Rare earth steel smelting method based on thin slab continuous casting and rolling production line

Abstract

The invention belongs to the technical field of steel smelting, and particularly relates to a rare earth steel smelting method based on a thin slab continuous casting and rolling production line. Comprising the following steps: (1) dynamic dephosphorization control: controlling the temperature of molten steel to be 1380-1410 DEG C, the content of FeO in slag to be 15-20%, the alkalinity of the slag to be 1.8-2.2 and the content of phosphorus at the end point to be less than or equal to 0.015% in a window period 0-6 minutes before converter smelting; and (2) converter end point carbon and oxygen regulation: the converter end point carbon content is greater than or equal to 0.08%, the end point oxygen content is less than or equal to 300ppm, and the end point inclusion density is less than or equal to 10 inclusions / mm < 2 >. Through a process chain of dynamic dephosphorization and oxygen reduction, gradient deoxidation and purification, step-by-step slag system optimization, rare earth sequential addition and whole-course protection casting, a forward circulation of molten steel cleanliness and rare earth stability is formed, the rare earth steel continuous casting problem is fundamentally solved, and technical innovation and economic breakthrough are achieved at the same time.

Description

Technical Field

[0001] The present invention belongs to the technical field of iron and steel smelting, and particularly relates to a method for smelting rare earth steel based on a thin slab continuous casting and rolling production line. Background Art

[0002] Rare earth elements in steel can refine the grain size, and at the same time can effectively improve the corrosion resistance and low-temperature toughness of steel. However, for rare earth steel added with rare earth elements, serious nozzle caking is likely to occur during continuous casting, which will cause production interruption and prevent the production from proceeding smoothly. At present, the smelting of rare earth steel lacks a systematic cleanliness control model, and the timing and parameters of dephosphorization, deoxidation, and rare earth treatment are not coordinated, resulting in poor stability and high cost in the continuous casting of rare earth steel.

[0003] The existing technology has the following problems:

[0004] 1. The contradiction between dephosphorization and end-point oxygen: In the traditional converter process, to control the phosphorus content (≤0.020%), it is necessary to reduce the end-point carbon (≤0.05%), resulting in a high end-point oxygen content (>500 ppm) and an increase in the amount of inclusions generated.

[0005] 2. Coarse deoxidation process: The end-point oxygen content of the conventional RH treatment is 100 - 300 ppm, and the oxidability of the LF refining slag is high (FeO + MnO > 1.5%), which cannot meet the requirements of rare earth steel for an ultra-low oxygen environment.

[0006] 3. Serious oxidation loss of rare earth: Existing patents (such as CN202110527109.0) mostly add rare earths in the later stage of RH or before calcium treatment, and the recovery rate is less than 50%, and the problem of caking caused by rare earth oxide inclusions has not been solved. Summary of the Invention

[0007] The purpose of the present invention is to provide a method for smelting rare earth steel based on a thin slab continuous casting and rolling production line to solve the problems existing in the prior art.

[0008] The technical solution adopted by the present invention to solve its technical problems is as follows:

[0009] A method for smelting rare earth steel based on a thin slab continuous casting and rolling production line includes the following steps:

[0010] (1) Dynamic dephosphorization control: In the time window of 0 - 6 minutes before the end of converter smelting, control the molten steel temperature to be 1380 - 1410 °C, the FeO content in the slag to be 15% - 20%, the slag basicity to be 1.8 - 2.2, and the end-point phosphorus content ≤ 0.015%;

[0011] (2) Converter end-point carbon and oxygen regulation: The end-point carbon content of the converter ≥ 0.08%, the end-point oxygen content ≤ 300 ppm, and the end-point inclusion density ≤ 10 pieces / mm 2 ;

[0012] (3) RH gradient deoxidation: Decarburization is carried out through RH vacuum treatment, and the oxygen content at the end point is ≤50 ppm, and high-carbon alloy is used for deoxidation;

[0013] (4) LF step-by-step deoxidation and slag system optimization: Silicon-manganese alloy is added in the pre-deoxidation stage for pre-deoxidation, controlling the aluminum content in the steel to be ≤100 ppm. After desulfurization, aluminum wire is added as a supplement. In the final deoxidation stage, aluminum pellets are added for final deoxidation, and the content of FeO + MnO in the final slag is ≤1.0%;

[0014] (5) Calcium treatment and rare earth addition timing control: Calcium treatment is carried out using anti-splash calcium wire. Rare earth alloy is added within 1 minute after the calcium treatment is completed, and argon is softly blown for 1 minute, with the argon flower diameter ≤50 mm, and then it is left to stand for 3 minutes;

[0015] (6) Continuous casting protective casting: Argon gas is used for sealed protection throughout the process, and the oxygen content in the tundish is ≤15 ppm.

[0016] Furthermore, in the dynamic dephosphorization control described in step (1), the basicity of the slag is adjusted by adding lime in batches:

[0017] 60%-70% of the total lime amount is added within the first 2 minutes, and the addition amount is ≥4.8 kg / ton of steel. The remaining lime is added within 4-6 minutes, and the fluctuation range of the total basicity is ≤±0.1.

[0018] Furthermore, the method for regulating the carbon and oxygen at the end of the converter in step (2) is as follows: In the late stage of blowing, a low lance position operation with a lance height of 0.8-1.2 m is adopted, and oxygen supply is stopped 2 minutes before the end point, and natural deoxidation is carried out using the remaining carbon.

[0019] Furthermore, during the RH gradient deoxidation described in step (3), the RH treatment time is 15-25 minutes, the vacuum degree is ≤100 Pa, and the decarburization is carried out until the carbon content of the molten steel is ≤0.12%;

[0020] The high-carbon alloy is a high-carbon ferromanganese alloy with C≥6.5%. The addition of the high-carbon ferromanganese alloy is carried out in two stages:

[0021] The first stage: 70% of the total amount is added within 5 minutes after the start of RH treatment;

[0022] The second stage: The remaining 30% is added when the carbon content is decarburized to C≤0.15%.

[0023] Furthermore, after the pre-deoxidation in step (4), 500-1000 kg of the casting residue of the same steel type is added, and lime is supplemented according to an addition amount of ≥8 kg / ton of steel, and the total number of lime supplements is ≤2 times.

[0024] Further, in the timing control of calcium treatment and rare earth addition in step (5), the wire feeding speed of calcium treatment is 2.5 - 3.5 m / s, the calcium addition amount is 0.02 - 0.04 kg per ton of molten steel, the particle size of rare earth alloy is 5 - 15 mm, the rare earth alloy is preheated to 200 - 300 °C before addition, and the addition position is at 1 / 3 height above the bottom of the ladle.

[0025] Further, in the continuous casting protective casting in step (6), the basicity of the tundish covering agent is 1.2 - 1.6, the liquid level fluctuation of the mold is ≤ ±2 mm, and the nozzle nodulation rate is < 1%.

[0026] Further, the composition of the tundish covering agent is:

[0027] CaO: 30 - 35%, SiO2: 25 - 30%, Al2O3: 15 - 20%, MgO: 5 - 8%, melting point ≤ 1300 °C, viscosity (1300 °C) ≤ 0.5 Pa·s.

[0028] The present invention has the following beneficial effects:

[0029] 1. Through the process chain of dynamic dephosphorization and deoxidation → gradient deoxidation and purification → step-by-step slag system optimization → rare earth timing addition → full-process protective casting, the present invention forms a positive cycle of molten steel cleanliness and rare earth stability, fundamentally solves the continuous casting problem of rare earth steel, and combines technological innovation and economic breakthrough.

[0030] 2. The oxygen content decreases step by step (300 ppm in converter → 50 ppm in RH → 20 ppm in LF → 15 ppm in continuous casting), and the total amount of inclusions decreases by 60%, thus achieving the effect of cleaning molten steel.

[0031] 3. The proportion of finished product oxide inclusions is ≤ 3%, the grain size is refined to ≥ 8 grades, the impact energy at -40 °C is increased to ≥ 100 J, and the rare earth recovery rate is 87%, enabling efficient utilization of rare earth.

[0032] 4. The nozzle nodulation rate is < 1%, the cost per ton of steel is reduced by 100 - 150 yuan, large-scale production of rare earth steel is realized, and the production process is stable. Specific embodiments

[0033] In order to make the purpose, technical solutions and advantages of the present invention clearer, the following further elaborates the present invention in detail with reference to specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0034] Taking the production of Q345BRE rare earth structural steel as an example, the hot metal conditions are C: 4.2%, Si: 0.45%, Mn: 0.25%, P: 0.13%, S: 0.035%, temperature 1280 °C, and the scrap ratio is 15% (where heavy scrap ≥ 80%).

[0035] A method for smelting rare earth steel based on a thin slab continuous casting and rolling production line, comprising the following steps:

[0036] (1) Dynamic dephosphorization control: In the first 0 - 2 minutes of converter smelting: Add 5.8 kg of lime per ton of steel (60% of the total amount), lance height 1.8 m, oxygen supply intensity 3.2 Nm 3 / (min·t); From 2 - 6 minutes: Supplementary addition of 3.9 kg of lime per ton of steel, lance height reduced to 1.2 m, temperature 1405 °C, slag composition: FeO = 17.2%, CaO / SiO2 = 2.1.

[0037] In the initial stage of converter smelting (0 - 6 minutes), by precisely controlling the temperature (1380 - 1410 °C), the FeO content in the slag (15% - 20%) and the alkalinity (1.8 - 2.2), a high - reactivity slag is formed to achieve efficient dephosphorization (dephosphorization rate ≥ 85%). Synergistic effect: The phosphorus content in the molten steel after dephosphorization ≤ 0.015%, creating conditions for high - carbon tapping at the end point (carbon ≥ 0.08%).

[0038] (2) Converter end - point carbon - oxygen regulation: In the final stage of blowing, a low - lance operation with a lance height of 0.8 - 1.2 m is adopted, oxygen supply is stopped 2 minutes before the end point, and natural deoxidation is carried out using the remaining carbon. End - point carbon 0.10%, oxygen 265 ppm, phosphorus 0.012%, tapping temperature 1655 °C, density of end - point inclusions ≤ 10 pieces / mm 2 . The high carbon content inhibits the dissolution of oxygen through the carbon - oxygen balance reaction, reducing the end - point oxygen content to ≤ 300 ppm and reducing the formation of inclusions such as ferrous oxide (FeO).

[0039] (3) RH gradient deoxidation: Decarburization is carried out through RH vacuum treatment, vacuum degree 75 Pa, RH treatment time 18 minutes, decarburization end - point: [C] = 0.10%, decarburization rate 0.008% / min. High - carbon ferromanganese alloy with C ≥ 6.5% is used for deoxidation. 85 kg is added within 5 minutes after the start of RH treatment, and the remaining 37 kg is supplemented at the 12th minute, end - point oxygen content 42 ppm.

[0040] In the RH vacuum treatment stage, through forced decarburization (end - point carbon ≤ 0.12%) combined with deoxidation using high - carbon alloys (such as high - carbon ferromanganese), the free oxygen in the molten steel is consumed in stages, reducing the oxygen content to ≤ 50 ppm. Synergistic effect: The vacuum environment (≤ 100 Pa) accelerates the decarburization reaction. At the same time, the carbon in the high - carbon alloy preferentially combines with oxygen, reducing the amount of deoxidation products and providing ultra - low - oxygen molten steel for subsequent processes.

[0041] (4)LF Stepwise Deoxidation and Slag System Optimization: In the pre-deoxidation stage, ferrosilicon manganese alloy (Si: 17.2%, Mn: 66.5%) is added at 16 kg per ton of steel for pre-deoxidation, and the aluminum content [Al] in the steel is 78 ppm. After pre-deoxidation, 900 kg of the casting residue slag of the same steel grade (composition: CaO 48%, SiO2 13%, Al2O3 6.5%) is added, and lime is supplemented at a dosage of ≥8 kg per ton of steel, with the total number of lime supplementation times ≤2 times. After desulfurization, aluminum wire is added, with a feeding amount of 1.2 kg per ton of steel. In the final deoxidation stage, aluminum pellets are added for final deoxidation. The particle size of the aluminum pellets is 1.5 - 2.5 mm, and the addition amount is 0.9 kg per ton of steel. The oxygen content in the molten steel is 19 ppm, the basicity of the final slag is 2.6, and the content of FeO + MnO in the final slag is 0.75%.

[0042] Pre-deoxidation with ferrosilicon manganese alloy (Si-Mn), controlling the aluminum content ≤100 ppm to avoid premature formation of high-melting-point Al2O3 inclusions; Slag system optimization: Introducing the casting residue slag of the same steel grade (500 - 1000 kg) to reduce the melting point of the slag system, and cooperating with high-alkalinity lime (the first batch ≥8 kg / t) to form a highly adsorptive final slag (FeO + MnO ≤1.0%); Final deoxidation: Supplementary addition of aluminum wire and final deoxidation with aluminum pellets to achieve deep deoxidation (molten steel oxygen ≤20 ppm), and the final slag adsorbs fine inclusions such as Al2O3. Synergistic effect: The stepwise deoxidation strategy reduces the risk of molten steel absorbing oxygen, and the highly adsorptive slag system effectively removes inclusions, improving the purity of molten steel.

[0043] (5)Timing Control of Calcium Treatment and Rare Earth Addition: Calcium treatment is carried out using anti-splash calcium wire, with a wire feeding speed of 3.2 m / s and a calcium addition amount of 0.035 kg per ton of steel. After treatment, [O] in the steel is 18 ppm. Rare earth alloy (particle size 5 - 15 mm, preheated at 250°C) is added at 55 s after the calcium treatment is completed, and the addition position is at 1 / 3 height above the bottom of the ladle. Soft blowing of argon gas for 1 min, with a flow rate of 45 NL / min and an argon flower diameter of 45 mm, and then standing for 3 min, with the uniformity of rare earth distribution being 96.3%.

[0044] Calcium treatment: Using anti-splash calcium wire (Ca ≥96%) to modify Al2O3 inclusions in the steel into low-melting-point CaO - Al2O3 - CaS composite phases to prevent nozzle blockage; Rare earth addition: Preheated rare earth alloy is added within 60 seconds after calcium treatment, using the "buffer effect" of calcium treatment to inhibit rare earth oxidation, and soft blowing of argon gas (argon flower ≤50 mm) to promote uniform dispersion of rare earth, and standing for 3 minutes to achieve uniform distribution of elements. Synergistic effect: Calcium treatment purifies molten steel and reduces its oxidability, and rare earth is efficiently dissolved in a low-oxygen environment (recovery rate ≥85%), avoiding the formation of large-sized inclusions.

[0045] (6)Continuous casting protective casting: Argon gas sealing protection is adopted throughout the process to reduce secondary oxidation during casting. The oxygen content in the tundish is ≤ 15 ppm. The composition of the tundish covering agent is: CaO 32%, SiO2 28%, Al2O3 18%, melting point 1285 °C, viscosity 0.45 Pa·s (1300 °C), basicity 1.4, the liquid level fluctuation in the mold is ±1.2 mm, and the nozzle caking rate is 0.7%.

[0046] The performance comparison of the obtained finished products and those obtained by the traditional process is shown in Table 1 below:

[0047] Table 1: Performance comparison of finished products

[0048] Test items This invention Traditional process Grain size Grade 9 (ASTM E112) Grade 6 Impact energy at -40°C 115 J (V-notch) 62J Corrosion weight loss rate <![CDATA[1.08g(m 2 ·h)- 1 (Salt spray test)]]> <![CDATA[1.85g(m 2 ·h)- 1 > Nozzle caking rate 0.7% (accumulative 14 furnaces) 5.3% (accumulative 5 furnaces)

[0049] The embodiments described above are only descriptions of the preferred embodiments of the present invention, and do not limit the concept and scope of the present invention. Without departing from the design concept of the present invention, various modifications and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope of the present invention.

[0050] The technologies, shapes, and structures not detailed in the present invention are all well-known technologies.

Claims

1. A rare earth steel smelting method based on a thin slab continuous casting and rolling production line, characterized in that: The following steps are involved: (1) Dynamic dephosphorization control: In the first 0-6min window period of converter smelting, the molten steel temperature is controlled to be 1380-1410℃, the FeO content in the slag is 15%-20%, the slag basicity is 1.8-2.2, and the final phosphorus content is ≤0.015%; (2) Control of carbon and oxygen at the converter end point: The converter end point carbon content is ≥ 0.08%, the end point oxygen content is ≤ 300 ppm, and the end point inclusion density is ≤ 10 / mm 2 ; (3) RH gradient deoxidation: decarburization is performed by RH vacuum treatment, the end point oxygen content is ≤50ppm, and high carbon alloy is used for deoxidation; (4) LF step-by-step deoxidation and slag system optimization: silicon-manganese alloy is added in the pre-deoxidation stage for pre-deoxidation, and the aluminum content in the steel is controlled to be ≤100ppm. Aluminum wire is added after desulfurization is completed, and aluminum particles are added in the final deoxidation stage for final deoxidation. The FeO+MnO content in the final slag is ≤1.0%; (5) Timing control of calcium treatment and rare earth addition: Use splash-proof calcium wire for calcium treatment, add rare earth alloy within 1 minute after calcium treatment, blow argon for 1 minute, the diameter of argon flower is ≤50mm, and then stand for 3 minutes; (6) Continuous casting protection casting: The whole process is sealed with argon gas for protection, and the oxygen content in the middle package is ≤15ppm.

2. The rare earth steel smelting method based on the thin slab continuous casting and rolling production line according to claim 1, characterized in that: In the dynamic dephosphorization control of step (1), the slag basicity is adjusted by adding lime in batches: Add 60%-70% of the total lime within the first 2 minutes, and the added amount should be ≥4.8kg / ton of steel. The remaining lime should be added within 4-6 minutes, and the total alkalinity fluctuation range should be ≤±0.

1.

3. The rare earth steel smelting method based on the thin slab continuous casting and rolling production line according to claim 1, characterized in that: The method for controlling the carbon and oxygen at the converter endpoint in step (2) is as follows: at the end of blowing, a low lance position of 0.8-1.2 m is used, oxygen supply is stopped 2 minutes before the end, and residual carbon is used for natural deoxidation.

4. The rare earth steel smelting method based on the thin slab continuous casting and rolling production line according to claim 1, characterized in that: During the RH gradient deoxidation in step (3), the RH treatment time is 15-25 min, the vacuum degree is ≤100 Pa, and the carbon content of the molten steel is decarburized to ≤0.12%; The high carbon alloy is a high carbon ferromanganese alloy with C ≥ 6.5%. The addition of high carbon ferromanganese alloy is carried out in two stages: Stage 1: Add 70% of the total amount within 5 min after the start of RH treatment; The second stage: when decarburization reaches C ≤ 0.15%, add the remaining 30%.

5. The rare earth steel smelting method based on the thin slab continuous casting and rolling production line according to claim 1, characterized in that: After the pre-deoxidation in step (4), 500-1000 kg of casting slag of the same type of steel is added, and ash is added in an amount of ≥8 kg / ton of steel, and the total number of ash additions is ≤2 times.

6. The rare earth steel smelting method based on the thin slab continuous casting and rolling production line according to claim 1, characterized in that: In the step (5), the calcium treatment and rare earth addition timing control are as follows: the calcium treatment feed line speed is 2.5-3.5 m / s, the calcium addition amount is 0.02-0.04 kg / ton of steel, the rare earth alloy particle size is 5-15 mm, the rare earth alloy is preheated to 200-300° C. before addition, and the addition position is 1 / 3 of the height above the bottom of the ladle.

7. The rare earth steel smelting method based on the thin slab continuous casting and rolling production line according to claim 1, characterized in that: In the continuous casting protection casting of step (6), the basicity of the tundish covering agent is 1.2-1.6.