melted in the induction furnace, nitrogen in the environment can easily enter the ferromanganese melt to cause nitrogen increase. This technology uses Ar gas to control the atmosphere in the induction furnace and adjust the silicon content in the ferromanganese melt to achieve the goal of controlling the increase in the nitrogen content of the melt during the melting and insulation of ferromanganese. This process for producing high manganese steel requires the installation of an induction furnace, which increases the complexity of the smelting process. In addition, the ferromanganese melt inevitably comes into contact with the atmosphere during the process of being added to the ladle, absorbing nitrogen in the environment, resulting in an increase in the nitrogen content of the melt.

[0005] In view of the above situation, it is urgent to develop a technology for producing ultra-low nitrogen and ultra-low sulfur steel, which can effectively reduce the amount of nitrogen added to the molten steel while controlling the sulfur content in the molten steel to meet the standards, so as to meet the requirements for nitrogen and sulfur content in ultra-low nitrogen and ultra-low sulfur steel. Summary of the invention

[0006] In view of the problem of increased nitrogen content in molten steel during desulfurization in the prior art, the purpose of the present invention is to provide an ultra-low nitrogen and ultra-low sulfur steel and a smelting method thereof, which can effectively reduce the amount of nitrogen added to the molten steel while controlling the sulfur content in the molten steel to meet the standard, thereby meeting the requirements for nitrogen and sulfur content in the ultra-low nitrogen and ultra-low sulfur steel.

[0007] To achieve the above object, the present invention adopts the following technical solution:

[0008] A first aspect of the present invention provides a method for smelting ultra-low nitrogen and ultra-low sulfur steel, comprising the following steps:

[0009] S1, hot metal pretreatment, deep desulfurization of hot metal to a sulfur content of ≤20ppm;

[0010] S2, primary refining, using oxygen blowing decarburization in the primary refining furnace, using bottom blowing argon to stir the molten steel throughout the process, adding slag making materials, alloys and deoxidizers into the ladle during the steel tapping process of the primary refining furnace, and adding limestone into the ladle before the steel tapping is completed;

[0011] S3, LF refining, uses bottom blowing argon to stir the molten steel throughout the process, and adds bauxite clinker and active lime to quickly make slag during the LF refining process;

[0012] S4, pouring. During the pouring of molten steel, the water inlet of the ladle and the tundish is sealed with argon gas.

[0013] Preferably, in step S2, the order of adding slag material, alloy and deoxidizer during the steel tapping process is as follows:

[0014] When the steel output reaches 1 / 5, 4-6 kg / t steel of active lime is added to the ladle. When the steel output reaches 1 / 3, alloy is added according to the lower limit of the finished product composition of ultra-low nitrogen and ultra-low sulfur steel. Before the end of steel output, 0.5-2 kg / t steel of limestone is added to the ladle, and aluminum and aluminum slag are added according to the acid-soluble aluminum content of 0.005-0.01 wt% in the molten steel.

[0015] Preferably, during the steel tapping process, the amount of aluminum slag added is 0.3-1.0 kg / t steel, and the amount of aluminum added is 0.3-0.5 kg / t steel.

[0016] Preferably, in step S2, the S content in the molten steel in the ladle after tapping is ≤100 ppm.

[0017] Preferably, in step S3, at the initial stage of LF refining, the flow rate of bottom blowing argon is controlled to be ≤3.0Nl / (min·t steel), and the power of the electrode is controlled to be ≤70kW / t steel; and at the later stage of LF refining, the flow rate of bottom blowing argon is controlled to be ≤7.0Nl / (min·t steel), and the power of the electrode is controlled to be ≤130kW / t steel.

[0018] Preferably, in step S3, at the initial stage of LF refining, 0.5-1.5 kg / t steel of bauxite clinker is added, and then 4-10 kg / t steel of active lime is added.

[0019] Preferably, the particle size of the bauxite clinker is 5 to 20 mm; the content of Al2O3 in the bauxite clinker is 70 to 80 wt%, the content of Fe2O3 is ≤2 wt%, and the content of TiO2 is ≤2 wt%.

[0020] Preferably, the particle size of the active lime is 5 to 30 mm; the CaO content in the active lime is ≥ 98%.

[0021] Preferably, the bauxite clinker and active lime are added as slag in batches, and the amount of slag added in each batch is ≤2kg / t steel.

[0022] Preferably, in step S3, the proportion of heats in which the nitrogen content in the molten steel is ≤50 ppm after the LF refining is greater than 98%.

[0023] Preferably, after LF refining is completed, hydrogen and inclusions in the molten steel are removed by vacuum treatment, and nitrogen in the steel is further removed.

[0024] Preferably, the vacuum treatment uses an RH vacuum furnace.

[0025] Preferably, in step S4, the composition of the molten steel satisfies: S≤50ppm, N≤50ppm, Mn: 00.1-3.5%, Al: 0.015-0.1%.

[0026] Preferably, in the molten steel, S≤20ppm, and N≤31ppm.

[0027] The second aspect of the present invention provides an ultra-low nitrogen and ultra-low sulfur steel obtained by the smelting method of ultra-low nitrogen and ultra-low sulfur steel as described in the first aspect of the present invention.

[0028] Compared with the existing production technology of ultra-low nitrogen and ultra-low sulfur steel, the present invention has the following beneficial effects:

[0029] 1. The present invention can effectively control the ultra-low nitrogen and ultra-low sulfur steel (sulfur content is less than 20ppm or even 10ppm) after LF refining, and the nitrogen content in the molten steel is ≤50ppm, with the proportion of the furnaces exceeding 98%;

[0030] 2. The present invention performs deep desulfurization during the pretreatment of molten steel, adds slag-making materials, alloys and deoxidizers into the ladle during the steel-making process to reduce nitrogen absorption by molten steel, sequentially adds bauxite clinker and active lime to quickly slag during the LF furnace refining process, and blows Ar gas from the bottom throughout the process to prevent nitrogen increase in molten steel during rapid desulfurization, and then uses Ar gas sealing during the pouring process to inhibit nitrogen absorption by molten steel, thereby finally meeting the nitrogen and sulfur content requirements of ultra-low nitrogen and ultra-low sulfur steel;

[0031] 3. The present invention uses bauxite clinker instead of traditional aluminum slag in the LF furnace refining process, which has a fast slag forming speed, low raw material cost and reduces the nitrogen content of liquid steel;

[0032] 4. The present invention can also use RH refining after LF furnace refining to remove hydrogen and inclusions in the molten steel, and further remove nitrogen in the molten steel, thereby obtaining an ultra-low nitrogen and ultra-low sulfur steel with a finished product sulfur content of less than 31 ppm and less than 20 ppm. DETAILED DESCRIPTION

[0033] In order to better understand the above technical solution of the present invention, the technical solution of the present invention is further described below in conjunction with embodiments.

[0034] For ultra-low nitrogen and ultra-low sulfur steel, the present invention adds active lime and limestone during the steelmaking process of the primary smelting furnace, replaces aluminum slag with bauxite clinker in the LF furnace, and adds slag in a predetermined order, so as to effectively reduce the nitrogen increase in the molten steel and meet the requirements of nitrogen and sulfur content of ultra-low nitrogen and ultra-low sulfur steel.

[0035] The ultra-low nitrogen and ultra-low sulfur steel smelting method of the present invention is applicable to the finished molten steel composition range (wt%): [S]≤0.005, [N]≤0.005, [Mn]=0.1-3.5, [Al]=0.015-0.1.

[0036] The smelting method of ultra-low nitrogen and ultra-low sulfur steel of the present invention is applicable to the following two process flows:

[0037] (1) Hot metal pretreatment - primary smelting in primary smelting furnace (converter or AOD furnace) - LF refining - pouring (mold casting or continuous casting);

[0038] (2) Hot metal pretreatment - primary smelting in primary smelting furnace (converter or AOD furnace) - LF refining - vacuum treatment (RH vacuum furnace) - pouring (mold casting or continuous casting);

[0039] The smelting method of ultra-low nitrogen and ultra-low sulfur steel of the present invention comprises the following steps:

[0040] S1, hot metal pretreatment, deep desulfurization of hot metal to a sulfur content of ≤20ppm;

[0041] Hot metal pretreatment is mainly for sulfur control, which is used to control the sulfur content in the hot metal within a lower range.

[0042] S2, primary refining, using oxygen blowing decarburization in the primary refining furnace, blowing argon gas at the bottom to stir the molten steel throughout the process, adding slag material, alloy and deoxidizer into the ladle during the steel tapping process of the primary refining furnace, and adding limestone into the ladle before the steel tapping is completed;

[0043] Specifically, oxygen blowing decarburization is carried out in a primary furnace (such as a converter or AOD furnace). During the primary refining process, a large amount of CO or CO2 gas escapes from the molten steel during the decarburization process, and most of the nitrogen in the molten steel is removed. At the same time, the amount of CO or CO2 gas escaping in the late and final stages of smelting is reduced, and there is a risk of nitrogen increase. In order to ensure the efficiency of nitrogen removal during the decarburization period, Ar gas bottom blowing and stirring are used throughout the process, and the bottom blowing is ensured to be intact. At the same time, at the end of the converter smelting, no deoxidizing alloys such as manganese and silicon are added to the furnace to ensure the oxygen content of the molten steel and reduce the nitrogen absorption of the molten steel during steelmaking.

[0044] During the steel-making process, slag-making materials, alloys and deoxidizers are added to the ladle; the slag-making materials can be active lime; the alloys are alloy components in the finished ultra-low nitrogen and ultra-low sulfur steel products, such as manganese, silicon and aluminum; the deoxidizers can be aluminum, ferrosilicon and ferromanganese. Since the molten steel flowing into the ladle during steel-making carries strong kinetic energy, it is conducive to the full mixing of the added materials and the molten steel, the heating of the slag and the partial melting into liquid slag. In order to cover the surface of the molten steel with liquid slag as soon as possible, active lime should be added as early as possible. For example, the alloy components in the finished product, such as manganese or ferromanganese, silicon or ferrosilicon, should be added after the furnace. At the end of steel-making, a small amount of limestone is added to form a weak oxidizing atmosphere with the decomposed CO2 to inhibit the absorption of nitrogen by the molten steel. It is particularly pointed out that the added limestone is limited to the addition of molten steel at the end of steel-making. If it is added to the molten steel too early, the gas generated by the decomposition of limestone is easily released in a concentrated manner, forming a slight explosion in the molten steel, creating safety risks. A small amount of aluminum and aluminum slag is added at the end of steel tapping, with the acid-soluble aluminum content in the molten steel being between 0.005 and 0.01%, thereby reducing the nitrogen increase in the molten steel.

[0045] During the steel tapping process, the order of adding slag-making material, alloy and deoxidizer into the ladle is as follows: when the steel tapping volume reaches 1 / 5, 4-6 kg / t steel of active lime is added into the ladle; when the steel tapping volume reaches 1 / 3, alloys such as manganese or ferromanganese, ferrosilicon, etc. are added according to the lower limit of the finished product composition of ultra-low nitrogen and ultra-low sulfur steel; before the end of steel tapping, 0.5-2 kg / t steel of limestone is added into the ladle in sequence, and aluminum and aluminum slag are added into the ladle according to the acid-soluble aluminum content of 0.005-0.01% in the molten steel. In a specific embodiment, 0.3-1.0 kg / t steel of aluminum slag and 0.3-0.5 kg / t steel of aluminum are added.

[0046] In the above process, the sulfur content of the added charge must be strictly controlled to ensure that the [S] of the molten steel in the ladle after steelmaking is ≤100ppm. On the one hand, this can meet the sulfur content requirements of the ultra-low carbon steel product; on the other hand, for ultra-low sulfur steel, it can reduce the desulfurization load during subsequent LF refining, thereby reducing the amount of nitrogen added to the molten steel during the LF furnace refining process.

[0047] S3, LF refining, argon is blown from the bottom to stir the molten steel throughout the whole process, and bauxite clinker and active lime are added during the LF refining process for rapid slagging;

[0048] Specifically, during the LF refining process, argon is blown from the bottom of the ladle to stir the molten steel, which not only promotes the uniformity of the temperature of the molten steel composition, but also fully mixes the molten steel and slag to improve the desulfurization efficiency of the molten steel. If the bottom blowing argon flow rate is too small, the required mixing effect cannot be achieved. If the argon flow rate is too large, the molten steel is easily exposed and comes into contact with the ambient gas, increasing the risk of nitrogen increase.

[0049] In the early stage of LF refining, the flow rate of bottom blowing argon is controlled to be ≤3.0Nl / (min·t steel), and the power of electrode is ≤70kW / t steel. In the late stage of LF refining, the flow rate of bottom blowing argon is controlled to be ≤7.0Nl / (min·t steel), and the power of electrode is ≤130kW / t steel.

[0050] Regarding LF refining and slag making, from the perspective of metallurgical effects, it is necessary to quickly remove sulfur from the molten steel to below the target value, while controlling the increase in nitrogen content in the molten steel; in this case, the LF furnace and part of the slag added after the furnace need to form liquid slag as soon as possible to increase the desulfurization rate and wrap the molten steel surface to prevent nitrogen increase in the molten steel.

[0051] In the early stage of LF refining, 0.5-1.5kg / t steel of bauxite clinker is added, and the particle size of bauxite clinker is 5-20mm; the content of Al2O3 in the bauxite clinker is 70-80wt%, the content of Fe2O3 is ≤2wt%, and the content of TiO2 is ≤2wt%. Subsequently, 4-10kg / t steel of active lime is added, and the particle size of the active lime is 5-30mm; the content of CaO in the active lime is ≥98%. In particular, bauxite clinker and active lime are used as slag materials, which need to be added in batches, and the amount of slag added in each batch is ≤2kg / t steel, so as to prevent the slag materials from accumulating on the surface of the molten steel, which is not conducive to power supply and slag melting.

[0052] Through the above process, the proportion of heats with nitrogen content ≤50ppm in molten steel after LF refining is greater than 98%.

[0053] For steel types with strict requirements on hydrogen content or inclusions in steel, after LF refining, it is necessary to remove hydrogen and inclusions in the molten steel through vacuum treatment, and further remove nitrogen in the steel; in a preferred embodiment, the vacuum treatment adopts an RH vacuum furnace.

[0054] S4, pouring. During the pouring of molten steel, the water inlet of the ladle and the tundish is sealed with argon gas.

[0055] Specifically, after the above treatment, the composition of the molten steel meets the following requirements: S content is less than 20ppm, N content is less than 50ppm; in a preferred embodiment, N content is less than 31ppm (molten steel subjected to vacuum treatment after LF refining); the temperature of the molten steel meets the following requirements: the lower limit of the temperature meets the requirement that the molten steel is in liquid state and has good fluidity before the molten steel is poured, and the upper limit cannot be too high, otherwise energy will be wasted and the cooling water consumption will be too large during solidification. After that, the molten steel is poured, and the flow of molten steel at the water inlet of the ladle and the tundish generates negative pressure. For this reason, argon gas sealing is used to inhibit nitrogen absorption by the molten steel, reduce nitrogen increase in the molten steel, and finally obtain an ultra-low nitrogen and ultra-low sulfur steel product with a finished nitrogen content of less than 50ppm and an S content of less than 20ppm.

[0056] The present invention also provides an ultra-low nitrogen and ultra-low sulfur steel prepared by the above smelting method, the composition of which satisfies: S≤50ppm, N≤50ppm, Mn: 00.1-3.5%, Al: 0.015-0.1%. In a preferred embodiment, in the ultra-low nitrogen and ultra-low sulfur steel, S≤20ppm, N≤31ppm.

[0057] The ultra-low nitrogen and ultra-low sulfur steel and the smelting method thereof of the present invention are further introduced below with reference to specific examples.

[0058] Take cold-rolled ultra-high strength steel with main components [%C]=0.01-0.1, [%Si]≤0.5, [%Mn]=0.5-3.5, [%Al]=0.02-0.1, [%Ti]=0.01-0.08, [%P]≤0.015, [%S]≤0.002, [%N]≤0.0035 as an example.

[0059] Example

[0060] The process path adopted in this embodiment is: molten iron desulfurization - converter top and bottom combined blowing smelting - steel tapping (part of the slag) - LF furnace refining (heating, slag making, deoxidation, desulfurization, alloying) - RH refining (degassing, adjusting composition) - continuous casting - hot rolling.

[0061] Taking a typical heat (S1) using the smelting method of the present invention as an example, the specific process is as follows:

[0062] (1) Hot metal pretreatment: Hot metal is desulfurized to a sulfur content of 8 ppm.

[0063] (2) Primary smelting in converter: converter slag making, oxygen supply, bottom blowing Ar gas stirring.

[0064] Tapping from converter: When the tapping volume reaches 1 / 5, add 5.5kg / t of active lime to the ladle. When the tapping volume reaches 1 / 3, add manganese, followed by 0.5kg / t of ferrosilicon. Before the tapping is completed, add 1.2kg / t of limestone, 0.5kg / t of aluminum slag, and 0.35kg / t of aluminum to the ladle in sequence. The nitrogen content of the ladle steel liquid is 18ppm, the phosphorus content of the ladle steel liquid is 95ppm, and the sulfur content is 70ppm.

[0065] (3) LF refining: At the beginning of LF refining, 0.7kg / t steel of bauxite clinker is added, followed by 4.2kg / t steel of active lime; bauxite clinker and active lime are used as slag materials and need to be added in batches, with the amount of slag added in each batch being ≤2kg / t steel. At the beginning of refining, the bottom blowing gas flow rate is controlled at 2.2Nl / (min·t steel) and the electrode power is 60kW / t steel. At the end of power-on, the power is 120kW / t steel, and the bottom blowing gas flow rate is controlled at 5.8Nl / (min·t steel). At the end of LF refining, the liquid nitrogen content of ladle steel is 42ppm, the phosphorus content of ladle steel is 101ppm, and the sulfur content is 12ppm.

[0066] (4) RH vacuum treatment: RH vacuum furnace is used for vacuum treatment to remove hydrogen and inclusions in the molten steel, and further remove nitrogen from the molten steel. After the vacuum treatment, the nitrogen content in the molten steel is 27ppm.

[0067] (5) During pouring, during the pouring of molten steel with qualified composition and temperature, Ar gas is used to seal the water inlet of the ladle and the tundish; the nitrogen content of the molten steel in the tundish is 28ppm; and the final product can have a nitrogen content of less than 31ppm and a sulfur content of less than 20ppm, which is an ultra-low nitrogen and ultra-low sulfur steel product;

[0068] Heats S2-S6 adopt the same process flow as that of heat S1. The materials added during the smelting process and the control parameters are shown in Table 1. The sulfur content and nitrogen content of the molten steel during the smelting process are shown in Table 2.

[0069] Comparative Example

[0070] A typical heat (S7) of comparative smelting is adopted, and the specific process is as follows:

[0071] (1) Hot metal pretreatment: Hot metal is desulfurized to a sulfur content of 7 ppm.

[0072] (2) Primary smelting in converter: converter slag making, oxygen supply, bottom blowing Ar gas stirring.

[0073] Converter tapping: When the tapping volume reaches 1 / 5, add 2.5 / t of active lime to the ladle, and when the tapping volume reaches 1 / 3, add 2 / 3 of manganese. Before the tapping is completed, add 1.5kg / t of aluminum slag and 1kg / t of aluminum to the ladle. The nitrogen content of the ladle steel liquid is 30ppm, the phosphorus content of the ladle steel liquid is 100ppm, and the sulfur content is 74ppm.

[0074] (3) LF refining: At the beginning of LF refining, 1.2 kg / t steel of aluminum slag is added. Subsequently, 7.6 kg / t steel of active lime is added. At the beginning of refining, the bottom blowing gas flow rate is controlled to be 4 Nl / (min·t steel), and the electrode power is 80 kW / t steel. At the end of power-on, the power is 150 kW / t steel, and the bottom blowing gas flow rate is controlled to be 8.5 Nl / (min·t steel) at the end of power-on. At the end of LF refining, the nitrogen content of the ladle steel liquid is 75 ppm, the phosphorus content of the ladle steel liquid is less than 105 ppm, and the sulfur content is 10 ppm.

[0075] (4) RH vacuum treatment: RH vacuum treatment is carried out in an RH vacuum furnace to remove hydrogen and inclusions from the molten steel, and further remove nitrogen from the molten steel. The nitrogen content of the molten steel after vacuum treatment is 42ppm

[0076] (5) Pouring: During the pouring of molten steel with qualified composition and temperature, Ar gas is used to seal the water inlet of the ladle and the tundish. The nitrogen content of the molten steel in the tundish is 45ppm.

[0077] In Table 1 and Table 2, furnaces S1-S6 are implementation cases in the actual production process of the present invention, and furnaces S7-S12 are traditional ultra-low sulfur and low nitrogen refining processes. Table 1 shows the materials added in the smelting process and the control parameters in the LF process in Example 1, Example 2 and the comparative example; Table 2 shows the S content and N content in the molten steel during the smelting process. Combined with what is shown in Table 2, the present invention performs desulfurization and nitrogen control treatment on the molten steel, and at the end of LF refining, the nitrogen content of ultra-low sulfur steel (sulfur content less than 20ppm or even 10ppm) in the molten steel is ≤50ppm in more than 98% of the furnaces; after RH degassing and refining, a finished product with a nitrogen content of less than 31ppm and a sulfur content of less than 20ppm of ultra-low nitrogen and ultra-low sulfur steel products can be obtained; during LF refining, the present invention uses bauxite clinker instead of traditional aluminum slag, and the slag forming speed is fast, the raw material cost per ton of steel is reduced by 1.5 yuan, and the nitrogen content of the molten steel is reduced.

[0078] Table 1 Parameters during smelting

[0079]

[0080] Table 2 S content and N content in molten steel during smelting

[0081]

[0082]

[0083] It can be seen from the embodiments and comparative examples that compared with the traditional ultra-low sulfur and ultra-low nitrogen steel smelting process, the present invention can enable ultra-low nitrogen and ultra-low sulfur steel to obtain a lower finished nitrogen content while ensuring that the sulfur content of the molten steel is less than 20 ppm and without increasing the smelting cost.

[0084] In summary, the smelting method of ultra-low nitrogen and ultra-low sulfur steel of the present invention does not require modification of existing equipment. It only needs to use conventional primary smelting furnaces, LF furnaces, etc., use a small amount of limestone after the furnace, and use bauxite clinker instead of aluminum slag in the LF furnace. Adjust the relevant process steps and control the process parameters to achieve the established metallurgical goals. Compared with the traditional ultra-low sulfur and ultra-low nitrogen steel refining process, the present invention can obtain a lower finished product nitrogen content while ensuring that the sulfur content of the molten steel is less than 20ppm and does not increase the smelting cost; it is suitable for ultra-low nitrogen and ultra-low sulfur steel smelting, and has great application value in steel mills.

[0085] It should be noted that those skilled in the art should recognize that the above embodiments are only used to illustrate the present invention and are not intended to limit the present invention. As long as they are within the spirit of the present invention, any changes or modifications to the above embodiments will fall within the scope of the claims of the present invention.

Claims

1. A method for smelting ultra-low nitrogen and ultra-low sulfur steel, characterized in that: The following steps are involved: S1, hot metal pretreatment, deep desulfurization of hot metal to a sulfur content of ≤20ppm; S2, primary refining, using oxygen blowing decarburization in the primary refining furnace, blowing argon gas at the bottom to stir the molten steel throughout the process, adding slag material, alloy and deoxidizer into the ladle during the steel tapping process of the primary refining furnace, and adding limestone into the ladle before the steel tapping is completed; S3, LF refining, argon is blown from the bottom to stir the molten steel throughout the whole process, and bauxite clinker and active lime are added during the LF refining process for rapid slagging; S4, pouring. During the pouring of molten steel, the water inlet of the ladle and the tundish is sealed with argon gas.

2. The smelting method of ultra-low nitrogen and ultra-low sulfur steel according to claim 1, characterized in that: In step S2, the order of adding slag material, alloy and deoxidizer during the steel tapping process is as follows: When the steel output reaches 1 / 5, 4-6 kg / t steel of active lime is added to the ladle. When the steel output reaches 1 / 3, alloy is added according to the lower limit of the finished product composition of ultra-low nitrogen and ultra-low sulfur steel. Before the end of steel output, 0.5-2 kg / t steel of limestone is added to the ladle, and aluminum and aluminum slag are added according to the acid-soluble aluminum content of 0.005-0.01 wt% in the molten steel.

3. The smelting method of ultra-low nitrogen and ultra-low sulfur steel according to claim 2, characterized in that: During the steel tapping process, the amount of aluminum slag added is 0.3-1.0 kg / t steel, and the amount of aluminum added is 0.3-0.5 kg / t steel.

4. The smelting method of ultra-low nitrogen and ultra-low sulfur steel according to claim 1, characterized in that: In the step S2, the S content in the molten steel in the ladle after tapping is ≤100ppm.

5. The smelting method of ultra-low nitrogen and ultra-low sulfur steel according to claim 1, characterized in that: In the step S3, at the initial stage of LF refining, the flow rate of bottom blowing argon is controlled to be ≤3.0Nl / (min·t steel), and the power of the electrode is controlled to be ≤70kW / t steel. At the later stage of LF refining, the flow rate of bottom blowing argon is controlled to be ≤7.0Nl / (min·t steel), and the power of the electrode is controlled to be ≤130kW / t steel.

6. The smelting method of ultra-low nitrogen and ultra-low sulfur steel according to claim 1, characterized in that: In step S3, at the initial stage of LF refining, 0.5-1.5 kg / t steel of bauxite clinker is added, and then 4-10 kg / t steel of active lime is added.

7. The smelting method of ultra-low nitrogen and ultra-low sulfur steel according to claim 6, characterized in that: The particle size of the bauxite clinker is 5 to 20 mm; the content of Al2O3 in the bauxite clinker is 70 to 80 wt%, the content of Fe2O3 is ≤2 wt%, and the content of TiO2 is ≤2 wt%.

8. The method for smelting ultra-low nitrogen and ultra-low sulfur steel according to claim 6, characterized in that: The particle size of the active lime is 5-30 mm; the content of CaO in the active lime is ≥98%.

9. The method for smelting ultra-low nitrogen and ultra-low sulfur steel according to claim 6, characterized in that: The bauxite clinker and active lime are added as slag in batches, and the amount of slag added in each batch is ≤2kg / t steel.

10. The method for smelting ultra-low nitrogen and ultra-low sulfur steel according to claim 1, characterized in that: In the step S3, after the LF refining is completed, the proportion of heats with nitrogen content of ≤50 ppm in the molten steel is greater than 98%.

11. The method for smelting ultra-low nitrogen and ultra-low sulfur steel according to claim 1, characterized in that: After LF refining, hydrogen and inclusions in the molten steel are removed by vacuum treatment, and nitrogen in the steel is further removed.

12. The method for smelting ultra-low nitrogen and ultra-low sulfur steel according to claim 11, characterized in that: The vacuum treatment adopts an RH vacuum furnace.

13. The smelting method of ultra-low nitrogen and ultra-low sulfur steel according to claim 1, characterized in that: In the step S4, the composition of the molten steel satisfies: S≤50ppm, N≤50ppm, Mn: 00.1-3.5%, Al: 0.015-0.1%.

14. The smelting method of ultra-low nitrogen and ultra-low sulfur steel according to claim 12, characterized in that: In the molten steel, S≤20ppm, N≤31ppm.

15. An ultra-low nitrogen and ultra-low sulfur steel obtained by the smelting method of ultra-low nitrogen and ultra-low sulfur steel as described in any one of claims 1 to 14.

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