Continuous casting production method for medium carbon steel large round billet

Abstract

The invention discloses a continuous casting production method of a medium carbon steel large round billet, which is characterized in that electromagnetic stirring is carried out in the following three solidification stages in the continuous casting process of the large round billet: (1) electromagnetic stirring is carried out at the middle lower part of a crystallizer, and the value range of magnetic field intensity distribution along the center line of the crystallizer is 0-10mT; (2) electromagnetic stirring is conducted at the position where the solidification rate of the casting blank ranges from 65% to 75%, and the value range of magnetic field intensity distribution ranges from 7 mT to 16 mT; and (3) electromagnetic stirring is conducted at the position where the solidification rate of the casting blank is 85%-90%, and the value range of magnetic field intensity distribution is 10-25 mT. The method acts on the casting blank in the appropriate solidification process, appropriate electromagnetic stirring force is applied, and macrosegregation of the large round blank is improved; the electromagnetic field is used for acting on three casting blanks in different solidification stages, on one hand, the flow field in the crystallizer can be improved, and the impact depth is reduced; and on the other hand, electromagnetic stirring is used in a casting flow area to reduce positive segregation of a CET area, the grain size of an equiaxed crystal area is refined, then the macrosegregation degree of a casting blank is reduced, and the carbon range is reduced.

Description

Technical Field

[0001] This invention relates to a continuous casting method, and more particularly to a method for continuous casting of medium carbon steel large round billets. Background Technology

[0002] Macroscopic segregation in continuously cast billets is a typical internal quality defect. Severe macroscopic segregation will affect the uniformity of the final rolled product's microstructure, thereby impacting the steel's service performance. With the development of the domestic economy, the use of large-sized round billets is gradually increasing, and their service environments are more complex, such as ultra-supercritical high-pressure boilers, ultra-low temperature wind power flanges, and high-speed train wheels and axles. Therefore, the requirements for quality control of large round billets, especially in terms of homogeneity, are becoming more stringent.

[0003] Currently, the main method for improving macroscopic segregation in continuous casting of large round billets is to use external electromagnetic equipment to stir the molten steel, thereby reducing superheat and increasing the equiaxed crystal ratio. Although electromagnetic stirring is widely used in continuous casting, and different manufacturers are conducting trial and error experiments to find the optimal stirring parameters such as current and frequency, previous patents or literature have not provided effective quantitative parameters for the magnetic field strength and installation position at different locations on the large round billet. These are the two most critical factors affecting the internal quality of the billet when using electromagnetic stirring equipment.

[0004] The dissertation "Research on the Control of Macroscopic Carbon Segregation in Continuously Cast Large Round Billets" (Doctoral Dissertation, Iron and Steel Research Institute, 2019) proposes a multi-stage electromagnetic stirring method, which uses three electromagnetic devices—"electromagnetic stirring in the crystallizer + electromagnetic stirring in the casting stream + electromagnetic stirring at the end of solidification"—to stir the molten steel. The core of this method is to control the positive segregation in the columnar-to-equiaxed crystal transformation (CET) region by using electromagnetic stirring in the casting stream, thereby reducing the carbon gradient in the cross-section of the large round billet. However, this method does not discuss the magnetic field strength at different locations.

[0005] Publication No. CN114029463A discloses a method for solving the segregation of large round billets in special steel. It uses an "electromagnetic vortex flow + electromagnetic stirring in the crystallizer" approach to act on the molten steel, while simultaneously employing an involute submerged entry nozzle to improve macroscopic segregation. The main theoretical basis of this method is to significantly increase the equiaxed and CET regions and reduce the columnar crystal regions through electromagnetic vortex flow technology, thereby reducing the carbon gradient of the large round billet cross-section. However, it does not discuss the synergistic effect of secondary cooling zone stirring and end-stage stirring.

[0006] The study "Research on the Position of Electromagnetic Stirring at the End of Continuous Casting of 82B Steel Based on Superheat Change" (Journal of Iron and Steel Research, 30(2018), 9:716-722.) suggests that the reasonable installation position of the end-stage electric stirrer is when the solidification rate of the billet is 0.70-0.80. However, the study "Optimization of Electromagnetic Stirring Position at the End of Solidification and Its Influence on the Internal Quality of High Carbon Steel Continuously Cast Round Billets" (Hot Working Technology, 47(2018), 9:34-38.) suggests that the reasonable installation position of the end-stage electric stirrer is when the central solid fraction is 0.10-0.20.

[0007] Previous research and process methods primarily sought optimal process parameters by altering the current and frequency of the electromagnetic stirring equipment. However, different electromagnetic devices, and even the same device at different stages of service, can produce varying electromagnetic field strengths, even with the same current and frequency. Therefore, when the electromagnetic field strength changes, the applied process parameters become ineffective.

[0008] Furthermore, different manufacturers hold differing opinions on the installation locations of the electric agitator and the final agitator in the secondary cooling zone, resulting in varying effects. This indicates that there is still controversy surrounding these two issues. The two-phase zone width and viscosity also differ among different steel grades; therefore, the effectiveness of electromagnetic stirring in continuous casting needs to be considered in conjunction with the specific steel grade and production process. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to provide a method for continuous casting of medium carbon steel large round billets that can effectively improve macroscopic segregation.

[0010] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: during the continuous casting process of large round billets, electromagnetic stirring is performed in the following three solidification stages:

[0011] (1) Electromagnetic stirring is performed in the lower part of the crystallizer, and the magnetic field strength distribution along the center line of the crystallizer ranges from 0 to 10 mT;

[0012] (2) Electromagnetic stirring is performed at a position where the solidification rate of the billet is 65% to 75%, and the magnetic field strength distribution ranges from 7 to 16 mT;

[0013] (3) Electromagnetic stirring is performed at a position where the solidification rate of the billet is 85% to 90%, and the magnetic field strength distribution ranges from 10 to 25 mT.

[0014] Furthermore, during the continuous casting process, the liquid-solid phase boundary of the two-phase region is determined by fs = 0.7.

[0015] Furthermore, during the continuous casting process, the casting speed is 0.21–0.30 m / min, and the superheat is 35–45 °C.

[0016] The beneficial effects of the above technical solution are as follows: This invention employs an electric agitator in the crystallizer and two electromagnetic stirring devices in the casting flow, acting on the billet during a suitable solidification process to apply appropriate electromagnetic stirring force, thereby improving macroscopic segregation in the large round billet. Simultaneously, an electromagnetic field acts on three billets at different solidification stages, which on the one hand improves the flow field within the crystallizer and reduces the impact depth; on the other hand, electromagnetic stirring in the casting flow region reduces positive segregation in the CET zone, refines the grain size in the equiaxed grain zone, and thus reduces the degree of macroscopic segregation in the billet, lowering the carbon gradient. This invention can significantly improve the macroscopic segregation of the billet, increase the uniformity of the billet composition, and control the cross-sectional carbon gradient of the resulting medium-carbon steel large round billet to within 0.045%. Attached Figure Description

[0017] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0018] Figure 1 This is a schematic diagram of the electromagnetic stirring position described in this invention;

[0019] Figure 2 This is a schematic diagram of the carbon range detection and sampling of the cross-section of the large round billet described in this invention.

[0020] In the diagram: the crystallizer electric stirring position is 100; the first stirring position of the casting stream is 200; and the second stirring position of the casting stream is 300. Detailed Implementation

[0021] This method for continuous casting of medium carbon steel large round billets, during the continuous casting process of large round billets... Figure 1 As shown, electromagnetic stirring is performed during the following three solidification stages of the cast billet, and an electromagnetic field is applied to each of these three solidification stages simultaneously during the electromagnetic stirring:

[0022] (1) Electrostatic stirring of the crystallizer: The lower part of the crystallizer is taken as the electrostatic stirring position 100 of the crystallizer. Electromagnetic stirring is performed at the electrostatic stirring position 100 of the crystallizer. The magnetic field strength distribution along the center line of the crystallizer height direction is in the range of 0 to 10 mT.

[0023] (2) First stirring of the casting flow: The position of the casting flow stirring position 200 is the position of 65% to 75% of the solidification rate of the billet. Electromagnetic stirring is performed at the first stirring position 200 of the casting flow. The entire range of this position is covered by the magnetic field. The magnetic field strength distribution ranges from 7 to 16 mT.

[0024] (3) Secondary stirring of the casting flow: The position of the casting flow is 300 when the solidification rate of the billet is 85% to 90%. Electromagnetic stirring is performed at the casting flow secondary stirring position 300. The entire range of this position is covered by the magnetic field, and the magnetic field strength distribution ranges from 10 to 25 mT.

[0025] This continuous casting method for medium carbon steel large round billets uses fs = 0.7 to determine the liquid-solid phase boundary in the two-phase region; that is, when fs ≤ 0.7, it is the liquid phase, and when fs > 0.7, it is the solid phase. This continuous casting method uses a straight-through submerged entry nozzle, a casting speed of 0.21–0.30 m / min, a superheat of 35–45 °C, and a secondary cooling water flow rate of 0.10–0.16 L / kg. This continuous casting method is particularly suitable for producing medium carbon steel large round billets with a cross-section of φ600 mm.

[0026] Example 1: The specific method for continuous casting of medium carbon steel large round billets is as follows.

[0027] (1) The continuous casting section is φ600mm, the steel grade is 42CrMo, and the main components are shown in Table 1.

[0028] Table 1: Main components of the billet in Example 1 (wt%)

[0029] element C Si Mn P S Cr Mo content 0.43 0.32 0.55 0.012 0.004 0.98 0.22

[0030] (2) In the continuous casting process of large round billets, an electromagnetic field is applied to the lower part of the crystallizer, with the magnetic field strength distribution along the centerline of the height direction ranging from 0 to 5 mT; an electromagnetic field is applied to the position where the solidification rate of the billet is 65% to 75% for the first stirring of the casting stream, with the magnetic field strength distribution ranging from 7 to 16 mT; an electromagnetic field is applied to the position where the solidification rate is 85% to 90% for the second stirring of the casting stream, with the electromagnetic field strength ranging from 10 to 17 mT. A straight-through submerged entry nozzle is used, the casting speed is 0.21 m / min, the superheat is 40℃, and the secondary cooling water flow rate is 0.10 L / kg.

[0031] (3) Use a 5mm drill bit to sample drill chips at typical locations on the cross-section of the cast billet, such as... Figure 2 The cross-section of the round billet shown has a radius R of 300 mm. Points 1, 9, 10, and 17 are on the surface (approximately 20 mm from the surface); points 2, 8, 11, and 16 are at 1 / 4 R from the surface; points 3, 7, 12, and 15 are at 1 / 2 R from the surface; points 4, 6, 13, and 14 are at 3 / 4 R from the surface; and point 5 is the center point, totaling 17 points. The carbon-sulfur sample from these locations was analyzed using a carbon-sulfur analyzer to obtain the carbon content. The results are shown in Table 2.

[0032] Table 2: Actual Detection Results of Carbon Element in Example 1 (wt%)

[0033] Serial Number 1 2 3 4 5 6 7 8 9 content 0.419 0.425 0.462 0.433 0.441 0.417 0.438 0.42 0.426 Serial Number 10 11 12 13 14 15 16 17 content 0.429 0.425 0.459 0.441 0.437 0.461 0.42 0.424

[0034] As shown in Table 2, the carbon range of the 42CrMo large round billet obtained in this embodiment is 0.045%.

[0035] Example 2: The specific method for continuous casting of large round carbon steel billets in this paper is as follows.

[0036] (1) The continuous casting section is φ600mm, the steel grade is 50 steel, and the main components are shown in Table 3.

[0037] Table 3: Main components of the test billet / wt%

[0038] element C Si Mn P S content 0.49 0.33 0.52 0.018 0.003

[0039] (2) In the continuous casting process of large round billets, an electromagnetic field is applied to the lower part of the crystallizer, with the magnetic field strength distribution along the centerline of the height direction ranging from 6 to 10 mT; an electromagnetic field is applied to the position where the solidification rate of the billet is 65% to 75% for the first stirring of the casting stream, with the magnetic field strength distribution ranging from 7 to 16 mT; an electromagnetic field is applied to the position where the solidification rate is 85% to 90% for the second stirring of the casting stream, with the electromagnetic field strength ranging from 10 to 25 mT. A straight-through submerged entry nozzle is used, the casting speed is 0.30 m / min, the superheat is 35℃, and the secondary cooling water flow rate is 0.16 L / kg.

[0040] (3) Use a 5mm drill bit to sample drill chips at typical locations on the cross-section of the cast billet, such as... Figure 2 As shown in Table 4, the chip samples were analyzed using a carbon-sulfur analyzer.

[0041] Table 4: Actual Detection Results of Carbon Element in Example 2 (wt%)

[0042] Serial Number 1 2 3 4 5 6 7 8 9 content 0.489 0.480 0.513 0.493 0.484 0.488 0.496 0.475 0.495 Serial Number 10 11 12 13 14 15 16 17 content 0.476 0.474 0.516 0.479 0.473 0.503 0.488 0.486

[0043] As shown in Table 4, the carbon range of the 50 steel round billet obtained in this embodiment is 0.043%.

[0044] Example 3: The specific method for continuous casting of medium carbon steel large round billets is as follows.

[0045] (1) The continuous casting section is φ600mm, the steel grade is 4130X, and the main components are shown in Table 5.

[0046] Table 5: Main components of the test billet / wt%

[0047] element C Si Mn P S Cr Mo Al content 0.30 0.30 0.78 0.012 0.002 1.0 0.23 0.022

[0048] (2) In the continuous casting process of large round billets, an electromagnetic field is applied to the lower part of the crystallizer, with the magnetic field strength distribution along the centerline of the height direction ranging from 6 to 10 mT; an electromagnetic field is applied to the position where the solidification rate of the billet is 65% to 75% for the first stirring of the casting stream, with the magnetic field strength distribution ranging from 7 to 16 mT; an electromagnetic field is applied to the position where the solidification rate is 85% to 90% for the second stirring of the casting stream, with the electromagnetic field strength ranging from 18 to 25 mT. A straight-through submerged entry nozzle is used, the casting speed is 0.25 m / min, the superheat is 45℃, and the secondary cooling water flow rate is 0.13 L / kg.

[0049] (3) Use a 5mm drill bit to sample drill chips at typical locations on the cross-section of the cast billet, such as... Figure 2 As shown in Table 6, the chip samples were analyzed using a carbon-sulfur analyzer.

[0050] Table 6: Actual Detection Results of Carbon Element in Example 3 (wt%)

[0051] Serial Number 1 2 3 4 5 6 7 8 9 content 0.294 0.304 0.313 0.286 0.289 0.282 0.318 0.283 0.301 Serial Number 10 11 12 13 14 15 16 17 content 0.295 0.284 0.325 0.284 0.290 0.321 0.285 0.296

[0052] As shown in Table 6, the carbon range of the 4130X large round billet obtained in this embodiment is 0.043%.

Claims

1. A method of continuous casting of a large round bloom of medium carbon steel, characterized in that, In the continuous casting process of the large round billet, electromagnetic stirring is carried out in the following three solidification stages: (1) electromagnetic stirring is carried out at the lower part of the crystallizer, and the magnetic field intensity distribution along the center line of the crystallizer is in the range of 0-10 mT; (2) electromagnetic stirring is carried out at the position where the solidification rate of the billet is 65%-75%, and the magnetic field intensity distribution is in the range of 7-16 mT; (3) electromagnetic stirring is carried out at the position where the solidification rate of the billet is 85%-90%, and the magnetic field intensity distribution is in the range of 10-25 mT.

2. A method of continuous casting of a large round billet of medium carbon steel according to claim 1, characterized in that: In the continuous casting process, the liquid-solid phase boundary of the two-phase region is determined by fs=0.

7.

3. A method for continuous casting of a large round billet of a medium carbon steel according to claim 1 or 2, characterized in that: In the continuous casting process, the pulling speed is 0.21-0.30 m / min, and the superheat is 35-45℃.

Images