A method for improving center carbon segregation and cross-sectional carbon distribution uniformity of continuous casting large round billets

Abstract

The present application relates to a method for improving center carbon segregation and cross-section carbon distribution uniformity of continuous casting large round billet, and belongs to the technical field of continuous casting of steel. In view of the problem that the existing unidirectional electromagnetic stirring technology cannot effectively solve the problem of large carbon difference of cross-section of large round billet, the core of the present application lies in adopting crystallizer alternating electromagnetic stirring technology, that is, by controlling the electromagnetic stirrer to alternately operate in the positive and negative directions according to a certain period, breaking the stable unidirectional rotating flow field, enhancing the stirring of the center area molten steel, and promoting the uniform distribution of solute elements. The present application can significantly eliminate the stirring dead zone, weaken the band segregation, thereby reducing the carbon difference of the cross-section of the casting billet by more than 40%, greatly improving the internal quality uniformity of the large round billet, and meeting the harsh requirements of high-end equipment manufacturing on base materials.

Description

Technical Field

[0001] This invention relates to the field of continuous casting technology for metallic materials, and particularly to the field of continuous casting technology for steel, specifically to a method for improving the uniformity of carbon segregation in the center and carbon distribution in the cross section of continuously cast large round billets (typically ≥ Φ380 mm in diameter). Background Technology

[0002] Continuously cast large round billets are the base material for manufacturing key components such as high-end wind turbine bearings, large marine crankshafts, high-pressure boiler tubes, train wheels, and axles. They require extremely high uniformity in internal quality. Carbon segregation is one of the most common internal defects in continuously cast billets, especially in large-section round billets. Due to the difficulty of feeding the molten steel at the end of solidification, the flow of molten steel enriched with solute between dendrites leads to severe carbon enrichment in the center of the billet and in the V-shaped segregation zone.

[0003] The carbon gradient of the billet cross section (i.e., the difference between the highest and lowest carbon content in the cross section) is a key indicator for measuring the degree of carbon segregation. An excessively large carbon gradient will lead to uneven microstructure and properties, large differences in hardness, and even cracks during subsequent heat treatment of the forgings, seriously affecting the fatigue life and safety reliability of the product.

[0004] Currently, in the steel industry, when producing continuously cast large round billets (typically ≥ Φ380mm in diameter), the ladle on the continuous casting ladle turret is usually connected to a long nozzle. The molten steel from the ladle is diverted to the tundish through the long nozzle, and then distributed to the crystallizer through multiple internal submerged entry nozzles installed in the tundish. To improve the solidification process of the molten steel in the crystallizer and enhance the quality of the continuously cast round billets, the industry commonly uses a ring-shaped electromagnetic stirrer (M-EMS) installed around the copper tubes of the crystallizer. Traditional M-EMS uses continuous rotational stirring in a single direction (clockwise or counterclockwise). By generating a rotating magnetic field within the crystallizer, it drives the molten steel to rotate, breaking up primary dendrites and expanding the equiaxed crystal zone, thereby mitigating central porosity and shrinkage cavities to some extent.

[0005] However, for large round billets, traditional unidirectional continuous M-EMS has the following limitations: 1. Stirring dead zone: Under a unidirectional rotating flow field, the molten steel flow velocity in the central region of the billet is low, forming a relatively stagnant "stirring dead zone", which has a limited effect on improving central segregation.

[0006] 2. Banded segregation: Strong unidirectional rotation may cause the enriched liquid at the solidification front to be concentrated and pushed to a certain area, which in turn forms a new, regular banded segregation, deteriorates the uniformity of carbon distribution in the cross section, and results in a persistently high carbon range.

[0007] 3. Bright white band: The stirring intensity and the formation of bright white band (negative segregation band) are contradictory, and it is difficult to achieve both with a single stirring parameter.

[0008] Therefore, developing a novel electromagnetic stirring method that can effectively break the stirring dead zone, homogenize the cross-sectional composition, and significantly reduce the carbon polarity is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0009] To address the aforementioned problems, this invention discloses a method for improving carbon segregation at the center and uniformity of carbon distribution across the cross-section of continuously cast large round billets. The specific technical solution is as follows: A method for improving the uniformity of carbon segregation at the center and carbon distribution across the cross-section of a continuously cast large round billet, wherein the diameter of the continuously cast large round billet is φ380-800mm, and alternating electromagnetic stirring is performed on the molten steel during the crystallization stage of continuous casting; the alternating electromagnetic stirring refers to the electromagnetic stirrer switching between forward and reverse rotation according to a preset cycle, wherein the stirring duration in a single direction of the alternating electromagnetic stirring, i.e., the alternation cycle T, is 10 seconds to 60 seconds, and there is a current interruption of 1 second to 10 seconds during the switching process between forward and reverse rotation.

[0010] Furthermore, the steel grade of the continuously cast large round billet is medium carbon chromium-molybdenum steel, wherein the mass content of carbon, chromium and molybdenum elements is: C: 0.30-0.55%, Cr: 0.80-1.80%, Mo: 0.15-0.60%.

[0011] Furthermore, the alternation period T is as follows: when 380≤φ<500mm, then 10≤T<25 seconds; when 500≤φ<700mm, then 25 seconds≤T<40 seconds; when 700≤φ≤800mm, then 40 seconds≤T≤60 seconds.

[0012] Furthermore, the electromagnetic stirring current and frequency of the crystallizer are as follows: when 380≤φ<500mm, the electromagnetic stirring current of the crystallizer is 250-340A and the frequency is 1.5-2.5Hz; when 500≤φ≤800mm, the electromagnetic stirring current of the crystallizer is 150-260A and the frequency is 1.0-1.5Hz.

[0013] Furthermore, the superheat and drawing speed are as follows: the superheat of the molten steel in the tundish is 10-40℃; when 380≤φ<450mm, the drawing speed is 0.50-0.58m / min; when 450≤φ<500mm, the drawing speed is 0.40-0.48m / min; when 500≤φ<600mm, the drawing speed is 0.32-0.40m / min; when 600≤φ<700mm, the drawing speed is 0.28-0.34m / min; when 700≤φ<800mm, the drawing speed is 0.21-0.25m / min; and when φ=800mm, the drawing speed is 0.16-0.20m / min.

[0014] Furthermore, the stirring intensity is controlled to cause a fluctuation of 2mm to 10mm in the meniscus of the molten steel in the crystallizer.

[0015] Furthermore, the electromagnetic stirrer is arranged around the crystallizer, and the upper surface of the electromagnetic stirrer is 400-450mm away from the horizontal position of the upper flange of the crystallizer.

[0016] The continuously cast large round billet was prepared by the above-mentioned method to improve the uniformity of carbon segregation in the center and carbon distribution in the cross section.

[0017] Furthermore, the carbon difference from the surface layer to 1 / 2R (radius) of the cross-section of the continuously cast large round billet is no greater than 0.025%, and the carbon difference from the surface layer to 3 / 4R (radius) is no greater than 0.050%.

[0018] The beneficial effects of this invention are: 1. This invention breaks the symmetry of the flow field and eliminates the stirring dead zone: By periodically changing the rotation direction of the molten steel, the stable and symmetrical flow field formed by traditional unidirectional stirring is completely broken. This unsteady flow field can generate stronger turbulent kinetic energy, effectively penetrating the "stirring dead zone" in the center of the billet, so that the enriched molten steel in this area is fully stirred and mixed, thereby significantly improving the central carbon segregation.

[0019] 2. This invention homogenizes the cross-sectional composition and reduces the carbon gradient: alternating forward and reverse stirring prevents the enriched solute liquid from being continuously pushed to a fixed area of ​​the solidified shell, promoting a more dispersed and uniform distribution of solute elements across the entire cross-section of the billet. This directly leads to a significant reduction in the carbon gradient of the billet's cross-section.

[0020] 3. This invention weakens banded segregation: The alternating flow field disrupts the conditions for the formation of banded segregation, making the bright white band of negative segregation blurry and discontinuous, thus reducing its negative impact.

[0021] 4. This invention expands the equiaxed crystal region: strong alternating convection effectively breaks down dendrites, increases the number of crystal nuclei, promotes the growth of equiaxed crystals, and thus further inhibits the formation of channel segregation. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the electromagnetic stirring operation of the continuous casting round billet crystallizer of the present invention.

[0023] Figure 2 This is a comparative test result diagram of the carbon difference from the surface layer to 1 / 2R (radius) of the cross-section of a Φ380mm 42CrMo4 continuously cast round billet in an electromagnetically stirred crystallizer according to Embodiment 1 of the present invention. Figure 3This is a diagram showing the results of a comparative test on the carbon difference from the surface layer to 3 / 4R (radius) of a Φ380mm 42CrMo4 continuous casting round billet crystallizer using electromagnetic stirring in Embodiment 1 of the present invention.

[0024] Figure 4 This is a comparative test result diagram of the carbon difference from the surface layer to 1 / 2R (radius) of the cross-section of a Φ600mm 50CrMo steel continuously cast round billet in an electromagnetically stirred crystallizer according to Embodiment 2 of the present invention. Figure 5 This is a diagram showing the comparative test results of the carbon difference from the surface layer of the cross-section of a Φ600mm 50CrMo steel continuous casting round billet to 3 / 4R (radius) in an electromagnetic stirring mold for crystallizer, according to Embodiment 2 of the present invention. Detailed Implementation

[0025] The present invention will be further illustrated below with reference to the accompanying drawings and specific embodiments. It should be understood that the following specific embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0026] See Figure 1 This invention provides a specific connection layout example: the intelligent control center is equipped with a human-machine interface touch screen for setting the parameters and displaying the status of the electromagnetic stirrer; the PLC center is equipped with a dedicated controller for controlling the operating parameters of the electromagnetic stirrer and issuing timing control commands to drive the three-phase frequency converter to control the operating status of the electromagnetic stirrer (M-EMS).

[0027] Molten steel from the tundish is injected into the crystallizer assembly through an immersion nozzle. The molten steel is periodically stirred in both directions by an electromagnetic stirrer in the crystallizer, and finally enters the continuous casting machine to be cast into a large round billet.

[0028] Example 1

[0029] Take the production of Φ380mm 42CrMo4 steel round billets as an example.

[0030] 1. Equipment Configuration: As attached Figure 1 As shown, the continuous casting system includes a tundish, a crystallizer, an electromagnetic stirrer for the crystallizer, and a frequency conversion control system connected to the electromagnetic stirrer. This frequency conversion control system is programmed to implement alternating stirring logic.

[0031] 2. Process parameters: Throwing speed: 0.54 m / min Superheat: 28℃ Electromagnetic stirring mode: alternating stirring 1) Alternating stirring parameter settings: Forward mixing time: 25 seconds Reverse mixing time: 25 seconds Forward / reverse switching interval: 5 seconds Stirring current: 300A (equivalent to causing a ripple of about 5mm on the meniscus) 2) Comparative experiment: Under the same process conditions, a comparison was made between traditional unidirectional continuous stirring (current 300A) and the alternating stirring of the present invention.

[0032] 3. Results: For the cross-sections of the cast billets produced by both methods, drill cuttings were sampled and analyzed at distances of 10mm, 20mm, 1 / 8R, 1 / 4R, 1 / 2R, and 3 / 4R from the surface of the billet. The carbon content at these points was statistically analyzed, and the results are attached. Figure 2 and 3 As shown.

[0033] Traditional unidirectional continuous stirring: The maximum carbon difference from the surface of the billet cross section to 1 / 2R (radius) is 0.054%, and the maximum carbon difference from the surface to 3 / 4R (radius) is 0.065%.

[0034] The alternating stirring of this invention results in a maximum carbon difference of 0.024% from the surface layer to 1 / 2R (radius) of the billet cross section, and a maximum carbon difference of 0.036% from the surface layer to 3 / 4R (radius).

[0035] The results show that, by using the alternating electromagnetic stirring method of the present invention, the carbon difference of the billet cross section was successfully reduced by 50% under the condition that other process conditions remained unchanged, and the carbon segregation level of the billet center was also improved from grade 1.5 (poor) to below grade 1.0 (excellent), which significantly improved the internal quality uniformity of the billet.

[0036] Example 2

[0037] Take the production of Φ600mm 50CrMo steel round billets as an example.

[0038] 1. Equipment Configuration: As attached Figure 1 As shown, the continuous casting system includes a tundish, a crystallizer, an electromagnetic stirrer for the crystallizer, and a frequency conversion control system connected to the electromagnetic stirrer. This frequency conversion control system is programmed to implement alternating stirring logic.

[0039] 2. Process parameters: Throwing speed: 0.32 m / min Superheat: 25℃ Electromagnetic stirring mode: alternating stirring 1) Alternating stirring parameter settings: Forward mixing time: 30 seconds Reverse mixing time: 30 seconds Forward / reverse switching interval: 4 seconds Stirring current: 250A (equivalent to causing a ripple of about 4mm on the meniscus) 2) Comparative experiment: Under the same process conditions, a comparison was made between traditional unidirectional continuous stirring (current 250A) and the alternating stirring of the present invention.

[0040] 3. Results: For the cross-sections of the cast billets produced by both methods, drill cuttings were sampled and analyzed at distances of 10mm, 20mm, 1 / 8R, 1 / 4R, 1 / 2R, and 3 / 4R from the surface of the billet. The carbon content at these points was statistically analyzed, and the results are attached. Figure 4 and 5 As shown.

[0041] Traditional unidirectional continuous stirring: The maximum carbon difference from the surface of the billet cross section to 1 / 2R (radius) is 0.059%, and the maximum carbon difference from the surface to 3 / 4R (radius) is 0.067%.

[0042] The alternating stirring of this invention results in a maximum carbon difference of 0.023% from the surface layer to 1 / 2R (radius) of the billet cross section and a maximum carbon difference of 0.032% from the surface layer to 3 / 4R (radius).

[0043] The results show that, by using the alternating electromagnetic stirring method of the present invention, the carbon difference of the billet cross section was reduced by more than 50% under the condition that other process conditions remained unchanged, and the carbon segregation level of the billet center was also improved from grade 1.5 (poor) to below grade 1.0 (excellent), which significantly improved the internal quality uniformity of the billet.

[0044] Those skilled in the art will understand that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. It should also be understood that terms such as those defined in general dictionaries should be understood to have the same meaning as in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless defined as herein.

[0045] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for improving the uniformity of carbon segregation at the center and carbon distribution across the cross-section of a continuously cast large round billet, characterized in that, The diameter φ of the continuously cast large round billet is 380-800mm. During the crystallization stage of continuous casting, the molten steel is subjected to alternating electromagnetic stirring. The alternating electromagnetic stirring refers to the electromagnetic stirrer switching between forward and reverse rotation according to a preset cycle. The stirring duration in a single direction of the alternating electromagnetic stirring, i.e., the alternation cycle T, is 10 seconds to 60 seconds. During the switching between forward and reverse rotation, there is a current interval of 1 second to 10 seconds.

2. The method for improving the uniformity of carbon segregation at the center and the cross-sectional carbon distribution of continuously cast large round billets according to claim 1, characterized in that, The steel grade of the continuously cast large round billet is medium carbon chromium-molybdenum steel, wherein the mass content of carbon, chromium and molybdenum elements is: C: 0.30-0.55%, Cr: 0.80-1.80%, Mo: 0.15-0.60%.

3. The method for improving the uniformity of carbon segregation at the center and the cross-sectional carbon distribution of continuously cast large round billets according to claim 1, characterized in that, The alternation period T is as follows: when 380≤φ<500mm, then 10≤T<25 seconds; when 500≤φ<700mm, then 25 seconds≤T<40 seconds; when 700≤φ≤800mm, then 40 seconds≤T≤60 seconds.

4. The method for improving the uniformity of carbon segregation at the center and the cross-sectional carbon distribution of continuously cast large round billets according to claim 1, characterized in that, The electromagnetic stirring current and frequency of the crystallizer are as follows: when 380≤φ<500mm, the electromagnetic stirring current of the crystallizer is 250-340A and the frequency is 1.5-2.5Hz; when 500≤φ≤800mm, the electromagnetic stirring current of the crystallizer is 150-260A and the frequency is 1.0-1.5Hz.

5. The method for improving the uniformity of carbon segregation at the center and the cross-sectional carbon distribution of continuously cast large round billets according to claim 1, characterized in that, The superheat and drawing speed are as follows: the superheat of the molten steel in the ladle is 10-40℃; when 380≤φ<450mm, the drawing speed is 0.50-0.58m / min; when 450≤φ<500mm, the drawing speed is 0.40-0.48m / min; when 500≤φ<600mm, the drawing speed is 0.32-0.40m / min; when 600≤φ<700mm, the drawing speed is 0.28-0.34m / min; when 700≤φ<800mm, the drawing speed is 0.21-0.25m / min; and when φ=800mm, the drawing speed is 0.16-0.20m / min.

6. The method for improving the uniformity of carbon segregation at the center and the cross-sectional carbon distribution of continuously cast large round billets according to claim 1, characterized in that, The stirring intensity is controlled to produce a fluctuation of 2mm to 10mm in the meniscus of the molten steel in the crystallizer.

7. The method for improving the uniformity of carbon segregation at the center and carbon distribution across the cross section of a continuously cast large round billet according to claim 1, characterized in that, The electromagnetic stirrer is arranged around the crystallizer, and the upper surface of the electromagnetic stirrer is 400-450mm away from the horizontal position of the upper flange of the crystallizer.

8. A continuously cast large round billet prepared by the method described in any one of claims 1-7 for improving the uniformity of carbon segregation at the center and carbon distribution across the cross section of the continuously cast large round billet.

9. The continuously cast large round billet according to claim 8, characterized in that, The carbon difference between the surface layer and 1 / 2R (radius) of the cross-section of the continuously cast large round billet is no greater than 0.025%, and the carbon difference between the surface layer and 3 / 4R (radius) is no greater than 0.050%.

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