Smelting method of ultra-low nitrogen steel

Abstract

A smelting method of ultra-low nitrogen steel comprises the following steps: (1) pretreatment of molten iron: carrying out deep desulfurization on the molten iron by adopting a KR method until S is less than or equal to 0.0020 wt% and S in a steel ladle after tapping is less than or equal to 0.0012 wt%; (2) converter smelting, wherein Ar gas bottom blowing stirring is adopted in the whole process; (3) LF refining is conducted, the bottom blowing gas flow at the initial stage of refining is smaller than or equal to 3.0 Nl / (min.t steel), and the electrode electrifying power is smaller than or equal to 70 kW / t steel; the bottom blowing gas flow is smaller than or equal to 7.0 Nl / (min.t steel) in the later stage of electrification, and the electrification power is smaller than or equal to 130 kW / t steel; 4-10 kg of active lime is added to each ton of steel, and the content of CaO in ladle top slag is controlled to be 50-55 wt% after LF refining is finished; and (4) RH refining: starting a vacuum pump of an RH refining furnace for exhausting, before the pressure in a vacuum chamber is reduced to 1kPa, controlling the flow of circulating gas to be 6-8N1 / (min.t steel), reducing the pressure in the vacuum chamber to be less than or equal to 1kPa, and increasing the flow of the circulating gas to be 10-12N1 / (min.t steel). According to the method, on the premise that the sulfur content of the molten steel is smaller than 20 ppm and the smelting cost is not increased, the ultralow-nitrogen and ultralow-sulfur product with the nitrogen content smaller than 30 ppm can be obtained.

Description

TECHNICAL FIELD

[0001] The present application relates to a steelmaking process, in particular to a smelting method of ultra-low nitrogen steel. BACKGROUND

[0002] For high-end thick plate, pipeline products and cold-rolled ultra-high strength steel, KR-LD-LF-RH-CC process is generally used. The nitrogen content in the steel directly affects the quality of the billet, and then affects the hot delivery rate of the billet, the quality of the subsequent hot-rolled and cold-rolled products. In addition, in order to control the number of sulfides in the steel, the sulfur in the steel needs to be reduced to 20 ppm or even below 10 ppm. Under the condition of ensuring that the sulfur content of the finished product in the molten steel meets the requirements, how to stably control the nitrogen content in the steel below the specification at low cost is a challenge that modern steel plants generally face.

[0003] In the process of producing ultra-low sulfur and low nitrogen steel by KR-LD-LF-RH-CC process, desulfurization and nitrogen control of molten iron or molten steel are opposite. The change rule of sulfur content in molten steel is that KR removes sulfur in molten iron to a very low level, the sulfur content in molten steel in LF converter rises due to the influence of raw materials or desulfurization slag, and the sulfur in the steel is removed to below the specification content by LF furnace temperature rising and slagging. The change rule of nitrogen content in molten steel is that the nitrogen content in molten iron rises when KR removes sulfur, most of the nitrogen in molten steel is removed in the decarburization process of LD converter, the nitrogen content in molten steel rises sharply during tapping and LF furnace treatment, the nitrogen content in molten steel decreases during RH vacuum treatment, and then slightly rises during continuous casting.

[0004] The technical solution disclosed in Chinese patent publication No. CN101553583B uses aluminum combustion chemical heat to replace electric heating of LF furnace, thereby smelting ultra-low sulfur and ultra-low nitrogen steel with finished product sulfur below 10 ppm and finished product nitrogen less than 50 ppm at low cost. Equivalent lime 6-16 kg / t steel and Al 1.5-7 kg / t steel are added during post-furnace tapping. The molten steel is heated in the ladle by top-blown oxygen aluminum hot method, and auxiliary quantitative Ar gas is used to stir and mix the molten steel to remove sulfur in the molten steel while controlling the nitrogen increase in the molten steel. In fact, aluminum hot is used to adjust the temperature of the molten steel, which results in a large increase in slag amount. For dehydrogenated products with strict steel requirements, RH degassing refining treatment may not be possible.

[0005] The technical solution disclosed in Chinese patent publication No. CN105452504B is to produce high manganese steel by using a special device, and to control the atmosphere in the container by Ar gas while adjusting the silicon content in the melt to achieve the goal of controlling the increase of nitrogen content. SUMMARY

[0006] The purpose of the present application is to provide a smelting method of ultra-low nitrogen steel, which can obtain ultra-low nitrogen and ultra-low sulfur products with nitrogen content less than 30 ppm under the premise of ensuring that the sulfur content of the molten steel is less than 20 ppm and without increasing the smelting cost.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows:

[0008] A method for smelting ultra-low nitrogen steel includes: hot metal pretreatment, converter smelting, LF refining, and RH refining; wherein,

[0009] ① Hot metal pretreatment: The KR method is used to desulfurize the hot metal to S≤0.0020wt%, and the S in the ladle after tapping is ≤0.0012wt%.

[0010] ② Converter smelting

[0011] The converter uses Ar gas bottom blowing for stirring throughout the entire process;

[0012] Before tapping begins, argon gas is blown into the ladle at a flow rate of 6–10 Nl / (min·t steel) for 1–5 min. At the start of tapping, argon gas continues to be blown into the ladle at a flow rate of 1–5 Nl / (min·t steel). During tapping, slag-forming materials, alloys, or deoxidizers are added to the ladle. Activated lime is added, with the amount exceeding 35% of the total activated lime content. This total activated lime includes the amount added from tapping to the end of LF refining. At the end of tapping, aluminum or aluminum slag is added to maintain the acid-soluble aluminum content in the molten steel at 0.001–0.01 wt%.

[0013] ③LF Refining

[0014] In the initial stage of refining, the bottom-blown gas flow rate is controlled to be ≤3.0 Nl / (min·t steel), and the electrode energizing power is ≤70 kW / t steel; in the later stage of energizing, the bottom-blown gas flow rate is ≤7.0 Nl / (min·t steel), and the energizing power is ≤130 kW / t steel.

[0015] Add 4-10 kg / t of active lime to the steel, with a particle size of 5-30 mm and a composition of CaO ≥ 98 wt%. Control the CaO content in the top slag of the ladle at the end of LF refining to 50-55 wt%.

[0016] ④RH refining

[0017] In the RH refining furnace, the vacuum pump is started to exhaust the gas. As the pressure in the RH vacuum chamber decreases, hydrogen and nitrogen in the molten steel are continuously discharged in gaseous form. Before the pressure in the vacuum chamber drops to 1 kPa, the circulating gas flow rate is controlled at 6-8 Nl / (min·t steel) to quickly reduce the vacuum level in the vacuum chamber and increase the denitrification rate of the molten steel. When the pressure in the vacuum chamber drops to ≤1 kPa, the circulating gas flow rate is increased to 10-12 Nl / (min·t steel) to increase the reaction area for nitrogen removal under vacuum.

[0018] The final molten steel composition by weight percentage is: C 0.02~0.5%, Si≤1.0%, S≤0.0020%, Mn0.1~4.0%, Al 0.015~0.5%, Ti≤0.1%, N≤0.0030%, Ca≤0.0035%, with the balance including Fe and other unavoidable impurities.

[0019] Preferably, no deoxidizing alloy is added to the furnace during the final stage of converter smelting.

[0020] Preferably, 70-80% of the total amount of manganese is added during the tapping process and in the LF furnace, with the remainder added in the RH furnace.

[0021] Preferably, the finished ultra-low nitrogen steel contains silicon, and ferrosilicon should be added during the steelmaking process.

[0022] In the smelting method described in this invention:

[0023] ① Hot metal pretreatment

[0024] The high carbon and silicon content in molten iron facilitates sulfur removal. KR (Kinetic Refining) is used for deep desulfurization of the molten iron to achieve S ≤ 0.0020 wt%. To reduce the desulfurization load on the subsequent LF (Fan-Fried Furnace) furnace and thus reduce the amount of nitrogen added to the molten steel, the sulfur content of the converter charge is strictly controlled, ensuring that S ≤ 0.0012 wt% in the ladle after tapping.

[0025] ② Converter smelting

[0026] During the converter refining process, a large amount of CO or CO2 gas is generated and escapes from the molten steel during decarburization. Most of the nitrogen in the molten steel is removed by carrying it away. However, the amount of CO or CO2 gas escaping decreases in the later and final stages of smelting, posing a risk of nitrogen accumulation. To ensure efficient nitrogen removal during decarburization, Ar gas is used for bottom blowing and stirring throughout the entire converter process, and the bottom blowing is kept intact. At the end of the converter smelting process, no deoxidizing alloys such as manganese or silicon are added to the furnace to maintain the oxygen content of the molten steel and reduce nitrogen absorption in the tapped steel stream.

[0027] Before tapping begins, argon gas is blown into the bottom of the ladle at a flow rate of 6–10 Nl / (min·t steel) for 1–5 minutes to remove air from the ladle, reducing the chance of direct contact between the molten steel and air, and minimizing nitrogen absorption by the molten steel. Once tapping begins, argon gas is continued to be blown into the bottom of the ladle at a flow rate of 1–5 Nl / (min·t steel) to maintain a low nitrogen content in the ambient gas above the molten steel and to reduce the contact area between the molten steel and the ambient gas above it.

[0028] During tapping, slag-forming materials, alloys, or a small amount of deoxidizer are added to the ladle. The molten steel flowing into the ladle carries strong kinetic energy, which facilitates thorough mixing between the added materials and the molten steel, and causes the slag to heat up and partially melt into slag. To ensure that the surface of the molten steel is covered by slag as quickly as possible, active lime is added as early as possible, with the amount of lime added exceeding 35% of the total amount (lime added from tapping to the end of LF refining). Aluminum or aluminum slag is added at the end of tapping to keep the acid-soluble aluminum content of the molten steel between 0.001% and 0.01%, reducing nitrogen addition to the molten steel.

[0029] ③LF Refining

[0030] During LF refining, the flow rate of bottom-blown gas in the ladle and the electrode energizing power are controlled. In the initial stage of refining, the bottom-blown gas flow rate is controlled to ≤3.0 Nl / (min·t steel), and the electrode energizing power is controlled to ≤70 kW / t steel. In the later stage of energizing, the bottom-blown gas flow rate is controlled to ≤7.0 Nl / (min·t steel), and the energizing power is controlled to ≤130 kW / t steel. In the initial stage of LF refining, the lime is not fully melted, so the bottom-blown gas flow rate and energizing power should not be too high to reduce the area of ​​molten steel exposed to ambient gases and inhibit nitrogen accumulation. In the later stage of energizing, the lime is fully melted, so the bottom-blown argon flow rate is appropriately increased to ensure thorough mixing of slag and steel and improve desulfurization efficiency.

[0031] In the initial stage of LF refining, add 4-10 kg / t steel of active lime with a particle size of 5-30 mm and a CaO content of ≥98%. Control the CaO content in the ladle top slag at the end of LF refining to 50-55%.

[0032] Using the above methods, the proportion of heats with a liquid nitrogen content ≤48ppm at the end of LF refining of ultra-low sulfur steel is greater than 97%.

[0033] After refining in the LF furnace, the molten steel that meets the requirements for temperature and sulfur content enters the RH furnace.

[0034] ④RH refining

[0035] In the RH refining furnace, the vacuum pump is started to exhaust gas. As the pressure in the RH vacuum chamber decreases, hydrogen and nitrogen in the molten steel are continuously removed in gaseous form. Before the pressure in the vacuum chamber drops to 1 kPa, the circulating gas flow rate is controlled at 6-8 Nl / (min·t steel) to rapidly reduce the vacuum level and increase the denitrification rate of the molten steel. When the pressure in the vacuum chamber drops to ≤1 kPa, the circulating gas flow rate is increased to 10-12 Nl / (min·t steel) to increase the reaction area for nitrogen removal under vacuum.

[0036] Regarding the adjustment of molten steel composition, alloys or materials compatible with the finished product composition and content need to be added to meet the steel grade requirements. The added alloys and materials contain nitrogen to varying degrees; generally, nitrogen in alloys exists in a combined form, while in materials like carbon powder, nitrogen exists in molecular form. The nitrogen in the added alloys or materials dissolves in the molten steel, increasing its nitrogen content. Alloying elements such as carbon, niobium, vanadium, chromium, and titanium are adjusted in the LF furnace, with only minor adjustments made during the RH treatment, the amount of which is ≤30 ppm. Manganese is added at the end of the furnace and in the LF furnace, accounting for 70-80% of the total amount, with the remainder added in the RH furnace. In this way, nitrogen introduced by the materials can be effectively removed from the molten steel during the subsequent RH vacuum treatment, reducing the impact of the materials on the nitrogen content of the molten steel.

[0037] During the pouring of molten steel with qualified composition and temperature, negative pressure is generated at the ladle and tundish nozzles due to the flow of molten steel. Therefore, Ar gas sealing is used to suppress nitrogen absorption by the molten steel and reduce nitrogen accumulation. Ultimately, ultra-low nitrogen and ultra-low sulfur products with nitrogen content less than 30 ppm and sulfur content less than 20 ppm can be obtained.

[0038] Compared with existing ultra-low nitrogen steel refining technologies, the beneficial effects of this invention are:

[0039] To control the nitrogen content in molten steel, this invention develops a new smelting method: a new bottom-blowing argon gas, adjusted slag-forming and deoxidation alloying processes are used in the ladle after the converter to reduce the amount of nitrogen added to the molten steel. During LF treatment, the bottom-blowing argon gas and material input are controlled to reduce the amount of nitrogen added to the molten steel. During RH treatment, the circulating gas mode and alloy fine-tuning system are improved to increase the RH denitrification efficiency. Molten steel with qualified composition and temperature is then poured into standard billets.

[0040] Using the technology of this invention to perform desulfurization and nitrogen control refining on molten steel, the proportion of heats with nitrogen content ≤48ppm in ultra-low sulfur steel (sulfur content less than 20ppm or even 10ppm) after LF refining is greater than 97%; after RH degassing refining, ultra-low nitrogen and ultra-low sulfur products with nitrogen content less than 30ppm (sulfur content less than 20ppm) can be obtained.

[0041] Compared with traditional ultra-low sulfur and ultra-low nitrogen steel refining processes, this invention can achieve a lower nitrogen content (less than 30 ppm) in the finished product while ensuring that the sulfur content of the molten steel is less than 20 ppm and without increasing the smelting cost. Detailed Implementation

[0042] The present invention will be further described below with reference to the embodiments.

[0043] This invention takes a crack-sensitive thick plate with the following main components as an example: [%C] = 0.02-0.2, [%Si] ≤ 1.0, [%Mn] = 0.8-3.5, [%Al] = 0.02-0.1, [%Ti] = 0.01-0.08, [%P] ≤ 0.015, [%S] ≤ 0.003, and [%N] ≤ 0.0035. The process path adopted is as follows: hot metal desulfurization - converter top and bottom re-blowing smelting - tapping (partial slag) - LF furnace refining (heating, slag making, deoxidation, desulfurization, alloying) - RH refining (degassing, composition adjustment) - continuous casting - hot rolling.

[0044] Typical furnace runs and related process effects of smelting using the technology of this invention are shown in Tables 1 and 2, respectively.

[0045] As shown in Examples 1 to 5, for steel grades smelted by the LD-LF-RH-CC process, the technology of this invention can stably obtain ultra-low nitrogen steel with a nitrogen content of ≤30ppm.

[0046] Compared with the examples, the distribution of each material input in Comparative Examples 1a and 1b at the three positions of tapping after the furnace, LF furnace, and RH (including the control of the acid-soluble aluminum content of the molten steel at the end of tapping) is outside the range specified in this invention. The nitrogen content of the molten steel is 5-10 ppm higher before the start of LF, and thus the nitrogen content of the finished molten steel increases by about 5 ppm.

[0047] Compared with the examples, Comparative Examples 2a and 2b did not use bottom blowing argon gas operation after the furnace, and the nitrogen content of the molten steel was about 8 ppm higher before the start of LF, which in turn increased the nitrogen content of the finished molten steel by about 5 ppm.

[0048] Compared with the examples, the flow rate and power of the bottom-blown stirring gas in Comparative Examples 3a and 3b are outside the range specified in this invention. The increase in nitrogen content of molten steel during the LF refining process is 2 to 10 ppm, and the nitrogen content of the finished molten steel increases by about 8 ppm.

[0049] Compared with the examples, the H circulation gas flow rate in Comparative Examples 4a and 4b was kept constant throughout the process, which reduced the denitrification efficiency of the molten steel during the RH refining process, resulting in an increase of approximately 3 ppm in the nitrogen content of the finished molten steel.

[0050] Compared with the examples, the converter tapping process, LF refining and RH refining of Comparative Examples 5 and 6 all adopted conventional smelting operations. The nitrogen content of the liquid steel before LF started and the increase in nitrogen content of the liquid steel after LF refining both increased. The denitrification efficiency of RH refining decreased, resulting in an increase of about 20 ppm in the nitrogen content of the finished liquid steel.

[0051] Referring to Table 1, the embodiments of the present invention are compared with traditional refining processes for the same steel grades. The present invention employs a new bottom-blown argon, slag-forming, and deoxidizing alloying process in the converter ladle, reducing the amount of nitrogen added to the molten steel. During LF treatment, controlling the bottom-blown argon and material input reduces the amount of nitrogen added to the molten steel. During RH treatment, the circulating gas mode and alloy fine-tuning system are improved, increasing the RH denitrification efficiency.

[0052] Using the technology of this invention to perform desulfurization and nitrogen control refining on molten steel, the proportion of heats with nitrogen content ≤48ppm in ultra-low sulfur steel (sulfur content less than 20ppm or even 10ppm) after LF refining is greater than 97%; after RH degassing refining, ultra-low nitrogen products with nitrogen content less than 30ppm (sulfur content less than 20ppm) can be obtained.

[0053] In summary, compared with traditional ultra-low sulfur and ultra-low nitrogen steel refining processes, this invention can achieve a lower nitrogen content in the finished product while ensuring that the sulfur content of the molten steel is less than 20 ppm and without increasing smelting costs.

[0054] The ultra-low nitrogen and ultra-low sulfur steel refining method developed in this invention requires only conventional LF and RH refining equipment, with an argon pipeline after the furnace. Argon is blown from the ladle after the furnace, and the slag-forming and deoxidation alloying processes are adjusted. During LF treatment, the bottom-blown argon and material input are controlled, and during RH treatment, the circulating gas mode and alloy fine-tuning regime are improved to achieve the predetermined metallurgical target. Compared with traditional ultra-low sulfur and ultra-low nitrogen steel refining processes, this invention can obtain ultra-low nitrogen steel with a nitrogen content of less than 30 ppm while ensuring that the sulfur content of the molten steel is less than 20 ppm and without increasing smelting costs. It is applicable to the smelting of ultra-low nitrogen and ultra-low sulfur steel and has significant application value in steel plants.

[0055]

[0056]

[0057]

Claims

1. A method of smelting ultra-low nitrogen steel, characterized by comprising: hot metal pretreatment, converter smelting, LF refining, RH refining; wherein, ① hot metal pretreatment, KR method is used to deeply desulfurize the hot metal to S≤0.0020wt%, S≤0.0012wt% in the ladle after tapping; ② converter smelting converter adopts Ar gas bottom blowing agitation all the way; before the start of tapping, the ladle blows argon gas, the argon gas flow is controlled at 6-10 Nl / (min·t steel), the duration is 1-5 min; at the start of tapping, the ladle continues to blow argon gas at the bottom, the flow is adjusted to 1-5 Nl / (min·t steel); during the tapping process, the ladle is charged with slagging material, alloy or deoxidizer; active lime is added, the amount of active lime added is greater than 35% of the total amount of active lime, which includes the amount of active lime added from tapping to the end of LF refining; aluminum or aluminum slag is added at the end of tapping to make the acid-soluble aluminum content of the molten steel 0.001-0.01wt%; ③ LF refining in the early stage of refining, the bottom blowing gas flow is controlled to be ≤3.0 Nl / (min·t steel), and the electrode power is ≤70 kW / t steel; in the later stage of power-on, the bottom blowing gas flow is ≤7.0 Nl / (min·t steel), and the power-on power is ≤130 kW / t steel; 4-10 kg / t steel of active lime is added, the particle size of active lime is 5-30 mm, the composition specification is CaO≥98wt%, and the CaO content in the top slag of the ladle at the end of LF refining is controlled to be 50-55wt%; ④ RH refining in the RH refining furnace, the vacuum pump is started to exhaust, as the pressure in the RH vacuum chamber decreases, hydrogen and nitrogen in the molten steel are continuously discharged from the molten steel in the form of gas; before the pressure in the vacuum chamber is reduced to 1 kPa, the circulating gas flow is controlled to be 6-8 Nl / (min·t steel); when the pressure in the vacuum chamber is reduced to ≤1 kPa, the circulating gas flow is increased to 10-12 Nl / (min·t steel); the final composition of the molten steel is: C 0.02-0.5%, Si≤1.0%, S≤0.0020%, Mn 0.1-4.0%, Al 0.015-0.5%, Ti≤0.1%, N≤0.0030%, Ca≤0.0035%, the balance including Fe and other unavoidable impurities.

2. The method of smelting ultra-low nitrogen steel according to claim 1, characterized in that, no deoxidizing alloy is added in the converter at the end of smelting.

3. The method of smelting ultra-low nitrogen steel according to claim 1, characterized in that, 70-80% of the total amount of manganese is added during the tapping process and in the LF furnace, and the balance is completed in the RH furnace.

4. The method of smelting ultra-low nitrogen steel according to claim 1, characterized in that, silicon is contained in the composition of the finished ultra-low nitrogen steel, and silicon iron should be added during the tapping process.

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