Method for improving quality of continuous casting billet by using ultrasonic waves

Abstract

The invention belongs to the field of ferrous metallurgy, and particularly relates to a method for improving the quality of a continuous casting billet through ultrasonic waves. According to the method, a multi-directional focused ultrasonic field and solidification front edge dynamic regulation and control combined mode is adopted, and electromagnetic stirring phase difference control and cooperative control of a secondary cooling section (fan-shaped section) auxiliary ultrasonic vibrator are combined at the same time. Wherein the composite action mode of the longitudinal standing wave field and the transverse traveling wave field of the multidirectional focused ultrasonic wave field and the solidification front edge dynamic regulation and control mode act on the casting blank, so that the equiaxed crystal rate can be remarkably increased, and the combination of ultrasonic waves and electromagnetic stirring is beneficial to reducing the segregation degree of the casting blank; and the addition of nano aluminum oxide and magnesium oxide mixed particles in the casting powder is beneficial to improving sound wave conduction so as to reduce subcutaneous bubbles. The isometric crystal rate of the finally obtained continuous casting billet is increased to 30%-55%; the center segregation index of the finally obtained continuous casting billet is reduced to 1.05 or below; and the number of subcutaneous bubbles per square meter is less than 0.5.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy, and specifically relates to a method for improving the quality of continuously cast billets using ultrasonic waves. Background Technology

[0002] Continuous casting is currently the most widely used method for obtaining steel billets in industrial production, characterized by high production efficiency, low production cost, product diversification, and stable billet quality. The equiaxed crystal ratio, segregation degree, and subcutaneous bubble rate of continuously cast billets are important factors affecting billet quality.

[0003] The effects of equiaxed crystal ratio: (1) Improve internal density: A high equiaxed crystal ratio can effectively reduce the porosity and looseness inside the billet and improve the internal density of the billet. For example, in the production of some steels that require high strength and high toughness, a higher equiaxed crystal ratio can make the billet better withstand pressure and tension in subsequent processing and reduce the generation of internal defects; (2) Improve the uniformity of mechanical properties: The grain size and shape of equiaxed crystals are relatively consistent, making the mechanical properties of the billet more uniform in all directions, and it is not easy to have problems such as excessive local deformation or cracks, which is conducive to producing steel products with stable performance and reliable quality; (3) Reduce segregation: A higher equiaxed crystal ratio can make the solute elements diffuse and mix more fully during solidification, reduce the degree of segregation, and improve the quality uniformity of the billet. The effects of segregation: (1) Reduced mechanical properties: For example, in high-carbon steel billets, high-carbon central segregation will produce hard martensite and bainite structures during the cooling process, reducing the toughness and ductility of the steel, and making it prone to fracture during subsequent processing; (2) Caused internal cracks: Segregation is often accompanied by central porosity and central cracks, which further reduces the internal density of the billet. During solidification, the segregation of solute elements will lead to uneven solidification shrinkage, resulting in stress concentration inside the billet, which can easily cause internal cracks; Affected processing performance: During rolling, the hardness and strength of the segregated area may be different from other parts, resulting in uneven rolling force and easy rolling defects such as surface cracks and uneven thickness. In addition, segregation may also affect the weldability and heat treatment performance of the steel, making subsequent processing and use difficult. The effects of subcutaneous bubbles: (1) Surface defects: Subcutaneous bubbles will be oxidized on the surface of the heating furnace to form a decarburized layer, which cannot be welded during the rolling process, thus forming defects such as scaly folds, scars, and cracks on the surface of the rolled material, affecting the appearance quality and surface finish of the product. (2) Reduced internal quality: The metal structure around the bubbles may have stress concentration, which can easily cause crack propagation during subsequent processing, and even lead to defects such as delamination inside the billet, seriously affecting the mechanical properties and performance of the billet. For example, when producing thick plates, subcutaneous bubbles may cause the steel plate to fail the flaw detection process, affecting the quality and reliability of the steel plate. (3) Increased production costs: In order to eliminate subcutaneous bubble defects, the billet needs to be ground, which will increase production costs and production cycle. In addition, if the subcutaneous bubble defects are severe, the billet will be scrapped, which will also cause waste of raw materials and energy, further increasing production costs.

[0004] Current methods for increasing the equiaxed crystal ratio mainly include: 1) using weak cooling or air mist cooling in the secondary cooling zone to inhibit columnar crystal growth; 2) using electromagnetic stirring to break up growing columnar crystals; 3) using specially designed nozzles such as multi-hole rotating nozzles to reduce the temperature gradient between the surface and core of the billet, ultimately inhibiting columnar crystal development; 4) controlling low casting speed to allow the molten steel more time to solidify and nucleate in the crystallizer, which is conducive to equiaxed crystal growth. Current methods for reducing segregation include: 1) controlling the superheat of molten steel to avoid excessive solidification time and excessive segregation of solute elements; 2) using high-frequency, small-amplitude vibration to make the billet shell more uniform in the early stage of solidification, reducing the enrichment and segregation of solute elements at the solidification front; 3) using dynamic light reduction technology to promote the diffusion and uniform distribution of solute elements; 4) strengthening the stirring and flow control of the tundish. Current methods for reducing the occurrence rate of subcutaneous bubbles include: 1) optimizing the tundish baking regime; 2) strictly controlling the quality and use of protective slag; 3) improving the purity of molten steel.

[0005] Chinese patent CN113198992B, entitled "A Production Method and Apparatus for Crack-Sensitive Steel Continuous Casting Billets and the Application and Process of Ultrasonic Waves," describes ultrasonic treatment of molten steel upon its entry into the crystallizer. By controlling the casting temperature (TL±10~30℃) and ultrasonic parameters, it aims to reduce the probability of surface cracks in the continuously cast billets and improve crack resistance. However, the ultrasonic tool head in this application is applied to the surface of the molten steel in the crystallizer, and this invention does not improve the segregation problem during solidification.

[0006] Chinese patent application CN104014754A, entitled "A Method for Producing High Manganese Steel Using an Ultrasonic Vibration Continuous Casting Crystallizer," utilizes ultrasonic vibration generated in a horizontal continuous casting crystallizer to gradually shape the molten steel. The ultrasonic vibration force automatically "demolds" the initial billet shell from the inner wall of the crystallizer, thereby eliminating or reducing surface defects in the high manganese steel billet and promoting the development of isometric crystals in the billet. Chinese patent application CN104001881B, entitled "A Method for Producing Stainless Steel Based on an Ultrasonic Vibration Crystallizer," directly penetrates ultrasonic waves into the interior of the stainless steel billet, causing the newly formed metal dendrites inside the billet to break, thus forming numerous heterogeneous nucleation sites within the billet and promoting the development of isometric crystals. Both patents are applicable to horizontal continuous casting and cannot effectively suppress segregation and subcutaneous bubble problems. Summary of the Invention

[0007] To address the problems existing in the background technology, the present invention aims to provide a method for improving the quality of continuously cast billets using ultrasound. Through core hardware (arrangement angle, frequency matching, sound field mode) parameter control + intelligent control + auxiliary oscillator technology, the equiaxed crystal ratio of the final continuously cast billet can be effectively improved, the center segregation index can be reduced, and subcutaneous bubbles can be reduced.

[0008] To achieve the above objectives, the present invention employs the following technical solution:

[0009] A method for improving the quality of continuously cast billets using ultrasound specifically includes the following steps:

[0010] a) Install an array of ultrasonic generators in the crystallizer area and / or secondary cooling zone of the continuous casting machine;

[0011] b) Dynamically match the ultrasonic frequency according to the carbon content of the steel: f = (20 + 80 × C) 2 ) kHz, where C is the carbon content in wt%, and is linearly compensated for with superheat ΔT at 0.5 kHz / ℃, with a power density of 0.5~5 W / cm³. 2 Duty cycle of action time: 30%–80%;

[0012] c) During the solidification process of molten steel, a multi-directional focused ultrasonic field is applied to the billet through the ultrasonic generator array;

[0013] d) Monitor the solidification front position of the billet in real time and dynamically adjust the ultrasonic action area and output parameters to focus the ultrasonic energy in the two-phase region;

[0014] e) Combined field treatment with electromagnetic stirring device, the phase difference between ultrasonic and electromagnetic stirring is dynamically adjusted with the pulling speed: the phase difference Δφ is numerically = π / 4×(1+v / 2), the unit of Δφ is rad, v is the pulling speed, and the unit of v is m / min.

[0015] In step a), the ultrasonic generators in the crystallizer area are arranged in a ring array, with the transducer axis at an angle of 15° to 75° to the center line of the casting flow, and the transducer surface maintaining a distance of 50 to 300 mm from the molten steel surface.

[0016] In step c), the multi-directional focused ultrasonic field includes a combined action mode of longitudinal standing wave field and transverse traveling wave field, with the longitudinal sound pressure amplitude controlled at 0.5 to 2 MPa and the transverse acoustic velocity reaching 0.1 to 0.5 m / s.

[0017] In step d), a solidification front prediction model that fuses infrared thermal imager and ultrasonic echo data is used. When the solidification shell thickness growth rate is lower than a set threshold, the ultrasonic power density in the corresponding area is automatically increased.

[0018] In step e), the electromagnetic stirring device operates at a frequency of 2 to 10 Hz and has a magnetic induction intensity of 0.05 to 0.3 T.

[0019] It also includes setting an auxiliary ultrasonic transducer in the fan-shaped section, with the transducer in direct contact with the fan-shaped section of the billet. The transducer operates at a frequency of 300 to 1 MHz and has a pulse rise time of ≤50 ns, which is used to eliminate micro-cracks on the surface of the billet.

[0020] The ultrasonic generator adopts a pulse modulation mode with a pulse width of 1 to 10 ms, a repetition frequency of 10 to 100 Hz, and a pulse rise time of no more than 0.1 μs.

[0021] While applying the ultrasonic field, 0.1wt% to 0.5wt% of nano-alumina and magnesium oxide mixed particles are added to the protective slag as a sound wave transmission enhancement medium.

[0022] The final continuously cast billet has an equiaxed crystal ratio of 30% to 55%, a center segregation index of less than 1.05, and a subcutaneous bubble count of less than 0.5 per square meter.

[0023] Compared with the prior art, the beneficial effects of the present invention are:

[0024] This invention employs a combination of multi-directional focused ultrasonic fields and dynamic control of the solidification front, along with phase difference control via electromagnetic stirring and coordinated control of the secondary cooling section (fan-shaped section) assisted ultrasonic transducer. The combined effect of the longitudinal standing wave field and transverse traveling wave field of the multi-directional focused ultrasonic field, along with the dynamic control of the solidification front, significantly improves the equiaxed crystal ratio of the cast billet. The combination of ultrasonic waves and electromagnetic stirring helps reduce the degree of segregation in the cast billet. Adding a mixture of nano-alumina and magnesium oxide particles to the protective slag improves sound wave transmission, thereby reducing subcutaneous bubbles. Specifically, it has the following beneficial effects:

[0025] (1) The equiaxed crystal ratio of the final continuously cast billet is increased to 30%–55%;

[0026] (2) The center segregation index of the final continuously cast billet was reduced to below 1.05;

[0027] (3) The number of subcutaneous air bubbles per square meter is less than 0.5. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the specific implementation methods of this invention will be further described below in conjunction with the embodiments. The following embodiments are used to specifically illustrate the content of this invention. These embodiments are only general descriptions of the content of this invention and do not limit the content of this invention.

[0029] Comparative Example 1: Single-frequency ultrasonic treatment;

[0030] The steel grade is peritectic steel (C=0.16wt%), small square billet with a cross-sectional size of 160mm×160mm, drawing speed of 1.2~1.4m / min, and crystallizer water flow rate of 110~115m³ / min. 3 / h, secondary cooling section specific water flow rate 0.4~0.5 L / kg. Process conditions are no electromagnetic stirring, no nano-alumina and magnesium oxide mixed particles added to the protective slag. Specific steps are as follows: 1) Set up an ultrasonic generator array in the crystallizer area of ​​the continuous casting machine. The ultrasonic array arrangement angle in the crystallizer area is 45°, and the transducer surface is kept 150mm away from the molten steel surface. The ultrasonic field is a single frequency setting without dynamic adjustment. Specific setting parameters are: frequency = 60kHz (fixed value), power density = 5W / cm², duty cycle = 80%, casting speed 1.2m / min.

[0031] The technical results are as follows: the final slab has an equiaxed crystal ratio of 24%; a center segregation index of 1.20; and a subcutaneous bubble count of 1.2 per m².

[0032] Comparative Example 2: No ultrasonic treatment;

[0033] The steel grade is high-carbon steel (C=0.7wt%) slab with a cross-sectional dimension of 900mm×100mm. The casting speed is 1.8~2.0m / min. The process conditions are: no ultrasonic treatment, no electromagnetic stirring, and no addition of nano-alumina and magnesium oxide mixed particles to the protective slag. Adjustment is solely based on cooling intensity, with a crystallizer water flow rate of 110~115 m³ / min. 3 / h, the specific water flow rate of the secondary cooling section is 0.7~0.8 L / kg.

[0034] The technical results are as follows: the final slab has an equiaxed crystal ratio of 22%; a center segregation index of 1.30; and a subcutaneous bubble count of 1.6 per m².

[0035] Example 1: Ultrasonic treatment of peritectic steel billet;

[0036] The steel grade is peritectic steel small square billet (C=0.16wt%), with dimensions of 160mm×160mm, a drawing speed of 1.2~1.4m / min, and a crystallizer water flow rate of 110~115 m³ / min. 3 / h, the specific water volume of the secondary cooling section is 0.4~0.5 L / kg. The process conditions are to apply ultrasonic and electromagnetic stirring, and to add 0.5% by mass of nano-alumina and magnesium oxide mixed particles to the protective slag. The specific steps are as follows: (1) Arrange a ring ultrasonic array in the crystallizer area of ​​the continuous casting machine, and apply a multi-directional focused ultrasonic field to the billet through the ultrasonic generator array. The transducer axis is at a 15° angle to the center line of the casting flow, and the spacing is 300mm. The specific ultrasonic parameters are: frequency f=37kHz [according to f=20+80×0.16 2[Calculated with +0.5 × 30℃ (superheat)], power density = 5 W / cm³ 2 Duty cycle = 80%; longitudinal sound pressure amplitude of ultrasonic field = 0.5MPa, transverse sound velocity = 0.5m / s, ultrasonic pulse width = 1ms, repetition frequency = 100Hz, pulse rise time = 0.1μs. (2) Real-time monitoring of solidified shell thickness, when the growth rate is less than 0.8mm / min, automatically increase power density to 6W / cm. 2 (3) The electromagnetic stirring parameters are: frequency = 2Hz, magnetic induction intensity = 0.3T, and the phase difference between the ultrasonic wave and the electromagnetic stirring is = 2π / 5~17π / 40 [calculated according to Δφ = π / 4×(1+v / 2)]. (4) The frequency of the fan-shaped auxiliary ultrasonic transducer is 300kHz, and the rise time of the pulse front is 50ns.

[0037] The technical effects are as follows: the equiaxed crystal ratio of the final continuously cast billet is increased to 55%, the center segregation index is reduced to 1.02, and the number of subcutaneous bubbles per square meter is 0.4.

[0038] Example 2: Ultrasonic treatment of high-carbon steel continuous casting billets;

[0039] The steel grade is high-carbon steel slab (C=0.7wt%), with a cross-sectional dimension of 900mm×100mm, and the crystallizer water flow rate is 110~115 m³ / h. 3 / h, the specific water volume of the secondary cooling section is 0.7~0.8 L / kg. The process conditions are to apply ultrasonic and electromagnetic stirring, add 0.1% by mass of nano-alumina and magnesium oxide mixed particles to the protective slag, and the casting speed is 1.8~2.0m / min. The specific steps are as follows: (1) Arrange a ring ultrasonic array in the crystallizer area of ​​the continuous casting machine, and apply a multi-directional focused ultrasonic field to the billet through the ultrasonic generator array. The transducer axis is at a 75° angle to the center line of the casting flow, and the spacing is 50mm. The specific ultrasonic parameters are: frequency = 69kHz [according to f = 20 + 80 × 0.7 2 [Calculated using +0.5 × 20℃ (superheat)], power density = 0.5 W / cm³ 2 , Duty cycle = 30%; longitudinal sound pressure amplitude of ultrasonic field = 2MPa, transverse sound velocity = 0.1m / s, ultrasonic pulse width = 10ms, repetition frequency = 10Hz, pulse rise time = 0.08μs. (2) Real-time monitoring of solidified shell thickness, when the speed increase is less than 0.7mm / min, the power density is automatically increased to 3W / cm². (3) Electromagnetic stirring parameters are: frequency = 2Hz, magnetic induction intensity = 0.3T, phase difference between ultrasonic waves and electromagnetic stirring = 19π / 40~1π / 2 [calculated according to Δφ = π / 4×(1+v / 2)]. (4) The frequency of the fan-shaped auxiliary ultrasonic transducer is 500MHz, and the pulse rise time is 40ns.

[0040] The technical effects are as follows: the equiaxed crystal ratio of the final continuously cast billet is increased to 30%, the center segregation index is reduced to 1.03, and the number of subcutaneous bubbles per square meter is 0.3.

[0041] Example 3: Ultrasonic treatment of medium carbon steel continuous casting billets;

[0042] The steel grade is medium carbon steel small square billet (C=0.45wt%), with dimensions of 160mm×160mm, and the crystallizer water flow rate is 110~115m³ / h. 3 / h, the specific water volume of the secondary cooling section is 0.4~0.5L / kg, and the casting speed is 2.4~2.6m / min. The process conditions are to apply ultrasonic and electromagnetic stirring, add 0.3% by mass of nano-alumina and magnesium oxide mixed particles to the protective slag, and the casting speed is 2.0~2.2m / min. The specific steps are as follows: (1) Arrange a ring ultrasonic array in the crystallizer area of ​​the continuous casting machine, and apply a multi-directional focused ultrasonic field to the billet through the ultrasonic generator array. The transducer axis is at a 45° angle to the center line of the casting flow, and the spacing is 1800mm. The specific ultrasonic parameters are: frequency = 49kHz [according to f = 20 + 80 × 0.45 2 [Calculated with +0.5 × 25℃ (superheat)], power density = 3 W / cm³ 2 Duty cycle = 55%; longitudinal sound pressure amplitude of ultrasonic field = 1.3MPa, transverse sound velocity = 0.3m / s, ultrasonic pulse width = 5ms, repetition frequency = 60Hz, pulse rise time = 0.07μs. (2) Real-time monitoring of solidified shell thickness, when the growth rate is less than 0.75mm / min, automatically increasing the power density to 3.5W / cm. 2 (3) The electromagnetic stirring parameters are: frequency = 6Hz, magnetic induction intensity = 0.15T, and the phase difference between the ultrasonic wave and the electromagnetic stirring is = 1π / 2~21π / 40 [calculated according to Δφ = π / 4×(1+v / 2)]. (4) The frequency of the fan-shaped auxiliary ultrasonic transducer is 700kHz, and the rise time of the pulse front is 35ns.

[0043] The technical effects are as follows: the equiaxed crystal ratio of the final continuously cast billet is increased to 42%, the center segregation index is reduced to 1.04, and the number of subcutaneous bubbles per square meter is 0.4.

[0044] Table 1 Comparison of data from the examples and comparative examples.

[0045]

[0046] Comparative Example 1 demonstrates that the effect of single-frequency ultrasound is extremely limited. Comparative Example 2 shows that traditional processes cannot achieve the effect of this invention. Combined with the data in Table 1, it can be seen that after adopting this invention, the equiaxed crystal ratio, center segregation, and subcutaneous bubble problems of the continuously cast billet are significantly improved.

Claims

1. A method for improving the quality of a continuously cast billet by means of ultrasound, characterized in that, Specifically comprising the following method steps: a) arranging an ultrasonic generator array in the crystallizer area and / or secondary cooling section of a continuous casting machine; b) Dynamically match the ultrasonic frequency according to the carbon content of the steel: f = (20 + 80 × C) 2 ) kHz, where C is the carbon content in wt%, and is linearly compensated for with superheat ΔT at 0.5 kHz / ℃, with a power density of 0.5~5 W / cm³. 2 Duty cycle of action time: 30%–80%; c) applying a multi-directional focused ultrasonic field to the casting blank through the ultrasonic generator array during the solidification of the molten steel; d) monitoring the position of the solidification front of the casting blank in real time, and dynamically adjusting the ultrasonic action area and output parameters so that the ultrasonic energy is focused in the two-phase zone; e) combining the electromagnetic stirring device for composite field processing, and dynamically adjusting the phase difference between the ultrasonic wave and the electromagnetic stirring according to the casting speed: the phase difference Δφ is equal to π / 4×(1+v / 2) in numerical value, and the unit of Δφ is rad, and v is the casting speed, and the unit of v is m / min.

2. The method for improving the quality of a continuous casting billet using ultrasonic waves according to claim 1, characterized by: In the step a), the ultrasonic generator in the crystallizer area is arranged in a ring array, the axis of the transducer is at an angle of 15°-75° to the center line of the casting flow, and the surface of the transducer is kept at a distance of 50-300 mm from the molten steel surface.

3. The method for improving the quality of a continuously cast slab using ultrasonic waves according to claim 1, characterized in that: In the step c), the multi-directional focused ultrasonic field includes a complex action mode of longitudinal standing wave field and transverse traveling wave field, the longitudinal acoustic pressure amplitude is controlled at 0.5-2 MPa, and the transverse acoustic flow velocity reaches 0.1-0.5 m / s.

4. The method for improving the quality of a continuously cast slab using ultrasonic waves according to claim 1, characterized in that: In the step d), a solidification front prediction model is used to fuse the infrared thermal imaging and ultrasonic echo data, and when the solidification shell thickness growth rate is lower than the set threshold, the ultrasonic power density of the corresponding area is automatically enhanced.

5. The method for improving quality of a continuous casting billet using ultrasonic waves according to claim 1, characterized in that: In the step e), the working frequency of the electromagnetic stirring device is 2-10 Hz, and the magnetic induction intensity is 0.05-0.3 T.

6. The method for improving quality of a continuous casting billet using ultrasonic waves according to claim 1, wherein: An auxiliary ultrasonic vibrator is arranged in the sector, the vibrator is in direct contact with the casting blank of the sector, the working frequency of the vibrator is 300 kHz-700 kHz, and the pulse front rising time is ≤50 ns.

7. The method for improving quality of a continuous casting billet using ultrasonic waves according to claim 1, wherein: The ultrasonic generator adopts a pulse modulation mode, the pulse width is 1-10 ms, the repetition frequency is 10-100 Hz, and the pulse front rising time is not more than 0.1 μs.

8. The method for improving the quality of a continuously cast slab using ultrasonic waves according to claim 1, characterized in that: While applying the ultrasonic field, 0.1wt%-0.5wt% of mixed nanoparticles of alumina and magnesium oxide are added to the protective slag as a sound conduction enhancement medium.

9. The method for improving quality of a continuous casting billet using ultrasonic waves according to claim 1, wherein: The equiaxed crystal ratio of the final continuous casting blank is increased to 30%-55%, the center segregation index is reduced to less than 1.05, and the number of subcutaneous gas bubbles per square meter is less than 0.5.

Images