Method for reducing mixed casting and blank throwing of steel grade T63 and HRB400

Abstract

The invention belongs to the technical field of steel smelting, and particularly relates to a method for reducing mixed casting and blank throwing of steel grade T63 and HRB400. Comprising the following steps that (1) preparation before mixed pouring is conducted, specifically, the steel feeding temperature of T63 steel is increased by a refining furnace and controlled to be 1565-1570 DEG C, a water gap is replaced with a BN coating water gap of 19 mm, and after slag of a tundish is discharged to a slag layer of 20-40 mm, a double-layer covering agent is laid; (2) after pouring of the medium ladle is stopped, the liquid level is reduced to 200 mm, a T63 large ladle is replaced for pouring, air curtain protection is started synchronously, the pulling speed is reduced according to a pulling speed formula, and the liquid level is slowly increased to 500 mm; and (3) in the stage that the liquid level rises to 850 mm from 500 mm, temperature compensation is started, the temperature of the molten steel is maintained to be larger than or equal to 1540 DEG C, the pulling speed is further reduced to 70% of the initial value, and the fluctuation of the liquid level is maintained to be smaller than or equal to + / -10 mm. The length of a mixed casting transition area is reduced, the waste throwing amount is reduced, and the utilization rate of residual molten steel is increased.

Description

Technical Field

[0001] The present invention belongs to the technical field of steel smelting, and particularly relates to a method for reducing the waste slab caused by the mixed casting of steel grades T63 and HRB400. Background Art

[0002] There are significant differences between T63 steel bars and general steel bars in terms of strength, material saving rate, comprehensive cost, seismic performance, and ductility. The yield strength of T63 high-strength steel bars is ≥630 MPa, and the tensile strength is ≥790 MPa, while the strength design value of conventional construction steel HRB400 is 360 MPa. The strength design value of T63 steel bars reaches 545 MPa. The material saving rate of T63 steel bars compared to HRB400 steel bars can reach about 30%, and the comprehensive cost can be saved by about 12%. Using T63 steel bars can reduce the cross-sectional size and material consumption of components, reduce the self-weight of the structure, improve the seismic strength and the ability to resist shock waves, with excellent seismic performance, good ductility, and large safety reserves.

[0003] The continuous casting machine of a certain steel plant is 165×165MM - 8-strand 8-casting mold, with high production efficiency and strong controllability. Practice has proved that when producing steel grade T63, if a new pouring ladle is used or the secondary pouring start method is adopted, serious flocculation flow will occur, ultimately leading to production interruption, unplanned pouring stop, and having a significant impact on the company's production rhythm. To avoid unplanned pouring stop of the continuous casting machine, the mixed casting method of T63 and HRB400 steel grades is adopted. The current mixed casting method has the following problems:

[0004] 1. Severe turbulent diffusion: When the liquid level rapidly rises with a fixed drawing speed, the turbulence of the molten steel intensifies, and the mixed casting interface diffuses;

[0005] 2. Uncontrollable temperature decay: The temperature of the molten steel in the tundish decreases with the pouring time, the fluidity becomes worse, and the risk of flocculation flow is intensified;

[0006] 3. Interface oxidation inclusions: Traditional covering agents cannot effectively isolate the oxidation of the molten steel in the mixed casting area, resulting in an increase in interface inclusions and being forced to expand the waste rejection range. Summary of the Invention

[0007] The purpose of the present invention is to provide a method for reducing the waste slab caused by the mixed casting of steel grades T63 and HRB400 to solve the problems existing in the prior art.

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

[0009] A method for reducing the waste slab caused by the mixed casting of steel grades T63 and HRB400, comprising the following steps:

[0010] (1) Preparation before mixed casting: In the refining furnace, increase the ladle temperature of T63 steel grade to be controlled at 1565 - 1570 °C, replace the tundish nozzle with a 19 mm BN-coated nozzle. After the tundish slag is discharged to a slag layer of 20 - 40 mm, lay a double-layer covering flux.

[0011] (2) After the tundish stops casting, lower the liquid level to 200 mm, replace the T63 ladle and start casting, and simultaneously turn on the gas curtain protection. According to the casting speed formula, reduce the casting speed to make the liquid level slowly rise to 500 mm.

[0012] (3) During the stage when the liquid level rises from 500 mm to 850 mm, start temperature compensation, maintain the molten steel temperature ≥ 1540 °C, further reduce the casting speed to 70% of the initial value, and keep the liquid level fluctuation ≤ ±10 mm.

[0013] (4) Apply electromagnetic braking with a magnetic field intensity of 0.25 - 0.30 T throughout the process through the electromagnetic braking system.

[0014] Furthermore, in step (1), the inner wall bottom layer of the BN-coated nozzle is a plasma-sprayed BN layer, and the surface layer is a chemical vapor deposition BN layer.

[0015] Furthermore, the laying method of the double-layer covering flux in step (1) is: after the tundish slag is discharged, first lay a 20 mm thick high-alkalinity bottom covering flux, and then stack a 15 mm thick low-melting-point top covering flux.

[0016] Furthermore, the composition of the high-alkalinity bottom covering flux is CaO 45 - 50 wt%, SiO 2 30 - 35 wt%, Al 2 O 3 15 - 20 wt%, and the alkalinity is 1.3 - 1.5;

[0017] The composition of the low-melting-point top covering flux is Na 2 O 20 - 25 wt%, B 2 O 3 60 - 65 wt%, SiO 2 10 - 15 wt%.

[0018] Furthermore, in step (2), the gas composition of the gas curtain protection is a mixed gas of argon and nitrogen, and the volume ratio Ar:N 2 = 7:3, the flow rate is 5 - 8 L / min and the pressure is 0.2 - 0.3 MPa, and the gas curtain injection angle is 30° - 45° inclined downward to cover the tundish molten steel liquid level.

[0019] Furthermore, in step (2), the casting speed formula is: V(t) = V 0 ×[1 - 0.15×(H - 200) / 300];

[0020] Among them, V(t) is the dynamic casting speed, with the unit of m / min, and V 0 is the initial casting speed, with a value range of 1.2 - 1.5 m / min, and H is the real-time liquid level height, with the unit of mm.

[0021] Furthermore, the specific method of temperature compensation in step (3) is as follows: when the liquid level rises to 500 mm, CaC 2 particles are blown in through the tundish stopper rod for temperature compensation, and the addition amount is 0.2 - 0.5 kg / ton of molten steel. Meanwhile, Ce-La alloy rare earth wire is fed at a speed of 1.0 - 1.5 m / min. In the Ce-La alloy rare earth wire, Ce:La = 7:3, and the diameter is 2 mm.

[0022] The present invention has the following beneficial effects:

[0023] In the present invention, slag is discharged first to reduce impurities, and then a double-layer covering agent is added to isolate oxidation, forming a "clean interface + inert environment". The BN-coated nozzle reduces the flow resistance of molten steel, thereby suppressing turbulent slag entrainment and providing a stable foundation for mixed casting. When the liquid level drops to 200 mm, the residual amount of HRB400 in the tundish just meets the control requirements of the transition zone (about 24% of the total amount of mixed casting). At this time, the T63 ladle is opened for pouring. After the new molten steel is injected, HRB400 - T63 gradient mixing (instead of violent mixing) is formed, and the mixed casting interface is reduced through the dynamic casting speed - liquid level coupling control method. With temperature compensation (CaC 2 exothermic + Ce-La alloy modification), the fluidity of the molten steel in the mixed casting transition zone is maintained, and its performance reaches more than 95% of the base material, and it can be directly used as a composite material. At the same time, air is isolated through air curtain protection to inhibit the oxidation of molten steel (reducing the interfacial oxygen content). The air curtain injection angle forms an "air cushion effect", which cooperates with electromagnetic braking to reduce the impact force of the molten steel jet, reduce the flow rate fluctuation, reduce the heat loss of the molten steel, and improve the exothermic efficiency of CaC 2 particles. When the air curtain is opened, the casting speed is synchronously reduced to avoid the failure of the air curtain caused by violent liquid level fluctuations.

[0024] Compared with the existing process, the length of the transition zone is reduced from 3.0 - 3.5 m to 0.5 - 0.8 m, the scrap rejection amount is reduced from 60 - 80 tons / furnace to 10 - 15 tons / furnace, the interfacial oxygen content is reduced from 120 - 150 ppm to 30 - 40 ppm, and the utilization rate of the residual molten steel is increased from 40% to 90%. Specific Embodiments

[0025] In order to make the purpose, technical solutions and advantages of the present invention clearer, the following further elaborates on the present invention in combination with 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.

[0026] Taking the production of steel grade T63 on an 8-strand continuous caster with a size of 165×165mm as an example: A method for reducing the throwing of billets mixed with steel grade T63 and HRB400 includes the following steps:

[0027] (1) Preparation before mixed casting:

[0028] a. Nozzle replacement

[0029] The tapping temperature of steel grade T63 in the refining furnace is increased and controlled at 1565 - 1570°C, and the nozzle is replaced with a 19mm BN-coated nozzle. The inner wall bottom layer of the BN-coated nozzle is a plasma-sprayed BN layer, and the surface layer is a chemical vapor deposition BN layer.

[0030] Plasma spraying BN (boron nitride) on the nozzle substrate can ensure mechanical support by increasing wear resistance and corrosion resistance on the basis of the bonding strength with the nozzle substrate. The chemical vapor deposition BN layer is formed by depositing boron nitride on the bottom layer through chemical vapor deposition, reducing the surface roughness. The top layer structure formed is dense and pore-free, which can block the penetration of molten steel, avoid local melting damage caused by the attachment of inclusions such as alumina, and can reduce the flow resistance of molten steel, thereby suppressing turbulent slag entrainment.

[0031] b. Slag discharge and laying of covering agent

[0032] After the tundish slag is discharged to a slag layer thickness of 35mm, first lay a 20mm-thick high-alkalinity bottom covering agent, and then stack a 15mm-thick low-melting-point top covering agent. The high-alkalinity bottom covering agent consists of 48wt% CaO, 32wt% SiO 2 3, 20wt% Al 2 2O, with an alkalinity of 1.3 - 1.5; the low-melting-point top covering agent consists of 22wt% Na 3 2O, 63wt% B 2 2O, 15wt% SiO 2 2. 3 The high-alkalinity bottom covering agent has a high CaO content and can efficiently adsorb inclusions such as Al 2 2O, MnS, etc. in molten steel, forming a solid solution and floating to the slag layer. The high-alkalinity bottom covering agent remains in a semi-molten state at the tundish molten steel temperature (1540 - 1570°C), which can not only isolate air but also not completely melt and be involved in molten steel. At the same time, the high-alkalinity bottom covering agent has a low thermal conductivity (about 1.2W / m·K), reducing the temperature drop of molten steel from 2°C / min of the traditional covering agent to 0.8°C / min.

[0033] The high-alkalinity bottom covering agent has a relatively high CaO content and can efficiently adsorb inclusions such as Al 2 2O, MnS, etc. in molten steel, making them form a solid solution and floating to the slag layer. The high-alkalinity bottom covering agent remains in a semi-molten state at the tundish molten steel temperature (1540 - 1570°C), which can not only isolate air but also not completely melt and be involved in molten steel. At the same time, the high-alkalinity bottom covering agent has a low thermal conductivity (about 1.2W / m·K), reducing the temperature drop of molten steel from 2°C / min of the traditional covering agent to 0.8°C / min. 3 3

[0034] The low melting point property of the low melting point top covering agent enables it to quickly melt into a liquid state (viscosity 0.1 - 0.5 Pa·s) on the surface of the molten steel, thereby completely isolating oxygen penetration (oxygen diffusion coefficient reduced to 10 -12 m 2 / s), and then being able to quickly form a liquid protective layer. The B 2 O 3 in the low melting point top covering agent reacts with FeO in the molten steel to form a borate glass phase, thereby inhibiting secondary oxidation. At the same time, the liquid slag layer can automatically fill the cracks generated by the liquid level fluctuation, preventing the molten steel from being exposed, and making it have a certain self-healing performance.

[0035] (2) Liquid level 200 - 500 mm stage: After the tundish stops pouring, the liquid level drops to 200 mm, replace the T63 ladle and start pouring while synchronously turning on the gas curtain protection. The gas composition of the gas curtain protection is a mixed gas of argon and nitrogen, and the volume ratio Ar:N 2 = 7:3, the flow rate is 6 L / min and the pressure is 0.25 MPa, and the gas curtain spraying angle is 40° inclined downward to cover the molten steel surface in the tundish.

[0036] The initial drawing speed V 0 is 1.3 m / min. According to the drawing speed formula: V(t) = V 0 ×[1 - 0.15×(H - 200) / 300], dynamically adjust the drawing speed. When the liquid level rises to 500 mm, the drawing speed is reduced to 1.0 m / min.

[0037] (3) Liquid level 500 - 850 mm stage: Blow CaC 2 particles through the tundish stopper rod for temperature compensation, the addition amount is 0.4 kg / ton of molten steel. At the same time, feed the Ce - La alloy rare earth wire at a speed of 1.2 m / min. In the Ce - La alloy rare earth wire, Ce:La = 7:3, the diameter is 2 mm, and the feeding position is the symmetric points on both sides of the long nozzle. At the same time, reduce the drawing speed to 0.9 m / min and keep the liquid level fluctuation ≤ ±10 mm.

[0038] After blowing CaC 2 particles through the tundish stopper rod, it reacts with the molten steel and releases heat, thereby compensating for the temperature drop and maintaining the molten steel temperature ≥ 1540 °C. The rare earth elements can refine the grains and adsorb inclusions, reducing the grain size in the transition zone from 150 μm to below 50 μm. At the same time, using the extremely strong chemical activity of Ce and La, they preferentially react with impurities such as O and S in the molten steel to form high - melting - point compounds and make them float to the slag layer, reducing the number of inclusions in the steel.

[0039] (4) Apply electromagnetic braking with a magnetic field intensity of 0.25 T throughout the process through the electromagnetic braking system.

[0040] The working principle of the present invention is:

[0041] First, slag is discharged to reduce impurities, and then a double-layer covering agent is added to isolate oxidation to form a "clean interface + inert environment". The BN coated nozzle reduces the flow resistance of the molten steel, thereby inhibiting turbulent slag and providing a stable foundation for mixed casting. When the liquid level drops to 200mm, the residual amount of HRB400 in the tundish just meets the control requirements of the transition zone (accounting for about 24% of the total mixed casting). At this time, the T63 ladle pouring is started, and the new molten steel is injected to form a HRB400-T63 gradient mixture (rather than violent mixing), and the mixed casting interface is reduced through the dynamic pulling speed-liquid level coupling control method, and the temperature compensation (CaC 2 Heat release + Ce-La alloy modification) maintains the fluidity of molten steel in the transition zone of mixed casting, and makes its performance reach more than 95% of the parent material, which can be used directly as a composite material. At the same time, the air curtain is used to isolate the air and inhibit the oxidation of molten steel (reduce the oxygen content at the interface). The air curtain spray angle forms an "air cushion effect", which cooperates with electromagnetic braking to reduce the impact force of molten steel injection, reduce flow rate fluctuations, reduce heat loss of molten steel, and improve CaC 2 The heat release efficiency of particles. When the air curtain is turned on, the pulling speed is reduced synchronously to avoid the failure of the air curtain due to violent fluctuations in the liquid level.

[0042] Compared with the existing process, the length of the transition zone is reduced from 3.0-3.5m to 0.5-0.8m, the scrap amount is reduced from 60-80 tons / furnace to 10-15 tons / furnace, the interface oxygen content is reduced from 120-150ppm to 30-40ppm, and the utilization rate of residual molten steel is increased from 40% to 90%.

[0043] The above embodiments are only for describing the preferred implementation of the present invention, and are not intended to 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 ordinary persons in the art to the technical solution of the present invention should fall within the protection scope of the present invention.

[0044] The techniques, shapes, and structural parts not described in detail in the present invention are all well-known techniques.

Claims

1. A method for reducing the casting of mixed casting of steel grades T63 and HRB400, characterized in that: The following steps are involved: (1) Preparation before mixing and pouring: The refining furnace increases the temperature of T63 steel to 1565-1570℃, the nozzle is replaced with a 19mm BN coated nozzle, and after the slag is discharged from the tundish to a slag layer of 20-40mm, a double-layer covering agent is laid; (2) After the middle ladle stops pouring, the liquid level drops to 200 mm. The T63 ladle is replaced and pouring is started. The air curtain protection is turned on simultaneously. The pulling speed is reduced according to the pulling speed formula, so that the liquid level slowly rises to 500 mm. (3) When the liquid level rises from 500 mm to 850 mm, start temperature compensation to maintain the molten steel temperature ≥ 1540 °C, further reduce the casting speed to 70% of the initial value, and keep the liquid level fluctuation ≤ ± 10 mm; (4) The electromagnetic brake system applies electromagnetic braking with a magnetic field strength of 0.25-0.30T throughout the entire process.

2. The method for reducing the mixing of steel grades T63 and HRB400 and casting blanks according to claim 1 is characterized in that: The bottom layer of the inner wall of the BN coating nozzle in step (1) is a plasma sprayed BN layer, and the surface layer is a chemical vapor deposited BN layer.

3. The method for reducing the mixing of steel grades T63 and HRB400 in claim 1 is characterized in that: The laying method of the double-layer covering agent in step (1) is as follows: after the tundish is deslagging, a 20 mm thick high-basicity bottom layer covering agent is first laid, and then a 15 mm thick low-melting-point top layer covering agent is superimposed.

4. The method for reducing the mixing of steel grades T63 and HRB400 in claim 3 is characterized in that: The high basicity bottom layer covering agent comprises 45-50wt% CaO, 30-35wt% SiO2, 15-20wt% Al2O3, and a basicity of 1.3-1.5; The low melting point top layer covering agent comprises Na2O2 0-25wt%, B2O3 60-65wt%, and SiO2 10-15wt%.

5. The method for reducing the mixing of steel grades T63 and HRB400 in claim 1 is characterized in that: The gas component of the gas curtain protection in step (2) is a mixture of argon and nitrogen, with a volume ratio of Ar:N2=7:3, a flow rate of 5-8L / min and a pressure of 0.2-0.3MPa. The gas curtain spraying angle is 30°-45° tilted downward to cover the molten steel surface in the tundish.

6. The method for reducing the mixing of steel grades T63 and HRB400 in claim 1 is characterized in that: The pulling speed formula in step (2) is: V(t)=V0×[1-0.15×(H-200) / 300]; Among them, V(t) is the dynamic pulling speed, the unit is m / min, V0 is the initial pulling speed, the value is 1.2-1.5m / min, and H is the real-time liquid level height, the unit is mm.

7. The method for reducing the mixing of steel grades T63 and HRB400 in claim 1 is characterized in that: The specific method of temperature compensation in step (3) is as follows: when the liquid level rises to 500 mm, CaC2 particles are blown into the middle package plug rod for temperature compensation, and the added amount is 0.2-0.5 kg / ton of steel. At the same time, Ce-La alloy rare earth wire is fed at a speed of 1.0-1.5 m / min. The Ce:La=7:3 in the Ce-La alloy rare earth wire, and the diameter is 2 mm.